Disruption of Nectin-like 1 cell adhesion molecule leads to delayed axonal myelination in the CNS

Necl-1/CADM3 regulates cone synapse formation in the mouse retina

In vertebrates, retinal neural circuitry for visual perception is organized in specific layers. The outer plexiform layer is the first synaptic region in the visual pathway, where photoreceptor synaptic terminals connect with bipolar and horizontal cell processes. However, molecular mechanisms underlying cone synapse formation to mediate OFF pathways remain unknown. This study reveals that Necl-1/CADM3 is localized at S- and S/M-opsin-containing cones and dendrites of type 4 OFF cone bipolar cells (CBCs). In Necl-1-/- mouse retina, synapses between cones and type 4 OFF CBCs were dislocated, horizontal cell distribution became abnormal, and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors were dislocated. Necl-1-/- mice exhibited aberrant short-wavelength-light-elicited signal transmission from cones to OFF CBCs, which was rescued by AMPA receptor potentiator. Additionally, Necl-1-/- mice showed impaired optokinetic responses. These findings suggest that Necl-1 regulates cone synapse formation to mediate OFF cone pathways elicited by short-wavelength light in mouse retina.

SynCAMs in Normal Vertebrate Neural Development and Neuropsychiatric Disorders: from the Perspective of the OCAs

Neuronal synaptic junctions connect neurons to enable neuronal signal transmission in the nervous system. The proper establishment of synaptic connections required many adhesion molecules. Malfunctions of these adhesion molecules can result in neural development disorders and neuropsychiatric disorders. How specific synapses are established by various adhesion molecules for proper neural circuitry is a fundamental question of neuroscience. SynCAMs, also named CADMs, Necl, etc., are among the many adhesion proteins found in synapses. Here, we review the current understanding of the physical properties of SynCAMs and their roles in axon pathfinding, myelination, synaptogenesis, and synaptic plasticity. In addition, we discuss the involvement of SynCAMs in neuropsychiatric disorders. Finally, we propose that SynCAM functions can be better viewed and understood from the perspective of orientational cell adhesions (OCAs). In particular, we discuss the possibilities of how SynCAMs can be regulated at the cell-type specific expression, transcription variants, posttranslational modification, and subcellular localization to modulate the diversity of SynCAMs as OCA molecules. Being major components of the synapses, SynCAMs continue to be an important research topic of neuroscience, and many outstanding questions are waiting to be answered.

Homophilic interaction of cell adhesion molecule 3 coordinates retina neuroepithelial cell proliferation

Correct cell number generation is central to tissue development. However, in vivo roles of coordinated proliferation of individual neural progenitors in regulating cell numbers of developing neural tissues and the underlying molecular mechanism remain mostly elusive. Here, we showed that wild-type (WT) donor retinal progenitor cells (RPCs) generated significantly expanded clones in host retinae with G1-lengthening by p15 (cdkn2a/b) overexpression (p15+) in zebrafish. Further analysis showed that cell adhesion molecule 3 (cadm3) was reduced in p15+ host retinae, and overexpression of either full-length or ectodomains of Cadm3 in p15+ host retinae markedly suppressed the clonal expansion of WT donor RPCs. Notably, WT donor RPCs in retinae with cadm3 disruption recapitulated expanded clones that were found in p15+ retinae. More strikingly, overexpression of Cadm3 without extracellular ig1 domain in RPCs resulted in expanded clones and increased retinal total cell number. Thus, homophilic interaction of Cadm3 provides an intercellular mechanism underlying coordinated cell proliferation to ensure cell number homeostasis of the developing neuroepithelia.

Indispensable role of Nectin-like 4 in regulating synapse-related molecules, synaptic structure, and individual behavior

Nectin-like family members (Necls) are involved in synaptic organization. In contrast to that of Necl-2/CADM1/SynCAM1, which is critical in synaptic events, investigation of Necl-4/CADM4/SynCAM4 in synapses has largely lagged behind given the particularity of homophilic self-interactions compared to interactions with other Necls. We sought to further understand the role of Necl-4 in synapses and found that knockout of Necl-4 led to aberrant expression levels of proteins mediating synaptic function in cortex homogenates and augmented accumulation of ionotropic glutamate receptor in postsynaptic density fractions, although a compensatory effect of Necl-1 on the expression levels existed. Concurrently, we also found increased synaptic clefts in the cortex and simplified dendritic morphology of primary cultured cortical neurons. Experiments on individual behaviors suggested that compared to their wild-type littermates, Necl-4-KO mice exhibited impaired acquisition of spatial memory and working memory and enhanced behavioral despair and anxiety-like behavior. These findings suggest that Necl-4 mediates synaptic function and related behaviors through an indispensable role and offer a new perspective about collaboration and specialization among Necls.

CADM2 is implicated in impulsive personality and numerous other traits by genome- and phenome-wide association studies in humans and mice

Impulsivity is a multidimensional heritable phenotype that broadly refers to the tendency to act prematurely and is associated with multiple forms of psychopathology, including substance use disorders. We performed genome-wide association studies (GWAS) of eight impulsive personality traits from the Barratt Impulsiveness Scale and the short UPPS-P Impulsive Personality Scale (N = 123,509-133,517 23andMe research participants of European ancestry), and a measure of Drug Experimentation (N = 130,684). Because these GWAS implicated the gene CADM2, we next performed single-SNP phenome-wide studies (PheWAS) of several of the implicated variants in CADM2 in a multi-ancestral 23andMe cohort (N = 3,229,317, European; N = 579,623, Latin American; N = 199,663, African American). Finally, we produced Cadm2 mutant mice and used them to perform a Mouse-PheWAS ("MouseWAS") by testing them with a battery of relevant behavioral tasks. In humans, impulsive personality traits showed modest chip-heritability (~6-11%), and moderate genetic correlations (rg = 0.20-0.50) with other personality traits, and various psychiatric and medical traits. We identified significant associations proximal to genes such as TCF4 and PTPRF, and also identified nominal associations proximal to DRD2 and CRHR1. PheWAS for CADM2 variants identified associations with 378 traits in European participants, and 47 traits in Latin American participants, replicating associations with risky behaviors, cognition and BMI, and revealing novel associations including allergies, anxiety, irritable bowel syndrome, and migraine. Our MouseWAS recapitulated some of the associations found in humans, including impulsivity, cognition, and BMI. Our results further delineate the role of CADM2 in impulsivity and numerous other psychiatric and somatic traits across ancestries and species.

