New insights into fragile X syndrome: from molecules to neurobehaviors

Roles of fragile X mental retardation protein in dopaminergic stimulation-induced synapse-associated protein synthesis and subsequent alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-4-propionate (AMPA) receptor internalization

Fragile X syndrome, the most common form of inherited mental retardation, is caused by the absence of the RNA-binding protein fragile X mental retardation protein (FMRP). FMRP regulates local protein synthesis in dendritic spines. Dopamine (DA) is involved in the modulation of synaptic plasticity. Activation of DA receptors can regulate higher brain functions in a protein synthesis-dependent manner. Our recent study has shown that FMRP acts as a key messenger for DA modulation in forebrain neurons. Here, we demonstrate that FMRP is critical for DA D1 receptor-mediated synthesis of synapse-associated protein 90/PSD-95-associated protein 3 (SAPAP3) in the prefrontal cortex (PFC). DA D1 receptor stimulation induced dynamic changes of FMRP phosphorylation. The changes in FMRP phosphorylation temporally correspond with the expression of SAPAP3 after D1 receptor stimulation. Protein phosphatase 2A, ribosomal protein S6 kinase, and mammalian target of rapamycin are the key signaling molecules for FMRP linking DA D1 receptors to SAPAP3. Knockdown of SAPAP3 did not affect surface expression of alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-4-propionate (AMPA) GluR1 receptors induced by D1 receptor activation but impaired their subsequent internalization in cultured PFC neurons; the subsequent internalization of GluR1 was also impaired in Fmr1 knock-out PFC neurons, suggesting that FMRP may be involved in subsequent internalization of GluR1 through regulating the abundance of SAPAP3 after DA D1 receptor stimulation. Our study thus provides further insights into FMRP involvement in DA modulation and may help to reveal the molecular mechanisms underlying impaired learning and memory in fragile X syndrome.

Dysregulation of mTOR signaling in fragile X syndrome

Fragile X syndrome, the most common form of inherited mental retardation and leading genetic cause of autism, is caused by transcriptional silencing of the Fmr1 gene. The fragile X mental retardation protein (FMRP), the gene product of Fmr1, is an RNA binding protein that negatively regulates translation in neurons. The Fmr1 knock-out mouse, a model of fragile X syndrome, exhibits cognitive deficits and exaggerated metabotropic glutamate receptor (mGluR)-dependent long-term depression at CA1 synapses. However, the molecular mechanisms that link loss of function of FMRP to aberrant synaptic plasticity remain unclear. The mammalian target of rapamycin (mTOR) signaling cascade controls initiation of cap-dependent translation and is under control of mGluRs. Here we show that mTOR phosphorylation and activity are elevated in hippocampus of juvenile Fmr1 knock-out mice by four functional readouts: (1) association of mTOR with regulatory associated protein of mTOR; (2) mTOR kinase activity; (3) phosphorylation of mTOR downstream targets S6 kinase and 4E-binding protein; and (4) formation of eukaryotic initiation factor complex 4F, a critical first step in cap-dependent translation. Consistent with this, mGluR long-term depression at CA1 synapses of FMRP-deficient mice is exaggerated and rapamycin insensitive. We further show that the p110 subunit of the upstream kinase phosphatidylinositol 3-kinase (PI3K) and its upstream activator PI3K enhancer PIKE, predicted targets of FMRP, are upregulated in knock-out mice. Elevated mTOR signaling may provide a functional link between overactivation of group I mGluRs and aberrant synaptic plasticity in the fragile X mouse, mechanisms relevant to impaired cognition in fragile X syndrome.

Metabotropic glutamate receptor-mediated long-term depression: molecular mechanisms

The ability to modify synaptic transmission between neurons is a fundamental process of the nervous system that is involved in development, learning, and disease. Thus, synaptic plasticity is the ability to bidirectionally modify transmission, where long-term potentiation and long-term depression (LTD) represent the best characterized forms of plasticity. In the hippocampus, two main forms of LTD coexist that are mediated by activation of either N-methyl-d-aspartic acid receptors (NMDARs) or metabotropic glutamate receptors (mGluRs). Compared with NMDAR-LTD, mGluR-LTD is less well understood, but recent advances have started to delineate the underlying mechanisms. mGluR-LTD at CA3:CA1 synapses in the hippocampus can be induced either by synaptic stimulation or by bath application of the group I selective agonist (R,S)-3,5-dihydroxyphenylglycine. Multiple signaling mechanisms have been implicated in mGluR-LTD, illustrating the complexity of this form of plasticity. This review provides an overview of recent studies investigating the molecular mechanisms underlying hippocampal mGluR-LTD. It highlights the role of key molecular components and signaling pathways that are involved in the induction and expression of mGluR-LTD and considers how the different signaling pathways may work together to elicit a persistent reduction in synaptic transmission.

Biomarkers in the Study of Families of Individuals with Developmental Disabilities

Research during the past 20 years on families of children with developmental disabilities has yielded a rich body of knowledge about the stress of parenting a child with DD, and the risk and protective factors that result in profiles of family resilience vs. vulnerability at various stages of the family life course. Virtually all of this research has been based on data collected from self-report measures, and has focused on family interactions and relationships, and the psychosocial well-being of individual family members. The present chapter focuses on different sources of data, namely biomarkers, which have the potential to extend our understanding of the biological mechanisms by which the stress of parenting a child with developmental disabilities can take its toll on parents' physical and mental health. We focus on two examples: (1) variations in the FMR1 gene, FMRP, and FMR1 messenger RNA in mothers of children with fragile X syndrome and the association of these measures with maternal depression and anxiety; and (2) profiles of cortisol expression in mothers of children with disabilities and the association of cortisol with daily measures of caregiving stress.

Signals, synapses, and synthesis: how new proteins control plasticity

Localization of mRNAs to dendrites and local protein synthesis afford spatial and temporal regulation of gene expression and endow synapses with the capacity to autonomously alter their structure and function. Emerging evidence indicates that RNA binding proteins, ribosomes, translation factors and mRNAs encoding proteins critical to synaptic structure and function localize to neuronal processes. RNAs are transported into dendrites in a translationally quiescent state where they are activated by synaptic stimuli. Two RNA binding proteins that regulate dendritic RNA delivery and translational repression are cytoplasmic polyadenylation element binding protein and fragile X mental retardation protein (FMRP). The fragile X syndrome (FXS) is the most common known genetic cause of autism and is characterized by the loss of FMRP. Hallmark features of the FXS include dysregulation of spine morphogenesis and exaggerated metabotropic glutamate receptor-dependent long term depression, a cellular substrate of learning and memory. Current research focuses on mechanisms whereby mRNAs are transported in a translationally repressed state from soma to distal process and are activated at synaptic sites in response to synaptic signals.

