THE ROLE OF BK CHANNELS IN NEUROPATHOLOGY OF FRAGILE X SYNDROME

描述(由申请人提供)：编码脆性X智力低下蛋白(FMRP)的Fmr1基因转录沉默是导致精神残疾的最常见的遗传原因，称为脆性X综合征(FXS)。FXS还具有自闭症和癫痫高共病的特点。因此，广泛的研究工作集中在阐明FMRP在突触上的功能，以揭示FXS的分子基础。FMRP被认为是一种调节树突局部蛋白质合成的RNA结合蛋白，到目前为止，对FXS的研究主要集中在FMRP在树突功能中的翻译依赖作用上。然而，尽管经过了20年的深入研究，FXS的分子基础仍然不完全清楚。我们最近发现了FMRP在树突室以外的重要功能；最值得注意的是FMRP在调节动作电位(AP)波形和突触传递中的关键突触前作用。具体地说，我们的结果表明，FMRP通过以细胞自主的突触前方式调节AP持续时间来调节兴奋性海马神经元中神经递质的释放。FMRP的这些突触前活动是翻译不依赖的，并且是通过FMRP与大电导钙激活K+(BK)通道的相互作用而特异性地介导的。FMRP的缺失导致BK通道活性降低和AP过度增宽，进而导致突触前钙内流增加，神经递质释放增加，重复活动中的短期可塑性(STP)增加。基于信息论的分析表明，这些与FMRP缺失相关的缺陷导致突触传递信息的能力显着异常。我们观察到FMRP调节海马神经元和皮质锥体神经元的AP时程，提示FMRP对突触前功能的调节可能是一种广泛的现象，可能在FXS的病理生理学中发挥作用。事实上，我们目前的研究表明，针对BK通道功能的药物或遗传操作可以降低癫痫的易感性，并挽救因FMRP丢失而导致的几个突触和行为缺陷。然而，FMRP调节BK通道活性的机制以及FMRP-BK通道相互作用在FXS认知和行为异常中的作用尚不清楚。拟议的研究将从单渠道分析到行为分析，以解决这些关键问题。我们的结果有望为FXS的病理生理学提供重要的新见解，并为药物开发和干预开辟新的途径。

DESCRIPTION (provided by applicant): Transcriptional silencing of the Fmr1 gene encoding Fragile X mental retardation protein (FMRP) is the most common inheritable cause of mental disability, known as Fragile X syndrome (FXS). FXS is also characterized by a high co-morbidity with autism and epilepsy. Extensive research efforts have thus focused on elucidating the functions of FMRP at synapses to uncover the molecular basis of FXS. FMRP was identified as an RNA-binding protein regulating local protein synthesis in dendrites, and research to date on FXS has concentrated extensively on the translation-dependent roles of FMRP in dendritic function. Yet despite two decades of intensive studies the molecular basis of FXS remains incompletely understood. We have recently identified important functions of FMRP beyond the dendritic compartment; most notably the critical presynaptic roles of FMRP in regulating action potential (AP) waveform and synaptic transmission. Specifically, our results demonstrate that FMRP regulates neurotransmitter release in excitatory hippocampal neurons via modulation of AP duration in a cell-autonomous presynaptic manner. These presynaptic actions of FMRP are translation-independent and are mediated specifically by FMRP interaction with the large-conductance calcium-activated K+ (BK) channels. Loss of FMRP causes reduced BK channel activity and excessive AP broadening, leading in turn to elevated presynaptic calcium influx, increased neurotransmitter release and short-term plasticity (STP) during repetitive activity. Information theory-based analysis indicates that these defects associated with FMRP loss cause marked abnormalities in the ability of synapses to transmit information. Our observation that FMRP modulates AP duration both in the hippocampal and cortical pyramidal neurons suggests that BK channel-dependent regulation of presynaptic function by FMRP may be a widespread phenomenon that could play a role in the pathophysiology of FXS. Indeed, our current studies suggest that pharmacological or genetic manipulations targeting BK channel function can reduce seizure susceptibility and rescue several synaptic and behavioral deficits caused by FMRP loss. Yet the mechanisms by which FMRP modulates the activity of BK channels and the roles of FMRP-BK channel interaction in cognitive and behavioral abnormalities in FXS remain unknown. The proposed studies will span from single-channel to behavioral analyses to address these critical questions. Our results are expected to provide major new insights into pathophysiology of FXS and open new avenues for drug development and intervention.