Deafness and stria vascularis defects in S1P2 receptor-null mice

Epithelial cell extrusion requires the sphingosine-1-phosphate receptor 2 pathway

Apoptotic epithelial cells signal to neighboring cells to induce dying cell extrusion by releasing sphingosine-1-phosphate.

To maintain an intact barrier, epithelia eliminate dying cells by extrusion. During extrusion, a cell destined for apoptosis signals its neighboring cells to form and contract a ring of actin and myosin, which squeezes the dying cell out of the epithelium. Here, we demonstrate that the signal produced by dying cells to initiate this process is sphingosine-1-phosphate (S1P). Decreasing S1P synthesis by inhibiting sphingosine kinase activity or by blocking extracellular S1P access to its receptor prevented apoptotic cell extrusion. Extracellular S1P activates extrusion by binding the S1P₂ receptor in the cells neighboring a dying cell, as S1P₂ knockdown in these cells or its loss in a zebrafish mutant disrupted cell extrusion. Because live cells can also be extruded, we predict that this S1P pathway may also be important for driving delamination of stem cells during differentiation or invasion of cancer cells.

The role of sphingosine 1-phosphate in migration of osteoclast precursors; an application of intravital two-photon microscopy

Sphingosine-1-phosphate (S1P), a biologically active lysophospholipid that is enriched in blood, controls the trafficking of osteoclast precursors between the circulation and bone marrow cavities via G protein-coupled receptors, S1PRs. While S1PR1 mediates chemoattraction toward S1P in bone marrow, where S1P concentration is low, S1PR2 mediates chemorepulsion in blood, where the S1P concentration is high. The regulation of precursor recruitment may represent a novel therapeutic strategy for controlling osteoclast-dependent bone remodeling. Through intravital multiphoton imaging of bone tissues, we reveal that the bidirectional function of S1P temporospatially regulates the migration of osteoclast precursors within intact bone tissues. Imaging technologies have enabled in situ visualization of the behaviors of several players in intact tissues. In addition, intravital microscopy has the potential to be more widely applied to functional analysis and intervention.

S1P activates store-operated calcium entry via receptor- and non-receptor-mediated pathways in vascular smooth muscle cells

Sphingosine-1-phosphate (S1P) has been shown to modulate intracellular Ca(2+) through both G protein-coupled receptors and intracellular second messenger pathways. The precise mechanism by which S1P activates store-operated calcium entry (SOCE) in vascular smooth muscle cells (VSMCs) has not been fully characterized. Because sphingolipids and Ca(2+) modulate proliferation and constriction in VSMCs, characterizing the connection between S1P and SOCE may provide novel therapeutic targets for vascular diseases. We found that S1P triggered STIM1 puncta formation and SOCE in VSMCs. S1P-activated SOCE was inhibited by 2-aminoethoxydiphenyl borate (2-APB), diethylstilbestrol (DES), and gadolinium (Gd(3+)). SOCE was observed in VSMCs lacking either S1P(2) or S1P(3) receptors, suggesting that S1P acts via multiple signaling pathways. Indeed, both extracellular and intracellular S1P application increased the total internal reflection fluorescence signal in VSMCs cells transfected with STIM1-yellow fluorescent protein in a 2-APB-sensitive manner. These data, and the fact that 2-APB, DES, and Gd(3+) all inhibited S1P-induced cerebral artery constriction, suggest that SOCE modulates S1P-induced vasoconstriction in vivo. Finally, S1P-induced SOCE was larger in proliferative than in contractile VSMCs, correlating with increases in STIM1, Orai1, S1P(1), and S1P(3) receptor mRNA. These data demonstrate that S1P can act through both receptors and a novel intracellular pathway to activate SOCE. Because S1P-induced SOCE contributes to vessel constriction and is increased in proliferative VSMCs, it is likely that S1P/SOCE signaling in proliferative VSMCs may play a role in vascular dysfunction such as atherosclerosis and diabetes.

Selectivity and specificity of sphingosine-1-phosphate receptor ligands: caveats and critical thinking in characterizing receptor-mediated effects

Receptors for sphingosine-1-phosphate (S1P) have been identified only recently. Their medicinal chemistry is therefore still in its infancy, and few selective agonists or antagonists are available. Furthermore, the selectivity of S1P receptor agonists or antagonists is not well established. JTE-013 and BML-241 (also known as CAY10444), used extensively as specific S1P₂ and S1P₃ receptors antagonists respectively, are cases in point. When analyzing S1P-induced vasoconstriction in mouse basilar artery, we observed that JTE-013 inhibited not only the effect of S1P, but also the effect of U46619, endothelin-1 or high KCl; JTE-013 strongly inhibited responses to S1P in S1P² receptor knockout mice. Similarly, BML-241 has been shown to inhibit increases in intracellular Ca²⁺ concentration via P₂ receptor or α_1A-adrenoceptor stimulation and α_1A-adrenoceptor-mediated contraction of rat mesenteric artery, while it did not affect S1P₃-mediated decrease of forskolin-induced cyclic AMP accumulation. Another putative S1P_1/3 receptor antagonist, VPC23019, does not inhibit S1P₃-mediated vasoconstriction. With these examples in mind, we discuss caveats about relying on available pharmacological tools to characterize receptor subtypes.

