A decade of molecular studies of fragile X syndrome

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.

Fragile X related protein 1 clusters with ribosomes and messenger RNAs at a subset of dendritic spines in the mouse hippocampus

The formation and storage of memories in neuronal networks relies on new protein synthesis, which can occur locally at synapses using translational machinery present in dendrites and at spines. These new proteins support long-lasting changes in synapse strength and size in response to high levels of synaptic activity. To ensure that proteins are made at the appropriate time and location to enable these synaptic changes, messenger RNA (mRNA) translation is tightly controlled by dendritic RNA-binding proteins. Fragile X Related Protein 1 (FXR1P) is an RNA-binding protein with high homology to Fragile X Mental Retardation Protein (FMRP) and is known to repress and activate mRNA translation in non-neuronal cells. However, unlike FMRP, very little is known about the role of FXR1P in the central nervous system. To understand if FXR1P is positioned to regulate local mRNA translation in dendrites and at synapses, we investigated the expression and targeting of FXR1P in developing hippocampal neurons in vivo and in vitro. We found that FXR1P was highly expressed during hippocampal development and co-localized with ribosomes and mRNAs in the dendrite and at a subset of spines in mouse hippocampal neurons. Our data indicate that FXR1P is properly positioned to control local protein synthesis in the dendrite and at synapses in the central nervous system.

CAG repeat RNA as an auxiliary toxic agent in polyglutamine disorders

Over 20 genetic loci with abnormal expansions of short tandem repeats have been associated with human hereditary neurological diseases. Of these, specific trinucleotide repeats located in non-coding and coding regions of individual genes implicated in these disorders are strongly overrepresented. Expansions of CTG, CGG and CAG repeats are linked to, respectively, myotonic dystrophy type 1 (DM1), fragile X-associated tremor/ataxia syndrome (FXTAS), as well as Huntington's disease (HD) and a number of spinocerebellar ataxias (SCAs). Expanded CAG repeats in translated exons trigger the most disorders for which a protein gain-of-function mechanism has been proposed to explain neurodegeneration by polyglutamine-rich (poly-Q) proteins. However, the results of last years showed that RNA composed of mutated CAG repeats can also be toxic and contribute to pathogenesis of polyglutamine disorders through an RNA-mediated gain-of-function mechanism. This mechanism has been best characterized in the non-coding repeat disorder DM1 and is also implicated in several other diseases, such as FXTAS, spinocerebellar ataxia type 8 (SCA8), Huntington's disease-like 2 (HDL2), as well as in myotonic dystrophy type 2 (DM2), spinocerebellar ataxia type 10 (SCA10) and type 31 (SCA31). In this review, we summarize recent findings that emphasize the participation of coding mutant CAG repeat RNA in the pathogenesis of polyglutamine disorders, and we discuss the basis of an RNA gain-of-function model in non-coding diseases such as DM1, FXTAS and SCA8.

Conformational-dependent and independent RNA binding to the fragile x mental retardation protein

The interaction between the fragile X mental retardation protein (FMRP) and BC1 RNA has been the subject of controversy. We probed the parameters of RNA binding to FMRP in several ways. Nondenaturing agarose gel analysis showed that BC1 RNA transcripts produced by in vitro transcription contain a population of conformers, which can be modulated by preannealing. Accordingly, FMRP differentially binds to the annealed and unannealed conformer populations. Using partial RNase digestion, we demonstrate that annealed BC1 RNA contains a unique conformer that FMRP likely binds. We further demonstrate that this interaction is 100-fold weaker than that the binding of eEF-1A mRNA and FMRP, and that preannealing is not a general requirement for FMRP's interaction with RNA. In addition, binding does not require the N-terminal 204 amino acids of FMRP, methylated arginine residues and can be recapitulated by both fragile X paralogs. Altogether, our data continue to support a model in which BC1 RNA functions independently of FMRP.

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.

Observation of water molecules within the bimolecular d(G₃CT₄G₃C)₂G-quadruplex

G-Rich oligonucleotides with cytosine residues in their sequences can form G-quadruplexes where G-quartets are flanked by G·C Watson-Crick base pairs. In an attempt to probe the role of cations in stabilization of a structural element with two G·C base pairs stacked on a G-quartet, we utilized solution state nuclear magnetic resonance to study the folding of the d(G(3)CT(4)G(3)C) oligonucleotide into a G-quadruplex upon addition of (15)NH(4)(+) ions. Its bimolecular structure exhibits antiparallel strands with edge-type loops. Two G-quartets in the core of the structure are flanked by a couple of Watson-Crick G·C base pairs in a sheared arrangement. The topology is equivalent to the solution state structure of the same oligonucleotide in the presence of Na(+) and K(+) ions [Kettani, A., et al. (1998) J. Mol. Biol.282, 619, and Bouaziz, S., et al. (1998) J. Mol. Biol.282, 637). A single ammonium ion binding site was identified between adjacent G-quartets, but three sites were expected. The remaining potential cation binding sites between G-quartets and G·C base pairs are occupied by water molecules. This is the first observation of long-lived water molecules within a G-quadruplex structure. The flanking G·C base pairs adopt a coplanar arrangement and apparently do not require cations to neutralize unfavorable electrostatic interactions among proximal carbonyl groups. A relatively fast movement of ammonium ions from the inner binding site to bulk with the rate constants of 21 s(-1) was attributed to the lack of hydrogen bonds between adjacent G·C base pairs and the flexibility of the T(4) loops.

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.

