Protein phosphatase 2A regulatory subunit B56α limits phosphatase activity in the heart

Protein phosphatase 2A (PP2A) is a serine/threonine-selective holoenzyme composed of a catalytic, scaffolding, and regulatory subunit. In the heart, PP2A activity is requisite for cardiac excitation-contraction coupling and central in adrenergic signaling. We found that mice deficient in the PP2A regulatory subunit B56 alpha (1 of 13 regulatory subunits) had altered PP2A signaling in the heart that was associated with changes in cardiac physiology, suggesting that the B56 alpha regulatory subunit had an autoinhibitory role that suppressed excess PP2A activity. The increase in PP2A activity in the mice with reduced B56 alpha expression resulted in slower heart rates and increased heart rate variability, conduction defects, and increased sensitivity of heart rate to parasympathetic agonists. Increased PP2A activity in B56 alpha(+/-) myocytes resulted in reduced Ca2+ waves and sparks, which was associated with decreased phosphorylation (and thus decreased activation) of the ryanodine receptor RyR(2), an ion channel on intracellular membranes that is involved in Ca2+ regulation in cardiomyocytes. In line with an autoinhibitory role for B56 alpha, in vivo expression of B56 alpha in the absence of altered abundance of other PP2A subunits decreased basal phosphatase activity. Consequently, in vivo expression of B56 alpha suppressed parasympathetic regulation of heart rate and increased RyR(2) phosphorylation in cardiomyocytes. These data show that an integral component of the PP2A holoenzyme has an important inhibitory role in controlling PP2A enzyme activity in the heart.