Specific receptor for the opioid peptide dynorphin: structure--activity relationships

Antibodies expand the scope of angiotensin receptor pharmacology

G-protein-coupled receptors (GPCRs) are key regulators of human physiology and are the targets of many small-molecule research compounds and therapeutic drugs. While most of these ligands bind to their target GPCR with high affinity, selectivity is often limited at the receptor, tissue and cellular levels. Antibodies have the potential to address these limitations but their properties as GPCR ligands remain poorly characterized. Here, using protein engineering, pharmacological assays and structural studies, we develop maternally selective heavy-chain-only antibody ('nanobody') antagonists against the angiotensin II type I receptor and uncover the unusual molecular basis of their receptor antagonism. We further show that our nanobodies can simultaneously bind to angiotensin II type I receptor with specific small-molecule antagonists and demonstrate that ligand selectivity can be readily tuned. Our work illustrates that antibody fragments can exhibit rich and evolvable pharmacology, attesting to their potential as next-generation GPCR modulators.

Development of a Selective Peptide κ-Opioid Receptor Antagonist by Late-Stage Functionalization with Cysteine Staples

The κ-opioid receptor (KOR) is an attractive target for the development of novel drugs. KOR agonists are potentially safer pain medications, whereas KOR antagonists are promising drug candidates for the treatment of neuropsychiatric disorders. Hitherto, the vast majority of selective drug leads that have been developed for KOR are small molecules. In this study, novel peptide probes were designed by using an endogenous dynorphin A1-13 sequence as a template for peptide stapling via late-stage cysteine functionalization. Leveraging this strategy, we developed a stable and potent KOR antagonist, CSD-CH2(1,8)-NH2, with approximately 1000-fold improved selectivity for KOR over μ- and δ-opioid receptors. Its potent competitive KOR antagonism was verified in KOR-expressing cells, peripheral dorsal root ganglion neurons, and using the tail-flick and rotarod tests in mice. This work highlights the value of cysteine stapling to develop selective peptide probes to modulate central KOR function, as innovative peptide drug candidates for the treatment of KOR-related illnesses.

Antibodies Expand the Scope of Angiotensin Receptor Pharmacology

G protein-coupled receptors (GPCRs) are key regulators of human physiology and are the targets of many small molecule research compounds and therapeutic drugs. While most of these ligands bind to their target GPCR with high affinity, selectivity is often limited at the receptor, tissue, and cellular level. Antibodies have the potential to address these limitations but their properties as GPCR ligands remain poorly characterized. Here, using protein engineering, pharmacological assays, and structural studies, we develop maternally selective heavy chain-only antibody ("nanobody") antagonists against the angiotensin II type I receptor (AT1R) and uncover the unusual molecular basis of their receptor antagonism. We further show that our nanobodies can simultaneously bind to AT1R with specific small-molecule antagonists and demonstrate that ligand selectivity can be readily tuned. Our work illustrates that antibody fragments can exhibit rich and evolvable pharmacology, attesting to their potential as next-generation GPCR modulators.

Structures of the entire human opioid receptor family

Opioids are effective analgesics, but their use is beset by serious side effects, including addiction and respiratory depression, which contribute to the ongoing opioid crisis. The human opioid system contains four opioid receptors (μOR, δOR, κOR, and NOPR) and a set of related endogenous opioid peptides (EOPs), which show distinct selectivity toward their respective opioid receptors (ORs). Despite being key to the development of safer analgesics, the mechanisms of molecular recognition and selectivity of EOPs to ORs remain unclear. Here, we systematically characterize the binding of EOPs to ORs and present five structures of EOP-OR-Gi complexes, including β-endorphin- and endomorphin-bound μOR, deltorphin-bound δOR, dynorphin-bound κOR, and nociceptin-bound NOPR. These structures, supported by biochemical results, uncover the specific recognition and selectivity of opioid peptides and the conserved mechanism of opioid receptor activation. These results provide a structural framework to facilitate rational design of safer opioid drugs for pain relief.

Peptidomimetics and Their Applications for Opioid Peptide Drug Discovery

Despite various advantages, opioid peptides have been limited in their therapeutic uses due to the main drawbacks in metabolic stability, blood-brain barrier permeability, and bioavailability. Therefore, extensive studies have focused on overcoming the problems and optimizing the therapeutic potential. Currently, numerous peptide-based drugs are being marketed thanks to new synthetic strategies for optimizing metabolism and alternative routes of administration. This tutorial review briefly introduces the history and role of natural opioid peptides and highlights the key findings on their structure-activity relationships for the opioid receptors. It discusses details on opioid peptidomimetics applied to develop therapeutic candidates for the treatment of pain from the pharmacological and structural points of view. The main focus is the current status of various mimetic tools and the successful applications summarized in tables and figures.

Interactions between the κ opioid system, corticotropin-releasing hormone and oxytocin in partner loss

Selective adult social attachments, or ‘pair bonds’, represent central relationships for individuals in a number of social species, including humans. Loss of a pair mate has emotional consequences that may or may not diminish over time, and that often translate into impaired psychological and physical health. In this paper, we review the literature on the neuroendocrine mechanisms for the emotional consequences of partner loss, with a special focus on hypothesized interactions between oxytocin, corticotropin-releasing hormone and the κ opioid system.

This article is part of the theme issue ‘Interplays between oxytocin and other neuromodulators in shaping complex social behaviours’.

Opportunities and Challenges for In Silico Drug Discovery at Delta Opioid Receptors

The delta opioid receptor is a Gi-protein-coupled receptor (GPCR) with a broad expression pattern both in the central nervous system and the body. The receptor has been investigated as a potential target for a multitude of significant diseases including migraine, alcohol use disorder, ischemia, and neurodegenerative diseases. Despite multiple attempts, delta opioid receptor-selective molecules have not been translated into the clinic. Yet, the therapeutic promise of the delta opioid receptor remains and thus there is a need to identify novel delta opioid receptor ligands to be optimized and selected for clinical trials. Here, we highlight recent developments involving the delta opioid receptor, the closely related mu and kappa opioid receptors, and in the broader area of the GPCR drug discovery research. We focus on the validity and utility of the available delta opioid receptor structures. We also discuss the increased ability to perform ultra-large-scale docking studies on GPCRs, the rise in high-resolution cryo-EM structures, and the increased prevalence of machine learning and artificial intelligence in drug discovery. Overall, we pose that there are multiple opportunities to enable in silico drug discovery at the delta opioid receptor to identify novel delta opioid modulators potentially with unique pharmacological properties, such as biased signaling.

