Solution structure and basis for functional activity of stromal cell-derived factor-1; dissociation of CXCR4 activation from binding and inhibition of HIV-1

Chemoselective Copper-Mediated Modification of Selenocysteines in Peptides and Proteins

Highly valuable bioconjugated molecules must be synthesized through efficient, chemoselective chemical modifications of peptides and proteins. Herein, we report the chemoselective modification of peptides and proteins via a reaction between selenocysteine residues and aryl/alkyl radicals. In situ radical generation from hydrazine substrates and copper ions proceeds rapidly in an aqueous buffer at near neutral pH (5-8), providing a variety of Se-modified linear and cyclic peptides and proteins conjugated to aryl and alkyl molecules, and to affinity label tag (biotin). This chemistry opens a new avenue for chemical protein modifications.

The dimeric form of CXCL12 binds to atypical chemokine receptor 1

The pleiotropic chemokine CXCL12 is involved in diverse physiological and pathophysiological processes, including embryogenesis, hematopoiesis, leukocyte migration, and tumor metastasis. It is known to engage the classical receptor CXCR4 and the atypical receptor ACKR3. Differential receptor engagement can transduce distinct cellular signals and effects as well as alter the amount of free, extracellular chemokine. CXCR4 binds both monomeric and the more commonly found dimeric forms of CXCL12, whereas ACKR3 binds monomeric forms. Here, we found that CXCL12 also bound to the atypical receptor ACKR1 (previously known as Duffy antigen/receptor for chemokines or DARC). In vitro nuclear magnetic resonance spectroscopy and isothermal titration calorimetry revealed that dimeric CXCL12 bound to the extracellular N terminus of ACKR1 with low nanomolar affinity, whereas the binding affinity of monomeric CXCL12 was orders of magnitude lower. In transfected MDCK cells and primary human Duffy-positive erythrocytes, a dimeric, but not a monomeric, construct of CXCL12 efficiently bound to and internalized with ACKR1. This interaction between CXCL12 and ACKR1 provides another layer of regulation of the multiple biological functions of CXCL12. The findings also raise the possibility that ACKR1 can bind other dimeric chemokines, thus potentially further expanding the role of ACKR1 in chemokine retention and presentation.

Tuning the network charge of biohybrid hydrogel matrices to modulate the release of SDF-1

The delivery of chemotactic signaling molecules via customized biomaterials can effectively guide the migration of cells to improve the regeneration of damaged or diseased tissues. Here, we present a novel biohybrid hydrogel system containing two different sulfated glycosaminoglycans (sGAG)/sGAG derivatives, namely either a mixture of short heparin polymers (Hep-Mal) or structurally defined nona-sulfated tetrahyaluronans (9s-HA4-SH), to precisely control the release of charged signaling molecules. The polymer networks are described in terms of their negative charge, i.e. the anionic sulfate groups on the saccharides, using two parameters, the integral density of negative charge and the local charge distribution (clustering) within the network. The modulation of both parameters was shown to govern the release characteristics of the chemotactic signaling molecule SDF-1 and allows for seamless transitions between burst and sustained release conditions as well as the precise control over the total amount of delivered protein. The obtained hydrogels with well-adjusted release profiles effectively promote MSC migration in vitro and emerge as promising candidates for new treatment modalities in the context of bone repair and wound healing.

Biparatopic Protein Nanoparticles for the Precision Therapy of CXCR4+ Cancers

The accumulated molecular knowledge about human cancer enables the identification of multiple cell surface markers as highly specific therapeutic targets. A proper tumor targeting could significantly avoid drug exposure of healthy cells, minimizing side effects, but it is also expected to increase the therapeutic index. Specifically, colorectal cancer has a particularly poor prognosis in late stages, being drug targeting an appropriate strategy to substantially improve the therapeutic efficacy. In this study, we have explored the potential of the human albumin-derived peptide, EPI-X4, as a suitable ligand to target colorectal cancer via the cell surface protein CXCR4, a chemokine receptor overexpressed in cancer stem cells. To explore the potential use of this ligand, self-assembling protein nanoparticles have been generated displaying an engineered EPI-X4 version, which conferred a modest CXCR4 targeting and fast and high level of cell apoptosis in tumor CXCR4+ cells, in vitro and in vivo. In addition, when EPI-X4-based building blocks are combined with biologically inert polypeptides containing the CXCR4 ligand T22, the resulting biparatopic nanoparticles show a dramatically improved biodistribution in mouse models of CXCR4+ human cancer, faster cell internalization and enhanced target cell death when compared to the version based on a single ligand. The generation of biparatopic materials opens exciting possibilities in oncotherapies based on high precision drug delivery based on the receptor CXCR4.

Diurnal expression of CXC receptors 4 (CXCR4) and CXC chemokine ligand 12 (CXCL12) in Pelteobagrus vachellii

The CXC chemokine ligand 12/CXC receptor 4 ligand/receptor interaction is the most ancient chemokine system in vertebrates, and it plays a pivotal role in the immune system's response against bacterial infection. In the current study, 1211 bp CXCR4 and 937 bp CXCL12 genes, which encode 364 and 99 amino acids, respectively, were isolated. Within the 24-hour light/dark cycle, the maximum of CXCR4 in the intestine, spleen, and anterior kidney of Pelteobagrus vachellii occurs at 8:00, 16:00, and 16:00, respectively. The maximum of CXCL12 in the intestine, spleen, and anterior kidney of P. vachellii occurs at 20:00, 12:00, and 20:00, respectively. CXCR4 and CXCL12 expressions showed 24-hour variation, which contributed to understanding of the immune rhythm of the teleost.

GPR182 is an endothelium-specific atypical chemokine receptor that maintains hematopoietic stem cell homeostasis

G protein–coupled receptors (GPCRs) are important regulators of cellular and biological functions and are primary targets of therapeutic drugs. About 100 mammalian GPCRs are still considered orphan receptors because they lack a known endogenous ligand. We report the deorphanization of GPR182, which is expressed in endothelial cells of the microvasculature. We show that GPR182 is an atypical chemokine receptor, which binds CXCL10, 12, and 13. However, binding does not induce downstream signaling. Consistent with a scavenging function of GPR182, mice lacking GPR182 have increased plasma levels of chemokines. In line with the crucial role of CXCL12 in hematopoietic stem cell homeostasis, we found that loss of GPR182 results in increased egress of hematopoietic stem cells from the bone marrow.

G protein–coupled receptor 182 (GPR182) has been shown to be expressed in endothelial cells; however, its ligand and physiological role has remained elusive. We found GPR182 to be expressed in microvascular and lymphatic endothelial cells of most organs and to bind with nanomolar affinity the chemokines CXCL10, CXCL12, and CXCL13. In contrast to conventional chemokine receptors, binding of chemokines to GPR182 did not induce typical downstream signaling processes, including G_q- and G_i-mediated signaling or β-arrestin recruitment. GPR182 showed relatively high constitutive activity in regard to β-arrestin recruitment and rapidly internalized in a ligand-independent manner. In constitutive GPR182-deficient mice, as well as after induced endothelium-specific loss of GPR182, we found significant increases in the plasma levels of CXCL10, CXCL12, and CXCL13. Global and induced endothelium-specific GPR182-deficient mice showed a significant decrease in hematopoietic stem cells in the bone marrow as well as increased colony-forming units of hematopoietic progenitors in the blood and the spleen. Our data show that GPR182 is a new atypical chemokine receptor for CXCL10, CXCL12, and CXCL13, which is involved in the regulation of hematopoietic stem cell homeostasis.

