Signal Transduction Mechanisms Mediating the Regulation of Vascular G Protein-Coupled Receptors

Blood pressure homeostasis is controlled via a complex network of cell signaling mechanisms. Among the broad network of receptors and signaling molecules regulating blood vessel reactivity, members of the G protein-coupled receptor (GPCR) family are known to play a central role. GPCR activity represents a delicate, but coordinated balance between molecular mechanisms governing receptor signaling, desensitization, and re-sensitization. GPCR kinase 2 (GRK2) modulates multiple cellular responses through GPCR desensitization and alterations in GRK2 activity are considered to play an important role in the development of hypertension. The main premise of our study was to test whether the inhibition of GRK2 expression leads to alterations in vascular reactivity, vascular tone, and vascular smooth muscle cell (VSMC) signaling. Genetic knockdown of GRK2 expression results in a mouse that shows indications of intrauterine growth retardation phenotype and becomes spontaneously hypertensive at 8-12 weeks of age due to alterations in the balance between mechanisms regulating vasodilatation and vasoconstriction. The extensive loss of GRK2 expression favors an increased in vasoconstriction associated with an increase in peripheral resistance and this is likely due to the reduced Gαq/11-coupled receptor desensitization. The vasodilatation in response to GαS-coupled receptor stimulation was also enhanced, but the increases in vasoconstrictor mechanisms dominate the physiological phenotype. In addition, VSMCs cultured from shGRK2-knockdown mice demonstrate an altered ERK1/2 and Akt/PKB signaling with age, as well as age-dependent increases in cellular proliferation and migration responses linked to Gαq/11-coupled GPCR activation. Our results indicate that, as blood pressure increases in the shGRK2 mice, the expression of renal renin angiotensin system (RAS) components increases correspondingly and this has a strong impact on the regulation of both peripheral vascular resistance and sodium balance. The chronic activation of RAS also potentiates renal injury by inducing alteration in glomerular filtration rates and progression of renal fibrosis. Thereby, these intricate effects complement each other in the onset of hypertension. Finally, to enable definition of the role of Rab4GTPase on GPCRs re-sensitization, we have developed a vascular specific inhibitory Rab2S22N transgenic mouse and the documentation of the hypertensive phenotype is the first evidence for the existence of a causal relationship between alteration in Rab4 activity and vascular GPCR signaling. Taken together, our findings indicate that the balance between mechanisms regulating vascular tone is significantly modulated by intracellular regulatory proteins underlying GPCR signal transduction