Beta-arrestin-mediated activation of MAPK by inverse agonists reveals distinct active conformations for G protein-coupled receptors

Adrenoceptors: Receptors, Ligands and Their Clinical Uses, Molecular Pharmacology and Assays

The nine G protein-coupled adrenoceptor subtypes are where the endogenous catecholamines adrenaline and noradrenaline interact with cells. Since they are important therapeutic targets, over a century of effort has been put into developing drugs that modify their activity. This chapter provides an outline of how we have arrived at current knowledge of the receptors, their physiological roles and the methods used to develop ligands. Initial studies in vivo and in vitro with isolated organs and tissues progressed to cell-based techniques and the use of cloned adrenoceptor subtypes together with high-throughput assays that allow close examination of receptors and their signalling pathways. The crystal structures of many of the adrenoceptor subtypes have now been determined opening up new possibilities for drug development.

Know your molecule: pharmacological characterization of drug candidates to enhance efficacy and reduce late-stage attrition

Despite advances in chemical, computational and biological sciences, the rate of attrition of drug candidates in clinical development is still high. A key point in the small-molecule discovery process that could provide opportunities to help address this challenge is the pharmacological characterization of hit and lead compounds, culminating in the selection of a drug candidate. Deeper characterization is increasingly important, because the 'quality' of drug efficacy, at least for G protein-coupled receptors (GPCRs), is now understood to be much more than activation of commonly evaluated pathways such as cAMP signalling, with many more 'efficacies' of ligands that could be harnessed therapeutically. Such characterization is being enabled by novel assays to characterize the complex behaviour of GPCRs, such as biased signalling and allosteric modulation, as well as advances in structural biology, such as cryo-electron microscopy. This article discusses key factors in the assessments of the pharmacology of hit and lead compounds in the context of GPCRs as a target class, highlighting opportunities to identify drug candidates with the potential to address limitations of current therapies and to improve the probability of them succeeding in clinical development.

Muscarinic acetylcholine receptor-mediated phosphorylation of extracellular signal-regulated kinase in HSY salivary ductal cells involves distinct signaling pathways

OBJECTIVES

Typical agonists of G protein-coupled receptors (GPCRs), including muscarinic acetylcholine receptors (mAChRs), activate both G-protein and β-arrestin signaling systems, and are termed balanced agonists. In contrast, biased agonists selectively activate a single pathway, thereby offering therapeutic potential for the specific activation of that pathway. The mAChR agonists carbachol and pilocarpine are known to induce phosphorylation of extracellular signal-regulated kinase-1/2 (ERK1/2) via G-protein-dependent and -independent pathways, respectively. We investigated the involvement of β-arrestin and its downstream mechanisms in the ERK1/2 phosphorylation induced by carbachol and pilocarpine in the human salivary ductal cell line, HSY cells.

METHODS

HSY cells were stimulated with pilocarpine or carbachol, with or without various inhibitors. The cell lysates were analyzed by western blotting using the antibodies p44/p42MAPK and phosphor-p44/p42MAPK.

RESULTS

Western blot analysis revealed that carbachol elicited greater stimulation of ERK1/2 phosphorylation compared to pilocarpine. ERK1/2 phosphorylation was inhibited by atropine and gefitinib, suggesting that mAChR activation induces transactivation of epidermal growth factor receptors (EGFR). Moreover, inhibition of carbachol-mediated ERK1/2 phosphorylation was achieved by GF-109203X (a PKC inhibitor), a βARK1/GRK2 inhibitor, barbadin (a β-arrestin inhibitor), pitstop 2 (a clathrin inhibitor), and dynole 34-2 (a dynamin inhibitor). In contrast, pilocarpine-mediated ERK1/2 phosphorylation was only inhibited by barbadin (a β-arrestin inhibitor) and PP2 (a Src inhibitor).

CONCLUSION

Carbachol activates both G-protein and β-arrestin pathways, whereas pilocarpine exclusively activates the β-arrestin pathway. Additionally, downstream of β-arrestin, carbachol activates clathrin-dependent internalization, while pilocarpine activates Src.

Local anesthetics inhibit muscarinic acetylcholine receptor-mediated calcium responses and the recruitment of β-arrestin in HSY human parotid cells

OBJECTIVES

Local anesthetics act on G protein-coupled receptors (GPCRs); thus, their potential as allosteric modulators of GPCRs has attracted attention. Intracellular signaling via GPCRs involves both G-protein- and β-arrestin-mediated pathways. To determine the effects of local anesthetics on muscarinic acetylcholine receptors (mAChR), a family of GPCRs, we analyzed the effects of local anesthetics on mAChR-mediated Ca2+ responses and formation of receptor-β-arrestin complexes in the HSY human parotid cell line.

METHODS

Ca2+ responses were monitored by fura-2 spectrofluorimetry. Ligand-induced interactions between mAChR and β-arrestin were examined using a β-arrestin GPCR assay kit.

RESULTS

Lidocaine reduced mAChR-mediated Ca2+ responses but did not change the intracellular Ca2+ concentration in non-stimulated cells. The membrane-impermeant lidocaine analog QX314 and procaine inhibited mAChR-mediated Ca2+ responses, with EC50 values of 48.0 and 20.4 μM, respectively, for 50 μM carbachol-stimulated Ca2+ responses. In the absence of extracellular Ca2+, the pretreatment of cells with QX314 reduced carbachol-induced Ca2+ release, indicating that QX314 reduced Ca2+ release from intracellular stores. Lidocaine and QX314 did not affect store-operated Ca2+ entry as they did not alter the thapsigargin-induced Ca2+ response. QX314 and procaine reduced the carbachol-mediated recruitment of β-arrestin, and administration of procaine suppressed pilocarpine-induced salivary secretion in mice.

CONCLUSION

Local anesthetics, including QX314, act on mAChR to reduce carbachol-induced Ca2+ release from intracellular stores and the recruitment of β-arrestin. These findings support the notion that local anesthetics and their derivatives are starting points for the development of functional allosteric modulators of mAChR.

SHU9119 and MBP10 are biased ligands at the human melanocortin-4 receptor

The melanocortin-4 receptor (MC4R), a G protein-coupled receptor, is critically involved in regulating energy homeostasis as well as modulation of reproduction and sexual function. Two peptide antagonists (SHU9119 and MBP10) were derived from the endogenous agonist α-melanocyte stimulating hormone. But their pharmacology at human MC4R is not fully understood. Herein, we performed detailed pharmacological studies of SHU9119 and MBP10 on wild-type (WT) and six naturally occurring constitutively active MC4Rs. Both ligands had no or negligible agonist activity in Gαs-cAMP signaling on WT MC4R, but stimulated extracellular signal-regulated kinases 1 and 2 (ERK1/2) activation on WT and mutant MC4Rs. Mechanistic studies revealed that SHU9119 and MBP10 stimulated ERK1/2 signaling of MC4R by different mechanisms, with SHU9119-stimulated ERK1/2 signaling mediated by phosphatidylinositol 3-kinase (PI3K) and MBP10-initiated ERK1/2 activation through PI3K and β-arrestin. In summary, our studies demonstrated that SHU9119 and MBP10 were biased ligands for MC4R, preferentially activating ERK1/2 signaling through different mechanisms. SHU9119 acted as a biased ligand and MBP10 behaved as a biased allosteric modulator.

Bias translation: The final frontier?

Biased signalling is a natural result of GPCR allosteric function and should be expected from any and all synthetic and natural agonists. Therefore, it may be encountered in all agonist discovery projects and must be considered as a beneficial (or possible detrimental) feature of new candidate molecules. While bias is detected easily, the synoptic nature of GPCR signalling makes translation of simple in vitro bias to complex in vivo systems problematic. The practical outcome of this is a difficulty in predicting the therapeutic value of biased signalling due to the failure of translation of identified biased signalling to in vivo agonism. This is discussed in this review as well as some new ways forward to improve this translation process and better exploit this powerful pharmacologic mechanism.

