Molecular mechanisms of ceramide-mediated CD95 clustering

Unveiling enigmatic essence of Sphingolipids: A promising avenue for glaucoma treatment

Treatment of glaucoma, the leading cause of irreversible blindness, remains challenging. The apoptotic loss of retinal ganglion cells (RGCs) in glaucoma is the pathological hallmark. Current treatments often remain suboptimal as they aim to halt RGC loss secondary to reduction of intraocular pressure. The pathophysiological targets for exploring direct neuroprotective approaches, therefore are highly relevant. Sphingolipids have emerged as significant target molecules as they are not only the structural components of various cell constituents, but they also serve as signaling molecules that regulate molecular pathways involved in cell survival and death. Investigations have shown that a critical balance among various sphingolipid species, particularly the ceramide and sphingosine-1-phosphate play a role in deciding the fate of the cell. In this review we briefly discuss the metabolic interconversion of sphingolipid species to get an insight into "sphingolipid rheostat", the dynamic balance among metabolites. Further we highlight the role of sphingolipids in the key pathophysiological mechanisms that lead to glaucomatous loss of RGCs. Lastly, we summarize the potential drug candidates that have been investigated for their neuroprotective effects in glaucoma via their effects on sphingolipid axis.

Diabetic retinopathy is a ceramidopathy reversible by anti-ceramide immunotherapy

Diabetic retinopathy is a microvascular disease that causes blindness. Using acid sphingomyelinase knockout mice, we reported that ceramide generation is critical for diabetic retinopathy development. Here, in patients with proliferative diabetic retinopathy, we identify vitreous ceramide imbalance with pathologic long-chain C16-ceramides increasing and protective very long-chain C26-ceramides decreasing. C16-ceramides generate pro-inflammatory/pro-apoptotic ceramide-rich platforms on endothelial surfaces. To geo-localize ceramide-rich platforms, we invented a three-dimensional confocal assay and showed that retinopathy-producing cytokines TNFα and IL-1β induce ceramide-rich platform formation on retinal endothelial cells within seconds, with volumes increasing 2-logs, yielding apoptotic death. Anti-ceramide antibodies abolish these events. Furthermore, intravitreal and systemic anti-ceramide antibodies protect from diabetic retinopathy in standardized rodent ischemia reperfusion and streptozotocin models. These data support (1) retinal endothelial ceramide as a diabetic retinopathy treatment target, (2) early-stage therapy of non-proliferative diabetic retinopathy to prevent progression, and (3) systemic diabetic retinopathy treatment; and they characterize diabetic retinopathy as a "ceramidopathy" reversible by anti-ceramide immunotherapy.

Ceramides are fuel gauges on the drive to cardiometabolic disease

Ceramides are signals of fatty acid excess that accumulate when a cell's energetic needs have been met and its nutrient storage has reached capacity. As these sphingolipids accrue, they alter the metabolism and survival of cells throughout the body including in the heart, liver, blood vessels, skeletal muscle, brain, and kidney. These ceramide actions elicit the tissue dysfunction that underlies cardiometabolic diseases such as diabetes, coronary artery disease, metabolic-associated steatohepatitis, and heart failure. Here, we review the biosynthesis and degradation pathways that maintain ceramide levels in normal physiology and discuss how the loss of ceramide homeostasis drives cardiometabolic pathologies. We highlight signaling nodes that sense small changes in ceramides and in turn reprogram cellular metabolism and stimulate apoptosis. Finally, we evaluate the emerging therapeutic utility of these unique lipids as biomarkers that forecast disease risk and as targets of ceramide-lowering interventions that ameliorate disease.

Biological function, topology, and quantification of plasma membrane Ceramide

Over the past 30 years, a growing body of evidence has revealed the regulatory role of the lipid ceramide in various cellular functions. The structural diversity of ceramide, resulting in numerous species, and its distinct distribution within subcellular compartments may account for its wide range of functions. However, our ability to study the potential role of ceramide in specific subcellular membranes has been limited. Several works have shown mitochondrial, Golgi, and plasma membrane ceramide to mediate signaling pathways independently. These results have started to shift the focus on ceramide signaling research toward specific membrane pools. Nonetheless, the challenge arises from the substantial intracellular ceramide content, hindering efforts to quantify its presence in particular membranes. Recently, we have developed the first method capable of detecting and quantifying ceramide in the plasma membrane, leading to unexpected results such as detecting different pools of ceramide responding to drug concentration or time. This review summarizes the historical context that defined the idea of pools of ceramide, the studies on plasma membrane ceramide as a bioactive entity, and the tools available for its study, especially the new method to detect and, for the first time, quantify plasma membrane ceramide. We believe this method will open new avenues for researching sphingolipid signaling and metabolism.

Cryo-EM elucidates the uroplakin complex structure within liquid-crystalline lipids in the porcine urothelial membrane

The urothelium, a distinct epithelial tissue lining the urinary tract, serves as an essential component in preserving urinary tract integrity and thwarting infections. The asymmetric unit membrane (AUM), primarily composed of the uroplakin complex, constitutes a critical permeability barrier in fulfilling this role. However, the molecular architectures of both the AUM and the uroplakin complex have remained enigmatic due to the paucity of high-resolution structural data. In this study, we utilized cryo-electron microscopy to elucidate the three-dimensional structure of the uroplakin complex within the porcine AUM. While the global resolution achieved was 3.5 Å, we acknowledge that due to orientation bias, the resolution in the vertical direction was determined to be 6.3 Å. Our findings unveiled that the uroplakin complexes are situated within hexagonally arranged crystalline lipid membrane domains, rich in hexosylceramides. Moreover, our research rectifies a misconception in a previous model by confirming the existence of a domain initially believed to be absent, and pinpointing the accurate location of a crucial Escherichia coli binding site implicated in urinary tract infections. These discoveries offer valuable insights into the molecular underpinnings governing the permeability barrier function of the urothelium and the orchestrated lipid phase formation within the plasma membrane.

Ceramides and their roles in programmed cell death

Programmed cell death plays a crucial role in maintaining the homeostasis and integrity of multicellular organisms, and its dysregulation contributes to the pathogenesis of many diseases. Programmed cell death is regulated by a range of macromolecules and low-molecular messengers, including ceramides. Endogenous ceramides have different functions, that are influenced by their localization and the presence of their target molecules. This article provides an overview of the current understanding of ceramides and their impact on various types of programmed cell death, including apoptosis, anoikis, macroautophagy and mitophagy, and necroptosis. Moreover, it highlights the emergence of dihydroceramides as a new class of bioactive sphingolipids and their downstream targets as well as their future roles in cancer cell growth, drug resistance and tumor metastasis.

Sphingolipids: From structural components to signaling hubs

In late November 2019, Prof. Lina M. Obeid passed away from cancer, a disease she spent her life researching and studying its intricate molecular underpinnings. Along with her husband, Prof. Yusuf A. Hannun, Obeid laid down the foundations of sphingolipid biochemistry and oversaw its remarkable evolution over the years. Lipids are a class of macromolecules that are primarily associated with cellular architecture. In fact, lipids constitute the perimeter of the cell in such a way that without them, there cannot be cells. Hence, much of the early research on lipids identified the function of this class of biological molecules as merely structural. Nevertheless, unlike proteins, carbohydrates, and nucleic acids, lipids are elaborately diverse as they are not made up of monomers in polymeric forms. This diversity in structure is clearly mirrored by functional pleiotropy. In this chapter, we focus on a major subset of lipids, sphingolipids, and explore their historic rise from merely inert structural components of plasma membranes to lively and necessary signaling molecules that transmit various signals and control many cellular processes. We will emphasize the works of Lina Obeid since she was an integral pillar of the sphingolipid research world.

