Modulation of Amyloid-Beta Aggregation in the Endosomal/Lysosomal Pathway

Washington University, Saint Louis MO

Investigators

Abstract

DESCRIPTION (provided by applicant): The goal of this project is to investigate a potential mechanism by which amyloid-¿ peptides (A¿) form aggregates in the brain. This is directly relevant to the pathogenesis of Alzheimer's disease (AD). In vitro studies have shown that A¿ forms aggregates only at concentrations that are more than one thousand times higher than those measured in cerebrospinal fluid and in brain tissue. Even so, A¿ plaque formation is characteristic for AD and genetic evidence demonstrates that mutations that increase A¿ aggregation propensity lead to early onset AD. This apparent contradiction suggests that there must either be: 1) high local concentrations of A¿ somewhere in the brain, or 2) cellular and/or biochemical mechanisms which lower the concentration required for A¿ to aggregate. Growing evidence suggests that A¿ can accumulate in intraneuronal vesicles well before the first appearance of amyloid plaques. Our preliminary data suggest that A¿ can be endocytosed and trafficked through the endo/lysosomal pathway. Moreover, we have found that A¿ is concentrated in late endo/lysosomes to levels that would support A¿ aggregation. This occurs even when low, physiologically relevant concentrations of A¿ are added to the outside of the cell. Also, we know that certain features of the endo/lysosomal environment, such as low pH, favor A¿ aggregation. This makes the endo/lysosomal pathway particularly attractive for nascent A¿ aggregate formation in vivo. The hypothesis to be tested in this work is that the earliest stages of A¿ aggregation in vivo occur within neural cells during endo/lysosome maturation. In this proposal, we will investigate A¿ uptake from the extracellular space and the effect of trafficking along the endo/lysosomal pathway on its aggregation state. We will use modern fluorescence spectroscopic and microscopic methods to develop a detailed time course of the uptake, trafficking, and aggregation of A¿ along the endo/lysosomal pathway. These experiments will be combined with modern cell biology tools to interrogate how specific environmental factors of the endo/lysosomal pathway contribute to the aggregation of A¿. We speculate that endocytosis and trafficking of A¿ along the endo/lysosomal pathway represents a normal pathway for quality control and maintenance of extracellular A¿ levels. However, characteristics of the endo/lysosomal environment, such as low pH and locally high protein concentrations, favor A¿ aggregation. It is quite possible that the balance between A¿ aggregation and degradation along the endo/lysosomal pathway is a critical determinant in AD pathogenesis.