Metabolic Acidosis Increases Intracellular Calcium in Bone Cells Through Activation of the Proton Receptor OGR1

Abstract

Metabolic acidosis increases urine Ca without increasing intestinal absorption, leading to bone Ca loss. It is unclear how bone cells detect the increase in proton concentration. To determine which G protein-coupled proton sensing receptors are expressed in bone, PCR was performed, and products were detected for OGR1, TDAG8, G2A, and GPR4. We tested the hypothesis that the G protein-coupled proton sensor, OGR1, is an H+-sensing receptor in bone. To determine whether acid-induced bone resorption involves OGR1, we incubated mouse calvariae in neutral pH (NTL) or acidic (MET) medium ± the OGR1 inhibitor CuCl2. CuCl2 decreased MET-induced Ca efflux. We used fluorescent imaging of perfused bone cells to determine whether MET increases Cai. Perfusion with MET induced a rapid, flow-independent, increase in Cai in individual bone cells. To determine whether transfection of OGR1 into a heterologous cell type would increase Cai in response to H+, we perfused Chinese hamster ovary (CHO) cells transfected with mouse OGR1 cDNA. Perfusion with MET induced a rapid increase in Cai in OGR1-transfected CHO cells. These data indicate that OGR1 induces an increase in Cai in response to MET and is a prime candidate for an osteoblast proton sensor.

Bone Disease in Medullary Sponge Kidney and Effect of Potassium Citrate Treatment

“…Case reports involving children have described an association among MSK, growth retardation, and overt or incomplete dRTA (12,13). Incomplete dRTA typically causes no overt systemic acidosis but may cause recurrent positive acid loads in periods of increased protein intake or catabolic stress triggering alkali release from the bone and thus leading to a greater bone reabsorption (14); in fact, increased osteoblast and osteoclast activation has been described in idRTA (15), and the G proteincoupled proton sensor OGR1 was recently found to work as a H ϩ -sensing receptor in osteoblasts (16). Conversely, even normal individuals with normal renal acidifying activity have shown a close stoichiometry between positive H ϩ balance and negative calcium balance during the ingestion of excessive acid loads (17), which has prompted the view that incoming H ϩ titrates bone carbonates, thereby protecting serum bicarbonate concentrations at the expense of a slow bone dissolution.…”

Emerging roles for the pH-sensing G protein-coupled receptors in response to acidotic stress

“…72 The reduction in osteoblast activity may also reduce the gene expression of several matrix proteins such as COL1A, OPN, and MGP, which may contribute to increased osteoarthritis development. 71,[74][75][76] Two of the pH-sensing G protein-coupled receptors, GPR68 and GPR65, have been reported to sense acidosis in bone cells.…”

“…19 As osteoclast and osteoblast homeostasis may regulate systemic pH by controlling the release of minerals from bone, GPR68 has been speculated as a possible bone acidosis sensor. 19,76 In addition, GPR68 is expressed during osteoclastogenesis and may be involved in osteoclast differentiation. 77 Acidosis …”

“…81 N/A Regulates gene expression in response to acidosis in bone cells. 19,76,78,79 GPR68 activation by acidosis stimulates apoptosis in chondrocytes. 80 endocrine N/A Regulates inflammatory gene expression in insulin target tissues; insulin sensitivity is increased in GPR4-null mice.…”

“…79 GPR68 is also expressed in cultured neonatal calvariae osteoblasts and in response to acidosis may increase intracellular Ca 2+ release. 76 Furthermore, following the transfection of a heterologous GPR68 construct into Chinese hamster ovary cells, the response to acidosis stimulated the intracellular release of Ca 2+ as well.…”