Selective hydrolysis of a mitochondrial pool of sphingomyelin induces apoptosis

Sphingolipids as key mediators in folliculogenesis and female fertility

AIMS

This review investigates the regulatory functions of sphingolipids-particularly sphingosine-1-phosphate (S1P) and ceramide-in the process of ovarian folliculogenesis. It emphasizes how the dynamic balance between ceramide and S1P, orchestrated by enzymes like sphingomyelinase, ceramidase, and sphingosine kinase (SphK1/2), governs follicle development, survival, and reproductive potential.

MATERIALS AND METHODS

A comprehensive review of recent experimental and clinical studies was performed to examine the impact of sphingolipids and their metabolic enzymes on follicular activation, growth, and maturation. Special focus was placed on the signaling mechanisms mediated by S1P and ceramide in ovarian cells.

KEY FINDINGS

Sphingolipids are essential mediators of proliferation, differentiation, and programmed cell death during folliculogenesis. S1P enhances follicle survival and growth by activating S1P receptors (notably S1PR1 and S1PR3), which in turn stimulate PI3K/Akt/mTOR and ERK/MAPK signaling cascades. Additionally, S1P-induced calcium mobilization via Gq- and Gi-coupled pathways supports metabolic resilience and cellular longevity. Acting as a pro-survival factor, S1P is particularly influential during physiological events such as the LH surge. Conversely, ceramide-whether derived from sphingomyelin hydrolysis or de novo synthesis-accumulates in mitochondria and promotes apoptosis by activating Bax and suppressing Bcl-2 family proteins. The interplay between ceramide and S1P, often referred to as the "sphingolipid rheostat," plays a central role in determining follicular destiny.

SIGNIFICANCE

Modulating sphingolipid signaling and its enzymatic regulators holds promise as a therapeutic avenue for enhancing ovarian function and treating gynecological conditions. This review highlights the critical importance of maintaining a functional ceramide/S1P balance to preserve follicular viability and overall reproductive health.

Ceramides may Play a Central Role in the Pathogenesis of Alzheimer's Disease: a Review of Evidence and Horizons for Discovery

While several hypotheses have been proposed to explain the underlying mechanisms of Alzheimer's disease, none have been entirely satisfactory. Both genetic and non-genetic risk factors, such as infections, metabolic disorders and psychological stress, contribute to this debilitating disease. Multiple lines of evidence indicate that ceramides may be central to the pathogenesis of Alzheimer's disease. Tumor necrosis factor-α, saturated fatty acids and cortisol elevate the brain levels of ceramides, while genetic risk factors, such as mutations in APP, presenilin, TREM2 and APOE ε4, also elevate ceramide synthesis. Importantly, ceramides displace sphingomyelin and cholesterol from lipid raft-like membrane patches that connect the endoplasmic reticulum and mitochondria, disturbing mitochondrial oxidative phosphorylation and energy production. As a consequence, the flattening of lipid rafts alters the function of γ-secretase, leading to increased production of Aβ42. Moreover, ceramides inhibit the insulin-signaling cascade via at least three mechanisms, resulting in the activation of glycogen synthase kinase-3 β. Activation of this kinase has multiple consequences, as it further deteriorates insulin resistance, promotes the transcription of BACE1, causes hyperphosphorylation of tau and inhibits the transcription factor Nrf2. Functional Nrf2 prevents apoptosis, mediates anti-inflammatory activity and improves blood-brain barrier function. Thus, various seemingly unrelated Alzheimer's disease risk factors converge on ceramide production, whereas the elevated levels of ceramides give rise to the well-known pathological features of Alzheimer's disease. Understanding and targeting these mechanisms may provide a promising foundation for the development of novel preventive and therapeutic strategies.

Decoding ceramide function: how localization shapes cellular fate and how to study it

Recent studies emphasize that lipid synthesis, metabolism, and transport are crucial in modulating lipid function, underscoring the significance of lipid localization within the cell, in addition to their chemical structure. Ceramides stand out in this context because of their multifaceted roles in cellular processes. Here, we focus on the role of ceramides in apoptosis, senescence, and autophagy as these processes offer unique and contrasting perspectives on how ceramides function and can be intricately linked to their subcellular localization, providing critical insights into their complex biological interactions. Additionally, we highlight recent advancements in tools and techniques that have boosted our understanding of ceramide dynamics and different mechanisms of lipid functioning.

Neuroprotective effect of neuron-specific deletion of the C16 ceramide synthetic enzymes in an animal model of multiple sclerosis

Ceramide C16 is a sphingolipid detected at high levels in several neurodegenerative disorders, including multiple sclerosis (MS). It can be generated de novo or from the hydrolysis of other sphingolipids, such as sphingomyelin or through the recycling of sphingosine, in what is known as the salvage pathway. While the myelin damage occurring in MS suggests the importance of the hydrolytic and salvage pathways, the growing interest on the importance of diet in demyelinating disorders, prompted us to investigate the involvement of de novo ceramide C16 synthesis on disease severity. A diet rich in saturated fats such as palmitic acid, as found in many highly processed foods, provides substrates for the ceramide C16 synthetic enzymes ceramide synthase 6 (CERS6) and 5 (CERS5), which are expressed in the central nervous system. Using the experimental autoimmune encephalomyelitis (EAE) model of inflammatory demyelination, we show here that mice with CamK2a+ neuronal specific deletion of both CerS6 and CerS5 show a milder course of EAE than wild type mice, even when fed a diet enriched in palmitic acid. At a cellular level, neurons lacking both CerS6 and CerS5 are protected from the mitochondrial dysfunction arising from exposure to oxidative stress and palmitic acid in the medium. These data underscore the importance of a healthy diet avoiding processed foods for demyelinating disorders and identifies endogenous neuronal synthesis of ceramide C16 as an important determinant of disease severity.

The steady-state level of plasma membrane ceramide is regulated by neutral sphingomyelinase 2

During the last 30 years, an increasing number of cellular functions have been reported to be regulated by the lipid ceramide. The diversity in the ceramide structure, leading to tens of ceramide species and the discrete distribution based on subcellular topology, could explain the wide variety of functions attributed to this bioactive lipid. One of these pools of ceramide resides in the plasma membrane, and several works have suggested that an increase in plasma membrane ceramide (PMCer) in response to stimulation leads to cell death and modulates cell adhesion and migration. However, there is a limitation in studying PMCer content in this location primarily due to the inability to quantify its mass. Our group recently developed a method to specifically quantitate PMCer. In this work, we interrogate what sphingolipid metabolizing enzymes are responsible for modulating the basal levels of plasma membrane ceramide. An in-silico prediction and experimental confirmation found an almost perfect correlation between the endogenous expression levels of neutral sphingomyelinase (nSMase2) and the amount of plasma membrane ceramide in unstimulated cells. Manipulating the expression levels of nSMase2, but not other candidate enzymes of ceramide metabolism, profoundly affected PMCer. Moreover, a physiologic induction of nSMase2 during cell confluence resulted in a nSMase2-dependent dramatic increase in PMCer. Together, these results identify nSMase2 as the primary enzyme to regulate plasma membrane ceramide.

