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

A Systematic Structure-Function Characterization of a Human Mutation in Neurexin-3α Reveals an Extracellular Modulatory Sequence That Stabilizes Neuroligin-1 Binding to Enhance the Postsynaptic Properties of Excitatory Synapses

α-Neurexins are essential and highly expressed presynaptic cell-adhesion molecules that are frequently linked to neuropsychiatric and neurodevelopmental disorders. Despite their importance, how the elaborate extracellular sequences of α-neurexins contribute to synapse function is poorly understood. We recently characterized the presynaptic gain-of-function phenotype caused by a missense mutation in an evolutionarily conserved extracellular sequence of neurexin-3α (A687T) that we identified in a patient diagnosed with profound intellectual disability and epilepsy. The striking A687T gain-of-function mutation on neurexin-3α prompted us to systematically test using mutants whether the presynaptic gain-of-function phenotype is a consequence of the addition of side-chain bulk (i.e., A687V) or polar/hydrophilic properties (i.e., A687S). We used multidisciplinary approaches in mixed-sex primary hippocampal cultures to assess the impact of the neurexin-3αA687 residue on synapse morphology, function and ligand binding. Unexpectedly, neither A687V nor A687S recapitulated the neurexin-3α A687T phenotype. Instead, distinct from A687T, molecular replacement with A687S significantly enhanced postsynaptic properties exclusively at excitatory synapses and selectively increased binding to neuroligin-1 and neuroligin-3 without changing binding to neuroligin-2 or LRRTM2. Importantly, we provide the first experimental evidence supporting the notion that the position A687 of neurexin-3α and the N-terminal sequences of neuroligins may contribute to the stability of α-neurexin-neuroligin-1 trans-synaptic interactions and that these interactions may specifically regulate the postsynaptic strength of excitatory synapses.

Distinct functional domains of Dystroglycan regulate inhibitory synapse formation and maintenance in cerebellar Purkinje cells

Dystroglycan is a cell adhesion molecule that localizes to synapses throughout the nervous system. While Dystroglycan is required to maintain inhibitory synapses from cerebellar molecular layer interneurons (MLIs) onto Purkinje cells (PCs) whether initial synaptogenesis during development is dependent on Dystroglycan has not been examined. We show that conditional deletion of Dystroglycan from Purkinje cells prior to synaptogenesis results in impaired MLI:PC synapse formation and function due to reduced presynaptic inputs and abnormal postsynaptic GABAA receptor clustering. Using genetic manipulations that disrupt glycosylation of Dystroglycan or truncate its cytoplasmic domain, we show that Dystroglycan's role in synapse function requires both extracellular and intracellular interactions, whereas synapse formation requires only extracellular interactions. Together, these findings provide molecular insight into the mechanism of inhibitory synapse formation and maintenance in cerebellar cortex.

Astrocytic Neuroligin-3 influences gene expression and social behavior, but is dispensable for synapse number

Neuroligin-3 (Nlgn3) is an autism-associated cell-adhesion molecule that interacts with neurexins and is robustly expressed in both neurons and astrocytes. Neuronal Nlgn3 is an essential regulator of synaptic transmission but the function of astrocytic Nlgn3 is largely unknown. Given the high penetrance of Nlgn3 mutations in autism and the emerging role of astrocytes in neuropsychiatric disorders, we here asked whether astrocytic Nlgn3 might shape neural circuit properties in the cerebellum similar to neuronal Nlgn3. Imaging of tagged Nlgn3 protein produced by CRISPR/Cas9-mediated genome editing showed that Nlgn3 is enriched in the cell body but not the fine processes of cerebellar astrocytes (Bergmann glia). Astrocyte-specific knockout of Nlgn3 did not detectably alter the number of synapses, synaptic transmission, or astrocyte morphology in mouse cerebellum. However, spatial transcriptomic analyses revealed a significant shift in gene expression among multiple cerebellar cell types after the deletion of astrocytic Nlgn3. Hence, in contrast to neuronal Nlgn3, astrocytic Nlgn3 in the cerebellum is not involved in shaping synapses but may modulate gene expression in specific brain areas.

Combinatorial expression of neurexin genes regulates glomerular targeting by olfactory sensory neurons

Precise connectivity between specific neurons is essential for the formation of the complex neural circuitry necessary for executing intricate motor behaviors and higher cognitive functions. While trans -interactions between synaptic membrane proteins have emerged as crucial elements in orchestrating the assembly of these neural circuits, the synaptic surface proteins involved in neuronal wiring remain largely unknown. Here, using unbiased single-cell transcriptomic and mouse genetic approaches, we uncover that the neurexin family of genes enables olfactory sensory neuron (OSNs) axons to form appropriate synaptic connections with their mitral and tufted (M/T) cell synaptic partners, within the mammalian olfactory system. Neurexin isoforms are differentially expressed within distinct populations of OSNs, resulting in unique pattern of neurexin expression that is specific to each OSN type, and synergistically cooperate to regulate axonal innervation, guiding OSN axons to their designated glomeruli. This process is facilitated through the interactions of neurexins with their postsynaptic partners, including neuroligins, which have distinct expression patterns in M/T cells. Our findings suggest a novel mechanism underpinning the precise assembly of olfactory neural circuits, driven by the trans -interaction between neurexins and their ligands.

The developmental timing of spinal touch processing alterations predicts behavioral changes in genetic mouse models of autism spectrum disorders

Altered somatosensory reactivity is frequently observed among individuals with autism spectrum disorders (ASDs). Here, we report that although multiple mouse models of ASD exhibit aberrant somatosensory behaviors in adulthood, some models exhibit altered tactile reactivity as early as embryonic development, whereas in others, altered reactivity emerges later in life. Additionally, tactile overreactivity during neonatal development is associated with anxiety-like behaviors and social behavior deficits in adulthood, whereas tactile overreactivity that emerges later in life is not. The locus of circuit disruption dictates the timing of aberrant tactile behaviors, as altered feedback or presynaptic inhibition of peripheral mechanosensory neurons leads to abnormal tactile reactivity during neonatal development, whereas disruptions in feedforward inhibition in the spinal cord lead to touch reactivity alterations that manifest later in life. Thus, the developmental timing of aberrant touch processing can predict the manifestation of ASD-associated behaviors in mouse models, and differential timing of sensory disturbance onset may contribute to phenotypic diversity across individuals with ASD.

Distinct Alterations in Dendritic Spine Morphology in the Absence of β-Neurexins

Dendritic spines are essential for synaptic function because they constitute the postsynaptic compartment of the neurons that receives the most excitatory input. The extracellularly shorter variant of the presynaptic cell adhesion molecules neurexins, β-neurexin, has been implicated in various aspects of synaptic function, including neurotransmitter release. However, its role in developing or stabilizing dendritic spines as fundamental computational units of excitatory synapses has remained unclear. Here, we show through morphological analysis that the deletion of β-neurexins in hippocampal neurons in vitro and in hippocampal tissue in vivo affects presynaptic dense-core vesicles, as hypothesized earlier, and, unexpectedly, alters the postsynaptic spine structure. Specifically, we observed that the absence of β-neurexins led to an increase in filopodial-like protrusions in vitro and more mature mushroom-type spines in the CA1 region of adult knockout mice. In addition, the deletion of β-neurexins caused alterations in the spine head dimension and an increase in spines with perforations of their postsynaptic density but no changes in the overall number of spines or synapses. Our results indicate that presynaptic β-neurexins play a role across the synaptic cleft, possibly by aligning with postsynaptic binding partners and glutamate receptors via transsynaptic columns.