Potential pharmacological treatment of fragile X syndrome during adulthood

Fragile X syndrome (FXS) is the most common form of inherited mental retardation, characterized by moderate-to-severe mental retardation, attention deficits, and hyperactivity. This disease results from the expansion of a trinucleotide repeat (CGG) within the X-linked fragile X mental retardation 1 (FMR1) gene, which leads to the lack of the product of the FMR1 gene-fragile X mental retardation protein. Many mental disorders such as FXS and Rett syndrome are thought to originate during early developmental period, but recent findings have suggested the involvement of the processes in the adult nervous system. Here we outline our recent studies and initial clinical trials that may provide an approach to treat FXS in the adulthood.

Fragile X Mental Retardation Protein is Involved in Protein Synthesis-Dependent Collapse of Growth Cones Induced by Semaphorin-3A

Fragile X syndrome, the most frequent form of familial mental retardation, is caused by mutation of the Fmr1 gene. Fmr1 encodes the fragile X mental retardation protein (FMRP), an mRNA binding protein regulating local, postsynaptic mRNA translation along dendrites necessary for long-term synaptic plasticity. However, recent studies on FMRP localization in axons and growth cones suggest a possible function in the regulation of local protein synthesis needed for axon guidance. Here, we have demonstrated that FMRP is involved in axonal and growth cone responses induced by the axon guidance factor, Semaphorin-3A (Sema3A). In cultured hippocampal neurons from wild type mice, Sema3A-induced growth cone collapse was protein synthesis-dependent. In contrast, Sema3A-induced growth cone collapse was attenuated in Fmr1 knock-out (KO) neurons and insensitive to protein synthesis inhibitors, suggesting that FMRP is involved in protein synthesis-dependent growth cone collapse. Sema3A increased phosphorylation of eukaryotic initiation factor 4E (eIF4E), an indicator of local translation, in distal axons and growth cones of wild type, but not Fmr1 KO neurons. Furthermore, Sema3A rapidly induced a protein synthesis-dependent increase in levels of microtubule associated protein 1B (MAP1B) in distal axons of wild type neurons, but this response was attenuated in Fmr1 KO neurons. These results suggest a possible role of FMRP to regulate local translation and axonal protein localization in response to Sema3A. This study reveals a new link between FMRP and semaphorin signaling in vitro, and raises the possibility that FMRP may have a critical role in semaphorin signaling in axon guidance during brain development.

Site-selective scission of human genome by artificial restriction DNA cutter

By using an artificial restriction DNA cutter which is composed of Ce(iv)/EDTA and two pseudo-complementary peptide nucleic acid strands (pcPNAs), only one target site in the whole genome of human beings (one site in the X chromosome) was selectively hydrolyzed.

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

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

Ca2+/calmodulin-dependent protein kinase IV links group I metabotropic glutamate receptors to fragile X mental retardation protein in cingulate cortex

Fragile X syndrome is caused by a lack of fragile X mental retardation protein (FMRP) due to silencing of the FMR1 gene. The metabotropic glutamate receptors (mGluRs) in the central nervous system contribute to higher brain functions including learning/memory, persistent pain, and mental disorders. Our recent study has shown that activation of Group I mGluR up-regulated FMRP in anterior cingulate cortex (ACC), a key region for brain cognitive and executive functions; Ca(2+) signaling pathways could be involved in the regulation of FMRP by Group I mGluRs. In this study we demonstrate that stimulating Group I mGluRs activates Ca(2+)/calmodulin-dependent protein kinase IV (CaMKIV) in ACC neurons. In ACC neurons of adult mice overexpressing CaMKIV, the up-regulation of FMRP by stimulating Group I mGluR is enhanced. The enhancement occurs at the transcriptional level as the Fmr1 mRNA level was further elevated compared with wild-type mice. Using pharmacological approaches, we found that inhibition of CaMKIV could attenuate the up-regulation of FMRP by Group I mGluRs. CaMKIV contribute to the regulation of FMRP by Group I mGluRs probably through cyclic AMP-responsive element binding protein (CREB) activation, as manipulation of CaMKIV could simultaneously cause the change of CREB phosphorylation induced by Group I mGluR activation. Our study has provided strong evidence for CaMKIV as a molecular link between Group I mGluRs and FMRP in ACC neurons and may help us to elucidate the pathogenesis of fragile X syndrome.

FMR1: a gene with three faces

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

Whole-brain expression analysis of FMRP in adult monkey and its relationship to cognitive deficits in fragile X syndrome

Fragile X syndrome (FXS) is one of the most prevalent forms of heritable mental retardation and developmental delay in males. The syndrome is caused by the silencing of a single gene (fragile X mental retardation-1; FMR1) and the lack of expression of its protein product (fragile X mental retardation-1 protein; FMRP). Recent work has linked the high expression levels of FMRP in the magnocellular layers of lateral geniculate nucleus (M-LGN) of the visual system to a specific reduction of perceptual function known to be mediated by that neural structure. This finding has given rise to the intriguing notion that FMRP expression level may be used as an index of susceptibility of specific brain regions to the observed perceptual and cognitive deficits in FXS. We undertook a comprehensive expression profiling study of FMRP in the monkey to obtain further insight into the link between FMPR expression and the behavioural impact of its loss in FXS. We report here the first 3D whole-brain map of FMRP expression in the Old-World monkey and show that certain brain structures display high FMRP levels, such as the cerebellum, striatum, and temporal lobe structures. This finding provides support for the notion that FMRP expression loss is linked to behavioural and cognitive impairment associated with these structures. We argue that whole-brain FMRP expression mapping may be used to formulate and test new hypotheses about other forms of impairments in FXS that were not specifically examined in this study.

Translational regulation of the human achaete-scute homologue-1 by fragile X mental retardation protein

Fragile X syndrome is a common inherited cause of mental retardation that results from loss or mutation of the fragile X mental retardation protein (FMRP). In this study, we identified the mRNA of the basic helix-loop-helix transcription factor human achaete-scute homologue-1 (hASH1 or ASCL1), which is required for normal development of the nervous system and has been implicated in the formation of neuroendocrine tumors, as a new FMRP target. Using a double-immunofluorescent staining technique we detected an overlapping pattern of both proteins in the hippocampus, temporal cortex, subventricular zone, and cerebellum of newborn rats. Forced expression of FMRP and gene silencing by small interference RNA transfection revealed a positive correlation between the cellular protein levels of FMRP and hASH1. A luciferase reporter construct containing the 5'-untranslated region of hASH1 mRNA was activated by the full-length FMRP, but not by naturally occurring truncated FMR proteins, in transient co-transfections. The responsible cis-element was mapped by UV-cross-linking experiments and reporter mutagenesis assays to a (U)(10) sequence located in the 5'-untranslated region of the hASH1 mRNA. Sucrose density gradient centrifugation revealed that hASH1 transcripts were translocated into a translationally active polysomal fraction upon transient transfection of HEK293 cells with FMRP, thus indicating translational activation of hASH1 mRNA. In conclusion, we identified hASH1 as a novel downstream target of FMRP. Improved translation efficiency of hASH1 mRNA by FMRP may represent an important regulatory switch in neuronal differentiation.

