An essential role for the H218/AGR16/Edg-5/LP(B2) sphingosine 1-phosphate receptor in neuronal excitability

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.

Lysophospholipids and their receptors in the central nervous system

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P), two of the best-studied lysophospholipids, are known to influence diverse biological events, including organismal development as well as function and pathogenesis within multiple organ systems. These functional roles are due to a family of at least 11 G protein-coupled receptors (GPCRs), named LPA(1-6) and S1P(1-5), which are widely distributed throughout the body and that activate multiple effector pathways initiated by a range of heterotrimeric G proteins including G(i/o), G(12/13), G(q) and G(s), with actual activation dependent on receptor subtypes. In the central nervous system (CNS), a major locus for these signaling pathways, LPA and S1P have been shown to influence myriad responses in neurons and glial cell types through their cognate receptors. These receptor-mediated activities can contribute to disease pathogenesis and have therapeutic relevance to human CNS disorders as demonstrated for multiple sclerosis (MS) and possibly others that include congenital hydrocephalus, ischemic stroke, neurotrauma, neuropsychiatric disorders, developmental disorders, seizures, hearing loss, and Sandhoff disease, based upon the experimental literature. In particular, FTY720 (fingolimod, Gilenya, Novartis Pharma, AG) that becomes an analog of S1P upon phosphorylation, was approved by the FDA in 2010 as a first oral treatment for MS, validating this class of receptors as medicinal targets. This review will provide an overview and update on the biological functions of LPA and S1P signaling in the CNS, with a focus on results from studies using genetic null mutants for LPA and S1P receptors. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

Oral fingolimod rescues the functional deficits of synapses in experimental autoimmune encephalomyelitis

BACKGROUND AND PURPOSE Alterations of glutamate-mediated synaptic transmission occur early during neuroinflammatory insults, and lead to degenerative neuronal damage in multiple sclerosis (MS) and also in experimental autoimmune encephalomyelitis (EAE), which is a murine model of MS. Fingolimod is an effective orally active agent for the treatment of MS, affecting lymphocyte invasion of the brain. However, it is still unclear if fingolimod can be neuroprotective in this disorder. EXPERIMENTAL APPROACH Using neurophysiological recordings and morphological evaluation of dendritic integrity, we evaluated the effects of oral fingolimod on the clinical score of EAE mice in order to determine whether the compound was associated with preservation of synaptic transmission. KEY RESULTS Oral fingolimod prevented and reversed the pre- and postsynaptic alterations of glutamate transmission in EAE mice. These effects were associated with a clear amelioration of the clinical deterioration seen in EAE mice, and with a significant inhibition of neuronal dendritic pathology. Fingolimod did not alter the spontaneous excitatory postsynaptic currents in control animals, suggesting that only the pathological processes behind the inflammation-induced defects in glutamate transmission were modulated by this compound. CONCLUSIONS AND IMPLICATIONS The beneficial effects of fingolimod on the clinical, synaptic and dendritic abnormalities of murine EAE might correlate with the neuroprotective actions of this agent, as observed in MS patients. LINKED ARTICLE This article is commented on by Gillingwater, pp. 858-860 of this issue. To view this commentary visit http://dx.doi.org/10.1111/j.1476-5381.2011.01612.x.

Extracellular and intracellular sphingosine-1-phosphate in cancer

Sphingosine-1-phosphate (S1P) was first described as a signaling molecule over 20 years ago. Since then, great strides have been made to reveal its vital roles in vastly different cellular and disease processes. Initially, S1P was considered nothing more than the terminal point of sphingolipid metabolism; however, over the past two decades, a large number of reports have helped unveil its full potential as an important regulatory, bioactive sphingolipid metabolite. S1P has a plethora of physiological functions, due in part to its many sites of actions and its different pools, which are both intra- and extracellular. S1P plays pivotal roles in many physiological processes, including the regulation of cell growth, migration, autophagy, angiogenesis, and survival, and thus, not surprisingly, S1P has been linked to cancer. In this review, we will summarize the vast body of knowledge, highlighting the connection between S1P and cancer. We will also suggest new avenues for future research.

Insights into the pharmacological relevance of lysophospholipid receptors

The discovery of lysophospholipid (LP) 7-transmembrane, G protein-coupled receptors (GPCRs) that began in the 1990s, together with research into the functional roles of the major LPs known as lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P), have opened new research avenues into their biological processes and mechanisms. Major examples of LP signalling effects include embryogenesis, nervous system development, vascular development, uterine implantation, immune cell trafficking, and inflammatory reactions. LP signalling also influences the pathophysiology of many diseases including cancer, autoimmune and inflammatory diseases, which indicate that LP receptors may be attractive targets for pharmacological therapies. A key example of such a therapeutic agent is the S1P receptor modulator FTY720, which upon phosphorylation and continued drug exposure, acts as an S1P receptor functional antagonist. This compound (also known as fingolimod or Gilenya) has recently been approved by the FDA for the treatment of relapsing forms of multiple sclerosis. Continued basic and translational research on LP signalling should provide novel insights into both basic biological mechanisms, as well as novel therapeutic approaches to combat a range of human diseases.

