Regulation and functions of sphingosine kinases in the brain

MSC Promotes the Secretion of Exosomal lncRNA KLF3-AS1 to Regulate Sphk1 Through YY1-Musashi-1 Axis and Improve Cerebral Ischemia-Reperfusion Injury

Stroke remains the 3rd leading cause of long-term disability globally. Over the past decade, mesenchymal stem cell (MSC) transplantation has been proven as an effective therapy for ischemic stroke. However, the mechanism of MSC-derived exosomal lncRNAs during cerebral ischemia/reperfusion (I/R) remains ambiguous. The oxygen-glucose deprivation/reoxygenation (OGD/R) and middle cerebral artery occlusion (MCAO) rat model were generated. MSCs were isolated and characterized by flow cytometry and histochemical staining, and MSC exosomes were purified and characterized by transmission electron microscopy, flow cytometry and Western blot. Western blot, RT-qPCR and ELISA assay were employed to examine the expression or secretion of key molecules. CCK-8 and TUNEL assays were used to assess cell viability and apoptosis. RNA immunoprecipitation and RNA pull-down were used to investigate the direct association between krüppel-like factor 3 antisense RNA 1 (KLF3-AS1) and musashi-1(MSI1). Yin Yang 1 (YY1)-mediated transcriptional regulation was assessed by chromatin immunoprecipitation and luciferase assays. The histological changes and immunoreactivity of key molecules in brain tissues were examined by H&E and immunohistochemistry. MSCs were successfully isolated and exhibited directionally differential potentials. MSC exosomal KLF3-AS1 alleviated OGD/R-induced inflammation in SK-N-SH and SH-SY5Y cells via modulating Sphk1. Mechanistical studies showed that MSI1 positively regulated KLF3-AS1 expression through its direct binding to KLF3-AS1. YY1 was identified as a transcription activator of MSI1 in MSCs. Functionally, YY1/MSI1 axis regulated the release of MSC exosomal KLF3-AS1 to modulate sphingosine kinase 1 (Sphk1)/NF-κB pathway, thereby ameliorating OGD/R- or cerebral I/R-induced injury. MSCs promote the release of exosomal KLF3-AS1 to regulate Sphk1 through YY1/MSI axis and improve cerebral I/R injury.

The SphK1/S1P Axis Regulates Synaptic Vesicle Endocytosis via TRPC5 Channels

Sphingosine-1-phosphate (S1P), a bioactive sphingolipid concentrated in the brain, is essential for normal brain functions, such as learning and memory and feeding behaviors. Sphingosine kinase 1 (SphK1), the primary kinase responsible for S1P production in the brain, is abundant within presynaptic terminals, indicating a potential role of the SphK1/S1P axis in presynaptic physiology. Altered S1P levels have been highlighted in many neurologic diseases with endocytic malfunctions. However, it remains unknown whether the SphK1/S1P axis may regulate synaptic vesicle endocytosis in neurons. The present study evaluates potential functions of the SphK1/S1P axis in synaptic vesicle endocytosis by determining effects of a dominant negative catalytically inactive SphK1. Our data for the first time identify a critical role of the SphK1/S1P axis in endocytosis in both neuroendocrine chromaffin cells and neurons from mice of both sexes. Furthermore, our Ca2+ imaging data indicate that the SphK1/S1P axis may be important for presynaptic Ca2+ increases during prolonged stimulations by regulating the Ca2+ permeable TRPC5 channels, which per se regulate synaptic vesicle endocytosis. Collectively, our data point out a critical role of the regulation of TRPC5 by the SphK1/S1P axis in synaptic vesicle endocytosis.SIGNIFICANCE STATEMENT Sphingosine kinase 1 (SphK1), the primary kinase responsible for brain sphingosine-1-phosphate (S1P) production, is abundant within presynaptic terminals. Altered SphK1/S1P metabolisms has been highlighted in many neurologic disorders with defective synaptic vesicle endocytosis. However, whether the SphK1/S1P axis may regulate synaptic vesicle endocytosis is unknown. Here, we identify that the SphK1/S1P axis regulates the kinetics of synaptic vesicle endocytosis in neurons, in addition to controlling fission-pore duration during single vesicle endocytosis in neuroendocrine chromaffin cells. The regulation of the SphK1/S1P axis in synaptic vesicle endocytosis is specific since it has a distinguished signaling pathway, which involves regulation of Ca2+ influx via TRPC5 channels. This discovery may provide novel mechanistic implications for the SphK1/S1P axis in brain functions under physiological and pathologic conditions.

Therapeutic potential of the sphingosine kinase 1 inhibitor, PF-543

PF-543 is a sphingosine kinase 1（SPHK1）inhibitor developed by Pfizer and is currently considered the most potent selective SPHK1 inhibitor. SPHK1 catalyses the production of sphingosine 1-phosphate (S1P) from sphingosine. It is the rate-limiting enzyme of S1P production, and there is substantial evidence to support a very important role for sphingosine kinase in health and disease. This review is the first to summarize the role and mechanisms of PF-543 as an SPHK1 inhibitor in anticancer, antifibrotic, and anti-inflammatory processes, providing new therapeutic leads and ideas for future research and clinical trials.

Molecular and neuroimmune pharmacology of S1P receptor modulators and other disease-modifying therapies for multiple sclerosis

Multiple sclerosis (MS) is a neurological, immune-mediated demyelinating disease that affects people in the prime of life. Environmental, infectious, and genetic factors have been implicated in its etiology, although a definitive cause has yet to be determined. Nevertheless, multiple disease-modifying therapies (DMTs: including interferons, glatiramer acetate, fumarates, cladribine, teriflunomide, fingolimod, siponimod, ozanimod, ponesimod, and monoclonal antibodies targeting ITGA4, CD20, and CD52) have been developed and approved for the treatment of MS. All the DMTs approved to date target immunomodulation as their mechanism of action (MOA); however, the direct effects of some DMTs on the central nervous system (CNS), particularly sphingosine 1-phosphate (S1P) receptor (S1PR) modulators, implicate a parallel MOA that may also reduce neurodegenerative sequelae. This review summarizes the currently approved DMTs for the treatment of MS and provides details and recent advances in the molecular pharmacology, immunopharmacology, and neuropharmacology of S1PR modulators, with a special focus on the CNS-oriented, astrocyte-centric MOA of fingolimod.

An update on the use of sphingosine 1-phosphate receptor modulators for the treatment of relapsing multiple sclerosis

INTRODUCTION

Multiple sclerosis (MS) is an immune-mediated disorder of the CNS manifested by recurrent attacks of neurological symptoms (related to focal inflammation) and gradual disability accrual (related to progressive neurodegeneration and neuroinflammation). Sphingosine-1-phosphate-receptor (S1PR) modulators are a class of oral disease-modifying therapies (DMTs) for relapsing MS. The first S1PR modulator developed and approved for MS was fingolimod, followed by siponimod, ozanimod, and ponesimod. All are S1P analogues with different S1PR-subtype selectivity. They restrain the S1P-dependent lymphocyte egress from lymph nodes by binding the lymphocytic S1P-subtype-1-receptor. Depending on their pharmacodynamics and pharmacokinetics, they can also interfere with other biological functions.

AREAS COVERED

Our narrative review covers the PubMed English literature on S1PR modulators in MS until August 2022. We discuss their pharmacology, efficacy, safety profile, and risk management recommendations based on the results of phase II and III clinical trials. We briefly address their impact on the risk of infections and vaccines efficacy.

EXPERT OPINION

S1PR modulators decrease relapse rate and may modestly delay disease progression in people with relapsing MS. Aside their established benefit, their place and timing within the long-term DMT strategy in MS, as well as their immunological effects in the new and evolving context of the post-COVID-19 pandemic and vaccination campaigns warrant further study.

The role of sphingosine 1-phosphate metabolism in brain health and disease

Lipids are essential structural and functional components of the central nervous system (CNS). Sphingolipids are ubiquitous membrane components which were discovered in the brain in the late 19^th century. In mammals, the brain contains the highest concentration of sphingolipids in the body. Sphingosine 1-phosphate (S1P) derived from membrane sphingolipids evokes multiple cellular responses which, depending on its concentration and localization, make S1P a double-edged sword in the brain. In the present review we highlight the role of S1P in brain development and focus on the often contrasting findings regarding its contributions to the initiation, progression and potential recovery of different brain pathologies, including neurodegeneration, multiple sclerosis (MS), brain cancers, and psychiatric illnesses. A detailed understanding of the critical implications of S1P in brain health and disease may open the door for new therapeutic options. Thus, targeting S1P-metabolizing enzymes and/or signaling pathways might help overcome, or at least ameliorate, several brain illnesses.

