Arylacetamide-derived fluorescent probes: synthesis, biological evaluation, and direct fluorescent labeling of kappa opioid receptors in mouse microglial cells

Design, Synthesis, and Characterization of New δ Opioid Receptor-Selective Fluorescent Probes and Applications in Single-Molecule Microscopy of Wild-Type Receptors

The delta opioid receptor (δOR or DOR) is a G protein-coupled receptor (GPCR) showing a promising profile as a drug target for nociception and analgesia. Herein, we design and synthesize new fluorescent antagonist probes with high δOR selectivity that are ideally suited for single-molecule microscopy (SMM) applications in unmodified, untagged receptors. Using our new probes, we investigated wild-type δOR localization and mobility at low physiological receptor densities for the first time. Furthermore, we investigate the potential formation of δOR homodimers, as such a receptor organization might exhibit distinct pharmacological activity, potentially paving the way for innovative pharmacological therapies. Our findings indicate that the majority of δORs labeled with these probes exist as freely diffusing monomers on the cell surface in a simple cell model. This discovery advances our understanding of OR behavior and offers potential implications for future therapeutic research.

Chemical tools for the opioids

The opioids are potent and widely used pain management medicines despite also possessing severe liabilities that have fueled the opioid crisis. The pharmacological properties of the opioids primarily derive from agonism or antagonism of the opioid receptors, but additional effects may arise from specific compounds, opioid receptors, or independent targets. The study of the opioids, their receptors, and the development of remediation strategies has benefitted from derivatization of the opioids as chemical tools. While these studies have primarily focused on the opioids in the context of the opioid receptors, these chemical tools may also play a role in delineating mechanisms that are independent of the opioid receptors. In this review, we describe recent advances in the development and applications of opioid derivatives as chemical tools and highlight opportunities for the future.

Fluorescent opioid receptor ligands as tools to study opioid receptor function

Opioid receptors are divided into the three classical types: MOP(μ:mu), DOP(δ:delta) and KOP(κ:kappa) that are naloxone-sensitive and an additional naloxone-insensitive nociceptin/orphanin FQ(N/OFQ) peptide receptor(NOP). Studies to determine opioid receptor location and turnover variably rely on; (i) measuring receptor mRNA, (ii) genetically tagging receptors, (iii) labelling receptors with radioligands, (iv) use of antibodies in immunohistochemistry/Western Blotting or (v) measuring receptor function coupled with the use of selective antagonists. All have their drawbacks with significant issues relating to mRNA not necessarily predicting protein, poor antibody selectivity and utility of radiolabels in low expression systems. In this minireview we discuss use of fluorescently labelled opioid receptor ligands. To maintain the pharmacological properties of the corresponding parent ligand fluorescently labelled ligands must take into account fluorophore (brightness and propensity to bleach), linker length and chemistry, and site to which the linker (and hence probe) will be attached. Use of donor and acceptor fluorophores with spectral overlap facilitates use in FRET type assays to determine proximity of ligand or tagged receptor pairs. There is a wide range of probes of agonist and antagonist nature for all four opioid receptor types; caution is needed with agonist probes due to the possibility for internalization. We have produced two novel ATTO based probes; Dermorphin_ATTO488 (MOP) and N/OFQ_ATTO594 (NOP). These probes label MOP and NOP in a range of preparations and using N/OFQ_ATTO594 we demonstrate internalization and ligand-receptor interaction by FRET. Fluorescent opioid probes offer potential methodological advantages over more traditional use of antibodies and radiolabels.

Lighting Up the Plasma Membrane: Development and Applications of Fluorescent Ligands for Transmembrane Proteins

Despite the fact that transmembrane proteins represent the main therapeutic targets for decades, complete and in-depth knowledge about their biochemical and pharmacological profiling is not fully available. In this regard, target-tailored small-molecule fluorescent ligands are a viable approach to fill in the missing pieces of the puzzle. Such tools, coupled with the ability of high-precision optical techniques to image with an unprecedented resolution at a single-molecule level, helped unraveling many of the conundrums related to plasma proteins' life-cycle and druggability. Herein, we review the recent progress made during the last two decades in fluorescent ligand design and potential applications in fluorescence microscopy of voltage-gated ion channels, ligand-gated ion channels and G-coupled protein receptors.

