Biosynthesis of gangliosides containing C18:1 and C20:1 [3-14C]sphingosine after administrating [1-14C]palmitic acid and [1-14C]stearic acid to rat cerebellar granule cells in culture

GM1 Ganglioside Is A Key Factor in Maintaining the Mammalian Neuronal Functions Avoiding Neurodegeneration

Many species of ganglioside GM1, differing for the sialic acid and ceramide content, have been characterized and their physico-chemical properties have been studied in detail since 1963. Scientists were immediately attracted to the GM1 molecule and have carried on an ever-increasing number of studies to understand its binding properties and its neurotrophic and neuroprotective role. GM1 displays a well balanced amphiphilic behavior that allows to establish strong both hydrophobic and hydrophilic interactions. The peculiar structure of GM1 reduces the fluidity of the plasma membrane which implies a retention and enrichment of the ganglioside in specific membrane domains called lipid rafts. The dynamism of the GM1 oligosaccharide head allows it to assume different conformations and, in this way, to interact through hydrogen or ionic bonds with a wide range of membrane receptors as well as with extracellular ligands. After more than 60 years of studies, it is a milestone that GM1 is one of the main actors in determining the neuronal functions that allows humans to have an intellectual life. The progressive reduction of its biosynthesis along the lifespan is being considered as one of the causes underlying neuronal loss in aged people and severe neuronal decline in neurodegenerative diseases. In this review, we report on the main knowledge on ganglioside GM1, with an emphasis on the recent discoveries about its bioactive component.

Plasma 1-deoxysphingolipids are predictive biomarkers for type 2 diabetes mellitus

OBJECTIVE

Serine palmitoyltransferase (SPT) catalyzes the condensation of serine and palmitoyl coenzyme A, the first step in the de novo sphingolipid synthesis. Apart from these canonical substrates, SPT can also metabolize alanine and other acyl coenzyme As. This forms a spectrum of atypical sphingoid bases which are altered in the context of the metabolic syndrome (MetS) and type 2 diabetes mellitus (T2DM). We investigated whether atypical sphingolipids can be used as prospective markers to predict the incidence of T2DM.

RESEARCH DESIGN AND METHODS

Using liquid chromatography/mass spectrometry, we analyzed the sphingoid base profile in a prospective cohort with 339 individuals. All individuals were followed up for a period of 8 years.

RESULTS

Confirming earlier results, we found 1-deoxysphingolipids (1-deoxySLs) to be significantly elevated in patients with MetS, impaired fasting glucose, and T2DM. Patients who developed T2DM during the follow-up period (n=32) showed significantly higher 1-deoxySL levels at baseline compared with those who did not develop T2DM until the end of the study (n=70). 1-Deoxysphingosine levels were independent predictors for T2DM even after adjusting for glycated hemoglobin (standardized adjusted OR=2.1, CI 95% (1.19 to 3.71); p=0.010), MetS (standardized adjusted OR=1.97, CI 95% (1.13 to 3.43); p=0.017), and other risk factors such as age, sex, BMI, and lipid-lowering drugs. Similar results were observed for the 1-deoxysphinganine levels.

CONCLUSIONS

Our results support a novel role for 1-deoxySL as predictive biomarkers for the development of T2DM in risk patients and warrants further larger prospective trials in lower risk cohorts.

β-amyloid (25–35) enhances lipid metabolism and protein ubiquitination in cultured neurons

We investigated the effect of beta-amyloid (Abeta) (25-35), a cytotoxic fragment of Abeta peptide, on lipid metabolism and protein ubiquitination in cultured rat hippocampal neurons. After treatment with Abeta under conditions leading to apoptotis, as assessed by caspase activity assay, the total cell mass of lipids changed following a biphasic behavior, with an increase that reached a maximum after 16 hr of treatment, followed by a decrease. The increase at 16 hr was 15.3% in the case of phospholipids and 103.0% in the case of gangliosides and was due to enhanced biosynthesis as confirmed by increase of radioactivity incorporation (phospholipids +52.0%, gangliosides +193.1%) in cells fed with tritiated palmitic acid. No change with respect to cholesterol was observed. Strikingly, under these conditions, the ubiquitination state of cell proteins strongly increased. These effects were not observed with the (35-25) reverse sequence peptide. Similarly to Abeta, lactacystin treatment increased lipid synthesis and protein ubiquitination; only lactacystin, and not Abeta, induced a strong decrease of proteasome chimotrypsin activity. These results suggest that Abeta enhances protein ubiquitination, without inhibiting proteasomal activity, and lipid synthesis. These results may shed new light on the mechanisms of Abeta toxicity.

Ethanol-induced increase of sphingosine recycling for ganglioside biosynthesis: a study performed on cerebellar granule cells in culture

The effect of ethanol on ganglioside metabolism was assessed in cultured rat cerebellar granule cells. Cells were incubated in the presence of tritiated serine or galactose, and the synthesis of radioactive gangliosides was followed. The rate of de novo biosynthesis of gangliosides labeled in the oligosaccharide moiety (deriving from tritiated galactose) was not affected by the presence of ethanol. On the contrary, the biosynthesis of gangliosides labeled in the ceramide long chain base moiety (deriving from tritiated serine), dramatically decreased in the presence of alcohol. These results suggest that the gap between the extent of the biosynthesis of lipid and polar portions observed in the presence of ethanol, is filled by an increased recycling of sphingosine produced from ganglioside degradation. This hypothesis was confirmed by pulse-chase experiments with GM1 ganglioside, tritiated in the sphingosine moiety, and following radiolabeled gangliosides deriving from its metabolic processing. In fact, the radioactivity carried by gangliosides whose labeling could derive exclusively (GD1b + GT1b) or partially (GD1a) from the recycling of catabolic radiolabeled sphingosine, dramatically increased in ethanol-treated cells during the chase period. Taken together, these results suggest that ethanol increases ceramide sphingosine recycling for ganglioside biosynthesis.

