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Disrupted circuits in mouse models of autism spectrum disorder and intellectual disability
Abstract
Autism spectrum disorder (ASD) and intellectual disability (ID) are caused by a wide range of genetic mutations, a significant fraction of which reside in genes important for synaptic function. Studies have found that sensory, prefrontal, hippocampal, cerebellar, and striatal regions, as well as the circuits that connect them, are perturbed in mouse models of ASD and ID. Dissecting the disruptions in morphology and activity in these neural circuits might help us to understand the shared risk between the two disorders as well as their clinical heterogeneity. Treatments that target the balance between excitation and inhibition in these regions are able to reverse pathological phenotypes, elucidating this deficit as a commonality across models and opening new avenues for intervention.
... In this way it has been shown that in response to genetic manipulation of a number of ASD-associated genes mice can model impaired social behaviour (Silverman et al., 2010). Therefore, mice models of ASD have been used to elucidate which genes may co-ordinate social behaviours, how those genes may function in neural circuits that underpin social behaviours (Golden, Buxbaum and Rubeis, 2018) and possible therapeutic targets that could alleviate ASD-related symptoms (Kazdoba et al., 2016;Burket et al., 2011). ...
... This has helped to identify hyperexcitability of circuits during development and also increased activity of circuits that may underpin hypersensitivity of the animal to sensory stimuli (Gonçalves et al., 2013;Rotschafer and Razak, 2013;Gogolla et al., 2014). Understanding how mutations to risk genes may disrupt the activity of neural circuits has provided insights into the circuit level dysfunction in the aetiology of autism (Golden, Buxbaum and Rubeis, 2018). Methods utilising the implantation of electrodes into mice brains has been developed to monitor brain activity in freely moving animals and has identified circuits that are hyperactive during impaired social behaviours (Wang et al., 2016b). ...
... However, investigating determinants of defined neural circuits underpinning autism associated neuro-atypical behaviour is providing traction for discrete investigation of complex traits. Studies of distinct behaviours in mice has begun to unpick the effect of genetic disruption on molecular circuits and synapse function (Kim, Lim and Kaang, 2016;Golden, Buxbaum and Rubeis, 2018;Lee, Lee and Kim, 2017). ...
Autism spectrum disorder (ASD) is a neurodevelopmental disorder which is clinically characterised by core behavioural deficits including disruption to social behaviour. ASD has a clear genetic underpinning and hundreds of genes, with varying penetrance, have been implicated in its aetiology. Here I use the model organism C. elegans as an experimental platform to investigate the effect of genetic mutation on social behaviour.
To do this I utilised a social paradigm in C. elegans that is based on observing a complex, sensory-integrative, food leaving behaviour. Adult worms increasingly leave an otherwise replete food source as the number of progeny populating the environment increases. This behaviour is mediated by a progeny-derived social cue and hence represent an inter-organismal social interaction. In this thesis I first designed a bioinformatic pipeline to filter high risk ASD-associated genes to select those that could be investigated using the progeny-induced food leaving assay. I identified several candidate human genes by defining C. elegans orthologues which when mutated result in selective deficits in social behaviour. This work highlights that genetic determinants within synaptic, cell signalling, epigenetic modification and phospholipid metabolism functional domains have a role in the co-ordination of C. elegans social behaviour. Using a null mutant, I show that C. elegans social behaviour is dependent on the nlg-1 gene. I refine this approach by generating a mutation in C. elegans which is synonymous to a highly penetrant ASD-associated variant identified in humans. In doing this I used and optimised a CRISPR/Cas9 technique to precisely edit C. elegans genes.
Overall, the combined use of social behaviour analysis and genetic intervention in C. elegans provides a useful model to investigate the genetic determinants of autism. ASD is a complex disorder in which disruption within the biological system can influence different levels of biological organisation. Therefore, this thesis provides an avenue for future research to probe the effect of genetic disruption at different levels of the biological system to further understand the emergence of disrupted social behaviour.
... Answers to these questions are central to understanding cortical circuit development. Moreover, since neurodevelopmental disorders are associated with defects within cortical circuits (Del Pino et al., 2018;Foss-Feig et al., 2017;Golden et al., 2018;Mullins et al., 2016;Robertson and Baron-Cohen, 2017), insights into pyramidal circuit formation may also be relevant for understanding the mechanisms of diseases such as autism spectrum disorder. ...
... Insight into the timing and assembly of the first active pyramidal neuron circuits is also important in the context of neurodevelopmental disorders such as autism spectrum disorder and schizophrenia (Marín, 2016). Indeed, common circuit dysfunctions have been hypothesized to underlie the phenotypic similarities across the heterogeneous mixture of genetic abnormalities associated with these neurodevelopmental disorders (Courchesne and Pierce, 2005;Del Pino et al., 2018;Foss-Feig et al., 2017;Golden et al., 2018;Kaiser and Feng, 2015;Mullins et al., 2016;Robertson and Baron-Cohen, 2017). Further, patches of disorganized cortical tissue have been observed in brains of both children with autism as well as in mouse models of the disorder, around the time of birth (Choi et al., 2016;Dufour et al., 2022;Kim et al., 2017;Orosco et al., 2014;Peñagarikano et al., 2011;Rabelo et al., 2022;Stoner et al., 2014;Yim et al., 2017). ...
Cortical circuits are composed predominantly of pyramidal-to-pyramidal neuron connections, yet their assembly during embryonic development is not well understood. We show that embryonic layer 5 pyramidal neurons, identified through single cell transcriptomics, display two phases of circuit assembly in vivo. At E14.5, a multi-layered circuit motif, composed of a single layer 5 cell type, forms. This motif is transient, switching to a second circuit motif, involving all three types, by E17.5. In vivo targeted single cell recordings and two-photon calcium imaging of embryonic layer 5 neurons reveal that, in both phases, neurons have active somas and neurites, tetrodotoxin-sensitive voltage-gated conductances, and functional glutamatergic synapses. Embryonic layer 5 neurons strongly express autism-associated genes, and perturbing these genes disrupts the switch between the two motifs. Hence, layer 5 pyramidal neurons form transient active pyramidal-to-pyramidal circuits, at the inception of neocortex, and studying these circuits could yield insights into the etiology of autism.
... Genome-wide association studies on these patients indicate a genetic basis spanning hundreds of different risk genes (Abrahams et al., 2013;Iakoucheva et al., 2019). As a neurodevelopmental disorder diagnosed as early as 2 years of age (Hyman et al., 2020), ASD significantly impacts neural circuits across many brain regions including the cerebellum, somatosensory cortex, and prefrontal cortex (Golden et al., 2018). Within these brain regions, a common simplified model is that excitatory/ inhibitory (E/I) imbalance, presumably linked to GABAergic interneuron dysfunction, represents a neurophysiological hallmark of ASD (Rubenstein and Merzenich, 2003;Gogolla et al., 2009;Lee et al., 2017;Sohal and Rubenstein, 2019). ...
... ASD is a common neurodevelopmental disease involving different brain regions and a spectrum of behavioral deficits (Silverman et al., 2010;Golden et al., 2018). Emerging evidence from mouse models points to a role for interneuron-mediated functional deficits underlying these complex behaviors (Vogt et al., 2015;Paterno et al., 2020;Filice et al., 2020). ...
Impaired synaptic neurotransmission may underly circuit alterations contributing to behavioral autism spectrum disorder (ASD) phenotypes. A critical component of impairments reported in somatosensory and prefrontal cortex of ASD mouse models are parvalbumin (PV)-expressing fast-spiking interneurons. However, it remains unknown whether PV interneurons mediating hippocampal networks crucial to navigation and memory processing are similarly impaired. Using PV-labeled transgenic mice, a battery of behavioral assays, in vitro patch-clamp electrophysiology, and in vivo 32-channel silicon probe local field potential recordings, we address this question in a Cntnap2-null mutant mouse model representing a human ASD risk factor gene. Cntnap2−/− mice show a reduction in hippocampal PV interneuron density, reduced inhibitory input to CA1 pyramidal cells, deficits in spatial discrimination ability, and frequency-dependent circuit changes within the hippocampus, including alterations in gamma oscillations, sharp-wave ripples, and theta-gamma modulation. Our findings highlight hippocampal involvement in ASD and implicate interneurons as a potential therapeutical target.
... Future studies investing changes in E/I balance will require the identification of ASD-ID relevant neuronal subtypes in zebrafish. Many ASD-ID genes are expressed in glutamatergic excitatory or inhibitory GABAergic neurons, including Neurexin (NRX) and SHANK3, or Sodium Voltage-Gated Channels Alpha Subunit 1 (SCN1A) and CNTNAP2, respectively [185,191]. Several of these genes have been selectively knocked-out in specific neuronal subtypes to demonstrate that genetic mutations in those neuronal subtypes are sufficient to cause ASD-ID behavioral deficits [185,192]. For example, knocking-out Scn1a in forebrain GABAergic neurons recapitulated several Dravet syndrome phenotypes, including seizure susceptibility [192]. ...
... Importantly, alertness correlated brain regions showed similar activity patterns in mice, suggesting that the relationship between alertness circuits and behavior are conserved [249]. Furthermore, these conserved circuits and behaviors are important for ASD-ID research because arousal pathways have been implicated in ASD-ID [191,250,251]. This method could be strengthened by functionally testing circuits using lines from a Gal4 or Cre-Lox library [19,252]. ...
Autism spectrum disorder (ASD) and intellectual disability (ID) are neurodevelopmental disorders with overlapping diagnostic behaviors and risk factors. These include embryonic exposure to teratogens and mutations in genes that have important functions prenatally. Animal models, including rodents and zebrafish, have been essential in delineating mechanisms of neuropathology and identifying developmental critical periods, when those mechanisms are most sensitive to disruption. This review focuses on how the developmentally accessible zebrafish is contributing to our understanding of prenatal pathologies that set the stage for later ASD-ID behavioral deficits. We discuss the known factors that contribute prenatally to ASD-ID and the recent use of zebrafish to model deficits in brain morphogenesis and circuit development. We conclude by suggesting that a future challenge in zebrafish ASD-ID modeling will be to bridge prenatal anatomical and physiological pathologies to behavioral deficits later in life.
... Work from Marr (1969) and Albus (1971) used this structural map of the cerebellum, and at that time the quickly emerging details of its functional connectivity (Eccles, 1965), to postulate theories on its computational power over motor control. Given that the cerebellum has one neuronal population responsible for the output of its cortex, the Purkinje cell, and that this cell type is innervated by inputs in a predictable, reproducible pattern, the cerebellum is thought to execute a multitude of motor behaviors by modulating Purkinje cell spiking and the downstream consequences on cerebellar nuclear neuron firing (Marr, 1969;Gilbert, 1974;Ruigrok, 2011). It is also important to note that Purkinje cells project to distinct cerebellar output nuclei as revealed by the pattern of axonal projections, compartmental expression of molecules, and functional designation. ...
... While a reduction in gray matter volume has been found in the cerebellum, hippocampus, amygdala, and parietal lobe of children with autism spectrum disorder (ASD), others have reported an overall increase in brain volume as a hallmark feature of ASD (Carlisi et al., 2017;Riddle et al., 2017). Studies on the underlying genes associated with ASD pathogenesis suggest that a range of functions are disrupted such as synaptic maintenance and motor control (Antoine et al., 2018;Golden et al., 2018;Tatavarty et al., 2018). However, inconsistent structural findings and the seemingly endless number or combination of genetic causes have complicated the process of studying function in autism and other diseases. ...
The brain contains a large diversity of unique cell types that use specific genetic programs to control development and instruct the intricate wiring of sensory, motor, and cognitive brain regions. In addition to their cellular diversity and specialized connectivity maps, each region’s dedicated function is also expressed in their characteristic gross external morphologies. The folds on the surface of the cerebral cortex and cerebellum are classic examples. But, to what extent does structure relate to function and at what spatial scale? We discuss the mechanisms that sculpt functional brain maps and external morphologies. We also contrast the cryptic structural defects in conditions such as autism spectrum disorders to the overt microcephaly after Zika infections, taking into consideration that both diseases disrupt proper cognitive development. The data indicate that dynamic processes shape all brain areas to fit into jigsaw-like patterns. The patterns in each region reflect circuit connectivity, which ultimately supports local signal processing and accomplishes multi-areal integration of information processing to optimize brain functions.
... As an alternative, network-level rodent models can provide a useful preclinical framework to test different aetiological hypotheses. Whilst abnormal neural circuitry is a common neuropathological feature in preclinical models of other polygenic conditions such as autism spectrum disorder (ASD) (Golden et al., 2018), OCD is unique in the causal link that has been established between targeted circuit manipulations and OC-like behaviours in mice. ...
Obsessive-compulsive and related disorders (OCRD) is an emergent class of psychiatric illnesses that contribute substantially to the global mental health disease burden. In particular, the prototypical illness, obsessive-compulsive disorder (OCD), has a profoundly deleterious effect on the quality of life of those with lived experience. Both clinical and preclinical studies have investigated the genetic and environmental influences contributing to the pathogenesis of obsessive-compulsive and related disorders. Significant progress has been made in recent years in our understanding of the genetics of OCD, along with the critical role of common environmental triggers (e.g., stress). Some of this progress can be attributed to the sophistication of rodent models used in the field, particularly genetic mutant models, which demonstrate promising construct, face and predictive validity. However, there is a paucity of studies investigating how these genetic and environmental influences interact to precipitate the behavioural, cellular and molecular changes that occur in OCD. In this review, we assert that preclinical studies offer a unique opportunity to carefully manipulate environmental and genetic factors, and in turn to interrogate gene-environment interactions and relevant downstream sequelae. Such studies may serve to provide a mechanistic framework to build our understanding of the pathogenesis of complex neuropsychiatric diseases such as OCD. Furthermore, understanding gene-environment interactions and pathogenic mechanisms will facilitate precision medicine and other future approaches to enhance treatment, reduce side-effects of therapeutic interventions and improve the lives of those suffering from these devastating disorders.
