CYFIP1 coordinates mRNA translation and cytoskeleton remodeling to ensure proper dendritic spine formation

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

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

Deletion of the KH1 Domain of Fmr1 Leads to Transcriptional Alterations and Attentional Deficits in Rats

Fragile X syndrome (FXS) is a neurodevelopmental disorder caused by mutations in the FMR1 gene. It is a leading monogenic cause of autism spectrum disorder and inherited intellectual disability and is often comorbid with attention deficits. Most FXS cases are due to an expansion of CGG repeats leading to suppressed expression of fragile X mental retardation protein (FMRP), an RNA-binding protein involved in mRNA metabolism. We found that the previously published Fmr1 knockout rat model of FXS expresses an Fmr1 transcript with an in-frame deletion of exon 8, which encodes for the K-homology (KH) RNA-binding domain, KH1. Notably, 3 pathogenic missense mutations associated with FXS lie in the KH domains. We observed that the deletion of exon 8 in rats leads to attention deficits and to alterations in transcriptional profiles within the medial prefrontal cortex (mPFC), which map to 2 weighted gene coexpression network modules. These modules are conserved in human frontal cortex and enriched for known FMRP targets. Hub genes in these modules represent potential therapeutic targets for FXS. Taken together, these findings indicate that attentional testing might be a reliable cross-species tool for investigating FXS and identify dysregulated conserved gene networks in a relevant brain region.

Alzheimer's disease-related dysregulation of mRNA translation causes key pathological features with ageing

Alzheimer's disease (AD) is characterised by Aβ and tau pathology as well as synaptic degeneration, which correlates best with cognitive impairment. Previous work suggested that this pathological complexity may result from changes in mRNA translation. Here, we studied whether mRNA translation and its underlying signalling are altered in an early model of AD, and whether modelling this deficiency in mice causes pathological features with ageing. Using an unbiased screen, we show that exposure of primary neurons to nanomolar amounts of Aβ increases FMRP-regulated protein synthesis. This selective regulation of mRNA translation is dependent on a signalling cascade involving MAPK-interacting kinase 1 (Mnk1) and the eukaryotic initiation factor 4E (eIF4E), and ultimately results in reduction of CYFIP2, an FMRP-binding protein. Modelling this CYFIP2 reduction in mice, we find age-dependent Aβ accumulation in the thalamus, development of tau pathology in entorhinal cortex and hippocampus, as well as gliosis and synapse loss in the hippocampus, together with deficits in memory formation. Therefore, we conclude that early stages of AD involve increased translation of specific CYFIP2/FMRP-regulated transcripts. Since reducing endogenous CYFIP2 expression is sufficient to cause key features of AD with ageing in mice, we suggest that prolonged activation of this pathway is a primary step toward AD pathology, highlighting a novel direction for therapeutic targeting.

Dysregulated translational control in brain disorders: from genes to behavior

Control of protein synthesis (mRNA translation) is essential for proper brain development and function. Perturbations to the mechanisms governing mRNA translation have repeatedly been shown to constitute a neurodegenerative, neuropsychiatric, and neurodevelopmental disorder risk factor. Developing effective therapeutics for brain disorders will require a better understanding of the molecular mechanisms underlying the control of protein synthesis in brain function. Studies using transgenic animal models have been invaluable towards this end, providing exciting new insights into the genetic basis of brain disorders with hopeful prospects for new and effective treatment options.

Neurodevelopmental Disorders: From Genetics to Functional Pathways

Neurodevelopmental disorders (NDDs) are a class of disorders affecting brain development and function and are characterized by wide genetic and clinical variability. In this review, we discuss the multiple factors that influence the clinical presentation of NDDs, with particular attention to gene vulnerability, mutational load, and the two-hit model. Despite the complex architecture of mutational events associated with NDDs, the various proteins involved appear to converge on common pathways, such as synaptic plasticity/function, chromatin remodelers and the mammalian target of rapamycin (mTOR) pathway. A thorough understanding of the mechanisms behind these pathways will hopefully lead to the identification of candidates that could be targeted for treatment approaches.

Epilepsy- and intellectual disability-associated CYFIP2 interacts with both actin regulators and RNA-binding proteins in the neonatal mouse forebrain

Variants of the cytoplasmic FMR1-interacting protein 2 (CYFIP2) gene are associated with early-onset epileptic encephalopathy, intellectual disability, and developmental delay. However, the current understanding of the molecular functions of CYFIP2 is limited to those related to actin dynamics, and thus, the detailed mechanisms of CYFIP2-associated brain disorders remain largely unknown. Here, we isolated the neonatal forebrain CYFIP2 complex using newly generated Cyfip2-3×Flag knock-in mice, and performed mass spectrometry-based analyses to identify proteins in the complex. The CYFIP2 interactome, consisting of 140 proteins, contained not only the expected actin regulators but also 25 RNA-binding proteins (RBPs) including Argonaute proteins. Functionally, overexpression of brain disorder-associated CYFIP2 R87 variants, but not wild-type, inhibited stress granule formation in HeLa cells. Mechanistically, the CYFIP2 R87 variants formed intracellular clusters with Argonaute proteins under both basal and stress conditions, and thereby possibly preventing their assembly into stress granules. Beyond identifying CYFIP2 interactors in vivo, these results may provide novel insights for better understanding the molecular mechanisms of CYFIP2-associated brain disorders.

Autism and Schizophrenia-Associated CYFIP1 Regulates the Balance of Synaptic Excitation and Inhibition

Altered excitatory/inhibitory (E/I) balance is implicated in neuropsychiatric and neurodevelopmental disorders, but the underlying genetic etiology remains poorly understood. Copy number variations in CYFIP1 are associated with autism, schizophrenia, and intellectual disability, but its role in regulating synaptic inhibition or E/I balance remains unclear. We show that CYFIP1, and the paralog CYFIP2, are enriched at inhibitory postsynaptic sites. While CYFIP1 or CYFIP2 upregulation increases excitatory synapse number and the frequency of miniature excitatory postsynaptic currents (mEPSCs), it has the opposite effect at inhibitory synapses, decreasing their size and the amplitude of miniature inhibitory postsynaptic currents (mIPSCs). Contrary to CYFIP1 upregulation, its loss in vivo, upon conditional knockout in neocortical principal cells, increases expression of postsynaptic GABA_A receptor β2/3-subunits and neuroligin 3, enhancing synaptic inhibition. Thus, CYFIP1 dosage can bi-directionally impact inhibitory synaptic structure and function, potentially leading to altered E/I balance and circuit dysfunction in CYFIP1-associated neurological disorders.

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CYFIP1 and CYFIP2 are enriched at inhibitory synapses.

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CYFIP1 upregulation differentially disrupts inhibitory and excitatory synapses.

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Conditional loss of CYFIP1 alters neuroligin 3 and GABA_AR β-subunits expression.

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Loss of CYFIP1 increases inhibitory synaptic clusters and hence mIPSC amplitude.

Translational derepression of Elavl4 isoforms at their alternative 5' UTRs determines neuronal development

Neurodevelopment requires precise regulation of gene expression, including post-transcriptional regulatory events such as alternative splicing and mRNA translation. However, translational regulation of specific isoforms during neurodevelopment and the mechanisms behind it remain unknown. Using RNA-seq analysis of mouse neocortical polysomes, here we report translationally repressed and derepressed mRNA isoforms during neocortical neurogenesis whose orthologs include risk genes for neurodevelopmental disorders. We demonstrate that the translation of distinct mRNA isoforms of the RNA binding protein (RBP), Elavl4, in radial glia progenitors and early neurons depends on its alternative 5' UTRs. Furthermore, 5' UTR-driven Elavl4 isoform-specific translation depends on upstream control by another RBP, Celf1. Celf1 regulation of Elavl4 translation dictates development of glutamatergic neurons. Our findings reveal a dynamic interplay between distinct RBPs and alternative 5' UTRs in neuronal development and underscore the risk of post-transcriptional dysregulation in co-occurring neurodevelopmental disorders.

