Fragile X mental retardation protein is translated near synapses in response to neurotransmitter activation

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.

Synapses on demand require dendrites at the ready: how defining stages of dendritic development in vitro could inform studies of behaviorally driven information storage in the brain

Bill Greenough's work provides a framework for thinking about synaptogenesis not only as a key step in the initial wiring of neural systems according to a species typical plan (i.e., experience-expectant development), but also as a mechanism for storing information based an individual's unique experience over its lifetime (i.e., experience-dependent plasticity). Analysis of synaptic development in vitro brings a new opportunity to test the limits of expectant-expectant development at the level of the individual neuron. We analyzed dendritic growth, synapse formation, and the development of specialized cytoplasmic microdomains during development in cultured hippocampal neurons, to determine if the timing of each of these events is correlated. Taken together, the findings reported here support the hypotheses that (1) dendritic development is rate limiting in synapse formation and (2) synaptic circuits are assembled in a step-wise fashion consistent with a stage-specific shift from genomically pre-programmed to activity-dependent mechanisms.

Phenotypic changes in calbindin D28K immunoreactivity in the hippocampus of Fmr1 knockout mice

Fragile X syndrome (FXS), the most prevalent form of inherited mental retardation, is caused by the lack of FMRP (fragile mental retardation protein) as a result of the transcriptional silencing of the FMR1 gene. Here we analyze the immunohistochemical expression of the calbindin D28K protein in the hippocampus of Fmr1 knockout (KO) mice and compare it with that of their wildtype (WT) littermates. The spatial distribution pattern of calbindin-immunoreactive cells in the hippocampus was similar in WT and KO mice but for each age studied (ranging from 3.5-8 months) the dentate gyrus of Fmr1-KO mice showed a significant reduction in calbindin-immunoreactive granule cells. Also, the number of calbindin-immunoreactive cells was reduced in the CA1 pyramidal layer in KO mice compared to their WT littermates. In addition, Frm1-KO mice showed a group of calbindin-immunoreactive cells located only in the left CA3b subregion that was only sometimes observed in WT mice. Overall, the absence of FMRP results in a dysregulation of the calbindin protein expression in the hippocampus. This dysregulation is cell type- and time-dependent and as a consequence key elements of the hippocampal trisynaptic circuitry may lack calbindin in critical periods for normal memory/learning abilities to be achieved and may explain some of the FXS symptoms observed in the Fmr1-KO mouse model.

Neurodegeneration the RNA way

The expression, processing, transport and activities of both coding and non-coding RNAs play critical roles in normal neuronal function and differentiation. Over the past decade, these same pathways have come under scrutiny as potential contributors to neurodegenerative disease. Here we focus broadly on the roles of RNA and RNA processing in neurodegeneration. We first discuss a set of "RNAopathies", where non-coding repeat expansions drive pathogenesis through a surprisingly diverse set of mechanisms. We next explore an emerging class of "RNA binding proteinopathies" where redistribution and aggregation of the RNA binding proteins TDP-43 or FUS contribute to a potentially broad range of neurodegenerative disorders. Lastly, we delve into the potential contributions of alterations in both short and long non-coding RNAs to neurodegenerative illness.

Glycogen synthase kinase-3: a promising therapeutic target for fragile x syndrome

Recent advances in understanding the pathophysiological mechanisms contributing to fragile X syndrome (FXS) have increased optimism that drug interventions can provide significant therapeutic benefits. FXS results from inadequate expression of functional fragile X mental retardation protein (FMRP). FMRP may have several functions, but it is most well-established as an RNA binding protein that regulates translation, and it is thought that by this mechanism FMRP is capable of affecting numerous cellular processes by selectively regulating protein levels. The multiple cellular functions regulated by FMRP suggest that multiple interventions may be required for reversing the effects of deficient FMRP. Evidence that inhibitors of glycogen synthase kinase-3 (GSK3) may contribute to the therapeutic treatment of FXS is reviewed here. Lithium, a GSK3 inhibitor, improved function in the Drosophila model of FXS. In mice lacking FMRP expression (FX mice), GSK3 is hyperactive in several brain regions. Significant improvements in several FX-related phenotypes have been obtained in FX mice following the administration of lithium, and in some case other GSK3 inhibitors. These responses include normalization of heightened audiogenic seizure susceptibility and of hyperactive locomotor behavior, enhancement of passive avoidance learning retention and of sociability behaviors, and corrections of macroorchidism, neuronal spine density, and neural plasticity measured electrophysiologically as long term depression. A pilot clinical trial of lithium in patients with FXS also found improvements in several measures of behavior. Taken together, these findings indicate that lithium and other inhibitors of GSK3 are promising candidate therapeutic agents for treating FXS.