Ras and Rap signaling in synaptic plasticity and mental disorders

The Ras family GTPases (Ras, Rap1, and Rap2) and their downstream mitogen-activated protein kinases (ERK, JNK, and p38MAPK) and PI3K signaling cascades control various physiological processes. In neuronal cells, recent studies have shown that these parallel cascades signal distinct forms of AMPA-sensitive glutamate receptor trafficking during experience-dependent synaptic plasticity and adaptive behavior. Interestingly, both hypo- and hyperactivation of Ras/ Rap signaling impair the capacity of synaptic plasticity, underscoring the importance of a "happy-medium" dynamic regulation of the signaling. Moreover, accumulating reports have linked various genetic defects that either up- or down-regulate Ras/Rap signaling with several mental disorders associated with learning disability (e.g., Alzheimer's disease, Angelman syndrome, autism, cardio-facio-cutaneous syndrome, Coffin-Lowry syndrome, Costello syndrome, Cowden and Bannayan-Riley-Ruvalcaba syndromes, fragile X syndrome, neurofibromatosis type 1, Noonan syndrome, schizophrenia, tuberous sclerosis, and X-linked mental retardation), highlighting the necessity of happy-medium dynamic regulation of Ras/Rap signaling in learning behavior. Thus, the recent advances in understanding of neuronal Ras/Rap signaling provide a useful guide for developing novel treatments for mental diseases.

Narrative Skill in Boys with Fragile X Syndrome with and without Autism Spectrum Disorder

We examined recalled narratives of boys with fragile X syndrome with autism spectrum disorder (FXS-ASD; N=28) and without ASD (FXS-O; N=29), and compared them to those of boys with Down syndrome (DS; N=33) and typically developing boys (TD; N=39). Narratives were scored for mentions of macrostructural Story Grammar elements (Introduction, Relationship, Initiating Events, Internal Response, Attempts/Actions, and Ending). We found that narrative recall is predicted by short-term memory and nonverbal mental age levels in almost all groups (except TD), but not by expressive syntax or caregiver education. After adjusting for these covariates, there were no differences between the three groups with intellectual disability (ID). The FXS-ASD group, however, had significantly poorer performance than the TD group on the overall Story Grammar score, and both the FXS-O and FXS-ASD groups had lower Attempts/Actions scores than the TD group. We conclude that some form of narrative impairment may be associated with FXS, that this impairment may be shared by other forms of ID, and that the presence of ASD has a significantly detrimental effect on narrative recall.

Actin and Diseases of the Nervous System

Abnormal regulation of the actin cytoskeleton results in several pathological conditions affecting primarily the nervous system. Those of genetic origin arise during development, but others manifest later in life. Actin regulation is also affected profoundly by environmental factors that can have sustained consequences for the nervous system. Those consequences follow from the fact that the actin cytoskeleton is essential for a multitude of cell biological functions ranging from neuronal migration in cortical development and dendritic spine formation to NMDA receptor activity in learning and alcoholism. Improper regulation of actin, causing aggregation, can contribute to the neurodegeneration of amyloidopathies, such as Down's syndrome and Alzheimer's disease. Much progress has been made in understanding the molecular basis of these diseases.

TMS: using the theta-burst protocol to explore mechanism of plasticity in individuals with Fragile X syndrome and autism

Fragile X Syndrome (FXS), also known as Martin-Bell Syndrome, is a genetic abnormality found on the X chromosome. Individuals suffering from FXS display abnormalities in the expression of FMR1--a protein required for typical, healthy neural development. Recent data has suggested that the loss of this protein can cause the cortex to be hyperexcitable thereby affecting overall patterns of neural plasticity. In addition, Fragile X shows a strong comorbidity with autism: in fact, 30% of children with FXS are diagnosed with autism, and 2-5% of autistic children suffer from FXS. Transcranial Magnetic Stimulation (a non-invasive neurostimulatory and neuromodulatory technique that can transiently or lastingly modulate cortical excitability via the application of localized magnetic field pulses) represents a unique method of exploring plasticity and the manifestations of FXS within affected individuals. More specifically, Theta-Burst Stimulation (TBS), a specific stimulatory protocol shown to modulate cortical plasticity for a duration up to 30 minutes after stimulation cessation in healthy populations, has already proven an efficacious tool in the exploration of abnormal plasticity. Recent studies have shown the effects of TBS last considerably longer in individuals on the autistic spectrum--up to 90 minutes. This extended effect-duration suggests an underlying abnormality in the brain's natural plasticity state in autistic individuals, similar to the hyperexcitability induced by Fragile X Syndrome. In this experiment, utilizing single-pulse motor-evoked potentials (MEPs) as our benchmark, we will explore the effects of both intermittent and continuous TBS on cortical plasticity in individuals suffering from FXS and individuals on the Autistic Spectrum.

Fragile X mice: reduced long-term potentiation and N-Methyl-D-Aspartate receptor-mediated neurotransmission in dentate gyrus

Fragile X syndrome (FXS) is a monogenic mental retardation syndrome that frequently includes autism. The Fmr1-knockout (Fmr1-KO) mouse, like FXS-affected individuals, lacks the fragile X mental retardation protein (FMRP) and models autism as well as FXS. Limited human data and several mouse models have implicated the hippocampal dentate gyrus (DG) in autism. We therefore investigated whether the Fmr1-KO mouse exhibited functional changes in DG. We found diminished medial perforant path-granule cell long-term potentiation (LTP), complementing previous investigations of synaptic plasticity in Fmr1-KO demonstrating impaired LTP in CA1, neocortex, and amygdala and exaggerated long-term depression in CA1. We also found that peak amplitude of NMDA receptor-mediated excitatory postsynaptic currents (EPSCs) was smaller in Fmr1-KO than control. AMPA receptor-mediated EPSCs were comparable in the two strains, yielding a lower NMDA/AMPA ratio in Fmr1-KO mice and suggesting one mechanism by which absent FMRP might contribute to diminished LTP. The clinical hallmarks of autism include both excessive adherence to patterns and impaired detection of socially important patterns. The DG has a putative role in pattern separation (for time, space, and features) that has been attributed to granule cell number, firing rates, adult neurogenesis, and even perforant path LTP. DG also contributes to pattern completion in CA3 via its mossy fiber efferents, whose terminals include abundant FMRP in "fragile X granules." Together with the present data, these observations suggest that DG is a candidate region for further investigation in autism and that the Fmr1-KO model may be particularly apt.

dFMRP and Caprin, translational regulators of synaptic plasticity, control the cell cycle at the Drosophila mid-blastula transition