The Drosophila FMRP and LARK RNA-binding proteins function together to regulate eye development and circadian behavior

Fragile X syndrome (FXS) is the most common form of hereditary mental retardation. FXS patients have a deficit for the fragile X mental retardation protein (FMRP) that results in abnormal neuronal dendritic spine morphology and behavioral phenotypes, including sleep abnormalities. In a Drosophila model of FXS, flies lacking the dfmr1 protein (dFMRP) have abnormal circadian rhythms apparently as a result of altered clock output. In this study, we present biochemical and genetic evidence that dFMRP interacts with a known clock output component, the LARK RNA-binding protein. Our studies demonstrate physical interactions between dFMRP and LARK, that the two proteins are present in a complex in vivo, and that LARK promotes the stability of dFMRP. Furthermore, we show genetic interactions between the corresponding genes indicating that dFMRP and LARK function together to regulate eye development and circadian behavior.

Mechanisms of RNA-mediated disease

Recent mapping of functional sequence elements in the human genome has led to the realization that transcription is pervasive and that noncoding RNAs compose a significant portion of the transcriptome. Some dominantly inherited neurological disorders are associated with the expansion of microsatellite repeats in noncoding regions that result in the synthesis of pathogenic RNAs. Here, we review RNA gain-of-function mechanisms underlying three of these microsatellite expansion disorders to illustrate how some mutant RNAs cause disease.

Fragile X syndrome associated with tic disorders

Movement disorders other than late onset tremor-ataxia in association with fragile X syndrome, the most common identifiable cause of inherited mental retardation, seem to be rare. Here we describe five male patients from three unrelated families with fragile X syndrome that presented with motor and phonic tics. Clinically, 4 patients fulfilled diagnostic criteria for Gilles de la Tourette syndrome (GTS) while 1 patient would have been diagnosed with an adult onset tic disorder. However, in all patients onset of tics was considerably later than in typical GTS. Three patients had atypical tics and two patients reported waxing and waning of tic intensity over time. Four of the 5 patients showed clinical signs typical of fragile X syndrome, in particular dysmorphic features, learning difficulties and speech and language problems that required special treatment. All patients had co-morbidities common to both GTS and fragile X syndrome. We suggest considering fragile X syndrome in GTS complicated by co-morbidity with late onset of atypical tics, in particular when learning disability and dysmorphic features are present.

Limbic epileptogenesis in a mouse model of fragile X syndrome

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

Cap-dependent translation initiation and memory

It is widely accepted that changes in gene expression contribute to enduring modifications of synaptic strength and are required for long-term memory. This is an exciting, wide-open area of research at this moment, one of those areas where it is clear that important work is underway but where the surface has just been scratched in terms of our understanding. Much attention has been given to the mechanisms of gene transcription; however, the regulation of transcription is only one route of manipulating gene expression. Regulation of mRNA translation is another route, and is the ultimate step in the control of gene expression, enabling cells to regulate protein production without altering mRNA synthesis or transport. One of the main advantages of this mechanism over transcriptional control in the nucleus lies in the fact that it endows local sites with independent decision-making authority, a consideration that is of particular relevance in neurons with complex synapto-dendritic architecture. There are a growing number of groups that are taking on the challenge of identifying the mechanisms responsible for regulating the process of mRNA translation during synaptic plasticity and memory formation. In this chapter we will discuss what has been discovered with regard to the localization and regulation of mRNA translation during specific types of neuronal activity in the mammalian central nervous system. The data are most complete for cap-dependent translation; therefore, particular attention will be paid to the machinery that initiates cap-dependent translation and its regulation during synaptic plasticity as well as the behavioral phenotypes consequent to its dysregulation.

Fragile X mental retardation protein interactions with the microtubule associated protein 1B RNA

Fragile X mental retardation syndrome, the most common form of inherited mental retardation, is caused by the absence of the fragile X mental retardation protein (FMRP). FMRP has been shown to use its arginine-glycine-glycine (RGG) box to bind to a subset of RNA targets that form a G quadruplex structure. We performed a detailed analysis of the interactions between the FMRP RGG box and the microtubule associated protein 1B (MAP1B) mRNA, a relevant in vivo FMRP target. We show that MAP1B RNA forms an intramolecular G quadruplex structure, which is bound with high affinity and specificity by the FMRP RGG box. We determined that hydrophobic interactions are important in the FMRP RGG box-MAP1B RNA association, with minor contributions from electrostatic interactions. Our findings that at low protein:RNA ratios the RNA G quadruplex structure is slightly stabilized, whereas at high ratios is unfolded, suggest a mechanism by which the FMRP concentration variation in response to a neurotransmitter stimulation event could act as a regulatory switch for the protein function, from translation repressor to translation activator.

MAP1 structural organization in Drosophila: in vivo analysis of FUTSCH reveals heavy- and light-chain subunits generated by proteolytic processing at a conserved cleavage site

The MAP1 (microtubule-associated protein 1) family is a class of microtubule-binding proteins represented by mammalian MAP1A, MAP1B and the recently identified MAP1S. MAP1A and MAP1B are expressed in the nervous system and thought to mediate interactions of the microtubule-based cytoskeleton in neural development and function. The characteristic structural organization of mammalian MAP1s, which are composed of heavy- and light-chain subunits, requires proteolytic cleavage of a precursor polypeptide encoded by the corresponding map1 gene. MAP1 function in Drosophila appears to be fulfilled by a single gene, futsch. Although the futsch gene product is known to share several important functional properties with mammalian MAP1s, whether it adopts the same basic structural organization has not been addressed. Here, we report the identification of a Drosophila MAP1 light chain, LCf, produced by proteolytic cleavage of a futsch-encoded precursor polypeptide, and confirm co-localization and co-assembly of the heavy chain and LCf cleavage products. Furthermore, the in vivo properties of MAP1 proteins were further defined through precise MS identification of a conserved proteolytic cleavage site within the futsch-encoded MAP1 precursor and demonstration of light-chain diversity represented by multiple LCf variants. Taken together, these findings establish conservation of proteolytic processing and structural organization among mammalian and Drosophila MAP1 proteins and are expected to enhance genetic analysis of conserved MAP1 functions within the neuronal cytoskeleton.

