GPCR desensitization: Acute and prolonged phases

Is there a role for biogenic amine receptors in mediating β-phenylethylamine and RO5256390-induced vascular contraction?

BACKGROUND

Substantial evidence indicates trace amines can induce vasoconstriction independently of noradrenaline release. However, the mechanism underlying noradrenaline-independent vasoconstrictor responses to trace amines has not yet been established. This study evaluates the role of trace amine-associated receptor 1 (TAAR1) and other biogenic amine receptors in mediating β-phenylethylamine and the TAAR-1 selective agonist RO5256390-induced vasoconstriction.

METHODS

Vasoconstrictor responses to β-PEA and the TAAR1-selective agonist, RO5256390 were assessed in vitro in endothelium-denuded aortic rings and third-order mesenteric arteries of male Sprague Dawley rats.

RESULTS

β-PEA and RO5256390 induced concentration-dependent vasoconstriction of aortic rings but not third-order mesenteric arteries. Vasoconstrictor responses in aortic rings were insensitive to antagonists of 5-HT. The murine-selective TAAR1 antagonist, EPPTB, had no effect on either β-PEA or RO5256390-induced vasoconstriction. The α1-adrenoceptor antagonist, prazosin, and the α2-adrenoceptor antagonist, yohimbine, induced a shift of the β-PEA concentration response curve too small to be ascribed to antagonism of α1-or α2-adrenoceptors, respectively. The α2-adrenoceptor antagonist atipamezole had no effect on β-PEA or RO5256390-induced vasoconstriction.

CONCLUSION

Vasoconstrictor responses to trace amines are not mediated by classical biogenic amine neurotransmitter receptors. Insensitivity of β-PEA vasoconstrictor responses to EPPTB, may be explained by its low affinity for rat rather than murine TAAR1. Therefore, TAAR1 remains the most likely candidate receptor mediating vasoconstrictor responses to trace amines and that prazosin and yohimbine have low affinity for TAAR1.

Systemic and tissue-specific spexin response to acute treadmill exercise in rats

Spexin (SPX) is a 14-amino-acid peptide that plays an important role in the regulation of metabolism and energy homeostasis. It is well known that a variety of bioactive molecules released into the circulation by organs and tissues in response to acute and chronic exercise, known as exerkines, mediate the benefits of exercise by improving metabolic health. However, it is unclear whether acute exercise affects SPX levels in the circulation and peripheral tissues. This study aimed to determine whether acute treadmill exercise induces plasma SPX levels, as well as mRNA expression and immunostaining of SPX in skeletal muscle, adipose tissue, and liver. Male Sprague Dawley rats were divided into sedentary and acute exercise groups. Plasma, soleus (SOL), extensor digitorum longus (EDL), adipose tissue, and liver samples were collected at six time points (0, 1, 3, 6, 12, and 24 h) following 60 min of acute treadmill exercise at a speed of 25 m/min and 0 % grade. Acute exercise increased plasma SPX levels and induced mRNA expression of Spx in the SOL, EDL, and liver. Immunohistochemical analysis demonstrated that acute exercise led to a decrease in SPX immunostaining in the liver. Taken together, these findings suggest that SPX increases in response to acute exercise as a potential exerkine candidate, and the liver may be one of the sources of acute exercise-induced plasma SPX levels in rats. However, a comprehensive analysis is needed to fully elucidate the systemic response of SPX to acute exercise, as well as the tissue from which SPX is secreted.

Incretin therapy in feline diabetes mellitus - A review of the current state of research

Incretin hormones potentiate the glucose-induced insulin secretion following enteral nutrient intake. The best characterised incretin hormones are glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) which are produced in and secreted from the gut in response to nutrient ingestion. The property of incretins to enhance endogenous insulin secretion only at elevated blood glucose levels makes them interesting therapeutics for type 2 diabetes mellitus with a better safety profile than exogenous insulin. While incretin therapeutics (especially GLP-1 agonists, and more recently also GLP-1 / GIP dual agonists and other drugs that influence the incretin metabolism (e.g., dipeptidyl peptidase-4 (DPP-4) inhibitors)) are already widely used treatment options for human type 2 diabetes, these drugs are not yet approved for the therapy of feline diabetes mellitus. This review provides an introduction to incretins and feline diabetes mellitus in general and summarises the current study situation on incretins as therapeutics for feline diabetes mellitus to assess their possible future potential in feline medicine. Studies to date on the use of GLP-1 receptor agonists (GLP-1RA) in healthy cats largely confirm their insulinotropic effect known from other species. In diabetic cats, GLP-1RAs appear to significantly reduce glycaemic variability (GV, an indicator for the quality of glycaemic control), which is important for the management of the disease and prevention of long-term complications. However, for widespread use in feline diabetes mellitus, further studies are required that include larger numbers of diabetic cats, and that consider and test a possible need for dose adjustments to overweight and diabetic cats. Also evaluation of the outcome of GLP-1RA monotherapy will be neceessary.

MC4R Variants Modulate α-MSH and Setmelanotide Induced Cellular Signaling at Multiple Levels

CONTEXT

The melanocortin-4 receptor (MC4R) plays an important role in body weight regulation. Pathogenic MC4R variants are the most common cause of monogenic obesity.

OBJECTIVE

We have identified 17 MC4R variants in adult and pediatric patients with obesity. Here we aimed to functionally characterize these variants by analyzing 4 different aspects of MC4R signaling. In addition, we aimed to analyze the effect of setmelanotide, a potent MC4R agonist, on these MC4R variants.

MATERIALS AND METHODS

Cell surface expression and α-melanocyte stimulating hormone (α-MSH)- or setmelanotide-induced cAMP response, β-arrestin-2 recruitment, and ERK activation were measured in cells expressing either wild type or variant MC4R.

RESULTS

We found a large heterogeneity in the function of these variants. We identified variants with a loss of response for all studied MC4R signaling, variants with no cAMP accumulation or ERK activation but normal β-arrestin-2 recruitment, and variants with normal cAMP accumulation and ERK activation but decreased β-arrestin-2 recruitment, indicating disrupted desensitization and signaling mechanisms. Setmelanotide displayed a greater potency and similar efficacy as α-MSH and induced significantly increased maximal cAMP responses of several variants compared to α-MSH. Despite the heterogeneity in functional response, there was no apparent difference in the obesity phenotype in our patients.

CONCLUSION

We show that these obesity-associated MC4R variants affect MC4R signaling differently yet lead to a comparable clinical phenotype. Our results demonstrate the clinical importance of assessing the effect of MC4R variants on a range of molecular signaling mechanisms to determine their association with obesity, which may aid in improving personalized treatment.

