Label-free integrative pharmacology on-target of opioid ligands at the opioid receptor family

In vitro and in vivo pharmacological characterization of the synthetic opioid MT-45

MT-45 is a synthetic opioid that was developed in the 1970s as an analgesic compound. However, in recent years MT-45 has been associated with multiple deaths in Europe and has been included in the class of novel psychoactive substances known as novel synthetic opioids (NSOs). Little is known about the pharmaco-toxicological effects of MT-45. Therefore, we used a dynamic mass redistribution (DMR) assay to investigate the pharmacodynamic profile of this NSO in vitro compared with morphine. We then used in vivo studies to investigate the effect of the acute systemic administration of MT-45 (0.01-15 mg/kg i.p.) on motor and sensorimotor (visual, acoustic and tactile) responses, mechanical and thermal analgesia, muscle strength and body temperature in CD-1 male mice. Higher doses of MT-45 (6-30 mg/kg i.p.) were used to investigate cardiorespiratory changes (heart rate, respiratory rate, SpO₂ saturation and pulse distention). All effects of MT-45 were compared with those of morphine. In vitro DMR assay results demonstrated that at human recombinant opioid receptors MT-45 behaves as a potent selective mu agonist with a slightly higher efficacy than morphine. In vivo results showed that MT-45 progressively induces tail elevation at the lowest dose tested (0.01 mg/kg), increased mechanical and thermal antinociception (starting from 1 to 6 mg/kg), decreased visual sensorimotor responses (starting from 3 to 6 mg/kg) and reduced tactile responses, modulated motor performance and induced muscle rigidity at higher doses (15 mg/kg). In addition, at higher doses (15-30 mg/kg) MT-45 impaired the cardiorespiratory functions. All effects were prevented by the administration of the opioid receptor antagonist naloxone. These findings reveal the risks associated with the ingestion of opioids and the importance of studying these drugs and undertaking more clinical studies of the current molecules to better understand possible therapeutic interventions in the case of toxicity.

Systematic characterization of AT1 receptor antagonists with label-free dynamic mass redistribution assays

INTRODUCTION

In the drug discovery field, the binding affinities and binding kinetics of drug candidates are very important. Angiotensin II type 1 (AT1) receptor antagonists, e.g., candesartan, telmisartan, irbesartan, losartan and valsartan, show high affinities and long-lasting bindings to the receptor, making them preferred medications for hypertension treatment. However, the molecular binding properties of AT1 receptor antagonists are controversial.

METHODS

In this work, we established a profile to study the phenotypic properties of AT1 receptor antagonists with label-free dynamic mass redistribution (DMR) assays in native human cells. With noninvasive features, DMR assay were conducted in multiple formats. Eleven antagonists were systematically evaluated with angiotensin II as an agonist probe in the Hep G2 cell line, which endogenously expresses the AT1 receptor.

RESULTS

The IC₅₀ values to the AT1 receptor of individual antagonist varied with different incubation times. The antagonists showed competitive behavior with angiotensin II. Schild analysis was used to analyze the competitive behavior of the antagonist. All of the antagonist showed long-lasting possession of the AT1 receptor, except telmisartan. The systematic evaluation of the antagonists implied that 11 antagonists showed high binding affinity but distinct binding modes to AT1 receptor.

DISCUSSION

This study demonstrated that the DMR assay has great potential for determining the pharmacological parameters of ligands. This work may serve as guidance for other receptor and ligand assays.

Phenotypic assessment and ligand screening of ETA/ETB receptors with label-free dynamic mass redistribution assay

Endothelin receptors, consisting of two subtypes, ETA and ETB, are expressed in various tissues and widely regulate cardiovascular systems. The two receptors show distinct biological characteristics and are involved in different downstream pathways. Hence, to evaluate the ETA and ETB receptors on the same platform is helpful to display their pharmacological features. In this study, we developed a label-free dynamic mass redistribution (DMR) assay to investigate the phenotypic features of the ETA and ETB receptors in native cell lines. Meanwhile, specific agonists and antagonists were investigated for their pharmacological parameters. Results indicated that the DMR response of endothelin 1 (ET-1, an endogenous ETA/ETB agonist) was cell line dependent on ETA receptors and this ligand generated a biphasic dose-response curve in SH-SY5Y as well as PC3 cell lines. ET-1 and IRL 1620 (an ETB agonist) showed different DMR responses in U251 cells. IC₅₀ values of antagonists were consistent with the K_i values previously reported. Furthermore, a list of compounds was screened on the ETA and ETB receptor models established by the high-throughput DMR assays. This study demonstrated that the DMR assay had great potential in the phenotypic-based investigation and ligand screening of GPCRs.

