The interaction of highly helical structural mutants with the NOP receptor discloses the role of the address domain of nociceptin/orphanin FQ

A Multi-Angle Approach to Predict Peptide-GPCR Complexes: The N/OFQ-NOP System as a Successful AlphaFold Application Case Study

With nearly 700 structures solved and a growing number of customized structure prediction algorithms being developed at a fast pace, G protein-coupled receptors (GPCRs) are an optimal test case for validating new approaches for the prediction of receptor active state and ligand bioactive conformation complexes. In this study, we leveraged the availability of hundreds of peptide GPCRs in the active state and both classical homology and artificial intelligence (AI) based protein modeling combined with docking and AI-based peptide structure prediction approaches to predict the nociceptin/orphanin FQ-NOP receptor active state complex (N/OFQ-NOPa). The In Silico generated hypotheses were validated via the design, synthesis, and pharmacological characterization of novel linear N/OFQ(1-13)-NH2 analogues, leading to the discovery of a novel antagonist (3B; pKB = 6.63) bearing a single ring-constrained residue in place of the Gly2-Gly3 motif of the N/OFQ message sequence (FGGF). While the experimental validation was ongoing, the availability of the Cryo-EM structure of the predicted complex enabled us to unambiguously validate the generated hypotheses. To the best of our knowledge, this is the first example of a peptide-GPCR complex predicted with atomistic accuracy (full complex Cα RMSD < 1.0 Å) and of the N/OFQ message moiety being successfully modified with a rigid scaffold.

Pathogenic Variants in USH1G/SANS Alter Protein Interaction with Pre-RNA Processing Factors PRPF6 and PRPF31 of the Spliceosome

Pre-mRNA splicing is an essential process orchestrated by the spliceosome, a dynamic complex assembled stepwise on pre-mRNA. We have previously identified that USH1G protein SANS regulates pre-mRNA splicing by mediating the intranuclear transfer of the spliceosomal U4/U6.U5 tri-snRNP complex. During this process, SANS interacts with the U4/U6 and U5 snRNP-specific proteins PRPF31 and PRPF6 and regulates splicing, which is disturbed by variants of USH1G/SANS causative for human Usher syndrome (USH), the most common form of hereditary deaf-blindness. Here, we aim to gain further insights into the molecular interaction of the splicing molecules PRPF31 and PRPF6 to the CENTn domain of SANS using fluorescence resonance energy transfer assays in cells and in silico deep learning-based protein structure predictions. This demonstrates that SANS directly binds via two distinct conserved regions of its CENTn to the two PRPFs. In addition, we provide evidence that these interactions occur sequentially and a conformational change of an intrinsically disordered region to a short α-helix of SANS CENTn2 is triggered by the binding of PRPF6. Furthermore, we find that pathogenic variants of USH1G/SANS perturb the binding of SANS to both PRPFs, implying a significance for the USH1G pathophysiology.

Biased Agonism at Nociceptin/Orphanin FQ Receptors: A Structure Activity Study on N/OFQ(1-13)-NH2

Nociceptin/orphanin FQ (N/OFQ) controls different biological functions via selective stimulation of the N/OFQ peptide (NOP) receptor. The pleiotropic actions of N/OFQ may limit the development of NOP ligands as innovative drugs in different therapeutic areas. The pharmacological concept of functional selectivity (aka biased agonism) might be useful for amplifying beneficial actions and/or counteracting side effects. Thus, molecules with large bias factors toward G protein or β arrestin are required for investigating the translational value of NOP biased modulation. Herein, the biased behavior of a heterogeneous library of NOP-targeting peptide derivatives was evaluated in vitro with the aim to provide possible insights into the structural determinants that govern the selective activation of G protein versus β-arrestin. Our results demonstrate that lipidation of N/OFQ(1-13)-NH2 is a useful strategy for obtaining G protein biased agonists for the NOP receptor.

Nociceptin/Orphanin FQ and Urinary Bladder

Following identification as the endogenous ligand for the NOP receptor, nociceptin/orphanin FQ (N/OFQ) has been shown to control several biological functions including the micturition reflex. N/OFQ elicits a robust inhibitory effect on rat micturition by reducing the excitability of the afferent fibers. After intravesical administration N/OFQ increases urodynamic bladder capacity and volume threshold in overactive bladder patients but not in normal subjects. Moreover daily treatment with intravesical N/OFQ for 10 days significantly reduced urine leakage episodes. Different chemical modifications were combined into the N/OFQ sequence to generate Rec 0438 (aka UFP-112), a peptide NOP full agonist with high potency and selectivity and long-lasting duration of action. Rec 0438 mimicked the robust inhibitory effects of N/OFQ on rat micturition reflex; its action is solely due to NOP receptor stimulation, does not show tolerance liability after 2 weeks of treatment, and can be elicited by intravesical administration. Collectively the evidence summarized and discussed in this chapter strongly suggests that NOP agonists are promising innovative drugs to treat overactive bladder.