Transcriptomic profiling of microglia and astrocytes throughout aging

Background

Activation of microglia and astrocytes, a prominent hallmark of both aging and Alzheimer’s disease (AD), has been suggested to contribute to aging and AD progression, but the underlying cellular and molecular mechanisms are largely unknown.

Methods

We performed RNA-seq analyses on microglia and astrocytes freshly isolated from wild-type and APP-PS1 (AD) mouse brains at five time points to elucidate their age-related gene-expression profiles.

Results

Our results showed that from 4 months onward, a set of age-related genes in microglia and astrocytes exhibited consistent upregulation or downregulation (termed “age-up”/“age-down” genes) relative to their expression at the young-adult stage (2 months). And most age-up genes were more highly expressed in AD mice at the same time points. Bioinformatic analyses revealed that the age-up genes in microglia were associated with the inflammatory response, whereas these genes in astrocytes included widely recognized AD risk genes, genes associated with synaptic transmission or elimination, and peptidase-inhibitor genes.

Conclusions

Overall, our RNA-seq data provide a valuable resource for future investigations into the roles of microglia and astrocytes in aging- and amyloid-β-induced AD pathologies.

Association of Copy Number Variation of the 15q11.2 BP1-BP2 Region With Cortical and Subcortical Morphology and Cognition

Importance

Recurrent microdeletions and duplications in the genomic region 15q11.2 between breakpoints 1 (BP1) and 2 (BP2) are associated with neurodevelopmental disorders. These structural variants are present in 0.5% to 1.0% of the population, making 15q11.2 BP1-BP2 the site of the most prevalent known pathogenic copy number variation (CNV). It is unknown to what extent this CNV influences brain structure and affects cognitive abilities.

Objective

To determine the association of the 15q11.2 BP1-BP2 deletion and duplication CNVs with cortical and subcortical brain morphology and cognitive task performance.

Design, Setting, and Participants

In this genetic association study, T1-weighted brain magnetic resonance imaging were combined with genetic data from the ENIGMA-CNV consortium and the UK Biobank, with a replication cohort from Iceland. In total, 203 deletion carriers, 45 247 noncarriers, and 306 duplication carriers were included. Data were collected from August 2015 to April 2019, and data were analyzed from September 2018 to September 2019.

Main Outcomes and Measures

The associations of the CNV with global and regional measures of surface area and cortical thickness as well as subcortical volumes were investigated, correcting for age, age2, sex, scanner, and intracranial volume. Additionally, measures of cognitive ability were analyzed in the full UK Biobank cohort.

Results

Of 45 756 included individuals, the mean (SD) age was 55.8 (18.3) years, and 23 754 (51.9%) were female. Compared with noncarriers, deletion carriers had a lower surface area (Cohen d = -0.41; SE, 0.08; P = 4.9 × 10-8), thicker cortex (Cohen d = 0.36; SE, 0.07; P = 1.3 × 10-7), and a smaller nucleus accumbens (Cohen d = -0.27; SE, 0.07; P = 7.3 × 10-5). There was also a significant negative dose response on cortical thickness (β = -0.24; SE, 0.05; P = 6.8 × 10-7). Regional cortical analyses showed a localization of the effects to the frontal, cingulate, and parietal lobes. Further, cognitive ability was lower for deletion carriers compared with noncarriers on 5 of 7 tasks.

Conclusions and Relevance

These findings, from the largest CNV neuroimaging study to date, provide evidence that 15q11.2 BP1-BP2 structural variation is associated with brain morphology and cognition, with deletion carriers being particularly affected. The pattern of results fits with known molecular functions of genes in the 15q11.2 BP1-BP2 region and suggests involvement of these genes in neuronal plasticity. These neurobiological effects likely contribute to the association of this CNV with neurodevelopmental disorders.

Rho GTPase Regulators and Effectors in Autism Spectrum Disorders: Animal Models and Insights for Therapeutics

The Rho family GTPases are small G proteins that act as molecular switches shuttling between active and inactive forms. Rho GTPases are regulated by two classes of regulatory proteins, guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Rho GTPases transduce the upstream signals to downstream effectors, thus regulating diverse cellular processes, such as growth, migration, adhesion, and differentiation. In particular, Rho GTPases play essential roles in regulating neuronal morphology and function. Recent evidence suggests that dysfunction of Rho GTPase signaling contributes substantially to the pathogenesis of autism spectrum disorder (ASD). It has been found that 20 genes encoding Rho GTPase regulators and effectors are listed as ASD risk genes by Simons foundation autism research initiative (SFARI). This review summarizes the clinical evidence, protein structure, and protein expression pattern of these 20 genes. Moreover, ASD-related behavioral phenotypes in animal models of these genes are reviewed, and the therapeutic approaches that show successful treatment effects in these animal models are discussed.

Epigenome-wide association study for perceived discrimination among sub-Saharan African migrants in Europe - the RODAM study

Sub-Saharan African (SSA) migrants in Europe experience psychosocial stressors, such as perceived discrimination (PD). The effect of such a stressor on health could potentially be mediated via epigenetics. In this study we performed an epigenome-wide association study (EWAS) to assess the association between levels of PD with genome-wide DNA methylation profiles in SSA migrants. The Illumina 450 K DNA-methylation array was used on whole blood samples of 340 Ghanaian adults residing in three European cities from the cross-sectional Research on Obesity and Diabetes among African Migrants (RODAM) study. PD was assessed using sum scores of the Everyday Discrimination Scale (EDS). Differentially methylated positions and regions (DMPs and DMRs) were identified through linear regression analysis. Two hypo-methylated DMPs, namely cg13986138 (CYFIP1) and cg10316525(ANKRD63), were found to be associated with PD. DMR analysis identified 47 regions associated with the PD. To the best of our knowledge, this survey is the first EWAS for PD in first generation SSA migrants. We identified two DMPs associated with PD. Whether these associations underlie a consequence or causal effect within the scope of biological functionality needs additional research.

Persistent Cyfip1 Expression Is Required to Maintain the Adult Subventricular Zone Neurogenic Niche

Neural stem cells (NSCs) persist throughout life in the subventricular zone (SVZ) neurogenic niche of the lateral ventricles as Type B1 cells in adult mice. Maintaining this population of NSCs depends on the balance between quiescence and self-renewing or self-depleting cell divisions. Interactions between B1 cells and the surrounding niche are important in regulating this balance, but the mechanisms governing these processes have not been fully elucidated. The cytoplasmic FMRP-interacting protein (Cyfip1) regulates apical-basal polarity in the embryonic brain. Loss of Cyfip1 during embryonic development in mice disrupts the embryonic niche and affects cortical neurogenesis. However, a direct role for Cyfip1 in the regulation of adult NSCs has not been established. Here, we demonstrate that Cyfip1 expression is preferentially localized to B1 cells in the adult mouse SVZ. Loss of Cyfip1 in the embryonic mouse brain results in altered adult SVZ architecture and expansion of the adult B1 cell population at the ventricular surface. Furthermore, acute deletion of Cyfip1 in adult NSCs results in a rapid change in adherens junction proteins as well as increased proliferation and number of B1 cells at the ventricular surface. Together, these data indicate that Cyfip1 plays a critical role in the formation and maintenance of the adult SVZ niche; furthermore, deletion of Cyfip1 unleashes the capacity of adult B1 cells for symmetric renewal to increase the adult NSC pool.SIGNIFICANCE STATEMENT Neural stem cells (NSCs) persist in the subventricular zone of the lateral ventricles in adult mammals, and the size of this population is determined by the balance between quiescence and self-depleting or renewing cell division. The mechanisms regulating these processes are not fully understood. This study establishes that the cytoplasmic FMRP interacting protein 1 (Cyfip1) regulates NSC fate decisions in the adult subventricular zone and adult NSCs that are quiescent or typically undergo self-depleting divisions retain the ability to self-renew. These results contribute to our understanding of how adult NSCs are regulated throughout life and has potential implications for human brain disorders.