Cellular localization of metabotropic glutamate receptors in cortical tubers and subependymal giant cell tumors of tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder associated with cortical malformations (cortical tubers) and the development of glial tumors (subependymal giant-cell tumors, SGCTs). Expression of metabotropic glutamate receptor (mGluR) subtypes is developmentally regulated and several studies suggest an involvement of mGluR-mediated glutamate signaling in the regulation of proliferation and survival of neural stem-progenitor cells, as well as in the control of tumor growth. In the present study, we have investigated the expression and cell-specific distribution of group I (mGluR1, mGluR5), group II (mGluR2/3) and group III (mGluR4 and mGluR8) mGluR subtypes in human TSC specimens of both cortical tubers and SGCTs, using immunocytochemistry. Strong group I mGluR immunoreactivity (IR) was observed in the large majority of TSC specimens in dysplastic neurons and in giant cells within cortical tubers, as well as in tumor cells within SGCTs. In particular mGluR5 appeared to be most frequently expressed, whereas mGluR1alpha was detected in a subpopulation of neurons and giant cells. Cells expressing mGluR1alpha and mGluR5, demonstrate IR for phospho-S6 ribosomal protein (PS6), which is a marker of the mammalian target of rapamycin (mTOR) pathway activation. Group II and particularly group III mGluR IR was less frequently observed than group I mGluRs in dysplastic neurons and giant cells of tubers and tumor cells of SGCTs. Reactive astrocytes were mainly stained with mGluR5 and mGluR2/3. These findings expand our knowledge concerning the cellular phenotype in cortical tubers and in SGCTs and highlight the role of group I mGluRs as important mediators of glutamate signaling in TSC brain lesions. Individual mGluR subtypes may represent potential pharmacological targets for the treatment of the neurological manifestations associated with TSC brain lesions.

Modest alterations in patterns of motor neuron dendrite morphology in the Fmr1 knockout mouse model for fragile X

Fragile X, an inheritable form of mental retardation, is caused by the inactivation of a gene on the X chromosome, FMR1 which codes for an RNA binding protein, fragile X mental retardation protein. Loss of this protein is associated with reduced complexities of neuronal dendrites and alterations in spine morphology in a number of cortical brain regions, and these deficits may underlie the cognitive impairment observed in fragile X patients. Among the many symptoms of fragile X are altered motor functions, although the neuronal basis for these remains unclear. In this study we investigated whether knockout of Fmr1 in the mouse model of fragile X altered dendrite morphology in developing spinal cord motor neurons. We find that Fmr1 knockout leads to modest alterations in the distribution of dendritic arbor across the span of the motor neuron dendritic tree in 2- and 4-week-old mice, compared to wild-type controls, consistent with slower rates of extension and abnormal pruning of intermediate dendritic segments. These studies suggest that some motor deficits in fragile X patients may be due to abnormal maturation of dendritic patterning within spinal motor neurons, and suggest that strategies aimed at preventing motor impairment in fragile X patients may be targeted at motor functions during early development.

The fragile X continuum: new advances and perspectives

Fragile X syndrome is the world's most common hereditary cause of intellectual disability in men and to a lesser extent in women. The disorder is caused by the silencing of a single gene on the X chromosome, the Fragile X Mental Retardation Gene-1. A substantial body of research across the disciplines of molecular genetics, child psychiatry and developmental neuroscience bears testament to a decade of exciting and innovative science that has advanced our knowledge about the fragile X 'signature' or influence across cognitive and social development. The core aims of this review are to first discuss fragile X syndrome and premutation involvement in the context of current advances that demonstrate the dynamic nature of the genotype on phenotypic outcomes. Second, to discuss the implications of these recent advances for the development of clinical and educational interventions and resource tools that target specific phenotypic 'signatures' within the fragile X continuum.

Roles of calcium-stimulated adenylyl cyclase and calmodulin-dependent protein kinase IV in the regulation of FMRP by group I metabotropic glutamate receptors

The fragile X syndrome is caused by the lack of fragile X mental retardation protein (FMRP) attributable to silencing of the FMR1 gene. The metabotropic glutamate receptors (mGluRs) in the CNS contribute to different brain functions, including learning/memory, mental disorders, drug addiction, and persistent pain. Most of the previous studies have been focused on downstream targets of FMRP in hippocampal neurons, and fewer studies have been reported for the second-messenger signaling pathways between group I mGluRs and FMRP. Furthermore, no molecular study has been performed in the anterior cingulate cortex (ACC), a key region involved in high brain cognitive and executive functions. In this study, we demonstrate that activation of group I mGluR upregulated FMRP in ACC neurons of adult mice through the Ca(2+)-dependent signaling pathways. Using genetic approaches, we found that Ca(2+)/calmodulin-stimulated adenylyl cyclase 1 (AC1) and calcium/calmodulin-dependent kinase IV (CaMKIV) contribute to the upregulation of FMRP induced by stimulating group I mGluRs. The upregulation of FMRP occurs at the transcriptional level. The cAMP-dependent protein kinase is activated by stimulating group I mGluRs through AC1 in ACC neurons. Both AC1 and CaMKIV contribute to the regulation of FMRP by group I mGluRs probably through cAMP response element-binding protein activation. Our study has provided the first evidence for a molecular link between group I mGluRs and FMRP in ACC neurons and may help us to understand the pathogenesis of fragile X syndrome.

Group I metabotropic glutamate receptors: a role in neurodevelopmental disorders?

Group I metabotropic glutamate receptors (mGlu1 and mGlu5) are coupled to polyphosphoinositide hydrolysis and are involved in activity-dependent forms of synaptic plasticity, both during development and in the adult life. Group I mGlu receptors can also regulate proliferation, differentiation, and survival of neural stem/progenitor cells, which further support their role in brain development. An exaggerated response to activation of mGlu5 receptors may underlie synaptic dysfunction in Fragile X syndrome, the most common inherited form of mental retardation. In addition, group I mGlu receptors are overexpressed in dysplastic neurons of focal cortical dysplasia and hemimegaloencephaly, which are disorders of cortical development associated with chronic epilepsy. Drugs that block the activity of group I mGlu receptors (in particular, mGlu5 receptors) are potentially helpful for the treatment of Fragile X syndrome and perhaps other neurodevelopmental disorders.

Syntactic complexity during conversation of boys with fragile X syndrome and Down syndrome

PURPOSE

This study compared the syntax of boys who have fragile X syndrome (FXS) with and without autism spectrum disorder (ASD) with that of (a) boys who have Down syndrome (DS) and (b) typically developing (TD) boys.

METHOD

Thirty-five boys with FXS only, 36 boys with FXS with ASD, 31 boys with DS, and 46 TD boys participated. Conversational language samples were evaluated for utterance length and syntactic complexity (i.e., Index of Productive Syntax; H. S. Scarborough, 1990).

