Sphingosine-1-phosphate in the plasma compartment regulates basal and inflammation-induced vascular leak in mice

Spatiotemporal sphingosine-1-phosphate receptor 3 expression within the cerebral vasculature after ischemic stroke

Sphingosine-1-phosphate receptors (S1PRs) are promising therapeutic targets in cardiovascular disease, including ischemic stroke. However, important spatiotemporal information for alterations of S1PR expression is lacking. Here, we investigated the role of S1PR3 in ischemic stroke in rodent models and patient samples. We show that S1PR3 is acutely upregulated in perilesional reactive astrocytes after stroke, and that stroke volume and behavioral deficits are improved in mice lacking S1PR3. Further, we find that administration of an S1PR3 antagonist at 4-h post-stroke, but not at later timepoints, improves stroke outcome. Lastly, we observed higher plasma S1PR3 concentrations in experimental stroke and in patients with ischemic stroke. Together, our results establish S1PR3 as a potential drug target and biomarker in ischemic stroke.

How do sphingosine-1-phosphate affect immune cells to resolve inflammation?

Inflammation is an important immune response of the body. It is a physiological process of self-repair and defense against pathogens taken up by biological tissues when stimulated by damage factors such as trauma and infection. Inflammation is the main cause of high morbidity and mortality in most diseases and is the physiological basis of the disease. Targeted therapeutic strategies can achieve efficient toxicity clearance at the inflammatory site, reduce complications, and reduce mortality. Sphingosine-1-phosphate (S1P), a lipid signaling molecule, is involved in immune cell transport by binding to S1P receptors (S1PRs). It plays a key role in innate and adaptive immune responses and is closely related to inflammation. In homeostasis, lymphocytes follow an S1P concentration gradient from the tissues into circulation. One widely accepted mechanism is that during the inflammatory immune response, the S1P gradient is altered, and lymphocytes are blocked from entering the circulation and are, therefore, unable to reach the inflammatory site. However, the full mechanism of its involvement in inflammation is not fully understood. This review focuses on bacterial and viral infections, autoimmune diseases, and immunological aspects of the Sphks/S1P/S1PRs signaling pathway, highlighting their role in promoting intradial-adaptive immune interactions. How S1P signaling is regulated in inflammation and how S1P shapes immune responses through immune cells are explained in detail. We teased apart the immune cell composition of S1P signaling and the critical role of S1P pathway modulators in the host inflammatory immune system. By understanding the role of S1P in the pathogenesis of inflammatory diseases, we linked the genomic studies of S1P-targeted drugs in inflammatory diseases to provide a basis for targeted drug development.

A transcriptional network governing ceramide homeostasis establishes a cytokine-dependent developmental process

Transcriptional mechanisms controlling developmental processes establish and maintain proteomic networks, which can govern the levels of intracellular small molecules. Although dynamic changes in bioactive small molecules can link transcription factor and genome activity with cell state transitions, many mechanistic questions are unresolved. Using quantitative lipidomics and multiomics, we discover that the hematopoietic transcription factor GATA1 establishes ceramide homeostasis during erythroid differentiation by regulating genes encoding sphingolipid metabolic enzymes. Inhibiting a GATA1-induced sphingolipid biosynthetic enzyme, delta(4)-desaturase, or disrupting ceramide homeostasis with cell-permeable dihydroceramide or ceramide is detrimental to erythroid, but not myeloid, progenitor activity. Coupled with genetic editing-based rewiring of the regulatory circuitry, we demonstrate that ceramide homeostasis commissions vital stem cell factor and erythropoietin signaling by opposing an inhibitory protein phosphatase 2A-dependent, dual-component mechanism. Integrating bioactive lipids as essential components of GATA factor mechanisms to control cell state transitions has implications for diverse cell and tissue types.

S1P/S1PR signaling pathway advancements in autoimmune diseases

Sphingosine-1-phosphate (S1P) is a versatile sphingolipid that is generated through the phosphorylation of sphingosine by sphingosine kinase (SPHK). S1P exerts its functional effects by binding to the G protein-coupled S1P receptor (S1PR). This lipid mediator plays a pivotal role in various cellular activities. The S1P/S1PR signaling pathway is implicated in the pathogenesis of immune-mediated diseases, significantly contributing to the functioning of the immune system. It plays a crucial role in diverse physiological and pathophysiological processes, including cell survival, proliferation, migration, immune cell recruitment, synthesis of inflammatory mediators, and the formation of lymphatic and blood vessels. However, the full extent of the involvement of this signaling pathway in the development of autoimmune diseases remains to be fully elucidated. Therefore, this study aims to comprehensively review recent research on the S1P/S1PR axis in diseases related to autoimmunity.

Receptor-dependent effects of sphingosine-1-phosphate (S1P) in COVID-19: the black side of the moon

Severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) infection leads to hyper-inflammation and amplified immune response in severe cases that may progress to cytokine storm and multi-organ injuries like acute respiratory distress syndrome and acute lung injury. In addition to pro-inflammatory cytokines, different mediators are involved in SARS-CoV-2 pathogenesis and infection, such as sphingosine-1-phosphate (S1P). S1P is a bioactive lipid found at a high level in plasma, and it is synthesized from sphingomyelin by the action of sphingosine kinase. It is involved in inflammation, immunity, angiogenesis, vascular permeability, and lymphocyte trafficking through G-protein coupled S1P receptors. Reduction of the circulating S1P level correlates with COVID-19 severity. S1P binding to sphingosine-1-phosphate receptor 1 (S1PR1) elicits endothelial protection and anti-inflammatory effects during SARS-CoV-2 infection, by limiting excessive INF-α response and hindering mitogen-activated protein kinase and nuclear factor kappa B action. However, binding to S1PR2 opposes the effect of S1PR1 with vascular inflammation, endothelial permeability, and dysfunction as the concomitant outcome. This binding also promotes nod-like receptor pyrin 3 (NLRP3) inflammasome activation, causing liver inflammation and fibrogenesis. Thus, higher expression of macrophage S1PR2 contributes to the activation of the NLRP3 inflammasome and the release of pro-inflammatory cytokines. In conclusion, S1PR1 agonists and S1PR2 antagonists might effectively manage COVID-19 and its severe effects. Further studies are recommended to elucidate the potential conflict in the effects of S1P in COVID-19.

Sphingosine-1-phosphate related signalling pathways manipulating virus replication

Viruses can create a unique cellular environment that facilitates replication and transmission. Sphingosine kinases (SphKs) produce sphingosine-1-phosphate (S1P), a bioactive sphingolipid molecule that performs both physiological and pathological effects primarily by activating a subgroup of the endothelial differentiation gene family of G-protein coupled cell surface receptors known as S1P receptors (S1PR1-5). A growing body of evidence indicates that the SphK/S1P axis is crucial for regulating cellular activities in virus infections like respiratory viruses, enteroviruses, hepatitis viruses, herpes viruses, and arboviruses replicate. Depending on the type of virus, pro- or anti-viral activities of the SphK/S1P axis sometimes rely on the host immune system and sometimes directly through intracellular signalling pathways or cell proliferation. Recent research has shown novel roles of S1P and SphK in viral replication. Sphingosine kinase isoforms (SphK1 and SphK2) levels can be manipulated by several viruses to promote the effects that are expected. Regulation of cellular signalling pathways plays a significant role in the mechanism. The purpose of this review is to provide insight of the characters played by the SphK/S1P axis throughout diverse viral infection processes. We then assess potential therapeutic methods that are based on S1P signalling and metabolism during viral infections.

Signal Transduction and Gene Regulation in the Endothelium

Extracellular signals act on G-protein-coupled receptors (GPCRs) to regulate homeostasis and adapt to stress. This involves rapid intracellular post-translational responses and long-lasting gene-expression changes that ultimately determine cellular phenotype and fate changes. The lipid mediator sphingosine 1-phosphate (S1P) and its receptors (S1PRs) are examples of well-studied GPCR signaling axis essential for vascular development, homeostasis, and diseases. The biochemical cascades involved in rapid S1P signaling are well understood. However, gene-expression regulation by S1PRs are less understood. In this review, we focus our attention to how S1PRs regulate nuclear chromatin changes and gene transcription to modulate vascular and lymphatic endothelial phenotypic changes during embryonic development and adult homeostasis. Because S1PR-targeted drugs approved for use in the treatment of autoimmune diseases cause substantial vascular-related adverse events, these findings are critical not only for general understanding of stimulus-evoked gene regulation in the vascular endothelium, but also for therapeutic development of drugs for autoimmune and perhaps vascular diseases.

