An RGS for PI3K

Abstract

Immunoglobulin E (IgE) triggers the activation of the FcεRI immune receptor, which signals through immunoreceptor-based tyrosine activation motifs, in response to allergens. FcεRI activation on mast cells produces an increase in intracellular calcium concentration, which causes degranulation and the release of inflammatory mediators such as histamine. The signaling pathway from the FcεRI to calcium involves the tyrosine kinases Lyn and Syk, the adaptor proteins Gab2 and Grb2, phosphoinositide 3-kinase (PI3K), and phospholipase C-mediated production of inositol 1,4,5-trisphosphate (IP 3 ). G protein-coupled receptors, such as adenosine or prostaglandin receptors, may amplify IgE-mediated mast cell activation. Bansal et al . found that RGS13, a member of the R4 subfamily of regulator of G protein signaling (RGS) proteins, provided an inhibitory input into IgE-mediated mast cell degranulation through a mechanism involving the p85 subunit of PI3K instead of a G protein. RGS proteins are best known for attenuating G protein signaling through their actions as GTPase-activating proteins (GAPs) for specific Gα subunits and for interfering with the interactions between Gα subunits and downstream effectors. Bansal et al . studied bone marrow-derived mast cells (BMMCs) from Rgs13 knockout ( Rgs13 -KO) mice and found that antigen-elicited degranulation was greater and inhibition of G proteins with pertussis toxin did not alter this enhanced response, suggesting that G proteins were not mediating the effect. Furthermore, reconstitution of the Rgs13 -deficient BMMCs with mutant forms of RGS13 that could not interact with G proteins prevented the enhanced degranulation response. The Rgs13 -KO mice also exhibited elevated cutaneous allergic response compared with wild-type mice, yet did not show altered mast cell numbers or morphology. Using glutathione S -transferase fused to RGS13, the p85 subunit of PI3K was identified from lysates of BMMCs as a tyrosine-phosphorylated protein that interacted with RGS13. The interaction between RGS13 and p85 was direct and occurred with recombinant proteins, although it did require p85 tyrosine phosphorylation (achieved by incubation with Lyn). Rgs13 -deficient BMMCs showed increased production of IP 3 , phosphorylation of Akt, and increased intracellular calcium concentrations in response to antigen. Transduction of wild-type BMMCs with a cell-permeable RGS13 construct inhibited antigen-stimulated calcium flux and PI3K activity. Immunoprecipitation experiments with BMMCS transduced with the cell-permeable RGS13 indicated that RGS13 interfered with the recruitment of p85 to Gab2 of the FcεRI signaling complex. G. Bansal, Z. Xie, S. Rao, K. H. Nocka, K. M. Druey, Suppression of immunoglobulin E-mediated allergic responses by regulator of G protein signaling 13. Nat. Immunol. 9 , 73-80 (2008). [PubMed]