Neuroligin-3 is a neuronal adhesion protein at GABAergic and glutamatergic synapses

Somatostatin and parvalbumin inhibitory synapses onto hippocampal pyramidal neurons are regulated by distinct mechanisms

Excitation-inhibition balance is critical for optimal brain function, yet the mechanisms underlying the tuning of inhibition from different populations of inhibitory neurons are unclear. Here, we found evidence for two distinct pathways through which excitatory neurons cell-autonomously modulate inhibitory synapses. Synapses from parvalbumin-expressing interneurons onto hippocampal pyramidal neurons are regulated by neuronal firing, signaling through L-type calcium channels. Synapses from somatostatin-expressing interneurons are regulated by NMDA receptors, signaling through R-type calcium channels. Thus, excitatory neurons can cell-autonomously regulate their inhibition onto different subcellular compartments through their input (glutamatergic signaling) and their output (firing). Separately, while somatostatin and parvalbumin synapses onto excitatory neurons are both dependent on a common set of postsynaptic proteins, including gephyrin, collybistin, and neuroligin-2, decreasing neuroligin-3 expression selectively decreases inhibition from somatostatin interneurons, and overexpression of neuroligin-3 selectively enhances somatostatin inhibition. These results provide evidence that excitatory neurons can selectively regulate two distinct sets of inhibitory synapses.

Synaptic Neurexin Complexes: A Molecular Code for the Logic of Neural Circuits

Synapses are specialized junctions between neurons in brain that transmit and compute information, thereby connecting neurons into millions of overlapping and interdigitated neural circuits. Here, we posit that the establishment, properties, and dynamics of synapses are governed by a molecular logic that is controlled by diverse trans-synaptic signaling molecules. Neurexins, expressed in thousands of alternatively spliced isoforms, are central components of this dynamic code. Presynaptic neurexins regulate synapse properties via differential binding to multifarious postsynaptic ligands, such as neuroligins, cerebellin/GluD complexes, and latrophilins, thereby shaping the input/output relations of their resident neural circuits. Mutations in genes encoding neurexins and their ligands are associated with diverse neuropsychiatric disorders, especially schizophrenia, autism, and Tourette syndrome. Thus, neurexins nucleate an overall trans-synaptic signaling network that controls synapse properties, which thereby determines the precise responses of synapses to spike patterns in a neuron and circuit and which is vulnerable to impairments in neuropsychiatric disorders.

Proteolytic maturation of Drosophila Neuroligin 3 by tumor necrosis factor α-converting enzyme in the nervous system

BACKGROUND

The functions of autism-associated Neuroligins (Nlgs) are modulated by their post-translational modifications, such as proteolytic cleavage. A previous study has shown that there are different endogenous forms of DNlg3 in Drosophila, indicating it may undergo proteolytic processing. However, the molecular mechanism underlying DNlg3 proteolytic processing is unknown. Here, we report a novel proteolytic mechanism that is essential for DNlg3 maturation and function in the nervous system.

METHODS

Molecular cloning, cell culture, immunohistochemistry, western blotting and genetic studies were employed to map the DNlg3 cleavage region, identify the protease and characterize the cleavage manner. Behavior analysis, immunohistochemistry and genetic manipulations were employed to study the functions of different DNlg3 forms in the nervous system and neuromuscular junction (NMJs).

RESULTS

Tumor necrosis factor α-converting enzyme (TACE) cleaved DNlg3 exclusively at its extracellular acetylcholinesterase-like domain to generate the N-terminal fragment and the short membrane-anchored fragment (sDNlg3). DNlg3 was constitutively processed in an activity-independent manner. Interestingly, DNlg3 was cleaved intracellularly in the Golgi apparatus before it arrived at the cell surface, a unique cleavage mechanism that is distinct from 'conventional' ectodomain shedding of membrane proteins, including rodent Nlg1. Genetic studies showed that sDNlg3 was essential for maintaining proper locomotor activity in Drosophila.

CONCLUSIONS

Our results revealed a unique cleavage mechanism of DNlg3 and a neuron-specific role for DNlg3 maturation which is important in locomotor activity.

GENERAL SIGNIFICANCE

Our study provides a new insight into a cleavage mechanism of Nlgs maturation in the nervous system.

Developmental plasticity shapes synaptic phenotypes of autism-associated neuroligin-3 mutations in the calyx of Held

Neuroligins are postsynaptic cell-adhesion molecules that bind to presynaptic neurexins. Mutations in neuroligin-3 predispose to autism, but how such mutations affect synaptic function remains incompletely understood. Here we systematically examined the effect of three autism-associated mutations, the neuroligin-3 knockout, the R451C knockin, and the R704C knockin, on synaptic transmission in the calyx of Held, a central synapse ideally suited for high-resolution analyses of synaptic transmission. Surprisingly, germline knockout of neuroligin-3 did not alter synaptic transmission, whereas the neuroligin-3 R451C and R704C knockins decreased and increased, respectively, synaptic transmission. These puzzling results prompted us to ask whether neuroligin-3 mutant phenotypes may be reshaped by developmental plasticity. Indeed, conditional knockout of neuroligin-3 during late development produced a marked synaptic phenotype, whereas conditional knockout of neuroligin-3 during early development caused no detectable effect, mimicking the germline knockout. In canvassing potentially redundant candidate genes, we identified developmentally early expression of another synaptic neurexin ligand, cerebellin-1. Strikingly, developmentally early conditional knockout of cerebellin-1 only modestly impaired synaptic transmission, whereas in contrast to the individual single knockouts, developmentally early conditional double knockout of both cerebellin-1 and neuroligin-3 severely decreased synaptic transmission. Our data suggest an unanticipated mechanism of developmental compensation whereby cerebellin-1 and neuroligin-3 functionally occlude each other during development of calyx synapses. Thus, although acute manipulations more likely reveal basic gene functions, developmental plasticity can be a major factor in shaping the overall phenotypes of genetic neuropsychiatric disorders.

Structural Mechanism for Modulation of Synaptic Neuroligin-Neurexin Signaling by MDGA Proteins

Neuroligin-neurexin (NL-NRX) complexes are fundamental synaptic organizers in the central nervous system. An accurate spatial and temporal control of NL-NRX signaling is crucial to balance excitatory and inhibitory neurotransmission, and perturbations are linked with neurodevelopmental and psychiatric disorders. MDGA proteins bind NLs and control their function and interaction with NRXs via unknown mechanisms. Here, we report crystal structures of MDGA1, the NL1-MDGA1 complex, and a spliced NL1 isoform. Two large, multi-domain MDGA molecules fold into rigid triangular structures, cradling a dimeric NL to prevent NRX binding. Structural analyses guided the discovery of a broad, splicing-modulated interaction network between MDGA and NL family members and helped rationalize the impact of autism-linked mutations. We demonstrate that expression levels largely determine whether MDGAs act selectively or suppress the synapse organizing function of multiple NLs. These results illustrate a potentially brain-wide regulatory mechanism for NL-NRX signaling modulation.

Molecular Mechanism of MDGA1: Regulation of Neuroligin 2:Neurexin Trans-synaptic Bridges

Neuroligins and neurexins promote synapse development and validation by forming trans-synaptic bridges spanning the synaptic cleft. Select pairs promote excitatory and inhibitory synapses, with neuroligin 2 (NLGN2) limited to inhibitory synapses and neuroligin 1 (NLGN1) dominating at excitatory synapses. The cell-surface molecules, MAM domain-containing glycosylphosphatidylinositol anchor 1 (MDGA1) and 2 (MDGA2), regulate trans-synaptic adhesion between neurexins and neuroligins, impacting NLGN2 and NLGN1, respectively. We have determined the molecular mechanism of MDGA action. MDGA1 Ig1-Ig2 is sufficient to bind NLGN2 with nanomolar affinity; its crystal structure reveals an unusual locked rod-shaped array. In the crystal structure of the complex, two MDGA1 Ig1-Ig2 molecules each span the entire NLGN2 dimer. Site-directed mutagenesis confirms the observed interaction interface. Strikingly, Ig1 from MDGA1 binds to the same region on NLGN2 as neurexins do. Thus, MDGAs regulate the formation of neuroligin-neurexin trans-synaptic bridges by sterically blocking access of neurexins to neuroligins.

