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Analysis of human satellite cell dynamics on cultured adult skeletal muscle myofibers

Analysis of human satellite cell dynamics on cultured adult skeletal muscle myofibers Background: Maintaining stem cells in physiologically relevant states is necessary to understand cell and context- specific signalling paradigms and to understand complex interfaces between cells in situ. Understanding human stem cell function is largely based on tissue biopsies, cell culture, and transplantation into model organisms. Methods: Here, we describe a method to isolate post-mortem intact human muscle myofibers and culture muscle stem cells within the niche microenvironment to assay cellular dynamics, stem cell identity, stem cell hierarchy, and differentiation potential. Results: We show human myofiber culture maintains complex cell-cell contacts and extracellular niche composition during culture. Human satellite cells can be cultured at least 8 days, which represents a timepoint of activation, differentiation, and de novo human myofiber formation. We demonstrate that adult human muscle stem cells undergo apicobasal and planar cell divisions and express polarized dystrophin and EGFR. Furthermore, we validate that stimulation of the EGFR pathway stimulates the generation of myogenic progenitors and myogenic differentiation. Conclusions: This method provides proof of principle evidence for the use of human muscle to evaluate satellite cell dynamics and has applications in pre-clinical evaluation of therapeutics targeting muscle repair. Keywords: Human skeletal muscle, Satellite cell, Muscle stem cell, Myofiber culture Background Changes to satellite cell-intrinsic signalling, satellite Skeletal muscle is a complex tissue, responsible for cell-niche interactions, or the regenerative context in mobility, thermoregulation, and breathing. Skeletal conditions such as ageing or muscular dystrophy alter muscle maintenance, growth, and repair are facilitated the kinetics of muscle repair (reviewed in [2]). by muscle resident stem cells (satellite cells) which Improved modeling of human satellite cell dynamics reside within skeletal muscle, resting within a special- in a physiologically relevant context will improve our ized cleft underneath the basal lamina [1]. Muscle understanding of signalling pathways pertinent to homeostasis requires a balance between satellite cell muscle repair in humans. self-renewal and differentiation to facilitate efficient With age, progressive loss of satellite cell number is repair over time and in response to injury [2]. observed with functional decline in skeletal muscle [3, 4]. In aged mice, extrinsic changes to soluble ligands in the niche [5–7], increased fibrosis [8], and intrinsic changes in * Correspondence: mrudnicki@ohri.ca 1 satellite cells such as constitutively active p38α/β signalling, Sprott Center for Stem Cell Research, Regenerative Medicine Program, Ottawa Hospital Research Institute, 501 Smyth Road, Ottawa, ON K1H 8L6, or elevated JAK-STAT signalling alter cell fate, reduces re- Canada generative capacity in muscle [9–12] and biases aged satel- Department of Cellular and Molecular Medicine, Faculty of Medicine, lite cells to asymmetric modes of division producing University of Ottawa, Ottawa, ON, Canada Full list of author information is available at the end of the article committed progenitors and exhausting the muscle stem cell © The Author(s). 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Feige et al. Skeletal Muscle (2021) 11:1 Page 2 of 14 pool [10]. Chemical and physical cues present in young and progenitors. Consequently, regeneration is impaired con- healthy muscle act to balance satellite cell quiescence, self- tributing to disease progression [40]. Dystrophin expres- renewal, and asymmetric division to maintain the satellite sion has been noted in dispersed fetal human myogenic cell pool in a state amenable to rapid activation in response cells [41], but has yet to been observed in adult muscle to injury [9, 10, 13]. Understanding if these processes are stem cells in situ. conserved or altered in human satellite cells is important to Dystrophin-mediated signalling influencing satellite develop methods improving endogenous satellite cell- cell polarity is one mechanism promoting myogenic mediated repair. progenitor formation. We have previously shown that Applying laboratory insights to human disease is activation of the EGFR pathway can stimulate the forma- failure-prone, where roughly 1 in 10 pharmaceuticals tion of apicobasal-oriented mitotic centrosomes in a entering clinical trials gain approval by the FDA [14] dystrophin-independent manner in dystrophin-deficient with efficacy being a major concern [15]. The value of mice [23]. Mechanistically, polarized phosphorylated animal models for predicting clinical responses to ther- EGFR can recruit the mitotic centrosome regulator apy remains controversial and partially explained by Aurora Kinase A to facilitate apicobasal-oriented mitotic publication biases, methodological flaws in animal exper- divisions and the formation of myogenic progeny iments, non-transparent data reporting, and fundamental through asymmetric divisions. Both dystrophin and differences in human and animal physiology limiting EGFR-mediated satellite cell polarity are influenced by generalizability of results [16, 17]. the myofiber 3D microenvironment, where apicobasal- Improved methods investigating satellite cell biology oriented satellite cell divisions can be observed myofiber such as satellite cell isolation [18], satellite cell trans- culture in situ and in vivo in response to injury [22, 42]. plantation [19, 20], and myofiber culture [21] have Whether this pathway similarly functions in human improved our understanding of satellite cell heterogen- muscle stem cells has remained an unanswered eity [22–24], hierarchy [24–26], regenerative capacity question. [10, 27, 28], and the therapeutic potential of augmenting Myofiber culture [21] has led to multiple seminal dis- muscle stem cell repair [23, 29, 30]. coveries in mouse satellite cell biology [10, 22, 23, 28, 40]. Development of novel 3D culture systems [31], bio- We hypothesized human myofiber isolation could be feas- engineering approaches modeling human muscle [32, 33], ible and provide insight into fundamental differences in isolation of primary muscle cells from human cadavers human and mouse satellite cell biology. Common [34], humanized mouse models of muscular dystrophy methods of isolating human muscle such as punch biopsy [35], and advancements in iPSC-derived myogenic cells are not suitable to culture as sarcolemma membrane dam- [36, 37] have uncovered unique aspects of human muscle age results in calcium overload and myofiber hyper con- disease; however, no system robustly recapitulates the traction [43]. Maintaining healthy myofibers amenable for complexity in the adult human myofiber chemical com- culture requires tendon to tendon isolation followed by position, physical composition, or cellular composition enzymatic digestion of the extracellular matrix to release [33]. In model organisms, culturing myofibers harboring single muscle fibers [21], where healthy non-contracted satellite cells maintains extracellular matrix composition myofibers are cultured to maintain endogenous niche [1], myofiber rigidity [38], and endogenous niche interac- interactions. tions [22] to provide a more relevant ex vivo culture para- Here, we report that by utilizing primary human digm [2]. myofibers, we can model human muscle stem cell Understanding the satellite cell-intrinsic changes dynamics in a chemically, physically, and cellularly occurring with disease provides important insight into relevant context. This method is amendable to model treatments addressing the etiology of muscle disease. human muscle resident stem cells including satellite Duchenne’s muscular dystrophy (DMD) is a devasting cells and in the future feasibly other resident cell lethal disease where loss of dystrophin leads to severe types in muscle such as fibroadipogenic precursors, deficit in myofiber structural integrity [39]. Our lab mesenchymal cells, fibroblast, pericyte, endothelial made the seminal discovery that in mice, muscle stem cells, and tenocytes [44, 45]. Myofibers can be pre- cells also express dystrophin where it is localized to- pared within 3 h of surgical excision and can be cul- gether with the dystrophin-associated glycoprotein com- tured ex vivo for at least 8 days which reflects plex at the basal cortex of a portion of dividing cells satellite cell expansion, differentiation, and de novo against the basal lamina. Dystrophin recruits Par1b/ human myofiber formation. The ability to directly Mark2 to establish polarity that is required for asymmet- assay genetic pathways in human satellite cells in a ric apicobasal cell divisions. In the absence of dys- relevant context provides an exciting opportunity for trophin, loss of polarity leads to a significant reduction pre-clinical testing and to develop causative relation- in asymmetric divisions and reduced generation of ships in human satellite cell signalling. Feige et al. Skeletal Muscle (2021) 11:1 Page 3 of 14 We show adult human satellite cells undergo apicoba- Using blunt tipped micro scissors and blunt tipped twee- sal and planar cell divisions and express polarized dys- zers, fascicle boundaries are gently retracted and peri- trophin and EGFR. Moreover, we report that stimulation mysium is cut to free fascicle bundles continuing along of the EGFR pathway augments satellite cell generation the fascicle boundaries without freeing bundles from the of myogenic progenitors. Therefore, this method holds tendon. Fascicles are further dissociated to bundles con- the potential to accelerate therapeutic development by taining ~ 200 muscle fibers by measuring with a preci- evaluating genetic pathways relevant to human muscle sion micro ruler (TDI) to ensure ~ 2 mm in diameter disease. bundles are prepared. Using sharp micro scissors, tendons are cut to release free myofiber bundles avoid- Methods ing injury to myofibers. Myofiber bundles are then Experimental subjects placed in excess filtered myofiber culture media This study was approved by the Ottawa Hospital (DMEM, 110 mg Pyruvate, 20% FBS, 2.5ng/ml bFGF, 1% Research Ethics Board and informed consent was Chicken Embryo extract–25 ml/bundle) and cultured at obtained prior to proceeding. Samples were harvested 37 °C in normoxia. For EGF treatment, human recom- from patients that had already been consented for organ binant EGF was supplemented to the media at isolation donation through the Trillium Gift of Life Network fol- at 100 ng/mL where 1% BSA in PBS served as vehicle lowing neurological determination of death in compli- control. Media is changed every day. Following culture, ance with guidelines from The Ottawa Hospital myofibers are fixed by placement in excess warmed 4% Research Ethics Board. Donors did not present with paraformaldehyde for 5 min where excess 4% paraformal- cachexia or muscular dystrophy by independent chart dehyde is then injected directly within myofiber bundles review. using an insulin syringe. Bundles are fixed for 30 min in 4% paraformaldehyde. Bundles are then moved to 0.4% Histological analysis of muscle cross sections paraformaldehyde for 12 h followed by extensive wash- Minimum fiber Feret and myofiber surface area mea- ing. Samples are maintained in PBS containing Pro- surements were performed using the semi-automated Clin950 at 4 °C for long-term storage. For analysis of full- SMASH software plugin for MATLAB 2015a described length bundles, individual myofibers may be isolated by previously [46]. Total myofiber count per cross section retraction using blunt tipped tweezers along the length of was verified by manual validation of SMASH myofiber a myofiber bundle. For cross-sectional analysis of cul- masks and original images. Myofiber types were counted tured bundles, bundles are frozen whole prior to fixing or manually across each cross section studied. fixed bundles are segmented, hydrated for 12 h in a sucrose gradient and frozen by imbedding in OCT and Human Psoas minor isolation freezing by nitrogen cooled isopentane. For bundle ana- Immediately after the organ retrieval process was lysis, fixed cultured myofiber bundles are manually disso- completed, the psoas minor muscle was exposed ciated to 5–15 myofiber bundles and segmented to using Deaver retractors to visualize tendinous inser- length amendable to microscopic analysis and to improve tions into the iliopectineal arch. A graphic overview antibody penetration. of theprocedureisprovidedin Fig. 1a. The distal as- pect of the psoas tendon was cut, and the psoas dis- Immunostaining and antibodies sected to itsoriginonthe vertebralbodiesand cut Myofibers and myofiber bundles are processed by identi- immediately adjacent to the vertebra. It is critical to cal means. Fixed myofiber samples are washed in PBS minimize tension on the Psoas minor during dissec- and permeabilized in 0.4% Triton-X100 for 1 h with tion. The free Psoas minor was the immediately rocking. Samples are washed in PBS containing 125 mM placed into sterile, cold transport media (DMEM, 110 glycine 3× in