Background: Adrenal glands are essential endocrine organs composed of two embryological distinct tissues. Morphological changes during their development are well described, but less understood with regard to their molecular mechanisms. To identify proteins and pathways, which drive the initial steps of the specification of the endocrine function of the adrenal gland, rat’s adrenal glands were isolated at different embryonic days (E): E14, E16, E18, E19 and postnatal day 1 (P1). Results: The alteration of the proteome during the stages E16, E19 and P1 was investigated by combining two dimensional gel electrophoresis and mass spectrometric analysis. Out of 594 excised protein spots, 464 spots were identified, resulting in 203 non-redundant proteins. The ontogenic classification of the identified proteins according to their molecular function resulted in 10 different categories, whereas the classification of their biological processes resulted in 19 different groups. This gives an insight into the complex mechanisms underlying adrenal gland development. Interestingly, the expression of retinoic acid pathway proteins was decreased during the development of the adrenal gland, suggesting that this pathway is only important at early stages. On the other hand, key proteins of the cholesterol synthesis increased their expression significantly at E19 revealing the initiation of the endocrine specialization of the adrenal glands. Conclusions: This study presents the first comprehensive wide proteome analysis of three different stages of embryonic adrenal gland development. The identified proteins, which were expressed in early stages of development, will shed light on the molecular mechanisms underlying embryonic development of the adrenal gland. Keywords: Adrenal gland, Proteomics, Prenatal development Background pH, and glucose homeostasis through the synthesis of cor- Adrenal glands (AGs) are complex endocrine organs, ticosteroids, such as cortisol [3,4]. which are vitally necessary. Each gland is composed of The development of the adrenal gland begins with the two embryological distinct tissues, the mesodermal cortex migration of neural crest cells toward the coelomic cav- and the medulla, arising from the neural crest ectoderm. ity wall and forms a thickening at the medial side of the The adrenal medulla produces catecholamine, such as epi- fetal cortex anlage of the adrenal gland. When the fetal nephrine (adrenaline) [1,2], in the classic immediate stress cortex surrounds these cells, they start to differentiate response (fight or flight), whereas the adrenal cortex regu- into catecholamine secreting chromaffine cells of the ad- lates the long-term stress response by modulating salt, renal medulla. Later, further mesenchymal cells detach from the mesothelium and surround the fetal cortex, * Correspondence: email@example.com building the permanent cortex [5,6]. Department of Nephrology and Rheumatology, University Medical Center The adrenal cortex, the outer 90% of the gland, syn- Goettingen, Georg-August University Goettingen, Robert-Koch-Strasse 40, thesizes corticosteroid hormones from cholesterol. Its D-37075 Goettingen, Germany Full list of author information is available at the end of the article © 2015 Dihazi et al.; licensee BioMed Central. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. 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. Dihazi et al. Proteome Science (2015) 13:6 Page 2 of 13 major secretions are cortisol, an important metabolic fixed number of genes. Moreover, gene expression changes, hormone, androgens such as androstenedione and dehy- which result in alteration in mRNA level, do not necessar- droepiandrostenedione (DHEA), and aldosterone, a hor- ily result in protein expression or activity modification. mone regulating water and electrolyte concentrations Proteome investigation might help to overcome the limita- [5,6]. The adrenal medulla, the inner 10% of the gland is tions of the transcriptomic analysis and deliver important the main source of the catecholamines epinephrine (adren- insights into the molecular mechanisms of AG develop- aline) and norepinephrine (noradrenaline). ment on the proteome level. For these reasons we decided The adrenal gland is an organ, which undergoes major to use proteomics methods to investigate changes in the postnatal developmental modifications [7-9]. Recently, a protein expression during AG embryonic development. differential proteomic analysis of adrenal gland during We performed a comparative proteomic analysis of two postnatal development focusing on regulated proteins, prenatal stages (embryonic day 16 (E16) and 19 (E19)) as which are involved in steroidogenesis, cell proliferation, well as of newborn rats (postnatal day 1 (P1)). Here we and cholesterol synthesis  was published. report some of the molecular modifications identified, The underlying molecular