Pharmacological Properties of Peptides Derived from Stromal Cell-Derived Factor 1: Study on Human Polymorphonuclear Cells

Different contributions of chemokine N-terminal features attest to a different ligand binding mode and a bias towards activation of ACKR3/CXCR7 compared with CXCR4 and CXCR3

BACKGROUND AND PURPOSE

Chemokines and their receptors form an intricate interaction and signalling network that plays critical roles in various physiological and pathological cellular processes. The high promiscuity and apparent redundancy of this network makes probing individual chemokine/receptor interactions and functional effects, as well as targeting individual receptor axes for therapeutic applications, challenging. Despite poor sequence identity, the N-terminal regions of chemokines, which play a key role in their activity and selectivity, contain several conserved features. Thus far little is known regarding the molecular basis of their interactions with typical and atypical chemokine receptors or the conservation of their contributions across chemokine-receptor pairs.

EXPERIMENTAL APPROACH

We used a broad panel of chemokine variants and modified peptides derived from the N-terminal region of chemokines CXCL12, CXCL11 and vCCL2, to compare the contributions of various features to binding and activation of their shared receptors, the two typical, canonical G protein-signalling receptors, CXCR4 and CXCR3, as well as the atypical scavenger receptor CXCR7/ACKR3, which shows exclusively arrestin-dependent activity.

KEY RESULTS

We provide molecular insights into the plasticity of the ligand-binding pockets of these receptors, their chemokine binding modes and their activation mechanisms. Although the chemokine N-terminal region is a critical determinant, neither the most proximal residues nor the N-loop are essential for binding and activation of ACKR3, as distinct from binding and activation of CXCR4 and CXCR3.

CONCLUSION AND IMPLICATIONS

These results suggest a different interaction mechanism between this atypical receptor and its ligands and illustrate its strong propensity to activation.

Development of Mimokines, chemokine N terminus-based CXCR4 inhibitors optimized by phage display and rational design

The chemokine receptor CXCR4 (C-X-C chemokine receptor type 4 also known as fusin or CD184 (cluster of differentiation 184)) is implicated in various biological and pathological processes of the hematopoietic and immune systems. CXCR4 is also one of the major coreceptors for HIV-1 entry into target cells and is overexpressed in many cancers, supporting cell survival, proliferation, and migration. CXCR4 is thus an extremely relevant drug target. Among the different strategies to block CXCR4, chemokine-derived peptide inhibitors hold great therapeutic potential. In this study, we used the N-terminus of vCCL2/vMIPII, a viral CXCR4 antagonist chemokine, as a scaffold motif to engineer and select CXCR4 peptide inhibitors, called Mimokines, which imitate the chemokine-binding mode but display an enhanced receptor affinity, antiviral properties, and receptor selectivity. We first engineered a Mimokine phage displayed library based on the first 21 residues of vCCL2, in which cysteine 11 and 12 were fully randomized and screened it against purified CXCR4 stabilized in liposomes. We identified Mimokines displaying up to 4-fold higher affinity for CXCR4 when compared to the reference peptide and fully protected MT-4 cells against HIV-1 infection. These selected Mimokines were then subjected to dimerization, D-amino acid, and aza-β3-amino acid substitution to further enhance their potency and selectivity. Optimized Mimokines exhibited up to 120-fold enhanced CXCR4 binding (range of 20 nM) and more than 200-fold improved antiviral properties (≤ 1 μM) compared to the parental Mimokines. Interestingly, these optimized Mimokines also showed up to 25-fold weaker affinity for ACKR3/CXCR7 and may therefore serve as lead compounds for further development of more selective CXCR4 peptide inhibitors and probes.

