Lysoglycosphingolipids have the ability to induce cell death through direct PI3K inhibition.

Sphingolipidoses are inherited metabolic disorders associated with glycosphingolipids accumulation, neurodegeneration, and neuroinflammation leading to severe neurological symptoms. Lysoglycosphingolipids (lysoGSLs), also known to accumulate in the tissues of sphingolipidosis patients, exhibit cytotoxicity. LysoGSLs are the possible pathogenic cause, but the mechanisms are still unknown in detail. Here, we first show that lysoGSLs are potential inhibitors of phosphoinositide 3-kinase (PI3K) to reduce cell survival signaling. We found that phosphorylated Akt was commonly reduced in fibroblasts from patients with sphingolipidoses, including GM1/GM2 gangliosidoses and Gaucher's disease, suggesting the contribution of lysoGSLs to the pathogenesis. LysoGSLs caused cell death and decreased the level of phosphorylated Akt as in the patient fibroblasts. Extracellularly administered lysoGM1 permeated the cell membrane to diffusely distribute in the cytoplasm. LysoGM1 and lysoGM2 also inhibited the production of phosphatidylinositol-(3,4,5)-triphosphate and the translocation of Akt from the cytoplasm to the plasma membrane. We also predicted that lysoGSLs could directly bind to the catalytic domain of PI3K by in silico docking study, suggesting that lysoGSLs could inhibit PI3K by directly interacting with PI3K in the cytoplasm. Furthermore, we revealed that the increment of lysoGSLs amounts in the brain of sphingolipidosis model mice correlated with the neurodegenerative progression. Our findings suggest that the down-regulation of PI3K/Akt signaling by direct interaction of lysoGSLs with PI3K in the brains is a neurodegenerative mechanism in sphingolipidoses. Moreover, we could propose the intracellular PI3K activation or inhibition of lysoGSLs biosynthesis as novel therapeutic approaches for sphingolipidoses because lysoGSLs should be cell death mediators by directly inhibiting PI3K, especially in neurons.

The expanding boundaries of sphingolipid lysosomal storage diseases; insights from Niemann-Pick disease type C.

Lysosomal storage diseases are inborn errors of metabolism that arise due to loss of function mutations in genes encoding lysosomal enzymes, protein co-factors or lysosomal membrane proteins. As a consequence of the genetic defect, lysosomal function is impaired and substrates build up in the lysosome leading to 'storage'. A sub group of these disorders are the sphingolipidoses in which sphingolipids accumulate in the lysosome. In this review, I will discuss how the study of these rare lysosomal disorders reveals unanticipated links to other rare and common human diseases using Niemann-Pick disease type C as an example.

Sphingolipids and their role in health and disease in the central nervous system.

Sphingolipids (SLs) are lipids derived from sphingosine, and their metabolism involves a broad and complex network of reactions. Although SLs are widely distributed in the body, it is well known that they are present in high concentrations within the central nervous system (CNS). Under physiological conditions, their abundance and distribution in the CNS depend on brain development and cell type. Consequently, SLs metabolism impairment may have a significant impact on the normal CNS function, and has been associated with several disorders, including sphingolipidoses, Parkinson's, and Alzheimer's. This review summarizes the main SLs characteristics and current knowledge about synthesis, catabolism, regulatory pathways, and their role in physiological and pathological scenarios in the CNS.

Rare Diseases in Glycosphingolipid Metabolism.