Dissecting the role of the S1P/S1PR axis in health and disease

Sphingosine-1-phosphate (S1P) is a pleiotropic sphingophospholipid generated from the phosphorylation of sphingosine by sphingosine kinases (SPHKs). S1P has been experimentally demonstrated to modulate an array of cellular processes such as cell proliferation, cell survival, cell invasion, vascular maturation, and angiogenesis by binding with any of the five known G-protein-coupled sphingosine 1 phosphate receptors (S1P1-5) on the cell surface in an autocrine as well as a paracrine manner. Recent studies have shown that the S1P receptors (S1PRs) and SPHKs are the key targets for modulating the pathophysiological consequences of various debilitating diseases, such as cancer, sepsis, rheumatoid arthritis, ulcerative colitis, and other related illnesses. In this article, we recapitulate these novel discoveries relative to the S1P/S1PR axis, necessary for the proper maintenance of health, as well as the induction of tumorigenic, angiogenic, and inflammatory stimuli that are vital for the development of various diseases, and the novel therapeutic tools to modulate these responses in oral biology and medicine.

International Union of Basic and Clinical Pharmacology. LXXVIII. Lysophospholipid receptor nomenclature

Lysophospholipids are cell membrane-derived lipids that include both glycerophospholipids such as lysophosphatidic acid (LPA) and sphingoid lipids such as sphingosine 1-phosphate (S1P). These and related molecules can function in vertebrates as extracellular signals by binding and activating G protein-coupled receptors. There are currently five LPA receptors, along with a proposed sixth (LPA₁-LPA₆), and five S1P receptors (S1P₁-S1P₅). A remarkably diverse biology and pathophysiology has emerged since the last review, driven by cloned receptors and targeted gene deletion ("knockout") studies in mice, which implicate receptor-mediated lysophospholipid signaling in most organ systems and multiple disease processes. The entry of various lysophospholipid receptor modulatory compounds into humans through clinical trials is ongoing and may lead to new medicines that are based on this signaling system. This review incorporates IUPHAR Nomenclature Committee guidelines in updating the nomenclature for lysophospholipid receptors ( http://www.iuphar-db.org/DATABASE/FamilyMenuForward?familyId=36).

Roles of sphingosine-1-phosphate signaling in angiogenesis

Sphingosine-1-phosphate (S1P) is a blood-borne lipid mediator with pleiotropic biological activities. S1P acts via the specific cell surface G-protein-coupled receptors, S1P_1-5. S1P₁ and S1P₂ were originally identified from vascular endothelial cells (ECs) and smooth muscle cells, respectively. Emerging evidence shows that S1P plays crucial roles in the regulation of vascular functions, including vascular formation, barrier protection and vascular tone via S1P₁, S1P₂ and S1P₃. In particular, S1P regulates vascular formation through multiple mechanisms; S1P exerts both positive and negative effects on angiogenesis and vascular maturation. The positive and negative effects of S1P are mediated by S1P₁ and S1P₂, respectively. These effects of S1P₁ and S1P₂ are probably mediated by the S1P receptors expressed in multiple cell types including ECs and bone-marrow-derived cells. The receptor-subtype-specific, distinct effects of S1P favor the development of novel therapeutic tactics for antitumor angiogenesis in cancer and therapeutic angiogenesis in ischemic diseases.

Tumor necrosis factor-α enhances microvascular tone and reduces blood flow in the cochlea via enhanced sphingosine-1-phosphate signaling

BACKGROUND AND PURPOSE

We sought to demonstrate that tumor necrosis factor (TNF)-α, via sphingosine-1-phosphate signaling, has the potential to alter cochlear blood flow and thus, cause ischemic hearing loss.

METHODS

We performed intravital fluorescence microscopy to measure blood flow and capillary diameter in anesthetized guinea pigs. To measure capillary diameter ex vivo, capillary beds from the gerbil spiral ligament were isolated from the cochlear lateral wall and maintained in an organ bath. Isolated gerbil spiral modiolar arteries, maintained and transfected in organ culture, were used to measure calcium sensitivity (calcium-tone relationship). In a clinical study, a total of 12 adult patients presenting with typical symptoms of sudden hearing loss who were not responsive or only partially responsive to prednisolone treatment were identified and selected for etanercept treatment. Etanercept (25 mg s.c.) was self-administered twice a week for 12 weeks.

RESULTS

TNF-α induced a proconstrictive state throughout the cochlear microvasculature, which reduced capillary diameter and cochlear blood flow in vivo. In vitro isolated preparations of the spiral modiolar artery and spiral ligament capillaries confirmed these observations. Antagonizing sphingosine-1-phosphate receptor 2 subtype signaling (by 1 μmol/L JTE013) attenuated the effects of TNF-α in all models. TNF-α activated sphingosine kinase 1 (Sk1) and induced its translocation to the smooth muscle cell membrane. Expression of a dominant-negative Sk1 mutant (Sk1(G82D)) eliminated both baseline spiral modiolar artery calcium sensitivity and TNF-α effects, whereas a nonphosphorylatable Sk1 mutant (Sk1(S225A)) blocked the effects of TNF-α only. A small group of etanercept-treated, hearing loss patients recovered according to a 1-phase exponential decay (half-life=1.56 ± 0.20 weeks), which matched the kinetics predicted for a vascular origin.