The Crosstalk between FcεRI and Sphingosine Signaling in Allergic Inflammation

Sphingolipid molecules have recently attracted attention as signaling molecules in allergic inflammation diseases. Sphingosine-1-phosphate (S1P) is synthesized by two isoforms of sphingosine kinases (SPHK 1 and SPHK2) and is known to be involved in various cellular processes. S1P levels reportedly increase in allergic inflammatory diseases, such as asthma and anaphylaxis. FcεRI signaling is necessary for allergic inflammation as it can activate the SPHKs and increase the S1P level; once S1P is secreted, it can bind to the S1P receptors (S1PRs). The role of S1P signaling in various allergic diseases is discussed. Increased levels of S1P are positively associated with asthma and anaphylaxis. S1P can either induce or suppress allergic skin diseases in a context-dependent manner. The crosstalk between FcεRI and S1P/SPHK/S1PRs is discussed. The roles of the microRNAs that regulate the expression of the components of S1P signaling in allergic inflammatory diseases are also discussed. Various reports suggest the role of S1P in FcεRI-mediated mast cell (MC) activation. Thus, S1P/SPHK/S1PRs signaling can be the target for developing anti-allergy drugs.

Postnatal deletion of Spns2 prevents neuroinflammation without compromising blood vascular functions

Protein Spinster homolog 2 (Spns2) is a sphingosine-1-phosphate (S1P) transporter that releases S1P to regulate lymphocyte egress and trafficking. Global deletion of Spns2 (Spns2^-/-) has been shown to reduce disease severity in several autoimmune disease models. To examine whether Spns2 could be exploited as a drug target, we generated and characterized the mice with postnatal knockout of Spns2 (Spns2-Mx1Cre). Our results showed that Spns2-Mx1Cre mice had significantly low number of lymphocytes in blood and lymphoid organs similar to Spns2^-/- mice. Lymph but not plasma S1P levels were significantly reduced in both groups of knockout mice. Our lipidomic results also showed that Spns2 releases different S1P species into lymph. Interestingly, lymphatic vessels in the lymph nodes (LNs) of Spns2^-/- and Spns2-Mx1Cre mice exhibited morphological defects. The structures of high endothelial venules (HEV) in the LNs of Spns2-Mx1Cre mice were disorganized. These results indicate that lack of Spns2 affects both S1P secretion and LN vasculatures. Nevertheless, blood vasculature of these Spns2 deficient mice was not different to controls under homeostasis and vascular insults. Importantly, Spns2-Mx1Cre mice were resistant to multiple sclerosis in experimental autoimmune encephalomyelitis (EAE) models with significant reduction of pathogenic Th17 cells in the central nervous system (CNS). This study suggests that pharmacological inhibition of Spns2 may be exploited for therapeutic applications in treatment of neuroinflammation.

A Potential Interplay between HDLs and Adiponectin in Promoting Endothelial Dysfunction in Obesity

Obesity is an epidemic public health problem that has progressively worsened in recent decades and is associated with low-grade chronic inflammation (LGCI) in metabolic tissues and an increased risk of several diseases. In particular, LGCI alters metabolism and increases cardiovascular risk by impairing endothelial function and altering the functions of adiponectin and high-density lipoproteins (HDLs). Adiponectin is an adipokine involved in regulating energy metabolism and body composition. Serum adiponectin levels are reduced in obese individuals and negatively correlate with chronic sub-clinical inflammatory markers. HDLs are a heterogeneous and complex class of lipoproteins that can be dysfunctional in obesity. Adiponectin and HDLs are strictly interdependent, and the maintenance of their interplay is essential for vascular function. Since such a complex network of interactions is still overlooked in clinical settings, this review aims to highlight the mechanisms involved in the impairment of the HDLs/adiponectin axis in obese patients to predict the risk of cardiovascular diseases and activate preventive countermeasures. Here, we provide a narrative review of the role of LGCI in altering HDLs, adiponectin and endothelial functions in obesity to encourage new studies about their synergic effects on cardiovascular health and disease.

Sphingosine 1-phosphate receptor-targeted therapeutics in rheumatic diseases

Sphingosine 1-phosphate (S1P), which acts via G protein-coupled S1P receptors (S1PRs), is a bioactive lipid essential for vascular integrity and lymphocyte trafficking. The S1P-S1PR signalling axis is a key component of the inflammatory response in autoimmune rheumatic diseases. Several drugs that target S1PRs have been approved for the treatment of multiple sclerosis and inflammatory bowel disease and are under clinical testing for patients with systemic lupus erythematosus (SLE). Preclinical studies support the hypothesis that targeting the S1P-S1PR axis would be beneficial to patients with SLE, rheumatoid arthritis (RA) and systemic sclerosis (SSc) by reducing pathological inflammation. Whereas most preclinical research and development efforts are focused on reducing lymphocyte trafficking, protective effects of circulating S1P on endothelial S1PRs, which maintain the vascular barrier and enable blood circulation while dampening leukocyte extravasation, have been largely overlooked. In this Review, we take a holistic view of S1P-S1PR signalling in lymphocyte and vascular pathobiology. We focus on the potential of S1PR modulators for the treatment of SLE, RA and SSc and summarize the rationale, pathobiology and evidence from preclinical models and clinical studies. Improved understanding of S1P pathobiology in autoimmune rheumatic diseases and S1PR therapeutic modulation is anticipated to lead to efficacious and safer management of these diseases.

Pathophysiological, Cellular, and Molecular Events of the Vascular System in Anaphylaxis

Anaphylaxis is a systemic hypersensitivity reaction that can be life threatening. Mechanistically, it results from the immune activation and release of a variety of mediators that give rise to the signs and symptoms of this pathological event. For years, most of the research in anaphylaxis has focused on the contribution of the immune component. However, approaches that shed light on the participation of other cellular and molecular agents are necessary. Among them, the vascular niche receives the various signals (e.g., histamine) that elicit the range of anaphylactic events. Cardiovascular manifestations such as increased vascular permeability, vasodilation, hypotension, vasoconstriction, and cardiac alterations are crucial in the pathophysiology of anaphylaxis and are highly involved to the development of the most severe cases. Specifically, the endothelium, vascular smooth muscle cells, and their molecular signaling outcomes play an essential role downstream of the immune reaction. Therefore, in this review, we synthesized the vascular changes observed during anaphylaxis as well as its cellular and molecular components. As the risk of anaphylaxis exists both in clinical procedures and in routine life, increasing our knowledge of the vascular physiology and their molecular mechanism will enable us to improve the clinical management and how to treat or prevent anaphylaxis.

Key Message

Anaphylaxis, the most severe allergic reaction, involves a variety of immune and non-immune molecular signals that give rise to its pathophysiological manifestations. Importantly, the vascular system is engaged in processes relevant to anaphylactic events such as increased vascular permeability, vasodilation, hypotension, vasoconstriction, and decreased cardiac output. The novelty of this review focuses on the fact that new studies will greatly improve the understanding of anaphylaxis when viewed from a vascular molecular angle and specifically from the endothelium. This knowledge will improve therapeutic options to treat or prevent anaphylaxis.

Potential Pathophysiological Mechanisms Underlying Multiple Organ Dysfunction in Cytokine Release Syndrome

In recent decades, many serious respiratory infections have broken out all over the world, including SARS-CoV, MERS, and COVID-19. They are characterized by strong infectivity, rapid disease progression, high mortality, and poor prognosis. Excessive immune system activation results in cytokine hypersecretion, which is an important reason for the aggravation of symptoms, and can spread throughout the body leading to systemic multiple organ dysfunction, namely, cytokine release syndrome (CRS). Although many diseases related to CRS have been identified, the mechanism of CRS is rarely mentioned clearly. This review is intended to clarify the pathogenetic mechanism of CRS in the deterioration of related diseases, describe the important signaling pathways and clinical pathophysiological characteristics of CRS, and provide ideas for further research and development of specific drugs for corresponding targets to treat CRS.

Sphingosine 1-phosphate and its regulatory role in vascular endothelial cells

Sphingosine 1-phosphate (S1P) is a bioactive metabolite of sphingomyelin. S1P activates a series of signaling cascades by acting on its receptors S1PR1-3 on endothelial cells (ECs), which plays an important role in endothelial barrier maintenance, anti-inflammation, antioxidant and angiogenesis, and thus is considered as a potential therapeutic biomarker for ischemic stroke, sepsis, idiopathic pulmonary fibrosis, cancers, type 2 diabetes and cardiovascular diseases. We presently review the levels of S1P in those vascular and vascular-related diseases. Plasma S1P levels were reduced in various inflammation-related diseases such as atherosclerosis and sepsis, but were increased in other diseases including type 2 diabetes, neurodegeneration, cerebrovascular damages such as acute ischemic stroke, Alzheimer's disease, vascular dementia, angina, heart failure, idiopathic pulmonary fibrosis, community-acquired pneumonia, and hepatocellular carcinoma. Then, we highlighted the molecular mechanism by which S1P regulated EC biology including vascular development and angiogenesis, inflammation, permeability, and production of reactive oxygen species (ROS), nitric oxide (NO) and hydrogen sulfide (H₂S), which might provide new ways for exploring the pathogenesis and implementing individualized therapy strategies for those diseases.