Oxytocin and vasopressin signaling in health and disease

Neurohypophysial peptides are ancient and evolutionarily highly conserved neuropeptides that regulate many crucial physiological functions in vertebrates and invertebrates. The human neurohypophysial oxytocin/vasopressin (OT/VP) signaling system with its four receptors has become an attractive drug target for a variety of diseases, including cancer, pain, cardiovascular indications, and neurological disorders. Despite its promise, drug development faces hurdles, including signaling complexity, selectivity and off-target concerns, translational interspecies differences, and inefficient drug delivery. In this review we dive into the complexity of the OT/VP signaling system in health and disease, provide an overview of relevant pharmacological probes, and discuss the latest trends in therapeutic lead discovery and drug development.

Propranolol Promotes Monocyte-to-Macrophage Differentiation and Enhances Macrophage Anti-Inflammatory and Antioxidant Activities by NRF2 Activation

Adrenergic pathways represent the main channel of communication between the nervous system and the immune system. During inflammation, blood monocytes migrate within tissue and differentiate into macrophages, which polarize to M1 or M2 macrophages with tissue-damaging or -reparative properties, respectively. This study investigates whether the β-adrenergic receptor (β-AR)-blocking drug propranolol modulates the monocyte-to-macrophage differentiation process and further influences macrophages in their polarization toward M1- and M2-like phenotypes. Six-day-human monocytes were cultured with M-CSF in the presence or absence of propranolol and then activated toward an M1 pro-inflammatory state or an M2 anti-inflammatory state. The chronic exposure of monocytes to propranolol during their differentiation into macrophages promoted the increase in the M1 marker CD16 and in the M2 markers CD206 and CD163 and peroxisome proliferator-activated receptor ɣ expression. It also increased endocytosis and the release of IL-10, whereas it reduced physiological reactive oxygen species. Exposure to the pro-inflammatory conditions of propranolol-differentiated macrophages resulted in an anti-inflammatory promoting effect. At the molecular level, propranolol upregulated the expression of the oxidative stress regulators NRF2, heme oxygenase-1 and NQO1. By contributing to regulating macrophage activities, propranolol may represent a novel anti-inflammatory and immunomodulating compound with relevant therapeutic potential in several inflammatory diseases.

State-dependent dynamics of extramembrane domains in the β2 -adrenergic receptor

G protein-coupled receptors (GPCRs) are membrane-bound signaling proteins that play an essential role in cellular signaling processes. Due to their intrinsic function of transmitting internal signals in response to external cues, these receptors are adapted to be highly dynamic in nature. The β2 -adrenergic receptor (β2 AR) is a representative member of the family that has been extensively analyzed in terms of its structure and activation. Although the structure of the transmembrane domain has been characterized in the different functional states of the receptor, the conformational dynamics of the extramembrane domains, especially the intrinsically disordered regions are still emerging. In this study, we analyze the state-dependent dynamics of extramembrane domains of β2 AR using atomistic molecular dynamics simulations. We introduce a parameter, the residue excess dynamics that allows us to better quantify receptor dynamics. Using this measure, we show that the dynamics of the extramembrane domains are sensitive to the receptor state. Interestingly, the ligand-bound intermediateR ' state shows the maximal dynamics compared to either the active R*G or inactive R states. Ligand binding appears to be correlated with high residue excess dynamics that are dampened upon G protein coupling. The intracellular loop-3 (ICL3) domain has a tendency to flip towards the membrane upon ligand binding, which could contribute to receptor "priming." We highlight an important ICL1-helix-8 interplay that is broken in the ligand-bound state but is retained in the active state. Overall, our study highlights the importance of characterizing the functional dynamics of the GPCR loop domains.

Vasopressin V2 receptor, tolvaptan, and ERK1/2 phosphorylation in the renal collecting duct

Tolvaptan, a vasopressin antagonist selective for the V2-subtype vasopressin receptor (V2R), is widely used in the treatment of hyponatremia and autosomal-dominant polycystic kidney disease (ADPKD). Its effects on signaling in collecting duct cells have not been fully characterized. Here, we perform RNA-seq in a collecting duct cell line (mpkCCD). The data show that tolvaptan inhibits the expression of mRNAs that were previously shown to be increased in response to vasopressin including aquaporin-2, but also reveals mRNA changes that were not readily predictable and suggest off-target actions of tolvaptan. One such action is activation of the MAPK kinase (ERK1/ERK2) pathway. Prior studies have shown that ERK1/ERK2 activation is essential in the regulation of a variety of cellular and physiological processes and can be associated with cell proliferation. In immunoblotting experiments, we demonstrated that ERK1/ERK2 phosphorylation in mpkCCD cells was significantly reduced by vasopressin, in contrast to the increases seen in non-collecting-duct cells overexpressing V2R in prior studies. We also found that tolvaptan has a strong effect to increase ERK1/ERK2 phosphorylation in the presence of vasopressin and that tolvaptan's effect to increase ERK1/ERK2 phosphorylation is absent in mpkCCD cells in which both protein kinase A (PKA)-catalytic subunits have been deleted. Thus, it appears that the tolvaptan effect to increase ERK activation is PKA-dependent and is not due to an off-target effect of tolvaptan. We conclude that in cells expressing V2R at endogenous levels: 1) vasopressin decreases ERK1/ERK2 activation; 2) in the presence of vasopressin, tolvaptan increases ERK1/ERK2 activation; and 3) these effects are PKA-dependent.NEW & NOTEWORTHY Vasopressin is a key hormone that regulates the function of the collecting duct of the kidney. ERK1 and ERK2 are enzymes that play key roles in physiological regulation in all cells. The authors used collecting duct cell cultures to investigate the effects of vasopressin and the vasopressin receptor antagonist tolvaptan on ERK1 and ERK2 phosphorylation and activation.

Apelin/APJ system protects placental trophoblasts from hypoxia-induced oxidative stress through activating PI3K/Akt signaling pathway in preeclampsia

BACKGROUND

Preeclampsia is a placentally induced syndrome with diverse clinical presentation that currently has no cure. Oxidative stress is a potent inducer of placental dysfunction. The apelin receptor (APJ) system is a pleiotropic pathway with a potential for therapeutic targeting in preeclampsia. This study examines the alteration of circulating apelin levels and placental APJ expression in preeclampsia and investigates whether apelin/APJ system can protect placental trophoblast from hypoxia-induced oxidative stress injury through PI3K/AKT signaling pathway.

RESULTS

Our results confirmed that maternal apelin concentration was increased in women with preeclampsia, but APJ expression was reduced in the preeclamptic placentas. Apelin-13 treatment not only specifically attenuated CoCl2-induced superoxide production, but also prevented CoCl2-induced reduction of SOD activity and SOD1 expression. In addition, apelin-13 suppressed CoCl2-induced apoptosis by increasing the expression of bcl-2/bax ratio and by decreasing the expression of active caspase-3 in placental trophoblasts. Furthermore, we found that apelin-13 binding APJ activated the PI3K and AKT kinases and inhibition of PI3K kinase significantly blocked the anti-oxidative effects of apelin-13 in placental trophoblasts.

CONCLUSIONS

Decrease of placental APJ expression is associated with oxidative stress-induced placental dysfunction in preeclampsia, and increased circulating apelin could be a moderately successful marker to differentiate subjects with preeclampsia from healthy pregnant women. Inhibition of superoxide production and caspase-3 cleavage, together with upregulation of SOD activity/expression and bcl-2/bax ratio, could be the potential molecular mechanisms by which apelin-13/APJ protects placental trophoblasts from oxidative stress injury.

