A decade of molecular studies of fragile X syndrome

Fragile X mental retardation protein is associated with translating polyribosomes in neuronal cells

Fragile X mental retardation protein (FMRP) is an RNA binding protein encoded by the gene FMR1, whose expression is impaired in patients with fragile X mental retardation. The association of FMRP with polyribosomes in non-neural cell lines has previously suggested that FMRP is involved in translational regulation. However, the relevance of these studies to neuronal function has been questioned by the finding that FMRP in brain is not associated with polyribosomes, but is part of small ribonucleo-protein complexes that do not appear to include ribosomes. Here we optimize methods to analyze brain polyribosomes, allowing us to definitively demonstrate that FMRP forms complexes with cortical brain polyribosomes. Moreover, we demonstrate in neuroblastoma cells that the FMRP-polyribosome complexes are sensitive to puromycin, a drug that targets actively translating ribosomes. These data indicate that FMRP associates with functional polyribosomes in neurons.

FMRP and its target RNAs: fishing for the specificity

Learning and memory difficulties observed in patients with fragile X syndrome, as well as in a mouse model for the syndrome, are partially due to impaired translational regulation of neuronal mRNAs encoding key molecules for the synaptic structure and function. There has been intense interest in characterizing the mRNAs that are regulated by the fragile X mental retardation protein (FMRP) in the neuronal cell. A large number of candidate FMRP-interacting mRNAs has been identified over the last few years and three models have been described so far to explain the specificity of these interactions. Here, we report our vision on how they could work in the same and/or in different pathways and suggest that the three mechanisms may not be mutually exclusive.

The fragile X protein controls microtubule-associated protein 1B translation and microtubule stability in brain neuron development

The fragile X mental retardation protein (FMRP) is a selective RNA-binding protein implicated in regulating translation of its mRNA ligands. The absence of FMRP results in fragile X syndrome, one of the leading causes of inherited mental retardation. Delayed dendritic spine maturation was found in fragile X mental retardation patients as well as in Fmr1 knockout (KO) mice, indicating the functional requirement of FMRP in synaptic development. However, the biochemical link between FMRP deficiency and the neuronal impairment during brain development has not been defined. How FMRP governs normal synapse development in the brain remains elusive. We report here that the developmentally programmed FMRP expression represses the translation of microtubule associated protein 1B (MAP1B) and is required for the accelerated decline of MAP1B during active synaptogenesis in neonatal brain development. The lack of FMRP results in misregulated MAP1B translation and delayed MAP1B decline in the Fmr1 KO brain. Furthermore, the aberrantly elevated MAP1B protein expression leads to abnormally increased microtubule stability in Fmr1 KO neurons. Together, these results indicate that FMRP plays critical roles in controlling cytoskeleton organization during neuronal development, and the abnormal microtubule dynamics is a conceivable underlying factor for the pathogenesis of fragile X mental retardation.

Gene structure and expression of the 5'-(CGG)(n)-3'-binding protein (CGGBP1)

The human nuclear 5'-(CGG)(n)-3'-binding protein (CGGBP1) influences the expression of the fragile X mental retardation (FMR1) gene by specifically interacting with the 5'-(CGG)(n>5)-3' repeat in its 5' untranslated region. Here, we show that CGGBP1 binds to 5'-(CGG)(n)-3' repeats with n>or=5 and to interrupted repeats. The genomic and mRNA organization of the human and murine CGGBP1 genes was studied and the human gene was mapped to chromosome 3p. Due to alternative polyadenylation, mRNAs of 1.2 and 4.5 kb are transcribed at varying ratios in human and murine cells and in embryonic, fetal, and adult tissues. The human and the murine genes, including promoters and large parts of the untranslated regions, are highly conserved. A sequence of 235 nucleotides 5' upstream of CGGBP1 is essential for promoter activity in transfection experiments. Complete in vitro methylation inactivates the promoter, which is unmethylated in human cells as shown by bisulfite genomic sequencing.

The mGluR theory of fragile X mental retardation

Many of the diverse functional consequences of activating group 1 metabotropic glutamate receptors require translation of pre-existing mRNA near synapses. One of these consequences is long-term depression (LTD) of transmission at hippocampal synapses. Loss of fragile X mental retardation protein (FMRP), the defect responsible for fragile X syndrome in humans, increases LTD in mouse hippocampus. This finding is consistent with the growing evidence that FMRP normally functions as a repressor of translation of specific mRNAs. Here we present a theory that can account for diverse neurological and psychiatric aspects of fragile X syndrome, based on the assumption that many of the protein-synthesis-dependent functions of metabotropic receptors are exaggerated in fragile X syndrome. The theory suggests new directions for basic research as well as novel therapeutic approaches for the treatment of humans with fragile X, the most frequent inherited cause of mental retardation and an identified cause of autism.

Annotation: Deconstructing the attention deficit in fragile X syndrome: a developmental neuropsychological approach

BACKGROUND

Fragile X syndrome is one of the world's leading hereditary causes of developmental delay in males. The past decade has witnessed an explosion of research that has begun to unravel the condition at its various levels: from the genetic and brain levels to the cognitive level, and then to the environmental and behavioural levels. Our aim in this review is to attempt to integrate some of the extensive body of knowledge to move the research a step closer to understanding how the dynamics of atypical development can influence the specific cognitive and behavioural end-states frequently observed in children and adolescents with fragile X syndrome.

METHODS

We conducted a review of the current neuropsychological and neuropsychiatric approaches that have attempted to delineate the pattern of 'spared' and 'impaired' functions associated with the phenotype.

