Cell-Type-Specific Translation Profiling Reveals a Novel Strategy for Treating Fragile X Syndrome

Exploring microbiota-gut-brain axis biomarkers linked to autism spectrum disorder in prenatally chlorpyrifos-exposed Fmr1 knock-out and wild-type male rats

Fmr1 (fragile X messenger ribonucleoprotein 1)-knockout (KO) rats, modeling the human Fragile X Syndrome (FXS), are of particular interest for exploring the ASD-like phenotype in preclinical studies. Gestational exposure to chlorpyrifos (CPF) has been associated with ASD diagnosis in humans and ASD-like behaviors in rodents and linked to the microbiota-gut-brain axis. In this study, we have used both Fmr1-KO and wild-type male rats (F2 generation) at postnatal days (PND) 7 and 40 obtained after F1 pregnant females were randomly exposed to 1 mg/kg/mL/day of CPF or vehicle. A nuclear magnetic resonance (NMR) metabolomics approach together with gene expression profiles of these F2 generation rats were employed to analyze different brain regions (such as prefrontal cortex, hippocampus, and cerebellum), whole large intestine (at PND7) and gut content (PND40). The statistical comparison of each matrix spectral profile unveiled tissue-specific metabolic fingerprints. Significant variations in some biomarker levels were detected among brain tissues of different genotypes, including taurine, myo-inositol, and 3-hydroxybutyric acid, and exposure to CPF induced distinct metabolic alterations, particularly in serine and myo-inositol. Additionally, this study provides a set of metabolites associated with gastrointestinal dysfunction in ASD, encompassing several amino acids, choline-derived compounds, bile acids, and sterol molecules. In terms of gene expression, genotype and gestational exposure to CPF had only minimal effects on decarboxylase 2 (gad2) and cholinergic receptor muscarinic 2 (chrm2) genes.

Translational modulator ISRIB alleviates synaptic and behavioral phenotypes in Fragile X syndrome

Fragile X syndrome (FXS) is caused by the loss of fragile X messenger ribonucleoprotein (FMRP), a translational regulator that binds the transcripts of proteins involved in synaptic function and plasticity. Dysregulated protein synthesis is a central effect of FMRP loss, however, direct translational modulation has not been leveraged in the treatment of FXS. Thus, we examined the effect of the translational modulator integrated stress response inhibitor (ISRIB) in treating synaptic and behavioral symptoms of FXS. We show that FMRP loss dysregulates synaptic protein abundance, stabilizing dendritic spines through increased PSD-95 levels while preventing spine maturation through reduced glutamate receptor accumulation, thus leading to the formation of dense, immature dendritic spines, characteristic of FXS patients and Fmr1 knockout (KO) mice. ISRIB rescues these deficits and improves social recognition in Fmr1 KO mice. These findings highlight the therapeutic potential of targeting core translational mechanisms in FXS and neurodevelopmental disorders more broadly.

Striatal insights: a cellular and molecular perspective on repetitive behaviors in pathology

Animals often behave repetitively and predictably. These repetitive behaviors can have a component that is learned and ingrained as habits, which can be evolutionarily advantageous as they reduce cognitive load and the expenditure of attentional resources. Repetitive behaviors can also be conscious and deliberate, and may occur in the absence of habit formation, typically when they are a feature of normal development in children, or neuropsychiatric disorders. They can be considered pathological when they interfere with social relationships and daily activities. For instance, people affected by obsessive-compulsive disorder, autism spectrum disorder, Huntington's disease and Gilles de la Tourette syndrome can display a wide range of symptoms like compulsive, stereotyped and ritualistic behaviors. The striatum nucleus of the basal ganglia is proposed to act as a master regulator of these repetitive behaviors through its circuit connections with sensorimotor, associative, and limbic areas of the cortex. However, the precise mechanisms within the striatum, detailing its compartmental organization, cellular specificity, and the intricacies of its downstream connections, remain an area of active research. In this review, we summarize evidence across multiple scales, including circuit-level, cellular, and molecular dimensions, to elucidate the striatal mechanisms underpinning repetitive behaviors and offer perspectives on the implicated disorders. We consider the close relationship between behavioral output and transcriptional changes, and thereby structural and circuit alterations, including those occurring through epigenetic processes.

Variable expression of MECP2, CDKL5, and FMR1 in the human brain: Implications for gene restorative therapies

MECP2, CDKL5, and FMR1 are three X-linked neurodevelopmental genes associated with Rett, CDKL5-, and fragile-X syndrome, respectively. These syndromes are characterized by distinct constellations of severe cognitive and neurobehavioral anomalies, reflecting the broad but unique expression patterns of each of the genes in the brain. As these disorders are not thought to be neurodegenerative and may be reversible, a major goal has been to restore expression of the functional proteins in the patient's brain. Strategies have included gene therapy, gene editing, and selective Xi-reactivation methodologies. However, tissue penetration and overall delivery to various regions of the brain remain challenging for each strategy. Thus, gaining insights into how much restoration would be required and what regions/cell types in the brain must be targeted for meaningful physiological improvement would be valuable. As a step toward addressing these questions, here we perform a meta-analysis of single-cell transcriptomics data from the human brain across multiple developmental stages, in various brain regions, and in multiple donors. We observe a substantial degree of expression variability for MECP2, CDKL5, and FMR1 not only across cell types but also between donors. The wide range of expression may help define a therapeutic window, with the low end delineating a minimum level required to restore physiological function and the high end informing toxicology margin. Finally, the inter-cellular and inter-individual variability enable identification of co-varying genes and will facilitate future identification of biomarkers.

Growth-suppressor microRNAs mediate synaptic overgrowth and behavioral deficits in Fragile X mental retardation protein deficiency

Abnormal neuronal and synapse growth is a core pathology resulting from deficiency of the Fragile X mental retardation protein (FMRP), but molecular links underlying the excessive synthesis of key synaptic proteins remain incompletely defined. We find that basal brain levels of the growth suppressor let-7 microRNA (miRNA) family are selectively lowered in FMRP-deficient mice and activity-dependent let-7 downregulation is abrogated. Primary let-7 miRNA transcripts are not altered in FMRP-deficiency and posttranscriptional misregulation occurs downstream of MAPK pathway induction and elevation of Lin28a, a let-7 biogenesis inhibitor. Neonatal restoration of brain let-7 miRNAs corrects hallmarks of FMRP-deficiency, including dendritic spine overgrowth and social and cognitive behavioral deficits, in adult mice. Blockade of MAPK hyperactivation normalizes let-7 miRNA levels in both brain and peripheral blood plasma from Fmr1 KO mice. These results implicate dysregulated let-7 miRNA biogenesis in the pathogenesis of FMRP-deficiency, and highlight let-7 miRNA-based strategies for future biomarker and therapeutic development.