... A proteomic study of cortical synapses noted smaller differences in Fmr1 KO mice after 3 weeks of age (Tang et al., 2015), and both Shank3 and Syngap1 show changes in interaction partners throughout development (Li et al., 2016(Li et al., , 2017. Moreover, we focused on hippocampal synapses, but there are important roles for the neocortex, striatum, and cerebellum in the pathophysiology of ASD animal models (Golden et al., 2018). Fractionation of the PSD limits our findings to synaptic profiles in glutamatergic neurons, but ASD risk genes also influence interneuron development (Cntnap2; Peñagarikano et al., 2011) and oligodendrocyte maturation (Pten; Lee et al., 2019). ...
Specific and effective treatments for autism spectrum disorder (ASD) are lacking due to a poor understanding of disease mechanisms. Here we test the idea that similarities between diverse ASD mouse models are caused by deficits in common molecular pathways at neuronal synapses. To do this, we leverage the availability of multiple genetic models of ASD that exhibit shared synaptic and behavioral deficits and use quantitative mass spectrometry with isobaric tandem mass tagging (TMT) to compare their hippocampal synaptic proteomes. Comparative analyses of mouse models for Fragile X syndrome ( Fmr1 knockout), cortical dysplasia focal epilepsy syndrome ( Cntnap2 knockout), PTEN hamartoma tumor syndrome ( Pten haploinsufficiency), ANKS1B syndrome ( Anks1b haploinsufficiency), and idiopathic autism (BTBR+) revealed several common altered cellular and molecular pathways at the synapse, including changes in oxidative phosphorylation, and Rho family small GTPase signaling. Functional validation of one of these aberrant pathways, Rac1 signaling, confirms that the ANKS1B model displays altered Rac1 activity counter to that observed in other models, as predicted by the bioinformatic analyses. Overall similarity analyses reveal clusters of synaptic profiles, which may form the basis for molecular subtypes that explain genetic heterogeneity in ASD despite a common clinical diagnosis. Our results suggest that ASD-linked susceptibility genes ultimately converge on common signaling pathways regulating synaptic function and propose that these points of convergence are key to understanding the pathogenesis of this disorder.
... The former can be explained by abnormal circuitry (Golden et al., 2018), notably in the indirect pathway downstream from D2-receptor expressing medium spiny neurons (MSNs). The subthalamic nucleus (STN), an excitatory nucleus in the indirect pathway downstream from the D2-MSNs, is a key factor in the normal functioning of the basal ganglia. ...
There is an increased incidence of autism among the children of women who take the anti-epileptic, mood stabilizing drug, valproic acid (VPA) during pregnancy, moreover, exposure to VPA in utero causes autistic-like symptoms in rodents and non-human primates. Analysis of RNAseq data ob-tained from fetal mouse brains 3 hr after VPA administration revealed that VPA significantly [p(FDR) ≤ 0.025] increased or decreased the expression of approximately 7,300 genes. No significant sex dif-ferences in VPA-induced gene expression were observed. Expression of genes associated with neurodevelopmental disorders such as autism as well as neurogenesis, axon growth and synapto-genesis, GABAergic, glutaminergic and dopaminergic synaptic transmission, perineuronal nets, and circadian rhythms was dysregulated by VPA. Moreover, expression of 400 autism risk genes was significantly altered by VPA. In addition, expression of 247 genes that have been reported to play fundamental roles in the development of the nervous system, but are not linked to autism by GWAS, was significantly increased or decreased by VPA. The goal of this study was to identify mouse genes that are: (a) significantly up- or down-regulated by VPA in the fetal brain and (b) known to be associated with autism and/or to play a role in embryonic neurodevelopmental processes, perturbation of which has the potential to alter brain connectivity in the postnatal and adult brain. The set of genes meeting these criteria provides potential targets for future hypothesis-driven approaches to elucidating the proximal underlying causes of defective brain connectivity in neuro-developmental disorders such as autism.
... From a clinical perspective, aberrant neural circuits have been observed to participate in the initiation and progression of a variety of neurological disorders, such as autism spectrum disorder (ASD) and neurodegenerative diseases, through pathological neuronal activities and abnormal axon guidance protein changes (Van Battum et al., 2015;Sudarov et al., 2018;Barth and Ray, 2019;Moussa and Wester, 2022). Taking ASD as an example, an imbalance in the ratio of excitation to inhibition within cortical circuits has been hypothesized as a specific developmental mechanism (Golden et al., 2018;Sohal and Rubenstein, 2019). Therefore, it is reasonable to focus on and further reveal regionspecific neuronal diversity, which supports us to better understand the complexity of neural circuits that underlie autistic phenotypes or others. ...
Neural circuits are characterized as interconnecting neuron networks connected by synapses. Some kinds of gene expression and/or functional changes of neurons and synaptic connections may result in aberrant neural circuits, which has been recognized as one crucial pathological mechanism for the onset of many neurological diseases. Gradual advances in single-cell sequencing approaches with strong technological advantages, as exemplified by high throughput and increased resolution for live cells, have enabled it to assist us in understanding neuronal diversity across diverse brain regions and further transformed our knowledge of cellular building blocks of neural circuits through revealing numerous molecular signatures. Currently published transcriptomic studies have elucidated various neuronal subpopulations as well as their distribution across prefrontal cortex, hippocampus, hypothalamus, and dorsal root ganglion, etc. Better characterization of brain region-specific circuits may shed light on new pathological mechanisms involved and assist in selecting potential targets for the prevention and treatment of specific neurological disorders based on their established roles. Given diverse neuronal populations across different brain regions, we aim to give a brief sketch of current progress in understanding neuronal diversity and neural circuit complexity according to their locations. With the special focus on the application of single-cell sequencing, we thereby summarize relevant region-specific findings. Considering the importance of spatial context and connectivity in neural circuits, we also discuss a few published results obtained by spatial transcriptomics. Taken together, these single-cell sequencing data may lay a mechanistic basis for functional identification of brain circuit components, which links their molecular signatures to anatomical regions, connectivity, morphology, and physiology. Furthermore, the comprehensive characterization of neuron subtypes, their distributions, and connectivity patterns via single-cell sequencing is critical for understanding neural circuit properties and how they generate region-dependent interactions in different context.
... Rodent models have enabled mechanistic insights into the genetic causes and circuit-level manifestations of single-gene neurodevelopmental disorders (NDDs) [1][2][3][4][5][6][7]. As mechanism-based treatments are developed for NDDs, the effectiveness of such treatments is often first tested preclinically by assessing improvements in mouse behavior. ...
Angelman syndrome (AS) is a neurodevelopmental disorder caused by loss of expression of the maternal copy of the UBE3A gene. Individuals with AS have a multifaceted behavioral phenotype consisting of deficits in motor function, epilepsy, cognitive impairment, sleep abnormalities, as well as other comorbidities. Effectively modeling this behavioral profile and measuring behavioral improvement will be crucial for the success of ongoing and future clinical trials. Foundational studies have defined an array of behavioral phenotypes in the AS mouse model. However, no single behavioral test is able to fully capture the complex nature of AS—in mice, or in children. We performed multidimensional analysis (principal component analysis + k-means clustering) to quantify the performance of AS model mice (n = 148) and wild-type littermates (n = 138) across eight behavioral domains. This approach correctly predicted the genotype of mice based on their behavioral profile with ~95% accuracy, and remained effective with reasonable sample sizes (n = ~12–15). Multidimensional analysis was effective using different combinations of behavioral inputs and was able to detect behavioral improvement as a function of treatment in AS model mice. Overall, multidimensional behavioral analysis provides a tool for evaluating the effectiveness of preclinical treatments for AS. Multidimensional analysis of behavior may also be applied to rodent models of related neurodevelopmental disorders, and may be particularly valuable for disorders where individual behavioral tests are less reliable than in AS.
... The etiology of neurological problems is varied and complex, ranging from environmental factors such as alcohol abuse and viral infections to genetic alterations (77). Affected subjects can present gross morphological abnormalities in the nervous system including microcephaly, cortical malformations such as lissencephaly (i.e., absence of normal folds in the cerebral cortex), as well as more subtle changes to structure of neural circuits and synapses (77)(78)(79)(80)(81). ...
Protein disulfide isomerases (PDIs) constitute a family of oxidoreductases promoting redox protein folding and quality control in the endoplasmic reticulum (ER). PDIs catalyze disulfide bond formation, isomerization and reduction, operating in concert with molecular chaperones to fold secretory cargoes in addition to directing misfolded proteins to be refolded or degraded. Importantly, PDIs are emerging as key components of the proteostasis network, integrating protein folding status with central surveillance mechanisms to balance proteostasis according to cellular needs. Recent advances in the field driven by the generation of new mouse models, human genetic studies, and omics methodologies, in addition to interventions using small molecules and gene therapy, have revealed the significance of PDIs to the physiology of the nervous system and their implications in pathologies, ranging from neurodevelopmental conditions to neurodegenerative diseases and traumatic injuries. Here, we review the principles of redox protein folding in the ER with a focus on current evidence linking genetic mutations and biochemical alterations to PDIs in the etiology of neurological conditions.
... The genes that lead to the E-I imbalance through their effect on neurotransmission are often implicated in both ASD and intellectual disability [12][13][14]. People with ASD and comorbid intellectual disability are more likely to have epilepsy than autistic individuals without intellectual disability [15,16], possibly because of more severe disturbances of the E-I balance. ...
Background
Altered neuronal excitation–inhibition (E–I) balance is strongly implicated in ASD. However, it is not known whether the direction and degree of changes in the E–I ratio in individuals with ASD correlates with intellectual disability often associated with this developmental disorder. The spectral slope of the aperiodic 1/f activity reflects the E–I balance at the scale of large neuronal populations and may uncover its putative alternations in individuals with ASD with and without intellectual disability.
Methods
Herein, we used magnetoencephalography (MEG) to test whether the 1/f slope would differentiate ASD children with average and below–average (< 85) IQ. MEG was recorded at rest with eyes open/closed in 49 boys with ASD aged 6–15 years with IQ ranging from 54 to 128, and in 49 age-matched typically developing (TD) boys. The cortical source activity was estimated using the beamformer approach and individual brain models. We then extracted the 1/f slope by fitting a linear function to the log–log-scale power spectra in the high-frequency range.
Results
The global 1/f slope averaged over all cortical sources demonstrated high rank-order stability between the two conditions. Consistent with previous research, it was steeper in the eyes-closed than in the eyes-open condition and flattened with age. Regardless of condition, children with ASD and below-average IQ had flatter slopes than either TD or ASD children with average or above-average IQ. These group differences could not be explained by differences in signal-to-noise ratio or periodic (alpha and beta) activity.
Limitations
Further research is needed to find out whether the observed changes in E–I ratios are characteristic of children with below-average IQ of other diagnostic groups.
Conclusions
The atypically flattened spectral slope of aperiodic activity in children with ASD and below-average IQ suggests a shift of the global E–I balance toward hyper-excitation. The spectral slope can provide an accessible noninvasive biomarker of the E–I ratio for making objective judgments about treatment effectiveness in people with ASD and comorbid intellectual disability.
... Autism spectrum disorder (ASD) encompasses a heterogenous set of multifactorial 27 challenges in neurodevelopment, classified according to three fundamental features; 28 compromised social correspondence skills, delayed dialect development, and raised 29 stereotyped alternately tedium practices (Mohn et al. 2014;Golden et al. 2017;30 Abrahams and Geschwind 2008;Geschwind 2008;Sudhof 2008;Zoghbi 2003). 31 ASD is believed to occur due to the complicated processes that involve numerous (Zumbrunn et al. 2001;Akiyama and Kawasaki 2006) and as 174 an mRNA-binding protein with several of its targets involved in brain development 175 (Preitner et al. 2014). ...
Autism spectrum disorder (ASD) development is a highly multifaceted process as evidenced by the complexity of the factors involved in the etiology of ASD, including genetic and nongenetic factors. Several forms of ASD result from genetic alterations in genes that regulate the process of protein synthesis. A growing body of evidence suggests that abnormal synaptic protein synthesis might contribute to ASD and ASD-like clinical features. Several reports of different mutated genes responsible for ASD cases and genetic models have emerged, revealing dysregulation of many crucial signaling pathways. In this chapter, the authors summarize the various factors described to contribute to ASD, both genetic and nongenetic, and their association with WNT, SHH, RA, FGF, and BMP/TGF-β signaling pathways. In addition, the authors discuss the scope for additional research for a better understanding of the pathophysiology of ASD in the context of disrupted signaling pathways, which could help open the doors to identify possible gene targets and novel therapeutic strategies.KeywordsASDAutismSignaling pathwaysGenesTherapeutic targetsDevelopmental mechanisms
... The genes that lead to the E-I imbalance through their effect on neurotransmission are often implicated in both ASD and intellectual disability (ID) (11)(12)(13). People with ASD and comorbid intellectual disability are more likely to have epilepsy than autistic individuals without intellectual disability (14,15), possibly because of more severe disturbances of the E-I balance. ...
