The C. elegans L1CAM homologue LAD-2 functions as a coreceptor in MAB-20/Sema2 mediated axon guidance

Nematode Extracellular Protein Interactome Expands Connections between Signaling Pathways

Multicellularity was accompanied by the emergence of new classes of cell surface and secreted proteins. The nematode C. elegans is a favorable model to study cell surface interactomes, given its well-defined and stereotyped cell types and intercellular contacts. Here we report our C. elegans extracellular interactome dataset, the largest yet for an invertebrate. Most of these interactions were unknown, despite recent datasets for flies and humans, as our collection contains a larger selection of protein families. We uncover new interactions for all four major axon guidance pathways, including ectodomain interactions between three of the pathways. We demonstrate that a protein family known to maintain axon locations are secreted receptors for insulins. We reveal novel interactions of cystine-knot proteins with putative signaling receptors, which may extend the study of neurotrophins and growth-factor-mediated functions to nematodes. Finally, our dataset provides insights into human disease mechanisms and how extracellular interactions may help establish connectomes.

The hereditary spastic paraplegias

The hereditary spastic paraplegias (HSPs) are a group of more than 90 genetic disorders in which lower extremity spasticity and weakness are either the primary neurologic impairments ("uncomplicated HSP") or when accompanied by other neurologic deficits ("complicated HSP"), important features of the clinical syndrome. Various genetic types of HSP are inherited such as autosomal dominant, autosomal recessive, X-linked, and maternal (mitochondrial) traits. Symptoms that begin in early childhood may be nonprogressive and resemble spastic diplegic cerebral palsy. Symptoms that begin later, typically progress insidiously over a number of years. Genetic testing is able to confirm the diagnosis for many subjects. Insights from gene discovery indicate that abnormalities in diverse molecular processes underlie various forms of HSP, including disturbance in axon transport, endoplasmic reticulum morphogenesis, vesicle transport, lipid metabolism, and mitochondrial function. Pathologic studies in "uncomplicated" HSP have shown axon degeneration particularly involving the distal ends of corticospinal tracts and dorsal column fibers. Treatment is limited to symptom reduction including amelioration of spasticity, reducing urinary urgency, proactive physical therapy including strengthening, stretching, balance, and agility exercise.

An olfactory-interneuron circuit that drives stress-induced avoidance behavior in C. elegans

Physiological stress represents a drastic change of internal state and can drive avoidance behavior, but the neural circuits are incompletely defined. Here, we characterize a sensory-interneuron circuit for mitochondrial stress-induced avoidance behavior in C. elegans. The olfactory sensory neurons and the AIY interneuron are essential, with the olfactory neurons acting upstream of AIY. Unlike pathogen avoidance, stress-induced avoidance does not require AIB, AIZ or RIA interneurons. Ablation or inhibition of the head motor neurons SMDD/V alters the worm's locomotion and reduces avoidance. These findings substantiate our understanding of the circuit mechanisms that underlie learned avoidance behavior triggered by mitochondrial stress.

Inferring a spatial code of cell-cell interactions across a whole animal body

Cell-cell interactions shape cellular function and ultimately organismal phenotype. Interacting cells can sense their mutual distance using combinations of ligand-receptor pairs, suggesting the existence of a spatial code, i.e., signals encoding spatial properties of cellular organization. However, this code driving and sustaining the spatial organization of cells remains to be elucidated. Here we present a computational framework to infer the spatial code underlying cell-cell interactions from the transcriptomes of the cell types across the whole body of a multicellular organism. As core of this framework, we introduce our tool cell2cell, which uses the coexpression of ligand-receptor pairs to compute the potential for intercellular interactions, and we test it across the Caenorhabditis elegans' body. Leveraging a 3D atlas of C. elegans' cells, we also implement a genetic algorithm to identify the ligand-receptor pairs most informative of the spatial organization of cells across the whole body. Validating the spatial code extracted with this strategy, the resulting intercellular distances are negatively correlated with the inferred cell-cell interactions. Furthermore, for selected cell-cell and ligand-receptor pairs, we experimentally confirm the communicatory behavior inferred with cell2cell and the genetic algorithm. Thus, our framework helps identify a code that predicts the spatial organization of cells across a whole-animal body.

The regulatory landscape of neurite development in Caenorhabditis elegans

Neuronal communication requires precise connectivity of neurite projections (axons and dendrites). Developing neurites express cell-surface receptors that interpret extracellular cues to enable correct guidance toward, and connection with, target cells. Spatiotemporal regulation of neurite guidance molecule expression by transcription factors (TFs) is critical for nervous system development and function. Here, we review how neurite development is regulated by TFs in the Caenorhabditis elegans nervous system. By collecting publicly available transcriptome and ChIP-sequencing data, we reveal gene expression dynamics during neurite development, providing insight into transcriptional mechanisms governing construction of the nervous system architecture.

Bma-LAD-2, an Intestinal Cell Adhesion Protein, as a Potential Therapeutic Target for Lymphatic Filariasis

Lymphatic filariasis is a debilitating disease that afflicts over 70 million people worldwide. It is caused by the parasitic nematodes Wuchereria bancrofti, Brugia malayi, and Brugia timori. Despite substantial success, efforts to eliminate LF will likely require more time and resources than predicted. Identifying new drug and vaccine targets in adult filariae could help elimination efforts. This study’s aim was to evaluate intestinal proteins in adult Brugia malayi worms as possible therapeutic targets. Using short interfering RNA (siRNA), we successfully targeted four candidate gene transcripts: Bma-Serpin, Bma-ShTK, Bma-Reprolysin, and Bma-LAD-2. Of those, Bma-LAD-2, an immunoglobulin superfamily cell adhesion molecule (IgSF CAM), was determined to be essential for adult worm survival. We observed a 70.42% knockdown in Bma-LAD-2 transcript levels 1 day post-siRNA incubation and an 87.02% reduction in protein expression 2 days post-siRNA incubation. This inhibition of Bma-LAD-2 expression resulted in an 80% decrease in worm motility over 6 days, a 93.43% reduction in microfilaria release (Mf) by day 6 post-siRNA incubation, and a dramatic decrease in (4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction. Transmission electron microscopy revealed the loss of microvilli and unraveling of mitochondrial cristae in the intestinal epithelium of Bma-LAD-2 siRNA-treated worms. Strikingly, Bma-LAD-2 siRNA-treated worms exhibited an almost complete loss of pseudocoelomic fluid. A luciferase immunoprecipitation system assay did not detect anti-Bma-LAD-2 IgE in the serum of 30 LF patients, indicating that LF exposure does not result in IgE sensitization to this antigen. These results indicate that Bma-LAD-2 is an essential protein for adult Brugia malayi and may be an effective therapeutic target.