Sphingolipidoses is a cluster of genetic rare disorders regarding glycosphingolipid metabolism, classified as lysosomal storage disorders (LSD). Here, we focus on eight inheritable diseases, including GM1 gangliosidosis, GM2 gangliosidosis, Fabry disease, Gaucher's disease, metachromatic leukodystrophy, Krabbe disease, Niemann-Pick disease A and B, and Farber disease. Mostly, pathogenic mutations in the key enzyme are loss-function, resulting in accumulation of substrates and deficiency of products. Thus, cellular overload of substrates causes lipotoxicity, which is deleterious to cellular and organ function. In the terms of clinical manifestations in sphingolipidoses, multiple systems and organs, especially central nervous system (CNS) are usually affected. As for diagnosis strategy, enzymatic activity assay and genetic sequencing are helpful. Up till now, limited treatment approaches have approved for treating sphingolipidoses, with some potential strategies for further evaluation. In general, enzyme replacement therapy (ERT), substrate reduction therapy (SRT), and molecular chaperones are feasible choices for enzyme deficiency disorders, but these therapies are limited to relieve CNS lesions and symptoms due to prevention from blood-brain barrier. Other possible treatments such as gene therapy, bone marrow transplantation (BMT), and hematopoietic stem cell transplantation (HSCT) need further evaluation.

Genetic defects in the sphingolipid degradation pathway and their effects on microglia in neurodegenerative disease.

Sphingolipids, which function as plasma membrane lipids and signaling molecules, are highly enriched in neuronal and myelin membranes in the nervous system. They are degraded in lysosomes by a defined sequence of enzymatic steps. In the related group of disorders, the sphingolipidoses, mutations in the genes that encode the individual degradative enzymes cause lysosomal accumulation of sphingolipids and often result in severe neurodegenerative disease. Here we review the information indicating that microglia, which actively clear sphingolipid-rich membranes in the brain during development and homeostasis, are directly affected by these mutations and promote neurodegeneration in the sphingolipidoses. We also identify parallels between the sphingolipidoses and more common forms of neurodegeneration, which both exhibit evidence of defective sphingolipid clearance in the nervous system.

Emerging mechanisms of drug-induced phospholipidosis.

Drug-induced phospholipidosis is a lysosomal storage disorder characterized by excessive accumulation of phospholipids. Its cellular mechanism is still not well understood, but it is known that cationic amphiphilic drugs can induce it. These drugs have a hydrophilic amine head group that can be protonated in the endolysosomal compartment. As cationic amphiphiles, they are trapped in lysosomes, where they interfere with negatively charged intralysosomal vesicles, the major platforms of cellular sphingolipid degradation. Metabolic principles observed in sphingolipid and phospholipid catabolism and inherited sphingolipidoses are of great importance for lysosomal function and physiological lipid turnover at large. Therefore, we also propose intralysosomal vesicles as major platforms for degradation of lipids and phospholipids reaching them by intracellular pathways like autophagy and endocytosis. Phospholipids are catabolized as components of vesicle surfaces by protonated, positively charged phospholipases, electrostatically attracted to the negatively charged vesicles. Model experiments suggest that progressively accumulating cationic amphiphilic drugs inserting into the vesicle membrane with their hydrophobic molecular moieties disturb and attenuate the main mechanism of lipid degradation as discussed here. By compensating the negative surface charge, cationic enzymes are released from the surface of vesicles and proteolytically degraded, triggering a progressive lipid storage and the formation of inactive lamellar bodies.

Inhibition of the IGF-1-PI3K-Akt-mTORC2 pathway in lipid rafts increases neuronal vulnerability in a genetic lysosomal glycosphingolipidosis.

Glycosphingolipid (GSL) accumulation is implicated in the neuropathology of several lysosomal conditions, such as Krabbe disease, and may also contribute to neuronal and glial dysfunction in adult-onset conditions such as Parkinson's disease, Alzheimer's disease and multiple sclerosis. GSLs accumulate in cellular membranes and disrupt their structure; however, how membrane disruption leads to cellular dysfunction remains unknown. Using authentic cellular and animal models for Krabbe disease, we provide a mechanism explaining the inactivation of lipid raft (LR)-associated IGF-1-PI3K-Akt-mTORC2, a pathway of crucial importance for neuronal function and survival. We show that psychosine, the GSL that accumulates in Krabbe disease, leads to a dose-dependent LR-mediated inhibition of this pathway by uncoupling IGF-1 receptor phosphorylation from downstream Akt activation. This occurs by interfering with the recruitment of PI3K and mTORC2 to LRs. Akt inhibition can be reversed by sustained IGF-1 stimulation, but only during a time window before psychosine accumulation reaches a threshold level. Our study shows a previously unknown connection between LR-dependent regulation of mTORC2 activity at the cell surface and a genetic neurodegenerative disease. Our results show that LR disruption by psychosine desensitizes cells to extracellular growth factors by inhibiting signal transmission from the plasma membrane to intracellular compartments. This mechanism serves also as a mechanistic model to understand how alterations of the membrane architecture by the progressive accumulation of lipids undermines cell function, with potential implications in other genetic sphingolipidoses and adult neurodegenerative conditions. This article has an associated First Person interview with the first author of the paper.