BACKGROUND
An altered balance of neuronal excitation and inhibition (E-I balance) might be implicated in the co-occurrence of autism and intellectual disability, but this hypothesis has never been tested. E-I balance changes can be estimated from the spectral slope of the aperiodic 1/f neural activity. Herein, we used magnetoencephalography (MEG) to test whether the 1/f slope would differentiate ASD children with and without intellectual disability.
METHODS
MEG was recorded at rest with eyes open/closed in 49 boys with ASD aged 6-15 years with a broad range of IQs, and in 49 age-matched typically developing (TD) boys. The cortical source activity was estimated using the LCMV beamformer approach. We then extracted the 1/f slope by fitting a linear function in to the log-log-scale power spectra in the high-frequency range.
RESULTS
The grand averaged 1/f slope was steeper in the eyes closed than in the eyes open condition, but had high rank-order stability between them. In line with the previous research, the slope flattened with age. Children with ASD and below-average (<85) IQ had flatter slopes than either TD or ASD children with average IQ. These group differences could not be explained by differences in signal-to-noise ratio or periodic (alpha and beta) activity.
CONCLUSIONS
The atypically flattened spectral slope of aperiodic activity in children with ASD and below-average IQ suggests a shift of the global E-I balance toward hyper-excitation. The spectral slope can provide an accessible non-invasive biomarker of the E-I ratio for translational research and making objective judgments about treatment effectiveness.
... A large number of risk genes that are associated with ASD pathogenesis have been implicated in an altered axonal growth, an imbalance of the neuronal network excitation/ inhibition ratio, and an altered long-term synaptic plasticity in the corticostriatal pathway (Fuccillo, 2016;Golden et al., 2018;Kuo & Liu, 2019;Shepherd, 2013). The high-confidence ASD-associated genes include: FMR1, KMT2A, GRIN2B, SCN2A, NLGN1, NLGN3, MET, CNTNAP2, FOXP2, TSHZ3, SHANK3, PTEN, CHD8, MECP2, DYRK1A, RELN, FOXP1, SYNGAP1, and NRXN (Li & Pozzo-Miller, 2019;Nakanishi et al., 2019;Verma et al., 2019). ...
Striatal medium-sized spiny neurons (MSNs) integrate dopamine signals mainly through the cAMP signaling pathway. Dopamine D1 or D2 receptors trigger an increase or a decrease in cAMP levels, respectively. My thesis focuses on how phosphodiesterases (PDEs), which degrade cAMP, are involved in the integration of dopamine signals in the striatum. I used genetically-encoded FRET biosensors to monitor cAMP level in real time in individual living neurons in striatal brain slice preparations. I used selective inhibitors to determine the function of each PDE. PDE1B, which is activated by calcium-calmodulin, appears as a detector of the coincidence of dopamine and glutamate signals, which is critical in the regulation of synaptic plasticity involved in reward-based learning. PDE10A shows the most prominent activity, efficiently degrading both high and low cAMP levels. PDE10A activity is required to allow for PKA de-activation, and therefore needed to transduce a dopamine signal through D2 receptors into a decrease in PKA-dependent phosphorylation. PDE2A and PDE4 appeared to degrade only high levels of cAMP, preventing large increases in cAMP. PDE2A, which activity can be increased by cGMP, also appears as a detector of dopamine and NO coincidence. Understanding PDE functions can highlight their potential as therapeutic targets in CNS pathologies. As an example, we showed an increased PDE2A function in the hippocampus of a mouse model of Fragile X syndrome. Besides the cAMP/PKA pathway, dopamine D2 receptors is reported to activate non-canonical pathways. Attempts to use biosensors for Akt and ERK pathways did not provide conclusive data.
... The generation of a relevant disease specific knockout model for autistic traits/behaviors has been difficult because of the genetically heterogeneous nature of ASD. A number of rodent models (knockout, humanized knock-in mice, and Cre-loxP) for rare variations (de novo and CNV) detected in ASD have been used to understand the etiology of ASD [108][109][110][111]. Cadps2-knockout model showed decreased brain-derived neurotrophic factor (BDNF) release from neocortical and cerebellar neurons [112]. ...
Autism Spectrum Disorder (ASD) comprises a heterogeneous group of neurodevelopmental disorders with a strong heritable genetic component. At present, ASD is diagnosed solely by behavioral criteria. Advances in genomic analysis have contributed to numerous candidate genes for the risk of ASD, where rare mutations and s common variants contribute to its susceptibility. Moreover, studies show rare de novo variants, copy number variation and single nucleotide polymorphisms (SNPs) also impact neurodevelopment signaling. Exploration of rare and common variants involved in common dysregulated pathways can provide new diagnostic and therapeutic strategies for ASD. Contributions of current innovative molecular strategies to understand etiology of ASD will be explored which are focused on whole exome sequencing (WES), whole genome sequencing (WGS), microRNA, long non-coding RNAs and CRISPR/Cas9 models. Some promising areas of pharmacogenomic and endophenotype directed therapies as novel personalized treatment and prevention will be discussed.
... The balance between excitatory synapses and inhibitory synapses is a key factor controlling neural circuit functions and can be disrupted by various genetic and environmental risk factors associated with ASD (Yizhar et al, 2011;Bateup et al, 2013;Selimbeyoglu et al, 2017). However, the E/ I imbalance in ASD needs to be interpreted in specific brain regions, cell types, and synaptic connections relevant to defined behavioral outcomes (Rothwell et al, 2014;Golden et al, 2018;Sohal & Rubenstein, 2019). ZIKV-affected offspring mice exhibited decreased expression of parvalbumin (PV) and calretinin (CAL), which label different subtypes of interneurons (Kepecs & Fishell, 2014). ...
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.
... Intellectual disability (ID) or cognitive impairment is characterized by significant limitations in cognitive functions, including reasoning, learning, memory, and adaptive behaviors, which co-occur with many neuropsychiatric disorders, including autism spectrum disorder (ASD) and schizophrenia (Morgan et al., 2008;Matson and Shoemaker, 2009). Cognitive impairments in these disorders have been commonly linked to dysfunction within hippocampal and cortical circuits (O'Tuathaigh et al., 2007;Kvajo et al., 2008;Golden et al., 2018); however, whether converging neurobiological mechanisms underlie cognitive impairments across disorders has not been established. This issue has an important implication: if common mechanisms can be identified, then therapeutic approaches capable of treating cognitive impairments in a subset of neuropsychiatric disorders may be developed. ...
Neuropsychiatric disorders are often accompanied by cognitive impairments/intellectual disability (ID). It is not clear whether there are converging mechanisms underlying these debilitating impairments. We found that many autism and schizophrenia risk genes are expressed in the anterodorsal subdivision (AD) of anterior thalamic nuclei, which has reciprocal connectivity with learning and memory structures. CRISPR-Cas9 knockdown of multiple risk genes selectively in AD thalamus led to memory deficits. While the AD is necessary for contextual memory encoding, the neighboring anteroventral subdivision (AV) regulates memory specificity. These distinct functions of AD and AV are mediated through their projections to retrosplenial cortex, using differential mechanisms. Furthermore, knockdown of autism and schizophrenia risk genes PTCHD1, YWHAG, or HERC1 from AD led to neuronal hyperexcitability, and normalization of hyperexcitability rescued memory deficits in these models. This study identifies converging cellular to circuit mechanisms underlying cognitive deficits in a subset of neuropsychiatric disease models.
... We observed a strong seizure susceptibility phenotype in Arid1b +/− mice (Fig. 6). Arid1b +/− mice were functionally susceptible to PTZ-induced seizures, related to GABAergic neuron plasticity, we attribute this to an excitatory-inhibition imbalance, as with other genetic models of NDD [67][68][69]. This hypothesis corroborates histological analysis by Jung et al. [20]. ...
Background
One of the causal mechanisms underlying neurodevelopmental disorders (NDDs) is chromatin modification and the genes that regulate chromatin. AT-rich interactive domain 1B (ARID1B) , a chromatin modifier, has been linked to autism spectrum disorder and to affect rare and inherited genetic variation in a broad set of NDDs.
Methods
A novel preclinical mouse model of Arid1b deficiency was created and validated to characterize and define neuroanatomical, behavioral and transcriptional phenotypes. Neuroanatomy was assessed ex vivo in adult animals and in vivo longitudinally from birth to adulthood. Behavioral testing was also performed throughout development and tested all aspects of motor, learning, sociability, repetitive behaviors, seizure susceptibility, and general milestones delays.
Results
We validated decreased Arid1b mRNA and protein in Arid1b +/− mice, with signatures of increased axonal and synaptic gene expression, decreased transcriptional regulator and RNA processing expression in adult Arid1b +/− cerebellum. During neonatal development, Arid1b +/− mice exhibited robust impairments in ultrasonic vocalizations (USVs) and metrics of developmental growth. In addition, a striking sex effect was observed neuroanatomically throughout development. Behaviorally, as adults, Arid1b +/− mice showed low motor skills in open field exploration and normal three-chambered approach. Arid1b +/− mice had learning and memory deficits in novel object recognition but not in visual discrimination and reversal touchscreen tasks. Social interactions in the male–female social dyad with USVs revealed social deficits on some but not all parameters. No repetitive behaviors were observed. Brains of adult Arid1b +/− mice had a smaller cerebellum and a larger hippocampus and corpus callosum. The corpus callosum increase seen here contrasts previous reports which highlight losses in corpus callosum volume in mice and humans.
Limitations
The behavior and neuroimaging analyses were done on separate cohorts of mice, which did not allow a direct correlation between the imaging and behavioral findings, and the transcriptomic analysis was exploratory, with no validation of altered expression beyond Arid1b .
Conclusions
This study represents a full validation and investigation of a novel model of Arid1b +/− haploinsufficiency throughout development and highlights the importance of examining both sexes throughout development in NDDs.
... Importantly, data also indicate that Shank2 deficiency in the hippocampus and striatum contribute critically to the phenotypic impairments in social and motor behaviors (Schmeisser et al. 2012;Won et al. 2012). Overall, in mouse models of ASD, sensory, prefrontal, hippocampal, cerebellar, and striatal regions, as well as the circuits that connect them are perturbed (Golden et al. 2018). These circuits differentially contribute to ASD-related behaviors, a pattern that has emerged across several major ASD mutant mouse lines (Rothwell et al. 2014;Bey et al. 2018;Zerbi et al. 2018;Guo et al. 2019). ...
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.
... Genetic mutations and environmental risk factors resulting in the presentation of repetitive behaviors in rodent models involve multiple cellular, molecular, and network factors. The majority of ASD alterations involve excitatory glutamatergic, inhibitory GABAergic, serotonergic and dopaminergic neurons, receptors, neurotransmitters, neuronal migration, and spine densities resulting in changes in signaling pathways and synaptic activity which may converge on common neural circuits (Golden et al., 2018). ...
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.
... We note that cortical alterations have also emerged in mouse [86][87][88] and stem cell models (reviewed in [89]) of CNVs associated with ASD risk. We will not discuss the circuit changes and behavioral outcomes of the rodent models with mutations in ASD risk genes, which are discussed elsewhere [90][91][92]. We will primarily focus on the cellular and molecular functions of high-risk genes in (1) neurogenesis, including both progenitor proliferation and differentiation into neurons; (2) migration of the newborn projection neurons to the cortical plate; (3) cell specification of projection neurons, defining cortical lamination; (4) morphogenesis and synaptogenesis of projection neurons; (5) migration and development of cortical GABAergic interneurons ( Figure 1, Table 1). ...
Autism spectrum disorder (ASD) is a prevalent neurodevelopmental disorder characterized by impairments in social communication and social interaction, and the presence of repetitive behaviors and/or restricted interests. In the past few years, large-scale whole-exome sequencing and genome-wide association studies have made enormous progress in our understanding of the genetic risk architecture of ASD. While showing a complex and heterogeneous landscape, these studies have led to the identification of genetic loci associated with ASD risk. The intersection of genetic and transcriptomic analyses have also begun to shed light on functional convergences between risk genes, with the mid-fetal development of the cerebral cortex emerging as a critical nexus for ASD. In this review, we provide a concise summary of the latest genetic discoveries on ASD. We then discuss the studies in postmortem tissues, stem cell models, and rodent models that implicate recently identified ASD risk genes in cortical development.
... We observed a strong seizure susceptibility phenotype in Arid1b +/mice ( Figure 7). Arid1b +/mice were functionally susceptible to PTZ-induced seizures, related to GABAergic neuron plasticity, we attribute this to an excitatory-inhibition imbalance, as with other genetic models of NDD [72][73][74]. This hypothesis corroborates histological analysis by Jung et al. [19]. ...
Background - One of the causal mechanisms underlying neurodevelopmental disorders (NDDs) is chromatin modification, and the genes that regulate chromatin. AT-Rich Interactive Domain 1B (ARID1B) , a chromatin modifier, has been shown to be reduced in autism spectrum disorder (ASD) and to affect rare and inherited genetic variation in a broad set of NDDs.
Methods - A novel preclinical mouse model of Arid1b deficiency was created validated to characterize and define neuroanatomical, behavioural and transcriptional phenotypes. Neuroanatomy was assess ex vivo in adult animals and in vivo longitudinally from birth to adulthood. Behavioural testing was also performed throughout development and tested all aspects of motor, learning, sociability, repetitive behaviours, seizure susceptibility and general milestones.
