A historical perspective of the glycosphingolipids and sphingolipidoses

Ceramides and other bioactive sphingolipid backbones in health and disease: lipidomic analysis, metabolism and roles in membrane structure, dynamics, signaling and autophagy

Sphingolipids are comprised of a backbone sphingoid base that may be phosphorylated, acylated, glycosylated, bridged to various headgroups through phosphodiester linkages, or otherwise modified. Organisms usually contain large numbers of sphingolipid subspecies and knowledge about the types and amounts is imperative because they influence membrane structure, interactions with the extracellular matrix and neighboring cells, vesicular traffic and the formation of specialized structures such as phagosomes and autophagosomes, as well as participate in intracellular and extracellular signaling. Fortunately, "sphingolipidomic" analysis is becoming feasible (at least for important subsets such as all of the backbone "signaling" subspecies: ceramides, ceramide 1-phosphates, sphingoid bases, sphingoid base 1-phosphates, inter alia) using mass spectrometry, and these profiles are revealing many surprises, such as that under certain conditions cells contain significant amounts of "unusual" species: N-mono-, di-, and tri-methyl-sphingoid bases (including N,N-dimethylsphingosine); 3-ketodihydroceramides; N-acetyl-sphingoid bases (C2-ceramides); and dihydroceramides, in the latter case, in very high proportions when cells are treated with the anticancer drug fenretinide (4-hydroxyphenylretinamide). The elevation of DHceramides by fenretinide is befuddling because the 4,5-trans-double bond of ceramide has been thought to be required for biological activity; however, DHceramides induce autophagy and may be important in the regulation of this important cellular process. The complexity of the sphingolipidome is hard to imagine, but one hopes that, when partnered with other systems biology approaches, the causes and consequences of the complexity will explain how these intriguing compounds are involved in almost every aspect of cell behavior and the malfunctions of many diseases.

The egghead gene is required for compartmentalization in Drosophila optic lobe development

The correct targeting of photoreceptor neurons (R-cells) in the developing Drosophila visual system requires multiple guidance systems in the eye-brain complex as well as the precise organization of the target area. Here, we report that the egghead (egh) gene, encoding a glycosyltransferase, is required for a compartment boundary between lamina glia and lobula cortex, which is necessary for appropriate R1-R6 innervation of the lamina. In the absence of egh, R1-R6 axons form a disorganized lamina plexus and some R1-R6 axons project abnormally to the medulla instead of the lamina. Mosaic analysis demonstrates that this is not due to a loss of egh function in the eye or in the neurons and glia of the lamina. Rather, as indicated by clonal analysis and cell-specific genetic rescue experiments, egh is required in cells of the lobula complex primordium which transiently abuts the lamina and medulla in the developing larval brain. In the absence of egh, perturbation of sheath-like glial processes occurs at the boundary region delimiting lamina glia and lobula cortex, and inappropriate invasion of lobula cortex cells across this boundary region disrupts the pattern of lamina glia resulting in inappropriate R1-R6 innervation. This finding underscores the importance of the lamina/lobula compartment boundary in R1-R6 axon targeting.

Recent advances in the biochemistry and genetics of sphingolipidoses

Sphingolipidoses are a subgroup of lysosomal storage diseases. They are defined as disorders caused by a genetic defect in catabolism of sphingosine-containing lipids. Catabolism of these lipids involves enzymes and activator proteins. After the discovery of lysosomes by de Duve and the demonstration of the first defective lysosomal enzyme by Hers in 1963, the first enzyme deficiency for sphingolipidoses was characterized in 1965 and all the defective enzymes were demonstrated in the last three decades. In 1984, the first activator protein was found and it expanded the concept of sphingolipidoses. In the following years, many researches have been undertaken to understand the molecular basis of these diseases, the mechanism of pathogenesis, the mechanism of lysosomal digestion of glycosphingolipids (GSLs) and the functional domains of lysosomal enzymes. New hypotheses and theories have been put forward for the mechanism of lysosomal digestion and pathogenesis. However, although much has been done, the pathogenesis of sphingolipidoses has not been fully elucidated. Mouse models of these diseases have facilitated the elucidation of pathogenesis and the development of therapeutic strategies for these diseases, which are not treatable at present except for Fabry and type 1 Gaucher disease. The purpose of this review is to collect information on the recent researches related to sphingolipidoses. The review includes the hydrolysis of GSLs in lysosome, mechanism of hydrolysis, pathogenesis and genetics of sphingolipidoses, a brief mouse model and therapeutic strategies of these diseases.