pH-Sensing G Protein-Coupled Receptor OGR1 (GPR68) Expression and Activation Increases in Intestinal Inflammation and Fibrosis

Abstract

Local extracellular acidification occurs at sites of inflammation. Proton-sensing ovarian cancer G-protein-coupled receptor 1 (OGR1, also known as GPR68) responds to decreases in extracellular pH. Our previous studies show a role for OGR1 in the pathogenesis of mucosal inflammation, suggesting a link between tissue pH and immune responses. Additionally, pH-dependent signalling is associated with the progression of intestinal fibrosis. In this study, we aimed to investigate OGR1 expression and OGR1-mediated signalling in patients with inflammatory bowel disease (IBD). Our results show that OGR1 expression significantly increased in patients with IBD compared to non-IBD patients, as demonstrated by qPCR and immunohistochemistry (IHC). Paired samples from non-inflamed and inflamed intestinal areas of IBD patients showed stronger OGR1 IHC staining in inflamed mucosal segments compared to non-inflamed mucosa. IHC of human surgical samples revealed OGR1 expression in macrophages, granulocytes, endothelial cells, and fibroblasts. OGR1-dependent inositol phosphate (IP) production was significantly increased in CD14+ monocytes from IBD patients compared to healthy subjects. Primary human and murine fibroblasts exhibited OGR1-dependent IP formation, RhoA activation, F-actin, and stress fibre formation upon an acidic pH shift. OGR1 expression and signalling increases with IBD disease activity, suggesting an active role of OGR1 in the pathogenesis of IBD.

Bicarbonate secretion and acid/base sensing by the intestine

“…Inflammation promotes glycolytic energy production, which is attributed to the increased demand for energy by infiltrating immune cells and to local hypoxia of the mucosal tissue, which results in increased formation of lactate and protons [ 37 , 95 , 122 ]. The acidification activates GPR4 and GPR65 with severe consequences.…”

“…GPR68 was found to be increased in the mucosa of patients with IBD and inflamed segments showed higher abundance of GPR68 than uninflamed mucosa [ 37 ]. Short-term expression of GPR68 is induced by the proinflammatory cytokine TNF, which was also shown to function as a major mediator for IBD-associated inflammation.…”

Role of pH-sensing receptors in colitis

“…A role of OGR1 in inflammation was described early on, and several studies have shown a crucial role for OGR1 in the expression of inflammatory and tissue remodeling factors under acidic conditions [ 49 , 83 ]. In IBD patients, intestinal tissue shows increased expression of OGR1, especially in inflamed areas, compared to the intestinal mucosa of healthy individuals [ 22 , 25 ]. Furthermore, the expression of OGR1 correlates with higher clinical scores in IBD patients, suggesting that OGR1 has a clinically relevant pro-inflammatory effect that could be targeted for treatment [ 22 , 25 ].…”

“…In IBD patients, intestinal tissue shows increased expression of OGR1, especially in inflamed areas, compared to the intestinal mucosa of healthy individuals [ 22 , 25 ]. Furthermore, the expression of OGR1 correlates with higher clinical scores in IBD patients, suggesting that OGR1 has a clinically relevant pro-inflammatory effect that could be targeted for treatment [ 22 , 25 ]. In a murine model of spontaneous colitis, OGR1 perpetuated intestinal inflammation through the expression of IL-6, TNFα, IL-8, and SPARC [ 22 ], a collagen-binding protein that mediates fibrosis [ 6 , 7 ], while Ogr1 -deficiency was protective [ 22 , 25 ].…”

“…OGR1 expression and OGR1-dependent signaling were observed in primary human and murine intestinal fibroblasts, and proton-activated OGR1-mediated signaling increases filamentous actin (F-actin) stress fibers at acidic pH in vitro [ 25 ]. Gpr4 expression has also been reported in primary intestinal fibroblasts [ 143 ], and fibroblasts found in other organs, e.g., rat kidney [ 111 ].…”