The adhesion family of G protein-coupled receptors (aGPCRs) comprises 33 members in human, several of which are distinctly expressed and functionally involved in polymorphonuclear cells (PMNs). As former work indicated the possible presence of the aGPCR GPR97 in granulocytes, we studied its cellular distribution, molecular structure, signal transduction, and biological function in PMNs. RNA sequencing and mass-spectrometry revealed abundant RNA and protein expression of ADGRG3/GPR97 in granulocyte precursors and terminally differentiated neutrophilic, eosinophilic, and basophilic granulocytes. Using a newly generated GPR97-specific monoclonal antibody, we confirmed that endogenous GPR97 is a proteolytically processed, dichotomous, N-glycosylated receptor. GPR97 was detected in tissue-infiltrating PMNs and upregulated during systemic inflammation. Antibody ligation of GPR97 increased neutrophil reactive oxygen species production and proteolytic enzyme activity, which is accompanied by an increase in mitogen-activated protein kinases and IκBα phosphorylation. In-depth analysis of the GPR97 signaling cascade revealed a possible switch from basal Gαs/cAMP-mediated signal transduction to a Gαi-induced reduction in cAMP levels upon mutation-induced activation of the receptor, in combination with an increase in downstream effectors of Gβγ, such as SRE and NF-κB. Finally, ligation of GPR97 increased the bacteria uptake and killing activity of neutrophils. We conclude that the specific presence of GPR97 regulates antimicrobial activity in human granulocytes.

Neutrophils play essential anti-microbial and inflammatory roles in host defense, however, their activities require tight regulation as dysfunction often leads to detrimental inflammatory and autoimmune diseases. Here we show that the adhesion molecule GPR97 allosterically activates CD177-associated membrane proteinase 3 (mPR3), and in conjugation with several protein interaction partners leads to neutrophil activation in humans. Crystallographic and deletion analysis of the GPR97 extracellular region identified two independent mPR3-binding domains. Mechanistically, the efficient binding and activation of mPR3 by GPR97 requires the macromolecular CD177/GPR97/PAR2/CD16b complex and induces the activation of PAR2, a G protein-coupled receptor known for its function in inflammation. Triggering PAR2 by the upstream complex leads to strong inflammatory activation, prompting anti-microbial activities and endothelial dysfunction. The role of the complex in pathologic inflammation is underscored by the finding that both GPR97 and mPR3 are upregulated on the surface of disease-associated neutrophils. In summary, we identify a PAR2 activation mechanism that directs neutrophil activation, and thus inflammation. The PR3/CD177/GPR97/PAR2/CD16b protein complex, therefore, represents a potential therapeutic target for neutrophil-mediated inflammatory diseases.

Neutrophils play essential anti-microbial and inflammatory roles in host defense, however their activities are tightly regulated as neutrophil dysfunction often leads to detrimental inflammatory and autoimmune diseases. Here, we identified a novel PR3/CD177/GPR97/PAR2/CD16b interactome as a critical modulator of neutrophil activation. Using multiple approaches, we deorphanized GPR97, a human neutrophil-restricted adhesion G protein-coupled receptor (aGPCR), as the interacting protein and allosteric activator of CD177-associated membrane proteinase 3 (mPR3). Structural and deletion analysis of the GPR97 extracellular region disclosed two independent mPR3-binding domains. The efficient binding and activation of mPR3 by GPR97 required a macromolecular CD177/GPR97/PAR2/CD16b interactome and resulted in the transactivation of PAR2, another GPCR. GPR97-mediated PAR2 transactivation in neutrophils elicited strong inflammatory activation, triggering anti-microbial activities and endothelial dysfunction. Altogether, we identify a novel aGPCR-GPCR transactivation mechanism that directs neutrophil activation and inflammation. The PR3/CD177/GPR97/PAR2/CD16b interactome represents a potential therapeutic target for neutrophil-mediated inflammatory diseases.