GPCR Signaling Regulation: The Role of GRKs and Arrestins

Arrestin-3-assisted activation of JNK3 mediates dopaminergic behavioral sensitization

In rodents with unilateral ablation of neurons supplying dopamine to the striatum, chronic treatment with the dopamine precursor L-DOPA induces a progressive increase of behavioral responses, a process known as behavioral sensitization. This sensitization is blunted in arrestin-3 knockout mice. Using virus-mediated gene delivery to the dopamine-depleted striatum of these mice, we find that the restoration of arrestin-3 fully rescues behavioral sensitization, whereas its mutant defective in c-Jun N-terminal kinase (JNK) activation does not. A 25-residue arrestin-3-derived peptide that facilitates JNK3 activation in cells, expressed ubiquitously or selectively in direct pathway striatal neurons, also fully rescues sensitization, whereas an inactive homologous arrestin-2-derived peptide does not. Behavioral rescue is accompanied by the restoration of JNK3 activity, as reflected by JNK-dependent phosphorylation of the transcription factor c-Jun in the dopamine-depleted striatum. Thus, arrestin-3-assisted JNK3 activation in direct pathway neurons is a critical element of the molecular mechanism underlying sensitization upon dopamine depletion and chronic L-DOPA treatment.

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

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

Engineered mini-G proteins block the internalization of cognate GPCRs and disrupt downstream intracellular signaling

Mini-G proteins are engineered, thermostable variants of Gα subunits designed to stabilize G protein-coupled receptors (GPCRs) in their active conformations. Because of their small size and ease of use, they are popular tools for assessing GPCR behaviors in cells, both as reporters of receptor coupling to Gα subtypes and for cellular assays to quantify compartmentalized signaling at various subcellular locations. Here, we report that overexpression of mini-G proteins with their cognate GPCRs disrupted GPCR endocytic trafficking and associated intracellular signaling. In cells expressing the Gαs-coupled GPCR glucagon-like peptide 1 receptor (GLP-1R), coexpression of mini-Gs, a mini-G protein derived from Gαs, blocked β-arrestin 2 recruitment and receptor internalization and disrupted endosomal GLP-1R signaling. These effects did not involve changes in receptor phosphorylation or lipid nanodomain segregation. Moreover, we found that mini-G proteins derived from Gαi and Gαq also inhibited the internalization of GPCRs that couple to them. Finally, we developed an alternative intracellular signaling assay for GLP-1R using a nanobody specific for active Gαs:GPCR complexes (Nb37) that did not affect GLP-1R internalization. Our results have important implications for designing methods to assess intracellular GPCR signaling.

An overview of circulating and urinary biomarkers capable of predicting the transition of acute kidney injury to chronic kidney disease

INTRODUCTION

Acute kidney injury (AKI) defined by a substantial decrease in kidney function within hours to days and is often irreversible with higher risk to chronic kidney disease (CKD) transition.

AREAS COVERED

The authors discuss the diagnostic and predictive utilities of serum and urinary biomarkers on AKI and on the risk of AKI-to-CKD progression. The authors focus on the relevant literature covering evidence of circulating and urinary biomarkers' capability to predict the transition of AKI to CKD.

EXPERT OPINION

Based on the different modalities of serum and urinary biomarkers, multiple biomarker panel seems to be potentially useful to distinguish between various types of AKI, to detect the severity and the risk of AKI progression, to predict the clinical outcome and evaluate response to the therapy. Serum/urinary neutrophil gelatinase-associated lipocalin (NGAL), serum/urinary uromodulin, serum extracellular high mobility group box-1 (HMGB-1), serum cystatin C and urinary liver-type fatty acid-binding protein (L-FABP) were the most effective in the prediction of AKI-to-CKD transition regardless of etiology and the presence of critical state in patients. The current clinical evidence on the risk assessments of AKI progression is mainly based on the utility of combination of functional, injury and stress biomarkers, mainly NGAL, L-FABP, HMGB-1 and cystatin C.

The roles of RGS proteins in cardiometabolic disease

G protein-coupled receptors (GPCRs) are the most prominent receptors on the surface of the cell and play a central role in the regulation of cardiac and metabolic functions. GPCRs transmit extracellular stimuli to the interior of the cells by activating one or more heterotrimeric G proteins. The duration and intensity of G protein-mediated signalling are tightly controlled by a large array of intracellular mediators, including the regulator of G protein signalling (RGS) proteins. RGS proteins selectively promote the GTPase activity of a subset of Gα subunits, thus serving as negative regulators in a pathway-dependent manner. In the current review, we summarise the involvement of RGS proteins in cardiometabolic function with a focus on their tissue distribution, mechanisms of action and dysregulation under various disease conditions. We also discuss the potential therapeutic applications for targeting RGS proteins in treating cardiometabolic conditions and current progress in developing RGS modulators. 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.

G protein-coupled receptor endocytosis generates spatiotemporal bias in β-arrestin signaling

The stabilization of different active conformations of G protein-coupled receptors is thought to underlie the varying efficacies of biased and balanced agonists. Here, profiling the activation of signal transducers by angiotensin II type 1 receptor (AT1R) agonists revealed that the extent and kinetics of β-arrestin binding exhibited substantial ligand-dependent differences, which were lost when receptor internalization was inhibited. When AT1R endocytosis was prevented, even weak partial agonists of the β-arrestin pathway acted as full or near-full agonists, suggesting that receptor conformation did not exclusively determine β-arrestin recruitment. The ligand-dependent variance in β-arrestin translocation was much larger at endosomes than at the plasma membrane, showing that ligand efficacy in the β-arrestin pathway was spatiotemporally determined. Experimental investigations and mathematical modeling demonstrated how multiple factors concurrently shaped the effects of agonists on endosomal receptor-β-arrestin binding and thus determined the extent of functional selectivity. Ligand dissociation rate and G protein activity had particularly strong, internalization-dependent effects on the receptor-β-arrestin interaction. We also showed that endocytosis regulated the agonist efficacies of two other receptors with sustained β-arrestin binding: the V2 vasopressin receptor and a mutant β2-adrenergic receptor. In the absence of endocytosis, the agonist-dependent variance in β-arrestin2 binding was markedly diminished. Our results suggest that endocytosis determines the spatiotemporal bias in GPCR signaling and can aid in the development of more efficacious, functionally selective compounds.

Robust GRK2/3/6-dependent desensitization of oxytocin receptor in neurons

Oxytocin plays critical roles in the brain as a neuromodulator, regulating social and other affective behavior. However, the regulatory mechanisms controlling oxytocin receptor (OXTR) signaling in neurons remain unexplored. In this study, we have identified robust and rapid-onset desensitization of OXTR response in multiple regions of the mouse brain. Both cell autonomous spiking response and presynaptic activation undergo similar agonist-induced desensitization. G-protein-coupled receptor kinases (GRK) GRK2, GRK3, and GRK6 are recruited to the activated OXTR in neurons, followed by recruitment of β-arrestin-1 and -2. Neuronal OXTR desensitization was impaired by suppression of GRK2/3/6 kinase activity but remained unaltered with double knockout of β-arrestin-1 and -2. Additionally, we observed robust agonist-induced internalization of neuronal OXTR and its Rab5-dependent recruitment to early endosomes, which was impaired by GRK2/3/6 inhibition. This work defines distinctive aspects of the mechanisms governing OXTR desensitization and internalization in neurons compared to prior studies in heterologous cells.