A conserved neuropeptide system links head and body motor circuits to enable adaptive behavior

Neuromodulators promote adaptive behaviors that are often complex and involve concerted activity changes across circuits that are often not physically connected. It is not well understood how neuromodulatory systems accomplish these tasks. Here, we show that the Caenorhabditis elegans NLP-12 neuropeptide system shapes responses to food availability by modulating the activity of head and body wall motor neurons through alternate G-protein coupled receptor (GPCR) targets, CKR-1 and CKR-2. We show ckr-2 deletion reduces body bend depth during movement under basal conditions. We demonstrate CKR-1 is a functional NLP-12 receptor and define its expression in the nervous system. In contrast to basal locomotion, biased CKR-1 GPCR stimulation of head motor neurons promotes turning during local searching. Deletion of ckr-1 reduces head neuron activity and diminishes turning while specific ckr-1 overexpression or head neuron activation promote turning. Thus, our studies suggest locomotor responses to changing food availability are regulated through conditional NLP-12 stimulation of head or body wall motor circuits.

Re-Expression of Poly/Oligo-Sialylated Adhesion Molecules on the Surface of Tumor Cells Disrupts Their Interaction with Immune-Effector Cells and Contributes to Pathophysiological Immune Escape

Glycans linked to surface proteins are the most complex biological macromolecules that play an active role in various cellular mechanisms. This diversity is the basis of cell-cell interaction and communication, cell growth, cell migration, as well as co-stimulatory or inhibitory signaling. Our review describes the importance of neuraminic acid and its derivatives as recognition elements, which are located at the outermost positions of carbohydrate chains linked to specific glycoproteins or glycolipids. Tumor cells, especially from solid tumors, mask themselves by re-expression of hypersialylated neural cell adhesion molecule (NCAM), neuropilin-2 (NRP-2), or synaptic cell adhesion molecule 1 (SynCAM 1) in order to protect themselves against the cytotoxic attack of the also highly sialylated immune effector cells. More particularly, we focus on α-2,8-linked polysialic acid chains, which characterize carrier glycoproteins such as NCAM, NRP-2, or SynCam-1. This characteristic property correlates with an aggressive clinical phenotype and endows them with multiple roles in biological processes that underlie all steps of cancer progression, including regulation of cell-cell and/or cell-extracellular matrix interactions, as well as increased proliferation, migration, reduced apoptosis rate of tumor cells, angiogenesis, and metastasis. Specifically, re-expression of poly/oligo-sialylated adhesion molecules on the surface of tumor cells disrupts their interaction with immune-effector cells and contributes to pathophysiological immune escape. Further, sialylated glycoproteins induce immunoregulatory cytokines and growth factors through interactions with sialic acid-binding immunoglobulin-like lectins. We describe the processes, which modulate the interaction between sialylated carrier glycoproteins and their ligands, and illustrate that sialic acids could be targets of novel therapeutic strategies for treatment of cancer and immune diseases.

The Prop1-like homeobox gene unc-42 specifies the identity of synaptically connected neurons

Many neuronal identity regulators are expressed in distinct populations of cells in the nervous system, but their function is often analyzed only in specific isolated cellular contexts, thereby potentially leaving overarching themes in gene function undiscovered. We show here that the Caenorhabditis elegans Prop1-like homeobox gene unc-42 is expressed in 15 distinct sensory, inter- and motor neuron classes throughout the entire C. elegans nervous system. Strikingly, all 15 neuron classes expressing unc-42 are synaptically interconnected, prompting us to investigate whether unc-42 controls the functional properties of this circuit and perhaps also the assembly of these neurons into functional circuitry. We found that unc-42 defines the routes of communication between these interconnected neurons by controlling the expression of neurotransmitter pathway genes, neurotransmitter receptors, neuropeptides, and neuropeptide receptors. Anatomical analysis of unc-42 mutant animals reveals defects in axon pathfinding and synaptic connectivity, paralleled by expression defects of molecules involved in axon pathfinding, cell-cell recognition, and synaptic connectivity. We conclude that unc-42 establishes functional circuitry by acting as a terminal selector of functionally connected neuron types. We identify a number of additional transcription factors that are also expressed in synaptically connected neurons and propose that terminal selectors may also function as 'circuit organizer transcription factors' to control the assembly of functional circuitry throughout the nervous system. We hypothesize that such organizational properties of transcription factors may be reflective of not only ontogenetic, but perhaps also phylogenetic trajectories of neuronal circuit establishment.

