Identification and mechanism of G protein-biased ligands for chemokine receptor CCR1

Insight into structural properties of viral G protein-coupled receptors and their role in the viral infection: IUPHAR Review 41

G protein-coupled receptors (GPCRs) are pivotal in cellular signalling and drug targeting. Herpesviruses encode GPCRs (vGPCRs) to manipulate cellular signalling, thereby regulating various aspects of the virus life cycle, such as viral spreading and immune evasion. vGPCRs mimic host chemokine receptors, often with broader signalling and high constitutive activity. This review focuses on the recent advancements in structural knowledge about vGPCRs, with an emphasis on molecular mechanisms of action and ligand binding. The structures of US27 and US28 from human cytomegalovirus (HCMV) are compared to their closest human homologue, CX3CR1. Contrasting US27 and US28, the homotrimeric UL78 structure (HCMV) reveals more distance to chemokine receptors. Open reading frame 74 (ORF74; Kaposi's sarcoma-associated herpesvirus) is compared to CXCRs, whereas BILF1 (Epstein-Barr virus) is discussed as a putative lipid receptor. Furthermore, the roles of vGPCRs in latency and lytic replication, reactivation, dissemination and immune evasion are reviewed, together with their potential as drug targets for virus infections and virus-related diseases.

Structural and functional determination of peptide versus small molecule ligand binding at the apelin receptor

We describe a structural and functional study of the G protein-coupled apelin receptor, which binds two endogenous peptide ligands, apelin and Elabela/Toddler (ELA), to regulate cardiovascular development and function. Characterisation of naturally occurring apelin receptor variants from the UK Genomics England 100,000 Genomes Project, and AlphaFold2 modelling, identifies T892.64 as important in the ELA binding site, and R1684.64 as forming extensive interactions with the C-termini of both peptides. Base editing to introduce an R/H1684.64 variant into human stem cell-derived cardiomyocytes demonstrates that this residue is critical for receptor binding and function. Additionally, we present an apelin receptor crystal structure bound to the G protein-biased, small molecule agonist, CMF-019, which reveals a deeper binding mode versus the endogenous peptides at lipophilic pockets between transmembrane helices associated with GPCR activation. Overall, the data provide proof-of-principle for using genetic variation to identify key sites regulating receptor-ligand engagement.

Biased signaling in GPCRs: Structural insights and implications for drug development

G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors in humans, playing a crucial role in regulating diverse cellular processes and serving as primary drug targets. Traditional drug design has primarily focused on ligands that uniformly activate or inhibit GPCRs. However, the concept of biased agonism-where ligands selectively stabilize distinct receptor conformations, leading to unique signaling outcomes-has introduced a paradigm shift in therapeutic development. Despite the promise of biased agonists to enhance drug efficacy and minimize side effects, a comprehensive understanding of the structural and biophysical mechanisms underlying biased signaling is essential. Recent advancements in GPCR structural biology have provided unprecedented insights into ligand binding, conformational dynamics, and the molecular basis of biased signaling. These insights, combined with improved techniques for characterizing ligand efficacy, have driven the development of biased ligands for several GPCRs, including opioid, angiotensin, and adrenergic receptors. This review synthesizes these developments, from mechanisms to drug discovery in biased signaling, emphasizing the role of structural insights in the rational design of next-generation biased agonists with superior therapeutic profiles. Ultimately, these advances hold the potential to revolutionize GPCR-targeted drug discovery, paving the way for more precise and effective treatments.

Structural basis for ligand recognition of the human hydroxycarboxylic acid receptor HCAR3

Hydroxycarboxylic acid receptor 3 (HCAR3), a class A G-protein-coupled receptor, is an important cellular energy metabolism sensor with a key role in the regulation of lipolysis in humans. HCAR3 is deeply involved in many physiological processes and serves as a valuable target for the treatment of metabolic diseases, tumors, and immune diseases. Here, we report four cryoelectron microscopy (cryo-EM) structures of human HCAR3-Gi1 complexes with or without agonists: the endogenous ligand 3-hydroxyoctanoic acid, the drug niacin, the highly subtype-specific agonist compound 5c (4-(n-propyl)amino-3-nitrobenzoic acid), and the apo form. Together with mutagenesis and functional analyses, we revealed the recognition mechanisms of HCAR3 for different agonists. In addition, the key residues that determine the ligand selectivity between HCAR2 and HCAR3 were also illuminated. Overall, these findings provide a structural basis for the ligand recognition, activation, and selectivity and G-protein coupling mechanisms of HCAR3, which contribute to the design of HCAR3-targeting drugs with high efficacy and selectivity.

Ligand bias at the muscarinic acetylcholine receptor family: Opportunities and challenges

Muscarinic acetylcholine receptors (mAChRs) are G protein-coupled receptors (GPCRs) that are activated by the endogenous neurotransmitter, acetylcholine (ACh). Disruption of mAChR signalling has been associated with a variety of neurological disorders and non-neurological diseases. Consequently, the development of agonists and antagonists of the mAChRs has been a major avenue in drug discovery. Unfortunately, mAChR ligands are often associated with on-target side effects for two reasons. The first reason is due to the high sequence conservation at the orthosteric ACh binding site among all five receptor subtypes (M1-M5), making on-target subtype selectivity a major challenge. The second reason is due to on-target side effects of mAChR drugs that are associated with the pleiotropic nature of mAChR signalling at the level of a single mAChR subtype. Indeed, there is growing evidence that within the myriad of signalling events produced by mAChR ligands, some will have therapeutic benefits, whilst others may promote cholinergic side effects. This paradigm of drug action, known as ligand bias or biased agonism, is an attractive feature for next-generation mAChR drugs, as it holds the promise of developing drugs devoid of on-target adverse effects. Although relatively simple to detect and even quantify in vitro, ligand bias, as observed in recombinant systems, does not always translate to in vivo systems, which remains a major hurdle in GPCR drug discovery, including the mAChR family. Here we report recent studies that have attempted to detect and quantify ligand bias at the mAChR family, and briefly discuss the challenges associated with biased agonist drug development. This article is part of the Special Issue on "Ligand Bias".

Structural Rearrangement of the AT1 Receptor Modulated by Membrane Thickness and Tension

Membrane-embedded mechanosensitive (MS) proteins, including ion channels and G-protein coupled receptors (GPCRs), are essential for the transduction of external mechanical stimuli into biological signals. The angiotensin II type 1 (AT1) receptor plays many important roles in cardiovascular regulation and is associated with diseases such as hypertension and congestive heart failure. The membrane-mediated activation of the AT1 receptor is not well understood, despite this being one of the most widely studied GPCRs within the context of biased agonism. Here, we use extensive molecular dynamics (MD) simulations to characterize the effect of the local membrane environment on the activation of the AT1 receptor. We show that membrane thickness plays an important role in the stability of active and inactive states of the receptor, as well as the dynamic interchange between states. Furthermore, our simulation results show that membrane tension is effective in driving large-scale structural changes in the inactive state such as the outward movement of transmembrane helix 6 to stabilize intermediate active-like conformations. We conclude by comparing our simulation observations with AlphaFold 2 predictions, as a proxy to experimental structures, to provide a framework for how membrane mediated stimuli can facilitate activation of the AT1 receptor through the β-arrestin signaling pathway.

