High-performance liquid chromatography analysis of ganglioside carbohydrates at the picomole level after ceramide glycanase digestion and fluorescent labeling with 2-aminobenzamide

Use of Fluorescent 2-AB to Explore the Bidirectional Transport Mechanism of Pseudostellaria heterophylla Polysaccharides across Caco-2 Cells

Polysaccharides are abundant in natural resources and perform numerous physiological functions. Polysaccharide structures often lack chromophore groups; thus, current analytical methods cannot distinguish polysaccharide metabolites in the body or polysaccharide prototypes in biological samples. Thus, the measurement of polysaccharides in blood, bodily fluid, and cell-culture medium is difficult. Our early-stage research resulted in the isolation of two homogeneous polysaccharides from Pseudostellaria heterophylla, PHP_0.5MSC-F and PHP_H-1-2, which have anti-hyperglycemia and insulin resistance improvement effects for type 2 diabetes. In this study, the reducing terminal sugars of PHP_0.5MSC-F and PHP_H-1-2 were labeled with 2-aminobenzamide (2-AB) to prepare novel fluorescent probes for HPLC-coupled fluorescence detection (HPLC-FLD). Quantitative analysis was performed in reference to T40, and the detection limit for PHP_0.5MSC-F was found to be 8.84 μg/mL with a linear range of 29.45-683.28 μg/mL. In reference to T70, the detection limit for PHP_H-1-2 was found to be 13.89 μg/mL with a linear range of 46.29-462.76 μg/mL. This method was used to measure the bidirectional transport of polysaccharides across caco-2 cells from apical to basolateral (AP→BL) or from basolateral to apical (BL→AP) directions and to evaluate the polysaccharide bioavailability. The drug absorption capacity was determined based on the apparent permeability coefficient (Papp), and the Papp values for the two polysaccharides were found to be greater than 1 × 10^-6 cm/s, which suggests easy absorption. Regarding bidirectional transport, the AP→BL Papp values were greater than the BL→AP values; thus, PHP_0.5MSC-F and PHP_H-1-2 mainly underwent passive transference. The two membrane permeable polysaccharides were not P-gp efflux transporter substrates. The absorption mechanism of PHP_0.5MSC-F complies with passive diffusion under a concentration gradient, whereas PHP_H-1-2 mainly utilizes a clathrin-mediated endocytic pathway to enter caco-2 cells. This innovative HPLC-FLD method can help to track polysaccharide internalization in vitro and in vivo to facilitate cellular uptake and biodistribution exploration.

Profiling of glycosphingolipids with SCDase digestion and HPLC-FLD-MS

Lipid components of cells and tissues feature a large diversity of structures that present a challenging problem for molecular analysis. Glycolipids from mammalian cells contain glycosphingolipids (GSLs) as their major glycolipid component, and these structures vary in the identity of the glycan headgroup as well as the structure of the fatty acid and sphingosine (Sph) tails. Analysis of intact GSLs is challenging due to the low abundance of these species. Here, we develop a new strategy for the analysis of lyso-GSL (l-GSL), GSL that retain linkage of the glycan headgroup with the Sph base. The analysis begins with digestion of a GSL sample with sphingolipid ceramide N-deacylase (SCDase), followed by labelling with an amine-reactive fluorophore. The sample was then analyzed by HPLC-FLD-MS and quantitated by addition of an external standard. This method was compared to analysis of GSL glycans after cleavage by an Endoglycoceramidase (EGCase) enzyme and labeling with a fluorophore (2-anthranilic acid, 2AA). The two methods are complementary, with EGCase providing improved signal (due to fewer species) and SCDase providing analysis of lyso-GSL. Importantly the SCDase method provides Sph composition of GSL species. We demonstrate the method on cultured human cells (Jurkat T cells) and tissue homogenate (porcine brain).

High-throughput and high-sensitivity N-Glycan profiling: A platform for biopharmaceutical development and disease biomarker discovery

Protein glycosylation contributes to critical biological function of glycoproteins. Glycan analysis is essential for the production of biopharmaceuticals as well as for the identification of disease biomarkers. However, glycans are highly heterogeneous, which has considerably hampered the progress of glycomics. Here, we present an improved 96-well plate format platform for streamlined glycan profiling that takes advantage of rapid glycoprotein denaturation, deglycosylation, fluorescent derivatization, and on-matrix glycan clean-up. This approach offers high sensitivity with consistent identification and quantification of diverse N-glycans across multiple samples on a high-throughput scale. We demonstrate its capability for N-glycan profiling of glycoproteins from various sources, including two recombinant monoclonal antibodies produced from Chinese Hamster Ovary cells, EG2-hFc and rituximab, polyclonal antibodies purified from human serum, and total glycoproteins from human serum. Combined with the complementary information obtained by sequential digestion from exoglycosidase arrays, this approach allows the detection and identification of multiple N-glycans in these complex biological samples. The reagents, workflow, and Hydrophilic interaction liquid chromatography with fluorescence detection (HILIC-FLD), are simple enough to be implemented into a straightforward user-friendly setup. This improved technology provides a powerful tool in support of rapid advancement of glycan analysis for biopharmaceutical development and biomarker discovery for clinical disease diagnosis.

Separation and preparation of N-glycans based on ammonia-catalyzed release method

The study of carbohydrates requires large amounts of glycans. N-Glycans can be synthesized but generating large quantities of N-glycans with diverse structures remains difficult. In this study, we aimed to obtain large amounts of glycans using an optimized procedure. Two types of reductive N-glycans were released from chicken egg albumin (ovalbumin) and soy protein using an ammonia catalysis method and labeled with benzenesulfonyl hydrazide (BSH). After preliminary separation by preparative HPLC, N-glycan-BSH components were de-labeled separately and reducing N-glycans were recovered. The de-labeled reducing N-glycans were derived with different labeling reagents and further separated and purified with two/multi-dimensional HPLC for various studies. We selected the bifunctional reagent 2-amino-N-(2-aminoethyl)-benzamide (AEAB) as a labeling reagent combined with C18 column for two-dimensional HPLC separation. A total of 21 and 8 N-glycan-AEAB conjugates were obtained from ovalbumin and soy protein, respectively. A reactive primary alkylamine of N-glycan-AEAB conjugates can be effectively immobilized on microarray surfaces, allowing for subsequent functional studies of glycans.