Modulation of dADAR-dependent RNA editing by the Drosophila fragile X mental retardation protein

Loss of FMR1 gene function results in fragile X syndrome (FXS), the most common heritable form of intellectual disability. The protein encoded from this locus (FMRP) is an RNA binding protein thought to primarily act as a translational regulator, however recent studies implicate FMRP in other mechanisms of gene regulation. Here, we demonstrate that the Drosophila fragile X homolog (dFMR1) biochemically interacts with the A-to-I RNA editing enzyme, dADAR. We found that dAdar and dfmr1 mutant larvae exhibit distinct morphological neuromuscular junction (NMJ) defects. Epistasis experiments based on these phenotypic differences suggest that dAdar acts downstream of dfmr1 and that dFMR1 modulates dADAR activity. Furthermore, sequence analyses revealed that loss or overexpression of dFMR1 affects editing efficiency on certain dADAR targets with defined roles in synaptic transmission. These results link dFMR1 with the RNA editing pathway and suggest that proper NMJ synaptic architecture requires modulation of dADAR activity by dFMR1.

Comprehensive motor testing in Fmr1-KO mice exposes temporal defects in oromotor coordination

Fragile X syndrome (FXS; MIM #300624), a well-recognized form of inherited human mental retardation is caused, in most cases, by a CGG trinucleotide repeat expansion in the 5'-untranslated region of FMR1, resulting in reduced expression of the fragile X mental retardation protein (FMRP). Clinical features include macroorchidism, anxiety, mental retardation, motor coordination, and speech articulation deficits. The Fmr1 knockout (Fmr1-KO) mouse, a mouse model for FXS, has been shown to replicate the macroorchidism, cognitive deficits, and neuroanatomical abnormalities found in human FXS. Here we asked whether Fmr1-KO mice also display appendicular and oromotor deficits comparable to the ataxia and dysarthric speech seen in FXS patients. We employed standard motor tests for balance and appendicular motor coordination, and used a novel long-term fluid-licking assay to investigate oromotor function in Fmr1-KO mice and their wild-type (WT) littermates. Fmr1-KO mice performed equally well as their WT littermates on standard motor tests, with the exception of a raised-beam task. However, Fmr1-KO mice had a significantly slower licking rhythm than their WT littermates. Deficits in rhythmic fluid-licking in Fmr1-KO mice have been linked to cerebellar pathologies. It is believed that balance and motor coordination deficits in FXS patients are caused by cerebellar neurophathologies. The neuronal bases of speech articulation deficits in FXS patients are currently unknown. It is yet to be established whether similar neuronal circuits control rhythmic fluid-licking pattern in mice and speech articulation movement in humans.

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.

Amygdala regulation of fear and emotionality in fragile X syndrome

Fear is a universal response to a threat to one's body or social status. Disruption in the detection and response of the brain's fear system is commonly observed in a variety of neurodevelopmental disorders, including fragile X syndrome (FXS), a brain disorder characterized by variable cognitive impairment and behavioral disturbances such as social avoidance and anxiety. The amygdala is highly involved in mediating fear processing, and increasing evidence supports the idea that inhibitory circuits play a key role in regulating the flow of information associated with fear conditioning in the amygdala. Here, we review the known and potential importance of amygdala fear circuits in FXS, and how developmental studies are critical to understand the formation and function of neuronal circuits that modulate amygdala-based behaviors.

Overview of glutamatergic neurotransmission in the nervous system

This introductory article to the special edition on glutamate neurotransmission in neuropsychiatric disorders provides an overview of glutamate neurotransmitter system physiology and pharmacology. Glutamate was only relatively recently recognized as the major excitatory neurotransmitter in the mammalian brain, in part due to its ubiquitous nature and diverse metabolic roles within the CNS. The extremely high concentration of glutamate in brain tissue paired with its excitotoxic potential requires tight physiological regulation of extracellular glutamate levels and receptor signaling in order to assure optimal excitatory neurotransmission but limits excitotoxic damage. In order to achieve this high level of control, the system has developed a complex physiology with multiple regulatory processes modulating glutamate metabolism, release, receptor signaling, and uptake. The basic physiology of the various regulatory components of the system including the rich receptor pharmacology is briefly reviewed. Potential contributions from each of the system's components to the pathophysiology of neuropsychiatric illnesses are briefly discussed, as are the many new pharmacological targets for drug development provided by the system, especially as they pertain to the proceeding preclinical and clinical articles in this issue.

miR-124a is required for hippocampal axogenesis and retinal cone survival through Lhx2 suppression

MicroRNA-124a (miR-124a) is the most abundant microRNA expressed in the vertebrate CNS. Despite past investigations into the role of miR-124a, inconsistent results have left the in vivo function of miR-124a unclear. We examined the in vivo function of miR-124a by targeted disruption of Rncr3 (retinal non-coding RNA 3), the dominant source of miR-124a. Rncr3(-/-) mice exhibited abnormalities in the CNS, including small brain size, axonal mis-sprouting of dentate gyrus granule cells and retinal cone cell death. We found that Lhx2 is an in vivo target mRNA of miR-124a. We also observed that LHX2 downregulation by miR-124a is required for the prevention of apoptosis in the developing retina and proper axonal development of hippocampal neurons. These results suggest that miR-124a is essential for the maturation and survival of dentate gyrus neurons and retinal cones, as it represses Lhx2 translation.