Regulation of cellular and systemic sphingolipid homeostasis

One hundred and fifty years ago, Johann Thudichum described sphingolipids as unusual "Sphinx-like" lipids from the brain. Today, we know that thousands of sphingolipid molecules mediate many essential functions in embryonic development and normal physiology. In addition, sphingolipid metabolism and signalling pathways are dysregulated in a wide range of pathologies, and therapeutic agents that target sphingolipids are now used to treat several human diseases. However, our understanding of sphingolipid regulation at cellular and organismal levels and their functions in developmental, physiological and pathological settings is rudimentary. In this Review, we discuss recent advances in sphingolipid pathways in different organelles, how secreted sphingolipid mediators modulate physiology and disease, progress in sphingolipid-targeted therapeutic and diagnostic research, and the trans-cellular sphingolipid metabolic networks between microbiota and mammals. Advances in sphingolipid biology have led to a deeper understanding of mammalian physiology and may lead to progress in the management of many diseases.

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.

Lipid unsaturation promotes BAX and BAK pore activity during apoptosis

BAX and BAK are proapoptotic members of the BCL2 family that directly mediate mitochondrial outer membrane permeabilition (MOMP), a central step in apoptosis execution. However, the molecular architecture of the mitochondrial apoptotic pore remains a key open question and especially little is known about the contribution of lipids to MOMP. By performing a comparative lipidomics analysis of the proximal membrane environment of BAK isolated in lipid nanodiscs, we find a significant enrichment of unsaturated species nearby BAK and BAX in apoptotic conditions. We then demonstrate that unsaturated lipids promote BAX pore activity in model membranes, isolated mitochondria and cellular systems, which is further supported by molecular dynamics simulations. Accordingly, the fatty acid desaturase FADS2 not only enhances apoptosis sensitivity, but also the activation of the cGAS/STING pathway downstream mtDNA release. The correlation of FADS2 levels with the sensitization to apoptosis of different lung and kidney cancer cell lines by co-treatment with unsaturated fatty acids supports the relevance of our findings. Altogether, our work provides an insight on how local lipid environment affects BAX and BAK function during apoptosis.

Ceramides and mitochondrial homeostasis

Lipotoxicity arises from the accumulation of lipid intermediates in non-adipose tissue, precipitating cellular dysfunction and death. Ceramide, a toxic byproduct of excessive free fatty acids, has been widely recognized as a primary contributor to lipotoxicity, mediating various cellular processes such as apoptosis, differentiation, senescence, migration, and adhesion. As the hub of lipid metabolism, the excessive accumulation of ceramides inevitably imposes stress on the mitochondria, leading to the disruption of mitochondrial homeostasis, which is typified by adequate ATP production, regulated oxidative stress, an optimal quantity of mitochondria, and controlled mitochondrial quality. Consequently, this review aims to collate current knowledge and facts regarding the involvement of ceramides in mitochondrial energy metabolism and quality control, thereby providing insights for future research.

Mysterious sphingolipids: metabolic interrelationships at the center of pathophysiology

Metabolic pathways are complex and intertwined. Deficiencies in one or more enzymes in a given pathway are directly linked with genetic diseases, most of them having devastating manifestations. The metabolic pathways undertaken by sphingolipids are diverse and elaborate with ceramide species serving as the hubs of sphingolipid intermediary metabolism and function. Sphingolipids are bioactive lipids that serve a multitude of cellular functions. Being pleiotropic in function, deficiency or overproduction of certain sphingolipids is associated with many genetic and chronic diseases. In this up-to-date review article, we strive to gather recent scientific evidence about sphingolipid metabolism, its enzymes, and regulation. We shed light on the importance of sphingolipid metabolism in a variety of genetic diseases and in nervous and immune system ailments. This is a comprehensive review of the state of the field of sphingolipid biochemistry.

Interplay of lipid metabolism and inflammation in podocyte injury

Podocytes are critical for maintaining permselectivity of the glomerular filtration barrier, and podocyte injury is a major cause of proteinuria in various primary and secondary glomerulopathies. Lipid dysmetabolism and inflammatory activation are the distinctive hallmarks of podocyte injury. Lipid accumulation and lipotoxicity trigger cytoskeletal rearrangement, insulin resistance, mitochondrial oxidative stress, and inflammation. Subsequently, inflammation promotes the progression of glomerulosclerosis and renal fibrosis via multiple pathways. These data suggest that lipid dysmetabolism positively or negatively regulates inflammation during podocyte injury. In this review, we summarize recent advances in the understanding of lipid metabolism and inflammation, and highlight the potential association between lipid metabolism and podocyte inflammation.

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.

Mitochondrial electron transport chain, ceramide, and coenzyme Q are linked in a pathway that drives insulin resistance in skeletal muscle

Insulin resistance (IR) is a complex metabolic disorder that underlies several human diseases, including type 2 diabetes and cardiovascular disease. Despite extensive research, the precise mechanisms underlying IR development remain poorly understood. Previously we showed that deficiency of coenzyme Q (CoQ) is necessary and sufficient for IR in adipocytes and skeletal muscle (Fazakerley et al., 2018). Here, we provide new insights into the mechanistic connections between cellular alterations associated with IR, including increased ceramides, CoQ deficiency, mitochondrial dysfunction, and oxidative stress. We demonstrate that elevated levels of ceramide in the mitochondria of skeletal muscle cells result in CoQ depletion and loss of mitochondrial respiratory chain components, leading to mitochondrial dysfunction and IR. Further, decreasing mitochondrial ceramide levels in vitro and in animal models (mice, C57BL/6J) (under chow and high-fat diet) increased CoQ levels and was protective against IR. CoQ supplementation also rescued ceramide-associated IR. Examination of the mitochondrial proteome from human muscle biopsies revealed a strong correlation between the respirasome system and mitochondrial ceramide as key determinants of insulin sensitivity. Our findings highlight the mitochondrial ceramide-CoQ-respiratory chain nexus as a potential foundation of an IR pathway that may also play a critical role in other conditions associated with ceramide accumulation and mitochondrial dysfunction, such as heart failure, cancer, and aging. These insights may have important clinical implications for the development of novel therapeutic strategies for the treatment of IR and related metabolic disorders.

