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

Lysophosphatidylserine: A Signaling Lipid with Implications in Human Diseases

Lysophosphatidylserine (lyso-PS) has emerged as yet another important signaling lysophospholipid in mammals, and deregulation in its metabolism has been directly linked to an array of human autoimmune and neurological disorders. It has an indispensable role in several biological processes in humans, and therefore, cellular concentrations of lyso-PS are tightly regulated to ensure optimal signaling and functioning in physiological settings. Given its biological importance, the past two decades have seen an explosion in the available literature toward our understanding of diverse aspects of lyso-PS metabolism and signaling and its association with human diseases. In this Review, we aim to comprehensively summarize different aspects of lyso-PS, such as its structure, biodistribution, chemical synthesis, and SAR studies with some synthetic analogs. From a biochemical perspective, we provide an exhaustive coverage of the diverse biological activities modulated by lyso-PSs, such as its metabolism and the receptors that respond to them in humans. We also briefly discuss the human diseases associated with aberrant lyso-PS metabolism and signaling and posit some future directions that may advance our understanding of lyso-PS-mediated mammalian physiology.

Structural basis for lysophosphatidylserine recognition by GPR34

GPR34 is a recently identified G-protein coupled receptor, which has an immunomodulatory role and recognizes lysophosphatidylserine (LysoPS) as a putative ligand. Here, we report cryo-electron microscopy structures of human GPR34-Gi complex bound with one of two ligands bound: either the LysoPS analogue S3E-LysoPS, or M1, a derivative of S3E-LysoPS in which oleic acid is substituted with a metabolically stable aromatic fatty acid surrogate. The ligand-binding pocket is laterally open toward the membrane, allowing lateral entry of lipidic agonists into the cavity. The amine and carboxylate groups of the serine moiety are recognized by the charged residue cluster. The acyl chain of S3E-LysoPS is bent and fits into the L-shaped hydrophobic pocket in TM4-5 gap, and the aromatic fatty acid surrogate of M1 fits more appropriately. Molecular dynamics simulations further account for the LysoPS-regioselectivity of GPR34. Thus, using a series of structural and physiological experiments, we provide evidence that chemically unstable 2-acyl LysoPS is the physiological ligand for GPR34. Overall, we anticipate the present structures will pave the way for development of novel anticancer drugs that specifically target GPR34.

Nano-encapsulated Cu(II) complex as a promising insecticidal for Aedes aegypti (Diptera: Culicidae)

Nanoparticle (NP) research is an area of scientific interest with high potential for application in biomedical, optical, and electronic fields. Due to their relatively large surface area compared to their mass, NPs can be more chemically reactive and change their reactive strength or other properties. NP-based drug delivery systems provide transport and an effective and controlled way to release the drugs. This work aimed to study the solubility and biological activity of nano-encapsulated copper metal complexes for the induction of toxicity and mortality in larvae of Aedes aegypti mosquitoes. After the nano-encapsulated metal complexes were prepared, the efficiency of this incorporation was determined by electron paramagnetic resonance, and toxicity bioassays were performed. The polymeric-based PLGA NPs encapsulating metal complexes exhibited high toxicity and specificity for target organisms (insect vectors, i.e., A. aegypti), with relatively less environmental impact and long-term control of their breeding.

Synthesis and Biological Evaluation of Lysophosphatidic Acid Analogues Using Conformational Restriction and Bioisosteric Replacement Strategies