Mass spectrometric evidence for the mechanism of free-radical depolymerization of various types of glycosaminoglycans

Glycosaminoglycans (GAGs) are large, complex carbohydrate molecules that interact with a wide range of proteins involved in physiological and pathological processes. Several naturally derived GAGs have emerged as potentially useful therapeutics in clinical applications. Natural polysaccharides, however, generally have high molecular weights with a degree of polydispersity, making it difficult to investigate their structural properties. In this study, we establish a free-radical-mediated micro-reaction system and use hydrophilic interaction chromatography (HILIC)-Fourier transform mass spectrometry (FTMS) to profile the degraded products of various types of GAGs, heparin, chondroitin sulfate A, NS-heparosan, and oversulfated chondroitin sulfate (OSCS), to reveal the free-radical degradation mechanism of GAGs. The results show that the bulk fragments of GAGs generated by free-radical degradation can maintain their basic structural units and sulfate substituents. In addition, an abundance of oligomers modified with oxidation at their reducing ends or by dehydration also appeared. We discovered that these modifications were related in terms of the degree of sulfation and the α- or β-linkage of HexNY (Y = SO₃^- or Ac), and especially that the different linkage of the disaccharide unit is the main factor in modification. In addition, the method based on micro-free-radical reaction and HILIC-FTMS is both effective and sensitive, thus suggesting its broad practical value for the structural characterization and in the biological structure-function studies of GAGs.

Approach for Profiling of Glycosphingolipid Glycosylation by Multiplexed Capillary Gel Electrophoresis Coupled to Laser-Induced Fluorescence Detection To Identify Cell-Surface Markers of Human Pluripotent Stem Cells and Derived Cardiomyocytes

Application of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) as tissue transplants in regenerative medicine depends on cell-surface marker-based characterization and/or purification. Glycosphingolipids (GSLs) are a family of highly diverse surface-exposed biomolecules that have been neglected as potential surface markers for hiPSC-CMs due to significant analytical challenges. Here, we describe the development of a novel and high-throughput-compatible workflow for the analysis of GSL-derived glycans based on ceramide glycanase digestion, 8-aminopyrene-1,3,6-trisulfonic acid (APTS) labeling, and multiplexed capillary gel electrophoresis coupled to laser-induced fluorescence detection (xCGE-LIF). GSL glycans were detected with highly reproducible migration times after repeated analysis by xCGE-LIF. We built up a migration time database comprising 38 different glycan species, and we showed exemplarily that as few as 10 pg of fucosyl lactotetra was detectable. GSL glycan profiling could be performed with 10⁵ human induced pluripotent stem cells, and we quantitatively dissected global alterations of GSL glycosylation of human induced pluripotent stem cells (hiPSCs) and hiPSC-CMs by employing xCGE-LIF. In our study, we observed a general switch from complex GSLs with lacto- and globo-series core structures comprising the well-known human pluripotent stem cell marker stage-specific embryonic antigen 3 (SSEA3) and SSEA4 in hiPSCs toward the simple gangliosides GM3 and GD3 in hiPSC-CMs. This is the first description of GM3 and GD3 being highly abundant GSLs on the cell surface of stem cell-derived cardiomyocytes.

Mass spectrometric quantification of plasma glycosphingolipids in human GM3 ganglioside deficiency

Background

Among Amish communities of North America, biallelic mutations of ST3GAL5 (c.694C > T) eliminate synthesis of GM3 and its derivative downstream a- and b-series gangliosides. Systemic ganglioside deficiency is associated with infantile onset psychomotor retardation, slow brain growth, intractable epilepsy, deafness, and cortical visual impairment. We developed a robust quantitative assay to simultaneously characterize glycan and ceramide moieties of plasma glycosphingolipids (GSLs) among ST3GAL5 c.694C > T homozygotes (n = 8), their heterozygous siblings (n = 24), and wild type control (n = 19) individuals.

Methods

Following extraction and saponification of total plasma lipids, GSLs were purified on a tC18 cartridge column, permethylated, and subjected to nanospray ionization mass spectrometry utilizing neutral loss scanning and data-dependent acquisition. Plasma GSLs were quantified against appropriate synthetic standards.

Results

Our method demonstrated linearity from 5 to 250 μl of plasma. Recovery of synthetic GSLs spiked into plasma was 99-104% with no matrix interference. Quantitative plasma GSL profiles discriminated among ST3GAL5 genotypes: GM3 and GD3 were undetectable in ST3GAL5 c.694C > T homozygotes, who had markedly elevated lactosylceramide (19.17 ± 4.20 nmol/ml) relative to heterozygous siblings (9.62 ± 2.46 nmol/ml) and wild type controls (6.55 ± 2.16 nmol/ml). Children with systemic ganglioside deficiency had a distinctive shift in ceramide composition toward higher mass species.

Conclusions

Our quantitative glycolipidomics method discriminates among ST3GAL5 c.694C > T genotypes, can reveal subtle structural heterogeneity, and represents a useful new strategy to diagnose and monitor GSL disorders in humans.

Production of Rare Phyto-Ceramides from Abundant Food Plant Residues

Ceramides (Cers) are major components of the outermost layer of the skin, the stratum corneum, and play a crucial role in permeability barrier functions. Alterations in Cer composition causing skin diseases are compensated with semisynthetic skin-identical Cers. Plants constitute new resources for Cer production as they contain glucosylceramides (GluCers) as major components. GluCers were purified from industrial waste plant materials, apple pomace (Malus domestica), wheat germs (Triticum sp.), and coffee grounds (Coffea sp.), with GluCer contents of 28.9 mg, 33.7 mg, and 4.4 mg per 100 g of plant material. Forty-five species of GluCers (1-45) were identified with different sphingoid bases, saturated or monounsaturated α-hydroxy fatty acids (C15-28), and β-glucose as polar headgroup. Three main GluCers were hydrolyzed by a recombinant human glucocerebrosidase to produce phyto-Cers (46-48). These studies showed that rare and expensive phyto-Cers can be obtained from industrial food plant residues.

Quantitative Glycomics: A Combined Analytical and Bioinformatics Approach

Glycosylation is one of the most common and essential protein modifications. Glycans conjugated to biomolecules modulate the function of such molecules through both direct recognition of glycan structures and indirect mechanisms that involve the control of protein turnover rates, stability, and conformation. The biological attributes of glycans in numerous biological processes and implications in a number of diseases highlight the necessity for comprehensive characterization of protein glycosylation. This chapter reviews cutting-edge methods and tools developed to facilitate quantitative glycomics. This chapter highlights the different methods employed for the release and purification of glycans from biological samples. The most effective labeling methods developed for sensitive quantitative glycomics are also described and discussed. The chromatographic approaches that have been used effectively in glycomics are also highlighted.

Comprehensive Profiling of Glycosphingolipid Glycans Using a Novel Broad Specificity Endoglycoceramidase in a High-Throughput Workflow

The biological function of glycosphingolipids (GSLs) is largely determined by their glycan headgroup moiety. This has placed a renewed emphasis on detailed GSL headgroup structural analysis. Comprehensive profiling of GSL headgroups in biological samples requires the use of endoglycoceramidases with broad substrate specificity and a robust workflow that enables their high-throughput analysis. We present here the first high-throughput glyco-analytical platform for GSL headgroup profiling. The workflow features enzymatic release of GSL glycans with a novel broad-specificity endoglycoceramidase I (EGCase I) from Rhodococcus triatomea, selective glycan capture on hydrazide beads on a robotics platform, 2AB-fluorescent glycan labeling, and analysis by UPLC-HILIC-FLD. R. triatomea EGCase I displayed a wider specificity than known EGCases and was able to efficiently hydrolyze gangliosides, globosides, (n)Lc-type GSLs, and cerebrosides. Our workflow was validated on purified GSL standard lipids and was applied to the characterization of GSLs extracted from several mammalian cell lines and human serum. This study should facilitate the analytical workflow in functional glycomics studies and biomarker discovery.