Lysophosphatidic Acid Receptor 3 (LPA3): Signaling and Phosphorylation Sites

LPA3 receptors were expressed in TREx HEK 293 cells, and their signaling and phosphorylation were studied. The agonist, lysophosphatidic acid (LPA), increased intracellular calcium and ERK phosphorylation through pertussis toxin-insensitive processes. Phorbol myristate acetate, but not LPA, desensitizes LPA3-mediated calcium signaling, the agonists, and the phorbol ester-induced LPA3 internalization. Pitstop 2 (clathrin heavy chain inhibitor) markedly reduced LPA-induced receptor internalization; in contrast, phorbol ester-induced internalization was only delayed. LPA induced rapid β-arrestin-LPA3 receptor association. The agonist and the phorbol ester-induced marked LPA3 receptor phosphorylation, and phosphorylation sites were detected using mass spectrometry. Phosphorylated residues were detected in the intracellular loop 3 (S221, T224, S225, and S229) and in the carboxyl terminus (S321, S325, S331, T333, S335, Y337, and S343). Interestingly, phosphorylation sites are within sequences predicted to constitute β-arrestin binding sites. These data provide insight into LPA3 receptor signaling and regulation.

Arrestins: A Small Family of Multi-Functional Proteins

The first member of the arrestin family, visual arrestin-1, was discovered in the late 1970s. Later, the other three mammalian subtypes were identified and cloned. The first described function was regulation of G protein-coupled receptor (GPCR) signaling: arrestins bind active phosphorylated GPCRs, blocking their coupling to G proteins. It was later discovered that receptor-bound and free arrestins interact with numerous proteins, regulating GPCR trafficking and various signaling pathways, including those that determine cell fate. Arrestins have no enzymatic activity; they function by organizing multi-protein complexes and localizing their interaction partners to particular cellular compartments. Today we understand the molecular mechanism of arrestin interactions with GPCRs better than the mechanisms underlying other functions. However, even limited knowledge enabled the construction of signaling-biased arrestin mutants and extraction of biologically active monofunctional peptides from these multifunctional proteins. Manipulation of cellular signaling with arrestin-based tools has research and likely therapeutic potential: re-engineered proteins and their parts can produce effects that conventional small-molecule drugs cannot.

Illuminating the neuropeptide Y4 receptor and its ligand pancreatic polypeptide from a structural, functional, and therapeutic perspective

The neuropeptide Y4 receptor (Y4R), a rhodopsin-like G protein-coupled receptor (GPCR) and the hormone pancreatic polypeptide (PP) are members of the neuropeptide Y family consisting of four receptors (Y1R, Y2R, Y4R, Y5R) and three highly homologous peptide ligands (neuropeptide Y, peptide YY, PP). In this family, the Y4R is of particular interest as it is the only subtype with high affinity to PP over NPY. The Y4R, as a mediator of PP signaling, has a pivotal role in appetite regulation and energy homeostasis, offering potential avenues for the treatment of metabolic disorders such as obesity. PP as anorexigenic peptide is released postprandial from the pancreas in response to food intake, induces satiety signals and contributes to hamper excessive food intake. Moreover, this system was also described to be associated with different types of cancer: overexpression of Y4R have been found in human adenocarcinoma cells, while elevated levels of PP are related to the development of pancreatic endocrine tumors. The pharmacological relevance of the Y4R advanced the search for potent and selective ligands for this receptor subtype, which will be significantly progressed through the elucidation of the active state PP-Y4R cryo-EM structure. This review summarizes the development of novel PP-derived ligands, like Obinepitide as dual Y2R/Y4R agonist in clinical trials or UR-AK86c as small hexapeptide agonist with picomolar affinity, as well as the first allosteric modulators that selectively target the Y4R, e.g. VU0506013 as potent Y4R positive allosteric modulator or (S)-VU0637120 as allosteric antagonist. Here, we provide valuable insights into the complex physiological functions of the Y4R and PP and the pharmacological relevance of the system in appetite regulation to open up new avenues for the development of tool compounds for targeted therapies with potential applications in metabolic disorders.

Cryo-electron microscopy for GPCR research and drug discovery in endocrinology and metabolism

G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors, with many GPCRs having crucial roles in endocrinology and metabolism. Cryogenic electron microscopy (cryo-EM) has revolutionized the field of structural biology, particularly regarding GPCRs, over the past decade. Since the first pair of GPCR structures resolved by cryo-EM were published in 2017, the number of GPCR structures resolved by cryo-EM has surpassed the number resolved by X-ray crystallography by 30%, reaching >650, and the number has doubled every ~0.63 years for the past 6 years. At this pace, it is predicted that the structure of 90% of all human GPCRs will be completed within the next 5-7 years. This Review highlights the general structural features and principles that guide GPCR ligand recognition, receptor activation, G protein coupling, arrestin recruitment and regulation by GPCR kinases. The Review also highlights the diversity of GPCR allosteric binding sites and how allosteric ligands could dictate biased signalling that is selective for a G protein pathway or an arrestin pathway. Finally, the authors use the examples of glycoprotein hormone receptors and glucagon-like peptide 1 receptor to illustrate the effect of cryo-EM on understanding GPCR biology in endocrinology and metabolism, as well as on GPCR-related endocrine diseases and drug discovery.

ArreSTick motif controls β-arrestin-binding stability and extends phosphorylation-dependent β-arrestin interactions to non-receptor proteins

The binding and function of β-arrestins are regulated by specific phosphorylation motifs present in G protein-coupled receptors (GPCRs). However, the exact arrangement of phosphorylated amino acids responsible for establishing a stable interaction remains unclear. We employ a 1D sequence convolution model trained on GPCRs with established β-arrestin-binding properties. With this approach, amino acid motifs characteristic of GPCRs that form stable interactions with β-arrestins can be identified, a pattern that we name "arreSTick." Intriguingly, the arreSTick pattern is also present in numerous non-receptor proteins. Using proximity biotinylation assay and mass spectrometry analysis, we demonstrate that the arreSTick motif controls the interaction between many non-receptor proteins and β-arrestin2. The HIV-1 Tat-specific factor 1 (HTSF1 or HTATSF1), a nuclear transcription factor, contains the arreSTick pattern, and its subcellular localization is influenced by β-arrestin2. Our findings unveil a broader role for β-arrestins in phosphorylation-dependent interactions, extending beyond GPCRs to encompass non-receptor proteins as well.

LPA3 Receptor Phosphorylation Sites: Roles in Signaling and Internalization

Lysophosphatidic acid (LPA) type 3 (LPA3) receptor mutants were generated in which the sites detected phosphorylated were substituted by non-phosphorylatable amino acids. Substitutions were made in the intracellular loop 3 (IL3 mutant), the carboxyl terminus (Ctail), and both domains (IL3/Ctail). The wild-type (WT) receptor and the mutants were expressed in T-REx HEK293 cells, and the consequences of the substitutions were analyzed employing different functional parameters. Agonist- and LPA-mediated receptor phosphorylation was diminished in the IL3 and Ctail mutants and essentially abolished in the IL3/Ctail mutant, confirming that the main phosphorylation sites are present in both domains and their role in receptor phosphorylation eliminated by substitution and distributed in both domains. The WT and mutant receptors increased intracellular calcium and ERK 1/2 phosphorylation in response to LPA and PMA. The agonist, Ki16425, diminished baseline intracellular calcium, which suggests some receptor endogenous activity. Similarly, baseline ERK1/2 phosphorylation was diminished by Ki16425. An increase in baseline ERK phosphorylation was detected in the IL3/Ctail mutant. LPA and PMA-induced receptor interaction with β-arrestin 2 and LPA3 internalization were severely diminished in cells expressing the mutants. Mutant-expressing cells also exhibit increased baseline proliferation and response to different stimuli, which were inhibited by the antagonist Ki16425, suggesting a role of LPA receptors in this process. Migration in response to different attractants was markedly increased in the Ctail mutant, which the Ki16425 antagonist also attenuated. Our data experimentally show that receptor phosphorylation in the distinct domains is relevant for LPA3 receptor function.

