Synthesis, modeling and functional activity of substituted styrene-amides as small-molecule CXCR7 agonists

Structural visualization of small molecule recognition by CXCR3 uncovers dual-agonism in the CXCR3-CXCR7 system

Chemokine receptors are critically involved in multiple physiological and pathophysiological processes related to immune response mechanisms. Most chemokine receptors are prototypical GPCRs although some also exhibit naturally-encoded signaling-bias toward β-arrestins (βarrs). C-X-C type chemokine receptors, namely CXCR3 and CXCR7, constitute a pair wherein the former is a prototypical GPCR while the latter exhibits selective coupling to βarrs despite sharing a common natural agonist: CXCL11. Moreover, CXCR3 and CXCR7 also recognize small molecule agonists suggesting a modular orthosteric ligand binding pocket. Here, we determine cryo-EM structures of CXCR3 in an Apo-state and in complex with small molecule agonists biased toward G-proteins or βarrs. These structural snapshots uncover an allosteric network bridging the ligand-binding pocket to intracellular side, driving the transducer-coupling bias at this receptor. Furthermore, structural topology of the orthosteric binding pocket also allows us to discover and validate that selected small molecule agonists of CXCR3 display robust agonism at CXCR7. Collectively, our study offers molecular insights into signaling-bias and dual agonism in the CXCR3-CXCR7 system with therapeutic implications.

The role of CXCL12/CXCR4/CXCR7 axis in cognitive impairment associated with neurodegenerative diseases

Neurodegenerative diseases, including Alzheimer's Disease (AD), Parkinson's Disease (PD), Multiple Sclerosis (MS), and Amyotrophic Lateral Sclerosis (ALS), are characterized by progressive neuronal loss and cognitive impairment (CI). The: Cysteine-X-cysteine chemokine ligand 12(CXCL12)/CXC chemokine receptor type 4 (CXCR4)/CXC chemokine receptor type 7 (CXCR7) axis has emerged as a critical molecular pathway in the development of CI in these disorders. This review explores the role of this axis in the pathogenesis of CI across these neurodegenerative diseases, synthesizing current evidence and its implications for targeted therapies. In AD, dysregulation of this axis contributes to amyloid-β accumulation and tau hyperphosphorylation, leading to synaptic dysfunction and cognitive decline. PD studies reveal that CXCL12/CXCR4 signaling influences dopaminergic neuron survival and microglial activation, affecting cognitive function. In MS, the axis modulates neuroinflammation and demyelination processes, impacting cognitive performance. ALS research indicates that the CXCL12/CXCR4/CXCR7 pathway is involved in motor neuron degeneration and associated cognitive deficits. Across these diseases, the axis influences neuroinflammation, synaptic plasticity, and neuronal survival through various signaling cascades, including PI3K/AKT, MAPK, and JAK/STAT pathways. Emerging evidence suggests that modulating this axis could provide neuroprotective effects and potentially alleviate cognitive symptoms. This review highlights the potential of the CXCL12/CXCR4/CXCR7 axis as a therapeutic target for addressing CI in neurodegenerative diseases. It also underscores the need for further research to fully elucidate its role and develop effective interventions, potentially leading to improved clinical management strategies for these devastating disorders.

Regulation of the chemokine receptors CXCR4 and ACKR3 by receptor activity-modifying proteins

The chemokine CXCL12 and its two cognate receptors-CXCR4 and ACKR3-are key players in various homeostatic and pathophysiological processes, including embryonic development, autoimmune diseases, tissue repair, and cancer. Recent reports identified an interaction of CXCR4 and ACKR3 with receptor activity-modifying proteins (RAMPs), and RAMP3 has been shown to facilitate ACKR3's recycling properties. Yet, the functional effects of RAMPs on the CXCL12 signaling axis remain largely elusive. Here, we characterize the effects of RAMPs on CXCR4 and ACKR3 function. We show that, in the absence of a ligand, RAMPs do not affect the cell membrane localization or constitutive internalization of the two receptors. RAMP3 inhibits ligand-stimulated internalization of ACKR3, which retains the receptor at the membrane and inhibits its ability to scavenge CXCL12. In addition, while cAMP inhibition by CXCR4 is unaffected by RAMPs, basal and ligand-stimulated β-arrestin recruitment to both CXCR4 and ACKR3 is reduced in the presence of RAMP3 due to complex formation at the cell surface. The effects on ACKR3 are observed for chemokine, small molecule, and peptide agonists as well as for a N-terminal truncated receptor variant, suggesting that RAMP regulation involves contacts with the transmembrane domain of the receptor. Taken together, our results show that RAMPs regulate the CXCL12 signaling axis by directly interfering with receptor function. These findings could have direct implications for the interplay between receptors in vivo as well as future drug design in the therapeutic targeting of the CXCL12 signaling axis.

Deficient GATA6-CXCR7 signaling leads to bicuspid aortic valve

The cardiac outflow tract (OFT) transiently links the ventricles to the aortic sac and forms the arterial valves. Abnormalities in these valves, such as bicuspid aortic valve (BAV), are common congenital anomalies. GATA6-inactivating variants cause cardiac OFT defects and BAV, but their mechanisms are unclear. We generated Gata6STOP/+ mice using CRISPR-Cas9, which show highly penetrant BAV (70%) and membranous ventricular septal defects (43%). These mice exhibited decreased proliferation and increased ISL1-positive progenitor cells in the OFT, indicating abnormal cardiovascular differentiation. Gata6 deletion with the Mef2cCre driver line recapitulated Gata6STOP/+ phenotypes, indicating a cell-autonomous role for Gata6 in the second heart field. Gata6STOP/+ mice showed reduced OFT length and caliber, associated with deficient cardiac neural crest cell contribution, which may cause valvulo-septal defects. RNA-sequencing analysis showed depletion in pathways related to cell proliferation and migration, highlighting Cxcr7 (also known as Ackr3) as a candidate gene. Reduced mesenchymal cell migration and invasion were observed in Gata6STOP/+ OFT tissue. CXCR7 agonists reduced mesenchymal cell migration and increased invasion in wild-type but not in Gata6STOP/+ explants, indicating the GATA6-dependent role of CXCR7 in OFT development and its potential link to BAV.

Novel Small-Molecule Atypical Chemokine Receptor 3 Agonists: Design, Synthesis, and Pharmacological Evaluation for Antiplatelet Therapy

ACKR3, an atypical chemokine receptor, has been associated with prothrombotic events and the development of cardiovascular events. We designed, synthesized, and evaluated a series of novel small molecule ACKR3 agonists. Extensive structure-activity relationship studies resulted in several promising agonists with potencies ranging from the low micromolar to nanomolar range, for example, 23 (EC50 = 111 nM, Emax = 95%) and 27 (EC50 = 69 nM, Emax = 82%) in the β-arrestin-recruitment assay. These compounds are selective for ACKR3 versus ACKR2, CXCR3, and CXCR4. Several agonists were subjected to investigations of their P-selectin expression reduction in the flow cytometry experiments. In particular, compounds 23 and 27 showed the highest potency for platelet aggregation inhibition, up to 80% and 97%, respectively. The most promising compounds, especially 27, exhibited good solubility, metabolic stability, and no cytotoxicity, suggesting a potential tool compound for the treatment of platelet-mediated thrombosis.

Bilayer lipids modulate ligand binding to atypical chemokine receptor 3

Chemokine receptors belong to the large class of G protein-coupled receptors (GPCRs) and are involved in a number of (patho)physiological processes. Previous studies highlighted the importance of membrane lipids for modulating GPCR structure and function. However, the underlying mechanisms of how lipids regulate GPCRs are often poorly understood. Here, we report that anionic lipid bilayers increase the binding affinity of the chemokine CXCL12 for the atypical chemokine receptor 3 (ACKR3) by modulating the CXCL12 binding kinetics. Notably, the anionic bilayer favors CXCL12 over the more positively charged chemokine CXCL11, which we explained by bilayer interactions orienting CXCL12 but not CXCL11 for productive ACKR3 binding. Furthermore, our data suggest a stabilization of active ACKR3 conformations in anionic bilayers. Taken together, the described regulation of chemokine selectivity of ACKR3 by the lipid bilayer proposes an extended version of the classical model of chemokine binding including the lipid environment of the receptor.

