Cell adhesion and neurite outgrowth are promoted by neurofascin NF155 and inhibited by NF186

Genomic signatures of high-altitude adaptation and chromosomal polymorphism in geladas

Primates have adapted to numerous environments and lifestyles, but very few species are native to high elevations. Here, we investigated high-altitude adaptations in the gelada (Theropithecus gelada), a monkey endemic to the Ethiopian Plateau. We examined genome-wide variation in conjunction with measurements of hematological and morphological traits. Our new gelada reference genome is highly intact and assembled at chromosome-length levels. Unexpectedly, we identified a chromosomal polymorphism in geladas that could potentially contribute to reproductive barriers between populations. Compared to baboons at low altitude, we found that high-altitude geladas exhibit significantly expanded chest circumferences, potentially allowing for greater lung surface area for increased oxygen diffusion. We identified gelada-specific amino acid substitutions in the alpha-chain subunit of adult hemoglobin but found that gelada hemoglobin does not exhibit markedly altered oxygenation properties compared to lowland primates. We also found that geladas at high altitude do not exhibit elevated blood hemoglobin concentrations, in contrast to the normal acclimatization response to hypoxia in lowland primates. The absence of altitude-related polycythemia suggests that geladas are able to sustain adequate tissue-oxygen delivery despite environmental hypoxia. Finally, we identified numerous genes and genomic regions exhibiting accelerated rates of evolution, as well as gene families exhibiting expansions in the gelada lineage, potentially reflecting altitude-related selection. Our findings lend insight into putative mechanisms of high-altitude adaptation while suggesting promising avenues for functional hypoxia research.

Neuronal post-developmentally acting SAX-7S/L1CAM can function as cleaved fragments to maintain neuronal architecture in C. elegans

Whereas remarkable advances have uncovered mechanisms that drive nervous system assembly, the processes responsible for the lifelong maintenance of nervous system architecture remain poorly understood. Subsequent to its establishment during embryogenesis, neuronal architecture is maintained throughout life in the face of the animal's growth, maturation processes, the addition of new neurons, body movements, and aging. The C. elegans protein SAX-7, homologous to the vertebrate L1 protein family of neural adhesion molecules, is required for maintaining the organization of neuronal ganglia and fascicles after their successful initial embryonic development. To dissect the function of sax-7 in neuronal maintenance, we generated a null allele and sax-7S-isoform-specific alleles. We find that the null sax-7(qv30) is, in some contexts, more severe than previously described mutant alleles, and that the loss of sax-7S largely phenocopies the null, consistent with sax-7S being the key isoform in neuronal maintenance. Using a sfGFP::SAX-7S knock-in, we observe sax-7S to be predominantly expressed across the nervous system, from embryogenesis to adulthood. Yet, its role in maintaining neuronal organization is ensured by post-developmentally acting SAX-7S, as larval transgenic sax-7S(+) expression alone is sufficient to profoundly rescue the null mutants' neuronal maintenance defects. Moreover, the majority of the protein SAX-7 appears to be cleaved, and we show that these cleaved SAX-7S fragments together, not individually, can fully support neuronal maintenance. These findings contribute to our understanding of the role of the conserved protein SAX-7/L1CAM in long-term neuronal maintenance, and may help decipher processes that go awry in some neurodegenerative conditions.

Neurofascin antibodies in chronic inflammatory demyelinating polyradiculoneuropathy: from intrinsic genetic background to clinical manifestations

