Perinuclear Signaling and Cardiac Hypertrophy

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Stimulation of the b-adrenergic receptor (bAR) during the flight-or-fight response increases contractility through protein kinase A (PKA)-stimulated Ca2+ fluxes at the sarcoplasmic reticulum (SR). However, chronic stress on the heart leads to long-term stimulation of the bAR by increased circulating catecholamines, resulting in pathological remodeling due to Ca2+-mediated gene transcription. These divergent responses to bAR activation and Ca2+ signaling suggests that multiples pools of bAR-regulated PKA activity exist in the cardiac myocyte. We suggest that internal bARs at the nuclear envelope activate a PKA-stimulated perinuclear Ca2+ signals that induce pathological gene transcription without affecting cardiac contractility. Therefore, reagents that can block nuclear bAR and attenuate perinuclear Ca2+ transients should prevent disease initiation and/or progression without affecting contractility. Our preliminary data finds that PKA bound to the nuclear envelope located scaffolding protein muscle A-kinase anchoring protein b (mAKAPb) responds to internal bARs to initiate perinuclear Ca2+ transients that stimulate pathological gene transcription. The central hypothesis of this proposal is that targeting mAKAPb signalosomes will modulate nuclear b-AR-mediated PKA and Ca2+ responses that induce pathological disease, without inhibiting, and in fact, maintaining cardiac contractility. Specific Aim 1: Stimulation of a nuclear pool of Ca2+ is required for induction of cardiac hypertrophy. In this Aim, we will buffer the mAKAPb-stimulated perinuclear Ca2+ pool using a specifically localized Ca2+ binging protein in order to determine the importance of this pool for induction of cardiac hypertrophy in adult rat ventricular myoytes. Using adeno-associated virus to deliver the mAKAPb-targeted Ca2+ “buffer” to the cardiac myocytes in vivo, we will determine the effect on pathological remodeling induced by pressure overload and catecholamine infusion. Importantly, the effect of buffering perinuclear Ca2+ on contractility will be investigated. Specific Aim 2: Inhibition of mAKAPb-bound PKA blocks induction of cardiac hypertrophy. This Aim will use tools that selectively modulate only mAKAPb-bound PKA to demonstrate that the associated kinase is both necessary and sufficient for induction of cardiac hypertrophy in vitro and in vivo, while not regulating contractility. Specific Aim 3: A nuclear bAR receptor is responsible for cardiac hypertrophy. Using pharmacological inhibitors, mAKAP-targeted nanobodies that inhibit specifically nuclear b-AR activation, and siRNA against specific b-AR subtypes, we will test the hypothesis that internal, nuclear located bARs are responsible for induction of cardiac hypertrophy. However, modulating these receptors should not affect contractility. Through these Aims, this proposal will define a novel signaling compartment orchestrated by mAKAPb that is required for pathological gene transcription and induction of cardiac disease, but does not affect contractility. Furthermore, completion of this project will reveal how targeting mAKAPb signalosomes can be therapeutically beneficial in the prevention of cardiac remodeling and heart failure.