The TRPV1 Receptor Is Up-Regulated by Sphingosine 1-Phosphate and Is Implicated in the Anandamide-Dependent Regulation of Mitochondrial Activity in C2C12 Myoblasts

The sphingosine 1-phosphate (S1P) and endocannabinoid (ECS) systems comprehend bioactive lipids widely involved in the regulation of similar biological processes. Interactions between S1P and ECS have not been so far investigated in skeletal muscle, where both systems are active. Here, we used murine C2C12 myoblasts to investigate the effects of S1P on ECS elements by qRT-PCR, Western blotting and UHPLC-MS. In addition, the modulation of the mitochondrial membrane potential (ΔΨm), by JC-1 and Mitotracker Red CMX-Ros fluorescent dyes, as well as levels of protein controlling mitochondrial function, along with the oxygen consumption were assessed, by Western blotting and respirometry, respectively, after cell treatment with methanandamide (mAEA) and in the presence of S1P or antagonists to endocannabinoid-binding receptors. S1P induced a significant increase in TRPV1 expression both at mRNA and protein level, while it reduced the protein content of CB2. A dose-dependent effect of mAEA on ΔΨm, mediated by TRPV1, was evidenced; in particular, low doses were responsible for increased ΔΨm, whereas a high dose negatively modulated ΔΨm and cell survival. Moreover, mAEA-induced hyperpolarization was counteracted by S1P. These findings open new dimension to S1P and endocannabinoids cross-talk in skeletal muscle, identifying TRPV1 as a pivotal target.

An Update on Sphingosine-1-Phosphate and Lysophosphatidic Acid Receptor Transcripts in Rodent Olfactory Mucosa

Olfactory neurons connect the external environment and the brain, allowing the translocation of materials from the nasal cavity into the brain. The olfactory system is involved in SARS-CoV-2 infections; early in the pandemic declared in 2020, a loss of the sense of smell was found in many infected patients. Attention has also been focused on the role that the olfactory epithelium appears to play in the entry of the SARS-CoV-2 virus into the brain. Specifically, SARS-CoV-2 enters cells via the angiotensin-converting enzyme 2 protein (ACE2), which is found on supporting cells in the olfactory epithelium. The intranasal administration of sphingosine has been proposed to prevent the binding of SARS-CoV-2 to ACE2. Further, sphingosine-1-phosphate (S1P) receptors appear to facilitate the entry of SARS-CoV-2 into the brain. The goal of these studies was to characterize S1P receptor expression status in rodent olfactory mucosa. The expression of receptors for a related sphingolipid, lysophosphatidic acid (LPA), was also assessed. The results confirm previous reports of S1P1 and S1P3 receptor expression, as well as LPA receptor 1, in mouse olfactory mucosa; moreover, they extend the previous findings to identify additional S1P and LPA receptor transcripts in rat and mouse olfactory mucosa, as well as in cultured olfactory neurons. These findings may enhance the utility of rodent models in identifying agonists and/or antagonists of S1P and LPA receptors that may block the entry of SARS-CoV-2 and other viruses into nasal epithelial cells, and prevent transmission from the nasal cavity into the brain.

Neuronal contact upregulates astrocytic sphingosine-1-phosphate receptor 1 to coordinate astrocyte-neuron cross communication

Astrocytes, the most abundant glial cells in the mammalian brain, directly associate with and regulate neuronal processes and synapses and are important regulators of brain development. Yet little is known of the molecular mechanisms that control the establishment of astrocyte morphology and the bi-directional communication between astrocytes and neurons. Here we show that neuronal contact stimulates expression of S1PR1, the receptor for the bioactive sphingolipid metabolite sphingosine-1-phosphate (S1P), on perisynaptic astrocyte processes and that S1PR1 drives astrocyte morphological complexity and morphogenesis. Moreover, the S1P/S1PR1 axis increases neuronal contact-induced expression of astrocyte secreted synaptogenic factors SPARCL1 and thrombospondin 4 that are involved in neural circuit assembly. Our findings have uncovered new functions for astrocytic S1PR1 signaling in regulation of bi-directional astrocyte-neuron crosstalk at the nexus of astrocyte morphogenesis and synaptogenesis.

Therapeutic Potential of SphK1 Inhibitors Based on Abnormal Expression of SphK1 in Inflammatory Immune Related-Diseases

Sphingosine kinase 1(SphK1) a key enzyme that catalyzes the conversion of sphingosine (Sph) to sphingosine 1-phosphate (S1P), so as to maintain the dynamic balance of sphingolipid-rheostat in cells and participate in cell growth and death, proliferation and migration, vasoconstriction and remodeling, inflammation and metabolism. The normal expression of SphK1 maintains the balance of physiological and pathological states, which is reflected in the regulation of inflammatory factor secretion, immune response in traditional immune cells and non-traditional immune cells, and complex signal transduction. However, abnormal SphK1 expression and activity are found in various inflammatory and immune related-diseases, such as hypertension, atherosclerosis, Alzheimer’s disease, inflammatory bowel disease and rheumatoid arthritis. In view of the therapeutic potential of regulating SphK1 and its signal, the current research is aimed at SphK1 inhibitors, such as SphK1 selective inhibitors and dual SphK1/2 inhibitor, and other compounds with inhibitory potency. This review explores the regulatory role of over-expressed SphK1 in inflammatory and immune related-diseases, and investigate the latest progress of SphK1 inhibitors and the improvement of disease or pathological state.

Chemical and genetic rescue of in vivo progranulin-deficient lysosomal and autophagic defects

In 2006, GRN mutations were first linked to frontotemporal dementia (FTD), the leading cause of non-Alzheimer dementias. While much research has been dedicated to understanding the genetic causes of the disease, our understanding of the mechanistic impacts of GRN deficiency has only recently begun to take shape. With no known cure or treatment available for GRN-related FTD, there is a growing need to rapidly advance genetic and/or small-molecule therapeutics for this disease. This issue is complicated by the fact that, while lysosomal dysfunction seems to be a key driver of pathology, the mechanisms linking a loss of GRN to a pathogenic state remain unclear. In our attempt to address these key issues, we have turned to the nematode, Caenorhabditis elegans, to model, study, and find potential therapies for GRN-deficient FTD. First, we show that the loss of the nematode GRN ortholog, pgrn-1, results in several behavioral and molecular defects, including lysosomal dysfunction and defects in autophagic flux. Our investigations implicate the sphingolipid metabolic pathway in the regulation of many of the in vivo defects associated with pgrn-1 loss. Finally, we utilized these nematodes as an in vivo tool for high-throughput drug screening and identified two small molecules with potential therapeutic applications against GRN/pgrn-1 deficiency. These compounds reverse the biochemical, cellular, and functional phenotypes of GRN deficiency. Together, our results open avenues for mechanistic and therapeutic research into the outcomes of GRN-related neurodegeneration, both genetic and molecular.

Recent Insights into the Interplay of Alpha-Synuclein and Sphingolipid Signaling in Parkinson's Disease

Molecular studies have provided increasing evidence that Parkinson’s disease (PD) is a protein conformational disease, where the spread of alpha-synuclein (ASN) pathology along the neuraxis correlates with clinical disease outcome. Pathogenic forms of ASN evoke oxidative stress (OS), neuroinflammation, and protein alterations in neighboring cells, thereby intensifying ASN toxicity, neurodegeneration, and neuronal death. A number of evidence suggest that homeostasis between bioactive sphingolipids with opposing function—e.g., sphingosine-1-phosphate (S1P) and ceramide—is essential in pro-survival signaling and cell defense against OS. In contrast, imbalance of the “sphingolipid biostat” favoring pro-oxidative/pro-apoptotic ceramide-mediated changes have been indicated in PD and other neurodegenerative disorders. Therefore, we focused on the role of sphingolipid alterations in ASN burden, as well as in a vast range of its neurotoxic effects. Sphingolipid homeostasis is principally directed by sphingosine kinases (SphKs), which synthesize S1P—a potent lipid mediator regulating cell fate and inflammatory response—making SphK/S1P signaling an essential pharmacological target. A growing number of studies have shown that S1P receptor modulators, and agonists are promising protectants in several neurological diseases. This review demonstrates the relationship between ASN toxicity and alteration of SphK-dependent S1P signaling in OS, neuroinflammation, and neuronal death. Moreover, we discuss the S1P receptor-mediated pathways as a novel promising therapeutic approach in PD.