Results - Brains of adult Arid1b+/- mice had a smaller cerebellum and a larger hippocampus and corpus callosum. These results stand in contrast to previously reported data highlighting losses in corpus callosum volume. In addition, a striking sex dependence was observed throughout development; males had an early emergence of this neuroanatomical phenotype at postnatal day 7, whereas females had a delayed emergence around postnatal day 40. Behaviourally, during neonatal development, Arid1b+/- mice exhibited robust impairments in ultrasonic vocalizations (USVs) and metrics of developmental growth. As adults, Arid1b+/- mice showed low motor skills in open field exploration and normal three chambered approach. Arid1b+/- mice had learning and memory deficits in novel object recognition but not in visual discrimination and reversal touchscreen tasks. Social interactions in the male-female social dyad with USVs revealed social deficits on some but not all parameters. No repetitive behaviours were observed.
Limitations – The behaviour and the neuroimaging analysis were done on separate cohorts of mice, which does not allow a direct correlation between the imaging and behavioural findings.
Conclusions – This study represents a full investigation of Arid1b+/- haploinsufficiency throughout development and highlights the importance of examining both sexes throughout development in NDDs.
... Sensory deficits have also been replicated in FXS models 35 and many of the brain regions that were perturbed in our study are implicated in sensory perception. To begin with, three regions that are involved in olfaction, the piriform cortex, olfactory bulb, and anterior commissure were altered in Fmr1-Δ exon 8 rats. ...
Fragile X syndrome (FXS) is a neurodevelopmental disorder that is caused by mutations in the FMR1 gene. Neuroanatomical alterations have been reported in both male and female individuals with FXS, yet the morphological underpinnings of these alterations have not been elucidated. In the current study, we found structural changes in both male and female rats that model FXS, some of which are similarly impaired in both sexes, including the superior colliculus and periaqueductal gray, and others that show sex-specific changes. The splenium of the corpus callosum, for example, was only impaired in males. We also found reduced axonal caliber in the splenium, offering a mechanism for its structural changes. Furthermore, we found that overall, male rats have higher brain-wide diffusion than female rats. Our results provide insight into which brain regions are vulnerable to a loss of Fmr1 expression and reveal an impairment at the level of the axon that could cause structural changes in white matter regions.
... Subtle differences in rate-limiting steps in synaptic transmission in different neuronal subtypes, together with the organization of brain micro-circuits, provide a starting point to explain this. Among the main developmental symptoms, seizures, intellectual disability, and autism spectrum disorder (ASD) are common and have all been linked to dysregulation of the excitation/inhibition (E/I) balance in brain circuits (Golden et al., 2018). At first glance, the SNARE machinery appears to be universally required in all synapses, glutamatergic (excitation) and GABAergic (inhibition). ...
Neuronal SNAREs and their key regulators together drive synaptic vesicle exocytosis and synaptic transmission as a single integrated membrane fusion machine. Human pathogenic mutations have now been reported for all eight core components, but patients are diagnosed with very different neurodevelopmental syndromes. We propose to unify these syndromes, based on etiology and mechanism, as “SNAREopathies.” Here, we review the strikingly diverse clinical phenomenology and disease severity and the also remarkably diverse genetic mechanisms. We argue that disease severity generally scales with functional redundancy and, conversely, that the large effect of mutations in some SNARE genes is the price paid for extensive integration and exceptional specialization. Finally, we discuss how subtle differences in components being rate limiting in different types of neurons helps to explain the main symptoms.
... We have speculated that SERT, 5-HT 1A receptors, NLGN2, and GABA receptors may form a complex to spatially co-organize inhibitory control mechanisms that regulate 5-HT neuron excitability (Ye et al., 2016), which is diminished in the SERT Ala56 mouse (Veenstra-VanderWeele et al., 2012). Studies of GABA receptor agonist stimulation of 5-HT neurons from WT and SERT Ala56 mice are needed to test this idea, and if the model is corroborated, would add an additional dimension to the suggested contribution of an excitation/inhibition imbalance in ASD (Yizhar et al., 2011;Nelson and Valakh, 2015;Golden et al., 2018), and bridge the latter model to evidence of altered serotonergic signaling in ASD. ...
Altered serotonin (5-HT) signaling is associated with multiple brain disorders, including major depressive disorder (MDD), obsessive-compulsive disorder (OCD), and autism spectrum disorder (ASD). The presynaptic, high-affinity 5-HT transporter (SERT) tightly regulates 5-HT clearance after release from serotonergic neurons in the brain and enteric nervous systems, among other sites. Accumulating evidence suggests that SERT is dynamically regulated in distinct activity states as a result of environmental and intracellular stimuli, with regulation perturbed by disease-associated coding variants. Our lab identified a rare, hypermorphic SERT coding substitution, Gly56Ala, in subjects with ASD, finding that the Ala56 variant stabilizes a high-affinity outward-facing conformation (SERT∗) that leads to elevated 5-HT uptake in vitro and in vivo. Hyperactive SERT Ala56 appears to preclude further activity enhancements by p38α mitogen-activated protein kinase (MAPK) and can be normalized by pharmacological p38α MAPK inhibition, consistent with SERT Ala56 mimicking, constitutively, a high-activity conformation entered into transiently by p38α MAPK activation. We hypothesize that changes in SERT-interacting proteins (SIPs) support the shift of SERT into the SERT∗ state which may be captured by comparing the composition of SERT Ala56 protein complexes with those of wildtype (WT) SERT, defining specific interactions through comparisons of protein complexes recovered using preparations from SERT–/– (knockout; KO) mice. Using quantitative proteomic-based approaches, we identify a total of 459 SIPs, that demonstrate both SERT specificity and sensitivity to the Gly56Ala substitution, with a striking bias being a loss of SIP interactions with SERT Ala56 compared to WT SERT. Among this group are previously validated SIPs, such as flotillin-1 (FLOT1) and protein phosphatase 2A (PP2A), whose functions are believed to contribute to SERT microdomain localization and regulation. Interestingly, our studies nominate a number of novel SIPs implicated in ASD, including fragile X mental retardation 1 protein (FMR1) and SH3 and multiple ankyrin repeat domains protein 3 (SHANK3), of potential relevance to long-standing evidence of serotonergic contributions to ASD. Further investigation of these SIPs, and the broader networks they engage, may afford a greater understanding of ASD as well as other brain and peripheral disorders associated with perturbed 5-HT signaling.
... We observed a strong seizure susceptibility phenotype in Arid1b +/mice by numerous metrics including reduced latency to loss of righting, tonic hindlimb extension, and faster death. As are Arid1b +/mice were functionally susceptible to PTZ-induced seizures, related to GABAergic neuron plasticity, we attribute this to an excitatory-inhibition imbalance, as other new genetic models of NDD have also exhibited [85][86][87] . ...
One of the causal mechanisms underlying neurodevelopmental disorders (NDDs) is chromatin modification, and genes that regulate chromatin modify and control events regulating the formation of neural connections. AT-Rich Interactive Domain 1B (ARID1B), a chromatin modifier, has been shown to be reduced in autism spectrum disorder (ASD) and to affect rare and inherited genetic variation in a broad set of NDDs. For this work, a novel preclinical mouse model of Arid1b deficiency was created and molecularly validated to characterize and define neuroanatomical, behavioural and transcriptional phenotypes. Brains of adult Arid1b +/-mice had a smaller cerebellum along with a larger hippocampus and corpus callosum. In addition, a notable sex dependence was observed throughout development; males had an early emergence of the neuroanatomical phenotype around postnatal day 7, whereas females had a delayed emergence of the phenotype around postnatal day 40. Behavioural assays relevant to NDD were conducted during neonatal development and adulthood to evaluate general health, anxiety-like, motor, cognitive, and social behaviours in Arid1b +/-mice. During neonatal development, Arid1b +/-mice exhibited robust impairments in ultrasonic vocalizations (USVs) and metrics of developmental growth. As adults, Arid1b +/-mice showed low motor skills in open field exploration and normal three chambered approach. Arid1b +/-mice had learning and memory deficits in novel object recognition but surprisingly not in visual discrimination and reversal touchscreen tasks. Social interactions in the male-female social dyad with USVs revealed social deficits on some but not all parameters. No repetitive behaviours were observed. This study represents a full investigation of Arid1b +/-haploinsufficiency throughout development and highlights the importance of examining both sexes throughout development in NDDs.
... Il semblerait que la maturation des épines joue un rôle sur la plasticité synaptique et qu'en effet certaines formes d'apprentissage augmenteraient le nombre d'épines dendritiques dont la morphologie serait associée à des phénomènes cellulaires nécessaires à la mémorisation (von Bohlen Und Halbach, 2010). En plus d'une altération de la morphologie de la synapse, il a été également observé dans plusieurs modèles de DI que les circuits neuronaux et la balance d'excitation/inhibition au sein de ce réseau sont dérégulés, ce qui a pour conséquence des défauts de mémorisation et d'apprentissage (Golden, Buxbaum, & De Rubeis, 2018). ...
La synapse est le lieu de communication entre les neurones à l'origine de nos capacités cognitives. Les mutations des gènes codant pour des protéines synaptiques sont responsables des maladies neurodéveloppementales appelées synaptopathies, recouvrant un large spectre de pathologies, de la déficience intellectuelle aux troubles du spectre autistique. Cependant, il est actuellement établi que les neurones ne sont pas les seuls acteurs au niveau de la synapse. Les astrocytes jouent également un rôle essentiel dans la mise en place du réseau neuronal et le fonctionnement de la synapse. Ils assurent aussi l'homéostasie ionique synaptique et sont capables de sécréter des glio-transmetteurs qui modulent l'activité synaptique. Oligophrénine-1 (OPHN1) est un gène associé à la déficience intellectuelle liée à l'X chez l'Homme. OPHN1 est une protéine synaptique dont les fonctions neuronales sont bien connues. La protéine peut directement interagir avec le cytosquelette d'actine et joue un rôle dans la formation et la maturation des épines dendritiques. Cette protéine est aussi exprimée dans les astrocytes mais sa fonction astrocytaire n'est pas connue. A l'aide d'un modèle KO de souris pour Ophn1, nous avons mis en évidence les conséquences de l'absence d'Ophn1 dans les astrocytes. Nous avons démontré que la délétion d'OPHN1 altère la migration et la morphologie des astrocytes in vitro. Sachant qu'Ophn1 est capable d'inactiver la GTPase RhoA, nous avons utilisé un inhibiteur de la voie RhoA/ROCK pour retrouver un phénotype de migration normal. In vivo nous avons choisi un modèle de cicatrisation gliale cortical afin de pouvoir observer la migration et la morphologie des astrocytes au niveau de la cicatrice. Nous avons observé que la délétion d'Ophn1 altérait la cicatrisation gliale et que les astrocytes à proximité de la cicatrice était moins ramifiés. L'ensemble de ces résultats nous permet de constater que les astrocytes sont altérés dans notre modèle murin de déficience intellectuelle liée à l'X. De plus, le KO conditionel astrocytaire mis en place nous permettra à l'avenir d'étudier les conséquences de la perte d'OPHN1 uniquement dans les astrocytes, et de comprendre la contribution astrocytaire dans la physiopathologie de cette maladie neuro-développementale.
... We observed a strong seizure susceptibility phenotype in Arid1b +/mice by numerous metrics including reduced latency to loss of righting, tonic hindlimb extension, and faster death. As are Arid1b +/mice were functionally susceptible to PTZ-induced seizures, related to GABAergic neuron plasticity, we attribute this to an excitatory-inhibition imbalance, as other new genetic models of NDD have also exhibited [85][86][87] . ...
One of the causal mechanisms underlying neurodevelopmental disorders (NDDs) is chromatin modification, and genes that regulate chromatin modify and control events regulating the formation of neural connections. AT-Rich Interactive Domain 1B (ARID1B), a chromatin modifier, has been shown to be reduced in autism spectrum disorder (ASD) and to affect rare and inherited genetic variation in a broad set of NDDs. For this work, a novel preclinical mouse model of Arid1b deficiency was created and molecularly validated to characterize and define neuroanatomical, behavioural and transcriptional phenotypes. Brains of adult Arid1b+/- mice had a smaller cerebellum along with a larger hippocampus and corpus callosum. In addition, a notable sex dependence was observed throughout development; males had an early emergence of the neuroanatomical phenotype around postnatal day 7, whereas females had a delayed emergence of the phenotype around postnatal day 40. Behavioural assays relevant to NDD were conducted during neonatal development and adulthood to evaluate general health, anxiety-like, motor, cognitive, and social behaviours in Arid1b+/- mice. During neonatal development, Arid1b+/- mice exhibited robust impairments in ultrasonic vocalizations (USVs) and metrics of developmental growth. As adults, Arid1b+/- mice showed low motor skills in open field exploration and normal three chambered approach. Arid1b+/- mice had learning and memory deficits in novel object recognition but surprisingly not in visual discrimination and reversal touchscreen tasks. Social interactions in the male-female social dyad with USVs revealed social deficits on some but not all parameters. No repetitive behaviours were observed. This study represents a full investigation of Arid1b+/- haploinsufficiency throughout development and highlights the importance of examining both sexes throughout development in NDDs.
... A large number of risk genes that are associated with ASD pathogenesis have been identified (Nakanishi, Anderson, & Takumi, 2019;Verma, Paul, Amrapali Vishwanath, Vaidya, & Clement, 2019). Studies on ASD-related genes have indicated an altered axonal growth, an imbalance in neural network excitation/inhibition, and an impaired synaptic plasticity in the corticostriatal pathway (Fuccillo, 2016;Golden, Buxbaum, & De Rubeis, 2018;Kuo & Liu, 2019;Shepherd, 2013). Below we first discuss the current findings of corticostriatal dysfunction in experimental models of high-confidence ASD-associated genes ( Table 1). ...
