Autism-associated neuroligin-3 mutations commonly impair striatal circuits to boost repetitive behaviors

Reversing neural circuit and behavior deficit in mice exposed to maternal inflammation by Zika virus

Zika virus (ZIKV) infection during pregnancy is linked to various developmental brain disorders. Infants who are asymptomatic at birth might have postnatal neurocognitive complications. However, animal models recapitulating these neurocognitive phenotypes are lacking, and the circuit mechanism underlying behavioral abnormalities is unknown. Here, we show that ZIKV infection during mouse pregnancy induces maternal immune activation (MIA) and leads to autistic-like behaviors including repetitive self-grooming and impaired social memory in offspring. In the medial prefrontal cortex (mPFC), ZIKV-affected offspring mice exhibit excitation and inhibition imbalance and increased cortical activity. This could be explained by dysregulation of inhibitory neurons and synapses, and elevated neural activity input from mPFC-projecting ventral hippocampus (vHIP) neurons. We find structure alterations in the synaptic connections and pattern of vHIP innervation of mPFC neurons, leading to hyperconnectivity of the vHIP-mPFC pathway. Decreasing the activity of mPFC-projecting vHIP neurons with a chemogenetic strategy rescues social memory deficits in ZIKV offspring mice. Our studies reveal a hyperconnectivity of vHIP to mPFC projection driving social memory deficits in mice exposed to maternal inflammation by ZIKV.

Mice with an autism-associated R451C mutation in neuroligin-3 show a cautious but accurate response style in touchscreen attention tasks

One of the earliest identifiable features of autism spectrum disorder (ASD) is altered attention. Mice expressing the ASD-associated R451C mutation in synaptic adhesion protein neuroligin-3 (NL3) exhibit impaired reciprocal social interactions and repetitive and restrictive behaviours. The role of this mutation in attentional abnormalities has not been established. We assessed attention in male NL3^R451C mice using two well-established tasks in touchscreen chambers. In the 5-choice serial reaction task, rodents were trained to attend to light stimuli that appear in any one of five locations. While no differences between NL3^R451C and WT mice were seen in accuracy or omissions, slower response times and quicker reward collection latencies were seen across all training and probe trials. In the rodent continuous-performance test, animals were required to discriminate, and identify a visual target pattern over multiple distractor stimuli. NL3^R451C mice displayed enhanced ability to attend to stimuli when task-load was low during training and baseline but lost this advantage when difficulty was increased by altering task parameters in probe trials. NL3^R451C mice made less responses to the distractor stimuli, exhibiting lower false alarm rates during all training stages and in probe trials. Slower response times and quicker reward latencies were consistently seen in NL3^R451C mice in the rCPT. Slower response times are a major cognitive phenotype reported in ASD patients and are indicative of slower processing speed. Enhanced attention has been shown in a subset of ASD patients and we have demonstrated this phenotype also exists in the NL3^R451C mouse model.

Inhibition of Elevated Ras-MAPK Signaling Normalizes Enhanced Motor Learning and Excessive Clustered Dendritic Spine Stabilization in the MECP2-Duplication Syndrome Mouse Model of Autism

The inflexible repetitive behaviors and "insistence on sameness" seen in autism imply a defect in neural processes controlling the balance between stability and plasticity of synaptic connections in the brain. It has been proposed that abnormalities in the Ras-ERK/MAPK pathway, a key plasticity-related cell signaling pathway known to drive consolidation of clustered synaptic connections, underlie altered learning phenotypes in autism. However, a link between altered Ras-ERK signaling and clustered dendritic spine plasticity has yet to be explored in an autism animal model in vivo The formation and stabilization of dendritic spine clusters is abnormally increased in the MECP2-duplication syndrome mouse model of syndromic autism, suggesting that ERK signaling may be increased. Here, we show that the Ras-ERK pathway is indeed hyperactive following motor training in MECP2-duplication mouse motor cortex. Pharmacological inhibition of ERK signaling normalizes the excessive clustered spine stabilization and enhanced motor learning behavior in MECP2-duplication mice. We conclude that hyperactive ERK signaling may contribute to abnormal clustered dendritic spine consolidation and motor learning in this model of syndromic autism.

Proteomics Study on the Cerebrospinal Fluid of Patients with Encephalitis

OBJECTIVE

Label-free quantitative proteomics was applied to analyze differentially expressed proteins (DEPs) in the cerebrospinal fluid (CSF) of patients with encephalitis. The database was used to screen for possible biomarkers in encephalitis, followed by validation and preliminary investigation of the role of some DEPs in the pathogenesis of encephalitis using enzyme-linked immunosorbent assay (ELISA).

METHODS

We performed label-free quantitative proteomics on 16 cerebrospinal fluid samples (EM group, encephalitis with mental and behavioral disorders patients, n = 5; NED group, encephalitis without mental and behavioral disorders patients, n = 6; N group, healthy individuals, n = 5). The extracted CSF proteins were examined by mass spectrometry and enzymatic digestion and detected using protein profiling and data analysis. Interproscan was used to perform Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis of the DEPs. ELISA was used to verify the changes in the levels of some DEPs in the CSF.

RESULTS

A total of 941 proteins were found to be significantly differentially expressed, including 250 upregulated DEPs and 691 downregulated DEPs. GO analysis suggested that there were six enriched functions that intersect among the EM, NED, and N groups, including synapse organization, membrane, integral component of membrane, membrane part, G-protein-coupled receptor signaling pathway, and transmembrane signaling receptor activity. KEGG analysis revealed that there were three signaling pathways that intersect among the EM, NED, and N groups, including fructose and mannose metabolism, inositol phosphate metabolism, and Jak-STAT signaling pathway. Furthermore, four downregulated encephalitis-related neurological synapse proteins were identified after screening for differentially expressed proteins, including NRXN3, NFASC, LRRC4B, and NLGN2. The result of ELISA further verified that the expression of NLGN2 and LRRC4B was obviously higher in the NED group than in the N group.

CONCLUSIONS

These findings demonstrated that NLGN2 and LRRC4B proteins were upregulated in the NED group and could be potential biomarkers for the diagnosis of encephalitis, but still needs a lot of multiomics studies to be used in clinical.

An Autism-Associated Neuroligin-3 Mutation Affects Developmental Synapse Elimination in the Cerebellum

Neuroligin is a postsynaptic cell-adhesion molecule that is involved in synapse formation and maturation by interacting with presynaptic neurexin. Mutations in neuroligin genes, including the arginine to cystein substitution at the 451st amino acid residue (R451C) of neuroligin-3 (NLGN3), have been identified in patients with autism spectrum disorder (ASD). Functional magnetic resonance imaging and examination of post-mortem brain in ASD patients implicate alteration of cerebellar morphology and Purkinje cell (PC) loss. In the present study, we examined possible association between the R451C mutation in NLGN3 and synaptic development and function in the mouse cerebellum. In NLGN3-R451C mutant mice, the expression of NLGN3 protein in the cerebellum was reduced to about 10% of the level of wild-type mice. Elimination of redundant climbing fiber (CF) to PC synapses was impaired from postnatal day 10–15 (P10–15) in NLGN3-R451C mutant mice, but majority of PCs became mono-innervated as in wild-type mice after P16. In NLGN3-R451C mutant mice, selective strengthening of a single CF relative to the other CFs in each PC was impaired from P16, which persisted into juvenile stage. Furthermore, the inhibition to excitation (I/E) balance of synaptic inputs to PCs was elevated, and calcium transients in the soma induced by strong and weak CF inputs were reduced in NLGN3-R451C mutant mice. These results suggest that a single point mutation in NLGN3 significantly influences the synapse development and refinement in cerebellar circuitry, which might be related to the pathogenesis of ASD.