Unraveling the Role of Neuroligin3 in Autism Spectrum Disorders: Pathophysiological Insights and Targeted Therapies

Autism Spectrum Disorder is a neurodevelopmental disorder characterized by impaired social and communication skills, repetitive behaviors, and/or restricted interests with a prevalence of as high as 1% of children. Autism spectrum has strongly associated with genetic factors and exhibits wide clinical and heterogeneous genetic architecture. Most genes associated with Autism are involved in neuronal and synaptic development. The neuroligin3, the sex-linked gene on the X chromosome, was the first gene to be associated with a monogenic form of Autism. Neuroligin3 is a postsynaptic cell adhesion protein involved in synapse transmission, brain formation, and neuronal development. In this review, we provide recent findings on different mutations in the Neuroligin3 gene linked to Autism spectrum disorder and their molecular pathway effect. We also give the behavioral, and synaptic alterations reported in the Neuroligin3 animal model of Autism and the potential therapeutic strategies targeting the biological processes and the main symptoms of autism spectrum disorder. In addition, we discuss the use of novel technologies like induced pluripotent stem cells from Autistic patients that have the potential to differentiate in human neurons and therefore have a variety of applications in therapy and biomedical studies to search specific biomarkers, and develop systems for screening chemical molecules in human cells to discover target therapies.

Purkinje-cell-specific MeCP2 deficiency leads to motor deficits and autistic-like behavior due to aberrations in PTP1B-TrkB-SK signaling

Patients with Rett syndrome suffer from a loss-of-function mutation of the Mecp2 gene, which results in various symptoms including autistic traits and motor deficits. Deletion of Mecp2 in the brain mimics part of these symptoms, but the specific function of methyl-CpG-binding protein 2 (MeCP2) in the cerebellum remains to be elucidated. Here, we demonstrate that Mecp2 deletion in Purkinje cells (PCs) reduces their intrinsic excitability through a signaling pathway comprising the small-conductance calcium-activated potassium channel PTP1B and TrkB, the receptor of brain-derived neurotrophic factor. Aberration of this cascade, in turn, leads to autistic-like behaviors as well as reduced vestibulocerebellar motor learning. Interestingly, increasing activity of TrkB in PCs is sufficient to rescue PC dysfunction and abnormal motor and non-motor behaviors caused by Mecp2 deficiency. Our findings highlight how PC dysfunction may contribute to Rett syndrome, providing insight into the underlying mechanism and paving the way for rational therapeutic designs.

Multiple N-linked glycosylation sites critically modulate the synaptic abundance of neuroligin isoforms

In recent years, elegant glycomic and glycoproteomic approaches have revealed an intricate glycosylation profile of mammalian brain with enormous spatial and temporal diversities. Nevertheless, at a cellular level, it is unclear how these post-translational modifications affect various proteins to influence crucial neuronal properties. Here, we have investigated the impact of N-linked glycosylation on neuroligins (NLGNs), a class of cell-adhesion molecules that play instructive roles in synapse organization. We found that endogenous NLGN proteins are differentially glycosylated across several regions of murine brain in a sex-independent but isoform-dependent manner. In both rodent primary neurons derived from brain sections and human neurons differentiated from stem cells, all NLGN variants were highly enriched with multiple N-glycan subtypes, which cumulatively ensured their efficient trafficking to the cell surface. Removal of these N-glycosylation residues only had a moderate effect on NLGNs' stability or expression levels but particularly enhanced their retention at the endoplasmic reticulum. As a result, the glycosylation-deficient NLGNs exhibited considerable impairments in their dendritic distribution and postsynaptic accumulation, which in turn, virtually eliminated their ability to recruit presynaptic terminals and significantly reduced NLGN overexpression-induced assemblies of both glutamatergic and GABAergic synapse structures. Therefore, our results highlight an essential mechanistic contribution of N-linked glycosylations in facilitating the appropriate secretory transport of a major synaptic cell-adhesion molecule and promoting its cellular function in neurons.

Synapse organizers as molecular codes for synaptic plasticity

Synapse organizing proteins are multifaceted molecules that coordinate the complex processes of brain development and plasticity at the level of individual synapses. Their importance is demonstrated by the major brain disorders that emerge when their function is compromised. The mechanisms whereby the various families of organizers govern synapses are diverse, but converge on the structure, function, and plasticity of synapses. Therefore, synapse organizers regulate how synapses adapt to ongoing activity, a process central for determining the developmental trajectory of the brain and critical to all forms of cognition. Here, we explore how synapse organizers set the conditions for synaptic plasticity and the associated molecular events, which eventually link to behavioral features of neurodevelopmental and neuropsychiatric disorders. We also propose central questions on how synapse organizers influence network function through integrating nanoscale and circuit-level organization of the brain.

IL-4 shapes microglia-dependent pruning of the cerebellum during postnatal development

Interleukin-4 (IL-4) is a type 2 cytokine with pleiotropic functions in adaptive immunity, allergies, and cognitive processes. Here, we show that low levels of IL-4 in the early postnatal stage delineate a critical period in which microglia extensively prune cerebellar neurons. Elevating the levels of this cytokine via peripheral injection, or using a mouse model of allergic asthma, leads to defective pruning, permanent increase in cerebellar granule cells, and circuit alterations. These animals also show a hyperkinetic and impulsive-like phenotype, reminiscent of attention-deficit hyperactivity disorder (ADHD). These alterations are blocked in Il4rαfl/fl::Cx3cr1-CreER mice, which are deficient in IL-4 receptor signaling in microglia. These findings demonstrate a previously unknown role for IL-4 during a neuroimmune critical period of cerebellar maturation and provide a first putative mechanism for the comorbidity between allergic disease and ADHD observed in humans.

Organization of Purkinje cell development by neuronal MEGF11 in cerebellar granule cells

As cerebellar granule cells (GCs) coordinate the formation of regular cerebellar networks during postnatal development, molecules in GCs are expected to be involved. Here, we test the effects of the knockdown (KD) of multiple epidermal growth factor-like domains protein 11 (MEGF11), which is a homolog of proteins mediating astrocytic phagocytosis but is substantially increased at the later developmental stages of GCs on cerebellar development. MEGF11-KD in GCs of developing mice results in abnormal cerebellar structures, including extensively ectopic Purkinje cell (PC) somas, and in impaired motor functions. MEGF11-KD also causes abnormally asynchronous synaptic release from GC axons, parallel fibers, before the appearance of abnormal cerebellar structures. Interestingly, blockade of this abnormal synaptic release restores most of the cerebellar structures. Thus, apart from phagocytic functions of its related homologs in astrocytes, MEGF11 in GCs promotes proper PC development and cerebellar network formation by regulating immature synaptic transmission.

Neurexin-3 subsynaptic densities are spatially distinct from Neurexin-1 and essential for excitatory synapse nanoscale organization in the hippocampus

Proteins critical for synaptic transmission are non-uniformly distributed and assembled into regions of high density called subsynaptic densities (SSDs) that transsynaptically align in nanocolumns. Neurexin-1 and neurexin-3 are essential presynaptic adhesion molecules that non-redundantly control NMDAR- and AMPAR-mediated synaptic transmission, respectively, via transsynaptic interactions with distinct postsynaptic ligands. Despite their functional relevance, fundamental questions regarding the nanoscale properties of individual neurexins, their influence on the subsynaptic organization of excitatory synapses and the mechanisms controlling how individual neurexins engage in precise transsynaptic interactions are unknown. Using Double Helix 3D dSTORM and neurexin mouse models, we identify neurexin-3 as a critical presynaptic adhesion molecule that regulates excitatory synapse nano-organization in hippocampus. Furthermore, endogenous neurexin-1 and neurexin-3 form discrete and non-overlapping SSDs that are enriched opposite their postsynaptic ligands. Thus, the nanoscale organization of neurexin-1 and neurexin-3 may explain how individual neurexins signal in parallel to govern different synaptic properties.

Overexpression of the autism candidate gene Cyfip1 pathologically enhances olivo-cerebellar signaling in mice

Cyfip1, the gene encoding cytoplasmic FMR1 interacting protein 1, has been of interest as an autism candidate gene for years. A potential role in autism spectrum disorder (ASD) is suggested by its location on human chromosome 15q11-13, an instable region that gives rise to a variety of copy number variations associated with syndromic autism. In addition, the CYFIP1 protein acts as a binding partner to Fragile X Messenger Ribonucleoprotein (FMRP) in the regulation of translation initiation. Mutation of FMR1, the gene encoding FMRP, causes Fragile X syndrome, another form of syndromic autism. Here, in mice overexpressing CYFIP1, we study response properties of cerebellar Purkinje cells to activity of the climbing fiber input that originates from the inferior olive and provides an instructive signal in sensorimotor input analysis and plasticity. We find that CYFIP1 overexpression results in enhanced localization of the synaptic organizer neurexin 1 (NRXN1) at climbing fiber synaptic input sites on Purkinje cell primary dendrites and concomitant enhanced climbing fiber synaptic transmission (CF-EPSCs) measured using whole-cell patch-clamp recordings from Purkinje cells in vitro. Moreover, using two-photon measurements of GCaMP6f-encoded climbing fiber signals in Purkinje cells of intact mice, we observe enhanced responses to air puff stimuli applied to the whisker field. These findings resemble our previous phenotypic observations in a mouse model for the human 15q11-13 duplication, which does not extend to the Cyfip1 locus. Thus, our study demonstrates that CYFIP1 overexpression shares a limited set of olivo-cerebellar phenotypes as those resulting from an increased number of copies of non-overlapping genes located on chromosome 15q11-13.

