Although opioid analgesics are indispensable in treating pain, these drugs are accompanied by life-threatening side effects. While clinically relevant opioid drugs target the µ opioid receptor (MOR), a heterodimer between the MOR and the δ opioid receptor (DOR) has emerged as another target to develop safer analgesics. Although some heterodimer-preferring agonists have been reported so far, it is still difficult to activate the MOR/DOR heterodimer selectively in the presence of MOR or DOR monomers/homodimers. To gain insights to develop selective agonists for MOR/DOR, herein we prepared analogs of CYM51010, one of the reported heterodimer-preferring agonists, and collected structure-activity relationship information. We found that the ethoxycarbonyl group was needed for the activity for the heterodimer, although this group could be substituted with functional groups with similar sizes, such as an ethoxycarbonyl group. As for the acetylaminophenyl group, not a type of substituent, but rather a substituent located at a specific position (para-position) was essential for the activity. Changing the linker length between the acetylaminophenyl group and the piperidine moiety also had deleterious effects on the activity. On the other hand, the substitution of the acetylamino group with a trifluoroacetylamino group and the substitution of the phenethyl group with a benzyl group diminished the activities for the monomers/homodimers while keeping the activity for MOR/DOR, which enhanced the selectivity. Our findings herein will play an important role in developing selective agonists for MOR/DOR and for elucidating the physiological roles of this heterodimer in analgesic processes and in the establishment of side effects.

The GPCR signalling cascade is a key pathway responsible for the signal transduction of a multitude of physical and chemical stimuli, including light, odorants, neurotransmitters and hormones. Understanding the structural and functional properties of the GPCR cascade requires direct observation of signalling processes in high spatial and temporal resolution, with minimal perturbation to endogenous systems. Optical microscopy and spectroscopy techniques are uniquely suited to this purpose because they excel at multiple spatial and temporal scales and can be used in living objects. Here, we review recent developments in microscopy and spectroscopy technologies which enable new insights into GPCR signalling. We focus on advanced techniques with high spatial and temporal resolution, single-molecule methods, labelling strategies and approaches suitable for endogenous systems and large living objects. This review aims to assist researchers in choosing appropriate microscopy and spectroscopy approaches for a variety of applications in the study of cellular signalling.

Currently, G protein-coupled receptors (GPCRs) constitute a significant group of membrane-bound receptors representing more than 30% of therapeutic targets. Fluorine is commonly used in designing highly active biological compounds, as evidenced by the steadily increasing number of drugs by the Food and Drug Administration (FDA). Herein, we identified and analyzed 898 target-based F-containing isomeric analog sets for SAR analysis in the ChEMBL database-FiSAR sets active against 33 different aminergic GPCRs comprising a total of 2163 fluorinated (1201 unique) compounds. We found 30 FiSAR sets contain activity cliffs (ACs), defined as pairs of structurally similar compounds showing significant differences in affinity (≥50-fold change), where the change of fluorine position may lead up to a 1300-fold change in potency. The analysis of matched molecular pair (MMP) networks indicated that the fluorination of aromatic rings showed no clear trend toward a positive or negative effect on affinity. Additionally, we propose an in silico workflow (including induced-fit docking, molecular dynamics, quantum polarized ligand docking, and binding free energy calculations based on the Generalized-Born Surface-Area (GBSA) model) to score the fluorine positions in the molecule.

G protein-coupled receptors (GPCRs) are a large family of integral membrane proteins which conduct a wide range of biological roles and represent significant drug targets. Most biophysical and structural studies of GPCRs have been conducted on detergent-solubilised receptors, and it is clear that detergents can have detrimental effects on GPCR function. Simultaneously, there is increasing appreciation of roles for specific lipids in modulation of GPCR function. Lipid nanoparticles such as nanodiscs and styrene maleic acid lipid particles (SMALPs) offer opportunities to study integral membrane proteins in lipid environments, in a form that is soluble and amenable to structural and biophysical experiments. Here, we review the application of lipid nanoparticle technologies to the study of GPCRs, assessing the relative merits and limitations of each system. We highlight how these technologies can provide superior platforms to detergents for structural and biophysical studies of GPCRs and inform on roles for protein-lipid interactions in GPCR function.

