Mouse SPNS2 functions as a sphingosine-1-phosphate transporter in vascular endothelial cells

S1P control of endothelial integrity

Sphingosine 1-phosphate (S1P), a lipid mediator produced by sphingolipid metabolism, promotes endothelial cell spreading, vascular maturation/stabilization, and barrier function. S1P is present at high concentrations in the circulatory system, whereas in tissues its levels are low. This so-called vascular S1P gradient is essential for S1P to regulate much physiological and pathophysiological progress such as the modulation of vascular permeability. Cellular sources of S1P in blood has only recently begun to be identified. In this review, we summarize the current understanding of S1P in regulating vascular integrity. In particular, we discuss the recent discovery of the endothelium-protective functions of HDL-bound S1P which is chaperoned by apolipoprotein M.

Sphingosine-1-phosphate: a master regulator of lymphocyte egress and immunity

Sphingosine-1-phosphate (S1P) is a central factor responsible for lymphocyte distribution in the body. S1P is able to control the integrity of various effector cell populations within many lymphoid tissues by directing lymphocyte egress. In this review, we give an overview of the generation and degradation of S1P in specific lymphoid microenvironments. Furthermore, we discuss, sometimes contradictory, the functions of the five S1P receptors on different cells in diverse tissues and give an idea of additional counteracting chemotactic signals for lymphocyte immigration and emigration. We focus special attention to recent discoveries of S1P-specific transporters, like spinster-homolog-2 and the active secretion of S1P by endothelial cells, erythrocytes and platelets. In addition, we describe the microanatomical structures as well as entry and egress routes into lymphoid organs which lymphocytes use for efficient trafficking. Finally, we give an overview of open questions regarding the regulation of lymphocyte homing from primary lymphoid organs to secondary lymphoid organs and back again.

Sphingosine phosphate lyase regulates myogenic differentiation via S1P receptor-mediated effects on myogenic microRNA expression

S1P lyase (SPL) catalyzes the irreversible degradation of sphingosine-1-phosphate (S1P), a bioactive lipid whose signaling activities regulate muscle differentiation, homeostasis, and satellite cell (SC) activation. By regulating S1P levels, SPL also controls SC recruitment and muscle regeneration, representing a potential therapeutic target for muscular dystrophy. We found that SPL is induced during myoblast differentiation. To investigate SPL's role in myogenesis at the cellular level, we generated and characterized a murine myoblast SPL-knockdown (SPL-KD) cell line lacking SPL. SPL-KD cells accumulated intracellular and extracellular S1P and failed to form myotubes under conditions that normally stimulate myogenic differentiation. Under differentiation conditions, SPL-KD cells also demonstrated delayed induction of 3 myogenic microRNAs (miRNAs), miR-1, miR-206, and miR-486. SPL-KD cells successfully differentiated when treated with an S1P1 agonist, S1P2 antagonist, and combination treatments, which also increased myogenic miRNA levels. SPL-KD cells transfected with mimics for miR-1 or miR-206 also overcame the differentiation block. Thus, we show for the first time that the S1P/SPL/S1P-receptor axis regulates the expression of a number of miRNAs, thereby contributing to myogenic differentiation.

Genome editing using artificial site-specific nucleases in zebrafish

Zebrafish is a model vertebrate suitable for genetic analysis. Forward genetic analysis via chemical mutagenesis screening has established a variety of zebrafish mutants that are defective in various types of organogenesis, and the genes responsible for the individual mutants have been identified from genome mapping. On the other hand, reverse genetic analysis via targeted gene disruption using embryonic stem (ES) cells (e.g., knockout mouse) can uncover gene functions by investigating the phenotypic effects. However, this approach is mostly limited to mice among the vertebrate models because of the difficulty in establishing ES cells. Recently, new gene targeting technologies, such as the transcription activator-like effector nucleases (TALEN) and clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 systems, have been developed: that can directly introduce genome modifications at the targeted genomic locus. Here, we summarize these new and powerful genome editing techniques for the study of zebrafish.

