Novel Molecular Mechanism Involving α 1D (Cav1.3) L-Type Calcium Channel in Autoimmune-Associated Sinus Bradycardia

Abstract

Background— Congenital heart block (CHB) is an autoimmune disease that affects fetuses/infants born to mothers with anti-Ro/La antibodies (positive IgG). Although the hallmark of CHB is complete atrioventricular block, sinus bradycardia has been reported recently in animal models of CHB. Interestingly, knockout of the neuroendocrine α 1D Ca channel in mice results in significant sinus bradycardia and atrioventricular block, a phenotype reminiscent to that seen in CHB. Here, we tested the hypothesis that the α 1D Ca channel is a novel target for positive IgG. Methods and Results— Reverse transcription–polymerase chain reaction, confocal indirect immunostaining, and Western blot data established the expression of the α 1D Ca channel in the human fetal heart. The effect of positive IgG on α 1D Ca current ( I Ca-L ) was characterized in heterologous expression systems (tsA201 cells and Xenopus oocytes) because of the unavailability of α 1D -specific modulators. α 1D I Ca-L activated at negative potentials (between −60 and −50 mV). Positive IgG inhibited α 1D I Ca-L in both expression systems. This inhibition was rescued by a Ca channel activator, Bay K8644. No effect on α 1D I Ca-L was observed with negative IgG and denatured positive IgG. Western blot data showed that positive IgG binds directly to α 1D Ca channel protein. Conclusions— The data are the first to demonstrate (1) expression of the α 1D Ca channel in human fetal heart, (2) inhibition of α 1D I Ca-L by positive IgG, and (3) direct cross-reactivity of positive IgG with the α 1D Ca channel protein. Given that α 1D I Ca-L activates at voltages within the pacemaker’s diastolic depolarization, inhibition of α 1D I Ca-L in part may account for autoimmune-associated sinus bradycardia. In addition, Bay K8644 rescue of α 1D I Ca-L inhibition opens new directions in the development of pharmacotherapeutic approaches in the management of CHB.