Myelin sheath injury and repairment after subarachnoid hemorrhage

Subarachnoid hemorrhage (SAH) can lead to damage to the myelin sheath in white matter. Through classification and analysis of relevant research results, the discussion in this paper provides a deeper understanding of the spatiotemporal change characteristics, pathophysiological mechanisms and treatment strategies of myelin sheath injury after SAH. The research progress for this condition was also systematically reviewed and compared related to myelin sheath in other fields. Serious deficiencies were identified in the research on myelin sheath injury and treatment after SAH. It is necessary to focus on the overall situation and actively explore different treatment methods based on the spatiotemporal changes in the characteristics of the myelin sheath, as well as the initiation, intersection and common action point of the pathophysiological mechanism, to finally achieve accurate treatment. We hope that this article can help researchers in this field to further clarify the challenges and opportunities in the current research on myelin sheath injury and treatment after SAH.

Synaptic Cell Adhesion Molecule 3 (SynCAM3) Deletion Promotes Recovery from Spinal Cord Injury by Limiting Glial Scar Formation

Synaptic cell adhesion molecules (SynCAMs) play an important role in the formation and maintenance of synapses and the regulation of synaptic plasticity. SynCAM3 is expressed in the synaptic cleft of the central nervous system (CNS) and is involved in the connection between axons and astrocytes. We hypothesized that SynCAM3 may be related to the astrocytic scar (glial scar, the most important factor of CNS injury treatment) through extracellular matrix (ECM) reconstitution. Thus, we investigated the influence of the selective removal of SynCAM3 on the outcomes of spinal cord injury (SCI). SynCAM3 knock-out (KO) mice were subjected to moderate compression injury of the lower thoracic spinal cord using wild-type (WT) (C57BL/6JJc1) mice as controls. Single-cell RNA sequencing analysis over time, quantitative real-time polymerase chain reaction (qRT-PCR) analysis, and immunohistochemistry (IHC) showed reduced scar formation in SynCAM3 KO mice compared to WT mice. SynCAM3 KO mice showed improved functional recovery from SCI by preventing the transformation of reactive astrocytes into scar-forming astrocytes, resulting in improved ECM reconstitution at four weeks after injury. Our findings suggest that SynCAM3 could be a novel therapeutic target for SCI.

Differential Contribution of Cadm1-Cadm3 Cell Adhesion Molecules to Peripheral Myelinated Axons

Cell adhesion proteins of the Cadm (SynCAM/Necl) family regulate myelination and the organization of myelinated axons. In the peripheral nervous system (PNS), intercellular contact between Schwann cells and their underlying axons is believed to be mediated by binding of glial Cadm4 to axonal Cadm3 or Cadm2. Nevertheless, given that distinct neurons express different combinations of the Cadm proteins, the identity of the functional axonal ligand for Cadm4 remains to be determined. Here, we took a genetic approach to compare the phenotype of Cadm4 null mice, which exhibit abnormal distribution of Caspr and Kv1 potassium channels, with mice lacking different combinations of Cadm1-Cadm3 genes. We show that in contrast to mice lacking the single Cadm1, Cadm2, or Cadm3 genes, genetic ablation of all three phenocopies the abnormalities detected in the absence of Cadm4. Similar defects were observed in double mutant mice lacking Cadm3 and Cadm2 (i.e., Cadm3 ^-/- /Cadm2 ^-/-) or Cadm3 and Cadm1 (i.e., Cadm3 ^-/- /Cadm1 ^-/-), but not in mice lacking Cadm1 and Cadm2 (i.e., Cadm1 ^-/- /Cadm2 ^-/-). Furthermore, axonal organization abnormalities were also detected in Cadm3 null mice that were heterozygous for the two other axonal Cadms. Our results identify Cadm3 as the main axonal ligand for glial Cadm4, and reveal that its absence could be compensated by the combined action of Cadm2 and Cadm1.SIGNIFICANCE STATEMENT Myelination by Schwann cells enables fast conduction of action potentials along motor and sensory axons. In these nerves, Schwann cell-axon contact is mediated by cell adhesion molecules of the Cadm family. Cadm4 in Schwann cells regulates axonal ensheathment and myelin wrapping, as well as the organization of the axonal membrane, but the identity of its axonal ligands is not clear. Here, we reveal that Cadm mediated axon-glia interactions depend on a hierarchical adhesion code that involves multiple family members. Our results provide important insights into the molecular mechanisms of axon-glia communication, and the function of Cadm proteins in PNS myelin.

Role of glia in optic nerve

Glial cells are critically important for maintenance of neuronal activity in the central nervous system (CNS), including the optic nerve (ON). However, the ON has several unique characteristics, such as an extremely high myelination level of retinal ganglion cell (RGC) axons throughout the length of the nerve (with virtually all fibers myelinated by 7 months of age in humans), lack of synapses and very narrow geometry. Moreover, the optic nerve head (ONH) - a region where the RGC axons exit the eye - represents an interesting area that is morphologically distinct in different species. In many cases of multiple sclerosis (demyelinating disease of the CNS) vision problems are the first manifestation of the disease, suggesting that RGCs and/or glia in the ON are more sensitive to pathological conditions than cells in other parts of the CNS. Here, we summarize current knowledge on glial organization and function in the ON, focusing on glial support of RGCs. We cover both well-established concepts on the important role of glial cells in ON health and new findings, including novel insights into mechanisms of remyelination, microglia/NG2 cell-cell interaction, astrocyte reactivity and the regulation of reactive astrogliosis by mitochondrial fragmentation in microglia.

Axo-glial interdependence in peripheral nerve development

During the development of the peripheral nervous system, axons and myelinating Schwann cells form a unique symbiotic unit, which is realized by a finely tuned network of molecular signals and reciprocal interactions. The importance of this complex interplay becomes evident after injury or in diseases in which aspects of axo-glial interaction are perturbed. This Review focuses on the specific interdependence of axons and Schwann cells in peripheral nerve development that enables axonal outgrowth, Schwann cell lineage progression, radial sorting and, finally, formation and maintenance of the myelin sheath.

