Autism-associated mutations in ProSAP2/Shank3 impair synaptic transmission and neurexin-neuroligin-mediated transsynaptic signaling

Decrease of GSK-3β Activity in the Anterior Cingulate Cortex of Shank3b -/- Mice Contributes to Synaptic and Social Deficiency

Social deficiency is one of the core syndromes of autism spectrum disorders (ASD), for which the underlying developmental mechanism still remains elusive. Anterior cingulate cortex (ACC) plays a key role in integrating social information and regulating social behavior. Recent studies have indicated that synaptic dysfunction in ACC is essential for ASD social defects. In the present study, we investigated the development of synapses and the roles of glycogen synthase kinase 3β (GSK-3β), which mediates multiple synaptic signaling pathways in ACC by using Shank3b^−/− mice (a widely used ASD mouse model). Our data revealed that Shank3b mutation abolished the social induced c-Fos expression in ACC. From 4 weeks post-birth, neurons in Shank3b^−/− ACC exhibited an obvious decrease in spine density and stubby spines. The length and thickness of post-synaptic density (PSD), the expression of vesicular glutamate transporter 2 (vGlut2) and glutamate receptor 2 (GluR2), and the frequency of miniature excitatory post-synaptic currents (mEPSCs) were significantly reduced in Shank3b^−/−ACC. Interestingly, the levels of phosphorylated GSK-3β (Ser9), which inhibits the activity of GSK-3β, decreased along the same time course as the levels of GluR2 increased in ACC during development. Shank3b mutation leads to a dramatic increase of pGSK-3β (Ser9), and decrease of pPSD95 (a substrate of GSK-3β) and GluR2. Local delivery of AAV expressing constitutively active GSK-3β restored the expression of GluR2, increased the spine density and the number of mature spines. More importantly, active GSK-3β significantly promoted the social activity of Shank3b^−/− mice. These data, in together, indicate that decrease of GSK-3β activity in ACC may contribute to the synaptic and social defects of Shank3b^−/− mice. Enhancing GSK-3β activity may be utilized to treat ASD in the future.

GABAA receptor availability is not altered in adults with autism spectrum disorder or in mouse models

Preliminary studies have suggested that γ-aminobutyric acid type A (GABA_A) receptors, and potentially the GABA_A α5 subtype, are deficient in autism spectrum disorder (ASD). However, prior studies have been confounded by the effects of medications, and these studies did not compare findings across different species. We measured both total GABA_A and GABA_A α5 receptor availability in two positron emission tomography imaging studies. We used the tracer [¹¹C]flumazenil in 15 adults with ASD and in 15 control individuals without ASD and the tracer [¹¹C]Ro15-4513 in 12 adults with ASD and in 16 control individuals without ASD. All participants were free of medications. We also performed autoradiography, using the same tracers, in three mouse models of ASD: the Cntnap2 knockout mouse, the Shank3 knockout mouse, and mice carrying a 16p11.2 deletion. We found no differences in GABA_A receptor or GABA_A α5 subunit availability in any brain region of adults with ASD compared to those without ASD. There were no differences in GABA_A receptor or GABA_A α5 subunit availability in any of the three mouse models. However, adults with ASD did display altered performance on a GABA-sensitive perceptual task. Our data suggest that GABA_A receptor availability may be normal in adults with ASD, although GABA signaling may be functionally impaired.

Overlapping and unique toxic effects of three alternative antifouling biocides (Diuron, Irgarol 1051®, Sea-Nine 211®) on non-target marine fish

The use of alternative biocides has increased due to their economic and ecological relevance. Although data regarding the toxicity of commercial alternative biocides in marine organisms are accumulating, little is known about their toxic pathways or mechanisms. To compare the toxic effects of commercial alternative biocides on non-target pelagic fish (flounder) embryos, we investigated the adverse effects of developmental malformation and transcriptional changes. Three biocides including Diuron, Irgarol 1051^® and Sea-Nine 211^® produced a largely overlapping suite of developmental malformations, including tail-fin fold defects and dorsal body axis curvature. In our test, the potencies of these biocides were ranked in the following order with respect to malformation and mortalities: Sea-Nine 211^® > Irgarol 1051^® > Diuron. Consistent with the toxicity rankings, the expression of genes related to heart formation was greater in embryonic flounder exposed to Sea-Nine 211^® than in those exposed to Irgarol 1051^® or Diuron, while expression of genes related to fin malformation was greater in the Irgarol 1051^® exposure group. In analyses of differential gene expression (DEG) profiles (fold change of genes with a cutoff P < 0.05) by high-throughput sequencing (RNA-seq), genes associated with nervous system development, transmembrane transport activity, and muscle cell development were significantly changed commonly. Embryos exposed to Diuron showed changes related to cellular protein localization, whereas genes associated with immune system processes were up-regulated significantly in embryos exposed to Irgarol 1051^®. Genes related to actin filament organization and embryonic morphogenesis were up-regulated in embryos exposed to Sea-Nine 211^®. Overall, our study provides a better understanding of the overlapping and unique developmental toxic effects of three commercial booster biocides through transcriptomic analyses in a non-target species, embryonic flounder.

SAP97-mediated rescue of NMDA receptor surface distribution in a neuronal model of Huntington's disease

Huntington's disease (HD) is a genetic neurodegenerative disorder caused by an expansion of the CAG repeat tract in the HTT gene, leading to motor, cognitive, and psychiatric symptoms. At the cellular level, NMDA-type glutamate receptors are upregulated at glutamatergic extrasynaptic sites in HD, triggering cell death signaling pathways and driving HD neurodegeneration. Extrasynaptic and synaptic glutamate receptor trafficking and surface distribution are regulated by the α and β N-terminal isoforms of SAP97, a postsynaptic density protein localized at glutamatergic synapses. Here we examined the surface distribution of NMDARs and AMPARs in a cellular model of HD, and whether the manipulation of individual SAP97 isoforms can regulate receptor distribution in diseased neurons. Using dSTORM super-resolution imaging, we reveal that mutant HTT drives the elevation of extrasynaptic NMDAR clusters located 100-500 nm from the postsynaptic density. This was accompanied by a decline in synaptic NMDAR-mediated currents while surface NMDAR-mediated currents remained unchanged. These effects were induced within 3 days of mutant HTT expression in rat hippocampal neurons in vitro, and were specific for NMDARs and not observed with AMPARs. Intriguingly, upregulation of either α- or βSAP97 expression increased synaptic and/or perisynaptic NMDAR localization and prevented the shift of NMDARs to extrasynaptic sites in mutant HTT neurons. This was accompanied by the rescue of normal synaptic NMDAR-mediated currents. Taken together, our high-resolution data reveals plasticity in surface NMDAR localization driven by mutant HTT and identifies the similar but independent roles of SAP97 N-terminal isoforms in maintaining normal synaptic function in pathological states.

Perineuronal Nets and Their Role in Synaptic Homeostasis

Extracellular matrix (ECM) molecules that are released by neurons and glial cells form perineuronal nets (PNNs) and modulate many neuronal and glial functions. PNNs, whose structure is still not known in detail, surround cell bodies and dendrites, which leaves free space for synapses to come into contact. A reduction in the expression of many neuronal ECM components adversely affects processes that are associated with synaptic plasticity, learning, and memory. At the same time, increased ECM activity, e.g., as a result of astrogliosis following brain damage or in neuroinflammation, can also have harmful consequences. The therapeutic use of enzymes to attenuate elevated neuronal ECM expression after injury or in Alzheimer’s disease has proven to be beneficial by promoting axon growth and increasing synaptic plasticity. Yet, severe impairment of ECM function can also lead to neurodegeneration. Thus, it appears that to ensure healthy neuronal function a delicate balance of ECM components must be maintained. In this paper we review the structure of PNNs and their components, such as hyaluronan, proteoglycans, core proteins, chondroitin sulphate proteoglycans, tenascins, and Hapln proteins. We also characterize the role of ECM in the functioning of the blood-brain barrier, neuronal communication, as well as the participation of PNNs in synaptic plasticity and some clinical aspects of perineuronal net impairment. Furthermore, we discuss the participation of PNNs in brain signaling. Understanding the molecular foundations of the ways that PNNs participate in brain signaling and synaptic plasticity, as well as how they change in physiological and pathological conditions, may help in the development of new therapies for many degenerative and inflammatory diseases of the brain.

