Arrestin recruitment and signaling by G protein-coupled receptor heteromers

Apelin receptor dimer: Classification, future prospects, and pathophysiological perspectives

Apelin receptor (APJ), a member of the class A family of G protein-coupled receptor (GPCR), plays a crucial role in regulating cardiovascular and central nervous systems function. APJ influences the onset and progression of various diseases such as hypertension, atherosclerosis, and cerebral stroke, making it an important target for drug development. Our preliminary findings indicate that APJ can form homodimers, heterodimers, or even higher-order oligomers, which participate in different signaling pathways and have distinct functions compared with monomers. APJ homodimers can serve as neuroprotectors against, and provide new pharmaceutical targets for vascular dementia (VD). This review article aims to summarize the structural characteristics of APJ dimers and their roles in physiology and pathology, as well as explore their potential pharmacological applications.

Exploiting Cell-Based Assays to Accelerate Drug Development for G Protein-Coupled Receptors

G protein-coupled receptors (GPCRs) are relevant targets for health and disease as they regulate various aspects of metabolism, proliferation, differentiation, and immune pathways. They are implicated in several disease areas, including cancer, diabetes, cardiovascular diseases, and mental disorders. It is worth noting that about a third of all marketed drugs target GPCRs, making them prime pharmacological targets for drug discovery. Numerous functional assays have been developed to assess GPCR activity and GPCR signaling in living cells. Here, we review the current literature of genetically encoded cell-based assays to measure GPCR activation and downstream signaling at different hierarchical levels of signaling, from the receptor to transcription, via transducers, effectors, and second messengers. Singleplex assay formats provide one data point per experimental condition. Typical examples are bioluminescence resonance energy transfer (BRET) assays and protease cleavage assays (e.g., Tango or split TEV). By contrast, multiplex assay formats allow for the parallel measurement of multiple receptors and pathways and typically use molecular barcodes as transcriptional reporters in barcoded assays. This enables the efficient identification of desired on-target and on-pathway effects as well as detrimental off-target and off-pathway effects. Multiplex assays are anticipated to accelerate drug discovery for GPCRs as they provide a comprehensive and broad identification of compound effects.

Buprenorphine for Chronic Pain Management: a Narrative Review

PURPOSE OF REVIEW

The aim of this review is to educate healthcare professionals regarding buprenorphine for the use of opioid use disorder (OUD) as well as for chronic pain management. This review provides physicians and practitioners with updated information regarding the distinct characteristics and intricacies of prescribing buprenorphine.

RECENT FINDINGS

Buprenorphine is approved by the US Food and Drug Administration (FDA) for acute pain, chronic pain, opioid use disorder (OUD), and opioid dependence. When compared to most other opioids, buprenorphine offers superior patient tolerability, an excellent half-life, and minimal respiratory depression. Buprenorphine does have notable side effects as well as pharmacokinetic properties that require special attention, especially if patients require future surgical interventions. Many physicians are not trained to initiate or manage patients on buprenorphine. However, buprenorphine offers a potentially safer alternative for medication management for patients who require chronic opioid therapy for pain or have OUD. This review provides updated information on buprenorphine for both chronic pain and OUD.

A dual nociceptin and mu opioid receptor agonist exhibited robust antinociceptive effect with decreased side effects

The compelling demand of a consummate analgesic medication without addiction is rising due to the clinical mistreatment. Additionally, the series of severe untoward effects usually deterred the utilization while coping with serious pain. As a possible turning point, we revealed that compound 14 is a dual agonist of mu opioid receptor (MOR) and nociceptin-orphanin FQ opioid peptide (NOP) receptor in this study. More importantly, compound 14 achieves pain relieving at very small doses, meanwhile, reduces several unwanted side effects such as constipation, reward, tolerance and withdrawal effects. Here, we evaluated the antinociception and side effects of this novel compound from wild type and humanized mice to further develop a safer prescription analgesic drug.