Past and present of beta arrestins: A new perspective on insulin secretion and effect

BACKGROUND

Beta arrestins had been known as intracellular adaptors that uncouple and inactivate the G protein-coupled receptors that they interact with. Their roles as signal initiators for some receptors have recently been recognized.

SCOPE OF REVIEW

In this review, we focused on their role in mediating metabolic modulation primarily in relation to insulin signaling. Commenced by the upstream receptor, they seem to act like intracellular hubs that divert the metabolic profile of the cell. The amount of metabolic substrates in circulation and their usage/deposition by tissues are controlled by the contribution of all systems in the organism. This control is enabled by the release of hormones such as insulin, glucagon and glucagon-like peptide-1. Intriguingly, some ligands -either agonists or antagonists-of different classes of receptors have preferential properties mediated by β arrestins. This is not surprizing considering that substrate supply and usage should parallel physiological function such as hormone release or muscle contraction.

MAJOR CONCLUSIONS

Available data indicate that β arrestins conduct the regulatory role in insulin secretion and action. They may be good candidates to target when the upstream signal demands the function that may compromise the cell. An example is carvedilol that is protective by preventing the stimulatory effects of excessive catecholamines, stimulates mitochondrial function and has preferential clinical outcomes in metabolic disorders.

[Role of the sympathetic nervous system in chronic inflammation]

In this review article the current model of the interaction between the sympathetic nervous system (SNS) and the immune system in the context of chronic inflammation is presented. Mechanisms in the interaction between the SNS and the immune system are shown, which are similar for all disease entities: 1) the biphasic effect of the sympathetic system on the inflammatory response with a proinflammatory, stimulating effect before and during the activation of the immune system (early) and a more inhibitory effect in late phases of immune activation (chronic). 2) The interruption of communication between immune cells and the brain by withdrawal of sympathetic nerve fibers from areas of inflammation, such as the spleen, lymph nodes or peripheral foci of inflammation. 3) The local replacement of catecholamines by neurotransmitter-producing cells to fine-tune the local immune response independently of the brain. 4) Increased activity of the SNS due to an imbalance of the autonomic nervous system at the systemic level, which provides an explanation for known disease sequelae and comorbidities due to the long duration of chronic inflammatory reactions, such as increased cardiovascular risk with hypertension, diabetes mellitus and catabolic metabolism. The understanding of neuroimmune interactions can lead to new therapeutic approaches, e.g., a stimulation of beta-adrenergic and even more an inhibition of alpha-adrenergic receptors or a restoration of the autonomic balance in the context of arthritis ) can make an anti-inflammatory contribution (more influence of the vagus nerve); however, in order to translate the theoretical findings into clinical action that is beneficial for the patient, controlled interventional studies are required.

Profiling of basal and ligand-dependent GPCR activities by means of a polyvalent cell-based high-throughput platform

Representing the most attractive and successful druggable receptors of the proteome, GPCRs regulate a myriad of physiological and pathophysiological functions. Although over half of present pharmaceuticals target GPCRs, the advancement of drug discovery is hampered by a lack of adequate screening tools, the majority of which are limited to probing agonist-induced G-protein and β-arrestin-2-mediated events as a measure of receptor activation. Here, we develop Tango-Trio, a comprehensive cell-based high-throughput platform comprising cumate-inducible expression of transducers, capable of the parallelized profiling of both basal and agonist-dependent GPCR activities. We capture the functional diversity of GPCRs, reporting β-arrestin-1/2 couplings, selectivities, and receptor internalization signatures across the GPCRome. Moreover, we present the construction of cumate-induced basal activation curves at approximately 200 receptors, including over 50 orphans. Overall, Tango-Trio's robustness is well-suited for the functional characterization and screening of GPCRs, especially for parallel interrogation, and is a valuable addition to the pharmacological toolbox.

Chronic Effects of the Sympathetic Nervous System in Inflammatory Models

The immune system is embedded in a network of regulatory systems to keep homeostasis in case of an immunologic challenge. Neuroendocrine immunologic research has revealed several aspects of these interactions over the past decades, e.g., between the autonomic nervous system and the immune system. This review will focus on evidence revealing the role of the sympathetic nervous system (SNS) in chronic inflammation, like colitis, multiple sclerosis, systemic sclerosis, lupus erythematodes, and arthritis with a focus on animal models supported by human data. A theory of the contribution of the SNS in chronic inflammation will be presented that spans these disease entities. One major finding is the biphasic nature of the sympathetic contribution to inflammation, with proinflammatory effects until the point of disease outbreak and mainly anti-inflammatory influence thereafter. Since sympathetic nerve fibers are lost from sites of inflammation during inflammation, local cells and immune cells achieve the capability to endogenously produce catecholamines to fine-tune the inflammatory response independent of brain control. On a systemic level, it has been shown across models that the SNS is activated in inflammation as opposed to the parasympathetic nervous system. Permanent overactivity of the SNS contributes to many of the known disease sequelae. One goal of neuroendocrine immune research is defining new therapeutic targets. In this respect, it will be discussed that at least in arthritis, it might be beneficial to support β-adrenergic and inhibit α-adrenergic activity besides restoring autonomic balance. Overall, in the clinical setting, we now need controlled interventional studies to successfully translate the theoretical knowledge into benefits for patients.

Investigation of metal modulation of oxytocin structure receptor-mediated signaling

Oxytocin is a 9-amino acid peptide hormone. Since its discovery in 1954, it has most commonly been studied in relation to its role in stimulating parturition and lactation. However, it is now known that oxytocin has a widely diverse set of functions throughout the body including neuromodulation, bone growth, and inflammation. Previous research has suggested that divalent metal ions may be required for oxytocin activity, but the exact metal species and specific pathways have yet to be fully elucidated. In this work, we focus on characterizing copper and zinc bound forms of oxytocin and related analogs through far-UV circular dichroism. We report that Cu(ii) and Zn(ii) bind uniquely to oxytocin and all analogs investigated. Furthermore, we investigate how these metal bound forms may affect downstream signaling of MAPK activation upon receptor binding. We find that both Cu(ii) and Zn(ii) bound oxytocin attenuates the activation of the MAPK pathway upon receptor binding relative to oxytocin alone. Interestingly, we observed that Zn(ii) bound forms of linear oxytocin facilitate increased MAPK signaling. This study lays the foundation for future work on elucidating the metal effects on oxytocin's diverse bioactivity.

CNTNAP4 signaling regulates osteosarcoma disease progression

Improved treatment strategies for sarcoma rely on clarification of the molecular mediators of disease progression. Recently, we reported that the secreted glycoprotein NELL-1 modulates osteosarcoma (OS) disease progression in part via altering the sarcomatous extracellular matrix (ECM) and cell-ECM interactions. Of known NELL-1 interactor proteins, Contactin-associated protein-like 4 (Cntnap4) encodes a member of the neurexin superfamily of transmembrane molecules best known for its presynaptic functions in the central nervous system. Here, CRISPR/Cas9 gene deletion of CNTNAP4 reduced OS tumor growth, sarcoma-associated angiogenesis, and pulmonary metastases. CNTNAP4 knockout (KO) in OS tumor cells largely phenocopied the effects of NELL-1 KO, including reductions in sarcoma cell attachment, migration, and invasion. Further, CNTNAP4 KO cells were found to be unresponsive to the effects of NELL-1 treatment. Transcriptomic analysis combined with protein phospho-array demonstrated notable reductions in the MAPK/ERK signaling cascade with CNTNAP4 deletion, and the ERK1/2 agonist isoproterenol restored cell functions among CNTNAP4 KO tumor cells. Finally, human primary cells and tissues in combination with sequencing datasets confirmed the significance of CNTNAP4 signaling in human sarcomas. In summary, our findings demonstrate the biological importance of NELL-1/CNTNAP4 signaling axis in disease progression of human sarcomas and suggest that targeting the NELL-1/CNTNAP4 signaling pathway represents a strategy with potential therapeutic benefit in sarcoma patients.

