Heterodimerization of g protein-coupled receptors: specificity and functional significance

Olfactory receptors in neural regeneration in the central nervous system

Olfactory receptors are crucial for detecting odors and play a vital role in our sense of smell, influencing behaviors from food choices to emotional memories. These receptors also contribute to our perception of flavor and have potential applications in medical diagnostics and environmental monitoring. The ability of the olfactory system to regenerate its sensory neurons provides a unique model to study neural regeneration, a phenomenon largely absent in the central nervous system. Insights gained from how olfactory neurons continuously replace themselves and reestablish functional connections can provide strategies to promote similar regenerative processes in the central nervous system, where damage often results in permanent deficits. Understanding the molecular and cellular mechanisms underpinning olfactory neuron regeneration could pave the way for developing therapeutic approaches to treat spinal cord injuries and neurodegenerative diseases like Alzheimer's disease. Olfactory receptors are found in almost any cell of every organ/tissue of the mammalian body. This ectopic expression provides insights into the chemical structures that can activate olfactory receptors. In addition to odors, olfactory receptors in ectopic expression may respond to endogenous compounds and molecules produced by mucosal colonizing microbiota. The analysis of the function of olfactory receptors in ectopic expression provides valuable information on the signaling pathway engaged upon receptor activation and the receptor's role in proliferation and cell differentiation mechanisms. This review explores the ectopic expression of olfactory receptors and the role they may play in neural regeneration within the central nervous system, with particular attention to compounds that can activate these receptors to initiate regenerative processes. Evidence suggests that olfactory receptors could serve as potential therapeutic targets for enhancing neural repair and recovery following central nervous system injuries.

Mice mortality induced by dexmedetomidine is non-alpha2-adrenergic receptor-dependent

Dexmedetomidine (DMED) is widely used in sedative anesthetics in clinical settings. Currently, there are no therapeutic interventions available for the lethal toxicity induced by DMED. Administered at doses ranging from 25 to 100 mg/kg intraperitoneally (i.p.), DMED exhibited a dose-dependent fatality in mice. The 50 % lethal dose (LD50) was determined to be 48.5 mg/kg within 24 h and 44.8 mg/kg within 7 days. Alpha2-adrenergic receptor antagonists, namely yohimbine and atipamezole, demonstrated no mitigating effect on the lethal toxicity induced by DMED at the dose of 44.8 mg/kg. Conversely, the administration of atipamezole and yohimbine increased the mortality associated with DMED. By contrast, the imidazoline receptor antagonist idazoxan and the opioid receptor antagonist naloxone significantly attenuated the mortality induced by DMED, thereby prolonging the median survival time following administration of DMED at 44.8 mg/kg (i.p.). Furthermore, the immune modulator imiquimod, calcium sensitizer levosimendan, and TAAR1 antagonist RO5212773 decreased acute mortality without impacting chronic mortality induced by DMED. Histopathological analysis revealed characteristic lung alterations, including capillary hemorrhage, widened alveolar septa, fusion of pulmonary alveoli, and inflammatory cell infiltrate upon DMED exposure. Pretreatment with idazoxan or naloxone inhibited these pathological changes induced by DMED, while atipamezole or yohimbine pretreatment exacerbated lung damage. Elevated levels of serum creatine kinase and myoglobin were noted following DMED administration. However, pretreatment with idazoxan or naloxone decreased the rise of serum creatine kinase and myoglobin. Collectively, these results highlighted the involvement of imidazoline receptors and opioid receptor in DMED mortality.

Dabigatran Combined With Benztropine Ameliorates Cobalt Chloride-Induced Parkinsonism in Rats, Restores Protease-Activated Receptor 1 (PAR1), and Mitigates Oxidative Stress

BACKGROUND

The presumed implication of thromboembolic and oxidative stress pathways in parkinsonism guided the current research toward the exploration of the anticoagulant dabigatran etexilate (DE) as a thrombin inhibitor in the cobalt chloride (CoCl2)-induced parkinsonism (CIP) model, a model of significance to industrial toxins-related health issues.

METHODS

Oral CoCl2 (12.5 mg/kg) was administered daily for 60 days, with the introduction of benztropine mesylate (BM) (10 mg/kg) and/or DE (3 mg/kg) on day 31. Rearing, postural instability, and pasta handling were evaluated, followed by histopathologic examination of the substantia nigra (SN) and striatum (STR). The expressions of brain dopamine receptor 2 (D2 ), adenosine receptor 1 (A1) and 2A (A2A), and protease-activated receptor 1 (PAR1), as well as the brain levels of dopamine (DA), endothelin 1 (ET1), malondialdehyde (MDA), and glutathione (GSH), were assessed.

RESULTS

BM+DE restored the number of rears to the control level, compared to being reduced in the CIP model. BM+DE restored the first, second, third, and average displacement distances to the control level, compared to being reduced in the CIP model. BM+DE was superior to either BM or DE in restoring the time to finish eating pasta and the number of adjustments of forepaws while eating to control levels after being affected in the CIP model. BM+DE restored DA to the control level and was superior to DE in restoring D2  to the control level. BM+DE was superior to BM in restoring A1 and A2A , increasing A1/A2A beyond the control level. BM+DE was superior to BM in restoring PAR1 and ET1 to control levels. BM+DE was superior to BM in restoring MDA to the control level and was superior to both BM and DE in increasing GSH beyond the control level. BM+DE exhibited the highest percentage of preserved neurons in SN, which was negatively correlated with MDA.

CONCLUSION

BM+DE offers a therapeutic potential for parkinsonism triggered by chronic exposure to CoCl2. The implication of thrombin-related factors and oxidative stress in the modulation of the dopaminergic-adenosinergic crosstalk is plausible.

Protein-Protein Interactions of Odorant and G Protein-Coupled Receptors: A BRET Analysis Approach

Odorant receptors (ORs), the largest subfamily of G protein-coupled receptors, are pivotal not only for olfactory detection in the nose but also for various biological functions in extra-nasal tissues. The functional characterization of ORs presents significant challenges, especially due to the difficulty of achieving their surface expression in in vitro systems. Research indicates that interactions between ORs and other receptors, as well as OR multimerization, are crucial for their membrane localization and the modulation of signaling pathways. Utilizing the bioluminescence resonance energy transfer (BRET) assay, a technique for investigating protein-protein interactions, our study quantifies the interactions between ORs and various receptors. This approach provides novel insights into OR molecular dynamics, emphasizing the role of OR multimerization. By exploring into these complexities, our work aims to enhance the understanding of cellular signaling mechanisms and promote the development of innovative therapeutic strategies.

LPA1-mediated inhibition of CXCR4 attenuates CXCL12-induced signaling and cell migration

BACKGROUND

G protein-coupled receptor heteromerization is believed to exert dynamic regulatory impact on signal transduction. CXC chemokine receptor 4 (CXCR4) and its ligand CXCL12, both of which are overexpressed in many cancers, play a pivotal role in metastasis. Likewise, lysophosphatidic acid receptor 1 (LPA1) is implicated in cancer cell proliferation and migration. In our preliminary study, we identified LPA1 as a prospective CXCR4 interactor. In the present study, we investigated in detail the formation of the CXCR4-LPA1 heteromer and characterized the unique molecular features and function of this heteromer.

METHODS

We employed bimolecular fluorescence complementation, bioluminescence resonance energy transfer, and proximity ligation assays to demonstrate heteromerization between CXCR4 and LPA1. To elucidate the distinctive molecular characteristics and functional implications of the CXCR4-LPA1 heteromer, we performed various assays, including cAMP, BRET for G protein activation, β-arrestin recruitment, ligand binding, and transwell migration assays.

