The resilient synapse: insights from genetic interference of synaptic cell adhesion molecules

Decoding the Nectin Interactome: Implications for Brain Development, Plasticity, and Neurological Disorders

The nectin family of cell adhesion molecules (CAMs) comprising nectins and nectin-like molecules has emerged as a key regulator of various pivotal neural processes, including neuronal development, migration, synapse formation, and plasticity. Nectins engage in homophilic and heterophilic interactions to mediate cell-cell adhesion, contributing to the establishment and maintenance of neural circuits. Their extracellular domains facilitate trans-synaptic interactions, while intracellular domains participate in signaling cascades influencing cytoskeletal dynamics and synaptic function. The exhibition of distinct localization patterns in neurons, astrocytes, and the blood-brain barrier underscores their diverse roles in the brain. The dysregulation of nectins has been implicated in several neurological disorders, such as neurodevelopmental disorders, depression, schizophrenia, and Alzheimer's disease. This review examines the structural and functional characteristics of nectins and their distribution and molecular mechanisms governing neural connectivity and cognition. It further discusses experimental studies unraveling nectin-mediated pathophysiology and potential therapeutic interventions targeting nectin-related pathways. Collectively, this comprehensive analysis highlights the significance of nectins in brain development, function, and disorders, paving the way for future research directions and clinical implications.

Dystroglycan Mediates Clustering of Essential GABAergic Components in Cerebellar Purkinje Cells

Muscle dystrophin–glycoprotein complex (DGC) links the intracellular cytoskeleton to the extracellular matrix. In neurons, dystroglycan and dystrophin, two major components of the DGC, localize in a subset of GABAergic synapses, where their function is unclear. Here we used mouse models to analyze the specific role of the DGC in the organization and function of inhibitory synapses. Loss of full-length dystrophin in mdx mice resulted in a selective depletion of the transmembrane β-dystroglycan isoform from inhibitory post-synaptic sites in cerebellar Purkinje cells. Remarkably, there were no differences in the synaptic distribution of the extracellular α-dystroglycan subunit, of GABA_A receptors and neuroligin 2. In contrast, conditional deletion of the dystroglycan gene from Purkinje cells caused a disruption of the DGC and severely impaired post-synaptic clustering of neuroligin 2, GABA_A receptors and scaffolding proteins. Accordingly, whole-cell patch-clamp analysis revealed a significant reduction in the frequency and amplitude of spontaneous IPSCs recorded from Purkinje cells. In the long-term, deletion of dystroglycan resulted in a significant decrease of GABAergic innervation of Purkinje cells and caused an impairment of motor learning functions. These results show that dystroglycan is an essential synaptic organizer at GABAergic synapses in Purkinje cells.

Diverse roles for glycosaminoglycans in neural patterning

The nervous system coordinates the functions of most multicellular organisms and their response to the surrounding environment. Its development involves concerted cellular interactions, including migration, axon guidance, and synapse formation. These processes depend on the molecular constituents and structure of the extracellular matrices (ECM). An essential component of ECMs are proteoglycans, i.e., proteins containing unbranched glycan chains known as glycosaminoglycans (GAGs). A defining characteristic of GAGs is their enormous molecular diversity, created by extensive modifications of the glycans during their biosynthesis. GAGs are widely expressed, and their loss can lead to catastrophic neuronal defects. Despite their importance, we are just beginning to understand the function and mechanisms of GAGs in neuronal development. In this review, we discuss recent evidence suggesting GAGs have specific roles in neuronal patterning and synaptogenesis. We examine the function played by the complex modifications present on GAG glycans and their roles in regulating different aspects of neuronal patterning. Moreover, the review considers the function of proteoglycan core proteins in these processes, stressing their likely role as co-receptors of different signaling pathways in a redundant and context-dependent manner. We conclude by discussing challenges and future directions toward a better understanding of these fascinating molecules during neuronal development. Developmental Dynamics 247:54-74, 2018. © 2017 Wiley Periodicals, Inc.

