Overlapping functions of the cell adhesion molecules Nr-CAM and L1 in cerebellar granule cell development

Junctional Adhesion Molecule (JAM)-C recruitment of Pard3 and drebrin to cell contacts initiates neuron-glia recognition and layer-specific cell sorting in developing cerebella

Sorting maturing neurons into distinct layers is critical for brain development, with disruptions leading to neurological disorders and pediatric cancers. Lamination coordinates where, when, and how cells interact, facilitating events that direct migrating neurons to their destined positions within emerging neural networks and control the wiring of connections in functional circuits. While the role of adhesion molecule expression and presentation in driving adhesive recognition during neuronal migration along glial fibers is recognized, the mechanisms by which the spatial arrangement of these molecules on the cell surface dictates adhesive specificity and translates contact-based external cues into intracellular responses like polarization and cytoskeletal organization remain largely unexplored. We used the cerebellar granule neuron (CGN) system to demonstrate that JAM-C receptor cis-binding on the same cell and trans-binding to neighboring cells controls the recruitment of the Pard3 polarity protein and drebrin microtubule-actin crosslinker at CGN to glial adhesion sites, complementing previous studies that showed Pard3 controls JAM-C exocytic surface presentation. Leveraging advanced imaging techniques, specific probes for cell recognition, and analytical methods to dissect adhesion dynamics, our findings reveal: 1) JAM-C cis or trans mutants result in reduced adhesion formation between CGNs and cerebellar glia, 2) these mutants exhibit delayed recruitment of Pard3 at the adhesion sites, and 3) CGNs with JAM-C mutations experience postponed sorting and entry into the cerebellar molecular layer (ML). By developing a conditional system to image adhesion components from two different cells simultaneously, we made it possible to investigate the dynamics of cell recognition on both sides of neuron-glial contacts and the subsequent recruitment of proteins required for CGN migration. This system and an approach that calculates local correlation based on convolution kernels at the cell adhesions site revealed that CGN to CGN JAM recognition preferentially recruits higher levels of Pard3 and drebrin than CGN to glia JAM recognition. The long latency time of CGNs in the inner external germinal layer (EGL) can be attributed to the combined strength of CGN-CGN contacts and the less efficient Pard3 recruitment by CGN-BG contacts, acting as gatekeepers to ML entry. As CGNs eventually transition to glia binding for radial migration, our research demonstrates that establishing permissive JAM-recognition sites on glia via cis and trans interactions of CGN JAM-C serves as a critical temporal checkpoint for sorting at the EGL to ML boundary. This mechanism integrates intrinsic and extrinsic cellular signals, facilitating heterotypic cell sorting into the ML and dictating the precise spatial organization within the cerebellar architecture.

NrCAM-deficient mice exposed to chronic stress exhibit disrupted latent inhibition, a hallmark of schizophrenia

The neuronal cell adhesion molecule (NrCAM) is widely expressed and has important physiological functions in the nervous system across the lifespan, from axonal growth and guidance to spine and synaptic pruning, to organization of proteins at the nodes of Ranvier. NrCAM lies at the core of a functional protein network where multiple targets (including NrCAM itself) have been associated with schizophrenia. Here we investigated the effects of chronic unpredictable stress on latent inhibition, a measure of selective attention and learning which shows alterations in schizophrenia, in NrCAM knockout (KO) mice and their wild-type littermate controls (WT). Under baseline experimental conditions both NrCAM KO and WT mice expressed robust latent inhibition (p = 0.001). However, following chronic unpredictable stress, WT mice (p = 0.002), but not NrCAM KO mice (F < 1), expressed latent inhibition. Analyses of neuronal activation (c-Fos positive counts) in key brain regions relevant to latent inhibition indicated four types of effects: a single hit by genotype in IL cortex (p = 0.0001), a single hit by stress in Acb-shell (p = 0.031), a dual hit stress x genotype in mOFC (p = 0.008), vOFC (p = 0.020), and Acb-core (p = 0.032), and no effect in PrL cortex (p > 0.141). These results indicating a pattern of differential effects of genotype and stress support a complex stress × genotype interaction model and a role for NrCAM in stress-induced pathological behaviors relevant to schizophrenia and other psychiatric disorders.

Ankyrin B promotes developmental spine regulation in the mouse prefrontal cortex

Postnatal regulation of dendritic spine formation and refinement in cortical pyramidal neurons is critical for excitatory/inhibitory balance in neocortical networks. Recent studies have identified a selective spine pruning mechanism in the mouse prefrontal cortex mediated by class 3 Semaphorins and the L1 cell adhesion molecules, neuron-glia related cell adhesion molecule, Close Homolog of L1, and L1. L1 cell adhesion molecules bind Ankyrin B, an actin-spectrin adaptor encoded by Ankyrin2, a high-confidence gene for autism spectrum disorder. In a new inducible mouse model (Nex1Cre-ERT2: Ank2flox: RCE), Ankyrin2 deletion in early postnatal pyramidal neurons increased spine density on apical dendrites in prefrontal cortex layer 2/3 of homozygous and heterozygous Ankyrin2-deficient mice. In contrast, Ankyrin2 deletion in adulthood had no effect on spine density. Sema3F-induced spine pruning was impaired in cortical neuron cultures from Ankyrin B-null mice and was rescued by re-expression of the 220 kDa Ankyrin B isoform but not 440 kDa Ankyrin B. Ankyrin B bound to neuron-glia related CAM at a cytoplasmic domain motif (FIGQY1231), and mutation to FIGQH inhibited binding, impairing Sema3F-induced spine pruning in neuronal cultures. Identification of a novel function for Ankyrin B in dendritic spine regulation provides insight into cortical circuit development, as well as potential molecular deficiencies in autism spectrum disorder.

Ankyrin B Promotes Developmental Spine Regulation in the Mouse Prefrontal Cortex

Postnatal regulation of dendritic spine formation and refinement in cortical pyramidal neurons is critical for excitatory/inhibitory balance in neocortical networks. Recent studies have identified a selective spine pruning mechanism in the mouse prefrontal cortex (PFC) mediated by class 3 Semaphorins and the L1-CAM cell adhesion molecules Neuron-glia related CAM (NrCAM), Close Homolog of L1 (CHL1), and L1. L1-CAMs bind Ankyrin B (AnkB), an actin-spectrin adaptor encoded by Ankyrin2 ( ANK2 ), a high confidence gene for autism spectrum disorder (ASD). In a new inducible mouse model (Nex1Cre-ERT2: Ank2 flox : RCE), Ank2 deletion in early postnatal pyramidal neurons increased spine density on apical dendrites in PFC layer 2/3 of homozygous and heterozygous Ank2 -deficient mice. In contrast, Ank2 deletion in adulthood had no effect on spine density. Sema3F-induced spine pruning was impaired in cortical neuron cultures from AnkB-null mice and was rescued by re-expression of the 220 kDa AnkB isoform but not 440 kDa AnkB. AnkB bound to NrCAM at a cytoplasmic domain motif (FIGQY 1231 ), and mutation to FIGQH inhibited binding, impairing Sema3F-induced spine pruning in neuronal cultures. Identification of a novel function for AnkB in dendritic spine regulation provides insight into cortical circuit development, as well as potential molecular deficiencies in ASD.

Bi-allelic loss of function variant in the NRCAM gene is associated with motor-predominant axonal polyneuropathy; the second report

BACKGROUND

The role of biallelic variants in the NRCAM gene underlying a neurodevelopmental disorder has been defined recently. The phenotype is mainly recognized by varying severity of global developmental delay/intellectual disability, hypotonia, spasticity, and peripheral neuropathy.

METHODS

Here, we describe a patient with an initial diagnosis of motor-predominant axonal polyneuropathy or a form of distal SMA. Whole-exome sequencing (WES), in parallel with WES-based CNV detection and assessment of homozygosity runs, was performed to identify this patient's possible genetic cause.

RESULTS

Whole exome sequencing revealed a homozygous variant, c.73C > T (p.Gln25*), in the NRCAM gene, while the patient manifests a mild range of phenotypes compared to NRCAM-related disorder. He presented only motor-predominant axonal polyneuropathy with no other signs of central nervous system involvement.