RESULTS

We observed that CXCR4 forms heteromers with LPA1 in recombinant HEK293A cells and the human breast cancer cell line MDA-MB-231. Coexpression of LPA1 with CXCR4 reduced CXCL12-mediated cAMP inhibition, ERK activation, Gαi/o activation, and β-arrestin recruitment, while CXCL12 binding to CXCR4 remained unaffected. In contrast, CXCR4 had no impact on LPA1-mediated signaling. The addition of lysophosphatidic acid (LPA) further hindered CXCL12-induced Gαi/o recruitment to CXCR4. LPA or alkyl-OMPT inhibited CXCL12-induced migration in various cancer cells that endogenously express both CXCR4 and LPA1. Conversely, CXCL12-induced calcium signaling and migration were increased in LPAR1 knockout cells, and LPA1-selective antagonists enhanced CXCL12-induced Gαi/o signaling and cell migration in the parental MDA-MB-231 cells but not in LPA1-deficient cells. Ultimately, complete inhibition of cell migration toward CXCL12 and alkyl-OMPT was only achieved in the presence of both CXCR4 and LPA1 antagonists.

CONCLUSIONS

The presence and impact of CXCR4-LPA1 heteromers on CXCL12-induced signaling and cell migration have been evidenced across various cell lines. This discovery provides crucial insights into a valuable regulatory mechanism of CXCR4 through heteromerization. Moreover, our findings propose a therapeutic potential in combined CXCR4 and LPA1 inhibitors for cancer and inflammatory diseases associated with these receptors, simultaneously raising concerns about the use of LPA1 antagonists alone for such conditions. Video Abstract.

Detecting and measuring of GPCR signaling - comparison of human induced pluripotent stem cells and immortal cell lines

G protein-coupled receptors (GPCRs) are a large family of transmembrane proteins that play a major role in many physiological processes, and thus GPCR-targeted drug development has been widely promoted. Although research findings generated in immortal cell lines have contributed to the advancement of the GPCR field, the homogenous genetic backgrounds, and the overexpression of GPCRs in these cell lines make it difficult to correlate the results with clinical patients. Human induced pluripotent stem cells (hiPSCs) have the potential to overcome these limitations, because they contain patient specific genetic information and can differentiate into numerous cell types. To detect GPCRs in hiPSCs, highly selective labeling and sensitive imaging techniques are required. This review summarizes existing resonance energy transfer and protein complementation assay technologies, as well as existing and new labeling methods. The difficulties of extending existing detection methods to hiPSCs are discussed, as well as the potential of hiPSCs to expand GPCR research towards personalized medicine.

The olfactory Olfr-78/51E2 receptor interacts with the adenosine A2A receptor. Effect of menthol and 1,8-cineole on A2A receptor-mediated signaling

Heteromer formation is unknown for the olfactory family of G protein-coupled receptors (GPCRs). We here identified, in a heterologous system, heteromers formed by the adenosine A_2A receptor (A_2AR), which is a target for neuroprotection, and an olfactory receptor. A_2AR interacts with the receptor family 51, subfamily E, member 2 (OR51E2), the human ortholog of the mouse Olfr-78, whose mRNA is differentially expressed in activated microglia treated with adenosine receptor ligands. Bioluminescence resonance energy transfer (BRET) assays were performed in HEK-293T cells expressing the human version of the receptors, OR51E2 and A_2AR, fused, respectively, to Renilla luciferase (RLuc) and the yellow fluorescent protein (YFP). BRET data was consistent with a receptor-receptor interaction whose consequences at the functional level were measured by cAMP level determination in CHO cells. Results showed an olfactory receptor-mediated partial blockade of G_s coupling to the A_2AR, i.e., the effect of the A_2AR selective agonist on intracellular levels of cAMP was significantly reduced. Two odorants, menthol and 1,8-cineole, which failed to show G_olf-mediated OR51E2 activation because they did not increase cytosolic cAMP levels, reduced the BRET readings in cells expressing A_2AR-YFP and OR51E2-Rluc, most likely suggesting a conformational change of at least one receptor. These odorants led to an almost complete block of A_2AR coupling to G_s.

Phosphatidylserine exposure modulates adhesion GPCR BAI1 (ADGRB1) signaling activity

Brain-specific angiogenesis inhibitor 1 (BAI1; also called ADGRB1 or B1) is an adhesion G protein-coupled receptor known from studies on macrophages to bind to phosphatidylserine (PS) on apoptotic cells via its N-terminal thrombospondin repeats. A separate body of work has shown that B1 regulates postsynaptic function and dendritic spine morphology via signaling pathways involving Rac and Rho. However, it is unknown if PS binding by B1 has any effect on the receptor's signaling activity. To shed light on this subject, we studied G protein-dependent signaling by B1 in the absence and presence of coexpression with the PS flippase ATP11A in human embryonic kidney 293T cells. ATP11A expression reduced the amount of PS exposed extracellularly and also strikingly reduced the signaling activity of coexpressed full-length B1 but not a truncated version of the receptor lacking the thrombospondin repeats. Further experiments with an inactive mutant of ATP11A showed that the PS flippase function of ATP11A was required for modulation of B1 signaling. In coimmunoprecipitation experiments, we made the surprising finding that ATP11A not only modulates B1 signaling but also forms complexes with B1. Parallel studies in which PS in the outer leaflet was reduced by an independent method, deletion of the gene encoding the endogenous lipid scramblase anoctamin 6 (ANO6), revealed that this manipulation also markedly reduced B1 signaling. These findings demonstrate that B1 signaling is modulated by PS exposure and suggest a model in which B1 serves as a PS sensor at synapses and in other cellular contexts.

VEGF receptor heterodimers and homodimers are differentially expressed in neuronal and endothelial cell types

PURPOSE

We have previously reported that VEGF-B is more potent than VEGF-A in mediating corneal nerve growth in vitro and in vivo, and this stimulation of nerve growth appears to be different from stimulation of angiogenesis by these same ligands, at least in part due to differences in VEGF receptor activation. VEGF signaling may be modulated by a number of factors including receptor number or the formation of receptor hetero- vs. homodimers. In endothelial cells, VEGF receptor heterodimer (VEGR1/R2) activation after ligand binding and subsequent phosphorylation alters the activation of downstream signaling cascades. However, our understanding of these processes in neuronal cell types remains unclear. The purpose of this study was to identify the presence and distribution of VEGF Receptor-Ligand interactions in neuronal cells as compared to endothelial cells.

METHODS

PC12 (rat neuronal cell line), MAEC (mouse aortic endothelial cell line), MVEC (mouse venous endothelial cell line) and HUVEC (human umbilical venous endothelial cell line; control group) were used. Cells were acutely stimulated either with VEGF-A (50 ng/μL) or VEGF-B (50 ng/μL) or "vehicle" (PBS; control group). We also isolated mouse trigeminal ganglion cells from thy1-YFP neurofluorescent mice. After treatment, cells were used as follows: (i) One group was fixed in 4% paraformaldehyde and processed for VEGFR1 and VEGFR2 immunostaining and visualized using confocal fluorescence microscopy and Total Internal Reflection (TIRF) microscopy; (ii) the second group was harvested in cell lysis buffer (containing anti-protease / anti-phosphatase cocktail), lysed and processed for immunoprecipitation (IP; Thermo Fisher IP kit) and immunoblotting (IB; LI-COR® Systems). Immunoprecipitated proteins were probed either with anti-VEGFR1 or anti-VEGFR2 IgG antibodies to evaluate VEGFR1-R2-heterodimerization; (iii) a third group of cells was also processed for Duolink Proximity Ligation Assay (PLA; Sigma) to assess the presence and distribution of VEGF-receptor homo- and heterodimers in neuronal and endothelial cells.

