Fragile X protein controls the efficacy of mRNA transport in Drosophila neurons

A non-canonical start codon in the Drosophila fragile X gene yields two functional isoforms

Fragile X syndrome is caused by the loss of expression of the fragile X mental retardation protein (FMRP). As a RNA binding protein, FMRP functions in translational regulation, localization, and stability of its neuronal target transcripts. The Drosophila homologue, dFMR1, is well conserved in sequence and function with respect to human FMRP. Although dFMR1 is known to express two main isoforms, the mechanism behind production of the second, more slowly migrating isoform has remained elusive. Furthermore, it remains unknown whether the two isoforms may also contribute differentially to dFMR1 function. We have found that this second dFMR1 isoform is generated through an alternative translational start site in the dfmr1 5'UTR. This 5'UTR coding sequence is well conserved in the melanogaster group. Translation of the predominant, smaller form of dFMR1 (dFMR1-S(N)) begins at a canonical start codon (ATG), whereas translation of the minor, larger form (dFMR1-L(N)) begins upstream at a non-canonical start codon (CTG). To assess the contribution of the N-terminal extension toward dFMR1 activity, we generated transgenic flies that exclusively express either dFMR1-S(N) or dFMR1-L(N). Expression analyses throughout development revealed that dFMR1-S(N) is required for normal dFMR1-L(N) expression levels in adult brains. In situ expression analyses showed that either dFMR1-S(N) or dFMR1-L(N) is individually sufficient for proper dFMR1 localization in the nervous system. Functional studies demonstrated that both dFMR1-S(N) and dFMR1-L(N) can function independently to rescue dfmr1 null defects in synaptogenesis and axon guidance. Thus, dfmr1 encodes two functional isoforms with respect to expression and activity throughout neuronal development.

BDNF and TrkB in neuronal differentiation of Fmr1-knockout mouse

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

Drosophila FMRP regulates microtubule network formation and axonal transport of mitochondria

Fragile X syndrome, the most common form of inherited mental retardation, is caused by the absence of the fragile X mental retardation protein FMRP. The RNA-binding FMRP represses translation of the microtubule (MT)-associated protein 1B (MAP1B) during synaptogenesis in the brain of the neonatal mouse. However, the effect of FMRP on MTs remains unclear. Mounting evidence shows that the structure and the function of FMRP are well conserved across species from Drosophila to human. From a genetic screen, we identified spastin as a dominant suppressor of rough eye caused by dfmr1 over-expression. spastin encodes an MT-severing protein, and its mutations cause neurodegenerative hereditary spastic paraplegia. Epistatic and biochemical analyses revealed that dfmr1 acts upstream of or in parallel with spastin in multiple processes, including synapse development, locomotive behaviour and MT network formation. Immunostaining showed that both loss- and gain-of-function mutations of dfmr1 result in an apparently altered MT network. Western analysis revealed that the levels of α-tubulin and acetylated MTs remained normal in dfmr1 mutants, but increased significantly when dfmr1 was over-expressed. To examine the consequence of the aberrant MTs in dfmr1 mutants, we analysed the MT-dependent mitochondrial transport and found that the number of mitochondria and the flux of mitochondrial transport are negatively regulated by dfmr1. These results demonstrate that dFMRP plays a crucial role in controlling MT formation and mitochondrial transport. Thus, defective MTs and abnormal mitochondrial transport might account for, at least partially, the pathogenesis of fragile X mental retardation.

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.

Bicaudal-D regulates fragile X mental retardation protein levels, motility, and function during neuronal morphogenesis

The expression of the RNA-binding factor Fragile X mental retardation protein (FMRP) is disrupted in the most common inherited form of cognitive deficiency in humans. FMRP controls neuronal morphogenesis by mediating the translational regulation and localization of a large number of mRNA targets, and these functions are closely associated with transport of FMRP complexes within neurites by microtubule-based motors. However, the mechanisms that link FMRP to motors and regulate its transport are poorly understood. Here we show that FMRP is complexed with Bicaudal-D (BicD) through a domain in the latter protein that mediates linkage of cargoes with the minus-end-directed motor dynein. We demonstrate in Drosophila that the motility and, surprisingly, levels of FMRP protein are dramatically reduced in BicD mutant neurons, leading to a paucity of FMRP within processes. We also provide functional evidence that BicD and FMRP cooperate to control dendritic morphogenesis in the larval nervous system. Our findings open new perspectives for understanding localized mRNA functions in neurons.