Neuropeptide System Regulation of Prefrontal Cortex Circuitry: Implications for Neuropsychiatric Disorders

Neuropeptides, a diverse class of signaling molecules in the nervous system, modulate various biological effects including membrane excitability, synaptic transmission and synaptogenesis, gene expression, and glial cell architecture and function. To date, most of what is known about neuropeptide action is limited to subcortical brain structures and tissue outside of the central nervous system. Thus, there is a knowledge gap in our understanding of neuropeptide function within cortical circuits. In this review, we provide a comprehensive overview of various families of neuropeptides and their cognate receptors that are expressed in the prefrontal cortex (PFC). Specifically, we highlight dynorphin, enkephalin, corticotropin-releasing factor, cholecystokinin, somatostatin, neuropeptide Y, and vasoactive intestinal peptide. Further, we review the implication of neuropeptide signaling in prefrontal cortical circuit function and use as potential therapeutic targets. Together, this review summarizes established knowledge and highlights unknowns of neuropeptide modulation of neural function underlying various biological effects while offering insights for future research. An increased emphasis in this area of study is necessary to elucidate basic principles of the diverse signaling molecules used in cortical circuits beyond fast excitatory and inhibitory transmitters as well as consider components of neuropeptide action in the PFC as a potential therapeutic target for neurological disorders. Therefore, this review not only sheds light on the importance of cortical neuropeptide studies, but also provides a comprehensive overview of neuropeptide action in the PFC to serve as a roadmap for future studies in this field.

Opioid ligands addressing unconventional binding sites and more than one opioid receptor subtype

Opioid receptors (ORs) represent one of the most significant groups of G-protein coupled receptor (GPCR) drug targets and also act as prototypical models for GPCR function. In a constant effort to develop drugs with less side effects, and tools to explore the ORs nature and function, various (poly)pharmacological ligand design approaches have been performed. That is, besides classical ligands, a great number of bivalent ligands (i.e. aiming on two distinct OR subtypes), univalent heteromer-selective ligands and bitopic and allosteric ligands have been synthesized for the ORs. The scope of our review is to present the most important of the aforementioned ligands, highlight their properties and exhibit the current state-of-the-art pallet of promising drug candidates or useful molecular tools for the ORs.

Investigation of Tumor Cells and Receptor-Ligand Simulation Models for the Development of PET Imaging Probes Targeting PSMA and GRPR and a Possible Crosstalk between the Two Receptors

Prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPR) have both been used in nuclear medicine as targets for molecular imaging and therapy of prostate (PCa) and breast cancer (BCa). Three bioconjugate probes, the PSMA specific: [⁶⁸Ga]Ga-1, ((HBED-CC)-Ahx-Lys-NH-CO-NH Glu or PSMA-11), the GRPR specific: [⁶⁸Ga]Ga-2, ((HBED-CC)-4-amino-1-carboxymethyl piperidine-[D-Phe⁶, Sta¹³]BN(6-14), a bombesin (BN) analogue), and 3 (the BN analogue: 4-amino-1-carboxymethyl piperidine-[(R)-Phe⁶, Sta¹³]BN(6-14) connected with the fluorescent dye, BDP-FL), were synthesized and tested in vitro with PCa and BCa cell lines, more specifically, with PCa cells, PC-3 and LNCaP, with BCa cells, T47D, MDA-MB-231, and with the in-house created PSMA-overexpressing PC-3^(PSMA), T47D^(PSMA), and MDA-MB-231^(PSMA). In addition, biomolecular simulations were conducted on the association of 1 and 2 with PSMA and GRPR. The PSMA overexpression resulted in an increase of cell-bound radioligand [⁶⁸Ga]Ga-1 (PSMA) for PCa and BCa cells and also of [⁶⁸Ga]Ga-2 (GRPR), especially in those cell lines already expressing GRPR. The results were confirmed by fluorescence-activated cell sorting with a PE-labeled PSMA-specific antibody and the fluorescence tracer 3. The docking calculations and molecular dynamics simulations showed how 1 enters the PSMA funnel region and how pharmacophore Glu-urea-Lys interacts with the arginine patch, the S1', and S1 subpockets by forming hydrogen and van der Waals bonds. The chelating moiety of 1, that is, HBED-CC, forms additional stabilizing hydrogen bonding and van der Waals interactions in the arene-binding site. Ligand 2 is diving into the GRPR transmembrane (TM) helical cavity, thereby forming hydrogen bonds through its amidated end, water-mediated hydrogen bonds, and π-π interactions. Our results provide valuable information regarding the molecular mechanisms involved in the interactions of 1 and 2 with PSMA and GRPR, which might be useful for the diagnostic imaging and therapy of PCa and BCa.

Pharmacology of Kappa Opioid Receptors: Novel Assays and Ligands

The present study investigated the in vitro pharmacology of the human kappa opioid receptor using multiple assays, including calcium mobilization in cells expressing chimeric G proteins, the dynamic mass redistribution (DMR) label-free assay, and a bioluminescence resonance energy transfer (BRET) assay that allows measurement of receptor interaction with G protein and β-arrestin 2. In all assays, dynorphin A, U-69,593, and [D-Pro10]dyn(1-11)-NH2 behaved as full agonists with the following rank order of potency [D-Pro10]dyn(1-11)-NH2 > dynorphin A ≥ U-69,593. [Dmt1,Tic2]dyn(1-11)-NH2 behaved as a moderate potency pure antagonist in the kappa-β-arrestin 2 interaction assay and as low efficacy partial agonist in the other assays. Norbinaltorphimine acted as a highly potent and pure antagonist in all assays except kappa-G protein interaction, where it displayed efficacy as an inverse agonist. The pharmacological actions of novel kappa ligands, namely the dynorphin A tetrameric derivative PWT2-Dyn A and the palmitoylated derivative Dyn A-palmitic, were also investigated. PWT2-Dyn A and Dyn A-palmitic mimicked dynorphin A effects in all assays showing similar maximal effects but 3–10 fold lower potency. In conclusion, in the present study, multiple in vitro assays for the kappa receptor have been set up and pharmacologically validated. In addition, PWT2-Dyn A and Dyn A-palmitic were characterized as potent full agonists; these compounds are worthy of further investigation in vivo for those conditions in which the activation of the kappa opioid receptor elicits beneficial effects e.g. pain and pruritus.

Analgesic Opioid Ligand Discovery Based on Nonmorphinan Scaffolds Derived from Natural Sources

Strong opioid analgesics, including morphine, are the mainstays for treating moderate to severe acute pain and alleviating chronic cancer pain. However, opioid-related adverse effects, including nausea or vomiting, sedation, respiratory depression, constipation, pruritus (itch), analgesic tolerance, and addiction and abuse liability, are problematic. In addition, the use of opioids to relieve chronic noncancer pain is controversial due to the "opioid crisis" characterized by opioid misuse or abuse and escalating unintentional death rates due to respiratory depression. Hence, considerable research internationally has been aimed at the "Holy Grail" of the opioid analgesic field, namely the discovery of novel and safer opioid analgesics with improved opioid-related adverse effects. In this Perspective, medicinal chemistry strategies are addressed, where structurally diverse nonmorphinan-based opioid ligands derived from natural sources were deployed as lead molecules. The current state of play, clinical or experimental status, and novel opioid ligand discovery approaches are elaborated in the context of retaining analgesia with improved safety and reduced adverse effects, especially addiction liability.

