A direct role for FMRP in activity-dependent dendritic mRNA transport links filopodial-spine morphogenesis to fragile X syndrome

MicroRNAs in psychiatric and neurodevelopmental disorders

Abnormalities in microRNA (miRNA)-mediated gene regulation have been observed in a variety of human diseases, especially in cancer. Here, we provide an account of newly emerging connections between miRNAs with various psychiatric and neurodevelopmental disorders, including recent findings of miRNA dysregulation in the 22q11.2 microdeletion syndrome, a well-established genetic risk factor for schizophrenia. miRNAs appear to be components of both the genetic architecture of these complex phenotypes as well as integral parts of the biological pathways that mediate the effects of primary genetic deficits. Therefore, they may contribute to both genetic heterogeneity and phenotypic variation of psychiatric and neurodevelopmental disorders and could serve as novel therapeutic targets.

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.

Dendritically localized transcripts are sorted into distinct ribonucleoprotein particles that display fast directional motility along dendrites of hippocampal neurons

Localization of mRNAs to postsynaptic sites and their subsequent translation is thought to contribute to synapse-specific plasticity. However, the direct visualization of dendritic RNA transport in living neurons remains a major challenge. Here, we analyze the transport of Alexa-labeled RNAs microinjected into mature hippocampal neurons. We show that microinjected MAP2 and CaMKIIalpha RNAs form particles that localize into dendrites as their endogenous counterparts. In contrast, nonlocalizing RNAs or truncated CaMKIIalpha, lacking the dendritic targeting element, remain in the cell body. Furthermore, our microinjection approach allowed us to identify a novel dendritically localized RNA, Septin7. Time-lapse videomicroscopy of neurons injected with CaMKIIalpha and Septin7 RNAs demonstrates fast directional movement along the dendrites of hippocampal neurons, with similar kinetics to Staufen1 ribonucleoprotein particles (RNPs). Coinjection and simultaneous visualization of two RNAs, as well as double detection of the corresponding endogenous RNAs, reveal that neuronal transcripts are differentially sorted in dendritic RNPs.

Microtubule-dependent mRNA transport in fungi

The localization and local translation of mRNAs constitute an important mechanism to promote the correct subcellular targeting of proteins. mRNA localization is mediated by the active transport of mRNPs, large assemblies consisting of mRNAs and associated factors such as RNA-binding proteins. Molecular motors move mRNPs along the actin or microtubule cytoskeleton for short-distance or long-distance trafficking, respectively. In filamentous fungi, microtubule-based long-distance transport of vesicles, which are involved in membrane and cell wall expansion, supports efficient hyphal growth. Recently, we discovered that the microtubule-mediated transport of mRNAs is essential for the fast polar growth of infectious filaments in the corn pathogen Ustilago maydis. Combining in vivo UV cross-linking and RNA live imaging revealed that the RNA-binding protein Rrm4, which constitutes an integral part of the mRNP transport machinery, mediates the transport of distinct mRNAs encoding polarity factors, protein synthesis factors, and mitochondrial proteins. Moreover, our results indicate that microtubule-dependent mRNA transport is evolutionarily conserved from fungi to higher eukaryotes. This raises the exciting possibility of U. maydis as a model system to uncover basic concepts of long-distance mRNA transport.

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

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

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.

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.

Understanding neuronal connectivity through the post-transcriptional toolkit

Post-transcriptional regulatory mechanisms have emerged as a critical component underlying the diversification and spatiotemporal control of the proteome during the establishment of precise neuronal connectivity. These mechanisms have been shown to be important for virtually all stages of assembling a neural network, from neurite guidance, branching, and growth to synapse morphogenesis and function. From the moment a gene is transcribed, it undergoes a series of post-transcriptional regulatory modifications in the nucleus and cytoplasm until its final deployment as a functional protein. Initially, a message is subjected to extensive structural regulation through alternative splicing, which is capable of greatly expanding the protein repertoire by generating, in some cases, thousands of functionally distinct isoforms from a single gene locus. Then, RNA packaging into neuronal transport granules and recognition by RNA-binding proteins and/or microRNAs is capable of restricting protein synthesis to selective locations and under specific input conditions. This ability of the post-transcriptional apparatus to expand the informational content of a cell and control the deployment of proteins in both spatial and temporal dimensions is a feature well adapted for the extreme morphological properties of neural cells. In this review, we describe recent advances in understanding how post-transcriptional regulatory mechanisms refine the proteomic complexity required for the assembly of intricate and specific neural networks.