Individual differences in stereotypy and neuron subtype translatome with TrkB deletion

Motor stereotypies occurring in early-onset neuropsychiatric diseases are associated with dysregulated basal ganglia direct-pathway activity. Disruptions in network connectivity through impaired neuronal structure have been implicated in both rodents and humans. However, the neurobiological mechanisms leading to direct-pathway neuron disconnectivity in stereotypy remain poorly understood. We have a mouse line with Tropomyosin receptor kinase B (TrkB) receptor deletion from D1-expressing cells (D1-Cre-flTrkB) in which a subset of animals shows repetitive rotations and head tics with juvenile onset. Here we demonstrate these behaviors may be associated with abnormal direct-pathway activity by reducing rotations using chemogenetic inhibition of dorsal striatum D1-medium spiny neurons (D1-MSNs) in both juvenile and young-adult mice. Taking advantage of phenotypical differences in animals with similar genotypes, we then interrogated the D1-MSN specific translatome associated with repetitive behavior by using RNA sequencing of ribosome-associated mRNA. Detailed translatome analysis followed by multiplexed gene expression assessment revealed profound alterations in neuronal projection and synaptic structure related genes in stereotypy mice. Examination of neuronal morphology demonstrated dendritic atrophy and dendritic spine loss in dorsal striatum D1-MSNs from mice with repetitive behavior. Together, our results uncover phenotype-specific molecular alterations in D1-MSNs that relate to morphological adaptations in mice displaying stereotypy behavior.

Facilitating mGluR4 activity reverses the long-term deleterious consequences of chronic morphine exposure in male mice

Understanding the neurobiological underpinnings of abstinence from drugs of abuse is critical to allow better recovery and ensure relapse prevention in addicted subjects. By comparing the long-term transcriptional consequences of morphine and cocaine exposure, we identified the metabotropic glutamate receptor subtype 4 (mGluR4) as a promising pharmacological target in morphine abstinence. We evaluated the behavioral and molecular effects of facilitating mGluR4 activity in abstinent mice. Transcriptional regulation of marker genes of medium spiny neurons (MSNs) allowed best discriminating between 4-week morphine and cocaine abstinence in the nucleus accumbens (NAc). Among these markers, Grm4, encoding mGluR4, displayed down-regulated expression in the caudate putamen and NAc of morphine, but not cocaine, abstinent mice. Chronic administration of the mGluR4 positive allosteric modulator (PAM) VU0155041 (2.5 and 5 mg/kg) rescued social behavior, normalized stereotypies and anxiety and blunted locomotor sensitization in morphine abstinent mice. This treatment improved social preference but increased stereotypies in cocaine abstinent mice. Finally, the beneficial behavioral effects of VU0155041 treatment in morphine abstinent mice were correlated with restored expression of key MSN and neural activity marker genes in the NAc. This study reports that chronic administration of the mGluR4 PAM VU0155041 relieves long-term deleterious consequences of morphine exposure. It illustrates the neurobiological differences between opiate and psychostimulant abstinence and points to pharmacological repression of excessive activity of D2-MSNs in the NAc as a promising therapeutic lever in drug addiction.

Safety and efficacy of genetic MECP2 supplementation in the R294X mouse model of Rett syndrome

Rett syndrome is a neurodevelopmental disorder caused predominantly by loss-of-function mutations in MECP2, encoding transcriptional modulator methyl-CpG-binding protein 2 (MeCP2). Although no disease-modifying therapies exist at this time, some proposed therapeutic strategies aim to supplement the mutant allele with a wild-type allele producing typical levels of functional MeCP2, such as gene therapy. Because MECP2 is a dosage-sensitive gene, with both loss and gain of function causing disease, these approaches must achieve a narrow therapeutic window to be both safe and effective. While MeCP2 supplementation rescues RTT-like phenotypes in mouse models, the tolerable threshold of MeCP2 is not clear, particularly for partial loss-of-function mutations. We assessed the safety of genetically supplementing full-length human MeCP2 in the context of the R294X allele, a common partial loss-of-function mutation retaining DNA-binding capacity. We assessed the potential for adverse effects from MeCP2 supplementation of a partial loss-of-function mutant and the potential for dominant negative interactions between mutant and full-length MeCP2. In male hemizygous R294X mice, MeCP2 supplementation rescued RTT-like behavioral phenotypes and did not elicit behavioral evidence of excess MeCP2. In female heterozygous R294X mice, RTT-specific phenotypes were similarly rescued. However, MeCP2 supplementation led to evidence of excess MeCP2 activity in a motor coordination assay, suggesting that the underlying motor circuitry is particularly sensitive to MeCP2 dosage in females. These results show that genetic supplementation of full-length MeCP2 is safe in males and largely so females. However, careful consideration of risk for adverse motor effects may be warranted for girls and women with RTT.

Of mice and men - and guinea pigs?

This year marks the twentieth anniversary of the publication of the first draft of the human genome and its broad availability to the scientific community. In parallel, the annotation of the mouse genome led to the identification and analysis of countless genes by means of genetic manipulation. Today, when comparing both genomes, it might surprise that some genes are still seeking their respective homologs in either species. In this review, we aim at raising awareness for the remarkable differences between the researcher's favorite rodents, i.e., mice and rats, when it comes to the generation of rodent research models regarding genes with a particular delicate localization, namely the pseudoautosomal region on both sex chromosomes. Many of these genes are of utmost clinical relevance in humans and still miss a rodent disease model giving their absence in mice and rats or low sequence similarity compared to humans. The abundance of rodents within mammals prompted us to investigate different branches of rodents leading us to the re-discovery of the guinea pig as a mammalian research model for a distinct group of genes.

Behavioral Tests for Mouse Models of Autism: An Argument for the Inclusion of Cerebellum-Controlled Motor Behaviors

Mouse models of Autism Spectrum Disorder (ASD) have been interrogated using a variety of behavioral tests in order to understand the symptoms of ASD. However, the hallmark behaviors that are classically affected in ASD - deficits in social interaction and communication as well as the occurrence of repetitive behaviors - do not have direct murine equivalents. Thus, it is critical to identify the caveats that come with modeling a human disorder in mice. The most commonly used behavioral tests represent complex cognitive processes based on largely unknown brain circuitry. Motor impairments provide an alternative, scientifically rigorous approach to understanding ASD symptoms. Difficulties with motor coordination and learning - seen in both patients and mice - point to an involvement of the cerebellum in ASD pathology. This brain area supports types of motor learning that are conserved throughout vertebrate evolution, allowing for direct comparisons of functional abnormalities between humans with autism and ASD mouse models. Studying simple motor behaviors provides researchers with clearly interpretable results. We describe and evaluate methods used on mouse behavioral assays designed to test for social, communicative, perseverative, anxious, nociceptive, and motor learning abnormalities. We comment on the effectiveness and validity of each test based on how much information its results give, as well as its relevance to ASD, and will argue for an inclusion of cerebellum-supported motor behaviors in the phenotypic description of ASD mouse models. LAY SUMMARY: Mouse models of Autism Spectrum Disorder help us gain insight about ASD symptoms in human patients. However, there are many differences between mice and humans, which makes interpreting behaviors challenging. Here, we discuss a battery of behavioral tests for specific mouse behaviors to explore whether each test does indeed evaluate the intended measure, and whether these tests are useful in learning about ASD.

Cerebellar Dysfunction in Autism Spectrum Disorders: Deriving Mechanistic Insights from an Internal Model Framework

Autism spectrum disorders (ASD) are highly prevalent neurodevelopmental disorders; however, the neurobiological mechanisms underlying disordered behavior in ASD remain poorly understood. Notably, individuals with ASD have demonstrated difficulties generating implicitly derived behavioral predictions and adaptations. Although many brain regions are involved in these processes, the cerebellum contributes an outsized role to these behavioral functions. Consistent with this prominent role, cerebellar dysfunction has been increasingly implicated in ASD. In this review, we will utilize the foundational, theoretical contributions of the late neuroscientist Masao Ito to establish an internal model framework for the cerebellar contribution to ASD-relevant behavioral predictions and adaptations. Additionally, we will also explore and then apply his key experimental contributions towards an improved, mechanistic understanding of the contribution of cerebellar dysfunction to ASD.