Differential modulation of sensory response dynamics by cilia structure and intraflagellar transport within and across chemosensory neurons

Sensory neurons contain morphologically diverse primary cilia that are built by intraflagellar transport (IFT) and house sensory signaling molecules. Since both ciliary structural and signaling proteins are trafficked via IFT, it has been challenging to decouple the contributions of IFT and cilia structure to neuronal responses. By acutely inhibiting IFT without altering cilia structure and vice versa , here we describe the differential roles of ciliary trafficking and sensory ending morphology in shaping chemosensory responses in C. elegans. We show that a minimum cilium length but not continuous IFT is necessary for a subset of responses in the ASH nociceptive neurons. In contrast, neither cilia nor continuous IFT are necessary for odorant responses in the AWA olfactory neurons. Instead, continuous IFT differentially modulates response dynamics in AWA. Upon acute inhibition of IFT, cilia-destined odorant receptors are shunted to ectopic branches emanating from the cilia base. Spatial segregation of receptors in these branches from a cilia-restricted regulatory kinase results in odorant desensitization defects, highlighting the importance of precise organization of signaling molecules at sensory endings in regulating response dynamics. We also find that adaptation of AWA responses upon repeated exposure to an odorant is mediated by IFT-driven removal of its cognate receptor, whereas adaptation to a second odorant is regulated via IFT-independent mechanisms. Our results reveal unexpected complexity in the contribution of IFT and cilia organization to the regulation of responses even within a single chemosensory neuron type, and establish a critical role for these processes in the precise modulation of olfactory behaviors.

Voltage Sensors Embedded in G Protein-Coupled Receptors

Some signaling processes mediated by G protein-coupled receptors (GPCRs) are modulated by membrane potential. In recent years, increasing evidence that GPCRs are intrinsically voltage-dependent has accumulated. A recent publication challenged the view that voltage sensors are embedded in muscarinic receptors. Herein, we briefly discuss the evidence that supports the notion that GPCRs themselves are voltage-sensitive proteins and an alternative mechanism that suggests that voltage-gated sodium channels are the voltage-sensing molecules involved in such processes.

TOR signaling regulates GPCR levels on the plasma membrane and suppresses the Saccharomyces cerevisiae mating pathway

Saccharomyces cerevisiae respond to mating pheromone through the GPCRs Ste2 and Ste3, which promote growth of a mating projection in response to ligand binding. This commitment to mating is nutritionally and energetically taxing, and so we hypothesized that the cell may suppress mating signaling during starvation. We set out to investigate negative regulators of the mating pathway in nutritionally depleted environments. Here, we report that nutrient deprivation led to loss of Ste2 from the plasma membrane. Recapitulating this effect with nitrogen starvation led us to hypothesize that it was due to TORC1 signaling. Rapamycin inhibition of TORC1 impacted membrane levels of all yeast GPCRs. Inhibition of TORC1 also dampened mating pathway output. Deletion analysis revealed that TORC1 repression leads to α-arrestin-directed CME through TORC2-Ypk1 signaling. We then set out to determine whether major downstream effectors of the TOR complexes also downregulate pathway output during mating. We found that autophagy contributes to pathway downregulation through analysis of strains lacking ATG8 . We also show that Ypk1 significantly reduced pathway output. Thus, both autophagy machinery and TORC2-Ypk1 signaling serve as attenuators of pheromone signaling during mating. Altogether, we demonstrate that the stress-responsive TOR complexes coordinate GPCR endocytosis and reduce the magnitude of pheromone signaling, in ligand-independent and ligand-dependent contexts.

One Sentence Summary

TOR signaling regulates the localization of all Saccharomyces cerevisiae GPCRs during starvation and suppress the mating pathway in the presence and absence of ligand.

Adrenoceptor Desensitization: Current Understanding of Mechanisms

G-protein coupled receptors (GPCRs) transduce a wide range of extracellular signals. They are key players in the majority of biologic functions including vision, olfaction, chemotaxis, and immunity. However, as essential as most of them are to body function and homeostasis, overactivation of GPCRs has been implicated in many pathologic diseases such as cancer, asthma, and heart failure (HF). Therefore, an important feature of G protein signaling systems is the ability to control GPCR responsiveness, and one key process to control overstimulation involves initiating receptor desensitization. A number of steps are appreciated in the desensitization process, including cell surface receptor phosphorylation, internalization, and downregulation. Rapid or short-term desensitization occurs within minutes and involves receptor phosphorylation via the action of intracellular protein kinases, the binding of β-arrestins, and the consequent uncoupling of GPCRs from their cognate heterotrimeric G proteins. On the other hand, long-term desensitization occurs over hours to days and involves receptor downregulation or a decrease in cell surface receptor protein level. Of the proteins involved in this biologic phenomenon, β-arrestins play a particularly significant role in both short- and long-term desensitization mechanisms. In addition, β-arrestins are involved in the phenomenon of biased agonism, where the biased ligand preferentially activates one of several downstream signaling pathways, leading to altered cellular responses. In this context, this review discusses the different patterns of desensitization of the α 1-, α 2- and the β adrenoceptors and highlights the role of β-arrestins in regulating physiologic responsiveness through desensitization and biased agonism. SIGNIFICANCE STATEMENT: A sophisticated network of proteins orchestrates the molecular regulation of GPCR activity. Adrenoceptors are GPCRs that play vast roles in many physiological processes. Without tightly controlled desensitization of these receptors, homeostatic imbalance may ensue, thus precipitating various diseases. Here, we critically appraise the mechanisms implicated in adrenoceptor desensitization. A better understanding of these mechanisms helps identify new druggable targets within the GPCR desensitization machinery and opens exciting therapeutic fronts in the treatment of several pathologies.

Regulation of the pro-inflammatory G protein-coupled receptor GPR84

GPR84 is an understudied rhodopsin-like class A G protein-coupled receptor, which is arousing particular interest from a therapeutic perspective. Not least this reflects that gpr84 expression is significantly up-regulated following acute inflammatory stimuli and in inflammatory diseases, and that receptor activation plays a role in regulating pro-inflammatory responses and migration of cells of the innate immune system such as neutrophils, monocytes, macrophages and microglia. Although most physiological responses of GPR84 reflect receptor coupling to Gαi/o-proteins, several studies indicate that agonist-activated GPR84 can recruit arrestin adaptor proteins and this regulates receptor internalisation and desensitisation. To date, little is known on the patterns of either basal or ligand regulated GPR84 phosphorylation and how these might control these processes. Here, we consider what is known about the regulation of GPR84 signalling with a focus on how G protein receptor kinase-mediated phosphorylation regulates arrestin protein recruitment and receptor function. LINKED ARTICLES: This article is part of a themed issue GPR84 Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.10/issuetoc.

Rational Design of Drugs Targeting G-Protein-Coupled Receptors: Ligand Search and Screening

G protein-coupled receptors (GPCRs) are transmembrane proteins that participate in many physiological processes and represent major pharmacological targets. Recent advances in structural biology of GPCRs have enabled the development of drugs based on the receptor structure (structure-based drug design, SBDD). SBDD utilizes information about the receptor-ligand complex to search for suitable compounds, thus expanding the chemical space of possible receptor ligands without the need for experimental screening. The review describes the use of structure-based virtual screening (SBVS) for GPCR ligands and approaches for the functional testing of potential drug compounds, as well as discusses recent advances and successful examples in the application of SBDD for the identification of GPCR ligands.