The molecular mechanisms driving the conserved metazoan developmental shift referred to as the mid-blastula transition (MBT) remain mysterious. Typically, cleavage divisions give way to longer asynchronous cell cycles with the acquisition of a gap phase. In Drosophila, rapid synchronous nuclear divisions must pause at the MBT to allow the formation of a cellular blastoderm through a special form of cytokinesis termed cellularization. Drosophila Fragile X mental retardation protein (dFMRP; FMR1), a transcript-specific translational regulator, is required for cellularization. The role of FMRP has been most extensively studied in the nervous system because the loss of FMRP activity in neurons causes the misexpression of specific mRNAs required for synaptic plasticity, resulting in mental retardation and autism in humans. Here, we show that in the early embryo dFMRP associates specifically with Caprin, another transcript-specific translational regulator implicated in synaptic plasticity, and with eIF4G, a key regulator of translational initiation. dFMRP and Caprin collaborate to control the cell cycle at the MBT by directly mediating the normal repression of maternal Cyclin B mRNA and the activation of zygotic frühstart mRNA. These findings identify two new targets of dFMRP regulation and implicate conserved translational regulatory mechanisms in processes as diverse as learning, memory and early embryonic development.

AGG interruptions in (CGG)(n) DNA repeat tracts modulate the structure and thermodynamics of non-B conformations in vitro

The trinucleotide repeat sequence CGG/CCG is known to expand in the human genome. This expansion is the primary pathogenic signature of fragile X syndrome, which is the most common form of inherited mental retardation. It has been proposed that formation of non-B conformations by the repetitive sequence contributes to the expansion mechanism. It is also known that the CGG/CCG repeat sequence of healthy individuals, which is not prone to expansion, contains AGG/CCT interruptions every 8-11 CGG/CCG repeats. Using DNA containing 19 or 39 CGG repeats, we have found that both the position and number of interruptions modulate the non-B conformation adopted by the repeat sequence. Analysis by chemical probes revealed larger loops and the presence of bulges for sequences containing interruptions. Additionally, using optical analysis and calorimetry, the effect of these structural changes on the thermodynamic stability of the conformation has been quantified. Notably, changing even one nucleotide, as occurs when CGG is replaced with an AGG interruption, causes a measurable decrease in the stability of the conformation adopted by the repeat sequence. These results provide insight into the role interruptions may play in preventing expansion in vivo and also contribute to our understanding of the relationship between non-B conformations and trinucleotide repeat expansion.

Taurine regulation of short term synaptic plasticity in fragile X mice

BACKGROUND

Fragile X Syndrome is the most common known genetic cause of autism. The Fmr1-KO mouse, lacks the fragile X mental retardation protein (FMRP), and is used as a model of the syndrome. The core behavioral deficits of autism may be conceptualized either as excessive adherence to patterns as seen in repetitive actions and aberrant language, or as insensitivity to subtle but socially important changes in patterns. The hippocampus receives information from the entorhinal cortex and plays a crucial role in the processing of patterned information. To gain more insight into the physiological function of FMRP and the neuronal mechanisms underlying fragile X syndrome, we examined the electrophysiological response of the hippocampus to pair pulse stimulation as a measure of patterned information processing and how it is affected in the Fmr1-KO mouse.

METHODS

In this study, we used paired-pulse stimulation of the afferent perforant path and recorded from the CA1 region of the hippocampus. Two-month-old FVB/NJ male mice and age-matched Fmr1-KO mice were used in this study. Hippocampal slices were prepared, equilibrated in artificial cerebrospinal fluid (aCSF), and excitatory post synaptic potentials (EPSPs) measured by stimulating the perforant path of the dentate gyrus (DG) while recording from the molecular layer of CA1. Stimulation occurred by setting current and pulse width to evoke a fixed percentage of maximal EPSP amplitude. This stimulation paradigm allowed us to examine the processing capabilities of the hippocampus as a function of increasing interstimulus intervals (ISI) and how taurine, a GABAA receptor agonist, affects such information processing.

RESULTS

We found that hippocampal slices from wild type (WT) showed pair-pulse facilitation at ISI of 100-300 ms whereas slices from Fmr1-KO brains showed a consistent pair-pulse depression at a comparable ISI. Addition of 10 muM taurine to WT slices resulted in a drastic decrease of the peak response to the second stimulus, resulting in an initial depression at 100 ms ISI followed by potentiation at higher ISI (150 ms and above). In the presence of taurine, the amplitude of the second response remained significantly lower than in its absence. Fmr1-KO mice however, were completely insensitive to taurine application and pair-pulse stimulation always resulted in a depression of the response to the second stimulus.

CONCLUSIONS

Previously we reported that Fmr1-KO mice have reduced beta subunits of the GABAA receptors. We also showed as well as others that taurine acts as an agonist or a modulator for GABAA receptors. Therefore, the insensitivity of Fmr1-KO slices to taurine application could be due to the reduced binding sites on the GABAA receptors in the Fmr1-KO mice.

Gene expression profiling identifies WNT7A as a possible candidate gene for decreased cancer risk in fragile X syndrome patients

BACKGROUND AND AIMS

Although sporadic cases of cancer in patients with fragile X syndrome (FXS) have been reported, extensive studies carried out in Denmark and Finland concluded that cancer incidence in these patients is lower than in the general population. On the other hand, the FMR1 protein, which is involved in the translation process, is absent in FXS patients. Hence, it is reasonable to assume that these patients exhibit an abnormal expression of some proteins involved in regulating tumor suppressor genes and/or oncogenes, thus explaining its decreased cancer frequency. We undertook this study to analyze the expression of oncogenes and tumor suppressor genes in fragile X syndrome patients.

METHODS

Molecular analysis of the FMR1 gene was achieved in 10 male patients and controls. Total RNA from peripheral blood was used to evaluate expression of oncogenes and tumor suppressor genes included in a 10,000 gene microarray library. Quantitative real-time PCR was utilized to confirm genes with differential expression.

RESULTS

Among 27 genes showing increased expression in FXS patients, only eight genes exhibited upregulation in at least 50% of them. Among these, ARMCX2 and PPP2R5C genes are tumor suppressor related. Likewise, 23/65 genes showed decreased expression in >50% of patients. Among them, WNT7A gene is a ligand of the beta-catenin pathway, which is widely related to oncogenic processes. Decreased expression of WNT7A was confirmed by quantitative RT-PCR. Expression of c-Myc, c-Jun, cyclin-D and PPARdelta genes, as target of the beta-catenin pathway, was moderately reduced in FXS patients.