Lysophosphatidic acid (LPA) is a key player in many physiological and pathophysiological processes. The biological activities of LPA are mediated through interactions with-at least-six subtypes of G-protein-coupled receptors (GPCRs) named LPA1-6. Developing a pharmacological tool molecule that activates LPA subtype receptors selectively will allow a better understanding of their specific physiological roles. Here, we designed and synthesized conformationally restricted 25 1-oleoyl LPA analogues MZN-001 to MZN-025 by incorporating its glycerol linker into dihydropyran, tetrahydropyran, and pyrrolidine rings and variating the lipophilic chain. The agonistic activities of these compounds were evaluated using the TGFα shedding assay. Overall, the synthesized analogues exhibited significantly reduced agonistic activities toward LPA1, LPA2, and LPA6, while demonstrating potent activities toward LPA3, LPA4, and LPA5 compared to the parent LPA. Specifically, MZN-010 showed more than 10 times greater potency (EC50 = 4.9 nM) than the standard 1-oleoyl LPA (EC50 = 78 nM) toward LPA5 while exhibiting significantly lower activity on LPA1, LPA2, and LPA6 and comparable potency toward LPA3 and LPA4. Based on the MZN-010 scaffold, we synthesized additional analogues with improved selectivity and potency toward LPA5. Compound MZN-021, which contains a saturated lipophilic chain, exhibited 50 times more potent activity (EC50 = 1.2 nM) than the natural LPA against LPA5 with over a 45-fold higher selectivity when compared to those of other LPA receptors. Thus, MZN-021 was found to be a potent and selective LPA5 agonist. The findings of this study could contribute to broadening the current knowledge about the stereochemical and three-dimensional arrangement of LPA pharmacophore components inside LPA receptors and paving the way toward synthesizing other subtype-selective pharmacological probes.

Cryo-EM structures of human GPR34 enable the identification of selective antagonists

GPR34 is a functional G-protein-coupled receptor of Lysophosphatidylserine (LysoPS), and has pathogenic roles in numerous diseases, yet remains poorly targeted. We herein report a cryo-electron microscopy (cryo-EM) structure of GPR34 bound with LysoPS (18:1) and Gi protein, revealing a unique ligand recognition mode with the negatively charged head group of LysoPS occupying a polar cavity formed by TM3, 6 and 7, and the hydrophobic tail of LysoPS residing in a lateral open hydrophobic groove formed by TM3-5. Virtual screening and subsequent structural optimization led to the identification of a highly potent and selective antagonist (YL-365). Design of fusion proteins allowed successful determination of the challenging cryo-EM structure of the inactive GPR34 complexed with YL-365, which revealed the competitive binding of YL-365 in a portion of the orthosteric binding pocket of GPR34 and the antagonist-binding-induced allostery in the receptor, implicating the inhibition mechanism of YL-365. Moreover, YL-365 displayed excellent activity in a neuropathic pain model without obvious toxicity. Collectively, this study offers mechanistic insights into the endogenous agonist recognition and antagonist inhibition of GPR34, and provides proof of concept that targeting GPR34 represents a promising strategy for disease treatment.

Concise Synthesis of Glycerophospholipids

Glycerophospholipids are major components of cellular membranes and provide important signaling molecules. Besides shaping membrane properties, some bind to specific receptors to activate biological pathways. Untangling the roles of individual glycerophospholipids requires clearly defined molecular species, a challenge that can be best addressed through chemical synthesis. However, glycerophospholipid syntheses are often lengthy due to the contrasting polarities found within these lipids. We now report a general strategy to quickly access glycerophospholipids via opening of a phosphate triester epoxide with carboxylic acids catalyzed by Jacobsen's Co(salen) complex. We show that this method can be applied to a variety of commercially available fatty acids, photoswitchable fatty acids, and other carboxylic acids to provide the corresponding glycerophosphate derivatives.

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.

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.

Current Knowledge on the Biology of Lysophosphatidylserine as an Emerging Bioactive Lipid

Lysophosphatidylserine (LysoPS) is an emerging lysophospholipid (LPL) mediator, which acts through G protein-coupled receptors, like lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P). LysoPS is detected in various tissues and cells and thought to be produced mainly by the deacylation of phosphatidylserine. LysoPS has been known to stimulate degranulation of mast cells. Recently, four LysoPS-specific G protein-coupled receptors (GPCRs) were identified. These GPCRs belong to the P2Y family which covers receptors for nucleotides and LPLs and are predominantly expressed in immune cells such as lymphocytes and macrophages. Studies on knockout mice of these GPCRs have revealed that LysoPS has immune-modulatory functions. Up-regulation of a LysoPS-producing enzyme, PS-specific phospholipase A1, was frequently observed in situations where the immune system is activated including autoimmune diseases and organ transplantations. Therefore, modulation of LysoPS signaling appears to be a promising method for providing therapies for the treatment of immune diseases. In this review, we summarize the biology of LysoPS-producing enzymes and receptors, recent developments in LysoPS signal modulators, and prospects for future therapeutic applications.

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.

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.