NOP-Targeted Peptide Ligands

The nociceptin/orphanin FQ (N/OFQ)-N/OFQ peptide (NOP) receptor system is widely distributed at both the peripheral and central level where it modulates important biological functions with increasing therapeutic implications. This chapter wants to provide a comprehensive and updated overview focused on the available structure-activity relationship studies on NOP receptor peptide ligands developed through different rational approaches. Punctual modifications and cyclizations of the N/OFQ sequence have been properly combined furnishing potent NOP selective ligands with different pharmacological activities (full and partial agonists, pure antagonists) and enhanced metabolic stability in vivo. The screening of peptide libraries provided a second family of NOP ligands that have been successfully optimized. Moreover, recent findings suggest the possibility to apply different multimerization strategies for the realization of multi-target NOP/opioid receptor ligands or tetrabranched N/OFQ derivatives with extraordinarily prolonged duration of action in vivo. The diverse approaches led to the identification of important pharmacological tools along with drug candidates currently in clinical development such as Rec 0438 (aka UFP-112) for the treatment of overactive bladder and SER 100 (aka ZP120) for the clinical management of systolic hypertension.

Structure- and conformation-activity studies of nociceptin/orphanin FQ receptor dimeric ligands

The peptide nociceptin/orphanin FQ (N/OFQ) and the N/OFQ receptor (NOP) constitute a neuropeptidergic system that modulates various biological functions and is currently targeted for the generation of innovative drugs. In the present study dimeric NOP receptor ligands with spacers of different lengths were generated using both peptide and non-peptide pharmacophores. The novel compounds (12 peptide and 7 nonpeptide ligands) were pharmacologically investigated in a calcium mobilization assay and in the mouse vas deferens bioassay. Both structure- and conformation-activity studies were performed. Results demonstrated that dimerization did not modify the pharmacological activity of both peptide and non-peptide pharmacophores. Moreover, when dimeric compounds were obtained with low potency peptide pharmacophores, dimerization recovered ligand potency. This effect depends on the doubling of the C-terminal address sequence rather than the presence of an additional N-terminal message sequence or modifications of peptide conformation.

Nociceptin/Orphanin FQ Receptor Structure, Signaling, Ligands, Functions, and Interactions with Opioid Systems

The NOP receptor (nociceptin/orphanin FQ opioid peptide receptor) is the most recently discovered member of the opioid receptor family and, together with its endogenous ligand, N/OFQ, make up the fourth members of the opioid receptor and opioid peptide family. Because of its more recent discovery, an understanding of the cellular and behavioral actions induced by NOP receptor activation are less well developed than for the other members of the opioid receptor family. All of these factors are important because NOP receptor activation has a clear modulatory role on mu opioid receptor-mediated actions and thereby affects opioid analgesia, tolerance development, and reward. In addition to opioid modulatory actions, NOP receptor activation has important effects on motor function and other physiologic processes. This review discusses how NOP pharmacology intersects, contrasts, and interacts with the mu opioid receptor in terms of tertiary structure and mechanism of receptor activation; location of receptors in the central nervous system; mechanisms of desensitization and downregulation; cellular actions; intracellular signal transduction pathways; and behavioral actions with respect to analgesia, tolerance, dependence, and reward. This is followed by a discussion of the agonists and antagonists that have most contributed to our current knowledge. Because NOP receptors are highly expressed in brain and spinal cord and NOP receptor activation sometimes synergizes with mu receptor-mediated actions and sometimes opposes them, an understanding of NOP receptor pharmacology in the context of these interactions with the opioid receptors will be crucial to the development of novel therapeutics that engage the NOP receptor.

Helix-constrained nociceptin peptides are potent agonists and antagonists of ORL-1 and nociception