Immunolocalization of Sphingolipid Catabolism Enzymes along the Nephron: Novel Early Urinary Biomarkers of Renal Damage

The objective of this study was to investigate whether the activity of enzymes involved in sphingolipid catabolism could be biomarkers to predict early renal damage in streptozotocin (STZ)-induced diabetic rats and Angiotensin II (Ang II)-induced hypertension rats. Diabetic and hypertensive rats had no changes in plasma creatinine concentration. However, transmission electron microscopy (TEM) analysis showed slight ultrastructural changes in the glomeruli and tubular epithelial cells from diabetic and hypertensive rats. Our results show that the acid sphingomyelinase (aSMase) and neutral sphingomyelinase (nSMase) activity increased in the urine of diabetic rats and decreased in hypertensive rats. Only neutral ceramidase (nCDase) activity increased in the urine of diabetic rats. Furthermore, the immunofluorescence demonstrated positive staining for the nSMase, nCDase, and sphingosine kinase (SphK1) in glomerular mesangial cells, proximal tubule, ascending thin limb of the loop of Henle, thick ascending limb of Henle's loop, and principal cells of the collecting duct in the kidney. In conclusion, our results suggest that aSMase and nCDase activity in urine could be a novel predictor of early slight ultrastructural changes in the nephron, aSMase and nCDase as glomerular injury biomarkers, and nSMase as a tubular injury biomarker in diabetic and hypertensive rats.

Sphingolipids in mitochondria-from function to disease

Sphingolipids are not only structural components of cellular membranes but also play vital roles in cell signaling and modulation of cellular processes. Within mitochondria, sphingolipids exert diverse effects on mitochondrial dynamics, energy metabolism, oxidative stress, and cell death pathways. In this review, we summarize literature addressing the crucial role of sphingolipids in mitochondria, highlighting their impact on mitochondrial dynamics, cellular bioenergetics, and important cell processes including apoptosis and mitophagy.

Purification and Characterization of Mitochondrial Mg2+-Independent Sphingomyelinase from Rat Brain

Sphingomyelinase (SMase) catalyzes ceramide production from sphingomyelin. Ceramides are critical in cellular responses such as apoptosis. They enhance mitochondrial outer membrane permeabilization (MOMP) through self-assembly in the mitochondrial outer membrane to form channels that release cytochrome c from intermembrane space (IMS) into the cytosol, triggering caspase-9 activation. However, the SMase involved in MOMP is yet to be identified. Here, we identified a mitochondrial Mg2+-independent SMase (mt-iSMase) from rat brain, which was purified 6,130-fold using a Percoll gradient, pulled down with biotinylated sphingomyelin, and subjected to Mono Q anion exchange. A single peak of mt-iSMase activity was eluted at a molecular mass of approximately 65 kDa using Superose 6 gel filtration. The purified enzyme showed optimal activity at pH of 6.5 and was inhibited by dithiothreitol and Mg2+, Mn2+, N2+, Cu2+, Zn2+, Fe2+, and Fe3+ ions. It was also inhibited by GW4869, which is a non-competitive inhibitor of Mg2+-dependent neutral SMase 2 (encoded by SMPD3), that protects against cytochrome c release-mediated cell death. Subfractionation experiments showed that mt-iSMase localizes in the IMS of the mitochondria, implying that mt-iSMase may play a critical role in generating ceramides for MOMP, cytochrome c release, and apoptosis. These data suggest that the purified enzyme in this study is a novel SMase.

Mitochondrial electron transport chain, ceramide and Coenzyme Q are linked in a pathway that drives insulin resistance in skeletal muscle

Insulin resistance (IR) is a complex metabolic disorder that underlies several human diseases, including type 2 diabetes and cardiovascular disease. Despite extensive research, the precise mechanisms underlying IR development remain poorly understood. Here, we provide new insights into the mechanistic connections between cellular alterations associated with IR, including increased ceramides, deficiency of coenzyme Q (CoQ), mitochondrial dysfunction, and oxidative stress. We demonstrate that elevated levels of ceramide in the mitochondria of skeletal muscle cells results in CoQ depletion and loss of mitochondrial respiratory chain components, leading to mitochondrial dysfunction and IR. Further, decreasing mitochondrial ceramide levels in vitro and in animal models (under chow and high fat diet) increased CoQ levels and was protective against IR. CoQ supplementation also rescued ceramide-associated IR. Examination of the mitochondrial proteome from human muscle biopsies revealed a strong correlation between the respirasome system and mitochondrial ceramide as key determinants of insulin sensitivity. Our findings highlight the mitochondrial Ceramide-CoQ-respiratory chain nexus as a potential foundation of an IR pathway that may also play a critical role in other conditions associated with ceramide accumulation and mitochondrial dysfunction, such as heart failure, cancer, and aging. These insights may have important clinical implications for the development of novel therapeutic strategies for the treatment of IR and related metabolic disorders.

Ceramides and ceramide synthases in cancer: Focus on apoptosis and autophagy

Different studies corroborate a role for ceramide synthases and their downstream products, ceramides, in modulation of apoptosis and autophagy in the context of cancer. These mechanisms of regulation, however, appear to be context dependent in terms of ceramides' fatty acid chain length, subcellular localization, and the presence or absence of their downstream targets. Our current understanding of the role of ceramide synthases and ceramides in regulation of apoptosis and autophagy could be harnessed to pioneer the development of new treatments to activate or inhibit a single type of ceramide synthase, thereby regulating the apoptosis induction or cross talk of apoptosis and autophagy in cancer cells. Moreover, the apoptotic function of ceramide suggests that ceramide analogues can pave the way for the development of novel cancer treatments. Therefore, in the current review paper we discuss the impact of ceramide synthases and ceramides in regulation of apoptosis and autophagy in context of different types of cancers. We also briefly introduce the latest information on ceramide synthase inhibitors, their application in diseases including cancer therapy, and discuss approaches for drug discovery in the field of ceramide synthase inhibitors. We finally discussed strategies for developing strategies to use lipids and ceramides analysis in biological fluids for developing early biomarkers for cancer.

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.