Intracellular signaling pathways of muscarinic acetylcholine receptor-mediated detrusor muscle contractions

Acetylcholine plays an essential role in the regulation of detrusor muscle contractions, and antimuscarinics are widely used in the management of overactive bladder syndrome. However, several adverse effects limit their application and patients' compliance. Thus, this study aimed to further analyze the signal transduction of M2 and M3 receptors in the murine urinary bladder to eventually find more specific therapeutic targets. Experiments were performed on adult male wild-type, M2, M3, M2/M3, or Gαq/11 knockout (KO), and pertussis toxin (PTX)-treated mice. Contraction force and RhoA activity were measured in the urinary bladder smooth muscle (UBSM). Our results indicate that carbamoylcholine (CCh)-induced contractions were associated with increased activity of RhoA and were reduced in the presence of the Rho-associated kinase (ROCK) inhibitor Y-27632 in UBSM. CCh-evoked contractile responses and RhoA activation were markedly reduced in detrusor strips lacking either M2 or M3 receptors and abolished in M2/M3 KO mice. Inhibition of Gαi-coupled signaling by PTX treatment shifted the concentration-response curve of CCh to the right and diminished RhoA activation. CCh-induced contractile responses were markedly decreased in Gαq/11 KO mice; however, RhoA activation was unaffected. In conclusion, cholinergic detrusor contraction and RhoA activation are mediated by both M2 and M3 receptors. Furthermore, whereas both Gαi and Gαq/11 proteins mediate UBSM contraction, the activation at the RhoA-ROCK pathway appears to be linked specifically to Gαi. These findings may aid the identification of more specific therapeutic targets for bladder dysfunctions.NEW & NOTEWORTHY Muscarinic acetylcholine receptors are of utmost importance in physiological regulation of micturition and also in the development of voiding disorders. We demonstrate that the RhoA-Rho-associated kinase (ROCK) pathway plays a crucial role in contractions induced by cholinergic stimulation in detrusor muscle. Activation of RhoA is mediated by both M2 and M3 receptors as well as by Gi but not Gq/11 proteins. The Gi-RhoA-ROCK pathway may provide a novel therapeutic target for overactive voiding disorders.

Excitatory neuron-specific suppression of the integrated stress response contributes to autism-related phenotypes in fragile X syndrome

Dysregulation of protein synthesis is one of the key mechanisms underlying autism spectrum disorder (ASD). However, the role of a major pathway controlling protein synthesis, the integrated stress response (ISR), in ASD remains poorly understood. Here, we demonstrate that the main arm of the ISR, eIF2α phosphorylation (p-eIF2α), is suppressed in excitatory, but not inhibitory, neurons in a mouse model of fragile X syndrome (FXS; Fmr1-/y). We further show that the decrease in p-eIF2α is mediated via activation of mTORC1. Genetic reduction of p-eIF2α only in excitatory neurons is sufficient to increase general protein synthesis and cause autism-like behavior. In Fmr1-/y mice, restoration of p-eIF2α solely in excitatory neurons reverses elevated protein synthesis and rescues autism-related phenotypes. Thus, we reveal a previously unknown causal relationship between excitatory neuron-specific translational control via the ISR pathway, general protein synthesis, and core phenotypes reminiscent of autism in a mouse model of FXS.

Improvement of sensory deficits in fragile X mice by increasing cortical interneuron activity after the critical period

Changes in the function of inhibitory interneurons (INs) during cortical development could contribute to the pathophysiology of neurodevelopmental disorders. Using all-optical in vivo approaches, we find that parvalbumin (PV) INs and their immature precursors are hypoactive and transiently decoupled from excitatory neurons in postnatal mouse somatosensory cortex (S1) of Fmr1 KO mice, a model of fragile X syndrome (FXS). This leads to a loss of parvalbumin INs (PV-INs) in both mice and humans with FXS. Increasing the activity of future PV-INs in neonatal Fmr1 KO mice restores PV-IN density and ameliorates transcriptional dysregulation in S1, but not circuit dysfunction. Critically, administering an allosteric modulator of Kv3.1 channels after the S1 critical period does rescue circuit dynamics and tactile defensiveness. Symptoms in FXS and related disorders could be mitigated by targeting PV-INs.

Increased degradation of FMRP contributes to neuronal hyperexcitability in tuberous sclerosis complex

Autism spectrum disorder (ASD) is a highly prevalent neurodevelopmental disorder, but new therapies have been impeded by a lack of understanding of the pathological mechanisms. Tuberous sclerosis complex (TSC) and fragile X syndrome are associated with alterations in the mechanistic target of rapamycin (mTOR) and fragile X messenger ribonucleoprotein 1 (FMRP), which have been implicated in the development of ASD. Previously, we observed that transcripts associated with FMRP were down-regulated in TSC2-deficient neurons. In this study, we find that FMRP turnover is dysregulated in TSC2-deficient rodent primary neurons and human induced pluripotent stem cell (iPSC)-derived neurons and is dependent on the E3 ubiquitin ligase anaphase-promoting complex. We also demonstrate that overexpression of FMRP can partially rescue hyperexcitability in TSC2-deficient iPSC-derived neurons. These data indicate that FMRP dysregulation represents an important pathological mechanism in the development of abnormal neuronal activity in TSC and illustrate a molecular convergence between these two neurogenetic disorders.

Cell-type-specific disruption of cortico-striatal circuitry drives repetitive patterns of behavior in fragile X syndrome model mice

Individuals with fragile X syndrome (FXS) are frequently diagnosed with autism spectrum disorder (ASD), including increased risk for restricted and repetitive behaviors (RRBs). Consistent with observations in humans, FXS model mice display distinct RRBs and hyperactivity that are consistent with dysfunctional cortico-striatal circuits, an area relatively unexplored in FXS. Using a multidisciplinary approach, we dissect the contribution of two populations of striatal medium spiny neurons (SPNs) in the expression of RRBs in FXS model mice. Here, we report that dysregulated protein synthesis at cortico-striatal synapses is a molecular culprit of the synaptic and ASD-associated motor phenotypes displayed by FXS model mice. Cell-type-specific translational profiling of the FXS mouse striatum reveals differentially translated mRNAs, providing critical information concerning potential therapeutic targets. Our findings uncover a cell-type-specific impact of the loss of fragile X messenger ribonucleoprotein (FMRP) on translation and the sequence of neuronal events in the striatum that drive RRBs in FXS.

mRNA transport, translation, and decay in adult mammalian central nervous system axons

Localized mRNA translation regulates synapse function and axon maintenance, but how compartment-specific mRNA repertoires are regulated is largely unknown. We developed an axonal transcriptome capture method that allows deep sequencing of metabolically labeled mRNAs from retinal ganglion cell axon terminals in mouse. Comparing axonal-to-somal transcriptomes and axonal translatome-to-transcriptome enables genome-wide visualization of mRNA transport and translation and unveils potential regulators tuned to each process. FMRP and TDP-43 stand out as key regulators of transport, and experiments in Fmr1 knockout mice validate FMRP's role in the axonal transportation of synapse-related mRNAs. Pulse-and-chase experiments enable genome-wide assessment of mRNA stability in axons and reveal a strong coupling between mRNA translation and decay. Measuring the absolute mRNA abundance per axon terminal shows that the adult axonal transcriptome is stably maintained by persistent transport. Our datasets provide a rich resource for unique insights into RNA-based mechanisms in maintaining presynaptic structure and function in vivo.

Excessive proteostasis contributes to pathology in fragile X syndrome

In fragile X syndrome (FX), the leading monogenic cause of autism, excessive neuronal protein synthesis is a core pathophysiology; however, an overall increase in protein expression is not observed. Here, we tested whether excessive protein synthesis drives a compensatory rise in protein degradation that is protective for FX mouse model (Fmr1^-/y) neurons. Surprisingly, although we find a significant increase in protein degradation through ubiquitin proteasome system (UPS), this contributes to pathological changes. Normalizing proteasome activity with bortezomib corrects excessive hippocampal protein synthesis and hyperactivation of neurons in the inferior colliculus (IC) in response to auditory stimulation. Moreover, systemic administration of bortezomib significantly reduces the incidence and severity of audiogenic seizures (AGS) in the Fmr1^-/y mouse, as does genetic reduction of proteasome, specifically in the IC. Together, these results identify excessive activation of the UPS pathway in Fmr1^-/y neurons as a contributor to multiple phenotypes that can be targeted for therapeutic intervention.

