Neurotrophic Natural Products

Neurotrophins (NGF, BDNF, NT3, NT4) can decrease cell death, induce differentiation, as well as sustain the structure and function of neurons, which make them promising therapeutic agents for the treatment of neurodegenerative disorders. However, neurotrophins have not been very effective in clinical trials mostly because they cannot pass through the blood-brain barrier owing to being high-molecular-weight proteins. Thus, neurotrophin-mimic small molecules, which stimulate the synthesis of endogenous neurotrophins or enhance neurotrophic actions, may serve as promising alternatives to neurotrophins. Small-molecular-weight natural products, which have been used in dietary functional foods or in traditional medicines over the course of human history, have a great potential for the development of new therapeutic agents against neurodegenerative diseases such as Alzheimer's disease. In this contribution, a variety of natural products possessing neurotrophic properties such as neurogenesis, neurite outgrowth promotion (neuritogenesis), and neuroprotection are described, and a focus is made on the chemistry and biology of several neurotrophic natural products.

Neuronal Ganglioside and Glycosphingolipid (GSL) Metabolism and Disease : Cascades of Secondary Metabolic Errors Can Generate Complex Pathologies (in LSDs)

Glycosphingolipids (GSLs) are a diverse group of membrane components occurring mainly on the surfaces of mammalian cells. They and their metabolites have a role in intercellular communication, serving as versatile biochemical signals (Kaltner et al, Biochem J 476(18):2623-2655, 2019) and in many cellular pathways. Anionic GSLs, the sialic acid containing gangliosides (GGs), are essential constituents of neuronal cell surfaces, whereas anionic sulfatides are key components of myelin and myelin forming oligodendrocytes. The stepwise biosynthetic pathways of GSLs occur at and lead along the membranes of organellar surfaces of the secretory pathway. After formation of the hydrophobic ceramide membrane anchor of GSLs at the ER, membrane-spanning glycosyltransferases (GTs) of the Golgi and Trans-Golgi network generate cell type-specific GSL patterns for cellular surfaces. GSLs of the cellular plasma membrane can reach intra-lysosomal, i.e. luminal, vesicles (ILVs) by endocytic pathways for degradation. Soluble glycoproteins, the glycosidases, lipid binding and transfer proteins and acid ceramidase are needed for the lysosomal catabolism of GSLs at ILV-membrane surfaces. Inherited mutations triggering a functional loss of glycosylated lysosomal hydrolases and lipid binding proteins involved in GSL degradation cause a primary lysosomal accumulation of their non-degradable GSL substrates in lysosomal storage diseases (LSDs). Lipid binding proteins, the SAPs, and the various lipids of the ILV-membranes regulate GSL catabolism, but also primary storage compounds such as sphingomyelin (SM), cholesterol (Chol.), or chondroitin sulfate can effectively inhibit catabolic lysosomal pathways of GSLs. This causes cascades of metabolic errors, accumulating secondary lysosomal GSL- and GG- storage that can trigger a complex pathology (Breiden and Sandhoff, Int J Mol Sci 21(7):2566, 2020).

Microbial lectome versus host glycolipidome: How pathogens exploit glycosphingolipids to invade, dupe or kill

Glycosphingolipids (GSLs) are ubiquitous components of the cell membranes, found across several kingdoms of life, from bacteria to mammals, including humans. GSLs are a subclass of major glycolipids occurring in animal lipid membranes in clusters named “lipid rafts.” The most crucial functions of GSLs include signal transduction and regulation as well as participation in cell proliferation. Despite the mainstream view that pathogens rely on protein–protein interactions to survive and thrive in their hosts, many also target the host lipids. In particular, multiple pathogens produce adhesion molecules or toxins that bind GSLs. Attachment of pathogens to cell surface receptors is the initial step in infections. Many mammalian pathogens have evolved to recognize GSL-derived receptors. Animal glycosphingolipidomes consist of multiple types of GSLs differing in terminal glycan and ceramide structures in a cell or tissue-specific manner. Interspecies differences in GSLs dictate host specificity as well as cell and tissue tropisms. Evolutionary pressure exerted by pathogens on their hosts drives changes in cell surface glycoconjugates, including GSLs, and has produced a vast number of molecules and interaction mechanisms. Despite that abundance, the role of GSLs as pathogen receptors has been largely overlooked or only cursorily discussed. In this review, we take a closer look at GSLs and their role in the recognition, cellular entry, and toxicity of multiple bacterial, viral and fungal pathogens.

Investigation on the surface-active and antimicrobial properties of a natural glycolipid product

Glycolipids are a group of sugar-containing lipids with versatile functions. In this study, a natural glycolipid product was obtained from soy lecithin, and its emulsifying, oil-gelling, antibacterial and antiviral properties were investigated. A silica-based extraction method on a preparative scale was used to recover the glycolipid product (GLP) from soy lecithin. The GLP consisted of three different glycolipid classes: acylated sterol glucoside (64.16%), sterol glucoside (25.57%) and cerebroside (6.71%). As an emulsifier, the GLP was able to form a stable water-in-oil emulsion. The GLP exhibited a good oil-gelling property, capable of gelling rapeseed oil at a concentration of 6%. For the investigated microorganisms (Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus), the GLP did not show any antibacterial effects. The GLP exerted antiviral activity against lentivirus, but not adenovirus. The results of this study help in enriching the knowledge on the properties of naturally occurring glycolipids, which may find potential applications in the food, pharmaceutical and related industries.