Neuronal function and dysfunction of CYFIP2: from actin dynamics to early infantile epileptic encephalopathy

The cytoplasmic FMR1-interacting protein family (CYFIP1 and CYFIP2) are evolutionarily conserved proteins originally identified as binding partners of the fragile X mental retardation protein (FMRP), a messenger RNA (mRNA)-binding protein whose loss causes the fragile X syndrome. Moreover, CYFIP is a key component of the heteropentameric WAVE regulatory complex (WRC), a critical regulator of neuronal actin dynamics. Therefore, CYFIP may play key roles in regulating both mRNA translation and actin polymerization, which are critically involved in proper neuronal development and function. Nevertheless, compared to CYFIP1, neuronal function and dysfunction of CYFIP2 remain largely unknown, possibly due to the relatively less well established association between CYFIP2 and brain disorders. Despite high amino acid sequence homology between CYFIP1 and CYFIP2, several in vitro and animal model studies have suggested that CYFIP2 has some unique neuronal functions distinct from those of CYFIP1. Furthermore, recent whole-exome sequencing studies identified de novo hot spot variants of CYFIP2 in patients with early infantile epileptic encephalopathy (EIEE), clearly implicating CYFIP2 dysfunction in neurological disorders. In this review, we highlight these recent investigations into the neuronal function and dysfunction of CYFIP2, and also discuss several key questions remaining about this intriguing neuronal protein.

Evidence for a Contribution of the Nlgn3/Cyfip1/Fmr1 Pathway in the Pathophysiology of Autism Spectrum Disorders

Autism Spectrum Disorders (ASD) are characterized by heterogeneity both in their presentation and their genetic aetiology. In order to discover points of convergence common to different cases of ASD, attempts were made to identify the biological pathways genes associated with ASD contribute to. Many of these genes were found to play a role in neuronal and synaptic development and function. Among these genes are FMR1, CYFIP1 and NLGN3, all present at the synapse and reliably linked to ASD. In this review, we evaluate the evidence for the contribution of these genes to the same biological pathway responsible for the regulation of structural and physiological plasticity. Alterations in dendritic spine density and turnover, as well as long-term depression (LTD), were found in mouse models of mutations of all three genes. This overlap in the phenotypes associated with these mouse models likely arises from the molecular interaction between the protein products of FMR1, CYFIP1, and NLG3. A number of other proteins linked to ASD are also likely to participate in these pathways, resulting in further downstream effects. Overall, a synaptic pathway centered around FMR1, CYFIP1, and NLG3 is likely to contribute to the phenotypes associated with structural and physiological plasticity characteristic of ASD.

Translation from the Ribosome to the Clinic: Implication in Neurological Disorders and New Perspectives from Recent Advances

De novo protein synthesis by the ribosome and its multitude of co-factors must occur in a tightly regulated manner to ensure that the correct proteins are produced accurately at the right time and, in some cases, also in the proper location. With novel techniques such as ribosome profiling and cryogenic electron microscopy, our understanding of this basic biological process is better than ever and continues to grow. Concurrently, increasing attention is focused on how translational regulation in the brain may be disrupted during the progression of various neurological disorders. In fact, translational dysregulation is now recognized as the de facto pathogenic cause for some disorders. Novel mechanisms including ribosome stalling, ribosome-associated quality control, and liquid-liquid phase separation are closely linked to translational regulation, and may thus be involved in the pathogenic process. The relationships between translational dysregulation and neurological disorders, as well as the ways through which we may be able to reverse those detrimental effects, will be examined in this review.

Tiam1 is Critical for Glutamatergic Synapse Structure and Function in the Hippocampus

Mounting evidence suggests numerous glutamatergic synapse subtypes exist in the brain, and that these subtypes are likely defined by unique molecular regulatory mechanisms. Recent work has identified substantial divergence of molecular composition between commonly studied Schaffer collateral synapses and perforant path-dentate gyrus (DG) synapses of the hippocampus. However, little is known about the molecular mechanisms that may confer unique properties to perforant path-DG synapses. Here we investigate whether the RhoGEF (Rho guanine-nucleotide exchange factor) protein Tiam1 plays a unique role in the regulation of glutamatergic synapses in dentate granule neurons using a combination of molecular, electrophysiological, and imaging approaches in rat entorhino-hippocampal slices of both sexes. We find that inhibition of Tiam1 function in dentate granule neurons reduces synaptic AMPA receptor function and causes dendritic spines to adopt an elongated filopodia-like morphology. We also find that Tiam1's support of perforant path-DG synapse function is dependent on its GEF domain and identify a potential role for the auto-inhibitory PH domain of Tiam1 in regulating Tiam1 function at these synapses. In marked contrast, reduced Tiam1 expression in CA1 pyramidal neurons produced no effect on glutamatergic synapse development. Together, these data identify a critical role for Tiam1 in the hippocampus and reveal a unique Tiam1-mediated molecular program of glutamatergic synapse regulation in dentate granule neurons.SIGNIFICANCE STATEMENT Several lines of evidence independently point to the molecular diversity of glutamatergic synapses in the brain. Rho guanine-nucleotide exchange factor (RhoGEF) proteins as powerful modulators of glutamatergic synapse function have also become increasingly appreciated in recent years. Here we investigate the synaptic regulatory role of the RhoGEF protein Tiam1, whose expression appears to be remarkably enriched in granule neurons of the dentate gyrus. We find that Tiam1 plays a critical role in the development of glutamatergic perforant path-dentate gyrus synapses, but not in commonly studied in Schaffer collateral-CA1 synapses. Together, these data reveal a unique RhoGEF-mediated molecular program of glutamatergic synapse regulation in dentate granule neurons.

A Cyfip2-Dependent Excitatory Interneuron Pathway Establishes the Innate Startle Threshold

Sensory experiences dynamically modify whether animals respond to a given stimulus, but it is unclear how innate behavioral thresholds are established. Here, we identify molecular and circuit-level mechanisms underlying the innate threshold of the zebrafish startle response. From a forward genetic screen, we isolated five mutant lines with reduced innate startle thresholds. Using whole-genome sequencing, we identify the causative mutation for one line to be in the fragile X mental retardation protein (FMRP)-interacting protein cyfip2. We show that cyfip2 acts independently of FMRP and that reactivation of cyfip2 restores the baseline threshold after phenotype onset. Finally, we show that cyfip2 regulates the innate startle threshold by reducing neural activity in a small group of excitatory hindbrain interneurons. Thus, we identify a selective set of genes critical to establishing an innate behavioral threshold and uncover a circuit-level role for cyfip2 in this process.

Using forward genetics, electrophysiology, and combined behavior and Ca²⁺ imaging in zebrafish, Marsden et al. show that cyfip2 regulates the acoustic startle threshold by controlling the activity of excitatory spiral fiber interneurons.

