Heterotrimeric G Protein Signaling In Allergic Inflammat

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Abstract

Mast cells and T lymphocytes, two cell types integral to development of an allergic response and asthma, express numerous inflammation-generating receptors coupled to heterotrimeric G proteins (GPCRs). The purpose of this study is to understand mechanisms of intracellular G-protein-coupled signal transduction in these cells and subsequent pathways to inflammation. In particular, the project focuses on the control of G protein activity in inflammatory processes by a novel family of regulators of G protein signaling (RGS proteins), which inhibit G alpha subunits 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, little is known about the physiological role of these proteins in native mammalian systems. RGS13, a GAP for Gi and Gq, was found to be expressed in murine and human mast cells and B lymphocytes. RGS13 deficient mice have been generated. A vector was constructed that replaces the RGS13 gene with LacZ, and positive beta-galactosidase staining was observed in cultured, bone marrow-derived mast cells from knockout mice. Unexpectedly, RGS13 inhibited cAMP generation induced by stimulation of a Gs-coupled receptor and by forskolin, a direct activator adenylyl cyclase, in a fibroblast cell line. RGS13 was detected in mouse mast cells and transformed mouse mast cell lines by immunoblotting. A highly homologous RGS protein, RGS16, is highly expressed in activated T lymphocytes. Its role in T cell activation and chemotaxis was studied in transgenic mice that overexpress RGS16 in CD4+ T lymphocytes. RGS16 transgenic mice express 2-3 fold higher levels of RGS16 in resting CD4+ and CD8+ cells than wild type controls. In a mouse model of allergic airway inflammation induced by ovalbumin (OVA), RGS16 transgenic T lymphocytes exhibited abnormal activation responses. Lymphocytes in lung fluid and spleen secreted increased amounts of IL-4, IL-5, and IL-13 cytokines after OVA challenge. This effect was replicated in vitro after recall with OVA. In addition, transgenic T lymphocytes bearing certain chemokine receptors (CCR3, CCR5, and CXCR4) adoptively transferred to OVA challenged and sensitized mice migrated in reduced amounts to the site of airway inflammation and accumulated in spleen and lymph nodes as compared to wild type controls. RGS16 was shown to inhibit G-alpha 13-evoked cytoskeletal and transcriptional responses in an epithelial cell line. RGS16 bound G-alpha 13 directly, inducing G-alpha 13 redistribution at the membrane and decreased binding to a downstream effector.