CONCLUSIONS

TNF-α indeed reduces cochlear blood flow via activation of vascular sphingosine-1-phosphate signaling. This integrates hearing loss into the family of ischemic microvascular pathologies, with implications for risk stratification, diagnosis, and treatment.

A microfluidic platform for probing small artery structure and function

Although pathologic changes to the structure and function of small blood vessels are hallmarks of various cardiovascular diseases, limitations of conventional investigation methods (i.e. pressure myography) have prohibited a comprehensive understanding of the underlying mechanisms. We developed a microfluidic device to facilitate assessment of resistance artery structure and function under physiological conditions (37 degrees C, 45 mmHg transmural pressure). The platform allows for on-chip fixation, long-term culture and fully automated acquisition of up to ten dose-response sequences of intact mouse mesenteric artery segments (diameter approximately 250 micrometres and length approximately 1.5 mm) in a well-defined microenvironment. Even abluminal application of phenylephrine or acetylcholine (homogeneous condition) yielded dose-response relationships virtually identical to conventional myography. Unilateral application of phenylephrine (heterogeneous condition) limited constriction to the drug-exposed side, suggesting a lack of circumferential communication. The microfluidic platform allows us to address new fundamental biological questions, replaces a manually demanding procedure with a scalable approach and may enable organ-based screens to be routinely performed during drug development.

The influences of sphingolipid metabolites on gentamicin-induced hair cell loss of the rat cochlea

Sphingolipid metabolites inducing ceramide, sphingosine, and sphingosine-1-phosphate (S1P) play important roles in the regulation of cell proliferation, survival, and death. Aminoglycoside antibiotics including gentamicin induce inner ear hair cell loss and sensorineural hearing loss. Apoptotic cell death is considered to play a key role in this injury. The present study was designed to investigate the possible involvement of ceramide and S1P in hair cell death due to gentamicin. In addition, the effects of other metabolites of ceramide, gangliosides GM1 (GM1) and GM3 (GM3), on gentamicin ototoxicity were also investigated. Basal turn organ of Corti explants from p3 to p5 rats were maintained in tissue culture and exposed to 20 or 35μM gentamicin for 48h. The effects of ceramide, S1P, GM1, and GM3 on gentamicin-induced hair cell loss were examined. Gentamicin-induced hair cell loss was increased by ceramide but was decreased by S1P. GM1 and GM3 exhibited protective effects against gentamicin-induced hair cell death at the limited concentrations. These results indicate that ceramide enhances gentamicin ototoxicity by promoting apoptotic hair cell death, and that S1P, GM1, and GM3 act as cochlear protectants. In conclusion, sphingolipid metabolites influence the apoptotic reaction of hair cells to gentamicin ototoxicity.

Sphingosine 1-phosphate receptor 2 signals through leukemia-associated RhoGEF (LARG), to promote smooth muscle cell differentiation

OBJECTIVE

The goals of this study were to identify the signaling pathway by which sphingosine 1-phosphate (S1P) activates RhoA in smooth muscle cells (SMC) and to evaluate the contribution of this pathway to the regulation of SMC phenotype.

METHODS AND RESULTS

Using a combination of receptor-specific agonists and antagonists we identified S1P receptor 2 (S1PR2) as the major S1P receptor subtype that regulates SMC differentiation marker gene expression. Based on the known coupling properties of S1PR2 and our demonstration that overexpression of Galpha(12) or Galpha(13) increased SMC-specific promoter activity, we next tested whether the effects of S1P in SMC were mediated by the regulator of G protein-signaling-Rho guanine exchange factors (RGS-RhoGEFs) (leukemia-associated RhoGEF [LARG], PDZ-RhoGEF [PRG], RhoGEF [p115]). Although each of the RGS-RhoGEFs enhanced actin polymerization, myocardin-related transcription factor-A nuclear localization, and SMC-specific promoter activity when overexpressed in 10T1/2 cells, LARG exhibited the most robust effect and was the only RGS-RhoGEF activated by S1P in SMC. Importantly, siRNA-mediated depletion of LARG significantly inhibited the activation of RhoA and SMC differentiation marker gene expression by S1P. Knockdown of LARG had no effect on SMC proliferation but promoted SMC migration as measured by scratch wound and transwell assays.

CONCLUSIONS

These data indicate that S1PR2-dependent activation of RhoA in SMC is mediated by LARG and that this signaling mechanism promotes the differentiated SMC phenotype.

Sphingosine kinase 1 and sphingosine-1-phosphate receptor 2 are vital to recovery from anaphylactic shock in mice

Sphingosine kinase 1 (SphK1) and SphK2 are ubiquitous enzymes that generate sphingosine-1-phosphate (S1P), a ligand for a family of G protein-coupled receptors (S1PR1-S1PR5) with important functions in the vascular and immune systems. Here we explore the role of these kinases and receptors in recovery from anaphylaxis in mice. We found that Sphk2-/- mice had a rapid recovery from anaphylaxis. In contrast, Sphk1-/- mice showed poor recovery from anaphylaxis and delayed histamine clearance. Injection of S1P into Sphk1-/- mice increased histamine clearance and promoted recovery from anaphylaxis. Adoptive cell transfer experiments demonstrated that SphK1 activity was required in both the hematopoietic and nonhematopoietic compartments for recovery from anaphylaxis. Mice lacking the S1P receptor S1PR2 also showed a delay in plasma histamine clearance and a poor recovery from anaphylaxis. However, S1P did not promote the recovery of S1pr2-/- mice from anaphylaxis, whereas S1pr2+/- mice showed partial recovery. Unlike Sphk2-/- mice, Sphk1-/- and S1pr2-/- mice had severe hypotension during anaphylaxis. Thus, SphK1-produced S1P regulates blood pressure, histamine clearance, and recovery from anaphylaxis in a manner that involves S1PR2. This suggests that specific S1PR2 agonists may serve to counteract the vasodilation associated with anaphylactic shock.