Dynamic Cross Talk between S1P and CXCL12 Regulates Hematopoietic Stem Cells Migration, Development and Bone Remodeling

Hematopoietic stem cells (HSCs) are mostly retained in a quiescent non-motile mode in their bone marrow (BM) niches, shifting to a migratory cycling and differentiating state to replenish the blood with mature leukocytes on demand. The balance between the major chemo-attractants CXCL12, predominantly in the BM, and S1P, mainly in the blood, dynamically regulates HSC recruitment to the circulation versus their retention in the BM. During alarm situations, stress-signals induce a decrease in CXCL12 levels in the BM, while S1P levels are rapidly and transiently increased in the circulation, thus favoring mobilization of stem cells as part of host defense and repair mechanisms. Myeloid cytokines, including G-CSF, up-regulate S1P signaling in the BM via the PI3K pathway. Induced CXCL12 secretion from stromal cells via reactive oxygen species (ROS) generation and increased S1P₁ expression and ROS signaling in HSCs, all facilitate mobilization. Bone turnover is also modulated by both CXCL12 and S1P, regulating the dynamic BM stromal microenvironment, osteoclasts and stem cell niches which all functionally express CXCL12 and S1P receptors. Overall, CXCL12 and S1P levels in the BM and circulation are synchronized to mutually control HSC motility, leukocyte production and osteoclast/osteoblast bone turnover during homeostasis and stress situations.

Sphingosine 1-phosphate receptors are essential mediators of eyelid closure during embryonic development

The fetal development of the mammalian eyelid involves the expansion of the epithelium over the developing cornea, fusion into a continuous sheet covering the eye, and a splitting event several weeks later that results in the formation of the upper and lower eyelids. Recent studies have revealed a significant number of molecular signaling components that are essential mediators of eyelid development. Receptor-mediated sphingosine 1-phosphate (S1P) signaling is known to influence diverse biological processes, but its involvement in eyelid development has not been reported. Here, we show that two S1P receptors, S1P2 and S1P3, are collectively essential mediators of eyelid closure during murine development. Homozygous deletion of the gene encoding either receptor has no apparent effect on eyelid development, but double-null embryos are born with an "eyes open at birth" defect due to a delay in epithelial sheet extension. Both receptors are expressed in the advancing epithelial sheet during the critical period of extension. Fibroblasts derived from double-null embryos have a deficient response to epidermal growth factor, suggesting that S1P2 and S1P3 modulate this essential signaling pathway during eyelid closure.

Sphingosine 1-phosphate is a key metabolite linking sphingolipids to glycerophospholipids

The sphingolipid metabolite sphingosine 1-phosphate (S1P) is a well-known lipid mediator. As a lipid mediator, S1P must be present in extracellular space and bind to its cell surface receptors (S1P1-5). However, most S1P, synthesized intracellularly, is metabolized without being released into extracellular space, in other words, without functioning as a lipid mediator in the vast majority of cells except those supplying plasma and lymph S1P such as blood cells and endothelial cells. Instead, intracellular S1P plays an important role as an intermediate of the sole sphingolipid-to-glycerophospholipid metabolic pathway. The degradation of S1P by S1P lyase is the first irreversible reaction (committed step) of this pathway. This metabolic pathway is conserved in eukaryotes from yeast to human, indicating its much older origin than the function of S1P as a lipid mediator, which is found to be present only in vertebrates and chordates. The sphingolipid-to-glycerophospholipid metabolism takes place ubiquitously in mammalian tissues, and its defect causes an aberration of several tissue functions as well as abnormal lipid metabolism. Although this metabolic pathway has been known for over four decades, only recently the precise reactions and enzymes involved in this pathway have been revealed. This review will focus on the recent advances in our understanding of the sphingolipid metabolic pathway via S1P and its physiological and pathological roles. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Sphingosine-1-phosphate receptor 2

Sphingosine-1-phosphate (S1P) is a potent bioactive sphingolipid involved in cell proliferation, angiogenesis, inflammation and malignant transformation among other functions. S1P acts either directly on intracellular targets or activates G protein-coupled receptors, specifically five S1P receptors (S1PRs). The identified S1PRs differ in cellular and tissue distribution, and each is coupled to specific G proteins, which mediate unique functions. Here, we describe functional characteristics of all five receptors, emphasizing S1PR2, which is critical in the immune, nervous, metabolic, cardiovascular, musculoskeletal, and renal systems. This review also describes the role of this receptor in tumor growth and metastasis and suggests potential therapeutic avenues that exploit S1PR2.

Targeting the sphingosine-1-phosphate axis in cancer, inflammation and beyond

The bioactive lipid sphingosine-1-phosphate (S1P) is involved in multiple cellular signalling systems and has a pivotal role in the control of immune cell trafficking. As such, S1P has been implicated in disorders such as cancer and inflammatory diseases. This Review discusses the ways in which S1P might be therapeutically targeted - for example, via the development of chemical inhibitors that target the generation, transport and degradation of S1P and via the development of specific S1P receptor agonists. We also highlight recent conflicting results observed in preclinical studies targeting S1P and discuss ongoing clinical trials in this field.