excess buffer until no appearance of deter- mg/ml Pyruvate), and maintained on ice during gent remains. Samples are blocked using TrueBlack as transport. per manufacturer’s instructions. Samples are then blocked in blocking buffer containing 5% normal donkey Human myofiber preparation serum for 3 h at room temperature to overnight at 4 °C In a sterile environment, Psoas muscle is placed in an with rocking. Samples are washed in PBS and primary appropriate vessel buffered with ice and containing antibodies are applied for 24 h at room temperature with enough transport media to submerge. A photographic rocking. Samples are washed in excess PBS 5× 30 min overview of the procedure is presented in Fig. 1a and each with rocking. Secondary antibodies are applied 24 h S1A. Using a dissecting microscope and blunt micro at room temperature with rocking in the dark. Samples scissors, adipose tissue, and free epimysium is dissected are washed in excess PBS for 30 min with rocking and to visualize muscle fascicle tendon-tendon organization. DAPI is applied for 1 h with rocking. Samples are 20 Feige et al. Skeletal Muscle (2021) 11:1 Page 4 of 14 Psoas minor Psoas minor T12 L1 L2 L3 L4 L5 Psoas major Psoas major 20mm 10mm 1mm p<0.005 C 400 DE 3.0 100% F p<0.005 D0 Intact D0 Contracted D8 Cultured 80% 2.0 60% d0 Intact 200 1000 d0 Contracted 40% 1.0 d8 Cultured 20% 0 0 0% 0 d8 Human Psoas p<0.005 HI J 80 40 Mouse EDL 60 Mouse Psoas 30 Human Psoas 40 40 0 0 0 10uM 0 30 60 90 120 K Avg minimum fiber Feret (um) Human Psoas minor 1mm 1mm Fig. 1 Human Psoas minor muscle is amenable for myofiber culture. a Graphic overview of anatomical dissection of Psoas minor muscle from organ donors. b Photographic overview of myofiber bundle isolation from surgically removed human Psoas minor muscle. Quantification of c single myofibers per bundle and d myofiber bundle diameter, whiskers represent min and max values. e Myofiber viability as quantified by presence of hypercontracted myofibers. f Representative images and quantification of myofiber sarcomere spacing from intact, contracted, and cultured myofibers stained with α-Actinin (green). g Representative image and quantification of human Psoas fiber type stained for type I myofibers (blue), type IIa myofibers (green), type IIx myofibers (red), and wheat germ agglutinin (White). h Quantification of average minimum fiber Feret and i minimum myofiber Feret proportion of myofibers from human Psoas myofibers compared to mouse Extensor digitorum longus and mouse Psoas muscles using SMASH software. j Quantification of human Psoas myofiber length compared to mouse Extensor digitorum longus and mouse Psoas muscles. k Representative image of a single isolated human Psoas minor myofiber stained with DAPI (blue). c, d, f Error bars represent means ± SD, e, g–j Error bars represent means ± SEM; p values are listed. c, d n = 22 myofiber bundles, e n =4 biological replicates, f n = averages from 12 to 19 myofibers per condition, g n = 5 biological replicates. h–j n = 3 biological replicates Type 1 Type 2a Type 2x mEDL mPSOAS hPSOAS mEDL mPSOAS hPSOAS DAPI Fiber number / bundle MYH7 MYH2 MYH1 WGA Bundle diameter (mm) % of myofibers Myofiber viability per bundle (%) Avg minimum fiber Feret (um) ACTN Proportion of myofibers (%) A-actinin spacing (nm) Fiber Length (mm) Feige et al. Skeletal Muscle (2021) 11:1 Page 5 of 14 washed in excess PBS 5× 30 min each with rocking in presented images represent maximum intensity projec- the dark. Samples are then passed through a serial 20– tions unless stated otherwise. Myofibers partially encom- 80% glycerol series and mounted onto slides in glycerol passed in Z-stacks were excluded form analysis. mounting media containing 0.1 M n-propyl gallate. Antibodies used in the study are as follows: Mouse Quantification and statistical analysis anti-Pax7 (1:2, DSHB, Cat. no. Ab528428; RRID: AB_ Compiled data are expressed as mean ± standard devi- 528428), rat anti-Laminin (1:1000, Sigma, Cat. no. ation (SD) or mean ± standard error of the mean (SEM) L0663; RRID: AB_477153), Mouse anti-Dystrophin (1: as stated. Experiments were performed with a minimum 500, DSHB, Cat. no. MANEX1011B; RRID:AB_1157876), of three biological replicates unless stated otherwise. For Rabbit anti-p-EGFR (1:250, Cell Signaling technology, statistical comparisons of two conditions, the Student’s t Cat. no. 3777S; RRID: AB_2096270), Mouse anti-M- test was used. Data is presented as paired (mean (control Cadherin (1:500, BD Biosciences Cat. no. 611101, RRID: vs treatment)) for direct comparison of biologically AB_398414), Rabbit anti-MyoD (1:500, Abcam, Cat. no, matched samples and unpaired (mean control vs mean ab133627), Mouse anti-Syndecan-4 (1:500, Santa Cruz treatment) where appropriate. Statistical testing was per- Biotechnology Cat. no. sc-12766; RRID:AB_628314), formed for each histogram where appropriate and was Rabbit anti-Annexin-5 (1:500, Abcam, Cat. no. ab14196, reported where significant. p values are provided for RRID:AB_300979), Mouse anti-MyoG (1:500, Novus, each statistical test performed. No data were removed as Cat. no. MAB66861; RRID:AB_10973343), Mouse anti- outliers. Experimental design incorporated user blinding α-Actinin (1:1000, Sigma, Cat. no. A7732; RRID:AB_ when possible. Statistical analysis was performed in 2221571), Mouse anti-MHC (1:1000, DSHB, Cat. no. MF GraphPad Prism or Microsoft Excel. 20; AB_2147781), Mouse anti-MyH2 (1:100, DSHB, Cat. no. SC-71; AB_2147165), Mouse anti-MyH1 (1:100, Key resources DSHB, Cat. no. 6H1; AB_2314830), Mouse anti-MyH4 Key resources are listed in Table S2. (1:100, DSHB, Cat. no. BF-F3; RRID: AB_2266724), Mouse anti-MyH7 (1:100, DSHB, Cat. no. BA-F8; RRID: Results AB_10572253), Rat anti-Perlecan (1:500, NSU Biorea- Human Psoas muscle is amenable for satellite cell culture gents, Cat. no. V2600; RRID:AB_2119238), Rabbit anti- in situ Ki67 (1:1000, Abcam, Cat. no. ab15580; RRID:AB_ To evaluate our hypothesis that human myofibers could 443209), and Wheat Germ Agglutinin Alexa 488 conju- be cultured in a laboratory setting, we decided to isolate gate (1:1000, Fisher, Cat. no. W11261). primary human tissue from neurological determination of death organ donors following ethics approval and in- Scanning electron microscopy formed consent. We evaluated muscle groups for suit- Samples were washed with water prior to dehydration in ability in myofiber culture where surgical access to both a35–100% graded series of ethanol. Samples were crit- tendinous insertions is feasible and where myofibers are ical point dried in dry 100% ethanol using liquid CO2 as short to facilitate manipulation in a laboratory setting transition fluid. CO2 was exchanged at 5-min intervals with common plasticware and reasonable reagent for 8 rounds followed by a final 3 h release. Samples volumes. We identified the small muscles of the hand, were maintained under dust-free desiccation following including the Abductor or Flexor Pollicis Brevis, the critical point drying. Samples were mounted on Flexor Digitorum Brevis of the foot or the Pronator aluminum stages using double-sided carbon tape. Quadratus of the forearm would be ideal candidates due Mounted samples were sputter coated with gold for 1 to their size and accessibility; however, clinical availabil- min (~ 10 nm) and stored under dust-free desiccation ity of these muscles limited laboratory testing. We rea- prior to imaging on a phenom Pro-X scanning electron soned that an alternative strategy would involve isolating microscope at an accelerating voltage of 15 kV. muscle with large angles of pennation, where individual myofibers are aligned at an angle oblique to the longitu- Imaging and analysis dinal angle of muscle contraction. These muscles pro- Human Psoas myofiber bundles were analyzed on a vide a mechanical advantage over shorter contraction Leica TCS SP8 confocal microscope equipped with HC distances and possess shorter myofiber fascicles attach- PL APO 20× IMM CORR objective with HyD and PMT ing to fibrous aponeuroses running along the periphery detectors. Filters and detectors were set to maximal of the muscle [47]. These muscle types would also allow bandwidth and sensitivity to limit bleed through between myofiber isolation from partial muscle dissections. channels. Tile scans were stitched directly following We identified the human Psoas muscle as a moder- acquisition using the Leica LAS AF software. Manual ately pennate muscle attached to the T12 vertebrae and image analysis was performed using ImageJ and the Lacunar ligament (Fig. 1a) with some variable Feige et al. Skeletal Muscle (2021) 11:1 Page 6 of 14 presence within populations [48, 49], as a good candi- generally lack glycolytic myosin heavy chain type IIb date to isolate myofibers. We obtained post-mortem (MyHC IIb) muscle fibers in favor of type IIx [50]. muscle samples from neurologically determined Additionally, myofiber type has an impact on satellite deceased organ donors (2♀,1♂) with a mean age of 61 cell response to exercise, where satellite cells resident to ± 9.4 years (range 50–68) (Table S1). Donors did not oxidative MyHC type I human muscles show augmented have muscular dystrophy and did not exhibit muscle expansion following aerobic training [51]. We hypothe- cachexia. Muscle samples were 24.35 ± 13.56 g in mass sized that myofiber type would influence satellite cell and 11.42 ± 2.15 cm in length and isolated within 2–4h fate in human Psoas myofiber cultures and could con- from cardiac death. found translating signalling pathways identified in mice Mouse myofiber isolation requires enzymatic digestion to human satellite cells. of the extracellular components from isolated mouse We evaluated myofiber type and histological profiles Extensor digitorum longus muscle to release single myo- of human Psoas myofiber bundles, mouse Extensor fibers amenable for culture. To test if human Psoas myo- digitorum longus (EDL) and mouse Psoas myofibers to fibers could be prepared similarly, we subjected better correlate difference in rodent and human satellite myofiber isolated Psoas biopsies to collagenase digestion. cell biology. Isolated human Psoas myofiber bundles Isolated Psoas muscle was resistant to digestion using were composed of mixed 36.5 ± 3.1% slow (type I) and collagenase type 1, type 2, type 3, type 4, type 5, type 6, 63.5 ± 6% fast (type IIa, IIx) myofibers compared to 14.4 type 7, or elastase buffers due to the thick perimysium, ± 2% type IIa and 62.5 ± 1.2% type IIb fast twitch mouse endomysium, extracellular matrix (ECM), and vascular Psoas or 11 ± 1.2% type IIa and 72.9 ± 0.7% type IIb networks present (data not shown). Thus, the established EDL muscles (Figure S1G). Human Psoas myofibers do methods of myofiber isolation used in mouse studies are not significantly differ in minimum fiber Feret (Fig. 1h) not appropriate for human myofiber isolation. or myofiber surface area (Figure S1I) compared to Therefore, we manually dissected myofiber bundles mouse Psoas or EDL fibers where a subset of human from Psoas samples from tendon to tendon (Figure S1A). Psoas myofibers are hypertrophic (Fig. 1I, S1J). Human Manually dissected myofibril bundles contained 220 ± 63 myofibers however are roughly 10-fold longer than myofibers (Fig. 1c) and were 1.9 ± 0.5 mm in diameter mouse Psoas or EDL myofibers (Fig. 1j–k) (36.4 ± 0.4 (Fig. 1d). Myofiber bundles displayed heterogeneous via- mm versus 3.34 ± 0.05 mm and 3.60 ± 0.12 mm). Taken bility, where excess tension during surgical excision or together, this data suggests that human Psoas myofibers processing resulted in samples containing hyper display distinct histological characteristics from mouse contracted myofibers which were excluded from further Psoas or EDL muscle likely due to requirements for hip analysis (Figure S1A). Successful myofiber preparations flexion and posture in an erect position. contained intact myofibers where 83 ± 5% of myofibers within a bundle did not exhibit any signs of hyper contrac- Human satellite cells expand in myofiber culture tion or injury (Fig. 1e). Injured myofibers can be distin- Satellite cells remain mitotically quiescent but are guished into hypercontracted myofibers, moderately poised to activate and enter the cell cycle in response injured and minor injured subgroups. Hypercontracted to extrinsic cues such as exercise or trauma [2]. In ro- myofibers (Figure S1B) show bisected myofiber segmenta- dents, this can be achieved by experimental models of tion within an extracellular matrix scaffold while myofi- injury, muscle digestion for stem cell isolation [52, 53] bers with moderate damage (Figure S1C) exhibit or in the case of myofiber preparation, digestion with widespread disorganization of sarcomeric banding and sig- collagenase and exposure to growth factors in cell cul- nificant autofluorescence. Myofibers with minor damage ture [21]. To evaluate if human satellite cells from (Figure S1D) exhibit focal autofluorescence and invagin- Psoas myofiber cultures spontaneously activate in vitro, ation of the extracellular matrix and maintain myofiber- we evaluated basal satellite cell numbers immediately cell-ECM contact. Quantification of sarcomere spacing following surgical excision and during culture where we through α-Actinin staining (Fig. 1f, S1E-F) showed