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 receptors mediate neuromodulatory functions in the CNS

Sphingosine-1-phosphate (S1P) is a ubiquitous, lipophilic cellular mediator that acts in part by activation of G-protein-coupled receptor. Modulation of S1P signaling is an emerging pharmacotherapeutic target for immunomodulatory drugs. Although multiple S1P receptor types exist in the CNS, little is known about their function. Here, we report that S1P stimulated G-protein activity in the CNS, and results from [³⁵S]GTPγS autoradiography using the S1P₁-selective agonist SEW2871 and the S1P_1/3-selective antagonist VPC44116 show that in several regions a majority of this activity is mediated by S1P₁ receptors. S1P receptor activation inhibited glutamatergic neurotransmission as determined by electrophysiological recordings in cortical neurons in vitro , and this effect was mimicked by SEW2871 and inhibited by VPC44116. Moreover, central administration of S1P produced in vivo effects resembling the actions of cannabinoids, including thermal antinociception, hypothermia, catalepsy and hypolocomotion, but these actions were independent of CB₁ receptors. At least one of the central effects of S1P, thermal antinociception, is also at least partly S1P₁ receptor mediated because it was produced by SEW2871 and attenuated by VPC44116. These results indicate that CNS S1P receptors are part of a physiologically relevant and widespread neuromodulatory system, and that the S1P₁ receptor contributes to S1P-mediated antinociception.     

Sphingosine-1-phosphate phosphatases

Sphingosine-1-phosphate is a potent proliferative, survival, and morphogenetic factor, acting as an extracellular ligand for the EDG family of G-protein-coupled receptors and possibly intracellularly through as yet, unidentified targets. It is produced within most, if not all cells by phosphorylation of sphingosine, and is an abundant serum lipid that is released from activated platelets. Sphingosine and sphingosine-1-phosphate are in dynamic equilibrium with each other due to the activities of sphingosine kinase and sphingosine-1-phosphate phosphatase (SPPase). Several SPPase genes have now been cloned, first from yeast and more recently from mammalian cells. By sequence homology, these enzymes can be classified as a subset of membrane bound, Type 2 lipid phosphohydrolases that contain conserved residues within three domains predicted to be at the active site of the enzyme. Outside of the consensus motif, there is very little homology between SPPases and the other type 2 lipid phosphohydrolases in the LPP/PAP family. Type 2 phosphatase activity is Mg+-independent and insensitive to N-ethylmaleimide, and substrate specificity is broad for LPP enzymes, whereas SPPases are highly selective for sphingolipid substrates. SPPase activity in yeast and mammalian cells regulates intracellular sphingosine-1-phosphate levels, and also alters the levels of sphingosine and ceramide, two other signaling molecules that often oppose the actions of sphingosine-1-phosphate. Thus, loss of SPPase in yeast results in high sphingosine-1-phosphate levels and cells are more resistant to stress, and in mammalian cells, overexpression of SPPase elevates ceramide levels and provokes apoptosis.     

Sphingosine-1-phosphate receptors: receptor specificity versus functional redundancy

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid that has recently been shown to bind cell surface S1P receptors (previously called endothelial differentiation gene (Edg) receptors), which are members of the G-protein-coupled family of receptors. Signaling via S1P is a complex process, as cells usually express a number of these receptors on their surfaces. Many of the S1P receptors share common G-proteins, invoking the question of how these receptors are specific in their actions. This review describes the coupling pathways of S1P receptors, and highlights the in vitro and in vivo evidence for the "uniqueness" of each receptor in activating downstream signaling pathways, taking the effect of S1P on migration as an example.     