RESULTS

After controlling for nonverbal mental age and maternal education levels, the 2 FXS groups did not differ in utterance length or syntactic complexity. The FXS groups and the DS group produced shorter, less complex utterances overall and less complex noun phrases, verb phrases, and sentence structures than did the TD boys. The FXS with ASD group and the DS group, but not the FXS-only group, produced less complex questions/negations than did the TD group. Compared with the DS group, both FXS groups produced longer, more complex utterances overall, but on the specific complexity measures, they scored higher only on questions/negations.

CONCLUSION

Boys with FXS and DS have distinctive language profiles. Although both groups demonstrated syntactic delays, boys with DS showed greater delays.

New implications for the QUAKING RNA binding protein in human disease

The use of spontaneously occurring mouse models has proved to be a valuable tool throughout the years to delineate the signals required for nervous system development. This is especially true in the field of myelin biology, with a large number of different models available. The quaking viable mouse models dysmyelination in the nervous system and links the QUAKING RNA binding proteins to myelination and cell fate decisions. In this Mini-Review, we highlight the biological functions attributed to this KH-type RNA binding protein and the recent achievements linking it to human disorders.

In vitro and in cellulo evidences for association of the survival of motor neuron complex with the fragile X mental retardation protein

Spinal muscular atrophy (SMA) is caused by reduced levels of the survival of motor neuron (SMN) protein. Although the SMN complex is essential for assembly of spliceosomal U small nuclear RNPs, it is still not understood why reduced levels of the SMN protein specifically cause motor neuron degeneration. SMN was recently proposed to have specific functions in mRNA transport and translation regulation in neuronal processes. The defective protein in Fragile X mental retardation syndrome (FMRP) also plays a role in transport of mRNPs and in their translation. Therefore, we examined possible relationships of SMN with FMRP. We observed granules containing both transiently expressed red fluorescent protein(RFP)-tagged SMN and green fluorescent protein(GFP)-tagged FMRP in cell bodies and processes of rat primary neurons of hypothalamus in culture. By immunoprecipitation experiments, we detected an association of FMRP with the SMN complex in human neuroblastoma SH-SY5Y cells and in murine motor neuron MN-1 cells. Then, by in vitro experiments, we demonstrated that the SMN protein is essential for this association. We showed that the COOH-terminal region of FMRP, as well as the conserved YG box and the region encoded by exon 7 of SMN, are required for the interaction. Our findings suggest a link between the SMN complex and FMRP in neuronal cells.

Dynamic association of the fragile X mental retardation protein as a messenger ribonucleoprotein between microtubules and polyribosomes

The fragile X mental retardation protein (FMRP) is a selective RNA-binding protein that regulates translation and plays essential roles in synaptic function. FMRP is bound to specific mRNA ligands, actively transported into neuronal processes in a microtubule-dependent manner, and associated with polyribosomes engaged in translation elongation. However, the biochemical relationship between FMRP-microtubule association and FMRP-polyribosome association remains elusive. Here, we report that although the majority of FMRP is incorporated into elongating polyribosomes in the soluble cytoplasm, microtubule-associated FMRP is predominantly retained in translationally dormant, polyribosome-free messenger ribonucleoprotein (mRNP) complexes. Interestingly, FMRP-microtubule association is increased when mRNPs are dynamically released from polyribosomes as a result of inhibiting translation initiation. Furthermore, the I304N mutant FMRP that fails to be incorporated into polyribosomes is associated with microtubules in mRNP particles and transported into neuronal dendrites in a microtubule-dependent, 3,5-dihydroxyphenylglycine-stimulated manner with similar kinetics to that of wild-type FMRP. Hence, polyribosome-free FMRP-mRNP complexes travel on microtubules and wait for activity-dependent translational derepression at the site of function. The dual participation of FMRP in dormant mRNPs and polyribosomes suggests distinct roles of FMRP in dendritic transport and translational regulation, two distinct phases that control local protein production to accommodate synaptic plasticity.

Whole genome microarray analysis of gene expression in subjects with fragile X syndrome

PURPOSE

Fragile X syndrome, the most common inherited form of human mental retardation, arises as a consequence of a large expansion of a CGG trinucleotide repeat in 5' untranslated region of the fragile X mental retardation 1 (FMR1) gene located on the X chromosome. Although the FMR1 gene was cloned 15 years ago, the mechanisms that cause fragile X syndrome remain to be elucidated. Multiple studies have identified proteins that potentially interact with FMRP, the product of FMR1, and differentially expressed genes in an Fmr1 knockout mouse. To assess the impact of fragile X syndrome on gene expression in humans and to attempt to identify disturbed genes and gene interactive pathways relevant to fragile X syndrome, we performed gene expression microarray analysis using RNA isolated from lymphoblastoid cells derived from males with fragile X syndrome with and similarly aged control males.

METHODS

We used whole genome microarrays consisting of 57,000 probes to analyze global changes to the transcriptome in readily available lymphoblastoid cell lines derived from males with fragile X syndrome and healthy comparison males with normal intelligence. We verified the differential expression of several of these genes with known biological function relevant to fragile X syndrome using quantitative reverse transcription polymerase chain reaction using RNA from lymphoblastoid cells from fragile X syndrome and control males as well as RNA from human brain tissue (frontal cortex) of other affected fragile X syndrome males.

RESULTS

We identified more than 90 genes that had significant differences in probe intensity of at least 1.5-fold with a false discovery rate of 5% in cells from males with fragile X syndrome relative to comparison males. The list of 90 differentially expressed genes contained an overrepresentation of genes involved in signaling (e.g., UNC13B [-3.3-fold change in expression in lymphoblasts by quantitative reverse transcription polymerase chain reaction), GABRD [+2.0-fold change] EEF1A2 [+4.3-fold change]), morphogenesis (e.g., MAP1B [-7.5-fold change], ACCN1 [-8.0-fold change]), and neurodevelopment and function (e.g., PPP1R9B [+3.5-fold change], HES1 [+2.8-fold change]).

CONCLUSIONS

These genes may represent members of candidate networks disturbed by the loss of FMR1 and consequently fragile X mental retardation protein function, thus lending support for altered fragile X mental retardation protein function resulting in an abnormal transcriptome. Further analyses of the genes, especially those that have been identified in multiple studies, are warranted to develop a more integrated description of the alterations in gene processing that lead to fragile X syndrome.