Harmonization of L1CAM expression facilitates axon outgrowth and guidance of a motor neuron

Brain development requires precise regulation of axon outgrowth, guidance and termination by multiple signaling and adhesion molecules. How the expression of these neurodevelopmental regulators is transcriptionally controlled is poorly understood. The Caenorhabditis elegans SMD motor neurons terminate axon outgrowth upon sexual maturity and partially retract their axons during early adulthood. Here we show that C-terminal binding protein 1 (CTBP-1), a transcriptional corepressor, is required for correct SMD axonal development. Loss of CTBP-1 causes multiple defects in SMD axon development: premature outgrowth, defective guidance, delayed termination and absence of retraction. CTBP-1 controls SMD axon guidance by repressing the expression of SAX-7, an L1 cell adhesion molecule (L1CAM). CTBP-1-regulated repression is crucial because deregulated SAX-7/L1CAM causes severely aberrant SMD axons. We found that axonal defects caused by deregulated SAX-7/L1CAM are dependent on a distinct L1CAM, called LAD-2, which itself plays a parallel role in SMD axon guidance. Our results reveal that harmonization of L1CAM expression controls the development and maturation of a single neuron.

ngn-1/neurogenin Activates Transcription of Multiple Terminal Selector Transcription Factors in the Caenorhabditis elegans Nervous System

Proper nervous system development is required for an organism's survival and function. Defects in neurogenesis have been linked to neurodevelopmental disorders such as schizophrenia and autism. Understanding the gene regulatory networks that orchestrate neural development, specifically cascades of proneural transcription factors, can better elucidate which genes are most important during early neurogenesis. Neurogenins are a family of deeply conserved factors shown to be both necessary and sufficient for the development of neural subtypes. However, the immediate downstream targets of neurogenin are not well characterized. The objective of this study was to further elucidate the role of ngn-1/neurogenin in nervous system development and to identify its downstream transcriptional targets, using the nematode Caenorhabditis elegans as a model for this work. We found that ngn-1 is required for axon outgrowth, nerve ring architecture, and neuronal cell fate specification. We also showed that ngn-1 may have roles in neuroblast migration and epithelial integrity during embryonic development. Using RNA sequencing and comparative transcriptome analysis, we identified eight transcription factors (hlh-34/NPAS1, unc-42/PROP1, ceh-17/PHOX2A, lim-4/LHX6, fax-1/NR2E3, lin-11/LHX1, tlp-1/ZNF503, and nhr-23/RORB) whose transcription is activated, either directly or indirectly, by ngn-1 Our results show that ngn-1 has a role in transcribing known terminal regulators that establish and maintain cell fate of differentiated neural subtypes and confirms that ngn-1 functions as a proneural transcription factor in C. elegans neurogenesis.

Origin and evolution of plexins, semaphorins, and Met receptor tyrosine kinases

The transition from unicellular to multicellular organisms poses the question as to when genes that regulate cell-cell interactions emerged during evolution. The receptor and ligand pairing of plexins and semaphorins regulates cellular interactions in a wide range of developmental and physiological contexts. We surveyed here genomes of unicellular eukaryotes and of non-bilaterian and bilaterian Metazoa and performed phylogenetic analyses to gain insight into the evolution of plexin and semaphorin families. Remarkably, we detected plexins and semaphorins in unicellular choanoflagellates, indicating their evolutionary origin in a common ancestor of Choanoflagellida and Metazoa. The plexin domain structure is conserved throughout all clades; in contrast, semaphorins are structurally diverse. Choanoflagellate semaphorins are transmembrane proteins with multiple fibronectin type III domains following the N-terminal Sema domain (termed Sema-FN). Other previously not yet described semaphorin classes include semaphorins of Ctenophora with tandem immunoglobulin domains (Sema-IG) and secreted semaphorins of Echinoderamata (Sema-SP, Sema-SI). Our study also identified Met receptor tyrosine kinases (RTKs), which carry a truncated plexin extracellular domain, in several bilaterian clades, indicating evolutionary origin in a common ancestor of Bilateria. In addition, a novel type of Met-like RTK with a complete plexin extracellular domain was detected in Lophotrochozoa and Echinodermata (termed Met-LP RTK). Our findings are consistent with an ancient function of plexins and semaphorins in regulating cytoskeletal dynamics and cell adhesion that predates their role as axon guidance molecules.

A sensory-motor neuron type mediates proprioceptive coordination of steering in C. elegans via two TRPC channels

Animal locomotion is mediated by a sensory system referred to as proprioception. Defects in the proprioceptive coordination of locomotion result in uncontrolled and inefficient movements. However, the molecular mechanisms underlying proprioception are not fully understood. Here, we identify two transient receptor potential cation (TRPC) channels, trp-1 and trp-2, as necessary and sufficient for proprioceptive responses in C. elegans head steering locomotion. Both channels are expressed in the SMDD neurons, which are required and sufficient for head bending, and mediate coordinated head steering by sensing mechanical stretches due to the contraction of head muscle and orchestrating dorsal head muscle contractions. Moreover, the SMDD neurons play dual roles to sense muscle stretch as well as to control muscle contractions. These results demonstrate that distinct locomotion patterns require dynamic and homeostatic modulation of feedback signals between neurons and muscles.

Proprioception provides the nervous system with feedback about body posture in animals and is essential for the generation of coherent locomotive behaviors, such as walking, running, or crawling. However, little is known about the identity of proprioceptive receptors that sense body movement to regulate locomotion and the extent to which proprioception modulates sensorimotor coordination. Here, we analyze the molecular mechanisms that control head steering locomotion of Caenorhabditis elegans. We show that this movement is regulated by the transient receptor potential cation (TRPC) channels TRP-1 and TRP-2 and the SMDD proprioceptive neurons. We observe that mutant animals for both channels are defective in head steering locomotion and that ectopic expression of TRP-1 or TPR-2 in a C. elegans chemosensory neuron confers head bending–dependent responses, suggesting roles for these channels in proprioception. We also find that SMDD neurons are both necessary and sufficient to generate head steering locomotion via the two channels. Moreover, we demonstrate that the proprioceptive system mediates locomotion coordination by desynchronizing activities in motor systems. We conclude that two TRPC channels in collaboration with the proprioceptive receptor SMDD neurons control head steering in worms during forward locomotion.

Comprehensive single-cell transcriptional profiling of a multicellular organism

To resolve cellular heterogeneity, we developed a combinatorial indexing strategy to profile the transcriptomes of single cells or nuclei, termed sci-RNA-seq (single-cell combinatorial indexing RNA sequencing). We applied sci-RNA-seq to profile nearly 50,000 cells from the nematode Caenorhabditis elegans at the L2 larval stage, which provided >50-fold "shotgun" cellular coverage of its somatic cell composition. From these data, we defined consensus expression profiles for 27 cell types and recovered rare neuronal cell types corresponding to as few as one or two cells in the L2 worm. We integrated these profiles with whole-animal chromatin immunoprecipitation sequencing data to deconvolve the cell type-specific effects of transcription factors. The data generated by sci-RNA-seq constitute a powerful resource for nematode biology and foreshadow similar atlases for other organisms.