Sphingosine-1-phosphate and lipid phosphohydrolases

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid that acts as both an extracellular ligand for the endothelial differentiation gene-1 (EDG-1) G-protein coupled receptor (GPCR) family and as an intracellular messenger. Cellular levels of S1P are low and tightly regulated in a spatial-temporal manner not only by sphingosine kinase (SPHK) but also by degradation catalyzed by S1P lyase, specific S1P phosphohydrolases, and by general lipid phosphate phosphohydrolases (LPPs). LPPs are characterized as magnesium-independent, insensitive to inhibition by N-ethylmaleimide (NEM) and possessing broad substrate specificity with a variety of phosphorylated lipids, including S1P, phosphatidic acid (PA), and lysophosphatidic acid (LPA). LPPs contain three highly conserved domains that define a phosphohydrolase superfamily. Recently, several specific S1P phosphohydrolases have been identified in yeast and mammalian cells. Phylogenetic and biochemical analyses indicate that these enzymes constitute a new subset of the LPP family. As further evidence, S1P phosphohydrolases exhibit high specificity for phosphorylated sphingoid bases. Enforced expression of S1P phosphohydrolase alters the cellular levels of sphingolipid metabolites in yeast and mammalian cells, increasing sphingosine and ceramide, bioactive sphingolipids that often have opposing biological actions to S1P. By regulating the cellular ratio between ceramide/sphingosine and S1P, S1P phosphohydrolase is poised to be a critical factor in cell survival/cell death decisions. Indeed, expression of S1P phosphohydrolase in mammalian cells increases apoptosis, whereas deletion of S1P phosphohydrolases in yeast correlates with resistance to heat stress. In this review, we discuss the role of phosphohydrolases in the metabolism of S1P and how turnover of S1P can regulate sphingolipid metabolites signaling.     

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, S1P(1), S1P(2) and S1P(3). S1P(1) 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, S1P(2), 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, S1P(1) is upregulated to mediate local production of platelet-derived growth factor, which is a key player in vascular remodeling. S1P(3) expressed on endothelial cells also mediates chemotaxis toward S1P and vasorelaxation via NO production in certain vascular bed, playing protective roles for vascular integrity. S1P(3) expressed on VSMCs and cardiac sinoatrial node cells mediates vasopressor and negative chronotropic effect, respectively. In addition, S1P(3), together with S1P(2) 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 S1P(3) 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.     

Sphingosine-1-phosphate as a mediator involved in development of fibrotic diseases

Fibrosis is a pathological process characterized by massive deposition of extracellular matrix (ECM) such as type I/III collagens and fibronectin that are secreted by an expanded pool of myofibroblasts, which are phenotypically altered fibroblasts with more contractile, proliferative, migratory and secretory activities. Fibrosis occurs in various organs including the lung, heart, liver and kidney, resulting in loss of normal tissue architecture and functions. Myofibroblasts could originate from multiple sources including tissue-resident fibroblasts, epithelial and endothelial cells through mechanisms of epithelial/endothelial-mesenchymal transition (EMT/EndMT), and bone marrow-derived circulating progenitors called fibrocytes. Emerging evidence in recent years shows that sphingosine-1-phosphate (S1P) acts on several types of target cells and is engaged in pro-fibrotic inflammatory process and fibrogenic process through multiple mechanisms, which include vascular permeability change, leukocyte infiltration, and migration, proliferation and myofibroblast differentiation of fibroblasts. Many of these S1P actions are receptor subtype-specific. In these actions, S1P has multiple cross-talks with other cytokines, particularly transforming growth factor-β (TGFβ), which plays a major role in fibrosis. The cross-talks include the regulation of S1P production through altered expression and activity of sphingosine kinases in fibrotic lesions, altered expression of S1P receptors, and S1P receptor-mediated transactivation of TGFβ signaling pathway. These cross-talks may give rise to a feed-forward, amplifying loop between S1P and TGFβ, and possibly with other cytokines in stimulating fibrogenesis. Another lysophospholipid mediator lysophosphatidic acid has also been recently implicated in fibrosis. The lysophospholipid signaling pathways represent novel, promising therapeutic targets for treating refractory fibrotic diseases. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.     

