GPR68, a proton-sensing GPCR, mediates acid-induced visceral nociception
Background & Aims: Localised acidification from immune cell infiltration and heightened glycolysis contributes to colitis pathology by activating acid-sensing receptors such as GPR68, a proton-sensing GPCR expressed on immune and stromal cells. Single-cell RNAseq analysis revealed GPR68 is also expressed in colonic sensory neurons, prompting us to investigate its role in acid-induced colonic nociception. Methods: Expression of GPR68 in colonic nociceptors and tissue from people with colitis was confirmed by in silico analysis of our RNAseq databases. Its contribution to disease activity was assessed using the acute dextran sulphate sodium (DSS) model of colitis. Acid-evoked sensory signalling was evaluated via colonic afferent recordings and Ca2+; imaging in DRG neurons from wild-type and GPR68-/- mice, supported by pharmacological studies using Ogerin (a GPR68 positive allosteric modulator) and Ogremorphin (a GPR68 antagonist). Results: RNAseq analysis showed GPR68 is robustly expressed in Trpv1+; colonic nociceptors and upregulated in tissue from people with inflammatory bowel disease, consistent with reduced disease activity in DSS-treated GPR68-/- mice. Genetic deletion of GPR68 abolished colonic afferent responses to acid, which were also attenuated by Ogremorphin and enhanced by Ogerin. In Ca2+;-free buffer, DRG neurons from GPR68-/- mice or those pre-treated with Ogremorphin showed significantly reduced acid-evoked intracellular Ca2+; responses. By contrast the colonic afferent and DRG Ca2+ response (in Ca2+-containing buffer) to capsaicin was comparable between tissue from wild-type and GPR68-/- mice highlighting the involvement of divergent proton-dependent cellular signalling cascades. Conclusions: These findings identify GPR68 as a key mediator of acid-induced colonic nociception and highlight its potential as a therapeutic target for the treatment of pain in colitis.