Metabolism of glucosyl [13H]ceramide by human skin fibroblasts from normal and glucosylceramidotic subjects

Glucosylceramide transfer from lysosomes--the missing link in molecular pathology of glucosylceramidase deficiency: a hypothesis based on existing data

Gaucher disease (GD), deficiency of acid glucosylceramidase (GlcCer-ase) is characterized by deficient degradation of beta-glucosylceramide (GlcCer). It is well known that, in GD, the lysosomal accumulation of uncleaved GlcCer is limited to macrophages, which are gradually converted to storage cells with well known cytology--Gaucher cells (GCs). On the basis of previous studies of the disorder and of a comparison with other lysosomal enzymopathies affecting degradation of the GlcCer-based glycosphingolipid series, it is hypothesized that in other cell types (i.e. non-macrophage cells) the uncleaved GlcCer, in GlcCer-ase deficiency, is transferred to other cell compartments, where it may be processed and even accumulated to various degrees. The consequence of the abnormal extralysosomal load may differ according to the cell type and compartment targeted and may be influenced by genetically determined factors, by a number of acquired conditions, including the current metabolic situation. The sequelae of the uncleaved GlcCer extralysosomal transfer may range from probably innocent or positive stimulatory, to the much more serious, in which it interferes with a variety of cell functions, and in extreme cases, can lead to cell death. This alternative processing of uncleaved GlcCer may help to explain tissue alterations seen in GD that have, so far, resisted explanation based simply on the presence of GCs. Paralysosomal alternative processing may thus go a long way towards filling a long-standing gap in the understanding of the molecular pathology of the disorder. The impact of this alternative process will most likely be inversely proportional to the level of residual GlcCer-ase activity. Lysosomal sequestration of GlcCer in these cells is either absent or in those exceptional cases where it does occur, it is exceptional and rudimentary. It is suggested that paralysosomal alternative processing of uncleaved GlcCer is the main target for enzyme replacement therapy. The mechanism responsible for GlcCer transfer remains to be elucidated. It may also help in explaining the so far unclear origin of glucosylsphingosine (GlcSph) and define the mutual relation between these two processes.

High-resolution loading tests in the study of genetic heterogeneity in gangliosidosis fibroblasts

GM1- and GM2-gangliosides were isolated from brain and radiolabelled. The labelled moieties were localized by hydrolysis with lysosomal enzymes, followed by thin-layer chromatography of the products. High-resolution loading tests with labelled gangliosides were developed and found to differentiate infantile and juvenile forms of GM1- and GM2-gangliosidoses as well as the identification of B, O and AB types of GM2-gangliosidosis.

Administration of pyrene lipids by receptor-mediated endocytosis and their degradation in skin fibroblasts

Sphingomyelin and seven glycosphingolipids were labeled with the fluorescent probe pyrene and administered into cultured fibroblasts by receptor-mediated endocytosis. For this purpose pyrene sphingomyelin or mixtures of pyrene glycolipid and unlabeled sphingomyelin were dispersed as small, unilamellar liposomes. Apolipoprotein E was then added and the receptor for this ligand on the cell surface was utilized for uptake of the liposomes and their transport to the lysosomes, where the respective pyrene lipids were degraded. Following incubation with each of the respective pyrene lipids, only the administered compound and the pyrene ceramide were present; intermediate hydrolysis products were not detected. This indicated that, in skin fibroblasts, the lysosomal ceramidase was limiting and controlled the rate of total degradation of the pyrene sphingolipids.

Acid beta-glucosidase: enzymology and molecular biology of Gaucher disease

Human lysosomal beta-glucosidase (D-glucosyl-acylsphingosine glucohydrolase, EC 3.2.1.45) is a membrane-associated enzyme that cleaves the beta-glucosidic linkage of glucosylceramide (glucocerebroside), its natural substrate, as well as synthetic beta-glucosides. Experiments with cultured cells suggest that in vivo this glycoprotein requires interaction with negatively charged lipids and a small acidic protein, SAP-2, for optimal glucosylceramide hydrolytic rates. In vitro, detergents (Triton X-100 or bile acids) or negatively charged ganglioside or phospholipids and one of several "activator proteins" increase hydrolytic rate of lipid and water-soluble substrates. Using such in vitro assay systems and active site-directed covalent inhibitors, kinetic and structural properties of the active site have been elucidated. The defective activity of this enzyme leads to the variants of Gaucher disease, the most prevalent lysosomal storage disease. The nonneuronopathic (type 1) and neuronopathic (types 2 and 3) variants of this inherited (autosomal recessive) disease but panethnic, but type 1 is most prevalent in the Ashkenazi Jewish population. Several missense mutations, identified in the structural gene for lysosomal beta-glucosidase from Gaucher disease patients, are presumably casual to the specifically altered posttranslational oligosaccharide processing or stability of the enzyme as well as the altered in vitro kinetic properties of the residual enzyme from patient tissues.