Building branched tissue structures: from single cell guidance to coordinated construction

Branched networks are ubiquitous throughout nature, particularly found in tissues that require large surface area within a restricted volume. Many tissues with a branched architecture, such as the vasculature, kidney, mammary gland, lung and nervous system, function to exchange fluids, gases and information throughout the body of an organism. The generation of branched tissues requires regulation of branch site specification, initiation and elongation. Branching events often require the coordination of many cells to build a tissue network for material exchange. Recent evidence has emerged suggesting that cell cooperativity scales with the number of cells actively contributing to branching events. Here, we compare mechanisms that regulate branching, focusing on how cell cohorts behave in a coordinated manner to build branched tissues.This article is part of the themed issue 'Systems morphodynamics: understanding the development of tissue hardware'.

Protein Tyrosine Phosphatase δ Mediates the Sema3A-Induced Cortical Basal Dendritic Arborization through the Activation of Fyn Tyrosine Kinase

Leukocyte common antigen-related (LAR) class protein tyrosine phosphatases (PTPs) are critical for axonal guidance; however, their relation to specific guidance cues is poorly defined. We here show that PTP-3, a LAR homolog in Caenorhabditis elegans, is involved in axon guidance regulated by Semaphorin-2A-signaling. PTPδ, one of the vertebrate LAR class PTPs, participates in the Semaphorin-3A (Sema3A)-induced growth cone collapse response of primary cultured dorsal root ganglion neurons from Mus musculus embryos. In vivo, however, the contribution of PTPδ in Sema3A-regualted axon guidance was minimal. Instead, PTPδ played a major role in Sema3A-dependent cortical dendritic growth. Ptpδ^-/- and Sema3a^-/- mutant mice exhibited poor arborization of basal dendrites of cortical layer V neurons. This phenotype was observed in both male and female mutants. The double-heterozygous mutants, Ptpδ^+/-; Sema3a^+/-, also showed a similar phenotype, indicating the genetic interaction. In Ptpδ^-/- brains, Fyn and Src kinases were hyperphosphorylated at their C-terminal Tyr527 residues. Sema3A-stimulation induced dephosphorylation of Tyr527 in the dendrites of wild-type cortical neurons but not of Ptpδ^-/- Arborization of cortical basal dendrites was reduced in Fyn^-/- as well as in Ptpδ^+/-; Fyn^+/- double-heterozygous mutants. Collectively, PTPδ mediates Sema3A-signaling through the activation of Fyn by C-terminal dephosphorylation.SIGNIFICANCE STATEMENT The relation of leukocyte common antigen-related (LAR) class protein tyrosine phosphatases (PTPs) and specific axon guidance cues is poorly defined. We show that PTP-3, a LAR homolog in Caenorhabditis elegans, participates in Sema2A-regulated axon guidance. PTPδ, a member of vertebrate LAR class PTPs, is involved in Sema3A-regulated cortical dendritic growth. In Sema3A signaling, PTPδ activates Fyn and Src kinases by dephosphorylating their C-terminal Tyr residues. This is the first evidence showing that LAR class PTPs participate in Semaphorin signaling in vivo.

An Aversive Response to Osmotic Upshift in Caenorhabditis elegans

Environmental osmolarity presents a common type of sensory stimulus to animals. While behavioral responses to osmotic changes are important for maintaining a stable intracellular osmolarity, the underlying mechanisms are not fully understood. In the natural habitat of Caenorhabditis elegans, changes in environmental osmolarity are commonplace. It is known that the nematode acutely avoids shocks of extremely high osmolarity. Here, we show that C. elegans also generates gradually increased aversion of mild upshifts in environmental osmolarity. Different from an acute avoidance of osmotic shocks that depends on the function of a transient receptor potential vanilloid channel, the slow aversion to osmotic upshifts requires the cGMP-gated sensory channel subunit TAX-2. TAX-2 acts in several sensory neurons that are exposed to body fluid to generate the aversive response through a motor network that underlies navigation. Osmotic upshifts activate the body cavity sensory neuron URX, which is known to induce aversion upon activation. Together, our results characterize the molecular and cellular mechanisms underlying a novel sensorimotor response to osmotic stimuli and reveal that C. elegans engages different behaviors and the underlying mechanisms to regulate responses to extracellular osmolarity.

Worms on the spectrum - C. elegans models in autism research

The small non-parasitic nematode Caenorhabditis elegans is widely used in neuroscience thanks to its well-understood development and lineage of the nervous system. Furthermore, C. elegans has been used to model many human developmental and neurological conditions to better understand disease mechanisms and identify potential therapeutic strategies. Autism spectrum disorder (ASD) is the most prevalent of all neurodevelopmental disorders, and the C. elegans system may provide opportunities to learn more about this complex disorder. Since basic cell biology and biochemistry of the C. elegans nervous system is generally very similar to mammals, cellular or molecular phenotypes can be investigated, along with a repertoire of behaviours. For instance, worms have contributed greatly to the understanding of mechanisms underlying mutations in genes coding for synaptic proteins such as neuroligin and neurexin. Using worms to model neurodevelopmental disorders like ASD is an emerging topic that harbours great, untapped potential. This review summarizes the numerous contributions of C. elegans to the field of neurodevelopment and introduces the nematode system as a potential research tool to study essential roles of genes associated with ASD.