Autism Spectrum Disorder-Related Syndromes: Modeling with Drosophila and Rodents

Whole exome analyses have identified a number of genes associated with autism spectrum disorder (ASD) and ASD-related syndromes. These genes encode key regulators of synaptogenesis, synaptic plasticity, cytoskeleton dynamics, protein synthesis and degradation, chromatin remodeling, transcription, and lipid homeostasis. Furthermore, in silico studies suggest complex regulatory networks among these genes. Drosophila is a useful genetic model system for studies of ASD and ASD-related syndromes to clarify the in vivo roles of ASD-associated genes and the complex gene regulatory networks operating in the pathogenesis of ASD and ASD-related syndromes. In this review, we discuss what we have learned from studies with vertebrate models, mostly mouse models. We then highlight studies with Drosophila models. We also discuss future developments in the related field.

Synapse-to-Nucleus Signaling in Neurodegenerative and Neuropsychiatric Disorders

Synapse-to-nucleus signaling is critical for converting signals received at synapses into transcriptional programs essential for cognition, memory, and emotion. This neuronal mechanism usually involves activity-dependent translocation of synaptonuclear factors from synapses to the nucleus resulting in regulation of transcriptional programs underlying synaptic plasticity. Acting as synapse-to-nucleus messengers, amyloid precursor protein intracellular domain associated-1 protein, cAMP response element binding protein (CREB)-regulated transcription coactivator-1, Jacob, nuclear factor kappa-light-chain-enhancer of activated B cells, RING finger protein 10, and SH3 and multiple ankyrin repeat domains 3 play essential roles in synapse remodeling and plasticity, which are considered the cellular basis of memory. Other synaptic proteins, such as extracellular signal-regulated kinase, calcium/calmodulin-dependent protein kinase II gamma, and CREB2, translocate from dendrites or cytosol to the nucleus upon synaptic activity, suggesting that they could contribute to synapse-to-nucleus signaling. Notably, some synaptonuclear factors converge on the transcription factor CREB, indicating that CREB signaling is a key hub mediating integration of synaptic signals into transcriptional programs required for neuronal function and plasticity. Although major efforts have been focused on identification and regulatory mechanisms of synaptonuclear factors, the relevance of synapse-to-nucleus communication in brain physiology and pathology is still unclear. Recent evidence, however, indicates that synaptonuclear factors are implicated in neuropsychiatric, neurodevelopmental, and neurodegenerative disorders, suggesting that uncoupling synaptic activity from nuclear signaling may prompt synapse pathology, contributing to a broad spectrum of brain disorders. This review summarizes current knowledge of synapse-to-nucleus signaling in neuron survival, synaptic function and plasticity, and memory. Finally, we discuss how altered synapse-to-nucleus signaling may lead to memory and emotional disturbances, which is relevant for clinical and therapeutic strategies in neurodegenerative and neuropsychiatric diseases.

Enhanced Glutamatergic Currents at Birth in Shank3 KO Mice

Autism spectrum disorders (ASD) are neurodevelopmental disorders induced by genetic and environmental factors. In our recent studies, we showed that the GABA developmental shifts during delivery and the second postnatal week are abolished in two rodent models of ASD. Maternal treatment around birth with bumetanide restored the GABA developmental sequence and attenuated the autism pathogenesis in offspring. Clinical trials conducted in parallel confirmed the usefulness of bumetanide treatment to attenuate the symptoms in children with ASD. Collectively, these observations suggest that an alteration of the GABA developmental sequence is a hallmark of ASD. Here, we investigated whether similar alterations occur in the Shank3 mouse model of ASD. We report that in CA3 pyramidal neurons, the driving force and inhibitory action of GABA are not different in naïve and Shank3-mutant age-matched animals at birth and during the second postnatal week. In contrast, the frequency of spontaneous excitatory postsynaptic currents is already enhanced at birth and persists through postnatal day 15. Therefore, in CA3 pyramidal neurons of Shank3-mutant mice, glutamatergic but not GABAergic activity is affected at early developmental stages, hence reflecting the heterogeneity of mechanisms underlying the pathogenesis of ASD.

Synaptopathology Involved in Autism Spectrum Disorder

Autism spectrum disorder (ASD) encompasses a group of multifactorial neurodevelopmental disorders characterized by impaired social communication, social interaction and repetitive behaviors. ASD affects 1 in 59 children, and is about 4 times more common among boys than among girls. Strong genetic components, together with environmental factors in the early stage of development, contribute to the pathogenesis of ASD. Multiple studies have revealed that mutations in genes like NRXN, NLGN, SHANK, TSC1/2, FMR1, and MECP2 converge on common cellular pathways that intersect at synapses. These genes encode cell adhesion molecules, scaffolding proteins and proteins involved in synaptic transcription, protein synthesis and degradation, affecting various aspects of synapses including synapse formation and elimination, synaptic transmission and plasticity. This suggests that the pathogenesis of ASD may, at least in part, be attributed to synaptic dysfunction. In this article, we will review major genes and signaling pathways implicated in synaptic abnormalities underlying ASD, and discuss molecular, cellular and functional studies of ASD experimental models.

Shank and Zinc Mediate an AMPA Receptor Subunit Switch in Developing Neurons

During development, pyramidal neurons undergo dynamic regulation of AMPA receptor (AMPAR) subunit composition and density to help drive synaptic plasticity and maturation. These normal developmental changes in AMPARs are particularly vulnerable to risk factors for Autism Spectrum Disorders (ASDs), which include loss or mutations of synaptic proteins and environmental insults, such as dietary zinc deficiency. Here, we show how Shank2 and Shank3 mediate a zinc-dependent regulation of AMPAR function and subunit switch from GluA2-lacking to GluA2-containing AMPARs. Over development, we found a concomitant increase in Shank2 and Shank3 with GluA2 at synapses, implicating these molecules as potential players in AMPAR maturation. Since Shank activation and function require zinc, we next studied whether neuronal activity regulated postsynaptic zinc at glutamatergic synapses. Zinc was found to increase transiently and reversibly with neuronal depolarization at synapses, which could affect Shank and AMPAR localization and activity. Elevated zinc induced multiple functional changes in AMPAR, indicative of a subunit switch. Specifically, zinc lengthened the decay time of AMPAR-mediated synaptic currents and reduced their inward rectification in young hippocampal neurons. Mechanistically, both Shank2 and Shank3 were necessary for the zinc-sensitive enhancement of AMPAR-mediated synaptic transmission and act in concert to promote removal of GluA1 while enhancing recruitment of GluA2 at pre-existing Shank puncta. These findings highlight a cooperative local dynamic regulation of AMPAR subunit switch controlled by zinc signaling through Shank2 and Shank3 to shape the biophysical properties of developing glutamatergic synapses. Given the zinc sensitivity of young neurons and its dependence on Shank2 and Shank3, genetic mutations and/or environmental insults during early development could impair synaptic maturation and circuit formation that underlie ASD etiology.

