Elucidation of CXCR7-mediated signaling events and inhibition of CXCR4-mediated tumor cell transendothelial migration by CXCR7 ligands

Precision Culture Scaling to Establish High-Throughput Vasculogenesis Models

Hydrogel-based 3D cell cultures can recapitulate (patho)physiological phenomena ex vivo. However, due to their complex multifactorial regulation, adapting these tissue and disease models for high-throughput screening workflows remains challenging. In this study, a new precision culture scaling (PCS-X) methodology combines statistical techniques (design of experiment and multiple linear regression) with automated, parallelized experiments and analyses to customize hydrogel-based vasculogenesis cultures using human umbilical vein endothelial cells and retinal microvascular endothelial cells. Variations of cell density, growth factor supplementation, and media composition are systematically explored to induce vasculogenesis in endothelial mono- and cocultures with mesenchymal stromal cells or retinal microvascular pericytes in 384-well plate formats. The developed cultures are shown to respond to vasculogenesis inhibitors in a compound- and dose-dependent manner, demonstrating the scope and power of PCS-X in creating parallelized tissue and disease models for drug discovery and individualized therapies.

Chemokines as Prognostic Factor in Colorectal Cancer Patients: A Systematic Review and Meta-Analysis

Chemokines orchestrate many aspects of tumorigenic processes such as angiogenesis, apoptosis and metastatic spread, and related receptors are expressed on tumor cells as well as on inflammatory cells (e.g., tumor-infiltrating T cells, TILs) in the tumor microenvironment. Expressional changes of chemokines and their receptors in solid cancers are common and well known, especially in affecting colorectal cancer patient outcomes. Therefore, the aim of this current systematic review and meta-analysis was to classify chemokines as a prognostic biomarker in colorectal cancer patients. A systematic literature search was conducted in PubMed, CENTRAL and Web of Science. Information on the chemokine expression of 25 chemokines in colorectal cancer tissue and survival data of the patients were investigated. The hazard ratio of overall survival and disease-free survival with chemokine expression was examined. The risk of bias was analyzed using Quality in Prognosis Studies. Random effects meta-analysis was performed to determine the impact on overall respectively disease survival. For this purpose, the pooled hazard ratios (HR) and their 95% confidence intervals (CI) were used for calculation. Twenty-five chemokines were included, and the search revealed 5556 publications. A total of thirty-one publications were included in this systematic review and meta-analysis. Overexpression of chemokine receptor CXCR4 was associated with both a significantly reduced overall survival (HR = 2.70, 95%-CI: 1.57 to 4.66, p = 0.0003) as well as disease-free survival (HR = 2.68, 95%-CI: 1.41 to 5.08, p = 0.0026). All other chemokines showed either heterogeneous results or few studies were available. The overall risk of bias for CXCR4 was rated low. At the current level of evidence, this study demonstrates that CXCR4 overexpression in patients with colorectal cancer is associated with a significantly diminished overall as well as disease-free survival. Summed up, this systematic review and meta-analysis reveals CXCR4 as a promising prognostic biomarker. Nevertheless, more evidence is needed to evaluate CXCR4 and its antagonists serving as new therapeutic targets.

Atypical chemokine receptors in the immune system

Leukocyte migration is a fundamental component of innate and adaptive immune responses as it governs the recruitment and localization of these motile cells, which is crucial for immune cell priming, effector functions, memory responses and immune regulation. This complex cellular trafficking system is controlled to a large extent via highly regulated production of secreted chemokines and the restricted expression of their membrane-tethered G-protein-coupled receptors. The activity of chemokines and their receptors is also regulated by a subfamily of molecules known as atypical chemokine receptors (ACKRs), which are chemokine receptor-like molecules that do not couple to the classical signalling pathways that promote cell migration in response to chemokine ligation. There has been a great deal of progress in understanding the biology of these receptors and their functions in the immune system in the past decade. Here, we describe the contribution of the various ACKRs to innate and adaptive immune responses, focussing specifically on recent progress. This includes recent findings that have defined the role for ACKRs in sculpting extracellular chemokine gradients, findings that broaden the spectrum of chemokine ligands recognized by these receptors, candidate new additions to ACKR family, and our increasing understanding of the role of these receptors in shaping the migration of innate and adaptive immune cells.

Extracellular vesicles derived from SARS-CoV-2 M-protein-induced triple negative breast cancer cells promoted the ability of tissue stem cells supporting cancer progression

Introduction

SARS-CoV-2 infection increases the risk of worse outcomes in cancer patients, including those with breast cancer. Our previous study reported that the SARS-CoV-2 membrane protein (M-protein) promotes the malignant transformation of triple-negative breast cancer cells (triple-negative BCC).

Methods

In the present study, the effects of M-protein on the ability of extracellular vesicles (EV) derived from triple-negative BCC to regulate the functions of tissue stem cells facilitating the tumor microenvironment were examined.

Results

Our results showed that EV derived from M-protein-induced triple-negative BCC (MpEV) significantly induced the paracrine effects of adipose tissue-derived mesenchymal stem cells (ATMSC) on non-aggressive BCC, promoting the migration, stemness phenotypes, and in vivo metastasis of BCC, which is related to PGE2/IL1 signaling pathways, in comparison to EV derived from normal triple-negative BCC (nEV). In addition to ATMSC, the effects of MpEV on endothelial progenitor cells (EPC), another type of tissue stem cells, were examined. Our data suggested that EPC uptaking MpEV acquired a tumor endothelial cell-like phenotype, with increasing angiogenesis and the ability to support the aggressiveness and metastasis of non-aggressive BCC.

Discussion

Taken together, our findings suggest the role of SARS-CoV-2 M-protein in altering the cellular communication between cancer cells and other non-cancer cells inside the tumor microenvironment via EV. Specifically, M-proteins induced the ability of EV derived from triple-negative BCC to promote the functions of non-cancer cells, such as tissue stem cells, in tumorigenesis.

Disruption of placental ACKR3 impairs growth and hematopoietic development of offspring

ACKR3 scavenges and degrades the stem cell recruiting chemokine CXCL12, which is essential for proper embryonic and, in particular, haematopoietic development. Here, we demonstrate strong expression of ACKR3 on trophoblasts. Using a maternally administered pharmacological blocker and Cre-mediated genetic approaches, we demonstrate that trophoblast ACKR3 is essential for preventing movement of CXCL12 from the mother to the embryo, with elevated plasma CXCL12 levels being detected in embryos from ACKR3-blocker-treated mothers. Mice born to mothers treated with the blocker are lighter and shorter than those born to vehicle-treated mothers and, in addition, display profound anaemia associated with a markedly reduced bone marrow haematopoietic stem cell population. Importantly, although the haematopoietic abnormalities are corrected as mice age, our studies reveal a postnatal window during which offspring of ACKR3-blocker-treated mice are unable to mount effective inflammatory responses to inflammatory/infectious stimuli. Overall, these data demonstrate that ACKR3 is essential for preventing CXCL12 transfer from mother to embryo and for ensuring properly regulated CXCL12 control over the development of the haematopoietic system.

