Ceramide in stem cell differentiation and embryo development: novel functions of a topological cell-signaling lipid and the concept of ceramide compartments

Sphingolipids accumulate in aged muscle, and their reduction counteracts sarcopenia

Age-related muscle dysfunction and sarcopenia are major causes of physical incapacitation in older adults and currently lack viable treatment strategies. Here we find that sphingolipids accumulate in mouse skeletal muscle upon aging and that both genetic and pharmacological inhibition of sphingolipid synthesis prevent age-related decline in muscle mass while enhancing strength and exercise capacity. Inhibition of sphingolipid synthesis confers increased myogenic potential and promotes protein synthesis. Within the sphingolipid pathway, we show that accumulation of dihydroceramides is the culprit disturbing myofibrillar homeostasis. The relevance of sphingolipid pathways in human aging is demonstrated in two cohorts, the UK Biobank and Helsinki Birth Cohort Study in which gene expression-reducing variants of SPTLC1 and DEGS1 are associated with improved and reduced fitness of older individuals, respectively. These findings identify sphingolipid synthesis inhibition as an attractive therapeutic strategy for age-related sarcopenia and co-occurring pathologies.

Aryl hydrocarbon receptor (AhR) reveals evidence of antagonistic pleiotropy in the regulation of the aging process

The antagonistic pleiotropy hypothesis is a well-known evolutionary theory to explain the aging process. It proposes that while a particular gene may possess beneficial effects during development, it can exert deleterious properties in the aging process. The aryl hydrocarbon receptor (AhR) has a significant role during embryogenesis, but later in life, it promotes several age-related degenerative processes. For instance, AhR factor (i) controls the pluripotency of stem cells and the stemness of cancer stem cells, (ii) it enhances the differentiation of embryonal stem cells, especially AhR signaling modulates the differentiation of hematopoietic stem cells and progenitor cells, (iii) it also stimulates the differentiation of immunosuppressive Tregs, Bregs, and M2 macrophages, and finally, (iv) AhR signaling participates in the differentiation of many peripheral tissues. On the other hand, AhR signaling is involved in many processes promoting cellular senescence and pathological processes, e.g., osteoporosis, vascular dysfunction, and the age-related remodeling of the immune system. Moreover, it inhibits autophagy and aggravates extracellular matrix degeneration. AhR signaling also stimulates oxidative stress, promotes excessive sphingolipid synthesis, and disturbs energy metabolism by catabolizing NAD⁺ degradation. The antagonistic pleiotropy of AhR signaling is based on the complex and diverse connections with major signaling pathways in a context-dependent manner. The major regulatory steps include, (i) a specific ligand-dependent activation, (ii) modulation of both genetic and non-genetic responses, (iii) a competition and crosstalk with several transcription factors, such as ARNT, HIF-1α, E2F1, and NF-κB, and (iv) the epigenetic regulation of target genes with binding partners. Thus, not only mTOR signaling but also the AhR factor demonstrates antagonistic pleiotropy in the regulation of the aging process.

Metabolite alteration analysis of acetaminophen-induced liver injury using a mass microscope

Acetaminophen (APAP)-induced liver injury (APAP-ILI), which occurs during APAP overdose, has been extensively studied. The production of N-acetyl-p-benzoquinone imine (NAPQI), the reactive metabolite of APAP, primarily contributes to liver injury. However, the mechanism underlying APAP-ILI has not been fully characterized. For further clarification, it is important to consider drug localization and endogenous substances in the injured liver. Herein, we show the localization of NAPQI metabolites and the injury site-specific changes in endogenous substances in the rat liver following APAP overdose using a mass microscope. Our results of on-tissue derivatization matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) showed that the glutathione metabolite of APAP, a detoxified metabolite of NAPQI, localized in the damaged central vein region in the rat liver following APAP administration. Moreover, in the conventional MALDI-MSI, the intensities of some phospholipids, phosphocreatine, and ceramides decreased or increased in the damaged regions compared with those in non-damaged regions. Phosphocreatine was localized in the damaged cells, whereas its related mitochondrial creatine kinase was localized in the non-damaged cells. These results are expected to contribute to further elucidation of the mechanisms underlying APAP-ILI. Our findings illustrate the localization of NAPQI-related metabolites and endogenous molecules associated with APAP-ILI, which may be related to apoptosis or metabolic adaptation ultimately protecting the cells. As MALDI-MSI can analyze and differentiate regions with tissue damage, it is a valuable tool for analyzing the mechanism underlying drug-induced liver injury to identify novel biomarkers.

Bioactive Lipid Signaling in Cardiovascular Disease, Development, and Regeneration

Cardiovascular disease (CVD) remains a leading cause of death globally. Understanding and characterizing the biochemical context of the cardiovascular system in health and disease is a necessary preliminary step for developing novel therapeutic strategies aimed at restoring cardiovascular function. Bioactive lipids are a class of dietary-dependent, chemically heterogeneous lipids with potent biological signaling functions. They have been intensively studied for their roles in immunity, inflammation, and reproduction, among others. Recent advances in liquid chromatography-mass spectrometry techniques have revealed a staggering number of novel bioactive lipids, most of them unknown or very poorly characterized in a biological context. Some of these new bioactive lipids play important roles in cardiovascular biology, including development, inflammation, regeneration, stem cell differentiation, and regulation of cell proliferation. Identifying the lipid signaling pathways underlying these effects and uncovering their novel biological functions could pave the way for new therapeutic strategies aimed at CVD and cardiovascular regeneration.