Aplospojaveedins A-C, unusual sulfur-containing alkaloids produced by the endophytic fungus Aplosporella javeedii using OSMAC strategy

Three sulfur-containing alkaloids aplospojaveedins A-C (1-3) with a hitherto undescribed carbon skeleton comprising octahy-dronaphthalene, α, β-unsaturated lactam and glycine-cysteine moieties were isolated from Aplosporella javeedii. Their structures were elucidated by 1D and 2D NMR spectroscopy, HR-MS, X-ray diffraction analysis, DFT-NMR and TDDFT-ECD calculations. A plausible biosynthetic pathway and putative targets are described. The blind docking suggested that 1-3 may have functional effects on several putative targets such as the GPCR cannabinoid receptor 2 or the integrin α5β1 complex.

Structural insights into CXCR4 modulation and oligomerization

Activation of the chemokine receptor CXCR4 by its chemokine ligand CXCL12 regulates diverse cellular processes. Previously reported crystal structures of CXCR4 revealed the architecture of an inactive, homodimeric receptor. However, many structural aspects of CXCR4 remain poorly understood. Here, we use cryo-electron microscopy to investigate various modes of human CXCR4 regulation. CXCL12 activates CXCR4 by inserting its N terminus deep into the CXCR4 orthosteric pocket. The binding of US Food and Drug Administration-approved antagonist AMD3100 is stabilized by electrostatic interactions with acidic residues in the seven-transmembrane-helix bundle. A potent antibody blocker, REGN7663, binds across the extracellular face of CXCR4 and inserts its complementarity-determining region H3 loop into the orthosteric pocket. Trimeric and tetrameric structures of CXCR4 reveal modes of G-protein-coupled receptor oligomerization. We show that CXCR4 adopts distinct subunit conformations in trimeric and tetrameric assemblies, highlighting how oligomerization could allosterically regulate chemokine receptor function.

Progress on the development of Class A GPCR-biased ligands

Class A G protein-coupled receptors (GPCRs) continue to garner interest for their essential roles in cell signalling and their importance as drug targets. Although numerous drugs in the clinic target these receptors, over 60% GPCRs remain unexploited. Moreover, the adverse effects triggered by the available unbiased GPCR modulators, limit their use and therapeutic value. In this context, the elucidation of biased signalling has opened up new pharmacological avenues holding promise for safer therapeutics. Functionally selective ligands favour receptor conformations facilitating the recruitment of specific effectors and the modulation of the associated pathways. This review surveys the current drug discovery landscape of GPCR-biased modulators with a focus on recent advances. Understanding the biological effects of this preferential coupling is at different stages depending on the Class A GPCR family. Therefore, with a focus on individual GPCR families, we present a compilation of the functionally selective modulators reported over the past few years. In doing so, we dissect their therapeutic relevance, molecular determinants and potential clinical applications.

Structural analysis of the human C5a-C5aR1 complex using cryo-electron microscopy

The complement system is a complex network of proteins that plays a crucial role in the innate immune response. One important component of this system is the C5a-C5aR1 complex, which is critical in the recruitment and activation of immune cells. In-depth investigation of the activation mechanism as well as biased signaling of the C5a-C5aR1 system will facilitate the elucidation of C5a-mediated pathophysiology. In this study, we determined the structure of C5a-C5aR1-Gi complex at a high resolution of 3 Å using cryo-electron microscopy (Cryo-EM). Our results revealed the binding site of C5a, which consists of a polar recognition region on the extracellular side and an amphipathic pocket within the transmembrane domain. Furthermore, we found that C5a binding induces conformational changes of C5aR1, which subsequently leads to the activation of G protein signaling pathways. Notably, a key residue (M265) located on transmembrane helix 6 (TM6) was identified to play a crucial role in regulating the recruitment of β-arrestin driven by C5a. This study provides more information about the structure and function of the human C5a-C5aR1 complex, which is essential for the proper functioning of the complement system. The findings of this study can also provide a foundation for the design of new pharmaceuticals targeting this receptor with bias or specificity.

Signaling Modulation Mediated by Ligand Water Interactions with the Sodium Site at μOR

The mu opioid receptor (μOR) is a target for clinically used analgesics. However, adverse effects, such as respiratory depression and physical dependence, necessitate the development of alternative treatments. Recently we reported a novel strategy to design functionally selective opioids by targeting the sodium binding allosteric site in μOR with a supraspinally active analgesic named C6guano. Presently, to improve systemic activity of this ligand, we used structure-based design, identifying a new ligand named RO76 where the flexible alkyl linker and polar guanidine guano group is swapped with a benzyl alcohol, and the sodium site is targeted indirectly through waters. A cryoEM structure of RO76 bound to the μOR-Gi complex confirmed that RO76 interacts with the sodium site residues through a water molecule, unlike C6guano which engages the sodium site directly. Signaling assays coupled with APEX based proximity labeling show binding in the sodium pocket modulates receptor efficacy and trafficking. In mice, RO76 was systemically active in tail withdrawal assays and showed reduced liabilities compared to those of morphine. In summary, we show that targeting water molecules in the sodium binding pocket may be an avenue to modulate signaling properties of opioids, and which may potentially be extended to other G-protein coupled receptors where this site is conserved.

Molecular mechanism of distinct chemokine engagement and functional divergence of the human Duffy antigen receptor

The Duffy antigen receptor is a seven-transmembrane (7TM) protein expressed primarily at the surface of red blood cells and displays strikingly promiscuous binding to multiple inflammatory and homeostatic chemokines. It serves as the basis of the Duffy blood group system in humans and also acts as the primary attachment site for malarial parasite Plasmodium vivax and pore-forming toxins secreted by Staphylococcus aureus. Here, we comprehensively profile transducer coupling of this receptor, discover potential non-canonical signaling pathways, and determine the cryoelectron microscopy (cryo-EM) structure in complex with the chemokine CCL7. The structure reveals a distinct binding mode of chemokines, as reflected by relatively superficial binding and a partially formed orthosteric binding pocket. We also observe a dramatic shortening of TM5 and 6 on the intracellular side, which precludes the formation of the docking site for canonical signal transducers, thereby providing a possible explanation for the distinct pharmacological and functional phenotype of this receptor.

Cryo-EM Structure and Biochemical Analysis of the Human Chemokine Receptor CCR8

The C-C motif chemokine receptor 8 (CCR8) is a class A G-protein-coupled receptor that has emerged as a promising therapeutic target in cancer and autoimmune diseases. In the present study, we solved the cryo-electron microscopy (cryo-EM) structure of the human CCR8-Gi complex in the absence of a ligand at 2.58 Å. Structural analysis and comparison revealed that our apo CCR8 structure undergoes some conformational changes and is similar to that in the CCL1-CCR8 complex structure, indicating an active state. In addition, the key residues of CCR8 involved in the recognition of LMD-009, a potent nonpeptide agonist, were investigated by mutating CCR8 and testing the calcium flux induced by LMD-009-CCR8 interaction. Three mutants of CCR8, Y1133.32A, Y1724.64A, and E2867.39A, showed a dramatically decreased ability in mediating calcium mobilization, indicating their key interaction with LMD-009 and key roles in activation. These structural and biochemical analyses enrich molecular insights into the agonism and activation of CCR8 and will facilitate CCR8-targeted therapy.