Excess ribosomal protein production unbalances translation in a model of Fragile X Syndrome

Dysregulated protein synthesis is a core pathogenic mechanism in Fragile X Syndrome (FX). The mGluR Theory of FX predicts that pathological synaptic changes arise from the excessive translation of mRNAs downstream of mGlu_1/5 activation. Here, we use a combination of CA1 pyramidal neuron-specific TRAP-seq and proteomics to identify the overtranslating mRNAs supporting exaggerated mGlu_1/5 -induced long-term synaptic depression (mGluR-LTD) in the FX mouse model (Fmr1^−/y). Our results identify a significant increase in the translation of ribosomal proteins (RPs) upon mGlu_1/5 stimulation that coincides with a reduced translation of long mRNAs encoding synaptic proteins. These changes are mimicked and occluded in Fmr1^−/y neurons. Inhibiting RP translation significantly impairs mGluR-LTD and prevents the length-dependent shift in the translating population. Together, these results suggest that pathological changes in FX result from a length-dependent alteration in the translating population that is supported by excessive RP translation.

Dysregulated protein synthesis is key contributor to Fragile X syndrome. Here the authors identify a relationship between ribosome expression and the translation of long mRNAs that contributes to synaptic weakening in a model of Fragile X syndrome.

Single cell transcriptomics reveals dysregulated cellular and molecular networks in a fragile X syndrome model

Despite advances in understanding the pathophysiology of Fragile X syndrome (FXS), its molecular basis is still poorly understood. Whole brain tissue expression profiles have proved surprisingly uninformative, therefore we applied single cell RNA sequencing to profile an FMRP deficient mouse model with higher resolution. We found that the absence of FMRP results in highly cell type specific gene expression changes that are strongest among specific neuronal types, where FMRP-bound mRNAs were prominently downregulated. Metabolic pathways including translation and respiration are significantly upregulated across most cell types with the notable exception of excitatory neurons. These effects point to a potential difference in the activity of mTOR pathways, and together with other dysregulated pathways, suggest an excitatory-inhibitory imbalance in the Fmr1-knock out cortex that is exacerbated by astrocytes. Our data demonstrate that FMRP loss affects abundance of key cellular communication genes that potentially affect neuronal synapses and provide a resource for interrogating the biological basis of this disorder.

Fragile X syndrome is a leading genetic cause of inherited intellectual disability and autism spectrum disorder. It results from the inactivation of a single gene, FMR1 and hence the loss of its encoded protein FMRP. Despite decades of intensive research, we still lack an overview of the molecular and biological consequences of the disease. Using single cell RNA sequencing, we profiled cells from the brain of healthy mice and of knock-out mice lacking the FMRP protein, a common model for this disease, to identify molecular changes that happen across different cell types. We find neurons are the most impacted cell type, where genes in multiple pathways are similarly impacted. This includes transcripts known to be bound by FMRP, which are collectively decreased only in neurons but not in other cell types. Our results show how the loss of FMRP affects the intricate interactions between different brain cell types, which could provide new perspectives to the development of therapeutic interventions.

eIF2-dependent translation initiation: Memory consolidation and disruption in Alzheimer's disease

Memory storage is a conserved survivability feature, present in virtually any complex species. During the last few decades, much effort has been devoted to understanding how memories are formed and which molecular switches define whether a memory should be stored for a short or a long period of time. Among these, de novo protein synthesis is known to be required for the conversion of short- to long-term memory. There are a number translational control pathways involved in synaptic plasticity and memory consolidation, including the phosphorylation of the eukaryotic initiation factor 2 alpha (eIF2α), which has emerged as a critical molecular switch for long-term memory consolidation. In this review, we discuss findings pertaining to the requirement of de novo protein synthesis to memory formation, how local dendritic and axonal translation is regulated in neurons, and how these can influence memory consolidation. We also highlight the importance of eIF2α-dependent translation initiation to synaptic plasticity and memory formation. Finally, we contextualize how aberrant phosphorylation of eIF2α contributes to Alzheimer's disease (AD) pathology and how preventing disruption of eIF2-dependent translation may be a therapeutic avenue for preventing and/or restoring memory loss in AD.

An Optimized Protocol for the Mapping of Cell Type-Specific Ribosome-Associated Transcript Isoforms from Small Mouse Brain Regions

Over the past years, technological advances in transcriptomics provided deep insights into gene expression programs and their role in tissue organization and cellular functions. The isolation of ribosome-associated transcripts is a powerful approach for deep profiling of cell type-specific transcripts, and particularly well-suited for quantitative analysis of transcript isoforms. This method employs conditional ribosome epitope-tagging in genetically defined cell types, followed by affinity-isolation of ribosome-associated mRNAs. Advantages of this approach are twofold: first, the method enables rapid retrieval of mRNAs without tissue dissociation and cell sorting steps. Second, capturing of ribosome-associated mRNAs, enriches for transcripts recruited for active translation, therefore providing an approximation to the cellular translatome. Here, we describe one application of this method for the identification of the transcriptome of excitatory neuronal cells (mitral and tufted cells) of the mouse olfactory bulb, through RiboTag isolation from the vGlut2-IRES-cre mouse line as genetic driver of endogenously tagged ribosome expression.

A Double Jeopardy: Loss of FMRP Results in DSB and Down-regulated DNA Repair

Our understanding of the molecular functions of the nucleocytoplasmic FMRP protein, which, if absent or dysfunctional, causes the fragile X syndrome (FXS), largely revolves around its involvement in protein translation regulation in the cytoplasm. Recent studies have begun honing in on the nuclear and genomic functions of FMRP. We have shown that during DNA replication stress, cells derived from FXS patients sustain increased level of R-loop formation and DNA double strand breaks. Here, we describe a transcriptomic analysis of these cells in order to identify those genes most impacted by the loss of FMRP with and without replication stress. We show that FMRP loss causes transcriptomic changes previously reported in untreated conditions. Importantly, we also show that replication stress, in addition to causing excess of DSB, results in down-regulation of transcription in virtually all DNA repair pathways. This finding suggests that despite normal DNA damage response, FXS patient-derived cells experience R-loop-induced DNA breakage as well as impaired DNA repair functions, effectively a double jeopardy. We suggest that it is imperative to deepen the understanding of the nuclear functions, particularly a genome protective function, of FMRP, which will lead to discoveries of novel therapeutic interventions for the FXS.

FMRP regulates mRNAs encoding distinct functions in the cell body and dendrites of CA1 pyramidal neurons

Neurons rely on translation of synaptic mRNAs in order to generate activity-dependent changes in plasticity. Here, we develop a strategy combining compartment-specific crosslinking immunoprecipitation (CLIP) and translating ribosome affinity purification (TRAP) in conditionally tagged mice to precisely define the ribosome-bound dendritic transcriptome of CA1 pyramidal neurons. We identify CA1 dendritic transcripts with differentially localized mRNA isoforms generated by alternative polyadenylation and alternative splicing, including many that have altered protein-coding capacity. Among dendritic mRNAs, FMRP targets were found to be overrepresented. Cell-type-specific FMRP-CLIP and TRAP in microdissected CA1 neuropil revealed 383 dendritic FMRP targets and suggests that FMRP differentially regulates functionally distinct modules in CA1 dendrites and cell bodies. FMRP regulates ~15–20% of mRNAs encoding synaptic functions and 10% of chromatin modulators, in the dendrite and cell body, respectively. In the absence of FMRP, dendritic FMRP targets had increased ribosome association, consistent with a function for FMRP in synaptic translational repression. Conversely, downregulation of FMRP targets involved in chromatin regulation in cell bodies suggests a role for FMRP in stabilizing mRNAs containing stalled ribosomes in this compartment. Together, the data support a model in which FMRP regulates the translation and expression of synaptic and nuclear proteins within different compartments of a single neuronal cell type.