signifi- analyzed an average 1.8 ± 1.1 mm length of myofibers cantly shorter sarcomere banding in contracted myofibers, per experiment (Figure S2A). whereas healthy myofibers can be maintained in culture Human Psoas muscle has generally more satellite cells for at least 8 days without myofiber contraction or loss of per mm (15.7 ± 3.0) compared to mouse Psoas (9.5 ± sarcomere disorganization. Together, this data suggests 0.5) or EDL myofiber cross-sections (10.0 ± 0.36) follow- that human Psoas myofiber bundles can be isolated and ing immunofluorescence staining for Pax7 (Fig. 2a). We maintained in culture without consequence to myofiber observed by immunofluorescence staining for Pax7 that integrity. human Psoas myofibers with centrally located nuclei Mouse and human myofibers exhibit different histo- possess significantly increased (75.1%, 8.67 ± 1.6 versus logical characteristics, where human skeletal muscle 10um 10um 10um Feige et al. Skeletal Muscle (2021) 11:1 Page 7 of 14 AB C D Mouse D0 Human D0 Human D0 Human Psoas 100 100 80 80 60 60 40 40 20 20 0 0 10um d0 d0 EF Human D0 Human D4 Human D8 10um 10um 10um GH I J p<0.005 150 40 50 100 p=0.02 p=0.03 p=0.03 40 80 30 60 20 40 10 20 0 0 0 d0 d4 d8 d0 d4 d8 d0 d4 d8 d0 d4 d8 Fig. 2 Human satellite cells expand in culture. a Representative image and quantification of satellite cell density from human Psoas minor muscle cross sections stained with DAPI (blue), Perlecan (green), Pax7 (red), and Laminin (white). b Representative images of mouse and human satellite cells in the niche at isolation stained for DAPI (blue), Laminin (green), dystrophin (red) and Pax7 (white). c Representative image and quantification of human satellite cells expressing Syndecan-4 (SDC4) at isolation stained with DAPI (blue), Syndecan-4 (green), and Pax7 (red). d Quantification of human satellite cells expressing M-Cadherin (MCAD) at isolation. e Representative scanning electron micrograph of cultured human myofibers (day 8) showing maintenance of myofiber extracellular matrix composition and cell-cell contacts. f Representative images of human myofibers in culture from day 0, day 4, and day 8 stained with DAPI (blue), Ki67 (green), Pax7 (red), and dystrophin (white). Quantification of g total nuclei per mm of myofiber, h satellite cells expressing Ki67 per millimeter of myofiber, i percentage of satellite cells per total nuclei per mm of myofiber, j proportion of satellite cells expressing Ki67 on myofibers. a, c, d, g–j Error bars represent mean ± SEM; a n = 3 biological replicates for mouse EDL, mouse Psoas, n = 5 biological replicates from human Psoas. c, d, g–j n = 3 biological replicates 15.2 ± 3.1) satellite cells per millimeter of myofiber double labelled with Pax7 and Ki67 to assess human sat- (Figure S2B-C) possibly due to prior repair in vivo. ellite cell proliferation. We observed no statistical Human Psoas satellite cells reside within the niche at change in total nuclei per millimeter of myofiber follow- isolation (Fig. 2b), where 86.5 ± 5.25% of human satellite ing 8 days in culture (Fig. 2g) with a concomitant cells express the cell surface marker Syndecan-4 (Fig. 2c, increase in proliferating satellite cells (0.017 ± 0.02 ver- S2D, F) and 34.8 ± 7.6% heterogeneously express M- sus 23.8 ± 4.3 per millimeter myofiber day 8) (Fig. 2h) Cadherin (Fig. 2d, S2G). Most satellite cells maintain and other cell types (0.7 ± 0.2 versus 7.6 ± 0.8 per milli- Syndecan-4 expression in culture (84.1.0 ± 6.7%) meter myofiber day 8) (Figure S2I) while the number of (Figure S2F) and do not express the apoptotic Ki67-satellite cell falls (9.18 ± 1.9 versus 2.06 ± 0.74 per marker Annexin-5 (Figure S2D). millimeter myofiber day 8). Following 8 days in culture, To evaluate myofiber integrity and maintenance of human satellite cells make up 30.0 ± 3.7% of total nuclei cell-cell and ECM interactions throughout culture, myo- along myofibers (Fig. 2I). As the number of activated fiber bundles were subject to scanning electron micros- satellite cells heterogeneously increases at day 4 (Fig. 2h, copy. Electron micrographs (Fig. 2e) show myofiber S2I), the first satellite cell divisions likely occur between bundles exhibit no apparent tearing or porosity and day 3 and day 4 in culture (Fig. 2h, i), where the bio- maintain complex cell-cell and cell-ECM interactions logical variance between samples is greater than that be- following 8 days in culture. This data suggests human tween myofibers (Figure S2K). Following 8 days in satellite cells expand locally in response to prior trauma culture, most satellite cells (91.8 ± 1.2%) express Ki67 and express and maintain Syndecan-4 expression (Fig. 2j). Taken together, this data suggests human satel- throughout culture. lite cells actively proliferate in culture along with other To directly assess satellite cell expansion in culture, we resident cell types however with slower activation kinet- quantified satellite cell numbers throughout culture and ics than that of mouse satellite cells. mEDL mPSOAS hPSOAS DAPI PLC Pax7 LAM Pax7 per mm DAPI LAM Dmd Pax7 DAPI Ki67 Pax7 Dmd Total Nuclei /mm fiber Ki67+ Pax7+ Nuclei DAPI SDC4 Pax7 /mm fiber % Pax7+ Nuclei /mm fiber % SCD4+ satellite cells % Pax7+ Ki67+ % MCAD+ satellite cells Feige et al. Skeletal Muscle (2021) 11:1 Page 8 of 14 EGF stimulation promotes myogenic progenitor asymmetric cell division to orient the satellite cell mi- formation in human Psoas myofiber culture totic spindle in an apical-basal orientation [23]. To To evaluate myogenic differentiation in Psoas myofiber evaluate if human Psoas satellite cells integrate EGFR culture, we examined myofibers by immunofluorescence signalling, cultures were treated with EGF ligand staining for Myogenin (MyoG), a transcription factor throughout culture to stimulate EGFR activation. Fol- expressed at the onset of differentiation [54]. Culturing lowing EGFR stimulation, myofibers were stained for the Psoas myofibers for 8 days results in significant presence presence of phosphorylated EGFR, where most satellite of MyoG-expressing cells (20.8 ± 1.8 per millimeter cells express p-EGFR (Fig. 3d, Figure S3D), and its ex- myofiber) concomitant with proliferation of satellite pression can become locally restricted as observed in ac- cells, where we do not observe co-labelling of Pax7 and tivated mouse satellite cells [23]. This data suggests that MyoG by immunofluorescence (Fig. 3a–c, S3A). the EGFR signalling pathway is activated following EGF Additionally, by 8 days in culture, we can observe de treatment in human satellite cells in a manner analogous novo myofiber formation occurring characterized by to mouse satellite cells. multiple organized and aligned myocytes expressing To evaluate if EGFR augmented the production of MyoG (Figure S3B-C) suggesting human Psoas myofiber myogenic progeny through the promotion of asymmetric culture may represent a paradigm to model human satel- division as found previously in mouse muscle [23], we lite cell activation, differentiation, and myofiber forma- examined fibers by immunofluorescence for Pax7, tion. This data indicates that by 8 days in culture, MyoG, and Ki67. Quantifying total nuclei per millimeter human Psoas satellite cells expand in number and of fiber show no significant change following 8 days of express differentiation markers including MyoG. EGF treatment (Figure S3H); however, treatment with We have previously established that mouse satellite EGF results in an appreciable increase in the number of cells express EGFR that is polarized before an proliferating satellite cells per millimeter of myofiber A Human myofiber Day 8 BC 40 40 p=0.008 10 10 0 0 d0 d8 d0 d8 EF G 50 8 50 40 40 30 30 10 10 0 0 d8 d8 d8 d8 d8 d8 EGF EGF EGF Fig. 3 Human satellite cell expansion and differentiation can be tuned in situ. a Representative image (magnification from Figure S3A) of human satellite cells and Myogenin (MyoG) expressing differentiating progenitors cultured on 8-day human Psoas minor myofiber cultures stained with DAPI (blue), MyoG (green), and Pax7 (red). Quantification of b number of Pax7-expressing cells per millimeter of myofiber, c MyoG-expressing cells per millimeter of myofiber. d Representative images of human satellite cells expressing phosphorylated active EGFR (p-EGFR) stained with DAPI (blue), p-EGFR (green), Pax7 (red), and dystrophin (white). White arrow denotes localized p-EGFR expression. Quantification of e satellite cells expressing Ki67 per millimeter of myofiber and f Ki67-negative satellite cells per millimeter of myofiber following EGF treatment or vehicle control of human myofibers. g Quantification of number of MyoG-expressing cells per millimeter of myofiber following EGF treatment of human myofibers. b, c, e–g Error bars represent means ± SD (EGF) and means ± SEM (control); b, c n = 3 biological replicates. d–g n = 3 biological replicates control, n = 2 biological replicates EGF DAPI pEGFR Pax7 Dmd DAPI MyoG Pax7 Pax7+ cells per mm myofiber Ki67+ Pax7+ Nuclei MyoG+ cells per mm fiber per mm myofiber Ki67- Pax7+ nuclei per mm fiber MyoG+ cells per mm myofiber Feige et al. Skeletal Muscle (2021) 11:1 Page 9 of 14 (56.7% paired, 78% unpaired, n = 3 control, n = 2 EGF) examined myofibers by immunofluorescence staining (Fig. 3e, S3F) and an increased proportion of satellite throughout culture. Interestingly, we observed rare apico- cells per myofiber (29% increase unpaired, 15% increase basal and planar-oriented satellite cell doublets expressing paired, n = 3 control, n = 2 EGF) (Figure S3G). Interest- Pax7 and residing in the niche from samples fixed at isola- ingly, EGF treatment increased the number of Ki67- tion and stained with Pax7, Perlecan, and Dystrophin negative satellite cells per millimeter of myofiber (161% (Fig. 4). As these cells are occupying the same niche space, increase unpaired, 118% increase paired, n = 3 control, n we believe this is not due to random cell migration along = 2 EGF) following 8 days in culture treated with EGF the myofiber, suggesting that homeostatic repair mecha- or vehicle control (Fig. 3f, S3H). However, the propor- nisms may undergo either mode of division. tion of Pax7/Ki67 expressing cells was similar following We additionally examined culture day 3 to day 4, a EGF treatment (92% control vs 89% EGF) suggesting any time point reflecting the first division in human satellite effect as a mitogen was negligible. Concomitant with an cells in culture. Strikingly, staining for the protein dys- increase in proliferating satellite cells, EGF treatment re- trophin (DMD), which can be polarized in mouse satel- sulted in an appreciable increase in MyoG-expressing lite cells to facilitate asymmetric division [40], shows cells per millimeter of myofiber (69% increase unpaired, strong expression and polarity on the basal surface of a 78% increase paired n = 3 control, n = 2 EGF) (Fig. 3g, subset of cultured satellite cells (Fig. 5a), along with a S3J). Therefore, we conclude that activation of EGFR subset of satellite cells expressing non-polar dystrophin signalling in muscle stem cells promotes the generation (Fig. 5b). We observe asynchronous expression of DMD of progenitors, likely by stimulating asymmetric divi- in satellite cells along myofibers, where cells in close sions, similarly between human and mouse. proximity can express DMD perhaps due to region- specific cues present within the myofiber microenviron- Human satellite cells undergo apicobasal and planar cell ment. This suggests that human satellite cells can divisions and express polarized dystrophin polarize dystrophin to their basal surface interfacing To evaluate the possibility of human satellite cells under- with the extracellular matrix in an identical manner to going apical-basal-oriented asymmetric division, we activated mouse satellite cells. AB Apicobasal Planar DAPI PLC Pax7 Dmd DAPI PLC Pax7 Dmd DAPI PLC Pax7 Dmd Fig. 4 Human satellite cells can orient division angles. Representative images of a planar and b apicobasal-oriented human satellite cells in the niche at isolation stained with DAPI (blue), Perlecan (green), Pax7 (red), and dystrophin (white). n = 3 biological replicates Feige et al. Skeletal Muscle (2021) 11:1 Page 10 of 14 Polar B Non-Polar C Diffuse Fig. 5 Human satellite cells express polarized dystrophin in culture. Representative image of human satellite cells cultured for four days and stained with DAPI (blue), Pax7 (red), and dystrophin (green) showing a polarized dystrophin localization, b non-polar dystrophin localization, and c diffuse dystrophin staining. n = 3 biological replicates Discussion essential to maintaining muscle repair over a lifetime. Evaluating human stem cell dynamics in a relevant con- Asymmetric stem cell division is one method to bal- text is critical to model biological phenomena and ance stem cell maintenance and the production of generalize results to benefit human health. New methods myogenic progeny, where following division one to improve the pre-clinical evaluation of therapeutic daughter cell maintains its stem cell state and one strategies to augment endogenous stem cell activity hold differentiates down the myogenic lineage. Asymmetric promise to improve regenerative medicine outcomes in cell division is established through cells integrating