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- The Company of Biologists
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- © 2021 The Company of Biologists. All rights reserved.
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- 0950-1991
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- 0950-1991
- DOI
- 10.1242/dev.107995
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Abstract
© 2014. Published by The Company of Biologists Ltd | Development (2014) 141, 1749-1756 doi:10.1242/dev.107995 RESEARCH ARTICLE Functional phylogenetic analysis of LGI proteins identifies an interaction motif crucial for myelination 1,2 1 1 1 1 3 4 Linde Kegel , Martine Jaegle , Siska Driegen , Eerik Aunin , Kris Leslie , Yuko Fukata , Masahiko Watanabe , 3 1,2, Masaki Fukata and Dies Meijer * ABSTRACT wide variety of neurological disorders. LGI1 mutations are found in patients with autosomal dominant lateral temporal lobe epilepsy The cellular interactions that drive the formation and maintenance of (ADLTE; OMIM 600512) characterized by auditory auras the insulating myelin sheath around axons are only partially (Kalachikov et al., 2002; Morante-Redolat et al., 2002), and auto- understood. Leucine-rich glioma-inactivated (LGI) proteins play antibodies against LGI1 have been found in sera from patients with important roles in nervous system development and mutations in limbic encephalitis and associated neurological diseases (Irani et al., their genes have been associated with epilepsy and amyelination. 2010; Lai et al., 2010; Ohkawa et al., 2013). More recently, a Their function involves interactions with ADAM22 and ADAM23 cell mutation in Lgi2 was found associated with a juvenile form of surface receptors, possibly in apposing membranes, thus attenuating remitting epilepsy in an Italian breed of water dogs (Seppala et al., cellular interactions. LGI4-ADAM22 interactions are required for 2011). The fourth member of this protein family, LGI4, has been axonal sorting and myelination in the developing peripheral nervous associated with childhood convulsions and absence epilepsy system (PNS). Functional analysis revealed that, despite their high (Guetal., 2004; Ishiietal.,2010).Moreover, targeted and homology and affinity for ADAM22, LGI proteins are functionally clp spontaneous (claw paw mice; Lgi4 ) mutations in Lgi4 in mice distinct. To dissect the key residues in LGI proteins required for cause severe congenital hypomyelination of the developing peripheral coordinating axonal sorting and myelination in the developing PNS, nervous system (PNS) and premature death (Bermingham et al., 2006; we adopted a phylogenetic and computational approach and Henry et al., 1991; Nishino et al., 2010; Ozkaynak et al., 2010). demonstrate that the mechanism of action of LGI4 depends on a Genetic and biochemical evidence suggests that the mechanism cluster of three amino acids on the outer surface of the LGI4 protein, of action of LGI proteins involves binding to a subset of cell thus providing a structural basis for the mechanistic differences in surface receptors belonging to the ADAM (a disintegrin and LGI protein function in nervous system development and evolution. metalloproteinase) family, i.e. ADAM11, ADAM22 and ADAM23 KEY WORDS: Evolution and development, Myelination, Leucine-rich (Kegel et al., 2013; Novak, 2004; Seals and Courtneidge, 2003). In glioma-inactivated, ADAM23, Schwann cell, Mouse particular, LGI1 and LGI4 bind to cell surface expressed ADAM22 and ADAM23, and both proteins co-precipitate from brain lysates INTRODUCTION with ADAM22 or ADAM23 antibodies (Fukata et al., 2006; Ogawa Dynamic cell-cell interactions determine key morphological and et al., 2010; Ozkaynak et al., 2010; Sagane et al., 2008). It has also functional properties of the nervous system during ontogeny been suggested that LGI1 contributes to trans-synaptic interactions and adult life. The formation and dynamics of the neuronal and synaptic strength in hippocampal neurons through binding synapse and the elaboration of the myelin sheath that surrounds the postsynaptic ADAM22 and presynaptic ADAM23 (Fukata et al., majority of axons during the postnatal development of the vertebrate 2010; Yokoi et al., 2012). nervous system probably illustrate this most profoundly. However, LGI4 function has been primarily analyzed in the context of the relatively little is known about the molecular mechanisms that developing PNS where it is secreted from Schwann cells and binds to coordinate such interactions. ADAM22 in the axonal membrane (Nishino et al., 2010; Ozkaynak One class of molecules that plays an important role in cellular et al., 2010). The interaction between LGI4 and ADAM22 is required interactions in nervous system development and function is the for timely axonal sorting and myelination (Bermingham et al., 2006; leucine-rich glioma-inactivated (LGI) protein family (reviewed by Henry et al., 1991; Sagane et al., 2005), developmental processes that Kegel et al., 2013). LGI proteins are secreted glycoproteins that involve extensive interactions between the axonal and Schwann cell consist of a leucine-rich repeat (LRR) domain and a so-called membranes. ADAM23 is expressed in the PNS and in particular in epilepsy-associated or epitempin (EPTP) domain (Gu et al., 2002). Schwann cells (Dhaunchak et al., 2010), but its role in axonal sorting Both protein domains are generally involved in protein-protein and myelination has not been explored. The high degree of homology interactions. LGI genes and proteins have been