MRGPRB2/X2 and the analogous effects of its agonist and antagonist in DSS-induced colitis in mice

The mast cell receptor Mrgprb2, a mouse orthologue of human Mrgprx2, is known as an inflammatory receptor and its elevated expression is associated with various diseases such as ulcerative colitis. We aimed to elucidate the role of Mrgprb2/x2 and the effect of its ligands on a chemically induced murine colitis model. We showed that in Mrgprb2-/- mice, there is a differential regulation of cytokine releases in the blood plasma and severe colonic damages after DSS treatment. Unexpectedly, we demonstrated that known Mrgprb2/x2 agonists (peptide P17, P17 analogues and CST-14) and antagonist (GE1111) similarly increased the survival rate of WT mice subjected to 4% DSS-induced colitis, ameliorated the colonic damages of 2.5% DSS-induced colitis, restored major protein mRNA expression involved in colon integrity, reduced CD68+ and F4/80+ immune cell infiltration and restored cytokine levels. Collectively, our findings highlight the eminent role of Mrpgrb2/x2 in conferring a beneficial effect in the colitis model, and this significance is demonstrated by the heightened severity of colitis with altered cytokine releases and inflammatory immune cell infiltration observed in the Mrgprb2 knockout mice. Elevated expression of Mrgprb2 in WT colitis murine models may represent the organism's adaptive protective mechanism since Mrgprb2 knockout results in severe colitis. On the other hand, both agonist and antagonist of Mrgprb2 analogously mitigated the severity of colitis in DSS-induced colitis model by altering Mrgprb2 expression, immune cell infiltration and inflammatory cytokine releases.

M1/M4 receptors as potential therapeutic treatments for schizophrenia: A comprehensive study

Muscarinic acetylcholine receptors (mAChRs) play a significant role in the pathophysiology of schizophrenia. Although activating mAChRs holds potential in addressing the full range of schizophrenia symptoms, clinical application of many non-selective mAChR agonists in cognitive deficits, positive and negative symptoms is hindered by peripheral side effects (gastrointestinal disturbances and cardiovascular effects) and dosage restrictions. Ligands binding to the allosteric sites of mAChRs, particularly the M1 and M4 subtypes, demonstrate activity in improving cognitive function and amelioration of positive and negative symptoms associated with schizophrenia, enhancing our understanding of schizophrenia. The article aims to critically examine current design concepts and clinical advancements in synthesizing and designing small molecules targeting M1/M4, providing theoretical insights and empirical support for future research in this field.

Function and regulation of GPR84 in human neutrophils

Human neutrophils are components of the innate immune system and are the most abundant white blood cells in the circulation. They are professional phagocytes and express several G protein-coupled receptors (GPCRs), which are essential for proper neutrophil functions. So far, the two formyl peptide receptors, FPR1 and FPR2, have been the most extensively studied group of neutrophil GPCRs, but recently, a new group, the free fatty acid (FFA) receptors, has attracted growing attention. Neutrophils express two FFA receptors, GPR84 and FFA2, which sense medium- and short-chain fatty acids respectively, and display similar activation profiles. The exact pathophysiological role of GPR84 is not yet fully understood, but it is generally regarded as a pro-inflammatory receptor that mediates neutrophil activation. In this review, we summarize current knowledge of how GPR84 affects human neutrophil functions and discuss the regulatory mechanisms that control these responses, focusing on the similarities and differences in comparison to the two FPRs and FFA2. LINKED ARTICLES: This article is part of a themed issue GPR84 Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.10/issuetoc.

Fungal alkaloid malbrancheamide reorients the lipid binding domain of GRK5

G protein-coupled receptors (GPCRs) are the largest group of receptors involved in various types of signaling. GPCR signaling is regulated via receptor phosphorylation by G protein-coupled receptor kinases 5 (GRK5). Calmodulin (CaM), a universal Ca2+ sensor, inhibits receptor phosphorylation by binding to GRK5. However, the inhibitor malbrancheamide (MBC), which binds at CaM C-lobe, allows for receptor phosphorylation. To understand the phosphorylation mechanism by GRK5, we carried out a MD simulation of the CaM/GRK5 complex in the presence and absence of the MBC inhibitor. The lipid binding domain (LBD) of GRK5 adopted different positions in the presence and absence of inhibitor. Furthermore, the inhibitor MBC restricted the movement of the N-lobe tether (NLT) loop, probably blocking the autophosphorylation of GRK5.

G protein-coupled receptors (GPCRs): advances in structures, mechanisms, and drug discovery

G protein-coupled receptors (GPCRs), the largest family of human membrane proteins and an important class of drug targets, play a role in maintaining numerous physiological processes. Agonist or antagonist, orthosteric effects or allosteric effects, and biased signaling or balanced signaling, characterize the complexity of GPCR dynamic features. In this study, we first review the structural advancements, activation mechanisms, and functional diversity of GPCRs. We then focus on GPCR drug discovery by revealing the detailed drug-target interactions and the underlying mechanisms of orthosteric drugs approved by the US Food and Drug Administration in the past five years. Particularly, an up-to-date analysis is performed on available GPCR structures complexed with synthetic small-molecule allosteric modulators to elucidate key receptor-ligand interactions and allosteric mechanisms. Finally, we highlight how the widespread GPCR-druggable allosteric sites can guide structure- or mechanism-based drug design and propose prospects of designing bitopic ligands for the future therapeutic potential of targeting this receptor family.

The role of G protein-coupled receptor kinases in GLP-1R β-arrestin recruitment and internalisation

The glucagon-like peptide 1 receptor (GLP-1R) is a validated clinical target for the treatment of type 2 diabetes and obesity. Unlike most G protein-coupled receptors (GPCRs), the GLP-1R undergoes an atypical mode of internalisation that does not require β-arrestins. While differences in GLP-1R trafficking and β-arrestin recruitment have been observed between clinically used GLP-1R agonists, the role of G protein-coupled receptor kinases (GRKs) in affecting these pathways has not been comprehensively assessed. In this study, we quantified the contribution of GRKs to agonist-mediated GLP-1R internalisation and β-arrestin recruitment profiles using cells where endogenous β-arrestins, or non-visual GRKs were knocked out using CRISPR/Cas9 genome editing. Our results confirm the previously established atypical β-arrestin-independent mode of GLP-1R internalisation and revealed that GLP-1R internalisation is dependent on the expression of GRKs. Interestingly, agonist-mediated GLP-1R β-arrestin 1 and β-arrestin 2 recruitment were differentially affected by endogenous GRK knockout with β-arrestin 1 recruitment more sensitive to GRK knockout than β-arrestin 2 recruitment. Moreover, individual overexpression of GRK2, GRK3, GRK5 or GRK6 in a newly generated GRK2/3/4/5/6 HEK293 cells, rescued agonist-mediated β-arrestin 1 recruitment and internalisation profiles to similar levels, suggesting that there is no specific GRK isoform that drives these pathways. This study advances mechanistic understanding of agonist-mediated GLP-1R internalisation and provides novel insights into how GRKs may fine-tune GLP-1R signalling.

Histamine H1 Receptor-Mediated JNK Phosphorylation Is Regulated by Gq Protein-Dependent but Arrestin-Independent Pathways

Arrestins are known to be involved not only in the desensitization and internalization of G protein-coupled receptors but also in the G protein-independent activation of mitogen-activated protein (MAP) kinases, such as extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK), to regulate cell proliferation and inflammation. Our previous study revealed that the histamine H1 receptor-mediated activation of ERK is dually regulated by Gq proteins and arrestins. In this study, we investigated the roles of Gq proteins and arrestins in the H1 receptor-mediated activation of JNK in Chinese hamster ovary (CHO) cells expressing wild-type (WT) human H1 receptors, the Gq protein-biased mutant S487TR, and the arrestin-biased mutant S487A. In these mutants, the Ser487 residue in the C-terminus region of the WT was truncated (S487TR) or mutated to alanine (S487A). Histamine significantly stimulated JNK phosphorylation in CHO cells expressing WT and S487TR but not S487A. Histamine-induced JNK phosphorylation in CHO cells expressing WT and S487TR was suppressed by inhibitors against H1 receptors (ketotifen and diphenhydramine), Gq proteins (YM-254890), and protein kinase C (PKC) (GF109203X) as well as an intracellular Ca2+ chelator (BAPTA-AM) but not by inhibitors against G protein-coupled receptor kinases (GRK2/3) (cmpd101), β-arrestin2 (β-arrestin2 siRNA), and clathrin (hypertonic sucrose). These results suggest that the H1 receptor-mediated phosphorylation of JNK is regulated by Gq-protein/Ca2+/PKC-dependent but GRK/arrestin/clathrin-independent pathways.