Dietary Zinc Supplementation Prevents Autism Related Behaviors and Striatal Synaptic Dysfunction in Shank3 Exon 13-16 Mutant Mice

The SHANK family of synaptic proteins (SHANK1–3) are master regulators of the organizational structure of excitatory synapses in the brain. Mutations in SHANK1–3 are prevalent in patients with autism spectrum disorders (ASD), and loss of one copy of SHANK3 causes Phelan-McDermid Syndrome, a syndrome in which Autism occurs in >80% of cases. The synaptic stability of SHANK3 is highly regulated by zinc, driving the formation of postsynaptic protein complexes and increases in excitatory synaptic strength. As ASD-associated SHANK3 mutations retain responsiveness to zinc, here we investigated how increasing levels of dietary zinc could alter behavioral and synaptic deficits that occur with ASD. We performed behavioral testing together with cortico-striatal slice electrophysiology on a Shank3^−/− mouse model of ASD (Shank3^{ex13–1616−/−}), which displays ASD-related behaviors and structural and functional deficits at striatal synapses. We observed that 6 weeks of dietary zinc supplementation in Shank3^{ex13–16−/−} mice prevented ASD-related repetitive and anxiety behaviors and deficits in social novelty recognition. Dietary zinc supplementation also increased the recruitment of zinc sensitive SHANK2 to synapses, reduced synaptic transmission specifically through N-methyl-D-aspartate (NMDA)-type glutamate receptors, reversed the slowed decay tau of NMDA receptor (NMDAR)-mediated currents and occluded long term potentiation (LTP) at cortico-striatal synapses. These data suggest that alterations in NMDAR function underlie the lack of NMDAR-dependent cortico-striatal LTP and contribute to the reversal of ASD-related behaviors such as compulsive grooming. Our data reveal that dietary zinc alters neurological function from synapses to behavior, and identifies dietary zinc as a potential therapeutic agent in ASD.

Functional Relevance of Missense Mutations Affecting the N-Terminal Part of Shank3 Found in Autistic Patients

Genetic defects in SHANK genes are associated with autism. Deletions and truncating mutations suggest haploinsufficiency for Shank3 as a major cause of disease which may be analyzed in appropriate Shank deficient mouse models. Here we will focus on the functional analysis of missense mutations found in SHANK genes. The relevance of most of these mutations for Shank function, and their role in autism pathogenesis is unclear. This is partly due to the fact that mutations spare the most well studied functional domains of Shank3, such as the PDZ and SAM domains, or the short proline-rich motifs which are required for interactions with postsynaptic partners Homer, Cortactin, dynamin, IRSp53 and Abi-1. One set of mutations affects the N-terminal part, including the highly conserved SPN domain and ankyrin repeats. Functional analysis from several groups has indicated that these mutations (e.g., R12C; L68P; R300C, and Q321R) interfere with the critical role of Shank3 for synapse formation. More recently the structural analysis of the SPN-ARR module has begun to shed light on the molecular consequences of mutations in the SPN of Shank3. The SPN was identified as a Ras association domain, with high affinities for GTP-bound, active forms of Ras and Rap. The two autism related mutations in this part of the protein, R12C and L68P, both abolish Ras binding. Further work is directed at identifying the consequences of Ras binding to Shank proteins at postsynaptic sites.

Altered Amygdala Excitation and CB1 Receptor Modulation of Aggressive Behavior in the Neuroligin-3R451C Mouse Model of Autism

Understanding neuronal mechanisms underlying aggression in patients with autism spectrum disorder (ASD) could lead to better treatments and prognosis. The Neuroligin-3 (NL3)^R451C mouse model of ASD has a heightened aggressive phenotype, however the biological mechanisms underlying this behavior are unknown. It is well established that NL3^R451C mice have imbalanced excitatory and inhibitory synaptic activity in the hippocampus and somatosensory cortex. The amygdala plays a role in modulating aggressive behavior, however potential changes in synaptic activity in this region have not previously been assessed in this model. We investigated whether aggressive behavior is robustly present in mice expressing the R451C mutation, following back-crossing onto a congenic background strain. Endocannabinoids influence social interaction and aggressive behavior, therefore we also studied the effects of cannabinoid receptor 1 (CB1) agonist on NL3^R451C mice. We report that NL3^R451C mice have increased amplitude of miniature excitatory postsynaptic currents (EPSCs) with a concomitant decrease in the amplitude of inhibitory postsynaptic currents (IPSCs) in the basolateral amygdala. Importantly, we demonstrated that NL3^R451C mice bred on a C57Bl/6 background strain exhibit an aggressive phenotype. Following non-sedating doses (0.3 and 1.0 mg/kg) of the CB1 receptor agonist WIN55,212-2 (WIN), we observed a significant reduction in aggressive behavior in NL3^R451C mice. These findings demonstrate altered synaptic activity in the basolateral amygdala and suggest that the NL3^R451C mouse model is a useful preclinical tool to understand the role of CB1 receptor function in aggressive behavior.

Genetic variability in scaffolding proteins and risk for schizophrenia and autism-spectrum disorders: a systematic review

Scaffolding proteins represent an evolutionary solution to controlling the specificity of information transfer in intracellular networks. They are highly concentrated in complexes located in specific subcellular locations. One of these complexes is the postsynaptic density of the excitatory synapses. There, scaffolding proteins regulate various processes related to synaptic plasticity, such as glutamate receptor trafficking and signalling, and dendritic structure and function. Most scaffolding proteins can be grouped into 4 main families: discs large (DLG), discs-large-associated protein (DLGAP), Shank and Homer. Owing to the importance of scaffolding proteins in postsynaptic density architecture, it is not surprising that variants in the genes that code for these proteins have been associated with neuropsychiatric diagnoses, including schizophrenia and autism-spectrum disorders. Such evidence, together with the clinical, neurobiological and genetic overlap described between schizophrenia and autism-spectrum disorders, suggest that alteration of scaffolding protein dynamics could be part of the pathophysiology of both. However, despite the potential importance of scaffolding proteins in these psychiatric conditions, no systematic review has integrated the genetic and molecular data from studies conducted in the last decade. This review has the following goals: to systematically analyze the literature in which common and/or rare genetic variants (single nucleotide polymorphisms, single nucleotide variants and copy number variants) in the scaffolding family genes are associated with the risk for either schizophrenia or autism-spectrum disorders; to explore the implications of the reported genetic variants for gene expression and/or protein function; and to discuss the relationship of these genetic variants to the shared genetic, clinical and cognitive traits of schizophrenia and autism-spectrum disorders.

Dendritic spine actin cytoskeleton in autism spectrum disorder

Dendritic spines are small actin-rich protrusions from neuronal dendrites that form the postsynaptic part of most excitatory synapses. Changes in the shape and size of dendritic spines correlate with the functional changes in excitatory synapses and are heavily dependent on the remodeling of the underlying actin cytoskeleton. Recent evidence implicates synapses at dendritic spines as important substrates of pathogenesis in neuropsychiatric disorders, including autism spectrum disorder (ASD). Although synaptic perturbations are not the only alterations relevant for these diseases, understanding the molecular underpinnings of the spine and synapse pathology may provide insight into their etiologies and could reveal new drug targets. In this review, we will discuss recent findings of defective actin regulation in dendritic spines associated with ASD.

Glutamate and GABA in autism spectrum disorder-a translational magnetic resonance spectroscopy study in man and rodent models

Autism spectrum disorder (ASD) is a pervasive neurodevelopmental syndrome with a high human and economic burden. The pathophysiology of ASD is largely unclear, thus hampering development of pharmacological treatments for the core symptoms of the disorder. Abnormalities in glutamate and GABA signaling have been hypothesized to underlie ASD symptoms, and may form a therapeutic target, but it is not known whether these abnormalities are recapitulated in humans with ASD, as well as in rodent models of the disorder. We used translational proton magnetic resonance spectroscopy ([1H]MRS) to compare glutamate and GABA levels in adult humans with ASD and in a panel of six diverse rodent ASD models, encompassing genetic and environmental etiologies. [1H]MRS was performed in the striatum and the medial prefrontal cortex, of the humans, mice, and rats in order to allow for direct cross-species comparisons in specific cortical and subcortical brain regions implicated in ASD. In humans with ASD, glutamate concentration was reduced in the striatum and this was correlated with the severity of social symptoms. GABA levels were not altered in either brain region. The reduction in striatal glutamate was recapitulated in mice prenatally exposed to valproate, and in mice and rats carrying Nlgn3 mutations, but not in rodent ASD models with other etiologies. Our findings suggest that glutamate/GABA abnormalities in the corticostriatal circuitry may be a key pathological mechanism in ASD; and may be linked to alterations in the neuroligin-neurexin signaling complex.