mechanisms of the prenatal and discuss the subjacent functions of the differentially development of the adrenal gland are still not completely expressed proteins. understood. To unravel the protein expression changes that accompany the AG embryonic development, two Results main approaches might be considered: the analysis of Preparation and HE staining of the adrenal glands the transcriptome and proteome investigation. Tran- To identify and characterize proteins and pathways in- scriptomic analysis is a powerful method, widely used volved in the development of the adrenal gland, rats for comparison of gene expression of different biological were obtained at different stages of embryonic (E14, samples. Nonetheless, mRNA translation might be influ- E16, E18 and E19) and neonatal (P1) development and enced by many factors, which can have a profound im- the AGs were obtained (Figure 1A). The HE staining of pact on the amount of protein synthesized. Additionally, the adrenal gland from E19 embryos showed the two post-transcriptional and post-translational events, which distinct tissues of this endocrine organ, the mesodermal play important roles in embryonic development, increase cortex, which is the zone of steroid-synthesizing cells the diversity of proteins that can be synthesized from a and the medulla, which consists of cells arising from the E19 adrenal gland E16 adrenal gland kidney kidney adrenal cortex kidney adrenal medulla adrenal gland 4x 10x Figure 1 Adrenal gland preparation. A: Embryos were removed from the mother at different days of development. The adrenal glands were excised from the embryos as well as from the newborn rats (P1). B: HE staining of a paraffin section of a rat embryo at E19 showing the kidney and the adrenal gland, enclosing the adrenal cortex and the adrenal medulla. Dihazi et al. Proteome Science (2015) 13:6 Page 3 of 13 neural crest ectoderm and is responsible for synthesis To gain more information on the biological mecha- and secretion of catecholamines (Figure 1B). nisms of the identified proteins in embryonic AGs, we combined DAVID bioinformatics with information on Mapping of embryonic adrenal glands proteome the putative function of the protein found in the UniProt Proteins were extracted and purified from adrenal glands and GenBank databases. Thereby, we were able to anno- homogenates, and separated by 2-D gel electrophoresis tate all 203 gene products. An analysis of the molecular as described in material and methods. The 2-DE analysis function of the identified proteins based on the Gene was performed using immobilized pI gradients (IPG) (pI Ontology (GO) terms allowed the classification of the 5–8) in the first dimension, and SDS-PAGE in the sec- proteins into ten different categories (Figure 4A); ap- ond dimension. To investigate protein pattern differ- proximately half of these proteins were classified as ences between E16 and E19/P1, three independent 2-DE binding proteins (48%, 180 proteins) and about one- images of each protein extract from three independent fourth were involved in catalytic activities (28%, 106 pro- biological replicates were selected for comparative and teins). The rest of the proteins were classified in other statistical analyses. We assessed the degree of 2-DE tech- categories with less than 5% (20 proteins) per group nical variation by determining the coefficient of variation e.g. transcription regulation, enzyme activity and anti- (CV) for all spot volumes on a gel for all technical repli- oxidant activity. The ontological classification of the cates. Protein spot volumes were determined for all identified proteins according to their postulated involve- matched spots in an experiment set using Delt2D soft- ment in biological processes resulted in 19 different ware. We observed only minimal differences in CV categories revealing the complexity of mechanisms values from replicate gels from the same embryonic stage, that drives embryonic development of adrenal glands more than 85% of the matched spots have a CV% < 20%. (Figure 4B). To investigate the biological variation that can result from the use of different pregnant female animals for AG isola- Protein expression changes during embryonic tion, CV% was determined for AG 2-DE from same em- development: alteration in cholesterol metabolism and bryonic stage isolated from different pregnant female. retinoic acid pathway proteins Depending on the embryonic stage, between 940 and A total of 168 non-redundant proteins were identified in 1150 protein spots were detected on 2-DE gels. The CV% E16, 190 proteins in E19, and 195 proteins in P1 (Figure 5). values were below 10% for 25–31% of the spots and below Comparative analysis of the protein expression with 20% for 75–86% of the spots. regards to the embryonic stage showed that 163 proteins The spots detected by image analysis in the Flamingo- were present in