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.

Defects in translational regulation contributing to human cognitive and behavioral disease

Recent data suggest that the levels of many synaptic proteins may be tightly controlled by the opposing processes of new translation and protein turnover in neurons. Alterations in this balance or in the levels of such dosage-sensitive proteins that result in altered stoichiometry of protein complexes at developing and remodeling synapses may underlie several human cognitive diseases including Fragile X Syndrome, autism spectrum disorders, Angelman syndrome and non-syndromic mental retardation. While a significant amount is known about the transduction of membrane signals to the translational apparatus through kinase cascades acting on general translation factors, much less is understood about how such signals may influence the activity of mRNA-specific regulators, their mechanisms of action and the specific sets of mRNAs they regulate. New approaches to the unbiased in vivo identification of maps of binding sites for these proteins on mRNA is expected to greatly increase our understanding of this crucial level of regulation in neuronal development and function.

Fragile X-associated disorders: a clinical overview

Fragile X Syndrome (FraX) is the most common inherited cause of learning disability worldwide. FraX is an X-linked neuro-developmental disorder involving an unstable trinucleotide repeat expansion of cytosine guanine guanine (CGG). Individuals with the full mutation of FraX have >200 GG repeats with premutation carriers having 55-200 GG repeats. A wide spectrum of physical, behavioural, cognitive, psychiatric and medical problems have been associated with both full mutation and premutation carriers of FraX. In this review, we detail the clinical profile and examine the aetiology, epidemiology, neuropathology, neuroimaging findings and possible management strategies for individuals with both the full mutation and premutation of FraX.

The FRAXopathies: definition, overview, and update

The fragile X syndrome, fragile X tremor ataxia syndrome, and premature ovarian insufficiency are conditions related to the X chromosome folate-sensitive fragile site FRAXA. Therefore, we propose that they are considered as a family of disorders under the general designation of FRAXopathies. The present review will outline the main clinical and molecular features of these disorders, with special emphasis on the pathogenic mechanisms that lead to distinct phenotypes, starting from related mutations. The understanding of these mechanisms is already generating promising therapeutic approaches.

Sleep and synaptic homeostasis: structural evidence in Drosophila

The functions of sleep remain elusive, but a strong link exists between sleep need and neuronal plasticity. We tested the hypothesis that plastic processes during wake lead to a net increase in synaptic strength and sleep is necessary for synaptic renormalization. We found that, in three Drosophila neuronal circuits, synapse size or number increases after a few hours of wake and decreases only if flies are allowed to sleep. A richer wake experience resulted in both larger synaptic growth and greater sleep need. Finally, we demonstrate that the gene Fmr1 (fragile X mental retardation 1) plays an important role in sleep-dependent synaptic renormalization.

Descartins, the memory molecules

Investigation of protein translation at the synapse, using functioning synaptic particles termed synaptoneurosomes, has led to identification of Fragile X protein as a key synaptic component. In its absence, some key mRNAs are translated more diffusely in the cell, and more slowly. Recent studies have implicated ERK (extracellular receptor regulated kinase) as a central factor in regulating the kinetics of translation at the synapse.

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.

Modulation of dendritic spines and synaptic function by Rac1: a possible link to Fragile X syndrome pathology

Rac1, a protein of the Rho GTPase subfamily, has been implicated in neuronal and spine development as well as the formation of synapses with appropriate partners. Dendrite and spine abnormalities have been implicated in several psychiatric disorders such as Fragile X syndrome, where neurons show a high density of long, thin, and immature dendritic spines. Although abnormalities in dendrites and spines have been correlated with impaired cognitive abilities in mental retardation, the causes of these malformations are not yet well understood. Fragile X syndrome is the most common type of inherited mental retardation caused by the absence of FMRP protein, a RNA-binding protein implicated in the regulation of mRNA translation and transport, leading to protein synthesis. We suggest that FMRP might act as a negative regulator on the synthesis of Rac1. Maintaining an optimal level of Rac1 and facilitating the reorganization of the cytoskeleton likely leads to normal neuronal morphology during activity-dependent plasticity. In our study, we first demonstrated that Rac1 is not only associated but necessary for normal spine development and long-term synaptic plasticity. We further showed that, in Fmr1 knockout mice, lack of FMRP induces an overactivation of Rac1 in the mouse brain and other organs that have been shown to be altered in Fragile X syndrome. In those animals, pharmacological manipulation of Rac1 partially reverses their altered long-term plasticity. Thus, regulation of Rac1 may provide a functional link among deficient neuronal morphology, aberrant synaptic plasticity and cognition impairment in Fragile X syndrome.