Loss of Neuroligin3 specifically downregulates retinal GABAAα2 receptors without abolishing direction selectivity

The postsynaptic adhesion proteins Neuroligins (NLs) are essential for proper synapse function, and their alterations are associated with a variety of neurodevelopmental disorders. It is increasingly clear that each NL isoform occupies specific subsets of synapses and is able to regulate the function of discrete networks. Studies of NL2 and NL4 in the retina in particular have contributed towards uncovering their role in inhibitory synapse function. In this study we show that NL3 is also predominantly expressed at inhibitory postsynapses in the retinal inner plexiform layer (IPL), where it colocalizes with both GABAA- and glycinergic receptor clusters in a 3:2 ratio. In the NL3 deletion-mutant (knockout or KO) mouse, we uncovered a dramatic reduction of the number of GABAAα2-subunit containing GABAA receptor clusters at the IPL. Retinal activity was thereafter assessed in KO and wild-type (WT) littermates by multi-electrode-array recordings of the output cells of retina, the retinal ganglion cells (RGCs). RGCs in the NL3 KO showed reduced spontaneous activity and an altered response to white noise stimulation. Moreover, upon application of light flashes, the proportion of cells firing at light offset (OFF RGCs) was significantly lower in the NL3 KO compared to WT littermates, whereas the relative number of cells firing at light onset (ON RGCs) increased. Interestingly, although GABAAα2-bearing receptors have been related to direction-selective circuits of the retina, features of direction selective-retinal ganglion cells recorded remained unperturbed in the NL3 KO. Together our data underscore the importance of NL3 for the integrity of specific GABAAergic retinal circuits and identifies NL3 as an important regulator of retinal activity.

Inter-Synaptic Lateral Diffusion of GABAA Receptors Shapes Inhibitory Synaptic Currents

The lateral mobility of neurotransmitter receptors has been shown to tune synaptic signals. Here we report that GABAA receptors (GABAARs) can diffuse between adjacent dendritic GABAergic synapses in long-living desensitized states, thus laterally spreading "activation memories" between inhibitory synapses. Glutamatergic activity limits this inter-synaptic diffusion by trapping GABAARs at excitatory synapses. This novel form of activity-dependent hetero-synaptic interplay is likely to modulate dendritic synaptic signaling.

Synapse Formation in Monosynaptic Sensory-Motor Connections Is Regulated by Presynaptic Rho GTPase Cdc42

Spinal reflex circuit development requires the precise regulation of axon trajectories, synaptic specificity, and synapse formation. Of these three crucial steps, the molecular mechanisms underlying synapse formation between group Ia proprioceptive sensory neurons and motor neurons is the least understood. Here, we show that the Rho GTPase Cdc42 controls synapse formation in monosynaptic sensory-motor connections in presynaptic, but not postsynaptic, neurons. In mice lacking Cdc42 in presynaptic sensory neurons, proprioceptive sensory axons appropriately reach the ventral spinal cord, but significantly fewer synapses are formed with motor neurons compared with wild-type mice. Concordantly, electrophysiological analyses show diminished EPSP amplitudes in monosynaptic sensory-motor circuits in these mutants. Temporally targeted deletion of Cdc42 in sensory neurons after sensory-motor circuit establishment reveals that Cdc42 does not affect synaptic transmission. Furthermore, addition of the synaptic organizers, neuroligins, induces presynaptic differentiation of wild-type, but not Cdc42-deficient, proprioceptive sensory neurons in vitro Together, our findings demonstrate that Cdc42 in presynaptic neurons is required for synapse formation in monosynaptic sensory-motor circuits.

SIGNIFICANCE STATEMENT

Group Ia proprioceptive sensory neurons form direct synapses with motor neurons, but the molecular mechanisms underlying synapse formation in these monosynaptic sensory-motor connections are unknown. We show that deleting Cdc42 in sensory neurons does not affect proprioceptive sensory axon targeting because axons reach the ventral spinal cord appropriately, but these neurons form significantly fewer presynaptic terminals on motor neurons. Electrophysiological analysis further shows that EPSPs are decreased in these mice. Finally, we demonstrate that Cdc42 is involved in neuroligin-dependent presynaptic differentiation of proprioceptive sensory neurons in vitro These data suggest that Cdc42 in presynaptic sensory neurons is essential for proper synapse formation in the development of monosynaptic sensory-motor circuits.

Differential role of GABAA receptors and neuroligin 2 for perisomatic GABAergic synapse formation in the hippocampus

Perisomatic GABAergic synapses onto hippocampal pyramidal cells arise from two populations of basket cells with different neurochemical and functional properties. The presence of the dystrophin-glycoprotein complex in their postsynaptic density (PSD) distinguishes perisomatic synapses from GABAergic synapses on dendrites and the axon-initial segment. Targeted deletion of neuroligin 2 (NL2), a transmembrane protein interacting with presynaptic neurexin, has been reported to disrupt postsynaptic clustering of GABA_A receptors (GABA_AR) and their anchoring protein, gephyrin, at perisomatic synapses. In contrast, targeted deletion of Gabra2 disrupts perisomatic clustering of gephyrin, but not of α1-GABA_AR, NL2, or dystrophin/dystroglycan. Unexpectedly, conditional deletion of Dag1, encoding dystroglycan, selectively prevents the formation of perisomatic GABAergic synapses from basket cells expressing cholecystokinin. Collectively, these observations suggest that multiple mechanisms regulate formation and molecular composition of the GABAergic PSD at perisomatic synapses. Here, we further explored this issue by investigating the effect of targeted deletion of Gabra1 and NL2 on the dystrophin-glycoprotein complex and on perisomatic synapse formation, using immunofluorescence analysis with a battery of GABAergic pre- and postsynaptic markers. We show that the absence of α1-GABA_AR increases GABAergic synapses containing the α2 subunit, without affecting the clustering of dystrophin and NL2; in contrast, the absence of NL2 produces highly variable effects postsynaptically, not restricted to perisomatic synapses and being more severe for the GABA_AR subunits and gephyrin than dystrophin. Altogether, the results confirm the importance of NL2 as organizer of the GABAergic PSD and unravel distinct roles for α1- and α2-GABA_ARs in the formation of GABAergic circuits in close interaction with the dystrophin-glycoprotein complex.

Unique versus Redundant Functions of Neuroligin Genes in Shaping Excitatory and Inhibitory Synapse Properties