Silencing the porcine iGb3s gene does not affect Galα3Gal levels or measures of anticipated pig-to-human and pig-to-primate acute rejection

BACKGROUND

The Galα(1,3)Gal epitope (α-GAL), created by α-1,3-glycosyltransferase-1 (GGTA1), is a major xenoantigen causing hyperacute rejection in pig-to-primate and pig-to-human xenotransplantation. In response, GGTA1 gene-deleted pigs have been generated. However, it is unclear whether there is a residual small amount of α-Gal epitope expressed in GGTA1(-/-) pigs. Isoglobotrihexosylceramide synthase (iGb3s), another member of the glycosyltransferase family, catalyzes the synthesis of isoglobo-series glycosphingolipids with an α-GAL-terminal disaccharide (iGb3), creating the possibility that iGb3s may be a source of α-GAL epitopes in GGTA1(-/-) animals. The objective of this study was to examine the impact of silencing the iGb3s gene (A3GalT2) on pig-to-primate and pig-to-human immune cross-reactivity by creating and comparing GGTA1(-/-) pigs to GGTA1(-/-) - and A3GalT2(-/-) -double-knockout pigs.

METHODS

We used the CRISPR/Cas 9 system to target the GGTA1 and A3GalT2 genes in pigs. Both GGTA1 and A3GalT2 genes are functionally inactive in humans and baboons. CRISPR-treated cells used directly for somatic cell nuclear transfer produced single- and double-gene-knockout piglets in a single pregnancy. Once grown to maturity, the glycosphingolipid profile (including iGb3) was assayed in renal tissue by normal-phase liquid chromatography. In addition, peripheral blood mononuclear cells (PBMCs) were subjected to (i) comparative cross-match cytotoxicity analysis against human and baboon serum and (ii) IB4 staining for α-GAL/iGb3.

RESULTS

Silencing of the iGb3s gene significantly modulated the renal glycosphingolipid profile and iGb3 was not detected. Moreover, the human and baboon serum PBMC cytotoxicity and α-GAL/iGb3 staining were unchanged by iGb3s silencing.

CONCLUSIONS

Our data suggest that iGb3s is not a contributor to antibody-mediated rejection in pig-to-primate or pig-to-human xenotransplantation. Although iGb3s gene silencing significantly changed the renal glycosphingolipid profile, the effect on Galα3Gal levels, antibody binding, and cytotoxic profiles of baboon and human sera on porcine PBMCs was neutral.

Elevation in sphingomyelin synthase activity is associated with increases in amyloid-beta peptide generation

A pathological hallmark of Alzheimer’s disease (AD) is the presence of amyloid-beta peptide (Aβ) plaques in the brain. Aβ is derived from a sequential proteolysis of the transmenbrane amyloid precursor protein (APP), a process which is dependent on the distribution of lipids present in the plasma membrane. Sphingomyelin is a major membrane lipid, however its role in APP processing is unclear. Here, we assessed the expression of sphingomyelin synthase (SGMS1; the gene responsible for sphingomyelin synthesis) in human brain and found that it was significantly elevated in the hippocampus of AD brains, but not in the cerebellum. Secondly, we assessed the impact of altering SGMS activity on Aβ generation. Inhibition of SGMS activity significantly reduced the level of Aβ in a dose- and time dependent manner. The decrease in Aβ level occurred without changes in APP expression or cell viability. These results when put together indicate that SGMS activity impacts on APP processing to produce Aβ and it could be a contributing factor in Aβ pathology associated with AD.

Systematic comparison of reverse phase and hydrophilic interaction liquid chromatography platforms for the analysis of N-linked glycans

Due to the hydrophilic nature of glycans, reverse phase chromatography has not been widely used as a glycomic separation technique coupled to mass spectrometry. Other approaches such as hydrophilic interaction chromatography and porous graphitized carbon chromatography are often employed, though these strategies frequently suffer from decreased chromatographic resolution, long equilibration times, indefinite retention, and column bleed. Herein, it is shown that, through an efficient hydrazone formation derivatization of N-linked glycans (~4 h of additional sample preparation time which is carried out in parallel), numerous experimental and practical advantages are gained when analyzing the glycans by online reverse phase chromatography. These benefits include an increased number of glycans detected, increased peak capacity of the separation, and the ability to analyze glycans on the identical liquid chromatography-mass spectrometry platform commonly used for proteomic analyses. The data presented show that separation of derivatized N-linked glycans by reverse phase chromatography significantly out-performs traditional separation of native or derivatized glycans by hydrophilic interaction chromatography. Furthermore, the movement to a more ubiquitous separation technique will afford numerous research groups the opportunity to analyze both proteomic and glycomic samples on the same platform with minimal time and physical change between experiments, increasing the efficiency of "multiomic" biological approaches.

Preparation and characterization of EGCase I, applicable to the comprehensive analysis of GSLs, using a rhodococcal expression system

Endoglycoceramidase (EGCase) is a glycosidase capable of hydrolyzing the β -glycosidic linkage between the oligosaccharides and ceramides of glycosphingolipids (GSLs). Three molecular species of EGCase differing in specificity were found in the culture fluid of Rhodococcus equi (formerly Rhodococcus sp. M-750) and designated EGCase I, II, and III. This study describes the molecular cloning of EGCase I and characterization of the recombinant enzyme, which was highly expressed in a rhodococcal expression system using Rhodococcus erythropolis. Kinetic analysis revealed the turnover number (k(cat)) (k(cat)) of the recombinant EGCase I to be 22- and 1,200-fold higher than that of EGCase II toward GM1a and Gb3Cer, respectively, although the K(m) of both enzymes was almost the same for these substrates. Comparison of the three-dimensional structure of EGCase I (model) and EGCase II (crystal) indicated that a flexible loop hangs over the catalytic cleft of EGCase II but not EGCase I. Deletion of the loop from EGCase II increased the k(cat) of the mutant enzyme, suggesting that the loop is a critical factor affecting the turnover of substrates and products in the catalytic region. Recombinant EGCase I exhibited broad specificity and good reaction efficiency compared with EGCase II, making EGCase I well-suited to a comprehensive analysis of GSLs.