Unveiling Liquid-Crystalline Lipids in the Urothelial Membrane through Cryo-EM

The urothelium, a distinct epithelial tissue lining the urinary tract, serves as an essential component in preserving urinary tract integrity and thwarting infections. The asymmetric unit membrane (AUM), primarily composed of the uroplakin complex, constitutes a critical permeability barrier in fulfilling this role. However, the molecular architectures of both the AUM and the uroplakin complex have remained enigmatic due to the paucity of high-resolution structural data. In this study, we utilized cryo-electron microscopy to elucidate the three-dimensional structure of the uroplakin complex within the porcine AUM. While the global resolution achieved was 3.5 Å, we acknowledge that due to orientation bias, the resolution in the vertical direction was determined to be 6.3 Å. Our findings unveiled that the uroplakin complexes are situated within hexagonally arranged crystalline lipid membrane domains, rich in hexosylceramides. Moreover, our research rectifies a misconception in a previous model by confirming the existence of a domain initially believed to be absent, and pinpointing the accurate location of a crucial Escherichia coli binding site implicated in urinary tract infections. These discoveries offer valuable insights into the molecular underpinnings governing the permeability barrier function of the urothelium and the orchestrated lipid phase formation within the plasma membrane.

Unveiling Liquid-Crystalline Lipids in the Urothelial Membrane through Cryo-EM

The urothelium, a distinct epithelial tissue lining the urinary tract, serves as an essential component in preserving urinary tract integrity and thwarting infections. The asymmetric unit membrane (AUM), primarily composed of the uroplakin complex, constitutes a critical permeability barrier in fulfilling this role. However, the molecular architectures of both the AUM and the uroplakin complex have remained enigmatic due to the paucity of high-resolution structural data. In this investigation, we employed cryo-electron microscopy to elucidate the three-dimensional structure of the uroplakin complex embedded within the porcine AUM at a resolution of 3.5 Å. Our findings unveiled that the uroplakin complexes are situated within hexagonally arranged crystalline lipid membrane domains, rich in hexosylceramides. Moreover, our research rectifies a misconception in a previous model by confirming the existence of a domain initially believed to be absent, and pinpointing the accurate location of a crucial Escherichia coli binding site implicated in urinary tract infections. These discoveries offer valuable insights into the molecular underpinnings governing the permeability barrier function of the urothelium and the orchestrated lipid phase formation within the plasma membrane.

Engineered Lipidic Nanomaterials Inspired by Sphingomyelin Metabolism for Cancer Therapy

Sphingomyelin (SM) and its metabolites are crucial regulators of tumor cell growth, differentiation, senescence, and programmed cell death. With the rise in lipid-based nanomaterials, engineered lipidic nanomaterials inspired by SM metabolism, corresponding lipid targeting, and signaling activation have made fascinating advances in cancer therapeutic processes. In this review, we first described the specific pathways of SM metabolism and the roles of their associated bioactive molecules in mediating cell survival or death. We next summarized the advantages and specific applications of SM metabolism-based lipidic nanomaterials in specific cancer therapies. Finally, we discussed the challenges and perspectives of this emerging and promising SM metabolism-based nanomaterials research area.

Sphingolipids in thyroid eye disease

Graves' disease (GD) is caused by an autoimmune formation of autoantibodies and autoreactive T-cells against the thyroid stimulating hormone receptor (TSHR). The autoimmune reaction does not only lead to overstimulation of the thyroid gland, but very often also to an immune reaction against antigens within the orbital tissue leading to thyroid eye disease, which is characterized by activation of orbital fibroblasts, orbital generation of adipocytes and myofibroblasts and increased hyaluronan production in the orbit. Thyroid eye disease is the most common extra-thyroidal manifestation of the autoimmune Graves' disease. Several studies indicate an important role of sphingolipids, in particular the acid sphingomyelinase/ceramide system and sphingosine 1-phosphate in thyroid eye disease. Here, we discuss how the biophysical properties of sphingolipids contribute to cell signaling, in particular in the context of thyroid eye disease. We further review the role of the acid sphingomyelinase/ceramide system in autoimmune diseases and its function in T lymphocytes to provide some novel hypotheses for the pathogenesis of thyroid eye disease and potentially allowing the development of novel treatments.

Ablation of Sphingosine Kinase 1 Protects Cornea from Neovascularization in a Mouse Corneal Injury Model

The purpose of this study was to investigate the role of sphingosine kinase 1 (SphK1), which generates sphingosine-1-phosphate (S1P), in corneal neovascularization (NV). Wild-type (WT) and Sphk1 knockout (Sphk1^−/−) mice received corneal alkali-burn treatment to induce corneal NV by placing a 2 mm round piece of Whatman No. 1 filter paper soaked in 1N NaOH on the center of the cornea for 20 s. Corneal sphingolipid species were extracted and identified using liquid chromatography/mass spectrometry (LC/MS). The total number of tip cells and those positive for ethynyl deoxy uridine (EdU) were quantified. Immunocytochemistry was done to examine whether pericytes were present on newly forming blood vessels. Cytokine signaling and angiogenic markers were compared between the two groups using multiplex assays. Data were analyzed using appropriate statistical tests. Here, we show that ablation of SphK1 can significantly reduce NV invasion in the cornea following injury. Corneal sphingolipid analysis showed that total levels of ceramides, monohexosyl ceramides (HexCer), and sphingomyelin were significantly elevated in Sphk^−/− corneas compared to WT corneas, with a comparable level of sphingosine among the two genotypes. The numbers of total and proliferating endothelial tip cells were also lower in the Sphk1^−/− corneas following injury. This study underscores the role of S1P in post-injury corneal NV and raises further questions about the roles played by ceramide, HexCer, and sphingomyelin in regulating corneal NV. Further studies are needed to unravel the role played by bioactive sphingolipids in maintenance of corneal transparency and clear vision.

The key role of sphingolipid metabolism in cancer: New therapeutic targets, diagnostic and prognostic values, and anti-tumor immunotherapy resistance

Biologically active sphingolipids are closely related to the growth, differentiation, aging, and apoptosis of cancer cells. Some sphingolipids, such as ceramides, are favorable metabolites in the sphingolipid metabolic pathway, usually mediating antiproliferative responses, through inhibiting cancer cell growth and migration, as well as inducing autophagy and apoptosis. However, other sphingolipids, such as S1P, play the opposite role, which induces cancer cell transformation, migration and growth and promotes drug resistance. There are also other sphingolipids, as well as enzymes, played potentially critical roles in cancer physiology and therapeutics. This review aimed to explore the important roles of sphingolipid metabolism in cancer. In this article, we summarized the role and value of sphingolipid metabolism in cancer, including the distribution of sphingolipids, the functions, and their relevance to cancer diagnosis and prognosis. We also summarized the known and potential antitumor targets present in sphingolipid metabolism, analyzed the correlation between sphingolipid metabolism and tumor immunity, and summarize the antitumor effects of natural compounds based on sphingolipids. Through the analysis and summary of sphingolipid antitumor therapeutic targets and immune correlation, we aim to provide ideas for the development of new antitumor drugs, exploration of new therapeutic means for tumors, and study of immunotherapy resistance mechanisms.