TAK-861, a potent, orally available orexin receptor 2-selective agonist, produces wakefulness in monkeys and improves narcolepsy-like phenotypes in mouse models

Narcolepsy type 1 (NT1) is associated with severe loss of orexin neurons and characterized by symptoms including excessive daytime sleepiness and cataplexy. Current medications indicated for NT1 often show limited efficacy, not addressing the full spectrum of symptoms, demonstrating a need for novel drugs. We discovered a parenteral orexin receptor 2 (OX2R) agonist, danavorexton, and an orally available OX2R agonist, TAK-994; both improving NT1 phenotypes in mouse models and individuals with NT1. However, danavorexton has limited oral availability and TAK-994 has a risk of off-target liver toxicity. To avoid off-target-based adverse events, a highly potent molecule with low effective dose is preferred. Here, we show that a novel OX2R-selective agonist, TAK-861 [N-{(2S,3R)-4,4-Difluoro-1-(2-hydroxy-2-methylpropanoyl)-2-[(2,3',5'-trifluoro[1,1'-biphenyl]-3-yl)methyl]pyrrolidin-3-yl}ethanesulfonamide], activates OX2R with a half-maximal effective concentration of 2.5 nM and promotes wakefulness at 1 mg/kg in mice and monkeys, suggesting ~ tenfold higher potency and lower effective dosage than TAK-994. Similar to TAK-994, TAK-861 substantially ameliorates wakefulness fragmentation and cataplexy-like episodes in orexin/ataxin-3 and orexin-tTA;TetO DTA mice (NT1 mouse models). Compared with modafinil, TAK-861 induces highly correlated brain-wide neuronal activation in orexin-tTA;TetO DTA mice, suggesting efficient wake-promoting effects. Thus, TAK-861 has potential as an effective treatment for individuals with hypersomnia disorders including narcolepsy, potentially with a favorable safety profile.

Developmental Changes in the Excitation-Contraction Mechanisms of the Ventricular Myocardium and Their Sympathetic Regulation in Small Experimental Animals

The developmental changes in the excitation-contraction mechanisms of the ventricular myocardium of small animals (guinea pig, rat, mouse) and their sympathetic regulation will be summarized. The action potential duration monotonically decreases during pre- and postnatal development in the rat and mouse, while in the guinea pig it decreases during the fetal stage but turns into an increase just before birth. Such changes can be attributed to changes in the repolarizing potassium currents. The T-tubule and the sarcoplasmic reticulum are scarcely present in the fetal cardiomyocyte, but increase during postnatal development. This causes a developmental shift in the Ca2+ handling from a sarcolemma-dependent mechanism to a sarcoplasmic reticulum-dependent mechanism. The sensitivity for beta-adrenoceptor-mediated positive inotropy decreases during early postnatal development, which parallels the increase in sympathetic nerve innervation. The alpha-adrenoceptor-mediated inotropy in the mouse changes from positive in the neonate to negative in the adult. This can be explained by the change in the excitation-contraction mechanism mentioned above. The shortening of the action potential duration enhances trans-sarcolemmal Ca2+ extrusion by the Na+-Ca2+ exchanger. The sarcoplasmic reticulum-dependent mechanism of contraction in the adult allows Na+-Ca2+ exchanger activity to cause negative inotropy, a mechanism not observed in neonatal myocardium. Such developmental studies would provide clues towards a more comprehensive understanding of cardiac function.

Heterogeneity of tethered agonist signaling in adhesion G protein-coupled receptors

Adhesion G protein-coupled receptor (aGPCR) signaling influences development and homeostasis in a wide range of tissues. In the current model for aGPCR signaling, ligand binding liberates a conserved sequence that acts as an intramolecular, tethered agonist (TA), yet this model has not been evaluated systematically for all aGPCRs. Here, we assessed the TA-dependent activities of all 33 aGPCRs in a suite of transcriptional reporter, G protein activation, and β-arrestin recruitment assays using a new fusion protein platform. Strikingly, only ∼50% of aGPCRs exhibited robust TA-dependent activation, and unlike other GPCR families, aGPCRs showed a notable preference for G12/13 signaling. AlphaFold2 predictions assessing TA engagement in the predicted intramolecular binding pocket aligned with the TA dependence of the cellular responses. This dataset provides a comprehensive resource to inform the investigation of all human aGPCRs and for targeting aGPCRs therapeutically.

Cell-cell communication: new insights and clinical implications

Multicellular organisms are composed of diverse cell types that must coordinate their behaviors through communication. Cell-cell communication (CCC) is essential for growth, development, differentiation, tissue and organ formation, maintenance, and physiological regulation. Cells communicate through direct contact or at a distance using ligand-receptor interactions. So cellular communication encompasses two essential processes: cell signal conduction for generation and intercellular transmission of signals, and cell signal transduction for reception and procession of signals. Deciphering intercellular communication networks is critical for understanding cell differentiation, development, and metabolism. First, we comprehensively review the historical milestones in CCC studies, followed by a detailed description of the mechanisms of signal molecule transmission and the importance of the main signaling pathways they mediate in maintaining biological functions. Then we systematically introduce a series of human diseases caused by abnormalities in cell communication and their progress in clinical applications. Finally, we summarize various methods for monitoring cell interactions, including cell imaging, proximity-based chemical labeling, mechanical force analysis, downstream analysis strategies, and single-cell technologies. These methods aim to illustrate how biological functions depend on these interactions and the complexity of their regulatory signaling pathways to regulate crucial physiological processes, including tissue homeostasis, cell development, and immune responses in diseases. In addition, this review enhances our understanding of the biological processes that occur after cell-cell binding, highlighting its application in discovering new therapeutic targets and biomarkers related to precision medicine. This collective understanding provides a foundation for developing new targeted drugs and personalized treatments.

Structural basis of positive allosteric modulation of metabotropic glutamate receptor activation and internalization

The metabotropic glutamate receptors (mGluRs) are neuromodulatory family C G protein coupled receptors which assemble as dimers and allosterically couple extracellular ligand binding domains (LBDs) to transmembrane domains (TMDs) to drive intracellular signaling. Pharmacologically, mGluRs can be targeted at the LBDs by glutamate and synthetic orthosteric compounds or at the TMDs by allosteric modulators. Despite the potential of allosteric compounds as therapeutics, an understanding of the functional and structural basis of their effects is limited. Here we use multiple approaches to dissect the functional and structural effects of orthosteric versus allosteric ligands. We find, using electrophysiological and live cell imaging assays, that both agonists and positive allosteric modulators (PAMs) can drive activation and internalization of group II and III mGluRs. The effects of PAMs are pleiotropic, boosting the maximal response to orthosteric agonists and serving independently as internalization-biased agonists across mGluR subtypes. Motivated by this and intersubunit FRET analyses, we determine cryo-electron microscopy structures of mGluR3 in the presence of either an agonist or antagonist alone or in combination with a PAM. These structures reveal PAM-driven re-shaping of intra- and inter-subunit conformations and provide evidence for a rolling TMD dimer interface activation pathway that controls G protein and beta-arrestin coupling.