Surveying nonvisual arrestins reveals allosteric interactions between functional sites

Arrestins are important scaffolding proteins that are expressed in all vertebrate animals. They regulate cell-signaling events upon binding to active G-protein coupled receptors (GPCR) and trigger endocytosis of active GPCRs. While many of the functional sites on arrestins have been characterized, the question of how these sites interact is unanswered. We used anisotropic network modeling (ANM) together with our covariance compliment techniques to survey all the available structures of the nonvisual arrestins to map how structural changes and protein-binding affect their structural dynamics. We found that activation and clathrin binding have a marked effect on arrestin dynamics, and that these dynamics changes are localized to a small number of distant functional sites. These sites include α-helix 1, the lariat loop, nuclear localization domain, and the C-domain β-sheets on the C-loop side. Our techniques suggest that clathrin binding and/or GPCR activation of arrestin perturb the dynamics of these sites independent of structural changes.

How Carvedilol activates β2-adrenoceptors

Carvedilol is among the most effective β-blockers for improving survival after myocardial infarction. Yet the mechanisms by which carvedilol achieves this superior clinical profile are still unclear. Beyond blockade of β1-adrenoceptors, arrestin-biased signalling via β2-adrenoceptors is a molecular mechanism proposed to explain the survival benefits. Here, we offer an alternative mechanism to rationalize carvedilol's cellular signalling. Using primary and immortalized cells genome-edited by CRISPR/Cas9 to lack either G proteins or arrestins; and combining biological, biochemical, and signalling assays with molecular dynamics simulations, we demonstrate that G proteins drive all detectable carvedilol signalling through β2ARs. Because a clear understanding of how drugs act is imperative to data interpretation in basic and clinical research, to the stratification of clinical trials or to the monitoring of drug effects on the target pathway, the mechanistic insight gained here provides a foundation for the rational development of signalling prototypes that target the β-adrenoceptor system.

Elabela Peptide: An Emerging Target in Therapeutics

Elabela, a bioactive micropeptide, is recognized as the second endogenous ligand for the Apelin receptor and is widely distributed in different tissues and organs. Elabela plays an important role in various physiological processes, such as blood pressure control, heart morphogenesis, apoptosis, angiogenesis, cell proliferation, migration, etc. Elabela is also implicated in pathological conditions, like cardiac dysfunctions, heart failure, hypertension, kidney diseases, cancer and CNS disorders. The association of Elabela with these disease conditions makes it a potential target for their therapy. This review summarizes the physiological role of Elabela peptide as well as its implication in various disease conditions.

β2 -Adrenoceptor agonist profiling reveals biased signalling phenotypes for the β2 -adrenoceptor with possible implications for the treatment of asthma

BACKGROUND AND PURPOSE

β-Adrenoceptor agonists relieve airflow obstruction by activating β2 -adrenoceptors, which are G protein-coupled receptors (GPCRs) expressed on human airway smooth muscle (HASM) cells. The currently available β-adrenoceptor agonists are balanced agonists, however, and signal through both the stimulatory G protein (Gs )- and β-arrestin-mediated pathways. While Gs signalling is beneficial and promotes HASM relaxation, β-arrestin activation is associated with reduced Gs efficacy. In this context, biased ligands that selectively promote β2 -adrenoceptor coupling to Gs signalling represent a promising strategy to treat asthma. Here, we examined several β-adrenoceptor agonists to identify Gs -biased ligands devoid of β-arrestin-mediated effects.

EXPERIMENTAL APPROACH

Gs -biased ligands for the β2 -adrenoceptor were identified by high-throughput screening and then evaluated for Gs interaction, Gi interaction, cAMP production, β-arrestin interaction, GPCR kinase (GRK) phosphorylation of the receptor, receptor trafficking, ERK activation, and functional desensitization of the β2 -adrenoceptor.

KEY RESULTS

We identified ractopamine, dobutamine, and higenamine as Gs -biased agonists that activate the Gs /cAMP pathway upon β2 -adrenoceptor stimulation while showing minimal Gi or β-arrestin interaction. Furthermore, these compounds did not induce any receptor trafficking and had reduced GRK5-mediated phosphorylation of the β2 -adrenoceptor. Finally, we observed minimal physiological desensitization of the β2 -adrenoceptor in primary HASM cells upon treatment with biased agonists.

CONCLUSION AND IMPLICATIONS

Our work demonstrates that Gs -biased signalling through the β2 -adrenoceptor may prove to be an effective strategy to promote HASM relaxation in the treatment of asthma. Such biased compounds may also be useful in identifying the molecular mechanisms that determine biased signalling and in design of safer drugs.

Propranolol Administration Modulates Neural Activity in the Hippocampal Hilus During Fear Retrieval

Altered fear learning is a strong behavioral component of anxiety disorders such as post-traumatic stress disorder (PTSD). Recent efforts have attempted to combine exposure therapies with drugs that target fear memory retrieval and memory reconsolidation, in order to improve treatment efficacy. The noradrenergic (NA) signaling system is of particular interest, due to its role in regulating the stress response and its involvement in fear and learning processes. Importantly, propranolol (P), a non-selective β-adrenergic antagonist, has shown the potential in decreasing exaggerated fear in both humans and animal models. In a previous study, we utilized an activity-dependent tagging murine model to determine the neural mechanisms by which propranolol attenuates learned fear. We found that propranolol acutely decreased memory trace reactivation specifically in the dorsal dentate gyrus (dDG), but not in CA3 or CA1. Here, we extended our previous study by investigating whether propranolol additionally altered activity in the hilus, a polymorphic layer that consists of neurons, mossy cells, and GABAergic interneurons. We found that propranolol acutely reduced overall hilar activity in both the dorsal and ventral hilus. Moreover, we report that propranolol significantly altered the activity of parvalbumin (PV)⁺ cells in the ventral (vDG), but not dorsal DG (dDG). Together, these results suggest that a β-adrenergic blockade may affect the activity of excitatory and inhibitory cell types in the hilar layer of the DG, and that these alterations may contribute to manipulating fear memory traces.

Effector membrane translocation biosensors reveal G protein and βarrestin coupling profiles of 100 therapeutically relevant GPCRs

The recognition that individual GPCRs can activate multiple signaling pathways has raised the possibility of developing drugs selectively targeting therapeutically relevant ones. This requires tools to determine which G proteins and βarrestins are activated by a given receptor. Here, we present a set of BRET sensors monitoring the activation of the 12 G protein subtypes based on the translocation of their effectors to the plasma membrane (EMTA). Unlike most of the existing detection systems, EMTA does not require modification of receptors or G proteins (except for G_s). EMTA was found to be suitable for the detection of constitutive activity, inverse agonism, biased signaling and polypharmacology. Profiling of 100 therapeutically relevant human GPCRs resulted in 1500 pathway-specific concentration-response curves and revealed a great diversity of coupling profiles ranging from exquisite selectivity to broad promiscuity. Overall, this work describes unique resources for studying the complexities underlying GPCR signaling and pharmacology.