The brain has over 100 billion neurons that together form vast networks to relay electrical signals. A neuron receives electrical signals from other neurons via branch-like structures known as dendrites. The signals then travel into the cell body of the neuron. If their sum reaches a threshold, they fire a new signal through a single outgoing projection known as the axon, which is connected to the dendrites of other neurons.

A single neuron has thousands of dendrites that each receive inputs from different axons, and it is thought that the strengthening and weakening of these dendritic connections enables us to learn and store memories.

Dendrites are filled with molecules known as messenger ribonucleic acids (mRNAs) that act as templates to make proteins. Axonal signals reaching the dendrites can trigger these mRNAs to make new proteins that strengthen or weaken the connections between the two neurons, which is believed to be necessary for generating long-term memories.

A protein called FMRP is found in both the cell body and dendrites and is able to bind to and regulate the ability of mRNAs to make proteins. A loss of the gene encoding FMRP is the most common cause of inherited intellectual disability and autism in humans, but it remains unclear precisely what role this protein plays in learning and memory.

Hale et al. used genetic and bioinformatics approaches to specifically study mRNAs in the dendrites and the cell body of a specific type of neuron involved in memory in mice. The experiments revealed that FMRP played different roles in the dendrites and cell body. In the dendrites, FMRP interacted with mRNAs encoding proteins that can change how the neuron responds to a signal from a neighboring neuron and may alter how strong the connections between the neurons are. On the other hand, FMRP in the cell body modulated the activities of mRNAs encoding proteins that in turn regulate the activities of genes.

These findings change the way we think about how memory may work by suggesting that groups of mRNAs encoding proteins with certain activities are found in distinct parts of a single neuron. These observations offer new ways to approach intellectual disabilities and autism spectrum disorder.

Chronic alcohol drinking persistently suppresses thalamostriatal excitation of cholinergic neurons to impair cognitive flexibility

Exposure to addictive substances impairs flexible decision making. Cognitive flexibility is mediated by striatal cholinergic interneurons (CINs). However, how chronic alcohol drinking alters cognitive flexibility through CINs remains unclear. Here, we report that chronic alcohol consumption and withdrawal impaired reversal of instrumental learning. Chronic alcohol consumption and withdrawal also caused a long-lasting (21 days) reduction of excitatory thalamic inputs onto CINs and reduced pause responses of CINs in the dorsomedial striatum (DMS). CINs are known to inhibit glutamatergic transmission in dopamine D1 receptor–expressing medium spiny neurons (D1-MSNs) but facilitate this transmission in D2-MSNs, which may contribute to flexible behavior. We discovered that chronic alcohol drinking impaired CIN-mediated inhibition in D1-MSNs and facilitation in D2-MSNs. Importantly, in vivo optogenetic induction of long-term potentiation of thalamostriatal transmission in DMS CINs rescued alcohol-induced reversal learning deficits. These results demonstrate that chronic alcohol drinking reduces thalamic excitation of DMS CINs, compromising their regulation of glutamatergic transmission in MSNs, which may contribute to alcohol-induced impairment of cognitive flexibility. These findings provide a neural mechanism underlying inflexible drinking in alcohol use disorder.

FMRP-Driven Neuropathology in Autistic Spectrum Disorder and Alzheimer's disease: A Losing Game

Fragile X mental retardation protein (FMRP) is an RNA binding protein (RBP) whose absence is essentially associated to Fragile X Syndrome (FXS). As an RNA Binding Protein (RBP), FMRP is able to bind and recognize different RNA structures and the control of specific mRNAs is important for neuronal synaptic plasticity. Perturbations of this pathway have been associated with the autistic spectrum. One of the FMRP partners is the APP mRNA, the main protagonist of Alzheimer’s disease (AD), thereby regulating its protein level and metabolism. Therefore FMRP is associated to two neurodevelopmental and age-related degenerative conditions, respectively FXS and AD. Although these pathologies are characterized by different features, they have been reported to share a number of common molecular and cellular players. The aim of this review is to describe the double-edged sword of FMRP in autism and AD, possibly allowing the elucidation of key shared underlying mechanisms and neuronal circuits. As an RBP, FMRP is able to regulate APP expression promoting the production of amyloid β fragments. Indeed, FXS patients show an increase of amyloid β load, typical of other neurological disorders, such as AD, Down syndrome, Parkinson’s Disease, etc. Beyond APP dysmetabolism, the two neurodegenerative conditions share molecular targets, brain circuits and related cognitive deficits. In this review, we will point out the potential common neuropathological pattern which needs to be addressed and we will hopefully contribute to clarifying the complex phenotype of these two neurorological disorders, in order to pave the way for a novel, common disease-modifying therapy.

Identification and functional modelling of plausibly causative cis-regulatory variants in a highly-selected cohort with X-linked intellectual disability

Identifying causative variants in cis-regulatory elements (CRE) in neurodevelopmental disorders has proven challenging. We have used in vivo functional analyses to categorize rigorously filtered CRE variants in a clinical cohort that is plausibly enriched for causative CRE mutations: 48 unrelated males with a family history consistent with X-linked intellectual disability (XLID) in whom no detectable cause could be identified in the coding regions of the X chromosome (chrX). Targeted sequencing of all chrX CRE identified six rare variants in five affected individuals that altered conserved bases in CRE targeting known XLID genes and segregated appropriately in families. Two of these variants, FMR1CRE and TENM1CRE, showed consistent site- and stage-specific differences of enhancer function in the developing zebrafish brain using dual-color fluorescent reporter assay. Mouse models were created for both variants. In male mice Fmr1CRE induced alterations in neurodevelopmental Fmr1 expression, olfactory behavior and neurophysiological indicators of FMRP function. The absence of another likely causative variant on whole genome sequencing further supported FMR1CRE as the likely basis of the XLID in this family. Tenm1CRE mice showed no phenotypic anomalies. Following the release of gnomAD 2.1, reanalysis showed that TENM1CRE exceeded the maximum plausible population frequency of a XLID causative allele. Assigning causative status to any ultra-rare CRE variant remains problematic and requires disease-relevant in vivo functional data from multiple sources. The sequential and bespoke nature of such analyses renders them time-consuming and challenging to scale for routine clinical use.

Loss of Tsc1 in cerebellar Purkinje cells induces transcriptional and translation changes in FMRP target transcripts

Tuberous sclerosis complex (TSC) is a genetic disorder that is associated with multiple neurological manifestations. Previously, we demonstrated that Tsc1 loss in cerebellar Purkinje cells (PCs) can cause altered social behavior in mice. Here, we performed detailed transcriptional and translational analyses of Tsc1-deficient PCs to understand the molecular alterations in these cells. We found that target transcripts of the Fragile X Mental Retardation Protein (FMRP) are reduced in mutant PCs with evidence of increased degradation. Surprisingly, we observed unchanged ribosomal binding for many of these genes using translating ribosome affinity purification. Finally, we found that multiple FMRP targets, including SHANK2, were reduced, suggesting that compensatory increases in ribosomal binding efficiency may be unable to overcome reduced transcript levels. These data further implicate dysfunction of FMRP and its targets in TSC and suggest that treatments aimed at restoring the function of these pathways may be beneficial.