How to bake a brain: yeast as a model neuron

More than 30 years ago Dan Koshland published an inspirational essay presenting the bacterium as a model neuron (Koshland, Trends Neurosci 6:133-137, 1983). In the article he argued that there are several similarities between neurons and bacterial cells in "how signals are processed within a cell or how this processing machinery can be modified to produce plasticity". He then explored the bacterial chemosensory system to emphasize its attributes that are analogous to information processing in neurons. In this review, we wish to expand Koshland's original idea by adding the yeast cell to the list of useful models of a neuron. The fact that yeasts and neurons are specialized versions of the eukaryotic cell sharing all principal components sets the stage for a grand evolutionary tinkering where these components are employed in qualitatively different tasks, but following analogous molecular logic. By way of example, we argue that evolutionarily conserved key components involved in polarization processes (from budding or mating in Saccharomyces cervisiae to neurite outgrowth or spinogenesis in neurons) are shared between yeast and neurons. This orthologous conservation of modules makes S. cervisiae an excellent model organism to investigate neurobiological questions. We substantiate this claim by providing examples of yeast models used for studying neurological diseases.

Asymmetric N-cadherin expression results in synapse dysfunction, synapse elimination, and axon retraction in cultured mouse neurons

Synapse elimination and pruning of axon collaterals are crucial developmental events in the refinement of neuronal circuits. While a control of synapse formation by adhesion molecules is well established, the involvement of adhesion molecules in developmental synapse loss is poorly characterized. To investigate the consequences of mis-match expression of a homophilic synaptic adhesion molecule, we analysed an asymmetric, exclusively postsynaptic expression of N-cadherin. This was induced by transfecting individual neurons in cultures of N-cadherin knockout mouse neurons with a N-cadherin expression vector. 2 days after transfection, patch-clamp analysis of AMPA receptor-mediated miniature postsynaptic currents revealed an impaired synaptic function without a reduction in the number of presynaptic vesicle clusters. Long-term asymmetric expression of N-cadherin for 8 days subsequently led to synapse elimination as indicated by a loss of colocalization of presynaptic vesicles and postsynaptic PSD95 protein. We further studied long-term asymmetric N-cadherin expression by conditional, Cre-induced knockout of N-cadherin in individual neurons in cultures of N-cadherin expressing cortical mouse neurons. This resulted in a strong retraction of axonal processes in individual neurons that lacked N-cadherin protein. Moreover, an in vivo asymmetric expression of N-cadherin in the developmentally transient cortico-tectal projection was indicated by in-situ hybridization with layer V neurons lacking N-cadherin expression. Thus, mis-match expression of N-cadherin might contribute to selective synaptic connectivity.

Analysis of adhesion molecules and basement membrane contributions to synaptic adhesion at the Drosophila embryonic NMJ

Synapse formation and maintenance crucially underlie brain function in health and disease. Both processes are believed to depend on cell adhesion molecules (CAMs). Many different classes of CAMs localise to synapses, including cadherins, protocadherins, neuroligins, neurexins, integrins, and immunoglobulin adhesion proteins, and further contributions come from the extracellular matrix and its receptors. Most of these factors have been scrutinised by loss-of-function analyses in animal models. However, which adhesion factors establish the essential physical links across synaptic clefts and allow the assembly of synaptic machineries at the contact site in vivo is still unclear. To investigate these key questions, we have used the neuromuscular junction (NMJ) of Drosophila embryos as a genetically amenable model synapse. Our ultrastructural analyses of NMJs lacking different classes of CAMs revealed that loss of all neurexins, all classical cadherins or all glutamate receptors, as well as combinations between these or with a Laminin deficiency, failed to reveal structural phenotypes. These results are compatible with a view that these CAMs might have no structural role at this model synapse. However, we consider it far more likely that they operate in a redundant or well buffered context. We propose a model based on a multi-adaptor principle to explain this phenomenon. Furthermore, we report a new CAM-independent adhesion mechanism that involves the basement membranes (BM) covering neuromuscular terminals. Thus, motorneuronal terminals show strong partial detachment of the junction when BM-to-cell surface attachment is impaired by removing Laminin A, or when BMs lose their structural integrity upon loss of type IV collagens. We conclude that BMs are essential to tie embryonic motorneuronal terminals to the muscle surface, lending CAM-independent structural support to their adhesion. Therefore, future developmental studies of these synaptic junctions in Drosophila need to consider the important contribution made by BM-dependent mechanisms, in addition to CAM-dependent adhesion.

