Gangliosides in normal human serum. Concentration, pattern and transport by lipoproteins

Brain-derived gangliosides prime human platelet aggregation and induce platelet-leukocyte aggregate formation

BACKGROUND

Platelet activation and interaction with leukocytes are crucial in inflammation. Gangliosides, sialic acid-containing glycosphingolipids, have been linked to different inflammatory conditions related to cardio- and neurodegenerative disorders. The role of gangliosides in platelet and leukocyte function, although reported, still needs further investigation.

OBJECTIVES

We aimed to study the role of gangliosides in platelet activation and platelet-leukocyte interaction in vitro.

METHODS

Platelet activation was studied through aggregometry in platelet-rich plasma from apparently healthy human volunteers. Signaling protein phosphorylation was analyzed by immunoblotting. Platelet P-selectin expression and platelet-leukocyte aggregate formation were measured by flow cytometry.

RESULTS

The gangliosides monosialoganglioside GM1, disialoganglioside GD1a, and trisialoganglioside GT1b did not induce by themselves any platelet aggregation. Conversely, when preincubated with platelets, they potentiate platelet aggregation induced by submaximal adenosine diphosphate and collagen concentrations and increased P-selectin expression. Incubation of platelets with free sialic acid and the soluble part of monosialoganglioside GM1 induced a similar potentiating effect on platelet aggregation but not on platelet P-selectin expression. Consistently, analyzing the signaling protein phosphorylation, only the entire gangliosides activated extracellular stimuli-responsive kinase 1/2 suggesting that a complete ganglioside is crucial for its action on platelets. Both the priming effect on platelet aggregation and ERK1/2 activation were prevented by aspirin. Moreover, incubation of citrated whole blood with gangliosides induced platelet-leukocyte aggregate formation accompanied by increased expression of granulocyte and monocyte CD11b compared with untreated blood, suggesting a primary leukocyte activation.

CONCLUSION

Gangliosides may act in vitro both on platelet and leukocyte activation and on their interaction. The observed effects might contribute to inflammatory processes in clinical conditions.

Unraveling the complexity of glycosphingolipidome: the key role of mass spectrometry in the structural analysis of glycosphingolipids

Glycosphingolipids (GSL) are a highly heterogeneous class of lipids representing the majority of the sphingolipid category. GSL are fundamental constituents of cellular membranes that have key roles in various biological processes, such as cellular signaling, recognition, and adhesion. Understanding the structural complexity of GSL is pivotal for unraveling their functional significance in a biological context, specifically their crucial role in the pathophysiology of various diseases. Mass spectrometry (MS) has emerged as a versatile and indispensable tool for the structural elucidation of GSL enabling a deeper understanding of their complex molecular structures and their key roles in cellular dynamics and patholophysiology. Here, we provide a thorough overview of MS techniques tailored for the analysis of GSL, emphasizing their utility in probing GSL intricate structures to advance our understanding of the functional relevance of GSL in health and disease. The application of tandem MS using diverse fragmentation techniques, including novel ion activation methodologies, in studying glycan sequences, linkage positions, and fatty acid composition is extensively discussed. Finally, we address current challenges, such as the detection of low-abundance species and the interpretation of complex spectra, and offer insights into potential solutions and future directions by improving MS instrumentation for enhanced sensitivity and resolution, developing novel ionization techniques, or integrating MS with other analytical approaches for comprehensive GSL characterization.

Comprehensive Glycan Analysis of Sphingolipids in Human Serum/Plasma

Glycosphingolipids (GSLs) are glycolipids with ceramide and carbohydrate head groups that play an important role in numerous biological processes. Previously, we performed GSL-glycan analysis of various cell lines and virus-infected cells using a glycoblotting approach. Recently, we developed several methods for sialic acid linkage-specific chemical modification to distinguish sialylated glycan isomers by mass spectrometry. In this chapter, we describe a method for analyzing GSL-glycans in human serum/plasma using glycoblotting combined with aminolysis-SALSA (sialic acid linkage-specific alkylamidation) and lactone-driven ester-to-amide derivatization (LEAD)-SALSA for comprehensive and detailed structural glycomics.

Interorgan Metabolism of Ganglioside Is Altered in Type 2 Diabetes

GM3 is implicated in cell signaling, inflammation and insulin resistance. The intestinal mucosa metabolizes ganglioside and provides gangliosides for uptake by peripheral tissues. Gangliosides downregulate acute and chronic inflammatory signals. It is likely that transport of intestinal derived gangliosides to other tissues impact the same signals characteristic of inflammatory change in other chronic conditions such as Type 2 Diabetes (T2DM). The postprandial ceramide composition of GM3 and other gangliosides in plasma and chylomicrons has not been examined in T2DM. The present study assessed if diet or T2DM alters ganglioside components in plasma and chylomicrons secreted from the intestinal mucosa after a meal. GD1, GD3, and GM3 content of chylomicrons and plasma was determined by LC/triple quad MS in non-diabetic (control) and T2DM individuals in the fasting and postprandial state after 2 days of consuming a low or high fat diet in a randomized blinded crossover design. Diet fat level did not alter baseline plasma or chylomicron ganglioside levels. Four hours after the test meal, plasma monounsaturated GD3 was 75% higher, plasma saturated GD3 was 140% higher and plasma polyunsaturated GM3 30% lower in diabetic subjects compared to control subjects. At 4 h, chylomicron GD1 was 50% lower in T2DM compared to controls. The proportion of d34:1 in GD3 was more abundant and d36:1 in GD1 less abundant in T2DM compared to control subjects at 4 h. The present study indicates that T2DM alters ceramide composition of ganglioside available for uptake by peripheral tissues.

Deficiency of the frontotemporal dementia gene GRN results in gangliosidosis

Haploinsufficiency of GRN causes frontotemporal dementia (FTD). The GRN locus produces progranulin (PGRN), which is cleaved to lysosomal granulin polypeptides. The function of lysosomal granulins and why their absence causes neurodegeneration are unclear. Here we discover that PGRN-deficient human cells and murine brains, as well as human frontal lobes from GRN-mutation FTD patients have increased levels of gangliosides, glycosphingolipids that contain sialic acid. In these cells and tissues, levels of lysosomal enzymes that catabolize gangliosides were normal, but levels of bis(monoacylglycero)phosphates (BMP), lipids required for ganglioside catabolism, were reduced with PGRN deficiency. Our findings indicate that granulins are required to maintain BMP levels to support ganglioside catabolism, and that PGRN deficiency in lysosomes leads to gangliosidosis. Lysosomal ganglioside accumulation may contribute to neuroinflammation and neurodegeneration susceptibility observed in FTD due to PGRN deficiency and other neurodegenerative diseases.

Aberrant Ganglioside Functions to Underpin Dysregulated Myelination, Insulin Signalling, and Cytokine Expression: Is There a Link and a Room for Therapy?