Primary cilia control cell alignment and patterning in bone development via ceramide-PKCζ-β-catenin signaling

Intraflagellar transport (IFT) proteins are essential for cilia assembly and function. IFT protein mutations lead to ciliopathies, which manifest as variable skeletal abnormalities. However, how IFT proteins regulate cell alignment during bone development is unknown. Here, we show that the deletion of IFT20 in osteoblast lineage using Osterix-Cre and inducible type I Collagen-CreERT cause a compromised cell alignment and a reduced bone mass. This finding was validated by the disorganized collagen fibrils and decreased bone strength and stiffness in IFT20-deficient femurs. IFT20 maintains cilia and cell alignment in osteoblasts, as the concentric organization of three-dimensional spheroids was disrupted by IFT20 deletion. Mechanistically, IFT20 interacts with the ceramide-PKCζ complex to promote PKCζ phosphorylation in cilia and induce the apical localization of β-catenin in osteoblasts, both of which were disrupted in the absence of IFT20. These results reveal that IFT20 regulates polarity and cell alignment via ceramide-pPKCζ-β-catenin signaling during bone development.

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.

Lipid Alterations during Zebrafish Embryogenesis Revealed by Dynamic Mass Spectrometry Profiling with C=C Specificity

Lipids exert substantial influences on vertebrate embryogenesis, but their metabolic dynamics at detailed structural levels remains elusive, primarily owing to the lack of a tool capable of resolving their huge structural diversity. Herein, we present the first large-scale and spatiotemporal monitoring of unsaturated lipids with C=C specificity in single developing zebrafish embryos enabled by photochemical derivatization and tandem mass spectrometry (MS). The lipid isomer composition was found extremely stable in yolk throughout embryogenesis, while notable differences in ratios of C=C location (e.g., PC 16:0_16:1 (7) vs. 16:0_16:1 (9)) and fatty acyl composition isomers (e.g., PC 16:1_18:1 vs. 16:0_18:2) were unveiled between blastomeres and yolk from zygote to 4 h post fertilization (hpf). From 24 hpf onwards, lipid isomer compositions in embryo head and tail evolved distinctively with development, suggesting a meticulously regulated lipid remodeling essential for cell division and differentiation. This work has laid the foundation for functional studies of structurally defined lipids in vertebrate embryology.

Loss of SMPD4 Causes a Developmental Disorder Characterized by Microcephaly and Congenital Arthrogryposis

Sphingomyelinases generate ceramide from sphingomyelin as a second messenger in intracellular signaling pathways involved in cell proliferation, differentiation, or apoptosis. Children from 12 unrelated families presented with microcephaly, simplified gyral pattern of the cortex, hypomyelination, cerebellar hypoplasia, congenital arthrogryposis, and early fetal/postnatal demise. Genomic analysis revealed bi-allelic loss-of-function variants in SMPD4, coding for the neutral sphingomyelinase-3 (nSMase-3/SMPD4). Overexpression of human Myc-tagged SMPD4 showed localization both to the outer nuclear envelope and the ER and additionally revealed interactions with several nuclear pore complex proteins by proteomics analysis. Fibroblasts from affected individuals showed ER cisternae abnormalities, suspected for increased autophagy, and were more susceptible to apoptosis under stress conditions, while treatment with siSMPD4 caused delayed cell cycle progression. Our data show that SMPD4 links homeostasis of membrane sphingolipids to cell fate by regulating the cross-talk between the ER and the outer nuclear envelope, while its loss reveals a pathogenic mechanism in microcephaly.

Mammalian sphingoid bases: Biophysical, physiological and pathological properties

Sphingoid bases encompass a group of long chain amino alcohols which form the essential structure of sphingolipids. Over the last years, these amphiphilic molecules were moving more and more into the focus of biomedical research due to their role as bioactive molecules. In fact, free sphingoid bases interact with specific receptors and target molecules and have been associated with numerous biological and physiological processes. In addition, they can modulate the biophysical properties of biological membranes. Several human diseases are related to pathological changes in the structure and metabolism of sphingoid bases. Yet, the mechanisms underlying their biological and pathophysiological actions remain elusive. Within this review, we aimed to summarize the current knowledge on the biochemical and biophysical properties of the most common sphingoid bases and to discuss their importance in health and disease.

Sphingomyelin Breakdown in T Cells: Role of Membrane Compartmentalization in T Cell Signaling and Interference by a Pathogen

Sphingolipids are major components of cellular membranes, and at steady-state level, their metabolic fluxes are tightly controlled. On challenge by external signals, they undergo rapid turnover, which substantially affects the biophysical properties of membrane lipid and protein compartments and, consequently, signaling and morphodynamics. In T cells, external cues translate into formation of membrane microdomains where proximal signaling platforms essential for metabolic reprograming and cytoskeletal reorganization are organized. This review will focus on sphingomyelinases, which mediate sphingomyelin breakdown and ensuing ceramide release that have been implicated in T-cell viability and function. Acting at the sphingomyelin pool at the extrafacial or cytosolic leaflet of cellular membranes, acid and neutral sphingomyelinases organize ceramide-enriched membrane microdomains that regulate T-cell homeostatic activity and, upon stimulation, compartmentalize receptors, membrane proximal signaling complexes, and cytoskeletal dynamics as essential for initiating T-cell motility and interaction with endothelia and antigen-presenting cells. Prominent examples to be discussed in this review include death receptor family members, integrins, CD3, and CD28 and their associated signalosomes. Progress made with regard to experimental tools has greatly aided our understanding of the role of bioactive sphingolipids in T-cell biology at a molecular level and of targets explored by a model pathogen (measles virus) to specifically interfere with their physiological activity.

