Sphingosine 1-phosphate analogue recognition and selectivity at S1P4 within the endothelial differentiation gene family of receptors

O-Cyclic Phytosphingosine-1-Phosphate Protects against Motor Dysfunctions and Glial Cell Mediated Neuroinflammation in the Parkinson's Disease Mouse Models

O-cyclic phytosphingosine-1-phosphate (cPS1P) is a novel and chemically synthesized sphingosine metabolite derived from phytosphingosine-1-phosphate (S1P). This study was undertaken to unveil the potential neuroprotective effects of cPS1P on two different mouse models of Parkinson's disease (PD). The study used 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and neuron specific enolase promoter human alpha-synuclein (NSE-hαSyn) Korl transgenic mice. MPTP was injected for five consecutive days and cPS1P was injected for alternate days for six weeks intraperitoneally. We performed behavioral tests and analyzed the immunohistochemistry and immunofluorescence staining in the substantia nigra pars compacta (SNpc) and the striatum. The behavior tests showed a significant reduction in the motor functions in the PD models, which was reversed with the administration of cPS1P. In addition, both PD-models showed reduced expression of the sphingosine-1-phosphate receptor 1 (S1PR1), and α-Syn which was restored with cPS1P treatment. In addition, administration of cPS1P restored dopamine-related proteins such as tyrosine hydroxylase (TH), vesicular monoamine transporter 2 (VMAT2), and dopamine transporter (DAT). Lastly, neuroinflammatory related markers such as glial fibrillary acidic protein (GFAP), ionized calcium-binding adapter protein-1 (Iba-1), c-Jun N-terminal kinases (JNK), nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB), tumor necrosis factor-alpha (TNF-α), and interleukin 1 beta (IL-1β) were all reduced after cPS1P administration. The overall findings supported the notion that cPS1P protects against dopamine depletion, neuroinflammation, and PD-associated symptoms.

The noncanonical chronicles: Emerging roles of sphingolipid structural variants

Sphingolipids (SPs) are structurally diverse and represent one of the most quantitatively abundant classes of lipids in mammalian cells. In addition to their structural roles, many SP species are known to be bioactive mediators of essential cellular processes. Historically, studies have focused on SP species that contain the canonical 18‑carbon, mono-unsaturated sphingoid backbone. However, increasingly sensitive analytical technologies, driven by advances in mass spectrometry, have facilitated the identification of previously under-appreciated, molecularly distinct SP species. Many of these less abundant species contain noncanonical backbones. Interestingly, a growing number of studies have identified clinical associations between these noncanonical SPs and disease, suggesting that there is functional significance to the alteration of SP backbone structure. For example, associations have been found between SP chain length and cardiovascular disease, pain, diabetes, and dementia. This review will provide an overview of the processes that are known to regulate noncanonical SP accumulation, describe the clinical correlations reported for these molecules, and review the experimental evidence for the potential functional implications of their dysregulation. It is likely that further scrutiny of noncanonical SPs may provide new insight into pathophysiological processes, serve as useful biomarkers for disease, and lead to the design of novel therapeutic strategies.

Sphingosine-1-phosphate (S1P) analog phytosphingosine-1-phosphate (P1P) improves the in vitro maturation efficiency of porcine oocytes via regulation of oxidative stress and apoptosis

Phytosphingosine-1-phosphate (P1P) is a signaling sphingolipid that regulates various physiological activities. However, little is known about the effect of P1P in the context of reproduction. Thus, we aimed to investigate the influence of P1P on oocyte maturation during porcine in vitro maturation (IVM). Here, we report the expression of S1PR1-3 among P1P receptors (S1PR1-4) in cumulus cells and oocytes. When P1P was administered at concentrations of 10, 50, 100, and 1,000 nM during IVM, the metaphase II rate was significantly increased in the 1,000 nM (1 μM) P1P treatment group. Maturation rate improvement by P1P supplementation was observed only in the presence of epidermal growth factor (EGF). Oocytes under the influence of P1P showed decreased intracellular reactive oxygen species levels but no significant differences in glutathione levels. In our molecular studies, P1P treatment upregulated gene expression involved in cumulus expansion (Has2 and EGF), antioxidant enzymes (SOD3 and Cat), and developmental competence (Oct4) while activating extracellular signal-regulated kinase1/2 and Akt signaling. P1P treatment also influenced oocyte survival by shifting the ratio of Bcl-2 to Bax while inactivating JNK signaling. We further demonstrated that oocytes matured with P1P displayed significantly higher developmental competence (cleavage and blastocyst [BL] formation rate) and greater BL quality (total cell number and the ratio of apoptotic cells) when activated via parthenogenetic activation (PA) and in vitro fertilization. Despite the low levels of endogenous P1P found in animals, exogenous P1P influenced animal reproduction, as shown by increased porcine oocyte maturation as well as preimplantation embryo development. This study and its findings are potentially relevant for both human and animal-assisted reproduction.

O-cyclic phytosphingosine-1-phosphate stimulates HIF1α-dependent glycolytic reprogramming to enhance the therapeutic potential of mesenchymal stem cells

O-cyclic phytosphingosine-1-phosphate (cP1P) is a novel chemically synthesized sphingosine metabolite derived from phytosphingosine-1-phosphate. Although structurally similar to sphingosine-1-phosphate (S1P), its biological properties in stem cells remain to be reported. We investigated the effect of cP1P on the therapeutic potential of mesenchymal stem cells (MSCs) and their regulatory mechanism. We found that, under hypoxia, cP1P suppressed MSC mitochondrial dysfunction and apoptosis. Metabolic data revealed that cP1P stimulated glycolysis via the upregulation of glycolysis-related genes. cP1P-induced hypoxia-inducible factor 1 alpha (HIF1α) plays a key role for MSC glycolytic reprogramming and transplantation efficacy. The intracellular calcium-dependent PKCα/mammalian target of the rapamycin (mTOR) signaling pathway triggered by cP1P regulated HIF1α translation via S6K1, which is critical for HIF1 activation. Furthermore, the cP1P-activated mTOR pathway induced bicaudal D homolog 1 expression, leading to HIF1α nuclear translocation. In conclusion, cP1P enhances the therapeutic potential of MSC through mTOR-dependent HIF1α translation and nuclear translocation.