CONCLUSIONS

This study is the second report of an association between biallelic NRCAM gene variants and a Mendelian disorder. The obtained clinical data, together with the molecular findings in this patient, expands the clinical and molecular spectrum of NRCAM-related disorder and highlights its phenotypic complexity. Although patients with loss of function variants in this gene have previously presented severe clinical features, we show that type of the pathogenic variant does not necessarily determine the severity of this phenotype.

Multi-omic profiling reveals the ataxia protein sacsin is required for integrin trafficking and synaptic organization

Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is a childhood-onset cerebellar ataxia caused by mutations in SACS, which encodes the protein sacsin. Cellular ARSACS phenotypes include mitochondrial dysfunction, intermediate filament disorganization, and progressive death of cerebellar Purkinje neurons. It is unclear why the loss of sacsin causes these deficits or why they manifest as cerebellar ataxia. Here, we perform multi-omic profiling in sacsin knockout (KO) cells and identify alterations in microtubule dynamics and mislocalization of focal adhesion (FA) proteins, including multiple integrins. Deficits in FA structure, signaling, and function can be rescued by targeting PTEN, a negative regulator of FA signaling. ARSACS mice possess mislocalization of ITGA1 in Purkinje neurons and synaptic disorganization in the deep cerebellar nucleus (DCN). The sacsin interactome reveals that sacsin regulates interactions between cytoskeletal and synaptic adhesion proteins. Our findings suggest that disrupted trafficking of synaptic adhesion proteins is a causal molecular deficit in ARSACS.

Endoglycan Regulates Purkinje Cell Migration by Balancing Cell-Cell Adhesion

The importance of cell adhesion molecules for the development of the nervous system has been recognized many decades ago. Functional in vitro and in vivo studies demonstrated a role of cell adhesion molecules in cell migration, axon growth and guidance, as well as synaptogenesis. Clearly, cell adhesion molecules have to be more than static glue making cells stick together. During axon guidance, cell adhesion molecules have been shown to act as pathway selectors but also as a means to prevent axons going astray by bundling or fasciculating axons. We identified Endoglycan as a negative regulator of cell-cell adhesion during commissural axon guidance across the midline. The presence of Endoglycan allowed commissural growth cones to smoothly navigate the floor-plate area. In the absence of Endoglycan, axons failed to exit the floor plate and turn rostrally. These observations are in line with the idea of Endoglycan acting as a lubricant, as its presence was important, but it did not matter whether Endoglycan was provided by the growth cone or the floor-plate cells. Here, we expand on these observations by demonstrating a role of Endoglycan during cell migration. In the developing cerebellum, Endoglycan was expressed by Purkinje cells during their migration from the ventricular zone to the periphery. In the absence of Endoglycan, Purkinje cells failed to migrate and, as a consequence, cerebellar morphology was strongly affected. Cerebellar folds failed to form and grow, consistent with earlier observations on a role of Purkinje cells as Shh deliverers to trigger granule cell proliferation.

Loss of Function of the Neural Cell Adhesion Molecule NrCAM Regulates Differentiation, Proliferation and Neurogenesis in Early Postnatal Hypothalamic Tanycytes

Hypothalamic tanycytes are neural stem and progenitor cells, but little is known of how they are regulated. Here we provide evidence that the cell adhesion molecule, NrCAM, regulates tanycytes in the adult niche. NrCAM is strongly expressed in adult mouse tanycytes. Immunohistochemical and in situ hybridization analysis revealed that NrCAM loss of function leads to both a reduced number of tanycytes and reduced expression of tanycyte-specific cell markers, along with a small reduction in tyrosine hydroxylase-positive arcuate neurons. Similar analyses of NrCAM mutants at E16 identify few changes in gene expression or cell composition, indicating that NrCAM regulates tanycytes, rather than early embryonic hypothalamic development. Neurosphere and organotypic assays support the idea that NrCAM governs cellular homeostasis. Single-cell RNA sequencing (scRNA-Seq) shows that tanycyte-specific genes, including a number that are implicated in thyroid hormone metabolism, show reduced expression in the mutant mouse. However, the mild tanycyte depletion and loss of markers observed in NrCAM-deficient mice were associated with only a subtle metabolic phenotype.

Bi-allelic variants in neuronal cell adhesion molecule cause a neurodevelopmental disorder characterized by developmental delay, hypotonia, neuropathy/spasticity

Cell adhesion molecules are membrane-bound proteins predominantly expressed in the central nervous system along principal axonal pathways with key roles in nervous system development, neural cell differentiation and migration, axonal growth and guidance, myelination, and synapse formation. Here, we describe ten affected individuals with bi-allelic variants in the neuronal cell adhesion molecule NRCAM that lead to a neurodevelopmental syndrome of varying severity; the individuals are from eight families. This syndrome is characterized by developmental delay/intellectual disability, hypotonia, peripheral neuropathy, and/or spasticity. Computational analyses of NRCAM variants, many of which cluster in the third fibronectin type III (Fn-III) domain, strongly suggest a deleterious effect on NRCAM structure and function, including possible disruption of its interactions with other proteins. These findings are corroborated by previous in vitro studies of murine Nrcam-deficient cells, revealing abnormal neurite outgrowth, synaptogenesis, and formation of nodes of Ranvier on myelinated axons. Our studies on zebrafish nrcama^Δ mutants lacking the third Fn-III domain revealed that mutant larvae displayed significantly altered swimming behavior compared to wild-type larvae (p < 0.03). Moreover, nrcama^Δ mutants displayed a trend toward increased amounts of α-tubulin fibers in the dorsal telencephalon, demonstrating an alteration in white matter tracts and projections. Taken together, our study provides evidence that NRCAM disruption causes a variable form of a neurodevelopmental disorder and broadens the knowledge on the growing role of the cell adhesion molecule family in the nervous system.

Impact of Neurofascin on Chronic Inflammatory Demyelinating Polyneuropathy via Changing the Node of Ranvier Function: A Review

The effective conduction of action potential in the peripheral nervous system depends on the structural and functional integrity of the node of Ranvier and paranode. Neurofascin (NF) plays an important role in the conduction of action potential in a saltatory manner. Two subtypes of NF, NF186, and NF155, are involved in the structure of the node of Ranvier. In patients with chronic inflammatory demyelinating polyneuropathy (CIDP), anti-NF antibodies are produced when immunomodulatory dysfunction occurs, which interferes with the conduction of action potential and is considered the main pathogenic factor of CIDP. In this study, we describe the assembling mechanism and anatomical structure of the node of Ranvier and the necessary cell adhesion molecules for its physiological function. The main points of this study are that we summarized the recent studies on the role of anti-NF antibodies in the changes in the node of Ranvier function and its impact on clinical manifestations and analyzed the possible mechanisms underlying the pathogenesis of CIDP.

Neural cell adhesion molecule is required for ventricular conduction system development

The most distal portion of the ventricular conduction system (VCS) contains cardiac Purkinje cells (PCs), which are essential for synchronous activation of the ventricular myocardium. Contactin-2 (CNTN2), a member of the immunoglobulin superfamily of cell adhesion molecules (IgSF-CAMs), was previously identified as a marker of the VCS. Through differential transcriptional profiling, we discovered two additional highly enriched IgSF-CAMs in the VCS: NCAM-1 and ALCAM. Immunofluorescence staining showed dynamic expression patterns for each IgSF-CAM during embryonic and early postnatal stages, but ultimately all three proteins became highly enriched in mature PCs. Mice deficient in NCAM-1, but not CNTN2 or ALCAM, exhibited defects in PC gene expression and VCS patterning, as well as cardiac conduction disease. Moreover, using ST8sia2 and ST8sia4 knockout mice, we show that inhibition of post-translational modification of NCAM-1 by polysialic acid leads to disrupted trafficking of sarcolemmal intercalated disc proteins to junctional membranes and abnormal expansion of the extracellular space between apposing PCs. Taken together, our data provide insights into the complex developmental biology of the ventricular conduction system.

