Heterogeneous expression of the atypical chemokine receptor ACKR3 in glioblastoma patient-derived tissue samples and cell cultures.

Glioblastoma (GBM) is the most aggressive glial tumor of the adult brain, associated with invariably fatal outcome, and a deeper understanding of the underlying malignant mechanisms is necessary to address the current therapeutic failure. We previously demonstrated the role of the CXCL12/CXCR4 axis in GBM cell migration and resistance to ionizing radiation. The atypical chemokine receptor ACKR3, responsible for CXCL12 scavenging, was previously suggested as additional important player in the context of GBM. Following validation of the detection tools, we observed that ACKR3 is expressed within GBM patient tumor tissue, distributed in diverse cell types. In contrast to CXCR4, ACKR3 expression in patient-derived stem-like cells (GSCs) remains however low, while ACKR3 gene expression by tumor cells appears to be modulated by the in-vivo environment. Using overexpression models, we also showed that in vitro ACKR3 had no significant direct effect on cell proliferation or invasion. Altogether, these results suggest that in vitro ACKR3 plays a minor role in malignant GBM cell biology and that its expression is possibly regulated by in-vivo influences. The subtle and multifaceted functions ACKR3 could exert in GBM should therefore only be tackled within a comprehensive tumor microenvironment considering tumoral but also non-tumoral cells.

GRK specificity and Gßγ dependency determines the potential of a GPCR for arrestin-biased agonism.

G protein-coupled receptors (GPCRs) are mainly regulated by GPCR kinase (GRK) phosphorylation and subsequent ß-arrestin recruitment. The ubiquitously expressed GRKs are classified into cytosolic GRK2/3 and membrane-tethered GRK5/6 subfamilies. GRK2/3 interact with activated G protein ßγ-subunits to translocate to the membrane. Yet, this need was not linked as a factor for bias, influencing the effectiveness of ß-arrestin-biased agonist creation. Using multiple approaches such as GRK2/3 mutants unable to interact with Gßγ, membrane-tethered GRKs and G protein inhibitors in GRK2/3/5/6 knockout cells, we show that G protein activation will precede GRK2/3-mediated ß-arrestin2 recruitment to activated receptors. This was independent of the source of free Gßγ and observable for Gs-, Gi- and Gq-coupled GPCRs. Thus, ß-arrestin interaction for GRK2/3-regulated receptors is inseparably connected with G protein activation. We outline a theoretical framework of how GRK dependence on free Gßγ can determine a GPCR's potential for biased agonism. Due to this inherent cellular mechanism for GRK2/3 recruitment and receptor phosphorylation, we anticipate generation of ß-arrestin-biased ligands to be mechanistically challenging for the subgroup of GPCRs exclusively regulated by GRK2/3, but achievable for GRK5/6-regulated receptors, that do not demand liberated Gßγ. Accordingly, GRK specificity of any GPCR is foundational for developing arrestin-biased ligands.

New pairings and deorphanization among the atypical chemokine receptor family - physiological and clinical relevance.

Atypical chemokine receptors (ACKRs) form a small subfamily of receptors (ACKR1-4) unable to trigger G protein-dependent signaling in response to their ligands. They do, however, play a crucial regulatory role in chemokine biology by capturing, scavenging or transporting chemokines, thereby regulating their availability and signaling through classical chemokine receptors. ACKRs add thus another layer of complexity to the intricate chemokine-receptor interaction network. Recently, targeted approaches and screening programs aiming at reassessing chemokine activity towards ACKRs identified several new pairings such as the dimeric CXCL12 with ACKR1, CXCL2, CXCL10 and CCL26 with ACKR2, the viral broad-spectrum chemokine vCCL2/vMIP-II, a range of opioid peptides and PAMP-12 with ACKR3 as well as CCL20 and CCL22 with ACKR4. Moreover, GPR182 (ACKR5) has been lately proposed as a new promiscuous atypical chemokine receptor with scavenging activity notably towards CXCL9, CXCL10, CXCL12 and CXCL13. Altogether, these findings reveal new degrees of complexity of the chemokine network and expand the panel of ACKR ligands and regulatory functions. In this minireview, we present and discuss these new pairings, their physiological and clinical relevance as well as the opportunities they open for targeting ACKRs in innovative therapeutic strategies.

GPR182 is a broadly scavenging atypical chemokine receptor influencing T-independent immunity.

