Abstracts

Rationale: Large-conductance calcium- and voltage-activated potassium channels (so called "BK channels", KCNMA1 gene) are expressed in many tissues including the CNS. BK channels regulate action potential repolarization and excitability of neurons. The conventional role of this potassium channel in reducing excitable has been questioned due to the fact that a human BK channel gain-of-function mutation, D434G, causes epilepsy. Affected family members have generalized epilepsy and paroxysmal dyskinesia. The mechanisms by which a calcium-activated potassium channels cause epilepsy is not understood. Methods: In order to understand the mechanism underlying the paradoxical increase in excitability and seizures in BK potassium channel gain-of-function, we have generated transgenic mice expressing the BK channel D434G mutation (D369G in mice) under regulation of a bacterial artificial chromosome containing the KCNMA1 transcriptional control region. In addition, we included a loxP-flanked transcriptional terminator in the 5' untranslated sequence to regulate D369G transgene expression in a Cre-recombinase dependent fashion. The mice have been studied with regard to transgene expression, behavior, and electrophysiology (video-eeg monitoring). Results: Immunostaining for BK channels revealed transgene expression patterns similar to the native gene including the hippocampus and cortex, albeit at markedly reduced expression levels. Nevertheless, we found that D369G transgenic mice have handling-induced generalized tonic-clonic seizures lasting for approximately 30-45 seconds duration. Seizures initiate with a stargazer-type head lifting, followed by a flagpole tail, and then forelimb tonic contraction, and hindlimb clonus movements. The reflex seizures are evoked similar to those seen in the EL mouse, and are induced by handling of mice (generally by lifting the mice by their tail) but not other stimuli such as strobe lighting or other visual stimuli, environment changes, or low or high temperature stress. Handling-evoked seizures occur reliably only in mature mice (greater than 3 months of age). In addition to the seizure phenotype, we find that D369G mice mated to forebrain specific EMX-cre drivers, have extreme fear behavior during handling and also evident using the open field test. Ongoing experiments will utilize more specific cre-driver lines to determine which regions of the brain are necessary for seizures. Conclusions: The D369G transgenic mice demonstrate that BK channel gain-of-function is indeed sufficient to cause spontaneous seizures. We have however found no evidence of mice display phenotypes reminiscent of movement disorder. The mice should provide an opportunity in future studies to identify the mechanisms that underly paradoxical increases in excitability and epilepsy in a potassium channel gain-of-function. Funding: National Science Foundation grant 1456862 to RB.