Nociceptin (orphanin FQ) is a 17-residue neuropeptide hormone with roles in both nociception and analgesia. It is an opioid-like peptide that binds to and activates the G-protein-coupled receptor opioid receptor-like-1 (ORL-1, NOP, orphanin FQ receptor, kappa-type 3 opioid receptor) on central and peripheral nervous tissue, without activating classic delta-, kappa-, or mu-opioid receptors or being inhibited by the classic opioid antagonist naloxone. The three-dimensional structure of ORL-1 was recently published, and the activation mechanism is believed to involve capture by ORL-1 of the high-affinity binding, prohelical C-terminus. This likely anchors the receptor-activating N-terminus of nociception nearby for insertion in the membrane-spanning helices of ORL-1. In search of higher agonist potency, two lysine and two aspartate residues were strategically incorporated into the receptor-binding C-terminus of the nociceptin sequence and two Lys(i)→Asp(i+4) side chain-side chain condensations were used to generate lactam cross-links that constrained nociceptin into a highly stable α-helix in water. A cell-based assay was developed using natively expressed ORL-1 receptors on mouse neuroblastoma cells to measure phosphorylated ERK as a reporter of agonist-induced receptor activation and intracellular signaling. Agonist activity was increased up to 20-fold over native nociceptin using a combination of this helix-inducing strategy and other amino acid modifications. An NMR-derived three-dimensional solution structure is described for a potent ORL-1 agonist derived from nociceptin, along with structure-activity relationships leading to the most potent known α-helical ORL-1 agonist (EC₅₀ 40 pM, pERK, Neuro-2a cells) and antagonist (IC₅₀ 7 nM, pERK, Neuro-2a cells). These α-helix-constrained mimetics of nociceptin(1-17) had enhanced serum stability relative to unconstrained peptide analogues and nociceptin itself, were not cytotoxic, and displayed potent thermal analgesic and antianalgesic properties in rats (ED₅₀ 70 pmol, IC₅₀ 10 nmol, s.c.), suggesting promising uses in vivo for the treatment of pain and other ORL-1-mediated responses.

Structure of the nociceptin/orphanin FQ receptor in complex with a peptide mimetic

Members of the opioid receptor family of G-protein-coupled receptors (GPCRs) are found throughout the peripheral and central nervous system, where they have key roles in nociception and analgesia. Unlike the 'classical' opioid receptors, δ, κ and μ (δ-OR, κ-OR and μ-OR), which were delineated by pharmacological criteria in the 1970s and 1980s, the nociceptin/orphanin FQ (N/OFQ) peptide receptor (NOP, also known as ORL-1) was discovered relatively recently by molecular cloning and characterization of an orphan GPCR. Although it shares high sequence similarity with classical opioid GPCR subtypes (∼60%), NOP has a markedly distinct pharmacology, featuring activation by the endogenous peptide N/OFQ, and unique selectivity for exogenous ligands. Here we report the crystal structure of human NOP, solved in complex with the peptide mimetic antagonist compound-24 (C-24) (ref. 4), revealing atomic details of ligand-receptor recognition and selectivity. Compound-24 mimics the first four amino-terminal residues of the NOP-selective peptide antagonist UFP-101, a close derivative of N/OFQ, and provides important clues to the binding of these peptides. The X-ray structure also shows substantial conformational differences in the pocket regions between NOP and the classical opioid receptors κ (ref. 5) and μ (ref. 6), and these are probably due to a small number of residues that vary between these receptors. The NOP-compound-24 structure explains the divergent selectivity profile of NOP and provides a new structural template for the design of NOP ligands.

Development of nociceptin receptor (NOP) agonists and antagonists

The nociceptin opioid (NOP) receptor is the most recently discovered member of the family of the opioid receptors; its endogenous agonist is the peptide nociceptin. Due to the subsequent elucidation of its physiological role in both central and peripheral nervous system and in some non-neural tissues, there is a rapidly growing interest in the pharmacological application of substances active on this receptor. Despite the current clinical use of a morphinane-based NOP/MOP mixed ligand (buprenorphine) as an analgesic and in the treatment of drug addictions, so far just a few clinical trials have been made with selective NOP ligands. However, the perspective of their utilization is rapidly growing. Agonists can find applications in the treatment of neuropathic pain, anxiety, cough, drug addition, urinary incontinence, anorexia, congestive heart failure, hypertension; and antagonists for pain, depression, Parkinson's disease, obesity, and as memory enhancers. Besides peptide ligands, which are still subjected to many pharmacological investigations, many different chemical classes of NOP ligands have been discovered: piperidines, nortropanes, spiropiperidines, 4-amino-quinolines and quinazolines, and others. The new advances in establishing structure-activity relationships, also with the help of modeling studies, can permit the development of more active and selective molecules.

UFP-112 a potent and long-lasting agonist selective for the Nociceptin/Orphanin FQ receptor

Nociceptin/orphanin FQ (N/OFQ) controls several biological functions via selective activation of the N/OFQ peptide receptor (NOP). [(pF)Phe(4) Aib(7) Arg(14) Lys(15) ]N/OFQ-NH(2) (UFP-112) is an NOP receptor ligand designed using a combination of several chemical modifications in the same peptide sequence that increase NOP receptor affinity/potency and/or reduce susceptibility to enzymatic degradation. In the present review article, we summarize data from the literature and present original findings on the in vitro and in vivo pharmacological features of UFP-112. Moreover, important biological actions and possible therapeutic indications of NOP receptor agonists are discussed based on the results obtained with UFP-112 and compared with other peptide and nonpeptide NOP receptor ligands.