Gangliosides are molecules widely present in the plasma membranes of mammalian cells, participating in a variety of processes, including protein organization, transmembrane signalling and cell adhesion. Gangliosides are abundant in the grey matter of the brain, where they are critically involved in postnatal neural development and function. The common precursor of the majority of brain gangliosides, GM3, is formed by the sialylation of lactosylceramide, and four derivatives of its a- and b-series, GM1, GD1a, GD1b and GT1b, constitute 95% of all the brain gangliosides. Impairments in ganglioside metabolism due to genetic abnormalities of GM-synthases are associated with severe neurological disorders. Apart from that, the latest genome-wide association and translational studies suggest a role of genes involved in brain ganglioside synthesis in less pervasive psychiatric disorders. Remarkably, the most recent animal studies showed that abnormal ganglioside functions result in dysregulated neuroinflammation, aberrant myelination and altered insulin receptor signalling. At the same time, these molecular features are well established as accompanying developmental psychiatric disorders such as attention-deficit hyperactivity disorder (ADHD) and autism spectrum disorders (ASD). This led us to hypothesize a role of deficient ganglioside function in developmental neuropsychiatric disorders and warrants further gene association clinical studies addressing this question. Here, we critically review the literature to discuss this hypothesis and focus on the recent studies on ST3GAL5-deficient mice. In addition, we elaborate on the therapeutic potential of various anti-inflammatory remedies for treatment of developmental neuropsychiatric conditions related to aberrant ganglioside functions.

Pathophysiological Significance of GM3 Ganglioside Molecular Species With a Particular Attention to the Metabolic Syndrome Focusing on Toll-Like Receptor 4 Binding

GM3 ganglioside, the first molecule in ganglioside family biosynthesis, is formed by transfer of sialic acid to lactosylceramide. Several dozen GM3 molecular species exist, based on diversity of ceramide structures. Among ceramide structures composed of sphingosine and fatty acids, there is a great diversity resulting from different combinations of chain length, hydroxylation, and unsaturation of fatty acid chains. Expression patterns of GM3 species in serum vary during pathogenesis of metabolic syndrome. Physiological activity of each species, and significance of the variability, are poorly understood. Our studies revealed that GM3 species with differing fatty acid structures act as pro- or anti-inflammatory endogenous Toll-like receptor 4 (TLR4) ligands. Very long-chain fatty acid (VLCFA) and α-hydroxyl VLCFA GM3 variants strongly enhanced TLR4 activation. In contrast, long-chain fatty acid (LCFA) and ω-9 unsaturated VLCFA GM3 variants suppressed TLR4 activation. GM3 interacted with extracellular TLR4/myeloid differentiation factor 2 (MD-2) complex, thereby promoting dimerization/oligomerization. In obesity and metabolic syndrome, VLCFA-variant GM3 species were elevated in serum and adipose tissue, whereas LCFA-variant species were reduced, and such imbalances were correlated with disease progression. Our findings summarized in this review demonstrate that GM3 molecular species are disease-related endogenous TLR4 ligands and modulate homeostatic and pathogenic innate immune responses.

[Homeostatic and Pathophysiological Regulation of Toll-like Receptor 4 Signaling by GM3 Ganglioside Molecular Species]

Chronic inflammation plays an important role in the pathogenesis of obesity and metabolic disorders. In obesity, pattern-recognition receptors in innate immune system, such as Toll-like receptor 4 (TLR4), cause chronic inflammation through prolonged activation by various endogenous ligands, including fatty acids and its metabolites. Gangliosides and other glycosphingolipids are important metabolites of fatty acids and saccharides. GM3, the simplest ganglioside comprising α2,3-sialyllactose, is expressed in insulin-sensitive peripheral tissues such as liver and adipose tissue, and furthermore secreted abundantly into serum. It has been shown that GM3 regulates the signal transduction of insulin receptor in adipose tissue as a component of membrane microdomains, and elevation in GM3 level causes insulin resistance. However, the homeostatic and pathophysiological functions of extracellularly secreted GM3 are poorly understood. We recently reported that GM3 species with differing fatty acid structures act as pro- and anti-inflammatory endogenous TLR4 ligands. GM3 with very long-chain fatty acid (VLCFA) and α-hydroxyl VLCFA strongly enhanced TLR4 activation. Conversely, GM3 with long-chain fatty acid (LCFA) and ω-9 unsaturated VLCFA inhibited TLR4 activation, counteracting the VLCFA species. GM3 interacted with the extracellular complex of TLR4 and promoted dimerization/oligomerization. In obesity and metabolic disorders, VLCFA species were increased in serum and adipose tissue, whereas LCFA species was relatively decreased; their imbalances were correlated to disease progression. Our findings suggest that GM3 species are disease-related endogenous TLR4 ligands, and "glycosphingolipid sensing" by TLR4 controls the homeostatic and pathological roles of innate immune signaling.

Hexosaminidase A (HEXA) regulates hepatic sphingolipid and lipoprotein metabolism in mice

Hexosaminidase A (HexA), a heterodimer consisting of HEXA and HEXB, converts the ganglioside sphingolipid GM2 to GM3 by removing a terminal N-acetyl-d-galactosamine. HexA enzyme deficiency in humans leads to GM2 accumulation in cells, particularly in neurons, and is associated with neurodegeneration. While HexA and sphingolipid metabolism have been extensively investigated in the context of neuronal lipid metabolism, little is known about the metabolic impact of HexA and ganglioside degradation in other tissues. Here, we focussed on the role of HexA in the liver, which is a major regulator of systemic lipid metabolism. We find that hepatic Hexa expression is induced by lipid availability and increased in the presence of hepatic steatosis, which is associated with increased hepatic GM3 content. To assess the impact of HEXA on hepatic lipid metabolism, we used an adeno-associated virus to overexpress HEXA in the livers of high-fat diet fed mice. HEXA overexpression was associated with increased hepatic GM3 content and increased expression of enzymes involved in the degradation of glycated sphingolipids, ultimately driving sphingomyelin accumulation in the liver. In addition, HEXA overexpression led to substantial proteome remodeling in cell surface lipid rafts, which was associated with increased VLDL processing and secretion, hypertriglyceridemia and ectopic lipid accumulation in peripheral tissues. This study established an important role of HEXA in modulating hepatic sphingolipid and lipoprotein metabolism.