Conditional deletion of neurexins dysregulates neurotransmission from dopamine neurons

Midbrain dopamine (DA) neurons are key regulators of basal ganglia functions. The axonal domain of these neurons is highly complex, with a large subset of non-synaptic release sites and a smaller subset of synaptic terminals from which in addition to DA, glutamate or GABA are also released. The molecular mechanisms regulating the connectivity of DA neurons and their neurochemical identity are unknown. An emerging literature suggests that neuroligins, trans-synaptic cell adhesion molecules, regulate both DA neuron connectivity and neurotransmission. However, the contribution of their major interaction partners, neurexins (Nrxns), is unexplored. Here, we tested the hypothesis that Nrxns regulate DA neuron neurotransmission. Mice with conditional deletion of all Nrxns in DA neurons (DAT::NrxnsKO) exhibited normal basic motor functions. However, they showed an impaired locomotor response to the psychostimulant amphetamine. In line with an alteration in DA neurotransmission, decreased levels of the membrane DA transporter (DAT) and increased levels of the vesicular monoamine transporter (VMAT2) were detected in the striatum of DAT::NrxnsKO mice, along with reduced activity-dependent DA release. Strikingly, electrophysiological recordings revealed an increase of GABA co-release from DA neuron axons in the striatum of these mice. Together, these findings suggest that Nrxns act as regulators of the functional connectivity of DA neurons.

The developmental timing of spinal touch processing alterations and its relation to ASD-associated behaviors in mouse models

Altered somatosensory reactivity is frequently observed among individuals with autism spectrum disorders (ASDs). Here, we report that while multiple mouse models of ASD exhibit aberrant somatosensory behaviors in adulthood, some models exhibit altered tactile reactivity as early as embryonic development, while in others, altered reactivity emerges later in life. Additionally, tactile over-reactivity during neonatal development is associated with anxiety-like behaviors and social interaction deficits in adulthood, whereas tactile over-reactivity that emerges later in life is not. The locus of circuit disruption dictates the timing of aberrant tactile behaviors: altered feedback or presynaptic inhibition of peripheral mechanosensory neurons leads to abnormal tactile reactivity during neonatal development, while disruptions in feedforward inhibition in the spinal cord lead to touch reactivity alterations that manifest later in life. Thus, the developmental timing of aberrant touch processing can predict the manifestation of ASD-associated behaviors in mouse models, and differential timing of sensory disturbance onset may contribute to phenotypic diversity across individuals with ASD.

Critical periods and Autism Spectrum Disorders, a role for sleep

Brain development relies on both experience and genetically defined programs. Time windows where certain brain circuits are particularly receptive to external stimuli, resulting in heightened plasticity, are referred to as "critical periods". Sleep is thought to be essential for normal brain development. Importantly, studies have shown that sleep enhances critical period plasticity and promotes experience-dependent synaptic pruning in the developing mammalian brain. Therefore, normal plasticity during critical periods depends on sleep. Problems falling and staying asleep occur at a higher rate in Autism Spectrum Disorder (ASD) relative to typical development. In this review, we explore the potential link between sleep, critical period plasticity, and ASD. First, we review the importance of critical period plasticity in typical development and the role of sleep in this process. Next, we summarize the evidence linking ASD with deficits in synaptic plasticity in rodent models of high-confidence ASD gene candidates. We then show that the high-confidence rodent models of ASD that show sleep deficits also display plasticity deficits. Given how important sleep is for critical period plasticity, it is essential to understand the connections between synaptic plasticity, sleep, and brain development in ASD. However, studies investigating sleep or plasticity during critical periods in ASD mouse models are lacking. Therefore, we highlight an urgent need to consider developmental trajectory in studies of sleep and plasticity in neurodevelopmental disorders.

Astrocytic Neuroligins Are Not Required for Synapse Formation or a Normal Astrocyte Cytoarchitecture

Astrocytes exert multifarious roles in the formation, regulation, and function of synapses in the brain, but the mechanisms involved remain unclear. Interestingly, astrocytes abundantly express neuroligins, postsynaptic adhesion molecules that bind to presynaptic neurexins. A pioneering recent study reported that loss-of-function of neuroligins in astrocytes impairs excitatory synapse formation and astrocyte morphogenesis. This study suggested a crucial synaptic function for astrocytic neuroligins but was puzzling given that constitutive neuroligin deletions do not decrease excitatory synapse numbers. Thus, we here examined the function of astrocytic neuroligins using a rigorous conditional genetic approach with deletion of all major neuroligins (Nlgn1-3) in astrocytes. Our results show that early postnatal deletion of neuroligins from astrocytes has no effect on cortical or hippocampal synapses and does not alter the cytoarchitecture of astrocytes. Thus, astrocytic neuroligins are unlikely to shape synapse formation or astrocyte development but may perform other important functions in astrocytes.

Neuroligin-2 controls the establishment of fast GABAergic transmission in adult-born granule cells

GABAergic inhibition is critical for the precision of neuronal spiking and the homeostatic regulation of network activity in the brain. Adult neurogenesis challenges network homeostasis because new granule cells (GCs) integrate continuously in the functional dentate gyrus. While developing, adult-born GCs undergo a transient state of enhanced excitability due to the delayed maturation of perisomatic GABAergic inhibition by parvalbumin interneurons (PV-INs). The mechanisms underlying this delayed synaptic maturation remain unknown. We examined the morphology and function of synapses formed by PV-INs onto new GCs over a 2-month interval in young adult mice, and investigated the influence of the synaptic adhesion molecule neuroligin-2 (NL2). Perisomatic appositions of PV-IN terminals onto new GCs were conspicuous at 2 weeks and continued to grow in size to reach a plateau over the fourth week. Postsynaptic knockdown of NL2 by expression of a short-hairpin RNA (shNL2) in new GCs resulted in smaller size of synaptic contacts, reduced area of perisomatic appositions of the vesicular GABA transporter VGAT, and the number of presynaptic active sites. GCs expressing shNL2 displayed spontaneous GABAergic responses with decreased frequency and amplitude, as well as slower kinetics compared to control GCs. In addition, postsynaptic responses evoked by optogenetic stimulation of PV-INs exhibited slow kinetics, increased paired-pulse ratio and coefficient of variation in GCs with NL2 knockdown, suggesting a reduction in the number of active synapses as well as in the probability of neurotransmitter release (Pr ). Our results demonstrate that synapses formed by PV-INs on adult-born GCs continue to develop beyond the point of anatomical growth, and require NL2 for the structural and functional maturation that accompanies the conversion into fast GABAergic transmission.

Regulation of Expression of Extracellular Matrix Proteins by Differential Target Multiplexed Spinal Cord Stimulation (SCS) and Traditional Low-Rate SCS in a Rat Nerve Injury Model

There is limited research on the association between the extracellular matrix (ECM) and chronic neuropathic pain. The objective of this study was twofold. Firstly, we aimed to assess changes in expression levels and the phosphorylation of ECM-related proteins due to the spared nerve injury (SNI) model of neuropathic pain. Secondly, two modalities of spinal cord stimulation (SCS) were compared for their ability to reverse the changes induced by the pain model back toward normal, non-injury levels. We identified 186 proteins as ECM-related and as having significant changes in protein expression among at least one of the four experimental groups. Of the two SCS treatments, the differential target multiplexed programming (DTMP) approach reversed expression levels of 83% of proteins affected by the pain model back to levels seen in uninjured animals, whereas a low-rate (LR-SCS) approach reversed 67%. There were 93 ECM-related proteins identified in the phosphoproteomic dataset, having a combined 883 phosphorylated isoforms. DTMP back-regulated 76% of phosphoproteins affected by the pain model back toward levels found in uninjured animals, whereas LR-SCS back-regulated 58%. This study expands our knowledge of ECM-related proteins responding to a neuropathic pain model as well as providing a better perspective on the mechanism of action of SCS therapy.