Inimitable Impacts of Ceramides on Lipid Rafts Formed in Artificial and Natural Cell Membranes

Ceramide is the simplest precursor of sphingolipids and is involved in a variety of biological functions ranging from apoptosis to the immune responses. Although ceramide is a minor constituent of plasma membranes, it drastically increases upon cellular stimulation. However, the mechanistic link between ceramide generation and signal transduction remains unknown. To address this issue, the effect of ceramide on phospholipid membranes has been examined in numerous studies. One of the most remarkable findings of these studies is that ceramide induces the coalescence of membrane domains termed lipid rafts. Thus, it has been hypothesised that ceramide exerts its biological activity through the structural alteration of lipid rafts. In the present article, we first discuss the characteristic hydrogen bond functionality of ceramides. Then, we showed the impact of ceramide on the structures of artificial and cell membranes, including the coalescence of the pre-existing lipid raft into a large patch called a signal platform. Moreover, we proposed a possible structure of the signal platform, in which sphingomyelin/cholesterol-rich and sphingomyelin/ceramide-rich domains coexist. This structure is considered to be beneficial because membrane proteins and their inhibitors are separately compartmentalised in those domains. Considering the fact that ceramide/cholesterol content regulates the miscibility of those two domains in model membranes, the association and dissociation of membrane proteins and their inhibitors might be controlled by the contents of ceramide and cholesterol in the signal platform.

Lipid larceny: channelizing host lipids for establishing successful pathogenesis by bacteria

Lipids are complex organic compounds made up of carbon, oxygen, and hydrogen. These play a diverse and intricate role in cellular processes like membrane trafficking, protein sorting, signal transduction, and bacterial infections. Both Gram-positive bacteria (Staphylococcus sp., Listeria monocytogenes, etc.) and Gram-negative bacteria (Chlamydia sp., Salmonella sp., E. coli, etc.) can hijack the various host-lipids and utilize them structurally as well as functionally to mount a successful infection. The pathogens can deploy with various arsenals to exploit host membrane lipids and lipid-associated receptors as an attachment for toxins' landing or facilitate their entry into the host cellular niche. Bacterial species like Mycobacterium sp. can also modulate the host lipid metabolism to fetch its carbon source from the host. The sequential conversion of host membrane lipids into arachidonic acid and prostaglandin E2 due to increased activity of cPLA-2 and COX-2 upon bacterial infection creates immunosuppressive conditions and facilitates the intracellular growth and proliferation of bacteria. However, lipids' more debatable role is that they can also be a blessing in disguise. Certain host-lipids, especially sphingolipids, have been shown to play a crucial antibacterial role and help the host in combating the infections. This review shed light on the detailed role of host lipids in bacterial infections and the current understanding of the lipid in therapeutics. We have also discussed potential prospects and the need of the hour to help us cope in this race against deadly pathogens and their rapidly evolving stealthy virulence strategies.

Cholesterol and sphingomyelin are critical for Fcγ receptor-mediated phagocytosis of Cryptococcus neoformans by macrophages

Cryptococcus neoformans is a fungal pathogen that causes life-threatening meningoencephalitis in lymphopenic patients. Pulmonary macrophages comprise the first line of host defense upon inhalation of fungal spores by aiding in clearance but can also potentially serve as a niche for their dissemination. Given that macrophages play a key role in the outcome of a cryptococcal infection, it is crucial to understand factors that mediate phagocytosis of C. neoformans. Since lipid rafts (high-order plasma membrane domains enriched in cholesterol and sphingomyelin [SM]) have been implicated in facilitating phagocytosis, we evaluated whether these ordered domains govern macrophages' ability to phagocytose C. neoformans. We found that cholesterol or SM depletion resulted in significantly deficient immunoglobulin G (IgG)-mediated phagocytosis of fungus. Moreover, repletion of macrophage cells with a raft-promoting sterol (7-dehydrocholesterol) rescued this phagocytic deficiency, whereas a raft-inhibiting sterol (coprostanol) significantly decreased IgG-mediated phagocytosis of C. neoformans. Using a photoswitchable SM (AzoSM), we observed that the raft-promoting conformation (trans-AzoSM) resulted in efficient phagocytosis, whereas the raft-inhibiting conformation (cis-AzoSM) significantly but reversibly blunted phagocytosis. We observed that the effect on phagocytosis may be facilitated by Fcγ receptor (FcγR) function, whereby IgG immune complexes crosslink to FcγRIII, resulting in tyrosine phosphorylation of FcR γ-subunit (FcRγ), an important accessory protein in the FcγR signaling cascade. Correspondingly, cholesterol or SM depletion resulted in decreased FcRγ phosphorylation. Repletion with 7-dehydrocholesterol restored phosphorylation, whereas repletion with coprostanol showed FcRγ phosphorylation comparable to unstimulated cells. Together, these data suggest that lipid rafts are critical for facilitating FcγRIII-mediated phagocytosis of C. neoformans.

Geniposide ameliorates chronic unpredictable mild stress induced depression-like behavior through inhibition of ceramide-PP2A signaling via the PI3K/Akt/GSK3β axis

BACKGROUND

Depression is a severe mental disorder. Unfortunately, more than half of patients with major depression disorder cannot achieve remission after initial treatment with an antidepressant. Geniposide, a bioactive iridoid glycoside isolated from Gardenia jasminoides Ellis, can ameliorate depressive-like behaviors in mice. However, the underlying mechanism is still not very clear.

METHODS

The pharmacological methods including ELISA, immunofluorescence, and Western blot were used to investigate the role of geniposide on chronic unpredictable mild stress (CUMS)-induced depression mice.

RESULTS

In this study, we found that geniposide could inhibit CUMS-induced depressive-like behaviors in mice. Geniposide is able to reduce the levels of ceramide and lower the activity of acid sphingomyelinase (ASM) in hippocampus; besides, ASM inhibitor (amitriptyline) can decrease the concentration of ceramide and ameliorate depressive-like behaviors of mice. Moreover, geniposide can also alleviate CUMS-induced hippocampal neuronal apoptosis and increase the phosphorylated form of PI3K, Akt, and GSK3β. Additionally, PI3K inhibitor (LY294002) can also abolish the neuroprotective effect of geniposide on hippocampal neurons in vitro.

CONCLUSIONS

These results indicate that geniposide exert a potential antidepressant-like effect on CUMS mice, and its effect might be associated with activated PI3K/Akt/GSK3β signaling, reduced the level of ceramide and hippocampal neuron apoptosis.

Compartmentalization of Sphingolipid metabolism: Implications for signaling and therapy

Sphingolipids (SLs) are a family of bioactive lipids implicated in a variety of cellular processes, and whose levels are controlled by an interlinked network of enzymes. While the spatial distribution of SL metabolism throughout the cell has been understood for some time, the implications of this for SL signaling and biological outcomes have only recently begun to be fully explored. In this review, we outline the compartmentalization of SL metabolism and describe advances in tools for investigating and probing compartment-specific SL functions. We also briefly discuss the implications of SL compartmentalization for cell signaling and therapeutic approaches to targeting the SL network.