Conformational dynamics underlying atypical chemokine receptor 3 activation

Atypical Chemokine Receptor 3 (ACKR3) belongs to the G protein-coupled receptor family but it does not signal through G proteins. The structural properties that govern the functional selectivity and the conformational dynamics of ACKR3 activation are poorly understood. Here, we combined hydrogen/deuterium exchange mass spectrometry, site-directed mutagenesis, and molecular dynamics simulations to examine the binding mode and mechanism of action of ACKR3 ligands of different efficacies. Our results show that activation or inhibition of ACKR3 is governed by intracellular conformational changes of helix 6, intracellular loop 2, and helix 7, while the DRY motif becomes protected during both processes. Moreover, we identified the binding sites and the allosteric modulation of ACKR3 upon β-arrestin 1 binding. In summary, this study highlights the structure-function relationship of small ligands, the binding mode of β-arrestin 1, the activation dynamics, and the atypical dynamic features in ACKR3 that may contribute to its inability to activate G proteins.

Second generation Al18F-labeled D-amino acid peptide for CXCR4 targeted molecular imaging

BACKGROUND

The C-X-C chemokine receptor type 4 (CXCR4) is overexpressed in many cancers, e.g. multiple myeloma and acute leukemia, yet solely [68Ga]PentixaFor is used for clinical PET imaging. The aim of this study was to develop and assess a second generation Al18F-labeled D-amino acid peptide based on the viral macrophage inflammatory protein II for CXCR4 targeted molecular imaging.

METHODS

We designed a library of monomer and multimer constructs and evaluated their binding affinity for human and mouse CXCR4. Based on these results, we selected the best vector molecule for development of an Al18F-labeled ligand, [18F]AlF-NOTA-2xDV1(c11sc12s), which was further evaluated in a cell-based binding assay to assess its binding properties and specificity for CXCR4. Next, pharmacokinetics and tumor uptake of [18F]AlF-NOTA-2xDV1(c11sc12s) were evaluated in naïve mice and mice with xenografts derived from U87.CXCR4 cells. Finally, we performed an imaging study in a non-human primate to assess the in vivo distribution of this novel radioligand in a species closely related to humans.

RESULTS

The lead ligand AlF-NOTA-2xDV1(c11sc12s) showed six-fold higher affinity for human CXCR4 compared to Ga-Pentixafor. The corresponding radiotracer was obtained in a good radiochemical yield of 40.1 ± 13.5 % (n = 4) and apparent molar activity of 20.4 ± 3.3 MBq/nmol (n = 4) after optimization. In U87.CD4.CXCR4 cell binding assays, the total bound fraction of [18F]AlF-NOTA-(2×)DV1(c11sc12s) was 32.4 ± 1.8 %. This fraction could be reduced by 82.5 % in the presence of 75 μM AMD3100. In naïve mice, [18F]AlF-NOTA-2xDV1(c11sc12s) accumulated in organs expressing mouse CXCR4, e.g. the liver (SUVmean (mean standardized uptake value) 75 min p.i. 11.7 ± 0.6), which was blockable by co-injecting AMD3100 (5 mg/kg). In U87.CXCR4 xenografted tumor mice, the tumor uptake of [18F]AlF-NOTA-2xDV1(c11sc12s) remained low (SUVmean 0.5 ± 0.1), but was reduced by co-administration of AMD3100. Surprisingly, [18F]AlF-NOTA-2xDV1(c11sc12s) exhibited a similar biodistribution in a non-human primate as in mice indicating off-target binding of [18F]AlF-NOTA-2xDV1(c11sc12s) in liver tissue. We confirmed that [18F]AlF-NOTA-2xDV1(c11sc12s) is taken up by hepatocytes using in vitro studies and that the uptake can be blocked with AMD3100 and rifampicin, a potent organic anion-transporting-polypeptide (OATP)1B1 and OATP1B3 inhibitor.

CONCLUSION

The second generation D-peptide AlF-NOTA-2xDV1(c11sc12s) showed high affinity for human CXCR4 and the corresponding radiotracer was produced in good radiochemical yields. However, [18F]AlF-NOTA-2xDV1(c11sc12s) is not specific for CXCR4 and is also a substrate for OATP1B1 and/or OATP1B3, known to mediate hepatic uptake. Therefore, D-amino acid peptides, based on the viral macrophage inflammatory protein II, are not the prefered vector molecule for the development of CXCR4 targeting molecular imaging tools.

Pharmacological Characterization and Radiolabeling of VUF15485, a High-Affinity Small-Molecule Agonist for the Atypical Chemokine Receptor ACKR3

Atypical chemokine receptor 3 (ACKR3), formerly referred to as CXCR7, is considered to be an interesting drug target. In this study, we report on the synthesis, pharmacological characterization and radiolabeling of VUF15485, a new ACKR3 small-molecule agonist, that will serve as an important new tool to study this β-arrestin-biased chemokine receptor. VUF15485 binds with nanomolar affinity (pIC50 = 8.3) to human ACKR3, as measured in [125I]CXCL12 competition binding experiments. Moreover, in a bioluminescence resonance energy transfer-based β-arrestin2 recruitment assay VUF15485 acts as a potent ACKR3 agonist (pEC50 = 7.6) and shows a similar extent of receptor activation compared with CXCL12 when using a newly developed, fluorescence resonance energy transfer-based ACKR3 conformational sensor. Moreover, the ACKR3 agonist VUF15485, tested against a (atypical) chemokine receptor panel (agonist and antagonist mode), proves to be selective for ACKR3. VUF15485 labeled with tritium at one of its methoxy groups ([3H]VUF15485), binds ACKR3 saturably and with high affinity (K d = 8.2 nM). Additionally, [3H]VUF15485 shows rapid binding kinetics and consequently a short residence time (<2 minutes) for binding to ACKR3. The selectivity of [3H]VUF15485 for ACKR3, was confirmed by binding studies, whereupon CXCR3, CXCR4, and ACKR3 small-molecule ligands were competed for binding against the radiolabeled agonist. Interestingly, the chemokine ligands CXCL11 and CXCL12 are not able to displace the binding of [3H]VUF15485 to ACKR3. The radiolabeled VUF15485 was subsequently used to evaluate its binding pocket. Site-directed mutagenesis and docking studies using a recently solved cryo-EM structure propose that VUF15485 binds in the major and the minor binding pocket of ACKR3. SIGNIFICANCE STATEMENT: The atypical chemokine receptor atypical chemokine receptor 3 (ACKR3) is considered an interesting drug target in relation to cancer and multiple sclerosis. The study reports on new chemical biology tools for ACKR3, i.e., a new agonist that can also be radiolabeled and a new ACKR3 conformational sensor, that both can be used to directly study the interaction of ACKR3 ligands with the G protein-coupled receptor.

Crosstalk between purinergic receptor P2Y11 and chemokine receptor CXCR7 is regulated by CXCR4 in human macrophages

P2Y11 is a G protein-coupled ATP receptor that activates IL-1 receptor (IL-1R) in a cyclic AMP dependent manner. In human macrophages, P2Y11/IL-1R crosstalk with CCL20 as a prime target is controlled by phosphodiesterase 4 (PDE4), which mediates breakdown of cyclic AMP. Here, we used gene expression analysis to identify activation of CXCR4 and CXCR7 as a hallmark of P2Y11 signaling. We found that PDE4 inhibition with rolipram boosts P2Y11/IL-1R-induced upregulation of CXCR7 expression and CCL20 production in an epidermal growth factor receptor dependent manner. Using an astrocytoma cell line, naturally expressing CXCR7 but lacking CXCR4, P2Y11/IL-1R activation effectively induced and CXCR7 agonist TC14012 enhanced CCL20 production even in the absence of PDE4 inhibition. Moreover, CXCR7 depletion by RNA interference suppressed CCL20 production. In macrophages, the simultaneous activation of P2Y11 and CXCR7 by their respective agonists was sufficient to induce CCL20 production with no need of PDE4 inhibition, as CXCR7 activation increased its own and eliminated CXCR4 expression. Finally, analysis of multiple CCL chemokines in the macrophage secretome revealed that CXCR4 inactivation and CXCR7 activation selectively enhanced P2Y11/IL-1R-mediated secretion of CCL20. Altogether, our data establish CXCR7 as an integral component of the P2Y11/IL-1R-initiated signaling cascade and CXCR4-associated PDE4 as a regulatory checkpoint.