S1P(2), the G protein-coupled receptor for sphingosine-1-phosphate, negatively regulates tumor angiogenesis and tumor growth in vivo in mice

Sphingosine-1-phosphate (S1P) has been implicated in tumor angiogenesis by acting through the G(i)-coupled chemotactic receptor S1P(1). Here, we report that the distinct receptor S1P(2) is responsible for mediating the G(12/13)/Rho-dependent inhibitory effects of S1P on Akt, Rac, and cell migration, thereby negatively regulating tumor angiogenesis and tumor growth. By using S1P(2)(LacZ/+) mice, we found that S1P(2) was expressed in both tumor and normal blood vessels in many organs, in both endothelial cells (EC) and vascular smooth muscle cells, as well as in tumor-associated, CD11b-positive bone marrow-derived cells (BMDC). Lewis lung carcinoma or B16 melanoma cells implanted in S1P(2)-deficient (S1P(2)(-/-)) mice displayed accelerated tumor growth and angiogenesis with enhanced association of vascular smooth muscle cells and pericytes. S1P(2)(-/-) ECs exhibited enhanced Rac activity, Akt phosphorylation, cell migration, proliferation, and tube formation in vitro. Coinjection of S1P(2)(-/-) ECs and tumor cells into wild-type mice also produced a relative enhancement of tumor growth and angiogenesis in vivo. S1P(2)(-/-) mice were also more efficient at recruiting CD11b-positive BMDCs into tumors compared with wild-type siblings. Bone marrow chimera experiments revealed that S1P(2) acted in BMDCs to promote tumor growth and angiogenesis. Our results indicate that, in contrast to endothelial S1P(1), which stimulates tumor angiogenesis, S1P(2) on ECs and BMDCs mediates a potent inhibition of tumor angiogenesis, suggesting a novel therapeutic tactic for anticancer treatment.

Targeted disruption of the S1P2 sphingosine 1-phosphate receptor gene leads to diffuse large B-cell lymphoma formation

S1P(2) sphingosine 1-phosphate receptor signaling can regulate proliferation, survival, morphology, and migration in many cell types in vitro. Here, we report that S1P(2)(-/-) mice develop clonal B-cell lymphomas with age, such that approximately half of the animals display this neoplasm by 1.5 to 2 years of age. Histologic, immunophenotypic, and molecular analyses revealed a uniform tumor phenotype with features of germinal center (GC)-derived diffuse large B-cell lymphoma (DLBCL). Tumor formation was preceded by increases in GC B cells and CD69(+) T cells, as well as an increased formation of spontaneous GCs, suggesting that S1P(2) loss may promote lymphomagenesis in part by disrupting GC B-cells homeostasis. With the sole exception of rare lung tumors, the effect of S1P(2) gene disruption is remarkably restricted to DLBCL. In humans, 28 of 106 (26%) DLBCL samples were found to harbor multiple somatic mutations in the 5' sequences of the S1P(2) gene. Mutations displayed features resembling those generated by the IgV-associated somatic hypermutation mechanism, but were not detected at significant levels in normal GC B cells, indicating a tumor-associated aberrant function. Collectively, our data suggest that S1P(2) signaling may play a critical role in suppressing DLBCL formation in vivo. The high incidence of DLBCL in S1P(2)(-/-) mice, its onset at old age, and the relative lack of other neoplasms identify these mice as a novel, and potentially valuable, model for this highly prevalent and aggressive human malignancy.

The phosphorylation motif at serine 225 governs the localization and function of sphingosine kinase 1 in resistance arteries

OBJECTIVE

The purpose of this study was to characterize a phosphorylation motif at serine 225 as a molecular switch that regulates the pressure-dependent activation of sphingosine kinase 1 (Sk1) in resistance artery smooth muscle cells.

METHODS AND RESULTS

In isolated hamster gracilis muscle resistance arteries, pressure-dependent activation/translocation of Sk1 by ERK1/2 was critically dependent on its serine 225 phosphorylation site. Specifically, expression of Sk1(S225A) reduced resting and myogenic tone, resting Ca(2+), pressure-induced Ca(2+) elevations, and Ca(2+) sensitivity. The lack of function of the Sk1(S225A) mutant could not be entirely overcome by forced localization to the plasma membrane via a myristoylation/palmitylation motif; the membrane anchor also significantly inhibited the function of the wild-type Sk1 enzyme. In both cases, Ca(2+) sensitivity and myogenic tone were attenuated, whereas Ca(2+) handling was normalized/enhanced. These discrete effects are consistent with cell surface receptor-mediated effects (Ca(2+) sensitivity) and intracellular effects of S1P (Ca(2+) handling). Accordingly, S1P(2) receptor inhibition (1 micromol/L JTE013) attenuated myogenic tone without effect on Ca(2+).