Immune responses highly depend on the effective trafficking of immune cells into and within secondary lymphoid organs (SLOs). Atypical chemokine receptors (ACKRs) scavenge chemokines to eliminate them from the extracellular space, thereby generating gradients that guide leukocytes. In contrast to canonical chemokine receptors, ACKRs do not induce classical intracellular signaling that results in cell migration. Recently, the closest relative of ACKR3, GPR182, has been partially deorphanized as a potential novel ACKR. We confirm and extend previous studies by identifying further ligands that classify GPR182 as a broadly scavenging chemokine receptor. We validate the "atypical" nature of the receptor, wherein canonical G-protein-dependent intracellular signaling is not activated following ligand stimulation. However, ß-arrestins are required for ligand-independent internalization and chemokine scavenging whereas the C-terminus is in part dispensable. In the absence of GPR182 in vivo, we observed elevated chemokine levels in the serum but also in SLO interstitium. We also reveal that CXCL13 and CCL28, which do not bind any other ACKR, are bound and efficiently scavenged by GPR182. Moreover, we found a cooperative relationship between GPR182 and ACKR3 in regulating serum CXCL12 levels, and between GPR182 and ACKR4 in controlling CCL20 levels. Furthermore, we unveil a new phenotype in GPR182-KO mice, in which we observed a reduced marginal zone (MZ), both in size and in cellularity, and thus in the T-independent antibody response. Taken together, we and others have unveiled a novel, broadly scavenging chemokine receptor, which we propose should be named ACKR5.

Nanoluciferase-based complementation assay for systematic profiling of GPCR-GRK interactions.

G protein-coupled receptor kinases (GRKs) are a family of seven soluble receptor-modifying enzymes which are essential regulators of GPCR activity. Following agonist-induced receptor activation and G protein dissociation, GRKs prime the receptor for desensitization through phosphorylation of its C terminus, which subsequently allows arrestins to bind and initiate the receptor internalization process. While GRKs constitute key GPCR-interacting proteins, to date, no method has been put forward to readily and systematically determine the preference of a specific GPCR towards the seven different GRKs (GRK1-7). This chapter describes a simple and standardized approach for systematic profiling of GRK1-7-GPCR interactions relying on the complementation of the split Nanoluciferase (NanoBiT). When applied to a set of GPCRs (MOR, 5-HT1A, B2AR, CXCR3, AVPR2, CGRPR), including two intrinsically ß-arrestin-biased receptors (ACKR2 and ACKR3), this methodology yields highly reproducible results highlighting different GRK recruitment profiles. Using this assay, further characterization of MOR, a crucial target in the development of analgesics, reveals not only its GRK fingerprint but also related kinetics and activity of various ligands for a single GRK.

Nanoluciferase-based methods to monitor activation, modulation and trafficking of atypical chemokine receptors.

Chemokines regulate directed cell migration, proliferation and survival and are key components in various physiological and pathological processes. They exert their functions by interacting with seven-transmembrane domain receptors that signal through G proteins (GPCRs). Atypical chemokine receptors (ACKRs) play important roles in the chemokine-receptor network by regulating chemokine bioavailability for the classical receptors through chemokine sequestration, scavenging or transport. Currently, this subfamily of receptors comprises four members: ACKR1, ACKR2, ACKR3 and ACKR4. They differ notably from the classical chemokine receptors by their inability to elicit G protein-mediated signaling, which precludes the use of classical assays relying on the activation of G proteins and related downstream secondary messengers to investigate ACKRs. There is therefore a need for alternative approaches to monitor ACKR activation, modulation and trafficking. This chapter details sensitive and versatile methods based on Nanoluciferase Binary Technology (NanoBiT) and Nanoluciferase Bioluminescence Resonance Energy Transfer (NanoBRET) to monitor ACKR2 and ACKR3 activity through the measurement of ß-arrestin and GRK recruitment, and receptor trafficking, including internalization and delivery to early endosomes.

The Extended N-Terminal Domain Confers Atypical Chemokine Receptor Properties to CXCR3-B.

The chemokine receptor CXCR3 plays a critical role in immune cell recruitment and activation. CXCR3 exists as two main isoforms, CXCR3-A and CXCR3-B, resulting from alternative splicing. Although the two isoforms differ only by the presence of an N-terminal extension in CXCR3-B, they have been attributed divergent functional effects on cell migration and proliferation. CXCR3-B is the more enigmatic isoform and the mechanisms underlying its function and signaling remain elusive. We therefore undertook an in-depth cellular and molecular comparative study of CXCR3-A and CXCR3-B, investigating their activation at different levels of the signaling cascades, including G protein coupling, ß-arrestin recruitment and modulation of secondary messengers as well as their downstream gene response elements. We also compared the subcellular localization of the two isoforms and their trafficking under resting and stimulated conditions along with their ability to internalize CXCR3-related chemokines. Here, we show that the N-terminal extension of CXCR3-B drastically affects receptor features, modifying its cellular localization and preventing G protein coupling, while preserving ß-arrestin recruitment and chemokine uptake capacities. Moreover, we demonstrate that gradual truncation of the N terminus leads to progressive recovery of surface expression and G protein coupling. Our study clarifies the molecular basis underlying the divergent effects of CXCR3 isoforms, and emphasizes the ß-arrestin-bias and the atypical nature of CXCR3-B.