conditions such as Duchenne’s muscular dystrophy. extrinsic environmental cues to restrict cell fate deter- Here, we developed a novel system to evaluate human minants in a polarized manner such that when a cell satellite cell fate choices in a relevant context to interro- orients its mitotic centrosomes parallel with internally gate human satellite cell biology and evaluate pre- polarized cell fate determinants, daughter cells will re- clinical therapeutics in improving muscle regeneration ceive discrete cellular contents [2]. Typically, in (Fig. 6). mouse satellite cells, the daughter cell maintaining A balance in satellite cell proliferation, production niche interactions with the basal lamina maintains its of myogenic progeny, and return to quiescence is stem cell nature. Establishment of an apical-basal- DAPI Dmd Pax7 DAPI Dmd Pax7 DAPI Dmd Pax7 DAPI Dmd Pax7 Feige et al. Skeletal Muscle (2021) 11:1 Page 11 of 14 Fig. 6 Graphical outline of human Psoas culture. Schematic diagram summarizing the procedure for the characterization of human satellite cell dynamics on cultured intact myofibers from the human Psoas muscle oriented mitotic spindle is in part facilitated by the studies exploring other resident cell types such as PAR polarity complex, where we have previously fibroblast, pericyte, and fibroadipogenic precursor shown EGFR is spatially restricted before mitotic divi- cells will better evaluate the specificity of EGF in cul- sions to orient centrosomes through recruitment of ture. Additionally, studies exploring growth factor and Aurora kinase A and spindle assembly [23]. We hy- oxygen penetration in myofiber bundles through pothesized that as human Psoas myofibers maintain staining of growth factors and incubation in oxygen myofiber and extracellular matrix composition that sensing compounds such as Hypoxy Probe labels human satellite cells in culture could integrate three- would further evaluate the potential effect of nutrient dimensional external cues to influence cell fate. availability on heterogeneity in satellite cell activity Our findings support that human Psoas myofiber within myofiber bundles. Studies exploring non- cultures provide a new opportunity to culture human myogenic cell death occurring in culture day 4–8may satellite cells to explore fate choices during satellite provide insight into the cellular dynamics along a cell activation and differentiation. In our system, myo- myofiber during early muscle repair. fiber integrity is maintained (Fig. 1, S1) as well as cell The observation that EGF treatment increases the polarity cues (Figs. 2b, e, 4,and 5) where treatment number of non-proliferative satellite cells following with EGF results in augmented production of myo- treatment (Fig. 3f) suggests that the effect of EGF on genic progeny (Fig. 3e–g). As EGF treatment also in- satellite cells may not be acting as a general mitogen. creases the number of non-satellite cells expressing Taken with an increase in proliferating satellite cells Ki67 (Figure S3I), it is possible that EGF influences (Fig. 3e) and increased formation of myogenic pro- cell survival or is acting as a mitogen on the varied geny (Fig. 3g), supports the hypothesis that EGF cell populations within myofiber cultures. Further treatment stimulates asymmetric division resulting in Feige et al. Skeletal Muscle (2021) 11:1 Page 12 of 14 augmented production of non-cycling satellite stem conserved mechanism first discovered in mouse. Future cells (Pax7+Ki67−) and rapidly proliferating myogenic studies with increased experimental size exploring the satellite cells similar to that observed in mouse [23]. kinetics of cell cycle entry and quantification of division Further studies are required to increase biological angles following the first satellite cell division (likely days sample sizes to fully delineate the role of EGF on hu- 2–3) will shed important insight into the extent of sym- man satellite cells. Taken together, the human Psoas metric and asymmetric satellite cell divisions occurring muscle provides an exciting tool to explore in niche in human muscle. satellite cell biology and facilitate translation of pre- clinical therapeutics from studies in model organisms Conclusion to humans. Human myofiber culture provides an exciting opportun- Previous studies have developed methods to assess ity to evaluate pre-clinical drug efficacy in a relevant hu- human satellite cell expansion in vitro from primary tis- man context. This method provides an opportunity to sue [55] through the culture of hypercontracted human assay satellite cell heterogeneity, stem cell potential, myofiber fragments from punch biopsies. Myofiber frag- stem cell hierarchy, and activation kinetics with the po- ments contained 1–8 bisected myofibers 2–3mmin tential to therapeutically interrogate pathways of interest. length and by 10 days in culture, 80% of nuclei were Additionally, this method provides an additional tool to Pax7+ with a significant amount of Desmin expressing validate phenomena observed in other model organisms, cells within the myofiber [55]. Transplantation of whole validate lead compounds for drug discovery and testing, myofiber fragments resulted in limited engraftment into reduce animal model use, and accelerate the evaluation mouse recipients. Differences in timepoints and muscle of therapeutics improving human health. We envision groups assessed between our study and others [55] limit this technique will aid in generalizing pre-clinical strat- comparisons; however, in our hands’ injury to the myofi- egies into the clinical arena and may in the long term be ber through hyper contraction or bisection results in dis- appropriate as a personalized therapeutic tool. organized myofiber sarcomeres (Fig. 1f) and altered myofiber-ECM interactions (Figure S1B-C), where only Supplementary Information minor focal damage is tolerated along myofibers to The online version contains supplementary material available at https://doi. org/10.1186/s13395-020-00256-z. maintain myofiber-cell-ECM interactions (Figure S1D). Typically, hypercontracted fibers are discarded in experi- Additional file 1: Supplemental figures related to figures 1-3, patient in- ments using mouse myofibers due to the abnormal formation used in this study and key resource table. Figure S1: Myofi- behavior of satellite cells [56]. Additionally, cell polarity bers from human Psoas muscle can be maintained in situ, Related to Fig. is maintained in Psoas myofiber culture (Figs. 3d and 5) 1. A) Photographic overview of human Psoas minor myofiber bundle iso- lation showing expanded images of intact myofiber bundles (panel 9) and by 8 days ~ 30% of cells express Pax7 and ~ 20% and hypercontracted myofiber bundles (panel 10). Representative images represent committed progeny. Differences between the of B) hypercontracted myofibers and C) myofibers with moderate dam- models could be attributed to differential activation cues, age stained for DAPI (Blue), α-Actinin (Green) and Myosin heavy chain (MF20, Red). D) Representative image of myofibers with minor damage non-satellite cell survival, differential activation in differ- stained for DAPI (Blue), Dystrophin (Green), Laminin (White) and IgG ent muscle groups, or limitations in our study including (Red). E) Representative images of single myofiber sarcomeres from intact, exogenous culture of myofibers, modest sample size, or contracted and cultured myofibers stained with α-actinin (Green) show- ing representative histograms of staining intensity and sarcomere spa- differences in outbred human donors. This suggests cing. F) Representative image of disorganized sarcomeres from injured human Psoas myofiber culture may reflect a model of myofibers stained with α-Actinin (Green) and MF20 (Red). G) Representa- homeostatic turnover or response to minor injuries such tive images and quantification of myofiber type from mouse Extensor digitorum longus and mouse Psoas muscle stained with Type 1 myofi- as load-induced trauma or de-innervation, while human bers (Blue), Type 2a myofibers (Green), Type 2b myofibers (Red) and myofiber fragments may represent a paradigm of rapid Wheat germ agglutinin (White). H) Representative image of human Psoas satellite cell activation in response to widespread myofi- muscle cross sections stained with Laminin (Red) with I) quantification of average myofiber surface area and (J) myofiber surface area proportion ber damage. from human Psoas myofibers compared to mouse Extensor digitorum Our findings provide proof-of-principle evidence to longus and mouse psoas muscles using SMASH software. K) Representa- support that in addition to the mature myofiber, human tive image and quantification of mouse Extensor digitorum longus and mouse psoas myofiber lengths from isolated single myofibers. (K) Error satellite cells express polarized dystrophin during satel- bars represent mean ± SD, (G-J) Error bars represent mean ±SEM; (G, I-J) lite cell activation and that human myofiber culture rep- n = 3 biological replicates, (K) n = 40 myofibers per condition. Figure resents a novel paradigm to explore human satellite cell S2: Human satellite cells expand in situ, Related to Fig. 2. A) Quantifica- tion of average length of myofiber analyzed per experiment, whiskers self-renewal and myogenic differentiation. We further represent min and max. B) Representative image of human myofibers validate that human satellite cells can undergo planar showing centrally located nuclei stained with DAPI (Blue), Ki67 (Green), and apicobasal-oriented divisions and demonstrate the Pax7 (Red) and Dystrophin (White) and C) quantification of satellite cells per mm myofiber present at isolation on centrally nucleated fibers (CNF). EGFR pathway to be of pre-clinical interest to augment D) Representative image of myofibers stained with DAPI (Blue), SDC4 the production of myogenic progeny through a Feige et al. Skeletal Muscle (2021) 11:1 Page 13 of 14 Ethics approval and consent to participate (Green) Pax7 (Red) and Annexin-5 (White) with E) bisected myofibers This study was approved by the Ottawa Hospital Research Ethics Board serving as positive control stained for Annexin-5 (White) DAPI (Blue) and under the protocol number 20150544-01H. Pax7 (Red). F) Quantification of satellite cells expressing SDC4 at day 8 in culture. G) Representative image of satellite cells expressing M-Cadherin after isolation stained for DAPI (Blue), MCAD (Green) and Pax7 (Red). H) Consent for publication Representative image of satellite cell expansion on myofibers following 8 Not applicable. days in culture stained with DAPI (Blue), Ki67 (Green), Pax7 (Red) and Dys- trophin (White) and quantification of I) Ki67 expression non-satellite cells per mm of myofiber, J) number of KI67 negative satellite cells per mm of Competing interests myofiber and K) Ki67 expressing satellite cells per mm of myofiber across M.A.R is CSO and Founder of Satellos Bioscience. P.F. and E.T. declare no samples (s#). (A, C, K) Error bars represent mean ± SD, (F, I-K) Error bars competing interests. represent mean ± SEM; (A) n = 351 myofibers. (C) n = averages from 20 (non-CNF) and 9 (CNF) myofibers. (F, I-K) n = 3 biological replicates. (K) n Author details = averages from 4-22 myofibers, where individual data points represent 1 Sprott Center for Stem Cell Research, Regenerative Medicine Program, individual myofibers. Figure S3: Myofiber culture unveils unique regen- Ottawa Hospital Research Institute, 501 Smyth Road, Ottawa, ON K1H 8L6, erative phenomena, Related to Fig. 3. Representative images of A) Repre- Canada. Department of Cellular and Molecular Medicine, Faculty of sentative image of cultured myofiber bundle stained for DAPI (Blue), 3 Medicine, University of Ottawa, Ottawa, ON, Canada. Department of MyoG (Green) and Pax7 (Red) (also presented in Figure 3A for reference). Medicine, Faculty of Medicine, University of Ottawa, Ottawa, ON, Canada. B) Representative image of myogenic progenitors and C) in situ de novo 4 Department of Surgery, Division of Neurosurgery, Faculty of Medicine, myofiber repair from fibers stained with DAPI (Blue), MyoG (Green) and University of Ottawa, Ottawa, ON, Canada. Ottawa Hospital Research MyoD (Red) where white dotted arrows outline the myocyte alignment. Institute, Neuroscience Program, Ottawa, ON, Canada. D) Representative images of cultured myofiber bundles stained for DAPI (Blue), pEGFR (Green) and Pax7 (Red). Quantification of E) total nuclei per Received: 3 September 2020 Accepted: 6 December 2020 mm of myofiber and across samples. F) Quantification of human satellite cells expressing Ki67 or Ki67 negative per mm of fiber across samples fol- lowing culture in control or EGF containing media. G) Quantification pro- portion of nuclei expressing pax7 per myofiber. Quantification of H) References proportion of satellite cells (Pax7+) stained negative for Ki67 and I) pro- 1. Bentzinger CF, Wang YX, Dumont NA, Rudnicki MA. Cellular dynamics in the portion of non-satellite cells (Pax7-) expressing Ki67 following culture in muscle satellite cell niche. EMBO Rep. 2013;14(12):1062–72. control or EGF containing media. 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Analysis of human satellite cell dynamics on cultured adult skeletal muscle myofibers