Interactions of the G quartet forming semaphorin 3F RNA with the RGG box domain of the fragile X protein family

Fragile X syndrome, the most common cause of inherited mental retardation, is caused by the transcriptional silencing of the fmr1 gene due to an unstable expansion of a CGG trinucleotide repeat and its subsequent hypermethylation in its 5′ UTR. This gene encodes for the fragile X mental retardation protein (FMRP), an RNA-binding protein that has been shown to use its RGG box domain to bind to G quartet-forming RNA. In this study, we performed a detailed analysis of the interactions between the FMRP RGG box domain and one of its proposed RNA targets, human semaphorin 3F (S3F) RNA by using biophysical methods such as fluorescence, UV and circular dichroism spectroscopy. We show that this RNA forms a G quartet-containing structure, which is recognized with high affinity and specificity by the FMRP RGG box. In addition, we analyzed the interactions of human S3F RNA with the RGG box and RG cluster of the two FMRP autosomal paralogs, the FXR1P and FXR2P. We found that this RNA is bound with high affinity and specificity only by the FXR1P RGG box, but not by the FXR2P RG cluster. Both FMRP and FXR1P RGG box are able to unwind the G quartet structure of S3F RNA, however, the peptide concentrations required in this process are very different: a ratio of 1:6 RNA:FMRP RGG box versus 1:2 RNA:FXR1P RGG box.

Discourse skills of boys with fragile X syndrome in comparison to boys with Down syndrome

PURPOSE

This study compared the conversational discourse skills of boys who have fragile X syndrome with and without autism spectrum disorder (ASD) with those of boys with Down syndrome and boys who are typically developing.

METHOD

Participants were boys who have fragile X syndrome with (n = 26) and without (n = 28) ASD, boys with Down syndrome (n = 29), and boys who are typically developing (n = 22). Turns during an examiner-child interaction consisting of structured and semistructured activities were coded for the boys' ability to maintain a topic of conversation and the frequency of perseveration.

RESULTS

The results revealed that boys who had both fragile X and ASD produced significantly more noncontingent discourse than did boys who had only fragile X, boys with Down syndrome, or typically developing boys. This finding was observed regardless of whether the topic was maintained or changed and whether the turn type was a response or initiation. Regardless of autism status, boys with fragile X used more perseveration than did boys in the other groups.

CONCLUSION

These findings suggest that some aspects of the conversational discourse difficulties attributed to fragile X syndrome may be a function of the high rate of comorbidity between fragile X and autism, whereas some difficulties may be characteristic of fragile X syndrome.

Language comprehension in boys with fragile X syndrome and boys with Down syndrome

BACKGROUND

Fragile X syndrome (FXS) is the most common known inherited cause of intellectual disability, yet very few studies have explored the language comprehension skills of children with FXS. We examined the receptive vocabulary, grammatical morphology and syntax skills of boys with FXS (who were additionally classified as having autism, autism spectrum, or no autism) and compared them to boys with Down syndrome (DS) and typically developing (TD) boys at similar non-verbal developmental levels.

METHODS

The Vocabulary, Grammatical Morphology, and Elaborated Phrases and Sentences subtests of the Test for Auditory Comprehension of Language - 3rd Edition (TACL-3) were administered annually up to three times to assess the language comprehension skills of 35 boys with FXS without autism, 24 boys with FXS with autism spectrum, 19 boys with FXS with autism, 45 boys with DS and 40 TD boys at similar non-verbal cognitive levels.

RESULTS

After controlling for non-verbal cognition and maternal education levels, we found that the three groups of boys with FXS did not differ from each other but scored lower than the TD boys in language comprehension. The boys with DS scored lower in language comprehension than boys with FXS without autism and TD boys. For all of the groups, scores for receptive vocabulary, grammatical morphology and syntax did not differ.

CONCLUSIONS

Boys with FXS and boys with DS differed in receptive language levels, demonstrating unique language profiles for each syndrome. Language comprehension appears to be an important area to target in assessment and intervention for both populations.

Delineation of early attentional control difficulties in fragile X syndrome: focus on neurocomputational changes

Fragile X syndrome (FXS) is due to the silencing of a single X-linked gene and it is associated with striking attentional difficulties. As FXS is well characterised at the cellular level, the condition provides a unique opportunity to investigate how a genetic dysfunction can impact on the development of neurocomputational properties relevant to attention. Thirteen young boys with FXS and 13 mental-age-matched typically developing controls performed a touch-screen-based search task that manipulated the similarity between targets and distractors and their heterogeneity in size. Search speed, path and errors were recorded as multiple measures of performance. Children did not differ in overall search speed or path when searching amongst distractors, but striking error patterns distinguished children with FXS from controls. Firstly, although clear markers of previously found targets remained on screen, children with FXS perseverated on touching previous hits more than typically developing controls, consistent with the well-documented inhibitory deficits in adults with the disorder. Secondly, they could accurately discriminate single target-distractor pairs, but, when searching a complex display, they touched distractors more often than control children when distractors were similar to targets and especially so when these were infrequent, highlighting difficulties in judging relative size and allocate attentional weight independently of stimulus frequency. Thirdly, their performance was also characterised by inaccuracies in pointing, suggesting additional motor control deficits. Taken together, the findings suggest that fragile X syndrome affects the early development of multiple processes contributing to efficient attentional selection, as would be predicted from an understanding of the neurocomputational changes associated with the disorder.

Substitution of critical isoleucines in the KH domains of Drosophila fragile X protein results in partial loss-of-function phenotypes

Fragile X mental retardation proteins (FMRP) are RNA-binding proteins that interact with a subset of cellular RNAs. Several RNA-binding domains have been identified in FMRP, but the contribution of these individual domains to FMRP function in an animal model is not well understood. In this study, we have generated flies with point mutations in the KH domains of the Drosophila melanogaster fragile X gene (dfmr1) in the context of a genomic rescue fragment. The substitutions of conserved isoleucine residues within the KH domains with asparagine are thought to impair binding of RNA substrates and perhaps the ability of FMRP to assemble into mRNP complexes. The mutants were analyzed for defects in development and behavior that are associated with deletion null alleles of dfmr1. We find that these KH domain mutations result in partial loss of function or no significant loss of function for the phenotypes assayed. The phenotypes resulting from these KH domain mutants imply that the capacities of the mutant proteins to bind RNA and form functional mRNP complexes are not wholly disrupted and are consistent with biochemical models suggesting that RNA-binding domains of FMRP can function independently.

Splicing regulation in neurologic disease

The importance of alternative splicing in the regulation of diverse biological processes is reflected in the growing list of human diseases associated with known or suspected splicing defects. It is becoming evident that alternative splicing plays a particularly important role in neurologic disease, which is perhaps not surprising given the important role splicing plays in generating complexity and function in the brain. This review considers the evidence that defects in regulation of splicing may underlie many types of human neurologic diseases.

