The multifaceted role of Fragile X-Related Protein 1 (FXR1) in cellular processes: an updated review on cancer and clinical applications

RNA-binding proteins (RBPs) modulate the expression level of several target RNAs (such as mRNAs) post-transcriptionally through interactions with unique binding sites in the 3'-untranslated region. There is mounting information that suggests RBP dysregulation plays a significant role in carcinogenesis. However, the function of FMR1 autosomal homolog 1(FXR1) in malignancies is just beginning to be unveiled. Due to the diversity of their RNA-binding domains and functional adaptability, FXR1 can regulate diverse transcript processing. Changes in FXR1 interaction with RNA networks have been linked to the emergence of cancer, although the theoretical framework defining these alterations in interaction is insufficient. Alteration in FXR1 expression or localization has been linked to the mRNAs of cancer suppressor genes, cancer-causing genes, and genes involved in genomic expression stability. In particular, FXR1-mediated gene regulation involves in several cellular phenomena related to cancer growth, metastasis, epithelial-mesenchymal transition, senescence, apoptosis, and angiogenesis. FXR1 dysregulation has been implicated in diverse cancer types, suggesting its diagnostic and therapeutic potential. However, the molecular mechanisms and biological effects of FXR1 regulation in cancer have yet to be understood. This review highlights the current knowledge of FXR1 expression and function in various cancer situations, emphasizing its functional variety and complexity. We further address the challenges and opportunities of targeting FXR1 for cancer diagnosis and treatment and propose future directions for FXR1 research in oncology. This work intends to provide an in-depth review of FXR1 as an emerging oncotarget with multiple roles and implications in cancer biology and therapy.

FMRP Long-Range Transport and Degradation Are Mediated by Dynlrb1 in Sensory Neurons

The fragile X messenger ribonucleoprotein 1 (FMRP) is a multifunctional RNA-binding protein implicated in human neurodevelopmental and neurodegenerative disorders. FMRP mediates the localization and activity-dependent translation of its associated mRNAs through the formation of phase-separated condensates that are trafficked by microtubule-based motors in axons. Axonal transport and localized mRNA translation are critical processes for long-term neuronal survival and are closely linked to the pathogenesis of neurological diseases. FMRP dynein-mediated axonal trafficking is still largely unexplored but likely to constitute a key process underlying FMRP spatiotemporal translational regulation. Here, we show that dynein light chain roadblock 1 (Dynlrb1), a subunit of the dynein complex, is a critical regulator of FMRP function. In sensory axons, FMRP associates with endolysosomal organelles, likely through annexin A11, and is retrogradely trafficked by the dynein complex in a Dynlrb1-dependent manner. Moreover, Dynlrb1 silencing induced FMRP granule accumulation and repressed the translation of microtubule-associated protein 1b, one of its primary mRNA targets. Our findings suggest that Dynlrb1 regulates FMRP function through the control of its transport and targeted degradation.

Early Administration of the Phytocannabinoid Cannabidivarin Prevents the Neurobehavioral Abnormalities Associated with the Fmr1-KO Mouse Model of Fragile X Syndrome

Phytocannabinoids, including the non-addictive cannabis component cannabidivarin (CBDV), have been reported to hold therapeutic potential in several neurodevelopmental disorders (NDDs). Nonetheless, the therapeutic value of phytocannabinoids for treating Fragile X syndrome (FXS), a major NDD, remains unexplored. Here, we characterized the neurobehavioral effects of CBDV at doses of 20 or 100 mg/kg in the Fmr1-knockout (Fmr1-KO) mouse model of FXS using two temporally different intraperitoneal regimens: subchronic 10-day delivery during adulthood (Study 1: rescue treatment) or chronic 5-week delivery at adolescence (Study 2: preventive treatment). Behavioral tests assessing FXS-like abnormalities included anxiety, locomotor, cognitive, social and sensory alterations. Expression of inflammatory and plasticity markers was investigated in the hippocampus and prefrontal cortex. When administered during adulthood (Study 1), the effects of CBDV were marginal, rescuing at the lower dose only the acoustic hyper-responsiveness of Fmr1-KO mice and at both doses their altered hippocampal expression of neurotrophins. When administered during adolescence (Study 2), CBDV at both doses prevented the cognitive, social and acoustic alterations of adult Fmr1-KO mice and modified the expression of several inflammatory brain markers in both wild-type littermates and mutants. These findings warrant the therapeutic potential of CBDV for preventing neurobehavioral alterations associated with FXS, highlighting the relevance of its early administration.

Multifaceted functions of RNA-binding protein vigilin in gene silencing, genome stability, and autism-related disorders

RNA-binding proteins (RBPs) are emerging as important players in regulating eukaryotic gene expression and genome stability. Specific RBPs have been shown to mediate various chromatin-associated processes ranging from transcription to gene silencing and DNA repair. One of the prominent classes of RBPs is the KH domain-containing proteins. Vigilin, an evolutionarily conserved KH domain-containing RBP has been shown to be associated with diverse biological processes like RNA transport and metabolism, sterol metabolism, chromosome segregation, and carcinogenesis. We have previously reported that vigilin is essential for heterochromatin-mediated gene silencing in fission yeast. More recently, we have identified that vigilin in humans plays a critical role in efficient repair of DNA double-stranded breaks and functions in homology-directed DNA repair. In this review, we highlight the multifaceted functions of vigilin and discuss the findings in the context of gene expression, genome organization, cancer, and autism-related disorders.

Amyloid Properties of the FXR1 Protein Are Conserved in Evolution of Vertebrates

Functional amyloids are fibrillary proteins with a cross-β structure that play a structural or regulatory role in pro- and eukaryotes. Previously, we have demonstrated that the RNA-binding FXR1 protein functions in an amyloid form in the rat brain. This RNA-binding protein plays an important role in the regulation of long-term memory, emotions, and cancer. Here, we evaluate the amyloid properties of FXR1 in organisms representing various classes of vertebrates. We show the colocalization of FXR1 with amyloid-specific dyes in the neurons of amphibians, reptiles, and birds. Moreover, FXR1, as with other amyloids, forms detergent-resistant insoluble aggregates in all studied animals. The FXR1 protein isolated by immunoprecipitation from the brains of different vertebrate species forms fibrils, which show yellow-green birefringence after staining with Congo red. Our data indicate that in the evolution of vertebrates, FXR1 acquired amyloid properties at least 365 million years ago. Based on the obtained data, we discuss the possible role of FXR1 amyloid fibrils in the regulation of vital processes in the brain of vertebrates.