Multifunctional Enkephalin Analogs with a New Biological Profile: MOR/DOR Agonism and KOR Antagonism

In our previous studies, we developed a series of mixed MOR/DOR agonists that are enkephalin-like tetrapeptide analogs with an N-phenyl-N-piperidin-4-ylpropionamide (Ppp) moiety at the C-terminus. Further SAR study on the analogs, initiated by the findings from off-target screening, resulted in the discovery of LYS744 (6, Dmt-DNle-Gly-Phe(p-Cl)-Ppp), a multifunctional ligand with MOR/DOR agonist and KOR antagonist activity (GTPγS assay: IC₅₀ = 52 nM, I_max = 122% cf. IC₅₀ = 59 nM, I_max = 100% for naloxone) with nanomolar range of binding affinity (K_i = 1.3 nM cf. K_i = 2.4 nM for salvinorin A). Based on its unique biological profile, 6 is considered to possess high therapeutic potential for the treatment of chronic pain by modulating pathological KOR activation while retaining analgesic efficacy attributed to its MOR/DOR agonist activity.

Developing Cyclic Opioid Analogues: Fluorescently Labeled Bioconjugates of Biphalin

The development of bioconjugates is of pivotal importance in medicinal chemistry due to their potential applications as therapeutic agents to improve the targeting of specific diseases, decrease toxicity, or control drug release. In this work we achieved the synthesis and characterization of three novel opioid peptides fluorescently labeled, analogues of cyclic biphalin derivatives, namely 1D, 1C, and 2C. Among them, compound 1D, containing a dansyl-maleimide motif, exhibited an excellent binding affinity and functional potency for the δ-opioid receptor (DOR). 1D also demonstrated a strong fluorescence emission spectrum ranging from 300 to 700 nm. These features could be highly desirable for medical and biological applications needed for targeting the DOR, including in vivo imaging, and as a lead for the design of fluorescent probes.

Selective and Wash-Resistant Fluorescent Dihydrocodeinone Derivatives Allow Single-Molecule Imaging of μ-Opioid Receptor Dimerization

μ-Opioid receptors (μ-ORs) play a critical role in the modulation of pain and mediate the effects of the most powerful analgesic drugs. Despite extensive efforts, it remains insufficiently understood how μ-ORs produce specific effects in living cells. We developed new fluorescent ligands based on the μ-OR antagonist E-p-nitrocinnamoylamino-dihydrocodeinone (CACO), that display high affinity, long residence time and pronounced selectivity. Using these ligands, we achieved single-molecule imaging of μ-ORs on the surface of living cells at physiological expression levels. Our results reveal a high heterogeneity in the diffusion of μ-ORs, with a relevant immobile fraction. Using a pair of fluorescent ligands of different color, we provide evidence that μ-ORs interact with each other to form short-lived homodimers on the plasma membrane. This approach provides a new strategy to investigate μ-OR pharmacology at single-molecule level.

Neurobiological Effects of Morphine after Spinal Cord Injury

Opioids and non-steroidal anti-inflammatory drugs are used commonly to manage pain in the early phase of spinal cord injury (SCI). Despite its analgesic efficacy, however, our studies suggest that intrathecal morphine undermines locomotor recovery and increases lesion size in a rodent model of SCI. Similarly, intravenous (IV) morphine attenuates locomotor recovery. The current study explores whether IV morphine also increases lesion size after a spinal contusion (T12) injury and quantifies the cell types that are affected by early opioid administration. Using an experimenter-administered escalating dose of IV morphine across the first seven days post-injury, we quantified the expression of neuron, astrocyte, and microglial markers at the injury site. SCI decreased NeuN expression relative to shams. In subjects with SCI treated with IV morphine, virtually no NeuN⁺ cells remained across the rostral-caudal extent of the lesion. Further, whereas SCI per se increased the expression of astrocyte and microglial markers (glial fibrillary acidic protein and OX-42, respectively), morphine treatment decreased the expression of these markers. These cellular changes were accompanied by attenuation of locomotor recovery (Basso, Beattie, Bresnahan scores), decreased weight gain, and the development of opioid-induced hyperalgesia (increased tactile reactivity) in morphine-treated subjects. These data suggest that morphine use is contraindicated in the acute phase of a spinal injury. Faced with a lifetime of intractable pain, however, simply removing any effective analgesic for the management of SCI pain is not an ideal option. Instead, these data underscore the critical need for further understanding of the molecular pathways engaged by conventional medications within the pathophysiological context of an injury.

