P041 G protein-independent signalling by the chemokine scavenger receptor D6 is required for its chemokine scavenging activity

Introduction Chemokine receptors include a set of molecules referred to as “atypical” chemokine receptors (ACR) because unable to induce directional cell migration and elicit conventional GPCR signalling. As no signaling activity has been observed after ACR engagement by their ligands, they are presently considered “silent” receptors. D6 is an ACR with scavenger activity on inflammatory CC chemokines with a non-redundant role for appropriate regulation of innate immunity and inflammation. Methods D6 expression was evaluated by FACS and confocal microscopy. Chemokine degradation was measured by ELISA test. Protein phosphorylation levels were detected by western blot. Results Our previous data showed that, differently from CCR5, ligand induces D6 up-regulation on plasma membrane through a rapid mobilization of receptor from recycling endosomes, thus improving its scavenging performance. Here we report that in cells expressing D6 or CCR5, ligand engagement causes a massive actin cytoskeleton reorganization and changes receptors colocalization with actin filaments in a completely opposite fashion. Both receptors signal to actin cytoskeleton by phosphorylating cofilinA through the Rac1-PAK1-LIMK1-dependent pathway. However, opposite to CCR5, D6 activates this pathway via a beta-arrestin-dependent, G protein-independent mechanism. Inhibition of each component of this signaling cascade completely abrogates D6 adaptive upregulation and scavenging activity. Conclusion D6, and possibly ACR in general, are signaling receptors exerting their regulatory functions on inflammation and immunity via the activation of a distinct signaling pathway.