The Functional Roles of pH-sensing G protein-coupled receptors in Intestinal Inflammation

Abstract

The inflammatory microenvironment in inflammatory bowel disease (IBD) is complex, replete with microbial byproducts, complement, leukocytes, and resulting inflammatory cytokines. Parallel to these microenvironmental factors are protons, which are produced in excess due to altered metabolism of infiltrated leukocytes and local ischemia. Immune cells and intestinal microvasculature exist in the acidic, inflamed microenvironment and in turn alter their function in response to the acidic pH. Currently, only little is known how cells sense extracellular acidity and subsequently alter the inflammatory response. Recently, a class of proton-sensing G protein-coupled receptors (GPCRs) have emerged as functional pH-sensors, expressed in either leukocytes or vasculature, and are capable of altering immune cell inflammatory programs in response to acidic pH. These family members include GPR4, OGR1 (GPR68), TDAG8 (GPR65), and G2A (GPR132). Our group has uncovered a novel role for GPR4 in mediating endothelial cell (EC) inflammation in response to acidic pH. GPR4 activation in ECs have resulted in increased vascular adhesion molecule expression and functionally mediates leukocyte-EC interactions which are essential for the leukocyte extravasation process. Proton-sensors GPR65 and GPR68, however, are not expressed in ECs but are highly expressed in myeloid and lymphoid cells. GPR65 and GPR68, therefore, has been shown to mediate both pro- and anti-inflammatory responses in leukocytes in response to acidic pH. GPR132, however, has been described as a promiscuous GPCR, capable of responding to protons, bioactive lipids, and oxidized free fatty acids. Evidence suggests GPR132 is highly expressed in immune cells and plays important roles in immunity and the inflammatory response. Several lines of evidence suggest loss of pH homeostasis is associated IBD and could correspond to the degree of inflammation. For these reasons we sought to investigate the functional roles of pH- sensors GPR4, GPR65, and GPR132 in the regulation of intestinal inflammation. We utilized the acute and chronic dextran sulfate sodium (DSS)- induced experimental colitis mouse model with GPR4-null, GPR65-null, or GPR132-null mice. Our results indicate GPR4 contributes to intestinal inflammation in both acute and chronic DSS-induced colitis models likely though mediating leukocyte infiltration into the intestinal mucosa. Furthermore, a novel GPR4 antagonist was capable of inhibiting acute intestinal inflammation, suggesting GPR4 could be a valuable therapeutic target in colitis. Conversely, GPR65 reduces intestinal inflammation in the chronic DSS-induced colitis model. In vitro studies using bone marrow derived macrophages suggest GPR65 regulates macrophage functions. Lastly, GPR132 genetic deficiency was evaluated in two generations of GPR132 knockout mice in intestinal inflammation. Our results suggest GPR132 functions to reduce intestinal inflammation in the DSS-induced colitis mouse model. Further studies need to be performed to fully evaluate the role and mechanism of GPR132 in intestinal inflammation. Overall, this dissertation work highlights the emerging roles of pH-sensing GPCRs in the regulation of intestinal inflammation and implicates these receptors as valuable therapeutic targets in the remediation of intestinal inflammation.