Targeting Chronic Inflammation of the Digestive System in Cancer Prevention: Modulators of the Bioactive Sphingolipid Sphingosine-1-Phosphate Pathway

Incidence of gastrointestinal (GI) cancers is increasing, and late-stage diagnosis makes these cancers difficult to treat. Chronic and low-grade inflammation are recognized risks for most GI cancers. The GI mucosal immune system maintains healthy homeostasis and signalling molecules made from saturated fats, bioactive sphingolipids, play essential roles in healthy GI immunity. Sphingosine-1-phosphate (S1P), a bioactive sphingolipid, is a key mediator in a balanced GI immune response. Disruption in the S1P pathway underlies systemic chronic metabolic inflammatory disorders, including diabetes and GI cancers, providing a strong rationale for using modulators of the S1P pathway to treat pathological inflammation. Here, we discuss the effects of bioactive sphingolipids in immune homeostasis with a focus on S1P in chronic low-grade inflammation associated with increased risk of GI carcinogenesis. Contemporary information on S1P signalling involvement in cancers of the digestive system, from top to bottom, is reviewed. Further, we discuss the use of novel S1P receptor modulators currently in clinical trials and their potential as first-line drugs in the clinic for chronic inflammatory diseases. Recently, ozanimod (Zeposia^TM) and etrasimod have been approved for clinical use to treat ulcerative colitis and eosinophilic oesophagitis, respectively, which may have longer term benefits in reducing risk of GI cancers.

Erythrocytes increase endogenous sphingosine 1-phosphate levels as an adaptive response to SARS-CoV-2 infection

Low plasma levels of the signaling lipid metabolite sphingosine 1-phosphate (S1P) are associated with disrupted endothelial cell (EC) barriers, lymphopenia and reduced responsivity to hypoxia. Total S1P levels were also reduced in 23 critically ill patients with coronavirus disease 2019 (COVID-19), and the two main S1P carriers, serum albumin (SA) and high-density lipoprotein (HDL) were dramatically low. Surprisingly, we observed a carrier-changing shift from SA to HDL, which probably prevented an even further drop in S1P levels. Furthermore, intracellular S1P levels in red blood cells (RBCs) were significantly increased in COVID-19 patients compared with healthy controls due to up-regulation of S1P producing sphingosine kinase 1 and down-regulation of S1P degrading lyase expression. Cell culture experiments supported increased sphingosine kinase activity and unchanged S1P release from RBC stores of COVID-19 patients. These observations suggest adaptive mechanisms for maintenance of the vasculature and immunity as well as prevention of tissue hypoxia in COVID-19 patients.

Sphingosine-1-Phosphate Signaling in Ischemic Stroke: From Bench to Bedside and Beyond

Sphingosine-1-phosphate (S1P) signaling is being increasingly recognized as a strong modulator of immune cell migration and endothelial function. Fingolimod and other S1P modulators in ischemic stroke treatment have shown promise in emerging experimental models and small-scale clinical trials. In this article, we will review the current knowledge of the role of S1P signaling in brain ischemia from the aspects of inflammation and immune interventions, sustaining endothelial functions, regulation of blood-brain barrier integrity, and functional recovery. We will then discuss the current and future therapeutic perspectives of targeting S1P for the treatment of ischemic stroke. Mechanism studies would help to bridge the gap between preclinical studies and clinical practice. Future success of bench-to-bedside translation shall be based on in depth understanding of S1P signaling during stroke and on the ability to have a fine temporal and spatial regulation of the signal pathway.

Plasma Lipid Profiling Contributes to Untangle the Complexity of Moyamoya Arteriopathy

Moyamoya arteriopathy (MA) is a rare cerebrovascular disorder characterized by ischemic/hemorrhagic strokes. The pathophysiology is unknown. A deregulation of vasculogenic/angiogenic/inflammatory pathways has been hypothesized as a possible pathophysiological mechanism. Since lipids are implicated in modulating neo-vascularization/angiogenesis and inflammation, their deregulation is potentially involved in MA. Our aim is to evaluate angiogenic/vasculogenic/inflammatory proteins and lipid profile in plasma of MA patients and control subjects (healthy donors HD or subjects with atherosclerotic cerebrovascular disease ACVD). Angiogenic and inflammatory protein levels were measured by ELISA and a complete lipidomic analysis was performed on plasma by mass spectrometry. ELISA showed a significant decrease for MMP-9 released in plasma of MA. The untargeted lipidomic analysis showed a cumulative depletion of lipid asset in plasma of MA as compared to HD. Specifically, a decrease in membrane complex glycosphingolipids peripherally circulating in MA plasma with respect to HD was observed, likely suggestive of cerebral cellular recruitment. The quantitative targeted approach demonstrated an increase in free sphingoid bases, likely associated with a deregulated angiogenesis. Our findings indicate that lipid signature could play a central role in MA and that a detailed biomarker profile may contribute to untangle the complex, and still obscure, pathogenesis of MA.

Signaling through the S1P-S1PR Axis in the Gut, the Immune and the Central Nervous System in Multiple Sclerosis: Implication for Pathogenesis and Treatment

Sphingosine 1-phosphate (S1P) is a signaling molecule with complex biological functions that are exerted through the activation of sphingosine 1-phosphate receptors 1–5 (S1PR1–5). S1PR expression is necessary for cell proliferation, angiogenesis, neurogenesis and, importantly, for the egress of lymphocytes from secondary lymphoid organs. Since the inflammatory process is a key element of immune-mediated diseases, including multiple sclerosis (MS), S1PR modulators are currently used to ameliorate systemic immune responses. The ubiquitous expression of S1PRs by immune, intestinal and neural cells has significant implications for the regulation of the gut–brain axis. The dysfunction of this bidirectional communication system may be a significant factor contributing to MS pathogenesis, since an impaired intestinal barrier could lead to interaction between immune cells and microbiota with a potential to initiate abnormal local and systemic immune responses towards the central nervous system (CNS). It appears that the secondary mechanisms of S1PR modulators affecting the gut immune system, the intestinal barrier and directly the CNS, are coordinated to promote therapeutic effects. The scope of this review is to focus on S1P−S1PR functions in the cells of the CNS, the gut and the immune system with particular emphasis on the immunologic effects of S1PR modulation and its implication in MS.

PET Study of Sphingosine-1-phosphate Receptor 1 Expression in Response to S. aureus Infection

Sphingosine-1-phosphate receptor 1 (S1PR1) plays a crucial role in infectious diseases. Targeting S1PR1 provides protection against pathogens, such as influenza viruses. This study is aimed at investigating S1PR1 in response to bacterial infection by assessing S1PR1 expression in S. aureus-infected mice. A rodent local muscle bacterial infection model was developed by injecting S. aureus to the lower hind limb of Balb/c mice. The changes of S1PR1 expression in response to bacterial infection and blocking treatment were assessed using ex vivo biodistribution and in vivo positron emission tomography (PET) after intravenous injection of an S1PR1-specific radiotracer [18F]TZ4877. The specificity of [18F]TZ4877 was assessed using S1PR1-specific antagonist, NIBR-0213, and S1PR1-specific DsiRNA pretreated the animals. Immunohistochemical studies were performed to confirm the increase of S1PR1 expression in response to infection. Ex vivo biodistribution data showed that the uptake of [18F]TZ4877 was increased 30.6%, 54.3%, 74.3%, and 115.3% in the liver, kidney, pancreas, and thymus of the infected mice, respectively, compared to that in normal control mice, indicating that S1PR1 is involved in the early immune response to bacterial infection. NIBR-0213 or S1PR1-specific DsiRNA pretreatment reduced the tissue uptake of [18F]TZ4877, suggesting that uptake of [18F]TZ4877 is specific. Our PET/CT study data also confirmed that infected mice have increased [18F]TZ4877 uptake in several organs comparing to that in normal control mice. Particularly, compared to control mice, a 39% increase of [18F]TZ4877 uptake was observed in the infected muscle of S. aureus mice, indicating that S1PR1 expression was directly involved in the inflammatory response to infection. Overall, our study suggested that S1PR1 plays an important role in the early immune response to bacterial infection. The uptake of [18F]TZ4877 is tightly correlated with the S1R1 expression in response to S. aureus infection. PET with S1PR1-specific radiotracer [18F]TZ4877 could provide a noninvasive tool for detecting the early S1PR1 immune response to infectious diseases.