The emerging role of FTY720 as a sphingosine 1-phosphate analog for the treatment of ischemic stroke: The cellular and molecular mechanisms

Finding novel and effective drugs for the treatment of ischemic stroke is warranted because there is not a definitive treatment for this prevalent disease. Due to the relevance between the sphingosine 1-phosphate (S1P) receptor and several neurological diseases including ischemic stroke, it seems that fingolimod (FTY720), as an agonist of S1P receptor, can be a useful therapeutic strategy in these patients. FTY720 is the first oral drug approved by the US food and drug administration for the treatment of multiple sclerosis. Three important mechanisms for neuroprotective effects of FTY720 have been described. First, the functional antagonistic mechanism that is associated with lymphopenia and reduced lymphocytic inflammation. This effect results from the down-regulation and degradation of lymphocytes' S1P receptors, which inhibits lymph node lymphocytes from entering the bloodstream. Second, a functional agonistic activity that is mediated through direct effects via targeting S1P receptors on the membrane of various cells including neurons, microglia, oligodendrocytes, astrocytes, and endothelial cells of blood vessels in the central nervous system (CNS), and the third, receptor-independent mechanisms that are displayed by binding to specific cellular proteins that modulate intracellular signaling pathways or affect epigenetic transcriptions. Therefore, we review these mechanisms in more detail and describe the animal model and in clinical trial studies that support these three mechanisms for the neuroprotective action of FTY720 in ischemic stroke.

Targeting the Sphingosine Kinase/Sphingosine-1-Phosphate Signaling Axis in Drug Discovery for Cancer Therapy

Sphingolipid metabolites have emerged as critical players in the regulation of various physiological processes. Ceramide and sphingosine induce cell growth arrest and apoptosis, whereas sphingosine-1-phosphate (S1P) promotes cell proliferation and survival. Here, we present an overview of sphingolipid metabolism and the compartmentalization of various sphingolipid metabolites. In addition, the sphingolipid rheostat, a fine metabolic balance between ceramide and S1P, is discussed. Sphingosine kinase (SphK) catalyzes the synthesis of S1P from sphingosine and modulates several cellular processes and is found to be essentially involved in various pathophysiological conditions. The regulation and biological functions of SphK isoforms are discussed. The functions of S1P, along with its receptors, are further highlighted. The up-regulation of SphK is observed in various cancer types and is also linked to radio- and chemoresistance and poor prognosis in cancer patients. Implications of the SphK/S1P signaling axis in human pathologies and its inhibition are discussed in detail. Overall, this review highlights current findings on the SphK/S1P signaling axis from multiple angles, including their functional role, mechanism of activation, involvement in various human malignancies, and inhibitor molecules that may be used in cancer therapy.

SPHK2-Generated S1P in CD11b+ Macrophages Blocks STING to Suppress the Inflammatory Function of Alveolar Macrophages

Acute lung injury (ALI) is a lethal inflammatory lung disorder whose incidence is on the rise. Alveolar macrophages normally act to resolve inflammation, but when dysregulated they can provoke ALI. We demonstrate that monocyte-derived macrophages (CD11b⁺ macrophages) recruited into the airspace upregulate the anti-inflammatory function of alveolar macrophages by suppressing their stimulator of type 1 interferon gene (STING) signaling. Depletion of CD11b⁺ macrophages in mice (macrophage^dep mice) after endotoxin or after Pseudomonas aeruginosa causes expansion of the inflammatory alveolar macrophage population, leading to neutrophil accumulation, irreversible loss of lung vascular barrier function, and lethality. We show that CD11b⁺ macrophages suppress alveolar macrophage-STING signaling via sphingosine kinase-2 (SPHK2) generation of sphingosine-1-phosphate (S1P). Thus, adoptive transfer of wild-type (WT) or STING^−/−, but not SPHK2^−/−, CD11b monocytes from murine bone marrow into injured macrophage^dep mice rescue anti-inflammatory alveolar macrophages and reverse lung vascular injury. SPHK2-induced S1P generation in CD11b⁺ macrophages has the potential to educate alveolar macrophages to resolve ALI.

Joshi et al. demonstrate an essential role of SPHK2⁺ monocyte-derived CD11b⁺ macrophages, which are recruited to the airspace, in promoting anti-inflammatory function of alveolar macrophages during lung injury. They show that S1P generated by recruited SPHK2⁺-CD11b⁺ macrophages suppresses STING signaling in alveolar macrophages to resolve inflammatory injury.

Abundant oleoyl-lysophosphatidylethanolamine in brain stimulates neurite outgrowth and protects against glutamate toxicity in cultured cortical neurons

Lysophosphatidylethanolamines (LPEs) are bioactive lysophospholipids that have been suggested to play important roles in several biological processes. We performed a quantitative analysis of LPE species and showed their composition in mouse brain. We examined the roles of oleoyl-LPE (18:1 LPE), which is one of the abundant LPE species in brain. In cultured cortical neurons, application of 18:1 LPE stimulated neurite outgrowth. The effect of 18:1 LPE on neurite outgrowth was inhibited by Gq/11 inhibitor YM-254890, phospholipase C (PLC) inhibitor U73122, protein kinase C (PKC) inhibitor Go6983, or mitogen-activated protein kinase (MAPK) inhibitor U0126. Additionally, 18:1 LPE increased the phosphorylation of MAPK/extracellular signal-regulated kinase 1/2. These results suggest that the action of 18:1 LPE on neurite outgrowth is mediated by the Gq/11/PLC/PKC/MAPK pathway. Moreover, we found that application of 18:1 LPE protects neurons from glutamate-induced excitotoxicity. This effect of 18:1 LPE was suppressed by PKC inhibitor Go6983. These results suggest that 18:1 LPE protects neurons from glutamate toxicity via PKC inhibitor Go6983-sensitive PKC subtype. Collectively, our results demonstrated that 18:1 LPE stimulates neurite outgrowth and protects against glutamate toxicity in cultured cortical neurons. Our findings provide insights into the physiological or pathological roles of 18:1 LPE in the brain.

UPLC-Q-TOF/MS-Based Serum Metabolomics Reveals the Anti-Ischemic Stroke Mechanism of Nuciferine in MCAO Rats

Nuciferine is an aporphine alkaloid monomer that is extracted from the leaves of the lotus species Nymphaea caerulea and Nelumbo nucifera Gaertn. Nuciferine was reported to treat cerebrovascular diseases. However, the potential mechanism of the neuroprotective effects of nuciferine at the metabolomics level is still not unclear. The present research used neurological score, infarct volume, cerebral water content, and ultraperformance liquid chromatography to quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF/MS)-based serum metabolomics to elucidate the anti-ischemic stroke effect and mechanisms of nuciferine. The results showed that nuciferine significantly improved neurological deficit scores and ameliorated cerebral edema and infarction. Multivariate data analysis methods were used to examine the differences in serum endogenous metabolism between groups, and the biomarkers of nuciferine on ischemic stroke were identified. Approximately 19 metabolites and 7 metabolic pathways associated with nuciferine on treatment of stroke were found, which indicated that nuciferine exerted a positive therapeutic action on cerebral ischemic by regulating metabolism. These results provided some data support for the study of anti-stroke effect of nuciferine from the perspective of metabolomics.

Evidence for Altered Metabolism of Sphingosine-1-Phosphate in the Corpus Callosum of Patients with Schizophrenia

The disturbed integrity of myelin and white matter, along with dysregulation of the lipid metabolism, may be involved in schizophrenia pathophysiology. Considering the crucial role of sphingolipids in neurodevelopment, particularly in oligodendrocyte differentiation and myelination, we examined the role of sphingolipid dynamics in the pathophysiology of schizophrenia. We performed targeted mass spectrometry-based analysis of sphingolipids from the cortical area and corpus callosum of postmortem brain samples from patients with schizophrenia and controls. We observed lower sphingosine-1-phosphate (S1P) levels, specifically in the corpus callosum of patients with schizophrenia, but not in major depressive disorder or bipolar disorder, when compared with the controls. Patient data and animal studies showed that antipsychotic intake did not contribute to the lowered S1P levels. We also found that lowered S1P levels in the corpus callosum of patients with schizophrenia may stem from the upregulation of genes for S1P-degrading enzymes; higher expression of genes for S1P receptors suggested a potential compensatory mechanism for the lowered S1P levels. A higher ratio of the sum of sphingosine and ceramide to S1P, which can induce apoptosis and cell-cycle arrest, was also observed in the samples of patients with schizophrenia than in controls. These results suggest that an altered S1P metabolism may underlie the deficits in oligodendrocyte differentiation and myelin formation, leading to the structural and molecular abnormalities of white matter reported in schizophrenia. Our findings may pave the way toward a novel therapeutic strategy.

