Single-molecule imaging reveals dimerization/oligomerization of CXCR4 on plasma membrane closely related to its function

Identifying and Assessing Putative Allosteric Sites and Modulators for CXCR4 Predicted through Network Modeling and Site Identification by Ligand Competitive Saturation

The chemokine receptor CXCR4 is a critical target for the treatment of several cancer types and HIV-1 infections. While orthosteric and allosteric modulators have been developed targeting its extracellular or transmembrane regions, the intramembrane region of CXCR4 may also include allosteric binding sites suitable for the development of allosteric drugs. To investigate this, we apply the Gaussian Network Model (GNM) to the monomeric and dimeric forms of CXCR4 to identify residues essential for its local and global motions located in the hinge regions of the protein. Residue interaction network (RIN) analysis suggests hub residues that participate in allosteric communication throughout the receptor. Mutual residues from the network models reside in regions with a high capacity to alter receptor dynamics upon ligand binding. We then investigate the druggability of these potential allosteric regions using the site identification by ligand competitive saturation (SILCS) approach, revealing two putative allosteric sites on the monomer and three on the homodimer. Two screening campaigns with Glide and SILCS-Monte Carlo docking using FDA-approved drugs suggest 20 putative hit compounds including antifungal drugs, anticancer agents, HIV protease inhibitors, and antimalarial drugs. In vitro assays considering mAB 12G5 and CXCL12 demonstrate both positive and negative allosteric activities of these compounds, supporting our computational approach. However, in vivo functional assays based on the recruitment of β-arrestin to CXCR4 do not show significant agonism and antagonism at a single compound concentration. The present computational pipeline brings a new perspective to computer-aided drug design by combining conformational dynamics based on network analysis and cosolvent analysis based on the SILCS technology to identify putative allosteric binding sites using CXCR4 as a showcase.

The β2-adrenergic receptor associates with CXCR4 multimers in human cancer cells

While the existence and functional role of class C G-protein-coupled receptors (GPCR) dimers is well established, there is still a lack of consensus regarding class A and B GPCR multimerization. This lack of consensus is largely due to the inherent challenges of demonstrating the presence of multimeric receptor complexes in a physiologically relevant cellular context. The C-X-C motif chemokine receptor 4 (CXCR4) is a class A GPCR that is a promising target of anticancer therapy. Here, we investigated the potential of CXCR4 to form multimeric complexes with other GPCRs and characterized the relative size of the complexes in a live-cell environment. Using a bimolecular fluorescence complementation (BiFC) assay, we identified the β2 adrenergic receptor (β2AR) as an interaction partner. To investigate the molecular scale details of CXCR4-β2AR interactions, we used a time-resolved fluorescence spectroscopy method called pulsed-interleaved excitation fluorescence cross-correlation spectroscopy (PIE-FCCS). PIE-FCCS can resolve membrane protein density, diffusion, and multimerization state in live cells at physiological expression levels. We probed CXCR4 and β2AR homo- and heteromultimerization in model cell lines and found that CXCR4 assembles into multimeric complexes larger than dimers in MDA-MB-231 human breast cancer cells and in HCC4006 human lung cancer cells. We also found that β2AR associates with CXCR4 multimers in MDA-MB-231 and HCC4006 cells to a higher degree than in COS-7 and CHO cells and in a ligand-dependent manner. These results suggest that CXCR4-β2AR heteromers are present in human cancer cells and that GPCR multimerization is significantly affected by the plasma membrane environment.

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.

Single-Molecule Imaging Reveals Differential AT1R Stoichiometry Change in Biased Signaling

G protein-coupled receptors (GPCRs) represent promising therapeutic targets due to their involvement in numerous physiological processes mediated by downstream G protein- and β-arrestin-mediated signal transduction cascades. Although the precise control of GPCR signaling pathways is therapeutically valuable, the molecular details for governing biased GPCR signaling remain elusive. The Angiotensin II type 1 receptor (AT1R), a prototypical class A GPCR with profound implications for cardiovascular functions, has become a focal point for biased ligand-based clinical interventions. Herein, we used single-molecule live-cell imaging techniques to evaluate the changes in stoichiometry and dynamics of AT1R with distinct biased ligand stimulations in real time. It was revealed that AT1R existed predominantly in monomers and dimers and underwent oligomerization upon ligand stimulation. Notably, β-arrestin-biased ligands induced the formation of higher-order aggregates, resulting in a slower diffusion profile for AT1R compared to G protein-biased ligands. Furthermore, we demonstrated that the augmented aggregation of AT1R, triggered by activation from each biased ligand, was completely abrogated in β-arrestin knockout cells. These findings furnish novel insights into the intricate relationship between GPCR aggregation states and biased signaling, underscoring the pivotal role of molecular behaviors in guiding the development of selective therapeutic agents.