Endothelial barrier function is co-regulated at vessel bifurcations by fluid forces and sphingosine-1-phosphate

Sphingosine-1-phosphate (S1P) is a bioactive sphingolipid mediator of endothelial barrier function. Prior studies have implicated mechanical stimulation due to intravascular laminar shear stress in co-regulating S1P signaling in endothelial cells (ECs). Yet, vascular networks in vivo consist of vessel bifurcations, and this geometry generates hemodynamic forces at the bifurcation point distinct from laminar shear stress. However, the role of these forces at vessel bifurcations in regulating S1P-dependent endothelial barrier function is not known. In this study, we implemented a microfluidic platform that recapitulates the flow dynamics of vessel bifurcations with in situ quantification of the permeability of microvessel analogues. Co-application of S1P with impinging bifurcated fluid flow, which is characterized by approximately zero shear stress and 38 dyn•cm-2 stagnation pressure at the vessel bifurcation point, promotes vessel stabilization. Similarly, co-treatment of S1P with 3 dyn•cm-2 laminar shear stress is also protective of endothelial barrier function. Moreover, it is shown that vessel stabilization due to bifurcated fluid flow and laminar shear stress is dependent on S1P receptor 1 or 2 signaling. Collectively, these findings demonstrate the endothelium-protective function of fluid forces at vessel bifurcations and their involvement in coordinating S1P-dependent regulation of vessel permeability.

SPNS2 enables T cell egress from lymph nodes during an immune response

T cell expression of sphingosine 1-phosphate (S1P) receptor 1 (S1PR1) enables T cell exit from lymph nodes (LNs) into lymph, while endothelial S1PR1 expression regulates vascular permeability. Drugs targeting S1PR1 treat autoimmune disease by trapping pathogenic T cells within LNs, but they have adverse cardiovascular side effects. In homeostasis, the transporter SPNS2 supplies lymph S1P and enables T cell exit, while the transporter MFSD2B supplies most blood S1P and supports vascular function. It is unknown whether SPNS2 remains necessary to supply lymph S1P during an immune response, or whether in inflammation other compensatory transporters are upregulated. Here, using a model of dermal inflammation, we demonstrate that SPNS2 supplies the S1P that guides T cells out of LNs with an ongoing immune response. Furthermore, deletion of Spns2 is protective in a mouse model of multiple sclerosis. These results support the therapeutic potential of SPNS2 inhibitors to achieve spatially specific modulation of S1P signaling.

Barrier maintenance by S1P during inflammation and sepsis

Sphingosine 1-phosphate (S1P) is a multifaceted lipid signaling molecule that activates five specific G protein-coupled S1P receptors. Despite the fact that S1P is known as one of the strongest barrier-enhancing molecules for two decades, no medical application is available yet. The reason for this lack of translation into clinical practice may be the complex regulatory network of S1P signaling, metabolism and transportation.In this review, we will provide an overview about the physiology and the network of S1P signaling with the focus on endothelial barrier maintenance in inflammation. We briefly describe the physiological role of S1P and the underlying S1P signaling in barrier maintenance, outline differences of S1P signaling and metabolism in inflammatory diseases, discuss potential targets and compounds for medical intervention, and summarize our current knowledge regarding the role of S1P in the maintenance of specialized barriers like the blood-brain barrier and the placenta.

Role of the SphK-S1P-S1PRs pathway in invasion of the nervous system by SARS-CoV-2 infection

Global spread of the severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) is still ongoing. Before an effective vaccine is available, the development of potential treatments for resultant coronavirus disease 2019 (COVID‐19) is crucial. One of the disease hallmarks is hyper‐inflammatory responses, which usually leads to a severe lung disease. Patients with COVID‐19 also frequently suffer from neurological symptoms such as acute diffuse encephalomyelitis, brain injury and psychiatric complications. The metabolic pathway of sphingosine‐1‐phosphate (S1P) is a dynamic regulator of various cell types and disease processes, including the nervous system. It has been demonstrated that S1P and its metabolic enzymes, regulating neuroinflammation and neurogenesis, exhibit important functions during viral infection. S1P receptor 1 (S1PR1) analogues including AAL‐R and RP‐002 inhibit pathophysiological responses at the early stage of H1N1 virus infection and then play a protective role. Fingolimod (FTY720) is an S1P receptor modulator and is being tested for treating COVID‐19. Our review provides an overview of SARS‐CoV‐2 infection and critical role of the SphK‐S1P‐SIPR pathway in invasion of SARS‐CoV‐2 infection, particularly in the central nervous system (CNS). This may help design therapeutic strategies based on the S1P‐mediated signal transduction, and the adjuvant therapeutic effects of S1P analogues to limit or prevent the interaction between the host and SARS‐CoV‐2, block the spread of the SARS‐CoV‐2, and consequently treat related complications in the CNS.

Endothelial S1P1 Signaling Counteracts Infarct Expansion in Ischemic Stroke

RATIONALE

Cerebrovascular function is critical for brain health, and endogenous vascular protective pathways may provide therapeutic targets for neurological disorders. S1P (Sphingosine 1-phosphate) signaling coordinates vascular functions in other organs, and S1P1 (S1P receptor-1) modulators including fingolimod show promise for the treatment of ischemic and hemorrhagic stroke. However, S1P1 also coordinates lymphocyte trafficking, and lymphocytes are currently viewed as the principal therapeutic target for S1P1 modulation in stroke.

OBJECTIVE

To address roles and mechanisms of engagement of endothelial cell S1P1 in the naive and ischemic brain and its potential as a target for cerebrovascular therapy.

METHODS AND RESULTS

Using spatial modulation of S1P provision and signaling, we demonstrate a critical vascular protective role for endothelial S1P1 in the mouse brain. With an S1P1 signaling reporter, we reveal that abluminal polarization shields S1P1 from circulating endogenous and synthetic ligands after maturation of the blood-neural barrier, restricting homeostatic signaling to a subset of arteriolar endothelial cells. S1P1 signaling sustains hallmark endothelial functions in the naive brain and expands during ischemia by engagement of cell-autonomous S1P provision. Disrupting this pathway by endothelial cell-selective deficiency in S1P production, export, or the S1P1 receptor substantially exacerbates brain injury in permanent and transient models of ischemic stroke. By contrast, profound lymphopenia induced by loss of lymphocyte S1P1 provides modest protection only in the context of reperfusion. In the ischemic brain, endothelial cell S1P1 supports blood-brain barrier function, microvascular patency, and the rerouting of blood to hypoperfused brain tissue through collateral anastomoses. Boosting these functions by supplemental pharmacological engagement of the endothelial receptor pool with a blood-brain barrier penetrating S1P1-selective agonist can further reduce cortical infarct expansion in a therapeutically relevant time frame and independent of reperfusion.

CONCLUSIONS

This study provides genetic evidence to support a pivotal role for the endothelium in maintaining perfusion and microvascular patency in the ischemic penumbra that is coordinated by S1P signaling and can be harnessed for neuroprotection with blood-brain barrier-penetrating S1P1 agonists.

Erythrocyte sphingosine kinase regulates intraerythrocytic development of Plasmodium falciparum

The sphingolipid pool is key regulator of vital cellular functions in Plasmodium falciparum a causative agent for deadly malaria. Erythrocytes, the host for asexual stage of Plasmodium, are major reservoir for Sphingosine-1-phosphate (S1P). Erythrocyte possesses Sphingosine kinase (SphK) that catalyzed its biosynthesis from sphingosine (Sph). Since, Plasmodium lacks SphK homologous protein it can be envisaged that it co-opts sphingolipids from both intraerythrocytic as well as extracellular pools for its growth and development. Herein, by sphingosine-NBD probing, we report that infected erythrocytes imports Sph from extracellular pool, which is converted to S1P and thereby taken by P. falciparum. Next, by targeting of the SphK through specific inhibitor N,N-Dimethylsphingosine DMS, we show a reduction in erythrocyte endogenous S1P pool and SphK-phosphorylation that led to inhibition in growth and development of ring stage P. falciparum. Owing to the role of S1P in erythrocyte glycolysis we analyzed uptake of NBD-Glucose and production of lactate in DMS treated and untreated plasmodium. DMS treatment led to decreased glycolysis in Plasmodium. Interestingly the host free Plasmodium did not show any effect on glycolysis with DMS treatment indicating its host-mediated effect. Further to understand the in-vivo anti-plasmodial effects of exogenous and endogenous erythrocyte S1P level, Sphingosine-1-phosphate lyase (S1PL) inhibitor (THI), S1P and SphK-1 inhibitor (DMS), were used in Plasmodium berghei ANKA (PbA) mice model. DMS treatment led to reduction of endogenous S1P conferred significant decrease in parasite load, whereas the plasma level S1P modulated by (THI) and exogenous S1P have no effect on growth of Plasmodium. This suggested erythrocyte endogenous S1P pool is important for Plasmodium growth whereas the plasma level S1P has no effect. Altogether, this study provides insight on cellular processes regulated by S1P in P. falciparum and highlights the novel mechanistically distinct molecular target i.e. SphK-1.

