Role of TRIP8b in epilepsy

描述(申请人提供)：难于内科和外科治疗的癫痫发作对许多癫痫患者的生活造成负面影响，并导致显著的发病率和死亡率，但治疗难治性癫痫发作的基本机制仍然难以捉摸。在不同类型的遗传性和获得性癫痫中控制大脑兴奋性的候选之一是超极化激活的环核苷酸门控(HCN)通道(h通道)蛋白家族。H通道介导超极化激活电流(Ih)，这是控制神经元兴奋性的关键。四个亚单位，hcn1-4，结合在一起形成h通道，在整个大脑中有不同的表达。小鼠的HCN2基因突变会导致类似于人类失神癫痫的癫痫发作。HCN1和HCN2也与颞叶癫痫(TLE)有关，TLE是导致医学上难治性癫痫的最常见原因。在TLE大鼠模型中，Ih在海马树突中表达下调，这一变化导致兴奋性增加，并可能有助于这些癫痫动物癫痫发作倾向的增加。TLE中Ih的减少可以通过h通道的错误定位来解释，该通道远离远端的树突，进入胞体内的亚细胞室。这种h通道转运缺陷与HCN1与含有四肽重复序列(TPR)的Rab8b相互作用蛋白(TRIP8b)之间的相互作用减少有关。TRIP8b存在于多种选择性剪接变异体中，对h通道的转运和功能具有“上调”或“下调”的作用，但主要的脑TRIP8b亚型上调h电流密度和hcn1表面表达。由于有证据表明TRIP8b在控制h通道功能中起关键作用，我们通过操纵编码TRIP8b的基因产生了三个不同的小鼠系，1)完全敲除TRIP8b，2)条件性敲除TRIP8b，以及3)选择性删除外显子，将表达限制为“上调”TRIP8b亚型。初步研究表明，完全敲除TRIP8b会导致小鼠癫痫失神。我们假设TRIP8b小鼠的癫痫是由于h通道偏离质膜而导致丘脑和皮质h通道病变所致。我们将通过完成以下具体目标来解决这一假说：1)确定丘脑和皮质中TRIP8b的区域特异性消除是否导致失神癫痫，以及2)证明h通道表面表达水平控制失神发作的易感性。该项目将利用遗传学、生物化学、免疫组织化学和电生理学工具，通过编码TRIP8b基因突变的小鼠来表征癫痫的机制，主要目标是表征一种以前未知的癫痫原因，为未来开发新的癫痫治疗方法奠定基础。

DESCRIPTION (provided by applicant): Seizures refractory to medical and surgical management negatively affect the lives of many patients with epilepsy and lead to significant morbidity and mortality, but basic mechanisms underlying therapy-resistant seizures remain elusive. One candidate for controlling brain excitability in different types of genetic and acquired epilepsy is the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel (h channel) family of proteins. h channels mediate hyperpolarization-activated current, (Ih), which is critical for control of neuronal excitability. Four subunits, HCN1-4, combine to form h channels and are expressed differentially throughout the brain. Mutation of the Hcn2 gene in mice results in seizures resembling those in human absence epilepsy. HCN1 and HCN2 have also been implicated in temporal lobe epilepsy (TLE), the most common cause of medically refractory seizures. In a rat model of TLE, Ih is downregulated in hippocampal dendrites, a change that leads to increased excitability and may contribute to increased seizure propensity in these epileptic animals. Reduced Ih in TLE can be explained by mislocalization of h channels away from distal dendrites and into subcellular compartments within the soma. This h channel trafficking defect is associated with reduced interaction between HCN1 and tetratricopeptide repeat (TPR)-containing Rab8b interacting protein (TRIP8b), the h channel auxiliary subunit in brain. TRIP8b exists in multiple alternative splice variants with "upregulating" or "downregulating" effects on h channel trafficking and function, but the predominant brain TRIP8b isoforms upregulate Ih current density and HCN1 surface expression. Because of evidence that TRIP8b plays a critical role in controlling h channel function, we generated three distinct lines of mice with manipulations in the gene encoding TRIP8b, 1) Total knockout of TRIP8b, 2) conditional knockout of TRIP8b, and 3) selective deletion of exons, limiting expression to "upregulating" TRIP8b isoform. Preliminary studies reveal that total knockout of TRIP8b leads to absence epilepsy in mice. We hypothesize that epilepsy in TRIP8b mice results from thalamic and cortical h channelopathy due to mislocalization of h channels away from the plasma membrane. We will address this hypothesis by completing the following specific aims: 1) to determine if region-specific elimination of TRIP8b in thalamus and cortex leads to absence epilepsy, and 2) to demonstrate that h channel surface expression levels control susceptibility to absence seizures. This project will utilize genetic, biochemical, immunohistochemical and electrophysiological tools to characterize the mechanisms of epilepsy in mice with mutations of the gene encoding TRIP8b gene, with the overarching goal of characterizing a previously unknown cause of epilepsy to build the foundation for future development of novel epilepsy treatments.