Neuroligins are evolutionarily conserved postsynaptic cell adhesion molecules that interact with presynaptic neurexins. Neurons express multiple neuroligin isoforms that are targeted to specific synapses, but their synaptic functions and mechanistic redundancy are not completely understood. Overexpression or RNAi-mediated knockdown of neuroligins, respectively, causes a dramatic increase or decrease in synapse density, whereas genetic deletions of neuroligins impair synapse function with only minor effects on synapse numbers, raising fundamental questions about the overall physiological role of neuroligins. Here, we have systematically analyzed the effects of conditional genetic deletions of all major neuroligin isoforms (i.e., NL1, NL2, and NL3), either individually or in combinations, in cultured mouse hippocampal and cortical neurons. We found that conditional genetic deletions of neuroligins caused no change or only a small change in synapses numbers, but strongly impaired synapse function. This impairment was isoform specific, suggesting that neuroligins are not functionally redundant. Sparse neuroligin deletions produced phenotypes comparable to those of global deletions, indicating that neuroligins function in a cell-autonomous manner. Mechanistically, neuroligin deletions decreased the synaptic levels of neurotransmitter receptors and had no effect on presynaptic release probabilities. Overexpression of neuroligin-1 in control or neuroligin-deficient neurons increased synaptic transmission and synapse density but not spine numbers, suggesting that these effects reflect a gain-of-function mechanism; whereas overexpression of neuroligin-3, which, like neuroligin-1 is also targeted to excitatory synapses, had no comparable effect. Our data demonstrate that neuroligins are required for the physiological organization of neurotransmitter receptors in postsynaptic specializations and suggest that they do not play a major role in synapse formation.SIGNIFICANCE STATEMENT Human neuroligin genes have been associated with autism, but the cellular functions of different neuroligins and their molecular mechanisms remain incompletely understood. Here, we performed comparative analyses in cultured mouse neurons of all major neuroligin isoforms, either individually or in combinations, using conditional knockouts. We found that neuroligin deletions did not affect synapse numbers but differentially impaired excitatory or inhibitory synaptic functions in an isoform-specific manner. These impairments were due, at least in part, to a decrease in synaptic distribution of neurotransmitter receptors upon deletion of neuroligins. Conversely, the overexpression of neuroligin-1 increased synapse numbers but not spine numbers. Our results suggest that various neuroligin isoforms perform unique postsynaptic functions in organizing synapses but are not essential for synapse formation or maintenance.

Posttranslational modifications of neuroligins regulate neuronal and glial signaling

This review covers the dynamic regulation of neuroligin isoforms, focusing on posttranslational events including phosphorylation, glycosylation and activity-dependent cleavage. There is a growing literature on how phosphorylation confers an isoform-specific level of modulation affecting a variety of protein interactions. In addition, recent studies describe activity-dependent proteolytic cleavage of neuroligins, revealing a broader role for neuroligins than just synaptic 'glue'. Interesting new research implicates the cleaved extracellular fragments of neuroligins in promoting glioma. These reports on cell signaling mediated by the cleavage products of neuroligins suggest novel and important roles for neuroligins in neuro-glial signaling.

Autism spectrum disorder-associated genes and the development of dentate granule cells

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by severe clinical symptoms such as the deficiency of the social communication, repetitive and stereotyped behaviors, and restricted interests. Although complex genetic and environmental factors are thought to contribute to the development of ASD, the precise etiologies are largely unknown. Neuroanatomical observations have been made of developmental abnormalities in different brain regions, including dentate gyrus of hippocampus, which is widely accepted as the center for learning and memory. However, little is known about what roles ASD-associated genes play in the development of hippocampal dentate granule cells. In this article, we summarized functions and pathophysiological significance of 6 representative ASD-associated genes, SEMA5A, PTEN, NLGN, EN-2, FMR1, and MECP2, by focusing on the development of dentate gyrus. We then introduced a recently developed gene transfer method directed to neonatal dentate granule cells. This new method will be useful for elucidating physiological as well as pathophysiological significance of ASD-associated genes in the development of hippocampal formation.

APLP1 Is a Synaptic Cell Adhesion Molecule, Supporting Maintenance of Dendritic Spines and Basal Synaptic Transmission

The amyloid precursor protein (APP), a key player in Alzheimer's disease, belongs to the family of synaptic adhesion molecules (SAMs) due to its impact on synapse formation and synaptic plasticity. These functions are mediated by both the secreted APP ectodomain that acts as a neurotrophic factor and full-length APP forming trans-cellular dimers. Two homologs of APP exist in mammals: the APP like proteins APLP1 and APLP2, exhibiting functions that partly overlap with those of APP. Here we tested whether APLP1 and APLP2 also show features of SAMs. We found that all three family members were upregulated during postnatal development coinciding with synaptogenesis. We observed presynaptic and postsynaptic localization of all APP family members and could show that heterologous expression of APLP1 or APLP2 in non-neuronal cells induces presynaptic differentiation in contacting axons of cocultured neurons, similar to APP and other SAMs. Moreover, APP/APLPs all bind to synaptic-signaling molecules, such as MINT/X11. Furthermore, we report that aged APLP1 knock-out mice show impaired basal transmission and a reduced mEPSC frequency, likely resulting from reduced spine density. This demonstrates an essential nonredundant function of APLP1 at the synapse. Compared to APP, APLP1 exhibits increased trans-cellular binding and elevated cell-surface levels due to reduced endocytosis. In conclusion, our results establish that APLPs show typical features of SAMs and indicate that increased surface expression, as observed for APLP1, is essential for proper synapse formation in vitro and synapse maintenance in vivoSIGNIFICANCE STATEMENT According to the amyloid-cascade hypothesis, Alzheimer's disease is caused by the accumulation of Aβ peptides derived from sequential cleavage of the amyloid precursor protein (APP) by β-site APP cleaving enzyme 1 (BACE1) and γ-secretase. Here we show that all mammalian APP family members (APP, APLP1, and APLP2) exhibit synaptogenic activity, involving trans-synaptic dimerization, similar to other synaptic cell adhesion molecules, such as Neuroligin/Neurexin. Importantly, our study revealed that the loss of APLP1, which is one of the major substrates of BACE1, causes reduced spine density in aged mice. Because some therapeutic interventions target APP processing (e.g., BACE inhibitors), those strategies may alter APP/APLP physiological function. This should be taken into account for the development of pharmaceutical treatments of Alzheimer's disease.

Conditional ablation of neuroligin-1 in CA1 pyramidal neurons blocks LTP by a cell-autonomous NMDA receptor-independent mechanism

Neuroligins are postsynaptic cell-adhesion molecules implicated in autism and other neuropsychiatric disorders. Despite extensive work, the role of neuroligins in synapse function and plasticity, especially N-methyl-d-aspartate (NMDA) receptor (NMDAR)-dependent long-term potentiation (LTP), remains unclear. To establish which synaptic functions unequivocally require neuroligins, we analyzed single and triple conditional knockout (cKO) mice for all three major neuroligin isoforms (NL1-NL3). We inactivated neuroligins by stereotactic viral expression of Cre-recombinase in hippocampal CA1 region pyramidal neurons at postnatal day 0 (P0) or day 21 (P21) and measured synaptic function, synaptic plasticity and spine numbers in acute hippocampal slices 2-3 weeks later. Surprisingly, we find that ablation of neuroligins in newborn or juvenile mice only modestly impaired basal synaptic function in hippocampus and caused no alteration in postsynaptic spine numbers. However, triple cKO of NL1-NL3 or single cKO of NL1 impaired NMDAR-mediated excitatory postsynaptic currents and abolished NMDAR-dependent LTP. Strikingly, the NL1 cKO also abolished LTP elicited by activation of L-type Ca²⁺-channels during blockade of NMDARs. These findings demonstrate that neuroligins are generally not essential for synapse formation in CA1 pyramidal neurons but shape synaptic properties and that NL1 specifically is required for LTP induced by postsynaptic Ca²⁺-elevations, a function which may contribute to the pathophysiological role of neuroligins in brain disorders.

Emerging Synaptic Molecules as Candidates in the Etiology of Neurological Disorders

Synapses are complex structures that allow communication between neurons in the central nervous system. Studies conducted in vertebrate and invertebrate models have contributed to the knowledge of the function of synaptic proteins. The functional synapse requires numerous protein complexes with specialized functions that are regulated in space and time to allow synaptic plasticity. However, their interplay during neuronal development, learning, and memory is poorly understood. Accumulating evidence links synapse proteins to neurodevelopmental, neuropsychiatric, and neurodegenerative diseases. In this review, we describe the way in which several proteins that participate in cell adhesion, scaffolding, exocytosis, and neurotransmitter reception from presynaptic and postsynaptic compartments, mainly from excitatory synapses, have been associated with several synaptopathies, and we relate their functions to the disease phenotype.