all three stages, whereas the other proteins stained gels are presented in Figure 2A for E19 and in were uniquely found in one or two stages (Figure 5). Additional file 1: Figure S1 for E16 (A) and P1 (B). The Several proteins, which were identified in all stages, gels were analyzed by Delta 2D Version 4.3 (Decodon, showed significant expression regulation between E16 Braunschweig). The 2-DE gel images of the different and P1. An examination of all regulated proteins revealed stages were overlaid and carefully compared. Evaluation 48 (31 non-redundant) proteins for which expression was of the 2-DE maps of the three stages revealed high simi- increased in P1 (Table 1) and 48 proteins (36 non- larity (>95% spots overlapping) in the protein pattern be- redundant) for which expression was decreased in tween the late embryonic (E19) and newborn (P1) stages, stageP1(Table2). whereas the adrenal glands from the early embryonic stage The ontological classification of proteins for which ex- (E16) showed different protein patterns than E19 and P1 pression was significantly altered in AGs during embry- (Figure 2B and Additional file 1: Figure S1C, D). To obtain onic development, was performed according to their an overview of the existing proteins in the AG at the dif- molecular function (Figure 6A, 6B). Although the regu- ferent stages, more than 500 protein spots were excised lated proteins were divided into two groups; proteins for from the 2-DE gels and further processed for identifica- which expression was increased (Table 1) and proteins tion. Protein spots, which were present at all three investi- for which expression was decreased (Table 2), they partly gated AG stages, were only excised for identification from belong to the same categories of molecular function. Six one of the gels. proteins for which expression was decreased and eight The excised spots were in-gel digested with trypsin and for which expression was increased are involved in lipid processed for mass spectrometric analysis (MS/MS). Pro- binding. Three of the proteins, which expression was in- teins were identified by the sequence databases search creased, are involved in fatty acid binding (Alb, Fabp3, using Mascot. A total of 464 protein spots were identified and Fabp6) (Figure 6A), whereas three of the proteins resulting in 203 non-redundant proteins (Additional file 2: for which expression was decreased, are involved in ret- Table S1). Some of the proteins were identified in multiple inol binding (Rbp1, Crabp1, and Crabp2) (Figure 6B). spots, as shown in Figure 3. The expression of some of these proteins was confirmed Dihazi et al. Proteome Science (2015) 13:6 Page 4 of 13 Hsp90b1 Hsp90b1 Hnrnpk Stip1 Gmps Pdia3 Pdia3 Pdia3 Pdia3 Star Atp5b Aldh2 Vim Hspd1 Hnrnpf Dnaja1 Hnrnpk Nudc Pdha1 Strap Actb Akr1b1 Actb Eef1d Akr1b1 Akr1e2 Txnl1 Rplp0 Rplp0 Hnrnpdl Cotl1 Anxa5 Akr1e2 Nmral1 Gfap Farsa Hibadh Calb2 Ca2 Ca1 Ctsb Eif4e Psmd Uchl3 Gsto1 Idi1 Star Pgam1 Star Star Star Glo 1 Apoa1 Tpi1 Ran Psma6 Prdx3 Psmb1 Mesdc2 Apoa1 Prdx3 Prdx3 Cmpk1 Mesdc2 Aprt Atp5h Stmn1 Dut Prdx5 Arpc5 Sod1 Nme2 Nme1 Sumo2 Hist1h2ba Rbp1 Lgals1 Ddt Hbb Txn Cstb Txn Plec1 Fabp6 Fabp6 Fabp3 Mtpn Gfap Eif1b Hrsp12 Mtpn Hspe1 Txn S100a11 S100a10 Cox5a Dyn1rb1 B E 19 E 23 Hsp90b1 Hsp90b1 Hnrnpk Hspa9 Hspa9 Stip1 Gmps Lmnb1 Alb Alb Alb Alb Alb Hspd1 Hspd1 Lmnb1 Hspd1 Hspd1 Hspd1 Pdia3 P4hb Pdia3 Pdia3 Pdia3 Pdia3 Star Fgb Gfap Atp5b Vim Aldh2 Aldh2 Hspd1 Aldh2 Pdia6 Hnrnpf Eno1 Dld Aldh2 Hnrnpk Dnaja1 Nudc Bckdha Pdha1 Hspa8 Actb Apoa4 Eno1 Idh1 Akr1b1 Hsp9 Farsa Eno1 Acads Nudc Strap Ivd Actb Eif3g Farsa Ddah1 Ctsd Actb Ctsd Gapdh Akr1a1 Strap Prdx3 Akr1b1 Akr1b1 Rgn Actb Akr1b1 Akr1e2 Akr1b1 Eef1d Eif3g Rplp0 Anxa3 Txnl1 Eif3i Rplp0 Hnrnpdl Pdhb Gapdh Gnb1 Cotl1 Akr1e2 Anxa5 Krt73 Akr1b1 Hnrpdl Nmral1 Ca2 Phb Krt6a Vdac2 Gfap Ddah2 Krt6a Ca1 Hibadh Farsa Cbr1 Cbr1 Ca2 Calb2 Ca1 Ctsb Capzb Ywhaz Eif4e Tpi1 Psmd Gsto1 Calr Uchl3 Pgam1 Canx Pgam1 Etfb Erp29 Gsto1 Idi1 Tpi1 Uchl3 Star Set Star Pgam1 Hspa9 Star Mesdc2 Star Prdx6 Echs1 Cotl1 Psma6 Rasl2-9 Glo 1 Apoa1 Park7 Ech1 Ran Psma6 Ndufv2 Apoa1 Apoa1 Prdx3 Atp5h Mesdc2 Prdx3 Psmb1 Prdx3 Psma2 Sod2 Psmb4 Atp5a1 Pebp1 Gstp1 Apoa1 Tagln2 Apoa1 Prdx3 Psma6 Farsa Prdx3 Prdx2 Tuba1a Prdx3 Prdx3 Cmpk1 Psmb6 Aprt Fth1 Atp5h Farsa Farsa Calr Stmn1 Dut Prdx5 Hist1h4b Arpc5 Nme1 Sod1 Nme1 Sumo2 Nme2 Hist1h2ba Ube2n Stmn1 Nme2 Rbp1 Lgals1 Ddt Hbb Farsa Hbb Cstb Farsa Txn Txn Cstb Fabp6 Ndufa5 Fabp6 Fabp6 Plec1 Fabp6 Hrsp12 Fabp3 Mtpn Gfap Farsa Eif1b Mtpn Hrsp12 Hspe1 Cox5a Txn S100a11 Cox5a S100a10 Dyn1rb1 Farsa Figure 2 (See legend on next page.) Dihazi et al. Proteome Science (2015) 13:6 Page 5 of 13 (See figure on previous page.) Figure 2 2-DE gel of the adrenal gland. A: 2-DE reference map of proteins extracted from the adrenal gland of an E19 embryo. 