Leucine 434 is essential for docosahexaenoic acid-induced augmentation of L-glutamate transporter current.

Astrocytic excitatory amino acid transporter 2 (EAAT2) plays a major role in removing the excitatory neurotransmitter L-glutamate (L-Glu) from synaptic clefts in the forebrain to prevent excitotoxicity. Polyunsaturated fatty acids such as docosahexaenoic acid (DHA, 22:6 n-3) enhance synaptic transmission, and their target molecules include EAATs. Here, we aimed to investigate the effect of DHA on EAAT2 and identify the key amino acid for DHA/EAAT2 interaction by electrophysiological recording of L-Glu-induced current in Xenopus oocytes transfected with EAATs, their chimeras, and single mutants. DHA transiently increased the amplitude of EAAT2 but tended to decrease that of excitatory amino acid transporter subtype 1 (EAAT1), another astrocytic EAAT. Single mutation of leucine (Leu) 434 to alanine (Ala) completely suppressed the augmentation by DHA, while mutation of EAAT1 Ala 435 (corresponding to EAAT2 Leu434) to Leu changed the effect from suppression to augmentation. Other polyunsaturated fatty acids (docosapentaenoic acid, eicosapentaenoic acid, arachidonic acid, and α-linolenic acid) similarly augmented the EAAT2 current and suppressed the EAAT1 current. Finally, our docking analysis suggested the most stable docking site is the lipid crevice of EAAT2, in close proximity to the L-Glu and sodium binding sites, suggesting that the DHA/Leu434 interaction might affect the elevator-like slide and/or the shapes of the other binding sites. Collectively, our results highlight a key molecular detail in the DHA-induced regulation of synaptic transmission involving EAATs.

Structural insights into ligand recognition by the lysophosphatidic acid receptor LPA6.

Lysophosphatidic acid (LPA) is a bioactive lipid composed of a phosphate group, a glycerol backbone, and a single acyl chain that varies in length and saturation. LPA activates six class A G-protein-coupled receptors to provoke various cellular reactions. Because LPA signalling has been implicated in cancer and fibrosis, the LPA receptors are regarded as promising drug targets. The six LPA receptors are subdivided into the endothelial differentiation gene (EDG) family (LPA1-LPA3) and the phylogenetically distant non-EDG family (LPA4-LPA6). The structure of LPA1 has enhanced our understanding of the EDG family of LPA receptors. By contrast, the functional and pharmacological characteristics of the non-EDG family of LPA receptors have remained unknown, owing to the lack of structural information. Although the non-EDG LPA receptors share sequence similarity with the P2Y family of nucleotide receptors, the LPA recognition mechanism cannot be deduced from the P2Y1 and P2Y12 structures because of the large differences in the chemical structures of their ligands. Here we determine the 3.2 Å crystal structure of LPA6, the gene deletion of which is responsible for congenital hair loss, to clarify the ligand recognition mechanism of the non-EDG family of LPA receptors. Notably, the ligand-binding pocket of LPA6 is laterally open towards the membrane, and the acyl chain of the lipid used for the crystallization is bound within this pocket, indicating the binding mode of the LPA acyl chain. Docking and mutagenesis analyses also indicated that the conserved positively charged residues within the central cavity recognize the phosphate head group of LPA by inducing an inward shift of transmembrane helices 6 and 7, suggesting that the receptor activation is triggered by this conformational rearrangement.

Isosteric Replacement of Ester Linkage of Lysophospholipids with Heteroaromatic Rings Retains Potency and Subtype Selectivity.

Our group has reported various derivatives of lysophosphatidylserine (LysoPS) as potent and subtype-selective agonists for G-protein-coupled receptors (GPCRs). However, the ester linkage between the glycerol moiety and fatty acid or fatty acid surrogate is present in all of them. In order to develop these LysoPS analogs as drug candidates, appropriate pharmacokinetic consideration is essential. Here, we found that the ester bond of LysoPS is highly susceptible to metabolic degradation in mouse blood. Accordingly, we examined isosteric replacement of the ester linkage with heteroaromatic rings. The resulting compounds showed excellent retention of potency and receptor subtype selectivity, as well as increased metabolic stability in vitro.

Suspected cholestatic liver injury induced by favipiravir in a patient with COVID-19.