Reelin central fragment supplementation improves cognitive deficits in a mouse model of Fragile X Syndrome

Fragile X Syndrome (FXS) is the most common form of inherited intellectual disability and is characterized by autistic behaviors, childhood seizures, and deficits in learning and memory. FXS has a loss of function of the FMR1 gene that leads to a lack of Fragile X Mental Retardation Protein (FMRP) expression. FMRP is critical for synaptic plasticity, spatial learning, and memory. Reelin is a large extracellular glycoprotein essential for synaptic plasticity and numerous neurodevelopmental processes. Reduction in Reelin signaling is implicated as a contributing factor in disease etiology in several neurological disorders, including schizophrenia, and autism. However, the role of Reelin in FXS is poorly understood. We demonstrate a reduction in Reelin in Fmr1 knock-out (KO) mice, suggesting that a loss of Reelin activity may contribute to FXS. We demonstrate here that Reelin signaling enhancement via a single intracerebroventricular injection of the Reelin central fragment into Fmr1 KO mice can profoundly rescue cognitive deficits in hidden platform water maze and fear conditioning, as well as hyperactivity during the open field. Improvements in behavior were associated with rescued levels of post synaptic marker in Fmr1 KO mice when compared to controls. These data suggest that increasing Reelin signaling in FXS could offer a novel therapeutic for improving cognition in FXS.

Rg1 Promotes the Proliferation and Adipogenic Differentiation of Human Adipose-Derived Stem Cells via FXR1/Lnc-GAS5-AS1 Pathway

BACKGROUND

Human adipose-derived stem cells (hASCs) play an important role in regenerative medicine.

OBJECTIVE

Exploring the mechanism of Rg1 in the promotion of the proliferation and adipogenic differentiation of hASCs is important in regenerative medicine research.

METHODS

To observe ginsenoside Rg1 in promoting the proliferation and adipogenic differentiation of hASCs, Rg1 medium at different concentrations was established and tested using the cell counting kit-8 (CCK-8) assay, oil red O staining, alizarin red, and alcian blue. Compared to the control, differentially expressed genes (DEGs) were screened via DEG analysis, which was carried out in the Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses. To explore the relationship among mRNA, long non-coding RNA (lncRNA) and microRNA (miRNA), we constructed a competing endogenous RNA (ceRNA) network.

RESULTS

In this study, Rg1 was observed to promote the proliferation and adipogenic differentiation of hASCs. Additionally, enriched BPs and KEGG pathways may be involved in the promotion process, where FXR1 and Lnc-GAS5-AS1 were found to be regulatory factors. The regulatory network suggested that Rg1 could regulate the adipocytokine signaling pathway and IL-17 signaling pathway via FXR1 and Lnc-GAS5-AS1, which served as the mechanism encompassing the promotion of Rg1 on the proliferation and adipogenic differentiation of hASCs.

CONCLUSION

A comprehensive transcriptional regulatory network related to the promotion ability of Rg1 was constructed, revealing mechanisms regarding Rg1's promotion of the proliferation and adipogenic differentiation of hASCs. The present study provides a theoretical basis for optimizing the function of hASCs.

Widespread Alterations in Translation Elongation in the Brain of Juvenile Fmr1 Knockout Mice

FMRP (fragile X mental retardation protein) is a polysome-associated RNA-binding protein encoded by Fmr1 that is lost in fragile X syndrome. Increasing evidence suggests that FMRP regulates both translation initiation and elongation, but the gene specificity of these effects is unclear. To elucidate the impact of Fmr1 loss on translation, we utilize ribosome profiling for genome-wide measurements of ribosomal occupancy and positioning in the cortex of 24-day-old Fmr1 knockout mice. We find a remarkably coherent reduction in ribosome footprint abundance per mRNA for previously identified, high-affinity mRNA binding partners of FMRP and an increase for terminal oligopyrimidine (TOP) motif-containing genes canonically controlled by mammalian target of rapamycin-eIF4E-binding protein-eIF4E binding protein-eukaryotic initiation factor 4E (mTOR-4E-BP-eIF4E) signaling. Amino acid motif- and gene-level analyses both show a widespread reduction of translational pausing in Fmr1 knockout mice. Our findings are consistent with a model of FMRP-mediated regulation of both translation initiation through eIF4E and elongation that is disrupted in fragile X syndrome.

Platelets as a surrogate disease model of neurodevelopmental disorders: Insights from Fragile X Syndrome

Fragile X Syndrome (FXS) is the most common inherited form of intellectual disability and the leading monogenic cause of autism spectrum disorders (ASD). Despite a large number of therapeutics developed in past years, there is currently no targeted treatment approved for FXS. In fact, translation of the positive and very promising preclinical findings from animal models to human subjects has so far fallen short owing in part to the low predictive validity of the Fmr1 ko mouse, an overly simplistic model of the complex human disease. This issue stresses the critical need to identify new surrogate human peripheral cell models of FXS, which may in fact allow for the identification of novel and more efficient therapies. Of all described models, blood platelets appear to be one of the most promising and appropriate disease models of FXS, in part owing to their close biochemical similarities with neurons. Noteworthy, they also recapitulate some of FXS neuron's core molecular dysregulations, such as hyperactivity of the MAPK/ERK and PI3K/Akt/mTOR pathways, elevated enzymatic activity of MMP9 and decreased production of cAMP. Platelets might therefore help furthering our understanding of FXS pathophysiology and might also lead to the identification of disease-specific biomarkers, as was shown in several psychiatric disorders such as schizophrenia and Alzheimer's disease. Moreover, there is additional evidence suggesting that platelet signaling may assist with prediction of cognitive phenotype and could represent a potent readout of drug efficacy in clinical trials. Globally, given the neurobiological overlap between different forms of intellectual disability, platelets may be a valuable window to access the molecular underpinnings of ASD and other neurodevelopmental disorders (NDD) sharing similar synaptic plasticity defects with FXS. Platelets are indeed an attractive model for unraveling pathophysiological mechanisms involved in NDD as well as to search for diagnostic and therapeutic biomarkers.

