Involvement of sphingosine-1-phosphate in glutamate secretion in hippocampal neurons

Dysregulation of sphingolipid metabolism in pain

Pain is a clinical condition that is currently of great concern and is often caused by tissue or nerve damage or occurs as a concomitant symptom of a variety of diseases such as cancer. Severe pain seriously affects the functional status of the body. However, existing pain management programs are not fully satisfactory. Therefore, there is a need to delve deeper into the pathological mechanisms underlying pain generation and to find new targets for drug therapy. Sphingolipids (SLs), as a major component of the bilayer structure of eukaryotic cell membranes, also have powerful signal transduction functions. Sphingolipids are abundant, and their intracellular metabolism constitutes a huge network. Sphingolipids and their various metabolites play significant roles in cell proliferation, differentiation, apoptosis, etc., and have powerful biological activities. The molecules related to sphingolipid metabolism, mainly the core molecule ceramide and the downstream metabolism molecule sphingosine-1-phosphate (S1P), are involved in the specific mechanisms of neurological disorders as well as the onset and progression of various types of pain, and are closely related to a variety of pain-related diseases. Therefore, sphingolipid metabolism can be the focus of research on pain regulation and provide new drug targets and ideas for pain.

Extracellular α-synuclein impairs sphingosine 1-phosphate receptor type 3 (S1PR3)-regulated lysosomal delivery of cathepsin D in HeLa cells

α-Synuclein (α-Syn)-positive intracellular fibrillar protein deposits, known as Lewy bodies, are thought to be involved in the pathogenesis of Parkinson's disease (PD). Although recent lines of evidence suggested that extracellular α-Syn secreted from pathogenic neurons contributes to the propagation of PD pathology, the precise mechanism of action remains unclear. We have reported that extracellular α-Syn caused sphingosine 1-phosphate (S1P) receptor type 1 (S1PR1) uncoupled from Gi and inhibited downstream G-protein signaling in SH-SY5Y cells, although its patho/physiological role remains to be clarified. Here we show that extracellular α-Syn caused S1P receptor type 3 (S1PR3) uncoupled from G protein in HeLa cells. Further studies indicated that α-Syn treatment reduced cathepsin D activity while enhancing the secretion of immature pro-cathepsin D into cell culture medium, suggesting that lysosomal delivery of cathepsin D was disturbed. Actually, extracellular α-Syn attenuated the retrograde trafficking of insulin-like growth factor-II/mannose 6-phosphate (IGF-II/M6P) receptor, which is under the regulation of S1PR3. These findings shed light on the understanding of dissemination of the PD pathology, that is, the mechanism underlying how extracellular α-Syn secreted from pathogenic cells causes lysosomal dysfunction of the neighboring healthy cells, leading to propagation of the disease.

Siponimod Attenuates Neuronal Cell Death Triggered by Neuroinflammation via NFκB and Mitochondrial Pathways

Multiple sclerosis (MS) is the most common autoimmune demyelinating disease of the central nervous system (CNS), consisting of heterogeneous clinical courses varying from relapsing-remitting MS (RRMS), in which disability is linked to bouts of inflammation, to progressive disease such as primary progressive MS (PPMS) and secondary progressive MS (SPMS), in which neurological disability is thought to be linked to neurodegeneration. As a result, successful therapeutics for progressive MS likely need to have both anti-inflammatory and direct neuroprotective properties. The modulation of sphingosine-1-phosphate (S1P) receptors has been implicated in neuroprotection in preclinical animal models. Siponimod/BAF312, the first oral treatment approved for SPMS, may have direct neuroprotective benefits mediated by its activity as a selective (S1P receptor 1) S1P1 and (S1P receptor 5) S1P5 modulator. We showed that S1P1 was mainly present in cortical neurons in lesioned areas of the MS brain. To gain a better understanding of the neuroprotective effects of siponimod in MS, we used both rat neurons and human-induced pluripotent stem cell (iPSC)-derived neurons treated with the neuroinflammatory cytokine tumor necrosis factor-alpha (TNF-α). Cell survival/apoptotic assays using flow cytometry and IncuCyte live cell analyses showed that siponimod decreased TNF-α induced neuronal cell apoptosis in both rat and human iPSCs. Importantly, a transcriptomic analysis revealed that mitochondrial oxidative phosphorylation, NFκB and cytokine signaling pathways contributed to siponimod's neuroprotective effects. Our data suggest that the neuroprotection of siponimod/BAF312 likely involves the relief of oxidative stress in neuronal cells. Further studies are needed to explore the molecular mechanisms of such interactions to determine the relationship between mitochondrial dysfunction and neuroinflammation/neurodegeneration.

Presynaptic Release-Regulating Sphingosine 1-Phosphate 1/3 Receptors in Cortical Glutamatergic Terminals: Adaptations in EAE Mice and Impact of Therapeutic FTY720

This study provides evidence of the existence of presynaptic inhibitory sphingosine-1-phosphate receptor 1 (S1P1R) and facilitatory S1P3R in cortical nerve endings (synaptosomes) of healthy mice. The conclusion relies on the findings that (i) the S1P1R agonist CS-2100 (0.1-30 nM) inhibits the 12 mM KCl-evoked glutamate exocytosis (quantified as the release of [3H]D-aspartate) while the S1P3R allosteric agonist CYM-5541 potentiates it and (ii) these effects are inhibited by the S1P1R antagonist Ex 26 (30-300 nM) and the S1P3R antagonist TY-52156 (100-1000 nM), respectively. Confocal microscopy and western blot analysis confirmed the presence of S1P1R and S1P3R proteins in cortical glutamatergic synaptosomes, which were scarcely accessible to biotin in a biotinylation study. Then, we demonstrated that S1P1R and S1P3R densities and their release activity are amplified in cortical synaptosomes of mice suffering from experimental autoimmune encephalomyelitis (EAE), despite receptors maintain their preferential internal distribution. Receptor changes recover following chronic oral therapeutic FTY720 (0.03 mg/Kg/day). These results improve our knowledge of the role of presynaptic release-regulating S1P1Rs and S1P3Rs controlling glutamate transmission in the CNS also unravelling functional adaptations during EAE that recover following chronic FTY720. In a whole, these findings provide new information on the central neuroprotectant activities of FTY720.

A novel co-target of ACY1 governing plasma membrane translocation of SphK1 contributes to inflammatory and neuropathic pain

Previous studies validate that inhibiting sodium channel 1.8 (Nav1.8) effectively relieves inflammatory and neuropathic pain. However, Nav1.8 blockers have cardiac side effects in addition to analgesic effects. Here, we constructed a spinal differential protein expression profile using Nav1.8 knockout mice to screen common downstream proteins of Nav1.8 in inflammatory and neuropathic pain. We found that aminoacylase 1 (ACY1) expression was increased in wild-type mice compared to Nav1.8 knockout mice in both pain models. Moreover, spinal ACY1 overexpression induced mechanical allodynia in naive mice, while ACY1 suppression alleviated inflammatory and neuropathic pain. Further, ACY1 could interact with sphingosine kinase 1 and promote its membrane translocation, resulting in sphingosine-1-phosphate upregulation and the activation of glutamatergic neurons and astrocytes. In conclusion, ACY1 acts as a common downstream effector protein of Nav1.8 in inflammatory and neuropathic pain and could be a new and precise therapeutic target for chronic pain.