Optical control of sphingosine-1-phosphate formation and function

Sphingosine-1-phosphate (S1P) plays important roles as a signaling lipid in a variety of physiological and pathophysiological processes. S1P signals via a family of G protein-coupled receptors (S1P_1–5) and intracellular targets. Here, we report on photoswitchable analogs of S1P and its precursor sphingosine, respectively termed PhotoS1P and PhotoSph. PhotoS1P enables optical control of S1P_1–3, shown through electrophysiology and Ca²⁺ mobilization assays. We evaluated PhotoS1Pin vivo, where it reversibly controlled S1P₃-dependent pain hypersensitivity in mice. The hypersensitivity induced by PhotoS1P is comparable to that induced by S1P. PhotoS1P is uniquely suited for the study of S1P biology in cultured cells and in vivo because it exhibits prolonged metabolic stability compared to the rapidly metabolized S1P. Using lipid mass spectrometry analysis, we constructed a metabolic map of PhotoS1P and PhotoSph. The formation of these photoswitchable lipids was found to be light-dependent, providing a novel approach to optically probe sphingolipid biology.

Dose-dependent effect of aspirin on the level of sphingolipids in human blood

PURPOSE

Aspirin is an antiplatelet drug which is commonly used in secondary prevention in ischemic heart disease and cerebrovascular events, and in newly diagnosed myocardial infarction. The aim of the present study was to examine effect of aspirin on the level of selected sphingolipid intermediates in plasma, erythrocytes and platelets.

MATERIAL AND METHOD

Forty two healthy volunteers participated in the study. They were divided into two groups. In one group aspirin was given orally, daily, for one week in a dose of 75 mg (n=25). The subjects from the second group received one 300 mg dose of the drug (n=17). In both groups the blood was taken 4h after the last dose of aspirin. The following sphingolipid intermediates were quantified using high-pressure liquid chromatography: sphinganine, sphingosine, sphingosine-1-phosphate (S1P), sphinganine-1-phosphate (SA1P) and ceramide.

RESULTS

It was found that lower dose of aspirin increased the level of S1P and ceramide in erythrocytes (by 23 and 37%, respectively) having no effect on plasma and platelet sphingolipid levels. Higher dose of the drug reduced S1P and SA1P concentration in the plasma (by 16 and 10%, respectively).

CONCLUSION

We conclude that aspirin interferes with sphingolipid metabolism in blood and that this effect depends on a dose of the drug. Since S1P is a potent cardioprotectant, the reduction in its plasma concentration after the loading dose of aspirin could be undesired side effect of the drug.

Towards selective lysophospholipid GPCR modulators

G-protein-coupled receptors (GPCRs) that recognize the lysophospholipids (LPLs) are grouped into two phylogenetically distinct families: the endothelial differentiation gene (Edg) and non-Edg GPCRs. Owing to their more recent identification, and hindered by a lack of selective pharmacological tools, our understanding of the functions and signaling pathways of the non-Edg GPCRs is still in its infancy. Targeting the non-conserved allosteric binding sites of the LPL GPCRs shows particular promise for the development of selective modulators by structure-based drug design. However, only one Edg GPCR (S1PR1) structure has been determined to date, and it has low sequence identity with the non-Edg GPCRs (<20%). Thus, a representative structure of a non-Edg GPCR remains a pressing objective for selective structure-based drug design. Obtaining selective modulators targeting the non-Edg receptors would help to unravel the biology behind these novel GPCRs and potentially will support therapeutic treatment of diseases such as cancer, inflammation, and neuropsychiatric disorders.

The organization of the sphingosine 1-phosphate signaling system

The understanding of the role of the sphingosine 1-phosphate signaling system in immunology and host defense has deepened exponentially over the past 12 years since the discovery that lymphocyte egress was reversibly modulated by sphingosine 1-phosphate receptors, and with the development of fingolimod, a prodrug of a nonselective S1P receptor agonist, for therapeutic use in the treatment of relapsing, remitting multiple sclerosis. Innovative genetic and chemical approaches, together with structural biology, now provide a more detailed molecular understanding of a regulated lysophospholipid ligand with a variety of autocrine, paracrine, and systemic effects in physiology and pathology, based upon selective interactions with a high affinity and selective evolutionary cluster of G-protein-coupled receptors.

Structural biology of the S1P1 receptor

The sphingosine 1 phosphate receptor family has been studied widely since the initial discovery of its first member, endothelium differentiation gene 1. Since this initial discovery, the family has been renamed and the primary member of the family, the S1P1 receptor, has been targeted for a variety of disease indications and successfully drugged for the treatment of patients with relapsing multiple sclerosis. Recently, the three-dimensional structure of the S1P1 receptor has been determined by X-ray crystallography and the specifics of the sphingosine 1 phosphate ligand binding pocket mapped. Key structural features for the S1P1 receptor will be reviewed and the potential binding modes of additional pharmacologically active agents against the receptor will be analyzed in an effort to better understand the structural basis of important receptor-ligand interactions.

Sphingosine-1-phosphate and its receptors: structure, signaling, and influence

The sphingosine-1-phosphate (S1P) receptor signaling system has biological and medical importance and is the first lipid G protein-coupled receptor (GPCR) structure to be solved to 2.8-Å resolution. S1P binds to five high-affinity GPCRs generating multiple downstream signals that play essential roles in vascular development and endothelial integrity, control of cardiac rhythm, and routine oral treatment of multiple sclerosis. Genetics, chemistry, and now structural biology have advanced this integrated biochemical system. The S1P receptors have a novel N-terminal fold that occludes access to the binding pocket from the extracellular environment as well as orthosteric and bitopic ligands with very different physicochemical properties. S1P receptors and metabolizing enzymes have been deleted, inducibly deleted, and knocked in as tagged or altered receptors in mice. An array of genetic models allows analysis of integrated receptor function in vivo. We can now directly understand causal relationships among protein expression, signal, and control points in physiology and pathology.