Small Molecule Fluorescent Ligands for the Atypical Chemokine Receptor 3 (ACKR3)

The atypical chemokine receptor 3 (ACKR3) is a receptor that induces cancer progression and metastasis in multiple cell types. Therefore, new chemical tools are required to study the role of ACKR3 in cancer and other diseases. In this study, fluorescent probes, based on a series of small molecule ACKR3 agonists, were synthesized. Three fluorescent probes, which showed specific binding to ACKR3 through a luminescence-based NanoBRET binding assay (pKd ranging from 6.8 to 7.8) are disclosed. Due to their high affinity at the ACKR3, we have shown their application in both competition binding experiments and confocal microscopy studies showing the cellular distribution of this receptor.

Molecular insights into intrinsic transducer-coupling bias in the CXCR4-CXCR7 system

Chemokine receptors constitute an important subfamily of G protein-coupled receptors (GPCRs), and they are critically involved in a broad range of immune response mechanisms. Ligand promiscuity among these receptors makes them an interesting target to explore multiple aspects of biased agonism. Here, we comprehensively characterize two chemokine receptors namely, CXCR4 and CXCR7, in terms of their transducer-coupling and downstream signaling upon their stimulation by a common chemokine agonist, CXCL12, and a small molecule agonist, VUF11207. We observe that CXCR7 lacks G-protein-coupling while maintaining robust βarr recruitment with a major contribution of GRK5/6. On the other hand, CXCR4 displays robust G-protein activation as expected but exhibits significantly reduced βarr-coupling compared to CXCR7. These two receptors induce distinct βarr conformations even when activated by the same agonist, and CXCR7, unlike CXCR4, fails to activate ERK1/2 MAP kinase. We also identify a key contribution of a single phosphorylation site in CXCR7 for βarr recruitment and endosomal localization. Our study provides molecular insights into intrinsic-bias encoded in the CXCR4-CXCR7 system with broad implications for drug discovery.

Discovery and Development of First-in-Class ACKR3/CXCR7 Superagonists for Platelet Degranulation Modulation

The atypical chemokine receptor 3 (ACKR3), formerly known as CXC-chemokine receptor 7 (CXCR7), has been postulated to regulate platelet function and thrombus formation. Herein, we report the discovery and development of first-in-class ACKR3 agonists, which demonstrated superagonistic properties with E_max values of up to 160% compared to the endogenous reference ligand CXCL12 in a β-arrestin recruitment assay. Initial in silico screening using an ACKR3 homology model identified two hits, C10 (EC₅₀ 19.1 μM) and C11 (EC₅₀ = 11.4 μM). Based on these hits, extensive structure-activity relationship studies were conducted by synthesis and testing of derivatives. It resulted in the identification of the novel thiadiazolopyrimidinone-based compounds 26 (LN5972, EC₅₀ = 3.4 μM) and 27 (LN6023, EC₅₀ = 3.5 μM). These compounds are selective for ACKR3 versus CXCR4 and show metabolic stability. In a platelet degranulation assay, these agonists effectively reduced P-selectin expression by up to 97%, suggesting potential candidates for the treatment of platelet-mediated thrombosis.

Effects of Small Molecule Ligands on ACKR3 Receptors

Chemokines such as stromal derived factor 1 and their G protein coupled receptors are well-known regulators of the development and functions of numerous tissues. C-X-C motif chemokine ligand 12 (CXCL12) has two receptors: C-X-C chemokine motif receptor 4 (CXCR4) and atypical chemokine receptor 3 (ACKR3). ACKR3 has been described as an atypical "biased" receptor because it does not appear to signal through G proteins and, instead, signals solely through the β-arrestin pathway. In support of this conclusion, we have shown that ACKR3 is unable to signal through any of the known mammalian G α isoforms and have generated a comprehensive map of the G α activation by CXCL12/CXCR4. We also synthesized a series of small molecule ligands which acted as selective agonists for ACKR3 as assessed by their ability to recruit β-arrestin to the receptor. Using select point mutations, we studied the molecular characteristics that determine the ability of small molecules to activate ACKR3 receptors, revealing a key role for the deeper binding pocket composed of residues in the transmembrane domains of ACKR3. The development of more selective ACKR3 ligands should allow us to better appreciate the unique roles of ACKR3 in the CXCL12/CXCR4/ACKR3-signaling axis and better understand the structural determinants for ACKR3 activation. SIGNIFICANCE STATEMENT: We are interested in the signaling produced by the G protein coupled receptor atypical chemokine receptor 3 (ACKR3), which signals atypically. In this study, novel selective ligands for ACKR3 were discovered and the site of interactions between these small molecules and ACKR3 was defined. This work will help to better understand the unique signaling roles of ACKR3.

Structures of atypical chemokine receptor 3 reveal the basis for its promiscuity and signaling bias

Both CXC chemokine receptor 4 (CXCR4) and atypical chemokine receptor 3 (ACKR3) are activated by the chemokine CXCL12 yet evoke distinct cellular responses. CXCR4 is a canonical G protein-coupled receptor (GPCR), whereas ACKR3 is intrinsically biased for arrestin. The molecular basis for this difference is not understood. Here, we describe cryo-EM structures of ACKR3 in complex with CXCL12, a more potent CXCL12 variant, and a small-molecule agonist. The bound chemokines adopt an unexpected pose relative to those established for CXCR4 and observed in other receptor-chemokine complexes. Along with functional studies, these structures provide insight into the ligand-binding promiscuity of ACKR3, why it fails to couple to G proteins, and its bias toward β-arrestin. The results lay the groundwork for understanding the physiological interplay of ACKR3 with other GPCRs.

Small Molecule-mediated Insulin Hypersecretion Induces Transient ER Stress Response and Loss of Beta Cell Function

Pancreatic islet beta cells require a fine-tuned endoplasmic reticulum (ER) stress response for normal function; abnormal ER stress contributes to diabetes pathogenesis. Here, we identified a small molecule, SW016789, with time-dependent effects on beta cell ER stress and function. Acute treatment with SW016789 potentiated nutrient-induced calcium influx and insulin secretion, while chronic exposure to SW016789 transiently induced ER stress and shut down secretory function in a reversible manner. Distinct from the effects of thapsigargin, SW016789 did not affect beta cell viability or apoptosis, potentially due to a rapid induction of adaptive genes, weak signaling through the eIF2α kinase PERK, and lack of oxidative stress gene Txnip induction. We determined that SW016789 acted upstream of voltage-dependent calcium channels (VDCCs) and potentiated nutrient- but not KCl-stimulated calcium influx. Measurements of metabolomics, oxygen consumption rate, and G protein-coupled receptor signaling did not explain the potentiating effects of SW016789. In chemical cotreatment experiments, we discovered synergy between SW016789 and activators of protein kinase C and VDCCs, suggesting involvement of these pathways in the mechanism of action. Finally, chronically elevated calcium influx was required for the inhibitory impact of SW016789, as blockade of VDCCs protected human islets and MIN6 beta cells from hypersecretion-induced dysfunction. We conclude that beta cells undergoing this type of pharmacological hypersecretion have the capacity to suppress their function to mitigate ER stress and avoid apoptosis. These results have the potential to uncover beta cell ER stress mitigation factors and add support to beta cell rest strategies to preserve function.