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.

Visualizing Sphingosine-1-Phosphate Receptor 1(S1P1) Signaling During Central Nervous System De- and Remyelination

Multiple sclerosis (MS) is an inflammatory-demyelinating disease of the central nervous system (CNS) mediated by aberrant auto-reactive immune responses. The current immune-modulatory therapies are unable to protect and repair immune-mediated neural tissue damage. One of the therapeutic targets in MS is the sphingosine-1-phosphate (S1P) pathway which signals via sphingosine-1-phosphate receptors 1-5 (S1P1-5). S1P receptors are expressed predominantly on immune and CNS cells. Considering the potential neuroprotective properties of S1P signaling, we utilized S1P1-GFP (Green fluorescent protein) reporter mice in the cuprizone-induced demyelination model to investigate in vivo S1P - S1P1 signaling in the CNS. We observed S1P1 signaling in a subset of neural stem cells in the subventricular zone (SVZ) during demyelination. During remyelination, S1P1 signaling is expressed in oligodendrocyte progenitor cells in the SVZ and mature oligodendrocytes in the medial corpus callosum (MCC). In the cuprizone model, we did not observe S1P1 signaling in neurons and astrocytes. We also observed β-arrestin-dependent S1P1 signaling in lymphocytes during demyelination and CNS inflammation. Our findings reveal β-arrestin-dependent S1P1 signaling in oligodendrocyte lineage cells implying a role of S1P1 signaling in remyelination.

Atorvastatin Improves the Propionic Acid-Induced Autism in Rats: The Roles of Sphingosine-1-Phosphate and Anti-inflammatory Action

Purpose The aim of this study is to investigate the benefits of atorvastatin on the propionic acid-induced autism model via increasing sphingosine-1-phosphate and anti-inflammatory actions with imaging and brain tissue investigations. Materials and methods Twenty-five mg/kg/day/rat of propionic acid (PPA) was administered intraperitoneally to 20 male Wistar rats, and 10 male Wistar rats were fed orally. Study groups were designed as follows: Group 1: Control Group (orally fed control, n=10); Group 2 (PPA+saline, n=10); Group 3 (PPA+Atorvastatin, n=10). The brain biochemical and histopathology assessments and magnetic resonance (MR) imaging were conducted across groups in order to compare them. Results The PPA+Atorvastatin group was found to have significantly lower levels of brain malondialdehyde, IL-2 level, IL-17, tumor necrosis factor-alpha (TNF-α), and lactate compared to the PPA+saline group. The PPA+Atorvastatin group had higher levels of nerve growth factor and nuclear factor erythroid 2-related factor 2 (NRF-2) and sphingosine-1-phosphate. In histopathology assessments, the PPA+Atorvastatin group was found to have significantly higher neuronal counts of CA1 and CA2 in the hippocampus, and Purkinje cells in the cerebellum. Conclusions Current findings suggest that atorvastatin increases sphingosine-1-phosphate levels and decreases inflammatory actions which characterize the autism rodent model implemented in this study. These preliminary results have to be confirmed by further experimental and clinical studies.

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.

Lipids and Secretory Vesicle Exocytosis

In recent years, the number of studies implicating lipids in the regulation of synaptic vesicle exocytosis has risen considerably. It has become increasingly clear that lipids such as phosphoinositides, lysophospholipids, cholesterol, arachidonic acid and myristic acid play critical regulatory roles in the processes leading up to exocytosis. Lipids may affect membrane fusion reactions by altering the physical properties of the membrane, recruiting key regulatory proteins, concentrating proteins into exocytic "hotspots" or by modulating protein functions allosterically. Discrete changes in phosphoinositides concentration are involved in multiple trafficking events including exocytosis and endocytosis. Lipid-modifying enzymes such as the DDHD2 isoform of phospholipase A1 were recently shown to contribute to memory acquisition via dynamic modifications of the brain lipid landscape. Considering the increasing reports on neurodegenerative disorders associated with aberrant intracellular trafficking, an improved understanding of the control of lipid pathways is physiologically and clinically significant and will afford unique insights into mechanisms and therapeutic methods for neurodegenerative diseases. Consequently, this chapter will discuss the different classes of lipids, phospholipase enzymes, the evidence linking them to synaptic neurotransmitter release and how they act to regulate key steps in the multi-step process leading to neuronal communication and memory acquisition.

O-Cyclic Phytosphingosine-1-Phosphate Protects against Motor Dysfunctions and Glial Cell Mediated Neuroinflammation in the Parkinson's Disease Mouse Models

O-cyclic phytosphingosine-1-phosphate (cPS1P) is a novel and chemically synthesized sphingosine metabolite derived from phytosphingosine-1-phosphate (S1P). This study was undertaken to unveil the potential neuroprotective effects of cPS1P on two different mouse models of Parkinson's disease (PD). The study used 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and neuron specific enolase promoter human alpha-synuclein (NSE-hαSyn) Korl transgenic mice. MPTP was injected for five consecutive days and cPS1P was injected for alternate days for six weeks intraperitoneally. We performed behavioral tests and analyzed the immunohistochemistry and immunofluorescence staining in the substantia nigra pars compacta (SNpc) and the striatum. The behavior tests showed a significant reduction in the motor functions in the PD models, which was reversed with the administration of cPS1P. In addition, both PD-models showed reduced expression of the sphingosine-1-phosphate receptor 1 (S1PR1), and α-Syn which was restored with cPS1P treatment. In addition, administration of cPS1P restored dopamine-related proteins such as tyrosine hydroxylase (TH), vesicular monoamine transporter 2 (VMAT2), and dopamine transporter (DAT). Lastly, neuroinflammatory related markers such as glial fibrillary acidic protein (GFAP), ionized calcium-binding adapter protein-1 (Iba-1), c-Jun N-terminal kinases (JNK), nuclear factor kappa-light-chain-enhancer of activated B cells (NF-kB), tumor necrosis factor-alpha (TNF-α), and interleukin 1 beta (IL-1β) were all reduced after cPS1P administration. The overall findings supported the notion that cPS1P protects against dopamine depletion, neuroinflammation, and PD-associated symptoms.

Sphingosine 1-Phoshpate Receptors are Located in Synapses and Control Spontaneous Activity of Mouse Neurons in Culture

Sphingosine-1-phosphate (S1P) is best known for its roles as vascular and immune regulator. Besides, it is also present in the central nervous system (CNS) where it can act as neuromodulator via five S1P receptors (S1PRs), and thus control neurotransmitter release. The distribution of S1PRs in the active zone and postsynaptic density of CNS synapses remains unknown. In the current study, we investigated the localization of S1PR1-5 in synapses of the mouse cortex. Cortical nerve terminals purified in a sucrose gradient were endowed with all five S1PRs. Further subcellular fractionation of cortical nerve terminals revealed S1PR2 and S1PR4 immunoreactivity in the active zone of presynaptic nerve terminals. Interestingly, only S1PR2 and S1PR3 immunoreactivity was found in the postsynaptic density. All receptors were present outside the active zone of nerve terminals. Neurons in the mouse cortex and primary neurons in culture showed immunoreactivity against all five S1PRs, and Ca²⁺ imaging revealed that S1P inhibits spontaneous neuronal activity in a dose-dependent fashion. When testing selective agonists for each of the receptors, we found that only S1PR1, S1PR2 and S1PR4 control spontaneous neuronal activity. We conclude that S1PR2 and S1PR4 are located in the active zone of nerve terminals and inhibit neuronal activity. Future studies need to test whether these receptors modulate stimulation-induced neurotransmitter release.

