A polybasic motif in alternatively spliced KChIP2 isoforms prevents Ca2 regulation of Kv4 channels

Abstract

The Kv4 family of A-type voltage-gated K channels regulates the excitability in hippocampal pyramidal neuron dendrites and are key determinants of dendritic integration, spike timing-Dependent plasticity, long-term potentiation, and learning. Kv4.2 channel expression is down-regulated following hippocampal seizures and in epilepsy, suggesting A-type currents as therapeutic targets. In addition to pore-forming Kv4 subunits, modulatory auxiliary subunits called K channel–interacting proteins (KChIPs) modulate Kv4 expression and activity and are required to recapitulate native hippocampal A-type currents in heterologous expression systems. KChIP mRNAs contain multiple start sites and alternative exons that generate considerable N-terminal variation and functional diversity in shaping Kv4 currents. As members of the EF-hand domain– containing neuronal Ca2 sensor protein family, KChIP auxiliary proteins may convey Ca2 sensitivity upon Kv4 channels; however, to what degree intracellular Ca2 regulates KChIP–Kv4.2 complexes is unclear. To answer this question, we expressed KChIP2 with Kv4.2 in HEK293T cells, and, with whole-cell patch– clamp electrophysiology, measured an 1.5-fold increase in Kv4.2 current density in the presence of elevated intracellular Ca2. Intriguingly, the Ca2 regulation of Kv4 current was specific to KChIP2b and KChIP2c splice isoforms that lack a putative polybasic domain that is present in longer KChIP2a1 and KChIP2a isoforms. Site-Directed acidification of the basic residues within the polybasic motif of KChIP2a1 rescued Ca2-mediated regulation of Kv4 current density. These results support divergent Ca2 regulation of Kv4 channels mediated by alternative splicing of KChIP2 isoforms. They suggest that distinct KChIP–Kv4 interactions may differentially control excitability and function of hippocampal dendrites.