Identification of P2Y receptors involved in oleamide-suppressing inflammatory responses in murine microglia and human dendritic cells

Microglia, a type of immune cell in the central nervous system, are involved in inflammation leading to neurodegenerative diseases. We previously identified oleamide from fermented dairy products as a neuroprotective compound suppressing microglial inflammation. Oleamide is an endocannabinoid and displays anti-inflammatory activity via the cannabinoid-2 (CB2) receptor; however, the mechanism underlying this anti-inflammatory activity has not been fully elucidated. Here, we found that the suppressive effect of oleamide on microglial tumor necrosis factor-α (TNF-α) production was canceled by inhibitors of G-protein-coupled receptor (GPCR) downstream signaling but not by a CB2 antagonist, suggesting that GPCRs other than CB2 are involved in the anti-inflammatory effects of oleamide. An extensive screen for GPCRs using a transforming growth factor-α shedding assay system identified P2Y1, P2Y4, P2Y6, P2Y10, and P2Y11 as candidates for the oleamide target. P2Y1 and P2Y10 agonists suppressed microglial TNF-α production, while a pan P2 receptor antagonist canceled the suppressive effect. Furthermore, we observed a relationship between the P2Y1 agonistic activities and the suppressive activities of oleamide and its analogs. Taken together, our results suggest that, in addition to CB2, P2Y type receptors are the potential targets of oleamide, and P2Y1 plays a role in the suppression of microglial inflammatory responses by oleamide. (200/200 words)

The G protein-coupled receptor GPR34 - The past 20 years of a grownup

Research on GPR34, which was discovered in 1999 as an orphan G protein-coupled receptor of the rhodopsin-like class, disclosed its physiologic relevance only piece by piece. Being present in all recent vertebrate genomes analyzed so far it seems to improve the fitness of species although it is not essential for life and reproduction as GPR34-deficient mice demonstrate. However, closer inspection of macrophages and microglia, where it is mainly expressed, revealed its relevance in immune cell function. Recent data clearly demonstrate that GPR34 function is required to arrest microglia in the M0 homeostatic non-phagocytic phenotype. Herein, we summarize the current knowledge on its evolution, genomic and structural organization, physiology, pharmacology and relevance in human diseases including neurodegenerative diseases and cancer, which accumulated over the last 20 years.

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.

Palladium-Catalyzed α-Stereoselective O-Glycosylation of O(3)-Acylated Glycals

Pd(MeCN)₂Cl₂ enables the α-stereoselective catalytic synthesis of 2,3-unsaturated O-glycosides from O(3)-acylated glycals without the requirement for additives to preactivate either donor or nucleophile. Mechanistic studies suggest that, unlike traditional (η3-allyl)palladium-mediated processes, the reaction proceeds via an alkoxy-palladium intermediate that increases the proton acidity and oxygen nucleophilicity of the alcohol. The method is exemplified with the synthesis of a range of glycosides and glycoconjugates of synthetic utility.

Phospholipid localization implies microglial morphology and function via Cdc42 in vitro

Under a quiescent state, microglia exhibit a ramified shape, rather than the amoeboid-like morphology following injury or inflammation. The manipulation of microglial morphology in vitro has not been very successful, which has impeded the progress of microglial studies. We demonstrate that lysophosphatidylserine (LysoPS), a kind of lysophospholipids, rapidly and substantially alters the morphology of primary cultured microglia to an in vivo-like ramified shape in a receptor independent manner. This mechanism is mediated by Cdc42 activity. LysoPS is incorporated into the plasma membrane and converted to phosphatidylserine (PS) via the Lands' cycle. The accumulated PS on the membrane recruits Cdc42. Both Cdc42 and PS colocalize predominantly in primary and secondary processes, but not in peripheral branches or tips of microglia. Along with the morphological changes LysoPS suppresses inflammatory cytokine production and NF-kB activity. The present study provides a tool to manipulate a microglial phenotype from an amoeboid to a fully ramified in vitro, which certainly contributes to studies exploring microglial physiology and pathology.

Pd-catalyzed direct oxidative mono-aroyloxylation of O-aralkyl substituted acetoxime ethers

A highly site-selective palladium-catalyzed ortho-mono-aroyloxylation of O-aralkyl substituted acetoxime ethers via direct Csp²–H bond activation has been developed with simple exo-acetoxime as a directing group.