PP2A-B56alpha is a key determinant of cardiac protein phosphorylation and functional responses to beta-adrenergic signalling

B56alpha is a protein phosphatase 2A (PP2A) regulatory subunit which modulates the heart’s inotropic response to acute beta-adrenergic receptor (beta-AR) stimulation, although knowledge of underlying molecular mechanisms is limited. In this study, mice deficient for B56alpha and wildtype controls received an intraperitoneal injection of isoproterenol (0.1 mg/kg) to activate beta-AR signalling in vivo. Hearts were explanted two minutes post-injection and processed for quantitative phosphoproteomics. We identified >200 proteins that were differentially phosphorylated between genotypes, including 26 hyperphosphorylated proteins harbouring a B56 binding motif (i.e. putative substrates). Gene Ontology enrichment analysis revealed ‘regulation of release of sequestered Ca2+ into cytosol by sarcoplasmic reticulum’, ‘cardiac muscle contraction’ and ‘cardiac muscle hypertrophy’ as key processes likely to be impacted by B56alpha deficiency. In vitro, loss of B56alpha in cardiomyocytes blunted acute isoproterenol-induced increases in intracellular calcium transient amplitude, confirming that B56alpha plays a key role in calcium handling. In vivo, loss of B56alpha protected mice from developing systolic dysfunction in response to sustained isoproterenol infusion (60 mg/kg/day for 14 days), despite comparable increases in heart mass. These findings reaffirm a key role for B56alpha as a mediator of physiologically important cardiac responses to beta-AR stimulation and reveal potential new molecular mechanisms for this regulatory function, including putative cardiac B56alpha substrates.