A Scintillation Proximity Assay for Real-Time Kinetic Analysis of Chemokine-Chemokine Receptor Interactions

Chemokine receptors are extensively involved in a broad range of physiological and pathological processes, making them attractive drug targets. However, despite considerable efforts, there are very few approved drugs targeting this class of seven transmembrane domain receptors to date. In recent years, the importance of including binding kinetics in drug discovery campaigns was emphasized. Therefore, kinetic insight into chemokine-chemokine receptor interactions could help to address this issue. Moreover, it could additionally deepen our understanding of the selectivity and promiscuity of the chemokine-chemokine receptor network. Here, we describe the application, optimization and validation of a homogenous Scintillation Proximity Assay (SPA) for real-time kinetic profiling of chemokine-chemokine receptor interactions on the example of ACKR3 and CXCL12. The principle of the SPA is the detection of radioligand binding to receptors reconstituted into nanodiscs by scintillation light. No receptor modifications are required. The nanodiscs provide a native-like environment for receptors and allow for full control over bilayer composition and size. The continuous assay format enables the monitoring of binding reactions in real-time, and directly accounts for non-specific binding and potential artefacts. Minor adaptations additionally facilitate the determination of equilibrium binding metrics, making the assay a versatile tool for the study of receptor-ligand interactions.

Stromal cell-derived factor-1 exerts opposing roles through CXCR4 and CXCR7 in angiotensin II-induced adventitial remodeling

Recent studies have emphasized the role of vascular adventitia inflammation and immune response in hypertension. It has been reported that stromal cell-derived factor-1 (SDF-1) plays various biological functions through its receptors C-X-C motif chemokine receptor 4 (CXCR4) and CXCR7 in tumor growth and tissue repair. However, it is unclear that whether SDF-1/CXCR4/CXCR7 axis is involved in hypertensive vascular remodeling. In the present study, the involvement of SDF-1/CXCR4/CXCR7 axis was evaluated with lentivirus-mediated shRNA of SDF-1 and CXCR7, CXCR4 antagonist AMD3100 and CXCR7 agonist VUF11207 in angiotensin II (AngII)-induced hypertensive mice and in cultured adventitial fibroblasts (AFs). Results showed that AngII infusion markedly increased SDF-1 expressed in vascular adventitia, but not in media and endothelium. Importantly, blockade of SDF-1/CXCR4 axis strikingly potentiated AngII-induced adventitial thickening and fibrosis, as indicated by enhanced collagen I deposition. In contrast, CXCR7 shRNA largely attenuated AngII-induced adventitial thickness and fibrosis, whereas CXCR7 activation with VUF11207 significantly potentiated AngII-induced adventitial thickening and fibrosis. In consistent with these in vivo study, CXCR4 inhibition with AMD3100 and CXCR7 activation with VUF11207 aggravated AngII-induced inflammation, proliferation and migration in cultured AFs. In summary, these results suggested that SDF-1 exerted opposing effects through CXCR4 and CXCR7 in AngII-induced vascular adventitial remodeling.

C‑X‑C receptor 7 agonist acts as a C‑X‑C motif chemokine ligand 12 inhibitor to ameliorate osteoclastogenesis and bone resorption

The C-X-C receptor (CXCR) 7 agonist, VUF11207, is a chemical compound that binds specifically to CXCR7, and negatively regulates C-X-C motif chemokine ligand 12 (CXCL12) and CXCR4-induced cellular events. Lipopolysaccharide (LPS) can induce inflammatory cytokines and pathological bone loss. LPS also induces expression of CXCL12, enhancing sensitivity to receptor activator of NF-κB ligand (RANKL) and tumor necrosis factor-α (TNF-α) in vivo. RANKL and TNF-α induce the differentiation of osteoclasts into osteoclast precursors and bone resorption. The current study was performed to examine the effects of a CXCR7 agonist on osteoclastogenesis and bone resorption induced by LPS in vivo. In addition, the mechanisms underlying these in vivo effects were investigated by in vitro experiments. Eight-week-old male C57BL/6J mice were subcutaneously injected over the calvariae with LPS alone or LPS and CXCR7 agonist. After sacrifice, the number of osteoclasts and the bone resorption area were measured. In vitro experiments were performed to investigate the effects of CXCL12 and CXCR7 agonist on osteoclastogenesis induced by RANKL and TNF-α. Mice injected with LPS and CXCR7 agonist showed significantly reduced osteoclastogenesis and bone resorption compared with mice injected with LPS alone. Moreover, the CXCR7 agonist inhibited CXCL12 enhancement of RANKL- and TNF-α-induced osteoclastogenesis in vitro. Thus, CXCR7 agonist inhibited LPS-induced osteoclast-associated cytokines, such as RANKL and TNF-α, as well as RANKL- and TNF-α-induced osteoclastogenesis in vitro by modulating CXCL12-mediated enhancement of osteoclastogenesis. In conclusion, CXCR7 agonist reduced CXCL12-mediated osteoclastogenesis and bone resorption.

Atypical Roles of the Chemokine Receptor ACKR3/CXCR7 in Platelet Pathophysiology

The manifold actions of the pro-inflammatory and regenerative chemokine CXCL12/SDF-1α are executed through the canonical GProteinCoupledReceptor CXCR4, and the non-canonical ACKR3/CXCR7. Platelets express CXCR4, ACKR3/CXCR7, and are a vital source of CXCL12/SDF-1α themselves. In recent years, a regulatory impact of the CXCL12-CXCR4-CXCR7 axis on platelet biogenesis, i.e., megakaryopoiesis, thrombotic and thrombo-inflammatory actions have been revealed through experimental and clinical studies. Platelet surface expression of ACKR3/CXCR7 is significantly enhanced following myocardial infarction (MI) in acute coronary syndrome (ACS) patients, and is also associated with improved functional recovery and prognosis. The therapeutic implications of ACKR3/CXCR7 in myocardial regeneration and improved recovery following an ischemic episode, are well documented. Cardiomyocytes, cardiac-fibroblasts, endothelial lining of the blood vessels perfusing the heart, besides infiltrating platelets and monocytes, all express ACKR3/CXCR7. This review recapitulates ligand induced differential trafficking of platelet CXCR4-ACKR3/CXCR7 affecting their surface availability, and in regulating thrombo-inflammatory platelet functions and survival through CXCR4 or ACKR3/CXCR7. It emphasizes the pro-thrombotic influence of CXCL12/SDF-1α exerted through CXCR4, as opposed to the anti-thrombotic impact of ACKR3/CXCR7. Offering an innovative translational perspective, this review also discusses the advantages and challenges of utilizing ACKR3/CXCR7 as a potential anti-thrombotic strategy in platelet-associated cardiovascular disorders, particularly in coronary artery disease (CAD) patients post-MI.

Emerging Roles of the Atypical Chemokine Receptor 3 (ACKR3) in Cardiovascular Diseases

Chemokines, and their receptors play a crucial role in the pathophysiology of cardiovascular diseases (CVD). Chemokines classically mediate their effects by binding to G-protein-coupled receptors. The discovery that chemokines can also bind to atypical chemokine receptors (ACKRs) and initiate alternative signaling pathways has changed the paradigm regarding chemokine-related functions. Among these ACKRs, several studies have highlighted the exclusive role of ACKR3, previously known as C-X-C chemokine receptor type 7 (CXCR7), in CVD. Indeed, ACKR3 exert atheroprotective, cardioprotective and anti-thrombotic effects through a wide range of cells including endothelial cells, platelets, inflammatory cells, fibroblasts, vascular smooth muscle cells and cardiomyocytes. ACKR3 functions as a scavenger receptor notably for the pleiotropic chemokine CXCL12, but also as a activator of different pathways such as β-arrestin-mediated signaling or modulator of CXCR4 signaling through the formation of ACKR3-CXCR4 heterodimers. Hence, a better understanding of the precise roles of ACKR3 may pave the way towards the development of novel and improved therapeutic strategies for CVD. Here, we summarize the structural determinant characteristic of ACKR3, the molecules targeting this receptor and signaling pathways modulated by ACKR3. Finally, we present and discuss recent findings regarding the role of ACKR3 in CVD.

