Characterization of recombinant human CXCR4 in insect cells: role of extracellular domains and N-glycosylation in ligand binding

Neutrophils disrupt B-1a cell homeostasis by targeting Siglec-G to exacerbate sepsis

B-1a cells, an innate-like cell population, are crucial for pathogen defense and the regulation of inflammation through their release of natural IgM and IL-10. In sepsis, B-1a cell numbers are decreased in the peritoneal cavity as they robustly migrate to the spleen. Within the spleen, migrating B-1a cells differentiate into plasma cells, leading to alterations in their original phenotype and functionality. We discovered a key player, sialic acid-binding immunoglobulin-like lectin-G (Siglec-G), which is expressed predominantly on B-1a cells and negatively regulates B-1a cell migration to maintain homeostasis. Siglec-G interacts with CXCR4/CXCL12 to modulate B-1a cell migration. Neutrophils aid B-1a cell migration via neutrophil elastase (NE)-mediated Siglec-G cleavage. Human studies revealed increased NE expression in septic patients. We identified an NE cleavage sequence in silico, leading to the discovery of a decoy peptide that protects Siglec-G, preserves peritoneal B-1a cells, reduces inflammation, and enhances sepsis survival. The role of Siglec-G in inhibiting B-1a cell migration to maintain their inherent phenotype and function is compromised by NE in sepsis, offering valuable insights into B-1a cell homeostasis. Employing a small decoy peptide to prevent NE-mediated Siglec-G cleavage has emerged as a promising strategy to sustain peritoneal B-1a cell homeostasis, alleviate inflammation, and ultimately improve outcomes in sepsis patients.

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.

Treatment of B-cell precursor acute lymphoblastic leukemia with the Galectin-1 inhibitor PTX008

Background

Drug resistance of B-cell precursor acute lymphoblastic leukemia (BP-ALL) cells is conferred by both intrinsic and extrinsic factors, which could be targeted to promote chemo-sensitization. Our previous studies showed that Galectin-3, a lectin that clusters galactose-modified glycoproteins and that has both an intracellular and extracellular location, protects different subtypes of BP-ALL cells against chemotherapy. Galectin-1 is related to Galectin-3 and its expression was previously reported to be restricted to the MLL subtype of BP-ALL.

Methods and results

Here, we report that Galectin-1 is expressed at different levels in and on different subclasses of BP-ALLs. Bone marrow plasma also contains high levels of Galectin-1. PTX008 is an allosteric inhibitor which inhibits Galectin-1 but not Galectin-3-mediated agglutination. The compound reduces migration of BP-ALL cells to CXCL12 and OP9 stromal cells and inhibits fibronectin-mediated adhesion. It also affects cell cycle progression of BCP-ALL cells. PTX008 is cytostatic for BP-ALL cells even when these are co-cultured with protective stroma, and can sensitize ALL cells to vincristine chemotherapy in vitro and in mice.

Conclusions

PTX008 inhibits multiple functions that contribute to BP-ALL survival. The effects of Galectin-1 inhibition on both BP-ALL cell proliferation and migration suggest both the leukemia cells as well as the microenvironment that protects these cells may be targeted.

Electronic supplementary material

The online version of this article (10.1186/s13046-018-0721-7) contains supplementary material, which is available to authorized users.

Proteolytic Origin of the Soluble Human IL-6R In Vivo and a Decisive Role of N-Glycosylation

Signaling of the cytokine interleukin-6 (IL-6) via its soluble IL-6 receptor (sIL-6R) is responsible for the proinflammatory properties of IL-6 and constitutes an attractive therapeutic target, but how the sIL-6R is generated in vivo remains largely unclear. Here, we use liquid chromatography–mass spectrometry to identify an sIL-6R form in human serum that originates from proteolytic cleavage, map its cleavage site between Pro-355 and Val-356, and determine the occupancy of all O- and N-glycosylation sites of the human sIL-6R. The metalloprotease a disintegrin and metalloproteinase 17 (ADAM17) uses this cleavage site in vitro, and mutation of Val-356 is sufficient to completely abrogate IL-6R proteolysis. N- and O-glycosylation were dispensable for signaling of the IL-6R, but proteolysis was orchestrated by an N- and O-glycosylated sequon near the cleavage site and an N-glycan exosite in domain D1. Proteolysis of an IL-6R completely devoid of glycans is significantly impaired. Thus, glycosylation is an important regulator for sIL-6R generation.

Interleukin-6 (IL-6) is a cytokine secreted by our body upon infection or trauma to stimulate the immune system response. IL-6 is partially responsible for fever and triggers inflammation in many diseases. It activates its target cells via the membrane-bound IL-6 receptor (IL-6R), and soluble forms of this receptor (sIL-6R) are present in high amounts in the serum of healthy individuals and mediate the inflammatory response in all cells of the human body. However, it remains unclear how the soluble form of this cytokine is generated in humans. In this study, we isolate sIL-6R from human serum and show that the majority is produced via cleavage of the membrane-bound IL-6R by a protease. We identify the exact cleavage site and find that it is identical to a cleavage site used by the metalloprotease ADAM17. We further show that glycosylation, a post-transcriptional modification, is dispensable for the transport and biological function of IL-6R and map the occupancy of all O- and N-glycosylation sites. However, we find that only a single N-glycan is critically involved in the regulation of proteolysis by ADAM17 and conclude that glycosylation is an important regulator for sIL-6R generation.