150 μg proteins were loaded on an 11-cm IPG strip with a linear pH gradient pI 5–8 for IEF, 12% SDS-polyacrylamide gels were used for the SDS-PAGE. Proteins were stained with Flamingo fluorescent gel stain. Identified spots were assigned a gene name. B:Anoverlay of the 2D gel images of the adrenal glands of E19 embryo and newborn. The identified spots are labeled with the gene names. by Western blot analysis (6C, D). The expression of pro- as evidenced by different spots with similar molecular teins, which are involved in transporter activity, espe- weightsbut different pIs(Additional file 3: Figure S2). cially cholesterol transporter activity, was increased in the The proteins involved in the retinoic acid pathway, es- adrenal gland during embryonic development (Apoa1, pecially Crabp1, Crapb2, Aldh1a1 and Aldh1a2 were Apoa4, and Star) (Figure 6A, 6C). Among these proteins, detected in the adrenal medulla shown by immunofluor- the expression of Star (steroidogenic acute regulatory pro- escence staining of histological sections (Figure 7A, 7B). tein, mitochondrial) an essential cholesterol transporting protein, which is expressed exclusively in adrenal cortex, Discussion was significantly affected. Star is a protein, which plays a To date, the growth and function of the adrenal gland key role in steroid hormone synthesis by enhancing the me- have been predominantly investigated in the postnatal tabolism of cholesterol into pregnenolone. Star was barely and adult stages . Detailed studies of the molecular expressed in early stage of adrenal gland development (E16), mechanisms involved in embryonic development of the whereas in later stage (E19) the expression increased signifi- AGs are still missing. In order to elucidate the molecular cantly. The protein seems to be posttranslationally modified mechanisms associated with the embryonic development AB CD Figure 3 Temporal change in the expression of selected proteins. Graphs represent enlargement of the gel regions of interest showing protein spots found to be differentially expressed in AG from different embryonic stages. The protein expression quantification for selected proteins is given in form of bar diagrams. On the y-axis the relative intensity of spot is given on the x-axis the corresponding embryonic stage from which the AGs were collected. Results are given as the means ± SD of the percentage volume of spot as quantified by Delta 2D software. All the proteins shown, present significant expression changes during AG development (P < 0.05). Three different spots (Spot 1, 2 and 3) were identified as Star (A), two spots (Spot 1 and 2) were identified as Crabp1 (B), two spots (Spot 1 and 2) were identified as Apoa1 (C), and two spots (Spot 1 and 2) were identified as Cfl1 (D). Western blot analyses confirming the protein expression changes are provided. Dihazi et al. Proteome Science (2015) 13:6 Page 6 of 13 Figure 4 Pie charts of the obtained categories after analysis with DAVID bioinformatics. A: Classification of all identified proteins in the AG according to their molecular function using David Bioinformatics. Most of the proteins were found to be involved in binding functions or catalytic activity. B: Distribution of all identified proteins according to their involvement in biological processes. of the AG, we investigated the proteome changes be- Phgdh) decreases in the course of development (Table 2). tween early and late embryonic stage of development of In contrast, the expression of proteins, which are involved the AGs. The data obtained from this study suggest de- in lipid metabolic process (Apoa4, Tpi1, Ech1, Apoa1, Star, velopmental stage specific pathway activation. Taking Prdx6, Fabp3, Atp5a1, Fabp6), steroid metabolic process the biological and functional classification of the pro- (Apoa1, Apoa4, Star, Fabp6), response to stress (Star, Phb, teins, which were differentially expressed during the de- Prdx3, Cotl1, Apoa4, Apoa1, Alb, Aldh2, Pebp1, Ctsd, velopment into account, we could observe, that expression Hspe1, Hspd1), cell differentiation (Actb, Apoa1, Apoa4, of the proteins involved in biological processes like re- Star, Pebp1, Prdx3, Calr), neurogenesis (Actb, Apoa1, sponse to stress (Ube2n, Gfap, Vcp, Pkm2, Akr1b1, Prdx5, Apoa4, Calr, Star), and neuron development/differenti- Hspa4), cell cycle process (Phgdh, Stmn1, Tubb5), transport ation (Actb, Apoa1, Apoa4) increases in the course of (Khsrp,Crabp1,Crabp2,Clic1,Cfl1,Rbp1, Vcp),gene ex- development (Table 1). pression (Fubp1, Hnrpdl, Khsrp), cell differentiation (Cfl1, The proteomics data revealed interesting aspects in Phgdh, Stmn1), and embryonic development (Crabp2, Cfl1, the AG embryonic development. First, the expression of Dihazi et al. Proteome Science (2015) 13:6 Page 7 of 13 Figure 5 Venn diagram of the distribution of the identified proteins in the adrenal gland at the different days of development. At day E16 a total of 168 proteins were identified, of which 4 were only present at day