βγ G-proteins, but not regulators of G-protein signaling 4, modulate opioid-induced respiratory rate depression

Opioid medications are the mainstay of pain management but present substantial side-effects such as respiratory depression which can be lethal with overdose. Most opioid drugs, such as fentanyl, act on opioid receptors such as the G-protein-coupled µ-opioid receptors (MOR). G-protein-coupled receptors activate pertussis toxin-sensitive G-proteins to inhibit neuronal activity. Binding of opioid ligands to MOR and subsequent activation G proteins βγ is modulated by regulator of G-protein signaling (RGS). The roles of G-proteins βγ and RGS in MOR-mediated inhibition of the respiratory network are not known. Using rodent models to pharmacologically modulate G-protein signaling, we aim to determine the roles of βγ G-proteins and RGS4. We showed that inhibition of βγ G-proteins using gallein perfused in the brainstem circuits regulating respiratory depression by opioid drugs results in complete reversal of respiratory depression. Blocking of RGS4 using CCG55014 did not change the respiratory depression induced by MOR activation despite co-expression of RGS4 and MORs in the brainstem. Our results suggest that neuronal inhibition by opioid drugs is mediated by G-proteins, but not by RGS4, which supports the concept that βγ G-proteins could be molecular targets to develop opioid overdose antidotes without the risks of re-narcotization often found with highly potent opioid drugs. On the other hand, RGS4 mediates opioid analgesia, but not respiratory depression, and RGS4 may be molecular targets to develop pain therapies without respiratory liability.

Brain Structure and Function: Insights from Chemical Neuroanatomy

We present a brief historical and epistemological outline of investigations on the brain’s structure and functions. These investigations have mainly been based on the intermingling of chemical anatomy, new techniques in the field of microscopy and computer-assisted morphometric methods. This intermingling has enabled extraordinary investigations to be carried out on brain circuits, leading to the development of a new discipline: “brain connectomics”. This new approach has led to the characterization of the brain’s structure and function in physiological and pathological conditions, and to the development of new therapeutic strategies. In this context, the conceptual model of the brain as a hyper-network with a hierarchical, nested architecture, arranged in a “Russian doll” pattern, has been proposed. Our investigations focused on the main characteristics of the modes of communication between nodes at the various miniaturization levels, in order to describe the brain’s integrative actions. Special attention was paid to the nano-level, i.e., to the allosteric interactions among G protein-coupled receptors organized in receptor mosaics, as a promising field in which to obtain a new view of synaptic plasticity and to develop new, more selective drugs. The brain’s multi-level organization and the multi-faceted aspects of communication modes point to an emerging picture of the brain as a very peculiar system, in which continuous self-organization and remodeling take place under the action of external stimuli from the environment, from peripheral organs and from ongoing integrative actions.

Cannabinoids and the placenta: Receptors, signaling and outcomes

Cannabis use during pregnancy is increasing. The improvement of pregnancy-related symptoms including morning sickness and management of mood and stress are among the most reported reasons for its use. Δ9-tetrahydrocannabinol (Δ9-THC) and cannabidiol (CBD) are the most abundant cannabinoids found within the cannabis flower. The concentration of these components has drastically increased in the past 20 years. Additionally, many edibles contain only one cannabinoid and are marketed to achieve a specific goal, meaning there are an increasing number of pregnancies that are exposed to isolated cannabinoids. Both Δ9-THC and CBD cross the placenta and can impact the fetus directly, but the receptors through which cannabinoids act are also expressed throughout the placenta, suggesting that the effects of in-utero cannabinoid exposure may include indirect effects from the placenta. In-utero cannabis research focuses on short and long-term fetal health and development; however, these studies include little to no placenta analysis. Prenatal cannabinoid exposure is linked to small for gestational age and fetal growth-restricted babies. Compromised placental development is also associated with fetal growth restriction and the few studies (clinical and animal models) that included placental analysis, identify changes in placental vasculature and function in these cannabinoid-exposed pregnancies. In vitro studies further support cannabinoid impact on cell function in the different populations that comprise the placenta. In this article, we aim to summarize how phytocannabinoids can impact placental development and function. Specifically, the cannabinoids and their actions at the different receptors are described, with receptor localization throughout the human and murine placenta discussed. Findings from studies that included placental analysis and how cannabinoid signaling may modulate critical developmental processing including cell proliferation, angiogenesis and migration are described. Considering the current research, prenatal cannabinoid exposure may significantly impact placental development, and, as such, identifying windows of placental vulnerability for each cannabinoid will be critical to elucidate the etiology of fetal outcome studies.

