Microdomain Localization and Trafficking of BACE1

This proposal investigates BACE1, one of the enzymes critical for the production of Alzheimer’s disease amyloid beta production. We are interested in deciphering the molecular principles of BACE1 trafficking in lipid rafts and non-raft domains using biochemical and live cell imaging approaches in order to elucidate the cell biology of BACE1 processing of amyloid precursor protein (APP). Our studies will uncover novel and significant basic insights into cellular processes that modulate amyloid beta production, thus contributing to the development of strategies aimed at reducing amyloid beta burden.

Introduction
Alzheimer’s disease (AD) is the major cause of dementia in the elderly. Toxic amyloid beta peptides accumulate in the brains of individuals with AD. Production and accumulation of amyloid beta are central events in AD pathogenesis.

Hypothesis and Specific Aims
Our proposal seeks to investigate the regulation of amyloid beta production. Sequential cell division of amyloid precursor protein (APP) by BACE1 and gamma secretase generates amyloid beta peptides. Our investigation focuses on BACE1. We seek to investigate membrane localization and movement of BACE1 in cultured cells and hippocampal neurons. Our first aim is to characterize BACE1 association with specialized cholesterol-rich microdomains of cellular membranes, called lipid rafts, which play important roles in amyloid beta production. Our second aim is to explore the dynamics of BACE1 movement through the cells using live cell imaging strategies.

Long-term Goals
A better understanding of localization and dynamics of BACE1 movement in cells will shed more light on the mechanisms involved in amyloid beta production. Information stemming from our biochemical, molecular and cellular investigations will contribute to the development of novel and rational strategies for therapeutic intervention for AD aimed at reducing amyloid beta burden.

Progress Updates

Dr. Thinakaran’s team performed biochemical analyses using experimental BACE1 mutants and chimeric proteins to understand whether association with lipid rafts (which are specialized areas of the fatty envelope surrounding the cell) could alter APP processing in a manner that affects the generation of toxic beta-amyloid peptides. From their previous studies, it appears that lipid raft association per se is not required for beta-amyloid production in non-nerve cells. However, the team found that raft association seems to play an important role in delivering BACE1 to specific locations within cultured nerve cells. These results suggest that the attachment of fatty acid chains to BACE1 (i.e. post-translational lipid modification) and the association of BACE1 to lipid rafts could potentially influence APP processing and beta-amyloid production in nerve cells.

In a second line of investigation, the team used live cell imaging and high-speed video microscopy analysis to study how BACE1 is transported within cultured cell lines and in cultured hippocampal nerve cells. Their studies show that BACE1 is largely localized in membrane organelles involved in the recycling of cell surface proteins. By specifically labeling the BACE1 protein that was exposed to the cell surface, they were able to follow the movement of BACE1 inside neurons in real time. These studies have provided novel insights into neuronal trafficking of BACE1, and have identified novel regulators involved in axonal transport of BACE1. Dr. Thinakaran’s future studies will determine the significance and effect of the BACE1 axonal transport on APP processing and beta-amyloid production in nerve cells.