Delineation of the genetic and clinical spectrum of Phelan-McDermid syndrome caused by SHANK3 point mutations

Background

Phelan-McDermid syndrome (PMS) is a neurodevelopmental disorder characterized by psychiatric and neurological features. Most reported cases are caused by 22q13.3 deletions, leading to SHANK3 haploinsufficiency, but also usually encompassing many other genes. While the number of point mutations identified in SHANK3 has increased in recent years due to large-scale sequencing studies, systematic studies describing the phenotype of individuals harboring such mutations are lacking.

Methods

We provide detailed clinical and genetic data on 17 individuals carrying mutations in SHANK3. We also review 60 previously reported patients with pathogenic or likely pathogenic SHANK3 variants, often lacking detailed phenotypic information.

Results

SHANK3 mutations in our cohort and in previously reported cases were distributed throughout the protein; the majority were truncating and all were compatible with de novo inheritance. Despite substantial allelic heterogeneity, four variants were recurrent (p.Leu1142Valfs*153, p.Ala1227Glyfs*69, p.Arg1255Leufs*25, and c.2265+1G>A), suggesting that these are hotspots for de novo mutations. All individuals studied had intellectual disability, and autism spectrum disorder was prevalent (73%). Severe speech deficits were common, but in contrast to individuals with 22q13.3 deletions, the majority developed single words, including 41% with at least phrase speech. Other common findings were consistent with reports among individuals with 22q13.3 deletions, including hypotonia, motor skill deficits, regression, seizures, brain abnormalities, mild dysmorphic features, and feeding and gastrointestinal problems.

Conclusions

Haploinsufficiency of SHANK3 resulting from point mutations is sufficient to cause a broad range of features associated with PMS. Our findings expand the molecular and phenotypic spectrum of PMS caused by SHANK3 point mutations and suggest that, in general, speech impairment and motor deficits are more severe in the case of deletions. In contrast, renal abnormalities associated with 22q13.3 deletions do not appear to be related to the loss of SHANK3.

Common Ribs of Inhibitory Synaptic Dysfunction in the Umbrella of Neurodevelopmental Disorders

The term neurodevelopmental disorder (NDD) is an umbrella term used to group together a heterogeneous class of disorders characterized by disruption in cognition, emotion, and behavior, early in the developmental timescale. These disorders are heterogeneous, yet they share common behavioral symptomatology as well as overlapping genetic contributors, including proteins involved in the formation, specialization, and function of synaptic connections. Advances may arise from bridging the current knowledge on synapse related factors indicated from both human studies in NDD populations, and in animal models. Mounting evidence has shown a link to inhibitory synapse formation, specialization, and function among Autism, Angelman, Rett and Dravet syndromes. Inhibitory signaling is diverse, with numerous subtypes of inhibitory interneurons, phasic and tonic modes of inhibition, and the molecular and subcellular diversity of GABA_A receptors. We discuss common ribs of inhibitory synapse dysfunction in the umbrella of NDD, highlighting alterations in the developmental switch to inhibitory GABA, dysregulation of neuronal activity patterns by parvalbumin-positive interneurons, and impaired tonic inhibition. Increasing our basic understanding of inhibitory synapses, and their role in NDDs is likely to produce significant therapeutic advances in behavioral symptom alleviation for interrelated NDDs.

CRISPR/Cas9-induced shank3b mutant zebrafish display autism-like behaviors

Background

Human genetic and genomic studies have supported a strong causal role of SHANK3 deficiency in autism spectrum disorder (ASD). However, the molecular mechanism underlying SHANK3 deficiency resulting in ASD is not fully understood. Recently, the zebrafish has become an attractive organism to model ASD because of its high efficiency of genetic manipulation and robust behavioral phenotypes. The orthologous gene to human SHANK3 is duplicated in the zebrafish genome and has two homologs, shank3a and shank3b. Previous studies have reported shank3 morphants in zebrafish using the morpholino method. Here, we report the generation and characterization of shank3b mutant zebrafish in larval and adult stages using the CRISPR/Cas9 genome editing technique.

Methods

CRISPR/Cas9 was applied to generate a shank3b loss-of-function mutation (shank3b-/- ) in zebrafish. A series of morphological measurements, behavioral tests, and molecular analyses were performed to systematically characterize the behavioral and molecular changes in shank3b mutant zebrafish.

Results

shank3b-/- zebrafish exhibited abnormal morphology in early development. They showed reduced locomotor activity both as larvae and adults, reduced social interaction and time spent near conspecifics, and significant repetitive swimming behaviors. Additionally, the levels of both postsynaptic homer1 and presynaptic synaptophysin were significantly reduced in the adult brain of shank3b-deficient zebrafish.

Conclusions

We generated the first inheritable shank3b mutant zebrafish model using CRISPR/Cas9 gene editing approach. shank3b-/- zebrafish displayed robust autism-like behaviors and altered levels of the synaptic proteins homer1 and synaptophysin. The versatility of zebrafish as a model for studying neurodevelopment and conducting drug screening will likely have a significant contribution to future studies of human SHANK3 function and ASD.

Developmental social communication deficits in the Shank3 rat model of phelan-mcdermid syndrome and autism spectrum disorder

Mutations in the SHANK3 gene have been discovered in autism spectrum disorder (ASD), and the intellectual disability, Phelan-McDermid Syndrome. This study leveraged a new rat model of Shank3 deficiency to assess complex behavioral phenomena, unique to rats, which display a richer social behavior repertoire than mice. Uniquely detectable emissions of ultrasonic vocalizations (USV) in rats serve as situation-dependent affective signals and accomplish important communicative functions. We report, for the first time, a call and response acoustic playback assay of bidirectional social communication in juvenile Shank3 rats. Interestingly, we found that Shank3-deficient null males did not demonstrate the enhanced social approach behavior typically exhibited following playback of pro-social USV. Concomitantly, we discovered that emission of USV in response to playback was not genotype-dependent and emitted response calls were divergent in meaning. This is the first report of these socially relevant responses using a genetic model of ASD. A comprehensive and empirical analysis of vigorous play during juvenile reciprocal social interactions further revealed fewer bouts and reduced durations of time spent playing by multiple key parameters, including reduced anogenital sniffing and allogrooming. We further discovered that male null Shank3-deficient pups emitted fewer isolation-induced USV than Shank3 wildtype controls. Postnatal whole brain anatomical phenotyping was applied to visualize anatomical substrates that underlie developmental phenotypes. The data presented here lend support for the important role of Shank3 in social communication, the core symptom domain of ASD. By increasing the number of in vivo functional outcome measures, we improved the likelihood for identifying and moving forward with medical interventions. Autism Res 2018, 11: 587-601. © 2018 International Society for Autism Research, Wiley Periodicals, Inc.

LAY SUMMARY

Clinically relevant outcomes are required to demonstrate the utility of therapeutics. We introduce findings in a rat model, and assess the impact of mutations in Shank3, an autism risk gene. We found that males with deficient expression of Shank3 did not demonstrate typical responses in a bi-directional social communication test and that social interaction was lower on key parameters. Outcome measures reported herein extend earlier results in mice and capture responses to acoustic calls, which is analogous to measuring receptive and expressive communication.

