Date Published: January 31, 2017
Publisher: Public Library of Science
Author(s): Lara Waldschmidt, Vera Junkereit, Robert Bähring, Mark S. Shapiro.
The transient outward current (Ito) in cardiomyocytes is largely mediated by Kv4 channels associated with Kv Channel Interacting Protein 2 (KChIP2). A knockout model has documented the critical role of KChIP2 in Ito expression. The present study was conducted to characterize in both sexes the dependence of Ito properties, including current magnitude, inactivation kinetics, recovery from inactivation and voltage dependence of inactivation, on the number of functional KChIP2 alleles. For this purpose we performed whole-cell patch-clamp experiments on isolated left ventricular cardiomyocytes from male and female mice which had different KChIP2 genotypes; i.e., wild-type (KChIP2+/+), heterozygous knockout (KChIP2+/-) or complete knockout of KChIP2 (KChIP2-/-). We found in both sexes a KChIP2 gene dosage effect (i.e., a proportionality between number of alleles and phenotype) on Ito magnitude, however, concerning other Ito properties, KChIP2+/- resembled KChIP2+/+. Only in the total absence of KChIP2 (KChIP2-/-) we observed a slowing of Ito kinetics, a slowing of recovery from inactivation and a negative shift of a portion of the voltage dependence of inactivation. In a minor fraction of KChIP2-/- myocytes Ito was completely lost. The distinct KChIP2 genotype dependences of Ito magnitude and inactivation kinetics, respectively, seen in cardiomyocytes were reproduced with two-electrode voltage-clamp experiments on Xenopus oocytes expressing Kv4.2 and different amounts of KChIP2. Our results corroborate the critical role of KChIP2 in controlling Ito properties. They demonstrate that the Kv4.2/KChIP2 interaction in cardiomyocytes is highly dynamic, with a clear KChIP2 gene dosage effect on Kv4 channel surface expression but not on inactivation gating.
A transient outward current (Ito) causes the initial (phase 1) repolarization of the action potential in ventricular cardiomyocytes , thereby controlling Ca2+ entry and excitation-contraction coupling . A large portion of Ito is mediated by voltage-dependent K+ (Kv) channels belonging to the Kv4 subfamily [3–7] associated with cytoplasmic Kv Channel Interacting Proteins (KChIPs; [6, 8, 9]). Different genes are coding for Kv4.1, Kv4.2 and Kv4.3 channel subtypes, respectively, with two splice variants of Kv4.3 . There are also different KChIP genes coding for KChIP1, KChIP2, KChIP3 and KChIP4, respectively, with different splice variants for each gene [11, 12]. The molecular correlates of Ito expression in cardiomyocytes have been extensively studied in several species (reviewed in ). Generally, there is abundant Kv4.2 and/or Kv4.3 expression and abundant KChIP2 expression, with different relative amounts depending on species, cardiac tissue and transmural cell layer .
We have shown in murine left ventricular myocytes a distinct KChIP2 gene dosage effect on Ito magnitude (i.e., amplitude and density). Inactivation properties (i.e., inactivation kinetics, kinetics of recovery from inactivation and voltage dependence of inactivation), on the other hand, were found to be similar in KChIP2+/+ and KChIP2+/- myocytes. Only in KChIP2-/- myocytes inactivation and recovery from inactivation were slowed and a portion of the voltage dependence of inactivation was shifted to more negative potentials. No qualitative differences concerning the KChIP2 genotype-dependent remodeling have been observed between male and female myocytes.