RESULTS

TIRF and fluorescence confocal microscopy revealed the presence of VEGFR1 co-localized with VEGFR2 in endothelial and PC12 neuronal cells. Cell lysates immunoprecipitated with anti-VEGFR1 further validated the existence of VEGFR1-R2 heterodimers in PC12 neuronal cells. Neuronal cells showed higher levels of VEGFR1-R2 heterodimers as compared to endothelial cells whereas endothelial cells showed higher VEGFR2-R2 homodimers compared to neuronal cells as demonstrated by Duolink PLA. Levels of VEGFR1-R1 homodimers were very low in neuronal and endothelial cells.

CONCLUSIONS

Differences in VEGF Receptor homo- and heterodimer distribution may explain the differential role of VEGF ligands in neuronal versus endothelial cell types. This may in turn influence VEGF activity and regulation of neuronal cell homeostasis.

Synthesis of a donor-acceptor heterodimer via trifunctional completive self-sorting

Selective self-assembly of heterodimers consisting of two non-identical subunits plays important roles in Nature but is rarely encountered in synthetic supramolecular systems. Here we show that photocleavage of a donor–acceptor porphyrin complex produces an heterodimeric structure with surprising selectivity. The system forms via a multi-step sequence that starts with an oxidative ring opening, which produces an equimolar mixture of two isomeric degradation products (zinc(II) bilatrien-abc-ones, BTOs). These two isomers are susceptible to water addition, yielding the corresponding zinc(II) 15-hydroxybiladien-ab-ones (HBDOs). However, in the photocleavage experiment only one HBDO isomer is formed, and it quantitatively combines with the remaining BTO isomer. The resulting heterodimer is stabilized by a Zn–O coordination bond and extended dispersion interactions between the overlapping π-surfaces of the monomers. The observed selectivity can be seen as a case of completive self-sorting, simultaneously controlled by three types of complementary interactions.

The preparation of heterodimeric structures via self-assembly processes is challenging. Here, the authors report the photooxidation of a donor–acceptor metalloporphyrin, which enables a self-sorting process that yields an heterodimer quantitatively.

Dysfunctional Heteroreceptor Complexes as Novel Targets for the Treatment of Major Depressive and Anxiety Disorders

Among mental diseases, major depressive disorder (MDD) and anxiety deserve a special place due to their high prevalence and their negative impact both on society and patients suffering from these disorders. Consequently, the development of novel strategies designed to treat them quickly and efficiently, without or at least having limited side effects, is considered a highly important goal. Growing evidence indicates that emerging properties are developed on recognition, trafficking, and signaling of G-protein coupled receptors (GPCRs) upon their heteromerization with other types of GPCRs, receptor tyrosine kinases, and ionotropic receptors such as N-methyl-D-aspartate (NMDA) receptors. Therefore, to develop new treatments for MDD and anxiety, it will be important to identify the most vulnerable heteroreceptor complexes involved in MDD and anxiety. This review focuses on how GPCRs, especially serotonin, dopamine, galanin, and opioid heteroreceptor complexes, modulate synaptic and volume transmission in the limbic networks of the brain. We attempt to provide information showing how these emerging concepts can contribute to finding new ways to treat both MDD and anxiety disorders.

Metabotropic Glutamate Receptor 5 Antagonism Reduces Pathology and Differentially Improves Symptoms in Male and Female Heterozygous zQ175 Huntington’s Mice

Huntington’s disease (HD) is an inherited autosomal dominant neurodegenerative disorder that leads to progressive motor and cognitive impairment. There are currently no available disease modifying treatments for HD patients. We have previously shown that pharmacological blockade of metabotropic glutamate receptor 5 (mGluR5) signaling rescues motor deficits, improves cognitive impairments and mitigates HD neuropathology in male zQ175 HD mice. Mounting evidence indicates that sex may influence HD progression and we have recently reported a sex-specific pathological mGluR5 signaling in Alzheimer’s disease (AD) mice. Here, we compared the outcomes of treatment with the mGluR5 negative allosteric modulator CTEP (2-chloro-4-[2-[2,5-dimethyl-1-[4-(trifluoromethoxy)phenyl]imidazol-4-yl]ethynyl]pyridine) in both male and female symptomatic zQ175 mice. We found that female zQ175 mice required a longer treatment duration with CTEP than male mice to show improvement in their rotarod performance. Unlike males, chronic CTEP treatment did not improve the grip strength nor reverse the cognitive decline of female zQ175 mice. However, CTEP reduced the number of huntingtin aggregates, improved neuronal survival and decreased microglia activation in the striatum of both male and female zQ175 mice. Together, our results indicate that mGluR5 antagonism can reduce HD neuropathology in both male and female zQ175 HD mice, but sex has a clear impact on the efficacy of the treatment and must be taken into consideration for future HD drug development.

Identification of small molecule allosteric modulators that act as enhancers/disrupters of rhodopsin oligomerization

The elongated cilia of the outer segment of rod and cone photoreceptor cells can contain concentrations of visual pigments of up to 5 mM. The rod visual pigments, G protein–coupled receptors called rhodopsins, have a propensity to self-aggregate, a property conserved among many G protein–coupled receptors. However, the effect of rhodopsin oligomerization on G protein signaling in native cells is less clear. Here, we address this gap in knowledge by studying rod phototransduction. As the rod outer segment is known to adjust its size proportionally to overexpression or reduction of rhodopsin expression, genetic perturbation of rhodopsin cannot be used to resolve this question. Therefore, we turned to high-throughput screening of a diverse library of 50,000 small molecules and used a novel assay for the detection of rhodopsin dimerization. This screen identified nine small molecules that either disrupted or enhanced rhodopsin dimer contacts in vitro. In a subsequent cell-free binding study, we found that all nine compounds decreased intrinsic fluorescence without affecting the overall UV-visible spectrum of rhodopsin, supporting their actions as allosteric modulators. Furthermore, ex vivo electrophysiological recordings revealed that a disruptive, hit compound #7 significantly slowed down the light response kinetics of intact rods, whereas compound #1, an enhancing hit candidate, did not substantially affect the photoresponse kinetics but did cause a significant reduction in light sensitivity. This study provides a monitoring tool for future investigation of the rhodopsin signaling cascade and reports the discovery of new allosteric modulators of rhodopsin dimerization that can also alter rod photoreceptor physiology.

Functional Selectivity of Dopamine D1 Receptor Signaling: Retrospect and Prospect

Research progress on dopamine D₁ receptors indicates that signaling no longer is limited to G protein-dependent cyclic adenosine monophosphate phosphorylation but also includes G protein-independent β-arrestin-related mitogen-activated protein kinase activation, regulation of ion channels, phospholipase C activation, and possibly more. This review summarizes recent studies revealing the complexity of D₁ signaling and its clinical implications, and suggests functional selectivity as a promising strategy for drug discovery to magnify the merit of D₁ signaling. Functional selectivity/biased receptor signaling has become a major research front because of its potential to improve therapeutics through precise targeting. Retrospective pharmacological review indicated that many D₁ ligands have some degree of mild functional selectivity, and novel compounds with extreme bias at D₁ signaling were reported recently. Behavioral and neurophysiological studies inspired new methods to investigate functional selectivity and gave insight into the biased signaling of several drugs. Results from recent clinical trials also supported D₁ functional selectivity signaling as a promising strategy for discovery and development of better therapeutics.