CONCLUSIONS

Results suggest that this diminished expression of the WNT7A gene may be related to a supposed protection of FXS patients to develop cancer.

Regulation of dendritic development by BDNF requires activation of CRTC1 by glutamate

Dendritic growth is essential for the establishment of a functional nervous system. Among extrinsic signals that control dendritic development, substantial evidence indicates that BDNF regulates dendritic morphology. However, little is known about the underlying mechanisms by which BDNF controls dendritic growth. In this study, we show that the MAPK signaling pathway and the transcription factor cAMP response element-binding protein (CREB) mediate the effects of BDNF on dendritic length and complexity. However, phosphorylation of CREB alone is not sufficient for the stimulation of dendritic growth by BDNF. Thus, using a mutant form of CREB unable to bind CREB-regulated transcription coactivator (CRTC1), we demonstrate that this effect also requires a functional interaction between CREB and CRTC1. Moreover, inhibition of CRTC1 expression by shRNA-mediated knockdown abolished BDNF-induced dendritic growth of cortical neurons. Interestingly, we found that nuclear translocation of CRTC1 results from activation of NMDA receptors by glutamate, a process that is essential for the effects of BDNF on dendritic development. Together, these data identify a previously unrecognized mechanism by which CREB and the coactivator CRTC1 mediate the effects of BDNF on dendritic growth.

Biological roles of translin and translin-associated factor-X: RNA metabolism comes to the fore

Translin, and its binding partner protein TRAX (translin-associated factor-X) are a paralogous pair of conserved proteins, which have been implicated in a broad spectrum of biological activities, including cell growth regulation, mRNA processing, spermatogenesis, neuronal development/function, genome stability regulation and carcinogenesis, although their precise role in some of these processes remains unclear. Furthermore, translin (with or without TRAX) has nucleic-acid-binding activity and it is apparent that controlling nucleic acid metabolism and distribution are central to the biological role(s) of this protein and its partner TRAX. More recently, translin and TRAX have together been identified as enhancer components of an RNAi (RNA interference) pathway in at least one organism and this might provide critical insight into the biological roles of this enigmatic partnership. In the present review we discuss the biological and the biochemical properties of these proteins that indicate that they play a central and important role in eukaryotic cell biology.

Transcranial magnetic stimulation provides means to assess cortical plasticity and excitability in humans with fragile x syndrome and autism spectrum disorder

Fragile X Syndrome (FXS) is the most common heritable cause of intellectual disability. In vitro electrophysiologic data from mouse models of FXS suggest that loss of fragile X mental retardation protein affects intracortical excitability and synaptic plasticity. Specifically, the cortex appears hyperexcitable, and use-dependent long-term potentiation (LTP) and long-term depression (LTD) of synaptic strength are abnormal. Though animal models provide important information, FXS and other neurodevelopmental disorders are human diseases and as such translational research to evaluate cortical excitability and plasticity must be applied in the human. Transcranial magnetic stimulation paradigms have recently been developed to non-invasively investigate cortical excitability using paired pulse stimulation, as well as LTP- and LTD-like synaptic plasticity in response to theta burst stimulation (TBS) in vivo in the human. TBS applied on consecutive days can be used to measure metaplasticity (the ability of the synapse to undergo a second plastic change following a recent induction of plasticity). The current study investigated intracortical inhibition, plasticity and metaplasticity in full mutation females with FXS, participants with autism spectrum disorders (ASD), and neurotypical controls. Results suggest that intracortical inhibition is normal in participants with FXS, while plasticity and metaplasticity appear abnormal. ASD participants showed abnormalities in plasticity and metaplasticity, as well as heterogeneity in intracortical inhibition. Our findings highlight the utility of non-invasive neurophysiological measures to translate insights from animal models to humans with neurodevelopmental disorders, and thus provide direct confirmation of cortical dysfunction in patients with FXS and ASD.

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.

Hypothesis: a role for fragile X mental retardation protein in mediating and relieving microRNA-guided translational repression?

MicroRNA (miRNA)-guided messenger RNA (mRNA) translational repression is believed to be mediated by effector miRNA-containing ribonucleoprotein (miRNP) complexes harboring fragile X mental retardation protein (FMRP). Recent studies documented the nucleic acid chaperone properties of FMRP and characterized its role and importance in RNA silencing in mammalian cells. We propose a model in which FMRP could facilitate miRNA assembly on target mRNAs in a process involving recognition of G quartet structures. Functioning within a duplex miRNP, FMRP may also mediate mRNA targeting through a strand exchange mechanism, in which the miRNA* of the duplex is swapped for the mRNA. Furthermore, FMRP may contribute to the relief of miRNA-guided mRNA repression through a reverse strand exchange reaction, possibly initiated by a specific cellular signal, that would liberate the mRNA for translation. Suboptimal utilization of miRNAs may thus account for some of the molecular defects in patients with the fragile X syndrome.

Dicer-derived microRNAs are utilized by the fragile X mental retardation protein for assembly on target RNAs

In mammalian cells, fragile X mental retardation protein (FMRP) has been reported to be part of a microRNA (miRNA)-containing effector ribonucleoprotien (RNP) complex believed to mediate translational control of specific mRNAs. Here, using recombinant proteins, we demonstrate that human FMRP can act as a miRNA acceptor protein for the ribonuclease Dicer and facilitate the assembly of miRNAs on specific target RNA sequences. The miRNA assembler property of FMRP was abrogated upon deletion of its single-stranded (ss) RNA binding K-homology domains. The requirement of FMRP for efficient RNA interference (RNAi) in vivo was unveiled by reporter gene silencing assays using various small RNA inducers, which also supports its involvement in an ss small interfering RNA (siRNA)-containing RNP (siRNP) effector complex in mammalian cells. Our results define a possible role for FMRP in RNA silencing and may provide further insight into the molecular defects in patients with the fragile X syndrome.

