Structural and Computational Insights into Dynamics and Intermediate States of Orexin 2 Receptor Signaling.

Orexin 2 receptor (OX2R) is a G protein-coupled receptor (GPCR) whose activation is crucial to regulation of the sleep-wake cycle. Recently, inactive and active state structures were determined from X-ray crystallography and cryo-electron microscopy single particle analysis, and the activation mechanisms have been discussed based on these static data. GPCRs have multiscale intermediate states during activation, and insights into these dynamics and intermediate states may aid the precise control of intracellular signaling by ligands in drug discovery. Molecular dynamics (MD) simulations are used to investigate dynamics induced in response to thermal perturbations, such as structural fluctuations of main and side chains. In this study, we proposed collective motions of the TM domain during activation by performing 30 independent microsecond-scale MD simulations for various OX2R systems and applying relaxation mode analysis. The analysis results suggested that TM3 had a vertical structural movement relative to the membrane surface during activation. In addition, we extracted three characteristic amino acid residues on TM3, i.e., Q1343.32, V1423.40, and R1523.50, which exhibited large conformational fluctuations. We quantitatively evaluated the changes in their equilibrium during activation in relation to the movement of TM3. We also discuss the regulation of ligand binding recognition and intracellular signal selectivity by changes in the equilibrium of Q1343.32 and R1523.50, respectively, according to MD simulations and GPCR database. Additionally, the OX2R-Gi signaling complex is stabilized in the conformation resembling a non-canonical (NC) state, which was previously proposed as an intermediate state during activation of neurotensin 1 receptor. Insights into the dynamics and intermediate states during activation gained from this study may be useful for developing biased agonists for OX2R.

Interaction modes of human orexin 2 receptor with selective and nonselective antagonists studied by NMR spectroscopy.

Orexin neuropeptides have many physiological roles in the sleep-wake cycle, feeding behavior, reward demands, and stress responses by activating cognitive receptors, the orexin receptors (OX1R and OX2R), distributed in the brain. There are only subtle differences between OX1R and OX2R in the orthosteric site, which has hindered the rational development of subtype-selective antagonists. In this study, we utilized solution-state NMR to capture the structural plasticity of OX2R labeled with 13CH3-ε-methionine in complex with antagonists. Mutations in the orthosteric site allosterically affected the intracellular tip of TM6. Ligand exchange experiments with the subtype-selective EMPA and the nonselective suvorexant identified three methionine residues that were substantially perturbed. The NMR spectra suggested that the suvorexant-bound state exhibited more structural plasticity than the EMPA-bound state, which has not been foreseen from the close similarity of their crystal structures, providing insights into dynamic features to be considered in understanding the ligand recognition mode.

Identification and Characterization of Cannabidiol as an OX1R Antagonist by Computational and In Vitro Functional Validation.

The potential, multifaceted therapeutic profile of cannabidiol (CBD), a major constituent derived from the Cannabis sativa plant, covers a wide range of neurological and psychiatric disorders, ranging from anxiety to pediatric epilepsy and drug addiction. However, the molecular targets responsible for these effects have been only partially identified. In this view, the involvement of the orexin system, the key regulator in arousal and the sleep/wake cycle, and in motivation and reward processes, including drug addiction, prompted us to explore, using computational and experimental approaches, the possibility that CBD could act as a ligand of orexin receptors, orexin 1 receptor of type 1 (OX1R) and type 2 (OX2R). Ligand-binding assays showed that CBD is a selective ligand of OX1R in the low micromolar range (Ki 1.58 ± 0.2 µM) while in vitro functional assays, carried out by intracellular calcium imaging and mobilization assays, showed that CBD acts as an antagonist at this receptor. Finally, the putative binding mode of CBD has been inferred by molecular docking and molecular dynamics simulations and its selectivity toward the OX1R subtype rationalized at the molecular level. This study provides the first evidence that CBD acts as an OX1R antagonist, supporting its potential use in addictive disorders and/or body weight regulation.

Orexins/Hypocretins and Cancer: A Neuropeptide as Emerging Target.