Modulating β-arrestin 2 recruitment at the δ- and μ-opioid receptors using peptidomimetic ligands

μ-Opioid receptor agonists provide potent and effective acute analgesia; however, their therapeutic window narrows considerably upon repeated administration, such as required for treating chronic pain. In contrast, bifunctional μ/δ opioid agonists, such as the endogenous enkephalins, have potential for treating both acute and chronic pain. However, enkephalins recruit β-arrestins, which correlate with certain adverse effects at μ- and δ-opioid receptors. Herein, we identify the C-terminus of Tyr-ψ[(Z)CF[double bond, length as m-dash]CH]-Gly-Leu-enkephalin, a stable enkephalin derivative, as a key site to regulate bias of both δ- and μ-opioid receptors. Using in vitro assays, substitution of the Leu⁵ carboxylate with amides (NHEt, NMe₂, N^CyPr) reduced β-arrestin recruitment efficacy through both the δ-opioid and μ-opioid, while retaining affinity and cAMP potency. For this series, computational studies suggest key ligand-receptor interactions that might influence bias. These findings should enable the discovery of a range of tool compounds with previously unexplored biased μ/δ opioid agonist pharmacological profiles.

Genome Mining-Based Discovery of Blenny Fish-Derived Peptides Targeting the Mouse κ-Opioid Receptor

Over the past years, peptides have attracted increasing interest for G protein-coupled receptor (GPCR) drug discovery and development. Peptides occupy a unique chemical space that is not easily accessible for small molecules and antibodies and provide advantages over these ligand classes such as lower toxicity and higher selectivity. The κ-opioid receptor (KOR) is a prototypic GPCR and an appealing therapeutic target for the development of safer and more effective analgesics. Recently, peptides have emerged as analgesic drug candidates with improved side effect profiles. We have previously identified plant-derived peptides, which activate KOR. Based on this precedent, here we relied on publicly available databases to discover novel KOR peptide ligands by genome mining. Using human preprodynorphin as a query, we identified blenny fish-derived peptides, referred to as blenniorphins, capable of binding to and activating KOR with nanomolar affinity and potency, respectively. Additionally, the blenniorphins altered β-arrestin-2 recruitment at the KOR. Our study demonstrates the utility of genome mining to identify peptide GPCR ligands with intriguing pharmacological properties and unveils the potential of blenny fishes as a source for novel KOR ligands.

Morphine alkaloids: History, biology, and synthesis

This chapter provides a short overview of the history of morphine since it's isolation by Sertürner in 1805. The biosynthesis of the title alkaloid as well as all total and formal syntheses of morphine and codeine published after 1996 are discussed in detail. The last section of this chapter provides a detailed overview of medicinally relevant derivatives of the title alkaloid.

Plant-Derived Cyclotides Modulate κ-Opioid Receptor Signaling

Cyclotides are plant-derived disulfide-rich peptides comprising a cyclic cystine knot, which confers remarkable stability against thermal, proteolytic, and chemical degradation. They represent an emerging class of G protein-coupled receptor (GPCR) ligands. In this study, utilizing a screening approach of plant extracts and pharmacological analysis we identified cyclotides from Carapichea ipecacuanha to be ligands of the κ-opioid receptor (KOR), an attractive target for developing analgesics with reduced side effects and therapeutics for multiple sclerosis (MS). This prompted us to verify whether [T20K]kalata B1, a cyclotide in clinical development for the treatment of MS, is able to modulate KOR signaling. T20K bound to and fully activated KOR in the low μM range. We then explored the ability of T20K to allosterically modulate KOR. Co-incubation of T20K with KOR ligands resulted in positive allosteric modulation in functional cAMP assays by altering either the efficacy of dynorphin A1-13 or the potency and efficacy of U50,488 (a selective KOR agonist), respectively. In addition, T20K increased the basal response upon cotreatment with U50,488. In the bioluminescence resonance energy transfer assay T20K negatively modulated the efficacy of U50,488. This study identifies cyclotides capable of modulating KOR and highlights the potential of plant-derived peptides as an opportunity to develop cyclotide-based KOR modulators.

Design of a Stable Cyclic Peptide Analgesic Derived from Sunflower Seeds that Targets the κ-Opioid Receptor for the Treatment of Chronic Abdominal Pain

The rising opioid crisis has become a worldwide societal and public health burden, resulting from the abuse of prescription opioids. Targeting the κ-opioid receptor (KOR) in the periphery has emerged as a powerful approach to develop novel pain medications without central side effects. Inspired by the traditional use of sunflower (Helianthus annuus) preparations for analgesic purposes, we developed novel stabilized KOR ligands (termed as helianorphins) by incorporating different dynorphin A sequence fragments into a cyclic sunflower peptide scaffold. As a result, helianorphin-19 selectively bound to and fully activated the KOR with nanomolar potency. Importantly, helianorphin-19 exhibited strong KOR-specific peripheral analgesic activity in a mouse model of chronic visceral pain, without inducing unwanted central effects on motor coordination/sedation. Our study provides a proof of principle that cyclic peptides from plants may be used as templates to develop potent and stable peptide analgesics applicable via enteric administration by targeting the peripheral KOR for the treatment of chronic abdominal pain.

Kappa Opioid Receptor Mediated Differential Regulation of Serotonin and Dopamine Transporters in Mood and Substance Use Disorder

Dynorphin (DYN) is an endogenous neurosecretory peptide which exerts its activity by binding to the family of G protein-coupled receptors, namely the kappa opioid receptor (KOR). Opioids are associated with pain, analgesia, and drug abuse, which play a central role in mood disorders with monoamine neurotransmitter interactions. Growing evidence demonstrates the cellular signaling cascades linked to KOR-mediated monoamine transporters regulation in cell models and native brain tissues. This chapter will review DYN/KOR role in mood and addiction in relevance to dopaminergic and serotonergic neurotransmissions. Also, we discuss the recent findings on KOR-mediated differential regulation of serotonin and dopamine transporters (SERT and DAT). These findings led to a better understanding of the role of DYN/KOR system in aminergic neurotransmission via its modulatory effect on both amine release and clearance. Detailed knowledge of these processes at the molecular level enables designing novel pharmacological reagents to target transporter motifs to treat mood and addiction and reduce unwanted side effects such as aversion, dysphoria, sedation, and psychomimesis.