The evidence for and consequences of metabotropic glutamate receptor heterodimerization

Metabotropic glutamate receptors (mGluRs) are an essential component of the mammalian central nervous system. These receptors modulate neuronal excitability in response to extracellular glutamate through the activation of intracellular heterotrimeric G proteins. Like most other class C G protein-coupled receptors, mGluRs function as obligate dimer proteins, meaning they need to form dimer complexes before becoming functional receptors. All mGluRs possess the ability to homodimerize, but studies over the past ten years have demonstrated these receptors are also capable of forming heterodimers in specific patterns. These mGluR heterodimers appear to have their own unique biophysical behavior and pharmacology with both native and synthetic compounds with few rules having been identified that allow for prediction of the consequences of any particular mGluR pair forming heterodimers. Here, we review the relevant literature demonstrating the existence and consequences of mGluR heterodimerization. By collecting biophysical and pharmacological data of several mGluR heterodimers we demonstrate the lack of generalizable behavior of these complexes indicating that each individual dimeric pair needs to be investigated independently. Additionally, by combining sequence alignment and structural analysis, we propose that interactions between the β4-A Helix Loop and the D Helix in the extracellular domain of these receptors are the structural components that dictate heterodimerization compatibility. Finally, we discuss the potential implications of mGluR heterodimerization from the viewpoints of further developing our understanding of neuronal physiology and leveraging mGluRs as a therapeutic target for the treatment of pathophysiology.

Modeling the Heterodimer Interfaces of Melatonin Receptors

Melatonin receptors are Class A G protein-coupled receptors (GPCRs) that regulate a plethora of physiological activities in response to the rhythmic secretion of melatonin from the pineal gland. Melatonin is a key regulator in the control of circadian rhythm and has multiple functional roles in retinal physiology, memory, immunomodulation and tumorigenesis. The two subtypes of human melatonin receptors, termed MT₁ and MT₂, utilize overlapping signaling pathways although biased signaling properties have been reported in some cellular systems. With the emerging concept of GPCR dimerization, melatonin receptor heterodimers have been proposed to participate in system-biased signaling. Here, we used computational approaches to map the dimerization interfaces of known heterodimers of melatonin receptors, including MT₁/MT₂, MT₁/GPR50, MT₂/GPR50, and MT₂/5-HT_2C. By homology modeling and membrane protein docking analyses, we have identified putative preferred interface interactions within the different pairs of melatonin receptor dimers and provided plausible structural explanations for some of the unique pharmacological features of specific heterodimers previously reported. A thorough understanding of the molecular basis of melatonin receptor heterodimers may enable the development of new therapeutic approaches against aliments involving these heterodimeric receptors.

The macrophage odorant receptor Olfr78 mediates the lactate-induced M2 phenotype of tumor-associated macrophages

Expression and function of odorant receptors (ORs), which account for more than 50% of G protein-coupled receptors, are being increasingly reported in nonolfactory sites. However, ORs that can be targeted by drugs to treat diseases remain poorly identified. Tumor-derived lactate plays a crucial role in multiple signaling pathways leading to generation of tumor-associated macrophages (TAMs). In this study, we hypothesized that the macrophage OR Olfr78 functions as a lactate sensor and shapes the macrophage-tumor axis. Using Olfr78 ^+/+ and Olfr78 ^-/- bone marrow-derived macrophages with or without exogenous Olfr78 expression, we demonstrated that Olfr78 sensed tumor-derived lactate, which was the main factor in tumor-conditioned media responsible for generation of protumoral M2-TAMs. Olfr78 functioned together with Gpr132 to mediate lactate-induced generation of protumoral M2-TAMs. In addition, syngeneic Olfr78-deficient mice exhibited reduced tumor progression and metastasis together with an increased anti- versus protumoral immune cell population. We propose that the Olfr78-lactate interaction is a therapeutic target to reduce and prevent tumor progression and metastasis.

Synergistic activation of thrombin and angiotensin II receptors revealed by bioluminescence resonance energy transfer

We recently reported a physical interaction between the angiotensin II (AngII) receptor (AT1R) and thrombin receptor (PAR1) in HEK293 cells using bioluminescence resonance energy transfer (BRET) technology. This was characterized by thrombin trans-activating AT1R and the synergistic responses of the AT1R-PAR1 complex. Here, we investigated the other face of the coin by examining the effect of AT1R on PAR1 activity using BRET. AngII/AT1R did not promote PAR1 activation in the absence of thrombin. However, the combination of thrombin and AngII resulted in their synergistic/allosteric action. Moreover, AngII/AT1R potentiated the maximal thrombin responses, suggesting specific conformational changes within the AT1R-PAR1 complex. Overall, our data confirm the functional AT1R-PAR1 interplay and further support the implication of both AT1R and PAR1 protomers in their synergistic interaction as previously reported.

Apelin receptor homodimer inhibits apoptosis in vascular dementia

Apelin receptor (APJ), a member of family A of the G protein-coupled receptors (GPCRs), is a potential pharmaceutical target for diseases of the nervous system. Our previous work revealed that human APJ can form a homodimer that has different functional characteristics than the monomer. To investigate the effects of APJ homodimers on neuroprotection in vascular dementia (VD), we established VD model in rats and treated the animals by injecting apelin-13 into the lateral ventricle. In addition, we established an oxygen-glucose deprivation/reoxygenation (OGD/R) model in SH-SY5Y cells treated with apelin-13. After apelin-13 stimulation in the VD rat, the level of APJ and APJ homodimer were elevated. Furthermore, APJ homodimer decreased the level of cleaved caspase-3 and cleaved caspase-9 via the Gα_i3 and Gα_q signaling pathway, thereby increasing the number of neurons and inhibiting apoptosis. Consequently, APJ homodimers may serve as a unique mechanism for neuroprotection against VD and provide new pharmaceutical targets for VD.

G protein-coupled purinergic P2Y receptor oligomerization: Pharmacological changes and dynamic regulation

P2Y receptors (P2YRs) are a δ group of rhodopsin-like G protein-coupled receptors (GPCRs) with many essential functions in physiology and pathology, such as platelet aggregation, immune responses, neuroprotective effects, inflammation, and cellular proliferation. Thus, they are among the most researched therapeutic targets used for the clinical treatment of diseases (e.g., the antithrombotic drug clopidogrel and the dry eye treatment drug diquafosol). GPCRs transmit signals as dimers to increase the diversity of signalling pathways and pharmacological activities. Many studies have frequently confirmed dimerization between P2YRs and other GPCRs due to their functions in cardiovascular and cerebrovascular processes in vivo and in vitro. Recently, some P2YR dimers that dynamically balance physiological functions in the body were shown to be involved in effective signal transduction and exert pathological responses. In this review, we summarize the types, pharmacological changes, and active regulators of P2YR-related dimerization, and delineate new functions and pharmacological activities of P2YR-related dimers, which may be a novel direction to improve the effectiveness of medications.

Multiple GPCR Functional Assays Based on Resonance Energy Transfer Sensors

G protein-coupled receptors (GPCRs) represent one of the largest membrane protein families that participate in various physiological and pathological activities. Accumulating structural evidences have revealed how GPCR activation induces conformational changes to accommodate the downstream G protein or β-arrestin. Multiple GPCR functional assays have been developed based on Förster resonance energy transfer (FRET) and bioluminescence resonance energy transfer (BRET) sensors to monitor the conformational changes in GPCRs, GPCR/G proteins, or GPCR/β-arrestin, especially over the past two decades. Here, we will summarize how these sensors have been optimized to increase the sensitivity and compatibility for application in different GPCR classes using various labeling strategies, meanwhile provide multiple solutions in functional assays for high-throughput drug screening.

