Sensory stimulation increases cortical expression of the fragile X mental retardation protein in vivo

Glial-mediated dysregulation of neurodevelopment in Fragile X Syndrome

Astrocytes are highly involved in a multitude of developmental processes that are known to be dysregulated in Fragile X Syndrome. Here, we examine these processes individually and review the roles astrocytes play in contributing to the pathology of this syndrome. As a growing area of interest in the field, new and exciting insight is continually emerging. Understanding these glial-mediated roles is imperative for elucidating the underlying molecular mechanisms at play, not only in Fragile X Syndrome, but also other ASD-related disorders. Understanding these roles will be central to the future development of effective, clinically-relevant treatments of these disorders.

Peripheral Fragile X messenger ribonucleoprotein is required for the timely closure of a critical period for neuronal susceptibility in the ventral cochlear nucleus

Alterations in neuronal plasticity and critical periods are common across neurodevelopmental diseases, including Fragile X syndrome (FXS), the leading single-gene cause of autism. Characterized with sensory dysfunction, FXS is the result of gene silencing of Fragile X messenger ribonucleoprotein 1 (FMR1) and loss of its product, Fragile X messenger ribonucleoprotein (FMRP). The mechanisms underlying altered critical period and sensory dysfunction in FXS are obscure. Here, we performed genetic and surgical deprivation of peripheral auditory inputs in wildtype and Fmr1 knockout (KO) mice across ages and investigated the effects of global FMRP loss on deafferentation-induced neuronal changes in the ventral cochlear nucleus (VCN) and auditory brainstem responses. The degree of neuronal cell loss during the critical period was unchanged in Fmr1 KO mice. However, the closure of the critical period was delayed. Importantly, this delay was temporally coincidental with reduced hearing sensitivity, implying an association with sensory inputs. Functional analyses further identified early-onset and long-lasting alterations in signal transmission from the spiral ganglion to the VCN, suggesting a peripheral site of FMRP action. Finally, we generated conditional Fmr1 KO (cKO) mice with selective deletion of FMRP in spiral ganglion but not VCN neurons. cKO mice recapitulated the delay in the VCN critical period closure in Fmr1 KO mice, confirming an involvement of cochlear FMRP in shaping the temporal features of neuronal critical periods in the brain. Together, these results identify a novel peripheral mechanism of neurodevelopmental pathogenesis.

Axonal and presynaptic FMRP: Localization, signal, and functional implications

Fragile X mental retardation protein (FMRP) binds a selected set of mRNAs and proteins to guide neural circuit assembly and regulate synaptic plasticity. Loss of FMRP is responsible for Fragile X syndrome, a neuropsychiatric disorder characterized with auditory processing problems and social difficulty. FMRP actions in synaptic formation, maturation, and plasticity are site-specific among the four compartments of a synapse: presynaptic and postsynaptic neurons, astrocytes, and extracellular matrix. This review summarizes advancements in understanding FMRP localization, signals, and functional roles in axons and presynaptic terminals.

Fmr1 exon 14 skipping in late embryonic development of the rat forebrain

BACKGROUND

Fragile X syndrome, the major cause of inherited intellectual disability among men, is due to deficiency of the synaptic functional regulator FMR1 protein (FMRP), encoded by the FMRP translational regulator 1 (FMR1) gene. FMR1 alternative splicing produces distinct transcripts that may consequently impact FMRP functional roles. In transcripts without exon 14 the translational reading frame is shifted. For deepening current knowledge of the differential expression of Fmr1 exon 14 along the rat nervous system development, we conducted a descriptive study employing quantitative RT-PCR and BLAST of RNA-Seq datasets.

RESULTS

We observed in the rat forebrain progressive decline of total Fmr1 mRNA from E11 to P112 albeit an elevation on P3; and exon-14 skipping in E17-E20 with downregulation of the resulting mRNA. We tested if the reduced detection of messages without exon 14 could be explained by nonsense-mediated mRNA decay (NMD) vulnerability, but knocking down UPF1, a major component of this pathway, did not increase their quantities. Conversely, it significantly decreased FMR1 mRNA having exon 13 joined with either exon 14 or exon 15 site A.

CONCLUSIONS

The forebrain in the third embryonic week of the rat development is a period with significant skipping of Fmr1 exon 14. This alternative splicing event chronologically precedes a reduction of total Fmr1 mRNA, suggesting that it may be part of combinatorial mechanisms downregulating the gene's expression in the late embryonic period. The decay of FMR1 mRNA without exon 14 should be mediated by a pathway different from NMD. Finally, we provide evidence of FMR1 mRNA stabilization by UPF1, likely depending on FMRP.

Dynamics of the fragile X mental retardation protein correlates with cellular and synaptic properties in primary auditory neurons following afferent deprivation

Afferent activity dynamically regulates neuronal properties and connectivity in the central nervous system. The Fragile X mental retardation protein (FMRP) is an RNA-binding protein that regulates cellular and synaptic properties in an activity-dependent manner. Whether and how FMRP level and localization are regulated by afferent input remains sparsely examined and how such regulation is associated with neuronal response to changes in sensory input is unknown. We characterized changes in FMRP level and localization in the chicken nucleus magnocellularis (NM), a primary cochlear nucleus, following afferent deprivation by unilateral cochlea removal. We observed rapid (within 2 hr) aggregation of FMRP immunoreactivity into large granular structures in a subset of deafferented NM neurons. Neurons that exhibited persistent FMRP aggregation at 12-24 hr eventually lost cytoplasmic Nissl substance, indicating cell death. A week later, FMRP expression in surviving neurons regained its homeostasis, with a slightly reduced immunostaining intensity and enhanced heterogeneity. Correlation analyses under the homeostatic status (7-14 days) revealed that neurons expressing relatively more FMRP had a higher capability of maintaining cell body size and ribosomal activity, as well as a better ability to detach inactive presynaptic terminals. Additionally, the intensity of an inhibitory postsynaptic protein, gephyrin, was reduced following deafferentation and was positively correlated with FMRP intensity, implicating an involvement of FMRP in synaptic dynamics in response to reduced afferent inputs. Collectively, this study demonstrates that afferent input regulates FMRP expression and localization in ways associated with multiple types of neuronal responses and synaptic rearrangements.