Mammalian sphingoid bases: Biophysical, physiological and pathological properties

Sphingoid bases encompass a group of long chain amino alcohols which form the essential structure of sphingolipids. Over the last years, these amphiphilic molecules were moving more and more into the focus of biomedical research due to their role as bioactive molecules. In fact, free sphingoid bases interact with specific receptors and target molecules, and have been associated with numerous biological and physiological processes. In addition, they can modulate the biophysical properties of biological membranes. Several human diseases are related to pathological changes in the structure and metabolism of sphingoid bases. Yet, the mechanisms underlying their biological and pathophysiological actions remain elusive. Within this review, we aimed to summarize the current knowledge on the biochemical and biophysical properties of the most common sphingoid bases and to discuss their importance in health and disease.

The crosstalk between glycosphingolipids and neural stem cells

Until a few years ago, the majority of cell functions were envisioned as the result of protein and DNA activity. The cell membranes were considered as a mere structure of support and/or separation. In the last years, the function of cell membranes has, however, received more attention and their components of lipid nature have also been depicted as important cell mediators and the membrane organization was described as an important determinant for membrane-anchored proteins activity. In particular, because of their high diversity, glycosphingolipids offer a wide possibility of regulation. Specifically, the role of glycosphingolipids, in the fine-tuning of neuron activity, has recently received deep attention. For their pivotal role in vertebrate and mammals neural development, neural stem cells regulation is of main interest especially concerning their further functions in neurological pathology progression and treatment. Glycosphingolipids expression present a developmental regulation. In this view, glycosphingolipids can hold an important role in neural stem cells features because of their heterogeneity and their consequent capacity for eclectic interaction with other cell components.

Sphingolipids in neurodegeneration (with focus on ceramide and S1P)

For many decades, research on sphingolipids associated with neurodegenerative disease focused on alterations in glycosphingolipids, particularly glycosylceramides (cerebrosides), sulfatides, and gangliosides. This seemed quite natural since many of these glycolipids are constituents of myelin and accumulated in lipid storage diseases (sphingolipidoses) resulting from enzyme deficiencies in glycolipid metabolism. With the advent of recognizing ceramide and its derivative, sphingosine-1-phosphate (S1P), as key players in lipid cell signaling and regulation of cell death and survival, research focus shifted toward these two sphingolipids. Ceramide and S1P are invoked in a plethora of cell biological processes participating in neurodegeneration such as ER stress, autophagy, dysregulation of protein and lipid transport, exosome secretion and neurotoxic protein spreading, neuroinflammation, and mitochondrial dysfunction. Hence, it is timely to discuss various functions of ceramide and S1P in neurodegenerative disease and to define sphingolipid metabolism and cell signaling pathways as potential targets for therapy.

Sphingolipids and lipid rafts: Novel concepts and methods of analysis

About twenty years ago, the functional lipid raft model of the plasma membrane was published. It took into account decades of research showing that cellular membranes are not just homogenous mixtures of lipids and proteins. Lateral anisotropy leads to assembly of membrane domains with specific lipid and protein composition regulating vesicular traffic, cell polarity, and cell signaling pathways in a plethora of biological processes. However, what appeared to be a clearly defined entity of clustered raft lipids and proteins became increasingly fluid over the years, and many of the fundamental questions about biogenesis and structure of lipid rafts remained unanswered. Experimental obstacles in visualizing lipids and their interactions hampered progress in understanding just how big rafts are, where and when they are formed, and with which proteins raft lipids interact. In recent years, we have begun to answer some of these questions and sphingolipids may take center stage in re-defining the meaning and functional significance of lipid rafts. In addition to the archetypical cholesterol-sphingomyelin raft with liquid ordered (Lo) phase and the liquid-disordered (Ld) non-raft regions of cellular membranes, a third type of microdomains termed ceramide-rich platforms (CRPs) with gel-like structure has been identified. CRPs are "ceramide rafts" that may offer some fresh view on the membrane mesostructure and answer several critical questions for our understanding of lipid rafts.

Traceless synthesis of ceramides in living cells reveals saturation-dependent apoptotic effects

Mammalian cells synthesize thousands of distinct lipids, yet the function of many of these lipid species is unknown. Ceramides, a class of sphingolipid, are implicated in several cell-signaling pathways but poor cell permeability and lack of selectivity in endogenous synthesis pathways have hampered direct study of their effects. Here we report a strategy that overcomes the inherent biological limitations of ceramide delivery by chemoselectively ligating lipid precursors in vivo to yield natural ceramides in a traceless manner. Using this method, we uncovered the apoptotic effects of several ceramide species and observed differences in their apoptotic activity based on acyl-chain saturation. Additionally, we demonstrate spatiotemporally controlled ceramide synthesis in live cells through photoinitiated lipid ligation. Our in situ lipid ligation approach addresses the long-standing problem of lipid-specific delivery and enables the direct study of unique ceramide species in live cells.

