p63RhoGEF Couples G_αq/11-Mediated Signaling to Ca²⁺ Sensitization of Vascular Smooth Muscle Contractility

Rationale: In normal and diseased vascular smooth muscle (SM), the RhoA pathway, which is activated by multiple agonists through G protein-coupled receptors (GPCRs), plays a central role in regulating basal tone and peripheral resistance. This occurs through inhibition of myosin light chain phosphatase, leading to increased phosphorylation of the myosin regulatory light chain. Although it is thought that specific agonists and GPCRs may couple to distinct RhoA guanine nucleotide exchange factors (GEFs), thus raising the possibility of selective targeting of specific GEFs for therapeutic use, this notion is largely unexplored for SM contraction.

Objective: We examine whether p63RhoGEF, known to couple specifically to Gα_q/11 in vitro, is functional in blood vessels as a mediator of RhoA activation and if it is selectively activated by Gα_q/11 coupled agonists.

Methods and Results: We find that p63RhoGEF is present across SM tissues and demonstrate that silencing of the endogenous p63RhoGEF in mouse portal vein inhibits contractile force induced by endothelin-1 to a greater extent than the predominantly Gα_12/13-mediated thromboxane analog U46619. This is because endothelin-1 acts on Gα_q/11 as well as Gα_12/13. Introduction of the exogenous isolated pleckstrin-homology (PH) domain of p63RhoGEF (residues 331–580) into permeabilized rabbit portal vein inhibited Ca²⁺ sensitized force and activation of RhoA, when phenylephrine was used as an agonist. This reinforces the results based on endothelin-1, because phenylephrine is thought to act exclusively through Gα_q/11.

Conclusion: We demonstrate that p63RhoGEF selectively couples Gα_q/11 but not Gα_12/13, to RhoA activation in blood vessels and cultured cells and thus mediates the physiologically important Ca²⁺ sensitization of force induced with Gα_q/11-coupled agonists. Our results suggest that signaling through p63RhoGEF provides a novel mechanism for selective regulation of blood pressure.