Ganglioside Composition Distinguishes Anaplastic Ganglioglioma Tumor Tissue from Peritumoral Brain Tissue: Complementary Mass Spectrometry and Thin-Layer Chromatography Evidence

Gangliosides serve as antitumor therapy targets and aberrations in their composition strongly correlate with tumor growth and invasiveness. Anaplastic ganglioglioma is a rare, poorly characterized, malignant neuronal–glial tumor type. We present the first comparative characterization of ganglioside composition in anaplastic ganglioglioma vs. peritumoral and healthy brain tissues by combining mass spectrometry and thin-layer chromatography. Anaplastic ganglioglioma ganglioside composition was highly distinguishable from both peritumoral and healthy tissue despite having five to six times lower total content. Ten out of twelve MS-identified ganglioside classes, defined by unique glycan residues, were represented by a large number and considerable abundance of individual species with different fatty acid residues (C16–C24) in ceramide portions. The major structurally identified class was tumor-associated GD3 (>50%) with 11 species; GD3 (d18:1/24:0) being the most abundant. The dominant sphingoid base residue in ganglioside ceramides was sphingosine (d18:1), followed by eicosasphingosine (d20:1). The peritumoral tissue ganglioside composition was estimated as normal. Specific ganglioside composition and large variability of ganglioside ceramide structures determined in anaplastic ganglioglioma demonstrate realistic ganglioside expression patterns and correspond to the profile of high-grade malignancy brain tumors.

Simulating the enzymes of ganglioside biosynthesis with Glycologue

Gangliosides are an important class of sialylated glycosphingolipids linked to ceramide that are a component of the mammalian cell surface, especially those of the central nervous system, where they function in intercellular recognition and communication. We describe an in silico method for determining the metabolic pathways leading to the most common gangliosides, based on the known enzymes of their biosynthesis. A network of 41 glycolipids is produced by the actions of the 10 enzymes included in the model. The different ganglioside nomenclature systems in common use are compared and a systematic variant of the widely used Svennerholm nomenclature is described. Knockouts of specific enzyme activities are used to simulate congenital defects in ganglioside biosynthesis, and altered ganglioside status in cancer, and the effects on network structure are predicted. The simulator is available at the Glycologue website, https://glycologue.org/.

Glycosphingolipids and neuroinflammation in Parkinson's disease

Parkinson's disease is a progressive neurodegenerative disease characterized by the loss of dopaminergic neurons of the nigrostriatal pathway and the formation of neuronal inclusions known as Lewy bodies. Chronic neuroinflammation, another hallmark of the disease, is thought to play an important role in the neurodegenerative process. Glycosphingolipids are a well-defined subclass of lipids that regulate crucial aspects of the brain function and recently emerged as potent regulators of the inflammatory process. Deregulation in glycosphingolipid metabolism has been reported in Parkinson's disease. However, the interrelationship between glycosphingolipids and neuroinflammation in Parkinson's disease is not well known. This review provides a thorough overview of the links between glycosphingolipid metabolism and immune-mediated mechanisms involved in neuroinflammation in Parkinson's disease. After a brief presentation of the metabolism and function of glycosphingolipids in the brain, it summarizes the evidences supporting that glycosphingolipids (i.e. glucosylceramides or specific gangliosides) are deregulated in Parkinson's disease. Then, the implications of these deregulations for neuroinflammation, based on data from human inherited lysosomal glycosphingolipid storage disorders and gene-engineered animal studies are outlined. Finally, the key molecular mechanisms by which glycosphingolipids could control neuroinflammation in Parkinson's disease are highlighted. These include inflammasome activation and secretion of pro-inflammatory cytokines, altered calcium homeostasis, changes in the blood-brain barrier permeability, recruitment of peripheral immune cells or production of autoantibodies.

CHARMM-GUI Membrane Builder for Complex Biological Membrane Simulations with Glycolipids and Lipoglycans

Glycolipids (such as glycoglycerolipids, glycosphingolipids, and glycosylphosphatidylinositol) and lipoglycans (such as lipopolysaccharides (LPS), lipooligosaccharides (LOS), mycobacterial lipoarabinomannan, and mycoplasma lipoglycans) are typically found on the surface of cell membranes and play crucial roles in various cellular functions. Characterizing their structure and dynamics at the molecular level is essential to understand their biological roles, but systematic generation of glycolipid and lipoglycan structures is challenging because of great variations in lipid structures and glycan sequences (i.e., carbohydrate types and their linkages). To facilitate the generation of all-atom glycolipid/LPS/LOS structures, we have developed Glycolipid Modeler and LPS Modeler in CHARMM-GUI ( http://www.charmm-gui.org ), a web-based interface that simplifies building of complex biological simulation systems. In addition, we have incorporated these modules into Membrane Builder so that users can readily build a complex symmetric or asymmetric biological membrane system with various glycolipids and LPS/LOS. These tools are expected to be useful in innovative and novel glycolipid/LPS/LOS modeling and simulation research by easing tedious and intricate steps in modeling complex biological systems and shall provide insight into structures, dynamics, and underlying mechanisms of complex glycolipid-/LPS-/LOS-containing biological membrane systems.

