Enantiomerically Pure Quinoline-Based κ-Opioid Receptor Agonists: Chemoenzymatic Synthesis and Pharmacological Evaluation.

Racemic K -opioid receptor (KOR) agonist 2-(3,4-dichlorophenyl)-1-[(4aRS,8SR,8aSR)-8-(pyrrolidin-1-yl)-3,4,4a,5,6,7,8,8a-octahydroquinolin-1(2H)-yl]ethan-1-one ((±)-4) was prepared in a diastereoselective synthesis. The first key step of the synthesis was the diastereoselective hydrogenation of the silyl ether of 1,2,3,4-tetrahydroquinoin-8-ol ((±)-9) to afford cis,cis-configured perhydroquinoline derivative (±)-10. Removal of the TBDMS protecting group led to a ß-aminoalcohol that reacted with SO2 Cl2 to form an oxathiazolidine. Nucleophilic substitution with pyrrolidine resulted in the desired cis,trans-configured perhydroquinoline upon inversion of the configuration. In order to obtain enantiomerically pure KOR agonists 4 (99.8 % ee) and ent-4 (99.0 % ee), 1,2,3,4-tetrahydroquinolin-8-ols (R)-8 (99.1 % ee) and (S)-8 (98.4 % ee) were resolved by an enantioselective acetylation catalyzed by Amano lipase PS-IM. The absolute configuration was determined by CD spectroscopy. The 4aR,8S,8aS-configured enantiomer 4 showed sub-nanomolar KOR affinity (Ki =0.81 nM), which is more than 200 times higher than the KOR affinity of its enantiomer ent-4. In the cAMP assay and the Tango ß-arrestin-2 recruitment assay, 4 behaved as a KOR agonist. Upon incubation of human macrophages, human dendritic cells, and mouse myeloid immune cells with 4, the number of cells expressing co-stimulatory receptor CD86 and proinflammatory cytokines interleukin 6 and tumor necrosis factor α was significantly reduced; this indicates the strong anti-inflammatory activity of 4. The anti-inflammatory effects correlated well with the KOR affinity: (4aR,8S,8aS)-4 was slightly more potent than the racemic mixture (±)-4, and the distomer ent-4 was almost inactive.

Nanobody-enabled monitoring of kappa opioid receptor states.

Recent studies show that GPCRs rapidly interconvert between multiple states although our ability to interrogate, monitor and visualize them is limited by a relative lack of suitable tools. We previously reported two nanobodies (Nb39 and Nb6) that stabilize distinct ligand- and efficacy-delimited conformations of the kappa opioid receptor. Here, we demonstrate via X-ray crystallography a nanobody-targeted allosteric binding site by which Nb6 stabilizes a ligand-dependent inactive state. As Nb39 stabilizes an active-like state, we show how these two state-dependent nanobodies can provide real-time reporting of ligand stabilized states in cells in situ. Significantly, we demonstrate that chimeric GPCRs can be created with engineered nanobody binding sites to report ligand-stabilized states. Our results provide both insights regarding potential mechanisms for allosterically modulating KOR with nanobodies and a tool for reporting the real-time, in situ dynamic range of GPCR activity.

Development of Novel Quinoxaline-Based κ-Opioid Receptor Agonists for the Treatment of Neuroinflammation.

Neuroinflammatory disorders, such as multiple sclerosis or experimental autoimmune encephalomyelitis (EAE), an established mouse model mimicking part of the human pathology, are characterized by inflammatory infiltrates containing T helper 1 (TH1) and TH17 cells, which cause demyelination and neurodegeneration. Disease onset and perpetuation are mediated by peripherally generated autoreactive T cells infiltrating into the central nervous system, where they are restimulated by antigen-presenting cells. Here, we show that newly designed peripherally active, potent, and selective κ-opioid receptor (KOR) agonists comprising the ethylenediamine KOR pharmacophore in a perhydroquinoxaline scaffold exhibit potent anti-inflammatory capacities in primary antigen presenting cells as well as T cells. In the EAE model, the secondary amine 12 and the triazole 14 were able to ameliorate disease severity and to delay disease onset by blocking effector T cell activation. Importantly, the beneficial effects were mediated via signaling through KOR because off-target effects were excluded by using KOR-deficient mouse mutants.

5-HT2C Receptor Structures Reveal the Structural Basis of GPCR Polypharmacology.

Drugs frequently require interactions with multiple targets-via a process known as polypharmacology-to achieve their therapeutic actions. Currently, drugs targeting several serotonin receptors, including the 5-HT2C receptor, are useful for treating obesity, drug abuse, and schizophrenia. The competing challenges of developing selective 5-HT2C receptor ligands or creating drugs with a defined polypharmacological profile, especially aimed at G protein-coupled receptors (GPCRs), remain extremely difficult. Here, we solved two structures of the 5-HT2C receptor in complex with the highly promiscuous agonist ergotamine and the 5-HT2A-C receptor-selective inverse agonist ritanserin at resolutions of 3.0 Å and 2.7 Å, respectively. We analyzed their respective binding poses to provide mechanistic insights into their receptor recognition and opposing pharmacological actions. This study investigates the structural basis of polypharmacology at canonical GPCRs and illustrates how understanding characteristic patterns of ligand-receptor interaction and activation may ultimately facilitate drug design at multiple GPCRs.

