Effects and regulation of phosphatase inhibitor-1 in the heart and its therapeutic potential in heart failure

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Control of protein phosphorylation/dephosphorylation is an integral component of intracellular signal transduction. Phosphatase inhibitor-1 (I-1) is a prototypical mediator of cross-talk between kinases and phosphatases. Activated cAMP-dependent protein kinase (PKA) phosphorylates I-1 at Thr-35, converting it into a potent inhibitor of phosphatase-1. Inhibition of phosphatase-1 subsequently enhances PKA-mediated protein phosphorylation. We showed that adenovirally overexpression of I-1 does indeed amplify ß-agonist responses in adult rat cardiac myocytes. This was associated with increased phosphorylation of phospholamban (PLB), a key regulator of cardiac function. Conversely, in samples from failing human hearts we observed that downregulation and dephosphorylation of I-1 at Thr-35 was associated with decreased phosphorylation of PLB. In a recent study, phosphorylation of I-1 at Ser-67 has been reported to reduce its ability to inhibit phosphatase-1. Interestingly, a rise in intracellular Ca2+ could trigger the dephosphorylation of both phosphorylation sites by the same Ca2+-activated phosphatase 2B (calcineurin). The aim of this project is (i) to test whether overexpression of I-1 has different effects on phosphorylation of various PKA substrates (e.g. PLB as opposed to the ryanodine receptor), (ii) to evaluate I-1´s regulation by kinases/phosphatases in cardiac myocytes, (iii) to characterize I-1 overexpressing mice (cardiac specific I-1 TG), (iv) to generate and to characterize transgenic mice overexpressing mutated I-1 (Ser-67 replaced by Ala on an I-1 knockout background) under control of a regulatable promoter (Tet-Off system), and (v) to cross-breed I-1 TG mice with mice models with contractile dysfunction. The results are expected to extend our understanding of I-1´s role in cardiac myocyte signaling and contribute to answering the question whether I-1 may constitute a potential target in treating the depressed function of failing hearts.