Emerging concepts of ganglioside metabolism

Gangliosides (GGs) are sialic acid-containing glycosphingolipids (GSLs) and major membrane components enriched on cellular surfaces. Biosynthesis of mammalian GGs starts at the cytosolic leaflet of endoplasmic reticulum (ER) membranes with the formation of their hydrophobic ceramide anchors. After intracellular ceramide transfer to Golgi and trans-Golgi network (TGN) membranes, anabolism of GGs, as well as of other GSLs, is catalyzed by membrane-spanning glycosyltransferases (GTs) along the secretory pathway. Combined activity of only a few promiscuous GTs allows for the formation of cell-type-specific glycolipid patterns. Following an exocytotic vesicle flow to the cellular plasma membranes, GGs can be modified by metabolic reactions at or near the cellular surface. For degradation, GGs are endocytosed to reach late endosomes and lysosomes. Whereas membrane-spanning enzymes of the secretory pathway catalyze GSL and GG formation, a cooperation of soluble glycosidases, lipases and lipid-binding cofactors, namely the sphingolipid activator proteins (SAPs), act as the main players of GG and GSL catabolism at intralysosomal luminal vesicles (ILVs).

CD1: A Singed Cat of the Three Antigen Presentation Systems

Contrary to general view that the MHC Class I and II are the kapellmeisters of recognition and response to antigens, there is another big player in that part of immunity, represented by CD1 glycoproteins. In contrast to MHC Class I or II, which present peptides, CD1 molecules present lipids. Humans express five CD1 proteins (CD1a-e), four of which (CD1a-d) are trafficked to the cell surface, where they may display lipid antigens to T-cell receptors. This interaction may lead to both non-cognate and cognate T cell help to B cells, the latter eliciting anti-lipid antibody response. All CD1 proteins can bind a broad range of structurally different exogenous and endogenous lipids, but each shows a preference to one or more lipid classes. This unorthodox binding behavior is the result of elaborate architectures of CD1 binding clefts and distinct intracellular trafficking routes. Together, these features make CD1 system a versatile player in immune response, sitting at the crossroads of innate and adaptive immunity. While CD1 system may be involved in numerous infectious, inflammatory, and autoimmune diseases, its involvement may lead to opposite outcomes depending on different pathologies. Despite these ambiguities and complexity, CD1 system draws growing attention and continues to show glimmers of therapeutic potential. In this review, we summarize the current knowledge about CD1 proteins, their structures, lipid-binding profiles, and roles in immunity, and evaluate the role of CD1 proteins in eliciting humoral immune response.

Human Cancer Antigen Globo H Is a Cell-Surface Ligand for Human Ribonuclease 1

Pancreatic-type ribonucleases are secretory enzymes that catalyze the cleavage of RNA. Recent efforts have endowed the homologues from cow (RNase A) and human (RNase 1) with toxicity for cancer cells, leading to a clinical trial. The basis for the selective toxicity of ribonuclease variants for cancerous versus noncancerous cells has, however, been unclear. A screen for RNase A ligands in an array of mammalian cell-surface glycans revealed strong affinity for a hexasaccharide, Globo H, that is a tumor-associated antigen and the basis for a vaccine in clinical trials. The affinity of RNase A and RNase 1 for immobilized Globo H is in the low micromolar-high nanomolar range. Moreover, reducing the display of Globo H on the surface of human breast adenocarcinoma cells with a small-molecule inhibitor of biosynthesis or a monoclonal antibody antagonist decreases the toxicity of an RNase 1 variant. Finally, heteronuclear single quantum coherence (HSQC) NMR spectroscopy showed that RNase 1 interacts with Globo H by using residues that are distal from the enzymic active site. The discovery that a systemic human ribonuclease binds to a moiety displayed on human cancer cells links two clinical paradigms and suggests a mechanism for innate resistance to cancer.

Affinity of monoclonal antibodies for Globo-series glycans

Globo-series glycans are human cell-surface carbohydrates that include stem-cell marker SSEA-4 and cancer-cell antigen Globo H. These two hexasaccharides differ only in their terminal saccharide: N-acetylneuraminic acid in SSEA-4 and L-fucose in Globo H. Herein, we evaluated the affinity of the monoclonal antibodies α-SSEA-4 and α-GH for the glycans SSEA-4 and Globo H. Using fluorescence polarization, we find that the two monoclonal antibodies have affinity for their cognate glycan in the low nanomolar range, and have negligible affinity for the non-cognate glycan. Using surface plasmon resonance, we find that each cognate affinity is ∼20-fold greater if the glycan is immobilized on a surface rather than free in solution. We conclude that the terminal saccharide plays a dominant role in the ability of monoclonal antibodies to recognize these Globo-series glycans and that the extraordinary specificity of these antibodies supports their use for identifying and sorting stem-cells (α-SSEA-4) and as an agent in cancer immunotherapy (α-GH).

Pharmacological chaperones as therapeutics for lysosomal storage diseases

Lysosomal enzymes are responsible for the degradation of a wide variety of glycolipids, oligosaccharides, proteins, and glycoproteins. Inherited mutations in the genes that encode these proteins can lead to reduced stability of newly synthesized lysosomal enzymes. While often catalytically competent, the mutated enzymes are unable to efficiently pass the quality control mechanisms of the endoplasmic reticulum, resulting in reduced lysosomal trafficking, substrate accumulation, and cellular dysfunction. Pharmacological chaperones (PCs) are small molecules that bind and stabilize mutant lysosomal enzymes, thereby allowing proper cellular translocation. Such compounds have been shown to increase enzyme activity and reduce substrate burden in a number of preclinical models and clinical studies. In this Perspective, we review several of the lysosomal diseases for which PCs have been studied and the SAR of the various classes of molecules.