Plasticity of dendritic spines: Molecular function and dysfunction in neurodevelopmental disorders

Dendritic spines are tiny postsynaptic protrusions from a dendrite that receive most of the excitatory synaptic input in the brain. Functional and structural changes in dendritic spines are critical for synaptic plasticity, a cellular model of learning and memory. Conversely, altered spine morphology and plasticity are common hallmarks of human neurodevelopmental disorders, such as intellectual disability and autism. The advances in molecular and optical techniques have allowed for exploration of dynamic changes in structure and signal transduction at single-spine resolution, providing significant insights into the molecular regulation underlying spine structural plasticity. Here, I review recent findings on: how synaptic stimulation leads to diverse forms of spine structural plasticity; how the associated biochemical signals are initiated and transmitted into neuronal compartments; and how disruption of single genes associated with neurodevelopmental disorders can lead to abnormal spine structure in human and mouse brains. In particular, I discuss the functions of the Ras superfamily of small GTPases in spatiotemporal regulation of the actin cytoskeleton and protein synthesis in dendritic spines. Multiple lines of evidence implicate disrupted Ras signaling pathways in the spine structural abnormalities observed in neurodevelopmental disorders. Both deficient and excessive Ras activities lead to disrupted spine structure and deficits in learning and memory. Dysregulation of spine Ras signaling, therefore, may play a key role in the pathogenesis of multiple neurodevelopmental disorders with distinct etiologies.

Estimating the effect size of the 15Q11.2 BP1-BP2 deletion and its contribution to neurodevelopmental symptoms: recommendations for practice

Background

The 15q11.2 deletion is frequently identified in the neurodevelopmental clinic. Case–control studies have associated the 15q11.2 deletion with neurodevelopmental disorders, and clinical case series have attempted to delineate a microdeletion syndrome with considerable phenotypic variability. The literature on this deletion is extensive and confusing, which is a challenge for genetic counselling. The aim of this study was to estimate the effect size of the 15q11.2 deletion and quantify its contribution to neurodevelopmental disorders.

Methods

We performed meta-analyses on new and previously published case–control studies and used statistical models trained in unselected populations with cognitive assessments. We used new (n=241) and previously published (n=150) data from a clinically referred group of deletion carriers. 15q11.2 duplications (new n=179 and previously published n=35) were used as a neutral control variant.

Results

The deletion decreases IQ by 4.3 points. The estimated ORs and respective frequencies in deletion carriers for intellectual disabilities, schizophrenia and epilepsy are 1.7 (3.4%), 1.5 (2%) and 3.1 (2.1%), respectively. There is no increased risk for heart malformations and autism. In the clinically referred group, the frequency and nature of symptoms in deletions are not different from those observed in carriers of the 15q11.2 duplication suggesting that most of the reported symptoms are due to ascertainment bias.

Conclusions

We recommend that the deletion should be classified as ‘pathogenic of mild effect size’. Since it explains only a small proportion of the phenotypic variance in carriers, it is not worth discussing in the developmental clinic or in a prenatal setting.

Domain-Specific Cognitive Impairments in Humans and Flies With Reduced CYFIP1 Dosage

BACKGROUND

Deletions encompassing a four-gene region on chromosome 15 (BP1-BP2 at 15q11.2), seen at a population frequency of 1 in 500, are associated with increased risk for schizophrenia, epilepsy, and other common neurodevelopmental disorders. However, little is known in terms of how these common deletions impact cognition.

METHODS

We used a Web-based tool to characterize cognitive function in a novel cohort of adult carriers and their noncarrier family members. Results from 31 carrier and 38 noncarrier parents from 40 families were compared with control data from 6530 individuals who self-registered on the Lumosity platform and opted in to participate in research. We then examined aspects of sensory and cognitive function in flies harboring a mutation in Cyfip, the homologue of one of the genes within the deletion. For the fly studies, 10 or more groups of 50 individuals per genotype were included.

RESULTS

Our human studies revealed profound deficits in grammatical reasoning, arithmetic reasoning, and working memory in BP1-BP2 deletion carriers. No such deficits were observed in noncarrier spouses. Our fly studies revealed deficits in associative and nonassociative learning despite intact sensory perception.

CONCLUSIONS

Our results provide new insights into outcomes associated with BP1-BP2 deletions and call for a discussion on how to appropriately communicate these findings to unaffected carriers. Findings also highlight the utility of an online tool in characterizing cognitive function in a geographically distributed population.

De novo hotspot variants in CYFIP2 cause early-onset epileptic encephalopathy

OBJECTIVE

The cytoplasmic fragile X mental retardation 1 interacting proteins 2 (CYFIP2) is a component of the WASP-family verprolin-homologous protein (WAVE) regulatory complex, which is involved in actin dynamics. An obvious association of CYFIP2 variants with human neurological disorders has never been reported. Here, we identified de novo hotspot CYFIP2 variants in neurodevelopmental disorders and explore the possible involvement of the CYFIP2 mutants in the WAVE signaling pathway.

METHODS

We performed trio-based whole-exome sequencing (WES) in 210 families and case-only WES in 489 individuals with epileptic encephalopathies. The functional effect of CYFIP2 variants on WAVE signaling was evaluated by computational structural analysis and in vitro transfection experiments.

RESULTS

We identified three de novo CYFIP2 variants at the Arg87 residue in 4 unrelated individuals with early-onset epileptic encephalopathy. Structural analysis indicated that the Arg87 residue is buried at an interface between CYFIP2 and WAVE1, and the Arg87 variant may disrupt hydrogen bonding, leading to structural instability and aberrant activation of the WAVE regulatory complex. All mutant CYFIP2 showed comparatively weaker interactions to the VCA domain than wild-type CYFIP2. Immunofluorescence revealed that ectopic speckled accumulation of actin and CYFIP2 was significantly increased in cells transfected with mutant CYFIP2.

INTERPRETATION

Our findings suggest that de novo Arg87 variants in CYFIP2 have gain-of-function effects on the WAVE signaling pathway and are associated with severe neurological disorders. Ann Neurol 2018;83:794-806.

The autism- and schizophrenia-associated protein CYFIP1 regulates bilateral brain connectivity and behaviour

Copy-number variants of the CYFIP1 gene in humans have been linked to autism spectrum disorders (ASD) and schizophrenia (SCZ), two neuropsychiatric disorders characterized by defects in brain connectivity. Here, we show that CYFIP1 plays an important role in brain functional connectivity and callosal functions. We find that Cyfip1-heterozygous mice have reduced functional connectivity and defects in white matter architecture, similar to phenotypes found in patients with ASD, SCZ and other neuropsychiatric disorders. Cyfip1-deficient mice also present decreased myelination in the callosal axons, altered presynaptic function, and impaired bilateral connectivity. Finally, Cyfip1 deficiency leads to abnormalities in motor coordination, sensorimotor gating and sensory perception, which are also known neuropsychiatric disorder-related symptoms. These results show that Cyfip1 haploinsufficiency compromises brain connectivity and function, which might explain its genetic association to neuropsychiatric disorders.

Cyfip1 haploinsufficient rats show white matter changes, myelin thinning, abnormal oligodendrocytes and behavioural inflexibility

The biological basis of the increased risk for psychiatric disorders seen in 15q11.2 copy number deletion is unknown. Previous work has shown disturbances in white matter tracts in human carriers of the deletion. Here, in a novel rat model, we recapitulated low dosage of the candidate risk gene CYFIP1 present within the 15q11.2 interval. Using diffusion tensor imaging, we first showed extensive white matter changes in Cyfip1 mutant rats, which were most pronounced in the corpus callosum and external capsule. Transmission electron microscopy showed that these changes were associated with thinning of the myelin sheath in the corpus callosum. Myelin thinning was independent of changes in axon number or diameter but was associated with effects on mature oligodendrocytes, including aberrant intracellular distribution of myelin basic protein. Finally, we demonstrated effects on cognitive phenotypes sensitive to both disruptions in myelin and callosal circuitry.