Mechanisms of HIV-1 evasion to the antiviral activity of chemokine CXCL12 indicate potential links with pathogenesis

HIV-1 infects CD4 T lymphocytes (CD4TL) through binding the chemokine receptors CCR5 or CXCR4. CXCR4-using viruses are considered more pathogenic, linked to accelerated depletion of CD4TL and progression to AIDS. However, counterexamples to this paradigm are common, suggesting heterogeneity in the virulence of CXCR4-using viruses. Here, we investigated the role of the CXCR4 chemokine CXCL12 as a driving force behind virus virulence. In vitro, CXCL12 prevents HIV-1 from binding CXCR4 and entering CD4TL, but its role in HIV-1 transmission and propagation remains speculative. Through analysis of thirty envelope glycoproteins (Envs) from patients at different stages of infection, mostly treatment-naïve, we first interrogated whether sensitivity of viruses to inhibition by CXCL12 varies over time in infection. Results show that Envs resistant (RES) to CXCL12 are frequent in patients experiencing low CD4TL levels, most often late in infection, only rarely at the time of primary infection. Sensitivity assays to soluble CD4 or broadly neutralizing antibodies further showed that RES Envs adopt a more closed conformation with distinct antigenicity, compared to CXCL12-sensitive (SENS) Envs. At the level of the host cell, our results suggest that resistance is not due to improved fusion or binding to CD4, but owes to viruses using particular CXCR4 molecules weakly accessible to CXCL12. We finally asked whether the low CD4TL levels in patients are related to increased pathogenicity of RES viruses. Resistance actually provides viruses with an enhanced capacity to enter naive CD4TL when surrounded by CXCL12, which mirrors their situation in lymphoid organs, and to deplete bystander activated effector memory cells. Therefore, RES viruses seem more likely to deregulate CD4TL homeostasis. This work improves our understanding of the pathophysiology and the transmission of HIV-1 and suggests that RES viruses' receptors could represent new therapeutic targets to help prevent CD4TL depletion in HIV+ patients on cART.

Strategies and progress in CXCR4-targeted anti-HIV therapeutic development

The acquired immunodeficiency syndrome (AIDS), caused by the human immunodeficiency virus (HIV), has been a global public health challenge for several decades. The majority of HIV infection is caused by the human immunodeficiency virus type 1 (HIV-1) which enters and infects a host cell via the cell surface proteins of CD4 as the primary receptor, and chemokine receptors CXCR4 or CCR5 as the co-receptor-then undergoing replication using the cell's intracellular machinery. Whereas many drugs targeting CCR5-mediated entry or HIV-1 replication via reverse transcriptase or proteases have long been used clinically, agents targeting CXCR4 are yet to be advanced to clinical application. Here in this review we highlight some of the strategies for and progress made in the discovery of novel small molecules, peptides, and larger molecules that target CXCR4, and their future prospects for translation into the clinic as a new class of anti-HIV therapeutics.

A nanoluciferase biosensor to investigate endogenous chemokine secretion and receptor binding

Secreted chemokines are critical mediators of cellular communication that elicit intracellular signaling by binding membrane-bound receptors. Here we demonstrate the development and use of a sensitive real-time approach to quantify secretion and receptor binding of native chemokines in live cells to better understand their molecular interactions and function. CRISPR/Cas9 genome editing was used to tag the chemokine CXCL12 with the nanoluciferase fragment HiBiT. CXCL12 secretion was subsequently monitored and quantified by luminescence output. Binding of tagged CXCL12 to either chemokine receptors or membrane glycosaminoglycans could be monitored due to the steric constraints of nanoluciferase complementation. Furthermore, binding of native CXCL12-HiBiT to AlexaFluor488-tagged CXCR4 chemokine receptors could also be distinguished from glycosaminoglycan binding and pharmacologically analyzed using BRET. These live cell approaches combine the sensitivity of nanoluciferase with CRISPR/Cas9 genome editing to detect, quantify, and monitor binding of low levels of native secreted proteins in real time.

A Multipurpose First-in-Human Study With the Novel CXCR7 Antagonist ACT-1004-1239 Using CXCL12 Plasma Concentrations as Target Engagement Biomarker

The C-X-C chemokine receptor 7 (CXCR7) has evolved as a promising, druggable target mainly in the immunology and oncology fields modulating plasma concentrations of its ligands CXCL11 and CXCL12 through receptor-mediated internalization. This "scavenging" activity creates concentration gradients of these ligands between blood vessels and tissues that drive directional cell migration. This randomized, double-blind, placebo-controlled first-in-human study assessed the safety, tolerability, pharmacokinetics, and pharmacodynamics of ACT-1004-1239, a first-in-class drug candidate small-molecule CXCR7 antagonist. Food effect and absolute bioavailability assessments were also integrated in this multipurpose study. Healthy male subjects received single ascending oral doses of ACT-1004-1239 (n = 36) or placebo (n = 12). At each of six dose levels (1-200 mg), repeated blood sampling was done over 144 hours for pharmacokinetic/pharmacodynamic assessments using CXCL11 and CXCL12 as biomarkers of target engagement. ACT-1004-1239 was safe and well tolerated up to the highest tested dose of 200 mg. CXCL12 plasma concentrations dose-dependently increased and more than doubled compared with baseline, indicating target engagement, whereas CXCL11 concentrations remained unchanged. An indirect-response pharmacokinetic/pharmacodynamic model well described the relationship between ACT-1004-1239 and CXCL12 concentrations across the full dose range, supporting once-daily dosing for future clinical studies. At doses ≥ 10 mg, time to reach maximum plasma concentration ranged from 1.3 to 3.0 hours and terminal elimination half-life from 17.8 to 23.6 hours. The exposure increase across the dose range was essentially dose-proportional and no relevant food effect on pharmacokinetics was determined. The absolute bioavailability was 53.0% based on radioactivity data after oral vs. intravenous ¹⁴ C-radiolabeled microtracer administration of ACT-1004-1239. Overall, these comprehensive data support further clinical development of ACT-1004-1239.

Submolecular probing of the complement C5a receptor-ligand binding reveals a cooperative two-site binding mechanism

A current challenge to produce effective therapeutics is to accurately determine the location of the ligand-biding site and to characterize its properties. So far, the mechanisms underlying the functional activation of cell surface receptors by ligands with a complex binding mechanism remain poorly understood due to a lack of suitable nanoscopic methods to study them in their native environment. Here, we elucidated the ligand-binding mechanism of the human G protein-coupled C5a receptor (C5aR). We discovered for the first time a cooperativity between the two orthosteric binding sites. We found that the N-terminus C5aR serves as a kinetic trap, while the transmembrane domain acts as the functional site and both contributes to the overall high-affinity interaction. In particular, Asp282 plays a key role in ligand binding thermodynamics, as revealed by atomic force microscopy and steered molecular dynamics simulation. Our findings provide a new structural basis for the functional and mechanistic understanding of the GPCR family that binds large macromolecular ligands.

The CXCL12-CXCR4 Signaling Axis Plays a Key Role in Cancer Metastasis and is a Potential Target for Developing Novel Therapeutics against Metastatic Cancer

Metastasis is the main cause of death in cancer patients; there is currently no effective treatment for cancer metastasis. This is primarily due to our insufficient understanding of the metastatic mechanisms in cancer. An increasing number of studies have shown that the C-X-C motif chemokine Ligand 12 (CXCL12) is overexpressed in various tissues and organs. It is a key niche factor that nurtures the pre-metastatic niches (tumorigenic soil) and recruits tumor cells (oncogenic "seeds") to these niches, thereby fostering cancer cell aggression and metastatic capabilities. However, the C-X-C motif chemokine Receptor 4 (CXCR4) is aberrantly overexpressed in various cancer stem/progenitor cells and functions as a CXCL12 receptor. CXCL12 activates CXCR4 as well as multiple downstream multiple tumorigenic signaling pathways, promoting the expression of various oncogenes. Activation of the CXCL12-CXCR4 signaling axis promotes Epithelial-Mesenchymal Transition (EMT) and mobilization of cancer stem/progenitor cells to pre-metastatic niches. It also nurtures cancer cells with high motility, invasion, and dissemination phenotypes, thereby escalating multiple proximal or distal cancer metastasis; this results in poor patient prognosis. Based on this evidence, recent studies have explored either CXCL12- or CXCR4-targeted anti-cancer therapeutics and have achieved promising results in the preclinical trials. Further exploration of this new strategy and its potent therapeutics effect against metastatic cancer through the targeting of the CXCL12- CXCR4 signaling axis may lead to a novel therapy that can clean up the tumor microenvironment ("soil") and kill the cancer cells, particularly the cancer stem/progenitor cells ("seeds"), in cancer patients. Ultimately, this approach has the potential to effectively treat metastatic cancer.