CONCLUSIONS

Translocation and precise subcellular positioning of Sk1 is essential for full Sk1 function; and two distinct S1P pools, proposed to be intra- and extracellular, contribute to the maintenance of vascular tone.

Sphingosine 1-phosphate receptor signaling

The sphingosine 1-phosphate (S1P) receptor signaling system is a productive model system. A hydrophobic zwitterionic lysophospholipid ligand with difficult physical properties interacts with five high-affinity G protein-coupled receptors to generate multiple downstream signals. These signals modulate homeostasis and pathology on a steep agonist concentration-response curve. Ligand presence is essential for vascular development and endothelial integrity, while acute increases in ligand concentrations result in cardiac death. Understanding this integrated biochemical system has exemplified the impact of both genetics and chemistry. Developing specific tools with defined biochemical properties for the reversible modulation of signals in real time has been essential to complement insights gained from genetic approaches that may be irreversible and compensated. Despite its knife-edge between life and death, this system, based in part on receptor subtype-selectivity and in part on differential attenuation of deleterious signals, now appears to be on the cusp of meaningful therapy for multiple sclerosis.

Sphingosine 1-phosphate: a regulator of arterial lesions

Sphingosine-1 phosphate (S1P) is a bioactive sphingolipid that is critical in the development of blood vessels, and in the adult regulates vascular functions including vascular tone, endothelial integrity, and angiogenesis. Further, S1P may regulate arterial lesions in disease and after injury by controlling leukocyte recruitment and smooth muscle cell functions.

Chemical modulators of sphingosine-1-phosphate receptors as barrier-oriented therapeutic molecules

Biological barriers regulate the passage of cells, pathogens, fluids, nutrients, ions and signalling molecules between anatomical compartments during homeostasis and disease. Yet strategies that allow for reversible therapeutic modulation of these barriers are still in their infancy. The enhancement or protection of natural barriers is desirable in conditions such as acute respiratory distress syndrome or ischaemia-reperfusion injuries, whereas a temporary disruption could facilitate the penetration of drugs across such barriers. This Review discusses the role of sphingosine-1-phosphate receptors in the regulation and protection of biological barriers, and the potential of therapeutic strategies that target this receptor family.

Regulation of vascular physiology and pathology by the S1P2 receptor subtype

Sphingosine-1-phosphate (S1P) is now recognized as a lipid mediator that acts via G-protein-coupled receptors. S1P receptors couple to various heterotrimeric G-proteins and regulate downstream targets and ultimately cell behaviour. The prototypical S1P1 receptor is known to couple to Gi and regulates angiogenesis, vascular development, and immune cell trafficking. In this review, we focus our attention on the S1P2 receptor, which has a unique G-protein-coupling property in that it preferentially activates the G(12/13) pathway. Recent studies indicate that the S1P2 receptor regulates critical intracellular signalling pathways, such as Rho GTPase, the phosphatase PTEN, and VE-cadherin-based adherens junctions. Analysis of mutant mice has revealed the critical role of this receptor in inner ear physiology, heart and vascular development, vascular remodelling, and vascular tone, permeability, and angiogenesis in vertebrates. These studies suggest that selective modulation of S1P2 receptor function by pharmacological tools may be useful in a variety of pathological conditions.

Expression patterns of the lysophospholipid receptor genes during mouse early development

Lysophospholipids (LPs) such as lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are known to mediate various biological responses, including cell proliferation, migration, and differentiation. To better understand the role of these lipids in mammalian early development, we applied whole-mount in situ hybridization techniques to E8.5 to E12.5 mouse embryos. We determined the expression patterns of the following LP receptor genes, which belong to the G protein-coupled receptor (GPCR) family: EDG1 to EDG8 (S1P1 to S1P5 and LPA1 to LPA3), LPA4 (GPR23/P2Y9), and LPA5 (GPR92). We found that the S1P/LPA receptor genes exhibit overlapping expression patterns in a variety of organ primordia, including the developing brain and cardiovascular system, presomitic mesoderm and somites, branchial arches, and limb buds. These results suggest that multiple receptor systems for LPA/S1P lysophospholipids may be functioning during organogenesis.

Lysophospholipid activation of G protein-coupled receptors

One of the major lipid biology discoveries in last decade was the broad range of physiological activities of lysophospholipids that have been attributed to the actions of lysophospholipid receptors. The most well characterized lysophospholipids are lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P). Documented cellular effects of these lipid mediators include growth-factor-like effects on cells, such as proliferation, survival, migration, adhesion, and differentiation. The mechanisms for these actions are attributed to a growing family of 7-transmembrane, G protein-coupled receptors (GPCRs). Their pathophysiological actions include immune modulation, neuropathic pain modulation, platelet aggregation, wound healing, vasopressor activity, and angiogenesis. Here we provide a brief introduction to receptor-mediated lysophospholipid signaling and physiology, and then discuss potential therapeutic roles in human diseases.

