Ancillary subunits KChIP2c and DPP6 differentially modulate the inhibition of Kv4.2 channels by riluzole

In native tissue, Kv4.2 channels associate with the ancillary subunits Kv channels interacting proteins (KChIPs) and dipeptidyl peptidase-related proteins (DPPs) to evoke rapidly activating/inactivating currents in the heart (I_to) and brain (I_A). Despite extensive knowledge of Kv4.2 biophysical modulation by auxiliary subunits, the pharmacological effects, especially those related to the co-expressed subunit and the state-dependent drug binding, remain unknown. Here, we investigated the effects of co-expressing KChIP2c or DPP6 on the pharmacological inhibition of Kv4.2 channels by riluzole. Riluzole inhibited Kv4.2, Kv4.2/DPP6, and Kv4.2/KChIP2c channels in a voltage-independent manner, with potency ranked as Kv4.2/DPP6 > Kv4.2 > Kv4.2/KChIP2c. Additionally, to a dissimilar extent, riluzole inhibited the channels from the closed state, left-shifted the inactivation curves, and enhanced the closed-state inactivation (differently modifying the rate constants of this latter). More divergent effects were observed: the inactivation kinetics was accelerated in Kv4.2 and Kv4.2/KChIP2c but not in Kv4.2/DPP6; only in Kv4.2/KChIP2c, the activation curve was left-shifted and the recovery from inactivation was decelerated; and the closed-state inactivation developed faster in Kv4.2 and Kv4.2/DPP6 but was slower in Kv4.2/KChIP2c channels. Notably, inhibition from the closed-inactivated state was more rapid than from the closed state for the three channels. We conclude that riluzole can elicit differential effects on native Kv4.2 channels depending on the presence of distinct ancillary subunits. These findings contribute to our understanding of the interplay between auxiliary subunits and pharmacological regulation of α-subunits of ion channels, highlighting the role of the former by modulating the organ-specific effects of channel-interacting drugs.