Mechanistic regulation of SPHK1 expression and translocation by EMAP II in pulmonary smooth muscle cells

Phosphorylation of sphingosine by sphingosine kinase 1 (SPHK1) produces the bioactive sphingolipid sphingosine-1-phosphate (S1P), a microvascular and immuno-modulator associated with vascular remodeling in pulmonary arterial hypertension (PAH). The low intracellular concentration of S1P is under tight spatial-temporal control. Molecular mechanisms that mediate S1P burden and S1P regulation of vascular remodeling are poorly understood. Similarities between two early response pro-inflammatory cytokine gene transcript activation profiles, S1P and Endothelial Monocyte Activating Polypeptide II (EMAP II), suggested a strategic link between their signaling pathways. We determined that EMAP II triggers a bimodal phosphorylation, transcriptional regulation and membrane translocation of SPHK1 through a common upstream process in both macrophages and pulmonary artery smooth muscle cells (PASMCs). EMAP II initiates a dual function of ERK1/2: phosphorylation of SPHK1 and regulation of the transcription factor EGR1 that induces expression of SPHK1. Activated ERK1/2 induces a bimodal phosphorylation of SPHK1 which reciprocally increases S1P levels. This identified common upstream signaling mechanism between a protein and a bioactive lipid initiates cell specific downstream signaling representing a multifactorial mechanism that contributes to inflammation and PASMC proliferation which are cardinal histopathological phenotypes of PAH.

Identification of a 22 bp DNA cis Element that Plays a Critical Role in Colony Stimulating Factor 1-Dependent Transcriptional Activation of the SPHK1 Gene

Sphingosine-1-phosphate (S1P) is an anabolic clastokine. Colony Stimulating Factor 1 (CSF1) induces expression of the rate limiting enzyme required for S1P synthesis, sphingosine kinase 1 (SPHK1) in bone in vivo, and in osteoclasts in vitro. To study the mechanism of CSF1-induced SPHK1 gene expression, a 2608 bp fragment of the murine SPHK1 gene (- 2497 to + 111 bp relative to the transcription start site) was cloned and transfected into pZen cells (murine fibroblasts engineered to express c-fms). SPHK1 promoter activity was assessed using a dual-luciferase reporter assay system. By analyzing a series of 5'-deletions, a CSF1-responsive region was identified in the region - 1250 to - 1016 bp. To define putative DNA binding site(s) in this fragment, two biotin-labeled fragments that completely overlapped this region were generated, one 163 bp in length (- 1301 to - 1139) and one 169 bp in length (- 1157 to - 989). EMSAs revealed the 163 bp fragment as the target for protein binding. Using EMSAs, the nuclear protein binding region was further narrowed to an 85 bp fragment, (- 1223 to - 1139). Using a series of unlabeled DNA sequences as "cold competitors" in EMSAs, a 22 bp sequence is identified as the smallest fragment that could successfully compete away protein binding. The same 22 bp sequence also competed DNA binding in EMSAs using nuclear protein isolated from primary murine osteoclasts. A full-length wild-type SPHK1 promoter and an SPHK1 promoter in which the ATGGGGG motif was mutated were subsequently expressed in pZen cells. Mutating this ATGGGGG motif nearly completely abrogated the ability of CSF1 to activate the promoter. Although two transcription factors, KLF6 and Sp1 have been reported to bind to this sequence, supershift EMSAs failed to detect either among the proteins bound to the 85 bp DNA fragment.

Potential sphingosine-1-phosphate-related therapeutic targets in the treatment of cerebral ischemia reperfusion injury

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid that regulates lymphocyte trafficking, glial cell activation, vasoconstriction, endothelial barrier function, and neuronal death pathways in the brain. Research has increasingly implicated S1P in the pathology of cerebral ischemia reperfusion (IR) injury. As a high-affinity agonist of S1P receptor, fingolimod exhibits excellent neuroprotective effects against ischemic challenge both in vivo and in vitro. By summarizing recent progress on how S1P participates in the development of brain IR injury, this review identifies potential therapeutic targets for the treatment of brain IR injury.

Sphingosine-1-Phosphate in the Tumor Microenvironment: A Signaling Hub Regulating Cancer Hallmarks

As a key hub of malignant properties, the cancer microenvironment plays a crucial role intimately connected to tumor properties. Accumulating evidence supports that the lysophospholipid sphingosine-1-phosphate acts as a key signal in the cancer extracellular milieu. In this review, we have a particular focus on glioblastoma, representative of a highly aggressive and deleterious neoplasm in humans. First, we highlight recent advances and emerging concepts for how tumor cells and different recruited normal cells contribute to the sphingosine-1-phosphate enrichment in the cancer microenvironment. Then, we describe and discuss how sphingosine-1-phosphate signaling contributes to favor cancer hallmarks including enhancement of proliferation, stemness, invasion, death resistance, angiogenesis, immune evasion and, possibly, aberrant metabolism. We also discuss the potential of how sphingosine-1-phosphate control mechanisms are coordinated across distinct cancer microenvironments. Further progress in understanding the role of S1P signaling in cancer will depend crucially on increasing knowledge of its participation in the tumor microenvironment.

Neuronal and glial DNA methylation and gene expression changes in early epileptogenesis

BACKGROUND AND AIMS

Mesial Temporal Lobe Epilepsy is characterized by progressive changes of both neurons and glia, also referred to as epileptogenesis. No curative treatment options, apart from surgery, are available. DNA methylation (DNAm) is a potential upstream mechanism in epileptogenesis and may serve as a novel therapeutic target. To our knowledge, this is the first study to investigate epilepsy-related DNAm, gene expression (GE) and their relationship, in neurons and glia.

METHODS

We used the intracortical kainic acid injection model to elicit status epilepticus. At 24 hours post injection, hippocampi from eight kainic acid- (KA) and eight saline-injected (SH) mice were extracted and shock frozen. Separation into neurons and glial nuclei was performed by flow cytometry. Changes in DNAm and gene expression were measured with reduced representation bisulfite sequencing (RRBS) and mRNA-sequencing (mRNAseq). Statistical analyses were performed in R with the edgeR package.

RESULTS

We observed fulminant DNAm- and GE changes in both neurons and glia at 24 hours after initiation of status epilepticus. The vast majority of these changes were specific for either neurons or glia. At several epilepsy-related genes, like HDAC11, SPP1, GAL, DRD1 and SV2C, significant differential methylation and differential gene expression coincided.

CONCLUSION

We found neuron- and glia-specific changes in DNAm and gene expression in early epileptogenesis. We detected single genetic loci in several epilepsy-related genes, where DNAm and GE changes coincide, worth further investigation. Further, our results may serve as an information source for neuronal and glial alterations in both DNAm and GE in early epileptogenesis.

Evaluation of binding and inhibition mechanism of dietary phytochemicals with sphingosine kinase 1: Towards targeted anticancer therapy

Sphingosine kinase 1 (SphK1) has recently gained attention as a potential drug target for its association with cancer and other inflammatory diseases. Here, we have investigated the binding affinity of dietary phytochemicals viz., ursolic acid, capsaicin, DL-α tocopherol acetate, quercetin, vanillin, citral, limonin and simvastatin with the SphK1. Docking studies revealed that all these compounds bind to the SphK1 with varying affinities. Fluorescence binding and isothermal titration calorimetric measurements suggested that quercetin and capsaicin bind to SphK1 with an excellent affinity, and significantly inhibits its activity with an admirable IC50 values. The binding mechanism of quercetin was assessed by docking and molecular dynamics simulation studies for 100 ns in detail. We found that quercetin acts as a lipid substrate competitive inhibitor, and it interacts with important residues of active-site pocket through hydrogen bonds and other non-covalent interactions. Quercetin forms a stable complex with SphK1 without inducing any significant conformational changes in the protein structure. In conclusion, we infer that quercetin and capsaicin provide a chemical scaffold to develop potent and selective inhibitors of SphK1 after required modifications for the clinical management of cancer.

