MARVEL domain containing CMTM4 affects CXCR4 trafficking

The MARVEL proteins CMTM4 and CMTM6 control PD-L1, thereby influencing tumor immunity. We found that defective zebrafish cmtm4 slowed the development of the posterior lateral line (pLL) by altering the Cxcr4b gradient across the pLL primordium (pLLP). Analysis in mammalian cells uncovered that CMTM4 interacted with CXCR4, altering its glycosylation pattern, but did not affect internalization or degradation of CXCR4 in the absence of its ligand CXCL12. Synchronized release of CXCR4 from the endoplasmic reticulum revealed that CMTM4 slowed CXCR4 trafficking from the endoplasmic reticulum to the plasma membrane without affecting overall cell surface expression. Altered CXCR4 trafficking reduced ligand-induced CXCR4 degradation and affected AKT but not ERK1/2 activation. CMTM4 expression, in contrast to that of CXCR4, correlated with the survival of patients with renal cell cancer in the TCGA cohort. Furthermore, we observed that cmtm4 depletion promotes the separation of cells from the pLLP cell cluster in zebrafish embryos. Collectively, our findings indicate that CMTM4 exerts general roles in the biosynthetic pathway of cell surface molecules and seems to affect CXCR4-dependent cell migration.

Demystifying the CXCR4 conundrum in cancer biology: Beyond the surface signaling paradigm

The oncogenic chemokine duo CXCR4-CXCL12/SDF-1 (C-X-C Receptor 4-C-X-C Ligand 12/ Stromal-derived factor 1) has been the topic of intense scientific disquisitions since Muller et al., in her ground-breaking research, described this axis as a critical determinant of organ-specific metastasis in breast cancer. Elevated CXCR4 levels correlate with distant metastases, poor prognosis, and unfavourable outcomes in most solid tumors. Therapeutic impediment of the axis in clinics with Food and Drug Administration (FDA) approved inhibitors like AMD3100 or Plerixafor yield dubious results, contrary to pre-clinical developments. Clinical trials entailing inhibition of CXCR7 (C-X-C Receptor 7), another convicted chemokine receptor that exhibits affinity for CXCL12, reveal outcomes analogous to that of CXCR4-CXCL12 axis blockade. Of note, the cellular CXCR4 knockout phenotype varies largely from that of inhibitor treatments. These shaky findings pique great curiosity to delve further into the realm of this infamous chemokine receptor to provide a probable explanation. A multitude of recent reports suggests the presence of an increased intracellular CXCR4 pool in various cancers, both cytoplasmic and nuclear. This intracellular CXCR4 protein reserve seems active as it correlates with vital tumor attributes, viz. prognosis, aggressiveness, metastasis, and disease-free survival. Diminishing this entire intracellular CXCR4 load apart from the surface signals looks encouraging from a therapeutic point of view. Transcending beyond the classically accepted concept of ligand-mediated surface signaling, this review sheds new light on plausible associations of intracellularly compartmentalised CXCR4 with various aspects of tumorigenesis. Besides, this review also puts forward a comprehensive account of CXCR4 regulation in different cancers.

Development of a Novel SPR Assay to Study CXCR4-Ligand Interactions

G protein-coupled receptors (GPCRs) are involved in a plethora of different diseases. Consequently, these proteins are considered as an important class of drug targets. Measuring detailed kinetic information on these types of proteins has been challenging. Surface plasmon resonance (SPR) can provide this information, however, the use of SPR on GPCRs remains a complex issue. Here, we report an SPR assay to investigate the interactions between the full-length chemokine receptor CXCR4 and nanobody-Fc (Nb-Fc) ligands. Nb-Fcs consist of two monovalent VHH domains fused with an Fc domain of a human IgG molecule. The CXCR4 protein used in this assay was produced with a C-terminal 10x-histidine tag and was immobilized on a nitrilotriacetic acid chip. In order to verify the sensitivity and effectiveness of this assay, the results were compared to data obtained from cellular assays as well as from another SPR assay using CXCR4 virus-like particles (VLPs). CXCR4 remained intact and stable for at least 12 h, and the kinetic results correlated well with both the cellular assays and the VLP SPR assay results. Apart from determining the binding kinetics of Nb-Fc with CXCR4, our results contributed to understanding CXCR4 interaction dynamics. In conclusion, this assay provides a viable experimental platform that has high potential to be expanded for studying other molecules as well as other histidine-tagged GPCRs.