Hedgehog signaling and Tre1 regulate actin dynamics through PI(4,5)P2 to direct migration of Drosophila embryonic germ cells

The Tre1 G-protein coupled receptor (GPCR) was discovered to be required for Drosophila germ cell (GC) coalescence almost two decades ago, yet the molecular events both upstream and downstream of Tre1 activation remain poorly understood. To gain insight into these events, we describe a bona fide null allele and both untagged and tagged versions of Tre1. We find that the primary defect with complete Tre1 loss is the failure of GCs to properly navigate, with GC mis-migration occurring from early stages. We find that Tre1 localizes with F-actin at the migration front, along with PI(4,5)P₂; dPIP5K, an enzyme that generates PI(4,5)P₂; and dWIP, a protein that binds activated Wiskott-Aldrich syndrome protein (WASP), which stimulates F-actin polymerization. We show that Tre1 is required for polarized accumulation of F-actin, PI(4,5)P₂, and dPIP5K. Smoothened also localizes with F-actin at the migration front, and Hh, through Smo, increases levels of Tre1 at the plasma membrane and Tre1’s association with dPIP5K.

Kim et al. uncover molecular and cellular events upstream and downstream of the Tre1 G-protein coupled receptor (GPCR), which is required for germ cell navigation in Drosophila. Hedgehog signaling through Smoothened localizes Tre1 to activate F-actin assembly through dPIP5K, PI(4,5)P₂, and WASP.

Interplay Between Angiotensin II Type 1 Receptor and Thrombin Receptor Revealed by Bioluminescence Resonance Energy Transfer Assay

The key hormone of the renin-angiotensin system (RAS), angiotensin II (AngII), and thrombin are known to play major roles in the vascular system and its related disorders. Previous studies reported connections between AngII and thrombin in both physiological and pathophysiological models. However, the molecular mechanisms controlling such interplay at the level of their receptors belonging to the family of G protein-coupled receptors (GPCRs) are not fully understood. In this study, we investigated the functional interaction between the AngII type 1 receptor (AT1R) and the thrombin receptor [or protease-activated receptor 1 (PAR1)] in human embryonic kidney 293 (HEK293) cells. For this, we used various bioluminescence resonance energy transfer (BRET) proximity-based assays to profile the coupling to the heterotrimeric Gαq protein, β-arrestin recruitment, and receptor internalization and trafficking in intact cells. The overall dose-response and real-time kinetic BRET data demonstrated the specific molecular proximity between AT1R and PAR1 resulting in their functional interaction. This was characterized by thrombin inducing BRET increase within AT1R/Gαq and AT1R/β-arrestin pairs and synergistic effects observed upon the concomitant activation of both receptors suggesting a positive allosteric interaction. The BRET data corroborated with the data on the downstream Gαq/inositol phosphate pathway. Moreover, the selective pharmacological blockade of the receptors revealed the implication of both AT1R and PAR1 protomers in such a synergistic interaction and the possible transactivation of AT1R by PAR1. Interestingly, the positive action of PAR1 on AT1R activation was contrasted with its apparent inhibition of AT1R internalization and its endosomal trafficking. Finally, BRET saturation and co-immunoprecipitation assays supported the physical AT1-PAR1 interaction in HEK293 cells. Our study reveals for the first time the functional interaction between AT1R and PAR1 in vitro characterized by a transactivation and positive allosteric modulation of AT1R and inhibition of its desensitization and internalization. This finding may constitute the molecular basis of the well-known interplay between RAS and thrombin. Thus, our data should lead to revising some findings on the implication of RAS and thrombin in vascular physiology and pathophysiology revealing the importance to consider the functional and pharmacological interaction between AT1R and thrombin receptors.

The role of endothelin B receptor in bone modelling during orthodontic tooth movement: a study on ETB knockout rats

The endothelin system has an important role in bone modelling during orthodontic tooth movement (OTM); however, little is known about the involvement of endothelin B receptors (ET_B) in this process. The aim of this study was to evaluate the role of ET_B in bone modelling during OTM using ET_B knockout rats (ET_B-KO). Thirty-two male rats were divided into 4 groups (n = 8 per group): the ET_B-KO appliance group, ET_B-KO control group, wild type (ET_B-WT) appliance group, and ET_B-WT control group. The appliance consisted of a super-elastic closed-coil spring placed between the first and second left maxillary molar and the incisors. Tooth movement was measured on days 0 and 35, and maxillary alveolar bone volume, osteoblast, and osteoclast volume were determined histomorphometrically on day 35 of OTM. Next, we determined the serum endothelin 1 (ET-1) level and gene expression levels of the osteoclast activity marker cathepsin K and osteoblast activity markers osteocalcin and dentin matrix acidic phosphoprotein 1 (DMP1) on day 35. The ET_B-KO appliance group showed significantly lower osteoblast activity, diminished alveolar bone volume and less OTM than the ET_B-WT appliance group. Our results showed that ET_B is involved in bone modelling in the late stage of OTM.

A biophysical perspective on receptor-mediated virus entry with a focus on HIV

As part of their entry and infection strategy, viruses interact with specific receptor molecules expressed on the surface of target cells. The efficiency and kinetics of the virus-receptor interactions required for a virus to productively infect a cell is determined by the biophysical properties of the receptors, which are in turn influenced by the receptors' plasma membrane (PM) environments. Currently, little is known about the biophysical properties of these receptor molecules or their engagement during virus binding and entry. Here we review virus-receptor interactions focusing on the human immunodeficiency virus type 1 (HIV), the etiological agent of acquired immunodeficiency syndrome (AIDS), as a model system. HIV is one of the best characterised enveloped viruses, with the identity, roles and structure of the key molecules required for infection well established. We review current knowledge of receptor-mediated HIV entry, addressing the properties of the HIV cell-surface receptors, the techniques used to measure these properties, and the macromolecular interactions and events required for virus entry. We discuss some of the key biophysical principles underlying receptor-mediated virus entry and attempt to interpret the available data in the context of biophysical mechanisms. We also highlight crucial outstanding questions and consider how new tools might be applied to advance understanding of the biophysical properties of viral receptors and the dynamic events leading to virus entry.

Exceptional diversity of opsin expression patterns in Neogonodactylus oerstedii (Stomatopoda) retinas

Stomatopod crustaceans possess some of the most complex animal visual systems, including at least 16 spectrally distinct types of photoreceptive units (e.g., assemblages of photoreceptor cells). Here we fully characterize the set of opsin genes expressed in retinal tissues and determine expression patterns of each in the stomatopod Neogonodactylus oerstedii Using a combination of transcriptome and RACE sequencing, we identified 33 opsin transcripts expressed in each N. oerstedii eye, which are predicted to form 20 long-wavelength-sensitive, 10 middle-wavelength-sensitive, and three UV-sensitive visual pigments. Observed expression patterns of these 33 transcripts were highly unusual in five respects: 1) All long-wavelength and short/middle-wavelength photoreceptive units expressed multiple opsins, while UV photoreceptor cells expressed single opsins; 2) most of the long-wavelength photoreceptive units expressed at least one middle-wavelength-sensitive opsin transcript; 3) the photoreceptors involved in spatial, motion, and polarization vision expressed more transcripts than those involved in color vision; 4) there is a unique opsin transcript that is expressed in all eight of the photoreceptive units devoted to color vision; and 5) expression patterns in the peripheral hemispheres of the eyes suggest visual specializations not previously recognized in stomatopods. Elucidating the expression patterns of all opsin transcripts expressed in the N. oerstedii retina reveals the potential for previously undocumented functional diversity in the already complex stomatopod eye and is a first step toward understanding the functional significance of the unusual abundance of opsins found in many arthropod species' visual systems.