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Copyright © The Author(s) 2021
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2044-5040
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10.1186/s13395-020-00256-z
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Abstract

Background: Maintaining stem cells in physiologically relevant states is necessary to understand cell and context- specific signalling paradigms and to understand complex interfaces between cells in situ. Understanding human stem cell function is largely based on tissue biopsies, cell culture, and transplantation into model organisms. Methods: Here, we describe a method to isolate post-mortem intact human muscle myofibers and culture muscle stem cells within the niche microenvironment to assay cellular dynamics, stem cell identity, stem cell hierarchy, and differentiation potential. Results: We show human myofiber culture maintains complex cell-cell contacts and extracellular niche composition during culture. Human satellite cells can be cultured at least 8 days, which represents a timepoint of activation, differentiation, and de novo human myofiber formation. We demonstrate that adult human muscle stem cells undergo apicobasal and planar cell divisions and express polarized dystrophin and EGFR. Furthermore, we validate that stimulation of the EGFR pathway stimulates the generation of myogenic progenitors and myogenic differentiation. Conclusions: This method provides proof of principle evidence for the use of human muscle to evaluate satellite cell dynamics and has applications in pre-clinical evaluation of therapeutics targeting muscle repair. Keywords: Human skeletal muscle, Satellite cell, Muscle stem cell, Myofiber culture Background Changes to satellite cell-intrinsic signalling, satellite Skeletal muscle is a complex tissue, responsible for cell-niche interactions, or the regenerative context in mobility, thermoregulation, and breathing. Skeletal conditions such as ageing or muscular dystrophy alter muscle maintenance, growth, and repair are facilitated the kinetics of muscle repair (reviewed in [2]). by muscle resident stem cells (satellite cells) which Improved modeling of human satellite cell dynamics reside within skeletal muscle, resting within a special- in a physiologically relevant context will improve our ized cleft underneath the basal lamina [1]. Muscle understanding of signalling pathways pertinent to homeostasis requires a balance between satellite cell muscle repair in humans. self-renewal and differentiation to facilitate efficient With age, progressive loss of satellite cell number is repair over time and in response to injury [2]. observed with functional decline in skeletal muscle [3, 4]. In aged mice, extrinsic changes to soluble ligands in the niche [5–7], increased fibrosis [8], and intrinsic changes in * Correspondence: mrudnicki@ohri.ca 1 satellite cells such as constitutively active p38α/β signalling, Sprott Center for Stem Cell Research, Regenerative Medicine Program, Ottawa Hospital Research Institute, 501 Smyth Road, Ottawa, ON K1H 8L6, or elevated JAK-STAT signalling alter cell fate, reduces re- Canada generative capacity in muscle [9–12] and biases aged satel- Department of Cellular and Molecular Medicine, Faculty of Medicine, lite cells to asymmetric modes of division producing University of Ottawa, Ottawa, ON, Canada Full list of author information is available at the end of the article committed progenitors and exhausting the muscle stem cell © The Author(s). 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated in a credit line to the data. Feige et al. Skeletal Muscle (2021) 11:1 Page 2 of 14 pool [10]. Chemical and physical cues present in young and progenitors. Consequently, regeneration is impaired con- healthy muscle act to balance satellite cell quiescence, self- tributing to disease progression [40]. Dystrophin expres- renewal, and asymmetric division to maintain the satellite sion has been noted in dispersed fetal human myogenic cell pool in a state amenable to rapid activation in response cells [41], but has yet to been observed in adult muscle to injury [9, 10, 13]. Understanding if these processes are stem cells in situ. conserved or altered in human satellite cells is important to Dystrophin-mediated signalling influencing satellite develop methods improving endogenous satellite cell- cell polarity is one mechanism promoting myogenic mediated repair. progenitor formation. We have previously shown that Applying laboratory insights to human disease is activation of the EGFR pathway can stimulate the forma- failure-prone, where roughly 1 in 10 pharmaceuticals tion of apicobasal-oriented mitotic centrosomes in a entering clinical trials gain approval by the FDA [14] dystrophin-independent manner in dystrophin-deficient with efficacy being a major concern [15]. The value of mice [23]. Mechanistically, polarized phosphorylated animal models for predicting clinical responses to ther- EGFR can recruit the mitotic centrosome regulator apy remains controversial and partially explained by Aurora Kinase A to facilitate apicobasal-oriented mitotic publication biases, methodological flaws in animal exper- divisions and the formation of myogenic progeny iments, non-transparent data reporting, and fundamental through asymmetric divisions. Both dystrophin and differences in human and animal physiology limiting EGFR-mediated satellite cell polarity are influenced by generalizability of results [16, 17]. the myofiber 3D microenvironment, where apicobasal- Improved methods investigating satellite cell biology oriented satellite cell divisions can be observed myofiber such as satellite cell isolation [18], satellite cell trans- culture in situ and in vivo in response to injury [22, 42]. plantation [19, 20], and myofiber culture [21] have Whether this pathway similarly functions in human improved our understanding of satellite cell heterogen- muscle stem cells has remained an unanswered eity [22–24], hierarchy [24–26], regenerative capacity question. [10, 27, 28], and the therapeutic potential of augmenting Myofiber culture [21] has led to multiple seminal dis- muscle stem cell repair [23, 29, 30]. coveries in mouse satellite cell biology [10, 22, 23, 28, 40]. Development of novel 3D culture systems [31], bio- We hypothesized human myofiber isolation could be feas- engineering approaches modeling human muscle [32, 33], ible and provide insight into fundamental differences in isolation of primary muscle cells from human cadavers human and mouse satellite cell biology. Common [34], humanized mouse models of muscular dystrophy methods of isolating human muscle such as punch biopsy [35], and advancements in iPSC-derived myogenic cells are not suitable to culture as sarcolemma membrane dam- [36, 37] have uncovered unique aspects of human muscle age results in calcium overload and myofiber hyper con- disease; however, no system robustly recapitulates the traction [43]. Maintaining healthy myofibers amenable for complexity in the adult human myofiber chemical com- culture requires tendon to tendon isolation followed by position, physical composition, or cellular composition enzymatic digestion of the extracellular matrix to release [33]. In model organisms, culturing myofibers harboring single muscle fibers [21], where healthy non-contracted satellite cells maintains extracellular matrix composition myofibers are cultured to maintain endogenous niche [1], myofiber rigidity [38], and endogenous niche interac- interactions. tions [22] to provide a more relevant ex vivo culture para- Here, we report that by utilizing primary human digm [2]. myofibers, we can model human muscle stem cell Understanding the satellite cell-intrinsic changes dynamics in a chemically, physically, and cellularly occurring with disease provides important insight into relevant context. This method is amendable to model treatments addressing the etiology of muscle disease. human muscle resident stem cells including satellite Duchenne’s muscular dystrophy (DMD) is a devasting cells and in the future feasibly other resident cell lethal disease where loss of dystrophin leads to severe types in muscle such as fibroadipogenic precursors, deficit in myofiber structural integrity [39]. Our lab mesenchymal cells, fibroblast, pericyte, endothelial made the seminal discovery that in mice, muscle stem cells, and tenocytes [44, 45]. Myofibers can be pre- cells also express dystrophin where it is localized to- pared within 3 h of surgical excision and can be cul- gether with the dystrophin-associated glycoprotein com- tured ex vivo for at least 8 days which reflects plex at the basal cortex of a portion of dividing cells satellite cell expansion, differentiation, and de novo against the basal lamina. Dystrophin recruits Par1b/ human myofiber formation. The ability to directly Mark2 to establish polarity that is required for asymmet- assay genetic pathways in human satellite cells in a ric apicobasal cell divisions. In the absence of dys- relevant context provides an exciting opportunity for trophin, loss of polarity leads to a significant reduction pre-clinical testing and to develop causative relation- in asymmetric divisions and reduced generation of ships in human satellite cell signalling. Feige et al. Skeletal Muscle (2021) 11:1 Page 3 of 14 We show adult human satellite cells undergo apicoba- Using blunt tipped micro scissors and blunt tipped twee- sal and planar cell divisions and express polarized dys- zers, fascicle boundaries are gently retracted and peri- trophin and EGFR. Moreover, we report that stimulation mysium is cut to free fascicle bundles continuing along of the EGFR pathway augments satellite cell generation the fascicle boundaries without freeing bundles from the of myogenic progenitors. Therefore, this method holds tendon. Fascicles are further dissociated to bundles con- the potential to accelerate therapeutic development by taining ~ 200 muscle fibers by measuring with a preci- evaluating genetic pathways relevant to human muscle sion micro ruler (TDI) to ensure ~ 2 mm in diameter disease. bundles are prepared. Using sharp micro scissors, tendons are cut to release free myofiber bundles avoid- Methods ing injury to myofibers. Myofiber bundles are then Experimental subjects placed in excess filtered myofiber culture media This study was approved by the Ottawa Hospital (DMEM, 110 mg Pyruvate, 20% FBS, 2.5ng/ml bFGF, 1% Research Ethics Board and informed consent was Chicken Embryo extract–25 ml/bundle) and cultured at obtained prior to proceeding. Samples were harvested 37 °C in normoxia. For EGF treatment, human recom- from patients that had already been consented for organ binant EGF was supplemented to the media at isolation donation through the Trillium Gift of Life Network fol- at 100 ng/mL where 1% BSA in PBS served as vehicle lowing neurological determination of death in compli- control. Media is changed every day. Following culture, ance with guidelines from The Ottawa Hospital myofibers are fixed by placement in excess warmed 4% Research Ethics Board. Donors did not present with paraformaldehyde for 5 min where excess 4% paraformal- cachexia or muscular dystrophy by independent chart dehyde is then injected directly within myofiber bundles review. using an insulin syringe. Bundles are fixed for 30 min in 4% paraformaldehyde. Bundles are then moved to 0.4% Histological analysis of muscle cross sections paraformaldehyde for 12 h followed by extensive wash- Minimum fiber Feret and myofiber surface area mea- ing. Samples are maintained in PBS containing Pro- surements were performed using the semi-automated Clin950 at 4 °C for long-term storage. For analysis of full- SMASH software plugin for MATLAB 2015a described length bundles, individual myofibers may be isolated by previously [46]. Total myofiber count per cross section retraction using blunt tipped tweezers along the length of was verified by manual validation of SMASH myofiber a myofiber bundle. For cross-sectional analysis of cul- masks and original images. Myofiber types were counted tured bundles, bundles are frozen whole prior to fixing or manually across each cross section studied. fixed bundles are segmented, hydrated for 12 h in a sucrose gradient and frozen by imbedding in OCT and Human Psoas minor isolation freezing by nitrogen cooled isopentane. For bundle ana- Immediately after the organ retrieval process was lysis, fixed cultured myofiber bundles are manually disso- completed, the psoas minor muscle was exposed ciated to 5–15 myofiber bundles and segmented to using Deaver retractors to visualize tendinous inser- length amendable to microscopic analysis and to improve tions into the iliopectineal arch. A graphic overview antibody penetration. of theprocedureisprovidedin Fig. 1a. The distal as- pect of the psoas tendon was cut, and the psoas dis- Immunostaining and antibodies sected to itsoriginonthe vertebralbodiesand cut Myofibers and myofiber bundles are processed by identi- immediately adjacent to the vertebra. It is critical to cal means. Fixed myofiber samples are washed in PBS minimize tension on the Psoas minor during dissec- and permeabilized in 0.4% Triton-X100 for 1 h with tion. The free Psoas minor was the immediately rocking. Samples are washed