Homeostatic and pathogenic roles of the GM3 ganglioside

Two decades ago, we achieved molecular cloning of ganglioside GM3 synthase (GM3S; ST3GAL5), the enzyme responsible for initiating biosynthesis of complex gangliosides. The efforts of our research group since then have been focused on clarifying the physiological and pathological roles of gangliosides, particularly GM3. This review summarizes our long-term studies on the roles of GM3 in insulin resistance and adipogenesis in adipose tissues, cholesterol uptake in intestine, and leptin resistance in hypothalamus. We hypothesized that GM3 plays a role in innate immune function of macrophages and demonstrated that molecular species of GM3 with differing acyl-chain structures and modifications functioned as pro- and anti-inflammatory endogenous Toll-like receptor 4 (TLR4) modulators in macrophages. Very-long-chain and α-hydroxy GM3 species enhanced TLR4 activation, whereas long-chain and unsaturated GM3 species counteracted this effect. Lipidomic analyses of serum and adipose tissues revealed that imbalances between such pro- and anti-inflammatory GM3 species promoted progression of metabolic disorders. GM3 thus functions as a physiological regulatory factor controlling the balance between homeostatic and pathological states. Ongoing studies based on these findings will clarify the mechanisms underlying ganglioside-dependent control of energy homeostasis and innate immune responses.

Comprehensive Glycomic Approach Reveals Novel Low-Molecular-Weight Blood Group-Specific Glycans in Serum and Cerebrospinal Fluid

ABO blood antigens on the human red blood cell membrane as well as different cells in various human tissues have been thoroughly studied. Anti-A and -B antibodies of IgM are present in serum/plasma, but blood group-specific glyco-antigens have not been extensively described. In this study, we performed comprehensive and quantitative serum glycomic analyses of various glycoconjugates and free oligosaccharides in all blood groups. Our comprehensive glycomic approach revealed that blood group-specific antigens in serum/plasma are predominantly present on glycosphingolipids on lipoproteins rather than glycoproteins. Expression of the ABO antigens on glycosphingolipids depends not only on blood type but also on secretor status. Blood group-specific glycans in serum/plasma were classified as type I, whereas those on RBCs had different structures including hexose and hexosamine residues. Analysis of free oligosaccharides revealed that low-molecular-weight blood group-specific glycans, commonly containing lacto-N-difucotetraose, were expressed in serum/plasma according to blood group. Furthermore, comprehensive glycomic analysis in human cerebrospinal fluid showed that many kinds of free oligosaccharides were highly expressed, and low-molecular-weight blood group-specific glycans, which existed in plasma from the same individuals, were present. Our findings provide the first evidence for low-molecular-weight blood group-specific glycans in both serum/plasma and cerebrospinal fluid.

Neuraminidases 1 and 3 Trigger Atherosclerosis by Desialylating Low-Density Lipoproteins and Increasing Their Uptake by Macrophages

Background Chronic vascular disease atherosclerosis starts with an uptake of atherogenic modified low-density lipoproteins (LDLs) by resident macrophages, resulting in formation of arterial fatty streaks and eventually atheromatous plaques. Increased plasma sialic acid levels, increased neuraminidase activity, and reduced sialic acid LDL content have been previously associated with atherosclerosis and coronary artery disease in human patients, but the mechanism underlying this association has not been explored. Methods and Results We tested the hypothesis that neuraminidases contribute to development of atherosclerosis by removing sialic acid residues from glycan chains of the LDL glycoprotein and glycolipids. Atherosclerosis progression was investigated in apolipoprotein E and LDL receptor knockout mice with genetic deficiency of neuraminidases 1, 3, and 4 or those treated with specific neuraminidase inhibitors. We show that desialylation of the LDL glycoprotein, apolipoprotein B 100, by human neuraminidases 1 and 3 increases the uptake of human LDL by human cultured macrophages and by macrophages in aortic root lesions in Apoe^-/- mice via asialoglycoprotein receptor 1. Genetic inactivation or pharmacological inhibition of neuraminidases 1 and 3 significantly delays formation of fatty streaks in the aortic root without affecting the plasma cholesterol and LDL levels in Apoe^-/- and Ldlr^-/- mouse models of atherosclerosis. Conclusions Together, our results suggest that neuraminidases 1 and 3 trigger the initial phase of atherosclerosis and formation of aortic fatty streaks by desialylating LDL and increasing their uptake by resident macrophages.

Homeostatic and pathogenic roles of GM3 ganglioside molecular species in TLR4 signaling in obesity

Innate immune signaling via TLR4 plays critical roles in pathogenesis of metabolic disorders, but the contribution of different lipid species to metabolic disorders and inflammatory diseases is less clear. GM3 ganglioside in human serum is composed of a variety of fatty acids, including long‐chain (LCFA) and very‐long‐chain (VLCFA). Analysis of circulating levels of human serum GM3 species from patients at different stages of insulin resistance and chronic inflammation reveals that levels of VLCFA‐GM3 increase significantly in metabolic disorders, while LCFA‐GM3 serum levels decrease. Specific GM3 species also correlates with disease symptoms. VLCFA‐GM3 levels increase in the adipose tissue of obese mice, and this is blocked in TLR4‐mutant mice. In cultured monocytes, GM3 by itself has no effect on TLR4 activation; however, VLCFA‐GM3 synergistically and selectively enhances TLR4 activation by LPS/HMGB1, while LCFA‐GM3 and unsaturated VLCFA‐GM3 suppresses TLR4 activation. GM3 interacts with the extracellular region of TLR4/MD2 complex to modulate dimerization/oligomerization. Ligand‐molecular docking analysis supports that VLCFA‐GM3 and LCFA‐GM3 act as agonist and antagonist of TLR4 activity, respectively, by differentially binding to the hydrophobic pocket of MD2. Our findings suggest that VLCFA‐GM3 is a risk factor for TLR4‐mediated disease progression.

A Review of Lipidomics of Cardiovascular Disease Highlights the Importance of Isolating Lipoproteins

Cutting-edge lipidomic profiling measures hundreds or even thousands of lipids in plasma and is increasingly used to investigate mechanisms of cardiovascular disease (CVD). In this review, we introduce lipidomic techniques, describe distributions of lipids across lipoproteins, and summarize findings on the association of lipids with CVD based on lipidomics. The main findings of 16 cohort studies were that, independent of total and high-density lipoprotein cholesterol (HDL-c), ceramides (d18:1/16:0, d18:1/18:0, and d18:1/24:1) and phosphatidylcholines (PCs) containing saturated and monounsaturated fatty acyl chains are positively associated with risks of CVD outcomes, while PCs containing polyunsaturated fatty acyl chains (PUFA) are inversely associated with risks of CVD outcomes. Lysophosphatidylcholines (LPCs) may be positively associated with risks of CVD outcomes. Interestingly, the distributions of the identified lipids vary across lipoproteins: LPCs are primarily contained in HDLs, ceramides are mainly contained in low-density lipoproteins (LDLs), and PCs are distributed in both HDLs and LDLs. Thus, the potential mechanism behind previous findings may be related to the effect of the identified lipids on the biological functions of HDLs and LDLs. Only eight studies on the lipidomics of HDL and non-HDL particles and CVD outcomes have been conducted, which showed that higher triglycerides (TAGs), lower PUFA, lower phospholipids, and lower sphingomyelin content in HDLs might be associated with a higher risk of coronary heart disease (CHD). However, the generalizability of these studies is a major concern, given that they used case-control or cross-sectional designs in hospital settings, included a very small number of participants, and did not correct for multiple testing or adjust for blood lipids such as HDL-c, low-density lipoprotein cholesterol (LDL-c), or TAGs. Overall, findings from the literature highlight the importance of research on lipidomics of lipoproteins to enhance our understanding of the mechanism of the association between the identified lipids and the risk of CVD and allow the identification of novel lipid biomarkers in HDLs and LDLs, independent of HDL-c and LDL-c. Lipidomic techniques show the feasibility of this exciting research direction, and the lack of high-quality epidemiological studies warrants well-designed prospective cohort studies.