associated with a between LGI1 and LGI4 and their high affinity for ADAM22 and ADAM23 suggests that a common mechanism might underlie their 1 respective functions in the CNS and PNS. Erasmus University Medical Center, Biomedical Sciences, Departments of Genetics and Cell Biology, Rotterdam 3015GE, The Netherlands. Centre for In this study, we test this hypothesis and show that LGI4 Neuroregeneration, University of Edinburgh, Chancellor’s Building, 49 Little France functions in the PNS through a distinct mechanism that does not Crescent, Edinburgh EH16 4SB, UK. Division of Membrane Physiology, require ADAM23. We further show that the function of LGI4 is Department of Cell Physiology, National Institute for Physiological Sciences, Okazaki, Aichi 444-8787, Japan. Department of Anatomy, Hokkaido University unique in that none of the other LGI family members is able to elicit Graduate School of Medicine, Kita-ku, Sapporo 060-8638, Japan. myelination. Here, we have adopted a general integrated approach that includes phylogenetic, computational and functional analysis, *Author for correspondence ([email protected]) to identify interaction motifs in LGI proteins required for Received 14 January 2014; Accepted 21 February 2014 coordinating myelination in the developing PNS. We demonstrate DEVELOPMENT RESEARCH ARTICLE Development (2014) 141, 1749-1756 doi:10.1242/dev.107995 that the unique function of LGI4 depends on small set of amino Mouse Genome Informatics), a major cell adhesion molecule involved acids that present a novel interaction surface in these proteins. The in paranodal junction formation and stability (Fig. 1D; reviewed by data presented here have implications for our understanding of LGI Salzer et al., 2008). Interestingly, ADAM23 protein accumulates in the protein function in development and evolution. juxtaparanodal domain, abutting the paranodal junction (Fig. 1D, WT), the same site at which shaker-type potassium channels, RESULTS ADAM22, CASPR2 (CNTNAP2) and TAG-1 (CNTN2) accumulate ADAM23 is expressed in the PNS but is not required for (Ogawa et al., 2010; Rasband, 2011). Microscopic examination of Δ1/Δ1 axonal sorting and myelination semi-thin transverse sections of sciatic nerves of Adam23 mice Adam23 is widely expressed in the CNS and in neurons and did not reveal any abnormalities, in marked contrast with the severe Schwann cells (SCs) of the PNS (Dhaunchak et al., 2010; hypomyelinating phenotype observed in the sciatic nerve of Adam22 Goldsmith et al., 2004; Ozkaynak et al., 2010; Sagane et al., null mice of the same age (Fig. 1E). Thus, it is unlikely that ADAM23 1999). Indeed, an ADAM23-specific antibody detects a 70 kD is mechanistically involved in LGI4-ADAM22 interactions that govern protein in extracts from mouse postnatal day (P) 12 sciatic nerve myelin formation in the developing peripheral nerve. (Fig. 1A; MSN) and in membrane extracts of cultured rat Schwann cells (Fig. 1A, RSC). This band corresponds to the glycosylated, LGI4, but not LGI1, 2 or 3, facilitates myelination in the PNS mature form of ADAM23 (Goldsmith et al., 2004). The fact that LGI4-ADAM22 interactions in the PNS do not require To test whether ADAM23 is mechanistically involved in LGI4- ADAM23 for myelination to occur in the PNS suggests that ADAM22-mediated interactions in PNS development, we generated a LGI4-ADAM22 interactions are mechanistically distinct from the Lox mouse line with an Adam23 floxed allele (Adam23 ;Fig.1B).A LGI1-ADAM22/ADAM23 interactions involved in excitatory Lox/Lox germline-expressing Cre mouse was crossed with Adam23 mice synapse maturation and strength in the CNS. This difference Δ1/Δ1 Δ1/Δ1 and fully deleted (Adam23 ) mice were examined. Adam23 cannot be explained by affinity differences of LGI1 and LGI4 for were born at expected frequencies, but all animals developed a severe the ADAM22 and ADAM23 receptors, as both proteins bind avidly tremor in the second week of postnatal life and died by P15, in to both receptors (Owuor et al., 2009; Sagane et al., 2008). To accordance with previous reports on gene-trap Adam23 mutant mice explore potential functional differences between these LGI proteins, (Mitchell et al., 2001; Owuor et al., 2009). Western blot analysis of we performed genetic complementation experiments in primary sciatic nerves dissected from P12 mice revealed the presence of the sensory neuron-SC cultures derived from dorsal root ganglia (DRG) Δ1/Δ1 clp/clp clp/+ ADAM23 protein in wild-type, but not in Adam23 ,mice of Lgi4 and Lgi4 (referred to as clp/clp and clp/+, (Fig. 1C). Levels of the major myelin proteins MPZ and MBP were respectively, in the following text) mouse embryos at embryonic day Δ1/Δ1 unaffected in Adam23 sciatic nerves, indicating that there is (E) 13. Control clp/+ cultures exhibit robust myelination, whereas myelin formation. Normal paranodal junctions were formed in the clp/clp cultures do not myelinate (Bermingham et al., 2006). absence of ADAM23 as judged by staining for CASPR (CNTNAP1 – Cultures were retrovirally transduced with different Lgi cDNA Fig. 1. Adam23 is expressed in the peripheral nervous system but is not required for axonal sorting and myelination. (A) Western blot analysis of membrane proteins extracted from P12 mouse sciatic nerve (MSN) or cultured rat Schwann cells (RSC) reveals that ADAM23 is expressed as a 70-kD protein. (B) The mouse Adam23 locus spans over 250 kb of genomic DNA with <25 exons. The structures of the wild-type, conditional and null alleles of Δ1/Δ1 Adam23 are depicted. (C) Western blot analysis