The corticostriatal pathway that carries sensory, motor, and limbic information to the striatum plays a critical role in motor control, action selection, and reward. Dysfunction of this pathway is associated with many neurological and psychiatric disorders. Corticostriatal synapses have unique features in their cortical origins and striatal targets. In this review, we first describe axonal growth and synaptogenesis in the corticostriatal pathway during development, and then summarize the current understanding of the molecular bases of synaptic transmission and plasticity at mature corticostriatal synapses. Genes associated with autism spectrum disorder (ASD) have been implicated in axonal growth abnormalities, imbalance of the synaptic excitation/inhibition ratio, and altered long‐term synaptic plasticity in the corticostriatal pathway. Here, we review a number of ASD‐associated high‐confidence genes, including FMR1, KMT2A, GRIN2B, SCN2A, NLGN1, NLGN3, MET, CNTNAP2, FOXP2, TSHZ3, SHANK3, PTEN, CHD8, MECP2, DYRK1A, RELN, FOXP1, SYNGAP1, and NRXN, and discuss their relevance to proper corticostriatal function.
... Autism spectrum disorders (ASD) are brain developmental disorders characterized by social deficits and repetitive behaviors. A large number of genetic mutations associated with ASD have been identified (Rosti et al., 2014;De Rubeis and Buxbaum, 2015;de la Torre-Ubieta et al., 2016;McDermott et al., 2018), and potential mechanisms underlying ASD have been identified (Südhof, 2008;Bourgeron, 2009Bourgeron, , 2015Kleijer et al., 2014;Barak and Feng, 2016;Hulbert and Jiang, 2016;Golden et al., 2018). However, knock-in animals carrying heterozygous ASD-risk mutations from individuals with ASD have been less thoroughly investigated. ...
Mutations in Tbr1, a high-confidence ASD (autism spectrum disorder)-risk gene encoding the transcriptional regulator TBR1, have been shown to induce diverse ASD-related molecular, synaptic, neuronal, and behavioral dysfunctions in mice. However, whether Tbr1 mutations derived from autistic individuals cause similar dysfunctions in mice remains unclear. Here we generated and characterized mice carrying the TBR1-K228E de novo mutation identified in human ASD and identified various ASD-related phenotypes. In heterozygous mice carrying this mutation (Tbr1 +/K228E mice), levels of the TBR1-K228E protein, which is unable to bind target DNA, were strongly increased. RNA-Seq analysis of the Tbr1 +/K228E embryonic brain indicated significant changes in the expression of genes associated with neurons, astrocytes, ribosomes, neuronal synapses, and ASD risk. The Tbr1 +/K228E neocortex also displayed an abnormal distribution of parvalbumin-positive interneurons, with a lower density in superficial layers but a higher density in deep layers. These changes were associated with an increase in inhibitory synaptic transmission in layer 6 pyramidal neurons that was resistant to compensation by network activity. Behaviorally, Tbr1 +/K228E mice showed decreased social interaction, increased self-grooming, and modestly increased anxiety-like behaviors. These results suggest that the human heterozygous TBR1-K228E mutation induces ASD-related transcriptomic, protein, neuronal, synaptic, and behavioral dysfunctions in mice.
... Animal Models Support the Metabolic Syndrome -Hyperexcitability Hypothesis of ASD Mouse models of ASD exhibit altered molecular and cellular mechanisms that converge on metabolism and neuronal network excitability. These models have been reviewed recently from a behavioral perspective, and exhibit social withdrawal, anxiety, repetitive behaviors, and/or cognitive inflexibility [43,44]. Here, we focus on mouse models of ASD that provide evidence of hyperexcitability in the pathology of ASD. ...
We review evidence that suggests a role for excessive consumption of energy-dense foods, particularly fructose, and consequent obesity and insulin resistance (metabolic syndrome) in the recent increase in prevalence of autism spectrum disorders (ASD). Maternal insulin resistance, obesity, and diabetes may predispose offspring to ASD by mechanisms involving chronic activation of anabolic cellular pathways and a lack of metabolic switching to ketosis resulting in a deficit in GABAergic signaling and neuronal network hyperexcitability. Metabolic reprogramming by epigenetic DNA and chromatin modifications may contribute to alterations in gene expression that result in ASD. These mechanistic insights suggest that interventions that improve metabolic health such as intermittent fasting and exercise may ameliorate developmental neuronal network abnormalities and consequent behavioral manifestations in ASD.
... Autism spectrum disorder (ASD) is a multifactorial neurodevelopmental disorder characterized by impaired social interactions and elevated repetitive behaviors, in which various circuits in the sensory, prefrontal, hippocampal, cerebellar, striatal, and other midbrain regions are perturbed [1,2]. Compared to de novo non-coding variations, the de novo coding variants have a strong association with ASD as shown by the whole-genome sequence association (WGSA) studies in 519 ASD families [3]. ...
Background:
The development of an autistic brain is a highly complex process as evident from the involvement of various genetic and non-genetic factors in the etiology of the autism spectrum disorder (ASD). Despite being a multifactorial neurodevelopmental disorder, autistic patients display a few key characteristics, such as the impaired social interactions and elevated repetitive behaviors, suggesting the perturbation of specific neuronal circuits resulted from abnormal signaling pathways during brain development in ASD. A comprehensive review for autistic signaling mechanisms and interactions may provide a better understanding of ASD etiology and treatment.
Main body:
Recent studies on genetic models and ASD patients with several different mutated genes revealed the dysregulation of several key signaling pathways, such as WNT, BMP, SHH, and retinoic acid (RA) signaling. Although no direct evidence of dysfunctional FGF or TGF-β signaling in ASD has been reported so far, a few examples of indirect evidence can be found. This review article summarizes how various genetic and non-genetic factors which have been reported contributing to ASD interact with WNT, BMP/TGF-β, SHH, FGF, and RA signaling pathways. The autism-associated gene ubiquitin-protein ligase E3A (UBE3A) has been reported to influence WNT, BMP, and RA signaling pathways, suggesting crosstalk between various signaling pathways during autistic brain development. Finally, the article comments on what further studies could be performed to gain deeper insights into the understanding of perturbed signaling pathways in the etiology of ASD.
Conclusion:
The understanding of mechanisms behind various signaling pathways in the etiology of ASD may help to facilitate the identification of potential therapeutic targets and design of new treatment methods.
... Another pathologic feature associated with ASD is an imbalance of excitatory to inhibitory neural circuits [60]. Kenet et al. [91] reported that rats developmentally exposed to NDL PCB 95 at 6 mg/kg/day in the maternal diet during gestation and lactation exhibited profoundly abnormal development of the primary auditory cortex (A1) without impairment of hearing sensitivity and brainstem auditory responses of pups. ...
Halogenated organic compounds are pervasive in natural and built environments. Despite restrictions on the production of many of these compounds in most parts of the world through the Stockholm Convention on Persistent Organic Pollutants (POPs), many “legacy” compounds, including polychlorinated biphenyls (PCBs), are routinely detected in human tissues where they continue to pose significant health risks to highly exposed and susceptible populations. A major concern is developmental neurotoxicity, although impacts on neurodegenerative outcomes have also been noted. Here, we review human studies of prenatal and adult exposures to PCBs and describe the state of knowledge regarding outcomes across domains related to cognition (e.g., IQ, language, memory, learning), attention, behavioral regulation and executive function, and social behavior, including traits related to attention-deficit hyperactivity disorder (ADHD) and autism spectrum disorders (ASD). We also review current understanding of molecular mechanisms underpinning these associations, with a focus on dopaminergic neurotransmission, thyroid hormone disruption, calcium dyshomeostasis, and oxidative stress. Finally, we briefly consider contemporary sources of organohalogens that may pose human health risks via mechanisms of neurotoxicity common to those ascribed to PCBs.
... The first neurons formed are those that are closest to the ventricle (layer VI neurons) and the last formed are those neurons located in the most superficial layers (layers II-III neurons) (Florio and Huttner, 2014;Molyneaux et al., 2007). Of note, perturbations in sensory regions and in the circuits that connect these regions have been described in several ASD mouse models (Golden et al., 2018). We previously showed that production of the early-born cortical neurons is enhanced in Dyrk1a +/embryos (Najas et al., 2015). ...
Autism spectrum disorders are early onset neurodevelopmental disorders characterized by deficits in social communication and restricted repetitive behaviors, yet they are quite heterogeneous in terms of their genetic basis and phenotypic manifestations. Recently, de novo pathogenic mutations in DYRK1A, a chromosome 21 gene associated to neuropathological traits of Down syndrome, have been identified in patients presenting a recognizable syndrome included in the autism spectrum. These mutations produce DYRK1A kinases with partial or complete absence of the catalytic domain, or they represent missense mutations located within this domain. Here, we undertook an extensive biochemical characterization of the DYRK1A missense mutations reported to date and show that most of them, but not all, result in enzymatically dead DYRK1A proteins. We also show that haploinsufficient Dyrk1a+/- mutant mice mirror the neurological traits associated with the human pathology, such as defective social interactions, stereotypic behaviors and epileptic activity. These mutant mice present altered proportions of excitatory and inhibitory neocortical neurons and synapses. Moreover, we provide evidence that alterations in the production of cortical excitatory neurons are contributing to these defects. Indeed, by the end of the neurogenic period, the expression of developmental regulated genes involved in neuron differentiation and/or activity is altered. Therefore, our data indicate that altered neocortical neurogenesis could critically affect the formation of cortical circuits, thereby contributing to the neuropathological changes in DYRK1A haploinsufficiency syndrome.
... Animal models lacking both Shank2 and Shank3 will therefore be of interest to examine whether loss of both SHANK proteins further impairs the zinc sensitivity of these behaviors. It will also be of importance to determine whether increases in dietary zinc can prevent ASD behavioral deficits in mouse models expressing other ASD mutations in SHANKs (e.g., Wang et al., 2011;Schmeisser et al., 2012;Won et al., 2012;Mei et al., 2016;Zhou et al., 2016) and also in other synaptic proteins (Ebert and Greenberg, 2013;Chen et al., 2014;Golden et al., 2018), to identify the breadth of dietary zinc as a potential therapeutic strategy, and the involvement of zinc-responsive SHANKs in restoring normal behaviors. ...
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 (Shank3ex13–1616−/−), which displays ASD-related behaviors and structural and functional deficits at striatal synapses. We observed that 6 weeks of dietary zinc supplementation in Shank3ex13–16−/− 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.
Individuals with mutations in a single copy of the SHANK3 gene present with social-interaction deficits. Although social behavior in mice depends on olfaction, mice with mutations in a single copy of the Shank3 gene do not have olfactory deficits in simple odor identification tasks (Drapeau et al. 2018). Here we tested olfaction in mice with mutations in a single copy of the Shank3 gene (Peça et al. 2011) using a complex odor-task and imaging in awake mice. Average glomerular responses in the olfactory bulb of Shank3B +/− were correlated with WT mice. However, there was increased trial-to-trial variability in the odor responses for Shank3B +/− mice. Simulations demonstrated that this increased variability could affect odor detection in novel environments. In order to test whether performance was affected by the increased variability, we tested target odor recognition in the presence of novel background odors using a recently developed task (Y. Li et al. 2023). Head-fixed mice were trained to detect target odors in the presence of known background odors. Performance was tested using catch trials where the known background odors were replaced by novel background odors. We compared the performance of 8 Shank3B +/− mice (5 males, 3 females) on this task with 6 WT mice (3 males, 3 females). Performance for known background odors and learning rates were similar between Shank3B +/− and WT mice. However, when tested with novel background odors, Shank3B +/− mice performance dropped to almost chance levels. Thus, haploinsufficiency of the Shank3 gene causes a specific deficit in odor detection in novel environments. Our results are discussed in the context of other Shank3 mouse models and have implications for understanding olfactory function in neurodevelopmental disorders.
Significance Statement
People and mice with mutations in a single copy in the synaptic gene Shank3 show features seen in autism spectrum disorders, including social interaction deficits. Although mice social behavior uses olfaction, mice with mutations in a single copy of Shank3 have so far not shown olfactory deficits when tested using simple tasks. Here we used a recently developed task to show that these mice could identify odors in the presence of known background odors as well as wild-type mice. However, their performance fell below wild-type mice when challenged with novel background odors. This deficit was also previously reported in the Cntnap2 mouse model of autism suggesting that odor detection in novel backgrounds is a general deficit across mouse models of autism.
Cortical circuits are composed predominantly of pyramidal-to-pyramidal neuron connections, yet their assembly during embryonic development is not well understood. We show that mouse embryonic Rbp4-Cre cortical neurons, transcriptomically closest to layer 5 pyramidal neurons, display two phases of circuit assembly in vivo. At E14.5, they form a multi-layered circuit motif, composed of only embryonic near-projecting-type neurons. By E17.5, this transitions to a second motif involving all three embryonic types, analogous to the three adult layer 5 types. In vivo patch clamp recordings and two-photon calcium imaging of embryonic Rbp4-Cre neurons reveal active somas and neurites, tetrodotoxin-sensitive voltage-gated conductances, and functional glutamatergic synapses, from E14.5 onwards. Embryonic Rbp4-Cre neurons strongly express autism-associated genes and perturbing these genes interferes with the switch between the two motifs. Hence, pyramidal neurons form active, transient, multi-layered pyramidal-to-pyramidal circuits at the inception of neocortex, and studying these circuits could yield insights into the etiology of autism.