Two C-terminal isoforms of Aplysia tachykinin-related peptide receptors exhibit phosphorylation-dependent and phosphorylation-independent desensitization mechanisms

Diversity, a hallmark of G protein-coupled receptor (GPCR) signaling, partly stems from alternative splicing of a single gene generating more than one isoform for a receptor. Additionally, receptor responses to ligands can be attenuated by desensitization upon prolonged or repeated ligand exposure. Both phenomena have been demonstrated and exemplified by the deuterostome tachykinin signaling system, although the role of phosphorylation in desensitization remains a subject of debate. Here, we describe the signaling system for tachykinin-related peptides (TKRPs) in a protostome, mollusk Aplysia. We cloned the Aplysia TKRP precursor, which encodes three TKRPs (apTKRP-1, apTKRP-2a, and apTKRP-2b) containing the FXGXR-amide motif. In situ hybridization and immunohistochemistry showed predominant expression of TKRP mRNA and peptide in the cerebral ganglia. TKRPs and their posttranslational modifications were observed in extracts of central nervous system ganglia using mass spectrometry. We identified two Aplysia TKRP receptors (apTKRPRs), named apTKRPR-A and apTKRPR-B. These receptors are two isoforms generated through alternative splicing of the same gene and differ only in their intracellular C termini. Structure-activity relationship analysis of apTKRP-2b revealed that both C-terminal amidation and conserved residues of the ligand are critical for receptor activation. C-terminal truncates and mutants of apTKRPRs suggested that there is a C-terminal phosphorylation-independent desensitization for both receptors. Moreover, apTKRPR-B also exhibits phosphorylation-dependent desensitization through the phosphorylation of C-terminal Ser/Thr residues. This comprehensive characterization of the Aplysia TKRP signaling system underscores the evolutionary conservation of the TKRP and TK signaling systems, while highlighting the intricacies of receptor regulation through alternative splicing and differential desensitization mechanisms.

Apelin modulates inflammation and leukocyte recruitment in experimental autoimmune encephalomyelitis

Demyelination due to autoreactive T cells and inflammation in the central nervous system are principal features of multiple sclerosis (MS), a chronic and highly disabling human disease affecting brain and spinal cord. Here, we show that treatment with apelin, a secreted peptide ligand for the G protein-coupled receptor APJ/Aplnr, is protective in experimental autoimmune encephalomyelitis (EAE), an animal model of MS. Apelin reduces immune cell entry into the brain, delays the onset and reduces the severity of EAE. Apelin affects the trafficking of leukocytes through the lung by modulating the expression of cell adhesion molecules that mediate leukocyte recruitment. In addition, apelin induces the internalization and desensitization of its receptor in endothelial cells (ECs). Accordingly, protection against EAE major outcomes of apelin treatment are phenocopied by loss of APJ/Aplnr function, achieved by EC-specific gene inactivation in mice or knockdown experiments in cultured primary endothelial cells. Our findings highlight the importance of the lung-brain axis in neuroinflammation and indicate that apelin targets the transendothelial migration of immune cells into the lung during acute inflammation.

Sex differences in CGRP-induced vasodilation of human middle meningeal arteries but not human coronary arteries: implications for migraine

BACKGROUND

Migraine prevalence and levels of calcitonin gene-related peptide (CGRP), a peptide involved in migraine pathophysiology, differ between men and women, and appear to be affected by changes in sex hormones. The present study investigated the sex-specific responses to CGRP in human isolated arteries.

METHODS

CGRP-induced relaxation of 62 (28 men and 34 women) human isolated middle meningeal arteries (HMMA) and 139 (69 men and 70 women) human isolated coronary arteries (HCA) was compared between men and women in groups <50 years and ≥50 years of age as a proxy for pre- and postmenopausal status in women, as well as matched-age groups for men.

RESULTS

In HCA, no differences were observed between male and female tissue, or between the different age groups. However, in HMMA, the maximum response was significantly smaller and CGRP was less potent in females <50 compared with males <50 years of age. No differences were observed between the older age groups.

CONCLUSIONS

Sex differences were observed for CGRP-induced relaxation of HMMA, but not HCA. These differences could arise from differential receptor expression in the vascular beds combined with the effect of sex hormones on CGRP and subsequent receptor desensitization.

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.

G Protein-Coupled Receptors: A Century of Research and Discovery

GPCRs (G protein-coupled receptors), also known as 7 transmembrane domain receptors, are the largest receptor family in the human genome, with ≈800 members. GPCRs regulate nearly every aspect of human physiology and disease, thus serving as important drug targets in cardiovascular disease. Sharing a conserved structure comprised of 7 transmembrane α-helices, GPCRs couple to heterotrimeric G-proteins, GPCR kinases, and β-arrestins, promoting downstream signaling through second messengers and other intracellular signaling pathways. GPCR drug development has led to important cardiovascular therapies, such as antagonists of β-adrenergic and angiotensin II receptors for heart failure and hypertension, and agonists of the glucagon-like peptide-1 receptor for reducing adverse cardiovascular events and other emerging indications. There continues to be a major interest in GPCR drug development in cardiovascular and cardiometabolic disease, driven by advances in GPCR mechanistic studies and structure-based drug design. This review recounts the rich history of GPCR research, including the current state of clinically used GPCR drugs, and highlights newly discovered aspects of GPCR biology and promising directions for future investigation. As additional mechanisms for regulating GPCR signaling are uncovered, new strategies for targeting these ubiquitous receptors hold tremendous promise for the field of cardiovascular medicine.

Kinetic Model for the Desensitization of G Protein-Coupled Receptor

Desensitization of G-protein-coupled receptors (GPCR) is a general regulatory mechanism adopted by biological organisms against overstimulation of G protein signaling. Although the details of the mechanism are extensively studied, it is not easy to gain an overarching understanding of the process constituted by a multitude of molecular events with vastly differing time scales. To offer a semiquantitative yet predictive understanding of the mechanism, we formulate a kinetic model for the G protein signaling and desensitization by considering essential biochemical steps from ligand binding to receptor internalization. The internalization, followed by receptor depletion from the plasma membrane, attenuates the downstream signal. Together with the kinetic model and its full numerics of the expression derived for the dose-response relation, an approximated form of the expression clarifies the role played by the individual biochemical processes and allows us to identify four distinct regimes for the downregulation that emerge from the balance between phosphorylation, dephosphorylation, and the cellular level of β-arrestin.

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.

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.

Pharmacological Advances in Incretin-Based Polyagonism: What We Know and What We Don't

The prevalence of obesity continues to rise in both adolescents and adults, in parallel obesity is strongly associated with the increased incidence of type 2 diabetes, heart failure, certain types of cancer, and all-cause mortality. In relation to obesity, many pharmacological approaches of the past have tried and failed to combat the rising obesity epidemic, particularly due to insufficient efficacy or unacceptable side effects. However, while the history of antiobesity medication is plagued by failures and disappointments, we have witnessed over the last 10 years substantial progress, particularly in regard to biochemically optimized agonists at the receptor for glucagon-like peptide-1 (GLP-1R) and unimolecular coagonists at the receptors for GLP-1 and the glucose-dependent insulinotropic polypeptide (GIP). Although the GIP receptor:GLP-1R coagonists are being heralded as premier pharmacological tools for the treatment of obesity and diabetes, uncertainty remains as to why these drugs testify superiority over best-in-class GLP-1R monoagonists. Particularly with regard to GIP, there remains great uncertainty if and how GIP acts on systems metabolism and if the GIP system should be activated or inhibited to improve metabolic outcome in adjunct to GLP-1R agonism. In this review, we summarize recent advances in GLP-1- and GIP-based pharmacology and discuss recent findings and open questions related to how the GIP system affects systemic energy and glucose metabolism.