Cesarean section delivery is a risk factor of autism-related behaviors in mice

Cesarean section (C/S) is one way of delivering babies, and is chosen when mothers or babies are facing problems or life-threatening conditions during pregnancy. Many meta-analyses have suggested an etiological relationship between C/S delivery and autism spectrum disorders (ASDs). However, as a risk factor for ASDs, C/S delivery has not yet been well studied. Because C/S deliveries have been increasing, it is very important to investigate the causal association between C/S and ASDs. Here, using three approaches, we showed experimentally that C/S delivery induced ASD-like traits in offspring mice, and that some of these changes were ameliorated by one-time oxytocin (OXT) treatment. Treatment with OXT receptor antagonists before natural delivery also induced ASD-related behaviors. Moreover, wild-type mice born to OXT-KO dams showed similar changes. Thus, insufficient OXT exposure from dams to offspring during delivery may be a trigger for ASD-related behaviors.

Pharmacological inhibition of the primary endocannabinoid producing enzyme, DGL-α, induces autism spectrum disorder-like and co-morbid phenotypes in adult C57BL/J mice

Accumulating evidence links dysfunction in the endocannabinoid system (ECS) with the pathology of neurodevelopmental disorders, particularly autism spectrum disorder (ASD). Variants in ECS genes CNR1 and DAGLA are associated with neurological phenotypes in humans. The endocannabinoids (eCBs), 2-AG and AEA, which act at the primary cannabinoid receptor (CB1), mediate behaviors relevant to neurodevelopmental disorders. The overlap between these eCBs is poorly understood. Most ECS studies have focused on stress responses, anxiety, and epilepsy, however, its role in social behavior and communication has only recently come under investigation. This represents a critical gap in our understanding of the ECS and its relationship to ASD. Furthermore, the increasing prevalence of ASD and a lack of therapeutics emphasize a crucial need for novel therapeutic targets. To this aim, we used an inhibitor of the eCB producing enzyme DGL-α, DO34, and the CB1 inverse agonist, rimonabant, to evaluate the role of the primary eCB, 2-AG, in ASD. Adult male C57BL/6J mice were used in a series of behavioral paradigms which assessed social behavior, social communication, repetitive behaviors, anxiety and locomotor activity. DO34 and rimonabant increased anxiety-like behavior, while only DO34 induced hyperactivity, social deficits, and repetitive self-grooming behavior. These data indicate that reduced 2-AG bioavailability, or CB1 inhibition, each induce unique respective behavioral phenotypes relevant to neurodevelopmental disorders, particularly ASD. This suggests fundamental differences in CB1 signaling via 2-AG and the CB1 receptor itself, particularly for social behaviors, and that 2-AG signaling may represent a target for the development of novel therapeutics. LAY SUMMARY: Endocannabinoids play a critical role in the developing nervous system. Alterations in the endocannabinoid system are linked to neurodevelopmental disorders. Studies suggest these variants may play a critical role in the core symptoms of autism spectrum disorder. In this study, pharmacological inhibition of the primary endocannabinoid producing enzyme, DGL-α, induced a constellation of deficits in behavioral domains associated with autism.

The Emergence of Network Activity Patterns in the Somatosensory Cortex - An Early Window to Autism Spectrum Disorders

Across mammalian species, patterned activity in neural populations is a prominent feature of developing sensory cortices. Numerous studies have long appreciated the diversity of these patterns, characterizing their differences in spatial and temporal dynamics. In the murine somatosensory cortex, neuronal co-activation is thought to guide the formation of sensory maps and prepare the cortex for sensory processing after birth. While pioneering studies deftly utilized slice electrophysiology and unit recordings to characterize correlated activity, a detailed understanding of the underlying circuits remains poorly understood. More recently, advances in in vivo calcium imaging in awake mouse pups and increasing genetic tractability of neuronal types have allowed unprecedented manipulation of circuit components at select developmental timepoints. These novel approaches have proven fundamental in uncovering the identity of neurons engaged in correlated activity during development. In particular, recent studies have highlighted interneurons as key in refining the spatial extent and temporal progression of patterned activity. Here, we discuss how emergent synchronous activity across the first postnatal weeks is shaped by underlying gamma aminobutyric acid (GABA)ergic contributors in the somatosensory cortex. Further, the importance of participation in specific activity patterns per se for neuronal maturation and perdurance will be of particular highlight in this survey of recent literature. Finally, we underscore how aberrant neuronal synchrony and disrupted inhibitory interneuron activity underlie sensory perturbations in neurodevelopmental disorders, particularly Autism Spectrum Disorders (ASDs), emphasizing the importance of future investigative approaches that incorporate the spatiotemporal features of patterned activity alongside the cellular components to probe disordered circuit assembly.

Incorporation of a Biocompatible Nanozyme in Cellular Antioxidant Enzyme Cascade Reverses Huntington's Like Disorder in Preclinical Model

The potentiality of nano-enzymes in therapeutic use has directed contemporary research to develop a substitute for natural enzymes, which are suffering from several disadvantages including low stability, high cost, and difficulty in storage. However, inherent toxicity, inefficiency in the physiological milieu, and incompatibility to function in cellular enzyme networks limit the therapeutic use of nanozymes in living systems. Here, it is shown that citrate functionalized manganese-based biocompatible nanoscale material (C-Mn₃ O₄ NP) efficiently mimics glutathione peroxidase (GPx) enzyme in the physiological milieu and easily incorporates into the cellular multienzyme cascade for H₂ O₂ scavenging. A detailed computational study reveals the mechanism of the nanozyme action. The in vivo therapeutic efficacy of C-Mn₃ O₄ nanozyme is further established in a preclinical animal model of Huntington's disease (HD), a prevalent progressive neurodegenerative disorder, which has no effective medication to date. Management of HD in preclinical animal trial using a biocompatible (non-toxic) nanozyme as a part of the metabolic network may uncover a new paradigm in nanozyme based therapeutic strategy.

Copy number variants in neurexin genes: phenotypes and mechanisms

Neurexins are central to trans-synaptic cell adhesion and signaling during synapse specification and maintenance. The past two decades of human genetics research have identified structural variations in the neurexin gene family, in particular NRXN1 copy number variants (CNVs), implicated in multiple neuropsychiatric and developmental disorders. The heterogeneity and reduced penetrance of NRXN1 deletions, in addition to the pleiotropic, circuit-specific functions of NRXN1, present substantial obstacles to understanding how compromised NRXN1 function predisposes individuals to neuropsychiatric disorders. Here, we provide an updated review of NRXN1 genetics in disease, followed by recently published work using both human induced pluripotent stem cell (iPSC) derived systems and animal models to understand the mechanisms of disease pathophysiology. Finally, we suggest our outlook on how the field should progress to improve our understanding of neurexin mediated disease pathogenesis. We believe that understanding how structural genetic variants in NRXN1 contribute to disease pathophysiology requires parallel approaches in iPSC and mouse model systems, each leveraging their unique strengths — analysis of genetic interactions and background effects in iPSCs and neural circuit and behavioral analysis in mice.

Canonical versus non-canonical transsynaptic signaling of neuroligin 3 tunes development of sociality in mice

Neuroligin 3 (NLGN3) and neurexins (NRXNs) constitute a canonical transsynaptic cell-adhesion pair, which has been implicated in autism. In autism spectrum disorder (ASD) development of sociality can be impaired. However, the molecular mechanism underlying NLGN3-mediated social development is unclear. Here, we identify non-canonical interactions between NLGN3 and protein tyrosine phosphatase δ (PTPδ) splice variants, competing with NRXN binding. NLGN3-PTPδ complex structure revealed a splicing-dependent interaction mode and competition mechanism between PTPδ and NRXNs. Mice carrying a NLGN3 mutation that selectively impairs NLGN3-NRXN interaction show increased sociability, whereas mice where the NLGN3-PTPδ interaction is impaired exhibit impaired social behavior and enhanced motor learning, with imbalance in excitatory/inhibitory synaptic protein expressions, as reported in the Nlgn3 R451C autism model. At neuronal level, the autism-related Nlgn3 R451C mutation causes selective impairment in the non-canonical pathway. Our findings suggest that canonical and non-canonical NLGN3 pathways compete and regulate the development of sociality.