Over 20 years ago, orexin neuropeptides (Orexin-A/hypocretin-1 and Orexin-B/hypocretins-2) produced from the same precursor in hypothalamus were identified. These two neurotransmitters and their receptors (OX1R and OX1R), present in the central and peripheral nervous system, play a major role in wakefulness but also in drug addiction, food consumption, homeostasis, hormone secretion, reproductive function, lipolysis and blood pressure regulation. With respect to these biological functions, orexins were involved in various pathologies encompassing narcolepsy, neurodegenerative diseases, chronic inflammations, metabolic syndrome and cancers. The expression of OX1R in various cancers including colon, pancreas and prostate cancers associated with its ability to induce a proapoptotic activity in tumor cells, suggested that the orexins/OX1R system could have a promising therapeutic role. The present review summarizes the relationship between cancers and orexins/OX1R system as an emerging target.

Identification of a novel conserved signaling motif in CD200 receptor required for its inhibitory function.

The inhibitory signaling of CD200 receptor 1 (CD200R) has been attributed to its NPxY signaling motif. However, NPxY-motifs are present in multiple protein families and are mostly known to mediate protein trafficking between subcellular locations rather than signaling. Therefore, we investigated whether additional motifs specify the inhibitory function of CD200R. We performed phylogenetic analysis of the intracellular domain of CD200R in mammals, birds, bony fish, amphibians and reptiles. Indeed, the tyrosine of the NPxY-motif is fully conserved across species, in line with its central role in CD200R signaling. In contrast, P295 of the NPxY-motif is not conserved. Instead, a conserved stretch of negatively charged amino acids, EEDE279, and two conserved residues P285 and K292 in the flanking region prior to the NPxY-motif are required for CD200R mediated inhibition of p-Erk, p-Akt308, p-Akt473, p-rpS6 and LPS-induced IL-8 secretion. Altogether, we show that instead of the more common NPxY-motif, CD200R signaling can be assigned to a unique signaling motif in mammals defined by: EEDExxPYxxYxxKxNxxY.

Structures of active-state orexin receptor 2 rationalize peptide and small-molecule agonist recognition and receptor activation.

Narcolepsy type 1 (NT1) is a chronic neurological disorder that impairs the brain's ability to control sleep-wake cycles. Current therapies are limited to the management of symptoms with modest effectiveness and substantial adverse effects. Agonists of the orexin receptor 2 (OX2R) have shown promise as novel therapeutics that directly target the pathophysiology of the disease. However, identification of drug-like OX2R agonists has proven difficult. Here we report cryo-electron microscopy structures of active-state OX2R bound to an endogenous peptide agonist and a small-molecule agonist. The extended carboxy-terminal segment of the peptide reaches into the core of OX2R to stabilize an active conformation, while the small-molecule agonist binds deep inside the orthosteric pocket, making similar key interactions. Comparison with antagonist-bound OX2R suggests a molecular mechanism that rationalizes both receptor activation and inhibition. Our results enable structure-based discovery of therapeutic orexin agonists for the treatment of NT1 and other hypersomnia disorders.

Allosteric interactions via the orthosteric ligand binding sites in a constitutive G-protein-coupled receptor homodimer.

I interpret some recent data to indicate that co-operative effects take place between the (identical) orthosteric binding sites in a G-protein-coupled receptor dimer. In the current study, the reasonability of this concept was tested by creating a mathematical model. The model is composed of a symmetrical constitutive receptor dimer in which the protomers are able to affect each other allosterically, and it includes binding, receptor activation and signal amplification steps. The model was utilized for analyses of previous data as well as simulations of predicted behaviour. The model demonstrates the behaviour stated in the hypotheses, i.e. even an apparently neutral receptor ligand can allosterically affect agonist binding or receptor activation by binding to the normal orthosteric ligand binding site. Therewith the speculated allosteric action originating from the orthosteric binding site of the dimeric receptor is a realistic possibility. The results of the simulations and curve fitting constitute a reasonable starting point for further studies, and the model can be utilized to design meaningful experiments to investigate these questions.

Role of orexin receptors within the dentate gyrus in antinociception induced by chemical stimulation of the lateral hypothalamus in an animal model of inflammatory pain.