Dissociation of ERK signalling inhibition from the anti-amyloidogenic action of synthetic ceramide analogues

Inhibition of GSL (glycosphingolipid) synthesis reduces Aβ (amyloid β-peptide) production in vitro. Previous studies indicate that GCS (glucosylceramide synthase) inhibitors modulate phosphorylation of ERK1/2 (extracellular-signal-regulated kinase 1/2) and that the ERK pathway may regulate some aspects of Aβ production. It is not clear whether there is a causative relationship linking GSL synthesis inhibition, ERK phosphorylation and Aβ production. In the present study, we treated CHO cells (Chinese-hamster ovary cells) and SH-SY5Y neuroblastoma cells, that both constitutively express human wild-type APP (amyloid precursor protein) and process this to produce Aβ, with GSL-modulating agents to explore this relationship. We found that three related ceramide analogue GSL inhibitors, based on the PDMP (D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol) structure, reduced cellular Aβ production and in all cases this was correlated with inhibition of pERK (phosphorylated ERK) formation. Importantly, the L-threo enantiomers of these compounds (that are inferior GSL synthesis inhibitors compared with the D-threo-enantiomers) also reduced ERK phosphorylation to a similar extent without altering Aβ production. Inhibition of ERK activation using either PD98059 [2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one] or U0126 (1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio] butadiene) had no impact on Aβ production, and knockdown of endogenous GCS using small interfering RNA reduced cellular GSL levels without suppressing Aβ production or pERK formation. Our data suggest that the alteration in pERK levels following treatment with these ceramide analogues is not the principal mechanism involved in the inhibition of Aβ generation and that the ERK signalling pathway does not play a crucial role in processing APP through the amyloidogenic pathway.

Shotgun glycomics: a microarray strategy for functional glycomics

Major challenges of glycomics are to characterize a glycome and identify functional glycans as ligands for glycan-binding proteins (GBPs). To address these issues we have developed a general strategy termed shotgun glycomics. We focus on glycosphingolipids (GSLs), a challenging class of glycoconjugates recognized by toxins, antibodies, and GBPs. We derivatized GSLs extracted from cells with a heterobifunctional fluorescent tag suitable for covalent immobilization. Fluorescent GSLs were separated by multidimensional chromatography, quantified, and coupled to glass slides to create GSL shotgun microarrays. The microarrays were interrogated with cholera toxin, antibodies, and sera from patients with Lyme disease to identify biologically relevant GSLs that were subsequently characterized by mass spectrometry. Shotgun glycomics incorporating GSLs and potentially glycoprotein-derived glycans provides an approach to accessing the complex glycomes of animal cells and offers a strategy for focusing structural analyses on functionally significant glycans.

Glycosphingolipid storage leads to the enhanced degradation of the B cell receptor in Sandhoff disease mice

Glycosphingolipid storage diseases are a group of inherited metabolic diseases in which glycosphingolipids accumulate due to their impaired lysosomal breakdown. Splenic B cells isolated from NPC1, Sandhoff, GM1-gangliosidosis and Fabry disease mouse models showed large (20- to 30-fold) increases in disease specific glycosphingolipids and up to a 4-fold increase in cholesterol. The magnitude of glycosphingolipid storage was in the order NPC1 > Sandhoff approximately GM1 gangliosidosis > Fabry. Except for Fabry disease, glycosphingolipid storage led to an increase in the lysosomal compartment and altered glycosphingolipid trafficking. In order to investigate the consequences of storage on B cell function, the levels of surface expression of B cell IgM receptor and its associated components were quantitated in Sandhoff B cells, since they are all raft-associated on activation. Both the B cell receptor, CD21 and CD19 had decreased cell surface expression. In contrast, CD40 and MHC II, surface receptors that do not associate with lipid rafts, were unchanged. Using a pulse chase biotinylation procedure, surface B cell receptors on a Sandhoff lymphoblast cell line were found to have a significantly decreased half-life. Increased co-localization of fluorescently conjugated cholera toxin and lysosomes was also observed in Sandhoff B cells. Glycosphingolipid storage leads to the enhanced formation of lysosomal lipid rafts, altered endocytic trafficking and increased degradation of the B cell receptor.

Altered sphingolipid metabolism induced by tumor hypoxia - new vistas in glycolipid tumor markers

Uncontrolled growth of malignant cells produces hypoxic regions in locally advanced tumors. Recently we showed that tumor hypoxia-induced transcription of multiple genes involved in glycan synthesis, leading to expression of useful glycolipid tumor markers, such as gangliosides having N-glycolyl sialic acid. Our subsequent studies indicated that the ceramide portion of glycolipids, as well as their glycan moiety, was also significantly affected by hypoxia. Tumor hypoxia-induced marked accumulation of sphinganine (dihydrosphingosine) long-chain base, and significant reduction of unsaturated very long-chain fatty acids in the ceramide moiety. Mass-spectrometry, which yields information on both glycan- and ceramide moieties, is expected to be clinically useful in detecting such distinct molecular species of cancer-associated glycolipids having combined alteration in both glycan- and ceramide moieties.

Sphingolipidomics: methods for the comprehensive analysis of sphingolipids

Sphingolipids comprise a highly diverse and complex class of molecules that serve as both structural components of cellular membranes and signaling molecules capable of eliciting apoptosis, differentiation, chemotaxis, and other responses in mammalian cells. Comprehensive or "sphingolipidomic" analyses (structure specific, quantitative analyses of all sphingolipids, or at least all members of a critical subset) are required in order to elucidate the role(s) of sphingolipids in a given biological context because so many of the sphingolipids in a biological system are inter-converted structurally and metabolically. Despite the experimental challenges posed by the diversity of sphingolipid-regulated cellular responses, the detection and quantitation of multiple sphingolipids in a single sample has been made possible by combining classical analytical separation techniques such as high-performance liquid chromatography (HPLC) with state-of-the-art tandem mass spectrometry (MS/MS) techniques. As part of the Lipid MAPS consortium an internal standard cocktail was developed that comprises the signaling metabolites (i.e. sphingoid bases, sphingoid base-1-phosphates, ceramides, and ceramide-1-phosphates) as well as more complex species such as mono- and di-hexosylceramides and sphingomyelin. Additionally, the number of species that can be analyzed is growing rapidly with the addition of fatty acyl Co-As, sulfatides, and other complex sphingolipids as more internal standards are becoming available. The resulting LC-MS/MS analyses are one of the most analytically rigorous technologies that can provide the necessary sensitivity, structural specificity, and quantitative precision with high-throughput for "sphingolipidomic" analyses in small sample quantities. This review summarizes historical and state-of-the-art analytical techniques used for the identification, structure determination, and quantitation of sphingolipids from free sphingoid bases through more complex sphingolipids such as sphingomyelins, lactosylceramides, and sulfatides including those intermediates currently considered sphingolipid "second messengers". Also discussed are some emerging techniques and other issues remaining to be resolved for the analysis of the full sphingolipidome.