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.

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.

Enhanced endocannabinoid signaling elevates neuronal excitability in fragile X syndrome

Fragile X syndrome (FXS) results from deficiency of fragile X mental retardation protein (FMRP). FXS is the most common heritable form of mental retardation, and is associated with the occurrence of seizures. Factors responsible for initiating FXS-related hyperexcitability are poorly understood. Many protein-synthesis-dependent functions of group I metabotropic glutamate receptors (Gp1 mGluRs) are exaggerated in FXS. Gp1 mGluR activation can mobilize endocannabinoids (eCBs) in the hippocampus and thereby increase excitability, but whether FMRP affects eCBs is unknown. We studied Fmr1 knock-out (KO) mice lacking FMRP to test the hypothesis that eCB function is altered in FXS. Whole-cell evoked IPSCs (eIPSCs) and field potentials were recorded in the CA1 region of acute hippocampal slices. Three eCB-mediated responses were examined: depolarization-induced suppression of inhibition (DSI), mGluR-initiated eCB-dependent inhibitory short-term depression (eCB-iSTD), and eCB-dependent inhibitory long-term depression (eCB-iLTD). Low concentrations of a Gp1 mGluR agonist produced larger eCB-mediated responses in Fmr1 KO mice than in wild-type (WT) mice, without affecting DSI. Western blots revealed that levels of mGluR1, mGluR5, or cannabinoid receptor (CB1R) were unchanged in Fmr1 KO animals, suggesting that the coupling between mGluR activation and eCB mobilization was enhanced by FMRP deletion. The increased susceptibility of Fmr1 KO slices to eCB-iLTD was physiologically relevant, since long-term potentiation of EPSP-spike (E-S) coupling induced by the mGluR agonist was markedly larger in Fmr1 KO mice than in WT animals. Alterations in eCB signaling could contribute to the cognitive dysfunction associated with FXS.

A simple, high-throughput assay for Fragile X expanded alleles using triple repeat primed PCR and capillary electrophoresis

Population screening has been proposed for Fragile X syndrome to identify premutation carrier females and affected newborns. We developed a PCR-based assay capable of quickly detecting the presence or absence of an expanded FMR1 allele with high sensitivity and specificity. This assay combines a triplet repeat primed PCR with high-throughput automated capillary electrophoresis. We evaluated assay performance using archived samples sent for Fragile X diagnostic testing representing a range of Fragile X CGG-repeat expansions. Two hundred five previously genotyped samples were tested with the new assay. Data were analyzed for the presence of a trinucleotide "ladder" extending beyond 55 repeats, which was set as a cut-off to identify expanded FMR1 alleles. We identified expanded FMR1 alleles in 132 samples (59 premutation, 71 full mutation, 2 mosaics) and normal FMR1 alleles in 73 samples. We found 100% concordance with previous results from PCR and Southern blot analyses. In addition, we show feasibility of using this assay with DNA extracted from dried-blood spots. Using a single PCR combined with high-throughput fragment analysis on the automated capillary electrophoresis instrument, we developed a rapid and reproducible PCR-based laboratory assay that meets many of the requirements for a first-tier test for population screening.

Anatomical phenotyping in a mouse model of fragile X syndrome with magnetic resonance imaging

Fragile X Syndrome (FXS) is the most common single gene cause of inherited mental impairment, and cognitive deficits can range from simple learning disabilities to mental retardation. Human FXS is caused by a loss of the Fragile X Mental Retardation Protein (FMRP). The fragile X knockout (FX KO) mouse also shows a loss of FMRP, as well as many of the physical and behavioural characteristics of human FXS. This work aims to characterize the anatomical changes between the FX KO and a corresponding wild type mouse. Significant volume decreases were found in two regions within the deep cerebellar nuclei, namely the nucleus interpositus and the fastigial nucleus, which may be caused by a loss of neurons as indicated by histological analysis. Well-known links between these nuclei and previously established behavioural and physical characteristics of FXS are discussed. The loss of FMRP has a significant effect on these two nuclei, and future studies of FXS should evaluate the biochemical, physiological, and behavioral consequences of alterations in these key nuclei.

Dysregulation of mTOR signaling in fragile X syndrome

Fragile X syndrome, the most common form of inherited mental retardation and leading genetic cause of autism, is caused by transcriptional silencing of the Fmr1 gene. The fragile X mental retardation protein (FMRP), the gene product of Fmr1, is an RNA binding protein that negatively regulates translation in neurons. The Fmr1 knock-out mouse, a model of fragile X syndrome, exhibits cognitive deficits and exaggerated metabotropic glutamate receptor (mGluR)-dependent long-term depression at CA1 synapses. However, the molecular mechanisms that link loss of function of FMRP to aberrant synaptic plasticity remain unclear. The mammalian target of rapamycin (mTOR) signaling cascade controls initiation of cap-dependent translation and is under control of mGluRs. Here we show that mTOR phosphorylation and activity are elevated in hippocampus of juvenile Fmr1 knock-out mice by four functional readouts: (1) association of mTOR with regulatory associated protein of mTOR; (2) mTOR kinase activity; (3) phosphorylation of mTOR downstream targets S6 kinase and 4E-binding protein; and (4) formation of eukaryotic initiation factor complex 4F, a critical first step in cap-dependent translation. Consistent with this, mGluR long-term depression at CA1 synapses of FMRP-deficient mice is exaggerated and rapamycin insensitive. We further show that the p110 subunit of the upstream kinase phosphatidylinositol 3-kinase (PI3K) and its upstream activator PI3K enhancer PIKE, predicted targets of FMRP, are upregulated in knock-out mice. Elevated mTOR signaling may provide a functional link between overactivation of group I mGluRs and aberrant synaptic plasticity in the fragile X mouse, mechanisms relevant to impaired cognition in fragile X syndrome.

