Fragile X mental retardation protein deficiency leads to excessive mGluR5-dependent internalization of AMPA receptors

Fragile X syndrome: causes, diagnosis, mechanisms, and therapeutics

Fragile X syndrome (FXS) is the most frequent form of inherited intellectual disability and is also linked to other neurologic and psychiatric disorders. FXS is caused by a triplet expansion that inhibits expression of the FMR1 gene; the gene product, FMRP, regulates mRNA metabolism in the brain and thus controls the expression of key molecules involved in receptor signaling and spine morphology. While there is no definitive cure for FXS, the understanding of FMRP function has paved the way for rational treatment designs that could potentially reverse many of the neurobiological changes observed in FXS. Additionally, behavioral, pharmacological, and cognitive interventions can raise the quality of life for both patients and their families.

Activation of 5-HT7 serotonin receptors reverses metabotropic glutamate receptor-mediated synaptic plasticity in wild-type and Fmr1 knockout mice, a model of Fragile X syndrome

BACKGROUND

Fragile X syndrome (FXS) is a genetic cause of intellectual disability and autism. Fmr1 knockout (Fmr1 KO) mice, an animal model of FXS, exhibit spatial memory impairment and synapse malfunctioning in the hippocampus, with abnormal enhancement of long-term depression mediated by metabotropic glutamate receptors (mGluR-LTD). The neurotransmitter serotonin (5-HT) modulates hippocampal-dependent learning through serotonin 1A (5-HT1A) and serotonin 7 (5-HT7) receptors; the underlying mechanisms are unknown.

METHODS

We used electrophysiology to test the effects of 5-HT on mGluR-LTD in wild-type and Fmr1 KO mice and immunocytochemistry and biotinylation assay to study related changes of 2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl)propanoic acid (AMPA) glutamate receptor surface expression.

RESULTS

Application of 5-HT or 8-OH-DPAT (a mixed 5-HT1A/5-HT7 agonist) reversed mGluR-LTD in hippocampal slices. Reversal of mGluR-LTD by 8-OH-DPAT persisted in the presence of the 5-HT1A receptor antagonist WAY-100635, was abolished by SB-269970 (5-HT7 receptor antagonist), and was mimicked by LP-211, a novel selective 5-HT7 receptor agonist. Consistently, 8-OH-DPAT decreased mGluR-mediated reduction of AMPA glutamate receptor 2 (GluR2) subunit surface expression in hippocampal slices and cultured hippocampal neurons, an effect mimicked by LP-211 and blocked by SB-269970. In Fmr1 KO mice, mGluR-LTD was abnormally enhanced; similarly to wild-type, 8-OH-DPAT reversed mGluR-LTD and decreased mGluR-induced reduction of surface AMPA receptors, an effect antagonized by SB-269970.

CONCLUSIONS

Serotonin 7 receptor activation reverses metabotropic glutamate receptor-induced AMPA receptor internalization and LTD both in wild-type and in Fmr1 KO mice, correcting excessive mGluR-LTD. Therefore, selective activation of 5-HT7 receptors may represent a novel strategy in the therapy of FXS.

Chronic administration of AFQ056/Mavoglurant restores social behaviour in Fmr1 knockout mice

Fragile X syndrome is caused by lack of FMR1 protein (FMRP) leading to severe symptoms, including intellectual disability, hyperactivity and autistic-like behaviour. FMRP is an RNA binding protein involved in the regulation of translation of specific target mRNAs upon stimulation of metabotropic glutamate receptor 5 (mGluR5) at the synapse. The absence of FMRP leads to enhanced activity of mGluR5 signal transduction pathways. Many conflicting results have been reported regarding social behaviour deficits in Fmr1 knockout mice, and little is known about the involvement of mGluR5 pathways on social behaviour. In this study, a three-chambered task was used to determine sociability and preference for social novelty in Fmr1 knockout mice. Disruption of Fmr1 functioning resulted in enhanced interaction with stranger mouse during sociability while no significant changes were observed during preference for social novelty assay. Chronic administration of a specific mGluR5 antagonist, AFQ056/Mavoglurant, was able to restore sociability behaviour of Fmr1 knockout mice to levels of wild type littermates. These results support the importance of mGluR5 signalling pathways on social interaction behaviour and that AFQ056/Mavoglurant might be useful as potential therapeutic intervention to rescue various behavioural aspects of the fragile X phenotype.

Fragile X syndrome: mechanistic insights and therapeutic avenues regarding the role of potassium channels

Fragile X syndrome (FXS) is a common form of mental disability and one of the known causes of autism. The mutation responsible for FXS is a large expansion of the trinucleotide CGG repeats that leads to DNA methylation of the fragile X mental retardation gene 1 (FMR1) and transcriptional silencing, resulting in the absence of fragile X mental retardation protein (FMRP), an mRNA binding protein. Although it is widely known that FMRP is critical for metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD), which has provided a general theme for developing pharmacological drugs for FXS, specific downstream targets of FMRP may also be of therapeutic value. Since alterations in potassium channel expression level or activity could underlie neuronal network defects in FXS, here we describe recent findings on how these channels might be altered in mouse models of FXS and the possible therapeutic avenues for treating FXS.

Roles of CREB in the regulation of FMRP by group I metabotropic glutamate receptors in cingulate cortex

Background

Fragile X syndrome is caused by lack of fragile X mental retardation protein (FMRP) due to silencing of the FMR1 gene. The metabotropic glutamate receptors (mGluRs) in the central nervous system contribute to higher brain functions including learning/memory, mental disorders and persistent pain. The transcription factor cyclic AMP-responsive element binding protein (CREB) is involved in important neuronal functions, such as synaptic plasticity and neuronal survival. Our recent study has shown that stimulation of Group I mGluRs upregulated FMRP and activated CREB in anterior cingulate cortex (ACC), a key region for brain cognitive and executive functions, suggesting that activation of Group I mGluRs may upregulate FMRP through CREB signaling pathway.

Results

In this study, we demonstrate that CREB contributes to the regulation of FMRP by Group I mGluRs. In ACC neurons of adult mice overexpressing dominant active CREB mutant, the upregulation of FMRP by stimulating Group I mGluR is enhanced compared to wild-type mice. However, the regulation of FMRP by Group I mGluRs is not altered by overexpression of Ca²⁺-insensitive mutant form of downstream regulatory element antagonist modulator (DREAM), a transcriptional repressor involved in synaptic transmission and plasticity.

Conclusion

Our study has provided further evidence for CREB involvement in regulation of FMRP by Group I mGluRs in ACC neurons, and may help to elucidate the pathogenesis of fragile X syndrome.