Engineered adhesion molecules drive synapse organization

In multicellular organisms, cell-adhesion molecules connect cells into tissues and mediate intercellular signaling between these cells. In vertebrate brains, synaptic cell-adhesion molecules (SAMs) guide the formation, specification, and plasticity of synapses. Some SAMs, when overexpressed in cultured neurons or in heterologous cells co-cultured with neurons, drive formation of synaptic specializations onto the overexpressing cells. However, genetic deletion of the same SAMs from neurons often has no effect on synapse numbers, but frequently severely impairs synaptic transmission, suggesting that most SAMs control the function and plasticity of synapses (i.e., organize synapses) instead of driving their initial establishment (i.e., make synapses). Since few SAMs were identified that mediate initial synapse formation, it is difficult to develop methods that enable experimental control of synaptic connections by targeted expression of these SAMs. To overcome this difficulty, we engineered novel SAMs from bacterial proteins with no eukaryotic homologues that drive synapse formation. We named these engineered adhesion proteins "Barnoligin" and "Starexin" because they were assembled from parts of Barnase and Neuroligin-1 or of Barstar and Neurexin3β, respectively. Barnoligin and Starexin robustly induce the formation of synaptic specializations in a specific and directional manner in cultured neurons. Synapse formation by Barnoligin and Starexin requires both their extracellular Barnase- and Barstar-derived interaction domains and their Neuroligin- and Neurexin-derived intracellular signaling domains. Our findings support a model of synapse formation whereby trans-synaptic interactions by SAMs drive synapse organization via adhesive interactions that activate signaling cascades.

Neurexins and their ligands at inhibitory synapses

Since the discovery of neurexins (Nrxns) as essential and evolutionarily conserved synaptic adhesion molecules, focus has largely centered on their functional contributions to glutamatergic synapses. Recently, significant advances to our understanding of neurexin function at GABAergic synapses have revealed that neurexins can play pleiotropic roles in regulating inhibitory synapse maintenance and function in a brain-region and synapse-specific manner. GABAergic neurons are incredibly diverse, exhibiting distinct synaptic properties, sites of innervation, neuromodulation, and plasticity. Different classes of GABAergic neurons often express distinct repertoires of Nrxn isoforms that exhibit differential alternative exon usage. Further, Nrxn ligands can be differentially expressed and can display synapse-specific localization patterns, which may contribute to the formation of a complex trans-synaptic molecular code that establishes the properties of inhibitory synapse function and properties of local circuitry. In this review, we will discuss how Nrxns and their ligands sculpt synaptic inhibition in a brain-region, cell-type and synapse-specific manner.

NMDA receptor hypofunction underlies deficits in parvalbumin interneurons and social behavior in neuroligin 3 R451C knockin mice

Neuroligins (NLs), a family of postsynaptic cell-adhesion molecules, have been associated with autism spectrum disorder. We have reported that dysfunction of the medial prefrontal cortex (mPFC) leads to social deficits in an NL3 R451C knockin (KI) mouse model of autism. However, the underlying molecular mechanism remains unclear. Here, we find that N-methyl-D-aspartate receptor (NMDAR) function and parvalbumin-positive (PV+) interneuron number and expression are reduced in the mPFC of the KI mice. Selective knockdown of NMDAR subunit GluN1 in the mPFC PV+ interneuron decreases its intrinsic excitability. Restoring NMDAR function by its partial agonist D-cycloserine rescues the PV+ interneuron dysfunction and social deficits in the KI mice. Interestingly, early D-cycloserine administration at adolescence prevents adult KI mice from social deficits. Together, our results suggest that NMDAR hypofunction and the resultant PV+ interneuron dysfunction in the mPFC may constitute a central node in the pathogenesis of social deficits in the KI mice.

Computational models of neurotransmission at cerebellar synapses unveil the impact on network computation

The neuroscientific field benefits from the conjoint evolution of experimental and computational techniques, allowing for the reconstruction and simulation of complex models of neurons and synapses. Chemical synapses are characterized by presynaptic vesicle cycling, neurotransmitter diffusion, and postsynaptic receptor activation, which eventually lead to postsynaptic currents and subsequent membrane potential changes. These mechanisms have been accurately modeled for different synapses and receptor types (AMPA, NMDA, and GABA) of the cerebellar cortical network, allowing simulation of their impact on computation. Of special relevance is short-term synaptic plasticity, which generates spatiotemporal filtering in local microcircuits and controls burst transmission and information flow through the network. Here, we present how data-driven computational models recapitulate the properties of neurotransmission at cerebellar synapses. The simulation of microcircuit models is starting to reveal how diverse synaptic mechanisms shape the spatiotemporal profiles of circuit activity and computation.

Distinct neurexin-cerebellin complexes control AMPA- and NMDA-receptor responses in a circuit-dependent manner

At CA1→subiculum synapses, alternatively spliced neurexin-1 (Nrxn1SS4+) and neurexin-3 (Nrxn3SS4+) enhance NMDA-receptors and suppress AMPA-receptors, respectively, without affecting synapse formation. Nrxn1SS4+ and Nrxn3SS4+ act by binding to secreted cerebellin-2 (Cbln2) that in turn activates postsynaptic GluD1 receptors. Whether neurexin-Cbln2-GluD1 signaling has additional functions besides regulating NMDA- and AMPA-receptors, and whether such signaling performs similar roles at other synapses, however, remains unknown. Here, we demonstrate using constitutive Cbln2 deletions in mice that at CA1→subiculum synapses, Cbln2 performs no additional developmental roles besides regulating AMPA- and NMDA-receptors. Moreover, low-level expression of functionally redundant Cbln1 did not compensate for a possible synapse-formation function of Cbln2 at CA1→subiculum synapses. In exploring the generality of these findings, we examined the prefrontal cortex where Cbln2 was recently implicated in spinogenesis, and the cerebellum where Cbln1 is known to regulate parallel-fiber synapses. In the prefrontal cortex, Nrxn1SS4+-Cbln2 signaling selectively controlled NMDA-receptors without affecting spine or synapse numbers, whereas Nrxn3SS4+-Cbln2 signaling had no apparent role. In the cerebellum, conversely, Nrxn3SS4+-Cbln1 signaling regulated AMPA-receptors, whereas now Nrxn1SS4+-Cbln1 signaling had no manifest effect. Thus, Nrxn1SS4+- and Nrxn3SS4+-Cbln1/2 signaling complexes differentially control NMDA- and AMPA-receptors in different synapses in diverse neural circuits without regulating synapse or spine formation.

Sensory Over-responsivity and Aberrant Plasticity in Cerebellar Cortex in a Mouse Model of Syndromic Autism

Background

Patients with autism spectrum disorder often show altered responses to sensory stimuli as well as motor deficits, including an impairment of delay eyeblink conditioning, which involves integration of sensory signals in the cerebellum. Here, we identify abnormalities in parallel fiber (PF) and climbing fiber (CF) signaling in the mouse cerebellar cortex that may contribute to these pathologies.

Methods

We used a mouse model for the human 15q11-13 duplication (patDp/+) and studied responses to sensory stimuli in Purkinje cells from awake mice using two-photon imaging of GCaMP6f signals. Moreover, we examined synaptic transmission and plasticity using in vitro electrophysiological, immunohistochemical, and confocal microscopic techniques.

Results

We found that spontaneous and sensory-evoked CF-calcium transients are enhanced in patDp/+ Purkinje cells, and aversive movements are more severe across sensory modalities. We observed increased expression of the synaptic organizer NRXN1 at CF synapses and ectopic spread of these synapses to fine dendrites. CF-excitatory postsynaptic currents recorded from Purkinje cells are enlarged in patDp/+ mice, while responses to PF stimulation are reduced. Confocal measurements show reduced PF+CF-evoked spine calcium transients, a key trigger for PF long-term depression, one of several plasticity types required for eyeblink conditioning learning. Long-term depression is impaired in patDp/+ mice but is rescued on pharmacological enhancement of calcium signaling.

