Molecular Mechanisms of GPCR Signaling: A Structural Perspective

Single nucleotide variations encoding missense mutations in G protein-coupled receptors may contribute to autism

Autism is a neurodevelopmental condition with a range of symptoms that vary in intensity and severity from person to person. Genetic sequencing has identified thousands of genes containing mutations in autistic individuals, which may contribute to the development of autistic symptoms. Several of these genes encode G protein-coupled receptors (GPCRs), which are cell surface expressed proteins that transduce extracellular messages to the intracellular space. Mutations in GPCRs can impact their function, resulting in aberrant signalling within cells and across neurotransmitter systems in the brain. This review summarises the current knowledge on autism-associated single nucleotide variations encoding missense mutations in GPCRs and the impact of these genetic mutations on GPCR function. For some autism-associated mutations, changes in GPCR expression levels, ligand affinity, potency and efficacy have been observed. However, for many the functional consequences remain unknown. Thus, further work to characterise the functional impacts of the genetically identified mutations is required. LINKED ARTICLES: This article is part of a themed issue Therapeutic Targeting of G Protein-Coupled Receptors: hot topics from the Australasian Society of Clinical and Experimental Pharmacologists and Toxicologists 2021 Virtual Annual Scientific Meeting. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.14/issuetoc.

A Study on the Robustness and Stability of Explainable Deep Learning in an Imbalanced Setting: The Exploration of the Conformational Space of G Protein-Coupled Receptors

G-protein coupled receptors (GPCRs) are transmembrane proteins that transmit signals from the extracellular environment to the inside of the cells. Their ability to adopt various conformational states, which influence their function, makes them crucial in pharmacoproteomic studies. While many drugs target specific GPCR states to exert their effects-thereby regulating the protein's activity-unraveling the activation pathway remains challenging due to the multitude of intermediate transformations occurring throughout this process, and intrinsically influencing the dynamics of the receptors. In this context, computational modeling, particularly molecular dynamics (MD) simulations, may offer valuable insights into the dynamics and energetics of GPCR transformations, especially when combined with machine learning (ML) methods and techniques for achieving model interpretability for knowledge generation. The current study builds upon previous work in which the layer relevance propagation (LRP) technique was employed to interpret the predictions in a multi-class classification problem concerning the conformational states of the β2-adrenergic (β2AR) receptor from MD simulations. Here, we address the challenges posed by class imbalance and extend previous analyses by evaluating the robustness and stability of deep learning (DL)-based predictions under different imbalance mitigation techniques. By meticulously evaluating explainability and imbalance strategies, we aim to produce reliable and robust insights.

Chemogenetic Signaling in Space and Time: Considerations for Designing Neuroscience Experiments Using DREADDs

The use of designer receptors exclusively activated by designer drugs (DREADDs) has led to significant advances in our understanding of the neural circuits that govern behavior. By allowing selective control over cellular activity and signaling, DREADDs have become an integral tool for defining the pathways and cellular phenotypes that regulate sleep, pain, motor activity, goal-directed behaviors, and a variety of other processes. In this review, we provide a brief overview of DREADDs and discuss notable discoveries in the neurosciences with an emphasis on circuit mechanisms. We then highlight methodological approaches to achieve pathway specific activation of DREADDs. Finally, we discuss spatial and temporal constraints of DREADDs signaling and how these features can be incorporated into experimental designs to precisely dissect circuits of interest.

From orphan to oncogene: The role of GPR35 in cancer and immune modulation

G protein-coupled receptors (GPCRs) are well-studied and the most traceable cell surface receptors for drug discovery. One of the intriguing members of this family is G protein-coupled receptors 35 (GPR35), which belongs to the class A rhodopsin-like family of GPCRs identified over two decades ago. GPR35 presents interesting features such as ubiquitous expression and distinct isoforms. Moreover, functional and genome-wide association studies on its widespread expression have linked GPR35 with pathophysiological disease progression. Various pieces of evidence have been accumulated regarding the independent or endogenous ligand-dependent role of GPR35 in cancer progression and metastasis. In the current scenario, the relationship of this versatile receptor and its putative endogenous ligands for the activation of oncogenic signal transduction pathways at the cellular level is an active area of research. These intriguing features offered by GPR35 make it an oncological target, justifying its uniqueness at the physiological and pathophysiological levels concerning other GPCRs. For pharmacologically targeting receptor-induced signaling, few potential competitive antagonists have been discovered that offer high selectivity at a human level. In addition to its fascinating features, targeting GPR35 at rodent and human orthologue levels is distinct, thus contributing to the sub-species selectivity. Strategies to modulate these issues will help us understand and truly target GPR35 at the therapeutic level. In this article, we have provided prospects on each topic mentioned above and suggestions to overcome the challenges. This review discusses the molecular mechanism and signal transduction pathways activated by endogenous ligands or spontaneous auto-activation of GPR35 that contributes towards disease progression. Furthermore, we have highlighted the GPR35 structure, ubiquitous expression, its role in immunomodulation, and at the pathophysiological level, especially in cancer, indicating its status as a versatile receptor. Subsequently, we discussed the various proposed ligands and their mechanism of interaction with GPR35. Additionally, we have summarized the GPR35 antagonist that provides insights into the opportunities for therapeutically targeting this receptor.

Modelling lipid rafts formation through chemo-mechanical interplay triggered by receptor-ligand binding

Cell membranes, mediator of many biological mechanisms from adhesion and metabolism up to mutation and infection, are highly dynamic and heterogeneous environments exhibiting a strong coupling between biochemical events and structural re-organisation. This involves conformational changes induced, at lower scales, by lipid order transitions and by the micro-mechanical interplay of lipids with transmembrane proteins and molecular diffusion. Particular attention is focused on lipid rafts, ordered lipid microdomains rich of signalling proteins, that co-localise to enhance substance trafficking and activate different intracellular biochemical pathways. In this framework, the theoretical modelling of the dynamic clustering of lipid rafts implies a full multiphysics coupling between the kinetics of phase changes and the mechanical work performed by transmembrane proteins on lipids, involving the bilayer elasticity. This mechanism produces complex interspecific dynamics in which membrane stresses and chemical potentials do compete by determining different morphological arrangements, alteration in diffusive walkways and coalescence phenomena, with a consequent influence on both signalling potential and intracellular processes. Therefore, after identifying the leading chemo-mechanical interactions, the present work investigates from a modelling perspective the spatio-temporal evolution of raft domains to theoretically explain co-localisation and synergy between proteins' activation and raft formation, by coupling diffusive and mechanical phenomena to observe different morphological patterns and clustering of ordered lipids. This could help to gain new insights into the remodelling of cell membranes and could potentially suggest mechanically based strategies to control their selectivity, by orienting intracellular functions and mechanotransduction.

Assessing Protein-Protein Docking Protocols: Case Studies of G-Protein-Coupled Receptor Interactions

The interactions of G-protein-coupled receptors (GPCRs) with other proteins are critical in several cellular processes but resolving their structural dynamics remains challenging. An increasing number of GPCR complexes have been experimentally resolved but others including receptor variants are yet to be characterized, necessitating computational predictions of their interactions. Although integrative approaches with multi-scale simulations would provide rigorous estimates of their conformational dynamics, protein-protein docking remains a first tool of choice of many researchers due to the availability of open-source programs and easy to use web servers with reasonable predictive power. Protein-protein docking algorithms have limited ability to consider protein flexibility, environment effects, and entropy contributions and are usually a first step towards more integrative approaches. The two critical steps of docking: the sampling and scoring algorithms have improved considerably and their performance has been validated against experimental data. In this chapter, we provide an overview and generalized protocol of a few docking protocols using GPCRs as test cases. In particular, we demonstrate the interactions of GPCRs with extracellular protein ligands and an intracellular protein effectors (G-protein) predicted from docking approaches and test their limitations. The current chapter will help researchers critically assess docking protocols and predict experimentally testable structures of GPCR complexes.