Xenopus laevis neuronal cell adhesion molecule (nrcam): plasticity of a CAM in the developing nervous system

Neuron-glial-related cell adhesion molecule (NRCAM) is a neuronal cell adhesion molecule of the L1 immunoglobulin superfamily, which plays diverse roles during nervous system development including axon growth and guidance, synapse formation, and formation of the myelinated nerve. Perturbations in NRCAM function cause a wide variety of disorders, which can affect wiring and targeting of neurons, or cause psychiatric disorders as well as cancers through abnormal modulation of signaling events. In the present study, we characterize the Xenopus laevis homolog of nrcam. Expression of Xenopus nrcam is most abundant along the dorsal midline throughout the developing brain and in the outer nuclear layer of the retina.

The Genetics of Axon Guidance and Axon Regeneration in Caenorhabditis elegans

The correct wiring of neuronal circuits depends on outgrowth and guidance of neuronal processes during development. In the past two decades, great progress has been made in understanding the molecular basis of axon outgrowth and guidance. Genetic analysis in Caenorhabditis elegans has played a key role in elucidating conserved pathways regulating axon guidance, including Netrin signaling, the slit Slit/Robo pathway, Wnt signaling, and others. Axon guidance factors were first identified by screens for mutations affecting animal behavior, and by direct visual screens for axon guidance defects. Genetic analysis of these pathways has revealed the complex and combinatorial nature of guidance cues, and has delineated how cues guide growth cones via receptor activity and cytoskeletal rearrangement. Several axon guidance pathways also affect directed migrations of non-neuronal cells in C. elegans, with implications for normal and pathological cell migrations in situations such as tumor metastasis. The small number of neurons and highly stereotyped axonal architecture of the C. elegans nervous system allow analysis of axon guidance at the level of single identified axons, and permit in vivo tests of prevailing models of axon guidance. C. elegans axons also have a robust capacity to undergo regenerative regrowth after precise laser injury (axotomy). Although such axon regrowth shares some similarities with developmental axon outgrowth, screens for regrowth mutants have revealed regeneration-specific pathways and factors that were not identified in developmental screens. Several areas remain poorly understood, including how major axon tracts are formed in the embryo, and the function of axon regeneration in the natural environment.

An extrasynaptic GABAergic signal modulates a pattern of forward movement in Caenorhabditis elegans

As a common neurotransmitter in the nervous system, γ-aminobutyric acid (GABA) modulates locomotory patterns in both vertebrates and invertebrates. However, the signaling mechanisms underlying the behavioral effects of GABAergic modulation are not completely understood. Here, we demonstrate that a GABAergic signal in C. elegans modulates the amplitude of undulatory head bending through extrasynaptic neurotransmission and conserved metabotropic receptors. We show that the GABAergic RME head motor neurons generate undulatory activity patterns that correlate with head bending and the activity of RME causally links with head bending amplitude. The undulatory activity of RME is regulated by a pair of cholinergic head motor neurons SMD, which facilitate head bending, and inhibits SMD to limit head bending. The extrasynaptic neurotransmission between SMD and RME provides a gain control system to set head bending amplitude to a value correlated with optimal efficiency of forward movement.

An extrasynaptic GABAergic signal modulates a pattern of forward movement in Caenorhabditis elegans

As a common neurotransmitter in the nervous system, γ-aminobutyric acid (GABA) modulates locomotory patterns in both vertebrates and invertebrates. However, the signaling mechanisms underlying the behavioral effects of GABAergic modulation are not completely understood. Here, we demonstrate that a GABAergic signal in C. elegans modulates the amplitude of undulatory head bending through extrasynaptic neurotransmission and conserved metabotropic receptors. We show that the GABAergic RME head motor neurons generate undulatory activity patterns that correlate with head bending and the activity of RME causally links with head bending amplitude. The undulatory activity of RME is regulated by a pair of cholinergic head motor neurons SMD, which facilitate head bending, and inhibits SMD to limit head bending. The extrasynaptic neurotransmission between SMD and RME provides a gain control system to set head bending amplitude to a value correlated with optimal efficiency of forward movement.

DOI:http://dx.doi.org/10.7554/eLife.14197.001

Confirmation of a non-synonymous SNP in PNPLA8 as a candidate causal mutation for Weaver syndrome in Brown Swiss cattle

Background

Bovine progressive degenerative myeloencephalopathy (Weaver syndrome) is a neurodegenerative disorder in Brown Swiss cattle that is characterized by progressive hind leg weakness and ataxia, while sensorium and spinal reflexes remain unaffected. Although the causal mutation has not been identified yet, an indirect genetic test based on six microsatellite markers and consequent exclusion of Weaver carriers from breeding have led to the complete absence of new cases for over two decades. Evaluation of disease status by imputation of 41 diagnostic single nucleotide polymorphisms (SNPs) and a common haplotype published in 2013 identified several suspected carriers in the current breeding population, which suggests a higher frequency of the Weaver allele than anticipated. In order to prevent the reemergence of the disease, this study aimed at mapping the gene that underlies Weaver syndrome and thus at providing the basis for direct genetic testing and monitoring of today’s Braunvieh/Brown Swiss herds.

Results

Combined linkage/linkage disequilibrium mapping on Bos taurus chromosome (BTA) 4 based on Illumina Bovine SNP50 genotypes of 43 Weaver-affected, 31 Weaver carrier and 86 Weaver-free animals resulted in a maximum likelihood ratio test statistic value at position 49,812,384 bp. The confidence interval (0.853 Mb) determined by the 2-LOD drop-off method was contained within a 1.72-Mb segment of extended homozygosity. Exploitation of whole-genome sequence data from two official Weaver carriers and 1145 other bulls that were sequenced in Run4 of the 1000 bull genomes project showed that only a non-synonymous SNP (rs800397662) within the PNPLA8 gene at position 49,878,773 bp was concordant with the Weaver carrier status. Targeted SNP genotyping confirmed this SNP as a candidate causal mutation for Weaver syndrome. Genotyping for the candidate causal mutation in a random sample of 2334 current Braunvieh animals suggested a frequency of the Weaver allele of 0.26 %.

Conclusions

Through combined use of exhaustive sequencing data and SNP genotyping results, we were able to provide evidence that supports the non-synonymous mutation at position 49,878,773 bp as the most likely causal mutation for Weaver syndrome. Further studies are needed to uncover the exact mechanisms that underlie this syndrome.

