Non-canonical Role of CaVα2δ1 in Cardiac Hypertrophy

Abstract

Excessive neurohormonal activity contributes to the progression of pathological hypertrophy and subsequent transition to heart failure. Although increased diastolic Ca2+ represents a hallmark feature of heart failure, the role of L-type CaV1.2 channels in this process remains controversial. CaV1.2 is an oligomeric protein complex formed by the distinctive pore-forming subunit CaVα1 with auxiliary subunits (CaVβ, calmodulin, and CaVα2δ1). CaVα2δ1 is a large extracellular GPI-anchored protein that facilitates channel activation at physiological voltages and stimulates (≈ 5-fold) peak current density. The present study examined the time-dependent changes in the expression of CaV auxiliary subunits in an in vitro model of cardiac hypertrophy. Cultured 1-day old neonatal rat ventricular cardiomyocytes (NRVMs) were treated for 24-48 h with 1 μM norepinephrine or 50 nM endothelin-1. In the presence of norepinephrine or endothelin-1, cardiac troponin-T protein levels and cell surface area (confocal imaging) were increased 1.8 ± 0.3-fold (n=3) and 1.5 ± 0.3-fold (n=4), respectively. Analysis of the CaV1.2 subunits revealed that CaVα2δ1 protein levels were 1.4 ± 0.15-fold higher (n=3) in hypertrophic cardiomyocytes compared to untreated NRVMs without a concomitant change in the expression of CaVα1 and CaVβ subunits. CaVα2δ1 was identified at the plasma membrane and unexpectedly detected at the perinuclear region co-localized with calreticulin. The increased expression of CaVα2δ1 protein levels persisted in hypertrophic NRVMs treated with 1 μM nifedipine. Lastly, confocal imaging identified modest CaV1.2 and CaVα2δ1 immunoreactivity in ventricular fibroblasts. Experiments are currently examining the identity of CaVα2δ1 protein partners in NRVMs. Collectively, these data demonstrate that the heavily glycosylated CaVα2δ1 protein is upregulated in hypertrophic NRVMs and localization at the perinuclear region may contribute in part to the progression of pathological cardiac hypertrophy via a potential non-canonical paradigm.