Understanding the biological activity of amyloid proteins in vitro: from inhibited cellular MTT reduction to altered cellular cholesterol homeostatis.

1. MTT is taken into the cell through endocytosis and is reduced and accumulated in a population of acidic vesicles. Reduced MTT formazan is exocytosed to form needle-like formazan crystals at the cell surface. Mitochondria are unlikely to play a significant role in cellular MTT reduction. 2. Amyloid fibrils inhibit cellular MTT reduction indirectly by enhancing MTT formazan exocytosis. All protein fibrils with beta-pleated sheet structure which have been examined enhance MTT formazan exocytosis and induce neurotoxicity. 3. Cellular free cholesterol regulates the exocytosis of the intracellular MTT formazan-transporting vesicles and these vesicles may be involved in cellular cholesterol homeostasis. 4. Amyloid fibrils inhibit cholesterol esterification and alter the distribution of free cholesterol in neurons. 5. Amyloid fibril-induced alterations in cellular cholesterol metabolism and vesicle trafficking may contribute to neurodegeneration in vivo.