in PBS containing 125 mM placed into sterile, cold transport media (DMEM, 110 glycine 3× in excess buffer until no appearance of deter- mg/ml Pyruvate), and maintained on ice during gent remains. Samples are blocked using TrueBlack as transport. per manufacturer’s instructions. Samples are then blocked in blocking buffer containing 5% normal donkey Human myofiber preparation serum for 3 h at room temperature to overnight at 4 °C In a sterile environment, Psoas muscle is placed in an with rocking. Samples are washed in PBS and primary appropriate vessel buffered with ice and containing antibodies are applied for 24 h at room temperature with enough transport media to submerge. A photographic rocking. Samples are washed in excess PBS 5× 30 min overview of the procedure is presented in Fig. 1a and each with rocking. Secondary antibodies are applied 24 h S1A. Using a dissecting microscope and blunt micro at room temperature with rocking in the dark. Samples scissors, adipose tissue, and free epimysium is dissected are washed in excess PBS for 30 min with rocking and to visualize muscle fascicle tendon-tendon organization. DAPI is applied for 1 h with rocking. Samples are 20 Feige et al. Skeletal Muscle (2021) 11:1 Page 4 of 14 Psoas minor Psoas minor T12 L1 L2 L3 L4 L5 Psoas major Psoas major 20mm 10mm 1mm p<0.005 C 400 DE 3.0 100% F p<0.005 D0 Intact D0 Contracted D8 Cultured 80% 2.0 60% d0 Intact 200 1000 d0 Contracted 40% 1.0 d8 Cultured 20% 0 0 0% 0 d8 Human Psoas p<0.005 HI J 80 40 Mouse EDL 60 Mouse Psoas 30 Human Psoas 40 40 0 0 0 10uM 0 30 60 90 120 K Avg minimum fiber Feret (um) Human Psoas minor 1mm 1mm Fig. 1 Human Psoas minor muscle is amenable for myofiber culture. a Graphic overview of anatomical dissection of Psoas minor muscle from organ donors. b Photographic overview of myofiber bundle isolation from surgically removed human Psoas minor muscle. Quantification of c single myofibers per bundle and d myofiber bundle diameter, whiskers represent min and max values. e Myofiber viability as quantified by presence of hypercontracted myofibers. f Representative images and quantification of myofiber sarcomere spacing from intact, contracted, and cultured myofibers stained with α-Actinin (green). g Representative image and quantification of human Psoas fiber type stained for type I myofibers (blue), type IIa myofibers (green), type IIx myofibers (red), and wheat germ agglutinin (White). h Quantification of average minimum fiber Feret and i minimum myofiber Feret proportion of myofibers from human Psoas myofibers compared to mouse Extensor digitorum longus and mouse Psoas muscles using SMASH software. j Quantification of human Psoas myofiber length compared to mouse Extensor digitorum longus and mouse Psoas muscles. k Representative image of a single isolated human Psoas minor myofiber stained with DAPI (blue). c, d, f Error bars represent means ± SD, e, g–j Error bars represent means ± SEM; p values are listed. c, d n = 22 myofiber bundles, e n =4 biological replicates, f n = averages from 12 to 19 myofibers per condition, g n = 5 biological replicates. h–j n = 3 biological replicates Type 1 Type 2a Type 2x mEDL mPSOAS hPSOAS mEDL mPSOAS hPSOAS DAPI Fiber number / bundle MYH7 MYH2 MYH1 WGA Bundle diameter (mm) % of myofibers Myofiber viability per bundle (%) Avg minimum fiber Feret (um) ACTN Proportion of myofibers (%) A-actinin spacing (nm) Fiber Length (mm) Feige et al. Skeletal Muscle (2021) 11:1 Page 5 of 14 washed in excess PBS 5× 30 min each with rocking in presented images represent maximum intensity projec- the dark. Samples are then passed through a serial 20– tions unless stated otherwise. Myofibers partially encom- 80% glycerol series and mounted onto slides in glycerol passed in Z-stacks were excluded form analysis. mounting media containing 0.1 M n-propyl gallate. Antibodies used in the study are as follows: Mouse Quantification and statistical analysis anti-Pax7 (1:2, DSHB, Cat. no. Ab528428; RRID: AB_ Compiled data are expressed as mean ± standard devi- 528428), rat anti-Laminin (1:1000, Sigma, Cat. no. ation (SD) or mean ± standard error of the mean (SEM) L0663; RRID: AB_477153), Mouse anti-Dystrophin (1: as stated. Experiments were performed with a minimum 500, DSHB, Cat. no. MANEX1011B; RRID:AB_1157876), of three biological replicates unless stated otherwise. For Rabbit anti-p-EGFR (1:250, Cell Signaling technology, statistical comparisons of two conditions, the Student’s t Cat. no. 3777S; RRID: AB_2096270), Mouse anti-M- test was used. Data is presented as paired (mean (control Cadherin (1:500, BD Biosciences Cat. no. 611101, RRID: vs treatment)) for direct comparison of biologically AB_398414), Rabbit anti-MyoD (1:500, Abcam, Cat. no, matched samples and unpaired (mean control vs mean ab133627), Mouse anti-Syndecan-4 (1:500, Santa Cruz treatment) where appropriate. Statistical testing was per- Biotechnology Cat. no. sc-12766; RRID:AB_628314), formed for each histogram where appropriate and was Rabbit anti-Annexin-5 (1:500, Abcam, Cat. no. ab14196, reported where significant. p values are provided for RRID:AB_300979), Mouse anti-MyoG (1:500, Novus, each statistical test performed. No data were removed as Cat. no. MAB66861; RRID:AB_10973343), Mouse anti- outliers. Experimental design incorporated user blinding α-Actinin (1:1000, Sigma, Cat. no. A7732; RRID:AB_ when possible. Statistical analysis was performed in 2221571), Mouse anti-MHC (1:1000, DSHB, Cat. no. MF GraphPad Prism or Microsoft Excel. 20; AB_2147781), Mouse anti-MyH2 (1:100, DSHB, Cat. no. SC-71; AB_2147165), Mouse anti-MyH1 (1:100, Key resources DSHB, Cat. no. 6H1; AB_2314830), Mouse anti-MyH4 Key resources are listed in Table S2. (1:100, DSHB, Cat. no. BF-F3; RRID: AB_2266724), Mouse anti-MyH7 (1:100, DSHB, Cat. no. BA-F8; RRID: Results AB_10572253), Rat anti-Perlecan (1:500, NSU Biorea- Human Psoas muscle is amenable for satellite cell culture gents, Cat. no. V2600; RRID:AB_2119238), Rabbit anti- in situ Ki67 (1:1000, Abcam, Cat. no. ab15580; RRID:AB_ To evaluate our hypothesis that human myofibers could 443209), and Wheat Germ Agglutinin Alexa 488 conju- be cultured in a laboratory setting, we decided to isolate gate (1:1000, Fisher, Cat. no. W11261). primary human tissue from neurological determination of death organ donors following ethics approval and in- Scanning electron microscopy formed consent. We evaluated muscle groups for suit- Samples were washed with water prior to dehydration in ability in myofiber culture where surgical access to both a35–100% graded series of ethanol. Samples were crit- tendinous insertions is feasible and where myofibers are ical point dried in dry 100% ethanol using liquid CO2 as short to facilitate manipulation in a laboratory setting transition fluid. CO2 was exchanged at 5-min intervals with common plasticware and reasonable reagent for 8 rounds followed by a final 3 h release. Samples volumes. We identified the small muscles of the hand, were maintained under dust-free desiccation following including the Abductor or Flexor Pollicis Brevis, the critical point drying. Samples were mounted on Flexor Digitorum Brevis of the foot or the Pronator aluminum stages using double-sided carbon tape. Quadratus of the forearm would be ideal candidates due Mounted samples were sputter coated with gold for 1 to their size and accessibility; however, clinical availabil- min (~ 10 nm) and stored under dust-free desiccation ity of these muscles limited laboratory testing. We rea- prior to imaging on a phenom Pro-X scanning electron soned that an alternative strategy would involve isolating microscope at an accelerating voltage of 15 kV. muscle with large angles of pennation, where individual myofibers are aligned at an angle oblique to the longitu- Imaging and analysis dinal angle of muscle contraction. These muscles pro- Human Psoas myofiber bundles were analyzed on a vide a mechanical advantage over shorter contraction Leica TCS SP8 confocal microscope equipped with HC distances and possess shorter myofiber fascicles attach- PL APO 20× IMM CORR objective with HyD and PMT ing to fibrous aponeuroses running along the periphery detectors. Filters and detectors were set to maximal of the muscle [47]. These muscle types would also allow bandwidth and sensitivity to limit bleed through between myofiber isolation from partial muscle dissections. channels. Tile scans were stitched directly following We identified the human Psoas muscle as a moder- acquisition using the Leica LAS AF software. Manual ately pennate muscle attached to the T12 vertebrae and image analysis was performed using ImageJ and the Lacunar ligament (Fig. 1a) with some variable Feige et al. Skeletal Muscle (2021) 11:1 Page 6 of 14 presence within populations [48, 49], as a good candi- generally lack glycolytic myosin heavy chain type IIb date to isolate myofibers. We obtained post-mortem (MyHC IIb) muscle fibers in favor of type IIx [50]. muscle samples from neurologically determined Additionally, myofiber type has an impact on satellite deceased organ donors (2♀,1♂) with a mean age of 61 cell response to exercise, where satellite cells resident to ± 9.4 years (range 50–68) (Table S1). Donors did not oxidative MyHC type I human muscles show augmented have muscular dystrophy and did not exhibit muscle expansion following aerobic training [51]. We hypothe- cachexia. Muscle samples were 24.35 ± 13.56 g in mass sized that myofiber type would influence satellite cell and 11.42 ± 2.15 cm in length and isolated within 2–4h fate in human Psoas myofiber cultures and could con- from cardiac death. found translating signalling pathways identified in mice Mouse myofiber isolation requires enzymatic digestion to human satellite cells. of the extracellular components from isolated mouse We evaluated myofiber type and histological profiles Extensor digitorum longus muscle to release single myo- of human Psoas myofiber bundles, mouse Extensor fibers amenable for culture. To test if human Psoas myo- digitorum longus (EDL) and mouse Psoas myofibers to fibers could be prepared similarly, we subjected better correlate difference in rodent and human satellite myofiber isolated Psoas biopsies to collagenase digestion. cell biology. Isolated human Psoas myofiber bundles Isolated Psoas muscle was resistant to digestion using were composed of mixed 36.5 ± 3.1% slow (type I) and collagenase type 1, type 2, type 3, type 4, type 5, type 6, 63.5 ± 6% fast (type IIa, IIx) myofibers compared to 14.4 type 7, or elastase buffers due to the thick perimysium, ± 2% type IIa and 62.5 ± 1.2% type IIb fast twitch mouse endomysium, extracellular matrix (ECM), and vascular Psoas or 11 ± 1.2% type IIa and 72.9 ± 0.7% type IIb networks present (data not shown). Thus, the established EDL muscles (Figure S1G). Human Psoas myofibers do methods of myofiber isolation used in mouse studies are not significantly differ in minimum fiber Feret (Fig. 1h) not appropriate for human myofiber isolation. or myofiber surface area (Figure S1I) compared to Therefore, we manually dissected myofiber bundles mouse Psoas or EDL fibers where a subset of human from Psoas samples from tendon to tendon (Figure S1A). Psoas myofibers are hypertrophic (Fig. 1I, S1J). Human Manually dissected myofibril bundles contained 220 ± 63 myofibers however are roughly 10-fold longer than myofibers (Fig. 1c) and were 1.9 ± 0.5 mm in diameter mouse Psoas or EDL myofibers (Fig. 1j–k) (36.4 ± 0.4 (Fig. 1d). Myofiber bundles displayed heterogeneous via- mm versus 3.34 ± 0.05 mm and 3.60 ± 0.12 mm). Taken bility, where excess tension during surgical excision or together, this data suggests that human Psoas myofibers processing resulted in samples containing hyper display distinct histological characteristics from mouse contracted myofibers which were excluded from further Psoas or EDL muscle likely due to requirements for hip analysis (Figure S1A). Successful myofiber preparations flexion and posture in an erect position. contained intact myofibers where 83 ± 5% of myofibers within a bundle did not exhibit any signs of hyper contrac- Human satellite cells expand in myofiber culture tion or injury (Fig. 1e). Injured myofibers can be distin- Satellite cells remain mitotically quiescent but are guished into hypercontracted myofibers, moderately poised to activate and enter the cell cycle in response injured and minor injured subgroups. Hypercontracted to extrinsic cues such as exercise or trauma [2]. In ro- myofibers (Figure S1B) show bisected myofiber segmenta- dents, this can be achieved by experimental models of tion within an extracellular matrix scaffold while myofi- injury, muscle digestion for stem cell isolation [52, 53] bers with moderate damage (Figure S1C) exhibit or in the case of myofiber preparation, digestion with widespread disorganization of sarcomeric banding and sig- collagenase and exposure to growth factors in cell cul- nificant autofluorescence. Myofibers with minor damage ture [21]. To evaluate if human satellite cells from (Figure S1D) exhibit focal autofluorescence and invagin- Psoas myofiber cultures spontaneously activate in vitro, ation of the extracellular matrix and maintain myofiber- we evaluated basal satellite cell numbers immediately cell-ECM contact. Quantification of sarcomere spacing following