Since Gβγ binding is a critical prerequisite for Gβγ-GRK2-PI3K-mediated GPCR desensitization, several approaches have been explored to interdict pathologic Gβγ interactions, including Gβγ-GRK2-PI3K interaction. The first reported approach exploited GRK2, which possesses three general domains, including an N-terminal RGS and protein recognition domain, a central kinase domain, and a C-terminal region encoding the Gβγ binding domain. To study the role of Gβγ signaling and interactions, the C-terminal 194 amino acids encoding the GRK2 Gβγ binding domain (βARKct) was expressed in cells as a Gβγ peptide inhibitor, where it attenuated homologous β-AR desensitization in a GPCR-specific manner [24
]. βARKct expression attenuated β-AR desensitization without disrupting normal signaling. Subsequently, transgenic mice were created with myocardial-targeted expression of βARKct, which demonstrated enhanced basal cardiac function and response to isoproterenol [18
]. Mating of the cardiac-targeted βARKct mice with the cardiac-targeted GRK2 overexpressing mice normalized cardiac function. Subsequent data also demonstrated that βARKct disrupts recruitment not only of GRK2, but also the GRK2-PI3K complex [13
]. These data provided direct evidence that the mechanism responsible for the phenotype in these mice was βARKct inhibition of GPCR-initiated, Gβγ-mediated signaling [18
To determine the role of Gβγ-mediated signaling and protein-protein interactions in the pathogenesis of HF, the cardioprotective potential of βARKct has been assessed in numerous animal models of HF. The data repeatedly demonstrate a salutary, cardioprotective effect of βARKct both by cardiac-restricted transgenesis, as well as through adenoviral delivery, in various models of both ischemic and non-ischemic HF [20
]. Notably, βARKct has not only normalized cardiac function, but has also normalized several aspects of β-AR signaling [22
]. Furthermore, βARKct was shown to be synergistic with β-AR blockers (standard medical therapy for HF) in the cardiac calsequestrin overexpressor (CSQ) mouse model of HF [28
]. We have previously demonstrated that βARKct-mediated normalization of cardiac function in two genetically engineered animal models of HF is accompanied by normalization of cardiac gene expression in a large scale gene expression profiling study [26
]. βARKct has also been shown to normalize contractile function of failing human cardiac mycoytes [29
]. Importantly, bARKct can also ameliorate enhancement of ischemia-reperfusion injury in cardiac GRK2 transgenic mice [74
]. Interestingly, βARKct has demonstrated efficacy in other cardiovascular diseases, including vascular restensosis and hypertension [30
]. Thus, inhibition of Gβγ signaling and protein-protein interactions represents a promising therapeutic target in the treatment of HF.
The therapeutic potential of targeting Gβγ signaling in HF pathogenesis has been subsequently validated by directly comparing βARKct to truncated phosducin [34
], a Gβγ binding protein discovered in retinal cell membranes following activation of the GPCR transducin. Like GRK2 and PI3K, cytosolic phosducin is recruited to membrane Gβγ subunits upon GPCR activation, and inhibits subsequent Gβγ signaling, including GRK2 recruitment [35
]. Viral gene delivery of βARKct or a ~200 amino acid N-terminally truncated phosducin to the pacing-induced HF rabbit heart equally normalized cardiac function. Both peptides normalized isolated failing cardiomyocyte contractility, with mildly differential effects on β-AR signaling [34
]. provides a partial list of the effects of gain and loss of function of Gβγ-mediated signaling effectors in the heart.
A partial list of gain and loss of function of Gβγ-mediated signaling effectors in the heart.