Activation of Kappa Opioid Receptor Regulates the Hypothermic Response to Calorie Restriction and Limits Body Weight Loss

Mammals maintain a nearly constant core body temperature (T_b) by balancing heat production and heat dissipation. This comes at a high metabolic cost that is sustainable if adequate calorie intake is maintained. When nutrients are scarce or experimentally reduced such as during calorie restriction (CR), endotherms can reduce energy expenditure by lowering T_b [1-6]. This adaptive response conserves energy, limiting the loss of body weight due to low calorie intake [7-10]. Here we show that this response is regulated by the kappa opioid receptor (KOR). CR is associated with increased hypothalamic levels of the endogenous opioid Leu-enkephalin, which is derived from the KOR agonist precursor dynorphin [11]. Pharmacological inhibition of KOR, but not of the delta or the mu opioid receptor subtypes, fully blocked CR-induced hypothermia and increased weight loss during CR independent of calorie intake. Similar results were seen with DIO mice subjected to CR. In contrast, inhibiting KOR did not change T_b in animals fed ad libitum (AL). Chemogenetic inhibition of KOR neurons in the hypothalamic preoptic area reduced the CR-induced hypothermia, whereas chemogenetic activation of prodynorphin-expressing neurons in the arcuate or the parabrachial nucleus lowered T_b. These data indicate that KOR signaling is a pivotal regulator of energy homeostasis and can affect body weight during dieting by modulating T_b and energy expenditure.

Pharmacological profile of the neuropeptide S receptor: Dynamic mass redistribution studies

Neuropeptide S (NPS) is the endogenous ligand of the neuropeptide S receptor (NPSR). NPS modulates several biological functions including anxiety, wakefulness, pain, and drug abuse. The aim of this study was the investigation of the pharmacological profile of NPSR using the dynamic mass redistribution (DMR) assay. DMR is a label-free assay that offers a holistic view of cellular responses after receptor activation. HEK293 cells stably transfected with the murine NPSR (HEK293mNPSR) have been used. To investigate the nature of the NPS-evoked DMR signaling, FR900359 (Gq inhibitor), pertussis toxin (Gi inhibitor), and rolipram (phosphodiesterase inhibitor) were used. To determine the pharmacology of NPSR, several selective ligands (agonists, partial agonists, antagonists) have been tested. NPS, through selective NPSR activation, evoked a robust DMR signal with potency in the nanomolar range. This signal was predominantly, but not completely, blocked by FR900359, suggesting the involvement of the Gq-dependent signaling cascade. NPSR ligands (agonists and antagonists) displayed potency values in DMR experiments similar, but not identical, to those reported in the literature. Furthermore, partial agonists produced a higher efficacy in DMR than in calcium experiments. DMR can be successfully used to study the pharmacology and signaling properties of novel NPSR ligands. This innovative approach will likely increase the translational value of in vitro pharmacological studies.

NOP receptor pharmacological profile - A dynamic mass redistribution study

The Nociceptin/Orphanin FQ (N/OFQ) peptide NOP receptor is coupled to pertussis toxin (PTX)-sensitive G proteins (G_i/o) whose activation leads to the inhibition of both cAMP production and calcium channel activity, and to the stimulation of potassium currents. The label free dynamic mass redistribution (DMR) approach has been demonstrated useful for investigating the pharmacological profile of G protein-coupled receptors. Herein, we employ DMR technology to systematically characterize the pharmacology of a large panel of NOP receptor ligands. These are of peptide and non-peptide nature and display varying degrees of receptor efficacy, ranging from full agonism to pure antagonism. Using Chinese hamster ovary (CHO) cells expressing the human NOP receptor we provide rank orders of potency for full and partial agonists as well as apparent affinities for selective antagonists. We find the pharmacological profile of NOP receptor ligands to be similar but not identical to values reported in the literature using canonical assays for G_i/o-coupled receptors. Our data demonstrate that holistic label-free DMR detection can be successfully used to investigate the pharmacology of the NOP receptor and to characterize the cellular effects of novel NOP receptor ligands.