Evolution of the Autism-Associated Neuroligin-4 Gene Reveals Broad Erosion of Pseudoautosomal Regions in Rodents

Variants in genes encoding synaptic adhesion proteins of the neuroligin family, most notably neuroligin-4, are a significant cause of autism spectrum disorders in humans. Although human neuroligin-4 is encoded by two genes, NLGN4X and NLGN4Y, that are localized on the X-specific and male-specific regions of the two sex chromosomes, the chromosomal localization and full genomic sequence of the mouse Nlgn4 gene remain elusive. Here, we analyzed the neuroligin-4 genes of numerous rodent species by direct sequencing and bioinformatics, generated complete drafts of multiple rodent neuroligin-4 genes, and examined their evolution. Surprisingly, we find that the murine Nlgn4 gene is localized to the pseudoautosomal region (PAR) of the sex chromosomes, different from its human orthologs. We show that the sequence differences between various neuroligin-4 proteins are restricted to hotspots in which rodent neuroligin-4 proteins contain short repetitive sequence insertions compared with neuroligin-4 proteins from other species, whereas all other protein sequences are highly conserved. Evolutionarily, these sequence insertions initiate in the clade eumuroidea of the infraorder myomorpha and are additionally associated with dramatic changes in noncoding sequences and gene size. Importantly, these changes are not exclusively restricted to neuroligin-4 genes but reflect major evolutionary changes that substantially altered or even deleted genes from the PARs of both sex chromosomes. Our results show that despite the fact that the PAR in rodents and the neuroligin-4 genes within the rodent PAR underwent massive evolutionary changes, neuroligin-4 proteins maintained a highly conserved core structure, consistent with a substantial evolutionary pressure preserving its physiological function.

Exogenous l-lactate promotes astrocyte plasticity but is not sufficient for enhancing striatal synaptogenesis or motor behavior in mice

l-Lactate is an energetic and signaling molecule that may be produced through astrocyte-specific aerobic glycolysis and is elevated in striatal muscle during intensive exercise. l-Lactate has been shown to promote neurotrophic gene expression through astrocytes within the hippocampus, however, its role in neuroplasticity within the striatum remains unknown. This study sought to investigate the role of peripheral sources of l-lactate in promoting astrocyte-specific gene expression and morphology as well as its role in neuroplasticity within the striatum of healthy animals. Using in vitro primary astrocyte cell culture, administration of l-lactate increased the expression of the neurotrophic factors Bdnf, Gdnf, Cntf, and the immediate early gene cFos. l-Lactate's promotion of neurotrophic factor expression was mediated through the lactate receptor HCAR1 since application of the HCAR1 agonist 3,5-DHBA also increased expression of Bdnf in primary astrocytes. Similar to our previous report demonstrating exercise-induced changes in astrocytic structure within the striatum, l-lactate administration to healthy mice led to increased astrocyte morphological complexity as well as astrocyte-specific neurotrophic expression within the striatum. Our study failed to demonstrate an effect of peripheral l-lactate on synaptogenesis or motor behavior. Insufficient levels and/or inadequate delivery of l-lactate through regional cerebral blood flow within the striatum may account for the lack of these benefits. Taken together, these novel findings suggest a potential framework that links peripheral l-lactate production within muscle and intensive exercise with neuroplasticity of specific brain regions through astrocytic function.

Neurexin1⍺ differentially regulates synaptic efficacy within striatal circuits

Mutations in genes essential for synaptic function, such as the presynaptic adhesion molecule Neurexin1α (Nrxn1α), are strongly implicated in neuropsychiatric pathophysiology. As the input nucleus of the basal ganglia, the striatum integrates diverse excitatory projections governing cognitive and motor control, and its impairment may represent a recurrent pathway to disease. Here, we test the functional relevance of Nrxn1α in striatal circuits by employing optogenetic-mediated afferent recruitment of dorsal prefrontal cortical (dPFC) and parafascicular thalamic connections onto dorsomedial striatal (DMS) spiny projection neurons (SPNs). For dPFC-DMS circuits, we find decreased synaptic strength specifically onto indirect pathway SPNs in both Nrxn1α+/- and Nrxn1α-/- mice, driven by reductions in neurotransmitter release. In contrast, thalamic excitatory inputs to DMS exhibit relatively normal excitatory synaptic strength despite changes in synaptic N-methyl-D-aspartate receptor (NMDAR) content. These findings suggest that dysregulation of Nrxn1α modulates striatal function in an input- and target-specific manner.

Optogenetic Approaches to Understand the Neural Circuit Mechanism of Social Deficits Seen in Autism Spectrum Disorders

Individuals with neurodevelopmental disorders, such as autism spectrum disorders (ASDs), are diagnosed based on nonquantitative objective parameters such as behavioral phenotypes. It is still unclear how any neural mechanism affects such behavioral phenotypes in these patients. In human genetics, a large number of genetic abnormalities including single nucleotide variation (SNV) and copy number variation (CNV) have been found in individuals with ASDs. It is thought that influence of such variations converges on dysfunction of neural circuit resulting in common behavioral phenotypes of ASDs such as deficits in social communication and interaction. Recent studies suggest that an excitatory/inhibitory (E/I) imbalanced state, which induces disruption of neural circuit activities, is one of the pathophysiological abnormalities in ASD brains. To assess the causal relationship between brain abnormalities and behavioral deficits, we can take advantage of optogenetics with animal models of ASDs that recapitulate human genetic mutations. Here, we review optogenetics studies being utilized to dissect neural circuit mechanisms associated with social deficits in model mice of ASD. Optogenetic manipulation of disrupted neural activities would help us understand how neural circuits affect behavioral deficits observed in ASDs.

Synaptic recognition molecules in development and disease

Synaptic connectivity patterns underlie brain functions. How recognition molecules control where and when neurons form synapses with each other, therefore, is a fundamental question of cellular neuroscience. This chapter delineates adhesion and signaling complexes as well as secreted factors that contribute to synaptic partner recognition in the vertebrate brain. The sections follow a developmental perspective and discuss how recognition molecules (1) guide initial synaptic wiring, (2) provide for the rejection of incorrect partner choices, (3) contribute to synapse specification, and (4) support the removal of inappropriate synapses once formed. These processes involve a rich repertoire of molecular players and key protein families are described, notably the Cadherin and immunoglobulin superfamilies, Semaphorins/Plexins, Leucine-rich repeat containing proteins, and Neurexins and their binding partners. Molecular themes that diversify these recognition systems are defined and highlighted throughout the text, including the neuron-type specific expression and combinatorial action of recognition factors, alternative splicing, and post-translational modifications. Methodological innovations advancing the field such as proteomic approaches and single cell expression studies are additionally described. Further, the chapter highlights the importance of choosing an appropriate brain region to analyze synaptic recognition factors and the advantages offered by laminated structures like the hippocampus or retina. In a concluding section, the profound disease relevance of aberrant synaptic recognition for neurodevelopmental and psychiatric disorders is discussed. Based on the current progress, an outlook is presented on research goals that can further advance insights into how recognition molecules provide for the astounding precision and diversity of synaptic connections.

Selective postnatal excitation of neocortical pyramidal neurons results in distinctive behavioral and circuit deficits in adulthood

In genetic and pharmacological models of neurodevelopmental disorders, and human data, neural activity is altered within the developing neocortical network. This commonality begs the question of whether early enhancement in excitation might be a common driver, across etiologies, of characteristic behaviors. We tested this concept by chemogenetically driving cortical pyramidal neurons during postnatal days 4–14. Hyperexcitation of Emx1-, but not dopamine transporter-, parvalbumin-, or Dlx5/6-expressing neurons, led to decreased social interaction and increased grooming activity in adult animals. In vivo optogenetic interrogation in adults revealed decreased baseline but increased stimulus-evoked firing rates of pyramidal neurons and impaired recruitment of inhibitory neurons. Slice recordings in adults from prefrontal cortex layer 5 pyramidal neurons revealed decreased intrinsic excitability and increased synaptic E/I ratio. Together these results support the prediction that enhanced pyramidal firing during development, in otherwise normal cortex, can selectively drive altered adult circuit function and maladaptive changes in behavior.