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.

Fragile X protein controls neural stem cell proliferation in the Drosophila brain

Fragile X syndrome (FXS) is the most common form of inherited mental retardation and is caused by the loss of function for Fragile X protein (FMRP), an RNA-binding protein thought to regulate synaptic plasticity by controlling the localization and translation of specific mRNAs. We have recently shown that FMRP is required to control the proliferation of the germline in Drosophila. To determine whether FMRP is also required for proliferation during brain development, we examined the distribution of cell cycle markers in dFmr1 brains compared with wild-type throughout larval development. Our results indicate that the loss of dFmr1 leads to a significant increase in the number of mitotic neuroblasts (NB) and BrdU incorporation in the brain, consistent with the notion that FMRP controls proliferation during neurogenesis. Developmental studies suggest that FMRP also inhibits neuroblast exit from quiescence in early larval brains, as indicated by misexpression of Cyclin E. Live imaging experiments indicate that by the third instar larval stage, the length of the cell cycle is unaffected, although more cells are found in S and G2/M in dFmr1 brains compared with wild-type. To determine the role of FMRP in neuroblast division and differentiation, we used Mosaic Analysis with a Repressible Marker (MARCM) approaches in the developing larval brain and found that single dFmr1 NB generate significantly more neurons than controls. Our results demonstrate that FMRP is required during brain development to control the exit from quiescence and proliferative capacity of NB as well as neuron production, which may provide insights into the autistic component of FXS.

Fragile X mental retardation protein has a unique, evolutionarily conserved neuronal function not shared with FXR1P or FXR2P

Fragile X syndrome (FXS), resulting solely from the loss of function of the human fragile X mental retardation 1 (hFMR1) gene, is the most common heritable cause of mental retardation and autism disorders, with syndromic defects also in non-neuronal tissues. In addition, the human genome encodes two closely related hFMR1 paralogs: hFXR1 and hFXR2. The Drosophila genome, by contrast, encodes a single dFMR1 gene with close sequence homology to all three human genes. Drosophila that lack the dFMR1 gene (dfmr1 null mutants) recapitulate FXS-associated molecular, cellular and behavioral phenotypes, suggesting that FMR1 function has been conserved, albeit with specific functions possibly sub-served by the expanded human gene family. To test evolutionary conservation, we used tissue-targeted transgenic expression of all three human genes in the Drosophila disease model to investigate function at (1) molecular, (2) neuronal and (3) non-neuronal levels. In neurons, dfmr1 null mutants exhibit elevated protein levels that alter the central brain and neuromuscular junction (NMJ) synaptic architecture, including an increase in synapse area, branching and bouton numbers. Importantly, hFMR1 can, comparably to dFMR1, fully rescue both the molecular and cellular defects in neurons, whereas hFXR1 and hFXR2 provide absolutely no rescue. For non-neuronal requirements, we assayed male fecundity and testes function. dfmr1 null mutants are effectively sterile owing to disruption of the 9+2 microtubule organization in the sperm tail. Importantly, all three human genes fully and equally rescue mutant fecundity and spermatogenesis defects. These results indicate that FMR1 gene function is evolutionarily conserved in neural mechanisms and cannot be compensated by either FXR1 or FXR2, but that all three proteins can substitute for each other in non-neuronal requirements. We conclude that FMR1 has a neural-specific function that is distinct from its paralogs, and that the unique FMR1 function is responsible for regulating neuronal protein expression and synaptic connectivity.

