Phospholamban ablation in hearts expressing the high affinity SERCA2b isoform normalizes global Ca2+ homeostasis but not Ca2+-dependent hypertrophic signaling

Abstract

Cardiomyocytes from failing hearts exhibit reduced levels of the sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA) and/or increased activity of the endogenous SERCA inhibitor phospholamban. The resulting reduction in the Ca2+ affinity of SERCA impairs SR Ca2+ cycling in this condition. We have previously investigated the physiological impact of increasing the Ca2+ affinity of SERCA by substituting SERCA2a with the higher affinity SERCA2b pump. When phospholamban was also ablated, these double knockouts (DKO) exhibited a dramatic reduction in total SERCA levels, severe hypertrophy, and diastolic dysfunction. We presently examined the role of cardiomyocyte Ca2+ homeostasis in both functional and structural remodeling in these hearts. Despite the low SERCA levels in DKO, we observed near-normal Ca2+ homeostasis with rapid Ca2+ reuptake even at high Ca2+ loads and stimulation frequencies. Well-preserved global Ca2+ homeostasis in DKO was paradoxically associated with marked activation of the Ca2+-dependent nuclear factor of activated T-cell-calcineurin pathway known to trigger hypertrophy. No activation of the MAP kinase signaling pathway was detected. These findings suggest that local changes in Ca2+ homeostasis may play an important signaling role in DKO, perhaps due to reduced microdomain Ca2+ buffering by SERCA2b. Furthermore, alterations in global Ca2+ homeostasis can also not explain impaired in vivo diastolic function in DKO. Taken together, our results suggest that normalizing global cardiomyocyte Ca2+ homeostasis does not necessarily protect against hypertrophy and heart failure development and that excessively increasing SERCA Ca2+ affinity may be detrimental.