Deprogramming metabolism in pancreatic cancer with a bi-functional GPR55 inhibitor and biased β2 adrenergic agonist

Metabolic reprogramming contributes to oncogenesis, tumor growth, and treatment resistance in pancreatic ductal adenocarcinoma (PDAC). Here we report the effects of (R,S′)-4′-methoxy-1-naphthylfenoterol (MNF), a GPR55 antagonist and biased β₂-adrenergic receptor (β₂-AR) agonist on cellular signaling implicated in proliferation and metabolism in PDAC cells. The relative contribution of GPR55 and β₂-AR in (R,S′)-MNF signaling was explored further in PANC-1 cells. Moreover, the effect of (R,S′)-MNF on tumor growth was determined in a PANC-1 mouse xenograft model. PANC-1 cells treated with (R,S′)-MNF showed marked attenuation in GPR55 signal transduction and function combined with increased β₂-AR/Gα_s/adenylyl cyclase/PKA signaling, both of which contributing to lower MEK/ERK, PI3K/AKT and YAP/TAZ signaling. (R,S′)-MNF administration significantly reduced PANC-1 tumor growth and circulating l-lactate concentrations. Global metabolic profiling of (R,S′)-MNF-treated tumor tissues revealed decreased glycolytic metabolism, with a shift towards normoxic processes, attenuated glutamate metabolism, and increased levels of ophthalmic acid and its precursor, 2-aminobutyric acid, indicative of elevated oxidative stress. Transcriptomics and immunoblot analyses indicated the downregulation of gene and protein expression of HIF-1α and c-Myc, key initiators of metabolic reprogramming in PDAC. (R,S′)-MNF treatment decreased HIF-1α and c-Myc expression, attenuated glycolysis, shifted fatty acid metabolism towards β-oxidation, and suppressed de novo pyrimidine biosynthesis in PANC-1 tumors. The results indicate a potential benefit of combined GPR55 antagonism and biased β₂-AR agonism in PDAC therapy associated with the deprogramming of altered cellular metabolism.

Community Guidelines for GPCR Ligand Bias: IUPHAR Review XX

G protein-coupled receptors modulate a plethora of physiological processes and mediate the effects of one-third of FDA-approved drugs. Depending on which ligand activates a receptor, it can engage different intracellular transducers. This 'biased signaling' paradigm requires that we now characterize physiological signaling not just by receptors but by ligand-receptor pairs. Ligands eliciting biased signaling may constitute better drugs with higher efficacy and fewer adverse effects. However, ligand bias is very complex, making reproducibility and description challenging. Here, we provide guidelines and terminology for any scientists to design and report ligand bias experiments. The guidelines will aid consistency and clarity, as the basic receptor research and drug discovery communities continue to advance our understanding and exploitation of ligand bias. Scientific insight, biosensors, and analytical methods are still evolving and should benefit from and contribute to the implementation of the guidelines, together improving translation from in vitro to disease-relevant in vivo models.

"ADPKD-omics": determinants of cyclic AMP levels in renal epithelial cells

The regulation of cyclic adenosine monophosphate (cAMP) levels in kidney epithelial cells is important in at least 2 groups of disorders, namely water balance disorders and autosomal dominant polycystic kidney disease. Focusing on the latter, we review genes that code for proteins that are determinants of cAMP levels in cells. We identify which of these determinants are expressed in the 14 kidney tubule segments using recently published RNA-sequencing and protein mass spectrometry data ("autosomal dominant polycystic kidney disease-omics"). This includes G protein-coupled receptors, adenylyl cyclases, cyclic nucleotide phosphodiesterases, cAMP transporters, cAMP-binding proteins, regulator of G protein-signaling proteins, G protein-coupled receptor kinases, arrestins, calcium transporters, and calcium-binding proteins. In addition, compartmentalized cAMP signaling in the primary cilium is discussed, and a specialized database of the proteome of the primary cilium of cultured "IMCD3" cells is provided as an online resource (https://esbl.nhlbi.nih.gov/Databases/CiliumProteome/). Overall, this article provides a general resource in the form of a curated list of proteins likely to play roles in determination of cAMP levels in kidney epithelial cells and, therefore, likely to be determinants of progression of autosomal dominant polycystic kidney disease.

Propranolol and low-dose isoproterenol ameliorate insulin resistance, enhance β-arrestin2 signaling, and reduce cardiac remodeling in high-fructose, high-fat diet-fed mice: Comparative study with metformin

AIMS

β-Arrestin2 signaling has emerged as a promising therapeutic target for the management of insulin resistance and related complications. Moreover, recent studies have shown that certain G protein-coupled receptor (GPCR) ligands can modulate β-arrestin2 signaling. The current study examined the effects of the β-blocker propranolol and a low dose of the agonist isoproterenol (L-D-ISOPROT) on β-arrestin2 signaling, insulin resistance, and cardiac remodeling in high-fructose, high-fat diet (HFrHFD)-fed mice. In addition, the effects of these agents were compared to those of the clinical antidiabetic agent, metformin.

MATERIALS AND METHODS

Insulin resistance was induced by HFrHFD feeding for 16 weeks. Mice were then randomly allocated to groups receiving propranolol, L-D-ISOPROT, metformin, or vehicle (control) for 4 weeks starting on week 13 of HFrHFD feeding. Survival rate, body weight, visceral fat weight, blood glucose, serum insulin, insulin resistance index, hepatic β-arrestin2 signaling, heart weight, left and right ventricular thicknesses, cardiac fibrosis severity, serum endothelin-1, cardiac cardiotrophin-1, and cardiac β-arrestin2 signaling were then compared among groups.

KEY FINDINGS

HFrHFD for 16 weeks significantly increased insulin resistance index, cardiac fibrosis area, and serum endothelin-1, and reduced hepatic β-arrestin2 signaling, cardiac cardiotrophin-1, and cardiac β-arrestin2 signaling without significant changes in survival rate, body weight, visceral fat weight, heart weight, or left and right ventricular thicknesses. All three drugs reduced insulin resistance and cardiac remodeling parameters and enhanced β-arrestin2 signaling with variable efficacies.

SIGNIFICANCE

Propranolol and L-D-ISOPROT, like metformin, can reduce insulin-resistance and cardiac remodeling in HFrHFD-fed mice, possibly by upregulating β-arrestin2 signaling activity. Therefore, β-arrestin2-signaling modulation might be a promising strategy for insulin-resistance treatment.

Ivy leaves dry extract EA 575® mediates biased β2-adrenergic receptor signaling

BACKGROUND

β₂-adrenergic receptor (β₂-AR) stimulation activates the G protein/cAMP pathway, which is opposed by the GRK2/β-arrestin 2 pathway. The latter is undesirable in the treatment of respiratory diseases.

HYPOTHESIS/PURPOSE

EA 575® is capable of mediating a biased β₂-adrenergic signaling pathway.

METHODS

The impact of the ivy leaves dry extract EA 575® on β₂-adrenergic signaling was tested in a dynamic mass redistribution assay in HEK wild-type and in HEK β-arrestin knock-out cells. cAMP formation and recruitment of β-arrestin 2 were investigated using GloSensor™ and PathHunter® assays, respectively. NFκB transcriptional activity was determined in both HEK wild-type as well as HEK β-arrestin knock-out cells.

RESULTS

EA 575® inhibits the recruitment of β-arrestin 2 and thereby enhances G protein/cAMP signaling under β₂-stimulating conditions, as evidenced by a corresponding increase in cAMP formation. While β₂-AR-mediated inhibition of NFκB transcriptional activity is β-arrestin-dependent, EA 575® leads to significant inhibition of NFκB transcriptional activity in β-arrestin knock-out cells and thus via a β-arrestin-independent signaling pathway.

CONCLUSION

EA 575® is the first active phytopharmaceutical ingredient for which biased β₂-adrenergic activation has been described. This shift towards G protein/cAMP signaling provides the molecular basis for the clinically proven efficacy of EA 575® in the treatment of lower respiratory tract diseases. In this light, EA 575® could potentially reduce β-arrestin-mediated adverse effects in new combinatorial therapeutic approaches.