Mechanisms regulating input-output function and plasticity of neurons in the absence of FMRP

The function of brain circuits relies on high-fidelity information transfer within neurons. Synaptic inputs arrive primarily at dendrites, where they undergo integration and summation throughout the somatodendritic domain, ultimately leading to the generation of precise patterns of action potentials. Emerging evidence suggests that the ability of neurons to transfer synaptic information and modulate their output is impaired in a number of neurodevelopmental disorders including Fragile X Syndrome. In this review we summarise recent findings that have revealed the pathophysiological and plasticity mechanisms that alter the ability of neurons in sensory and limbic circuits to reliably code information in the absence of FMRP. We examine which aspects of this transform may result directly from the loss of FMRP and those that a result from compensatory or homeostatic alterations to neuronal function. Dissection of the mechanisms leading to altered input-output function of neurons in the absence of FMRP and their effects on regulating neuronal plasticity throughout development could have important implications for potential therapies for Fragile X Syndrome, including directing the timing and duration of different treatment options.

FUS-ALS mutants alter FMRP phase separation equilibrium and impair protein translation

FUsed in Sarcoma (FUS) is a multifunctional RNA binding protein (RBP). FUS mutations lead to its cytoplasmic mislocalization and cause the neurodegenerative disease amyotrophic lateral sclerosis (ALS). Here, we use mouse and human models with endogenous ALS-associated mutations to study the early consequences of increased cytoplasmic FUS. We show that in axons, mutant FUS condensates sequester and promote the phase separation of fragile X mental retardation protein (FMRP), another RBP associated with neurodegeneration. This leads to repression of translation in mouse and human FUS-ALS motor neurons and is corroborated in vitro, where FUS and FMRP copartition and repress translation. Last, we show that translation of FMRP-bound RNAs is reduced in vivo in FUS-ALS motor neurons. Our results unravel new pathomechanisms of FUS-ALS and identify a novel paradigm by which mutations in one RBP favor the formation of condensates sequestering other RBPs, affecting crucial biological functions, such as protein translation.

Reciprocal control of translation and transcription in autism spectrum disorder

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by deficits in social communication and the presence of restricted patterns of interest and repetitive behaviors. ASD is genetically heterogeneous and is believed to be caused by both inheritable and de novo gene variations. Studies have revealed an extremely complex genetic landscape of ASD, favoring the idea that mutations in different clusters of genes interfere with interconnected downstream signaling pathways and circuitry, resulting in aberrant behavior. In this review, we describe a select group of candidate genes that represent both syndromic and non-syndromic forms of ASD and encode proteins that are important in transcriptional and translational regulation. We focus on the interplay between dysregulated translation and transcription in ASD with the hypothesis that dysregulation of each synthetic process triggers a feedback loop to act on the other, which ultimately exacerbates ASD pathophysiology. Finally, we summarize findings from interdisciplinary studies that pave the way for the investigation of the cooperative impact of different genes and pathways underlying the development of ASD.

Genetic removal of p70 S6K1 corrects coding sequence length-dependent alterations in mRNA translation in fragile X syndrome mice

Loss of the fragile X mental retardation protein (FMRP) causes fragile X syndrome (FXS). FMRP is widely thought to repress protein synthesis, but its translational targets and modes of control remain in dispute. We previously showed that genetic removal of p70 S6 kinase 1 (S6K1) corrects altered protein synthesis as well as synaptic and behavioral phenotypes in FXS mice. In this study, we examined the gene specificity of altered messenger RNA (mRNA) translation in FXS and the mechanism of rescue with genetic reduction of S6K1 by carrying out ribosome profiling and RNA sequencing on cortical lysates from wild-type, FXS, S6K1 knockout, and double knockout mice. We observed reduced ribosome footprint (RF) abundance in the majority of differentially translated genes in the cortices of FXS mice. We used molecular assays to discover evidence that the reduction in RF abundance reflects an increased rate of ribosome translocation, which is captured as a decrease in the number of translating ribosomes at steady state and is normalized by inhibition of S6K1. We also found that genetic removal of S6K1 prevented a positive-to-negative gradation of alterations in translation efficiencies (RF/mRNA) with coding sequence length across mRNAs in FXS mouse cortices. Our findings reveal the identities of dysregulated mRNAs and a molecular mechanism by which reduction of S6K1 prevents altered translation in FXS.

The molecular biology of FMRP: new insights into fragile X syndrome

Fragile X mental retardation protein (FMRP) is the product of the fragile X mental retardation 1 gene (FMR1), a gene that - when epigenetically inactivated by a triplet nucleotide repeat expansion - causes the neurodevelopmental disorder fragile X syndrome (FXS). FMRP is a widely expressed RNA-binding protein with activity that is essential for proper synaptic plasticity and architecture, aspects of neural function that are known to go awry in FXS. Although the neurophysiology of FXS has been described in remarkable detail, research focusing on the molecular biology of FMRP has only scratched the surface. For more than two decades, FMRP has been well established as a translational repressor; however, recent whole transcriptome and translatome analyses in mouse and human models of FXS have shown that FMRP is involved in the regulation of nearly all aspects of gene expression. The emerging mechanistic details of the mechanisms by which FMRP regulates gene expression may offer ways to design new therapies for FXS.

Nicotinic regulation of local and long-range input balance drives top-down attentional circuit maturation

Cognitive function depends on frontal cortex development; however, the mechanisms driving this process are poorly understood. Here, we identify that dynamic regulation of the nicotinic cholinergic system is a key driver of attentional circuit maturation associated with top-down frontal neurons projecting to visual cortex. The top-down neurons receive robust cholinergic inputs, but their nicotinic tone decreases following adolescence by increasing expression of a nicotinic brake, Lynx1 Lynx1 shifts a balance between local and long-range inputs onto top-down frontal neurons following adolescence and promotes the establishment of attentional behavior in adulthood. This key maturational process is disrupted in a mouse model of fragile X syndrome but was rescued by a suppression of nicotinic tone through the introduction of Lynx1 in top-down projections. Nicotinic signaling may serve as a target to rebalance local/long-range balance and treat cognitive deficits in neurodevelopmental disorders.

Cyfip1 Regulates SynGAP1 at Hippocampal Synapses

In humans, copy number variations in CYFIP1 appear to have sweeping physiological and structural consequences in the brain, either producing or altering the severity of intellectual disability, autism, and schizophrenia. Independently, SynGAP1 haploinsufficiency produces intellectual disability and, frequently, autism. Cyfip1 inhibits protein translation and promotes actin polymerization, and SynGAP1 is a synaptically localized Ras/Rap GAP. While these proteins are clearly distinct, studies investigating their functions in mice have shown that each regulates the maturation of synapses in the hippocampus and haploinsufficiency for either produces an exaggerated form of mGluR-dependent long-term depression, suggesting that some signaling pathways converge. In this study, we examined how Cyfip1 haploinsufficiency impacts SynGAP1 levels and localization, as well as potential sites for mechanistic interaction in mouse hippocampus. The data show that synaptic, but not total, levels of SynGAP1 in Cyfip1 ^+/- mice were abnormally low during early postnatal development and in adults. This may be in response to a shift in the balance of kinases that activate SynGAP1 as levels of Cdk5 were reduced and those of activated CaMKII were maintained in Cyfip1 ^+/- mice compared to wild-type mice. Alternatively, this could reflect altered actin dynamics as Rac1 activity in Cyfip1 ^+/- hippocampus was boosted significantly compared to wild-type mice, and levels of synaptic F-actin were generally enhanced due in part to an increase in the activity of the WAVE regulatory complex. Decreased synaptic SynGAP1 coupled with a CaMKII-mediated bias toward Rap1 inactivation at synapses is also consistent with increased levels of synaptic GluA2, increased AMPA receptor-mediated responses to stimulation, and increased levels of synaptic mGluR1/5 compared to wild-type mice. Collectively, our data suggest that Cyfip1 regulates SynGAP1 and the two proteins work coordinately at synapses to appropriately direct actin polymerization and GAP activity.