Altered intrinsic properties and bursting activities of neurons in layer IV of somatosensory cortex from Fmr-1 knockout mice

Neuroadaptations and alterations in neuronal excitability are critical in brain maturation and many neurological diseases. Fragile X syndrome (FXS) is a pervasive neurodevelopmental disorder characterized by extensive synaptic and circuit dysfunction. It is still unclear about the alterations in intrinsic excitability of individual neurons and their link to hyperexcitable circuitry. In this study, whole cell patch-clamp recordings were employed to characterize the membrane and firing properties of layer IV cells in slices of the somatosensory cortex of Fmr-1 knockout (KO) mice. These cells generally exhibited a regular spiking (RS) pattern, while there were significant increases in the number of cells that adopted intrinsic bursting (IB) compared with age-matched wild type (WT) cells. The cells subgrouped according to their firing patterns and maturation differed significantly in membrane and discharge properties between KO and WT. The changes in the intrinsic properties were consistent with highly facilitated discharges in KO cells induced by current injection. Spontaneous activities of RS neurons driven by local network were also increased in the KO cells, especially in neonate groups. Under an epileptiform condition mimicked by omission of Mg(2+) in extracellular solution, these RS neurons from KO mice were more likely to switch to burst discharges. Analysis on bursts revealed that the KO cells tended to form burst discharges and even severe events manifested as seizure-like ictal discharges. These results suggest that alterations in intrinsic properties in individual neurons are involved in the abnormal excitability of cortical circuitry and possibly account for the pathogenesis of epilepsy in FXS.

Increasing our understanding of human cognition through the study of Fragile X Syndrome

Fragile X Syndrome (FXS) is considered the most common form of inherited intellectual disability. It is caused by reductions in the expression level or function of a single protein, the Fragile X Mental Retardation Protein (FMRP), a translational regulator which binds to approximately 4% of brain messenger RNAs. Accumulating evidence suggests that FXS is a complex disorder of cognition, involving interactions between genetic and environmental influences, leading to difficulties in acquiring key life skills including motor skills, language, and proper social behaviors. Since many FXS patients also present with one or more features of autism spectrum disorders (ASDs), insights gained from studying the monogenic basis of FXS could pave the way to a greater understanding of underlying features of multigenic ASDs. Here we present an overview of the FXS and FMRP field with the goal of demonstrating how loss of a single protein involved in translational control affects multiple stages of brain development and leads to debilitating consequences on human cognition. We also focus on studies which have rescued or improved FXS symptoms in mice using genetic or therapeutic approaches to reduce protein expression. We end with a brief description of how deficits in translational control are implicated in FXS and certain cases of ASDs, with many recent studies demonstrating that ASDs are likely caused by increases or decreases in the levels of certain key synaptic proteins. The study of FXS and its underlying single genetic cause offers an invaluable opportunity to study how a single gene influences brain development and behavior.

BDNF in fragile X syndrome

Fragile X syndrome (FXS) is a monogenic disorder that is caused by the absence of FMR1 protein (FMRP). FXS serves as an excellent model disorder for studies investigating disturbed molecular mechanisms and synapse function underlying cognitive impairment, autism, and behavioral disturbance. Abnormalities in dendritic spines and synaptic transmission in the brain of FXS individuals and mouse models for FXS indicate perturbations in the development, maintenance, and plasticity of neuronal network connectivity. However, numerous alterations are found during the early development in FXS, including abnormal differentiation of neural progenitors and impaired migration of newly born neurons. Several aspects of FMRP function are modulated by brain-derived neurotrophic factor (BDNF) signaling. Here, we review the evidence of the role for BDNF in the developing and adult FXS brain. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.

Sleep and synaptic homeostasis: structural evidence in Drosophila

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

Cellular stress-induced up-regulation of FMRP promotes cell survival by modulating PI3K-Akt phosphorylation cascades

Background

Fragile X syndrome (FXS), the most commonly inherited mental retardation and single gene cause of autistic spectrum disorder, occurs when the Fmr1 gene is mutated. The product of Fmr1, fragile X linked mental retardation protein (FMRP) is widely expressed in HeLa cells, however the roles of FMRP within HeLa cells were not elucidated, yet. Interacting with a diverse range of mRNAs related to cellular survival regulatory signals, understanding the functions of FMRP in cellular context would provide better insights into the role of this interesting protein in FXS. Using HeLa cells treated with etoposide as a model, we tried to determine whether FMRP could play a role in cell survival.

Methods

Apoptotic cell death was induced by etoposide treatment on Hela cells. After we transiently modulated FMRP expression (silencing or enhancing) by using molecular biotechnological methods such as small hairpin RNA virus-induced knock down and overexpression using transfection with FMRP expression vectors, cellular viability was measured using propidium iodide staining, TUNEL staining, and FACS analysis along with the level of activation of PI3K-Akt pathway by Western blot. Expression level of FMRP and apoptotic regulator BcL-xL was analyzed by Western blot, RT-PCR and immunocytochemistry.