Mutational Analysis of Atypical Chemokine Receptor 3 (ACKR3/CXCR7) Interaction with Its Chemokine Ligands CXCL11 and CXCL12

Atypical chemokine receptors do not mediate chemotaxis or G protein signaling, but they recruit arrestin. They also efficiently scavenge their chemokine ligands, thereby contributing to gradient maintenance and termination. ACKR3, also known as CXCR7, binds and degrades the constitutive chemokine CXCL12, which also binds the canonical receptor CXCR4, and CXCL11, which also binds CXCR3. Here we report comprehensive mutational analysis of the ACKR3 interaction with its chemokine ligands, using 30 substitution mutants. Readouts are radioligand binding competition, arrestin recruitment, and chemokine scavenging. Our results suggest different binding modes for both chemokines. CXCL11 depends on the ACKR3 N terminus and some extracellular loop (ECL) positions for primary binding, ECL residues mediate secondary binding and arrestin recruitment potency. CXCL12 binding required key residues Asp-179^4.60 and Asp-275^6.58 (residue numbering follows the Ballesteros-Weinstein scheme), with no evident involvement of N-terminal residues, suggesting an uncommon mode of receptor engagement. Mutation of residues corresponding to CRS2 in CXCR4 (positions Ser-103^2.63 and Gln-301^7.39) increased CXCL11 binding, but reduced CXCL12 affinity. Mutant Q301E^7.39 did not recruit arrestin. Mutant K118A^3.26 in ECL1 showed moderate baseline arrestin recruitment with ablation of ligand-induced responses. Substitutions that affected CXCL11 binding also diminished scavenging. However, detection of reduced CXCL12 scavenging by mutants with impaired CXCL12 affinity required drastically reduced receptor expression levels, suggesting that scavenging pathways can be saturated and that CXCL12 binding exceeds scavenging at higher receptor expression levels. Arrestin recruitment did not correlate with scavenging; although Q301E^7.39 degraded chemokines in the absence of arrestin, S103D^2.63 had reduced CXCL11 scavenging despite intact arrestin responses.

CXCR4 signaling in health and disease

Chemokines and chemokine receptors regulate multiple processes such morphogenesis, angiogenesis and immune responses. Among the chemokine receptors, CXCR4 stands out for its pleiotropic roles as well as for its involvement in several pathological conditions, including immune diseases, viral infections and cancer. For these reasons, CXCR4 represents a crucial target in drug development. In this review, we discuss of CXCR4 receptor properties and signaling in health and diseases, focusing on the WHIM syndrome, an inherited immunodeficiency caused by mutations of the CXCR4 gene.

Mechanisms regulating cell membrane localization of the chemokine receptor CXCR4 in human hepatocarcinoma cells

Hepatocellular carcinoma (HCC) cells with a mesenchymal phenotype show an asymmetric subcellular distribution of the chemokine receptor CXCR4, which is required for cell migration and invasion. In this work we examine the mechanisms that regulate the intracellular trafficking of CXCR4 in HCC cells. Results indicate that HCC cells present CXCR4 at the cell surface, but most of this protein is in endomembranes colocalizing with markers of the Golgi apparatus and recycling endosomes. The presence of high protein levels of CXCR4 present at the cell surface correlates with a mesenchymal-like phenotype and a high autocrine activation of the Transforming Growth Factor-beta (TGF-β) pathway. CXCR4 traffics along the Golgi/exocyst/plasma membrane pathway and requires EXOC4 (Sec8) component of the exocyst complex. HCC cells use distinct mechanisms for the CXCR4 internalization such as dynamin-dependent endocytosis and macropinocytosis. Regardless of the endocytic mechanisms, colocalization of CXCR4 and Rab11 is observed, which could be involved not only in receptor recycling but also in its post-Golgi transport. In summary, this work highlights membrane trafficking pathways whose pharmacological targeting could subsequently result in the inactivation of one of the main guiding mechanisms used by metastatic cells to colonize secondary organs and tissues.