Staphylococcus aureus α-Toxin Induces Acid Sphingomyelinase Release From a Human Endothelial Cell Line

Staphylococcus aureus (S. aureus) is well known to express a plethora of toxins of which the pore-forming hemolysin A (α-toxin) is the best-studied cytolysin. Pore-forming toxins (PFT) permeabilize host membranes during infection thereby causing concentration-dependent effects in host cell membranes ranging from disordered ion fluxes to cytolysis. Host cells possess defense mechanisms against PFT attack, resulting in endocytosis of the breached membrane area and delivery of repair vesicles to the insulted plasma membrane as well as a concurrent release of membrane repair enzymes. Since PFTs from several pathogens have been shown to recruit membrane repair components, we here investigated whether staphylococcal α-toxin is able to induce these mechanisms in endothelial cells. We show that S. aureus α-toxin induced increase in cytosolic Ca²⁺ in endothelial cells, which was accompanied by p38 MAPK phosphorylation. Toxin challenge led to increased endocytosis of an extracellular fluid phase marker as well as increased externalization of LAMP1-positive membranes suggesting that peripheral lysosomes are recruited to the insulted plasma membrane. We further observed that thereby the lysosomal protein acid sphingomyelinase (ASM) was released into the cell culture medium. Thus, our results show that staphylococcal α-toxin triggers mechanisms in endothelial cells, which have been implicated in membrane repair after damage of other cell types by different toxins.

Molecular Mechanisms of Lipid Metabolism Disorders in Infectious Exacerbations of Chronic Obstructive Pulmonary Disease

Exacerbations largely determine the character of the progression and prognosis of chronic obstructive pulmonary disease (COPD). Exacerbations are connected with changes in the microbiological landscape in the bronchi due to a violation of their immune homeostasis. Many metabolic and immune processes involved in COPD progression are associated with bacterial colonization of the bronchi. The objective of this review is the analysis of the molecular mechanisms of lipid metabolism and immune response disorders in the lungs in COPD exacerbations. The complex role of lipid metabolism disorders in the pathogenesis of some infections is only beginning to be understood, however, there are already fewer and fewer doubts even now about its significance both in the pathogenesis of infectious exacerbations of COPD and in general in the progression of the disease. It is shown that the lipid rafts of the plasma membranes of cells are involved in many processes related to the detection of pathogens, signal transduction, the penetration of pathogens into the cell. Smoking disrupts the normally proceeded processes of lipid metabolism in the lungs, which is a part of the COPD pathogenesis.

Sphingolipids as critical players in retinal physiology and pathology

Sphingolipids have emerged as bioactive lipids involved in the regulation of many physiological and pathological processes. In the retina, they have been established to participate in numerous processes, such as neuronal survival and death, proliferation and migration of neuronal and vascular cells, inflammation, and neovascularization. Dysregulation of sphingolipids is therefore crucial in the onset and progression of retinal diseases. This review examines the involvement of sphingolipids in retinal physiology and diseases. Ceramide (Cer) has emerged as a common mediator of inflammation and death of neuronal and retinal pigment epithelium cells in animal models of retinopathies such as glaucoma, age-related macular degeneration (AMD), and retinitis pigmentosa. Sphingosine-1-phosphate (S1P) has opposite roles, preventing photoreceptor and ganglion cell degeneration but also promoting inflammation, fibrosis, and neovascularization in AMD, glaucoma, and pro-fibrotic disorders. Alterations in Cer, S1P, and ceramide 1-phosphate may also contribute to uveitis. Notably, use of inhibitors that either prevent Cer increase or modulate S1P signaling, such as Myriocin, desipramine, and Fingolimod (FTY720), preserves neuronal viability and retinal function. These findings underscore the relevance of alterations in the sphingolipid metabolic network in the etiology of multiple retinopathies and highlight the potential of modulating their metabolism for the design of novel therapeutic approaches.

Analysis of Lipids in Ceramide-Enriched Membrane Domains

Ceramide can be generated on cell surfaces by the activity of the acid sphingomyelinase. The unique biophysical properties of ceramide result in the self-formation of small ceramide-enriched membrane domains that spontaneously fuse to large ceramide-enriched membrane macrodomains. The present chapter describes how these domains can be labeled and thereby visualized in cells. Further, the chapter provides protocols how ceramide and sphingosine can be quantified on the surface of cells and organs.

Non-Thermal Plasma Couples Oxidative Stress to TRAIL Sensitization through DR5 Upregulation

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) induces apoptosis in various tumor cells without affecting most normal cells. Despite being in clinical testing, novel strategies to induce TRAIL-mediated apoptosis are in need to overcome cancer cell unresponsiveness and resistance. Plasma-activated medium (PAM) markedly stimulates reactive oxygen/nitrogen species (ROS/RNS)-dependent apoptosis in cancer cells. We investigate the capability of PAM and TRAIL (PAM/TRAIL) combination therapy to overcome TRAIL resistance and improve the anticancer efficacy of TRAIL. The combinatorial treatment of PAM and TRAIL shows synergistic effects on growth inhibition in TRAIL-resistant cancer cells via augmented apoptosis by two attributes. DR5 (TRAIL-R2) transcription by CHOP is upregulated in a PAM-generated ROS/RNS-dependent manner, and PAM itself upregulates PTEN expression mediated by suppression of miR-425 which is involved in Akt inactivation, leading to increased apoptosis induction. Treatment of cancer cell lines with the antioxidant N-acetylcysteine reduces the extent of membrane dysfunction and the expression of both CHOP-DR5 and miR-425-PTEN axes, attenuating PAM/TRAIL-induced cancer cell apoptosis. These data suggest that PAM/TRAIL treatment is a novel approach to sensitizing cancer cells to TRAIL-induced apoptosis and overcoming TRAIL resistance. PAM is a promising candidate for further investigations as a chemotherapeutic sensitizer in the treatment of cancer.

On the role of sphingolipids in cell survival and death

Sphingolipids, universal components of biological membranes of all eukaryotic organisms, from yeasts to mammals, in addition of playing a structural role, also play an important part of signal transduction pathways. They participate or, also, ignite several fundamental subcellular signaling processes but, more in general, they directly contribute to key biological activities such as cell motility, growth, senescence, differentiation as well as cell fate, i.e., survival or death. The sphingolipid metabolic pathway displays an intricate network of reactions that result in the formation of multiple sphingolipids, including ceramide, and sphingosine-1-phosphate. Different sphingolipids, that have key roles in determining cell fate, can induce opposite effects: as a general rule, sphingosine-1-phosphate promotes cell survival and differentiation, whereas ceramide is known to induce apoptosis. Furthermore, together with cholesterol, sphingolipids also represent the basic lipid component of lipid rafts, cholesterol- and sphingolipid-enriched membrane microdomains directly involved in cell death and survival processes. In this review, we briefly describe the characteristics of sphingolipids and lipid membrane microdomains. In particular, we will consider the involvement of various sphingolipids per se and of lipid rafts in apoptotic pathway, both intrinsic and extrinsic, in nonapoptotic cell death, in autophagy, and in cell differentiation. In addition, their roles in the most common physiological and pathological contexts either as pathogenetic elements or as biomarkers of diseases will be considered. We would also hint how the manipulation of sphingolipid metabolism could represent a potential therapeutic target to be investigated and functionally validated especially for those diseases for which therapeutic options are limited or ineffective.