Ligand-Directed Signaling at the Delta Opioid Receptor

Delta opioid receptors (δORs) regulate a number of physiological functions, and agonists for this receptor are being pursued for the treatment of mood disorders, chronic pain, and migraine. A major challenge to the development of these compounds is that, like many G-protein coupled receptors (GPCRs), agonists at the δOR can induce very different signaling and receptor trafficking events. This concept, known as ligand-directed signaling, functional selectivity, or biased agonism, can result in different agonists producing highly distinct behavioral consequences. In this chapter, we highlight the in vitro and in vivo evidence for ligand-directed signaling and trafficking at the δOR. A number of biological implications of agonist-directed signaling at the δOR have been demonstrated. Importantly, ligand-specific effects can impact both acute behavioral effects of delta agonists, as well as the long-term adaptations induced by chronic drug treatment. A better understanding of the specific signaling cascades that regulate these differential behavioral effects would help to guide rational drug design, ultimately resulting in δOR agonists with fewer adverse effects.

Synthesis and Opioid Activity of Tyr1 -ψ[(Z)CF=CH]-Gly2 and Tyr1 -ψ[(S)/(R)-CF3 CH-NH]-Gly2 Leu-enkephalin Fluorinated Peptidomimetics

We describe the design, synthesis, and opioid activity of fluoroalkene (Tyr1 -ψ[(Z)CF=CH]-Gly2 ) and trifluoroethylamine (Tyr1 -ψ[(S)/(R)-CF3 CH-NH]-Gly2 ) analogues of the endogenous opioid neuropeptide, Leu-enkephalin. The fluoroalkene peptidomimetic exhibited low nanomolar functional activity (5.0±2 nm and 60±15 nm for δ- and μ-opioid receptors, respectively) with a μ/δ-selectivity ratio that mimics that of the natural peptide. However, the trifluoroethylamine peptidomimetics, irrespective of stereochemistry, did not activate the opioid receptors, which suggest that bulky CF3 substituents are not tolerated at this position.

Discovery of new muscarinic acetylcholine receptor antagonists from Scopolia tangutica

Scopolia tangutica (S. tangutica) is a traditional Chinese medicinal plant used for antispasmodics, anesthesia, analgesia and sedation. Its pharmacological activities are mostly associated with the antagonistic activity at muscarinic acetylcholine receptors (mAchRs) of several known alkaloids such as atropine and scopolamine. With our recent identification of four hydroxycinnamic acid amides from S. tangutica, we hypothesized that this plant may contain previously unidentified alkaloids that may also contribute to its in vivo effect. Herein, we used a bioassay-guided multi-dimension separation strategy to discover novel mAchR antagonists from S. tangutica. The core of this approach is to use label-free cell phenotypic assay to first identify active fractions, and then to guide purification of active ligands. Besides four tropanes and six cinnamic acid amides that have been previously isolated from S. tangutica, we recently identified two new tropanes, one new cinnamic acid amide, and nine other compounds. Six tropane compounds purified from S. tangutica for the first time were confirmed to be competitive antagonists of muscarinic receptor 3 (M3), including the two new ones 8 and 12 with IC₅₀ values of 1.97 μM and 4.47 μM, respectively. Furthermore, the cinnamic acid amide 17 displayed 15-fold selectivity for M1 over M3 receptors. These findings will be useful in designing lead compounds for mAchRs and elucidating mechanisms of action of S. tangutica.

