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

Abstract

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 B56α (1 of 13 regulatory subunits) had altered PP2A signaling in the heart that was associated with changes in cardiac physiology, suggesting that the B56α regulatory subunit had an autoinhibitory role that suppressed excess PP2A activity. The increase in PP2A activity in the mice with reduced B56α 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 B56α+/− myocytes resulted in reduced Ca2+ waves and sparks, which was associated with decreased phosphorylation (and thus decreased activation) of the ryanodine receptor RyR2, an ion channel on intracellular membranes that is involved in Ca2+ regulation in cardiomyocytes. In line with an autoinhibitory role for B56α, in vivo expression of B56α in the absence of altered abundance of other PP2A subunits decreased basal phosphatase activity. Consequently, in vivo expression of B56α suppressed parasympathetic regulation of heart rate and increased RyR2 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.

Roles and regulation of protein phosphatase 2A (PP2A) in the heart

“…In fact, many cardiac phosphoproteins display multiple phosphorylation sites [16]. The central role of kinase/phosphatase balance for normal cardiac function is clearly illustrated by defects in protein phosphorylation in both congenital and acquired forms of human disease including arrhythmia and heart failure [19–21]. As noted above, due to their broad suppression of kinase activity, beta-blockers have been a mainstay for treatment of cardiac diseases since their discovery in the 1960's [4–6].…”

“…For example, PP2A reduces cAMP-dependent protein kinase A (PKA)-dependent phosphorylation of Cav1.2, resulting in decreased I Ca,L and ultimately reduced inward (depolarizing) current [37]. Activities of PP2A and PP1 are further critical for the dephosphorylation of multiple residues of RyR2, the primary calcium-release channel of the myocyte sarcoplasmic reticulum(SR).Whereas PKA-(via RyR2 Serine 2808) and Ca 2+ /calmodulin-dependent kinase (CaMKII)-(via RyR2 Serine 2814) dependent phosphorylation favor increased RyR2 channel open probability and Ca 2+ release [38,39], PP2A and PP1 reverse this process resulting in reduced RyR2 activity [21]. Notably, altered regulation of this axis resulting in RyR2 hyperphosphorylation and increased RyR2 open probability is directly linked to potentially fatal human catecholaminergic polymorphic ventricular tachycardia (CPVT) [40,41].…”

“…As noted above, PP2A is targeted to the cardiac dyad by the regulatory subunits PR130 and B56α,where it modulates RyR2 phosphorylation [60–62]. Interestingly, B56α that is trafficked to the cardiac dyad by ankyrin-B [43,63] appears to act as an auto-inhibitor of PP2A activity at the dyad causing increased phosphorylation at RyR2 Ser2808 and Ser2814 [21]. Other PP2A populations are targeted to the nucleus via B56γ [64].…”

“…Cardiac phenotypes observed in models of altered cardiac PP2A-Cα expression are summarized in Table 2. Finally, it is important to note that both in vivo and in vitro studies have illustrated that select subunit activity is critical for mature PP2A holoenzyme function [21,60,62,67]. However, more critical are the relative local levels of PP2A subunits.…”

“…However, more critical are the relative local levels of PP2A subunits. For example, while one might predict that loss of the PP2A regulatory subunit would decrease PP2A activity [60, 62], other structural and functional studies suggest that it instead may increase enzymatic activity [21,67,71]. Further, chronic overexpression or deletion of a single subunit, while acutely altering PP2A activity, may result in transcriptional remodeling to compensate for altered signaling.…”