Transport and processing of beta-hexosaminidase in normal and mucolipidosis-II cultured fibroblasts. Effect of monensin and nigericin

Two mechanisms for the recapture of extracellular GM2 activator protein: evidence for a major secretory form of the protein

The GM2 activator protein is a small monomeric protein containing a single site for Asn-linked glycosylation. Its only proven in vivo function is to act as a substrate specific cofactor for the hydrolysis of GM2 ganglioside by lysosomal beta-hexosaminidase A. However, we and others have shown it can act as a general glycolipid transporter at neutral pH in vitro. Any other possible in vivo functions would require that some of the newly synthesized activator molecules not be targeted to the lysosome. The lysosomal targeting mechanism for the activator has not been conclusively identified. While earlier reports suggested that it is likely through the mannose-6-phosphate receptor, another more recent report demonstrated that deficient human cells could recapture nonglycosylated, bacterially produced activator, suggesting its use of an alternate targeting pathway. Here, we demonstrate that the mannose-6-phosphate pathway is likely the major intracellular, biosynthetic route to the lysosome, as well as a high affinity recapture pathway for the endocytosis of activator protein from extracellular fluids. Additionally, we show that there exists a second lower affinity recapture pathway that requires its native protein structure, is carbohydrate independent, and likely does not involve its ability to bind glycosphingolipids in the plasma membrane. Finally, we document that the pool of newly synthesized precursor activator protein contains a majority of molecules with a complex-type oligosaccharide, which cannot contain a functional mannose-6-phosphate targeting signal. These molecules makeup the secreted forms of the protein in normal human fibroblasts.

Niemann-Pick disease: coupling and uncoupling of inhibited sphingomyelinase activity and exogenous cholesterol esterification in fibroblasts by ionophore treatment

In order to elucidate a biochemical relationship between sphingomyelin and cholesterol metabolisms, we examined the effects of several ionophores (monensin, nigericin, A23187, ionomycin, lasalocid) on sphingomyelinase activity and cholesterol esterification in cultured human fibroblasts. Phase-contrast microscopy showed the presence of foamy cells with monensin and nigericin treatments only. Electron microscopic examination revealed lamellated membranous bodies and cytoplasmic vacuoles in cells treated with monensin and nigericin. Monensin and nigericin treatments led to reduction of acid sphingomyelinase activity and disturbance of the esterification of lipoprotein-derived cholesterol in cultured fibroblasts, which is compatible with the biochemical changes of Niemann-Pick disease, type C. A23187, ionomycin, and lasalocid treatments showed only sphingomyelinase reduction in treated fibroblasts. Experimental models in this culture system could be produced in these ways, mimicking subtypes of Niemann-Pick disease, type A, B and type C.

Effects of flavonoids on enzyme secretion and endocytosis in normal and mucolipidosis II fibroblasts

The effect of flavonoids on beta-hexosaminidase transport and endocytosis has been studied in cultured human skin fibroblasts. In mucolipidosis II fibroblast cultures, characterized by their preferential secretion of most newly synthesized hydrolases, quercetin and phloretin (200 microM) inhibited beta-hexosaminidase synthesis as well as total culture-associated enzyme activity. Taxifolin induced a 2.4-fold increase in the total enzyme activity without altering the intra- and extracellular distribution of the enzyme. Rutin, although less effective, also stimulated an overall increase in total enzyme. The flavonoid effects were all concentration-dependent. Very little effect was observed in either the distribution or the total beta-hexosaminidase activity in normal fibroblast cultures. Taxifolin and hesperitin inhibited receptor-mediated endocytosis of beta-hexosaminidase by fibroblasts up to 50% of control uptake. Naringin, quercetin, and phloretin moderately inhibited uptake by 30% while rutin and fisetin had no effect. The results demonstrate that certain naturally occurring flavonoids affect the secretion of lysosomal enzymes as well as their endocytosis by fibroblasts. Since most individuals ingest up to one gram per day of these substances, flavonoids may prove to have significant effects on normal lysosomal enzyme physiology.

Intracellular transport of high molecular weight intermediates of acid alpha-glucosidase in human fibroblasts

Two transient, high molecular weight precursors of human acid alpha-glucosidase were detected by immune precipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The high molecular weight precursors were rapidly converted into lower molecular weight forms corresponding to previously identified intermediates of acid alpha-glucosidase. An accumulation of these precursors was observed in fibroblasts treated with monensin and nigericin, suggesting that these precursors are intermediates of acid alpha-glucosidase undergoing transport through the Golgi complex.

Immunocytochemical localization of beta-hexosaminidase and electron-microscopic characterization of human fibroblasts following treatment with monensin and nigericin

Immunocytochemical localization of beta-hexosaminidase in cultured human skin fibroblasts was performed in the presence or absence of the Na+/K+ ionophores monensin and nigericin. In the presence of monensin, beta-hexosaminidase accumulated in the periphery of swollen vesicles in the paranuclear region of fibroblasts from normal individuals and from patients with mucolipidosis II. Nigericin-treated cells had more extensive vacuolization of the cytoplasm and the localization of the enzyme was more diffuse within these vacuoles. Morphological studies at the ultrastructral level indicated that a perturbation of the Golgi region occurred during ionophore treatment. These findings suggest that beta-hexosaminidase in ionophore-treated fibroblasts is trapped in a time- and dose-dependent manner in the paranuclear region presumed to be the swollen cisternae of the Golgi region, or adjacent vesicles derived from the Golgi region. Furthermore, fibroblasts are more sensitive to perturbation by nigericin than by monensin at similar concentrations and exposure times. These data support biochemical findings that the two ionophores differentially inhibit the transport of lysosomal enzymes in the Golgi region.