Glycosphingolipids
From LipidomicsWiki
Contents |
Basics
Glycosphingolipids (GSLs), amphipathic compounds consisting of sugar and Ceramide(Cer) moieties, are ubiquitous components of the plasma membrane of all vertebrate cells. GSLs are considered to be receptors for bacteria, viruses and their toxins (e.g. Vibrio cholera, HIV), modulators of cell growth and differentiation, organizers of cellular attachment to matrices, play roles in cell-cell adhesion and receptor mediated signal transduction. Further glycolipids have been identified as tumor or differentiation antigens using monoclonal antibodies. More than 400 species of GSLs possessing different sugar structures have been reported, although only seven monosaccharides have mainly been found in vertebrate GSLs. GSLs show heterogeneity not only in their sugar chain but also in their Cer moieties. The biological significance of Cerheterogeneity is still not well understood. However, the structure of Cer, especially the fatty acid moieties, could influence the localization and functions of GSLs on the plasma membrane, possibly by direct interaction with cholesterol, phospholipids, and the transmembrane domains of receptor proteins. It is noteworthy that free Cerderived from GSLs, could mediate intracellular signal transduction.
Structures
LIPID MAPS sphingolipids classes and subclasses
Natural sources
Nomenclature
Sphingolipid classes and subclasses
Biophysical Properties
Biosynthesis
GSL de novo biosynthesis starts with the formation of Cer to which the individual saccharide units are linked one after the other by stepwise additions. The Cer assigned to higher GSLs is glucosylated by a glucosyl-transferase [Paul et al., 1996], whereas that assigned to sphingomyelin flip-flops to the lumenal side of the stack, where its conversion to sphingomyelin occurs [Diringer et al., 1972].
The Cer moiety acting as a precursor of Gal-ceramide and sulphatide remains in the endoplasmic reticulum, turns to the lumenal side of the membrane, is galactosylated by the action of a ceramide galactosyl-transferase [Schulte et al., 1993], and then transferred to the lumenal Golgi stack. Here, it is submitted to sulphation by a sulphate transferase involving PAPS (3'-phosphoadenosine, 5'-phosphosulphate) [Sunderam et al., 1992]. Probably, Gal-ceramide is sialosylated to GM4 ganglioside at the same site by the action of a sialyl-transferase (SAT II) [Kolter et al., 2002].
Glc-ceramide turns, by a yet uncharacterized flippase, to the lumenal side of the cis-Golgi stack, where further glycosylations take place (see the scheme of ganglioside biosynthesis). The first glycosylation, specifically catalysed by lactosyl-ceramide synthase is galactosylation of Glc-ceramide to lactosyl-ceramide (Lac-ceramide) [Nomurova et al., 1998]. Lac-ceramide is sialosylated to GM3 ganglioside, GM3 to GD3, and GD3 to GT3. The three involved sialyl-transferase (SAT I, SAT II and SAT III) specifically recognize the acceptor substrate [Huwiler et al., 2000; Kolter et al., 2002]. GM3, GD3 and GT3 are the starting points for the "a-series", "b-series" and "c-series" gangliosides, respectively. Along each series, non-specific N-acetylgalactosaminyl-transferase, galactosyl-transferase and sialyl-transferase (SAT IV) introduce in sequence a residue of N-acetylgalactosamine, galactose and sialic acid, respectively, giving rise to more complex gangliosides. Further sialosylations can be accomplished by sialosyl-transferase V (SAT V). From lact-ceramide, a further series of glycosphingolipids ("O-series") can originate from the sequential action of N-acetylgalactosaminyl-transferase, galactosyl-transferase and sialyl-transferase IV and V, producing GA2 (asialo-GM2), GA1 (asialo-GM1), and gangliosides GM1b, GD1c and GD1.
It should be noted that SAT I, IV and V catalyse the formation of 2 3 sialosyl linkage to galactose, SAT II and III of 2 8 sialosyl linkage to sialic acid, and a yet uncharacterized sialyl-transferase (SAT X, possibly SAT V) of 2 6 sialosyl linkage to N-acetylgalactosamine. There is some evidence for a gradient distribution of the glycosyl-transferases in the Golgi system, with earlier glycosylations prevailing in the cis/medial Golgi and later glycosylations in the trans-Golgi/trans -Golgi network [Kolter et al., 2002]. This would implicate a flow (possibly vesicular) of intermediates from one Golgi stack to the following one (as in the case of glycoproteins). However, there are cases of glycosyl-transferases that constitute multi-component complexes [Giraudo et al., 2001] where the product of the first enzyme is immediately processed by the following enzyme till it reaches the final product. This evidence would support the hypothesis [Roseman 1970] that a multi-glycosyl-transferase complex is responsible for the synthesis of each individual ganglioside.