There are bunch of autoantibodies, particularly autoantibodies against proteins located at the node of Ranvier, have been discovered and transformed the clinical management of chronic inflammatory demyelinating polyradiculoneuropathy (CIDP). Neurofascin (NF) plays an important role in both the nodal and paranodal regions of the node of Ranvier. In this review, we focus on the two characteristic forms of neurofascin: NF186 and NF155, comparing the similarities and differences between them, reviewing the current knowledge on genetic backgrounds, pathogenesis, clinical manifestations, and management of patients with anti-neurofascin positive CIDP. Autoantibodies against neurofascin were mainly IgG4 isotype. Mutation of NFASC gene in human causes severe neurodevelopment disorders, and HLA DRB1*15 may be a strong risk factor for the development of anti-NF155 antibodies. Motor impairment, sensory ataxia, and tremor were the typical presentations of patients with anti-NF155+ CIDP, while tetraplegia and cranial nerve involvement were more common in patients with anti-NF186+ CIDP. Recent studies have depicted a relatively clear picture of anti-NF155+ CIDP, and the strong clinical correlation of NF186 with CIDP remains unclear. The genetic background of neurofascin will assist in future explorations.

Methylomic changes in response to micronutrient supplementation and MTHFR genotype

Exposure times and dosage required for dietary components to modify DNA methylation patterns are largely unknown.

AIM

This exploratory research represents the first genome-wide analysis of DNA methylation changes during a randomized-controlled-trial (RCT) for dietary supplementation with broad spectrum vitamins, minerals and amino acids in humans.

METHODS

Genome-wide changes in methylation from paired, peripheral blood samples were assessed using the Infinium Methylation EPIC 850 K array.

RESULTS

Methylation increased at 84% of the most significant differentially methylated CpGs; however, none showed significance after adjustment for genome-wide testing.

CONCLUSION

Micronutrient supplementation is unlikely to have a substantial biological effect on DNA methylation over 10 weeks; however, the trend toward hypermethylation that we observed is likely to become more marked with longer exposure periods.

Characterization of Somatically-Eliminated Genes During Development of the Sea Lamprey (Petromyzon marinus)

The sea lamprey (Petromyzon marinus) is a basal vertebrate that undergoes developmentally programmed genome rearrangements (PGRs) during early development. These events facilitate the elimination of ∼20% of the genome from the somatic cell lineage, resulting in distinct somatic and germline genomes. Thus far only a handful of germline-specific genes have been definitively identified within the estimated 500 Mb of DNA that is deleted during PGR, although a few thousand germline-specific genes are thought to exist. To improve our understanding of the evolutionary/developmental logic of PGR, we generated computational predictions to identify candidate germline-specific genes within a new transcriptomic dataset derived from adult germline and the early embryonic stages during which PGR occurs. Follow-up validation studies identified 44 germline-specific genes and further characterized patterns of transcription and DNA loss during early embryogenesis. Expression analyses reveal that many of these genes are differentially expressed during early embryogenesis and presumably function in the early development of the germline. Ontology analyses indicate that many of these germline-specific genes play known roles in germline development, pluripotency, and oncogenesis (when misexpressed). These studies provide support for the theory that PGR serves to segregate molecular functions related to germline development/pluripotency in order to prevent their potential misexpression in somatic cells. This larger set of eliminated genes also allows us to extend the evolutionary/developmental breadth of this theory, as some deleted genes (or their gnathostome homologs) appear to be associated with the early development of somatic lineages, perhaps through the evolution of novel functions within gnathostome lineages.

Neurofascin-155 IgG4 in chronic inflammatory demyelinating polyneuropathy

OBJECTIVE

We report the clinical and serologic features of Japanese patients with chronic inflammatory demyelinating polyneuropathy (CIDP) displaying anti-neurofascin-155 (NF155) immunoglobulin G4 (IgG4) antibodies.

METHODS

In sera from 533 patients with CIDP, anti-NF155 IgG4 antibodies were detected by ELISA. Binding of IgG antibodies to central and peripheral nerves was tested.