Autism Spectrum Disorders (ASD) are a group of neurodevelopmental disorders characterized by repetitive behaviors, lack of social interaction and communication. CC2D1A is identified in patients as an autism risk gene. Recently, we suggested that heterozygous Cc2d1a mice exhibit impaired autophagy in the hippocampus. We now report the analysis of autophagy markers (Lc3, Beclin and p62) in different regions hippocampus, prefrontal cortex, hypothalamus and cerebellum, with an overall decrease in autophagy and changes in Beclin-1/p62 ratio in the hippocampus. We observed sex-dependent variations in transcripts and protein expression levels. Moreover, our analyses suggest that alterations in autophagy initiated in Cc2d1a heterozygous parents are variably transmitted to offspring, even when the offspring's genotype is wild type. Aberration in the autophagy mechanism may indirectly contribute to induce synapse alteration in the ASD brain.
Autism spectrum disorders (ASD) are neurodevelopmental disorders characterized by social and repetitive symptoms. A key feature of ASD is early-life manifestations of symptoms, indicative of early pathophysiological mechanisms. In mouse models of ASD, increasing evidence indicates that there are early pathophysiological mechanisms that can be corrected early to prevent phenotypic defects in adults, overcoming the disadvantage of the short-lasting effects that characterize adult-initiated treatments. In addition, the results from gene restorations indicate that ASD-related phenotypes can be rescued in some cases even after the brain has fully matured. These results suggest that we need to consider both temporal and mechanistic aspects in studies of ASD models, and carefully compare genetic and non-genetic corrections. Here, we summarize the early and late corrections in mouse models of ASD by genetic and pharmacological interventions, and discuss how to better integrate these results to ensure efficient and long-lasting corrections for eventual clinical translation.
Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterised by a triad of behavioural impairments and includes disruption in social behaviour. ASD has a clear genetic underpinning and hundreds of genes are implicated in its aetiology. However, how single penetrant genes disrupt activity of neural circuits which lead to affected behaviours is only beginning to be understood and less is known about how low penetrant genes interact to disrupt emergent behaviours. Investigations are well served by experimental approaches that allow tractable investigation of the underpinning genetic basis of circuits that control behaviours that operate in the biological domains that are neuro-atypical in autism. The model organism C . elegans provides an experimental platform to investigate the effect of genetic mutations on behavioural outputs including those that impact social biology. Here we use progeny-derived social cues that modulate C . elegans food leaving to assay genetic determinants of social behaviour. We used the SAFRI Gene database to identify C . elegans orthologues of human ASD associated genes. We identified a number of mutants that displayed selective deficits in response to progeny. The genetic determinants of this complex social behaviour highlight the important contribution of synaptopathy and implicates genes within cell signalling, epigenetics and phospholipid metabolism functional domains. The approach overlaps with a growing number of studies that investigate potential molecular determinants of autism in C . elegans . However, our use of a complex, sensory integrative, emergent behaviour provides routes to enrich new or underexplored biology with the identification of novel candidate genes with a definable role in social behaviour.
NMR offers the unique potential to holistically screen hundreds of metabolites and has already proved to be a powerful technique able to provide a global picture of a wide range of metabolic processes underlying complex and multifactorial diseases, such as neurodegenerative and neurodevelopmental diseases. The aim of this study was to apply an NMR-based metabolomics approach to explore brain metabolic changes in both male and female rats induced by prenatal exposure to two chemicals associated with autism disorders-the organophosphorus pesticide chlorpyrifos (CPF) and the antiepileptic drug valproic acid (VPA)-at different postnatal ages. Depending on the age and on the brain region (hippocampus and cerebellum), several metabolites were shown to be significantly affected by exposure to both compounds. The evaluation of the spectral profiles revealed that the nervous-system-specific metabolite N-acetylaspartate (NAA), amino acid neurotransmitters (e.g., glutamate, glutamine, GABA, glycine), pyroglutamic acid, unsaturated fatty acids, and choline-based compounds are discriminant biomarkers. Additionally, metabolic changes varied as a function of age, but importantly not of sex.
Impaired synaptic function is a common phenotype in animal models for autism spectrum disorder (ASD), and ASD risk genes are enriched for synaptic function. Here we leverage the availability of multiple ASD mouse models exhibiting synaptic deficits and behavioral correlates of ASD and use quantitative mass spectrometry with isobaric tandem mass tagging (TMT) to compare the hippocampal synaptic proteomes from 7 mouse models. We identified common altered cellular and molecular pathways at the synapse, including changes in Rho family small GTPase signaling, suggesting that it may be a point of convergence in ASD. Comparative analyses also revealed clusters of synaptic profiles, with similarities observed among models for Fragile X syndrome (Fmr1 knockout), PTEN hamartoma tumor syndrome (Pten haploinsufficiency), and the BTBR+ model of idiopathic ASD. Opposing changes were found in models for cortical dysplasia focal epilepsy syndrome (Cntnap2 knockout), Phelan McDermid syndrome (Shank3 InsG3680), Timothy syndrome (Cacna1c G406R), and ANKS1B syndrome (Anks1b haploinsufficiency), which were similar to each other. We propose that these clusters of synaptic profiles form the basis for molecular subtypes that explain genetic heterogeneity in ASD despite a common clinical diagnosis. Drawn from an internally controlled survey of the synaptic proteome across animal models, our findings support the notion that synaptic dysfunction in the hippocampus is a shared mechanism of disease in ASD, and that Rho GTPase signaling may be an important pathway leading to disease phenotypes in autism.
This chapter concludes the book with a discussion of current and future directions in research and their potential to guide innovation in prevention and treatment of self-injury. We focus on three areas of potential promise. The first is primary and secondary prevention of the development and maintenance of chronic self-injurious behavior by early intervention in childhood. The second is identification of cross-syndrome processes in the neurobiology of genetic syndromes associated with self-injury. Particular emphasis is placed on impairments in inhibitory processes, with research examining both functional connectivity in relevant networks and synaptic development and functioning in inhibitory cortical and hippocampal interneurons. The third is the potential for basic and translational research in learning processes to increase the effectiveness and robustness of behavioral interventions, with particular emphasis on the utility of behavioral momentum theory in understanding the basic behavioral processes, including behavioral reinstatement, renewal, and resurgence, which underlie relapse following successful intervention.
Autism is a neurodevelopmental disorder characterised by deficits in social interaction, social communication, sensory abnormalities, stereotyped and repetitive behaviours, and restricted interests. Autism spectrum disorder (ASD) has proven difficult to study because of the diversity of its behavioural symptoms, complex genetics and lack of ubiquitous biomarkers. Progress in the development of heuristic animal models has allowed investigators to model some of the core symptoms of autism, including deficits in social interactions, aspects of social communication and stereotyped, repetitive behaviours. Specific hypotheses can now be formulated and tested in mouse, rat and non‐human primate models. Translational applications of animal models have advanced the discovery of therapeutic interventions for treating the core features of ASD. This article describes some of the major behavioural approaches and animal models designed to study neurodevelopmental disorders, with a focus on ASDs.
Key Concepts
• Autism is a complex disorder whose underlying biology is being studied using rodent and non‐human primate animal models.
• Rodents and non‐human primates are social animals with rich and complex behavioural repertories that make them useful for modelling neurodevelopmental disorders.
• Many of the behaviours that reflect core deficits in autism, including social communication, social interaction, motor stereotypies and repetitive behaviours, can be modelled in rodents and non‐human primates.
• Inbred strains of mice can be used to discover genes important for the expression of complex behaviours and can provide insights into the neurobiology of social interactions and social communication.
• Mice and rats with targeted mutations in risk genes and deletions identified in people with autism provide research tools for investigating mechanistic substrates of the disorder and for discovering therapeutic interventions.
• Animal models of neurodevelopmental disorders, while important and useful, typically only model specific aspects of a disorder rather that recapitulating its full symptomatology.
Objective:
To compare current analgesia and sedation management practices between critically ill children with pre-existing cognitive impairment and critically ill neurotypical children, including possible indicators of therapeutic efficacy.
Study design:
This study used secondary analysis of prospective data from the RESTORE clinical trial, with 2449 children admitted to the pediatric intensive care unit and receiving mechanical ventilation for acute respiratory failure. Subjects with a baseline Pediatric Cerebral Performance Category ≥3 were defined as subjects with cognitive impairment, and differences between groups were explored using regression methods accounting for pediatric intensive care unit as a cluster variable.
Results:
This study identified 412 subjects (17%) with cognitive impairment. Compared with neurotypical subjects, subjects with cognitive impairment were older (median, years, 6.2 vs 1.4; P < .001) with more severe pediatric acute respiratory distress syndrome (40% vs 33%; P = .009). They received significantly lower cumulative doses of opioids (median, mg/kg, 14.2 vs 16.2; P < .001) and benzodiazepines (10.6 vs 14.4; P < .001). Three nonverbal subjects with cognitive impairment received no analgesia or sedation. Subjects with cognitive impairment were assessed as having more study days awake and calm and fewer study days with an episode of pain. They were less likely to be assessed as having inadequate pain/sedation management or unplanned endotracheal/invasive tube removal. Subjects with cognitive impairment had more documented iatrogenic withdrawal symptoms than neurotypical subjects.
Conclusions:
Subjects with cognitive impairment in this study received less medication, but it is unclear whether they have authentically lower analgesic and/or sedative requirements or are vulnerable to inadequate assessment of discomfort because of the lack of validated assessment tools. We recommend the development of pain and sedation assessment tools specific to this patient population.
Autism is a complex neurodevelopmental condition, and little is known about its neurobiology. Much of autism research has focused on the social, communication and cognitive difficulties associated with the condition. However, the recent revision of the diagnostic criteria for autism has brought another key domain of autistic experience into focus: sensory processing. Here, we review the properties of sensory processing in autism and discuss recent computational and neurobiological insights arising from attention to these behaviours. We argue that sensory traits have important implications for the development of animal and computational models of the condition. Finally, we consider how difficulties in sensory processing may relate to the other domains of behaviour that characterize autism.
Mutations in MECP2 cause Rett syndrome (RTT), an X-linked neurological disorder characterized by regressive loss of neurodevelopmental milestones and acquired psychomotor deficits. However, the cellular heterogeneity of the brain impedes an understanding of how MECP2 mutations contribute to RTT. Here we developed a Cre-inducible method for cell-type-specific biotin tagging of MeCP2 in mice. Combining this approach with an allelic series of knock-in mice carrying frequent RTT-associated mutations (encoding T158M and R106W) enabled the selective profiling of RTT-associated nuclear transcriptomes in excitatory and inhibitory cortical neurons. We found that most gene-expression changes were largely specific to each RTT-associated mutation and cell type. Lowly expressed cell-type-enriched genes were preferentially disrupted by MeCP2 mutations, with upregulated and downregulated genes reflecting distinct functional categories. Subcellular RNA analysis in MeCP2-mutant neurons further revealed reductions in the nascent transcription of long genes and uncovered widespread post-transcriptional compensation at the cellular level. Finally, we overcame X-linked cellular mosaicism in female RTT models and identified distinct gene-expression changes between neighboring wild-type and mutant neurons, providing contextual insights into RTT etiology that support personalized therapeutic interventions.
Deficits in social cognition are the defining characteristic of autism spectrum disorder (ASD). Social cognition requires the integration of several neural circuits in a time-sensitive fashion, so impairments in social interactions could arise as a result of alterations in network connectivity. Electroencephalography (EEG) has revealed abnormalities in event related potentials (ERPs) evoked by auditory and visual sensory stimuli in humans with ASD, indicating disruption of neural connectivity. Similar abnormalities in sensory-evoked ERPs have been observed in animal models of ASD, suggesting that ERPs have the potential to provide a translational biomarker of the disorder. People with ASD also have abnormal ERPs in response to auditory and visual social stimuli, demonstrating functional disruption of the social circuit. To assess the integrity of the social circuit and characterize biomarkers of circuit dysfunction, novel EEG paradigms that use social stimuli to induce ERPs should be developed for use in animal models. The identification of a socially-relevant ERP that is consistent in animal models and humans would facilitate the development of pharmacological treatment strategies for the social impairments in ASD and other neuropsychiatric disorders.
Haploinsufficiency of SHANK3, encoding the synapse scaffolding protein SHANK3, leads to a highly penetrant form of autism spectrum disorder. How SHANK3 insufficiency affects specific neural circuits and how this is related to specific symptoms remains elusive. Here we used shRNA to model Shank3 insufficiency in the ventral tegmental area of mice. We identified dopamine (DA) and GABA cell-type-specific changes in excitatory synapse transmission that converge to reduce DA neuron activity and generate behavioral deficits, including impaired social preference. Administration of a positive allosteric modulator of the type 1 metabotropic glutamate receptors mGluR1 during the first postnatal week restored DA neuron excitatory synapse transmission and partially rescued the social preference defects, while optogenetic DA neuron stimulation was sufficient to enhance social preference. Collectively, these data reveal the contribution of impaired ventral tegmental area function to social behaviors and identify mGluR1 modulation during postnatal development as a potential treatment strategy.
Human neuroimaging studies suggest that aberrant neural connectivity underlies behavioural
deficits in autism spectrum disorders (ASDs), but the molecular and neural circuit
mechanisms underlying ASDs remain elusive. Here, we describe a complete knockout mouse
model of the autism-associated Shank3 gene, with a deletion of exons 4–22 (De4–22). Both
mGluR5-Homer scaffolds and mGluR5-mediated signalling are selectively altered in striatal
neurons. These changes are associated with perturbed function at striatal synapses, abnormal
brain morphology, aberrant structural connectivity and ASD-like behaviour. In vivo recording
reveals that the cortico-striatal-thalamic circuit is tonically hyperactive in mutants, but
becomes hypoactive during social behaviour. Manipulation of mGluR5 activity attenuates
excessive grooming and instrumental learning differentially, and rescues impaired striatal
synaptic plasticity in De4–22 / mice. These findings show that deficiency of Shank3 can
impair mGluR5-Homer scaffolding, resulting in cortico-striatal circuit abnormalities that
underlie deficits in learning and ASD-like behaviours. These data suggest causal links
between genetic, molecular, and circuit mechanisms underlying the pathophysiology of ASDs.