Frequent spontaneous seizures followed by spatial working memory/anxiety deficits in mice lacking sphingosine 1-phosphate receptor 2

The diverse physiological effects of sphingosine 1-phosphate (S1P) are mostly mediated by its five cognate G protein-coupled receptors, S1P(1)-S1P(5), which have attracted much attention as future drug targets. To gain insight into S1P(2)-mediated signaling, we analyzed frequent spontaneous seizures in S1P(2)-deficient (S1P(2)(-/-)) mice obtained after several backcrosses onto a C57BL/6N background. Full-time video recording of 120 S1P(2)(-/-) mice identified 420 seizures both day and night between postnatal days 25 and 45, which were accompanied by high-voltage synchronized cortical discharges and a series of typical episodes: wild run, tonic-clonic convulsion, freezing, and, occasionally, death. Nearly 40% of 224 S1P(2)(-/-) mice died after such seizures, while the remaining 60% of the mice survived to adulthood; however, approximately half of the deliveries from S1P(2)(-/-) pregnant mice resulted in neonatal death. In situ hybridization revealed exclusive s1p(2) expression in the hippocampal pyramidal/granular neurons of wild-type mice, and immunohistochemistry/microarray analyses identified enhanced gliosis in the whole hippocampus and its neighboring neocortex in seizure-prone adult S1P(2)(-/-) mice. Seizure-prone adult S1P(2)(-/-) mice displayed impaired spatial working memory in the eight-arm radial maze test and increased anxiety in the elevated plus maze test, whereas their passive avoidance learning memory performance in the step-through test and hippocampal long-term potentiation was indistinguishable from that of wild-type mice. Our findings suggest that blockade of S1P(2) signaling may cause seizures/hippocampal insults and impair some specific central nervous system functions.

Pharmacological relevance and potential of sphingosine 1-phosphate in the vascular system

Sphingosine-1-phosphate (S1P) was identified as a crucial molecule for regulating immune responses, inflammatory processes as well as influencing the cardiovascular system. S1P mediates differentiation, proliferation and migration during vascular development and homoeostasis. S1P is a naturally occurring lipid metabolite and is present in human blood in nanomolar concentrations. S1P is not only involved in physiological but also in pathophysiological processes. Therefore, this complex signalling system is potentially interesting for pharmacological intervention. Modulation of the system might influence inflammatory, angiogenic or vasoregulatory processes. S1P activates G-protein coupled receptors, namely S1P(1-5) , whereas only S1P(1-3) is present in vascular cells. S1P can also act as an intracellular signalling molecule. This review highlights the pharmacological potential of S1P signalling in the vascular system by giving an overview of S1P-mediated processes in endothelial cells (ECs) and vascular smooth muscle cells (VSMCs). After a short summary of S1P metabolism and signalling pathways, the role of S1P in EC and VSMC proliferation and migration, the cause of relaxation and constriction of arterial blood vessels, the protective functions on endothelial apoptosis, as well as the regulatory function in leukocyte adhesion and inflammatory responses are summarized. This is followed by a detailed description of currently known pharmacological agonists and antagonists as new tools for mediating S1P signalling in the vasculature. The variety of effects influenced by S1P provides plenty of therapeutic targets currently under investigation for potential pharmacological intervention.

Loss of sphingosine kinase 1/S1P signaling impairs cell growth and survival of neurons and progenitor cells in the developing sensory ganglia

BACKGROUND

Lysophospholipids such as lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are important signaling molecules that can regulate a wide range of cellular responses. We discovered that Sphingosine kinase 1 (Sphk1), a key enzyme that converts sphingosine to S1P, is expressed in neurons and progenitor cells in nascent trigeminal and dorsal root ganglia during mouse embryogenesis.

METHODS AND FINDINGS

Sphk1 null mouse embryos do not display overt deficits owing to compensation by Sphk2. Thus, we analyzed embryos that are deficient in both Sphk1 and Sphk2 (which essentially eliminates S1P function) in order to investigate the role(s) of Sphk1 during sensory ganglia formation. While animals lacking 1-3 alleles of Sphk1 and Sphk2 had no obvious phenotype, embryos without both genes displayed clear developmental defects. The complete absence of Sphk1 and Sphk2 resulted in trigeminal and dorsal root ganglia with fewer neurons and progenitor cells. The profound loss in cell number could be attributed to a decrease in cell proliferation as well as an increase in apoptosis. Furthermore, Sphk1/2 double mutants displayed an overall reduction in other sphingolipids as well as an imbalance of S1P/sphingosine and S1P/ceramide ratio, thereby favoring cell death and reducing cell growth.

CONCLUSIONS

Together, these results provide strong in vivo evidence that sphingosine kinase/S1P signaling plays an important role in regulating early events during development of sensory ganglia.

The impact of bioactive lipids on cardiovascular development

Lysophospholipids comprise a group of bioactive molecules with multiple biological functions. The cardinal members of this signalling molecule group are sphingosylphosphorylcholine (SPC), lysophosphatidic acid (LPA), and sphingosine 1-phosphate (S1P) which are, at least in part, homologous to each other. Bioactive lipids usually act via G-protein coupled receptors (GPCRs), but can also function as direct intracellular messengers. Recently, it became evident that bioactive lipids play a role during cellular differentiation development. SPC induces mesodermal differentiation of mouse ES cells and differentiation of promyelocytic leukemia cells, by a mechanism being critically dependent on MEK-ERK signalling. LPA stimulates the clonal expansion of neurospheres from neural stem/progenitor cells and induces c-fos via activation of mitogen- and stress-activated protein kinase 1 (MSK1) in ES cells. S1P acts on hematopoietic progenitor cells as a chemotactic factor and has also been found to be critical for cardiac and skeletal muscle regeneration. Furthermore, S1P promotes cardiogenesis and similarly activates Erk signalling in mouse ES cells. Interestingly, S1P may also act to maintain human stem cell pluripotency. Both LPA and S1P positively regulate the proliferative capacity of murine ES cells. In this paper we will focus on the differential and developmental impact of lysophospholipids on cardiovascular development.