Anatomy and Cell Biology of Autism Spectrum Disorder: Lessons from Human Genetics

Until recently autism spectrum disorder (ASD) was regarded as a neurodevelopmental condition with unknown causes and pathogenesis. In the footsteps of the revolution of genome technologies and genetics, and with its high degree of heritability, ASD became the first neuropsychiatric disorder for which clues towards molecular and cellular pathogenesis were uncovered by genetic identification of susceptibility genes. Currently several hundreds of risk genes have been assigned, with a recurrence below 1% in the ASD population. The multitude and diversity of known ASD genes has extended the clinical notion that ASD comprises very heterogeneous conditions ranging from severe intellectual disabilities to mild high-functioning forms. The results of genetics have allowed to pinpoint a limited number of cellular and molecular processes likely involved in ASD including protein synthesis, signal transduction, transcription/chromatin remodelling and synaptic function all playing an essential role in the regulation of synaptic homeostasis during brain development. In this context, we highlight the role of protein synthesis as a key process in ASD pathogenesis as it might be central in synaptic deregulation and a potential target for intervention. These current insights should lead to a rational design of interventions in molecular and cellular pathways of ASD pathogenesis that may be applied to affected individuals in the future.

Neuroligin 2 R215H Mutant Mice Manifest Anxiety, Increased Prepulse Inhibition, and Impaired Spatial Learning and Memory

Neuroligin 2 (NLGN2) is a postsynaptic adhesion protein that plays an essential role in synaptogenesis and function of inhibitory neuron. We previously identified a missense mutation R215H of the NLGN2 in a patient with schizophrenia. This missense mutation was shown to be pathogenic in several cell-based assays. The objective of this study was to better understand the behavioral consequences of this mutation in vivo. We generated a line of transgenic mice carrying this mutation using a recombinant-based method. The mice were subjected to a battery of behavioral tests including open field locomotor activity assay, prepulse inhibition (PPI) assay, accelerated rotarod test, novel location and novel recognition tests, elevated plus-maze (EPM) test, and Morris water maze test. The transgenic animals were viable and fertile, but the Nlgn2 R215H knock-in (KI) homozygous mice showed growth retardation, anxiety-like behavior, increased PPI, and impaired spatial learning and memory. There was no significant interaction between sex and genotype in most behavioral tests; however, we observed a significant interaction between sex and genotype in EPM test in this study. Also, we found that the Nlgn2 R215H homozygous KI mice did not express the NLGN2 protein, resembling Nlgn2 knockout mice. Our results demonstrate that Nlgn2 R215H KI homozygous mice manifest several behavioral abnormalities similar to those found in psychiatric patients carrying NLGN2 mutations, indicating that dysfunction of NLGN2 contributes to the pathogenesis of certain psychiatric symptoms commonly present in various mental disorders, not limited to schizophrenia.

Global Analysis of Protein Expression of Inner Ear Hair Cells

The mammalian inner ear (IE) subserves auditory and vestibular sensations via highly specialized cells and proteins. Sensory receptor hair cells (HCs) are necessary for transducing mechanical inputs and stimulating sensory neurons by using a host of known and as yet unknown protein machinery. To understand the protein composition of these unique postmitotic cells, in which irreversible protein degradation or damage can lead to impaired hearing and balance, we analyzed IE samples by tandem mass spectrometry to generate an unbiased, shotgun-proteomics view of protein identities and abundances. By using Pou4f3/eGFP-transgenic mice in which HCs express GFP driven by Pou4f3, we FACS purified a population of HCs to analyze and compare the HC proteome with other IE subproteomes from sensory epithelia and whole IE. We show that the mammalian HC proteome comprises hundreds of uniquely or highly expressed proteins. Our global proteomic analysis of purified HCs extends the existing HC transcriptome, revealing previously undetected gene products and isoform-specific protein expression. Comparison of our proteomic data with mouse and human databases of genetic auditory/vestibular impairments confirms the critical role of the HC proteome for normal IE function, providing a cell-specific pool of candidates for novel, important HC genes. Several proteins identified exclusively in HCs by proteomics and verified by immunohistochemistry map to human genetic deafness loci, potentially representing new deafness genes.

SIGNIFICANCE STATEMENT

Hearing and balance rely on specialized sensory hair cells (HCs) in the inner ear (IE) to convey information about sound, acceleration, and orientation to the brain. Genetically and environmentally induced perturbations to HC proteins can result in deafness and severe imbalance. We used transgenic mice with GFP-expressing HCs, coupled with FACS sorting and tandem mass spectrometry, to define the most complete HC and IE proteome to date. We show that hundreds of proteins are uniquely identified or enriched in HCs, extending previous gene expression analyses to reveal novel HC proteins and isoforms. Importantly, deafness-linked proteins were significantly enriched in HCs, suggesting that this in-depth proteomic analysis of IE sensory cells may hold potential for deafness gene discovery.

Cell adhesion molecules and sleep

Cell adhesion molecules (CAMs) play essential roles in the central nervous system, where some families are involved in synaptic development and function. These synaptic adhesion molecules (SAMs) are involved in the regulation of synaptic plasticity, and the formation of neuronal networks. Recent findings from studies examining the consequences of sleep loss suggest that these molecules are candidates to act in sleep regulation. This review highlights the experimental data that lead to the identification of SAMs as potential sleep regulators, and discusses results supporting that specific SAMs are involved in different aspects of sleep regulation. Further, some potential mechanisms by which SAMs may act to regulate sleep are outlined, and the proposition that these molecules may serve as molecular machinery in the two sleep regulatory processes, the circadian and homeostatic components, is presented. Together, the data argue that SAMs regulate the neuronal plasticity that underlies sleep and wakefulness.

Distinct roles for extracellular and intracellular domains in neuroligin function at inhibitory synapses

Neuroligins (NLGNs) are postsynaptic cell adhesion molecules that interact trans-synaptically with neurexins to mediate synapse development and function. NLGN2 is only at inhibitory synapses while NLGN3 is at both excitatory and inhibitory synapses. We found that NLGN3 function at inhibitory synapses in rat CA1 depends on the presence of NLGN2 and identified a domain in the extracellular region that accounted for this functional difference between NLGN2 and 3 specifically at inhibitory synapses. We further show that the presence of a cytoplasmic tail (c-tail) is indispensible, and identified two domains in the c-tail that are necessary for NLGN function at inhibitory synapses. These domains point to a gephyrin-dependent mechanism that is disrupted by an autism-associated mutation at R705 and a gephyrin-independent mechanism reliant on a putative phosphorylation site at S714. Our work highlights unique and separate roles for the extracellular and intracellular regions in specifying and carrying out NLGN function respectively.

DOI:http://dx.doi.org/10.7554/eLife.19236.001

Distortion of the normal function of synaptic cell adhesion molecules by genetic variants as a risk for autism spectrum disorders

Synaptic cell adhesion molecules (SCAMs) are a functional category of cell adhesion molecules that connect pre- and postsynapses by the protein-protein interaction via their extracellular cell adhesion domains. Countless numbers of common genetic variants and rare mutations in SCAMs have been identified in the patients with autism spectrum disorders (ASDs). Among these, NRXN and NLGN family proteins cooperatively function at synaptic terminals both of which genes are strongly implicated as risk genes for ASDs. Knock-in mice carrying a single rare point mutation of NLGN3 (NLGN3 R451C) discovered in the patients with ASDs display a deficit in social interaction and an enhancement of spatial learning and memory ability reminiscent of the clinical phenotype of ASDs. NLGN4 knockout (KO) and NRXN2α KO mice also show a deficit in sociability as well as some specific neuropsychiatric behaviors. In this review, we selected NRXNs/NLGNs, CNTNAP2/CNTNAP4, CNTN4, ITGB3, and KIRREL3 as strong ASD risk genes based on SFARI score and summarize the protein structures, functions at synapses, representative discoveries in human genetic studies, and phenotypes of the mutant model mice in light of the pathophysiology of ASDs.