BACKGROUND: To conduct an in vitro investigation into the effect of different concentrations of levocetirizine hydrochloride on the growth of human dermal papilla cells (hDPCs) the underlying mechanisms involved.
METHODS: hDPCs were cultured in Dulbecco\'s Modified Eagle Medium (DMEM) containing different concentrations of levocetirizine hydrochloride for 48 h. The growth of hDPCs was observed by immunofluorescence staining, and the cell proliferation was detected by MTT assay. After the hDPCs were cultured in DMEM containing 1, 10, 100, 1,000, and 10,000 ng/mL levocetirizine hydrochloride for 48 h, the mRNA expressions of cyclooxygenase 2 (COX-2), prostaglandin D2 synthase (PTGDS), prostaglandin E2 (PGE2), prostaglandin F2α (PGF2α), G protein-coupled receptor 44 (GPR44), protein kinase B (AKT), and glycogen synthase kinase 3β (GSK3β) were determined by real-time fluorescence-based quantitative polymerase chain reaction (PCR), and the protein expressions of PTGDS, phosphorylated protein kinase B (pAKT), and phosphorylated glycogen synthase kinase 3β (pGSK3β) were detected by Western blotting. After the hDPCs were cultured in DMEM containing 1, 10, 100, 1,000, 10,000 ng/mL levocetirizine hydrochloride for 24 h, the secretion levels of prostaglandin D2 (PGD2) and PGD2 receptor (PGD2R) in the culture supernatant were determined by enzyme-linked immunosorbent assay (ELISA). One-way analysis of variance (ANOVA) was performed using SPSS 17.0 software, and the LSD-t test was used for pairwise comparisons.
RESULTS: Immunofluorescence staining showed that hDPCs in the 100 ng/mL group grew well, with over 90% confluency. Methyl thiazolyl tetrazolium (MTT) method showed that the proliferation rate of hDPCs significantly differed between different levocetirizine hydrochloride groups and the blank control group (F=42.22, P<0.05), while the proliferation rate was significantly higher in the 100 ng/mL group (115.80%±5.10%) than in the blank control group (100%) (t=28.26, P<0.05). The relative mRNA expressions of COX-2, PGF2a, PTGDS, GPR44, and AKT showed significant differences in different levocetirizine hydrochloride groups (the F values were 1.97, 3.66, 2.17, 2.66, and 7.32, respectively; all P<0.05), whereas the mRNA expressions of PGE2 and GSK3β showed no significant difference (F=0.87, F=1.19, respectively; both P>0.05). The mRNA expressions of COX-2, PTGDS, and GPR44 in the 100 ng/mL group (0.840.08, 0.810.10, and 0.85±0.09, respectively) were significantly lower than those in the blank control group (t=1.97, t=2.17, and t=2.65, respectively; all P<0.05), whereas the mRNA expressions of PGF2α and AKT in the 100 ng/mL group (1.96±0.25 and 1.74±0.32, respectively) were significantly higher than those in the blank control group (t=3.662 and t=7.325, respectively; both P<0.05). There were significant differences in the levels of PTGDS, pAKT, pGSK3β, PGD2, and PGD2R proteins between the different levocetirizine hydrochloride groups (the F values were 11.84, 3.89, 4.07, 66.15, and 44.33, respectively). The protein expressions of PTGDS, PGD2, and PGD2R in the 100 ng/mL group (0.32±0.05, 141.62±5.44, and 215.08±9.55, respectively) were significantly lower than those in the blank control group (0.73±0.06, 180.08±6.15, and 273.24±3.18, respectively) (the t values were 5.66, 45.07, and 92.05, respectively; all P<0.05), whereas the protein expressions of pAKT and pGSK3β in the 100 ng/mL group (0.59±0.05 and 0.46±0.03, respectively) were significantly higher than those in the blank control group (0.46±0.02 and 0.35±0.042, respectively) (t=16.59, t=7.73, respectively; both P<0.05).
CONCLUSIONS: Levocetirizine hydrochloride may promote the growth and proliferation of hDPC in vitro by inhibiting the PGD2-GPR44 pathway and activating the AKT signaling pathway.

The neuropeptide galanin functions via three G-protein coupled receptors, Gal1-3-R. Both Gal1-R and 2-R are involved in pain signaling at the spinal level. Here a Gal2-R-EGFP transgenic (TG) mouse was generated and studied in pain tests and by characterizing Gal2-R expression in both sensory ganglia and spinal cord. After peripheral spared nerve injury, mechanical allodynia developed and was ipsilaterally similar between wild type (WT) and TG mice. A Gal2-R-EGFP-positive signal was primarily observed in small and medium-sized dorsal root ganglion (DRG) neurons and in spinal interneurons and processes. No significant difference in size distribution of DRG neuronal profiles was found between TG and WT mice. Both percentage and fluorescence intensity of Gal2-R-EGFP-positive neuronal profiles were overall significantly upregulated in ipsilateral DRGs as compared to contralateral DRGs. There was an ipsilateral reduction in substance P-positive and calcitonin gene-related peptide (CGRP)-positive neuronal profiles, and this reduction was more pronounced in TG as compared to WT mice. Moreover, Gal2-R-EGFP partly co-localized with three pain-related neuropeptides, CGRP, neuropeptide Y and galanin, both in intact and injured DRGs, and with galanin also in local neurons in the superficial dorsal horn. Taken together, the present results provide novel information on the localization and phenotype of DRG and spinal neurons expressing the second galanin receptor, Gal2-R, and on phenotypic changes following peripheral nerve injury. Gal2-R may also be involved in autoreceptor signaling.