Objective:
We aimed to delineate the neurodevelopmental spectrum associated with SYNGAP1 mutations and to investigate genotype-phenotype correlations.
Methods:
We sequenced the exome or screened the exons of SYNGAP1 in a total of 251 patients with neurodevelopmental disorders. Molecular and clinical data from patients with SYNGAP1 mutations from other centres were also collected, focusing on developmental aspects and the associated epilepsy phenotype. A review of SYNGAP1 mutations published in the literature was also performed.
Results:
We describe 17 unrelated affected individuals carrying 13 different novel loss-of-function SYNGAP1 mutations. Developmental delay was the first manifestation of SYNGAP1-related encephalopathy; intellectual disability became progressively obvious and was associated with autistic behaviours in eight patients. Hypotonia and unstable gait were frequent associated neurological features. With the exception of one patient who experienced a single seizure, all patients had epilepsy, characterised by falls or head drops due to atonic or myoclonic seizures, (myoclonic) absences and/or eyelid myoclonia. Triggers of seizures were frequent (n=7). Seizures were pharmacoresistant in half of the patients. The severity of the epilepsy did not correlate with the presence of autistic features or with the severity of cognitive impairment. Mutations were distributed throughout the gene, but spared spliced 3' and 5' exons. Seizures in patients with mutations in exons 4-5 were more pharmacoresponsive than in patients with mutations in exons 8-15.
Conclusions:
SYNGAP1 encephalopathy is characterised by early neurodevelopmental delay typically preceding the onset of a relatively recognisable epilepsy comprising generalised seizures (absences, myoclonic jerks) and frequent triggers.
A cardinal feature of early stages of human brain development centres on the sensory, cognitive, and emotional experiences that shape neuronal-circuit formation and refinement. Consequently, alterations in these processes account for many psychiatric and neurodevelopmental disorders. Neurodevelopment disorders affect 3-4% of the world population. The impact of these disorders presents a major challenge to clinicians, geneticists, and neuroscientists. Mutations that cause neurodevelopmental disorders are commonly found in genes encoding proteins that regulate synaptic function. Investigation of the underlying mechanisms using gain or loss of function approaches has revealed alterations in dendritic spine structure, function, and plasticity, consequently modulating the neuronal circuit formation and thereby raising the possibility of neurodevelopmental disorders resulting from synaptopathies. One such gene, SYNGAP1 (Synaptic Ras-GTPase-activating protein has been shown to cause Intellectual Disability with comorbid Autism Spectrum Disorder and epilepsy in children. SYNGAP1 is a negative regulator of Ras, Rap and of AMPA receptor trafficking to the postsynaptic membrane, thereby regulating not only synaptic plasticity, but also neuronal homeostasis. Recent studies on the neurophysiology of SYNGAP1, using Syngap1 mouse models, have provided deeper insights into how downstream signalling proteins and synaptic plasticity are regulated by SYNGAP1. This knowledge has led to a better understanding of the function of SYNGAP1 and suggests a potential target during critical period of development when the brain is more susceptible to therapeutic intervention.
Phelan-McDermid syndrome or 22q13.3 deletion syndrome is a rare neurodevelopmental disorder characterized by generalized developmental delay, intellectual disability, absent or delayed speech, seizures, autism spectrum disorder, neonatal hypotonia, physical dysmorphic features, and recurrent medical comorbidities. Individuals with Phelan-McDermid syndrome have terminal deletions of the chromosomal region 22q13.3 encompassing SHANK3, a gene encoding a structural component of excitatory synapses indispensable for proper synaptogenesis and neuronal physiology, or point mutations within the gene. Here, we review the clinical aspects of the syndrome and the genetic findings shedding light onto the underlying etiology. We also provide an overview on the evidence from genetic studies and mouse models that supports SHANK3 haploinsufficiency as a major contributor of the neurobehavioral manifestations of Phelan-McDermid syndrome. Finally, we discuss how all these discoveries are uncovering the pathophysiology of Phelan-McDermid syndrome and are being translated into clinical trials for novel therapeutics ameliorating the core symptoms of the disorder.
Autism spectrum disorder (ASD) and intellectual disability (ID) can be caused by mutations in a large number of genes. One example is SHANK3 on the terminal end of chromosome 22q. Loss of one functional copy of SHANK3 results in 22q13 deletion syndrome or Phelan-McDermid syndrome (PMS) and causes a monogenic form of ASD and/or ID with a frequency of 0.5% to 2% of cases. SHANK3 is the critical gene in this syndrome, and its loss results in disruption of synaptic function. With chromosomal microarray analyses now a standard of care in the assessment of ASD and developmental delay, and with the emergence of whole exome and whole genome sequencing in this context, identification of PMS in routine clinical settings will increase significantly. However, PMS remains a rare disorder, and the majority of physicians have never seen a case. While there is agreement about core deficits of PMS, there have been no established parameters to guide evaluation and medical monitoring of the syndrome. Evaluations must include a thorough history and physical and dysmorphology examination. Neurological deficits, including the presence of seizures and structural brain abnormalities should be assessed as well as motor deficits. Endocrine, renal, cardiac, and gastrointestinal problems all require assessment and monitoring in addition to the risk of recurring infections, dental and vision problems, and lymphedema. Finally, all patients should have cognitive, behavioral, and ASD evaluations. The objective of this paper is to address this gap in the literature and establish recommendations to assess the medical, genetic, and neurological features of PMS.
The genetic architecture of autism spectrum disorder involves the interplay of common and rare variants and their impact on hundreds of genes. Using exome sequencing, here we show that analysis of rare coding variation in 3,871 autism cases and 9,937 ancestry-matched or parental controls implicates 22 autosomal genes at a false discovery rate (FDR) < 0.05, plus a set of 107 autosomal genes strongly enriched for those likely to affect risk (FDR < 0.30). These 107 genes, which show unusual evolutionary constraint against mutations, incur de novo loss-of-function mutations in over 5% of autistic subjects. Many of the genes implicated encode proteins for synaptic formation, transcriptional regulation and chromatin-remodelling pathways. These include voltage-gated ion channels regulating the propagation of action potentials, pacemaking and excitability-transcription coupling, as well as histone-modifying enzymes and chromatin remodellers-most prominently those that mediate post-translational lysine methylation/demethylation modifications of histones.
Background
Considerable clinical heterogeneity has been well documented amongst individuals with autism spectrum disorders (ASD). However, little is known about the biological mechanisms underlying phenotypic diversity. Genetic studies have established a strong causal relationship between ASD and molecular defects in the SHANK3 gene. Individuals with various defects of SHANK3 display considerable clinical heterogeneity. Different lines of Shank3 mutant mice with deletions of different portions of coding exons have been reported recently. Variable synaptic and behavioral phenotypes have been reported in these mice, which makes the interpretations for these data complicated without the full knowledge of the complexity of the Shank3 transcript structure.
Methods
We systematically examined alternative splicing and isoform-specific expression of Shank3 across different brain regions and developmental stages by regular RT-PCR, quantitative real time RT-PCR (q-PCR), and western blot. With these techniques, we also investigated the effects of neuronal activity and epigenetic modulation on alternative splicing and isoform-specific expression of Shank3. We explored the localization and influence on dendritic spine development of different Shank3 isoforms in cultured hippocampal neurons by cellular imaging.
Results
The Shank3 gene displayed an extensive array of mRNA and protein isoforms resulting from the combination of multiple intragenic promoters and extensive alternative splicing of coding exons in the mouse brain. The isoform-specific expression and alternative splicing of Shank3 were brain-region/cell-type specific, developmentally regulated, activity-dependent, and involved epigenetic regulation. Different subcellular distribution and differential effects on dendritic spine morphology were observed for different Shank3 isoforms.
Conclusions
Our results indicate a complex transcriptional regulation of Shank3 in mouse brains. Our analysis of select Shank3 isoforms in cultured neurons suggests that different Shank3 isoforms have distinct functions. Therefore, the different types of SHANK3 mutations found in patients with ASD and different exonic deletions of Shank3 in mutant mice are predicted to disrupt selective isoforms and result in distinct dysfunctions at the synapse with possible differential effects on behavior. Our comprehensive data on Shank3 transcriptional regulation thus provides an essential molecular framework to understand the phenotypic diversity in SHANK3 causing ASD and Shank3 mutant mice.
Dysfunction in sensory information processing is a hallmark of many neurological disorders, including autism spectrum disorders, schizophrenia and Rett syndrome (RTT). Using mouse models of RTT, a monogenic disorder caused by mutations in MECP2, we found that the large-scale loss of MeCP2 from forebrain GABAergic interneurons led to deficits in auditory event-related potentials and seizure manifestation, whereas the restoration of MeCP2 in specific classes of interneurons ameliorated these deficits.
Repetitive Transcranial Magnetic Stimulation (rTMS)/ Deep-brain Magnetic Stimulation (DMS) is an effective therapy for various neuropsychiatric disorders including major depression disorder. The molecular and cellular mechanisms underlying the impacts of rTMS/DMS on the brain are not yet fully understood.
Here we studied the effects of deep-brain magnetic stimulation to brain on the molecular and cellular level. We examined the adult hippocampal neurogenesis and hippocampal synaptic plasticity of rodent under stress conditions with deep-brain magnetic stimulation treatment. We found that DMS promotes adult hippocampal neurogenesis significantly and facilitates the development of adult new-born neurons. Remarkably, DMS exerts anti-depression effects in the learned helplessness mouse model and rescues hippocampal long-term plasticity impaired by restraint stress in rats. Moreover, DMS alleviates the stress response in a mouse model for Rett syndrome and prolongs the life span of these animals dramatically.
Deep-brain magnetic stimulation greatly facilitates adult hippocampal neurogenesis and maturation, also alleviates depression and stress-related responses in animal models.
Critical periods of developmental plasticity contribute to the refinement of neural connections that broadly shape brain development. These windows of plasticity are thought to be important for the maturation of perception, language, and cognition. Synaptic properties in cortical regions that underlie critical periods influence the onset and duration of windows, although it remains unclear how mechanisms that shape synapse development alter critical-period properties. In this study, we demonstrate that inactivation of a single copy of syngap1, which causes a surprisingly common form of sporadic, non-syndromic intellectual disability with autism in humans, induced widespread early functional maturation of excitatory connections in the mouse neocortex. This accelerated functional maturation was observed across distinct areas and layers of neocortex and directly influenced the duration of a critical-period synaptic plasticity associated with experience-dependent refinement of cortical maps. These studies support the idea that genetic control over synapse maturation influences the duration of critical-period plasticity windows. These data also suggest that critical-period duration links synapse maturation rates to the development of intellectual ability.
The PSD-95/SAP90 family of proteins has recently been implicated in the organization of synaptic structure. Here, we describe the isolation of a novel Ras-GTPase activating protein, SynGAP, that interacts with the PDZ domains of PSD-95 and SAP102 in vitro and in vivo. SynGAP is selectively expressed in brain and is highly enriched at excitatory synapses, where it is present in a large macromolecular complex with PSD-95 and the NMDA receptor. SynGAP stimulates the GTPase activity of Ras, suggesting that it negatively regulates Ras activity at excitatory synapses. Ras signaling at the postsynaptic membrane may be involved in the modulation of excitatory synaptic transmission by NMDA receptors and neurotrophins. These results indicate that SynGAP may play an important role in the modulation of synaptic plasticity.
Mutations that cause intellectual disability (ID) and autism spectrum disorder (ASD) are commonly found in genes that encode for synaptic proteins. However, it remains unclear how mutations that disrupt synapse function impact intellectual ability. In the SYNGAP1 mouse model of ID/ASD, we found that dendritic spine synapses develop prematurely during the early postnatal period. Premature spine maturation dramatically enhanced excitability in the developing hippocampus, which corresponded with the emergence of behavioral abnormalities. Inducing SYNGAP1 mutations after critical developmental windows closed had minimal impact on spine synapse function, whereas repairing these pathogenic mutations in adulthood did not improve behavior and cognition. These data demonstrate that SynGAP protein acts as a critical developmental repressor of neural excitability that promotes the development of life-long cognitive abilities. We propose that the pace of dendritic spine synapse maturation in early life is a critical determinant of normal intellectual development.
Silent synapses, or excitatory synapses that lack functional α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs), are thought to be critical for regulation of neuronal circuits and synaptic plasticity. Here, we report that SynGAP, an excitatory synapse-specific RasGAP, regulates AMPAR trafficking, silent synapse number, and excitatory synaptic transmission in hippocampal and cortical cultured neurons. Overexpression of SynGAP in neurons results in a remarkable depression of AMPAR-mediated miniature excitatory postsynaptic currents, a significant reduction in synaptic AMPAR surface expression, and a decrease in the insertion of AMPARs into the plasma membrane. This change is specific for AMPARs because no change is observed in synaptic NMDA receptor expression or total synapse density. In contrast to these results, synaptic transmission is increased in neurons from SynGAP knockout mice as well as in neuronal cultures treated with SynGAP small interfering RNA. In addition, activation of the extracellular signal-regulated kinase, ERK, is significantly decreased in SynGAP-overexpressing neurons, whereas P38 mitogen-activated protein kinase (MAPK) signaling is potentiated. Furthermore, ERK activation is up-regulated in neurons from SynGAP knockout mice, whereas P38 MAPK function is depressed. Taken together, these data suggest that SynGAP plays a critical role in the regulation of neuronal MAPK signaling, AMPAR membrane trafficking, and excitatory synaptic transmission.
