LOSS OF ENDOSOMAL LYSOSOMAL MEMBRANE IMPERMEABILITY IS AN EARLY EVENTIN AMYLOID A-BETA-1-42 PATHOGENESIS/

Previous studies have implicated the failure to degrade aggregated A b eta 1-42 in late endosomes or secondary lysosomes as a mechanism for t he accumulation of B-amyloid in Alzheimer's disease. We examined the c onsequences of intracellular accumulation of A beta 1-42 on the integr ity of the endosomal/lysosomal compartment by monitoring Lucifer Yello w fluorescence and the release of lysosomal hydrolases into the solubl e, cytosolic fraction. In control cells, the Lucifer Yellow fluorescen ce is observed as punctate staining in a perinuclear distribution with no apparent cytoplasmic fluorescence, consistent with its localizatio n in late endosomes or secondary lysosomes, After incubation with A be ta 1-42 for 6 hr, a loss of lysosomal membrane impermeability is obser ved as evidenced by redistribution of the fluorescence to a diffuse, c ytoplasmic pattern, The loss of lysosomal membrane impermeability is c orrelated with A beta 1-42 accumulation, since incubation of the cells with the nonaccumulating isoform of amyloid, A beta 1-40, does not in duce leakage. The same results were obtained using the release of solu ble lysosomal hydrolases, cathepsin D and beta-hexosaminidase, into th e cytosol as an assay for the leakage of lysosomal contents. Together, our results suggest that the loss of lysosomal membrane impermeabilit y may be an early event in AP pathogenesis, and provide an explanation for the miscompartmentalization of extracellular and cytoplasmic comp onents observed in Alzheimer's disease (AD), The release of hydrolases may further cause the breakdown of the cytoskeleton and the blebbing of the plasma membrane, and the leakage of heparan sulfate glycosamino glycans from the lysosome may ultimately promote the assembly of tau i nto neurofibrillary tangles (NFT), (C) 1998 Wiley-Liss,Inc.