The oligodendrocyte growth cone and its actin cytoskeleton: A fundamental element for progenitor cell migration and CNS myelination

Cells of the oligodendrocyte (OLG) lineage engage in highly motile behaviors that are crucial for effective central nervous system (CNS) myelination. These behaviors include the guided migration of OLG progenitor cells (OPCs), the surveying of local environments by cellular processes extending from differentiating and pre-myelinating OLGs, and during the process of active myelin wrapping, the forward movement of the leading edge of the myelin sheath's inner tongue along the axon. Almost all of these motile behaviors are driven by actin cytoskeletal dynamics initiated within a lamellipodial structure that is located at the tip of cellular OLG/OPC processes and is structurally as well as functionally similar to the neuronal growth cone. Accordingly, coordinated stoichiometries of actin filament (F-actin) assembly and disassembly at these OLG/OPC growth cones have been implicated in directing process outgrowth and guidance, and the initiation of myelination. Nonetheless, the functional importance of the OLG/OPC growth cone still remains to be fully understood, and, as a unique aspect of actin cytoskeletal dynamics, F-actin depolymerization and disassembly start to predominate at the transition from myelination initiation to myelin wrapping. This review provides an overview of the current knowledge about OLG/OPC growth cones, and it proposes a model in which actin cytoskeletal dynamics in OLG/OPC growth cones are a main driver for morphological transformations and motile behaviors. Remarkably, these activities, at least at the later stages of OLG maturation, may be regulated independently from the transcriptional gene expression changes typically associated with CNS myelination.

Coordinated internodal and paranodal adhesion controls accurate myelination by oligodendrocytes

Elazar et al. show that reduced axoglial adhesion at both the paranodal junction and the internodes results in the formation of multimyelinated axons. Their findings demonstrate that accurate ensheathment by oligodendrocytes depends on the coordinated action of these different adhesion systems.

Oligodendrocyte–axon contact is mediated by several cell adhesion molecules (CAMs) that are positioned at distinct sites along the myelin unit, yet their role during myelination remains unclear. Cadm4 and its axonal receptors, Cadm2 and Cadm3, as well as myelin-associated glycoprotein (MAG), are enriched at the internodes below the compact myelin, whereas NF155, which binds the axonal Caspr/contactin complex, is located at the paranodal junction that is formed between the axon and the terminal loops of the myelin sheath. Here we report that Cadm4-, MAG-, and Caspr-mediated adhesion cooperate during myelin membrane ensheathment. Genetic deletion of either Cadm4 and MAG or Cadm4 and Caspr resulted in the formation of multimyelinated axons due to overgrowth of the myelin away from the axon and the forming paranodal junction. Consequently, these mice displayed paranodal loops either above or underneath compact myelin. Our results demonstrate that accurate placement of the myelin sheath by oligodendrocytes requires the coordinated action of internodal and paranodal CAMs.

Exploring the involvement of Tac2 in the mouse hippocampal stress response through gene networking

Tachykinin 2 (Tac2) is expressed in a number of areas throughout the brain, including the hippocampus. However, knowledge about its function has been only well explored in the hypothalamus in the context of reproductive health. In this study, we identified and validated increased hippocampal Tac2 mRNA expression in response to chronic mild stress in mice. Expression quantitative trait locus (eQTL) analysis showed Tac2 is cis-regulated in the hippocampus. Using a systems genetics approach, we constructed a Tac2 co-expression network to better understand the relationship between Tac2 and the hippocampal stress response. Our network identified 69 total genes associated with Tac2, several of which encode major neuropeptides involved in hippocampal stress signaling as well as critical genes for producing neural plasticity, indicating that Tac2 is involved in these processes. Pathway analysis for the member of Tac2 gene network revealed a strong connection between Tac2 and neuroactive ligand-receptor interaction, calcium signaling pathway, as well as cardiac muscle contraction. In addition, we also identified 46 stress-related phenotypes, specifically fear conditioning response, that were significantly correlated with Tac2 expression. Our results provide evidence for Tac2 as a strong candidate gene who likely plays a role in hippocampal stress processing and neural plasticity.

Structure of the heterophilic interaction between the nectin-like 4 and nectin-like 1 molecules

Nectin-like (Necl) molecules are Ca²⁺-independent Ig-like transmembrane cell adhesion molecules that participate in junctions between different cell types. The specific cell-cell adhesions mediated by Necl proteins are important in neural development and have been implicated in neurodegenerative diseases. Here, we present the crystal structure of the mouse Necl-4 full ectodomain and the structure of the heterophilic Necl ectodomain complex formed by the mNecl-4 and mNecl-1 ectodomains. We demonstrate that, while the ectodomain of mNecl-4 is monomeric, it forms a stable heterodimer with Ig1 of mNecl-1, with an affinity significantly higher than that observed for self-dimerization of the mNecl-1 ectodomain. We validated our structural characterizations by performing a surface plasmon resonance assay and an Fc fusion protein binding assay in mouse primary dorsal root ganglia neurites and Schwann cells and identified a selection of residues important for heterophilic interactions. Finally, we proposed a model of Necl binding specificity that involves an induced-fit conformational change at the dimerization interface.

Axoglial Adhesion by Cadm4 Regulates CNS Myelination

The initiation of axoglial contact is considered a prerequisite for myelination, yet the role cell adhesion molecules (CAMs) play in mediating such interactions remains unclear. To examine the function of axoglial CAMs, we tested whether enhanced CAM-mediated adhesion between OLs and neurons could affect myelination. Here we show that increased expression of a membrane-bound extracellular domain of Cadm4 (Cadm4dCT) in cultured oligodendrocytes results in the production of numerous axoglial contact sites that fail to elongate and generate mature myelin. Transgenic mice expressing Cadm4dCT were hypomyelinated and exhibit multiple myelin abnormalities, including myelination of neuronal somata. These abnormalities depend on specific neuron-glial interaction as they were not observed when these OLs were cultured alone, on nanofibers, or on neurons isolated from mice lacking the axonal receptors of Cadm4. Our results demonstrate that tightly regulated axon-glia adhesion is essential for proper myelin targeting and subsequent membrane wrapping and lateral extension.