Summary: The cell adhesion molecule NCAM-1 and its post-translational modification by polysialylation are required for normal formation and function of the specialized ventricular conduction system.

Temporal Regulation of Dendritic Spines Through NrCAM-Semaphorin3F Receptor Signaling in Developing Cortical Pyramidal Neurons

Neuron-glial related cell adhesion molecule NrCAM is a newly identified negative regulator of spine density that genetically interacts with Semaphorin3F (Sema3F), and is implicated in autism spectrum disorders (ASD). To investigate a role for NrCAM in spine pruning during the critical adolescent period when networks are established, we generated novel conditional, inducible NrCAM mutant mice (Nex1Cre-ERT2: NrCAMflox/flox). We demonstrate that NrCAM functions cell autonomously during adolescence in pyramidal neurons to restrict spine density in the visual (V1) and medial frontal cortex (MFC). Guided by molecular modeling, we found that NrCAM promoted clustering of the Sema3F holoreceptor complex by interfacing with Neuropilin-2 (Npn2) and PDZ scaffold protein SAP102. NrCAM-induced receptor clustering stimulated the Rap-GAP activity of PlexinA3 (PlexA3) within the holoreceptor complex, which in turn, inhibited Rap1-GTPase and inactivated adhesive β1 integrins, essential for Sema3F-induced spine pruning. These results define a developmental function for NrCAM in Sema3F receptor signaling that limits dendritic spine density on cortical pyramidal neurons during adolescence.

Neuropsychopathology of Autism Spectrum Disorder: Complex Interplay of Genetic, Epigenetic, and Environmental Factors

Autism spectrum disorder (ASD) is a complex heterogeneous consortium of pervasive development disorders (PDD) which ranges from atypical autism, autism, and Asperger syndrome affecting brain in the developmental stage. This debilitating neurodevelopmental disorder results in both core as well as associated symptoms. Core symptoms observed in autistic patients are lack of social interaction, pervasive, stereotyped, and restricted behavior while the associated symptoms include irritability, anxiety, aggression, and several comorbid disorders.ASD is a polygenic disorder and is multifactorial in origin. Copy number variations (CNVs) of several genes that regulate the synaptogenesis and signaling pathways are one of the major factors responsible for the pathogenesis of autism. The complex integration of various CNVs cause mutations in the genes which code for molecules involved in cell adhesion, voltage-gated ion-channels, scaffolding proteins as well as signaling pathways (PTEN and mTOR pathways). These mutated genes are responsible for affecting synaptic transmission by causing plasticity dysfunction responsible, in turn, for the expression of ASD.Epigenetic modifications affecting DNA transcription and various pre-natal and post-natal exposure to a variety of environmental factors are also precipitating factors for the occurrence of ASD. All of these together cause dysregulation of glutamatergic signaling as well as imbalance in excitatory: inhibitory pathways resulting in glial cell activation and release of inflammatory mediators responsible for the aberrant social behavior which is observed in autistic patients.In this chapter we review and provide insight into the intricate integration of various genetic, epigenetic, and environmental factors which play a major role in the pathogenesis of this disorder and the mechanistic approach behind this integration.

α1ACT Is Essential for Survival and Early Cerebellar Programming in a Critical Neonatal Window

Postnatal cerebellar development is a precisely regulated process involving well-orchestrated expression of neural genes. Neurological phenotypes associated with CACNA1A gene defects have been increasingly recognized, yet the molecular principles underlying this association remain elusive. By characterizing a dose-dependent CACNA1A gene deficiency mouse model, we discovered that α1ACT, as a transcription factor and secondary protein of CACNA1A mRNA, drives dynamic gene expression networks within cerebellar Purkinje cells and is indispensable for neonatal survival. Perinatal loss of α1ACT leads to motor dysfunction through disruption of neurogenesis and synaptic regulatory networks. However, its elimination in adulthood has minimal effect on the cerebellum. These findings shed light on the critical role of α1ACT in facilitating neuronal development in both mice and humans and support a rationale for gene therapies for calcium-channel-associated cerebellar disorders. Finally, we show that bicistronic expression may be common to the voltage-gated calcium channel (VGCC) gene family and may help explain complex genetic syndromes.

Neuronal cell adhesion molecule regulating neural systems underlying addiction

Aims

The human NRCAM gene is associated with polysubstance use. Nrcam knockout mice do not acquire a preference for addictive substances. We aimed to elucidate the role of Nrcam in specific neural circuits underlying congenital preference for substances and the acquisition of addiction.

Methods

We analyzed gene expression patterns of neural molecules to find a common addiction pathway dependent on Nrcam function. We examined monoaminergic, glutamatergic, and GABAergic systems in the brains of Nrcam knockout mice following treatment with methamphetamine (METH) or saline (SAL) using micro‐array gene expression analysis, which was replicated using TaqMan gene expression analysis. To find a common addiction pathway, we examined similarities and differences between the expression patterns of molecules in METH‐treated mice and in Nrcam knockout mice treated with cocaine (COC).

Results

Glutaminase expression in brain was reduced in Nrcam heterozygous mice after METH and COC treatment, consistent with our previous study. Metabotropic glutamate receptor 2 expression was reduced in Nrcam heterozygous mice that received either METH or COC treatment. Several other molecules could act in independent addiction pathways involving METH or COC. We also found that GABA receptor subunit g2 expression was reduced in Nrcam heterozygous mice that underwent SAL treatment, and that METH treatment attenuated this reduction.

Conclusion

Nrcam differentially regulates glutamatergic and GABAergic molecules in naive brains and in brains of animals with acquired addiction. Elucidating the complex neural mechanisms underlying polysubstance use will uncover biological features of addiction and may contribute to the development of effective pharmaceutical treatments.

The human/mice NRCAM is involved in specific neural circuits underlying congenital preference for substances and the acquisition of addiction. Mice Nrcam differentially regulates glutamatergic and GABAergic molecules in naive brains and in brains of animals with acquired addiction. Elucidating the complex neural mechanisms underlying polysubstance use will uncover biological features of addiction and may contribute to the development of effective pharmaceutical treatments.

High N-glycan multiplicity is critical for neuronal adhesion and sensitizes the developing cerebellum to N-glycosylation defect

Proper brain development relies highly on protein N-glycosylation to sustain neuronal migration, axon guidance and synaptic physiology. Impairing the N-glycosylation pathway at early steps produces broad neurological symptoms identified in congenital disorders of glycosylation. However, little is known about the molecular mechanisms underlying these defects. We generated a cerebellum specific knockout mouse for Srd5a3, a gene involved in the initiation of N-glycosylation. In addition to motor coordination defects and abnormal granule cell development, Srd5a3 deletion causes mild N-glycosylation impairment without significantly altering ER homeostasis. Using proteomic approaches, we identified that Srd5a3 loss affects a subset of glycoproteins with high N-glycans multiplicity per protein and decreased protein abundance or N-glycosylation level. As IgSF-CAM adhesion proteins are critical for neuron adhesion and highly N-glycosylated, we observed impaired IgSF-CAM-mediated neurite outgrowth and axon guidance in Srd5a3 mutant cerebellum. Our results link high N-glycan multiplicity to fine-tuned neural cell adhesion during mammalian brain development.

Promotion of differentiation in developing mouse cerebellar granule cells by a cell adhesion molecule BT-IgSF

Brain- and testis-specific immunoglobulin superfamily (BT-IgSF) (also known as IgSF11), one of the immunoglobulin superfamily proteins, is a cell adhesion molecule, expressed in the developing cerebellum. We hypothesized that BT-IgSF might have some function in the development of cerebellum, although the physiological roles of BT-IgSF in the cerebellum remain unclear. To investigate the role of BT-IgSF in the development of mouse cerebellum, we first determined the presence of BT-IgSF in the newborn mouse cerebellum; its expression level was found to be much higher than that in the adults. BT-IgSF was abundantly expressed in the molecular layer, where cerebellar granule cell precursors (CGCPs) are in the differentiation stage during migration. We subsequently analyzed the effects of BT-IgSF-knockdown and -overexpression on the proliferation and differentiation of primary cultured CGCPs. BT-IgSF suppressed the proliferation of CGCPs, and promoted their differentiation into cerebellar granule cells. Taken together, our results suggested that BT-IgSF is one of the important cell adhesion molecules that regulate the developmentof mouse cerebellum.

