Heterotrimeric G Protein Signaling In Allergic Inflammation

National Institute Of Allergy And Infectious Diseases

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Abstract

Mast cells (MCs), granulocytes, and lymphocytes are integral to the development of an allergic response. Allergic inflammation may also be generated through activation of receptors coupled to heterotrimeric G proteins (GPCRs). The purpose of this study is to understand mechanisms of G protein-mediated signal transduction in immune cells, with a focus on GPCR-mediated trafficking of leukocytes to sites of allergic inflammation. GPCRs activate a core pathway of heterotrimeric G proteins, which bind guanosine triphosphate (GTP) in exchange for guanosine diphosphate (GDP). The GTP-bound form of the G protein alpha subunit induces downstream signaling cascades, including intracellular calcium flux responsible for MC/basophil degranulation. This project focuses on a family of regulators of G protein signaling (RGS proteins), which inhibit the function of G alpha-i and G alpha-q, but not G alpha-s, proteins by increasing their GTPase activity. G alpha subunits oscillate between GDP- (inactive) and GTP- (active) bound forms based on ligand occupancy of the associated receptor. The GTPase accelerating (GAP) activity of RGS proteins limits the time of interaction of active G-alpha and its effectors, resulting in desensitization of GCPR signaling. Despite a growing body of knowledge concerning the biochemical mechanisms of RGS action, relatively little is known about the physiological role of these proteins in allergic inflammation. A major area of investigation is the recruitment of inflammatory leukocytes to sites of inflammation. Chemokines are a major class of compounds acting on leukocyte GPCRs, which orchestrate immune cell trafficking, and RGS proteins including RGS5, RGS13, and RGS16 inhibit chemokine signaling by desensitizing GPCR signals. In collaborative work with Dr. Khasawneh (University of Texas El Paso), we demonstrated a critical negative regulatory function of both RGS10 and RGS16 in platelet activation. Platelets from RGS10 or RGS16-deficient mice exhibited enhanced aggregration, granule secretion, and adhesion in response to GPCR ligands thrombin and CXCL12 as well as collagen. Patients with undefined immunodeficiencies and novel mutations in G proteins and/or RGS proteins are being characterized in collaborative studies with Drs. Orange and Su. A final area of investigation is the role of RGS5 in neutrophil trafficking. Using Rgs5-/- mice, we discovered in 2017 that neutrophils deficient in RGS5 traffic abnormally to sites of inflammation in lung and peritoneum. RGS5-deficient neutrophils transferred into WT recipient mice trafficked abnormally compared to WT neutrophils, suggesting that these abnormalities are neutrophil-intrinsic. Neutrophils isolated from Rgs5 gene deleted mice display enhanced chemotaxis and adhesion to chemokine and exaggerated chemokine signaling. Current studies are aimed at understanding the molecular mechanisms underlying this phenotype.