Sphingosine 1-phosphate in coagulation and inflammation

Sphingosine 1-phosphate (S1P) is a lipid mediator produced from sphingomyelin by the sequential enzymatic actions of sphingomyelinase, ceramidase, and sphingosine kinase. Five subtypes of cell surface G-protein-coupled receptors, S1P(1-5), mediate the actions of S1P in various organs systems, most notably cardiovascular, immune, and central nervous systems. S1P is enriched in blood and lymph but is present at much lower concentrations in interstitial fluids of tissues. This vascular S1P gradient is important for the regulation of trafficking of various immune cells. FTY720, which was recently approved for the treatment of relapsing-remitting multiple sclerosis, potently sequesters lymphocytes into lymph nodes by functionally antagonizing the activity of the S1P(1) receptor. S1P also plays critical roles in the vascular barrier integrity, thereby regulating inflammation, tumor metastasis, angiogenesis, and atherosclerosis. Recent studies have also revealed the involvement of S1P signaling in coagulation and in tumor necrosis factor α-mediated signaling. This review highlights the importance of S1P signaling in these inflammatory processes as well as the contribution of each receptor subtype, which exhibits both cooperative and redundant functions.

The Sphingosine-1-phospate receptor 1 mediates S1P action during cardiac development

BACKGROUND

Sphingosine-1-phosophate (S1P) is a biologically active sphingolipid metabolite that influences cellular events including differentiation, proliferation, and migration. S1P acts through five distinct cell surface receptors designated S1P1-5R, with S1P1R having the highest expression level in the developing heart. S1P1R is critical for vascular maturation, with its loss leading to embryonic death by E14.5; however, its function during early cardiac development is not well known. Our previous studies demonstrated that altered S1P levels adversely affects atrioventricular (AV) canal development in vitro, with reduced levels leading to cell death and elevated levels inhibiting cell migration and endothelial to mesenchymal cell transformation (EMT).

RESULTS

We determined, by real-time PCR analysis, that S1P1R was expressed at least 10-fold higher than other S1P receptors in the developing heart. Immunohistochemical analysis revealed S1P1R protein expression in both endothelial and myocardial cells in the developing atrium and ventricle. Using AV canal cultures, we observed that treatment with either FTY720 (an S1P1,3,4,5R agonist) or KRP203 (an S1P1R-specific agonist) caused similar effects on AV canal cultures as S1P treatment, including induction of cell rounding, inhibition of cell migration, and inhibition of EMT. In vivo, morphological analysis of embryonic hearts at E10.5 revealed that S1P1R-/- hearts were malformed with reduced myocardial tissue. In addition to reduced myocardial tissue, E12.5 S1P1R-/- hearts had disrupted morphology of the heart wall and trabeculae, with thickened and disorganized outer compact layer and reduced fibronectin (FN) deposition compared to S1P1R+/+ littermates. The reduced myocardium was accompanied by a decrease in cell proliferation but not an increase in apoptosis.

CONCLUSIONS

These data indicate that S1P1R is the primary mediator of S1P action in AV canal cultures and that loss of S1P1R expression in vivo leads to malformed embryonic hearts, in part due to reduced fibronectin expression and reduced cell proliferation.

Expression of LPP3 in Bergmann glia is required for proper cerebellar sphingosine-1-phosphate metabolism/signaling and development

Bioactive lipids serve as intracellular and extracellular mediators in cell signaling in normal and pathological conditions. Here we describe that an important regulator of some of these lipids, the lipid phosphate phosphatase-3 (LPP3), is abundantly expressed in specific plasma membrane domains of Bergmann glia (BG), a specialized type of astrocyte with key roles in cerebellum development and physiology. Mice selectively lacking expression of LPP3/Ppap2b in the nervous system are viable and fertile but exhibit defects in postnatal cerebellum development and modifications in the cytoarchitecture and arrangement of BG with a mild non-progressive motor coordination defect. Lipid and gene profiling studies in combination with pharmacological treatments suggest that most of these effects are associated with alterations in sphingosine-1-phosphate (S1P) metabolism and signaling. Altogether our data indicate that LPP3 participates in several aspects of neuron-glia communication required for proper cerebellum development.