RESULTS

Anti-NF155 IgG4 antibodies were identified in 38 patients (7%) with CIDP, but not in disease controls or normal participants. These patients were younger at onset as compared to 100 anti-NF155-negative patients with CIDP. Twenty-eight patients (74%) presented with sensory ataxia, 16 (42%) showed tremor, 5 (13%) presented with cerebellar ataxia associated with nystagmus, 3 (8%) had demyelinating lesions in the CNS, and 20 of 25 (80%) had poor response to IV immunoglobulin. The clinical features of the antibody-positive patients were statistically more frequent as compared to negative patients with CIDP (n = 100). Anti-NF155 IgG antibodies targeted similarly central and peripheral paranodes.

CONCLUSION

Anti-NF155 IgG4 antibodies were associated with a subgroup of patients with CIDP showing a younger age at onset, ataxia, tremor, CNS demyelination, and a poor response to IV immunoglobulin. The autoantibodies may serve as a biomarker to improve patients' diagnosis and guide treatments.

Perlecan is recruited by dystroglycan to nodes of Ranvier and binds the clustering molecule gliomedin

Fast neural conduction requires accumulation of Na(+) channels at nodes of Ranvier. Dedicated adhesion molecules on myelinating cells and axons govern node organization. Among those, specific laminins and dystroglycan complexes contribute to Na(+) channel clustering at peripheral nodes by unknown mechanisms. We show that in addition to facing the basal lamina, dystroglycan is found near the nodal matrix around axons, binds matrix components, and participates in initial events of nodogenesis. We identify the dystroglycan-ligand perlecan as a novel nodal component and show that dystroglycan is required for the selective accumulation of perlecan at nodes. Perlecan binds the clustering molecule gliomedin and enhances clustering of node of Ranvier components. These data show that proteoglycans have specific roles in peripheral nodes and indicate that peripheral and central axons use similar strategies but different molecules to form nodes of Ranvier. Further, our data indicate that dystroglycan binds free matrix that is not organized in a basal lamina.

The synaptic proteome

Synapses are focal hot spots for signal transduction and plasticity in the brain. A synapse comprises an axon terminus, the presynapse, the synaptic cleft containing extracellular matrix proteins as well as adhesion molecules, and the postsynaptic density as target structure for chemical signaling. The proteomes of the presynaptic and postsynaptic active zones control neurotransmitter release and perception. These tasks demand short- and long-term structural and functional dynamics of the synapse mediated by its proteinaceous inventory. This review addresses subcellular fractionation protocols and the related proteomic approaches to the various synaptic subcompartments with an emphasis on the presynaptic active zone (PAZ). Furthermore, it discusses major constituents of the PAZ including the amyloid precursor protein family members. Numerous proteins regulating the rearrangement of the cytoskeleton are indicative of the functional and structural dynamics of the pre- and postsynapse. The identification of protein candidates of the synapse provides the basis for further analyzing the interaction of synaptic proteins with their targets, and the effect of their deletion opens novel insights into the functional role of these proteins in neuronal communication. The knowledge of the molecular interactome is also a prerequisite for understanding numerous neurodegenerative diseases.

The Proteome of the Murine Presynaptic Active Zone

The proteome of the presynaptic active zone controls neurotransmitter release and the short- and long-term structural and functional dynamics of the nerve terminal. The proteinaceous inventory of the presynaptic active zone has recently been reported. This review will evaluate the subcellular fractionation protocols and the proteomic approaches employed. A breakthrough for the identification of the proteome of the presynaptic active zone was the successful employment of antibodies directed against a cytosolic epitope of membrane integral synaptic vesicle proteins for the immunopurification of synaptic vesicles docked to the presynaptic plasma membrane. Combining immunopurification and subsequent analytical mass spectrometry, hundreds of proteins, including synaptic vesicle proteins, components of the presynaptic fusion and retrieval machinery, proteins involved in intracellular and extracellular signaling and a large variety of adhesion molecules, were identified. Numerous proteins regulating the rearrangement of the cytoskeleton are indicative of the functional and structural dynamics of the presynapse. This review will critically discuss both the experimental approaches and prominent protein candidates identified. Many proteins have not previously been assigned to the presynaptic release sites and may be directly involved in the short- and long-term structural modulation of the presynaptic compartment. The identification of proteinaceous constituents of the presynaptic active zone provides the basis for further analyzing the interaction of presynaptic proteins with their targets and opens novel insights into the functional role of these proteins in neuronal communication.