Long-Range Coupled Motions Underlie Ligand Recognition by a Chemokine Receptor

Chemokines are unusual class-A G protein-coupled receptor agonists because of their large size (∼10 kDa) and binding at two distinct receptor sites: N-terminal domain (Site-I, unique to chemokines) and a groove defined by extracellular loop/transmembrane helices (Site-II, shared with all small molecule class-A ligands). Structures and sequence analysis reveal that the receptor N-terminal domains (N-domains) are flexible and contain intrinsic disorder. Using a hybrid NMR-MD approach, we characterized the role of Site-I interactions for the CXCL8-CXCR1 pair. NMR data indicate that the CXCR1 N-domain becomes structured on binding and that the binding interface is extensive with 30% CXCL8 residues participating in this initial interaction. MD simulations indicate that CXCL8 bound at Site-I undergoes extensive reorganization on engaging Site-II with several residues initially engaged at Site-I also engaging at Site-II. We conclude that structural plasticity of Site-I interactions plays an active role in driving ligand recognition by a chemokine receptor.

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Structural plasticity governs chemokine-receptor interactions

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Receptor N-terminal domain captures the chemokine by a fly-casting mechanism

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Crosstalk between two distinct binding sites determines recognition and function

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A hybrid NMR-MD approach provides crucial insights into receptor binding mechanism

Designed CXCR4 mimic acts as a soluble chemokine receptor that blocks atherogenic inflammation by agonist-specific targeting

Targeting a specific chemokine/receptor axis in atherosclerosis remains challenging. Soluble receptor-based strategies are not established for chemokine receptors due to their discontinuous architecture. Macrophage migration-inhibitory factor (MIF) is an atypical chemokine that promotes atherosclerosis through CXC-motif chemokine receptor-4 (CXCR4). However, CXCR4/CXCL12 interactions also mediate atheroprotection. Here, we show that constrained 31-residue-peptides ('msR4Ms') designed to mimic the CXCR4-binding site to MIF, selectively bind MIF with nanomolar affinity and block MIF/CXCR4 without affecting CXCL12/CXCR4. We identify msR4M-L1, which blocks MIF- but not CXCL12-elicited CXCR4 vascular cell activities. Its potency compares well with established MIF inhibitors, whereas msR4M-L1 does not interfere with cardioprotective MIF/CD74 signaling. In vivo-administered msR4M-L1 enriches in atherosclerotic plaques, blocks arterial leukocyte adhesion, and inhibits atherosclerosis and inflammation in hyperlipidemic Apoe-/- mice in vivo. Finally, msR4M-L1 binds to MIF in plaques from human carotid-endarterectomy specimens. Together, we establish an engineered GPCR-ectodomain-based mimicry principle that differentiates between disease-exacerbating and -protective pathways and chemokine-selectively interferes with atherosclerosis.

Chemical mutagenesis of a GPCR ligand: Detoxifying "inflammo-attraction" to direct therapeutic stem cell migration

While inflammatory chemokines, constitutively produced by pathologic regions, are pivotal for attracting reparative stem cells, one would certainly not want to further “inflame” a diseased brain by instilling such molecules. Exploiting the fact that receptors for such cytokines (G protein-coupled receptors [GPCR]) possess two “pockets”—one for binding, the other for signaling—we created a synthetic GPCR-agonist that maximizes interaction with the former and narrows that with the latter. Homing is robust with no inflammation. The peptide successfully directed the integration of human induced pluripotent stem cell derivatives (known to have muted migration) in a model of a prototypical neurodegenerative condition, ameliorating symptomatology.

A transplanted stem cell’s engagement with a pathologic niche is the first step in its restoring homeostasis to that site. Inflammatory chemokines are constitutively produced in such a niche; their binding to receptors on the stem cell helps direct that cell’s “pathotropism.” Neural stem cells (NSCs), which express CXCR4, migrate to sites of CNS injury or degeneration in part because astrocytes and vasculature produce the inflammatory chemokine CXCL12. Binding of CXCL12 to CXCR4 (a G protein-coupled receptor, GPCR) triggers repair processes within the NSC. Although a tool directing NSCs to where needed has been long-sought, one would not inject this chemokine in vivo because undesirable inflammation also follows CXCL12–CXCR4 coupling. Alternatively, we chemically “mutated” CXCL12, creating a CXCR4 agonist that contained a strong pure binding motif linked to a signaling motif devoid of sequences responsible for synthetic functions. This synthetic dual-moity CXCR4 agonist not only elicited more extensive and persistent human NSC migration and distribution than did native CXCL 12, but induced no host inflammation (or other adverse effects); rather, there was predominantly reparative gene expression. When co-administered with transplanted human induced pluripotent stem cell-derived hNSCs in a mouse model of a prototypical neurodegenerative disease, the agonist enhanced migration, dissemination, and integration of donor-derived cells into the diseased cerebral cortex (including as electrophysiologically-active cortical neurons) where their secreted cross-corrective enzyme mediated a therapeutic impact unachieved by cells alone. Such a “designer” cytokine receptor-agonist peptide illustrates that treatments can be controlled and optimized by exploiting fundamental stem cell properties (e.g., “inflammo-attraction”).

CXCL12-PLGA/Pluronic Nanoparticle Internalization Abrogates CXCR4-Mediated Cell Migration

Chemokine-induced chemotaxis mediates physiological and pathological immune cell trafficking, as well as several processes involving cell migration. Among them, the role of CXCL12/CXCR4 signaling in cancer and metastasis is well known, and CXCR4 has been often targeted with small molecule-antagonists or short CXCL12-derived peptides to limit the pathological processes of cell migration and invasion. To reduce CXCR4-mediated chemotaxis, we adopted a different approach. We manufactured poly(lactic acid-co-glycolic acid) (PLGA)/Pluronic F127 nanoparticles through microfluidics-assisted nanoprecipitation and functionalized them with streptavidin to docking a biotinylated CXCL12 to be exposed on the nanoparticle surface. Our results show that CXCL12-decorated nanoparticles are non-toxic and do not induce inflammatory cytokine release in THP-1 monocytes cultured in fetal bovine and human serum-supplemented media. The cell internalization of our chemokine receptor-targeting particles increases in accordance with CXCR4 expression in FBS/medium. We demonstrated that CXCL12-decorated nanoparticles do not induce cell migration on their own, but their pre-incubation with THP-1 significantly decreases CXCR4⁺-cell migration, thereby antagonizing the chemotactic action of CXCL12. The use of biodegradable and immune-compatible chemokine-mimetic nanoparticles to reduce cell migration opens the way to novel antagonists with potential application in cancer treatments and inflammation.

Monitoring Allosteric Interactions with CXCR4 Using NanoBiT Conjugated Nanobodies

Camelid single-domain antibody fragments (nanobodies) offer the specificity of an antibody in a single 15-kDa immunoglobulin domain. Their small size allows for easy genetic manipulation of the nanobody sequence to incorporate protein tags, facilitating their use as biochemical probes. The nanobody VUN400, which recognizes the second extracellular loop of the human CXCR4 chemokine receptor, was used as a probe to monitor specific CXCR4 conformations. VUN400 was fused via its C terminus to the 11-amino-acid HiBiT tag (VUN400-HiBiT) which complements LgBiT protein, forming a full-length functional NanoLuc luciferase. Here, complemented luminescence was used to detect VUN400-HiBiT binding to CXCR4 receptors expressed in living HEK293 cells. VUN400-HiBiT binding to CXCR4 could be prevented by orthosteric and allosteric ligands, allowing VUN400-HiBiT to be used as a probe to detect allosteric interactions with CXCR4. These data demonstrate that the high specificity offered by extracellular targeted nanobodies can be utilized to probe receptor pharmacology.

