Lipid and glycolipid mediators in CD95-induced apoptotic signaling

Synthesis of ganglioside GD3 and its comparison with bovine GD3 with regard to oligodendrocyte apoptosis mitochondrial damage

2,3-Dehydroneuraminic acid derivative 5 was transformed in five efficient steps into sialyl donor 2, which has a phenylthio group on the beta-side of the 3-position for anchimeric assistance and a diethyl phosphite residue as leaving group at the anomeric carbon. The known GM3 intermediate 10 was transformed into the 4b,4c,8c-O-unprotected acceptor 3, which was then allowed to react with 2 by using TMSOTf as catalyst and acetonitrile as solvent to afford the desired tetrasaccharide 12, which has an alpha(2-8)-linkage between two neuraminic acid residues. Removal of the phenylthio group gave intermediate 13, which was transformed into O-tetraosyl trichloroacetimidate 16 as glycosyl donor. Application of the azidosphingosine glycosylation procedure furnished GD3 (1) in high overall yield. Comparison of synthetic GD3 with bovine-brain-derived GD3 showed that there were similar effects in GD3-triggered uncoupling of mitochondrial respiration and in induction of apoptosis in oligodendrocytes.

Human antibodies against a purified glucosylceramide from Cryptococcus neoformans inhibit cell budding and fungal growth

A major ceramide monohexoside (CMH) was purified from lipidic extracts of Cryptococcus neoformans. This molecule was analyzed by high-performance thin-layer chromatography (HPTLC), gas chromatography coupled with mass spectrometry, and fast atom bombardment-mass spectrometry. The cryptococcal CMH is a beta-glucosylceramide, with the carbohydrate residue attached to 9-methyl-4,8-sphingadienine in amidic linkage to 2-hydroxyoctadecanoic acid. Sera from patients with cryptococcosis and a few other mycoses reacted with the cryptococcal CMH. Specific antibodies were purified from patients' sera by immunoadsorption on the purified glycolipid followed by protein G affinity chromatography. The purified antibodies to CMH (mainly immunoglobulin G1) bound to different strains and serological types of C. neoformans, as shown by flow cytofluorimetry and immunofluorescence labeling. Transmission electron microscopy of yeasts labeled with immunogold-antibodies to CMH and immunostaining of isolated cell wall lipid extracts separated by HPTLC showed that the cryptococcal CMH predominantly localizes to the fungal cell wall. Confocal microscopy revealed that the beta-glucosylceramide accumulates mostly at the budding sites of dividing cells with a more disperse distribution at the cell surface of nondividing cells. The increased density of sphingolipid molecules seems to correlate with thickening of the cell wall, hence with its biosynthesis. The addition of human antibodies to CMH to cryptococcal cultures of both acapsular and encapsulated strains of C. neoformans inhibited cell budding and cell growth. This process was complement-independent and reversible upon removal of the antibodies. The present data suggest that the cryptococcal beta-glucosylceramide is a fungal antigen that plays a role on the cell wall synthesis and yeast budding and that antibodies raised against this component are inhibitory in vitro.

Phospholipid signaling in apoptosis: peroxidation and externalization of phosphatidylserine

The role of phospholipids in apoptosis signaling and the relationship between oxidation of phosphatidylserine and its redistribution in the plasma membrane were studied. A novel method for detection of site-specific phospholipid peroxidation based on the use of cis-parinaric acid as a reporter molecule metabolically integrated into membrane phospholipids in living cells was employed. When several tissue culture cell lines and different exogenous oxidants were used, the relationship between the oxidation of phosphatidylserine and apoptosis has been revealed. The plasma membrane was the preferred site of phosphatidylserine oxidation in cells. It was shown that selective oxidation of phosphatidylserine precedes its translocation from the inside to the outside surface of the plasma membrane during apoptosis. A model is proposed in which cytochrome c released from mitochondria by oxidative stress binds to phosphatidylserine located at the cytoplasmic surface of the plasma membrane and induces its oxidation. Interaction of peroxidized phosphatidylserine with aminophospholipid translocase causes inhibition of the enzyme relevant to phosphatidylserine externalization.