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.

Constitutive activation of S1P receptors at the trans-Golgi network is required for surface transport carrier formation

The importance of the G-protein βγ subunits in the regulation of cargo transport from the trans-Golgi network (TGN) to the plasma membrane (PM) is well accepted; however, the molecular mechanism underlying the G-protein activation at the TGN remains unclear. We show here that sphingosine 1-phosphate (S1P) receptors at the PM were trafficked to the TGN in response to a surface transport cargo, temperature-sensitive vesicular stomatitis virus glycoprotein tagged with green fluorescent protein accumulation in the Golgi. The receptor internalization occurred in an S1P-independent manner but required phosphorylation by G-protein receptor kinase 2 and β-arrestin association before internalization. Continuously activated S1P receptors in a manner dependent on S1P at the TGN kept transmitting G-protein signals including the βγ subunits supply necessary for transport carrier formation at the TGN destined for the PM.

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S1P receptors traffic from the PM to Golgi in a surface cargo-dependent manner

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S1PR trafficking follows GRK2-dependent phosphorylation and β-arrestin binding

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S1PRs at the Golgi are continuously activated by S1P while sending G-protein signals

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S1PR/Gβγ signals at the Golgi are indispensable for surface transport carrier formation

Lessons from S1P receptor targeting in multiple sclerosis

Sphingosine 1-phosphate (S1P) is a potent bioactive sphingolipid binding to specific G protein-coupled receptors expressed in several organs. The relevance of S1P-S1P receptor axis in the pathophysiology of immune and nervous systems has encouraged the development of S1P receptor modulators for the treatment of neurological, autoimmune and/or inflammatory disorders. Currently, four S1P receptor modulators are approved drugs for multiple sclerosis (MS), an inflammatory disorder of the central nervous system. As main pharmacologic effect, these treatments induce lymphopenia due to the loss of responsiveness to S1P gradients guiding lymphocyte egress from lymphoid organs into the bloodstream. Recent data point to immunological effects of the S1P modulators beyond the inhibition of lymphocyte trafficking. Further, these drugs may cross the blood-brain barrier and directly target CNS resident cells expressing S1P receptors. Here we review the role of S1P signalling in neuroimmunology at the light of the evidences generated from the study of the mechanism of action of S1P receptor modulators in MS and integrate this information with findings derived from neuroinflammatory animal models and in vitro observations. These insights can direct the application of therapeutic approaches targeting S1P receptors in other disease areas.

Preclinical and Clinical Evidence for the Involvement of Sphingosine 1-Phosphate Signaling in the Pathophysiology of Vascular Cognitive Impairment

Sphingosine 1-phosphates (S1Ps) are bioactive lipids that mediate a diverse range of effects through the activation of cognate receptors, S1P₁-S1P₅. Scrutiny of S1P-regulated pathways over the past three decades has identified important and occasionally counteracting functions in the brain and cerebrovascular system. For example, while S1P₁ and S1P₃ mediate proinflammatory effects on glial cells and directly promote endothelial cell barrier integrity, S1P₂ is anti-inflammatory but disrupts barrier integrity. Cumulatively, there is significant preclinical evidence implicating critical roles for this pathway in regulating processes that drive cerebrovascular disease and vascular dementia, both being part of the continuum of vascular cognitive impairment (VCI). This is supported by clinical studies that have identified correlations between alterations of S1P and cognitive deficits. We review studies which proposed and evaluated potential mechanisms by which such alterations contribute to pathological S1P signaling that leads to VCI-associated chronic neuroinflammation and neurodegeneration. Notably, S1P receptors have divergent but overlapping expression patterns and demonstrate complex interactions. Therefore, the net effect produced by S1P represents the cumulative contributions of S1P receptors acting additively, synergistically, or antagonistically on the neural, vascular, and immune cells of the brain. Ultimately, an optimized therapeutic strategy that targets S1P signaling will have to consider these complex interactions.

Integrative Analysis of Metabolomic and Transcriptomic Profiles Uncovers Biological Pathways of Feed Efficiency in Pigs

Feed efficiency (FE) is an economically important trait. Thus, reliable predictors would help to reduce the production cost and provide sustainability to the pig industry. We carried out metabolome-transcriptome integration analysis on 40 purebred Duroc and Landrace uncastrated male pigs to identify potential gene-metabolite interactions and explore the molecular mechanisms underlying FE. To this end, we applied untargeted metabolomics and RNA-seq approaches to the same animals. After data quality control, we used a linear model approach to integrate the data and find significant differently correlated gene-metabolite pairs separately for the breeds (Duroc and Landrace) and FE groups (low and high FE) followed by a pathway over-representation analysis. We identified 21 and 12 significant gene-metabolite pairs for each group. The valine-leucine-isoleucine biosynthesis/degradation and arginine-proline metabolism pathways were associated with unique metabolites. The unique genes obtained from significant metabolite-gene pairs were associated with sphingolipid catabolism, multicellular organismal process, cGMP, and purine metabolic processes. While some of the genes and metabolites identified were known for their association with FE, others are novel and provide new avenues for further research. Further validation of genes, metabolites, and gene-metabolite interactions in larger cohorts will elucidate the regulatory mechanisms and pathways underlying FE.

The ceramide-S1P pathway as a druggable target to alleviate peripheral neuropathic pain

Introduction: Neuropathic pain disorders are diverse, and the currently available therapies are ineffective in the majority of cases. Therefore, there is a major need for gaining novel mechanistic insights and developing new treatment strategies for neuropathic pain. Areas covered: We performed an in-depth literature search on the molecular mechanisms and systemic importance of the ceramide-to-S1P rheostat regulating neuron function and neuroimmune interactions in the development of neuropathic pain. Expert opinion: The S1P receptor modulator FTY720 (fingolimod, Gilenya®), LPA receptor antagonists and several mechanistically related compounds in clinical development raise great expectations for treating neuropathic pain disorders. Research on S1P receptors, S1P receptor modulators or SPHK inhibitors with distinct selectivity, pharmacokinetics and safety must provide more mechanistic insight into whether they may qualify as useful treatment options for neuropathic pain disorders. The functional relevance of genetic variations within the ceramide-to-S1P rheostat should be explored for an enhanced understanding of neuropathic pain pathogenesis. The ceramide-to-S1P rheostat is emerging as a critically important regulator hub of neuroimmune interactions along the pain pathway, and improved mechanistic insight is required to develop more precise and effective drug treatment options for patients suffering from neuropathic pain disorders.