Role of Ceramidases in Sphingolipid Metabolism and Human Diseases

Human pathologies such as Alzheimer’s disease, type 2 diabetes-induced insulin resistance, cancer, and cardiovascular diseases have altered lipid homeostasis. Among these imbalanced lipids, the bioactive sphingolipids ceramide and sphingosine-1 phosphate (S1P) are pivotal in the pathophysiology of these diseases. Several enzymes within the sphingolipid pathway contribute to the homeostasis of ceramide and S1P. Ceramidase is key in the degradation of ceramide into sphingosine and free fatty acids. In humans, five different ceramidases are known—acid ceramidase, neutral ceramidase, and alkaline ceramidase 1, 2, and 3—which are encoded by five different genes (ASAH1, ASAH2, ACER1, ACER2, and ACER3, respectively). Notably, the neutral ceramidase N-acylsphingosine amidohydrolase 2 (ASAH2) shows considerable differences between humans and animals in terms of tissue expression levels. Besides, the subcellular localization of ASAH2 remains controversial. In this review, we sum up the results obtained for identifying gene divergence, structure, subcellular localization, and manipulating factors and address the role of ASAH2 along with other ceramidases in human diseases.

Feeding Stimulates Sphingosine-1-Phosphate Mobilization in Mouse Hypothalamus

Previous studies have shown that the sphingolipid-derived mediator sphingosine-1-phosphate (S1P) reduces food intake by activating G protein-coupled S1P receptor-1 (S1PR1) in the hypothalamus. Here, we examined whether feeding regulates hypothalamic mobilization of S1P and other sphingolipid-derived messengers. We prepared lipid extracts from the hypothalamus of C57Bl6/J male mice subjected to one of four conditions: free feeding, 12 h fasting, and 1 h or 6 h refeeding. Liquid chromatography/tandem mass spectrometry was used to quantify various sphingolipid species, including sphinganine (SA), sphingosine (SO), and their bioactive derivatives SA-1-phosphate (SA1P) and S1P. In parallel experiments, transcription of S1PR1 (encoded in mice by the S1pr1 gene) and of key genes of sphingolipid metabolism (Sptlc2, Lass1, Sphk1, Sphk2) was measured by RT-PCR. Feeding increased levels of S1P (in pmol-mg⁻¹ of wet tissue) and SA1P. This response was accompanied by parallel changes in SA and dihydroceramide (d18:0/18:0), and was partially (SA1P) or completely (S1P) reversed by fasting. No such effects were observed with other sphingolipid species targeted by our analysis. Feeding also increased transcription of Sptlc2, Lass1, Sphk2, and S1pr1. Feeding stimulates mobilization of endogenous S1PR1 agonists S1P and SA1P in mouse hypothalamus, via a mechanism that involves transcriptional up-regulation of de novo sphingolipid biosynthesis. The results support a role for sphingolipid-mediated signaling in the central control of energy balance.

Quantification of 3D Brain Microangioarchitectures in an Animal Model of Krabbe Disease

We performed a three-dimensional (3D) analysis of the microvascular network of the cerebral cortex of twitcher mice (an authentic model of Krabbe disease) using a restricted set of indexes that are able to describe the arrangement of the microvascular tree in CD31-stained sections. We obtained a near-linear graphical "fingerprint" of the microangioarchitecture of wild-type and twitcher animals that describes the amounts, spatial dispersion, and spatial relationships of adjacent classes of caliber-filtered microvessels. We observed significant alterations of the microangioarchitecture of the cerebral cortex of twitcher mice, whereas no alterations occur in renal microvessels, which is keeping with the observation that kidney is an organ that is not affected by the disease. This approach may represent an important starting point for the study of the microvascular changes that occur in the central nervous system (CNS) under different physiopathological conditions.

Extracellular and intracellular sphingosine-1-phosphate distinctly regulates exocytosis in chromaffin cells

Sphingosine-1-phosphate (S1P) is an essential bioactive sphingosine lipid involved in many neurological disorders. Sphingosine kinase 1 (SphK1), a key enzyme for S1P production, is concentrated in presynaptic terminals. However, the role of S1P/SphK1 signaling in exocytosis remains elusive. By detecting catecholamine release from single vesicles in chromaffin cells, we show that a dominant negative SphK1 (SphK1^DN ) reduces the number of amperometric spikes and increases the duration of foot, which reflects release through a fusion pore, implying critical roles for S1P in regulating the rate of exocytosis and fusion pore expansion. Similar phenotypes were observed in chromaffin cells obtained from SphK1 knockout mice compared to those from wild-type mice. In addition, extracellular S1P treatment increased the number of amperometric spikes, and this increase, in turn, was inhibited by a selective S1P3 receptor blocker, suggesting extracellular S1P may regulate the rate of exocytosis via activation of S1P3. Furthermore, intracellular S1P application induced a decrease in foot duration of amperometric spikes in control cells, indicating intracellular S1P may regulate fusion pore expansion during exocytosis. Taken together, our study represents the first demonstration that S1P regulates exocytosis through distinct mechanisms: extracellular S1P may modulate the rate of exocytosis via activation of S1P receptors while intracellular S1P may directly control fusion pore expansion during exocytosis. OPEN SCIENCE BADGES: This article has received a badge for *Open Materials* because it provided all relevant information to reproduce the study in the manuscript. The complete Open Science Disclosure form for this article can be found at the end of the article. More information about the Open Practices badges can be found at https://cos.io/our-services/open-science-badges/.

Activation of Sphingosine-1-Phosphate Receptor 1 in the Spinal Cord Produces Mechanohypersensitivity Through the Activation of Inflammasome and IL-1β Pathway

Sphingosine-1-phosphate (S1P) receptor 1 subtype (S1PR1) activation by its ligand S1P in the dorsal horn of the spinal cord causes mechanohypersensitivity. The cellular and molecular pathways remain poorly understood. We report that the activation of S1PR1 with an intrathecal injection of the highly selective S1PR1 agonist SEW2871 led to the development of mechanoallodynia by activating the nod-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome (increased expression of NLRP3, cleaved caspase 1 and mature IL-1β) in the dorsal horn of the spinal cord. The functional S1PR1 antagonist FTY720 blocked NLRP3 activation and IL-1β production. Moreover, inhibiting IL-10 signaling with an intrathecal injection of an anti-IL-10 antibody attenuated the beneficial effects exerted by FTY720. This finding suggests that disrupting S1PR1 signaling engages beneficial IL-10-dependent pathways. Notably, we found that mice with astrocyte-specific deletions of S1pr1 did not develop mechanoallodynia after intrathecal injection of SEW2871 and exhibited decreased levels of cleaved caspase 1, identifying astrocytes as a key cellular locus for S1PR1 activity. Our findings provide novel mechanistic insights on how S1PR1 activation in the spinal cord contributes to the development of nociception while identifying the cellular substrate for these activities. PERSPECTIVE: This study is the first to link the activation of NLRP3 and IL-1β signaling in the spinal cord and S1PR1 signaling in astrocytes to the development of S1PR1-evoked mechanoallodynia. These findings provide critical basic science insights to support the development of therapies targeted toward S1PR1.

Measurement of the Spatial Distribution of S1P in Small Quantities of Tissues: Development and Application of a Highly Sensitive LC-MS/MS Method Combined with Laser Microdissection

Sphingosine-1-phosphate (S1P) acts as an extracellular signaling molecule with diverse biological functions. Tissues appear to have an S1P gradient, which is functionally relevant in the biological significance of S1P, although its existence has not been measured directly. Here, we report a highly sensitive method to determine the distribution of S1P, using a column-switching LC-MS/MS system combined with laser microdissection (LMD). Column switching using narrow core Capcell Pak C18 analytical and trap columns with 0.3 mm inner diameter improved the performance of the LC-MS/MS system. The calibration curve of S1P showed good linearity (r>0.999) over the range of 0.05–10 nM (1–200 fmol/injection). The accuracy of the method was confirmed by measuring S1P-spiked laser microdissected mice tissue sections. To evaluate our S1P analytical method, we quantified S1P extracted from micro-dissected mouse brain and spleen. These results show that this method can measure low S1P concentrations and determine S1P distribution in tissue microenvironments.