Simultaneous quantification of glucosylceramide and galactosylceramide by normal-phase HPLC using O-phtalaldehyde derivatives prepared with sphingolipid ceramide N-deacylase

We report here a method of simultaneously quantifying glucosylceramide (GlcCer) and galactosylceramide (GalCer) by normal-phase HPLC using O-phtalaldehyde derivatives. Treatment with sphingolipid ceramide N-deacylase which converts the cerebrosides in the sample to their lyso-forms was followed by the quantitative labeling of free NH(2) groups of the lyso-cerebrosides with O-phtalaldehyde. Using this method, 14.1 pmol of GlcCer and 10.4 pmol of GalCer, and 108.1 pmol of GlcCer and 191.1 pmol of GalCer were detected in zebrafish embryos and RPMI 1864 cells, respectively, while 22.2 pmol of GlcCer but no GalCer was detected in CHOP cells using cell lysate containing 100 microg of protein. Linearity for the determination of each cerebroside was observed from 50 to 400 microg of protein under the conditions used, which corresponds to approximately 10(3) to 10(5) RPMI cells and 5 to 80 zebrafish embryos. The present method clearly revealed that the treatment of RPMI cells with a GlcCer synthase inhibitor P4 resulted in a marked decrease in GlcCer but not GalCer, concomitantly with a significant decrease in the GlcCer synthase activity. On the other hand, GlcCer but not GalCer increased 2-fold when an acid glucocerebrosidase inhibitor CBE was injected into zebrafish embryos. Interestingly, the treatment of CHOP cells with ciclosporin A increased GlcCer possibly due to the inhibition of LacCer synthase. A significant increase in levels of GlcCer in fibroblasts from patients with Gaucher disease was clearly shown by the method. The proposed method is useful for the determination of GlcCer and GalCer levels in various biological samples.

Development of a single column method for the separation of lipid- and protein-derived oligosaccharides

Fluorescent labeling of oligosaccharides with anthranilic acid (2-aminobenzoic acid; 2AA), or 2-aminobenzamide (2AB) permits the rapid, sensitive analysis of structures present in cells and tissues. Normal-phase (NP)/hydrophilic interaction chromatography (HILIC) is commonly used to separate fluorophore-derivatized oligosaccharides. Column elution is expressed as glucose units (GU) following calculation of relative retention when compared to an external glucose oligomer standard. However, there is significant overlap between sialylated and neutral oligosaccharides. Normal-phase anion-exchange (NP-AE) HPLC can separate differing classes of oligosaccharides according to the number of charged residues, but relative retention times in GU cannot be calculated across the entire gradient. We have overcome this difficulty by use of a Dionex AS11 column that combines both hydrophilic interaction and anion-exchange chromatographies, termed HIAX, which enables the calculation of GU values for oligosaccharides that carry sialylated or other negatively charged groups. The same method may also be employed for 2AB and other fluorophore-labeled oligosaccharides. Additionally, the same HPLC eluants are used for the differing HPLC columns. Therefore, analysis of HILIC- or HIAX-separated fluorophore-labeled oligosaccharides can be performed using a single HPLC system with a single set of eluents following a simple column change.

Two-dimensional HPLC separation with reverse-phase-nano-LC-MS/MS for the characterization of glycan pools after labeling with 2-aminobenzamide

N-Glycans, O-glycans, glycolipid glycan chains, and other oligosaccharide pools released from various natural sources often represent very complex mixtures. Various tandem mass spectrometric techniques may be applied to glycan pools, resulting in valuable structural information. For a very detailed analysis of these pools, however, the following approach may be preferable: In a first step, reducing-end oligosaccharides are labeled with the aromatic tag 2-aminobenzamide (2-AB). The 2-AB glycans are separated by analytical-scale, normal-phase (NP)-HPLC (first dimension) and peak-fractionated using fluorescence detection. Peak fractions are analyzed by nano-LC-ESI-IT-MS/MS using a conventional reverse-phase (RP) nanocolumn (second dimension). Chromatography may be monitored by measuring the UV absorbance of the AB tag. Tandem mass spectrometry may be performed on deprotonated species (negative-ion mode), on proton adducts, as well as on sodium adducts (positive-ion mode). This approach has the following particular advantages: (1) the combination of the two HPLC dimensions usually separates isobaric species from each other, thereby allowing the tandem mass spectrometric characterization of individual glycan structures; (2) the AB-mass tag helps with the unambiguous assignment of fragment ions; (3) the second-dimension RP-nano-LC-MS/MS analyses can be performed in many mass spectrometric laboratories, as only standard equipment is needed. Alternatively, for a less in-depth characterization of complex glycan pools, the RP-nano-LC-MS/MS technique may be used as a stand-alone technique. In conclusion, the presented methods allow the detailed mass spectrometric characterization of complex N-glycan pools released from various biological sources.

Reduction of plasma glycosphingolipid levels has no impact on atherosclerosis in apolipoprotein E-null mice

Glycosphingolipids (GSLs) have been implicated as potential atherogenic lipids. Studies in apolipoprotein E-null (apoE(-/-)) mice indicate that exacerbated tissue GSL accumulation resulting from alpha-galactosidase deficiency promotes atherosclerosis, whereas the serine palmitoyl transferase inhibitor myriocin (which reduces plasma and tissue levels of several sphingolipids, including sphingomyelin, ceramide, sphingosine-1-phosphate, and GSLs) inhibits atherosclerosis. It is not clear whether GSL synthesis inhibition per se has an impact on atherosclerosis. To address this issue, apoE(-/-) mice maintained on a high-fat diet were treated with a potent glucosylceramide synthesis inhibitor, d-threo-1-ethylendioxyphenyl-2-palmitoylamino-3-pyrrolidino-propanol (EtDO-P4), 10 mg/kg/day for 94 days, and lesion development was compared in mice that were treated with vehicle only. EtDO-P4 reduced plasma GSL concentration by approximately 50% but did not affect cholesterol or triglyceride levels. Assessment of atherosclerotic lesions at four different sites indicated that EtDO-P4 had no significant impact on lesion area. Thus, despite the previously observed positive correlations between plasma and aortic GSL concentrations and the development of atherosclerosis, and the in vitro evidence implying that GSLs may be pro-atherogenic, our current data indicate that inhibition of GSL synthesis does not inhibit atherosclerosis in vivo.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update covering the period 2001-2002

This review is the second update of the original review on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates that was published in 1999. It covers fundamental aspects of the technique as applied to carbohydrates, fragmentation of carbohydrates, studies of specific carbohydrate types such as those from plant cell walls and those attached to proteins and lipids, studies of glycosyl-transferases and glycosidases, and studies where MALDI has been used to monitor products of chemical synthesis. Use of the technique shows a steady annual increase at the expense of older techniques such as FAB. There is an increasing emphasis on its use for examination of biological systems rather than on studies of fundamental aspects and method development and this is reflected by much of the work on applications appearing in tabular form.