Genome-wide identification of microRNAs in decidual natural killer cells from patients with unexplained recurrent spontaneous abortion

PROBLEM

This study revealed miRNA regulation and functional microarray-based profiles of miRNAs in the natural killer (NK) cells of the decidual tissue obtained from patients with unexplained recurrent spontaneous abortion (URSA).

METHOD OF STUDY

Patients with URSA were categorized based on the occurrence of at least two or more successive spontaneous abortions between 7th and 10th gestational week. Total RNA was isolated from the NK cells of the decidual tissue obtained from patients with induced abortion at about the 8th gestational week. The potential contribution of regulatory miRNAs to a genetic predisposition to URSA was characterized by comparison with healthy and fertile controls and bioinformatics analyses.

RESULTS

Analysis of the miRNA expression profiles identified 50 miRNAs that were differentially expressed, including one down-regulated and 49 up-regulated miRNAs in the URSA group. MiRNA-Gene-Network, miRNA-GO-Network and miRNA-Gene-TF-Network were constructed. The key miRNAs, genes, GOs and core TFs in the network were determined.

CONCLUSION

Our results suggest that a close relationship exists between the aberrant miRNAs and URSA. Furthermore, these findings support the notion that altered expression of miRNAs may contribute to the clinical diagnosis of URSA and the potential to develop novel strategies for therapeutic targets against URSA.

The Rules of Attraction in Central Nervous System Myelination

The wrapping of myelin around axons is crucial for the development and function of the central nervous system (CNS) of vertebrates, greatly regulating the conduction of action potentials. Oligodendrocytes, the myelinating glia of the CNS, have an intrinsic tendency to wrap myelin around any permissive structure in vitro, but in vivo, myelin is targeted with remarkable specificity only to certain axons. Despite the importance of myelination, the mechanisms by which oligodendrocytes navigate a complex milieu that includes many types of cells and their cellular projections and select only certain axons for myelination remains incompletely understood. In this Mini-review, I highlight recent studies that shed light on the molecular and cellular rules governing CNS myelin targeting.

Mutation of the cellular adhesion molecule NECL2 is associated with neuromyelitis optica spectrum disorder

AIMS

To investigate the association of the Nectin/Necl family genes with the risk of developing NMOSD.

METHODS

Whole-exome sequencing was performed on two familial NMOSD cases and two unaffected family members. Additionally, 106 patients with sporadic NMOSD and 212 healthy controls (HCs) underwent screening for mutant Necl2. Finally, the molecular weight and cellular localization of mutant NECL2 was examined in transfected HeLa cells.

RESULTS

We identified a novel deletion mutation in Necl2 (c.1052_1060delCCACCACCA; p. Thr351_Thr353del), which was associated with disease manifestation in the NMOSD familial cases. The frequency at which the mutation occurred in patients with sporadic NMOSD was significantly higher than for HCs (5.7% and 0, respectively; p<0.01). The mutation was located in the extracellular domain close to the transmembrane region, at a point in the protein sequence characterized by threonine enrichment. The mutant NECL2 had a lower molecular weight and exhibited defective trafficking to the cell surface.

CONCLUSIONS

Our results suggest that the Necl2 mutation identified herein may be associated with the risk of developing NMOSD. Furthermore, mutated NECL2 may play a role in the pathogenesis of the disease, potentially through its roles in axonal regeneration and/or via neuron-glia interactions that are relevant to myelination.

VCAM1 acts in parallel with CD69 and is required for the initiation of oligodendrocyte myelination

Oligodendrocytes differentiate to wrap their plasma membranes around axons, forming the myelin sheath. A neuronal cue is one of the regulator elements controlling this process. Here, we demonstrate that VCAM1, which plays a key role throughout the immune system, is also expressed in oligodendrocytes, where it regulates the initiation of myelination. VCAM1 knockout mice exhibit reduced myelin thickness. Decreased myelin thickness is also observed in mutant mice of α4 integrin, which is a neuronal VCAM1 ligand. Furthermore, CD69 is identified as one of the transcripts downregulated when VCAM1 is knocked down in oligodendrocytes. Knockdown of CD69 in mice indicates its role in myelination. Therefore, VCAM1 contributes not only to the initiation of myelination but also to its regulation through controlling the abundance of CD69, demonstrating that an intercellular molecule whose primary role is in the immune system can also play an unexpected role in the CNS.

The vascular cell adhesion molecule VCAM1 plays a role in the immune system but is also expressed in oligodendrocytes. Here, the authors find VCAM1 interacts with neuronal α4 integrin to regulate oligodendrocyte differentiation and thereby myelination, an effect mediated by downstream CD69 signalling.

Cadm3 (Necl-1) interferes with the activation of the PI3 kinase/Akt signaling cascade and inhibits Schwann cell myelination in vitro

Axo-glial interactions are critical for myelination and the domain organization of myelinated fibers. Cell adhesion molecules belonging to the Cadm family, and in particular Cadm3 (axonal) and its heterophilic binding partner Cadm4 (Schwann cell), mediate these interactions along the internode. Using targeted shRNA-mediated knockdown, we show that the removal of axonal Cadm3 promotes Schwann cell myelination in the in vitro DRG neuron/Schwann cell myelinating system. Conversely, over-expressing Cadm3 on the surface of DRG neuron axons results in an almost complete inability by Schwann cells to form myelin segments. Axons of superior cervical ganglion (SCG) neurons, which do not normally support the formation of myelin segments by Schwann cells, express higher levels of Cadm3 compared to DRG neurons. Knocking down Cadm3 in SCG neurons promotes myelination. Finally, the extracellular domain of Cadm3 interferes in a dose-dependent manner with the activation of ErbB3 and of the pro-myelinating PI3K/Akt pathway, but does not interfere with the activation of the Mek/Erk1/2 pathway. While not in direct contradiction, these in vitro results shed lights on the apparent lack of phenotype that was reported from in vivo studies of Cadm3^-/- mice. Our results suggest that Cadm3 may act as a negative regulator of PNS myelination, potentially through the selective regulation of the signaling cascades activated in Schwann cells by axonal contact, and in particular by type III Nrg-1. Further analyses of peripheral nerves in the Cadm^-/- mice will be needed to determine the exact role of axonal Cadm3 in PNS myelination. GLIA 2016;64:2247-2262.