Analysis by substituted cysteine scanning mutagenesis of the fourth transmembrane domain of the CXCR4 receptor in its inactive and active state

The chemokine SDF-1 (CXCL12) selectively binds to CXCR4, a member of the G protein-coupled receptor (GPCR) superfamily. In this study, we used the substituted-cysteine accessibility method (SCAM) to identify specific residues of the fourth transmembrane domain (TM4) that contribute to the formation of the binding pocket of CXCR4 in its inactive and active state. We successively substituted each residue from E179((4.68)) to K154((4.43)) with cysteine and expressed the mutants in COS-7 cells. Mutant receptors were then alkylated with methanethiosulfonate-ethylammonium (MTSEA), and binding inhibition was monitored using the CXCR4 antagonist FC131 [cyclo(-D-Tyr(1)-Arg(2)-Arg(3)-Nal(4)-Gly(5)-)], which displays anti-HIV activity. MTSEA treatment resulted in a significant reduction of FC131 binding to D171C((4.60)) and P170C((4.59)). To assess TM4 accessibility in an active state of CXCR4, TM4 cysteine mutants were transposed within the constitutively active mutant N119S((3.35)). MTSEA treatment of TM4 mutants N119S-S178C((4.67)), N119S-V177C((4.66)) and N119S-I173C((4.62)) resulted in a significant reduction in FC131 binding. Protection assays using FC131 prior to MTSEA treatment significantly reduced the alkylation of all MTSEA-sensitive mutants. The accessibility of the D171C((4.60)) and P170C((4.59)) residues suggests that they are oriented towards a water-accessible area of the binding pocket of CXCR4. S178C((4.67)), V177C((4.66)) and I173C((4.62)) showed binding inhibition only in an N119S((3.35)) background. Taken together our results suggest that TM4 and ECL2 undergo conformational changes during CXCR4 activation and also demonstrate how TM4 is an important feature for the binding of anti-HIV compounds.

Quantification of ligand binding to G-protein coupled receptors on cell membranes by ellipsometry

G-protein-coupled receptors (GPCRs) are prime drug targets and targeted by approximately 60% of current therapeutic drugs such as β-blockers, antipsychotics and analgesics. However, no biophysical methods are available to quantify their interactions with ligand binding in a native environment. Here, we use ellipsometry to quantify specific interactions of receptors within native cell membranes. As a model system, the GPCR-ligand CXCL12α and its receptor CXCR4 are used. Human-derived Ishikawa cells were deposited onto gold coated slides via Langmuir-Schaefer film deposition and interactions between the receptor CXCR4 on these cells and its ligand CXCL12α were detected via total internal reflection ellipsometry (TIRE). This interaction could be inhibited by application of the CXCR4-binding drug AMD3100. Advantages of this approach are that it allows measurement of interactions in a lipid environment without the need for labelling, protein purification or reconstitution of membrane proteins. This technique is potentially applicable to a wide variety of cell types and their membrane receptors, providing a novel method to determine ligand or drug interactions targeting GPCRs and other membrane proteins.

Sf9 cells: a versatile model system to investigate the pharmacological properties of G protein-coupled receptors

The Sf9 cell/baculovirus expression system is widely used for high-level protein expression, often with the purpose of purification. However, proteins may also be functionally expressed in the defined Sf9 cell environment. According to the literature, the pharmacology of G-protein-coupled receptors (GPCRs) functionally reconstituted in Sf9 cells is similar to the receptor properties in mammalian cells. Sf9 cells express both recombinant GPCRs and G-proteins at much higher levels than mammalian cells. Sf9 cells can be grown in suspension culture, providing an inexpensive way of obtaining large protein amounts. Co-infection with various baculoviruses allows free combination of GPCRs with different G-proteins. The absence of constitutively active receptors in Sf9 cells provides an excellent signal-to background ratio in functional assays, allowing the detection of agonist-independent receptor activity and of small ligand-induced signals including partial agonistic and inverse agonistic effects. Insect cell Gα(i)-like proteins mostly do not couple productively to mammalian GPCRs. Thus, unlike in mammalian cells, Sf9 cells do not require pertussis toxin treatment to obtain a Gα(i)-free environment. Co-expression of GPCRs with Gα(i1), Gα(i2), Gα(i3) or Gα(o) in Sf9 cells allows the generation of a selectivity profile for these Gα(i/o)-isoforms. Additionally, GPCR-G-protein combinations can be compared with defined 1:1 stoichiometry by expressing GPCR-Gα fusion proteins. Sf9 cells can also be employed for ligand screening in medicinal chemistry programs, using radioligand binding assays or functional assays, like the steady-state GTPase- or [(35)S]GTPγS binding assay. This review shows that Sf9 cells are a versatile model system to investigate the pharmacological properties of GPCRs.