Results

An increased FMRP expression was measured in etoposide-treated HeLa cells, which was induced by PI3K-Akt activation. Without FMRP expression, cellular defence mechanism via PI3K-Akt-Bcl-xL was weakened and resulted in an augmented cell death by etoposide. In addition, FMRP over-expression lead to the activation of PI3K-Akt signalling pathway as well as increased FMRP and BcL-xL expression, which culminates with the increased cell survival in etoposide-treated HeLa cells.

Conclusions

Taken together, these results suggest that FMRP expression is an essential part of cellular survival mechanisms through the modulation of PI3K, Akt, and Bcl-xL signal pathways.

Histone deacetylases suppress CGG repeat-induced neurodegeneration via transcriptional silencing in models of fragile X tremor ataxia syndrome

Fragile X Tremor Ataxia Syndrome (FXTAS) is a common inherited neurodegenerative disorder caused by expansion of a CGG trinucleotide repeat in the 5'UTR of the fragile X syndrome (FXS) gene, FMR1. The expanded CGG repeat is thought to induce toxicity as RNA, and in FXTAS patients mRNA levels for FMR1 are markedly increased. Despite the critical role of FMR1 mRNA in disease pathogenesis, the basis for the increase in FMR1 mRNA expression is unknown. Here we show that overexpressing any of three histone deacetylases (HDACs 3, 6, or 11) suppresses CGG repeat-induced neurodegeneration in a Drosophila model of FXTAS. This suppression results from selective transcriptional repression of the CGG repeat-containing transgene. These findings led us to evaluate the acetylation state of histones at the human FMR1 locus. In patient-derived lymphoblasts and fibroblasts, we determined by chromatin immunoprecipitation that there is increased acetylation of histones at the FMR1 locus in pre-mutation carriers compared to control or FXS derived cell lines. These epigenetic changes correlate with elevated FMR1 mRNA expression in pre-mutation cell lines. Consistent with this finding, histone acetyltransferase (HAT) inhibitors repress FMR1 mRNA expression to control levels in pre-mutation carrier cell lines and extend lifespan in CGG repeat-expressing Drosophila. These findings support a disease model whereby the CGG repeat expansion in FXTAS promotes chromatin remodeling in cis, which in turn increases expression of the toxic FMR1 mRNA. Moreover, these results provide proof of principle that HAT inhibitors or HDAC activators might be used to selectively repress transcription at the FMR1 locus.

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.

Critical period plasticity is disrupted in the barrel cortex of FMR1 knockout mice

Alterations in sensory processing constitute prominent symptoms of fragile X syndrome; however, little is known about how disrupted synaptic and circuit development in sensory cortex contributes to these deficits. To investigate how the loss of fragile X mental retardation protein (FMRP) impacts the development of cortical synapses, we examined excitatory thalamocortical synapses in somatosensory cortex during the perinatal critical period in Fmr1 knockout mice. FMRP ablation resulted in dysregulation of glutamatergic signaling maturation. The fraction of silent synapses persisting to later developmental times was increased; there was a temporal delay in the window for synaptic plasticity, while other forms of developmental plasticity were not altered in Fmr1 knockout mice. Our results indicate that FMRP is required for the normal developmental progression of synaptic maturation, and loss of this important RNA binding protein impacts the timing of the critical period for layer IV synaptic plasticity.

Fragile X mental retardation protein in learning-related synaptic plasticity

Fragile X syndrome (FXS) is caused by a lack of the fragile X mental retardation protein (FMRP) due to silencing of the Fmr1 gene. As an RNA binding protein, FMRP is thought to contribute to synaptic plasticity by regulating plasticity-related protein synthesis and other signaling pathways. Previous studies have mostly focused on the roles of FMRP within the hippocampus--a key structure for spatial memory. However, recent studies indicate that FMRP may have a more general contribution to brain functions, including synaptic plasticity and modulation within the prefrontal cortex. In this brief review, we will focus on recent studies reported in the prefrontal cortex, including the anterior cingulate cortex (ACC). We hypothesize that alterations in ACC-related plasticity and synaptic modulation may contribute to various forms of cognitive deficits associated with FXS.

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.

Activity-dependent modulation of neural circuit synaptic connectivity

In many nervous systems, the establishment of neural circuits is known to proceed via a two-stage process; (1) early, activity-independent wiring to produce a rough map characterized by excessive synaptic connections, and (2) subsequent, use-dependent pruning to eliminate inappropriate connections and reinforce maintained synapses. In invertebrates, however, evidence of the activity-dependent phase of synaptic refinement has been elusive, and the dogma has long been that invertebrate circuits are “hard-wired” in a purely activity-independent manner. This conclusion has been challenged recently through the use of new transgenic tools employed in the powerful Drosophila system, which have allowed unprecedented temporal control and single neuron imaging resolution. These recent studies reveal that activity-dependent mechanisms are indeed required to refine circuit maps in Drosophila during precise, restricted windows of late-phase development. Such mechanisms of circuit refinement may be key to understanding a number of human neurological diseases, including developmental disorders such as Fragile X syndrome (FXS) and autism, which are hypothesized to result from defects in synaptic connectivity and activity-dependent circuit function. This review focuses on our current understanding of activity-dependent synaptic connectivity in Drosophila, primarily through analyzing the role of the fragile X mental retardation protein (FMRP) in the Drosophila FXS disease model. The particular emphasis of this review is on the expanding array of new genetically-encoded tools that are allowing cellular events and molecular players to be dissected with ever greater precision and detail.