Dietary supplementation of omega-3 fatty acids rescues fragile X phenotypes in Fmr1-Ko mice

Omega-3 polyunsaturated fatty acids (n-3 PUFAs) are known to critically influence brain development and functions. Dietary supplementation with n-3 PUFAs has been suggested as a non-pharmacological therapy for a number of developmental disorders, e.g., autistic spectrum disorders (ASD), but human studies so far have led to conflicting results. Furthermore, it has been hypothesized that the therapeutic impact of n-3 PUFAs on these disorders might be explained by their anti-inflammatory properties and their promoting effects on synaptic function and plasticity, but no clear evidence has been produced in this direction. We evaluated the impact of n-3 PUFA dietary supplementation in a mouse model of fragile X syndrome (FXS), i.e., a major developmental disease and the most frequent monogenic cause of ASD. Fmr1-KO and wild-type mice were provided with a diet enriched or not with n-3 PUFAs from weaning until adulthood when they were tested for multiple FXS-like behaviors. The brain expression of several cytokines and of brain-derived neurotrophic factor (BDNF) was concomitantly assessed as inflammatory and synaptic markers. n-3 PUFA supplementation rescued most of the behavioral abnormalities displayed by Fmr1-KO mice, including alterations in emotionality, social interaction and non-spatial memory, although not their deficits in social recognition and spatial memory. n-3 PUFAs also rescued most of the neuroinflammatory imbalances of KOs, but had a limited impact on their BDNF deficits. These results demonstrate that n-3 PUFAs dietary supplementation, although not a panacea, has a considerable therapeutic value for FXS and potentially for ASD, suggesting a major mediating role of neuroinflammatory mechanisms.

Deregulation of Fragile X-related protein 1 by the lipodystrophic lamin A p.R482W mutation elicits a myogenic gene expression program in preadipocytes

The nuclear lamina is implicated in the regulation of various nuclear functions. Several laminopathy-causing mutations in the LMNA gene, notably the p.R482W substitution linked to familial partial lipodystrophy type 2 (FPLD2), are clustered in the immunoglobulin fold of lamin A. We report a functional association between lamin A and fragile X-related protein 1 (FXR1P), a protein of the fragile X-related family involved in fragile X syndrome. Searching for proteins differentially interacting with the immunoglobulin fold of wild-type and R482W mutant lamin A, we identify FXR1P as a novel component of the lamin A protein network. The p.R482W mutation abrogates interaction of FXR1P with lamin A. Fibroblasts from FPLD2 patients display elevated levels of FXR1P and delocalized FXR1P. In human adipocyte progenitors, deregulation of lamin A expression leads to FXR1P up-regulation, impairment of adipogenic differentiation and induction of myogenin expression. FXR1P overexpression also stimulates a myogenic gene expression program in these cells. Our results demonstrate a cross-talk between proteins hitherto implicated in two distinct mesodermal pathologies. We propose a model where the FPLD2 lamin A p.R482W mutation elicits, through up-regulation of FXR1P, a remodeling of an adipogenic differentiation program into a myogenic program.

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.

Long-lasting effects of minocycline on behavior in young but not adult Fragile X mice

Fragile X Syndrome (FXS) is the most common single-gene inherited form of intellectual disability with behaviors characteristic of autism. People with FXS display childhood seizures, hyperactivity, anxiety, developmental delay, attention deficits, and visual-spatial memory impairment, as well as a propensity for obsessive-compulsive disorder. Several of these aberrant behaviors and FXS-associated synaptic irregularities also occur in "fragile X mental retardation gene" knock-out (Fmr1 KO) mice. We previously reported that minocycline promotes the maturation of dendritic spines - postsynaptic sites for excitatory synapses - in the developing hippocampus of Fmr1 KO mice, which may underlie the beneficial effects of minocycline on anxiolytic behavior in young Fmr1 KO mice. In this study, we compared the effectiveness of minocycline treatment in young and adult Fmr1 KO mice, and determined the dependence of behavioral improvements on short-term versus long-term minocycline administration. We found that 4- and 8-week-long treatments significantly reduced locomotor activity in both young and adult Fmr1 KO mice. Some behavioral improvements persisted in young mice post-treatment, but in adults the beneficial effects were lost soon after minocycline treatment was stopped. We also show, for the first time, that minocycline treatment partially attenuates the number and severity of audiogenic seizures in Fmr1 KO mice. This report provides further evidence that minocycline treatment has immediate and long-lasting benefits on FXS-associated behaviors in the Fmr1 KO mouse model.

A novel role for the RNA-binding protein FXR1P in myoblasts cell-cycle progression by modulating p21/Cdkn1a/Cip1/Waf1 mRNA stability

The Fragile X-Related 1 gene (FXR1) is a paralog of the Fragile X Mental Retardation 1 gene (FMR1), whose absence causes the Fragile X syndrome, the most common form of inherited intellectual disability. FXR1P plays an important role in normal muscle development, and its absence causes muscular abnormalities in mice, frog, and zebrafish. Seven alternatively spliced FXR1 transcripts have been identified and two of them are skeletal muscle-specific. A reduction of these isoforms is found in myoblasts from Facio-Scapulo Humeral Dystrophy (FSHD) patients. FXR1P is an RNA–binding protein involved in translational control; however, so far, no mRNA target of FXR1P has been linked to the drastic muscular phenotypes caused by its absence. In this study, gene expression profiling of C2C12 myoblasts reveals that transcripts involved in cell cycle and muscular development pathways are modulated by Fxr1-depletion. We observed an increase of p21—a regulator of cell-cycle progression—in Fxr1-knocked-down mouse C2C12 and FSHD human myoblasts. Rescue of this molecular phenotype is possible by re-expressing human FXR1P in Fxr1-depleted C2C12 cells. FXR1P muscle-specific isoforms bind p21 mRNA via direct interaction with a conserved G-quadruplex located in its 3′ untranslated region. The FXR1P/G-quadruplex complex reduces the half-life of p21 mRNA. In the absence of FXR1P, the upregulation of p21 mRNA determines the elevated level of its protein product that affects cell-cycle progression inducing a premature cell-cycle exit and generating a pool of cells blocked at G0. Our study describes a novel role of FXR1P that has crucial implications for the understanding of its role during myogenesis and muscle development, since we show here that in its absence a reduced number of myoblasts will be available for muscle formation/regeneration, shedding new light into the pathophysiology of FSHD.