Exploring In Vivo Dynamics of Bovine Milk Derived Gangliosides

Gangliosides are glycosphingolipids present in mammalian cell membranes, playing important structural and functional roles. Human studies on the health benefits of gangliosides are increasing, but knowledge gaps regarding ganglioside analysis exist. The study aimed to investigate blood sample type (serum/plasma), storage conditions, diurnal, day-to-day variation and acute effects of consuming bovine-derived gangliosides on circulating monosialylated gangliosides. Seventy-one women (18–40 yrs, 20–≤30.0 kg/m²) were enrolled and 61 completed the intervention. They visited the clinic three times following overnight fasting. Serum/plasma gangliosides were analyzed over 2 h (visit-1), 8 h (visit-2) and 8 h following either zero or high ganglioside meals (visit-3). Samples stored at −20 °C and −70 °C were analyzed at 3-, 6-, 12- and 18-months. Plasma and serum GM3-gangliosides did not differ, plasma GM3 did not change diurnally, from day-to-day, in response to a high vs. low ganglioside meal or after 7-days low ganglioside vs. habitual diet (P > 0.05). GM3 concentrations were lower in samples stored at −70 °C vs. −20 °C from 6-months onwards and decreased over time with lowest levels at 12- and 18-months stored at −70 °C. In conclusion, either serum/plasma stored at −20- or −70 °C for up to 6 months, are acceptable for GM3-ganglioside analysis. Blood samples can be collected at any time of the day and participants do not have to be in the fasted state.

Dietary Polar Lipids and Cognitive Development: A Narrative Review

Polar lipids are amphiphilic lipids with a hydrophilic head and a hydrophobic tail. Polar lipids mainly include phospholipids and sphingolipids. They are structural components of neural tissues, with the peak rate of accretion overlapping with neurodevelopmental milestones. The critical role of polar lipids in cognitive development is thought to be mediated through the regulation of signal transduction, myelination, and synaptic plasticity. Animal products (egg, meat, and dairy) are the major dietary sources of polar lipids for children and adults, whereas human milk and infant formula provide polar lipids to infants. Due to the differences observed in both concentration and proportion of polar lipids in human milk, the estimated daily intake in infants encompasses a wide range. In addition, health authorities define neither intake recommendations nor guidelines for polar lipid intake. However, adequate intake is defined for 2 nutrients that are elements of these polar lipids, namely choline and DHA. To date, limited studies exist on the brain bioavailability of dietary polar lipids via either placental transfer or the blood-brain barrier. Nevertheless, due to their role in pre- and postnatal development of the brain, there is a growing interest for the use of gangliosides, which are sphingolipids, as a dietary supplement for pregnant/lactating mothers or infants. In line with this, supplementing gangliosides and phospholipids in wild-type animals and healthy infants does suggest some positive effects on cognitive performance. Whether there is indeed added benefit of supplementing polar lipids in pregnant/lactating mothers or infants requires more clinical research. In this article, we report findings of a review of the state-of-the-art evidence on polar lipid supplementation and cognitive development. Dietary sources, recommended intake, and brain bioavailability of polar lipids are also discussed.

Serum GM3(d18:1-16:0) and GM3(d18:1-24:1) levels may be associated with lymphoma: An exploratory study with haematological diseases

GM3 (monosialodihexosylganglioside) is a type of ganglioside, which is a molecule composed of ceramide and oligosaccharide containing one or more sialic acids. Since GM3 is abundantly expressed in blood cells, we investigated the association between GM3 molecular species and haematological diseases. We measured the serum levels of seven GM3 molecular species in subjects with various haematological diseases (n = 52) and healthy subjects (n = 24) using a liquid chromatography tandem-mass spectrometry technique as an exploratory study. In all the subjects with haematological diseases, GM3(d18:1-16:0) were inversely correlated with the erythrocytes counts. Regarding the difference in serum GM3 molecular species levels among each haematological diseases and healthy subjects, the levels of GM3(d18:1-16:0) and GM3(d18:1-24:1) were higher in the lymphoid neoplasm group than healthy subjects. Principal component analyses also revealed that the GM3(d18:1-16:0) and GM3(d18:1-24:1) levels were significant contributing factors for discriminating the lymphoid neoplasm group. Moreover, in the lymphoid neoplasm group, the GM3(d18:1-16:0) levels were significantly and positively correlated with the levels of C-reactive protein, soluble interleukin-2 receptor, and lactate dehydrogenase. In conclusion, in our exploratory study with haematological diseases, GM3 molecular species showed different distribution among disease groups, and serum GM3(d18:1-16:0) and GM3(d18:1-24:1) might be associated with lymphoma.

Recessive GM3 synthase deficiency: Natural history, biochemistry, and therapeutic frontier

GM3 synthase, encoded by ST3GAL5, initiates synthesis of all downstream cerebral gangliosides. Here, we present biochemical, functional, and natural history data from 50 individuals homozygous for a pathogenic ST3GAL5 c.862C>T founder allele (median age 8.1, range 0.7-30.5 years). GM3 and its derivatives were undetectable in plasma. Weight and head circumference were normal at birth and mean Apgar scores were 7.7 ± 2.0 (1 min) and 8.9 ± 0.5 (5 min). Somatic growth failure, progressive microcephaly, global developmental delay, visual inattentiveness, and dyskinetic movements developed within a few months of life. Infantile-onset epileptic encephalopathy was characterized by a slow, disorganized, high-voltage background, poor state transitions, absent posterior rhythm, and spike trains from multiple independent cortical foci; >90% of electrographic seizures were clinically silent. Hearing loss affected cochlea and central auditory pathways and 76% of children tested failed the newborn hearing screen. Development stagnated early in life; only 13 (26%) patients sat independently (median age 30 months), three (6%) learned to crawl, and none achieved reciprocal communication. Incessant irritability, often accompanied by insomnia, began during infancy and contributed to high parental stress. Despite catastrophic neurological dysfunction, neuroimaging showed only subtle or no destructive changes into late childhood and hospitalizations were surprisingly rare (0.2 per patient per year). Median survival was 23.5 years. Our observations corroborate findings from transgenic mice which indicate that gangliosides might have a limited role in embryonic neurodevelopment but become vital for postnatal brain growth and function. These results have critical implications for the design and implementation of ganglioside restitution therapies.