The complexities of ligand/receptor interactions: Exploring the role of molecular vibrations and quantum tunnelling

Molecular vibrations and quantum tunneling may link ligand binding to the function of pharmacological receptors. The well-established lock-and-key model explains a ligand's binding and recognition by a receptor; however, a general mechanism by which receptors translate binding into activation, inactivation, or modulation remains elusive. The Vibration Theory of Olfaction was proposed in the 1930s to explain this subset of receptor-mediated phenomena by correlating odorant molecular vibrations to smell, but a mechanism was lacking. In the 1990s, inelastic electron tunneling was proposed as a plausible mechanism for translating molecular vibration to odorant physiology. More recently, studies of ligands' vibrational spectra and the use of deuterated ligand analogs have provided helpful information to study this admittedly controversial hypothesis in metabotropic receptors other than olfactory receptors. In the present work, based in part on published experiments from our laboratory using planarians as an experimental organism, I will present a rationale and possible experimental approach for extending this idea to ligand-gated ion channels.

Evoked Weibel-Palade Body Exocytosis Modifies the Endothelial Cell Surface by Releasing a Substrate-Selective Phosphodiesterase

Weibel Palade bodies (WPB) are lysosome-related secretory organelles of endothelial cells. Commonly known for their main cargo, the platelet and leukocyte receptors von-Willebrand factor (VWF) and P-selectin, WPB play a crucial role in hemostasis and inflammation. Here, the authors identify the glycerophosphodiester phosphodiesterase domain-containing protein 5 (GDPD5) as a WPB cargo protein and show that GDPD5 is transported to WPB following uptake from the plasma membrane via an unique endocytic transport route. GDPD5 cleaves GPI-anchored, plasma membrane-resident proteins within their GPI-motif, thereby regulating their local activity. The authors identify a novel target of GDPD5 , the complement regulator CD59, and show that it is released from the endothelial surface by GDPD5 following WPB exocytosis. This results in increased deposition of complement components and can enhance local inflammatory and thrombogenic responses. Thus, stimulus-induced WPB exocytosis can modify the endothelial cell surface by GDPD5-mediated selective release of a subset of GPI-anchored proteins.

Melanocortin agonism in a social context selectively activates nucleus accumbens in an oxytocin-dependent manner

Social deficits are debilitating features of many psychiatric disorders, including autism. While time-intensive behavioral therapy is moderately effective, there are no pharmacological interventions for social deficits in autism. Many studies have attempted to treat social deficits using the neuropeptide oxytocin for its powerful neuromodulatory abilities and influence on social behaviors and cognition. However, clinical trials utilizing supplementation paradigms in which exogenous oxytocin is chronically administered independent of context have failed. An alternative treatment paradigm suggests pharmacologically activating the endogenous oxytocin system during behavioral therapy to enhance the efficacy of therapy by facilitating social learning. To this end, melanocortin receptor agonists like Melanotan II (MTII), which induces central oxytocin release and accelerates formation of partner preference, a form of social learning, in prairie voles, are promising pharmacological tools. To model pharmacological activation of the endogenous oxytocin system during behavioral therapy, we administered MTII prior to social interactions between male and female voles. We assessed its effect on oxytocin-dependent activity in brain regions subserving social learning using Fos expression as a proxy for neuronal activation. In non-social contexts, MTII only activated hypothalamic paraventricular nucleus, a primary site of oxytocin synthesis. However, during social interactions, MTII selectively increased oxytocin-dependent activation of nucleus accumbens, a site critical for social learning. These results suggest a mechanism for the MTII-induced acceleration of partner preference formation observed in previous studies. Moreover, they are consistent with the hypothesis that pharmacologically activating the endogenous oxytocin system with a melanocortin agonist during behavioral therapy has potential to facilitate social learning.

Hypertensive Heart Disease: Mechanisms, Diagnosis and Treatment

Hypertensive heart disease (HHD) presents a substantial global health burden, spanning a spectrum from subtle cardiac functional alterations to overt heart failure. In this comprehensive review, we delved into the intricate pathophysiological mechanisms governing the onset and progression of HHD. We emphasized the significant role of neurohormonal activation, inflammation, and metabolic remodeling in HHD pathogenesis, offering insights into promising therapeutic avenues. Additionally, this review provided an overview of contemporary imaging diagnostic tools for precise HHD severity assessment. We discussed in detail the current potential treatments for HHD, including pharmacologic, lifestyle, and intervention devices. This review aimed to underscore the global importance of HHD and foster a deeper understanding of its pathophysiology, ultimately contributing to improved public health outcomes.

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.

Cellular forgetting, desensitisation, stress and ageing in signalling networks. When do cells refuse to learn more?

Recent findings show that single, non-neuronal cells are also able to learn signalling responses developing cellular memory. In cellular learning nodes of signalling networks strengthen their interactions e.g. by the conformational memory of intrinsically disordered proteins, protein translocation, miRNAs, lncRNAs, chromatin memory and signalling cascades. This can be described by a generalized, unicellular Hebbian learning process, where those signalling connections, which participate in learning, become stronger. Here we review those scenarios, where cellular signalling is not only repeated in a few times (when learning occurs), but becomes too frequent, too large, or too complex and overloads the cell. This leads to desensitisation of signalling networks by decoupling signalling components, receptor internalization, and consequent downregulation. These molecular processes are examples of anti-Hebbian learning and 'forgetting' of signalling networks. Stress can be perceived as signalling overload inducing the desensitisation of signalling pathways. Ageing occurs by the summative effects of cumulative stress downregulating signalling. We propose that cellular learning desensitisation, stress and ageing may be placed along the same axis of more and more intensive (prolonged or repeated) signalling. We discuss how cells might discriminate between repeated and unexpected signals, and highlight the Hebbian and anti-Hebbian mechanisms behind the fold-change detection in the NF-κB signalling pathway. We list drug design methods using Hebbian learning (such as chemically-induced proximity) and clinical treatment modalities inducing (cancer, drug allergies) desensitisation or avoiding drug-induced desensitisation. A better discrimination between cellular learning, desensitisation and stress may open novel directions in drug design, e.g. helping to overcome drug resistance.

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.

Exploring the effects of dietary inulin in rainbow trout fed a high-starch, 100% plant-based diet

BACKGROUND

High dietary carbohydrates can spare protein in rainbow trout (Oncorhynchus mykiss) but may affect growth and health. Inulin, a prebiotic, could have nutritional and metabolic effects, along with anti-inflammatory properties in teleosts, improving growth and welfare. We tested this hypothesis in rainbow trout by feeding them a 100% plant-based diet, which is a viable alternative to fishmeal and fish oil in aquaculture feeds. In a two-factor design, we examined the impact of inulin (2%) as well as the variation in the carbohydrates (CHO)/plant protein ratio on rainbow trout. We assessed the influence of these factors on zootechnical parameters, plasma metabolites, gut microbiota, production of short-chain fatty acids and lactic acid, as well as the expression of free-fatty acid receptor genes in the mid-intestine, intermediary liver metabolism, and immune markers in a 12-week feeding trial.

RESULTS

The use of 2% inulin did not significantly change the fish intestinal microbiota, but interestingly, the high CHO/protein ratio group showed a change in intestinal microbiota and in particular the beta diversity, with 21 bacterial genera affected, including Ralstonia, Bacillus, and 11 lactic-acid producing bacteria. There were higher levels of butyric, and valeric acid in groups fed with high CHO/protein diet but not with inulin. The high CHO/protein group showed a decrease in the expression of pro-inflammatory cytokines (il1b, il8, and tnfa) in liver and a lower expression of the genes coding for tight-junction proteins in mid-intestine (tjp1a and tjp3). However, the 2% inulin did not modify the expression of plasma immune markers. Finally, inulin induced a negative effect on rainbow trout growth performance irrespective of the dietary carbohydrates.