Systematic Assessment of Human CCR7 Signalling Using NanoBRET Biosensors Points towards the Importance of the Cellular Context

The human CC chemokine receptor 7 (CCR7) is activated by two natural ligands, CC chemokine ligand 19 (CCL19) and 21 (CCL21). The CCL19-CCL21-CCR7 axis has been extensively studied in vitro, but there is still debate over whether CCL21 is an overall weaker agonist or if the axis displays biased signalling. In this study, we performed a systematic analysis at the transducer level using NanoBRET-based methodologies in three commonly used cellular backgrounds to evaluate pathway and ligand preferences, as well as ligand bias and the influence of the cellular system thereon. We found that both CCL19 and CCL21 activated all cognate G proteins and some non-cognate couplings in a cell-type-dependent manner. Both ligands recruited β-arrestin1 and 2, but the potency was strongly dependent on the cellular system. Overall, CCL19 and CCL21 showed largely conserved pathway preferences, but small differences were detected. However, these differences only consolidated in a weak ligand bias. Together, these data suggest that CCL19 and CCL21 share mostly overlapping, weakly biased, transducer profiles, which can be influenced by the cellular context.

Functional diversification of cell signaling by GPCR localization

G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors and a critical class of regulators of mammalian physiology. Also known as seven transmembrane receptors (7TMs), GPCRs are ubiquitously expressed and versatile, detecting a diverse set of endogenous stimuli, including odorants, neurotransmitters, hormones, peptides, and lipids. Accordingly, GPCRs have emerged as the largest class of drug targets, accounting for upward of 30% of all prescription drugs. The view that ligand-induced GPCR responses originate exclusively from the cell surface has evolved to reflect accumulating evidence that receptors can elicit additional waves of signaling from intracellular compartments. These events in turn shape unique cellular and physiological outcomes. Here, we discuss our current understanding of the roles and regulation of compartmentalized GPCR signaling.

β-Arrestin-independent endosomal cAMP signaling by a polypeptide hormone GPCR

Many G protein-coupled receptors (GPCRs) initiate a second phase of stimulatory heterotrimeric G protein (Gs)-coupled cAMP signaling after endocytosis. The prevailing current view is that the endosomal signal is inherently β-arrestin-dependent because β-arrestin is necessary for receptor internalization and, for some GPCRs, to prolong the endosomal signal. Here we revise this view by showing that the vasoactive intestinal peptide receptor 1 (VIPR1), a secretin-family polypeptide hormone receptor, does not require β-arrestin to internalize or to generate an endosomal signal. β-Arrestin instead resolves the plasma membrane and endosomal signaling phases into sequential cAMP peaks by desensitizing the plasma membrane phase without affecting the endosomal phase. This appears to occur through the formation of functionally distinct VIPR1-β-arrestin complexes at each location that differ in their phosphorylation dependence. We conclude that endosomal GPCR signaling can occur in the absence of β-arrestin and that β-arrestin sculpts the spatiotemporal profile of cellular GPCR-G protein signaling through location-specific remodeling of GPCR-β-arrestin complexes.

CMV-encoded GPCRs in infection, disease, and pathogenesis

G protein coupled receptors (GPCRs) are seven-transmembrane domain proteins that modulate cellular processes in response to external stimuli. These receptors represent the largest family of membrane proteins, and in mammals, their signaling regulates important physiological functions, such as vision, taste, and olfaction. Many organisms, including yeast, slime molds, and viruses encode GPCRs. Cytomegaloviruses (CMVs) are large, betaherpesviruses, that encode viral GPCRs (vGPCRs). Human CMV (HCMV) encodes four vGPCRs, including UL33, UL78, US27, and US28. Each of these vGPCRs, as well as their rodent and primate orthologues, have been investigated for their contributions to viral infection and disease. Herein, we discuss how the CMV vGPCRs function during lytic and latent infection, as well as our understanding of how they impact viral pathogenesis.

GPCR kinases differentially modulate biased signaling downstream of CXCR3 depending on their subcellular localization

Some G protein-coupled receptors (GPCRs) demonstrate biased signaling such that ligands of the same receptor exclusively or preferentially activate certain downstream signaling pathways over others. This phenomenon may result from ligand-specific receptor phosphorylation by GPCR kinases (GRKs). GPCR signaling can also exhibit location bias because GPCRs traffic to and signal from subcellular compartments in addition to the plasma membrane. Here, we investigated whether GRKs contributed to location bias in GPCR signaling. GRKs translocated to endosomes after stimulation of the chemokine receptor CXCR3 or other GPCRs in cultured cells. GRK2, GRK3, GRK5, and GRK6 showed distinct patterns of recruitment to the plasma membrane and to endosomes depending on the identity of the biased ligand used to activate CXCR3. Analysis of engineered forms of GRKs that localized to either the plasma membrane or endosomes demonstrated that biased CXCR3 ligands elicited different signaling profiles that depended on the subcellular location of the GRK. Each GRK exerted a distinct effect on the regulation of CXCR3 engagement of β-arrestin, internalization, and activation of the downstream effector kinase ERK. Our work highlights a role for GRKs in location-biased GPCR signaling and demonstrates the complex interactions between ligands, GRKs, and cellular location that contribute to biased signaling.

The Role of G Protein-Coupled Receptors and Receptor Kinases in Pancreatic β-Cell Function and Diabetes

Type 2 diabetes (T2D) mellitus has emerged as a major global health concern that has accelerated in recent years due to poor diet and lifestyle. Afflicted individuals have high blood glucose levels that stem from the inability of the pancreas to make enough insulin to meet demand. Although medication can help to maintain normal blood glucose levels in individuals with chronic disease, many of these medicines are outdated, have severe side effects, and often become less efficacious over time, necessitating the need for insulin therapy. G protein-coupled receptors (GPCRs) regulate many physiologic processes, including blood glucose levels. In pancreatic β cells, GPCRs regulate β-cell growth, apoptosis, and insulin secretion, which are all critical in maintaining sufficient β-cell mass and insulin output to ensure euglycemia. In recent years, new insights into the signaling of incretin receptors and other GPCRs have underscored the potential of these receptors as desirable targets in the treatment of diabetes. The signaling of these receptors is modulated by GPCR kinases (GRKs) that phosphorylate agonist-activated GPCRs, marking the receptor for arrestin binding and internalization. Interestingly, genome-wide association studies using diabetic patient cohorts link the GRKs and arrestins with T2D. Moreover, recent reports show that GRKs and arrestins expressed in the β cell serve a critical role in the regulation of β-cell function, including β-cell growth and insulin secretion in both GPCR-dependent and -independent pathways. In this review, we describe recent insights into GPCR signaling and the importance of GRK function in modulating β-cell physiology. SIGNIFICANCE STATEMENT: Pancreatic β cells contain a diverse array of G protein-coupled receptors (GPCRs) that have been shown to improve β-cell function and survival, yet only a handful have been successfully targeted in the treatment of diabetes. This review discusses recent advances in our understanding of β-cell GPCR pharmacology and regulation by GPCR kinases while also highlighting the necessity of investigating islet-enriched GPCRs that have largely been unexplored to unveil novel treatment strategies.