Internal water channel formation in CXCR4 is crucial for Gi-protein coupling upon activation by CXCL12

Chemokine receptor CXCR4 is a major drug target for numerous diseases because of its involvement in the regulation of cell migration and the developmental process. In this study, atomic-level molecular dynamics simulations were used to determine the activation mechanism and internal water formation of CXCR4 in complex with chemokine CXCL12 and Gi-protein. The results indicated that CXCL12-bound CXCR4 underwent transmembrane 6 (TM6) outward movement and a decrease in tyrosine toggle switch by eliciting the breakage of hydrophobic layer to form a continuous internal water channel. In the GDP-bound Gαi-protein state, the rotation and translation of the α5-helix of Gαi-protein toward the cytoplasmic pocket of CXCR4 induced an increase in interdomain distance for GDP leaving. Finally, an internal water channel formation model was proposed based on our simulations for CXCL12-bound CXCR4 in complex with Gαi-protein upon activation for downstream signaling. This model could be useful in anticancer drug development.

High-Throughput Screening of a Functional Human CXCL12-CXCR4 Signaling Axis in a Genetically Modified S. cerevisiae: Discovery of a Novel Up-Regulator of CXCR4 Activity

CXCL12 activates CXCR4 and is involved in embryogenesis, hematopoiesis, and angiogenesis. It has pathological roles in HIV-1, WHIM disease, cancer, and autoimmune diseases. An antagonist, AMD3100, is used for the release of CD34+ hematopoietic stem cells from the bone marrow for autologous transplantation for lymphoma or multiple myeloma patients. Adverse effects are tolerated due to its short-term treatment, but AMD3100 is cardiotoxic in clinical studies for HIV-1. In an effort to determine whether Saccharomyces cerevisiae expressing a functional human CXCR4 could be used as a platform for identifying a ligand from a library of less ∼1,000 compounds, a high-throughput screening was developed. We report that 2-carboxyphenyl phosphate (fosfosal) up-regulates CXCR4 activation only in the presence of CXCL12. This is the first identification of a compound that increases CXCR4 activity by any mechanism. We mapped the fosfosal binding site on CXCL12, described its mechanism of action, and studied its chemical components, salicylate and phosphate, to conclude that they synergize to achieve the functional effect.

Aberrant CXCR4 Signaling at Crossroad of WHIM Syndrome and Waldenstrom's Macroglobulinemia

Given its pleiotropic functions, including its prominent role in inflammation, immune responses and cancer, the C-X-C chemokine receptor type 4 (CXCR4) has gained significant attention in recent years and has become a relevant target in drug development. Although the signaling properties of CXCR4 have been extensively studied, several aspects deserve deeper investigations. Mutations in the C-term tail of the CXCR4 gene cause WHIM syndrome, a rare congenital immunodeficiency associated by chronic leukopenia. Similar mutations have also been recently identified in 30% of patients affected by Waldenstrom’s macroglobulinaemia, a B-cell neoplasia with bone marrow accumulation of malignant cells. An ample body of work has been generated to define the impact of WHIM mutations on CXCR4 signaling properties and evaluate their role on pathogenesis, diagnosis, and response to therapy, although the identity of disease-causing signaling pathways and their relevance for disease development in different genetic variants are still open questions. This review discusses the current knowledge on biochemical properties of CXCR4 mutations to identify their prototypic signaling profile potentially useful to highlighting novel opportunities for therapeutic intervention.

Relevance of the CXCR4/CXCR7-CXCL12 axis and its effect in pathophysiological conditions

The impact of the C-X-C receptor (CXCR) 7 and its close co-player CXCR4 in different physiological and pathophysiological processes has been extensively investigated within the last decades. Following activation by their shared ligand C-X-C ligand (CXCL) 12, both chemokine receptors can induce various routes of cell signaling and/or scavenge CXCL12 from the extracellular environment. This contributes to organ development and maintenance of homeostasis. Alterations of the CXCR4/CXCR7-CXCL12 axis have been detected in diseases such as cancer, central nervous system and cardiac disorders, and autoimmune diseases. These alterations include changes of the expression pattern, distribution, or downstream effects. The progression of the diseases can be regulated in preclinical models by the use of various modulators suggesting that this axis serves as a promising therapeutic target. It is therefore of great interest to investigate CXCR4/CXCR7/CXCL12 modulators in clinical development, with several CXCR4 and CXCL12 modulators such as plerixafor, ulocuplumab, balixafortide, and olaptesed pegol having already reached this stage. An overview is presented of the most important diseases whose outcomes can be positively or negatively regulated by the CXCR4/CXCR7-CXCL12 axis and summarizes preclinical and clinical data of modulators of that axis. Contrary to CXCR4 and CXCL12 modulators, CXCR7 modulators have, thus far, not been extensively studied. Therefore, more (pre)clinical investigations are needed.

Functional anatomy of the full-length CXCR4-CXCL12 complex systematically dissected by quantitative model-guided mutagenesis

Because of their prominent roles in development, cancer, and HIV, the chemokine receptor CXCR4 and its ligand CXCL12 have been the subject of numerous structural and functional studies, but the determinants of ligand binding, selectivity, and signaling are still poorly understood. Here, building on our latest structural model, we used a systematic mutagenesis strategy to dissect the functional anatomy of the CXCR4-CXCL12 complex. Key charge swap mutagenesis experiments provided evidence for pairwise interactions between oppositely charged residues in the receptor and chemokine, confirming the accuracy of the predicted orientation of the chemokine relative to the receptor and providing insight into ligand selectivity. Progressive deletion of N-terminal residues revealed an unexpected contribution of the receptor N terminus to chemokine signaling. This finding challenges a longstanding "two-site" hypothesis about the essential features of the receptor-chemokine interaction in which the N terminus contributes only to binding affinity. Our results suggest that although the interaction of the chemokine N terminus with the receptor-binding pocket is the key driver of signaling, the signaling amplitude depends on the extent to which the receptor N terminus binds the chemokine. Together with systematic characterization of other epitopes, these data enable us to propose an experimentally consistent structural model for how CXCL12 binds CXCR4 and initiates signal transmission through the receptor transmembrane domain.

Structural basis of CXC chemokine receptor 2 activation and signalling

Chemokines and their receptors mediate cell migration, which influences multiple fundamental biological processes and disease conditions such as inflammation and cancer¹. Although ample effort has been invested into the structural investigation of the chemokine receptors and receptor-chemokine recognition^2-4, less is known about endogenous chemokine-induced receptor activation and G-protein coupling. Here we present the cryo-electron microscopy structures of interleukin-8 (IL-8, also known as CXCL8)-activated human CXC chemokine receptor 2 (CXCR2) in complex with G_i protein, along with a crystal structure of CXCR2 bound to a designed allosteric antagonist. Our results reveal a unique shallow mode of binding between CXCL8 and CXCR2, and also show the interactions between CXCR2 and G_i protein. Further structural analysis of the inactive and active states of CXCR2 reveals a distinct activation process and the competitive small-molecule antagonism of chemokine receptors. In addition, our results provide insights into how a G-protein-coupled receptor is activated by an endogenous protein molecule, which will assist in the rational development of therapeutics that target the chemokine system for better pharmacological profiles.

Design, synthesis, and biological characterization of a new class of symmetrical polyamine-based small molecule CXCR4 antagonists

CXCR4, a well-studied coreceptor of human immunodeficiency virus type 1 (HIV-1) entry, recognizes its cognate ligand SDF-1α (also named CXCL12) which plays many important roles, including regulating immune cells, controlling hematopoietic stem cells, and directing cancer cells migration. These pleiotropic roles make CXCR4 an attractive target to mitigate human disorders. Here a new class of symmetrical polyamines was designed and synthesized as potential small molecule CXCR4 antagonists. Among them, a representative compound 21 (namely HF50731) showed strong CXCR4 binding affinity (mean IC₅₀ = 19.8 nM) in the CXCR4 competitive binding assay. Furthermore, compound 21 significantly inhibited SDF-1α-induced calcium mobilization and cell migration, and blocked HIV-1 infection via antagonizing CXCR4 coreceptor function. The structure-activity relationship analysis, site-directed mutagenesis, and molecular docking were conducted to further elucidate the binding mode of compound 21, suggesting that compound 21 could primarily occupy the minor subpocket of CXCR4 and partially bind in the major subpocket by interacting with residues W94, D97, D171, and E288. Our studies provide not only new insights for the fragment-based design of small molecule CXCR4 antagonists for clinical applications, but also a new and effective molecular probe for CXCR4-targeting biological studies.