RESULTS

The profile of findings suggests that fragile X syndrome should not be viewed merely as a catalogue of spared and impaired cognitive functions or modules. Instead, there appears to be a process of almost gradual modularisation whereby cognitive mechanisms become domain specific as a function of development itself (Karmiloff-Smith, 1992). The results of a decade of intense research point towards an early weakness in one or more components of executive control rather than single, static higher-level deficits (e.g., spatial cognition, speech processing). This weakness affects both the development of more complex functions and current performance.

CONCLUSIONS

The prevailing tendency to interpret developmental disorders in terms of fixed damage to distinct modular functions needs to be reconsidered. We offer this review as an example of an alternative approach, attempting to identify an initial deficit and its consequences for the course of development. Through better definition of the cognitive and behavioural phenotype, in combination with current progress in brain imaging techniques and molecular studies, the next decade should continue to hold exciting promise for fragile X syndrome and other neurodevelopmental disorders.

Epstein-Barr virus transformation of human lymphoblastoid cells from patients with fragile X syndrome induces variable changes on CGG repeats size and promoter methylation

BACKGROUND

Our understanding of fragile X syndrome can be improved by reversing the expression of the silenced fragile X mental retardation 1 (FMR1) gene in immortalized cells from these patients. Epstein-Barr virus (EBV) infection has been extensively used to transform B cells into a permanent lymphoblastoid cell line.

METHODS

We immortalized B lymphocytes from three different fragile X patients and one normal male. We analyzed the CGG triplet repeats and methylation status of the FMR1 and interferon (IFN)-gamma promoter. We also assayed FMR1 mRNA levels by real-time PCR and FMR1 protein (FMRP) by Western blot.

RESULTS

We observed that EBV transformation may induce the instability of CGG repeats and DNA demethylation that can lead to the modification of mRNA expression.

CONCLUSIONS

EBV transformation may induce variable changes in the genome that can lead to the misinterpretations of experimental data obtained from these cells. Thus, periodic testing of DNA from immortalized cells should be routinely undertaken to detect undesired effects.

Biochemical evidence for the association of fragile X mental retardation protein with brain polyribosomal ribonucleoparticles

Fragile X syndrome is caused by the absence of the fragile X mental retardation protein (FMRP). This RNA-binding protein is widely expressed in human and mouse tissues, and it is particularly abundant in the brain because of its high expression in neurons, where it localizes in the cell body and in granules throughout dendrites. Although FMRP is thought to regulate trafficking of repressed mRNA complexes and to influence local protein synthesis in synapses, it is not known whether it has additional functions in the control of translation in the cell body. Here, we have used recently developed approaches to investigate whether FMRP is associated with the translation apparatus. We demonstrate that, in the brain, FMRP is present in actively translating polyribosomes, and we show that this association is acutely sensitive to the type of detergent required to release polyribosomes from membranous structures. In addition, proteomic analyses of purified brain polyribosomes reveal the presence of several RNA-binding proteins that, similarly to FMRP, have been previously localized in neuronal granules. Our findings highlight the complex roles of FMRP both in actively translating polyribosomes and in repressed trafficking ribonucleoparticle granules.

Altered cortical synaptic morphology and impaired memory consolidation in forebrain- specific dominant-negative PAK transgenic mice

Molecular and cellular mechanisms for memory consolidation in the cortex are poorly known. To study the relationships between synaptic structure and function in the cortex and consolidation of long-term memory, we have generated transgenic mice in which catalytic activity of PAK, a critical regulator of actin remodeling, is inhibited in the postnatal forebrain. Cortical neurons in these mice displayed fewer dendritic spines and an increased proportion of larger synapses compared to wild-type controls. These alterations in basal synaptic morphology correlated with enhanced mean synaptic strength and impaired bidirectional synaptic modifiability (enhanced LTP and reduced LTD) in the cortex. By contrast, spine morphology and synaptic plasticity were normal in the hippocampus of these mice. Importantly, these mice exhibited specific deficits in the consolidation phase of hippocampus-dependent memory. Thus, our results provide evidence for critical relationships between synaptic morphology and bidirectional modifiability of synaptic strength in the cortex and consolidation of long-term memory.

Fragile X mental retardation syndrome: from pathogenesis to diagnostic issues

The Fragile X (FRAXA) syndrome is the most common cause of familial (monogenic) mental retardation and is widespread in human populations. This syndrome is characterised by an unusual mode of transmission for an X-linked disease. In affected families, one frequently finds clinically normal transmitting males, whose daughters - also clinically normal - have a high risk of having affected children. The risk of developing the disease (penetrance) thus appears to increase in successive generations of the same family through maternal transmission. As shown by molecular cloning of the fragile X locus, Fragile X mutations are unstable expansions of a CGG trinucleotide repeat, located in the first exon (non-protein-coding) of the FMR1 gene (for Fragile X Mental Retardation). Two main types of mutation are observed in affected families. A full mutation is found in patients with mental retardation and corresponds to large expansions of the repeat. Premutations are moderate expansions and are found in normal transmitting males and in the majority of clinically normal carrier females. About 15% of patients show a mosaic pattern consisting of both full mutations and premutations. Although analysis of the CGG expansion has led to the establishment of reliable tests for diagnosis and genetic counseling of Fragile X syndrome, care must be exercised to use these tools to answer the concerns of the families and avoid doing harm. In our opinion, testing in children should be restricted to those who show a developmental delay, cognitive deficits and/or abnormal behavior evocative of the syndrome. A carrier diagnosis in a girl who is clinically normal should probably only be performed at an age where she can understand the consequences for family planning and the options of prenatal diagnosis. When testing children with borderline cognitive deficits, a positive diagnosis should be used to improve educational strategies for the children - and not to stigmatise them.