The Drosophila fragile X mental retardation gene regulates sleep need

Sleep need is affected by developmental stage and neuronal plasticity, but the underlying mechanisms remain unclear. The fragile X mental retardation gene Fmr1, whose loss-of-function mutation causes the most common form of inherited mental retardation in humans, is involved in synaptogenesis and synaptic plasticity, and its expression depends on both developmental stage and waking experience. Fmr1 is highly conserved across species and Drosophila mutants carrying dFmr1 loss-of-function or gain-of-function mutations are well characterized: amorphs have overgrown dendritic trees with larger synaptic boutons, developmental defects in pruning, and enhanced neurotransmission, while hypermorphs show opposite defects, including dendritic and axonal underbranching and loss of synapse differentiation. We find here that dFmr1 amorphs are long sleepers and hypermorphs are short sleepers, while both show increased locomotor activity and shortened lifespan. Both amorphs and hypermorphs also show abnormal sleep homeostasis, with impaired waking performance and no sleep rebound after sleep deprivation. An impairment in the circadian regulation of sleep cannot account for the altered sleep phenotype of dFmr1 mutants, nor can an abnormal activation of glutamatergic metabotropic receptors. Moreover, overexpression of dFmr1 throughout the mushroom bodies is sufficient to reduce sleep. Finally, dFmr1 protein levels are modulated by both developmental stage and behavioral state, with increased expression immediately after eclosure and after prolonged wakefulness. Thus, dFmr1 expression dose-dependently affects both sleep and synapses, suggesting that changes in sleep time in dFmr1 mutants may derive from changes in synaptic physiology.

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.

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.

Limbic epileptogenesis in a mouse model of fragile X syndrome

Fragile X syndrome (FXS), caused by silencing of the Fmr1 gene, is the most common form of inherited mental retardation. Epilepsy is reported to occur in 20-25% of individuals with FXS. However, no overall increased excitability has been reported in Fmr1 knockout (KO) mice, except for increased sensitivity to auditory stimulation. Here, we report that kindling increased the expressions of Fmr1 mRNA and protein in the forebrain of wild-type (WT) mice. Kindling development was dramatically accelerated in Fmr1 KO mice, and Fmr1 KO mice also displayed prolonged electrographic seizures during kindling and more severe mossy fiber sprouting after kindling. The accelerated rate of kindling was partially repressed by inhibiting N-methyl-D-aspartic acid receptor (NMDAR) with MK-801 or mGluR5 receptor with 2-methyl-6-(phenylethynyl)-pyridine (MPEP). The rate of kindling development in WT was not effected by MPEP, however, suggesting that FMRP normally suppresses epileptogenic signaling downstream of metabolic glutamate receptors. Our findings reveal that FMRP plays a critical role in suppressing limbic epileptogenesis and predict that the enhanced susceptibility of patients with FXS to epilepsy is a direct consequence of the loss of an important homeostatic factor that mitigates vulnerability to excessive neuronal excitation.

Expression of fragile X mental retardation protein within the vocal control system of developing and adult male zebra finches

Individuals with fragile X syndrome (FXS) are cognitively impaired and have marked speech delays and deficits. Our goal was to characterize expression of fragile X mental retardation protein (FMRP), encoded by Fmr1 fragile X mental retardation 1 gene or transcript (FMR1), in an animal model that learns to vocalize, namely the zebra finch Taeniopygia guttata (Tgu). We cloned and sequenced the zebra finch ortholog of FMR1 (TguFmr1) and developed an antibody that recognizes TguFmrp specifically. TguFmrp has structural features similar to its human ortholog FMRP. Because FXS patients exhibit sensorimotor deficits, we examined TguFmrp expression prior to, during, and after sensorimotor song learning in zebra finches. We found that TguFmrp is expressed throughout the brain and in four major song nuclei of the male zebra finch brain, primarily in neurons. Additionally, prior to sensorimotor learning, we observed elevated TguFmrp expression in the robust nucleus of the arcopallium (RA) of post-hatch day 30 males, compared with the surrounding telencephalon, suggesting a preparation for this stage of song learning. Finally, we observed variable TguFmrp expression in the RA of adolescent and adult males: in some males it was elevated and in others it was comparable to the surrounding telencephalon. In summary, we have characterized the zebra finch ortholog of FMRP and found elevated levels in the premotor nucleus RA at a key developmental stage for vocal learning.

Drosophila fragile X mental retardation protein developmentally regulates activity-dependent axon pruning

Fragile X Syndrome (FraX) is a broad-spectrum neurological disorder with symptoms ranging from hyperexcitability to mental retardation and autism. Loss of the fragile X mental retardation 1 (fmr1) gene product, the mRNA-binding translational regulator FMRP, causes structural over-elaboration of dendritic and axonal processes, as well as functional alterations in synaptic plasticity at maturity. It is unclear, however, whether FraX is primarily a disease of development, a disease of plasticity or both: a distinction that is vital for engineering intervention strategies. To address this crucial issue, we have used the Drosophila FraX model to investigate the developmental function of Drosophila FMRP (dFMRP). dFMRP expression and regulation of chickadee/profilin coincides with a transient window of late brain development. During this time, dFMRP is positively regulated by sensory input activity, and is required to limit axon growth and for efficient activity-dependent pruning of axon branches in the Mushroom Body learning/memory center. These results demonstrate that dFMRP has a primary role in activity-dependent neural circuit refinement during late brain development.

Fragile X mental retardation protein induces synapse loss through acute postsynaptic translational regulation

Fragile X syndrome, as well as other forms of mental retardation and autism, is associated with altered dendritic spine number and structure. Fragile X syndrome is caused by loss-of-function mutations in Fragile X mental retardation protein (FMRP), an RNA-binding protein that regulates protein synthesis in vivo. It is unknown whether FMRP plays a direct, cell-autonomous role in regulation of synapse number, function, or maturation. Here, we report that acute postsynaptic expression of FMRP in Fmr1 knock-out (KO) neurons results in a decrease in the number of functional and structural synapses without an effect on their synaptic strength or maturational state. Similarly, neurons endogenously expressing FMRP (wild-type) have fewer synapses than neighboring Fmr1 KO neurons. An intact K homology domain 2 (KH2) RNA-binding domain and dephosphorylation of FMRP at S500 were required for the effects of FMRP on synapse number, indicating that FMRP interaction with RNA and translating polyribosomes leads to synapse loss.

