Effect of Small-Molecules on S4 Movement and Gating of Normal and Mutated IKM Channels

Abstract

Abnormal or synchronous neuronal activity in the brain leads to epileptic seizures that, when repeated or prolonged, can cause neuronal damage resulting in delayed psychomotor development, intellectual disability and other neurological disorders. One of the major potassium current in neurons is the muscarine-regulated M-current. The M-current, formed by the heteromerization of KCNQ2 and KCNQ3 channels (IKM), activates in the time frame of action potential initiation, thereby its crucial role in controlling neuronal excitability. The slow kinetics of activation and deactivation of the IKM (KCNQ2/KCNQ3) channel regulates the membrane potential and impede repetitive neuronal firing. Inherited mutations in the IKM channel cause a wide spectrum of early-onset epileptic disorders ranging from benign familial neonatal seizures to severe epileptic encephalopathies. I here study the molecular mechanisms by which a set of epileptic-inducing mutations in KCNQ2 and KCNQ3 cause malfunction of the IKM channel. I use a fluorescence assay, voltage clamp fluorometry (VCF), to measure simultaneously voltage sensor movement and gate opening during IKM channel activation in these mutations. The IKM channel is an attractive pharmacological target to treat epilepsy, because increasing the M-current stabilizes the resting and subthreshold membrane potential, thereby reducing membrane excitability. Lipophilic compounds, such as polyunsaturated fatty acids (PUFAs), have been shown to favorably affect neuronal function. In particular, PUFAs have been shown to improve the outcome of epilepsy, therefore constituting very promising anti-epileptic agents. However, the molecular mechanism of action of PUFAs is unknown. Based on the molecular mechanism for each epileptic-inducing IKM channel mutation, I show that different PUFAs shift the voltage dependence of the IKM channel mutations, suggesting that PUFAs should, at least partly, reverse the effects of some epilepsy-causing mutations and restore wild type-like IKM currents in these mutations.