Platelet ACKR3/CXCR7 Favors Anti-Platelet Lipids over an Atherothrombotic Lipidome and Regulates Thrombo-inflammation

Platelet ACKR3/CXCR7 surface expression is enhanced and influences prognosis in coronary artery disease-(CAD) patients, who exhibit a distinct atherothrombotic platelet lipidome. Current investigation validates the potential of ACKR3/CXCR7 in regulating thrombo-inflammatory response, through its impact on the platelet lipidome. CAD patients-(n=230) with enhanced platelet-ACKR3/CXCR7 expression exhibited reduced aggregation. Pharmacological CXCR7-agonist-(VUF11207) significantly reduced pro-thrombotic platelet response in blood from ACS patients-(n=11) ex vivo. CXCR7-agonist administration reduced thrombotic functions and thrombo-inflammatory platelet-leukocyte interactions post myocardial infarction-(MI) and arterial injury in vivo. ACKR3/CXCR7-ligation did not affect surface availability of GPIbα, GPV, GPVI, GPIX, αv-integrin, β3-integrin, coagulation profile-(APTT, PT), bleeding time, plasma-dependent thrombin generation-(thrombinoscopy) or clot formation-(thromboelastography), but counteracted activation-induced phosphatidylserine exposure and procoagulant platelet-assisted thrombin generation. Targeted-(micro-UHPLC-ESI-QTrap-MS/MS) and untargeted-(UHPLC-ESI-QTOF-MS/MS) lipidomics analysis revealed that ACKR3/CXCR7-ligation favored generation of anti-thrombotic lipids-(dihomo-γ-linolenic acid-DGLA, 12-hydroxyeicosatrienoic acid-12-HETrE) over cyclooxygenase-COX-1-(thromboxane-TxA2), or 12-lipoxygenase-LOX-(12-HETE) metabolized pro-thrombotic, and phospholipase derived atherogenic-(lysophosphatidylcholine-LPC) lipids, in healthy subjects and CAD patients, contrary to anti-platelet therapy. Through 12-HETrE, ACKR3/CXCR7-ligation coordinated with Gαs-coupled prostacyclin receptor-(IP) to trigger cAMP-PKA mediated platelet inhibition. ACKR3/CXCR7-ligation reduced generation of lipid agonists-(arachidonic acid-AA,TxA2), lipid signaling intermediates-(lyophosphatidylinositol-LPI, diacylglycerol-DG), which affected calcium mobilization, intracellular signaling, consequently platelet interaction with physiological matrices and thrombo-inflammatory secretion-(IL1β,IFN-γ,TGF-β,IL-8), emphasizing its functional dichotomy from pro-thrombotic CXCR4. Moreover, CXCR7-agonist regulated heparin-induced thrombocytopenia-(HIT)-sera/IgG-induced platelet and neutrophil activation, heparin induced platelet aggregation-(HIPA), generation of COX-1-(TxA2), 12-LOX-(12-HETE) derived thrombo-inflammatory lipids, platelet-neutrophil aggregate formation, and thrombo-inflammatory secretion (sCD40L, IL-1β, IFN-γ, TNF-α, sP-selectin, IL-8, tissue factor-TF) ex vivo. Therefore, ACKR3/CXCR7 may offer a novel therapeutic strategy in acute/chronic thrombo-inflammation exaggerated cardiovascular pathologies, and CAD.

Discovery and evaluation of non-basic small molecule modulators of the atypical chemokine receptor CXCR7

The atypical chemokine receptor C-X-C chemokine receptor type 7 (CXCR7) is an attractive therapeutic target for a variety of cardiac and immunological diseases. As a strategy to mitigate known risks associated with the development of higher molecular weight, basic compounds, a series of pyrrolidinyl-azolopyrazines were identified as promising small-molecule CXCR7 modulators. Using a highly enabled parallel medicinal chemistry strategy, structure-activity relationship studies geared towards a reduction in lipophilicity and incorporation of saturated heterocycles led to the identification of representative tool compound 20. Notably, compound 20 maintained good potency against CXCR7 with a suitable balance of physicochemical properties to support in vivo pharmacokinetic studies.

Atypical opioid receptors: unconventional biology and therapeutic opportunities

Endogenous opioid peptides and prescription opioid drugs modulate pain, anxiety and stress by activating four opioid receptors, namely μ (mu, MOP), δ (delta, DOP), κ (kappa, KOP) and the nociceptin/orphanin FQ receptor (NOP). Interestingly, several other receptors are also activated by endogenous opioid peptides and influence opioid-driven signaling and biology. However, they do not meet the criteria to be recognized as classical opioid receptors, as they are phylogenetically distant from them and are insensitive to classical non-selective opioid receptor antagonists (e.g. naloxone). Nevertheless, accumulating reports suggest that these receptors may be interesting alternative targets, especially for the development of safer analgesics. Five of these opioid peptide-binding receptors belong to the family of G protein-coupled receptors (GPCRs)-two are members of the Mas-related G protein-coupled receptor X family (MrgX1, MrgX2), two of the bradykinin receptor family (B_1, B₂), and one is an atypical chemokine receptor (ACKR3). Additionally, the ion channel N-methyl-d-aspartate receptors (NMDARs) are also activated by opioid peptides. In this review, we recapitulate the implication of these alternative receptors in opioid-related disorders and discuss their unconventional biology, with members displaying signaling to scavenging properties. We provide an overview of their established and emerging roles and pharmacology in the context of pain management, as well as their clinical relevance as alternative targets to overcome the hurdles of chronic opioid use. Given the involvement of these receptors in a wide variety of functions, including inflammation, chemotaxis, anaphylaxis or synaptic transmission and plasticity, we also discuss the challenges associated with the modulation of both their canonical and opioid-driven signaling.

Ackr3-Venus knock-in mouse lights up brain vasculature

The atypical chemokine receptor 3, ACKR3, is a G protein-coupled receptor, which does not couple to G proteins but recruits βarrestins. At present, ACKR3 is considered a target for cancer and cardiovascular disorders, but less is known about the potential of ACKR3 as a target for brain disease. Further, mouse lines have been created to identify cells expressing the receptor, but there is no tool to visualize and study the receptor itself under physiological conditions. Here, we engineered a knock-in (KI) mouse expressing a functional ACKR3-Venus fusion protein to directly detect the receptor, particularly in the adult brain. In HEK-293 cells, native and fused receptors showed similar membrane expression, ligand induced trafficking and signaling profiles, indicating that the Venus fusion does not alter receptor signaling. We also found that ACKR3-Venus enables direct real-time monitoring of receptor trafficking using resonance energy transfer. In ACKR3-Venus knock-in mice, we found normal ACKR3 mRNA levels in the brain, suggesting intact gene transcription. We fully mapped receptor expression across 14 peripheral organs and 112 brain areas and found that ACKR3 is primarily localized to the vasculature in these tissues. In the periphery, receptor distribution aligns with previous reports. In the brain there is notable ACKR3 expression in endothelial vascular cells, hippocampal GABAergic interneurons and neuroblast neighboring cells. In conclusion, we have generated Ackr3-Venus knock-in mice with a traceable ACKR3 receptor, which will be a useful tool to the research community for interrogations about ACKR3 biology and related diseases.

Biased agonism at chemokine receptors

In the human chemokine system, interactions between the approximately 50 known endogenous chemokine ligands and 20 known chemokine receptors (CKRs) regulate a wide range of cellular functions and biological processes including immune cell activation and homeostasis, development, angiogenesis, and neuromodulation. CKRs are a family of G protein-coupled receptors (GPCR), which represent the most common and versatile class of receptors in the human genome and the targets of approximately one third of all Food and Drug Administration-approved drugs. Chemokines and CKRs bind with significant promiscuity, as most CKRs can be activated by multiple chemokines and most chemokines can activate multiple CKRs. While these ligand-receptor interactions were previously regarded as redundant, it is now appreciated that many chemokine:CKR interactions display biased agonism, the phenomenon in which different ligands binding to the same receptor signal through different pathways with different efficacies, leading to distinct biological effects. Notably, these biased responses can be modulated through changes in ligand, receptor, and or the specific cellular context (system). In this review, we explore the biochemical mechanisms, functional consequences, and therapeutic potential of biased agonism in the chemokine system. An enhanced understanding of biased agonism in the chemokine system may prove transformative in the understanding of the mechanisms and consequences of biased signaling across all GPCR subtypes and aid in the development of biased pharmaceuticals with increased therapeutic efficacy and safer side effect profiles.