Epitope Mapping of an Anti-Mouse CCR8 Monoclonal Antibody C8Mab-2 Using Flow Cytometry

The C-C motif chemokine receptor 8 (CCR8) is highly and selectively expressed in regulatory T (Treg) cells and is associated with tumor progression. The massive accumulation of Treg cells into tumors suppresses the effector function of CD8+ cells against tumor cells. Therefore, selective depletion of Treg cells using anti-CCR8 monoclonal antibodies (mAbs) reinvigorates antitumor immune responses and improves responses to cancer immunotherapy. Previously, we developed an anti-mouse CCR8 (mCCR8) mAb, C8Mab-2, using the Cell-Based Immunization and Screening method. In this study, the binding epitope of C8Mab-2 was investigated using flow cytometry. The mCCR8 extracellular domain-substituted mutant analysis showed that C8Mab-2 recognizes the N-terminal region (1-33 amino acids) of mCCR8. Next, 1×alanine (or glycine) scanning and 2×alanine (or glycine) scanning were conducted in the N-terminal region. The results revealed that the 17-DFFTAP-22 sequence is important for the recognition by C8Mab-2, and Thr20 is a central amino acid of the epitope. These results revealed the involvement of the N-terminus of mCCR8 in the recognition by C8Mab-2.

Conformational dynamics underlying atypical chemokine receptor 3 activation

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

Fluorophore-Labeled Pyrrolones Targeting the Intracellular Allosteric Binding Site of the Chemokine Receptor CCR1

In this study, we describe the structure-based development of the first fluorescent ligands targeting the intracellular allosteric binding site (IABS) of the CC chemokine receptor type 1 (CCR1), a G protein-coupled receptor (GPCR) that has been pursued as a drug target in inflammation and immune diseases. Starting from previously reported intracellular allosteric modulators of CCR1, tetramethylrhodamine (TAMRA)-labeled ligands were designed, synthesized, and tested for their suitability as fluorescent tracers to probe binding to the IABS of CCR1. In the course of these studies, we developed LT166 (12) as a highly versatile fluorescent CCR1 ligand, enabling cell-free as well as cellular NanoBRET-based binding studies in a nonradioactive and high-throughput manner. Besides the detection of intracellular allosteric ligands by direct competition with 12, we were also able to monitor the binding of extracellular antagonists due to their positive cooperative binding with 12. Thereby, we provide a straightforward and nonradioactive method to easily distinguish between ligands binding to the IABS of CCR1 and extracellular negative modulators. Further, we applied 12 for the identification of novel chemotypes for intracellular CCR1 inhibition that feature high binding selectivity for CCR1 over CCR2. For one of the newly identified intracellular CCR1 ligands (i.e., 23), we were able to show CCR1 over CCR2 selectivity also on a functional level and demonstrated that this compound inhibits basal β-arrestin recruitment to CCR1, thereby acting as an inverse agonist. Thus, our fluorescent CCR1 ligand 12 represents a highly promising tool for future studies of CCR1-targeted pharmacology and drug discovery.

Crosstalk among proximal tubular cells, macrophages, and fibroblasts in acute kidney injury: single-cell profiling from the perspective of ferroptosis

The link between ferroptosis, a form of cell death mediated by iron and acute kidney injury (AKI) is recently gaining widespread attention. However, the mechanism of the crosstalk between cells in the pathogenesis and progression of acute kidney injury remains unexplored. In our research, we performed a non-negative matrix decomposition (NMF) algorithm on acute kidney injury single-cell RNA sequencing data based specifically focusing in ferroptosis-associated genes. Through a combination with pseudo-time analysis, cell-cell interaction analysis and SCENIC analysis, we discovered that proximal tubular cells, macrophages, and fibroblasts all showed associations with ferroptosis in different pathways and at various time. This involvement influenced cellular functions, enhancing cellular communication and activating multiple transcription factors. In addition, analyzing bulk expression profiles and marker genes of newly defined ferroptosis subtypes of cells, we have identified crucial cell subtypes, including Egr1 + PTC-C1, Jun + PTC-C3, Cxcl2 + Mac-C1 and Egr1 + Fib-C1. All these subtypes which were found in AKI mice kidneys and played significantly distinct roles from those of normal mice. Moreover, we verified the differential expression of Egr1, Jun, and Cxcl2 in the IRI mouse model and acute kidney injury human samples. Finally, our research presented a novel analysis of the crosstalk of proximal tubular cells, macrophages and fibroblasts in acute kidney injury targeting ferroptosis, therefore, contributing to better understanding the acute kidney injury pathogenesis, self-repairment and acute kidney injury-chronic kidney disease (AKI-CKD) progression.

Dual fluorescence reporter mice for Ccl3 transcription, translation, and intercellular communication

Chemokines guide immune cells during their response against pathogens and tumors. Various techniques exist to determine chemokine production, but none to identify cells that directly sense chemokines in vivo. We have generated CCL3-EASER (ErAse, SEnd, Receive) mice that simultaneously report for Ccl3 transcription and translation, allow identifying Ccl3-sensing cells, and permit inducible deletion of Ccl3-producing cells. We infected these mice with murine cytomegalovirus (mCMV), where Ccl3 and NK cells are critical defense mediators. We found that NK cells transcribed Ccl3 already in homeostasis, but Ccl3 translation required type I interferon signaling in infected organs during early infection. NK cells were both the principal Ccl3 producers and sensors of Ccl3, indicating auto/paracrine communication that amplified NK cell response, and this was essential for the early defense against mCMV. CCL3-EASER mice represent the prototype of a new class of dual fluorescence reporter mice for analyzing cellular communication via chemokines, which may be applied also to other chemokines and disease models.

Synergistic antitumor activity by dual blockade of CCR1 and CXCR2 expressed on myeloid cells within the tumor microenvironment

BACKGROUND

Chemokine signaling within the tumor microenvironment can promote tumor progression. Although CCR1 and CXCR2 on myeloid cells could be involved in tumor progression, it remains elusive what effect would be observed if both of those are blocked.

METHODS

We employed two syngeneic colorectal cancer mouse models: a transplanted tumor model and a liver metastasis model. We generated double-knockout mice for CCR1 and CXCR2, and performed bone marrow (BM) transfer experiments in which sub-lethally irradiated wild-type mice were reconstituted with BM from either wild-type, Ccr1-/-, Cxcr2-/- or Ccr1-/-Cxcr2-/- mice.

RESULTS

Myeloid cells that express MMP2, MMP9 and VEGF were accumulated around both types of tumors through CCR1- and CXCR2-mediated pathways. Mice reconstituted with Ccr1-/-Cxcr2-/- BM exhibited the strongest suppression of tumor growth and liver metastasis compared with other three groups. Depletion of CCR1+CXCR2+ myeloid cells led to a higher frequency of CD8+ T cells, whereas the numbers of Ly6G+ neutrophils, FOXP3+ Treg cells and CD31+ endothelial cells were significantly decreased. Furthermore, treatment with a neutralizing anti-CCR1 mAb to mice reconstituted with Cxcr2-/- BM significantly suppressed tumor growth and liver metastasis.

