Interdependent allosteric FFA2R modulators synergistically induce functional selective activation and desensitization in neutrophils

The non-activating allosteric modulator AZ1729, specific for free fatty acid receptor 2 (FFA2R), transfers the orthosteric FFA2R agonists propionate and the P2Y2R specific agonist ATP into activating ligands that trigger an assembly of the neutrophil superoxide generating NADPH-oxidase. The homologous priming effect on the propionate response and the heterologous receptor cross-talk sensitized ATP response mediated by AZ1729 are functional characteristics shared with Cmp58, another non-activating allosteric FFA2R modulator. In addition, AZ1729 also turned Cmp58 into a potent activator of the superoxide generating neutrophil NADPH-oxidase, and in agreement with the allosteric modulation concept, the effect was reciprocal in that Cmp58 turned AZ1729 into a potent activating allosteric agonist. The activation signals down-stream of FFA2R when stimulated by the two interdependent allosteric modulators were biased in that, unlike for orthosteric agonists, the two complementary modulators together triggered an activation of the NADPH-oxidase, but not any transient rise in the cytosolic concentration of free Ca2+. In addition, following AZ1729/Cmp58 activation, signaling by the desensitized FFA2Rs was functionally selective in that the orthosteric agonist propionate could still induce a transient rise in intracellular Ca2+. The novel neutrophil activation and receptor down-stream signaling pattern mediated by the two cross-sensitizing allosteric FFA2R modulators represents a new regulatory mechanism that controls receptor signaling. Significance StatementA novel activation mechanism of a G-protein coupled free fatty acid receptor (FFA2R), is synergistically triggered by two otherwise non-activating allosteric modulators in the absence of orthosteric agonists. The receptor down-stream signaling proved to be functionally selective (biased); a superoxide generating enzyme is assembled and activated without involvement of the Ca2+ signaling pathway. The novel activation mechanism and the receptor down-stream signaling pattern mediated by the two cross-sensitizing allosteric FFA2R modulators, represents a new regulatory mechanism for control of GPCR-signaling.