An emerging paradigm of CXCL12 involvement in the metastatic cascade

The chemokine CXCL12, also known as stromal cell-derived factor 1 (SDF1), has emerged as a pivotal regulator in the intricate molecular networks driving cancer progression. As an influential factor in the tumor microenvironment, CXCL12 plays a multifaceted role that spans beyond its traditional role as a chemokine inducing invasion and metastasis. Indeed, CXCL12 has been assigned functions related to epithelial-to-mesenchymal transition, cancer cell stemness, angiogenesis, and immunosuppression, all of which are currently viewed as specialized biological programs contributing to the "metastatic cascade" among other cancer hallmarks. Its interaction with its cognate receptor, CXCR4, initiates a cascade of events that not only shapes the metastatic potential of tumor cells but also defines the niches within the secondary organs that support metastatic colonization. Given the profound implications of CXCL12 in the metastatic cascade, understanding its mechanistic underpinnings is of paramount importance for the targeted elimination of rate-limiting steps in the metastatic process. This review aims to provide a comprehensive overview of the current knowledge surrounding the role of CXCL12 in cancer metastasis, especially its molecular interactions rationalizing its potential as a therapeutic target.

COL3A1-positive endothelial cells influence LUAD prognosis and regulate LUAD carcinogenesis by NCL-PI3K-AKT axis

BACKGROUND

Lung adenocarcinoma (LUAD), as the most common type of lung cancer, poses a significant threat to public health. Tumor heterogeneity plays a crucial role in carcinogenesis, which could be largely deciphered by next-generation sequencing (NGS).

METHODS

We obtained and screened single-cell RNA sequencing (scRNA-seq) data from 16 LUAD samples, and endothelial cells (ECs) were grouped into three clusters. The origin of EC differentiation was explored by pseudo-time analysis. CellChat analysis was used to detect potential communication between ECs and malignant cells, and gene regulatory network analysis was used to identify changes in transcription factor activity. We explored the prognosis of specific ECs clusters and their effects on the tumor microenvironment (TME) at the bulk transcriptome level. 5-Ethynyl-2'- deoxyuridine (EdU) and Ki-67 staining were conducted to study the proliferative phenotype of LUAD cell lines. Western blotting targeting the phosphorylation of PI3K-AKT proteins was utilized for determination of the downstream pathway of NCL.

RESULTS

COL3A1-positive ECs showed the highest crosstalk interaction with malignant cells, indicating that they have important effects on driving LUAD carcinogenesis. Vascular endothelial growth factor (VEGF) signaling pathway was identified as the main signaling pathway, mediating signal transduction from malignant cells. The TME-related genes of COL3A1-positive ECs were significantly more highly expressed. COL3A1-positive ECs showed unique metabolic and immune characteristics, as well as highly activated metabolic signaling pathways and inflammatory responses. Importantly, LUAD patients with low COL3A1-positive ECs scores displayed an inferior prognosis outcome and a higher risk of metastasis. The key target gene NCL, which is involved in the interaction between epithelial cells and cancer cells, has been identified through screening. Flow cytometry showed that knockdown of NCL prompted the apoptosis of A549 and NCI-H1299. Western blotting showed that knockdown of NCL decreased the phosphorylation of AKT and PI3K, which identified the downstream pathway of NCL.

CONCLUSIONS

COL3A1-positive ECs have important effects on the development of LUAD and the formation of an immune microenvironment. Furthermore, we identified a key target gene, NCL, which is involved in the interaction between endothelial cells and cancer cells. NCL also affected the apoptosis and proliferation in LUAD through the PI3K-AKT pathway.

Stem-like signatures in human meningioma cells are under the control of CXCL11/CXCL12 chemokine activity

BACKGROUND

Meningiomas are mainly benign brain tumors, although about 20% of histologically benign cases are clinically aggressive and recur after resection. We hypothesize that meningioma brain invasiveness and recurrence may be related to the presence of cancer stem cells and their high responsiveness to the CXCL12-CXCR4/CXCR7 chemokine axis. The aim of this study was to isolate meningioma stem cells from human samples, characterize them for biological features related to malignant behavior, and to identify the role of CXCR4/CXCR7 in these processes.

METHODS

Meningioma stem cells were isolated from patient-derived primary cultures in stem cell-permissive conditions, and characterized for phenotype, self-renewal, proliferation and migration rates, vasculogenic mimicry (VM), and in vivo tumorigenesis, in comparison with differentiated meningioma cells and stem-like cells isolated from normal meninges. These cell populations were challenged with CXCL12 and CXCL11 and receptor antagonists to define the chemokine role in stem cell-related functions.

RESULTS

Stem-like cells isolated from meningioma cultures display higher proliferation and migration rates, and VM, as compared to meningioma non-stem cells or cells isolated from normal meninges and were the only tumorigenic population in vivo. In meningioma cells, these stem-like functions were under the control of the CXCR4/CXCR7 chemokine axis.

CONCLUSIONS

We report a role for CXCL11 and CXCL12 in the control of malignant features in stem-like cells isolated from human meningioma, providing a possible basis for the aggressive clinical behavior observed in subsets of these tumors. CXCR4/CXCR7 antagonists might represent a useful approach for meningioma at high risk of recurrence and malignant progression.

Chemokine receptor CXCR7 activates Aurora Kinase A and promotes neuroendocrine prostate cancer growth

CXCR7 is an atypical chemokine receptor that recruits β-arrestin (ARRB2) and internalizes into clathrin-coated intracellular vesicles where the complex acts as a scaffold for cytoplasmic kinase assembly and signal transduction. Here, we report that CXCR7 was elevated in the majority of prostate cancer (PCa) cases with neuroendocrine features (NEPC). CXCR7 markedly induced mitotic spindle and cell cycle gene expression. Mechanistically, we identified Aurora Kinase A (AURKA), a key regulator of mitosis, as a novel target that was bound and activated by the CXCR7-ARRB2 complex. CXCR7 interacted with proteins associated with microtubules and golgi, and, as such, the CXCR7-ARRB2-containing vesicles trafficked along the microtubules to the pericentrosomal golgi apparatus, where the complex interacted with AURKA. Accordingly, CXCR7 promoted PCa cell proliferation and tumor growth, which was mitigated by AURKA inhibition. In summary, our study reveals a critical role of CXCR7-ARRB2 in interacting and activating AURKA, which can be targeted by AURKA inhibitors to benefit a subset of patients with NEPC.

Targeting CXCL9/10/11-CXCR3 axis: an important component of tumor-promoting and antitumor immunity

Chemokines are chemotactic-competent molecules composed of a family of small cytokines, playing a key role in regulating tumor progression. The roles of chemokines in antitumor immune responses are of great interest. CXCL9, CXCL10, and CXCL11 are important members of chemokines. It has been widely investigated that these three chemokines can bind to their common receptor CXCR3 and regulate the differentiation, migration, and tumor infiltration of immune cells, directly or indirectly affecting tumor growth and metastasis. Here, we summarize the mechanism of how the CXCL9/10/11-CXCR3 axis affects the tumor microenvironment, and list the latest researches to find out how this axis predicts the prognosis of different cancers. In addition, immunotherapy improves the survival of tumor patients, but some patients show drug resistance. Studies have found that the regulation of CXCL9/10/11-CXCR3 on the tumor microenvironment is involved in the process of changing immunotherapy resistance. Here we also describe new approaches to restoring sensitivity to immune checkpoint inhibitors through the CXCL9/10/11-CXCR3 axis.

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, β-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.