E16, 1 was present at day E16 and E19 and 163 were present at all three examined developmental days. At day E19 a total of 190 proteins were identified, of which three were only present on the given stage, whereas 23 were present at day E19 and P1. Of the 195 proteins, which were identified in the newborn rat, 9 were only present at this embryonic stage. key proteins of the RA-pathway decreases in the course These spots may represent four different isoforms of the of AG development suggesting that this pathway is only protein. Three of these were verified by 2-D Western important in early stages of development. Second, pro- blot analysis. Apoa1, Apoa4 and Fabp6 appeared in late teins, which are important in the steroid biosynthetic stage of AG development, Apoa1 participates in the re- process or sterol transporter activity, like Apoa1, Apoa4, verse transport of cholesterol from tissues to the liver Star, Fabp3 or Fabp6 were hardly expressed in early for excretion by promoting cholesterol efflux from tis- stage, whereas their expression increased in the course sues and by acting as a cofactor for the lecithin choles- of development of the adrenal gland. Star (Steroidogenic terol acyltransferase (LCAT). Apoa4 may have a role in acute regulatory protein, mitochondrial) stimulates the chylomicrons and VLDL secretion and catabolism. It is regulated production of steroid hormones in the adrenal also required for efficient activation of lipoprotein lipase cortex and gonads by facilitating the delivery of choles- by ApoC-II, which is a potent activator of LCAT. Apoa4 terol to the inner mitochondrial membrane , which is a major component of HDL and chylomicrons. The is the rate-limiting step in the production of steroid hor- expression of proteins involved in steroid metabolic mones [12,13]. Production of steroid hormones is one of process and cholesterol synthesis increased significantly the main functions of the adrenal cortex. Whereas Star in later stages suggesting that the embryonic development of was not detected in the early embryonic stage (E16), four the adrenal gland is accompanied by coordinated metabolic spots were identified as Star in later stages (E19, P1). modifications that facilitate thefunctionalroleof the gland. Dihazi et al. Proteome Science (2015) 13:6 Page 8 of 13 Table 1 Proteins, which expression increased throughout Table 2 Proteins, which expression decreased throughout the embryonic development of the adrenal gland the embryonic development of the adrenal gland Increased E16/P1 p-value Increased E16/P1 p-value Decreased E16/P1 p-value Decreased E16/P1 p-value proteins proteins proteins proteins Actb 0.25 0.0013 Hnrnpk 0.21 0.0381 Akr1b1 6.4 0.0002 Khsrp 2.6 0.0268 Actb 0.29 0.0062 Hnrnpk 0.33 0.0410 Anxa3 2.3 0.0012 Khsrp 3.1 0.0052 Akr1e2 0.31 0.0105 Hrsp12 0.04 0.0013 Arpc5 2.0 0.0405 Krt6a 2.8 0.0306 Alb 0.01 0.0141 Hspa9 0.40 0.0044 Cct3 3.9 0.0052 Krt6a 3.3 0.0188 Alb 0.06 0.0236 Hspa9 0.40 0.0182 Cfl1 3.7 0.0061 Krt73 4.1 0.0471 Aldh2 0.30 0.0394 Hspa9 0.41 0.0381 Cfl1 <10 0.0012 Lmnb1 2.2 0.0089 Aldh2 0.52 0.0103 Hspd1 0.40 0.0416 Clic1 2.1 0.0026 Pdia3 2.4 0.0165 Apoa1 0.06 0.0416 Hspe1 0.04 0.0249 Crabp1 <10 0.0003 Pdia3 2.5 0.00401 Apoa1 0.17 0.0495 Krt73 0.50 0.0007 Crabp1 <10 0.0019 Pdia3 3.6 0.00171 Apoa1 0.17 0.0210 Mesdc2 0.19 0.0156 Crabp2 7.3 0.0001 Pdia3 2.0 0.0413 Apoa4 >0.01 0.0150 Pebp1 0.31 0.0031 Dhfr 2.2 0.0072 Phgdh 2.4 0.0021 Atp5a1 0.39 0.0021 Pebp1 0.43 0.0461 Eef1a1 <10 0.0028 Pkm2 2.3 0.0346 Ca2 0.15 0.0043 Phb 0.46 0.0037 Fscn1 6.5 0.00174 Prdx5 2.6 0.0059 Calr 0.04 0.0021 Prdx3 0.01 0.0441 Fscn1 7.3 0.0083 Rbp1 3.8 0.0002 Cmpk1 0.43 0.0071 Prdx3 0.35 0.0134 Fubp1 3.1 0.0373 Sept11 2.7 0.0073 Cotl1 0.16 0.0092 Prdx3 0.12 0.0023 Gfap 2.9 0.0367 Stip1 2.1 0.0144 Cox5a 0.19 0.0115 Prdx3 0.17 0.0372 Gmps 3.4 0.0210 Stip1 2.3 0.0185 Cox5a 0.22 0.0003 Prdx3 0.26 0.0501 Gnb1 2.1 0.0209 Stmn1 2.0 0.0027 Ctsd 0.20 0.0046 Star 0.42 0.0019 Gnb211 3.6 0.0196 Stmn1 3.7 0.0003 Eno1 0.17 0.0064 Star 0.52 0.0021 Hnrnph1 2.1 0.0303 Tubb5 <10 0.0001 Fabp3 0.42 0.0223 Tpi1 0.46 0.0291 Hnrnph1 2.7 0.0415 Ube2n 2.2 0.0221 Fabp6 0.14 0.0069 Tpi1 0.33 0.0063 Hnrpdl 2.2 0.0046 Vcp 2.5 0.0031 Fabp6 0.30 0.0022 Tuba1a >0.01 0.0002 Hsp90b1 4.4 0.0275 Vim 2.5 0.0001 Hibadh 0.35 0.0070 Txn 0.10 0.0012 Hspa4 3.7 0.0342 Vim 3.7 0.0012 Another important aspect of the embryonic develop- of the protein patterns is an issue when using this tech- ment of AGs seems to be the slowdown of the RA- nique and the range of molecular weight that is resolv- pathway as revealed by the down-regulation of the key able by the method is limited, affecting the detection of proteins of this pathway in the late prenatal stage. Pro- very large and very small proteins. Hydrophobic, highly teins, which are involved in retinoic acid pathway, like acidic or highly basic proteins are poorly detected result- cellular retinoic acid binding