The impact of RGS and other G-protein regulatory proteins on Gαi-mediated signaling in immunity

Abstract

Leukocyte chemoattractant receptors are members of the G-protein coupled receptor (GPCR) family. Signaling downstream of these receptors directs the localization, positioning and homeostatic trafficking of leukocytes; as well as their recruitment to, and their retention at, inflammatory sites. Ligand induced changes in the molecular conformation of chemoattractant receptors results in the engagement of heterotrimeric G-proteins, which promotes α subunits to undergo GTP/GDP exchange. This results in the functionally release of βγ subunits from the heterotrimers, thereby activating downstream effector molecules, which initiate leukocyte polarization, gradient sensing, and directional migration. Pertussis toxin ADP ribosylates Gαi subunits and prevents chemoattractant receptors from triggering Gαi nucleotide exchange. The use of pertussis toxin revealed the essential importance of Gαi subunit nucleotide exchange for chemoattractant receptor signaling. More recent studies have identified a range of regulatory mechanisms that target these receptors and their associated heterotrimeric G-proteins, thereby helping to control the magnitude, kinetics, and duration of signaling. A failure in these regulatory pathways can lead to impaired receptor signaling and immunopathology. The analysis of mice with targeted deletions of Gαi isoforms as well as some of these G-protein regulatory proteins is providing insights into their roles in chemoattractant receptor signaling.

Endothelin-1 promotes vascular smooth muscle cell migration across the artery wall: a mechanism contributing to vascular remodelling and intimal hyperplasia in giant-cell arteritis

“…As G-coupled receptors, ET-1 receptors may activate heterotrimeric G proteins which have important roles in integrin inside-out and outside-in signalling 31. Pertussis toxin induces ADP-ribosylation of several Gα i subunits inhibiting their activity 32. As shown in figure 3C, ET-1-induced Y397 FAK phosphorylation was abrogated by pertussis toxin confirming the participation of heterotrimeric G proteins in ET-1-induced FAK activation.…”

The putative Gαq-inhibiting pepducin P4Pal10 distinctly modulates function of the Gαi-coupled receptors FPR2 and FFA2R in neutrophils

“…The conformational changes in the receptor, initiated by the binding of an agonist, promotes an exchange of GDP for GTP, which leads to dissociation and activation of the Gα subunit and a start of the G-protein dependent signaling cascade (3). The large structural similarities between G-proteins makes it hard to determine the precise identity of a signaling partner, and two bacterial toxins (pertussis toxin from Bordetella pertussis and cholera toxin from Vibro cholera ) have for long been the only tools available to investigate receptor coupling to Gαi (sensitive to pertussis toxin) and Gαs (sensitive to cholera toxin), respectively (5, 6). New and more efficient inhibitors of specific G-protein subunits have been eagerly awaited and small molecule inhibitors of Gαq are now available (7, 8).…”

Gαi2 Signaling Regulates Inflammasome Priming and Cytokine Production by Biasing Macrophage Phenotype Determination

“…Apart from the classical GPCR/G-protein signaling paradigm, there exist several regulatory proteins that modulate G-protein signaling. The regulators of G-protein signaling (RGS) family consists of more than 30 members and is defined by the presence of an RGS domain (5,6). RGS proteins act as GTPase accelerating proteins (GAPs) for specific Ga subunits.…”

“…RGS proteins act as GTPase accelerating proteins (GAPs) for specific Ga subunits. They enhance the intrinsic GTPase activity of Ga by stabilizing the GTPase transition state, accelerating the intrinsic Ga GTPase activity by as much as 100-fold (6,7). In this way, RGS proteins limit the duration of GPCR-mediated G-protein signaling, and G-proteins genetically modified to be RGS-insensitive proteins show higher basal levels of signaling (6).…”