Glycosphingolipid synthesis requires FAPP2 transfer of glucosylceramide

The molecular machinery responsible for the generation of transport carriers moving from the Golgi complex to the plasma membrane relies on a tight interplay between proteins and lipids. Among the lipid-binding proteins of this machinery, we previously identified the four-phosphate adaptor protein FAPP2, the pleckstrin homology domain of which binds phosphatidylinositol 4-phosphate and the small GTPase ARF1. FAPP2 also possesses a glycolipid-transfer-protein homology domain. Here we show that human FAPP2 is a glucosylceramide-transfer protein that has a pivotal role in the synthesis of complex glycosphingolipids, key structural and signalling components of the plasma membrane. The requirement for FAPP2 makes the whole glycosphingolipid synthetic pathway sensitive to regulation by phosphatidylinositol 4-phosphate and ARF1. Thus, by coupling the synthesis of glycosphingolipids with their export to the cell surface, FAPP2 emerges as crucial in determining the lipid identity and composition of the plasma membrane.

Glycome mapping on DNA sequencing equipment

Here we provide a detailed protocol for the analysis of protein-linked glycans on DNA sequencing equipment. This protocol satisfies the glyco-analytical needs of many projects and can form the basis of 'glycomics' studies, in which robustness, high throughput, high sensitivity and reliable quantification are of paramount importance. The protocol routinely resolves isobaric glycan stereoisomers, which is much more difficult by mass spectrometry (MS). Earlier methods made use of polyacrylamide gel-based sequencers, but we have now adapted the technique to multicapillary DNA sequencers, which represent the state of the art today. In addition, we have integrated an option for HPLC-based fractionation of highly anionic 8-amino-1,3,6-pyrenetrisulfonic acid (APTS)-labeled glycans before rapid capillary electrophoretic profiling. This option facilitates either two-dimensional profiling of complex glycan mixtures and exoglycosidase sequencing, or MS analysis of particular compounds of interest rather than of the total pool of glycans in a sample.

Severe endothelial dysfunction in the aorta of a mouse model of Fabry disease; partial prevention by N-butyldeoxynojirimycin treatment

OBJECTIVE

Fabry disease results from alpha-gala-ctosidase A deficiency and is characterized by the lysosomal accumulation of globotriaosylceramide. Globotriaosylceramide storage predominantly affects endothelial cells, altering vascular wall morphology and vasomotor function. Our objective was to investigate aortic globotriaosylceramide levels, morphology and function in a mouse model of Fabry disease, and the effect of substrate reduction therapy, using the glycosphingolipid biosynthesis inhibitor N-butyldeoxynojirimycin.

METHODS AND RESULTS

Mice used were C57BL/6J and alpha-galactosidase A knockout (Fabry). We show progressive accumulation of aortic globotriaosylceramide throughout the lifespan of untreated Fabry mice (55-fold elevation at 2 months increasing to 187-fold by 19 months), localized to endothelial and vascular smooth-muscle cells; there was no effect on vascular wall morphology in young Fabry mice. In old mice, storage resulted in intimal thickening. Endothelial function declined with age in Fabry mouse aorta. Aortae from N-butyldeoxynojirimycin-treated Fabry mice at 19 months of age had reduced endothelial globotriaosylceramide storage, fewer morphological abnormalities and less severe vasomotor dysfunction compared with untreated littermates.

CONCLUSION

We provide evidence of a novel vascular phenotype in the Fabry mouse that has relevance to vascular disease in Fabry patients. N-Butyldeoxynojirimycin treatment partially prevented the phenotype in the Fabry mouse by reducing endothelial globotriaosylceramide storage.

New insights into glycosphingolipid functions--storage, lipid rafts, and translocators

Glycosphingolipids are key components of eukaryotic cellular membranes. Through their propensity to form lipid rafts, they are important in membrane transport and signaling. At the cell surface, they are required for caveolar-mediated endocytosis, a process required for the action of many glycosphingolipid-binding toxins. Glycosphingolipids also exist intracellularly, on both leaflets of organelle membranes. It is expected that dissecting the mechanisms of cell pathology seen in the glycosphingolipid storage diseases, where lysosomal glycosphingolipid degradation is defective, will reveal their functions. Disrupted cation gradients in Mucolipidosis type IV disease are interlinked with glycosphingolipid storage, defective rab 7 function, and the activation of autophagy. Relationships between drug translocators and glycosphingolipid synthesis are also discussed. Mass spectrometry of cell lines defective in drug transporters reveal clear differences in glycosphingolipid mass and fatty acid composition. The potential roles of glycosphingolipids in lipid raft formation, endocytosis, and cationic gradients are discussed.

Inhibition of atherosclerosis by the serine palmitoyl transferase inhibitor myriocin is associated with reduced plasma glycosphingolipid concentration

Glycosphingolipids (GSL) have been implicated as potential atherogenic lipids. Inhibition of hepatic serine palmitoyl transferase (SPT) reduces plasma sphingomyelin (SM) levels in the absence of changes in cholesterol or triglyceride (TG) concentration and this leads to a reduction of atherosclerosis in apolipoprotein-E gene knockout (apoE(-/-)) mice. The possibility that the reduced atherosclerosis resulting from SPT inhibition is associated with decreases in plasma GSL concentration has not been examined and was the primary aim of this investigation. We show that intraperitoneal delivery of the SPT inhibitor myriocin for 9 weeks inhibits atherosclerosis in apoE(-/-) mice fed a high fat diet. Lesion inhibition was most pronounced at the aortic arch and distal sites of the thoracic and abdominal aorta. There was also a trend towards a reduction in lesion area at the aortic root. Myriocin treatment resulted in significant reductions in both plasma SM and GSL concentration of 42% and 25%, as assessed by enzymatic and HPLC methods, respectively. Moreover, SM and GSL concentrations were significantly correlated, indicating that SPT inhibition suppresses the synthesis of both these sphingolipids concomitantly. The inhibition of atherosclerosis induced by myriocin was not associated with changes in plasma cholesterol or TG concentrations or lipoprotein profiles as determined by FPLC. These data indicate that therapeutic reduction of plasma SM and/or GSL concentrations may offer a novel treatment for atherosclerosis.

The association of Shiga-like toxin with detergent-resistant membranes is modulated by glucosylceramide and is an essential requirement in the endoplasmic reticulum for a cytotoxic effect

Receptor-mediated internalization to the endoplasmic reticulum (ER) and subsequent retro-translocation to the cytosol are essential sequential processes required for the productive intoxication of susceptible mammalian cells by Shiga-like toxin-1 (SLTx). Recently, it has been proposed that the observed association of certain ER-directed toxins and viruses with detergent-resistant membranes (DRM) may provide a general mechanism for their retrograde transport to endoplasmic reticulum (ER). Here, we show that DRM recruitment of SLTx bound to its globotriosylceramide (Gb(3)) receptor is mediated by the availability of other glycosphingolipids. Reduction in glucosylceramide (GlcCer) levels led to complete protection against SLTx and a reduced cell surface association of bound toxin with DRM. This reduction still allowed efficient binding and transport of the toxin to the ER. However, toxin sequestration within DRM of the ER was abolished under reduced GlcCer conditions, suggesting that an association of toxin with lipid microdomains or rafts in the ER (where these are defined by detergent insolubility) is essential for a later step leading to or involving retro-translocation of SLTx across the ER membrane. In support of this, we show that a number of ER residents, proteins intimately involved in the process of ER dislocation of misfolded proteins, are present in DRM.