of P12 sciatic nerve extracts from wild-type and Adam23 null (A23 ) mice demonstrates the specificity of the ADAM23 antibody. Expression levels of the major myelin proteins, myelin protein zero (MPZ) and myelin basic protein (MBP) and acetylated alpha tubulin, which is mainly axonal, are not altered in Adam23 null nerves. (D) Longitudinal sections of cryopreserved P12 nerves immunolabeled with antibodies against ADAM23 (red) and CASPR (green). (E) Transverse semi-thin section of sciatic nerves at P12, isolated from wild type, Adam23 null Δ1/Δ1 neo/neo (Adam23 ) and Adam22 null (Adam22 ) stained with PPD. The number and thickness of myelin rings around axons in Adam23 null animals does not differ from wild-type animals, whereas only few axons are thinly myelinated in Adam22 null mice at this stage of development. Scale bars: 10 μm. DEVELOPMENT RESEARCH ARTICLE Development (2014) 141, 1749-1756 doi:10.1242/dev.107995 Fig. 2. Lgi4 is the only Lgi member to genetically complement myelination in clp/clp cultures. (A) Retroviral transduction of Lgi4 in clp/clp cultures restores myelination. Clp/+ and clp/clp cultures were infected with retrovirus containing Lgi1-4 and maintained in myelinating conditions for 19 days and then stained for MPZ and neurofilament (NFM). Infected Schwann cells express GFP as the retroviral construct carries an IRES-GFP cassette. Clp/+ cultures express normal levels of MPZ (a-d) whereas Lgi1-3-infected clp/clp cultures show no MPZ expression (e-g). Numerous myelin segments are evident in Lgi4 infected clp/clp cultures (h). More than six cultures of each genotype were infected per construct. Scale bar: 100 µm. (B) LGI proteins are secreted and can be recovered from the tissue culture supernatant of clp/clp cultures. LGI proteins were purified from tissue culture supernatant three days after infection. Proteins were detected by western blot using a V5 antibody. expression constructs (Lgi1, 2, 3 and 4). Transduction of clp/clp modern day bony fishes and mammals, we sought to identify specific cultures with Lgi4 restores myelination, as shown by expression of amino acids within the LGI4 protein that contribute to its myelin- MPZ, the major protein in compact myelin (Fig. 2Ad). This is in line promoting function. Comparative genomics has so far failed to with our previous observations (Bermingham et al., 2006). By identify a clear Lgi4 homolog among the five Lgi genes in the contrast, none of the other Lgi family members (Lgi1, 2 or 3) genomes of zebrafish and other teleost fish, leading some authors to restored myelination in clp/clp cultures (Fig. 2Ae-g) despite high- suggest that Lgi4 is most closely related to an ancestral Lgi1 gene level LGI protein expression, as shown by western blot (Fig. 2B). (Gu et al., 2005). On the basis of a much larger dataset, others suggested it is most closely related to an ancestral Lgi3 gene The PNS-specific function of LGI4 requires the LRR domain (Leonardi et al., 2011). To shed more light on the ancestry of the Lgi4 but crucially depends on its EPTP domain gene and to identify the functional homolog of LGI4 in the zebrafish To establish whether the LGI4-specific myelination function is a genome, we cloned the five zebrafish Lgi cDNAs and investigated property of the LRR domain or the EPTP domain, we generated and which of the five Lgi proteins can restore myelination in clp/clp tested domain deletion constructs (Fig. 3A). Previously, it was shown cultures. Immunoprecipitation experiments showed that zebrafish that LGI1 binds ADAM22 through its EPTP domain (Fukata et al., Lgi1a and Lgi1b directly bind the ectodomain of mouse ADAM22 2006). We found that this was also true for LGI4 (supplementary (supplementary material Fig. S2A) and ADAM23 (supplementary material Fig. S1A,B). However, no myelination was found in clp/clp material Fig. S2B). By contrast, zebrafish Lgi2a, Lgi2b and Lgi3 did EPTP LRR cultures when the LGI4 domain or the LGI4 domain was not bind (supplementary material Fig. S2) and expression of Lgi2a, expressed (Fig. 2Be,f), indicating that both domains are required for Lgi2b and Lgi3 in clp/clp cultures did not restore myelination (Fig. 4). LGI4 function. It is possible that LGI4 is tethered to the axonal However, both Lgi1a and Lgi1b did restore myelination in clp/clp membrane through its EPTP-dependent binding to ADAM22, cultures, strongly suggesting that they are the functional homologs of whereas the LRR domain engages in other specific interactions. LGI4 in zebrafish. These data are in line with the suggestion that the We next examined whether the LRR domain of LGI4 contributes Lgi4 gene arose in tetrapods from an ancestral Lgi1 gene and rapidly unique properties. We exchanged the LRR and EPTP domains of acquired an essential function in the nervous system. By contrast, L4E3 L3E4 LGI3 and LGI4 to generate LGI and LGI (Fig. 3A). Similar these functions remained associated with the duplicated Lgi1 genes in domain swapping constructs were created by exchanging the teleost fish (Gu et al., 2005) and with the single Lgi1 gene in lobe LRR and EPTP domains of LGI1 and LGI4 (supplementary finned fish, such as the coelacanth (supplementary material Fig. S4). material Fig. S1C). These chimeric proteins are efficiently expressed and secreted, and bind to cell surface-expressed Phylogenetic comparison and structural modeling identifies ADAM22 (supplementary material Fig. S1D,E and Fig. S3). a three amino acid cluster on the surface of the EPTP domain Transduction of these expression constructs into clp/clp cultures The