A Presynaptic Function of Shank Protein in Drosophila

Human genetic studies support that loss-of-function mutations in the SH3 domain and ankyrin repeat containing family proteins (SHANK1-3), the large synaptic scaffolding proteins enriched at the postsynaptic density of excitatory synapses, are causative for autism spectrum disorder and other neuropsychiatric disorders in humans. To better understand the in vivo functions of Shank and facilitate dissection of neuropathology associated with SHANK mutations in human, we generated multiple mutations in the Shank gene, the only member of the SHANK family in Drosophila melanogaster Both male and female Shank null mutants were fully viable and fertile with no apparent morphological or developmental defects. Expression analysis revealed apparent enrichment of Shank in the neuropils of the CNS. Specifically, Shank coexpressed with another PSD scaffold protein, Homer, in the calyx of mushroom bodies in the brain. Consistent with high expression in mushroom body calyces, Shank mutants show an abnormal calyx structure and reduced olfactory acuity. These morphological and functional phenotypes were fully rescued by pan-neuronal reexpression of Shank, and only partially rescued by presynaptic but no rescue by postsynaptic reexpression of Shank. Our findings thus establish a previously unappreciated presynaptic function of Shank.SIGNIFICANCE STATEMENT Mutations in SHANK family genes are causative for idiopathic autism spectrum disorder. To understand the neural function of Shank, a large scaffolding protein enriched at the postsynaptic densities, we examined the role of Drosophila Shank in synapse development at the peripheral neuromuscular junctions and the central mushroom body calyx. Our results demonstrate that, in addition to its conventional postsynaptic function, Shank also acts presynaptically in synapse development in the brain. This study offers novel insights into the synaptic role of Shank.

Transcellular Nanoalignment of Synaptic Function

At each of the brain's vast number of synapses, the presynaptic nerve terminal, synaptic cleft, and postsynaptic specialization form a transcellular unit to enable efficient transmission of information between neurons. While we know much about the molecular machinery within each compartment, we are only beginning to understand how these compartments are structurally registered and functionally integrated with one another. This review will describe the organization of each compartment and then discuss their alignment across pre- and postsynaptic cells at a nanometer scale. We propose that this architecture may allow for precise synaptic information exchange and may be modulated to contribute to the remarkable plasticity of brain function.

Transcellular Nanoalignment of Synaptic Function

At each of the brain's vast number of synapses, the presynaptic nerve terminal, synaptic cleft, and postsynaptic specialization form a transcellular unit to enable efficient transmission of information between neurons. While we know much about the molecular machinery within each compartment, we are only beginning to understand how these compartments are structurally registered and functionally integrated with one another. This review will describe the organization of each compartment and then discuss their alignment across pre- and postsynaptic cells at a nanometer scale. We propose that this architecture may allow for precise synaptic information exchange and may be modulated to contribute to the remarkable plasticity of brain function.

Parkin Deficiency Reduces Hippocampal Glutamatergic Neurotransmission by Impairing AMPA Receptor Endocytosis

Mutations in the gene encoding Parkin, an E3 ubiquitin ligase, lead to juvenile-onset Parkinson's disease by inducing the selective death of midbrain dopaminergic neurons. Accumulating evidence indicates that Parkin also has an important role in excitatory glutamatergic neurotransmission, although its precise mechanism of action remains unclear. Here, we investigate Parkin's role at glutamatergic synapses of rat hippocampal neurons. We find that Parkin-deficient neurons exhibit significantly reduced AMPA receptor (AMPAR)-mediated currents and cell-surface expression, and that these phenotypes result from decreased postsynaptic expression of the adaptor protein Homer1, which is necessary for coupling AMPAR endocytic zones with the postsynaptic density. Accordingly, Parkin loss of function leads to the reduced density of postsynaptic endocytic zones and to impaired AMPAR internalization. These findings demonstrate a novel and essential role for Parkin in glutamatergic neurotransmission, as a stabilizer of postsynaptic Homer1 and the Homer1-linked endocytic machinery necessary for maintaining normal cell-surface AMPAR levels.

SIGNIFICANCE STATEMENT

Mutations in Parkin, a ubiquitinating enzyme, lead to the selective loss of midbrain dopaminergic neurons and juvenile-onset Parkinson's disease (PD). Parkin loss of function has also been shown to alter hippocampal glutamatergic neurotransmission, providing a potential explanation for PD-associated cognitive impairment. However, very little is known about Parkin's specific sites or mechanisms of action at glutamatergic synapses. Here, we show that Parkin deficiency leads to decreased AMPA receptor-mediated activity due to disruption of the postsynaptic endocytic zones required for maintaining proper cell-surface AMPA receptor levels. These findings demonstrate a novel role for Parkin in synaptic AMPA receptor internalization and suggest a Parkin-dependent mechanism for hippocampal dysfunction that may explain cognitive deficits associated with some forms of PD.

Shank3 Is Part of a Zinc-Sensitive Signaling System That Regulates Excitatory Synaptic Strength

Shank3 is a multidomain scaffold protein localized to the postsynaptic density of excitatory synapses. Functional studies in vivo and in vitro support the concept that Shank3 is critical for synaptic plasticity and the trans-synaptic coupling between the reliability of presynaptic neurotransmitter release and postsynaptic responsiveness. However, how Shank3 regulates synaptic strength remains unclear. The C terminus of Shank3 contains a sterile alpha motif (SAM) domain that is essential for its postsynaptic localization and also binds zinc, thus raising the possibility that changing zinc levels modulate Shank3 function in dendritic spines. In support of this hypothesis, we find that zinc is a potent regulator of Shank3 activation and dynamics in rat hippocampal neurons. Moreover, we show that zinc modulation of synaptic transmission is Shank3 dependent. Interestingly, an autism spectrum disorder (ASD)-associated variant of Shank3 (Shank3(R87C)) retains its zinc sensitivity and supports zinc-dependent activation of AMPAR-mediated synaptic transmission. However, elevated zinc was unable to rescue defects in trans-synaptic signaling caused by the R87C mutation, implying that trans-synaptic increases in neurotransmitter release are not necessary for the postsynaptic effects of zinc. Together, these data suggest that Shank3 is a key component of a zinc-sensitive signaling system, regulating synaptic strength that may be impaired in ASD.

SIGNIFICANCE STATEMENT

Shank3 is a postsynaptic protein associated with neurodevelopmental disorders such as autism and schizophrenia. In this study, we show that Shank3 is a key component of a zinc-sensitive signaling system that regulates excitatory synaptic transmission. Intriguingly, an autism-associated mutation in Shank3 partially impairs this signaling system. Therefore, perturbation of zinc homeostasis may impair, not only synaptic functionality and plasticity, but also may lead to cognitive and behavioral abnormalities seen in patients with psychiatric disorders.

Drosophila Studies on Autism Spectrum Disorders

In the past decade, numerous genes associated with autism spectrum disorders (ASDs) have been identified. These genes encode key regulators of synaptogenesis, synaptic function, and synaptic plasticity. Drosophila is a prominent model system for ASD studies to define novel genes linked to ASDs and decipher their molecular roles in synaptogenesis, synaptic function, synaptic plasticity, and neural circuit assembly and consolidation. Here, we review Drosophila studies on ASD genes that regulate synaptogenesis, synaptic function, and synaptic plasticity through modulating chromatin remodeling, transcription, protein synthesis and degradation, cytoskeleton dynamics, and synaptic scaffolding.