Conclusions

Our findings suggest that this genetic abnormality causes a pathological inflation of CF signaling, possibly resulting from enhanced NRXN1 expression, with consequences for the representation of sensory stimuli by the CF input and for PF synaptic organization and plasticity.

Cochlear ribbon synapse maturation requires Nlgn1 and Nlgn3

Hearing depends on precise synaptic transmission between cochlear inner hair cells and spiral ganglion neurons through afferent ribbon synapses. Neuroligins (Nlgns) facilitate synapse maturation in the brain, but they have gone unstudied in the cochlea. We report Nlgn3 and Nlgn1 knockout (KO) cochleae have fewer ribbon synapses and have impaired hearing. Nlgn3 KO is more vulnerable to noise trauma with limited activity at high frequencies one day after noise. Furthermore, Nlgn3 KO cochleae have a 5-fold reduction in synapse number compared to wild type after two weeks of recovery. Double KO cochlear phenotypes are more prominent than the KOs, for example, 5-fold smaller synapses, 25% reduction in synapse density, and 30% less synaptic output. These observations indicate Nlgn3 and Nlgn1 are essential to cochlear ribbon synapse maturation and function.

In vivo nanoscopic landscape of neurexin ligands underlying anterograde synapse specification

Excitatory synapses are formed and matured by the cooperative actions of synaptic organizers, such as neurexins (Nrxns), neuroligins (Nlgns), LRRTMs, and Cbln1. Recent super-resolution nanoscopy developments have revealed that many synaptic organizers, as well as glutamate receptors and glutamate release machinery, exist as nanoclusters within synapses. However, it is unclear how such nanodomains interact with each other to organize excitatory synapses in vivo. By applying X10 expansion microscopy to epitope tag knockin mice, we found that Cbln1, Nlgn1, and LRRTM1, which share Nrxn as a common presynaptic receptor, form overlapping or separate nanodomains depending on Nrxn with or without a sequence encoded by splice site 4. The size and position of glutamate receptor nanodomains of GluD1, NMDA, and AMPA receptors were regulated by Cbln1, Nlgn1, and LRRTM1 nanodomains, respectively. These findings indicate that Nrxns anterogradely regulate the postsynaptic nanoscopic architecture of glutamate receptors through competition and coordination of Nrxn ligands.

Neuroligin-3 confines AMPA receptors into nanoclusters, thereby controlling synaptic strength at the calyx of Held synapses

The subsynaptic organization of postsynaptic neurotransmitter receptors into nanoclusters that are aligned with presynaptic release sites is essential for the high fidelity of synaptic transmission. However, the mechanisms controlling the nanoscale organization of neurotransmitter receptors in vivo remain incompletely understood. Here, we deconstructed the role of neuroligin-3 (Nlgn3), a postsynaptic adhesion molecule linked to autism, in organizing AMPA-type glutamate receptors in the calyx of Held synapse. Deletion of Nlgn3 lowered the amplitude and slowed the kinetics of AMPA receptor-mediated synaptic responses. Super-resolution microscopy revealed that, unexpectedly, these impairments in synaptic transmission were associated with an increase in the size of postsynaptic PSD-95 and AMPA receptor nanoclusters but a decrease of the densities in these clusters. Modeling showed that a dilution of AMPA receptors into larger nanocluster volumes decreases synaptic strength. Nlgn3, likely by binding to presynaptic neurexins, thus is a key organizer of AMPA receptor nanoclusters that likely acts via PSD-95 adaptors to optimize the fidelity of synaptic transmission.

Regulation of Inhibitory Signaling at the Receptor and Cellular Level; Advances in Our Understanding of GABAergic Neurotransmission and the Mechanisms by Which It Is Disrupted in Epilepsy

Inhibitory signaling in the brain organizes the neural circuits that orchestrate how living creatures interact with the world around them and how they build representations of objects and ideas. Without tight control at multiple points of cellular engagement, the brain’s inhibitory systems would run down and the ability to extract meaningful information from excitatory events would be lost leaving behind a system vulnerable to seizures and to cognitive decline. In this review, we will cover many of the salient features that have emerged regarding the dynamic regulation of inhibitory signaling seen through the lens of cell biology with an emphasis on the major building blocks, the ligand-gated ion channel receptors that are the first transduction point when the neurotransmitter GABA is released into the synapse. Epilepsy association will be used to indicate importance of key proteins and their pathways to brain function and to introduce novel areas for therapeutic intervention.

Modified climbing fiber/Purkinje cell synaptic connectivity in the cerebellum of the neonatal phencyclidine model of schizophrenia

Synaptogenesis and neural network remodeling are at their maximum during the perinatal period of human brain development. Perturbations of this highly sensitive stage might underlie the etiology of neurodevelopmental disorders. Subchronic neonatal administration of phencyclidine, a drug of abuse, has been used to model schizophrenia in rodents. In this model, we found specific long-term synaptic changes in Purkinje cells and transient gene expression changes in the cerebellum. While transient increased neuronal activity in the cerebellum, induced using chemogenetics, reproduces some phencyclidine-induced molecular changes, it is insufficient to reproduce the long-term synaptic effects. Our results show the complex mechanism of action of phencyclidine on the development of neuronal connectivity and further highlight the potential contribution of cerebellar defects in psychiatric diseases.

Environmental perturbations during the first years of life are a major factor in psychiatric diseases. Phencyclidine (PCP), a drug of abuse, has psychomimetic effects, and neonatal subchronic administration of PCP in rodents leads to long-term behavioral changes relevant for schizophrenia. The cerebellum is increasingly recognized for its role in diverse cognitive functions. However, little is known about potential cerebellar changes in models of schizophrenia. Here, we analyzed the characteristics of the cerebellum in the neonatal subchronic PCP model. We found that, while the global cerebellar cytoarchitecture and Purkinje cell spontaneous spiking properties are unchanged, climbing fiber/Purkinje cell synaptic connectivity is increased in juvenile mice. Neonatal subchronic administration of PCP is accompanied by increased cFos expression, a marker of neuronal activity, and transient modification of the neuronal surfaceome in the cerebellum. The largest change observed is the overexpression of Ctgf, a gene previously suggested as a biomarker for schizophrenia. This neonatal increase in Ctgf can be reproduced by increasing neuronal activity in the cerebellum during the second postnatal week using chemogenetics. However, it does not lead to increased climbing fiber/Purkinje cell connectivity in juvenile mice, showing the complexity of PCP action. Overall, our study shows that administration of the drug of abuse PCP during the developmental period of intense cerebellar synaptogenesis and circuit remodeling has long-term and specific effects on Purkinje cell connectivity and warrants the search for this type of synaptic changes in psychiatric diseases.

Deletion of Calsyntenin-3, an atypical cadherin, suppresses inhibitory synapses but increases excitatory parallel-fiber synapses in cerebellum

Cadherins contribute to the organization of nearly all tissues, but the functions of several evolutionarily conserved cadherins, including those of calsyntenins, remain enigmatic. Puzzlingly, two distinct, non-overlapping functions for calsyntenins were proposed: As postsynaptic neurexin ligands in synapse formation, or as presynaptic kinesin adaptors in vesicular transport. Here, we show that, surprisingly, acute CRISPR-mediated deletion of calsyntenin-3 in mouse cerebellum in vivo causes a large decrease in inhibitory synapse, but a robust increase in excitatory parallel-fiber synapses in Purkinje cells. As a result, inhibitory synaptic transmission was suppressed, whereas parallel-fiber synaptic transmission was enhanced in Purkinje cells by the calsyntenin-3 deletion. No changes in the dendritic architecture of Purkinje cells or in climbing-fiber synapses were detected. Sparse selective deletion of calsyntenin-3 only in Purkinje cells recapitulated the synaptic phenotype, indicating that calsyntenin-3 acts by a cell-autonomous postsynaptic mechanism in cerebellum. Thus, by inhibiting formation of excitatory parallel-fiber synapses and promoting formation of inhibitory synapses in the same neuron, calsyntenin-3 functions as a postsynaptic adhesion molecule that regulates the excitatory/inhibitory balance in Purkinje cells.