Modified Akuamma Alkaloids with Increased Potency at the Mu-opioid Receptor

Akuammine (1) and pseudoakuammigine (2) are indole alkaloids found in the seeds of the akuamma tree (Picralima nitida). Both alkaloids are weak agonists of the mu opioid receptor (μOR); however, they produce minimal effects in animal models of antinociception. To probe the interactions of 1 and 2 at the opioid receptors, we have prepared a collection of 22 semisynthetic derivatives. Evaluation of this collection at the μOR and kappa opioid receptor (κOR) revealed structural-activity relationship trends and derivatives with improved potency at the μOR. Most notably, the introduction of a phenethyl moiety to the N1 of 2 produces a 70-fold increase in potency and a 7-fold increase in selectivity for the μOR. The in vitro potency of this compound resulted in increased efficacy in the tail-flick and hot-plate assays of antinociception. The improved potency of these derivatives highlights the promise of exploring natural product scaffolds to probe the opioid receptors.

The role of G protein-coupled receptor in neutrophil dysfunction during sepsis-induced acute respiratory distress syndrome

Sepsis is defined as a life-threatening dysfunction due to a dysregulated host response to infection. It is a common and complex syndrome and is the leading cause of death in intensive care units. The lungs are most vulnerable to the challenge of sepsis, and the incidence of respiratory dysfunction has been reported to be up to 70%, in which neutrophils play a major role. Neutrophils are the first line of defense against infection, and they are regarded as the most responsive cells in sepsis. Normally, neutrophils recognize chemokines including the bacterial product N-formyl-methionyl-leucyl-phenylalanine (fMLP), complement 5a (C5a), and lipid molecules Leukotriene B4 (LTB4) and C-X-C motif chemokine ligand 8 (CXCL8), and enter the site of infection through mobilization, rolling, adhesion, migration, and chemotaxis. However, numerous studies have confirmed that despite the high levels of chemokines in septic patients and mice at the site of infection, the neutrophils cannot migrate to the proper target location, but instead they accumulate in the lungs, releasing histones, DNA, and proteases that mediate tissue damage and induce acute respiratory distress syndrome (ARDS). This is closely related to impaired neutrophil migration in sepsis, but the mechanism involved is still unclear. Many studies have shown that chemokine receptor dysregulation is an important cause of impaired neutrophil migration, and the vast majority of these chemokine receptors belong to the G protein-coupled receptors (GPCRs). In this review, we summarize the signaling pathways by which neutrophil GPCR regulates chemotaxis and the mechanisms by which abnormal GPCR function in sepsis leads to impaired neutrophil chemotaxis, which can further cause ARDS. Several potential targets for intervention are proposed to improve neutrophil chemotaxis, and we hope that this review may provide insights for clinical practitioners.

Membrane estrogen receptor and follicle-stimulating hormone receptor

Follicle-stimulating hormone (FSH) and estrogens are fundamental to support reproductive functions. Beside the well-known FSH membrane receptor (FSHR), a G protein-coupled estrogen receptor (GPER) has been found, over the last two decades, in several tissues. It may trigger rapid, non-genomic responses of estradiol, activating proliferative and survival stimuli. The two receptors were co-characterized in the ovary, where they modulate different intracellular signaling cascades, according to the expression level and developmental stage of ovarian follicles. Moreover, they may physically interact to form heteromeric assemblies, suggestive of a new mode of action to regulate FSH-specific signals, and likely determining the follicular fate between atresia and dominance. The knowledge of FSH and estrogen membrane receptors provides a new, deeper level of comprehension of human reproduction.

Biased Activation Mechanism Induced by GPCR Heterodimerization: Observations from μOR/δOR Dimers