Dynorphins in Development and Disease: Implications for Cardiovascular Disease

It is well-established that cardiovascular disease continues to represent a growing health problem and significant effort has been made to elucidate the underlying mechanisms. In this review, we report on past and recent high impact publications in the field of intracrine network signaling, focusing specifically on opioids and their interrelation with key modulators of the cardiovascular system and the onset of related disease. We present an overview of studies outlining the scope of cardiovascular and cerebrovascular processes that are affected by opioids, including heart function, ischemia, reperfusion, and blood flow. Specific emphasis is placed on the importance of dynorphin molecules in cerebrovascular and cardiovascular regulation. Evidence suggests that excessive or insufficient dynorphin could make an important contribution to cardiovascular physiology, yet numerous paradoxical observations frequently impede a clear understanding of the role of dynorphin. Thus, we argue that dynorphin-mediated signaling events for which an immediate regulatory effect is disputed should not be dismissed as unimportant, as they may play a role in cross-talk with other signaling networks. Finally, we consider the most recent evidence on the role of dynorphin during cardiovascular-related inflammation and on the potential value of endogenous and exogenous inhibitors of kappa-opioid receptor, a major dynorphin A receptor, to limit or prevent cardiovascular disease and its related sequelae.

Synthesis and Assessment of Fused β-Carboline Derivatives as Kappa Opioid Receptor Agonists

The synthesis of 5-formyl-6-aryl-6H-indolo[3,2,1-de][1,5] naphthyridine-2-carboxylates by reaction between 1-formyl-9H-β-carbolines and cinnamaldehydes in the presence of pyrrolidine in water with microwave irradiation is described. Pharmacophoric modification of the formyl group offered several new fused β-carboline derivatives, which were investigated for their κ-opioid receptor (KOR) agonistic activity. Two compounds 4 a and 4 c produced appreciable agonist activity on KOR with EC₅₀ values of 46±19 and 134±9 nM, respectively. Moreover, compound-induced KOR signaling studies suggested both compounds to be extremely G-protein-biased agonists. The analgesic effect of 4 a was validated by the increase in tail flick latency in mice in a time-dependent manner, which was completely blocked by the KOR-selective antagonist norBNI. Moreover, unlike U50488, an unbiased full KOR agonist, 4 a did not induce sedation. The docking of 4 a with the human KOR was studied to rationalize the result.

Conformational Constraint between Aromatic Residue Side Chains in the "Message" Sequence of the Peptide Arodyn Using Ring Closing Metathesis Results in a Potent and Selective Kappa Opioid Receptor Antagonist

Kappa opioid receptor (KOR) antagonists have recently shown potential for treating drug addiction and mood disorders. The linear acetylated dynorphin A analog arodyn (Ac[Phe^1,2,3,Arg⁴,d-Ala⁸]dynorphin A-(1-11)NH₂), synthesized in our laboratory, demonstrated potent and selective KOR antagonism. Cyclization of arodyn could potentially stabilize the bioactive conformation and enhance its metabolic stability. The cyclization strategy employed involved ring closing metathesis between adjacent meta- or para-substituted Tyr(allyl) residues in the "message" sequence that were predicted in a docking study to yield analogs that would bind to the KOR with binding poses similar to arodyn. Consistent with the modeling, the resulting analogs retained KOR affinity similar to arodyn; the peptides involving cyclization between para O-allyl groups also retained high KOR selectivity, with one analog exhibiting KOR antagonist potency (K_B = 15 nM) similar to arodyn. These promising cyclized analogs with constrained aromatic residues represent novel leads for further exploration of KOR pharmacology.

Induced dysregulation of ACE2 by SARS-CoV-2 plays a key role in COVID-19 severity

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), the cause of COVID-19, is reported to increase the rate of mortality worldwide. COVID-19 is associated with acute respiratory symptoms as well as blood coagulation in the vessels (thrombosis), heart attack and stroke. Given the requirement of angiotensin converting enzyme 2 (ACE2) receptor for SARS-CoV-2 entry into host cells, here we discuss how the downregulation of ACE2 in the COVID-19 patients and virus-induced shift in ACE2 catalytic equilibrium, change the concentrations of substrates such as angiotensin II, apelin-13, dynorphin-13, and products such as angiotensin (1–7), angiotensin (1–9), apelin-12, dynorphin-12 in the human body. Substrates accumulation ultimately induces inflammation, angiogenesis, thrombosis, neuronal and tissue damage while diminished products lead to the loss of the anti-inflammatory, anti-thrombotic and anti-angiogenic responses. In this review, we focus on the viral-induced imbalance between ACE2 substrates and products which exacerbates the severity of COVID-19. Considering the roadmap, we propose multiple therapeutic strategies aiming to rebalance the products of ACE2 and to ameliorate the symptoms of the disease.

High affinity binding of SARS-CoV-2 spike protein enhances ACE2 carboxypeptidase activity

The novel severe acute respiratory syndrome coronavirus (SARS-CoV-2) has emerged to a pandemic and caused global public health crisis. Human angiotensin-converting enzyme 2(ACE2) was identified as the entry receptor for SARS-CoV-2. As a carboxypeptidase, ACE2 cleaves many biological substrates besides angiotensin II to control vasodilatation and vascular permeability. Given the nanomolar high affinity between ACE2 and SARS-CoV-2 spike protein, we investigated how this interaction would affect the enzymatic activity of ACE2. Surprisingly, SARS-CoV-2 trimeric spike protein increased ACE2 proteolytic activity ∼3-10 fold against model peptide substrates, such as caspase-1 substrate and Bradykinin-analog. The enhancement in ACE2 enzymatic function was mediated by the binding of SARS-CoV-2 spike RBD domain. These results highlighted the potential for SARS-CoV-2 infection to enhance ACE2 activity, which may be relevant to the cardiovascular symptoms associated with COVID-19.

Discovery of SHR0687, a Highly Potent and Peripheral Nervous System-Restricted KOR Agonist

Analgesics with no abuse liability are highly demanded in the market. KOR agonists have been proved to be strong analgesics without MOR agonist-related side effects, such as respiratory depression, tolerance, and dependence liability; however, activation of KOR in the central nervous system (CNS) may cause sedation and anxiety. It has been reported that peripheral KOR activation produces comparable bioactivity without CNS-related side effects. Herein, we designed and synthesized a novel tetrapeptide (SHR0687), which was shown to be a highly potent KOR agonist with excellent selectivity over other opioid receptors, such as MOR and DOR. In addition, SHR0687 displayed favorable PK profiles across species, as well as robust in vivo efficacy in a rat carrageenan-induced pain model. Notably, SHR0687 exhibited negligible blood-brain barrier penetration, which was meaningful in minimizing CNS-related side effects.