The pathophysiology of fragile X (and what it teaches us about synapses)

Fragile X is the most common known inherited cause of intellectual disability and autism, and it typically results from transcriptional silencing of FMR1 and loss of the encoded protein, FMRP (fragile X mental retardation protein). FMRP is an mRNA-binding protein that functions at many synapses to inhibit local translation stimulated by metabotropic glutamate receptors (mGluRs) 1 and 5. Recent studies on the biology of FMRP and the signaling pathways downstream of mGluR1/5 have yielded deeper insight into how synaptic protein synthesis and plasticity are regulated by experience. This new knowledge has also suggested ways that altered signaling and synaptic function can be corrected in fragile X, and human clinical trials based on this information are under way.

New perspectives on the biology of fragile X syndrome

Fragile X syndrome (FXS) is a trinucleotide repeat disorder caused by a CGG repeat expansion in FMR1, and loss of its protein product FMRP. Recent studies have provided increased support for the role of FMRP in translational repression via ribosomal stalling and the microRNA pathway. In neurons, particular focus has been placed on identifying the signaling pathways such as PI3K and mTOR downstream of group 1 metabotropic glutamate receptors (mGluR1/5) that regulate FMRP. New evidence also suggests that loss of FMRP causes presynaptic dysfunction and abnormal adult neurogenesis. In addition, studies on FXS stem cells especially induced pluripotent stem (iPS) cells and new sequencing efforts hold out promise for deeper understanding of the silencing process and mutation spectrum of FMR1.

Activity-dependent neurotransmitter respecification

For many years it has been assumed that the identity of the transmitters expressed by neurons is stable and unchanging. Recent work, however, shows that electrical activity can respecify neurotransmitter expression during development and in the mature nervous system, and an understanding is emerging of the molecular mechanisms underlying activity-dependent transmitter respecification. Changes in postsynaptic neurotransmitter receptor expression accompany and match changes in transmitter specification, thus enabling synaptic transmission. The functional roles of neurotransmitter respecification are beginning to be understood and appear to involve homeostatic synaptic regulation, which in turn influences behaviour. Activation of this novel form of plasticity by sensorimotor stimuli may provide clinical benefits.

Sapap3 deletion causes mGluR5-dependent silencing of AMPAR synapses

Synaptic transmission mediated by AMPA-type glutamate receptors (AMPARs) is regulated by scaffold proteins in the postsynaptic density. SAP90/PSD-95-associated protein 3 (SAPAP3) is a scaffold protein that is highly expressed in striatal excitatory synapses. While loss of SAPAP3 is known to cause obsessive-compulsive disorder-like behaviors in mice and reduce extracellular field potentials in the striatum, the mechanism by which SAPAP3 regulates excitatory neurotransmission is largely unknown. This study demonstrates that Sapap3 deletion reduces AMPAR-mediated synaptic transmission in striatal medium spiny neurons (MSNs) through postsynaptic endocytosis of AMPARs. Striatal MSNs in Sapap3 KO mice have fewer synapses with AMPAR activity and a higher proportion of silent synapses. We further find that increased metabotropic glutamate receptor 5 (mGluR5) activity in Sapap3 KO mice underlies the decrease in AMPAR synaptic transmission and excessive synapse silencing. These findings suggest a model whereby the normal role of SAPAP3 is to inhibit mGluR5-driven endocytosis of AMPARs. The results of this study provide the first evidence for the mechanism by which the SAPAP family of scaffold proteins regulates AMPAR synaptic activity.

Taking STEPs forward to understand fragile X syndrome

A priority of fragile X syndrome (FXS) research is to determine the molecular mechanisms underlying the functional, behavioral, and structural deficits in humans and in the FXS mouse model. Given that metabotropic glutamate receptor (mGluR) long-term depression (LTD) is exaggerated in FXS mice, considerable effort has focused on proteins that regulate this form of synaptic plasticity. STriatal-Enriched protein tyrosine Phosphatase (STEP) is a brain-specific phosphatase implicated as an "LTD protein" because it mediates AMPA receptor internalization during mGluR LTD. STEP also promotes NMDA receptor endocytosis and inactivates ERK1/2 and Fyn, thereby opposing synaptic strengthening. We hypothesized that dysregulation of STEP may contribute to the pathophysiology of FXS. We review how STEP's expression and activity are regulated by dendritic protein synthesis, ubiquitination, proteolysis, and phosphorylation. We also discuss implications for STEP in FXS and other disorders, including Alzheimer's disease. As highlighted here, pharmacological interventions targeting STEP may prove successful for FXS.

Molecular and genetic analysis of the Drosophila model of fragile X syndrome

The Drosophila genome contains most genes known to be involved in heritable disease. The extraordinary genetic malleability of Drosophila, coupled to sophisticated imaging, electrophysiology, and behavioral paradigms, has paved the way for insightful mechanistic studies on the causes of developmental and neurological disease as well as many possible interventions. Here, we focus on one of the most advanced examples of Drosophila genetic disease modeling, the Drosophila model of Fragile X Syndrome, which for the past decade has provided key advances into the molecular, cellular, and behavioral defects underlying this devastating disorder. We discuss the multitude of RNAs and proteins that interact with the disease-causing FMR1 gene product, whose function is conserved from Drosophila to human. In turn, we consider FMR1 mechanistic relationships in non-neuronal tissues (germ cells and embryos), peripheral motor and sensory circuits, and central brain circuits involved in circadian clock activity and learning/memory.

Fragile X syndrome and targeted treatment trials

Work in recent years has revealed an abundance of possible new treatment targets for fragile X syndrome (FXS). The use of animal models, including the fragile X knockout mouse which manifests a phenotype very similar to FXS in humans, has resulted in great strides in this direction of research. The lack of Fragile X Mental Retardation Protein (FMRP) in FXS causes dysregulation and usually overexpression of a number of its target genes, which can cause imbalances of neurotransmission and deficits in synaptic plasticity. The use of metabotropic glutamate receptor (mGluR) blockers and gamma amino-butyric acid (GABA) agonists have been shown to be efficacious in reversing cellular and behavioral phenotypes, and restoring proper brain connectivity in the mouse and fly models. Proposed new pharmacological treatments and educational interventions are discussed in this chapter. In combination, these various targeted treatments show promising preliminary results in mitigating or even reversing the neurobiological abnormalities caused by loss of FMRP, with possible translational applications to other neurodevelopmental disorders including autism.