Human iPSC-based models highlight defective glial and neuronal differentiation from neural progenitor cells in metachromatic leukodystrophy

The pathological cascade leading from primary storage to neural cell dysfunction and death in metachromatic leukodystrophy (MLD) has been poorly elucidated in human-derived neural cell systems. In the present study, we have modeled the progression of pathological events during the differentiation of patient-specific iPSCs to neuroepithelial progenitor cells (iPSC-NPCs) and mature neurons, astrocytes, and oligodendrocytes at the morphological, molecular, and biochemical level. We showed significant sulfatide accumulation and altered sulfatide composition during the differentiation of MLD iPSC-NPCs into neuronal and glial cells. Changes in sulfatide levels and composition were accompanied by the expansion of the lysosomal compartment, oxidative stress, and apoptosis. The neuronal and glial differentiation capacity of MLD iPSC-NPCs was significantly impaired. We showed delayed appearance and/or reduced levels of oligodendroglial and astroglial markers as well as reduced number of neurons and disorganized neuronal network. Restoration of a functional Arylsulfatase A (ARSA) enzyme in MLD cells using lentiviral-mediated gene transfer normalized sulfatide levels and composition, globally rescuing the pathological phenotype. Our study points to MLD iPSC-derived neural progeny as a useful in vitro model to assess the impact of ARSA deficiency along NPC differentiation into neurons and glial cells. In addition, iPSC-derived neural cultures allowed testing the impact of ARSA reconstitution/overexpression on disease correction and, importantly, on the biology and functional features of human NPCs, with important therapeutic implications.

Increased liver tumor formation in neutral sphingomyelinase-2-deficient mice

Sphingolipids are key signaling lipids in cancer. Genome-wide studies have identified neutral SMase-2 (nSMase2), an enzyme generating ceramide from SM, as a potential repressor for hepatocellular carcinoma. However, little is known about the sphingolipids regulated by nSMase2 and their roles in liver tumor development. We discovered growth of spontaneous liver tumors in 27.3% (9 of 33) of aged male nSMase2-deficient (fro/fro) mice. Lipidomics analysis showed a marked increase of SM in the tumor. Unexpectedly, tumor tissues presented with more than a 7-fold increase of C₁₆-ceramide, concurrent with upregulation of ceramide synthase 5. The fro/fro liver tumor, but not adjacent tissue, exhibited substantial accumulation of lipid droplets, suggesting that nSMase2 deficiency is associated with tumor growth and increased neutral lipid generation in the tumor. Tumor tissue expressed significantly increased levels of CD133 and EpCAM mRNA, two markers of liver cancer stem-like cells (CSCs) and higher levels of phosphorylated signal transducer and activator of transcription 3, an essential regulator of stemness. CD133(+) cells showed strong labeling for SM and ceramide. In conclusion, these results suggest that SMase-2 deficiency plays a role in the survival or proliferation of CSCs, leading to spontaneous tumors, which is associated with tumor-specific effects on lipid homeostasis.

Novel function of ceramide for regulation of mitochondrial ATP release in astrocytes

We reported that amyloid β peptide (Aβ₄₂) activated neutral SMase 2 (nSMase2), thereby increasing the concentration of the sphingolipid ceramide in astrocytes. Here, we show that Aβ₄₂ induced mitochondrial fragmentation in wild-type astrocytes, but not in nSMase2-deficient cells or astrocytes treated with fumonisin B1 (FB1), an inhibitor of ceramide synthases. Unexpectedly, ceramide depletion was concurrent with rapid movements of mitochondria, indicating an unknown function of ceramide for mitochondria. Using immunocytochemistry and super-resolution microscopy, we detected ceramide-enriched and mitochondria-associated membranes (CEMAMs) that were codistributed with microtubules. Interaction of ceramide with tubulin was confirmed by cross-linking to N-[9-(3-pent-4-ynyl-3-H-diazirine-3-yl)-nonanoyl]-D-erythro-sphingosine (pacFACer), a bifunctional ceramide analog, and binding of tubulin to ceramide-linked agarose beads. Ceramide-associated tubulin (CAT) translocated from the perinuclear region to peripheral CEMAMs and mitochondria, which was prevented in nSMase2-deficient or FB1-treated astrocytes. Proximity ligation and coimmunoprecipitation assays showed that ceramide depletion reduced association of tubulin with voltage-dependent anion channel 1 (VDAC1), an interaction known to block mitochondrial ADP/ATP transport. Ceramide-depleted astrocytes contained higher levels of ATP, suggesting that ceramide-induced CAT formation leads to VDAC1 closure, thereby reducing mitochondrial ATP release, and potentially motility and resistance to Aβ₄₂ Our data also indicate that inhibiting ceramide generation may protect mitochondria in Alzheimer's disease.