Translational control during early development

Translational control of specific messenger RNAs, which themselves are often asymmetrically localized within the cytoplasm of a cell, underlies many events in germline development, and in embryonic axis specification. This comprehensive, but by no means exhaustive, review attempts to present a picture of the present state of knowledge about mechanisms underlying mRNA localization and translational control of specific mRNAs that are mediated by trans-acting protein factors. While RNA localization and translational control are widespread in evolution and have been studied in many experimental systems, this article will focus mainly on three particularly well-characterized systems: Drosophila, Caenorhabditis elegans, and Xenopus. In keeping with the overall theme of this volume, instances in which translational control factors have been linked to human disease states will also be discussed.

Lighting up mRNA localization in Drosophila oogenesis

The asymmetric localization of four maternal mRNAs - gurken, bicoid, oskar and nanos - in the Drosophila oocyte is essential for the development of the embryonic body axes. Fluorescent imaging methods are now being used to visualize these mRNAs in living tissue, allowing dynamic analysis of their behaviors throughout the process of localization. This review summarizes recent findings from such studies that provide new insight into the elaborate cellular mechanisms that are used to transport mRNAs to different regions of the oocyte and to maintain their localized distributions during oogenesis.

Dynamics of axonal mRNA transport and implications for peripheral nerve regeneration

Locally generating new proteins in subcellular regions provide means to spatially and temporally modify protein content in polarized cells. Recent years have seen resurgence of the concept that axonal processes of neurons can locally synthesize proteins. Experiments from a number of groups have now shown that axonal protein synthesis helps to initiate growth, provides a means to respond to guidance cues, and generates retrograde signaling complexes. Additionally, there is increasing evidence that locally synthesized proteins provide functions beyond injury responses and growth in the mature peripheral nervous system. A key regulatory event in this translational regulation is moving the mRNA templates into the axonal compartment. Transport of mRNAs into axons is a highly regulated and specific process that requires interaction of RNA binding proteins with specific cis-elements or structures within the mRNAs. mRNAs are transported in ribonucleoprotein particles that interact with microtubule motor proteins for long-range axonal transport and likely use microfilaments for short-range movement in the axons. The mature axon is able to recruit mRNAs into translation with injury and possibly other stimuli, suggesting that mRNAs can be stored in a dormant state in the distal axon until needed. Axotomy triggers a shift in the populations of mRNAs localized to axons, indicating a dynamic regulation of the specificity of the axonal transport machinery. In this review, we discuss how axonal mRNA transport and localization are regulated to achieve specific changes in axonal RNA content in response to axonal stimuli.

Temporal requirements of the fragile x mental retardation protein in modulating circadian clock circuit synaptic architecture

Loss of fragile X mental retardation 1 (FMR1) gene function is the most common cause of inherited mental retardation and autism spectrum disorders, characterized by attention disorder, hyperactivity and disruption of circadian activity cycles. Pursuit of effective intervention strategies requires determining when the FMR1 product (FMRP) is required in the regulation of neuronal circuitry controlling these behaviors. In the well-characterized Drosophila disease model, loss of the highly conserved dFMRP causes circadian arrhythmicity and conspicuous abnormalities in the circadian clock circuitry. Here, a novel Sholl Analysis was used to quantify over-elaborated synaptic architecture in dfmr1-null small ventrolateral neurons (sLN_vs), a key subset of clock neurons. The transgenic Gene-Switch system was employed to drive conditional neuronal dFMRP expression in the dfmr1-null mutant background in order to dissect temporal requirements within the clock circuit. Introduction of dFMRP during early brain development, including the stages of neurogenesis, neuronal fate specification and early pathfinding, provided no rescue of dfmr1 mutant phenotypes. Similarly, restoring normal dFMRP expression in the adult failed to restore circadian circuit architecture. In sharp contrast, supplying dFMRP during a transient window of very late brain development, wherein synaptogenesis and substantial subsequent synaptic reorganization (e.g. use-dependent pruning) occur, provided strong morphological rescue to reestablish normal sLN_vs synaptic arbors. We conclude that dFMRP plays a developmentally restricted role in sculpting synaptic architecture in these neurons that cannot be compensated for by later reintroduction of the protein at maturity.