Lysophospholipid receptors in vertebrate development, physiology, and pathology

Lysophospholipid (LP) research has experienced a period of renaissance with the discovery of the lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) receptors in the late 1990s. Vertebrate LP receptors regulate embryogenesis, vascular development, neurogenesis, uterine development, oocyte survival, immune cell trafficking and inflammatory reactions. LP signaling is important in cancer, autoimmunity and inflammatory diseases. Research on LP biology has contributed to the development of a first-generation S1P receptor modulator that has entered phase III clinical trials for the treatment of multiple sclerosis. Further basic research on LP signaling is anticipated to lead to novel therapeutic tools to combat various human diseases.

Sphingosine-1-phosphate synthesis and functions in mast cells

Sphingolipids are not only major lipid components of all eukaryotic cell membranes, but they also comprise an important family of bioactive signaling molecules that regulate a diverse array of biological responses. The sphingolipid metabolite sphingosine-1-phosphate (S1P), is a key regulator of immune responses. Cellular levels of S1P are determined by the balance between its synthesis, involving two sphingosine kinases (SphK1 and SphK2), and its degradation, involving S1P lyase and S1P phosphatases. S1P mainly signals through its cell-surface receptors and may also have intracellular functions. S1P has important functions in mast cells - the major effectors of allergic responses. Antigen triggering of IgE receptors on mast cells activates both SphKs resulting in the production of S1P that is released and regulates and amplifies mast cell functions, including degranulation as well as cytokine and chemokine release.

Biological roles of lysophospholipid receptors revealed by genetic null mice: an update

Two lysophospholipids (LPs), lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P), are known to affect various cellular events. Their actions are mediated by binding to at least ten bona fide high-affinity G protein-coupled receptors referred to as LPA1-5 and S1P1-5. These LPs are expressed throughout the body and are involved in a range of biological activities including normal development, as well as functioning in most organ systems. A growing number of biological functions have been uncovered in vivo using single- or multiple-null mice for each LP receptor. This review will focus on findings from in vivo as well as in vitro studies using genetic null mice for the LP receptors, LPA1,2,3 and S1P1,2,3,5, and for the LP producing enzymes, autotaxin and sphingosine kinase 1/2.

Sphingolipids in the lungs

Sphingolipids such as sphingosine-1-phosphate (S1P), ceramide, or sphingomyelin are essential constituents of plasma membranes and regulate many (patho)physiological cellular responses inducing apoptosis and cell survival, vascular permeability, mast cell activation, and airway smooth muscle functions. The complexity of sphingolipid biology is generated by a great variety of compounds, diverse receptors, and often antagonistic functions of different sphingolipids. For instance, apoptosis is promoted by ceramide and prevented by S1P, and pulmonary vascular permeability is increased by S1P2/3 receptors and by ceramide, whereas S1P1 receptors stabilize barrier integrity. Several enzymes of the sphingolipid metabolism respond to external stimuli such as sphingomyelinase isoenzymes that are activated by many stress stimuli and the sphingosine kinase isoenzymes that are activated by allergens. The past years have provided increasing evidence that these processes contribute to pulmonary disorders including asthma, chronic obstructive pulmonary disease, acute lung injury, and cystic fibrosis. Sphingolipid metabolism offers several novel therapeutic targets for the treatment of lung diseases such as emphysema, asthma, cystic fibrosis, respiratory tract infection, sepsis, and acute lung injury.

Oncogenic mutations in GNAQ occur early in uveal melanoma

PURPOSE

Early/initiating oncogenic mutations have been identified for many cancers, but such mutations remain unidentified in uveal melanoma (UM). An extensive search for such mutations was undertaken, focusing on the RAF/MEK/ERK pathway, which is often the target of initiating mutations in other types of cancer.

METHODS

DNA samples from primary UMs were analyzed for mutations in 24 potential oncogenes that affect the RAF/MEK/ERK pathway. For GNAQ, a stimulatory alpha(q) G-protein subunit which was recently found to be mutated in UMs, resequencing was expanded to include 67 primary UMs and 22 peripheral blood samples. GNAQ status was analyzed for association with clinical, pathologic, chromosomal, immunohistochemical, and transcriptional features.

RESULTS

Activating mutations at codon 209 were identified in GNAQ in 33 (49%) of 67 primary UMs, including 2 (22%) of 9 iris melanomas and 31 (54%) of 58 posterior UMs. No mutations were found in the other 23 potential oncogenes. GNAQ mutations were not found in normal blood DNA samples. Consistent with GNAQ mutation being an early or initiating event, this mutation was not associated with any clinical, pathologic, or molecular features associated with late tumor progression.

CONCLUSIONS

GNAQ mutations occur in about half of UMs, representing the most common known oncogenic mutation in this cancer. The presence of this mutation in tumors at all stages of malignant progression suggests that it is an early event in UM. Mutations in this G-protein-coupled receptor provide new insights into UM pathogenesis and could lead to new therapeutic possibilities.

Sphingosine kinase/sphingosine 1-phosphate signalling in central nervous system

Sphingolipids were once regarded as inert structural components of cell membranes. Now these metabolites are generally believed to be important bioactive molecules that control a wide repertoire of cellular processes such as proliferation and survival of cells. Along with these ubiquitous cell functions observed in many peripheral tissues sphingolipid metabolites, especially sphingosine 1-phosphate, exert important neuron-specific functions such as regulation of neurotransmitter release. This review summarizes physiological and pathological roles of sphingolipid metabolites emphasizing the role of sphingosine 1-phosphate in the central nervous system.