Dissecting influenza virus pathogenesis uncovers a novel chemical approach to combat the infection

The cytokine storm is an aggressive immune response characterized by the recruitment of inflammatory leukocytes and exaggerated levels of cytokines and chemokines at the site of infection. Here we review evidence that cytokine storm directly contributes to the morbidity and mortality resulting from influenza virus infection and that sphingosine-1-phosphate (S1P) receptor agonists can abort cytokine storms providing significant protection against pathogenic human influenza viral infections. In experiments using murine models and the human pathogenic 2009 influenza viruses, S1P1 receptor agonist alone reduced deaths from influenza virus by over 80% as compared to lesser protection (50%) offered by the antiviral neuraminidase inhibitor oseltamivir. Optimal protection of 96% was achieved by combined therapy with the S1P1 receptor agonist and oseltamivir. The functional mechanism of S1P receptor agonist(s) action and the predominant role played by pulmonary endothelial cells as amplifiers of cytokine storm during influenza infection are described.

Integrating the puzzle pieces: the current atomistic picture of phospholipid-G protein coupled receptor interactions

A compelling question of how phospholipids interact with their target receptors has been of interest since the first receptor-mediated effects were reported. The recent report of a crystal structure for the S1P(1) receptor in complex with an antagonist phospholipid provides interesting perspective on the insights that had previously been gained through structure-activity studies of the phospholipids, as well as modeling and mutagenesis studies of the receptors. This review integrates these varied lines of investigation in the context of their various contributions to our current understanding of phospholipid-receptor interactions. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

Myocardial infarction differentially alters sphingolipid levels in plasma, erythrocytes and platelets of the rat

Three bioactive sphingolipids, namely sphingosine-1-phosphate (S1P), ceramide (CER) and sphingosine (SPH) were shown to be involved in ischemia/reperfusion injury of the heart. S1P is a powerful cardioprotectant, CER activates apoptosis and SPH in a low dose is cardioprotective whereas in a high dose is cardiotoxic. The aim of the present study was to examine effects of experimental myocardial infarction on the level of selected sphingolipids in plasma, erythrocytes and platelets in the rat. Myocardial infarction was produced in male Wistar rats by ligation of the left coronary artery. Blood was taken from the abdominal aorta at 1, 6 and 24 h after the ligation. Plasma, erythrocytes and platelets were isolated and S1P, dihydrosphingosine-1-phosphate (DHS1P), SPH, dihydrosphingosine (DHS) and CER were quantified by means of an Agilent 6460 triple quadrupole mass spectrometer using positive ion electrospray ionization source with multiple reaction monitoring. The infarction reduced the plasma level of S1P, DHS1P, SPH and DHS but increased the level of total CER. In erythrocytes, there was a sharp elevation in the level of SPH and DHS early after the infarction and a reduction after 24 h whereas the level of S1P, DHS1P and total CER gradually increased. In platelets, the level of each of the examined compounds profoundly decreased 1 and 6 h after the infarction and partially normalized in 24 h. The results obtained clearly show that experimental heart infarction in rats produces deep changes in metabolism of sphingolipids in the plasma, platelets and erythrocytes.

Phytosphingosine-1-phosphate represses the hydrogen peroxide-induced activation of c-Jun N-terminal kinase in human dermal fibroblasts through the phosphatidylinositol 3-kinase/Akt pathway

Dermal fibroblasts are differentiated mesenchymal cells that regulate the extracellular matrix through the production of dermis components. Dermal fibroblasts can be damaged by reactive oxygen species induced by ultraviolet rays and chemicals. In addition to its effects on the dermis, oxidative stress poses a major threat to organisms and is believed to play an essential role in many disease processes. In this study, we show that human dermal fibroblasts (HDFs) express sphingosine-1-phosphate (S1P) receptors S1P(1), S1P(2), and S1P(3). In addition, cell viability of HDFs is increased by phytosphingosine-1-phosphate (PhS1P) via regulation of the Jun N-terminal kinase (JNK)/Akt pathway. Interestingly, regulation of the JNK/Akt pathway by PhS1P attenuated H(2)O(2)-induced cell growth arrest. Together, our data indicate that PhS1P attenuates H(2)O(2)-induced growth arrest through regulation of the signal molecules Akt and JNK, and suggest that PhS1P may have value as an anti-aging material in cosmetics and medicine.

Comparative modeling of lipid receptors

Comparative modeling is a powerful technique to generate models of proteins from families already represented by members with experimentally characterized three-dimensional structures. The method is particularly important for modeling membrane-bound receptors in the G Protein-Coupled Receptor (GPCR) family, such as many of the lipid receptors (such as the cannabinoid, prostanoid, lysophosphatidic acid, sphingosine 1-phosphate, and eicosanoid receptor family members), as these represent particularly challenging targets for experimental structural characterization methods. Although challenging modeling targets, these receptors have been linked to therapeutic indications that vary from nociception to cancer, and thus are of interest as therapeutic targets. Accurate models of lipid receptors are therefore valuable tools in the drug discovery and optimization phases of therapeutic development. This chapter describes the construction and evaluation of comparative structural models of lipid receptors beginning with the selection of template structures.

Discovery, synthesis and SAR analysis of novel selective small molecule S1P4-R agonists based on a (2Z,5Z)-5-((pyrrol-3-yl)methylene)-3-alkyl-2-(alkylimino)thiazolidin-4-one chemotype

High affinity and selective S1P(4) receptor (S1P(4)-R) small molecule agonists may be important proof-of-principle tools used to clarify the receptor biological function and effects to assess the therapeutic potential of the S1P(4)-R in diverse disease areas including treatment of viral infections and thrombocytopenia. A high-throughput screening campaign of the Molecular Libraries-Small Molecule Repository was carried out by our laboratories and identified (2Z,5Z)-5-((1-(2-fluorophenyl)-2,5-dimethyl-1H-pyrrol-3-yl)methylene)-3-methyl-2-(methylimino) thiazolidin-4-one as a promising S1P(4)-R agonist hit distinct from literature S1P(4)-R modulators. Rational chemical modifications of the hit allowed the identification of a promising lead molecule with low nanomolar S1P(4)-R agonist activity and exquisite selectivity over the other S1P(1-3,5)-Rs family members. The lead molecule herein disclosed constitutes a valuable pharmacological tool to explore the effects of the S1P(4)-R signaling cascade and elucidate the molecular basis of the receptor function.