distant evolutionary relationship between LGI4 and Lgi1a/b revealed that the LRR domains of LGI3 (Fig. 3C) and LGI1 provides us with a saturated mutation analysis of LGI4 function in (supplementary material Fig. S1D,E) are interchangeable with the the PNS. Selective pressure will have retained those amino acid LGI4 LRR domain, thus demonstrating that the unique myelin- residues crucial for the myelination-promoting activity of LGI4 and promoting function of LGI4 depends on its EPTP domain. Lgi1a/b. Visual inspection of LGI protein alignments identified We further delineated the LGI4 EPTP functional domain by three amino acids within the second half of the EPTP domain creating and testing an additional chimeric construct in which only that were identical in LGI4 proteins from mammals and Lgi1 the last four repeat motifs of the LGI4 EPTP domain were retained proteins from zebrafish, but were different in the other LGI E4(4-7) [LGI3 ]. This chimeric protein fully restores myelination in proteins (highlighted in yellow in Fig. 5A and supplementary clp/clp cultures (Fig. 3Cf), further structurally localizing LGI4 material Fig. S4). A structure for the LGI4 EPTP domain was function to the carboxyl-terminal half of the EPTP domain. predicted on the basis of its high primary amino acid sequence homology with the WDR5 protein using the online HHpred tool Functional analysis identifies the zebrafish homolog of (Soding et al., 2005). Mapping the position of these three amino mammalian Lgi4 acids onto this structure shows that their side chains are solvent We reasoned that the protein domain responsible for LGI4 specificity exposed and form a potential interaction interface at the side of the must be conserved in other vertebrates. Taking advantage of the more slightly conical, donut-shaped EPTP domain (Fig. 5B). The three than 800 million years of independent evolutionary history between amino acids (RR-M) contribute two positive charges (the two DEVELOPMENT RESEARCH ARTICLE Development (2014) 141, 1749-1756 doi:10.1242/dev.107995 which the identity of the three amino acids was changed into those found in LGI4, i.e. N422R, Q423R and S464M (Fig. 5A,C). RR-M LGI1 was found to be secreted and to bind ADAM22 (Fig. 5C; supplementary material Fig. S3), indicating that the amino acid replacements do not affect the proper processing and folding of the RR-M RR-M LGI1 protein. Transduction of clp/clp cultures with Lgi1 restored myelination (Fig. 5D), demonstrating that these amino acid side chains determine LGI4 specificity and contribute to an interaction interface that is crucial for LGI4 function in PNS myelination. As mammalian LGI1 is normally glycosylated at position N422 (Sirerol-Piquer et al., 2006), it is possible that these sugar side chains obstruct a potential myelin-promoting interface. The simple destruction of this glycosylation may unmask it, and, thus, we N422Q generated a LGI1 mutant protein. A previous study showed that mutating this N-linked glycosylation site does not affect the proper processing or secretion of LGI1 (Sirerol-Piquer et al., 2006). N422Q Indeed, we found that LGI1 is normally secreted from transfected cells. clp/clp and clp/+ DRG co-cultures were N422Q transduced with LGI1 . Myelination was observed in clp/+, but not in clp/clp, cultures, showing that removal of the sugar modifications on this side of the LGI1 protein does not unmask a latent myelin-promoting activity. Therefore, the three amino acids we identified here are an essential part of an interaction interface that is functionally significant for PNS myelination. DISCUSSION LGI proteins play important roles in key developmental and Fig. 3. The myelin-promoting function of Lgi4 depends on the carboxyl functional aspects of the vertebrate nervous system, but their terminal half of the EPTP domain. (A) Schematic of the constructs used for infection in B and C. Lgi4 domains are indicated in green, Lgi3 domains are mechanism of action is still poorly understood. Here, we have indicated in orange. Red oval represents a V5 and 6xHis tag. (B) LGI4 function investigated whether a common or distinct mechanism(s) underlies cannot be reduced to its LRR or EPTP domain. Clp/+ and clp/clp co-cultures LGI protein function during nervous system development. We LRR EPTP were infected with retrovirus containing Lgi4 or Lgi4 and maintained in demonstrate here that LGI4 functions in the peripheral nervous myelinating conditions for 19 days and then stained for MPZ (myelin) and NFM system through a distinct mechanism that involves a unique (neurons). Infected Schwann cells express GFP as the retroviral construct interaction surface on the outer circumference of the EPTP domain. carries an IRES-GFP cassette. Clp/+ cultures (a-c) and Lgi4-infected clp/clp LRR EPTP In contrast to the proposed models for LGI1 function in the central cultures (d) express normal levels of MPZ, whereas Lgi4 (e) and Lgi4 (f) infected clp/clp cultures show no MPZ expression. (C) LGI4 function is nervous system, LGI4 does not require the ADAM23 receptor to drive associated with its EPTP domain. Clp/+ and clp/clp cultures were infected with myelination in the PNS. A model in which LGI4 links the apposing L4E3 L3E4 E4(4-7) retrovirus containing Lgi , Lgi or Lgi3 . Clp/+ co-cultures (a-c) axonal membrane and Schwann cell membrane through interaction L4E3 express normal levels of MPZ, whereas Lgi -infected clp/clp cultures show L3E4 with ADAM22 and ADAM23, respectively, seems therefore unlikely. no MPZ expression (d). Numerous myelin segments are evident