The proteome of the presynaptic active zone from mouse brain

Neurotransmitter release as well as the structural and functional dynamics of the presynaptic active zone is controlled by proteinaceous components. Here we describe for the first time an experimental approach for the isolation of the presynaptic active zone from individual mouse brains, a prerequisite for understanding the functional inventory of the presynaptic protein network and for the later analysis of changes occurring in mutant mice. Using a monoclonal antibody against the ubiquitous synaptic vesicle protein SV2 we immunopurified synaptic vesicles docked to the presynaptic plasma membrane. Enrichment studies by means of Western blot analysis and mass spectrometry identified 485 proteins belonging to an impressive variety of functional categories. Our data suggest that presynaptic active zones represent focal hot spots that are not only involved in the regulation of neurotransmitter release but also in multiple structural and functional alterations the adult nerve terminal undergoes during neural activity in adult CNS. They furthermore open new avenues for characterizing alterations in the active zone proteome of mutant mice and their corresponding controls, including the various mouse models of neurological diseases.

Quantitative proteomics for identifying biomarkers for tuberculous meningitis

INTRODUCTION

Tuberculous meningitis is a frequent extrapulmonary disease caused by Mycobacterium tuberculosis and is associated with high mortality rates and severe neurological sequelae. In an earlier study employing DNA microarrays, we had identified genes that were differentially expressed at the transcript level in human brain tissue from cases of tuberculous meningitis. In the current study, we used a quantitative proteomics approach to discover protein biomarkers for tuberculous meningitis.

METHODS

To compare brain tissues from confirmed cased of tuberculous meningitis with uninfected brain tissue, we carried out quantitative protein expression profiling using iTRAQ labeling and LC-MS/MS analysis of SCX fractionated peptides on Agilent's accurate mass QTOF mass spectrometer.

RESULTS AND CONCLUSIONS

Through this approach, we identified both known and novel differentially regulated molecules. Those described previously included signal-regulatory protein alpha (SIRPA) and protein disulfide isomerase family A, member 6 (PDIA6), which have been shown to be overexpressed at the mRNA level in tuberculous meningitis. The novel overexpressed proteins identified in our study included amphiphysin (AMPH) and neurofascin (NFASC) while ferritin light chain (FTL) was found to be downregulated in TBM. We validated amphiphysin, neurofascin and ferritin light chain using immunohistochemistry which confirmed their differential expression in tuberculous meningitis. Overall, our data provides insights into the host response in tuberculous meningitis at the molecular level in addition to providing candidate diagnostic biomarkers for tuberculous meningitis.

A phylogenetic analysis of the L1 family of neural cell adhesion molecules

L1-type genes form one of several distinct gene families that encode adhesive proteins, which are predominantly expressed in developing and mature metazoan nervous systems. These proteins have a multitude of different important cellular functions in neuronal and glial cells. L1-type gene products are transmembrane proteins with a characteristic extracellular domain structure consisting of six immunoglobulin and three to five fibronectin type III protein folds. As reported here, L1-type proteins can be identified in most metazoan phyla with the notable exception of Porifera (sponges). This puts the origin of L1-type genes at a point in time when primitive cellular neural networks emerged, approximately 1,200 to 1,500 million years ago. Subsequently, several independent gene duplication events generated multiple paralogous L1-type genes in some phyla, allowing for a considerable diversification of L1 structures and the emergence of new functional features and molecular interactions. One such evolutionary newer feature is the appearance of RGD integrin-binding motifs in some vertebrate L1 family members.