Acid sphingomyelinase regulates TH 2 cytokine release and bronchial asthma

BACKGROUND

Allergic diseases and especially allergic asthma are widespread diseases with high prevalence in childhood, but also in adults. Acid sphingomyelinase (ASM) is a key regulator of the sphingolipid pathway. Previous studies defined the association of ASM with the pathogenesis of T_H 1-directed lung diseases like cystic fibrosis and acute lung injury. Here, we define the role of ASM in T_H 2-regulated allergic bronchial asthma.

METHODS

To determine the role of Asm under baseline conditions, wild-type (WT) and Asm^-/- mice were ventilated with a flexiVent setup and bronchial hyperresponsiveness was determined using acetylcholine. Flow cytometry and cytokine measurements in bronchoalveolar lavage fluid and lung tissue were followed by in vitro T_H 2 differentiations with cells from WT and Asm^-/- mice and blockade of Asm with amitriptyline. As proof of principle, we conducted an ovalbumin-induced model of asthma in WT- and Asm^-/-  mice.

RESULTS

At baseline, Asm^-/- mice showed better lung mechanics, but unaltered bronchial hyperresponsiveness. Higher numbers of Asm^-/- T cells in bronchoalveolar lavage fluid released lower levels of IL-4 and IL-5, and these results were paralleled by decreased production of typical T_H 2 cytokines in Asm^-/- T lymphocytes in vitro. This phenotype could be imitated by incubation of T cells with amitriptyline. In the ovalbumin asthma model, Asm^-/- animals were protected from high disease activity and showed better lung functions and lower levels of eosinophils and T_H 2 cytokines.

CONCLUSION

Asm deficiency could induce higher numbers of T_H 2 cells in the lung, but those cells release decreased T_H 2 cytokine levels. Hereby, Asm^-/- animals are protected from bronchial asthma, which possibly offers novel therapeutic strategies, for example, with ASM blockade.

Non-pharmacological factors that determine drug use and addiction

Based on their pharmacological properties, psychoactive drugs are supposed to take control of the natural reward system to finally drive compulsory drug seeking and consumption. However, psychoactive drugs are not used in an arbitrary way as pure pharmacological reinforcement would suggest, but rather in a highly specific manner depending on non-pharmacological factors. While pharmacological effects of psychoactive drugs are well studied, neurobiological mechanisms of non-pharmacological factors are less well understood. Here we review the emerging neurobiological mechanisms beyond pharmacological reinforcement which determine drug effects and use frequency. Important progress was made on the understanding of how the character of an environment and social stress determine drug self-administration. This is expanded by new evidence on how behavioral alternatives and opportunities for drug instrumentalization generate different patterns of drug choice. Emerging evidence suggests that the neurobiology of non-pharmacological factors strongly determines pharmacological and behavioral drug action and may, thus, give rise for an expanded system's approach of psychoactive drug use and addiction.

Glycosphingolipid metabolism and polycystic kidney disease

Sphingolipids and glycosphingolipids are classes of structurally and functionally important lipids that regulate multiple cellular processes, including membrane organization, proliferation, cell cycle regulation, apoptosis, transport, migration, and inflammatory signalling pathways. Imbalances in sphingolipid levels or subcellular localization result in dysregulated cellular processes and lead to the development and progression of multiple disorders, including polycystic kidney disease. This review will describe metabolic pathways of glycosphingolipids with a focus on the evidence linking glycosphingolipid mediated regulation of cell signalling, lipid microdomains, cilia, and polycystic kidney disease. We will discuss molecular mechanisms of glycosphingolipid dysregulation and their impact on cystogenesis. We will further highlight how modulation of sphingolipid metabolism can be translated into new approaches for the treatment of polycystic kidney disease and describe current clinical studies with glucosylceramide synthase inhibitors in Autosomal Dominant Polycystic Kidney Disease.

The Transfer of Sphingomyelinase Contributes to Drug Resistance in Multiple Myeloma

Multiple myeloma (MM) is well-known for the development of drug resistance, leading to relapse. Therefore, finding novel treatment strategies remains necessary. By performing a lipidomics assay on MM patient plasma, we aimed to identify new targets. We observed a dysregulation in the sphingolipid metabolism, with the upregulation of several ceramides and downregulation of sphingomyelin. This imbalance suggests an increase in sphingomyelinase, the enzyme responsible for hydrolyzing sphingomyelin into ceramide. We confirmed the upregulation of acid sphingomyelinase (ASM) in primary MM cells. Furthermore, we observed an increase in ASM expression in MM cell lines treated with melphalan or bortezomib, as well as in their exosomes. Exosomes high in ASM content were able to transfer the drug-resistant phenotype to chemosensitive cells, hereby suggesting a tumor-protective role for ASM. Finally, inhibition of ASM by amitriptyline improved drug sensitivity in MM cell lines and primary MM cells. In summary, this study is the first to analyze differences in plasma lipid composition of MM patients and match the observed differences to an upregulation of ASM. Moreover, we demonstrate that amitriptyline is able to inhibit ASM and increase sensitivity to anti-myeloma drugs. This study, therefore, provides a rational to include ASM-targeting-drugs in combination strategies in myeloma patients.

Acid sphingomyelinase - a regulator of canonical transient receptor potential channel 6 (TRPC6) activity

Recent investigations propose the acid sphingomyelinase (ASM)/ceramide system as a novel target for antidepressant action. ASM catalyzes the breakdown of the abundant membrane lipid sphingomyelin to the lipid messenger ceramide. This ASM-induced lipid modification induces a local shift in membrane properties, which influences receptor clustering and downstream signaling. Canonical transient receptor potential channels 6 (TRPC6) are non-selective cation channels located in the cell membrane that play an important role in dendritic growth, synaptic plasticity and cognition in the brain. They can be activated by hyperforin, an ingredient of the herbal remedy St. John's wort for treatment of depression disorders. Because of their role in the context of major depression, we investigated the crosstalk between the ASM/ceramide system and TRPC6 ion channels in a pheochromocytoma cell line 12 neuronal cell model (PC12 rat pheochromocytoma cell line). Ca²⁺ imaging experiments indicated that hyperforin-induced Ca²⁺ influx through TRPC6 channels is modulated by ASM activity. While antidepressants, known as functional inhibitors of ASM activity, reduced TRPC6-mediated Ca²⁺ influx, extracellular application of bacterial sphingomyelinase rebalanced TRPC6 activity in a concentration-related way. This effect was confirmed in whole-cell patch clamp electrophysiology recordings. Lipidomic analyses revealed a decrease in very long chain ceramide/sphingomyelin molar ratio after ASM inhibition, which was connected with changes in the abundance of TRPC6 channels in flotillin-1-positive lipid rafts as visualized by western blotting. Our data provide evidence that the ASM/ceramide system regulates TRPC6 channels likely by controlling their recruitment to specific lipid subdomains and thereby fine-tuning their physical properties.