Tumour associated endothelial cells: origin, characteristics and role in metastasis and anti-angiogenic resistance

Tumour progression and metastasis remain the leading causes of cancer-related death worldwide. Tumour angiogenesis is essential for tumour progression. The vasculature surrounding tumours is not only a transport channel for nutrients, oxygen, and metabolites, but also a pathway for metastasis. There is a close interaction between tumour cells and endothelial cells in the tumour microenvironment. Recent studies have shown that tumour-associated endothelial cells have different characteristics from normal vascular endothelial cells, play an important role in tumour progression and metastasis, and are expected to be a key target for cancer therapy. This article reviews the tissue and cellular origin of tumour-associated endothelial cells and analyses the characteristics of tumour-associated endothelial cells. Finally, it summarises the role of tumour-associated endothelial cells in tumour progression and metastasis and the prospects for their use in clinical anti-angiogenic therapy.

Recent advance of small-molecule drugs for clinical treatment of multiple myeloma

Multiple myeloma (MM) is a hematologic neoplasm of plasma cells that is currently deemed incurable. Despite the introduction of novel immunomodulators and proteasome inhibitors, MM remains a challenging disease with high rates of relapse and refractoriness. The management of refractory and relapsed MM patients remains a formidable task, primarily due to the emergence of multiple drug resistance. Consequently, there is an urgent need for novel therapeutic agents to address this clinical challenge. In recent years, a significant amount of research has been dedicated to the discovery of novel therapeutic agents for the treatment of MM. The clinical utilization of proteasome inhibitor carfilzomib and immunomodulator pomalidomide has been successively introduced. As basic research continues to advance, novel therapeutic agents, including panobinostat, a histone deacetylase inhibitor, and selinexor, a nuclear export inhibitor, have progressed to the clinical trial and application phase. This review aims to furnish a comprehensive survey of the clinical applications and synthetic pathways of select drugs, with the intention of imparting valuable insights for future drug research and development geared towards MM.

Evaluation of Atypical Chemokine Receptor Expression in T Cell Subsets

Chemokines are molecules that pertain to a family of small cytokines and can generate cell chemotaxis through the interaction with their receptors. Chemokines can trigger signaling via conventional G-protein-coupled receptors or through atypical chemokine receptors. Currently, four atypical chemokine receptors have been are described (ACKR1, ACKR2, ACKR3 and ACKR4). ACKRs are expressed in various cells and tissues, including T lymphocytes. These receptors' main function is related to the internalization and degradation of chemokines, as well as to the inflammation control. However, the expression of these receptors in human T lymphocytes is unclear in the literature. The objective of this study was to evaluate the expression of ACKRs in different subpopulations of T lymphocytes. For this, peripheral blood from healthy donors was used to analyze the expression of ACKR2, ACKR3 and ACKR4 by immunophenotyping CD4, CD8 T lymphocytes and, in their subsets, naive, transition and memory. Results obtained in this study demonstrated that ACKR2, ACKR3 and ACKR4 receptors were expressed by T lymphocytes subsets in different proportions. These receptors are highly expressed in the cytoplasmic milieu of all subsets of T lymphocytes, therefore suggesting that their expression in plasma membrane is regulated after transcription, and it must be dependent on a stimulus, which was not identified in our study. Thus, regarding ACKRs function as scavenger receptors, at least for the ACKR3, this function does not impair the chemotaxis exert for their ligand compared to the typical counterpart receptor.

Atypical chemokine receptors: emerging therapeutic targets in cancer

Atypical chemokine receptors (ACKRs) regulate the availability of chemokines via chemokine scavenging, while also having the capacity to elicit downstream function through β-arrestin coupling. This contrasts with conventional chemokine receptors that directly elicit immune cell migration through G protein-coupled signaling. The significance of ACKRs in cancer biology has previously been poorly understood, but recent findings have highlighted the multifaceted role of these receptors in tumorigenesis and immune response modulation within the tumor microenvironment (TME). Additionally, recent research has expanded our understanding of the function of several receptors including GPR182, CCRL2, GPR1, PITPNM3, and C5aR2 that share similarities with the ACKR family. In this review, we discuss these recent developments, and highlight the opportunities and challenges of pharmacologically targeting ACKRs in cancer.

Atypical CXCL12 signaling enhances neutrophil migration by modulating nuclear deformability

To reach inflamed tissues from the circulation, neutrophils must overcome physical constraints imposed by the tissue architecture, such as the endothelial barrier or the three-dimensional (3D) interstitial space. In these microenvironments, neutrophils are forced to migrate through spaces smaller than their own diameter. One of the main challenges for cell passage through narrow gaps is the deformation of the nucleus, the largest and stiffest organelle in cells. Here, we showed that chemokines, the extracellular signals that guide cell migration in vivo, modulated nuclear plasticity to support neutrophil migration in restricted microenvironments. Exploiting microfabricated devices, we found that the CXC chemokine CXCL12 enhanced the nuclear pliability of mouse bone marrow–derived neutrophils to sustain their migration in 3D landscapes. This previously uncharacterized function of CXCL12 was mediated by the atypical chemokine receptor ACKR3 (also known as CXCR7), required protein kinase A (PKA) activity, and induced chromatin compaction, which resulted in enhanced cell migration in 3D. Thus, we propose that chemical cues regulate the nuclear plasticity of migrating leukocytes to optimize their motility in restricted microenvironments.

Rethinking the chemokine cascade in brain metastasis: Preventive and therapeutic implications

Brain metastasis (BrM) is one of the major causes of death in cancer patients and is associated with an estimated 10-40 % of total cancer cases. The survival rate of brain metastatic patients has not improved due to intratumor heterogeneity, the survival adaptations of brain homing metastatic cells, and the lack of understanding of underlying molecular mechanisms that limit the availability of effective therapies. The heterogeneous population of immune cells and tumor-initiating cells or cancer stem cells in the tumor microenvironment (TME) release various factors, such as chemokines that upon binding to their cognate receptors enhance tumor growth at primary sites and help tumor cells metastasize to the brain. Furthermore, brain metastatic sites have unique heterogeneous microenvironment that fuels cancer cells in establishing BrM. This review explores the crosstalk of chemokines with the heterogeneous TME during the progression of BrM and recognizes potential therapeutic approaches. We also discuss and summarize different targeted, immunotherapeutic, chemotherapeutic, and combinatorial strategies (with chemo-/immune- or targeted-therapies) to attenuate chemokines mediated BrM.

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.

Colorectal Cancer: The Contribution of CXCL12 and Its Receptors CXCR4 and CXCR7

Simple Summary

Many signaling pathways are involved in cancer progression, and among these pathways, the CXCL12 axis and its two receptors CXCR4 and CXCR7 are well described for many cancers. This review presents the current knowledge on the role played by each of the actors of this axis in colorectal cancer and on its consideration in the development of new therapeutic strategies.

Abstract

Colorectal cancer is one of the most common cancers, and diagnosis at late metastatic stages is the main cause of death related to this cancer. This progression to metastasis is complex and involves different molecules such as the chemokine CXCL12 and its two receptors CXCR4 and CXCR7. The high expression of receptors in CRC is often associated with a poor prognosis and aggressiveness of the tumor. The interaction of CXCL12 and its receptors activates signaling pathways that induce chemotaxis, proliferation, migration, and cell invasion. To this end, receptor inhibitors were developed, and their use in preclinical and clinical studies is ongoing. This review provides an overview of studies involving CXCR4 and CXCR7 in CRC with an update on their targeting in anti-cancer therapies.