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BL-OG allows chemogenetic activation and optogenetic interrogation in the same animal

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Developmental hyperexcitation in normal mice leads to neurodevelopmental disorders

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In these mice adult neurons show reduced baseline activity and increased excitability

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Reduced activity-triggered coherence and altered oscillations in cortex and striatum

Genetic influences of autism candidate genes on circuit wiring and olfactory decoding

Olfaction supports a multitude of behaviors vital for social communication and interactions between conspecifics. Intact sensory processing is contingent upon proper circuit wiring. Disturbances in genetic factors controlling circuit assembly and synaptic wiring can lead to neurodevelopmental disorders, such as autism spectrum disorder (ASD), where impaired social interactions and communication are core symptoms. The variability in behavioral phenotype expression is also contingent upon the role environmental factors play in defining genetic expression. Considering the prevailing clinical diagnosis of ASD, research on therapeutic targets for autism is essential. Behavioral impairments may be identified along a range of increasingly complex social tasks. Hence, the assessment of social behavior and communication is progressing towards more ethologically relevant tasks. Garnering a more accurate understanding of social processing deficits in the sensory domain may greatly contribute to the development of therapeutic targets. With that framework, studies have found a viable link between social behaviors, circuit wiring, and altered neuronal coding related to the processing of salient social stimuli. Here, the relationship between social odor processing in rodents and humans is examined in the context of health and ASD, with special consideration for how genetic expression and neuronal connectivity may regulate behavioral phenotypes.

Neural Mechanisms Underlying Repetitive Behaviors in Rodent Models of Autism Spectrum Disorders

Autism spectrum disorder (ASD) is comprised of several conditions characterized by alterations in social interaction, communication, and repetitive behaviors. Genetic and environmental factors contribute to the heterogeneous development of ASD behaviors. Several rodent models display ASD-like phenotypes, including repetitive behaviors. In this review article, we discuss the potential neural mechanisms involved in repetitive behaviors in rodent models of ASD and related neuropsychiatric disorders. We review signaling pathways, neural circuits, and anatomical alterations in rodent models that display robust stereotypic behaviors. Understanding the mechanisms and circuit alterations underlying repetitive behaviors in rodent models of ASD will inform translational research and provide useful insight into therapeutic strategies for the treatment of repetitive behaviors in ASD and other neuropsychiatric disorders.

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.

Constraint-Induced Movement Therapy Promotes Neural Remodeling and Functional Reorganization by Overcoming Nogo-A/NgR/RhoA/ROCK Signals in Hemiplegic Cerebral Palsy Mice

Background. Little is known about the induction of functional and brain structural reorganization in hemiplegic cerebral palsy (HCP) by constraint-induced movement therapy (CIMT). Objective. We aimed to explore the specific molecular mechanism of functional and structural plasticity related to CIMT in HCP. Methods. The mice were divided into a control group and HCP groups with different interventions (unconstraint-induced movement therapy [UNCIMT], CIMT or siRNA-Nogo-A [SN] treatment): the HCP, HCP+UNCIMT, HCP+CIMT, HCP+SN, and HCP+SN+CIMT groups. Rotarod and front-limb suspension tests, immunohistochemistry, Golgi-Cox staining, transmission electron microscopy, and Western blot analyses were applied to measure motor function, neurons and neurofilament density, dendrites/axon areas, myelin integrity, and Nogo-A/NgR/RhoA/ROCK expression in the motor cortex. Results. The mice in the HCP+CIMT group had better motor function, greater neurons and neurofilament density, dendrites/axon areas, myelin integrity, and lower Nogo-A/NgR/RhoA/ROCK expression in the motor cortex than the HCP and HCP+UNCIMT groups (P < .05). Moreover, the expression of Nogo-A/NgR/RhoA/ROCK, the improvement of neural remodeling and motor function of mice in the HCP+SN group were similar to those in the HCP+CIMT group (P > .05). The neural remodeling and motor function of the HCP+SN+CIMT group were significantly greater than those in the HCP+SN and HCP+CIMT groups (P < .05). Motor function were positively correlated with the density of neurons (r = 0.450 and 0.309, respectively; P < .05) and neurofilament (r = 0.717 and 0.567, respectively; P < .05). Conclusions. CIMT might promote the remodeling of neurons, neurofilament, dendrites/axon areas, and myelin in the motor cortex by partially inhibiting the Nogo-A/NgR/RhoA/ROCK pathway, thereby promoting the improvement of motor function in HCP mice.

Neuroligin-2 as a central organizer of inhibitory synapses in health and disease

Postsynaptic organizational protein complexes play central roles both in orchestrating synapse formation and in defining the functional properties of synaptic transmission that together shape the flow of information through neuronal networks. A key component of these organizational protein complexes is the family of synaptic adhesion proteins called neuroligins. Neuroligins form transsynaptic bridges with presynaptic neurexins to regulate various aspects of excitatory and inhibitory synaptic transmission. Neuroligin-2 (NLGN2) is the only member that acts exclusively at GABAergic inhibitory synapses. Altered expression and mutations in NLGN2 and several of its interacting partners are linked to cognitive and psychiatric disorders, including schizophrenia, autism, and anxiety. Research on NLGN2 has fundamentally shaped our understanding of the molecular architecture of inhibitory synapses. Here, we discuss the current knowledge on the molecular and cellular functions of mammalian NLGN2 and its role in the neuronal circuitry that regulates behavior in rodents and humans.

The Parvalbumin Hypothesis of Autism Spectrum Disorder

The prevalence of autism spectrum disorder (ASD)-a type of neurodevelopmental disorder-is increasing and is around 2% in North America, Asia, and Europe. Besides the known genetic link, environmental, epigenetic, and metabolic factors have been implicated in ASD etiology. Although highly heterogeneous at the behavioral level, ASD comprises a set of core symptoms including impaired communication and social interaction skills as well as stereotyped and repetitive behaviors. This has led to the suggestion that a large part of the ASD phenotype is caused by changes in a few and common set of signaling pathways, the identification of which is a fundamental aim of autism research. Using advanced bioinformatics tools and the abundantly available genetic data, it is possible to classify the large number of ASD-associated genes according to cellular function and pathways. Cellular processes known to be impaired in ASD include gene regulation, synaptic transmission affecting the excitation/inhibition balance, neuronal Ca2+ signaling, development of short-/long-range connectivity (circuits and networks), and mitochondrial function. Such alterations often occur during early postnatal neurodevelopment. Among the neurons most affected in ASD as well as in schizophrenia are those expressing the Ca2+-binding protein parvalbumin (PV). These mainly inhibitory interneurons present in many different brain regions in humans and rodents are characterized by rapid, non-adaptive firing and have a high energy requirement. PV expression is often reduced at both messenger RNA (mRNA) and protein levels in human ASD brain samples and mouse ASD (and schizophrenia) models. Although the human PVALB gene is not a high-ranking susceptibility/risk gene for either disorder and is currently only listed in the SFARI Gene Archive, we propose and present supporting evidence for the Parvalbumin Hypothesis, which posits that decreased PV level is causally related to the etiology of ASD (and possibly schizophrenia).

Molecular mechanisms for targeted ASD treatments

The possibility to generate construct valid animal models enabled the development and testing of therapeutic strategies targeting the core features of autism spectrum disorders (ASDs). At the same time, these studies highlighted the necessity of identifying sensitive developmental time windows for successful therapeutic interventions. Animal and human studies also uncovered the possibility to stratify the variety of ASDs in molecularly distinct subgroups, potentially facilitating effective treatment design. Here, we focus on the molecular pathways emerging as commonly affected by mutations in diverse ASD-risk genes, on their role during critical windows of brain development and the potential treatments targeting these biological processes.