Therapeutic strategies in fragile X syndrome: dysregulated mGluR signaling and beyond

Fragile X syndrome (FXS) is an inherited neurodevelopmental disease caused by loss of function of the fragile X mental retardation protein (FMRP). In the absence of FMRP, signaling through group 1 metabotropic glutamate receptors is elevated and insensitive to stimulation, which may underlie many of the neurological and neuropsychiatric features of FXS. Treatment of FXS animal models with negative allosteric modulators of these receptors and preliminary clinical trials in human patients support the hypothesis that metabotropic glutamate receptor signaling is a valuable therapeutic target in FXS. However, recent research has also shown that FMRP may regulate diverse aspects of neuronal signaling downstream of several cell surface receptors, suggesting a possible new route to more direct disease-targeted therapies. Here, we summarize promising recent advances in basic research identifying and testing novel therapeutic strategies in FXS models, and evaluate their potential therapeutic benefits. We provide an overview of recent and ongoing clinical trials motivated by some of these findings, and discuss the challenges for both basic science and clinical applications in the continued development of effective disease mechanism-targeted therapies for FXS.

In vivo neuronal function of the fragile X mental retardation protein is regulated by phosphorylation

Fragile X syndrome (FXS), caused by loss of the Fragile X Mental Retardation 1 (FMR1) gene product (FMRP), is the most common heritable cause of intellectual disability and autism spectrum disorders. It has been long hypothesized that the phosphorylation of serine 500 (S500) in human FMRP controls its function as an RNA-binding translational repressor. To test this hypothesis in vivo, we employed neuronally targeted expression of three human FMR1 transgenes, including wild-type (hFMR1), dephosphomimetic (S500A-hFMR1) and phosphomimetic (S500D-hFMR1), in the Drosophila FXS disease model to investigate phosphorylation requirements. At the molecular level, dfmr1 null mutants exhibit elevated brain protein levels due to loss of translational repressor activity. This defect is rescued for an individual target protein and across the population of brain proteins by the phosphomimetic, whereas the dephosphomimetic phenocopies the null condition. At the cellular level, dfmr1 null synapse architecture exhibits increased area, branching and bouton number. The phosphomimetic fully rescues these synaptogenesis defects, whereas the dephosphomimetic provides no rescue. The presence of Futsch-positive (microtubule-associated protein 1B) supernumerary microtubule loops is elevated in dfmr1 null synapses. The human phosphomimetic restores normal Futsch loops, whereas the dephosphomimetic provides no activity. At the behavioral level, dfmr1 null mutants exhibit strongly impaired olfactory associative learning. The human phosphomimetic targeted only to the brain-learning center restores normal learning ability, whereas the dephosphomimetic provides absolutely no rescue. We conclude that human FMRP S500 phosphorylation is necessary for its in vivo function as a neuronal translational repressor and regulator of synaptic architecture, and for the manifestation of FMRP-dependent learning behavior.

Fragile x syndrome

Recent data from a national survey highlighted a significant difference in obesity rates in young fragile X males (31%) compared to age matched controls (18%). Fragile X syndrome (FXS) is the most common cause of intellectual disability in males and the most common single gene cause of autism. This X-linked disorder is caused by an expansion of a trinucleotide CGG repeat (>200) on the promotor region of the fragile X mental retardation 1 gene (FMR1). As a result, the promotor region often becomes methylated which leads to a deficiency or absence of the FMR1 protein (FMRP). Common characteristics of FXS include mild to severe cognitive impairments in males but less severe cognitive impairment in females. Physical features of FXS include an elongated face, prominent ears, and post-pubertal macroorchidism. Severe obesity in full mutation males is often associated with the Prader-Willi phenotype (PWP) which includes hyperphagia, lack of satiation after meals, and hypogonadism or delayed puberty; however, there is no deletion at 15q11-q13 nor uniparental maternal disomy. Herein, we discuss the molecular mechanisms leading to FXS and the Prader-Willi phenotype with an emphasis on mouse FMR1 knockout studies that have shown the reversal of weight increase through mGluR antagonists. Finally, we review the current medications used in treatment of FXS including the atypical antipsychotics that can lead to weight gain and the research regarding the use of targeted treatments in FXS that will hopefully have a significantly beneficial effect on cognition and behavior without weight gain.

Molecular mechanisms of fragile X syndrome: a twenty-year perspective

Fragile X syndrome (FXS) is a common form of inherited intellectual disability and is one of the leading known causes of autism. The mutation responsible for FXS is a large expansion of the trinucleotide CGG repeat in the 5' untranslated region of the X-linked gene FMR1. This expansion leads to DNA methylation of FMR1 and to transcriptional silencing, which results in the absence of the gene product, FMRP, a selective messenger RNA (mRNA)-binding protein that regulates the translation of a subset of dendritic mRNAs. FMRP is critical for mGluR (metabotropic glutamate receptor)-dependent long-term depression, as well as for other forms of synaptic plasticity; its absence causes excessive and persistent protein synthesis in postsynaptic dendrites and dysregulated synaptic function. Studies continue to refine our understanding of FMRP's role in synaptic plasticity and to uncover new functions of this protein, which have illuminated therapeutic approaches for FXS.

The psychiatric presentation of fragile x: evolution of the diagnosis and treatment of the psychiatric comorbidities of fragile X syndrome

Fragile X syndrome (FXS) is the leading inherited cause of mental retardation and autism spectrum disorders worldwide. It presents with a distinct behavioral phenotype which overlaps significantly with that of autism. Unlike autism and most common psychiatric disorders, the neurobiology of fragile X is relatively well understood. Lack of the fragile X mental retardation protein causes dysregulation of synaptically driven protein synthesis, which in turn causes global disruption of synaptic plasticity. Thus, FXS can be considered a disorder of synaptic plasticity, and a developmental disorder in the purest sense: mutation of the FMR1 (fragile X mental retardation 1) gene results in abnormal synaptic development in response to experience. Accumulation of this abnormal synaptic development, over time, leads to a characteristic and surprisingly consistent behavioral phenotype of attention deficit, hyperactivity, impulsivity, multiple anxiety symptoms, repetitive/perseverative/stereotypic behaviors, unstable affect, aggression, and self-injurious behavior. Many features of the behavioral and psychiatric phenotype of FXS follow a developmental course, waxing and waning over the life span. In most cases, symptoms present as a mixed clinical picture, not fitting established diagnostic categories. There have been many clinical trials in fragile X subjects, but no placebo-controlled trials of adequate size or methodology utilizing the most commonly prescribed psychiatric medications. However, large and well-designed trials of investigational agents which target the underlying pathology of FXS have recently been completed or are under way. While the literature offers little guidance to the clinician treating patients with FXS today, potentially disease-modifying treatments may be available in the near future.