Targeting GPR65 alleviates hepatic inflammation and fibrosis by suppressing the JNK and NF-κB pathways

BACKGROUND

G-protein coupled receptors (GPCRs) are recognized as attractive targets for drug therapy. However, it remains poorly understood how GPCRs, except for a few chemokine receptors, regulate the progression of liver fibrosis. Here, we aimed to reveal the role of GPR65, a proton-sensing receptor, in liver fibrosis and to elucidate the underlying mechanism.

METHODS

The expression level of GPR65 was evaluated in both human and mouse fibrotic livers. Furthermore, Gpr65-deficient mice were treated with either bile duct ligation (BDL) for 21 d or carbon tetrachloride (CCl4) for 8 weeks to investigate the role of GPR65 in liver fibrosis. A combination of experimental approaches, including Western blotting, quantitative real-time reverse transcription‑polymerase chain reaction (qRT-PCR), and enzyme-linked immunosorbent assay (ELISA), confocal microscopy and rescue studies, were used to explore the underlying mechanisms of GPR65's action in liver fibrosis. Additionally, the therapeutic potential of GPR65 inhibitor in the development of liver fibrosis was investigated.

RESULTS

We found that hepatic macrophages (HMs)-enriched GPR65 was upregulated in both human and mouse fibrotic livers. Moreover, knockout of Gpr65 significantly alleviated BDL- and CCl4-induced liver inflammation, injury and fibrosis in vivo, and mouse bone marrow transplantation (BMT) experiments further demonstrated that the protective effect of Gpr65 knockout is primarily mediated by bone marrow-derived macrophages (BMMs). Additionally, in vitro data demonstrated that Gpr65 silencing and GPR65 antagonist inhibited, while GPR65 overexpression and application of GPR65 endogenous and exogenous agonists enhanced the expression and release of tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6) and transforming growth factor-β (TGF-β), all of which subsequently promoted the activation of hepatic stellate cells (HSCs) and the damage of hepatocytes (HCs). Mechanistically, GPR65 overexpression, the acidic pH and GPR65 exogenous agonist induced up-regulation of TNF-α and IL-6 via the Gαq-Ca2+-JNK/NF-κB pathways, while promoted the expression of TGF-β through the Gαq-Ca2+-MLK3-MKK7-JNK pathway. Notably, pharmacological GPR65 inhibition retarded the development of inflammation, HCs injury and fibrosis in vivo.

CONCLUSIONS

GPR65 is a major regulator that modulates the progression of liver fibrosis. Thus, targeting GPR65 could be an effective therapeutic strategy for the prevention of liver fibrosis.

Analgesic Activity of 5-Acetamido-2-Hydroxy Benzoic Acid Derivatives and an In-Vivo and In-Silico Analysis of Their Target Interactions

The design, synthesis, and evaluation of novel non-steroidal anti-inflammatory drugs (NSAIDs) with better activity and lower side effects are big challenges today. In this work, two 5-acetamido-2-hydroxy benzoic acid derivatives were proposed, increasing the alkyl position (methyl) in an acetamide moiety, and synthesized, and their structural elucidation was performed using 1H NMR and 13C NMR. The changes in methyl in larger groups such as phenyl and benzyl aim to increase their selectivity over cyclooxygenase 2 (COX-2). These 5-acetamido-2-hydroxy benzoic acid derivatives were prepared using classic methods of acylation reactions with anhydride or acyl chloride. Pharmacokinetics and toxicological properties were predicted using computational tools, and their binding affinity (kcal/mol) with COX-2 receptors (Mus musculus and Homo sapiens) was analyzed using docking studies (PDB ID 4PH9, 5KIR, 1PXX and 5F1A). An in-silico study showed that 5-acetamido-2-hydroxy benzoic acid derivates have a better bioavailability and binding affinity with the COX-2 receptor, and in-vivo anti-nociceptive activity was investigated by means of a writhing test induced by acetic acid and a hot plate. PS3, at doses of 20 and 50 mg/kg, reduced painful activity by 74% and 75%, respectively, when compared to the control group (20 mg/kg). Regarding the anti-nociceptive activity, the benzyl showed reductions in painful activity when compared to acetaminophen and 5-acetamido-2-hydroxy benzoic acid. However, the proposed derivatives are potentially more active than 5-acetamido-2-hydroxy benzoic acid and they support the design of novel and safer derivative candidates. Consequently, more studies need to be conducted to evaluate the different pharmacological actions, the toxicity of possible metabolites that can be generated, and their potential use in inflammation and pain therapy.

The NPXXY Motif Regulates β-Arrestin Recruitment by the CB1 Cannabinoid Receptor

Background: Activation of signaling effectors by G-protein coupled receptors (GPCRs) depends on different molecular mechanisms triggered by conserved amino acid residues. Although studies have focused on the G-protein signaling state, the mechanism for β-arrestin signaling by CB1 is not yet well defined. Studies have indicated that transmembrane helix 7 (TMH7) and the highly conserved NPXXY motif can be subject to different conformational changes in response to biased ligands and could therefore participate in a molecular mechanism to trigger β-arrestin recruitment. Objective: To investigate the effect of mutations in the NPXXY motif on different signaling pathways activated by the CB1 receptor. Materials and Methods: Point mutations of the NPXXY motif and associated residues were generated in the CB1 receptor using site-directed mutagenesis and transfection into HEK-293 cells. Signaling by wild-type and mutant receptors was analyzed by quantifying inhibition of cAMP, and by β-arrestin recruitment assays. Results: We found that N7.49 and Y7.53 are essential for β-arrestin recruitment by CB1. N7.49A and Y7.53F impair β-arrestin signaling, with no effect on G-protein signaling. We found a regulatory role for residue I2.43; I2.43 interacts with Y7.53, affecting its positioning. Reducing steric bulk at I2.43 (I2.43A) enhances β-arrestin1 recruitment, while introducing a polar residue (I2.43T) reduces β-arrestin recruitment. Conclusions: These findings point to a novel mechanism for β-arrestin recruitment, implicating amino acids in the NPXXY motif as critical for the putative β-arrestin biased conformational state of Class A GPCRs.

Genetic variants of G-protein coupled receptors associated with pubertal disorders

Background

The human hypothalamic-pituitary-gonadal (HPG) axis is the regulatory center for pubertal development. This axis involves six G-protein coupled receptors (GPCRs) encoded by KISS1R, TACR3, PROKR2, GNRHR, LHCGR, and FSHR.

Methods

Previous studies have identified several rare variants of the six GPCR genes in patients with pubertal disorders. In vitro assays and animal studies have provided information on the function of wild-type and variant GPCRs.

Main Findings

Of the six GPCRs, those encoded by KISS1R and TACR3 are likely to reside at the top of the HPG axis. Several loss-of-function variants in the six genes were shown to cause late/absent puberty. In particular, variants in KISS1R, TACR3, PROKR2, and GNRHR lead to hypogonadotropic hypogonadism in autosomal dominant, recessive, and oligogenic manners. Furthermore, a few gain-of-function variants of KISS1R, PROKR2, and LHCGR have been implicated in precocious puberty. The human HPG axis may contain additional GPCRs.

Conclusion

The six GPCRs in the HPG axis govern pubertal development through fine-tuning of hormone secretion. Rare sequence variants in these genes jointly account for a certain percentage of genetic causes of pubertal disorders. Still, much remains to be clarified about the molecular network involving the six GPCRs.