Role of the transmembrane domain 4/extracellular loop 2 junction of the human gonadotropin-releasing hormone receptor in ligand binding and receptor conformational selection

Recent crystal structures of G protein-coupled receptors (GPCRs) show the remarkable structural diversity of extracellular loop 2 (ECL2), implying its potential role in ligand binding and ligand-induced receptor conformational selectivity. Here we have applied molecular modeling and mutagenesis studies to the TM4/ECL2 junction (residues Pro(174(4.59))-Met(180(4.66))) of the human gonadotropin-releasing hormone (GnRH) receptor, which uniquely has one functional type of receptor but two endogenous ligands in humans. We suggest that the above residues assume an α-helical extension of TM4 in which the side chains of Gln(174(4.60)) and Phe(178(4.64)) face toward the central ligand binding pocket to make H-bond and aromatic contacts with pGlu(1) and Trp(3) of both GnRH I and GnRH II, respectively. The interaction between the side chains of Phe(178(4.64)) of the receptor and Trp(3) of the GnRHs was supported by reciprocal mutations of the interacting residues. Interestingly, alanine mutations of Leu(175(4.61)), Ile(177(4.63)), and Met(180(4.66)) decreased mutant receptor affinity for GnRH I but, in contrast, increased affinity for GnRH II. This suggests that these residues make intramolecular or intermolecular contacts with residues of transmembrane (TM) domain 3, TM5, or the phospholipid bilayer, which couple the ligand structure to specific receptor conformational switches. The marked decrease in signaling efficacy of I177A and F178A also indicates that IIe(177(4.63)) and Phe(178(4.64)) are important in stabilizing receptor-active conformations. These findings suggest that the TM4/ECL2 junction is crucial for peptide ligand binding and, consequently, for ligand-induced receptor conformational selection.

FTY720 (Gilenya) phosphate selectivity of sphingosine 1-phosphate receptor subtype 1 (S1P1) G protein-coupled receptor requires motifs in intracellular loop 1 and transmembrane domain 2

FTY720 phosphate (FTY720P) is a high potency agonist for all the endothelial differentiation gene family sphingosine 1-phosphate (S1P) receptors except S1P receptor subtype 2 (S1P(2)). To map the distinguishing features of S1P(2) ligand recognition, we applied a computational modeling-guided mutagenesis strategy that was based on the high degree of sequence homology between S1P(1) and S1P(2). S1P(2) point mutants of the ligand-binding pocket were characterized. The head group-interacting residues Arg3.28, Glu3.29, and Lys7.34 were essential for activation. Mutation of residues Ala3.32, Leu3.36, Val5.41, Phe6.44, Trp6.48, Ser7.42, and Ser7.46, predicted to interact with the S1P hydrophobic tail, impaired activation by S1P. Replacing individual or multiple residues in the ligand-binding pocket of S1P(2) with S1P(1) sequence did not impart activation by FTY720P. Chimeric S1P(1)/S1P(2) receptors were generated and characterized for activation by S1P or FTY720P. The S1P(2) chimera with S1P(1) sequence from the N terminus to transmembrane domain 2 (TM2) was activated by FTY720P, and the S1P(2)(IC1-TM2)(S1P1) domain insertion chimera showed S1P(1)-like activation. Twelve residues in this domain, distributed in four motifs a-d, differ between S1P(1) and S1P(2). Insertion of (78)RPMYY in motif b alone or simultaneous swapping of five other residues in motifs c and d from S1P(1) into S1P(2) introduced FTY720P responsiveness. Molecular dynamics calculations indicate that FTY720P binding selectivity is a function of the entropic contribution to the binding free energy rather than enthalpic contributions and that preferred agonists retain substantial flexibility when bound. After exposure to FTY720P, the S1P(2)(IC1-TM2)(S1P1) receptor recycled to the plasma membrane, indicating that additional structural elements are required for the selective degradative trafficking of S1P(1).

Translational aspects of sphingosine 1-phosphate biology

Sphingosine 1-phosphate (S1P) is a bioactive lipid that has both physiological and pathophysiological roles. It regulates cellular processes such as proliferation, migration, survival and differentiation and affects all organ systems. S1P not only activates S1P-specific receptors to initiate cellular signalling pathways but also directly regulates specific intracellular target proteins. The therapeutic opportunities surrounding S1P signalling are numerous and exemplified by the recent approval of FTY720 (a sphingosine analogue, Gilenya™) for the treatment of relapsing multiple sclerosis. A major focus of research is to develop small-molecule antagonists/agonists/inhibitors that are specific to the different S1P receptor subtypes and the enzymes that regulate S1P levels. This review describes fundamental aspects of S1P biology with an emphasis on the translational potential of intervention therapeutics.