proteins (Crabp1, Crabp2) ing in less information on proteome. Moreover, the fact and the retinol binding protein (Rbp1), were found to be that AGs are constituted from two types of tissue, it might expressed in early stages in the AG, but could not be de- be more informative to resolve the proteome of the two tected in late embryonic stages and new born rats. Rbp1 parts separately. This requires additional methods for cell is binding and transporting retinol in the cell. Once in separation or tissue microdissection, which will result in the cell, retinol is converted into retinoic acid. Crabp1 severely reduced amount of usable sample. This will re- and Crabp2 are important for the transport of retinoic quire an even higher number of embryos to be involved in acid from the cytosol to the nucleus, where it serves as a the study and/or more sensitive analytical methods. ligand for nuclear retinoic acid receptors (RARs) that directly regulate gene transcription [14-16] especially of Conclusions genes that modulate the overall development of the em- In summary, this study provides preliminary proteomic bryo. The down-regulation of the RA-pathway proteins maps of AGs embryonic development and highlights the during embryonic development suggests the restricted embryonic stage specific pathway modulation. The RA- importance of this pathway in the AG development. pathway seems to be important at the initial steps of the There are several limitations arising from the 2-D elec- AG development, whereas the molecular changes in trophoresis as a separation method. The reproducibility later stages revealed an increased importance of the Dihazi et al. Proteome Science (2015) 13:6 Page 9 of 13 AB CD Figure 6 Pie charts and diagrams of regulated proteins. A: Distribution of the proteins, which were increased during the development of the adrenal gland, according to their molecular function. B: The diagram shows the protein intensity based on Western blot analysis. C: Distribution of the proteins, which were decreased during the development of the adrenal gland, according to their molecular function. D: The diagram shows the protein intensity based on Western blot analysis. regulation of steroid hormones synthesis as a first step according to the German animal care and ethics legisla- towards the endocrine function of this organ. Additional tion (NIH standards) and were approved by the local gov- investigations are still needed to elucidate the specific ernment authorities. role of the single proteins in the AG development and maturation. Protein extraction The protein extraction for 2-D gel electrophoresis was Methods performed as described previously . A single AG, espe- Animals cially from embryonic stage E14 and E16 (200 – 300 μm Wistar Han rats were kept under 12:12 h cycle of light diameter), will not deliver enough protein for 2-DE ana- with ad libitum access to food and drink. Pregnant rat lysis. Embryos from the same pregnant rat females have females were used to collect the embryos at different the same genetic background and the AGs from these em- embryonic stages: embryonic day 14 (E14), 16 (E16) 18 bryos can be pooled together for proteomic analyses. The (E18) and 19 (E19), and newborn (P1). The adrenal AGs from embryos at the same embryonic stage and from glands (AG) were dissected from these embryos as well the same female (between 14–17 embryos) were pooled, as from neonatal pups. To prepare the AG protein ex- the lysis buffer (9.5 M urea, 2% CHAPS (w/v), 2% ampho- tracts, 60 AGs were used in the E14, 90 in E16, 50 in lytes (w/v), 1% DTT) was added and the samples were E18, 60 in the E19 extracts, and 30 AGs were used for the vortexed. Thereafter, the samples were incubated for P1 extract. All experimental procedures were performed 30 min at 4°C. For removing the cell debris, centrifugation Dihazi et al. Proteome Science (2015) 13:6 Page 10 of 13 Figure 7 Immunohistological fluorescence staining of the adrenal gland at day E19. The fluorescence staining shows a higher expression of the proteins Crabp1 and Crabp2 (A) and Aldh1a1 and Aldh1a2 (B) in the adrenal medulla than in the adrenal cortex. The slides were analyzed on an immunofluorescence Zeiss Axiophot microscope (Carl Zeiss, Jena, Germany) using the AnalySIS software (Soft Imaging Systems, Leinfelden, Germany). was carried out for 30 min at 13,000 ×g and 4°C. The according to Bradford . Bovine serum albumin supernatant was recentrifuged at 13,000 ×g and 4°C for an (Sigma, Steinheim, Germany) was used as standard. additional 30 min to get maximal purity. The pellet was discarded, and the resulting samples were used immedi- ately or stored at −80°C until use. 2D gel electrophoresis (2-DE) To assure for high data quality 2-DE, five biological repli- cates consisting of five pregnant rats (each 14–17 embryos) Protein precipitation for every embryonic stage were prepared. For embryonic To reduce the salt contamination and to enrich the pro- AGs isolated from embryos collected from the same teins, methanol-chloroform-precipitation according to mother at least three independent experimental replicates Wessel and Flugge  was performed. Briefly, 0.4 ml from each embryonic stage as well as from newborn pups methanol (100%) was added to 0.1 ml protein sample were performed. IPG strips (11 cm, pI 5–8) were passively and mixed together. 