SDF-1α induces differential trafficking of CXCR4-CXCR7 involving cyclophilin A, CXCR7 ubiquitination and promotes platelet survival

Platelet-derived SDF-1α (CXCL12) mediates inflammatory and regenerative mechanisms. The present study characterizes the effect of SDF-1α ligation in platelets. SDF-1α (0-100 μM) dose and time dependently caused internalization of its receptor CXCR4 (28.9 ± 1.6 vs. 16.1 ± 1.9 in SDF-1α-treated platelets), coupled to the surface externalization of CXCR7 (65.5 ± 8 vs. 162.8 ± 27.6 following SDF-1α treatment), both in vitro and in vivo. This was inhibited in the presence of AMD3100 (100 μM), CXCR4 blocking and vesicular transport inhibitors (brefeldin A, 10 μM; rapamycin, 100 nM). SDF-1α/CXCR-4-mediated CXCR7 translocation was significantly reduced by inhibitors of ERK1/2-(U0126-10 μM) and cyclophilinA (CyPA)-(NIM811-10 μM) by 28 and 46%, respectively. Further, SDF-1α-induced downstream phosphorylation of Erk1/2 led to CyPA-dependent ubiquitination of CXCR7, which is essential for its surface translocation. CyPA-PPIase-activity inhibitor NIM-811, Erk1/2, and E1-ligase inhibitor-(PYR-41-25 μM) significantly abolished SDF-1α-driven CXCR7 ubiquitination and subsequent surface translocation. SDF-1α induced CXCR7 ubiquitination, and its surface exposure was observed in wild-type murine platelets, but not in CyPA-deficient platelets. SDF-1α/CXCR4-CyPA-dependent CXCR7 translocation and its subsequent ligation attenuated activation-induced apoptosis both in vitro and when administered in vivo. This antiapoptotic effect of SDF-1α was abrogated by blocking CXCR7, also significantly affected in Cypa(-/-) platelets. Thus, we decipher a novel mechanism, whereby SDF-1α regulates relative receptor availability in circulating platelets and exerts its prosurvival benefits.-Chatterjee, M., Seizer, P., Borst, O., Schönberger, T., Mack, A., Geisler, T., Langer, H. F., May, A. E., Vogel, S., Lang, F., Gawaz, M. SDF-1α induces differential trafficking of CXCR4-CXCR7 involving cyclophilin A, CXCR7 ubiquitination and promotes platelet survival.

Marked structural and functional heterogeneity in CXCR4: separation of HIV-1 and SDF-1alpha responses

CXCR4, the chemotactic cell receptor for SDF-1alpha, is essential for immune trafficking and HIV infection. CXCR4 is remarkably heterogeneous and the purpose of this study was to better identify the isoforms expressed by cells and compare their structure and function. We found that cells express either a predominant isoform or multiple isoforms. These were best resolved on SDS-PAGE using sucrose-gradient-fractionated, triton-insoluble, membrane extracts. We hypothesized that glycosyl modification may underpin some of this heterogeneity and that cell isoform(s) differences may underscore CXCR4's multiple cell functions. A comparison of wild-type (WT) and dual N-linked glycosylation site, N11A/N176A, mutant CXCR4 expressed in 3T3 and HEK-293 cells served to implicate variabilities in glycosylation and oligomerization in almost half of the isoforms. Immunoprecipitation of CXCR4 revealed monomer and dimer non-glycosylated forms of 34 kDa and 68 kDa from the N11A/N176A mutant, compared with glycosylated 40 kDa and 47 kDa and 73 kDa and 80 kDa forms from WT. The functional specificity of isoform action was also implicated because, despite CEMT4 cells expressing high levels of CXCR4 and 11 different isoforms, a single 83 kDa form was found to bind gp120 for HIV-1 IIIB infection. Furthermore, comparative studies found that in contrast to SDF-1alpha-responsive Nalm-6 cells that expressed similar levels of a single isoform, CEMT4 cells did not show a Ca(++) flux or a chemotactic response to SDF-1alpha. Thus, CXCR4 can differ both structurally and functionally between cells, with HIV-1 infection and chemotaxis apparently mediated by different isoforms. This separation of structure and function has implications for understanding HIV-1 entry and SDF-1alpha responses and may indicate therapeutic possibilities.