Implication of Endoplasmic Reticulum Stress in Autism Spectrum Disorder

Autism spectrum disorder (ASD) is categorized as a neurodevelopmental disorder according to the Diagnostic and Statistical Manual of Disorders, Fifth Edition and is defined as a congenital impairment of the central nervous system. ASD may be caused by a chromosomal abnormality or gene mutation. However, these etiologies are insufficient to account for the pathogenesis of ASD. Therefore, we propose that the etiology and pathogenesis of ASD are related to the stress of the endoplasmic reticulum (ER). ER stress, induced by valproic acid, increased in ASD mouse model, characterized by an unfolded protein response that is activated by this stress. The inhibition of neurite outgrowth and expression of synaptic factors are observed in ASD. Similarly, ER stress suppresses the neurite outgrowth and expression of synaptic factors. Additionally, hyperplasia of the brain is observed in patients with ASD. ER stress also enhances neuronal differentiation. Synaptic factors, such as cell adhesion molecule and shank, play important roles in the formation of neural circuits. Thus, ER stress is associated with the abnormalities of neuronal differentiation, neurite outgrowth, and synaptic protein expression. ER stress elevates the expression of the ubiquitin-protein ligase HRD1 for the degradation of unfolded proteins. HRD1 expression significantly increased in the middle frontal cortex in the postmortem of patients with ASD. Moreover, HRD1 silencing improved the abnormalities induced by ER stress. Because other ubiquitin ligases are related with neurite outgrowth, ER stress may be related to the pathogenesis of neuronal developmental diseases via abnormalities of neuronal differentiation or maturation.

Genome-wide Methyl-Seq analysis of blood-brain targets of glucocorticoid exposure

Chronic exposure to glucocorticoids (GCs) can lead to psychiatric complications through epigenetic mechanisms such as DNA methylation (DNAm). We sought to determine whether epigenetic changes in a peripheral tissue can serve as a surrogate for those in a relatively inaccessible tissue such as the brain. DNA extracted from the hippocampus and blood of mice treated with GCs or vehicle solution was assayed using a genome-wide DNAm platform (Methyl-Seq) to identify differentially methylated regions (DMRs) induced by GC treatment. We observed that ∼70% of the DMRs in both tissues lost methylation following GC treatment. Of the 3,095 DMRs that mapped to the same genes in both tissues, 1,853 DMRs underwent DNAm changes in the same direction. Interestingly, only 209 DMRs (<7%) overlapped in genomic coordinates between the 2 tissues, suggesting tissue-specific differences in GC-targeted loci. Pathway analysis showed that the DMR-associated genes were members of pathways involved in metabolism, immune function, and neurodevelopment. Also, changes in cell type composition of blood and brain were examined by fluorescence-activated cell sorting. Separation of the cortex into neuronal and non-neuronal fractions and the leukocytes into T-cells, B-cells, and neutrophils showed that GC-induced methylation changes primarily occurred in neurons and T-cells, with the blood tissue also undergoing a shift in the proportion of constituent cell types while the proportion of neurons and glia in the brain remained stable. From the current pilot study, we found that despite tissue-specific epigenetic changes and cellular heterogeneity, blood can serve as a surrogate for GC-induced changes in the brain.

N-terminal SAP97 isoforms differentially regulate synaptic structure and postsynaptic surface pools of AMPA receptors

The location and density of postsynaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors is controlled by scaffolding proteins within the postsynaptic density (PSD). SAP97 is a PSD protein with two N-terminal isoforms, α and β, that have opposing effects on synaptic strength thought to result from differential targeting of AMPA receptors into distinct synaptic versus extrasynaptic locations, respectively. In this study, we have applied dSTORM super resolution imaging in order to localize the synaptic and extrasynaptic pools of AMPA receptors in neurons expressing α or βSAP97. Unexpectedly, we observed that both α and βSAP97 enhanced the localization of AMPA receptors at synapses. However, this occurred via different mechanisms: αSAP97 increased PSD size and consequently the number of receptor binding sites, whilst βSAP97 increased synaptic receptor cluster size and surface AMPA receptor density at the PSD edge and surrounding perisynaptic sites without changing PSD size. αSAP97 also strongly enlarged presynaptic active zone protein clusters, consistent with both presynaptic and postsynaptic enhancement underlying the previously observed αSAP97-induced increase in AMPA receptor-mediated currents. In contrast, βSAP97-expressing neurons increased the proportion of immature filopodia that express higher levels of AMPA receptors, decreased the number of functional presynaptic terminals, and also reduced the size of the dendritic tree and delayed the maturation of mushroom spines. Our data reveal that SAP97 isoforms can specifically regulate surface AMPA receptor nanodomain clusters, with βSAP97 increasing extrasynaptic receptor domains at peri-synaptic and filopodial sites. Moreover, βSAP97 negatively regulates synaptic maturation both structurally and functionally. These data support diverging presynaptic and postsynaptic roles of SAP97 N-terminal isoforms in synapse maturation and plasticity. As numerous splice isoforms exist in other major PSD proteins (e.g., Shank, PSD95, and SAP102), this alternative splicing may result in individual PSD proteins having divergent functional and structural roles in both physiological and pathophysiological synaptic states.

SHANK proteins limit integrin activation by directly interacting with Rap1 and R-Ras

SHANK3, a synaptic scaffold protein and actin regulator, is widely expressed outside of the central nervous system with predominantly unknown function. Solving the structure of the SHANK3 N-terminal region revealed that the SPN-domain is an unexpected Ras-association domain with high affinity for GTP-bound Ras and Rap G-proteins. The role of Rap1 in integrin activation is well established but the mechanisms to antagonize it remain largely unknown. Here, we show that SHANK1 and SHANK3 act as integrin activation inhibitors by sequestering active Rap1 and R-Ras via the SPN-domain and thus limiting their bioavailability at the plasma membrane. Consistently, SHANK3 silencing triggers increased plasma membrane Rap1 activity, cell spreading, migration and invasion. Autism-related mutations within the SHANK3 SPN-domain (R12C and L68P) disrupt G-protein interaction and fail to counteract integrin activation along the Rap1/RIAM/talin axis in cancer cells and neurons. Altogether, we establish SHANKs as critical regulators of G-protein signalling and integrin-dependent processes.

A Novel Human CAMK2A Mutation Disrupts Dendritic Morphology and Synaptic Transmission, and Causes ASD-Related Behaviors

Characterizing the functional impact of novel mutations linked to autism spectrum disorder (ASD) provides a deeper mechanistic understanding of the underlying pathophysiological mechanisms. Here we show that a de novo Glu183 to Val (E183V) mutation in the CaMKIIα catalytic domain, identified in a proband diagnosed with ASD, decreases both CaMKIIα substrate phosphorylation and regulatory autophosphorylation, and that the mutated kinase acts in a dominant-negative manner to reduce CaMKIIα-WT autophosphorylation. The E183V mutation also reduces CaMKIIα binding to established ASD-linked proteins, such as Shank3 and subunits of l-type calcium channels and NMDA receptors, and increases CaMKIIα turnover in intact cells. In cultured neurons, the E183V mutation reduces CaMKIIα targeting to dendritic spines. Moreover, neuronal expression of CaMKIIα-E183V increases dendritic arborization and decreases both dendritic spine density and excitatory synaptic transmission. Mice with a knock-in CaMKIIα-E183V mutation have lower total forebrain CaMKIIα levels, with reduced targeting to synaptic subcellular fractions. The CaMKIIα-E183V mice also display aberrant behavioral phenotypes, including hyperactivity, social interaction deficits, and increased repetitive behaviors. Together, these data suggest that CaMKIIα plays a previously unappreciated role in ASD-related synaptic and behavioral phenotypes.SIGNIFICANCE STATEMENT Many autism spectrum disorder (ASD)-linked mutations disrupt the function of synaptic proteins, but no single gene accounts for >1% of total ASD cases. The molecular networks and mechanisms that couple the primary deficits caused by these individual mutations to core behavioral symptoms of ASD remain poorly understood. Here, we provide the first characterization of a mutation in the gene encoding CaMKIIα linked to a specific neuropsychiatric disorder. Our findings demonstrate that this ASD-linked de novo CAMK2A mutation disrupts multiple CaMKII functions, induces synaptic deficits, and causes ASD-related behavioral alterations, providing novel insights into the synaptic mechanisms contributing to ASD.