Neuroligins Are Selectively Essential for NMDAR Signaling in Cerebellar Stellate Interneurons

Neuroligins are postsynaptic cell-adhesion molecules that contribute to synapse specification. However, many other postsynaptic cell-adhesion molecules are known and the relative contributions of neuroligins versus other such molecules in different types of synapses and neurons remains largely unknown. Here, we have studied the role of neuroligins in cerebellar stellate interneurons that participate in a well defined circuit that converges on Purkinje cells as the major output neurons of cerebellar cortex. By crossing triple conditional knock-out (cKO) mice targeting all three major neuroligins [neuroligin-1 to neuroligin-3 (NL123)] with parvalbumin-Cre (PV-Cre) transgenic mice, we deleted neuroligins from inhibitory cerebellar interneurons and Purkinje cells, allowing us to study the effects of neuroligin deletions on cerebellar stellate cell synapses by electrophysiology in acute slices. PV-Cre/NL123 cKO mice did not exhibit gross alterations of cerebellar structure or cerebellar interneuron morphology. Strikingly, electrophysiological recordings in stellate cells from these PV-Cre/NL123 cKO mice revealed a large decrease in NMDAR-mediated excitatory synaptic responses, which, in stellate cells, are largely extrasynaptic, without a change in AMPA-receptor-mediated responses. Parallel analyses in PV-Cre/NL1 mice that are single NL1 cKO mice uncovered the same phenotype, demonstrating that NL1 is responsible for recruiting extrasynaptic NMDARs. Moreover, we observed only a modest impairment in inhibitory synaptic responses in stellate cells lacking NL123 despite a nearly complete suppression of inhibitory synaptic transmission in Purkinje cells by the same genetic manipulation. Our results suggest that, unlike other types of neurons investigated, neuroligins are selectively essential in cerebellar stellate interneurons for enabling the function of extrasynaptic NMDARs.

SIGNIFICANCE STATEMENT

Neuroligins are postsynaptic cell-adhesion molecules genetically linked to autism. However, the contributions of neuroligins to interneuron functions remain largely unknown. Here, we analyzed the role of neuroligins in cerebellar stellate interneurons. We deleted neuroligin-1, neuroligin-2, and neuroligin-3, the major cerebellar neuroligin isoforms, from stellate cells in triple NL123 conditional knock-out mice and analyzed synaptic responses by acute slice electrophysiology. We find that neuroligins are selectively essential for extrasynaptic NMDAR-mediated signaling, but dispensable for both AMPAR-mediated and inhibitory synaptic transmission. Our results reveal a critical and selective role for neuroligins in the regulation of NMDAR responses in cerebellar stellate interneurons.

Distinct Defects in Synaptic Differentiation of Neocortical Neurons in Response to Prenatal Valproate Exposure

Autism spectrum disorders (ASDs) are a heterogeneous group of neurodevelopmental disorders characterized by impairments in social interactions and stereotyped behaviors. Valproic acid (VPA) is frequently used to treat epilepsy and bipolar disorders. When taken during pregnancy, VPA increases the risk of the unborn child to develop an ASD. In rodents, in utero VPA exposure can precipitate behavioral phenotypes related to ASD in the offspring. Therefore, such rodent models may allow for identification of synaptic pathophysiology underlying ASD risk. Here, we systematically probed alterations in synaptic proteins that might contribute to autism-related behavior in the offspring of in utero VPA-exposed mice. Moreover, we tested whether direct VPA exposure of cultured neocortical neurons may recapitulate the molecular alterations seen in vivo. VPA-exposed neurons in culture exhibit a significant increase in the number of glutamatergic synapses accompanied by a significant decrease in the number of GABAergic synapses. This shift in excitatory/inhibitory balance results in substantially increased spontaneous activity in neuronal networks arising from VPA-exposed neurons. Pharmacological experiments demonstrate that the alterations in GABAergic and glutamatergic synaptic proteins and structures are largely caused by inhibition of histone deacetylases. Therefore, our study highlights an epigenetic mechanism underlying the synaptic pathophysiology in this ASD model.

Control of neuronal synapse specification by a highly dedicated alternative splicing program

Alternative RNA splicing represents a central mechanism for expanding the coding power of genomes. Individual RNA-binding proteins can control alternative splicing choices in hundreds of RNA transcripts, thereby tuning amounts and functions of large numbers of cellular proteins. We found that the RNA-binding protein SLM2 is essential for functional specification of glutamatergic synapses in the mouse hippocampus. Genome-wide mapping revealed a markedly selective SLM2-dependent splicing program primarily consisting of only a few target messenger RNAs that encode synaptic proteins. Genetic correction of a single SLM2-dependent target exon in the synaptic recognition molecule neurexin-1 was sufficient to rescue synaptic plasticity and behavioral defects in Slm2 knockout mice. These findings uncover a highly selective alternative splicing program that specifies synaptic properties in the central nervous system.

Age-associated Cognitive Decline: Insights into Molecular Switches and Recovery Avenues

Age-associated cognitive decline is an inevitable phenomenon that predisposes individuals for neurological and psychiatric disorders eventually affecting the quality of life. Scientists have endeavored to identify the key molecular switches that drive cognitive decline with advancing age. These newly identified molecules are then targeted as recovery of cognitive aging and related disorders. Cognitive decline during aging is multi-factorial and amongst several factors influencing this trajectory, gene expression changes are pivotal. Identifying these genes would elucidate the neurobiological underpinnings as well as offer clues that make certain individuals resilient to withstand the inevitable age-related deteriorations. Our laboratory has focused on this aspect and investigated a wide spectrum of genes involved in crucial brain functions that attribute to senescence induced cognitive deficits. We have recently identified master switches in the epigenome regulating gene expression alteration during brain aging. Interestingly, these factors when manipulated by chemical or genetic strategies successfully reverse the age-related cognitive impairments. In the present article, we review findings from our laboratory and others combined with supporting literary evidences on molecular switches of brain aging and their potential as recovery targets.

Pathogenic mechanism of an autism-associated neuroligin mutation involves altered AMPA-receptor trafficking

Neuroligins are postsynaptic cell-adhesion molecules that bind to presynaptic neurexins. Although the general synaptic role of neuroligins is undisputed, their specific functions at a synapse remain unclear, even controversial. Moreover, many neuroligin gene mutations were associated with autism, but the pathophysiological relevance of these mutations is often unknown, and their mechanisms of action uninvestigated. Here, we examine the synaptic effects of an autism-associated neuroligin-4 substitution (called R704C), which mutates a cytoplasmic arginine residue that is conserved in all neuroligins. We show that the R704C mutation, when introduced into neuroligin-3, enhances the interaction between neuroligin-3 and AMPA receptors, increases AMPA-receptor internalization and decreases postsynaptic AMPA-receptor levels. When introduced into neuroligin-4, conversely, the R704C mutation unexpectedly elevated AMPA-receptor-mediated synaptic responses. These results suggest a general functional link between neuroligins and AMPA receptors, indicate that both neuroligin-3 and -4 act at excitatory synapses but perform surprisingly distinct functions, and demonstrate that the R704C mutation significantly impairs the normal function of neuroligin-4, thereby validating its pathogenicity.

Behavioral phenotypes of genetic mouse models of autism

More than a hundred de novo single gene mutations and copy-number variants have been implicated in autism, each occurring in a small subset of cases. Mutant mouse models with syntenic mutations offer research tools to gain an understanding of the role of each gene in modulating biological and behavioral phenotypes relevant to autism. Knockout, knockin and transgenic mice incorporating risk gene mutations detected in autism spectrum disorder and comorbid neurodevelopmental disorders are now widely available. At present, autism spectrum disorder is diagnosed solely by behavioral criteria. We developed a constellation of mouse behavioral assays designed to maximize face validity to the types of social deficits and repetitive behaviors that are central to an autism diagnosis. Mouse behavioral assays for associated symptoms of autism, which include cognitive inflexibility, anxiety, hyperactivity, and unusual reactivity to sensory stimuli, are frequently included in the phenotypic analyses. Over the past 10 years, we and many other laboratories around the world have employed these and additional behavioral tests to phenotype a large number of mutant mouse models of autism. In this review, we highlight mouse models with mutations in genes that have been identified as risk genes for autism, which work through synaptic mechanisms and through the mTOR signaling pathway. Robust, replicated autism-relevant behavioral outcomes in a genetic mouse model lend credence to a causal role for specific gene contributions and downstream biological mechanisms in the etiology of autism.