Differential activity and selectivity of N-terminal modified CXCL12 chemokines at the CXCR4 and ACKR3 receptors

Chemokines play critical roles in numerous physiologic and pathologic processes through their action on seven-transmembrane (TM) receptors. The N-terminal domain of chemokines, which is a key determinant of signaling via its binding within a pocket formed by receptors' TM helices, can be the target of proteolytic processing. An illustrative case of this regulatory mechanism is the natural processing of CXCL12 that generates chemokine variants lacking the first two N-terminal residues. Whereas such truncated variants behave as antagonists of CXCR4, the canonical G protein-coupled receptor of CXCL12, they are agonists of the atypical chemokine receptor 3 (ACKR3/CXCR7), suggesting the implication of different structural determinants in the complexes formed between CXCL12 and its two receptors. Recent analyses have suggested that the CXCL12 N-terminus first engages the TM helices of ACKR3 followed by the receptor N-terminus wrapping around the chemokine core. Here we investigated the first stage of ACKR3-CXCL12 interactions by comparing the activity of substituted or N-terminally truncated variants of CXCL12 toward CXCR4 and ACKR3. We showed that modification of the first two N-terminal residues of the chemokine (K1R or P2G) does not alter the ability of CXCL12 to activate ACKR3. Our results also identified the K1R variant as a G protein-biased agonist of CXCR4. Comparative molecular dynamics simulations of the complexes formed by ACKR3 either with CXCL12 or with the P2G variant identified interactions between the N-terminal 2-4 residues of CXCL12 and a pocket formed by receptor's TM helices 2, 6, and 7 as critical determinants for ACKR3 activation.

Structural Insights Into How Proteoglycans Determine Chemokine-CXCR1/CXCR2 Interactions: Progress and Challenges

Proteoglycans (PGs), present in diverse environments, such as the cell membrane surface, extracellular milieu, and intracellular granules, are fundamental to life. Sulfated glycosaminoglycans (GAGs) are covalently attached to the core protein of proteoglycans. PGs are complex structures, and are diverse in terms of amino acid sequence, size, shape, and in the nature and number of attached GAG chains, and this diversity is further compounded by the phenomenal diversity in GAG structures. Chemokines play vital roles in human pathophysiology, from combating infection and cancer to leukocyte trafficking, immune surveillance, and neurobiology. Chemokines mediate their function by activating receptors that belong to the GPCR class, and receptor interactions are regulated by how, when, and where chemokines bind GAGs. GAGs fine-tune chemokine function by regulating monomer/dimer levels and chemotactic/haptotactic gradients, which are also coupled to how they are presented to their receptors. Despite their small size and similar structures, chemokines show a range of GAG-binding geometries, affinities, and specificities, indicating that chemokines have evolved to exploit the repertoire of chemical and structural features of GAGs. In this review, we summarize the current status of research on how GAG interactions regulate ELR-chemokine activation of CXCR1 and CXCR2 receptors, and discuss knowledge gaps that must be overcome to establish causal relationships governing the impact of GAG interactions on chemokine function in human health and disease.

Crosslinking-guided geometry of a complete CXC receptor-chemokine complex and the basis of chemokine subfamily selectivity

Chemokines and their receptors are orchestrators of cell migration in humans. Because dysregulation of the receptor-chemokine system leads to inflammation and cancer, both chemokines and receptors are highly sought therapeutic targets. Yet one of the barriers for their therapeutic targeting is the limited understanding of the structural principles behind receptor-chemokine recognition and selectivity. The existing structures do not include CXC subfamily complexes and lack information about the receptor distal N-termini, despite the importance of the latter in signaling, regulation, and bias. Here, we report the discovery of the geometry of the complex between full-length CXCR4, a prototypical CXC receptor and driver of cancer metastasis, and its endogenous ligand CXCL12. By comprehensive disulfide cross-linking, we establish the existence and the structure of a novel interface between the CXCR4 distal N-terminus and CXCL12 β1-strand, while also recapitulating earlier findings from nuclear magnetic resonance, modeling and crystallography of homologous receptors. A cross-linking-informed high-resolution model of the CXCR4-CXCL12 complex pinpoints the interaction determinants and reveals the occupancy of the receptor major subpocket by the CXCL12 proximal N terminus. This newly found positioning of the chemokine proximal N-terminus provides a structural explanation of CXC receptor-chemokine selectivity against other subfamilies. Our findings challenge the traditional two-site understanding of receptor-chemokine recognition, suggest the possibility of new affinity and signaling determinants, and fill a critical void on the structural map of an important class of therapeutic targets. These results will aid the rational design of selective chemokine-receptor targeting small molecules and biologics with novel pharmacology.

Self-healing DNA-based injectable hydrogels with reversible covalent linkages for controlled drug delivery

Injectable hydrogels represent a valuable tool for the delivery of therapeutic molecules aimed to restore the functionality of damaged tissues. In this study, we report the design of a nanocomposite DNA-based hydrogel crosslinked with oxidized alginate (OA) via the formation of reversible imine linkages. The formulated hydrogel functioned as an injectable carrier for the sustained delivery of a small molecule drug, simvastatin. The degree of oxidation of alginate and the concentration of silicate-based nanoparticles (nSi) were varied to modulate the rheological properties of the hydrogels. Specifically, the formulations consisting of OA with higher degree of oxidation displayed the highest value of storage moduli, yield stress, yield strain, and rapid recovery after removal of cyclic stress. The hydrogel formulations exhibited self-healing and shear-thinning properties due to the reversible nature of the covalent imine bonds formed between the aldehyde groups of OA and the amine groups present in the DNA nucleotides. Moreover, the incorporation of charged nSi further enhanced the shear strength of the formulated hydrogels by establishing electrostatic interactions with the phosphate groups of the DNA network. The optimized hydrogel was able to promote the sustained release of simvastatin for more than a week. The bioactivity of the released drug was confirmed by testing its ability to induce osteogenic differentiation and migration of human adipose-derived stem cells in vitro. Overall, the results obtained from this study demonstrate that DNA could be used as a natural biopolymer to fabricate self-healing injectable hydrogels with sustained release properties for minimally invasive therapeutic approaches. STATEMENT OF SIGNIFICANCE: Dynamic covalent chemistry, especially Schiff base reactions have emerged as a promising route for the formation of injectable hydrogels. Our study demonstrated the development of a DNA-based self-healing hydrogel formed via Schiff base reaction occurring at physiological conditions. The hydrogels functioned as sustained delivery vehicles for the hydrophobic drug simvastatin, which requires a polymeric carrier for controlled delivery of therapeutic concentrations of the drug without exhibiting cytotoxic effects. Presently available hydrogel-based drug delivery systems encounter major challenges for the delivery of hydrophobic drugs due to the hydrophilic nature of the base matrix. Our strategy presents a platform technology for the design of minimally invasive approaches for the sustained delivery of hydrophobic drugs similar to simvastatin.

