Effects of Small Molecule Ligands on ACKR3 Receptors^S

Chemokines such as stromal derived factor 1 and their G protein coupled receptors are well-known regulators of the development and functions of numerous tissues. C-X-C motif chemokine ligand 12 (CXCL12) has two receptors: C-X-C chemokine motif receptor 4 (CXCR4) and atypical chemokine receptor 3 (ACKR3). 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 b-arrestin pathway. In support of this conclusion, we have shown that ACKR3 is unable to signal through any of the known mammalian G_a isoforms and have generated a comprehensive map of the G_a 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 b-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.