Postgenomic global analysis of translational control induced by oncogenic signaling

It is commonly assumed that developmental and oncogenic signaling achieve their phenotypic effects primarily by directly regulating the transcriptional profile of cells. However, there is growing evidence that the direct effect on transcription may be overshadowed by differential effects on the translational efficiency of specific existing mRNA species. Global analysis of this effect using microarrays indicates that this mechanism of controlling protein production provides a highly specific, robust, and rapid response to oncogenic and developmental stimuli. The mRNAs so affected encode proteins involved in cell-cell interaction, signal transduction, and growth control. Furthermore, a large number of transcription factors capable of secondarily rearranging the transcriptional profile of the cell are controlled at this level as well. To what degree this translational control is either necessary or sufficient for tumor formation or maintenance remains to be determined.

The fragile X mental retardation protein has nucleic acid chaperone properties

The fragile X syndrome is the most common cause of inherited mental retardation resulting from the absence of the fragile X mental retardation protein (FMRP). FMRP contains two K-homology (KH) domains and one RGG box that are landmarks characteristic of RNA-binding proteins. In agreement with this, FMRP associates with messenger ribonucleoparticles (mRNPs) within actively translating ribosomes, and is thought to regulate translation of target mRNAs, including its own transcript. To investigate whether FMRP might chaperone nucleic acid folding and hybridization, we analysed the annealing and strand exchange activities of DNA oligonucleotides and the enhancement of ribozyme-directed RNA substrate cleavage by FMRP and deleted variants relative to canonical nucleic acid chaperones, such as the cellular YB-1/p50 protein and the retroviral nucleocapsid protein HIV-1 NCp7. FMRP was found to possess all the properties of a potent nucleic acid chaperone, requiring the KH motifs and RGG box for optimal activity. These findings suggest that FMRP may regulate translation by acting on RNA-RNA interactions and thus on the structural status of mRNAs.

Sensorimotor gating abnormalities in young males with fragile X syndrome and Fmr1-knockout mice

Fragile X syndrome (FXS) is the most common single gene (FMR1) disorder affecting cognitive and behavioral function in humans. This syndrome is characterized by a cluster of abnormalities including lower IQ, attention deficits, impairments in adaptive behavior and increased incidence of autism. Here, we show that young males with FXS have profound deficits in prepulse inhibition (PPI), a basic marker of sensorimotor gating that has been extensively studied in rodents. Importantly, the magnitude of the PPI impairments in the fragile X children predicted the severity of their IQ, attention, adaptive behavior and autistic phenotypes. Additionally, these measures were highly correlated with each other, suggesting that a shared mechanism underlies this complex phenotypic cluster. Studies in Fmr1-knockout mice also revealed sensorimotor gating and learning abnormalities. However, PPI and learning were enhanced rather than reduced in the mutants. Therefore, these data show that mutations of the FMR1 gene impact equivalent processes in both humans and mice. However, since these phenotypic changes are opposite in direction, they also suggest that murine compensatory mechanisms following loss of FMR1 function differ from those in humans.

Short tandem repeats are associated with diverse mRNAs encoding membrane-targeted proteins

Within the genomes of multicellular organisms, short tandem repeating sequences (STRs) are ubiquitous, yet usage patterns remain obscure. The repeats (AC)n and (GU)n appear frequently in the untranslated regions (UTRs) of messenger RNAs (mRNAs). To investigate STR usage patterns, we used three approaches: (1) comparisons of individual mRNA database sequences including annotations and linked references, (2) statistical analysis of complete, UTR databases and (3) study of a large gene family, the aquaporins. Among 500 (AC)n- or (GU)n-containing mRNAs, 58 (12%) had known functions. Of these, 50 (86%) encoded proteins whose activities involved membranes or lipids, including integral membrane proteins, peripheral membrane proteins, ion channels, lipid enzymes, receptors and secreted proteins. A control sequence (AU)n also occurred in mRNAs, but only 5% encoded membrane-related functions. Investigation of all reported 3' UTR sequences, demonstrated that the STR (AC)n was 9 times more common in mRNAs encoding membrane functions than in the total UTR database (P < 0.001). Similarly, (GU)n was 8 times more common in membrane-function mRNAs than in the total database (P < 0.001). These observations suggest that (AC)n and (GU)n may be UTR signals for some mRNAs encoding membrane-targeted proteins.

Effects of ethanol on cultured embryonic neurons from the cerebral cortex of the rat

Fetal alcohol syndrome is a serious disorder that causes lifelong learning, memory, and behavioral problems. In the current study, we determined the ethanol concentrations that produced detrimental effects on the development of embryonic cortical neurons because mental capacity seems to be proportional to the level of dendritic arborization. Neurons from fetal rat cortices were grown in culture in close proximity to a glial plane. The cells were treated with concentrations of ethanol ranging from 450 nM to 45 mM, and neurite outgrowth was subsequently quantified. A significant decrease in dendritic branching was observed at ethanol concentrations as low as 45 microM after 6 days of ethanol exposure in vitro, whereas changes in primary neurite outgrowth were observed at an ethanol concentration of 4.5 microM. This finding is of particular interest as it seems to indicate that occasional ethanol exposure is detrimental to cortical development at very low concentrations of ethanol.