Opioid-induced redistribution of 6TM and 7TM μ opioid receptors: A hypothesized mechanistic facilitator model of opioid-induced hyperalgesia

Opioids are still the most popular form of pain treatment, but many unavoidable side effects make opioids a big challenge in effective pain management. Opioid-induced hyperalgesia (OIH), a paradoxical phenomenon, portrays an increased sensitivity to harmful stimuli caused by opioid exposure. Changes in the neural modulation are considered a major contributor to the development of OIH. Activation of opioid receptors (ORs) and corresponding downstream molecules are the vital composition of functional performance of opioids. Increasing interests were proposed of the interaction between ORs and other neural transmitter systems such as glutamatergic, GABAergic and adrenergic ones to the genesis of OIH. G protein coupled μ-opioid receptor (MOR) was studied comprehensively on its role in the development of OIH. In addition to the relationship between MOR and other neurotransmitter receptors, a new intracellular MOR that has six transmembrane (6TM) domains was identified, and found to perform a pro-nociceptive task in contrast to the counterpart 7TM isoform. A mechanistic model of OIH in which both 6TM and 7TM MORs undergoing membrane redistribution upon opioid exposure is proposed which eventually facilitates the neurons more sensitive to nociceptive stimulation than that of the preceding opioid exposure.

Compound annotation with real time cellular activity profiles to improve drug discovery

INTRODUCTION

In the past decade, a range of innovative strategies have been developed to improve the productivity of pharmaceutical research and development. In particular, compound annotation, combined with informatics, has provided unprecedented opportunities for drug discovery.

AREAS COVERED

In this review, a literature search from 2000 to 2015 was conducted to provide an overview of the compound annotation approaches currently used in drug discovery. Based on this, a framework related to a compound annotation approach using real-time cellular activity profiles for probe, drug, and biology discovery is proposed.

EXPERT OPINION

Compound annotation with chemical structure, drug-like properties, bioactivities, genome-wide effects, clinical phenotypes, and textural abstracts has received significant attention in early drug discovery. However, these annotations are mostly associated with endpoint results. Advances in assay techniques have made it possible to obtain real-time cellular activity profiles of drug molecules under different phenotypes, so it is possible to generate compound annotation with real-time cellular activity profiles. Combining compound annotation with informatics, such as similarity analysis, presents a good opportunity to improve the rate of discovery of novel drugs and probes, and enhance our understanding of the underlying biology.

Label-free functional selectivity assays

G protein-coupled receptors (GPCRs) represent the largest class of drug targets. Ligand-directed functional selectivity or biased agonism opens new possibility for discovering GPCR drugs with better efficacy and safety profiles. However, quantification of ligand bias is challenging. Herein, we present five different label-free dynamic mass redistribution (DMR) approaches to assess ligand bias acting at the β2-adrenergic receptor (β2AR). Multiparametric analysis of the DMR agonist profiles reveals divergent pharmacology of a panel of β2AR agonists. DMR profiling using catechol as a conformational probe detects the presence of multiple conformations of the β2AR. DMR assays under microfluidics, together with chemical biology tools, discover ligand-directed desensitization of the receptor. DMR antagonist reverse assays manifest biased antagonism. DMR profiling using distinct probe-modulated cells detects the biased agonism in the context of self-referenced pharmacological activity map.

Opioid receptor desensitization: mechanisms and its link to tolerance

Opioid receptors (OR) are part of the class A of G-protein coupled receptors and the target of the opiates, the most powerful analgesic molecules used in clinic. During a protracted use, a tolerance to analgesic effect develops resulting in a reduction of the effectiveness. So understanding mechanisms of tolerance is a great challenge and may help to find new strategies to tackle this side effect. This review will summarize receptor-related mechanisms that could underlie tolerance especially receptor desensitization. We will focus on the latest data obtained on molecular mechanisms involved in opioid receptor desensitization: phosphorylation, receptor uncoupling, internalization, and post-endocytic fate of the receptor.