Comprehensive study of rodent olfactory tissue lipid composition

The peripheral olfactory tissue (OT) plays a primordial role in the detection and transduction of olfactory information. Recent proteomic and transcriptomic studies have provided valuable insight into proteins and RNAs expressed in this tissue. Paradoxically, there is little information regarding the lipid composition of mammalian OT. To delve further into this issue, using a set of complementary state-of-the-art techniques, we carried out a comprehensive analysis of OT lipid composition in rats and mice fed with standard diets. The results showed that phospholipids are largely predominant, the major classes being phosphatidylcholine and phosphatidylethanolamine. Two types of plasmalogens, plasmenyl-choline and plasmenyl-ethanolamine, as well as gangliosides were also detected. With the exception of sphingomyelin, substantial levels of n-3 polyunsaturated fatty acids, mainly docosahexaenoic acid (22:6n-3; DHA), were found in the different phospholipid classes. These findings demonstrate that the rodent OT shares several features in common with other neural tissues, such as the brain and retina.

Biology of GM3 Ganglioside

Since the successful molecular cloning in 1998 of GM3 synthase (GM3S, ST3GAL5), the enzyme responsible for initiating biosynthesis of all complex gangliosides, the efforts of our research group have been focused on clarifying the physiological and pathological implications of gangliosides, particularly GM3. We have identified isoforms of GM3S proteins having distinctive lengths of N-terminal cytoplasmic tails, and found that these cytoplasmic tails define subcellular localization, stability, and in vivo activity of GM3S isoforms. Our studies of the molecular pathogenesis of type 2 diabetes, focused on interaction between insulin receptor and GM3 in membrane microdomains, led to a novel concept: type 2 diabetes and certain other lifestyle-related diseases are membrane microdomain disorders resulting from aberrant expression of gangliosides. This concept has enhanced our understanding of the pathophysiological roles of GM3 and related gangliosides in various diseases involving chronic inflammation, such as insulin resistance, leptin resistance, and T-cell function and immune disorders (e.g., allergic asthma). We also demonstrated an essential role of GM3 in murine and human auditory systems; a common pathological feature of GM3S deficiency is deafness. This is the first direct link reported between gangliosides and auditory functions.

Monosialyl Ganglioside GM3 Decreases Apolipoprotein B-100 Secretion in Liver Cells

Some sialic acid-containing glycolipids are known to regulate development of atherosclerosis with accumulated plasma apolipoprotein B-100 (Apo-B)-containing lipoproteins, because Apo-B as an atherogenic apolipoprotein is assembled mainly in VLDL and LDL. Previously, we have elucidated that disialyl GD3 promotes the microsomal triglyceride transfer protein (MTP) gene expression and secretion of triglyceride (TG)-assembled ApoB, claiming the GD3 role in ApoB lipoprotein secretion in liver cells. In the synthetic pathway of gangliosides, GD3 is synthesized by addition of a sialic acid residue to GM3. Thus, there should be some regulatory links between GM3 and GD3. In this study, exogenous and endogenous monosialyl GM3 has been examined how GM3 plays a role in ApoB secretion in Chang liver cells in a view point of MTP and ApoB degradation in the same cells. The level of GM3 ganglioside in the GM3 synthase gene-transfected cells was increased in the cell extract, but not in the medium. In addition, GM3 synthase gene-transfected cells showed a diminished secretion of TG-enriched ApoB with a lower content of TG in the medium. Exogenous GM3 treatment for 24 h exerted a dose dependent inhibitory effect on ApoB secretion together with TG, while a liver-specific albumin was unchanged, indicating that GM3 effect is limited to ApoB secretion. GM3 decreased the mRNA level of MTP gene, too. ApoB protein assembly dysregulated by GM3 indicates the impaired ApoB secretion is caused by a proteasome-dependent pathway. Treatment with small interfering RNAs (siRNAs) decreased ApoB secretion, but GM3-specific antibody did not. These results indicate that plasma membrane associated GM3 inhibits ApoB secretion, lowers development of atherosclerosis by decreasing the secretion of TG-enriched ApoB containing lipoproteins, suggesting that GM3 is an inhibitor of ApoB and TG secretion in liver cells. J. Cell. Biochem. 118: 2168-2181, 2017. © 2017 Wiley Periodicals, Inc.

Plasma Lipidomic Profiles Improve on Traditional Risk Factors for the Prediction of Cardiovascular Events in Type 2 Diabetes Mellitus

BACKGROUND

Clinical lipid measurements do not show the full complexity of the altered lipid metabolism associated with diabetes mellitus or cardiovascular disease. Lipidomics enables the assessment of hundreds of lipid species as potential markers for disease risk.

METHODS

Plasma lipid species (310) were measured by a targeted lipidomic analysis with liquid chromatography electrospray ionization-tandem mass spectrometry on a case-cohort (n=3779) subset from the ADVANCE trial (Action in Diabetes and Vascular Disease: Preterax and Diamicron-MR Controlled Evaluation). The case-cohort was 61% male with a mean age of 67 years. All participants had type 2 diabetes mellitus with ≥1 additional cardiovascular risk factors, and 35% had a history of macrovascular disease. Weighted Cox regression was used to identify lipid species associated with future cardiovascular events (nonfatal myocardial infarction, nonfatal stroke, and cardiovascular death) and cardiovascular death during a 5-year follow-up period. Multivariable models combining traditional risk factors with lipid species were optimized with the Akaike information criteria. C statistics and NRIs were calculated within a 5-fold cross-validation framework.

RESULTS

Sphingolipids, phospholipids (including lyso- and ether- species), cholesteryl esters, and glycerolipids were associated with future cardiovascular events and cardiovascular death. The addition of 7 lipid species to a base model (14 traditional risk factors and medications) to predict cardiovascular events increased the C statistic from 0.680 (95% confidence interval [CI], 0.678-0.682) to 0.700 (95% CI, 0.698-0.702; P<0.0001) with a corresponding continuous NRI of 0.227 (95% CI, 0.219-0.235). The prediction of cardiovascular death was improved with the incorporation of 4 lipid species into the base model, showing an increase in the C statistic from 0.740 (95% CI, 0.738-0.742) to 0.760 (95% CI, 0.757-0.762; P<0.0001) and a continuous net reclassification index of 0.328 (95% CI, 0.317-0.339). The results were validated in a subcohort with type 2 diabetes mellitus (n=511) from the LIPID trial (Long-Term Intervention With Pravastatin in Ischemic Disease).