Favipiravir is an antiviral drug that is expected to have a therapeutic effect on SARS-CoV2 infection. Teratogenicity and hyperuricemia are known as the main side effects of favipiravir, but little is known about other side effects. This report describes a case of cholestatic liver injury induced by favipiravir. A 73-year-old Japanese with a history of alcoholic hepatitis was infected with SARS-CoV2. Drug therapy was instituted with lopinavir/ritonavir combined with interferon ß-1b. However, his condition worsened despite additional support with continuous hemodiafiltration and veno-venous extracorporeal membrane oxygenation. We suspected complications of bacterial pneumonia and started favipiravir in addition to antimicrobial therapy. Favipiravir was administered at 6000 mg/day on the first day and 2400 mg/day for the second and subsequent days for 14 days. After the initiation of antibiotics, transaminase and total bilirubin were elevated, suggesting a transient cholestasic liver dysfunction. The liver dysfunction in this case may have been triggered by antibacterial treatment, and high dose of favipiravir may have promoted the deterioration of liver function. Monitoring of liver function is vital and close attention should be paid when using favipiravir at high doses or in patients with impaired liver function.

Membrane Phospholipid Analogues as Molecular Rulers to Probe the Position of the Hydrophobic Contact Point of Lysophospholipid Ligands on the Surface of G-Protein-Coupled Receptor during Membrane Approach.

When lipid mediators bind to G-protein-coupled receptors (GPCRs), the ligand first enters the lipid bilayer, then diffuses laterally in the cell membrane to make hydrophobic contact with the receptor protein, and finally enters the receptor's binding pocket. In this process, the location of the hydrophobic contact point on the surface of the receptor has been little discussed even in cases in which the crystal structure has been determined, because the ligand binding pocket is buried inside the transmembrane (TM) domains. Here, we coupled an activator ligand to a series of membrane phospholipid surrogates, which constrain the depth of entry of the ligand into the lipid bilayer. Consequently, via measurement of the receptor-activating activity as a function of the depth of entry into the membrane, these surrogates can be used as molecular rulers to estimate the location of the hydrophobic contact point on the surface of GPCR. We focused on lysophosphatidylserine (LysoPS) receptor GPR34 and prepared a series of simplified membrane-lipid-surrogate-conjugated lysophospholipid analogues by attaching alkoxy amine chains of varying lengths to the hydrophobic tail of a potent GPR34 agonist. As expected, the activity of these lipid-conjugated LysoPS analogues was dependent on chain length. The predicted contact position matches the position of the terminal benzene ring of a nonlipidic ligand that protrudes between TMs 4 and 5 of the receptor. We further found that the nature of the terminal hydrophilic functional group of the conjugated membrane lipid surrogate strongly influences the activity, suggesting that lateral hydrophilic contact of LysoPS analogues with the receptor's surface is also crucial for ligand-GPCR binding.

Non-naturally Occurring Helical Molecules Can Interfere with p53-MDM2 and p53-MDMX Protein-Protein Interactions.

We have discovered that ß-amino acid homooligomers with cis- or trans-amide conformation can fold themselves into highly ordered helices. Moreover, unlike α-amino acid peptides, which are significantly stabilized by intramolecular hydrogen bonding, these helical structures are autogenous conformations that are stable without the aid of hydrogen bonding and irrespective of solvent (protic/aprotic/halogenated) or temperature. A structural overlap comparison of helical cis/trans bicyclic ß-proline homooligomers with typical α-helix structure of α-amino acid peptides reveals clear differences of pitch and diameter per turn. Bridgehead substituents of the present homooligomers point outwards from the helical surface. We were interested to know whether such non-naturally occurring divergent helical molecules could mimic α-helix structures. In this study, we show that bicyclic ß-proline oligomer derivatives inhibit p53-MDM2 and p53-MDMX protein-protein interactions, exhibiting MDM2-antagonistic and MDMX-antagonistic activities.

Exploration of LPS2 agonist binding modes using the combination of a new hydrophobic scaffold and homology modeling.

Lysophosphatidylserine (LysoPS) is an endogenous pan-agonist of three G-protein coupled receptors (GPCRs): LPS1/GPR34, LPS2/P2Y10, and LPS3/GPR174, and we previously reported a series of LysoPS-based agonists of these receptors. Interestingly, we found that LPS1 agonist activity was very sensitive to structural change at the hydrophobic fatty acid moiety, whereas LPS2 agonist activity was not. Here, to probe the molecular basis of LPS2 agonist binding, we developed a new class of hydrophobic fatty acid surrogates having a biphenyl-ether scaffold. The LPS2 agonist activity of these compounds proved sensitive to molecular modification of the hydrophobic skeleton. Thus, we next constructed an LPS2 model by homology modeling and docking/molecular dynamics (MD) simulation, and validated it by means of SAR studies together with point mutations of selected receptor amino-acid residues. The putative ligand-binding site of LPS2 is Γ-shaped, with a hydrophilic site horizontally embedded in the receptor transmembrane helix bundles and a perpendicular hydrophobic groove adjoining transmembrane domains 4 and 5 that is open to the membrane bilayer. The binding poses of LPS2 agonists to this site are consistent with easy incorporation of various kinds of fatty acid surrogates. Structural development based on this model afforded a series of potent and selective LPS2 full agonists, which showed enhanced in vitro actin stress fiber formation effect.