The Atypical Chemerin Receptor GPR1 Displays Different Modes of Interaction with β-Arrestins in Humans and Mice with Important Consequences on Subcellular Localization and Trafficking

Atypical chemokine receptors (ACKRs) have emerged as a subfamily of chemokine receptors regulating the local bioavailability of their ligands through scavenging, concentration, or transport. The biological roles of ACKRs in human physiology and diseases are often studied by using transgenic mouse models. However, it is unknown whether mouse and human ACKRs share the same properties. In this study, we compared the properties of the human and mouse atypical chemerin receptor GPR1 and showed that they behave differently regarding their interaction with β-arrestins. Human hGPR1 interacts with β-arrestins as a result of chemerin stimulation, whereas its mouse orthologue mGPR1 displays a strong constitutive interaction with β-arrestins in basal conditions. The constitutive interaction of mGPR1 with β-arrestins is accompanied by a redistribution of the receptor from the plasma membrane to early and recycling endosomes. In addition, β-arrestins appear mandatory for the chemerin-induced internalization of mGPR1, whereas they are dispensable for the trafficking of hGPR1. However, mGPR1 scavenges chemerin and activates MAP kinases ERK1/2 similarly to hGPR1. Finally, we showed that the constitutive interaction of mGPR1 with β-arrestins required different structural constituents, including the receptor C-terminus and arginine 3.50 in the second intracellular loop. Altogether, our results show that sequence variations within cytosolic regions of GPR1 orthologues influence their ability to interact with β-arrestins, with important consequences on GPR1 subcellular distribution and trafficking.

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.

CXCR7 Antagonism Reduces Acute Lung Injury Pathogenesis

Loss of control in the trafficking of immune cells to the inflamed lung tissue contributes to the pathogenesis of life-threatening acute lung injury (ALI) and its more severe form, acute respiratory distress syndrome (ARDS). Targeting CXCR7 has been proposed as a potential therapeutic approach to reduce pulmonary inflammation; however, its role and its crosstalk with the two chemokine receptors CXCR3 and CXCR4 via their shared ligands CXCL11 and CXCL12 is not yet completely understood. The present paper aimed to characterize the pathological role of the CXCR3/CXCR4/CXCR7 axis in a murine model of ALI. Lipopolysaccharide (LPS) inhalation in mice resulted in the development of key pathologic features of ALI/ARDS, including breathing dysfunctions, alteration in the alveolar capillary barrier, and lung inflammation. LPS inhalation induced immune cell infiltration into the bronchoalveolar space, including CXCR3⁺ and CXCR4⁺ cells, and enhanced the expression of the ligands of these two chemokine receptors. The first-in-class CXCR7 antagonist, ACT-1004-1239, increased levels of CXCL11 and CXCL12 in the plasma without affecting their levels in inflamed lung tissue, and consequently reduced CXCR3⁺ and CXCR4⁺ immune cell infiltrates into the bronchoalveolar space. In the early phase of lung inflammation, characterized by a massive influx of neutrophils, treatment with ACT-1004-1239 significantly reduced the LPS-induced breathing pattern alteration. Both preventive and therapeutic treatment with ACT-1004-1239 reduced lung vascular permeability and decreased inflammatory cell infiltrates. In conclusion, these results demonstrate a key pathological role of CXCR7 in ALI/ARDS and highlight the clinical potential of ACT-1004-1239 in ALI/ARDS pathogenesis.

PET Imaging Radiotracers of Chemokine Receptors

Chemokines and chemokine receptors have been recognized as critical signal components that maintain the physiological functions of various cells, particularly the immune cells. The signals of chemokines/chemokine receptors guide various leukocytes to respond to inflammatory reactions and infectious agents. Many chemokine receptors play supportive roles in the differentiation, proliferation, angiogenesis, and metastasis of diverse tumor cells. In addition, the signaling functions of a few chemokine receptors are associated with cardiac, pulmonary, and brain disorders. Over the years, numerous promising molecules ranging from small molecules to short peptides and antibodies have been developed to study the role of chemokine receptors in healthy states and diseased states. These drug-like candidates are in turn exploited as radiolabeled probes for the imaging of chemokine receptors using noninvasive in vivo imaging, such as positron emission tomography (PET). Recent advances in the development of radiotracers for various chemokine receptors, particularly of CXCR4, CCR2, and CCR5, shed new light on chemokine-related cancer and cardiovascular research and the subsequent drug development. Here, we present the recent progress in PET radiotracer development for imaging of various chemokine receptors.

CXCL12-CXCR4/CXCR7 Axis in Colorectal Cancer: Therapeutic Target in Preclinical and Clinical Studies

Chemokines are chemotactic cytokines that promote cancer growth, metastasis, and regulate resistance to chemotherapy. Stromal cell-derived factor 1 (SDF1) also known as C-X-C motif chemokine 12 (CXCL12), a prognostic factor, is an extracellular homeostatic chemokine that is the natural ligand for chemokine receptors C-X-C chemokine receptor type 4 (CXCR4), also known as fusin or cluster of differentiation 184 (CD184) and chemokine receptor type 7 (CXCR7). CXCR4 is the most widely expressed rhodopsin-like G protein coupled chemokine receptor (GPCR). The CXCL12–CXCR4 axis is involved in tumor growth, invasion, angiogenesis, and metastasis in colorectal cancer (CRC). CXCR7, recently termed as atypical chemokine receptor 3 (ACKR3), is amongst the G protein coupled cell surface receptor family that is also commonly expressed in a large variety of cancer cells. CXCR7, like CXCR4, regulates immunity, angiogenesis, stem cell trafficking, cell growth and organ-specific metastases. CXCR4 and CXCR7 are expressed individually or together, depending on the tumor type. When expressed together, CXCR4 and CXCR7 can form homo- or hetero-dimers. Homo- and hetero-dimerization of CXCL12 and its receptors CXCR4 and CXCR7 alter their signaling activity. Only few drugs have been approved for clinical use targeting CXCL12-CXCR4/CXCR7 axis. Several CXCR4 inhibitors are in clinical trials for solid tumor treatment with limited success whereas CXCR7-specific inhibitors are still in preclinical studies for CRC. This review focuses on current knowledge of chemokine CXCL12 and its receptors CXCR4 and CXCR7, with emphasis on targeting the CXCL12–CXCR4/CXCR7 axis as a treatment strategy for CRC.

Role of Atypical Chemokines and Chemokine Receptors Pathways in the Pathogenesis of COPD

Chronic obstructive pulmonary disease (COPD) represents a heightened inflammatory response in the lung generally resulting from tobacco smoking-induced recruitment and activation of inflammatory cells and/or activation of lower airway structural cells. Several mediators can modulate activation and recruitment of these cells, particularly those belonging to the chemokines (conventional and atypical) family. There is emerging evidence for complex roles of atypical chemokines and their receptors (such as high mobility group box 1 (HMGB1), antimicrobial peptides, receptor for advanced glycosylation end products (RAGE) or toll-like receptors (TLRs)) in the pathogenesis of COPD, both in the stable disease and during exacerbations. Modulators of these pathways represent potential novel therapies for COPD and many are now in preclinical development. Inhibition of only a single atypical chemokine or receptor may not block inflammatory processes because there is redundancy in this network. However, there are many animal studies that encourage studies for modulating the atypical chemokine network in COPD. Thus, few pharmaceutical companies maintain a significant interest in developing agents that target these molecules as potential antiinflammatory drugs. Antibody-based (biological) and small molecule drug (SMD)-based therapies targeting atypical chemokines and/or their receptors are mostly at the preclinical stage and their progression to clinical trials is eagerly awaited. These agents will most likely enhance our knowledge about the role of atypical chemokines in COPD pathophysiology and thereby improve COPD management.

