Abstract 1057: Specific Targeting of Gi Signaling in Normal and Dysfunctional Myocardium

Abstract

Evidence has accumulated to suggest that cardiomyocyte apoptosis contributes significantly to the progression of heart failure. A salient characteristic of the failing heart is an upregulation of the adenylyl cyclase inhibitory G protein alpha subunit, Galpha(i2); however this upregulation has yet to be established as beneficial or deleterious. Of note, G protein-coupled receptor (GPCR) signaling through Gi has been implicated in the cardioprotective properties of adenosine, opioids, and bradykinin; however to date there has been no in vivo evidence that increased Gi in ischemic myocardium is cardioprotective. We have created a novel molecular tool to specifically address this question and other Gi-related mechanisms in the heart. We have developed a Gi inhibitor, GiCT - the carboxyl-terminal 63 amino acids of Galphai(i2), which represents the region of Gi that interacts specifically with GPCRs. Gi selectivity of GiCT has been shown in vitro as Gi signals are inhibited while Gs and Gq signals are not. Thus, a class-specific Gi inhibitor has been developed. We next created a transgenic mouse line with inducible, cardiac-specific expression of GiCT (Tg-GiCT mice). At baseline, Tg-GiCT mice display a structural and functional phenotype consistent with non-transgenic (nTg) mice. However, when subjected to stress in the form of myocardial ischemia/reperfusion (I/R) injury, Tg-GiCT mice demonstrate a significant increase in infarct size as compared to nTg mice. Furthermore, Tg-GiCT mice demonstrate increased myocardial apoptosis in response to I/R, with a resultant decrease in contractile performance. These results indicate first that ischemic damage induces apoptotic signals that can be countered by GPCR signaling specifically through Gi. Moreover, the beneficial survival effects of Gi signaling on the prevention of myocardial apoptosis in ischemia appears to outweigh any potential detrimental effects that Gi upregulation may have on contractility through inhibition of cAMP. Overall, our results demonstrate the in vivo utility of GiCT - to elucidate the novel finding that upregulation of Gi seen in dysfunctional myocardium induces survival signals and appears to be an adaptive mechanism post-ischemia to protect myocytes from apoptosis.