A perinuclear calcium compartment regulates cardiac myocyte hypertrophy

The pleiotropic Ca2+/calmodulin-dependent phosphatase calcineurin is a key regulator of pathological cardiac myocyte hypertrophy. The selective activation of hypertrophic calcineurin signaling under stress conditions has been attributed to compartmentation of Ca2+ signaling in cardiac myocytes. Here, perinuclear signalosomes organized by the scaffold protein muscle A-Kinase Anchoring Protein beta (mAKAP beta/AKAP6 beta) are shown to orchestrate local Ca2+ transients, inducing calcineurin-dependent NFATc nuclear localization and myocyte hypertrophy in response to beta-adrenergic receptor activation. Fluorescent biosensors for Ca2+ and calcineurin and protein kinase A (PKA) activity, both diffusely expressed and localized by nesprin-1 alpha to the nuclear envelope, are used to define an autonomous mAKAP beta signaling compartment in adult and neonatal rat ventricular myocytes. Notably, beta-adrenergic-stimulated perinuclear Ca2+ and PKA and CaN activity transients depended upon mAKAP beta expression, while Ca2+ elevation and PKA and CaN activity in the cytosol were mAKAP beta independent. Buffering perinuclear cAMP and Ca2+ prevented calcineurin-dependent NFATc nuclear translocation and myocyte hypertrophy, without affecting cardiac myocyte contractility. Additional findings suggest that the perinuclear Ca2+ transients were mediated by signalosome-associated ryanodine receptors regulated by local PKA phosphorylation. These results demonstrate the existence of a functionally independent Ca2+ signaling compartment in the cardiac myocyte regulating hypertrophy and provide a premise for targeting mAKAP beta signalosomes to prevent selectively cardiac hypertrophy in disease.