Electronic supplementary material

The online version of this article (doi:10.1186/s12711-016-0201-5) contains supplementary material, which is available to authorized users.

EFN-4 functions in LAD-2-mediated axon guidance in Caenorhabditis elegans

During development of the nervous system, growing axons rely on guidance molecules to direct axon pathfinding. A well-characterized family of guidance molecules are the membrane-associated ephrins, which together with their cognate Eph receptors, direct axon navigation in a contact-mediated fashion. InC. elegans, the ephrin-Eph signaling system is conserved and is best characterized for their roles in neuroblast migration during early embryogenesis. This study demonstrates a role for the C. elegans ephrin EFN-4 in axon guidance. We provide both genetic and biochemical evidence that is consistent with the C. elegans divergent L1 cell adhesion molecule LAD-2 acting as a non-canonical ephrin receptor to EFN-4 to promote axon guidance. We also show that EFN-4 probably functions as a diffusible factor because EFN-4 engineered to be soluble can promote LAD-2-mediated axon guidance. This study thus reveals a potential additional mechanism for ephrins in regulating axon guidance and expands the repertoire of receptors by which ephrins can signal.

The Caenorhabditis elegans Ephrin EFN-4 Functions Non-cell Autonomously with Heparan Sulfate Proteoglycans to Promote Axon Outgrowth and Branching

The Eph receptors and their cognate ephrin ligands play key roles in many aspects of nervous system development. These interactions typically occur within an individual tissue type, serving either to guide axons to their terminal targets or to define boundaries between the rhombomeres of the hindbrain. We have identified a novel role for the Caenorhabditis elegans ephrin EFN-4 in promoting primary neurite outgrowth in AIY interneurons and D-class motor neurons. Rescue experiments reveal that EFN-4 functions non-cell autonomously in the epidermis to promote primary neurite outgrowth. We also find that EFN-4 plays a role in promoting ectopic axon branching in a C. elegans model of X-linked Kallmann syndrome. In this context, EFN-4 functions non-cell autonomously in the body-wall muscle and in parallel with HS modification genes and HSPG core proteins. This is the first report of an epidermal ephrin providing a developmental cue to the nervous system.

The C-terminal binding protein (CTBP-1) regulates dorsal SMD axonal morphology in Caenorhabditis elegans

C-terminal binding proteins (CtBPs) are transcriptional co-repressors which cooperate with a variety of transcription factors to repress gene expression. Caenorhabditis elegans CTBP-1 expression has been observed in the nervous system and hypodermis. In C. elegans, CTBP-1 regulates several processes including Acute Functional Tolerance to ethanol and functions in the nervous system to modulate both lifespan and expression of a lipase gene called lips-7. Incorrect structure and/or function of the nervous system can lead to behavioral changes. Here, we demonstrate reduced exploration behavior in ctbp-1 mutants. Our examination of a subset of neurons involved in regulating locomotion revealed that the axonal morphology of dorsal SMD (SMDD) neurons is altered in ctbp-1 mutants at the fourth larval (L4) stage. Expressing CTBP-1 under the control of the endogenous ctbp-1 promoter rescued both the exploration behavior phenotype and defective SMDD axon structure in ctbp-1 mutants at the L4 stage. Interestingly, the pre-synaptic marker RAB-3 was found to localize to the mispositioned portion of SMDD axons in a ctbp-1 mutant. Further analysis of SMDD axonal morphology at days 1, 3 and 5 of adulthood revealed that the number of ctbp-1 mutants showing an SMDD axonal morphology defect increases in early adulthood and the observed defect appears to be qualitatively more severe. CTBP-1 is prominently expressed in the nervous system with weak expression detected in the hypodermis. Surprisingly, solely expressing CTBP-1a in the nervous system or hypodermis did not restore correct SMDD axonal structure in a ctbp-1 mutant. Our results demonstrate a role for CTBP-1 in exploration behavior and the regulation of SMDD axonal morphology in C. elegans.

The Adhesion Molecule KAL-1/anosmin-1 Regulates Neurite Branching through a SAX-7/L1CAM-EGL-15/FGFR Receptor Complex

Neurite branching is essential for correct assembly of neural circuits, yet it remains a poorly understood process. For example, the neural cell adhesion molecule KAL-1/anosmin-1, which is mutated in Kallmann syndrome, regulates neurite branching through mechanisms largely unknown. Here, we show that KAL-1/anosmin-1 mediates neurite branching as an autocrine co-factor with EGL-17/FGF through a receptor complex consisting of the conserved cell adhesion molecule SAX-7/L1CAM and the fibroblast growth factor receptor EGL-15/FGFR. This protein complex, which appears conserved in humans, requires the immunoglobulin (Ig) domains of SAX-7/L1CAM and the FN(III) domains of KAL-1/anosmin-1 for formation in vitro as well as function in vivo. The kinase domain of the EGL-15/FGFR is required for branching, and genetic evidence suggests that ras-mediated signaling downstream of EGL-15/FGFR is necessary to effect branching. Our studies establish a molecular pathway that regulates neurite branching during development of the nervous system.

Skin-derived cues control arborization of sensory dendrites in Caenorhabditis elegans

Sensory dendrites depend on cues from their environment to pattern their growth and direct them toward their correct target tissues. Yet, little is known about dendrite-substrate interactions during dendrite morphogenesis. Here, we describe MNR-1/menorin, which is part of the conserved Fam151 family of proteins and is expressed in the skin to control the elaboration of "menorah"-like dendrites of mechanosensory neurons in Caenorhabditis elegans. We provide biochemical and genetic evidence that MNR-1 acts as a contact-dependent or short-range cue in concert with the neural cell adhesion molecule SAX-7/L1CAM in the skin and through the neuronal leucine-rich repeat transmembrane receptor DMA-1 on sensory dendrites. Our data describe an unknown pathway that provides spatial information from the skin substrate to pattern sensory dendrite development nonautonomously.