Lysophospholipids and Their G-Coupled Protein Signaling in Alzheimer's Disease: From Physiological Performance to Pathological Impairment

Lysophospholipids (LPLs) are bioactive signaling lipids that are generated from phospholipase-mediated hydrolyzation of membrane phospholipids (PLs) and sphingolipids (SLs). Lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are two of the best-characterized LPLs which mediate a variety of cellular physiological responses via specific G-protein coupled receptor (GPCR) mediated signaling pathways. Considerable evidence now demonstrates the crucial role of LPA and S1P in neurodegenerative diseases, especially in Alzheimer’s disease (AD). Dysfunction of LPA and S1P metabolism can lead to aberrant accumulation of amyloid-β (Aβ) peptides, the formation of neurofibrillary tangles (NFTs), neuroinflammation and ultimately neuronal death. Summarizing LPA and S1P signaling profile may aid in profound health and pathological processes. In the current review, we will introduce the metabolism as well as the physiological roles of LPA and S1P in maintaining the normal functions of the nervous system. Given these pivotal functions, we will further discuss the role of dysregulation of LPA and S1P in promoting AD pathogenesis.

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.

The Sphkl/SlP pathway regulates angiogenesis via NOS/NO synthesis following cerebral ischemia-reperfusion

Aims

Sphingosine kinase 1 (Sphk1) and the signaling molecule sphingosine‐1‐phosphate (S1P) are known to be key regulators of a variety of important biological processes, such as neovascularization. Nitric oxide (NO) is also known to play a role in vasoactive properties, whether Sphk1/S1P signaling is able to alter angiogenesis in the context of cerebral ischemia‐reperfusion injury (IRI), and whether such activity is linked with NO production, however, remains uncertain.

Methods

We used immunofluorescence to detect the expression of Sphk1 and NOS in cerebral epithelial cells (EC) after IR or oxygen‐glucose deprivation (OGDR). Western blotting was used to detect the Sphk1 and NOS protein levels in brain tissues or HBMECs. Adenovirus transfection was used to inhibit Sphk1 and NOS. An NO kit was used to detect NO contents in brain tissues and epithelial cells. Tube formation assays were conducted to measure angiogenesis.

Results

We determined that EC used in a model of cerebral IRI expressed Sphk1, and that inhibiting this expression led to decreased expression of two isoforms of NO synthase (eNOS and iNOS), as well as to decrease neovascularization density and NO production following injury. In HBMECs, knocking down Sphk1 markedly reduced NO production owing to reduced eNOS activity, and inhibiting eNOS directly similarly decreased NO production in a manner which could be reversed via exogenously treating cells with S1P. We further found that knocking down Sphk1 reduced HBMEC eNOS expression, in addition to decreasing the adhesion, migration, and tube formation abilities of these cells under OGDR conditions.

Conclusions

Based on these results, we therefore postulate that Sphk1/S1P signaling is able to mediate angiogenesis following cerebral IRI via the regulation of eNOS activity and NO production. As such, targeting these pathways may potentially represent a novel means of improving patient prognosis in those suffering from cerebral IRI.

Sphingosine Kinase 2 Potentiates Amyloid Deposition but Protects against Hippocampal Volume Loss and Demyelination in a Mouse Model of Alzheimer's Disease

Sphingosine 1-phosphate (S1P) is a potent vasculoprotective and neuroprotective signaling lipid, synthesized primarily by sphingosine kinase 2 (SK2) in the brain. We have reported pronounced loss of S1P and SK2 activity early in Alzheimer's disease (AD) pathogenesis, and an inverse correlation between hippocampal S1P levels and age in females, leading us to speculate that loss of S1P is a sensitizing influence for AD. Paradoxically, SK2 was reported to mediate amyloid β (Aβ) formation from amyloid precursor protein (APP) in vitro To determine whether loss of S1P sensitizes to Aβ-mediated neurodegeneration, we investigated whether SK2 deficiency worsens pathology and memory in male J20 (PDGFB-APP_SwInd) mice. SK2 deficiency greatly reduced Aβ content in J20 mice, associated with significant improvements in epileptiform activity and cross-frequency coupling measured by hippocampal electroencephalography. However, several key measures of APP_SwInd-dependent neurodegeneration were enhanced on the SK2-null background, despite reduced Aβ burden. These included hippocampal volume loss, oligodendrocyte attrition and myelin loss, and impaired performance in Y-maze and social novelty memory tests. Inhibition of the endosomal cholesterol exporter NPC1 greatly reduced sphingosine phosphorylation in glial cells, linking loss of SK2 activity and S1P in AD to perturbed endosomal lipid metabolism. Our findings establish SK2 as an important endogenous regulator of both APP processing to Aβ, and oligodendrocyte survival, in vivo These results urge greater consideration of the roles played by oligodendrocyte dysfunction and altered membrane lipid metabolic flux as drivers of neurodegeneration in AD.SIGNIFICANCE STATEMENT Genetic, neuropathological, and functional studies implicate both Aβ and altered lipid metabolism and/or signaling as key pathogenic drivers of Alzheimer's disease. In this study, we first demonstrate that the enzyme SK2, which generates the signaling lipid S1P, is required for Aβ formation from APP in vivo Second, we establish a new role for SK2 in the protection of oligodendrocytes and myelin. Loss of SK2 sensitizes to Aβ-mediated neurodegeneration by attenuating oligodendrocyte survival and promoting hippocampal atrophy, despite reduced Aβ burden. Our findings support a model in which Aβ-independent sensitizing influences such as loss of neuroprotective S1P are more important drivers of neurodegeneration than gross Aβ concentration or plaque density.

Age-Dependent Changes to Sphingolipid Balance in the Human Hippocampus are Gender-Specific and May Sensitize to Neurodegeneration

The greatest risk factor for developing Alzheimer's disease (AD) is aging. The major genetic risk factor for AD is the ɛ4 allele of the APOE gene, encoding the brain's major lipid transport protein, apolipoprotein E (ApoE). The research community is yet to decipher why the ApoE4 variant pre-disposes to AD, and how aging causes the disease. Studies have shown deregulated levels of sphingolipids, including decreased levels of the neuroprotective signaling lipid sphingosine 1-phosphate (S1P), and increased ceramide content, in brain tissue and serum of people with pre-clinical or very early AD. In this study we investigated whether sphingolipid levels are affected as a function of age or APOE genotype, in the hippocampus of neurologically normal subjects over the age of 65. Lipids were quantified in 80 postmortem tissue samples using liquid chromatography tandem mass spectrometry (LC-MS/MS). Sphingolipid levels were not significantly affected by the presence of one ɛ4 or ɛ2 allele. However, ceramide, sphingomyelin, and sulfatide content was very significantly correlated with age in the hippocampus of males. On the other hand, S1P, normalized to its non-phosphorylated precursor sphingosine, was inversely correlated with age in females. Our results therefore establish gender-specific differences in sphingolipid metabolism in the aging human brain. Ceramide is a pro-apoptotic lipid, and heavily implicated as a driver of insulin resistance in metabolic tissues. S1P is a neuroprotective lipid that supports glutamatergic neurotransmission. Increasing ceramide and decreasing S1P levels may contribute significantly to a pro-neurodegenerative phenotype in the aging brain.

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/.