Sphingosine-1-phosphate regulation of mammalian development

Sphingosine-1-phosphate (S1P) was first identified as a lysophospholipid metabolite whose formation is required for the irreversible degradation of sphingolipids. Years later, it was discovered that S1P is a bioactive lipid that provokes varied cell responses by acting through cell-surface receptors to drive cell signaling. More recent findings in model organisms have now established that S1P metabolism and signaling are integrated into many physiological systems. We describe here the surprising breadth of function of S1P in mammalian development and the underlying biologic processes that S1P regulates.

Sphingosine 1-phosphate chemical biology

A dozen years ago, the term 'S1P' (sphingosine 1-phosphate) was not in the lexicons of scientific literature databases. By early 2008, this query term retrieved well over 1000 citations from PubMed - about 225 of these appeared in 2007. Indeed, S1P is arguably the most heavily studied lipid molecule at present. What happened to distinguish S1P among many other signaling lipids? We believe that the seminal event was the linking of the investigational drug, FTY720 (fingolimod), to S1P signaling. This realization profoundly altered understanding of S1P biology, revealing both that S1P is prominent in lymphocyte trafficking and that mimicking S1P signaling with an agonist drug can modulate the immune system to considerable therapeutic benefit. Neither fact was known prior to FTY720; indeed, this molecule is testament to the power of chemical biology. In this communication, we attempt to summarize progress to date in S1P chemical biology.

Lysophospholipid signaling in the function and pathology of the reproductive system

BACKGROUND

Lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are two prominent signaling lysophospholipids (LPs) exerting their functions through a group of G protein-coupled receptors (GPCRs). This review covers current knowledge of the LP signaling in the function and pathology of the reproductive system.

METHODS

PubMed was searched up to May 2008 for papers on lysophospholipids/LPA/S1P/LPC/SPC in combination with each part of the reproductive system, such as testis/ovary/uterus.

RESULTS

LPA and SIP are found in significant amounts in serum and other biological fluids. To date, 10 LP receptors have been identified, including LPA(1-5) and S1P(1-5). In vitro and in vivo studies from the past three decades have demonstrated or suggested the physiological functions of LP signaling in reproduction, such as spermatogenesis, male sexual function, ovarian function, fertilization, early embryo development, embryo spacing, implantation, decidualization, pregnancy maintenance and parturition, as well as pathological roles in ovary, cervix, mammary gland and prostate cancers.

CONCLUSIONS

Receptor knock-out and other studies indicate tissue-specific and receptor-specific functions of LP signaling in reproduction. More comprehensive studies are required to define mechanisms of LP signaling and explore the potential use as a therapeutic target.

Regulation and functions of sphingosine kinases in the brain

It has long been known that sphingolipids, especially sphingomyelin, a principal component of myelin, are highly enriched in the central nervous system and are structural components of all eukaryotic cell membranes. In the last few years, substantial evidence has accumulated from studies of many types of cells demonstrating that in addition to their structural roles, their breakdown products form a new class of signaling molecules with potent and myriad regulatory effects on essentially every cell in the body. While the sphingolipid metabolites sphingosine and its precursor ceramide have been associated with cell growth arrest and apoptosis, sphingosine-1-phosphate (S1P) enhances proliferation, differentiation, and cell survival as well as regulates many physiological and pathological processes. The relative levels of these three interconvertible sphingolipid metabolites, and thus cell fate, are strongly influenced by the activity of sphingosine kinases, of which there are two isoforms, designated SphK1 and SphK2, the enzymes that phosphorylate sphingosine to produce S1P. Not much is yet known of the importance of S1P in the central nervous system. Therefore, this review is focused on current knowledge of regulation of SphK1 and SphK2 on both transcriptional and post-translational levels and the functions of these isozymes and their product S1P and its receptors in the central nervous system.

Sphingosine-1-phosphate signaling and biological activities in the cardiovascular system

The plasma lysophospholipid mediator sphingosine-1-phosphate (S1P) is produced exclusively by sphingosine kinase (SPHK) 1 and SPHK2 in vivo, and plays diverse biological and pathophysiological roles by acting largely through three members of the G protein-coupled S1P receptors, S1P1, S1P2 and S1P3. S1P1 expressed on endothelial cells mediates embryonic vascular maturation and maintains vascular integrity by contributing to eNOS activation, inhibiting vascular permeability and inducing endothelial cell chemotaxis via Gi-coupled mechanisms. By contrast, S1P2, is expressed in high levels on vascular smooth muscle cells (VSMCs) and certain types of tumor cells, inhibiting Rac and cell migration via a G(12/13)-and Rho-dependent mechanism. In rat neointimal VSMCs, S1P1 is upregulated to mediate local production of platelet-derived growth factor, which is a key player in vascular remodeling. S1P3 expressed on endothelial cells also mediates chemotaxis toward S1P and vasorelaxation via NO production in certain vascular bed, playing protective roles for vascular integrity. S1P3 expressed on VSMCs and cardiac sinoatrial node cells mediates vasopressor and negative chronotropic effect, respectively. In addition, S1P3, together with S1P2 and SPHK1, is suggested to play a protective role against acute myocardial ischemia. However, our recent work indicates that overexpressed SPHK1 is involved in cardiomyocyte degeneration and fibrosis in vivo, in part through S1P activation of the S1P3 signaling. We also demonstrated that exogenously administered S1P accelerates neovascularization and blood flow recovery in ischemic limbs, suggesting its usefulness for angiogenic therapy. These results provide evidence for S1P receptor subtype-specific pharmacological intervention as a novel therapeutic approach to cardiovascular diseases and cancer.