RNA localization in neurite morphogenesis and synaptic regulation: current evidence and novel approaches

It is now generally accepted that RNA localization in the central nervous system conveys important roles both during development and in the adult brain. Of special interest is protein synthesis located at the synapse, as this potentially confers selective synaptic modification and has been implicated in the establishment of memories. However, the underlying molecular events are largely unknown. In this review, we will first discuss novel findings that highlight the role of RNA localization in neurons. We will focus on the role of RNA localization in neurotrophin signaling, axon outgrowth, dendrite and dendritic spine morphogenesis as well as in synaptic plasticity. Second, we will briefly present recent work on the role of microRNAs in translational control in dendrites and its implications for learning and memory. Finally, we discuss recent approaches to visualize RNAs in living cells and their employment for studying RNA trafficking in neurons.

Murine hippocampal neurons expressing Fmr1 gene premutations show early developmental deficits and late degeneration

Premutation CGG repeat expansions (55-200 CGG repeats; preCGG) within the fragile X mental retardation 1 (FMR1) gene give rise to the neurodegenerative disorder, fragile X-associated tremor/ataxia syndrome (FXTAS), primary ovarian insufficiency and neurodevelopmental problems. Morphometric analysis of Map2B immunofluorescence reveals that neurons cultured from heterozygous female mice with preCGG repeats in defined medium display shorter dendritic lengths and fewer branches between 7 and 21 days in vitro compared with wild-type (WT) littermates. Although the numbers of synapsin and phalloidin puncta do not differ from WT, preCGG neurons possess larger puncta. PreCGG neurons display lower viability, and express elevated stress protein as they mature. PreCGG neurons have inherently different patterns of growth, dendritic complexity and synaptic architecture discernable early in the neuronal trajectory to maturation, and may reflect a cellular basis for the developmental component of the spectrum of clinical involvement in carriers of premutation alleles. The reduced viability of preCGG neurons is consistent with the mRNA toxicity and neurodegeneration associated with FXTAS.

Neuropeptide Release is Impaired in a Mouse Model of Fragile X Mental Retardation Syndrome

Fragile X syndrome (FXS), an inherited disorder characterized by mental retardation and autismlike behaviors, is caused by the failure to transcribe the gene for fragile X mental retardation protein (FMRP), a translational regulator and transporter of select mRNAs. FXS model mice (Fmr1 KO mice) exhibit impaired neuropeptide release. Release of biogenic amines does not differ between wild-type (WT) and Fmr1 KO mice. Rab3A, an mRNA cargo of FMRP involved in the recruitment of vesicles, is decreased by ∼50% in synaptoneurosomes of Fmr1 KO mice; however, the number of dense-core vesicles (DCVs) does not differ between WT and Fmr1 KO mice. Therefore, deficits associated with FXS may reflect this aberrant vesicle release, specifically involving docking and fusion of peptidergic DCVs, and may lead to defective maturation/maintenance of synaptic connections.

Single mRNA tracking in live cells

Asymmetric distribution of mRNA is a prevalent phenomenon observed in diverse cell types. The posttranscriptional movement and localization of mRNA provides an important mechanism to target certain proteins to specific cytoplasmic regions of their function. Recent technical advances have enabled real-time visualization of single mRNA molecules in living cells. Studies analyzing the motion of individual mRNAs have shed light on the complex RNA transport system. This chapter presents an overview of general approaches for single particle tracking and some methodologies that are used for single mRNA detection.