Cytoplasmic intron sequence-retaining transcripts can be dendritically targeted via ID element retrotransposons

RNA precursors give rise to mRNA after splicing of intronic sequences traditionally thought to occur in the nucleus. Here, we show that intron sequences are retained in a number of dendritically-targeted mRNAs, by using microarray and Illumina sequencing of isolated dendritic mRNA as well as in situ hybridization. Many of the retained introns contain ID elements, a class of SINE retrotransposon. A portion of these SINEs confers dendritic targeting to exogenous and endogenous transcripts showing the necessity of ID-mediated mechanisms for the targeting of different transcripts to dendrites. ID elements are capable of selectively altering the distribution of endogenous proteins, providing a link between intronic SINEs and protein function. As such, the ID element represents a common dendritic targeting element found across multiple RNAs. Retention of intronic sequence is a more general phenomenon than previously thought and plays a functional role in the biology of the neuron, partly mediated by co-opted repetitive sequences.

Clustered coding variants in the glutamate receptor complexes of individuals with schizophrenia and bipolar disorder

Current models of schizophrenia and bipolar disorder implicate multiple genes, however their biological relationships remain elusive. To test the genetic role of glutamate receptors and their interacting scaffold proteins, the exons of ten glutamatergic 'hub' genes in 1304 individuals were re-sequenced in case and control samples. No significant difference in the overall number of non-synonymous single nucleotide polymorphisms (nsSNPs) was observed between cases and controls. However, cluster analysis of nsSNPs identified two exons encoding the cysteine-rich domain and first transmembrane helix of GRM1 as a risk locus with five mutations highly enriched within these domains. A new splice variant lacking the transmembrane GPCR domain of GRM1 was discovered in the human brain and the GRM1 mutation cluster could perturb the regulation of this variant. The predicted effect on individuals harbouring multiple mutations distributed in their ten hub genes was also examined. Diseased individuals possessed an increased load of deleteriousness from multiple concurrent rare and common coding variants. Together, these data suggest a disease model in which the interplay of compound genetic coding variants, distributed among glutamate receptors and their interacting proteins, contribute to the pathogenesis of schizophrenia and bipolar disorders.

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.

Dual regulation of fragile X mental retardation protein by group I metabotropic glutamate receptors controls translation-dependent epileptogenesis in the hippocampus

Group I metabotropic glutamate receptors (mGluRs) stimulation activates translation-dependent epileptogenesis in the hippocampus. This translation is regulated by repressors, including BC1 RNA and fragile X mental retardation protein (FMRP). Recent data indicate that group I mGluR stimulation exerts bidirectional control over FMRP level by activating translation and ubiquitin-proteasome system (UPS)-dependent proteolysis for the up- and downregulation of the protein, respectively. At present, the temporal relationship of translation and proteolysis on FMRP and their interplay for group I mGluR-mediated translation and epileptogenesis are unknown. We addressed these issues by using mouse hippocampal slices. Agonist [(S)-3,5-dihydroxyphenylglycine (DHPG)] stimulation of group I mGluRs caused a biphasic change in FMRP level. An initial decrease (within 10 min) was followed by an increase at 30 min. When slices were pretreated with translation inhibitor (anisomycin or cycloheximide), group I mGluRs elicited a sustained decrease in FMRP. This decrease was prevented by a proteasome inhibitor [Z-Leu-Leu-Leu-CHO (MG-132)]. When slices were pretreated with MG-132 alone, DHPG no longer elicited any change in FMRP. MG-132 also suppressed increase in other proteins, including postsynaptic density-95 and α-calcium/calmodulin-dependent protein kinase II, normally elicited by group I mGluR stimulation. Physiological experiments showed that proteasome inhibitor suppressed group I mGluR-induced prolonged synchronized discharges. However, proteasome inhibitor did not affect group I mGluR-induced prolonged synchronized discharges in Fmr1(-/-) preparations, where functional FMRP is absent. The results suggest that constitutive FMRP in hippocampal cells acts as a brake on group I mGluR-mediated translation and epileptogenesis. FMRP downregulation via UPS removes this brake enabling group I mGluR-mediated translation and epileptogenesis.

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.

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.

Regulatory BC1 RNA and the fragile X mental retardation protein: convergent functionality in brain

Background

BC RNAs and the fragile X mental retardation protein (FMRP) are translational repressors that have been implicated in the control of local protein synthesis at the synapse. Work with BC1 and Fmr1 animal models has revealed that phenotypical consequences resulting from the absence of either BC1 RNA or FMRP are remarkably similar. To establish functional interactions between BC1 RNA and FMRP is important for our understanding of how local protein synthesis regulates neuronal excitability.