Label-free monitoring of μ-opioid receptor-mediated signaling

In this study, we used a combination of traditional signaling investigation approaches, bioluminescence resonance energy transfer (BRET) biosensors, and the label-free approach surface plasmon resonance (SPR) spectroscopy to monitor the signaling cascades of the μ-opioid receptor (MOP). In human embryonic kidney cells stably expressing a Flag-tagged version of human MOP, we compared the signals triggered by the noninternalizing and internalizing MOP agonists morphine and DAMGO (Tyr-D-Ala-Gly-N-methyl-Phe-Gly-ol), respectively. We studied three major and well described components of MOP signaling: receptor internalization, G protein coupling, and activation of extracellular signal-regulated kinase ERK1/ERK2. Our results show that morphine and DAMGO display different profiles of receptor internalization and a similar ability to trigger the phosphorylation of ERK1/ERK2. Our SPR analyses revealed that morphine and DAMGO evoke similar SPR signatures and that Gαi, cAMP-dependent pathways, and ERK1/ERK2 have key roles in morphine- and DAMGO-mediated signaling. Most interestingly, we found that the so-called MOP neutral antagonists CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2)), naloxone, and naltrexone behave like partial agonists. Even more intriguing, BRET experiments indicate that CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2)) induces similar conformational changes as naltrexone at the Gαi-βγ interface, whereas it appears as an inverse agonist based on its SPR response thus indicating distinct signaling mechanisms for the two ligands. Taken together, our results support the usefulness of label-free methods such as SPR to study whole-cell responses and signaling cascades triggered by G protein-coupled receptors and complement the conventional approaches by revealing cellular responses that would have been otherwise undetectable.

Label-free cell phenotypic profiling decodes the composition and signaling of an endogenous ATP-sensitive potassium channel

Current technologies for studying ion channels are fundamentally limited because of their inability to functionally link ion channel activity to cellular pathways. Herein, we report the use of label-free cell phenotypic profiling to decode the composition and signaling of an endogenous ATP-sensitive potassium ion channel (K_ATP) in HepG2C3A, a hepatocellular carcinoma cell line. Label-free cell phenotypic agonist profiling showed that pinacidil triggered characteristically similar dynamic mass redistribution (DMR) signals in A431, A549, HT29 and HepG2C3A, but not in HepG2 cells. Reverse transcriptase PCR, RNAi knockdown, and K_ATP blocker profiling showed that the pinacidil DMR is due to the activation of SUR2/Kir6.2 K_ATP channels in HepG2C3A cells. Kinase inhibition and RNAi knockdown showed that the pinacidil activated K_ATP channels trigger signaling through Rho kinase and Janus kinase-3, and cause actin remodeling. The results are the first demonstration of a label-free methodology to characterize the composition and signaling of an endogenous ATP-sensitive potassium ion channel.

A label-free optical biosensor with microfluidics identifies an intracellular signalling wave mediated through the β(2)-adrenergic receptor

The canonical model of G protein-coupled receptor (GPCR) signalling states that it is solely initiated at the cell surface. In recent years, a handful of evidence has started emerging from high-resolution molecular assays that the internalized receptors can mediate the third wave of signalling, besides G protein- and β-arrestin-mediated signalling both initiating at the cell surface. However, little is known about the functional consequences of distinct waves of GPCR signalling, in particular, at the whole cell system level. We here report the development of label-free biosensor antagonist reverse assays and their use to differentiate the signalling waves of an endogenous β2-adrenergic receptor (β2-AR) in A431 cells. Results showed that the persistent agonist treatment activated the β2-ARs, leading to a long-term sustained dynamic mass redistribution (DMR) signal, a whole cell phenotypic response. Under the persistent treatment scheme in microplates, a panel of known β-blockers all dose-dependently and completely reversed the DMR signal of epinephrine at a relatively low dose (10 nM), except for sotalol which partially reversed the DMR. Under the perfusion conditions with microfluidics, the subsequent perfusion with sotalol only reversed the DMR induced by epinephrine or isoproterenol at 10 nM, but not at 10 μM. Furthermore, the degree of the DMR reversion by sotalol was found to be in an opposite relation with the duration of the initial agonist treatment. Together, these results suggest that the hydrophilic antagonist sotalol is constrained outside the cells throughout the assays, and the early signalling wave initiated at the cell surface dominates the DMR induced by epinephrine or isoproterenol at relatively low doses, while a secondary and late signalling wave is initiated once the receptors are internalized and contributes partially to the long-term sustainability of the DMR of epinephrine or isoproterenol at high doses.