Evaluating the association between the SHANK3 gene and bipolar disorder

BACKGROUND

Previous studies have shown that the genes involved in synaptic density and plasticity were downregulated in bipolar disorder (BD) postmortem brains. The SHANK3 (SH3 and multiple ankyrin repeat domain 3) protein is a scaffold protein enriched in the postsynaptic density of excitatory synapses, and recent findings were suggestive of a possible role of SHANK3 in the development of BD. In this study, we aimed to evaluate the genetic association between the gene encoding SHANK3 (SHANK3) and BD in Han Chinese.

METHODS

Ten variants in SHANK3 were genotyped among 1482 individuals with or without BD. We measured the mRNA expression level of SHANK3 in patients with BD and controls. We then performed an eQTL analysis.

RESULTS

In our sample set, there were no significant differences in allele, genotype and haplotype frequencies between the BD and control groups. Our results showed no significant difference in the level of peripheral SHANK3 expression between the BD patients and healthy controls. Further eQTL analysis showed that rs9616915 has functional effect on SHANK3 expression in the hippocampus.

CONCLUSION

This study does not provide evidence for a major role of SHANK3 in the pathogenesis of BD.

Actin-Dependent Alterations of Dendritic Spine Morphology in Shankopathies

Shank proteins (Shank1, Shank2, and Shank3) act as scaffolding molecules in the postsynaptic density of many excitatory neurons. Mutations in SHANK genes, in particular SHANK2 and SHANK3, lead to autism spectrum disorders (ASD) in both human and mouse models. Shank3 proteins are made of several domains-the Shank/ProSAP N-terminal (SPN) domain, ankyrin repeats, SH3 domain, PDZ domain, a proline-rich region, and the sterile alpha motif (SAM) domain. Via various binding partners of these domains, Shank3 is able to bind and interact with a wide range of proteins including modulators of small GTPases such as RICH2, a RhoGAP protein, and βPIX, a RhoGEF protein for Rac1 and Cdc42, actin binding proteins and actin modulators. Dysregulation of all isoforms of Shank proteins, but especially Shank3, leads to alterations in spine morphogenesis, shape, and activity of the synapse via altering actin dynamics. Therefore, here, we highlight the role of Shank proteins as modulators of small GTPases and, ultimately, actin dynamics, as found in multiple in vitro and in vivo models. The failure to mediate this regulatory role might present a shared mechanism in the pathophysiology of autism-associated mutations, which leads to dysregulation of spine morphogenesis and synaptic signaling.

Dysfunctional cerebellar Purkinje cells contribute to autism-like behaviour in Shank2-deficient mice

Loss-of-function mutations in the gene encoding the postsynaptic scaffolding protein SHANK2 are a highly penetrant cause of autism spectrum disorders (ASD) involving cerebellum-related motor problems. Recent studies have implicated cerebellar pathology in the aetiology of ASD. Here we evaluate the possibility that cerebellar Purkinje cells (PCs) represent a critical locus of ASD-like pathophysiology in mice lacking Shank2. Absence of Shank2 impairs both PC intrinsic plasticity and induction of long-term potentiation at the parallel fibre to PC synapse. Moreover, inhibitory input onto PCs is significantly enhanced, most prominently in the posterior lobe where simple spike (SS) regularity is most affected. Using PC-specific Shank2 knockouts, we replicate alterations of SS regularity in vivo and establish cerebellar dependence of ASD-like behavioural phenotypes in motor learning and social interaction. These data highlight the importance of Shank2 for PC function, and support a model by which cerebellar pathology is prominent in certain forms of ASD.

Alleviation of N-Methyl-D-Aspartate Receptor-Dependent Long-Term Depression via Regulation of the Glycogen Synthase Kinase-3β Pathway in the Amygdala of a Valproic Acid-Induced Animal Model of Autism

The amygdala plays crucial roles in socio-emotional behavior and cognition, both of which are abnormal in autism spectrum disorder (ASD). Valproic acid (VPA)-exposed rat offspring have demonstrated ASD phenotypes and amygdala excitatory/inhibitory imbalance. However, the role of glutamatergic synapses in this imbalance remains unclear. In this study, we used a VPA-induced ASD-like model to assess glutamatergic synapse-dependent long-term depression (LTD) and depotentiation (DPT) in the amygdala. We first confirmed that the VPA-exposed offspring exhibited sociability deficits, anxiety, depression-like behavior, and abnormal nociception thresholds. Then, electrophysiological examination showed a significantly decreased paired-pulse ratio in the amygdala. In addition, both NMDA-dependent LTD and DPT were absent from the amygdala. Furthermore, we found that the levels of glycogen synthase kinase3β (GSK-3β) phosphorylation and β-catenin were significantly higher in the amygdala of the experimental animals than in the controls. Local infusion of phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin into the amygdala reversed the increased phosphorylation level and impaired social behavior. Taken together, the results suggested that NMDA receptor-related synaptic plasticity is dysfunctional in VPA-exposed offspring. In addition, GSK-3β in the amygdala is critical for synaptic plasticity at the glutamatergic synapses and is related to social behavior. Its role in the underlying mechanism of ASD merits further investigation.

The spectrum of epilepsy and electroencephalographic abnormalities due to SHANK3 loss-of-function mutations

OBJECTIVE

The coincidence of autism with epilepsy is 27% in those individuals with intellectual disability.¹ Individuals with loss-of-function mutations in SHANK3 have intellectual disability, autism, and variably, epilepsy.^2-5 The spectrum of seizure semiologies and electroencephalography (EEG) abnormalities has never been investigated in detail. With the recent report that SHANK3 mutations are present in approximately 2% of individuals with moderate to severe intellectual disabilities and 1% of individuals with autism, determining the spectrum of seizure semiologies and electrographic abnormalities will be critical for medical practitioners to appropriately counsel the families of patients with SHANK3 mutations.

METHODS

A retrospective chart review was performed of all individuals treated at the Blue Bird Circle Clinic for Child Neurology who have been identified as having either a chromosome 22q13 microdeletion encompassing SHANK3 or a loss-of-function mutation in SHANK3 identified through whole-exome sequencing. For each subject, the presence or absence of seizures, seizure semiology, frequency, age of onset, and efficacy of therapy were determined. Electroencephalography studies were reviewed by a board certified neurophysiologist. Neuroimaging was reviewed by both a board certified pediatric neuroradiologist and child neurologist.

RESULTS

There is a wide spectrum of seizure semiologies, frequencies, and severity in individuals with SHANK3 mutations. There are no specific EEG abnormalities found in our cohort, and EEG abnormalities were present in individuals diagnosed with epilepsy and those without history of a clinical seizure.

SIGNIFICANCE

All individuals with a mutation in SHANK3 should be evaluated for epilepsy due to the high prevalence of seizures in this population. The most common semiology is atypical absence seizure, which can be challenging to identify due to comorbid intellectual disability in individuals with SHANK3 mutations; however, no consistent seizure semiology, neuroimaging findings, or EEG findings were present in the majority of individuals with SHANK3 mutations.