in Lgi E4(4-7) However, we found that ADAM23 accumulates at the juxtaparanode (e) and Lgi3 (f) infected clp/clp cultures. More than six cultures of each (JXP) of myelinated axons, which is characterized by a high density of genotype were infected per construct. Scale bars: 100 µm. shaker type potassium channels and the cell adhesion molecules arginine R-groups) and a hydrogen acceptor (the sulfur atom of CASPR2, TAG-1 and ADAM22. The accumulation of Kv1 channels methionine) to this putative interaction surface. at the JXP depends on CASPR2 and TAG-1 interactions in the To assess the relevance of this putative three amino acid (RR-M apposing membranes and interactions between the CASPR2 motif) interaction surface for the myelin-promoting activity of LGI4, cytoplasmic domain and Protein 4.1B (EPB4.1L3 – Mouse Genome RR-M we generated a mouse Lgi1 expression cassette (Lgi1 ; Fig. 5C) in Informatics) that link the complex to the axonal cytoskeleton. The Fig. 4. lgi1a and lgi1b are functional homologs of mammalian LGI4. (A) Clp/+ and clp/clp cultures were infected with retroviruses containing zebrafish Lgi proteins and maintained in myelinating conditions for 19 days and then stained for MPZ (myelin) and NFM (neurons). Infected Schwann cells express GFP. Clp/+ cultures show normal amounts of myelin figures (a-e). lgi1a and lgi1b restore myelination in clp/clp cultures (f,g). lgi2a, lgi2b and lgi3 infected clp/clp cultures do not show any myelin (h-j). More than six cultures of each genotype were infected per construct. Scale bar: 100 µm. (B) The zebrafish Lgi proteins are recovered from the conditioned medium of the cultures shown in A three days after infection and detected by western blot using a V5 antibody. DEVELOPMENT RESEARCH ARTICLE Development (2014) 141, 1749-1756 doi:10.1242/dev.107995 myelination in our in vitro myelination system derived from LGI4 mutant mice. Thus, ADAM22-ligand binding per se is not sufficient to elicit the biologically relevant response in this system. Differential outcome of different ligands binding to the same receptor is not an uncommon phenomenon. For example, binding of NGF or NT3 (also known as NTF3) to the TrkA receptor (NTRK1) has common but also different biological effects that correlate with different ligand-induced receptor membrane mobility and internalization (Marchetti et al., 2013). One other example is provided by the Type I interferons (16 different forms), which elicit different physiological responses through the same heterodimeric receptors IFNAR1 and IFNAR2 (Thomas et al., 2011). Interferons bind to this receptor through common ‘anchor points’ that are interspersed among ligand-specific interactions that tune ligand affinities, a mechanism referred to as ‘ligand proofreading’. This mechanism involves conformational changes in the receptor. The crystal structure of the ADAM22 ectodomain has been resolved and it was suggested that considerable movement between the metalloproteinase-like (M) domain and the disintegrin/cysteine- rich/EGF-like (DCE) domain might provide a basis for ligand discrimination and/or ligand-induced conformational changes (Liu et al., 2009). It is thus conceivable that part of the specificity of the LGI4-ADAM22 interaction results from such ligand-induced conformational adaptation. As crystal structures for LGI proteins or the LGI-ADAM ectodomain complex are not available, such a mechanism remains highly speculative. Computational modeling of LGI1 structure led Leonardi and colleagues to suggest that LGI proteins bind to the ADAM receptor through the top surface of its EPTP domain (Leonardi et al., 2011). Theinteraction surface wehaveidentified hereinLGI4 maps to the circumference of the EPTP domain and is thus distinct from the prospective ADAM22/23 interaction interfaces common to all LGI proteins. The seven-bladed propeller structure of the EPTP domain provides a stable scaffold to present multiple interaction surfaces and our results underscore its versatility. The seven-bladed propeller structure is found in a wide range of proteins involved in different cellular processes and, in general, it functions as a multivalent interaction hub (Xu and Min, 2011). For example, the WDR5 protein, to which the EPTP domain of LGI proteins is most closely related, is part of the mammalian Trithorax (TrxG) complex. It interacts with the Fig. 5. LGI4-specific function depends on a conserved three amino acid motif. (A) LGI protein alignments identified three amino acids within histone H3 tail and the RBBP5 core complex subunit through the top the second half of the EPTP domain that were identical between LGI4 and bottom part of its seven-bladed propeller, effectively presenting proteins from mammals (mouse: Mm, Mus musculus; opossum: the histone H3 tail for K4 methylation by the MLL1 histone Md, Monodelphis domestica) and Lgi1 proteins from zebrafish methyltransferase (also known as KMT2A) (Avdic et al., 2011). (Dr, Danio rerio) and coelacanth (Lc, Latimeria chalumnae), but were Likewise, LGI4 function in the PNS appears to depend on interactions different in other LGI proteins. Chicken: Gg, Gallus gallus; Chinese softshell with the ADAM22 receptor through the top surface of the EPTP turtle: Ps, Pelodiscus sinensis. (B) A structure for the LGI4 EPTP domain domain and possibly with accessory proteins through the surface on was predicted on the basis of its high primary amino acid sequence homology with the WDR5 protein using the online HHpred tool (Soding the side, created by the amino acid side chains of propeller blades 