Genetics of human prefrontal function

Evolution of the prefrontal cortex was an essential precursor to civilization. During the past decade, it became increasingly obvious that human prefrontal function is under substantial genetic control. In particular, heritability studies of frontal lobe-related neuropsychological function, electrophysiology and neuroimaging have greatly improved our insight. Moreover, the first genes that are relevant for prefrontal function such as catechol-O-methyltransferase (COMT) are currently discovered. In this review, we summarize the present knowledge on the genetics of human prefrontal function. For historical reasons, we discuss the genetics of prefrontal function within the broader concept of general cognitive ability (intelligence). Special emphasis is also given to methodological concerns that need to be addressed when conducting research on the genetics of prefrontal function in humans.

Chipping away at brain function: mining for insights with microarrays

The impact of microarray studies on neurobiology has been limited because, with the exception of a few outstanding papers, most reports provide little more than lists of genes, often leaving the reader at a loss to understand which and how many of the identified transcripts will be true positives with significant biological impact. However, some recent papers have offered considerable biological insight by providing independent in vivo confirmation of the roles of candidate genes, offering a glimpse of the potential power of microarrays in neurobiological research.

Biochemical and genetic interaction between the fragile X mental retardation protein and the microRNA pathway

Fragile X syndrome is caused by a loss of expression of the fragile X mental retardation protein (FMRP). FMRP is a selective RNA-binding protein which forms a messenger ribonucleoprotein (mRNP) complex that associates with polyribosomes. Recently, mRNA ligands associated with FMRP have been identified. However, the mechanism by which FMRP regulates the translation of its mRNA ligands remains unclear. MicroRNAs are small noncoding RNAs involved in translational control. Here we show that in vivo mammalian FMRP interacts with microRNAs and the components of the microRNA pathways including Dicer and the mammalian ortholog of Argonaute 1 (AGO1). Using two different Drosophila melanogaster models, we show that AGO1 is critical for FMRP function in neural development and synaptogenesis. Our results suggest that FMRP may regulate neuronal translation via microRNAs and links microRNAs with human disease.

RNA interference: A new mechanism by which FMRP acts in the normal brain? What can Drosophila teach us?

Fragile X syndrome is the most common heritable form of mental retardation caused by loss-of-function mutations in the FMR1 gene. The FMR1 gene encodes an RNA-binding protein that associates with translating ribosomes and acts as a negative translational regulator. Recent work in Drosophila melanogaster has shown that the fly homolog of FMR1 (dFMR1) plays an important role in regulating neuronal morphology, which may underlie the observed deficits in behaviors of dFMR1 mutant flies. Biochemical analysis has revealed that dFMR1 forms a complex that includes ribosomal proteins and, surprisingly, Argonaute2 (AGO2), an essential component of the RNA-induced silencing complex (RISC) that mediates RNA interference (RNAi) in Drosophila. dFMR1 also associates with Dicer, another essential processing enzyme of the RNAi pathway. Moreover, both a micro-RNA (miRNA) and short interfering RNAs (siRNAs) can coimmunoprecipitate with dFMR1. Together these findings suggest that dFMR1 functions in an RNAi-related apparatus to regulate the expression of its target genes at the level of translation. These findings raise the possibility that Fragile X syndrome may be the result of a protein synthesis abnormality caused by a defect in an RNAi-related apparatus. Because the core mechanisms of complex behaviors such as learning and memory and circadian rhythms appear to be conserved, studies of Fragile X syndrome using Drosophila as a model provide an economy-of-scale for identifying biological processes that likely underlie the abnormal morphology of dendritic spines and behavioral disturbances observed in Fragile X patients.

The fragile X mental retardation protein, FMRP, recognizes G-quartets

Fragile X mental retardation is a disease caused by the loss of function of a single RNA-binding protein, FMRP. Identifying the RNA targets recognized by FMRP is likely to reveal much about its functions in controlling some aspects of memory and behavior. Recent evidence suggests that one of the predominant RNA motifs recognized by the FMRP protein is an intramolecular G-quartet and that the RGG box of FMRP mediates this interaction. Searching databases of mRNA sequence information, as well as compiled sequences of predicted FMRP targets based on biochemical identification, has revealed that many of these predicted FMRP targets contain intramolecular G-quartets. Interestingly, many of the G-quartet containing RNA targets encode proteins involved in neuronal development and synaptic function. Defects in the metabolism of this set of RNAs, presumably in the translation of their protein products, is likely to underlie the behavioral and cognitive changes seen in the disease.

A muscleblind knockout model for myotonic dystrophy

The neuromuscular disease myotonic dystrophy (DM) is caused by microsatellite repeat expansions at two different genomic loci. Mutant DM transcripts are retained in the nucleus together with the muscleblind (Mbnl) proteins, and these abnormal RNAs somehow interfere with pre-mRNA splicing regulation. Here, we show that disruption of the mouse Mbnl1 gene leads to muscle, eye, and RNA splicing abnormalities that are characteristic of DM disease. Our results support the hypothesis that manifestations of DM can result from sequestration of specific RNA binding proteins by a repetitive element expansion in a mutant RNA.