Biased signaling due to oligomerization of the G protein-coupled platelet-activating factor receptor

G protein-coupled receptors (GPCRs) are important drug targets that mediate various signaling pathways by activating G proteins and engaging β-arrestin proteins. Despite its importance for the development of therapeutics with fewer side effects, the underlying mechanism that controls the balance between these signaling modes of GPCRs remains largely unclear. Here, we show that assembly into dimers and oligomers can largely influence the signaling mode of the platelet-activating factor receptor (PAFR). Single-particle analysis results show that PAFR can form oligomers at low densities through two possible dimer interfaces. Stabilization of PAFR oligomers through cross-linking increases G protein activity, and decreases β-arrestin recruitment and agonist-induced internalization significantly. Reciprocally, β-arrestin prevents PAFR oligomerization. Our results highlight a mechanism involved in the control of receptor signaling, and thereby provide important insights into the relationship between GPCR oligomerization and downstream signaling.

Targeting CXCR4 and CD47 Receptors: An Overview of New and Old Molecules for a Biological Personalized Anticancer Therapy

Biological therapy, with its multifaceted applications, has revolutionized the treatment of tumors, mainly due to its ability to exclusively target cancer cells and reduce the adverse effects on normal tissues. This review focuses on the therapies targeting the CXCR4 and CD47 receptors. We surveyed the results of early clinical trials testing compounds classified as nonpeptides, small peptides, CXCR4 antagonists or specific antibodies whose activity reduces or completely blocks the intracellular signaling pathways and cell proliferation. We then examined antibodies and fusion proteins against CD47, the receptor that acts as a “do not eat me” signal to phagocytes escaping immune surveillance. Despite these molecules being tested in early clinical trials, some drawbacks are emerging that impair their use in practice. Finally, we examined the ImmunoGenic Surrender mechanism that involves crosstalk and co-internalization of CXCR4 and CD47 upon engagement of CXCR4 by ligands or other molecules. The favorable effect of such compounds is dual as CD47 surface reduction impact on the immune response adds to the block of CXCR4 proliferative potential. These results suggest that a combination of different therapeutic approaches has more beneficial effects on patients’ survival and may pave the way for new accomplishments in personalized anticancer therapy.

Dimerization of the Peptide CXCR4-Antagonist on Macromolecular and Supramolecular Protraction Arms Affords Increased Potency and Enhanced Plasma Stability

Peptides are prime drug candidates due to their high specificity of action but are disadvantaged by low proteolytic stability. Here, we focus on the development of stabilized analogues of EPI-X4, an endogenous peptide antagonist of CXCR4. We synthesized macromolecular peptide conjugates and performed side-by-side comparison with their albumin-binding counterparts and considered monovalent conjugates, divalent telechelic conjugates, and Y-shaped peptide dimers. All constructs were tested for competition with the CXCR4 antibody-receptor engagement, inhibition of receptor activation, and inhibition of the CXCR4-tropic human immunodeficiency virus infection. We found that the Y-shaped conjugates were more potent than the parent peptide and at the same time more stable in human plasma, with a favorable outlook for translational studies.

Carbon Dot Blinking Fingerprint Uncovers Native Membrane Receptor Organizations via Deep Learning

Oligomeric organization of G protein-coupled receptors is proposed to regulate receptor signaling and function, yet rapid and precise identification of the oligomeric status especially for native receptors on a cell membrane remains an outstanding challenge. By using blinking carbon dots (CDs), we now develop a deep learning (DL)-based blinking fingerprint recognition method, named deep-blinking fingerprint recognition (BFR), which allows automatic classification of CD-labeled receptor organizations on a cell membrane. This DL model integrates convolutional layers, long-short-term memory, and fully connected layers to extract time-dependent blinking features of CDs and is trained to a high accuracy (∼95%) for identifying receptor organizations. Using deep blinking fingerprint recognition, we found that CXCR4 mainly exists as 87.3% monomers, 12.4% dimers, and <1% higher-order oligomers on a HeLa cell membrane. We further demonstrate that the heterogeneous organizations can be regulated by various stimuli at different degrees. The receptor-binding ligands, agonist SDF-1α and antagonist AMD3100, can induce the dimerization of CXCR4 to 33.1 and 20.3%, respectively. In addition, cytochalasin D, which inhibits actin polymerization, similarly prompts significant dimerization of CXCR4 to 30.9%. The multi-pathway organization regulation will provide an insight for understanding the oligomerization mechanism of CXCR4 as well as for elucidating their physiological functions.

