Molecular Basis of Voltage Activation of an Epilepsy-Causing Mutation in the S4 of KCNQ3 Channel

Abstract

Epilepsy is a clinically devastating disease affecting more than 50 million people worldwide. Epilepsy is characterized by abnormal neuronal activity in the brain that can cause delayed psychomotor development, mental retardation or even death. At present, more than 30% of patients suffering from epilepsy do not benefit from available anti-epileptic drugs. Therefore, there is an ongoing need to understand the molecular basis of the disease to design treatments. One of the major potassium currents in neurons is the muscarine-regulated M-current (IKM). The IKM, formed by the heteromerization of KCNQ2 and KCNQ3 channels, activates in the time frame of action potential initiation, thereby controlling neuronal excitability. We here study the molecular mechanisms by which the epileptic-inducing mutation R230C in KCNQ3 causes channel malfunction. For instance, KCNQ3-R230C is assumed to lock the voltage sensor (S4) of KCNQ3 channel in the activated state, resulting in a constitutively open channel. We use voltage clamp fluorometry to understand how R230C affects the voltage sensor and the gate of KCNQ3 channel. We show that R230C, contrary to what was previously assumed, allows S4 movement in KCNQ3-R230C channels. R230C shifts the closing and S4 movement of KCNQ3 to extreme negative potentials, such that at physiological voltage range (−80 mV to +40 mV), the channel is always open. By comparing the functional properties of substituting R230 by alanine, cysteine or histidine, we found that the main functional effect of this mutation seems to be the loss of the positive charge. Thus, either by external application of MTSEA that converts R230C to a charged, lysine-like residue or by protonation of a substituted lysine at position R230 the voltage dependence of channel activation and the equilibrium of the S4 movement are shifted to positive values.