Age-dependent and selective impairment of long-term potentiation in the anterior piriform cortex of mice lacking the fragile X mental retardation protein

Synaptic function and plasticity were studied in mice lacking the fragile X mental retardation protein (FMRP), a model for the fragile X mental retardation syndrome. Associational connections were studied in slices of anterior piriform (olfactory) cortex, and Schaffer-commissural synapses were studied in slices of hippocampus. Knock-out (KO) mice lacking FMRP were compared with congenic C57BL/6J wild-type (WT) controls. Input-output curves and paired-pulse plasticity were not significantly altered in KO compared with WT mice in either the olfactory cortex or hippocampus. Long-term potentiation (LTP) induced by theta burst stimulation in the anterior piriform cortex was normal in KO mice aged < 6 months but was impaired in KO mice aged > 6 months. The deficit in LTP was significant in mice aged 6-12 months and more pronounced in mice aged 12-18 months. Similar differences between WT and KO mice were seen whether LTP was induced in the presence or absence of a GABAA receptor blocker. Postsynaptic responses to patterned burst stimulation in KO mice showing impaired LTP were not significantly different from those in WT mice, suggesting that the LTP deficit was not caused by alterations in circuit properties. No differences in hippocampal LTP were observed in WT and KO mice at any ages. The results indicate that FMRP deficiency is associated with an age-dependent and region-selective impairment in long-term synaptic plasticity.

Fragile X mental retardation protein levels increase following complex environment exposure in rat brain regions undergoing active synaptogenesis

Fragile X mental retardation protein (FMRP), which is absent in fragile X syndrome, is synthesized in vitro in response to neurotransmitter activation. Humans and mice lacking FMRP exhibit abnormal dendritic spine development, suggesting that this protein plays an important role in synaptic plasticity. Previously, our laboratory demonstrated increased FMRP immunoreactivity in visual cortex of rats exposed to complex environments (EC) and in motor cortex of rats trained on motor-skill tasks compared with animals reared individually in standard laboratory housing (IC). Here, we use immunohistochemistry to extend those findings by investigating FMRP levels in visual cortex and hippocampal dentate gyrus of animals exposed to EC or IC. Rats exposed to EC for 20 days exhibited increased FMRP immunoreactivity in visual cortex compared with animals housed in standard laboratory caging. In the dentate gyrus, animals exposed to EC for 20 days had higher FMRP levels than animals exposed to EC for 5 or 10 days. In light of possible antibody crossreactivity with closely related proteins FXR1P and FXR2P, FMRP immunoreactivity in the posterior-dorsal one-third of cerebral cortex was also examined by Western blotting following 20 days of EC exposure. FMRP levels were greater in EC animals, whereas levels of FXR1P and FXR2P were unaffected by experience. These results provide further evidence for behaviorally induced alteration of FMRP expression in contrast to its homologues, extend previous findings suggesting regulation of its expression by synaptic activity, and support the theories associating FMRP expression with alteration of synaptic structure both in development and later in the life-cycle.

From mRNP trafficking to spine dysmorphogenesis: the roots of fragile X syndrome

The mental retardation protein FMRP is involved in the transport of mRNAs and their translation at synapses. Patients with fragile X syndrome, in whom FMRP is absent or mutated, show deficits in learning and memory that might reflect impairments in the translational regulation of a subset of neuronal mRNAs. The study of FMRP provides important insights into the regulation and functions of local protein synthesis in the neuronal periphery, and increases our understanding of how these functions can produce specific effects at individual synapses.

Regulating fragile X gene transcription in the brain and beyond

The past several years have seen remarkable growth in our understanding of the molecular processes underlying fragile X syndrome (FXS). Many studies have provided new insights into the regulation of Fmr1 gene expression and the potential function of its protein product. It is now known that the promoter elements modulating Fmr1 transcription involve a complex array of both cis and trans factors. Moreover, recent studies of epigenetic modification of chromatin have provided novel clues to unlocking the mysteries behind the regulation of Fmr1 expression. Here, we review the latest findings on the regulation of Fmr1 transcription.

Fragile X mental retardation protein is necessary for neurotransmitter-activated protein translation at synapses

Fragile X mental retardation is caused by absence of the RNA-binding protein fragile X mental retardation protein (FMRP), encoded by the FMR1 gene. There is increasing evidence that FMRP regulates transport and modulates translation of some mRNAs. We studied neurotransmitter-activated synaptic protein synthesis in fmr1-knockout mice. Synaptoneurosomes from knockout mice did not manifest accelerated polyribosome assembly or protein synthesis as it occurs in wild-type mice upon stimulation of group I metabotropic glutamate receptors. Direct activation of protein kinase C did not compensate in the knockout mouse, indicating that the FMRP-dependent step is further along the signaling pathway. Visual cortices of young knockout mice exhibited a lower proportion of dendritic spine synapses containing polyribosomes than did the cortices of wild-type mice, corroborating this finding in vivo. This deficit in rapid neurotransmitter-controlled local translation of specific proteins may contribute to morphological and functional abnormalities observed in patients with fragile X syndrome.