GPCRs regulate multiple intracellular signaling cascades. Biasedly activating one signaling pathway over the others provides additional clinical utility to optimize GPCR-based therapies. GPCR heterodimers possess different functions from their monomeric states, including their selectivity to different transducers. However, the biased signaling mechanism induced by the heterodimerization remains unclear. Motivated by the issue, we select an important GPCR heterodimer (μOR/δOR heterodimer) as a case and use microsecond Gaussian accelerated molecular dynamics simulation coupled with potential of mean force and protein structure network (PSN) to probe mechanisms regarding the heterodimerization-induced constitutive β-arrestin activity and efficacy change of the agonist DAMGO. The results show that only the lowest energy state of the μOR/δOR heterodimer, which adopts a slightly outward shift of TM6 and an ICL2 conformation close to the receptor core, can selectively accommodate β-arrestins. PSN further reveals important roles of H8, ICL1, and ICL2 in regulating the constitutive β-arrestin-biased activity for the apo μOR/δOR heterodimer. In addition, the heterodimerization can allosterically alter the binding mode of DAMGO mainly by means of W7.35. Consequently, DAMGO transmits the structural signal mainly through TM6 and TM7 in the dimer, rather than TM3 similar to the μOR monomer, thus changing the efficacy of DAMGO from a balanced agonist to the β-arrestin-biased one. On the other side, the binding of DAMGO to the heterodimer can stabilize μOR/δOR heterodimers through a stronger interaction of TM1/TM1 and H8/H8, accordingly enhancing the interaction of μOR with δOR and the binding affinity of the dimer to the β-arrestin. The agonist DAMGO does not change main compositions of the regulation network from the dimer interface to the transducer binding pocket of the μOR protomer, but induces an increase in the structural communication of the network, which should contribute to the enhanced β-arrestin coupling. Our observations, for the first time, reveal the molecular mechanism of the biased signaling induced by the heterodimerization for GPCRs, which should be beneficial to more comprehensively understand the GPCR bias signaling.

Biased signaling due to oligomerization of the G protein-coupled platelet-activating factor receptor

G protein-coupled receptors (GPCRs) are important drug targets that mediate various signaling pathways by activating G proteins and engaging β-arrestin proteins. Despite its importance for the development of therapeutics with fewer side effects, the underlying mechanism that controls the balance between these signaling modes of GPCRs remains largely unclear. Here, we show that assembly into dimers and oligomers can largely influence the signaling mode of the platelet-activating factor receptor (PAFR). Single-particle analysis results show that PAFR can form oligomers at low densities through two possible dimer interfaces. Stabilization of PAFR oligomers through cross-linking increases G protein activity, and decreases β-arrestin recruitment and agonist-induced internalization significantly. Reciprocally, β-arrestin prevents PAFR oligomerization. Our results highlight a mechanism involved in the control of receptor signaling, and thereby provide important insights into the relationship between GPCR oligomerization and downstream signaling.

G protein coupled pattern recognition receptors expressed in neutrophils : Recognition, activation/modulation, signaling and receptor regulated functions

Neutrophils, the most abundant white blood cell in human blood, express receptors that recognize damage/microbial associated pattern molecules of importance for cell recruitment to sites of inflammation. Many of these receptors belong to the family of G protein coupled receptors (GPCRs). These receptor-proteins span the plasma membrane in expressing cells seven times and the down-stream signaling rely in most cases on an activation of heterotrimeric G proteins. The GPCRs expressed in neutrophils recognize a number of structurally diverse ligands (activating agonists, allosteric modulators, and inhibiting antagonists) and share significant sequence homologies. Studies of receptor structure and function have during the last 40 years generated important information on GPCR biology in general; this knowledge aids in the overall understanding of general pharmacological principles, governing regulation of neutrophil function and inflammatory processes, including novel leukocyte receptor activities related to ligand recognition, biased/functional selective signaling, allosteric modulation, desensitization, and reactivation mechanisms as well as communication (receptor transactivation/cross-talk) between GPCRs. This review summarizes the recent discoveries and pharmacological hallmarks with focus on some of the neutrophil expressed pattern recognition GPCRs. In addition, unmet challenges, including recognition by the receptors of diverse ligands and how biased signaling mediate different biological effects are described/discussed.

Opioid signaling and design of analgesics

Clinical treatment of acute to severe pain relies on the use of opioids. While their potency is significant, there are considerable side effects that can negatively affect patients. Their rise in usage has correlated with the current opioid epidemic in the United States, which has led to more than 70,000 deaths per year (Volkow and Blanco, 2021). Opioid-related drug development aims to make target compounds that show strong potency but with diminished side effects. Research into pharmaceuticals that could act as potential alternatives to current pains medications has relied on mechanistic insights of opioid receptors, a class of G-protein coupled receptors (GPCRs), and biased agonism, a common phenomenon among pharmaceutical compounds where downstream effects can be altered at the same receptor via different agonists. Opioids function typically by binding to an active site on the extracellular portion of opioid receptors. Once activated, the opioid receptor initiates a G-protein signaling pathway and/or the β-arrestin2 pathway. The proposed concept for the development of safe analgesics around mu and kappa opioid receptor subtypes has focused on not recruiting β-arrestin2 (biased agonism) and/or having low efficacy at the receptor (partial agonism). By altering chemical motifs on a common scaffold, chemists can take advantage of biased agonism as well as create compounds with low intrinsic efficacy for the desired treatments. This review will focus on ligands with bias profile, signaling aspects of the receptor and probe into the structural basis of receptor that leads to bias and/or partial agonism.