CONCLUSIONS

With a 100% plant-based diet, inclusion of high levels of carbohydrates could be a promising way for fish nutrition in aquaculture through a protein sparing effect whereas the supplementation of 2% inulin does not appear to improve the use of CHO when combined with a 100% plant-based diet.

Hypothalamic free fatty acid receptor-1 regulates whole-body energy balance

OBJECTIVE

Free fatty acid receptor-1 (FFAR1) is a medium- and long-chain fatty acid sensing G protein-coupled receptor that is highly expressed in the hypothalamus. Here, we investigated the central role of FFAR1 on energy balance.

METHODS

Central FFAR1 agonism and virogenic knockdown were performed in mice. Energy balance studies, infrared thermographic analysis of brown adipose tissue (BAT) and molecular analysis of the hypothalamus, BAT, white adipose tissue (WAT) and liver were carried out.

RESULTS

Pharmacological stimulation of FFAR1, using central administration of its agonist TUG-905 in diet-induced obese mice, decreases body weight and is associated with increased energy expenditure, BAT thermogenesis and browning of subcutaneous WAT (sWAT), as well as reduced AMP-activated protein kinase (AMPK) levels, reduced inflammation, and decreased endoplasmic reticulum (ER) stress in the hypothalamus. As FFAR1 is expressed in distinct hypothalamic neuronal subpopulations, we used an AAV vector expressing a shRNA to specifically knockdown Ffar1 in proopiomelanocortin (POMC) neurons of the arcuate nucleus of the hypothalamus (ARC) of obese mice. Our data showed that knockdown of Ffar1 in POMC neurons promoted hyperphagia and body weight gain. In parallel, these mice developed hepatic insulin resistance and steatosis.

CONCLUSIONS

FFAR1 emerges as a new hypothalamic nutrient sensor regulating whole body energy balance. Moreover, pharmacological activation of FFAR1 could provide a therapeutic advance in the management of obesity and its associated metabolic disorders.

Hepatic β-arrestins: potential roles in liver health and disease

Β-arrestins are intracellular scaffolding proteins that have multifaceted roles in different types of disorders. In this review article, we gave a summary about the discovery, characterization and classification of these proteins and their intracellular functions. Moreover, this review article focused on the hepatic expression of β-arrestins and their hepatocellular distribution and function in each liver cell type. Also, we showed that β-arrestins are key regulators of distinct types of hepatic disorders. On the other hand, we addressed some important points that have never been studied before regarding the role of β-arrestins in certain types of hepatic disorders which needs more research efforts to cover.

Therapeutic Potential for Beta-3 Adrenoreceptor Agonists in Peripheral Arterial Disease and Diabetic Foot Ulcers

Annually, peripheral arterial disease is estimated to cost over USD 21 billion and diabetic foot disease an estimated at USD 9-13 billion. Mirabegron is a TGA-approved beta-3 adrenoreceptor agonist, shown to be safe and effective in the treatment of overactive bladder syndrome by stimulating bladder smooth muscle relaxation. In this review, we discuss the potential use of beta-3 adrenoreceptor agonists as therapeutic agents repurposed for peripheral arterial disease and diabetic foot ulcers. The development of both conditions is underpinned by the upregulation of oxidative stress pathways and consequential inflammation and hypoxia. In oxidative stress, there is an imbalance of reactive oxygen species and nitric oxide. Endothelial nitric oxide synthase becomes uncoupled in disease states, producing superoxide and worsening oxidative stress. Agonist stimulation of the beta-3 adrenoreceptor recouples and activates endothelial nitric oxide synthase, increasing the production of nitric oxide. This reduces circulating reactive oxygen species, thus decreasing redox modification and dysregulation of cellular proteins, causing downstream smooth muscle relaxation, improved endothelial function and increased angiogenesis. These mechanisms lead to endothelial repair in peripheral arterial disease and an enhanced perfusion in hypoxic tissue, which will likely improve the healing of chronic ulcers.

Proximity Interactome Analysis of Super Conserved Receptors Expressed in the Brain Identifies EPB41L2, SLC3A2, and LRBA as Main Partners

The Super-Conserved Receptors Expressed in the Brain (SREBs) form a subfamily of orphan G protein-coupled receptors, highly conserved in evolution and characterized by a predominant expression in the brain. The signaling pathways activated by these receptors (if any) are presently unclear. Given the strong conservation of their intracellular loops, we used a BioID2 proximity-labeling assay to identify protein partners of SREBs that would interact with these conserved domains. Using streptavidin pull-down followed by mass spectrometry analysis, we identified the amino acid transporter SLC3A2, the AKAP protein LRBA, and the 4.1 protein EPB41L2 as potential interactors of these GPCRs. Using co-immunoprecipitation experiments, we confirmed the physical association of these proteins with the receptors. We then studied the functional relevance of the interaction between EPB41L2 and SREB1. Immunofluorescence microscopy revealed that SREB1 and EPB41L2 co-localize at the plasma membrane and that SREB1 is enriched in the β-catenin-positive cell membranes. siRNA knockdown experiments revealed that EPB41L2 promotes the localization of SREB1 at the plasma membrane and increases the solubilization of SREB1 when using detergents, suggesting a modification of its membrane microenvironment. Altogether, these data suggest that EPB41L2 could regulate the subcellular compartmentalization of SREBs and, as proposed for other GPCRs, could affect their stability or activation.

Discovery and Characterization of Novel CNS-Penetrant GPR55 Agonists

From our NETSseq-derived human brain transcriptomics data, we identified GPR55 as a potential molecular target for the treatment of motor symptoms in patients with Parkinson's disease. From a high-throughput screen, we identified and optimized agonists with nanomolar potency against both human and rat GPR55. We discovered compounds with either strong or limited β-arrestin signaling and receptor desensitization, indicating biased signaling. A compound that showed minimal GPR55 desensitization demonstrated a reduction in firing frequency of medium spiny neurons cultured from rat striatum but did not reverse motor deficits in a rat hypolocomotion model. Further profiling of several desensitizing and non-desensitizing lead compounds showed that they are selective over related cannabinoid receptors CB1 and CB2 and that unbound brain concentrations well above the respective GPR55 EC50 can be readily achieved following oral administration. The novel brain-penetrant GPR55 agonists disclosed can be used to probe the role of this receptor in the brain.

The effect of N-arachidonoylethanolamide administration on energy and fat metabolism of early lactating dairy cows

The aim of the study was to investigate the effect of N-arachidonoylethanolamide (AEA), an endocannabinoid with orexigenic characteristics, on plasma endocannabinoid concentrations, feed intake, energy balance, lipomobilisation, and hepatic lipid metabolism of early-lactating dairy cows. The experiment involved 10 pairs of Holstein half-sibling cows (end of 2nd-3rd pregnancy). Half-sibs of each pair were randomly assigned to either AEA (n = 10) or control (CON) group (n = 10). From day 1 to 30 postpartum, the AEA group received 5 intraperitoneal injections per week of 3 µg/kg body weight AEA and the CON group 0.9% NaCl. In week 1-3 postpartum, AEA administration had no effect on dry matter intake, body weight, or lipomobilisation, but increased plasma triglyceride concentration on d 21 p.p. and mRNA abundances of genes related to hepatic triglyceride synthesis. In week 4 postpartum, the AEA group showed reduced feed intake and whole-body carbohydrate oxidation, but increased whole-body fat oxidation and hepatic lipid accumulation, likely as a result of a counter-regulatory leptin increase. In conclusion, the present study shows a tissue-specific AEA insensitivity and may point to a leptin-controlled regulation of the ECS in early-lactation.

