Full pharmacological efficacy of a novel S1P1 agonist that does not require S1P-like headgroup interactions.

Strong evidence exists for interactions of zwitterionic phosphate and amine groups in sphingosine-1 phosphate (S1P) to conserved Arg and Glu residues present at the extracellular face of the third transmembrane domain of S1P receptors. The contribution of Arg(120) and Glu(121) for high-affinity ligand-receptor interactions is essential, because single-point R(120)A or E(121)A S1P(1) mutants neither bind S1P nor transduce S1P function. Because S1P receptors are therapeutically interesting, identifying potent selective agonists with different binding modes and in vivo efficacy is of pharmacological importance. Here we describe a modestly water-soluble highly selective S1P(1) agonist [2-(4-(5-(3,4-diethoxyphenyl)-1,2,4-oxadiazol-3-yl)-2,3-dihydro-1H-inden-1-yl amino) ethanol (CYM-5442)] that does not require Arg(120) or Glu(121) residues for activating S1P(1)-dependent p42/p44 mitogen-activated protein kinase phosphorylation, which defines a new hydrophobic pocket in S1P(1). CYM-5442 is a full agonist in vitro for S1P(1) internalization, phosphorylation, and ubiquitination. It is noteworthy that CYM-5442 was a full agonist for induction and maintenance of S1P(1)-dependent blood lymphopenia, decreasing B lymphocytes by 65% and T lymphocytes by 85% of vehicle. Induction of CYM-5442 lymphopenia was dose- and time-dependent, requiring serum concentrations in the 50 nM range. In vitro measures of S1P(1) activation by CYM-5442 were noncompetitively inhibited by a specific S1P(1) antagonist [(R)-3-amino-(3-hexylphenylamino)-4-oxobutylphosphonic acid (W146)], competitive for S1P, 2-amino-2-(4-octylphenethyl)propane-1,3-diol (FTY720-P), and 5-[4-phenyl-5-(trifluoromethyl)-2-thienyl]-3-[3-(trifluoromethyl)phenyl]-1,2, 4-oxadiazole (SEW2871). In addition, lymphopenia induced by CYM-5442 was reversed by W146 administration or upon pharmacokinetic agonist clearance. Pharmacokinetics in mice also indicated that CYM-5442 partitions significantly in central nervous tissue. These data show that CYM-5442 activates S1P(1)-dependent pathways in vitro and to levels of full efficacy in vivo through a hydrophobic pocket separate from the orthosteric site of S1P binding that is headgroup-dependent.

S1P1-selective in vivo-active agonists from high-throughput screening: off-the-shelf chemical probes of receptor interactions, signaling, and fate.

The essential role of the sphingosine 1-phosphate (S1P) receptor S1P(1) in regulating lymphocyte trafficking was demonstrated with the S1P(1)-selective nanomolar agonist, SEW2871. Despite its lack of charged headgroup, the tetraaromatic compound SEW2871 binds and activates S1P(1) through a combination of hydrophobic and ion-dipole interactions. Both S1P and SEW2871 activated ERK, Akt, and Rac signaling pathways and induced S1P(1) internalization and recycling, unlike FTY720-phosphate, which induces receptor degradation. Agonism with receptor recycling is sufficient for alteration of lymphocyte trafficking by S1P and SEW2871. S1P(1) modeling and mutagenesis studies revealed that residues binding the S1P headgroup are required for kinase activation by both S1P and SEW2871. Therefore, SEW2871 recapitulates the action of S1P in all the signaling pathways examined and overlaps in interactions with key headgroup binding receptor residues, presumably replacing salt-bridge interactions with ion-dipole interactions.

Defective membrane interactions of familial Parkinson's disease mutant A30P alpha-synuclein.

alpha-Synuclein (alpha-Syn) is an abundant presynaptic protein of unknown function, which has been implicated in the pathogenesis of Parkinson's disease. Alpha-Syn has been suggested to play a role in lipid transport and synaptogenesis, and growing evidence suggests that alpha-Syn interactions with cellular membranes are physiologically important. In the current study, we demonstrate that the familial Parkinson's disease-linked A30P mutant alpha-Syn is defective in binding to phospholipid vesicles in vitro as determined by vesicle ultracentrifugation, circular dichroism spectroscopy, and low-angle X-ray diffraction. Interestingly, our data also suggest that alpha-Syn may bind to the lipid vesicles as a dimer, which suggest that this species could be a physiologically relevant and functional entity. In contrast, the naturally occurring murine A53T substitution, which is also linked to Parkinson's disease, displayed a normal membrane-binding activity that was comparable to wild-type alpha-Syn. A double mutant A53T/A30P alpha-Syn showed defective membrane binding similar to the A30P protein, indicating that the proline mutation is dominant in terms of impairing the membrane-binding activity. With these observations, we suggest that the A53T and A30P mutants may have different physiological consequences in vivo and could possibly contribute to early onset Parkinson's disease via unique mechanisms.

Novel selective allosteric and bitopic ligands for the S1P(3) receptor.