Sphingosine kinase 1 regulates tumor necrosis factor-mediated RANTES induction through p38 mitogen-activated protein kinase but independently of nuclear factor κB activation

Sphingosine kinase 1 (SK1) produces the pro-survival sphingolipid sphingosine 1-phosphate and has been implicated in inflammation, proliferation, and angiogenesis. Recent studies identified TRAF2 as a sphingosine 1-phosphate target, implicating SK1 in activation of the NF-κB pathway, but the functional consequences of this connection on gene expression are unknown. Here, we find that loss of SK1 potentiates induction of the chemokine RANTES (regulated on activation, normal T cell expressed and secreted; also known as CCL5) in HeLa cells stimulated with TNF-α despite RANTES induction being highly dependent on the NF-κB pathway. Additionally, we find that SK1 is not required for TNF-induced IKK phosphorylation, IκB degradation, nuclear translocation of NF-κB subunits, and transcriptional NF-κB activity. In contrast, loss of SK1 prevented TNF-induced phosphorylation of p38 MAPK, and inhibition of p38 MAPK, like SK1 knockdown, also potentiates RANTES induction. Finally, in addition to RANTES, loss of SK1 also potentiated the induction of multiple chemokines and cytokines in the TNF response. Taken together, these data identify a potential and novel anti-inflammatory function of SK1 in which chemokine levels are suppressed through SK1-mediated activation of p38 MAPK. Furthermore, in this system, activation of NF-κB is dissociated from SK1, suggesting that the interaction between these pathways may be more complex than currently thought.

Molecular and physiological functions of sphingosine 1-phosphate transporters

Sphingosine 1-phosphate (S1P) is a lipid mediator that plays important roles in diverse cellular functions such as cell proliferation, differentiation and migration. S1P is synthesized inside the cells and subsequently released to the extracellular space, where it binds to specific receptors that are located on the plasma membranes of target cells. Accumulating recent evidence suggests that S1P transporters including SPNS2 mediate S1P release from the cells and are involved in the physiological functions of S1P. In this review, we discuss recent advances in our understanding of the mechanism and physiological functions of S1P transporters. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Sphingolipid regulation of ezrin, radixin, and moesin proteins family: implications for cell dynamics

A key but poorly studied domain of sphingolipid functions encompasses endocytosis, exocytosis, cellular trafficking, and cell movement. Recently, the ezrin, radixin and moesin (ERM) family of proteins emerged as novel potent targets regulated by sphingolipids. ERMs are structural proteins linking the actin cytoskeleton to the plasma membrane, also forming a scaffold for signaling pathways that are used for cell proliferation, migration and invasion, and cell division. Opposing functions of the bioactive sphingolipid ceramide and sphingosine-1-phosphate (S1P), contribute to ERM regulation. S1P robustly activates whereas ceramide potently deactivates ERM via phosphorylation/dephosphorylation, respectively. This recent dimension of cytoskeletal regulation by sphingolipids opens up new avenues to target cell dynamics, and provides further understanding of some of the unexplained biological effects mediated by sphingolipids. In addition, these studies are providing novel inroads into defining basic mechanisms of regulation and action of bioactive sphingolipids. This review describes the current understanding of sphingolipid regulation of the cytoskeleton, it also describes the biologies in which ERM proteins have been involved, and finally how these two large fields have started to converge. This article is part of a Special Issue entitled New Frontiers in Sphingolipid Biology.

Roles, regulation and inhibitors of sphingosine kinase 2

The bioactive sphingolipids ceramide, sphingosine and sphingosine-1-phosphate (S1P) are important signalling molecules that regulate a diverse array of cellular processes. Most notably, the balance of the levels of these three sphingolipids in cells, termed the 'sphingolipid rheostat', can dictate cell fate, where ceramide and sphingosine enhance apoptosis and S1P promotes cell survival and proliferation. The sphingosine kinases (SKs) catalyse the production of S1P from sphingosine and are therefore central regulators of the sphingolipid rheostat and attractive targets for cancer therapy. Two SKs exist in humans: SK1 and SK2. SK1 has been extensively studied and there is a large body of evidence to demonstrate its role in promoting cell survival, proliferation and neoplastic transformation. SK1 is also elevated in many human cancers which appears to contribute to carcinogenesis, chemotherapeutic resistance and poor patient outcome. SK2, however, has not been as well characterized, and there are contradictions in the key physiological functions that have been proposed for this isoform. Despite this, many studies are now emerging that implicate SK2 in key roles in a variety of diseases, including the development of a range of solid tumours. Here, we review the literature examining SK2, its physiological and pathophysiological functions, the current knowledge of its regulation, and recent developments in targeting this complex enzyme.