Control of dendritic development by the Drosophila fragile X-related gene involves the small GTPase Rac1

Fragile X syndrome is caused by loss-of-function mutations in the fragile X mental retardation 1 gene. How these mutations affect neuronal development and function remains largely elusive. We generated specific point mutations or small deletions in the Drosophila fragile X-related (Fmr1) gene and examined the roles of Fmr1 in dendritic development of dendritic arborization (DA) neurons in Drosophila larvae. We found that Fmr1 could be detected in the cell bodies and proximal dendrites of DA neurons and that Fmr1loss-of-function mutations increased the number of higher-order dendritic branches. Conversely, overexpression of Fmr1 in DA neurons dramatically decreased dendritic branching. In dissecting the mechanisms underlying Fmr1 function in dendrite development, we found that the mRNA encoding small GTPase Rac1 was present in the Fmr1-messenger ribonucleoprotein complexes in vivo. Mosaic analysis with a repressor cell marker (MARCM) and overexpression studies revealed that Rac1 has a cell-autonomous function in promoting dendritic branching of DA neurons. Furthermore, Fmr1 and Rac1 genetically interact with each other in controlling the formation of fine dendritic branches. These findings demonstrate that Fmr1 affects dendritic development and that Rac1 is partially responsible for mediating this effect.

Phosphorylation influences the translation state of FMRP-associated polyribosomes

Fragile X mental retardation protein, FMRP, is absent in patients with fragile X syndrome, a common form of mental retardation. FMRP is a nucleocytoplasmic RNA binding protein that is primarily associated with polyribosomes. FMRP is believed to be a translational repressor and may regulate the translation of certain mRNAs at the base of dendritic spines in neurons. However, little is known about the regulation of FMRP. Using mass spectrometry and site-directed mutagenesis, we show that FMRP is phosphorylated between residues 483 and 521, N-terminal to the RGG box, both in murine brain and in cultured cells. Primary phosphorylation occurs on the highly conserved serine 499, which triggers hierarchical phosphorylation of nearby serines. FMRP is phosphorylated within 2-4 h of synthesis, however, phosphorylation has no effect on the half-life of the protein. In contrast to the Drosophila ortholog dFxr, the phosphorylation status of mammalian FMRP does not influence its association with specific mRNAs in vivo. However, we find unphosphorylated FMRP associated with actively translating polyribosomes while a fraction of phosphorylated FMRP is associated with apparently stalled polyribosomes. Our data suggest that the phosphorylation may regulate FMRP and that the release of FMRP-induced translational suppression may involve a dephosphorylation signal.

Distribution of CGG repeat sizes within the fragile X mental retardation 1 (FMR1) homologue in a non-human primate population

Fragile X syndrome, the most common inherited form of mental retardation, arises in individuals with more than 200 CGG repeats in the 5' untranslated region of the fragile X mental retardation 1 ( FMR1) gene. Although CGG repeat numbers comparable to those found in the normal human population are found in various non-human primates, neither the within-species size variation nor the propensity for expansion of the CGG repeat has been described for any non-human primate species. The allele distribution has now been determined for FMR1 (homologue) CGG repeats of 265 unrelated founder females of Macaca mulatta monkeys. Among 530 X chromosomes, at least 26 distinct repeat lengths were identified, ranging from 16 to 54 CGG repeats. Of these alleles 79% have between 25 and 33 CGG repeats. Detailed examination of the CGG region revealed a conserved G (CGG)(2 )G interruption, although in no case was an AGG trinucleotide detected. Two animals carried borderline premutation alleles with 54 CGG repeats, within the region of marginal instability for humans. Thus, M. mulatta may be useful as an animal model for the study of fragile X syndrome.

Model of autism: increased ratio of excitation/inhibition in key neural systems

Autism is a severe neurobehavioral syndrome, arising largely as an inherited disorder, which can arise from several diseases. Despite recent advances in identifying some genes that can cause autism, its underlying neurological mechanisms are uncertain. Autism is best conceptualized by considering the neural systems that may be defective in autistic individuals. Recent advances in understanding neural systems that process sensory information, various types of memories and social and emotional behaviors are reviewed and compared with known abnormalities in autism. Then, specific genetic abnormalities that are linked with autism are examined. Synthesis of this information leads to a model that postulates that some forms of autism are caused by an increased ratio of excitation/inhibition in sensory, mnemonic, social and emotional systems. The model further postulates that the increased ratio of excitation/inhibition can be caused by combinatorial effects of genetic and environmental variables that impinge upon a given neural system. Furthermore, the model suggests potential therapeutic interventions.

A microRNA array reveals extensive regulation of microRNAs during brain development

Several hundred microRNAs (miRNAs) have recently been cloned from a wide range of organisms across phylogeny. Despite the high degree of conservation of miRNAs, their functions in general, and in mammals particularly, are just beginning to be defined. Here we show that an oligonucleotide DNA array can be successfully used for the simultaneous analysis of miRNA expression profiles from tissues or cells. From a subset of miRNAs expressed in the brain we designed an oligonucleotide array spotted with probes specific for 44 mature miRNAs. These arrays demonstrated precise regulation of miRNA expression at mammalian brain developmental epochs. About 20% of the probed miRNAs changed significantly in their expression during normal brain development, and two of them, miR-9 and miR-131, were dysregulated in presenilin-1 null mice exhibiting severe brain developmental defects. Transcripts with regulated expression patterns on the arrays were validated by Northern blots. Additionally, a bioinformatic analysis of developmentally regulated miRNAs suggested potential mRNA targets. The arrays also revealed miRNAs distributed to translating polyribosomes in primary neurons where they are likely to modulate translation. Therefore, oligonucleotide arrays provide a new tool for studying miRNA expression in a variety of biological and pathobiological settings. Creating clusters of coexpressed miRNAs will contribute to understanding their regulation, functions, and discovery of mRNA targets.