Sphingosine 1-phosphate receptor 4 uses HER2 (ERBB2) to regulate extracellular signal regulated kinase-1/2 in MDA-MB-453 breast cancer cells

We demonstrate here that the bioactive lipid sphingosine 1-phosphate (S1P) uses sphingosine 1-phosphate receptor 4 (S1P(4)) and human epidermal growth factor receptor 2 (HER2) to stimulate the extracellular signal regulated protein kinase 1/2 (ERK-1/2) pathway in MDA-MB-453 cells. This was based on several lines of evidence. First, the S1P stimulation of ERK-1/2 was abolished by JTE013, which we show here is an S1P(2/4) antagonist and reduced by siRNA knockdown of S1P(4). Second, the S1P-stimulated activation of ERK-1/2 was almost completely abolished by a HER2 inhibitor (ErbB2 inhibitor II) and reduced by siRNA knockdown of HER2 expression. Third, phyto-S1P, which is an S1P(4) agonist, stimulated ERK-1/2 activation in an S1P(4)- and HER2-dependent manner. Fourth, FTY720 phosphate, which is an agonist at S1P(1,3,4,5) but not S1P(2) stimulated activation of ERK-1/2. Fifth, S1P stimulated the tyrosine phosphorylation of HER2, which was reduced by JTE013. HER2 which is an orphan receptor tyrosine kinase is the preferred dimerization partner of the EGF receptor. However, EGF-stimulated activation of ERK-1/2 was not affected by siRNA knockdown of HER2 or by ErbB2 (epidermal growth factor receptor 2 (or HER2)) inhibitor II in MDA-MB-453 cells. Moreover, S1P-stimulated activation of ERK-1/2 does not require an EGF receptor. Thus, S1P and EGF function in a mutually exclusive manner. In conclusion, the magnitude of the signaling gain on the ERK-1/2 pathway produced in response to S1P can be increased by HER2 in MDA-MB-453 cells. The linkage of S1P with an oncogene suggests that S1P and specifically S1P(4) may have an important role in breast cancer progression.

Sphingosine 1-phosphate regulation of extracellular signal-regulated kinase-1/2 in embryonic stem cells

Recent evidence suggests that sphingosine 1-phosphate (S1P) regulates self-renewal of human embryonic stem (ES) cells and differentiation of mouse embryoid bodies (derived from mouse ES cells) to cardiomyocytes. We have investigated the role of S1P in regulating ERK-1/2 signaling in mouse ES cells. In this regard, we found that both mouse ES-D3 and CGR8 cells express S1P(1), S1P(2), S1P(3), and S1P(5) but lack S1P(4). The treatment of ES cells with S1P induced the activation of ERK-1/2 via a mechanism that was not mediated by S1P(1), S1P(2), or S1P(3). This was based on: (i) the failure of S1P(1), S1P(2), or S1P(3) antagonists to inhibit S1P-stimulated ERK-1/2 activation and (ii) the failure of SEW 2871 (S1P(1) receptor agonist) to stimulate ERK-1/2 activation. The treatment of ES cells with phytosphingosine 1-phosphate (phyto-S1P), which we show here is an agonist of the S1P(5) receptor, stimulated ERK-1/2 activation. These findings therefore suggest that S1P(5) may mediate the effects of S1P in terms of regulating ERK-1/2 signaling in ES cells. The S1P-dependent activation of ERK-1/2 was sensitive to inhibition by pertussis toxin (uncouples the G-protein, G(i) from GPCR), bisindolylmaleimide I (PKC inhibitor), and PP2 (c-Src inhibitor), but was not reduced by LY29004 (PI3K inhibitor) suggesting that S1P uses G(i)-, PKC-, and c-Src-dependent mechanisms to activate the ERK-1/2 pathway in ES cells.

Unique ligand selectivity of the GPR92/LPA5 lysophosphatidate receptor indicates role in human platelet activation

Lysophosphatidic acid (LPA) is a ligand for LPA(1-3) of the endothelial differentiation gene family G-protein-coupled receptors, and LPA(4-8) is related to the purinergic family G-protein-coupled receptor. Because the structure-activity relationship (SAR) of GPR92/LPA(5) is limited and whether LPA is its preferred endogenous ligand has been questioned in the literature, in this study we applied a combination of computational and experimental site-directed mutagenesis of LPA(5) residues predicted to interact with the headgroup of LPA. Four residues involved in ligand recognition in LPA(5) were identified as follows: R2.60N mutant abolished receptor activation, whereas H4.64E, R6.62A, and R7.32A greatly reduced receptor activation. We also investigated the SAR of LPA(5) using LPA analogs and other non-lysophospholipid ligands. SAR revealed that the rank order of agonists is alkyl glycerol phosphate > LPA > farnesyl phosphates >> N-arachidonoylglycine. These results confirm LPA(5) to be a bona fide lysophospholipid receptor. We also evaluated several compounds with previously established selectivity for the endothelial differentiation gene receptors and found several that are LPA(5) agonists. A pharmacophore model of LPA(5) binding requirements was developed for in silico screening, which identified two non-lipid LPA(5) antagonists. Because LPA(5) transcripts are abundant in human platelets, we tested its antagonists on platelet activation and found that these non-lipid LPA(5) antagonists inhibit platelet activation. The present results suggest that selective inhibition of LPA(5) may provide a basis for future anti-thrombotic therapies.

Ligand-binding pocket shape differences between sphingosine 1-phosphate (S1P) receptors S1P1 and S1P3 determine efficiency of chemical probe identification by ultrahigh-throughput screening

We have studied the sphingosine 1-phosphate (S1P) receptor system to better understand why certain molecular targets within a closely related family are much more tractable when identifying compelling chemical leads. Five medically important G-protein-coupled receptors for S1P regulate heart rate, coronary artery caliber, endothelial barrier integrity, and lymphocyte trafficking. Selective S1P receptor agonist probes would be of great utility to study receptor subtype-specific function. Through systematic screening of the same libraries, we identified novel selective agonist chemotypes for each of the S1P1 and S1P3 receptors. Ultrahigh-throughput screening (uHTS) for S1P1 was more effective than that for S1P3, with many selective, low nanomolar hits of proven mechanism emerging. Receptor structure modeling and ligand docking reveal differences between the receptor binding pockets, which are the basis for subtype selectivity. Novel selective agonists interact primarily in the hydrophobic pocket of the receptor in the absence of headgroup interactions. Chemistry-space and shape-based analysis of the screening libraries in combination with the binding models explain the observed differential hit rates and enhanced efficiency for lead discovery for S1P1 versus S1P3 in this closely related receptor family.

