Gating currents from neuronal Kv7 channels

Abstract

The Kv7 family of voltage-gated K+ channels consists of five members (Kv7.1- Kv7.5), each showing specific tissue distribution and physiological role. Mutations in Kv7.1 cause the long QT syndrome, whereas Kv7.4 mutations underlie a rare form of slowly progressive deafness (DFNA2); finally, mutations in Kv7.2 and Kv7.3 genes have been identified in families affected by Benign Familial Neonatal Seizures. Disease-causing mutations often affect residues in the voltage-sensing domain of Kv7 subunits. In the present study, we have characterized the ionic and gating current of homomeric neuronally-expressed Kv7 channels (Kv7.2-Kv7.5) using the cut-open oocyte voltage clamp. The ionic and gating current were recorded at 18°C and also at 28°C to speed up gating charge movement. Increasing the recording temperature from 18°C to 28°C caused an accelerated activation/deactivation kinetics of the ionic currents in all homomeric Kv7 channels (the Q10 for the activation kinetics at 0 mV was 3.8, 4.1, 8.3, and 2.8 for Kv7.2, Kv7.3, Kv7.4 and Kv7.5). Moreover, currents carried by Kv7.4 channels (and less so Kv7.2) also showed a significant increase in their maximal value. Gating currents were only resolved in Kv7.4 and Kv7.5 channels, possibly because of an higher membrane channel density; ON gating charges at saturated potential (+40mV) amounted to 1.34±0.34 (Kv7.4) and 0.79±0.20 nC (Kv7.5). At 28°C, Kv7.4 gating currents had the following salient properties: 1) similar time integral of ON and OFF, indicating charge conservation; 2) a left-shift in the V1/2 of the ON gating charge when compared to ionic currents; 3) a rising phase in the OFF gating charge after depolarizations to values >0 mV. These results represent the first description of Kv7.4 and Kv7.5 gating currents and may help to clarify the molecular consequence of disease-causing mutations affecting channel gating. Supported by NIH GM30376 and Telethon GGP07125.