Ceramide analogue C2-cer induces a loss in insulin sensitivity in muscle cells through the salvage/recycling pathway

Ceramides have been shown to play a major role in the onset of skeletal muscle insulin resistance and therefore in the prevalence of type 2 diabetes (T2D). However, many of the studies involved in the discovery of deleterious ceramide actions used a non-physiological cell-permeable short-chain ceramide analogue, the C2-ceramide (C2-cer). In the present study, we determined how C2-cer promotes insulin resistance in muscle cells. We demonstrate that C2-cer enters the salvage/recycling pathway and becomes de-acylated, yielding sphingosine, re-acylation of which depends on the availability of long chain fatty acids provided by the lipogenesis pathway in muscle cells. Importantly, we show these salvaged ceramides are actually responsible for the inhibition of insulin signaling induced by C2-cer. Interestingly, we also show that the exogenous and endogenous mono-unsaturated fatty acid oleate prevents C2-cer to be recycled into endogenous ceramide species in a diacylglycerol O-acyltransferase 1 (DGAT1)-dependent mechanism, which forces free fatty acid metabolism towards triacylglyceride production. Altogether, the study highlights for the first time that C2-cer induces a loss in insulin sensitivity through the salvage/recycling pathway in muscle cells. This study also validates C2-cer as a convenient tool to decipher mechanisms by which long-chain ceramides mediate insulin resistance in muscle cells and suggests that in addition to the de novo ceramide synthesis, recycling of ceramide could contribute to muscle insulin resistance observed in obesity and T2D.

A simple, highly sensitive, and facile method to quantify ceramide at the plasma membrane

The role of ceramide in biological functions is typically based on the elevation of cellular ceramide, measured by LC-MS in the total cell lysate. However, it has become increasingly appreciated that ceramide in different subcellular organelles regulates specific functions. In the plasma membrane, changes in ceramide levels might represent a small percentage of the total cellular ceramide, evading MS detection but playing a critical role in cell signaling. Importantly, there are currently no efficient techniques to quantify ceramide in the plasma membrane. Here, we developed a method to measure the mass of ceramide in the plasma membrane using a short protocol that is based on the hydrolysis of plasma membrane ceramide into sphingosine by the action of exogenously applied bacterial recombinant neutral ceramidase. Plasma membrane ceramide content can then be determined by measuring the newly generated sphingosine at a stoichiometry of 1:1. A key step of this protocol is the chemical fixation of cells to block cellular sphingolipid metabolism, especially of sphingosine to sphingosine 1-phosphate. We confirmed that chemical fixation does not disrupt the lipid composition at the plasma membrane, which remains intact during the time of the assay. We illustrate the power of the approach by applying this protocol to interrogate the effects of the chemotherapeutic compound doxorubicin. Here we distinguished two pools of ceramide, depending on the doxorubicin concentration, consolidating different reports. In summary, we have developed the first approach to quantify ceramide in the plasma membrane, allowing the study of new avenues in sphingolipid compartmentalization and function.

Sphingolipids and Atherosclerosis: The Dual Role of Ceramide and Sphingosine-1-Phosphate

Sphingolipids are bioactive molecules that play either pro- and anti-atherogenic roles in the formation and maturation of atherosclerotic plaques. Among SLs, ceramide and sphingosine-1-phosphate showed antithetic properties in regulating various molecular mechanisms and have emerged as novel potential targets for regulating the development of atherosclerosis. In particular, maintaining the balance of the so-called ceramide/S1P rheostat is important to prevent the occurrence of endothelial dysfunction, which is the trigger for the entire atherosclerotic process and is strongly associated with increased oxidative stress. In addition, these two sphingolipids, together with many other sphingolipid mediators, are directly involved in the progression of atherogenesis and the formation of atherosclerotic plaques by promoting the oxidation of low-density lipoproteins (LDL) and influencing the vascular smooth muscle cell phenotype. The modulation of ceramide and S1P levels may therefore allow the development of new antioxidant therapies that can prevent or at least impair the onset of atherogenesis, which would ultimately improve the quality of life of patients with coronary artery disease and significantly reduce their mortality.

Vascular Smooth Muscle Cell Neutral Sphingomyelinase 2 in the Release of Exosomes and Vascular Calcification

Vascular calcification (VC) is the pathological precipitation of calcium salts in the walls of blood vessels. It is a risk factor for cardiovascular events and their associated mortality. VC can be observed in a variety of cardiovascular diseases and is most prominent in diseases that are associated with dysregulated mineral homeostasis such as in chronic kidney disease. Local factors and mechanisms underlying VC are still incompletely understood, but it is appreciated that VC is a multifactorial process in which vascular smooth muscle cells (VSMCs) play an important role. VSMCs participate in VC by releasing extracellular vesicles (EVs), the extent, composition, and propensity to calcify of which depend on VSMC phenotype and microenvironment. Currently, no targeted therapy is available to treat VC. In-depth knowledge of molecular players of EV release and the understanding of their mechanisms constitute a vital foundation for the design of pharmacological treatments to combat VC effectively. This review highlights our current knowledge of VSMCs in VC and focuses on the biogenesis of exosomes and the role of the neutral Sphingomyelinase 2 (nSMase2).

Ceramide de novo synthesis in non-alcoholic fatty liver disease: Pathogenic mechanisms and therapeutic perspectives

Non-alcoholic fatty liver disease (NAFLD) is the most common chronic liver disease, and its advanced form non-alcoholic steatohepatitis (NASH) may progress to cirrhosis and hepatocellular carcinoma. Ceramides have been shown to exacerbate NAFLD development through enhancing insulin resistance, reactive oxygen species production, liver steatosis, lipotoxicity and hepatocyte apoptosis, and eventually causing hepatic inflammation and fibrosis. Emerging evidence indicates that ceramide production in NAFLD is predominantly attributed to activation of the de novo synthesis pathway of ceramides in hepatocytes. More importantly, pharmacological modulation of ceramide de novo synthesis in preclinical studies seems efficacious for the treatment of NAFLD. In this review, we provide an overview of the pathogenic mechanisms of ceramides in NAFLD, discuss recent advances and challenges in pharmacological interventions targeting ceramide de novo synthesis, and propose some research directions in the field.

You aren't IMMUNE to the ceramides that accumulate in cardiometabolic disease

Obesity leads to persistent increases in immune responses that contribute to cardiometabolic pathologies such as diabetes and cardiovascular disease. Pro-inflammatory macrophages infiltrate the expanding fat mass, which leads to increased production of cytokines such as tumor necrosis factor-alpha. Moreover, saturated fatty acids enhance signaling through the toll-like receptors involved in innate immunity. Herein we discuss the evidence that ceramides-which are intermediates in the biosynthetic pathway that produces sphingolipids-are essential intermediates that link these inflammatory signals to impaired tissue function. We discuss the mechanisms linking these immune insults to ceramide production and review the numerous ceramide actions that alter cellular metabolism, induce oxidative stress, and stimulate apoptosis. Lastly, we evaluate the correlation of ceramides in humans with inflammation-linked cardiometabolic disease and discuss preclinical studies which suggest that ceramide-lowering interventions may be an effective strategy to treat or prevent such maladies.