Lysophospholipid interactions with protein targets

Bioactive lysophospholipids include lysophosphatidic acid (LPA), sphingosine 1-phosphate (S1P), cyclic-phosphatidic acid (CPA) and alkyl glycerolphosphate (AGP). These lipid mediators stimulate a variety of responses that include cell survival, proliferation, migration, invasion, wound healing, and angiogenesis. Responses to lysophospholipids depend upon interactions with biomolecular targets in the G protein-coupled receptor (GPCR) and nuclear receptor families, as well as enzymes. Our current understanding of lysophospholipid interactions with these targets is based on a combination of lysophospholipid analog structure activity relationship studies as well as more direct structural characterization techniques such as X-ray crystallography, nuclear magnetic resonance (NMR) spectroscopy, and experimentally-validated molecular modeling. The direct structural characterization studies are the focus of this review, and provide the insight necessary to stimulate structure-based therapeutic lead discovery efforts in the future.

Subtype-specific residues involved in ligand activation of the endothelial differentiation gene family lysophosphatidic acid receptors

Lysophosphatidic acid (LPA) is a ligand for three endothelial differentiation gene family G protein-coupled receptors, LPA(1-3). We performed computational modeling-guided mutagenesis of conserved residues in transmembrane domains 3, 4, 5, and 7 of LPA(1-3) predicted to interact with the glycerophosphate motif of LPA C18:1. The mutants were expressed in RH7777 cells, and the efficacy (E(max)) and potency (EC(50)) of LPA-elicited Ca(2+) transients were measured. Mutation to alanine of R3.28 universally decreased both the efficacy and potency in LPA(1-3) and eliminated strong ionic interactions in the modeled LPA complexes. The alanine mutation at Q3.29 decreased modeled interactions and activation in LPA(1) and LPA(2) more than in LPA(3). The mutation W4.64A had no effect on activation and modeled LPA interaction of LPA(1) and LPA(2) but reduced the activation and modeled interactions of LPA(3). The R5.38A mutant of LPA(2) and R5.38N mutant of LPA(3) showed diminished activation by LPA; however, in LPA(1) the D5.38A mutation did not, and mutation to arginine enhanced receptor activation. In LPA(2), K7.36A decreased the potency of LPA; in LPA(1) this same mutation increased the E(max). In LPA(3), R7.36A had almost no effect on receptor activation; however, the mutation K7.35A increased the EC(50) in response to LPA 10-fold. In LPA(1-3), the mutation Q3.29E caused a modest increase in EC(50) in response to LPA but caused the LPA receptors to become more responsive to sphingosine 1-phosphate (S1P). Surprisingly micromolar concentrations of S1P activated the wild type LPA(2) and LPA(3) receptors, indicating that S1P may function as a weak agonist of endothelial differentiation gene family LPA receptors.

Molecular recognition in the sphingosine 1-phosphate receptor family

Computational modeling and its application in ligand screening and ligand receptor interaction studies play important roles in structure-based drug design. A series of sphingosine 1-phosphate (S1P) receptor ligands with varying potencies and receptor selectivities were docked into homology models of the S1P(1-5) receptors. These studies provided molecular insights into pharmacological trends both across the receptor family as well as at single receptors. This study identifies ligand recognition features that generalize across the S1P receptor family, features unique to the S1P(4) and S1P(5) receptors, and suggests significant structural differences of the S1P(2) receptor. Docking results reveal a previously unknown sulfur-aromatic interaction between the S1P(4) C5.44 sulfur atom and the phenyl ring of benzimidazole as well as pi-pi interaction between F3.33 of S1P(1,4,5) and aromatic ligands. The findings not only confirm the importance of a cation-pi interaction between W4.64 and the ammonium of S1P at S1P(4) but also predict the same interaction at S1P(5). S1P receptor models are validated for pharmacophore development including database mining and new ligand discovery and serve as tools for ligand optimization to improve potency and selectivity.

Peptide design and structural characterization of a GPCR loop mimetic

G protein-coupled receptors (GPCRs) control fundamental aspects of human physiology and behaviors. Knowledge of their structures, especially for the loop regions, is limited and has principally been obtained from homology models, mutagenesis data, low resolution structural studies, and high resolution studies of peptide models of receptor segments. We developed an alternate methodology for structurally characterizing GPCR loops, using the human S1P(4) first extracellular loop (E1) as a model system. This methodology uses computational peptide designs based on transmembrane domain (TM) model structures in combination with CD and NMR spectroscopy. The characterized peptides contain segments that mimic the self-assembling extracellular ends of TM 2 and TM 3 separated by E1, including residues R3.28(121) and E3.29(122) that are required for sphingosine 1-phosphate (S1P) binding and receptor activation in the S1P(4) receptor. The S1P(4) loop mimetic peptide interacted specifically with an S1P headgroup analog, O-phosphoethanolamine (PEA), as evidenced by PEA-induced perturbation of disulfide cross-linked coiled-coil first extracellular loop mimetic (CCE1a) (1)H and (15)N backbone amide chemical shifts. CCE1a was capable of weakly binding PEA near biologically relevant residues R29 and E30, which correspond to R3.28 and E3.29 in the full-length S1P(4) receptor, confirming that it has adopted a biologically relevant conformation. We propose that the combination of coiled-coil TM replacement and conformational stabilization with an interhelical disulfide bond is a general design strategy that promotes native-like structure for loops derived from GPCRs.

The lysophosphatidic acid type 2 receptor is required for protection against radiation-induced intestinal injury

BACKGROUND & AIMS

We recently identified lysophosphatidic acid (LPA) as a potent antiapoptotic agent for the intestinal epithelium. The objective of the present study was to evaluate the effect of octadecenyl thiophosphate (OTP), a novel rationally designed, metabolically stabilized LPA mimic, on radiation-induced apoptosis of intestinal epithelial cells in vitro and in vivo.

METHODS

The receptors and signaling pathways activated by OTP were examined in IEC-6 and RH7777 cell lines and wild-type and LPA(1) and LPA(2) knockout mice exposed to different apoptotic stimuli.