Sphingosine-1-Phosphate Metabolism and Signaling in Kidney Diseases

In the past few decades, sphingolipids and sphingolipid metabolites have gained attention because of their essential role in the pathogenesis and progression of kidney diseases. Studies in models of experimental and clinical nephropathies have described accumulation of sphingolipids and sphingolipid metabolites, and it has become clear that the intracellular sphingolipid composition of renal cells is an important determinant of renal function. Proper function of the glomerular filtration barrier depends heavily on the integrity of lipid rafts, which include sphingolipids as key components. In addition to contributing to the structural integrity of membranes, sphingolipid metabolites, such as sphingosine-1-phosphate (S1P), play important roles as second messengers regulating biologic processes, such as cell growth, differentiation, migration, and apoptosis. This review will focus on the role of S1P in renal cells and how aberrant extracellular and intracellular S1P signaling contributes to the pathogenesis and progression of kidney diseases.

Lysolipids in Vascular Development, Biology, and Disease

Membrane phospholipid metabolism forms lysophospholipids, which possess unique biochemical and biophysical properties that influence membrane structure and dynamics. However, lysophospholipids also function as ligands for G-protein-coupled receptors that influence embryonic development, postnatal physiology, and disease. The 2 most well-studied species-lysophosphatidic acid and S1P (sphingosine 1-phosphate)-are particularly relevant to vascular development, physiology, and cardiovascular diseases. This review summarizes the role of lysophosphatidic acid and S1P in vascular developmental processes, endothelial cell biology, and their roles in cardiovascular disease processes. In addition, we also point out the apparent connections between lysophospholipid biology and the Wnt (int/wingless family) pathway, an evolutionarily conserved fundamental developmental signaling system. The discovery that components of the lysophospholipid signaling system are key genetic determinants of cardiovascular disease has warranted current and future research in this field. As pharmacological approaches to modulate lysophospholipid signaling have entered the clinical sphere, new findings in this field promise to influence novel therapeutic strategies in cardiovascular diseases.

Bioactive sphingolipids: Advancements and contributions from the laboratory of Dr. Lina M. Obeid

Sphingolipids and their synthetic enzymes have emerged as critical mediators in numerous diseases including inflammation, aging, and cancer. One enzyme in particular, sphingosine kinase (SK) and its product sphingosine-1-phosphate (S1P), has been extensively implicated in these processes. SK catalyzes the phosphorylation of sphingosine to S1P and exists as two isoforms, SK1 and SK2. In this review, we will discuss the contributions from the laboratory of Dr. Lina M. Obeid that have defined the roles for several bioactive sphingolipids in signaling and disease with an emphasis on her work defining SK1 in cellular fates and pathobiologies including proliferation, senescence, apoptosis, and inflammation.

Emerging Pharmacological Treatments for Cerebral Edema: Evidence from Clinical Studies

Cerebral edema, a common and often fatal companion to most forms of acute central nervous system disease, has been recognized since the time of ancient Egypt. Unfortunately, our therapeutic armamentarium remains limited, in part due to historic limitations in our understanding of cerebral edema pathophysiology. Recent advancements have led to a number of clinical trials for novel therapeutics that could fundamentally alter the treatment of cerebral edema. In this review, we discuss these agents, their targets, and the data supporting their use, with a focus on agents that have progressed to clinical trials.

Targeting the SphK-S1P-SIPR Pathway as a Potential Therapeutic Approach for COVID-19

The world is currently experiencing the worst health pandemic since the Spanish flu in 1918-the COVID-19 pandemic-caused by the coronavirus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). This pandemic is the world's third wake-up call this century. In 2003 and 2012, the world experienced two major coronavirus outbreaks, SARS-CoV-1 and Middle East Respiratory syndrome coronavirus (MERS-CoV), causing major respiratory tract infections. At present, there is neither a vaccine nor a cure for COVID-19. The severe COVID-19 symptoms of hyperinflammation, catastrophic damage to the vascular endothelium, thrombotic complications, septic shock, brain damage, acute disseminated encephalomyelitis (ADEM), and acute neurological and psychiatric complications are unprecedented. Many COVID-19 deaths result from the aftermath of hyperinflammatory complications, also referred to as the "cytokine storm syndrome", endotheliitus and blood clotting, all with the potential to cause multiorgan dysfunction. The sphingolipid rheostat plays integral roles in viral replication, activation/modulation of the immune response, and importantly in maintaining vasculature integrity, with sphingosine 1 phosphate (S1P) and its cognate receptors (SIPRs: G-protein-coupled receptors) being key factors in vascular protection against endotheliitus. Hence, modulation of sphingosine kinase (SphK), S1P, and the S1P receptor pathway may provide significant beneficial effects towards counteracting the life-threatening, acute, and chronic complications associated with SARS-CoV-2 infection. This review provides a comprehensive overview of SARS-CoV-2 infection and disease, prospective vaccines, and current treatments. We then discuss the evidence supporting the targeting of SphK/S1P and S1P receptors in the repertoire of COVID-19 therapies to control viral replication and alleviate the known and emerging acute and chronic symptoms of COVID-19. Three clinical trials using FDA-approved sphingolipid-based drugs being repurposed and evaluated to help in alleviating COVID-19 symptoms are discussed.

S1P lyase inhibition protects against sepsis by promoting disease tolerance via the S1P/S1PR3 axis

Background

One-third of all deaths in hospitals are caused by sepsis. Despite its demonstrated prevalence and high case fatality rate, antibiotics remain the only target-oriented treatment option currently available. Starting from results showing that low-dose anthracyclines protect against sepsis in mice, we sought to find new causative treatment options to improve sepsis outcomes.

Methods

Sepsis was induced in mice, and different treatment options were evaluated regarding cytokine and biomarker expression, lung epithelial cell permeability, autophagy induction, and survival benefit. Results were validated in cell culture experiments and correlated with patient samples.

Findings

Effective low-dose epirubicin treatment resulted in substantial downregulation of the sphingosine 1-phosphate (S1P) degrading enzyme S1P lyase (SPL). Consequent accumulation and secretion of S1P in lung parenchyma cells stimulated the S1P-receptor type 3 (S1PR3) and mitogen-activated protein kinases p38 and ERK, reducing tissue damage via increased disease tolerance. The protective effects of SPL inhibition were absent in S1PR3 deficient mice. Sepsis patients showed increased expression of SPL, stable expression of S1PR3, and increased levels of mucin-1 and surfactant protein D as indicators of lung damage.

Interpretation

Our work highlights a tissue-protective effect of SPL inhibition in sepsis due to activation of the S1P/S1PR3 axis and implies that SPL inhibitors and S1PR3 agonists might be potential therapeutics to protect against sepsis by increasing disease tolerance against infections.

Funding

This study was supported by the Center for Sepsis Control and Care (CSCC), the German Research Foundation (DFG), RTG 1715 (to M. H. G. and I. R.) and the National Institutes of Health, Grant R01GM043880 (to S. S.).

Murine platelet production is suppressed by S1P release in the hematopoietic niche, not facilitated by blood S1P sensing

The bioactive lipid mediator sphingosine 1-phosphate (S1P) was recently assigned critical roles in platelet biology: whereas S1P₁ receptor-mediated S1P gradient sensing was reported to be essential for directing proplatelet extensions from megakaryocytes (MKs) toward bone marrow sinusoids, MK sphingosine kinase 2 (Sphk2)-derived S1P was reported to further promote platelet shedding through receptor-independent intracellular actions, and platelet aggregation through S1P₁ Yet clinical use of S1P pathway modulators including fingolimod has not been associated with risk of bleeding or thrombosis. We therefore revisited the role of S1P in platelet biology in mice. Surprisingly, no reduction in platelet counts was observed when the vascular S1P gradient was ablated by impairing S1P provision to plasma or S1P degradation in interstitial fluids, nor when gradient sensing was impaired by S1pr1 deletion selectively in MKs. Moreover, S1P₁ expression and signaling were both undetectable in mature MKs in situ, and MK S1pr1 deletion did not affect platelet aggregation or spreading. When S1pr1 deletion was induced in hematopoietic progenitor cells, platelet counts were instead significantly elevated. Isolated global Sphk2 deficiency was associated with thrombocytopenia, but this was not replicated by MK-restricted Sphk2 deletion and was reversed by compound deletion of either Sphk1 or S1pr2, suggesting that this phenotype arises from increased S1P export and S1P₂ activation secondary to redistribution of sphingosine to Sphk1. Consistent with clinical observations, we thus observe no essential role for S1P₁ in facilitating platelet production or activation. Instead, S1P restricts megakaryopoiesis through S1P₁, and can further suppress thrombopoiesis through S1P₂ when aberrantly secreted in the hematopoietic niche.