Charge Conversion Polymer–Liposome Complexes to Overcome the Limitations of Cationic Liposomes in Mitochondrial-Targeting Drug Delivery

Mitochondrial-targeting therapy is considered an important strategy for cancer treatment. (3-Carboxypropyl) triphenyl phosphonium (CTPP) is one of the candidate molecules that can drive drugs or nanomedicines to target mitochondria via electrostatic interactions. However, the mitochondrial-targeting effectiveness of CTPP is low. Therefore, pH-sensitive polymer–liposome complexes with charge-conversion copolymers and CTPP-containing cationic liposomes were designed for efficiently delivering an anti-cancer agent, ceramide, into cancer cellular mitochondria. The charge-conversion copolymers, methoxypoly(ethylene glycol)-block-poly(methacrylic acid-g-histidine), were anionic and helped in absorbing and shielding the positive charges of cationic liposomes at pH 7.4. In contrast, charge-conversion copolymers became neutral in order to depart from cationic liposomes and induced endosomal escape for releasing cationic liposomes into cytosol at acidic endosomes. The experimental results reveal that these pH-sensitive polymer–liposome complexes could rapidly escape from MCF-7 cell endosomes and target MCF-7 mitochondria within 3 h, thereby leading to the generation of reactive oxygen species and cell apoptosis. These findings provide a promising solution for cationic liposomes in cancer mitochondrial-targeting drug delivery.

Reduced ER-mitochondria connectivity promotes neuroblastoma multidrug resistance

Most cancer deaths result from progression of therapy resistant disease, yet our understanding of this phenotype is limited. Cancer therapies generate stress signals that act upon mitochondria to initiate apoptosis. Mitochondria isolated from neuroblastoma cells were exposed to tBid or Bim, death effectors activated by therapeutic stress. Multidrug‐resistant tumor cells obtained from children at relapse had markedly attenuated Bak and Bax oligomerization and cytochrome c release (surrogates for apoptotic commitment) in comparison with patient‐matched tumor cells obtained at diagnosis. Electron microscopy identified reduced ER–mitochondria‐associated membranes (MAMs; ER–mitochondria contacts, ERMCs) in therapy‐resistant cells, and genetically or biochemically reducing MAMs in therapy‐sensitive tumors phenocopied resistance. MAMs serve as platforms to transfer Ca²⁺ and bioactive lipids to mitochondria. Reduced Ca²⁺ transfer was found in some but not all resistant cells, and inhibiting transfer did not attenuate apoptotic signaling. In contrast, reduced ceramide synthesis and transfer was common to resistant cells and its inhibition induced stress resistance. We identify ER–mitochondria‐associated membranes as physiologic regulators of apoptosis via ceramide transfer and uncover a previously unrecognized mechanism for cancer multidrug resistance.

Changes in metabolomics and lipidomics in brain tissue and their correlations with the gut microbiome after chronic food-derived arsenic exposure in mice

Arsenic can cause neurodegenerative diseases of the brain, but the definite mechanism is still unknown. In this study, to discuss the disturbances on brain metabolome and lipidome under subchronic arsenic exposure, we treated mice with the arsenic-containing feed (concentration of total arsenic = 30 mg/kg) prepared in accordance with the proportion of rice arsenicals for 16 weeks and performed metabolomics and lipidomics studies respectively using UHPLC-Triple-TOF-MS/MS and UHPLC-Q Exactive Focus MS/MS on mice brain. In addition, the distributions of arsenical metabolites along the feed-gut-blood-brain chain were analyzed by ICP-MS and HPLC-ICP-MS, and fecal microbial variations were investigated by 16 s sequencing. The data showed that although only a tiny amount of arsenic (DMA=0.101 mg/kg, uAs=0.071 mg/kg) enters the brain through the blood-brain barrier, there were significant changes in brain metabolism, including 118 metabolites and 17 lipids. These different metabolites were involved in 30 distinct pathways, including glycometabolism, and metabolisms of lipid, nucleic acid, and amino acid were previously reported to be correlated with neurodegenerative diseases. Additionally, these different metabolites were significantly correlated with 12 gut bacterial OTUs, among which Lachnospiraceae, Muribaculaceae, Ruminococcaceae, and Erysipelotrichaceae were also previously reported to be related to the distortion of metabolism, indicating that the disturbance of metabolism in the brain may be associated with the disturbance of gut microbes induced by arsenic. Thus, the current study demonstrated that the brain metabolome and lipidome were significantly disturbed under subchronic arsenic exposure, and the disturbances also significantly correlated with some gut microbiome and may be associated with neurodegenerative diseases. Although preliminary, the results shed some light on the pathophysiology of arsenic-caused neurodegenerative diseases.

The unfolding role of ceramide in coordinating retinoid-based cancer therapy

Sphingolipid-mediated regulation in cancer development and treatment is largely ceramide-centered with the complex sphingolipid metabolic pathways unfolding as attractive targets for anticancer drug discovery. The dynamic interconversion of sphingolipids is tightly controlled at the level of enzymes and cellular compartments in response to endogenous or exogenous stimuli, such as anticancer drugs, including retinoids. Over the past two decades, evidence emerged that retinoids owe part of their potency in cancer therapy to modulation of sphingolipid metabolism and ceramide generation. Ceramide has been proposed as a 'tumor-suppressor lipid' that orchestrates cell growth, cell cycle arrest, cell death, senescence, autophagy, and metastasis. There is accumulating evidence that cancer development is promoted by the dysregulation of tumor-promoting sphingolipids whereas cancer treatments can kill tumor cells by inducing the accumulation of endogenous ceramide levels. Resistance to cancer therapy may develop due to a disrupted equilibrium between the opposing roles of tumor-suppressor and tumor-promoter sphingolipids. Despite the undulating effect and complexity of sphingolipid pathways, there are emerging opportunities for a plethora of enzyme-targeted therapeutic interventions that overcome resistance resulting from perturbed sphingolipid pathways. Here, we have revisited the interconnectivity of sphingolipid metabolism and the instrumental role of ceramide-biosynthetic and degradative enzymes, including bioactive sphingolipid products, how they closely relate to cancer treatment and pathogenesis, and the interplay with retinoid signaling in cancer. We focused on retinoid targeting, alone or in combination, of sphingolipid metabolism nodes in cancer to enhance ceramide-based therapeutics. Retinoid and ceramide-based cancer therapy using novel strategies such as combination treatments, synthetic retinoids, ceramide modulators, and delivery formulations hold promise in the battle against cancer.

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.