Novel and atypical pathways for serotonin signaling

Serotonin (5-HT) appeared billions of years before 5-HT receptors and synapses. It is thus not surprising that 5-HT can control biological processes independently of its receptors. One example is serotonylation, which consists of covalent binding of 5-HT to the primary amine of glutamine. Over the past 20 years, serotonylation has been involved in the regulation of many signaling mechanisms. One of the most striking examples is the recent evidence that serotonylation of histone H3 constitutes an epigenetic mark. However, the pathophysiological role of histone H3 serotonylation remains to be discovered. All but one of the 5-HT receptors are G-protein-coupled receptors (GPCRs). The signaling pathways they control are finely tuned, and new, unexpected regulatory mechanisms are being uncovered continuously. Some 5-HT receptors (5-HT_2C, 5-HT₄, 5-HT₆, and 5-HT₇) signal through mechanisms that require neither G-proteins nor β-arrestins, the two classical and almost universal GPCR signal transducers. 5-HT₆ receptors are constitutively activated via their association with intracellular GPCR-interacting proteins (GIPs), including neurofibromin 1, cyclin-dependent kinase 5 (Cdk5), and G-protein-regulated inducer of neurite outgrowth 1 (GPRIN1). Interactions of 5-HT₆ receptor with Cdk5 and GPRIN1 are not concomitant but occur sequentially and play a key role in dendritic tree morphogenesis. Furthermore, 5-HT₆ receptor-mediated G-protein signaling in neurons is different in the cell body and primary cilium, where it is modulated by smoothened receptor activation. Finally, 5-HT_2A receptors form heteromers with mGlu₂ metabotropic glutamate receptors. This heteromerization results in a specific phosphorylation of mGlu₂ receptor on a serine residue (Ser⁸⁴³) upon agonist stimulation of 5-HT_2A or mGlu₂ receptor. mGlu₂ receptor phosphorylation on Ser⁸⁴³ is an essential step in engagement of G_i/o signaling not only upon mGlu₂ receptor activation but also following 5-HT_2A receptor activation, and thus represents a key molecular event underlying functional crosstalk between both receptors.

Functional characterization of a loss-of-function mutant I324M of arginine vasopressin receptor 2 in X-linked nephrogenic diabetes insipidus

X-linked nephrogenic diabetes insipidus (X-linked NDI) is a rare inherited disease mainly caused by lost-of-function mutations in human AVPR2 gene encoding arginine vasopressin receptor 2 (V2R). Our focus of the current study is on exploration of the functional and biochemical properties of Ile324Met (I324M) mutation identified in a pedigree showing as typical recessive X-linked NDI. We demonstrated that I324M mutation interfered with the conformation of complex glycosylation of V2R. Moreover, almost all of the I324M-V2R failed to express on the cell surface due to being captured by the endoplasmic reticulum control system. We further examined the signaling activity of DDAVP-medicated cAMP and ERK1/2 pathways and the results revealed that the mutant receptor lost the ability in response to DDAVP stimulation contributed to the failure of accumulation of cAMP and phosphorylated ERK1/2. Based on the characteristics of molecular defects of I324M mutant, we selected two reagents (SR49059 and alvespimycin) to determine whether the functions of I324M-V2R can be restored and we found that both compounds can significantly “rescue” I324M mutation. Our findings may provide further insights for understanding the pathogenic mechanism of AVPR2 gene mutations and may offer some implications on development of promising treatments for patients with X-linked NDI.

Subtype selective fluorescent ligands based on ICI 118,551 to study the human β2-adrenoceptor in CRISPR/Cas9 genome-edited HEK293T cells at low expression levels

Fluorescent ligand technologies have proved to be powerful tools to improve our understanding of ligand‐receptor interactions. Here we have characterized a small focused library of nine fluorescent ligands based on the highly selective β₂‐adrenoceptor (β₂AR) antagonist ICI 118,551. The majority of fluorescent ICI 118,551 analogs had good affinity for the β₂AR (pK_D >7.0) with good selectivity over the β₁AR (pK_D <6.0). The most potent and selective ligands being 8c (ICI 118,551‐Gly‐Ala‐BODIPY‐FL‐X; β₂AR pK_D 7.48), 9c (ICI 118,551‐βAla‐βAla‐BODIPY‐FL‐X; β₂AR pK_D 7.48), 12a (ICI 118,551‐PEG‐BODIPY‐X‐630/650; β₂AR pK_D 7.56), and 12b (ICI 118,551‐PEG‐BODIPY‐FL; β₂AR pK_D 7.42). 9a (ICI 118,551‐βAla‐βAla‐BODIPY‐X‐630/650) had the highest affinity at recombinant β₂ARs (pK_D 7.57), but also exhibited significant binding affinity to the β₁AR (pK_D 6.69). Nevertheless, among the red fluorescent ligands, 9a had the best imaging characteristics in recombinant HEK293 T cells and labeling was mostly confined to the cell surface. In contrast, 12a showed the highest propensity to label intracellular β₂ARs in HEK293 T cell expressing exogenous β₂ARs. This suggests that a combination of the polyethylene glycol (PEG) linker and the BODIPY‐X‐630/650 makes this ICI 118,551 derivative particularly susceptible to crossing the cell membrane to access the intracellular β₂ARs. We have also used these ligands in combination with CRISPR/Cas9 genome‐edited HEK293 T cells to undertake for the first time real‐time ligand binding to native HEK293 T β₂ARs at low native receptor expression levels. These studies provided quantitative data on ligand‐binding characteristics but also allowed real‐time visualization of the ligand‐binding interactions in genome‐edited cells using NanoBRET luminescence imaging.

Atypical p38 Signaling, Activation, and Implications for Disease

The mitogen-activated protein kinase (MAPK) p38 is an essential family of kinases, regulating responses to environmental stress and inflammation. There is an ever-increasing plethora of physiological and pathophysiological conditions attributed to p38 activity, ranging from cell division and embryonic development to the control of a multitude of diseases including retinal, cardiovascular, and neurodegenerative diseases, diabetes, and cancer. Despite the decades of intense investigation, a viable therapeutic approach to disrupt p38 signaling remains elusive. A growing body of evidence supports the pathological significance of an understudied atypical p38 signaling pathway. Atypical p38 signaling is driven by a direct interaction between the adaptor protein TAB1 and p38α, driving p38 autophosphorylation independent from the classical MKK3 and MKK6 pathways. Unlike the classical MKK3/6 signaling pathway, atypical signaling is selective for just p38α, and at present has only been characterized during pathophysiological stimulation. Recent studies have linked atypical signaling to dermal and vascular inflammation, myocardial ischemia, cancer metastasis, diabetes, complications during pregnancy, and bacterial and viral infections. Additional studies are required to fully understand how, when, where, and why atypical p38 signaling is induced. Furthermore, the development of selective TAB1-p38 inhibitors represents an exciting new opportunity to selectively inhibit pathological p38 signaling in a wide array of diseases.

The Effects of Apelin and Elabela Ligands on Apelin Receptor Distinct Signaling Profiles

Apelin and Elabela are endogenous peptide ligands for Apelin receptor (APJ), a widely expressed G protein-coupled receptor. They constitute a spatiotemporal dual ligand system to control APJ signal transduction and function. We investigated the effects of Apelin-13, pGlu₁-apelin-13, Apelin-17, Apelin-36, Elabela-21 and Elabela-32 peptides on APJ signal transduction. Whether different ligands are biased to different APJ mediated signal transduction pathways was studied. We observed the different changes of G protein dependent and β-arrestin dependent signaling pathways after APJ was activated by six peptide ligands. We demonstrated that stimulation with APJ ligands resulted in dose-dependent increases in both G protein dependent [cyclic AMP (cAMP), Ca²⁺ mobilization, and the early phase extracellular related kinase (ERK) activation] and β-arrestin dependent [GRKs, β-arrestin 1, β-arrestin 2, and β2 subunit of the clathrin adaptor AP2] signaling pathways. However, the ligands exhibited distinct signaling profiles. Elabela-32 showed a >1000-fold bias to the β-statin-dependent signaling pathway. These data provide that Apelin-17 was biased toward β-arrestin dependent signaling. Eabela-21 and pGlu₁-Apelin-13 exhibited very distinct activities on the G protein dependent pathway. The activity profiles of these ligands could be valuable for the development of drugs with high selectivity for specific APJ downstream signaling pathways.