Decreased ACKR3 (CXCR7) function causes oculomotor synkinesis in mice and humans

Oculomotor synkinesis is the involuntary movement of the eyes or eyelids with a voluntary attempt at a different movement. The chemokine receptor CXCR4 and its ligand CXCL12 regulate oculomotor nerve development; mice with loss of either molecule have oculomotor synkinesis. In a consanguineous family with congenital ptosis and elevation of the ptotic eyelid with ipsilateral abduction, we identified a co-segregating homozygous missense variant (c.772G>A) in ACKR3, which encodes an atypical chemokine receptor that binds CXCL12 and functions as a scavenger receptor, regulating levels of CXCL12 available for CXCR4 signaling. The mutant protein (p.V258M) is expressed and traffics to the cell surface but has a lower binding affinity for CXCL12. Mice with loss of Ackr3 have variable phenotypes that include misrouting of the oculomotor and abducens nerves. All embryos show oculomotor nerve misrouting, ranging from complete misprojection in the midbrain, to aberrant peripheral branching, to a thin nerve, which aberrantly innervates the lateral rectus (as seen in Duane syndrome). The abducens nerve phenotype ranges from complete absence, to aberrant projections within the orbit, to a normal trajectory. Loss of ACKR3 in the midbrain leads to downregulation of CXCR4 protein, consistent with reports that excess CXCL12 causes ligand-induced degradation of CXCR4. Correspondingly, excess CXCL12 applied to ex vivo oculomotor slices causes axon misrouting, similar to inhibition of CXCR4. Thus, ACKR3, through its regulation of CXCL12 levels, is an important regulator of axon guidance in the oculomotor system; complete loss causes oculomotor synkinesis in mice, while reduced function causes oculomotor synkinesis in humans.

The multilayered complexity of the chemokine receptor system

The chemokines receptor family are membrane-expressed class A-specific seven-transmembrane receptors linked to G proteins. Through interaction with the corresponding ligands, the chemokines, they induce a wide variety of cellular responses including cell polarization, movement, immune and inflammatory responses, as well as the prevention of HIV-1 infection. Like a Russian matryoshka doll, the chemokine receptor system is more complex than initially envisaged. This review focuses on the mechanisms that contribute to this dazzling complexity and how they modulate the signaling events triggered by chemokines. The chemokines and their receptors exist as monomers, dimers and oligomers, their expression pattern is highly regulated, and the ligands can bind distinct receptors with similar affinities. The use of novel imaging-based technologies, particularly real-time imaging modalities, has shed new light on the very dynamic conformations that chemokine receptors adopt depending on the cellular context, and that affect chemokine-mediated responses. This complex scenario presents both challenging and exciting opportunities for drug discovery.

Tailored Lipoprotein-Like miRNA Delivery Nanostructure Suppresses Glioma Stemness and Drug Resistance through Receptor-Stimulated Macropinocytosis

Glioma initiating cells (GICs) function as the seed for the propagation and relapse of glioma. Designing a smart and efficient strategy to target the GICs and to suppress the multiple signaling pathways associated with stemness and chemoresistance is essential to achieving a cancer cure. Inspired by the metabolic difference in endocytosis between GICs, differentiated glioma cells, and normal cells, a tailored lipoprotein-like nanostructure is developed to amplify their internalization into GICs through receptor-stimulated macropinocytosis. As CXCR4 is highly expressed on GICs and glioma tumor sites, meanwhile, the activation of CXCR4 induces the receptor-stimulated macropinocytosis pathway in GICs, this CXCR4 receptor-stimulated lipoprotein-like nanoparticle (SLNP) achieves efficient accumulation in GICs in vitro and in vivo. By carrying microRNA-34a in the core, this tailored SLNP reduces sex-determining region Y-box 2 and Notch1 expression, powerfully inhibits GICs stemness and chemoresistance, and significantly prolongs the survival of GICs-bearing mice. Taken together, a tailored lipoprotein-based nanostructure realizes efficient GICs accumulation and therapeutic effect through receptor-stimulated macropinocytosis, providing a powerful nanoplatform for RNA interference drugs to combat glioma.

Stimulating Macropinocytosis for Intracellular Nucleic Acid and Protein Delivery: A Combined Strategy with Membrane-Lytic Peptides To Facilitate Endosomal Escape

Delivery of biomacromolecules via endocytic pathways requires the efficient accumulation of cargo molecules into endosomes, followed by their release to the cytosol. We propose a unique intracellular delivery strategy for bioactive molecules using a new potent macropinocytosis-inducing peptide derived from stromal-derived factor 1α (SN21). This peptide allowed extracellular materials to enter cells through the activation of macropinocytosis. To provide the ability to release internalized cargoes from endosomes, we conjugated SN21 with membrane-lytic peptides. The combination of a macropinocytosis-inducing peptide and a membrane-lytic peptide successfully delivered functional siRNA and proteins, which include antibodies, Cre recombinase, and an artificial transcription regulator protein having a transcription activator-like effector (TALE) motif. This study shows the feasibility of combining the physiological stimulation of macropinocytosis with the physicochemical disruption of endosomes as a strategy for intracellular delivery.

Degranulation of human cytotoxic lymphocytes is a major source of proteolytically active soluble CD26/DPP4

Dipeptidyl peptidase 4 (DPP4, CD26) is a serine protease detected on several immune cells and on epithelial cells of various organs. Besides the membrane-bound enzyme, a catalytically active soluble form (sCD26/DPP4) is detected in several body fluids. Both variants cleave off dipeptides from the N-termini of various chemokines, neuropeptides, and hormones. CD26/DPP4 plays a fundamental role in the regulation of blood glucose levels by inactivating insulinotropic incretins and CD26/DPP4 inhibitors are thus routinely used in diabetes mellitus type 2 therapy to improve glucose tolerance. Such inhibitors might also prevent the CD26/DPP4-mediated inactivation of the T-cell chemoattractant CXCL10 released by certain tumors and thus improve anti-tumor immunity and immunotherapy. Despite its implication in the regulation of many (patho-)physiological processes and its consideration as a biomarker and therapeutic target, the cellular source of sCD26/DPP4 remains highly debated and mechanisms of its release are so far unknown. In line with recent reports that activated T lymphocytes could be a major source of sCD26/DPP4, we now demonstrate that CD26/DPP4 is stored in secretory granules of several major human cytotoxic lymphocyte populations and co-localizes with effector proteins such as granzymes, perforin, and granulysin. Upon stimulation, vesicular CD26/DPP4 is rapidly translocated to the cell surface in a Ca2+-dependent manner. Importantly, activation-induced degranulation leads to a massive release of proteolytically active sCD26/DPP4. Since activated effector lymphocytes serve as a major source of sCD26/DPP4, these results might explain the observed disease-associated alterations of sCD26/DPP4 serum levels and also indicate a so far unknown role of CD26/DPP4 in lymphocyte-mediated cytotoxicity.

Cross-kingdom mimicry of the receptor signaling and leukocyte recruitment activity of a human cytokine by its plant orthologs

Human macrophage migration-inhibitory factor (MIF) is an evolutionarily-conserved protein that has both extracellular immune-modulating and intracellular cell-regulatory functions. MIF plays a role in various diseases, including inflammatory diseases, atherosclerosis, autoimmunity, and cancer. It serves as an inflammatory cytokine and chemokine, but also exhibits enzymatic activity. Secreted MIF binds to cell-surface immune receptors such as CD74 and CXCR4. Plants possess MIF orthologs but lack the associated receptors, suggesting functional diversification across kingdoms. Here, we characterized three MIF orthologs (termed MIF/d-dopachrome tautomerase-like proteins or MDLs) of the model plant Arabidopsis thaliana Recombinant Arabidopsis MDLs (AtMDLs) share similar secondary structure characteristics with human MIF, yet only have minimal residual tautomerase activity using either p-hydroxyphenylpyruvate or dopachrome methyl ester as substrate. Site-specific mutagenesis suggests that this is due to a distinct amino acid difference at the catalytic cavity-defining residue Asn-98. Surprisingly, AtMDLs bind to the human MIF receptors CD74 and CXCR4. Moreover, they activate CXCR4-dependent signaling in a receptor-specific yeast reporter system and in CXCR4-expressing human HEK293 transfectants. Notably, plant MDLs exert dose-dependent chemotactic activity toward human monocytes and T cells. A small molecule MIF inhibitor and an allosteric CXCR4 inhibitor counteract this function, revealing its specificity. Our results indicate cross-kingdom conservation of the receptor signaling and leukocyte recruitment capacities of human MIF by its plant orthologs. This may point toward a previously unrecognized interplay between plant proteins and the human innate immune system.