N-linked glycosylation is essential for the stability but not the signaling function of the interleukin-6 signal transducer glycoprotein 130

N-Linked glycosylation is an important determinant of protein structure and function. The interleukin-6 signal transducer glycoprotein 130 (gp130) is a common co-receptor for cytokines of the interleukin (IL)-6 family and is N-glycosylated at 9 of 11 potential sites. Whereas N-glycosylation of the extracellular domains D1-D3 of gp130 has been shown to be dispensable for binding of the gp130 ligand IL-6 and its cognate receptor in vitro, the role of the N-linked glycans on domains D4 and D6 is still unclear. We have mutated the asparagines of all nine functional N-glycosylation sites of gp130 to glutamine and systematically analyzed the consequences of deleted N-glycosylation (dNG) in both cellular gp130 and in a soluble gp130-IgG1-Fc fusion protein (sgp130Fc). Our results show that sgp130Fc-dNG is inherently unstable and degrades rapidly under conditions that do not harm wild-type sgp130Fc. Consistently, the bulk of cellular gp130-dNG is not transported to the plasma membrane but is degraded in the proteasome. However, the small quantities of gp130-dNG, which do reach the cell surface, are still able to activate the key gp130 signaling target signal transducer and activator of transcription-3 (STAT3) upon binding of the agonistic complex of IL-6 and soluble IL-6 receptor. In conclusion, N-linked glycosylation is required for the stability but not the signal-transducing function of gp130.

CXCR4 and matrix metalloproteinase-1 are elevated in breast carcinoma-associated fibroblasts and in normal mammary fibroblasts exposed to factors secreted by breast cancer cells

The complex molecular communications that occur between neoplastic and stromal cells within the tumor microenvironment play an integral role in breast cancer pathogenesis. Carcinoma-associated fibroblasts (CAF) produce tumor-enhancing factors and have been strongly implicated in breast cancer development. Similar to the way in which tumors have been compared with "wounds that never heal," CAFs have been equated to activated fibroblasts, which are present in inflammatory environments, in which they aid in wound healing through tissue remodeling and repair. Matrix metalloproteinase-1 (MMP-1) and G protein-coupled receptor, CXCR4, are elevated in these activated fibroblasts, in which they facilitate angiogenesis and matrix degradation, processes that are also vital to breast cancer metastasis. In this study, we investigated MMP-1 and CXCR4 expression in normal human mammary fibroblasts (HMF) exposed to soluble breast cancer factors. Historically, elevated CXCR4 expression is associated with breast cancer cells. However, we show that soluble factors secreted by SUM102 breast cancer cells stimulated the expression of MMP-1 and CXCR4 in HMFs. As a result, these stromal cells acquired an invasive and migratory phenotype. To confirm the clinical relevancy of our findings, we analyzed CAFs obtained from primary breast cancers. These cells also displayed elevated MMP-1 and CXCR4 levels compared with counterpart fibroblasts, and were more invasive and migratory. Together, our data suggest that soluble breast cancer factors initiate the transdifferentiation of normal HMFs to tumor-promoting CAFs, and that through the induction of MMP-1 and CXCR4 levels, these cells exhibit an invasive and migratory phenotype.

N-glycans modulate the activation of gp130 in mouse embryonic neural precursor cells

gp130 is a ubiquitously expressed glycoprotein and signal transducer of interleukin 6 family of cytokines. It has been reported that gp130 has 11 potential N-glycosylation sites in the extracellular domain, and nine of them are actually N-glycosylated. However, the structure and functional role of the carbohydrate chains carried by gp130 are totally unknown. In this study, we examined the functional role of N-glycans of gp130 in mouse neuroepithelial cells. In neuroepithelial cells treated with tunicamycin, an N-glycosylation inhibitor, unglycosylated form of gp130 was detected. The unglycosylated gp130 was not phosphorylated in response to leukemia inhibitory factor stimulation. Although the unglycosylated gp130 was found to be expressed on the cell surface, it could not form a heterodimer with leukemia inhibitory factor receptor. These results suggest that N-glycans are required for the activation, but not for the localization, of gp130 in neuroepithelial cells.

Functional reconstitution of the human chemokine receptor CXCR4 with G(i)/G (o)-proteins in Sf9 insect cells