Decreased expression of the GABAA receptor in fragile X syndrome

After our initial discovery of under expression of the GABA(A) receptor delta subunit in a genome wide screening for differentially expressed mRNAs in brain of fragile X mice, a validated model for fragile X mental retardation syndrome, we analyzed expression of the 17 remaining subunits of the GABA(A) receptor using real-time PCR. We confirmed nearly 50% under expression of the delta subunit and found a significant 35%-50% reduction in expression of 7 additional subunit mRNAs, namely alpha(1), alpha(3), and alpha(4), beta(1) and beta(2) and gamma(1) and gamma(2), in fragile X mice compared to wild-type littermates. In concordance with previous results, under expression was found in cortex, but not in cerebellum. Moreover, decreased expression of specific GABA(A) receptor subunits in fragile X syndrome seems to be an evolutionary conserved hallmark since in the fragile X fly (Drosophila melanogaster) model we also found almost 50% under expression of all 3 subunits which make up the invertebrate GABA receptor, namely Grd, Rdl and Lcch3. In addition, we demonstrated a direct correlation between the amount of dFmrp and the expression of the GABA receptor subunits Rdl and Grd. Our results add evidence to previous observations of an altered GABAergic system in fragile X syndrome. Because GABA(A) receptors are the major inhibitory receptors in brain, involved in anxiety, depression, insomnia, learning and memory and epilepsy, processes also disturbed in fragile X patients, the well described GABA(A) receptor pharmacology might open new powerful opportunities for treatment of the behavioral and epileptic phenotype associated with fragile X syndrome.

Effect of CX516, an AMPA-modulating compound, on cognition and behavior in fragile X syndrome: a controlled trial

A Phase II, 4-week randomized, double-blind, placebo-controlled clinical trial was conducted to evaluate the safety and efficacy of the Ampakine compound CX516 as a potential treatment for the underlying disorder in fragile X syndrome (FXS). After baseline screening, subjects with FXS (n = 49) underwent a 1-week placebo lead-in and then were randomized to study drug or placebo for a 4-week period. Cognitive and behavioral outcome measures were administered prior to treatment, at the end of treatment, and 2 weeks posttreatment. There were minimal side effects, no significant changes in safety parameters, and no serious adverse events. There was a 12.5% frequency of allergic rash in the CX516 group and 1 subject developed a substantial rash. There was also no significant improvement in memory, the primary outcome measure, or in secondary measures of language, attention/executive function, behavior, and overall functioning in CX516-treated subjects compared to placebo. This study did demonstrate that many outcome measures were reproducible in this test-retest setting for the FXS population, yet some were too difficult or variable. Adult subjects with FXS were able to complete an intensive clinical trial, and some valid outcome measures were identified for future FXS trial design. Problems with potency of CX516 in other studies have suggested dosing may have been inadequate for therapeutic effect and thus it remains unclear whether modulation of AMPA-mediated neurotransmission is a viable therapeutic strategy for the treatment of FXS.

Dynamic translational and proteasomal regulation of fragile X mental retardation protein controls mGluR-dependent long-term depression

Genetic deletion of fragile X mental retardation protein (FMRP) has been shown to enhance mGluR-dependent long-term depression (LTD). Herein, we demonstrate that mGluR-LTD induces a transient, translation-dependent increase in FMRP that is rapidly degraded by the ubiquitin-proteasome pathway. Moreover, proteasome inhibitors abolished mGluR-LTD, and LTD was absent in mice that overexpress human FMRP. Neither translation nor proteasome inhibitors blocked the augmentation of mGluR-LTD in FMRP-deficient mice. In addition, mGluR-LTD is associated with rapid increases in the protein levels of FMRP target mRNAs in wild-type mice. Interestingly, the basal levels of these proteins were elevated and their synthesis was improperly regulated during mGluR-LTD in FMRP-deficient mice. Our findings indicate that hippocampal mGluR-LTD requires the rapid synthesis and degradation of FMRP and that mGluR-LTD triggers the synthesis of FMRP binding mRNAs. These findings indicate that the translation, ubiquitination, and proteolysis of FMRP functions as a dynamic regulatory system for controlling synaptic plasticity.

mRNA specific subcellular localization represents a crucial step for fine-tuning of gene expression in mammalian cells

mRNA subcellular distribution and translational control are key player mechanisms for post-transcriptional gene expression regulation. In the last decade it has become increasingly clear that these processes are associated with various human diseases. Understanding the interconnected multistep process of mRNA localization and its involvement in organelle biogenesis and in the overall spatial structure of eukaryotic cells will be an important step towards the long-term goal of curing individual molecular defects. In a recent issue, Russo et al. [The 3'-untranslated region directs ribosomal protein-encoding mRNAs to specific cytoplasmic regions, Biochim. Biophys. Acta, Mol. Cell Res. 1763 (8) (2006) 833-843] reported interesting findings on the mechanisms that direct mRNAs encoding different ribosomal proteins to specific cytoplasmic regions in human cells.

X-linked mental retardation: many genes for a complex disorder

X-linked mental retardation (XLMR) is a common cause of moderate to severe intellectual disability in males. XLMR is very heterogeneous, and about two-thirds of patients have clinically indistinguishable non-syndromic (NS-XLMR) forms, which has greatly hampered their molecular elucidation. A few years ago, international consortia overcame this impasse by collecting DNA and cell lines from large cohorts of XLMR families, thereby paving the way for the systematic study of the molecular causes of XLMR. Mutations in known genes might already account for 50% of the families with NS-XLMR, and various genes have been pinpointed that seem to be of particular diagnostic importance. Eventually, even therapy of XLMR might become possible, as suggested by the unexpected plasticity of the neuronal wiring in the brain, and the recent successful drug treatment of a fly model for fragile X syndrome.

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.

Advanced microscopic imaging methods to investigate cortical development and the etiology of mental retardation

Studies on human patients and animal models of disease have shown that disruptions in prenatal and early postnatal brain development are a root cause of mental retardation. Since proper brain development is achieved by a strict spatiotemporal control of neurogenesis, cell migration, and patterning of synapses, abnormalities in one or more of these events during prenatal development can lead to cognitive dysfunction after birth. Many of underlying causes of mental retardation must therefore be studied in developing brains. To aid in this research, live imaging using laser scanning microscopy (LSM) has recently allowed neuroscientists to delve deeply into the complex three-dimensional environment of the living brain to record dynamic cellular events over time. This review will highlight recent examples of how LSM is being applied to elucidate both normal and abnormal cortical development.