Probing GPCR Dimerization Using Peptides

G protein-coupled receptors (GPCRs) are the largest class of membrane proteins and the most common and extensively studied pharmacological target. Numerous studies over the last decade have confirmed that GPCRs do not only exist and function in their monomeric form but in fact, have the ability to form dimers or higher order oligomers with other GPCRs, as well as other classes of receptors. GPCR oligomers have become increasingly attractive to investigate as they have the ability to modulate the pharmacological responses of the receptors which in turn, could have important functional roles in diseases, such as cancer and several neurological & neuropsychiatric disorders. Despite the growing evidence in the field of GPCR oligomerisation, the lack of structural information, as well as targeting the 'undruggable' protein-protein interactions (PPIs) involved in these complexes, has presented difficulties. Outside the field of GPCRs, targeting PPIs has been widely studied, with a variety of techniques being investigated; from small-molecule inhibitors to disrupting peptides. In this review, we will demonstrate several physiologically relevant GPCR dimers and discuss an array of strategies and techniques that can be employed when targeting these complexes, as well as provide ideas for future development.

Polyfunctionality of the CXCR4/CXCL12 axis in health and disease: Implications for therapeutic interventions in cancer and immune-mediated diseases

Historically the chemokine receptor CXCR4 and its canonical ligand CXCL12 are associated with the bone marrow niche and hematopoiesis. However, CXCL12 exhibits broad tissue expression including brain, thymus, heart, lung, liver, kidney, spleen, and bone marrow. CXCR4 can be considered as a node which is integrating and transducing inputs from a range of ligand-receptor interactions into a responsive and divergent network of intracellular signaling pathways that impact multiple cellular processes such as proliferation, migration, and stress resistance. Dysregulation of the CXCR4/CXCL12 axis and consequent fundamental cellular processes, are associated with a panoply of disease. This review frames the polyfunctionality of the receptor at a molecular, physiological, and pathophysiological levels. Transitioning our perspective of this axis from a single gene/protein:single function model to a polyfunctional signaling cascade highlights the potential for finer therapeutic intervention and cautions against a reductionist approach.

Deep-Learning-Assisted Single-Molecule Tracking on a Live Cell Membrane

Single-molecule fluorescence imaging is a powerful tool to study protein function by tracking molecular position and distribution, but the precise and rapid identification of dynamic molecules remains challenging due to the heterogeneous distribution and interaction of proteins on the live cell membrane. We now develop a deep-learning (DL)-assisted single-molecule imaging method that can precisely distinguish the monomer and complex for rapid and real-time tracking of protein interaction. This DL-based model, which comprises convolutional layers, max pooling layers, and fully connected layers, is trained to reach an accuracy of >98% for identifying monomer and complex. We use this method to investigate the dynamic process of chemokine receptor CXCR4 on the live cell membrane during the early signaling stage. The results show that, upon ligand activation, the CXCR4 undergoes a dynamic process of forming a receptor complex. We further demonstrate that the CXCR4 complex tends to be internalized at 2.5-fold higher rate into the cell interior than the monomer via the clathrin-dependent pathway. This study is the first example to scrutinize the early signaling process of CXCR4 at the single-molecule level on the live cell membrane. We envision that this DL-assisted imaging method would be a broadly useful technique to study more protein families for elucidating their physiological and pathological functions.

At the Bench: Pre-clinical evidence for multiple functions of CXCR4 in cancer

Signaling through chemokine receptor, C-X-C chemokine receptor type 4 (CXCR4) regulates essential processes in normal physiology, including embryogenesis, tissue repair, angiogenesis, and trafficking of immune cells. Tumors co-opt many of these fundamental processes to directly stimulate proliferation, invasion, and metastasis of cancer cells. CXCR4 signaling contributes to critical functions of stromal cells in cancer, including angiogenesis and multiple cell types in the tumor immune environment. Studies in animal models of several different types of cancers consistently demonstrate essential functions of CXCR4 in tumor initiation, local invasion, and metastasis to lymph nodes and distant organs. Data from animal models support clinical observations showing that integrated effects of CXCR4 on cancer and stromal cells correlate with metastasis and overall poor prognosis in >20 different human malignancies. Small molecules, Abs, and peptidic agents have shown anticancer efficacy in animal models, sparking ongoing efforts at clinical translation for cancer therapy. Investigators also are developing companion CXCR4-targeted imaging agents with potential to stratify patients for CXCR4-targeted therapy and monitor treatment efficacy. Here, pre-clinical studies demonstrating functions of CXCR4 in cancer are reviewed.