The chemokine stromal cell-derived factor-1alpha (SDF-1alpha) binds to the chemokine receptor CXCR4 that couples to pertussis toxin-sensitive G-proteins of the G(i)/G(o)-family. CXCR4 plays a role in the pathogenesis of autoimmune diseases, human immunodeficiency virus infection and various tumors, fetal development as well as endothelial progenitor and T-cell recruitment. To this end, most CXCR4 studies have focused on the cellular level. The aim of this study was to establish a reconstitution system for the human CXCR4 that allows for the analysis of receptor/G-protein coupling at the membrane level. We wished to study specifically constitutive CXCR4 activity and the G-protein-specificity of CXCR4. We co-expressed N- and C-terminally epitope-tagged human CXCR4 with various G(i)/G(o)-proteins and regulator of G-protein signaling (RGS)-proteins in Sf9 insect cells. Expression of CXCR4, G-proteins, and RGS-proteins was verified by immunoblotting. CXCR4 coupled more effectively to Galpha(i1) and Galpha(i2) than to Galpha(i3) and Galpha(o) and insect cell G-proteins as assessed by SDF-1alpha-stimulated high-affinity steady-state GTP hydrolysis. The RGS-proteins RGS4 and GAIP enhanced SDF-1alpha-stimulated GTP hydrolysis. SDF-1alpha stimulated [(35)S]guanosine 5'-[gamma-thio]triphosphate (GTPgammaS) binding to Galpha(i2). RGS4 did not enhance GTPgammaS binding. Na(+) salts of halides did not reduce basal GTPase activity. The bicyclam, 1-[[1,4,8,11-tetrazacyclotetradec-1-ylmethyl)phenyl]methyl]-1,4,8,11-tetrazacyclotetradecane (AMD3100), acted as CXCR4 antagonist but was devoid of inverse agonistic activity. Halides reduced the maximum SDF-1alpha-stimulated GTP hydrolysis in the order of efficacy I(-) > Br(-) > Cl(-). In addition, salts reduced the potency of SDF-1alpha at activating GTP hydrolysis. From our data, we conclude the following: (1) Sf9 cells are a suitable system for expression of functionally intact human CXCR4; (2) Human CXCR4 couples effectively to Galpha(i1) and Galpha(i2); (3) There is no evidence for constitutive activity of CXCR4; (4) RGS-proteins enhance agonist-stimulated GTP hydrolysis, showing that GTP hydrolysis becomes rate-limiting in the presence of SDF-1alpha; (5) By analogy to previous observations made for the beta(2)-adrenoceptor coupled to G(s), the inhibitory effects of halides on agonist-stimulated GTP hydrolysis may be due to increased GDP-affinity of G(i)-proteins, reducing the efficacy of CXCR4 at stimulating nucleotide exchange.

Regulation of CXCR4 signaling

The chemokine receptor CXCR4 belongs to the large superfamily of G protein-coupled receptors, and is directly involved in a number of biological processes including organogenesis, hematopoiesis, and immune response. Recent evidence has highlighted the role of CXCR4 in a variety of diseases including HIV, cancer, and WHIM syndrome. Importantly, the involvement of CXCR4 in cancer metastasis and WHIM syndrome appears to be due to dysregulation of the receptor leading to enhanced signaling. Herein we review what is currently known regarding the regulation of CXCR4 and how dysregulation contributes to disease progression.

In vitro reconstitution and preparative purification of complexes between the chemokine receptor CXCR4 and its ligands SDF-1alpha, gp120-CD4 and AMD3100

CXCR4 belongs to the family of G protein-coupled receptors and mediates the various developmental and regulatory effects of the chemokine SDF-1alpha. In addition, CXCR4 acts as a co-receptor along with CD4 for the HIV-1 viral glycoprotein gp120. Recently, there has also been a small molecule described that antagonizes both SDF-1 and gp120 binding to CXCR4. The structural and mechanistic basis for this dual recognition ability of CXCR4 is unknown largely due to the technical challenges of biochemically producing the components of the various complexes. We expressed the human CXCR4 receptor using a modified baculovirus expression vector that facilitates a single step antibody affinity purification of CXCR4 to >80% purity from Hi5 cells. The recombinant receptor undergoes N-linked glycosylation, tyrosine sulfation and is recognized by the 12G5 conformation specific antibody against human CXCR4. We are able to purify CXCR4 alone as well as complexed with its endogenous ligand SDF-1, its viral ligand gp120, and a small molecule antagonist AMD3100 by ion-exchange chromatography. We anticipate that the expression and purification scheme described in this paper will facilitate structure-function studies aimed at elucidating the molecular basis for CXCR4 recognition of its endogenous chemokine and viral ligands.

Chemokine receptor internalization and intracellular trafficking

The internalization and intracellular trafficking of chemokine receptors have important implications for the cellular responses elicited by chemokine receptors. The major pathway by which chemokine receptors internalize is the clathrin-mediated pathway, but some receptors may utilize lipid rafts/caveolae-dependent internalization routes. This review discusses the current knowledge and controversies regarding these two different routes of endocytosis. The functional consequences of internalization and the regulation of chemokine receptor recycling will also be addressed. Modifications of chemokine receptors, such as palmitoylation, ubiquitination, glycosylation, and sulfation, may also impact trafficking, chemotaxis and signaling. Finally, this review will cover the internalization and trafficking of viral and decoy chemokine receptors.

Chemokine receptor CCR5: insights into structure, function, and regulation

CC chemokine receptor 5 (CCR5) is a seven-transmembrane, G protein-coupled receptor (GPCR) which regulates trafficking and effector functions of memory/effector T-lymphocytes, macrophages, and immature dendritic cells. It also serves as the main coreceptor for the entry of R5 strains of human immunodeficiency virus (HIV-1, HIV-2). Chemokine binding to CCR5 leads to cellular activation through pertussis toxin-sensitive heterotrimeric G proteins as well as G protein-independent signalling pathways. Like many other GPCR, CCR5 is regulated by agonist-dependent processes which involve G protein coupled receptor kinase (GRK)-dependent phosphorylation, beta-arrestin-mediated desensitization and internalization. This review discusses recent advances in the elucidation of the structure and function of CCR5, as well as the complex mechanisms that regulate CCR5 signalling and cell surface expression.