Seven decades of "advances"

The field of carbohydrate science, as documented in the 70 volumes of Advances in Carbohydrate Chemistry (and Biochemistry) during the years 1944 through 2014, is surveyed. Subject areas detailed in individual volumes cover a broad range to include fundamental structural studies, synthesis, reactivity, mechanisms, analytical methodology, enzymology, biological and medicinal applications, food technology, and industrial and commercial aspects. The contributions of many prominent research leaders in the carbohydrate field are recorded in biographical memoirs. Stages in the development of internationally accepted systems for naming carbohydrate structures and for their graphical depiction are noted, and indexing questions for retrieval of data are addressed.

Ganglioside biochemistry

Gangliosides are sialic acid-containing glycosphingolipids. They occur especially on the cellular surfaces of neuronal cells, where they form a complex pattern, but are also found in many other cell types. The paper provides a general overview on their structures, occurrence, and metabolism. Key functional, biochemical, and pathobiochemical aspects are summarized.

Functional roles of gangliosides in neurodevelopment: an overview of recent advances

Gangliosides are sialic acid-containing glycosphingolipids that are most abundant in the nervous system. They are localized primarily in the outer leaflets of plasma membranes and participated in cell-cell recognition, adhesion, and signal transduction and are integral components of cell surface microdomains or lipid rafts along with proteins, sphingomyelin and cholesterol. Ganglioside-rich lipid rafts play an important role in signaling events affecting neural development and the pathogenesis of certain diseases. Disruption of gangloside synthase genes in mice induces developmental defects and neural degeneration. Targeting ganglioside metabolism may represent a novel therapeutic strategy for intervention in certain diseases. In this review, we focus on recent advances on metabolic and functional studies of gangliosides in normal brain development and in certain neurological disorders.

Physicochemical characterization of natural-like branched-chain glycosides toward formation of hexosomes and vesicles

Synthetic branched-chain glycolipids have become of great interest in biomimicking research, since they provide a suitable alternative for natural glycolipids, which are difficult to extract from natural resources. Therefore, branched-chain glycolipids obtained by direct syntheses are of utmost interest. In this work, two new branched-chain glycolipids are presented, namely, 2-hexyldecyl β(α)-D-glucoside (2-HDG) and 2-hexyldecyl β(α)-D-maltoside (2-HDM) based on glucose and maltose, respectively. The self-assembly properties of these glycolipids have been studied, observing the phase behavior under thermotropic and lyotropic conditions. Due to their amphiphilic characteristics, 2-HDG and 2-HDM possess rich phase behavior in dry form and in aqueous dispersions. In the thermotropic study, 2-HDG formed a columnar hexagonal liquid crystalline phase, whereas in a binary aqueous system, 2-HDG formed an inverted hexagonal liquid crystalline phase in equilibrium with excess aqueous solution. Furthermore, aqueous dispersions of the hexagonal liquid crystal could be obtained, dispersions known as hexosomes. On the other hand, 2-HDM formed a lamellar liquid crystalline phase (smectic A) in thermotropic conditions, whereas multilamellar vesicles have been observed in equilibrium with aqueous media. Surprisingly, 2-HDM mixed with sodium dodecyl sulfate or aerosol OT induced the formation of more stable unilamellar vesicles. Thus, the branched-chain glycolipids 2-HDG and 2-HDM not only provided alternative nonionic surfactants with rich phase behavior and versatile nanostructures, but also could be used as new drug carrier systems in the future.

Lipidomics of glycosphingolipids

Glycosphingolipids (GSLs) contain one or more sugars that are attached to a sphingolipid moiety, usually to a ceramide, but in rare cases also to a sphingoid base. A large structural heterogeneity results from differences in number, identity, linkage, and anomeric configuration of the carbohydrate residues, and also from structural differences within the hydrophobic part. GSLs form complex cell-type specific patterns, which change with the species, the cellular differentiation state, viral transformation, ontogenesis, and oncogenesis. Although GSL structures can be assigned to only a few series with a common carbohydrate core, their structural variety and the complex pattern are challenges for their elucidation and quantification by mass spectrometric techniques. We present a general overview of the application of lipidomics for GSL determination. This includes analytical procedures and instrumentation together with recent correlations of GSL molecular species with human diseases. Difficulties such as the structural complexity and the lack of standard substances for complex GSLs are discussed.

Lipid classification, structures and tools

The study of lipids has developed into a research field of increasing importance as their multiple biological roles in cell biology, physiology and pathology are becoming better understood. The Lipid Metabolites and Pathways Strategy (LIPID MAPS) consortium is actively involved in an integrated approach for the detection, quantitation and pathway reconstruction of lipids and related genes and proteins at a systems-biology level. A key component of this approach is a bioinformatics infrastructure involving a clearly defined classification of lipids, a state-of-the-art database system for molecular species and experimental data and a suite of user-friendly tools to assist lipidomics researchers. Herein, we discuss a number of recent developments by the LIPID MAPS bioinformatics core in pursuit of these objectives.

Structures, biosynthesis, and functions of gangliosides--an overview

Gangliosides are sialic acid-containing glycosphingolipids that are most abundant in the nervous system. Heterogeneity and diversity of the structures in their carbohydrate chains are characteristic hallmarks of these lipids; so far, 188 gangliosides with different carbohydrate structures have been identified in vertebrates. The molecular structural complexity increases manifold if one considers heterogeneity in the lipophilic components. The expression levels and patterns of brain gangliosides are known to change drastically during development. In cells, gangliosides are primarily, but not exclusively, localized in the outer leaflets of plasma membranes and are integral components of cell surface microdomains with sphingomyelin and cholesterol from which they participate in cell-cell recognition, adhesion, and signal transduction. In this brief review, we discuss the structures, metabolism and functions of gangliosides.