4 et al., 2005). Mapping the position of the three identical amino acids onto and 5. Alternatively, these side chains in blades 4 and 5 provide this structure shows that their side chains are solvent-exposed and form a specific additional ADAM22 contacts to fine tune the interaction and potential interaction interface at the outer surface of the EPTP domain. confer specificity (Fig. 5C). As ADAM22 is also an integrin receptor, (C,D) LGI1 mutant construct containing the identical amino acids the binding of LGI ligand may potentiate interactions with integrins (LGI1 RR-M) binds ADAM22 overexpressing HeLa cells (C) and stimulates on the adaxonal Schwann cell membrane, thereby stimulating myelination in clp/clp cultures (D). More than six cultures of each genotype were infected per construct. Scale bar: 100 µm. migration of the Schwann cell lamellipodia over the axonal surface to initiate myelination. presence of the ADAM22 and ADAM23 receptors in apposing In addition to the interaction platform provided by the EPTP membranes of the JXP is compatible with the suggestion that they domain, LGI proteins contain an LRR domain that is engaged in contribute to the formation or stability of the JXP complexes, possibly protein-protein interactions. Indeed, the LRR domain of LGI4 is in an LGI4-dependent fashion, a hypothesis that will be the subject of essential for full LGI4 activity, as the LGI4 EPTP domain by itself is future experiments. secreted but does not restore myelination in LGI4-deficient clp/clp Despite the fact that both LGI4 and LGI1 bind with relative high cultures (Fig. 3). Our experiments did not reveal a specific role for affinity to the ADAM22 receptor, only LGI4 is capable of restoring the LGI4 LRR domain, as that of LGI1 or LGI3 could replace it DEVELOPMENT RESEARCH ARTICLE Development (2014) 141, 1749-1756 doi:10.1242/dev.107995 obtained through Addgene), which was modified to extend the LGI open without loss of myelin-promoting activity. What generic role the reading frame with a V5 and 6xHis tag. Furthermore, all LGI open reading LRR domain fulfills in LGI4 function, or in any of the other LGI frames start with a signal peptide derived from the mouse kappa light chain proteins, is unclear. The demonstration that LGI proteins can immunoglobulin gene replacing the LGI protein’s own signal peptide. dimerize through their LRR domains might be of functional Adam22-Fc constructs have been previously described (Ozkaynaket al., 2010). significance (Fukata et al., 2006). Our functional phylogenetic analysis allows us to reconstruct the Virus production evolutionary history of this interaction surface in ancestral LGI HEK293T cells were co-transfected with pcl-ECO packaging vector proteins. Lgi genes are only found in the genomes of chordates (with (IMGENEX Corporation) and pBMN-LGI-IRES-GFP constructs, using the exception of urochordates), suggesting that they evolved in the the polyethyleneimine (PEI) method (Boussif et al., 1995). Virus was early vertebrate lineage 525 million years ago. The primitive extant collected in DMEM/F10, 10% heat-inactivated fetal calf serum (FCS) and chordates Amphioxus (Brachiostoma floridae) and Lamprey Penicillin + Streptomycin (Invitrogen) (PS) and was ready for use 2 days (Petromyzon marinus) have one Lgi gene that encodes proteins that after transfection. When virus was not used directly, it was snap frozen in miss the RR-M motif identified here (supplementary material Fig. S4). liquid nitrogen and stored at −80°C. Viral supernatants were titred using By contrast, all extant jawed-vertebrates (gnasthostomes) have LGI primary rat Schwann cells. proteins that carry this motif. The evolution of this motif then coincides with the proposed first appearance of compact myelin in the Primary cell culture and virus infection peripheral nervous system of jawed vertebrates ∼425 million years DRGs were isolated from individually genotyped E13.5 mouse embryos (Kleitman et al., 2002) and dissociated in trypsin. Cells were plated onto Matrigel ago (Yoshida and Colman, 1996). A duplication event of an ancestral (BD Biosciences)/poly-D-lysine (Sigma)-coated 18 mm coverslips (Thermo Lgi1 gene in the early tetrapod lineage then gave rise to the Lgi1 and Scientific) and maintained in Neurobasal medium (Invitrogen) supplemented Lgi4 genes present in the genomes of all modern day tetrapods, dating with B27 (Invitrogen), Glutamax (Invitrogen), 4 g/l D-glucose, 100 ng/ml NGF the birth of the Lgi4 gene to ∼365 million years before present day. On (Harlan), 10% horse serum (Invitrogen) and PS. The next day, medium was the basis of this reconstruction and our complementation assays, we replaced with BN medium [BME medium (Invitrogen) containing insulin- predict that the Lgi1a/b proteins in zebrafish are expressed in Schwann transferrin-selenium-A supplement (Invitrogen), Glutamax, 0.2% bovine serum cells and promote sorting and myelination of peripheral nerve fibers. albumin (BSA), 4 g/l D-glucose, 100 ng/ml NGF, PS] for one day. The Our analysis also emphasizes the importance of combining functional following day, cultures were infected with retrovirus in BN medium data as generated here with computational methods to reconstruct supplemented with 3.75 µg/ml Polybrene (Sigma) and incubated overnight. reliable evolutionary relationships. The virus-containing medium was replaced with fresh BN medium and cultures were incubated for another day before medium was switched to M1 medium [MEM (Invitrogen), 3% FCS, 100 ng/ml NGF, PS]. One week after DRG MATERIALS AND METHODS isolation, myelination was induced by refreshing co-cultures every other day Mice clp/+ with MEM (Invitrogen), 10% FCS, 100 ng/ml NGF, PS, 50 µg/ml ascorbic acid C57BL/6J clp/+ (Lgi4 ) mice were originally obtained from the Jackson (Sigma) for 19 days, after which the cultures were fixed in 4% paraformaldehyde Laboratory and the clp allele has subsequently been maintained on a FvB/N (PFA)/4% sucrose in PBS for 15 min at 4°C. During the culture period, and C57BL/6 background. Genotypes were determined by PCR using the conditioned medium from the cultures was collected to assess the expression of Expand Long Template PCR system (Roche) and the following primers: 0 0 0 the LGI proteins by western blotting. 