Neuronal cell adhesion molecule (NrCAM) is expressed by sensory cells in the cochlea and is necessary for proper cochlear innervation and sensory domain patterning during development

BACKGROUND

In the cochlea, auditory development depends on precise patterns of innervation by afferent and efferent nerve fibers, as well as a stereotyped arrangement of hair and supporting cells. Neuronal cell adhesion molecule (NrCAM) is a homophilic cell adhesion molecule that controls diverse aspects of nervous system development, but the function of NrCAM in cochlear development is not well understood.

RESULTS

Throughout cochlear innervation, NrCAM is detectable on spiral ganglion neuron (SGN) afferent and olivocochlear efferent fibers, and on the membranes of developing hair and supporting cells. Neonatal Nrcam-null cochleae show errors in type II SGN fasciculation, reduced efferent innervation, and defects in the stereotyped packing of hair and supporting cells. Nrcam loss also leads to dramatic changes in the profiles of presynaptic afferent and efferent synaptic markers at the time of hearing onset. Despite these numerous developmental defects, Nrcam-null adults do not show defects in auditory acuity, and by postnatal day 21, the developmental deficits in ribbon synapse distribution and sensory domain structure appear to have been corrected.

CONCLUSIONS

NrCAM is expressed by several neural and sensory epithelial subtypes within the developing cochlea, and the loss of Nrcam confers numerous, but nonpermanent, developmental defects in innervation and sensory domain patterning. Developmental Dynamics 247:934-950, 2018. © 2018 Wiley Periodicals, Inc.

Molecular guidance cues in the development of visual pathway

70%–80% of our sensory input comes from vision. Light hit the retina at the back of our eyes and the visual information is relayed into the dorsal lateral geniculate nuclei (dLGN) and primary visual cortex (V1) thereafter, constituting the image-forming visual circuit. Molecular cues are one of the key factors to guide the wiring and refinement of the image-forming visual circuit during pre- and post-embryonic stages. Distinct molecular cues are involved in different developmental stages and nucleus, suggesting diverse guidance mechanisms. In this review, we summarize molecular guidance cues throughout the image-forming visual circuit, including chiasm determination, eye-specific segregation and refinement in the dLGN, and at last the reciprocal connections between the dLGN and V1.

The immunoglobulin-like superfamily member IGSF3 is a developmentally regulated protein that controls neuronal morphogenesis

The establishment of a functional brain depends on the fine regulation and coordination of many processes, including neurogenesis, differentiation, dendritogenesis, axonogenesis, and synaptogenesis. Proteins of the immunoglobulin-like superfamily (IGSF) are major regulators during this sequence of events. Different members of this class of proteins play nonoverlapping functions at specific developmental time-points, as shown in particular by studies of the cerebellum. We have identified a member of the little studied EWI subfamily of IGSF, the protein IGSF3, as a membrane protein expressed in a neuron specific- and time-dependent manner during brain development. In the cerebellum, it is transiently found in membranes of differentiating granule cells, and is particularly concentrated at axon terminals. There it co-localizes with other IGSF proteins with well-known functions in cerebellar development: TAG-1 and L1. Functional analysis shows that IGSF3 controls the differentiation of granule cells, more precisely axonal growth and branching. Biochemical experiments demonstrate that, in the developing brain, IGSF3 is in a complex with the tetraspanin TSPAN7, a membrane protein mutated in several forms of X-linked intellectual disabilities. In cerebellar granule cells, TSPAN7 promotes axonal branching and the size of TSPAN7 clusters is increased by downregulation of IGSF3. Thus IGSF3 is a novel regulator of neuronal morphogenesis that might function through interactions with multiple partners including the tetraspanin TSPAN7. This developmentally regulated protein might thus be at the center of a new signaling pathway controlling brain development. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 75-92, 2017.

Cerebrospinal Fluid NrCAM is not a Suitable Biomarker to Discriminate between Dementia Disorders--A Pilot Study

Neuronal Cell Adhesion Molecule (NrCAM) is a proposed new cerebrospinal fluid (CSF) biomarker in Alzheimer's disease (AD). In this pilot study, we aimed to validate and extend previous results and measured NrCAM by ELISA in CSF of patients with AD, frontotemporal dementia, dementia with Lewy bodies, and non-demented controls. NrCAM levels were comparable in all groups, but correlated positively with total tau and phosphorylated tau levels. Furthermore, NrCAM had no significant additional diagnostic value when combined with amyloid-β42, total tau, and phosphorylated tau proteins. Therefore, NrCAM is not a suitable CSF biomarker to differentiate between dementia groups.

Neural cell adhesion molecule NrCAM regulates Semaphorin 3F-induced dendritic spine remodeling

Neuron-glial related cell adhesion molecule (NrCAM) is a regulator of axon growth and repellent guidance, and has been implicated in autism spectrum disorders. Here a novel postsynaptic role for NrCAM in Semaphorin3F (Sema3F)-induced dendritic spine remodeling was identified in pyramidal neurons of the primary visual cortex (V1). NrCAM localized to dendritic spines of star pyramidal cells in postnatal V1, where it was coexpressed with Sema3F. NrCAM deletion in mice resulted in elevated spine densities on apical dendrites of star pyramidal cells at both postnatal and adult stages, and electron microscopy revealed increased numbers of asymmetric synapses in layer 4 of V1. Whole-cell recordings in cortical slices from NrCAM-null mice revealed increased frequency of mEPSCs in star pyramidal neurons. Recombinant Sema3F-Fc protein induced spine retraction on apical dendrites of wild-type, but not NrCAM-null cortical neurons in culture, while re-expression of NrCAM rescued the spine retraction response. NrCAM formed a complex in brain with Sema3F receptor subunits Neuropilin-2 (Npn-2) and PlexinA3 (PlexA3) through an Npn-2-binding sequence (TARNER) in the extracellular Ig1 domain. A trans heterozygous genetic interaction test demonstrated that Sema3F and NrCAM pathways interacted in vivo to regulate spine density in star pyramidal neurons. These findings reveal NrCAM as a novel postnatal regulator of dendritic spine density in cortical pyramidal neurons, and an integral component of the Sema3F receptor complex. The results implicate NrCAM as a contributor to excitatory/inhibitory balance in neocortical circuits.

Singular localization of sodium channel β4 subunit in unmyelinated fibres and its role in the striatum

Voltage-gated Na(+) channel β-subunits are multifunctional molecules that modulate Na(+) channel activity and regulate cell adhesion, migration and neurite outgrowth. β-subunits including β4 are known to be highly concentrated in the nodes of Ranvier and axon initial segments in myelinated axons. Here we show diffuse β4 localization in striatal projection fibres using transgenic mice that express fluorescent protein in those fibres. These axons are unmyelinated, forming large, inhibitory fibre bundles. Furthermore, we report β4 dimer expression in the mouse brain, with high levels of β4 dimers in the striatal projection fascicles, suggesting a specific role of β4 in those fibres. Scn4b-deficient mice show a resurgent Na(+) current reduction, decreased repetitive firing frequency in medium spiny neurons and increased failure rates of inhibitory postsynaptic currents evoked with repetitive stimulation, indicating an in vivo channel regulatory role of β4 in the striatum.