Capturing Peptide-GPCR Interactions and Their Dynamics

Many biological functions of peptides are mediated through G protein-coupled receptors (GPCRs). Upon ligand binding, GPCRs undergo conformational changes that facilitate the binding and activation of multiple effectors. GPCRs regulate nearly all physiological processes and are a favorite pharmacological target. In particular, drugs are sought after that elicit the recruitment of selected effectors only (biased ligands). Understanding how ligands bind to GPCRs and which conformational changes they induce is a fundamental step toward the development of more efficient and specific drugs. Moreover, it is emerging that the dynamic of the ligand–receptor interaction contributes to the specificity of both ligand recognition and effector recruitment, an aspect that is missing in structural snapshots from crystallography. We describe here biochemical and biophysical techniques to address ligand–receptor interactions in their structural and dynamic aspects, which include mutagenesis, crosslinking, spectroscopic techniques, and mass-spectrometry profiling. With a main focus on peptide receptors, we present methods to unveil the ligand–receptor contact interface and methods that address conformational changes both in the ligand and the GPCR. The presented studies highlight a wide structural heterogeneity among peptide receptors, reveal distinct structural changes occurring during ligand binding and a surprisingly high dynamics of the ligand–GPCR complexes.

High affinity binding of SARS-CoV-2 spike protein enhances ACE2 carboxypeptidase activity

A novel coronavirus (SARS-CoV-2) has emerged to a global pandemic and caused significant damages to public health. Human angiotensin-converting enzyme 2(ACE2) was identified as the entry receptor for SARS-CoV-2. As a carboxypeptidase, ACE2 cleaves many biological substrates besides Ang II to control vasodilatation and permeability. Given the nanomolar high affinity between ACE2 and SARS-CoV-2 spike protein, we wonder how this interaction would affect the enzymatic activity of ACE2. Surprisingly, SARS-CoV-2 trimeric spike protein increased ACE2 proteolytic activity ~3-10 fold when fluorogenic caspase-1 substrate and Bradykinin-analog peptides were used to characterize ACE2 activity. In addition, the enhancement was mediated by ACE2 binding of RBD domain of SARS-CoV-2 spike. These results highlighted the altered activity of ACE2 during SARS-CoV-2 infection and would shed new lights on the pathogenesis of COVID-19 and its complications for better treatments.

Dynorphin and its role in alcohol use disorder

The dynorphin / kappa opioid receptor (KOR) system has been implicated in many aspects that influence neuropsychiatric disorders. Namely, this system modulates neural circuits that primarily regulate reward seeking, motivation processing, stress responsivity, and pain sensitivity, thus affecting the development of substance and alcohol use disorder (AUD). The effects of this system are often bidirectional and depend on projection targets. To date, a majority of the studies focusing on this system have examined the KOR function using agonists and antagonists. Indeed, there are studies that have examined prodynorphin and dynorphin levels by measuring mRNA and tissue content levels; however, static levels of the neuropeptide and its precursor do not explain complete and online function of the peptide as would be explained by measuring dynorphin transmission in real time. New and exciting methods using optogenetics, chemogenetics, genetic sensors, fast scan cyclic voltammetry are now being developed to detect various neuropeptides with a focus on opioid peptides, including dynorphin. In this review we discuss studies that examine dynorphin projections in areas involved in AUD, its functional involvement in AUD and vulnerability to develop AUD at various ages. Moreover, we discuss dynorphin's role in promoting AUD by dysregulation motivation circuits and how advancements in opioid peptide detection will further our understanding.

Biased Ligands at the Kappa Opioid Receptor: Fine-Tuning Receptor Pharmacology

The kappa opioid receptor (KOR) is a G protein-coupled receptor (GPCR) that can signal through multiple signaling pathways. KOR agonists are known to relieve pain and itch, as well as induce dysphoria, sedation, hallucinations, and diuresis. As is the case with many other GPCRs, specific signaling pathways downstream of the KOR have been linked to certain physiological responses induced by the receptor. Those studies motivated the search and discovery of a number of KOR ligands that preferentially activate one signaling pathway over another. Such compounds are termed functionally selective or biased ligands, and may present a way of inducing desired receptor effects with reduced adverse reactions. In this chapter, I review the molecular intricacies of KOR signaling and discuss the studies that have used biased signaling through the KOR as a way to selectively modulate in vivo physiology.

Understanding Peptide Binding in Class A G Protein-Coupled Receptors

Many physiologic processes are controlled through the activation of G protein-coupled receptors (GPCRs) by regulatory peptides, making peptide GPCRs particularly useful targets for major human diseases such as diabetes and cancer. Peptide GPCRs are also being evaluated as next-generation targets for the development of novel antiparasite agents and insecticides in veterinary medicine and agriculture. Resolution of crystal structures for several peptide GPCRs has advanced our understanding of peptide-receptor interactions and fueled interest in correlating peptide heterogeneity with receptor-binding properties. In this review, the knowledge of recently crystalized peptide-GPCR complexes, previously accumulated peptide structure-activity relationship studies, receptor mutagenesis, and sequence alignment are integrated to better understand peptide binding to the transmembrane cavity of class A GPCRs. Using SAR data, we show that peptide class A GPCRs can be divided into groups with distinct hydrophilic residues. These characteristic residues help explain the preference of a receptor to bind the C-terminal free carboxyl group, the C-terminal amidated group, or the N-terminal ammonium group of peptides.

Structure-Activity Relationship Studies of 6α- and 6β-Indolylacetamidonaltrexamine Derivatives as Bitopic Mu Opioid Receptor Modulators and Elaboration of the "Message-Address Concept" To Comprehend Their Functional Conversion

Structure-activity relationship (SAR) studies of numerous opioid ligands have shown that introduction of a methyl or ethyl group on the tertiary amino group at position 17 of the epoxymorphinan skeleton generally results in a mu opioid receptor (MOR) agonist while introduction of a cyclopropylmethyl group typically leads to an antagonist. Furthermore, it has been shown that introduction of heterocyclic ring systems at position 6 can favor antagonism. However, it was reported that 17-cyclopropylmethyl-3,14β-dihydroxy-4,5α-epoxy-6β-[(2'-indolyl)acetamido]morphinan (INTA), which bears a cyclopropylmethyl group at position 17 and an indole ring at position 6, acted as a MOR agonist. We herein report a SAR study on INTA with a series of its complementary derivatives to understand how introduction of an indole moiety with α or β linkage at position 6 of the epoxymorphinan skeleton may influence ligand function. Interestingly, one of INTA derivatives, compound 15 (NAN) was identified as a MOR antagonist both in vitro and in vivo. Molecular modeling studies revealed that INTA and NAN may interact with different domains of the MOR allosteric binding site. In addition, INTA may interact with W293 and N150 residues found in the orthosteric site to stabilize MOR activation conformation while NAN does not. These results suggest that INTA and NAN may be bitopic ligands and the type of allosteric interactions with the MOR influence their functional activity. These insights along with our enriched comprehension of the "message-address" concept will to benefit future ligand design.