Altered neuroligin expression is involved in social deficits in a mouse model of the fragile X syndrome

The fragile X syndrome (FXS) is the most common form of inherited mental retardation. Caused by a transcriptional silencing of the fragile X mental retardation protein (FMRP), a mRNA binding protein itself, misregulated translation is thought to be the leading cause of the fragile X syndrome. Interestingly, recent results indicated several neuroligin interacting proteins to be affected by this misregulation, including neurexin1 and PSD95, which have also been implicated in autism spectrum disorders. Using co-immunoprecipitation assays and RT-PCR, FMRP is shown to interact with neuroligin1- and 2-mRNA, while no interaction with neuroligin3-mRNA is observed. In line with FMRP's role in translation regulation, Western blot as well as immunohistochemistry analysis reveal changes in protein expression levels suggesting impaired synaptic function. As increasing evidence indicates neuroligin expression to be critical for synapse maturation and function, consequences of impaired neuroligin1 expression in FXS are assessed by overexpressing HA-neuroligin1 in FMR1-/- mice, a model for FXS. Behavioural assessments demonstrate that enhanced neuroligin1 expression improves social behaviour in FMR1-/- mice, whereas no positive effect on learning and memory is seen. These results provide for the first time evidence for an involvement of a neuroligin-neurexin protein network in core symptoms of FXS.

GPCR signalling to the translation machinery

G protein-coupled receptors (GPCRs) are involved in most physiological processes, many of them being engaged in fully differentiated cells. These receptors couple to transducers of their own, primarily G proteins and beta-arrestins, which launch intracellular signalling cascades. Some of these signalling events regulate the translational machinery to fine-tune general cell metabolism or to alter protein expression pattern. Though extensively documented for tyrosine kinase receptors, translational regulation by GPCRs is still poorly appreciated. The objective of this review paper is to address the following questions: i) is there a "GPCR signature" impacting on the translational machinery, and ultimately on the type of mRNA translated? ii) are the regulatory networks involved similar as those utilized by tyrosine kinase receptors? In particular, we will discuss the specific features of translational control mediated by GPCRs and highlight the intrinsic properties of GPCRs these mechanisms could rely on.

Dynein and kinesin regulate stress-granule and P-body dynamics

Stress granules (SGs) and P-bodies (PBs) are related cytoplasmic structures harboring silenced mRNAs. SGs assemble transiently upon cellular stress, whereas PBs are constitutive and are further induced by stress. Both foci are highly dynamic, with messenger ribonucleoproteins (mRNPs) and proteins rapidly shuttling in and out. Here, we show that impairment of retrograde transport by knockdown of mammalian dynein heavy chain 1 (DHC1) or bicaudal D1 (BicD1) inhibits SG formation and PB growth upon stress, without affecting protein-synthesis blockage. Conversely, impairment of anterograde transport by knockdown of kinesin-1 heavy chain (KIF5B) or kinesin light chain 1 (KLC1) delayed SG dissolution. Strikingly, SG dissolution is not required to restore translation. Simultaneous knockdown of dynein and kinesin reverted the effect of single knockdowns on both SGs and PBs, suggesting that a balance between opposing movements driven by these molecular motors governs foci formation and dissolution. Finally, we found that regulation of SG dynamics by dynein and kinesin is conserved in Drosophila.

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.

Kinesin superfamily motor proteins and intracellular transport

Intracellular transport is fundamental for cellular function, survival and morphogenesis. Kinesin superfamily proteins (also known as KIFs) are important molecular motors that directionally transport various cargos, including membranous organelles, protein complexes and mRNAs. The mechanisms by which different kinesins recognize and bind to specific cargos, as well as how kinesins unload cargo and determine the direction of transport, have now been identified. Furthermore, recent molecular genetic experiments have uncovered important and unexpected roles for kinesins in the regulation of such physiological processes as higher brain function, tumour suppression and developmental patterning. These findings open exciting new areas of kinesin research.

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.

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.

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.