Methodology/Principal Findings

We generated BC1−/− Fmr1−/− double knockout (dKO) mice. We examined such animals, lacking both BC1 RNA and FMRP, in comparison with single knockout (sKO) animals lacking either one repressor. Analysis of neural phenotypical output revealed that at least three attributes of brain functionality are subject to control by both BC1 RNA and FMRP: neuronal network excitability, epileptogenesis, and place learning. The severity of CA3 pyramidal cell hyperexcitability was significantly higher in BC1−/− Fmr1−/− dKO preparations than in the respective sKO preparations, as was seizure susceptibility of BC1−/− Fmr1−/− dKO animals in response to auditory stimulation. In place learning, BC1−/− Fmr1−/− dKO animals were severely impaired, in contrast to BC1−/− or Fmr1−/− sKO animals which exhibited only mild deficits.

Conclusions/Significance

Our data indicate that BC1 RNA and FMRP operate in sequential-independent fashion. They suggest that the molecular interplay between two translational repressors directly impacts brain functionality.

The role of fragile X mental retardation protein in major mental disorders

Fragile X mental retardation protein (FMRP) is highly enriched in neurons and binds to approximately 4% of mRNAs in mammalian brain. Its loss is a hallmark of fragile X syndrome (FXS), the most common form of mental retardation. In this review we discuss the mutation in the fragile X mental retardation-1 gene (FMR1), that leads to FXS, the role FMRP plays in neuronal cells, experiments from our own laboratory that demonstrate reductions of FMRP in additional psychiatric disorders (autism, schizophrenia, bipolar disorder, and major depressive disorder), and potential therapies to ameliorate the loss of FMRP. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

Drosophila as a tool for personalized medicine: a primer

The goal of personalized medicine is to treat each patient with the best drug: optimal therapeutic benefit with minimal side effects. The genomic revolution is rapidly identifying the genetic contribution to the diseased state as well as its contribution to drug efficacy and toxicity. The ability to perform genome-wide studies has led to an overwhelming number of candidate genes and/or their associated variants; however, understanding which are of therapeutic importance is becoming the greatest unmet need in the personalized medicine field. A related issue is the need to improve our methods of identifying and characterizing therapeutic drugs in the context of the complex genomic landscape of the intact body. Drosophila have proven to be a powerful tool for understanding the basic biological mechanisms of human development. This article will review Drosophila as a whole animal tool for gene and drug discovery. We will examine how Drosophila can be used to both sort through the myriad of hits coming from human genome-wide scans and to dramatically improve the early steps in pharmaceutical drug development.

Open-label add-on treatment trial of minocycline in fragile X syndrome

Background

Fragile X syndrome (FXS) is a disorder characterized by a variety of disabilities, including cognitive deficits, attention-deficit/hyperactivity disorder, autism, and other socio-emotional problems. It is hypothesized that the absence of the fragile X mental retardation protein (FMRP) leads to higher levels of matrix metallo-proteinase-9 activity (MMP-9) in the brain. Minocycline inhibits MMP-9 activity, and alleviates behavioural and synapse abnormalities in fmr1 knockout mice, an established model for FXS. This open-label add-on pilot trial was conducted to evaluate safety and efficacy of minocycline in treating behavioural abnormalities that occur in humans with FXS.

Methods

Twenty individuals with FXS, ages 13-32, were randomly assigned to receive 100 mg or 200 mg of minocycline daily. Behavioural evaluations were made prior to treatment (baseline) and again 8 weeks after daily minocycline treatment. The primary outcome measure was the Aberrant Behaviour Checklist-Community Edition (ABC-C) Irritability Subscale, and the secondary outcome measures were the other ABC-C subscales, clinical global improvement scale (CGI), and the visual analog scale for behaviour (VAS). Side effects were assessed using an adverse events checklist, a complete blood count (CBC), hepatic and renal function tests, and antinuclear antibody screen (ANA), done at baseline and at 8 weeks.

Results

The ABC-C Irritability Subscale scores showed significant improvement (p < 0.001), as did the VAS (p = 0.003) and the CGI (p < 0.001). The only significant treatment-related side effects were minor diarrhea (n = 3) and seroconversion to a positive ANA (n = 2).

Conclusions

Results from this study demonstrate that minocycline provides significant functional benefits to FXS patients and that it is well-tolerated. These findings are consistent with the fmr1 knockout mouse model results, suggesting that minocycline modifies underlying neural defects that account for behavioural abnormalities. A placebo-controlled trial of minocycline in FXS is warranted.

Trial registration

ClinicalTrials.gov Open-Label Trial NCT00858689.

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.