New insights into the role of brain-derived neurotrophic factor in synaptic plasticity

Substantial evidence indicates that brain-derived neurotrophic factor (BDNF) plays a crucial role in synaptic plasticity. Long-lasting synaptic plasticity is restricted to active synapses and requires new protein synthesis. Recent work has identified local protein synthesis as an important source for new protein during the expression of enduring synaptic plasticity. This review discusses recent progress in understanding the mechanisms that restrict the action of BDNF to active synapses and by which BDNF mediates chemical and structural modifications of individual synapses, placing an emphasis on the role of local protein synthesis in these processes.

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.

Translating nociceptor sensitivity: the role of axonal protein synthesis in nociceptor physiology

The increased sensitivity of peripheral pain-sensing neurons, or nociceptors, is a major cause of the sensation of pain that follows injury. This plasticity is thought to contribute to the maintenance of chronic pain states. Although we have a broad knowledge of the factors that stimulate changes in nociceptor sensitivity, the cellular mechanisms that underlie this plasticity are still poorly understood; however, they are likely to involve changes in gene expression required for the phenotypic and functional changes seen in nociceptive neurons after injury. While the regulation of gene expression at the transcriptional level has been studied extensively, the regulation of protein synthesis, which is also a tightly controlled process, has only recently received more attention. Despite the established role of protein synthesis in the plasticity of neuronal cell bodies and dendrites, little attention has been paid to the role of translation control in mature undamaged axons. In this regard, several recent studies have demonstrated that the control of protein synthesis within the axonal compartment is crucial for the normal function and regulation of sensitivity of nociceptors. Pathways and proteins regulating this process, such as the mammalian target of rapamycin signaling cascade and the fragile X mental retardation protein, have recently been identified. We review here recent evidence for the regulation of protein synthesis within a nociceptor's axonal compartment and its contribution to this neuron's plasticity. We believe that an increased understanding of this process will lead to the identification of novel targets for the treatment of chronic pain.

Disruption of normal cytoskeletal dynamics may play a key role in the pathogenesis of epilepsy

Epilepsy, a common disease affecting 1% to 2% of the population, is characterized by seizures, hyperexcitability at synapses, and aberrant extension of neurons following seizures. Much work has been done on the role of synaptic components in the pathogenesis of epilepsy, but relatively little attention has been given to the potential role of the cytoskeleton. The neuronal cytoskeleton consists of microtubules, actin filaments, intermediate filaments, and associated proteins. A number of mutations in both microtubule-associated proteins (MAPs) and actin-binding proteins, as well as altered expression levels of several cytoskeletal proteins, are known to be involved in epilepsy. These changes will affect the dynamics of the neuronal cytoskeleton and therefore are likely to contribute to the pathogenesis of epilepsy through mechanisms such as increased neurotrophic support to neurons and increased sprouting of mossy fibers. These changes may also contribute to hyperexcitability of neurons through an as yet unidentified mechanism.

A study of the ultrastructure of fragile-X-related proteins

Fragile-X-related proteins form a family implicated in RNA metabolism. Their sequence is composed of conserved N-terminal and central regions which contain Tudor and KH domains and of a divergent C-terminus with motifs rich in arginine and glycine residues. The most widely studied member of the family is probably FMRP (fragile X mental retardation protein), since absence or mutation of this protein in humans causes fragile X syndrome, the most common cause of inherited mental retardation. Understanding the structural properties of FMRP is essential for correlating it with its functions. The structures of isolated domains of FMRP have been reported, but nothing is yet known with regard to the spatial arrangement of the different modules, partly because of difficulties in producing both the full-length protein and its multidomain fragments in quantities, purities and monodispersity amenable for structural studies. In the present study, we describe how we have produced overlapping recombinant fragments of human FMRP and its paralogues which encompass the evolutionary conserved region. We have studied their behaviour in solution by complementary biochemical and biophysical techniques, identified the regions which promote self-association and determined their overall three-dimensional shape. The present study paves the way to further studies and rationalizes the existing knowledge on the self-association properties of these proteins.