Sphingosine-1-phosphate receptor-2 deficiency leads to inhibition of macrophage proinflammatory activities and atherosclerosis in apoE-deficient mice

Sphingosine-1-phosphate (S1P) is a biologically active sphingolipid that has pleiotropic effects in a variety of cell types including ECs, SMCs, and macrophages, all of which are central to the development of atherosclerosis. It may therefore exert stimulatory and inhibitory effects on atherosclerosis. Here, we investigated the role of the S1P receptor S1PR2 in atherosclerosis by analyzing S1pr2-/- mice with an Apoe-/- background. S1PR2 was expressed in macrophages, ECs, and SMCs in atherosclerotic aortas. In S1pr2-/-Apoe-/- mice fed a high-cholesterol diet for 4 months, the area of the atherosclerotic plaque was markedly decreased, with reduced macrophage density, increased SMC density, increased eNOS phosphorylation, and downregulation of proinflammatory cytokines compared with S1pr2+/+Apoe-/- mice. Bone marrow chimera experiments indicated a major role for macrophage S1PR2 in atherogenesis. S1pr2-/-Apoe-/- macrophages showed diminished Rho/Rho kinase/NF-κB (ROCK/NF-κB) activity. Consequently, they also displayed reduced cytokine expression, reduced oxidized LDL uptake, and stimulated cholesterol efflux associated with decreased scavenger receptor expression and increased cholesterol efflux transporter expression. S1pr2-/-Apoe-/- ECs also showed reduced ROCK and NF-κB activities, with decreased MCP-1 expression and elevated eNOS phosphorylation. Pharmacologic S1PR2 blockade in S1pr2+/+Apoe-/- mice diminished the atherosclerotic plaque area in aortas and modified LDL accumulation in macrophages. We conclude therefore that S1PR2 plays a critical role in atherogenesis and may serve as a novel therapeutic target for atherosclerosis.

Plasma membrane sphingomyelin hydrolysis increases hippocampal neuron excitability by sphingosine-1-phosphate mediated mechanisms

Proteins that control the excitability of neurons, including voltage-dependent ion channels and neurotransmitter receptors, reside in a membrane lipid environment that includes sphingomyelin, but the influence of the metabolism of this lipid on excitability is unknown. Sphingomyelin in the plasma membrane can be cleaved by neutral sphingomyelinases (nSMase) to generate ceramides and sphingosine-1-phosphate (S1P) which have been shown to play a variety of roles in cellular signaling processes. We found that application of nSMase to hippocampal slices results in a selective enhancement in the population spike amplitude, resulting in fEPSP-PS potentiation of the CA3-CA1 schaeffer collateral synapse. Single cell recordings showed that nSMase activity increases action potential frequency in CA1 neurons in a reversible manner. Additional current clamp recordings showed that nSMase reduces the slow after-hyperpolarization after a burst of action potentials. Mass spectrometry-based measurements demonstrated that nSMase activity induces a rapid increase in the levels of ceramides and S1P in cells in hippocampal slices. The ability of nSMase to increase CA1 neuron excitability was blocked by an inhibitor of sphingosine kinase, the enzyme that converts ceramide to S1P. Moreover, direct intracellular application of S1P to CA1 neurons increased action potential firing. Our findings suggest roles for sphingomyelin metabolism and S1P in the positive regulation of the excitability of hippocampal neurons.

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.

Synaptic PRG-1 modulates excitatory transmission via lipid phosphate-mediated signaling

Plasticity related gene-1 (PRG-1) is a brain-specific membrane protein related to lipid phosphate phosphatases, which acts in the hippocampus specifically at the excitatory synapse terminating on glutamatergic neurons. Deletion of prg-1 in mice leads to epileptic seizures and augmentation of EPSCs, but not IPSCs. In utero electroporation of PRG-1 into deficient animals revealed that PRG-1 modulates excitation at the synaptic junction. Mutation of the extracellular domain of PRG-1 crucial for its interaction with lysophosphatidic acid (LPA) abolished the ability to prevent hyperexcitability. As LPA application in vitro induced hyperexcitability in wild-type but not in LPA(2) receptor-deficient animals, and uptake of phospholipids is reduced in PRG-1-deficient neurons, we assessed PRG-1/LPA(2) receptor-deficient animals, and found that the pathophysiology observed in the PRG-1-deficient mice was fully reverted. Thus, we propose PRG-1 as an important player in the modulatory control of hippocampal excitability dependent on presynaptic LPA(2) receptor signaling.

Differential expression of sphingosine-1-phosphate receptors 1-5 in the developing nervous system

Sphingosine-1-phosphate (S1P) binds to G protein-coupled receptors and can regulate a wide range of cellular functions. In a previous study, we isolated two key enzymes in the S1P pathway that were expressed in migrating neural crest cells. To determine if S1P receptors are present in neural crest cells or peripheral nervous system, we examine the expression patterns of S1P receptors (S1pr1-5) in mouse, and s1pr1 and s1pr3 in chick embryos. Here, we present a comprehensive expression analysis of these receptors using in situ hybridizations, which provide spatiotemporal information. We showed that S1pr2 was expressed in migrating cranial neural crest cells and enteric neurons. S1pr1 was prominently expressed in the neuroepithelium whereas S1pr4 and S1pr5 were in neurons at later stages. On the contrary, S1pr3 was predominantly detected in non-neuronal cells within and surrounding neural structures. We also described novel expression sites for S1P receptors in the developing nervous system.