Common mechanisms of excitatory and inhibitory imbalance in schizophrenia and autism spectrum disorders

Autism Spectrum Disorders (ASD) and Schizophrenia (SCZ) are cognitive disorders with complex genetic architectures but overlapping behavioral phenotypes, which suggests common pathway perturbations. Multiple lines of evidence implicate imbalances in excitatory and inhibitory activity (E/I imbalance) as a shared pathophysiological mechanism. Thus, understanding the molecular underpinnings of E/I imbalance may provide essential insight into the etiology of these disorders and may uncover novel targets for future drug discovery. Here, we review key genetic, physiological, neuropathological, functional, and pathway studies that suggest alterations to excitatory/inhibitory circuits are keys to ASD and SCZ pathogenesis.

Neuroligins Sculpt Cerebellar Purkinje-Cell Circuits by Differential Control of Distinct Classes of Synapses

Neuroligins are postsynaptic cell-adhesion molecules that bind presynaptic neurexins and are genetically linked to autism. Neuroligins are proposed to organize synaptogenesis and/or synaptic transmission, but no systematic analysis of neuroligins in a defined circuit is available. Here, we show that conditional deletion of all neuroligins in cerebellar Purkinje cells caused loss of distal climbing-fiber synapses and weakened climbing-fiber but not parallel-fiber synapses, consistent with alternative use of neuroligins and cerebellins as neurexin ligands for the excitatory climbing-fiber versus parallel-fiber synapses. Moreover, deletion of neuroligins increased the size of inhibitory basket/stellate-cell synapses but simultaneously severely impaired their function. Multiple neuroligin isoforms differentially contributed to climbing-fiber and basket/stellate-cell synapse functions, such that inhibitory synapse-specific neuroligin-2 was unexpectedly essential for maintaining normal climbing-fiber synapse numbers. Using systematic analyses of all neuroligins in a defined neural circuit, our data thus show that neuroligins differentially contribute to various Purkinje-cell synapses in the cerebellum in vivo.

Excitatory/Inhibitory Balance and Circuit Homeostasis in Autism Spectrum Disorders

Autism spectrum disorders (ASDs) and related neurological disorders are associated with mutations in many genes affecting the ratio between neuronal excitation and inhibition. However, understanding the impact of these mutations on network activity is complicated by the plasticity of these networks, making it difficult in many cases to separate initial deficits from homeostatic compensation. Here we explore the contrasting evidence for primary defects in inhibition or excitation in ASDs and attempt to integrate the findings in terms of the brain's ability to maintain functional homeostasis.

Developmental expression of the neuroligins and neurexins in fragile X mice

Neuroligins and neurexins are transsynaptic proteins involved in the maturation of glutamatergic and GABAergic synapses. Research has identified synaptic proteins and function as primary contributors to the development of fragile X syndrome. Fragile X mental retardation protein (FMRP), the protein that is lacking in fragile X syndrome, binds neuroligin-1 and -3 mRNA. Using in situ hybridization, we examined temporal and spatial expression patterns of neuroligin (NLGN) and neurexin (NRXN) mRNAs in the somatosensory (S1) cortex and hippocampus in wild-type (WT) and fragile X knockout (FMR1-KO) mice during the first 5 weeks of postnatal life. Genotype-based differences in expression included increased NLGN1 mRNA in CA1 and S1 cortex, decreased NLGN2 mRNA in CA1 and dentate gyrus (DG) regions of the hippocampus, and increased NRXN3 mRNA in CA1, DG, and S1 cortex between female WT and FMR1-KO mice. In male mice, decreased expression of NRXN3 mRNA was observed in CA1 and DG regions of FMR1-KO mice. Sex differences in hippocampal expression of NLGN2, NRXN1, NRXN2, and NRXN3 mRNAs and in S1 cortex expression of NRXN3 mRNAs were observed WT mice, whereas sex differences in NLGN3, NRXN1, NRXN2, and NRXN3 mRNA expression in the hippocampus and in NLGN1, NRXN2 and NRXN3 mRNA expression in S1 cortex were detected in FMR1-KO mice. These results provide a neuroanatomical map of NLGN and NRXN expression patterns over postnatal development in WT and FMR1-KO mice. The differences in developmental trajectory of these synaptic proteins could contribute to long-term differences in CNS wiring and synaptic function.

The Sorting Receptor SorCS1 Regulates Trafficking of Neurexin and AMPA Receptors

The formation, function, and plasticity of synapses require dynamic changes in synaptic receptor composition. Here, we identify the sorting receptor SorCS1 as a key regulator of synaptic receptor trafficking. Four independent proteomic analyses identify the synaptic adhesion molecule neurexin and the AMPA glutamate receptor (AMPAR) as major proteins sorted by SorCS1. SorCS1 localizes to early and recycling endosomes and regulates neurexin and AMPAR surface trafficking. Surface proteome analysis of SorCS1-deficient neurons shows decreased surface levels of these, and additional, receptors. Quantitative in vivo analysis of SorCS1-knockout synaptic proteomes identifies SorCS1 as a global trafficking regulator and reveals decreased levels of receptors regulating adhesion and neurotransmission, including neurexins and AMPARs. Consequently, glutamatergic transmission at SorCS1-deficient synapses is reduced due to impaired AMPAR surface expression. SORCS1 mutations have been associated with autism and Alzheimer disease, suggesting that perturbed receptor trafficking contributes to synaptic-composition and -function defects underlying synaptopathies.

The cellular and molecular landscape of neuroligins

A fundamental physical interaction exists across the synapse. It is mediated by synaptic adhesion molecules, and is among the earliest and most indispensable of molecular events occurring during synaptogenesis. The regulation of adhesion molecules and their interactions with other synaptic proteins likely affect not only on synapse formation but also on ongoing synaptic function. We review research on one major family of postsynaptic adhesion molecules, neuroligins, which bind to their presynaptic partner neurexin across the synaptic cleft. We move from a structural overview to the broad cellular and synaptic context of neuroligins, intermolecular interactions, and molecular modifications that occur within a synapse. Finally, we examine evidence concerning the physiological functions of neuroligin in a cell and highlight areas requiring further investigation.

Axo-glial communication through neurexin-neuroligin signaling regulates myelination and oligodendrocyte differentiation

Axonal transsynaptic signaling between presynaptic neurexin (NX) and postsynaptic neuroligin (NL) is essential for many properties of synaptic connectivity. Here, we demonstrate the existence of a parallel axo-glial signaling pathway between axonal NX and oligodendritic (OL) NL3. We show that this pathway contributes to the regulation of myelinogenesis, the maintenance of established myelination, and the differentiation state of the OL using in vitro models. We first confirm that NL3 mRNA and protein are expressed in OLs and in OL precursors. We then show that OLs in culture form contacts with non-neuronal cells exogenously expressing NL3's binding partners NX1α or NX1β. Conversely, blocking axo-glial NX-NL3 signaling by saturating NX with exogenous soluble NL protein (NL-ECD) disrupts interactions between OLs and axons in both in vitro and ex vivo assays. Myelination by OLs is tied to their differentiation state, and we find that blocking NX-NL signaling with soluble NL protein also caused OL differentiation to stall at an immature stage. Moreover, in vitro knockdown of NL3 in OLs with siRNAs stalls their development and reduces branching complexity. Interestingly, inclusion of an autism related mutation in the NL blocking protein attenuates these effects; OLs differentiate and the dynamics of OL-axon signaling occur normally as this peptide does not disrupt NX-NL3 axo-glial interactions. Our findings provide evidence not only for a new pathway in axo-glial communication, they also potentially explain the correlation between altered white matter and autism. GLIA 2015;63:2023-2039.