CONCLUSIONS

The improvement in the prediction of cardiovascular events, above traditional risk factors, demonstrates the potential of plasma lipid species as biomarkers for cardiovascular risk stratification in diabetes mellitus.

CLINICAL TRIAL REGISTRATION

URL: https://clinicaltrials.gov. Unique identifier: NCT00145925.

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.

Plasmatic Ganglioside Profile and Age-Related Macular Degeneration: A Case-Control Study

PURPOSE

Gangliosides are glycosphingolipids that are particularly abundant in the nervous system, including the retina. However, their precise role in this tissue and its pathologies remain poorly understood. The objective of the present study was to characterize the ganglioside profile of human plasma and to determine whether it is affected in age-related macular degeneration (AMD).

METHODS

Eighty-three subjects were included: control subjects (n = 25), atrophic AMD patients (n = 27) and exudative AMD patients (n = 31). For each subject, gangliosides were extracted from plasma and analyzed by liquid chromatography coupled to mass spectrometry.

RESULTS

GM3 appeared to be by far the major ganglioside of human plasma, associated with GD3. No specific ganglioside class was detected in the plasma of AMD patients. Fourteen molecular species of GM3 and 9 species of GD3, accounting for the variability of the ceramide moiety of the ganglioside molecule, were identified and characterized. Analyses revealed no significant differences in the proportion of these species between control, atrophic and exudative AMD patient groups. Total GM3 levels did not differ either.

CONCLUSION

Although gangliosides are considered important for the retina's structure and function, it seems that circulating gangliosides are not associated with the retinal damage occurring during the course of AMD.

Identification of Ganglioside GM3 Molecular Species in Human Serum Associated with Risk Factors of Metabolic Syndrome

Serum GM3 molecular species were quantified in 125 Japanese residents using tandem mass spectrometry multiple reaction monitoring. Individuals were categorized by the presence or absence of metabolic disease risk factors including visceral fat accumulation, hyperglycemia and dyslipidemia. A total of 23 GM3 molecular species were measured, of these, eight were found to be significantly elevated in individuals with visceral fat accumulation and metabolic disease, defined as the presence of hyperglycemia and dyslipidemia. All of the GM3 molecular species were composed of the sphingoid base sphingosine (d18:1 (Δ4)) and, interestingly, six of the eight elevated GM3 molecular species contained a hydroxylated ceramide moiety. The hydroxylated GM3 species were, in order of decreasing abundance, d18:1-h24:0 ≈ d18:1-h24:1 > d18:1-h22:0 » d18:1-h20:0 > d18:1-h21:0 > d18:1-h18:1. Univariate and multiple linear regression analyses were conducted using a number of clinical health variables associated with obesity, type 2 diabetes, metabolic disease, atherosclerosis and hypertension. GM3(d18:1-h24:1) was identified as the best candidate for metabolic screening, proving to be significantly correlated with intima-media thickness, used for the detection of atherosclerotic disease in humans, and a number of metabolic disease risk factors including autotaxin, LDL-c and homeostatic model assessment insulin resistance (HOMA-IR).

Increased sialidase activity in serum of cancer patients: Identification of sialidase and inhibitor activities in human serum

Aberrant sialylation in glycoproteins and glycolipids is a characteristic feature of malignancy. Human sialidases, which catalyze the removal of sialic acid residues from glycoconjugates, have been implicated in cancer progression. They have been detected in a wide variety of human cells and tissues, but few studies have focused on their existence in human serum. Among the four types identified to date, we previously demonstrated that plasma membrane-associated ganglioside sialidase (NEU3) is markedly upregulated in various human cancers, including examples in the colon and prostate. Here, using a sensitive assay method, we found a significant increase of sialidase activity in the serum of patients with prostate cancer compared with that in healthy subjects having low activity, if any. Activity was apparent with gangliosides as substrates, but only to a very limited extent with 4-methylumbelliferyl sialic acid, a good synthetic substrate for sialidases other than human NEU3. The serum sialidase was also almost entirely immunoprecipitated with anti-NEU3 antibody, but not with antibodies for other sialidases. Interestingly, sera additionally contained inhibitory activity against the sialidase and also against recombinant human NEU3. The sialidase and inhibitor activities could be separated by exosome isolation and by hydrophobic column chromatography. The serum sialidase was assessed by a sandwich ELISA method using two anti-NEU3 antibodies. The results provide strong evidence that the serum sialidase is, in fact, NEU3, and this subtype may, therefore, be a potential utility for novel diagnosis of human cancers.

Galectin-3 drives glycosphingolipid-dependent biogenesis of clathrin-independent carriers

Several cell surface molecules including signalling receptors are internalized by clathrin-independent endocytosis. How this process is initiated, how cargo proteins are sorted and membranes are bent remains unknown. Here, we found that a carbohydrate-binding protein, galectin-3 (Gal3), triggered the glycosphingolipid (GSL)-dependent biogenesis of a morphologically distinct class of endocytic structures, termed clathrin-independent carriers (CLICs). Super-resolution and reconstitution studies showed that Gal3 required GSLs for clustering and membrane bending. Gal3 interacted with a defined set of cargo proteins. Cellular uptake of the CLIC cargo CD44 was dependent on Gal3, GSLs and branched N-glycosylation. Endocytosis of β1-integrin was also reliant on Gal3. Analysis of different galectins revealed a distinct profile of cargoes and uptake structures, suggesting the existence of different CLIC populations. We conclude that Gal3 functionally integrates carbohydrate specificity on cargo proteins with the capacity of GSLs to drive clathrin-independent plasma membrane bending as a first step of CLIC biogenesis.

Serum sialic acids levels according to the severity of liver cirrhosis

BACKGROUND

The sialylation of serum proteins and lipids changes in liver diseases of different etiologies and could change the total sialic acid (TSA), lipid-bound SA (LSA), and free SA (FSA) levels in the sera. However, little is known of the relationship of serum SAs concentrations and the severity of liver disease. Therefore, the aim of this study was to investigate the SAs concentrations (TSA, LSA, and FSA) in liver cirrhosis in relation with the severity of liver disease.

METHODS

Tested group consisted of 91 consecutive patients suffering from liver cirrhosis. For each patient, the Child-Pugh score was calculated. TSA and LSA were determined by the enzymatic method on microplate reader, and FSA using the thiobarbituric method.

RESULTS

Among the SA forms, only the serum FSA level in liver cirrhosis appears to be different according to the severity of liver damage evaluated by the Child-Pugh score. It was the highest in score C, and was higher than that in scores B and A. The elevated levels of FSA significantly positively correlated with the Child-Pugh score.

CONCLUSION

In conclusion, the sialylation of serum proteins and lipids changes in liver cirrhosis, but only the serum concentrations of FSA are stage-related and reflect the severity of liver disease.