Modulators of CXCR4 and CXCR7/ACKR3 Function

The two G protein-coupled receptors (GPCRs) C-X-C chemokine receptor type 4 (CXCR4) and atypical chemokine receptor 3 (ACKR3) are part of the class A chemokine GPCR family and represent important drug targets for human immunodeficiency virus (HIV) infection, cancer, and inflammation diseases. CXCR4 is one of only three chemokine receptors with a US Food and Drug Administration approved therapeutic agent, the small-molecule modulator AMD3100. In this review, known modulators of the two receptors are discussed in detail. Initially, the structural relationship between receptors and ligands is reviewed on the basis of common structural motifs and available crystal structures. To date, no atypical chemokine receptor has been crystallized, which makes ligand design and predictions for these receptors more difficult. Next, the selectivity, receptor activation, and the resulting ligand-induced signaling output of chemokines and other peptide ligands are reviewed. Binding of pepducins, a class of lipid-peptides whose basis is the internal loop of a GPCR, to CXCR4 is also discussed. Finally, small-molecule modulators of CXCR4 and ACKR3 are reviewed. These modulators have led to the development of radio- and fluorescently labeled tool compounds, enabling the visualization of ligand binding and receptor characterization both in vitro and in vivo. SIGNIFICANCE STATEMENT: To investigate the pharmacological modulation of CXCR4 and ACKR3, significant effort has been focused on the discovery and development of a range of ligands, including small-molecule modulators, pepducins, and synthetic peptides. Imaging tools, such as fluorescent probes, also play a pivotal role in the field of drug discovery. This review aims to provide an overview of the aforementioned modulators that facilitate the study of CXCR4 and ACKR3 receptors.

Evasins: Tick Salivary Proteins that Inhibit Mammalian Chemokines

Ticks are hematophagous arachnids that parasitize mammals and other hosts, feeding on their blood. Ticks secrete numerous salivary factors that enhance host blood flow or suppress the host inflammatory response. The recruitment of leukocytes, a hallmark of inflammation, is regulated by chemokines, which activate chemokine receptors on the leukocytes. Ticks target this process by secreting glycoproteins called Evasins, which bind to chemokines and prevent leukocyte recruitment. This review describes the recent discovery of numerous Evasins produced by ticks, their classification into two structural and functional classes, and the efficacy of Evasins in animal models of inflammatory diseases. The review also proposes a standard nomenclature system for Evasins and discusses the potential of repurposing or engineering Evasins as therapeutic anti-inflammatory agents.

Structural basis of chemokine and receptor interactions: Key regulators of leukocyte recruitment in inflammatory responses

In response to infection or injury, the body mounts an inflammatory immune response in order to neutralize pathogens and promote tissue repair. The key effector cells for these responses are the leukocytes (white blood cells), which are specifically recruited to the site of injury. However, dysregulation of the inflammatory response, characterized by the excessive migration of leukocytes to the affected tissues, can also lead to chronic inflammatory diseases. Leukocyte recruitment is regulated by inflammatory mediators, including an important family of small secreted chemokines and their corresponding G protein-coupled receptors expressed in leukocytes. Unsurprisingly, due to their central role in the leukocyte inflammatory response, chemokines and their receptors have been intensely investigated and represent attractive drug targets. Nonetheless, the full therapeutic potential of chemokine receptors has not been realized, largely due to the complexities in the chemokine system. The determination of chemokine-receptor structures in recent years has dramatically shaped our understanding of the molecular mechanisms that underpin chemokine signaling. In this review, we summarize the contemporary structural view of chemokine-receptor recognition, and describe the various binding modes of peptide and small-molecule ligands to chemokine receptors. We also provide some perspectives on the implications of these data for future research and therapeutic development. IMPORTANCE STATEMENT: Given their central role in the leukocyte inflammatory response, chemokines and their receptors are considered as important regulators of physiology and viable therapeutic targets. In this review, we provide a summary of the current understanding of chemokine: chemokine-receptor interactions that have been gained from structural studies, as well as their implications for future drug discovery efforts.

Binding of Dickkopf-3 to CXCR7 Enhances Vascular Progenitor Cell Migration and Degradable Graft Regeneration

Supplemental Digital Content is available in the text.

Rationale:

Vascular progenitor cells play key roles in physiological and pathological vascular remodeling—a process that is crucial for the regeneration of acellular biodegradable scaffolds engineered as vital strategies against the limited availability of healthy autologous vessels for bypass grafting. Therefore, understanding the mechanisms driving vascular progenitor cells recruitment and differentiation could help the development of new strategies to improve tissue-engineered vessel grafts and design drug-targeted therapy for vessel regeneration.

Objective:

In this study, we sought to investigate the role of Dkk3 (dickkopf-3), recently identified as a cytokine promotor of endothelial repair and smooth muscle cell differentiation, on vascular progenitor cells cell migration and vascular regeneration and to identify its functional receptor that remains unknown.

Methods and Results:

Vascular stem/progenitor cells were isolated from murine aortic adventitia and selected for the Sca-1 (stem cell antigen-1) marker. Dkk3 induced the chemotaxis of Sca-1+ cells in vitro in transwell and wound healing assays and ex vivo in the aortic ring assay. Functional studies to identify Dkk3 receptor revealed that overexpression or knockdown of chemokine receptor CXCR7 (C-X-C chemokine receptor type 7) in Sca-1+ cells resulted in alterations in cell migration. Coimmunoprecipitation experiments using Sca-1+ cell extracts treated with Dkk3 showed the physical interaction between DKK3 and CXCR7, and specific saturation binding assays identified a high-affinity Dkk3-CXCR7 binding with a dissociation constant of 14.14 nmol/L. Binding of CXCR7 by Dkk3 triggered the subsequent activation of ERK1/2 (extracellular signal-regulated kinases 1/2)-, PI3K (phosphatidylinositol 3-kinase)/AKT (protein kinase B)-, Rac1 (Ras-related C3 botulinum toxin substrate 1)-, and RhoA (Ras homolog gene family, member A)-signaling pathways involved in Sca-1+ cell migration. Tissue-engineered vessel grafts were fabricated with or without Dkk3 and implanted to replace the rat abdominal aorta. Dkk3-loaded tissue-engineered vessel grafts showed efficient endothelization and recruitment of vascular progenitor cells, which had acquired characteristics of mature smooth muscle cells. CXCR7 blocking using specific antibodies in this vessel graft model hampered stem/progenitor cell recruitment into the vessel wall, thus compromising vascular remodeling.

Conclusions:

We provide a novel and solid evidence that CXCR7 serves as Dkk3 receptor, which mediates Dkk3-induced vascular progenitor migration in vitro and in tissue-engineered vessels, hence harnessing patent grafts resembling native blood vessels.

A Novel CXCR4 Targeting Protein SDF-1/54 as an HIV-1 Entry Inhibitor

CXC chemokine receptor 4 (CXCR4) is a co-receptor for HIV-1 entry into target cells. Its natural ligand, the chemokine SDF-1, inhibits viral entry mediated by this receptor. However, the broad expression pattern of CXCR4 and its critical roles in various physiological and pathological processes indicate that the direct application of SDF-1 as an entry inhibitor might have severe consequences. Previously, we constructed an effective SDF-1 mutant, SDF-1/54, by deleting the α-helix of the C-terminal functional region of SDF-1. Of note, SDF-1/54 shows remarkable decreased chemotoxic ability, but maintains a similar binding affinity to CXCR4, suggesting SDF-1/54 might better serve as a CXCR4 inhibitor. Here, we found that SDF-1/54 exhibited potent antiviral activity against various X4 HIV-1 strains, including the infectious clone HIV-1 NL4-3, laboratory-adapted strain HIV-1 IIIB, clinical isolates and even drug-resistant strains. By using time-of-addition assay, non-infectious and infectious cell–cell fusion assay and CXCR4 internalization assay, we demonstrated SDF-1/54 is an HIV-1 entry inhibitor. A combination of SDF-1/54 with several antiretroviral drugs exhibited potent synergistic anti-HIV-1 activity. Moreover, SDF-1/54 was stable and its anti-HIV-1 activity was not significantly affected by the presence of seminal fluid, vaginal fluid simulant and human serum albumin. SDF-1/54 showed limited in vitro cytotoxicity to lymphocytes and vaginal epithelial cells. Based on these findings, SDF-1/54 could have a therapeutic potential as an HIV-1 entry inhibitor.