Structural Characterization of Receptor-Receptor Interactions in the Allosteric Modulation of G Protein-Coupled Receptor (GPCR) Dimers

G protein-coupled receptor (GPCR) oligomerization, while contentious, continues to attract the attention of researchers. Numerous experimental investigations have validated the presence of GPCR dimers, and the relevance of dimerization in the effectuation of physiological functions intensifies the attractiveness of this concept as a potential therapeutic target. GPCRs, as a single entity, have been the main source of scrutiny for drug design objectives for multiple diseases such as cancer, inflammation, cardiac, and respiratory diseases. The existence of dimers broadens the research scope of GPCR functions, revealing new signaling pathways that can be targeted for disease pathogenesis that have not previously been reported when GPCRs were only viewed in their monomeric form. This review will highlight several aspects of GPCR dimerization, which include a summary of the structural elucidation of the allosteric modulation of class C GPCR activation offered through recent solutions to the three-dimensional, full-length structures of metabotropic glutamate receptor and γ-aminobutyric acid B receptor as well as the role of dimerization in the modification of GPCR function and allostery. With the growing influence of computational methods in the study of GPCRs, we will also be reviewing recent computational tools that have been utilized to map protein-protein interactions (PPI).

Understanding the impacts of cellular environments on ligand binding of membrane receptors by computational simulations

Binding of cell surface receptors with their extracellular ligands initiates various intracellular signaling pathways. However, our understanding of the cellular functions of these receptors is very limited due to the fact that in vivo binding between ligands and receptors has only been successfully measured in a very small number of cases. In living cells, receptors are anchored on surfaces of the plasma membrane, which undergoes thermal undulations. Moreover, it has been observed in various systems that receptors can be organized into oligomers prior to ligand binding. It is not well understood how these cellular factors play roles in regulating the dynamics of ligand-receptor interactions. Here, we tackled these problems by using a coarse-grained kinetic Monte Carlo simulation method. Using this method, we demonstrated that the membrane undulations cause a negative effect on ligand-receptor interactions. We further found that the preassembly of membrane receptors on the cell surface can not only accelerate the kinetics of ligand binding but also reduce the noises during the process. In general, our study highlights the importance of membrane environments in regulating the function of membrane receptors in cells. The simulation method can be potentially applied to specific receptor systems involved in cell signaling.

Workflows of the Single-Molecule Imaging Analysis in Living Cells: Tutorial Guidance to the Measurement of the Drug Effects on a GPCR

Single-molecule imaging (SMI) is a powerful method to measure the dynamics of membrane proteins on the cell membrane. The single-molecule tracking (SMT) analysis provides information about the diffusion dynamics, the oligomer size distribution, and the particle density change. The affinity and on/off-rate of a protein-protein interaction can be estimated from the dual-color SMI analysis. However, it is difficult for trainees to determine quantitative information from the SMI movies. The present protocol guides the detailed workflows to measure the drug-activated dynamics of a G protein-coupled receptor (GPCR) and metabotropic glutamate receptor 3 (mGluR3), by using the total internal reflection fluorescence microscopy (TIRFM). This tutorial guidance comprises an open-source software, named smDynamicsAnalyzer, with which one can easily analyze the SMT dataset by just following the workflows after building a designated folder structure ( https://github.com/masataka-yanagawa/IgorPro8-smDynamicsAnalyzer ).

Evaluation of histone deacetylase inhibitor substituted zinc and indium phthalocyanines for chemo- and photodynamic therapy

In this study, we synthesized and characterized 3-hydroxypyridin-2-thione (3-HPT) bearing zinc (ZnPc-1 and ZnPc-2) and indium (InPc-1 and InPc-2) phthalocyanine (Pc) derivatives, either non-peripherally or peripherally substituted as photosensitizer (PS) agents and evaluated their anti-cancer efficacy on two breast cancer cell lines, MDA-MB-231 and MCF-7 as well as a human endothelial cell line, HUVEC. Our results indicated different localization patterns between ZnPcs and InPcs in addition to enhanced effects on the mitochondrial network for InPcs. Moreover, peripheral or non-peripheral substitution of HDACi moieties altered cellular localization between ZnPc-1 and ZnPc-2, leading to increased IC₅₀ values along with decreased anti-cancer activity for non-peripheral substitution. When considering the compounds' differential effects in vitro, our data indicates that further research is required to determine the ideal Pcs for anti-cancer PDT treatments since the core metals of the compounds have affected the cellular localization, and positioning of the chemotherapeutic residues may inhibit cellular penetrance.