Metadynamics simulations leveraged by statistical analyses and artificial intelligence-based tools to inform the discovery of G protein-coupled receptor ligands

G Protein-Coupled Receptors (GPCRs) are a large family of membrane proteins with pluridimensional signaling profiles. They undergo ligand-specific conformational changes, which in turn lead to the differential activation of intracellular signaling proteins and the consequent triggering of a variety of biological responses. This conformational plasticity directly impacts our understanding of GPCR signaling and therapeutic implications, as do ligand-specific kinetic differences in GPCR-induced transducer activation/coupling or GPCR-transducer complex stability. High-resolution experimental structures of ligand-bound GPCRs in the presence or absence of interacting transducers provide important, yet limited, insights into the highly dynamic process of ligand-induced activation or inhibition of these receptors. We and others have complemented these studies with computational strategies aimed at characterizing increasingly accurate metastable conformations of GPCRs using a combination of metadynamics simulations, state-of-the-art algorithms for statistical analyses of simulation data, and artificial intelligence-based tools. This minireview provides an overview of these approaches as well as lessons learned from them towards the identification of conformational states that may be difficult or even impossible to characterize experimentally and yet important to discover new GPCR ligands.

Gαs and Gαq/11 protein coupling bias of two AVPR2 mutants (R68W and V162A) that cause nephrogenic diabetes insipidus

Loss-of-function mutations of the arginine vasopressin receptor 2 gene (AVPR2) cause Nephrogenic diabetes insipidus (NDI). AVPR2 is a kind of G protein coupled receptor (GPCR) and mainly couples with Gαs protein leading to cAMP accumulation in the cell as a secondary messenger. Recent studies showed that some AVPR2 mutations could cause biased Gαq/11 protein coupling rather than Gαs. Investigation into the characterization of biased receptors may give insights into the relationship between the conformational change of the receptor because of the mutation and related downstream signaling. In this study, R68W and V162A were analyzed to whether they show a bias to Gαs or Gαq/11 proteins. Their functionality in terms of cAMP production via Gαs protein coupling was decreased compared to the wild-type receptor. On the other hand, they showed the ability to couple with Gαq/11 protein and make Ca²⁺ mobilization at different levels in the cell. R68W showed bias to coupling with Gαq/11 protein rather than V162A and wild-type receptor. Studies about the Gα protein coupling bias of mutant AVPR2s may broaden our understanding of the relationship between the changed conformation of the receptor and consequently activated signaling pathways, and also may shed light on the development of more effective new therapeutics.

Structure-guided optimization of light-activated chimeric G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) are the largest human receptor family and involved in virtually every physiological process. One hallmark of their function is specific coupling to selected signaling pathways. The ability to tune this coupling would make development of receptors with new capabilities possible. Complexes of GPCRs and G-proteins have recently been resolved at high resolution, but this information was in only few cases harnessed for rational receptor engineering. Here, we demonstrate structure-guided optimization of light-activated OptoXRs. Our hypothesis was that incorporation of GPCR-Gα contacts would lead to improved coupling. We first evaluated structure-based alignments for chimeric receptor fusion. We then show in a light-activated β₂AR that including Gα contacts increased signaling 7- to 20-fold compared with other designs. In turn, contact elimination diminished function. Finally, this platform allowed optimization of a further OptoXR and spectral tuning. Our work exemplifies structure-based OptoXR development for targeted cell and network manipulation.

Activation and allosteric regulation of the orphan GPR88-Gi1 signaling complex

GPR88 is an orphan class A G-protein-coupled receptor that is highly expressed in the striatum and regulates diverse brain and behavioral functions. Here we present cryo-EM structures of the human GPR88-Gi1 signaling complex with or without a synthetic agonist (1R, 2R)-2-PCCA. We show that (1R, 2R)-2-PCCA is an allosteric modulator binding to a herein identified pocket formed by the cytoplasmic ends of transmembrane segments 5, 6, and the extreme C terminus of the α5 helix of Gi1. We also identify an electron density in the extracellular orthosteric site that may represent a putative endogenous ligand of GPR88. These structures, together with mutagenesis studies and an inactive state model obtained from metadynamics simulations, reveal a unique activation mechanism for GPR88 with a set of distinctive structure features and a water-mediated polar network. Overall, our results provide a structural framework for understanding the ligand binding, activation and signaling mechanism of GPR88, and will facilitate the innovative drug discovery for neuropsychiatric disorders and for deorphanization of this receptor.

Identification of a tachykinin receptor and its implication in carbohydrate and lipid homeostasis in Rhodnius prolixus, a chagas disease vector

Neuropeptides and their receptors are fundamentally important in regulating many physiological and behavioural processes in insects. In this work, we have identified, cloned, and sequenced the tachykinin receptor (Rhopr-TKR) from Rhodnius prolixus, a vector of Chagas disease. The receptor is a G protein-coupled receptor belonging to the Rhodopsin Family A. The total length of the open reading frame of the Rhopr-TKR transcript is 1110 bp, which translates into a receptor of 338 amino acids. Fluorescent in-situ RNA-hybridization (FISH) for the Rhopr-TKR transcript shows a signal in a group of six bilaterally paired neurons in the protocerebrum of the brain, localized in a similar region as the insulin producing cells. To examine the role of tachykinin signaling in lipid and carbohydrate homeostasis we used RNA interference. Downregulation of the Rhopr-TKR transcript led to a decrease in the size of blood meal consumed and a significant increase in circulating carbohydrate and lipid levels. Further investigation revealed a close relationship between tachykinin and insulin signaling since the downregulation of the Rhopr-TKR transcript negatively affected the transcript expression for insulin-like peptide 1 (Rhopr-ILP1), insulin-like growth factor (Rhopr-IGF) and insulin receptor 1 (Rhopr-InR1) in both the central nervous system and fat body. Taken together, these findings suggest that tachykinin signaling regulates lipid and carbohydrate homeostasis via the insulin signaling pathway.

Development of enhanced conformational sampling methods to probe the activation landscape of GPCRs

G protein-coupled receptors (GPCRs) make up the largest superfamily of integral membrane proteins and play critical signal transduction roles in many physiological processes. Developments in molecular biology, biophysical, biochemical, pharmacological, and computational techniques aimed at these important therapeutic targets are beginning to provide unprecedented details on the structural as well as functional basis of their pleiotropic signaling mediated by G proteins, β arrestins, and other transducers. This pleiotropy presents a pharmacological challenge as the same ligand-receptor interaction can cause a therapeutic effect as well as an undesirable on-target side-effect through different downstream pathways. GPCRs don't function as simple binary on-off switches but as finely tuned shape-shifting machines described by conformational ensembles, where unique subsets of conformations may be responsible for specific signaling cascades. X-ray crystallography and more recently cryo-electron microscopy are providing snapshots of some of these functionally-important receptor conformations bound to ligands and/or transducers, which are being utilized by computational methods to describe the dynamic conformational energy landscape of GPCRs. In this chapter, we review the progress in computational conformational sampling methods based on molecular dynamics and discrete sampling approaches that have been successful in complementing biophysical and biochemical studies on these receptors in terms of their activation mechanisms, allosteric effects, actions of biased ligands, and effects of pathological mutations. Some of the sampled simulation time scales are beginning to approach receptor activation time scales. The list of conformational sampling methods and example uses discussed is not exhaustive but includes representative examples that have pushed the limits of classical molecular dynamics and discrete sampling methods to describe the activation energy landscape of GPCRs.

Extracellular crosslinking mass spectrometry reveals HLA class I - HLA class II interactions on the cell surface

Human Leukocyte Antigen (HLA) complexes are critical cell-surface protein assemblies that facilitate T-cell surveillance of almost all cell types in the body. While T-cell receptor binding to HLA class I and class II complexes is well-described with detailed structural information, the nature of cis HLA interactions within the plasma membrane of the surveyed cells remains to be better characterized, as protein-protein interactions in the membrane environment are technically challenging to profile. Here we performed extracellular chemical crosslinking on intact antigen presenting cells to specifically elucidate protein-protein interactions present in the external plasma membrane. We found that the crosslink dataset was dominated by inter- and intra-protein crosslinks involving HLA molecules, which enabled not only the construction of an HLA-centric plasma membrane protein interaction map, but also revealed multiple modes of HLA class I - HLA class II interactions with further structural modeling based on crosslinker distance restraints. Collectively, our data demonstrate that HLA molecules colocalize and can be densely packed on the plasma membrane.