Analysis of carbohydrates in plants by high-performance anion-exchange chromatography coupled with electrospray mass spectrometry

A mass spectrometer was coupled to high-performance anion-exchange chromatography (HPAEC) with the help of electrochemical neutralization of the eluent and post-column addition of lithium chloride for carbohydrate analysis. Parallel selective channels (single ion monitoring) were used to decrease the detection limits and separate unresolved peaks. The mass specific detection allowed the simultaneous analysis of a wide range of sugar alcohols, mono-, di- and oligosaccharides. Carbohydrates extracted from leaves of poplar submitted to drought stress were analyzed using pulsed amperometric detection (PAD), then mass spectrometry. It allowed the confirmation of peak attribution and the identification of salicin as a major compound in the extracts. Different responses to water deficit and re-hydration were obtained for several carbohydrates, suggesting different roles in osmoprotection processes.

Glycosphingolipid Accumulation Inhibits Cholesterol Efflux via the ABCA1/Apolipoprotein A-I Pathway

Cellular glycosphingolipid (GSL) storage is known to promote cholesterol accumulation. Although physical interactions between GSLs and cholesterol are thought to cause intracellular cholesterol "trapping," it is not known whether cholesterol homeostatic mechanisms are also impaired under these conditions. ApoA-I-mediated cholesterol efflux via ABCA1 (ATP-binding cassette transporter A1) is a key regulator of cellular cholesterol balance. Here, we show that apoA-I-mediated cholesterol efflux was inhibited (by up to 53% over 8 h) when fibroblasts were treated with lactosylceramide or the glucocerebrosidase inhibitor conduritol B epoxide. Furthermore, apoA-I-mediated cholesterol efflux from fibroblasts derived from patients with genetic GSL storage diseases (Fabry disease, Sandhoff disease, and GM1 gangliosidosis) was impaired compared with control cells. Conversely, apoA-I-mediated cholesterol efflux from fibroblasts and cholesterol-loaded macrophage foam cells was dose-dependently stimulated (by up to 6-fold over 8 h) by the GSL synthesis inhibitor 1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP). Unexpectedly, a structurally unrelated GSL synthesis inhibitor, N-butyldeoxynojirimycin, was unable to stimulate apoA-I-mediated cholesterol efflux despite achieving similar GSL depletion. PDMP was found to up-regulate ABCA1 mRNA and protein expression, thereby identifying a contributing mechanism for the observed acceleration of cholesterol efflux to apoA-I. This study reveals a novel defect in cellular cholesterol homeostasis induced by GSL storage and identifies PDMP as a new agent for enhancing cholesterol efflux via the ABCA1/apoA-I pathway.

Analysis of fluorescently labeled glycosphingolipid-derived oligosaccharides following ceramide glycanase digestion and anthranilic acid labeling

Interest in cellular glycosphingolipid (GSL) function has necessitated the development of a rapid and sensitive method to both analyze and characterize the full complement of structures present in various cells and tissues. An optimized method to characterize oligosaccharides released from glycosphingolipids following ceramide glycanase digestion has been developed. The procedure uses the fluorescent compound anthranilic acid (2-aminobenzoic acid; 2-AA) to label oligosaccharides prior to analysis using normal-phase high-performance liquid chromatography. The labeling procedure is rapid, selective, and easy to perform and is based on the published method of Anumula and Dhume [Glycobiology 8 (1998) 685], originally used to analyze N-linked oligosaccharides. It is less time consuming than a previously published 2-aminobenzamide labeling method [Anal. Biochem. 298 (2001) 207] for analyzing GSL-derived oligosaccharides, as the fluorescent labeling is performed on the enzyme reaction mixture. The purification of 2-AA-labeled products has been improved to ensure recovery of oligosaccharides containing one to four monosaccharide units, which was not previously possible using the Anumula and Dhume post-derivatization purification procedure. This new approach may also be used to analyze both N- and O-linked oligosaccharides.

Glycosphingolipid structural analysis and glycosphingolipidomics

Sphingosines, or sphingoids, are a family of naturally occurring long-chain hydrocarbon derivatives sharing a common 1,3-dihydroxy-2-amino-backbone motif. The majority of sphingolipids, as their derivatives are collectively known, can be found in cell membranes in the form of amphiphilic conjugates, each composed of a polar head group attached to an N-acylated sphingoid, or ceramide. Glycosphingolipids (GSLs), which are the glycosides of either ceramide or myo-inositol-(1-O)-phosphoryl-(O-1)-ceramide, are a structurally and functionally diverse sphingolipid subclass; GSLs are ubiquitously distributed among all eukaryotic species and are found in some bacteria. Since GSLs are secondary metabolites, direct and comprehensive analysis (metabolomics) must be considered an essential complement to genomic and proteomic approaches for establishing the structural repertoire within an organism and deducing its possible functional roles. The glycosphingolipidome clearly comprises an important and extensive subset of both the glycome and the lipidome, but the complexities of GSL structure, biosynthesis, and function form the outlines of a considerable analytical problem, especially since their structural diversity confers by extension an enormous variability with respect to physicochemical properties. This chapter covers selected developments and applications of techniques in mass spectrometric (MS) that have contributed to GSL structural analysis and glycosphingolipidomics since 1990. Sections are included on basic characteristics of ionization and fragmentation of permethylated GSLs and of lithium-adducted nonderivatized GSLs under positive-ion electrospray ionization mass spectrometry (ESI-MS) and collision-induced mass spectrometry (CID-MS) conditions; on the analysis of sulfatides, mainly using negative-ion techniques; and on selected applications of ESI-MS and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) to emerging GSL structural, functional, and analytical issues. The latter section includes a particular focus on evolving techniques for analysis of gangliosides, GSLs containing sialic acid, as well as on characterizations of GSLs from selected nonmammalian eukaryotes, such as dipterans, nematodes, cestodes, and fungi. Additional sections focus on the issue of whether it is better to leave GSLs intact or remove the ceramide; on development and uses of thin-layer chromatography (TLC) blotting and TLC-MS techniques; and on emerging issues of high-throughput analysis, including the use of flow injection, liquid chromatography mass spectrometry (LC-MS), and capillary electrophoresis mass spectrometry (CE-MS).

Electrospray ionization ion trap mass spectrometry for structural characterization of oligosaccharides derivatized with 2-aminobenzamide

The use of electrospray ionization (ESI) quadrupole ion trap mass spectrometry and reversed-phase high-performance liquid chromatography (HPLC) for the characterization of 2-aminobenzamide (2AB)-labeled oligosaccharides and N-linked protein oligosaccharide mixtures is described. The major signals were obtained under these conditions from the [M+Na]+ ions for all 2AB-derivatized oligosaccharides. Under collision-induced dissociation, sodiated molecular species generated in the ESI mode yield simple and predictable mass spectra. Tandem mass spectrometry (MS/MS) experiments with orders higher than two offer a number of ways to enhance MS/MS spectra and to derive information not present in MS and MS2 spectra. Information on composition, sequence, branching and, to some extent, interglycosidic linkages can be deduced from fragments resulting from the cleavage of glycosidic bonds and from weak cross-ring cleavage products. Reversed-phase HPLC and derivatization by reductive amination using 2-aminobenzamide were finally applied to characterize a glycan pool enzymatically released from glycoproteins.