The distribution of repressive histone modifications on silenced FMR1 alleles provides clues to the mechanism of gene silencing in fragile X syndrome

Fragile X syndrome (FXS) is the most common heritable cause of intellectual disability and the most common known cause of autism. Most cases of FXS result from the expansion of a CGG·CCG repeat in the 5' UTR of the FMR1 gene that leads to gene silencing. It has previously been shown that silenced alleles are associated with histone H3 dimethylated at lysine 9 (H3K9Me2) and H3 trimethylated at lysine 27 (H3K27Me3), modified histones typical of developmentally repressed genes. We show here that these alleles are also associated with elevated levels of histone H3 trimethylated at lysine 9 (H3K9Me3) and histone H4 trimethylated at lysine 20 (H4K20Me3). All four of these modified histones are present on exon 1 of silenced alleles at levels comparable to that seen on pericentric heterochromatin. The two groups of histone modifications show a different distribution on fragile X alleles: H3K9Me2 and H3K27Me3 have a broad distribution, whereas H3K9Me3 and H4K20Me3 have a more focal distribution with the highest level of these marks being present in the vicinity of the repeat. This suggests that the trigger for gene silencing may be local to the repeat itself and perhaps involves a mechanism similar to that involved in the formation of pericentric heterochromatin.

Altered mRNA transport, docking, and protein translation in neurons lacking fragile X mental retardation protein

Fragile X syndrome is caused by the absence of functional fragile X mental retardation protein (FMRP), an RNA binding protein. The molecular mechanism of aberrant protein synthesis in fmr1 KO mice is closely associated with the role of FMRP in mRNA transport, delivery, and local protein synthesis. We show that GFP-labeled Fmr1 and CaMKIIalpha mRNAs undergo decelerated motion at 0-40 min after group I mGluR stimulation, and later recover at 40-60 min. Then we investigate targeting of mRNAs associated with FMRP after neuronal stimulation. We find that FMRP is synthesized closely adjacent to stimulated mGluR5 receptors. Moreover, in WT neurons, CaMKIIalpha mRNA can be delivered and translated in dendritic spines within 10 min in response to group I mGluR stimulation, whereas KO neurons fail to show this response. These data suggest that FMRP can mediate spatial mRNA delivery for local protein synthesis in response to synaptic stimulation.

Fragile X syndrome and model organisms: identifying potential routes of therapeutic intervention

Fragile X syndrome (FXS) is a cognitive disorder caused by silencing of the fragile X mental retardation 1 gene (FMR1). Since the discovery of the gene almost two decades ago, most scientific contributions have focused on identifying the molecular function of the fragile X mental retardation protein (FMRP) and understanding how absence of FMR1 gene expression gives rise to the disease phenotypes. The use of model organisms has allowed rapid progression in the FXS field and has given insight into the molecular basis of the disease. The mouse and fly FXS models have enabled studies to identify potential targets and pathways for pharmacological treatment. Here, we briefly review the two primary FXS model systems and describe how studies in these organisms have led us closer to therapeutic treatments for patients afflicted with FXS.

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.

Short- and long-term memory are modulated by multiple isoforms of the fragile X mental retardation protein

The diversity of protein isoforms arising from alternative splicing is thought to modulate fine-tuning of synaptic plasticity. Fragile X mental retardation protein (FMRP), a neuronal RNA binding protein, exists in isoforms as a result of alternative splicing, but the contribution of these isoforms to neural plasticity are not well understood. We show that two isoforms of Drosophila melanogaster FMRP (dFMR1) have differential roles in mediating neural development and behavior functions conferred by the dfmr1 gene. These isoforms differ in the presence of a protein interaction module that is related to prion domains and is functionally conserved between FMRPs. Expression of both isoforms is necessary for optimal performance in tests of short- and long-term memory of courtship training. The presence or absence of the protein interaction domain may govern the types of ribonucleoprotein (RNP) complexes dFMR1 assembles into, with different RNPs regulating gene expression in a manner necessary for establishing distinct phases of memory formation.

Recombinant bacterial expression and purification of human fragile X mental retardation protein isoform 1

The loss of expression of the fragile X mental retardation protein (FMRP) leads to fragile X syndrome. FMRP has two types of RNA binding domains, two K-homology domains and an arginine-glycine-glycine box domain, and it is proposed to act as a translation regulator of specific messenger RNA. The interest to produce sufficient quantities of pure recombinant FMRP for biochemical and biophysical studies is high. However, the recombinant bacterial expression of FMRP has had limited success, and subsequent recombinant eukaryotic and in vitro expression has also resulted in limited success. In addition, the in vitro and eukaryotic expression systems may produce FMRP which is posttranslationally modified, as phosphorylation and arginine methylation have been shown to occur on FMRP. In this study, we have successfully isolated the conditions for recombinant expression, purification and long-term storage of FMRP using Escherichia coli, with a high yield. The expression of FMRP using E. coli renders the protein devoid of the posttranslational modifications of phosphorylation and arginine methylation, allowing the study of the direct effects of these modifications individually and simultaneously. In order to assure that FMRP retained activity throughout the process, we used fluorescence spectroscopy to assay the binding activity of the FMRP arginine-glycine-glycine box for the semaphorin 3F mRNA and confirmed that FMRP remained active.

