Chronic ß-adrenergic stimulation reverses depressed Ca handling in mice overexpressing inhibitor-2 of protein phosphatase 1.

RATIONALE: A higher expression/activity of type 1 serine/threonine protein phosphatase 1 (PP1) may contribute to dephosphorylation of cardiac regulatory proteins triggering the development of heart failure. OBJECTIVE: Here, we tested the putatively protective effects of PP1 inhibitor-2 (I2) overexpression using a heart failure model induced by chronic ß-adrenergic stimulation. METHODS AND RESULTS: Transgenic (TG) and wild-type (WT) mice were subjected to isoprenaline (ISO) or isotonic NaCl solution supplied via osmotic minipumps for 7 days. I2 overexpression was associated with a depressed PP1 activity. Basal contractility was unchanged in catheterized mice and isolated cardiomyocytes between TGNaCl and WTNaCl. TGISO mice exhibited more fibrosis and a higher expression of hypertrophy marker proteins as compared to WTISO. After acute administration of ISO, the contractile response was accompanied by a higher sensitivity in TGISO as compared to WTISO. In contrast to basal contractility, the peak amplitude of [Ca]i and SR Ca load were reduced in TGNaCl as compared to WTNaCl. These effects were normalized to WT levels after chronic ISO stimulation. Cardiomyocyte relaxation and [Ca]i decay kinetics were hastened in TGISO as compared to WTISO, which can be explained by a higher phospholamban phosphorylation at Ser16. Chronic catecholamine stimulation was followed by an enhanced expression of GSK3ß, whereas the phosphorylation at Ser9 was lower in TG as compared to the corresponding WT group. This resulted in a higher I2 phosphorylation that may reactivate PP1. CONCLUSION: Our findings suggest that the basal desensitization of ß-adrenergic signaling and the depressed Ca handling in TG by inhibition of PP1 is restored by a GSK3ß-dependent phosphorylation of I2.

Cardiac function is regulated by B56α-mediated targeting of protein phosphatase 2A (PP2A) to contractile relevant substrates.

Dephosphorylation of important myocardial proteins is regulated by protein phosphatase 2A (PP2A), representing a heterotrimer that is comprised of catalytic, scaffolding, and regulatory (B) subunits. There is a multitude of B subunit family members directing the PP2A holoenzyme to different myocellular compartments. To gain a better understanding of how these B subunits contribute to the regulation of cardiac performance, we generated transgenic (TG) mice with cardiomyocyte-directed overexpression of B56α, a phosphoprotein of the PP2A-B56 family. The 2-fold overexpression of B56α was associated with an enhanced PP2A activity that was localized mainly in the cytoplasm and myofilament fraction. Contractility was enhanced both at the whole heart level and in isolated cardiomyocytes of TG compared with WT mice. However, peak amplitude of [Ca]i did not differ between TG and WT cardiomyocytes. The basal phosphorylation of cardiac troponin inhibitor (cTnI) and the myosin-binding protein C was reduced by 26 and 35%, respectively, in TG compared with WT hearts. The stimulation of ß-adrenergic receptors by isoproterenol (ISO) resulted in an impaired contractile response of TG hearts. At a depolarizing potential of -5 mV, the ICa,L current density was decreased by 28% after administration of ISO in TG cardiomyocytes. In addition, the ISO-stimulated phosphorylation of phospholamban at Ser(16) was reduced by 27% in TG hearts. Thus, the increased PP2A-B56α activity in TG hearts is localized to specific subcellular sites leading to the dephosphorylation of important contractile proteins. This may result in higher myofilament Ca(2+) sensitivity and increased basal contractility in TG hearts. These effects were reversed by ß-adrenergic stimulation.