RESULTS

OTP was more efficacious than LPA in reducing gamma irradiation-, camptothecin-, or tumor necrosis factor alpha/cycloheximide-induced apoptosis and caspase-3-8, and caspase-9 activity in the IEC-6 cell line. In RH7777 cells lacking LPA receptors, OTP selectively protected LPA(2) but not LPA(1) and LPA(3) transfectants. In C57BL/6 and LPA(1) knockout mice exposed to 15 Gy gamma irradiation, orally applied OTP reduced the number of apoptotic bodies and activated caspase-3-positive cells but was ineffective in LPA(2) knockout mice. OTP, with higher efficacy than LPA, enhanced intestinal crypt survival in C57BL/6 mice but was without any effect in LPA(2) knockout mice. Intraperitoneally administered OTP reduced death caused by lethal dose (LD)(100/30) radiation by 50%.

CONCLUSIONS

Our data indicate that OTP is a highly effective antiapoptotic agent that engages similar prosurvival pathways to LPA through the LPA(2) receptor subtype.

Sphingosine 1-phosphate pKa and binding constants: intramolecular and intermolecular influences

The dissociation constant for an ionizable ligand binding to a receptor is dependent on its charge and therefore on its environmentally-influenced pKa value. The pKa values of sphingosine 1-phosphate (S1P) were studied computationally in the context of the wild type S1P1 receptor and the following mutants: E3.29Q, E3.29A, and K5.38A. Calculated pKa values indicate that S1P binds to S1P1 and its site mutants with a total charge of -1, including a +1 charge on the ammonium group and a -2 charge on the phosphate group. The dissociation constant of S1P binding to these receptors was studied as well. The models of wild type and mutant proteins originated from an active receptor model that was developed previously. We used ab initio RHF/6-31+G(d) to optimize our models in aqueous solution, where the solvation energy derivatives are represented by conductor-like polarizable continuum model (C-PCM) and integral equation formalism polarizable continuum model (IEF-PCM). Calculation of the dissociation constant for each mutant was determined by reference to the experimental dissociation constant of the wild type receptor. The computed dissociation constants of the E3.29Q and E3.29A mutants are three to five orders of magnitude higher than those for the wild type receptor and K5.38A mutant, indicating vital contacts between the S1P phosphate group and the carboxylate group of E3.29. Computational dissociation constants for K5.38A, E3.29A, and E3.29Q mutants were compared with experimentally determined binding and activation data. No measurable binding of S1P to the E3.29A and E3.29Q mutants was observed, supporting the critical contacts observed computationally. These results validate the quantitative accuracy of the model.

Identification of the hydrophobic ligand binding pocket of the S1P1 receptor

Sphingosine 1-phosphate (S1P), a naturally occurring sphingolipid mediator and also a second messenger with growth factor-like actions in almost every cell type, is an endogenous ligand of five G protein-coupled receptors (GPCRs) in the endothelial differentiation gene family. The lack of GPCR crystal structures sets serious limitations to rational drug design and in silico searches for subtype-selective ligands. Here we report on the experimental validation of a computational model of the ligand binding pocket of the S1P1 GPCR surrounding the aliphatic portion of S1P. The extensive mutagenesis-based validation confirmed 18 residues lining the hydrophobic ligand binding pocket, which, combined with the previously validated three head group-interacting residues, now complete the mapping of the S1P ligand recognition site. We identified six mutants (L3.43G/L3.44G, L3.43E/L3.44E, L5.52A, F5.48G, V6.40L, and F6.44G) that maintained wild type [32P]S1P binding with abolished ligand-dependent activation by S1P. These data suggest a role for these amino acids in the conformational transition of S1P1 to its activated state. Three aromatic mutations (F5.48Y, F6.44G, and W6.48A) result in differential activation, by S1P or SEW2871, indicating that structural differences between the two agonists can partially compensate for differences in the amino acid side chain. The now validated ligand binding pocket provided us with a pharmacophore model, which was used for in silico screening of the NCI, National Institutes of Health, Developmental Therapeutics chemical library, leading to the identification of two novel nonlipid agonists of S1P1.

Identification of Leu276 of the S1P1 receptor and Phe263 of the S1P3 receptor in interaction with receptor specific agonists by molecular modeling, site-directed mutagenesis, and affinity studies

Sphingosine-1-phosphate (S1P) receptor agonists are novel immunosuppressive agents. The selectivity of S1P1 against S1P3 is strongly correlated with lymphocyte sequestration and minimum acute toxicity and bradycardia. This study describes molecular modeling, site-directed mutagenesis, and affinity studies exploring the molecular basis for selectivity between S1P1 and S1P3 receptors. Computational models of human S1P1 and S1P3 receptors bound with two nonselective agonists or two S1P1-selective agonists were developed based on the X-ray crystal structure of bovine rhodopsin. The models predict that S1P1 Leu276 and S1P3 Phe263 contribute to the S1P1/S1P3 selectivity of the two S1P1-selective agonists. These residues were subjected to site-directed mutagenesis. The wild-type and mutant S1P receptors were expressed in Chinese hamster ovary cells and examined for their abilities to bind to and be activated by agonists in vitro. The results indicate that the mutations have minimal effects on the activities of the two nonselective agonists, although they have dramatic effects on the S1P1-selective agonists. These studies provide a fundamental understanding of how these two receptor-selective agonists bind to the S1P1 and S1P3 receptors, which should aid development of more selective S1P1 receptor agonists with immunosuppressive properties and improved safety profiles.

Distribution and dynamic changes of sphingolipids in blood in response to platelet activation

BACKGROUND

Sphingolipids are signaling molecules in a range of biological processes. While sphingosine-1-phosphate (S1P) is thought to be abundantly stored in platelets and released upon stimulation, knowledge about the distribution and function of other sphingolipids in blood is lacking.

OBJECTIVES

To analyze the sphingolipid content of blood components with special emphasis on dynamic changes in platelets.