Targeted treatments for fragile X syndrome

Fragile X syndrome (FXS) is the most common identifiable genetic cause of intellectual disability and autistic spectrum disorders (ASD), with up to 50% of males and some females with FXS meeting criteria for ASD. Autistic features are present in a very high percent of individuals with FXS, even those who do not meet full criteria for ASD. Recent major advances have been made in the understanding of the neurobiology and functions of FMRP, the FMR1 (fragile X mental retardation 1) gene product, which is absent or reduced in FXS, largely based on work in the fmr1 knockout mouse model. FXS has emerged as a disorder of synaptic plasticity associated with abnormalities of long-term depression and long-term potentiation and immature dendritic spine architecture, related to the dysregulation of dendritic translation typically activated by group I mGluR and other receptors. This work has led to efforts to develop treatments for FXS with neuroactive molecules targeted to the dysregulated translational pathway. These agents have been shown to rescue molecular, spine, and behavioral phenotypes in the FXS mouse model at multiple stages of development. Clinical trials are underway to translate findings in animal models of FXS to humans, raising complex issues about trial design and outcome measures to assess cognitive change that might be associated with treatment. Genes known to be causes of ASD interact with the translational pathway defective in FXS, and it has been hypothesized that there will be substantial overlap in molecular pathways and mechanisms of synaptic dysfunction between FXS and ASD. Therefore, targeted treatments developed for FXS may also target subgroups of ASD, and clinical trials in FXS may serve as a model for the development of clinical trial strategies for ASD and other cognitive disorders.

Fragile X syndrome: an update on developing treatment modalities

Intellectual disability (ID; mental retardation) is considered an immutable condition. Current medical practices are aimed at relieving symptoms and not at altering the underlying cognitive deficits. Scientific advancements from the past decade have led to the exciting possibility that ID may now be treatable. Moreover, pharmaceutical therapies targeting the most common form of inherited ID, Fragile X syndrome (FXS), may become the new benchmark for central nervous system (CNS) drug discovery: seeking cures for neurodevelopmental disorders.

The Role of Striatal-Enriched Protein Tyrosine Phosphatase (STEP) in Cognition

Striatal-enriched protein tyrosine phosphatase (STEP) has recently been implicated in several neuropsychiatric disorders with significant cognitive impairments, including Alzheimer's disease, schizophrenia, and fragile X syndrome. A model has emerged by which STEP normally opposes the development of synaptic strengthening and that disruption in STEP activity leads to aberrant synaptic function. We review the mechanisms by which STEP contributes to the etiology of these and other neuropsychiatric disorders. These findings suggest that disruptions in STEP activity may be a common mechanism for cognitive impairments in diverse illnesses.

Gender-specific effect of Mthfr genotype and neonatal vigabatrin interaction on synaptic proteins in mouse cortex

The enzyme methylenetetrahydrofolate reductase (MTHFR) is a part of the homocysteine and folate metabolic pathways, affecting the methylations of DNA, RNA, and proteins. Mthfr deficiency was reported as a risk factor for neurodevelopmental disorders such as autism spectrum disorder and schizophrenia. Neonatal disruption of the GABAergic system is also associated with behavioral outcomes. The interaction between the epigenetic influence of Mthfr deficiency and neonatal exposure to the GABA potentiating drug vigabatrin (GVG) in mice has been shown to have gender-dependent effects on mice anxiety and to have memory impairment effects in a gender-independent manner. Here we show that Mthfr deficiency interacts with neonatal GABA potentiation to alter social behavior in female, but not male, mice. This impairment was associated with a gender-dependent enhancement of proteins implicated in excitatory synapse plasticity in the female cortex. Reelin and fragile X mental retardation 1 protein (FMRP) levels and membrane GluR1/GluR2 ratios were elevated in wild-type mice treated neonatally with GVG and in Mthfr+/- mice treated with saline, but not in Mthfr+/- mice treated with GVG, compared with control groups (wild type treated with saline). A minor influence on the levels of these proteins was observed in male mice cortices, possibly due to high basal protein levels. Interaction between gender, genotype, and treatment was also observed in the GABA pathway. In female mice, GABA Aα2/gephyrin ratios were suppressed in all test groups; in male mice, a genotype-specific enhancement of GABA Aα2/gephyrin was observed. The lack of an effect on either reln or Fmr1 transcription suggests post-transcriptional regulation of these genes. Taken together, these findings suggest that Mthfr deficiency may interact with neonatal GABA potentiation in a gender-dependent manner to interrupt synaptic function. This may illustrate a possible mechanism for the epigenetic involvement of Mthfr deficiency in neurodevelopmental disorders.

Fragile X mental retardation protein regulates protein expression and mRNA translation of the potassium channel Kv4.2

A prominent characteristic of the inherited intellectual impairment disease fragile X syndrome (FXS) is neuronal hyperexcitability, resulting in a variety of symptoms, such as hyperactivity, increased sensitivity to sensory stimuli, and a high incidence of epileptic seizures. These symptoms account for a significant part of the disease pattern, but the underlying molecular mechanisms of neuronal hyperexcitability in FXS remain poorly understood. FXS is caused by loss of expression of fragile X mental retardation protein (FMRP), which regulates synaptic protein synthesis and is a key player to limit signaling pathways downstream of metabotropic glutamate receptors 1/5 (mGlu1/5). Recent findings suggest that FMRP might also directly regulate voltage-gated potassium channels. Here, we show that total and plasma membrane protein levels of Kv4.2, the major potassium channel regulating hippocampal neuronal excitability, are reduced in the brain of an FXS mouse model. Antagonizing mGlu5 activity with 2-methyl-6-(phenylethynyl)-pyridine (MPEP) partially rescues reduced surface Kv4.2 levels in Fmr1 knock-out (KO) mice, suggesting that excess mGlu1/5 signal activity contributes to Kv4.2 dysregulation. As an additional mechanism, we show that FMRP is a positive regulator of Kv4.2 mRNA translation and protein expression and associates with Kv4.2 mRNA in vivo and in vitro. Our results suggest that absence of FMRP-mediated positive control of Kv4.2 mRNA translation, protein expression, and plasma membrane levels might contribute to excess neuronal excitability in Fmr1 KO mice, and thus imply a potential mechanism underlying FXS-associated epilepsy.