Muscle development is a complex process controlled by the timely expression of genes encoding crucial regulators of the muscle cell precursors called myoblasts. We know from previous studies that inactivation of the Fragile X related 1 (FXR1) gene in various animal models (mouse, frog, and zebrafish) causes muscular and cardiac abnormalities. Also, FXR1P is reduced in a human myopathy called Fascio-Scapulo Humeral Dystrophy (FSHD), suggesting its critical role in muscle that findings presented in this study contribute to elucidating. Cell-cycle arrest is a prerequisite to differentiation of myoblasts into mature myotubes, which will form the muscle. One key regulator is the p21/Cdkn1a/Cip1/Waf1 protein, which commands myoblasts to stop proliferating, and this action is particularly important during muscle regeneration. In this study, we have identified FXR1P as a novel regulator of p21 expression. We show that FXR1P absence in mouse myoblasts and FSHD-derived myopathic myoblasts increases abnormally the levels of p21, causing a premature cell cycle exit of myoblasts. Our study predicts that FXR1P absence leads to a reduced number of myoblasts available for muscle formation and regeneration. This explains the drastic effects of FXR1 inactivation on muscle and brings a better understanding of the molecular/cellular bases of FSHD.

Characterization of fragile X mental retardation protein recruitment and dynamics in Drosophila stress granules

The RNA-binding protein Fragile X Mental Retardation (FMRP) is an evolutionarily conserved protein that is particularly abundant in the brain due to its high expression in neurons. FMRP deficiency causes fragile X mental retardation syndrome. In neurons, FMRP controls the translation of target mRNAs in part by promoting dynamic transport in and out neuronal RNA granules. We and others have previously shown that upon stress, mammalian FMRP dissociates from translating polysomes to localize into neuronal-like granules termed stress granules (SG). This localization of FMRP in SG is conserved in Drosophila. Whether FMRP plays a key role in SG formation, how FMRP is recruited into SG, and whether its association with SG is dynamic are currently unknown. In contrast with mammalian FMRP, which has two paralog proteins, Drosophila FMR1 (dFMRP) is encoded by a single gene that has no paralog. Using this genetically simple model, we assessed the role of dFMRP in SG formation and defined the determinants required for its recruitment in SG as well as its dynamics in SG. We show that dFMRP is dispensable for SG formation in vitro and ex vivo. FRAP experiments showed that dFMRP shuttles in and out SG. The shuttling activity of dFMRP is mediated by a protein-protein interaction domain located at the N-terminus of the protein. This domain is, however, dispensable for the localization of dFMRP in SG. This localization of dFMRP in SG requires the KH and RGG motifs which are known to mediate RNA binding, as well as the C-terminal glutamine/asparagine rich domain. Our studies thus suggest that the mechanisms controlling the recruitment of FMRP into SG and those that promote its shuttling between granules and the cytosol are uncoupled. To our knowledge, this is the first demonstration of the regulated shuttling activity of a SG component between RNA granules and the cytosol.

Characterization of Fragile X Mental Retardation Protein granules formation and dynamics in Drosophila

FMRP is an evolutionarily conserved protein that is highly expressed in neurons and its deficiency causes fragile X mental retardation syndrome. FMRP controls the translation of target mRNAs in part by promoting their dynamic transport in neuronal RNA granules. We have previously shown that high expression of mammalian FMRP induces formation of granules termed FMRP granules. These RNA granules are reminiscent of neuronal granules, of stress granules, as well as of the recently described in vitro-assembled granules. In contrast with mammalian FMRP, which has two paralog proteins, Drosophila FMRP (dFMRP) is encoded by a single gene that has no paralog. Using this genetically simple organism, we investigated formation and dynamics of FMRP granules. We found that increased expression of dFMRP in Drosophila cells induces the formation of dynamic dFMRP RNA granules. Mutagenesis studies identified the N-terminal protein–protein domain of dFMRP as a key determinant for FMRP granules formation. The RGG RNA binding motif of dFMRP is dispensable for dFMRP granules formation since its deletion does not prevent formation of those granules. Deletion of the RGG motif reduced, however, dFMRP trafficking between FMRP granules and the cytosol. Similarly, deletion of a large part of the KH RNA binding motif of dFMRP had no effect on formation of dFMRP-granules, but diminished the shuttling activity of dFMRP. Our results thus suggest that the mechanisms controlling formation of RNA granules and those promoting their dynamics are uncoupled. This study opens new avenues to further elucidate the molecular mechanisms controlling FMRP trafficking with its associated mRNAs in and out of RNA granules.

Fragile X mental retardation protein interacts with the RNA-binding protein Caprin1 in neuronal RiboNucleoProtein complexes [corrected]

Fragile X syndrome is caused by the absence of the Fragile X Mental Retardation Protein (FMRP), an RNA-binding protein. FMRP is associated with messenger RiboNucleoParticles (mRNPs) present in polyribosomes and its absence in neurons leads to alteration in synaptic plasticity as a result of translation regulation defects. The molecular mechanisms by which FMRP plays a role in translation regulation remain elusive. Using immunoprecipitation approaches with monoclonal Ab7G1-1 and a new generation of chicken antibodies, we identified Caprin1 as a novel FMRP-cellular partner. In vivo and in vitro evidence show that Caprin1 interacts with FMRP at the level of the translation machinery as well as in trafficking neuronal granules. As an RNA-binding protein, Caprin1 has in common with FMRP at least two RNA targets that have been identified as CaMKIIα and Map1b mRNAs. In view of the new concept that FMRP species bind to RNA regardless of known structural motifs, we propose that protein interactors might modulate FMRP functions.