STDP and Mental Retardation: Dysregulation of Dendritic Excitability in Fragile X Syndrome

Development of cognitive function requires the formation and refinement of synaptic networks of neurons in the brain. Morphological abnormalities of synaptic spines occur throughout the brain in a wide variety of syndromic and non-syndromic disorders of mental retardation (MR). In both neurons from human post-mortem tissue and mouse models of retardation, the changes observed in synaptic spine and dendritic morphology can be subtle, in the range of 10-20% alterations for spine protrusion length and density. Functionally, synapses in hippocampus and cortex show deficits in long-term potentiation (LTP) and long-term depression (LTD) in an array of neurodevelopmental disorders including Down's, Angelman, Fragile X and Rett syndrome. Recent studies have shown that in principle the machinery for synaptic plasticity is in place in these synapses, but that significant alterations in spike-timing-dependent plasticity (STDP) induction rules exist in cortical synaptic pathways of Fragile X MR syndrome. In this model, the threshold for inducing timing-dependent long-term potentiation (tLTP) is increased in these synapses. Increased postsynaptic activity can overcome this threshold and induce normal levels of tLTP. In this review, we bring together recent studies investigating STDP in neurodevelopmental learning disorders using Fragile X syndrome as a model and we argue that alterations in dendritic excitability underlie deficits seen in STDP. Known and candidate dendritic mechanisms that may underlie the plasticity deficits are discussed. Studying STDP in monogenic MR syndromes with clear deficits in information processing at the cognitive level also provides the field with an opportunity to make direct links between cognition and processing rules at the synapse during development.

Genetic controls balancing excitatory and inhibitory synaptogenesis in neurodevelopmental disorder models

Proper brain function requires stringent balance of excitatory and inhibitory synapse formation during neural circuit assembly. Mutation of genes that normally sculpt and maintain this balance results in severe dysfunction, causing neurodevelopmental disorders including autism, epilepsy and Rett syndrome. Such mutations may result in defective architectural structuring of synaptic connections, molecular assembly of synapses and/or functional synaptogenesis. The affected genes often encode synaptic components directly, but also include regulators that secondarily mediate the synthesis or assembly of synaptic proteins. The prime example is Fragile X syndrome (FXS), the leading heritable cause of both intellectual disability and autism spectrum disorders. FXS results from loss of mRNA-binding FMRP, which regulates synaptic transcript trafficking, stability and translation in activity-dependent synaptogenesis and plasticity mechanisms. Genetic models of FXS exhibit striking excitatory and inhibitory synapse imbalance, associated with impaired cognitive and social interaction behaviors. Downstream of translation control, a number of specific synaptic proteins regulate excitatory versus inhibitory synaptogenesis, independently or combinatorially, and loss of these proteins is also linked to disrupted neurodevelopment. The current effort is to define the cascade of events linking transcription, translation and the role of specific synaptic proteins in the maintenance of excitatory versus inhibitory synapses during neural circuit formation. This focus includes mechanisms that fine-tune excitation and inhibition during the refinement of functional synaptic circuits, and later modulate this balance throughout life. The use of powerful new genetic models has begun to shed light on the mechanistic bases of excitation/inhibition imbalance for a range of neurodevelopmental disease states.

Abnormal mGlu 5 receptor/endocannabinoid coupling in mice lacking FMRP and BC1 RNA

Transcriptional silencing of the gene encoding the fragile X mental retardation protein (FMRP) causes fragile X syndrome (FXS). FMRP acts as a translational repressor at central synapses, and molecular and synaptic plasticity studies have shown that the absence of this protein alters metabotropic glutamate 5 receptors (mGlu5Rs)-mediated signaling. In the striatum of mice lacking FMRP, we found enhanced activity of diacylglycerol lipase (DAGL), the enzyme limiting 2-arachidonoylglicerol (2-AG) synthesis, associated with altered sensitivity of GABA synapses to the mobilization of this endocannabinoid by mGlu5R stimulation with DHPG. Mice lacking another repressor of synaptic protein synthesis, BC1 RNA, also showed potentiated mGlu5R-driven 2-AG responses, indicating that both FMRP and BC1 RNA act as physiological constraints of mGlu5R/endocannabinoid coupling at central synapses. The effects of FMRP ablation on DAGL activity and on DHPG-mediated inhibition of GABA synapses were enhanced by simultaneous genetic inactivation of FMRP and BC1 RNA. In double FMRP and BC1 RNA lacking mice, striatal levels of 2-AG were also enhanced compared with control animals and to single mutants. Our data indicate for the first time that mGlu5R-driven endocannabinoid signaling in the striatum is under the control of both FMRP and BC1 RNA. The abnormal mGlu5R/2-AG coupling found in FMRP-KO mice emphasizes the involvement of mGlu5Rs in the synaptic defects of FXS, and identifies the modulation of the endocannabinoid system as a novel target for the treatment of this severe neuropsychiatric disorder.