The Relationship between Dendritic Branch Dynamics and CPEB-Labeled RNP Granules Captured in Vivo

Cytoplasmic Polyadenylation Element Binding protein (CPEB) is an RNA binding protein involved in dendritic delivery of mRNA and activity-dependent, polyadenylation-induced translation of mRNAs in the dendritic arbor. CPEB affects learning and memory and impacts neuronal morphological and synaptic plasticity. In neurons, CPEB is concentrated in ribonucleoprotein (RNP) granules that distribute throughout the dendritic arbor and localize near synapses, suggesting that the trafficking of RNP granules is important for CPEB function. We tagged full-length CPEB and an inactive mutant CPEB with fluorescent proteins, then imaged rapid dendritic branch dynamics and RNP distribution using two-photon time-lapse microscopy of neurons in the optic tectum of living Xenopus laevis tadpoles. Though the inactive CPEB mutant transports mRNA in the dendritic arbor, its expression interferes with CPEB-dependent translation because it is incapable of activity-triggered mRNA polyadenylation. In dendrites, the distributions of the active and inactive CPEB-containing RNP granules do not differ; the RNP granules are dense and their positions do not correlate with sites of rapid dendritic branch dynamics or the eventual fate of the dendritic branches. Because CPEB's sensitivity to activity-dependent signaling does not alter its dendritic distribution, it indicates that active sites in the dendritic arbor are not targeted for RNP granule localization. Nevertheless, inactive CPEB accumulates in granules in terminal dendritic branches, supporting the hypothesis that upon activation CPEB and its mRNA cargo are released from granules and are then available for dendritic translation.

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.

The Arc of synaptic memory

The immediate early gene Arc is emerging as a versatile, finely tuned system capable of coupling changes in neuronal activity patterns to synaptic plasticity, thereby optimizing information storage in the nervous system. Here, we attempt to overview the Arc system spanning from transcriptional regulation of the Arc gene, to dendritic transport, metabolism, and translation of Arc mRNA, to post-translational modification, localization, and degradation of Arc protein. Within this framework we discuss the function of Arc in regulation of actin cytoskeletal dynamics underlying consolidation of long-term potentiation (LTP) and regulation of AMPA-type glutamate receptor endocytosis underlying long-term depression (LTD) and homeostatic plasticity. Behaviorally, Arc has a key role in consolidation of explicit and implicit forms of memory, with recent work implicating Arc in adaptation to stress as well as maladaptive plasticity connected to drug addiction. Arc holds considerable promise as a “master regulator” of protein synthesis-dependent forms of synaptic plasticity, but the mechanisms that modulate and switch Arc function are only beginning to be elucidated.

Requirement for protein synthesis at developing synapses

Activity and protein synthesis act cooperatively to generate persistent changes in synaptic responses. This forms the basis for enduring memory in adults. Activity also shapes neural circuits developmentally, but whether protein synthesis plays a congruent function in this process is poorly understood. Here, we show that brief periods of global or local protein synthesis inhibition decrease the synaptic vesicles available for fusion and increase synapse elimination. Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is a critical target; its levels are controlled by rapid turnover, and blocking its activity or knocking it down recapitulates the effects of protein synthesis inhibition. Mature presynaptic terminals show decreased sensitivity to protein synthesis inhibition, and resistance coincides with a developmental switch in regulation from CaMKII to PKA (protein kinase A). These findings demonstrate a novel mechanism regulating presynaptic activity and synapse elimination during development, and suggest that protein translation acts coordinately with activity to selectively stabilize appropriate synaptic interactions.

Ultrastructural analysis of the functional domains in FMRP using primary hippocampal mouse neurons

Fragile X syndrome is caused by lack of the protein FMRP. FMRP mediates mRNA binding, dendritic mRNA transport and translational control at spines. We examined the role of functional domains of FMRP in neuronal RNA-granule formation and dendritic transport using different FMRP variants, including the mutant FMRP_I304N and the splice-variant FMRP_Iso12. Both variants are absent from dendritic RNA-granules in Fmr1 knockout neurons. Co-transfection experiments showed that wild-type FMRP recruits both FMRP variants into dendritic RNA-granules. Co-transfection of FXR2, an FMRP homologue, also resulted in redistribution of both variants into dendritic RNA-granules. Furthermore, the capacity of the variants to transport their mRNAs and the mRNA localization of an FMR1 construct containing silent point-mutations affecting only the G-quartet-structure were investigated. In conclusion, we show that wild-type FMRP and FXR2P are able to recruit FMRP variants into RNA-granules and that the G-quartet-structure in FMR1 mRNA is not essential for its incorporation in RNA-granules.