Investigation of amygdala volume in men with the fragile X premutation

Premutation fragile X carriers have a CGG repeat expansion (55 to 200 repeats) in the promoter region of the fragile X mental retardation 1 (FMR1) gene. Amygdala dysfunction has been observed in premutation symptomatology, and recent research has suggested the amygdala as an area susceptible to the molecular effects of the premutation. The current study utilizes structural magnetic resonance imaging (MRI) to examine the relationship between amygdala volume, CGG expansion size, FMR1 mRNA, and psychological symptoms in male premutation carriers without FXTAS compared with age and IQ matched controls. No significant between group differences in amygdala volume were found. However, a significant negative correlation between amygdala volume and CGG was found in the lower range of CGG repeat expansions, but not in the higher range of CGG repeat expansions.

Unusual subcellular confinement of the fragile X mental retardation protein (FMRP) in circulating human platelets: complete polyribosome dissociation

FMRP, a RNA-binding protein, was shown in association with polyribosomes in every cell types studied so far, suggesting a ubiquitous role as a translational regulator. Platelets are known for their limited protein synthesis potential. However, current investigations put forward that RNA metabolism is more developed than previously thought. Unexpectedly, our results provide evidence that FMRP, in platelets, is not constitutively associated with heavy particles, such as polyribosomes, and possesses a sedimentation coefficient of less than 10S contrasting with values of 150 to 500S as reported in other cell types. In summary, this report brings to light platelets as a simple human biological system to delineate novel FMRP functions as well as strengthening our comprehension of the pathophysiology of the fragile X syndrome which results from the absence of FMRP.

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

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

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

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

MicroRNAs as a molecular basis for mental retardation, Alzheimer's and prion diseases

MicroRNAs (miRNAs) are small, approximately 21- to 23-nucleotide (nt) non-coding RNA species that act as key regulators of gene expression along a central and well-defined cellular process known as RNA silencing, and involving the recognition and translational control of specific messenger RNA (mRNAs). Generated through the well-orchestrated and sequential processing of miRNA precursor molecules, mature miRNAs are subsequently incorporated into miRNA-containing ribonucleoprotein effector complexes to regulate mRNA translation through the recognition of specific binding sites of imperfect complementarity located mainly in the 3' untranslated region. Predicted to regulate up to 90% of the genes in humans, miRNAs may thus control cellular processes in all cells and tissues of the human body. Likely to play a central role in health and disease, a dysfunctional miRNA-based regulation of gene expression may represent the main etiologic factor underlying diseases affecting major organs, such as the brain. In this review article, the molecular mechanisms underlying the role and function of miRNAs in the regulation of genes involved in neurological and neurodegenerative diseases will be discussed, with a focus on the fragile X syndrome, Alzheimer's disease (AD) and prion disease.

Protein components of the microRNA pathway and human diseases

MicroRNAs (miRNAs) are key regulators of messenger RNA (mRNA) translation known to be involved in a wide variety of cellular processes. In fact, their individual importance is reflected in the diseases that may arise upon the loss, mutation or dysfunction of specific miRNAs. It has been appreciated only recently that diseases may also develop when the protein components of the miRNA machinery itself are affected. The core enzymes of the major protein complexes involved in miRNA biogenesis and function, such as the ribonucleases III (RNases III) Drosha and Dicer as well as Argonaute 2 (Ago2), appear to be essential. However, the accessory proteins of the miRNA pathway, such as the DiGeorge syndrome critical region gene 8 (DGCR8) protein, Exportin-5 (Exp-5), TAR RNA binding protein (TRBP) and fragile X mental retardation protein (FMRP), are each related, in various ways, to specific genetic diseases.

A mouse model of fragile X syndrome exhibits heightened arousal and/or emotion following errors or reversal of contingencies

This study was designed to further assess cognitive and affective functioning in a mouse model of Fragile X syndrome (FXS), the Fmr1(tm1Cgr) or Fmr1 "knockout" (KO) mouse. Male KO mice and wild-type littermate controls were tested on learning set and reversal learning tasks. The KO mice were not impaired in associative learning, transfer of learning, or reversal learning, based on measures of learning rate. Analyses of videotapes of the reversal learning task revealed that both groups of mice exhibited higher levels of activity and wall-climbing during the initial sessions of the task than during the final sessions, a pattern also seen for trials following an error relative to those following a correct response. Notably, the increase in both behavioral measures seen early in the task was significantly more pronounced for the KO mice than for controls, as was the error-induced increase in activity level. This pattern of effects suggests that the KO mice reacted more strongly than controls to the reversal of contingencies and pronounced drop in reinforcement rate, and to errors in general. This pattern of effects is consistent with the heightened emotional reactivity frequently described for humans with FXS.

Methylation regulates the intracellular protein-protein and protein-RNA interactions of FMRP

FMRP, the fragile X mental retardation protein, is an RNA-binding protein that interacts with approximately 4% of fetal brain mRNA. We have recently shown that a methyltransferase (MT) co-translationally methylates FMRP in vitro and that methylation modulates the ability of FMRP to bind mRNA. Here, we recapitulate these in vitro data in vivo, demonstrating that methylation of FMRP affects its ability to bind to FXR1P and regulate the translation of FMRP target mRNAs. Additionally, using double-label fluorescence confocal microscopy, we identified a subpopulation of FMRP-containing small cytoplasmic granules that are distinguishable from larger stress granules. Using the oxidative-stress induced accumulation of abortive pre-initiation complexes as a measure of the association of FMRP with translational components, we have demonstrated that FMRP associates with ribosomes during initiation and, more importantly, that methylation regulates this process by influencing the ratio of FMRP-homodimer-containing mRNPs to FMRP-FXR1P-heterodimer-containing mRNPs. These data suggest a vital role for methylation in normal FMRP functioning.