FMRP links optimal codons to mRNA stability in neurons

Fragile X syndrome (FXS) is caused by inactivation of the FMR1 gene and loss of encoded FMRP, an RNA binding protein that represses translation of some of its target transcripts. Here we use ribosome profiling and RNA sequencing to investigate the dysregulation of translation in the mouse brain cortex. We find that most changes in ribosome occupancy on hundreds of mRNAs are largely driven by dysregulation in transcript abundance. Many down-regulated mRNAs, which are mostly responsible for neuronal and synaptic functions, are highly enriched for FMRP binding targets. RNA metabolic labeling demonstrates that, in FMRP-deficient cortical neurons, mRNA down-regulation is caused by elevated degradation and is correlated with codon optimality. Moreover, FMRP preferentially binds mRNAs with optimal codons, suggesting that it stabilizes such transcripts through direct interactions via the translational machinery. Finally, we show that the paradigm of genetic rescue of FXS-like phenotypes in FMRP-deficient mice by deletion of the Cpeb1 gene is mediated by restoration of steady-state RNA levels and consequent rebalancing of translational homeostasis. Our data establish an essential role of FMRP in codon optimality-dependent mRNA stability as an important factor in FXS.

High neural activity accelerates the decline of cognitive plasticity with age in Caenorhabditis elegans

The ability to learn progressively declines with age. Neural hyperactivity has been implicated in impairing cognitive plasticity with age, but the molecular mechanisms remain elusive. Here, we show that chronic excitation of the Caenorhabditis elegans O₂-sensing neurons during ageing causes a rapid decline of experience-dependent plasticity in response to environmental O₂ concentration, whereas sustaining lower activity of O₂-sensing neurons retains plasticity with age. We demonstrate that neural activity alters the ageing trajectory in the transcriptome of O₂-sensing neurons, and our data suggest that high-activity neurons redirect resources from maintaining plasticity to sustaining continuous firing. Sustaining plasticity with age requires the K⁺-dependent Na⁺/Ca²⁺ (NCKX) exchanger, whereas the decline of plasticity with age in high-activity neurons acts through calmodulin and the scaffold protein Kidins220. Our findings demonstrate directly that the activity of neurons alters neuronal homeostasis to govern the age-related decline of neural plasticity and throw light on the mechanisms involved.

A Review of Molecular Imaging of Glutamate Receptors

Molecular imaging with positron emission tomography (PET) and single photon emission computed tomography (SPECT) is a well-established and important in vivo technique to evaluate fundamental biological processes and unravel the role of neurotransmitter receptors in various neuropsychiatric disorders. Specific ligands are available for PET/SPECT studies of dopamine, serotonin, and opiate receptors, but corresponding development of radiotracers for receptors of glutamate, the main excitatory neurotransmitter in mammalian brain, has lagged behind. This state of affairs has persisted despite the central importance of glutamate neurotransmission in brain physiology and in disorders such as stroke, epilepsy, schizophrenia, and neurodegenerative diseases. Recent years have seen extensive efforts to develop useful ligands for molecular imaging of subtypes of the ionotropic (N-methyl-D-aspartate (NMDA), kainate, and AMPA/quisqualate receptors) and metabotropic glutamate receptors (types I, II, and III mGluRs). We now review the state of development of radioligands for glutamate receptor imaging, placing main emphasis on the suitability of available ligands for reliable in vivo applications. We give a brief account of the radiosynthetic approach for selected molecules. In general, with the exception of ligands for the GluN2B subunit of NMDA receptors, there has been little success in developing radiotracers for imaging ionotropic glutamate receptors; failure of ligands for the PCP/MK801 binding site in vivo doubtless relates their dependence on the open, unblocked state of the ion channel. Many AMPA and kainite receptor ligands with good binding properties in vitro have failed to give measurable specific binding in the living brain. This may reflect the challenge of developing brain-penetrating ligands for amino acid receptors, compounded by conformational differences in vivo. The situation is better with respect to mGluR imaging, particularly for the mGluR5 subtype. Several successful PET ligands serve for investigations of mGluRs in conditions such as schizophrenia, depression, substance abuse and aging. Considering the centrality and diversity of glutamatergic signaling in brain function, we have relatively few selective and sensitive tools for molecular imaging of ionotropic and metabotropic glutamate receptors. Further radiopharmaceutical research targeting specific subtypes and subunits of the glutamate receptors may yet open up new investigational vistas with broad applications in basic and clinical research.

Single-cell and spatial transcriptomics enables probabilistic inference of cell type topography

The field of spatial transcriptomics is rapidly expanding, and with it the repertoire of available technologies. However, several of the transcriptome-wide spatial assays do not operate on a single cell level, but rather produce data comprised of contributions from a – potentially heterogeneous – mixture of cells. Still, these techniques are attractive to use when examining complex tissue specimens with diverse cell populations, where complete expression profiles are required to properly capture their richness. Motivated by an interest to put gene expression into context and delineate the spatial arrangement of cell types within a tissue, we here present a model-based probabilistic method that uses single cell data to deconvolve the cell mixtures in spatial data. To illustrate the capacity of our method, we use data from different experimental platforms and spatially map cell types from the mouse brain and developmental heart, which arrange as expected.

Alma Andersson et al. present a probabilistic framework that integrates single-cell and bulk spatial transcriptomics in order to spatially map cell types onto their respective tissues. They apply their method to the developing human heart and mouse brain to demonstrate the power of the technique.

Astroglial FMRP deficiency cell-autonomously up-regulates miR-128 and disrupts developmental astroglial mGluR5 signaling

The loss of fragile X mental retardation protein (FMRP) causes fragile X syndrome (FXS), the most common inherited intellectual disability. How the loss of FMRP alters protein expression and astroglial functions remains essentially unknown. Here we showed that selective loss of astroglial FMRP in vivo up-regulates a brain-enriched miRNA, miR-128-3p, in mouse and human FMRP-deficient astroglia, which suppresses developmental expression of astroglial metabotropic glutamate receptor 5 (mGluR5), a major receptor in mediating developmental astroglia to neuron communication. Selective in vivo inhibition of miR-128-3p in FMRP-deficient astroglia sufficiently rescues decreased mGluR5 function, while astroglial overexpression of miR-128-3p strongly and selectively diminishes developmental astroglial mGluR5 signaling. Subsequent transcriptome and proteome profiling further suggests that FMRP commonly and preferentially regulates protein expression through posttranscriptional, but not transcriptional, mechanisms in astroglia. Overall, our study defines an FMRP-dependent cell-autonomous miR pathway that selectively alters developmental astroglial mGluR5 signaling, unveiling astroglial molecular mechanisms involved in FXS pathogenesis.

Sustained correction of associative learning deficits after brief, early treatment in a rat model of Fragile X Syndrome

Fragile X Syndrome (FXS) is one of the most common monogenic forms of autism and intellectual disability. Preclinical studies in animal models have highlighted the potential of pharmaceutical intervention strategies for alleviating the symptoms of FXS. However, whether treatment strategies can be tailored to developmental time windows that define the emergence of particular phenotypes is unknown. Similarly, whether a brief, early intervention can have long-lasting beneficial effects, even after treatment cessation, is also unknown. To address these questions, we first examined the developmental profile for the acquisition of associative learning in a rat model of FXS. Associative memory was tested using a range of behavioral paradigms that rely on an animal's innate tendency to explore novelty. Fmr1 knockout (KO) rats showed a developmental delay in their acquisition of object-place recognition and did not demonstrate object-place-context recognition paradigm at any age tested (up to 23 weeks of age). Treatment of Fmr1 KO rats with lovastatin between 5 and 9 weeks of age, during the normal developmental period that this associative memory capability is established, prevents the emergence of deficits but has no effect in wild-type animals. Moreover, we observe no regression of cognitive performance in the FXS rats over several months after treatment. This restoration of the normal developmental trajectory of cognitive function is associated with the sustained rescue of both synaptic plasticity and altered protein synthesis. The findings provide proof of concept that the impaired emergence of the cognitive repertoire in neurodevelopmental disorders may be prevented by brief, early pharmacological intervention.