Pinceau organization in the cerebellum requires distinct functions of neurofascin in Purkinje and basket neurons during postnatal development

Basket axon collaterals synapse onto the Purkinje soma/axon initial segment (AIS) area to form specialized structures, the pinceau, which are critical for normal cerebellar function. Mechanistic details of how the pinceau become organized during cerebellar development are poorly understood. Loss of cytoskeletal adaptor protein Ankyrin G (AnkG) results in mislocalization of the cell adhesion molecule Neurofascin (Nfasc) at the Purkinje AIS and abnormal organization of the pinceau. Loss of Nfasc in adult Purkinje neurons leads to slow disorganization of the Purkinje AIS and pinceau morphology. Here, we used mouse conditional knock-out techniques to show that selective loss of Nfasc, specifically in Purkinje neurons during early development, prevented maturation of the AIS and resulted in loss of Purkinje neuron spontaneous activity and pinceau disorganization. Loss of Nfasc in both Purkinje and basket neurons caused abnormal basket axon collateral branching and targeting to Purkinje soma/AIS, leading to extensive pinceau disorganization, Purkinje neuron degeneration, and severe ataxia. Our studies reveal that the Purkinje Nfasc is required for AIS maturation and for maintaining stable contacts between basket axon terminals and the Purkinje AIS during pinceau organization, while the basket neuron Nfasc in combination with Purkinje Nfasc is required for proper basket axon collateral outgrowth and targeting to Purkinje soma/AIS. Thus, cerebellar pinceau organization requires coordinated mechanisms involving specific Nfasc functions in both Purkinje and basket neurons.

The axon initial segment in nervous system disease and injury

The axon initial segment (AIS), with its dense clusters of voltage-gated ion channels decorating the axonal membrane, regulates action potential initiation and modulation. The AIS also functions as a barrier to maintain axodendritic polarity, and its precise axonal location contributes to the fine-tuning of neuronal excitability. Therefore, it is not surprising that mutations in AIS-related genes, disruption of the molecular organization of the AIS and altered AIS ion channel expression, function, location and/or density are emerging as key players in neurological disorders. Here, we consider the role of the AIS in nervous system disease and injury.

Neurofascin: a switch between neuronal plasticity and stability

Neurofascin (NF) is a cell surface protein belonging to the immunoglobulin superfamily (IgSF). Different polypeptides of 186, 180, 166 and 155 kDa are generated by alternative splicing. Expression of these isoforms is temporally and spatially regulated and can be roughly grouped into embryonic, adult and glial expression. NF interacts with many different interaction partners both extra- and intracellularly. Interactions of NF166 and NF180 selectively regulate mechanisms of plasticity like neurite outgrowth and the formation postsynaptic components. By contrast, NF155 and NF186 confer stabilization of neural structures by interaction with voltage-gated sodium channels and ankyrinG at axon initial segments (AIS) or nodes of Ranvier as well as neuron-glia interactions at the paranodes. Alternatively spliced isoforms of neurofascin may therefore balance dynamic and stabilizing mechanisms of the CNS.

The cell adhesion molecule neurofascin stabilizes axo-axonic GABAergic terminals at the axon initial segment

Cell adhesion molecules regulate synapse formation and maintenance via transsynaptic contact stabilization involving both extracellular interactions and intracellular postsynaptic scaffold assembly. The cell adhesion molecule neurofascin is localized at the axon initial segment of granular cells in rat dentate gyrus, which is mainly targeted by chandelier cells. Lentiviral shRNA-mediated knockdown of neurofascin in adult rat brain indicates that neurofascin regulates the number and size of postsynaptic gephyrin scaffolds, the number of GABA(A) receptor clusters as well as presynaptic glutamate decarboxylase-positive terminals at the axon initial segment. By contrast, overexpression of neurofascin in hippocampal neurons increases gephyrin cluster size presumably via stimulation of fibroblast growth factor receptor 1 signaling pathways.