CONCLUSION

Dual blockade of CCR1 and CXCR2 pathways in myeloid cells could be an effective therapy against colorectal cancer.

Structural perspectives on chemokine receptors

Chemokine receptors are integral to the immune system and prime targets in drug discovery that have undergone extensive structural elucidation in recent years. We outline a timeline of these structural achievements, discuss the intracellular negative allosteric modulation of chemokine receptors, analyze the mechanisms of orthosteric receptor activation, and report on the emerging concept of biased signaling. Additionally, we highlight differences of G-protein binding among chemokine receptors. Intracellular allosteric modulators in chemokine receptors interact with a conserved motif within transmembrane helix 7 and helix 8 and exhibit a two-fold inactivation mechanism that can be harnessed for drug-discovery efforts. Chemokine recognition is a multi-step process traditionally explained by a two-site model within chemokine recognition site 1 (CRS1) and CRS2. Recent structural studies have extended our understanding of this complex mechanism with the identification of CRS1.5 and CRS3. CRS3 is implicated in determining ligand specificity and surrounds the chemokine by almost 180°. Within CRS3 we identified the extracellular loop 2 residue 45.51 as a key interaction mediator for chemokine binding. Y2917.43 on the other hand was shown in CCR1 to be a key determinant of signaling bias which, along with specific chemokine-dependent phosphorylation ensembles at the G-protein coupled receptors (GPCR's) C-terminus, seems to play a pivotal role in determining the direction of signal bias in GPCRs.

G Protein-Coupled Receptor-Ligand Pose and Functional Class Prediction

G protein-coupled receptor (GPCR) transmembrane protein family members play essential roles in physiology. Numerous pharmaceuticals target GPCRs, and many drug discovery programs utilize virtual screening (VS) against GPCR targets. Improvements in the accuracy of predicting new molecules that bind to and either activate or inhibit GPCR function would accelerate such drug discovery programs. This work addresses two significant research questions. First, do ligand interaction fingerprints provide a substantial advantage over automated methods of binding site selection for classical docking? Second, can the functional status of prospective screening candidates be predicted from ligand interaction fingerprints using a random forest classifier? Ligand interaction fingerprints were found to offer modest advantages in sampling accurate poses, but no substantial advantage in the final set of top-ranked poses after scoring, and, thus, were not used in the generation of the ligand-receptor complexes used to train and test the random forest classifier. A binary classifier which treated agonists, antagonists, and inverse agonists as active and all other ligands as inactive proved highly effective in ligand function prediction in an external test set of GPR31 and TAAR2 candidate ligands with a hit rate of 82.6% actual actives within the set of predicted actives.

Benchmarking AlphaFold-Generated Structures of Chemokine-Chemokine Receptor Complexes

AlphaFold and AlphaFold-Multimer have become two essential tools for the modeling of unknown structures of proteins and protein complexes. In this work, we extensively benchmarked the quality of chemokine-chemokine receptor structures generated by AlphaFold-Multimer against experimentally determined structures. Our analysis considered both the global quality of the model, as well as key structural features for chemokine recognition. To study the effects of template and multiple sequence alignment parameters on the results, a new prediction pipeline called LIT-AlphaFold (https://github.com/LIT-CCM-lab/LIT-AlphaFold) was developed, allowing extensive input customization. AlphaFold-Multimer correctly predicted differences in chemokine binding orientation and accurately reproduced the unique binding orientation of the CXCL12-ACKR3 complex. Further, the predictions of the full receptor N-terminus provided insights into a putative chemokine recognition site 0.5. The accuracy of chemokine N-terminus binding mode prediction varied between complexes, but the confidence score permitted the distinguishing of residues that were very likely well positioned. Finally, we generated a high-confidence model of the unsolved CXCL12-CXCR4 complex, which agreed with experimental mutagenesis and cross-linking data.

Molecular Determinant Underlying Selective Coupling of Primary G-Protein by Class A GPCRs

G-protein-coupled receptors (GPCRs) transmit downstream signals predominantly via G-protein pathways. However, the conformational basis of selective coupling of primary G-protein remains elusive. Histamine receptors H2R and H3R couple with Gs- or Gi-proteins respectively. Here, three cryo-EM structures of H2R-Gs and H3R-Gi complexes are presented at a global resolution of 2.6-2.7 Å. These structures reveal the unique binding pose for endogenous histamine in H3R, wherein the amino group interacts with E2065.46 of H3R instead of the conserved D1143.32 of other aminergic receptors. Furthermore, comparative analysis of the H2R-Gs and H3R-Gi complexes reveals that the structural geometry of TM5/TM6 determines the primary G-protein selectivity in histamine receptors. Machine learning (ML)-based structuromic profiling and functional analysis of class A GPCR-G-protein complexes illustrate that TM5 length, TM5 tilt, and TM6 outward movement are key determinants of the Gs and Gi/o selectivity among the whole Class A family. Collectively, the findings uncover the common structural geometry within class A GPCRs that determines the primary Gs- and Gi/o-coupling selectivity.

Structure-based design of non-hypertrophic apelin receptor modulator

Apelin is a key hormone in cardiovascular homeostasis that activates the apelin receptor (APLNR), which is regarded as a promising therapeutic target for cardiovascular disease. However, adverse effects through the β-arrestin pathway limit its pharmacological use. Here, we report cryoelectron microscopy (cryo-EM) structures of APLNR-Gi1 complexes bound to three agonists with divergent signaling profiles. Combined with functional assays, we have identified "twin hotspots" in APLNR as key determinants for signaling bias, guiding the rational design of two exclusive G-protein-biased agonists WN353 and WN561. Cryo-EM structures of WN353- and WN561-stimulated APLNR-G protein complexes further confirm that the designed ligands adopt the desired poses. Pathophysiological experiments have provided evidence that WN561 demonstrates superior therapeutic effects against cardiac hypertrophy and reduced adverse effects compared with the established APLNR agonists. In summary, our designed APLNR modulator may facilitate the development of next-generation cardiovascular medications.

Unveiling the structural mechanisms of nonpeptide ligand recognition and activation in human chemokine receptor CCR8

The human CC chemokine receptor 8 (CCR8) is an emerging therapeutic target for cancer immunotherapy and autoimmune diseases. Understanding the molecular recognition of CCR8, particularly with nonpeptide ligands, is valuable for drug development. Here, we report three cryo-electron microscopy structures of human CCR8 complexed with Gi trimers in the ligand-free state or activated by nonpeptide agonists LMD-009 and ZK 756326. A conserved Y1.39Y3.32E7.39 motif in the orthosteric binding pocket is shown to play a crucial role in the chemokine and nonpeptide ligand recognition. Structural and functional analyses indicate that the lack of conservation in Y1143.33 and Y1724.64 among the CC chemokine receptors could potentially contribute to the selectivity of the nonpeptide ligand binding to CCR8. These findings present the characterization of the molecular interaction between a nonpeptide agonist and a chemokine receptor, aiding the development of therapeutics targeting related diseases through a structure-based approach.