The pathophysiology of opioid-induced respiratory depression

Opiates, such as morphine, and synthetic opioids, such as fentanyl, constitute a class of drugs acting on opioid receptors which have been used therapeutically and recreationally for centuries. Opioid drugs have strong analgesic properties and are used to treat moderate to severe pain, but also present side effects including opioid dependence, tolerance, addiction, and respiratory depression, which can lead to lethal overdose if not treated. This chapter explores the pathophysiology, the neural circuits, and the cellular mechanisms underlying opioid-induced respiratory depression and provides a translational perspective of the most recent research. The pathophysiology discussed includes the effects of opioid drugs on the respiratory system in patients, as well as the animal models used to identify underlying mechanisms. Using a combination of gene editing and pharmacology, the neural circuits and molecular pathways mediating neuronal inhibition by opioids are examined. By using pharmacology and neuroscience approaches, new therapies to prevent or reverse respiratory depression by opioid drugs have been identified and are currently being developed. Considering the health and economic burden associated with the current opioid epidemic, innovative research is needed to better understand the side effects of opioid drugs and to discover new therapeutic solutions to reduce the incidence of lethal overdoses.

Buprenorphine: Far Beyond the "Ceiling"

Chronic pain, including neuropathic pain, represents an untreated disease with important repercussions on the quality of life and huge costs on the national health system. It is well known that opioids are the most powerful analgesic drugs, but they represent the second or third line in neuropathic pain, that remain difficult to manage. Moreover, these drugs show several side effects that limit their use. In addition, opioids possess addictive properties that are associated with misuse and drug abuse. Among available opioids compounds, buprenorphine has been suggested advantageous for a series of clinical reasons, including the effectiveness in neuropathic pain. Some properties are partly explained by its unique pharmacological characteristics. However, questions on the dynamic profile remain to be answered. Pharmacokinetics optimization strategies, and additional potentialities, are still to be explored. In this paper, we attempt to conceptualize the potential undiscovered dynamic profile of buprenorphine.

β-Arrestin 2 (ARRB2) Polymorphism is Associated With Adverse Consequences of Chronic Heroin Use

BACKGROUND AND OBJECTIVES

β-arrestin 2 is an intracellular protein recruited during the activation of G-protein-coupled receptors. In preclinical studies, β-arrestin 2 has been implicated in µ-opioid receptor desensitization and internalization and the development of opioid tolerance and dependence. The present study investigated relationships between variants in the gene encoding β-arrestin 2 (ARRB2) and clinically relevant phenotypes among individuals with opioid use disorder (OUD). We hypothesized that ARRB2 variants would be associated with the negative effects of long-term heroin use.

METHODS

Chronic heroin users (N = 201; n = 103 African American; n = 98 Caucasian) were genotyped for ARRB2 r1045280 (synonymous, also affecting binding motif of transcription factor GTF2IRD1), rs2036657 (3'UTR) and rs3786047 (intron) and assessed for the past-month frequency of use, injection use, and lifetime duration of heroin use, number of heroin quit-attempts, and heroin use-related consequences.

RESULTS

Lifetime heroin-use consequences (especially occupational and health-related) were significantly lower for African American ARRB2 r1045280 C-allele carriers compared with the TT genotype. There was no significant genotype difference in the Caucasian group. ARRB2 rs2036657 was in strong linkage disequilibrium with rs1045280.

DISCUSSION AND CONCLUSIONS

These results, consistent with extant data, illustrate a role for ancestry-dependent allelic variation in ARRB2 r1045280 on heroin-use consequences. The ARRB2 r1045280 C-allele played a protective role in African-descent participants.