Homophilic adhesion mechanism of neurofascin, a member of the L1 family of neural cell adhesion molecules

The L1 family neural cell adhesion molecules play key roles in specifying the formation and remodeling of the neural network, but their homophilic interaction that mediates adhesion is not well understood. We report two crystal structures of a dimeric form of the headpiece of neurofascin, an L1 family member. The four N-terminal Ig-like domains of neurofascin form a horseshoe shape, akin to several other immunoglobulin superfamily cell adhesion molecules such as hemolin, axonin, and Dscam. The neurofascin dimer, captured in two crystal forms with independent packing patterns, reveals a pair of horseshoes in trans-synaptic adhesion mode. The adhesion interaction is mediated mostly by the second Ig-like domain, which features an intermolecular β-sheet formed by the joining of two individual GFC β-sheets and a large but loosely packed hydrophobic cluster. Mutagenesis combined with gel filtration assays suggested that the side chain hydrogen bonds at the intermolecular β-sheet are essential for the homophilic interaction and that the residues at the hydrophobic cluster play supplementary roles. Our structures reveal a conserved homophilic adhesion mode for the L1 family and also shed light on how the pathological mutations of L1 affect its structure and function.

Novel forms of neurofascin 155 in the central nervous system: alterations in paranodal disruption models and multiple sclerosis

Stability of the myelin-axon unit is achieved, at least in part, by specialized paranodal junctions comprised of the neuronal heterocomplex of contactin and contactin-associated protein and the myelin protein neurofascin 155. In multiple sclerosis, normal distribution of these proteins is altered, resulting in the loss of the insulating myelin and consequently causing axonal dysfunction. Previously, this laboratory reported that mice lacking the myelin-enriched lipid sulphatide are characterized by a progressive deterioration of the paranodal structure. Here, it is shown that this deterioration is preceded by significant loss of neurofascin 155 clustering at the myelin paranode. Interestingly, prolonged electrophoretic separation revealed the existence of two neurofascin 155 bands, neurofascin 155 high and neurofascin 155 low, which are readily observed following N-linked deglycosylation. Neurofascin 155 high is observed at 7 days of age and reaches peak expression at one month of age, while neurofascin 155 low is first observed at 14 days of age and constantly increases until 5 months of age. Studies using conditional neurofascin knockout mice indicated that neurofascin 155 high and neurofascin 155 low are products of the neurofascin gene and are exclusively expressed by oligodendrocytes within the central nervous system. Neurofascin 155 high is a myelin paranodal protein while the distribution of neurofascin 155 low remains to be determined. While neurofascin 155 high levels are significantly reduced in the sulphatide null mice at 15 days, 30 days and 4 months of age, neurofascin 155 low levels remain unaltered. Although maintained at normal levels, neurofascin 155 low is incapable of preserving paranodal structure, thus indicating that neurofascin 155 high is required for paranodal stability. Additionally, comparisons between neurofascin 155 high and neurofascin 155 low in human samples revealed a significant alteration, specifically in multiple sclerosis plaques.

Analysis of non-canonical fibroblast growth factor receptor 1 (FGFR1) interaction reveals regulatory and activating domains of neurofascin

Fibroblast growth factor receptors (FGFRs) are important for many different mechanisms, including cell migration, proliferation, differentiation, and survival. Here, we show a new link between FGFR1 and the cell adhesion molecule neurofascin, which is important for neurite outgrowth. After overexpression in HEK293 cells, embryonal neurofascin isoform NF166 was able to associate with FGFR1, whereas the adult isoform NF186, differing from NF166 in additional extracellular sequences, was deficient. Pharmacological inhibitors and overexpression of dominant negative components of the FGFR signaling pathway pointed to the activation of FGFR1 after association with neurofascin in neurite outgrowth assays in chick tectal neurons and rat PC12-E2 cells. Both extra- and intracellular domains of embryonal neurofascin isoform NF166 were able to form complexes with FGFR1 independently. However, the cytosolic domain was both necessary and sufficient for the activation of FGFR1. Cytosolic serine residues 56 and 100 were shown to be essential for the neurite outgrowth-promoting activity of neurofascin, whereas both amino acid residues were dispensable for FGFR1 association. In conclusion, the data suggest a neurofascin intracellular domain, which activates FGFR1 for neurite outgrowth, whereas the extracellular domain functions as an additional, regulatory FGFR1 interaction domain in the course of development.