T-Cell Accumulation in the Hypertensive Brain: A Role for Sphingosine-1-Phosphate-Mediated Chemotaxis

Hypertension is considered the major modifiable risk factor for the development of cognitive impairment. Because increased blood pressure is often accompanied by an activation of the immune system, the concept of neuro-inflammation gained increasing attention in the field of hypertension-associated neurodegeneration. Particularly, hypertension-associated elevated circulating T-lymphocyte populations and target organ damage spurred the interest to understanding mechanisms leading to inflammation-associated brain damage during hypertension. The present study describes sphingosine-1-phosphate (S1P) as major contributor to T-cell chemotaxis to the brain during hypertension-associated neuro-inflammation and cognitive impairment. Using Western blotting, flow cytometry and mass spectrometry approaches, we show that hypertension stimulates a sphingosine kinase 1 (SphK1)-dependent increase of cerebral S1P concentrations in a mouse model of angiotensin II (AngII)-induced hypertension. The development of a distinct S1P gradient between circulating blood and brain tissue associates to elevated CD3+ T-cell numbers in the brain. Inhibition of S1P₁-guided T-cell chemotaxis with the S1P receptor modulator FTY720 protects from augmentation of brain CD3 expression and the development of memory deficits in hypertensive WT mice. In conclusion, our data highlight a new approach to the understanding of hypertension-associated inflammation in degenerative processes of the brain during disease progression.

Sphingolipids modulate docking, Ca2+ sensitivity and membrane fusion of native cortical vesicles

Docking, priming, and membrane fusion of secretory vesicles (i.e. regulated exocytosis) requires lipids and proteins. Sphingolipids, in particular, sphingosine and sphingosine-1-phosphate, have been implicated in the modulation of exocytosis. However, the specific exocytotic steps that sphingolipids modulate and the enzymes that regulate sphingolipid concentrations on native secretory vesicle membranes remain unknown. Here we use tightly coupled functional and molecular analyses of fusion-ready cell surface complexes and cortical vesicles isolated from oocytes to assess the role of sphingolipids in the late, Ca²⁺-triggered steps of exocytosis. The molecular changes resulting from treatments with sphingolipid modifying compounds coupled with immunoblotting analysis revealed the presence of sphingosine kinase on native vesicles; the presence of a sphingosine-1-phosphate phosphatase is also indicated. Changes in sphingolipid concentrations on vesicles altered their docking/priming, Ca²⁺-sensitivity, and ability to fuse, indicating that sphingolipid concentrations are tightly regulated and maintained at optimal levels and ratios to ensure efficient exocytosis.

Essential Role of Sphingosine Kinase 2 in the Regulation of Cargo Contents in the Exosomes from K562 Cells

Sphingosine 1-phosphate (S1P) is a bioactive phosphorylated product of sphingosine catalyzed by sphingosine kinase (SphK) and implicated in diverse cellular functions including vesicular trafficking. In the present study we have shown the importance of one of the subtypes of SphK, SphK2, in the regulation of cargo content in exosomes released from human myeloid leukemia K562 cells. First, SphK2 has been shown to localize with N-Rh-PE-positive late endosomes in the cells. Next, siRNA-mediated knockdown of Sphk2 but not SphK1 resulted in a reduction of cargo content in purified exosomes. The involvement of SphK2 in this phenomenon was further investigated by pharmacological approaches. When cells were treated with N,N-dimethylsphingosine (DMS), one of the most frequently used inhibitors for SphK, cargo contents in purified exosomes were enhanced unexpectedly. Finally, it has been shown that DMS has a potency to stimulate SphK2 activity depending on the substrate sphingosine- and the inhibitor-doses as estimated by in vitro assay systems using a purified SphK2. These findings suggest that SphK2/S1P signaling plays an important role in the regulation of cargo content in exosomes in K562 cells.

Sphingosine 1-Phosphate Receptor Subtype 1 as a Therapeutic Target for Brain Trauma

Traumatic brain injury (TBI) provokes secondary pathological mechanisms, including ischemic and inflammatory processes. The new research in sphingosine 1-phosphate (S1P) receptor modulators has opened the door for an effective mechanism of reducing central nervous system (CNS) inflammatory lesion activity. Thus, the aim of this study was to characterize the immunomodulatory effect of the functional S1PR1 antagonist, siponimod, in phase III clinical trials for autoimmune disorders and of the competitive sphingosine 1-phosphate receptor subtype 1 (S1PR1) antagonist, TASP0277308, in pre-clinical development in an in vivo model of TBI in mice. We used the well-characterized model of TBI caused by controlled cortical impact. Mice were injected intraperitoneally with siponimod or TASP0277308 (1 mg/kg) at 1 and 4 h post-trauma. Our results demonstrated that these agents exerted significant beneficial effects on TBI pre-clinical scores in term of anti-inflammatory and immunomodulatory effects, in particular, attenuation of astrocytes and microglia activation, cytokines release, and rescue of the reduction of adhesion molecules (i.e., occludin and zonula occludens-1). Moreover, these compounds were able to decrease T-cell activation visible by reduction of CD4⁺ and CD8⁺, reduce the lesioned area (measured by 2,3,5-triphenyltetrazolium chloride staining), and to preserve tissue architecture, microtubule stability, and neural plasticity. Moreover, our findings provide pre-clinical evidence for the use of low-dose oral S1PR1 antagonists as neuroprotective strategies for TBI and broaden our understanding of the underlying S1PR1-driven neuroinflammatory processes in the pathophysiology of TBI. Altogether, our results showed that blocking the S1PR1 axis is an effective therapeutic strategy to mitigate neuropathological effects engaged in the CNS by TBI.

Extracellular α-synuclein drives sphingosine 1-phosphate receptor subtype 1 out of lipid rafts, leading to impaired inhibitory G-protein signaling

α-Synuclein (α-Syn)-positive intracytoplasmic inclusions, known as Lewy bodies, are thought to be involved in the pathogenesis of Lewy body diseases, such as Parkinson's disease (PD). Although growing evidence suggests that cell-to-cell transmission of α-Syn is associated with the progression of PD and that extracellular α-Syn promotes formation of inclusion bodies, its precise mechanism of action in the extracellular space remains unclear. Here, as indicated by both conventional fractionation techniques and FRET-based protein-protein interaction analysis, we demonstrate that extracellular α-Syn causes expulsion of sphingosine 1-phosphate receptor subtype 1 (S1P₁R) from the lipid raft fractions. S1P₁R regulates vesicular trafficking, and its expulsion involved α-Syn binding to membrane-surface gangliosides. Consequently, the S1P₁R became refractory to S1P stimulation required for activating inhibitory G-protein (G_i) in the plasma membranes. Moreover, the extracellular α-Syn also induced uncoupling of the S1P₁R on internal vesicles, resulting in the reduced amount of CD63 molecule (CD63) in the lumen of multivesicular endosomes, together with a decrease in CD63 in the released exosomes from α-Syn-treated cells. Furthermore, cholesterol-depleting agent-induced S1P₁R expulsion from the rafts also resulted in S1P₁R uncoupling. Taken together, these results suggest that extracellular α-Syn-induced expulsion of S1P₁R from lipid rafts promotes the uncoupling of S1P₁R from G_i, thereby blocking subsequent G_i signals, such as inhibition of cargo sorting into exosomal vesicles in multivesicular endosomes. These findings help shed additional light on PD pathogenesis.

