Differential proton sensitivity of related G protein-coupled receptors T cell death-associated gene 8 and G2A expressed in immune cells

Abstract

G2A, T cell death-associated gene 8 (TDAG8), ovarian cancer G protein-coupled receptor 1 (OGR1), and G protein-coupled receptor 4 (GPR4) form a group of structurally related G protein-coupled receptors (GPCRs) originally proposed to bind proinflammatory lipids. More recent studies have challenged the identification of lipid agonists for these GPCRs and have suggested that they function primarily as proton sensors. We compared the ability of these four receptors to modulate pH-dependent responses by using transiently transfected cell lines. In accordance with previously published reports, OGR1 was found to evoke strong pH-dependent responses as measured by inositol phosphate accumulation. We also confirmed the pH-dependent cAMP production by GPR4 and TDAG8. However, we found the activity of the human G2A receptor and its mouse homolog to be significantly less sensitive to pH fluctuations as measured by inositol phosphate and cAMP accumulation. Sequence homology analysis indicated that, with one exception, the histidine residues that were previously shown to be important for pH sensing by OGR1, GPR4, and TDAG8 were not conserved in the G2A receptor. We further addressed the pH-sensing properties of G2A and TDAG8 in a cellular context where these receptors are coexpressed. In thymocytes and splenocytes explanted from receptor-deficient mice, TDAG8 was found to be critical for pH-dependent cAMP production. In contrast, G2A was found to be dispensable for this process. We conclude that members of this GPCR group exhibit differential sensitivity to extracellular protons, and that expression of TDAG8 by immune cells may regulate responses in acidic microenvironments.