Oxidative stress reveals heterogeneity of FMRP granules in PC12 cell neurites

PC12 cells are a well-known model of parasympathetic neurons. They have also been used to study the dynamics of heterologously expressed fragile X mental retardation (FMRP) granule trafficking down neurites. Here, we demonstrate that undifferentiated and differentiated PC12 cells harbor endogenous FMRP-containing granules. These granules are not stress granules because they do not associate with an authentic stress granule marker protein T-cell internal antigen 1 (TIA-1). Treatment with sodium arsenite induces stress granule formation in undifferentiated and differentiated PC12 cells. In NGF-treated cells, FMRP-containing stress granules are observed in the soma, neurites and growth cones by co-immunostaining with anti-TIA-1 antibody. These data demonstrate that all three microdomains respond similarly to oxidative stress. Nevertheless, we find significantly less co-localization of FMRP and TIA-1 and FMRP and its homologs in the neurites of differentiated PC12 cells treated with sodium arsenite than in the soma or growth cones. The heterogeneity of these granules suggests that FMRP has multiple roles in neurites.

The structure of the N-terminal domain of the fragile X mental retardation protein: a platform for protein-protein interaction

FMRP, whose lack of expression causes the X-linked fragile X syndrome, is a modular RNA binding protein thought to be involved in posttranslational regulation. We have solved the structure in solution of the N-terminal domain of FMRP (NDF), a functionally important region involved in multiple interactions. The structure consists of a composite fold comprising two repeats of a Tudor motif followed by a short alpha helix. The interactions between the three structural elements are essential for the stability of the NDF fold. Although structurally similar, the two repeats have different dynamic and functional properties. The second, more flexible repeat is responsible for interacting both with methylated lysine and with 82-FIP, one of the FMRP nuclear partners. NDF contains a 3D nucleolar localization signal, since destabilization of its fold leads to altered nucleolar localization of FMRP. We suggest that the NDF composite fold determines an allosteric mechanism that regulates the FMRP functions.

The nuclear MicroSpherule protein 58 is a novel RNA-binding protein that interacts with fragile X mental retardation protein in polyribosomal mRNPs from neurons

The fragile X syndrome, the leading cause of inherited mental retardation, is due to the inactivation of the fragile mental retardation 1 gene (FMR1) and the subsequent absence of its gene product FMRP. This RNA-binding protein is thought to control mRNA translation and its absence in fragile X cells leads to alteration in protein synthesis. In neurons, FMRP is thought to repress specific mRNAs during their transport as silent ribonucleoparticles (mRNPs) from the cell body to the distant synapses which are the sites of local synthesis of neuro-specific proteins. The mechanism by which FMRP sorts out its different mRNAs targets might be tuned by the intervention of different proteins. Using a yeast two-hybrid system, we identified MicroSpherule Protein 58 (MSP58) as a novel FMRP-cellular partner. In cell cultures, we found that MSP58 is predominantly present in the nucleus where it interacts with the nuclear isoform of FMRP. However, in neurons but not in glial cells, MSP58 is also present in the cytoplasmic compartment, as well as in neurites, where it co-localizes with FMRP. Biochemical evidence is given that MSP58 is associated with polyribosomal poly(A)+ mRNPs. We also show that MSP58, similar to FMRP, is present on polyribosomes prepared from synaptoneurosomes and that it behaves as an RNA-binding protein with a high affinity to the G-quartet structure. We propose that this novel cellular partner for FMRP escorts FMRP-containing mRNP from the nucleus and nucleolus to the somato-dendritic compartment where it might participate in neuronal translation regulation.

Fragile X mental retardation protein (FMRP) binds specifically to the brain cytoplasmic RNAs BC1/BC200 via a novel RNA-binding motif

Fragile X mental retardation protein (FMRP), the protein responsible for the fragile X syndrome, is an RNA-binding protein involved in localization and translation of neuronal mRNAs. One of the RNAs known to interact with FMRP is the dendritic non-translatable brain cytoplasmic RNA 1 BC1 RNA that works as an adaptor molecule linking FMRP and some of its regulated mRNAs. Here, we showed that the N terminus of FMRP binds strongly and specifically to BC1 and to its potential human analog BC200. This region does not contain a motif known to specifically recognize RNA and thus constitutes a new RNA-binding motif. We further demonstrated that FMRP recognition involves the 5' stem loop of BC1 and that this is the region that exhibits complementarity to FMRP target mRNAs, raising the possibility that FMRP plays a direct role in BC1/mRNA annealing.

The RNA-binding protein fragile X-related 1 regulates somite formation in Xenopus laevis

Fragile X-related 1 protein (FXR1P) is a member of a small family of RNA-binding proteins that includes the Fragile X mental retardation 1 protein (FMR1P) and the Fragile X-related 2 protein (FXR2P). These proteins are thought to transport mRNA and to control their translation. While FMR1P is highly expressed in neurons, substantial levels of FXR1P are found in striated muscles and heart, which are devoid of FMRP and FXR2P. However, little is known about the functions of FXR1P. We have isolated cDNAs for Xenopus Fxr1 and found that two specific splice variants are conserved in evolution. Knockdown of xFxr1p in Xenopus had highly muscle-specific effects, normal MyoD expression being disrupted, somitic myotomal cell rotation and segmentation being inhibited, and dermatome formation being abnormal. Consistent with the absence of the long muscle-specific xFxr1p isoform during early somite formation, these effects could be rescued by both the long and short mRNA variants. Microarray analyses showed that xFxr1p depletion affected the expression of 129 known genes of which 50% were implicated in muscle and nervous system formation. These studies shed significant new light on Fxr1p function(s).

Phosphorylation of p68 RNA helicase regulates RNA binding by the C-terminal domain of the protein

We previously reported ATPase, RNA unwinding, and RNA-binding activities of recombinant p68 RNA helicase that was expressed in Escherichia coli. Huang et al. The recombinant protein bound both single-stranded (ss) and double-stranded (ds) RNAs. To further characterize the substrate RNA binding by p68 RNA helicase, we expressed and purified the recombinant N-terminal and C-terminal domains of the protein. RNA-binding property and protein phosphorylation of the recombinant domains of p68 were analyzed. Our data demonstrated that the C-terminal domain of p68 RNA helicase bound ssRNA. More interestingly, the C-terminal domain was a target of protein kinase C (PKC). Phosphorylation of the C-terminal domain of p68 abolished its RNA binding. Based on our observations, we propose that the C-terminal domain is an RNA substrate binding site for p68. The protein phosphorylation by PKC regulates the RNA binding of p68 RNA helicase, which consequently controls the enzymatic activities of the protein.

