Lectin-mediated uptake of lysosomal hydrolases by genetically deficient human fibroblasts

Enzyme replacement in Fabry endothelial cells and fibroblasts: uptake experiments and electron microscopical studies

Endothelial cells are of particular interest for therapeutic strategies in Fabry's disease, because the accumulation of glycosphingolipids in the vascular endothelium as a result of alpha-galactosidase A (alpha-galA) deficiency is responsible for the major clinical manifestations of the disease. Electron microscopical observations of cultured endothelial cells obtained from the umbilical vein of a hemizygous Fabry fetus showed that the glycosphingolipids are deposited as lamellar material in the lysosomes, as has been found previously for cultured fibroblasts and many different tissues. Mannose 6-phosphate (man 6-P)-receptor mediated and Concanavalin A (ConA)-mediated uptake of purified alpha-galA was attempted in the endothelial cells as well as in cultured fibroblasts from the same fetus. Our results on high-uptake alpha-galA indicate that the endothelial cells do not internalize alpha-galA via the man 6-P receptor. Immunofluorescence studies after addition of the receptor antibody to the cells support the theory that they have no or very few man 6-P receptors on the surface. Morphological studies did not show lysosomal changes which could suggest that the enzyme is taken up into the endothelial cells; however, we found reproducible modifications of the lysosomes in Fabry fibroblasts after incubation with high-uptake alpha-galA. Cell-associated alpha-galA activity was found in both cell types, when the enzyme was added to cells preincubated with ConA; but the lectin treatment by itself induced considerable ultrastructural changes in the cytoplasm, which obscured a possible effect by the enzyme.

ConA-mediated binding and uptake of purified alpha-galactosidase A in Fabry fibroblasts

In most human tissues there are at least two different alpha-galactosidases, A and B. The former is deficient in patients hemizygous for Fabry disease. We have isolated it from human placenta and found that it was labile even at culture conditions, but was stabilized after binding to concanavalin A (conA). The alpha-galactosidase activity was markedly increased in Fabry fibroblasts when these were treated with conA and exposed to alpha-galA at 37 degrees C. The maximum activity was obtained after 1/2-2 h of incubation and was maintained for at least 4 h. The binding and uptake of conA into Fabry cells was followed by microscopical studies of fluorescein-labelled conA. We assume that alpha-galA is taken up by endocytosis of the enzyme-conA complex.

Use of lectins and polyethylene glycol for fusion of glycolipid-containing liposomes with eukaryotic cells

Efficient fusion of phospholipid vesicles with monolayer cultures of eukaryotic cells was accomplished by attaching glycolipid-containing vesicles to the cell surface by using a lectin displaying binding for both the cell surface and the glycolipid, followed by treatment with polyethylene glycol. Fusion was inferred from the transfer of fluorescent lipid analog probes embedded in the vesicle membrane over the entire cell surface and of fluoresceinated proteins from the aqueous space of the vesicle to the cytoplasm of the cell. Fluorescence recovery after photobleaching showed that both the injected membrane and the cytoplasmic markers were mobile. Two different lectin--glycolipid combinations [Ricinus communis agglutinin I-lactosylcerebroside and concanavalin A-tetradecyl- (or hexadecyl-) maltobionamide] were used to promote attachment of lipid vesicles before polyethylene glycol-induced fusion with BG-9 human fibroblasts, NIL-8M2 hamster cells, or L-929 mouse cells. In the absence of lectin or polyethylene glycol, fusion was negligible. However, when both the lectin and the glycol were used, a dramatic increase in the transfer of both vesicle membrane and aqueous space markers from the liposomes to the cells occurred.

Long-term intracellular retention of hexosaminidase A by Tay-Sachs disease brain and lung cells in vitro

Enzyme-replacement treatment for metabolic storage disorders has been widely studied using model cell culture systems. This study determines the long-term fate of human hexosaminidase A supplied to Tay-Sachs disease brain and lung cells. Hex A retention studies showed that the incorporated Hex A is retained in undiminished quantity by TSD lung cells maintained in stationary culture for 14 days. Tay-Sachs disease brain cells similarly followed for 28 days in stationary culture showed an initial reduction in Hex A for 3 days, after which the Hex A level stabilized and remained relatively constant for the next 25 days. Hexosaminidase B isoenzyme was found to accumulate in both cell lines during extended cultivation, despite the observation that significant amounts were excreted into the extracellular environment. The demonstration of long-term intracellular retention of exogenously supplied therapeutic enzyme by the target cells offers additional evidence for the feasibility of an enzyme-replacement approach for study and treatment of lysosomal storage disorders.

Enzyme replacement treatment for Tay-Sachs disease brain cells in culture utilizing concanavalin A-mediated hexosaminidase A uptake: biochemical and morphological evidence of GM2 mobilization

When Concanavalin A (Con A) is bound to the cell membrane, it functions as an artificial enzyme receptor, mediating the binding and intracellular incorporation of significant amounts of exogenous hexosaminidase A (Hex A) into Tay-Sachs disease (TSD) glia cells. The treated cells retained almost 50% of incorporated Hex A activity after 3 days incubation in Hex A free medium. Hex A was released from Con A within the cell and was available as free enzyme. Biochemical analysis of gangliosides in Con A and Hex A treated cells depicted a greater than 50% reduction in stored GM2 ganglioside and a fourfold reduction in GM2 label (14C) when compared to controls. Ultrastructural evidence of GM2 breakdown is presented which supports the biochemical and labeling studies.

Tay-Sachs disease brain cells in culture: mobilization of stored GM2 after concanavalin A-mediated uptake of hexosaminidase A

A human Tay-Sachs disease (TSD) fetal-brain-cell line is a useful model for the disease since the cells lack hexosaminidase A and accumulate the ganglioside, GM2. This brain-cell line was used to assess the effect of hexosaminidase A treatment on GM2 storage material. Entry of placental hexosaminidase A into the cells was obtained by pretreatment of the cultures with concanavalin A. Cells were analyzed periodically during six days. During the course of the experiment, GM2 in the cells decreased by approximately 50%. A substantial amount of hexosaminidase A was maintained in the cultures throughuot the experiment. This strategy was successful in mobilizing stored GM2 in TSD brain-cell cultures. Therefore, the activating factor needed for hexosaminidas A activity must be present in TSD-cultured brain cells.