Sphingolipids and neuronal degeneration in lysosomal storage disorders.

Ceramide, sphingomyelin, and glycosphingolipids (both neutral and acidic) are characterized by the presence in the lipid moiety of an aliphatic base known as sphingosine. Altogether, they are called sphingolipids and are particularly abundant in neuronal plasma membranes, where, via interactions with the other membrane lipids and membrane proteins, they play a specific role in modulating the cell signaling processes. The metabolic pathways determining the plasma membrane sphingolipid composition are thus the key point for functional changes of the cell properties. Unnatural changes of the neuronal properties are observed in sphingolipidoses, lysosomal storage diseases occurring when a lysosomal sphingolipid hydrolase is not working, leading to the accumulation of the substrate and to its distribution to all the cell membranes interacting with lysosomes. Moreover, secondary accumulation of sphingolipids is a common trait of other lysosomal storage diseases. This article is part of the Special Issue "Lysosomal Storage Disorders".

Niemann-Pick type C disease: The atypical sphingolipidosis.

Niemann-Pick type C (NPC) disease is a lysosomal storage disorder resulting from mutations in either the NPC1 (95%) or NPC2 (5%) genes. NPC typically presents in childhood with visceral lipid accumulation and complex progressive neurodegeneration characterized by cerebellar ataxia, dysphagia, and dementia, resulting in a shortened lifespan. While cholesterol is widely acknowledged as the principal storage lipid in NPC, multiple species of sphingolipids accumulate as well. This accumulation of sphingolipids led to the initial assumption that NPC disease was caused by a deficiency in a sphingolipid catabolism enzyme, similar to sphingomyelinase deficiencies with which it shares a family name. It took about half a century to determine that NPC was in fact caused by a cholesterol trafficking defect, and still as we approach a century after the initial identification of the disease, the mechanisms by which sphingolipids accumulate remain poorly understood. Here we focus on the defects of sphingolipid catabolism in the endolysosomal compartment and how they contribute to the biology and pathology observed in NPC disease. This review highlights the need for further work on understanding and possibly developing treatments to correct the accumulation of sphingolipids in addition to cholesterol in this currently untreatable disease.

Dysregulation of autophagy as a common mechanism in lysosomal storage diseases.

The lysosome plays a pivotal role between catabolic and anabolic processes as the nexus for signalling pathways responsive to a variety of factors, such as growth, nutrient availability, energetic status and cellular stressors. Lysosomes are also the terminal degradative organelles for autophagy through which macromolecules and damaged cellular components and organelles are degraded. Autophagy acts as a cellular homeostatic pathway that is essential for organismal physiology. Decline in autophagy during ageing or in many diseases, including late-onset forms of neurodegeneration is considered a major contributing factor to the pathology. Multiple lines of evidence indicate that impairment in autophagy is also a central mechanism underlying several lysosomal storage disorders (LSDs). LSDs are a class of rare, inherited disorders whose histopathological hallmark is the accumulation of undegraded materials in the lysosomes due to abnormal lysosomal function. Inefficient degradative capability of the lysosomes has negative impact on the flux through the autophagic pathway, and therefore dysregulated autophagy in LSDs is emerging as a relevant disease mechanism. Pathology in the LSDs is generally early-onset, severe and life-limiting but current therapies are limited or absent; recognizing common autophagy defects in the LSDs raises new possibilities for therapy. In this review, we describe the mechanisms by which LSDs occur, focusing on perturbations in the autophagy pathway and present the latest data supporting the development of novel therapeutic approaches related to the modulation of autophagy.