Cellular effects of deoxynojirimycin analogues: inhibition of N-linked oligosaccharide processing and generation of free glucosylated oligosaccharides

In the accompanying paper [Mellor, Neville, Harvey, Platt, Dwek and Butters (2004) Biochem. J. 381, 861-866] we treated HL60 cells with N-alk(en)yl-deoxynojirimycin (DNJ) compounds to inhibit glucosphingolipid (GSL) biosynthesis and identified a number of non-GSL-derived, small, free oligosaccharides (FOS) most likely produced due to inhibition of the oligosaccharide-processing enzymes a-glucosidases I and II. When HL60 cells were treated with concentrations of N-alk(en)ylated DNJ analogues that inhibited GSL biosynthesis completely, N-butyl- and N-nonyl-DNJ inhibited endoplasmic reticulum (ER) glucosidases I and II, but octadecyl-DNJ did not, probably due to the lack of ER lumen access for this novel, long-chain derivative. Glucosidase inhibition resulted in the appearance of free Glc1-3Man structures, which is evidence of Golgi glycoprotein endomannosidase processing of oligosaccharides with retained glucose residues. Additional large FOS was also detected in cells following a 16 h treatment with N-butyl- and N-nonyl-DNJ. When these FOS structures (>30, including >20 species not present in control cells) were characterized by enzyme digests and MALDI-TOF (matrix-assisted laser-desorption ionization-time-of-flight) MS, all were found to be polymannose-type oligosaccharides, of which the majority were glucosylated and had only one reducing terminal GlcNAc (N-acetylglucosamine) residue (FOS-GlcNAc1), demonstrating a cytosolic location. These results support the proposal that the increase in glucosylated FOS results from enzyme-mediated cytosolic cleavage of oligosaccharides from glycoproteins exported from the ER because of misfolding or excessive retention. Importantly, the present study characterizes the cellular properties of DNJs further and demonstrates that side-chain modifications allow selective inhibition of protein and lipid glycosylation pathways. This represents the most detailed characterization of the FOS structures arising from ER a-glucosidase inhibition to date.

Cellular effects of deoxynojirimycin analogues: uptake, retention and inhibition of glycosphingolipid biosynthesis

Deoxynojirimycin (DNJ) analogues are inhibitors of ceramide glucosyltransferase (CGT), which catalyses the first step in the glucosphingolipid (GSL) biosynthetic pathway. We have synthesized a series of DNJ analogues to study the contribution of N-alk(en)yl side chains (C4, C9 or C18) to the behaviour of these analogues in cultured HL60 cells. When cells were treated for 16 h at non-cytotoxic concentrations of inhibitor, a 40-50% decrease in GSL levels was measured by HPLC analysis of GSL-derived oligosaccharides following ceramide glycanase digestion of GSL and 2-aminobenzamide labelling of the released oligosaccharides. Using a novel technique for short-term [14C]galactose labelling of cellular GSL, we used compound inhibition of GSL biosynthesis as a marker for compound uptake into cells. Surprisingly, the uptake of all three of the DNJ analogues was extremely rapid and was not dependent upon the length of the N-alk(en)yl moiety. Compound uptake occurred in less than 1 min, as shown by the complete inhibition of GSL labelling in cells treated with all the DNJ analogues. Greatly increased cellular retention of N-cis-13-octadecenyl-DNJ was observed relative to the shorter-chain compounds, N-butyl-DNJ and N-nonyl-DNJ, as indicated by complete inhibition of CGT 24 h after removal of inhibitor from the culture medium. The present study further characterizes the properties of N-alk(en)ylated DNJs, and demonstrates that increasing the length of the side chain is a simple way of improving imino sugar retention and therefore inhibitory efficacy for CGT in cultured cells.

Inhibition of glucosylceramide synthase does not reverse drug resistance in cancer cells

The multidrug-resistant cancer cell lines NCI/AdR(RES) and MES-SA/DX-5 have higher glycolipid levels and higher P-glycoprotein expression than the chemosensitive cell lines MCF7-wt and MES-SA. Inhibiting glycolipid biosynthesis by blocking glucosylceramide synthase has been proposed to reverse drug resistance in MDR cells by causing an increased accumulation of proapoptotic ceramide during treatment of cells with cytotoxic drugs. We treated both multidrug-resistant cell lines with the glucosylceramide synthase inhibitors PDMP (d-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol), C9DGJ (N-nonyl-deoxygalactonojirimycin) or C4DGJ (N-butyl-deoxygalactonojirimycin). PDMP achieved a significant reversal of drug resistance in agreement with previous reports. However, the N-alkylated iminosugars C9DGJ and C4DGJ, which are more selective glucosylceramide synthase inhibitors than PDMP, failed to cause any reversal of drug resistance despite depleting glycolipids to the same extent as PDMP. Our results suggest that (a) inhibition of glucosylceramide synthase does not reverse multidrug resistance and (b) the chemosensitization achieved by PDMP cannot be caused by inhibition of glucosylceramide synthase alone.

Modulation of THP-1 macrophage and cholesterol-loaded foam cell apolipoprotein E levels by glycosphingolipids

Macrophages synthesize and secrete apolipoprotein E (apoE) constitutively. This process is upregulated under conditions of cholesterol loading. The response to cholesterol is antiatherogenic as it is believed to promote cholesterol efflux from the artery wall. The concentration of lactosyl ceramide (LacCer), a glycosphingolipid recently discovered to regulate cellular signaling, proliferation, and expression of adhesion molecules, is also increased in atherosclerotic tissues. Here we have investigated the effect of exogenous LacCer on macrophage apoE levels. We show that increasing macrophage LacCer levels sevenfold led to reductions in cellular and secreted apoE (15 and 30%, respectively, over a 24-h period) as determined by enzyme-linked immunosorbent assay. A similar effect was also induced by glucosyl ceramide (GlcCer) but not by ganglioside species. When macrophages were converted to cholesterol-loaded foam cells by incubation with acetylated LDL, the resulting increase in cellular apoE levels was inhibited by 26% when the cells were subsequently enriched with LacCer. After metabolic labeling of cellular glycosphingolipids with [14C]palmitate, we also discovered that high-density lipoprotein (HDL) stimulates the efflux of glycosphingolipids from foam cells. These data imply that LacCer and GlcCer may be proatherogenic due to the suppression of macrophage apoE production. Furthermore, the efflux of glycosphingolipids from macrophage foam cells to HDL could indicate a potential pathway for their removal from the artery wall and subsequent delivery to the liver.