Expansion of Sphingosine Kinase and Sphingosine-1-Phosphate Receptor Function in Normal and Cancer Cells: From Membrane Restructuring to Mediation of Estrogen Signaling and Stem Cell Programming

Sphingolipids, sphingolipid metabolizing enzymes, and their receptors network are being recognized as part of the signaling mechanisms, which govern breast cancer cell growth, migration, and survival during chemotherapy treatment. Approximately 70% of breast cancers are estrogen receptor (ER) positive and, thus, rely on estrogen signaling. Estrogen activates an intracellular network composed of many cytoplasmic and nuclear mediators. Some estrogen effects can be mediated by sphingolipids. Estrogen activates sphingosine kinase 1 (SphK1) and amplifies the intracellular concentration of sphingosine-1-phosphate (S1P) in breast cancer cells during stimulation of proliferation and survival. Specifically, Estrogen activates S1P receptors (S1PR) and induces growth factor receptor transactivation. SphK, S1P, and S1PR expression are causally associated with endocrine resistance and progression to advanced tumor stages in ER-positive breast cancers in vivo. Recently, the network of SphK/S1PR was shown to promote the development of ER-negative cancers and breast cancer stem cells, as well as stimulating angiogenesis. Novel findings confirm and broaden our knowledge about the cross-talk between sphingolipids and estrogen network in normal and malignant cells. Current S1PRs therapeutic inhibition was indicated as a promising chemotherapy approach in non-responsive and advanced malignancies. Considering that sphingolipid signaling has a prominent role in terminally differentiated cells, the impact should be considered when designing specific SphK/S1PR inhibitors. This study analyzes the dynamic of the transformation of sphingolipid axis during a transition from normal to pathological condition on the level of the whole organism. The sphingolipid-based mediation and facilitation of global effects of estrogen were critically accented as a bridging mechanism that should be explored in cancer prevention.

DHHC5-mediated palmitoylation of S1P receptor subtype 1 determines G-protein coupling

Sphingosine 1-phosphate (S1P) is a pleiotropic lipid mediator involved in the regulation of immune cell trafficking and vascular permeability acting mainly through G-protein-coupled S1P receptors (S1PRs). However, mechanism underlying how S1PRs are coupled with G-proteins remains unknown. Here we have uncovered that palmitoylation of a prototypical subtype S1P₁R is prerequisite for subsequent inhibitory G-protein (Gi) coupling. We have identified DHHC5 as an enzyme for palmitoylation of S1P₁R. Under basal conditions, S1P₁R was functionally associated with DHHC5 in the plasma membranes (PM) and was fully palmitoylated, enabling Gi coupling. Upon stimulation, the receptor underwent internalisation leaving DHHC5 in PM, resulting in depalmitoylation of S1P₁R. We also revealed that while physiological agonist S1P-induced endocytosed S1P₁R readily recycled back to PM, pharmacological FTY720-P-induced endocytosed S1P₁R-positive vesicles became associated with DHHC5 in the later phase, persistently transmitting Gi signals there. This indicates that FTY720-P switches off the S1P signal in PM, while switching on its signal continuously inside the cells. We propose that DHHC5-mediated palmitoylation of S1P₁R determines Gi coupling and its signalling in a spatio/temporal manner.

Application of High-Throughput Assays to Examine Phospho-Modulation of the Late Steps of Regulated Exocytosis

Abstract: Regulated exocytosis enables a range of physiological functions including neurotransmission, and the late steps (i.e., docking, priming and Ca²⁺-triggered membrane fusion) are modulated by a highly conserved set of proteins and lipids. Many of the molecular components and biochemical interactions required have been identified; the precise mechanistic steps they modulate and the biochemical interactions that need to occur across steps are still the subject of intense investigation. Particularly, although the involvement of phosphorylation in modulating exocytosis has been intensively investigated over the past three decades, it is unclear which phosphorylation events are a conserved part of the fundamental fusion mechanism and/or serve as part of the physiological fusion machine (e.g., to modulate Ca²⁺ sensitivity). Here, the homotypic fusion of cortical vesicles was monitored by utilizing new high-throughput, cost-effective assays to assess the influence of 17 small molecule phospho-modulators on docking/priming, Ca²⁺ sensitivity and membrane fusion. Specific phosphatases and casein kinase 2 are implicated in modulating the Ca²⁺ sensitivity of fusion, whereas sphingosine kinase is implicated in modulating the ability of vesicles to fuse. These results indicate the presence of multiple kinases and phosphatases on the vesicles and critical phosphorylation sites on vesicle membrane proteins and lipids that directly influence late steps of regulated exocytosis.

Crosstalk between sphingolipids and vitamin D3: potential role in the nervous system

Sphingolipids are both structural and bioactive compounds. In particular, ceramide and sphingosine 1-phosphate regulate cell fate, inflammation and excitability. 1-α,25-dihydroxyvitamin D3 (1,25(OH)2 D3 ) is known to play an important physiological role in growth and differentiation in a variety of cell types, including neural cells, through genomic actions mediated by its specific receptor, and non-genomic effects that result in the activation of specific signalling pathways. 1,25(OH)2 D3 and sphingolipids, in particular sphingosine 1-phosphate, share many common effectors, including calcium regulation, growth factors and inflammatory cytokines, but it is still not known whether they can act synergistically. Alterations in the signalling and concentrations of sphingolipids and 1,25(OH)2 D3 have been found in neurodegenerative diseases and fingolimod, a structural analogue of sphingosine, has been approved for the treatment of multiple sclerosis. This review, after a brief description of the role of sphingolipids and 1,25(OH)2 D3 , will focus on the potential crosstalk between sphingolipids and 1,25(OH)2 D3 in neural cells.

Regional changes in CNS and retinal glycerophospholipid profiles with age: a molecular blueprint

We present here a quantitative molecular blueprint of the three major glycerophospholipid (GPL) classes, phosphatidylcholine (PC), phosphatidylserine (PS), and phosphatidylethanolamine (PE), in retina and six regions of the brain in C57Bl6 mice at 2, 10, and 26 months of age. We found an age-related increase in molecular species containing saturated and monoenoic FAs and an overall decrease in the longer-chain PUFA molecular species across brain regions, with loss of DHA-containing molecular species as the most consistent and dramatic finding. Although we found very-long-chain PUFAs (VLC-PUFAs) (C28) in PC in the retina, no detectable levels were found in any brain region at any of the ages examined. All brain regions (except hippocampus and retina) showed a significant increase with age in PE plasmalogens. All three retina GPLs had di-PUFA molecular species (predominantly 44:12), which were most abundant in PS (∼30%). In contrast, low levels of di-PUFA GPL (1-2%) were found in all regions of the brain. This study provides a regional and age-related assessment of the brain's lipidome with a level of detail, inclusion, and quantification that has not heretofore been published.