Conceptual and experimental issues in biased agonism

In this review, we discuss the theoretical and experimental foundations for assessing agonism in the context of signalling bias in GPCRs. We show that the formulation of efficacy in classical receptor theory and the definition of ligand-induced allosteric effect in chemical thermodynamics are coincident measures of agonism, only if we recognize that the classical model cannot be considered as a mechanistic description of the physicochemical events underlying ligand-receptor signalling. It represents instead a mathematical tool, fortuitously capable of extracting efficacy information from concentration-dependent functional data, where both ligand-dependent and ligand-independent information are present. We also assert that dissecting efficacy from affinity, as originally advocated in classical theory, is imperative for understanding the molecular property underlying agonism, and the biased agonism that leads to preferential formation of diverse GPCR-transducer complexes. Finally, we argue that beyond the assumed translational value of functional selectivity (i.e. signalling bias), the identification of ligands with true bias of efficacy is of fundamental importance for unravelling the conformational space that determines the complex functional chemistry of GPCRs.

Biased signaling in naturally occurring mutations of G protein-coupled receptors associated with diverse human diseases

G protein-coupled receptors (GPCRs) play critical roles in transmitting a variety of extracellular signals into the cells and regulate diverse physiological functions. Naturally occurring mutations that result in dysfunctions of GPCRs have been known as the causes of numerous diseases. Significant progresses have been made in elucidating the pathophysiology of diseases caused by mutations. The multiple intracellular signaling pathways, such as G protein-dependent and β-arrestin-dependent signaling, in conjunction with recent advances on biased agonism, have broadened the view on the molecular mechanism of disease pathogenesis. This review aims to briefly discuss biased agonism of GPCRs (biased ligands and biased receptors), summarize the naturally occurring GPCR mutations that cause biased signaling, and propose the potential pathophysiological relevance of biased mutant GPCRs associated with various endocrine diseases.

Biased agonism at β-adrenergic receptors

The β-adrenergic receptors (βARs) include three subtypes, β₁, β₂ and β₃. These receptors are widely expressed and regulate numerous physiological processes including cardiovascular and metabolic functions and airway tone. The βARs are also important targets in the treatment of many diseases including hypertension, heart failure and asthma. In some cases, the use of current βAR ligands to treat a disease is suboptimal and can lead to severe side effects. One strategy to potentially improve such treatments is the development of biased agonists that selectively regulate a subset of βAR signaling pathways and responses. Here we discuss the compounds identified to date that preferentially activate a G_s- or β-arrestin-mediated signaling pathway through βARs. Mechanistic insight on how these compounds bias signaling sheds light on the potential development of even more selective compounds that should have increased utility in treating disease.

Biased signaling as allosteric probe dependence

Signaling 'bias' is a phenomenon whereby the natural allosteric probe dependence of seven transmembrane receptors allows different receptor conformations (stabilized by different agonists) to activate some signaling pathways (coupled to pleiotropically coupled receptors) more than others at the expense of those other pathways. There are a number of relevant scenarios where such an activity could be therapeutically beneficial therefore there are practical reasons why this property of receptors should be exploited. This paper discusses recent ideas around attempts to harness this potentially useful idea and also the limitations around the current methods available to do so. Specifically, the determination of a quantitative value for the receptor bias of a given agonist that may translate to useful in vivo has been particularly elusive and studies need to be directed to solving this problem.

Novel Insights into Beta 2 Adrenergic Receptor Function in the rd10 Model of Retinitis Pigmentosa

Background: In retinitis pigmentosa (RP), inherited rod death is followed by cone loss and blindness. Why cones die is still a matter of consideration. Here, we investigate the pathogenic role of the sympathetic transmission in the rd10 mouse model of RP. Methods: Retinal levels of beta adrenergic receptor (BAR) 2 and norepinephrine (NE) were measured. After administration of the BAR1/2 blocker propranolol or the hypoxia-inducible factor (HIF)-1 activator dimethyloxalylglycine (DMOG), retinal levels of HIF-1α, BAR2 or proteins involved in BAR2 desensitization were also measured. In DMOG treated mice, expression and localization of BAR2, inflammatory markers and cone arrestin were determined. Finally, rd10 mice were subjected to electroretinogram (ERG) analysis to assess rod and cone function. Results: In the rd10 retina, BAR2 overexpression and NE accumulation were found, with BAR2 immunoreactivity localized to Müller cells. BAR2 overexpression was likely due to desensitization defects. Upregulated levels of BAR2 were drastically reduced by propranolol that also restored desensitization defects. Due to the low level of HIF-1 consequent to the hyperoxic environment in the rd10 retina, we hypothesized a link between HIF-1 and BAR2. HIF-1α stabilization with DMOG resulted in i. increased HIF-1α accumulation, ii. decreased BAR2 levels, iii. restored desensitization processes, iv. reduced expression of inflammatory markers and v. increased cone survival without improved retinal function. Conclusions: Our results support a pathogenic role of the sympathetic system in RP that might help to understand why rd10 mice show a positive response to BAR blockers.

How Do Molecular Dynamics Data Complement Static Structural Data of GPCRs

G protein-coupled receptors (GPCRs) are implicated in nearly every physiological process in the human body and therefore represent an important drug targeting class. Advances in X-ray crystallography and cryo-electron microscopy (cryo-EM) have provided multiple static structures of GPCRs in complex with various signaling partners. However, GPCR functionality is largely determined by their flexibility and ability to transition between distinct structural conformations. Due to this dynamic nature, a static snapshot does not fully explain the complexity of GPCR signal transduction. Molecular dynamics (MD) simulations offer the opportunity to simulate the structural motions of biological processes at atomic resolution. Thus, this technique can incorporate the missing information on protein flexibility into experimentally solved structures. Here, we review the contribution of MD simulations to complement static structural data and to improve our understanding of GPCR physiology and pharmacology, as well as the challenges that still need to be overcome to reach the full potential of this technique.

Key phosphorylation sites in GPCRs orchestrate the contribution of β-Arrestin 1 in ERK1/2 activation

β-arrestins (βarrs) are key regulators of G protein-coupled receptor (GPCR) signaling and trafficking, and their knockdown typically leads to a decrease in agonist-induced ERK1/2 MAP kinase activation. Interestingly, for some GPCRs, knockdown of βarr1 augments agonist-induced ERK1/2 phosphorylation although a mechanistic basis for this intriguing phenomenon is unclear. Here, we use selected GPCRs to explore a possible correlation between the spatial positioning of receptor phosphorylation sites and the contribution of βarr1 in ERK1/2 activation. We discover that engineering a spatially positioned double-phosphorylation-site cluster in the bradykinin receptor (B₂ R), analogous to that present in the vasopressin receptor (V₂ R), reverses the contribution of βarr1 in ERK1/2 activation from inhibitory to promotive. An intrabody sensor suggests a conformational mechanism for this role reversal of βarr1, and molecular dynamics simulation reveals a bifurcated salt bridge between this double-phosphorylation site cluster and Lys²⁹⁴ in the lariat loop of βarr1, which directs the orientation of the lariat loop. Our findings provide novel insights into the opposite roles of βarr1 in ERK1/2 activation for different GPCRs with a direct relevance to biased agonism and novel therapeutics.