Cellular stress promotes NOD1/2-dependent inflammation via the endogenous metabolite sphingosine-1-phosphate

Cellular stress has been associated with inflammation, yet precise underlying mechanisms remain elusive. In this study, various unrelated stress inducers were employed to screen for sensors linking altered cellular homeostasis and inflammation. We identified the intracellular pattern recognition receptors NOD1/2, which sense bacterial peptidoglycans, as general stress sensors detecting perturbations of cellular homeostasis. NOD1/2 activation upon such perturbations required generation of the endogenous metabolite sphingosine-1-phosphate (S1P). Unlike peptidoglycan sensing via the leucine-rich repeats domain, cytosolic S1P directly bound to the nucleotide binding domains of NOD1/2, triggering NF-κB activation and inflammatory responses. In sum, we unveiled a hitherto unknown role of NOD1/2 in surveillance of cellular homeostasis through sensing of the cytosolic metabolite S1P. We propose S1P, an endogenous metabolite, as a novel NOD1/2 activator and NOD1/2 as molecular hubs integrating bacterial and metabolic cues.

Sphingolipids as a Culprit of Mitochondrial Dysfunction in Insulin Resistance and Type 2 Diabetes

Insulin resistance is defined as a complex pathological condition of abnormal cellular and metabolic response to insulin. Obesity and consumption of high-fat diet lead to ectopic accumulation of bioactive lipids in insulin-sensitive tissues. Intracellular lipid accumulation is regarded as one of the major factors in the induction of insulin resistance and type 2 diabetes (T2D). A significant number of studies have described the involvement of ceramides and other sphingolipids in the inhibition of insulin-signaling pathway in both skeletal muscles and the liver. Adverse effects of sphingolipid accumulation have recently been linked to the activation of protein kinase Cζ (PKCζ) and protein phosphatase 2A (PP2A), which, in turn, negatively affect phosphorylation of serine/threonine kinase Akt [also known as protein kinase B (PKB)], leading to decreased glucose uptake in skeletal muscles as well as increased gluconeogenesis and glycogenolysis in the liver. Sphingolipids, in addition to their direct impact on the insulin signaling pathway, may be responsible for other negative aspects of diabetes, namely mitochondrial dysfunction and deficiency. Mitochondrial health, which is characterized by appropriate mitochondrial quantity, oxidative capacity, controlled oxidative stress, undisturbed respiratory chain function, adenosine triphosphate (ATP) production and mitochondrial proliferation through fission and fusion, is impaired in the skeletal muscles and liver of T2D subjects. Recent findings suggest that impaired mitochondrial function may play a key role in the development of insulin resistance. Mitochondria stay in contact with the endoplasmic reticulum (ER), Golgi membranes and mitochondria-associated membranes (MAM) that are the main places of sphingolipid synthesis. Moreover, mitochondria are capable of synthesizing ceramide though ceramide synthase (CerS) activity. Recently, ceramides have been demonstrated to negatively affect mitochondrial respiratory chain function and fission/fusion activity, which is also a hallmark of T2D. Despite a significant correlation between sphingolipids, mitochondrial dysfunction, insulin resistance and T2D, this subject has not received much attention compared to the direct effect of sphingolipids on the insulin signaling pathway. In this review, we focus on the current state of scientific knowledge regarding the involvement of sphingolipids in the induction of insulin resistance by inhibiting mitochondrial function.

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

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

On the nature of ceramide-mitochondria interactions - Dissection using comprehensive mitochondrial phenotyping

Sphingolipids are a unique class of lipids owing to their non-glycerol-containing backbone, ceramide, that is constructed from a long-chain aliphatic amino alcohol, sphinganine, to which a fatty acid is attached via an amide bond. Ceramide plays a star role in the initiation of apoptosis by virtue of its interactions with mitochondria, a control point for a downstream array of signaling cascades culminating in apoptosis. Many pathways converge on mitochondria to elicit mitochondrial outer membrane permeabilization (MOMP), a step that corrupts bioenergetic service. Although much is known regarding ceramides interaction with mitochondria and the ensuing cell signal transduction cascades, how ceramide impacts the elements of mitochondrial bioenergetic function is poorly understood. The objective of this review is to introduce the reader to sphingolipid metabolism, present a snapshot of mitochondrial respiration, elaborate on ceramides convergence on mitochondria and the upstream players that collaborate to elicit MOMP, and introduce a mitochondrial phenotyping platform that can be of utility in dissecting the fine-points of ceramide impact on cellular bioenergetics.

The Role of Ceramides in Diabetes and Cardiovascular Disease Regulation of Ceramides by Adipokines

Metabolic dysfunction is intertwined with the pathophysiology of both diabetes and cardiovascular disease. Recently, one particular lipid class has been shown to influence the development and sustainment of these diseases: ceramides. As a subtype of sphingolipids, these species are particularly central to many sphingolipid pathways. Increased levels of ceramides are known to correlate with impaired cardiovascular and metabolic health. Furthermore, the interaction between ceramides and adipokines, most notably adiponectin and leptin, appears to play a role in the pathophysiology of these conditions. Adiponectin appears to counteract the detrimental effects of elevated ceramides, largely through activation of the ceramidase activity of its receptors. Elevated ceramides appear to worsen leptin resistance, which is an important phenomenon in the pathophysiology of obesity and metabolic syndrome.

A global lipid map defines a network essential for Zika virus replication

Zika virus (ZIKV), an arbovirus of global concern, remodels intracellular membranes to form replication sites. How ZIKV dysregulates lipid networks to allow this, and consequences for disease, is poorly understood. Here, we perform comprehensive lipidomics to create a lipid network map during ZIKV infection. We find that ZIKV significantly alters host lipid composition, with the most striking changes seen within subclasses of sphingolipids. Ectopic expression of ZIKV NS4B protein results in similar changes, demonstrating a role for NS4B in modulating sphingolipid pathways. Disruption of sphingolipid biosynthesis in various cell types, including human neural progenitor cells, blocks ZIKV infection. Additionally, the sphingolipid ceramide redistributes to ZIKV replication sites, and increasing ceramide levels by multiple pathways sensitizes cells to ZIKV infection. Thus, we identify a sphingolipid metabolic network with a critical role in ZIKV replication and show that ceramide flux is a key mediator of ZIKV infection.