People with a genetic deletion of the 15q11.2 locus are at increased risk for psychiatric disorders and white matter disturbances, but the gene(s) responsible are unclear. Here, the authors show that low dosage of CYFIP1, present in the human 15q11.2 region, alters white matter structure and cognition in rats.

Axon-Axon Interactions Regulate Topographic Optic Tract Sorting via CYFIP2-Dependent WAVE Complex Function

The axons of retinal ganglion cells (RGCs) are topographically sorted before they arrive at the optic tectum. This pre-target sorting, typical of axon tracts throughout the brain, is poorly understood. Here, we show that cytoplasmic FMR1-interacting proteins (CYFIPs) fulfill non-redundant functions in RGCs, with CYFIP1 mediating axon growth and CYFIP2 specifically involved in axon sorting. We find that CYFIP2 mediates homotypic and heterotypic contact-triggered fasciculation and repulsion responses between dorsal and ventral axons. CYFIP2 associates with transporting ribonucleoprotein particles in axons and regulates translation. Axon-axon contact stimulates CYFIP2 to move into growth cones where it joins the actin nucleating WAVE regulatory complex (WRC) in the periphery and regulates actin remodeling and filopodial dynamics. CYFIP2’s function in axon sorting is mediated by its binding to the WRC but not its translational regulation. Together, these findings uncover CYFIP2 as a key regulatory link between axon-axon interactions, filopodial dynamics, and optic tract sorting.

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CYFIP1 and CYFIP2 serve non-redundant functions in retinal axon growth and guidance

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CYFIP2 regulates growth cone filopodial dynamics and axon-axon responses

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CYFIP2 interacts with RNPs and the WRC in distinct cellular compartments

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Axon sorting is mediated by CYFIP2’s interaction with the WRC

Cyfip1 Haploinsufficiency Does Not Alter GABAA Receptor δ-Subunit Expression and Tonic Inhibition in Dentate Gyrus PV+ Interneurons and Granule Cells

Copy number variation (CNV) at chromosomal region 15q11.2 is linked to increased risk of neurodevelopmental disorders including autism and schizophrenia. A significant gene at this locus is cytoplasmic fragile X mental retardation protein (FMRP) interacting protein 1 (CYFIP1). CYFIP1 protein interacts with FMRP, whose monogenic absence causes fragile X syndrome (FXS). Fmrp knock-out has been shown to reduce tonic GABAergic inhibition by interacting with the δ-subunit of the GABAA receptor (GABAAR). Using in situ hybridization (ISH), qPCR, Western blotting techniques, and patch clamp electrophysiology in brain slices from a Cyfip1 haploinsufficient mouse, we examined δ-subunit mediated tonic inhibition in the dentate gyrus (DG). In wild-type (WT) mice, DG granule cells (DGGCs) responded to the δ-subunit-selective agonist THIP with significantly increased tonic currents. In heterozygous mice, no significant difference was observed in THIP-evoked currents in DGGCs. Phasic GABAergic inhibition in DGGC was also unaltered with no difference in properties of spontaneous IPSCs (sIPSCs). Additionally, we demonstrate that DG granule cell layer (GCL) parvalbumin-positive interneurons (PV+-INs) have functional δ-subunit-mediated tonic GABAergic currents which, unlike DGGC, are also modulated by the α1-selective drug zolpidem. Similar to DGGC, both IPSCs and THIP-evoked currents in PV+-INs were not different between Cyfip1 heterozygous and WT mice. Supporting our electrophysiological data, we found no significant change in hippocampal δ-subunit mRNA expression or protein level and no change in α1/α4-subunit mRNA expression. Thus, Cyfip1 haploinsufficiency, mimicking human 15q11.2 microdeletion syndrome, does not alter hippocampal phasic or tonic GABAergic inhibition, substantially differing from the Fmrp knock-out mouse model.

CYFIP1 overexpression increases fear response in mice but does not affect social or repetitive behavioral phenotypes

Background

CYFIP1, a protein that interacts with FMRP and regulates protein synthesis and actin dynamics, is overexpressed in Dup15q syndrome as well as autism spectrum disorder (ASD). While CYFIP1 heterozygosity has been rigorously studied due to its loss in 15q11.2 deletion, Prader-Willi and Angelman syndrome, the effects of CYFIP1 overexpression, as is observed in patients with CYFIP1 duplication, are less well understood.

Methods

We developed and validated a mouse model of human CYFIP1 overexpression (CYFIP1 OE) using qPCR and western blot analysis. We performed a large battery of behavior testing on these mice, including ultrasonic vocalizations, three-chamber social assay, home-cage behavior, Y-maze, elevated plus maze, open field test, Morris water maze, fear conditioning, prepulse inhibition, and the hot plate assay. We also performed RNA sequencing and analysis on the basolateral amygdala.

Results

Extensive behavioral testing in CYFIP1 OE mice reveals no changes in the core behaviors related to ASD: social interactions and repetitive behaviors. However, we did observe mild learning deficits and an exaggerated fear response. Using RNA sequencing of the basolateral amygdala, a region associated with fear response, we observed changes in pathways related to cytoskeletal regulation, oligodendrocytes, and myelination. We also identified GABA-A subunit composition changes in basolateral amygdala neurons, which are essential components of the neural fear conditioning circuit.

Conclusion

Overall, this research identifies the behavioral and molecular consequences of CYFIP1 overexpression and how they contribute to the variable phenotype seen in Dup15q syndrome and in ASD patients with excess CYFIP1.

How can memories last for days, years, or a lifetime? Proposed mechanisms for maintaining synaptic potentiation and memory

With memory encoding reliant on persistent changes in the properties of synapses, a key question is how can memories be maintained from days to months or a lifetime given molecular turnover? It is likely that positive feedback loops are necessary to persistently maintain the strength of synapses that participate in encoding. Such feedback may occur within signal-transduction cascades and/or the regulation of translation, and it may occur within specific subcellular compartments or within neuronal networks. Not surprisingly, numerous positive feedback loops have been proposed. Some posited loops operate at the level of biochemical signal-transduction cascades, such as persistent activation of Ca²⁺/calmodulin kinase II (CaMKII) or protein kinase Mζ. Another level consists of feedback loops involving transcriptional, epigenetic and translational pathways, and autocrine actions of growth factors such as BDNF. Finally, at the neuronal network level, recurrent reactivation of cell assemblies encoding memories is likely to be essential for late maintenance of memory. These levels are not isolated, but linked by shared components of feedback loops. Here, we review characteristics of some commonly discussed feedback loops proposed to underlie the maintenance of memory and long-term synaptic plasticity, assess evidence for and against their necessity, and suggest experiments that could further delineate the dynamics of these feedback loops. We also discuss crosstalk between proposed loops, and ways in which such interaction can facilitate the rapidity and robustness of memory formation and storage.

Reciprocal White Matter Changes Associated With Copy Number Variation at 15q11.2 BP1-BP2: A Diffusion Tensor Imaging Study

BACKGROUND

The 15q11.2 BP1-BP2 cytogenetic region has been associated with learning and motor delays, autism, and schizophrenia. This region includes a gene that codes for the cytoplasmic FMR1 interacting protein 1 (CYFIP1). The CYFIP1 protein is involved in actin cytoskeletal dynamics and interacts with the fragile X mental retardation protein. Absence of fragile X mental retardation protein causes fragile X syndrome. Because abnormal white matter microstructure has been reported in both fragile X syndrome and psychiatric disorders, we looked at the impact of 15q11.2 BP1-BP2 dosage on white matter microstructure.