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.

Microexons: at the nexus of nervous system development, behaviour and autism spectrum disorder

The discovery and characterization of a network of highly conserved neuronal microexons has provided fundamental new insight into mechanisms underlying nervous system development and function, as well as an important basis for pathway convergence in autism spectrum disorder. In the past few years, considerable progress has been made in comprehensively determining the repertoires of factors that control neuronal microexons. These results have illuminated molecular mechanisms that activate the splicing of microexons, including those that control gene expression programs critical for neurogenesis, as well as synaptic protein translation and neuronal activity. Remarkably, individual disruption of specific microexons in these pathways results in autism-like phenotypes and cognitive impairment in mice. This review discusses these findings and their implications for delivering new therapeutic strategies for neurological disorders.

Disrupted inhibitory plasticity and homeostasis in Fragile X syndrome

Fragile X Syndrome (FXS) is a neurodevelopmental disorder instigated by the absence of a key translation regulating protein, Fragile X Mental Retardation Protein (FMRP). The loss of FMRP in the CNS leads to abnormal synaptic development, disruption of critical periods of plasticity, and an overall deficiency in proper sensory circuit coding leading to hyperexcitable sensory networks. However, little is known about how this hyperexcitable environment affects inhibitory synaptic plasticity. Here, we show that in vivo layer 2/3 of the primary somatosensory cortex of the Fmr1 KO mouse exhibits basal hyperexcitability and an increase in neuronal firing rate suppression during whisker activation. This aligns with our in vitro data that indicate an increase in GABAergic spontaneous activity, a faulty mGluR-mediated inhibitory input and impaired inhibitory plasticity processes. Specifically, we find that mGluR activation sensitivity is overall diminished in the Fmr1 KO mouse leading to both a decreased spontaneous inhibitory postsynaptic input to principal cells and a disrupted form of inhibitory long-term depression (I-LTD). These data suggest an adaptive mechanism that acts to homeostatically counterbalance the cortical hyperexcitability observed in FXS.

Measuring mRNA translation in neuronal processes and somata by tRNA-FRET

In neurons, the specific spatial and temporal localization of protein synthesis is of great importance for function and survival. Here, we visualized tRNA and protein synthesis events in fixed and live mouse primary cortical culture using fluorescently-labeled tRNAs. We were able to characterize the distribution and transport of tRNAs in different neuronal sub-compartments and to study their association with the ribosome. We found that tRNA mobility in neural processes is lower than in somata and corresponds to patterns of slow transport mechanisms, and that larger tRNA puncta co-localize with translational machinery components and are likely the functional fraction. Furthermore, chemical induction of long-term potentiation (LTP) in culture revealed up-regulation of mRNA translation with a similar effect in dendrites and somata, which appeared to be GluR-dependent 6 h post-activation. Importantly, measurement of protein synthesis in neurons with high resolutions offers new insights into neuronal function in health and disease states.

Selective inhibition of glycogen synthase kinase 3α corrects pathophysiology in a mouse model of fragile X syndrome

Fragile X syndrome is caused by FMR1 gene silencing and loss of the encoded fragile X mental retardation protein (FMRP), which binds to mRNA and regulates translation. Studies in the Fmr1-/y mouse model of fragile X syndrome indicate that aberrant cerebral protein synthesis downstream of metabotropic glutamate receptor 5 (mGluR5) signaling contributes to disease pathogenesis, but clinical trials using mGluR5 inhibitors were not successful. Animal studies suggested that treatment with lithium might be an alternative approach. Targets of lithium include paralogs of glycogen synthase kinase 3 (GSK3), and nonselective small-molecule inhibitors of these enzymes improved disease phenotypes in a fragile X syndrome mouse model. However, the potential therapeutic use of GSK3 inhibitors has been hampered by toxicity arising from inhibition of both α and β paralogs. Recently, we developed GSK3 inhibitors with sufficient paralog selectivity to avoid a known toxic consequence of dual inhibition, that is, increased β-catenin stabilization. We show here that inhibition of GSK3α, but not GSK3β, corrected aberrant protein synthesis, audiogenic seizures, and sensory cortex hyperexcitability in Fmr1-/y mice. Although inhibiting either paralog prevented induction of NMDA receptor-dependent long-term depression (LTD) in the hippocampus, only inhibition of GSK3α impaired mGluR5-dependent and protein synthesis-dependent LTD. Inhibition of GSK3α additionally corrected deficits in learning and memory in Fmr1-/y mice; unlike mGluR5 inhibitors, there was no evidence of tachyphylaxis or enhanced psychotomimetic-induced hyperlocomotion. GSK3α selective inhibitors may have potential as a therapeutic approach for treating fragile X syndrome.

Widespread Alterations in Translation Elongation in the Brain of Juvenile Fmr1 Knockout Mice

FMRP (fragile X mental retardation protein) is a polysome-associated RNA-binding protein encoded by Fmr1 that is lost in fragile X syndrome. Increasing evidence suggests that FMRP regulates both translation initiation and elongation, but the gene specificity of these effects is unclear. To elucidate the impact of Fmr1 loss on translation, we utilize ribosome profiling for genome-wide measurements of ribosomal occupancy and positioning in the cortex of 24-day-old Fmr1 knockout mice. We find a remarkably coherent reduction in ribosome footprint abundance per mRNA for previously identified, high-affinity mRNA binding partners of FMRP and an increase for terminal oligopyrimidine (TOP) motif-containing genes canonically controlled by mammalian target of rapamycin-eIF4E-binding protein-eIF4E binding protein-eukaryotic initiation factor 4E (mTOR-4E-BP-eIF4E) signaling. Amino acid motif- and gene-level analyses both show a widespread reduction of translational pausing in Fmr1 knockout mice. Our findings are consistent with a model of FMRP-mediated regulation of both translation initiation through eIF4E and elongation that is disrupted in fragile X syndrome.

FMRP Control of Ribosome Translocation Promotes Chromatin Modifications and Alternative Splicing of Neuronal Genes Linked to Autism

Silencing of FMR1 and loss of its gene product, FMRP, results in fragile X syndrome (FXS). FMRP binds brain mRNAs and inhibits polypeptide elongation. Using ribosome profiling of the hippocampus, we find that ribosome footprint levels in Fmr1-deficient tissue mostly reflect changes in RNA abundance. Profiling over a time course of ribosome runoff in wild-type tissue reveals a wide range of ribosome translocation rates; on many mRNAs, the ribosomes are stalled. Sucrose gradient ultracentrifugation of hippocampal slices after ribosome runoff reveals that FMRP co-sediments with stalled ribosomes, and its loss results in decline of ribosome stalling on specific mRNAs. One such mRNA encodes SETD2, a lysine methyltransferase that catalyzes H3K36me3. Chromatin immunoprecipitation sequencing (ChIP-seq) demonstrates that loss of FMRP alters the deployment of this histone mark. H3K36me3 is associated with alternative pre-RNA processing, which we find occurs in an FMRP-dependent manner on transcripts linked to neural function and autism spectrum disorders.

