Alternate routes of calcium entry mediating pathological cardiac hypertrophy

Pathological cardiac hypertrophy is associated with an increased risk of heart failure and cardiovascular mortality. As hypertrophy affects tens of millions of Americans, inhibition of hypertrophy is one of the major therapeutic goals of cardiology. The induction of hypertrophy requires calcium (Ca2+) -regulated gene expression. However, it is not known how myocytes distinguish between the Ca2+ signals that regulate contraction and those that lead to cardiac hypertrophy. We hypothesized that myocytes achieve Ca2+ signaling specificity by spatially segregating the Ca2+ fluxes that trigger contraction from those that induce hypertrophy, perhaps by employing different ion channels for each process. While Ca2+ flux through the CaV1.2 channel is known to regulate myocyte contraction, no calcium channel has yet been shown to specifically activate hypertrophy.; To identify such a channel, we used in vitro neonatal rat ventricular myocytes to screen an RNA-interference (RNAi) library for ion channels that mediate Ca2+-dependent gene expression in response to hypertrophic stimuli. We identified several ion channels that are selectively linked to hypertrophic gene expression, including transient receptor potential C3 (TrpC3). RNAi-mediated knockdown of TrpC3 decreases expression of hypertrophy-associated genes such as the A- and B-type natriuretic peptides (ANP and BNP) in response to numerous hypertrophic stimuli, while TrpC3 overexpression increases BNP expression. Furthermore, stimuli that induce hypertrophy dramatically increase TrpC3 mRNA levels. Importantly, whereas TrpC3-knockdown strongly reduces gene expression associated with hypertrophy, it has a negligible effect on cell size and on myocyte beating. These results suggest that Ca2+ influx through TrpC3 channels specifically increases transcription of genes associated with hypertrophy but does not regulate the signaling pathways that control cell size or contraction. As we also found TrpC3 channels to have a distinct intracellular localization from that of CaV1.2, our findings support the spatial segregation model of myocyte Ca2+ specificity. Additionally, our results suggest that TrpC3 may represent an important therapeutic target for the treatment of cardiac hypertrophy and heart failure.