Context-Dependent Signaling of CXC Chemokine Receptor 4 and Atypical Chemokine Receptor 3

Abstract

G protein-coupled receptors (GPCRs) are regulated by complex molecular mechanisms, both in physiologic and pathologic conditions, and their signaling can be intricate. Many factors influence their signaling behavior, including the type of ligand that activates the GPCR, the presence of interacting partners, the kinetics involved, or their location. The two CXC-type chemokine receptors, CXC chemokine receptor 4 (CXCR4) and atypical chemokine receptor 3 (ACKR3), both members of the GPCR superfamily, are important and established therapeutic targets in relation to cancer, human immunodeficiency virus infection, and inflammatory diseases. Therefore, it is crucial to understand how the signaling of these receptors works to be able to specifically target them. In this review, we discuss how the signaling pathways activated by CXCR4 and ACKR3 can vary in different situations. G protein signaling of CXCR4 depends on the cellular context, and discrepancies exist depending on the cell lines used. ACKR3, as an atypical chemokine receptor, is generally reported to not activate G proteins but can broaden its signaling spectrum upon heteromerization with other receptors, such as CXCR4, endothelial growth factor receptor, or the a 1-adrenergic receptor (a 1-AR). Also, CXCR4 forms heteromers with CC chemokine receptor (CCR) 2, CCR5, the Na 1 / H 1 exchanger regulatory factor 1, CXCR3, a 1-AR, and the opioid receptors, which results in differential signaling from that of the monomeric subunits. In addition, CXCR4 is present on membrane rafts but can go into the nucleus during cancer progression, probably acquiring different signaling properties. In this review, we also provide an overview of the currently known critical amino acids involved in CXCR4 and ACKR3 signaling.

The atypical chemokine receptor ACKR3/CXCR7 is a broad-spectrum scavenger for opioid peptides

“…How this dual ligand scavenging function impacts on each other remains to be elucidated. Moreover, although the results of the present study add another piece to the growing body of evidence that ACKR3 is unable to trigger G protein signaling, it cannot be excluded that, in certain cell types or cellular contexts, ACKR3 could induce G protein-dependent or -independent signaling 27 . Finally, in analogy to its regulatory function within the chemokine system (when co-expressed with CXCR4) the ability of ACKR3 to heterodimerize with classical opioid receptors and to modulate their signaling properties remain to be investigated 85 .…”

“…It plays crucial roles in neuronal and cardiovascular development and in the migration of hematopoietic stem cells 24,25 . The functions of ACKR3 are proposed to mainly rely on arrestin recruitment, which is indicative of ACKR3 activation, while its signaling capacity remains highly debated and may be cell-context-dependent [26][27][28][29] . ACKR3 binds two endogenous chemokines, CXCL12 and CXCL11, which also activate the classical chemokine receptors CXCR4 and CXCR3 30 , respectively, as well as the virus-encoded CC chemokine vMIP-II/vCCL2 31 .…”

Natural and engineered chemokine (C-X-C motif) receptor 4 agonists prevent acute respiratory distress syndrome after lung ischemia–reperfusion injury and hemorrhage

“…Furthermore, the natural CXCR4 agonists CXCL12 and ubiquitin are proteolytically processed in the systemic circulation, leading to their inactivation or the generation of CXCR4 antagonists, which will result in altered ligand binding kinetics at the receptor and modulate subsequent signaling events 24,41,42 . These complex mechanisms likely correspond to the observations that signaling and functions of CXCR4 and its agonists depend on the cellular context and the specific pathophysiological environment 34,38 . Thus, context-dependent consequences of CXCR4 activation with CXCL12 may explain that CXCL12 administration desensitized phenylephrine-mediated vasoconstriction of isolated arteries and reduced blood pressure in a model of isolated hemorrhagic shock in spontaneously breathing rats, whereas CXCL12 sensitized the phenylephrine induced blood pressure response in normal rats and stabilized hemodynamics in the present ARDS model 34,37 .…”

“…Thus, direct protective effects of CXCR4 agonists on lung endothelial barrier function as well as improvement of cardiovascular function with subsequently reduced fluid resuscitation requirements upon CXCR4 activation likely contributed to the beneficial effects that we observed in the present study. CXCR4 and ACKR3, however, are known to hetero-oligomerize with each other and with several other G protein coupled receptors, which likely affects their signaling properties depending on the available repertoire of interacting receptor partners 34,35,[38][39][40] . Furthermore, the natural CXCR4 agonists CXCL12 and ubiquitin are proteolytically processed in the systemic circulation, leading to their inactivation or the generation of CXCR4 antagonists, which will result in altered ligand binding kinetics at the receptor and modulate subsequent signaling events 24,41,42 .…”

CRISPR-Mediated Protein Tagging with Nanoluciferase to Investigate Native Chemokine Receptor Function and Conformational Changes

“…The function of CXCR4 can be influenced by spatiotemporal factors such as cellular compartmentalization (Wysoczynski et al, 2005) as well as by the absence, presence, or relative stoichiometry of interacting partners found in the cellular proteome (Heuninck et al, 2019). Here, we observed no difference in the binding affinity of CXCL12-AF647 for NLuc/CXCR4, despite the differences in NanoBRET ligand binding assays used, i.e., high or low expression of CXCR4, the absence or presence of ACKR3, or where the effects of agonist-induced internalization of CXCR4 were removed, indicating that the variables we tested have little impact on CXCL12 binding to CXCR4.…”