FMRP has a cell-type-specific role in CA1 pyramidal neurons to regulate autism-related transcripts and circadian memory

Loss of the RNA binding protein FMRP causes Fragile X Syndrome (FXS), the most common cause of inherited intellectual disability, yet it is unknown how FMRP function varies across brain regions and cell types and how this contributes to disease pathophysiology. Here we use conditional tagging of FMRP and CLIP (FMRP cTag CLIP) to examine FMRP mRNA targets in hippocampal CA1 pyramidal neurons, a critical cell type for learning and memory relevant to FXS phenotypes. Integrating these data with analysis of ribosome-bound transcripts in these neurons revealed CA1-enriched binding of autism-relevant mRNAs, and CA1-specific regulation of transcripts encoding circadian proteins. This contrasted with different targets in cerebellar granule neurons, and was consistent with circadian defects in hippocampus-dependent memory in Fmr1 knockout mice. These findings demonstrate differential FMRP-dependent regulation of mRNAs across neuronal cell types that may contribute to phenotypes such as memory defects and sleep disturbance associated with FXS.

Audiogenic Seizures in the Fmr1 Knock-Out Mouse Are Induced by Fmr1 Deletion in Subcortical, VGlut2-Expressing Excitatory Neurons and Require Deletion in the Inferior Colliculus

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability and the leading monogenetic cause of autism. One symptom of FXS and autism is sensory hypersensitivity (also called sensory over-responsivity). Perhaps related to this, the audiogenic seizure (AGS) is arguably the most robust behavioral phenotype in the FXS mouse model-the Fmr1 knock-out (KO) mouse. Therefore, the AGS may be considered a mouse model of sensory hypersensitivity. Hyperactive circuits are hypothesized to underlie dysfunction in a number of brain regions in patients with FXS and Fmr1 KO mice, and the AGS may be a result of this. But the specific cell types and brain regions underlying AGSs in the Fmr1 KO are unknown. We used conditional deletion or expression of Fmr1 in different cell populations to determine whether Fmr1 deletion in those cells was sufficient or necessary, respectively, for the AGS phenotype in males. Our data indicate that Fmr1 deletion in glutamatergic neurons that express vesicular glutamate transporter 2 (VGlut2) and are located in subcortical brain regions is sufficient and necessary to cause AGSs. Furthermore, the deletion of Fmr1 in glutamatergic neurons of the inferior colliculus is necessary for AGSs. When we demonstrate necessity, we show that Fmr1 expression in either the larger population of VGlut2-expressing glutamatergic neurons or the smaller population of inferior collicular glutamatergic neurons-in an otherwise Fmr1 KO mouse-eliminates AGSs. Therefore, targeting these neuronal populations in FXS and autism may be part of a therapeutic strategy to alleviate sensory hypersensitivity.SIGNIFICANCE STATEMENT Sensory hypersensitivity in fragile X syndrome (FXS) and autism patients significantly interferes with quality of life. Audiogenic seizures (AGSs) are arguably the most robust behavioral phenotype in the FXS mouse model-the Fmr1 knockout-and may be considered a model of sensory hypersensitivity in FXS. We provide the clearest and most precise genetic evidence to date for the cell types and brain regions involved in causing AGSs in the Fmr1 knockout and, more broadly, for any mouse mutant. The expression of Fmr1 in these same cell types in an otherwise Fmr1 knockout eliminates AGSs indicating possible cellular targets for alleviating sensory hypersensitivity in FXS and other forms of autism.

Targeting Muscarinic Acetylcholine Receptors for the Treatment of Psychiatric and Neurological Disorders

Muscarinic acetylcholine receptors (mAChR) play important roles in regulating complex behaviors such as cognition, movement, and reward, making them ideally situated as potential drug targets for the treatment of several brain disorders. Recent advances in the discovery of subtype-selective allosteric modulators for mAChRs has provided an unprecedented opportunity for highly specific modulation of signaling by individual mAChR subtypes in the brain. Recently, mAChR allosteric modulators have entered clinical development for Alzheimer's disease (AD) and schizophrenia, and have potential utility for other brain disorders. However, mAChR allosteric modulators can display a diverse array of pharmacological properties, and a more nuanced understanding of the mAChR will be necessary to best translate preclinical findings into successful clinical treatments.

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.

New pathologic mechanisms in nucleotide repeat expansion disorders

Tandem microsatellite repeats are common throughout the human genome and intrinsically unstable, exhibiting expansions and contractions both somatically and across generations. Instability in a small subset of these repeats are currently linked to human disease, although recent findings suggest more disease-causing repeats await discovery. These nucleotide repeat expansion disorders (NREDs) primarily affect the nervous system and commonly lead to neurodegeneration through toxic protein gain-of-function, protein loss-of-function, and toxic RNA gain-of-function mechanisms. However, the lines between these categories have blurred with recent findings of unconventional Repeat Associated Non-AUG (RAN) translation from putatively non-coding regions of the genome. Here we review two emerging topics in NREDs: 1) The mechanisms by which RAN translation occurs and its role in disease pathogenesis and 2) How nucleotide repeats as RNA and translated proteins influence liquid-liquid phase separation, membraneless organelle dynamics, and nucleocytoplasmic transport. We examine these topics with a particular eye on two repeats: the CGG repeat expansion responsible for Fragile X syndrome and Fragile X-associated Tremor Ataxia Syndrome (FXTAS) and the intronic GGGGCC repeat expansion in C9orf72, the most common inherited cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Our thesis is that these emerging disease mechanisms can inform a broader understanding of the native roles of microsatellites in cellular function and that aberrations in these native processes provide clues to novel therapeutic strategies for these currently untreatable disorders.

Bassoon proteinopathy drives neurodegeneration in multiple sclerosis

Multiple sclerosis (MS) is characterized by inflammatory insults that drive neuroaxonal injury. However, knowledge about neuron-intrinsic responses to inflammation is limited. By leveraging neuron-specific messenger RNA profiling, we found that neuroinflammation leads to induction and toxic accumulation of the synaptic protein bassoon (Bsn) in the neuronal somata of mice and patients with MS. Neuronal overexpression of Bsn in flies resulted in reduction of lifespan, while genetic disruption of Bsn protected mice from inflammation-induced neuroaxonal injury. Notably, pharmacological proteasome activation boosted the clearance of accumulated Bsn and enhanced neuronal survival. Our study demonstrates that neuroinflammation initiates toxic protein accumulation in neuronal somata and advocates proteasome activation as a potential remedy.

Altered steady state and activity-dependent de novo protein expression in fragile X syndrome

Whether fragile X mental retardation protein (FMRP) target mRNAs and neuronal activity contributing to elevated basal neuronal protein synthesis in fragile X syndrome (FXS) is unclear. Our proteomic experiments reveal that the de novo translational profile in FXS model mice is altered at steady state and in response to metabotropic glutamate receptor (mGluR) stimulation, but the proteins expressed differ under these conditions. Several altered proteins, including Hexokinase 1 and Ras, also are expressed in the blood of FXS model mice and pharmacological treatments previously reported to ameliorate phenotypes modify their abundance in blood. In addition, plasma levels of Hexokinase 1 and Ras differ between FXS patients and healthy volunteers. Our data suggest that brain-based de novo proteomics in FXS model mice can be used to find altered expression of proteins in blood that could serve as disease-state biomarkers in individuals with FXS.

Elevated protein synthesis, and dysregulated mGluR signalling, are documented in fragile X syndrome (FXS) Here the authors use proteomic analysis in a mouse model of FXS, and following mGluR5 stimulation, to identify potential biomarkers for the disease.