SCIENTIFIC SIGNIFICANCE

These first-in-human findings, which should be replicated, provide support for mechanistic investigations of ARRB2 and related intracellular signaling molecules in OUD etiology, treatment, and relapse prevention. (Am J Addict 2021;00:00-00).

The Roles of Orphan G Protein-Coupled Receptors in Autoimmune Diseases

G protein-coupled receptors (GPCRs) constitute the largest family of plasma membrane receptors in nature and mediate the effects of a variety of extracellular signals, such as hormone, neurotransmitter, odor, and light signals. Due to their involvement in a broad range of physiological and pathological processes and their accessibility, GPCRs are widely used as pharmacological targets of treatment. Orphan G protein-coupled receptors (oGPCRs) are GPCRs for which no natural ligands have been found, and they not only play important roles in various physiological functions, such as sensory perception, reproduction, development, growth, metabolism, and responsiveness, but are also closely related to many major diseases, such as central nervous system (CNS) diseases, metabolic diseases, and cancer. Recently, many studies have reported that oGPCRs play increasingly important roles as key factors in the occurrence and progression of autoimmune diseases. Therefore, oGPCRs are likely to become potential therapeutic targets and may provide a breakthrough in the study of autoimmune diseases. In this article, we focus on reviewing the recent research progress and clinical treatment effects of oGPCRs in three common autoimmune diseases: multiple sclerosis (MS), rheumatoid arthritis (RA), and systemic lupus erythematosus (SLE), shedding light on novel strategies for treatments.

Synthesis and Pharmacology of a Novel μ-δ Opioid Receptor Heteromer-Selective Agonist Based on the Carfentanyl Template

In this work, we studied a series of carfentanyl amide-based opioid derivatives targeting the mu opioid receptor (μOR) and the delta opioid receptor (δOR) heteromer as a credible novel target in pain management therapy. We identified a lead compound named MP135 that exhibits high G-protein activity at μ-δ heteromers compared to the homomeric δOR or μOR and low β-arrestin2 recruitment activity at all three. Furthermore, MP135 exhibits distinct signaling profile, as compared to the previously identified agonist targeting μ-δ heteromers, CYM51010. Pharmacological characterization of MP135 supports the utility of this compound as a molecule that could be developed as an antinociceptive agent similar to morphine in rodents. In vivo characterization reveals that MP135 maintains untoward side effects such as respiratory depression and reward behavior; together, these results suggest that optimization of MP135 is necessary for the development of therapeutics that suppress the classical side effects associated with conventional clinical opioids.

Development of Diphenethylamines as Selective Kappa Opioid Receptor Ligands and Their Pharmacological Activities

Among the opioid receptors, the kappa opioid receptor (KOR) has been gaining substantial attention as a promising molecular target for the treatment of numerous human disorders, including pain, pruritus, affective disorders (i.e., depression and anxiety), drug addiction, and neurological diseases (i.e., epilepsy). Particularly, the knowledge that activation of the KOR, opposite to the mu opioid receptor (MOR), does not produce euphoria or leads to respiratory depression or overdose, has stimulated the interest in discovering ligands targeting the KOR as novel pharmacotherapeutics. However, the KOR mediates the negative side effects of dysphoria/aversion, sedation, and psychotomimesis, with the therapeutic promise of biased agonism (i.e., selective activation of beneficial over deleterious signaling pathways) for designing safer KOR therapeutics without the liabilities of conventional KOR agonists. In this review, the development of new KOR ligands from the class of diphenethylamines is presented. Specifically, we describe the design strategies, synthesis, and pharmacological activities of differently substituted diphenethylamines, where structure–activity relationships have been extensively studied. Ligands with distinct profiles as potent and selective agonists, G protein-biased agonists, and selective antagonists, and their potential use as therapeutic agents (i.e., pain treatment) and research tools are described.