A reduced number of metabotropic glutamate subtype 5 receptors are associated with constitutive homer proteins in a mouse model of fragile X syndrome

Fragile X (FRAX) syndrome is a common inherited form of mental retardation resulting from the lack of fragile X mental retardation protein (FMRP) expression. The consequences of FMRP absence in the mechanism underlying mental retardation are unknown. Here, we tested the hypothesis that glutamate receptor (GluR) expression might be altered in FRAX syndrome. Initial in situ hybridization and Western blotting experiments did not reveal differences in mRNA levels and protein expression of AMPA and NMDA subunits and metabotropic glutamate subtype 5 (mGlu5) receptors between control and Fmr1 knock-out (KO) mice during postnatal development. However, a detergent treatment (1% Triton X-100) revealed a selective reduction of mGlu5 receptor expression in the detergent-insoluble fraction of synaptic plasma membranes (SPMs) from KO mice, with no difference in the expression of NR2A, GluR1, GluR2/3, GluR4, and Homer proteins. mGlu5 receptor expression was also lower in Homer immunoprecipitates from Fmr1 KO SPMs. Homer, but not NR2A, mGlu5, and GluR1, was found to be less tyrosine phosphorylated in Fmr1 KO than control mice. Our data indicate that, in FRAX syndrome, a reduced number of mGlu5 receptors are tightly linked to the constituents of postsynaptic density and, in particular, to the constitutive forms of Homer proteins, with possible consequent alterations in synaptic plasticity.

Deficits in trace fear memory and long-term potentiation in a mouse model for fragile X syndrome

Trace fear memory requires the activity of the anterior cingulate cortex (ACC) and is sensitive to attention-distracting stimuli. Fragile X syndrome is the most common form of mental retardation with many patients exhibiting attention deficits. Previous studies in fragile X mental retardation 1 (FMR1) knock-out (KO) mice, a mouse model for fragile X, focused mainly on hippocampal-dependent plasticity and spatial memory. We demonstrate that FMR1 knock-out mice show a defect in trace fear memory without changes in locomotion, anxiety, and pain sensitivity. Whole-cell path-clamp recordings in the ACC show that long-term potentiation (LTP) was completely abolished. A similar decrease in LTP was found in the lateral amygdala, another structure implicated in fear memory. No significant changes were found in basal synaptic transmission. This suggests that synaptic plasticity in the ACC and amygdala of FMR1 KO mice plays an important role in the expression of behavioral phenotypes similar to the symptoms of fragile X syndrome.

Lessons from fragile X regarding neurobiology, autism, and neurodegeneration

The fragile X mental retardation 1 gene (FMR1) mutation causes two disorders: fragile X syndrome (FXS) in those with the full mutation and the fragile X-associated tremor/ataxia syndrome (FXTAS) in some older individuals with the premutation. FXS is caused by a deficiency of the FMR1 protein (FMRP) leading to dysregulation of many genes that create a phenotype with ADHD, anxiety, and autism. FXTAS is caused by the elevation of FMR1-mRNA to levels 2 to 8 times normal in the premutation. This causes an RNA gain of function toxicity leading to brain atrophy, white matter disease, neuronal and astrocytic inclusion formation, and subsequent ataxia, intention tremor, peripheral neuropathy, and cognitive decline. The neurobiology and pathophysiology of FXS and FXTAS are described in detail.

Ablation of the Sam68 RNA binding protein protects mice from age-related bone loss

The Src substrate associated in mitosis of 68 kDa (Sam68) is a KH-type RNA binding protein that has been shown to regulate several aspects of RNA metabolism; however, its physiologic role has remained elusive. Herein we report the generation of Sam68-null mice by homologous recombination. Aged Sam68^−/− mice preserved their bone mass, in sharp contrast with 12-month-old wild-type littermates in which bone mass was decreased up to approximately 75%. In fact, the bone volume of the 12-month-old Sam68^−/− mice was virtually indistinguishable from that of 4-month-old wild-type or Sam68^−/− mice. Sam68^−/− bone marrow stromal cells had a differentiation advantage for the osteogenic pathway. Moreover, the knockdown of Sam68 using short hairpin RNA in the embryonic mesenchymal multipotential progenitor C3H10T1/2 cells resulted in more pronounced expression of the mature osteoblast marker osteocalcin when differentiation was induced with bone morphogenetic protein-2. Cultures of mouse embryo fibroblasts generated from Sam68^+/+ and Sam68^−/− littermates were induced to differentiate into adipocytes with culture medium containing pioglitazone and the Sam68^−/− mouse embryo fibroblasts shown to have impaired adipocyte differentiation. Furthermore, in vivo it was shown that sections of bone from 12-month-old Sam68^−/− mice had few marrow adipocytes compared with their age-matched wild-type littermate controls, which exhibited fatty bone marrow. Our findings identify endogenous Sam68 as a positive regulator of adipocyte differentiation and a negative regulator of osteoblast differentiation, which is consistent with Sam68 being a modulator of bone marrow mesenchymal cell differentiation, and hence bone metabolism, in aged mice.

Osteoporosis is a debilitating bone disease that is characterized by reduced bone mass and microarchitectural damage, which result in increased bone fragility and susceptibility to fracture. Peak bone mass, which is achieved by the age of 30 in humans, has been identified as a major determinant of resistance or susceptibility to osteoporosis. The authors generated mice deficient for the Sam68 RNA binding protein, a protein of unknown physiologic function. The mice develop normally and are protected against bone loss during aging. Age-related bone loss has long been associated with an increase in marrow adipocytes, which are derived from the same mesenchymal lineage as osteoblasts in bone marrow. The authors showed that Sam68 regulates the differentiation of this mesenchymal lineage, such that in its absence, osteoblasts continued to be generated in aging bone, leading to preservation of bone mass. This study identifies a physiologic role for Sam68 as a modulator of the bone marrow stem cell niche and hence of bone metabolism. The data identify Sam68 as a potential therapeutic target for the prevention and treatment of age-related bone loss.

Macrocephaly and the control of brain growth in autistic disorders

Autism is a childhood-onset neuropsychiatric disorder characterized by marked impairments in social interactions and communication, with restricted stereotypic and repetitive patterns of behavior, interests, and activities. Genetic epidemiology studies indicate that a strong genetic component exists to this disease, but these same studies also implicate significant environmental influence. The disorder also displays symptomatologic heterogeneity, with broad individual differences and severity on a graded continuum. In the search for phenotypes to resolve heterogeneity and better grasp autism's underlying biology, investigators have noted a statistical overrepresentation of macrocephaly, an indicator of enlarged brain volume. This feature is one of the most widely replicated biological findings in autism. What then does brain enlargement signify? One hypothesis invoked for the origin of macrocephaly is a reduction in neuronal pruning and consolidation of synapses during development resulting in an overabundance of neurites. An increase in generation of cells is an additional mechanism for macrocephaly, though it is less frequently discussed in the literature. Here, we review neurodevelopmental mechanisms regulating brain growth and highlight one underconsidered potential causal mechanism for autism and macrocephaly--an increase in neurogenesis and/or gliogenesis. We review factors known to control these processes with an emphasis on nuclear receptor activation as one signaling control that may be abnormal and contribute to increased brain volume in autistic disorders.