Sphingolipids as Emerging Mediators in Retina Degeneration

The sphingolipids ceramide (Cer), sphingosine-1-phosphate (S1P), sphingosine (Sph), and ceramide-1-phosphate (C1P) are key signaling molecules that regulate major cellular functions. Their roles in the retina have gained increasing attention during the last decade since they emerge as mediators of proliferation, survival, migration, neovascularization, inflammation and death in retina cells. As exacerbation of these processes is central to retina degenerative diseases, they appear as crucial players in their progression. This review analyzes the functions of these sphingolipids in retina cell types and their possible pathological roles. Cer appears as a key arbitrator in diverse retinal pathologies; it promotes inflammation in endothelial and retina pigment epithelium (RPE) cells and its increase is a common feature in photoreceptor death in vitro and in animal models of retina degeneration; noteworthy, inhibiting Cer synthesis preserves photoreceptor viability and functionality. In turn, S1P acts as a double edge sword in the retina. It is essential for retina development, promoting the survival of photoreceptors and ganglion cells and regulating proliferation and differentiation of photoreceptor progenitors. However, S1P has also deleterious effects, stimulating migration of Müller glial cells, angiogenesis and fibrosis, contributing to the inflammatory scenario of proliferative retinopathies and age related macular degeneration (AMD). C1P, as S1P, promotes photoreceptor survival and differentiation. Collectively, the expanding role for these sphingolipids in the regulation of critical processes in retina cell types and in their dysregulation in retina degenerations makes them attractive targets for treating these diseases.

Sphingolipids and acid ceramidase as therapeutic targets in cancer therapy

BACKGROUND

Sphingolipids have been shown to play a key part in cancer cell growth and death and have increasingly become the subject of novel anti-cancer therapies. Acid ceramidase, a sphingolipid enzyme, has an important role in the regulation of apoptosis. In this review we aim to assess the current evidence supporting the role of sphingolipids in cancer and the potential role that acid ceramidase may play in cancer treatment.

METHODS

A literature search was performed for published full text articles using the PubMed, Cochrane and Scopus databases using the search criteria string "acid ceramidase", "sphingolipid", "cancer". Additional papers were detected by scanning the references of relevant papers. A summary of the evidence for each cancer subgroup was then formed. Given the nature of the data extracted, no meta-analysis was performed.

RESULTS

Over expression of acid ceramidase has been demonstrated in a number of human cancers. In vitro data demonstrate that manipulation of acid ceramidase may present a useful therapeutic target. In the clinical setting, a number of drugs have been investigated with the ability to target acid ceramidase, with the most promising of those being small molecular inhibitors, such as LCL521.

CONCLUSION

The role of the sphingolipid pathway in cancer is becoming very clearly established by promoting ceramide accumulation in response to cancer or cellular stress. Acid ceramidase is over expressed in a variety of cancers and has a role as a potential target for inhibition by novel specific inhibitors or off-target effects of traditional anti-cancer agents. Further work is required to develop acid ceramidase inhibitors safe for progression to clinical trials.

Cisplatin-Membrane Interactions and Their Influence on Platinum Complexes Activity and Toxicity

Cisplatin and other platinum(II) analogs are widely used in clinical practice as anti-cancer drugs for a wide range of tumors. The primary mechanism by which they exert their action is through the formation of adducts with genomic DNA. However, multiple cellular targets by platinum(II) complexes have been described. In particular, the early events occurring at the plasma membrane (PM), i.e., platinum-membrane interactions seem to be involved in the uptake, cytotoxicity and cell-resistance to cisplatin. In fact, PM influences signaling events, and cisplatin-induced changes on membrane organization and fluidity were shown to activate apoptotic pathways. This review critically discusses the sequence of events caused by lipid membrane-platinum interactions, with emphasis on the mechanisms that lead to changes in the biophysical properties of the membranes (e.g., fluidity and permeability), and how these correlate with sensitivity and resistance phenotypes of cells to platinum(II) complexes.

Sphingolipids as targets for inhalation treatment of cystic fibrosis

Studies over the past several years have demonstrated the important role of sphingolipids in cystic fibrosis (CF), chronic obstructive pulmonary disease and acute lung injury. Ceramide is increased in airway epithelial cells and alveolar macrophages of CF mice and humans, while sphingosine is dramatically decreased. This increase in ceramide results in chronic inflammation, increased death of epithelial cells, release of DNA into the bronchial lumen and thereby an impairment of mucociliary clearance; while the lack of sphingosine in airway epithelial cells causes high infection susceptibility in CF mice and possibly patients. The increase in ceramide mediates an ectopic expression of β1-integrins in the luminal membrane of CF epithelial cells, which results, via an unknown mechanism, in a down-regulation of acid ceramidase. It is predominantly this down-regulation of acid ceramidase that results in the imbalance of ceramide and sphingosine in CF cells. Correction of ceramide and sphingosine levels can be achieved by inhalation of functional acid sphingomyelinase inhibitors, recombinant acid ceramidase or by normalization of β1-integrin expression and subsequent re-expression of endogenous acid ceramidase. These treatments correct pulmonary inflammation and prevent or treat, respectively, acute and chronic pulmonary infections in CF mice with Staphylococcus aureus and mucoid or non-mucoid Pseudomonas aeruginosa. Inhalation of sphingosine corrects sphingosine levels only and seems to mainly act against the infection. Many antidepressants are functional inhibitors of the acid sphingomyelinase and were designed for systemic treatment of major depression. These drugs could be repurposed to treat CF by inhalation.

Early activation of CD95 is limited and localized to the cytotoxic synapse

The cytotoxic synapse formed between cytotoxic T lymphocytes or natural killer cells expressing CD95L and target cells with CD95 on their surface is a key pathway for apoptosis induction by the immune system. Despite similarities with the immune synapse in antigen presenting cells, little is known about the role of the spatiotemporal organization of agonistic proteins/receptor interactions for CD95 signaling. Here, we have developed an artificial cytotoxic synapse to examine how mobility and geometry of an anti-CD95 agonistic antibody affect receptor aggregation and mobility, ie the first step of receptor activation. By measuring the distribution, diffusion coefficient, and fraction of immobile CD95 receptor in living cells, we show that at short times, the initial activation of CD95 occurs locally and is limited to the contact region of the cytotoxic synapse. This anisotropic activation of apoptotic signaling supports a role for confined interactions on the efficiency of signal transduction that may have implications for biomedical applications of extrinsic apoptosis induction.

Regulation of Staphylococcus aureus Infection of Macrophages by CD44, Reactive Oxygen Species, and Acid Sphingomyelinase

Aims: Staphylococcus aureus plays an important role in sepsis, pneumonia, and wound infections. Acid sphingomyelinase (Asm)-deficient mice are highly susceptible to pulmonary S. aureus infections. Here, we investigated the role of CD44 as a molecule that mediates important aspects of the infection of macrophages with S. aureus. Results: We showed that CD44 activation by S. aureus stimulated Asm via the formation of reactive oxygen species, resulting in ceramide release, clustering of CD44 in ceramide-enriched membrane platforms, CD44/Asm-dependent activation of Rho family GTPases, translocation of phospho-ezrin/radixin/moesin to the plasma-membrane, and a rapid rearrangement of the actin cytoskeleton with cortical actin polymerization. Genetic deficiency of CD44 or Asm abrogated these signaling events and thereby reduced internalization of S. aureus into macrophages by 60-80%. Asm-deficient macrophages also exhibited reduced fusion of phagosomes with lysosomes, which prevented intracellular killing of S. aureus in macrophages and thereby allowed internalized S. aureus to replicate and cause severe pneumonia. Innovation and Conclusion: The CD44-Asm-ceramide system plays an important role in the infection of macrophages with S. aureus. Antioxid. Redox Signal. 28, 916-934.