In silico analysis of crustacean hyperglycemic hormone family G protein-coupled receptor candidates

Ecdysteroid molting hormone synthesis is directed by a pair of molting glands or Y-organs (YOs), and this synthesis is inhibited by molt-inhibiting hormone (MIH). MIH is a member of the crustacean hyperglycemic hormone (CHH) neuropeptide superfamily, which includes CHH and insect ion transport peptide (ITP). It is hypothesized that the MIH receptor is a Class A (Rhodopsin-like) G protein-coupled receptor (GPCR). The YO of the blackback land crab, Gecarcinus lateralis, expresses 49 Class A GPCRs, three of which (Gl-CHHR-A9, -A10, and -A12) were provisionally assigned as CHH-like receptors. CrusTome, a transcriptome database assembled from 189 crustaceans and 12 ecdysozoan outgroups, was used to deorphanize candidate MIH/CHH GPCRs, relying on sequence homology to three functionally characterized ITP receptors (BNGR-A2, BNGR-A24, and BNGR-A34) in the silk moth, Bombyx mori. Phylogenetic analysis and multiple sequence alignments across major taxonomic groups revealed extensive expansion and diversification of crustacean A2, A24, and A34 receptors, designated CHH Family Receptor Candidates (CFRCs). The A2 clade was divided into three subclades; A24 clade was divided into five subclades; and A34 was divided into six subclades. The subclades were distinguished by conserved motifs in extracellular loop (ECL) 2 and ECL3 in the ligand-binding region. Eleven of the 14 subclades occurred in decapod crustaceans. In G. lateralis, seven CFRC sequences, designated Gl-CFRC-A2α1, -A24α, -A24β1, -A24β2, -A34α2, -A34β1, and -A34β2, were identified; the three A34 sequences corresponded to Gl-GPCR-A12, -A9, and A10, respectively. ECL2 in all the CFRC sequences had a two-stranded β-sheet structure similar to human Class A GPCRs, whereas the ECL2 of decapod CFRC-A34β1/β2 had an additional two-stranded β-sheet. We hypothesize that this second β-sheet on ECL2 plays a role in MIH/CHH binding and activation, which will be investigated further with functional assays.

Structural basis of antibody inhibition and chemokine activation of the human CC chemokine receptor 8

The C-C motif chemokine receptor 8 (CCR8) is a class A G-protein coupled receptor that has emerged as a promising therapeutic target in cancer. Targeting CCR8 with an antibody has appeared to be an attractive therapeutic approach, but the molecular basis for chemokine-mediated activation and antibody-mediated inhibition of CCR8 are not fully elucidated. Here, we obtain an antagonist antibody against human CCR8 and determine structures of CCR8 in complex with either the antibody or the endogenous agonist ligand CCL1. Our studies reveal characteristic antibody features allowing recognition of the CCR8 extracellular loops and CCL1-CCR8 interaction modes that are distinct from other chemokine receptor - ligand pairs. Informed by these structural insights, we demonstrate that CCL1 follows a two-step, two-site binding sequence to CCR8 and that antibody-mediated inhibition of CCL1 signaling can occur by preventing the second binding event. Together, our results provide a detailed structural and mechanistic framework of CCR8 activation and inhibition that expands our molecular understanding of chemokine - receptor interactions and offers insight into the development of therapeutic antibodies targeting chemokine GPCRs.

Structural basis for ligand recognition and signaling of hydroxy-carboxylic acid receptor 2

Hydroxycarboxylic acid receptors (HCAR1, HCAR2, and HCAR3) transduce Gi/o signaling upon biding to molecules such as lactic acid, butyric acid and 3-hydroxyoctanoic acid, which are associated with lipolytic and atherogenic activity, and neuroinflammation. Although many reports have elucidated the function of HCAR2 and its potential as a therapeutic target for treating not only dyslipidemia but also neuroimmune disorders such as multiple sclerosis and Parkinson's disease, the structural basis of ligand recognition and ligand-induced Gi-coupling remains unclear. Here we report three cryo-EM structures of the human HCAR2-Gi signaling complex, each bound with different ligands: niacin, acipimox or GSK256073. All three agonists are held in a deep pocket lined by residues that are not conserved in HCAR1 and HCAR3. A distinct hairpin loop at the HCAR2 N-terminus and extra-cellular loop 2 (ECL2) completely enclose the ligand. These structures also reveal the agonist-induced conformational changes propagated to the G-protein-coupling interface during activation. Collectively, the structures presented here are expected to help in the design of ligands specific for HCAR2, leading to new drugs for the treatment of various diseases such as dyslipidemia and inflammation.

IUPHAR themed review: Opioid efficacy, bias, and selectivity

Drugs acting at the opioid receptor family are clinically used to treat chronic and acute pain, though they represent the second line of treatment behind GABA analogs, antidepressants and SSRI's. Within the opioid family mu and kappa opioid receptor are commonly targeted. However, activation of the mu opioid receptor has side effects of constipation, tolerance, dependence, euphoria, and respiratory depression; activation of the kappa opioid receptor leads to dysphoria and sedation. The side effects of mu opioid receptor activation have led to mu receptor drugs being widely abused with great overdose risk. For these reasons, newer safer opioid analgesics are in high demand. For many years a focus within the opioid field was finding drugs that activated the G protein pathway at mu opioid receptor, without activating the β-arrestin pathway, known as biased agonism. Recent advances have shown that this may not be the way forward to develop safer analgesics at mu opioid receptor, though there is still some promise at the kappa opioid receptor. Here we discuss recent novel approaches to develop safer opioid drugs including efficacy vs bias and fine-tuning receptor activation by targeting sub-pockets in the orthosteric site, we explore recent works on the structural basis of bias, and we put forward the suggestion that Gα subtype selectivity may be an exciting new area of interest.

Revealing the signaling of complement receptors C3aR and C5aR1 by anaphylatoxins

The complement receptors C3aR and C5aR1, whose signaling is selectively activated by anaphylatoxins C3a and C5a, are important regulators of both innate and adaptive immune responses. Dysregulations of C3aR and C5aR1 signaling lead to multiple inflammatory disorders, including sepsis, asthma and acute respiratory distress syndrome. The mechanism underlying endogenous anaphylatoxin recognition and activation of C3aR and C5aR1 remains elusive. Here we reported the structures of C3a-bound C3aR and C5a-bound C5aR1 as well as an apo-C3aR structure. These structures, combined with mutagenesis analysis, reveal a conserved recognition pattern of anaphylatoxins to the complement receptors that is different from chemokine receptors, unique pocket topologies of C3aR and C5aR1 that mediate ligand selectivity, and a common mechanism of receptor activation. These results provide crucial insights into the molecular understanding of C3aR and C5aR1 signaling and structural templates for rational drug design for treating inflammation disorders.

Biased allosteric activation of ketone body receptor HCAR2 suppresses inflammation

Hydroxycarboxylic acid receptor 2 (HCAR2), modulated by endogenous ketone body β-hydroxybutyrate and exogenous niacin, is a promising therapeutic target for inflammation-related diseases. HCAR2 mediates distinct pathophysiological events by activating Gi/o protein or β-arrestin effectors. Here, we characterize compound 9n as a Gi-biased allosteric modulator (BAM) of HCAR2 and exhibit anti-inflammatory efficacy in RAW264.7 macrophages via a specific HCAR2-Gi pathway. Furthermore, four structures of HCAR2-Gi complex bound to orthosteric agonists (niacin or monomethyl fumarate), compound 9n, and niacin together with compound 9n simultaneously reveal a common orthosteric site and a unique allosteric site. Combined with functional studies, we decipher the action framework of biased allosteric modulation of compound 9n on the orthosteric site. Moreover, co-administration of compound 9n with orthosteric agonists could enhance anti-inflammatory effects in the mouse model of colitis. Together, our study provides insight to understand the molecular pharmacology of the BAM and facilitates exploring the therapeutic potential of the BAM with orthosteric drugs.