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.

Drosophila Fragile X protein controls cellular proliferation by regulating cbl levels in the ovary

FMRP is an RNA binding protein linked to the most common form of inherited mental retardation, Fragile X syndrome (FraX). In addition to severe cognitive deficits, FraX etiology includes postpubescent macroorchidism, which is thought to result from overproliferation. Using a Drosophila FraX model, we show that FMRP controls germline proliferation during oogenesis. dFmr1 null ovaries contain egg chambers with both fewer and supranumerary germ cells. The mutant germaria contain a significantly increased number of cyclin E and PhosphoHistone H3 positive cells, suggesting that loss of FMRP leads to defects in cell cycle progression. BrdU incorporation and flow cytometry data suggest that, in addition to proliferation, germline endoreplication and ploidy are also affected by the loss of FMRP during ovary development. Here we report that FMRP controls the levels of cbl mRNA in the ovary and that reducing cbl gene dosage by half rescues the dFmr1 oogenesis phenotypes. These data support a model whereby FMRP controls germline proliferation by regulating the expression of cbl in the developing ovary.

The fragile X mental retardation protein in circadian rhythmicity and memory consolidation

The control of new protein synthesis provides a means to locally regulate the availability of synaptic components necessary for dynamic neuronal processes. The fragile X mental retardation protein (FMRP), an RNA-binding translational regulator, is a key player mediating appropriate synaptic protein synthesis in response to neuronal activity levels. Loss of FMRP causes fragile X syndrome (FraX), the most commonly inherited form of mental retardation and autism spectrum disorders. FraX-associated translational dysregulation causes wide-ranging neurological deficits including severe impairments of biological rhythms, learning processes, and memory consolidation. Dysfunction in cytoskeletal regulation and synaptic scaffolding disrupts neuronal architecture and functional synaptic connectivity. The understanding of this devastating disease and the implementation of meaningful treatment strategies require a thorough exploration of the temporal and spatial requirements for FMRP in establishing and maintaining neural circuit function.

Regulation of mRNA transport and translation in axons

Movement of mRNAs into axons occurs by active transport by microtubules through the activity of molecular motor proteins. mRNAs are sequestered into granular-like particles, referred to as transport ribonucleoprotein particles (RNPs) that mediate transport into the axonal compartment. The interaction of mRNA binding proteins with targeted mRNA is a key event in regulating axonal mRNA localization and subsequent localized translation of mRNAs. Several growth-modulating stimuli have been shown to regulate axonal mRNA localization. These do so by activating specific intracellular signaling pathways that converge upon RNA binding proteins and other components of the transport RNP to regulate their activity specifically. Transport can be both positively and negatively regulated by individual stimuli with regard to individual mRNAs. Consequently, there is exquisite specificity for regulating the axon's composition of mRNAs and proteins that control expression in the axon. Finally, recent studies indicate that axotomy can also trigger changes in axonal mRNA composition by specifically shifting the populations of mRNAs that are transported into distal axons.

Fragile X syndrome: loss of local mRNA regulation alters synaptic development and function

Fragile X syndrome is the most common inherited form of cognitive deficiency in humans and perhaps the best-understood single cause of autism. A trinucleotide repeat expansion, inactivating the X-linked FMR1 gene, leads to the absence of the fragile X mental retardation protein. FMRP is a selective RNA-binding protein that regulates the local translation of a subset of mRNAs at synapses in response to activation of Gp1 metabotropic glutamate receptors (mGluRs) and possibly other receptors. In the absence of FMRP, excess and dysregulated mRNA translation leads to altered synaptic function and loss of protein synthesis-dependent plasticity. Recent evidence indicates the role of FMRP in regulated mRNA transport in dendrites. New studies also suggest a possible local function of FMRP in axons that may be important for guidance, synaptic development, and formation of neural circuits. The understanding of FMRP function at synapses has led to rationale therapeutic approaches.