Combinatorial Ganglioside Biosynthesis*

Ganglioside Structure and Function Sphingolipids are building blocks of eukaryotic cell surfaces (1). They anchor lipid-bound carbohydrates to the plasma membrane and contribute to the epidermal water permeability barrier of the skin. Glycolipid patterns can reach a high degree of complexity that constitutes a paradigm of combinatorial biosynthesis. During combinatorial biosynthesis, a variety of biomolecules is generated by the combination of different building blocks (2). A paradigm for this process is the humoral immune response that relies on somatic recombination of gene fragments encoding proteins of the immunoglobulin family. In contrast to the formation of antibodies, the formation of the glycolipid variety is controlled at the transcriptional and posttranscriptional level. Glycolipid patterns are generated by only a few enzymes that are expressed in a cell typeand development-dependent fashion. The limited substrate specificity of some of the glycosyltransferases involved allows combination of the carbohydrate building blocks. On the other hand, the large number of theoretically possible glycolipid structures that can be formed by combination of different carbohydrate building blocks, linkage positions, and stereochemistry is realized only to a certain extent. Only a few glycolipid series are actually found in evolutionary related species. A series is defined by sugar sequences that are part of a common carbohydrate core. Gangliosides are acidic glycosphingolipids that contain one or more sialic acid residues and are particularly prevalent on neuronal cells. They belong to the so-called ganglio-, lacto-, and neolacto-series (Fig. 1). Other glycolipid series found in human tissues are the globo-, isoglobo-, and muco-series, which are all derived from lactosylceramide (Fig. 1). Sialic acids can be attached to several positions within these core structures but also to galactosylceramide, a glycolipid of the gala-series. Additional heterogeneity in the glycolipids arises from variations within the ceramide moiety, which are due to different alkyl chain lengths, the degree of hydroxylation, and desaturation. Functional consequences of heterogeneities in the lipid component have been discussed (3). Moreover, the sialic moiety can be heterogeneous with respect to modifications of the N-acetylneuraminic acid by O-acetylation, N-glycolylation in some species, lactonization, and others (4). Glycolipid patterns vary between different cell types and change with the differentiation of the cell (5). Differences in glycolipid composition have even been found between different neuronal cell types (6). For example, 1,3-N-acetylglucosaminyltransferase expression, which leads to the formation of glycolipids of the lactoand neolacto-series, is highly regulated during embryonic development in mice and decreases after birth to undetectable levels in most cell types (7). In adult cerebellum, it is restricted to Purkinje cells (8). The biological function of this molecular diversity is unclear, but the vital role of glycosphingolipids has been demonstrated by the fact that their complete deficiency in mice leads to early embryonic lethality (see below). Gangliosides seem to be involved in cell-to-cell interactions (9) and are also reported to regulate the activities of receptor tyrosine kinases in the plasma membrane such as the receptors of epidermal growth factor (10), nerve growth factor (11), and insulin (12) and in this way might regulate cell signaling. Gangliosides are not homogeneously distributed on the cell surface; together with glycosylphosphatidylinositol-anchored proteins, sphingomyelin, and cholesterol, they can segregate into membrane domains (rafts), which might be the physiological surroundings of many membrane proteins (13). Several reports indicate that physiological processes like embryogenesis and differentiation of neuronal cells and leukocytes might be influenced by gangliosides (for review, see Ref. 14). Just as with their functions, the mechanisms by which glycosphingolipid diversity is controlled are also beginning to be understood. We report on the principles of the ganglioside-synthesizing machinery and on recent findings from genetically engineered animals that shed light on regulation and function of these complex lipid patterns.