SynCAMs - From axon guidance to neurodevelopmental disorders

Many cell adhesion molecules are located at synapses but only few of them can be considered synaptic cell adhesion molecules in the strict sense. Besides the Neurexins and Neuroligins, the LRRTMs (leucine rich repeat transmembrane proteins) and the SynCAMs/CADMs can induce synapse formation when expressed in non-neuronal cells and therefore are true synaptic cell adhesion molecules. SynCAMs (synaptic cell adhesion molecules) are a subfamily of the immunoglobulin superfamily of cell adhesion molecules. As suggested by their name, they were first identified as cell adhesion molecules at the synapse which were sufficient to trigger synapse formation. They also contribute to myelination by mediating axon-glia cell contacts. More recently, their role in earlier stages of neural circuit formation was demonstrated, as they also guide axons both in the peripheral and in the central nervous system. Mutations in SynCAM genes were found in patients diagnosed with autism spectrum disorders. The diverse functions of SynCAMs during development suggest that neurodevelopmental disorders are not only due to defects in synaptic plasticity. Rather, early steps of neural circuit formation are likely to contribute.

Regulating Axonal Responses to Injury: The Intersection between Signaling Pathways Involved in Axon Myelination and The Inhibition of Axon Regeneration

Following spinal cord injury (SCI), a multitude of intrinsic and extrinsic factors adversely affect the gene programs that govern the expression of regeneration-associated genes (RAGs) and the production of a diversity of extracellular matrix molecules (ECM). Insufficient RAG expression in the injured neuron and the presence of inhibitory ECM at the lesion, leads to structural alterations in the axon that perturb the growth machinery, or form an extraneous barrier to axonal regeneration, respectively. Here, the role of myelin, both intact and debris, in antagonizing axon regeneration has been the focus of numerous investigations. These studies have employed antagonizing antibodies and knockout animals to examine how the growth cone of the re-growing axon responds to the presence of myelin and myelin-associated inhibitors (MAIs) within the lesion environment and caudal spinal cord. However, less attention has been placed on how the myelination of the axon after SCI, whether by endogenous glia or exogenously implanted glia, may alter axon regeneration. Here, we examine the intersection between intracellular signaling pathways in neurons and glia that are involved in axon myelination and axon growth, to provide greater insight into how interrogating this complex network of molecular interactions may lead to new therapeutics targeting SCI.

Systematic substrate identification indicates a central role for the metalloprotease ADAM10 in axon targeting and synapse function

Metzincin metalloproteases have major roles in intercellular communication by modulating the function of membrane proteins. One of the proteases is the a-disintegrin-and-metalloprotease 10 (ADAM10) which acts as alpha-secretase of the Alzheimer's disease amyloid precursor protein. ADAM10 is also required for neuronal network functions in murine brain, but neuronal ADAM10 substrates are only partly known. With a proteomic analysis of Adam10-deficient neurons we identified 91, mostly novel ADAM10 substrate candidates, making ADAM10 a major protease for membrane proteins in the nervous system. Several novel substrates, including the neuronal cell adhesion protein NrCAM, are involved in brain development. Indeed, we detected mistargeted axons in the olfactory bulb of conditional ADAM10-/- mice, which correlate with reduced cleavage of NrCAM, NCAM and other ADAM10 substrates. In summary, the novel ADAM10 substrates provide a molecular basis for neuronal network dysfunctions in conditional ADAM10-/- mice and demonstrate a fundamental function of ADAM10 in the brain.

Developmental expression and function analysis of protein tyrosine phosphatase receptor type D in oligodendrocyte myelination

Receptor protein tyrosine phosphatases (RPTPs) are extensively expressed in the central nervous system (CNS), and have distinct spatial and temporal patterns in different cell types during development. Previous studies have demonstrated possible roles for RPTPs in axon outgrowth, guidance, and synaptogenesis. In the present study, our results revealed that protein tyrosine phosphatase, receptor type D (PTPRD) was initially expressed in mature neurons in embryonic CNS, and later in oligodendroglial cells at postnatal stages when oligodendrocytes undergo active axonal myelination process. In PTPRD mutants, oligodendrocyte differentiation was normal and a transient myelination delay occurred at early postnatal stages, indicating the contribution of PTPRD to the initiation of axonal myelination. Our results also showed that the remyelination process was not affected in the absence of PTPRD function after a cuprizone-induced demyelination in adult animals.

New insights on Schwann cell development

In the peripheral nervous system, Schwann cells are glial cells that are in intimate contact with axons throughout development. Schwann cells generate the insulating myelin sheath and provide vital trophic support to the neurons that they ensheathe. Schwann cell precursors arise from neural crest progenitor cells, and a highly ordered developmental sequence controls the progression of these cells to become mature myelinating or nonmyelinating Schwann cells. Here, we discuss both seminal discoveries and recent advances in our understanding of the molecular mechanisms that drive Schwann cell development and myelination with a focus on cell-cell and cell-matrix signaling events.

Nectins and nectin-like molecules in development and disease

Nectins and nectin-like molecules (Necls)/Cadms are Ca(2+)-independent immunoglobulin superfamily cell adhesion molecules, expressed in most cell types. Nectins mediate not only homotypic but also heterotypic cell-cell adhesion, in contrast to classic cadherins which participate only in homophilic adhesion. Nectins and Necls function in organogenesis of the eye, inner ear, tooth, and cerebral cortex and in a variety of developmental processes including spermatogenesis, axon guidance, synapse formation, and myelination. They are also involved in various diseases, such as viral infection, hereditary ectodermal dysplasia, Alzheimer's disease, autism spectrum disorder, and cancer. Thus, nectins and Necls are crucial for both physiology and pathology. This review summarizes recent advances in research on these cell adhesion molecules in development and pathogenesis.

