OGR1 (GPR68) and TDAG8 (GPR65) Have Antagonistic Effects in Models of Colonic Inflammation

Inhibition of proton sensor GPR68 suppresses endothelial dysfunction and acute lung injury caused by Staphylococcus aureus bacterial particles

Lung bacterial infections, including hospital-acquired pneumonia, remain a serious problem for public health. Endothelial cell (EC) exposure to heat-killed Staphylococcus aureus (HKSA) represents a clinical scenario of high titers of killed bacterial particles present in the host after antibiotic therapy, which triggers inflammatory cascades, cytokine storms, and EC dysfunction leading to acute lung injury (ALI). GPR68 is a member of the proton-sensing G protein-coupled receptor family. Acting as a pH sensor, GPR68 becomes activated upon pH reduction and contributes to pathologic cell responses by activating ER stress and unfolded protein response. This study investigated the role of GPR68 in HKSA-induced EC dysfunction and HKSA-induced ALI. HKSA robustly increased GPR68 mRNA levels in human pulmonary EC and directly stimulated GPR68 activity. A selective GPR68 small molecule inhibitor, OGM-8345, attenuated HKSA-induced EC permeability and protected cell junction integrity. OGM-8345 inhibited HKSA-induced activation of inflammatory genes TNF-α, IL-6, IL-8, IL-1β, and CXCL5 and decreased cytokine secretion by HKSA-challenged EC. Co-treatment with the GPR68 activator Ogerin or medium acidification to pH 6.5 augmented HKSA-induced EC dysfunction, which was rescued by OGM-8345. Intratracheal HKSA injection increased vascular leak and lung inflammation in mice which were monitored by lung Evans blue extravasation, increased cell and protein count in bronchoalveolar lavage, and mRNA expression of inflammatory genes. ALI and barrier dysfunction was attenuated by OGM-8345. We show for the first time the role of GPR68 in mediating HKSA-induced lung injury and the strong potential for OGM-8345 as a therapeutic treatment of bacterial pathogen-induced ALI associated with tissue acidification.

The landscape of new therapeutic opportunities for IBD

This chapter presents an overview of the emerging strategies to address the unmet needs in the management of inflammatory bowel diseases (IBD). IBD poses significant challenges, as over half of patients experience disease progression despite interventions, leading to irreversible complications, and a substantial proportion do not respond to existing therapies, such as biologics. To overcome these limitations, we describe a diverse array of novel therapeutic approaches. In the area of immune homeostasis restoration, the focus is on targeting cytokine networks, leukocyte trafficking, novel immune pathways, and cell therapies involving regulatory T cells and mesenchymal stem cells (MSC). Recognizing the critical role of impaired intestinal barrier integrity in IBD, we highlight therapies aimed at restoring barrier function and promoting mucosal healing, such as those targeting cell proliferation, tight junctions, and lipid mediators. Addressing the challenges posed by fibrosis and fistulas, we describe emerging targets for reversing fibrosis like kinase and cytokine inhibitors and nuclear receptor agonists, as well as the potential of MSC for fistulas. The restoration of a healthy gut microbiome, through strategies like fecal microbiota transplantation, rationally defined bacterial consortia, and targeted antimicrobials, is also highlighted. We also describe innovative approaches to gut-targeted drug delivery to enhance efficacy and minimize side effects. Reinforcing these advancements is the critical role of precision medicine, which emphasizes the use of multiomics analysis for the discovery of biomarkers to enable personalized IBD care. Overall, the emerging landscape of therapeutic opportunities for IBD holds great potential to surpass the therapeutic ceiling of current treatments.

The Roles of Proton-Sensing G-Protein-Coupled Receptors in Inflammation and Cancer

The precise regulation of pH homeostasis is crucial for normal physiology. However, in tissue microenvironments, it can be impacted by pathological conditions such as inflammation and cancer. Due to the overproduction and accumulation of acids (protons), the extracellular pH is characteristically more acidic in inflamed tissues and tumors in comparison to normal tissues. A family of proton-sensing G-protein-coupled receptors (GPCRs) has been identified as molecular sensors for cells responding to acidic tissue microenvironments. Herein, we review the current research progress pertaining to these proton-sensing GPCRs, including GPR4, GPR65 (TDAG8), and GPR68 (OGR1), in inflammation and cancer. Growing evidence suggests that GPR4 and GPR68 are mainly pro-inflammatory, whereas GPR65 is primarily anti-inflammatory, in various inflammatory disorders. Both anti- and pro-tumorigenic effects have been reported for this family of receptors. Moreover, antagonists and agonists targeting proton-sensing GPCRs have been developed and evaluated in preclinical models. Further research is warranted to better understand the roles of these proton-sensing GPCRs in pathophysiology and is required in order to exploit them as potential therapeutic targets for disease treatment.

Role of pH-sensing receptors in colitis

Low pH in the gut is associated with severe inflammation, fibrosis, and colorectal cancer (CRC) and is a hallmark of active inflammatory bowel disease (IBD). Subsequently, pH-sensing mechanisms are of interest for the understanding of IBD pathophysiology. Tissue hypoxia and acidosis-two contributing factors to disease pathophysiology-are linked to IBD, and understanding their interplay is highly relevant for the development of new therapeutic options. One member of the proton-sensing G protein-coupled receptor (GPCR) family, GPR65 (T-cell death-associated gene 8, TDAG8), was identified as a susceptibility gene for IBD in a large genome-wide association study. In response to acidic extracellular pH, GPR65 induces an anti-inflammatory response, whereas the two other proton-sensing receptors, GPR4 and GPR68 (ovarian cancer G protein-coupled receptor 1, OGR1), mediate pro-inflammatory responses. Here, we review the current knowledge on the role of these proton-sensing receptors in IBD and IBD-associated fibrosis and cancer, as well as colitis-associated cancer (CAC). We also describe emerging small molecule modulators of these receptors as therapeutic opportunities for the treatment of IBD.