Perioperative Considerations for Patients Exposed to Psychostimulants

Concerns regarding the perioperative management of acute psychostimulant intoxication have been recognized for decades, but novel and diverse substances in this class continue to be developed. Despite the similarities in mechanisms of action among psychostimulants, each subclass within this broad category has unique receptor specificity and different mechanisms that play a role in patient clinical presentation. These issues present challenges to anesthesia providers when caring for patients with either acute or chronic exposure to psychostimulants during the perioperative period. Challenges result from both physiological and psychological effects that influence the action of the primary anesthetic agent, adjuvant anesthetics, and analgesics used for perioperative management of pain. The epidemiology, pharmacology, and perioperative implications of psychostimulant use are presented for amphetamines and similar acting nonamphetamines, cocaine, and, finally, the mixed-action drugs known as entactogens that share stimulant and psychedelic properties. This information is then used as the foundation for safe and effective perioperative management of patients exposed to psychostimulants.

Advances in adrenergic receptors for the treatment of chronic obstructive pulmonary disease: 2023 update

INTRODUCTION

Strong scientific evidence and large experience support the use of β2-agonists for the symptomatic alleviation of COPD. Therefore, there is considerable effort in discovering highly potent and selective β2-agonists.

AREAS COVERED

Recent research on novel β2-agonists for the treatment of COPD. A detailed literature search was performed in two major databases (PubMed/MEDLINE and Scopus) up to September 2023."

EXPERT OPINION

Compounds that preferentially activate a Gs- or β-arrestin-mediated signaling pathway via β- adrenoceptors (ARs) are more innovative. Pepducins, which target the intracellular region of β2-AR to modulate receptor signaling output, have the most interesting profile from a pharmacological point of view. They stabilize the conformation of the β2-AR and influence its signaling by interacting with the intracellular receptor-G protein interface. New bifunctional drugs called muscarinic antagonist-β2 agonist (MABA), which have both muscarinic receptor (mAChR) antagonism and β2-agonist activity in the same molecule, are a new opportunity. However, all tested compounds have been shown to act predominantly as mAChR antagonists or β2-agonists. An intriguing idea is to utilize allosteric modulators that bind to β2-ARs at sites different than those bound by orthosteric ligands to augment or reduce the signaling transduced by the orthosteric ligand.

Dual role of the adhesion G-protein coupled receptor ADRGE5/CD97 in glioblastoma invasion and proliferation

CD97, an adhesion G-protein coupled receptor highly expressed in glioblastoma (GBM), consists of two noncovalently bound domains: the N-terminal fragment (NTF) and C-terminal fragment. The C-terminal fragment contains a GPCR domain that couples to Gα12/13, while the NTF interacts with extracellular matrix components and other receptors. We investigated the effects of changing CD97 levels and its function on primary patient-derived GBM stem cells (pdGSCs) in vitro and in vivo. We created two functional mutants: a constitutively active ΔNTF and the noncleavable dominant-negative H436A mutant. The CD97 knockdown in pdGSCs decreased, while overexpression of CD97 increased tumor size. Unlike other constructs, the ΔNTF mutant promoted tumor cell proliferation, but the tumors were comparable in size to those with CD97 overexpression. As expected, the GBM tumors overexpressing CD97 were very invasive, but surprisingly, the knockdown did not inhibit invasiveness and even induced it in noninvasive U87 tumors. Importantly, our results indicate that NTF was present in the tumor core cells but absent in the pdGSCs invading the brain. Furthermore, the expression of noncleavable H436A mutant led to large tumors that invade by sending massive protrusions, but the invasion of individual tumor cells was substantially reduced. These data suggest that NTF association with CD97 GPCR domain inhibits individual cell dissemination but not overall tumor invasion. However, NTF dissociation facilitates pdGSCs brain infiltration and may promote tumor proliferation. Thus, the interplay between two functional domains regulates CD97 activity resulting in either enhanced cell adhesion or stimulation of tumor cell invasion and proliferation.

How to fail with paired VNS therapy

Vagus nerve stimulation (VNS) has gained enormous traction as a promising bioelectronic therapy. In particular, the delivery of VNS paired with training to promote neural changes has demonstrated clinical success for stroke recovery and found far-reaching application in other domains, from autism to psychiatric disorders to normal learning. The success of paired VNS has been extensively documented. Here, we consider a more unusual question: why does VNS have such broad utility, and perhaps more importantly, when does VNS not work? We present a discussion of the concepts that underlie VNS therapy and an anthology of studies that describe conditions in which these concepts are violated and VNS fails. We focus specifically on the mechanisms engaged by implanted VNS, and how the parameters of stimulation, stimulation method, pharmacological manipulations, accompanying comorbidities, and specifics of concurrent training interact with these mechanisms to impact the efficacy of VNS therapy. As paired VNS therapy is increasing translated to clinical implementation, a clear understanding of the conditions in which it does, and critically, does not work is fundamental to the success of this approach.

Structural basis of allosteric modulation of metabotropic glutamate receptor activation and desensitization

The metabotropic glutamate receptors (mGluRs) are neuromodulatory family C G protein coupled receptors which assemble as dimers and allosterically couple extracellular ligand binding domains (LBDs) to transmembrane domains (TMDs) to drive intracellular signaling. Pharmacologically, mGluRs can be targeted either at the LBDs by glutamate and synthetic orthosteric compounds or at the TMDs by allosteric modulators. Despite the potential of allosteric TMD-targeting compounds as therapeutics, an understanding of the functional and structural basis of their effects on mGluRs is limited. Here we use a battery of approaches to dissect the distinct functional and structural effects of orthosteric versus allosteric ligands. We find using electrophysiological and live cell imaging assays that both agonists and positive allosteric modulators (PAMs) can drive activation and desensitization of mGluRs. The effects of PAMs are pleiotropic, including both the ability to boost the maximal response to orthosteric agonists and to serve independently as desensitization-biased agonists across mGluR subtypes. Conformational sensors reveal PAM-driven inter-subunit re-arrangements at both the LBD and TMD. Motivated by this, we determine cryo-electron microscopy structures of mGluR3 in the presence of either an agonist or antagonist alone or in combination with a PAM. These structures reveal PAM-driven re-shaping of intra- and inter-subunit conformations and provide evidence for a rolling TMD dimer interface activation pathway that controls G protein and beta-arrestin coupling.

Highlights

-Agonists and PAMs drive mGluR activation, desensitization, and endocytosis-PAMs are desensitization-biased and synergistic with agonists-Four combinatorial ligand conditions reveal an ensemble of full-length mGluR structures with novel interfaces-Activation and desensitization involve rolling TMD interfaces which are re-shaped by PAM.