3-Hydroxypyridin-2-thione bearing zinc and indium phthalocyanine derivatives, as photosensitizer agents have been synthesized and evaluated for their anti-cancer efficacy on two breast cancer cell lines, MDA-MB-231 and MCF-7 as well as a human endothelial cell line, HUVEC.

G protein-coupled receptor-G protein interactions: a single-molecule perspective

G protein-coupled receptors (GPCRs) regulate many cellular and physiological processes, responding to a diverse range of extracellular stimuli including hormones, neurotransmitters, odorants, and light. Decades of biochemical and pharmacological studies have provided fundamental insights into the mechanisms of GPCR signaling. Thanks to recent advances in structural biology, we now possess an atomistic understanding of receptor activation and G protein coupling. However, how GPCRs and G proteins interact in living cells to confer signaling efficiency and specificity remains insufficiently understood. The development of advanced optical methods, including single-molecule microscopy, has provided the means to study receptors and G proteins in living cells with unprecedented spatio-temporal resolution. The results of these studies reveal an unexpected level of complexity, whereby GPCRs undergo transient interactions among themselves as well as with G proteins and structural elements of the plasma membrane to form short-lived signaling nanodomains that likely confer both rapidity and specificity to GPCR signaling. These findings may provide new strategies to pharmaceutically modulate GPCR function, which might eventually pave the way to innovative drugs for common diseases such as diabetes or heart failure.

Endocytosis and Trafficking of Heparan Sulfate Proteoglycans in Triple-Negative Breast Cancer Cells Unraveled with a Polycationic Peptide

The process of heparan sulfate proteoglycan (HSPG) internalization has been described as following different pathways. The tumor-specific branched NT4 peptide has been demonstrated to bind HSPGs on the plasma membrane and to be internalized in tumor cell lines. The polycationic peptide has been also shown to impair migration of different cancer cell lines in 2D and 3D models. Our hypothesis was that HSPG endocytosis could affect two important phenomena of cancer development: cell migration and nourishment. Using NT4 as an experimental tool mimicking heparin-binding ligands, we studied endocytosis and trafficking of HSPGs in a triple-negative human breast cancer cell line, MDA-MB-231. The peptide entered cells employing caveolin- or clathrin-dependent endocytosis and macropinocytosis, in line with what is already known about HSPGs. NT4 then localized in early and late endosomes in a time-dependent manner. The peptide had a negative effect on CDC42-activation triggered by EGF. The effect can be explained if we consider NT4 a competitive inhibitor of EGF on HS that impairs the co-receptor activity of the proteoglycan, reducing EGFR activation. Reduction of the invasive migratory phenotype of MDA-MB-231 induced by NT4 can be ascribed to this effect. RhoA activation was damped by EGF in MDA-MB-231. Indeed, EGF reduced RhoA-GTP and NT4 did not interfere with this receptor-mediated signaling. On the other hand, the peptide alone determined a small but solid reduction in active RhoA in breast cancer cells. This result supports the observation of few other studies, showing direct activation of the GTPase through HSPG, not mediated by EGF/EGFR.

Interclass GPCR heteromerization affects localization and trafficking

Membrane trafficking processes regulate G protein-coupled receptor (GPCR) activity. Although class A GPCRs are capable of activating G proteins in a monomeric form, they can also potentially assemble into functional GPCR heteromers. Here, we showed that the class A serotonin 5-HT2A receptors (5-HT2ARs) affected the localization and trafficking of class C metabotropic glutamate receptor 2 (mGluR2) through a mechanism that required their assembly as heteromers in mammalian cells. In the absence of agonists, 5-HT2AR was primarily localized within intracellular compartments, and coexpression of 5-HT2AR with mGluR2 increased the intracellular distribution of the otherwise plasma membrane-localized mGluR2. Agonists for either 5-HT2AR or mGluR2 differentially affected trafficking through Rab5-positive endosomes in cells expressing each component of the 5-HT2AR-mGluR2 heterocomplex alone, or together. In addition, overnight pharmacological 5-HT2AR blockade with clozapine, but not with M100907, decreased mGluR2 density through a mechanism that involved heteromerization between 5-HT2AR and mGluR2. Using TAT-tagged peptides and chimeric constructs that are unable to form the interclass 5-HT2AR-mGluR2 complex, we demonstrated that heteromerization was necessary for the 5-HT2AR-dependent effects on mGluR2 subcellular distribution. The expression of 5-HT2AR also augmented intracellular localization of mGluR2 in mouse frontal cortex pyramidal neurons. Together, our data suggest that GPCR heteromerization may itself represent a mechanism of receptor trafficking and sorting.