The Complex Signaling Pathways of the Ghrelin Receptor

The ghrelin receptor (GhrR) is known for its strong orexigenic effects in pharmacological doses and has long been considered as a promising target for the treatment of obesity. Several antagonists have been developed to decrease the orexigenic signaling, but none of these have been approved for the treatment of obesity because of adverse effects and lack of efficacy. Heterodimerization and biased signaling are important concepts for G-protein coupled receptor (GPCR) signaling, and the influence of these aspects on the GhrR may be important for feeding behavior and obesity. GhrR has been described to heterodimerize with other GPCRs, such as the dopamine receptors 1 and 2, leading to a modulation of the signaling properties of both dimerization partners. Another complicating factor of GhrR-mediated signaling is its ability to activate several different signaling pathways on ligand stimulation. Importantly, some ligands have shown to be "biased" or "functionally selective," implying that the ligand favors a particular signaling pathway. These unique signaling properties could have a sizeable impact on the physiological functions of the GhrR system. Importantly, heterodimerization may explain why the GhrR is expressed in areas of the brain that are difficult for peptide ligands to access. One possibility is that the purpose of GhrR expression is to modulate the function of other receptors in addition to merely being independently activated. We suggest that a deeper understanding of the signaling properties of the GhrR will facilitate future drug discovery in the areas of obesity and weight management.

Adenosine Receptor A1-A2a Heteromers Regulate EAAT2 Expression and Glutamate Uptake via YY1-Induced Repression of PPARγ Transcription

Adenosine receptors A1 (A1AR) and A2a (A2aAR) play an important role in regulating glutamate uptake to avoid glutamate accumulation that causes excitotoxicity in the brain; however, the precise mechanism of the effects of A1AR and A2aAR is unclear. Herein, we report that expression of the A1AR protein in the astrocyte membrane and the level of intracellular glutamate were decreased, while expression of the A2aR protein was elevated in cells exposed to oxygen-glucose deprivation (OGD) conditions. Coimmunoprecipitation (Co-IP) experiments showed that A1AR interacts with A2aAR under OGD conditions. The activation of A1AR and inactivation of A2aAR by 2-chloro-N6-cyclopentyladenosine (CCPA) and SCH58251, respectively, partly reversed OGD-mediated glutamate uptake dysfunction, elevated EAAT2, and PPARγ protein levels, and suppressed the expression of Ying Yang 1 (YY1). Both the silencing of YY1 and the activation of PPARγ upregulated EAAT2 expression. Moreover, YY1 silencing elevated the PPARγ level under both normal and OGD conditions. Histone deacetylase (HDAC)1 was found to interact with YY1, and HDAC1 silencing improved PPARγ promoter activity. Taken together, our findings suggest that A1AR-A2aAR heteromers regulate EAAT2 expression and glutamate uptake through the YY1-mediated recruitment of HDAC1 to the PPARγ promoter region.

Modulation of P2Y11-related purinergic signaling in inflammation and cardio-metabolic diseases

Chronic inflammation is the hallmark of cardiovascular pathologies with a major role in both disease progression and occurrence of long-term complications. The massive release of ATP during the inflammatory process activates various purinergic receptors, including P2Y11. This receptor is less studied but ubiquitously expressed in all cells relevant for cardiovascular pathology: cardiomyocytes, fibroblasts, endothelial and immune cells. While several studies suggested a potential pro-inflammatory role for P2Y11 receptors, recent literature data are supportive of an anti-inflammatory profile characterized by the immunosuppression of dendritic cells, inhibition of fibroblast proliferation and of cytokines and ATP secretion. Moreover, modulation of its activity appears to mediate the positive inotropic effect of ATP and mitigate endothelial dysfunction, thus rendering this receptor a promising therapeutic target in the cardiovascular disease armamentarium. The aim of the present review is to summarize the current available knowledge on P2Y11-related purinergic signaling in the setting of inflammation and cardio-metabolic diseases.

Melatonin and the control of intraocular pressure

Melatonin is not only synthesized by the pineal gland but by several ocular structures. This natural indoleamine is of great importance for regulating several eye processes, among which pressure homeostasis is included. Glaucoma, the most prevalent eye disease, also known as the silent thief of vision, is a multifactorial pathology that is associated to age and, often, to intraocular hypertension (IOP). Indeed IOP is the only modifiable risk factor and as such medications are available to control it; however, novel medications are sought to minimize undesirable side effects. Melatonin and analogues decrease IOP in both normotensive and hypertensive eyes. Melatonin activates its cognate membrane receptors, MT₁ and MT₂, which are present in numerous ocular tissues, including the aqueous-humor-producing ciliary processes. Melatonin receptors belong to the superfamily of G-protein-coupled receptors and their activation would lead to different signalling pathways depending on the tissue. This review describes the molecular mechanisms underlying differential functionalities that are attributed to melatonin receptors. Accordingly, the current work highlights the important role of melatonin and its analogues in the healthy and in the glaucomatous eyes, with special attention to the control of intraocular pressure.

Extracellular Nucleotides and P2 Receptors in Renal Function

The understanding of the nucleotide/P2 receptor system in the regulation of renal hemodynamics and transport function has grown exponentially over the last 20 yr. This review attempts to integrate the available data while also identifying areas of missing information. First, the determinants of nucleotide concentrations in the interstitial and tubular fluids of the kidney are described, including mechanisms of cellular release of nucleotides and their extracellular breakdown. Then the renal cell membrane expression of P2X and P2Y receptors is discussed in the context of their effects on renal vascular and tubular functions. Attention is paid to effects on the cortical vasculature and intraglomerular structures, autoregulation of renal blood flow, tubuloglomerular feedback, and the control of medullary blood flow. The role of the nucleotide/P2 receptor system in the autocrine/paracrine regulation of sodium and fluid transport in the tubular and collecting duct system is outlined together with its role in integrative sodium and fluid homeostasis and blood pressure control. The final section summarizes the rapidly growing evidence indicating a prominent role of the extracellular nucleotide/P2 receptor system in the pathophysiology of the kidney and aims to identify potential therapeutic opportunities, including hypertension, lithium-induced nephropathy, polycystic kidney disease, and kidney inflammation. We are only beginning to unravel the distinct physiological and pathophysiological influences of the extracellular nucleotide/P2 receptor system and the associated therapeutic perspectives.

Heterodimerization of Mu Opioid Receptor Protomer with Dopamine D2 Receptor Modulates Agonist-Induced Internalization of Mu Opioid Receptor

The interplay between the dopamine (DA) and opioid systems in the brain is known to modulate the additive effects of substances of abuse. On one hand, opioids serve mankind by their analgesic properties, which are mediated via the mu opioid receptor (MOR), a Class A G protein-coupled receptor (GPCR), but on the other hand, they pose a potential threat by causing undesired side effects such as tolerance and dependence, for which the exact molecular mechanism is still unknown. Using human embryonic kidney 293T (HEK 293T) and HeLa cells transfected with MOR and the dopamine D₂ receptor (D₂R), we demonstrate that these receptors heterodimerize, using an array of biochemical and biophysical techniques such as coimmunoprecipitation (co-IP), bioluminescence resonance energy transfer (BRET¹), Fӧrster resonance energy transfer (FRET), and functional complementation of a split luciferase. Furthermore, live cell imaging revealed that D_2LR, when coexpressed with MOR, slowed down internalization of MOR, following activation with the MOR agonist [D-Ala2, N-MePhe4, Gly-ol]-enkephalin (DAMGO).