Lysosphingolipids and sphingolipidoses: Psychosine in Krabbe's disease

Until recently, lipids were considered inert building blocks of cellular membranes. This changed three decades ago when lipids were found to regulate cell polarity and vesicle transport, and the "lipid raft" concept took shape. The lipid-driven membrane anisotropy in form of "rafts" that associate with proteins led to the view that organized complexes of lipids and proteins regulate various cell functions. Disturbance of this organization can lead to cellular, tissue, and organ malfunction. Sphingolipidoses, lysosomal storage diseases that are caused by enzyme deficiencies in the sphingolipid degradation pathway, were found to be particularly detrimental to the brain. These enzyme deficiencies result in accumulation of sphingolipid metabolites in lysosomes, although it is not yet clear how this accumulation affects the organization of lipids in cellular membranes. Krabbe's disease (KD), or globoid cell leukodystrophy, was one of the first sphingolipidosis for which the raft concept offered a potential mechanism. KD is caused by mutations in the enzyme β-galactocerebrosidase; however, elevation of its substrate, galactosylceramide, is not observed or considered detrimental. Instead, it was found that a byproduct of galactosylceramide metabolism, the lysosphingolipid psychosine, is accumulated. The "psychosine hypothesis" has been refined by showing that psychosine disrupts lipid rafts and vesicular transport critical for the function of glia and neurons. The role of psychosine in KD is an example of how the disruption of sphingolipid metabolism can lead to elevation of a toxic lysosphingolipid, resulting in disruption of cellular membrane organization and neurotoxicity. © 2016 Wiley Periodicals, Inc.

Ceramide enhances COX-2 expression and VSMC contractile hyperreactivity via ER stress signal activation

Ceramide accumulation in blood vessels has been attributed to vascular dysfunction in progressive vascular complications in metabolic diseases. The present study showed that ceramide pretreatment promoted PE-induced vasoconstriction in rat endothelium-denuded vascular rings in a time- and dose-dependent manner. Endoplasmic reticulum (ER) stress inhibitors, 4-PBA and TUDCA, COX-2 inhibitors, Celecoxib and NS398, as well as PGE₂ receptor antagonist AH-6809 attenuated ceramide-promoted vascular hyperreactivity. Ceramide promoted the transcriptional and translational expression of COX-2 and BiP in VSMCs, which were blocked by the ER stress inhibitors, 4-PBA and TUDCA. These findings show that ceramide enhances PE-induced vascular smooth muscle constriction by mediation of the ER stress/COX-2/PGE₂ pathway. Therapeutic strategies targeted to reducing ER stress and COX-2 activation might be beneficial in attenuating vascular complications.

CHEMICAL COMPOUNDS

C₂-Ceramide (N-acetyl-d-erythro-sphingosine) CID:2662 Tauroursodeoxycholic Acid Sodium (TUDCA) CID:9848818 phenylephrine (PE) CID:6041.

Omics-based identification of the combined effects of idiosyncratic drugs and inflammatory cytokines on the development of drug-induced liver injury

The mechanisms of idiosyncratic drug-induced hepatotoxicity remain largely unclear. It has demonstrated that the drug idiosyncrasy is potentiated in the context of inflammation and intracellular ceramides may play a role in this process. To study the mechanisms, HepG2 cells were co-treated with high and low doses of three idiosyncratic (I) and three non-idiosyncratic (N) compounds, with (I+ and N+) or without (I- and N-) a cytokine mix. Microarray, lipidomics and flow cytometry were performed to investigate the genome-wide expression patterns, the intracellular ceramide levels and the induction of apoptosis. We found that all I+ treatments significantly influenced the immune response- and response to stimulus-associated gene ontology (GO) terms, but the induction of apoptotic pathways, which was confirmed by flow cytometry, only appeared to be induced after the high-dose treatment. The ceramide signaling-, ER stress-, NF-kB activation- and mitochondrial activity-related pathways were biologically involved in apoptosis induced by the high-dose I+. Additionally, genes participating in ceramide metabolism were significantly altered resulting in a measurable increase in ceramide levels. The increases in ceramide concentrations may induce ER stress and activate the JNK pathway by affecting the expression of the related genes, and eventually trigger the mitochondria-independent apoptosis in hepatocytes. Overall, our study provides a potential mechanism to explain the role of inflammation in idiosyncratic drug reactions.

Assessment of Membrane Fluidity Fluctuations during Cellular Development Reveals Time and Cell Type Specificity

Cell membrane is made up of a complex structure of lipids and proteins that diffuse laterally giving rise to what we call membrane fluidity. During cellular development, such as differentiation cell membranes undergo dramatic fluidity changes induced by proteins such as ARC and Cofilin among others. In this study we used the generalized polarization (GP) property of fluorescent probe Laurdan using two-photon microscopy to determine membrane fluidity as a function of time and for various cell lines. A low GP value corresponds to a higher fluidity and a higher GP value is associated with a more rigid membrane. Four different cell lines were monitored such as hN2, NIH3T3, HEK293 and L6 cells. Membrane fluidity was measured at 12h, 72h and 92 h. Our results show significant changes in membrane fluidity among all cell types at different time points. GP values tend to increase significantly within 92 h in hN2 cells and 72 h in NIH3T3 cells and only at 92 h in HEK293 cells. L6 showed a marked decrease in membrane fluidity at 72 h and starts to increase at 92 h. As expected, NIH3T3 cells have more rigid membrane at earlier time points. On the other hand, neurons tend to have the highest membrane fluidity at early time points emphasizing its correlation with plasticity and the need for this malleability during differentiation. This study sheds light on the involvement of membrane fluidity during neuronal differentiation and development of other cell lines.

