Characterization of a zebrafish (Danio rerio) sphingosine 1-phosphate receptor expressed in the embryonic brain

Dental regenerative therapy targeting sphingosine-1-phosphate (S1P) signaling pathway in endodontics

The establishment of regenerative therapy in endodontics targeting the dentin-pulp complex, cementum, periodontal ligament tissue, and alveolar bone will provide valuable information to preserve teeth. It is well known that the application of stem cells such as induced pluripotent stem cells, embryonic stem cells, and somatic stem cells is effective in regenerative medicine. There are many somatic stem cells in teeth and periodontal tissues including dental pulp stem cells (DPSCs), stem cells from the apical papilla, and periodontal ligament stem cells. Particularly, several studies have reported the regeneration of clinical pulp tissue and alveolar bone by DPSCs transplantation. However, further scientific issues for practical implementation remain to be addressed. Sphingosine-1-phosphate (S1P) acts as a bioactive signaling molecule that has multiple biological functions including cellular differentiation, and has been shown to be responsible for bone resorption and formation. Here we discuss a strategy for bone regeneration and a possibility for regenerative endodontics targeting S1P signaling pathway as one of approaches for induction of regeneration by improving the regenerative capacity of endogenous cells.

SCIENTIFIC FIELD OF DENTAL SCIENCE

Endodontology.

Apolipoprotein M Protects Against Lipopolysaccharide-Induced Acute Lung Injury via Sphingosine-1-Phosphate Signaling

It had been demonstrated that apolipoprotein M (apoM) is an important carrier of sphingosine-1-phosphate (S1P) in blood, and the S1P has critical roles in the pathogenesis of sepsis-induced acute lung injury (ALI). In the present study, we investigated whether apoM has beneficial effects in a mouse model after lipopolysaccharide (LPS)-induced ALI. Forty-eight mice were divided into two groups: male C57BL/6 wild-type (apoM^+/+) group (n = 24) and apoM gene-deficient (apoM^-/-) group (n = 24) and then randomly subdivided into four subgroups (n = 6 each) according to different intraperitoneal (i.p.) injection: control group, W146 group, LPS group, and LPS + W146 group. Serum levels of interleukin-1 beta (IL-1β) and mRNA levels of IL-1β, interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α), lung histology, wet/dry weight ratio, and immunohistochemistry were measured at 3 h after the baseline and compared in each group. Our results clearly demonstrated that IL-1β mRNA levels and other inflammatory biomarkers were significantly increased in the lungs of LPS-induced ALI apoM^-/- mice compared to those of the apoM^+/+ mice. Moreover, when apoM^+/+ mice were treated with W146, a S1P receptor (S1PR1) antagonist, these inflammatory biomarkers could be significantly upregulated by LPS-induced ALI. Therefore, it suggests that apoM-S1P-S1PR1 signaling might underlie the pathogenesis of ALI and apoM could have physiological benefits to alleviate LPS-induced ALI.

Sphingosine-1-phosphate/S1PR2-mediated signaling triggers Smad1/5/8 phosphorylation and thereby induces Runx2 expression in osteoblasts

Sphingosine-1-phosphate (S1P) is a signaling sphingolipid that also plays crucial roles in bone regeneration. Recently, we reported that the S1P receptors S1PR1 and S1PR2 were mainly expressed in osteoblast-like cells, and that the S1P/S1PR1 signaling pathway up-regulated osteoprotegerin and osteoblast differentiation. However, the involvement of S1P/S1PR2 signaling in osteoblast differentiation is not well understood. Here we investigate the role of S1P/S1PR2-mediated signaling in osteoblast differentiation and clarify the underlying signaling mechanisms. We found that an S1P/S1PR2/Gi-independent signaling pathway activated RhoA activity, leading to phosphorylation of Smad1/5/8 in mouse osteoblast-like MC3T3-E1 cells and primary osteoblasts. Furthermore, this signaling pathway promoted nuclear translocation of Smad4, and increased the amount of Smad6/7 protein in the nucleus. S1P also up-regulated runt-related transcription factor 2 (Runx2) expression through S1PR2/RhoA/ROCK/Smad1/5/8 signaling. Moreover, we found that S1P partially triggered S1PR2/RhoA/ROCK pathway leading to bone formation in vivo. These findings suggest that S1P induces RhoA activity, leading to the phosphorylation of Smad1/5/8, thereby promoting Runx2 expression and differentiation in osteoblasts. Our findings describe novel molecular mechanisms in S1P/S1PR2-mediated osteoblast differentiation that could aid future studies of bone regeneration.