Ceramide launches an acute anti-adhesion pro-migration cell signaling program in response to chemotherapy

Chemotherapy has been reported to upregulate sphingomylinases and increase cellular ceramide, often linked to the induction to cell death. In this work, we show that sublethal doses of doxorubicin and vorinostat still increased cellular ceramide, which was located predominantly at the plasma membrane. To interrogate possible functions of this specific pool of ceramide, we used recombinant enzymes to mimic physiological levels of ceramide at the plasma membrane upon chemotherapy treatment. Using mass spectrometry and network analysis, followed by experimental confirmation, the results revealed that this pool of ceramide acutely regulates cell adhesion and cell migration pathways with weak connections to commonly established ceramide functions (eg, cell death). Neutral sphingomyelinase 2 (nSMase2) was identified as responsible for the generation of plasma membrane ceramide upon chemotherapy treatment, and both ceramide at the plasma membrane and nSMase2 were necessary and sufficient to mediate these "side" effects of chemotherapy on cell adhesion and migration. This is the first time a specific pool of ceramide is interrogated for acute signaling functions, and the results define plasma membrane ceramide as an acute signaling effector necessary and sufficient for regulation of cell adhesion and cell migration under chemotherapeutical stress.

Mitochondria as intracellular signaling platforms in health and disease

Mitochondria, long viewed solely in the context of bioenergetics, are increasingly emerging as critical hubs for intracellular signaling. Due to their bacterial origin, mitochondria possess their own genome and carry unique lipid components that endow these organelles with specialized properties to help orchestrate multiple signaling cascades. Mitochondrial signaling modulates diverse pathways ranging from metabolism to redox homeostasis to cell fate determination. Here, we review recent progress in our understanding of how mitochondria serve as intracellular signaling platforms with a particular emphasis on lipid-mediated signaling, innate immune activation, and retrograde signaling. We further discuss how these signaling properties might potentially be exploited to develop new therapeutic strategies for a range of age-related conditions.

Sphingolipid distribution at mitochondria-associated membranes (MAMs) upon induction of apoptosis

The levels and composition of sphingolipids and related metabolites are altered in aging and in common disorders such as diabetes and cancers, as well as in neurodegenerative, cardiovascular, and respiratory diseases. Changes in sphingolipids have been implicated as being an essential step in mitochondria-driven cell death. However, little is known about the precise sphingolipid composition and modulation in mitochondria or related organelles. Here, we used LC-MS/MS to analyze the presence of key components of the ceramide metabolic pathway in vivo and in vitro in purified ER, mitochondria-associated membranes (MAMs), and mitochondria. Specifically, we analyzed the sphingolipids in the three pathways that generate ceramide: sphinganine in the de novo ceramide pathway, SM in the breakdown pathway, and sphingosine in the salvage pathway. We observed sphingolipid profiles in mouse liver, mouse brain, and a human glioma cell line (U251). We analyzed the quantitative and qualitative changes of these sphingolipids during staurosporine-induced apoptosis in U251 cells. Ceramide (especially C16-ceramide) levels increased during early apoptosis possibly through a conversion from mitochondrial sphinganine and SM, but sphingosine and lactosyl- and glycosyl-ceramide levels were unaffected. We also found that ceramide generation is enhanced in mitochondria when SM levels are decreased in the MAM. This decrease was associated with an increase in acid sphingomyelinase activity in MAM. We conclude that meaningful sphingolipid modifications occur in MAM, the mitochondria, and the ER during the early steps of apoptosis.

Sphingolipids and Mitochondrial Dynamic

For decades, sphingolipids have been related to several biological functions such as immune system regulation, cell survival, and proliferation. Recently, it has been reported that sphingolipids could be biomarkers in cancer and in other human disorders such as metabolic diseases. This is evidenced by the biological complexity of the sphingolipids associated with cell type-specific signaling and diverse sphingolipids molecules. As mitochondria dynamics have serious implications in homeostasis, in the present review, we focused on the relationship between sphingolipids, mainly ceramides and sphingosine-1-phosphate, and mitochondrial dynamics directed by fission, fusion, and mitophagy. There is evidence that the balances of ceramides (C18 and C16) and S1P, as well as the location of specific ceramide synthases in mitochondria, have roles in mitophagy and fission with an impact on cell fate and metabolism. However, signaling pathways controlling the sphingolipids metabolism and their location in mitochondria need to be better understood in order to propose new interventions and therapeutic strategies.

Reign in the membrane: How common lipids govern mitochondrial function

The lipids that make up biological membranes tend to be the forgotten molecules of cell biology. The paucity of data on these important entities likely reflects the difficulties of studying and understanding their biological roles, rather than revealing a lack of importance. Indeed, the lipid composition of biological membranes has a profound impact on a diverse array of cellular processes. The focus of this review is on the effects of different lipid classes on the function of mitochondria, particularly bioenergetics, in health and disease.

Sphingolipids in Alzheimer's disease, how can we target them?

Altered levels of sphingolipids and their metabolites in the brain, and the related downstream effects on neuronal homeostasis and the immune system, provide a framework for understanding mechanisms in neurodegenerative disorders and for developing new intervention strategies. In this review we will discuss: the metabolites of sphingolipids that function as second messengers; and functional aberrations of the pathway resulting in Alzheimer's disease (AD) pathophysiology. Focusing on the central product of the sphingolipid pathway ceramide, we describ approaches to pharmacologically decrease ceramide levels in the brain and we argue on how the sphingolipid pathway may represent a new framework for developing novel intervention strategies in AD. We also highlight the possible use of clinical and non-clinical drugs to modulate the sphingolipid pathway and sphingolipid-related biological cascades.

A lipid perspective on regulated cell death

Lipids are fundamental to life as structural components of cellular membranes and for signaling. They are also key regulators of different cellular processes such as cell division, proliferation, and death. Regulated cell death (RCD) requires the engagement of lipids and lipid metabolism for the initiation and execution of its killing machinery. The permeabilization of lipid membranes is a hallmark of RCD that involves, for each kind of cell death, a unique lipid profile. While the permeabilization of the mitochondrial outer membrane allows the release of apoptotic factors to the cytosol during apoptosis, permeabilization of the plasma membrane facilitates the release of intracellular content in other nonapoptotic types of RCD like necroptosis and ferroptosis. Lipids and lipid membranes are important accessory molecules required for the activation of protein executors of cell death such as BAX in apoptosis and MLKL in necroptosis. Peroxidation of membrane phospholipids and the subsequent membrane destabilization is a prerequisite to ferroptosis. Here, we discuss how lipids are essential players in apoptosis, the most common form of RCD, and also their role in necroptosis and ferroptosis. Altogether, we aim to highlight the contribution of lipids and membrane dynamics in cell death regulation.

Prokaryotic and Mitochondrial Lipids: A Survey of Evolutionary Origins

Mitochondria and bacteria share a myriad of properties since it is believed that the powerhouses of the eukaryotic cell have evolved from a prokaryotic origin. Ribosomal RNA sequences, DNA architecture and metabolism are strikingly similar in these two entities. Proteins and nucleic acids have been a hallmark for comparison between mitochondria and prokaryotes. In this chapter, similarities (and differences) between mitochondrial and prokaryotic membranes are addressed with a focus on structure-function relationship of different lipid classes. In order to be suitable for the theme of the book, a special emphasis is reserved to the effects of bioactive sphingolipids, mainly ceramide, on mitochondrial membranes and their roles in initiating programmed cell death.