N-linked glycosylation in the CXCR4 N-terminus inhibits binding to HIV-1 envelope glycoproteins

CXCR4 is a co-receptor along with CD4 for human immunodeficiency virus type 1 (HIV-1). We investigated the role of N-linked glycosylation in the N-terminus of CXCR4 in binding to HIV-1 gp120 envelope glycoproteins. Gp120s from CXCR4 (X4) and CCR5 (R5) using HIV-1 strains bound more efficiently to non-N-glycosylated than to N-glycosylated CXCR4 proteoliposomes in a CD4-dependent manner. Similar results were observed in binding studies using non-N-glycosylated or N-glycosylated CXCR4 expressed on cells. Mutation of the N-glycosylation site N11 in CXCR4 (N11Q-CXCR4) enhanced CD4-dependent binding of X4 and R5 gp120s and allowed more efficient entry of viruses pseudotyped with X4 or R5 HIV-1 envelope glycoproteins. However, the binding of R5 gp120 to N11Q-CXCR4 and entry of R5 HIV-1 viruses into cells expressing N11Q-CXCR4 were 20- and 100- to 1000-fold less efficient, respectively, than the levels achieved using X4 gp120 or X4 HIV-1 viruses. Binding of stromal cell-derived factor (SDF)-1alpha, the natural ligand of CXCR4, and SDF-1alpha-induced signaling were reduced by the N11Q mutation. These findings demonstrate that N-glycosylation at N11 inhibits the binding of CXCR4 to X4 and R5 HIV-1 gp120, and provide a better understanding of the structural elements of CXCR4 involved in HIV-1 Env-co-receptor interactions.

Molecular mapping of epitopes for interaction of HIV-1 as well as natural ligands with the chemokine receptors, CCR5 and CXCR4

OBJECTIVE

Mapping coreceptor epitopes used by the prototypic R5 and X4 strains, HIV-1BaL and HIV-1IIIB, in comparison with epitopes involved in the activation and signaling induced by the natural ligands, RANTES and SDF-1beta.

DESIGN

Receptor hybrids between CCR5 and CXCR4 were constructed.

METHODS

Using single-overlap and extension PCR, increasing portions of CCR5 were replaced with corresponding parts of CXCR4. Viral interaction with these constructs was monitored in infection experiments using stably transfected cell lines, and ligand-induced activation of cells transiently expressing the constructs was measured in terms of calcium fluxes.

RESULTS

SDF-1beta required an essentially complete CXCR4, whereas RANTES demanded both the N terminus and the first two extracellular loops of CCR5. HIV-1 infection experiments emphasized the importance of the CCR5 N terminus for infection with HIV-1BaL, whereas HIV-1IIIB was less demanding in its use of CXCR4.

CONCLUSION

This study, for the first time monitoring CCR5 and CXCR4 ligand activation and HIV-1 interaction concomitantly, indicates that ligands and virus use different receptor epitopes which, in turn, vary between the two receptors. One particular chimera (FC-4b), having its junctional region close to the conserved cysteine in ECL2, functioned as coreceptor for both HIV-1BaL and HIV-1IIIB, but was not activated with RANTES or SDF-1beta. The results provide a basis for tailoring drugs that block viral entry through the two major coreceptors without interfering with their physiological function.

Protein-disulfide isomerase-mediated reduction of two disulfide bonds of HIV envelope glycoprotein 120 occurs post-CXCR4 binding and is required for fusion

The human immunodeficiency virus (HIV) envelope (Env) glycoprotein (gp) 120 is a highly disulfide-bonded molecule that attaches HIV to the lymphocyte surface receptors CD4 and CXCR4. Conformation changes within gp120 result from binding and trigger HIV/cell fusion. Inhibition of lymphocyte surface-associated protein-disulfide isomerase (PDI) blocks HIV/cell fusion, suggesting that redox changes within Env are required. Using a sensitive assay based on a thiol reagent, we show that (i) the thiol content of gp120, either secreted by mammalian cells or bound to a lymphocyte surface enabling CD4 but not CXCR4 binding, was 0.5-1 pmol SH/pmol gp120 (SH/gp120), whereas that of gp120 after its interaction with a surface enabling both CD4 and CXCR4 binding was raised to 4 SH/gp120; (ii) PDI inhibitors prevented this change; and (iii) gp120 displaying 2 SH/gp120 exhibited CD4 but not CXCR4 binding capacity. In addition, PDI inhibition did not impair gp120 binding to receptors. We conclude that on average two of the nine disulfides of gp120 are reduced during interaction with the lymphocyte surface after CXCR4 binding prior to fusion and that cell surface PDI catalyzes this process. Disulfide bond restructuring within Env may constitute the molecular basis of the post-receptor binding conformational changes that induce fusion competence.