Sphingosine-1-phosphate receptor-1 controls venous endothelial barrier integrity in zebrafish

OBJECTIVE

Endothelial sphingosine-1-phosphate (S1P) receptor-1 (S1P(1)) affects different vascular functions, including blood vessel maturation and permeability. Here, we characterized the role of the zS1P(1) ortholog in vascular development in zebrafish.

METHODS AND RESULTS

zS1P(1) is expressed in dorsal aorta and posterior cardinal vein of zebrafish embryos at 24 to 30 hours postfertilization. zS1P(1) downregulation by antisense morpholino oligonucleotide injection causes early pericardial edema, lack of blood circulation, alterations of posterior cardinal vein structure, and late generalized edema. Also, zS1P(1) morphants are characterized by downregulation of vascular endothelial cadherin (VE-cadherin) and Eph receptor EphB4a expression and by disorganization of zonula occludens 1 junctions in posterior cardinal vein endothelium, with no alterations of dorsal aorta endothelium. VE-cadherin knockdown results in similar vascular alterations, whereas VE-cadherin overexpression is sufficient to rescue venous vascular integrity defects and EphB4a downregulation in zS1P(1) morphants. Finally, S1P(1) small interfering RNA transfection and the S1P(1) antagonist (R)-3-amino-(3-hexylphenylamino)-4-oxobutylphosphonic acid (W146) cause EPHB4 receptor down-modulation in human umbilical vein endothelial cells and the assembly of zonula occludens 1 intercellular contacts is prevented by the EPHB4 antagonist TNYL-RAW peptide in these cells.

CONCLUSIONS

The data demonstrate a nonredundant role of zS1P(1) in the regulation of venous endothelial barrier in zebrafish and identify a S1P(1)/VE-cadherin/EphB4a genetic pathway that controls venous vascular integrity.

Protection of LPS-induced murine acute lung injury by sphingosine-1-phosphate lyase suppression

A defining feature of acute lung injury (ALI) is the increased lung vascular permeability and alveolar flooding, which leads to associated morbidity and mortality. Specific therapies to alleviate the unremitting vascular leak in ALI are not currently clinically available; however, our prior studies indicate a protective role for sphingosine-1-phosphate (S1P) in animal models of ALI with reductions in lung edema. As S1P levels are tightly regulated by synthesis and degradation, we tested the hypothesis that inhibition of S1P lyase (S1PL), the enzyme that irreversibly degrades S1P via cleavage, could ameliorate ALI. Intratracheal instillation of LPS to mice enhanced S1PL expression, decreased S1P levels in lung tissue, and induced lung inflammation and injury. LPS challenge of wild-type mice receiving 2-acetyl-4(5)-[1(R),2(S),3(R),4-tetrahydroxybutyl]-imidazole to inhibit S1PL or S1PL(+/-) mice resulted in increased S1P levels in lung tissue and bronchoalveolar lavage fluids and reduced lung injury and inflammation. Moreover, down-regulation of S1PL expression by short interfering RNA (siRNA) in primary human lung microvascular endothelial cells increased S1P levels, and attenuated LPS-mediated phosphorylation of p38 mitogen-activated protein kinase and I-κB, IL-6 secretion, and endothelial barrier disruption via Rac1 activation. These results identify a novel role for intracellularly generated S1P in protection against ALI and suggest S1PL as a potential therapeutic target.