Functions of sphingolipid metabolism in mammals--lessons from genetic defects

Much is known about the pathways that control the biosynthesis, transport and degradation of sphingolipids. During the last two decades, considerable progress has been made regarding the roles this complex group of lipids play in maintaining membrane integrity and modulating responses to numerous signals. Further novel insights have been provided by the analysis of newly discovered genetic diseases in humans as well as in animal models harboring mutations in the genes whose products control sphingolipid metabolism and action. Through the description of the phenotypic consequences of genetic defects resulting in the loss of activity of the many proteins that synthesize, transport, bind, or degrade sphingolipids, this review summarizes the (patho)physiological functions of these lipids.

Essential role of sphingosine 1-phosphate receptor 2 in pathological angiogenesis of the mouse retina

Sphingosine 1-phosphate (S1P), a multifunctional lipid mediator that signals via the S1P family of G protein-coupled receptors (S1PR), regulates vascular maturation, permeability, and angiogenesis. In this study, we explored the role of S1P 2 receptor (S1P2R) in normal vascularization and hypoxia-triggered pathological angiogenesis of the mouse retina. S1P2R is strongly induced in ECs during hypoxic stress. When neonatal mice were subjected to ischemia-driven retinopathy, pathologic neovascularization in the vitreous chamber was suppressed in S1p2-/- mice concomitant with reduction in endothelial gaps and inflammatory cell infiltration. In addition, EC patterning and normal revascularization into the avascular zones of the retina were augmented. Reduced expression of the proinflammatory enzyme cyclooxygenase-2 (COX-2) and increased expression of eNOS were observed in the S1p2-/- mouse retina. S1P2R activation in ECs induced COX-2 expression and suppressed the expression of eNOS. These data identify the S1P2R-driven inflammatory process as an important molecular event in pathological retinal angiogenesis. We propose that antagonism of the S1P2R may be a novel therapeutic approach for the prevention and/or treatment of pathologic ocular neovascularization.

A novel method to quantify sphingosine 1-phosphate by immobilized metal affinity chromatography (IMAC)

Sphingosine 1-phosphate (S1P), a lysophospholipid mediator that signals through G protein-coupled receptors, regulates a wide plethora of biological responses such as angiogenesis and immune cell trafficking. Detection and quantification of S1P in biological samples is challenging due to its unique physicochemical nature and occurrence in trace quantities. In this report, we describe a new method to selectively enrich S1P and dihydro-S1P from biological samples by the Fe(3+) gel immobilized metal affinity chromatography (IMAC). The eluted S1P from IMAC was dephosphorylated, derivatized with o-phthalaldehyde (OPA), and detected by high-performance liquid chromatography (HPLC) coupled to a fluorescence detector. IMAC purification of S1P was linear for a wide range of S1P concentration. Using this assay, secretion of endogenous S1P from endothelial cells, fibroblasts and colon cancer cells was demonstrated. We also show that dihydro-S1P was the major sphingoid base phosphate secreted from HUVEC over expressed with Sphk1 cDNA. Pharmcological antagonists of ABC transporters, glyburide and MK-571 attenuated endogenous S1P release. This assay was also used to demonstrate that plasma S1P levels were not altered in mice deficient for ABC transporters, Abca1, Abca7 and Abcc1/Mrp1. IMAC-based affinity-enrichment coupled with a HPLC-based separation and detection system is a rapid and sensitive method to accurately quantify S1P.

Sphingosine 1-phosphate receptors in health and disease: mechanistic insights from gene deletion studies and reverse pharmacology

Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid that is critically involved in the embryonic development of the cardiovascular and central nervous systems. In the adult, S1P can produce cytoskeletal re-arrangements in many cell types to regulate immune cell trafficking, vascular homeostasis and cell communication in the central nervous system. S1P is contained in body fluids and tissues at different concentrations, and excessive production of the pleiotropic mediator at inflammatory sites may participate in various pathological conditions. Gene deletion studies and reverse pharmacology (techniques aiming to identify both ligands and function of receptors) provided evidence that many effects of S1P are mediated via five G-protein-coupled S1P receptor subtypes, and novel therapeutic strategies based on interaction with these receptors are being initiated. The prototype S1P receptor modulator, FTY720 (fingolimod), targets four of the five S1P receptor subtypes and may act at several levels to modulate lymphocyte trafficking via lymphocytic and endothelial S1P1 and, perhaps, other inflammatory processes through additional S1P receptor subtypes. A recently completed Phase II clinical trial suggested that the drug may provide an effective treatment of relapsing-remitting multiple sclerosis. FTY720 is currently being evaluated in larger-scale, longer-term, Phase III studies. This review provides an overview on S1P activities and S1P receptor function in health and disease, and summarizes the clinical experience with FTY720 in transplantation and multiple sclerosis.