Pain is a complex experience consisting of sensory, affective-motivational, and cognitive dimensions. Hence, identifying the multiple neural pathways subserving these functional aspects is a valuable task. The role of dentate gyrus (DG) as a relay station of neocortical afferents in the hippocampal formation (HF) in persistent pain is still controversial. The lateral hypothalamus (LH)-HF neural circuits are involved in numerous situations such as anxiety-like behavior, reward processing, feeding, orofacial as well as acute pain. Nonetheless, to our knowledge, the involvement of the LH-DG neural circuit in persistent pain has already remained unexplored. Adult male Wistar rats weighing 220-250 g were undergone stereotaxic surgery for unilateral implantation of two separate cannulae into the LH and DG. Intra-DG administration of the orexin-1 (OX1) and orexin-2 (OX2) receptor antagonists, SB334867 and TCS OX2 29, respectively, was performed 5 min before intra-LH microinjection of carbachol. Animals were then undergone the formalin test using 50 µl formalin injection (2.5%) into the plantar surface of the hind paw. Microinjection of SB334867 or TCS OX2 29 into the DG region attenuated the antinociceptive effect produced by carbachol microinjection into the LH. The preventive effect of SB334867 and TCS OX2 29 on intra-LH carbachol-induced antinociception was approximately equal in both early and late phases of formalin nociception. The results suggest a neural pathway from the LH to the DG, which contributes to the modulation of formalin-induced inflammatory pain through the recruitment of OX1 and OX2 receptors within the DG.

Discovery of attenuation effect of orexin 1 receptor to aversion of nalfurafine: Synthesis and evaluation of D-nor-nalfurafine derivatives and analyses of the three active conformations of nalfurafine.

The D-nor-nalfurafine derivatives, which were synthesized by contraction of the six-membered D-ring in nalfurafine (1), had no affinity for orexin 1 receptors (OX1Rs). The 17N-lone electron pair in 1 oriented toward the axial direction, while that of D-nor-derivatives was directed in the equatorial configuration. The axial lone electron pair can form a hydrogen bond with the 14-hydroxy group, which could push the 6-amide side chain toward the downward direction with respect to the C-ring. The resulting conformation would be an active conformation for binding with OX1R. The dual affinities of 1 for OX1R and κ opioid receptor (KOR) led us to elucidate the mechanism by which only 1 showed no aversion but U-50488H. Actually, 1 selectively induced severe aversion in OX1R knockout mice, but not in wild-type mice. These results well support that OX1R suppresses the aversion of 1. This is the elucidation of long period puzzle which 1 showed no aversion in KOR.

Structure-based development of a subtype-selective orexin 1 receptor antagonist.

Orexins are neuropeptides that activate the rhodopsin-like G protein-coupled receptors OX1R and OX2R. The orexin system plays an important role in the regulation of the sleep-wake cycle and the regulation of feeding and emotions. The nonselective orexin receptor antagonist suvorexant has been the first drug on the market targeting the orexin system and is prescribed for the treatment of insomnia. Subtype-selective OX1R antagonists are valuable tools to further investigate the functions and physiological role of the OX1R in vivo and promising lead compounds for the treatment of drug addiction, anxiety, pain or obesity. Starting from the OX1R and OX2R crystal structures bound to suvorexant, we exploited a single amino acid difference in the orthosteric binding site by using molecular docking and structure-based drug design to optimize ligand interactions with the OX1R while introducing repulsive interactions with the OX2R. A newly established enantiospecific synthesis provided ligands showing up to 75-fold selectivity for the OX1R over the OX2R subtype. The structure of a new OX1R antagonist with subnanomolar affinity (JH112) was determined by crystallography in complex with the OX1R and corresponded closely to the docking-predicted geometry. JH112 exhibits high selectivity over a panel of different GPCRs, is able to cross the blood-brain barrier and acts as slowly diffusing and insurmountable antagonist for Gq protein activation and in particular ß-arrestin-2 recruitment at OX1R. This study demonstrates the potential of structure-based drug design to develop more subtype-selective GPCR ligands with potentially reduced side effects and provides an attractive probe molecule and lead compound.

