Mechanism of increased BK channel activation from a channel mutation that causes epilepsy.
Concerted depolarization and Ca(2+) rise during neuronal action potentials activate large-conductance Ca(2+)- 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 Ca(2+)-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 Ca(2+)-dissociation constants (closed channel: 11.3-->5.2 microM; open channel: 0.92-->0.54 microM). The beta 4 subunit inhibits mutant channels through a slowing of activation kinetics. In physiological recording solutions, we established the Ca(2+) 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) microM: 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 Ca(2+) binding underlies greater contributions of BK current in the sharpening of action potentials for both alpha and alpha/beta 4 channels.
Wang
Bin
B
Department of Physiology, The University of Texas Health Science Center at San Antonio, San Antonio, TX 78229, USA.
Rothberg
Brad S
BS
Brenner
Robert
R
D000200:Action Potentials / Q000502:physiology; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002460:Cell Line; D004827:Epilepsy / Q000235:genetics*; D006801:Humans; D015640:Ion Channel Gating; D051036:Large-Conductance Calcium-Activated Potassium Channels / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D009154:Mutation; D011485:Protein Binding
D051036:Large-Conductance Calcium-Activated Potassium Channels; D002118:Calcium