Compressed Genotyping

Over the past three decades we have steadily increased our knowledge on the genetic basis of many severe disorders. Nevertheless, there are still great challenges in applying this knowledge routinely in the clinic, mainly due to the relatively tedious and expensive process of genotyping. Since the genetic variations that underlie the disorders are relatively rare in the population, they can be thought of as a sparse signal. Using methods and ideas from compressed sensing and group testing, we have developed a cost-effective genotyping protocol to detect carriers for severe genetic disorders. In particular, we have adapted our scheme to a recently developed class of high throughput DNA sequencing technologies. The mathematical framework presented here has some important distinctions from the 'traditional' compressed sensing and group testing frameworks in order to address biological and technical constraints of our setting.

Age-dependent cognitive impairment in a Drosophila fragile X model and its pharmacological rescue

Fragile X syndrome afflicts 1 in 2,500 individuals and is the leading heritable cause of mental retardation worldwide. The overriding clinical manifestation of this disease is mild to severe cognitive impairment. Age-dependent cognitive decline has been identified in Fragile X patients, although it has not been fully characterized nor examined in animal models. A Drosophila model of this disease has been shown to display phenotypes bearing similarity to Fragile X symptoms. Most notably, we previously identified naive courtship and memory deficits in young adults with this model that appear to be due to enhanced metabotropic glutamate receptor (mGluR) signaling. Herein we have examined age-related cognitive decline in the Drosophila Fragile X model and found an age-dependent loss of learning during training. We demonstrate that treatment with mGluR antagonists or lithium can prevent this age-dependent cognitive impairment. We also show that treatment with mGluR antagonists or lithium during development alone displays differential efficacy in its ability to rescue naive courtship, learning during training and memory in aged flies. Furthermore, we show that continuous treatment during aging effectively rescues all of these phenotypes. These results indicate that the Drosophila model recapitulates the age-dependent cognitive decline observed in humans. This places Fragile X in a category with several other diseases that result in age-dependent cognitive decline. This demonstrates a role for the Drosophila Fragile X Mental Retardation Protein (dFMR1) in neuronal physiology with regard to cognition during the aging process. Our results indicate that misregulation of mGluR activity may be causative of this age onset decline and strengthens the possibility that mGluR antagonists and lithium may be potential pharmacologic compounds for counteracting several Fragile X symptoms.

A mouse model of the human Fragile X syndrome I304N mutation

The mental retardation, autistic features, and behavioral abnormalities characteristic of the Fragile X mental retardation syndrome result from the loss of function of the RNA–binding protein FMRP. The disease is usually caused by a triplet repeat expansion in the 5′UTR of the FMR1 gene. This leads to loss of function through transcriptional gene silencing, pointing to a key function for FMRP, but precluding genetic identification of critical activities within the protein. Moreover, antisense transcripts (FMR4, ASFMR1) in the same locus have been reported to be silenced by the repeat expansion. Missense mutations offer one means of confirming a central role for FMRP in the disease, but to date, only a single such patient has been described. This patient harbors an isoleucine to asparagine mutation (I304N) in the second FMRP KH-type RNA–binding domain, however, this single case report was complicated because the patient harbored a superimposed familial liver disease. To address these issues, we have generated a new Fragile X Syndrome mouse model in which the endogenous Fmr1 gene harbors the I304N mutation. These mice phenocopy the symptoms of Fragile X Syndrome in the existing Fmr1–null mouse, as assessed by testicular size, behavioral phenotyping, and electrophysiological assays of synaptic plasticity. I304N FMRP retains some functions, but has specifically lost RNA binding and polyribosome association; moreover, levels of the mutant protein are markedly reduced in the brain specifically at a time when synapses are forming postnatally. These data suggest that loss of FMRP function, particularly in KH2-mediated RNA binding and in synaptic plasticity, play critical roles in pathogenesis of the Fragile X Syndrome and establish a new model for studying the disorder.

Missense mutations in human genes provide valuable insight into the genetic causes of disease. Fragile X Syndrome (FXS), a common genetic cause of autism and mental retardation, is usually caused by transcriptional silencing of the FMR1 gene. The potential importance of single patient with a missense mutation (I304N) in an RNA–binding domain of the Fragile X protein, FMRP, has been questioned in part because he has a confounding liver disease. We introduced the I304N mutation into the endogenous Fmr1 locus to create a mouse model of Fragile X Syndrome. We find that this mutation results in behavioral, electrophysiologic, and phenotypic features of the disease, loss of binding to RNA targets in the brain, and lower FMRP levels at a critical time during synapse formation. We conclude that loss of RNA binding and underexpression of FMRP are sufficient to cause the Fragile X Syndrome.

A distinct DNA-methylation boundary in the 5'- upstream sequence of the FMR1 promoter binds nuclear proteins and is lost in fragile X syndrome

We have discovered a distinct DNA-methylation boundary at a site between 650 and 800 nucleotides upstream of the CGG repeat in the first exon of the human FMR1 gene. This boundary, identified by bisulfite sequencing, is present in all human cell lines and cell types, irrespective of age, gender, and developmental stage. The same boundary is found also in different mouse tissues, although sequence homology between human and mouse in this region is only 46.7%. This boundary sequence, in both the unmethylated and the CpG-methylated modes, binds specifically to nuclear proteins from human cells. We interpret this boundary as carrying a specific chromatin structure that delineates a hypermethylated area in the genome from the unmethylated FMR1 promoter and protecting it from the spreading of DNA methylation. In individuals with the fragile X syndrome (FRAXA), the methylation boundary is lost; methylation has penetrated into the FMR1 promoter and inactivated the FMR1 gene. In one FRAXA genome, the upstream terminus of the methylation boundary region exhibits decreased methylation as compared to that of healthy individuals. This finding suggests changes in nucleotide sequence and chromatin structure in the boundary region of this FRAXA individual. In the completely de novo methylated FMR1 promoter, there are isolated unmethylated CpG dinucleotides that are, however, not found when the FMR1 promoter and upstream sequences are methylated in vitro with the bacterial M-SssI DNA methyltransferase. They may arise during de novo methylation only in DNA that is organized in chromatin and be due to the binding of specific proteins.