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.

Activation of sphingosine-1-phosphate receptor S1P5 inhibits oligodendrocyte progenitor migration

Sphingosine-1-phosphate (S1P) acts as an extracellular ligand for a family of G-protein coupled receptors that are crucial in cell migration. S1P5 is exclusively expressed in oligodendrocytes and oligodendrocyte precursor cells (OPCs), which migrate considerable distances during brain development. The current studies suggest a physiological role for S1P and S1P5 in regulation of OPC migration. mRNA expression levels of S1P2 and S1P5 are comparable in OPCs, but S1P binding specifically to the S1P5 receptor blocked OPC migration (IC50=29 nM). Thus, knocking down S1P5 using siRNA prevented the S1P-induced decrease in OPC migration, whereas knocking down S1P2 did not have any effect. S1P-induced modulation of OPC migration was insensitive to pertussis toxin, suggesting that S1P5-initiated signaling is not mediated by the G alpha(i)-protein coupled pathway. Furthermore, S1P5 appears to engage the G alpha(12/13) protein coupled Rho/ROCK signaling pathway to impede OPC migration. To modulate OPC motility, extracellular S1P could be derived from the export of intracellular S1P generated in response to glutamate treatment of OPCs. These studies suggest that S1P could be a part of the neuron-oligodendroglial communication network regulating OPC migration and may provide directional guidance cues for migrating OPCs in the developing brain.

Thalidomide-induced antiangiogenic action is mediated by ceramide through depletion of VEGF receptors, and is antagonized by sphingosine-1-phosphate

Thalidomide, which is clinically recognized as an efficient therapeutic agent for multiple myeloma, has been thought to exert antiangiogenic action through an unknown mechanism. We here show a novel mechanism of thalidomide-induced antiangiogenesis in zebrafish embryos. Thalidomide induces the defect of major blood vessels, which is demonstrated by their morphologic loss and confirmed by the depletion of vascular endothelial growth factor (VEGF) receptors such as neuropilin-1 and Flk-1. Transient increase of ceramide content through activation of neutral sphingomyelinase (nSMase) precedes thalidomide-induced vascular defect in the embryos. Synthetic cell permeable ceramide, N-acetylsphingosine (C2-ceramide) inhibits embryonic angiogenesis as well as thalidomide. The blockade of ceramide generation by antisense morpholino oligonucleotides for nSMase prevents thalidomide-induced ceramide generation and vascular defect. In contrast to ceramide, sphingosine-1-phosphate (S1P) inhibits nSMase-dependent ceramide generation and restores thalidomide-induced embryonic vascular defect with an increase of expression of VEGF receptors. In human umbilical vein endothelial cells (HUVECs), thalidomide-induced inhibition of cell growth, generation of ceramide through nSMase, and depletion of VEGF receptors are restored to the control levels by pretreatment with S1P. These results suggest that thalidomide-induced antiangiogenic action is regulated by the balance between ceramide and S1P signal.

Lysophosphatidic acid signaling controls cortical actin assembly and cytoarchitecture in Xenopus embryos

The mechanisms that control shape and rigidity of early embryos are not well understood, and yet are required for all embryonic processes to take place. In the Xenopus blastula, the cortical actin network in each blastomere is required for the maintenance of overall embryonic shape and rigidity. However, the mechanism whereby each cell assembles the appropriate pattern and number of actin filament bundles is not known. The existence of a similar network in each blastomere suggests two possibilities: cell-autonomous inheritance of instructions from the egg; or mutual intercellular signaling mediated by cell contact or diffusible signals. We show that intercellular signaling is required for the correct pattern of cortical actin assembly in Xenopus embryos, and that lysophosphatidic acid (LPA) and its receptors, corresponding to LPA1 and LPA2 in mammals, are both necessary and sufficient for this function.