Macrophage Migration Inhibitory Factor-CXCR4 Receptor Interactions: EVIDENCE FOR PARTIAL ALLOSTERIC AGONISM IN COMPARISON WITH CXCL12 CHEMOKINE

An emerging number of non-chemokine mediators are found to bind to classical chemokine receptors and to elicit critical biological responses. Macrophage migration inhibitory factor (MIF) is an inflammatory cytokine that exhibits chemokine-like activities through non-cognate interactions with the chemokine receptors CXCR2 and CXCR4, in addition to activating the type II receptor CD74. Activation of the MIF-CXCR2 and -CXCR4 axes promotes leukocyte recruitment, mediating the exacerbating role of MIF in atherosclerosis and contributing to the wealth of other MIF biological activities. Although the structural basis of the MIF-CXCR2 interaction has been well studied and was found to engage a pseudo-ELR and an N-like loop motif, nothing is known about the regions of CXCR4 and MIF that are involved in binding to each other. Using a genetic strain of Saccharomyces cerevisiae that expresses a functional CXCR4 receptor, site-specific mutagenesis, hybrid CXCR3/CXCR4 receptors, pharmacological reagents, peptide array analysis, chemotaxis, fluorescence spectroscopy, and circular dichroism, we provide novel molecular information about the structural elements that govern the interaction between MIF and CXCR4. The data identify similarities with classical chemokine-receptor interactions but also provide evidence for a partial allosteric agonist compared with CXCL12 that is possible due to the two binding sites of CXCR4.

Chemokine-Derived Peptides: Novel Antimicrobial and Antineoplasic Agents

Chemokines are a burgeoning family of chemotactic cytokines displaying a broad array of functions such as regulation of homeostatic leukocyte traffic and development, as well as activating the innate immune system. Their role in controlling early and late inflammatory stages is now well recognized. An improper balance either in chemokine synthesis or chemokine receptor expression contributes to various pathological disorders making chemokines and their receptors a useful therapeutic target. Research in this area is progressing rapidly, and development of novel agents based on chemokine/chemokine receptors antagonist functions are emerging as attractive alternative drugs. Some of these novel agents include generation of chemokine-derived peptides (CDP) with potential agonist and antagonist effects on inflammation, cancer and against bacterial infections. CDP have been generated mainly from N- and C-terminus chemokine sequences with subsequent modifications such as truncations or elongations. In this review, we present a glimpse of the different pharmacological actions reported for CDP and our current understanding regarding the potential use of CDP alone or as part of the novel therapies proposed in the treatment of microbial infections and cancer.

Neutralising properties of peptides derived from CXCR4 extracellular loops towards CXCL12 binding and HIV-1 infection

The chemokine receptor CXCR4 interacts with a single endogenous chemokine, CXCL12, and regulates a wide variety of physiological and pathological processes including inflammation and metastasis development. CXCR4 also binds the HIV-1 envelope glycoprotein, gp120, resulting in viral entry into host cells. Therefore, CXCR4 and its ligands represent valuable drug targets. In this study, we investigated the inhibitory properties of synthetic peptides derived from CXCR4 extracellular loops (ECL1-X4, ECL2-X4 and ECL3-X4) towards HIV-1 infection and CXCL12-mediated receptor activation. Among these peptides, ECL1-X4 displayed anti-HIV-1 activity against X4, R5/X4 and R5 viruses (IC50=24 to 76μM) in cell viability assay without impairing physiological CXCR4-CXCL12 signalling. In contrast, ECL2-X4 only inhibited X4 and R5/X4 strains, interfering with HIV-entry into cells. At the same time, ECL2-X4 strongly and specifically interacted with CXCL12, blocking its binding to CXCR4 and its second receptor, CXCR7 (IC50=20 and 100μM). Further analysis using mutated and truncated peptides showed that ECL2 of CXCR4 forms multiple contacts with the gp120 protein and the N-terminus of CXCL12. Chemokine neutralisation was mainly driven by four aspartates and the C-terminal residues of ECL2-X4. These results demonstrate that ECL2 represents an important structural determinant in CXCR4 activation. We identified the putative site for the binding of CXCL12 N-terminus and provided new structural elements to explain the recognition of gp120 and dimeric CXCR4 ligands.