5 -AGAGAGCCAAGACCTTGGCTAC-3 and 5 -GTCATCTGAAGGTCG- 0 clp AGGATGGC-3 (Lgi4 allele 550 bp and wt Lgi4 allele 750 bp). An LoxP Adam23 conditional knock-out allele (Adam23 ) was generated through Immunohistochemistry, western blotting and antibodies homologous recombination in embryonic stem cells (ESCs) using standard Transfected HEK293T or HeLa cells were fixed with 4% PFA/4% sucrose techniques (Jaegle et al., 2003). A mouse line was generated from a correctly in PBSfor 15 minat4°C,then washedwithPBS andblocked for at least LoxP/+ recombined ESC line and Adam23 mice were crossed with a transgenic 20 min at room temperature. Blocking solution contained 1% FBS, 0.2% mouse expressing Cre recombinase in the germline to generate offspring with fish skin gelatin (Sigma), 0.5% BSA in PBS. Primary antibody incubation Δ1/+ an Adam23 null allele (Adam23 ) in which the first exon plus promoter of was carried out at room temperature for 2 h or at 4°C overnight. Next day, Δ1/+ Adam23 were deleted. Adam23 mice were intercrossed to generate cells were extensively washed with PBS and incubated for 45 min at room Δ1/Δ1 Adam23 mice. Mice were genotyped by PCR using the following temperature with secondary antibody. Cells were mounted in Mowiol 0 0 0 primers: A23-S 5 -GATCCCAGTGGCTCAGTGCC-3 and A23-aS1 5 - containing DAPI for DNA staining. Samples were examined under a GATCTGGCAACAACTTGCCAAG-3 . The wild-type allele will yield a Zeiss AxioImager Z1 microscope (63× magnification) and images were Lox product of 256 bp and the LoxP allele (ADAM23 ) will yield a 300-bp captured with an AxioCam MRm digital camera, and analyzed with Δ1 product. The ADAM23-null allele (ADAM23 ) will yield a 421-bp PCR AxioVision software. Brightness and contrast levels were adjusted in product using the A23-S primer as above in combination with the following Adobe Photoshop CS4. 0 0 anti-sense primer: A23-aS2 5 -TGATTGCTCATTATGACAAGC-3 .Animal DRG cultures were essentially processed as described above. Additionally, experiments were performed according to protocols approved by the cultures were fixed with ice-cold methanol for 20 min in −20°C to facilitate independent Animal Experimentation Committee (DEC) and in compliance P0 staining. After fixation, cultures were washed with PBS/0.1% Triton with institutional guidelines at Erasmus University Medical Center. X-100 and blocked in 1% FBS, 0.2% fish skin gelatin, 0.5% BSA in PBS, for 1 h at room temperature. First and secondary antibody incubation and analysis DNA constructs was the same as described for HEK293T and HeLa cells, except after All mouse Lgi cDNAs were amplified from cDNA produced from incubation with antibodies cultures were washed five times with PBS/0.1% E13.5 DRG total RNA, cloned in Zero-Blunt TOPO vector (Invitrogen) and Triton X-100. Cultures were microscopically analyzed as described above fully sequenced. Zebrafish Lgi cDNAs were amplified from zebrafish embryos using 20× magnification. Western blot experiments were performed as at three days post fertilization. RNA was isolated using RNA-Bee reagent previously described (Jaegle et al., 2003). (Tel-Test) and converted into cDNA using Superscript III (Invitrogen) and Primary antibodies were from the following sources: chicken anti-MPZ cDNA was amplified using Phusion polymerase (Finnzymes). [PZO, Aves Labs; immunofluorescence (IF), 1:1000; western blot (WB), The different domain swap LGI expression constructs were created with 1:10,000], mouse anti-neurofilament M (2H3 monoclonal antibody developed overlap PCR. Point mutations were introduced using segments of DNA by Jessell and colleagues obtained from the Developmental Studies synthesized by Genscript. All Lgi expression constructs were cloned in the Hybridoma Bank, developed under the auspices of the National Institute of retroviral vector pBMN-IRES-GFP (G. Nolan, Stanford University, CA, USA, Child Health and Human Development and maintained by the Department DEVELOPMENT RESEARCH ARTICLE Development (2014) 141, 1749-1756 doi:10.1242/dev.107995 Supplementary material of biology, University of Iowa, Iowa City, IA, USA; IF, 1:100), rabbit anti-c- Supplementary material available online at Myc (SC-789, Santa Cruz Biotechnology; WB, 1:1000), mouse anti-V5 http://dev.biologists.org/lookup/suppl/doi:10.1242/dev.107995/-/DC1 [05025C5, Absea Biotechnology; WB, 1:100; and 05025D5, Absea Biotechnology; IF, 1:10], mouse-anti-MBP (SAB5300427, Sigma; WB, References 1:1000), mouse anti-acetylated alpha tubulin (clone C3B9, European Avdic, V., Zhang, P., Lanouette, S., Groulx, A., Tremblay, V., Brunzelle, J. and Collection of Cell Cultures; WB 1:10,000). The mouse anti-CASPR Couture, J.