Toward fulfilling the promise of molecular medicine in fragile X syndrome

Fragile X syndrome (FXS) is the most common inherited form of mental retardation and a leading known cause of autism. It is caused by loss of expression of the fragile X mental retardation protein (FMRP), an RNA-binding protein that negatively regulates protein synthesis. In neurons, multiple lines of evidence suggest that protein synthesis at synapses is triggered by activation of group 1 metabotropic glutamate receptors (Gp1 mGluRs) and that many functional consequences of activating these receptors are altered in the absence of FMRP. These observations have led to the theory that exaggerated protein synthesis downstream of Gp1 mGluRs is a core pathogenic mechanism in FXS. This excess can be corrected by reducing signaling by Gp1 mGluRs, and numerous studies have shown that inhibition of mGluR5, in particular, can ameliorate multiple mutant phenotypes in animal models of FXS. Clinical trials based on this therapeutic strategy are currently under way. FXS is therefore poised to be the first neurobehavioral disorder in which corrective treatments have been developed from the bottom up: from gene identification to pathophysiology in animals to novel therapeutics in humans. The insights gained from FXS and other autism-related single-gene disorders may also assist in identifying molecular mechanisms and potential treatment approaches for idiopathic autism.

Routes, destinations and delays: recent advances in AMPA receptor trafficking

Postsynaptic AMPA-type glutamate receptors (AMPARs) mediate most fast excitatory synaptic transmission and are crucial for many aspects of brain function, including learning, memory and cognition. The number, synaptic localization and subunit composition of synaptic AMPARs are tightly regulated by network activity and by the history of activity at individual synapses. Furthermore, aberrant AMPAR trafficking is implicated in neurodegenerative diseases. AMPARs therefore represent a prime target for drug development and the mechanisms that control their synaptic delivery, retention and removal are the subject of extensive research. Here, we review recent findings that have provided new insights into AMPAR trafficking and that might lead to the development of novel therapeutic strategies.

Fragile X mental retardation protein regulates heterosynaptic plasticity in the hippocampus

Silencing of a single gene, FMR1, is linked to a highly prevalent form of mental retardation, characterized by social and cognitive impairments, known as fragile X syndrome (FXS). The FMR1 gene encodes fragile X mental retardation protein (FMRP), which negatively regulates translation. Knockout of Fmr1 in mice results in enhanced long-term depression (LTD) induced by metabotropic glutamate receptor (mGluR) activation. Despite the evidence implicating FMRP in LTD, the role of FMRP in long-term potentiation (LTP) is less clear. Synaptic strength can be augmented heterosynaptically through the generation and sequestration of plasticity-related proteins, in a cell-wide manner. If heterosynaptic plasticity is altered in Fmr1 knockout (KO) mice, this may explain the cognitive deficits associated with FXS. We induced homosynaptic plasticity using the β-adrenergic receptor (β-AR) agonist, isoproterenol (ISO), which facilitated heterosynaptic LTP that was enhanced in Fmr1 KO mice relative to wild-type (WT) controls. To determine if enhanced heterosynaptic LTP in Fmr1 KO mouse hippocampus requires protein synthesis, we applied a translation inhibitor, emetine (EME). EME blocked homo- and heterosynaptic LTP in both genotypes. We also probed the roles of mTOR and ERK in boosting heterosynaptic LTP in Fmr1 KO mice. Although heterosynaptic LTP was blocked in both WT and KOs by inhibitors of mTOR and ERK, homosynaptic LTP was still enhanced following mTOR inhibition in slices from Fmr1 KO mice. Because mTOR will normally stimulate translation initiation, our results suggest that β-AR stimulation paired with derepression of translation results in enhanced heterosynaptic plasticity.

Altered sensitivity to excitotoxic cell death and glutamate receptor expression between two commonly studied mouse strains

Alterations in glutamatergic synapse function have been implicated in the pathogenesis of many different neurological disorders, including ischemia, epilepsy, Parkinson's disease, Alzheimer's disease, and Huntington's disease. While studying glutamate receptor function in juvenile Batten disease on the C57BL/6J and 129S6/S(v)E(v) mouse backgrounds, we noticed differences unlikely to be due to mutation difference alone. We report here that primary cerebellar granule cell cultures from C57BL/6J mice are more sensitive to N-methyl-D-aspartate (NMDA)-mediated cell death. Moreover, sensitivity to AMPA-mediated excitotoxicity is more variable and is dependent on the treatment conditions and age of the cultures. Glutamate receptor surface expression levels examined in vitro by in situ ELISA and in vivo by Western blot in surface cross-linked cerebellar samples indicated that these differences in sensitivity likely are due to strain-dependent differences in cell surface receptor expression levels. We propose that differences in glutamate receptor expression and in excitotoxic vulnerability should be taken into consideration in the context of characterizing disease models on the C57BL/6J and 129S6/S(v)E(v) mouse backgrounds.

Subregion-specific dendritic spine abnormalities in the hippocampus of Fmr1 KO mice

Fragile X syndrome (FXS) is the most common inherited form of mental retardation and is caused by the lack of fragile X mental retardation protein (FMRP). In the brain, spine abnormalities have been reported in both patients with FXS and Fmr1 knockout mice. This altered spine morphology has been linked to disturbed synaptic transmission related to altered signaling in the excitatory metabotropic glutamate receptor 5 (mGluR5) pathway. We investigated hippocampal protrusion morphology in adult Fmr1 knockout mice. Our results show a hippocampal CA1-specific altered protrusion phenotype, which was absent in the CA3 region of the hippocampus. This suggests a subregion-specific function of FMRP in synaptic plasticity in the brain.

AFQ056, a new mGluR5 antagonist for treatment of fragile X syndrome

Fragile X syndrome, the most common form of inherited intellectual disability, is caused by a lack of FMRP, which is the product of the Fmr1 gene. FMRP is an RNA-binding protein and a component of RNA-granules found in the dendrites of neurons. At the synapse, FMRP is involved in regulation of translation of specific target mRNAs upon stimulation of mGluR5 receptors. In this study, we test the effects of a new mGluR5 antagonist (AFQ056) on the prepulse inhibition of startle response in mice. We show that Fmr1 KO mice have a deficit in inhibition of the startle response after a prepulse and that AFQ056 can rescue this phenotype. We also studied the effect of AFQ056 on cultured Fmr1 KO hippocampal neurons; untreated neurons showed elongated spines and treatment resulted in shortened spines. These results suggest that AFQ056 might be a potent mGluR5 antagonist to rescue various aspects of the fragile X phenotype.