Hypothalamic tanycytes as mediators of maternally programmed seasonal plasticity

In mammals, maternal photoperiodic programming (MPP) provides a means whereby juvenile development can be matched to forthcoming seasonal environmental conditions.1,2,3,4 This phenomenon is driven by in utero effects of maternal melatonin5,6,7 on the production of thyrotropin (TSH) in the fetal pars tuberalis (PT) and consequent TSH receptor-mediated effects on tanycytes lining the 3rd ventricle of the mediobasal hypothalamus (MBH).8,9,10 Here we use LASER capture microdissection and transcriptomic profiling to show that TSH-dependent MPP controls the attributes of the ependymal region of the MBH in juvenile animals. In Siberian hamster pups gestated and raised on a long photoperiod (LP) and thereby committed to a fast trajectory for growth and reproductive maturation, the ependymal region is enriched for tanycytes bearing sensory cilia and receptors implicated in metabolic sensing. Contrastingly, in pups gestated and raised on short photoperiod (SP) and therefore following an over-wintering developmental trajectory with delayed sexual maturation, the ependymal region has fewer sensory tanycytes. Post-weaning transfer of SP-gestated pups to an intermediate photoperiod (IP), which accelerates reproductive maturation, results in a pronounced shift toward a ciliated tanycytic profile and formation of tanycytic processes. We suggest that tanycytic plasticity constitutes a mechanism to tailor metabolic development for extended survival in variable overwintering environments.

Basal interaction of the orphan receptor GPR101 with arrestins leads to constitutive internalization

GPR101 is an orphan G protein-coupled receptor that promotes growth hormone secretion in the pituitary. The microduplication of the GPR101 gene has been linked with the X-linked acrogigantism, or X-LAG, syndrome. This disease is characterized by excessive growth hormone secretion and abnormal rapid growth beginning early in life. Mechanistically, GPR101 induces growth hormone secretion through constitutive activation of multiple heterotrimeric G proteins. However, the full scope of GPR101 signaling remains largely elusive. Herein, we investigated the association of GPR101 to multiple transducers and uncovered an important basal interaction with Arrestin 2 (β-arrestin 1) and Arrestin 3 (β-arrestin 2). By using a GPR101 mutant lacking the C-terminus and cell lines with an Arrestin 2/3 null background, we show that the arrestin association leads to constitutive clathrin- and dynamin-mediated GPR101 internalization. To further highlight GPR101 intracellular fate, we assessed the colocalization of GPR101 with Rab protein markers. Internalized GPR101 was mainly colocalized with the early endosome markers, Rab5 and EEA-1, and to a lesser degree with the late endosome marker Rab7. However, GPR101 was not colocalized with the recycling endosome marker Rab11. These findings show that the basal arrestin recruitment by GPR101 C-terminal tail drives the receptor constitutive clathrin-mediated internalization. Intracellularly, GPR101 concentrates in the endosomal compartment and is degraded through the lysosomal pathway. In conclusion, we uncovered a constitutive intracellular trafficking of GPR101 that potentially represents an important layer of regulation of its signaling and function.

Endosomal signaling via cAMP in parathyroid hormone (PTH) type 1 receptor biology

Compartmentalization of GPCR signaling is an emerging topic that highlights the physiological relevance of spatial bias in signaling. The parathyroid hormone (PTH) type 1 receptor (PTH1R) was the first GPCR described to signal via heterotrimeric G-protein and cAMP from endosomes after β-arrestin mediated internalization, challenging the canonical GPCR signaling model which established that signaling is terminated by receptor internalization. More than a decade later, many other GPCRs have been shown to signal from endosomes via cAMP, and recent studies have proposed that location of cAMP generation impacts physiological outcomes of GPCR signaling. Here, we review the extensive literature regarding PTH1R endosomal signaling via cAMP, the mechanisms that regulate endosomal generation of cAMP, and the implications of spatial bias in PTH1R physiological functions.

The multifaceted functions of β-arrestins and their therapeutic potential in neurodegenerative diseases

Arrestins are multifunctional proteins that regulate G-protein-coupled receptor (GPCR) desensitization, signaling, and internalization. The arrestin family consists of four subtypes: visual arrestin1, β-arrestin1, β-arrestin2, and visual arrestin-4. Recent studies have revealed the multifunctional roles of β-arrestins beyond GPCR signaling, including scaffolding and adapter functions, and physically interacting with non-GPCR receptors. Increasing evidence suggests that β-arrestins are involved in the pathogenesis of a variety of neurodegenerative diseases, including Alzheimer's disease (AD), frontotemporal dementia (FTD), and Parkinson's disease (PD). β-arrestins physically interact with γ-secretase, leading to increased production and accumulation of amyloid-beta in AD. Furthermore, β-arrestin oligomers inhibit the autophagy cargo receptor p62/SQSTM1, resulting in tau accumulation and aggregation in FTD. In PD, β-arrestins are upregulated in postmortem brain tissue and an MPTP model, and the β2AR regulates SNCA gene expression. In this review, we aim to provide an overview of β-arrestin1 and β-arrestin2, and describe their physiological functions and roles in neurodegenerative diseases. The multifaceted roles of β-arrestins and their involvement in neurodegenerative diseases suggest that they may serve as promising therapeutic targets.

Target-based drug discovery: Applications of fluorescence techniques in high throughput and fragment-based screening

Target-based discovery of first-in-class therapeutics demands an in-depth understanding of the molecular mechanisms underlying human diseases. Precise measurements of cellular and biochemical activities are critical to gain mechanistic knowledge of biomolecules and their altered function in disease conditions. Such measurements enable the development of intervention strategies for preventing or treating diseases by modulation of desired molecular processes. Fluorescence-based techniques are routinely employed for accurate and robust measurements of in-vitro activity of molecular targets and for discovering novel chemical molecules that modulate the activity of molecular targets. In the current review, the authors focus on the applications of fluorescence-based high throughput screening (HTS) and fragment-based ligand discovery (FBLD) techniques such as fluorescence polarization (FP), Förster resonance energy transfer (FRET), fluorescence thermal shift assay (FTSA) and microscale thermophoresis (MST) for the discovery of chemical probe to exploring target's role in disease biology and ultimately, serve as a foundation for drug discovery. Some recent advancements in these techniques for compound library screening against important classes of drug targets, such as G-protein-coupled receptors (GPCRs) and GTPases, as well as phosphorylation- and acetylation-mediated protein-protein interactions, are discussed. Overall, this review presents a landscape of how these techniques paved the way for the discovery of small-molecule modulators and biologics against these targets for therapeutic benefits.

The MRGPR family of receptors in immunity

The discovery of Mas-related G protein-coupled receptors (Mrgprs) has opened a compelling chapter in our understanding of immunity and sensory biology. This family of receptors, with their unique expression and diverse ligands, has emerged as key players in inflammatory states and hold the potential to alleviate human diseases. This review will focus on the members of this receptor family expressed on immune cells and how they govern immune and neuro-immune pathways underlying various physiological and pathological states. Immune cell-specific Mrgprs have been shown to control a variety of manifestations, including adverse drug reactions, inflammatory conditions, bacterial immunity, and the sensing of environmental exposures like allergens and irritants.

Investigating G-protein coupled receptor signalling with light-emitting biosensors

G protein-coupled receptors (GPCRs) are the most frequent target of currently approved drugs and play a central role in both physiological and pathophysiological processes. Beyond the canonical understanding of GPCR signal transduction, the importance of receptor conformation, beta-arrestin (β-arr) biased signalling, and signalling from intracellular locations other than the plasma membrane is becoming more apparent, along with the tight spatiotemporal compartmentalisation of downstream signals. Fluorescent and bioluminescent biosensors have played a pivotal role in elucidating GPCR signalling events in live cells. To understand the mechanisms of action of the GPCR-targeted drugs currently available, and to develop new and better GPCR-targeted therapeutics, understanding these novel aspects of GPCR signalling is critical. In this review, we present some of the tools available to interrogate each of these features of GPCR signalling, we illustrate some of the key findings which have been made possible by these tools and we discuss their limitations and possible developments.

