Ca-v beta(2) transcription start site variants modulate calcium handling in newborn rat cardiomyocytes

Abstract

In the heart, the main pathway for calcium influx is mediated by L-type calcium channels, a multi-subunit complex composed of the pore-forming subunit Ca(V)1.2 and the auxiliary subunits Ca-V alpha(2)delta(1) and Ca-V beta(2). To date, five distinct Ca-V beta(2) transcriptional start site (TSS) variants (Ca-V beta(2a-e)) varying only in the composition and length of the N-terminal domain have been described, each of them granting distinct biophysical properties to the L-type current. However, the physiological role of these variants in Ca2+ handling in the native tissue has not been explored. Our results show that four of these variants are present in neonatal rat cardiomyocytes. The contribution of those Ca-V beta(2) TSS variants on endogenous L-type current and Ca2+ handling was explored by adenoviral-mediated overexpression of each Ca-V beta(2) variant in cultured newborn rat cardiomyocytes. As expected, all Ca-V beta(2) TSS variants increased L-type current density and produced distinctive changes on L-type calcium channel (LTCC) current activation and inactivation kinetics. The characteristics of the induced calcium transients were dependent on the TSS variant overexpressed. Moreover, the amplitude of the calcium transients varied depending on the subunit involved, being higher in cardiomyocytes transduced with Ca-V beta(2a) and smaller in Ca-V beta(2d). Interestingly, the contribution of Ca2+ influx and Ca2+ release on total calcium transients, as well as the sarcoplasmic calcium content, was found to be TSS-variant-dependent. Remarkably, determination of atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) messenger RNA (mRNA) abundance and cell size change indicates that Ca-V beta(2) TSS variants modulate the cardiomyocyte hypertrophic state. In summary, we demonstrate that expression of individual Ca-V beta(2) TSS variants regulates calcium handling in cardiomyocytes and, consequently, has significant repercussion in the development of hypertrophy.