Neuronal sphingolipidoses: Membrane lipids and sphingolipid activator proteins regulate lysosomal sphingolipid catabolism.

Glycosphingolipids and sphingolipids of cellular plasma membranes (PMs) reach luminal intra-lysosomal vesicles (LVs) for degradation mainly by pathways of endocytosis. After a sorting and maturation process (e.g. degradation of sphingomyelin (SM) and secretion of cholesterol), sphingolipids of the LVs are digested by soluble enzymes with the help of activator (lipid binding and transfer) proteins. Inherited defects of lipid-cleaving enzymes and lipid binding and transfer proteins cause manifold and fatal, often neurodegenerative diseases. The review summarizes recent findings on the regulation of sphingolipid catabolism and cholesterol secretion from the endosomal compartment by lipid modifiers, an essential stimulation by anionic membrane lipids and an inhibition of crucial steps by cholesterol and SM. Reconstitution experiments in the presence of all proteins needed, hydrolase and activator proteins, reveal an up to 10-fold increase of ganglioside catabolism just by the incorporation of anionic lipids into the ganglioside carrying membranes, whereas an additional incorporation of cholesterol inhibits GM2 catabolism substantially. It is suggested that lipid and other low molecular modifiers affect the genotype-phenotype relationship observed in patients with lysosomal diseases.

Acid ceramidase and the treatment of ceramide diseases: The expanding role of enzyme replacement therapy.

Ceramides are a diverse group of sphingolipids that play important roles in many biological processes. Acid ceramidase (AC) is one key enzyme that regulates ceramide metabolism. Early research on AC focused on the fact that it is the enzyme deficient in the rare genetic disorder, Farber Lipogranulomatosis. Recent research has revealed that deficiency of the same enzyme is responsible for a rare form of spinal muscular atrophy associated with myoclonic epilepsy (SMA-PME). Due to their diverse role in biology, accumulation of ceramides also has been implicated in the pathobiology of many other common diseases, including infectious lung diseases, diabetes, cancers and others. This has revealed the potential of AC as a therapy for many of these diseases. This review will focus on the biology of AC and the potential role of this enzyme in the treatment of human disease.

From sheep to mice to cells: tools for the study of the sphingolipidoses.

The sphingolipidoses are a group of inherited lysosomal storage diseases in which sphingolipids accumulate due to the defective activity of one or other enzymes involved in their degradation. For most of the sphingolipidoses, little is known about the molecular mechanisms that lead to disease, which has negatively impacted attempts to develop therapies for these devastating human diseases. Use of both genetically-modified animals, ranging from mice to larger mammals, and of novel cell culture systems, is of utmost importance in delineating the molecular mechanisms that cause pathophysiology, and in providing tools that enable testing the efficacy of new therapies. In this review, we discuss eight sphingolipidoses, namely Gaucher disease, Fabry disease, metachromatic leukodystrophy, Krabbe disease, Niemann-Pick diseases A and B, Farber disease, GM1 gangliosidoses, and GM2 gangliosidoses, and describe the tools that are currently available for their study. This article is part of a Special Issue entitled Tools to study lipid functions.

Beyond the cherry-red spot: Ocular manifestations of sphingolipid-mediated neurodegenerative and inflammatory disorders.

Sphingolipids are a ubiquitous membrane lipid present in every cell and found most abundantly in neural tissues. Disorders such as Tay-Sachs or Niemann-Pick disease are the most familiar examples of dysfunction in sphingolipid metabolism and are typically associated with neurodegeneration and ocular findings such as blindness. More recently, the role of bioactive sphingolipids has been established in a multitude of cellular events, including cell survival, growth, senescence and apoptosis, inflammation, and neovascularization. We discuss our current knowledge and understanding of sphingolipid metabolism and signaling in the pathogenesis of ocular diseases.

Metabolic and cellular bases of sphingolipidoses.