Pharmacological Characteristics of Porcine Orexin 2 Receptor and Mutants.

Orexin receptors (OXRs) play a critical regulatory role in central control of food intake, maintenance of sleeping states, energy metabolism, and neuroendocrine homeostasis. However, most previous studies have focused on the sleep-promoting functions of OXRs in human beings, while their potential value in enhancing food intake for livestock breeding has not been fully exploited. In this study, we successfully cloned porcine orexin 2 receptor (pOX2R) complementary DNA and constructed four pOX2R mutants (P10S, P11T, V308I, and T401I) by site-directed mutagenesis, and their functional expressions were further confirmed through Western blotting analysis. Pharmacological characteristics of pOX2R and their mutants were further investigated. These results showed that the P10S, P11T, and T401I mutants had decreased cAMP signaling with orexin A, whereas only the P11T mutant decreased under the stimulation of orexin B. Besides, only P10S displayed a decreased calcium release in response to both orexin ligands. Importantly, these mutants exhibited decreased phosphorylation levels of ERK1/2, p38, and CREB to some degree compared with wild-type pOX2R. Collectively, these findings highlight the critical role of these mutations in pOX2R signaling and expand our understanding of molecular and pharmacological characterization of pOX2R.

Detailed quantum mechanical studies on bioactive benzodiazepine derivatives and their adsorption over graphene sheets.

Estazolam (Z1) and related derivatives, adinazolam (Z2), alprazolam (Z3), 4-hydroxyalprazolam (Z4) and triazolam (Z5) have been studied by using various computational tools to analyze their geometry and spectral characteristics. The compounds were found to interact with graphene monolayer results shows that there is enhancement in various physico-chemical descriptors and surface enhanced Raman spectra (SERS). The various reactive descriptors obtained from the FMO analysis predict the reactive nature of the compound. The various lone pair/sigma to pi conjugation was analyzed using NBO formalism, which provides valuable information about intra molecular electron transfer which is vital in predicting the inherent stability of the molecule. Simulated electronic spectra using TD-DFT and CAM-B3LYP functional are discussed in detail with respect to electronic transitions and light harvesting efficiency. Suitability of candidates as a photo sensitizer in dye sensitized solar cells was studied and 4-Hydroxyalprazolam is identified as a suitable candidate. Nucleophilic and electrophilic regions of the molecules are identified using MESP, which adds to the reactivity information. It can be seen that the highest interaction energy has been obtained in the case of the Z5-graphene system, while the lowest interaction energy has been obtained in the case of the Z1-graphene system. Docking indicates that the ligands adsorbed over graphene also form stable complexes with the receptors as indicated by the high binding affinity energy values.

CD200 Checkpoint Reversal: A Novel Approach to Immunotherapy.

PURPOSE: Advances in immunotherapy have revolutionized care for some patients with cancer. However, current checkpoint inhibitors are associated with significant toxicity and yield poor responses for patients with central nervous system tumors, calling into question whether cancer immunotherapy can be applied to glioblastoma multiforme. We determined that targeting the CD200 activation receptors (CD200AR) of the CD200 checkpoint with a peptide inhibitor (CD200AR-L) overcomes tumor-induced immunosuppression. We have shown the clinical efficacy of the CD200AR-L in a trial in companion dogs with spontaneous high-grade glioma. Addition of the peptide to autologous tumor lysate vaccines significantly increased the median overall survival to 12.7 months relative to tumor lysate vaccines alone, 6.36 months. EXPERIMENTAL DESIGN: This study was developed to elucidate the mechanism of the CD200ARs and develop a humanized peptide inhibitor. We developed macrophage cell lines with each of four CD200ARs knocked out to determine their binding specificity and functional response. Using proteomics, we developed humanized CD200AR-L to explore their effects on cytokine/chemokine response, dendritic cell maturation and CMV pp65 antigen response in human CD14+ cells. GMP-grade peptide was further validated for activity. RESULTS: We demonstrated that the CD200AR-L specifically targets a CD200AR complex. Moreover, we developed and validated a humanized CD200AR-L for inducing chemokine response, stimulating immature dendritic cell differentiation and significantly enhanced an antigen-specific response, and determined that the use of the CD200AR-L downregulated the expression of CD200 inhibitory and PD-1 receptors. CONCLUSIONS: These results support consideration of a CD200AR-L as a novel platform for immunotherapy against multiple cancers including glioblastoma multiforme.