Neuroligin dependence of social behaviour in Caenorhabditis elegans provides a model to investigate an autism-associated gene

Autism spectrum disorder (ASD) is characterized by a triad of behavioural impairments including social behaviour. Neuroligin, a trans-synaptic adhesion molecule, has emerged as a penetrant genetic determinant of behavioural traits that signature the neuroatypical behaviours of autism. However, the function of neuroligin in social circuitry and the impact of genetic variation to this gene is not fully understood. Indeed, in animal studies designed to model autism, there remains controversy regarding the role of neuroligin dysfunction in the expression of disrupted social behaviours. The model organism, Caenorhabditis elegans, offers an informative experimental platform to investigate the impact of genetic variants on social behaviour. In a number of paradigms, it has been shown that inter-organismal communication by chemical cues regulates C. elegans social behaviour. We utilize this social behaviour to investigate the effect of autism-associated genetic variants within the social domain of the research domain criteria. We have identified neuroligin as an important regulator of social behaviour and segregate the importance of this gene to the recognition and/or processing of social cues. We also use CRISPR/Cas9 to edit an R-C mutation that mimics a highly penetrant human mutation associated with autism. C. elegans carrying this mutation phenocopy the behavioural dysfunction of a C. elegans neuroligin null mutant, thus confirming its significance in the regulation of animal social biology. This highlights that quantitative behaviour and precision genetic intervention can be used to manipulate discrete social circuits of the worm to provide further insight into complex social behaviour.

Cyclocreatine treatment ameliorates the cognitive, autistic and epileptic phenotype in a mouse model of Creatine Transporter Deficiency

Creatine Transporter Deficiency (CTD) is an inborn error of metabolism presenting with intellectual disability, behavioral disturbances and epilepsy. There is currently no cure for this disorder. Here, we employed novel biomarkers for monitoring brain function, together with well-established behavioral readouts for CTD mice, to longitudinally study the therapeutic efficacy of cyclocreatine (cCr) at the preclinical level. Our results show that cCr treatment is able to partially correct hemodynamic responses and EEG abnormalities, improve cognitive deficits, revert autistic-like behaviors and protect against seizures. This study provides encouraging data to support the potential therapeutic benefit of cyclocreatine or other chemically modified lipophilic analogs of Cr.

Autism spectrum disorder-like behavior caused by reduced excitatory synaptic transmission in pyramidal neurons of mouse prefrontal cortex

Autism spectrum disorder (ASD) is thought to result from deviation from normal development of neural circuits and synaptic function. Many genes with mutation in ASD patients have been identified. Here we report that two molecules associated with ASD susceptibility, contactin associated protein-like 2 (CNTNAP2) and Abelson helper integration site-1 (AHI1), are required for synaptic function and ASD-related behavior in mice. Knockdown of CNTNAP2 or AHI1 in layer 2/3 pyramidal neurons of the developing mouse prefrontal cortex (PFC) reduced excitatory synaptic transmission, impaired social interaction and induced mild vocalization abnormality. Although the causes of reduced excitatory transmission were different, pharmacological enhancement of AMPA receptor function effectively restored impaired social behavior in both CNTNAP2- and AHI1-knockdown mice. We conclude that reduced excitatory synaptic transmission in layer 2/3 pyramidal neurons of the PFC leads to impaired social interaction and mild vocalization abnormality in mice.

CNTNAP2 or AHI1 are autism-associated genes. Here the authors show using knockdown of the genes that this results in reduced excitatory synaptic transmission in layer 2/3 pyramidal neurons in the prefrontal cortex and is associated with impaired social interaction in mice.

The neuroligins and the synaptic pathway in Autism Spectrum Disorder

The genetics underlying autism spectrum disorder (ASD) is complex and heterogeneous, and de novo variants are found in genes converging in functional biological processes. Neuronal communication, including trans-synaptic signaling involving two families of cell-adhesion proteins, the presynaptic neurexins and the postsynaptic neuroligins, is one of the most recurrently affected pathways in ASD. Given the role of these proteins in determining synaptic function, abnormal synaptic plasticity and failure to establish proper synaptic contacts might represent mechanisms underlying risk of ASD. More than 30 mutations have been found in the neuroligin genes. Most of the resulting residue substitutions map in the extracellular, cholinesterase-like domain of the protein, and impair protein folding and trafficking. Conversely, the stalk and intracellular domains are less affected. Accordingly, several genetic animal models of ASD have been generated, showing behavioral and synaptic alterations. The aim of this review is to discuss the current knowledge on ASD-linked mutations in the neuroligin proteins and their effect on synaptic function, in various brain areas and circuits.

Hyperactive and impulsive behaviors of LMTK1 knockout mice

Lemur tail kinase 1 (LMTK1), previously called Apoptosis-Associated Tyrosine Kinase (AATYK), remains an uncharacterized Ser/Thr protein kinase that is predominantly expressed in the brain. It is recently reported that LMTK1A, an isoform of LMTK1, binds to recycling endosomes through its palmitoylation and regulates endosomal trafficking by suppressing the activity of Rab11 small GTPase. In neurons, knockdown or knockout of LMTK1 results in longer axons, greater branching of dendrites and increased number of spines, suggesting that LMTK1 plays a role in neuronal circuit formation. However, its in vivo function remained to be investigated. Here, we examined the brain structures and behaviors of LMTK1 knockout (KO) mice. LMTK1 was expressed in most neurons throughout the brain. The overall brain structure appeared to be normal in LMTK1 KO mice, but the numbers of synapses were increased. LMTK1 KO mice had a slight impairment in memory formation and exhibited distinct psychiatric behaviors such as hyperactivity, impulsiveness and high motor coordination without social interaction deficits. Some of these abnormal behaviors represent core features of attention deficit hyperactive disorder (ADHD), suggesting the possible involvement of LMTK1 in the pathogenesis of ADHD.

Setd1a Insufficiency in Mice Attenuates Excitatory Synaptic Function and Recapitulates Schizophrenia-Related Behavioral Abnormalities

SETD1A encodes a histone methyltransferase whose de novo mutations are identified in schizophrenia (SCZ) patients and confer a large increase in disease risk. Here, we generate Setd1a mutant mice carrying the frameshift mutation that closely mimics a loss-of-function variant of SCZ. Our Setd1a (+/-) mice display various behavioral abnormalities relevant to features of SCZ, impaired excitatory synaptic transmission in layer 2/3 (L2/3) pyramidal neurons of the medial prefrontal cortex (mPFC), and altered expression of diverse genes related to neurodevelopmental disorders and synaptic functions in the mPFC. RNAi-mediated Setd1a knockdown (KD) specifically in L2/3 pyramidal neurons of the mPFC only recapitulates impaired sociality among multiple behavioral abnormalities of Setd1a (+/-) mice. Optogenetics-assisted selective stimulation of presynaptic neurons combined with Setd1a KD reveals that Setd1a at postsynaptic site is essential for excitatory synaptic transmission. Our findings suggest that reduced SETD1A may attenuate excitatory synaptic function and contribute to the pathophysiology of SCZ.

An altered glial phenotype in the NL3R451C mouse model of autism

Autism Spectrum Disorder (ASD; autism) is a neurodevelopmental disorder characterised by deficits in social communication, and restricted and/or repetitive behaviours. While the precise pathophysiologies are unclear, increasing evidence supports a role for dysregulated neuroinflammation in the brain with potential effects on synapse function. Here, we studied characteristics of microglia and astrocytes in the Neuroligin-3 (NL3^R451C) mouse model of autism since these cell types are involved in regulating both immune and synapse function. We observed increased microglial density in the dentate gyrus (DG) of NL3^R451C mice without morphological differences. In contrast, WT and NL3^R451C mice had similar astrocyte density but astrocyte branch length, the number of branch points, as well as cell radius and area were reduced in the DG of NL3^R451C mice. Because retraction of astrocytic processes has been linked to altered synaptic transmission and dendrite formation, we assessed for regional changes in pre- and postsynaptic protein expression in the cortex, striatum and cerebellum in NL3^R451C mice. NL3^R451C mice showed increased striatal postsynaptic density 95 (PSD-95) protein levels and decreased cortical expression of synaptosomal-associated protein 25 (SNAP-25). These changes could contribute to dysregulated neurotransmission and cognition deficits previously reported in these mice.