Lysosomes are cellular stomachs. They degrade macromolecules and release their components as nutrients into the cytosol. Digestion of sphingolipids and other membrane lipids occurs at luminal intraendosomal vesicles and IMs (intraendosomal membranes). Sphingolipid and membrane digestion needs catabolic hydrolases with the help of lipid-binding proteins [SAPs (sphingolipid activator proteins)] and anionic lipids such as BMP [bis(monoacylglycero)phosphate]. Inherited defects of hydrolases or SAPs or uptake of cationic amphiphilic drugs cause lipid accumulation, eventually leading to death, especially in inherited sphingolipid storage diseases. IMs are formed during endocytosis and their lipid composition is adjusted for degradation. Their cholesterol content, which stabilizes membranes, decreases and the level of negatively charged BMP, which stimulates sphingolipid degradation, increases. At the level of late endosomes, cholesterol is transported out of the luminal vesicles preferentially by cholesterol-binding proteins, NPC (Niemann-Pick type C)-2 and NPC-1. Their defects lead to an endolysosomal accumulation of cholesterol and sphingolipids in Niemann-Pick type C disease. BMP and ceramide stimulate NPC-2-mediated cholesterol transfer, whereas sphingomyelin inhibits it. Anionic membrane lipids also activate sphingomyelin degradation by ASM (acid sphingomyelinase), facilitating cholesterol export by NPC-2. ASM is a non-specific phospholipase C and degrades more than 23 phospholipids. SAPs are membrane-perturbing proteins which solubilize lipids, facilitating glycolipid digestion by presenting them to soluble catabolic enzymes at acidic pH. High BMP and low cholesterol levels favour lipid extraction and membrane disintegration by saposin A and B. The simultaneous inherited defect of saposins A-D causes a severe membrane and sphingolipid storage disease, also disrupting the water permeability barrier of the skin.

Sustained activation of sphingomyelin synthase by 2-hydroxyoleic acid induces sphingolipidosis in tumor cells.

The mechanism of action of 2-hydroxyoleic acid (2OHOA), a potent antitumor drug, involves the rapid and specific activation of sphingomyelin synthase (SMS), leading to a 4-fold increase in SM mass in tumor cells. In the present study, we investigated the source of the ceramides required to sustain this dramatic increase in SM. Through radioactive and fluorescent labeling, we demonstrated that sphingolipid metabolism was altered by a 24 h exposure to 2OHOA, and we observed a consistent increase in the number of lysosomes and the presence of unidentified storage materials in treated cells. Mass spectroscopy revealed that different sphingolipid classes accumulated in human glioma U118 cells after exposure to 2OHOA, demonstrating a specific effect on C16-, C20-, and C22-containing sphingolipids. Based on these findings, we propose that the demand for ceramides required to sustain the SMS activation (ca. 200-fold higher than the basal level) profoundly modifies both sphingolipid and phospholipid metabolism. As the treatment is prolonged, tumor cells fail to adequately metabolize sphingolipids, leading to a situation resembling sphingolipidosis, whereby cell viability is compromised.

My journey into the world of sphingolipids and sphingolipidoses.

Analysis of lipid storage in postmortem brains of patients with amaurotic idiocy led to the recognition of five lysosomal ganglioside storage diseases and identification of their inherited metabolic blocks. Purification of lysosomal acid sphingomyelinase and ceramidase and analysis of their gene structures were the prerequisites for the clarification of Niemann-Pick and Farber disease. For lipid catabolism, intraendosomal vesicles are formed during the endocytotic pathway. They are subjected to lipid sorting processes and were identified as luminal platforms for cellular lipid and membrane degradation. Lipid binding glycoproteins solubilize lipids from these cholesterol poor membranes and present them to water-soluble hydrolases for digestion. Biosynthesis and intracellular trafficking of lysosomal hydrolases (hexosaminidases, acid sphingomyelinase and ceramidase) and lipid binding and transfer proteins (GM2 activator, saposins) were analyzed to identify the molecular and metabolic basis of several sphingolipidoses. Studies on the biosynthesis of glycosphingolipids yielded the scheme of Combinatorial Ganglioside Biosynthesis involving promiscuous glycosyltransferases. Their defects in mutagenized mice impair brain development and function.