The neurochemical basis for the treatment of autism spectrum disorders and Fragile X Syndrome

Autism spectrum disorders (ASD) and Fragile X Syndrome (FXS) are neurodevelopmental disorders that share overlapping behavioral characteristics. While FXS is known to result from a specific genetic mutation, the causes of the majority of cases of ASD are unknown. Animal models of FXS have revealed new insight into the cellular and biochemical changes that occur in the central nervous system in this disorder, while human genetic studies on individuals with autism have identified sets of genes that may increase susceptibility to the disorder. Together these discoveries suggest overlapping biochemical characteristics and reveal new directions for the potential development of pharmacological therapies that might prove useful in the treatment of both FXS and ASD. In particular, delayed synaptic maturation, abnormal synaptic structure and/or function and alterations in intracellular signaling pathways have been linked to the pathogenesis of FXS and ASD. Aberrations in GABA(A) receptor ion channels and the G-protein coupled metabotropic glutamate and GABA(B) transmitter systems are also linked to both disorders and these receptors are currently at the forefront of preclinical and clinical research into treatments for both autism and Fragile X Syndrome.

Glutamate receptor dysfunction and drug targets across models of autism spectrum disorders

There is strong evidence that metabotropic and ionotropic glutamate receptors are affected in autism spectrum disorders (ASD), but there are few candidate genes indicating involvement of these receptors. This suggests that glutamate receptor dysregulation may primarily be involved in the expression of ASD, but is an uncommon etiology. Directly implicated in models of fragile-X with ASD phenotypes is metabotropic glutamate receptor type 5 (mGluR5), which appears to be an effective pharmacologic target in a number of models of ASD. The review of other ASD models demonstrates that there is also evidence of a role for kainate, NMDA, and AMPA receptors in the neuropathophysiology of ASD, though the relationship between dysfunction in those receptors and ASD-associated phenotypes is not well understood. Current models indicate a way forward to delineate the role of glutamate receptors in ASD. Further development of preclinical models focusing on glutamate receptors may provide tools to target a clinically important subset of ASD symptoms.

Hypersensitivity to mGluR5 and ERK1/2 leads to excessive protein synthesis in the hippocampus of a mouse model of fragile X syndrome

Fragile X syndrome (FXS) is caused by loss of the FMR1 gene product FMRP (fragile X mental retardation protein), a repressor of mRNA translation. According to the metabotropic glutamate receptor (mGluR) theory of FXS, excessive protein synthesis downstream of mGluR5 activation causes the synaptic pathophysiology that underlies multiple aspects of FXS. Here, we use an in vitro assay of protein synthesis in the hippocampus of male Fmr1 knock-out (KO) mice to explore the molecular mechanisms involved in this core biochemical phenotype under conditions where aberrant synaptic physiology has been observed. We find that elevated basal protein synthesis in Fmr1 KO mice is selectively reduced to wild-type levels by acute inhibition of mGluR5 or ERK1/2, but not by inhibition of mTOR (mammalian target of rapamycin). The mGluR5-ERK1/2 pathway is not constitutively overactive in the Fmr1 KO, however, suggesting that mRNA translation is hypersensitive to basal ERK1/2 activation in the absence of FMRP. We find that hypersensitivity to ERK1/2 pathway activation also contributes to audiogenic seizure susceptibility in the Fmr1 KO. These results suggest that the ERK1/2 pathway, and other neurotransmitter systems that stimulate protein synthesis via ERK1/2, represent additional therapeutic targets for FXS.

Recurrent distal 7q11.23 deletion including HIP1 and YWHAG identified in patients with intellectual disabilities, epilepsy, and neurobehavioral problems

We report 26 individuals from ten unrelated families who exhibit variable expression and/or incomplete penetrance of epilepsy, learning difficulties, intellectual disabilities, and/or neurobehavioral abnormalities as a result of a heterozygous microdeletion distally adjacent to the Williams-Beuren syndrome region on chromosome 7q11.23. In six families with a common recurrent ∼1.2 Mb deletion that includes the Huntingtin-interacting protein 1 (HIP1) and tyrosine 3-monooxygenase/tryptophan 5-monooxygenase activation protein gamma (YWHAG) genes and that is flanked by large complex low-copy repeats, we identified sites for nonallelic homologous recombination in two patients. There were no cases of this ∼1.2 Mb distal 7q11.23 deletion copy number variant identified in over 20,000 control samples surveyed. Three individuals with smaller, nonrecurrent deletions (∼180-500 kb) that include HIP1 but not YWHAG suggest that deletion of HIP1 is sufficient to cause neurological disease. Mice with targeted mutation in the Hip1 gene (Hip1⁻(/)⁻) develop a neurological phenotype characterized by failure to thrive, tremor, and gait ataxia. Overall, our data characterize a neurodevelopmental and epilepsy syndrome that is likely caused by recurrent and nonrecurrent deletions, including HIP1. These data do not exclude the possibility that YWHAG loss of function is also sufficient to cause neurological phenotypes. Based on the current knowledge of Hip1 protein function and its proposed role in AMPA and NMDA ionotropic glutamate receptor trafficking, we believe that HIP1 haploinsufficiency in humans will be amenable to rational drug design for improved seizure control and cognitive and behavioral function.

The genetics of child psychiatric disorders: focus on autism and Tourette syndrome

Investigations into the genetics of child psychiatric disorders have finally begun to shed light on molecular and cellular mechanisms of psychopathology. The first strains of success in this notoriously difficult area of inquiry are the result of an increasingly sophisticated appreciation of the allelic architecture of common neuropsychiatric and neurodevelopmental disorders, the consolidation of large patient cohorts now beginning to reach sufficient size to power reliable studies, the emergence of genomic tools enabling comprehensive investigations of rare as well as common genetic variation, and advances in developmental neuroscience that are fueling the rapid translation of genetic findings.

BDNF and TrkB in neuronal differentiation of Fmr1-knockout mouse

Fragile X syndrome (FXS) is a common cause of inherited mental retardation and the best characterized form of autistic spectrum disorders. FXS is caused by the loss of functional fragile X mental retardation protein (FMRP), which leads to abnormalities in the differentiation of neural progenitor cells (NPCs) and in the development of dendritic spines and neuronal circuits. Brain-derived neurotrophic factor (BDNF) and its TrkB receptors play a central role in neuronal maturation and plasticity. We studied BDNF/TrkB actions in the absence of FMRP and show that an increase in catalytic TrkB expression in undifferentiated NPCs of Fmr1-knockout (KO) mice, a mouse model for FXS, is associated with changes in the differentiation and migration of neurons expressing TrkB in neurosphere cultures and in the developing cortex. Aberrant intracellular calcium responses to BDNF and ATP in subpopulations of differentiating NPCs combined with changes in the expression of BDNF and TrkB suggest cell subtype-specific alterations during early neuronal maturation in the absence of FMRP. Furthermore, we show that dendritic targeting of Bdnf mRNA was increased under basal conditions and further enhanced in cortical layer V and hippocampal CA1 neurons of Fmr1-KO mice by pilocarpine-induced neuronal activity represented by convulsive seizures, suggesting that BDNF/TrkB-mediated feedback mechanisms for strengthening the synapses were compromised in the absence of FMRP. Pilocarpine-induced seizures caused an accumulation of Bdnf mRNA transcripts in the most proximal segments of dendrites in cortical but not in hippocampal neurons of Fmr1-KO mice. In addition, BDNF protein levels were increased in the hippocampus but reduced in the cortex of Fmr1-KO mice in line with regional differences of synaptic plasticity in the brain of Fmr1-KO mice. Altogether, the present data suggest that alterations in the BDNF/TrkB signaling modulate brain development and impair synaptic plasticity in FXS.