Brain-wide structural and functional disruption in mice with oligodendrocyte-specific Nf1 deletion is rescued by inhibition of nitric oxide synthase

This study assessed the effects of myelin decompaction on motor behavior and brain-wide structural and functional connectivity, and the effect of nitric oxide synthase inhibition by N-nitro-l-arginine methyl ester (L-NAME) on these imaging measures. We report that inducible oligodendrocyte-specific inactivation of the Nf1 gene, which causes myelin decompaction, results in reduced initial motor coordination. Using diffusion-based magnetic resonance imaging (MRI), we show reduced myelin integrity, and using functional MRI, we show reduced functional connectivity in awake passive mice. L-NAME administration results in rescue of the pathology at the mesoscopic level, as measured using imaging procedures that can be directly applied to humans to study treatment efficacy in clinical trials.

Neurofibromin gene (NF1) mutation causes neurofibromatosis type 1 (NF1), a disorder in which brain white matter deficits identified by neuroimaging are common, yet of unknown cellular etiology. In mice, Nf1 loss in adult oligodendrocytes causes myelin decompaction and increases oligodendrocyte nitric oxide (NO) levels. Nitric oxide synthase (NOS) inhibitors rescue this pathology. Whether oligodendrocyte pathology is sufficient to affect brain-wide structure and account for NF1 imaging findings is unknown. Here we show that Nf1 gene inactivation in adult oligodendrocytes (Plp-Nf1^fl/+ mice) results in a motor coordination deficit. Magnetic resonance imaging in awake mice showed that fractional anisotropy is reduced in Plp-Nf1^fl/+ corpus callosum and that interhemispheric functional connectivity in the motor cortex is also reduced, consistent with disrupted myelin integrity. Furthermore, NOS-specific inhibition rescued both measures. These results suggest that oligodendrocyte defects account for aspects of brain dysfunction in NF1 that can be identified by neuroimaging and ameliorated by NOS inhibition.

A Novel Chd8 Mutant Mouse Displays Altered Ultrasonic Vocalizations and Enhanced Motor Coordination

Mutations in CHD8 are among the most common autism-causing genetic defects identified in human genomics studies. Therefore, many labs have attempted to model this disorder by generating mice with mutations in Chd8. Using a gene trap inserted after Exon 31, we created a novel Chd8 mutant mouse (Chd8^+/E31T ) and characterized its behavior on several different assays thought to have face validity for the human condition, attempting to model both the core symptoms (repetitive behaviors and social communication impairments) and common comorbidities (motor deficits, anxiety, and intellectual disability). We found that Chd8^+/E31T mice showed no difference compared to wild-type mice in amount of self-grooming, reproducing the negative finding most other studies have reported. Unlike some of the other published lines, Chd8^+/E31T mice did not show deficits in the three-chamber test for social novelty preference. A few studies have examined ultrasonic vocalizations in Chd8 mutant mice, but we are the first to report an increase in call length for adult mice. Additionally, we found that in contrast to previous published lines, Chd8^+/E31T mice displayed no anxiety-like behaviors or learning impairments but showed paradoxically significant improvement in motor function. The inconsistencies in behavioral phenotypes in the Chd8 mutant mice generated by different laboratories poses a challenge for modeling autism spectrum disorder and preclinical studies in mice going forward and warrants further investigation into the molecular consequences of the different mutations in Chd8 and the functional impact on behavior. LAY SUMMARY: Several different mouse models carrying mutations in the Chd8 gene have been created to study the effects of these autism-causing mutations in the laboratory. The current study characterizes a novel Chd8 mutant mouse model as well as summarizes data from previously published Chd8 mutant mice. The inconsistencies between different studies are concerning, but future research into the reasons why these inconsistencies occur may help us understand why patients with various mutations have different degrees of symptom severity. Autism Res 2020, 13: 1685-1697. © 2020 International Society for Autism Research and Wiley Periodicals LLC.

Neuroligins and Neurodevelopmental Disorders: X-Linked Genetics

Autism spectrum disorder (ASD) is a neurodevelopmental disorder that results in social-communication impairments, as well as restricted and repetitive behaviors. Moreover, ASD is more prevalent in males, with a male to female ratio of 4 to 1. Although the underlying etiology of ASD is generally unknown, recent advances in genome sequencing have facilitated the identification of a host of associated genes. Among these, synaptic proteins such as cell adhesion molecules have been strongly linked with ASD. Interestingly, many large genome sequencing studies exclude sex chromosomes, which leads to a shift in focus toward autosomal genes as targets for ASD research. However, there are many genes on the X chromosome that encode synaptic proteins, including strong candidate genes. Here, we review findings regarding two members of the neuroligin (NLGN) family of postsynaptic adhesion molecules, NLGN3 and NLGN4. Neuroligins have multiple isoforms (NLGN1-4), which are both autosomal and sex-linked. The sex-linked genes, NLGN3 and NLGN4, are both on the X chromosome and were among the first few genes to be linked with ASD and intellectual disability (ID). In addition, there is a less studied human neuroligin on the Y chromosome, NLGN4Y, which forms an X-Y pair with NLGN4X. We will discuss recent findings of these neuroligin isoforms regarding function at the synapse in both rodent models and human-derived differentiated neurons, and highlight the exciting challenges moving forward to a better understanding of ASD/ID.

Complex Interactions between Genes and Social Environment Cause Phenotypes Associated with Autism Spectrum Disorders in Mice

The etiology of autism spectrum disorders (ASDs) is a complex combination of genetic and environmental factors. Neuroligin3, a synaptic adhesion protein, and cytoplasmic FMR1 interacting protein 1 (CYFIP1), a regulator of protein translation and actin polymerization, are two proteins associated with ASDs that interact in neurons in vivo Here, we investigated the role of the Neuroligin3/CYFIP1 pathway in behavioral functioning and synapse formation in mice and found that it contributes to motor learning and synapse formation in males. Similar investigation in female mice revealed an absence of such phenotypes, suggesting that females are protected against mutations affecting this pathway. Previously, we showed that the social environment influences the behavior of male mice. We extended this finding and found that the transcriptome of wild-type mice housed with their mutant littermates, lacking Neuroligin3, differed from the transcriptome of wild-type mice housed together. Altogether, these results identify the role of the Neuroligin3/CYFIP1 pathway in male mouse behavior and highlight its sensitivity to social environment.

Rescue of oxytocin response and social behaviour in a mouse model of autism

One of the most fundamental challenges in developing treatments for autism-spectrum disorders is the heterogeneity of the condition. More than one hundred genetic mutations confer high risk for autism, with each individual mutation accounting for only a small fraction of autism cases^1–3. Subsets of risk genes can be grouped into functionally-related pathways, most prominently synaptic proteins, translational regulation, and chromatin modifications. To possibly circumvent this genetic complexity, recent therapeutic strategies have focused on the neuropeptides oxytocin and vasopressin^4–6 which regulate aspects of social behavior in mammals⁷. However, whether genetic risk factors might predispose to autism due to modification of oxytocinergic signaling remains largely unknown. Here, we report that an autism-associated mutation in the synaptic adhesion molecule neuroligin-3 (Nlgn3) results in impaired oxytocin signaling in dopaminergic neurons and in altered social novelty responses in mice. Surprisingly, loss of Nlgn3 is accompanied by a disruption of translation homeostasis in the ventral tegmental area. Treatment of Nlgn3^KO mice with a novel, highly specific, brain-penetrant inhibitor of MAP-kinase interacting kinases resets mRNA translation and restores oxytocin and social novelty responses. Thus, this work identifies an unexpected convergence between the genetic autism risk factor Nlgn3, translational regulation, and oxytocinergic signaling. Focus on such common core plasticity elements might provide a pragmatic approach to reduce the heterogeneity of autism. Ultimately, this would allow for mechanism-based stratification of patient populations to increase the success of therapeutic interventions.

