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.
Despite numerous existing medical and surgical treatments, seizures resistant to medical intervention remain a significant cause of disability and death in patients with epilepsy. Our project will characterize a new animal model with a mutation in a gene that leads to epilepsy, and will use cutting-edge techniques to understand why and how the gene mutation leads to seizures. From a public health perspective, the most important benefit of this project is the characterization of a previously unknown cause of epilepsy that will be the foundation for development of novel treatments to improve the lives of patients with epilepsy.