Sphingosine 1-phosphate (S1P) is a lysophospholipid signaling molecule that regulates important biological functions, including lymphocyte trafficking and vascular development, by activating G protein-coupled receptors for S1P, namely, S1P(1) through S1P(5). Here, we map the S1P(3) binding pocket with a novel allosteric agonist (CYM-5541), an orthosteric agonist (S1P), and a novel bitopic antagonist (SPM-242). With a combination of site-directed mutagenesis, ligand competition assay, and molecular modeling, we concluded that S1P and CYM-5541 occupy different chemical spaces in the ligand binding pocket of S1P(3). CYM-5541 allowed us to identify an allosteric site where Phe263 is a key gate-keeper residue for its affinity and efficacy. This ligand lacks a polar moiety, and the novel allosteric hydrophobic pocket permits S1P(3) selectivity of CYM-5541 within the highly similar S1P receptor family. However, a novel S1P(3)-selective antagonist, SPM-242, in the S1P(3) pocket occupies the ligand binding spaces of both S1P and CYM-5541, showing its bitopic mode of binding. Therefore, our coordinated approach with biochemical data and molecular modeling, based on our recently published S1P(1) crystal structure data in a highly conserved set of related receptors with a shared ligand, provides a strong basis for the successful optimization of orthosteric, allosteric, and bitopic modulators of S1P(3).

Crystal structure of a lipid G protein-coupled receptor.

The lyso-phospholipid sphingosine 1-phosphate modulates lymphocyte trafficking, endothelial development and integrity, heart rate, and vascular tone and maturation by activating G protein-coupled sphingosine 1-phosphate receptors. Here, we present the crystal structure of the sphingosine 1-phosphate receptor 1 fused to T4-lysozyme (S1P(1)-T4L) in complex with an antagonist sphingolipid mimic. Extracellular access to the binding pocket is occluded by the amino terminus and extracellular loops of the receptor. Access is gained by ligands entering laterally between helices I and VII within the transmembrane region of the receptor. This structure, along with mutagenesis, agonist structure-activity relationship data, and modeling, provides a detailed view of the molecular recognition and requirement for hydrophobic volume that activates S1P(1), resulting in the modulation of immune and stromal cell responses.

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.

Enhancement of capillary leakage and restoration of lymphocyte egress by a chiral S1P1 antagonist in vivo.

Sphingosine 1-phosphate (S1P, 1) regulates vascular barrier and lymphoid development, as well as lymphocyte egress from lymphoid organs, by activating high-affinity S1P1 receptors. We used reversible chemical probes (i) to gain mechanistic insights into S1P systems organization not accessible through genetic manipulations and (ii) to investigate their potential for therapeutic modulation. Vascular (but not airway) administration of the preferred R enantiomer of an in vivo-active chiral S1P1 receptor antagonist induced loss of capillary integrity in mouse skin and lung. In contrast, the antagonist did not affect the number of constitutive blood lymphocytes. Instead, alteration of lymphocyte trafficking and phenotype required supraphysiological elevation of S1P1 tone and was reversed by the antagonist. In vivo two-photon imaging of lymph nodes confirmed requirements for obligate agonism, and the data were consistent with the presence of a stromal barrier mechanism for gating lymphocyte egress. Thus, chemical modulation reveals differences in S1P-S1P1 'set points' among tissues and highlights both mechanistic advantages (lymphocyte sequestration) and risks (pulmonary edema) of therapeutic intervention.

S1P3 receptor-induced reorganization of epithelial tight junctions compromises lung barrier integrity and is potentiated by TNF.

Pulmonary pathologies including adult respiratory distress syndrome are characterized by disruption of pulmonary integrity and edema compromising respiratory function. Sphingosine 1-phosphate (S1P) is a lipid mediator synthesized and/or stored in mast cells, platelets, and epithelial cells, with production up-regulated by the proinflammatory cytokines IL-1 and TNF. S1P administration via the airways but not via the vasculature induces lung leakage. Using receptor-null mice, we show that S1P, acting on S1P3 receptor expressed on both type I and type II alveolar epithelial cells but not vascular endothelium, induces pulmonary edema by acute tight junction opening. WT but not S1P3-null mice showed disruption of pulmonary epithelial tight junctions and the appearance of paracellular gaps between epithelial cells by electron microscopy within 1 h of airways exposure to S1P. We further show by fluorescence microscopy that S1P induced rapid loss of ZO-1 reactivity, an essential component of the cytoplasmic plaque associated with tight junctions, as well as of the tetraspannin Claudin-18, an integral membrane organizer of tight junctions. S1P shows synergistic activity with the proinflammatory cytokine TNF, showing both pulmonary edema and mortality at subthreshold S1P doses. Specifically, preexposure of mice to subthreshold doses of TNF, which alone induced no lung edema, exacerbated S1P-induced edema and impaired survival. S1P, acting through S1P3, regulates epithelial integrity and acts additively with TNF in compromising respiratory barrier function. Because S1P3-null mice are resistant to S1P-induced pulmonary leakage, either alone or in the presence of TNF, S1P3 antagonism may be useful in protecting epithelial integrity in pulmonary disease.