The intracellular interactions of the L1 family of cell adhesion molecules

The L1 family of CAMs (cell adhesion molecules) has long aroused the interest of researchers, but primarily the extracellular interactions of these proteins have been elucidated. More recently, attention has turned to the intracellular signalling potentiated by transmembrane proteins and the cytoplasmic proteins with which they can interact. The present review brings up to date the current body of published knowledge for the intracellular interactions of L1-CAM family proteins and the potential importance of these interactions for the mechanisms of L1-CAM action.

The proteome of the presynaptic active zone: from docked synaptic vesicles to adhesion molecules and maxi-channels

The presynaptic proteome controls neurotransmitter release and the short and long term structural and functional dynamics of the nerve terminal. Using a monoclonal antibody against synaptic vesicle protein 2 we immunopurified a presynaptic compartment containing the active zone with synaptic vesicles docked to the presynaptic plasma membrane as well as elements of the presynaptic cytomatrix. Individual protein bands separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis were subjected to nanoscale-liquid chromatography electrospray ionization-tandem mass spectrometry. Combining this method with 2-dimensional benzyldimethyl-n-hexadecylammonium chloride/sodium dodecyl sulfate-polyacrylamide gel electrophoresis and matrix-assisted laser desorption ionization time of flight and immunodetection we identified 240 proteins comprising synaptic vesicle proteins, components of the presynaptic fusion and retrieval machinery, proteins involved in intracellular signal transduction, a large variety of adhesion molecules and proteins potentially involved in regulating the functional and structural dynamics of the pre-synapse. Four maxi-channels, three isoforms of voltage-dependent anion channels and the tweety homolog 1 were co-isolated with the docked synaptic vesicles. As revealed by in situ hybridization, tweety homolog 1 reveals a distinct expression pattern in the rodent brain. Our results add novel information to the proteome of the presynaptic active zone and suggest that in particular proteins potentially involved in the short and long term structural modulation of the mature presynaptic compartment deserve further detailed analysis.

The immunoglobulin superfamily of neuronal cell adhesion molecules: lessons from animal models and correlation with human disease

Neuronal cell adhesion molecules of the immunoglobulin superfamily (IgCAMs) play a crucial role in the formation of neural circuits at different levels: cell migration, axonal and dendritic targeting as well as synapse formation. Furthermore, in perinatal and adult life, neuronal IgCAMs are required for the formation and maintenance of specialized axonal membrane domains, synaptic plasticity and neurogenesis. Mutations in the corresponding human genes have been correlated to several human neuronal disorders. Perturbing neuronal IgCAMs in animal models provides powerful means to understand the molecular and cellular basis of such human disorders. In this review, we concentrate on the NCAM, L1 and contactin subfamilies of neuronal IgCAMs summarizing recent functional studies from model systems and highlighting their links to disease pathogenesis.