surgical excision and during culture where we through α-Actinin staining (Fig. 1f, S1E-F) showed signifi- analyzed an average 1.8 ± 1.1 mm length of myofibers cantly shorter sarcomere banding in contracted myofibers, per experiment (Figure S2A). whereas healthy myofibers can be maintained in culture Human Psoas muscle has generally more satellite cells for at least 8 days without myofiber contraction or loss of per mm (15.7 ± 3.0) compared to mouse Psoas (9.5 ± sarcomere disorganization. Together, this data suggests 0.5) or EDL myofiber cross-sections (10.0 ± 0.36) follow- that human Psoas myofiber bundles can be isolated and ing immunofluorescence staining for Pax7 (Fig. 2a). We maintained in culture without consequence to myofiber observed by immunofluorescence staining for Pax7 that integrity. human Psoas myofibers with centrally located nuclei Mouse and human myofibers exhibit different histo- possess significantly increased (75.1%, 8.67 ± 1.6 versus logical characteristics, where human skeletal muscle 10um 10um 10um Feige et al. Skeletal Muscle (2021) 11:1 Page 7 of 14 AB C D Mouse D0 Human D0 Human D0 Human Psoas 100 100 80 80 60 60 40 40 20 20 0 0 10um d0 d0 EF Human D0 Human D4 Human D8 10um 10um 10um GH I J p<0.005 150 40 50 100 p=0.02 p=0.03 p=0.03 40 80 30 60 20 40 10 20 0 0 0 d0 d4 d8 d0 d4 d8 d0 d4 d8 d0 d4 d8 Fig. 2 Human satellite cells expand in culture. a Representative image and quantification of satellite cell density from human Psoas minor muscle cross sections stained with DAPI (blue), Perlecan (green), Pax7 (red), and Laminin (white). b Representative images of mouse and human satellite cells in the niche at isolation stained for DAPI (blue), Laminin (green), dystrophin (red) and Pax7 (white). c Representative image and quantification of human satellite cells expressing Syndecan-4 (SDC4) at isolation stained with DAPI (blue), Syndecan-4 (green), and Pax7 (red). d Quantification of human satellite cells expressing M-Cadherin (MCAD) at isolation. e Representative scanning electron micrograph of cultured human myofibers (day 8) showing maintenance of myofiber extracellular matrix composition and cell-cell contacts. f Representative images of human myofibers in culture from day 0, day 4, and day 8 stained with DAPI (blue), Ki67 (green), Pax7 (red), and dystrophin (white). Quantification of g total nuclei per mm of myofiber, h satellite cells expressing Ki67 per millimeter of myofiber, i percentage of satellite cells per total nuclei per mm of myofiber, j proportion of satellite cells expressing Ki67 on myofibers. a, c, d, g–j Error bars represent mean ± SEM; a n = 3 biological replicates for mouse EDL, mouse Psoas, n = 5 biological replicates from human Psoas. c, d, g–j n = 3 biological replicates 15.2 ± 3.1) satellite cells per millimeter of myofiber double labelled with Pax7 and Ki67 to assess human sat- (Figure S2B-C) possibly due to prior repair in vivo. ellite cell proliferation. We observed no statistical Human Psoas satellite cells reside within the niche at change in total nuclei per millimeter of myofiber follow- isolation (Fig. 2b), where 86.5 ± 5.25% of human satellite ing 8 days in culture (Fig. 2g) with a concomitant cells express the cell surface marker Syndecan-4 (Fig. 2c, increase in proliferating satellite cells (0.017 ± 0.02 ver- S2D, F) and 34.8 ± 7.6% heterogeneously express M- sus 23.8 ± 4.3 per millimeter myofiber day 8) (Fig. 2h) Cadherin (Fig. 2d, S2G). Most satellite cells maintain and other cell types (0.7 ± 0.2 versus 7.6 ± 0.8 per milli- Syndecan-4 expression in culture (84.1.0 ± 6.7%) meter myofiber day 8) (Figure S2I) while the number of (Figure S2F) and do not express the apoptotic Ki67-satellite cell falls (9.18 ± 1.9 versus 2.06 ± 0.74 per marker Annexin-5 (Figure S2D). millimeter myofiber day 8). Following 8 days in culture, To evaluate myofiber integrity and maintenance of human satellite cells make up 30.0 ± 3.7% of total nuclei cell-cell and ECM interactions throughout culture, myo- along myofibers (Fig. 2I). As the number of activated fiber bundles were subject to scanning electron micros- satellite cells heterogeneously increases at day 4 (Fig. 2h, copy. Electron micrographs (Fig. 2e) show myofiber S2I), the first satellite cell divisions likely occur between bundles exhibit no apparent tearing or porosity and day 3 and day 4 in culture (Fig. 2h, i), where the bio- maintain complex cell-cell and cell-ECM interactions logical variance between samples is greater than that be- following 8 days in culture. This data suggests human tween myofibers (Figure S2K). Following 8 days in satellite cells expand locally in response to prior trauma culture, most satellite cells (91.8 ± 1.2%) express Ki67 and express and maintain Syndecan-4 expression (Fig. 2j). Taken together, this data suggests human satel- throughout culture. lite cells actively proliferate in culture along with other To directly assess satellite cell expansion in culture, we resident cell types however with slower activation kinet- quantified satellite cell numbers throughout culture and ics than that of mouse satellite cells. mEDL mPSOAS hPSOAS DAPI PLC Pax7 LAM Pax7 per mm DAPI LAM Dmd Pax7 DAPI Ki67 Pax7 Dmd Total Nuclei /mm fiber Ki67+ Pax7+ Nuclei DAPI SDC4 Pax7 /mm fiber % Pax7+ Nuclei /mm fiber % SCD4+ satellite cells % Pax7+ Ki67+ % MCAD+ satellite cells Feige et al. Skeletal Muscle (2021) 11:1 Page 8 of 14 EGF stimulation promotes myogenic progenitor asymmetric cell division to orient the satellite cell mi- formation in human Psoas myofiber culture totic spindle in an apical-basal orientation [23]. To To evaluate myogenic differentiation in Psoas myofiber evaluate if human Psoas satellite cells integrate EGFR culture, we examined myofibers by immunofluorescence signalling, cultures were treated with EGF ligand staining for Myogenin (MyoG), a transcription factor throughout culture to stimulate EGFR activation. Fol- expressed at the onset of differentiation [54]. Culturing lowing EGFR stimulation, myofibers were stained for the Psoas myofibers for 8 days results in significant presence presence of phosphorylated EGFR, where most satellite of MyoG-expressing cells (20.8 ± 1.8 per millimeter cells express p-EGFR (Fig. 3d, Figure S3D), and its ex- myofiber) concomitant with proliferation of satellite pression can become locally restricted as observed in ac- cells, where we do not observe co-labelling of Pax7 and tivated mouse satellite cells [23]. This data suggests that MyoG by immunofluorescence (Fig. 3a–c, S3A). the EGFR signalling pathway is activated following EGF Additionally, by 8 days in culture, we can observe de treatment in human satellite cells in a manner analogous novo myofiber formation occurring characterized by to mouse satellite cells. multiple organized and aligned myocytes expressing To evaluate if EGFR augmented the production of MyoG (Figure S3B-C) suggesting human Psoas myofiber myogenic progeny through the promotion of asymmetric culture may represent a paradigm to model human satel- division as found previously in mouse muscle [23], we lite cell activation, differentiation, and myofiber forma- examined fibers by immunofluorescence for Pax7, tion. This data indicates that by 8 days in culture, MyoG, and Ki67. Quantifying total nuclei per millimeter human Psoas satellite cells expand in number and of fiber show no significant change following 8 days of express differentiation markers including MyoG. EGF treatment (Figure S3H); however, treatment with We have previously established that mouse satellite EGF results in an appreciable increase in the number of cells express EGFR that is polarized before an proliferating satellite cells per millimeter of myofiber A Human myofiber Day 8 BC 40 40 p=0.008 10 10 0 0 d0 d8 d0 d8 EF G 50 8 50 40 40 30 30 10 10 0 0 d8 d8 d8 d8 d8 d8 EGF EGF EGF Fig. 3 Human satellite cell expansion and differentiation can be tuned in situ. a Representative image (magnification from Figure S3A) of human satellite cells and Myogenin (MyoG) expressing differentiating progenitors cultured on 8-day human Psoas minor myofiber cultures stained with DAPI (blue), MyoG (green), and Pax7 (red). Quantification of b number of Pax7-expressing cells per millimeter of myofiber, c MyoG-expressing cells per millimeter of myofiber. d Representative images of human satellite cells expressing phosphorylated active EGFR (p-EGFR) stained with DAPI (blue), p-EGFR (green), Pax7 (red), and dystrophin (white). White arrow denotes localized p-EGFR expression. Quantification of e satellite cells expressing Ki67 per millimeter of myofiber and f Ki67-negative satellite cells per millimeter of myofiber following EGF treatment or vehicle control of human myofibers. g Quantification of number of MyoG-expressing cells per millimeter of myofiber following EGF treatment of human myofibers. b, c, e–g Error bars represent means ± SD (EGF) and means ± SEM (control); b, c n = 3 biological replicates. d–g n = 3 biological replicates control, n = 2 biological replicates EGF DAPI pEGFR Pax7 Dmd DAPI MyoG Pax7 Pax7+ cells per mm myofiber Ki67+ Pax7+ Nuclei MyoG+ cells per mm fiber per mm myofiber Ki67- Pax7+ nuclei per mm fiber MyoG+ cells per mm myofiber Feige et al. Skeletal Muscle (2021) 11:1 Page 9 of 14 (56.7% paired, 78% unpaired, n = 3 control, n = 2 EGF) examined myofibers by immunofluorescence staining (Fig. 3e, S3F) and an increased proportion of satellite throughout culture. Interestingly, we observed rare apico- cells per myofiber (29% increase unpaired, 15% increase basal and planar-oriented satellite cell doublets expressing paired, n = 3 control, n = 2 EGF) (Figure S3G). Interest- Pax7 and residing in the niche from samples fixed at isola- ingly, EGF treatment increased the number of Ki67- tion and stained with Pax7, Perlecan, and Dystrophin negative satellite cells per millimeter of myofiber (161% (Fig. 4). As these cells are occupying the same niche space, increase unpaired, 118% increase paired, n = 3 control, n we believe this is not due to random cell migration along = 2 EGF) following 8 days in culture treated with EGF the myofiber, suggesting that homeostatic repair mecha- or vehicle control (Fig. 3f, S3H). However, the propor- nisms may undergo either mode of division. tion of Pax7/Ki67 expressing cells was similar following We additionally examined culture day 3 to day 4, a EGF treatment (92% control vs 89% EGF) suggesting any time point reflecting the first division in human satellite effect as a mitogen was negligible. Concomitant with an cells in culture. Strikingly, staining for the protein dys- increase in proliferating satellite cells, EGF treatment re- trophin (DMD), which can be polarized in mouse satel- sulted in an appreciable increase in MyoG-expressing lite cells to facilitate asymmetric division [40], shows cells per millimeter of myofiber (69% increase unpaired, strong expression and polarity on the basal surface of a 78% increase paired n = 3 control, n = 2 EGF) (Fig. 3g, subset of cultured satellite cells (Fig. 5a), along with a S3J). Therefore, we conclude that activation of EGFR subset of satellite cells expressing non-polar dystrophin signalling in muscle stem cells promotes the generation (Fig. 5b). We observe asynchronous expression of DMD of progenitors, likely by stimulating asymmetric divi- in satellite cells along myofibers, where cells in close sions, similarly between human and mouse. proximity can express DMD perhaps due to region- specific cues present within the myofiber microenviron- Human satellite cells undergo apicobasal and planar cell ment. This suggests that human satellite cells can divisions and express polarized dystrophin polarize dystrophin to their basal surface interfacing To evaluate the possibility of human satellite cells under- with the extracellular matrix in an identical manner to going apical-basal-oriented asymmetric division, we activated mouse satellite cells. AB Apicobasal Planar DAPI PLC Pax7 Dmd DAPI PLC Pax7 Dmd DAPI PLC Pax7 Dmd Fig. 4 Human satellite cells can orient division angles. Representative images of a planar and b apicobasal-oriented human satellite cells in the niche at isolation stained with DAPI (blue), Perlecan (green), Pax7 (red), and dystrophin (white). n = 3 biological replicates Feige et al. Skeletal Muscle (2021) 11:1 Page 10 of 14 Polar B Non-Polar C Diffuse Fig. 5 Human satellite cells express polarized dystrophin in culture. Representative image of human satellite cells cultured for four days and stained with DAPI (blue), Pax7 (red), and dystrophin (green) showing a polarized dystrophin localization, b non-polar dystrophin localization, and c diffuse dystrophin staining. n = 3 biological replicates Discussion essential to maintaining muscle repair over a lifetime. Evaluating human stem cell dynamics in a relevant con- Asymmetric stem cell division is one method to bal- text is critical to model biological phenomena and ance stem cell maintenance and the production of generalize results to benefit human health. New methods myogenic progeny, where following division one to improve the pre-clinical evaluation of therapeutic daughter cell maintains its stem cell state and one strategies to augment endogenous stem cell activity hold differentiates down the myogenic lineage. Asymmetric promise to