Subtext: De novo biosynthesis of the oligosaccharide moieties of gangliosides. Sialyl-transferases I, II and III (SAT I, SAT II, SAT III) specifically catalyse the reaction where they are placed; galactosyl-transferase I (GalTI) specifically catalyses the reaction Glc-Cer Gal-Glc-Cer; galactosyl-transferase II (GalTII) catalyses all the corresponding reactions horizontally placed on its line; N-acetylgalactosaminyl-transferase (GalNAcT) catalyses all the corresponding reactions horizontally placed on its line; sialyl-transferase IV and V (or X) (SAT IV and SAT V, or SAT X) catalyse all the reactions horizontally placed on their line, respectively. All formed linkages are -linkages, with the exception of sialyl linkages, that are .
The final products of glycosphingolipid biosynthesis are assumed to leave the Golgi stacks, or the trans-Golgi network, as budding vesicles and to reach and fuse with the plasma membrane. The fact that the saccharide chains of glycosphingolipids are built up in the lumenal side of Golgi membranes and exposed on the external leaflet of the plasma membrane favours the hypothesis of a vesicular transport of glycosphingolipids from the Golgi apparatus to the plasma membrane.
Metabolism
The biosynthesis of gangliosides (and glycosphingolipids in general) takes place on intracellular membranes (endoplasmic reticulum; Golgi apparatus) and is catalysed by membrane-bound enzymes. Transport of de novo produced compounds to the plasma membrane occurs via vesicles following the exocytotic membrane flow. Ganglioside (and glycosphingolipid, in general) degradation proceeds on the endocytotic route within the acidic compartments of the cell (late endosomes, lysosomes) by the action of hydrolytic enzymes (mostly soluble) assisted by activatory proteins and negatively charged lipids [Huwiler, 2000; Kolter 2002].
Degradation of GSL
Ganglioside catabolism consists of the sequential removal of individual sugar residues, starting from the non-reducing terminal unit, by(exo)glycohydrolases with the formation of ceramide, which is eventually split into long chain base and fatty acid by ceramidase [Huwiler et al., 2000; Kolter et al., 2002; Gatt 1966]. The flux of degradation occurs through the endocytosis-endosome-lysosome pathway, and all the enzymatic steps of the degradative process require an acidic pH inside the organelle. This condition is warranted by the action of a proton pump that brings H+ inside the organelle [Huwiler et al., 2000]. Since intralysosomal glycohydrolases are soluble and the inner face of the lysosomal membrane is rich in glycoconjugates resistant to these enzymes, it has been postulated that after endocytosis, a vesiculation process occurs at the level of the endosomal membrane leading to the formation of invaginating vesicles carrying gangliosides on their external layer. After endosome fusion with lysosomes, the gangliosides are exposed in these vesicles toward the lysosomal matrix, where the soluble glycohydrolases are located and are thus available for degradation [Huwiler et al., 2000]. The sequence of sugar removal from gangliosides is as follows (see the scheme given in Fig. 3). First, multi-sialogangliosides are transformed by sialidase to the corresponding mono-sialogangliosides GM1, and GM2, or Lac-ceramide (from GM3). From GM1, galactose is then removed to produce GM2, and from GM2, the N-acetylgalactosamine residue split off to form GM3, by the action of a (non-specific) β-galactosidase and β-N-acetylhexosaminidase, respectively. In some cells and animals, sialic acid is removed from GM1 and GM2 by a specific sialidase producing the corresponding asialoderivatives GA1 and GA2, that by the action of β-galactosidase and β-N-acetylhexosaminidase, are converted to Lac-ceramide by a double or single reaction. Lac-ceramide is then degraded to ceramide by the sequential action of a β-galactosidase and β-glucosidase. In vivo, intralysosomal degradation of most, if not all, glycosphingolipids requires, besides exoglycohydrolases, effector molecules, of protein nature, named "sphingolipid activator proteins (SAPs, or saposines)" [Huwiler et al., 2000]. Some saposines originate from a common precursor protein (prosaposin) by a selective proteolytic cleavage [Huwiler et al., 2000].
An alternative pathway for ganglioside (and, in general, glycosphingolipid) degradation consists of the splitting of the β-glucosidic linkage between glucose and ceramide, with the formation of ceramide and the oligosaccharide (Fig. 41). The enzymes catalysing this reaction, named "endoglycoceramidases" or "ceramide glycanases" [Ito et al., 1986; Zhou et al., 1989] appear to require, or to be markedly activated, by the presence of specific activator protein(s), seemingly soluble, whose action would be essential under in vivo conditions [Ito et al., 1991]. Endoglycoceramidases have been found to occur in some bacteria [Ito et al., 1986] and leeches [Zhou et al., 1989]. Although described to occur in lactating mammary glands of rodents [Basu et al., 1997], the presence of this enzyme in vertebrate, and, particularly, mammalian tissues, is yet to be definitely assessed.