METHODS

Combining a brain-wide voxel-based approach and a regional-based analysis, we analyzed diffusion tensor imaging data from healthy individuals with the deletion (n = 30), healthy individuals with the reciprocal duplication (n = 27), and IQ-matched control subjects with no large copy number variants (n = 19), recruited from a large genotyped population sample.

RESULTS

We found global mirror effects (deletion > control > duplication) on fractional anisotropy. The deletion group showed widespread increased fractional anisotropy when compared with duplication. Regional analyses revealed a greater effect size in the posterior limb of the internal capsule and a tendency for decreased fractional anisotropy in duplication.

CONCLUSIONS

These results show a reciprocal effect of 15q11.2 BP1-BP2 on white matter microstructure, suggesting that reciprocal chromosomal imbalances may lead to opposite changes in brain structure. Findings in the deletion overlap with previous white matter differences reported in fragile X syndrome patients, suggesting common pathogenic mechanisms derived from disruptions of cytoplasmic CYFIP1-fragile X mental retardation protein complexes. Our data begin to identify specific components of the 15q11.2 BP1-BP2 phenotype and neurobiological mechanisms of potential relevance to the increased risk for disorder.

A Synaptic Perspective of Fragile X Syndrome and Autism Spectrum Disorders

Altered synaptic structure and function is a major hallmark of fragile X syndrome (FXS), autism spectrum disorders (ASDs), and other intellectual disabilities (IDs), which are therefore classified as synaptopathies. FXS and ASDs, while clinically and genetically distinct, share significant comorbidity, suggesting that there may be a common molecular and/or cellular basis, presumably at the synapse. In this article, we review brain architecture and synaptic pathways that are dysregulated in FXS and ASDs, including spine architecture, signaling in synaptic plasticity, local protein synthesis, (m)RNA modifications, and degradation. mRNA repression is a powerful mechanism for the regulation of synaptic structure and efficacy. We infer that there is no single pathway that explains most of the etiology and discuss new findings and the implications for future work directed at improving our understanding of the pathogenesis of FXS and related ASDs and the design of therapeutic strategies to ameliorate these disorders.

Protein synthesis levels are increased in a subset of individuals with fragile X syndrome

Fragile X syndrome (FXS) is a monogenic form of intellectual disability and autism spectrum disorder caused by the absence of the fragile X mental retardation protein (FMRP). In biological models for the disease, this leads to upregulated mRNA translation and as a consequence, deficits in synaptic architecture and plasticity. Preclinical studies revealed that pharmacological interventions restore those deficits, which are thought to mediate the FXS cognitive and behavioral symptoms. Here, we characterized the de novo rate of protein synthesis in patients with FXS and their relationship with clinical severity. We measured the rate of protein synthesis in fibroblasts derived from 32 individuals with FXS and from 17 controls as well as in fibroblasts and primary neurons of 27 Fmr1 KO mice and 20 controls. Here, we show that levels of protein synthesis are increased in fibroblasts of individuals with FXS and Fmr1 KO mice. However, this cellular phenotype displays a broad distribution and a proportion of fragile X individuals and Fmr1 KO mice do not show increased levels of protein synthesis, having measures in the normal range. Because the same Fmr1 KO animal measures in fibroblasts predict those in neurons we suggest the validity of this peripheral biomarker. Our study offers a potential explanation for the comprehensive drug development program undertaken thus far yielding negative results and suggests that a significant proportion, but not all individuals with FXS, may benefit from the reduction of excessive levels of protein synthesis.

Rare copy number variations affecting the synaptic gene DMXL2 in neurodevelopmental disorders

BACKGROUND

Ultra-rare genetic variants, including non-recurrent copy number variations (CNVs) affecting important dosage-sensitive genes, are important contributors to the etiology of neurodevelopmental disorders (NDDs). Pairing family-based whole-genome sequencing (WGS) with detailed phenotype data can enable novel gene associations in NDDs.

METHODS

We performed WGS of six members from a three-generation family, where three individuals each had a spectrum of features suggestive of a NDD. CNVs and sequence-level variants were identified and further investigated in disease and control databases.

RESULTS

We identified a novel 252-kb deletion at 15q21 that overlaps the synaptic gene DMXL2 and the gene GLDN. The microdeletion segregated in NDD-affected individuals. Additional rare inherited and de novo sequence-level variants were found that may also be involved, including a missense change in GRIK5. Multiple CNVs and loss-of-function sequence variants affecting DMXL2 were discovered in additional unrelated individuals with a range of NDDs.

CONCLUSIONS

Disruption of DMXL2 may predispose to NDDs including autism spectrum disorder. The robust interpretation of private variants requires a multifaceted approach that incorporates multigenerational pedigrees and genome-wide and population-scale data.

Disruption of mTOR and MAPK pathways correlates with severity in idiopathic autism

The molecular signature underlying autism spectrum disorder remains largely unknown. This study identifies differential expression of mTOR and MAPK pathways in patients affected by mild and severe idiopathic autism. A total of 55 subjects were enrolled, of which 22 were typically developing individuals and 33 were patients aged between 3 and 11 years, with autism spectrum disorder. A detailed history, including physical examination, developmental evaluation, mental health history and autism diagnostic observation schedule were performed for each patient. Components of the mTOR and MAPK signalling pathways were analysed from peripheral blood at the protein level. Patients were then stratified according to their clinical phenotypes, and the molecular profiling was analysed in relation to the degree of autism severity. In this cohort of patients, we identified increased activity of mTOR and the MAPK pathways, key regulators of synaptogenesis and protein synthesis. Specifically, rpS6, p-eIF4E, TSC1 and p-MNK1 expression discriminated patients according to their clinical diagnosis, suggesting that components of protein synthesis signalling pathways might constitute a molecular signature of clinical severity in autism spectrum disorder.

Behavioral training rescues motor deficits in Cyfip1 haploinsufficiency mouse model of autism spectrum disorders

Deletions in the 15q11.2 region of the human genome are associated with neurobehavioral deficits, and motor development delay, as well as in some cases, symptoms of autism or schizophrenia. The cytoplasmic FMRP-interacting protein 1 (CYFIP1) is one of the four genes contained within this locus and has been associated with other genetic forms of autism spectrum disorders (ASD). In mice, Cyfip1 haploinsufficiency leads to alteration of dendritic spine morphology and defects in synaptic plasticity, two pathophysiological hallmarks of mouse models of ASD. At the behavioral level, however, Cyfip1 haploinsufficiency leads to minor phenotypes, not directly relevant for 15q11.2 deletion syndrome or ASD. A fundamental question is whether neuronal phenotypes caused by the mutation of Cyfip1 are relevant for the human condition. Here, we describe a synaptic cluster of ASD-associated proteins centered on CYFIP1 and the adhesion protein Neuroligin-3. Cyfip1 haploinsufficiency in mice led to decreased dendritic spine density and stability associated with social behavior and motor learning phenotypes. Behavioral training early in development resulted in alleviating the motor learning deficits caused by Cyfip1 haploinsufficiency. Altogether, these data provide new insight into the neuronal and behavioral phenotypes caused by Cyfip1 mutation and proof-of-concept for the development of a behavioral therapy to treat phenotypes associated with 15q11.2 syndromes and ASD.