Structure, function, and pathology of Neurexin-3

Neurexin-3 is primarily localized in the presynaptic membrane and forms complexes with various ligands located in the postsynaptic membrane. Neurexin-3 has important roles in synapse development and synapse functions. Neurexin-3 mediates excitatory presynaptic differentiation by interacting with leucine-rich-repeat transmembrane neuronal proteins. Meanwhile, neurexin-3 modulates the expression of presynaptic α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors and γ-aminobutyric acid A receptors by interacting with neuroligins at excitatory and inhibitory synapses. Numerous studies have documented the potential contribution of neurexin-3 to neurodegenerative and neuropsychiatric disorders, such as Alzheimer's disease, addiction behaviors, and other diseases, which raises hopes that understanding the mechanisms of neurexin-3 may hold the key to developing new strategies for related illnesses. This review comprehensively covers the literature to provide current knowledge of the structure, function, and clinical role of neurexin-3.

The Cerebellar Involvement in Autism Spectrum Disorders: From the Social Brain to Mouse Models

Autism spectrum disorders (ASD) are pervasive neurodevelopmental disorders that include a variety of forms and clinical phenotypes. This heterogeneity complicates the clinical and experimental approaches to ASD etiology and pathophysiology. To date, a unifying theory of these diseases is still missing. Nevertheless, the intense work of researchers and clinicians in the last decades has identified some ASD hallmarks and the primary brain areas involved. Not surprisingly, the areas that are part of the so-called “social brain”, and those strictly connected to them, were found to be crucial, such as the prefrontal cortex, amygdala, hippocampus, limbic system, and dopaminergic pathways. With the recent acknowledgment of the cerebellar contribution to cognitive functions and the social brain, its involvement in ASD has become unmistakable, though its extent is still to be elucidated. In most cases, significant advances were made possible by recent technological developments in structural/functional assessment of the human brain and by using mouse models of ASD. Mouse models are an invaluable tool to get insights into the molecular and cellular counterparts of the disease, acting on the specific genetic background generating ASD-like phenotype. Given the multifaceted nature of ASD and related studies, it is often difficult to navigate the literature and limit the huge content to specific questions. This review fulfills the need for an organized, clear, and state-of-the-art perspective on cerebellar involvement in ASD, from its connections to the social brain areas (which are the primary sites of ASD impairments) to the use of monogenic mouse models.

Mechanisms and Consequences of Cerebellar Purkinje Cell Disinhibition in a Mouse Model of Duchenne Muscular Dystrophy

Duchenne muscular dystrophy (DMD), the most common form of childhood muscular dystrophy, is caused by mutations in the dystrophin gene. In addition to debilitating muscle degeneration, patients display a range of cognitive deficits thought to result from the loss of dystrophin normally expressed in the brain. While the function of dystrophin in muscle tissue is well characterized, its role in the brain is still poorly understood. The highest expression of dystrophin in the mouse brain is in cerebellar Purkinje cells (PCs), where it colocalizes with GABAA receptor clusters. Using ex vivo electrophysiological recordings from connected molecular layer interneuron (MLI)-PC pairs, we investigated changes in inhibitory synaptic transmission caused by dystrophin deficiency. In male mdx mice (which lack long-form dystrophin), we found that responses at MLI-PC pairs were reduced by ∼60% because of both decreased quantal response amplitude and a reduced number of functional vesicle release sites. Using electron microscopy, we found significantly fewer and smaller anatomically defined inhibitory synapses contacting the soma of PCs in mdx mice, suggesting that dystrophin may play a critical role in synapse formation and/or maintenance. Functionally, we found reduced MLI-evoked pauses in PC firing in acute slices. In vivo recordings from awake mdx mice showed increased sensory-evoked simple spike firing in positively modulating PCs, consistent with reduced feedforward inhibition, but no change in negatively modulating PCs. These data suggest that dystrophin deficiency in PCs disrupts inhibitory signaling in the cerebellar circuit and PC firing patterns, potentially contributing to cognitive and motor deficits observed in mdx mice and DMD patients.SIGNIFICANCE STATEMENT Duchenne muscular dystrophy (DMD) is primarily characterized by progressive muscle weakening caused by genetic mutations in the gene for dystrophin. Dystrophin is also normally expressed in the CNS, and DMD patients experience a range of nonprogressive cognitive deficits. The pathophysiology of CNS neurons resulting from loss of dystrophin and the function of dystrophin in neurons are still poorly understood. Using cerebellar PCs as a model, we found that the loss of dystrophin specifically disrupts the number and strength of inhibitory synaptic connections, suggesting that dystrophin participates in formation and/or maintenance of these synapses. This work provides insight into the function of dystrophin in the CNS and establishes neuronal and synaptic dysfunction, which may underlie cognitive deficits in DMD.

Molecular self-avoidance in synaptic neurexin complexes

Synapses are thought to be organized by interactions of presynaptic neurexins with postsynaptic ligands, particularly with neuroligins and cerebellins. However, when a neuron forms adjacent pre- and postsynaptic specializations, as in dendrodendritic or axo-axonic synapses, nonfunctional cis neurexin/ligand interactions would be energetically favored. Here, we reveal an organizational principle for preventing synaptic cis interactions (“self-avoidance”). Using dendrodendritic synapses between mitral and granule cells in the olfactory bulb as a paradigm, we show that, owing to its higher binding affinity, cerebellin-1 blocks the cis interaction of neurexins with neuroligins, thereby enabling trans neurexin/neuroligin interaction. In mitral cells, ablating either cerebellin-1 or neuroligins severely impaired granule cell➔mitral cell synapses, as did overexpression of wild-type neurexins but not of mutant neurexins unable to bind to neuroligins. Our data uncover a molecular interaction network that organizes the self-avoidance of nonfunctional neurexin/ligand cis interactions, thus allowing assembly of physiological trans interactions.

Neurexin-3 defines synapse- and sex-dependent diversity of GABAergic inhibition in ventral subiculum

Ventral subiculum (vSUB) is integral to the regulation of stress and reward; however, the intrinsic connectivity and synaptic properties of the inhibitory local circuit are poorly understood. Neurexin-3 (Nrxn3) is highly expressed in hippocampal inhibitory neurons, but its function at inhibitory synapses has remained elusive. Using slice electrophysiology, imaging, and single-cell RNA sequencing, we identify multiple roles for Nrxn3 at GABAergic parvalbumin (PV) interneuron synapses made onto vSUB regular-spiking (RS) and burst-spiking (BS) principal neurons. Surprisingly, we find that intrinsic connectivity of vSUB and synaptic function of Nrxn3 in vSUB are sexually dimorphic. We reveal that PVs make preferential contact with RS neurons in male mice, but BS neurons in female mice. Furthermore, we determine that despite comparable Nrxn3 isoform expression in male and female PV neurons, Nrxn3 knockout impairs synapse density, postsynaptic strength, and inhibitory postsynaptic current (IPSC) amplitude at PV-RS synapses in males, but enhances presynaptic release and IPSC amplitude in females.