Extracellular α-synuclein induces sphingosine 1-phosphate receptor subtype 1 uncoupled from inhibitory G-protein leaving β-arrestin signal intact

Parkinson’s disease (PD) is the second most common neurodegenerative disorder. The presence of α-synuclein (α-Syn)-positive intracytoplasmic inclusions, known as Lewy bodies, is the cytopathological hallmark of PD. Increasing bodies of evidence suggest that cell-to-cell transmission of α-Syn plays a role in the progression of PD. Although extracellular α-Syn is known to cause abnormal cell motility, the precise mechanism remains elusive. Here we show that impairment of platelet-derived growth factor-induced cell motility caused by extracellular α-Syn is mainly attributed to selective inhibition of sphingosine 1-phosphate (S1P) signalling. Treatment of human neuroblastoma cells with recombinant α-Syn caused S1P type 1 (S1P₁) receptor-selective uncoupling from inhibitory G-protein (Gi) as determined by both functional and fluorescence resonance energy transfer (FRET)-based structural analyses. By contrast, α-Syn caused little or no effect on S1P₂ receptor-mediated signalling. Both wild-type and α-Syn(A53T), a mutant found in familiar PD, caused uncoupling of S1P₁ receptor, although α-Syn(A53T) showed stronger potency in uncoupling. Moreover, S1P₁ receptor-mediated β-arrestin signal was unaltered by α-Syn(A53T). These results suggest that exogenous α-Syn modulates S1P₁ receptor-mediated signalling from both Gi and β-arrestin signals into β-arrestin-biased signal. These findings uncovered a novel function of exogenous α-Syn in the cells.

Sphingosine 1-phosphate - A double edged sword in the brain

The physiological functions of sphingosine 1-phosphate (S1P) and its pathological roles in various diseases are increasingly being elucidated. Particularly, a growing body of literature has implicated S1P in the pathogenesis of brain related disorders. With the deciphering of more intricate aspects of S1P signalling, there is also a need to reconsider the notion of S1P only as a determinant of cell survival and proliferation. Further the concept of 'S1P-ceramide' balance as the controlling switch of cellular fate and functions needs to be refined. In this review, we focus on the brain related functions of S1P with special focus on its role in synaptic transmission, neuronal autophagy and neuroinflammation. The review also attempts to bring out the multi-faceted nature of S1P signalling aspects that makes it a 'double edged sword'. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.

Dysregulation of lipids in Alzheimer's disease and their role as potential biomarkers

The brain is highly enriched in lipids, and an intensive study of these lipids may be informative, not only of normal brain function but also of changes with age and in disease. In recent years, the development of highly sensitive mass spectrometry platforms and other high-throughput technologies has enabled the discovery of complex changes in the entire lipidome. This lipidomics approach promises to be a particularly useful tool for identifying diagnostic biomarkers for early detection of age-related neurodegenerative disease, such as Alzheimer's disease (AD), which has till recently been limited to protein- and gene-centric approaches. This review highlights known lipid changes affecting the AD brain and presents an update on the progress of lipid biomarker research in AD. Important considerations for designing large-scale lipidomics experiments are discussed to help standardize findings across different laboratories, as well as challenges associated with moving toward clinical application.

Prophylactic versus Therapeutic Fingolimod: Restoration of Presynaptic Defects in Mice Suffering from Experimental Autoimmune Encephalomyelitis

Fingolimod, the first oral, disease-modifying therapy for MS, has been recently proposed to modulate glutamate transmission in the central nervous system (CNS) of mice suffering from Experimental Autoimmune Encephalomyelitis (EAE) and in MS patients. Our study aims at investigating whether oral fingolimod recovers presynaptic defects that occur at different stages of disease in the CNS of EAE mice. In vivo prophylactic (0.3 mg/kg for 14 days, from the 7th day post immunization, d.p.i, the drug dissolved in the drinking water) fingolimod significantly reduced the clinical symptoms and the anxiety-related behaviour in EAE mice. Spinal cord inflammation, demyelination and glial cell activation are markers of EAE progression. These signs were ameliorated following oral fingolimod administration. Glutamate exocytosis was shown to be impaired in cortical and spinal cord terminals isolated from EAE mice at 21 ± 1 d.p.i., while GABA alteration emerged only at the spinal cord level. Prophylactic fingolimod recovered these presynaptic defects, restoring altered glutamate and GABA release efficiency. The beneficial effect occurred in a dose-dependent, region-specific manner, since lower (0.1-0.03 mg/kg) doses restored, although to a different extent, synaptic defects in cortical but not spinal cord terminals. A delayed reduction of glutamate, but not of GABA, exocytosis was observed in hippocampal terminals of EAE mice at 35 d.p.i. Therapeutic (0.3 mg/kg, from 21 d.p.i. for 14 days) fingolimod restored glutamate exocytosis in the cortex and in the hippocampus of EAE mice at 35 ± 1 d.p.i. but not in the spinal cord, where also GABAergic defects remained unmodified. These results improve our knowledge of the molecular events accounting for the beneficial effects elicited by fingolimod in demyelinating disorders.

Sphingosine 1-phosphate lyase ablation disrupts presynaptic architecture and function via an ubiquitin- proteasome mediated mechanism

The bioactive lipid sphingosine 1-phosphate (S1P) is a degradation product of sphingolipids that are particularly abundant in neurons. We have shown previously that neuronal S1P accumulation is toxic leading to ER-stress and an increase in intracellular calcium. To clarify the neuronal function of S1P, we generated brain-specific knockout mouse models in which S1P-lyase (SPL), the enzyme responsible for irreversible S1P cleavage was inactivated. Constitutive ablation of SPL in the brain (SPL^fl/fl/Nes) but not postnatal neuronal forebrain-restricted SPL deletion (SPL^fl/fl/CaMK) caused marked accumulation of S1P. Hence, altered presynaptic architecture including a significant decrease in number and density of synaptic vesicles, decreased expression of several presynaptic proteins, and impaired synaptic short term plasticity were observed in hippocampal neurons from SPL^fl/fl/Nes mice. Accordingly, these mice displayed cognitive deficits. At the molecular level, an activation of the ubiquitin-proteasome system (UPS) was detected which resulted in a decreased expression of the deubiquitinating enzyme USP14 and several presynaptic proteins. Upon inhibition of proteasomal activity, USP14 levels, expression of presynaptic proteins and synaptic function were restored. These findings identify S1P metabolism as a novel player in modulating synaptic architecture and plasticity.

The sphingosine-1-phosphate receptor: A novel therapeutic target for multiple sclerosis and other autoimmune diseases

Multiple sclerosis (MS) is a prototype autoimmune disease of the central nervous system (CNS). Currently, there is no drug that provides a cure for MS. To date, all immunotherapeutic drugs target relapsing remitting MS (RR-MS); it remains a daunting medical challenge in MS to develop therapy for secondary progressive MS (SP-MS). Since the approval of the non-selective sphingosine-1-phosphate (S1P) receptor modulator FTY720 (fingolimod [Gilenya®]) for RR-MS in 2010, there have been many emerging studies with various selective S1P receptor modulators in other autoimmune conditions. In this article, we will review how S1P receptor may be a promising therapeutic target for SP-MS and other autoimmune diseases such as psoriasis, polymyositis and lupus.