The G protein cascade amplification system couples with several receptors to sense/amplify the cellular signal, implying universal application. In order to explore whether GPCRs can trigger G protein signal amplification in tissues/cells from different species, bombykol receptor was isolated and purified from antennas of male Bombyx mori, which subsequently self-assembled on the cell membrane in rat taste buds/rat vomeronasa/catfish tentacles/taste bud tissues of rabbits/pig/cattle in those lacking endogenous bombykol receptor, followed by immobilization between two sheets of nucleopore membranes fixed by sodium alginate-starch gel, forming the sandwich-type sensing membrane, which in turn was immobilized on the glass-carbon electrode. Thus, bombykol receptor sensors were established with different tissues. The response current of bombykol receptor sensor toward bombykol was measured with an electrochemical workstation. Every bombykol receptor sensor could sense bombykol based on enzyme-substrate kinetics. The double reciprocal plot and the activation constant values of bombykol receptor sensors assembled with rat taste buds, rat vomeronasa, catfish tentacles, rabbit taste buds, pig taste buds, and cattle taste buds were calculated. Approximately 2-3 receptors could trigger the G protein cascade amplification system and achieve the maximum signal output. Moreover, the detection lower limit indicated that the bombykol receptor self-assembled on the cell membranes of different tissues that transmitted and amplified the bombykol signal with hypersensitivity. Also, cattle taste bud tissues served as an ideal system for heterogeneous GPCRs self-assembly and signal sensing/amplification. This sensing technique and method had promising potential in studies of biological pest control, sex pheromone detection, and receptor structure and function.

Multi-membrane spanning proteins, such as G protein-coupled receptors (GPCRs) and ion channels, are extremely difficult to purify as native proteins. Consequently, the generation of antibodies that recognize the native conformation can be challenging. By combining genetic immunization, phage display, and biopanning, we identified a panel of monovalent antibodies (nanobodies) targeting the vasoactive intestinal peptide receptor 1 (VPAC1) receptor. The nine unique nanobodies that were classified into four different families based on their CDR3 amino acid sequence and length, were highly specific for the human receptor and bind VPAC1 with moderate affinity. They all recognize a similar epitope localized in the extracellular N-terminal domain of the receptor and distinct from the orthosteric binding site. In agreement with binding studies, which showed that the nanobodies did not interfere with VIP binding, all nanobodies were devoid of any functional properties. However, we observed that the binding of two nanobodies was slightly increased in the presence of VPAC1 agonists [vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide-27 (PACAP-27)], but decreased in the presence of VPAC1 antagonist. As no evidence of allosteric activity was seen in VIP binding studies nor in functional assays, it is, therefore, possible that the two nanobodies may behave as very weak allosteric modulators of VPAC1, detectable only in some sensitive settings, but not in others. We demonstrated that the fluorescently labeled nanobodies detect VPAC1 on the surface of human leukocytes as efficiently as a reference mouse monoclonal antibody. We also developed a protocol allowing efficient detection of VPAC1 by immunohistochemistry in paraffin-embedded human gastrointestinal tissue sections. Thus, these nanobodies constitute new original tools to further investigate the role of VPAC1 in physiological and pathological conditions.

With the approach of the 30th year since the pioneering discovery of a cannabinoid receptor in rat brain (Devane et al., 1988), the field of cannabinoid pharmacology and physiology has impacted human physiology at multiple levels. The development of highly specific and potent orthosteric ligands, as well as the blossoming field of allosteric ligand development, has placed the endocannabinoid system in the forefront as a modulator of a multitude of physiologic processes. Reproducibility among laboratories is especially important due to the development of novel tools to investigate the role(s) of the endocannabinoid system in human physiology, and to clarify the roles for medicinal marijuana. Any definitive role in normal, or diseased states, must be satisfied through the demonstration of a specific receptor-mediated event. This chapter provides working protocols for the study of cannabinoid receptor-ligand binding, as well as immediate and downstream G protein-dependent signaling assays to assess receptor function.

Experiments are described that allowed cross-linking of analogs of a 13-amino acid peptide into the binding site of a model G protein-coupled receptor. Syntheses of peptide analogs that were used for photochemical or chemical cross-linking were carried out using solid-phase peptide synthesis. Chemical cross-linking utilized 3,4-dihydroxy-l-phenylalanine-incorporated peptides and subsequent periodate-mediated activation, whereas photochemical cross-linking was mediated by p-benzoyl-l-phenylalanine (Bpa)-labeled peptides and UV-initiated activation. Mass spectrometry was employed to locate the site(s) in the receptor that formed the cross-links to the ligand. We also describe a method called unnatural amino acid replacement that allowed capture of a peptide ligand into the receptor. In this method, the receptor was genetically modified by replacement of a natural amino acid with Bpa. The modified receptor was UV-irradiated to capture the ligand. The approaches described are applicable to other peptide-binding proteins and can reveal the ligand-binding site in atomic detail.