Fragile X mental retardation protein regulates the levels of scaffold proteins and glutamate receptors in postsynaptic densities

Functional absence of fragile X mental retardation protein (FMRP) causes the fragile X syndrome, a hereditary form of mental retardation characterized by a change in dendritic spine morphology. The RNA-binding protein FMRP has been implicated in regulating postsynaptic protein synthesis. Here we have analyzed whether the abundance of scaffold proteins and neurotransmitter receptor subunits in postsynaptic densities (PSDs) is altered in the neocortex and hippocampus of FMRP-deficient mice. Whereas the levels of several PSD components are unchanged, concentrations of Shank1 and SAPAP scaffold proteins and various glutamate receptor subunits are altered in both adult and juvenile knock-out mice. With the exception of slightly increased hippocampal SAPAP2 mRNA levels in adult animals, altered postsynaptic protein concentrations do not correlate with similar changes in total and synaptic levels of corresponding mRNAs. Thus, loss of FMRP in neurons appears to mainly affect the translation and not the abundance of particular brain transcripts. Semi-quantitative analysis of RNA levels in FMRP immunoprecipitates showed that in the mouse brain mRNAs encoding PSD components, such as Shank1, SAPAP1-3, PSD-95, and the glutamate receptor subunits NR1 and NR2B, are associated with FMRP. Luciferase reporter assays performed in primary cortical neurons from knock-out and wild-type mice indicate that FMRP silences translation of Shank1 mRNAs via their 3'-untranslated region. Activation of metabotropic glutamate receptors relieves translational suppression. As Shank1 controls dendritic spine morphology, our data suggest that dysregulation of Shank1 synthesis may significantly contribute to the abnormal spine development and function observed in brains of fragile X syndrome patients.

Dendritic mRNA targeting of Jacob and N-methyl-d-aspartate-induced nuclear translocation after calpain-mediated proteolysis

Jacob is a recently identified plasticity-related protein that couples N-methyl-d-aspartate receptor activity to nuclear gene expression. An expression analysis by Northern blot and in situ hybridization shows that Jacob is almost exclusively present in brain, in particular in the cortex and the limbic system. Alternative splicing gives rise to multiple mRNA variants, all of which exhibit a prominent dendritic localization in the hippocampus. Functional analysis in primary hippocampal neurons revealed that a predominant cis-acting dendritic targeting element in the 3'-untranslated region of Jacob mRNAs is responsible for dendritic mRNA localization. In the mouse brain, Jacob transcripts are associated with both the fragile X mental retardation protein, a well described trans-acting factor regulating dendritic mRNA targeting and translation, and the kinesin family member 5C motor complex, which is known to mediate dendritic mRNA transport. Jacob is susceptible to rapid protein degradation in a Ca(2+)- and Calpain-dependent manner, and Calpain-mediated clipping of the myristoylated N terminus of Jacob is required for its nuclear translocation after N-methyl-d-aspartate receptor activation. Our data suggest that local synthesis in dendrites may be necessary to replenish dendritic Jacob pools after truncation of the N-terminal membrane anchor and concomitant translocation of Jacob to the nucleus.

Application of aptamers and autofluorescent proteins for RNA visualization

The repertoire of RNAs transcribed and processed within living cells is of extraordinary complexity. With new types of RNA being identified, the need for tools to investigate the spatio-temporal aspects of processing and trafficking of these molecules has become more evident. To visualize RNA in living cells, autofluorescent proteins (AFPs) appear as a promising alternative to synthetic fluorescent compound based labels. While current fluorescent protein-based RNA labelings have provided many new insights into the biology of RNA regulation, further improvements and adaptations are desirable to make AFP labels as valuable in the RNA world as they have proven to be for protein tagging. This article reviews the achievements and existing challenges in engineering AFPs as efficient RNA tags for high resolution fluorescence microscopy in living cells.