Biased Opioid Ligands

Achieving effective pain management is one of the major challenges associated with modern day medicine. Opioids, such as morphine, have been the reference treatment for moderate to severe acute pain not excluding chronic pain modalities. Opioids act through the opioid receptors, the family of G-protein coupled receptors (GPCRs) that mediate pain relief through both the central and peripheral nervous systems. Four types of opioid receptors have been described, including the μ-opioid receptor (MOR), κ-opioid receptor (KOR), δ-opioid receptor (DOR), and the nociceptin opioid peptide receptor (NOP receptor). Despite the proven success of opioids in treating pain, there are still some inherent limitations. All clinically approved MOR analgesics are associated with adverse effects, which include tolerance, dependence, addiction, constipation, and respiratory depression. On the other hand, KOR selective analgesics have found limited clinical utility because they cause sedation, anxiety, dysphoria, and hallucinations. DOR agonists have also been investigated but they have a tendency to cause convulsions. Ligands targeting NOP receptor have been reported in the preclinical literature to be useful as spinal analgesics and as entities against substance abuse disorders while mixed MOR/NOP receptor agonists are useful as analgesics. Ultimately, the goal of opioid-related drug development has always been to design and synthesize derivatives that are equally or more potent than morphine but most importantly are devoid of the dangerous residual side effects and abuse potential. One proposed strategy is to take advantage of biased agonism, in which distinct downstream pathways can be activated by different molecules working through the exact same receptor. It has been proposed that ligands not recruiting β-arrestin 2 or showing a preference for activating a specific G-protein mediated signal transduction pathway will function as safer analgesic across all opioid subtypes. This review will focus on the design and the pharmacological outcomes of biased ligands at the opioid receptors, aiming at achieving functional selectivity.

Neutrophil Signaling That Challenges Dogmata of G Protein-Coupled Receptor Regulated Functions

Activation as well as recruitment of neutrophils, the most abundant leukocyte in human blood, to sites of infection/inflammation largely rely on surface-exposed chemoattractant receptors. These receptors belong to the family of 7-transmembrane domain receptors also known as G protein-coupled receptors (GPCRs) due to the fact that part of the downstream signaling relies on an activation of heterotrimeric G proteins. The neutrophil GPCRs share significant sequence homologies but bind many structurally diverse activating (agonistic) and inhibiting (antagonistic) ligands, ranging from fatty acids to purines, peptides, and lipopeptides. Recent structural and functional studies of neutrophil receptors have generated important information on GPCR biology in general; this knowledge aids in the overall understanding of general pharmacological principles, governing regulation of neutrophil function and inflammatory processes, including novel leukocyte receptor activities related to ligand recognition, biased/functional selective signaling, allosteric modulation, desensitization mechanisms and reactivation, and communication (cross-talk) between GPCRs. This review summarizes the recent discoveries and pharmacological hallmarks with focus on neutrophil GPCRs. In addition, unmet challenges are dealt with, including recognition by the receptors of diverse ligands and how biased signaling mediates different biological effects.

The G Protein-Coupled Receptor FFAR2 Promotes Internalization during Influenza A Virus Entry

Influenza A virus (IAV) coopts numerous host factors to complete its replication cycle. Here, we identify free fatty acid receptor 2 (FFAR2) as a cofactor for IAV entry into host cells. We found that downregulation of FFAR2 or Ffar2 expression significantly reduced the replication of IAV in A549 or RAW 264.7 cells. The treatment of A549 cells with small interfering RNA (siRNA) targeting FFAR2 or the FFAR2 pathway agonists 2-(4-chlorophenyl)-3-methyl-N-(thiazol-2-yl)butanamide (4-CMTB) and compound 58 (Cmp58) [(S)-2-(4-chlorophenyl)-3,3-dimethyl-N-(5-phenylthiazol-2-yl)butanamide] dramatically inhibited the nuclear accumulation of viral nucleoprotein (NP) at early time points postinfection, indicating that FFAR2 functions in the early stage of the IAV replication cycle. FFAR2 downregulation had no effect on the expression of sialic acid (SA) receptors on the cell membrane, the attachment of IAV to the SA receptors, or the activity of the viral ribonucleoprotein (vRNP) complex. Rather, the amount of internalized IAVs was significantly reduced in FFAR2-knocked-down or 4-CMTB- or Cmp58-treated A549 cells. Further studies showed that FFAR2 associated with β-arrestin1 and that β-arrestin1 interacted with the β2-subunit of the AP-2 complex (AP2B1), the essential adaptor of the clathrin-mediated endocytosis pathway. Notably, siRNA knockdown of either β-arrestin1 or AP2B1 dramatically impaired IAV replication, and AP2B1 knockdown or treatment with Barbadin, an inhibitor targeting the β-arrestin1/AP2B1 complex, remarkably decreased the amount of internalized IAVs. Moreover, we found that FFAR2 interacted with three G protein-coupled receptor (GPCR) kinases (i.e., GRK2, GRK5, and GRK6) whose downregulation inhibited IAV replication. Together, our findings demonstrate that the FFAR2 signaling cascade is important for the efficient endocytosis of IAV into host cells.IMPORTANCE To complete its replication cycle, IAV hijacks the host endocytosis machinery to invade cells. However, the underlying mechanisms of how IAV is internalized into host cells remain poorly understood, emphasizing the need to elucidate the role of host factors in IAV entry into cells. In this study, we identified FFAR2 as an important host factor for the efficient replication of both low-pathogenic and highly pathogenic IAV. We revealed that FFAR2 facilitates the internalization of IAV into target cells during the early stage of infection. Upon further characterization of the role of FFAR2-associated proteins in virus replication, we found that the FFAR2-β-arrestin1-AP2B1 signaling cascade is important for the efficient endocytosis of IAV. Our findings thus further our understanding of the biological details of IAV entry into host cells and establish FFAR2 as a potential target for antiviral drug development.