Vascular permeability in the fibrotic lung

Idiopathic pulmonary fibrosis (IPF) is thought to result from aberrant tissue repair processes in response to chronic or repetitive lung injury. The origin and nature of the injury, as well as its cellular and molecular targets, are likely heterogeneous, which complicates accurate pre-clinical modelling of the disease and makes therapeutic targeting a challenge. Efforts are underway to identify central pathways in fibrogenesis which may allow targeting of aberrant repair processes regardless of the initial injury stimulus. Dysregulated endothelial permeability and vascular leak have long been studied for their role in acute lung injury and repair. Evidence that these processes are of importance to the pathogenesis of fibrotic lung disease is growing. Endothelial permeability is increased in non-fibrosing lung diseases, but it resolves in a self-limited fashion in conditions such as bacterial pneumonia and acute respiratory distress syndrome. In progressive fibrosing diseases such as IPF, permeability appears to persist, however, and may also predict mortality. In this hypothesis-generating review, we summarise available data on the role of endothelial permeability in IPF and focus on the deleterious consequences of sustained endothelial hyperpermeability in response to and during pulmonary inflammation and fibrosis. We propose that persistent permeability and vascular leak in the lung have the potential to establish and amplify the pro-fibrotic environment. Therapeutic interventions aimed at recognising and "plugging" the leak may therefore be of significant benefit for preventing the transition from lung injury to fibrosis and should be areas for future research.

A G protein-biased S1P1 agonist, SAR247799, protects endothelial cells without affecting lymphocyte numbers

Endothelial dysfunction is a hallmark of tissue injury and is believed to initiate the development of vascular diseases. Sphingosine-1 phosphate receptor-1 (S1P₁) plays fundamental physiological roles in endothelial function and lymphocyte homing. Currently available clinical molecules that target this receptor are desensitizing and are essentially S1P₁ functional antagonists that cause lymphopenia. They are clinically beneficial in autoimmune diseases such as multiple sclerosis. In patients, several side effects of S1P₁ desensitization have been attributed to endothelial damage, suggesting that drugs with the opposite effect, namely, the ability to activate S1P_1, could help to restore endothelial homeostasis. We found and characterized a biased agonist of S1P₁, SAR247799, which preferentially activated downstream G protein signaling to a greater extent than β-arrestin and internalization signaling pathways. SAR247799 activated S1P₁ on endothelium without causing receptor desensitization and potently activated protection pathways in human endothelial cells. In a pig model of coronary endothelial damage, SAR247799 improved the microvascular hyperemic response without reducing lymphocyte numbers. Similarly, in a rat model of renal ischemia/reperfusion injury, SAR247799 preserved renal structure and function at doses that did not induce S1P₁-desensitizing effects, such as lymphopenia and lung vascular leakage. In contrast, a clinically used S1P₁ functional antagonist, siponimod, conferred minimal renal protection and desensitized S1P₁ These findings demonstrate that sustained S1P₁ activation can occur pharmacologically without compromising the immune response, providing a new approach to treat diseases associated with endothelial dysfunction and vascular hyperpermeability.

Inflammatory Conditions Disrupt Constitutive Endothelial Cell Barrier Stabilization by Alleviating Autonomous Secretion of Sphingosine 1-Phosphate

The breakdown of the endothelial cell (EC) barrier contributes significantly to sepsis mortality. Sphingosine 1-phosphate (S1P) is one of the most effective EC barrier-stabilizing signaling molecules. Stabilization is mainly transduced via the S1P receptor type 1 (S1PR1). Here, we demonstrate that S1P was autonomously produced by ECs. S1P secretion was significantly higher in primary human umbilical vein endothelial cells (HUVEC) compared to the endothelial cell line EA.hy926. Constitutive barrier stability of HUVEC, but not EA.hy926, was significantly compromised by the S1PR1 antagonist W146 and by the anti-S1P antibody Sphingomab. HUVEC and EA.hy926 differed in the expression of the S1P-transporter Spns2, which allowed HUVEC, but not EA.hy926, to secrete S1P into the extracellular space. Spns2 deficient mice showed increased serum albumin leakage in bronchoalveolar lavage fluid (BALF). Lung ECs isolated from Spns2 deficient mice revealed increased leakage of fluorescein isothiocyanate (FITC) labeled dextran and decreased resistance in electric cell-substrate impedance sensing (ECIS) measurements. Spns2 was down-regulated in HUVEC after stimulation with pro-inflammatory cytokines and lipopolysaccharides (LPS), which contributed to destabilization of the EC barrier. Our work suggests a new mechanism for barrier integrity maintenance. Secretion of S1P by EC via Spns2 contributed to constitutive EC barrier maintenance, which was disrupted under inflammatory conditions via the down-regulation of the S1P-transporter Spns2.

Sphingosine 1-phosphate and inflammation

AbstractSphingosine 1-phosphate (S1P), a sphingolipid mediator, regulates various cellular functions via high-affinity G protein-coupled receptors, S1P1-5. The S1P-S1P receptor signaling system plays important roles in lymphocyte trafficking and maintenance of vascular integrity, thus contributing to the regulation of complex inflammatory processes. S1P is enriched in blood and lymph while maintained low in intracellular or interstitial fluids, creating a steep S1P gradient that is utilized to facilitate efficient egress of lymphocytes from lymphoid organs. Blockage of the S1P-S1P receptor signaling system results in a marked decrease in circulating lymphocytes because of a failure of lymphocyte egress from lymphoid organs. This provides a basis of immunomodulatory drugs targeting S1P1 receptor such as FTY720, an immunosuppressive drug approved in 2010 as the first oral treatment for relapsing-remitting multiple sclerosis. The S1P-S1P receptor signaling system also plays important roles in maintenance of vascular integrity since it suppresses sprouting angiogenesis and regulates vascular permeability. Dysfunction of the S1P-S1P receptor signaling system results in various vascular defects, such as exaggerated angiogenesis in developing retina and augmented inflammation due to increased permeability. Endothelial-specific deletion of S1P1 receptor in mice fed high-fat diet leads to increased formation of atherosclerotic lesions. This review highlights the importance of the S1P-S1P receptor signaling system in inflammatory processes. We also describe our recent findings regarding a specific S1P chaperone, apolipoprotein M, that anchors to high-density lipoprotein and contributes to shaping the endothelial-protective and anti-inflammatory properties of high-density lipoprotein.

Serum sphingosine-1-phosphate levels and Sphingosine-1-Phosphate gene polymorphisms in acute respiratory distress syndrome: a multicenter prospective study

BACKGROUND

Sphingosine-1-phosphate (S1P) is a signaling phospholipid involved in pathophysiologic progression of acute respiratory distress syndrome (ARDS) through its roles in endothelial barrier function and immune modulation. We hypothesized that decreased serum S1P level is associated with the clinical outcomes of ARDS and polymorphisms in the S1P gene are associated with serum S1P levels.

METHODS

This multicenter prospective study includes ARDS patients and healthy blood donors as controls. Serum S1P levels were quantified using enzyme-linked immunosorbent assays. Eight tag single nucleotide polymorphisms (SNPs) in the S1P gene were detected, and their associations with S1P levels were evaluated.

RESULTS

A total of 121 ARDS patients and 100 healthy individuals were enrolled. Serum S1P levels were lower in ARDS patients than in controls (P < 0.001). Decreased S1P levels correlated with more organ dysfunction and higher Acute Physiology and Chronic Health Evaluation II scores. Changes in S1P levels in ARDS patients were associated with the clinical outcomes. The recessive model for SNP rs3743631 suggests that GG homozygote is associate with a higher risk for ARDS. The dominant model for SNP rs907045 suggests that AA or TA genotype might increase the risk for ARDS. In ARDS patients, the rs3743631 GG genotype showed lower S1P levels than those harboring AG and AA genotypes. The serum S1P levels of rs907045 AA or TA genotype patients were lower than those of TT genotype.

CONCLUSIONS

Serum S1P levels are dramatically decreased in ARDS patients. Reduced S1P levels are associated with worse clinical outcomes. There is a significant association between S1P rs3743631, rs907045 polymorphisms and susceptibility of ARDS.