Marginal Zone Formation Requires ACKR3 Expression on B Cells

The marginal zone (MZ) contributes to the highly organized spleen microarchitecture. We show that expression of atypical chemokine receptor 3 (ACKR3) defines two equal-sized populations of mouse MZ B cells (MZBs). ACKR3 is required for development of a functional MZ and for positioning of MZBs. Deletion of ACKR3 on B cells distorts the MZ, and MZBs fail to deliver antigens to follicles, reducing humoral responses. Reconstitution of MZ-deficient CD19^ko mice shows that ACKR3^- MZBs can differentiate into ACKR3⁺ MZBs, but not vice versa. The lack of a MZ is rescued by adoptive transfer of ACKR3-sufficient, and less by ACKR3-deficient, follicular B cells (FoBs); hence, ACKR3 expression is crucial for establishment of the MZ. The inability of CD19^ko mice to respond to T-independent antigen is rescued when ACKR3-proficient, but not ACKR3-deficient, FoBs are transferred. Accordingly, ACKR3-deficient FoBs are able to reconstitute the MZ if the niche is pre-established by ACKR3-proficient MZBs.

Upregulating CXCR7 accelerates endothelial progenitor cell-mediated endothelial repair by activating Akt/Keap-1/Nrf2 signaling in diabetes mellitus

Background

Endothelial progenitor cell (EPC) dysfunction contributes to vascular disease in diabetes mellitus. However, the molecular mechanism underlying EPC dysfunction and its contribution to delayed reendothelialization in diabetes mellitus remain unclear. Our study aimed to illustrate the potential molecular mechanism underlying diabetic EPC dysfunction in vivo and in vitro. Furthermore, we assessed the effect of EPC transplantation on endothelial regeneration in diabetic rats.

Methods

Late outgrowth EPCs were isolated from the bone marrow of rats for in vivo and in vitro studies. In vitro functional assays and Western blotting were conducted to reveal the association between C-X-C chemokine receptor type 7 (CXCR7) expression and diabetic EPC dysfunction. To confirm the association between cellular CXCR7 levels and EPC function, CXCR7 expression in EPCs was upregulated and downregulated via lentiviral transduction and RNA interference, respectively. Western blotting was used to reveal the potential molecular mechanism by which the Stromal-Derived Factor-1 (SDF-1)/CXCR7 axis regulates EPC function. To elucidate the role of the SDF-1/CXCR7 axis in EPC-mediated endothelial regeneration, a carotid artery injury model was established in diabetic rats. After the model was established, saline-treated, diabetic, normal, or CXCR7-primed EPCs were injected via the tail vein.

Results

Diabetic EPC dysfunction was associated with decreased CXCR7 expression. Furthermore, EPC dysfunction was mimicked by knockdown of CXCR7 in normal EPCs. However, upregulating CXCR7 expression reversed the dysfunction of diabetic EPCs. The SDF-1/CXCR7 axis positively regulated EPC function by activating the AKT-associated Kelch-like ECH-associated protein 1 (keap-1)/nuclear factor erythroid 2-related factor 2 (Nrf2) axis, which was reversed by blockade of AKT and Nrf2. Transplantation of CXCR7-EPCs accelerated endothelial repair and attenuated neointimal hyperplasia in diabetes mellitus more significantly than transplantation of diabetic or normal EPCs. However, the therapeutic effect of CXCR7-EPC transplantation on endothelial regeneration was reversed by knockdown of Nrf2 expression.

Conclusions

Dysfunction of diabetic EPCs is associated with decreased CXCR7 expression. Furthermore, the SDF-1/CXCR7 axis positively regulates EPC function by activating the AKT/keap-1/Nrf2 axis. CXCR7-primed EPCs might be useful for endothelial regeneration in diabetes-associated vascular disease.

CXCR4 and CXCR7 Signaling Pathways: A Focus on the Cross-Talk Between Cancer Cells and Tumor Microenvironment

The chemokine receptor 4 (CXCR4) and 7 (CXCR7) are G-protein-coupled receptors (GPCRs) activated through their shared ligand CXCL12 in multiple human cancers. They play a key role in the tumor/tumor microenvironment (TME) promoting tumor progression, targeting cell proliferation and migration, while orchestrating the recruitment of immune and stromal cells within the TME. CXCL12 excludes T cells from TME through a concentration gradient that inhibits immunoactive cells access and promotes tumor vascularization. Thus, dual CXCR4/CXCR7 inhibition will target different cancer components. CXCR4/CXCR7 antagonism should prevent the development of metastases by interfering with tumor cell growth, migration and chemotaxis and favoring the frequency of T cells in TME. Herein, we discuss the current understanding on the role of CXCL12/CXCR4/CXCR7 cross-talk in tumor progression and immune cells recruitment providing support for a combined CXCR4/CXCR7 targeting therapy. In addition, we consider emerging approaches that coordinately target both immune checkpoints and CXCL12/CXCR4/CXCR7 axis.

Lymphatic endothelial-cell expressed ACKR3 is dispensable for postnatal lymphangiogenesis and lymphatic drainage function in mice

Atypical chemokine receptor ACKR3 (formerly CXCR7) is a scavenging receptor that has recently been implicated in murine lymphatic development. Specifically, ACKR3-deficiency was shown to result in lymphatic hyperplasia and lymphedema, in addition to cardiac hyperplasia and cardiac valve defects leading to embryonic lethality. The lymphatic phenotype was attributed to a lymphatic endothelial cell (LEC)-intrinsic scavenging function of ACKR3 for the vascular peptide hormone adrenomedullin (AM), which is also important during postnatal lymphangiogenesis. In this study, we investigated the expression of ACKR3 in the lymphatic vasculature of adult mice and its function in postnatal lymphatic development and function. We show that ACKR3 is widely expressed in mature lymphatics and that it exerts chemokine-scavenging activity in cultured murine skin-derived LECs. To investigate the role of LEC-expressed ACKR3 in postnatal lymphangiogenesis and function during adulthood, we generated and validated a lymphatic-specific, inducible ACKR3 knockout mouse. Surprisingly, in contrast to the reported involvement of ACKR3 in lymphatic development, our analyses revealed no contribution of LEC-expressed ACKR3 to postnatal lymphangiogenesis, lymphatic morphology and drainage function.

Adipocyte-Specific ACKR3 Regulates Lipid Levels in Adipose Tissue

Dysfunctional adipose tissue (AT) may contribute to the pathology of several metabolic diseases through altered lipid metabolism, insulin resistance, and inflammation. Atypical chemokine receptor 3 (ACKR3) expression was shown to increase in AT during obesity, and its ubiquitous elimination caused hyperlipidemia in mice. Although these findings point towards a role of ACKR3 in the regulation of lipid levels, the role of adipocyte-specific ACKR3 has not yet been studied exclusively in this context. In this study, we established adipocyte- and hepatocyte-specific knockouts of Ackr3 in ApoE-deficient mice in order to determine its impact on lipid levels under hyperlipidemic conditions. We show for the first time that adipocyte-specific deletion of Ackr3 results in reduced AT triglyceride and cholesterol content in ApoE-deficient mice, which coincides with increased peroxisome proliferator-activated receptor-γ (PPAR-γ) and increased Angptl4 expression. The role of adipocyte ACKR3 in lipid handling seems to be tissue-specific as hepatocyte ACKR3 deficiency did not demonstrate comparable effects. In summary, adipocyte-specific ACKR3 seems to regulate AT lipid levels in hyperlipidemic Apoe^−/− mice, which may therefore be a significant determinant of AT health. Further studies are needed to explore the potential systemic or metabolic effects that adipocyte ACKR3 might have in associated disease models.