Regulation of Chlamydomonas flagella and ependymal cell motile cilia by ceramide-mediated translocation of GSK3

Cilia are important organelles formed by cell membrane protrusions; however, little is known about their regulation by membrane lipids. A novel, evolutionarily conserved activation mechanism for GSK3 by the sphingolipid (phyto)ceramide is characterized that is critical for ciliogenesis in Chlamydomonas and murine ependymal cells.

Cilia are important organelles formed by cell membrane protrusions; however, little is known about their regulation by membrane lipids. We characterize a novel activation mechanism for glycogen synthase kinase-3 (GSK3) by the sphingolipids phytoceramide and ceramide that is critical for ciliogenesis in Chlamydomonas and murine ependymal cells, respectively. We show for the first time that Chlamydomonas expresses serine palmitoyl transferase (SPT), the first enzyme in (phyto)ceramide biosynthesis. Inhibition of SPT in Chlamydomonas by myriocin led to loss of flagella and reduced tubulin acetylation, which was prevented by supplementation with the precursor dihydrosphingosine. Immunocytochemistry showed that (phyto)ceramide was colocalized with phospho–Tyr-216-GSK3 (pYGSK3) at the base and tip of Chlamydomonas flagella and motile cilia in ependymal cells. The (phyto)ceramide distribution was consistent with that of a bifunctional ceramide analogue UV cross-linked and visualized by click-chemistry–mediated fluorescent labeling. Ceramide depletion, by myriocin or neutral sphingomyelinase deficiency (fro/fro mouse), led to GSK3 dephosphorylation and defective flagella and cilia. Motile cilia were rescued and pYGSK3 localization restored by incubation of fro/fro ependymal cells with exogenous C24:1 ceramide, which directly bound to pYGSK3. Our findings suggest that (phyto)ceramide-mediated translocation of pYGSK into flagella and cilia is an evolutionarily conserved mechanism fundamental to the regulation of ciliogenesis.

Ambient ionisation mass spectrometry for lipid profiling and structural analysis of mammalian oocytes, preimplantation embryos and stem cells

Lipids play fundamental roles in mammalian embryo preimplantation development and cell fate. Triacylglycerol accumulates in oocytes and blastomeres as lipid droplets, phospholipids influence membrane functional properties, and essential fatty acid metabolism is important for maintaining the stemness of cells cultured in vitro. The growing impact that lipids have in the field of developmental biology makes analytical approaches to analyse structural information of great interest. This paper describes the concept and presents the results of lipid profiling by mass spectrometry (MS) of oocytes and preimplantation embryos, with special focus on ambient ionisation. Based on our previous experience with oocytes and embryos, we aim to convey that ambient MS is also valuable for stem cell differentiation analysis. Ambient ionisation MS allows the detection of a wide range of lipid classes (e.g. free fatty acids, cholesterol esters, phospholipids) in single oocytes, embryos and cell pellets, which are informative of in vitro culture impact, developmental and differentiation stages. Background on MS principles, the importance of underused MS scan modes for structural analysis of lipids, and statistical approaches used for data analysis are covered. We envisage that MS alone or in combination with other techniques will have a profound impact on the understanding of lipid metabolism, particularly in early embryo development and cell differentiation research.

Global metabolomic analysis of human saliva and plasma from healthy and diabetic subjects, with and without periodontal disease

Recent studies suggest that periodontal disease and type 2 diabetes mellitus are bi-directionally associated. Identification of a molecular signature for periodontitis using unbiased metabolic profiling could allow identification of biomarkers to assist in the diagnosis and monitoring of both diabetes and periodontal disease. This cross-sectional study identified plasma and salivary metabolic products associated with periodontitis and/or diabetes in order to discover biomarkers that may differentiate or demonstrate an interaction of these diseases. Saliva and plasma samples were analyzed from 161 diabetic and non-diabetic human subjects with a healthy periodontium, gingivitis and periodontitis. Metabolite profiling was performed using Metabolon's platform technology. A total of 772 metabolites were found in plasma and 475 in saliva. Diabetics had significantly higher levels of glucose and α-hydroxybutyrate, the established markers of diabetes, for all periodontal groups of subjects. Comparison of healthy, gingivitis and periodontitis saliva samples within the non-diabetic group confirmed findings from previous studies that included increased levels of markers of cellular energetic stress, increased purine degradation and glutathione metabolism through increased levels of oxidized glutathione and cysteine-glutathione disulfide, markers of oxidative stress, including increased purine degradation metabolites (e.g. guanosine and inosine), increased amino acid levels suggesting protein degradation, and increased ω-3 (docosapentaenoate) and ω-6 fatty acid (linoleate and arachidonate) signatures. Differences in saliva between diabetic and non-diabetic cohorts showed altered signatures of carbohydrate, lipid and oxidative stress exist in the diabetic samples. Global untargeted metabolic profiling of human saliva in diabetics replicated the metabolite signature of periodontal disease progression in non-diabetic patients and revealed unique metabolic signatures associated with periodontal disease in diabetics. The metabolites identified in this study that discriminated the periodontal groups may be useful for developing diagnostics and therapeutics tailored to the diabetic population.