Bifunctional opioid receptor ligands as novel analgesics

Prolonged treatment of chronic severe pain with opioid analgesics is frought with problematic adverse effects including tolerance, dependence, and life-threatening respiratory depression. Though these effects are mediated predominately through preferential activation of μ opioid peptide (μOP) receptors, there is an emerging appreciation that actions at κOP and δOP receptors contribute to the observed pharmacologic and behavioral profile of μOP receptor agonists and may be targeted simultaneously to afford improved analgesic effects. Recent developments have also identified the related nociceptin opioid peptide (NOP) receptor as a key modulator of the effects of μOP receptor signaling. We review here the available literature describing OP neurotransmitter systems and highlight recent drug and probe design strategies.

Alanine scan of the opioid peptide dynorphin B amide

To date structure-activity relationship (SAR) studies of the dynorphins (Dyn), endogenous peptides for kappa opioid receptors (KOR), have focused almost exclusively on Dyn A with minimal studies on Dyn B. While both Dyn A and Dyn B have identical N-terminal sequences, their C-terminal sequences differ, which could result in differences in pharmacological activity. We performed an alanine scan of the non-glycine residues up through residue 11 of Dyn B amide to explore the roles of these side chains in the activity of Dyn B. The analogs were synthesized by fluorenylmethyloxycarbonyl (Fmoc)-based solid phase peptide synthesis and evaluated for their opioid receptor affinities and opioid potency and efficacy at KOR. Similar to Dyn A the N-terminal Tyr¹ and Phe⁴ residues of Dyn B amide are critical for opioid receptor affinity and KOR agonist potency. The basic residues Arg⁶ and Arg⁷ contribute to the KOR affinity and agonist potency of Dyn B amide, while Lys¹⁰ contributes to the opioid receptor affinity, but not KOR agonist potency, of this peptide. Comparison to the Ala analogs of Dyn A (1-13) suggests that the basic residues in the C-terminus of both peptides contribute to KOR binding, but differences in their relative positions may contribute to the different pharmacological profiles of Dyn A and Dyn B. The other unique C-terminal residues in Dyn B amide also appear to influence the relative affinity of this peptide for KOR versus mu and delta opioid receptors. This SAR information may be applied in the design of new Dyn B analogs that could be useful pharmacological tools.

Caged Naloxone: Synthesis, Characterization, and Stability of 3- O-(4,5-Dimethoxy-2-nitrophenyl)carboxymethyl Naloxone (CNV-NLX)

The photolabile analogue of the broad-spectrum opioid antagonist naloxone, 3- O-(4,5-dimethoxy-2-nitrophenyl)carboxymethyl naloxone (also referred to as "caged naloxone", 3- O-(α-carboxy-6-nitroveratryl)naloxone, CNV-NLX), has been found to be a valuable biochemical probe. While the synthesis of CNV-NLX is simple, its characterization is complicated by the fact that it is produced as a mixture of α R,5 R,9 R,13 S,14 S and α S,5 R,9 R,13 S,14 S diastereomers. Using long-range and heteronuclear NMR correlations, the ¹H NMR and ¹³C NMR resonances of both diastereomers have been fully assigned, confirming the structures. Monitoring of solutions of CNV-NLX in saline buffer, in methanol, and in DMSO has shown CNV-NLX to be stable for over a week under fluorescent laboratory lights at room temperature. Exposure of such solutions to λ 365 nm from a hand-held UV lamp led to the formation of naloxone and CNV-related breakdown products.

Evaluation of prodynorphin gene polymorphisms and their association with heroin addiction in a sample of the southeast Iranian population

Genetic factors are supposed to account for about 30-50% of the predisposition to cocaine and heroin addiction. This study aims at investigating the effect of rs2281285, rs2235749, rs910080 and 68bp VNTR polymorphisms of prodynorphin (PDYN) gene on heroin dependence risk in a sample of the southeast Iranian population. This case-control study was done on 216 heroin dependence subjects and 219 healthy subjects. Genomic DNA was extracted from peripheral blood cells using salting out method. Genotyping of PDYN polymorphisms were performed using polymerase chain reaction (PCR) or PCR-RFLP method. The findings showed that PDYN rs910080 T>C variant significantly increased the risk of heroin dependence (OR=7.91, 95%CI=3.36-18.61, P<0.0001, CC vs TT; OR=7.53, 95%CI=3.30-17.16, P<0.0001, CC vs TT+TC; OR=1.75, 95%CI=1.33-2.32, p<0.0001, C vs T). The rs2235749 C>T, rs2281285 A>G and 68bp VNTR variants of PDYN gene were not associated with heroin dependence. Altogether, our results provide an association between rs910080 polymorphism of PDYN gene and risk of heroin dependence in a sample of the southeast Iranian population.

Computational insights into the subtype selectivity and "message-address-efficacy" mechanisms of opioid receptors through JDTic binding and unbinding

In drug design and discovery, binding affinity and selectivity are two basic properties of a drug candidate. Opioid receptors (ORs) are the main targets of strong analgesics. Like some other class A members of G-protein-coupled receptors (GPCRs), ORs exhibit complex selectivity on their ligands. The diversity of binding activity and selectivity among opioids has deeply attracted researchers for a long time. To investigate the subtype selectivity of μ, δ and κ ORs in detail, using the κ-selective antagonist JDTic as a probe, we performed a series of computational simulations, including molecular dynamics and metadynamics, on JDTic-μ/δ/κ-OR complexes. From the simulations, we found that the decisive factor of JDTic selectivity on the μ-subtype was the 2.63 position, which affected the efficacy of JDTic through changing the dynamics of the Q2.60 residue. In addition to the 2.63-position residue, the 7.35 position was the other crucial aspect of JDTic selectivity for the δ-subtype. Based on the results, we suggest a new concept, the "message-address-efficacy" hypothesis, to explain the relationships among the affinity, selectivity and function between ORs and opioids. Thus, all the detailed dynamics of JDTic-bound ORs might be helpful to deeply understand the subtype selectivity and binding mechanisms of other GPCRs.