The emerging role of synaptic cell-adhesion pathways in the pathogenesis of autism spectrum disorders

Recent advances in genetics and genomics have unveiled numerous cases of autism spectrum disorders (ASDs) associated with rare, causal genetic variations. These findings support a novel view of ASDs in which many independent, individually rare genetic variants, each associated with a very high relative risk, together explain a large proportion of ASDs. Although these rare variants impact diverse pathways, there is accumulating evidence that synaptic pathways, including those involving synaptic cell adhesion, are disrupted in some subjects with ASD. These findings provide insights into the pathogenesis of ASDs and enable the development of model systems with construct validity for specific causes of ASDs. In several neurodevelopmental disorders frequently associated with ASD, including fragile X syndrome, Rett syndrome and tuberous sclerosis, animal models have led to the development of new therapeutic approaches, giving rise to optimism with other causes of ASDs.

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.

FMR1: a gene with three faces

The FMR1 gene is involved in three different syndromes, the fragile X syndrome (FXS), premature ovarian insufficiency (POI) and the fragile X-associated tremor/ataxia syndrome (FXTAS) at older age. Fragile X syndrome is caused by an expansion of a CGG repeat above 200 units in the FMR1 gene resulting in the absence of the FMR1 mRNA and protein. The FMR1 protein is proposed to act as a regulator of mRNA transport and of translation of target mRNAs at the synapse. FXS is seen as a loss of function disorder. POI and FXTAS are found in individuals with an expanded repeat between 50 and 200 CGGs and are associated with increased FMR1 mRNA levels. The presence of elevated FMR1 mRNA in FXTAS suggests that FXTAS may represent a toxic RNA gain-of-function effect. The molecular basis of POI is yet unknown. The role of the FMR1 gene in these disorders is discussed.

Shank1 mRNA: dendritic transport by kinesin and translational control by the 5'untranslated region

Dendritic mRNA transport coupled with local regulation of translation enables neurons to selectively alter the protein composition of individual postsynaptic sites. We have analyzed dendritic localization of shank1 mRNAs; shank proteins (shank1-3) are scaffolding molecules of the postsynaptic density (PSD) of excitatory synapses, which are crucial for PSD assembly and the formation of dendritic spines. Live cell imaging demonstrates saltatory movements of shank1 mRNA containing granules along microtubules in both anterograde and retrograde directions. A population of brain messenger ribonucleoprotein particles (mRNPs) containing shank1 mRNAs associates with the cargo-binding domain of the motor protein KIF5C. Through expression of dominant negative proteins, we show that dendritic targeting of shank1 mRNA granules involves KIF5C and the KIF5-associated RNA-binding protein staufen1. While transport of shank1 mRNAs follows principles previously outlined for other dendritic transcripts, shank1 mRNAs are distinguished by their translational regulation. Translation is strongly inhibited by a GC-rich 5(')untranslated region; in addition, internal ribosomal entry sites previously detected in other dendritic transcripts are absent in the shank1 mRNA. A concept emerges from our data in which dendritic transport of different mRNAs occurs collectively via a staufen1- and KIF5-dependent pathway, whereas their local translation is controlled individually by unique cis-acting elements.

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.

mRNA localization: gene expression in the spatial dimension

The localization of mRNAs to subcellular compartments provides a mechanism for regulating gene expression with exquisite temporal and spatial control. Recent studies suggest that a large fraction of mRNAs localize to distinct cytoplasmic domains. In this Review, we focus on cis-acting RNA localization elements, RNA-binding proteins, and the assembly of mRNAs into granules that are transported by molecular motors along cytoskeletal elements to their final destination in the cell.

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.

Expression of the GABAergic system in animal models for fragile X syndrome and fragile X associated tremor/ataxia syndrome (FXTAS)

After our initial discovery of reduced expression of several subunits of the GABA(A) receptor in two different animal models for fragile X syndrome, a frequent form of inherited mental retardation, we analyzed further components of the GABAergic pathway. Interestingly, we found a down regulation of many additional elements of the GABA signalling system, strengthening our hypothesis of involvement of the GABAergic pathway in the pathophysiology of fragile X syndrome. This is of special interest with regard to new therapeutic opportunities for treatment of this disorder. Remarkably, under expression was predominantly observed in cortex, although some elements of the GABAergic system that are expressed presynaptically or in the glial cells were also down regulated in the cerebellum. Additionally, we assessed the GABAergic system in expanded CGG-repeat mice, a model for fragile X associated tremor/ataxia syndrome (FXTAS). This late onset neurodegenerative disorder occurs in carriers of the fragile X premutation (55-200 CGG repeats) and is completely distinct (from both clinical and molecular pathogenic perspectives) from the neurodevelopmental disorder fragile X syndrome. Here we found upregulation of many components of the GABAergic system in cerebellum, but not in cortex. This finding is consistent with the cerebellar phenotype of FXTAS patients and has implications for the mechanism causative of differential gene expression.