Genetics of early onset cognitive impairment

Intellectual disability (ID) is the leading socio-economic problem of health care, but compared to autism and schizophrenia, it has received very little public attention. Important risk factors for ID are malnutrition, cultural deprivation, poor health care, and parental consanguinity. In the Western world, fetal alcohol exposure is the most common preventable cause. Most severe forms of ID have genetic causes. Cytogenetically detectable and submicroscopic chromosomal rearrangements account for approximately 25% of all cases. X-linked gene defects are responsible in 10-12% of males with ID; to date, 91 of these defects have been identified. In contrast, autosomal gene defects have been largely disregarded, but due to coordinated efforts and the advent of next-generation DNA sequencing, this is about to change. As shown for Fra(X) syndrome, this renewed focus on autosomal gene defects will pave the way for molecular diagnosis and prevention, shed more light on the pathogenesis of ID, and reveal new opportunities for therapy.

Potential therapeutic interventions for fragile X syndrome

Fragile X syndrome (FXS) is caused by a lack of the fragile X mental retardation protein (FMRP); FMRP deficiency in neurons of patients with FXS causes intellectual disability (IQ<70) and several behavioural problems, including hyperactivity and autistic-like features. In the brain, no gross morphological malformations have been found, although subtle spine abnormalities have been reported. FXS has been linked to altered group I metabotropic glutamate receptor (mGluR)-dependent and independent forms of synaptic plasticity. Here, we discuss potential targeted therapeutic strategies developed to specifically correct disturbances in the excitatory mGluR and the inhibitory gamma-aminobutyric (GABA) receptor pathways that have been tested in animal models and/or in clinical trials with patients with FXS.

Delayed stabilization of dendritic spines in fragile X mice

Fragile X syndrome (FXS) causes mental impairment and autism through transcriptional silencing of the Fmr1 gene, resulting in the loss of the RNA-binding protein fragile X mental retardation protein (FMRP). Cortical pyramidal neurons in affected individuals and Fmr1 knock-out (KO) mice have an increased density of dendritic spines. The mutant mice also show defects in synaptic and experience-dependent circuit plasticity, which are known to be mediated in part by dendritic spine dynamics. We used in vivo time-lapse imaging with two-photon microscopy through cranial windows in male and female neonatal mice to test the hypothesis that dynamics of dendritic protrusions are altered in KO mice during early postnatal development. We find that layer 2/3 neurons from wild-type mice exhibit a rapid decrease in dendritic spine dynamics during the first 2 postnatal weeks, as immature filopodia are replaced by mushroom spines. In contrast, KO mice show a developmental delay in the downregulation of spine turnover and in the transition from immature to mature spine subtypes. Blockade of metabotropic glutamate receptor (mGluR) signaling, which reverses some adult phenotypes of KO mice, accentuated this immature protrusion phenotype in KO mice. Thus, absence of FMRP delays spine stabilization and dysregulated mGluR signaling in FXS may partially normalize this early synaptic defect.

Excess phosphoinositide 3-kinase subunit synthesis and activity as a novel therapeutic target in fragile X syndrome

Fragile X syndrome (FXS) is an inherited neurologic disease caused by loss of fragile X mental retardation protein (FMRP), which is hypothesized to mediate negative regulation of mRNA translation at synapses. A prominent feature of FXS animal models is exaggerated signaling through group 1 metabotropic glutamate receptors (gp1 mGluRs), and therapeutic strategies to treat FXS are targeted mainly at gp1 mGluRs. Recent studies, however, indicate that a variety of receptor-mediated signal transduction pathways are dysregulated in FXS, suggesting that FMRP acts on a common downstream signaling molecule. Here, we show that deficiency of FMRP results in excess activity of phosphoinositide 3-kinase (PI3K), a downstream signaling molecule of many cell surface receptors. In Fmr1 knock-out neurons, excess synaptic PI3K activity can be reduced by perturbation of gp1 mGluR-mediated signaling. Remarkably, increased PI3K activity was also observed in FMRP-deficient non-neuronal cells in the absence of gp1 mGluRs. Here, we show that FMRP regulates the synthesis and synaptic localization of p110beta, the catalytic subunit of PI3K. In wild type, gp1 mGluR activation induces p110beta translation, p110beta protein expression, and PI3K activity. In contrast, both p110beta protein synthesis and PI3K activity are elevated and insensitive to gp1 mGluR stimulation in Fmr1 knock-out. This suggests that dysregulated PI3K signaling may underlie the synaptic impairments in FXS. In support of this hypothesis, we show that PI3K antagonists rescue three FXS-associated phenotypes: dysregulated synaptic protein synthesis, excess AMPA receptor internalization, and increased spine density. Targeting excessive PI3K activity might thus be a potent therapeutic strategy for FXS.

Fragile X: leading the way for targeted treatments in autism

Two different mutations in the FMR1 gene may lead to autism. The full mutation, with >200 CGG repeats in the 5' end of FMR1, leads to hypermethylation and transcriptional silencing of FMR1, resulting in absence or deficiency of the protein product, FMRP. Deficiency of FMRP in the brain causes fragile X syndrome (FXS). Autism occurs in approximately 30% of those with FXS, and pervasive developmental disorders-not otherwise specified occur in an additional 30%. FMRP is an RNA binding protein that modulates receptor-mediated dendritic translation; deficiency leads to dysregulation of many proteins important for synaptic plasticity. Group I metabotropic glutamate receptor (mGluR1/5) activated translation is upregulated in FXS, and new targeted treatments that act on this system include mGluR5 antagonists and GABA agonists, which may reverse the cognitive and behavioral deficits in FXS. Matrix metalloproteinase 9 (MMP-9) is one of the proteins elevated in FXS, and minocycline reduces excess MMP-9 activity in the Fmr1 knockout mouse model of FXS. Both minocycline and mGluR5 antagonists are currently being evaluated in patients with FXS through controlled treatment trials. The premutation (55-200 CGG repeats) may also contribute to the mechanism of autism in approximately 10% of males and 2-3% of females. Premutations with <150 repeats exert cellular effects through a different molecular mechanism, one that involves elevated levels of FMR1 mRNA, CGG-mediated toxicity to neurons, early cell death, and fragile X-associated tremor/ataxia syndrome. In those with large premutations (150-200), lowered levels of FMRP also occur.