Reduction of Sphingosine Kinase 1 Phosphorylation and Activity in Plasmodium-Infected Erythrocytes

Sphingosine-1-phosphate (S1P), a bioactive lipid mediator is involved in an array of biological processes and linked to pathological manifestations. Erythrocyte is known as the major reservoir for S1P as they lack S1P-degrading enzymes (S1P lyase and S1P phosphohydrolase) and harbor sphingosine kinase-1 (SphK-1) essential for sphingosine conversion to S1P. Reduced S1P concentration in serum was correlated with disease severity in patients with Plasmodium falciparum and Plasmodium vivax infections. Herein, we aimed to identify the underlying mechanism and contribution of host erythrocytes toward depleted S1P levels in Plasmodium-infected patients vs. healthy individuals. The level and activity of SphK-1 were measured in vitro in both uninfected and cultured P. falciparum-infected erythrocytes. Infected erythrocytes demonstrated a significant decrease in SphK-1 level in a time-dependent manner. We found that 10–42 h post invasion (hpi), SphK1 level was predominantly reduced to ∼50% in rings, trophozoites, and schizonts compared to uninfected erythrocytes. We next analyzed the phosphorylation status of SphK-1, a modification responsible for its activity and S1P production, in both uninfected control and Plasmodium-infected erythrocytes. Almost ∼50% decrease in phosphorylation of SphK-1 was observed that could be corroborated with significant reduction in the production and release of S1P in infected erythrocytes. Serum S1P levels were studied in parallel in P. falciparum (N = 15), P. vivax (N = 36)-infected patients, and healthy controls (N = 6). The findings revealed that S1P concentration was significantly depleted in uncomplicated malaria cases and was found to be lowest in complicated malaria and thrombocytopenia in both P. falciparum and P. vivax-infected groups (^∗∗p < 0.01). The lower serum S1P level could be correlated with the reduced platelet count defining the role of S1P level in platelet formation. In conclusion, erythrocyte SphK-1 and S1P levels were studied in Plasmodium-infected individuals and erythrocytes that helped in characterizing the complications associated with malaria and thrombocytopenia, providing insights into the contribution of host erythrocyte biology in malaria pathogenesis. Finally, this study proposes the use of S1P and its analog as a novel adjunct therapy for malaria complications.

Mechanism of sphingosine 1-phosphate clearance from blood

The interplay of sphingosine 1-phosphate (S1P) synthetic and degradative enzymes as well as S1P exporters creates concentration gradients that are a fundamental to S1P biology. Extracellular S1P levels, such as in blood and lymph, are high relative to cellular S1P. The blood-tissue S1P gradient maintains endothelial integrity while local S1P gradients influence immune cell positioning. Indeed, the importance of S1P gradients was recognized initially when the mechanism of action of an S1P receptor agonist used as a medicine for multiple sclerosis was revealed to be inhibition of T-lymphocytes' recognition of the high S1P in efferent lymph. Furthermore, the increase in erythrocyte S1P in response to hypoxia influences oxygen delivery during high altitude acclimatization. However, understanding of how S1P gradients are maintained is incomplete. For example, S1P is synthesized but is only slowly metabolized by blood yet circulating S1P turns over quickly by an unknown mechanism. Prompted by the counterintuitive observation that blood S1P increases markedly in response to inhibition S1P synthesis (by sphingosine kinase 2 (SphK2)), we studied mice wherein several tissues were made deficient in either SphK2 or S1P degrading enzymes. Our data reveal a mechanism whereby S1P is de-phosphorylated at the hepatocyte surface and the resulting sphingosine is sequestered by SphK phosphorylation and in turn degraded by intracellular S1P lyase. Thus, we identify the liver as the primary site of blood S1P clearance and provide an explanation for the role of SphK2 in this process. Our discovery suggests a general mechanism whereby S1P gradients are shaped.

Sphingosine 1-phosphate-regulated transcriptomes in heterogenous arterial and lymphatic endothelium of the aorta

Despite the medical importance of G protein-coupled receptors (GPCRs), in vivo cellular heterogeneity of GPCR signaling and downstream transcriptional responses are not understood. We report the comprehensive characterization of transcriptomes (bulk and single-cell) and chromatin domains regulated by sphingosine 1-phosphate receptor-1 (S1PR1) in adult mouse aortic endothelial cells. First, S1PR1 regulates NFκB and nuclear glucocorticoid receptor pathways to suppress inflammation-related mRNAs. Second, S1PR1 signaling in the heterogenous endothelial cell (EC) subtypes occurs at spatially-distinct areas of the aorta. For example, a transcriptomically distinct arterial EC population at vascular branch points (aEC1) exhibits ligand-independent S1PR1/ß-arrestin coupling. In contrast, circulatory S1P-dependent S1PR1/ß-arrestin coupling was observed in non-branch point aEC2 cells that exhibit an inflammatory gene expression signature. Moreover, S1P/S1PR1 signaling regulates the expression of lymphangiogenic and inflammation-related transcripts in an adventitial lymphatic EC (LEC) population in a ligand-dependent manner. These insights add resolution to existing concepts of endothelial heterogeneity, GPCR signaling and S1P biology.

Sphingosine-1-Phosphate Enhances α1-Adrenergic Vasoconstriction via S1P2-G12/13-ROCK Mediated Signaling

Sphingosine-1-phosphate (S1P) has been implicated recently in the physiology and pathology of the cardiovascular system including regulation of vascular tone. Pilot experiments showed that the vasoconstrictor effect of S1P was enhanced markedly in the presence of phenylephrine (PE). Based on this observation, we hypothesized that S1P might modulate α₁-adrenergic vasoactivity. In murine aortas, a 20-minute exposure to S1P but not to its vehicle increased the E_max and decreased the EC₅₀ of PE-induced contractions indicating a hyperreactivity to α₁-adrenergic stimulation. The potentiating effect of S1P disappeared in S1P2 but not in S1P3 receptor-deficient vessels. In addition, smooth muscle specific conditional deletion of G_12/13 proteins or pharmacological inhibition of the Rho-associated protein kinase (ROCK) by Y-27632 or fasudil abolished the effect of S1P on α₁-adrenergic vasoconstriction. Unexpectedly, PE-induced contractions remained enhanced markedly as late as three hours after S1P-exposure in wild-type (WT) and S1P3 KO but not in S1P2 KO vessels. In conclusion, the S1P–S1P2–G_12/13–ROCK signaling pathway appears to have a major influence on α₁-adrenergic vasoactivity. This cooperativity might lead to sustained vasoconstriction when increased sympathetic tone is accompanied by increased S1P production as it occurs during acute coronary syndrome and stroke.

Emerging Connections of S1P-Metabolizing Enzymes with Host Defense and Immunity During Virus Infections

The sphingosine 1-phosphate (S1P) metabolic pathway is a dynamic regulator of multiple cellular and disease processes. Identification of the immune regulatory role of the sphingosine analog FTY720 led to the development of the first oral therapy for the treatment of an autoimmune disease, multiple sclerosis. Furthermore, inhibitors of sphingosine kinase (SphK), which mediate S1P synthesis, are being evaluated as a therapeutic option for the treatment of cancer. In conjunction with these captivating discoveries, S1P and S1P-metabolizing enzymes have been revealed to display vital functions during virus infections. For example, S1P lyase, which is known for metabolizing S1P, inhibits influenza virus replication by promoting antiviral type I interferon innate immune responses. In addition, both isoforms of sphingosine kinase have been shown to regulate the replication or pathogenicity of many viruses. Pro- or antiviral activities of S1P-metabolizing enzymes appear to be dependent on diverse virus–host interactions and viral pathogenesis. This review places an emphasis on summarizing the functions of S1P-metabolizing enzymes during virus infections and discusses the opportunities for designing pioneering antiviral drugs by targeting these host enzymes.

High-endothelial cell-derived S1P regulates dendritic cell localization and vascular integrity in the lymph node

While the sphingosine-1-phosphate (S1P)/sphingosine-1-phosphate receptor-1 (S1PR1) axis is critically important for lymphocyte egress from lymphoid organs, S1PR1-activation also occurs in vascular endothelial cells (ECs), including those of the high-endothelial venules (HEVs) that mediate lymphocyte immigration into lymph nodes (LNs). To understand the functional significance of the S1P/S1PR1-G_i axis in HEVs, we generated Lyve1;Spns2^Δ/Δ conditional knockout mice for the S1P-transporter Spinster-homologue-2 (SPNS2), as HEVs express LYVE1 during development. In these mice HEVs appeared apoptotic and were severely impaired in function, morphology and size; leading to markedly hypotrophic peripheral LNs. Dendritic cells (DCs) were unable to interact with HEVs, which was also observed in Cdh5^CRE-ERT2;S1pr1^Δ/Δ mice and wildtype mice treated with S1PR1-antagonists. Wildtype HEVs treated with S1PR1-antagonists in vitro and Lyve1-deficient HEVs show severely reduced release of the DC-chemoattractant CCL21 in vivo. Together, our results reveal that EC-derived S1P warrants HEV-integrity through autocrine control of S1PR1-G_i signaling, and facilitates concomitant HEV-DC interactions.