Bioactive compounds from the seeds of Amomum tsaoko Crevost et Lemaire, a Chinese spice as inhibitors of sphingosine kinases, SPHK1/2

Amomum tsaoko Crevost et Lemaire (Zingiberaceae), a traditional Chinese spice also known as “Caoguo” or “tsao-ko,” has been considered to have many health benefits. As part of our continuous efforts to screen natural resources exhibiting potential bioactivity, we examined the seeds of A. tsaoko and found that its EtOH extract inhibited sphingosine kinases 1 and 2 (SPHK1/2). Bioactivity-based analysis and chemical investigation of the EtOH extract led to the isolation and identification of four aliphatic alcohols (1–4), five fatty acids (5–9), 12 phenolics (10–21), and four terpenoids (22–25), including four new compounds, an acetylated aliphatic alcohol (2), a fatty acid (5), and two phenolics (10–11). In addition, compound 1 was isolated for the first time from natural sources in this study. The structures of all compounds were elucidated based on spectroscopic analysis, including 1D and/or 2D NMR and HR-ESIMS as well as LC/MS analysis. A recently developed method using competing enantioselective acylation (CEA) coupled with LC/MS analysis was applied for the assignment of absolute configuration of compound 5. The absolute configurations of compounds 10 and 11 were determined using ECD calculations. All of the compounds (1–25) isolated from the active fraction were evaluated for their SPHK1/2 inhibitory effects at the concentration of 10 μM. Aliphatic alcohols 2–4, fatty acids 7 and 9, and phenolic compounds 13–15 and 21 showed inhibition against the activity of SPHK1 up to 20% and aliphatic alcohols 2 and 4, fatty acid 8, and phenolic compounds 10, 11, 18, and 22 showed inhibition against the activity of SPHK2 up to 40% compared with the control. Compound 2 showed the highest potency to inhibit SPHK1 enzymatic activity, by 59.75%, and compound 22 showed the highest potency in inhibiting the activity of SPHK2, by 22.75%, in comparison with the control, where both exhibited higher inhibition compared to those of positive controls. Docking modeling studies were conducted to suggest the binding mode of 2 and 22 in the substrate-binding pocket of SPHK1 and SPHK2, respectively.

New bioactive compounds were identified from the seeds of Amomum tsaoko Crevost et Lemaire, a Chinese spice as inhibitors of sphingosine kinases, SPHK1/2.

Conjugated Bile Acids Promote Invasive Growth of Esophageal Adenocarcinoma Cells and Cancer Stem Cell Expansion via Sphingosine 1-Phosphate Receptor 2-Mediated Yes-Associated Protein Activation

Esophageal adenocarcinoma (EAC) is the sixth leading cause of cancer deaths worldwide and has been dramatically increasing in incidence over the past decade. Gastroesophageal reflux and Barrett esophagus are well-established risk factors for disease progression. Conjugated bile acids (CBAs), including taurocholate (TCA), represent the major bile acids in the gastroesophageal refluxate of advanced Barrett esophagus and EAC patients. Our previous studies suggested that CBA-induced activation of sphingosine 1-phosphate receptor 2 (S1PR2) plays a critical role in promoting cholangiocarcinoma cell invasive growth. However, the role of CBAs in EAC development and underlying mechanisms remains elusive. In the current study, we identified that the expression level of S1PR2 is correlated to invasiveness of EAC cells. TCA significantly promoted cell proliferation, migration, invasion, transformation, and cancer stem cell expansion in highly invasive EAC cells (OE-33 cells), but had less effect on the lower invasive EAC cells (OE-19 cells). Pharmacologic inhibition of S1PR2 with specific antagonist JTE-013 or knockdown of S1PR2 expression significantly reduced TCA-induced invasive growth of OE-33 cells, whereas overexpression of S1PR2 sensitized OE-19 cells to TCA-induced invasive growth. Furthermore, TCA-induced activation of S1PR2 was closely associated with YAP and β-catenin signaling pathways. In conclusion, CBA-induced activation of the S1PR2 signaling pathway is critically involved in invasive growth of EAC cells and represents a novel therapeutic target for EAC.

Emerging evidence for the modulation of exocytosis by signalling lipids

Membrane fusion is a key event in exocytosis of neurotransmitters and hormones stored in intracellular vesicles. In this process, soluble N‐ethylmaleimide sensitive factor attachment protein receptor (SNARE) proteins are essential components of the exocytotic molecular machinery, while lipids have been seen traditionally as structural elements. However, the so‐called signalling lipids, such as sphingosine and arachidonic acid, interact with SNAREs and directly modulate the frequency and mode of fusion events. Interestingly, recent work has proved that the sphingosine analogue FTY‐720, used in the treatment of multiple sclerosis, mimics the effects of signalling lipids. In the present Review, we discuss recent investigations suggesting that endogenous signalling lipids and synthetic analogues can modulate important physiological aspects of secretion, such as quantal release, vesicle recruitment into active sites, vesicle transport and even organelle fusion in the cytosol. Therefore, these compounds are far from being merely structural components of cellular membranes.

Deconvolution of Human Brain Cell Type Transcriptomes Unraveled Microglia-Specific Potential Biomarkers

Microglial cells form a context-dependent network of brain immunoeffector cells. Despite their indispensable roles, unresolved questions exist around biomarker discovery relevant to their cellular localization, self-renewing potential, and brain developmental dynamics. To resolve the existent gap in the annotation of candidate biomarkers, we conducted a meta-analysis of brain cells using available high-throughput data sets for deciphering microglia-specific expression profiles. We have identified 3,290 significant genes specific to microglia and further selected the top 20 dysregulated genes on the basis of p-value and log₂FC. To this list, we added 7 known microglia-specific markers making the candidate list comprising 27 genes for further downstream analyses. Next, we established a connectome of these potential markers with their putative protein partners, which demonstrated strong associations of upregulated genes like Dedicator of cytokinesis 2 (DOCK2) with early/mature microglial markers such as Sphingosine kinase 1 (SPHK1), CD68, and CD45. To elucidate their respective brain anatomical location, we deconvoluted the BrainSpan Atlas expression data. This analysis showed high expression of the majority of candidate genes in microglia-dense regions (Amygdala, Hippocampus, Striatum) in the postnatal brain. Furthermore, to decipher their localized expression across brain ages, we constructed a developmental dynamics map (DDM) comprising extensive gene expression profiles throughout prenatal to postnatal stages, which resulted in the discovery of novel microglia-specific gene signatures. One of the interesting readout from DDM is that all the microglia-dense regions exhibit dynamic regulation of few genes at 37 post conception week (pcw), the transition period between pre- and postnatal stages. To validate these findings and correlate them as potential biomarkers, we analyzed the expression of corresponding proteins in hESC-derived human microglia precursors. The cultured microglial precursors showed expression of Pentraxin 3 (PTX3) and SPHK1 as well as several known markers like CD68, Allograft inflammatory factor 1 (AIF1/IBA1). In summary, this study has furnished critical insights into microglia dynamics across human brain ages and cataloged potential transcriptomic fingerprints that can be further exploited for designing novel neurotherapeutics.

Sphingolipids role in the regulation of inflammatory response: From leukocyte biology to bacterial infection

Sphingolipids (SLs) are amphiphilic molecules mainly associated with the external leaflet of eukaryotic plasma membrane, and are structural membrane components with key signaling properties. Since the beginning of the last century, a large number of papers described the involvement of these molecules in several aspects of cell physiology and pathology. Several lines of evidence support the critical role of SLs in inflammatory diseases, by acting as anti- or pro-inflammatory mediators. They are involved in control of leukocyte activation and migration, and are recognized as essential players in host response to pathogenic infection. We propose here a critical overview of current knowledge on involvement of different classes of SLs in inflammation, focusing on the role of simple and complex SLs in pathogen-mediated inflammatory response.