Fragile-X Syndrome Is Associated With NMDA Receptor Hypofunction and Reduced Dendritic Complexity in Mature Dentate Granule Cells

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability. It is caused by the overexpansion of cytosine-guanine-guanine (CGG) trinucleotide in Fmr1 gene, resulting in complete loss of the fragile X mental retardation protein (FMRP). Previous studies using Fmr1 knockout (Fmr1 KO) mice have suggested that a N-methyl-D-aspartate receptors (NMDAR) hypofunction in the hippocampal dentate gyrus may partly contribute to cognitive impairments in FXS. Since activation of NMDAR plays an important role in dendritic arborization during neuronal development, we examined whether deficits in NMDAR function are associated with alterations in dendritic complexity in the hippocampal dentate region. The dentate granule cell layer (GCL) presents active postnatal neurogenesis, and consists of a heterogenous neuronal population with gradient ages from the superficial to its deep layer. Here, we show that neurons with multiple primary dendrites that reside in the outer GCL of Fmr1 KO mice display significantly smaller NMDAR excitatory post-synaptic currents (EPSCs) and a higher α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) to NMDA ratio in comparison to their wild-type counterparts. These deficits were associated with a significant decrease in dendritic complexity, with both dendritic length and number of intersections being significantly reduced. In contrast, although neurons with a single primary dendrite resided in the inner GCL of Fmr1 KO mice had a trend toward a reduction in NMDAR EPSCs and a higher AMPA/NMDA ratio, no alterations were found in dendritic complexity at this developmental stage. Our data indicate that the loss of FMRP causes NMDAR deficits and reduced dendritic complexity in granule neurons with multiple primary dendrites which are thought to be more mature in the GCL.

Plasticity of Spine Structure: Local Signaling, Translation and Cytoskeletal Reorganization

Dendritic spines are small protrusive structures on dendritic surfaces, and function as postsynaptic compartments for excitatory synapses. Plasticity of spine structure is associated with many forms of long-term neuronal plasticity, learning and memory. Inside these small dendritic compartments, biochemical states and protein-protein interactions are dynamically modulated by synaptic activity, leading to the regulation of protein synthesis and reorganization of cytoskeletal architecture. This in turn causes plasticity of structure and function of the spine. Technical advances in monitoring molecular behaviors in single dendritic spines have revealed that each signaling pathway is differently regulated across multiple spatiotemporal domains. The spatial pattern of signaling activity expands from a single spine to the nearby dendritic area, dendritic branch and the nucleus, regulating different cellular events at each spatial scale. Temporally, biochemical events are typically triggered by short Ca²⁺ pulses (~10–100 ms). However, these signals can then trigger activation of downstream protein cascades that can last from milliseconds to hours. Recent imaging studies provide many insights into the biochemical processes governing signaling events of molecular assemblies at different spatial localizations. Here, we highlight recent findings of signaling dynamics during synaptic plasticity and discuss their roles in long-term structural plasticity of dendritic spines.

Actin cytoskeleton dynamics in stem cells from autistic individuals

Several lines of indirect evidence, such as mutations or dysregulated expression of genes related to cytoskeleton, have suggested that cytoskeletal dynamics, a process essential for axons and dendrites development, is compromised in autism spectrum disorders (ASD). However, no study has yet examined whether cytoskeleton dynamics is functionally altered in cells from ASD patients. Here we investigated the regulation of actin cytoskeleton dynamics in stem cells from human exfoliated deciduous teeth (SHEDs) of 13 ASD patients and 8 control individuals by inducing actin filament depolymerization and then measuing their reconstruction upon activation of the RhoGTPases Rac, Cdc42 or RhoA. We observed that stem cells from seven ASD individuals (53%) presented altered dymanics of filament reconstruction, including a patient recently studied by our group whose iPSC-derived neuronal cells show shorten and less arborized neurites. We also report potentially pathogenic genetic variants that might be related to the alterations in actin repolymerization dynamics observed in some patient-derived cells. Our results suggest that, at least for a subgroup of ASD patients, the dynamics of actin polymerization is impaired, which might be ultimately leading to neuronal abnormalities.

An Intellectual Disability-Related Missense Mutation in Rac1 Prevents LTP Induction

The small GTPase Rac1 promotes actin polymerization and plays a critical and increasingly appreciated role in the development and plasticity of glutamatergic synapses. Growing evidence suggests that disruption of the Rac1 signaling pathway at glutamatergic synapses contributes to Autism Spectrum Disorder/intellectual disability (ASD/ID)-related behaviors seen in animal models of ASD/ID. Rac1 has also been proposed as a strong candidate of convergence for many factors implicated in the development of ASD/ID. However, the effects of ASD/ID-related mutations in Rac1 itself have not been explored in neurons. Here, we investigate a recently reported de novo missense mutation in Rac1 found in an individual with severe ID. Our modeling predicts that this mutation will strongly inhibit Rac1 activation by occluding Rac1's GTP binding pocket. Indeed, we find that this de novo mutation prevents Rac1 function and results in a selective reduction in synaptic AMPA receptor function. Furthermore, this mutation prevents the induction of long-term potentiation (LTP), the cellular mechanism underlying learning and memory formation. Together, our findings strongly suggest that this mutation contributes to the development of ID in this individual. This research demonstrates the importance of Rac1 in synaptic function and plasticity and contributes to a growing body of evidence pointing to dysregulation of actin polymerization at glutamatergic synapses as a contributing factor to ASD/ID.

Handling FMRP and its molecular partners: Structural insights into Fragile X Syndrome

Fragile X Mental Retardation Protein (FMRP) is a RNA-binding protein (RBP) known to control different steps of mRNA metabolism, even though its complete function is not fully understood yet. Lack or mutations of FMRP lead to Fragile X Syndrome (FXS), the most common form of inherited intellectual disability and a leading monogenic cause of autism spectrum disorder (ASD). It is well established that FMRP has a multi-domain architecture, a feature that allows this RBP to be engaged in a large interaction network with numerous proteins and mRNAs or non-coding RNAs. Insights into the three-dimensional (3D) structure of parts of its three domains (N-terminus, central domain and C-terminus) were obtained using Nuclear Magnetic Resonance and X-ray diffraction, but the complete 3D arrangement of each domain with respect to the others is still missing. Here, we review the structural features of FMRP and of the network of its protein and RNA interactions. Understanding these aspects is the first necessary step towards the design of novel compounds for new therapeutic interventions in FXS.

Transcriptomics analysis reveals new insights in E171-induced molecular alterations in a mouse model of colon cancer

Titanium dioxide as a food additive (E171) has been demonstrated to facilitate growth of chemically induced colorectal tumours in vivo and induce transcriptomic changes suggestive of an immune system impairment and cancer development. The present study aimed to investigate the molecular mechanisms behind the tumour stimulatory effects of E171 in combination with azoxymethane (AOM)/dextran sodium sulphate (DSS) and compare these results to a recent study performed under the same conditions with E171 only. BALB/c mice underwent exposure to 5 mg/kg_bw/day of E171 by gavage for 2, 7, 14, and 21 days. Whole genome mRNA microarray analyses on the distal colon were performed. The results show that E171 induced a downregulation of genes involved in the innate and adaptive immune system, suggesting impairment of this system. In addition, over time, signalling genes involved in colorectal cancer and other types of cancers were modulated. In relation to cancer development, effects potentially associated with oxidative stress were observed through modulation of genes related to antioxidant production. E171 affected genes involved in biotransformation of xenobiotics which can form reactive intermediates resulting in toxicological effects. These transcriptomics data reflect the early biological responses induced by E171 which precede tumour formation in an AOM/DSS mouse model.