Sphingolipid metabolites in neural signalling and function

Sphingolipid metabolites, such as ceramide, sphingosine, sphingosine-1-phosphate (S1P) and complex sphingolipids (gangliosides), are recognized as molecules capable of regulating a variety of cellular processes. The role of sphingolipid metabolites has been studied mainly in non-neuronal tissues. These studies have underscored their importance as signals transducers, involved in control of proliferation, survival, differentiation and apoptosis. In this review, we will focus on studies performed over the last years in the nervous system, discussing the recent developments and the current perspectives in sphingolipid metabolism and functions.

S1P 3 ‐mediated Akt activation and crosstalk with platelet‐derived growth factor receptor (PDGFR)

Akt plays a pivotal role in cell survival and tumorigenesis. We investigated the potential interaction between sphingosine-1-phosphate (S1P) and platelet-derived growth factor (PDGF) in the Akt signaling pathway. Using mouse embryonic fibroblasts (MEFs) from S1P receptor knockout mice, we show here that S1P3 was required for S473 phosphorylation of Akt by S1P. In addition, S1P-stimulated activation of Akt, but not ERK, was blocked by a PDGF receptor (PDGFR)-specific inhibitor, AG1296, suggesting a S1P3-mediated specific crosstalk between the Akt signaling pathways of S1P and PDGFR in MEFs. We investigated this crosstalk under different conditions and found that both Akt and ERK activation induced by S1P, but not lysophosphatidic acid (LPA), in HEY ovarian cancer cells required PDGFR but not epidermal growth factor receptor (EGFR) or insulin-like growth factor-I receptor (IGFR). Importantly, S1P induced a Gi-dependent tyrosine phosphorylation of PDGFR in HEY cells. This dependence on PDGFR in S1P-induced Akt activation was also observed in A2780, T47D, and HMEC-1 cells (which express S1P3), but not in PC-3 or GI-101A cells (which do not express S1P3), further supporting that S1P3 mediates the crosstalk between S1P and PDGFR. This is the first report demonstrating a unique interaction between S1P3 and PDGFR, in addition to demonstrating a specific role for S1P3 in S1P-induced Akt activation.

Embryonic brain expression analysis of lysophospholipid receptor genes suggests roles for s1p(1) in neurogenesis and s1p(1-3) in angiogenesis

In a comparison of embryonic brain expression patterns of lysophosphatidic acid and sphingosine 1-phosphate receptor genes (lpa(1-3) and s1p(1-5), respectively), transcripts detected by Northern blot were subsequently localized using in situ hybridization. We found striking s1p(1) expression adjacent to several ventricles. Near the lateral ventricle, s1p(1) expression was temporally and spatially coincident with neurogenesis and overlapped with lpa(1) in the neocortical area. We also observed a widespread diffuse pattern for lpa(2-3) and a scattered punctate pattern for s1p(1-3). The punctate pattern colocalized with vascular endothelial markers. Together, these results suggest that s1p(1) influences neurogenesis and s1p(1-3) influence angiogenesis in the developing brain.

Lysophospholipid receptors in the nervous system

The lysophospholipid mediators, lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P), are responsible for cell signaling in diverse pathways including survival, proliferation, motility, and differentiation. Most of this signaling occurs through an eight-member family of G-protein coupled receptors once known as the endothelial differentiation gene (EDG) family. More recently, the EDG receptors have been divided into two subfamilies: the lysophosphatidic acid subfamily, which includes LPA1, (EDG-2/VZG-1), LPA2 (EDG-4), and LPA3 (EDG-7), and the sphingosine-1-phosphate receptor subfamily, which includes S1P1 (EDG-1), S1P2 (EDG-5/H218/AGR16), S1P3 (EDG-3), S1P4 (EDG-6), and S1P5 (EDG-8/NRG-1). The ubiquitous expression of these receptors across species, coupled with their diverse cellular functions, has made lysophospholipid receptors an important focus of signal transduction research. Neuroscientists have recently begun to explore the role of lysophospholipid receptors in a number of cell types; this research has implicated these receptors in the survival, migration, and differentiation of cells in the mammalian nervous system.