Illuminating the Path to Target GPCR Structures and Functions

G-Protein-coupled receptors (GPCRs) are ubiquitous within eukaryotes, responsible for a wide array of physiological and pathological processes. Indeed, the fact that they are the most drugged target in the human genome is indicative of their importance. Despite the clear interest in GPCRs, most information regarding their activity has been so far obtained by analyzing the response from a "bulk medium". As such, this Perspective summarizes some of the common methods for this indirect observation. Nonetheless, by inspecting approaches applying super-resolution imaging, we argue that imaging is perfectly situated to obtain more detailed structural and spatial information, assisting in the development of new GPCR-targeted drugs and clinical strategies. The benefits of direct optical visualization of GPCRs are analyzed in the context of potential future directions in the field.

Functions of the CXCL12 Receptor ACKR3/CXCR7-What Has Been Perceived and What Has Been Overlooked

The CXCL12 system is central to the development of many organs and is further crucially engaged in pathophysiological processes underlying cancer, inflammation, and cardiovascular disorders. This disease-associated role presently focuses major interest on the two CXCL12 receptors, CXCR4 and atypical chemokine receptor 3 (ACKR3)/CXCR7, as promising therapeutic targets. Major obstacles in these ongoing efforts are confusing reports on the differential use of either ACKR3/CXCR7 and/or CXCR4 across various cells as well as on the specific function(s) of ACKR3/CXCR7. Although basically no doubts remain that CXCR4 represents a classic chemokine receptor, functions assigned to ACKR3/CXCR7 range from those of a strictly silent scavenger receptor eventually modulating CXCR4 signaling to an active and independent signaling receptor. In this review, we depict a thorough analysis of our present knowledge on different modes of organization and functions of the cellular CXCL12 system. We further highlight the potential role of ACKR3/CXCR7 as a "crosslinker" of different receptor systems. Finally, we discuss mechanisms with the potency to impinge on the cellular organization of the CXCL12 system and hence might represent additional future therapeutic targets. SIGNIFICANCE STATEMENT: Delineating the recognized functions of atypical chemokine receptor 3 and CXCR4 in CXCL12 signaling is central to the more detailed understanding of the role of the CXCL12 system in health and disease and will help to guide future research efforts.

Structural Complexity and Plasticity of Signaling Regulation at the Melanocortin-4 Receptor

The melanocortin-4 receptor (MC4R) is a class A G protein-coupled receptor (GPCR), essential for regulation of appetite and metabolism. Pathogenic inactivating MC4R mutations are the most frequent cause of monogenic obesity, a growing medical and socioeconomic problem worldwide. The MC4R mediates either ligand-independent or ligand-dependent signaling. Agonists such as α-melanocyte-stimulating hormone (α-MSH) induce anorexigenic effects, in contrast to the endogenous inverse agonist agouti-related peptide (AgRP), which causes orexigenic effects by suppressing high basal signaling activity. Agonist action triggers the binding of different subtypes of G proteins and arrestins, leading to concomitant induction of diverse intracellular signaling cascades. An increasing number of experimental studies have unraveled molecular properties and mechanisms of MC4R signal transduction related to physiological and pathophysiological aspects. In addition, the MC4R crystal structure was recently determined at 2.75 Å resolution in an inactive state bound with a peptide antagonist. Underpinned by structural homology models of MC4R complexes simulating a presumably active-state conformation compared to the structure of the inactive state, we here briefly summarize the current understanding and key players involved in the MC4R switching process between different activity states. Finally, these perspectives highlight the complexity and plasticity in MC4R signaling regulation and identify gaps in our current knowledge.

Monitoring Allosteric Interactions with CXCR4 Using NanoBiT Conjugated Nanobodies

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

Cholesterol-mediated oligomerization pathways of serotonin G-coupled receptor 5-HT2C