Spns2, a transporter of phosphorylated sphingoid bases, regulates their blood and lymph levels, and the lymphatic network

Sphingosine-1-phosphate (S1P), a ligand for 5 specific receptors, is a potent lipid mediator that plays important roles in lymphocyte trafficking and immune responses. S1P is produced inside cells and therefore must be secreted to exert its effects through these receptors. Spinster 2 (Spns2) is one of the cell surface transporters thought to secrete S1P. We have shown that Spns2 can export endogenous S1P from cells and also dihydro-S1P, which is active at all cell surface S1P receptors. Moreover, Spns2 mice have decreased levels of both of these phosphorylated sphingoid bases in blood, accompanied by increases in very long chain ceramide species, and have defective lymphocyte trafficking. Surprisingly, levels of S1P and dihydro-S1P were increased in lymph from Spns2 mice as well as in specific tissues, including lymph nodes, and interstitial fluid. Moreover, lymph nodes from Spns2 mice have aberrant lymphatic sinus that appeared collapsed, with reduced numbers of lymphocytes. Our data suggest that Spns2 is an S1P transporter in vivo that plays a role in regulation not only of blood S1P but also lymph node and lymph S1P levels and consequently influences lymphocyte trafficking and lymphatic vessel network organization.

Vasculogenic cytokines in wound healing

Chronic wounds represent a growing healthcare burden that particularly afflicts aged, diabetic, vasculopathic, and obese patients. Studies have shown that nonhealing wounds are characterized by dysregulated cytokine networks that impair blood vessel formation. Two distinct forms of neovascularization have been described: vasculogenesis (driven by bone-marrow-derived circulating endothelial progenitor cells) and angiogenesis (local endothelial cell sprouting from existing vasculature). Researchers have traditionally focused on angiogenesis but defects in vasculogenesis are increasingly recognized to impact diseases including wound healing. A more comprehensive understanding of vasculogenic cytokine networks may facilitate the development of novel strategies to treat recalcitrant wounds. Further, the clinical success of endothelial progenitor cell-based therapies will depend not only on the delivery of the cells themselves but also on the appropriate cytokine milieu to promote tissue regeneration. This paper will highlight major cytokines involved in vasculogenesis within the context of cutaneous wound healing.

Regulation of the sphingosine kinase/sphingosine 1-phosphate pathway

Sphingolipids have emerged as pleiotropic signaling molecules with roles in numerous cellular and biological functions. Defining the regulatory mechanisms governing sphingolipid metabolism is crucial in order to develop a complete understanding of the biological functions of sphingolipid metabolites. The sphingosine kinase/ sphingosine 1-phosphate pathway was originally thought to function in the irreversible breakdown of sphingoid bases; however, in the last few decades it has materialized as an extremely important signaling pathway involved in a plethora of cellular events contributing to both normal and pathophysiological events. Recognition of the SK/S1P pathway as a second messaging system has aided in the identification of many mechanisms of its regulation; however, a cohesive, global understanding of the regulatory mechanisms controlling the SK/S1P pathway is lacking. In this chapter, the role of the SK/S1P pathway as a second messenger is discussed, and its role in mediating TNF-α- and EGF-induced biologies is examined. This work provides a comprehensive look into the roles and regulation of the sphingosine kinase/ sphingosine 1-phosphate pathway and highlights the potential of the pathway as a therapeutic target.

Sphingosine 1-phosphate receptor signaling regulates proper embryonic vascular patterning

Sphingosine 1-phosphate (S1P) binds G-protein-coupled receptors (S1P(1-5)) to regulate a multitude of physiological effects, especially those in the vascular and immune systems. S1P receptors in the vascular system have been characterized primarily in mammals. Here, we report that the S1P receptors and metabolic enzymes are conserved in the genome of zebrafish Danio rerio. Bioinformatic analysis identified seven S1P receptor-like sequences in the zebrafish genome, including duplicated orthologs of receptors 3 and 5. Sphingolipidomic analysis detected erythrocyte and plasma S1P as well as high plasma ceramides and sphingosine. Morpholino-mediated knockdown of s1pr1 causes global and pericardial edema, loss of blood circulation, and vascular defects characterized by both reduced vascularization in intersegmental vessels, decreased proliferation of intersegmental and axial vessels, and hypersprouting in the caudal vein plexus. The s1pr2 gene was previously characterized as a regulator of cell migration and heart development, but its role in angiogenesis is not known. However, when expression of both s1pr1 and s1pr2 is suppressed, severely reduced vascular development of the intersegmental vessels was observed with doses of the s1pr1 morpholino that alone did not cause any discernible vascular defects, suggesting that s1pr1 and s1pr2 function cooperatively to regulate vascular development in zebrafish. Similarly, the S1P transporter, spns2, also cooperated with s1pr1. We propose that extracellular S1P acts through vascular S1P receptors to regulate vascular development.