Human prostacyclin receptor

Prostacyclin, a member of the eicosanoid family of lipid mediators, is the major product of arachidonic acid metabolism formed in the marcovascular endothelium. It is a potent vasodilator, antithrombotic, and antiplatelet agent that mediates it effects through a membrane-associated receptor termed the IP. Cloning of the cDNA for IP, from human and other species, indicated its membership of the G protein-coupled receptor superfamily and has allowed detailed examination of the signaling and regulatory pathways utilized by this receptor. This article examines the current state of knowledge of the IP, its signaling and regulation, and its biological role in vivo and examines the possible existence of multiple PGI2 receptor sites.

Infection of cells expressing CXCR4 mutants lacking N-glycosylation at the N-terminal extracellular domain is enhanced for R5X4-dualtropic human immunodeficiency virus type-1

BACKGROUND

Infection with human immunodeficiency virus type-1 (HIV-1) requires binding of the viral envelope gp120 to CD4 and to the CXCR4 coreceptor. Both, gp120 and CXCR4 are subject to N-glycosylation. Here we investigated the influence of the N-linked glycans g1 and g2 present on CXCR4 for HIV-1 infection.

METHODS

The two CXCR4 N-glycosylation sites g1 (NYT) and g2 (NVS) were mutated by changing the first or third amino acids N or T/S to Q and A respectively (g1; N11Q or T13A; g2, N176Q or S178A). Human osteosarcoma cells (GHOST) expressing human CD4 and the various CXCR4 glycosylation mutants were tested for infection using NL4-3-based viruses with X4, R5 or R5X4-tropism differing only in the V3 loop region.

RESULTS

All constructed cell lines expressing the various CXCR4 glycomutants showed similar permissiveness for the X4-monotropic virus and no change in the coreceptor specificity that allows infection of a CCR5-dependent R5-monotropic virus. Interestingly, the removal of glycan g1 significantly enhanced the permissiveness of GHOST cells for the R5X4 dualtropic virus. GHOST cells expressing the CXCR4-g1 or CXCR4-g1/2 mutants were infected at higher rates by the R5X4-dualtropic virus compared to cells expressing CXCR4-wt or CXCR4-g2 coreceptors.

CONCLUSION

Our present observations underscore a role for glycans present on the CXCR4 coreceptor in the entry process of HIV-1. The data will help to better understand the multifaceted mechanism of HIV infection and the selective forces which drive HIV-1 evolution from mono- to dual-tropism.

Structure, function, and inhibition of chemokines

Chemokines are the largest family of cytokines in human immunophysiology. These proteins are defined by four invariant cysteines and are categorized based on the sequence around the first two cysteines, which leads to two major and two minor subfamilies. Chemokines function by activating specific G protein-coupled receptors, which results in, among other functions, the migration of inflammatory and noninflammatory cells to the appropriate tissues or compartments within tissues. Some of these proteins and receptors have been implicated or shown to be involved in inflammation, autoimmune diseases, and infection by HIV-1. The three-dimensional structure of each monomer is virtually identical, but the quaternary structure of chemokines is different for each subfamily. Structure-function studies reveal several regions of chemokines to be involved in function, with the N-terminal region playing a dominant role. A number of proteins and small-molecule antagonists have been identified that inhibit chemokine activities. In this review, we discuss aspects of the structure, function, and inhibition of chemokines.

Role of N-glycans and SDF-1alpha on the coassociation of CD4 with CXCR4 at the plasma membrane of monocytic cells and blood lymphocytes

CXCR4 is a coreceptor, along with CD4, for human immunodeficiency virus type 1 (HIV-1). Trimolecular complexes between HIV-1 glycoprotein (gp)120, CD4 and CXCR4 constitute a prerequisite for HIV entry. We studied whether CD4 is associated with CXCR4 on CD4+ CXCR4+ cells. Using the conformation-dependent anti-CXCR4 mAb 12G5, CD4 was coimmunoprecipitated with CXCR4 from the membrane of U937 cells which support HIV-1(LAI) efficient infection, and from that of peripheral blood lymphocytes (PBL). CD4 association with CXCR4 increased upon PBL coculture for 5 days with autologous monocytes, decreased upon treatment of the cells or the CD4-CXCR4 complex with either N-glycanase or stromal cell derived factor-1alpha (SDF-1alpha) and was abolished by incubation of the cells with both, N-glycanase and SDF-1alpha. This indicates that glycans are partly involved in CD4 association with CXCR4 and may partly explain the inhibitory effect of SDF-1alpha on HIV infection.

