Glycosphingolipids: Structure, Localization and Metabolism |
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Glycosphingolipids (GSLs), amphipathic compounds consisting
of sugar and ceramide moieties, are ubiquitous components of
the plasma membrane of all vertebrate cells. GSLs are considered
to be receptors for microorganisms and their toxins, modulators
of cell growth and differentiation, and organizers of cellular
attachment to matrices. More than 400 species of GSLs possessing
different sugar structures have been reported, although only
seven monosaccharides have mainly been found in vertebrate GSLs.
The recent discovery of a novel derivative of sialic acid, deaminated
neuraminic acid (KDN), and a series of KDN-containing GSLs indicates
that novel and unusual structures of GSLs will likely be revealed
in future through further technological innovation. It is interesting
to note that alpha-galactosylceramide, which has never been found
in mammals, has been identified as a ligand for NKT cells. This
suggests that very minor but important GSLs are still to be demonstrated
in mammalian cells. GSLs show heterogeneity not only in their
sugar chain but also in their ceramide moieties. The biological
significance of ceramide heterogeneity is still not well understood.
However, the structure of ceramide, 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 ceramide derived from GSLs, like sphingomyelin,
could mediate intracellular signal transduction. The development of monoclonal antibodies specific to various GSLs has revealed detailed distribution of GSLs in tissues and on the cell surface. For example, in primary cultures of rat cerebellum, different and specific ganglioside (sialic acid-containing GSLs) species were found to be expressed on the surface of neurons, astrocytes and oligodendrocytes. GSLs show a non-uniform distribution in the plasma membrane, being restricted to the outer leaflet in which GSLs usually cluster together to form patches, although the possible presence of free GSLs in the cytosol still cannot be ruled out. Recently, GSLs were found to be maldistributed in cavities, so called 'caveolae,' on the exoplasmic membrane, where signal transduction-related proteins are also concentrated. This suggests that GSLs could mediate the signal transduction pathway through interaction with these signaling proteins. GSLs not only circulate between the plasma membrane and intracellular organs, but also move laterally over the exoplasmic membrane. Such migration could be conducted by the cholesterol-rich unit known as a 'raft.' In the pathway of GSL synthesis, the first step is transfer of glucose or galactose to a ceramide to produce glucosylceramide (GlcCer) or galactosylceramide (GalCer), respectively. This transfer reaction is catalyzed by UDP glucose: ceramide: glucosyltransferase (GlcT) and UDP-galactose:ceramide:galactosyltransferase (GalT), respectively. GlcT is completely different from GalT in both primary structure and localization. GlcT is a typical type III glycosyltransferase which possesses the transmembrane domain at its N-terminal, while GalT is a type I with the transmembrane domain at the C-terminal. The catalytic domain of GlcT is located on the cytosolic side of the Golgi membrane, while that of GalT is on the lumen side of the endoplasmic reticulum. It is of interest that the two enzymes show no homology, although GalT shows high homology with glucuronic acid transferase. mRNA of GlcT is ubiquitous in mammalian tissues, and GlcT gene seems to be a housekeeping gene. In contrast, the expression of RNA for GalT is restricted to specific organs such as the brain and kidney. The rule of extension of sugar chains for GSLs is common in all mammals, i.e. a monosaccharide is sequentially transferred to a GlcCer or GalCer from a nucleotide sugar by one of a series of specific glycosyl transferases, all of which are localized on the lumen side of the Golgi membrane. Thus GlcCer produced on the cytosolic side of the Golgi membrane must be transferred (flip-flopped) to the lumen side by a putative enzyme, 'flippase,' which has not yet been characterized. After recycling between the plasma membrane and intracellular organs, GSLs are finally transported to lysosomes where they are all hydrolyzed sequentially from the non-reducing end by exo-type glycosylhydrolases. All the hydrolysis reactions of GSLs in vivo seem to require specific activator proteins, although these can be replaced by certain detergents in vitro. The balance of synthesis and degradation of GSLs is completely regulated in the cell. If a glycosylhydrolase is lacking due to genetic deficiency, a GSL accumulates in the lysosomes and causes a serious disease. Although several disorders of GSL metabolism due to a lack of either a specific glycosidase or an activator protein have been elucidated, no specific disorder attributable to a lack of glycosyltransferase has yet been found. The biological functions of GSLs as well as the mechanism that regulates GSL metabolism in cells seems to still be a true mystery of the Sphinx, which is in fact the origin of the word 'Sphingolipid.' The recent remarkable progress that has been made in the gene cloning of glycosyltransferases and glycosylhydrolase should help provide an answer to this mystery. |
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| Makoto Ito (Faculty of Agriculture, Kyushu University) | |||||||||||
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| Jun.15, 1998 | |||||||||||
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