The N-terminus of the fragile X mental retardation protein contains a novel domain involved in dimerization and RNA binding

Fragile X syndrome, the most common cause of inherited mental retardation, is caused by the absence of the fragile X mental retardation protein (FMRP). The emerging picture is that FMRP is involved in repression of translation through a complex network of protein-protein and protein-RNA interactions. Very little structural information is, however, available for FMRP that could help to understand its function. In particular, no structural studies are available about the N-terminus of the protein, a highly conserved region which is involved in several molecular interactions. Here, we explore systematically the ability of the FMRP N-terminus to form independently folded units (domains). We produced deletion mutants and tested their fold and functional properties by mutually complementary biophysical and biochemical techniques. On the basis of our data, we conclude that the N-terminus contains a domain, that we named NDF, comprising the first 134 amino acids. Most interestingly, NDF comprises two copies of a newly identified Agenet motif. NDF is thermally stable and has a high content of beta structure. In addition to being able to bind to RNA and to recognize some of the FMRP interacting proteins, NDF forms stable dimers and is able to interact, although weakly, with the full-length protein. Our data provide conclusive evidence that NDF is a novel motif for protein-protein and protein-RNA interactions and contains a previously unidentified dimerization site.

RNA-mediated neurodegeneration caused by the fragile X premutation rCGG repeats in Drosophila

Fragile X syndrome carriers have FMR1 alleles, called premutations, with an intermediate number of 5' untranslated CGG repeats between patients (>200 repeats) and normal individuals (<60 repeats). A novel neurodegenerative disease has recently been appreciated in some premutation carriers. As no neurodegeneration is seen in fragile X patients, who do not express FMR1, we hypothesize that lengthened rCGG repeats of the premutation transcript may lead to neurodegeneration. Here, using Drosophila melanogaster, we show that 90 rCGG repeats alone are sufficient to cause neurodegeneration. This phenotype is neuron specific and rCGG repeat dosage sensitive. Although devoid of mutant protein, this neurodegeneration exhibits neuronal inclusion bodies that are Hsp70 and ubiquitin positive. Overexpression of Hsp70 could suppress the neurodegeneration. These results demonstrate that neurodegenerative phenotype associated with fragile X premutation is indeed caused by the lengthened rCGG repeats and provide the first in vivo experimental demonstration of RNA-mediated neurodegeneration.

The cationic porphyrin TMPyP4 destabilizes the tetraplex form of the fragile X syndrome expanded sequence d(CGG)n

Fragile X syndrome, the most common cause of inherited mental retardation, is instigated by dynamic expansion of a d(CGG) trinucleotide repeat in the 5'-untranslated region of the first exon of the FMR1 gene, resulting in its silencing. The expanded d(CGG)(n) tract readily folds into hairpin and tetraplex structures which may contribute to the blocking of FMR1 transcription. In this work, we report that the cationic porphyrin 5,10,15,20-tetra(N-methyl-4-pyridyl)porphin (TMPyP4) effectively destabilizes in vitro the G'2 bimolecular tetraplex structure of d(CGG)(n) while it stabilizes the G'2 tetraplex form of the telomeric sequence d(TTAGGG)(2). Similarly to TMPyP4, the hnRNP-related protein CBF-A also destabilizes G'2 tetrahelical d(CGG)(n) while binding and stabilizing tetraplex telomeric DNA. We report that relative to each agent individually, successive incubation of G'2 d(CGG)(n) with TMPyP4 followed by exposure to CBF-A results in a nearly additive extent of disruption of this tetraplex form of the repeat sequence. Our observations open up the prospect of unfolding secondary structures of the expanded FMR1 d(CGG)(n) tract of fragile X cells by their exposure to low molecular size drugs or to proteins such as TMPyP4 or CBF-A.

Methylation and acetylation in nervous system development and neurodegenerative disorders

The cytoarchitecture and cellular signaling mechanisms of the nervous system are complex, and this complexity is reflected at the molecular level with more genes being expressed in the nervous system than in any other tissue. Gene expression and protein function in neural cells can be regulated by methylation and acetylation. Studies of mice deficient in enzymes that control DNA methylation and of animals with a dietary deficiency of folate have established critical roles for methylation in development of the nervous system. Various neuronal proteins including histones and tubulin are regulated by acetylation which appears to serve important functions in the development, stability and plasticity of neuronal networks. Some inherited neurological disorders have recently been linked to mutations in genes that regulate DNA methylation, and alterations in DNA and protein methylation and/or acetylation have been documented in studies of age-related neurodegenerative disorders including Alzheimer's disease (AD), Parkinson's disease (PD) and Huntington's disease (HD). Manipulations of methylation and acetylation can affect the vulnerability of neurons to degeneration and apoptosis in experimental models of neurodegenerative disorders, suggesting a contribution to altered methylation and acetylation to the disease processes. Interestingly, dietary factors that influence DNA methylation may affect the risk of neurodegenerative disorders, for example, individuals with low dietary folate intake are at increased risk of Alzheimer's and Parkinson's diseases.

Transcribed short tandem repeats occur in couples with strongly preferred registers

Short tandem repeats (STRs) have been widely observed, but most STRs have no recognized organization or function. Here we show that for diverse mRNAs, 84% of (GC)(n) repeats were found unexpectedly coupled with another STR, (GU)(n). These STR couples exhibited preferred polarity and register. In 3(') untranslated mRNA sequences (UTRs) 100% of (GC)(n>6) repeats were tightly coupled with (GU)(n). For (GC)(n), stem folding energy correlated with the length and number of neighboring, non-folding (GU)(n) partners (p=0.014). Approximately 20% of (AU)(n>/=14) repeats were coupled with (GU)(n). The STR couple (AC)(n)(AG)(n) also exhibited polarity and register preferences. The sequence arrangement at STR-couple joints was conserved rigorously, suggesting that these sequences were under selection pressure. Some STR couples may function as mRNA processing landmarks, based on alternative transcript comparisons. These observations suggest that some transcribed STRs may be functional UTR signals with predictable organization and usage patterns.

