Abstract

Chemokines such as Stromal Derived Factor 1 (SDF-1 or CXCL12) and their G protein coupled receptors (GPCRs) are well known regulators of the development and functions of numerous tissues. CXCL12 has two receptors: CXCR4 and ACKR3 or CXCR7. ACKR3 has been described as an atypical "biased" receptor because it does not appear to signal through G proteins and instead, signals solely through the β-arrestin pathway. In support of this conclusion, we have shown that ACKR3 is unable to signal through any of the known mammalian G_α isoforms and have generated a comprehensive map of the G_α activation by CXCL12/CXCR4. We also synthesized a series of small molecule ligands which acted as selective agonists for ACKR3 as assessed by their ability to recruit β-arrestin to the receptor. Using select point mutations, we studied the molecular characteristics that determine the ability of small molecules to activate ACKR3 receptors, revealing a key role for the deeper binding pocket composed of residues in the transmembrane domains of ACKR3. The development of more selective ACKR3 ligands should allow us to better appreciate the unique roles of ACKR3 in the CXCL12/CXCR4/ACKR3 signaling axis and better understand the structural determinants for ACKR3 activation.

Significance Statement We are interested in the signaling produced by the G protein coupled receptor (GPCR) ACKR3 which signals atypically. In this study novel selective ligands for ACKR3 were discovered and the site of interactions between these small molecules and ACKR3 was defined. This work will help to better understand the unique signaling roles of ACKR3.