Tail engagement of arrestin at the glucagon receptor

Arrestins have pivotal roles in regulating G protein-coupled receptor (GPCR) signalling by desensitizing G protein activation and mediating receptor internalization1,2. It has been proposed that the arrestin binds to the receptor in two different conformations, 'tail' and 'core', which were suggested to govern distinct processes of receptor signalling and trafficking3,4. However, little structural information is available for the tail engagement of the arrestins. Here we report two structures of the glucagon receptor (GCGR) bound to β-arrestin 1 (βarr1) in glucagon-bound and ligand-free states. These structures reveal a receptor tail-engaged binding mode of βarr1 with many unique features, to our knowledge, not previously observed. Helix VIII, instead of the receptor core, has a major role in accommodating βarr1 by forming extensive interactions with the central crest of βarr1. The tail-binding pose is further defined by a close proximity between the βarr1 C-edge and the receptor helical bundle, and stabilized by a phosphoinositide derivative that bridges βarr1 with helices I and VIII of GCGR. Lacking any contact with the arrestin, the receptor core is in an inactive state and loosely binds to glucagon. Further functional studies suggest that the tail conformation of GCGR-βarr governs βarr recruitment at the plasma membrane and endocytosis of GCGR, and provides a molecular basis for the receptor forming a super-complex simultaneously with G protein and βarr to promote sustained signalling within endosomes. These findings extend our knowledge about the arrestin-mediated modulation of GPCR functionalities.

Structural basis for the ubiquitination of G protein βγ subunits by KCTD5/Cullin3 E3 ligase

G protein-coupled receptor (GPCR) signaling is precisely controlled to avoid overstimulation that results in detrimental consequences. Gβγ signaling is negatively regulated by a Cullin3 (Cul3)-dependent E3 ligase, KCTD5, which triggers ubiquitination and degradation of free Gβγ. Here, we report the cryo-electron microscopy structures of the KCTD5-Gβγ fusion complex and the KCTD7-Cul3 complex. KCTD5 in pentameric form engages symmetrically with five copies of Gβγ through its C-terminal domain. The unique pentameric assembly of the KCTD5/Cul3 E3 ligase places the ubiquitin-conjugating enzyme (E2) and the modification sites of Gβγ in close proximity and allows simultaneous transfer of ubiquitin from E2 to five Gβγ subunits. Moreover, we show that ubiquitination of Gβγ by KCTD5 is important for fine-tuning cyclic adenosine 3´,5´-monophosphate signaling of GPCRs. Our studies provide unprecedented insights into mechanisms of substrate recognition by unusual pentameric E3 ligases and highlight the KCTD family as emerging regulators of GPCR signaling.

Peptide G-Protein-Coupled Receptors and ErbB Receptor Tyrosine Kinases in Cancer

The ErbB RTKs (EGFR, HER2, HER3, and HER4) have been well-studied in cancer. EGFR, HER2, and HER3 stimulate cancer proliferation, principally by activating the phosphatidylinositol-3-kinase and extracellular signal-regulated kinase (ERK) pathways, resulting in increased cancer cell survival and proliferation. Cancer cells have high densities of the EGFR, HER2, and HER3 causing phosphorylation of tyrosine amino acids on protein substrates and tyrosine amino acids near the C-terminal of the RTKs. After transforming growth factor (TGF) α binds to the EGFR, homodimers or EGFR heterodimers form. HER2 forms heterodimers with the EGFR, HER3, and HER4. The EGFR, HER2, and HER3 are overexpressed in lung cancer patient tumors, and monoclonal antibodies (mAbs), such as Herceptin against HER2, are used to treat breast cancer patients. Patients with EGFR mutations are treated with tyrosine kinase inhibitors, such as gefitinib or osimertinib. Peptide GPCRs, such as NTSR1, are present in many cancers, and neurotensin (NTS) stimulates the growth of cancer cells. Lung cancer proliferation is impaired by SR48692, an NTSR1 antagonist. SR48692 is synergistic with gefitinib at inhibiting lung cancer growth. Adding NTS to lung cancer cells increases the shedding of TGFα, which activates the EGFR, or neuregulin-1, which activates HER3. The transactivation process is impaired by SRC, matrix metalloprotease, and reactive oxygen species inhibitors. While the transactivation process is complicated, it is fast and occurs within minutes after adding NTS to cancer cells. This review emphasizes the use of tyrosine kinase inhibitors and SR48692 to impair transactivation and cancer growth.

The In Vitro Contractile Response of Canine Pregnant Myometrium to Oxytocin and Denaverine Hydrochloride

In pregnant bitches, the response to oxytocin and denaverine hydrochloride in dystocia management is usually poor. To better understand the effect of both drugs on myometrial contractility, the circular and longitudinal muscle layers were examined in an organ bath. For each layer, three myometrial strips were stimulated twice, each with one of three oxytocin concentrations. The effect of denaverine hydrochloride was studied once in direct combination with oxytocin and alone with subsequent oxytocin administration. Contractions were recorded and evaluated for average amplitude, mean force, area under the curve (AUC), and frequency. Effects of different treatments were analyzed and compared within and between layers. In the circular layer, oxytocin significantly increased amplitude and mean force compared to untreated controls regardless of stimulation cycles or concentrations. In both layers, high oxytocin concentrations caused tonic contractions, while the lowest concentration created regular rhythmic contractions. Longitudinal layer tissue responded to oxytocin with a significantly decreased contractility when stimulated twice, presumably a sign of desensitization. Denaverine hydrochloride neither affected oxytocin induced contractions nor showed a priming effect to subsequent oxytocin. Thus, no benefit of denaverine hydrochloride on myometrial contractility was found in the organ bath. Our results suggest a better efficiency of low-dose oxytocin in canine dystocia management.

Altered Signaling and Desensitization Responses in PTH1R Mutants Associated with Eiken Syndrome

The parathyroid hormone receptor type 1 (PTH1R) is a G protein-coupled receptor that plays key roles in regulating calcium homeostasis and skeletal development via binding the ligands, PTH and PTH-related protein (PTHrP), respectively. Eiken syndrome is a rare disease of delayed bone mineralization caused by homozygous PTH1R mutations. Of the three mutations identified so far, R485X, truncates the PTH1R C-terminal tail, while E35K and Y134S alter residues in the receptor's amino-terminal extracellular domain. Here, using a variety of cell-based assays, we show that R485X increases the receptor's basal rate of cAMP signaling and decreases its capacity to recruit β-arrestin2 upon ligand stimulation. The E35K and Y134S mutations each weaken the binding of PTHrP leading to impaired β-arrestin2 recruitment and desensitization of cAMP signaling response to PTHrP but not PTH. Our findings support a critical role for interaction with β-arrestin in the mechanism by which the PTH1R regulates bone formation.