Synaptic cell adhesion

Chemical synapses are asymmetric intercellular junctions that mediate synaptic transmission. Synaptic junctions are organized by trans-synaptic cell adhesion molecules bridging the synaptic cleft. Synaptic cell adhesion molecules not only connect pre- and postsynaptic compartments, but also mediate trans-synaptic recognition and signaling processes that are essential for the establishment, specification, and plasticity of synapses. A growing number of synaptic cell adhesion molecules that include neurexins and neuroligins, Ig-domain proteins such as SynCAMs, receptor phosphotyrosine kinases and phosphatases, and several leucine-rich repeat proteins have been identified. These synaptic cell adhesion molecules use characteristic extracellular domains to perform complementary roles in organizing synaptic junctions that are only now being revealed. The importance of synaptic cell adhesion molecules for brain function is highlighted by recent findings implicating several such molecules, notably neurexins and neuroligins, in schizophrenia and autism.

Understanding the molecular diversity of GABAergic synapses

GABAergic synapses exhibit a high degree of subcellular and molecular specialization, which contrasts with their apparent simplicity in ultrastructural appearance. Indeed, when observed in the electron microscope, GABAergic synapses fit in the symmetric, or Gray’s type II category, being characterized by a relatively simple postsynaptic specialization. The inhibitory postsynaptic density cannot be readily isolated, and progress in understanding its molecular composition has lagged behind that of excitatory synapses. However, recent studies have brought significant progress in the identification of new synaptic proteins, revealing an unexpected complexity in the molecular machinery that regulates GABAergic synaptogenesis. In this article, we provide an overview of the molecular diversity of GABAergic synapses, and we consider how synapse specificity may be encoded by selective trans-synaptic interactions between pre- and postsynaptic adhesion molecules and secreted factors that reside in the synaptic cleft. We also discuss the importance of developing cataloguing tools that could be used to decipher the molecular diversity of synapses and to predict alterations of inhibitory transmission in the course of neurological diseases.

Cell adhesion, the backbone of the synapse: "vertebrate" and "invertebrate" perspectives

Synapses are asymmetric intercellular junctions that mediate neuronal communication. The number, type, and connectivity patterns of synapses determine the formation, maintenance, and function of neural circuitries. The complexity and specificity of synaptogenesis relies upon modulation of adhesive properties, which regulate contact initiation, synapse formation, maturation, and functional plasticity. Disruption of adhesion may result in structural and functional imbalance that may lead to neurodevelopmental diseases, such as autism, or neurodegeneration, such as Alzheimer's disease. Therefore, understanding the roles of different adhesion protein families in synapse formation is crucial for unraveling the biology of neuronal circuit formation, as well as the pathogenesis of some brain disorders. The present review summarizes some of the knowledge that has been acquired in vertebrate and invertebrate genetic model organisms.

Overexpression of the cell adhesion protein neuroligin-1 induces learning deficits and impairs synaptic plasticity by altering the ratio of excitation to inhibition in the hippocampus

Trans-synaptic cell-adhesion molecules have been implicated in regulating CNS synaptogenesis. Among these, the Neuroligin (NL) family (NLs 1-4) of postsynaptic adhesion proteins has been shown to promote the development and specification of excitatory versus inhibitory synapses. NLs form a heterophilic complex with the presynaptic transmembrane protein Neurexin (NRX). A differential association of NLs with postsynaptic scaffolding proteins and NRX isoforms has been suggested to regulate the ratio of excitatory to inhibitory synapses (E/I ratio). Using transgenic mice, we have tested this hypothesis by overexpressing NL1 in vivo to determine whether the relative levels of these cell adhesion molecules may influence synapse maturation, long-term potentiation (LTP), and/or learning. We found that NL1-overexpressing mice show significant deficits in memory acquisition, but not in memory retrieval. Golgi and electron microscopy analysis revealed changes in synapse morphology indicative of increased maturation of excitatory synapses. In parallel, electrophysiological examination indicated a shift in the synaptic activity toward increased excitation as well as impairment in LTP induction. Our results demonstrate that altered balance in the expression of molecules necessary for synapse specification and development (such as NL1) can lead to defects in memory formation and synaptic plasticity and outline the importance of rigidly controlled synaptic maturation processes.