NrCAM-regulating neural systems and addiction-related behaviors

We have previously shown that a haplotype associated with decreased NrCAM expression in brain is protective against addiction vulnerability for polysubstance abuse in humans and that Nrcam knockout mice do not develop conditioned place preferences for morphine, cocaine or amphetamine. In order to gain insight into NrCAM involvement in addiction vulnerability, which may involve specific neural circuits underlying behavioral characteristics relevant to addiction, we evaluated several behavioral phenotypes in Nrcam knockout mice. Consistent with a potential general reduction in motivational function, Nrcam knockout mice demonstrated less curiosity for novel objects and for an unfamiliar conspecific, showed also less anxiety in the zero maze. Nrcam heterozygote knockout mice reduced alcohol preference and buried fewer marbles in home cage. These observations provide further support for a role of NrCAM in substance abuse including alcoholism vulnerability, possibly through its effects on behavioral traits that may affect addiction vulnerability, including novelty seeking, obsessive compulsion and responses to aversive or anxiety-provoking stimuli. Additionally, in order to prove glutamate homeostasis hypothesis of addiction, we analyzed glutamatergic molecules regulated by NRCAM expression. Glutaminase appears to be involved in NrCAM-related molecular pathway in two different tissues from human and mouse. An inhibitor of the enzyme, prolyl-leucyl-glycinamide, treatment produced, at least, some of the phenotypes of mice shown in alcohol preference and in anxiety-like behavior. Thus, NrCAM could affect addiction-related behaviors via at least partially modulation of some glutamatergic pathways and neural function in brain.

Neuronal Ig/Caspr recognition promotes the formation of axoaxonic synapses in mouse spinal cord

Inhibitory microcircuits are wired with a precision that underlies their complex regulatory roles in neural information processing. In the spinal cord, one specialized class of GABAergic interneurons (GABApre) mediates presynaptic inhibitory control of sensory-motor synapses. The synaptic targeting of these GABAergic neurons exhibits an absolute dependence on proprioceptive sensory terminals, yet the molecular underpinnings of this specialized axoaxonic organization remain unclear. Here, we show that sensory expression of an NB2 (Contactin5)/Caspr4 coreceptor complex, together with spinal interneuron expression of NrCAM/CHL1, directs the high-density accumulation of GABAergic boutons on sensory terminals. Moreover, genetic elimination of NB2 results in a disproportionate stripping of inhibitory boutons from high-density GABApre-sensory synapses, suggesting that the preterminal axons of GABApre neurons compete for access to individual sensory terminals. Our findings define a recognition complex that contributes to the assembly and organization of a specialized GABAergic microcircuit.

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Sensory Ig/Caspr4 complex directs inhibitory synapse formation in mouse spinal cord

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Eliminating NB2 results in a reduced number of GABApre-sensory synapses

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Quantitative modeling suggests competition for formation of axoaxonic synapses

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Role for a contactin/Caspr complex in central synapse formation

Neuron glia-related cell adhesion molecule (NrCAM) promotes topographic retinocollicular mapping

NrCAM (Neuron-glial related cell adhesion molecule), a member of the L1 family of cell adhesion molecules, reversibly binds ankyrin and regulates axon growth, but it has not been studied for a role in retinotopic mapping. During development of retino-collicular topography, NrCAM was expressed uniformly in retinal ganglion cells (RGCs) along both mediolateral and anteroposterior retinal axes, and was localized on RGC axons within the optic tract and superior colliculus (SC). Anterograde tracing of RGC axons in NrCAM null mutant mice at P10, when the map resembles its mature form, revealed laterally displaced ectopic termination zones (eTZs) of axons from the temporal retina, indicating defective mediolateral topography, which is governed by ephrinB/EphBs. Axon tracing at P2 revealed that interstitial branch orientation of ventral-temporal RGC axons in NrCAM null mice was compromised in the medial direction, likely accounting for displacement of eTZs. A similar retinocollicular targeting defect in EphB mutant mice suggested that NrCAM and EphB interact to regulate mediolateral retino-collicular targeting. We found that EphB2 tyrosine kinase but not an EphB2 kinase dead mutant, phosphorylated NrCAM at a conserved tyrosine residue in the FIGQY ankyrin binding motif, perturbing ankyrin recruitment in NrCAM transfected HEK293 cells. Furthermore, the phosphorylation of NrCAM at FIGQY in SC was decreased in EphB1/3 and EphB1/2/3 null mice compared to WT, while phospho-FIGQY of NrCAM in SC was increased in EphB2 constitutively active (F620D/F620D) mice. These results demonstrate that NrCAM contributes to mediolateral retinocollicular axon targeting by regulating RGC branch orientation through a likely mechanism in which ephrinB/EphB phosphorylates NrCAM to modulate linkage to the actin cytoskeleton.

Transsynaptic coordination of synaptic growth, function, and stability by the L1-type CAM Neuroglian

Experiments in peripheral and central synapses reveal the regulatory mechanisms that enable trans-synaptic control of synapse development and maintenance by the L1-type CAM Neuroglian.

The precise control of synaptic connectivity is essential for the development and function of neuronal circuits. While there have been significant advances in our understanding how cell adhesion molecules mediate axon guidance and synapse formation, the mechanisms controlling synapse maintenance or plasticity in vivo remain largely uncharacterized. In an unbiased RNAi screen we identified the Drosophila L1-type CAM Neuroglian (Nrg) as a central coordinator of synapse growth, function, and stability. We demonstrate that the extracellular Ig-domains and the intracellular Ankyrin-interaction motif are essential for synapse development and stability. Nrg binds to Ankyrin2 in vivo and mutations reducing the binding affinities to Ankyrin2 cause an increase in Nrg mobility in motoneurons. We then demonstrate that the Nrg–Ank2 interaction controls the balance of synapse growth and stability at the neuromuscular junction. In contrast, at a central synapse, transsynaptic interactions of pre- and postsynaptic Nrg require a dynamic, temporal and spatial, regulation of the intracellular Ankyrin-binding motif to coordinate pre- and postsynaptic development. Our study at two complementary model synapses identifies the regulation of the interaction between the L1-type CAM and Ankyrin as an important novel module enabling local control of synaptic connectivity and function while maintaining general neuronal circuit architecture.

The function of neuronal circuits relies on precise connectivity, and processes like learning and memory involve refining this connectivity through the selective formation and elimination of synapses. Cell adhesion molecules (CAMs) that directly mediate cell–cell interactions at synaptic contacts are thought to mediate this structural synaptic plasticity. In this study, we used an unbiased genetic screen to identify the Drosophila L1-type CAM Neuroglian as a central regulator of synapse formation and maintenance. We show that the intracellular Ankyrin interaction motif, which links Neuroglian to the cytoskeleton, is an essential regulatory site for Neuroglian mobility, adhesion, and synaptic function. In motoneurons, the strength of Ankyrin binding directly controls the balance between synapse formation and maintenance. At a central synapse, however, a dynamic regulation of the Neuroglian–Ankyrin interaction is required to coordinate transsynaptic development. Our study identifies the interaction of the L1-type CAM with Ankyrin as a novel regulatory module enabling local and precise control of synaptic connectivity without altering general neuronal circuit architecture. This interaction is relevant for normal nervous system development and disease as mutations in L1-type CAMs cause mental retardation and psychiatric diseases in humans.

gdnf activates midline repulsion by Semaphorin3B via NCAM during commissural axon guidance

The Neurotrophic factor gdnf plays diverse developmental roles, supporting survival and also acting as a chemoattractant for axon and cell migration. We report that in the developing spinal cord, a focal source of gdnf is present in the floor plate (FP) where commissural axons cross the midline. Gdnf has no direct guidance properties but switches on the responsiveness of crossing commissural growth cones to the midline repellent Semaphorin3B by suppressing calpain-mediated processing of the Sema3B signaling coreceptor Plexin-A1. Analysis of single and double mutant mouse models indicates that although gdnf is the principal trigger of Sema3B midline repulsion, it acts with another FP cue, NrCAM. Finally, genetic and in vitro experiments provide evidence that this gdnf effect is RET independent and mediated by NCAM/GFRα1 signaling. This study identifies a regulator of midline crossing and reveals interplays between Semaphorin and gdnf signaling during axon guidance.