Serotonin (5-HT) receptors have been shown to homodimerize and heterodimerize with other G protein-coupled receptors (GPCRs), although the details of this process have not yet been elucidated. Here we use coarse-grained molecular dynamics on monomeric 5-HT2C receptors to predict the transmembrane (TM) helices involved in such associations. All these simulations were carried out both in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid bilayers and in mixed composition POPC-Cholesterol ones, to show whether the presence of cholesterol could directly influence and drive the dimeric association. The goal is to get insights on the self-assembly pathway leading to GPCRs 5-HT2C oligomerization, which is supposed to be the basis of its constitutional activity. From the analysis of the molecular dynamics trajectories, we observed the formation of 5-HT2C oligomers through self-assembly and we identified the main domains involved in the receptor dimerization. In particular, dimers and oligomers from the two different environments show TM4-TM5 and TM1-TM7-H8 as the preferential dimerization interfaces. Nevertheless, substantial differences arise for oligomers in POPC and in POPC-Chol membranes: in POPC-Chol the variability of dimers interfaces is strictly limited to the TM1-TM7-H8 and TM4-TM5 interfaces and the dimorphism depends on cholesterol that directly participates in its formation. These results are in agreement with both experimental evidences and other computational studies conducted on other GPCRs oligomerization.

The multilayered complexity of the chemokine receptor system

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

Characterization of heteromeric complexes between chemokine (C-X-C motif) receptor 4 and α1-adrenergic receptors utilizing intermolecular bioluminescence resonance energy transfer assays

Recently, we reported that chemokine (C-X-C motif) receptor 4 (CXCR4) heteromerizes with α₁-adrenergic receptors (AR) on the cell surface of vascular smooth muscle cells, through which the receptors cross-talk. Direct biophysical evidence for CXCR4:α₁-AR heteromers, however, is lacking. Here we utilized bimolecular luminescence/fluorescence complementation (BiLC/BiFC) combined with intermolecular bioluminescence resonance energy transfer (BRET) assays in HEK293T cells to evaluate CXCR4:α_1a/b/d-AR heteromerization. Atypical chemokine receptor 3 (ACKR3) and metabotropic glutamate receptor 1 (mGlu₁R) were utilized as controls. BRET between CXCR4-RLuc (Renilla reniformis) and enhanced yellow fluorescent protein (EYFP)-tagged ACKR3 or α_1a/b/d-ARs fulfilled criteria for constitutive heteromerization. BRET between CXCR4-RLuc and EYFP or mGlu₁R-EYFP were nonspecific. BRET₅₀ for CXCR4:ACKR3 and CXCR4:α_1a/b/d-AR heteromers were comparable. Stimulation of cells with phenylephrine increased BRET_max of CXCR4:α_1a/b/d-AR heteromers without affecting BRET₅₀; stimulation with CXCL12 reduced BRET_max of CXCR4:α_1a-AR heteromers, but did not affect BRET₅₀ or BRET_max/50 for CXCR4:α_1b/d-AR. A peptide analogue of transmembrane domain (TM) 2 of CXCR4 reduced BRET_max of CXCR4:α_1a/b/d-AR heteromers and increased BRET₅₀ of CXCR4:α_1a/b-AR interactions. A TM4 analogue of CXCR4 did not alter BRET. We observed CXCR4, α_1a-AR and mGlu₁R homodimerization by BiFC/BiLC, and heteromerization of homodimeric CXCR4 with proto- and homodimeric α_1a-AR by BiFC/BiLC BRET. BiFC/BiLC BRET for interactions between homodimeric CXCR4 and homodimeric mGlu₁R was nonspecific. Our findings suggest that the heteromerization affinity of CXCR4 for ACKR3 and α₁-ARs is comparable, provide evidence for conformational changes of the receptor complexes upon agonist binding and support the concept that proto- and oligomeric CXCR4 and α₁-ARs constitutively form higher-order hetero-oligomeric receptor clusters.

Context-Dependent Signaling of CXC Chemokine Receptor 4 and Atypical Chemokine Receptor 3

G protein-coupled receptors (GPCRs) are regulated by complex molecular mechanisms, both in physiologic and pathologic conditions, and their signaling can be intricate. Many factors influence their signaling behavior, including the type of ligand that activates the GPCR, the presence of interacting partners, the kinetics involved, or their location. The two CXC-type chemokine receptors, CXC chemokine receptor 4 (CXCR4) and atypical chemokine receptor 3 (ACKR3), both members of the GPCR superfamily, are important and established therapeutic targets in relation to cancer, human immunodeficiency virus infection, and inflammatory diseases. Therefore, it is crucial to understand how the signaling of these receptors works to be able to specifically target them. In this review, we discuss how the signaling pathways activated by CXCR4 and ACKR3 can vary in different situations. G protein signaling of CXCR4 depends on the cellular context, and discrepancies exist depending on the cell lines used. ACKR3, as an atypical chemokine receptor, is generally reported to not activate G proteins but can broaden its signaling spectrum upon heteromerization with other receptors, such as CXCR4, endothelial growth factor receptor, or the α ₁-adrenergic receptor (α ₁-AR). Also, CXCR4 forms heteromers with CC chemokine receptor (CCR) 2, CCR5, the Na⁺/H⁺ exchanger regulatory factor 1, CXCR3, α ₁-AR, and the opioid receptors, which results in differential signaling from that of the monomeric subunits. In addition, CXCR4 is present on membrane rafts but can go into the nucleus during cancer progression, probably acquiring different signaling properties. In this review, we also provide an overview of the currently known critical amino acids involved in CXCR4 and ACKR3 signaling.