Crosstalk between TLR2 and Sphk1 in microglia in the cerebral ischemia/reperfusion-induced inflammatory response

Stroke is associated with high morbidity and mortality, and much remains unknown about the injury-related mechanisms that occur following reperfusion. This study aimed to explore the roles of Toll-like receptor 2 (TLR2) and sphingosine kinase 1 (Sphk1) in microglial cells in inflammatory responses induced by cerebral ischemia/reperfusion (I/R). For this purpose, C57BL/6 mice were randomly divided into 4 groups as follows: the sham-operated group, the I/R group, the I/R group treated with TLR2 antibody, and the I/R group treated with N,N-dimethylsphingosine. Focal cerebral I/R was induced by middle cerebral artery occlusion. Double-labeling immunofluorescence was used to observe the protein expression of TLR2 and Sphk1 in the ischemic brain tissue. Quantitative polymerase chain reaction was performed to determine the mRNA levels of TLR2 and Sphkl in ischemic brain tissue. Enzyme-linked immunosorbent assay was carried out to detect the protein contents of interleukin (IL)-1β, tumor necrosis factor-α (TNF-α), IL-17 and IL-23 in ischemic brain tissue. The results revealed that I/R upregulated TLR2 and Sphk1 expression in microglial cells, and the inhibition of either TLR2 or Sphk1 inhibited the expression of the pro-inflammatory cytokines, IL-1β, TNF-α, IL-17 and IL-23. Notably, the inhibition of TLR2 activity also decreased Sphk1 expression. These results thus indicate that the activation of microglial cells, via a TLR2→Sphk1→pro-inflammatory cytokine (IL-1β, TNF-α, IL-17 and IL-23) pathway, may participate in I/R injury.

Sepsis-Associated Encephalopathy: The Blood-Brain Barrier and the Sphingolipid Rheostat

Sepsis is not only a significant cause of mortality worldwide but has particularly devastating effects on the central nervous system of survivors. It is therefore crucial to understand the molecular structure, physiology, and events involved in the pathogenesis of sepsis-associated encephalopathy, so that potential therapeutic advances can be achieved. A key determinant to the development of this type of encephalopathy is morphological and functional modification of the blood–brain barrier (BBB), whose function is to protect the CNS from pathogens and toxic threats. Key mediators of pathologic sequelae of sepsis in the brain include cytokines, including TNF-α, and sphingolipids, which are biologically active components of cellular membranes that possess diverse functions. Emerging data demonstrated an essential role for sphingolipids in the pulmonary vascular endothelium. This raises the question of whether endothelial stability in other organs systems such as the CNS may also be mediated by sphingolipids and their receptors. In this review, we will model the structure and vulnerability of the BBB and hypothesize mechanisms for therapeutic stabilization and repair following a confrontation with sepsis-induced inflammation.

Sphingosine 1-phosphate receptor 3 and RhoA signaling mediate inflammatory gene expression in astrocytes

Background

Sphingosine 1-phosphate (S1P) signals through G protein-coupled receptors to elicit a wide range of cellular responses. In CNS injury and disease, the blood-brain barrier is compromised, causing leakage of S1P from blood into the brain. S1P can also be locally generated through the enzyme sphingosine kinase-1 (Sphk1). Our previous studies demonstrated that S1P activates inflammation in murine astrocytes. The S1P₁ receptor subtype has been most associated with CNS disease, particularly multiple sclerosis. S1P₃ is most highly expressed and upregulated on astrocytes, however, thus we explored the involvement of this receptor in inflammatory astrocytic responses.

Methods

Astrocytes isolated from wild-type (WT) or S1P₃ knockout (KO) mice were treated with S1P₃ selective drugs or transfected with short interfering RNA to determine which receptor subtypes mediate S1P-stimulated inflammatory responses. Interleukin-6 (IL-6), and vascular endothelial growth factor A (VEGFa) messenger RNA (mRNA) and cyclooxygenase-2 (COX-2) mRNA and protein were assessed by q-PCR and Western blotting. Activation of RhoA was measured using SRE.L luciferase and RhoA implicated in S1P signaling by knockdown of Gα_12/13 proteins or by inhibiting RhoA activation with C3 exoenzyme. Inflammation was simulated by in vitro scratch injury of cultured astrocytes.

Results

S1P₃ was highly expressed in astrocytes and further upregulated in response to simulated inflammation. Studies using S1P₃ knockdown and S1P₃ KO astrocytes demonstrated that S1P₃ mediates activation of RhoA and induction of COX-2, IL-6, and VEGFa mRNA, with some contribution from S1P₂. S1P induces expression of all of these genes through coupling to the Gα_12/13 proteins which activate RhoA. Studies using S1P₃ selective agonists/antagonists as well as Fingolimod (FTY720) confirmed that stimulation of S1P₃ induces COX-2 expression in astrocytes. Simulated inflammation increased expression of Sphk1 and consequently activated S1P₃, demonstrating an autocrine pathway through which S1P is formed and released from astrocytes to regulate COX-2 expression.

Conclusions

S1P₃, through its ability to activate RhoA and its upregulation in astrocytes, plays a unique role in inducing inflammatory responses and should be considered as a potentially important therapeutic target for CNS disease progression.

Intracranial Injection of Dengue Virus Induces Interferon Stimulated Genes and CD8+ T Cell Infiltration by Sphingosine Kinase 1 Independent Pathways

We have previously reported that the absence of sphingosine kinase 1 (SK1) affects both dengue virus (DENV) infection and innate immune responses in vitro. Here we aimed to define SK1-dependancy of DENV-induced disease and the associated innate responses in vivo. The lack of a reliable mouse model with a fully competent interferon response for DENV infection is a challenge, and here we use an experimental model of DENV infection in the brain of immunocompetent mice. Intracranial injection of DENV-2 into C57BL/6 mice induced body weight loss and neurological symptoms which was associated with a high level of DENV RNA in the brain. Body weight loss and DENV RNA level tended to be greater in SK1^-/- compared with wildtype (WT) mice. Brain infection with DENV-2 is associated with the induction of interferon-β (IFN-β) and IFN-stimulated gene (ISG) expression including viperin, Ifi27l2a, IRF7, and CXCL10 without any significant differences between WT and SK1^-/- mice. The SK2 and sphingosine-1-phosphate (S1P) levels in the brain were unchanged by DENV infection or the lack of SK1. Histological analysis demonstrated the presence of a cellular infiltrate in DENV-infected brain with a significant increase in mRNA for CD8 but not CD4 suggesting this infiltrate is likely CD8⁺ but not CD4⁺ T-lymphocytes. This increase in T-cell infiltration was not affected by the lack of SK1. Overall, DENV-infection in the brain induces IFN and T-cell responses but does not influence the SK/S1P axis. In contrast to our observations in vitro, SK1 has no major influence on these responses following DENV-infection in the mouse brain.

Sphingosine-1-phosphate, regulated by FSH and VEGF, stimulates granulosa cell proliferation

Sphingosine-1-phosphate (S1P) is a bioactive polar sphingolipid which stimulates proliferation, growth and survival in various cell types. In the ovary S1P has been shown protect the granulosa cells and oocytes from insults such as oxidative stress and radiotherapy, and S1P concentrations are greater in healthy than atretic large follicles. Hence, we postulate that S1P is fundamental in follicle development and that it is activated in ovarian granulosa cells in response to FSH and VEGF. To test this hypothesis we set out: i) to evaluate the effect of FSH and VEGF on S1P synthesis in cultured bovine granulosa cells and ii) to analyse the effect of S1P on proliferation and survival of bovine granulosa cells in vitro. Seventy five thousand bovine granulosa cells from healthy medium-sized (4-7mm) follicles were cultured in 96-well plates in McCoy's 5a medium containing 10ng/mL of insulin and 1ng/mL of LR-IGF-I at 37°C in a 5% CO2/air atmosphere at 37°C. Granulosa cell production of S1P was tested in response to treatment with FSH (0, 0.1, 1 and 10ng/mL) and VEGF (0, 0.01, 0.1, 1, 10 and 100ng/mL) and measured by HPLC. Granulosa cells produced S1P at 48 and 96h, with the maximum production observed with 1ng/mL of FSH. Likewise, 0.01ng/mL of VEGF stimulated S1P production at 48, but not 96h of culture. Further, the granulosa cell expression of sphingosine kinase-1 (SK1), responsible for S1P synthesis, was demonstrated by Western blot after 48h of culture. FSH increased the expression of phosphorylated SK1 (P<0.05) and the addition of a SK1 inhibitor reduced the constitutive and FSH-stimulated S1P synthesis (P<0.05). Sphingosine-1-phosphate had a biphasic effect on granulosa cell number after culture. At low concentration S1P (0.1μM) increased granulosa cell number after 48h of culture (P<0.05) and the proportion of cells in the G2 and M phase of the cell cycle (P<0.05), whereas higher concentrations decreased cell number (10μM; P<0.05) by an increase (P<0.05) in the proportion of cells in apoptosis (hypodiploid cells). In addition, treatment with SK-178 suppressed the FSH- and VEGF-stimulated rise of the granulosa cells number (P<0.05). Interestingly, the effect of 0.1μM S1P on granulosa cell number and their proportion in G2/M phases is similar to that observed with 1ng/mL FSH. The results of this study are the first to demonstrate sphingosine-1-phosphate (S1P) synthesis in granulosa cells under the control of FSH and VEGF. The later achieved through the regulation of sphingosine kinase 1 expression. This S1P augments the proportion of cells in the G2/M phase of the cell cycle that translates in increased granulosa cell proliferation.