0.1 ml chloroform was added to the rehydrated for 12 h in 185 μl rehydration buffer (8 M urea, sample and the mixture was vortexed. Subsequently 1% CHAPS, 1% DTT, 0.2% ampholytes, and a trace of bro- 0.3 ml water was added and the solution was vortexed mophenol blue) containing 150 μg protein. The IEF step (R) and centrifuged at 13,000 ×g for 1 min. The aqueous was performed on the PROTEAN IEF Cell (Bio-Rad, layer was removed, and another 0.4 ml methanol (100%) Hercules, CA, USA). Temperature-controlled at 20°C, the was added to the rest of the chloroform and the inter- voltage was set to 500 V for 1 h, increased to 1000 V for phase with the precipitated proteins. The sample was 1 h, 2000 V for 1 h and left at 8000 V until a total of 50000 mixed and centrifuged for 2 min at 13,000 ×g and the Vhours was reached. Prior to SDS-PAGE, the IPG strips supernatant was removed. The pellet was vacuum dried were reduced for 20 min at room temperature in SDS and dissolved in lysis buffer. equilibration buffer containing 6 M urea, 30% glycerol, 2% Total protein concentration was determined using SDS 0.05 M Tris–HCl, and 2% DTT on a rocking table. Bio-Rad protein assay (Bio-Rad, Hercules, CA, USA) The strips were subsequently alkylated in the same solution Dihazi et al. Proteome Science (2015) 13:6 Page 11 of 13 with 2.5% iodoacetamide substituted for DTT, and a Hematoxylin solution (Merck) for 3 min. The slides were trace of bromophenol blue. For the SDS-PAGE 12% rinsed with water, and counterstained with 0.5% Eosin BisTris Criterion precast gels (Bio-Rad, Hercules, CA, G-solution (Merck) for 5 min. After 30 s of rinsing in USA) were used according to manufacturer’sinstruc- water, the slides were dehydrated in series of increas- tions. Thegelswererun at 150V for10min followed ing ethanol and xylene concentrations. The coverslips by 200 V until the bromophenol blue dye front had were mounted on the slides with Entellan Neu mount- reached the bottom of the gel. ing medium (Merck). Gel staining Immunohistochemistry For image analysis, 2-DE gels were fixed in a solution To monitor the expression of the selected proteins in containing 50% methanol and 12% acetic acid overnight the adrenal gland, indirect immunofluorescence staining and fluorescent stained with Flamingo fluorescent gel of proteins of interest was performed. The deparaffiniza- stain (Bio-Rad, Hercules, CA, USA) for minimum 5 h. tion of the sections was carried out as described above, Thereafter, gels were scanned at 50 μm resolution on a thereafter the slides were immersed into a staining dish Fuji FLA-5100 scanner using the Image Reader Software containing Antigen retrieval solution (18 mM citric acid, (Fuji). The digitalized images were analyzed using Delta 82 mM sodium citrate, pH 6.0) and warmed in a food 2D 4.3 (Decodon, Braunschweig, Germany). For protein steamer for 25 min. The slides were allowed to cool identification, 2-DE gels were additionally stained with down for 20 min before being washed with TBST for colloidal Coomassie blue, Roti-Blue (Roth, Karlsruhe, 5 min. Inactivation of endogenous peroxidase was per- Germany) overnight. formed with 3% hydrogen peroxide for 10 min at 37°C. After three successive washing steps with TBST, sections Protein identification were blocked with 10% goat serum for 60 min. The in- Manually excised gel plugs were digested as described cubation with the primary antibodies was carried out previously . After digestion the supernatant was re- overnight at 4°C in a humidified chamber. Molecular moved and saved, and the additional peptides were ex- Probes Alexa Fluor 647 goat anti-mouse IgG antibody or tracted with different acetonitrile/trifluoroacetic acid Alexa Fluor 647 goat anti-rabbit IgG (1:200) were used ratio under sonication. All supernatants were pooled to- as secondary antibodies. The incubation was performed gether, dried in a vacuum centrifuge, and dissolved in at room temperature for 60 min in the dark. The cover- 0.1% formic acid. The mass spectrometric sequencing slips were mounted on the slides using fluorescence was performed as described previously . Briefly, the mounting medium with DAPI (Vector Laboratories, tryptic peptides were subjected to mass spectrometric Inc., Burlingame, USA). Slides were analyzed on a Zeiss sequencing using a Q-TOF Ultima Global mass spec- Axiophot microscope (Carl Zeiss, Jena, Germany) using trometer (Micromass, Manchester, UK). the AnalySIS software (Soft Imaging Systems, Leinfelden, Processed data were searched against MSDB and Swiss- Germany). Prot databases through Mascot search engine using a peptide mass tolerance of 50 ppm (parts per million) Western blot analysis and fragment tolerance of 100 mmu (millimass unit). The validation of the 2-DE data was carried out using Protein identifications with at least two peptides se- Western blot analysis. To assure for the reproducibility quenced were considered significant. of the Western blot analysis, at least three biological and experimental replicates were performed. 