A comparative study of the performance of individuals with fragile X syndrome and Fmr1 knockout mice on Hebb-Williams mazes

Fragile X syndrome (FXS) is the most prevalent form of heritable mental retardation. It arises from a mutation in the FMR1 gene on the X chromosome that interferes with expression of fragile X mental retardation protein (FMRP) and leads to a wide range of behavioural and cognitive deficits. Previous studies have shown a deficit in basic visual perceptual processing as well as spatial abilities in FXS. How such a deficit may impact spatial navigation remains unknown. The current study extended previous research by evaluating spatial learning and memory using both virtual and physical versions of Hebb-Williams mazes, which allows for testing of humans and animals under comparable conditions. We compared the performance of individuals affected by FXS to typically developing individuals of equivalent mental age as well as the performance of Fmr1 knockout mice to wild-type control mice on the same maze problems. In human participants, performance of the comparison group improved across trials, showing expected significant decreases in both errors and latency. In contrast, the performance of the fragile X group remained at similar levels across trials. Although wild-type control mice made significantly fewer errors than the Fmr1 knockout mice, latencies were not statistically different between the groups. These findings suggest that affected humans and mice show similar spatial learning deficits attributable to the lack of FMRP. The implications of these data are discussed including the notion that Hebb-Williams mazes may represent a useful tool to examine the impact of pharmacological interventions on mitigating or reversing the symptoms associated with FXS.     

A study of FRAXE in mentally retarded individuals referred for fragile X syndrome (FRAXA) testing in the United Kingdom.

The folate-sensitive fragile site FRAXE is located in proximal Xq28 of the human X chromosome and lies approximately 600 kb distal to the fragile X syndrome (FRAXA) fragile site at Xq27.3. The cytogenetic expression of FRAXE is thought to be associated with mental handicap, but this is usually mild compared to that of the more common fragile X syndrome that is associated with the expression of the FRAXA fragile site. The exact incidence of FRAXE mental retardation is uncertain. We describe here the results of a U.K. survey designed to assess the frequency of FRAXE in a population of individuals referred for fragile X syndrome testing and found to be negative for expansion events at the FRAXA locus. No FRAXE expansion events were found in 362 cytogenetically negative males studied, and one expansion event was identified in a sample of 534 males for whom cytogenetic analyses were either unrecorded or not performed. Further FRAXE expansion events were detected in two related females known to be cytogenetically positive for a fragile site in Xq27.3-28. To gain insight into the FRAXE phenotype, the clinical details of the identified FRAXE male plus three other FRAXE individuals identified through previous referrals for fragile X syndrome testing are presented. For the population studied, we conclude that FRAXE mental retardation is a relatively rare but significant form of mental retardation for which genetic diagnosis would be appropriate.     

New insights into fragile X syndrome: from molecules to neurobehaviors

Fragile X syndrome - a common form of inherited mental retardation - is caused by the loss of the fragile X mental retardation 1 protein (FMRP). FMRP is an RNA-binding protein which forms a messenger ribonucleoprotein (mRNP) complex that associates with translating polyribosomes. It has been proposed that FMRP is involved in synaptic plasticity through the regulation of mRNA transportation and translation. Recent advances in the identification of the mRNA ligands that are bound by FMRP, the RNA sequence and structure required for FMRP-RNA interaction, and the physiological consequences of FMRP deficiency in the brain are important steps towards understanding the molecular pathogenesis of fragile X syndrome, and learning and memory in general.     

Fragile X mental retardation protein targets G quartet mRNAs important for neuronal function.

Loss of fragile X mental retardation protein (FMRP) function causes the fragile X mental retardation syndrome. FMRP harbors three RNA binding domains, associates with polysomes, and is thought to regulate mRNA translation and/or localization, but the RNAs to which it binds are unknown. We have used RNA selection to demonstrate that the FMRP RGG box binds intramolecular G quartets. This data allowed us to identify mRNAs encoding proteins involved in synaptic or developmental neurobiology that harbor FMRP binding elements. The majority of these mRNAs have an altered polysome association in fragile X patient cells. These data demonstrate that G quartets serve as physiologically relevant targets for FMRP and identify mRNAs whose dysregulation may underlie human mental retardation.     

