Baclofen and Other GABAB Receptor Agents Are Allosteric Modulators of the CXCL12 Chemokine Receptor CXCR4

CXCR4, a receptor for the chemokine CXCL12 (stromal-cell derived factor-1 alpha), is a G-protein-coupled receptor (GPCR), expressed in the immune and CNS and integrally involved in various neurological disorders. The GABA(B) receptor is also a GPCR that mediates metabotropic action of the inhibitory neurotransmitter GABA and is located on neurons and immune cells as well. Using diverse approaches, we report novel interaction between GABAB receptor agents and CXCR4 and demonstrate allosteric binding of these agents to CXCR4. First, both GABAB antagonists and agonists block CXCL12-elicited chemotaxis in human breast cancer cells. Second, a GABA(B) antagonist blocks the potentiation by CXCL12 of high-threshold Ca2+ channels in rat neurons. Third, electrophysiology in Xenopus oocytes and human embryonic kidney cell line 293 cells in which we coexpressed rat CXCR4 and the G-protein inward rectifier K+ (GIRK) channel showed that GABAB antagonist and agonist modified CXCL12-evoked activation of GIRK channels. To investigate whether GABA(B) ligands bind to CXCR4, we expressed this receptor in heterologous systems lacking GABA(B) receptors and performed competition binding experiments. Our fluorescent resonance energy transfer experiments suggest that GABA(B) ligands do not bind CXCR4 at the CXCL12 binding pocket suggesting allosteric modulation, in accordance with our electrophysiology experiments. Finally, using backscattering interferometry and lipoparticles containing only the CXCR4 receptor, we quantified the binding affinity for the GABA(B) ligands, confirming a direct interaction with the CXCR4 receptor. The effect of GABAergic agents on CXCR4 suggests new therapeutic potentials for neurological and immune diseases.