-F. (2011). Structural and biochemical insights into MLL1 core antibody was a kind gift of Dr E Peles (Weizmann Institute of Science, complex assembly. Structure 19, 101-108. Israel) and was used at 1:200 dilution. The ADAM23 antibody was raised Bermingham, J. R., Jr, Shearin, H., Pennington, J., O’Moore, J., Jaegle, M., in rabbits against a peptide corresponding to the 15 amino acid cytoplasmic tail Driegen, S., van Zon, A., Darbas, A., Ozkaynak, E., Ryu, E. J. et al. (2006). The claw paw mutation reveals a role for Lgi4 in peripheral nerve development. of the protein. The antibody was affinity purified and was used at a 1:200 Nat. Neurosci. 9, 76-84. dilution for western blots and at 1:50 dilution in immunohistochemistry. Boussif, O., Lezoualc’h, F., Zanta, M. A., Mergny, M. D., Scherman, D., The following secondary antibodies were used: Alexa594-conjugated donkey Demeneix, B. and Behr, J. P. (1995). A versatile vector for gene and anti-chicken and DyLight488-conjugated donkey anti-mouse (Jackson oligonucleotide transfer into cells in culture and in vivo: polyethylenimine. Proc. ImmunoResearch; IF, 1:300), Alexa488-conjugated donkey anti-mouse, Natl. Acad. Sci. U.S.A. 92, 7297-7301. Cy3-conjugated donkey anti-rabbit (Jackson ImmunoResearch; IF, 1:600), Dhaunchak, A. S., Huang, J. K., De Faria Junior, O., Roth, A. D., Pedraza, L., HRP-conjugated goat anti-rabbit (Dako; WB, 1:5000), goat anti-human (GE Antel, J. P., Bar-Or, A. and Colman, D. R. (2010). A proteome map of axoglial specializations isolated and purified from human central nervous system. Glia 58, Healthcare; WB, 1:5000), alkaline phosphatase-conjugated goat anti-chicken 1949-1960. (Aves Laboratory; WB, 1:1000), alkaline phosphatase-conjugated goat anti- Estable-Puig, J.F.,Bauer,W.C.andBlumberg,J.M.(1965).Paraphenylenediamine mouse (DAKO WB 1:1000), alkaline phosphatase-conjugated goat anti-rabbit staining of osmium-fixed plastic embedded tissue for light and phase microscopy. (DAKO; WB, 1:1000). J. Neuropathol. Exp. Neurol. 24,531-534. Fukata, Y., Adesnik, H., Iwanaga, T., Bredt, D. S., Nicoll, R. A. and Fukata, M. Immunoprecipitation (2006). Epilepsy-related ligand/receptor complex LGI1 and ADAM22 regulate synaptic transmission. Science 313, 1792-1795. For LGI-ADAM22 binding assays, media conditioned for 4 days with either Fukata, Y., Lovero, K. L., Iwanaga, T., Watanabe, A., Yokoi, N., Tabuchi, K., ADAM22-Fc or LGI-V5-His were collected and filtered. A portion of the Shigemoto, R., Nicoll, R. A. and Fukata, M. (2010). Disruption of LGI1-linked LGI-V5-His conditioned medium was adjusted to 20 mM imidazole and the synaptic complex causes abnormal synaptic transmission and epilepsy. Proc. His-tagged LGI proteins were bound to Nickel beads (Qiagen) overnight at Natl. Acad. Sci. U.S.A. 107, 3799-3804. 4°C (input). Conditioned medium from ADAM22-Fc transfected cells was Goldsmith, A. P., Gossage, S. J. and ffrench-Constant, C. (2004). ADAM23 is a adjusted with Tris pH 8 to a final concentration of 0.1 M and mixed with cell-surface glycoprotein expressed by central nervous system neurons. LGI-V5-His protein conditioned medium to allow binding overnight at 4°C. J. Neurosci. Res. 78, 647-658. Gu, W., Wevers, A., Schrö der, H., Grzeschik, K.-H., Derst, C., Brodtkorb, E., Adam22-Fc–LGI-V5-His protein complexes were bound to Protein A beads de Vos, R. and Steinlein, O. K. (2002). The LGI1 gene involved in lateral (RepliGen) and precipitated. Beads were washed in PBS/0.1% Triton X-100 temporal lobe epilepsy belongs to a new subfamily of leucine-rich repeat proteins. and bound proteins were separated on a 4-12% gradient SDS-PAGE gel FEBS Lett. 519, 71-76. (Invitrogen) and analyzed by western blotting. Gu, W., Sander, T., Becker, T. and Steinlein, O. K. (2004). Genotypic association LGI-V5-His proteins from conditioned media of retrovirally transduced of exonic LGI4 polymorphisms and childhood absence epilepsy. Neurogenetics 5, clp/clp cultures were precipitated using a V5 antibody (050210D11, Absea 41-44. Biotechnology) covalently coupled to Protein A beads (RepliGen). Bound Gu, W., Gibert, Y., Wirth, T., Elischer, A., Bloch, W., Meyer, A., Steinlein, O. K. and Begemann, G. (2005). Using gene-history and expression analyses to proteins were separated on a 4-12% gradient SDS-PAGE gel and analyzed assess the involvement of LGI genes in human disorders. Mol. Biol. Evol. 22, by western blotting using a V5 monoclonal antibody (clone 5C5). 2209-2216. Henry, E. W., Eicher, E. M. and Sidman, R. L. (1991). The mouse mutation claw Light microscopy paw: forelimb deformity and delayed myelination throughout the peripheral Mice were deeply anesthetized with sodium pentobarbital and transcardially nervous system. J. Hered. 82, 287-294. perfused with PBS followed by 4% PFA/1% glutaraldehyde in 0.1 M Irani, S. R., Alexander, S., Waters, P., Kleopa, K. A., Pettingill, P., Zuliani, L., Peles, E., Buckley, C., Lang, B. and Vincent, A. (2010). Antibodies to Kv1 phosphate buffer, pH 7.2. Sciatic nerves were dissected and placed in the potassium channel-complex proteins leucine-rich, glioma inactivated 1 protein same fixative at 4°C for at least 16 h. Tissues were washed with 0.1 M and contactin-associated protein-2 in limbic encephalitis, Morvan’s syndrome and phosphate buffer and osmicated overnight at 4°C in 1% osmium tetroxide/ acquired neuromyotonia. Brain 133, 2734-2748. ferricyanide in phosphate buffer. Tissues were embedded in plastic resin and Ishii, A., Zhang, B., Kaneko, S. and Hirose, S. (2010). Positive association 1 μm sections were cut, mounted and stained with paraphenylenediamine between benign familial infantile convulsions and LGI4. Brain Dev. 32, 538-543. 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