Ablation of Sphingosine 1-Phosphate Receptor Subtype 3 Impairs Hippocampal Neuron Excitability In vitro and Spatial Working Memory In vivo

Understanding the role of the bioactive lipid mediator sphingosine 1-phosphate (S1P) within the central nervous system has recently gained more and more attention, as it has been connected to major diseases such as multiple sclerosis and Alzheimer's disease. Even though much data about the functions of the five S1P receptors has been collected for other organ systems, we still lack a complete understanding for their specific roles, in particular within the brain. Therefore, it was the aim of this study to further elucidate the role of S1P receptor subtype 3 (S1P₃) in vivo and in vitro with a special focus on the hippocampus. Using an S1P₃ knock-out mouse model we applied a range of behavioral tests, performed expression studies, and whole cell patch clamp recordings in acute hippocampal slices. We were able to show that S1P₃ deficient mice display a significant spatial working memory deficit within the T-maze test, but not in anxiety related tests. Furthermore, S1p3 mRNA was expressed throughout the hippocampal formation. Principal neurons in area CA3 lacking S1P₃ showed significantly increased interspike intervals and a significantly decreased input resistance. Upon stimulation with S1P CA3 principal neurons from both wildtype and S1P3−/− mice displayed significantly increased evoked EPSC amplitudes and decay times, whereas rise times remained unchanged. These results suggest a specific involvement of S1P₃ for the establishment of spatial working memory and neuronal excitability within the hippocampus.

Sphingosine-1-Phosphate (S1P) Impacts Presynaptic Functions by Regulating Synapsin I Localization in the Presynaptic Compartment

Growing evidence indicates that sphingosine-1-P (S1P) upregulates glutamate secretion in hippocampal neurons. However, the molecular mechanisms through which S1P enhances excitatory activity remain largely undefined. The aim of this study was to identify presynaptic targets of S1P action controlling exocytosis. Confocal analysis of rat hippocampal neurons showed that S1P applied at nanomolar concentration alters the distribution of Synapsin I (SynI), a presynaptic phosphoprotein that controls the availability of synaptic vesicles for exocytosis. S1P induced SynI relocation to extrasynaptic regions of mature neurons, as well as SynI dispersion from synaptic vesicle clusters present at axonal growth cones of developing neurons. S1P-induced SynI relocation occurred in a Ca(2+)-independent but ERK-dependent manner, likely through the activation of S1P3 receptors, as it was prevented by the S1P3 receptor selective antagonist CAY1044 and in neurons in which S1P3 receptor was silenced. Our recent evidence indicates that microvesicles (MVs) released by microglia enhance the metabolism of endogenous sphingolipids in neurons and stimulate excitatory transmission. We therefore investigated whether MVs affect SynI distribution and whether endogenous S1P could be involved in the process. Analysis of SynI immunoreactivity showed that exposure to microglial MVs induces SynI mobilization at presynaptic sites and growth cones, whereas the use of inhibitors of sphingolipid cascade identified S1P as the sphingolipid mediating SynI redistribution. Our data represent the first demonstration that S1P induces SynI mobilization from synapses, thereby indicating the phosphoprotein as a novel target through which S1P controls exocytosis.

SIGNIFICANCE STATEMENT

Growing evidence indicates that the bioactive lipid sphingosine and its metabolite sphingosine-1-P (S1P) stimulate excitatory transmission. While it has been recently clarified that sphingosine influences directly the exocytotic machinery by activating the synaptic vesicle protein VAMP2 to form SNARE fusion complexes, the molecular mechanism by which S1P promotes neurotransmission remained largely undefined. In this study, we identify Synapsin I, a presynaptic phosphoprotein involved in the control of availability of synaptic vesicles for exocytosis, as the key target of S1P action. In addition, we provide evidence that S1P can be produced at mature axon terminals as well as at immature growth cones in response to microglia-derived signals, which may be important to stabilize nascent synapses and to restore or potentiate transmission.

Neural ablation of the PARK10 candidate Plpp3 leads to dopaminergic transmission deficits without neurodegeneration

Parkinson’s disease (PD) is a multifactorial neurodegenerative disorder, characterised by the progressive loss of midbrain dopaminergic neurons and a variety of motor symptoms. The gene coding for the phospholipid phosphatase 3, PLPP3 (formerly PPAP2B or LPP3), maps within the PARK10 locus, a region that has been linked with increased risk to late-onset PD. PLPP3 modulates the levels of a range of bioactive lipids controlling fundamental cellular processes within the central nervous system. Here we show that PLPP3 is enriched in astroglial cells of the adult murine ventral midbrain. Conditional inactivation of Plpp3 using a Nestin::Cre driver results in reduced mesencephalic levels of sphingosine-1-phosphate receptor 1 (S1P₁), a well-known mediator of pro-survival responses. Yet, adult PLPP3-deficient mice exhibited no alterations in the number of dopaminergic neurons or in the basal levels of striatal extracellular dopamine (DA). Potassium-evoked DA overflow in the striatum, however, was significantly decreased in mutant mice. Locomotor evaluation revealed that, although PLPP3-deficient mice exhibit motor impairment, this is not progressive or responsive to acute L-DOPA therapy. These findings suggest that disruption of Plpp3 during early neural development leads to dopaminergic transmission deficits in the absence of nigrostriatal degeneration, and without causing an age-related locomotor decline consistent with PD.

Sphingosine kinase 1/sphingosine-1-phosphate regulates the expression of interleukin-17A in activated microglia in cerebral ischemia/reperfusion

BACKGROUND

Microglial activation is one of the causative factors of neuroinflammation in cerebral ischemia/reperfusion (IR). Sphingosine kinase 1 (Sphk1), a key enzyme responsible for phosphorylating sphingosine into sphingosine-1-phosphate (S1P), plays an important role in the regulation of proinflammatory cytokines in activated microglia. Recent research demonstrated that S1P increased IL-17A-secretion and then worsened CNS (central nervous system) inflammation. Thus, in the present study, we sought to use microglial cells as the object of study to discuss the molecular mechanisms in Sphk1/S1P-regulated IL-17A-secretion in IR.

METHODS

We used immunofluorescence and confocal microscopy to detect whether Sphk1 is expressed in microglia after cerebral IR or oxygen-glucose deprivation (OGDR). Western blot analysis was used to estimate the total Sphk1 protein level at different time points after OGDR. To detect cytokine secretion in microglial supernatants in response to OGDR, we measured the concentration of IL-17A in the culture supernatants using an enzyme-linked immunosorbent assay (ELISA). To evaluate whether microglia subjected to OGDR exhibited neuronal injury, we used a commercially available terminal transferase-mediated deoxyuridine triphosphate-biotin nick end labeling (TUNEL) kit to detect apoptotic neurons.

RESULTS

Sphk1 was expressed in microglia in response to cerebral IR or OGDR at appointed time. Pre-injection with PF-543, an inhibitor of Sphk1, before IR clearly reduced the expression of Sphk1 in microglia relative to brain IR alone. The number of TUNEL-positive neurons was also decreased in the PF-543-pretreated animals before IR compared to the animals with IR alone. When S1P was administered in OGDR microglia, IL-17A expression and neuronal apoptosis were increased compared to OGDR alone and the administration of S1P alone. ELISA further confirmed the above results. Moreover, the inhibition of Sphk1 by siRNA reduced IL-17A production and relieved neuronal apoptosis in OGDR microglia.

CONCLUSION

These results indicated that Sphk1/S1P regulates the expression of IL-17A in activated microglia, inducing neuronal apoptosis in cerebral ischemia/reperfusion. The microglial Sphk1/S1P pathway may thus be a potential therapeutic target to control neuroinflammation in brain IR.