The C. elegans CDK8 Mediator module regulates axon guidance decisions in the ventral nerve cord and during dorsal axon navigation

Receptors expressed on the growth cone of outgrowing axons detect cues required for proper navigation. The pathway choices available to an axon are in part defined by the set of guidance receptors present on the growth cone. Regulated expression of receptors and genes controlling the localization and activity of receptors ensures that axons respond only to guidance cues relevant for reaching their targets. In genetic screens for axon guidance mutants, we isolated an allele of let-19/mdt-13, a component of the Mediator, a large ~30 subunit protein complex essential for gene transcription by RNA polymerase II. LET-19/MDT-13 is part of the CDK8 module of the Mediator. By testing other Mediator components, we found that all subunits of the CDK8 module as well as some other Mediator components are required for specific axon navigation decisions in a subset of neurons. Expression profiling demonstrated that let-19/mdt-13 regulates the expression of a large number of genes in interneurons. A mutation in the sax-3 gene, encoding a receptor for the repulsive guidance cue SLT-1, suppresses the commissure navigation defects found in cdk-8 mutants. This suggests that the CDK8 module specifically represses the SAX-3/ROBO pathway to ensure proper commissure navigation.

The contactin RIG-6 mediates neuronal and non-neuronal cell migration in Caenorhabditis elegans

Cell adhesion molecules of the Immunoglobulin Superfamily (IgCAMs) are key factors in nervous system formation. The contactin subgroup of IgCAMs consists of GPI-anchored glycoproteins implicated in axon outgrowth, guidance, fasciculation and neuronal differentiation. The mechanism by which contactins facilitate neuronal development is not understood. To gain insight into the function of contactins, we characterized RIG-6, the sole contactin of Caenorhabditis elegans. We show that the contactin RIG-6 is involved in excretory cell (EC) tubular elongation. We also show that RIG-6 mediates axon outgrowth and guidance along both the anterior-posterior and dorso-ventral axis, during C. elegans development. We find that optimal RIG-6 expression is critical for accurate mechanosensory neuron axon elongation and ventral nerve cord architecture. In addition, our data suggest that the cytoplasmic UNC-53/NAV2 proteins may contribute to relay signaling via contactins.

Regulation of adhesion by flexible ectodomains of IgCAMs

To perform their diverse biological functions the adhesion activities of the cell adhesion molecules of the immunoglobulin superfamily (IgCAMs) might be regulated by local clustering, proteolytical shedding of their ectodomains or rapid recycling to and from the plasma membrane. Another form of regulation of adhesion might be obtained through flexible ectodomains of IgCAMs which adopt distinct conformations and which in turn modulate their adhesion activity. Here, we discuss variations in the conformation of the extracellular domains of CEACAM1 and CAR that might influence their binding and signaling activities. Furthermore, we concentrate on alternative splicing of single domains and short segments in the extracellular regions of L1 subfamily members that might affect the organization of the N-terminal located Ig-like domains. In particular, we discuss variations of the linker sequence between Ig-like domains 2 and 3 (D2 and D3) that is required for the horseshoe conformation.

DBL-1, a TGF-β, is essential for Caenorhabditis elegans aversive olfactory learning

The TGF-β superfamily is conserved throughout metazoan, and its members play essential roles in development and disease. TGF-β has also been implicated in adult neural plasticity. However, the underlying mechanisms are not well understood. Here we report that DBL-1, a Caenorhabditis elegans TGF-β homolog known to control body morphology and immunity, is essential for aversive olfactory learning of potentially harmful bacteria food. We show that DBL-1 generated by the AVA command interneurons, which are critical for sensorimotor responses, regulates aversive olfactory learning, and that the activity of the type I TGF-β receptor SMA-6 in the hypodermis is needed during adulthood to generate olfactory plasticity. These spatial and temporal mechanisms are critical for the DBL-1 signaling to achieve its diverse functions in development and adult neural plasticity. Interestingly, aversive training decreases AVA calcium response, leading to an increase in the DBL-1 signal secreted from AVA, revealing an experience-dependent change that can underlie the role of TGF-β signaling in mediating plasticity.

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.

Semaphorin-1 and netrin signal in parallel and permissively to position the male ray 1 sensillum in Caenorhabditis elegans

Netrin and semaphorin axon guidance cues have been found to function in the genesis of several mammalian organs; however, little is known about the underlying molecular mechanisms involved. A genetic approach could help to reveal the underpinnings of these mechanisms. The most anterior ray sensillum (ray 1) in the Caenorhabditis elegans male tail is frequently displaced anterior to its normal position in smp-1/semaphorin-1a and plexin-1/plx-1 mutants. Here we report that UNC-6/netrin and its UNC-40/DCC receptor signal in parallel to SMP-1/semaphorin-1a and its PLX-1/plexin-1 receptor to prevent the anterior displacement of ray 1 and that UNC-6 plus SMP-1 signaling can account entirely for this function. We also report that mab-20/semaphorin-2a mutations, which prevent the separation of neighboring rays and cause ray fusions, suppress the anterior displacements of ray 1 caused by deficiencies in SMP-1 and UNC-6 signaling and this is independent of the ray fusion phenotype, whereas overexpression of UNC-40 and PLX-1 cause ray fusions. This suggests that for ray 1 positioning, a balance is struck between a tendency of SMP-1 and UNC-6 signaling to prevent ray 1 from moving away from ray 2 and a tendency of MAB-20/semaphorin-2a signaling to separate all rays from each other. Additional evidence suggests this balance involves the relative adhesion of the ray 1 structural cell to neighboring SET and hyp 7 hypodermal cells. This finding raises the possibility that changes in ray 1 positioning depend on passive movements caused by attachment to the elongating SET cell in opposition to the morphologically more stable hyp 7 cell. Several lines of evidence indicate that SMP-1 and UNC-6 function permissively in the context of ray 1 positioning.