Neuroligin-deficient mutants of C. elegans have sensory processing deficits and are hypersensitive to oxidative stress and mercury toxicity

Neuroligins are postsynaptic cell adhesion proteins that bind specifically to presynaptic membrane proteins called neurexins. Mutations in human neuroligin genes are associated with autism spectrum disorders in some families. The nematode Caenorhabditis elegans has a single neuroligin gene (nlg-1), and approximately a sixth of C. elegans neurons, including some sensory neurons, interneurons and a subset of cholinergic motor neurons, express a neuroligin transcriptional reporter. Neuroligin-deficient mutants of C. elegans are viable, and they do not appear deficient in any major motor functions. However, neuroligin mutants are defective in a subset of sensory behaviors and sensory processing, and are hypersensitive to oxidative stress and mercury compounds; the behavioral deficits are strikingly similar to traits frequently associated with autism spectrum disorders. Our results suggest a possible link between genetic defects in synapse formation or function, and sensitivity to environmental factors in the development of autism spectrum disorders.

The synaptic proteins neurexins and neuroligins are widely expressed in the vascular system and contribute to its functions

Unlike other neuronal counterparts, primary synaptic proteins are not known to be involved in vascular physiology. Here, we demonstrate that neurexins and neuroligins, which constitute large and complex families of fundamental players in synaptic activity, are produced and processed by endothelial and vascular smooth muscle cells throughout the vasculature. Moreover, they are dynamically regulated during vessel remodeling and form endogenous complexes in large vessels as well as in the brain. We used the chicken chorioallantoic membrane as a system to pursue functional studies and demonstrate that a monoclonal recombinant antibody against beta-neurexin inhibits angiogenesis, whereas exogenous neuroligin has a role in promoting angiogenesis. Finally, as an insight into the mechanism of action of beta-neurexin, we show that the anti-beta-neurexin antibody influences vessel tone in isolated chicken arteries. Our finding strongly supports the idea that even the most complex and plastic events taking place in the nervous system (i.e., synaptic activity) share molecular cues with the vascular system.

Polarized targeting of neurexins to synapses is regulated by their C-terminal sequences

Two families of cell-adhesion molecules, predominantly presynaptic neurexins and postsynaptic neuroligins, are important for the formation and functioning of synapses in the brain, and mutations in several genes encoding these transmembrane proteins have been found in autism patients. However, very little is known about how neurexins are targeted to synapses and which mechanisms regulate this process. Using various epitope-tagged neurexins in primary hippocampal neurons of wild-type and knock-out mice in vitro and in transgenic animals in vivo, we show that neurexins are trafficked throughout neurons via transport vesicles and the plasma membrane insertion of neurexins occurs preferentially in the axonal/synaptic compartment. We also observed that exit of neurexins from the ER/Golgi and correct targeting require their PDZ-binding motif at the C terminus, whereas two presumptive ER retention signals are inactive. The ubiquitous presence of neurexin-positive transport vesicles and absence of bassoon colabeling demonstrate that these carriers are not active zone precursor vesicles, but colocalization with CASK, RIM1alpha, and calcium channels suggests that they may carry additional components of the exocytotic machinery. Our data indicate that neurexins are delivered to synapses by a polarized and regulated targeting process that involves PDZ-domain mediated interactions, suggesting a novel pathway for the distribution of neurexins and other synaptic proteins.

Molecular mechanisms of presynaptic differentiation

Information processing in the nervous system relies on properly localized and organized synaptic structures at the correct locations. The formation of synapses is a long and intricate process involving multiple interrelated steps. Decades of research have identified a large number of molecular components of the presynaptic compartment. In addition to neurotransmitter-containing synaptic vesicles, presynaptic terminals are defined by cytoskeletal and membrane specializations that allow highly regulated exo- and endocytosis of synaptic vesicles and that maintain precise registration with postsynaptic targets. Functional studies at multiple levels have revealed complex interactions between the transport of vesicular intermediates, the presynaptic cytoskeleton, growth cone navigation, and synaptic targets. With the advent of finer anatomical, physiological, and molecular tools, great insights have been gained toward the mechanistic dissection of functionally redundant processes controlling the specificity and dynamics of synapses. This review highlights the recent findings pertaining to the cellular and molecular regulation of presynaptic differentiation.