Optic chiasm presentation of Semaphorin6D in the context of Plexin-A1 and Nr-CAM promotes retinal axon midline crossing

At the optic chiasm, retinal ganglion cells (RGCs) project ipsi- or contralaterally to establish the circuitry for binocular vision. Ipsilateral guidance programs have been characterized, but contralateral guidance programs are not well understood. Here, we identify a tripartite molecular system for contralateral RGC projections: Semaphorin6D (Sema6D) and Nr-CAM are expressed on midline radial glia and Plexin-A1 on chiasm neurons, and Plexin-A1 and Nr-CAM are also expressed on contralateral RGCs. Sema6D is repulsive to contralateral RGCs, but Sema6D in combination with Nr-CAM and Plexin-A1 converts repulsion to growth promotion. Nr-CAM functions as a receptor for Sema6D. Sema6D, Plexin-A1, and Nr-CAM are all required for efficient RGC decussation at the optic chiasm. These findings suggest a mechanism by which a complex of Sema6D, Nr-CAM, and Plexin-A1 at the chiasm midline alters the sign of Sema6D and signals Nr-CAM/Plexin-A1 receptors on RGCs to implement the contralateral RGC projection.

Sticky situations: recent advances in control of cell adhesion during neuronal migration

The migration of neurons along glial fibers from a germinal zone (GZ) to their final laminar positions is essential for morphogenesis of the developing brain; aberrations in this process are linked to profound neurodevelopmental and cognitive disorders. During this critical morphogenic movement, neurons must navigate complex migration paths, propelling their cell bodies through the dense cellular environment of the developing nervous system to their final destinations. It is not understood how neurons can successfully migrate along their glial guides through the myriad processes and cell bodies of neighboring neurons. Although much progress has been made in understanding the substrates (Fishell G, Hatten ME: Astrotactin provides a receptor system for CNS neuronal migration. Development 1991, 113:755; Elias LA, Wang DD, Kriegstein AR: Gap junction adhesion is necessary for radial migration in the neocortex. Nature 2007, 448:901; Anton ES, Kreidberg JA, Rakic P: Distinct functions of alpha3 and alpha. (v) integrin receptors in neuronal migration and laminar organization of the cerebral cortex. Neuron 1999, 22:277; Anton ES, Marchionni MA, Lee KF, Rakic P: Role of GGF/neuregulin signaling in interactions between migrating neurons and radial glia in the developing cerebral cortex. Development 1997, 124:3501), guidance mechanisms (Polleux F, Whitford KL, Dijkhuizen PA, Vitalis T, Ghosh A: Control of cortical interneuron migration by neurotrophins and PI3-kinase signaling. Development 2002, 129:3147; Zhou P, et al.: Polarized signaling endosomes coordinate BDNF-induced chemotaxis of cerebellar precursors. Neuron 2007, 55:53; Renaud J, et al.: Plexin-A2 and its ligand, Sema6A, control nucleus-centrosome coupling in migrating granule cells. Nat Neurosci 2008, 11:440), cytoskeletal elements (Schaar BT, McConnell SK: Cytoskeletal coordination during neuronal migration. Proc Natl Acad Sci U S A 2005, 102:13652; Tsai JW, Bremner KH, Vallee RB: Dual subcellular roles for LIS1 and dynein in radial neuronal migration in live brain tissue. Nat Neurosci 2007, 10:970; Solecki DJ, et al.: Myosin II motors and F-actin dynamics drive the coordinated movement of the centrosome and soma during CNS glial-guided neuronal migration. Neuron 2009, 63:63), and post-translational modifications (Patrick GN, Zhou P, Kwon YT, Howley PM, Tsai LH: p35, the neuronal-specific activator of cyclin-dependent kinase 5 (Cdk5) is degraded by the ubiquitin-proteasome pathway. J Biol Chem 1998, 273:24057; Suetsugu S, et al.: Regulation of actin cytoskeleton by mDab1 through N-WASP and ubiquitination of mDab1. Biochem J 2004, 384:1; Karakuzu O, Wang DP, Cameron S: MIG-32 and SPAT-3A are PRC1 homologs that control neuronal migration inCaenorhabditis elegans. Development 2009, 136:943) required for neuronal migration, we have yet to elucidate how neurons regulate their cellular interactions and adhesive specificity to follow the appropriate migratory pathways. Here I will examine recent developments in our understanding of the mechanisms controlling neuronal cell adhesion and how these mechanisms interact with crucial neurodevelopmental events, such as GZ exit, migration pathway selection, multipolar-to-radial transition, and final lamination.

Comparative genomics identification of a novel set of temporally regulated hedgehog target genes in the retina

The hedgehog (Hh) signaling pathway is involved in numerous developmental and adult processes with many links to cancer. In vertebrates, the activity of the Hh pathway is mediated primarily through three Gli transcription factors (Gli1, 2 and 3) that can serve as transcriptional activators or repressors. The identification of Gli target genes is essential for the understanding of the Hh-mediated processes. We used a comparative genomics approach using the mouse and human genomes to identify 390 genes that contained conserved Gli binding sites. RT-qPCR validation of 46 target genes in E14.5 and P0.5 retinal explants revealed that Hh pathway activation resulted in the modulation of 30 of these targets, 25 of which demonstrated a temporal regulation. Further validation revealed that the expression of Bok, FoxA1, Sox8 and Wnt7a was dependent upon Sonic Hh (Shh) signaling in the retina and their regulation is under positive and negative controls by Gli2 and Gli3, respectively. We also show using chromatin immunoprecipitation that Gli2 binds to the Sox8 promoter, suggesting that Sox8 is an Hh-dependent direct target of Gli2. Finally, we demonstrate that the Hh pathway also modulates the expression of Sox9 and Sox10, which together with Sox8 make up the SoxE group. Previously, it has been shown that Hh and SoxE group genes promote Müller glial cell development in the retina. Our data are consistent with the possibility for a role of SoxE group genes downstream of Hh signaling on Müller cell development.

The role of NrCAM in neural development and disorders--beyond a simple glue in the brain

NrCAM is a neuronal cell adhesion molecule of the L1 family of immunoglobulin super family. It plays a wide variety of roles in neural development, including cell proliferation and differentiation, axon growth and guidance, synapse formation, and the formation of the myelinated nerve structure. NrCAM functions in cell adhesion and modulates signaling pathways in neural development through multiple molecular interactions with guidance and other factors. Alterations in NrCAM structure/expression are associated with psychiatric disorders such as autism and drug addiction and with tumor progression. The mechanisms of NrCAM participation in development and how these might be perturbed in disorders are reviewed.

Overexpression of neurone glial-related cell adhesion molecule is an independent predictor of poor prognosis in advanced colorectal cancer

A downstream target of the Wnt pathway, neurone glial-related cell adhesion molecule (Nr-CAM) has recently been implicated in human cancer development. However, its role in colorectal cancer (CRC) pathobiology and clinical relevance remains unknown. In this study, we examined the clinical significance of Nr-CAM protein expression in a retrospective series of 428 CRCs using immunohistochemistry and tissue microarrays. Cox proportional hazards regression was used to calculate hazard ratios (HR) of mortality according to various clinicopathological features and molecular markers. All CRC samples were immunoreactive for Nr-CAM protein expression, compared to 10 ⁄ 245 (4%) matched normal tissue (P < 0.0001). Of 428 CRC samples, 97 (23%) showed Nr-CAM overexpression, which was significantly associated with nodal (P = 0.012) and distant (P = 0.039) metastasis, but not with extent of local invasion or tumor size. Additionally, Nr-CAM overexpression was associated with vascular invasion (P = 0.0029), p53 expression (P = 0.036), and peritoneal metastasis at diagnosis (P = 0.013). In a multivariate model adjusted for other clinicopathological predictors of survival, Nr-CAM overexpression correlated with a significant increase in disease-specific (HR 1.66; 95% confidence interval 1.11-2.47; P = 0.014) and overall mortality (HR 1.57; 95% confidence interval 1.07-2.30; P = 0.023) in advanced but not early stage disease. Notably, 5-fluorouracil-based chemotherapy conferred significant survival benefit to patients with tumors negative for Nr-CAM overexpression but not to those with Nr-CAM overexpressed tumors. In conclusion, Nr-CAM protein expression is upregulated in CRC tissues. Nr-CAM overexpression is an independent marker of poor prognosis among advanced CRC patients, and is a possible predictive marker for non-beneficence to 5-fluorouracil- based chemotherapy.