Switching Lysophosphatidylserine G Protein-Coupled Receptor Agonists to Antagonists by Acylation of the Hydrophilic Serine Amine.

Three human G protein-coupled receptors (GPCRs)-GPR34/LPS1, P2Y10/LPS2, and GPR174/LPS3-are activated specifically by lysophosphatidylserine (LysoPS), an endogenous hydrolysis product of a cell membrane component, phosphatidylserine (PS). LysoPS consists of l-serine, glycerol, and fatty acid moieties connected by phosphodiester and ester linkages. We previously generated potent and selective GPCR agonists by modification of the three modules and the ester linkage. Here, we show that a novel modification of the hydrophilic serine moiety, that is, N-acylations of the serine amine, converted a GPR174 agonist to potent GPR174 antagonists. Structural exploration of the amide functionality provided access to a range of activities from agonist to partial agonist to antagonist. The present study would provide a new strategy for the development of lysophospholipid receptor antagonists.

Non-naturally Occurring Regio Isomer of Lysophosphatidylserine Exhibits Potent Agonistic Activity toward G Protein-Coupled Receptors.

Lysophosphatidylserine (LysoPS), an endogenous ligand of G protein-coupled receptors, consists of l-serine, glycerol, and fatty acid moieties connected by phosphodiester and ester linkages, respectively. An ester linkage of phosphatidylserine can be hydrolyzed at the 1-position or at the 2-position to give 2-acyl lysophospholipid or 1-acyl lysophospholipid, respectively. 2-Acyl lysophospholipid is in nonenzymatic equilibrium with 1-acyl lysophospholipid in vivo. On the other hand, 3-acyl lysophospholipid is not found, at least in mammals, raising the question of whether the reason for this might be that the 3-acyl isomer lacks the biological activities of the other isomers. Here, to test this idea, we designed and synthesized a series of new 3-acyl lysophospholipids. Structure-activity relationship studies of more than 100 "glycol surrogate" derivatives led to the identification of potent and selective agonists for LysoPS receptors GPR34 and P2Y10. Thus, the non-natural 3-acyl compounds are indeed active and appear to be biologically orthogonal with respect to the physiologically relevant 1- and 2-acyl lysophospholipids.

Probing the Hydrophobic Binding Pocket of G-Protein-Coupled Lysophosphatidylserine Receptor GPR34/LPS1 by Docking-Aided Structure-Activity Analysis.

The ligands of certain G-protein-coupled receptors (GPCRs) have been identified as endogenous lipids, such as lysophosphatidylserine (LysoPS). Here, we analyzed the molecular basis of the structure-activity relationship of ligands of GPR34, one of the LysoPS receptor subtypes, focusing on recognition of the long-chain fatty acid moiety by the hydrophobic pocket. By introducing benzene ring(s) into the fatty acid moiety of 2-deoxy-LysoPS, we explored the binding site's preference for the hydrophobic shape. A tribenzene-containing fatty acid surrogate with modifications of the terminal aromatic moiety showed potent agonistic activity toward GPR34. Computational docking of these derivatives with a homology modeling/molecular dynamics-based virtual binding site of GPR34 indicated that a kink in the benzene-based lipid surrogates matches the L-shaped hydrophobic pocket of GPR34. A tetrabenzene-based lipid analogue bearing a bulky tert-butyl group at the 4-position of the terminal benzene ring exhibited potent GPR34 agonistic activity, validating the present hydrophobic binding pocket model.

Identification of lysophosphatidylthreonine with an aromatic fatty acid surrogate as a potent inducer of mast cell degranulation.

