Inverse Modulation of Neuronal K12.1 and K11.1 Channels by 4-Aminopyridine and NS1643.

The three members of the -like (Elk; K12.1-K12.3) family of voltage-gated K channels are predominantly expressed in neurons, but only little information is available on their physiological relevance. It was shown that K12.2 channels modulate excitability of hippocampal neurons, but no native current could be attributed to K12.1 and K12.3 subunits yet. This may appear somewhat surprising, given high expression of their mRNA transcripts in several brain areas. Native K12 currents may have been overlooked so far due to limited knowledge on their biophysical properties and lack of specific pharmacology. Except for K12.2, appropriate genetically modified mouse models have not been described; therefore, identification of K12-mediated currents in native cell types must rely on characterization of unique properties of the channels. We focused on recombinant human K12.1 to identify distinct properties of these channels. We found that K12.1 channels exhibited significant mode shift of activation, i.e., stabilization of the voltage sensor domain in a "relaxed" open state after prolonged channel activation. This mode shift manifested by a slowing of deactivation and, most prominently, a significant shift of voltage dependence to hyperpolarized potentials. In contrast to related K11.1, mode shift was not sensitive to extracellular Na, which allowed for discrimination between these isoforms. Sensitivity of K12.1 and K11.1 to the broad-spectrum K antagonist 4-aminopyridine was similar. However, 4-AP strongly activated K12.1 channels, but it was an inhibitor of K11 channels. Interestingly, the agonist of K11 channels NS1643 also differentially modulated the activity of these channels, i.e., NS1643 activated K11.1, but strongly inhibited K12.1 channels. Thus, these closely related channels are distinguished by inverse pharmacological profiles. In summary, we identified unique biophysical and pharmacological properties of K12.1 channels and established straightforward experimental protocols to characterize K12.1-mediated currents. Identification of currents in native cell types with mode shift that are activated through 4-AP and inhibited by NS1643 can provide strong evidence for contribution of K12.1 to whole cell currents.