Accelerated biosynthesis of neolacto-series glycosphingolipids in differentiated mouse embryonal carcinoma F9 cells detected by using dodecyl N-acetylglucosaminide as a saccharide primer

Using dodecyl N-acetylglucosaminide (GlcNAc-C12) as a saccharide primer, we investigated the biosynthetic changes of neolacto-series glycosphingolipids (GSLs) in mouse embryonal carcinoma F9 cells during differentiation induced by retinoic acid plus dibutyryl cyclic AMP (RA/dbcAMP). In the differentiated cells, the glycosylation of GlcNAc-C12 was greatly enhanced. The sugar compositions of glycosylated primers were assigned as Hex-GlcNAc, [Hex](2)-GlcNAc, [Hex](2)[HexNAc]-GlcNAc, and [NeuAc][Hex]-GlcNAc by liquid chromatography-tandem mass spectrometry. The detection of augmented biosynthesis of endogenous sialylparagloboside indicated that [NeuAc][Hex]-GlcNAc was predicted to be the non-reducing end trisaccharide of sialylparagloboside. The transcription of B3gnt5, B4galt1, Ggta1, Fut4 and St3gal6, encoding glycosyltransferases involved in the neolacto-series glycosphingolipids biosynthesis, was increased, whereas that of Fut9 and St6galI was decreased after RA/dbcAMP treatment. Furthermore, the sialyltransferase activity of ST3GalVI sialylating paragloboside was enhanced with the increase in St3gal6 expression. Since most stage-specific embryonic antigen-1 (SSEA-1) active determinants are carried by glycoproteins in F9 cells, the changes in glycolipid metabolism do not seem to be closely related to loss of cell surface SSEA-1 expression upon F9 differentiation. These results indicate that RA/dbcAMP treatment activates the biosynthesis of neolacto-series GSL and enhances sialylation of paragloboside in F9 cells with down-regulation of Fut9 expression.

Reflections on my career in analytical chemistry and biochemistry

My career has been focused in two major areas, analytical chemistry and biochemistry of complex lipids and glycoconjugates. Included here are the pioneering work on the gas chromatography of long-chain sphingolipid bases, carbohydrates, steroids and urinary organic acids. Mass spectrometry was utilized extensively in structural studies of sphingolipids, fatty acids, carbohydrates, steroids, urinary organic acids, polyisoprenoid alcohols, and juvenile hormone. Computer systems were developed for the acquisition and analysis of mass spectra, and were used for development of automated metabolic profiling of complex mixtures of metabolites. Fabry's disease was discovered to be a glycosphingolipidosis. Enzymes of lysosomal metabolism of glycosphingolipids were purified, characterized, and used in one of the first demonstrations of the feasibility of enzyme replacement therapy in a lysosomal storage disorder (Fabry's disease). Extracellular sialidases were studied to evaluate the hypothesis that they might be involved in the regulation of membrane growth factor receptors. The enzyme for hematoside synthesis was purified and characterized.

Sialic Acid and GM3 Ganglioside Expression in Papillomavirus-associated Urinary Bladder Tumours of Cattle with Chronic Enzootic Haematuria

This study was based on 30 papillomavirus-associated urinary bladder tumours from cattle with chronic haematuria, the animals having been kept since birth on pasture rich in bracken fern. The ganglioside content was assessed and compared with that of normal bovine urinary bladders, which was shown to be 28.6+/-3.3 (mean+/-SD) microg of lipid-bound sialic acid per gram of fresh tissue. In neoplastic bladder samples this value was higher but variable (120.9+/-80.6 in benign tumours, and 94.7+/-45.7 in malignant tumours). The main ganglioside, GM3, represented ca 75% of the total ganglioside mixture in normal tissues and 50-80% in tumour samples. GM1, GM2, GD1a, GD3 and FucGM1 were found as minor components. The study suggested that GM3 ganglioside may have a crucial role in "downregulation" of the metastatic potential of bovine urothelial cancers.

Efflux of sphingolipids metabolically labeled with [1-3H]sphingosine, L-[3-3H]serine and [9,10-3H]palmitic acid from normal cells in culture

The membrane complex lipids of human fibroblasts and differentiated rat cerebellar granule cells in culture were metabolically radiolabeled with [1-(3)H]sphingosine, L-[3-(3)H]serine and [9,10-(3)H]palmitic acid. A relevant efflux of radioactive sphingolipids and phosphatidylcholine was observed from cells to the culture medium in the presence of fetal calf serum. This event was independent of the concentration and structure of the metabolic precursor administered to cells, and it was linearly time-dependent. The radioactive lipid patterns present in the medium were different from those present in the cells. Radioactive sphingomyelin and ganglioside GM3 containing short acyl chains were the main species present in the medium from human fibroblasts, while sphingomyelin and GD3 ganglioside in that from neuronal cells. In the absence of proteins in the culture medium, the efflux of complex lipids was much lower than in the presence of serum, and the patterns of released molecules were again different from those of cells.