The fragile X mental retardation protein FMRP binds elongation factor 1A mRNA and negatively regulates its translation in vivo

Loss of the RNA-binding protein FMRP (fragile X mental retardation protein) leads to fragile X syndrome, the most common form of inherited mental retardation. Although some of the messenger RNA targets of this protein, including FMR1, have been ascertained, many have yet to be identified. We have found that Xenopus elongation factor 1A (EF-1A) mRNA binds tightly to recombinant human FMRP in vitro. Binding depended on protein determinants located primarily in the C-terminal end of hFMRP, but the hnRNP K homology domain influenced binding as well. When hFMRP was expressed in cultured cells, it dramatically reduced endogenous EF-1A protein expression but had no effect on EF-1A mRNA levels. In contrast, the translation of several other mRNAs, including those coding for dynamin and constitutive heat shock 70 protein, was not affected by the hFMRP expression. Most importantly, EF-1A mRNA and hFMR1 mRNA were coimmunoprecipitated with hFMRP. Finally, in fragile X lymphoblastoid cells in which hFMRP is absent, human EF-1A protein but not its corresponding mRNA is elevated compared with normal lymphoblastoid cells. These data suggest that hFMRP binds to EF-1A mRNA and also strongly argue that FMRP negatively regulates EF-1A expression in vivo.

Modelling brain diseases in mice: the challenges of design and analysis

Genetically engineered mice have been generated to model a variety of neurological disorders. Several of these models have provided valuable insights into the pathogenesis of the relevant diseases; however, they have rarely reproduced all, or even most, of the features observed in the corresponding human conditions. Here, we review the challenges that must be faced when attempting to accurately reproduce human brain disorders in mice, and discuss some of the ways to overcome them. Building on the knowledge gathered from the study of existing mutants, and on recent progress in phenotyping mutant mice, we anticipate better methods for preclinical interventional trials and significant advances in the understanding and treatment of neurological diseases.

Redistribution of syntaxin mRNA in neuronal cell bodies regulates protein expression and transport during synapse formation and long-term synaptic plasticity

Syntaxin has an important role in regulating vesicle docking and fusion essential for neurotransmitter release. Here, we demonstrate that the distribution of syntaxin mRNA in cell bodies of sensory neurons (SNs) of Aplysia maintained in cell culture is affected by synapse formation, synapse stabilization, and long-term facilitation (LTF) produced by 5-HT. The distribution of the mRNA in turn regulates expression and axonal transport of the protein. Syntaxin mRNA and protein accumulated at the axon hillock of SNs during the initial phase of synapse formation. Significant numbers of granules containing syntaxin were detected in the SN axon. When synaptic strength was stable, both mRNA and protein were targeted away from the axon hillock, and the number of syntaxin granules in the SN axon was reduced. Dramatic increases in mRNA and protein accumulation at the axon hillock and number of syntaxin granules in the SN axon were produced when cultures with stable connections were treated with 5-HT that evoked LTF. Anisomycin (protein synthesis inhibitor) or KT5720 (protein kinase A inhibitor) blocked LTF, accumulation of syntaxin mRNA and protein at the axon hillock, and the increase in syntaxin granules in SN axons. The results indicate that without significant effects on overall mRNA expression, both target interaction and 5-HT via activation of protein kinase A pathway regulate expression of syntaxin and its packaging for transport into axons by influencing the distribution of its mRNA in the SN cell body.

A role for myotonic dystrophy protein kinase in synaptic plasticity

Myotonic dystrophy (DM) is associated with an expanded triplet repeat in the 3'-untranslated region of the gene for myotonic dystrophy protein kinase (DMPK), which may reduce DMPK expression. It is unclear how reduced DMPK expression might contribute to the symptoms of DM because the normal function of DMPK is not yet understood. Thus we investigated the function of DMPK to gain insight into how reduced DMPK expression might lead to cognitive dysfunction in DM. We recently demonstrated a role for DMPK in modifying the cytoskeleton, and remodeling of the cytoskeleton is thought to be important for cognitive function. Therefore we hypothesized that DMPK might normally contribute to synaptic plasticity and cognitive function via an effect on actin cytoskeletal rearrangements. To test for involvement of DMPK in synaptic plasticity, we utilized the DMPK null mouse. This mouse showed no changes in baseline synaptic transmission in hippocampal area CA1, nor any changes in long-term synaptic potentiation (LTP) measured 3 h after induction. There was a significant decrease, however, in the decremental potentiation with a duration of 30-180 min that accompanies LTP. These results suggest a role for DMPK in synaptic plasticity that could be relevant to the cognitive dysfunction associated with DM.

Of mice and the fragile X syndrome

Fragile X syndrome is the most common cause of inherited mental retardation, and recently a number of mouse models have been generated to study the condition. Knockout of the gene associated with fragile X, Fmr1, results in mild, but consistent abnormalities, analogous to the clinical and pathological symptoms observed in human patients. Thus, many aspects of the syndrome can now be studied in mice, taking full advantage of the benefits of this model organism, including the short generation time and unlimited supply of tissue. The experimental data suggest that knockout of Fmr1 mildly disturbs a variety of processes in different brain regions.