Sphingosine-1-phosphate lyase in development and disease: sphingolipid metabolism takes flight

Sphingosine-1-phosphate lyase (SPL) is a highly conserved enzyme that catalyses the final step of sphingolipid degradation, namely the irreversible cleavage of the carbon chain at positions 2-3 of a long-chain base phosphate (LCBP), thereby yielding a long-chain aldehyde and phosphoethanolamine. LCBPs are potent signaling molecules involved in cell proliferation, survival, migration, cell-cell interactions and cell stress responses. Therefore, tight regulation of LCBP signaling is required for proper cell function, and perturbations of this system can lead to alterations in biological processes including development, reproduction and physiology. SPL is a key enzyme in regulating the intracellular and circulating levels of LCBPs and is, therefore, gaining attention as a putative target for pharmacological intervention. This review provides an overview of our current understanding of SPL structure and function, mechanisms involved in SPL regulation and the role of SPL in development and disease.     

Sphingosine-1-phosphate signaling in vasculogenesis and angiogenesis

Blood vessels either form de novo through the process of vasculogenesis or through angiogenesis that involves the sprouting and proliferation of endothelial cells in pre-existing blood vessels. A complex interactive network of signaling cascades downstream from at least three of the nine known G-protein-coupled sphingosine-1-phosphate (S1P) receptors act as a prime effector of neovascularization that occurs in embryonic development and in association with various pathologies. This review focuses on the current knowledge of the roles of S1P signaling in vasculogenesis and angiogenesis, with particular emphasis on vascular cell adhesion and motility responses.     

Sphingosine-1-phosphate lyase has a central role in the development of Dictyostelium discoideum

Sphingosine-1-phosphate, a product of sphingomyelin degradation, is an important element of signal transduction pathways that regulate cell proliferation and cell death. We have demonstrated additional roles for sphingosine-1-phosphate in growth and multicellular development. The specific disruption in Dictyostelium discoideum of the sphingosine-1-phosphate lyase gene, which encodes the enzyme that catalyzes sphingosine-1-phosphate degradation, results in a mutant strain with aberrant morphogenesis, as well as an increase in viability during stationary phase. The absence of sphingosine-1-phosphate lyase affects multiple stages throughout development, including the cytoskeletal architecture of aggregating cells, the ability to form migrating slugs, and the control of cell type-specific gene expression and terminal spore differentiation. This pleiotropic effect, which is due to the loss of sphingosine-1-phosphate lyase, establishes sphingolipids as pivotal regulatory molecules in a wide range of processes in multicellular development.     

Sphingosine-1-phosphate receptors: Zooming in on ligand-induced intracellular trafficking and its functional implications

Regulatory processes including receptor phosphorylation and intracellular trafficking, also referred to as receptor internalization, are important processes to terminate G protein-coupled receptor (GPCR) signaling. Compelling evidence now indicates that internalization of a receptor is not necessarily the endpoint of signaling, but can also be the beginning of the activation of intracellular signaling pathways.     

Sphingosine-1-phosphate: signaling inside and out

Ample evidence indicates that sphingosine-1-phosphate (SPP) can serve as an intracellular second messenger regulating calcium mobilization, and cell growth and survival. Moreover, the dynamic balance between levels of the sphingolipid metabolites, ceramide and SPP, and consequent regulation of opposing signaling pathways, is an important factor that determines whether a cell survives or dies. SPP has recently also been shown to be the ligand for the EDG-1 family of G-protein-coupled receptors, which now includes EDG-1, -3, -5, -6 and -8. SPP is thus a lipid mediator that has novel dual actions signaling inside and outside of the cell.     

Sphingosine-1-phosphate and oligodendrocytes: From cell development to the treatment of multiple sclerosis

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid that mediates a wide variety of biological effects in different cells and tissues. This review discusses the effects of S1P signaling in oligodendrocytes, the myelin making cells of the central nervous system (CNS). Results from different laboratories have uncovered direct actions of S1P at different maturational stages along the oligodendroglial lineage. There is also evidence for the existence in oligodendrocytes of interactions between S1P and signaling by factors which, like neurotrophin-3 (NT-3) and platelet-derived growth factor (PDGF), have profound effects on oligodendrocyte development and myelination. Moreover, S1P signaling in oligodendrocytes may not only play an important role during normal CNS development but also offer new therapeutic avenues to stimulate remyelination in demyelinating diseases like multiple sclerosis.     