Comparison of Orexin 1 and Orexin 2 Ligand Binding Modes Using X-ray Crystallography and Computational Analysis.

The orexin system, which consists of the two G protein-coupled receptors OX1 and OX2, activated by the neuropeptides OX-A and OX-B, is firmly established as a key regulator of behavioral arousal, sleep, and wakefulness and has been an area of intense research effort over the past two decades. X-ray structures of the receptors in complex with 10 new antagonist ligands from diverse chemotypes are presented, which complement the existing structural information for the system and highlight the critical importance of lipophilic hotspots and water molecules for these peptidergic GPCR targets. Learnings from the structural information regarding the utility of pharmacophore models and how selectivity between OX1 and OX2 can be achieved are discussed.

Essential structure of orexin 1 receptor antagonist YNT-707, part V: Structure-activity relationship study of the substituents on the 17-amino group.

The morphinan-type orexin 1 receptor (OX1R) antagonists such as YNT-707 (2) and YNT-1310 (3) show potent and extremely high selective antagonistic activity against OX1R. In the course of our studies of the essential structure of 2, we identified new scaffolds by simplification of the morphinan skeleton. However, the new chemical entities carrying the D-ring removed scaffold showed insufficient activity. To improve the activity of these derivatives, we investigated the effect of substituents mainly focused on the 17-nitrogen group. The 17-N-substituted derivatives, as well as the cyclic derivatives, were synthesized and examined the OX1R antagonistic activity. The assay results showed the interesting relationship between the OX1R antagonistic activity and the substituents on the 17-nitrogen: the antagonistic activity was increased as the bulkiness of 17-substituents increased. Finally, the 17-N-Boc derivative 14a showed the most potent OX1R antagonistic activity (Ki = 14.8 nM).

Oxadiazole Derivatives as Dual Orexin Receptor Antagonists: Synthesis, Structure-Activity Relationships, and Sleep-Promoting Properties in Rats.

The orexin system plays an important role in the regulation of wakefulness. Suvorexant, a dual orexin receptor antagonist (DORA) is approved for the treatment of primary insomnia. Herein, we outline our optimization efforts toward a novel DORA. We started our investigation with rac-[3-(5-chloro-benzooxazol-2-ylamino)piperidin-1-yl]-(5-methyl-2-[1,2,3]triazol-2-ylphenyl)methanone (3), a structural hybrid of suvorexant and a piperidine-containing DORA. During the optimization, we resolved liabilities such as chemical instability, CYP3A4 inhibition, and low brain penetration potential. Furthermore, structural modification of the piperidine scaffold was essential to improve potency at the orexin 2 receptor. This work led to the identification of (5-methoxy-4-methyl-2-[1,2,3]triazol-2-ylphenyl)-{(S)-2-[5-(2-trifluoromethoxyphenyl)-[1,2,4]oxadiazol-3-yl]pyrrolidin-1-yl}methanone (51), a potent, brain-penetrating DORA with in vivo efficacy similar to that of suvorexant in rats.

Essential structure of orexin 1 receptor antagonist YNT-707, part III: Role of the 14-hydroxy and the 3-methoxy groups in antagonistic activity toward the orexin 1 receptor in YNT-707 derivatives lacking the 4,5-epoxy ring.

Morphinan derivatives lacking the 4,5-epoxy ring were synthesized to examine the participation of the 14-OH group, the 3-OMe group, and the aromaticity of the A-ring in the activity and selectivity for the orexin 1 receptor (OX1R). The assay results and the conformational analyses of the 14-dehydrated and 14-H derivatives suggested that the orientations of the 6-amide side chain and the 17-benzenesulfonyl group would play important roles in the activity for OX1R. In the 6ß-derivatives, removal of the 3-OMe group and the reduction of the A-ring significantly decreased the activity toward the OX1R, but these changes did not affect the 6α-derivatives. These results indicate that the 3-OMe group and the A-ring would be essential structural moieties for the 6ß-derivatives.

