The DRF motif of CXCR6 as chemokine receptor adaptation to adhesion

Abstract

The CXC-chemokine receptor 6 (CXCR6) is a class A GTP-binding protein-coupled receptor (GPCRs) that mediates adhesion of leukocytes by interacting with the transmembrane cell surface-expressed chemokine ligand 16 (CXCL16), and also regulates leukocyte migration by interacting with the soluble shed variant of CXCL16. In contrast to virtually all other chemokine receptors with chemotactic activity, CXCR6 carries a DRF motif instead of the typical DRY motif as a key element in receptor activation and G protein coupling. In this work, modeling analyses revealed that the phenylalanine F3.51 in CXCR6 might have impact on intramolecular interactions including hydrogen bonds by this possibly changing receptor function. Initial investigations with embryonic kidney HEK293 cells and further studies with monocytic THP-1 cells showed that mutation of DRF into DRY does not influence ligand binding, receptor internalization, receptor recycling, and protein kinase B (AKT) signaling. Adhesion was slightly decreased in a time-dependent manner. However, CXCL16-induced calcium signaling and migration were increased. Vice versa, when the DRY motif of the related receptor CX3CR1 was mutated into DRF the migratory response towards CX3CL1 was diminished, indicating that the presence of a DRF motif generally impairs chemotaxis in chemokine receptors. Transmembrane and soluble CXCL16 play divergent roles in homeostasis, inflammation, and cancer, which can be beneficial or detrimental. Therefore, the DRF motif of CXCR6 may display a receptor adaptation allowing adhesion and cell retention by transmembrane CXCL16 but reducing the chemotactic response to soluble CXCL16. This adaptation may avoid permanent or uncontrolled recruitment of inflammatory cells as well as cancer metastasis.

The Role of CXCL16 in the Pathogenesis of Cancer and Other Diseases

“…CXCR6 requires Gα i for signal transduction and, therefore, is sensitive to pertussis toxin [ 12 , 40 ]. Activated CXCR6 leads to the activation of several signaling pathways, one causing calcium mobilization ( Figure 2 ) [ 7 , 12 , 25 ]. This, in response to sCXCL16, mediates the chemotaxis of the cells with activated CXCR6.…”

“…Importantly, in all other 7-transmembrane chemokine receptors, there is a DRY motif that is essential for receptor activation and signaling transduction to G proteins. In CXCR6, instead of a DRY motif, a DRF motif is present [ 25 ]. Nevertheless, it does not cause changes in the activation of the phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)→Akt/PKB axis and ligand binding, receptor internalization, or receptor recycling, but only increases cell adhesion to mCXCL16.…”

“…Nevertheless, it does not cause changes in the activation of the phosphatidylinositol-4,5-bisphosphate 3-kinase (PI3K)→Akt/PKB axis and ligand binding, receptor internalization, or receptor recycling, but only increases cell adhesion to mCXCL16. Also, this motif change reduces calcium signaling activated by CXCR6, which is associated with a reduction in chemotactic response to sCXCL16 [ 25 ]. This effect seems to be cell-specific [ 25 , 45 ].…”

CXCR6 by increasing retention of memory CD8 T cells in the ovarian tumor microenvironment promotes immunosurveillance and control of ovarian cancer

“…CXCR6 on tumor-specific T cells then engage with CXCL16 derived from the TME (produced by macrophages or tumor cells themselves) and facilitates T cell retention. The role of CXCR6 in T cell retention is supported by the previous demonstration that the unique DRF motif in CXCR6 makes it more suited for adhesion than chemotaxis to ligand CXCL16 (66) which exists in both transmembrane and soluble forms (67). We further propose that CXCR6 marks pre-Trm, cells that have increased propensity to become CXCR6 + CD103 + Trm cells, as CXCR6 mediated retention may increase the probability of T cell exposure to TGF-β, a cytokine proposed to induce CD103 + and Trm induction (42, 47, 68).…”

‘Crystal’ Clear? Lysophospholipid Receptor Structure Insights and Controversies

“…LPA 6 has a more conventional DRF motif. Mutation of DRF in the chemokine receptor CXCR6 to a canonical DRY leads to altered Gα protein coupling, which already occurs in a cell-type specific fashion [58, 59]. This may explain some difficulties encountered when attempting to measure activation of LPA 6 using traditional methodologies and cell lines [25, 32].…”