FMRP蛋白6种异构体与FXR1蛋白间的相互作用

脆性Ｘ综合征是最常见的遗传性智力低下疾病,其致病基因ＦＭＲ１存在复杂的选择剪接．ＦＭＲ１基因的功能及其选择剪接的生物学意义尚未阐明．ＦＭＲ１蛋白（ＦＭＲＰ）与脆性Ｘ相关蛋白１（ＦＸＲ１）可形成异源二聚体．采用酵母双杂交体系研究了由ＦＭＲ１第１２、１４、１５外显子不同选择剪接方式产生的６种ＦＭＲＰ异构体与ＦＸＲ１蛋白的相互作用,以期从蛋白质相互作用的角度探讨ＦＭＲ１基因选择剪接表达的生物学意义．结果表明各种异构体与ＦＸＲ１相互作用的强度随异构体蛋白肽链长度的增长而减弱．外显子１２、１４、１５的选择剪接虽然不能开关式控制ＦＭＲＰ与ＦＸＲ１的相互作用,但其Ｃ端亲水区在一定程度上影响相互作用的强弱．提示选择剪接对ＦＭＲＰ与ＦＸＲ１异源二聚体的稳定性产生影响．     

A survey of fragile X syndrome in a sample from Spanish Basque country

Fragile X syndrome is the most common inherited form of mental retardation. The syndrome is associated with a CGG repeat expansion in the 5'UTR of the first exon of the FMR1 gene. This gene maps to Xq27.3 and coincides with the cytogenetic fragile site (FRAXA). The present study deals with the prevalence of fragile X syndrome among individuals with mental retardation of unknown cause from institutions and special schools from the Spanish Basque Country. Results of cytogenetic and molecular studies, performed in a group of 134 unrelated individuals (92 males and 42 females) are presented. The cytogenetic marker at Xq27.3 was identified in 12 patients. Other chromosomal abnormalities were found in two cases that this and previous studies confirmed as Angelman and Prader-Willi syndromes. Two males, in whom the cytogenetic marker was identified, were found negative for FRAXA and FRAXE expansion at the molecular level. The present study shows that the frequency of the FRAXA full mutation in individuals of Spanish non-Basque origin is in the range of other Spanish populations. In the sample of Spanish Basque origin we have not found cytogenetic FRAXA site expression, and the CGG repeat size of FMR1 gene is in the normal range. The significance of these results are discussed.     

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

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

RNA and microRNAs in fragile X mental retardation

Fragile X syndrome is caused by the loss of an RNA-binding protein called FMRP (for fragile X mental retardation protein). FMRP seems to influence synaptic plasticity through its role in mRNA transport and translational regulation. Recent advances include the identification of mRNA ligands, FMRP-mediated mRNA transport and the neuronal consequence of FMRP deficiency. FMRP was also recently linked to the microRNA pathway. These advances provide mechanistic insight into this disorder, and into learning and memory in general.     

Deletion of FMR1 in Purkinje cells enhances parallel fiber LTD, enlarges spines, and attenuates cerebellar eyelid conditioning in Fragile X syndrome

Absence of functional FMRP causes Fragile X syndrome. Abnormalities in synaptic processes in the cerebral cortex and hippocampus contribute to cognitive deficits in Fragile X patients. So far, the potential roles of cerebellar deficits have not been investigated. Here, we demonstrate that both global and Purkinje cell-specific knockouts of Fmr1 show deficits in classical delay eye-blink conditioning in that the percentage of conditioned responses as well as their peak amplitude and peak velocity are reduced. Purkinje cells of these mice show elongated spines and enhanced LTD induction at the parallel fiber synapses that innervate these spines. Moreover, Fragile X patients display the same cerebellar deficits in eye-blink conditioning as the mutant mice. These data indicate that a lack of FMRP leads to cerebellar deficits at both the cellular and behavioral levels and raise the possibility that cerebellar dysfunctions can contribute to motor learning deficits in Fragile X patients.     

脆性X综合征致病机理与治疗

脆性X综合征(fragile X syndrome, FXS)是最常见的遗传性智力障碍疾病,主要是由于X染色体上脆性X智力低下基因1(fragile X-mental retardation gene 1, FMR1)5’端非翻译区CGG三核苷酸的重复扩增及其相邻部位CpG岛的异常甲基化而导致其编码产物脆性X智力低下蛋白(fragile X mental retardation protein, FMRP)的缺失引起。目前,基因诊断已成为FXS诊断的金标准,但临床治疗仍缺乏特异性。本文首先介绍了FMRP的结构与功能,剖析了FXS的致病机制,然后阐述了FXS中与FMRP表达相关的信号转导途径,深入探讨并总结了靶向干预FXS中信号通路、基因编辑逆转FMR1沉默以及靶向降解FXS异常表达蛋白的治疗策略。     

Fragile X mental retardation protein is required for chemically-induced long-term potentiation of the hippocampus in adult mice