Quantitative isoform-profiling of highly diversified recognition molecules

Complex biological systems rely on cell surface cues that govern cellular self-recognition and selective interactions with appropriate partners. Molecular diversification of cell surface recognition molecules through DNA recombination and complex alternative splicing has emerged as an important principle for encoding such interactions. However, the lack of tools to specifically detect and quantify receptor protein isoforms is a major impediment to functional studies. We here developed a workflow for targeted mass spectrometry by selected reaction monitoring that permits quantitative assessment of highly diversified protein families. We apply this workflow to dissecting the molecular diversity of the neuronal neurexin receptors and uncover an alternative splicing-dependent recognition code for synaptic ligands.

DOI:http://dx.doi.org/10.7554/eLife.07794.001

To create a protein, a gene is first copied to form an RNA molecule that contains regions known as introns and exons. Splicing removes the introns and joins the exons together to form a molecule of ‘messenger RNA’, which is translated into a protein.

Over the course of evolution, many groups—or families—of proteins have expanded and diversified their roles. One way in which this can occur is through a process known as alternative splicing, in which different exons can be included or excluded to generate the final messenger RNA. In this way, a single gene can produce a number of different proteins. These closely related proteins are known as isoforms.

The brain contains billions of neurons that communicate with one another across connections known as synapses. A family of proteins called neurexins helps neurons to form these synapses. Humans have three neurexin genes, which undergo extensive alternative splicing to produce thousands of protein isoforms. However, it is not known whether all of these isoforms are produced in neurons, as existing experimental techniques were not sensitive enough to easily distinguish one isoform from another.

A technique known as ‘selected reaction monitoring’ (or SRM for short) has recently emerged as a promising way to identify proteins. This allows proteins containing specific sequences to be separated out for analysis, in contrast to existing techniques that test randomly selected protein samples, which will result in most isoforms being missed. Schreiner, Simicevic et al. have now developed SRM further and show that this technique can detect the identity and amount of the neurexin isoforms present at synapses, including those that are only produced in very small quantities. Using SRM, Schreiner, Simicevic et al. demonstrate that neurexin isoforms differ in how they interact with synaptic receptors. Thus, alternative splicing of neurexins underlies a ‘recognition code’ at neuronal synapses.

In the future, this newly developed SRM method could be used to investigate isoforms in other protein families and tissues, and so may prove valuable for understanding how a wide range of cellular recognition processes work.

DOI:http://dx.doi.org/10.7554/eLife.07794.002

Hippocampal neuroligin-2 links early-life stress with impaired social recognition and increased aggression in adult mice

Early-life stress is a key risk factor for the development of neuropsychiatric disorders later in life. Neuronal cell adhesion molecules have been strongly implicated in the pathophysiology of psychiatric disorders and in modulating social behaviors associated with these diseases. Neuroligin-2 is a synaptic cell adhesion molecule, located at the postsynaptic membrane of inhibitory GABAergic synapses, and is involved in synaptic stabilization and maturation. Alterations in neuroligin-2 expression have previously been associated with changes in social behavior linked to psychiatric disorders, including schizophrenia and autism. In this study, we show that early-life stress, induced by limited nesting and bedding material, leads to impaired social recognition and increased aggression in adult mice, accompanied by increased expression levels of hippocampal neuroligin-2. Viral overexpression of hippocampal neuroligin-2 in adulthood mimics early-life stress-induced alterations in social behavior and social cognition. Moreover, viral knockdown of neuroligin-2 in the adult hippocampus attenuates the early-life stress-induced behavioral changes. Our results highlight the importance of neuroligin-2 in mediating early-life stress effects on social behavior and social cognition and its promising role as a novel therapeutic target for neuropsychiatric disorders.

In vivo clonal overexpression of neuroligin 3 and neuroligin 2 in neurons of the rat cerebral cortex: Differential effects on GABAergic synapses and neuronal migration

We studied the effect of clonal overexpression of neuroligin 3 (NL3) or neuroligin 2 (NL2) in the adult rat cerebral cortex following in utero electroporation (IUEP) at embryonic stage E14. Overexpression of NL3 leads to a large increase in vesicular gamma-aminobutyric acid (GABA) transporter (vGAT) and glutamic acid decarboxylase (GAD)65 in the GABAergic contacts that the overexpressing neurons receive. Overexpression of NL2 produced a similar effect but to a lesser extent. In contrast, overexpression of NL3 or NL2 after IUEP does not affect vesicular glutamate transporter 1 (vGlut1) in the glutamatergic contacts that the NL3 or NL2-overexpressing neurons receive. The NL3 or NL2-overexpressing neurons do not show increased innervation by parvalbumin-containing GABAergic terminals or increased parvalbumin in the same terminals that show increased vGAT. These results indicate that the observed increase in vGAT and GAD65 is not due to increased GABAergic innervation but to increased expression of vGAT and GAD65 in the GABAergic contacts that NL3 or NL2-overexpressing neurons receive. The majority of bright vGAT puncta contacting the NL3-overexpressing neurons have no gephyrin juxtaposed to them, indicating that many of these contacts are nonsynaptic. This contrasts with the majority of the NL2-overexpressing neurons, which show plenty of synaptic gephyrin clusters juxtaposed to vGAT. Besides having an effect on GABAergic contacts, overexpression of NL3 interferes with the neuronal radial migration, in the cerebral cortex, of the neurons overexpressing NL3.

The interplay between synaptic activity and neuroligin function in the CNS

Neuroligins (NLs) are postsynaptic transmembrane cell-adhesion proteins that play a key role in the regulation of excitatory and inhibitory synapses. Previous in vitro and in vivo studies have suggested that NLs contribute to synapse formation and synaptic transmission. Consistent with their localization, NL1 and NL3 selectively affect excitatory synapses, whereas NL2 specifically affects inhibitory synapses. Deletions or mutations in NL genes have been found in patients with autism spectrum disorders or mental retardations, and mice harboring the reported NL deletions or mutations exhibit autism-related behaviors and synapse dysfunction. Conversely, synaptic activity can regulate the phosphorylation, expression, and cleavage of NLs, which, in turn, can influence synaptic activity. Thus, in clinical research, identifying the relationship between NLs and synapse function is critical. In this review, we primarily discuss how NLs and synaptic activity influence each other.

Analysis of splice variants for the C. elegans orthologue of human neuroligin reveals a developmentally regulated transcript

Neuroligins are synaptic adhesion molecules and important determinants of synaptic function. They are expressed at postsynaptic sites and involved in synaptic organization through key extracellular and intracellular protein interactions. They undergo trans-synaptic interaction with presynaptic neurexins. Distinct neuroligins use differences in their intracellular domains to selectively recruit synaptic scaffolds and this plays an important role in how they encode specialization of synaptic function. Several levels of regulation including gene expression, splicing, protein translation and processing regulate the expression of neuroligin function. We have used in silico and cDNA analyses to investigate the mRNA splicing of the Caenorhabditis elegans orthologue nlg-1. Transcript analysis highlights the potential for gene regulation with respect to both temporal expression and splicing. We found nlg-1 splice variants with all the predicted exons are a minor species relative to major splice variants lacking exons 13 and 14, or 14 alone. These major alternatively spliced variants change the intracellular domain of the gene product NLG-1. Interestingly, exon 14 encodes a cassette with two distinct potential functional domains. One is a polyproline SH3 binding domain and the other has homology to a region encoding the binding site for the scaffolding protein gephyrin in mammalian neuroligins. This suggests differential splicing impacts on NLG-1 competence to recruit intracellular binding partners. This may have developmental relevance as nlg-1 exon 14 containing transcripts are selectively expressed in L2-L3 larvae. These results highlight a developmental regulation of C. elegans nlg-1 that could play a key role in the assembly of synaptic protein complexes during the early stages of nervous system development.

