Regulation of Kv4.3 Current by KChIP2 Splice Variants

Abstract

Background-The transient outward potassium current (I to ) encoded by the Kv4 family of potassium channels is important in the repolarization of cardiac myocytes. KChIPs are a recently identified group of Ca 2ϩ -binding accessory subunits that modulate Kv4-encoded currents. KChIP2 is the only family member expressed in the heart. Methods and Results-We previously cloned 2 novel splice variants of KChIP2 from human heart, named KChIP2S and KChIP2T. The transmural distribution of KChIP2 mRNA and protein in human and canine left ventricle was examined using kinetic RT-PCR and Western blots in the same tissues. A steep gradient of mRNA with greater KChIP2 expression in the epicardium was observed. However, no gradient of immunoreactive protein was observed. Immunocytochemistry reveals KChIP2 expression in the t-tubules and the nucleus. The predominant effects of all 3 KChIP2 splice variants on hKv4.3-encoded current are to increase the density, slow the current decay in a Ca 2ϩ -dependent manner, and hasten recovery from inactivation in a splice variant-specific fashion. Conclusions-A family of KChIP2 proteins is expressed in human hearts that exhibits differential modulation of hKv4.3 current in a Ca 2ϩ -dependent fashion. The effect of KChIP2 on the biophysical properties of expressed Kv4.3 current and the absence of a gradient of protein across the ventricular wall suggest that KChIP2 is either not a requisite component of human or canine ventricular I to or that its functional effect is being affected or additionally modified by other factors present in myocardial cells.

Three-dimensional structure of the KChIP1–Kv4.3 T1 complex reveals a cross-shaped octamer

“…Different KChIP isoforms affect α-subunit kinetics in distinct ways [41][42][43] . Most KChIP variability resides in the nonconserved N-terminal region absent from our structure.…”

“…Most KChIP variability resides in the nonconserved N-terminal region absent from our structure. It seems likely that this KChIP portion is responsible for the varied functional effects [41][42][43] . A truncated KChIP (KChIP2d) lacking the entire protein except for EF4 has been reported to affect channel properties 16,44 .…”

“…Exactly how KChIPs, T1 and the α subunit's transmembrane segments are mechanistically coupled remains to be explained. In light of the primacy of site 2 for modulating inactivation and recovery from inactivation, the interaction of KChIP with more than one T1 subunit offers a likely means by which KChIPs could influence gating.Different KChIP isoforms affect α-subunit kinetics in distinct ways [41][42][43] . Most KChIP variability resides in the nonconserved N-terminal region absent from our structure.…”

Post-transcriptional gene silencing of KChIP2 and Navβ1 in neonatal rat cardiac myocytes reveals a functional association between Na and Ito currents

“…This value was then normalized to mRNA levels measured from NRVMs transfected with either the non-silencing control siRNA or the control single base pair mismatch siRNA for the appropriate transcript [2]. For Western Blotting and co-immunoprecipitation, cell lysates were prepared as previously described [10].…”

“…Heterologous expression of Kv4 α-subunits generates currents similar, but not identical to native I to , suggesting that accessory subunits functionally contribute to the channel phenotype. K channel interacting protein (KChIP), specifically KChIP2 is expressed in the heart and modulates expressed Kv4 currents but again, does not reproduce the native I to with complete fidelity [2] [3]. These data suggest the existence of other factors in cardiac myocytes that are essential to complete functional expression of the native I to .…”