Genetic deletion of Cadm4 results in myelin abnormalities resembling Charcot-Marie-Tooth neuropathy

The interaction between myelinating Schwann cells and the axons they ensheath is mediated by cell adhesion molecules of the Cadm/Necl/SynCAM family. This family consists of four members: Cadm4/Necl4 and Cadm1/Necl2 are found in both glia and axons, whereas Cadm2/Necl3 and Cadm3/Necl1 are expressed by sensory and motor neurons. By generating mice lacking each of the Cadm genes, we now demonstrate that Cadm4 plays a role in the establishment of the myelin unit in the peripheral nervous system. Mice lacking Cadm4 (PGK-Cre/Cadm4(fl/fl)), but not Cadm1, Cadm2, or Cadm3, develop focal hypermyelination characterized by tomacula and myelin outfoldings, which are the hallmark of several Charcot-Marie-Tooth neuropathies. The absence of Cadm4 also resulted in abnormal axon-glial contact and redistribution of ion channels along the axon. These neuropathological features were also found in transgenic mice expressing a dominant-negative mutant of Cadm4 lacking its cytoplasmic domain in myelinating glia Tg(mbp-Cadm4dCT), as well as in mice lacking Cadm4 specifically in Schwann cells (DHH-Cre/Cadm4(fl/fl)). Consistent with these abnormalities, both PGK-Cre/Cadm4(fl/fl) and Tg(mbp-Cadm4dCT) mice exhibit impaired motor function and slower nerve conduction velocity. These findings indicate that Cadm4 regulates the growth of the myelin unit and the organization of the underlying axonal membrane.

On the resemblance of synapse formation and CNS myelination

The myelination of axons in the central nervous system (CNS) is essential for nervous system formation, function and health. CNS myelination continues well into adulthood, but not all axons become myelinated. Unlike the peripheral nervous system, where we know of numerous axon-glial signals required for myelination, we have a poor understanding of the nature or identity of such molecules that regulate which axons are myelinated in the CNS. Recent studies have started to elucidate cell behavior during myelination in vivo and indicate that the choice of which axons are myelinated is made prior to myelin sheath generation. Here we propose that interactions between axons and the exploratory processes of oligodendrocyte precursor cells (OPCs) lead to myelination and may be similar to those between dendrites and axons that prefigure and lead to synapse formation. Indeed axons and OPCs form synapses with striking resemblance to those of neurons, suggesting a similar mode of formation. We discuss families of molecules with specific functions at different stages of synapse formation and address studies that implicate the same factors during axon-OPC synapse formation and myelination. We also address the possibility that the function of such synapses might directly regulate the myelinating behavior of oligodendrocyte processes in vivo. In the future it may be of benefit to consider these similarities when taking a candidate-based approach to dissect mechanisms of CNS myelination.

Necl-4/SynCAM-4 is expressed in myelinating oligodendrocytes but not required for axonal myelination

The timing and progression of axonal myelination are precisely controlled by intercellular interactions between neurons and glia in development. Previous in vitro studies demonstrated that Nectin like 4 (Necl-4, also known as cell adhesion molecule Cadm-4 or SynCAM-4) plays an essential role in axonal myelination by Schwann cells in the peripheral nervous system (PNS). However, the role of Necl-4 protein in axonal myelination in the developing central nervous system (CNS) has remained unknown. In this study, we discovered upregulation of Necl-4 expression in mature oligodendrocytes at perinatal stages when axons undergo active myelination. We generated Necl4 gene knockout mice, but found that disruption of Necl-4 gene did not affect oligodendrocyte differentiation and myelin formation in the CNS. Surprisingly, disruption of Necl-4 had no significant effect on axonal myelination in the PNS either. Therefore, our results demonstrated that Necl-4 is dispensable for axonal myelination in the developing nervous system.

Hierarchical clustering of gene expression patterns in the Eomes + lineage of excitatory neurons during early neocortical development

BACKGROUND

Cortical neurons display dynamic patterns of gene expression during the coincident processes of differentiation and migration through the developing cerebrum. To identify genes selectively expressed by the Eomes + (Tbr2) lineage of excitatory cortical neurons, GFP-expressing cells from Tg(Eomes::eGFP) Gsat embryos were isolated to > 99% purity and profiled.

RESULTS

We report the identification, validation and spatial grouping of genes selectively expressed within the Eomes + cortical excitatory neuron lineage during early cortical development. In these neurons 475 genes were expressed ≥ 3-fold, and 534 genes ≤ 3-fold, compared to the reference population of neuronal precursors. Of the up-regulated genes, 328 were represented at the Genepaint in situ hybridization database and 317 (97%) were validated as having spatial expression patterns consistent with the lineage of differentiating excitatory neurons. A novel approach for quantifying in situ hybridization patterns (QISP) across the cerebral wall was developed that allowed the hierarchical clustering of genes into putative co-regulated groups. Forty four candidate genes were identified that show spatial expression with Intermediate Precursor Cells, 49 candidate genes show spatial expression with Multipolar Neurons, while the remaining 224 genes achieved peak expression in the developing cortical plate.

CONCLUSIONS

This analysis of differentiating excitatory neurons revealed the expression patterns of 37 transcription factors, many chemotropic signaling molecules (including the Semaphorin, Netrin and Slit signaling pathways), and unexpected evidence for non-canonical neurotransmitter signaling and changes in mechanisms of glucose metabolism. Over half of the 317 identified genes are associated with neuronal disease making these findings a valuable resource for studies of neurological development and disease.

Increased axonal expression of nectin-1 in multiple sclerosis plaques

Nectin-1 is a cell adhesion molecule that plays a role in interneuronal synapse formation, in axonal guidance during development and possibly in neuron-glia interactions. To better understand axonal changes in MS, nectin-1 expression was determined by immunohistochemistry in normal adult human cerebral white matter (n = 4) and in six MS plaques (three active and three inactive). The intensity of axonal nectin-1 expression was scored on a scale of 0 to 4+. In normal adult cerebral white matter, axons showed weak nectin-1 expression with a score of 1.25 ± 0.50. Axonal nectin-1 expression was significantly stronger within both active (score = 3.33 ± 0.289, p = 0.001) and inactive (score = 2.16 ± 0.29, p = 0.038) MS plaques than in normal white matter. Axons in white matter adjacent to MS plaques showed nectin-1 expression (score = 1.5 ± 0.50) that was not statistically different from normal controls (p = 0.542). These findings raise the possibility that increased expression of nectin-1 in MS lesions plays a role in the pathogenesis of MS through participation in axonal responses to injury and mediation of altered neuron-glia interactions relevant to myelination.