Differential Involvement of ACKR3 C-Tail in β-Arrestin Recruitment, Trafficking and Internalization

Background: The atypical chemokine receptor 3 (ACKR3) belongs to the superfamily of G protein-coupled receptors (GPCRs). Unlike classical GPCRs, this receptor does not activate G proteins in most cell types but recruits β-arrestins upon activation. ACKR3 plays an important role in cancer and vascular diseases. As recruitment of β-arrestins is triggered by phosphorylation of the C-terminal tail of GPCRs, we studied the role of different potential phosphorylation sites within the ACKR3 C-tail to further delineate the molecular mechanism of internalization and trafficking of this GPCR. Methods: We used various bioluminescence and fluorescence resonance energy transfer-based sensors and techniques in Human Embryonic Kidney (HEK) 293T cells expressing WT or phosphorylation site mutants of ACKR3 to measure CXCL12-induced recruitment of β-arrestins and G-protein-coupled receptor kinases (GRKs), receptor internalization and trafficking. Results: Upon CXCL12 stimulation, ACKR3 recruits both β-arrestin 1 and 2 with equivalent kinetic profiles. We identified interactions with GRK2, 3 and 5, with GRK2 and 3 being important for β-arrestin recruitment. Upon activation, ACKR3 internalizes and recycles back to the cell membrane. We demonstrate that β-arrestin recruitment to the receptor is mainly determined by a single cluster of phosphorylated residues on the C-tail of ACKR3, and that residue T³⁵² and in part S³⁵⁵ are important residues for β-arrestin1 recruitment. Phosphorylation of the C-tail appears essential for ligand-induced internalization and important for differential β-arrestin recruitment. GRK2 and 3 play a key role in receptor internalization. Moreover, ACKR3 can still internalize when β-arrestin recruitment is impaired or in the absence of β-arrestins, using alternative internalization pathways. Our data indicate that distinct residues within the C-tail of ACKR3 differentially regulate CXCL12-induced β-arrestin recruitment, ACKR3 trafficking and internalization.

Tissue factor pathway inhibitor upregulates CXCR7 expression and enhances CXCL12-mediated migration in chronic lymphocytic leukemia

The infiltration of chronic lymphocytic leukemia (CLL) cells into lymphoid organs correlates with disease severity. CXCL12 is a key chemotactic factor for the trafficking of CLL. Tissue factor pathway inhibitor (TFPI) is a serine protease inhibitor and plays a role in CXCL12-mediated hematopoietic stem cell homing. We aim to explore the role of TFPI in CXCL12-mediated migration of CLL cells. In this study, plasma TFPI concentrations were measured by ELISA. CLL cells were isolated from patients and used for trans-endothelial migration (TEM) assays. Quantitative RT-PCR and Western blotting were used to detect the expression of CXCR7, CXCR4 and β-catenin. Immunofluorescence and co-immunoprecipitation was used to detect the binding of TFPI and glypican-3 (GPC3). We found that plasma TFPI levels in CLL patients were higher than in healthy controls, particularly in the patients with advanced disease. TFPI enhanced CXCL12-mediated TEM of CLL cells by increasing the expression of the CXCL12 receptor CXCR7, but not of the CXCL12 receptor CXCR4. The effect of TFPI on TEM was abolished by the CXCR7 inhibitor, CCX771, while the CXCR4 inhibitor AMD3100 strongly increased TEM. TFPI co-localized with GPC3 on the cell surface. An antibody to GPC3, HS20, decreased CXCR7 expression and abolished the effect of TFPI on TEM. TFPI activated β-catenin and the Wnt/β-catenin inhibitor IWP4 repressed the effect of TFPI on CXCR7 expression and TEM. We conclude that TFPI may contribute to organ infiltration in CLL patients.

ACT-1004-1239, a first-in-class CXCR7 antagonist with both immunomodulatory and promyelinating effects for the treatment of inflammatory demyelinating diseases

Current strategies for the treatment of demyelinating diseases such as multiple sclerosis (MS) are based on anti-inflammatory or immunomodulatory drugs. Those drugs have the potential to reduce the frequency of new lesions but do not directly promote remyelination in the damaged central nervous system (CNS). Targeting CXCR7 (ACKR3) has been postulated as a potential therapeutic approach in demyelinating diseases, leading to both immunomodulation by reducing leukocyte infiltrates and promyelination by enhancing myelin repair. ACT-1004-1239 is a potent, selective, insurmountable, and orally available first-in-class CXCR7 receptor antagonist. The effect of ACT-1004-1239 was evaluated in the myelin oligodendrocyte glycoprotein (MOG)-induced experimental autoimmune encephalomyelitis (EAE) and the cuprizone-induced demyelination mouse models. In addition, ACT-1004-1239 was assessed in a rat oligodendrocyte precursor cell (OPC) differentiation assay in vitro. In the MOG-induced EAE model, ACT-1004-1239 treatment (10-100 mg/kg, twice daily, orally) showed a significant dose-dependent reduction in disease clinical scores, resulting in increased survival. At the highest dose tested (100 mg/kg, twice daily), ACT-1004-1239 delayed disease onset and significantly reduced immune cell infiltrates into the CNS and plasma neurofilament light chain concentration. Treatment with ACT-1004-1239 dose-dependently increased plasma CXCL12 concentration, which correlated with a reduction of the cumulative disease score. Furthermore, in the cuprizone model, ACT-1004-1239 treatment significantly increased the number of mature myelinating oligodendrocytes and enhanced myelination in vivo. In vitro, ACT-1004-1239 promoted the maturation of OPCs into myelinating oligodendrocytes. These results provide evidence that ACT-1004-1239 both reduces neuroinflammation and enhances myelin repair substantiating the rationale to explore its therapeutic potential in a clinical setting.

Beta-Arrestins and Receptor Signaling in the Vascular Endothelium

The vascular endothelium is the innermost layer of blood vessels and is a key regulator of vascular tone. Endothelial function is controlled by receptor signaling through G protein-coupled receptors, receptor tyrosine kinases and receptor serine-threonine kinases. The β-arrestins, multifunctional adapter proteins, have the potential to regulate all of these receptor families, although it is unclear as to whether they serve to integrate signaling across all of these different axes. Notably, the β-arrestins have been shown to regulate signaling by a number of receptors important in endothelial function, such as chemokine receptors and receptors for vasoactive substances such as angiotensin II, endothelin-1 and prostaglandins. β-arrestin-mediated signaling pathways have been shown to play central roles in pathways that control vasodilation, cell proliferation, migration, and immune function. At this time, the physiological impact of this signaling has not been studied in detail, but a deeper understanding of it could lead to the development of novel therapies for the treatment of vascular disease.