Cellular and molecular determinants targeting the Caenorhabditis elegans PHR protein RPM-1 to perisynaptic regions

Caenorhabditis elegans RPM-1 is a member of a conserved protein family, the PHR proteins, that includes human Pam, mouse Phr1, zebrafish Esrom, and Drosophila Highwire. PHR proteins play important roles in the development of the nervous system. In particular, mutations in rpm-1 cause a disruption of synaptic architecture, affecting the distribution of synaptic vesicles and the number of presynaptic densities. Using antibodies against RPM-1, we determined the localization of the endogenous RPM-1 protein in wild-type and in several mutants that affect synaptic development. Our analyses show that, in mature neurons, RPM-1 resides in a distinct region that is close to, but does not overlap with, the synaptic exo- and endocytosis domains. The localization of RPM-1 occurs independently of several proteins that function in the transport or assembly of synapse components, and its abundance is partially dependent on its binding partner the F-box protein FSN-1. RPM-1 has been shown to target the MAPKKK DLK-1 for degradation. We show that activated DLK-1 may be preferentially targeted for degradation. Furthermore, using transgene analysis, we identified a critical role of the conserved PHR domain of RPM-1 in its subcellular localization.

Transsynaptic modulation of the synaptic vesicle cycle by cell-adhesion molecules

Delicate control of the synaptic vesicle cycle is required to meet the demands imposed on synaptic transmission by the brain's complex information processing. In addition to intensively analyzed intrinsic regulation, extrinsic modulation of the vesicle cycle by the postsynaptic target neuron has become evident. Recent studies have demonstrated that several families of synaptic cell-adhesion molecules play a significant role in transsynaptic retrograde signaling. Different adhesion systems appear to specifically target distinct steps of the synaptic vesicle cycle. Signaling via classical cadherins regulates the recruitment of synaptic vesicles to the active zone. The neurexin/neuroligin system has been shown to modulate presynaptic release probability. In addition, reverse signaling via the EphB/ephrinB system plays an important role in the activity-dependent induction of long-term potentiation of presynaptic transmitter release. Moreover, the first hints of involvement of cell-adhesion molecules in vesicle endocytosis have been published. A general hypothesis is that specific adhesion systems might use different but parallel transsynaptic signaling pathways able to selectively modulate each step of the synaptic vesicle cycle in a tightly coordinated manner.

Synaptic adhesion molecules and PSD-95

Synaptic adhesion molecules are known to participate in various steps of synapse development including initial contacts between dendrites and axons, formation of early synapses, and their maturation and plastic changes. Notably, a significant subset of synaptic adhesion molecules associates with synaptic scaffolding proteins, suggesting that they may act in concert to couple trans-synaptic adhesion to molecular organization of synaptic proteins. Here, we describe an emerging group of synaptic adhesion molecules that directly interact with the abundant postsynaptic scaffold PSD-95, which include neuroligins, NGLs, SALMs, and ADAM22, and discuss how these proteins and PSD-95 act together to regulate synaptic development. PSD-95 may be one of the central organizers of synaptic adhesion that recruits diverse proteins to sites of synaptic adhesion, promotes trans-synaptic signaling, and couples neuronal activity with changes in synaptic adhesion.

Synaptic arrangement of the neuroligin/beta-neurexin complex revealed by X-ray and neutron scattering

Neuroligins are postsynaptic cell-adhesion proteins that associate with their presynaptic partners, the neurexins. Using small-angle X-ray scattering, we determined the shapes of the extracellular region of several neuroligin isoforms in solution. We conclude that the neuroligins dimerize via the characteristic four-helix bundle observed in cholinesterases, and that the connecting sequence between the globular lobes of the dimer and the cell membrane is elongated, projecting away from the dimer interface. X-ray scattering and neutron contrast variation data show that two neurexin monomers, separated by 107 A, bind at symmetric locations on opposite sides of the long axis of the neuroligin dimer. Using these data, we developed structural models that delineate the spatial arrangements of different neuroligin domains and their partnering molecules. As mutations of neurexin and neuroligin genes appear to be linked to autism, these models provide a structural framework for understanding altered recognition by these proteins in neurodevelopmental disorders.