Targeted Inhibition of Cardiomyocyte Gi Signaling Enhances Susceptibility to Apoptotic Cell Death in Response to Ischemic Stress

Abstract

Background-A salient characteristic of dysfunctional myocardium progressing to heart failure is an upregulation of the adenylyl cyclase inhibitory guanine nucleotide (G) protein ␣ subunit, G␣ i2 . It has not been determined conclusively whether increased Gi activity in the heart is beneficial or deleterious in vivo. Gi signaling has been implicated in the mechanism of cardioprotective agents; however, no in vivo evidence exists that any of the G␣ subunits are cardioprotective. We have created a novel molecular tool to specifically address the role of Gi proteins in normal and dysfunctional myocardium. Methods and Results-We have developed a class-specific Gi inhibitor peptide, GiCT, composed of the region of G␣ i2 that interacts specifically with G protein-coupled receptors. GiCT inhibits Gi signals specifically in vitro and in vivo, whereas Gs and Gq signals are not affected. In vivo expression of GiCT in transgenic mice effectively causes a "functional knockout" of cardiac G␣ i2 signaling. Inducible, cardiac-specific GiCT transgenic mice display a baseline phenotype consistent with nontransgenic mice. However, when subjected to ischemia/reperfusion injury, GiCT transgenic mice demonstrate a significant increase in infarct size compared with nontransgenic mice (from 36.9Ϯ2.5% to 50.9Ϯ4.3%). Mechanistically, this post-ischemia/reperfusion phenotype includes increased myocardial apoptosis and resultant decreased contractile performance. Conclusions-Overall, our results demonstrate the in vivo utility of GiCT to dissect specific mechanisms attributed to Gi signaling in stressed myocardium. Our results with GiCT indicate that upregulation of G␣ i2 is an adaptive protective response after ischemia to shield myocytes from apoptosis.

Myocardial expression, signaling, and function of GPR22: a protective role for an orphan G protein-coupled receptor

“…In addition, reduced or ablated GPR22 expression in this setting could result in the reduced activation of well-known G i -coupled survival pathways in heart cells, including G ␤␥ /phosphoinositide-3-kinase/Akt. Further support for this hypothesis comes from a recently published study (7) demonstrating that sustained inhibition of myocardial G i signaling in the setting of ischemic stress resulted in increased apoptosis and decreased contractile performance. Although cardiomyocyte apoptosis in our model was unconfirmed, this scenario is consistent with our observation of decreased contractile function in GPR22 Ϫ/Ϫ mice subjected to chronic TAC.…”

The small-molecule BGP-15 protects against heart failure and atrial fibrillation in mice

“…However, the content of G ai2 with IGF1R was significantly lower in ventricular tissue from the HF þ AF model but not BGP-15-treated mice. G ai has been shown to associate with IGF1R in other tissue/cell types 54,55 , and G ai2 provides protection in the heart 56,57 . Thus, this could represent one mechanism by which BGP-15 is mediating protection via IGF1R in the HF þ AF model.…”

Gi proteins mediate activation of the canonical hedgehog pathway in the myocardium

“…Sprague-Dawley rat pups (postnatal days [1][2][3] for the isolation of neonatal rat ventricular myocytes (NVRMs) were obtained from S-A Ace (Boyertown, PA). Transgenic cardiomyocytespecific inducible GiCT mice (GiCT/TTA) were obtained by breeding mice encoding a doxycyline-sensitive TTA transgene [␣-myosin heavy chain (MHC)-TTA] with mice encoding a TTA-driven ␣-MHC-GiCT transgene, as previously described (8). None of the transgenic animals showed any overt phenotype under nonstress conditions, but GiCT/TTA mice did have a reduced recovery of cardiac function after I/R injury (8).…”