Discovery of the Potent, Selective, Orally Available CXCR7 Antagonist ACT-1004-1239

The chemokine receptor CXCR7, also known as ACKR3, is a seven-transmembrane G-protein-coupled receptor (GPCR) involved in various pathologies such as neurological diseases, autoimmune diseases, and cancers. By binding and scavenging the chemokines CXCL11 and CXCL12, CXCR7 regulates their extracellular levels. From an original high-throughput screening campaign emerged hit 3 among others. The hit-to-lead optimization led to the discovery of a novel chemotype series exemplified by the trans racemic compound 11i. This series provided CXCR7 antagonists that block CXCL11- and CXCL12-induced ß-arrestin recruitment. Further structural modifications on the trisubstituted piperidine scaffold of 11i yielded compounds with high CXCR7 antagonistic activities and balanced ADMET properties. The effort described herein culminated in the discovery of ACT-1004-1239 (28f). Biological characterization of ACT-1004-1239 demonstrated that it is a potent, insurmountable antagonist. Oral administration of ACT-1004-1239 in mice up to 100 mg/kg led to a dose-dependent increase of plasma CXCL12 concentration.

The Role of the CXCL12/CXCR4/CXCR7 Chemokine Axis in Cancer

Chemokines are a family of small, secreted cytokines which regulate a variety of cell functions. The C-X-C motif chemokine ligand 12 (CXCL12) binds to C-X-C chemokine receptor type 4 (CXCR4) and C-X-C chemokine receptor type 7 (CXCR7). The interaction of CXCL12 and its receptors subsequently induces downstream signaling pathways with broad effects on chemotaxis, cell proliferation, migration, and gene expression. Accumulating evidence suggests that the CXCL12/CXCR4/CXCR7 axis plays a pivotal role in tumor development, survival, angiogenesis, metastasis, and tumor microenvironment. In addition, this chemokine axis promotes chemoresistance in cancer therapy via complex crosstalk with other pathways. Multiple small molecules targeting CXCR4/CXCR7 have been developed and used for preclinical and clinical cancer treatment. In this review, we describe the roles of the CXCL12/CXCR4/CXCR7 axis in cancer progression and summarize strategies to develop novel targeted cancer therapies.

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.

Dynamical modeling predicts an inflammation-inducible CXCR7+ B cell precursor with potential implications in lymphoid blockage pathologies

Background

The blockage at the early B lymphoid cell development pathway within the bone marrow is tightly associated with hematopoietic and immune diseases, where the disruption of basal regulatory networks prevents the continuous replenishment of functional B cells. Dynamic computational models may be instrumental for the comprehensive understanding of mechanisms underlying complex differentiation processes and provide novel prediction/intervention platforms to reinvigorate the system.

Methods

By reconstructing a three-module regulatory network including genetic transcription, intracellular transduction, and microenvironment communication, we have investigated the early B lineage cell fate decisions in normal and pathological settings. The early B cell differentiation network was simulated as a Boolean model and then transformed, using fuzzy logic, to a continuous model. We tested null and overexpression mutants to analyze the emergent behavior of the network. Due to its importance in inflammation, we investigated the effect of NFkB induction at different early B cell differentiation stages.

Results

While the exhaustive synchronous and asynchronous simulation of the early B cell regulatory network (eBCRN) reproduced the configurations of the hematopoietic progenitors and early B lymphoid precursors of the pathway, its simulation as a continuous model with fuzzy logics suggested a transient IL-7R⁺ ProB-to-Pre-B subset expressing pre-BCR and a series of dominant B-cell transcriptional factors. This conspicuous differentiating cell population up-regulated CXCR7 and reduced CXCR4 and FoxO1 expression levels. Strikingly, constant but intermediate NFkB signaling at specific B cell differentiation stages allowed stabilization of an aberrant CXCR7⁺ pre-B like phenotype with apparent affinity to proliferative signals, while under constitutive overactivation of NFkB, such cell phenotype was aberrantly exacerbated from the earliest stage of common lymphoid progenitors. Our mutant models revealed an abnormal delay in the BCR assembly upon NFkB activation, concomitant to sustained Flt3 signaling, down-regulation of Ebf1, Irf4 and Pax5 genes transcription, and reduced Ig recombination, pointing to a potential lineage commitment blockage.

Discussion

For the first time, an inducible CXCR7^hi B cell precursor endowed with the potential capability of shifting central lymphoid niches, is inferred by computational modeling. Its phenotype is compatible with that of leukemia-initiating cells and might be the foundation that bridges inflammation with blockage-related malignancies and a wide range of immunological diseases. Besides the predicted differentiation impairment, inflammation-inducible phenotypes open the possibility of newly formed niches colonized by the reported precursor. Thus, emergent bone marrow ecosystems are predicted following a pro-inflammatory induction, that may lead to hematopoietic instability associated to blockage pathologies.

Dynamic and Differential Expression of Duplicated Cxcr4/Cxcl12 Genes Facilitates Antiviral Response in Hexaploid Gibel Carp

Chemokine receptor cxcr4 and its ligand cxcl12 have evolved two paralogs in the teleost lineage. In this study, we have identified four duplicated cxcr4 and cxcl12 genes from hexaploid gibel carp, Carassius gibelio, respectively. Cgcxcr4bs and Cgcxcl12as were dynamically and differentially expressed in immune-related tissues, and significantly up-regulated in head kidney and spleen after crucian carp herpesvirus (CaHV) infection. Blocking Cxcr4/Cxcl12 axis by injecting AMD3100 brought more severe bleeding symptom and lower survival rate in CaHV-infected fish. AMD3100 treatment also suppressed the up-regulation of key antiviral genes in head kidney and spleen, and resulted in more acute replication of CaHV in vivo. Consistently, the similar suppression of up-regulated expression of key antiviral genes were also observed in CAB cells treated by AMD3100 after poly(I:C) stimulation. Finally, MAPK3 and JAK/STAT were identified as the possible pathways that CgCxcr4s and CgCxcl12s participate in to promote the antiviral response in vitro.

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

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

Dual-action CXCR4-targeting liposomes in leukemia: function blocking and drug delivery

CXC chemokine receptor 4 (CXCR4) is overexpressed by a broad range of hematological disorders, and its interaction with CXC chemokine ligand 12 (CXCL12) is of central importance in the retention and chemoprotection of neoplastic cells in the bone marrow and lymphoid organs. In this article, we describe the biological evaluation of a new CXCR4-targeting and -antagonizing molecule (BAT1) that we designed and show that, when incorporated into a liposomal drug delivery system, it can be used to deliver cancer therapeutics at high levels to chronic lymphocytic leukemia (CLL) cells. CXCR4 targeting and antagonism by BAT1 were demonstrated alone and following its incorporation into liposomes (BAT1-liposomes). Antagonism of BAT1 against the CXCR4/CXCL12 interaction was demonstrated through signaling inhibition and function blocking: BAT1 reduced ERK phosphorylation and cell migration to levels equivalent to those seen in the absence of CXCL12 stimulation (P < .001). Specific uptake of BAT1-liposomes and delivery of a therapeutic cargo to the cell nucleus was seen within 3 hours of incubation and induced significantly more CLL cell death after 24 hours than control liposomes (P = .004). The BAT1 drug-delivery system is modular, versatile, and highly clinically relevant, incorporating elements of proven clinical efficacy. The combined capabilities to block CXCL12-induced migration and intracellular signaling while simultaneously delivering therapeutic cargo mean that the BAT1-liposome drug-delivery system could be a timely and relevant treatment of a range of hematological disorders, particularly because the therapeutic cargo can be tailored to the disease being treated.