Living-Cell Diffracted X-ray Tracking Analysis Confirmed Internal Salt Bridge Is Critical for Ligand-Induced Twisting Motion of Serotonin Receptors

Serotonin receptors play important roles in neuronal excitation, emotion, platelet aggregation, and vasoconstriction. The serotonin receptor subtype 2A (5-HT_2AR) is a Gq-coupled GPCR, which activate phospholipase C. Although the structures and functions of 5-HT_2ARs have been well studied, little has been known about their real-time dynamics. In this study, we analyzed the intramolecular motion of the 5-HT_2AR in living cells using the diffracted X-ray tracking (DXT) technique. The DXT is a very precise single-molecular analytical technique, which tracks diffraction spots from the gold nanocrystals labeled on the protein surface. Trajectory analysis provides insight into protein dynamics. The 5-HT_2ARs were transiently expressed in HEK 293 cells, and the gold nanocrystals were attached to the N-terminal introduced FLAG-tag via anti-FLAG antibodies. The motions were recorded with a frame rate of 100 μs per frame. A lifetime filtering technique demonstrated that the unliganded receptors contain high mobility population with clockwise twisting. This rotation was, however, abolished by either a full agonist α-methylserotonin or an inverse agonist ketanserin. Mutation analysis revealed that the “ionic lock” between the DRY motif in the third transmembrane segment and a negatively charged residue of the sixth transmembrane segment is essential for the torsional motion at the N-terminus of the receptor.

GRK6 regulates the hemostatic response to injury through its rate-limiting effects on GPCR signaling in platelets

G protein-coupled receptors (GPCRs) mediate the majority of platelet activation in response to agonists. However, questions remain regarding the mechanisms that provide negative feedback toward activated GPCRs to limit platelet activation and thrombus formation. Here we provide the first evidence that GPCR kinase 6 (GRK6) serves this role in platelets, using GRK6-/- mice generated by CRISPR-Cas9 genome editing to examine the consequences of GRK6 knockout on GPCR-dependent signaling. Hemostatic thrombi formed in GRK6-/- mice are larger than in wild-type (WT) controls during the early stages of thrombus formation, with a rapid increase in platelet accumulation at the site of injury. GRK6-/- platelets have increased platelet activation, but in an agonist-selective manner. Responses to PAR4 agonist or adenosine 5'-diphosphate stimulation in GRK6-/- platelets are increased compared with WT littermates, whereas the response to thromboxane A2 (TxA2) is normal. Underlying these changes in GRK6-/- platelets is an increase in Ca2+ mobilization, Akt activation, and granule secretion. Furthermore, deletion of GRK6 in human MEG-01 cells causes an increase in Ca2+ response and PAR1 surface expression in response to thrombin. Finally, we show that human platelet activation in response to thrombin causes an increase in binding of GRK6 to PAR1, as well as an increase in the phosphorylation of PAR1. Deletion of GRK6 in MEG-01 cells causes a decrease in PAR1 phosphorylation. Taken together, these data show that GRK6 regulates the hemostatic response to injury through PAR- and P2Y12-mediated effects, helping to limit the rate of platelet activation during thrombus growth and prevent inappropriate platelet activation.

Trends in application of advancing computational approaches in GPCR ligand discovery

G protein-coupled receptors (GPCRs) comprise the most important superfamily of protein targets in current ligand discovery and drug development. GPCRs are integral membrane proteins that play key roles in various cellular signaling processes. Therefore, GPCR signaling pathways are closely associated with numerous diseases, including cancer and several neurological, immunological, and hematological disorders. Computer-aided drug design (CADD) can expedite the process of GPCR drug discovery and potentially reduce the actual cost of research and development. Increasing knowledge of biological structures, as well as improvements on computer power and algorithms, have led to unprecedented use of CADD for the discovery of novel GPCR modulators. Similarly, machine learning approaches are now widely applied in various fields of drug target research. This review briefly summarizes the application of rising CADD methodologies, as well as novel machine learning techniques, in GPCR structural studies and bioligand discovery in the past few years. Recent novel computational strategies and feasible workflows are updated, and representative cases addressing challenging issues on olfactory receptors, biased agonism, and drug-induced cardiotoxic effects are highlighted to provide insights into future GPCR drug discovery.

Influence of Bacillus thuringiensis and avermectins on gut physiology and microbiota in Colorado potato beetle: Impact of enterobacteria on susceptibility to insecticides

Gut physiology and the bacterial community play crucial roles in insect susceptibility to infections and insecticides. Interactions among Colorado potato beetle Leptinotarsa decemlineata (Say), its bacterial associates, pathogens and xenobiotics have been insufficiently studied. In this paper, we present our study of the survival, midgut histopathology, activity of digestive enzymes and bacterial communities of L. decemlineata larvae under the influence of Bacillus thuringiensis var. tenebrionis (morrissoni) (Bt), a natural complex of avermectins and a combination of both agents. Moreover, we estimated the impact of culturable enterobacteria on the susceptibility of the larvae to Bt and avermectins. An additive effect between Bt and avermectins was established regarding the mortality of the larvae. Both agents led to the destruction of midgut tissues, a decrease in the activity of alpha-amylases and alkaline proteinases, a decrease in the Spiroplasma leptinotarsae relative abundance and a strong elevation of Enterobacteriaceae abundance in the midgut. Moreover, an elevation of the enterobacterial CFU count was observed under the influence of Bt and avermectins, and the greatest enhancement was observed after combined treatment. Insects pretreated with antibiotics were less susceptible to Bt and avermectins, but reintroduction of the predominant enterobacteria Enterobacter ludwigii, Citrobacter freundii and Serratia marcescens increased susceptibility to both agents. We suggest that enterobacteria play an important role in the acceleration of Bt infection and avermectin toxicoses in L. decemlineata and that the additive effect between Bt and avermectin may be mediated by alterations in the bacterial community.

Fungus Metarhizium robertsii and neurotoxic insecticide affect gut immunity and microbiota in Colorado potato beetles

Fungal infections and toxicoses caused by insecticides may alter microbial communities and immune responses in the insect gut. We investigated the effects of Metarhizium robertsii fungus and avermectins on the midgut physiology of Colorado potato beetle larvae. We analyzed changes in the bacterial community, immunity- and stress-related gene expression, reactive oxygen species (ROS) production, and detoxification enzyme activity in response to topical infection with the M. robertsii fungus, oral administration of avermectins, and a combination of the two treatments. Avermectin treatment led to a reduction in microbiota diversity and an enhancement in the abundance of enterobacteria, and these changes were followed by the downregulation of Stat and Hsp90, upregulation of transcription factors for the Toll and IMD pathways and activation of detoxification enzymes. Fungal infection also led to a decrease in microbiota diversity, although the changes in community structure were not significant, except for the enhancement of Serratia. Fungal infection decreased the production of ROS but did not affect the gene expression of the immune pathways. In the combined treatment, fungal infection inhibited the activation of detoxification enzymes and prevented the downregulation of the JAK-STAT pathway caused by avermectins. The results of this study suggest that fungal infection modulates physiological responses to avermectins and that fungal infection may increase avermectin toxicosis by blocking detoxification enzymes in the gut.