Compartmentalized calcium dynamics in a C. elegans interneuron encode head movement

The confinement of neuronal activity to specific subcellular regions is a mechanism for expanding the computational properties of neurons. Although the circuit organization underlying compartmentalized activity has been studied in several systems, its cellular basis is still unknown. Here we characterize compartmentalized activity in Caenorhabditis elegans RIA interneurons, which have multiple reciprocal connections to head motor neurons and receive input from sensory pathways. We show that RIA spatially encodes head movement on a subcellular scale through axonal compartmentalization. This subcellular axonal activity is dependent on acetylcholine release from head motor neurons and is simultaneously present and additive with glutamate-dependent globally synchronized activity evoked by sensory inputs. Postsynaptically, the muscarinic acetylcholine receptor GAR-3 acts in RIA to compartmentalize axonal activity through the mobilization of intracellular calcium stores. The compartmentalized activity functions independently of the synchronized activity to modulate locomotory behaviour.

Semaphorin and Eph receptor signaling guide a series of cell movements for ventral enclosure in C. elegans

BACKGROUND

In the last stage of the Caenorhabditis elegans body wall closure, an open pocket in the epidermis is closed by the migration of marginal epidermal P/pocket cells to the ventral midline. The cellular and molecular mechanisms of this closure remain unknown.

RESULTS

Cells within the pocket align to form a bridge for migration of contralateral P cell pair P9/10 L,R (and neighboring P cells) to the midline. Bridge formation involves rearrangement of five sister pairs of PLX-2/plexin and VAB-1/Eph receptor expressing "plexin band" cells, of which three pairs form a scaffold for bridge assembly and two pairs form the bridge. Bridge formation requires VAB-1 kinase-dependent extension of presumptive bridge cells over scaffold cells toward the ventral midline. An unassembled vab-1 null mutant bridge obstructs P cell migration, which is largely overcome by plexin band expression of VAB-1 or VAB-1(delC) (a kinase deletion of VAB-1). VAB-1 also functions redundantly with MAB-20/semaphorin to prevent perdurant gaps between sister plexin band cells that block P cell migration.

CONCLUSIONS

The Eph receptor mediates cellular extensions required for bridge formation, independently facilitates P cell migration to the midline, and functions redundantly with PLX-2/plexin to prevent gaps in the bridge used for P9/10 cell migration in body wall closure.

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.

The Drosophila L1CAM homolog Neuroglian signals through distinct pathways to control different aspects of mushroom body axon development

The spatiotemporal integration of adhesion and signaling during neuritogenesis is an important prerequisite for the establishment of neuronal networks in the developing brain. In this study, we describe the role of the L1-type CAM Neuroglian protein (NRG) in different steps of Drosophila mushroom body (MB) neuron axonogenesis. Selective axon bundling in the peduncle requires both the extracellular and the intracellular domain of NRG. We uncover a novel role for the ZO-1 homolog Polychaetoid (PYD) in axon branching and in sister branch outgrowth and guidance downstream of the neuron-specific isoform NRG-180. Furthermore, genetic analyses show that the role of NRG in different aspects of MB axonal development not only involves PYD, but also TRIO, SEMA-1A and RAC1.

Diverse functions for the semaphorin receptor PlexinD1 in development and disease

Plexins are a family of single-pass transmembrane proteins that serve as cell surface receptors for Semaphorins during the embryonic development of animals. Semaphorin-Plexin signaling is critical for many cellular aspects of organogenesis, including cell migration, proliferation and survival. Until recently, little was known about the function of PlexinD1, the sole member of the vertebrate-specific PlexinD (PlxnD1) subfamily. Here we review novel findings about PlxnD1's roles in the development of the cardiovascular, nervous and immune systems and salivary gland branching morphogenesis and discuss new insights concerning the molecular mechanisms of PlxnD1 activity.

"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.

Cellular strategies of axonal pathfinding

Axons follow highly stereotyped and reproducible trajectories to their targets. In this review we address the properties of the first pioneer neurons to grow in the developing nervous system and what has been learned over the past several decades about the extracellular and cell surface substrata on which axons grow. We then discuss the types of guidance cues and their receptors that influence axon extension, what determines where cues are expressed, and how axons respond to the cues they encounter in their environment.

Extracellular interactome of the FGF receptor-ligand system: complexities and the relative simplicity of the worm

Fibroblast growth factors (FGFs) and their receptors (FGFRs) regulate a multitude of biological functions in embryonic development and in adult. A major question is how does one family of growth factors and their receptors control such a variety of functions? Classically, specificity was thought to be imparted by alternative splicing of the FGFRs, resulting in isoforms that bind specifically to a subset of the FGFs, and by different saccharide sequences in the heparan sulfate proteoglycan (HSPG) co-receptor. A growing number of noncanonical co-receptors such as integrins and neural cell adhesion molecule (NCAM) are now recognized as imparting additional complexity to classic FGFR signaling. This review will discuss the noncanonical FGFR ligands and speculate on the possibility that they provide additional and alternative means to determining the functional specificity of FGFR signaling. We will also discuss how invertebrate models such as C. elegans may advance our understanding of noncanonical FGFR signaling.

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.

Genetic interaction of Neuroglian and Semaphorin1a during guidance and synapse formation

We have previously demonstrated a function for Neuroglian and Semaphorin1a in Drosophila giant fiber circuit formation. Both molecules are required for guiding the giant fibers out of the brain and have distinct functions during giant synapse formation. In this study we characterized the effects of various combinations of Neuroglian and Semaphorin1a gain and loss of function backgrounds on giant fiber circuitry formation. We found that Neuroglian and Semaphorin1a genetically interact with each other during axon guidance as well as during synapse formation. Our experiments revealed that during pathfinding of the giant fibers out of the brain, Neuroglian function seems to be dependent on Semaphorin1a. In contrast, during giant fiber synapse formation we observed that Semaphorin1a signaling as a receptor can be altered by Neuroglian in the same cell. In summary, our findings suggest that Neuroglian and Semaphorin1a can regulate each other's function in cis and that the resultant signaling output is possibly different during guidance and synapse formation.