Mouse neurexin-1alpha deletion causes correlated electrophysiological and behavioral changes consistent with cognitive impairments

Deletions in the neurexin-1alpha gene were identified in large-scale unbiased screens for copy-number variations in patients with autism or schizophrenia. To explore the underlying biology, we studied the electrophysiological and behavioral phenotype of mice lacking neurexin-1alpha. Hippocampal slice physiology uncovered a defect in excitatory synaptic strength in neurexin-1alpha deficient mice, as revealed by a decrease in miniature excitatory postsynaptic current (EPSC) frequency and in the input-output relation of evoked postsynaptic potentials. This defect was specific for excitatory synaptic transmission, because no change in inhibitory synaptic transmission was observed in the hippocampus. Behavioral studies revealed that, compared with littermate control mice, neurexin-1alpha deficient mice displayed a decrease in prepulse inhibition, an increase in grooming behaviors, an impairment in nest-building activity, and an improvement in motor learning. However, neurexin-1alpha deficient mice did not exhibit any obvious changes in social behaviors or in spatial learning. Together, these data indicate that the neurexin-1alpha deficiency induces a discrete neural phenotype whose extent correlates, at least in part, with impairments observed in human patients.

Signals, synapses, and synthesis: how new proteins control plasticity

Localization of mRNAs to dendrites and local protein synthesis afford spatial and temporal regulation of gene expression and endow synapses with the capacity to autonomously alter their structure and function. Emerging evidence indicates that RNA binding proteins, ribosomes, translation factors and mRNAs encoding proteins critical to synaptic structure and function localize to neuronal processes. RNAs are transported into dendrites in a translationally quiescent state where they are activated by synaptic stimuli. Two RNA binding proteins that regulate dendritic RNA delivery and translational repression are cytoplasmic polyadenylation element binding protein and fragile X mental retardation protein (FMRP). The fragile X syndrome (FXS) is the most common known genetic cause of autism and is characterized by the loss of FMRP. Hallmark features of the FXS include dysregulation of spine morphogenesis and exaggerated metabotropic glutamate receptor-dependent long term depression, a cellular substrate of learning and memory. Current research focuses on mechanisms whereby mRNAs are transported in a translationally repressed state from soma to distal process and are activated at synaptic sites in response to synaptic signals.

Fragile X Mental Retardation Protein is Involved in Protein Synthesis-Dependent Collapse of Growth Cones Induced by Semaphorin-3A

Fragile X syndrome, the most frequent form of familial mental retardation, is caused by mutation of the Fmr1 gene. Fmr1 encodes the fragile X mental retardation protein (FMRP), an mRNA binding protein regulating local, postsynaptic mRNA translation along dendrites necessary for long-term synaptic plasticity. However, recent studies on FMRP localization in axons and growth cones suggest a possible function in the regulation of local protein synthesis needed for axon guidance. Here, we have demonstrated that FMRP is involved in axonal and growth cone responses induced by the axon guidance factor, Semaphorin-3A (Sema3A). In cultured hippocampal neurons from wild type mice, Sema3A-induced growth cone collapse was protein synthesis-dependent. In contrast, Sema3A-induced growth cone collapse was attenuated in Fmr1 knock-out (KO) neurons and insensitive to protein synthesis inhibitors, suggesting that FMRP is involved in protein synthesis-dependent growth cone collapse. Sema3A increased phosphorylation of eukaryotic initiation factor 4E (eIF4E), an indicator of local translation, in distal axons and growth cones of wild type, but not Fmr1 KO neurons. Furthermore, Sema3A rapidly induced a protein synthesis-dependent increase in levels of microtubule associated protein 1B (MAP1B) in distal axons of wild type neurons, but this response was attenuated in Fmr1 KO neurons. These results suggest a possible role of FMRP to regulate local translation and axonal protein localization in response to Sema3A. This study reveals a new link between FMRP and semaphorin signaling in vitro, and raises the possibility that FMRP may have a critical role in semaphorin signaling in axon guidance during brain development.

Fragile X mental retardation protein replacement restores hippocampal synaptic function in a mouse model of fragile X syndrome

Fragile X syndrome (FXS) is caused by a mutation that silences the fragile X mental retardation gene (FMR1), which encodes the fragile X mental retardation protein (FMRP). To determine whether FMRP replacement can rescue phenotypic deficits in a fmr1-knockout (KO) mouse model of FXS, we constructed an adeno-associated virus-based viral vector that expresses the major central nervous system (CNS) isoform of FMRP. Using this vector, we tested whether FMRP replacement could rescue the fmr1-KO phenotype of enhanced long-term depression (LTD), a form of synaptic plasticity that may be linked to cognitive impairments associated with FXS. Extracellular excitatory postsynaptic field potentials were recorded from CA3-CA1 synaptic contacts in hippocampal slices from wild-type (WT) and fmr1-KO mice in the presence of AP-5 and anisomycin. Paired-pulse low-frequency stimulation (PP-LFS)-induced LTD is enhanced in slices obtained from fmr1 KO compared with WT mice. Analyses of hippocampal synaptic function in fmr1-KO mice that received hippocampal injections of vector showed that the PP-LFS-induced LTD was restored to WT levels. These results indicate that expression of the major CNS isoform of FMRP alone is sufficient to rescue this phenotype and suggest that post-developmental protein replacement may have the potential to improve cognitive function in FXS.

Genetic modifiers of dFMR1 encode RNA granule components in Drosophila

Mechanisms of neuronal mRNA localization and translation are of considerable biological interest. Spatially regulated mRNA translation contributes to cell-fate decisions and axon guidance during development, as well as to long-term synaptic plasticity in adulthood. The Fragile-X Mental Retardation protein (FMRP/dFMR1) is one of the best-studied neuronal translational control molecules and here we describe the identification and early characterization of proteins likely to function in the dFMR1 pathway. Induction of the dFMR1 in sevenless-expressing cells of the Drosophila eye causes a disorganized (rough) eye through a mechanism that requires residues necessary for dFMR1/FMRP's translational repressor function. Several mutations in dco, orb2, pAbp, rm62, and smD3 genes dominantly suppress the sev-dfmr1 rough-eye phenotype, suggesting that they are required for dFMR1-mediated processes. The encoded proteins localize to dFMR1-containing neuronal mRNPs in neurites of cultured neurons, and/or have an effect on dendritic branching predicted for bona fide neuronal translational repressors. Genetic mosaic analyses indicate that dco, orb2, rm62, smD3, and dfmr1 are dispensable for translational repression of hid, a microRNA target gene, known to be repressed in wing discs by the bantam miRNA. Thus, the encoded proteins may function as miRNA- and/or mRNA-specific translational regulators in vivo.