Fragile X syndrome (FXS), a common form of inherited mental retardation, is caused by the lack of fragile X mental retardation protein (FMRP). The animal model of FXS, Fmr1 knockout mice, have deficits in the Morris water maze and trace fear memory tests, showing impairment in hippocampus-dependent learning and memory. However, results for synaptic long-term potentiation (LTP), a key cellular model for learning and memory, remain inconclusive in the hippocampus of Fmr1 knockout mice. Here, we demonstrate that FMRP is required for glycine induced LTP (Gly-LTP) in the CA1 of hippocampus. This form of LTP requires activation of post-synaptic NMDA receptors and metabotropic glutamateric receptors, as well as the subsequent activation of extracellular signal-regulated kinase (ERK) 1/2. However, paired-pulse facilitation was not affected by glycine treatment. Genetic deletion of FMRP interrupted the phosphorylation of ERK1/2, suggesting the possible role of FMRP in the regulation of the activity of ERK1/2. Our study provide strong evidences that FMRP participates in Gly-LTP in the hippocampus by regulating the phosphorylation of ERK1/2, and that improper regulation of these signaling pathways may contribute to the learning and memory deficits observed in FXS.     

The Fragile X Mental Retardation Protein in Circadian Rhythmicity and Memory Consolidation

The control of new protein synthesis provides a means to locally regulate the availability of synaptic components necessary for dynamic neuronal processes. The fragile X mental retardation protein (FMRP), an RNA-binding translational regulator, is a key player mediating appropriate synaptic protein synthesis in response to neuronal activity levels. Loss of FMRP causes fragile X syndrome (FraX), the most commonly inherited form of mental retardation and autism spectrum disorders. FraX-associated translational dysregulation causes wide-ranging neurological deficits including severe impairments of biological rhythms, learning processes, and memory consolidation. Dysfunction in cytoskeletal regulation and synaptic scaffolding disrupts neuronal architecture and functional synaptic connectivity. The understanding of this devastating disease and the implementation of meaningful treatment strategies require a thorough exploration of the temporal and spatial requirements for FMRP in establishing and maintaining neural circuit function.     

Subcellular Localization of Fragile X Mental Retardation Protein with the I304N Mutation in the RNA-Binding Domain in Cultured Hippocampal Neurons

1. Fragile X syndrome, the most common form of inherited mental retardation,iscaused by the lack or dysfunction of fragile X mental retardationprotein (FMRP). The I304N mutation in the RNA-binding domain of FMRP results in an exceptionally severe form of mental retardation.2. We have investigated the subcellular localization of FMRP and its I304N-mutated form in cultured hippocampal neurons and PC12 cells, using immunofluorescence microscopy. In PC12 cells, FMRP was predominantly localized to the cytoplasm and also to the processes after differentiation by NGF.3. In cultured hippocampal neurons, granular labeling was detected along the neuronal processes.4. Double-labeling with synaptophysin antibody revealed FMRP at synaptic sites in neurons.5. The I304N mutation did not appear to affect the transport of FMRP to dendrites or its localization at synaptic sites. Thus, FMRP is a synaptic protein and the severe phenotype observed in the patient with the I304N mutation is not produced by alterations in dendritic transport.     

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

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

Comprehensive Analysis of Ultrasonic Vocalizations in a Mouse Model of Fragile X Syndrome Reveals Limited, Call Type Specific Deficits

Fragile X syndrome (FXS) is a well-recognized form of inherited mental retardation, caused by a mutation in the fragile X mental retardation 1 (Fmr1) gene. The gene is located on the long arm of the X chromosome and encodes fragile X mental retardation protein (FMRP). Absence of FMRP in fragile X patients as well as in Fmr1 knockout (KO) mice results, among other changes, in abnormal dendritic spine formation and altered synaptic plasticity in the neocortex and hippocampus. Clinical features of FXS include cognitive impairment, anxiety, abnormal social interaction, mental retardation, motor coordination and speech articulation deficits. Mouse pups generate ultrasonic vocalizations (USVs) when isolated from their mothers. Whether those social ultrasonic vocalizations are deficient in mouse models of FXS is unknown. Here we compared isolation-induced USVs generated by pups of Fmr1-KO mice with those of their wild type (WT) littermates. Though the total number of calls was not significantly different between genotypes, a detailed analysis of 10 different categories of calls revealed that loss of Fmr1 expression in mice causes limited and call-type specific deficits in ultrasonic vocalization: the carrier frequency of flat calls was higher, the percentage of downward calls was lower and that the frequency range of complex calls was wider in Fmr1-KO mice compared to their WT littermates.