improve regenerative medicine outcomes in cell division is established through cells integrating conditions such as Duchenne’s muscular dystrophy. extrinsic environmental cues to restrict cell fate deter- Here, we developed a novel system to evaluate human minants in a polarized manner such that when a cell satellite cell fate choices in a relevant context to interro- orients its mitotic centrosomes parallel with internally gate human satellite cell biology and evaluate pre- polarized cell fate determinants, daughter cells will re- clinical therapeutics in improving muscle regeneration ceive discrete cellular contents [2]. Typically, in (Fig. 6). mouse satellite cells, the daughter cell maintaining A balance in satellite cell proliferation, production niche interactions with the basal lamina maintains its of myogenic progeny, and return to quiescence is stem cell nature. Establishment of an apical-basal- DAPI Dmd Pax7 DAPI Dmd Pax7 DAPI Dmd Pax7 DAPI Dmd Pax7 Feige et al. Skeletal Muscle (2021) 11:1 Page 11 of 14 Fig. 6 Graphical outline of human Psoas culture. Schematic diagram summarizing the procedure for the characterization of human satellite cell dynamics on cultured intact myofibers from the human Psoas muscle oriented mitotic spindle is in part facilitated by the studies exploring other resident cell types such as PAR polarity complex, where we have previously fibroblast, pericyte, and fibroadipogenic precursor shown EGFR is spatially restricted before mitotic divi- cells will better evaluate the specificity of EGF in cul- sions to orient centrosomes through recruitment of ture. Additionally, studies exploring growth factor and Aurora kinase A and spindle assembly [23]. We hy- oxygen penetration in myofiber bundles through pothesized that as human Psoas myofibers maintain staining of growth factors and incubation in oxygen myofiber and extracellular matrix composition that sensing compounds such as Hypoxy Probe labels human satellite cells in culture could integrate three- would further evaluate the potential effect of nutrient dimensional external cues to influence cell fate. availability on heterogeneity in satellite cell activity Our findings support that human Psoas myofiber within myofiber bundles. Studies exploring non- cultures provide a new opportunity to culture human myogenic cell death occurring in culture day 4–8may satellite cells to explore fate choices during satellite provide insight into the cellular dynamics along a cell activation and differentiation. In our system, myo- myofiber during early muscle repair. fiber integrity is maintained (Fig. 1, S1) as well as cell The observation that EGF treatment increases the polarity cues (Figs. 2b, e, 4,and 5) where treatment number of non-proliferative satellite cells following with EGF results in augmented production of myo- treatment (Fig. 3f) suggests that the effect of EGF on genic progeny (Fig. 3e–g). As EGF treatment also in- satellite cells may not be acting as a general mitogen. creases the number of non-satellite cells expressing Taken with an increase in proliferating satellite cells Ki67 (Figure S3I), it is possible that EGF influences (Fig. 3e) and increased formation of myogenic pro- cell survival or is acting as a mitogen on the varied geny (Fig. 3g), supports the hypothesis that EGF cell populations within myofiber cultures. Further treatment stimulates asymmetric division resulting in Feige et al. Skeletal Muscle (2021) 11:1 Page 12 of 14 augmented production of non-cycling satellite stem conserved mechanism first discovered in mouse. Future cells (Pax7+Ki67−) and rapidly proliferating myogenic studies with increased experimental size exploring the satellite cells similar to that observed in mouse [23]. kinetics of cell cycle entry and quantification of division Further studies are required to increase biological angles following the first satellite cell division (likely days sample sizes to fully delineate the role of EGF on hu- 2–3) will shed important insight into the extent of sym- man satellite cells. Taken together, the human Psoas metric and asymmetric satellite cell divisions occurring muscle provides an exciting tool to explore in niche in human muscle. satellite cell biology and facilitate translation of pre- clinical therapeutics from studies in model organisms Conclusion to humans. Human myofiber culture provides an exciting opportun- Previous studies have developed methods to assess ity to evaluate pre-clinical drug efficacy in a relevant hu- human satellite cell expansion in vitro from primary tis- man context. This method provides an opportunity to sue [55] through the culture of hypercontracted human assay satellite cell heterogeneity, stem cell potential, myofiber fragments from punch biopsies. Myofiber frag- stem cell hierarchy, and activation kinetics with the po- ments contained 1–8 bisected myofibers 2–3mmin tential to therapeutically interrogate pathways of interest. length and by 10 days in culture, 80% of nuclei were Additionally, this method provides an additional tool to Pax7+ with a significant amount of Desmin expressing validate phenomena observed in other model organisms, cells within the myofiber [55]. Transplantation of whole validate lead compounds for drug discovery and testing, myofiber fragments resulted in limited engraftment into reduce animal model use, and accelerate the evaluation mouse recipients. Differences in timepoints and muscle of therapeutics improving human health. We envision groups assessed between our study and others [55] limit this technique will aid in generalizing pre-clinical strat- comparisons; however, in our hands’ injury to the myofi- egies into the clinical arena and may in the long term be ber through hyper contraction or bisection results in dis- appropriate as a personalized therapeutic tool. organized myofiber sarcomeres (Fig. 1f) and altered myofiber-ECM interactions (Figure S1B-C), where only Supplementary Information minor focal damage is tolerated along myofibers to The online version contains supplementary material available at https://doi. org/10.1186/s13395-020-00256-z. maintain myofiber-cell-ECM interactions (Figure S1D). Typically, hypercontracted fibers are discarded in experi- Additional file 1: Supplemental figures related to figures 1-3, patient in- ments using mouse myofibers due to the abnormal formation used in this study and key resource table. Figure S1: Myofi- behavior of satellite cells [56]. Additionally, cell polarity bers from human Psoas muscle can be maintained in situ, Related to Fig. is maintained in Psoas myofiber culture (Figs. 3d and 5) 1. A) Photographic overview of human Psoas minor myofiber bundle iso- lation showing expanded images of intact myofiber bundles (panel 9) and by 8 days ~ 30% of cells express Pax7 and ~ 20% and hypercontracted myofiber bundles (panel 10). Representative images represent committed progeny. Differences between the of B) hypercontracted myofibers and C) myofibers with moderate dam- models could be attributed to differential activation cues, age stained for DAPI (Blue), α-Actinin (Green) and Myosin heavy chain (MF20, Red). D) Representative image of myofibers with minor damage non-satellite cell survival, differential activation in differ- stained for DAPI (Blue), Dystrophin (Green), Laminin (White) and IgG ent muscle groups, or limitations in our study including (Red). E) Representative images of single myofiber sarcomeres from intact, exogenous culture of myofibers, modest sample size, or contracted and cultured myofibers stained with α-actinin (Green) show- ing representative histograms of staining intensity and sarcomere spa- differences in outbred human donors. This suggests cing. F) Representative image of disorganized sarcomeres from injured human Psoas myofiber culture may reflect a model of myofibers stained with α-Actinin (Green) and MF20 (Red). G) Representa- homeostatic turnover or response to minor injuries such tive images and quantification of myofiber type from mouse Extensor digitorum longus and mouse Psoas muscle stained with Type 1 myofi- as load-induced trauma or de-innervation, while human bers (Blue), Type 2a myofibers (Green), Type 2b myofibers (Red) and myofiber fragments may represent a paradigm of rapid Wheat germ agglutinin (White). H) Representative image of human Psoas satellite cell activation in response to widespread myofi- muscle cross sections stained with Laminin (Red) with I) quantification of average myofiber surface area and (J) myofiber surface area proportion ber damage. from human Psoas myofibers compared to mouse Extensor digitorum Our findings provide proof-of-principle evidence to longus and mouse psoas muscles using SMASH software. K) Representa- support that in addition to the mature myofiber, human tive image and quantification of mouse Extensor digitorum longus and mouse psoas myofiber lengths from isolated single myofibers. (K) Error satellite cells express polarized dystrophin during satel- bars represent mean ± SD, (G-J) Error bars represent mean ±SEM; (G, I-J) lite cell activation and that human myofiber culture rep- n = 3 biological replicates, (K) n = 40 myofibers per condition. Figure resents a novel paradigm to explore human satellite cell S2: Human satellite cells expand in situ, Related to Fig. 2. A) Quantifica- tion of average length of myofiber analyzed per experiment, whiskers self-renewal and myogenic differentiation. We further represent min and max. B) Representative image of human myofibers validate that human satellite cells can undergo planar showing centrally located nuclei stained with DAPI (Blue), Ki67 (Green), and apicobasal-oriented divisions and demonstrate the Pax7 (Red) and Dystrophin (White) and C) quantification of satellite cells per mm myofiber present at isolation on centrally nucleated fibers (CNF). EGFR pathway to be of pre-clinical interest to augment D) Representative image of myofibers stained with DAPI (Blue), SDC4 the production of myogenic progeny through a Feige et al. Skeletal Muscle (2021) 11:1 Page 13 of 14 Ethics approval and consent to participate (Green) Pax7 (Red) and Annexin-5 (White) with E) bisected myofibers This study was approved by the Ottawa Hospital Research Ethics Board serving as positive control stained for Annexin-5 (White) DAPI (Blue) and under the protocol number 20150544-01H. Pax7 (Red). F) Quantification of satellite cells expressing SDC4 at day 8 in culture. G) Representative image of satellite cells expressing M-Cadherin after isolation stained for DAPI (Blue), MCAD (Green) and Pax7 (Red). H) Consent for publication Representative image of satellite cell expansion on myofibers following 8 Not applicable. days in culture stained with DAPI (Blue), Ki67 (Green), Pax7 (Red) and Dys- trophin (White) and quantification of I) Ki67 expression non-satellite cells per mm of myofiber, J) number of KI67 negative satellite cells per mm of Competing interests myofiber and K) Ki67 expressing satellite cells per mm of myofiber across M.A.R is CSO and Founder of Satellos Bioscience. P.F. and E.T. declare no samples (s#). (A, C, K) Error bars represent mean ± SD, (F, I-K) Error bars competing interests. represent mean ± SEM; (A) n = 351 myofibers. (C) n = averages from 20 (non-CNF) and 9 (CNF) myofibers. (F, I-K) n = 3 biological replicates. (K) n Author details = averages from 4-22 myofibers, where individual data points represent 1 Sprott Center for Stem Cell Research, Regenerative Medicine Program, individual myofibers. Figure S3: Myofiber culture unveils unique regen- Ottawa Hospital Research Institute, 501 Smyth Road, Ottawa, ON K1H 8L6, erative phenomena, Related to Fig. 3. Representative images of A) Repre- Canada. Department of Cellular and Molecular Medicine, Faculty of sentative image of cultured myofiber bundle stained for DAPI (Blue), 3 Medicine, University of Ottawa, Ottawa, ON, Canada. Department of MyoG (Green) and Pax7 (Red) (also presented in Figure 3A for reference). Medicine, Faculty of Medicine, University of Ottawa, Ottawa, ON, Canada. B) Representative image of myogenic progenitors and C) in situ de novo 4 Department of Surgery, Division of Neurosurgery, Faculty of Medicine, myofiber repair from fibers stained with DAPI (Blue), MyoG (Green) and University of Ottawa, Ottawa, ON, Canada. Ottawa Hospital Research MyoD (Red) where white dotted arrows outline the myocyte alignment. Institute, Neuroscience Program, Ottawa, ON, Canada. D) Representative images of cultured myofiber bundles stained for DAPI (Blue), pEGFR (Green) and Pax7 (Red). Quantification of E) total nuclei per Received: 3 September 2020 Accepted: 6 December 2020 mm of myofiber and across samples. F) Quantification of human satellite cells expressing Ki67 or Ki67 negative per mm of fiber across samples fol- lowing culture in control or EGF containing media. G) Quantification pro- portion of nuclei expressing pax7 per myofiber. Quantification of H) References proportion of satellite cells (Pax7+) stained negative for Ki67 and I) pro- 1. Bentzinger CF, Wang YX, Dumont NA, Rudnicki MA. Cellular dynamics in the portion of non-satellite cells (Pax7-) expressing Ki67 following culture in muscle satellite cell niche. EMBO Rep. 2013;14(12):1062–72. control or EGF containing media. 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