Neural circuit formation in the cerebellum is controlled by cell adhesion molecules of the Contactin family

Cell adhesion molecules of the immunoglobulin superfamily (IgSF CAMs) have been implicated in neural circuit formation in both the peripheral and the central nervous system. Several recent studies highlight a role of the Contactin group of IgSF CAMs in cerebellar development, in particular in the development of granule cells. Granule cells are the most numerous type of neurons in the nervous system and by forming a secondary proliferative zone in the cerebellum they provide an exception to the rule that neuronal precursors proliferate in the ventricular zone. Granule cells express Contactin-2, Contactin-1, and Contactin-6 in a sequential manner. Contactins are required for axon guidance, fasciculation, and synaptogenesis, and thus affect multiple steps in neural circuit formation in the developing cerebellum.

NrCAM deletion causes topographic mistargeting of thalamocortical axons to the visual cortex and disrupts visual acuity

NrCAM is a neural cell adhesion molecule of the L1 family that has been linked to autism spectrum disorders, a disease spectrum in which abnormal thalamocortical connectivity may contribute to visual processing defects. Here we show that NrCAM interaction with neuropilin-2 (Npn-2) is critical for semaphorin 3F (Sema3F)-induced guidance of thalamocortical axon subpopulations at the ventral telencephalon (VTe), an intermediate target for thalamic axon sorting. Genetic deletion of NrCAM or Npn-2 caused contingents of embryonic thalamic axons to misproject caudally in the VTe. The resultant thalamocortical map of NrCAM-null mutants showed striking mistargeting of motor and somatosensory thalamic axon contingents to the primary visual cortex, but retinogeniculate targeting and segregation were normal. NrCAM formed a molecular complex with Npn-2 in brain and neural cells, and was required for Sema3F-induced growth cone collapse in thalamic neuron cultures, consistent with a vital function for NrCAM in Sema3F-induced axon repulsion. NrCAM-null mice displayed reduced responses to visual evoked potentials recorded from layer IV in the binocular zone of primary visual cortex (V1), particularly when evoked from the ipsilateral eye, indicating abnormal visual acuity and ocularity. These results demonstrate that NrCAM is required for normal maturation of cortical visual acuity, and suggest that the aberrant projection of thalamic motor and somatosensory axons to the visual cortex in NrCAM-null mutant mice impairs cortical functions.

F3/contactin and TAG1 play antagonistic roles in the regulation of sonic hedgehog-induced cerebellar granule neuron progenitor proliferation

Modulation of the sonic hedgehog (SHH) pathway is a crucial factor in cerebellar morphogenesis. Stimulation of granule neuron progenitor (GNP) proliferation is a central function of SHH signalling, but how this is controlled locally is not understood. We show that two sequentially expressed members of the contactin (CNTN) family of adhesion molecules, TAG1 and F3, act antagonistically to control SHH-induced proliferation: F3 suppresses SHH-induced GNP proliferation and induces differentiation, whereas TAG1 antagonises F3. Production of GNPs in TAG1-null mice is delayed and reduced. F3 and TAG1 colocalise on GNPs with the related L1-like adhesion molecule NrCAM, and F3 fails to suppress the SHH-induced proliferation of NrCAM-deficient GNPs. We show that F3 and SHH both primarily affect a group of intermediate GNPs (IPs), which, though actively dividing, also express molecules associated with differentiation, including β-tubulin III (TuJ1) and TAG1. In vivo, intermediate progenitors form a discrete layer in the middle of the external germinal layer (mEGL), while F3 becomes expressed on the axons of postmitotic granule neurons as they leave the inner EGL (iEGL). We propose, therefore, that F3 acts as a localised signal in the iEGL that induces SHH-stimulated cells in the overlying mEGL to exit cell cycle and differentiate. By contrast, expression of TAG1 on GNPs antagonises this signal in the mEGL, preventing premature differentiation and sustaining GNP expansion in a paracrine fashion. Together, these findings indicate that CNTN and L1-like proteins play a significant role in modulating SHH-induced neuronal precursor proliferation.

"CRASH"ing with the worm: insights into L1CAM functions and mechanisms

The L1 family of cell adhesion molecules (L1CAMs) in vertebrates has long been studied for its roles in nervous system development and function. Members of this family have been associated with distinct neurological disorders that include CRASH, autism, 3p syndrome, and schizophrenia. The conservation of L1CAMs in Drosophila and Caenorhabditis elegans allows the opportunity to take advantage of these simple model organisms and their accessible genetic manipulations to dissect L1CAM functions and mechanisms of action. This review summarizes the discoveries of L1CAMs made in C. elegans, showcasing this simple model organism as a powerful system to uncover L1CAM mechanisms and roles in healthy and diseased states.

Rac1 regulates neuronal polarization through the WAVE complex

Neuronal migration and axon growth, key events during neuronal development, require distinct changes in the cytoskeleton. Although many molecular regulators of polarity have been identified and characterized, relatively little is known about their physiological role in this process. To study the physiological function of Rac1 in neuronal development, we have generated a conditional knock-out mouse, in which Rac1 is ablated in the whole brain. Rac1-deficient cerebellar granule neurons, which do not express other Rac isoforms, showed impaired neuronal migration and axon formation both in vivo and in vitro. In addition, Rac1 ablation disrupts lamellipodia formation in growth cones. The analysis of Rac1 effectors revealed the absence of the Wiskott-Aldrich syndrome protein (WASP) family verprolin-homologous protein (WAVE) complex from the plasma membrane of knock-out growth cones. Loss of WAVE function inhibited axon growth, whereas overexpression of a membrane-tethered WAVE mutant partially rescued axon growth in Rac1-knock-out neurons. In addition, pharmacological inhibition of the WAVE complex effector Arp2/3 also reduced axon growth. We propose that Rac1 recruits the WAVE complex to the plasma membrane to enable actin remodeling necessary for axon growth.

Role of the cytoplasmic domain of the L1 cell adhesion molecule in brain development

Mutations in the human L1CAM gene cause X-linked hydrocephalus and MASA (Mental retardation, Aphasia, Shuffling gait, Adducted thumbs) syndrome. In vitro studies have shown that the L1 cytoplasmic domain (L1CD) is involved in L1 trafficking, neurite branching, signaling, and interactions with the cytoskeleton. L1cam knockout (L1(KO)) mice have hydrocephalus, a small cerebellum, hyperfasciculation of corticothalamic tracts, and abnormal peripheral nerves. To explore the function of the L1CD, we made three new mice lines in which different parts of the L1CD have been altered. In all mutant lines L1 protein is expressed and transported into the axon. Interestingly, these new L1CD mutant lines display normal brain morphology. However, the expression of L1 protein in the adult is dramatically reduced in the two L1CD mutant lines that lack the ankyrin-binding region and they show defects in motor function. Therefore, the L1CD is not responsible for the major defects observed in L1(KO) mice, yet it is required for continued L1 protein expression and motor function in the adult.

A midline switch of receptor processing regulates commissural axon guidance in vertebrates

Commissural axon guidance requires complex modulations of growth cone sensitivity to midline-derived cues, but underlying mechanisms in vertebrates remain largely unknown. By using combinations of ex vivo and in vivo approaches, we uncovered a molecular pathway controlling the gain of response to a midline repellent, Semaphorin3B (Sema3B). First, we provide evidence that Semaphorin3B/Plexin-A1 signaling participates in the guidance of commissural projections at the vertebrate ventral midline. Second, we show that, at the precrossing stage, commissural neurons synthesize the Neuropilin-2 and Plexin-A1 Semaphorin3B receptor subunits, but Plexin-A1 expression is prevented by a calpain1-mediated processing, resulting in silencing commissural responsiveness. Third, we report that, during floor plate (FP) in-growth, calpain1 activity is suppressed by local signals, allowing Plexin-A1 accumulation in the growth cone and sensitization to Sema3B. Finally, we show that the FP cue NrCAM mediates the switch of Plexin-A1 processing underlying growth cone sensitization to Sema3B. This reveals pathway-dependent modulation of guidance receptor processing as a novel mechanism for regulating guidance decisions at intermediate targets.