Characterization of the real-time internalization of nine GPCRs reveals distinct dependence on arrestins and G proteins

G protein-coupled receptors (GPCRs) are seven transmembrane receptors that respond to external stimuli and undergo conformational changes to activate G proteins and modulate cellular processes leading to biological outcomes. To prevent overstimulation and prolonged exposure to stimuli, GPCRs are regulated by internalization. While the canonical GPCR internalization mechanism in mammalian cells is arrestin-dependent, clathrin-mediated endocytosis, more diverse GPCR internalization mechanisms have been described over the years. However, there is a lack of consistent methods used in the literature making it complicated to determine a receptor's internalization pathway. Here, we utilized a highly efficient time-resolved Förster resonance energy transfer (TR-FRET) internalization assay to determine the internalization profile of nine distinct GPCRs representing the GPCR classes A, B and C and with different G protein coupling profiles. This technique, coupled with clustered regularly interspaced palindromic repeats (CRISPR) engineered knockout cells allows us to effectively study the involvement of heterotrimeric G proteins and non-visual arrestins. We found that all the nine receptors internalized upon agonist stimulation in a concentration-dependent manner and six receptors showed basal internalization. Yet, there is no correlation between the receptor class and primary G protein coupling to the arrestin and G protein dependence for GPCR internalization. Overall, this study presents a platform for studying internalization that is applicable to most GPCRs and may even be extended to other membrane proteins. This method can be easily applicable to other endocytic machinery of interest and ultimately will lend itself towards the construction of comprehensive receptor internalization profiles.

Chronic opioid pain treatment converted to buprenorphine: A case series using a 3-step low-dose incremental dosing guideline

We report a 30-case series from the Pain Management Center at the Massachusetts General Hospital where we have applied a guideline to convert chronic treatment for pain from full agonist opioids (FAO) to buprenorphine (BUP). Of the patients, 24 (80 percent) elected to continue BUP over FAO. Five conversions were stopped for side effects (fatigue) and/or lack of sufficient pain reduction. One patient elected not to participate on the day that the conversion was to begin. There were no major adverse events. We conclude that conversion to BUP should be considered as an alternative to treat patients on chronic opioids for pain.

The Deubiquitylase Otub1 Regulates the Chemotactic Response of Splenic B Cells by Modulating the Stability of the γ-Subunit Gng2

The ubiquitin proteasome system performs the covalent attachment of lysine 48-linked polyubiquitin chains to substrate proteins, thereby targeting them for degradation, while deubiquitylating enzymes (DUBs) reverse this process. This posttranslational modification regulates key features both of innate and adaptative immunity, including antigen presentation, protein homeostasis and signal transduction. Here we show that loss of one of the most highly expressed DUBs, Otub1, results in changes in murine splenic B cell subsets, leading to a significant increase in marginal zone and transitional B cells and a concomitant decrease in follicular B cells. We demonstrate that Otub1 interacts with the γ-subunit of the heterotrimeric G protein, Gng2, and modulates its ubiquitylation status, thereby controlling Gng2 stability. Proximal mapping of Gng2 revealed an enrichment in partners associated with chemokine signaling, actin cytoskeleton and cell migration. In line with these findings, we show that Otub1-deficient B cells exhibit greater Ca2+ mobilization, F-actin polymerization and chemotactic responsiveness to Cxcl12, Cxcl13 and S1P in vitro, which manifests in vivo as altered localization of B cells within the spleen. Together, our data establishes Otub1 as a novel regulator of G-protein coupled receptor signaling in B cells, regulating their differentiation and positioning in the spleen.

Interplay of G-proteins and Serotonin in the Neuroimmunoinflammatory Model of Chronic Stress and Depression: A Narrative Review

INTRODUCTION

This narrative review addresses the clinical challenges in stress-related disorders such as depression, focusing on the interplay between neuron-specific and pro-inflammatory mechanisms at the cellular, cerebral, and systemic levels.

OBJECTIVE

We aim to elucidate the molecular mechanisms linking chronic psychological stress with low-grade neuroinflammation in key brain regions, particularly focusing on the roles of G proteins and serotonin (5-HT) receptors.

METHODS

This comprehensive review of the literature employs systematic, narrative, and scoping review methodologies, combined with systemic approaches to general pathology. It synthesizes current research on shared signaling pathways involved in stress responses and neuroinflammation, including calcium-dependent mechanisms, mitogen-activated protein kinases, and key transcription factors like NF-κB and p53. The review also focuses on the role of G protein-coupled neurotransmitter receptors (GPCRs) in immune and pro-inflammatory responses, with a detailed analysis of how 13 of 14 types of human 5-HT receptors contribute to depression and neuroinflammation.

RESULTS

The review reveals a complex interaction between neurotransmitter signals and immunoinflammatory responses in stress-related pathologies. It highlights the role of GPCRs and canonical inflammatory mediators in influencing both pathological and physiological processes in nervous tissue.

CONCLUSION

The proposed Neuroimmunoinflammatory Stress Model (NIIS Model) suggests that proinflammatory signaling pathways, mediated by metabotropic and ionotropic neurotransmitter receptors, are crucial for maintaining neuronal homeostasis. Chronic mental stress can disrupt this balance, leading to increased pro-inflammatory states in the brain and contributing to neuropsychiatric and psychosomatic disorders, including depression. This model integrates traditional theories on depression pathogenesis, offering a comprehensive understanding of the multifaceted nature of the condition.

Phosphorylation Sites of the Gastric Inhibitory Polypeptide Receptor (GIPR) Revealed by Trapped-Ion-Mobility Spectrometry Coupled to Time-of-Flight Mass Spectrometry (TIMS-TOF MS)

The gastric inhibitory polypeptide receptor (GIPR), a G protein-coupled receptor (GPCR) that regulates glucose metabolism and insulin secretion, is a target for the development of therapeutic agents to address type 2 diabetes and obesity. Signal transduction processes mediated by GPCR activation typically result in receptor phosphorylation, but very little is known about GIPR phosphorylation. Mass spectrometry (MS) is a powerful tool for detecting phosphorylation and other post-translational modifications of proteins and for identifying modification sites. However, applying MS methods to GPCRs is challenging because the native expression levels are low and the hydrophobicity of these proteins complicates isolation and enrichment. Here we use a widely available technique, trapped-ion-mobility spectrometry coupled to time-of-flight mass spectrometry (TIMS-TOF MS), to characterize the phosphorylation status of the GIPR. We identified eight serine residues that are phosphorylated, one in an intracellular loop and the remainder in the C-terminal domain. Stimulation with the native agonist GIP enhanced phosphorylation at four of these sites. For comparison, we evaluated tirzepatide (TZP), a dual agonist of the glucagon-like peptide-1 (GLP-1) receptor and the GIPR that has recently been approved for the treatment of type 2 diabetes. Stimulation with TZP enhanced phosphorylation at the same four sites that were enhanced with GIP; however, TZP also enhanced phosphorylation at a fifth site that is unique to this synthetic agonist. This work establishes an important and accessible tool for the characterization of signal transduction via the GIPR and reveals an unanticipated functional difference between GIP and TZP.

GPR146 regulates pulmonary vascular remodeling by promoting pulmonary artery smooth muscle cell proliferation through 5-lipoxygenase

The pathological feature of hypoxic pulmonary hypertension (PH) is pulmonary vascular remodeling (PVR), primarily attributed to the hyperproliferation and apoptosis resistance of pulmonary artery smooth muscle cells (PASMCs). Existing PH-targeted drugs have difficulties in reversing PVR. Therefore, it is vital to discover a new regulatory mechanism for PVR and develop new targeted drugs. G protein-coupled receptor 146 (GPR146) is believed to participate in this process. This study aimed to investigate the role of GPR146 in PASMCs during PH. We investigated the role of GPR146 in PVR and its underlying mechanism using hypoxic PASMCs and mouse model (Sugen 5416 (20 mg/kg)/hypoxia). In our recent study, we have observed a significant increase in the expression of GPR146 protein in animal models of PH as well as in patients diagnosed with pulmonary arterial hypertension (PAH). Through immunohistochemistry, we found that GPR146 was mainly localized in the smooth muscle and endothelial layers of the pulmonary vasculature. GPR146 deficiency induction exhibited protective effects against hypoxia-induced elevation of right ventricular systolic blood pressure (RVSP), right ventricular hypertrophy, and pulmonary vascular remodeling in mice. In particular, the deletion of GPR146 attenuated the hypoxia-triggered proliferation of PASMCs. Furthermore, 5-lipoxygenase (5-LO) was related to PH development. Hypoxia and overexpression of GPR146 increased 5-LO expression, which was reversed through GPR146 knockdown or siRNA intervention. Our study discovered that GPR146 exhibited high expression in the pulmonary vessels of pulmonary hypertension. Subsequent research revealed that GPR146 played a crucial role in the development of hypoxic PH by promoting lipid peroxidation and 5-LO expression. In conclusion, GPR146 may regulate pulmonary vascular remodeling by promoting PASMCs proliferation through 5-LO, which presents a feasible target for PH prevention and treatment.