Ceramide inhibits PKCθ by regulating its phosphorylation and translocation to lipid rafts in Jurkat cells

Protein kinase C theta (PKCθ) is a novel, calcium-independent member of the PKC family of kinases that was identified as a central player in T cell signaling and proliferation. Upon T cell activation by antigen-presenting cells, PKCθ gets phosphorylated and activated prior to its translocation to the immunological synapse where it couples with downstream effectors. PKCθ may be regulated by ceramide, a crucial sphingolipid that is known to promote differentiation, growth arrest, and apoptosis. To further investigate the mechanism, we stimulated human Jurkat T cells with either PMA or anti-CD3/anti-CD28 antibodies following induction of ceramide accumulation by adding exogenous ceramide, bacterial sphingomyelinase, or Fas ligation. Our results suggest that ceramide regulates the PKCθ pathway through preventing its critical threonine 538 (Thr538) phosphorylation and subsequent activation, thereby inhibiting the kinase's translocation to lipid rafts. Moreover, this inhibition is not likely to be a generic effect of ceramide on membrane reorganization. Other lipids, namely dihydroceramide, palmitate, and sphingosine, did not produce similar effects on PKCθ. Addition of the phosphatase inhibitors okadaic acid and calyculin A reversed the inhibition exerted by ceramide, and this suggests involvement of a ceramide-activated protein phosphatase. Such previously undescribed mechanism of regulation of PKCθ raises the possibility that ceramide, or one of its derivatives, and may prove valuable in novel therapeutic approaches for disorders involving autoimmunity or excessive inflammation-where PKCθ plays a critical role.

Unraveling the role of membrane microdomains during microbial infections

Infectious diseases pose major socioeconomic and health-related threats to millions of people across the globe. Strategies to combat infectious diseases derive from our understanding of the complex interactions between the host and specific bacterial, viral, and fungal pathogens. Lipid rafts are membrane microdomains that play important role in life cycle of microbes. Interaction of microbial pathogens with host membrane rafts influences not only their initial colonization but also their spread and the induction of inflammation. Therefore, intervention strategies aimed at modulating the assembly of membrane rafts and/or regulating raft-directed signaling pathways are attractive approaches for the. management of infectious diseases. The current review discusses the latest advances in terms of techniques used to study the role of membrane microdomains in various pathological conditions and provides updated information regarding the role of membrane rafts during bacterial, viral and fungal infections.

Suppression of Acid Sphingomyelinase Protects the Retina from Ischemic Injury

PURPOSE

Acid sphingomyelinase (ASMase) catalyzes the hydrolysis of sphingomyelin to ceramide and mediates multiple responses involved in inflammatory and apoptotic signaling. However, the role ASMase plays in ischemic retinal injury has not been investigated. The purpose of this study was to investigate how reduced ASMase expression impacts retinal ischemic injury.

METHODS

Changes in ceramide levels and ASMase activity were determined by high performance liquid chromatography-tandem mass spectrometry analysis and ASMase activity. Retinal function and morphology were assessed by electroretinography (ERG) and morphometric analyses. Levels of TNF-α were determined by ELISA. Activation of p38 MAP kinase was assessed by Western blot analysis.

RESULTS

In wild-type mice, ischemia produced a significant increase in retinal ASMase activity and ceramide levels. These increases were associated with functional deficits as measured by ERG analysis and significant structural degeneration in most retinal layers. In ASMase+/- mice, retinal ischemia did not significantly alter ASMase activity, and the rise in ceramide levels were significantly reduced compared to levels in retinas from wild-type mice. In ASMase+/- mice, functional and morphometric analyses of ischemic eyes revealed significantly less retinal degeneration than in injured retinas from wild-type mice. The ischemia-induced increase in retinal TNF-α levels was suppressed by the administration of the ASMase inhibitor desipramine, or by reducing ASMase expression.

CONCLUSIONS

Our results demonstrate that reducing ASMase expression provides partial protection from ischemic injury. Hence, the production of ceramide and subsequent mediators plays a role in the development of ischemic retinal injury. Modulating ASMase may present new opportunities for adjunctive therapies when treating retinal ischemic disorders.

Molecular mechanisms involved in the interaction of Neisseria meningitidis with cells of the human blood-cerebrospinal fluid barrier

Neisseria meningitidis is one of the most common aetiological agents of bacterial meningitis, affecting predominantly children and young adults. The interaction of N. meningitidis with human endothelial cells lining blood vessels of the blood-cerebrospinal fluid barrier (B-CSFB) is critical for meningitis development. In recent decades, there has been a significant increase in understanding of the molecular mechanisms involved in the interaction of N. meningitidis with brain vascular cells. In this review, we will describe how N. meningitidis adheres to the brain vasculature, may enter inside these cells, hijack receptor signalling pathways and alter host-cell responses in order to traverse the B-CSFB.

Ceramides and depression: A systematic review

BACKGROUND

Major depressive disorder is a significant contributor to global disability and mortality. The mechanisms of depression are vast and not fully understood, and as a result current treatment of depression is suboptimal. Aberrant sphingolipid metabolism has been observed in some cases of depression, specifically alterations in ceramide concentrations. The role of ceramides and other sphingolipids in depression is a novel concept. This review summarizes and evaluates the current state of evidence for a role of ceramides in depression pathophysiology and the potential for novel depression pharmacotherapies targeting ceramide metabolism.

METHODS

Medline, Embase, and PsycINFO databases were searched through October 2016 for English-language studies using combinations of the search terms: ceramide, depression, sphingolipid, and depressive symptoms.

RESULTS

Of the 489 articles screened, 14 were included in the qualitative synthesis of this review article. Pre-clinical and clinical evidence suggest that ceramide species may contribute to depression pathophysiology. In human studies, ceramides C18:0 and C20:0 are the species most strongly linked to depression. Evidence for altered ceramide metabolism in depression is present, but data for a causal role of ceramides in depression are lacking.

LIMITATIONS

This review was limited by potential reporting bias. Furthermore, a lack of specificity of which ceramides were altered in depression was common.

CONCLUSIONS

Pharmacotherapy targeting ceramide metabolism may be a novel treatment option for depression. A number of pharmacological targets exists for ceramide reduction and a number of currently approved medications inhibit ceramide production. More evidence, pre-clinical and clinical, is warranted to determine the extent and consistency of the role of ceramides in depression.

Role of caspases in CD95-induced biphasic activation of acid sphingomyelinase

Acid sphingomyelinase (A-SMase) plays an important role in the initiation of CD95 signaling by forming ceramide-enriched membrane domains that enable clustering and activation of the death receptors. In TNF-R1 and TRAIL-R1/R2 signaling, A-SMase also contributes to the lysosomal apoptosis pathway triggered by receptor internalization. Here, we investigated the molecular mechanism of CD95-mediated A-SMase activation, demonstrating that A-SMase is located in internalized CD95-receptosomes and is activated by the CD95/CD95L complex in a biphasic manner.Since several caspases have been described to be involved in the activation of A-SMase, we evaluated expression levels of caspase-8, caspase-7 and caspase-3 in CD95-receptosomes. The occurrence of cleaved caspase-8 correlated with the first peak of A-SMase activity and translocation of the A-SMase to the cell surface which could be blocked by the caspase-8 inhibitor IETD.Inhibition of CD95-internalization selectively reduced the second phase of A-SMase activity, suggesting a fusion between internalized CD95-receptosomes and an intracellular vesicular pool of A-SMase. Further analysis demonstrated that caspase-7 activity correlates with the second phase of the A-SMase activity, whereas active caspase-3 is present at early and late internalization time points. Blocking caspases-7/ -3 by DEVD reduced the second phase of A-SMase activation in CD95-receptosomes suggesting the potential role of caspase-7 or -3 for late A-SMase activation.In summary, we describe a biphasic A-SMase activation in CD95-receptosomes indicating (I.) a caspase-8 dependent translocation of A-SMase to plasma membrane and (II.) a caspase-7 and/or -3 dependent fusion of internalized CD95-receptosomes with intracellular A-SMase-containing vesicles.