Delta-opioid receptor analgesia is independent of microglial activation in a rat model of neuropathic pain

The analgesic effect of delta-opioid receptor (DOR) ligands in neuropathic pain is not diminished in contrast to other opioid receptor ligands, which lose their effectiveness as analgesics. In this study, we examine whether this effect is related to nerve injury-induced microglial activation. We therefore investigated the influence of minocycline-induced inhibition of microglial activation on the analgesic effects of opioid receptor agonists: morphine, DAMGO, U50,488H, DPDPE, Deltorphin II and SNC80 after chronic constriction injury (CCI) to the sciatic nerve in rats. Pre-emptive and repeated administration of minocycline (30 mg/kg, i.p.) over 7 days significantly reduced allodynia and hyperalgesia as measured on day 7 after CCI. The antiallodynic and antihyperalgesic effects of intrathecally (i.t.) administered morphine (10–20 µg), DAMGO (1–2 µg) and U50,488H (25–50 µg) were significantly potentiated in rats after minocycline, but no such changes were observed after DPDPE (10–20 µg), deltorphin II (1.5–15 µg) and SNC80 (10–20 µg) administration. Additionally, nerve injury-induced down-regulation of all types of opioid receptors in the spinal cord and dorsal root ganglia was not influenced by minocycline, which indicates that the effects of opioid ligands are dependent on other changes, presumably neuroimmune interactions. Our study of rat primary microglial cell culture using qRT-PCR, Western blotting and immunocytochemistry confirmed the presence of mu-opioid receptors (MOR) and kappa-opioid receptors (KOR), further we provide the first evidence for the lack of DOR on microglial cells. In summary, DOR analgesia is different from analgesia induced by MOR and KOR receptors because it does not dependent on injury-induced microglial activation. DOR agonists appear to be the best candidates for new drugs to treat neuropathic pain.

Exploring the neuroimmunopharmacology of opioids: an integrative review of mechanisms of central immune signaling and their implications for opioid analgesia

Vastly stimulated by the discovery of opioid receptors in the early 1970s, preclinical and clinical research was directed at the study of stereoselective neuronal actions of opioids, especially those played in their crucial analgesic role. However, during the past decade, a new appreciation of the non-neuronal actions of opioids has emerged from preclinical research, with specific appreciation for the nonclassic and nonstereoselective sites of action. Opioid activity at Toll-like receptors, newly recognized innate immune pattern recognition receptors, adds substantially to this unfolding story. It is now apparent from molecular and rodent data that these newly identified signaling events significantly modify the pharmacodynamics of opioids by eliciting proinflammatory reactivity from glia, the immunocompetent cells of the central nervous system. These central immune signaling events, including the release of cytokines and chemokines and the associated disruption of glutamate homeostasis, cause elevated neuronal excitability, which subsequently decreases opioid analgesic efficacy and leads to heightened pain states. This review will examine the current preclinical literature of opioid-induced central immune signaling mediated by classic and nonclassic opioid receptors. A unification of the preclinical pharmacology, neuroscience, and immunology of opioids now provides new insights into common mechanisms of chronic pain, naive tolerance, analgesic tolerance, opioid-induced hyperalgesia, and allodynia. Novel pharmacological targets for future drug development are discussed in the hope that disease-modifying chronic pain treatments arising from the appreciation of opioid-induced central immune signaling may become practical.

6-N,N-dimethylamino-2,3-naphthalimide: a new environment-sensitive fluorescent probe in delta- and mu-selective opioid peptides

A new environment-sensitive fluorophore, 6-N,N-(dimethylamino)-2,3-naphthalimide (6DMN) was introduced in the delta-selective opioid peptide agonist H-Dmt-Tic-Glu-NH(2) and in the mu-selective opioid peptide agonist endomorphin-2 (H-Tyr-Pro-Phe-Phe-NH(2)). Environment-sensitive fluorophores are a special class of chromophores that generally exhibit a low quantum yield in aqueous solution but become highly fluorescent in nonpolar solvents or when bound to hydrophobic sites in proteins or membranes. New fluorescent delta-selective irreversible antagonists (H-Dmt-Tic-Glu-NH-(CH(2))(5)-CO-Dap(6DMN)-NH(2) (1) and H-Dmt-Tic-Glu-Dap(6DMN)-NH(2) (2)) were identified as potential fluorescent probes showing good properties for use in studies of distribution and internalization of delta receptors by confocal laser scanning microscopy.