Discovery of Diphenylacetamides as CXCR7 Inhibitors with Novel β-Arrestin Antagonist Activity

The atypical chemokine receptor CXCR7 has been studied in various disease settings including immunological diseases and heart disease. Efforts to elucidate the role of CXCR7 have been limited by the lack of suitable chemical tools with a range of pharmacological profiles. A high-throughput screen was conducted to discover novel chemical matter with the potential to modulate CXCR7 receptor activity. This led to the identification of a series of diphenylacetamides confirmed in a CXCL12 competition assay indicating receptor binding. Further evaluation of this series revealed a lack of activity in the functional assay measuring β-arrestin recruitment. The most potent representative, compound 10 (K _i = 597 nM), was determined to be an antagonist in the β-arrestin assay (IC₅₀ = 622 nM). To our knowledge, this is the first reported small molecule β-arrestin antagonist for CXCR7, useful as an in vitro chemical tool to elucidate the effects of CXCL12 displacement with β-arrestin antagonism in models for diseases such as cardiac injury and suitable as starting point for hit optimization directed toward an in vivo tool compound for studying CXCR7 receptor pharmacology.

Two-Phase Dibromocyclopropanation of Unsaturated Alcohols Using Flow Chemistry

Dibromocyclopropanations are conventionally done by addition of dibromocarbene to alkenes under phase-transfer conditions in batch reactions using a strong base (50% NaOH (aq)), vigorous stirring and long reaction times. We have shown that cyclopropanation of unsaturated alcohols can be done under ambient conditions using continuous flow chemistry with 40% (w/w) NaOH (aq) as the base. The reactions were generally rapid; the yields were comparable to yields reported in the literature for the conventional batch reaction

Advances in CXCR7 Modulators

CXC chemokine receptor 7 (CXCR7) is a G-protein-coupled receptor that signals through the β-arrestin pathway. Its ligands include interferon-inducible T cell α chemoattractant (CXCL11) and stromal cell-derived factor-1 (CXCL12). It interacts with CXCR4, and the two are associated with various cancers, as well as other disease states such as coronary artery disease, stroke, inflammation and human immunodeficiency virus (HIV). Antibodies and small interfering RNA (siRNA) have shown the utility of antagonists of CXCR7 in these disease states. Although some small molecules were initially reported as antagonists due to their displayed activity, many function as agonists while still producing the desired pharmacologic effects. A potential reason for this contradiction is that effects may be due to elevated extracellular CXCL12 levels.

The diverse and complex roles of atypical chemokine receptors in cancer: From molecular biology to clinical relevance and therapy

Chemokines regulate directed cell migration, proliferation and survival and are key components in cancer biology. They exert their functions by interacting with seven-transmembrane domain receptors that signal through G proteins (GPCRs). A subgroup of four chemokine receptors known as the atypical chemokine receptors (ACKRs) has emerged as essential regulators of the chemokine functions. ACKRs play diverse and complex roles in tumor biology from tumor initiation to metastasis, including cancer cell proliferation, adherence to endothelium, epithelial-mesenchymal transition (EMT), extravasation from blood vessels, tumor-associated angiogenesis or protection from immunological responses. This chapter gives an overview on the established and emerging roles that the atypical chemokine receptors ACKR1, ACKR2, ACKR3 and ACKR4 play in the different phases of cancer development and dissemination, their clinical relevance, as well as on the hurdles to overcome in ACKRs targeting as cancer therapy.

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.

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.

Targeted Protein Internalization and Degradation by ENDosome TArgeting Chimeras (ENDTACs)

Targeted protein degradation has generated excitement in chemical biology and drug discovery throughout academia and industry. By hijacking the machinery responsible for protein degradation via the ubiquitin proteasome system (UPS), various cellular targets have been selectively degraded. However, since the tools used, often termed PROteolysis TArgeting Chimeras (PROTACs), hijack the intracellular quality control machinery, this technology can only access targets within the cell. Extracellular targets such as growth factors, cytokines, and chemokines bind to cell surface receptors, often initiating aberrant signaling in multiple diseases such as cancer and inflammation. However, efforts to develop small molecule inhibitors for these extracellular target proteins have been challenging. Herein, we developed a proof-of-concept approach to evaluate if extracellular proteins can be internalized and degraded via the receptor-mediated endolysosomal pathway. Using a heterodimeric molecule, termed "ENDosome TArgeting Chimera" (ENDTAC), internalization and degradation of an extracellular recombinant eGFP-HT7 fusion protein was achieved by hijacking the decoy GPCR receptor, CXCR7. This proof-of-concept study suggests that using ENDTACs to co-opt the endosomal-lysosomal degradation pathway, in contrast to PROTACs using the UPS, may provide an avenue for degrading extracellular targets such as cytokines. Overall, the technology described herein provides a novel expansion to the field of targeted protein degradation.

Nanobodies: New avenues for imaging, stabilizing and modulating GPCRs

The family of G protein-coupled receptors (GPCRs) is the largest class of membrane proteins and an important drug target due to their role in many (patho)physiological processes. Besides small molecules, GPCRs can be targeted by biologicals including antibodies and antibody fragments. This review describes the use of antibodies and in particular antibody fragments from camelid-derived heavy chain-only antibodies (nanobodies/VHHs/sdAbs) for detecting, stabilizing, modulating and therapeutically targeting GPCRs. Altogether, it becomes increasingly clear that the small size, structure and protruding antigen-binding loops of nanobodies are favorable features for the development of selective and potent GPCRs-binding molecules. This makes them attractive tools to modulate GPCR activity but also as targeting modalities for GPCR-directed therapeutics. In addition, these antibody-fragments are important tools in the stabilization of particular conformations of these receptors. Lastly, nanobodies, in contrast to conventional antibodies, can also easily be expressed intracellularly which render nanobodies important tools for studying GPCR function. Hence, GPCR-targeting nanobodies are ideal modalities to image, stabilize and modulate GPCR function.

Asymmetric Synthesis of a Potent CXCR7 Modulator Featuring a Hindered Tertiary β-Amino Amide Stereocenter

A practical and asymmetric synthesis of a small-molecule CXCR7 modulator featuring a highly functionalized and hindered tertiary β-amino amide framework is reported. The cornerstone of this strategy relied on the intermediacy of a reactive aziridinium species, which, following regioselective ring opening with cyanide, furnished the desired chiral β-tertiary amino nitrile for further elaboration. As a means of further highlighting this synthetic strategy, an expanded scope of hindered β-amino amide synthesis is also presented.

CXCR7 Targeting and Its Major Disease Relevance

Chemokine receptors are the target of small peptide chemokines. They play various important roles in physiological and pathological processes. CXCR7, later renamed ACKR3, is a non-classical seven transmembrane-spanning receptor whose function as a signaling or non-signaling scavenger/decoy receptor is currently under debate. Even for cell signaling mechanisms, there has been inconsistency on whether CXCR7 couples to G-proteins or β-arrestins. Several reasons may contribute to this uncertainty or controversy. In one hand, it has been neglected that CXCR7 has more than five natural ligands and unfortunately, most of the prior research only studied SDF-1 (CXCL12) and/or I-TAC (CXCL11); on the other hand, there are mounting evidence supporting ligand and tissue bias for receptor signaling, but limited such information is available for CXCR7. In this review we focus on summarizing the endogenous and exogenous ligands of CXCR7, the main diseases related to CXCR7 and the biased signaling events happening on CXCR7. These three aspects of CXCR7 pharmacologic properties may explain why the contradicting opinions of whether CXCR7 is a signaling or non-signaling receptor exist. Further, potential new direction and perspective for the study of CXCR7 biology and pharmacology are highlighted.

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.