Plasma membrane production of ceramide from ganglioside GM3 in human fibroblasts

Ceramide is a key lipid molecule necessary to regulate some cellular processes, including apoptosis and cell differentiation. In this context, its production has been shown to occur via sphingomyelin hydrolysis or sphingosine acylation. Here, we show that in human fibroblasts, plasma membrane ceramide is also produced from ganglioside GM3 by detachment of sugar units. Membrane-bound glycosylhydrolases have a role in this process. In fact, the production of ceramide from GM3 has been observed even under experimental conditions able to block endocytosis or lysosomal activity, and the overexpression of the plasma membrane ganglioside sialidase Neu3 corresponded to a higher production of ceramide in the plasma membrane. The increased activity of Neu3 was paralleled by an increase of GM3 synthase mRNA and GM3 synthase activity. Neu3-overexpressing fibroblasts were characterized by a reduced proliferation rate and higher basal number of apoptotic cells in comparison with wild-type cells. A similar behavior was observed when normal fibroblasts were treated with exogenous C2-ceramide.

Lack of ceramide generation and altered sphingolipid composition are associated with drug resistance in human ovarian carcinoma cells

PTX (Paclitaxel) is an antimitotic agent used in the treatment of a number of major solid tumours, particularly in breast and ovarian cancer. This study was undertaken to gain insight into the molecular alterations producing PTX resistance in ovarian cancer. PTX treatment is able to induce apoptosis in the human ovarian carcinoma cell line, CABA I. PTX-induced apoptosis in CABA I cells was accompanied by an increase in the cellular Cer (ceramide) levels and a decrease in the sphingomyelin levels, due to the activation of sphingomyelinases. The inhibition of acid sphingomyelinase decreased PTX-induced apoptosis. Under the same experimental conditions, PTX had no effect on Cer and sphingomyelin levels in the stable PTX-resistant ovarian carcinoma cell line, CABA-PTX.The acquisition of the PTX-resistant phenotype is accompanied by unique alterations in the complex sphingolipid pattern found on lipid extraction. In the drug-resistant cell line, the levels of sphingomyelin and neutral glycosphingolipids were unchanged compared with the drug-sensitive cell line. The ganglioside pattern in CABA I cells is more complex compared with that of CABA-PTX cells. Specifically, we found that the total ganglioside content in CABA-PTX cells was approximately half of that in CABA I cells, and GM3 ganglioside content was remarkably higher in the drug-resistant cell line. Taken together our findings indicate that: i) Cer generated by acid sphingomyelinase is involved in PTX-induced apoptosis in ovarian carcinoma cells, and PTX-resistant cells are characterized by their lack of increased Cer upon drug treatment, ii) PTX resistance might be correlated with an alteration in metabolic Cer patterns specifically affecting cellular ganglioside composition.

The membrane environment of endogenous cellular prion protein in primary rat cerebellar neurons

We studied the membrane environment of cellular prion protein in primary cultured rat cerebellar neurons differentiated in vitro. In these cells, about 45% of total cellular prion protein (corresponding to a 35-fold enrichment) is associated with a low-density, sphingolipid- and cholesterol-enriched membrane fraction, that can be separated by flotation on sucrose gradient. Biotinylation experiments indicated that almost all prion protein recovered in this fraction was exposed at the cell surface. Prion protein was efficiently separated from this fraction by a monoclonal antibody immuno-separation procedure. Under conditions designed to preserve lipid-mediated membrane organization, several proteins were found in the prion protein-enriched membrane domains (i.e. the non-receptor tyrosine kinases Lyn and Fyn and the neuronal glycosylphosphatidylinositol-anchored protein Thy-1). The prion protein-rich membrane domains contained, as well, about 50% of the sphingolipids, cholesterol and phosphatidylcholine present in the sphingolipid-enriched membrane fraction. All main sphingolipids, including sphingomyelin, neutral glycosphingolipids and gangliosides, were similarly enriched in the prion protein-rich membrane domains. Thus, prion protein plasma membrane environment in differentiated neurons resulted to be a complex entity, whose integrity requires a network of lipid-mediated non-covalent interactions.

Anti-tumor-promoting activity of simple models of galactoglycerolipids with branched and unsaturated acyl chains

Six new galactoglycerolipid analogs, in which one or two 4-methylpentanoyl or trans-2-butenoyl groups are linked to the 2-O-beta-D-galactosylglycerol skeleton, were tested for their anti-tumor-promoting activity using a short-term in vitro assay for Epstein-Barr virus early antigen (EBV-EA) activation. All these compounds were more active than their linear or saturated reference compounds in inhibiting the EBV activation promoted by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), the diester 1-O-(4-methylpentanoyl)-2-O-[6-O-(4-methylpentanoyl)-beta-D-galactopyranosyl]-sn-glycerol resulting the most active glycoglycerolipid analog till now tested. Four compounds (three butenoates and one 4-methylpentanoate), when tested in the in vivo two-stage carcinogenesis test, exhibited also inhibitory effects on mouse skin tumor promotion.

Sphingolipid uptake by cultured cells: complex aggregates of cell sphingolipids with serum proteins and lipoproteins are rapidly catabolized

Human fibroblasts, rat neurons, and murine neuroblastoma cells, cultured in the presence of fetal calf serum, were fed with [1-(3)H]sphingosine to radiolabel sphingolipids. The fate of cell sphingolipids, the release of sphingolipids in the culture medium, the interaction of sphingolipids with the proteins and lipoproteins of fetal calf serum, and the fate of sphingolipids taken up by the cells were investigated. For this latter purpose, the culture medium containing radioactive sphingolipids was delivered to nonlabeled cells. The presence of tritium at position 1 of sphingosine allowed us to follow the extent of sphingolipid catabolism by measuring the production of radioactive phosphatidylethanolamine and proteins by recycling the radioactive ethanolamine formed during sphingosine catabolism and the production of tritiated water. We confirmed that in cells the recycling of sphingosine occurred to a high extent and that only a minor portion of cell sphingolipids was catabolized to the small fragments of ethanolamine and water. Cell sphingolipids were released in the culture medium, where they formed large lipoproteic aggregates at a rate of about 12% per day. Released sphingolipids were taken up by the cells and catabolized to the sphingosine and then to ethanolamine, and recycling of sphingosine was not observed. This suggests that in the presence of fetal calf serum in the culture medium, exogenous sphingolipids directly reach the lysosomes, were they are entirely catabolized. Thus, the trafficking of sphingolipids from cells to the extracellular environment and from this to other cells does not allow the modification of the plasma membrane composition.