Pharmacology profile of F17464, a dopamine D3 receptor preferential antagonist

F17464 (N-(3-{4-[4-(8-Oxo-8H-[1,3]-dioxolo-[4,5-g]-chromen-7-yl)-butyl]-piperazin-1-yl}-phenyl)-methanesulfonamide, hydrochloride) is a new potential antipsychotic with a unique profile. The compound exhibits high affinity for the human dopamine receptor subtype 3 (hD3) (Ki = 0.17 nM) and the serotonin receptor subtype 1a (5-HT1a) (Ki = 0.16 nM) and a >50 fold lower affinity for the human dopamine receptor subtype 2 short and long form (hD2s/l) (Ki = 8.9 and 12.1 nM, respectively). [14C]F17464 dynamic studies show a slower dissociation rate from hD3 receptor (t1/2 = 110 min) than from hD2s receptor (t1/2 = 1.4 min) and functional studies demonstrate that F17464 is a D3 receptor antagonist, 5-HT1a receptor partial agonist. In human dopaminergic neurons F17464 blocks ketamine induced morphological changes, an effect D3 receptor mediated. In vivo F17464 target engagement of both D2 and 5-HT1a receptors is demonstrated in displacement studies in the mouse brain. F17464 increases dopamine release in the rat prefrontal cortex and mouse lateral forebrain - dorsal striatum and seems to reduce the effect of MK801 on % c-fos mRNA medium expressing neurons in cortical and subcortical regions. F17464 also rescues valproate induced impairment in a rat social interaction model of autism. All the neurochemistry and behavioural effects of F17464 are observed in the dose range 0.32-2.5 mg/kg i.p. in both rats and mice. The in vitro - in vivo pharmacology profile of F17464 in preclinical models is discussed in support of a therapeutic use of the compound in schizophrenia and autism.

The finger loop as an activation sensor in arrestin

The finger loop in the central crest of the receptor-binding site of arrestins engages the cavity between the transmembrane helices of activated G-protein-coupled receptors. Therefore, it was hypothesized to serve as the sensor that detects the activation state of the receptor. We performed comprehensive mutagenesis of the finger loop in bovine visual arrestin-1, generated mutant radiolabeled proteins by cell-free translation, and determined the effects of mutations on the in vitro binding of arrestin-1 to purified phosphorylated light-activated rhodopsin. This interaction is driven by two factors, rhodopsin activation and rhodopsin-attached phosphates. Therefore, the binding of arrestin-1 to light-activated unphosphorylated rhodopsin is low. To evaluate the role of the finger loop specifically in the recognition of the active receptor conformation, we tested the effects of these mutations in the context of truncated arrestin-1 that demonstrates much higher binding to unphosphorylated activated and phosphorylated inactive rhodopsin. The majority of finger loop residues proved important for arrestin-1 binding to light-activated rhodopsin, with six mutations affecting the binding exclusively to this form. Thus, the finger loop is the key element of arrestin-1 activation sensor. The data also suggest that arrestin-1 and its enhanced mutant bind various functional forms of rhodopsin differently.

In silico molecular docking and physicochemical property studies on effective phytochemicals targeting GPR116 for breast cancer treatment

G protein-coupled receptor 116 (GPR116), an orphan adhesion receptor, found an important role in cell adhesion and migration in eukaryotes. Abnormal expression of GPCR identified in various cancers turns focus of research community towards GPCR to identify the targeting drug against GPCR. Though GPR116 role was studied in progression of metastasis in triple-negative breast cancer (TNBC), unfortunately, still no drugs targeting GPR116 were identified. TNBC is a hormone-negative aggressive breast cancer found even in young women. Since TNBC has no target receptor for therapy, it would be desirable to target GPR116. Currently, chemotherapy is the only promising option for TNBC; however, these drugs cause chemoresistance. Hence this current study concentrated on finding drugable natural phytochemical ligands targeting GPR116 using in silico approach. Best docked ligand with target and active binding site amino acids were identified in molecular docking study. Pharmacokinetic properties (ADME) were assessed by Qikprop. Result showed that pharmacokinetics properties of natural phytochemicals were as good as existing chemotherapeutic cancer drugs. This study indicates that phytochemicals could be a promising target for GPR116. This in silico analysis facilitates further research to design the drug targeting GPR116 for treatment of TNBC.

G-Protein coupled receptors: structure and function in drug discovery

The G-protein coupled receptors (GPCRs) superfamily comprise similar proteins arranged into families or classes thus making it one of the largest in the mammalian genome. GPCRs take part in many vital physiological functions making them targets for numerous novel drugs. GPCRs share some distinctive features, such as the seven transmembrane domains, they also differ in the number of conserved residues in their transmembrane domain. Here we provide an introductory and accessible review detailing the computational advances in GPCR pharmacology and drug discovery. An overview is provided on family A-C GPCRs; their structural differences, GPCR signalling, allosteric binding and cooperativity. The dielectric constant (relative permittivity) of proteins is also discussed in the context of site-specific environmental effects.

The effects of diazinon on the cell types and gene expression of the olfactory epithelium and whole-body hormone concentrations in the Persian sturgeon (Acipenser persicus)

The olfactory function and imprinting of odorant information of the native stream play a critical role during the homing migration in fish. Pesticides may impair olfactory imprinting by altering olfaction and hormone functions. The present study aimed to determine how diazinon impacts olfactory epithelium morphology and cell composition, as well as hormone concentrations in Persian sturgeon (Acipenser persicus) during their lifetime in freshwater and, also during diazinon-free saltwater acclimation. Fingerlings were exposed to 0, 150, 300, and 450 μg·L^-1 of diazinon in freshwater for 7 days and then were transferred to diazinon-free saltwater by gradually increasing salinity up to 12 ppt. After diazinon exposure, the number of olfactory receptor cells (ORCs) and goblet cells (GCs) decreased and increased, respectively, and the expression of G-protein αolf (GPαolf) and calmodulin-dependent kinase II delta (CAMKIId) was down-regulated and up-regulated, respectively. Transferring the fish to diazinon-free saltwater (8 and 12 ppt) raised the number of ORCs, supporting cells (SCs), GCs, and GPαolf expression, and down-regulated CAMKIId without any significant differences among treatments. Exposure to diazinon increased whole-body cortisol at the high concentration, while decreased whole-body thyroxin (T4) and triiodothyronine (T3) in a dose-dependent manner. Although whole-body T4 and T3 increased in all the treatments after saltwater acclimation (8 and 12 ppt), the level of these hormones was lower in fish that had been exposed to diazinon than in the control. These results showed that diazinon can disrupt olfactory epithelium morphology and cell composition as well as hormone concentrations, which in turn may affect the olfactory imprinting in Persian sturgeon fingerlings.

Pituitary miRNAs target GHRHR splice variants to regulate GH synthesis by mediating different intracellular signalling pathways

Growth hormone (GH), whose synthesis and release are mainly regulated by intracellular signals mediated by growth hormone-releasing hormone receptor (GHRHR), is one of the major pituitary hormones and critical regulators of organism growth, metabolism, and immunoregulation. Pig GHRHR splice variants (SVs) may activate different signalling pathways via the variable C-terminal by alternative splicing, and SVs have the potential to change microRNA (miRNA) binding sites. In this study, we first confirmed the existence of pig GHRHR SVs (i.e., GHRHR, GHRHR SV1 and SV2) and demonstrated the inhibitory effects of critical pituitary miRNAs (i.e., let-7e and miR-328-5p) on GH synthesis and cell proliferation of primary pituitary cells. The SVs of GHRHR targeted by let-7e and miR-328-5p were predicted via bioinformatics analysis and verified by performing dual-luciferase reporter assays and detecting the expression of target transcripts. The differential responses of let-7e, and miR-328-5p to GH-releasing hormone and the changes in signalling pathways mediated by GHRHR suggested that let-7e and miR-328-5p were involved in GH synthesis mediated by GHRHR SVs, indicating that the two miRNAs played different roles by different ways. Finally, results showed that the protein coded by the GHRHR transcript regulated GH through the NO/NOS signalling pathway, whereas that coded by SV1 and SV2 regulated GH through the PKA/CREB signalling pathway, which was confirmed by the changes in signalling pathways after transfecting the expression vectors of GHRHR SVs to GH3 cells. To the best of our knowledge, this paper is the first to report pituitary miRNAs regulate GH synthesis by targeting the different SVs of GHRHR.