Congenital CNS hypomyelination in the Fig4 null mouse is rescued by neuronal expression of the PI(3,5)P(2) phosphatase Fig4

The plt (pale tremor) mouse carries a null mutation in the Fig4(Sac3) gene that results in tremor, hypopigmentation, spongiform degeneration of the brain, and juvenile lethality. FIG4 is a ubiquitously expressed phosphatidylinositol 3,5-bisphosphate phosphatase that regulates intracellular vesicle trafficking along the endosomal-lysosomal pathway. In humans, the missense mutation FIG4(I41T) combined with a FIG4 null allele causes Charcot-Marie-Tooth 4J disease, a severe form of peripheral neuropathy. Here we show that Fig4 null mice exhibit a dramatic reduction of myelin in the brain and spinal cord. In the optic nerve, smaller-caliber axons lack myelin sheaths entirely, whereas many large- and intermediate-caliber axons are myelinated but show structural defects at nodes of Ranvier, leading to delayed propagation of action potentials. In the Fig4 null brain and optic nerve, oligodendrocyte (OL) progenitor cells are present at normal abundance and distribution, but the number of myelinating OLs is greatly compromised. The total number of axons in the Fig4 null optic nerve is not reduced. Developmental studies reveal incomplete myelination rather than elevated cell death in the OL linage. Strikingly, there is rescue of CNS myelination and tremor in transgenic mice with neuron-specific expression of Fig4, demonstrating a non-cell-autonomous function of Fig4 in OL maturation and myelin development. In transgenic mice with global overexpression of the human pathogenic FIG4 variant I41T, there is rescue of the myelination defect, suggesting that the CNS of CMT4J patients may be protected from myelin deficiency by expression of the FIG4(I41T) mutant protein.

Adherens junctions in mammalian development, homeostasis and disease: lessons from mice

Mice have proven to be a particularly powerful model to study molecular mechanisms of development and disease. The reason for this is the close evolutionary relationship between rodents and humans, similarities in physiological mechanisms in mice and human, and the large number of techniques available to study gene functions in mice. A large number of mice mutations, either germ line, conditional or inducible, have been generated in the past years for adherens junctions components, and the number is still increasing. In this review we will discuss mice models that have contributed to understanding the developmental and physiological role of adherens junctions and their components in mammals and have revealed novel mechanistic aspects of how adherens junctions regulate morphogenesis and tissue homeostasis.

Molecular mechanisms regulating myelination in the peripheral nervous system

Glial cells and neurons are engaged in a continuous and highly regulated bidirectional dialog. A remarkable example is the control of myelination. Oligodendrocytes in the central nervous system (CNS) and Schwann cells (SCs) in the peripheral nervous system (PNS) wrap their plasma membranes around axons to organize myelinated nerve fibers that allow rapid saltatory conduction. The functionality of this system is critical, as revealed by numerous neurological diseases that result from deregulation of the system, including multiple sclerosis and peripheral neuropathies. In this review we focus on PNS myelination and present a conceptual framework that integrates crucial signaling mechanisms with basic SC biology. We will highlight signaling hubs and overarching molecular mechanisms, including genetic, epigenetic, and post-translational controls, which together regulate the interplay between SCs and axons, extracellular signals, and the transcriptional network.

Localization of Cadm2a and Cadm3 proteins during development of the zebrafish nervous system

Members of the Cadm/SynCAM/Necl/IGSF/TSLC family of cell adhesion molecules are known to have diverse functions during development of the nervous system, but information regarding their role during central nervous system (CNS) development in vivo is scarce. The rapid development of a relatively simple nervous system in larval zebrafish makes them a highly tractable model organism for studying gene function during nervous system development. An essential prerequisite for functional studies is a description of protein localization. To address this we have generated subtype-specific antibodies to two members of the zebrafish cell adhesion molecule family: cadm2a and cadm3. Using these novel antibodies we show that cadm3 and cadm2a are expressed throughout the nervous system of larval stage zebrafish. Particularly striking, and largely nonoverlapping expression of cadm2a and cadm3 is observed in the developing retina and spinal cord. Using in vitro binding assays we show that cadm2a and cadm3 bind heterophilically and preferentially to cadm1 and cadm4, respectively. These binding preferences are very similar to those seen for tetrapod Cadms but our study of protein localization suggests novel and diverse functions of cadms during nervous system development.

Schwann cell spectrins modulate peripheral nerve myelination

During peripheral nerve development, Schwann cells ensheathe axons and form myelin to enable rapid and efficient action potential propagation. Although myelination requires profound changes in Schwann cell shape, how neuron-glia interactions converge on the Schwann cell cytoskeleton to induce these changes is unknown. Here, we demonstrate that the submembranous cytoskeletal proteins αII and βII spectrin are polarized in Schwann cells and colocalize with signaling molecules known to modulate myelination in vitro. Silencing expression of these spectrins inhibited myelination in vitro, and remyelination in vivo. Furthermore, myelination was disrupted in motor nerves of zebrafish lacking αII spectrin. Finally, we demonstrate that loss of spectrin significantly reduces both F-actin in the Schwann cell cytoskeleton and the Nectin-like protein, Necl4, at the contact site between Schwann cells and axons. Therefore, we propose αII and βII spectrin in Schwann cells integrate the neuron-glia interactions mediated by membrane proteins into the actin-dependent cytoskeletal rearrangements necessary for myelination.

