TDAG8 Is a Proton-sensing and Psychosine-sensitive G-protein-coupled Receptor

Abstract

T cell death-associated gene 8 (TDAG8) has been reported to be a receptor for psychosine. Ovarian cancer G-protein-coupled receptor 1 (OGR1) and GPR4, Gprotein-coupled receptors (GPCRs) closely related to TDAG8, however, have recently been identified as protonsensing or extracellular pH-responsive GPCRs that stimulate inositol phosphate and cAMP production, respectively. In the present study, we examined whether TDAG8 senses extracellular pH change. In the several cell types that were transfected with TDAG8 cDNA, cAMP was markedly accumulated in response to neutral to acidic extracellular pH, with a peak response at approximately pH 7.0 -6.5. The pH effect was inhibited by copper ions and was reduced or lost in cells expressing mutated TDAG8 in which histidine residues were changed to phenylalanine. In the membrane fractions prepared from TDAG8-transfected cells, guanosine 5-O-(3-thiotriphosphate) binding activity and adenylyl cyclase activity were remarkably stimulated in response to neutral and acidic pH. The concentration-dependent effect of extracellular protons on cAMP accumulation was shifted to the right in the presence of psychosine. The inhibitory psychosine effect was also observed for pH-dependent actions in OGR1-and GPR4-expressing cells but not for prostaglandin E 2 -and sphingosine 1-phosphate-induced actions in any pH in native and sphingosine 1-phosphate receptor-expressing cells. Glucosylsphingosine and sphingosylphosphorylcholine similarly inhibited the pHdependent action, although to a lesser extent. Psychosinesensitive and pH-dependent cAMP accumulation was also observed in mouse thymocytes. We concluded that TDAG8 is one of the proton-sensing GPCRs coupling to adenylyl cyclase and psychosine, and its related lysosphingolipids behave as if they were antagonists against proteinsensing receptors, including TDAG8, GPR4, and OGR1. TDAG81 was initially cloned as an orphan GPCR, which is up-regulated during the programmed cell death of T lymphocytes (1-3). This gene product has recently been reported (4) to be a receptor for psychosine, a lysosphingolipid, which induces the formation of multinuclear cells. OGR1, which shares 41% identical amino acids with TDAG8, was initially reported (5) to be a receptor for sphingosylphosphorylcholine (SPC). GPR4 also shares homology with TDAG8 and was identified as a receptor for lysolipids, including lysophosphatidylcholine (LPC) and SPC (6). It has recently been reported (7), however, that OGR1 and GPR4 sense extracellular protons through histidine residues of receptors and are coupled to G-proteins to stimulate intracellular signaling pathways. Thus, OGR1 stimulation causes inositol phosphate production, and the subsequent mobilization of intracellular calcium and GPR4 stimulation induces cAMP accumulation, probably reflecting the activation of adenylyl cyclase in response to an extracellular pH change (7). These results raise the possibility that TDAG8 may also respond to extracellular pH change and stimulate intracellular signaling pathways.If TDAG8 is prov...

Identification of T Cell Death-associated Gene 8 (TDAG8) as a Novel Acid Sensing G-protein-coupled Receptor

“…Under the present assay conditions, we observed no effect of psychosine on TDAG8. The differences in the cAMP assay method, parental cell lines, and sublines, or intracellular localization of the expressed receptor may explain the discrepancy between our results and the previous reports (3,34). In addition, high concentrations of psychosine have been reported to possess a detergent-like hemolytic activity in vitro (35).…”

“…In addition, high concentrations of psychosine have been reported to possess a detergent-like hemolytic activity in vitro (35). Concerning calcium mobilization, Wang et al (34) and also we were unable to see the stimulatory effect of psychosine, whereas Im et al response. Further studies are needed to explain these differences and to determine whether or not TDAG8 works as a proton sensor in vivo.…”

“…While this article was under review, Wang et al (34) published their independent discovery of TDAG8 as a proton-sensing GPCR. They reported cAMP formation by acid treatment, which agrees with our data.…”

Gi-independent macrophage chemotaxis to lysophosphatidylcholine via the immunoregulatory GPCR G2A

“…As previously reported, 17 Intriguingly, recent studies demonstrated that G2A, as well as 3 related GPCRs-OGR1, GPR4, and TDAG8-can sense pH change to elicit downstream biologic activities such as inositol phosphate (IP) and cAMP formation. [40][41][42] LPC functions like an antagonist to decrease the pH-dependent activation of G2A in the IP assay. 41 A histidine (H174) in the human receptor plays a role in the proton-sensing function of G2A.…”

Extracellular Acidosis Induces Neutrophil Activation by a Mechanism Dependent on Activation of Phosphatidylinositol 3-Kinase/Akt and ERK Pathways

“…The original observations related to the ability of these receptors to recognize protons were conducted in osteosarcoma cells and primary human osteoblast precursors and showed that acidosis triggered inositol phosphate formation (28). Subsequent studies performed in a variety of transfected cell lines cultured at low values of extracellular pH (6.0 -7.0), found that OGR1 and G2A lead to the accumulation of inositolphosphate, whereas GPR4 and TDAG8 elicit cAMP formation (29,30). The expression and function of these receptors in human leukocytes has not been defined.…”