The influence of sphingosine-1-phosphate receptor signaling on lymphocyte trafficking: how a bioactive lipid mediator grew up from an "immature" vascular maturation factor to a "mature" mediator of lymphocyte behavior and function

Since the initial observations that highlighted the importance of lymphocyte trafficking for immune responses, the pathways utilized by B and T lymphocytes to recirculate and properly position themselves have been intensely studied. Most of the chemoattractants along with their cognate receptors that affect lymphocyte trafficking have been identified. Some of their functions are promotion of lymphocyte ingress into immune organs, localization of cells to specific regions within those organs, maintenance of lymphocyte basal motility in immune organs, facilitation of lymphocyte egress from these organs, and control of migration and homing of lymphocytes in the periphery. Since the seminal discovery that agonism of sphingosine-1-phosphate receptors evokes changes in lymphocyte homing and trafficking, considerable effort has been undertaken to characterize the mechanism utilized by these receptors to influence lymphocyte behavior. This review will focus on the influence of sphingosine-1-phosphate signaling system on lymphocyte localization, egress from lymph organs, and its effects on the lymphatic vasculature.

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 [(35)S]GTPgammaS autoradiography using the S1P(1)-selective agonist SEW2871 and the S1P(1/3)-selective antagonist VPC44116 show that in several regions a majority of this activity is mediated by S1P(1) 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 CB(1) receptors. At least one of the central effects of S1P, thermal antinociception, is also at least partly S1P(1) 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 S1P(1) receptor contributes to S1P-mediated antinociception.

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.

Integrating siRNA and protein-protein interaction data to identify an expanded insulin signaling network

Insulin resistance is one of the dominant symptoms of type 2 diabetes (T2D). Although the molecular mechanisms leading to this resistance are largely unknown, experimental data support that the insulin signaling pathway is impaired in patients who are insulin resistant. To identify novel components/modulators of the insulin signaling pathway, we designed siRNAs targeting over 300 genes and tested the effects of knocking down these genes in an insulin-dependent, anti-lipolysis assay in 3T3-L1 adipocytes. For 126 genes, significant changes in free fatty acid release were observed. However, due to off-target effects (in addition to other limitations), high-throughput RNAi-based screens in cell-based systems generate significant amounts of noise. Therefore, to obtain a more reliable set of genes from the siRNA hits in our screen, we developed and applied a novel network-based approach that elucidates the mechanisms of action for the true positive siRNA hits. Our analysis results in the identification of a core network underlying the insulin signaling pathway that is more significantly enriched for genes previously associated with insulin resistance than the set of genes annotated in the KEGG database as belonging to the insulin signaling pathway. We experimentally validated one of the predictions, S1pr2, as a novel candidate gene for T2D.

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.

Novel GPCR Screening Approach: Indirect Identification of S1P Receptor Agonists in Antagonist Screening Using a Calcium Assay

To elucidate the physiological function of sphingosine 1-phosphate receptors 1-3 (S1P1-3) we aimed to identify selective ligands for these GPCRs. S1P2 and S1P3 are coupled to Gq, and are, therefore, linked to the phospholipase C/IP3/calcium pathway. S1P1 is solely coupled to Gi and was artificially linked to calcium signaling by coexpression of Galpha 16. The three receptors desensitized on challenge of cells with an agonist (i.e., agonists appeared as antagonists in a second calcium measurement). We screened a compound library for inhibitors of S1P-stimulated calcium signals, and we could identify agonists and antagonists with a single measurement. Agonism and antagonism were confirmed by recording compound-and S1P-induced calcium signals from the same assay well. For the three receptors, we found a reciprocal correlation of agonism and "apparent" antagonism of agonists. In addition, agonists indirectly discovered by this approach do not promote calcium mobilization through endogenous GPCRs.

Sphingosine 1-phosphate modulates spinal nociceptive processing

Sphingosine 1-Phosphate (S1P) modulates various cellular functions such as apoptosis, cell differentiation, and migration. Although S1P is an abundant signaling molecule in the central nervous system, very little is known about its influence on neuronal functions. We found that S1P concentrations were selectively decreased in the cerebrospinal fluid of adult rats in an acute and an inflammatory pain model. Pharmacological inhibition of sphingosine kinases (SPHK) decreased basal pain thresholds and SphK2 knock-out mice, but not SphK1 knock-out mice, had a significant decrease in withdrawal latency. Intrathecal application of S1P or sphinganine 1-phosphate (dihydro-S1P) reduced the pain-related (nociceptive) behavior in the formalin assay. S1P and dihydro-S1P inhibited cyclic AMP (cAMP) synthesis, a key second messenger of spinal nociceptive processing, in spinal cord neurons. By combining fluorescence resonance energy transfer (FRET)-based cAMP measurements with Multi Epitope Ligand Cartography (MELC), we showed that S1P decreased cAMP synthesis in excitatory dorsal horn neurons. Accordingly, intrathecal application of dihydro-S1P abolished the cAMP-dependent phosphorylation of NMDA receptors in the outer laminae of the spinal cord. Taken together, the data show that S1P modulates spinal nociceptive processing through inhibition of neuronal cAMP synthesis.

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.

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.

