Defining the Molecular Mechanisms of Subtype Specific KCNQ2/3 Potassium Channel Activators

Abstract

Epilepsy comprises a diverse set of neurological disorders that impact more than 1% of individuals worldwide. Although more than 20 drugs are available to treat epilepsy, at least 30% of patients are resistant to pharmacotherapy and have severe uncontrollable epileptic episodes. This gap in care is often attributed to the similarity in the mechanisms of action of most anti-epileptic drugs, as they only affect a subset of possible molecular targets. Retigabine is the first approved anti-epileptic drug that acts by potentiating voltage-gated potassium channels, specifically targeting KCNQ channels that underlie the neuronal M-current. Retigabine is a ‘first in class’ drug with little specificity between KCNQ2-5, and efforts are ongoing to identify drugs with better subtype specificity and fewer side effects. The retigabine analog ICA069673 (ICA73) exhibits strong selectivity for KCNQ2 over KCNQ3, but appears to have a unique mechanism, because mutations that abolish retigabine effects do not affect ICA73. Patch clamp recordings from chimeras of KCNQ2 and KCNQ3 demonstrate that ICA73 interacts with the KCNQ2 voltage sensor (S1-S4) rather than the pore region (S5-S6) targeted by retigabine. KCNQ2 point mutants throughout the KCNQ2 voltage sensor were generated based on sequence differences between KCNQ2 and KCNQ3, and screened for ICA73-sensitivity. These experiments reveal two clusters of residues that underlie separable effects of ICA73: potentiation of peak currents, and a shift in the voltage-dependence of gating. Further exploration of KCNQ channel activators will accelerate our understanding of the molecular mechanisms of their unique effects, and the determinants of subtype specific drug binding to KCNQ channels.