Inv(X)(p21.1;q22.1) in a man with mental retardation, short stature, general muscle wasting, and facial dysmorphism: clinical study and mutation analysis of the NXF5 gene

We describe a 59-year-old male (patient A059) with moderate to severe mental retardation (MR) and a pericentric inversion of the X-chromosome: inv(X)(p21.1;q22.1). He had short stature, pectus excavatum, general muscle wasting, and facial dysmorphism. Until now, no other patients with similar clinical features have been described in the literature. Molecular analysis of both breakpoints led to the identification of a novel "Nuclear RNA export factor" (NXF) gene cluster on Xq22.1. Within this cluster, the NXF5 gene was interrupted with subsequent loss of gene expression. Hence, mutation analysis of the NXF5 and its neighboring homologue, the NXF2 gene was performed in 45 men with various forms of syndromic X-linked MR (XLMR) and in 70 patients with nonspecific XLMR. In the NXF5 gene four nucleotide changes: one intronic, two silent, and one missense (K23E), were identified. In the NXF2 gene two changes (one intronic and one silent) were found. Although none of these changes were causative mutations, we propose that NXF5 is a good candidate gene for this syndromic form of XLMR, given the suspected role of NXF proteins is within mRNA export/transport in neurons. Therefore, mutation screening of the NXF gene family in phenotypically identical patients is recommended.

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.

Animal models of mental retardation: from gene to cognitive function

About 2-3% of all children are affected by mental retardation, and genetic conditions rank among the leading causes of mental retardation. Alterations in the information encoded by genes that regulate critical steps of brain development can disrupt the normal course of development, and have profound consequences on mental processes. Genetically modified mouse models have helped to elucidate the contribution of specific gene alterations and gene-environment interactions to the phenotype of several forms of mental retardation. Mouse models of several neurodevelopmental pathologies, such as Down and Rett syndromes and X-linked forms of mental retardation, have been developed. Because behavior is the ultimate output of brain, behavioral phenotyping of these models provides functional information that may not be detectable using molecular, cellular or histological evaluations. In particular, the study of ontogeny of behavior is recommended in mouse models of disorders having a developmental onset. Identifying the role of specific genes in neuropathologies provides a framework in which to understand key stages of human brain development, and provides a target for potential therapeutic intervention.

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.

Fragile X syndrome, the Fragile X related proteins, and animal models

The Fragile X syndrome (FraX), which is characterized among other physical and neurologic impairments by mental retardation, is caused by the absence of the product of the FMR1 gene. The Fragile X Mental Retardation Protein (FMRP) is a member of a novel family of RNA-binding proteins. The latter includes two other proteins highly homologous with FMRP: the fragile X related proteins 1 and 2 (FXRP1 and FXRP2). Characterization of FXRPs, including their interaction with FMRP, will provide critical information about the mechanisms of action of FMRP and the role of this group of proteins in FMRP-deficient conditions such as FraX. Genetic manipulations of FMRP and the FXRPs should also provide valuable tools for investigating pathophysiology and gene therapies in FraX. The present review summarizes the strategies used for identifying the FXRPs, their chromosomal localization, molecular structure, and tissue distribution. It also reviews interactions between different members of this family of RNA-binding proteins. Animal models, both knockout and transgenic, of FMRP and the FXRPs are discussed. Phenotypic features of the FMR1 knockout mouse, the FMR1 transgenic rescue mouse, and other novel strategies for manipulating and delivering FMRP and FXRPs to the brain and other tissues are described.

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.

Triplet repeats, RNA secondary structure and toxic gain-of-function models for pathogenesis

Ten years after the discovery of human diseases caused by trinucleotide repeat expansions, searching for mechanistic links between gene mutation and pathological phenotype remains a fundamental and unsolved issue. Evidence accumulated so far indicates that the pathogenesis of repeat disorders is complex and multi-factorial. Diseases caused by CAG expansions coding for polyglutamine tracts have been extensively studied, and in most cases a toxic gain-of-function of the mutant protein was demonstrated. Most recently, tracking the effects of repeats along the pathway of gene expression is providing additional clues to understand how a triplet repeat expansion can cause disease. Expanded repeats form DNA secondary structures that confer genetic instability, and most likely contribute to alter the local chromatin configuration leading to transcriptional silencing. At the level of RNA, the expanded repeat may either interfere with processing of the primary transcript, resulting in deficit of the corresponding protein, or interact with RNA-binding proteins altering their normal activity. The latter mechanism, termed RNA gain-of-function, has no precedents in human genetics. Recent evidence suggests that expanded RNAs and associated RNA-binding proteins are potential contributors to the pathogenesis of several triplet repeat diseases.

NXF5, a novel member of the nuclear RNA export factor family, is lost in a male patient with a syndromic form of mental retardation

BACKGROUND

Although X-linked mental retardation (XLMR) affects 2%-3% of the human population, little is known about the underlying molecular mechanisms. Recent interest in this topic led to the identification of several genes for which mutations result in the disturbance of cognitive development.

RESULTS

We identified a novel gene that is interrupted by an inv(X)(p21.1;q22) in a male patient with a syndromic form of mental retardation. Molecular analysis of both breakpoint regions did not reveal an interrupted gene on Xp, but identified a novel nuclear RNA export factor (NXF) gene cluster, Xcen-NXF5-NXF2-NXF4-NXF3-Xqter, in which NXF5 is split by the breakpoint, leading to its functional nullisomy. The predicted NXF5 protein shows high similarity with the central part of the presumed mRNA nuclear export factor TAP/NXF1. Functional analysis of NXF5 demonstrates binding to RNA as well as to the RNA nuclear export-associated protein p15/NXT. In contrast to TAP/NXF1, overexpression studies localized NXF5 in the form of granules in the cell body and neurites of mature hippocampal neurons, suggesting a role in mRNA transport. The two newly identified mouse nxf homologs, nxf-a and nxf-b, which also map on X, show highest mRNA levels in the brain.