The importance of sphingolipids and reactive oxygen species in cardiovascular development

The heart is the first organ in the embryo to form. Its structural and functional complexity is the result of a thorough developmental program, where sphingolipids play an important role in cardiogenesis, heart maturation, angiogenesis, the regulation of vascular tone and vessel permeability. Sphingolipids are necessary for signal transduction and membrane microdomain formation. In addition, recent evidence suggests that sphingolipid metabolism is directly interconnected to the modulation of oxidative stress. However, cardiovascular development is highly sensitive to excessive reactive species production, and disturbances in sphingolipid metabolism can lead to abnormal development and cardiac disease. Therefore, in this review, we address the molecular link between sphingolipids and oxidative stress, connecting these pathways to cardiovascular development and cardiovascular disease.

MicroRNA transport: a new way in cell communication

MicroRNAs (miRNAs) can efficiently regulate gene expression by targeting mRNA to cause mRNA cleavage or translational repression. Growing evidence indicates that miRNAs exist not only in cells but also in a variety of body fluids, which stimulates substantial interest in the transport mechanism and regulating process of extracellular miRNAs. This article reviews the basic biogenesis of miRNAs in detail to explore the origin of extracellular miRNAs. Different miRNA transporters have been summarized (e.g., exosomes, microvesicles, apoptosis bodies, and RNA-binding proteins). In addition, we discuss the regulators affecting miRNA transport (e.g., ATP and ceramide) and the selection mechanism for different miRNA transporters. Studies about miRNA transporters and the transport mechanism are new and developing. With the progress of the research, new functions of extracellular miRNAs may be uncovered in the future.

p53 and Ceramide as Collaborators in the Stress Response

The sphingolipid ceramide mediates various cellular processes in response to several extracellular stimuli. Some genotoxic stresses are able to induce p53-dependent ceramide accumulation leading to cell death. However, in other cases, in the absence of the tumor suppressor protein p53, apoptosis proceeds partly due to the activity of this "tumor suppressor lipid", ceramide. In the current review, we describe ceramide and its roles in signaling pathways such as cell cycle arrest, hypoxia, hyperoxia, cell death, and cancer. In a specific manner, we are elaborating on the role of ceramide in mitochondrial apoptotic cell death signaling. Furthermore, after highlighting the role and mechanism of action of p53 in apoptosis, we review the association of ceramide and p53 with respect to apoptosis. Strikingly, the hypothesis for a direct interaction between ceramide and p53 is less favored. Recent data suggest that ceramide can act either upstream or downstream of p53 protein through posttranscriptional regulation or through many potential mediators, respectively.

Acid ceramidase as a therapeutic target in metastatic prostate cancer

Acid ceramidase (AC) catalyzes the hydrolysis of ceramide into sphingosine, in turn a substrate of sphingosine kinases that catalyze its conversion into the mitogenic sphingosine-1-phosphate. AC is expressed at high levels in several tumor types and has been proposed as a cancer therapeutic target. Using a model derived from PC-3 prostate cancer cells, the highly tumorigenic, metastatic, and chemoresistant clone PC-3/Mc expressed higher levels of the AC ASAH1 than the nonmetastatic clone PC-3/S. Stable knockdown of ASAH1 in PC-3/Mc cells caused an accumulation of ceramides, inhibition of clonogenic potential, increased requirement for growth factors, and inhibition of tumorigenesis and lung metastases. We developed de novo ASAH1 inhibitors, which also caused a dose-dependent accumulation of ceramides in PC-3/Mc cells and inhibited their growth and clonogenicity. Finally, immunohistochemical analysis of primary prostate cancer samples showed that higher levels of ASAH1 were associated with more advanced stages of this neoplasia. These observations confirm ASAH1 as a therapeutic target in advanced and chemoresistant forms of prostate cancer and suggest that our new potent and specific AC inhibitors could act by counteracting critical growth properties of these highly aggressive tumor cells.

An RNAi screen reveals intestinal regulators of branching morphogenesis, differentiation, and stem cell proliferation in planarians

Planarians grow and regenerate organs by coordinating proliferation and differentiation of pluripotent stem cells with remodeling of postmitotic tissues. Understanding how these processes are orchestrated requires characterizing cell-type-specific gene expression programs and their regulation during regeneration and homeostasis. To this end, we analyzed the expression profile of planarian intestinal phagocytes, cells responsible for digestion and nutrient storage/distribution. Utilizing RNA interference, we identified cytoskeletal regulators required for intestinal branching morphogenesis and a modulator of bioactive sphingolipid metabolism, ceramide synthase, required for the production of functional phagocytes. Additionally, we found that a gut-enriched homeobox transcription factor, nkx-2.2, is required for somatic stem cell proliferation, suggesting a niche-like role for phagocytes. Identification of evolutionarily conserved regulators of intestinal branching, differentiation, and stem cell dynamics demonstrates the utility of the planarian digestive system as a model for elucidating the mechanisms controlling postembryonic organogenesis.