Neurofascin regulates the formation of gephyrin clusters and their subsequent translocation to the axon hillock of hippocampal neurons

Little is known about the role of cell adhesion molecules (CAMs) in inhibitory synapse development. In particular, a functional link between CAMs and the clustering of postsynaptic scaffold component gephyrin, which is a critical determinant of gamma-aminobutyric acid A (GABA) receptor clustering, still needs to be elaborated. At early stages of inhibitory synapse formation, gephyrin and CAM neurofascin are diffusely expressed in the soma of hippocampal neurons. Subsequently, gephyrin clusters become localized to the axon hillock and neurofascin is observed all over the soma including the axon hillock suggesting a function for neurofascin in gephyrin clustering. Transfection of expression vectors for different isoforms and mutants of neurofascin revealed that neurofascin is required for the formation of gephyrin clusters presumably dependent on extracellular interactions. Furthermore, expression of neurofascin is necessary for the translocation of gephyrin clusters to the axon hillock of hippocampal neurons as shown by shRNA-mediated knockdown. In addition, overexpression of an embryonic neurofascin isoform is sufficient for functional rescue after knockdown of endogenous neurofascin.

Differential expression and functions of neuronal and glial neurofascin isoforms and splice variants during PNS development

The cell adhesion molecule neurofascin (NF) has a major neuronal isoform (NF186) containing a mucin-like domain followed by a fifth fibronectin type III repeat while these domains are absent from glial NF155. Neuronal NF isoforms lacking one or both of these domains are expressed transiently in embryonic dorsal root ganglia (DRG). These two domains are co-expressed in mature NF186, which peaks in expression prior to birth and then persists almost exclusively at nodes of Ranvier on myelinated axons. In contrast, glial NF155 is only detected postnatally with the onset of myelination. All these forms of NF bound homophilically and to Schwann cells but only the mature NF186 isoform inhibits cell adhesion, and this activity may be important in formation of the node of Ranvier. Schwann cells deficient in NF155 myelinated DRG axons in a delayed manner and they showed significantly decreased clustering of both NF and Caspr in regions where paranodes normally form. The combined results suggest that NF186 is expressed prenatally on DRG neurons and it may modulate their adhesive interactions with Schwann cells, which express NF155 postnatally and require it for development of axon-glial paranodal junctions.

Differential activities in adhesion and neurite growth of fibronectin type III repeats in the PTP-delta extracellular domain

The full-length extracellular domain (ECD) of protein tyrosine phosphatase delta (PTP-delta) functions as a ligand to promote cell adhesion and neurite outgrowth; this ECD contains three immunoglobulin (Ig) repeats and eight fibronectin type III (FN III) repeats. However, it is not known which regions of the ECD regulate its ligand functions. Therefore, we constructed and expressed a fusion protein of the PTP-delta ECD lacking FN III repeats 4-8, and tested this protein for neuronal adhesion and neurite-promoting ability. Compared to the full-length isoform, the truncated ECD was poorer at promoting adhesion, but a more potent promoter of neurite growth. The results suggest that distal FN III repeats of PTP-delta are important in adhesive functions, but dispensable for neurite outgrowth promotion. As the predominant isoform of PTP-delta during neural development (type D) also lacks distal FN III repeats, the functional properties we observe may be relevant to periods of axon extension, suggesting that splice variants of receptor PTPs play distinct roles in neural development.

A regulated switch of chick neurofascin isoforms modulates ligand recognition and neurite extension

Neural cell adhesion molecule neurofascin regulates the induction of neurite outgrowth, the establishment of synaptic connectivity and myelination. Neurofascin isoforms are generated by spatially and temporally controlled alternative splicing. Isoform NF166 is predominantly expressed in dorsal root ganglia from embryonal day 5 (E5) to E8, and a further neurofascin isoform NF185 appears at E9. Expression of neurofascin and its binding partner axonin-1 on sensory fibers implies functional interactions for neurite outgrowth. E7 sensory neurons require NF166-axonin-1 interactions for neurite extension, accordingly. The contribution of NF166-axonin-1 interaction for neurite outgrowth decreases in parallel with the appearance of NF185 on sensory neurons at E9. This finding may be explained by (1) alleviated intrinsic capability to use axonin-1 as a cellular receptor and (2) reduced binding of axonin-1 to NF185. Finally, NF166, but not NF185, serves as a cellular receptor for neurite induction via homophilic interactions with a neurofascin substrate.