International Union of Basic and Clinical Pharmacology. [corrected]. LXXXIX. Update on the extended family of chemokine receptors and introducing a new nomenclature for atypical chemokine receptors

Sixteen years ago, the Nomenclature Committee of the International Union of Pharmacology approved a system for naming human seven-transmembrane (7TM) G protein-coupled chemokine receptors, the large family of leukocyte chemoattractant receptors that regulates immune system development and function, in large part by mediating leukocyte trafficking. This was announced in Pharmacological Reviews in a major overview of the first decade of research in this field [Murphy PM, Baggiolini M, Charo IF, Hébert CA, Horuk R, Matsushima K, Miller LH, Oppenheim JJ, and Power CA (2000) Pharmacol Rev 52:145-176]. Since then, several new receptors have been discovered, and major advances have been made for the others in many areas, including structural biology, signal transduction mechanisms, biology, and pharmacology. New and diverse roles have been identified in infection, immunity, inflammation, development, cancer, and other areas. The first two drugs acting at chemokine receptors have been approved by the U.S. Food and Drug Administration (FDA), maraviroc targeting CCR5 in human immunodeficiency virus (HIV)/AIDS, and plerixafor targeting CXCR4 for stem cell mobilization for transplantation in cancer, and other candidates are now undergoing pivotal clinical trials for diverse disease indications. In addition, a subfamily of atypical chemokine receptors has emerged that may signal through arrestins instead of G proteins to act as chemokine scavengers, and many microbial and invertebrate G protein-coupled chemokine receptors and soluble chemokine-binding proteins have been described. Here, we review this extended family of chemokine receptors and chemokine-binding proteins at the basic, translational, and clinical levels, including an update on drug development. We also introduce a new nomenclature for atypical chemokine receptors with the stem ACKR (atypical chemokine receptor) approved by the Nomenclature Committee of the International Union of Pharmacology and the Human Genome Nomenclature Committee.

Design, synthesis, and functionalization of dimeric peptides targeting chemokine receptor CXCR4

The chemokine receptor CXCR4 is a critical regulator of inflammation and immune surveillance, and it is specifically implicated in cancer metastasis and HIV-1 infection. On the basis of the observation that several of the known antagonists remarkably share a C(2) symmetry element, we constructed symmetric dimers with excellent antagonistic activity using a derivative of a cyclic pentapeptide as monomer. To optimize the binding affinity, we investigated the influence of the distance between the monomers and the pharmacophoric sites in the synthesized constructs. The affinity studies in combination with docking computations support a two-site binding model. In a final step, 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) was introduced as chelator for (radio-)metals, thus allowing to exploit these compounds as a new group of CXCR4-binding peptidic probes for molecular imaging and endoradiotherapeutic purposes. Both the DOTA conjugates and some of their corresponding metal complexes retain good CXCR4 affinity, and one (68)Ga labeled compound was studied as PET tracer.

MIF-chemokine receptor interactions in atherogenesis are dependent on an N-loop-based 2-site binding mechanism

Macrophage migration inhibitory factor (MIF) is a cytokine that mediates inflammatory diseases. MIF promotes atherogenic leukocyte recruitment through a promiscuous, yet highly affine, interaction with CXCR2 and CXCR4. Binding to CXCR2 is dependent on a pseudo-(E)LR motif in MIF, but a second interaction site has been elusive. Here we identified an N-like loop in MIF, suggesting that MIF binding to CXCR2 follows the 2-site binding mode of bona fide chemokines. For MIF, the model predicts interactions between the N-like loop and the CXCR2 N domain (site 1) and pseudo-(E)LR and extracellular loops (ELs) of CXCR2 (site 2). Applying biophysical and peptide array analysis, we demonstrated an interaction between MIF and the CXCR2 N domain, which was pseudo-(E)LR independent. Peptide array analysis also indicated that the pseudo-(E)LR motif is responsible for MIF binding to EL2 and 3. Notably, peptides MIF-(40-49) and MIF-(47-56), representing N-like-loop-derived peptides, but not a scrambled control peptide, significantly blocked MIF/CXCR2 binding, MIF-mediated monocyte arrest under flow on aortic endothelial cells in vitro (IC(50): 1.24×10(-6) M), and MIF-dependent monocyte adhesion to atherosclerotic mouse carotid arteries in vivo. Thus, the N-like loop in MIF is critical for MIF's noncognate interaction with CXCR2 and proatherogenic functions. The 2-site binding model that explains chemokine receptor activation also applies to MIF.

