Howard Hughes Medical Institute Investigator
Familial Alzheimer's disease is a progressive neurodegenerative disorder. This condition is characterized by the accumulation of amyloidogenic peptide (Abeta42) in abnormal structures called neuritic plaques within the brain, leading to death of brain neurons. The mechanism that restricts accumulation of Abeta42 to nerve cells remains unexplained. Part of my lab effort is designed to understand on a molecular level the abnormal processing of amyloid precursor protein (APP) and accumulation of Abeta42 observed in Alzheimer's disease. Abeta42 is the product of proteolysis of a type I membrane protein, amyloid precursor protein, which is produced in a variety of tissues, but appears to generate amyloid only in the hippocampus and the amygdala. Proteolysis of APP is facilitated by a complex called the gamma-secretase. We study the traffic of amyloid precursor protein and PS1 using a cell-free vesicle budding reaction that recapitulates the first step in the transport of membrane proteins from the ER to the Golgi complex in mammalian cells. Mutations in PS1 retard the packaging of this protein and amyloid precursor protein into transport vesicles. This delay in transport may potentiate the cleavage of APP and accumulation of Abeta42 in the ER.
In 2012, we reported that APP traffics from the cell surface via endosomes to the TGN where gamma-secretase acts to generate Abeta peptide (Choy et al). In this itinerary, APP encounters another processing protease, beta secretase, which generates the substrate for gamma secretase. Since gamma cannot act on APP until the beta enzyme cleaves, much of the traffic of APP is directed from the TGN to the endosomal compartment in which the beta enzyme resides. The traffic of APP from the TGN to the endosome requires a clathrin assembly protein complex called AP-4. We have developed a cell-free biochemical reaction that reproduces the packaging of cargo proteins into transport vesicles that bud from the TGN. Our next project on the Alzheimer's effort will be an attempt to reconstitute APP packaging into transport vesicles in an AP-4-dependent reaction. If successful, this effort could uncover the essential components that govern the normal and abnormal processing of APP and thus the reason for the misprocessing that leads to amyloid peptide. Choy, R. W.-Y., Cheng, Z. and Schekman, R. (2012) Amyloid precursor protein (APP) traffics from the cell surface via endosomes for amyloid b(Ab) production in trans-Golgi network. Proc. Natl. Acad. Sci. USA 109, No. 30.