Hydrophilic labeling reagents of dipyrrylmethene-BF2 dyes for two-photon excited fluorometry: syntheses and photophysical characterization

Recently introduced bioaffinity assay technology, ArcDia TPX, is based on two-photon excited fluorescence (TPE) and it enables separation-free ultra-sensitive immunoassays from microvolumes. Here we present syntheses of novel two-photon excitable fluorescent labeling reagents which have been specially designed to be used as label molecules in the ArcDia TPX assay technique. The labeling reagents are based on dipyrrylmetheneboron difluoride (dipyrrylmethene-BF2) chromophore, which have been substituted with aryl, heteroaryl or arylalkenyl chemical groups to extend the pi-electron conjugation. These substitutions results in a series of dipyrrylmethene-BF2 fluorophores with different photophysical properties. Dipyrrylmethene-BF2 fluorophores have been further substituted with a dipeptide linker unit and finally activated as succinimidyl esters to enable specific coupling with primary amino groups. The dipeptide linker serves as a spacer arm between the label and a target, and enhances the solubility of the label in aqueous solutions. Study of the chemical and photophysical performance of the new labeling reagents is described. The new labeling reagents exhibit high fluorescence quantum yields, and molar absorption coefficients. The results show that the new labels with the hydrophilic dipeptide linker unit provide large two-photon excitation cross-sections, high fluorescence quantum efficiency and good solubility in aqueous solutions. The results suggest that the novel dipyrrylmethene-BF2 labels are highly applicable to bioaffinity assays based on two-photon excitation of fluorescence.

De novo identification of highly active fluorescent kappa opioid ligands from a rhodamine labeled tetrapeptide positional scanning library

Highly active fluorescent compounds having kappa opioid activity were identified following the screening in a kappa-specific radioligand binding assay of a positional scanning tetrapeptide combinatorial library in which every tetrapeptide was fluorescently labeled. Lissamine rhodamine B sulfonyl chloride was coupled to the N terminal of a mixture-based tetrapeptide positional scanning library made up of over 7.3 million tetrapeptides. Upon determination of the most active mixtures for each position of the library in the kappa binding assay, individual rhodamine labeled tetrapeptides were then synthesized and tested to determine their activities. Eight individual rhodamine labeled peptides were identified that were specific for the kappa opioid receptor, having binding affinities ranging from 5-20 nM. These peptides were poor inhibitors at the mu and delta receptors (K(i)>5,000 nM). Furthermore, neither rhodamine itself nor these same tetrapeptides lacking the N-terminal rhodamine had any significant activity at the kappa receptor, indicating that both the tetrapeptide sequence and the rhodamine moiety are required for receptor binding. This study has demonstrated that novel fluorescent compounds with intrinsic activity can be identified through the use of combinatorial chemistry.

Influence of linker unit on performance of palladium(II) coproporphyrin labelling reagent and its bioconjugates

In this paper we describe the preparation of a series of new phosphorescent labelling reagents, based on monosubstituted palladium(II) coproporphyrin-I and the isothiocyanato reactive group. The labelling reagents differ with respect to the chemical composition of the linker unit that combines the reactive group and the porphyrin chromophore. Altogether, seven different labelling reagents are prepared. The new labelling reagents are conjugated with monoclonal mouse IgG to yield label conjugates with variable degrees of conjugation. The effect is studied of linker unit on: (a) the conjugation reaction kinetics; (b) the biological activity of the resulting IgG conjugates; and (c) the efficiency of phosphorescence emission. The results show that an increase in the length of the linker unit has a positive effect on both the reactivity of the label and the biological activity of the resulting conjugates. In addition, the results indicate that the labels with the most hydrophilic linker units exhibit the highest phosphorescence emission efficiencies.

Binding and internalization of fluorescent opioid peptide conjugates in living cells