CXCR4, inhibitors and mechanisms of action

In this review, the author discusses recent advances in anti-HIV inhibitors, targeting CXCR4, including natural and modified chemokines, peptides and organic compounds, their mechanisms of action, and the molecular process of virus invasion of immune cells. Peptides with strong anti-HIV activity exhibit several common features, such as electrostatic charges, cyclization, beta-turns and dimerization induced by a sulphide bond. Organic compounds, such as cyclams, display a unique metal-mediated mechanism in the binding process to its target CXCR4. Understanding of their mechanisms of action may be useful for the design of more effective drugs. Consecutive interactions of viral glycoprotein gp120 with CD4 and the co-receptor, CXCR4 or another co-receptor CCR5 on the cell surface leads to virus invasion into host cells. The molecular details of the binding between HIV glycoproteins and the co-receptors also provide a basis for anti-HIV therapy.

Pain control by CXCR2 ligands through Ca2+-regulated release of opioid peptides from polymorphonuclear cells

Leukocytes counteract inflammatory pain by releasing opioid peptides, which bind to opioid receptors on peripheral sensory neurons. In the early phase of inflammation, polymorphonuclear cells (PMN) are the major source of opioids. Their recruitment is governed by ligands at the chemokine receptor CXCR2. Here, we examined whether chemokines can also induce opioid peptide secretion from PMN and thus inhibit inflammatory pain. In rats with hindpaw inflammation, intraplantar injection of CXCL2/3, but not of the CXCR4 ligand CXCL12, elicited naloxone-reversible (i.e., opioid receptor mediated) mechanical and thermal analgesia, which was abolished by systemic PMN depletion. Both CXCR1/2- and CXCR4-ligands induced PMN chemotaxis, but only CXCR1/2 ligands triggered opioid release from human and rat PMN in vitro. This release was unaltered by extracellular Ca2+ chelation, was mimicked by thapsigargin and was blocked by inhibitors of the inositol 1,4,5-triphosphate receptor (IP3) and by intracellular Ca2+ chelation, indicating that it required Ca2+ from intracellular but not extracellular sources. Furthermore, release was partially reduced by phosphoinositol-3-kinase (PI3K) inhibitors. Adoptive transfer of allogenic PMN into PMN-depleted rats reconstituted CXCL2/3-induced analgesia, which was inhibited by prior ex vivo chelation of intracellular Ca2+. These findings demonstrate that, beyond cell recruitment, CXCR2 ligands induce Ca2+-regulated opioid release from PMN and thereby inhibit inflammatory pain in vivo.

Characterization of the molecular pharmacology of AMD3100: a specific antagonist of the G-protein coupled chemokine receptor, CXCR4