Defective neuronal development in the mushroom bodies of Drosophila fragile X mental retardation 1 mutants

Fragile X mental retardation 1 (Fmr1) is a highly conserved gene with major roles in CNS structure and function. Its product, the RNA-binding protein FMRP, is believed to regulate translation of specific transcripts at postsynaptic sites in an activity-dependent manner. Hence, Fmr1 is central to the molecular mechanisms of synaptic plasticity required for normal neuronal maturation and cognitive ability. Mutations in its Drosophila ortholog, dfmr1, produce phenotypes of brain interneurons and axon terminals at the neuromuscular junction, as well as behavioral defects of circadian rhythms and courtship. We hypothesized that dfmr1 mutations would disrupt morphology of the mushroom bodies (MBs), highly plastic brain regions essential for many forms of learning and memory. We found developmental defects of MB lobe morphogenesis, of which the most common is a failure of beta lobes to stop at the brain midline. A similar recessive beta-lobe midline-crossing phenotype has been previously reported in the memory mutant linotte. The dfmr1 MB defects are highly sensitive to genetic background, which is reminiscent of mammalian fragile-X phenotypes. Mutations of dfmr1 also interact with one or more third-chromosome loci to promote alpha/beta-lobe maturation. These data further support the use of the Drosophila model system for study of hereditary cognitive disorders of humans.

The fragile X syndrome: from molecular genetics to neurobiology

Since the identification of the FMR1 gene basic research has been focused on the molecular characterization of the FMR1 gene product, the fragile X mental retardation protein (FMRP). Recent developments in fragile X research have provided new insights and knowledge about the physiological function of FMRP in the cell and the nerve cell in particular. Currently, compelling evidence suggests a role for FMRP in the transport/translation of dendritically localized mRNAs. In addition, the identification of some of the target mRNAs of FMRP have led to an increased interest in the neurobiology of the syndrome. This review highlights the role of FMRP in dendritic mRNA transport/translation in relation to synaptic plasticity, a molecular mechanism implicated in learning and memory.

Autism as a disorder of neural information processing: directions for research and targets for therapy

The broad variation in phenotypes and severities within autism spectrum disorders suggests the involvement of multiple predisposing factors, interacting in complex ways with normal developmental courses and gradients. Identification of these factors, and the common developmental path into which they feed, is hampered by the large degrees of convergence from causal factors to altered brain development, and divergence from abnormal brain development into altered cognition and behaviour. Genetic, neurochemical, neuroimaging, and behavioural findings on autism, as well as studies of normal development and of genetic syndromes that share symptoms with autism, offer hypotheses as to the nature of causal factors and their possible effects on the structure and dynamics of neural systems. Such alterations in neural properties may in turn perturb activity-dependent development, giving rise to a complex behavioural syndrome many steps removed from the root causes. Animal models based on genetic, neurochemical, neurophysiological, and behavioural manipulations offer the possibility of exploring these developmental processes in detail, as do human studies addressing endophenotypes beyond the diagnosis itself.

Metabotropic glutamate receptor activation regulates fragile x mental retardation protein and FMR1 mRNA localization differentially in dendrites and at synapses

Fragile X syndrome is caused by the absence of the mRNA-binding protein Fragile X mental retardation protein (FMRP), which may play a role in activity-regulated localization and translation of mRNA in dendrites and at synapses. We investigated whether neuronal activity and glutamatergic signals regulate trafficking of FMRP and its encoding Fmr1 mRNA into dendrites or at synapses. Using high-resolution fluorescence and digital imaging microscopy in cultured hippocampal neurons, FMRP and Fmr1 mRNA were localized in granules throughout dendrites and within spines. KCl depolarization rapidly increased FMRP and Fmr1 mRNA levels in dendrites. Metabotropic glutamate receptor (mGluR) activation, in particular mGluR5 activation, was necessary for localization of FMRP into dendrites. Blockade of either PKC or internal calcium prevented mGluR-dependent localization of both FMRP and Fmr1 mRNA in dendrites. The activity-dependent localization of FMRP was not dependent on protein synthesis. Fluorescence recovery after photobleaching analysis of live neurons transfected with enhanced green fluorescent protein-FMRP revealed increased granule trafficking in response to KCl depolarization. In contrast to its dendritic localization, mGluR activation diminished FMRP, but not Fmr1 mRNA, localization at synapses. These results demonstrate regulation of FMRP and Fmr1 mRNA trafficking in dendrites and synapses in response to specific glutamatergic signals.

Is fragile X mental retardation protein involved in activity-induced plasticity of dendritic spines?

Dendritic morphology of 2-week-old cultured neurons, taken from postnatal day 1 fragile X mental retardation gene1 knock out (FMR1-/-) mice hippocampus, were compared with cells taken from wild type mice. Under control conditions the FMR1-/- neurons displayed significantly lower spine densities compared to wild type neurons. Pharmacological stimulation of electrical activity, induced by bicuculline, caused a reduction in dendritic spine density in both the FMR1-/- and the wild type cells. In both groups, bicuculline induced a significant shrinkage of spines that were occupied by one or more synaptophysin-immunoreactive presynaptic terminals. The concentration of FMR1 in the wild type cultures was not affected by bicuculline treatment. These experiments indicate that FMR1 is not likely to be an essential factor in activity-modulated morphological plasticity of dendritic spines in cultured hippocampal neurons.

Somatosensory cortical barrel dendritic abnormalities in a mouse model of the fragile X mental retardation syndrome

The Fragile X mental retardation syndrome is the largest source of inherited mental retardation. The syndrome usually results from the transcriptional silencing of the fragile X mental retardation gene (FMR1). To date the most prominent reported neuronal abnormalities for the fragile X mental retardation syndrome include a higher density of long thin spines similar to those found in sensory deprived and developing tissue, suggesting a possible deficit in pruning of immature spines. Dendrites on spiny stellate cells in the inner 1/3 of the barrel wall in layer IV of the rodent somatosensory cortex have been shown to exhibit developmental pruning similar to that affecting spines. To determine if FMRP plays a role in dendritic development, these neurons were examined in two strains of adult FMRP knockout (FraX) mice. FraX mice in both strains exhibited a greater amount of septa-oriented dendritic material, a morphology consistent with pre-pruning status early in development. This observation suggests that FMRP could be necessary for normal developmentally regulated dendritic pruning.