Passing the Vascular Barrier: Endothelial Signaling Processes Controlling Extravasation

A central function of the vascular endothelium is to serve as a barrier between the blood and the surrounding tissue of the body. At the same time, solutes and cells have to pass the endothelium to leave or to enter the bloodstream to maintain homeostasis. Under pathological conditions, for example, inflammation, permeability for fluid and cells is largely increased in the affected area, thereby facilitating host defense. To appropriately function as a regulated permeability filter, the endothelium uses various mechanisms to allow solutes and cells to pass the endothelial layer. These include transcellular and paracellular pathways of which the latter requires remodeling of intercellular junctions for its regulation. This review provides an overview on endothelial barrier regulation and focuses on the endothelial signaling mechanisms controlling the opening and closing of paracellular pathways for solutes and cells such as leukocytes and metastasizing tumor cells.

Identification of ApoA4 as a sphingosine 1-phosphate chaperone in ApoM- and albumin-deficient mice

HDL-bound ApoM and albumin are protein chaperones for the circulating bioactive lipid, sphingosine 1-phosphate (S1P); in this role, they support essential extracellular S1P signaling functions in the vascular and immune systems. We previously showed that ApoM- and albumin-bound S1P exhibit differences in receptor activation and biological functions. Whether the physiological functions of S1P require chaperones is not clear. We examined ApoM-deficient, albumin-deficient, and double-KO (DKO) mice for circulatory S1P and its biological functions. In albumin-deficient mice, ApoM was upregulated, thus enabling S1P functions in embryonic development and postnatal adult life. The Apom:Alb DKO mice reproduced, were viable, and exhibited largely normal vascular and immune functions, which suggested sufficient extracellular S1P signaling. However, Apom:Alb DKO mice had reduced levels (∼25%) of plasma S1P, suggesting that novel S1P chaperones exist to mediate S1P functions. In this study, we report the identification of ApoA4 as a novel S1P binding protein. Recombinant ApoA4 bound to S1P, activated multiple S1P receptors, and promoted vascular endothelial barrier function, all reflective of its function as a S1P chaperone in the absence of ApoM and albumin. We suggest that multiple S1P chaperones evolved to support complex and essential extracellular signaling functions of this lysolipid mediator in a redundant manner.

Fighting the Fire: Mechanisms of Inflammatory Gene Regulation by the Glucocorticoid Receptor

For many decades, glucocorticoids have been widely used as the gold standard treatment for inflammatory conditions. Unfortunately, their clinical use is limited by severe adverse effects such as insulin resistance, cardiometabolic diseases, muscle and skin atrophies, osteoporosis, and depression. Glucocorticoids exert their effects by binding to the Glucocorticoid Receptor (GR), a ligand-activated transcription factor which both positively, and negatively regulates gene expression. Extensive research during the past several years has uncovered novel mechanisms by which the GR activates and represses its target genes. Genome-wide studies and mouse models have provided valuable insight into the molecular mechanisms of inflammatory gene regulation by GR. This review focusses on newly identified target genes and GR co-regulators that are important for its anti-inflammatory effects in innate immune cells, as well as mutations within the GR itself that shed light on its transcriptional activity. This research progress will hopefully serve as the basis for the development of safer immune suppressants with reduced side effect profiles.

Sphingosine 1-Phosphate Receptor 1 Signaling Maintains Endothelial Cell Barrier Function and Protects Against Immune Complex-Induced Vascular Injury

OBJECTIVE

Immune complex (IC) deposition activates polymorphonuclear neutrophils (PMNs), increases vascular permeability, and leads to organ damage in systemic lupus erythematosus and rheumatoid arthritis. The bioactive lipid sphingosine 1-phosphate (S1P), acting via S1P receptor 1 (S1P₁ ), is a key regulator of endothelial cell (EC) barrier function. This study was undertaken to investigate whether augmenting EC integrity via S1P₁ signaling attenuates inflammatory injury mediated by ICs.

METHODS

In vitro barrier function was assessed in human umbilical vein endothelial cells (HUVECs) by electrical cell-substrate impedance sensing. Phosphorylation of myosin light chain 2 (p-MLC-2) and VE-cadherin staining in HUVECs were assessed by immunofluorescence. A reverse Arthus reaction (RAR) was induced in the skin and lungs of mice with S1P₁ deleted from ECs (S1P₁ EC-knockout [ECKO] mice) and mice treated with S1P₁ agonists and antagonists.

RESULTS

S1P₁ agonists prevented loss of barrier function in HUVECs treated with IC-activated PMNs. S1P₁ ECKO and wild-type (WT) mice treated with S1P₁ antagonists had amplified RAR, whereas specific S1P₁ agonists attenuated skin and lung RAR in WT mice. ApoM-Fc, a novel S1P chaperone, mitigated EC cell barrier dysfunction induced by activated PMNs in vitro and attenuated lung RAR. Expression levels of p-MLC-2 and disruption of VE-cadherin, each representing manifestations of cell contraction and destabilization of adherens junctions, respectively, that were induced by activated PMNs, were markedly reduced by treatment with S1P₁ agonists and ApoM-Fc.

CONCLUSION

Our findings indicate that S1P₁ signaling in ECs modulates vascular responses to IC deposition. S1P₁ agonists and ApoM-Fc enhance the EC barrier, limit leukocyte escape from capillaries, and provide protection against inflammatory injury. The S1P/S1P₁ axis is a newly identified target to attenuate tissue responses to IC deposition and mitigate end-organ damage.

S1P-S1PR1 Signaling: the "Sphinx" in Osteoimmunology

The fundamental interaction between the immune and skeletal systems, termed as osteoimmunology, has been demonstrated to play indispensable roles in the maintenance of balance between bone resorption and formation. The pleiotropic sphingolipid metabolite, sphingosine 1-phosphate (S1P), together with its cognate receptor, sphingosine-1-phosphate receptor-1 (S1PR1), are known as key players in osteoimmunology due to the regulation on both immune system and bone remodeling. The role of S1P-S1PR1 signaling in bone remodeling can be directly targeting both osteoclastogenesis and osteogenesis. Meanwhile, inflammatory cell function and polarization in both adaptive immune (T cell subsets) and innate immune cells (macrophages) are also regulated by this signaling axis, suggesting that S1P-S1PR1 signaling could aslo indirectly regulate bone remodeling via modulating the immune system. Therefore, it could be likely that S1P-S1PR1 signaling might take part in the maintenance of continuous bone turnover under physiological conditions, while lead to the pathogenesis of bone deformities during inflammation. In this review, we summarized the immunological regulation of S1P-S1PR1 signal axis during bone remodeling with an emphasis on how osteo-immune regulators are affected by inflammation, an issue with relevance to chronical bone disorders such as rheumatoid arthritis, spondyloarthritis and periodontitis.

Loss of sphingosine 1-phosphate (S1P) in septic shock is predominantly caused by decreased levels of high-density lipoproteins (HDL)

Background

Sphingosine 1-phosphate (S1P) is a signaling lipid essential in regulating processes involved in sepsis pathophysiology, including endothelial permeability and vascular tone. Serum S1P is progressively reduced in sepsis patients with increasing severity. S1P function depends on binding to its carriers: serum albumin (SA) and high-density lipoproteins (HDL). The aim of this single-center prospective observational study was to determine the contribution of SA- and HDL-associated S1P (SA-S1P and HDL-S1P) to sepsis-induced S1P depletion in plasma with regard to identify future strategies to supplement vasoprotective S1P.

Methods

Sequential precipitation of lipoproteins was performed with plasma samples obtained from 100 ICU patients: surgical trauma (n = 20), sepsis (n = 63), and septic shock (n = 17) together with healthy controls (n = 7). Resultant fractions with HDL and SA were analyzed by liquid chromatography coupled to triple-quadrupole mass spectrometry (LC-MS/MS) for their S1P content.

Results

Plasma S1P levels significantly decreased with sepsis severity and showed a strong negative correlation with increased organ failure, quantified by the Sequential Organ Failure Assessment (SOFA) score (rho - 0.59, P < 0.001). In controls, total plasma S1P levels were 208 μg/L (187-216 μg/L). In trauma patients, we observed an early loss of SA-S1P (- 70%) with a concurrent increase of HDL-S1P (+ 20%), resulting in unaltered total plasma S1P with 210 μg/L (143-257 μg/L). The decrease of plasma S1P levels with increasing SOFA score in sepsis patients with 180.2 μg/L (123.3-253.0 μg/L) and in septic shock patients with 99.5 μg/L (80.2-127.2 μg/L) was mainly dependent on equivalent reductions of HDL and not SA as carrier protein. Thus, HDL-S1P contributed most to total plasma S1P in patients and progressively dropped with increasing SOFA score.

Conclusions

Reduced plasma S1P was associated with sepsis-induced organ failure. A constant plasma S1P level during the acute phase after surgery was maintained with increased HDL-S1P and decreased SA-S1P, suggesting the redistribution of plasma S1P from SA to HDL. The decrease of plasma S1P levels in patients with increasing sepsis severity was mainly caused by decreasing HDL and HDL-S1P. Therefore, strategies to reconstitute HDL-S1P rather than SA-S1P should be considered for sepsis patients.