Interplay between the Endogenous Opioid System and Proteasome Complex: Beyond Signaling

Intracellular signaling mechanisms underlying the opioid system regulation of nociception, neurotransmitters release, stress responses, depression, and the modulation of reward circuitry have been investigated from different points of view. The presence of the ubiquitin proteasome system (UPS) in the synaptic terminations suggest a potential role of ubiquitin-dependent mechanisms in the control of the membrane occupancy by G protein-coupled receptors (GPCRs), including those belonging to the opioid family. In this review, we focused our attention on the role played by the ubiquitination processes and by UPS in the modulation of opioid receptor signaling and in pathological conditions involving the endogenous opioid system. The collective evidence here reported highlights the potential usefulness of proteasome inhibitors in neuropathic pain, addictive behavior, and analgesia since these molecules can reduce pain behavioral signs, heroin self-administration, and the development of morphine analgesic tolerance. Moreover, the complex mechanisms involved in the effects induced by opioid agonists binding to their receptors include the ubiquitination process as a post-translational modification which plays a relevant role in receptor trafficking and degradation. Hence, UPS modulation may offer novel opportunities to control the balance between therapeutic versus adverse effects evoked by opioid receptor activation, thus, representing a promising druggable target.

Asymmetric Recruitment of β-Arrestin1/2 by the Angiotensin II Type I and Prostaglandin F2α Receptor Dimer

Initially identified as monomers, G protein-coupled receptors (GPCRs) can also form functional homo- and heterodimers that act as distinct signaling hubs for cellular signal integration. We previously found that the angiotensin II (Ang II) type 1 receptor (AT1R) and the prostaglandin F2α (PGF2α) receptor (FP), both important in the control of smooth muscle contractility, form such a functional heterodimeric complex in HEK 293 and vascular smooth muscle cells. Here, we hypothesize that both Ang II- and PGF2α-induced activation of the AT1R/FP dimer, or the parent receptors alone, differentially regulate signaling by distinct patterns of β-arrestin recruitment. Using BRET-based biosensors, we assessed the recruitment kinetics of β-arrestin1/2 to the AT1R/FP dimer, or the parent receptors alone, when stimulated by either Ang II or PGF2α. Using cell lines with CRISPR/Cas9-mediated gene deletion, we also examined the role of G proteins in such recruitment. We observed that Ang II induced a rapid, robust, and sustained recruitment of β-arrestin1/2 to AT1R and, to a lesser extent, the heterodimer, as expected, since AT1R is a strong recruiter of both β-arrestin subtypes. However, PGF2α did not induce such recruitment to FP alone, although it did when the AT1R is present as a heterodimer. β-arrestins were likely recruited to the AT1R partner of the dimer. Gα_q, Gα₁₁, Gα₁₂, and Gα₁₃ were all involved to some extent in PGF2α-induced β-arrestin1/2 recruitment to the dimer as their combined absence abrogated the response, and their separate re-expression was sufficient to partially restore it. Taken together, our data sheds light on a new mechanism whereby PGF2α specifically recruits and signals through β-arrestin but only in the context of the AT1R/FP dimer, suggesting that this may be a new allosteric signaling entity.