Annexin A1 interaction with the FPR2/ALX receptor: identification of distinct domains and downstream associated signaling.

Understanding how proresolving agonists selectively activate FPR2/ALX is a crucial step in the clarification of proresolution molecular networks that can be harnessed for the design of novel therapeutics for inflammatory disease. FPR2/ALX, a G protein-coupled receptor belonging to the formyl peptide receptor (FPR) family, conveys the biological functions of a variety of ligands, including the proresolution mediators annexin A1 (AnxA1) and lipoxin A(4), as well as the activating and proinflammatory protein serum amyloid A. FPR2/ALX is the focus of intense screening for novel anti-inflammatory therapeutics, and the small molecule compound 43 was identified as a receptor ligand. Here, we used chimeric FPR1 and FPR2/ALX clones (stably transfected in HEK293 cells) to identify the N-terminal region and extracellular loop II as the FPR2/ALX domain required for AnxA1-mediated signaling. Genomic responses were also assessed with domain-specific effects emerging, so the N-terminal region is required for AnxA1 induction of JAG1 and JAM3, whereas it is dispensable for modulation of SGPP2. By comparison, serum amyloid A non-genomic responses were reliant on extracellular loops I and II, whereas the small molecule compound 43 activated extracellular loop I with downstream signaling dependent on transmembrane region II. In desensitization experiments, the N-terminal region was dispensable for AnxA1-induced FPR2/ALX down-regulation in both the homologous and heterologous desensitization modes.