Shaky ground - The nature of metastable GPCR signalling complexes

How G protein-coupled receptors (GPCR) interact with one another remains an area of active investigation. Obligate dimers of class C GPCRs such as metabotropic GABA and glutamate receptors are well accepted, although whether this is a general feature of other GPCRs is still strongly debated. In this review, we focus on the idea that GPCR dimers and oligomers are better imagined as parts of larger metastable signalling complexes. We discuss the nature of functional oligomeric entities, their stabilities and kinetic features and how structural and functional asymmetries of such metastable entities might have implications for drug discovery. This article is part of the Special Issue entitled 'Receptor heteromers and their allosteric receptor-receptor interactions'.

Homo-molecular Fluorescence Complementation for Direct Visualization of Receptor Oligomerization in Living Cells

Cell surface receptor oligomerization is an attractive target process for drug screening. However, simple but reliable (and thus high-throughput) visualization methods for receptor oligomerization are still lacking. Herein, we report on a new single-construct homo-molecular fluorescence complementation (Homo-FC) probe, which shows strong fluorescence signals by oligomerization of fused receptors in living cells with unexpectedly low background signals. Importantly, this high signal-to-noise ratio was not affected by expression level variations of fused receptors. The Homo-FC probe was developed by optimized flopped fusion of split fragments of superfolder green fluorescence protein and subsequent surface charge engineering. Homo-FC reliably visualized the oligomerization of diverse natural receptors such as GPCR, EGFR, and even cytosolic DAI.

Remodeling our concept of chemokine receptor function: From monomers to oligomers

The chemokines direct leukocyte recruitment in both homeostatic and inflammatory conditions, and are therefore critical for immune reactions. By binding to members of the class A G protein-coupled receptors, the chemokines play an essential role in numerous physiological and pathological processes. In the last quarter century, the field has accumulated much information regarding the implications of these molecules in different immune processes, as well as mechanistic insight into the signaling events activated through their binding to their receptors. Here, we will focus on chemokine receptors and how new methodological approaches have underscored the role of their conformations in chemokine functions. Advances in biophysical-based techniques show that chemokines and their receptors act in very complex networks and therefore should not be considered isolated entities. In this regard, the chemokine receptors can form homo- and heterodimers as well as oligomers at the cell surface. These findings are changing our view as to how chemokines influence cell biology, identify partners that regulate chemokine function, and open new avenues for therapeutic intervention.

Watching Three-Dimensional Movements of Single Membrane Proteins in Lipid Bilayers

It is challenging to assess protein-membrane interactions because of the lack of appropriate tools to detect position changes of single proteins in the ∼4 nm range of biological membranes. We developed an assay recently, termed surface-induced fluorescence attenuation (SIFA). It is able to track both vertical and lateral dynamic motion of singly labeled membrane proteins in supported lipid bilayers. Similar to the FRET (fluorescence resonance energy transfer) principle, SIFA takes advantage of the energy transfer from a fluorophore to a light-absorbing surface to determine the distance at 2-8 nm away from the surface. By labeling a protein with a proper fluorophore and using graphene oxide as a two-dimensional quencher, we showed that SIFA is capable of monitoring three-dimensional movements of the fluorophore-labeled protein not only inside but also above the lipid bilayer atop the graphene oxide. Our data show that SIFA is a well-suited method to study the interplay between proteins and membranes.

The Dynamics of GPCR Oligomerization and Their Functional Consequences

The functional importance of G protein-coupled receptor (GPCR) oligomerization remains controversial. Although obligate dimers of class C GPCRs are well accepted, the generalizability of this phenomenon is still strongly debated with respect to other classes of GPCRs. In this review, we focus on understanding the organization and dynamics between receptor equivalents and their signaling partners in oligomeric receptor complexes, with a view toward integrating disparate viewpoints into a unified understanding. We discuss the nature of functional oligomeric entities, and how asymmetries in receptor structure and function created by oligomers might have implications for receptor function as allosteric machines and for future drug discovery.