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.

Cells lacking the fragile X mental retardation protein (FMRP) have normal RISC activity but exhibit altered stress granule assembly

The fragile X mental retardation protein (FMRP) is an RNA-binding protein involved in the mRNA metabolism. The absence of FMRP in neurons leads to alterations of the synaptic plasticity, probably as a result of translation regulation defects. The exact molecular mechanisms by which FMRP plays a role in translation regulation have remained elusive. The finding of an interaction between FMRP and the RNA interference silencing complex (RISC), a master of translation regulation, has suggested that both regulators could be functionally linked. We investigated here this link, and we show that FMRP exhibits little overlap both physically and functionally with the RISC machinery, excluding a direct impact of FMRP on RISC function. Our data indicate that FMRP and RISC are associated to distinct pools of mRNAs. FMRP, unlike RISC machinery, associates with the pool of mRNAs that eventually goes into stress granules upon cellular stress. Furthermore, we show that FMRP plays a positive role in this process as the lack of FMRP or a point mutant causing a severe fragile X alter stress granule formation. Our data support the proposal that FMRP plays a role in controlling the fate of mRNAs after translation arrest.

Ribonucleoprotein complexes in neurologic diseases

Ribonucleoprotein (RNP) complexes regulate the tissue-specific RNA processing and transport that increases the coding capacity of our genome and the ability to respond quickly and precisely to the diverse set of signals. This review focuses on three proteins that are part of RNP complexes in most cells of our body: TAR DNA-binding protein (TDP-43), the survival motor neuron protein (SMN), and fragile-X mental retardation protein (FMRP). In particular, the review asks the question why these ubiquitous proteins are primarily associated with defects in specific regions of the central nervous system? To understand this question, it is important to understand the role of genetic and cellular environment in causing the defect in the protein, as well as how the defective protein leads to misregulation of specific target RNAs. Two approaches for comprehensive analysis of defective RNA-protein interactions are presented. The first approach defines the RNA code or the collection of proteins that bind to a certain cis-acting RNA site in order to lead to a predictable outcome. The second approach defines the RNA map or the summary of positions on target RNAs where binding of a particular RNA-binding protein leads to a predictable outcome. As we learn more about the RNA codes and maps that guide the action of the dynamic RNP world in our brain, possibilities for new treatments of neurologic diseases are bound to emerge.

Interaction of USF1/USF2 and alpha-Pal/Nrf1 to Fmr-1 promoter increases in mouse brain during aging

Fragile X syndrome is caused due to silencing of FMR-1 gene transcription leading to loss of fragile X mental retardation protein (FMRP). To investigate whether the transcriptional mechanism is linked to aging, we have studied interaction of the transcription factors USF1/USF2 and alpha-Pal/Nrf1 to E-box and GC-box, respectively, in Fmr-1 promoter in the brain of young, adult, and old mouse using electrophoretic mobility shift assay (EMSA). Our data reveal that the interaction of these transcription factors to their respective promoter sequences increases in mouse brain as a function of age. The finding on the interaction of the above transcription factors to their cognate sequences is novel as the current investigation has been carried out in intact and aging mouse. The present finding is important in respect to age- and FMRP-dependent brain function.

Highways for mRNA transport

Two new studies reveal the role of microtubule polarity in the asymmetric localization of mRNAs. In this issue of Cell, Zimyanin et al. (2008) show that the asymmetric localization of oskar mRNA in fruit fly oocytes results from a slight bias in the direction of its transport. Meanwhile, Messitt et al. (2008) reporting in Developmental Cell find a subpopulation of microtubules that is critical for the asymmetric distribution of Vg1 mRNA in frog oocytes.