A G-protein coupled receptor 39 agonist stimulates proliferation of keratinocytes via an ERK-dependent pathway

Keratinocyte proliferation serves as a crucial process in skin wound healing. The zinc-sensing G-protein coupled receptor 39 (GPR39), which is highly expressed in keratinocytes, has been shown to promote skin wound healing. The aim of this study was to investigate the effect of GPR39 activation on proliferation of keratinocytes and its underlying mechanism using immortalized human keratinocytes (HaCaT) as an in vitro model. GPR39 was functionally expressed in HaCaT cells. BrdU proliferation assays showed that treatment with GPR39 agonist TC-G 1008 (100 nM and 1 μM) increased cell proliferation. TC-G 1008 also enhanced ERK phosphorylation in time- and concentration-dependent manners. This effect was suppressed by co-treatment with wortmannin (PI3K inhibitor) and U0126 (MKK inhibitor). Of note, neither inhibition of Gα_q-phospholipase C (PLC)-[Ca²⁺]_i nor Gα_s-PKA pathway affected GPR39 stimulation-induced ERK phosphorylation. Similarly, barbadin, an inhibitor of G-protein-independent β-arrestin pathway, did not suppress ERK phosphorylation induced by GPR39 activation. Of particular importance, wortmannin, U0126, and FR180204 (ERK inhibitor) abrogated the effect of TC-G 1008-induced cell proliferation. Taken together, this study reveals novel insights into the role of GPR39 in regulating keratinocyte proliferation via a PI3K-MKK-ERK-dependent mechanism. GPR39 agonists may be used in enhancing keratinocyte proliferation, which may be beneficial for the cutaneous wound treatment.

Phosphoproteomic Identification of Vasopressin/cAMP/Protein Kinase A-Dependent Signaling in Kidney

Water excretion by the kidney is regulated by the neurohypophyseal peptide hormone vasopressin through actions in renal collecting duct cells to regulate the water channel protein aquaporin-2. Vasopressin signaling is initiated by binding to a G-protein-coupled receptor called V2R, which signals through heterotrimeric G-protein subunit G_s α, adenylyl cyclase 6, and activation of the cAMP-regulated protein kinase (PKA). Signaling events coupling PKA activation and aquaporin-2 regulation were largely unknown until the advent of modern protein mass spectrometry techniques that allow proteome-wide quantification of protein phosphorylation changes (phosphoproteomics). This short review documents phosphoproteomic findings in collecting duct cells describing the response to V2R-selective vasopressin agonists and antagonists, the response to CRISPR-mediated deletion of PKA, results from in vitro phosphorylation studies using recombinant PKA, the response to the broad-spectrum kinase inhibitor H89 (N-[2-p-bromocinnamylamino-ethyl]-5-isoquinolinesulphonamide), and the responses underlying lithium-induced nephrogenic diabetes insipidus. These phosphoproteomic data sets have been made available online for modeling vasopressin signaling and signaling downstream from other G-protein-coupled receptors. SIGNIFICANCE STATEMENT: New developments in protein mass spectrometry are facilitating progress in identification of signaling networks. Using mass spectrometry, it is now possible to identify and quantify thousands of phosphorylation sites in a given cell type (phosphoproteomics). The authors describe the use of phosphoproteomics technology to identify signaling mechanisms downstream from a G-protein-coupled receptor, the vasopressin V2 subtype receptor, and its role of the regulation and dysregulation of water excretion in the kidney. Data from multiple phosphoproteomic data sets are provided as web-based resources.

Targeting GPCRs Against Cardiotoxicity Induced by Anticancer Treatments

Novel anticancer medicines, including targeted therapies and immune checkpoint inhibitors, have greatly improved the management of cancers. However, both conventional and new anticancer treatments induce cardiac adverse effects, which remain a critical issue in clinic. Cardiotoxicity induced by anti-cancer treatments compromise vasospastic and thromboembolic ischemia, dysrhythmia, hypertension, myocarditis, and cardiac dysfunction that can result in heart failure. Importantly, none of the strategies to prevent cardiotoxicity from anticancer therapies is completely safe and satisfactory. Certain clinically used cardioprotective drugs can even contribute to cancer induction. Since G protein coupled receptors (GPCRs) are target of forty percent of clinically used drugs, here we discuss the newly identified cardioprotective agents that bind GPCRs of adrenalin, adenosine, melatonin, ghrelin, galanin, apelin, prokineticin and cannabidiol. We hope to provoke further drug development studies considering these GPCRs as potential targets to be translated to treatment of human heart failure induced by anticancer drugs.

The Emerging Role of Gβ Subunits in Human Genetic Diseases

Environmental stimuli are perceived and transduced inside the cell through the activation of signaling pathways. One common type of cell signaling transduction network is initiated by G-proteins. G-proteins are activated by G-protein-coupled receptors (GPCRs) and transmit signals from hormones, neurotransmitters, and other signaling factors, thus controlling a number of biological processes that include synaptic transmission, visual photoreception, hormone and growth factors release, regulation of cell contraction and migration, as well as cell growth and differentiation. G-proteins mainly act as heterotrimeric complexes, composed of alpha, beta, and gamma subunits. In the last few years, whole exome sequencing and biochemical studies have shown causality of disease-causing variants in genes encoding G-proteins and human genetic diseases. This review focuses on the G-protein β subunits and their emerging role in the etiology of genetically inherited rare diseases in humans.

Rapid signalling responses via the G protein-coupled estrogen receptor, GPER, in a hippocampal cell line

The rapid non-genomic actions of 17β-estradiol in multiple tissues, including the nervous system, may involve the activation of the G-protein-coupled receptor, GPER. Different signalling pathways have been suggested to be activated by GPER in different cell lines and tissues. Controversially, GPER has also been suggested to be activated by the mineralocorticoid aldosterone, and by the non-steroidal diphenylacrylamide compound, STX, in some preparations. Evidence for the ability of the GPER agonist, G-1, and for aldosterone in the presence of the mineralocorticoid receptor antagonist, eplerenone, to potentiate forskolin-stimulated cyclic AMP levels in the hippocampal clonal cell line, mHippoE-18 is reviewed. The effects of both agents are blocked by the GPER antagonist G36, by PTX, (suggesting the involvement of Gi/o G proteins), by BAPTA-AM, (suggesting they are calcium sensitive), by wortmannin (suggesting an involvement of PI3Kinase) and by soluble amyloid-β peptides. STX also stimulates cyclic AMP levels in mHippoE-18 cells and these effects are blocked by G36 and PTX, as well as by amyloid-β peptides. This suggests that both aldosterone and STX may be capable of activating GPER in mHippoE-18 cells. Possible molecular mechanisms that may underlie these effects are discussed, together with possible forward directions for research on rapid non-genomic signalling by GPER, emphasising the importance of understanding the spatio-temporal aspects of its signalling in various tissues.

Aldosterone, STX and amyloid-β1-42 peptides modulate GPER (GPR30) signalling in an embryonic mouse hippocampal cell line (mHippoE-18)

The GPCR, GPER, mediates many of the rapid, non-genomic actions of 17β-estradiol in multiple tissues, including the nervous system. Controversially, it has also been suggested to be activated by aldosterone, and by the non-steroidal diphenylacrylamide compound, STX, in some preparations. Here, the ability of the GPER agonist, G-1, and aldosterone in the presence of the mineralocorticoid receptor antagonist, eplerenone, to potentiate forskolin-stimulated cyclic AMP levels in the hippocampal clonal cell line, mHippoE-18, are compared. Both stimulatory effects are blocked by the GPER antagonist G36, by PTX, (suggesting the involvement of Gi/o G proteins), by BAPTA-AM, (suggesting they are calcium sensitive), by wortmannin (suggesting an involvement of PI3Kinase) and by soluble amyloid-β peptides. STX also stimulates cyclic AMP levels in mHippoE-18 cells and these effects are blocked by G36 and PTX, as well as by amyloid-β peptides. This suggests that both aldosterone and STX may modulate GPER signalling in mHippoE-18 cells.