C(16)-Ceramide-induced F-actin regulation stimulates mouse embryonic stem cell migration: involvement of N-WASP/Cdc42/Arp2/3 complex and cofilin-1/α-actinin

Ceramide, a major structural element in the cellular membrane, is a key regulatory factor in various cellular behaviors that are dependent on ceramide-induced association of specific proteins. However, molecular mechanisms that regulate ceramide-induced embryonic stem cell (ESC) migration are still not well understood. Thus, we investigated the effect of ceramide on migration and its related signal pathways in mouse ESCs. Among ceramide species with different fatty acid chain lengths, C(16)-Cer increased migration of mouse ESCs in a dose- (≥1μM) and time-dependent (≥8h) manners, as determined by the cell migration assay. C(16)-Cer (10μM) increased protein-kinase C (PKC) phosphorylation. Subsequently, C(16)-Cer increased focal adhesion kinase (FAK) and Paxillin phosphorylation, which were inhibited by PKC inhibitor Bisindolylmaleimide I (1μM). When we examined for the downstream signaling molecules, C(16)-Cer activated small G protein (Cdc42) and increased the formation of complex with Neural Wiskott-Aldrich Syndrome Protein (N-WASP)/Cdc42/Actin-Related Protein 2/3 (Arp2/3). This complex formation was disrupted by FAK- and Paxillin-specific siRNAs. Furthermore, C(16)-Cer-induced increase of filamentous actin (F-actin) expression was inhibited by Cdc42-, N-WASP-, and Arp2/3-specific siRNAs, respectively. Indeed, C(16)-Cer increased cofilin-1/F-actin interaction or F-actin/α-actinin-1 and α-actinin-4 interactions in the cytoskeleton compartment, which was reversed by Cdc42-specific siRNA. Finally, C(16)-Cer-induced increase of cell migration was inhibited by knocking down each signal pathway-related molecules with siRNA or inhibitors. In conclusion, C(16)-Cer enhances mouse ESC migration through the regulation of PKC and FAK/Paxillin-dependent N-WASP/Cdc42/Arp2/3 complex formation as well as through promoting the interaction between cofilin-1 or α-actinin-1/-4 and F-actin.

Identification of spectral modifications occurring during reprogramming of somatic cells

Recent technological advances in cell reprogramming by generation of induced pluripotent stem cells (iPSC) offer major perspectives in disease modelling and future hopes for providing novel stem cells sources in regenerative medicine. However, research on iPSC still requires refining the criteria of the pluripotency stage of these cells and exploration of their equivalent functionality to human embryonic stem cells (ESC). We report here on the use of infrared microspectroscopy to follow the spectral modification of somatic cells during the reprogramming process. We show that induced pluripotent stem cells (iPSC) adopt a chemical composition leading to a spectral signature indistinguishable from that of embryonic stem cells (ESC) and entirely different from that of the original somatic cells. Similarly, this technique allows a distinction to be made between partially and fully reprogrammed cells. We conclude that infrared microspectroscopy signature is a novel methodology to evaluate induced pluripotency and can be added to the tests currently used for this purpose.

It's a lipid's world: bioactive lipid metabolism and signaling in neural stem cell differentiation

Lipids are often considered membrane components whose function is to embed proteins into cell membranes. In the last two decades, studies on brain lipids have unequivocally demonstrated that many lipids have critical cell signaling functions; they are called "bioactive lipids". Pioneering work in Dr. Robert Ledeen's laboratory has shown that two bioactive brain sphingolipids, sphingomyelin and the ganglioside GM1 are major signaling lipids in the nuclear envelope. In addition to derivatives of the sphingolipid ceramide, the bioactive lipids discussed here belong to the classes of terpenoids and steroids, eicosanoids, and lysophospholipids. These lipids act mainly through two mechanisms: (1) direct interaction between the bioactive lipid and a specific protein binding partner such as a lipid receptor, protein kinase or phosphatase, ion exchanger, or other cell signaling protein; and (2) formation of lipid microdomains or rafts that regulate the activity of a group of raft-associated cell signaling proteins. In recent years, a third mechanism has emerged, which invokes lipid second messengers as a regulator for the energy and redox balance of differentiating neural stem cells (NSCs). Interestingly, developmental niches such as the stem cell niche for adult NSC differentiation may also be metabolic compartments that respond to a distinct combination of bioactive lipids. The biological function of these lipids as regulators of NSC differentiation will be reviewed and their application in stem cell therapy discussed.

Introduction to the fractality principle of consciousness and the sentyon postulate

Recently, consciousness research has gained much attention. Indeed, the question at stake is significant: why is the brain not just a computing device, but generates a perception from within? Ambitious endeavors trying to simulate the entire human brain assume that the algorithm will do the trick: as soon as we assemble the brain in a computer and increase the number of operations per time, consciousness will emerge by itself. I disagree with this simplistic representation. My argument emerges from the "atomism paradox": the irreducible space of the consciously perceived world, the endospace is incompatible with the reducible and decomposable architecture of the brain or a computer. I will first discuss the fundamental challenges in current consciousness models and then propose a new model based on the fractality principle: "the whole is in each of its parts". This new model copes with the atomism paradox by implementing an iterative mapping of information from higher order brain structures to smaller scales on the cellular and molecular level, which I will refer to as "fractalization". This information fractalization gives rise to a new form of matter that is conscious ("bright matter"). Bright matter is composed of conscious particles or units named "sentyons". The internal fractality of these sentyons will close a loop (the "psychic loop") in a recurrent fractal neural network (RFNN) that allows for continuous and complete information transformation and sharing between higher order brain structures and the endpoint substrate of consciousness at the molecular level.