IBD-associated G protein-coupled receptor 65 variant compromises signalling and impairs key functions involved in inflammation

Small-molecule probe for IBD risk variant GPR65 I231L alters cytokine signaling networks through positive allosteric modulation

The proton-sensing heterotrimeric guanine nucleotide-binding protein-coupled receptor GPR65 is expressed in immune cells and regulates tissue homeostasis in response to decreased extracellular pH, which occurs in the context of inflammation and tumorigenesis. Genome-wide association studies linked GPR65 to several autoimmune and inflammatory diseases such as multiple sclerosis and inflammatory bowel disease (IBD). The loss-of-function GPR65 I231L IBD risk variant alters cellular metabolism, impairs protective tissue functions, and increases proinflammatory cytokine production. Hypothesizing that a small molecule designed to potentiate GPR65 at subphysiological pH could decrease inflammatory responses, we found positive allosteric modulators of GPR65 that engage and activate both human and mouse orthologs of the receptor. We observed that the chemical probe BRD5075 alters cytokine and chemokine programs in dendritic cells, establishing that immune signaling can be modulated by targeting GPR65. Our investigation offers improved chemical probes to further interrogate the biology of human GPR65 and its clinically relevant genetic variants.

Immunomodulation of Proton-activated G Protein-coupled Receptors in Inflammation

Proton-activated G protein-coupled receptors (GPCRs), initially discovered by Ludwig in 2003, are widely distributed in various tissues. These receptors have been found to modulate the immune system in several inflammatory diseases, including inflammatory bowel disease, atopic dermatitis, and asthma. Proton-activated GPCRs belong to the G protein-coupled receptor family and can detect alternations in extracellular pH. This detection triggers downstream signaling pathways within the cells, ultimately influencing the function of immune cells. In this review, we specifically focused on investigating the immune response of proton-activated GPCRs under inflammatory conditions.

Molecular basis of proton-sensing by G protein-coupled receptors

Three proton-sensing G protein-coupled receptors (GPCRs), GPR4, GPR65, and GPR68, respond to changes in extracellular pH to regulate diverse physiology and are implicated in a wide range of diseases. A central challenge in determining how protons activate these receptors is identifying the set of residues that bind protons. Here, we determine structures of each receptor to understand the spatial arrangement of putative proton sensing residues in the active state. With a newly developed deep mutational scanning approach, we determined the functional importance of every residue in proton activation for GPR68 by generating ~9,500 mutants and measuring effects on signaling and surface expression. This unbiased screen revealed that, unlike other proton-sensitive cell surface channels and receptors, no single site is critical for proton recognition in GPR68. Instead, a network of titratable residues extend from the extracellular surface to the transmembrane region and converge on canonical class A GPCR activation motifs to activate proton-sensing GPCRs. More broadly, our approach integrating structure and unbiased functional interrogation defines a new framework for understanding the rich complexity of GPCR signaling.

Recent advances in acid sensing by G protein coupled receptors

Changes in extracellular proton concentrations occur in a variety of tissues over a range of timescales under physiological conditions and also accompany virtually all pathologies, notably cancers, stroke, inflammation and trauma. Proton-activated, G protein coupled receptors are already partially active at physiological extracellular proton concentrations and their activity increases with rising proton concentrations. Their ability to monitor and report changes in extracellular proton concentrations and hence extracellular pH appears to be involved in a variety of processes, and it is likely to mirror and in some cases promote disease progression. Unsurprisingly, therefore, these pH-sensing receptors (pHR) receive increasing attention from researchers working in an expanding range of research areas, from cellular neurophysiology to systemic inflammatory processes. This review is looking at progress made in the field of pHRs over the past few years and also highlights outstanding issues.