The adhesion protein TAG-1 has a ganglioside environment in the sphingolipid-enriched membrane domains of neuronal cells in culture

We studied the interactions between gangliosides and proteins at the exoplasmic surface of the sphingolipid-enriched membrane domains by ganglioside photolabeling combined with cell surface biotin labeling. After cell photolabeling with radioactive photoactivable derivatives of GM3, GM1 and GD1b gangliosides, followed by cell surface biotin labeling, sphingolipid-enriched domains were prepared and immunoprecipitated with streptavidin-coupled beads, under experimental conditions preserving the integrity of the lipid domain. About 50% of the total radioactivity linked to proteins was associated with acylated tubulin, about 10% with a 135-kDa protein present as a series of species with pI ranging from 6.5 to 8.0, about 5% with a protein of about 70 kDa and with pI near to 6.5. By immunoprecipitation with streptavidin-coupled beads under conditions disrupting the integrity of the lipid domain, the 135 kDa protein was recovered in the immunoprecipitate, that did not contain tubulin. Thus, the 135 kDa protein has an exoplasmic domain, and it was then identified as the GPI-anchored neural cell adhesion molecule TAG-1. Remarkably, TAG-1 was cross-linked in a similar extent by the photoactivated ganglioside GM3, GM1 and GD1b. The three gangliosides bear different oligosaccharide chains, suggesting that the ganglioside/TAG-1 interaction is not specifically associated with the ganglioside oligosaccharide structure.

Altered sphingolipid metabolism in N-(4-hydroxyphenyl)-retinamide-resistant A2780 human ovarian carcinoma cells

In the present work, we studied the effects of fenretinide (N-(4-hydroxyphenyl)retinamide (HPR)), a hydroxyphenyl derivative of all-trans-retinoic acid, on sphingolipid metabolism and expression in human ovarian carcinoma A2780 cells. A2780 cells, which are sensitive to a pharmacologically achievable HPR concentration, become 10-fold more resistant after exposure to increasing HPR concentrations. Our results showed that HPR was able to induce a dose- and time-dependent increase in cellular ceramide levels in sensitive but not in resistant cells. This form of resistance in A2780 cells was not accompanied by the overexpression of multidrug resistance-specific proteins MDR1 P-glycoprotein and multidrug resistance-associated protein, whose mRNA levels did not differ in sensitive and resistant A2780 cells. HPR-resistant cells were characterized by an overall altered sphingolipid metabolism. The overall content in glycosphingolipids was similar in both cell types, but the expression of specific glycosphingolipids was different. Specifically, our findings indicated that glucosylceramide levels were similar in sensitive and resistant cells, but resistant cells were characterized by a 6-fold lower expression of lactosylceramide levels and by a 6-fold higher expression of ganglioside levels than sensitive cells. The main gangliosides from resistant A2780 cells were identified as GM3 and GM2. The possible metabolic mechanisms leading to this difference were investigated. Interestingly, the mRNA levels of glucosylceramide and lactosylceramide synthases were similar in sensitive and resistant cells, whereas GM3 synthase mRNA level and GM3 synthase activity were remarkably higher in resistant cells.

Inhibitory effect of stabilized analogues of glycoglycerolipids on Epstein-Barr virus activation and mouse skin tumor promotion

Nine new synthetic compounds, structurally related to the most active glycoglycerolipid analogues carrying a hexanoyl chain, were tested for their anti-tumor-promoting activity using a short-term in vitro assay for Epstein-Barr virus (EBV) activation. All these compounds, in which the ester function is replaced by different metabolically more stable groups, were almost as active as their ester reference compounds in inhibiting the EBV activation promoted by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). Two of these, devoid of any functionality on the lipophilic chain, when tested in an in vivo two-stage carcinogenesis test, exhibited marked inhibitory effects on mouse skin tumor promotion.

High pressure matrix-assisted laser desorption/ionization Fourier transform mass spectrometry for minimization of ganglioside fragmentation

Transiently elevating pressure in a matrix-assisted laser desorption/ionization Fourier transform mass spectrometry (MALDI-FTMS) source into the 1-10 mbar range during ionization decreases the metastable fragmentation of gangliosides. This allows detection of the molecular ion species without loss of the highly labile sialic acid residues. In these experiments, gangliosides with up to five sialic acids were ionized by MALDI and detected with the FTMS. In each case, when the high pressure collisional cooling was used, the singly charged molecular ion was the base peak in the spectra, both in the positive and negative ion modes, and minimal metastable fragmentation was observed. This result is promising, as the previously developed TLC separation methods can be coupled to MALDI-FTMS.

