ICAM5与RNA结合蛋白FMRP的动态关联机制及其对脆性X综合征树突棘成熟的影响

Spine dysmorphogenesis is a hallmark feature of Fragile X syndrome (FXS) in human and in Fmr1 KO mouse, where neurons show a high density of long, thin, and immature dendritic filopodia. Although abnormalities in dendrites and spines have been correlated with impaired cognitive abilities in mental retardation, the causes of these malformations are not yet well understood. Intercellular cell adhesion molecule-5 (ICAM5), initially called as telencephalin, is a member of the ICAM subfamily that is exclusively expressed in the telencephalon region of the brain. It has been implicated in neuronal and spine development as well as the formation and maintenance of dendritic filopodia and thereby slows spine maturation. We have reported that in postnatal development of Fmr1 KO mice, anomalous dendritic spines are concomitant to abundant ICAM5 expression in the neurons of FXS. Given that a loss of FMRP (Fragile mental retardation protein) causes the manifestation of synaptic defects, we hypothesize that ICAM5 may be a direct target of FMRP; and ICAM5 impairments may imply an underlying molecular mechanism in FXS. To answer these consequential questions, we will conduct a series of experiments using multiple molecular techniques including RIP, FISH, Time-lapse confocal imaging, and multiple molecular probes. Our results will likely point to a path toward an understanding of the underlying molecular mechanisms of spine dysmorphogenesis in FXS.

脆性X综合征 (Fragile X Syndrome，FXS) 的特征性病理改变是神经元上呈现大量未成熟的树突丝，而ICAM5是首个被发现阻碍树突棘成熟的细胞间粘附分子。本项目组已经首先证明 ICAM5蛋白在FXS小鼠神经元突触发育关键期表达异常增多，且与其树突棘成熟负相关。最近又发现ICAM5 mRNA与FMRP相互作用，而FMRP功能缺失正是FXS的核心致病因素，由此我们推测ICAM5 mRNA很可能是FMRP在神经元的直接靶标。因而本项目拟用Fmr1 KO小鼠和Thy1-YFP Tg小鼠，以及siRNA、RIP-Chip、FISH、多种荧光分子标记、活细胞激光共聚焦延时成像等技术，证明ICAM5 mRNA是FMRP的一个新靶标、FMRP调控ICAM5在树突膜局部的动态表达、ICAM5表达异常影响神经元树突棘正常发育。为探索FMRP的新功能、研究FXS智力缺陷的发病机制提供新方向。

脆性X综合征（FXS）是导致智力残疾的神经发育障碍，也是自闭症谱系障碍（ASD）的主要单基因病因，其中神经元显示异常的树突棘结构。通过本项目我们证明了ICAM5 mRNA是FMRP的直接靶分子，而且FMRP介导的ICAM5在树突局部异常表达是FXS树突棘发育障碍的重要分子机制。具体将获得了以下3个结论：1）在神经元内，FMRP靶向到ICAM5 mRNA，参与ICAM5的运输；在DHPG或者7,8-DHF刺激条件下，FMRP将ICAM5 mRNA运输到树突局部，并在树突局部表达。FMRP缺失影响ICAM5 mRNA在树突膜局部的翻译表达率；ICAM5在树突膜局部的翻译表达影响FXS神经元树突丝向树突棘的成熟过程。在FXS模型小鼠过表达FMRP或者桥粗ICAM5 能改善树突棘发育，矫正动物模型行为。