Whisker stimulation-dependent translation of FMRP in the barrel cortex requires activation of type I metabotropic glutamate receptors

Fragile X syndrome is a common inherited cause of mental retardation that results from the absence of the Fragile X Mental Retardation Protein (FMRP), an RNA binding protein thought to regulate translation of bound mRNAs, including its own. Previous studies in our laboratory have shown that FMRP expression increases in the barrel cortex of the rat after unilateral whisker stimulation, a model of experience dependent plasticity. This increase in protein is restricted to sub-cellular fractions enriched for synaptic or poly-ribosomal complexes. Here, we demonstrate that these increases are not accompanied by a change in FMR-1 mRNA levels and that they are blocked by the protein synthesis inhibitor cycloheximide in a dose dependent manner. Whisker stimulation dependent expression of FMRP is also abolished by pharmacological blockade of either NMDA receptors (MK-801, 0.25 mg/kg) or type I metabotropic glutamate receptors (AIDA, 5 mg/kg). In primary cortical neurons, activation of type I mGluRs leads to an increase in FMRP expression that is not effected by blockade of NMDA receptors. Taken together, these studies show that experience regulates FMRP production in vivo at the level of translation and supports a role for FMRP in metabotropic glutamate receptor mediated synaptic plasticity.

Fragile X mental retardation protein in plasticity and disease

Fragile X syndrome is the most common cause of mental retardation known to be inherited. The syndrome results from the suppressed expression of a single protein, the fragile X mental retardation protein (FMRP). Understanding the function and regulation of FMRP can, therefore, offer insights into both the pathophysiology of fragile X syndrome and the molecular mechanisms of learning and memory. We provide an overview of current concepts of how FMRP functions in the nervous system, with special emphasis on recent evidence that FMRP has a role in metabotropic glutamate receptor-activated protein translation and synaptic plasticity.

A decade of molecular studies of fragile X syndrome

Fragile X syndrome is one of the most common forms of inherited mental retardation. In most cases the disease is caused by the methylation-induced transcriptional silencing of the fragile X mental retardation 1 (FMR1) gene that occurs as a result of the expansion of a CGG repeat in the gene's 5'UTR and leads to the loss of protein product fragile X mental retardation protein (FMRP). FMRP is an RNA binding protein that associates with translating polyribosomes as part of a large messenger ribonucleoprotein (mRNP) and modulates the translation of its RNA ligands. Pathological studies from the brains of patients and from Fmr1 knockout mice show abnormal dendritic spines implicating FMRP in synapse formation and function. Evidence from both in vitro and in vivo neuronal studies indicates that FMRP is located at the synapse and the loss of FMRP alters synaptic plasticity. As synaptic plasticity has been implicated in learning and memory, analysis of synapse abnormalities in patients and Fmr1 knockout mice should prove useful in studying the pathogenesis of fragile X syndrome and understanding learning and cognition in general. If an appreciable portion of the total variance (in IQ) is due to sex linked genes, it is of more importance that a boy should have a clever mother than a clever father. Hogben 1932 (quoted in Lehrke 1974)

BDNF regulates the expression of fragile X mental retardation protein mRNA in the hippocampus

Both fragile X mental retardation protein (FMRP) and brain-derived neurotrophic factor (BDNF) are implicated in the maturation of neurons and in the higher cognitive functions. We have investigated whether FMRP and BDNF are reciprocally regulated in neurons. Exposure of cultured hippocampal neurons to BDNF, but not to NT-3, reduced FMR1 mRNA levels to 84.8% of control at 4 h and the levels were back to baseline by 24 h or 4 days. Furthermore, expression of FMR1 mRNA was reduced (82.4% of control) in vivo in the hippocampus of transgenic mice overexpressing TrkB receptors, and a small but significant (5.1%) decrease was also detected in FMRP protein levels. In contrast, the expression patterns of BDNF and TrkB mRNAs were not altered in FMRP-deficient mice compared to wild-type mice. Our data provide evidence that BDNF via TrkB signaling decreases FMRP expression and suggest a role for FMRP in BDNF-induced synaptic plasticity.

Advances in understanding of fragile X pathogenesis and FMRP function, and in identification of X linked mental retardation genes

The fragile X mental retardation syndrome is caused by large methylated expansions of a CGG repeat in the FMR1 gene that lead to the loss of expression of FMRP, an RNA-binding protein. FMRP is proposed to act as a regulator of mRNA transport or translation that plays a role in synaptic maturation and function. The recent observations of unexpected phenotypes in some carriers of fragile X premutations suggest a pathological role, in these individuals, of an abnormal FMR1 mRNA. FMRP was recently shown to interact preferentially with mRNAs containing a G quartet structure. Mouse and Drosophila models are used to decipher the function of FMRP, which was found to inhibit translation of some mRNA targets, but may be stimulatory in other cases. Proteins interacting with FMRP have been identified, and suggest a link with the Rac1 GTPase pathway that is important in neuronal maturation. Recent advances also include identification of other genes implicated in X-linked mental retardation.

Effects of Fragile X syndrome and an FMR1 knockout mouse model on forebrain neuronal cell biology

The neurological deficits exhibited by patients with Fragile X syndrome (FraX) have been attributed to the absence of the Fragile X Mental Retardation Protein (FMRP), the product of the FMR1 gene, which is nonfunctional in these individuals. While a great deal has been learned about FraX using non-invasive techniques and autopsy tissue from humans, the limited availability of subjects and specimens severely restricts the rate at which such data can be collected and the types of experimental questions posed. In view of these limitations, a transgenic mouse model of FraX has been constructed in which the FMR1 gene is selectively knocked out (KO) [Bakker et al. (1994) Cell 78:23-33]. These mice show molecular, morphological, and behavioral alterations consistent with phenotypes observed in FraX patients, making them good models to study the absence of FMRP expression.