Calorie restriction activates new adult born olfactory-bulb neurones in a ghrelin-dependent manner but acyl-ghrelin does not enhance subventricular zone neurogenesis

The ageing and degenerating brain show deficits in neural stem/progenitor cell (NSPC) plasticity that are accompanied by impairments in olfactory discrimination. Emerging evidence suggests that the gut hormone ghrelin plays an important role in protecting neurones, promoting synaptic plasticity and increasing hippocampal neurogenesis in the adult brain. In the present study, we investigated the role of ghrelin with respect to modulating adult subventricular zone (SVZ) NSPCs that give rise to new olfactory bulb (OB) neurones. We characterised the expression of the ghrelin receptor, growth hormone secretagogue receptor (GHSR), using an immunohistochemical approach in GHSR-eGFP reporter mice to show that GHSR is expressed in several regions, including the OB but not in the SVZ of the lateral ventricle. These data suggest that acyl-ghrelin does not mediate a direct effect on NSPC in the SVZ. Consistent with these findings, treatment with acyl-ghrelin or genetic silencing of GHSR did not alter NSPC proliferation within the SVZ. Similarly, using a bromodeoxyuridine pulse-chase approach, we show that peripheral treatment of adult rats with acyl-ghrelin did not increase the number of new adult-born neurones in the granule cell layer of the OB. These data demonstrate that acyl-ghrelin does not increase adult OB neurogenesis. Finally, we investigated whether elevating ghrelin indirectly, via calorie restriction (CR), regulated the activity of new adult-born cells in the OB. Overnight CR induced c-Fos expression in new adult-born OB cells but not in developmentally born cells, whereas neuronal activity was absent following re-feeding. These effects were not present in ghrelin^-/- mice, suggesting that adult-born cells are uniquely sensitive to changes in ghrelin mediated by fasting and re-feeding. In summary, ghrelin does not promote neurogenesis in the SVZ and OB; however, new adult-born OB cells are activated by CR in a ghrelin-dependent manner.

Advantages and shortcomings of cell-based electrical impedance measurements as a GPCR drug discovery tool

G Protein-Coupled Receptors (GPCRs) transduce extracellular signals and activate intracellular pathways, usually through activating associated G proteins. Due to their involvement in many human diseases, they are recognized worldwide as valuable drug targets. Many experimental approaches help identify small molecules that target GPCRs, including in vitro cell-based reporter assays and binding studies. Most cell-based assays use one signaling pathway or reporter as an assay readout. Moreover, they often require cell labeling or the integration of reporter systems. Over the last decades, cell-based electrical impedance biosensors have been explored for drug discovery. This label-free method holds many advantages over other cellular assays in GPCR research. The technology requires no cell manipulation and offers real-time kinetic measurements of receptor-mediated cellular changes. Instead of measuring the activity of a single reporter, the impedance readout includes information on multiple signaling events. This is beneficial when screening for ligands targeting orphan GPCRs since the signaling cascade(s) of the majority of these receptors are unknown. Due to its sensitivity, the method also applies to cellular models more relevant to disease, including patient-derived cell cultures. Despite its advantages, remaining issues regarding data comparability and interpretability has limited implementation of cell-based electrical impedance (CEI) in drug discovery. Future optimization must include both full exploitation of CEI response data using various ways of analysis as well as further exploration of its potential to detect biased activities early on in drug discovery. Here, we review the contribution of CEI technology to GPCR research, discuss its comparative benefits, and provide recommendations.

Shaky ground - The nature of metastable GPCR signalling complexes

How G protein-coupled receptors (GPCR) interact with one another remains an area of active investigation. Obligate dimers of class C GPCRs such as metabotropic GABA and glutamate receptors are well accepted, although whether this is a general feature of other GPCRs is still strongly debated. In this review, we focus on the idea that GPCR dimers and oligomers are better imagined as parts of larger metastable signalling complexes. We discuss the nature of functional oligomeric entities, their stabilities and kinetic features and how structural and functional asymmetries of such metastable entities might have implications for drug discovery. This article is part of the Special Issue entitled 'Receptor heteromers and their allosteric receptor-receptor interactions'.

Constraints on GPCR Heterodimerization Revealed by the Type-4 Induced-Association BRET Assay

G-protein-coupled receptors (GPCRs) comprise the largest and most pharmacologically important family of cell-surface receptors encoded by the human genome. In many instances, the distinct signaling behavior of certain GPCRs has been explained in terms of the formation of heteromers with, for example, distinct signaling properties and allosteric cross-regulation. Confirmation of this has, however, been limited by the paucity of reliable methods for probing heteromeric GPCR interactions in situ. The most widely used assays for GPCR stoichiometry, based on resonance energy transfer, are unsuited to reporting heteromeric interactions. Here, we describe a targeted bioluminescence resonance energy transfer (BRET) assay, called type-4 BRET, which detects both homo- and heteromeric interactions using induced multimerization of protomers within such complexes, at constant expression. Using type-4 BRET assays, we investigate heterodimerization among known GPCR homodimers: the CXC chemokine receptor 4 and sphingosine-1-phosphate receptors. We observe that CXC chemokine receptor 4 and sphingosine-1-phosphate receptors can form heterodimers with GPCRs from their immediate subfamilies but not with more distantly related receptors. We also show that heterodimerization appears to disrupt homodimeric interactions, suggesting the sharing of interfaces. Broadly, these observations indicate that heterodimerization results from the divergence of homodimeric receptors and will therefore likely be restricted to closely related homodimeric GPCRs.

Neurokinin-1 receptor-based bivalent drugs in pain management: The journey to nowhere?

Hybrid compounds (also known as chimeras, designed multiple ligands, bivalent compounds) are chemical units where two active components, usually possessing affinity and selectivity for distinct molecular targets, are combined as a single chemical entity. The rationale for using a chimeric approach is well documented as such novel drugs are characterized by their enhanced enzymatic stability and biological activity. This allows their use at lower concentrations, increasing their safety profile, particularly when considering undesirable side effects. In the group of synthetic bivalent compounds, drugs combining pharmacophores having affinities toward opioid and neurokinin-1 receptors have been extensively studied as potential analgesic drugs. Indeed, substance P is known as a major endogenous modulator of nociception both in the peripheral and central nervous systems. Hence, synthetic peptide fragments showing either agonism or antagonism at neurokinin 1 receptor were both assigned with analgesic properties. However, even though preclinical studies designated neurokinin-1 receptor antagonists as promising analgesics, early clinical studies revealed a lack of efficacy in human. Nevertheless, their molecular combination with enkephalin/endomorphin fragments has been considered as a valuable approach to design putatively promising ligands for the treatment of pain. This paper is aimed at summarizing a 20-year journey to the development of potent analgesic hybrid compounds involving an opioid pharmacophore and devoid of unwanted side effects. Additionally, the legitimacy of considering neurokinin-1 receptor ligands in the design of chimeric drugs is discussed.

US28: HCMV's Swiss Army Knife

US28 is one of four G protein coupled receptors (GPCRs) encoded by human cytomegalovirus (HCMV). The US28 protein (pUS28) is a potent signaling molecule that alters a variety of cellular pathways that ultimately alter the host cell environment. This viral GPCR is expressed not only in the context of lytic replication but also during viral latency, highlighting its multifunctional properties. pUS28 is a functional GPCR, and its manipulation of multiple signaling pathways likely impacts HCMV pathogenesis. Herein, we will discuss the impact of pUS28 on both lytic and latent infection, pUS28-mediated signaling and its downstream consequences, and the influence this viral GPCR may have on disease states, including cardiovascular disease and cancer. We will also discuss the potential for and progress towards exploiting pUS28 as a novel therapeutic to combat HCMV.

The Dynamics of GPCR Oligomerization and Their Functional Consequences

The functional importance of G protein-coupled receptor (GPCR) oligomerization remains controversial. Although obligate dimers of class C GPCRs are well accepted, the generalizability of this phenomenon is still strongly debated with respect to other classes of GPCRs. In this review, we focus on understanding the organization and dynamics between receptor equivalents and their signaling partners in oligomeric receptor complexes, with a view toward integrating disparate viewpoints into a unified understanding. We discuss the nature of functional oligomeric entities, and how asymmetries in receptor structure and function created by oligomers might have implications for receptor function as allosteric machines and for future drug discovery.