The dynamics of agonist-stimulated opioid receptor internalization and trafficking have been difficult to study in living cells in part because the available probes were inadequate. To overcome this obstacle, six new fluorescent opioid peptides were developed. Dermorphin (DERM), deltorphin (DELT), TIPP, and endomorphin were conjugated to BODIPY TR or Alexa Fluor 488, two fluorescent dyes with distinct hydrophobic properties. In membrane binding assays the fluorescent conjugates DERM-A488 or -BTR, DELT-A488 or -BTR, and TIPP-A488 displayed good binding affinity and selectivity for mu- and delta-opioid receptor subtypes. Furthermore, the fluorescent conjugates of dermorphin and deltorphin were biologically active as demonstrated by their ability to hyperpolarize locus coeruleus neurons (DERM-A488 or -BTR) and inhibit calcium currents in NG108-15 (DELT-A488). Both of these responses were antagonized by naloxone. In conjunction with confocal fluorescent microscopy the trafficking of these fluorescent ligands was monitored in real-time. The internalization of these ligands by mu- and delta-opioid receptors was found to be naloxone-sensitive and temperature-dependent. Interestingly, once these ligands were internalized the fluorescent puncta that formed became distributed in one of two patterns. In Chinese hamster ovary cells heterologously expressing either mu- or delta-opioid receptors the intracellular puncta were concentrated in the perinuclear region of the cell, whereas they were distributed throughout the cytoplasm in cells derived from either NG108-15 or SH-SY5Y cells. In summary, we have demonstrated that these novel, fluorescent opioid peptide conjugates permit real-time visual tracking of receptor-ligand complexes, including their internalization and trafficking, in living cells.

Fluorescein-labeled naloxone binding to mu opioid receptors on live Chinese hamster ovary cells using confocal fluorescent microscopy

A general method of confocal laser scanning microscopy was used to demonstrate specific binding of fluorescein-labeled naloxone (FNAL, 10-50 nM) to stably transfected mu opioid receptors on live Chinese hamster ovary cells. Nonspecific binding was visually indistinguishable from autofluorescence in cells with intact cell membranes. Fluorescent labeling of cell perimeters, not present in control nontransfected cells, reversed in transfected cells upon washout of FNAL or following the addition of either unlabeled naloxone (25 microM) or the mu specific antagonist CTOP (1 microM). The addition of the delta and kappa specific agonists DPDPE (1 microM) and U50488 (1 microM), respectively, failed to reverse the labeling. Further evidence of specific binding was obtained from kinetic experiments, where it was observed that only transfected cells showed a time-dependent exponential change in fluorescence that permitted estimation of association and dissociation binding rate constants of (5.8+/-0.5, mean+/-S.E.M.)x10(5) M(-1) s(-1) and (3.3+/-0.6)x10(-3) s(-1), respectively and a kinetically derived dissociation constant of 5.7+/-1.4 nM. These estimates were comparable to those obtained under similar conditions in radioligand binding experiments using [3H]-naloxone.

NTI4F: a non-peptide fluorescent probe selective for functional delta opioid receptors

A non-peptide fluorescent probe, NTI4F, has been developed for the delta (delta) opioid receptor. The probe is a potent delta-antagonist in the mouse vas deferens (MVD) smooth muscle assay and it binds to the delta opioid receptor with high affinity (Ki = 1 nM) and selectivity. Confocal microscopy indicates that the probe binds to Madin-Darby canine kidney (MDCK) cells transfected with the delta opioid receptor. This binding can be blocked by the delta opioid receptor antagonist, naltrindole (NTI), but not by morphine or ethylketazocine (EK) which are mu (mu)- and kappa (kappa)-selective ligands. This fluorescent probe should prove useful in the study of the distribution of the delta opioid receptor.

Synthesis and characterization of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-labeled fluorescent ligands for the mu opioid receptor

A series of opioid ligands utilizing the 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) fluorophores 4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene++ +-3-propionic acid or 4,4-difluoro-5-(4-phenyl-1,3-butadienyl)-4-bora-3a,4a-diaza- s-indacene-3-propionic acid were synthesized and characterized for their ability to act as a suitable fluorescent label for the mu opioid receptor. All compounds displaced the mu opioid receptor binding of [3H]Tyr-D-Ala-Gly-(Me)Phe-Gly-ol in monkey brain membranes with high affinity. The binding of fluorescent ligands to delta and kappa receptors was highly variable. 5,7-Dimethyl-BODIPY naltrexamine, "6-BNX," displayed subnanomolar affinities for the mu and kappa opioid receptors (Ki 0.07 and 0.43 nM, respectively) and nanomolar affinity at the delta (Ki 1.4 nM) receptor. Using fluorescence spectroscopy, the binding of 6-BNX in membranes from C6 glioma cells transfected with the cloned mu opioid receptor was investigated. In these membranes containing a high receptor density (10-80 pmol/mg protein), 6-BNX labeling was saturable, mu opioid specific, stereoselective (as determined with the isomers dextrorphan and levorphanol), and more than 90% specific. The results describe a series of newly developed fluorescent ligands for the mu opioid receptor and the use of one of these ligands as a label for the cloned mu receptor. These ligands provide a new approach for studying the structural and biophysical nature of opioid receptors.