Identification of a Salt Bridge That Is Functionally Important for Chemokine Receptor CXCR1 but not CXCR2

CXC chemokine receptors 1 (CXCR1) and 2 (CXCR2) have high sequence similarity and overlapping chemokine ligand profiles. Residue positions 3.32 and 7.39 are critical for signal transduction in the related CXCR4, and in these positions CXCR1 and CXCR2 contain oppositely charged residues (Lys3.32 and Glu7.39). Experimental and computed receptor structures reveal the possible formation of a salt bridge between transmembrane (TM) helices 3 and 7 via these two residues. To investigate the functional importance of Lys1173.32 and Glu2917.39 in CXCR1, along with the flanking Glu1183.33, we performed a signaling study on 16 CXCR1 mutants using two different CXCL8 isoforms. While single Ala-mutation (K1173.32A, E2917.39A) and charge reversal (K1173.32E, E2917.39K) resulted in nonfunctional receptors, double (K1173.32E-E2917.39K) and triple (K1173.32E-E1183.33A-E2917.39K) mutants rescued CXCR1 function. In contrast, the corresponding mutations did not affect the CXCR2 function to the same extent. Our findings show that the Lys3.32-Glu7.39 salt bridge between TM3 and -7 is functionally important for CXCR1 but not for CXCR2, meaning that signal transduction for these highly homologous receptors is not conserved.

Role of neutrophils in different stages of atherosclerosis

Neutrophils constitute the first line of defense in human immunity and can be attracted to inflamed and infected sites by various chemokines. As essential players in immune processes, neutrophils theoretically play integral roles in the course of chronic inflammation-induced atherosclerosis. However, because neutrophils are rarely found in atherosclerotic lesions, their involvement in the pathophysiological progression of atherosclerosis has been largely underestimated or ignored. Recent research has revealed convincing evidence showing the presence of neutrophils in atherosclerotic lesions and has revealed neutrophil contributions to different atherosclerosis stages in mice and humans. This review describes the underlying mechanisms of neutrophils in different stages of atherosclerosis and highlights potential neutrophil-targeted therapeutic strategies relevant to atherosclerosis. An in-depth understanding of neutrophils' roles in atherosclerosis pathology will promote exploration of new methods for the prevention and treatment of atherogenesis and atherothrombosis.

Development of tolerance to chemokine receptor antagonists: current paradigms and the need for further investigation

Chemokine G-protein coupled receptors are validated drug targets for many diseases, including cancer, neurological, and inflammatory disorders. Despite much time and effort spent on therapeutic development, very few chemokine receptor antagonists are approved for clinical use. Among potential reasons for the slow progress in developing chemokine receptor inhibitors, antagonist tolerance, a progressive reduction in drug efficacy after repeated administration, is likely to play a key role. The mechanisms leading to antagonist tolerance remain poorly understood. In many cases, antagonist tolerance is accompanied by increased receptor concentration on the cell surface after prolonged exposure to chemokine receptor antagonists. This points to a possible role of altered receptor internalization and presentation on the cell surface, as has been shown for agonist (primarily opioid) tolerance. In addition, examples of antagonist tolerance in the context of other G-protein coupled receptors suggest the involvement of noncanonical signal transduction in opposing the effects of the antagonists. In this review, we summarize the available progress and challenges in therapeutic development of chemokine receptor antagonists, describe the available knowledge about antagonist tolerance, and propose new avenues for future investigation of this important phenomenon. Furthermore, we highlight the modern methodologies that have the potential to reveal novel mechanisms leading to antagonist tolerance and to propel the field forward by advancing the development of potent "tolerance-free" antagonists of chemokine receptors.

Blood and CSF chemokines in Alzheimer's disease and mild cognitive impairment: a systematic review and meta-analysis

OBJECTIVE

Chemokines, which are chemotactic inflammatory mediators involved in controlling the migration and residence of all immune cells, are closely associated with brain inflammation, recognized as one of the potential processes/mechanisms associated with cognitive impairment. We aim to determine the chemokines which are significantly altered in Alzheimer's disease (AD) and mild cognitive impairment (MCI), as well as the respective effect sizes, by performing a meta-analysis of chemokines in cerebrospinal fluid (CSF) and blood (plasma or serum).

METHODS

We searched three databases (Pubmed, EMBASE and Cochrane library) for studies regarding chemokines. The three pairwise comparisons were as follows: AD vs HC, MCI vs healthy controls (HC), and AD vs MCI. The fold-change was calculated using the ratio of mean (RoM) chemokine concentration for every study. Subgroup analyses were performed for exploring the source of heterogeneity.

RESULTS

Of 2338 records identified from the databases, 61 articles comprising a total of 3937 patients with AD, 1459 with MCI, and 4434 healthy controls were included. The following chemokines were strongly associated with AD compared with HC: blood CXCL10 (RoM, 1.92, p = 0.039), blood CXCL9 (RoM, 1.78, p < 0.001), blood CCL27 (RoM, 1.34, p < 0.001), blood CCL15 (RoM, 1.29, p = 0.003), as well as CSF CCL2 (RoM, 1.19, p < 0.001). In the comparison of AD with MCI, there was significance for blood CXCL9 (RoM, 2.29, p < 0.001), blood CX3CL1 (RoM, 0.77, p = 0.017), and blood CCL1 (RoM, 1.37, p < 0.001). Of the chemokines tested, blood CX3CL1 (RoM, 2.02, p < 0.001) and CSF CCL2 (RoM, 1.16, p = 0.004) were significant for the comparison of MCI with healthy controls.

CONCLUSIONS

Chemokines CCL1, CCL2, CCL15, CCL27, CXCL9, CXCL10, and CX3CL1 might be most promising to serve as key molecular markers of cognitive impairment, although more cohort studies with larger populations are needed.

Staphylococcus aureus Infection Induces the Production of the Neutrophil Chemoattractants CXCL1, CXCL2, CXCL3, CXCL5, CCL3, and CCL7 by Murine Osteoblasts

Staphylococcus aureus is the principal causative agent of osteomyelitis, a serious bacterial infection of bone that is associated with progressive inflammatory damage. Bone-forming osteoblasts have increasingly been recognized to play an important role in the initiation and progression of detrimental inflammation at sites of infection and have been demonstrated to release an array of inflammatory mediators and factors that promote osteoclastogenesis and leukocyte recruitment following bacterial challenge. In the present study, we describe elevated bone tissue levels of the potent neutrophil-attracting chemokines CXCL1, CXCL2, CXCL3, CXCL5, CCL3, and CCL7 in a murine model of posttraumatic staphylococcal osteomyelitis. RNA sequencing (RNA-Seq) gene ontology analysis of isolated primary murine osteoblasts showed enrichment in differentially expressed genes involved in cell migration and chemokine receptor binding and chemokine activity following S. aureus infection, and a rapid increase in the expression of mRNA encoding CXCL1, CXCL2, CXCL3, CXCL5, CCL3, and CCL7, in these cells. Importantly, we have confirmed that such upregulated gene expression results in protein production with the demonstration that S. aureus challenge elicits the rapid and robust release of these chemokines by osteoblasts and does so in a bacterial dose-dependent manner. Furthermore, we have confirmed the ability of soluble osteoblast-derived chemokines to elicit the migration of a neutrophil-like cell line. As such, these studies demonstrate the robust production of CXCL1, CXCL2, CXCL3, CXCL5, CCL3, and CCL7 by osteoblasts in response to S. aureus infection, and the release of such neutrophil-attracting chemokines provides an additional mechanism by which osteoblasts could drive the inflammatory bone loss associated with staphylococcal osteomyelitis.

