Mechanism of Increased BK Channel Activation from a Channel Mutation that Causes Epilepsy

Concerted depolarization and Ca2+ rise during neuronal action potentials activate large-conductance Ca2+- and voltage-dependent K+ (BK) channels, whose robust K+ currents increase the rate of action potential repolarization. Gain-of-function BK channels in mouse knockout of the inhibitory beta 4 subunit and in a human mutation (alpha(D434G)) have been linked to epilepsy. Here, we investigate mechanisms underlying the gain-of-function effects of the equivalent mouse mutation (alpha(D369G)), its modulation by the beta 4 subunit, and potential consequences of the mutation on BK currents during action potentials. Kinetic analysis in the context of the Horrigan-Aldrich allosteric gating model revealed that changes in intrinsic and Ca2+-dependent gating largely account for the gain-of-function effects. D369G causes a greater than twofold increase in the closed-to-open equilibrium constant (6.6e(-7) -> 1.65e(-6)) and an approximate twofold decrease in Ca2+-dissociation constants (closed channel: 11.3 -> 5.2 mu M; open channel: 0.92 -> 0.54 mu M). The beta 4 subunit inhibits mutant channels through a slowing of activation kinetics. In physiological recording solutions, we established the Ca2+ dependence of current recruitment during action potential-shaped stimuli. D369G and beta 4 have opposing effects on BK current recruitment, where D369G reduces and beta 4 increases K-1/2 (K-1/2 mu M: alpha(WT) 13.7, alpha(D369G) 6.3, alpha(WT)/beta 4 24.8, and alpha(D369G)/beta 4 15.0). Collectively, our results suggest that the D369G enhancement of intrinsic gating and Ca2+ binding underlies greater contributions of BK current in the sharpening of action potentials for both alpha and alpha/beta 4 channels.