Kcnd2 Mutation Associated with Autism and Epilepsy Impairs Inactivation Gating in Kv4.2 K+ Channels

Abstract

Kv4 subunits underlie somatodendritic A-type K+ currents in neurons. A de novo Kv4.2 mutation, V404M, was identified by whole exome sequencing in a family with identical twins affected by autism and intractable seizures. V404 immediately follows the S6 PVP motif, which is involved in activation and inactivation gating. The effects of V404M on channel function were characterized in oocytes with or without KChIP3a and DPP10a, which associate with Kv4.2 in vivo. V404M currents reached peak amplitude more slowly and decayed more slowly and less completely than wild-type currents. Because the time to peak is determined by the rates of opening and inactivation, a longer time to peak likely reflects slower inactivation. The twins' genomes contain one wild-type and one mutant allele. To characterize channels containing both subunit types, wild type and V404M were co-expressed at a 1:1 ratio. Wild type/V404M currents resembled V404M alone indicating that the functional effects of V404M are dominant. When V404M was co-expressed with KChIP3a, little inactivation occurred during a 1 s pulse. Because KChIP3a prevents or inhibits open-state N-type inactivation, these results indicate that V404M significantly impairs closed-state inactivation. Co-expression with DPP10a confers fast open-state inactivation. However, slower closed-state inactivation was evident in V404M channels in the presence of DPP10a, indicating that V404M does not interfere with open-state inactivation conferred by DPP10a and that DPP10a does not mask the effect of V404M on closed-state inactivation. We conclude that the V404M mutation dramatically slows closed-state inactivation. In Kv4 channels, closed state inactivation likely results from uncoupling between the voltage sensor and the pore gate. We propose that replacing V404 with the larger methionine residue strengthens the physical interaction between the gate and voltage sensor so that opening is preferred over closed-state inactivation.