LPA1 is essential for lymphatic vessel development in zebrafish

Lysophosphatidic acid (LPA) has long been implicated in regulating vascular development via endothelial cell-expressed G protein-coupled receptors. However, because of a lack of notable vascular defects reported in LPA receptor knockout mouse studies, the regulation of vasculature by LPA receptors in vivo is still uncertain. Using zebrafish as a model, we studied the gene expression patterns and functions of an LPA receptor, LPA(1), during embryonic development, in particular, vascular formation. Whole-mount in situ hybridization experiments revealed that zebrafish lpa(1) (zlpa(1)) was ubiquitously expressed early in development, and its expression domains were later localized to the head region and the vicinity of the dorsal aorta. The expression of zlpa(1) surrounding the dorsal aorta suggests its role in vasculature development. Knocking down of zLPA(1) by injecting morpholino (MO) oligonucleotides at 0.625-1.25 ng per embryo resulted in the absence of thoracic duct and edema in pericardial sac and trunk in a dose-dependent manner. These zlpa(1)-MO-resulted defects could be specifically rescued by ectopic expression of zlpa(1). In addition, overexpression of vegf-c, a well-known lymphangiogenic factor, also partially ameliorated the inhibition of thoracic duct development. Taken together, these results demonstrate that LPA(1) is necessary for lymphatic vessel formation during embryonic development in zebrafish.

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.

"Inside-out" signaling of sphingosine-1-phosphate: therapeutic targets

Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid metabolite involved in many critical cellular processes including proliferation, survival, and migration, as well as angiogenesis and allergic responses. S1P levels inside cells are tightly regulated by the balance between its synthesis by sphingosine kinases and degradation. S1P is interconvertible with ceramide, which is a critical mediator of apoptosis. It has been postulated that the ratio between S1P and ceramide determines cell fate. Activation of sphingosine kinase by a variety of agonists increases intracellular S1P, which in turn can function intracellularly as a second messenger or be secreted out of the cell and act extracellularly by binding to and signaling through S1P receptors in autocrine and/or paracrine manners. Recent studies suggest that this "inside-out" signaling by S1P may play a role in many human diseases, including cancer, atherosclerosis, inflammation, and autoimmune disorders such as multiple sclerosis. In this review we summarize metabolism of S1P, mechanisms of sphingosine kinase activation, and S1P receptors and their downstream signaling pathways and examine relationships to multiple disease processes. In particular, we describe recent preclinical and clinical trials of therapies targeting S1P signaling, including 2-amino-2-propane-1,3-diol hydrochloride (FTY720, fingolimod), S1P receptor agonists, sphingosine kinase inhibitors, and anti-S1P monoclonal antibody.

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 kinase 1/S1P receptor signaling axis controls glial proliferation in mice with Sandhoff disease

Sphingosine-1-phosphate (S1P) is a lipid-signaling molecule produced by sphingosine kinase in response to a wide number of stimuli. By acting through a family of widely expressed G protein-coupled receptors, S1P regulates diverse physiological processes. Here we examined the role of S1P signaling in neurodegeneration using a mouse model of Sandhoff disease, a prototypical neuronopathic lysosomal storage disorder. When sphingosine kinase 1 (Sphk1) was deleted in Sandhoff disease mice, a milder disease course occurred, with decreased proliferation of glial cells and less-pronounced astrogliosis. A similar result of milder disease course and reduced astroglial proliferation was obtained by deletion of the gene for the S1P(3) receptor, a G protein-coupled receptor enriched in astrocytes. Our studies demonstrate a functional role of S1P synthesis and receptor expression in astrocyte proliferation leading to astrogliosis during the terminal stages of neurodegeneration in Sandhoff disease mice. Because astrocyte responses are involved in many types of neurodegeneration, the Sphk1/S1P receptor signaling axis may be generally important during the pathogenesis of neurodegenerative diseases.

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.

Sphingolipids and membrane biology as determined from genetic models

The importance of sphingolipids in membrane biology was appreciated early in the twentieth century when several human inborn errors of metabolism were linked to defects in sphingolipid degradation. The past two decades have seen an explosion of information linking sphingolipids with cellular processes. Studies have unraveled mechanistic details of the sphingolipid metabolic pathways, and these findings are being exploited in the development of novel therapies, some now in clinical trials. Pioneering work in yeast has laid the foundation for identifying genes encoding the enzymes of the pathways. The advent of the era of genomics and bioinformatics has led to the identification of homologous genes in other species and the subsequent creation of animal knock-out lines for these genes. Discoveries from these efforts have re-kindled interest in the role of sphingolipids in membrane biology. This review highlights some of the recent advances in understanding sphingolipids' roles in membrane biology as determined from genetic models.

Brain sphingosine-1-phosphate receptors: implication for FTY720 in the treatment of multiple sclerosis

Multiple sclerosis (MS) is an autoimmune, neurological disability with unknown etiology. The current therapies available for MS work by an immunomodulatory action, preventing T-cell- and macrophage-mediated destruction of brain-resident oligodendrocytes and axonal loss. Recently, FTY720 (fingolimod) was shown to significantly reduce relapse rates in MS patients and is currently in Phase III clinical trials. This drug attenuates trafficking of harmful T cells entering the brain by regulating sphingosine-1-phosphate (S1P) receptors. Here, we outline the direct roles that S1P receptors play in the central nervous system (CNS) and discuss additional modalities by which FTY720 may provide direct neuroprotection in MS.

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.