A complex structure of arrestin-2 bound to a G protein-coupled receptor

Arrestins comprise a family of signal regulators of G-protein-coupled receptors (GPCRs), which include arrestins 1 to 4. While arrestins 1 and 4 are visual arrestins dedicated to rhodopsin, arrestins 2 and 3 (Arr2 and Arr3) are β-arrestins known to regulate many nonvisual GPCRs. The dynamic and promiscuous coupling of Arr2 to nonvisual GPCRs has posed technical challenges to tackle the basis of arrestin binding to GPCRs. Here we report the structure of Arr2 in complex with neurotensin receptor 1 (NTSR1), which reveals an overall assembly that is strikingly different from the visual arrestin-rhodopsin complex by a 90° rotation of Arr2 relative to the receptor. In this new configuration, intracellular loop 3 (ICL3) and transmembrane helix 6 (TM6) of the receptor are oriented toward the N-terminal domain of the arrestin, making it possible for GPCRs that lack the C-terminal tail to couple Arr2 through their ICL3. Molecular dynamics simulation and crosslinking data further support the assembly of the Arr2‒NTSR1 complex. Sequence analysis and homology modeling suggest that the Arr2‒NTSR1 complex structure may provide an alternative template for modeling arrestin-GPCR interactions.

Integrating hydrogen-deuterium exchange mass spectrometry with molecular dynamics simulations to probe lipid-modulated conformational changes in membrane proteins

Biological membranes define the boundaries of cells and are composed primarily of phospholipids and membrane proteins. It has become increasingly evident that direct interactions of membrane proteins with their surrounding lipids play key roles in regulating both protein conformations and function. However, the exact nature and structural consequences of these interactions remain difficult to track at the molecular level. Here, we present a protocol that specifically addresses this challenge. First, hydrogen-deuterium exchange mass spectrometry (HDX-MS) of membrane proteins incorporated into nanodiscs of controlled lipid composition is used to obtain information on the lipid species that are involved in modulating the conformational changes in the membrane protein. Then molecular dynamics (MD) simulations in lipid bilayers are used to pinpoint likely lipid-protein interactions, which can be tested experimentally using HDX-MS. By bringing together the MD predictions with the conformational readouts from HDX-MS, we have uncovered key lipid-protein interactions implicated in stabilizing important functional conformations. This protocol can be applied to virtually any integral membrane protein amenable to classic biophysical studies and for which a near-atomic-resolution structure or homology model is available. This protocol takes ~4 d to complete, excluding the time for data analysis and MD simulations, which depends on the size of the protein under investigation.

Biased Signaling of CCL21 and CCL19 Does Not Rely on N-Terminal Differences, but Markedly on the Chemokine Core Domains and Extracellular Loop 2 of CCR7

Chemokine receptors play important roles in the immune system and are linked to several human diseases. Targeting chemokine receptors have so far shown very little success owing to, to some extent, the promiscuity of the immune system and the high degree of biased signaling within it. CCR7 and its two endogenous ligands display biased signaling and here we investigate the differences between the two ligands, CCL21 and CCL19, with respect to their biased activation of CCR7. We use bystander bioluminescence resonance energy transfer (BRET) based signaling assays and Transwell migration assays to determine (A) how swapping of domains between the two ligands affect their signaling patterns and (B) how receptor mutagenesis impacts signaling. Using chimeric ligands we find that the chemokine core domains are central for determining signaling outcome as the lack of β-arrestin-2 recruitment displayed by CCL21 is linked to its core domain and not N-terminus. Through a mutagenesis screen, we identify the extracellular domains of CCR7 to be important for both ligands and show that the two chemokines interact differentially with extracellular loop 2 (ECL-2). By using in silico modeling, we propose a link between ECL-2 interaction and CCR7 signal transduction. Our mutagenesis study also suggests a lysine in the top of TM3, K130^3.26, to be important for G protein signaling, but not β-arrestin-2 recruitment. Taken together, the bias in CCR7 between CCL19 and CCL21 relies on the chemokine core domains, where interactions with ECL-2 seem particularly important. Moreover, TM3 selectively regulates G protein signaling as found for other chemokine receptors.

Effects of Post-translational Modifications on Membrane Localization and Signaling of Prostanoid GPCR-G Protein Complexes and the Role of Hypoxia

G protein-coupled receptors (GPCRs) play a pivotal role in the adaptive responses to cellular stresses such as hypoxia. In addition to influencing cellular gene expression profiles, hypoxic microenvironments can perturb membrane protein localization, altering GPCR effector scaffolding and altering downstream signaling. Studies using proteomics approaches have revealed significant regulation of GPCR and G proteins by their state of post-translational modification. The aim of this review is to examine the effects of post-translational modifications on membrane localization and signaling of GPCR-G protein complexes, with an emphasis on vascular prostanoid receptors, and to highlight what is known about the effect of cellular hypoxia on these mechanisms. Understanding post-translational modifications of protein targets will help to define GPCR targets in treatment of disease, and to inform research into mechanisms of hypoxic cellular responses.

The Biological and Clinical Relevance of G Protein-Coupled Receptors to the Outcomes of Hematopoietic Stem Cell Transplantation: A Systematized Review

Hematopoietic stem cell transplantation (HSCT) remains the only curative treatment for several malignant and non-malignant diseases at the cost of serious treatment-related toxicities (TRTs). Recent research on extending the benefits of HSCT to more patients and indications has focused on limiting TRTs and improving immunological effects following proper mobilization and engraftment. Increasing numbers of studies report associations between HSCT outcomes and the expression or the manipulation of G protein-coupled receptors (GPCRs). This large family of cell surface receptors is involved in various human diseases. With ever-better knowledge of their crystal structures and signaling dynamics, GPCRs are already the targets for one third of the current therapeutic arsenal. The present paper assesses the current status of animal and human research on GPCRs in the context of selected HSCT outcomes via a systematized survey and analysis of the literature.

A free-energy landscape for the glucagon-like peptide 1 receptor GLP1R

G-protein-coupled receptors (GPCRs) are among the most important receptors in human physiology and pathology. They serve as master regulators of numerous key processes and are involved in as well as cause debilitating diseases. Consequently, GPCRs are among the most attractive targets for drug design and pharmaceutical interventions (>30% of drugs on the market). The glucagon-like peptide 1 (GLP-1) hormone receptor GLP1R is closely involved in insulin secretion by pancreatic β-cells and constitutes a major druggable target for the development of anti-diabetes and obesity agents. GLP1R structure was recently solved, with ligands, allosteric modulators and as part of a complex with its cognate G protein. However, the translation of this structural data into structure/function understanding remains limited. The current study functionally characterizes GLP1R with special emphasis on ligand and cellular partner binding interactions and presents a free-energy landscape as well as a functional model of the activation cycle of GLP1R. Our results should facilitate a deeper understanding of the molecular mechanism underlying GLP1R activation, forming a basis for improved development of targeted therapeutics for diabetes and related disorders.

Abnormalities in chemokine receptor recycling in chronic lymphocytic leukemia

In addition to their modulation through de novo expression and degradation, surface levels of chemokine receptors are tuned by their ligand-dependent recycling to the plasma membrane, which ensures that engaged receptors become rapidly available for further rounds of signaling. Dysregulation of this process contributes to the pathogenesis of chronic lymphocytic leukemia (CLL) by enhancing surface expression of chemokine receptors, thereby favoring leukemic cell accumulation in the protective niche of lymphoid organs. In this review, we summarize our current understanding of the process of chemokine receptor recycling, focusing on the impact of its dysregulation in CLL.

Design and biological assessment of membrane-tethering neuroprotective peptides derived from the pituitary adenylate cyclase-activating polypeptide type 1 receptor

BACKGROUND

The pituitary adenylate cyclase-activating polypeptide (PACAP) type 1 receptor (PAC1), a class B G protein-coupled receptor (GPCR), has emerged as a promising target for treating neurodegenerative conditions. Unfortunately, despite years of research, no PAC1-specific agonist has been discovered, as activity on two other GPCRs, VPAC1 and VPAC2, is retained with current analogs. Cell signaling is related to structural modifications in the intracellular loops (ICLs) of GPCRs. Thus, we hypothesized that peptides derived from the ICLs (called pepducins) of PAC1 might initiate, as allosteric ligands, signaling cascades after recognition of the parent receptor and modulation of its conformational landscape.