Determinants of Orexin Receptor Binding and Activation-A Molecular Dynamics Study.

We assess the stability of two previously suggested binding modes for the neuropeptide orexin-A in the OX2 receptor through extensive molecular dynamics simulations. As the activation determinants of the receptor remain unknown, we simulated an unliganded receptor and two small-molecular ligands, the antagonist suvorexant and the agonist Nag26 for comparison. Each system was simulated in pure POPC membrane as well as in the 25% cholesterol-POPC membrane. In total, we carried out 36 µs of simulations. Through this set of simulations, we report a stable binding mode for the C-terminus of orexin-A. In addition, we suggest interactions that would promote orexin receptor activation, as well as others that would stabilize the inactive state.

Investigation of New Orexin 2 Receptor Modulators Using In Silico and In Vitro Methods.

The neuropeptides, orexin A and orexin B (also known as hypocretins), are produced in hypothalamic neurons and belong to ligands for orphan G protein-coupled receptors. Generally, the primary role of orexins is to act as excitatory neurotransmitters and regulate the sleep process. Lack of orexins may lead to sleep disorder narcolepsy in mice, dogs, and humans. Narcolepsy is a neurological disorder of alertness characterized by a decrease of ability to manage sleep-wake cycles, excessive daytime sleepiness, and other symptoms, such as cataplexy, vivid hallucinations, and paralysis. Thus, the discovery of orexin receptors, modulators, and their causal implication in narcolepsy is the most important advance in sleep-research. The presented work is focused on the evaluation of compounds L1⁻L11 selected by structure-based virtual screening for their ability to modulate orexin receptor type 2 (OX2R) in comparison with standard agonist orexin-A together with their blood-brain barrier permeability and cytotoxicity. We can conclude that the studied compounds possess an affinity towards the OX2R. However, the compounds do not have intrinsic activity and act as the antagonists of this receptor. It was shown that L4 was the most potent antagonistic ligand to orexin A and displayed an IC50 of 2.2 µM, offering some promise mainly for the treatment of insomnia.

Localization of orexin B and orexin-2 receptor in the rat epididymis.

The peptides orexin A (OXA) and orexin B (OXB) derived from the proteolytic cleavage of a common precursor molecule, prepro-orexin, were originally described in the rat hypothalamus. Successively, they have been found in many other brain regions as well as in peripheral organs of mammals and other less evolved animals. The widespread localization of orexins accounts for the multiple activities that they exert in the body, including the regulation of energy homeostasis, feeding, metabolism, sleep and arousal, stress, addiction, and cardiovascular and endocrine functions. Both OXA and OXB peptides bind to two G-coupled receptors, orexin-1 (OX1R) and orexin-2 (OX2R) receptor, though with different binding affinity. Altered expression/activity of orexins and their receptors has been associated with a large number of human diseases. Though at present evidence highlighted a role for orexins and cognate receptors in mammalian reproduction, their central and/or local effects on gonadal functions remain poorly known. Here, we investigated the localization of OXB and OX2R in the rat epididymis. Immunohistochemical staining of sections from caput, corpus and cauda segments of the organ showed intense signals for both OXB and OX2R in the principal cells of the lining epithelium, while no staining was detected in the other cell types. Negative results were obtained from immunohistochemical analysis of hypothalamic and testicular tissues from OX2R knock-out mice (OX2R-/-) and OX1R/OX2R double knock-out (OX1R-/-; OX2R-/-) mice, thus demonstrating the specificity of the rabbit polyclonal anti-OX2R antibody used in our study. On contrary, the same antibody clearly showed the presence of OX2R in sections from hypothalamus and testis of normal mice and rats which are well known to express the receptor. Thus, our results provide the first definite evidence for the immunohistochemical localization of OXB and OX2R in the principal cells of rat epididymis.