Social behavior in prepubertal neurexin 1α deficient rats: A model of neurodevelopmental disorders

Loss-of-function mutations in the synaptic protein neurexin1α (NRXN1α) are associated with several neurodevelopmental disorders, including autism spectrum disorder (ASD), schizophrenia, and attention-deficit hyperactivity disorder (ADHD), and many of these disorders are defined by core deficits in social cognition. Mouse models of Nrxn1α deficiency are not amenable to studying aspects of social cognition because, in general, mice do not engage in complex social interactions such as social play or prosocial helping behaviors. Rats, on the contrary, engage in these complex, well-characterized social behaviors. Using the Nrxn1tm1Sage Sprague Dawley rat, we tested a range of cognitive and social behaviors in juveniles with haplo- or biallelic Nrxn1α mutation. We found a deficit in ultrasonic vocalizations (USVs) of male and female neonatal rats with Nrxn1α deficiency. A male-specific deficit in social play was observed in Nrxn1α-deficient juveniles, although sociability and social discrimination were unaltered. Nurturing behavior induced by exposure to pups was enhanced in male and female juveniles with biallelic Nrxn1α mutation. Performance in tasks of prosocial helping behavior and food retrieval indicated severe deficits in learning and cognition in juveniles with biallelic Nrxn1α mutation, and a less severe deficit in haploinsufficient rats, although Pavlovian learning was altered only in haploinsufficient males. We also observed a male-specific increase in mobility and object investigation in juveniles with complete Nrxn1α deficiency. Together, these observations more fully characterize the Nrxn1tm1Sage Sprague Dawley rat as a model for Nrxn1α-related neurodevelopmental disorders, and support a rationale for the juvenile rat as a more appropriate model for disorders that involve core deficits in complex social behaviors. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

Clinical Relevance and Prognostic Value of the Neuronal Protein Neuroligin 2 in Breast Cancer

Neuroligin 2 (NLGN2) is a well-recognized transmembrane scaffolding protein that functions in synapse development and neuronal signal transduction. It has recently been implicated in multiple diseases of peripheral ectodermal origin. However, the potential roles of NLGN2 in tumors remain ill-defined. The aim of this study was to determine the clinical relevance and prognostic value of NLGN2 in breast cancer. To this end, breast cancer datasets were extracted from TCGA and other public databases, and subjected to Kaplan-Meier potter for survival analysis, GEPIA2 for assessing the immunological relevance of NLGN2 and THPA for identifying its subcellular localization. The in-silico results were further validated by immunohistochemistry analysis of in-house tumor tissue specimens. NLGN2 was identified as a prognostic factor in breast cancer subtypes, and its high expression correlated to a favorable survival outcome. Moreover, NLGN2 overexpression in breast cancer was significantly associated with large tumor size, lymph node metastasis, late TNM stage, and high histological grade. Interestingly, there was a significant correlation between the expression level of NLGN2 and the immunomodulatory molecules, along with increased interstitial infiltration of lymphocytes. Furthermore, NLGN2 was predominantly localized in the mitochondria of breast cancer cells. In conclusion, NLGN2 has a prognostic role and immunoregulatory potential in breast cancer, and its functions likely have a mitochondrial basis. It is a promising therapeutic target in breast cancer and should be explored further.

Schizophrenia-associated LRRTM1 regulates cognitive behavior through controlling synaptic function in the mediodorsal thalamus

Reduced activity of the mediodorsal thalamus (MD) and abnormal functional connectivity of the MD with the prefrontal cortex (PFC) cause cognitive deficits in schizophrenia. However, the molecular basis of MD hypofunction in schizophrenia is not known. Here, we identified leucine-rich-repeat transmembrane neuronal protein 1 (LRRTM1), a postsynaptic cell-adhesion molecule, as a key regulator of excitatory synaptic function and excitation-inhibition balance in the MD. LRRTM1 is strongly associated with schizophrenia and is highly expressed in the thalamus. Conditional deletion of Lrrtm1 in the MD in adult mice reduced excitatory synaptic function and caused a parallel reduction in the afferent synaptic activity of the PFC, which was reversed by the reintroduction of LRRTM1 in the MD. Our results indicate that chronic reduction of synaptic strength in the MD by targeted deletion of Lrrtm1 functionally disengages the MD from the PFC and may account for cognitive, social, and sensorimotor gating deficits, reminiscent of schizophrenia.

Neuroligin-3: A Circuit-Specific Synapse Organizer That Shapes Normal Function and Autism Spectrum Disorder-Associated Dysfunction

Chemical synapses provide a vital foundation for neuron-neuron communication and overall brain function. By tethering closely apposed molecular machinery for presynaptic neurotransmitter release and postsynaptic signal transduction, circuit- and context- specific synaptic properties can drive neuronal computations for animal behavior. Trans-synaptic signaling via synaptic cell adhesion molecules (CAMs) serves as a promising mechanism to generate the molecular diversity of chemical synapses. Neuroligins (Nlgns) were discovered as postsynaptic CAMs that can bind to presynaptic CAMs like Neurexins (Nrxns) at the synaptic cleft. Among the four (Nlgn1-4) or five (Nlgn1-3, Nlgn4X, and Nlgn4Y) isoforms in rodents or humans, respectively, Nlgn3 has a heterogeneous expression and function at particular subsets of chemical synapses and strong association with non-syndromic autism spectrum disorder (ASD). Several lines of evidence have suggested that the unique expression and function of Nlgn3 protein underlie circuit-specific dysfunction characteristic of non-syndromic ASD caused by the disruption of Nlgn3 gene. Furthermore, recent studies have uncovered the molecular mechanism underlying input cell-dependent expression of Nlgn3 protein at hippocampal inhibitory synapses, in which trans-synaptic signaling of specific alternatively spliced isoforms of Nlgn3 and Nrxn plays a critical role. In this review article, we overview the molecular, anatomical, and physiological knowledge about Nlgn3, focusing on the circuit-specific function of mammalian Nlgn3 and its underlying molecular mechanism. This will provide not only new insight into specific Nlgn3-mediated trans-synaptic interactions as molecular codes for synapse specification but also a better understanding of the pathophysiological basis for non-syndromic ASD associated with functional impairment in Nlgn3 gene.

The long noncoding RNA Synage regulates synapse stability and neuronal function in the cerebellum

The brain is known to express many long noncoding RNAs (lncRNAs); however, whether and how these lncRNAs function in modulating synaptic stability remains unclear. Here, we report a cerebellum highly expressed lncRNA, Synage, regulating synaptic stability via at least two mechanisms. One is through the function of Synage as a sponge for the microRNA miR-325-3p, to regulate expression of the known cerebellar synapse organizer Cbln1. The other function is to serve as a scaffold for organizing the assembly of the LRP1-HSP90AA1-PSD-95 complex in PF-PC synapses. Although somewhat divergent in its mature mRNA sequence, the locus encoding Synage is positioned adjacent to the Cbln1 loci in mouse, rhesus macaque, and human, and Synage is highly expressed in the cerebella of all three species. Synage deletion causes a full-spectrum cerebellar ablation phenotype that proceeds from cerebellar atrophy, through neuron loss, on to synapse density reduction, synaptic vesicle loss, and finally to a reduction in synaptic activity during cerebellar development; these deficits are accompanied by motor dysfunction in adult mice, which can be rescued by AAV-mediated Synage overexpression from birth. Thus, our study demonstrates roles for the lncRNA Synage in regulating synaptic stability and function during cerebellar development.

The angiopoietin-Tie2 pathway regulates Purkinje cell dendritic morphogenesis in a cell-autonomous manner

Neuro-vascular communication is essential to synchronize central nervous system development. Here, we identify angiopoietin/Tie2 as a neuro-vascular signaling axis involved in regulating dendritic morphogenesis of Purkinje cells (PCs). We show that in the developing cerebellum Tie2 expression is not restricted to blood vessels, but it is also present in PCs. Its ligands angiopoietin-1 (Ang1) and angiopoietin-2 (Ang2) are expressed in neural cells and endothelial cells (ECs), respectively. PC-specific deletion of Tie2 results in reduced dendritic arborization, which is recapitulated in neural-specific Ang1-knockout and Ang2 full-knockout mice. Mechanistically, RNA sequencing reveals that Tie2-deficient PCs present alterations in gene expression of multiple genes involved in cytoskeleton organization, dendritic formation, growth, and branching. Functionally, mice with deletion of Tie2 in PCs present alterations in PC network functionality. Altogether, our data propose Ang/Tie2 signaling as a mediator of intercellular communication between neural cells, ECs, and PCs, required for proper PC dendritic morphogenesis and function.