Profound and redundant functions of arcuate neurons in obesity development

The current obesity epidemic faces a lack of mechanistic insights. It is known that the acute activity changes of a growing number of brain neurons rapidly alter feeding behaviour; however, how these changes translate to obesity development and the fundamental mechanism underlying brain neurons in controlling body weight remain elusive. Here, we show that chronic activation of hypothalamic arcuate GABAergic (GABA+), agouti-related protein (AgRP) neurons or arcuate non-AgRP GABA+ neurons leads to obesity, which is similar to the obese phenotype observed in ob/ob mice. Conversely, chronic inhibition of arcuate GABA+, but not AgRP, neurons reduces ageing-related weight gain and corrects ob/ob obesity. These results demonstrate that the modulation of Arc GABA+ neuron activity is a fundamental mechanism of body-weight regulation, and that arcuate GABA+ neurons are the major mediator of leptin action, with a profound and redundant role in obesity development.

Functional mosaic organization of neuroligins in neuronal circuits

Complex brain circuitry with feedforward and feedback systems regulates neuronal activity, enabling neural networks to process and drive the entire spectrum of cognitive, behavioral, sensory, and motor functions. Simultaneous orchestration of distinct cells and interconnected neural circuits is underpinned by hundreds of synaptic adhesion molecules that span synaptic junctions. Dysfunction of a single molecule or molecular interaction at synapses can lead to disrupted circuit activity and brain disorders. Neuroligins, a family of cell adhesion molecules, were first identified as postsynaptic-binding partners of presynaptic neurexins and are essential for synapse specification and maturation. Here, we review recent advances in our understanding of how this family of adhesion molecules controls neuronal circuit assembly by acting in a synapse-specific manner.

Autism-associated synaptic mutations impact the gut-brain axis in mice

Interactions between the gut microbiome and the brain affect mood and behaviour in health and disease. Using preclinical animal models, recent discoveries begin to explain how bacteria in the gut influence our mood as well as highlighting new findings relevant to autism. Autism-associated gene mutations known to alter synapse function in the CNS also affect inflammatory response and modify the enteric nervous system resulting in abnormal gastrointestinal motility and structure. Strikingly, these mutations additionally affect the gut microbiome in mice. This review describes the changes in gut physiology and microbiota in mouse models of autism with modified synapse function. The rationale for different regions of the gastrointestinal tract having variable susceptibility to dysfunction is also discussed. To dissect underlying biological mechanisms involving gut-brain axis dysfunction in preclinical models, a range of multidisciplinary approaches are required. This research will provide insights into the role of the gut-brain axis in health and neurodevelopmental disorders including autism.

Functional and molecular heterogeneity of D2R neurons along dorsal ventral axis in the striatum

Action control is a key brain function determining the survival of animals in their environment. In mammals, neurons expressing dopamine D2 receptors (D2R) in the dorsal striatum (DS) and the nucleus accumbens (Acb) jointly but differentially contribute to the fine regulation of movement. However, their region-specific molecular features are presently unknown. By combining RNAseq of striatal D2R neurons and histological analyses, we identified hundreds of novel region-specific molecular markers, which may serve as tools to target selective subpopulations. As a proof of concept, we characterized the molecular identity of a subcircuit defined by WFS1 neurons and evaluated multiple behavioral tasks after its temporally-controlled deletion of D2R. Consequently, conditional D2R knockout mice displayed a significant reduction in digging behavior and an exacerbated hyperlocomotor response to amphetamine. Thus, targeted molecular analyses reveal an unforeseen heterogeneity in D2R-expressing striatal neuronal populations, underlying specific D2R's functional features in the control of specific motor behaviors.

Forebrain excitatory neuron-specific SENP2 knockout mouse displays hyperactivity, impaired learning and memory, and anxiolytic-like behavior

Sentrin/SUMO-specific protease 2 (SENP2) is a member of SENPs family involved in maturation of SUMO precursors and deSUMOylation of specific target, and is highly expressed in the central nervous system (CNS). Although SENP2 has been shown to modulate embryonic development, fatty acid metabolism, atherosclerosis and epilepsy, the function of SENP2 in the CNS remains poorly understood. To address the role of SENP2 in the CNS and its potential involvement in neuropathology, we generated SENP2 conditional knockout mice by crossing floxed SENP2 mice with CaMKIIα-Cre transgenic mice. Behavioral tests revealed that SENP2 ablation induced hyper-locomotor activity, anxiolytic-like behaviors, spatial working memory impairment and fear-associated learning defect. In line with these observations, our RNA sequencing (RNA-seq) data identified a variety of differential expression genes that are particularly enriched in locomotion, learning and memory related biologic process. Taken together, our results indicated that SENP2 plays a critical role in emotional and cognitive regulation. This SENP2 conditional knockout mice model may help reveal novel mechanisms that underlie a variety of neuropsychiatric disorders associated with anxiety and cognition.

Elevated protein synthesis in microglia causes autism-like synaptic and behavioral aberrations

Mutations that inactivate negative translation regulators cause autism spectrum disorders (ASD), which predominantly affect males and exhibit social interaction and communication deficits and repetitive behaviors. However, the cells that cause ASD through elevated protein synthesis resulting from these mutations remain unknown. Here we employ conditional overexpression of translation initiation factor eIF4E to increase protein synthesis in specific brain cells. We show that exaggerated translation in microglia, but not neurons or astrocytes, leads to autism-like behaviors in male mice. Although microglial eIF4E overexpression elevates translation in both sexes, it only increases microglial density and size in males, accompanied by microglial shift from homeostatic to a functional state with enhanced phagocytic capacity but reduced motility and synapse engulfment. Consequently, cortical neurons in the mice have higher synapse density, neuroligins, and excitation-to-inhibition ratio compared to control mice. We propose that functional perturbation of male microglia is an important cause for sex-biased ASD.

Altered Caecal Neuroimmune Interactions in the Neuroligin-3R451C Mouse Model of Autism

The intrinsic nervous system of the gut interacts with the gut-associated lymphoid tissue (GALT) via bidirectional neuroimmune interactions. The caecum is an understudied region of the gastrointestinal (GI) tract that houses a large supply of microbes and is involved in generating immune responses. The caecal patch is a lymphoid aggregate located within the caecum that regulates microbial content and immune responses. People with Autism Spectrum Disorder (ASD; autism) experience serious GI dysfunction, including inflammatory disorders, more frequently than the general population. Autism is a highly prevalent neurodevelopmental disorder defined by the presence of repetitive behavior or restricted interests, language impairment, and social deficits. Mutations in genes encoding synaptic adhesion proteins such as the R451C missense mutation in neuroligin-3 (NL3) are associated with autism and impair synaptic transmission. We previously reported that NL3^R451C mice, a well-established model of autism, have altered enteric neurons and GI dysfunction; however, whether the autism-associated R451C mutation alters the caecal enteric nervous system and immune function is unknown. We assessed for gross anatomical changes in the caecum and quantified the proportions of caecal submucosal and myenteric neurons in wild-type and NL3^R451C mice using immunofluorescence. In the caecal patch, we assessed total cellular density as well as the density and morphology of Iba-1 labeled macrophages to identify whether the R451C mutation affects neuro-immune interactions. NL3^R451C mice have significantly reduced caecal weight compared to wild-type mice, irrespective of background strain. Caecal weight is also reduced in mice lacking Neuroligin-3. NL3^R451C caecal ganglia contain more neurons overall and increased numbers of Nitric Oxide (NO) producing neurons (labeled by Nitric Oxide Synthase; NOS) per ganglion in both the submucosal and myenteric plexus. Overall caecal patch cell density was unchanged however NL3^R451C mice have an increased density of Iba-1 labeled enteric macrophages. Macrophages in NL3^R451C were smaller and more spherical in morphology. Here, we identify changes in both the nervous system and immune system caused by an autism-associated mutation in Nlgn3 encoding the postsynaptic cell adhesion protein, Neuroligin-3. These findings provide further insights into the potential modulation of neural and immune pathways.