Upon various stimulations, mast cells (MCs) release a wide variety of chemical mediators stored in their cytoplasmic granules, which then initiates subsequent allergic reactions. Lysophosphatidylserine (LysoPS), a kind of lysophospholipid, potentiates the histamine release from MCs triggered by antigen stimulation. We previously showed through structure-activity studies of LysoPS analogs that LysoPS with a methyl group at the carbon of the serine residue, i.e., lysophosphatidylthreonine (LysoPT), is extremely potent in stimulating the MC degranulation. In this study, as our continuing study to identify more potent LysoPS analogs, we developed LysoPS analogs with fatty acid surrogates. We found that the substitution of oleic acid to an aromatic fatty acid surrogate (C3-pH-p-O-C11) in 2-deoxy-1-LysoPS resulted in significant increase in the ability to induce MCs degranulation compared with 2-deoxy-1-LysoPS with oleic acid. Conversion of the serine residue into the threonine residue further increased the activity of MC degranulation both in vitro and in vivo. The resulting super agonist, 2-deoxy-LysoPT with C3-pH-p-O-C11, will be a useful tool to elucidate the mechanisms of stimulatory effect of LysoPS on MC degranulation.

Conformational Constraint of the Glycerol Moiety of Lysophosphatidylserine Affords Compounds with Receptor Subtype Selectivity.

Lysophosphatidylserine (LysoPS) is an endogenous lipid mediator that specifically activates membrane proteins of the P2Y and its related families of G protein-coupled receptors (GPCR), GPR34 (LPS1), P2Y10 (LPS2), and GPR174 (LPS3). Here, in order to increase potency and receptor selectivity, we designed and synthesized LysoPS analogues containing the conformational constraints of the glycerol moiety. These reduced structural flexibility by fixation of the glycerol framework of LysoPS using a 2-hydroxymethyl-3-hydroxytetrahydropyran skeleton, and related structures identified compounds which exhibited high potency and selectivity for activation of GPR34 or P2Y10. Morphing of the structural shape of the 2-hydroxymethyl-3-hydroxytetrahydropyran skeleton into a planar benzene ring enhanced the P2Y10 activation potentcy rather than the GPR34 activation.

Structure-activity relationships of lysophosphatidylserine analogs as agonists of G-protein-coupled receptors GPR34, P2Y10, and GPR174.

Lysophosphatidylserine (LysoPS) is an endogenous lipid mediator generated by hydrolysis of membrane phospholipid phosphatidylserine. Recent ligand screening of orphan G-protein-coupled receptors (GPCRs) identified two LysoPS-specific human GPCRs, namely, P2Y10 (LPS2) and GPR174 (LPS3), which, together with previously reported GPR34 (LPS1), comprise a LysoPS receptor family. Herein, we examined the structure-activity relationships of a series of synthetic LysoPS analogues toward these recently deorphanized LysoPS receptors, based on the idea that LysoPS can be regarded as consisting of distinct modules (fatty acid, glycerol, and l-serine) connected by phosphodiester and ester linkages. Starting from the endogenous ligand (1-oleoyl-LysoPS, 1), we optimized the structure of each module and the ester linkage. Accordingly, we identified some structural requirements of each module for potency and for receptor subtype selectivity. Further assembly of individually structure-optimized modules yielded a series of potent and LysoPS receptor subtype-selective agonists, particularly for P2Y10 and GPR174.

Lysophosphatidylserine analogues differentially activate three LysoPS receptors.

Lysophosphatidylserine (1-oleoyl-2 R-lysophosphatidylserine, LysoPS) has been shown to have lipid mediator-like actions such as stimulation of mast cell degranulation and suppression of T lymphocyte proliferation, although the mechanisms of LysoPS actions have been elusive. Recently, three G protein-coupled receptors (LPS1/GPR34, LPS2/P2Y10 and LPS3/GPR174) were found to react specifically with LysoPS, raising the possibility that LysoPS serves as a lipid mediator that exerts its role through these receptors. Previously, we chemically synthesized a number of LysoPS analogues and evaluated them as agonists for mast-cell degranulation. Here, we used a transforming growth factor-α (TGFα) shedding assay to see if these LysoPS analogues activated the three LysoPS receptors. Modification of the serine moiety significantly reduced the ability of the analogues to activate the three LysoPS receptors, whereas modification of other parts resulted in loss of activity in receptor-specific manner. We found that introduction of methyl group to serine moiety (1-oleoyl-lysophosphatidylallothreonine) and removal of sn-2 hydroxyl group (1-oleoyl-2-deoxy-LysoPS) resulted in reduction of reactivity with LPS1 and LPS3, respectively. Accordingly, we synthesized a LysoPS analogue with the two modifications (1-oleoyl-2-deoxy-lysophosphatidylallothreonine) and found it to be an LPS2-selective agonist. These pharmacological tools will definitely help to identify the biological roles of these LysoPS receptors.