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Proton-sensing G-protein-coupled receptors


Blood pH is maintained in a narrow range around pH 7.4 mainly through regulation of respiration and renal acid extrusion1,2. The molecular mechanisms involved in pH homeostasis are not completely understood. Here we show that ovarian cancer G-protein-coupled receptor 1 (OGR1), previously described as a receptor for sphingosylphosphorylcholine3, acts as a proton-sensing receptor stimulating inositol phosphate formation. The receptor is inactive at pH 7.8, and fully activated at pH 6.8—site-directed mutagenesis shows that histidines at the extracellular surface are involved in pH sensing. We find that GPR4, a close relative of OGR1, also responds to pH changes, but elicits cyclic AMP formation. It is known that the skeleton participates in pH homeostasis as a buffering organ, and that osteoblasts respond to pH changes in the physiological range4, but the pH-sensing mechanism operating in these cells was hitherto not known. We detect expression of OGR1 in osteosarcoma cells and primary human osteoblast precursors, and show that these cells exhibit strong pH-dependent inositol phosphate formation. Immunohistochemistry on rat tissue sections confirms the presence of OGR1 in osteoblasts and osteocytes. We propose that OGR1 and GPR4 are proton-sensing receptors involved in pH homeostasis.

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Figure 1: IP formation experiments.
Figure 2: Sequence alignment and 3D model for OGR1.
Figure 3: pH-dependent activation of wild-type and mutated OGR1 receptor constructs following transient transfection into HEK293 cells.
Figure 4: cAMP formation experiments.
Figure 5: OGR1 in bone.

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We thank B. Wilmering-Wetter and H. Anklin for assistance in cell culture, IP formation and intracellular calcium assays, R. Brearley and M. Schäublin for support in molecular biology, and A Rebmann for IHC analysis. We also thank R. Bouhelal and I. Vranesic for advice concerning the fluorescence imaging plate reader, and J. Mosbacher for help with confocal microscopy.Authors' contributions The experiments shown in Figs 1, 3, 4, 5 were carried out by M.G.L., M.V., D.G. and K.S. J.A.G. performed the IHC on tissue sections. C.E.J. cloned GPR4 and performed expression profiling experiments. U.J. and H.H. participated in the cloning of OGR1, expression profiling, and experiments with lipid agonists. R.W. carried out receptor modelling.

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Correspondence to Klaus Seuwen.

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Ludwig, MG., Vanek, M., Guerini, D. et al. Proton-sensing G-protein-coupled receptors. Nature 425, 93–98 (2003).

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