Tracking sphingosine metabolism and transport in sphingolipidoses: NPC1 deficiency as a test case.

The late endosomal/lysosomal compartment (LE/LY) plays a key role in sphingolipid breakdown, with the last degradative step catalyzed by acid ceramidase. The released sphingosine can be converted to ceramide in the ER and transported by ceramide transfer protein (CERT) to the Golgi for conversion to sphingomyelin. The mechanism by which sphingosine exits LE/LY is unknown but Niemann-Pick C1 protein (NPC1) has been suggested to be involved. Here, we used sphingomyelin, ceramide and sphingosine labeled with [(3)H] in carbon-3 of the sphingosine backbone and targeted them to LE/LY in low-density lipoprotein (LDL) particles. These probes traced LE/LY sphingolipid degradation and recycling as suggested by (1) accumulation of [(3)H]-sphingomyelin-derived [(3)H]-ceramide and depletion of [(3)H]-sphingosine upon acid ceramidase depletion, and (2) accumulation of [(3)H]-sphingosine-derived [(3)H]-ceramide and attenuation of [(3)H]-sphingomyelin synthesis upon CERT depletion. NPC1 silencing did not result in the accumulation of [(3)H]-sphingosine derived from [(3)H]-sphingomyelin/LDL or [(3)H]-ceramide/LDL. Additional evidence against NPC1 playing a significant role in LE/LY sphingosine export was obtained in experiments using the [(3)H]-sphingolipids or a fluorescent sphingosine derivative in NPC1 knock-out cells. Instead, NPC1-deficient cells displayed an increased affinity for sphingosine independently of protein-mediated lipid transport. This likely contributes to the increased sphingosine content of NPC1 cells.

Cancer and sphingolipid storage disease therapy using novel synthetic analogs of sphingolipids.

Sphingolipid metabolites have become recognized for their participation in cell functions and signaling events that control a wide array of cellular activities. Two main sphingolipids, ceramide and sphingosine-1-phosphate, are involved in signaling pathways that regulate cell proliferation, apoptosis, motility, differentiation, angiogenesis, stress responses, protein synthesis, carbohydrate metabolism, and intracellular trafficking. Ceramide and S1P often exert opposing effects on cell survival, ceramide being pro-apoptotic and S1P generally promoting cell survival. Therefore, the conversion of one of these metabolites to the other by sphingolipid enzymes provides a vast network of regulation and provides a useful therapeutic target. Here we provide a survey of the current knowledge of the roles of sphingolipid metabolites in cancer and in lipid storage disease. We review our attempts to interfere with this network of regulation and so provide new treatments for a range of diseases. We synthesized novel analogs of sphingolipids which inhibit the hydrolysis of ceramide or its conversion to more complex sphingolipids. These analogs caused elevation of ceramide levels, leading to apoptosis of a variety of cancer cells. Administration of a synthetic analog to tumor-bearing mice resulted in reduction and even disappearance of the tumors. Therapies for sphingolipid storage diseases, such as Niemann-Pick and Gaucher diseases were achieved by two different strategies: inhibition of the biosynthesis of the substrate (substrate reduction therapy) and protection of the mutated enzyme (chaperone therapy). Sphingolipid metabolism was monitored by the use of novel fluorescent sphingolipid analogs. The results described in this review indicate that our synthetic analogs could be developed both as anticancer drugs and for the treatment of sphingolipid storage diseases.

A view on sphingolipids and disease.

Sphingolipid and glycosphingolipid levels and expression of sphingolipid metabolizing enzymes are altered in a variety of diseases or in response to drug treatment. Inherited defects of enzymes and other proteins required for the lysosomal degradation of these lipids lead to human sphingolipidoses. Also genetic defects that affect sphingolipid biosynthesis are known. Although the molecular details are often far from clear, (glyco)sphingolipids have been implicated to play a role in atherosclerosis, insulin resistance, cancer, and infections by pathogens. More general aspects of selected diseases are discussed.