The transporter Spns2 is required for secretion of lymph but not plasma sphingosine-1-phosphate

Plasma sphingosine-1-phosphate (S1P) regulates vascular permeability, and plasma and lymph S1P guide lymphocyte egress from lymphoid organs. S1P is made intracellularly, and little is known about how S1P is delivered into circulatory fluids. Here, we find that mice without the major facilitator superfamily transporter Spns2 have a profound reduction in lymph S1P, but only a minor decrease in plasma S1P. Spns2-deficient mice have a redistribution of lymphocytes from the spleen to lymph nodes and a loss of circulating lymphocytes, consistent with normal egress from the spleen directed by plasma S1P and blocked egress from lymph nodes directed by lymph S1P. Spns2 is needed in endothelial cells to supply lymph S1P and support lymphocyte circulation. As a differential requirement for lymph and blood S1P, Spns2 may be an attractive target for immune suppressive drugs.

Sphingolipids and atherosclerosis

The atherosclerotic lesion contains a high amount of sphingolipids, a large group of structurally diverse lipids that regulate distinct biological functions beyond their role as structural membrane components. Assessment of their role in atherogenesis has been enabled after genes that regulate their metabolism had been identified and facilitated by the more wide availability of mass spectrometry. Here we discuss recent mechanistic insights obtained in animal and epidemiological studies that have greatly enhanced our understanding of mechanisms how sphingolipids affect the atherosclerotic process.

The role of sphingosine 1-phosphate in immunity and sepsis

Sphingosine 1-phosphate (S1P) is a lipid metabolite with intra- and extracellular signalling properties. It activates five G protein-coupled cell surface receptors designated S1P-receptors type 1-5 (S1P1-5) that transmit extracellular signals into cells, and it modulates intracellular signalling as a cofactor. The analysis of sphingosine kinases (SphK) type 1 and 2, the key enzymes for S1P production, in different infection models point to an important role for the activation of different immune cells like macrophages, mast cells, and dendritic cells. S1P additionally influences local and systemic lymphocyte circulation and positioning, the vascular tone, and blood pressure. Modulation of S1P-mediated signalling pathways therefore results either in local immune cell activation or systemic immune suppression, or both. Pharmacological approaches that modulate certain S1P-mediated signalling pathways while leaving others untouched appear to be promising new avenues for next generation pharmaceuticals. This review summarizes current strategies to modulate S1P signalling for immune intervention with the clear focus on the specificity of the different principles applied. Known local and systemic effects of S1P on immunity are discussed as potential pharmaceutical targets to combat immune and autoimmune diseases and sepsis.

The functional roles of S1P in immunity

The lipid mediator sphingosine-1-phosphate (S1P) is generated within cells from sphingosine by two sphingosine kinases (SPHK1 and SPHK2). Intracellularly synthesized S1P is released into the extracellular fluid by S1P transporters, including SPNS2. Released S1P binds specifically to the G protein-coupled S1P receptors (S1PR1/S1P(1)-S1PR5/S1P(5)), which activate a diverse range of downstream signalling pathways. Recent studies have proposed that one of the central physiological functions of intercellular S1P signalling is in lymphocyte trafficking in vivo because genetic disruption of SPHK1/2, SPNS2 or S1PR1/S1P(1) in mice induces a lymphopenia phenotype. In this review, we discuss the current understanding of intercellular S1P signalling in the context of immunity.

Targeting the sphingosine kinase/sphingosine 1-phosphate pathway in disease: review of sphingosine kinase inhibitors

Sphingosine 1-phosphate (S1P) is an important bioactive sphingolipid metabolite that has been implicated in numerous physiological and cellular processes. Not only does S1P play a structural role in cells by defining the components of the plasma membrane, but in the last 20 years it has been implicated in various significant cell signaling pathways and physiological processes: for example, cell migration, survival and proliferation, cellular architecture, cell-cell contacts and adhesions, vascular development, atherosclerosis, acute pulmonary injury and respiratory distress, inflammation and immunity, and tumorogenesis and metastasis [1,2]. Given the wide variety of cellular and physiological processes in which S1P is involved, it is immediately obvious why the mechanisms governing S1P synthesis and degradation, and the manner in which these processes are regulated, are necessary to understand. In gaining more knowledge about regulation of the sphingosine kinase (SK)/S1P pathway, many potential therapeutic targets may be revealed. This review explores the roles of the SK/S1P pathway in disease, summarizes available SK enzyme inhibitors and examines their potential as therapeutic agents. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.