Status epilepticus causes a long-lasting redistribution of hippocampal cannabinoid type 1 receptor expression and function in the rat pilocarpine model of acquired epilepsy

Activation of the cannabinoid type 1 (CB1) receptor, a major G-protein-coupled receptor in brain, acts to regulate neuronal excitability and has been shown to mediate the anticonvulsant effects of cannabinoids in several animal models of seizure, including the rat pilocarpine model of acquired epilepsy. However, the long-term effects of status epilepticus on the expression and function of the CB1 receptor have not been described. Therefore, this study was initiated to evaluate the effect of status epilepticus on CB1 receptor expression, binding, and G-protein activation in the rat pilocarpine model of acquired epilepsy. Using immunohistochemistry, we demonstrated that status epilepticus causes a unique "redistribution" of hippocampal CB1 receptors, consisting of specific decreases in CB1 immunoreactivity in the dense pyramidal cell layer neuropil and dentate gyrus inner molecular layer, and increases in staining in the CA1-3 strata oriens and radiatum. In addition, this study demonstrates that the redistribution of CB1 receptor expression results in corresponding functional changes in CB1 receptor binding and G-protein activation using [3H] R+-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazin-yl](1-napthalen-yl)methanone mesylate (WIN55,212-2) and agonist-stimulated [35S]GTPgammaS autoradiography, respectively. The redistribution of CB1 receptor-mediated [35S]GTPgammaS binding was 1) attributed to an altered maximal effect (Emax) of WIN55,212-2 to stimulate [35S]GTPgammaS binding, 2) reversed by the CB1 receptor antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride (SR141716A), 3) confirmed by the use of other CB1 receptor agonists, and 4) not reproduced in other G-protein-coupled receptor systems examined. These results demonstrate that status epilepticus causes a unique and selective reorganization of the CB1 receptor system that persists as a permanent hippocampal neuronal plasticity change associated with the development of acquired epilepsy.

Metabolism and biological functions of two phosphorylated sphingolipids, sphingosine 1-phosphate and ceramide 1-phosphate

Sphingolipids are major lipid constituents of the eukaryotic plasma membrane. Without certain sphingolipids, cells and/or embryos cannot survive, indicating that sphingolipids possess important physiological functions that are not substituted for by other lipids. One such role may be signaling. Recent studies have revealed that some sphingolipid metabolites, such as long-chain bases (LCBs; sphingosine (Sph) in mammals), long-chain base 1-phosphates (LCBPs; sphingosine 1-phosphate (S1P) in mammals), ceramide (Cer), and ceramide 1-phosphate (C1P), act as signaling molecules. The addition of phosphate groups to LCB/Sph and Cer generates LCBP/S1P and C1P, respectively. These phospholipids exhibit completely different functions than those of their precursors. In this review, we describe recent advances in understanding the functions of LCBP/S1P and C1P in mammals and in the yeast Saccharomyces cerevisiae. Since LCB/Sph, LCBP/S1P, Cer, and C1P are mutually convertible, regulation of not only the total amount of the each lipid but also of the overall balance in cellular levels is important. Therefore, we describe in detail their metabolic pathways, as well as the genes involved in each reaction.

Sphingosine 1-phosphate (S1P) signaling is required for maintenance of hair cells mainly via activation of S1P2

Hearing requires the transduction of vibrational forces by specialized epithelial cells in the cochlea known as hair cells. The human ear contains a finite number of terminally differentiated hair cells that, once lost by noise-induced damage or toxic insult, can never be regenerated. We report here that sphingosine 1-phosphate (S1P) signaling, mainly via activation of its cognate receptor S1P2, is required for the maintenance of vestibular and cochlear hair cells in vivo. Two S1P receptors, S1P2 and S1P3, were found to be expressed in the cochlea by reverse transcription-PCR and in situ hybridization. Mice that are null for both these receptors uniformly display progressive cochlear and vestibular defects with hair cell loss, resulting in complete deafness by 4 weeks of age and, with complete penetrance, balance defects of increasing severity. This study reveals the previously unknown role of S1P signaling in the maintenance of cochlear and vestibular integrity and suggests a means for therapeutic intervention in degenerative hearing loss.

Tipping the gatekeeper: S1P regulation of endothelial barrier function

The lysophospholipid sphingosine 1-phosphate (S1P) is a pleiotropic signaling lipid present constitutively in plasma, and secreted locally at elevated concentrations at sites of inflammation. S1P maintains essential variable homeostatic functions in addition to inducing pathophysiology through the activation of five specific high-affinity G-protein-coupled receptors. Therefore, S1P can function as an extracellular rheostat regulating tonic and acutely evoked functions. Although S1P receptors can regulate lymphoid development and lymphocyte trafficking, and different opinions exist on the roles of receptor agonism and functional antagonism in regulating lymphocyte recirculation, this personal perspective highlights the pivotal control points regulated by constitutive and induced S1P receptor tone at vascular endothelial and lymphatic endothelial barriers, through which S1P agonism impacts on both innate and adaptive immunity. We also emphasize how specific, proof-of-concept chemical tools complement genetic approaches by enabling reversible perturbation of the S1P-S1P(1) receptor axis and, thus, clarifying in vivo mechanisms in the absence of developmental compensations.

