Calcium Regulation of Kv4-Kchip Ion Channel Complexes

Abstract

The Kv4 family of A-type voltage-gated K+ channels play a unique role in the dendrites of hippocampal pyramidal neurons. In particular, Kv4.2 channels are key determinants of dendritic excitability and integration, spike timing-dependent plasticity, long-term potentiation, and learning. Downregulation of Kv4.2 channel expression occurs following hippocampal seizures and in epilepsy suggesting A-type currents as targets for novel therapeutics. In addition to pore forming Kv4 subunits, native hippocampal A-type currents require modulatory auxiliary subunits known as K-channel interacting proteins (KChIPs). KChIPs function to enhance Kv4 expression and activity. KChIP auxiliary proteins may convey Ca2+ sensitivity upon Kv4 channels as they are members of the EF-hand domain containing neuronal Ca2+ sensor protein family; however, it is currently unclear to what degree intracellular Ca2+ regulates Kv4.2-KChIP complexes. To assess this possibility, we expressed KChIP2 with Kv4.2 in HEK293T cells and performed whole-cell patch clamp recordings in either nominal or 10 μM Ca2+. In the presence of 10 μM Ca2+, we measured a 1.5 fold increase in Kv4.2 current density. Intriguingly, Ca2+ regulation was specific to short splice isoforms of KChIP2, KChIP2b and 2c. KChIP mRNAs contain multiple start sites and alternative exons that generate considerable N-terminal variation and functional diversity in shaping Kv4 currents. Sequence alignment of the variable N-termini of KChIP2 isoforms suggest a novel domain limits Ca2+ sensitivity of the longer KChIP2 isoforms, KChIP2a1 and KChIP2a.