Probing compartment-specific sphingolipids with targeted bacterial sphingomyelinases and ceramidases

Sphingolipids contribute to the regulation of cell and tissue homeostasis, and disorders of sphingolipid metabolism lead to diseases such as inflammation, stroke, diabetes, and cancer. Sphingolipid metabolic pathways involve an array of enzymes that reside in specific subcellular organelles, resulting in the formation of many diverse sphingolipids with distinct molecular species based on the diversity of the ceramide (Cer) structure. In order to probe compartment-specific metabolism of sphingolipids in this study, we analyzed the Cer and SM species preferentially produced in the inner plasma membrane (PM), Golgi apparatus, ER, mitochondria, nucleus, and cytoplasm by using compartmentally targeted bacterial SMases and ceramidases. The results showed that the length of the acyl chain of Cer becomes longer according to the progress of Cer from synthesis in the ER to the Golgi apparatus, then to the PM. These findings suggest that each organelle shows different properties of SM-derived Cers consistent with its emerging distinct functions in vitro and in vivo.

Intricate role of mitochondrial lipid in mitophagy and mitochondrial apoptosis: its implication in cancer therapeutics

The efficacy of chemotherapy is mostly restricted by the drug resistance developed during the course of cancer treatment. Mitophagy, as a pro-survival mechanism, crucially maintains mitochondrial homeostasis and it is one of the mechanisms that cancer cells adopt for their progression. On the other hand, mitochondrial apoptosis, a precisely regulated form of cell death, acts as a tumor-suppressive mechanism by targeting cancer cells. Mitochondrial lipids, such as cardiolipin, ceramide, and sphingosine-1-phosphate, act as a mitophageal signal for the clearance of damaged mitochondria by interacting with mitophagic machinery as well as activate mitochondrial apoptosis via the release of cytochrome c into the cytoplasm. In the recent time, the lipid-mediated lethal mitophagy has also been used as an alternative approach to abolish the survival role of lipid in cancer. Therefore, by targeting mitochondrial lipids in cancer cells, the detailed mechanism linked to drug resistance can be unraveled. In this review, we precisely discuss the current knowledge about the multifaceted role of mitochondrial lipid in regulating mitophagy and mitochondrial apoptosis and its application in effective cancer therapy.

Lipotoxicty in yeast: a focus on plasma membrane signalling and membrane contact sites

Lipotoxicity is a pathophysiological process triggered by lipid overload. In metazoans, lipotoxicity is characterised by the ectopic deposition of lipids on organs other than adipose tissue. This leads to organ dysfunction, cell death, and is intimately linked to lipid-associated diseases such as cardiac dysfunction, atherosclerosis, stroke, hepatosteatosis, cancer and the metabolic syndrome. The molecules involved in eliciting lipotoxicity include FAs and their acyl-CoA derivatives, triacylglycerol (TG), diacylglycerol (DG), ceramides, acyl-carnitines and phospholipids. However, the cellular transport of toxic lipids through membrane contact sites (MCS) and vesicular mechanisms as well as lipid metabolism that progress lipotoxicity to the onset of disease are not entirely understood. Yeast has proven a useful model organism to study the molecular mechanisms of lipotoxicity. Recently, the Rim101 pathway, which senses alkaline pH and the lipid status at the plasmamembrane, has been connected to lipotoxicity. In this review article, we summarise recent research advances on the Rim101 pathway and MCS in the context of lipotoxicity in yeast and present a perspective for future research directions.

Visualizing bioactive ceramides

In the last 30 years, ceramides have been found to mediate a myriad of biological processes. Ceramides have been recognized as bioactive molecules and their metabolizing enzymes are attractive targets in cancer therapy and other diseases. The molecular mechanism of action of cellular ceramides are still not fully established, with insights into roles through modification of lipid rafts, creation of ceramide platforms, ceramide channels, or through regulation of direct protein effectors such as protein phosphatases and kinases. Recently, the 'Many Ceramides' hypothesis focuses on distinct pools of subcellular ceramides and ceramide species as potential defined bioactive entities. Traditional methods that measure changes in ceramide levels in the whole cell, such as mass spectrometry, fluorescent ceramide analogues, and ceramide antibodies, fail to differentiate specific bioactive species at the subcellular level. However, a few ceramide binding proteins have been reported, and a smaller subgroup within these, have been shown to translocate to ceramide-enriched membranes, revealing these localized pools of bioactive ceramides. In this review we want to discuss and consolidate these works and explore the possibility of defining these binding proteins as new tools are emerging to visualize bioactive ceramides in cells. Our goal is to encourage the scientific community to explore these ceramide partners, to improve techniques to refine the list of these binding partners, making possible the identification of specific domains that recognize and bind ceramides to be used to visualize the 'Many Ceramides' in the cell.

Metabolomic Analysis of Mouse Brain after a Transient Middle Cerebral Artery Occlusion by Mass Spectrometry Imaging

We performed metabolomic analyses of mouse brain using a transient middle cerebral artery occlusion (tMCAO) model with Matrix Assisted Laser Desorption/Ionization (MALDI)-mass spectrometry imaging (MSI) to reveal metabolite changes after cerebral ischemia. We selected and analyzed three metabolites, namely creatine (Cr), phosphocreatine (P-Cr), and ceramides (Cer), because these metabolites contribute to cell life and death. Eight-week-old male C57BL/6J mice were subjected to tMCAO via the intraluminal blockade of the middle cerebral artery (MCA) and reperfusion 60 min after the induction of ischemia. Each mouse was randomly assigned to one of the three groups; the groups were defined by the survival period after reperfusion: control, 1 h, and 24 h. Corrected samples were analyzed using MALDI-MSI. Results of MSI analysis showed the presence of several ionized substances and revealed spatial changes in some metabolites identified as precise substances, including Cr, P-Cr, Cer d18:1/18:0, phosphatidylcholine, L-glutamine, and L-histidine. Cr, P-Cr, and Cer d18:1/18:0 were changed after tMCAO, and P-Cr and Cer d18:1/18:0 accumulated over time in ischemic cores and surrounding areas following ischemia onset. The upregulation of P-Cr and Cer d18:1/18:0 was detected 1 h after tMCAO when no changes were evident on hematoxylin and eosin staining and immunofluorescence assay. P-Cr and Cer d18:1/18:0 can serve as neuroprotective therapies because they are biomarker candidates for cerebral ischemia.