METHODS

Three pepducins were synthesized and evaluated for their ability to 1) promote cell survival; 2) stimulate various signaling pathways associated with PAC1 activation; 3) modulate selectively PAC1, VPAC1 or VPAC2 activation; and 4) sustain mobility and prevent death of dopaminergic neurons in a zebrafish model of neurodegeneration.

RESULTS

Assays demonstrated that these molecules promote SH-SY5Y cell survival, a human neuroblastoma cell line expressing PAC1, and activate signaling via Gα_s and Gα_q, with distinct potencies and efficacies. Also, PAC1-Pep1 and PAC1-Pep2 activated selectively PAC1-mediated Gα_s stimulation. Finally, experiments, using a zebrafish neurodegeneration model, showed a neuroprotective action with all three pepducins and in particular, revealed the ability of PAC1-Pep1 and PAC1-Pep3 to preserve fish mobility and tyrosine hydroxylase expression in the brain.

CONCLUSION

We have developed the first neuroprotective pepducins derived from PAC1, a class B GPCR.

GENERAL SIGNIFICANCE

PAC1-derived pepducins represent attractive templates for the development of innovative neuroprotecting molecules.

Gi/o-Protein Coupled Receptors in the Aging Brain

Cells translate extracellular signals to regulate processes such as differentiation, metabolism and proliferation, via transmembranar receptors. G protein-coupled receptors (GPCRs) belong to the largest family of transmembrane receptors, with over 800 members in the human species. Given the variety of key physiological functions regulated by GPCRs, these are main targets of existing drugs. During normal aging, alterations in the expression and activity of GPCRs have been observed. The central nervous system (CNS) is particularly affected by these alterations, which results in decreased brain functions, impaired neuroregeneration, and increased vulnerability to neuropathologies, such as Alzheimer's and Parkinson diseases. GPCRs signal via heterotrimeric G proteins, such as G_o, the most abundant heterotrimeric G protein in CNS. We here review age-induced effects of GPCR signaling via the G_i/o subfamily at the CNS. During the aging process, a reduction in protein density is observed for almost half of the G_i/o-coupled GPCRs, particularly in age-vulnerable regions such as the frontal cortex, hippocampus, substantia nigra and striatum. G_i/o levels also tend to decrease with aging, particularly in regions such as the frontal cortex. Alterations in the expression and activity of GPCRs and coupled G proteins result from altered proteostasis, peroxidation of membranar lipids and age-associated neuronal degeneration and death, and have impact on aging hallmarks and age-related neuropathologies. Further, due to oligomerization of GPCRs at the membrane and their cooperative signaling, down-regulation of a specific G_i/o-coupled GPCR may affect signaling and drug targeting of other types/subtypes of GPCRs with which it dimerizes. G_i/o-coupled GPCRs receptorsomes are thus the focus of more effective therapeutic drugs aiming to prevent or revert the decline in brain functions and increased risk of neuropathologies at advanced ages.

The Endocannabinoid System as a Target in Cancer Diseases: Are We There Yet?

The endocannabinoid system (ECS) has been placed in the anti-cancer spotlight in the last decade. The immense data load published on its dual role in both tumorigenesis and inhibition of tumor growth and metastatic spread has transformed the cannabinoid receptors CB1 (CB1R) and CB2 (CB2R), and other members of the endocannabinoid-like system, into attractive new targets for the treatment of various cancer subtypes. Although the clinical use of cannabinoids has been extensively documented in the palliative setting, clinical trials on their application as anti-cancer drugs are still ongoing. As drug repurposing is significantly faster and more economical than de novo introduction of a new drug into the clinic, there is hope that the existing pharmacokinetic and safety data on the ECS ligands will contribute to their successful translation into oncological healthcare. CB1R and CB2R are members of a large family of membrane proteins called G protein-coupled receptors (GPCR). GPCRs can form homodimers, heterodimers and higher order oligomers with other GPCRs or non-GPCRs. Currently, several CB1R and CB2R-containing heteromers have been reported and, in cancer cells, CB2R form heteromers with the G protein-coupled chemokine receptor CXCR4, the G protein-coupled receptor 55 (GPR55) and the tyrosine kinase receptor (TKR) human V-Erb-B2 Avian Erythroblastic Leukemia Viral Oncogene Homolog 2 (HER2). These protein complexes possess unique pharmacological and signaling properties, and their modulation might affect the antitumoral activity of the ECS. This review will explore the potential of the endocannabinoid network in the anti-cancer setting as well as the clinical and ethical pitfalls behind it, and will develop on the value of cannabinoid receptor heteromers as potential new targets for anti-cancer therapies and as prognostic biomarkers.

The role of the 12(S)-HETE/GPR31/12-HETER axis in cancer and ischemia-reperfusion injury

The G protein-coupled receptors (GPCRs) constitute a large superfamily of seven transmembrane-spanning receptors that are activated by several classes of ligands, including bioactive lipids. GPCRs are attractive therapeutic targets for the treatment of human diseases, as they finely regulate a wide array of cellular functions. In this minireview, we summarized what is currently known about the G protein-coupled receptor GPR31/12-HETER. We highlighted, in particular, its structural similarity with human homologs, the biological functions of its recognized ligand 12(S)-hydroxyeicosatetraenoic acid (HETE), an arachidonic acid metabolite, and the role that GPR31/12-HETER-mediated signals play in cancer cell growth, invasion and metastasis, and in liver ischemia-reperfusion (IR) injury. Recent studies shed light and interest on the 12(S)-HETE/GPR31/12-HETER-activated signaling pathways and functions. The full spectrum of GPR31/12-HETER-mediated biological functions has yet to be characterized. Further studies are needed to identify other potential ligands, i.e. other than 12(S)-HETE. Another important remaining question is whether the multiple 12(S)-HETE-induced biological activities, including its role in diabetes, neurodegeneration, neuroprotection, and platelet function, occur via GPR31/12-HETER and/or involve the activation of other receptor molecules and pathways.

Structural biology of G protein-coupled receptor signaling complexes

G protein-coupled receptors (GPCRs) constitute the largest family of cell surface receptors that mediate numerous cell signaling pathways, and are targets of more than one-third of clinical drugs. Thanks to the advancement of novel structural biology technologies, high-resolution structures of GPCRs in complex with their signaling transducers, including G-protein and arrestin, have been determined. These 3D complex structures have significantly improved our understanding of the molecular mechanism of GPCR signaling and provided a structural basis for signaling-biased drug discovery targeting GPCRs. Here we summarize structural studies of GPCR signaling complexes with G protein and arrestin using rhodopsin as a model system, and highlight the key features of GPCR conformational states in biased signaling including the sequence motifs of receptor TM6 that determine selective coupling of G proteins, and the phosphorylation codes of GPCRs for arrestin recruitment. We envision the future of GPCR structural biology not only to solve more high-resolution complex structures but also to show stepwise GPCR signaling complex assembly and disassembly and dynamic process of GPCR signal transduction.

GPCR Signaling Regulation: The Role of GRKs and Arrestins

Every animal species expresses hundreds of different G protein-coupled receptors (GPCRs) that respond to a wide variety of external stimuli. GPCRs-driven signaling pathways are involved in pretty much every physiological function and in many pathologies. Therefore, GPCRs are targeted by about a third of clinically used drugs. The signaling of most GPCRs via G proteins is terminated by the phosphorylation of active receptor by specific kinases (GPCR kinases, or GRKs) and subsequent binding of arrestin proteins, that selectively recognize active phosphorylated receptors. In addition, GRKs and arrestins play a role in multiple signaling pathways in the cell, both GPCR-initiated and receptor-independent. Here we focus on the mechanisms of GRK- and arrestin-mediated regulation of GPCR signaling, which includes homologous desensitization and redirection of signaling to additional pathways by bound arrestins.