Functions and mechanisms of chemokine receptor 7 in tumors of the digestive system

Chemokine (C-X-C motif) receptor 7 (CXCR7), recently termed ACKR3, belongs to the G protein-coupled cell surface receptor family, binds to stromal cell-derived factor-1 [SDF-1, or chemokine (C-X-C motif) ligand 12] or chemokine (C-X-C motif) ligand 11, and is the most common chemokine receptor expressed in a variety of cancer cells. SDF-1 binds to its receptor chemokine (C-X-C motif) receptor 4 (CXCR4) and regulates cell proliferation, survival, angiogenesis and migration. In recent years, another new receptor for SDF-1, CXCR7, has been discovered, and CXCR7 has also been found to be expressed in a variety of tumor cells and tumor-related vascular endothelial cells. Many studies have shown that CXCR7 can promote the growth and metastasis of a variety of malignant tumor cells. Unlike CXCR4, CXCR7 exhibits a slight modification in the DRYLAIV motif and does not induce intracellular Ca²⁺ release following ligand binding, which is essential for recruiting and activating G proteins. CXCR7 is generally thought to work in three ways: (1) Recruiting β-arrestin 2; (2) Heterodimerizing with CXCR4; and (3) Acting as a “scavenger” of SDF-1, thus lowering the level of SDF-1 to weaken the activity of CXCR4. In the present review, the expression and role of CXCR7, as well as its prognosis in cancers of the digestive system, were investigated.

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.

The Role of Selected Chemokines and Their Receptors in the Development of Gliomas

Among heterogeneous primary tumors of the central nervous system (CNS), gliomas are the most frequent type, with glioblastoma multiforme (GBM) characterized with the worst prognosis. In their development, certain chemokine/receptor axes play important roles and promote proliferation, survival, metastasis, and neoangiogenesis. However, little is known about the significance of atypical receptors for chemokines (ACKRs) in these tumors. The objective of the study was to present the role of chemokines and their conventional and atypical receptors in CNS tumors. Therefore, we performed a thorough search for literature concerning our investigation via the PubMed database. We describe biological functions of chemokines/chemokine receptors from various groups and their significance in carcinogenesis, cancer-related inflammation, neo-angiogenesis, tumor growth, and metastasis. Furthermore, we discuss the role of chemokines in glioma development, with particular regard to their function in the transition from low-grade to high-grade tumors and angiogenic switch. We also depict various chemokine/receptor axes, such as CXCL8-CXCR1/2, CXCL12-CXCR4, CXCL16-CXCR6, CX3CL1-CX3CR1, CCL2-CCR2, and CCL5-CCR5 of special importance in gliomas, as well as atypical chemokine receptors ACKR1-4, CCRL2, and PITPMN3. Additionally, the diagnostic significance and usefulness of the measurement of some chemokines and their receptors in the blood and cerebrospinal fluid (CSF) of glioma patients is also presented.

Single-cell analysis of murine fibroblasts identifies neonatal to adult switching that regulates cardiomyocyte maturation

Cardiac maturation lays the foundation for postnatal heart development and disease, yet little is known about the contributions of the microenvironment to cardiomyocyte maturation. By integrating single-cell RNA-sequencing data of mouse hearts at multiple postnatal stages, we construct cellular interactomes and regulatory signaling networks. Here we report switching of fibroblast subtypes from a neonatal to adult state and this drives cardiomyocyte maturation. Molecular and functional maturation of neonatal mouse cardiomyocytes and human embryonic stem cell-derived cardiomyocytes are considerably enhanced upon co-culture with corresponding adult cardiac fibroblasts. Further, single-cell analysis of in vivo and in vitro cardiomyocyte maturation trajectories identify highly conserved signaling pathways, pharmacological targeting of which substantially delays cardiomyocyte maturation in postnatal hearts, and markedly enhances cardiomyocyte proliferation and improves cardiac function in infarcted hearts. Together, we identify cardiac fibroblasts as a key constituent in the microenvironment promoting cardiomyocyte maturation, providing insights into how the manipulation of cardiomyocyte maturity may impact on disease development and regeneration.

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

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

ACKR4 Recruits GRK3 Prior to β-Arrestins but Can Scavenge Chemokines in the Absence of β-Arrestins

Chemokines are essential for guiding cell migration. Atypical chemokine receptors (ACKRs) contribute to the cell migration process by binding, internalizing and degrading local chemokines, which enables the formation of confined gradients. ACKRs are heptahelical membrane spanning molecules structurally related to G-protein coupled receptors (GPCRs), but seem to be unable to signal through G-proteins upon ligand binding. ACKR4 internalizes the chemokines CCL19, CCL21, and CCL25 and is best known for shaping functional CCL21 gradients. Ligand binding to ACKR4 has been shown to recruit β-arrestins that has led to the assumption that chemokine scavenging relies on β-arrestin-mediated ACKR4 trafficking, a common internalization route taken by class A GPCRs. Here, we show that CCL19, CCL21, and CCL25 readily recruited β-arrestin1 and β-arrestin2 to human ACKR4, but found no evidence for β-arrestin-dependent or independent ACKR4-mediated activation of the kinases Erk1/2, Akt, or Src. However, we demonstrate that β-arrestins interacted with ACKR4 in the steady-state and contributed to the spontaneous trafficking of the receptor in the absence of chemokines. Deleting the C-terminus of ACKR4 not only interfered with the interaction of β-arrestins, but also with the uptake of fluorescently labeled cognate chemokines. We identify the GPCR kinase GRK3, and to a lesser extent GRK2, but not GRK4, GRK5, and GRK6, to be recruited to chemokine-stimulated ACKR4. We show that GRK3 recruitment proceded the recruitment of β-arrestins upon ACKR4 engagement and that GRK2/3 inhibition partially interfered with steady-state interaction and chemokine-driven recruitment of β-arrestins to ACKR4. Overexpressing β-arrestin2 accelerated the uptake of fluorescently labeled CCL19, indicating that β-arrestins contribute to the chemokine scavenging activity of ACKR4. By contrast, cells lacking β-arrestins were still capable to take up fluorescently labeled CCL19 demonstrating that β-arrestins are dispensable for chemokine scavenging by ACKR4.

ACKR3 Regulation of Neuronal Migration Requires ACKR3 Phosphorylation, but Not β-Arrestin

Phosphorylation of heptahelical receptors is thought to regulate G protein signaling, receptor endocytosis, and non-canonical signaling via recruitment of β-arrestins. We investigated chemokine receptor functionality under phosphorylation-deficient and β-arrestin-deficient conditions by studying interneuron migration in the embryonic cortex. This process depends on CXCL12, CXCR4, G protein signaling and on the atypical CXCL12 receptor ACKR3. We found that phosphorylation was crucial, whereas β-arrestins were dispensable for ACKR3-mediated control of CXCL12 levels in vivo. Cortices of mice expressing phosphorylation-deficient ACKR3 exhibited a major interneuron migration defect, which was accompanied by excessive activation and loss of CXCR4. Cxcl12-overexpressing mice mimicked this phenotype. Excess CXCL12 caused lysosomal CXCR4 degradation, loss of CXCR4 responsiveness, and, ultimately, similar motility defects as Cxcl12 deficiency. By contrast, β-arrestin deficiency caused only a subtle migration defect mimicked by CXCR4 gain of function. These findings demonstrate that phosphorylation regulates atypical chemokine receptor function without β-arrestin involvement in chemokine sequestration and non-canonical signaling.