SCA8 repeat expansion: large CTA/CTG repeat alleles are more common in ataxic patients, including those with SCA6

We analyzed the SCA8 CTA/CTG repeat in a large group of Japanese subjects. The frequency of large alleles (85-399 CTA/CTG repeats) was 1.9% in spinocerebellar ataxia (SCA), 0.4% in Parkinson disease, 0.3% in Alzheimer disease, and 0% in a healthy control group; the frequency was significantly higher in the group with SCA than in the control group. Homozygotes for large alleles were observed only in the group with SCA. In five patients with SCA from two families, a large SCA8 CTA/CTG repeat and a large SCA6 CAG repeat coexisted. Age at onset was correlated with SCA8 repeats rather than SCA6 repeats in these five patients. In one of these families, at least one patient showed only a large SCA8 CTA/CTG repeat allele, with no large SCA6 CAG repeat allele. We speculate that the presence of a large SCA8 CTA/CTG repeat allele influences the function of channels such as alpha(1A)-voltage-dependent calcium channel through changing or aberrant splicing, resulting in the development of cerebellar ataxia, especially in homozygous patients.

Sunrise at the synapse: the FMRP mRNP shaping the synaptic interface

Recent studies provide new insight into the mechanistic function of Fragile X Mental Retardation Protein (FMRP), paving the way to understanding the biological basis of Fragile X Syndrome. While it has been known for several years that there are spine defects associated with the absence of the mRNA binding protein FMRP, it has been unclear how its absence may lead to specific synaptic defects that underlie the learning and cognitive impairments in Fragile X. One hypothesis under study is that FMRP may play a key role in the regulation of dendritically localized mRNAs, at subsynaptic sites where regulation of local protein synthesis may influence synaptic structure and plasticity. This review highlights recent progress to identify the specific mRNA targets of FMRP and assess defects in mRNA regulation that occur in cells lacking FMRP. In addition, exciting new studies on Fmr1 knockout mice and mutant flies have begun to elucidate a key role for FMRP in synaptic growth, structure, and long-term plasticity.

The fragile X syndrome protein FMRP associates with BC1 RNA and regulates the translation of specific mRNAs at synapses

The Fragile X syndrome, which results from the absence of functional FMRP protein, is the most common heritable form of mental retardation. Here, we show that FMRP acts as a translational repressor of specific mRNAs at synapses. Interestingly, FMRP associates not only with these target mRNAs, but also with the dendritic, non-translatable RNA BC1. Blocking of BC1 inhibits the interaction of FMRP with its target mRNAs. Furthermore, BC1 binds directly to FMRP and can also associate, in the absence of any protein, with the mRNAs regulated by FMRP. This suggests a mechanism where BC1 could determine the specificity of FMRP function by linking the regulated mRNAs and FMRP. Thus, when FMRP is not present, loss of translational repression of specific mRNAs at synapses could result in synaptic dysfunction phenotype of Fragile X patients.

Experience effects on brain development: possible contributions to psychopathology

Researchers and clinicians are increasingly recognizing that psychological and psychiatric disorders are often developmentally progressive, and that diagnosis often represents a point along that progression that is defined largely by our abilities to detect symptoms. As a result, strategies that guide our searches for the root causes and etiologies of these disorders are beginning to change. This review describes interactions between genetics and experience that influence the development of psychopathologies. Following a discussion of normal brain development that highlights how specific cellular processes may be targeted by genetic or environmental factors, we focus on four disorders whose origins range from genetic (fragile X syndrome) to environmental (fetal alcohol syndrome) or a mixture of both factors (depression and schizophrenia). C.H. Waddington's canalization model (slightly modified) is used as a tool to conceptualize the interactive influences of genetics and experience in the development of these psychopathologies. Although this model was originally proposed to describe the 'canalizing' role of genetics in promoting normative development, it serves here to help visualize, for example, the effects of adverse (stressful) experience in the kindling model of depression, and the multiple etiologies that may underlie the development of schizophrenia. Waddington's model is also useful in understanding the canalizing influence of experience-based therapeutic approaches, which also likely bring about 'organic' changes in the brain. Finally, in light of increased evidence for the role of experience in the development and treatment of psychopathologies, we suggest that future strategies for identifying the underlying causes of these disorders be based less on the mechanisms of action of effective pharmacological treatments, and more on increased knowledge of the brain's cellular mechanisms of plastic change.

A Drosophila fragile X protein interacts with components of RNAi and ribosomal proteins

Fragile X syndrome is a common form of inherited mental retardation caused by the loss of FMR1 expression. The FMR1 gene encodes an RNA-binding protein that associates with translating ribosomes and acts as a negative translational regulator. In Drosophila, the fly homolog of the FMR1 protein (dFMR1) binds to and represses the translation of an mRNA encoding of the microtuble-associated protein Futsch. We have isolated a dFMR1-associated complex that includes two ribosomal proteins, L5 and L11, along with 5S RNA. The dFMR1 complex also contains Argonaute2 (AGO2) and a Drosophila homolog of p68 RNA helicase (Dmp68). AGO2 is an essential component for the RNA-induced silencing complex (RISC), a sequence-specific nuclease complex that mediates RNA interference (RNAi) in Drosophila. We show that Dmp68 is also required for efficient RNAi. We further show that dFMR1 is associated with Dicer, another essential component of the RNAi pathway, and microRNAs (miRNAs) in vivo, suggesting that dFMR1 is part of the RNAi-related apparatus. Our findings suggest a model in which the RNAi and dFMR1-mediated translational control pathways intersect in Drosophila. Our findings also raise the possibility that defects in an RNAi-related machinery may cause human disease.