Cryo-EM structure of the human chemerin receptor 1-Gi protein complex bound to the C-terminal nonapeptide of chemerin

Chemerin is a processed protein that acts on G protein-coupled receptors (GPCRs) for its chemotactic and adipokine activities. The biologically active chemerin (chemerin 21-157) results from proteolytic cleavage of prochemerin and uses its C-terminal peptide containing the sequence YFPGQFAFS for receptor activation. Here we report a high-resolution cryo-electron microscopy (cryo-EM) structure of human chemerin receptor 1 (CMKLR1) bound to the C-terminal nonapeptide of chemokine (C9) in complex with Gi proteins. C9 inserts its C terminus into the binding pocket and is stabilized through hydrophobic interactions involving its Y1, F2, F6, and F8, as well as polar interactions between G4, S9, and several amino acids lining the binding pocket of CMKLR1. Microsecond scale molecular dynamics simulations support a balanced force distribution across the whole ligand-receptor interface that enhances thermodynamic stability of the captured binding pose of C9. The C9 interaction with CMKLR1 is drastically different from chemokine recognition by chemokine receptors, which follow a two-site two-step model. In contrast, C9 takes an "S"-shaped pose in the binding pocket of CMKLR1 much like angiotensin II in the AT1 receptor. Our mutagenesis and functional analyses confirmed the cryo-EM structure and key residues in the binding pocket for these interactions. Our findings provide a structural basis for chemerin recognition by CMKLR1 for the established chemotactic and adipokine activities.

Prospect of acromegaly therapy: molecular mechanism of clinical drugs octreotide and paltusotine

Somatostatin receptor 2 (SSTR2) is highly expressed in neuroendocrine tumors and represents as a therapeutic target. Several peptide analogs mimicking the endogenous ligand somatostatin are available for clinical use, but poor therapeutic effects occur in a subset of patients, which may be correlated with subtype selectivity or cell surface expression. Here, we clarify the signal bias profiles of the first-generation peptide drug octreotide and a new-generation small molecule paltusotine by evaluating their pharmacological characteristics. We then perform cryo-electron microscopy analysis of SSTR2-Gi complexes to determine how the drugs activate SSTR2 in a selective manner. In this work, we decipher the mechanism of ligand recognition, subtype selectivity and signal bias property of SSTR2 sensing octreotide and paltusotine, which may aid in designing therapeutic drugs with specific pharmacological profiles against neuroendocrine tumors.

Mechanism of activation and biased signaling in complement receptor C5aR1

The complement system plays an important role in the innate immune response to invading pathogens. The complement fragment C5a is one of its important effector components and exerts diverse physiological functions through activation of the C5a receptor 1 (C5aR1) and associated downstream G protein and β-arrestin signaling pathways. Dysfunction of the C5a-C5aR1 axis is linked to numerous inflammatory and immune-mediated diseases, but the structural basis for activation and biased signaling of C5aR1 remains elusive. Here, we present cryo-electron microscopy structures of the activated wild-type C5aR1-G_i protein complex bound to each of the following: C5a, the hexapeptidic agonist C5a^pep, and the G protein-biased agonist BM213. The structures reveal the landscape of the C5a-C5aR1 interaction as well as a common motif for the recognition of diverse orthosteric ligands. Moreover, combined with mutagenesis studies and cell-based pharmacological assays, we deciphered a framework for biased signaling using different peptide analogs and provided insight into the activation mechanism of C5aR1 by solving the structure of C5aR1^I116A mutant-G_i signaling activation complex induced by C089, which exerts antagonism on wild-type C5aR1. In addition, unusual conformational changes in the intracellular end of transmembrane domain 7 and helix 8 upon agonist binding suggest a differential signal transduction process. Collectively, our study provides mechanistic understanding into the ligand recognition, biased signaling modulation, activation, and G_i protein coupling of C5aR1, which may facilitate the future design of therapeutic agents.

Ligand recognition mechanism of the human relaxin family peptide receptor 4 (RXFP4)

Members of the insulin superfamily regulate pleiotropic biological processes through two types of target-specific but structurally conserved peptides, insulin/insulin-like growth factors and relaxin/insulin-like peptides. The latter bind to the human relaxin family peptide receptors (RXFPs). Here, we report three cryo-electron microscopy structures of RXFP4-G_i protein complexes in the presence of the endogenous ligand insulin-like peptide 5 (INSL5) or one of the two small molecule agonists, compound 4 and DC591053. The B chain of INSL5 adopts a single α-helix that penetrates into the orthosteric pocket, while the A chain sits above the orthosteric pocket, revealing a peptide-binding mode previously unknown. Together with mutagenesis and functional analyses, the key determinants responsible for the peptidomimetic agonism and subtype selectivity were identified. Our findings not only provide insights into ligand recognition and subtype selectivity among class A G protein-coupled receptors, but also expand the knowledge of signaling mechanisms in the insulin superfamily.

Structural Understanding of Peptide-Bound G Protein-Coupled Receptors: Peptide-Target Interactions

The activation of G protein-coupled receptors (GPCRs) is triggered by ligand binding to their orthosteric sites, which induces ligand-specific conformational changes. Agonists and antagonists bound to GPCR orthosteric sites provide detailed information on ligand-binding modes. Among these, peptide ligands play an instrumental role in GPCR pharmacology and have attracted increased attention as therapeutic drugs. The recent breakthrough in GPCR structural biology has resulted in the remarkable availability of peptide-bound GPCR complexes. Despite the several structural similarities shared by these receptors, they exhibit distinct features in terms of peptide recognition and receptor activation. From this perspective, we have summarized the current status of peptide-bound GPCR structural complexes, largely focusing on the interactions between the receptor and its peptide ligand at the orthosteric site. In-depth structural investigations have yielded valuable insights into the molecular mechanisms underlying peptide recognition. This study would contribute to the discovery of GPCR peptide drugs with improved therapeutic effects.

Protein Design Strategies for the Structural–Functional Studies of G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) are an important family of membrane proteins responsible for many physiological functions in human body. High resolution GPCR structures are required to understand their molecular mechanisms and perform rational drug design, as GPCRs play a crucial role in a variety of diseases. That is difficult to obtain for the wild-type proteins because of their low stability. In this review, we discuss how this problem can be solved by using protein design strategies developed to obtain homogeneous stabilized GPCR samples for crystallization and cryoelectron microscopy.