CXCR7/ACKR3-targeting ligands interfere with X7 HIV-1 and HIV-2 entry and replication in human host cells

Chemokine receptors CCR5 and CXCR4 are considered the main coreceptors for initial HIV infection, replication and transmission, and subsequent AIDS progression. Over the years, other chemokine receptors, belonging to the family of G protein-coupled receptors, have also been identified as candidate coreceptors for HIV entry into human host cells. Amongst them, CXCR7, also known as atypical chemokine receptor 3 (ACKR3), was suggested as a coreceptor candidate capable of facilitating both HIV-1 and HIV-2 entry in vitro. In this study, a cellular infection model was established to further decipher the role of CXCR7 as an HIV coreceptor. Using this model, CXCR7-mediated viral entry was demonstrated for several clinical HIV isolates as well as laboratory strains. Of interest, the X4-tropic HIV-1 HE strain showed rapid adaptation towards CXCR7-mediated infection after continuous passaging on CD4- and CXCR7-expressing cells. Furthermore, we uncovered anti-CXCR7 monoclonal antibodies, small molecule CXCR7 inhibitors and the natural CXCR7 chemokine ligands as potent inhibitors of CXCR7 receptor-mediated HIV entry and replication. Even though the clinical relevance of CXCR7-mediated HIV infection remains poorly understood, our data suggest that divergent HIV-1 and HIV-2 strains can quickly adapt their coreceptor usage depending on the cellular environment, which warrants further investigation.

Lewis Acid Triggered Vinylcyclopropane-Cyclopentene Rearrangement

We report a mild Lewis acid induced isomerization of donor-acceptor cyclopropanes, containing an alkenyl moiety and diverse electron-withdrawing group(s) at the adjacent positions, into substituted cyclopentenes. We have found that 1,1,2-trisubstituted cyclopent-3-enes were exclusively obtained in yield of 51-99% when cyclopropanes with a 2-substituted alkenyl group as a donor underwent isomerization. For cyclopropanes bearing a trisubstituted alkenyl group either the corresponding cyclopent-3-enes or isomeric cyclopent-2-enes having two acceptor groups at the C(1) atom were formed, with the reaction selectivity being determined by the applied Lewis acid. We have shown that the reactivity of the donor-acceptor cyclopropane increases with the increase of the electron-donating character of (hetero)aromatic group attached to the alkenyl moiety. The synthetic utility of the developed methodology was also demonstrated through the synthesis of polysubstituted cyclopentane and piperidine derivatives.

Discovery of Potent and Orally Bioavailable Macrocyclic Peptide-Peptoid Hybrid CXCR7 Modulators

The chemokine receptor CXCR7 is an attractive target for a variety of diseases. While several small-molecule modulators of CXCR7 have been reported, peptidic macrocycles may provide advantages in terms of potency, selectivity, and reduced off-target activity. We produced a series of peptidic macrocycles that incorporate an N-linked peptoid functionality where the peptoid group enabled us to explore side-chain diversity well beyond that of natural amino acids. At the same time, theoretical calculations and experimental assays were used to track and reduce the polarity while closely monitoring the physicochemical properties. This strategy led to the discovery of macrocyclic peptide-peptoid hybrids with high CXCR7 binding affinities (K_i < 100 nM) and measurable passive permeability (P_app > 5 × 10^-6 cm/s). Moreover, bioactive peptide 25 (K_i = 9 nM) achieved oral bioavailability of 18% in rats, which was commensurate with the observed plasma clearance values upon intravenous administration.

CXCR7 Is Involved in Human Oligodendroglial Precursor Cell Maturation

Differentiation of oligodendroglial precursor cells (OPCs), a crucial prerequisite for central nervous system (CNS) remyelination in diseases such as Multiple Sclerosis (MS), is modulated by a multitude of extrinsic and intrinsic factors. In a previous study we revealed that the chemokine CXCL12 stimulates rodent OPC differentiation via activation of its receptor CXCR7. We could now demonstrate that CXCR7 is also expressed on NogoA- and Nkx2.2-positive oligodendroglial cells in human MS brains and that stimulation of cultured primary fetal human OPCs with CXCL12 promotes their differentiation as measured by surface marker expression and morphologic complexity. Pharmacological inhibition of CXCR7 effectively blocks these CXCL12-dependent effects. Our findings therefore suggest that a specific activation of CXCR7 could provide a means to promote oligodendroglial differentiation facilitating endogenous remyelination activities.

Mode of binding of the cyclic agonist peptide TC14012 to CXCR7: identification of receptor and compound determinants

The chemokine receptor CXCR7 is an atypical CXCL12 receptor that, as opposed to the classical CXCL12 receptor CXCR4, signals preferentially via the β-arrestin pathway and does not mediate chemotaxis. We previously reported that the cyclic peptide TC14012, a potent CXCR4 antagonist, also engaged CXCR7, albeit with lower potency. Surprisingly, the compound activated the CXCR7-arrestin pathway. The reason underlying the opposite effects of TC14012 on CXCR4 and CXCR7, and the mode of binding of TC14012 to CXCR7, remained unclear. The mode of binding of TC14012 to CXCR4 is known from cocrystallization of its analogue CVX15 with CXCR4. We here report the the mode of binding of TC14012 to CXCR7 by combining the use of compound analogues, receptor mutants, and molecular modeling. We find that the mode of binding of TC14012 to CXCR7 is indeed similar to that of CVX15 to CXCR4, with compound positions Arg2 and Arg14 engaging CXCR7 key residues D179(4.60) (on the tip of transmembrane domain 4) and D275(6.58) (on the tip of transmembrane domain 6), respectively. Interestingly, the TC14012 parent compound T140 is not a CXCR7 agonist, because of conformational constraints in its pharmacophore, which in TC14012 are relieved through C-terminal amidation. However, an engineered salt bridge between the CXCR7 ECL2 substitution R197D and compound residue Arg1 permitted T140 agonism by repositioning the compound in the binding pocket. In conclusion, our results show that the opposite effect of TC14012 on CXCR4 and CXCR7 is not explained by different binding modes. Rather, engagement of the interface between transmembrane domains and extracellular loops readily triggers CXCR7, but not CXCR4, activation.

Internal tandem duplication mutations in FLT3 gene augment chemotaxis to Cxcl12 protein by blocking the down-regulation of the Rho-associated kinase via the Cxcl12/Cxcr4 signaling axis

Internal tandem duplication mutations in the Flt3 gene (ITD-FLT3) enhance cell migration toward the chemokine Cxcl12, which is highly expressed in the therapy-protective bone marrow niche, providing a potential mechanism underlying the poor prognosis of ITD-FLT3(+) acute myeloid leukemia. We aimed to investigate the mechanisms linking ITD-FLT3 to increased cell migration toward Cxcl12. Classification of the expression of Cxcl12-regulated genes in ITD-FLT3(+) cells demonstrated that the enhanced migration of ITD-FLT3(+) cells toward Cxcl12 was associated with the differential expression of genes downstream of Cxcl12/Cxcr4, which are functionally distinct from those expressed in ITD-FLT3(-) cells but are independent of the Cxcr4 expression levels. Among these differentially regulated genes, the expression of Rock1 in the ITD-FLT3(+) cells that migrated toward Cxcl12 was significantly higher than in ITD-FLT3(-) cells that migrated toward Cxcl12. In ITD-FLT3(-) cells, Rock1 expression and Mypt1 phosphorylation were transiently up-regulated but were subsequently down-regulated by Cxcl12. In contrast, the presence of ITD-FLT3 blocked the Cxcl12-induced down-regulation of Rock1 and early Mypt1 dephosphorylation. Likewise, the FLT3 ligand counteracted the Cxcl12-induced down-regulation of Rock1 in ITD-FLT3(-) cells, which coincided with enhanced cell migration toward Cxcl12. Rock1 antagonists or Rock1 shRNA abolished the enhanced migration of ITD-FLT3(+) cells toward Cxcl12. Our findings demonstrate that ITD-FLT3 increases cell migration toward Cxcl12 by antagonizing the down-regulation of Rock1 expression. These findings suggest that the aberrant modulation of Rock1 expression and activity induced by ITD-FLT3 may enhance acute myeloid leukemia cell chemotaxis to the therapy-protective bone marrow niche, where Cxcl12 is abundantly expressed.