40 μg proteins Bioinformatics were separated by SDS-PAGE and transferred to Hybond The classification of the identified proteins according to ECL nitrocellulose membrane (GE Healthcare). Immuno- their main known/postulated functions was carried out detection was performed according to Towbin et al. . using DAVID bioinformatics [22,23]. This classification Briefly, membranes were blocked in 5% milk for 2 h at together with the official gene symbol (given in Additional room temperature, followed by overnight incubation at file 2: Table S1) was used to investigate and categorize the 4°C with diluted specific primary antibody. Mouse mono- gene ontology (GO)-annotations (biological processes and clonal anti-CRABP1 (1:1000) (abcam), rabbit anti-Cofilin molecular functions). (1:1000) (sigma), mouse monoclonal anti-StAR (1:250) (abcam) and mouse monoclonal anti-ß-actin (1:5000) Histochemistry (sigma) were used as primary antibodies. Molecular Paraffin embedded sections were first heated in an oven at Probes Alexa Fluor 647 goat anti-mouse IgG antibody 65°C for 1 h before undergoing several series of washes or Alexa Fluor 647 goat anti-rabbit IgG (1:2000) were of xylene and ethanol to deparaffinize/rehydrate. After used as secondary antibodies. Before imaging, the blots deparaffinization/rehydration, the slides were immersed in were dried in the dark. The blot membranes were Dihazi et al. Proteome Science (2015) 13:6 Page 12 of 13 scanned at 50 μm resolution on a Fuji FLA-5100 scan- Abbreviations AG: Adrenal gland; E16: Embryonic stage day 16; E19: Embryonic stage day ner (Fuji Photo) with single laser-emitting excitation 19; P1: Postnatal day 1; 2-DE: Two-dimensional gel electrophoresis; light at 635 nm. CV: Coefficient of variation; GO: Gene ontology. 2D Western blot analysis Competing interests 150 μg proteins were separated by isoelectric focusing The authors declare that they have no competing interests. and SDS-PAGE as described above and transferred to Hybond ECL nitrocellulose membrane (GE Healthcare). Authors’ contributions GD performed the majority of the experiments in the study, carried out the Immunodetection was performed as described above. sample preparation, the 2D electrophoresis, analyzed and interpreted the Mouse monoclonal anti-StAR (1:250) (abcam) was used data and drafted the manuscript; GM conceived of the study, and as primary antibodies. Molecular Probes Alexa Fluor 647 participated in its design; AA carried out the mass spectrometry analysis, ME contributed in the proteomics data generation; JW contributed in data goat anti-mouse IgG antibody (1:2000) was used as sec- interpretation and study design; HD conceived of the study, participated in ondary antibodies. Before imaging, the blots were dried its design, prepared the AG from the embryos, and coordinated and helped in the dark and scanned as described above. to draft the manuscript. All authors read and approved the final manuscript. Statistical analysis Acknowledgments The authors would like to thank Elke Brunst-Knoblich for her technical For 2-DE the digitalized images were analyzed; spot assistance. matching across gels and normalization were performed using Delta2D 4.3 (Decodon, Braunschweig, Germany). Author details Department of Nephrology and Rheumatology, University Medical Center Delta2D computes a 'spot quality' value for every detected Goettingen, Georg-August University Goettingen, Robert-Koch-Strasse 40, spot. This value shows how closely a spot represents the D-37075 Goettingen, Germany. Department of Clinical Chemistry, 'ideal' 3D Gaussian bell shape. Based on average spot Georg-August University Goettingen, Robert-Koch-Strasse 40, D-37075 Goettingen, Germany. volume ratio, spots whose relative expression is chan- ged at least 2-fold (increase or decrease) between the Received: 17 July 2014 Accepted: 15 January 2015 compared samples were considered to be significant. To analyze the significance of protein regulation, Student's t-test was performed, and statistical significance was as- References 1. Bürgi U. Normal and pathologic endocrinology of the adrenal glands. Helv sumed for P values less than 0.01. Chir Acta. 1989;56:307–14. 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Published: Feb 7, 2015
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