GM2-ganglioside metabolism in cultured human skin fibroblasts: unambiguous diagnosis of GM2-gangliosidosis

The metabolism of GM2-ganglioside was studied in situ using cultured skin fibroblasts from normal individuals and patients with different forms of GM2-gangliosidosis. [3H]Sphingosine-labeled GM2 was provided in the culture medium to confluent cells in 6-cm petri dishes. After 10 days, the cells were washed free of radioactivity and harvested by trypsinization. The cellular lipids were extracted and analyzed for radioactivity in GM2 and its metabolic products. In fibroblasts from healthy subjects, 50-60% of the total cellular radioactivity was found in the neutral glycosphingolipids, ceramide, sphingomyelin and fatty acids. Degradation of the labeled GM2 progressed rapidly via GM3, ceramide dihexoside and ceramide monohexoside with a build-up of radioactivity mainly in the ceramide pool of the cell. The labeled ceramide is also reutilized for the synthesis of ceramide trihexoside, globoside and sphingomyelin or is converted to fatty acid and incorporated in ester linkages. In contrast, cells from patients with GM2-gangliosidosis representing Tay-Sachs, Sandhoff and AB variant forms of the disease did not metabolize the ingested labeled GM2-like controls. Nearly all of the radioactivity was present in the ganglioside fraction in the lipid extracts from these cells and consisted of unhydrolyzed GM2. High-performance liquid chromatographic analysis of monosialogangliosides from cells grown without added labeled GM2 in the medium indicated accumulation of endogenously synthesized GM2 in cell lines from all patients with GM2 gangliosidosis compared to healthy controls. This approach provides a reliable tool for pre- and post-natal diagnosis of all forms of GM2-gangliosidosis without ambiguity.

Synthesis of a fluorescent derivative of glucosyl ceramide for the sensitive determination of glucocerebrosidase activity

A fluorescent derivative of glucosyl ceramide was synthesized by covalently linking a fluorescent fatty acid, 12-[N-methyl-N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)] aminododecanoic acid to the amino group of sphingosyl-1-O-beta-D-glucoside, glucosyl sphingosine. For hydrolysis by glucocerebrosidase, this substrate was dispersed in mixed micelles with Triton X-100 and sodium taurocholate or in unilamellar liposomes with phosphatidylcholine and the negatively charged lipid, dicetylphosphate. In either micellar or liposomal dispersions of the fluorescent substrate, reaction rates were linear with time and protein concentration, and saturation kinetics were observed. The rate of hydrolysis of this fluorescent substrate was equal to that obtained with radiolabeled glucosyl ceramide. The fluorescent glucosyl ceramide was used to determine glucocerebrosidase activity in extracts of human leukocytes, cultured skin fibroblasts, and various tissues as well as in partially purified splenic and placental glucocerebrosidase preparations. This fluorescent derivative of the natural substrate was not hydrolyzed by aryl beta-glucosidase(s), thereby facilitating the specific and reliable diagnosis of heterozygotes and homozygotes with Gaucher disease.

Comparison of synthetic and natural glucosylceramides as substrate for glucosylceramidase assay

Commercially available [3H]glucosylceramide is derived from spleen tissue of patients with Gaucher's disease. When such tritiated glucosylceramide was diluted with unlabelled glucosylceramide from different sources and used as the substrate for assays of glucosylceramidase, the apparent activities obtained differed drastically. When diluted with synthetic N-stearoyl- or N-lignoceroyl-glucosyldihydrosphingosine, the release of [3H]glucose was 4-5 times greater than when diluted with unlabelled glucosylceramide from Gaucher spleen, with either brain or fibroblast homogenate as the enzyme source. The Ki values of the synthetic substrates were 15-30 times greater than the Km for the natural mixture, indicating much lower affinity of the synthetic substrates for the enzyme. Although the reason for the reduction in affinity could not be identified, caution is required when the commercial [3H]glucosylceramide is to be diluted with unlabelled glucosylceramide.

Synthesis of glycolipids

The chemical syntheses of naturally occurring glycolipids derived from sphingosine bases and glycerol derivatives, and the syntheses of polyisoprenoid lipid intermediates and other miscellaneous glycolipids recorded up to the end of 1977 are reviewed.

Glycosphingolipid hydrolases: properties and molecular genetics

This is a review of the properties and molecular genetics of six lysosomal hydrolases: beta-galactosidase, hexosaminidases A and B, alpha-galactosidase, beta-glucosidase and alpha-fucosidase. Each enzyme is discussed with regards to isoenzymes and substrate specificity, subunit structure, genetic relationship of isoenzymes and genetic variants. The molecular genetics of human diseases caused by deficiencies of each enzyme are discussed.

A dialysis procedure for loading erythrocytes with enzymes and lipids

A dialysis procedure for hypotonic hemolysis has been developed in which erythrocytes can be loaded with water-soluble enzymes, detergent-solubilized enzymes (glucocerebrosidase) and detergent-dispersed glycolipid (glucocerebroside). The procedure allows approx. 40-50% of the added enzyme or glycolipid to be encapsulated. The final intracellular concentration of enzyme or glycolipid is about to the extracellular concentration. The loaded cells can be ingested by macrophage in vitro and the glucocerebroside partially degraded by lysosomal glucocerebrosidase. The use of this procedure for the investogation of Gaucher's disease is discussed.

Accumulation of lactosyl ceramide in leukocytes of patients with adult Gaucher's disease

Glycosphingolipids were isolated from the leukocytes of 10 patients with Type I, chronic nonneuronopathic (adult) Gaucher's disease and 12 normal subjects, by silicic acid column chromatography and thin-layer chromatography. Quantitation of the individual glycosyl ceramides was achieved by the determination of hexose and sphingosine content, using colorimetric and fluorometric procedures. Lactosyl ceramide, which is the main glycolipid in leukocytes of normal subjects, was significantly increased in the leukocytes of patients with Gaucher's disease. On the other hand, the amount of glycosyl ceramide, which is the main glycolipid accumulating in the reticuloendothelial cells of patients with Gaucher's disease, was similar in Gaucher and in control leukocytes.

Diagnosis of lipogranulomatosis (Farber disease) by use of cultured fibroblasts

The enzyme defect in Farber disease, a deficiency of acid ceramidase, has been demonstrated in cultured skin fibroblasts, which provides a means of confirming the diagnosis during life. The assay can also be performed using cultured amniotic fluid cells and is a potential tool for detection of carriers of the disease.