Differential Integration of DPLP Protein Variants Regulates Inactivation Kinetics of Neuronal A-Type Current

Abstract

Voltage-sensitive potassium channel complexes consisting of Kv4 pore-forming subunits and modulatory subunits mediate the neuronal subthreshold A-type current (ISA), a regulator of membrane excitability and action potential firing patterns. Both the KChIP (Kv channel-interacting proteins) and DPLP (dipeptidyl peptidase-like protein) modulatory subunits are expressed as N-terminal variants, and two DPLP variants (DPP10a, DPP6a) possess the unusual property of inducing similar fast inactivation kinetics of Kv4 channels in reconstitution studies. To investigate whether their effects are similar on native ISA, we characterized ISA from DPP10a-expressing cortical layer II/III pyramidal neurons (CtxPN) and DPP6a-expressing cerebellar granule neurons (CbGN) in rat brain slices. Surprisingly, CtxPN ISA and CbGN ISA differ significantly in their inactivation kinetics. CtxPN ISA undergo mono-exponential decay (τ = ∼8 ms); CbGN ISA, bi-exponential decay (at +36 mV: τ-1 = ∼11 ms, τ-2 = ∼120 ms). While CtxPN ISA resembles heterologously expressed Kv4.2+KChIP3a+DPP10a current, CbGN ISA is not recapitulated by the Kv4.2+KChIP3a+DPP6a current. Since CbGN reportedly also express robust levels of other DPP6 variants competing for Kv4 channels, we quantitated the levels of different DPP6 isoforms by qRT-PCR and determined their percentage contributions: DPP6a (DPP6-E) = ∼32%, DPP6b (DPP6-K) = ∼42%, DPP6c (DPP6-L) = ∼8%, and DPP6d (DPP6-S) = ∼18%. Since DPP6a and DPP6b variants together constitute the majority of the CbGN DPP6 species, we performed coexpression studies to investigate whether CbGN ISA is mediated by ternary complex channels with these DPP6 variants. We propose that relative ratios of DPLP auxiliary subunit isoforms likely contribute to the variability of ISA inactivation kinetics between neuronal populations.