Molecular identification of the Dyn/Kor system and its potential role in the reproductive axis of goldfish

To ascertain the significance of the dynorphin/kappa-opioid receptor (Dyn/Kor) system in fish reproduction, prodynorphin (pdyn) cDNA was cloned from goldfish. Two Dyn peptides (DynA and DynB) are present in the goldfish prodynorphin precursor. Both DynA and DynB are biologically active as they are able to functionally interact with the goldfish Kor expressed in cultured eukaryotic cells to suppress forskolin-induced CRE promoter activity. RT-PCR analysis showed that pdyn is widely expressed in brain regions, with the highest expression in hypothalamus. During ovarian development, hypothalamic pdyn and kor mRNA levels are lower in the early vitellogenic stage. Then the biological effects of Dyn peptides on salmon gonadotropin releasing hormone (sgnrh), luteinizing hormone beta (lhb) and follicle stimulating hormone beta (fshb) mRNA synthesis were further investigated in goldfish. Intraperitoneal injections of DynA and DynB significantly reduced hypothalamic sgnrh and pituitary lhb and fshb mRNA levels in male goldfish, but these two peptides only down-regulated sgnrh and lhb mRNA expression in female goldfish. In vitro studies revealed that DynA also decreased lhb mRNA levels in primary cultures of pituitary cells, indicating that this peptide can exert its actions at the pituitary level. Our findings suggest that the Dyn/Kor system plays a negative role in regulating the reproductive axis in goldfish.

Computational insights into the G-protein-biased activation and inactivation mechanisms of the μ opioid receptor

The μ opioid receptor (OR), a member of the class A subfamily of G-protein coupled receptors (GPCRs), is a major target for the treatment of pain. G-protein biased μ-OR agonists promise to be developed as analgesics. Thus, TRV130, the first representative μ-OR ligand with G-protein bias, has entered into phase III clinical trials. To identify the detailed G-protein-biased activation and inactivation mechanisms of the μ-OR, we constructed five μ-OR systems that were in complexes with the G-protein-biased agonists TRV130 and BU72, the antagonists β-FNA and naltrexone, as well as the free receptor. We performed a series of conventional molecular dynamics simulations and analyses of G-protein-biased activation and inactivation mechanisms of μ-OR. Our results, together with previously reported mutation results, revealed the operating mode of the activation switch composed of residues W^6.48 and Y^7.43 (Ballesteros/Weinstein numbering), the activity of which was responsible for down- and up-regulation, respectively, of the β-arrestin signaling, which in turn affected G-protein-biased activation of μ-OR. TRV130 was found to stabilize W^6.48 by interacting with Y^7.43. In addition, we obtained useful information regarding μ-OR-biased activation, such as strong stabilization of W^7.35 through a hydrophobic ring interaction in the TRV130 system. These findings may facilitate understanding of μ-OR biased activation and the design of new biased ligands for GPCRs.

Design, synthesis, and opioid activity of arodyn analogs cyclized by ring-closing metathesis involving Tyr(allyl)

Kappa (κ) opioid receptor selective antagonists are useful pharmacological tools in studying κ opioid receptors and have potential to be used as therapeutic agents for the treatment of a variety of diseases including mood disorders and drug addiction. Arodyn (Ac[Phe^1-3,Arg⁴,d-Ala⁸]Dyn A-(1-11)NH₂) is a linear acetylated dynorphin A (Dyn A) analog that is a potent and selective κ opioid receptor antagonist (Bennett et al. J Med Chem 2002;45:5617-5619) and prevents stress-induced reinstatement of cocaine-seeking behavior following central administration (Carey et al. Eur J Pharmacol 2007;569:84-89). To restrict its conformational mobility, explore possible bioactive conformations and potentially increase its metabolic stability we synthesized cyclic arodyn analogs on solid phase utilizing a novel ring-closing metathesis (RCM) reaction involving allyl-protected Tyr (Tyr(All)) residues. This approach preserves the aromatic functionality and directly constrains the side chains of one or more of the Phe residues. The novel cyclic arodyn analog 4 cyclized between Tyr(All) residues incorporated in positions 2 and 3 exhibited potent κ opioid receptor antagonism in the [³⁵S]GTPγS assay (K_B = 3.2 nM) similar to arodyn. Analog 3 cyclized between Tyr(All) residues in positions 1 and 2 also exhibited nanomolar κ opioid receptor antagonist potency (K_B = 27.5 nM) in this assay. These are the first opioid peptides cyclized via RCM involving aromatic residues, and given their promising pharmacological activity represent novel lead peptides for further exploration.

Evidence for the modulation of nociception in mice by central mast cells

BACKGROUND

Hyperalgesia that develops following nerve ligation corresponds temporally and in magnitude with the number of thalamic mast cells located contralateral to the ligature. We tested the possibility that mast cells modulate nociception centrally, similar to their role in the periphery.

METHODS

We examined the central effect of two hyperalgesic compounds that induce mast cell degranulation and of stabilized mast cells using cromolyn.

RESULTS

Thermal hyperalgesia (tail flick) induced by nerve growth factor (NGF, a neurotrophic compound) and mechanical hyperalgesia (von Frey) induced by dynorphin A (1-17) (opioid compound) each correlated with the per cent of thalamic mast cells that were degranulated. Degranulation of these mast cells by the central injection of compound 48/80, devoid of neurotrophic or opioid activity, was sufficient to recapitulate thermal hyperalgesia. Stabilization of mast cells by central injections of cromolyn produced no analgesic effect on baseline tail flick or von Frey fibre sensitivity, but inhibited thermal hyperalgesia produced by compound 48/80 and tactile hyperalgesia induced by dynorphin and by Freund's complete adjuvant. Finally, chemical nociception produced by the direct activation of nociceptors by formalin (phase I) was not inhibited by centrally injected cromolyn whereas chemical nociception dependent on central sensitization (formalin-phase II and acetic acid-induced abdominal stretches) was.

CONCLUSIONS

These convergent lines of evidence suggest that degranulation of centrally located mast cells sensitizes central nociceptive pathways leading to hyperalgesia and tonic chemical sensitivity.

SIGNIFICANCE

Hyperalgesia induced by spinal nerve ligation corresponds temporally and in magnitude with degranulation of thalamic mast cells. Here, we provide evidence that hyperalgesia induced by NGF, formalin and dynorphin also may depend on mast cell degranulation in the CNS whereas cromolyn, a mast cell stabilizer, blocks these effects in mice.

Structural Basis for Apelin Control of the Human Apelin Receptor

Apelin receptor (APJR) is a key regulator of human cardiovascular function and is activated by two different endogenous peptide ligands, apelin and Elabela, each with different isoforms diversified by length and amino acid sequence. Here we report the 2.6-Å resolution crystal structure of human APJR in complex with a designed 17-amino-acid apelin mimetic peptide agonist. The structure reveals that the peptide agonist adopts a lactam constrained curved two-site ligand binding mode. Combined with mutation analysis and molecular dynamics simulations with apelin-13 binding to the wild-type APJR, this structure provides a mechanistic understanding of apelin recognition and binding specificity. Comparison of this structure with that of other peptide receptors suggests that endogenous peptide ligands with a high degree of conformational flexibility may bind and modulate the receptors via a similar two-site binding mechanism.