Kinetics of CXCL12 binding to atypical chemokine receptor 3 reveal a role for the receptor N terminus in chemokine binding

Chemokines bind to membrane-spanning chemokine receptors, which signal through G proteins and promote cell migration. However, atypical chemokine receptor 3 (ACKR3) does not appear to couple to G proteins, and instead of directly promoting cell migration, it regulates the extracellular concentration of chemokines that it shares with the G protein-coupled receptors (GPCRs) CXCR3 and CXCR4, thereby influencing the responses of these receptors. Understanding how these receptors bind their ligands is important for understanding these different processes. Here, we applied association and dissociation kinetic measurements coupled to β-arrestin recruitment assays to investigate ACKR3:chemokine interactions. Our results showed that CXCL12 binding is unusually slow and driven by the interplay between multiple binding epitopes. We also found that the amino terminus of the receptor played a key role in chemokine binding and activation by preventing chemokine dissociation. It was thought that chemokines initially bind receptors through interactions between the globular domain of the chemokine and the receptor amino terminus, which then guides the chemokine amino terminus into the transmembrane pocket of the receptor to initiate signaling. On the basis of our kinetic data, we propose an alternative mechanism in which the amino terminus of the chemokine initially forms interactions with the extracellular loops and transmembrane pocket of the receptor, which is followed by the receptor amino terminus wrapping around the core of the chemokine to prolong its residence time. These data provide insight into how ACKR3 competes and cooperates with canonical GPCRs in its function as a scavenger receptor.

Structure-function guided modeling of chemokine-GPCR specificity for the chemokine XCL1 and its receptor XCR1

Chemokines interact with their G protein-coupled receptors (GPCRs) through a two-step, two-site mechanism and, through this interaction, mediate various homeostatic and immune response mechanisms. Upon initial recognition of the chemokine by the receptor, the amino terminus of the chemokine inserts into the orthosteric pocket of the GPCR, causing conformational changes that trigger intracellular signaling. There is considerable structural and functional evidence to suggest that the amino acid composition and length of the chemokine amino terminus is critical for GPCR activation, complementing the size and amino acid composition of the orthosteric pocket. However, very few structures of a native chemokine-receptor complex have been solved. Here, we used a hybrid approach that combines structure-function data with Rosetta modeling to describe key contacts within a chemokine-GPCR interface. We found that the extreme amino-terminal residues of the chemokine XCL1 (Val¹, Gly², Ser³, and Glu⁴) contribute a large fraction of the binding energy to its receptor XCR1, whereas residues near the disulfide bond-forming residue Cys¹¹ modulate XCR1 activation. Alterations in the XCL1 amino terminus changed XCR1 activation, as determined by assessing inositol triphosphate accumulation, intracellular calcium release, and directed cell migration. Computational analysis of XCL1-XCR1 interactions revealed functional contacts involving Glu⁴ of XCL1 and Tyr¹¹⁷ and Arg²⁷³ of XCR1. Subsequent mutation of Tyr¹¹⁷ and Arg²⁷³ led to diminished binding and activation of XCR1 by XCL1. These findings demonstrate the utility of a hybrid approach, using biological data and homology modeling, to study chemokine-GPCR interactions.

Structural and Biological Characterizations of Novel High-Affinity Fluorescent Probes with Overlapped and Distinctive Binding Regions on CXCR4

CXC-type chemokine receptor 4 (CXCR4) is well known as a co-receptor for cellular entry and infection of human immunodeficiency virus type 1 (HIV-1). As an important member of the G protein-coupled receptor (GPCR) family, CXCR4 also mediates a variety of cellular processes and functions, such as cell chemotaxis, proliferation, and calcium signal transductions. Identification and characterization of molecular ligands or probes of CXCR4 have been an intensive area of investigations as such ligands or probes are of significant clinical values for the studies and treatments of HIV-1 infection and other human diseases mediated by the receptor. The crystal structures of CXCR4 in complex with different ligands have revealed two distinctive binding regions or subpockets. Thus, understanding the interactions of diverse ligands with these distinctive CXCR4 binding regions has become vital for elucidating the relationship between binding modes and biological mechanisms of ligand actions. Peptidic CVX15 is the only ligand that has been validated to bind one of these distinctive binding regions (or so called the major subpocket) of CXCR4. Therefore, in this study, we developed an efficient probe system including two high-affinity peptidic fluorescent probes, designated as FITC-CVX15 and FITC-DV1, with the aim of targeting distinctive CXCR4 subpockets. We conducted rational design and chemical characterization of the two CXCR4-specific probes and examined their application in biological experiments including competitive binding assays, flow cytometry analysis, and confocal imaging. Especially these two probes were applied in parallel CXCR4 competitive binding assays to detect and analyze potential binding modes of diverse CXCR4 ligands, together with molecular docking and simulations. Our results have indicated that these peptidic fluorescent probe systems provide novel ligand detecting tools, as well as present a new approach for analyzing distinctive binding modes of diverse CXCR4 ligands.

Reversing the Tumor Target: Establishment of a Tumor Trap

Despite the tremendous progress made in the field of cancer therapy in recent years, certain solid tumors still cannot be successfully treated. Alongside classical treatments in the form of chemotherapy and/or radiotherapy, targeted treatments such as immunotherapy that cause fewer side effects emerge as new options in the clinics. However, these alternative treatments may not be useful for treating all types of cancers, especially for killing infiltrative and circulating tumor cells (CTCs). Recent advances pursue the trapping of these cancer cells within a confined area to facilitate their removal for therapeutic and diagnostic purposes. A good understanding of the mechanisms behind tumor cell migration may drive the design of traps that mimic natural tumor niches and guide the movement of the cancer cells. To bring this trapping idea into reality, strong efforts are being made to create structured materials that imitate myelinated fibers, blood vessels, or pre-metastatic niches and incorporate chemical cues such as chemoattractants or adhesive proteins. In this review, the different strategies used (or could be used) to trap tumor cells are described, and relevant examples of their performance are analyzed.

Oxidation State Dependent Conformational Changes of HMGB1 Regulate the Formation of the CXCL12/HMGB1 Heterocomplex

High-mobility Group Box 1 (HMGB1) is an abundant protein present in all mammalian cells and involved in several processes. During inflammation or tissue damage, HMGB1 is released in the extracellular space and, depending on its redox state, can form a heterocomplex with CXCL12. The heterocomplex acts exclusively via the chemokine receptor CXCR4 enhancing leukocyte recruitment.

Here, we used multi-microsecond molecular dynamics (MD) simulations to elucidate the effect of the disulfide bond on the structure and dynamics of HMGB1.

The results of the MD simulations show that the presence or lack of the disulfide bond between Cys23 and Cys45 modulates the conformational space explored by HMGB1, making the reduced protein more suitable to form a complex with CXCL12.

CXCL12 and Its Isoforms: Different Roles in Pancreatic Cancer?

CXCL12 is a chemokine that acts through CXCR4 and ACKR3 receptors and plays a physiological role in embryogenesis and haematopoiesis. It has an important role also in tumor development, since it is released by stromal cells of tumor microenvironment and alters the behavior of cancer cells. Many studies investigated the roles of CXCL12 in order to understand if it has an anti- or protumor role. In particular, it seems to promote tumor invasion, proliferation, angiogenesis, epithelial to mesenchymal transition (EMT), and metastasis in pancreatic cancer. Nevertheless, some evidence shows opposite functions; therefore research on CXCL12 is still ongoing. These discrepancies could be due to the presence of at least six CXCL12 splicing isoforms, each with different roles. Interestingly, three out of six variants have the highest levels of expression in the pancreas. Here, we report the current knowledge about the functions of this chemokine and then focus on pancreatic cancer. Moreover, we discuss the methods applied in recent studies in order to understand if they took into account the existence of the CXCL12 isoforms.