FACULTY

In my laboratory, we are interested in the cellular mechanisms of neurodegenerative diseases, especially Alzheimer’s disease. In particular, we are interested in the pathogenesis of amyloid b-protein (Ab) deposition in the brain, a process believed to be a seminal event in Alzheimer’s disease. Deposition of Ab, a proteolytic product of APP, is a morphological hallmark of Alzheimer’s disease. Our approach has been to study the processing, trafficking, and physiological function of b-amyloid precursor protein (APP). We have generally taken a cell biological approach to the examination of these questions.

APP TRAFFICKING. We had previously identified a constitutive secretory pathway of APP in transfected cells that precludes the formation of Ab because of a cleavage event within the Ab domain. We have now characterized a second APP trafficking pathway that involves the internalization and recycling of cell surface APP molecules. Processing in this pathway appears to be the major source of Ab released into medium. Therefore, we have been characterizing the regulation of the APP internalization pathway in neurons and nonneural cells to determine both the precise pathway of Ab production and the physiological role played by the processing of APP in this pathway. In our studies, we have found that the trafficking pathways of APP in neurons are complex. From presynaptic terminals, APP is internalized in vesicles that colocalize with synaptic vesicle markers but is sorted away from synaptic vesicles. This was the first demonstration of an axonal cell-surface protein that undergoes this sorting step. In addition, following internalization, APP is retrogradely transported back to the neuronal soma. But in cultured neurons, we have shown that APP is then transcytotically transported to the surface of the cell body. This was also the first demonstration of an endogenous neuronal protein to undergo transcytosis.

APP FUNCTION. The precise biological functions of APP are unclear. Most of the function attributed to APP to date concerns the secretory product. We have been more interested in the pool of molecules that is transported to the cell surface but is not secreted, in part because of our studies on APP trafficking pathways. We have recently showed that on the axonal surface, APP selectively colocalizes with the integrin family of adhesion molecules. We are in the process of understanding whether APP interacts directly with the integrins or functions in an integrin-like manner. In other studies, we have been examining the behavior of neurons cultured from transgenic mice deficient in APP expression. Future studies will examine the neuronal response of these null animals to various injuries in vivo.

FAMILIAL ALZHEIMER'S DISEASE MUTATIONS. A number of families have been identified in which the Alzheimer phenotype segregates in a fully penetrant autosomal dominant fashion and results in early onset of the disease. APP and a newly described gene family, presenilin, are responsible for the majority of these cases. We would like to understand the cellular mechanisms by which these mutations cause the disease phenotype. In the APP mutations, we have detected subtle abnormalities in APP trafficking. Studies are underway to determine the effects of presenilin mutations on APP processing. We are also examining the physiological function of the presenilins. Interestingly, the presenilins encode proteins of seven or more transmembrane domains and are most closely related to lin-12, a C. elegans protein that interacts with the Notch homolog during development. The Notch family of genes encode receptors for extracellular ligands that specify cell fate during development. In this regard, our initial studies show that presenilins appear to play a role in cellular development and neuronal cell death.