Ganglioside Metabolism in Health and Disease

Gangliosides (GGs) are cell type-specific sialic acid-containing glycosphingolipids (GSLs), which are enriched in mammalian brain. Defects in GSL metabolism cause fatal human diseases. GSLs are composed of a hydrophilic oligosaccharide linked in 1-O-position to a hydrophobic ceramide anchor, which itself is composed of a long-chain amino alcohol, the sphingoid base, and an amide-bound acyl chain. Biosynthesis of mammalian GGs and other GSLs starts with the formation of their hydrophobic ceramide anchor in the endoplasmic reticulum, followed by sequential glycosylation reactions along the secretory pathway, mainly at the luminal surface of Golgi and trans-Golgi network (TGN) membranes. Few membrane-anchored and often promiscuous glycosyltransferases allow the formation of cell type-specific glycolipid patterns in a combinatorial process. Inherited defects of these transferases therefore affect not only single structures but defined glycolipid series. GGs and other GSLs are thereafter transported by an exocytotic membrane flow to the plasma membrane where they are expressed in cell type-specific patterns, which can be modified by metabolic reactions at or near the cellular surface. Endocytosed (glyco)sphingolipids are degraded, together with other membrane lipids in a stepwise fashion by endolysosomal enzymes with the help of small lipid-binding proteins, the sphingolipid activator proteins (SAPs), at the surface of intraluminal lysosomal vesicles. Inherited defects in a sphingolipid-degrading enzyme or SAP cause the accumulation of the corresponding lipid substrates. Endolysosomal GSL degradation is strongly modified by the lipid components of the organelle microenvironments.