Detailed analysis of biased histamine H₄ receptor signalling by JNJ 7777120 analogues

BACKGROUND AND PURPOSE

The histamine H₄ receptor, originally thought to signal merely through Gαi proteins, has recently been shown to also recruit and signal via β-arrestin2. Following the discovery that the reference antagonist indolecarboxamide JNJ 7777120 appears to be a partial agonist in β-arrestin2 recruitment, we have identified additional biased hH₄R ligands that preferentially couple to Gαi or β-arrestin2 proteins. In this study, we explored ligand and receptor regions that are important for biased hH₄R signalling.

EXPERIMENTAL APPROACH

We evaluated a series of 48 indolecarboxamides with subtle structural differences for their ability to induce hH₄R-mediated Gαi protein signalling or β-arrestin2 recruitment. Subsequently, a Fingerprints for Ligands and Proteins three-dimensional quantitative structure-activity relationship analysis correlated intrinsic activity values with structural ligand requirements. Moreover, a hH₄R homology model was used to identify receptor regions important for biased hH₄R signalling.

KEY RESULTS

One indolecarboxamide (75) with a nitro substituent on position R7 of the aromatic ring displayed an equal preference for the Gαi and β-arrestin2 pathway and was classified as unbiased hH₄R ligand. The other 47 indolecarboxamides were β-arrestin2-biased agonists. Intrinsic activities of the unbiased as well as β-arrestin2-biased indolecarboxamides to induce β-arrestin2 recruitment could be correlated with different ligand features and hH₄R regions.

CONCLUSION AND IMPLICATIONS

Small structural modifications resulted in diverse intrinsic activities for unbiased (75) and β-arrestin2-biased indolecarboxamides. Analysis of ligand and receptor features revealed efficacy hotspots responsible for biased-β-arrestin2 recruitment. This knowledge is useful for the design of hH₄R ligands with biased intrinsic activities and aids our understanding of the mechanism of H₄R activation.

Silencing of CXCR7 gene represses growth and invasion and induces apoptosis in colorectal cancer through ERK and β-arrestin pathways

The CXC chemokine receptor 7 (CXCR7) has been reported to be involved in cell growth, metastasis and apoptosis in certain cancers. However, the function and molecular mechanisms of CXCR7 in human colorectal cancer (CRC) are still undefined. In the present study, sixty-eight cases of CRC tissues and corresponding adjacent non-cancer tissues (ANCT) were collected, and the expression of CXCR7 was assessed using immunohistochemistry (IHC) in biopsy samples. Furthermore, CXCR7 gene was silenced by small hairpin RNA-mediated lentiviral vector (Lv-shCXCR7), by transfection into human CRC cells (SW480 and HT-29). The levels of p-ERK, β-arrestin, proliferating cell nuclear antigen (PCNA), matrix metallopeptidase-2 (MMP-2) and caspase-3 (CAS-3) were detected by western blotting. Cell proliferative activities and invasive capability were respectively measured by MTT and Transwell assays. Cell apoptosis was analyzed by flow cytometry. The results demonstrated that CXCR7 expression was significantly upregulated in CRC tissues compared with the ANCT (54.4 vs. 36.8%, P=0.041), and correlated with Dukes staging and depth of invasion (P=0.007; P=0.002). Silencing of CXCR7 gene suppressed cell proliferation and invasion, and induced cell apoptosis in CRC cells with decreased expression of p-ERK, β-arrestin, PCNA and MMP-2 but increased expression of CAS-3. The tumor volumes in the SW480 subcutaneous tumor models treated with Lv-shCXCR7 were significantly smaller than those of the negative control (NC) and PBS groups (P<0.01). In conclusion, our findings indicate that upregulation of CXCR7 expression is associated with tumor invasion, and silencing of the CXCR7 gene represses the development of CRC cells through ERK and β-arrestin pathways, suggesting that CXCR7 may serve as a potential therapeutic target for the treatment of CRC.

CXCL12 modulation of CXCR4 and CXCR7 activity in human glioblastoma stem-like cells and regulation of the tumor microenvironment

Chemokines are crucial autocrine and paracrine players in tumor development. In particular, CXCL12, through its receptors CXCR4 and CXCR7, affects tumor progression by controlling cancer cell survival, proliferation and migration, and, indirectly, via angiogenesis or recruiting immune cells. Glioblastoma (GBM) is the most prevalent primary malignant brain tumor in adults and despite current multimodal therapies it remains almost incurable. The aggressive and recurrent phenotype of GBM is ascribed to high growth rate, invasiveness to normal brain, marked angiogenesis, ability to escape the immune system and resistance to standard of care therapies. Tumor molecular and cellular heterogeneity severely hinders GBM therapeutic improvement. In particular, a subpopulation of chemo- and radio-therapy resistant tumorigenic cancer stem-like cells (CSCs) is believed to be the main responsible for tumor cell dissemination to the brain. GBM cells display heterogeneous expression levels of CXCR4 and CXCR7 that are overexpressed in CSCs, representing a molecular correlate for the invasive potential of GBM. The microenvironment contribution in GBM development is increasingly emphasized. An interplay exists between CSCs, differentiated GBM cells, and the microenvironment, mainly through secreted chemokines (e.g., CXCL12) causing recruitment of fibroblasts, endothelial, mesenchymal and inflammatory cells to the tumor, via specific receptors such as CXCR4. This review covers recent developments on the role of CXCL12/CXCR4-CXCR7 networks in GBM progression and the potential translational impact of their targeting. The biological and molecular understanding of the heterogeneous GBM cell behavior, phenotype and signaling is still limited. Progress in the identification of chemokine-dependent mechanisms that affect GBM cell survival, trafficking and chemo-attractive functions, opens new perspectives for development of more specific therapeutic approaches that include chemokine-based drugs.

Chemokine receptor modeling: an interdisciplinary approach to drug design

Chemokines and their receptors are integral components of the immune response, regulating lymphocyte development, homing and trafficking, and playing a key role in the pathophysiology of many diseases. Chemokine receptors have, therefore, become the target for both small-molecule, peptide and antibody therapeutics. Chemokine receptors belong to the family of seven transmembrane receptor class A G protein-coupled receptors. The publication of the crystal structure of the archetypal class A seven transmembrane receptor protein rhodopsin, and other G protein-coupled receptors, including C-X-C chemokine receptor 4 and C-C chemokine receptor 5, provided the opportunity to create homology models of chemokine receptors. In this review, we describe an interdisciplinary approach to chemokine receptor modeling and the utility of this approach for structure-based drug design of chemokine receptor inhibitors.

Llama-derived single variable domains (nanobodies) directed against chemokine receptor CXCR7 reduce head and neck cancer cell growth in vivo

The chemokine receptor CXCR7, belonging to the membrane-bound G protein-coupled receptor superfamily, is expressed in several tumor types. Inhibition of CXCR7 with either small molecules or small interference (si)RNA has shown promising therapeutic benefits in several tumor models. With the increased interest and effectiveness of biologicals inhibiting membrane-bound receptors we made use of the "Nanobody platform" to target CXCR7. Previously we showed that Nanobodies, i.e. immunoglobulin single variable domains derived from naturally occurring heavy chain-only camelids antibodies, represent new biological tools to efficiently tackle difficult drug targets such as G protein-coupled receptors. In this study we developed and characterized highly selective and potent Nanobodies against CXCR7. Interestingly, the CXCR7-targeting Nanobodies displayed antagonistic properties in contrast with previously reported CXCR7-targeting agents. Several high affinity CXCR7-specific Nanobodies potently inhibited CXCL12-induced β-arrestin2 recruitment in vitro. A wide variety of tumor biopsies was profiled, showing for the first time high expression of CXCR7 in head and neck cancer. Using a patient-derived CXCR7-expressing head and neck cancer xenograft model in nude mice, tumor growth was inhibited by CXCR7-targeting Nanobody therapy. Mechanistically, CXCR7-targeting Nanobodies did not inhibit cell cycle progression but instead reduced secretion of the angiogenic chemokine CXCL1 from head and neck cancer cells in vitro, thus acting here as inverse agonists, and subsequent angiogenesis in vivo. Hence, with this novel class of CXCR7 inhibitors, we further substantiate the therapeutic relevance of targeting CXCR7 in head and neck cancer.