Age-related expression of Neurexin1 and Neuroligin3 is correlated with presynaptic density in the cerebral cortex and hippocampus of male mice

Neurexin1 (Nrxn1) and Neuroligin3 (Nlgn3) are cell adhesion proteins, which play an important role in synaptic plasticity that declines with advancing age. However, the expression of these proteins during aging has not been analyzed. In the present study, we have examined the age-related changes in the expression of these proteins in cerebral cortex and hippocampus of 10-, 30-, 50-, and 80-week-old male mice. Reverse transcriptase polymerase chain reaction (RT-PCR) analysis indicated that messenger RNA (mRNA) level of Nrxn1 and Nlgn3 significantly increased from 10 to 30 weeks and then decreased at 50 weeks in both the regions. However, in 80-week-old mice, Nrxn1 and Nlgn3 were further downregulated in cerebral cortex while Nrxn1 was downregulated and Nlgn3 was upregulated in hippocampus. These findings were corroborated by immunoblotting and immunofluorescence results. When the expression of Nrxn1 and Nlgn3 was correlated with presynaptic density marker synaptophysin, it was found that synaptophysin protein expression in cerebral cortex was high at 10 weeks and decreased gradually up to 80 weeks, whereas in hippocampus, it decreased until 50 weeks and then increased remarkably at 80 weeks. Furthermore, Pearson's correlation analysis showed that synaptophysin had a strong relation with Nrxn1 and Nlgn3 in cerebral cortex and with Nlgn3 in hippocampus. Thus, these findings showed that Nrxn1 and Nlgn3 are differentially expressed in cerebral cortex and hippocampus which might be responsible for alterations in synaptic plasticity during aging.

Heterogeneity and convergence: the synaptic pathophysiology of autism

Autism is a developmental disorder characterised by a high heterogeneity of clinical diagnoses and genetic associations. This heterogeneity is a challenge for the identification of the pathophysiology of the disease and for the development of new therapeutic strategies. New conceptual approaches are being used to try to challenge this complexity and gene cluster analysis studies suggest that the pathophysiology of autism is associated with a dysregulation of specific cellular mechanisms. This review will present the experimental evidence for a convergence of synaptic pathophysiology between syndromic and non-syndromic forms of autism, grouped under the generic term of autism spectrum disorders. In particular I will highlight the results from genetic mouse models identifying a convergence of dysregulation of the synaptic type I metabotropic glutamate receptor pathway in mouse models for autism spectrum disorders. These results help to build a new conceptual framework for the study of the synaptic phenotype of autism, which is important for the identification of new therapeutic strategies.

Perinatal exposure to bisphenol-A impairs spatial memory through upregulation of neurexin1 and neuroligin3 expression in male mouse brain

Bisphenol-A (BPA), a well known endocrine disruptor, impairs learning and memory in rodents. However, the underlying molecular mechanism of BPA induced impairment in learning and memory is not well known. As synaptic plasticity is the cellular basis of memory, the present study investigated the effect of perinatal exposure to BPA on the expression of synaptic proteins neurexin1 (Nrxn1) and neuroligin3 (Nlgn3), dendritic spine density and spatial memory in postnatal male mice. The pregnant mice were orally administered BPA (50 µg/kgbw/d) from gestation day (GD) 7 to postnatal day (PND) 21 and sesame oil was used as a vehicle control. In Morris water maze (MWM) test, BPA extended the escape latency time to locate the hidden platform in 8 weeks male mice. RT-PCR and Immunoblotting results showed significant upregulation of Nrxn1 and Nlgn3 expression in both cerebral cortex and hippocampus of 3 and 8 weeks male mice. This was further substantiated by in-situ hybridization and immunofluorescence techniques. BPA also significantly increased the density of dendritic spines in both regions, as analyzed by rapid Golgi staining. Thus our data suggest that perinatal exposure to BPA impairs spatial memory through upregulation of expression of synaptic proteins Nrxn1 and Nlgn3 and increased dendritic spine density in cerebral cortex and hippocampus of postnatal male mice.

Drosophila neuroligin3 regulates neuromuscular junction development and synaptic differentiation

Neuroligins (Nlgs) are a family of cell adhesion molecules thought to be important for synapse maturation and function. Mammalian studies have shown that different Nlgs have different roles in synaptic maturation and function. In Drosophila melanogaster, the roles of Drosophila neuroligin1 (DNlg1), neuroligin2, and neuroligin4 have been examined. However, the roles of neuroligin3 (dnlg3) in synaptic development and function have not been determined. In this study, we used the Drosophila neuromuscular junctions (NMJs) as a model system to investigate the in vivo role of dnlg3. We showed that DNlg3 was expressed in both the CNS and NMJs where it was largely restricted to the postsynaptic site. We generated dnlg3 mutants and showed that these mutants exhibited an increased bouton number and reduced bouton size compared with the wild-type (WT) controls. Consistent with alterations in bouton properties, pre- and postsynaptic differentiations were affected in dnlg3 mutants. This included abnormal synaptic vesicle endocytosis, increased postsynaptic density length, and reduced GluRIIA recruitment. In addition to impaired synaptic development and differentiation, we found that synaptic transmission was reduced in dnlg3 mutants. Altogether, our data showed that DNlg3 was required for NMJ development, synaptic differentiation, and function.

A combined transgenic proteomic analysis and regulated trafficking of neuroligin-2

Synapses, the basic units of communication in the brain, require complex molecular machinery for neurotransmitter release and reception. Whereas numerous components of excitatory postsynaptic sites have been identified, relatively few proteins are known that function at inhibitory postsynaptic sites. One such component is neuroligin-2 (NL2), an inhibitory synapse-specific cell surface protein that functions in cell adhesion and synaptic organization via binding to neurexins. In this study, we used a transgenic tandem affinity purification and mass spectrometry strategy to isolate and characterize NL2-associated complexes. Complexes purified from brains of transgenic His6-FLAG-YFP-NL2 mice showed enrichment in the Gene Ontology terms cell-cell signaling and synaptic transmission relative to complexes purified from wild type mice as a negative control. In addition to expected components including GABA receptor subunits and gephyrin, several novel proteins were isolated in association with NL2. Based on the presence of multiple components involved in trafficking and endocytosis, we showed that NL2 undergoes dynamin-dependent endocytosis in response to soluble ligand and colocalizes with VPS35 retromer in endosomes. Inhibitory synapses in brain also present a particular challenge for imaging. Whereas excitatory synapses on spines can be imaged with a fluorescent cell fill, inhibitory synapses require a molecular tag. We find the His6-FLAG-YFP-NL2 to be a suitable tag, with the unamplified YFP signal localizing appropriately to inhibitory synapses in multiple brain regions including cortex, hippocampus, thalamus, and basal ganglia. Altogether, we characterize NL2-associated complexes, demonstrate regulated trafficking of NL2, and provide tools for further proteomic and imaging studies of inhibitory synapses.

Preparation of synaptic plasma membrane and postsynaptic density proteins using a discontinuous sucrose gradient

Neuronal subcellular fractionation techniques allow the quantification of proteins that are trafficked to and from the synapse. As originally described in the late 1960's, proteins associated with the synaptic plasma membrane can be isolated by ultracentrifugation on a sucrose density gradient. Once synaptic membranes are isolated, the macromolecular complex known as the post-synaptic density can be subsequently isolated due to its detergent insolubility. The techniques used to isolate synaptic plasma membranes and post-synaptic density proteins remain essentially the same after 40 years, and are widely used in current neuroscience research. This article details the fractionation of proteins associated with the synaptic plasma membrane and post-synaptic density using a discontinuous sucrose gradient. Resulting protein preparations are suitable for western blotting or 2D DIGE analysis.