Role of chemokines in HPV-induced cancers

Human papillomaviruses (HPVs) cause cancers of the uterine cervix, oropharynx, anus, and vulvovaginal tract. Low-risk HPVs, such as HPV6 and 11, can also cause benign mucosal lesions including genital warts, and in patients with recurrent respiratory papillomatosis, lesions in the larynx, and on occasion, in the lungs. However, both high and less tumorigenic HPVs share a striking commonality in manipulating both innate and adaptive immune responses in HPV- infected keratinocytes, the natural host for HPV infection. In addition, immune/inflammatory cell infiltration into the tumor microenvironment influences cancer growth and prognosis, and this process is tightly regulated by different chemokines. Chemokines are small proteins and exert their biological effects by binding with G protein-coupled chemokine receptors (GPCRs) that are found on the surfaces of select target cells. Chemokines are not only involved in the establishment of a pro-tumorigenic microenvironment and organ-directed metastases but also involved in disease progression through enhancing tumor cell growth and proliferation. Therefore, having a solid grasp on chemokines and immune checkpoint modulators can help in the treatment of these cancers. In this review, we discuss the recent advances on the expression patterns and regulation of the main chemokines found in HPV-induced cancers, and their effects on both immune and non-immune cells in these lesions. Importantly, we also present the current knowledge of therapeutic interventions on the expression of specific chemokine and their receptors that have been shown to influence the development and progression of HPV-induced cancers.

Control of Gαq signaling dynamics and GPCR cross-talk by GRKs

Numerous processes contribute to the regulation of G protein-coupled receptors (GPCRs), but relatively little is known about rapid mechanisms that control signaling on the seconds time scale or regulate cross-talk between receptors. Here, we reveal that the ability of some GPCR kinases (GRKs) to bind Gα_q both drives acute signaling desensitization and regulates functional interactions between GPCRs. GRK2/3-mediated acute desensitization occurs within seconds, is rapidly reversible, and can occur upon local, subcellular activation. This rapid desensitization is kinase independent, insensitive to pharmacological inhibition, and generalizable across receptor families and effectors. We also find that the ability of GRK2 to bind G proteins also enables it to regulate the extent and timing of Gα_q-dependent signaling cross-talk between GPCRs. Last, we find that G protein/GRK2 interactions enable a novel form of GPCR trafficking cross-talk. Together, this work reveals potent forms of Gα_q-dependent GPCR regulation with wide-ranging pharmacological and physiological implications.

Eosinophils: A Friend or Foe in Human Health and Diseases

Background

Since their discovery, around 150 years, eosinophils research has been a field of changing perspective, and new directions are emerging since then.

Summary

Initially, eosinophils were perceived as terminally differentiated cytotoxic effector cells. Clearly, eosinophils are capable of playing functions other than immune responses, which is not surprising given their intricate interactions with pathogens as well as other circulating leukocytes. Attempts to comprehend the eosinophil biology and functions have yielded remarkable insights into their roles in human health and sickness. The use of FDA-approved eosinophils-targeting biologics has provided exciting opportunities to directly explore the contributions of eosinophils in disease etiology in humans.

Key Messages

In this review, we will focus on the eosinophils' lifecycle and discuss the current state of knowledge from mouse models and retrospective human studies demonstrating eosinophils' roles in the pathogenesis of human diseases such as asthma, cancer, and kidney disorders. Despite three recently approved anti-eosinophil agents, a number of key questions and challenges remain far from settled, thereby generating opportunity to further explore this enigmatic cell. A comprehensive understanding of eosinophils biology and function will surely aid in developing improved therapeutic strategies against eosinophils-associated disorders.

Molecular recognition of morphine and fentanyl by the human μ-opioid receptor

Morphine and fentanyl are among the most used opioid drugs that confer analgesia and unwanted side effects through both G protein and arrestin signaling pathways of μ-opioid receptor (μOR). Here, we report structures of the human μOR-G protein complexes bound to morphine and fentanyl, which uncover key differences in how they bind the receptor. We also report structures of μOR bound to TRV130, PZM21, and SR17018, which reveal preferential interactions of these agonists with TM3 side of the ligand-binding pocket rather than TM6/7 side. In contrast, morphine and fentanyl form dual interactions with both TM3 and TM6/7 regions. Mutations at the TM6/7 interface abolish arrestin recruitment of μOR promoted by morphine and fentanyl. Ligands designed to reduce TM6/7 interactions display preferential G protein signaling. Our results provide crucial insights into fentanyl recognition and signaling of μOR, which may facilitate rational design of next-generation analgesics.

Molecular insights into ligand recognition and activation of chemokine receptors CCR2 and CCR3

Chemokine receptors are a family of G-protein-coupled receptors with key roles in leukocyte migration and inflammatory responses. Here, we present cryo-electron microscopy structures of two human CC chemokine receptor-G-protein complexes: CCR2 bound to its endogenous ligand CCL2, and CCR3 in the apo state. The structure of the CCL2-CCR2-G-protein complex reveals that CCL2 inserts deeply into the extracellular half of the transmembrane domain, and forms substantial interactions with the receptor through the most N-terminal glutamine. Extensive hydrophobic and polar interactions are present between both two chemokine receptors and the Gα-protein, contributing to the constitutive activity of these receptors. Notably, complemented with functional experiments, the interactions around intracellular loop 2 of the receptors are found to be conserved and play a more critical role in G-protein activation than those around intracellular loop 3. Together, our findings provide structural insights into chemokine recognition and receptor activation, shedding lights on drug design targeting chemokine receptors.

Structural basis of GPCR coupling to distinct signal transducers: implications for biased signaling

Three classes of G-protein-coupled receptor (GPCR) partners - G proteins, GPCR kinases, and arrestins - preferentially bind active GPCRs. Our analysis suggests that the structures of GPCRs bound to these interaction partners available today do not reveal a clear conformational basis for signaling bias, which would have enabled the rational design of biased GRCR ligands. In view of this, three possibilities are conceivable: (i) there are no generalizable GPCR conformations conducive to binding a particular type of partner; (ii) subtle differences in the orientation of individual residues and/or their interactions not easily detectable in the receptor-transducer structures determine partner preference; or (iii) the dynamics of GPCR binding to different types of partners rather than the structures of the final complexes might underlie transducer bias.

Molecular recognition of formylpeptides and diverse agonists by the formylpeptide receptors FPR1 and FPR2

The formylpeptide receptors (FPRs) mediate pattern recognition of formylated peptides derived from invading pathogens or mitochondria from dead host cells. They can also sense other structurally distinct native peptides and even lipid mediators to either promote or resolve inflammation. Pharmacological targeting of FPRs represents a novel therapeutic approach in treating inflammatory diseases. However, the molecular mechanisms underlying FPR ligand recognition are elusive. We report cryo-EM structures of G_i-coupled FPR1 and FPR2 bound to a formylpeptide and G_i-coupled FPR2 bound to two synthetic peptide and small-molecule agonists. Together with mutagenesis data, our structures reveal the molecular mechanism of formylpeptide recognition by FPRs and structural variations of FPR1 and FPR2 leading to their different ligand preferences. Structural analysis also suggests that diverse FPR agonists sample a conserved activation chamber at the bottom of ligand-binding pockets to activate FPRs. Our results provide a basis for rational drug design on FPRs.