• trafficking
• Ras
• glutamate
• receptor
• plasticity
Autism spectrum disorders (ASDs) comprise a range of disorders that share a core of neurobehavioural deficits characterized by widespread abnormalities in social interactions, deficits in communication as well as restricted interests and repetitive behaviours. The neurological basis and circuitry mechanisms underlying these abnormal behaviours are poorly understood. SHANK3 is a postsynaptic protein, whose disruption at the genetic level is thought to be responsible for the development of 22q13 deletion syndrome (Phelan-McDermid syndrome) and other non-syndromic ASDs. Here we show that mice with Shank3 gene deletions exhibit self-injurious repetitive grooming and deficits in social interaction. Cellular, electrophysiological and biochemical analyses uncovered defects at striatal synapses and cortico-striatal circuits in Shank3 mutant mice. Our findings demonstrate a critical role for SHANK3 in the normal development of neuronal connectivity and establish causality between a disruption in the Shank3 gene and the genesis of autistic-like behaviours in mice.
Mutations in the X-linked MECP2 gene, which encodes the transcriptional regulator methyl-CpG-binding protein 2 (MeCP2), cause Rett syndrome and several neurodevelopmental disorders including cognitive disorders, autism, juvenile-onset schizophrenia and encephalopathy with early lethality. Rett syndrome is characterized by apparently normal early development followed by regression, motor abnormalities, seizures and features of autism, especially stereotyped behaviours. The mechanisms mediating these features are poorly understood. Here we show that mice lacking Mecp2 from GABA (γ-aminobutyric acid)-releasing neurons recapitulate numerous Rett syndrome and autistic features, including repetitive behaviours. Loss of MeCP2 from a subset of forebrain GABAergic neurons also recapitulates many features of Rett syndrome. MeCP2-deficient GABAergic neurons show reduced inhibitory quantal size, consistent with a presynaptic reduction in glutamic acid decarboxylase 1 (Gad1) and glutamic acid decarboxylase 2 (Gad2) levels, and GABA immunoreactivity. These data demonstrate that MeCP2 is critical for normal function of GABA-releasing neurons and that subtle dysfunction of GABAergic neurons contributes to numerous neuropsychiatric phenotypes.
Columns, Connections, and Correlations
What is the nature of interactions between neurons in neural circuits? The prevalent hypothesis suggests that dense local connectivity causes nearby cortical neurons to receive substantial amounts of common input, which in turn leads to strong correlations between them. Now two studies challenge this view, which impacts our fundamental understanding of coding in the cortex. Ecker et al. (p. 584 ) investigated the statistics of correlated firing in pairs of neurons from area V1 of awake macaque monkeys. In contrast to previous studies, correlations turned out to be very low, irrespective of the stimulus being shown to the animals, the distances of the recording sites, and the similarity of the neuron's receptive fields or response properties. In an accompanying modeling and recording paper, Renart et al. (p. 587 ) demonstrate how it is possible to have zero noise correlation, even among cells with common input.
Although autosomal forms of nonsyndromic mental retardation account for the majority of cases of mental retardation, the genes that are involved remain largely unknown. We sequenced the autosomal gene SYNGAP1, which encodes a ras GTPase-activating protein that is critical for cognition and synapse function, in 94 patients with nonsyndromic mental retardation. We identified de novo truncating mutations (K138X, R579X, and L813RfsX22) in three of these patients. In contrast, we observed no de novo or truncating mutations in SYNGAP1 in samples from 142 subjects with autism spectrum disorders, 143 subjects with schizophrenia, and 190 control subjects. These results indicate that SYNGAP1 disruption is a cause of autosomal dominant nonsyndromic mental retardation.
At excitatory synapses, the postsynaptic scaffolding protein postsynaptic density 95 (PSD-95) couples NMDA receptors (NMDARs) to the Ras GTPase-activating protein SynGAP. The close association of SynGAP and NMDARs suggests that SynGAP may have an important role in NMDAR-dependent activation of Ras signaling pathways, such as the MAP kinase pathway, and in synaptic plasticity. To explore this issue, we examined long-term potentiation (LTP), p42 MAPK (ERK2) signaling, and spatial learning in mice with a heterozygous null mutation of the SynGAP gene (SynGAP(-/+)). In SynGAP(-/+) mutant mice, the induction of LTP in the hippocampal CA1 region was strongly reduced in the absence of any detectable alteration in basal synaptic transmission and NMDAR-mediated synaptic currents. Although basal levels of activated ERK2 were elevated in hippocampal extracts from SynGAP(-/+) mice, NMDAR stimulation still induced a robust increase in ERK activation in slices from SynGAP(-/+) mice. Thus, although SynGAP may regulate the ERK pathway, its role in LTP most likely involves additional downstream targets. Consistent with this, the amount of potentiation induced by stimulation protocols that induce an ERK-independent form of LTP were also significantly reduced in slices from SynGAP(-/+) mice. An elevation of basal phospho-ERK2 levels and LTP deficits were also observed in SynGAP(-/+)/H-Ras(-)/- double mutants, suggesting that SynGAP may normally regulate Ras isoforms other than H-Ras. A comparison of SynGAP and PSD-95 mutants suggests that PSD-95 couples NMDARs to multiple downstream signaling pathways with very different roles in LTP and learning.
SynGAP is a brain-specific ras GTPase-activating protein that is an abundant component of the signaling complex associated with the NMDA-type glutamate receptor. We generated mutant mice lacking synGAP to study its physiological role. Homozygous mutant mice die in the first few days after birth; however, neurons from mutant embryos can be maintained in culture. Here, we report that spine and synapse formation are accelerated in cultured mutant neurons, and the spines of mature mutant neurons are significantly larger than those of wild type. Clusters of PSD-95 and subunits of AMPA-type and NMDA-type glutamate receptors accumulate in spines of mutant neurons by day 10 in vitro, whereas in wild-type neurons they are still mostly located in dendritic shafts. The frequency and amplitude of miniature EPSCs are larger in mutant neurons at day 10 in vitro, confirming that they have more functional synapses. At day 21 in vitro, the spines of mutant neurons remain significantly larger than those of wild type. The mutant phenotype at day 10 in vitro can be rescued by introduction of recombinant wild-type synGAP on day 9. In contrast, introduction of mutant synGAP with a mutated GAP domain or lacking the terminal domain that binds to PSD-95 does not rescue the mutant phenotype, indicating that both domains play a role in control of spine formation. Thus, the GAP activity of synGAP and its association with PSD-95 are important for normal regulation of spine and synapse formation in hippocampal neurons.
The roles of the hippocampus and prefrontal cortex (PFC) in memory processing - individually or in concert - are a major topic of interest in memory research. These brain areas have distinct and complementary roles in episodic memory, and their interactions are crucial for learning and remembering events. Considerable evidence indicates that the PFC and hippocampus become coupled via oscillatory synchrony that reflects bidirectional flow of information. Furthermore, newer studies have revealed specific mechanisms whereby neural representations in the PFC and hippocampus are mediated through direct connections or through intermediary regions. These findings suggest a model of how the hippocampus and PFC, along with their intermediaries, operate as a system that uses the current context of experience to retrieve relevant memories.
Recent theoretical accounts have proposed excitation (E) and inhibition (I) imbalance as a possible mechanistic, network-level hypothesis underlying neural and behavioral dysfunction across neurodevelopmental disorders, particularly autism spectrum disorder (ASD) and schizophrenia (SCZ). These two disorders share some overlap in their clinical presentation as well as convergence in their underlying genes and neurobiology. However, there are also clear points of dissociation in terms of phenotypes and putatively affected neural circuitry. Here we highlight emerging work from the clinical neuroscience literature examining neural correlates of E/I imbalance across children and adults with ASD and adults with both chronic and early-course SCZ. We discuss findings from diverse neuroimaging studies across distinct modalities, conducted with EEG, MEG, ¹H-MRS, and fMRI, including effects observed both during task and at rest. Throughout this review we discuss points of convergence and divergence in the ASD and SCZ literature, with a focus on disruptions in neural E/I balance. We also consider these findings in relation to predictions generated by theoretical neuroscience, particularly computational models predicting E/I imbalance across disorders. Finally, we discuss how human non-invasive neuroimaging can benefit from pharmacological challenge studies to reveal mechanisms in ASD and SCZ. Collectively, we attempt to shed light on shared and divergent neuroimaging effects across disorders with the goal of informing future research examining the mechanisms underlying the E/I imbalance hypothesis across neurodevelopmental disorders. We posit that such translational efforts are vital to facilitate development of neurobiologically informed treatment strategies across neuropsychiatric conditions.
SHANK3 proteins belong to the Shank/ProSAP family, which contain five-conserved domains - the ankyrin (ANK) repeated, the Src homology 3 (SH3), the PSD-95/Discs large/ZO-1 (PDZ), the proline-rich and the sterile alpha motif (SAM; Sheng and Kim, 2000). SHANK proteins are located at the postsynaptic density of excitatory synapses and indirectly link the metabotropic (mGluRs) and the ionotropic glutamate receptor (iGluRs: AMPARs, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors, and NMDARs, N-methyl-D-aspartate receptors) via different domains (Naisbitt et al., 1999; Tu et al., 1999). Through the SH3 domain, SHANK binds to glutamate receptor interacting protein (GRIP), linking AMPARs to the scaffold protein (Sheng and Kim, 2000). The PDZ domain binds to GKAP (Guanylate Kinase-Associated Protein) allowing the binding of SHANK to NMDAR (Naisbitt et al., 1999), while the proline-rich region contains the binding site for HOMER and links SHANK to mGluR. Although several mouse lines have been generated to investigate the neurobiology underlying SHANK3-related disorders, the results are quite puzzling and it is hard to reconcile basic evidences between models. One possible explanation for the observed discrepancies is that the distinct protein domains are involved in regulating synaptic transmission in a circuit-specific manner. This might underlie the heterogeneous synaptic and behavioral phenotypes observed across transgenic animals. This article is protected by copyright. All rights reserved.
Class I histone deacetylases (HDACs) Hdac1 and Hdac2 can associate together in protein complexes with transcriptional factors such as methyl-CpG-binding protein 2 (MeCP2). Given their high degree of sequence identity, we examined whether Hdac1 and Hdac2 were functionally redundant in mature mouse brain. We demonstrate that postnatal forebrain-specific deletion of both Hdac1 and Hdac2 in mice impacts neuronal survival and results in an excessive grooming phenotype caused by dysregulation of Sap90/Psd95-associated protein 3 (Sapap3; also known as Dlgap3) in striatum. Moreover, Hdac1- and Hdac2-dependent regulation of Sapap3 expression requires MECP2, the gene involved in the pathophysiology of Rett syndrome. We show that postnatal forebrain-specific deletion of Mecp2 causes excessive grooming, which is rescued by restoring striatal Sapap3 expression. Our results provide new insight into the upstream regulation of Sapap3 and establish the essential role of striatal Hdac1, Hdac2 and MeCP2 for suppression of repetitive behaviors.
Loss- and gain-of-function mutations in methyl-CpG-binding protein 2 (MECP2) underlie two distinct neurological syndromes with strikingly similar features, but the synaptic and circuit-level changes mediating these shared features are undefined. Here we report three novel signs of neural circuit dysfunction in three mouse models of MECP2 disorders (constitutive Mecp2 null, mosaic Mecp2+/−, and MECP2 duplication): abnormally elevated synchrony in the firing activity of hippocampal CA1 pyramidal neurons, an impaired homeostatic response to perturbations of excitatory-inhibitory balance, and decreased excitatory synaptic response in inhibitory neurons. Conditional mutagenesis studies revealed that MeCP2 dysfunction in excitatory neurons mediated elevated synchrony at baseline, while MeCP2 dysfunction in inhibitory neurons increased susceptibility to hypersynchronization in response to perturbations. Chronic forniceal deep brain stimulation (DBS), recently shown to rescue hippocampus-dependent learning and memory in Mecp2+/− (Rett) mice, also rescued all three features of hippocampal circuit dysfunction in these mice.
Fragile-X Syndrome (FXS) is the most common form of inherited intellectual disability and the leading genetic cause of autism spectrum disorder. FXS is caused by transcriptional silencing of the Fragile X Mental Retardation 1 (Fmr1) gene due to a CGG repeat expansion, resulting in the loss of Fragile X Mental Retardation Protein (FMRP). FMRP is involved in transcriptional regulation and trafficking of mRNA from the nucleus to the cytoplasm and distal sites both in pre- and post-synaptic terminals. Consequently, FXS is a multifaceted disorder associated with impaired synaptic plasticity. One region of the brain that is significantly impacted by the loss of FMRP is the hippocampus, a structure that plays a critical role in the regulation of mood and cognition. This review provides an overview of the neuropathology of Fragile-X Syndrome, highlighting how structural and synaptic deficits in hippocampal subregions, including the CA1 exhibiting exaggerated metabotropic glutamate receptor dependent long-term depression and the dentate gyrus displaying hypofunction of N-methyl-D-aspartate receptors, contribute to cognitive impairments associated with this neurodevelopmental disorder.