Changes in the lipid turnover, composition, and organization, as sphingolipid-enriched membrane domains, in rat cerebellar granule cells developing in vitro

In the present paper, we report on the properties of sphingolipid-enriched domains of rat cerebellar granule cells in culture at different stages of neuronal development. The major lipid components of these domains were glycerophospholipids and cholesterol. Glycerophospholipids were 45-75% and cholesterol 15-45% of total lipids of the domains. This corresponded to 5-17% of total cell glycerophospholipids and 15-45% of total cell cholesterol. Phosphatidylcholine, mainly dipalmitoylphosphatidylcholine, was 66-85% of all the glycerophospholipids associated with these domains. Consequently, the palmitoyl residue was significantly enriched in the domains. The surface occupied by these structures increased during development. 40-70% of cell sphingolipids segregated in sphingolipid-enriched membrane domains, with the maximum ganglioside density in fully differentiated neurons. A high content of ceramide was found in the domains of aging neurons. Then, the sphingolipid/glycerophospholipid molar ratio was more than doubled during the initial stage of development, whereas the cholesterol/glycerophospholipid molar ratio gradually decreased during in vitro differentiation. Phosphorylated phosphoinositides, which were scant in the domains of undifferentiated cells, dramatically increased during differentiation and aging in culture. Proteins were minor components of the domains (0.1-2.8% of all domain components). Phosphotyrosine-containing proteins were selectively recovered in the sphingolipid-enriched domain. Among these, Src family protein-tyrosine kinases, known to participate to the process of neuronal differentiation, were associated with the sphingolipid-enriched domains in a way specific for the type of kinase and for the developmental stage of the cell. Proteins belonging to other signaling pathways, such as phosphoinositide 3-kinase and its downstream target, Akt, were not associated with the domains.

Suppression of integrin expression and tumorigenicity by sulfation of lactosylceramide in 3LL Lewis lung carcinoma cells

To investigate the cellular functions of sulfated glycosphingolipids, we introduced the cerebroside sulfotransferase (CST) gene into J5 cells, a subclone of 3LL Lewis lung carcinoma cells. The J5 cells lack acidic glycosphingolipids but accumulate their common biosynthetic precursor, lactosylceramide. We established the stable CST transfectants, J5/CST-1 and J5/CST-2 clones, highly expressing sulfated lactosylceramide (SM3). Both clones exhibited more spherical morphology in comparison to mock transfectant, and their adhesiveness to fibronectin and laminin was significantly lower. The loss of cell-substratum interactions in these SM3-expressing cells could be attributed to decreased expression of integrins (alpha(5), alpha(6), and beta(1)) on the cell surface and their whole cellular levels. However, the levels of H-2K(b) and H-2D(b) antigens remained unchanged. Reverse transcriptase-polymerase chain reaction and Northern blot analyses for these integrins exhibited significant decrease of beta(1) gene expression in J5/CST-1 and 2, but there was no change in the levels of alpha(5) and alpha(6) transcripts. Deglycosylation by endoglycosidase H treatment clearly demonstrated that the precursor form of beta(1) integrin, possessing high mannose oligosaccharide chains, was preferentially decreased in the CST transfectants. These results demonstrate that endogenous SM3 negatively regulates beta(1) integrin expression at the transcriptional level, and the decrease of alpha integrin proteins in the CST transfectants was due to the post-transcriptional modification. We suggest the putative importance of the intracellular pre-beta(1) integrin pool for normal integrin maturation and subsequent function. Although the rates of cell proliferation in vitro for mock and CST transfectants were similar, tumorigenicity of J5/CST-1 and -2 cells inoculated into syngeneic C57/BL6 mice was greatly decreased or even absent. This was probably due to global loss of the efficient cell-matrix interactions, which are essential for the development of malignant tumors in vivo. Thus, we showed the evidence that cellular SM3 negatively regulates the cell-substratum interaction, resulting in the loss of tumorigenicity.

Sphingolipid-enriched membrane domains from rat cerebellar granule cells differentiated in culture. A compositional study

Sphingolipid-enriched membrane domains, characterized by a particular protein and lipid composition, have been detected in a variety of cells. However, limited data are available concerning these domains in neuronal cells. We analyzed the lipid and protein composition of a sphingolipid-enriched membrane fraction prepared from primary rat cerebellar granule cells differentiated in culture. Although the protein content of this fraction was only 1.4% of total cellular protein, 60% of the gangliosides, 67% of the sphingomyelin, 50% of the ceramide, and 40% of the cholesterol were located in this fraction. The protein pattern of the sphingolipid-enriched domain fraction was dramatically different from that associated with the cell homogenate. This fraction contained 25% of the tyrosine-phosphorylated proteins and was enriched in two proteins with apparent molecular masses of 135 and 15 kDa. 12% of cellular glycerophospholipids were located in the fraction, with phosphatidylcholine having the highest enrichment. The molar ratio between proteins, glycerophospholipids, cholesterol, sphingomyelin, ceramide and gangliosides in cerebellar granule cells was 1.6:41.6:6. 1:1.3:0.3:1 in the cell homogenate and 0.04:8.3:4.0:1.4:0.2:1 in the sphingolipid-enriched membrane fraction. These data indicate that selected proteins segregate with sphingolipids in specialized domains in the membrane of cultured neurons.

Inhibitory effects of monoacylated 2-O-beta-galactosylglycerols on Epstein-Barr virus activation: the significant role of the hexanoyl chain

Three series of monoacyl-2-O-beta-D-galactosylglycerols bearing an acyl chain of varying length, from C4 to C10, were studied due to their antitumor promoting effects on the activation of the Epstein-Barr virus early antigen (EBV-EA), such activation being induced by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). This study indicates that it is more the length of the acyl chain that is important for the activity, six carbon atoms resulting in maximum effect, rather than the position of the ester function and the nature of the sugar (galactose or glucose).