Unravelling the mechanism of neurotensin recognition by neurotensin receptor 1

The conformational ensembles of G protein-coupled receptors (GPCRs) include inactive and active states. Spectroscopy techniques, including NMR, show that agonists, antagonists and other ligands shift the ensemble toward specific states depending on the pharmacological efficacy of the ligand. How receptors recognize ligands and the kinetic mechanism underlying this population shift is poorly understood. Here, we investigate the kinetic mechanism of neurotensin recognition by neurotensin receptor 1 (NTS1) using 19F-NMR, hydrogen-deuterium exchange mass spectrometry and stopped-flow fluorescence spectroscopy. Our results indicate slow-exchanging conformational heterogeneity on the extracellular surface of ligand-bound NTS1. Numerical analysis of the kinetic data of neurotensin binding to NTS1 shows that ligand recognition follows an induced-fit mechanism, in which conformational changes occur after neurotensin binding. This approach is applicable to other GPCRs to provide insight into the kinetic regulation of ligand recognition by GPCRs.

Structure, function and drug discovery of GPCR signaling

G protein-coupled receptors (GPCRs) are versatile and vital proteins involved in a wide array of physiological processes and responses, such as sensory perception (e.g., vision, taste, and smell), immune response, hormone regulation, and neurotransmission. Their diverse and essential roles in the body make them a significant focus for pharmaceutical research and drug development. Currently, approximately 35% of marketed drugs directly target GPCRs, underscoring their prominence as therapeutic targets. Recent advances in structural biology have substantially deepened our understanding of GPCR activation mechanisms and interactions with G-protein and arrestin signaling pathways. This review offers an in-depth exploration of both traditional and recent methods in GPCR structure analysis. It presents structure-based insights into ligand recognition and receptor activation mechanisms and delves deeper into the mechanisms of canonical and noncanonical signaling pathways downstream of GPCRs. Furthermore, it highlights recent advancements in GPCR-related drug discovery and development. Particular emphasis is placed on GPCR selective drugs, allosteric and biased signaling, polyphamarcology, and antibody drugs. Our goal is to provide researchers with a thorough and updated understanding of GPCR structure determination, signaling pathway investigation, and drug development. This foundation aims to propel forward-thinking therapeutic approaches that target GPCRs, drawing upon the latest insights into GPCR ligand selectivity, activation, and biased signaling mechanisms.

Modulating signalling lifetime to optimise a prototypical animal opsin for optogenetic applications

Animal opsins are light activated G-protein-coupled receptors, capable of optogenetic control of G-protein signalling for research or therapeutic applications. Animal opsins offer excellent photosensitivity, but their temporal resolution can be limited by long photoresponse duration when expressed outside their native cellular environment. Here, we explore methods for addressing this limitation for a prototypical animal opsin (human rod opsin) in HEK293T cells. We find that the application of the canonical rhodopsin kinase (GRK1)/visual arrestin signal termination mechanism to this problem is complicated by a generalised suppressive effect of GRK1 expression. This attenuation can be overcome using phosphorylation-independent mutants of arrestin, especially when these are tethered to the opsin protein. We further show that point mutations targeting the Schiff base stability of the opsin can also reduce signalling lifetime. Finally, we apply one such mutation (E122Q) to improve the temporal fidelity of restored visual responses following ectopic opsin expression in the inner retina of a mouse model of retinal degeneration (rd1). Our results reveal that these two strategies (targeting either arrestin binding or Schiff-base hydrolysis) can produce more time-delimited opsin signalling under heterologous expression and establish the potential of this approach to improve optogenetic performance.

Competing Engagement of β-arrestin Isoforms Balances IGF1R/p53 Signaling and Controls Melanoma Cell Chemotherapeutic Responsiveness

Constraints on the p53 tumor suppressor pathway have long been associated with the progression, therapeutic resistance, and poor prognosis of melanoma, the most aggressive form of skin cancer. Likewise, the insulin-like growth factor type 1 receptor (IGF1R) is recognized as an essential coordinator of transformation, proliferation, survival, and migration of melanoma cells. Given that β-arrestin (β-arr) system critically governs the anti/pro-tumorigenic p53/IGF1R signaling pathways through their common E3 ubiquitin-protein ligase MDM2, we explore whether unbalancing this system downstream of IGF1R can enhance the response of melanoma cells to chemotherapy. Altering β-arr expression demonstrated that both β-arr1-silencing and β-arr2-overexpression (-β-arr1/+β-arr2) facilitated nuclear-to-cytosolic MDM2 translocation accompanied by decreased IGF1R expression, while increasing p53 levels, resulting in reduced cell proliferation/survival. Imbalance towards β-arr2 (-β-arr1/+β-arr2) synergizes with the chemotherapeutic agent, dacarbazine, in promoting melanoma cell toxicity. In both 3D spheroid models and in vivo in zebrafish models, this combination strategy, through dual IGF1R downregulation/p53 activation, limits melanoma cell growth, survival and metastatic spread. In clinical settings, analysis of the TCGA-SKCM patient cohort confirms β-arr1-/β-arr2+ imbalance as a metastatic melanoma vulnerability that may enhance therapeutic benefit. Our findings suggest that under steady-state conditions, IGF1R/p53-tumor promotion/suppression status-quo is preserved by β-arr1/2 homeostasis. Biasing this balance towards β-arr2 can limit the protumorigenic IGF1R activities while enhancing p53 activity, thus reducing multiple cancer-sustaining mechanisms. Combined with other therapeutics, this strategy improves patient responses and outcomes to therapies relying on p53 or IGF1R pathways.

IMPLICATIONS

Altogether, β-arrestin system bias downstream IGF1R is an important metastatic melanoma vulnerability that may be conductive for therapeutic benefit.

Neuroprotection and Beyond: The Central Role of CB1 and CB2 Receptors in Stroke Recovery

The endocannabinoid system, with its intricate presence in numerous cells, tissues, and organs, offers a compelling avenue for therapeutic interventions. Central to this system are the cannabinoid receptors 1 and 2 (CB1R and CB2R), whose ubiquity can introduce complexities in targeted treatments due to their wide-ranging physiological influence. Injuries to the central nervous system (CNS), including strokes and traumatic brain injuries, induce localized pro-inflammatory immune responses, termed neuroinflammation. Research has shown that compensatory immunodepression usually follows, and these mechanisms might influence immunity, potentially affecting infection risks in patients. As traditional preventive treatments like antibiotics face challenges, the exploration of immunomodulatory therapies offers a promising alternative. This review delves into the potential neuroprotective roles of the cannabinoid receptors: CB1R's involvement in mitigating excitotoxicity and CB2R's dual role in promoting cell survival and anti-inflammatory responses. However, the potential of cannabinoids to reduce neuroinflammation must be weighed against the risk of exacerbating immunodepression. Though the endocannabinoid system promises numerous therapeutic benefits, understanding its multifaceted signaling mechanisms and outcomes remains a challenge.