Glycomic analysis of high density lipoprotein shows a highly sialylated particle

Many of the functional proteins and lipids in high density lipoprotein (HDL) particles are potentially glycosylated, yet very little is known about the glycoconjugates of HDL. In this study, HDL was isolated from plasma by sequential micro-ultracentrifugation, followed by glycoprotein and glycolipid analysis. N-Glycans, glycopeptides, and gangliosides were extracted and purified followed by analysis with nano-HPLC Chip quadrupole time of flight mass spectrometry and MS/MS. HDL particles were found to be highly sialylated. Most of the N-glycans (∼90%) from HDL glycoproteins were sialylated with one or two neuraminic acids (Neu5Ac). The most abundant N-glycan was a biantennary complex type glycan with two sialic acids (Hexose₅HexNAc₄Neu5Ac₂) and was found in multiple glycoproteins using site-specific glycosylation analysis. The observed O-glycans were all sialylated, and most contained a core 1 structure with two Neu5Acs, including those that were associated with apolipoprotein CIII (ApoC-III) and fetuin A. GM3 (monosialoganglioside, NeuAc2–3Gal1–4Glc–Cer) and GD3 (disialoganglioside, NeuAc2–8NeuAc2–3Gal1–4Glc–Cer) were the major gangliosides in HDL. A 60% GM3 and 40% GD3 distribution was observed. Both GM3 and GD3 were composed of heterogeneous ceramide lipid tails, including d18:1/16:0 and d18:1/23:0. This report describes for the first time a glycomic approach for analyzing HDL, highlighting that HDL are highly sialylated particles.

GM3 and diabetes

We demonstrated the molecular pathogenesis of type 2 diabetes and insulin resistance focusing on the interaction between insulin receptor and GM3 ganglioside in adipocytes and propose a working hypothesis "metabolic disorders, such as type 2 diabetes, are membrane microdomain disorders caused by aberrant expression of gangliosides". It is expected that the development of novel diagnosis of metabolic syndrome by identifying the specific ganglioside species and a therapeutic strategy "membrane microdomain ortho-signaling therapy".

Circulating levels of ganglioside GM3 in metabolic syndrome: A pilot study

SUMMARY

BACKGROUND

Insulin resistance is a characteristic feature of metabolic syndrome. Ganglioside GM3 [α-Neu5Ac-(2-3)-β-Gal-(1-4)-β-Glc-(1-1)-ceramide] may impair insulin sensitivity in adipose tissue. We investigated the relationship between serum GM3 levels and adiposity indices, as well as between serum GM3 levels and metabolic risk variables.

METHODS

Study 1: we assessed serum GM3 levels in normal subjects and in patients with hyperglycemia and/or hyperlipidemia (HL). Study 2: we investigated the relationship between serum GM3 levels and metabolic risk variables in patients with type 2 diabetes.

RESULTS

Study 1: serum GM3 levels were higher in hyperglycemic patients (1.4-fold), hyperlipidemic patients (1.4-fold) and hyperglycemic patients with hyperlipidemia (1.6-fold), than in normal subjects. Study 2: serum GM3 levels were significantly increased in type 2 diabetic patients with severe obesity (visceral fat area (VFA) >200 cm(2), BMI > 30). The GM3 level was positively correlated with LDL-c (0.403, p = 0.012) in type 2 diabetes mellitus, but not affected by blood pressure. In addition, the high levels of small dense LDL (>10 mg/dL) were associated with the elevation of GM3.

CONCLUSIONS

Serum GM3 levels was affected by glucose and lipid metabolism abnormalities and by visceral obesity. GM3 may be a useful marker for severity of metabolic syndrome.

HDL-cholesterol in coronary artery disease risk: function or structure?

High-density lipoproteins (HDL) are inversely related with coronary artery disease (CAD) and HDL-cholesterol is the only standardized and reproducible parameter available to estimate plasma concentration of these lipoproteins. However, pharmacological interventions intended to increase HDL-cholesterol have not been consistently associated to an effective CAD risk reduction. Among patients with a myocardial infarction, 43 and 44% of men and women, respectively, had normal plasma levels of HDL-cholesterol, whereas genetic studies have failed to show a causal association between HDL-cholesterol and CAD risk. Instead, HDL functionality seems to be the target to be evaluated, but the existing methods are still poorly reproducible and far to be adapted to the clinical laboratory. HDL subclasses rise as a potential alternative for the evaluation of CAD risk; HDL subclasses are a surrogate of intravascular metabolism of these lipoproteins and probably of their functionality. Low levels of large HDL and increased proportions of small particles are the most remarkable features associated to an increased risk of type 2 diabetes mellitus (T2DM) or CAD. However, inflammation and other environmental factors are related with abnormal HDL structure, and, as a consequence, more prospective studies are needed to better support the clinical usefulness of HDL subclasses. New insights from proteome and lipidome profiles of HDL will provide potential HDL-related biomarkers in the coming years.

Unraveling the complexities of the HDL lipidome

Plasma high density lipoproteins (HDL) are small, dense, protein-rich particles compared with other lipoprotein classes; roughly half of total HDL mass is accounted for by lipid components. Phospholipids predominate in the HDL lipidome, accounting for 40-60% of total lipid, with lesser proportions of cholesteryl esters (30-40%), triglycerides (5-12%), and free cholesterol (5-10%). Lipidomic approaches have provided initial insights into the HDL lipidome with identification of over 200 individual molecular lipids species in normolipidemic HDL. Plasma HDL particles, however, reveal high levels of structural, compositional, and functional heterogeneity. Establishing direct relationships between HDL structure, composition, and atheroprotective functions bears the potential to identify clinically relevant HDL subpopulations. Furthermore, development of HDL-based therapies designed to target beneficial subspecies within the circulating HDL pool can be facilitated using this approach. HDL lipidomics can equally contribute to the identification of biomarkers of both normal and deficient HDL functionality, which may prove useful as biomarkers of cardiovascular risk. However, numerous technical issues remain to be addressed in order to make such developments possible. With all technical questions resolved, quantitative analysis of the molecular components of the HDL lipidome will contribute to expand our knowledge of cardiovascular and metabolic diseases.

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.

Cholesterol: its regulation and role in central nervous system disorders

Cholesterol is a major constituent of the human brain, and the brain is the most cholesterol-rich organ. Numerous lipoprotein receptors and apolipoproteins are expressed in the brain. Cholesterol is tightly regulated between the major brain cells and is essential for normal brain development. The metabolism of brain cholesterol differs markedly from that of other tissues. Brain cholesterol is primarily derived by de novo synthesis and the blood brain barrier prevents the uptake of lipoprotein cholesterol from the circulation. Defects in cholesterol metabolism lead to structural and functional central nervous system diseases such as Smith-Lemli-Opitz syndrome, Niemann-Pick type C disease, and Alzheimer's disease. These diseases affect different metabolic pathways (cholesterol biosynthesis, lipid transport and lipoprotein assembly, apolipoproteins, lipoprotein receptors, and signaling molecules). We review the metabolic pathways of cholesterol in the CNS and its cell-specific and microdomain-specific interaction with other pathways such as the amyloid precursor protein and discuss potential treatment strategies as well as the effects of the widespread use of LDL cholesterol-lowering drugs on brain functions.