Label-free drug discovery

Current drug discovery is dominated by label-dependent molecular approaches, which screen drugs in the context of a predefined and target-based hypothesis in vitro. Given that target-based discovery has not transformed the industry, phenotypic screen that identifies drugs based on a specific phenotype of cells, tissues, or animals has gained renewed interest. However, owing to the intrinsic complexity in drug-target interactions, there is often a significant gap between the phenotype screened and the ultimate molecular mechanism of action sought. This paper presents a label-free strategy for early drug discovery. This strategy combines label-free cell phenotypic profiling with computational approaches, and holds promise to bridge the gap by offering a kinetic and holistic representation of the functional consequences of drugs in disease relevant cells that is amenable to mechanistic deconvolution.

Dynamic mass redistribution analysis of endogenous β-adrenergic receptor signaling in neonatal rat cardiac fibroblasts

Label-free systems for the agnostic assessment of cellular responses to receptor stimulation have been shown to provide a sensitive method to dissect receptor signaling. β-adenergic receptors (βAR) are important regulators of normal and pathologic cardiac function and are expressed in cardiomyocytes as well as cardiac fibroblasts, where relatively fewer studies have explored their signaling responses. Using label-free whole cell dynamic mass redistribution (DMR) assays we investigated the response patterns to stimulation of endogenous βAR in primary neonatal rat cardiac fibroblasts (NRCF). The EPIC-BT by Corning was used to measure DMR responses in primary isolated NRCF treated with various βAR and EGFR ligands. Additional molecular assays for cAMP generation and receptor internalization responses were used to correlate the DMR findings with established βAR signaling pathways. Catecholamine stimulation of NRCF induced a concentration-dependent negative DMR deflection that was competitively blocked by βAR blockade and non-competitively blocked by irreversible uncoupling of Gs proteins. Subtype-selective βAR ligand profiling revealed a dominant role for β2AR in mediating the DMR responses, consistent with the relative expression levels of β2AR and β1AR in NRCF. βAR-mediated cAMP generation profiles revealed similar kinetics to DMR responses, each of which were enhanced via inhibition of cAMP degradation, as well as dynamin-mediated receptor internalization. Finally, G protein-independent βAR signaling through epidermal growth factor receptor (EGFR) was assessed, revealing a smaller but significant contribution of this pathway to the DMR response to βAR stimulation. Measurement of DMR responses in primary cardiac fibroblasts provides a sensitive readout for investigating endogenous βAR signaling via both G protein-dependent and –independent pathways.

Divergent label-free cell phenotypic pharmacology of ligands at the overexpressed β₂-adrenergic receptors

We present subclone sensitive cell phenotypic pharmacology of ligands at the β₂-adrenergic receptor (β₂-AR) stably expressed in HEK-293 cells. The parental cell line was transfected with green fluorescent protein (GFP)-tagged β₂-AR. Four stable subclones were established and used to profile a library of sixty-nine AR ligands. Dynamic mass redistribution (DMR) profiling resulted in a pharmacological activity map suggesting that HEK293 endogenously expresses functional G_i-coupled α₂-AR and G_s-coupled β₂-AR, and the label-free cell phenotypic activity of AR ligands are subclone dependent. Pathway deconvolution revealed that the DMR of epinephrine is originated mostly from the remodeling of actin microfilaments and adhesion complexes, to less extent from the microtubule networks and receptor trafficking, and certain agonists displayed different efficacy towards the cAMP-Epac pathway. We demonstrate that receptor signaling and ligand pharmacology is sensitive to the receptor expression level, and the organization of the receptor and its signaling circuitry.

Label-free cell phenotypic profiling identifies pharmacologically active compounds in two traditional Chinese medicinal plants

Label-free cell phenotypic profiling with three cell lines identified multiple pharmacologically active compounds including niacin in two TCM plants.