Ganglioside molecular species containing C18- and C20-sphingosine in mammalian nervous tissues and neuronal cell cultures

Gangliosides exist as a very complex mixture of species differing in both the hydrophilic and hydrophobic moieties. They are particularly abundant in the central nervous system (CNS), where they have been associated with development and maturation of the brain, neuritogenesis, synaptic transmission, memory formation and synaptic aging. Today, many data suggest that some of the effects exerted by gangliosides are due to interactions with proteins that participate in the transduction of signals through the membrane in membrane microdomains. A specific characteristic of CNS gangliosides is the structure of their long-chain base (LCB). In fact, considering all the mammalian cell sphingolipids, gangliosides, sulphatides, neutral glycosphingolipids, sphingomyelin and ceramides, it would seem that while the LCB with 18 carbons is the main component of all sphingolipids, only CNS gangliosides contain significant amounts of LCB with 20 carbons. C18-Sphingosine is always present in cell gangliosides; the individual ganglioside species containing C18-sphingosine increase during cell differentiation then remain constant during cell aging. Gangliosides containing C20-sphingosine are absent, or present only in traces, in undifferentiated cells but with the onset of cell differentiation they appear, their content slowly but continuously increasing throughout the life span. In this review we discuss the chemistry, physico-chemistry and metabolism of ganglioside species differing in LCB length and introduce the hypothesis that the varying ratio between C18- and C20-gangliosides during CNS development and aging can be instrumental in modulating membrane domain organisation and cell properties.

Conversion of dihydroceramide to ceramide occurs at the cytosolic face of the endoplasmic reticulum

Dihydroceramide desaturase is responsible for the introduction of the 4,5-trans double bond into ceramide. Here, we describe the localization of this enzyme in the endoplasmic reticulum (ER) using ER- and Golgi-enriched fractions from rat liver. Furthermore, enzyme topology was studied. Mild proteolysis of ER-derived vesicles under conditions which assure membrane integrity (latency of mannose 6-phosphatase was at least 91%) resulted in an up to 90% inactivation of dihydroceramide desaturase activity. This indicates a cytosolic orientation of dihydroceramide desaturase activity in the ER membrane.

Characterization of ceramide synthesis. A dihydroceramide desaturase introduces the 4,5-trans-double bond of sphingosine at the level of dihydroceramide

Ceramide (N-acylsphingosine) biosynthesis has been proposed to involve introduction of the 4,5-trans-double bond of sphingosine after synthesis of dihydroceramide (i.e. N-acylsphinganine). For the first time, the in vitro conversion of dihydroceramide to ceramide has been demonstrated using rat liver microsomes and N-[1-14C]octanoyl-D-erythro-sphinganine (st-H2Cer) and either NADH or NADPH as co-substrate; the apparent Km values for st-H2Cer and NADH were 340 and 120 microM, respectively. Molecular oxygen is required for enzymatic activity, and cyanide, divalent copper, as well as antibodies raised against cytochrome b5 are inhibitory, which suggests that this enzyme should be named dihydroceramide desaturase based on these similarities with the mechanism of delta9-desaturase (stearoyl-CoA desaturase). Factors that influenced the activity of dihydroceramide desaturase include the alkyl chain length of the sphingoid base (in the order C18 > C12 > C8) and fatty acid (C8 > C18); the stereochemistry of the sphingoid base (D-erythro- > L-threo-dihydroceramides); the nature of the headgroup, with the highest activity with dihydroceramide, but some (approximately 20%) activity with dihydroglucosylceramide, however); and the ability to utilize alternative reductants (ascorbic acid could substitute for a reduced pyridine nucleotide, but was inhibitory at higher concentrations). Dihydroceramide desaturase was inhibited by dithiothreitol, which suggests that it might be possible to alter ceramide synthesis by varying the thiol status of hepatocytes. Consistent with this hypothesis, when rat hepatocytes were cultured in varying concentrations of N-acetylcysteine (5 and 10 mM), there was a decrease in the relative incorporation of [14C]serine into [14C]ceramide. These studies have conclusively established the pathway of ceramide synthesis via desaturation of dihydroceramide and have uncovered several properties of this reaction that warrant further consideration for their relevance to both sphingolipid metabolism and signaling.

Age-related changes of the ganglioside long-chain base composition in rat cerebellum

The ganglioside mixture from the cerebellum of young, 6 month old and two years old rats, was fractionated by reversed phase high performance liquid chromatography, each ganglioside homogeneous in the oligosaccharide chain as well as in the long-chain base being subsequently quantified. Two long-chain bases, LCB, were components of the five major gangliosides GM1, GD1a, GD1b, GT1b and GQ1b, these being the C18:1 LCB and C20:1 LCB. The content of C20:1 ganglioside molecular species was lower than that of the C18:1 one. In very young animals, day 8, the C20:1 ganglioside species represented about 8% of the total ganglioside content, then they progressively increased and reached, at 2 years, about 42% of the total. C18:1 GD1a and C18:1 GT1b, were the major species in young animals and reached their highest content at day 29, being 1.45 and 1.28 nmol/mg protein, respectively. The content of these two species decreased in adult and old animals and at two years it was 0.71 and 0.82 nmol/mg protein, respectively.