Action potential-coupled Rho GTPase signaling drives presynaptic plasticity

In contrast to their postsynaptic counterparts, the contributions of activity-dependent cytoskeletal signaling to presynaptic plasticity remain controversial and poorly understood. To identify and evaluate these signaling pathways, we conducted a proteomic analysis of the presynaptic cytomatrix using in vivo biotin identification (iBioID). The resultant proteome was heavily enriched for actin cytoskeleton regulators, including Rac1, a Rho GTPase that activates the Arp2/3 complex to nucleate branched actin filaments. Strikingly, we find Rac1 and Arp2/3 are closely associated with synaptic vesicle membranes in adult mice. Using three independent approaches to alter presynaptic Rac1 activity (genetic knockout, spatially restricted inhibition, and temporal optogenetic manipulation), we discover that this pathway negatively regulates synaptic vesicle replenishment at both excitatory and inhibitory synapses, bidirectionally sculpting short-term synaptic depression. Finally, we use two-photon fluorescence lifetime imaging to show that presynaptic Rac1 activation is coupled to action potentials by voltage-gated calcium influx. Thus, this study uncovers a previously unrecognized mechanism of actin-regulated short-term presynaptic plasticity that is conserved across excitatory and inhibitory terminals. It also provides a new proteomic framework for better understanding presynaptic physiology, along with a blueprint of experimental strategies to isolate the presynaptic effects of ubiquitously expressed proteins.

The cell biology of synapse formation

In a neural circuit, synapses transfer information rapidly between neurons and transform this information during transfer. The diverse computational properties of synapses are shaped by the interactions between pre- and postsynaptic neurons. How synapses are assembled to form a neural circuit, and how the specificity of synaptic connections is achieved, is largely unknown. Here, I posit that synaptic adhesion molecules (SAMs) organize synapse formation. Diverse SAMs collaborate to achieve the astounding specificity and plasticity of synapses, with each SAM contributing different facets. In orchestrating synapse assembly, SAMs likely act as signal transduction devices. Although many candidate SAMs are known, only a few SAMs appear to have a major impact on synapse formation. Thus, a limited set of collaborating SAMs likely suffices to account for synapse formation. Strikingly, several SAMs are genetically linked to neuropsychiatric disorders, suggesting that impairments in synapse assembly are instrumental in the pathogenesis of neuropsychiatric disorders.

GluD1 is a signal transduction device disguised as an ionotropic receptor

Ionotropic glutamate delta receptors 1 (GluD1) and 2 (GluD2) exhibit the molecular architecture of postsynaptic ionotropic glutamate receptors, but assemble into trans-synaptic adhesion complexes by binding to secreted cerebellins that in turn interact with presynaptic neurexins^1-4. It is unclear whether neurexin-cerebellin-GluD1/2 assemblies serve an adhesive synapse-formation function or mediate trans-synaptic signalling. Here we show in hippocampal synapses, that binding of presynaptic neurexin-cerebellin complexes to postsynaptic GluD1 controls glutamate receptor activity without affecting synapse numbers. Specifically, neurexin-1-cerebellin-2 and neurexin-3-cerebellin-2 complexes differentially regulate NMDA (N-methyl-D-aspartate) receptors and AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors by activating distinct postsynaptic GluD1 effector signals. Of note, minimal GluD1 and GluD2 constructs containing only their N-terminal cerebellin-binding and C-terminal cytoplasmic domains, joined by an unrelated transmembrane region, fully control the levels of NMDA and AMPA receptors. The distinct signalling specificity of presynaptic neurexin-1 and neurexin-3^5,6 is encoded by their alternatively spliced splice site 4 sequences, whereas the regulatory functions of postsynaptic GluD1 are mediated by conserved cytoplasmic sequence motifs spanning 5-13 residues. Thus, GluDs are signalling molecules that regulate NMDA and AMPA receptors by an unexpected transduction mechanism that bypasses their ionotropic receptor architecture and directly converts extracellular neurexin-cerebellin signals into postsynaptic receptor responses.

Neurexins regulate presynaptic GABAB-receptors at central synapses

Diverse signaling complexes are precisely assembled at the presynaptic active zone for dynamic modulation of synaptic transmission and synaptic plasticity. Presynaptic GABA_B-receptors nucleate critical signaling complexes regulating neurotransmitter release at most synapses. However, the molecular mechanisms underlying assembly of GABA_B-receptor signaling complexes remain unclear. Here we show that neurexins are required for the localization and function of presynaptic GABA_B-receptor signaling complexes. At four model synapses, excitatory calyx of Held synapses in the brainstem, excitatory and inhibitory synapses on hippocampal CA1-region pyramidal neurons, and inhibitory basket cell synapses in the cerebellum, deletion of neurexins rendered neurotransmitter release significantly less sensitive to GABA_B-receptor activation. Moreover, deletion of neurexins caused a loss of GABA_B-receptors from the presynaptic active zone of the calyx synapse. These findings extend the role of neurexins at the presynaptic active zone to enabling GABA_B-receptor signaling, supporting the notion that neurexins function as central organizers of active zone signaling complexes.

Neurexins are evolutionarily conserved cell adhesion molecules that tune synapse formation and specification. Here the authors show that neurexins play similar roles in regulating presynaptic GABA_B receptors at multiple CNS synapses.

Stellate cell computational modeling predicts signal filtering in the molecular layer circuit of cerebellum

The functional properties of cerebellar stellate cells and the way they regulate molecular layer activity are still unclear. We have measured stellate cells electroresponsiveness and their activation by parallel fiber bursts. Stellate cells showed intrinsic pacemaking, along with characteristic responses to depolarization and hyperpolarization, and showed a marked short-term facilitation during repetitive parallel fiber transmission. Spikes were emitted after a lag and only at high frequency, making stellate cells to operate as delay-high-pass filters. A detailed computational model summarizing these physiological properties allowed to explore different functional configurations of the parallel fiber-stellate cell-Purkinje cell circuit. Simulations showed that, following parallel fiber stimulation, Purkinje cells almost linearly increased their response with input frequency, but such an increase was inhibited by stellate cells, which leveled the Purkinje cell gain curve to its 4 Hz value. When reciprocal inhibitory connections between stellate cells were activated, the control of stellate cells over Purkinje cell discharge was maintained only at very high frequencies. These simulations thus predict a new role for stellate cells, which could endow the molecular layer with low-pass and band-pass filtering properties regulating Purkinje cell gain and, along with this, also burst delay and the burst-pause responses pattern.

CA10 regulates neurexin heparan sulfate addition via a direct binding in the secretory pathway

Neurexins are presynaptic adhesion molecules that shape the molecular composition of synapses. Diversification of neurexins in numerous isoforms is believed to confer synapse-specific properties by engaging with distinct ligands. For example, a subset of neurexin molecules carry a heparan sulfate (HS) glycosaminoglycan that controls ligand binding, but how this post-translational modification is controlled is not known. Here, we observe that CA10, a ligand to neurexin in the secretory pathway, regulates neurexin-HS formation. CA10 is exclusively found on non-HS neurexin and CA10 expressed in neurons is sufficient to suppress HS addition and attenuate ligand binding and synapse formation induced by ligands known to recruit HS. This effect is mediated by a direct interaction in the secretory pathway that blocks the primary step of HS biosynthesis: xylosylation of the serine residue. NMR reveals that CA10 engages residues on either side of the serine that can be HS-modified, suggesting that CA10 sterically blocks xylosyltransferase access in Golgi. These results suggest a mechanism for the regulation of HS on neurexins and exemplify a new mechanism to regulate site-specific glycosylations.

Proper synaptic adhesion signaling in the control of neural circuit architecture and brain function

Trans-synaptic cell-adhesion molecules are critical for governing various stages of synapse development and specifying neural circuit properties via the formation of multifarious signaling pathways. Recent studies have pinpointed the putative roles of trans-synaptic cell-adhesion molecules in mediating various cognitive functions. Here, we review the literature on the roles of a diverse group of central synaptic organizers, including neurexins (Nrxns), leukocyte common antigen-related receptor protein tyrosine phosphatases (LAR-RPTPs), and their associated binding proteins, in regulating properties of specific type of synapses and neural circuits. In addition, we highlight the findings that aberrant synaptic adhesion signaling leads to alterations in the structures, transmission, and plasticity of specific synapses across diverse brain areas. These results seem to suggest that proper trans-synaptic signaling pathways by Nrxns, LAR-RPTPs, and their interacting network is likely to constitute central molecular complexes that form the basis for cognitive functions, and that these complexes are heterogeneously and complexly disrupted in many neuropsychiatric and neurodevelopmental disorders.