Patient-Oriented Perspective on Chemokine Receptor Expression and Function in Glioma.

Gliomas are severe brain malignancies, with glioblastoma (GBM) being the most aggressive one. Despite continuous efforts for improvement of existing therapies, overall survival remains poor. Over the last years, the implication of chemokines and their receptors in GBM development and progression has become more evident. Recently, large amounts of clinical data have been made available, prompting us to investigate chemokine receptors in GBM from a still-unexplored patient-oriented perspective. This study aims to highlight and discuss the involvement of chemokine receptors-CCR1, CCR5, CCR6, CCR10, CX3CR1, CXCR2, CXCR4, ACKR1, ACKR2, and ACKR3-most abundantly expressed in glioma patients based on the analysis of publicly available clinical datasets. Given the strong intratumoral heterogeneity characterizing gliomas and especially GBM, receptor expression was investigated by glioma molecular groups, by brain region distribution, emphasizing tissue-specific receptor functions, and by cell type enrichment. Our study constitutes a clinically relevant and patient-oriented guide that recapitulates the expression profile and the complex roles of chemokine receptors within the highly diversified glioma landscape. Additionally, it strengthens the importance of patient-derived material for development and precise amelioration of chemokine receptor-targeting therapies.

CXCL10 Is an Agonist of the CC Family Chemokine Scavenger Receptor ACKR2/D6.

Atypical chemokine receptors (ACKRs) are important regulators of chemokine functions. Among them, the atypical chemokine receptor ACKR2 (also known as D6) has long been considered as a scavenger of inflammatory chemokines exclusively from the CC family. In this study, by using highly sensitive ß-arrestin recruitment assays based on NanoBiT and NanoBRET technologies, we identified the inflammatory CXC chemokine CXCL10 as a new strong agonist ligand for ACKR2. CXCL10 is known to play an important role in the infiltration of immune cells into the tumour bed and was previously reported to bind to CXCR3 only. We demonstrated that ACKR2 is able to internalize and reduce the availability of CXCL10 in the extracellular space. Moreover, we found that, in contrast to CC chemokines, CXCL10 activity towards ACKR2 was drastically reduced by the dipeptidyl peptidase 4 (DPP4 or CD26) N-terminal processing, pointing to a different receptor binding pocket occupancy by CC and CXC chemokines. Overall, our study sheds new light on the complexity of the chemokine network and the potential role of CXCL10 regulation by ACKR2 in many physiological and pathological processes, including tumour immunology. Our data also testify that systematic reassessment of chemokine-receptor pairing is critically needed as important interactions may remain unexplored.

CCL20 is a novel ligand for the scavenging atypical chemokine receptor 4.

The chemokine CCL20 is broadly produced by endothelial cells in the liver, the lung, in lymph nodes and mucosal lymphoid tissues, and recruits CCR6 expressing leukocytes, particularly dendritic cells, mature B cells, and subpopulations of T cells. How CCL20 is systemically scavenged is currently unknown. Here, we identify that fluorescently labeled human and mouse CCL20 are efficiently taken-up by the atypical chemokine receptor ACKR4. CCL20 shares ACKR4 with the homeostatic chemokines CCL19, CCL21, and CCL25, although with a lower affinity. We demonstrate that all 4 human chemokines recruit ß-arrestin1 and ß-arrestin2 to human ACKR4. Similarly, mouse CCL19, CCL21, and CCL25 equally activate the human receptor. Interestingly, at the same chemokine concentration, mouse CCL20 did not recruit ß-arrestins to human ACKR4. Further cross-species analysis suggests that human ACKR4 preferentially takes-up human CCL20, whereas mouse ACKR4 similarly internalizes mouse and human CCL20. Furthermore, we engineered a fluorescently labeled chimeric chemokine consisting of the N-terminus of mouse CCL25 and the body of mouse CCL19, termed CCL25_19, which interacts with and is taken-up by human and mouse ACKR4.