Sphingosine-1-phosphate: dual messenger functions

The sphingolipid metabolite sphingosine-1-phosphate (S1P) is a serum-borne lipid that regulates many vital cellular processes. S1P is the ligand of a family of five specific G protein-coupled receptors that are differentially expressed in different tissues and regulate diverse cellular actions. Much less is known of the intracellular actions of S1P. It has been suggested that S1P may also function as an intracellular second messenger to regulate calcium mobilization, cell growth and suppression of apoptosis in response to a variety of extracellular stimuli. Dissecting the dual actions and identification of intracellular targets of S1P has been challenging, but there is ample evidence to suggest that the balance between S1P and ceramide and/or sphingosine levels in cells is an important determinant of cell fate.     

Sphingosine-1-phosphate signaling and its role in disease

The bioactive sphingolipid metabolite sphingosine-1-phosphate (S1P) is now recognized as a critical regulator of many physiological and pathophysiological processes, including cancer, atherosclerosis, diabetes and osteoporosis. S1P is produced in cells by two sphingosine kinase isoenzymes, SphK1 and SphK2. Many cells secrete S1P, which can then act in an autocrine or paracrine manner. Most of the known actions of S1P are mediated by a family of five specific G protein-coupled receptors. More recently, it was shown that S1P also has important intracellular targets involved in inflammation, cancer and Alzheimer's disease. This suggests that S1P actions are much more complex than previously thought, with important ramifications for development of therapeutics. This review highlights recent advances in our understanding of the mechanisms of action of S1P and its roles in disease.     

Sphingosine-1-phosphate signaling controlling osteoclasts and bone homeostasis

Bone is a dynamic organ that is continuously turned over during growth, even in adults. During bone remodeling, homeostasis is regulated by the balance between bone formation by osteoblasts and bone resorption by osteoclasts. However, in pathological conditions such as osteoporosis, osteopetrosis, arthritic joint destruction, and bone metastasis, this equilibrium is disrupted. Since osteoclasts are excessively activated in osteolytic diseases, the inhibition of osteoclast function has been a major therapeutic strategy. It has recently been demonstrated that 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. By means of intravital multiphoton imaging of bone tissues, we have recently revealed that the reciprocal action of S1P controls the migration of osteoclast precursors between bone tissues and blood stream. Imaging technologies have enabled us to visualize the in situ behaviors of different cell types in intact tissues. In this review we also discuss future perspectives on this new method in the field of bone biology and medical sciences in general. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.     

Sphingosine-1-phosphate mediates migration of mature dendritic cells

Sphingosine-1-phosphate (S1P) represents a potent modulator of diverse cellular activities, including lymphocyte trafficking and maintenance of lymphocyte homeostasis. The five known receptors for S1P (S1P(1-5)) belong to the family of G protein-coupled receptors. Upon binding S1P, they act downstream via heterotrimeric G proteins on members of the small GTPase family (Cdc42/Rac/Rho), evoking a S1P receptor-dependent activation pattern of Cdc42, Rac, and Rho, respectively. This, in turn, triggers cytoskeletal rearrangements determining cellular morphology and movement. In this study we investigated the effects of S1P on murine dendritic cells (DC). Mature DC, but not immature in vitro differentiated DC, were found to migrate to S1P, a phenomenon that correlated to the up-regulation of S1P1 and S1P3 in maturing DC. The same pattern of S1P receptor regulation could be observed in vivo on skin DC after their activation and migration into the lymph node. The migration-inducing effect of S1P could be severely hampered by application of the S1P analogon FTY720 in vitro and in vivo. A similar, yet more pronounced, block was observed upon preventing Cdc42/Rac and/or Rho activation by specific inhibitors. These results suggest that S1P-mediated signaling plays a pivotal role in the life cycle of DC.     

Sphingosine-1-phosphate: an enigmatic signalling lipid

The evolutionarily conserved actions of the sphingolipid metabolite, sphingosine-1-phosphate (S1P), in yeast, plants and mammals have shown that it has important functions. In higher eukaryotes, S1P is the ligand for a family of five G-protein-coupled receptors. These S1P receptors are differentially expressed, coupled to various G proteins, and regulate angiogenesis, vascular maturation, cardiac development and immunity, and are important for directed cell movement.