METHODS

Blood components from mice and humans were prepared by gradient centrifugation and analyzed by liquid chromatography-mass spectrometry. Additionally, murine platelets were activated in vitro and in vivo.

RESULTS

Isolated non-activated platelets of mice were devoid of S1P, but instead contained dihydrosphingosine-1-phosphate (dhS1P), along with a high concentration of ceramide. Activation of platelets in vitro led to a loss of dhS1P and an increase in sphingosine, accompanied by a reduction of ceramide content. Platelet activation in vivo led to an immediate and continuous rise of dhS1P in plasma, while S1P remained stable. The sphingolipid distribution of human blood was markedly different from mice. Human platelets contained dhS1P in addition to S1P.

CONCLUSIONS

Mouse platelets contain dhS1P instead of S1P. Platelet activation causes loss of dhS1P and breakdown of ceramide, implying ceramidase activation. Release of dhS1P from activated platelets might be a novel signaling pathway. Finally, the sphingolipid composition of mouse and human blood shows large differences, which must be considered when studying sphingolipid biology.

Protean agonism of the lysophosphatidic acid receptor-1 with Ki16425 reduces nerve growth factor-induced neurite outgrowth in pheochromocytoma 12 cells

We report here a novel role for the constitutively active lysophosphatidic acid receptor-1 (LPA(1)) receptor in providing Gbetagamma subunits for use by the Trk A receptor. This enhances the ability of nerve growth factor (NGF) to promote signalling and cell response. These conclusions were based on three lines of evidence. Firstly, the LPA(1) receptor was co-immunoprecipitated with the Trk A receptor from lysates, suggesting that these proteins form a complex. Secondly, Ki16425, a selective protean agonist of the LPA(1) receptor, decreased constitutive basal and LPA-induced LPA(1) receptor-stimulated GTPgammaS binding. Ki16425 reduced the LPA-induced activation of p42/p44 mitogen activated protein kinase (MAPK), while acting as a weak stimulator of p42/p44 MAPK on its own, properties typical of a protean agonist. Significantly, Ki16425 also reduced the NGF-induced stimulation of p42/p44 MAPK and inhibited NGF-stimulated neurite outgrowth. Thirdly, the over-expression of the C-terminal GRK-2 peptide, which sequesters Gbetagamma subunits, reduced the NGF-induced activation of p42/p44 MAPK. In contrast, the stimulation of PC12 cells with LPA leads to a predominant G(i)alpha2-mediated Trk A-independent activation of p42/p44 MAPK, where Gbetagamma subunits play a diminished role. These findings suggest a novel role for the constitutively active LPA(1) receptor in regulating NGF-induced neuronal differentiation.

Lysophospholipid receptors: signalling, pharmacology and regulation by lysophospholipid metabolism

The lysophospholipids, sphingosine-1-phosphate (S1P), lysophosphatidic acid (LPA), sphingosylphosphorylcholine (SPC) and lysophosphatidylcholine (LPC), activate diverse groups of G-protein-coupled receptors that are widely expressed and regulate decisive cellular functions. Receptors of the endothelial differentiation gene family are activated by S1P (S1P(1-5)) or LPA (LPA(1-3)); two more distantly related receptors are activated by LPA (LPA(4/5)); the GPR(3/6/12) receptors have a high constitutive activity but are further activated by S1P and/or SPC; and receptors of the OGR1 cluster (OGR1, GPR4, G2A, TDAG8) appear to be activated by SPC, LPC, psychosine and/or protons. G-protein-coupled lysophospholipid receptors regulate cellular Ca(2+) homoeostasis and the cytoskeleton, proliferation and survival, migration and adhesion. They have been implicated in development, regulation of the cardiovascular, immune and nervous systems, inflammation, arteriosclerosis and cancer. The availability of S1P and LPA at their G-protein-coupled receptors is regulated by enzymes that generate or metabolize these lysophospholipids, and localization plays an important role in this process. Besides FTY720, which is phosphorylated by sphingosine kinase-2 and then acts on four of the five S1P receptors of the endothelial differentiation gene family, other compounds have been identified that interact with more ore less selectivity with lysophospholipid receptors.

Sphingosine-1-phosphate stimulates rat primary chondrocyte proliferation

Rat primary chondrocytes express the sphingosine-1-phosphate (S1P) receptor, S1P(2), S1P(3), S1P(4), but not S1P(1). When chondrocytes were stimulated with S1P or phytosphingosine-1-phosphate (PhS1P, an S1P(1)- and S1P(4)-selective agonist), phospholipase C-mediated cytosolic calcium increase was dramatically induced. S1P and PhS1P also stimulated two kinds of mitogen-activated protein kinases, extracellular signal-regulated kinase (ERK) and p38 kinase in chondrocytes. In terms of the two phospholipids-mediated functional modulation of chondrocytes, S1P and PhS1P stimulated cellular proliferation. The two phospholipids-induced chondrocyte proliferations were almost completely blocked by PD98059 but not by SB203580, suggesting that ERK but not p38 kinase is essentially required for the proliferation. Pertussis toxin almost completely inhibited the two phospholipids-induced cellular proliferation and ERK activation, indicating the crucial role of G(i) protein. This study demonstrates the physiological role of two important phospholipids (S1P and PhS1P) on the modulation of rat primary chondrocyte proliferation, and the crucial role played by ERK in the process.

Structural characteristics of lysophosphatidic acid biological targets

Lysophosphatidic acid (LPA; 1-acyl-3-phosphoglycerol) exerts its biological activity through both extracellular and intracellular targets. Receptor targets include the cell-surface G-protein-coupled receptors LPA(1-4) and the nuclear PPAR-gamma (peroxisome-proliferator-activated receptor gamma). Enzyme targets include the secreted cancer cell motility factor, autotaxin, and the transmembrane phosphatases, LPP1-3 (where LPP stands for lipid phosphate phosphatase). Ion channel targets include the two pore domain ion channels in the TREK family, TREK-1, TREK-2 and TRAAK. Structural features of these targets and their interactions with LPA are reviewed.