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.

BACKGROUND.Fasting and NAD + -boosting compounds including NAD + precursor nicotinamide riboside (NR) confer anti-inflammatory effects. However, the underlying mechanisms and therapeutic potential are incompletely defined. METHODS.We explored the underlying biology in myeloid cells from healthy volunteers following in-vivo placebo or NR administration and subsequently tested the findings in-vitro in monocytes extracted from subjects with systemic lupus erythematosus (SLE).RESULTS. RNA sequencing of unstimulated and lipopolysaccharide (LPS)-activated monocytes implicate NR in the regulation of autophagy and type I interferon signaling. In primary monocytes NR blunts LPS-induced IFNb production and genetic or pharmacologic disruption of autophagy phenocopies this effect. Given NAD + is a co-enzyme in oxidoreductive reactions, metabolomics was performed and identified that NR increased inosine level. Inosine supplementation similarly blunts autophagy and IFNb release. Finally, as SLE exhibits type I interferon dysregulation, we assessed the NR effect on SLE patient monocytes and found that NR reduces autophagy and interferon-b release.CONCLUSION. We conclude that NR, in an NAD + -dependent manner and in part via inosinesignaling, mediates suppression of autophagy and attenuates type I interferon in myeloid cells and identifies NR as a potential adjunct for SLE management.

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