Epilepsy, the condition in which affected individuals suffer recurrent seizures, affects up to 2% of the population. Many familial epilepsy syndromes have been characterized, pointing to genetic factors that may predispose toward or underlie many cases of epilepsy. In addition to genetic approaches to identifying genes involved in human epilepsy syndromes, mouse models of epilepsy have further illuminated the important role of ion channels in epilepsy, and have demonstrated overlap between human and mouse epilepsy genes. One category of voltage-gated ion channels implicated in epilepsy is the hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channel, which underlies the hyperpolarization-activated current (In). HCN channels mediate rhythmic oscillations in neuronal membrane potential and play an important role in membrane excitability and dendritic signal integration. A spontaneous mutant mouse with a 4-nucleotide insertion in the C-terminus of the hyperpolarization-activated cyclic nucleotide-gated channel subunit 2 (HCN2) gene has recently been identified. This mutant, named apathetic, is ataxic, and exhibits brief behavioral arrest spells consistent with absence seizures and paroxysmal jumping behavior followed by post-ictal immobility that is consistent with myoclonic seizures. The apathetic mutation is predicted to produce a protein with a truncation of a large portion of the C-terminal tail of HCN2. Protein-protein interactions between ion channel C-termini and scaffolding proteins often regulate ion channel localization, and abnormalities of HCN channel subcellular distribution may play a role in epilepsy. This proposal will utilize EEG recording, behavioral monitoring, cell biological and biochemical techniques to test the hypothesis that truncation of the HCN2 C-terminus in apathetic mice causes epilepsy by mislocalization of HCN channels in thalamic neurons, as well as in cortical and hippocampal pyramidal neurons, resulting in abnormal excitability and epilepsy. The immediate aims of this proposal are to characterize a novel animal model of spontaneous genetic epilepsy and to add insight into the basic molecular mechanisms that underlie some forms of epilepsy, with the ultimate goal of identifying targets for new diagnostic testing and treatments for epilepsy. ? ? ?
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