Deafness and stria vascularis defects in S1P2 receptor-null mice

The S1P(2) receptor is a member of a family of G protein-coupled receptors that bind the extracellular sphingolipid metabolite sphingosine 1-phosphate with high affinity. The receptor is widely expressed and linked to multiple G protein signaling pathways, but its physiological function has remained elusive. Here we have demonstrated that S1P(2) receptor expression is essential for proper functioning of the auditory and vestibular systems. Auditory brainstem response analysis revealed that S1P(2) receptor-null mice were deaf by one month of age. These null mice exhibited multiple inner ear pathologies. However, some of the earliest cellular lesions in the cochlea were found within the stria vascularis, a barrier epithelium containing the primary vasculature of the inner ear. Between 2 and 4 weeks after birth, the basal and marginal epithelial cell barriers and the capillary bed within the stria vascularis of the S1P(2) receptor-null mice showed markedly disturbed structures. JTE013, an S1P(2) receptor-specific antagonist, blocked the S1P-induced vasoconstriction of the spiral modiolar artery, which supplies blood directly to the stria vascularis and protects its capillary bed from high perfusion pressure. Vascular disturbance within the stria vascularis is a potential mechanism that leads to deafness in the S1P(2) receptor-null mice.

Essential roles of sphingosine 1-phosphate/S1P1 receptor axis in the migration of neural stem cells toward a site of spinal cord injury

Neural stem/progenitor cells (NSPCs) migrate toward a damaged area of the central nervous system (CNS) for the purpose of limiting and/or repairing the damage. Although this migratory property of NSPCs could theoretically be exploited for cell-based therapeutics of CNS diseases, little is known of the mechanisms responsible for migratory responses of NSPCs. Here, we found that sphingosine 1-phosphate (Sph-1-P), a physiological lysophospholipid mediator, had a potent chemoattractant activity for NSPCs, in which, of Sph-1-P receptors, S1P(1) was abundantly expressed. Sph-1-P-induced NSPC migration was inhibited by the pretreatment with pertussis toxin, Y-27632 (a Rho kinase inhibitor), and VPC23019 (a competitive inhibitor of S1P(1) and S1P(3)). Sph-1-P does not act as intracellular mediator or in an autocrine manner, because [(3)H]sphingosine, incorporated into NSPCs, was mainly converted to ceramide and sphingomyeline intracellularly, and the stimulation-dependent formation and extracellular release of Sph-1-P were not observed. Further, Sph-1-P concentration in the spinal cord was significantly increased at 7 days after a contusion injury, due to accumulation of microglia and reactive astrocytes in the injured area. This locally increased Sph-1-P concentration contributed to the migration of in vivo transplanted NSPCs through its receptor S1P(1), given that lentiviral transduction of NSPCs with a short hairpin RNA interference for S1P(1) abolished in vivo NSPC migration toward the injured area. This is the first report to identify a physiological role for a lipid mediator in NSPC migration toward a pathological area of the CNS and further indicates that the Sph-1-P/S1P(1) pathway may have therapeutic potential for CNS injuries.

Vascular dysfunction in S1P2 sphingosine 1-phosphate receptor knockout mice

There is growing evidence that sphingosine 1-phosphate (S1P) plays an important role in regulating the development, morphology, and function of the cardiovascular system. There is little data, however, regarding the relative contribution of endogenous S1P and its cognate receptors (referred to as S1P(1-5)) to cardiovascular homeostasis. We used S1P(2) receptor knockout mice (S1P(2)(-/-)) to evaluate the role of S1P(2) in heart and vascular function. There were no significant differences in blood pressure between wild-type and S1P(2)(-/-) mice, measured in awake mice. Cardiac function, evaluated in situ by using a Millar catheter, was also not different in S1P(2)(-/-) mice under baseline or stimulated conditions. In vivo analysis of vascular function by flowmetry revealed decreases in mesenteric and renal resistance in S1P(2)(-/-) mice, especially during vasoconstriction with phenylephrine. In intact aortic rings, the concentration-force relations for both KCl and phenylephrine were right shifted in S1P(2)(-/-) mice, whereas the maximal isometric forces were not different. By contrast, in deendothelialized rings the concentration-force relations were not different but the maximal force was significantly greater in S1P(2)(-/-) aorta. Histologically, there were no apparent differences in vascular morphology. These data suggest that the S1P(2) receptor plays an important role in the function of the vasculature and is an important mediator of normal hemodynamics. This is mediated, at least in part, through an effect on the endothelium, but direct effects on vascular smooth muscle cannot be ruled out and require further investigation.

The S1P2 sphingosine 1-phosphate receptor is essential for auditory and vestibular function

Sphingosine 1-phosphate (S1P) is an endogenous growth factor with potent effects on many different cell types. Most of these effects are produced by activation of one or more of a family of G-protein coupled receptors. The S1P2 receptor can mediate S1P-induced proliferation, differentiation and survival in a wide variety of cells in culture. However, identifying essential in vivo functions for S1P2 has been hampered by its ubiquitous expression and the failure to detect any anatomical abnormalities in initial analyses of S1P2 knockout mice. We report here that all S1P2 knockout mice are profoundly deaf from postnatal day 22 and approximately half display a progressive loss of vestibular function with aging. Anatomically, both the auditory and vestibular systems appear to develop normally but then degrade. Morphological defects associated with hearing are first detected at 3 weeks postnatal as deformations of the organ of Corti/Nuel's space. By one year of age structures within the scala media are dramatically altered. The S1P2 knockout mice also display a loss of otoconia consistent with the vestibular impairment. The present data are the first to indicate that S1P signaling plays critical roles, in vivo, in auditory and vestibular functions. The data further establish that the S1P signaling occurs through the S1P2 receptor and makes an essential contribution to the structural maintenance of these systems, raising the possibility that properly targeted enhancement of this signaling may prove to be clinically beneficial.