Cold denaturation of yeast frataxin offers the clue to understand the effect of alcohols on protein stability

Although alcohols are well-known to be protein denaturants when present at high concentrations, their effect on proteins at low concentrations is much less well characterized. In this paper, we present a study of the effects of alcohols on protein stability using Yfh1, the yeast ortholog of the human protein frataxin. Exploiting the unusual property of this protein of undergoing cold denaturation around 0 degrees C without any ad hoc destabilization, we determined the stability curve on the basis of both high and low temperature unfolding in the presence of three commonly used alcohols: trifluoroethanol, ethanol, and methanol. In all cases, we observed an extended temperature range of protein stability as determined by a modest increase of the high temperature of unfolding but an appreciable decrease in the low temperature of unfolding. On the basis of simple thermodynamic considerations, we are able to interpret the literature on the effects of alcohols on proteins and to generalize our findings. We suggest that alcohols, at low concentration and physiological pH, stabilize proteins by greatly widening the range of temperatures over which the protein is stable. Our results also clarify the molecular mechanism of the interaction and validate the current theoretical interpretation of the mechanism of cold denaturation.

In vitro pharmacological characterisation of a novel cyclic nociceptin/orphanin FQ analogue c[Cys(7,10)]N/OFQ(1-13)NH (2)

Nociceptin/orphanin FQ (N/OFQ) is the endogenous 17 amino acid peptide ligand for the G(i)-protein-coupled N/OFQ receptor (NOP). In an attempt to improve the metabolic stability of N/OFQ, we have produced a truncated cyclic analogue with cysteine residues at positions 7 and 10, c[Cys(7,10)]N/OFQ(1-13)NH(2) (c[Cys(7,10)]). c[Cys(7,10)], the template N/OFQ(1-13)NH(2) and N/OFQ displaced the binding of [(3)H]N/OFQ to Chinese hamster ovary cells expressing recombinant human NOP (CHO(hNOP)) with pK ( i ) values of 9.98, 9.83 and 9.18, respectively. In addition, c[Cys(7,10)], N/OFQ(1-13)NH(2) and N/OFQ stimulated the binding of guanosine triphosphate gamma [(35)S] to CHO(hNOP) cells with pEC(50)/E (max) (stimulation factor) of 9.16/5.5, 9.11/4.9 and 8.35/5.5, respectively. c[Cys(7,10)], N/OFQ(1-13)NH(2) and N/OFQ inhibited forskolin-stimulated cyclic adenosine monophosphate (cAMP) formation with pEC(50) values of 10.08, 10.11 and 9.78, respectively. All ligands produced complete inhibition of cAMP formation. In both functional assays, c[Cys(7,10)] was a full agonist. In a series of metabolism experiments, incubation of 1 nM c[Cys(7,10)], N/OFQ(1-13)NH(2) and N/OFQ with a rat brain homogenate produced a time-dependent loss of peptide that was greatest for the native peptide N/OFQ. Amidation in N/OFQ(1-13)NH(2) produced some metabolic protection, but this was not significantly improved by further inclusion of c[Cys(7,10)]. In summary, c[Cys(7,10)] is a high-affinity, high-potency full agonist of the NOP receptor. However, we were unable to demonstrate clear metabolic protection.

Synthesis and biological activity of nociceptin/orphanin FQ analogues substituted in position 7 or 11 with Calpha,alpha-dialkylated amino acids

Previous structure-activity and NMR studies on nociceptin/orphanin FQ (N/OFQ) demonstrated that Aib substitution of Ala(7) and/or Ala(11) increases the peptide potency through an alpha helix structure induction mechanism. On these bases we synthesised and evaluated pharmacologically in the mouse vas deferens assay a series of N/OFQ-NH(2) analogues substituted in position 7 and 11 with Calpha,alpha-disubstituted cyclic, linear and branched amino acids. None of the 20 novel N/OFQ analogues produced better results than [Aib(7)]N/OFQ-NH(2). Thus, this substitution was combined with other chemical modifications known to modulate peptide potency and/or efficacy generating compound 21 [Nphe(1)Aib(7)Arg(14)Lys(15)]N/OFQ-NH(2) (coded as UFP-111), compound 22 [(pF)Phe(4)Aib(7)Arg(14)Lys(15)]N/OFQ-NH(2) (UFP-112) and compound 23 [Phe(1)Psi(CH(2)-NH)Gly(2)(pF)Phe(4)Aib(7)Arg(14)Lys(15)]N/OFQ-NH(2) (UFP-113). These novel peptides behaved as highly potent NOP receptor ligands showing full (UFP-112) and partial (UFP-113) agonist and pure antagonist (UFP-111) activities in a series of in vitro functional assays performed on pharmacological preparations expressing native as well as recombinant NOP receptors.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).