Agonist-induced CXCR4 and CB2 Heterodimerization Inhibits Gα13/RhoA-mediated Migration

G-protein-coupled receptor (GPCR) heterodimerization has emerged as a means by which alternative signaling entities can be created; yet, how receptor heterodimers affect receptor pharmacology remains unknown. Previous observations suggested a biochemical antagonism between GPCRs, CXCR4 and CB2 (CNR2), where agonist-bound CXCR4 and agonist-bound CB2 formed a physiologically nonfunctional heterodimer on the membrane of cancer cells, inhibiting their metastatic potential in vitro However, the reduced signaling entities responsible for the observed functional outputs remain elusive. This study now delineates the signaling mechanism whereby heterodimeric association between CXCR4 and CB2, induced by simultaneous agonist treatment, results in decreased CXCR4-mediated cell migration, invasion, and adhesion through inhibition of the Gα13/RhoA signaling axis. Activation of CXCR4 by its cognate ligand, CXCL12, stimulates Gα13 (GNA13), and subsequently, the small GTPase RhoA, which is required for directional cell migration and the metastatic potential of cancer cells. These studies in prostate cancer cells demonstrate decreased protein expression levels of Gα13 and RhoA upon simultaneous CXCR4/CB2 agonist stimulation. Furthermore, the agonist-induced heterodimer abrogated RhoA-mediated cytoskeletal rearrangement resulting in the attenuation of cell migration and invasion of an endothelial cell barrier. Finally, a reduction was observed in the expression of integrin α5 (ITGA5) upon heterodimerization, supported by decreased cell adhesion to extracellular matrices in vitro Taken together, the data identify a novel pharmacologic mechanism for the modulation of tumor cell migration and invasion in the context of metastatic disease.Implications: This study investigates a signaling mechanism by which GPCR heterodimerization inhibits cancer cell migration. Mol Cancer Res; 16(4); 728-39. ©2018 AACR.

Dopamine acting at D1-like, D2-like and α1-adrenergic receptors differentially modulates theta and gamma oscillatory activity in primary motor cortex

The loss of dopamine (DA) in Parkinson’s is accompanied by the emergence of exaggerated theta and beta frequency neuronal oscillatory activity in the primary motor cortex (M1) and basal ganglia. DA replacement therapy or deep brain stimulation reduces the power of these oscillations and this is coincident with an improvement in motor performance implying a causal relationship. Here we provide in vitro evidence for the differential modulation of theta and gamma activity in M1 by DA acting at receptors exhibiting conventional and non-conventional DA pharmacology. Recording local field potentials in deep layer V of rat M1, co-application of carbachol (CCh, 5 μM) and kainic acid (KA, 150 nM) elicited simultaneous oscillations at a frequency of 6.49 ± 0.18 Hz (theta, n = 84) and 34.97 ± 0.39 Hz (gamma, n = 84). Bath application of DA resulted in a decrease in gamma power with no change in theta power. However, application of either the D1-like receptor agonist SKF38393 or the D2-like agonist quinpirole increased the power of both theta and gamma suggesting that the DA-mediated inhibition of oscillatory power is by action at other sites other than classical DA receptors. Application of amphetamine, which promotes endogenous amine neurotransmitter release, or the adrenergic α1-selective agonist phenylephrine mimicked the action of DA and reduced gamma power, a result unaffected by prior co-application of D1 and D2 receptor antagonists SCH23390 and sulpiride. Finally, application of the α1-adrenergic receptor antagonist prazosin blocked the action of DA on gamma power suggestive of interaction between α1 and DA receptors. These results show that DA mediates complex actions acting at dopamine D1-like and D2-like receptors, α1 adrenergic receptors and possibly DA/α1 heteromultimeric receptors to differentially modulate theta and gamma activity in M1.

Recent Advances in the Realm of Allosteric Modulators for Opioid Receptors for Future Therapeutics

Opioids, and more specifically μ-opioid receptor (MOR) agonists such as morphine, have long been clinically used as therapeutics for severe pain states but often come with serious side effects such as addiction and tolerance. Many studies have focused on bringing about analgesia from the MOR with attenuated side effects, but its underlying mechanism is not fully understood. Recently, focus has been geared toward the design and elucidation of the orthosteric site with ligands of various biological profiles and mixed subtype opioid activities and selectivities, but targeting the allosteric site is an area of increasing interest. It has been shown that allosteric modulators play key roles in influencing receptor function such as its tolerance to a ligand and affect downstream pathways. There has been a high variance of chemical structures that provide allosteric modulation at a given receptor, but recent studies and reviews tend to focus on the altered cellular mechanisms instead of providing a more rigorous description of the allosteric ligand's structure-function relationship. In this review, we aim to explore recent developments in the structural motifs that potentiate orthosteric binding and their influences on cellular pathways in an effort to present novel approaches to opioid therapeutic design.

Regulation of G Protein-Coupled Receptors by Ubiquitination

G protein-coupled receptors (GPCRs) comprise the largest family of membrane receptors that control many cellular processes and consequently often serve as drug targets. These receptors undergo a strict regulation by mechanisms such as internalization and desensitization, which are strongly influenced by posttranslational modifications. Ubiquitination is a posttranslational modification with a broad range of functions that is currently gaining increased appreciation as a regulator of GPCR activity. The role of ubiquitination in directing GPCRs for lysosomal degradation has already been well-established. Furthermore, this modification can also play a role in targeting membrane and endoplasmic reticulum-associated receptors to the proteasome. Most recently, ubiquitination was also shown to be involved in GPCR signaling. In this review, we present current knowledge on the molecular basis of GPCR regulation by ubiquitination, and highlight the importance of E3 ubiquitin ligases, deubiquitinating enzymes and β-arrestins. Finally, we discuss classical and newly-discovered functions of ubiquitination in controlling GPCR activity.

Clarifying the Ghrelin System's Ability to Regulate Feeding Behaviours Despite Enigmatic Spatial Separation of the GHSR and Its Endogenous Ligand

Ghrelin is a hormone predominantly produced in and secreted from the stomach. Ghrelin is involved in many physiological processes including feeding, the stress response, and in modulating learning, memory and motivational processes. Ghrelin does this by binding to its receptor, the growth hormone secretagogue receptor (GHSR), a receptor found in relatively high concentrations in hypothalamic and mesolimbic brain regions. While the feeding and metabolic effects of ghrelin can be explained by the effects of this hormone on regions of the brain that have a more permeable blood brain barrier (BBB), ghrelin produced within the periphery demonstrates a limited ability to reach extrahypothalamic regions where GHSRs are expressed. Therefore, one of the most pressing unanswered questions plaguing ghrelin research is how GHSRs, distributed in brain regions protected by the BBB, are activated despite ghrelin’s predominant peripheral production and poor ability to transverse the BBB. This manuscript will describe how peripheral ghrelin activates central GHSRs to encourage feeding, and how central ghrelin synthesis and ghrelin independent activation of GHSRs may also contribute to the modulation of feeding behaviours.

Systematic protein-protein interaction mapping for clinically relevant human GPCRs

G‐protein‐coupled receptors (GPCRs) are the largest family of integral membrane receptors with key roles in regulating signaling pathways targeted by therapeutics, but are difficult to study using existing proteomics technologies due to their complex biochemical features. To obtain a global view of GPCR‐mediated signaling and to identify novel components of their pathways, we used a modified membrane yeast two‐hybrid (MYTH) approach and identified interacting partners for 48 selected full‐length human ligand‐unoccupied GPCRs in their native membrane environment. The resulting GPCR interactome connects 686 proteins by 987 unique interactions, including 299 membrane proteins involved in a diverse range of cellular functions. To demonstrate the biological relevance of the GPCR interactome, we validated novel interactions of the GPR37, serotonin 5‐HT4d, and adenosine ADORA2A receptors. Our data represent the first large‐scale interactome mapping for human GPCRs and provide a valuable resource for the analysis of signaling pathways involving this druggable family of integral membrane proteins.