L-Type Ca2+ Channel Cavb Subunits Associate with and Differentially Regulate the Cardiac Cav3.2 T-Type Ca2+ Channel Currents

Abstract

Low voltage activated T-type calcium channels (TTCC) play a pivotal role in the developing heart. Although the TTCC isoforms, Cav3.1 and Cav3.2, underlie cardiac TTCC current (ICa,T) and are expressed in atrial and ventricular myocytes during development, their expression and roles recede in the adult heart. However, previous studies have demonstrated the re-expression of ICa,T in pathological cardiac hypertrophy, suggesting that TTCCs contribute to the altered Ca2+ cycling and signaling in these pathological conditions. In addition, the reported altered expression of some Cavβ subunits (specifically β1 and β2 subunits) of the high voltage activated L-type calcium channels (LTCC) has been reported in hypertrophy and heart failure. We hypothesize that the altered expression of the Cavβ subunits in cardiac hypertrophy and heart failure results in the altered coupling and regulation of ICa,T. We found an increase in the Cavβ1 and Cavβ3 mRNA and Cavβ2 and Cavβ4 subunit isoform at the protein level in mouse ventricles in a transthoracic aortic constriction (TAC) induced pathological cardiac hypertrophy model. Whole-cell patch clamp electrophysiology using transiently transfected HEK293 cells revealed that co-expression of Cavβ1 or Cavβ2 with Cav3.2 channel isoform resulted in a significant increase in peak ICav3.2 and a rightward shift in the V1/2 of activation and significantly slower inactivation of ICav3.2. On the other hand, co-expression of Cavβ3 or Cavβ4 significantly reduced the peak ICav3.2. In contrast, co-expression of Cavβ isoforms did not alter ICav3.1. Furthermore, co-immunoprecipitation studies in transiently transfected HEK293 cells also demonstrated that the Cav3.2 channel separately co-immunoprecitated with anti-Cavβ1 or anti-Cavβ2 antibody. In conclusion, our data suggest that the Cavβ1 and Cavβ2 subunits of the LTCC may regulate ICav3.2 in cardiomyocytes during pathological cardiac hypertrophy.