Group II metabotropic glutamate receptor stimulation triggers production and release of Alzheimer's amyloid(beta)42 from isolated intact nerve terminals

Aberrant accumulation of amyloid beta (Abeta) oligomers may underlie the cognitive failure of Alzheimer's disease (AD). All species of Abeta peptides are produced physiologically during normal brain activity. Therefore, elucidation of mechanisms that interconnect excitatory glutamatergic neurotransmission, synaptic amyloid precursor protein (APP) processing and production of its metabolite, Abeta, may reveal synapse-specific strategies for suppressing the pathological accumulation of Abeta oligomers and fibrils that characterize AD. To study synaptic APP processing, we used isolated intact nerve terminals (cortical synaptoneurosomes) from TgCRND8 mice, which express a human APP with familial AD mutations. Potassium chloride depolarization caused sustained release from synaptoneurosomes of Abeta(42) as well as Abeta(40), and appeared to coactivate alpha-, beta- and gamma-secretases, which are known to generate a family of released peptides, including Abeta(40) and Abeta(42). Stimulation of postsynaptic group I metabotropic glutamate receptor (mGluRs) with DHPG (3,5-dihydroxyphenylglycine) induced a rapid accumulation of APP C-terminal fragments (CTFs) in the synaptoneurosomes, a family of membrane-bound intermediates generated from APP metabolized by alpha- and beta-secretases. Following stimulation with the group II mGluR agonist DCG-IV, levels of APP CTFs in the synaptoneurosomes initially increased but then returned to baseline by 10 min after stimulation. This APP CTF degradation phase was accompanied by sustained accumulation of Abeta(42) in the releasate, which was blocked by the group II mGluR antagonist LY341495. These data suggest that group II mGluR may trigger synaptic activation of all three secretases and that suppression of group II mGluR signaling may be a therapeutic strategy for selectively reducing synaptic generation of Abeta(42).

Roles of fragile X mental retardation protein in dopaminergic stimulation-induced synapse-associated protein synthesis and subsequent alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-4-propionate (AMPA) receptor internalization

Fragile X syndrome, the most common form of inherited mental retardation, is caused by the absence of the RNA-binding protein fragile X mental retardation protein (FMRP). FMRP regulates local protein synthesis in dendritic spines. Dopamine (DA) is involved in the modulation of synaptic plasticity. Activation of DA receptors can regulate higher brain functions in a protein synthesis-dependent manner. Our recent study has shown that FMRP acts as a key messenger for DA modulation in forebrain neurons. Here, we demonstrate that FMRP is critical for DA D1 receptor-mediated synthesis of synapse-associated protein 90/PSD-95-associated protein 3 (SAPAP3) in the prefrontal cortex (PFC). DA D1 receptor stimulation induced dynamic changes of FMRP phosphorylation. The changes in FMRP phosphorylation temporally correspond with the expression of SAPAP3 after D1 receptor stimulation. Protein phosphatase 2A, ribosomal protein S6 kinase, and mammalian target of rapamycin are the key signaling molecules for FMRP linking DA D1 receptors to SAPAP3. Knockdown of SAPAP3 did not affect surface expression of alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-4-propionate (AMPA) GluR1 receptors induced by D1 receptor activation but impaired their subsequent internalization in cultured PFC neurons; the subsequent internalization of GluR1 was also impaired in Fmr1 knock-out PFC neurons, suggesting that FMRP may be involved in subsequent internalization of GluR1 through regulating the abundance of SAPAP3 after DA D1 receptor stimulation. Our study thus provides further insights into FMRP involvement in DA modulation and may help to reveal the molecular mechanisms underlying impaired learning and memory in fragile X syndrome.

Spatially restricting gene expression by local translation at synapses

mRNA localization and regulated translation provide a means of spatially restricting gene expression within each of the thousands of subcellular compartments made by a neuron, thereby vastly increasing the computational capacity of the brain. Recent studies reveal that local translation is regulated by stimuli that trigger neurite outgrowth and/or collapse, axon guidance, synapse formation, pruning, activity-dependent synaptic plasticity, and injury-induced axonal regeneration. Impairments in the local regulation of translation result in aberrant signaling, physiology and morphology of neurons, and are linked to neurological disorders. This review highlights current advances in understanding how mRNA translation is repressed during transport and how local translation is activated by stimuli. We address the function of local translation in the context of fragile X mental retardation.