Novel Shank3 mutant exhibits behaviors with face validity for autism and altered striatal and hippocampal function

Mutations/deletions in the SHANK3 gene are associated with autism spectrum disorders and intellectual disability. Here, we present electrophysiological and behavioral consequences in novel heterozygous and homozygous mice with a transcriptional stop cassette inserted upstream of the PDZ domain-coding exons in Shank3 (Shank3^E13 ). Insertion of a transcriptional stop cassette prior to exon 13 leads to loss of the two higher molecular weight isoforms of Shank3. Behaviorally, both Shank3^E13 heterozygous (HET) and homozygous knockout (KO) mice display increased repetitive grooming, deficits in social interaction tasks, and decreased rearing. Shank3^E13 KO mice also display deficits in spatial memory in the Morris water maze task. Baseline hippocampal synaptic transmission and short-term plasticity are preserved in Shank3^E13 HET and KO mice, while both HET and KO mice exhibit impaired hippocampal long-term plasticity. Additionally, Shank3^E13 HET and KO mice display impaired striatal glutamatergic synaptic transmission. These results demonstrate for the first time in this novel Shank3 mutant that both homozygous and heterozygous mutation of Shank3 lead to behavioral abnormalities with face validity for autism along with widespread synaptic dysfunction. Autism Res 2017, 10: 42-65. © 2016 International Society for Autism Research, Wiley Periodicals, Inc.

Breaking down to build up: Neuroligin's C-terminal domain strengthens the synapse

The mechanisms by which neuroligin adhesion molecules modulate synaptic plasticity remain unclear. In this issue, Liu et al. (2016. J. Cell Biol.http://dx.doi.org/10.1083/jcb.201509023) demonstrate that neuroligin 1 promotes actin assembly associated with synaptic strengthening independent of adhesion, suggesting additional ways for neuroligins to contribute to neuronal development and disease pathology.

Altered thalamocortical development in the SAP102 knockout model of intellectual disability

Genetic mutations known to cause intellectual disabilities (IDs) are concentrated in specific sets of genes including both those encoding synaptic proteins and those expressed during early development. We have characterized the effect of genetic deletion of Dlg3, an ID-related gene encoding the synaptic NMDA-receptor interacting protein synapse-associated protein 102 (SAP102), on development of the mouse somatosensory cortex. SAP102 is the main representative of the PSD-95 family of postsynaptic MAGUK proteins during early development and is proposed to play a role in stabilizing receptors at immature synapses. Genetic deletion of SAP102 caused a reduction in the total number of thalamocortical (TC) axons innervating the somatosensory cortex, but did not affect the segregation of barrels. On a synaptic level SAP102 knockout mice display a transient speeding of NMDA receptor kinetics during the critical period for TC plasticity, despite no reduction in GluN2B-mediated component of synaptic transmission. These data indicated an interesting dissociation between receptor kinetics and NMDA subunit expression. Following the critical period NMDA receptor function was unaffected by loss of SAP102 but there was a reduction in the divergence of TC connectivity. These data suggest that changes in synaptic function early in development caused by mutations in SAP102 result in changes in network connectivity later in life.

The Shank3 Interaction Partner ProSAPiP1 Regulates Postsynaptic SPAR Levels and the Maturation of Dendritic Spines in Hippocampal Neurons

The postsynaptic density or PSD is a submembranous compartment containing a wide array of proteins that contribute to both morphology and function of excitatory glutamatergic synapses. In this study, we have analyzed functional aspects of the Fezzin ProSAP-interacting protein 1 (ProSAPiP1), an interaction partner of the well-known PSD proteins Shank3 and SPAR. Using lentiviral-mediated overexpression and knockdown of ProSAPiP1, we found that this protein is dispensable for the formation of both pre- and postsynaptic specializations per se. We further show that ProSAPiP1 regulates SPAR levels at the PSD and the maturation of dendritic spines. In line with previous findings on the ProSAPiP1 homolog PSD-Zip70, we conclude that Fezzins essentially contribute to the maturation of excitatory spine synapses.

Early hyperactivity and precocious maturation of corticostriatal circuits in Shank3B(-/-) mice

Some autistic individuals exhibit abnormal development of the caudate nucleus and associative cortical areas, suggesting potential dysfunction of cortico-basal ganglia (BG) circuits. Using optogenetic and electrophysiological approaches in mice, we identified a narrow postnatal period that is characterized by extensive glutamatergic synaptogenesis in striatal spiny projection neurons (SPNs) and a concomitant increase in corticostriatal circuit activity. SPNs during early development have high intrinsic excitability and respond strongly to cortical afferents despite sparse excitatory inputs. As a result, striatum and corticostriatal connectivity are highly sensitive to acute and chronic changes in cortical activity, suggesting that early imbalances in cortical function alter BG development. Indeed, a mouse model of autism with deletions in Shank3 (Shank3B(-/-)) shows early cortical hyperactivity, which triggers increased SPN excitatory synapse and corticostriatal hyperconnectivity. These results indicate that there is a tight functional coupling between cortex and striatum during early postnatal development and suggest a potential common circuit dysfunction that is caused by cortical hyperactivity.

Reduced sensory stimulation alters the molecular make-up of glutamatergic hair cell synapses in the developing cochlea

Neural activity during early development is known to alter innervation pathways in the central and peripheral nervous systems. We sought to examine how reduced sound-induced sensory activity in the cochlea affected the consolidation of glutamatergic synapses between inner hair cells (IHC) and the primary auditory neurons as these synapses play a primary role in transmitting sound information to the brain. A unilateral conductive hearing loss was induced prior to the onset of sound-mediated stimulation of the sensory hair cells, by rupturing the tympanic membrane and dislocating the auditory ossicles in the left ear of P11 mice. Auditory brainstem responses at P15 and P21 showed a 40-50-dB increase in thresholds for frequencies 8-32kHz in the dislocated ear relative to the control ear. Immunohistochemistry and confocal microscopy were subsequently used to examine the effect of this attenuation of sound stimulation on the expression of RIBEYE, which comprises the presynaptic ribbons, Shank-1, a postsynaptic scaffolding protein, and the GluA2/3 and 4 subunits of postsynaptic AMPA receptors. Our results show that dislocation did not alter the number of pre- or postsynaptic protein puncta. However, dislocation did increase the size of RIBEYE, GluA4, GluA2/3 and Shank-1 puncta, with postsynaptic changes preceding presynaptic changes. Our data suggest that a reduction in sound stimulation during auditory development induces plasticity in the molecular make-up of IHC glutamatergic synapses, but does not affect the number of these synapses. Up-regulation of synaptic proteins with sound attenuation may facilitate a compensatory increase in synaptic transmission due to the reduced sensory stimulation of the IHC.

Enlarged dendritic spines and pronounced neophobia in mice lacking the PSD protein RICH2

Background

The majority of neurons within the central nervous system receive their excitatory inputs via small, actin-rich protrusions called dendritic spines. Spines can undergo rapid morphological alterations according to synaptic activity. This mechanism is implicated in learning and memory formation as it is ultimately altering the number and distribution of receptors and proteins at the post-synaptic membrane, thereby regulating synaptic input. The Rho-family GTPases play an important role in regulating this spine plasticity by the interaction with cytoskeletal components and several signaling pathways within the spine compartment. Rho-GAP interacting CIP4 homologue2/RICH2 is a Rho-GAP protein regulating small GTPases and was identified as an interaction partner of the scaffolding protein SHANK3 at post-synaptic densities.

Results

Here, we characterize the loss of RICH2 in a novel mouse model. Our results show that RICH2 KO animals display a selective and highly significant fear of novel objects and increased stereotypic behavior as well as impairment of motor learning. We found an increase in multiple spine synapses in the hippocampus and cerebellum along with alterations in receptor composition and actin polymerization. Furthermore, we observed that the loss of RICH2 leads to a disinhibition of synaptic RAC1 in vivo.

Conclusions

The results are in line with the reported role of RAC1 activity being essential for activity-dependent spine enlargement. Since SHANK3 mutations are known to be causative for neuropsychiatric diseases of the Autism Spectrum (ASD), a disintegrated SHANK3/RICH2 complex at synaptic sites might at least in part be responsible for abnormal spine formation and plasticity in ASDs.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-016-0206-6) contains supplementary material, which is available to authorized users.