The alpha-glucosidase inhibitor 1-deoxynojirimycin blocks human immunodeficiency virus envelope glycoprotein-mediated membrane fusion at the CXCR4 binding step

1-Deoxynojirimycin (DNM) is a saccharide decoy that inhibits cellular alpha-glucosidase I-II activity. Treatment by DNM of human immunodeficiency virus (HIV)-infected lymphocyte cultures inhibits virus spread. The functional properties of the membrane-associated Env glycoprotein (Env) modified in the presence of DNM remain unclear because previous reports on this subject have essentially used recombinant soluble Envs whose properties differ notably from those of Env anchored on the surface of the virus. To model virus-associated Env synthesized in the presence of DNM, native Env was expressed at the surface of mammalian cells treated with DNM. As expected, its glycosylation pattern was altered in the presence of the inhibitor. Env was found able to bind CD4, whereas its ability to induce membrane fusion was abolished. The immunoreactivity of regions involved in interactions of Env with CXCR4 (V1, V2, C2, and V3) was modified and Env displayed altered interaction with this coreceptor. These results are consistent with the inhibition by DNM of virus entry at the Env/coreceptor interaction step. Finally, preliminary data indicate that suboptimal concentrations of DNM and natural or synthetic CXCR4 ligands used in combination potently inhibit the Env-mediated membrane fusion process. Altogether, our results suggest that DNM and its analogs deserve further investigation as anti-HIV agents in combination with experimental compounds targeting CXCR4 to inhibit each partner of this crucial step of HIV entry.

Sialylated O-glycans and sulfated tyrosines in the NH2-terminal domain of CC chemokine receptor 5 contribute to high affinity binding of chemokines

The chemokine receptor CCR5 plays an important role in leukocyte chemotaxis and activation, and also acts as a coreceptor for human and simian immunodeficiency viruses (HIV-1, HIV-2, and SIV). We provide evidence that CCR5 is O-glycosylated on serine 6 in the NH2 terminus. The O-linked glycans, particularly sialic acid moieties, significantly contribute to binding of the chemokine ligands. By contrast, removal of O-linked oligosaccharide exerted little effect on HIV-1 infection. Sulfation of specific tyrosine residues in the CCR5 NH2 terminus was important for efficient beta-chemokine binding. Thus, as has been observed for the binding of selectins and their ligands, O-linked carbohydrates and tyrosine sulfates play major roles in promoting the interaction of chemokines with CCR5. The resulting flexible arrays of negative charges on the CCR5 surface may allow specific, high-affinity interactions with diverse chemokine ligands. Although this is the first example of O-linked oligosaccharides and tyrosine sulfates playing a role in chemokine binding, the high density of serines, threonines and tyrosines in the N-termini of many CC chemokine receptors suggests that these posttranslational modifications may commonly contribute to chemokine binding.

Monocyte chemotactic protein-1 receptor CCR2B is a glycoprotein that has tyrosine sulfation in a conserved extracellular N-terminal region

Monocyte chemotactic protein-1 (MCP-1) binding to its receptor, CCR2B, plays an important role in a variety of diseases involving infection, inflammation, and/or injury. In our effort to understand the molecular basis of this interaction and its biological consequences, we recognized a conserved hexad of amino acids at the N-terminal extracellular domain of several chemokine receptors, including CCR2B. Human embryonic kidney 293 cells expressing Flag-tagged CCR2B containing site-directed mutations in this region, 21-26, including a consensus tyrosine sulfation site were used to determine MCP-1 binding and its biological consequences. The results showed that several of these amino acids are important for MCP-1 binding and consequent lamellipodium formation, chemotaxis, and signal transduction involving adenylate cyclase inhibition and Ca(2+) influx into cytoplasm. Mutations that prevented adenylate cyclase inhibition and Ca(2+) influx did not significantly inhibit lamellipodium formation and chemotaxis, suggesting that these signaling events are not involved in chemotaxis. CCR2B was found to be sulfated at Tyr(26); this sulfation was abolished by the substitution of Tyr with Ala and severely reduced by substitution of Asp(25), a part of the consensus sulfation site. The expressed CCR2B was found to be N:-glycosylated, as N:-glycosidase F treatment of the receptor or growth of the cells in tunicamycin reduced the receptor size to the same level, from 50 to 45 kDa. Thus, CCR2B is the first member of the CC chemokine receptor family shown to be a glycoprotein that is sulfated at the N-terminal Tyr. These post-translational modifications probably have significant biological functions.

Expression and functional characterization of mutant human CXCR4 in insect cells: role of cysteinyl and negatively charged residues in ligand binding

Human CXCR4 was expressed in Sf9 insect cells using the Bac-to-Bac baculovirus expression system. The recombinant receptor exhibited ligand binding activities with a K(d) value (3.3 nM) comparable to that of the native receptor. The role of four conserved cysteinyl residues was explored by site-directed mutagenesis. Each cysteine was individually changed to an alanine residue. All of the four mutants showed decreased ligand binding activity with increased K(d) values although comparable levels of receptor expression were observed. These results suggest that each of these four cysteinyl residues may be important for the ligand binding of the receptor. Evidence suggests that the ionic interaction may be involved in ligand binding. Point mutation of several relatively conserved acidic residues (Asp-10, Asp-262, Glu-275, and Glu-277) to an alanine residue greatly decreased the ligand binding activity and affinity. Since SDF-1alpha is a highly basic protein, these acidic residues may interact with the basic residues of SDF-1alpha by ionic pairing in addition to other molecular interactions and play an important role in ligand binding.