Investigating the function of KCNE4 in cardiac physiology
Ciampa, Erin Julia
KCNE4 is a potassium channel modulating protein that can dramatically inhibit distinct potassium currents, but we lack a clear understanding its mechanism for doing so and its physiologic significance. In this project we sought new understanding of the physiological functions of KCNE4, through identification of its protein interacting partners and assessment of the consequences of Kcne4 knockout in mouse cardiac physiology. A membrane-based yeast two-hybrid screen identified 20 putative interacting partners of KCNE4. The ‘hit’ with the most obvious potential for functional intersection with KCNE4 was calmodulin (CaM), a known ion channel modulator. We subsequently demonstrated a Ca2+-dependent biochemical interaction between KCNE4 and CaM and that KCNQ1 modulation by KCNE4 is impaired both upon mutation of the CaM-interaction site of KCNE4 and by acutely chelating intracellular calcium to displace CaM from KCNE4. These findings suggest a connection between the mechanism of KCNQ1 inhibition by KCNE4 and the activating effect of CaM on the channel. Whereas we had previously assumed that the inhibition of KCNQ1 by KCNE4 is caused by a direct effect of KCNE4 on the channel, these data introduce the new possibility that KCNE4 inhibits KCNQ1 by disrupting CaM-mediated activation. Analysis of a Kcne4-null mouse constituted another approach to investigating the physiologic significance of KCNE4. We hypothesized that Kcne4 may be an endogenous negative regulator of repolarizing currents in the cardiac action potential, and that Kcne4-null mice might display shortened repolarization time, with possible implications for arrhythmia susceptibility or excitation-contraction coupling. Electrocardiographic analysis revealed that Kcne4-null mice under isoflurane anesthesia have a shortened QTc interval compared with wild-type littermates. Our hypothesis was also supported by the observation of shortened action potential duration in isolated ventricular myocytes from Kcne4-null mice. Further, echocardiography studies demonstrated that conscious Kcne4-null mice have increased left ventricular internal diameter during diastole and systole and impaired myocardial contractility. Collectively, these studies suggest KCNE4 may contribute to cardiac physiology as a Ca2+-sensitive modulator of repolarizing currents, with possible downstream effects on excitation-contraction coupling and myocardial contractility.