Role of Ceramide in Apoptosis and Development of Insulin Resistance

This review presents data on the functional biochemistry of ceramide, one of the key sphingolipids with properties of a secondary messenger. Molecular mechanisms of the involvement of ceramide in apoptosis in pancreatic β-cells and its role in the formation of insulin resistance in pathogenesis of type 2 diabetes are reviewed. One of the main predispositions for the development of insulin resistance and diabetes is obesity, which is associated with ectopic fat deposition and significant increase in intracellular concentrations of cytotoxic ceramides. A possible approach to the restoration of tissue sensitivity to insulin in type 2 diabetes based on selective reduction of the content of cytotoxic ceramides is discussed.

Sphingolipid Organization in the Plasma Membrane and the Mechanisms That Influence It

Sphingolipids are structural components in the plasma membranes of eukaryotic cells. Their metabolism produces bioactive signaling molecules that modulate fundamental cellular processes. The segregation of sphingolipids into distinct membrane domains is likely essential for cellular function. This review presents the early studies of sphingolipid distribution in the plasma membranes of mammalian cells that shaped the most popular current model of plasma membrane organization. The results of traditional imaging studies of sphingolipid distribution in stimulated and resting cells are described. These data are compared with recent results obtained with advanced imaging techniques, including super-resolution fluorescence detection and high-resolution secondary ion mass spectrometry (SIMS). Emphasis is placed on the new insight into the sphingolipid organization within the plasma membrane that has resulted from the direct imaging of stable isotope-labeled lipids in actual cell membranes with high-resolution SIMS. Super-resolution fluorescence techniques have recently revealed the biophysical behaviors of sphingolipids and the unhindered diffusion of cholesterol analogs in the membranes of living cells are ultimately in contrast to the prevailing hypothetical model of plasma membrane organization. High-resolution SIMS studies also conflicted with the prevailing hypothesis, showing sphingolipids are concentrated in micrometer-scale membrane domains, but cholesterol is evenly distributed within the plasma membrane. Reductions in cellular cholesterol decreased the number of sphingolipid domains in the plasma membrane, whereas disruption of the cytoskeleton eliminated them. In addition, hemagglutinin, a transmembrane protein that is thought to be a putative raft marker, did not cluster within sphingolipid-enriched regions in the plasma membrane. Thus, sphingolipid distribution in the plasma membrane is dependent on the cytoskeleton, but not on favorable interactions with cholesterol or hemagglutinin. The alternate views of plasma membrane organization suggested by these findings are discussed.

Sphingolipids and mitochondrial apoptosis

The sphingolipid family of lipids modulate several cellular processes, including proliferation, cell cycle regulation, inflammatory signaling pathways, and cell death. Several members of the sphingolipid pathway have opposing functions and thus imbalances in sphingolipid metabolism result in deregulated cellular processes, which cause or contribute to diseases and disorders in humans. A key cellular process regulated by sphingolipids is apoptosis, or programmed cell death. Sphingolipids play an important role in both extrinsic and intrinsic apoptotic pathways depending on the stimuli, cell type and cellular response to the stress. During mitochondrial-mediated apoptosis, multiple pathways converge on mitochondria and induce mitochondrial outer membrane permeabilization (MOMP). MOMP results in the release of intermembrane space proteins such as cytochrome c and Apaf1 into the cytosol where they activate the caspases and DNases that execute cell death. The precise molecular components of the pore(s) responsible for MOMP are unknown, but sphingolipids are thought to play a role. Here, we review evidence for a role of sphingolipids in the induction of mitochondrial-mediated apoptosis with a focus on potential underlying molecular mechanisms by which altered sphingolipid metabolism indirectly or directly induce MOMP. Data available on these mechanisms is reviewed, and the focus and limitations of previous and current studies are discussed to present important unanswered questions and potential future directions.

Fas-Fas Ligand: Checkpoint of T Cell Functions in Multiple Sclerosis

Fas and Fas Ligand (FasL) are two molecules involved in the regulation of cell death. Their interaction leads to apoptosis of thymocytes that fail to rearrange correctly their T cell receptor (TCR) genes and of those that recognize self-antigens, a process called negative selection; moreover, Fas–FasL interaction leads to activation-induced cell death, a form of apoptosis induced by repeated TCR stimulation, responsible for the peripheral deletion of activated T cells. Both control mechanisms are particularly relevant in the context of autoimmune diseases, such as multiple sclerosis (MS), where T cells exert an immune response against self-antigens. This concept is well demonstrated by the development of autoimmune diseases in mice and humans with defects in Fas or FasL. In recent years, several new aspects of T cell functions in MS have been elucidated, such as the pathogenic role of T helper (Th) 17 cells and the protective role of T regulatory (Treg) cells. Thus, in this review, we summarize the role of the Fas–FasL pathway, with particular focus on its involvement in MS. We then discuss recent advances concerning the role of Fas–FasL in regulating Th17 and Treg cells’ functions, in the context of MS.

Acid Sphingomyelinase Promotes Endothelial Stress Response in Systemic Inflammation and Sepsis

The pathophysiology of sepsis involves activation of acid sphingomyelinase (SMPD1) with subsequent generation of the bioactive mediator ceramide. We herein evaluated the hypothesis that the enzyme exerts biological effects in endothelial stress response. Plasma-secreted sphingomyelinase activity, ceramide generation and lipid raft formation were measured in human microcirculatory endothelial cells (HMEC-1) stimulated with serum obtained from sepsis patients. Clustering of receptors relevant for signal transduction was studied by immuno staining. The role of SMPD1 for macrodomain formation was tested by pharmacological inhibition. To confirm the involvement of the stress enzyme, direct inhibitors (amino bisphosphonates) and specific downregulation of the gene was tested with respect to ADAMTS13 expression and cytotoxicity. Plasma activity and amount of SMPD1 were increased in septic patients dependent on clinical severity. Increased breakdown of sphingomyelin to ceramide in HMECs was observed following stimulation with serum from sepsis patients in vitro. Hydrolysis of sphingomyelin, clustering of receptor complexes, such as the CD95L/Fas-receptor, as well as formation of ceramide enriched macrodomains was abrogated using functional inhibitors (desipramine and NB6). Strikingly, the stimulation of HMECs with serum obtained from sepsis patients or mixture of proinflammatory cytokines resulted in cytotoxicity and ADAMTS13 downregulation which was abrogated using desipramine, amino bisphosphonates and genetic inhibitors. SMPD1 is involved in the dysregulation of ceramide metabolism in endothelial cells leading to macrodomain formation, cytotoxicity and downregulation of ADAMTS13 expression. Functional inhibitors, such as desipramine, are capable to improve endothelial stress response during sepsis and might be considered as a pharmacological treatment strategy to favor the outcome.