The heterotetrameric structure of the adenosine A1-dopamine D1 receptor complex: Pharmacological implication for restless legs syndrome

Dopaminergic and purinergic signaling play a pivotal role in neurological diseases associated with motor symptoms, including Parkinson's disease (PD), multiple sclerosis, amyotrophic lateral sclerosis, Huntington disease, Restless Legs Syndrome (RLS), spinal cord injury (SCI), and ataxias. Extracellular dopamine and adenosine exert their functions interacting with specific dopamine (DR) or adenosine (AR) receptors, respectively, expressed on the surface of target cells. These receptors are members of the family A of G protein-coupled receptors (GPCRs), which is the largest protein superfamily in mammalian genomes. GPCRs are target of about 40% of all current marketed drugs, highlighting their importance in clinical medicine. The striatum receives the densest dopamine innervations and contains the highest density of dopamine receptors. The modulatory role of adenosine on dopaminergic transmission depends largely on the existence of antagonistic interactions mediated by specific subtypes of DRs and ARs, the so-called A_2AR-D₂R and A₁R-D₁R interactions. Due to the dopamine/adenosine antagonism in the CNS, it was proposed that ARs and DRs could form heteromers in the neuronal cell surface. Therefore, adenosine can affect dopaminergic signaling through receptor-receptor interactions and by modulations in their shared intracellular pathways in the striatum and spinal cord. In this work we describe the allosteric modulations between GPCR protomers, focusing in those of adenosine and dopamine within the A₁R-D₁R heteromeric complex, which is involved in RLS. We also propose that the knowledge about the intricate allosteric interactions within the A₁R-D₁R heterotetramer, may facilitate the treatment of motor alterations, not only when the dopamine pathway is hyperactivated (RLS, chorea, etc.) but also when motor function is decreased (SCI, aging, PD, etc.).

The Role of Heat Shock Proteins in Regulating Receptor Signal Transduction

Heat shock proteins (Hsp) are a class of stress-inducible proteins that mainly act as molecular protein chaperones. This chaperone activity is diverse, including assisting in nascent protein folding and regulating client protein location and translocation within the cell. The main proteins within the Hsp family, particularly Hsp70 and Hsp90, also have a highly diverse and numerous set of protein clients, which when combined with the high expression levels of Hsp proteins (2%-6% of total protein content) establishes these molecules as "central regulators" of cell protein physiology. Among the client proteins, Hsps regulate numerous signal-transduction and receptor-regulatory kinases, and indeed directly regulate some receptors themselves. This also makes the Hsps, particularly Hsp90, central regulators of signal-transduction machinery, with important impacts on endogenous and drug ligand responses. Among these roles, Hsp90 in particular acts to maintain mature signaling kinases in a metastable conformation permissive for signaling activation. In this review, we will focus on the roles of the Hsps, with a special focus on Hsp90, in regulating receptor signaling and subsequent physiologic responses. We will also explore potential means to manipulate Hsp function to improve receptor-targeted therapies. Overall, Hsps are important regulators of receptor signaling that are receiving increasing interest and exploration, particularly as Hsp90 inhibitors progress toward clinical approval for the treatment of cancer. Understanding the complex interplay of Hsp regulation of receptor signaling may provide important avenues to improve patient treatment.

The role of α2-adrenergic receptors in hypertensive preeclampsia: A hypothesis

Preeclampsia, a major disorder of human pregnancy, manifests as persistent hypertension and proteinuria presenting after 20 weeks of pregnancy. Multiple systemic symptoms might be associated with preeclampsia including thrombocytopenia, liver impairment, pulmonary edema, and cerebral disturbances. However, vascular dysfunction remains the core pathological driver of preeclampsia. Defective placental implantation followed by dysfunctional placental spiral artery development promotes a hypoxic environment. Massive endothelial dysfunction characterized by reduced vasodilation, augmented vasoconstriction, and increased vascular permeability and inflammation ensues. Interestingly, the same signaling and inflammatory pathways implicated in preeclampsia appear to be shared with other vascular disorders involving alteration of α₂ -AR function. The role of α₂ -ARs in the regulation of microcirculatory function has long been recognized, thus raising the question of whether they are involved in the pathogenesis of vascular dysfunction in preeclampsia. Here, we review possible interplay between signaling and inflammatory pathways common to preeclampsia and α₂ -AR function/regulation. We speculate on the potential contribution of these receptors to the observed phenotype and the potential role for their pharmacological modulators as therapeutic interventions with preeclampsia.

Activated protein C, protease activated receptor 1, and neuroprotection

Protein C is a plasma serine protease zymogen whose active form, activated protein C (APC), exerts potent anticoagulant activity. In addition to its antithrombotic role as a plasma protease, pharmacologic APC is a pleiotropic protease that activates diverse homeostatic cell signaling pathways via multiple receptors on many cells. Engineering of APC by site-directed mutagenesis provided a signaling selective APC mutant with 3 Lys residues replaced by 3 Ala residues, 3K3A-APC, that lacks >90% anticoagulant activity but retains normal cell signaling activities. This 3K3A-APC mutant exerts multiple potent neuroprotective activities, which require the G-protein-coupled receptor, protease activated receptor 1. Potent neuroprotection in murine ischemic stroke models is linked to 3K3A-APC-induced signaling that arises due to APC's cleavage in protease activated receptor 1 at a noncanonical Arg46 site. This cleavage causes biased signaling that provides a major explanation for APC's in vivo mechanism of action for neuroprotective activities. 3K3A-APC appeared to be safe in ischemic stroke patients and reduced bleeding in the brain after tissue plasminogen activator therapy in a recent phase 2 clinical trial. Hence, it merits further clinical testing for its efficacy in ischemic stroke patients. Recent studies using human fetal neural stem and progenitor cells show that 3K3A-APC promotes neurogenesis in vitro as well as in vivo in the murine middle cerebral artery occlusion stroke model. These recent advances should encourage translational research centered on signaling selective APC's for both single-agent therapies and multiagent combination therapies for ischemic stroke and other neuropathologies.

Prediction of Molecular Mechanisms for LianXia NingXin Formula: A Network Pharmacology Study

Objectives: Network pharmacological methods were used to investigate the underlying molecular mechanisms of LianXia NingXin (LXNX) formula, a Chinese prescription, to treat coronary heart disease (CHD) and disease phenotypes (CHD related diseases and symptoms).

Methods: The different seed gene lists associated with the herbs of LXNX formula, the CHD co-morbid diseases and symptoms which were relieved by the LXNX formula (co-morbid diseases and symptoms) were curated manually from biomedical databases and published biomedical literatures. Module enrichment analysis was used to identify CHD-related disease modules in the protein–protein interaction (PPI) network which were also associated to the targets of LXNX formula (LXNX formula’s CHD modules). The molecular characteristics of LXNX formula’s CHD modules were investigated via functional enrichment analysis in terms of gene ontology and pathways. We performed shortest path analysis to explore the interactions between the drug targets of LXNX formula and CHD related disease phenotypes (e.g., co-morbid diseases and symptoms).

Results: We identified two significant CHD related disease modules (i.e., M146 and M203), which were targeted by the herbs of LXNX formula. Pathway and GO term functional analysis results indicated that G-protein coupled receptor signaling pathways (GPCR) of M146 and cellular protein metabolic process of M203 are important functional pathways for the respective module. This is further confirmed by the shortest path analysis between the drug targets of LXNX formula and the aforementioned disease modules. In addition, corticotropin releasing hormone (CRH) and natriuretic peptide precursor A (NPPA) are the only two LXNX formula target proteins with the low shortest path length (on average shorter than 3) to their respective CHD module and co-morbid disease and symptom gene groups.

Conclusion: G-protein coupled receptor signaling pathway and cellular protein metabolic process are the key LXNX formula’s pathways to treat CHD disease phenotypes, in which CRH and NPPA are the two key drug targets of LXNX formula. Further evidences from Chinese herb pharmacological databases indicate that Pinellia ternata (Banxia) has relatively strong adjustive functions on the two key targets.