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.

Physiopathological function of hematoside (GM3 ganglioside)

Since I was involved in the molecular cloning of GM3 synthase (SAT-I), which is the primary enzyme for the biosynthesis of gangliosides in 1998, my research group has been concentrating on our efforts to explore the physiological and pathological implications of gangliosides especially for GM3. During the course of study, we demonstrated the molecular pathogenesis of type 2 diabetes and insulin resistance focusing on the interaction between insulin receptor and gangliosides in membrane microdomains and propose a new concept: Life style-related diseases, such as type 2 diabetes, are a membrane microdomain disorder caused by aberrant expression of gangliosides. We also encountered an another interesting aspect indicating the indispensable role of gangliosides in auditory system. After careful behavioral examinations of SAT-I knockout mice, their hearing ability was seriously impaired with selective degeneration of the stereocilia of hair cells in the organ of Corti. This is the first observation demonstrating a direct link between gangliosides and hearing functions.

Inhibition of ganglioside biosynthesis as a novel therapeutic approach in insulin resistance

A new concept "Life style-related diseases, such as type 2 diabetes, are a membrane microdomain disorder caused by aberrant expression of gangliosides" has arisen. By examining this working hypothesis, we demonstrate the molecular pathogenesis of type 2 diabetes and insulin resistance focusing on the interaction between insulin receptor and gangliosides in microdomains microdomains and propose the new therapeutic strategy "membrane microdomain ortho-signaling therapy".

Lipid-bound sialic acid in alcoholics participates in increased level of total sialic acid

Serum total sialic acid (TSA) concentration is a sensitive marker of excessive alcohol consumption and is the sum of protein-bound sialic acid, lipid-bound sialic acid (LSA), and free sialic acid. The LSA is the fraction of SA attached to gangliosides that are transported in the blood by the lipoproteins. In this article, the effect of chronic alcohol consumption on the serum levels of LSA was evaluated. The objective of the study was to understand the mechanism of elevated serum TSA concentration during alcohol abuse. Additionally, the association of LSA with serum lipid profile was tested. For this purpose, the levels of LSA, TSA, lipids, lipoproteins, and apolipoproteins (apos) in the sera of 106 alcoholics were measured. The serum level of LSA in alcohol abusers was significantly elevated. This increase was because of the elevated level of LSA in patients drinking alcohol up to 2 days before sampling. The elevated level of LSA positively correlated with TSA, and also with biochemical indices of hepatocellular injury such as aspartate aminotransferase and gamma-glutamyltransferase, but did not correlate with any lipids, apos, and lipoproteins. The increase in LSA level is not related with the status of serum lipid profile but is related to the liver status estimated by the biochemical markers of liver cell damage. On the basis of our results, we conclude that the elevated level of LSA in alcohol abusers contributes to an increase in the serum concentration of TSA, and contrary to TSA, is affected by the status of liver cells.

Absence of oligodendroglial glucosylceramide synthesis does not result in CNS myelin abnormalities or alter the dysmyelinating phenotype of CGT-deficient mice

To examine the function of glycosphingolipids (GSLs) in oligodendrocytes, the myelinating cells of the central nervous system (CNS), mice were generated that lack oligodendroglial expression of UDP-glucose ceramide glucosyltransferase (encoded by Ugcg). These mice (Ugcg(flox/flox);Cnp/Cre) did not show any apparent clinical phenotype, their total brain and myelin extracts had normal GSL content, including ganglioside composition, and myelin abnormalities were not detected in their CNS. These data indicate that the elimination of gangliosides from oligodendrocytes is not detrimental to myelination. These mice were also used to asses the potential compensatory effect of hydroxyl fatty acid glucosylceramide (HFA-GlcCer) accumulation in UDP-galactose:ceramide galactosyltransferase (encoded by Cgt, also known as Ugt8a) deficient mice. At postnatal day 18, the phenotypic characteristics of the Ugcg(flox/flox);Cnp/Cre;Cgt(-/-) mutants, including the degree of hypomyelination, were surprisingly similar to that of Cgt(-/-) mice, suggesting that the accumulation of HFA-GlcCer in Cgt(-/-) mice does not modify their phenotype. These studies demonstrate that abundant, structurally intact myelin can form in the absence of glycolipids, which normally represent over 20% of the dry weight of myelin.

HDL lipids and insulin resistance

There is renewed interest in high-density lipoproteins (HDLs) due to recent findings linking atherosclerosis to the formation of dysfunctional HDL. This article focuses on the universe of HDL lipids and their potential protective or proinflammatory roles in vascular disease and insulin resistance. HDL carries a wide array of lipids including sterols, triglycerides, fat-soluble vitamins, and a large number of phospholipids, including phosphatidylcholine, sphingomyelin, and ceramide with many biological functions. Ceramide has been implicated in the pathogenesis of insulin resistance and has many proinflammatory properties. In contrast, sphingosine-1-phosphate, which is transported mainly in HDL, has anti-inflammatory properties that may be atheroprotective and may account for some of the beneficial effects of HDL. However, the complexity of the HDL lipidome is only beginning to reveal itself. The emergence of new analytical technologies should rapidly increase our understanding of the function of HDL lipids and their role in disease states.

Direct binding of gangliosides to Helicobacter pylori vacuolating cytotoxin (VacA) neutralizes its toxin activity

Gangliosides are target receptors for bacterial entry, yet those present in human milk exhibit a protective role against bacterial infection. Here, we show that treatment with ganglioside mixture at a concentration of 100 microg/mL resulted in significant inhibition of the vacuole formation activity of Helicobacter pylori vacuolating cytotoxin (VacA) in gastric epithelial cancer AZ-521 cells. All gangliosides (GM1, GM2, GM3, GD1a, GD1b, GD3 and GT1b) examined showed good neutralizing capacity against VacA. A pull-down assay was performed using lyso-GM1 coupled to Sepharose as the tagged polysaccharide polymer to capture VacA from H. pylori culture supernatant. GM1-VacA complexes were successfully precipitated, suggesting that GM1 binds directly to VacA. The hydrodynamic binding of lyso-GM1 and VacA measured by fluorescence correlation spectroscopy had a K(d) value of 190 nM. VacA also bound to lyso-GM1 at pH 2 corresponding to the physiological pH of human stomach. Collectively, these results showed that direct binding of H. pylori VacA to free gangliosides neutralizes the toxin activity of VacA. These findings offer an alternative insight into the role of gangliosides in VacA toxicity and the pathogenesis of H. pylori.