Mouse models of the fragile X premutation and fragile X-associated tremor/ataxia syndrome

Carriers of the fragile X premutation (FPM) have CGG trinucleotide repeat expansions of between 55 and 200 in the 5′-UTR of FMR1, compared to a CGG repeat length of between 5 and 54 for the general population. Carriers were once thought to be without symptoms, but it is now recognized that they can develop a variety of early neurological symptoms as well as being at risk for developing the late onset neurodegenerative disorder fragile X-associated tremor/ataxia syndrome (FXTAS). Several mouse models have contributed to our understanding of FPM and FXTAS, and findings from studies using these models are summarized here. This review also discusses how this information is improving our understanding of the molecular and cellular abnormalities that contribute to neurobehavioral features seen in some FPM carriers and in patients with FXTAS. Mouse models show much of the pathology seen in FPM carriers and in individuals with FXTAS, including the presence of elevated levels of Fmr1 mRNA, decreased levels of fragile X mental retardation protein, and ubiquitin-positive intranuclear inclusions. Abnormalities in dendritic spine morphology in several brain regions are associated with neurocognitive deficits in spatial and temporal memory processes, impaired motor performance, and altered anxiety. In vitro studies have identified altered dendritic and synaptic architecture associated with abnormal Ca²⁺ dynamics and electrical network activity. FPM mice have been particularly useful in understanding the roles of Fmr1 mRNA, fragile X mental retardation protein, and translation of a potentially toxic polyglycine peptide in pathology. Finally, the potential for using these and emerging mouse models for preclinical development of therapies to improve neurological function in FXTAS is considered.     

钙结合蛋白Calbindin在FMR1基因敲除小鼠脑组织中的表达

目的探讨脆性X综合征中钙结合蛋白Calbindin在神经元树突棘形态异常中的作用。方法我们选用FVB品系的FMR1基因敲除型(KO)和野生型(WT)小鼠,分别取新生1、3、5、7、10、14d,及成年(6周)的KO小鼠以及WT小鼠用免疫组织化学方法检测钙结合蛋白Calbindin在脑组织中的分布和表达情况,分别对大脑纹状皮质、海马、颞叶听区、梨状皮质、丘脑及小脑的免疫阳性显色细胞进行检测。结果在新生1d龄WT型及KO型小鼠中Calbindin免疫阳性细胞首先出现于梨状皮质和小脑皮质中,随着天龄的增长脑内其他各区逐渐出现Calbindin免疫阳性细胞的表达,且≤10dKO型小鼠Calbindin免疫阳性细胞平均光密度均显著高于WT型小鼠(P<0.05)。结论FMRP通过负性调节脑内钙结合蛋白Calbindin的表达,这推测与FMR1基因敲除小鼠神经元树突和树突棘形态异常有关。     

脆性X智力低下蛋白在大肠杆菌中的表达及纯化

目的:在大肠杆菌中对脆性X智力低下蛋白(FMRP)进行重组表达,获得高产量及高纯度的可溶性重组蛋白。方法:以人脑cDNA文库为模板,PCR扩增FMR1基因后将其克隆至融合表达载体pGEX-6P-1中,重组载体转化进入大肠杆菌BL21(DE3)pLysS,并用IPTG进行诱导表达。应用谷胱甘肽-琼脂糖凝胶4B进行亲和层析纯化GST-FMRP融合蛋白,并以SDS-PAGE电泳和Westernblot对融合蛋白的表达进行检测。结果:亲和纯化后的融合蛋白经FMRP及GST单抗分别进行Westernblot鉴定为GST-FMRP融合蛋白。结论:大肠杆菌中FMRPISO10的重组表达产量高,纯化得到可溶性重组蛋白,为FMRP抗体的制备及FXS检测试剂盒的开发奠定基础。     

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.     

Hippocampal pyramidal cells in adult Fmr1 knockout mice exhibit an immature-appearing profile of dendritic spines

Fragile X syndrome (FXS) is a common form of mental retardation caused by the absence of functional fragile X mental retardation protein (FMRP). FXS is associated with elevated density and length of dendritic spines, as well as an immature-appearing distribution profile of spine morphologies in the neocortex. Mice that lack FMRP (Fmr1 knockout mice) exhibit a similar phenotype in the neocortex, suggesting that FMRP is important for dendritic spine maturation and pruning. Examination of Golgi-stained pyramidal cells in hippocampal subfield CA1 of adult Fmr1 knockout mice reveals longer spines than controls and a morphology profile that, while essentially opposite of that described in the Fmr1 knockout neocortex, appears similarly immature. This finding strongly suggests that FMRP is required for the processes of spine maturation and pruning in multiple brain regions and that the specific pathology depends on the cellular context.