CONCLUSIONS

A novel member of the nuclear RNA export factor family is absent in a male patient with a syndromic form of mental retardation. Although we did not find direct evidence for the involvement of NXF5 in MR, the gene could be involved in development, possibly through a process in mRNA metabolism in neurons.

Annexin-1 is abnormally expressed in fragile X syndrome: two-dimensional electrophoresis study in lymphocytes

The search for targets of FMRP (the product of FMR1, the mutated gene in Fragile X syndrome) has predominantly focused on identifying transcripts that are regulated by this RNA-binding protein. This study introduces the use of two-dimensional gel electrophoresis (2D PAGE) as a novel approach for demonstrating changes in protein synthesis secondary to FMRP deficit. By a standardized 2D PAGE protocol, we studied leukocyte homogenates from 30 males with different patterns of FMR1 mutation and different levels of FMRP. Samples from these subjects were compared to those of 12 normal control males and eight subjects with other mental retardation-associated conditions (i.e., Rett and Down syndromes). We found an abnormal pattern of a major leukocytic protein, identified by 2D PAGE datasets and immunoblotting as annexin-1 (Anx-1). Anx-1 appeared in subjects with Fragile X as multiple rather than 1-2 spots, at approximately 37 kd, in the pI 5-7 range. The presence and intensity of this Anx-1 pattern was relatively independent of Anx-1 levels and inversely related to total and high MW FMRP immunoreactivities. Based on the 2D PAGE pattern, without obvious MW change, and on dephosphorylation assays, we concluded that Anx-1's abnormality represents an aberrant posttranslational modification other than phosphorylation. Comparisons of our data with published cytoskeletal protein 2D profiles suggest that Anx-1 may be abnormally acetylated and, consequently, incapable of establishing appropriate N-terminal protein-protein interactions. In addition to its peripheral anti-inflammatory function, Anx-1 mediates glucocorticoid inhibition of the hypothalamo-pituitary-adrenal axis. As the latter seems to be disrupted in Fragile X syndrome, the reported Anx-1 abnormality could be responsible for some aspects of the Fragile X neurobehavioral phenotype. Our data also emphasize the feasibility of using 2D PAGE for disclosing molecular abnormalities in Fragile X and other genetic disorders.

Alkylating agent and chromatin structure determine sequence context-dependent formation of alkylpurines

We determined the adduct maps of S(N)1 and S(N)2 alkylating agents in cultured human cells (in vivo) and in vitro to probe DNA-protein interactions along sequences of the promoter and exon 1 of the Fragile-X mental retardation 1 (FMR1) gene. Using ligation-mediated polymerase chain reaction (LMPCR), we compared the piperidine-sensitive alkylpurines sites generated by treating cultured cells (in vivo) and naked DNA (in vitro) with S(N)1 (N-methyl-N-nitrosourea, N-nitroso(acetoxymethyl)methylamine and 1-methyl-3-nitro-1-nitrosoguanidine) and S(N)2 alkylating agents (dimethyl sulfate (DMS), methane sulfonic acid methyl ester, iodo methane, diethyl sulfate, methane sulfonic acid ethyl ester and iodo ethane). The FMR1 promoter has four sites where DNA-protein interactions are observed. In these regions, the S(N)1 methylating agent reactions produced only hypo-reactive sites. In contrast, iodoalkane S(N)2 alkylating agents (MeI and EtI) reactions generated only hyper-reactive sites. Although there are hyper-reactive sites for the other S(N)2 reagents, the hyper-reactive site at +14 on the FMR1 map is more pronounced for the sulfate and sulfonate-derived alkylating agents than for the iodoalkanes. However, DMS modification in the presence of methyl sulfone, a compound that does not alkylate DNA, eliminates the hyper-reactive site observed at +14. This suggests that the electron-rich oxygen atoms of the sulfate and sulfonate-derived S(N)2 alkylating agent structure position the alkylating moiety to the neighboring N-7-guanine position to favor alkyl transfer to the guanine. Using KMnO(4) to probe for single-strand DNA, an unpaired cytosine base was detected at the 5'-side of the hyper- reactive guanine base at position +14, consistent with the formation of a local DNA single-strand bulge. In conclusion, we show that the sequence context-dependent formation of alkylpurines is determined by the chemical nature of the alkylating agent, the DNA sequence context, chromatin structure, and the presence of other non-reactive molecules that can inhibit alkylation.

The molecular genetics of autism

Autism is a severe neurodevelopmental disorder characterized by communication and social deficits and by stereotyped, repetitive behaviors. The syndrome of autism is highly heritable, is considered to be etiologically heterogeneous and is thought to be the result of multiple, interacting genes. It is more common than previously thought, and has a complex pattern of genetic transmission. From four recently completed genome-wide linkage screens of autism, distal 7q has emerged as the most prominent chromosomal region of interest. Additional support for 7q comes from autistic individuals with gross 7q cytologic abnormalities, and from linkage and association data in families with language and speech disorders. Chromosome 15q11-13 is also of interest because of numerous reports of macroscopic and molecular abnormalities in the region associated with Prader-Willi and Angelman syndromes. In this review, molecular aspects of these data, as well as future avenues of investigation, are discussed.

The nuclear chronicles: gene transcription and molecular traveling

The transfer and processing of an RNA transcript from its locus of transcription on chromatin through the nuclear membrane to its site of translation on cytoplasmic ribosomes is a long and complex journey involving numerous processes and interactions with various macromolecules. These various steps that regulate gene expression were the subject of the 9th Winternational Symposium of the Canadian Society of Biochemistry and Molecular & Cell Biology held at Manoir du Lac Delage, a small resort centre north of Québec City on February 12-15, 1999.Key words: nuclear pore, RNA transport, chromatin, RNA-binding proteins, nucleoporins.