A converging-methods approach to fragile X syndrome

Converging approaches across domains of brain anatomy, cell biology, and behavior indicate that Fragile X syndrome, arising from impaired expression of a single gene and protein, appears to involve an aberration of normal developmental processes. Synapse overproduction and selective elimination, or pruning, characterize normal brain development. In autopsy tissue from Fragile X patients and in a knockout mouse model of the disease, synapse overproduction appears to occur unaccompanied by synapse pruning and maturation, leaving an excess of immature spine synapses in place. The absence of the Fragile X protein seems to impair the synthesis of important proteins at synapses. The developmental outcome in Fragile X is a nervous system that is relatively disorganized, resulting in disrupted perceptual, and cognitive social, behavior.

Abnormal development of dendritic spines in FMR1 knock-out mice

Fragile X syndrome is caused by a mutation in the FMR1 gene leading to absence of the fragile X mental retardation protein (FMRP). Reports that patients and adult FMR1 knock-out mice have abnormally long dendritic spines of increased density suggested that the disorder might involve abnormal spine development. Because spine length, density, and motility change dramatically in the first postnatal weeks, we analyzed these properties in mutant mice and littermate controls at 1, 2, and 4 weeks of age. To label neurons, a viral vector carrying the enhanced green fluorescent protein gene was injected into the barrel cortex. Layer V neurons were imaged on a two-photon laser scanning microscope in fixed tissue sections. Analysis of >16,000 spines showed clear developmental patterns. Between 1 and 4 weeks of age, spine density increased 2.5-fold, and mean spine length decreased by 17% in normal animals. Early during cortical synaptogenesis, pyramidal cells in mutant mice had longer spines than controls. At 1 week, spine length was 28% greater in mutants than in controls. At 2 weeks, this difference was 10%, and at 4 weeks only 3%. Similarly, spine density was 33% greater in mutants than in controls at 1 week of age. At 2 or 4 weeks of age, differences were not detectable. The spine abnormality was not detected in neocortical organotypic cultures. The transient nature of the spine abnormality in the intact animal suggests that FMRP might play a role in the normal process of dendritic spine growth in coordination with the experience-dependent development of cortical circuits.

Synaptic regulation of protein synthesis and the fragile X protein

Protein synthesis occurs in neuronal dendrites, often near synapses. Polyribosomal aggregates often appear in dendritic spines, particularly during development. Polyribosomal aggregates in spines increase during experience-dependent synaptogenesis, e.g., in rats in a complex environment. Some protein synthesis appears to be regulated directly by synaptic activity. We use "synaptoneurosomes," a preparation highly enriched in pinched-off, resealed presynaptic processes attached to resealed postsynaptic processes that retain normal functions of neurotransmitter release, receptor activation, and various postsynaptic responses including signaling pathways and protein synthesis. We have found that, when synaptoneurosomes are stimulated with glutamate or group I metabotropic glutamate receptor agonists such as dihydroxyphenylglycine, mRNA is rapidly taken up into polyribosomal aggregates, and labeled methionine is incorporated into protein. One of the proteins synthesized is FMRP, the protein that is reduced or absent in fragile X mental retardation syndrome. FMRP has three RNA-binding domains and reportedly binds to a significant number of mRNAs. We have found that dihydroxyphenylglycine-activated protein synthesis in synaptoneurosomes is dramatically reduced in a knockout mouse model of fragile X syndrome, which cannot produce full-length FMRP, suggesting that FMRP is involved in or required for this process. Studies of autopsy samples from patients with fragile X syndrome have indicated that dendritic spines may fail to assume a normal mature size and shape and that there are more spines per unit dendrite length in the patient samples. Similar findings on spine size and shape have come from studies of the knockout mouse. Study of the development of the somatosensory cortical region containing the barrel-like cell arrangements that process whisker information suggests that normal dendritic regression is impaired in the knockout mouse. This finding suggests that FMRP may be required for the normal processes of maturation and elimination to occur in cerebral cortical development.

Abnormal dendritic spine characteristics in the temporal and visual cortices of patients with fragile-X syndrome: a quantitative examination

Fragile-X syndrome is a common form of mental retardation resulting from the inability to produce the fragile-X mental retardation protein. Qualitative examination of human brain autopsy material has shown that fragile-X patients exhibit abnormal dendritic spine lengths and shapes on parieto-occipital neocortical pyramidal cells. Similar quantitative results have been obtained in fragile-X knockout mice, that have been engineered to lack the fragile-X mental retardation protein. Dendritic spines on layer V pyramidal cells of human temporal and visual cortices stained using the Golgi-Kopsch method were investigated. Quantitative analysis of dendritic spine length, morphology, and number was carried out on patients with fragile-X syndrome and normal age-matched controls. Fragile-X patients exhibited significantly more long dendritic spines and fewer short dendritic spines than did control subjects in both temporal and visual cortical areas. Similarly, fragile-X patients exhibited significantly more dendritic spines with an immature morphology and fewer with a more mature type morphology in both cortical areas. In addition, fragile-X patients had a higher density of dendritic spines than did controls on distal segments of apical and basilar dendrites in both cortical areas. Long dendritic spines with immature morphologies and elevated spine numbers are characteristic of early development or a lack of sensory experience. The fact that these characteristics are found in fragile-X patients throughout multiple cortical areas may suggest a global failure of normal dendritic spine maturation and or pruning during development that persists throughout adulthood.