Investigation of adenosine A1 receptor-mediated β-arrestin 2 recruitment using a split-luciferase assay

Background: Adenosine A1 receptor (A1AR) plays a prominent role in neurological and cardiac diseases and inflammatory processes. Its endogenous ligand adenosine is known to be one of the key players in the sleep-wake cycle. Like other G protein-coupled receptors (GPCRs), stimulation of A1AR leads to the recruitment of arrestins in addition to the activation of G proteins. So far, little is known about the role of these proteins in signal transduction and regulation of A1AR compared to the activation of G proteins. In this work, we characterized a live cell assay for A1AR-mediated β-arrestin 2 recruitment. We have applied this assay to a set of different compounds that interact with this receptor. Methods: Based on NanoBit® technology, a protein complementation assay was developed in which the A1AR is coupled to the large part of the nanoluciferase (LgBiT), whereas its small part (SmBiT) is fused to the N-terminus of β-arrestin 2. Stimulation of A1AR results in the recruitment of β-arrestin 2 and subsequent complementation of a functional nanoluciferase. For comparison, corresponding data on the effect of receptor stimulation on intracellular cAMP levels were collected for some data sets using the GloSensor™ assay. Results: The assay gives highly reproducible results with a very good signal-to-noise ratio. Capadenoson, in contrast to adenosine, CPA, or NECA, shows only partial agonism in this assay with respect to the recruitment of β-arrestin 2, whereas it shows full agonism in the case of the inhibitory effect of A1AR on cAMP production. By using a GRK2 inhibitor, it becomes clear that the recruitment is at least partially dependent on the phosphorylation of the receptor by this kinase. Interestingly, this was also the first time that we demonstrate the A1AR-mediated recruitment of β-arrestin 2 by stimulation with a valerian extract. Conclusion: The presented assay is a useful tool for the quantitative study of A1AR-mediated β-arrestin 2 recruitment. It allows data collection for stimulatory, inhibitory, and modulatory substances and is also suitable for more complex substance mixtures such as valerian extract.

G protein-coupled receptors in neurodegenerative diseases and psychiatric disorders

Neuropsychiatric disorders are multifactorial disorders with diverse aetiological factors. Identifying treatment targets is challenging because the diseases are resulting from heterogeneous biological, genetic, and environmental factors. Nevertheless, the increasing understanding of G protein-coupled receptor (GPCR) opens a new possibility in drug discovery. Harnessing our knowledge of molecular mechanisms and structural information of GPCRs will be advantageous for developing effective drugs. This review provides an overview of the role of GPCRs in various neurodegenerative and psychiatric diseases. Besides, we highlight the emerging opportunities of novel GPCR targets and address recent progress in GPCR drug development.

Itch receptor MRGPRX4 interacts with the receptor activity-modifying proteins

Cholestatic itch is a severe and debilitating symptom in liver diseases with limited treatment options. The class A G protein-coupled receptor (GPCR) Mas-related GPCR subtype X4 (MRGPRX4) has been identified as a receptor for bile acids, which are potential cholestatic pruritogens. An increasing number of GPCRs have been shown to interact with receptor activity-modifying proteins (RAMPs), which can modulate different aspects of GPCR biology. Using a combination of multiplexed immunoassay and proximity ligation assay, we show that MRGPRX4 interacts with RAMPs. The interaction of MRGPRX4 with RAMP2, but not RAMP1 or 3, causes attenuation of basal and agonist-dependent signaling, which correlates with a decrease of MRGPRX4 cell surface expression as measured using a quantitative NanoBRET pulse-chase assay. Finally, we use AlphaFold Multimer to predict the structure of the MRGPRX4-RAMP2 complex. The discovery that RAMP2 regulates MRGPRX4 may have direct implications for future drug development for cholestatic itch.

An increase in the cytosolic concentration of free calcium ions activates the neutrophil NADPH-oxidase provided that the free fatty acid receptor 2 has been allosterically modulated

Signals generated by free fatty acid receptor 2 (FFA2R) can activate the neutrophil NADPH-oxidase without involvement of any orthosteric FFA2R agonist. The initiating signals may be generated by P2Y₂R, the receptor for ATP. An FFA2R specific allosteric modulator (PAM; Cmp58) was required for this response and used to investigate the mechanism by which signals generated by ATP/P2Y₂R activate an FFA2R dependent process. The P2Y₂R induced signal that together with the modulated FFA2R activates neutrophils, was generated downstream of the Gα_q containing G protein coupled to P2Y₂R. A rise in the cytosolic concentration of ionized calcium ([Ca²⁺]_i) was hypothesized to be the important signal. The hypothesis gained support from the finding that the modulator transferred the neutrophils to a Ca²⁺sensitive state. The rise in [Ca²⁺]_i induced by the Ca²⁺ specific ionophore ionomycin, activated the neutrophils provided that an allosteric modulator was bound to FFA2R. The activity of the superoxide generating NADPH-oxidase induced by ionomycin was rapidly terminated and the FFA2Rs could then no longer be activated by the FFA2R agonist propionate or by the signal generated by ATP/P2Y₂R. The non-responding state of FFA2R was, however, revoked by a cross-activating allosteric FFA2R modulator. The [Ca²⁺]_i mediated activation of neutrophils with their FFA2Rs allosterically modulated, represent a unique regulatory receptor crosstalk mechanism by which the activation potency of a G protein coupled receptor is controlled by a receptor-crosstalk signaling system operating from the cytosolic side of the plasma membrane.

Acute intrahippocampal administration of melanin-concentrating hormone impairs memory consolidation and decreases the expression of MCHR-1 and TrkB receptors

Interest in the role of melanin-concentrating hormone (MCH) in memory processes has increased in recent years, with some studies reporting memory-enhancing effects, while others report deleterious effects. Due to these discrepancies, this study seeks to provide new evidence about the role of MCH in memory consolidation and its relation with BDNF/TrkB system. To this end, in the first experiment, increased doses of MCH were acutely administered in both hippocampi to groups of male rats (25, 50, 200, and 500 ng). Microinjections were carried out immediately after finishing the sample trial of two hippocampal-dependent behavioral tasks: the Novel Object Recognition Test (NORT) and the modified Elevated Plus Maze (mEPM) test. Results indicated that a dose of 200 ng of MCH or higher impaired memory consolidation in both tasks. A second experiment was performed in which a dose of 200 ng of MCH was administered alone or co-administered with the MCHR-1 antagonist ATC-0175 at the end of the sample trial in the NORT. Results showed that MCH impaired memory consolidation, while the co-administration with ATC-0175 reverted this detrimental effect. Moreover, MCH induced a significant decrease in hippocampal MCHR-1 and TrkB expression with no modification in the expression of BDNF and NMDA receptor subunits NR1, NR2A, and NR2B. These results suggest that MCH in vivo elicits pro-amnesic effects in the rat hippocampus by decreasing the availability of its receptor and TrkB receptors, thus linking both endogenous systems to memory processes.

Lysophosphatidic acid receptor LPA1 trafficking and interaction with Rab proteins, as evidenced by Förster resonance energy transfer

LPA₁ internalization to endosomes was studied employing Förster Resonance Energy Transfer (FRET) in cells coexpressing the mCherry-lysophosphatidic acid LPA₁ receptors and distinct eGFP-tagged Rab proteins. Lysophosphatidic acid (LPA)-induced internalization was rapid and decreased afterward: phorbol myristate acetate (PMA) action was slower and sustained. LPA stimulated LPA₁-Rab5 interaction rapidly but transiently, whereas PMA action was rapid but sustained. Expression of a Rab5 dominant-negative mutant blocked LPA₁-Rab5 interaction and receptor internalization. LPA-induced LPA₁-Rab9 interaction was only observed at 60 min, and LPA₁-Rab7 interaction after 5 min with LPA and after 60 min with PMA. LPA triggered immediate but transient rapid recycling (i.e., LPA₁-Rab4 interaction), whereas PMA action was slower but sustained. Agonist-induced slow recycling (LPA₁-Rab11 interaction) increased at 15 min and remained at this level, whereas PMA action showed early and late peaks. Our results indicate that LPA₁ receptor internalization varies with the stimuli.