Fragile X syndrome and epilepsy

Fragile X syndrome (FXS) is one of the most prevalent mental retardations. It is mainly caused by the loss of fragile X mental retardation protein (FMRP). FMRP is an RNA binding protein and can regulate the translation of its binding RNA, thus regulate several signaling pathways. Many FXS patients show high susceptibility to epilepsy. Epilepsy is a chronic neurological disorder which is characterized by the recurrent appearance of spontaneous seizures due to neuronal hyperactivity in the brain. Both the abnormal activation of several signaling pathway and morphological abnormality that are caused by the loss of FMRP can lead to a high susceptibility to epilepsy. Combining with the research progresses on both FXS and epilepsy, we outlined the possible mechanisms of high susceptibility to epilepsy in FXS and tried to give a prospect on the future research on the mechanism of epilepsy that happened in other mental retardations.

The state of synapses in fragile X syndrome

Fragile X syndrome (FXS) is the most common inherited form of mental retardation and a leading genetic cause of autism. There is increasing evidence in both FXS and other forms of autism that alterations in synapse number, structure, and function are associated and contribute to these prevalent diseases. FXS is caused by loss of function of the Fmr1 gene, which encodes the RNA binding protein, fragile X mental retardation protein (FMRP). Therefore, FXS is a tractable model to understand synaptic dysfunction in cognitive disorders. FMRP is present at synapses where it associates with mRNA and polyribosomes. Accumulating evidence finds roles for FMRP in synapse development, elimination, and plasticity. Here, the authors review the synaptic changes observed in FXS and try to relate these changes to what is known about the molecular function of FMRP. Recent advances in the understanding of the molecular and synaptic function of FMRP, as well as the consequences of its loss, have led to the development of novel therapeutic strategies for FXS.

The FXG: a presynaptic fragile X granule expressed in a subset of developing brain circuits

The loss of Fragile X mental retardation protein (FMRP) causes Fragile X syndrome, the most common inherited mental retardation and single gene cause of autism. Although postsynaptic functions for FMRP are well established, potential roles at the presynaptic apparatus remain largely unexplored. Here, we characterize the expression of FMRP and its homologs, FXR1P and FXR2P, in the developing, mature and regenerating rodent nervous system, with a focus on presynaptic expression. As expected, FMRP is expressed in the somatodendritic domain in virtually all neurons. However, FMRP is also localized in discrete granules (Fragile X granules; FXGs) in a subset of brain regions including frontal cortex, hippocampal area CA3 and olfactory bulb glomeruli. Immunoelectron microscopy shows that FMRP is localized at presynaptic terminals and in axons within these FXG-rich regions. With the exception of the olfactory bulb, FXGs are prominent only in the developing brain. Experiments in regenerating olfactory circuits indicate that peak FXG expression occurs 2-4 weeks after neurogenesis, a period that correlates with synapse formation and refinement. Virtually all FXGs contain FXR2P, while region-selective subsets harbor FMRP and/or FXR1P. Genetic studies show that FXR2P is essential for FXG expression, while FMRP regulates FXG number and developmental profile. These findings suggest that Fragile X proteins play a distinct, presynaptic role during discrete developmental epochs in defined circuits of the mammalian CNS. We propose that the neurological defects in Fragile X syndrome, including the autistic features, could be due in part to the loss of FMRP function in presynaptic compartments.

Whole-brain expression analysis of FMRP in adult monkey and its relationship to cognitive deficits in fragile X syndrome

Fragile X syndrome (FXS) is one of the most prevalent forms of heritable mental retardation and developmental delay in males. The syndrome is caused by the silencing of a single gene (fragile X mental retardation-1; FMR1) and the lack of expression of its protein product (fragile X mental retardation-1 protein; FMRP). Recent work has linked the high expression levels of FMRP in the magnocellular layers of lateral geniculate nucleus (M-LGN) of the visual system to a specific reduction of perceptual function known to be mediated by that neural structure. This finding has given rise to the intriguing notion that FMRP expression level may be used as an index of susceptibility of specific brain regions to the observed perceptual and cognitive deficits in FXS. We undertook a comprehensive expression profiling study of FMRP in the monkey to obtain further insight into the link between FMPR expression and the behavioural impact of its loss in FXS. We report here the first 3D whole-brain map of FMRP expression in the Old-World monkey and show that certain brain structures display high FMRP levels, such as the cerebellum, striatum, and temporal lobe structures. This finding provides support for the notion that FMRP expression loss is linked to behavioural and cognitive impairment associated with these structures. We argue that whole-brain FMRP expression mapping may be used to formulate and test new hypotheses about other forms of impairments in FXS that were not specifically examined in this study.

The development of cortical columns: role of Fragile X mental retardation protein

Neuronal circuits in the brain are complex and precise. Here, I review aspects of the development of cortical columns in the rodent barrel cortex, focusing on the anatomical and functional data describing the maturation of ascending glutamatergic circuits. Projections from layer 4 to layer 3 develop into cortical columns with little macroscopic refinement. Depriving animals of normal sensory experience induces long-term synaptic depression but does not perturb this pattern of development. Mouse models of mental retardation can help us understand the mechanisms of development of cortical columns. Fmr1 knock-out (ko) mice, a model for Fragile X syndrome, lack Fragile X mental retardation protein (FMRP), a suppressor of translation present in synapses. Because FMRP expression is stimulated by neuronal activity, Fmr1-ko mice provide a model to survey the role of sensory input in brain development. Layer 4 to layer 3 projections are altered in multiple ways in the young mutant mice: connection rate is low and layer 4 cell axons are spatially diffuse. Sensory deprivation rescues the connection rate phenotype. The interaction of FMRP and neuronal activity in the development of cortical circuits is discussed.