The chemokine receptor CXCR4 is widely expressed on different cell types, is involved in leukocyte chemotaxis, and is a co-receptor for HIV. AMD3100 has been shown to be a CXCR4 receptor antagonist, and to block HIV infection of T-tropic, X4-using, virus in vitro and in vivo. AMD3100 is an effective mobilizer of hematopoietic stem cells and is being investigated in clinical trials in multiple myeloma and non-Hodgkins lymphoma patients. Using the CCRF-CEM T-cell line that constitutively expresses CXCR4 we confirmed that AMD3100 was an antagonist of SDF-1/CXCL12 ligand binding (IC50=651+/-37 nM). We have also shown that AMD3100 inhibits SDF-1 mediated GTP-binding (IC50=27+/-2.2 nM), SDF-1 mediated calcium flux (IC50=572+/-190 nM), and SDF-1 stimulated chemotaxis (IC50=51+/-17 nM). AMD3100 did not inhibit calcium flux against cells expressing CXCR3, CCR1, CCR2b, CCR4, CCR5 or CCR7 when stimulated with their cognate ligands, nor did it inhibit receptor binding of LTB4. AMD3100 did not, on its own, induce a calcium flux in the CCRF-CEM cells, which express multiple GPCRs including CXCR4, CCR4 and CCR7. Furthermore, AMD3100 neither stimulated GTP-binding, an assay for GPCR activation, in CEM cell membranes; nor chemotaxis of CCRF-CEM cells. These data therefore demonstrate that AMD3100 is a specific antagonist of CXCR4, is not cross-reactive with other chemokine receptors, and is not an agonist of CXCR4.

Bioluminescence resonance energy transfer reveals ligand-induced conformational changes in CXCR4 homo- and heterodimers

Homo- and heterodimerization have emerged as prominent features of G-protein-coupled receptors with possible impact on the regulation of their activity. Using a sensitive bioluminescence resonance energy transfer system, we investigated the formation of CXCR4 and CCR2 chemokine receptor dimers. We found that both receptors exist as constitutive homo- and heterodimers and that ligands induce conformational changes within the pre-formed dimers without promoting receptor dimer formation or disassembly. Ligands with different intrinsic efficacies yielded distinct bioluminescence resonance energy transfer modulations, indicating the stabilization of distinct receptor conformations. We also found that peptides derived from the transmembrane domains of CXCR4 inhibited activation of this receptor by blocking the ligand-induced conformational transitions of the dimer. Taken together, our data support a model in which chemokine receptor homo- and heterodimers form spontaneously and respond to ligand binding as units that undergo conformational changes involving both protomers even when only one of the two ligand binding sites is occupied.

Stroma Cell-Derived Factor 1α Mediates Desensitization of Human Neutrophil Respiratory Burst in Synovial Fluid from Rheumatoid Arthritic Patients

Classical chemoattractants such as fMLP or the complement factor C5a use G protein (Gi)-coupled receptors to stimulate both chemotaxis and production of reactive oxygen species (respiratory burst, RB) by polymorphonuclear leukocytes (PMN). The chemokine stroma cell-derived factor 1alpha (SDF1alpha) and its Gi-coupled receptor, CXCR4, regulate leukocyte trafficking and recruitment to the synovial fluid of rheumatoid arthritic patients (RA-SF). However, the role of SDF1alpha in the RB is unknown and was studied in this work in vitro with healthy PMN in the absence and presence of RA-SF. In healthy PMN, SDF1alpha failed to stimulate the RB, even though the p38 mitogen-activated protein kinase was activated to a similar level as in fMLP-stimulated PMN. In contrast, the SDF1alpha-mediated calcium transients and activation of phosphatidylinositol 3-kinase/Akt were partially deficient, while p44/42 mitogen-activated protein kinases were not activated. SDF1alpha actually desensitized weakly the fMLP-mediated RB of healthy PMN. This cross-inhibitory effect was amplified in PMN treated with RA-SF, providing a protection against the exacerbation of RB induced by C5a or fMLP. This SDF1alpha beneficial effect, which was prevented by the CXCR4 antagonist AMD3100, was associated with impairment of C5a- and fMLP-mediated early signaling events. Thus, although SDF1alpha promotes leukocyte emigration into rheumatoid synovium, our data suggest it cross-desensitizes the production of oxidant by primed PMN, a property that may be beneficial in the context of arthritis.