Impaired sociability and cognitive function in Nrcam-null mice

NRCAM (Neuronal Cell Adhesion Molecule) has an important role in axonal guidance and the organization of neural circuitry during brain development. Association analyses in human populations have identified NRCAM as a candidate gene for autism susceptibility. In the present study, we evaluated Nrcam-null mice for sociability, social novelty preference, and reversal learning as a model for the social deficits, repetitive behavior, and cognitive rigidity characteristic of autism. Prepulse inhibition of acoustic startle responses was also measured, to reflect sensorimotor-gating deficits in autism spectrum disorders. Assays for anxiety-like behavior in an elevated plus maze and open field, motor coordination, and olfactory ability in a buried food test were conducted to provide control measures for the interpretation of results. Overall, the loss of Nrcam led to behavioral alterations in sociability, acquisition of a spatial task, and reversal learning, dependent on sex. In comparison to male wild type mice, male Nrcam-null mutants had significantly decreased sociability in a three-chambered choice task. Low sociability in the male null mutants was not associated with changes in anxiety-like behavior, activity, or motor coordination. Male, but not female, Nrcam-null mice had small decreases in prepulse inhibition. Nrcam deficiency in female mice led to impaired acquisition of spatial learning in the Morris water maze task. Reversal learning deficits were observed in both male and female Nrcam-null mice. These results provide evidence that NRCAM mediates domains of function relevant to symptoms observed in autism.

Pathogenic human L1-CAM mutations reduce the adhesion-dependent activation of EGFR

L1-cell adhesion molecule (L1-CAM) belongs to a functionally conserved group of neural cell adhesion molecules that are implicated in many aspects of nervous system development. In many neuronal cells the adhesive function of L1-type CAMs induces cellular signaling processes that involves the activation of neuronal tyrosine protein kinases and among other functions regulates axonal growth and guidance. Mutations in the human L1-CAM gene are responsible for a complex neurodevelopmental condition, generally referred to as L1 syndrome. Several pathogenic L1-CAM mutations have been identified in humans that cause L1 syndrome in affected individuals without affecting the level of L1-CAM-mediated homophilic cell adhesion when tested in vitro. In this study, an analysis of two different pathogenic human L1-CAM molecules indicates that although both induce normal L1-CAM-mediated cell aggregation, they are defective in stimulating human epidermal growth factor receptor tyrosine kinase activity in vitro and are unable to rescue L1 loss-of-function conditions in a Drosophila transgenic model in vivo. These results indicate that the L1 syndrome-associated phenotype might involve the disruption of L1-CAM's functions at different levels. Either by reducing or abolishing L1-CAM protein expression, by interfering with L1-CAM's cell surface expression, by reducing L1-CAM's adhesive ability or by impeding further downstream adhesion-dependent signaling processes.

Concurrent Lpin1 and Nrcam mouse mutations result in severe peripheral neuropathy with transitory hindlimb paralysis

Peripheral neuropathy is a broad category of disorders with a diverse etiology, grouped together by their common pathogenic effect on the peripheral nervous system (PNS). Because of the heterogeneity observed to be responsible for these disorders, a forward genetics method of gene discovery was used to identify additional affected pathways. In this report, we describe the mutant mouse line 20884, generated by N-ethyl-N-nitrosourea mutagenesis, which is characterized by adult-onset transitory hindlimb paralysis. Linkage mapping revealed that two point mutations are responsible for the phenotype: a partial loss-of-function mutation in the gene for phosphatidate phosphatase Lpin1 and a truncation mutation in the gene that encodes the neuronal cell adhesion molecule NrCAM. To investigate how the 20884 Lpin1 and Nrcam mutations interact to produce the paralysis phenotype, the double mutant and both single mutants were analyzed by quantitative behavioral, histological, and electrophysiological means. The Lpin1(20884) mutant and the double mutant are characterized by similar levels of demyelination and aberrant myelin structures. Nevertheless, the double mutant exhibits more severe electrophysiological abnormalities than the Lpin1(20884) mutant. The Nrcam(20884) mutant is characterized by normal sciatic nerve morphology and a mild electrophysiological defect. Comparison of the double mutant phenotype with the two single mutants does not point to an additive relationship between the two defects; rather, the Lpin1(20884) and Nrcam(20884) defects appear to act synergistically to produce the 20884 phenotype. It is proposed that the absence of NrCAM in a demyelinating environment has a deleterious effect, possibly by impairing the process of remyelination.

Close homologue of adhesion molecule L1 promotes survival of Purkinje and granule cells and granule cell migration during murine cerebellar development

Several L1-related adhesion molecules, expressed in a well-coordinated temporospatial pattern during development, are important for fine tuning of specific cerebellar circuitries. We tested the hypothesis that CHL1, the close homologue of L1, abundantly expressed in the developing and adult cerebellum, is also required for normal cerebellar histogenesis. We found that constitutive ablation of CHL1 in mice caused significant loss (20-23%) of Purkinje and granule cells in the mature 2-month-old cerebellum. The ratio of stellate/basket interneurons to Purkinje cells was abnormally high (+38%) in CHL1-deficient (CHL1-/-) mice compared with wild-type (CHL1+/+) littermates, but the gamma-aminobutyric acid (GABA)ergic synaptic inputs to Purkinje cell bodies and dendrites were normal, as were numbers of Golgi interneurons, microglia, astrocytes, and Bergmann glia. Purkinje cell loss occurred before the first postnatal week and was associated with enhanced apoptosis, presumably as a consequence of CHL1 deficiency in afferent axons. In contrast, generation of granule cells, as indicated by in vivo analyses of cell proliferation and death, was unaffected in 1-week-old CHL1-/- mice, but numbers of migrating granule cells in the molecular layer were increased. This increase was likely related to retarded cell migration because CHL1-/- granule cells migrated more slowly than CHL1+/+ cells in vitro, and Bergmann glial processes guiding migration in vivo expressed CHL1 in wild-type mice. Granule cell deficiency in adult CHL1-/- mice appeared to result from decreased precursor cell proliferation after the first postnatal week. Our results indicate that CHL1 promotes Purkinje and granule cell survival and granule cell migration during cerebellar development.

IgCAMs redundantly control axon navigation in Caenorhabditis elegans

BACKGROUND

Cell adhesion molecules of the immunoglobulin superfamily (IgCAMs) form one of the largest and most diverse families of adhesion molecules and receptors in the nervous system. Many members of this family mediate contact and communication among neurons during development. The Caenorhabditis elegans genome contains a comparatively small number of IgCAMs, most of which are evolutionarily conserved and found across all animal phyla. Only some of these have been functionally characterized so far.

RESULTS

We systematically analyzed previously uncharacterized IgCAMs in C. elegans. Green fluorescent protein reporter constructs of 12 IgCAMs revealed that expression generally is not confined to a single tissue and that all tissues express at least one of the IgCAMs. Most IgCAMs were expressed in neurons. Within the nervous system significant overlap in expression was found in central components of the motor circuit, in particular the command interneurons, ventral cord motoneurons as well as motoneurons innervating head muscles. Sensory neurons are underrepresented among the cells expressing these IgCAMs. We isolated mutations for eight of the genes showing neuronal expression. Phenotypic analysis of single mutants revealed limited neuronal defects, in particular axon navigation defects in some of the mutants. Systematic genetic interaction studies uncovered two cases of functional overlap among three and four genes, respectively. A strain combining mutations in all eight genes is viable and shows no additional defects in the neurons that were analyzed, suggesting that genetic interactions among those genes are limited.

CONCLUSION

Genetic interactions involving multiple IgCAMs affecting axon outgrowth demonstrate functional overlap among IgCAMs during nervous system development.

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.

The role of cell adhesion molecules in axon growth and guidance

During development, axons elongate along the correct path toward their final targets. Growing axons maintain adhesive interactions with specific environmental cues via cell adhesion molecules (CAMs). The axon-environment adhesion must be dynamically controlled, both temporally and spatially, to enable the axons to navigate and migrate correctly. In this way, CAMs play a central role in mediating contact-dependent regulation of motile behavior of the axons. This chapter examines the mechanisms underlying how CAMs control axon growth and guidance, with a particular focus on intracellular signaling, trafficking, and interactions with the actin cytoskeleton.