Sphingosine kinase inhibition ameliorates chronic hypoperfusion-induced white matter lesions

White matter lesions (WML) are thought to contribute to vascular cognitive impairment in elderly patients. Growing evidence show that failure of myelin formation arising from the disruption of oligodendrocyte progenitor cell (OPC) differentiation is a cause of chronic vascular white matter damage. The sphingosine kinase (SphK)/sphingosine-1-phosphate (S1P) signaling pathway regulates oligodendroglia differentiation and function, and is known to be altered in hypoxia. In this study, we measured SphK, S1P as well as markers of WML, hypoxia and OPC (NG2) in a mouse bilateral carotid artery stenosis (BCAS) model of chronic cerebral hypoperfusion. Our results indicated that BCAS induced hypoxia inducible factor (HIF)-1α, Sphk2, S1P, and NG2 up-regulation together with accumulation of WML. In contrast, BCAS mice treated with the SphK inhibitor, SKI-II, showed partial reversal of SphK2, S1P and NG2 elevation and amelioration of WML. In an in vitro model of hypoxia, SKI-II reversed the suppression of OPC differentiation. Our study suggests a mechanism for hypoperfusion-associated WML involving HIF-1α-SphK2-S1P-mediated disruption of OPC differentiation, and proposes the SphK signaling pathway as a potential therapeutic target for white matter disease.

Rapid and profound rewiring of brain lipid signaling networks by acute diacylglycerol lipase inhibition

Diacylglycerol lipases (DAGLα and DAGLβ) convert diacylglycerol to the endocannabinoid 2-arachidonoylglycerol. Our understanding of DAGL function has been hindered by a lack of chemical probes that can perturb these enzymes in vivo. Here, we report a set of centrally active DAGL inhibitors and a structurally related control probe and their use, in combination with chemical proteomics and lipidomics, to determine the impact of acute DAGL blockade on brain lipid networks in mice. Within 2 h, DAGL inhibition produced a striking reorganization of bioactive lipids, including elevations in DAGs and reductions in endocannabinoids and eicosanoids. We also found that DAGLα is a short half-life protein, and the inactivation of DAGLs disrupts cannabinoid receptor-dependent synaptic plasticity and impairs neuroinflammatory responses, including lipopolysaccharide-induced anapyrexia. These findings illuminate the highly interconnected and dynamic nature of lipid signaling pathways in the brain and the central role that DAGL enzymes play in regulating this network.

Sphingosine kinase 2-deficiency mediated changes in spinal pain processing

Chronic pain is one of the most burdensome health issues facing the planet (as costly as diabetes and cancer combined), and in desperate need for new diagnostic targets leading to better therapies. The bioactive lipid sphingosine 1-phosphate (S1P) and its receptors have recently been shown to modulate nociceptive signaling at the level of peripheral nociceptors and central neurons. However, the exact role of S1P generating enzymes, in particular sphingosine kinase 2 (Sphk2), in nociception remains unknown. We found that both sphingosine kinases, Sphk1 and Sphk2, were expressed in spinal cord (SC) with higher levels of Sphk2 mRNA compared to Sphk1. All three Sphk2 mRNA-isoforms were present with the Sphk2.1 mRNA showing the highest relative expression. Mice deficient in Sphk2 (Sphk2^−/−) showed in contrast to mice deficient in Sphk1 (Sphk1^−/−) substantially lower spinal S1P levels compared to wild-type C57BL/6 mice. In the formalin model of acute peripheral inflammatory pain, Sphk2^−/− mice showed facilitation of nociceptive transmission during the late response, whereas responses to early acute pain, and the number of c-Fos immunoreactive dorsal horn neurons were not different between Sphk2^−/− and wild-type mice. Chronic peripheral inflammation (CPI) caused a bilateral increase in mechanical sensitivity in Sphk2^−/− mice. Additionally, CPI increased the relative mRNA expression of P₂X₄ receptor, brain-derived neurotrophic factor and inducible nitric oxide synthase in the ipsilateral SC of wild-type but not Sphk2^−/− mice. Similarly, Sphk2^−/− mice showed in contrast to wild-type no CPI-dependent increase in areas of the dorsal horn immunoreactive for the microglia marker Iba-1 and the astrocyte marker Glial fibrillary acidic protein (GFAP). Our results suggest that the tightly regulated cell signaling enzyme Sphk2 may be a key component for facilitation of nociceptive circuits in the CNS leading to central sensitization and pain memory formation.

Subcellular distribution of FTY720 and FTY720-phosphate in immune cells – another aspect of Fingolimod action relevant for therapeutic application

FTY720 (Fingolimod; Gilenya®) is an immune-modulatory prodrug which, after intracellular phosphorylation by sphingosine kinase 2 (SphK2) and export, mimics effects of the endogenous lipid mediator sphingosine-1-phosphate. Fingolimod has been introduced to treat relapsing-remitting multiple sclerosis. However, little has been published about the immune cell membrane penetration and subcellular distribution of FTY720 and FTY720-P. Thus, we applied a newly established LC-MS/MS method to analyze the subcellular distribution of FTY720 and FTY720-P in subcellular compartments of spleen cells of wild type, SphK1- and SphK2-deficient mice. These studies demonstrated that, when normalized to the original cell volume and calculated on molar basis, FTY720 and FTY720-P dramatically accumulated several hundredfold within immune cells reaching micromolar concentrations. The amount and distribution of FTY720 was differentially affected by SphK1- and SphK2-deficiency. On the background of recently described relevant intracellular FTY720 effects in the nanomolar range and the prolonged application in multiple sclerosis, this data showing a substantial intracellular accumulation of FTY720, has to be considered for benefit/risk ratio estimates.

Sphingosine kinase 1 mediates neuroinflammation following cerebral ischemia

Sphingosine kinases (Sphks) are the rate-limiting kinases in the generation of sphingosine-1-phosphate, which is a well-established intracellular pro-survival lipid mediator. Sphk2 has been reported to be protective following experimental stroke. We investigated the role of Sphk1 in cerebral ischemia using a mouse middle cerebral artery occlusion (MCAO) model and an in vitro glucose-oxygen deprivation (OGD) model. Sphk expression and activity were assessed in the ischemic brain with quantitative PCR (qPCR), Western blot, immunohistochemistry and enzyme-linked immunosorbent assay (ELISA). Pharmacological and gene knockdown approaches were utilized to investigate the effects of Sphk1 on stroke outcomes. The expression of Sphk1 but not that of Sphk2 was rapidly induced in the cortical penumbra over 96h after MCAO, and the microglia were one of the major cellular sources of Sphk1 induction. Consistently, Sphk activity was enhanced in the cortical penumbra. In contrast to the protective role of Sphk2, pharmacological inhibition and cortical knockdown of Sphk1 reduced infarction at 24 and 96h after reperfusion. Additionally, the Sphk1 inhibitor improved the neurological deficits at 96h after reperfusion. Mechanistically, Sphk1 inhibition and knockdown significantly attenuated MCAO-induced expression of inflammatory mediators in the cortical penumbra. Moreover, using a conditioned medium transfer approach, we demonstrated that OGD-treated neurons induced the expression of Sphk1 and pro-inflammatory mediators in primary microglia, and the microglial induction of pro-inflammatory mediators by ischemic neurons was blunted by Sphk1 inhibition. Taken together, our results indicate that Sphk1 plays an essential role in mediating post-stroke neuroinflammation.

The relationship between opioids and immune signalling in the spinal cord

Opioids are considered the gold standard for the treatment of moderate to severe pain. However, heterogeneity in analgesic efficacy, poor potency and side effects are associated with opioid use, resulting in dose limitations and suboptimal pain management. Traditionally thought to exhibit their analgesic actions via the activation of the neuronal G-protein-coupled opioid receptors, it is now widely accepted that neuronal activity of opioids cannot fully explain the initiation and maintenance of opioid tolerance, hyperalgesia and allodynia. In this review we will highlight the evidence supporting the role of non-neuronal mechanisms in opioid signalling, paying particular attention to the relationship of opioids and immune signalling.