Identification of the First Marine-Derived Opioid Receptor "Balanced" Agonist with a Signaling Profile That Resembles the Endorphins.

Opioid therapeutics are excellent analgesics, whose utility is compromised by dependence. Morphine (1) and its clinically relevant derivatives such as OxyContin (2), Vicodin (3), and Dilaudid (4) are "biased" agonists at the µ opioid receptor (OR), wherein they engage G protein signaling but poorly engage ß-arrestin and the endocytic machinery. In contrast, endorphins, the endogenous peptide agonists for ORs, are potent analgesics, show reduced liability for tolerance and dependence, and engage both G protein and ß-arrestin pathways as "balanced" agonists. We set out to determine if marine-derived alkaloids could serve as novel OR agonist chemotypes with a signaling profile distinct from morphine and more similar to the endorphins. Screening of 96 sponge-derived extracts followed by LC-MS-based purification to pinpoint the active compounds and subsequent evaluation of a mini library of related alkaloids identified two structural classes that modulate the ORs. These included the following: aaptamine (10), 9-demethyl aaptamine (11), demethyl (oxy)-aaptamine (12) with activity at the δ-OR (EC50: 5.1, 4.1, 2.3 µM, respectively) and fascaplysin (17), and 10-bromo fascaplysin (18) with activity at the µ-OR (EC50: 6.3, 4.2 µM respectively). An in vivo evaluation of 10 using δ-KO mice indicated its previously reported antidepressant-like effects are dependent on the δ-OR. Importantly, 17 functioned as a balanced agonist promoting both G protein signaling and ß-arrestin recruitment along with receptor endocytosis similar to the endorphins. Collectively these results demonstrate the burgeoning potential for marine natural products to serve as novel lead compounds for therapeutic targets in neuroscience research.

Analysis of carotenoid isomerase activity in a prototypical carotenoid cleavage enzyme, apocarotenoid oxygenase (ACO).

Carotenoid cleavage enzymes (CCEs) constitute a group of evolutionarily related proteins that metabolize a variety of carotenoid and non-carotenoid substrates. Typically, these enzymes utilize a non-heme iron center to oxidatively cleave a carbon-carbon double bond of a carotenoid substrate. Some members also isomerize specific double bonds in their substrates to yield cis-apocarotenoid products. The apocarotenoid oxygenase from Synechocystis has been hypothesized to represent one such member of this latter category of CCEs. Here, we developed a novel expression and purification protocol that enabled production of soluble, native ACO in quantities sufficient for high resolution structural and spectroscopic investigation of its catalytic mechanism. High performance liquid chromatography and Raman spectroscopy revealed that ACO exclusively formed all-trans products. We also found that linear polyoxyethylene detergents previously used for ACO crystallization strongly inhibited the apocarotenoid oxygenase activity of the enzyme. We crystallized the native enzyme in the absence of apocarotenoid substrate and found electron density in the active site that was similar in appearance to the density previously attributed to a di-cis-apocarotenoid intermediate. Our results clearly demonstrated that ACO is in fact a non-isomerizing member of the CCE family. These results indicate that careful selection of detergent is critical for the success of structural studies aimed at elucidating structures of CCE-carotenoid/retinoid complexes.

A nuclear ligand MRG15 involved in the proapoptotic activity of medicinal fungal galectin AAL (Agrocybe aegerita lectin).

BACKGROUND: We have previously reported a novel fungal galectin Agrocybe aegerita lectin (AAL) with apoptosis-induced activity and nuclear migration activity. The importance of nuclear localization for AAL's apoptosis-induced activity has been established by mutant study. However, the mechanism remains unclear. METHODS: We further investigated the mechanism using a previously reported carbohydrate recognition domain (CRD) mutant protein H59Q, which retained its nuclear localization activity but lost most of its apoptotic activity. The cell membrane-binding ability of recombinant AAL (rAAL) and H59Q was analyzed by FACS, and their cellular partners were identified by affinity chromatography and mass spectroscopy. Furthermore, the interaction of AAL and ligand was proved by mammalian two-hybrid and pull down assays. A knockdown assay was used to confirm the role of the ligand. RESULTS: The apoptotic activity of AAL could be blocked by lactose. Mutant H59Q retained comparable cell membrane-binding ability to rAAL. Four cellular binding partners of AAL in HeLa cells were identified: glucose-regulated protein 78 (GRP78); mortality factor 4-like protein 1 (MRG15); elongation factor 2 (EEF2); and heat shock protein 70 (Hsp70). CRD region of AAL was required for the interaction between AAL/mutant AAL and MRG15. MRG15 knockdown increased the cells' resistance to AAL treatment. CONCLUSION: MRG15 was a nuclear ligand for AAL in HeLa cells. These data implied the existence of a novel nuclear pathway for the antitumor activity of fungal galectin AAL. GENERAL SIGNIFICANCE: These findings provide a novel explanation of AAL bioactivity and contribute to the understanding of mushroom lectins' antitumor activity.