Rho GTPases in Intellectual Disability: From Genetics to Therapeutic Opportunities

Rho-class small GTPases are implicated in basic cellular processes at nearly all brain developmental steps, from neurogenesis and migration to axon guidance and synaptic plasticity. GTPases are key signal transducing enzymes that link extracellular cues to the neuronal responses required for the construction of neuronal networks, as well as for synaptic function and plasticity. Rho GTPases are highly regulated by a complex set of activating (GEFs) and inactivating (GAPs) partners, via protein:protein interactions (PPI). Misregulated RhoA, Rac1/Rac3 and cdc42 activity has been linked with intellectual disability (ID) and other neurodevelopmental conditions that comprise ID. All genetic evidences indicate that in these disorders the RhoA pathway is hyperactive while the Rac1 and cdc42 pathways are consistently hypoactive. Adopting cultured neurons for in vitro testing and specific animal models of ID for in vivo examination, the endophenotypes associated with these conditions are emerging and include altered neuronal networking, unbalanced excitation/inhibition and altered synaptic activity and plasticity. As we approach a clearer definition of these phenotype(s) and the role of hyper- and hypo-active GTPases in the construction of neuronal networks, there is an increasing possibility that selective inhibitors and activators might be designed via PPI, or identified by screening, that counteract the misregulation of small GTPases and result in alleviation of the cognitive condition. Here we review all knowledge in support of this possibility.

Dendritic spine actin cytoskeleton in autism spectrum disorder

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

Morphoregulatory functions of the RNA-binding motif protein 3 in cell spreading, polarity and migration

RNA-binding proteins are emerging as key regulators of transitions in cell morphology. The RNA-binding motif protein 3 (RBM3) is a cold-inducible RNA-binding protein with broadly relevant roles in cellular protection, and putative functions in cancer and development. Several findings suggest that RBM3 has morphoregulatory functions germane to its roles in these contexts. For example, RBM3 helps maintain the morphological integrity of cell protrusions during cell stress and disease. Moreover, it is highly expressed in migrating neurons of the developing brain and in cancer invadopodia, suggesting roles in migration. We here show that RBM3 regulates cell polarity, spreading and migration. RBM3 was present in spreading initiation centers, filopodia and blebs that formed during cell spreading in cell lines and primary myoblasts. Reducing RBM3 triggered exaggerated spreading, increased RhoA expression, and a loss of polarity that was rescued by Rho kinase inhibition and overexpression of CRMP2. High RBM3 expression enhanced the motility of cells migrating by a mesenchymal mode involving extension of long protrusions, whereas RBM3 knockdown slowed migration, greatly reducing the ability of cells to extend protrusions and impairing multiple processes that require directional migration. These data establish novel functions of RBM3 of potential significance to tissue repair, metastasis and development.

The Role of Actin Cytoskeleton in Dendritic Spines in the Maintenance of Long-Term Memory

Evidence indicates that long-term memory formation involves alterations in synaptic efficacy produced by modifications in neural transmission and morphology. However, it is not clear how such alterations induced by learning, that encode memory, are maintained over long period of time to preserve long-term memory. This is especially intriguing as the half-life of most of the proteins that underlie such changes is usually in the range of hours to days and these proteins may change their location over time. In this review we describe studies that indicate the involvement of dendritic spines in memory formation and its maintenance. These studies show that learning leads to changes in the number and morphology of spines. Disruption in spines morphology or manipulations that lead to alteration in their number after consolidation are associated with impairment in memory maintenance. We further ask how changes in dendritic spines morphology, induced by learning and reputed to encode memory, are maintained to preserve long-term memory. We propose a mechanism, based on studies described in the review, whereby the actin cytoskeleton and its regulatory proteins involved in the initial alteration in spine morphology induced by learning are also essential for spine structural stabilization that maintains long-term memory. In this model glutamate receptors and other synaptic receptors activation during learning leads to the creation of new actin cytoskeletal scaffold leading to changes in spines morphology and memory formation. This new actin cytoskeletal scaffold is preserved beyond actin and its regulatory proteins turnover and dynamics by active stabilization of the level and activity of actin regulatory proteins within these memory spines.

Phosphorylation of CYFIP2, a component of the WAVE-regulatory complex, regulates dendritic spine density and neurite outgrowth in cultured hippocampal neurons potentially by affecting the complex assembly

Actin dynamics is a critical mechanism underlying many cellular processes in neurons. The heteropentameric WAVE-regulatory complex (WRC), consisting of WAVE, CYFIP1/2, Nap, Abi, and HSPC300, is a key regulator of actin dynamics that activates the Arp2/3 complex to initiate actin polymerization and branching. The WRC is basally inactive because of intermolecular interactions among the components, which can be modulated by bindings of phospholipids and Rac1, and phosphorylations of WAVE and Abi. However, the phosphorylation of other components of WRC and their functional significance remain largely unknown. To address this issue, we focused on CYFIP1/2, in which we found two brain-specific phosphorylation sites (S582 of CYFIP2 and T1068/T1067 of CYFIP1/2) from a publicly available phosphoproteome database. To understand their functional effects, we overexpressed wild-type, phospho-blocking, or phospho-mimetic mutants of CYFIP2 in cultured hippocampal neurons, and found that only T1067A CYFIP2 decreased the density of stubby spines. Moreover, overexpression of wild-type CYFIP2 increased neurite length, but T1067A did not exert this effect. To understand the mechanism, we modeled CYFIP2 phosphorylation in the crystal structure of WRC and found that T1067 phosphorylation could weaken the interaction between CYFIP2 and Nap1 by inducing conformational changes of CYFIP2 α-helical bundles. In the co-immunoprecipitation assay, however, wild-type, T1067A, and T1067E CYFIP2 showed similar interaction levels to Nap1, suggesting that T1067 phosphorylation alone is not sufficient to disrupt the interaction. Considering that the activation of WRC requires disassembly of the complex, our results suggest that T1067 phosphorylation, together with other factors, could contribute toward the activation process.

CNV biology in neurodevelopmental disorders

Copy number variants (CNVs), characterized in recent years by cutting-edge technology, add complexity to our knowledge of the human genome. CNVs contribute not only to human diversity but also to different kinds of diseases including neurodevelopmental delay, autism spectrum disorder and neuropsychiatric diseases. Interestingly, many pathogenic CNVs are shared among these diseases. Studies suggest that pathophysiology of disease may not be simply attributed to a single driver gene within a CNV but also that multifactorial effects may be important. Gene expression and the resulting phenotypes may also be affected by epigenetic alteration and chromosomal structural changes. Combined with human genetics and systems biology, integrative research by multi-dimensional approaches using animal and cell models of CNVs are expected to further understanding of pathophysiological mechanisms of neurodevelopmental disorders and neuropsychiatric disorders.

Fundamental Elements in Autism: From Neurogenesis and Neurite Growth to Synaptic Plasticity

Autism spectrum disorder (ASD) is a set of neurodevelopmental disorders with a high prevalence and impact on society. ASDs are characterized by deficits in both social behavior and cognitive function. There is a strong genetic basis underlying ASDs that is highly heterogeneous; however, multiple studies have highlighted the involvement of key processes, including neurogenesis, neurite growth, synaptogenesis and synaptic plasticity in the pathophysiology of neurodevelopmental disorders. In this review article, we focus on the major genes and signaling pathways implicated in ASD and discuss the cellular, molecular and functional studies that have shed light on common dysregulated pathways using in vitro, in vivo and human evidence.

Highlights

Autism spectrum disorder (ASD) has a prevalence of 1 in 68 children in the United States.

ASDs are highly heterogeneous in their genetic basis.

ASDs share common features at the cellular and molecular levels in the brain.

Most ASD genes are implicated in neurogenesis, structural maturation, synaptogenesis and function.