Ligand stabilization of CXCR4/δ‐opioid receptor heterodimers reveals a mechanism for immune response regulation

Abstract

The CXCR4 chemokine receptor and the delta opioid receptor (DOR) are pertussis toxinsensitive G protein-coupled receptors (GPCR). Both are widely distributed in brain tissues and immune cells, and have key roles in inflammation processes and in pain sensation on proximal nerve endings. We show that in immune cells expressing CXCR4 and DOR, simultaneous addition of their ligands CXCL12 and [D-Pen2, DPen5]enkephalin does not trigger receptor function. This treatment does not affect ligand binding or receptor expression, nor does it promote heterologous desensitization. Our data indicate that CXCR4 and DOR form heterodimeric complexes that are dynamically regulated by the ligands. This is compatible with a model in which GPCR oligomerization leads to suppression of signaling, promoting a dominant negative effect. Knockdown of CXCR4 and DOR signaling by heterodimerization might have repercussions on physiological and pathological processes such as inflammation, pain sensation and HIV-1 infection.Supporting Information for this article is available at http://www.wiley-vch.de/contents/jc_2040/2008/37630_s.pdf See accompanying article: http://dx.doi.org/10.1002/eji200738101 IntroductionThe G protein-coupled receptors (GPCR) represent the largest, most diverse family of transmembrane receptors expressed in the body. They act through G proteins to regulate intracellular processes including cell adhesion, migration and proliferation [1]. Studies show that the GPCR can function as oligomers [2,3]. Perhaps their most striking feature is that GPCR form not only functional homo-oligomers, but can also associate with other GPCR to form hetero-oligomers. Homo-and hetero-oligomers differ in their pharmacological pro- files, ligand binding affinity, and/or internalization pathways [4][5][6]. GPCR hetero-oligomerization would thus enable generation of new types of signaling units. Opioid and chemokine receptors are members of the Ga i protein-linked GPCR. These receptors and their ligands are expressed in neurons and glial cells, and in immune system cells such as lymphocytes, monocytes and neutrophils [7]. They share the same microenvironment in many physiological situations and are essential for inflammation processes. Chemokine receptors promote immune cell migration to and adhesion at the inflammation site, whereas opioid receptors reduce pain sensation on proximal nerve endings. Opioid receptors can also regulate the immune response, as they alter antibody responses, cell-mediated immunity, phagocytic activity, adhesion and chemotaxis [8][9][10][11]. Opioids also prevent leukocyte movement toward chemokine gradients [12], and cross-desensitization is described between opioid and chemokine receptors [8,10]. The CCR5 chemokine receptor can form heterodimers with each of the three opioid receptor subtypes (l, d, and j) [8,10].CXCR4, the most widely expressed chemokine receptor [13], is crucial for correct lymphocyte trafficking [14], hematopoiesis, and development [15][16][17]. It is also a coreceptor for T-tropic HIV strains [1...

Slamming the DOR on chemokine receptor signaling: Heterodimerization silences ligand‐occupied CXCR4 and δ‐opioid receptors

“…In contrast to receptors that have been desensitized by internalization, the silent CXCR4/DOR complexes remain at the cell surface and appear to be available to readily reorganize as signaling-competent homodimers upon removal of one of the two ligands. Also, the profound impairment of CXCR4/DOR signaling and cellular responses observed by Pello et al [8] suggests that the distribution of homoand heterodimers is strongly biased in favor of heterodimers under conditions of stoichiometric receptor and saturating ligand levels. In their natural setting, immune cells may display different numbers of these receptors and will undoubtedly encounter varying proportions of ligands.…”

“…Therefore, proper controls and rigorous quantitative analysis are required in order to accurately describe dimerization dynamics in live cells [14][15]. Since the study by Pello et al [8] proposes a clear dynamic model of ligand-dependent receptor dimerization, more comprehensive studies will follow to verify the proposed mechanism in regulating GPCR signaling.…”

“…Whereas a single antagonist would inhibit function in a wide spectrum of cell types expressing the targeted GPCR, a combination of agonists intended to promote inhibition via heterodimerization could exert this effect more selectively in cells expressing both receptors. The in vivo feasibility of such combined therapy is supported by the demonstration by Pello et al [8] that inclusion of a DOR agonist can obviate the ability of CXCL12 to recruit monocytes to the site of injection. Systematic screening for potential heterodimeric GPCR interactions of co-expressed receptors might therefore be warranted.…”

“…In this issue of the European Journal of Immunology, Pello et al [8] reveal a new function of dimerization involving chemokine receptor CXCR4 and the d-opioid receptor (DOR), which are co-expressed on the surface of monocytes and other immune cells. The authors suggest that CXCR4 and DOR are capable of forming homodimers, as well as CXCR4/DOR heterodimers, in the plasma membrane.…”

Le baclofène est un modulateur allostérique du récepteur CXCR4

“…À l'aide de la technique TIRF, nous avons montré que les récepteurs CXCR4 ne forment pas de dimères lorsqu'ils sont exprimés à la membrane des ovocytes de xénope, contrairement à ce qui est décrit dans les cellules de mammifères [14,15]. Nous avons alors utilisé un autre système cellulaire, les cellules Nous avons d'abord pensé que les récep-teurs GABA B et CXCR4 pouvaient interagir dans la membrane des cellules : en effet, on sait que CXCR4 peut former des hétérodimères avec d'autres récepteurs couplés aux protéines G [12,13]. Toutefois, en collaboration avec une équipe américaine 1 nous avons fait exprimer à la membrane d'ovocytes de xénope les deux récepteurs marqués, l'un par un composé fluorescent émettant dans le rouge, l'autre par un composé fluorescent émettant dans le vert ; une analyse par microscopie TIRF (total internal reflexion fluorescence) nous a permis de constater que les deux types de récep-teurs ne sont pas colocalisés dans la membrane.…”