Chemical Probes Unravel an Antimicrobial Defense Response Triggered by Binding of the Human Opioid Dynorphin to a Bacterial Sensor Kinase

Host-microbe communication via small molecule signals is important for both symbiotic and pathogenic relationships, but is often poorly understood at the molecular level. Under conditions of host stress, levels of the human opioid peptide dynorphin are elevated, triggering virulence in the opportunistic pathogenic bacterium Pseudomonas aeruginosa via an unknown pathway. Here we apply a multilayered chemical biology strategy to unravel the mode of action of this putative interkingdom signal. We designed and applied dynorphin-inspired photoaffinity probes to reveal the protein targets of the peptide in live bacteria via chemical proteomics. ParS, a largely uncharacterized membrane sensor of a two-component system, was identified as the most promising hit. Subsequent full proteome studies revealed that dynorphin(1-13) induces an antimicrobial peptide-like response in Pseudomonas, with specific upregulation of membrane defense mechanisms. No such response was observed in a parS mutant, which was more susceptible to dynorphin-induced toxicity. Thus, P. aeruginosa exploits the ParS sensing machinery to defend itself against the host in response to dynorphin as a signal. This study highlights interkingdom communication as a potential essential strategy not only for induction of P. aeruginosa virulence but also for maintaining viability in the hostile environment of the host.

Inhibitory effects of dynorphin 3-14 on the lipopolysaccharide-induced toll-like receptor 4 signalling pathway

Dynorphin 1-17 (DYN 1-17) is biotransformed rapidly to a range of fragments in rodent inflamed tissue with dynorphin 3-14 (DYN 3-14) being the most stable and prevalent. DYN 1-17 has been shown previously to be involved in the regulation of inflammatory response following tissue injury, in which the biotransformation fragments of DYN 1-17 may possess similar features. This study investigated the effects of DYN 3-14 on lipopolysaccharide (LPS)-induced nuclear factor-kappaB/p65 (NF-κB/p65) nuclear translocation and the release of pro-inflammatory cytokines interleukin-1beta (IL-1β) and tumor necrosis factor-alpha (TNF-α) in differentiated THP-1 cells. Treatment with DYN 3-14 (10nM) resulted in 35% inhibition of the LPS-induced nuclear translocation of NF-κB/p65. Furthermore, DYN 3-14 modulated both IL-1β and TNF-α release; inhibiting IL-1β and paradoxically augmenting TNF-α release in a concentration-independent manner. A number of opioids have been implicated in the modulation of the toll-like receptor 4 (TLR4), highlighting the complexity of their immunomodulatory effects. To determine whether DYN 3-14 modulates TLR4, HEK-Blue™^-hTLR4 cells were stimulated with LPS in the presence of DYN 3-14. DYN 3-14 (10μM) inhibited TLR4 activation in a concentration-dependent fashion by suppressing the LPS signals around 300-fold lower than LPS-RS, a potent TLR4 antagonist. These findings indicate that DYN 3-14 is a potential TLR4 antagonist that alters cellular signaling in response to LPS and cytokine release, implicating a role for biotransformed endogenous opioid peptides in immunomodulation.

The efficacy of Dynorphin fragments at the κ, μ and δ opioid receptor in transfected HEK cells and in an animal model of unilateral peripheral inflammation

Dynorphin 1-17 is an endogenous peptide that is released at sites of inflammation by leukocytes, binding preferentially to κ-opioid receptors (KOP) to mediate nociception. We have previously shown that dynorphin 1-17 is rapidly biotransformed to smaller peptide fragments in inflamed tissue homogenate. This study aimed to determine the efficacy and potency of selected dynorphin fragments produced in an inflamed environment at the KOP, μ and δ-opioid receptors (MOP and DOP respectively) and in a model of inflammatory pain. Functional activity of Dynorphin 1-17 and fragments (1-6, 1-7 and 1-9) were screened over a range of concentrations against forskolin stimulated human embryonic kidney 293 (HEK) cells stably transfected with one of KOP, MOP or DOP. The analgesic activity of dynorphin 1-7 in a unilateral model of inflammatory pain was subsequently tested. Rats received unilateral intraplantar injections of Freund's Complete Adjuvant to induce inflammation. After six days rats received either dynorphin 1-7, 1-17 or the selective KOP agonist U50488H and mechanical allodynia determined. Dynorphin 1-7 and 1-9 displayed the greatest activity across all receptor subtypes, while dynorphin 1-7, 1-9 and 1-17 displaying a potent activation of both KOP and DOP evidenced by cAMP inihibition. Administration of dynorphin 1-7 and U50488H, but not dynorphin 1-17 resulted in a significant increase in paw pressure threshold at an equimolar dose suggesting the small peptide dynorphin 1-7 mediates analgesia. These results show that dynorphin fragments produced in an inflamed tissue homogenate have changed activity at the opioid receptors and that dynorphin 1-7 mediates analgesia.

Structure-Activity Relationships of [des-Arg7]Dynorphin A Analogues at the κ Opioid Receptor

Dynorphin A (Dyn A) is an endogenous ligand for the opioid receptors with preference for the κ opioid receptor (KOR), and its structure-activity relationship (SAR) has been extensively studied at the KOR to develop selective potent agonists and antagonists. Numerous SAR studies have revealed that the Arg⁷ residue is essential for KOR activity. In contrast, our systematic SAR studies on [des-Arg⁷]Dyn A analogues found that Arg⁷ is not a key residue and even deletion of the residue does not affect biological activities at the KOR. In addition, it was also found that [des-Arg⁷]Dyn A(1-9)-NH₂ is a minimum pharmacophore and its modification at the N-terminus leads to selective KOR antagonists. A lead ligand, 14, with high affinity and antagonist activity showed improved metabolic stability and could block antinociceptive effects of a KOR selective agonist, FE200665, in vivo, indicating high potential to treat KOR mediated disorders such as stress-induced relapse.

60 YEARS OF POMC: Lipotropin and beta-endorphin: a perspective

Many important fields of research had a humble origin. In the distant past, A J P Martin's discovery that amino acids could be separated by paper chromatography and Moore and Stein's use of columns for quantitative amino acid analysis provided the first steps towards the determination of structure in complex biologically active molecules. They opened the door to reveal the essential relationship that exists between structure and function. In molecular endocrinology, for example, striking advances have been made by chemists with their expertise in the identification of structure working with biologists who contributed valuable knowledge and experience. Advantage was gained from the convergence of different background, and it is notable that the whole is greater than the sum. In the determination of structure, it may be recalled that four of the world's great pioneers (Archibald Martin, Rodney Porter, Fred Sanger and Vincent du Vigneaud) were acknowledged for their fundamental contributions when individually they were awarded the Nobel Prize. They foresaw that the identification of structure would prove of outstanding importance in the future. Indeed, study of the structures of β-endorphin and enkephalin and the different forms of opiate activity they engender has led to a transformation in our understanding of chemical transmission in the brain.