Origins of epilepsy in fragile X syndrome

Fragile X syndrome is the leading heritable form of cognitive impairment and the leading known monogenic disorder associated with autism. Roughly one-quarter of children with this disorder have seizures, most of which are relatively benign and are resolved beyond childhood. Because of the prevalence of fragile X syndrome, numerous animal models have been developed and electrophysiological studies have taken place to investigate its pathogenesis. The investigations have yielded a wealth of information regarding the synaptic dysfunction that underlies the hyperexcitability and epileptiform features associated with this disorder.

Characterization and reversal of synaptic defects in the amygdala in a mouse model of fragile X syndrome

Fragile X syndrome (FXS), a common inherited form of mental impairment and autism, is caused by transcriptional silencing of the fragile X mental retardation 1 (FMR1) gene. Earlier studies have identified a role for aberrant synaptic plasticity mediated by the metabotropic glutamate receptors (mGluRs) in FXS. However, many of these observations are derived primarily from studies in the hippocampus. The strong emotional symptoms of FXS, on the other hand, are likely to involve the amygdala. Unfortunately, little is known about how exactly FXS affects synaptic function in the amygdala. Here, using whole-cell recordings in brain slices from adult Fmr1 knockout mice, we find mGluR-dependent long-term potentiation to be impaired at thalamic inputs to principal neurons in the lateral amygdala. Consistent with this long-term potentiation deficit, surface expression of the AMPA receptor subunit, GluR1, is reduced in the lateral amygdala of knockout mice. In addition to these postsynaptic deficits, lower presynaptic release was manifested by a decrease in the frequency of spontaneous miniature excitatory postsynaptic currents (mEPSCs), increased paired-pulse ratio, and slower use-dependent block of NMDA receptor currents. Strikingly, pharmacological inactivation of mGluR5 with 2-methyl-6-phenylethynyl-pyridine (MPEP) fails to rescue either the deficit in long-term potentiation or surface GluR1. However, the same acute MPEP treatment reverses the decrease in mEPSC frequency, a finding of potential therapeutic relevance. Therefore, our results suggest that synaptic defects in the amygdala of knockout mice are still amenable to pharmacological interventions against mGluR5, albeit in a manner not envisioned in the original hippocampal framework.

Interaction between Ephrins and mGlu5 metabotropic glutamate receptors in the induction of long-term synaptic depression in the hippocampus

We applied the group-I metabotropic glutamate (mGlu) receptor agonist, 3,5-dihydroxyphenylglycine (DHPG), to neonatal or adult rat hippocampal slices at concentrations (10 microM) that induced a short-term depression (STD) of excitatory synaptic transmission at the Schaffer collateral/CA1 synapses. DHPG-induced STD was entirely mediated by the activation of mGlu5 receptors because it was abrogated by the mGlu5 receptor antagonist, MPEP [2-methyl-6-(phenylethynyl)pyridine], but not by the mGlu1 receptor antagonist, CPCCOEt [7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester]. Knowing that ephrin-Bs functionally interact with group-I mGlu receptors (Calò et al., 2005), we examined whether pharmacological activation of ephrin-Bs could affect DHPG-induced STD. We activated ephrin-Bs using their cognate receptor, EphB1, under the form of a preclustered EphB1/Fc chimera. Addition of clustered EphB1/Fc alone to the slices induced a small but nondecremental depression of excitatory synaptic transmission, which differed from the depression induced by 10 microM DHPG. Surprisingly, EphB1/Fc-induced synaptic depression was abolished by MPEP (but not by CPCCOEt) suggesting that it required the endogenous activation of mGlu5 receptors. In addition, coapplication of DHPG and EphB1/Fc, resulted in a large and nondecremental long-term depression. The effect of clustered EphB1/Fc was specific because it was not mimicked by unclustered EphB1/Fc or clustered EphA1/Fc. These findings raise the intriguing possibility that changes in synaptic efficacy mediated by mGlu5 receptors are under the control of the ephrin/Eph receptor system, and that the neuronal actions of ephrins can be targeted by drugs that attenuate mGlu5 receptor signaling.

The Angelman Syndrome protein Ube3A regulates synapse development by ubiquitinating arc

Angelman Syndrome is a debilitating neurological disorder caused by mutation of the E3 ubiquitin ligase Ube3A, a gene whose mutation has also recently been associated with autism spectrum disorders (ASDs). The function of Ube3A during nervous system development and how Ube3A mutations give rise to cognitive impairment in individuals with Angleman Syndrome and ASDs are not clear. We report here that experience-driven neuronal activity induces Ube3A transcription and that Ube3A then regulates excitatory synapse development by controlling the degradation of Arc, a synaptic protein that promotes the internalization of the AMPA subtype of glutamate receptors. We find that disruption of Ube3A function in neurons leads to an increase in Arc expression and a concomitant decrease in the number of AMPA receptors at excitatory synapses. We propose that this deregulation of AMPA receptor expression at synapses may contribute to the cognitive dysfunction that occurs in Angelman Syndrome and possibly other ASDs.

Fragile X syndrome -- from genes to cognition

Fragile X syndrome (FXS), a single gene disorder with an expanded CGG allele on the X chromosome, is the most common form of inherited cognitive impairment. The cognitive deficit ranges from mild learning disabilities to severe intellectual disability. The phenotype includes hyperactivity, short attention span, emotional problems including anxiety, social avoidance, poor eye contact, and hyperarousal to sensory stimuli. Imaging studies in FXS have clarified the impact of the FMR1 mutation on brain development and function by documenting structural abnormalities, predominantly in the caudate nucleus and cerebellum, and functional deficits in the caudate, frontal-striatal circuits, and the limbic system. On the basis of current research results, a targeted treatment for FXS will be available in the near future. Currently, a number of psychopharmacological agents are helpful in treating many of the problems in FXS including hyperactivity, attention deficits, anxiety, episodic aggression, and hyperarousal. Although the targeted treatments aim at strengthening synaptic connections, it is essential that these treatments are combined with learning programs that address the cognitive deficits in FXS.