Sphingosine 1-phosphate type 1 receptor agonism inhibits transendothelial migration of medullary T cells to lymphatic sinuses.

Sphingosine 1-phosphate type 1 (S1P(1)) receptor agonists cause sequestration of lymphocytes in secondary lymphoid organs by a mechanism that is not well understood. One hypothesis proposes that agonists act as 'functional antagonists' by binding and internalizing S1P(1) receptors on lymphocytes; a second hypothesis proposes instead that S1P(1) agonists act on endothelial cells to prevent lymphocyte egress from lymph nodes. Here, two-photon imaging of living T cells in explanted lymph nodes after treatment with S1P(1) agonists or antagonists has provided insight into the mechanism by which S1P(1) agonists function. The selective S1P(1) agonist SEW2871 caused reversible slowing and 'log-jamming' of T cells between filled medullary cords and empty sinuses, whereas motility was unaltered in diffuse cortex. Removal or antagonist competition of SEW2871 permitted recovery of T cell motility in the parenchyma of the medulla and resumption of migration across the stromal endothelial barrier, leading to refilling of sinuses. Our results provide visualization of transendothelial migration of T cells into lymphatic sinuses and suggest that S1P(1) agonists act mainly on endothelial cell S1P(1) receptors to inhibit lymphocyte migration.

alpha-Synuclein-synaptosomal membrane interactions: implications for fibrillogenesis.

alpha-Synuclein exists in two different compartments in vivo-- correspondingly existing as two different forms: a membrane-bound form that is predominantly alpha-helical and a cytosolic form that is randomly structured. It has been suggested that these environmental and structural differences may play a role in aggregation propensity and development of pathological lesions observed in Parkinson's disease (PD). Such effects may be accentuated by mutations observed in familial PD kindreds. In order to test this hypothesis, wild-type and A53T mutant alpha-synuclein interactions with rat brain synaptosomal membranes were examined. Previous data has demonstrated that the A30P mutant has defective lipid binding and therefore was not examined in this study. Electron microscopy demonstrated that wild-type alpha-synuclein fibrillogenesis is accelerated in the presence of synaptosomal membranes whereas the A53T alpha-synuclein fibrillogenesis is inhibited under the same conditions. These results suggested that subtle sequence changes in alpha-synuclein could significantly alter interaction with membrane bilayers. Fluorescence and absorption spectroscopy using environment sensitive probes demonstrated variations in the inherent lipid properties in the presence and absence of alpha-synuclein. Addition of wild-type alpha-synuclein to synaptosomes did not significantly alter the membrane fluidity at either the fatty acyl chains or headgroup space, suggesting that synaptosomes have a high capacity for alpha-synuclein binding. In contrast, synaptosomal membrane fluidity was decreased by A53T alpha-synuclein binding with concomitant packing of the lipid headgroups. These results suggest that alterations in alpha-synuclein-lipid interactions may contribute to physiological changes detected in early onset PD.

Sphingosine 1-phosphate (S1P) receptor subtypes S1P1 and S1P3, respectively, regulate lymphocyte recirculation and heart rate.

Sphingosine 1-phosphate (S1P) influences heart rate, coronary artery caliber, endothelial integrity, and lymphocyte recirculation through five related high affinity G-protein-coupled receptors. Inhibition of lymphocyte recirculation by non-selective S1P receptor agonists produces clinical immunosuppression preventing transplant rejection but is associated with transient bradycardia. Understanding the contribution of individual receptors has been limited by the embryonic lethality of the S1P(1) knock-out and the unavailability of selective agonists or antagonists. A potent, S1P(1)-receptor selective agonist structurally unrelated to S1P was found to activate multiple signals triggered by S1P, including guanosine 5'-3-O-(thio)triphosphate binding, calcium flux, Akt and ERK1/2 phosphorylation, and stimulation of migration of S1P(1)- but not S1P(3)-expressing cells in vitro. The agonist also alters lymphocyte trafficking in vivo. Use of selective agonism together with deletant mice lacking S1P(3) receptor reveals that agonism of S1P(1) receptor alone is sufficient to control lymphocyte recirculation. Moreover, S1P(1) receptor agonist plasma levels are causally associated with induction and maintenance of lymphopenia. S1P(3), and not S1P(1), is directly implicated in sinus bradycardia. The sustained bradycardia induced by S1P receptor non-selective immunosuppressive agonists in wild-type mice is abolished in S1P(3)-/- mice, whereas S1P(1)-selective agonist does not produce bradycardia. Separation of receptor subtype usage for control of lymphocyte recirculation and heart rate may allow the identification of selective immunosuppressive S1P(1) receptor agonists with an enhanced therapeutic window. S1P(1)-selective agonists will be of broad utility in understanding cell functions in vitro, and vascular physiology in vivo, and the success of the chemical approach for S1P(1) suggests that selective tools for the resolution of function across this broad lipid receptor family are now possible.