The expression of TAG-1 in glial cells is sufficient for the formation of the juxtaparanodal complex and the phenotypic rescue of tag-1 homozygous mutants in the CNS

Myelinated fibers are organized into specialized domains that ensure the rapid propagation of action potentials and are characterized by protein complexes underlying axoglial interactions. TAG-1 (Transient Axonal Glycoprotein-1), a cell adhesion molecule of the Ig superfamily, is expressed by neurons as well as by myelinating glia. It is essential for the molecular organization of myelinated fibers as it maintains the integrity of the juxtaparanodal region through its interactions with Caspr2 and the voltage-gated potassium channels (VGKCs) on the axolemma. Since TAG-1 is the only known component of the juxtaparanodal complex expressed by the glial cell, it is important to clarify its role in the molecular organization of juxtaparanodes. For this purpose, we generated transgenic mice that exclusively express TAG-1 in oligodendrocytes and lack endogenous gene expression (Tag-1(-/-);plp(Tg(rTag-1))). Phenotypic analysis clearly demonstrates that glial TAG-1 is sufficient for the proper organization and maintenance of the juxtaparanodal domain in the CNS. Biochemical analysis shows that glial TAG-1 physically interacts with Caspr2 and VGKCs. Ultrastructural and behavioral analysis of Tag-1(-/-);plp(Tg(rTag-1)) mice shows that the expression of glial TAG-1 is sufficient to restore the axonal and myelin deficits as well as the behavioral defects observed in Tag-1(-/-) animals. Together, these data highlight the pivotal role of myelinating glia on axonal domain differentiation and organization.

Synaptic organizing complexes

A number of synaptogenic factors induce presynaptic or postsynaptic differentiation when presented to axons or dendrites. Many such factors participate in bidirectional trans-synaptic adhesion complexes. Axonal neurexins interacting in an isoform-specific code with multiple dendritic partners (neuroligins, LRRTMs, or Cbln-GluRδ), and axonal protein tyrosine phosphatase receptors interacting with dendritic NGL-3, nucleate local networks of high-affinity protein-protein interactions leading to aligned presynaptic and postsynaptic differentiation. Additional secreted target-derived factors such as fibroblast growth factors and glial-derived factors such as thrombospondin bind specific axonal or dendritic receptors stimulating signal transduction mechanisms to promote selective aspects of synapse development. Together with classical adhesion molecules and controlled by transcriptional cascades, these synaptogenic adhesion complexes and secreted factors organize the molecular composition and thus functional properties of central synapses.

Signals to promote myelin formation and repair

The myelin sheath wraps large axons in both the CNS and the PNS, and is a key determinant of efficient axonal function and health. Myelin is targeted in a series of diseases, notably multiple sclerosis (MS). In MS, demyelination is associated with progressive axonal damage, which determines the level of patient disability. The few treatments that are available for combating myelin damage in MS and related disorders, which largely comprise anti-inflammatory drugs, only show limited efficacy in subsets of patients. More-effective treatment of myelin disorders will probably be accomplished by early intervention with combinatorial therapies that target inflammation and other processes-for example, signaling pathways that promote remyelination. Indeed, evidence suggests that such pathways might be impaired in pathology and, hence, contribute to the failure of remyelination in such diseases. In this article, we review the molecular basis of signaling pathways that regulate myelination in the CNS and PNS, with a focus on signals that affect differentiation of myelinating glia. We also discuss factors such as extracellular molecules that act as modulators of these pathways. Finally, we consider the few preclinical and clinical trials of agents that augment this signaling.

Nectin-like molecules/SynCAMs are required for post-crossing commissural axon guidance

The Necl/SynCAM subgroup of immunoglobulin superfamily cell adhesion molecules has been implicated in late stages of neural circuit formation. They were shown to be sufficient for synaptogenesis by their trans-synaptic interactions. Additionally, they are involved in myelination, both in the central and the peripheral nervous system, by mediating adhesion between glia cells and axons. Here, we show that Necls/SynCAMs are also required for early stages of neural circuit formation. We demonstrate a role for Necls/SynCAMs in post-crossing commissural axon guidance in the developing spinal cord in vivo. Necl3/SynCAM2, the family member that has not been characterized functionally so far, plays a crucial role in this process. It is expressed by floorplate cells and interacts with Necls/SynCAMs expressed by commissural axons to mediate a turning response in post-crossing commissural axons.

On the biogenesis of myelin membranes: sorting, trafficking and cell polarity

In the central nervous system, a multilayered membrane layer known as the myelin sheath enwraps axons, and is required for optimal saltatory signal conductance. The sheath develops from membrane processes that extend from the plasma membrane of oligodendrocytes and displays a unique lipid and protein composition. Myelin biogenesis is carefully regulated, and multiple transport pathways involving a variety of endosomal compartments are involved. Here we briefly summarize how the major myelin proteins proteolipid protein and myelin basic protein reach the sheath, and highlight potential mechanisms involved, including the role of myelin specific lipids and cell polarity related transport pathways.

Myelin proteomics: molecular anatomy of an insulating sheath

Fast-transmitting vertebrate axons are electrically insulated with multiple layers of nonconductive plasma membrane of glial cell origin, termed myelin. The myelin membrane is dominated by lipids, and its protein composition has historically been viewed to be of very low complexity. In this review, we discuss an updated reference compendium of 342 proteins associated with central nervous system myelin that represents a valuable resource for analyzing myelin biogenesis and white matter homeostasis. Cataloging the myelin proteome has been made possible by technical advances in the separation and mass spectrometric detection of proteins, also referred to as proteomics. This led to the identification of a large number of novel myelin-associated proteins, many of which represent low abundant components involved in catalytic activities, the cytoskeleton, vesicular trafficking, or cell adhesion. By mass spectrometry-based quantification, proteolipid protein and myelin basic protein constitute 17% and 8% of total myelin protein, respectively, suggesting that their abundance was previously overestimated. As the biochemical profile of myelin-associated proteins is highly reproducible, differential proteome analyses can be applied to material isolated from patients or animal models of myelin-related diseases such as multiple sclerosis and leukodystrophies.

Myelination and regional domain differentiation of the axon

During evolution, as organisms increased in complexity and function, the need for the ensheathment and insulation of axons by glia became vital for faster conductance of action potentials in nerves. Myelination, as the process is termed, facilitates the formation of discrete domains within the axolemma that are enriched in ion channels, and macromolecular complexes consisting of cell adhesion molecules and cytoskeletal regulators. While it is known that glia play a substantial role in the coordination and organization of these domains, the mechanisms involved and signals transduced between the axon and glia, as well as the proteins regulating axo-glial junction formation remain elusive. Emerging evidence has shed light on the processes regulating myelination and domain differentiation, and key molecules have been identified that are required for their assembly and maintenance. This review highlights these recent findings, and relates their significance to domain disorganization as seen in several demyelinating disorders and other neuropathies.