Beta-amyloid precursor protein (BetaAPP) is an integral membrane protein that is ubiquitously expressed in mammalian cells, with highest levels in the brain. BetaAPP was identified and cloned because a 39-43 residue proteolytic fragment (Abeta) is the subunit of the amyloid filaments which accumulate as myriad extracellular ('senile') plaques in the brains of humans with Alzheimer's disease (AD). Subsequent studies of normal BetaAPP structure and function have revealed several interesting and unusual properties that have relevance to other single membrane-spanning proteins: a) betaAPP functions both as an apparent cell-anchored receptor and as a secreted derivative that acts upon other cells; b) proteolysis of betaAPP leads to regulated release of several different soluble derivatives, including hydrophobic peptides that include much of the transmembrane domain; c) betaAPP is phosphorylated solely on its ectodomain, leading to secretion of phosphorylated betaAPP; d) the large betaAPP ectodomain contains diverse structural motifs that give rise to several putative functions of the molecule; and e) betaAPP is a member of a highly conserved gene family, including homologs in Drosophila and C.elegans. Based on these findings and on extensive preliminary data, a series of integrated biochemical, cell biological and molecular biological experiments which address both the normal structure and function of betaAPP and the mechanisms of cerebral accumulation of its Abeta fragment is proposed.
Specific Aims are to: 1) Identify and clone the physiological receptor and/or counter-receptor for APPs/betaAPP and characterize its role in betaAPP function and the endocytosis and processing of betaAPP. Preliminary work has established assays for betaAPP functional activity in the picomolar range and the reagents needed for expression cloning (or purification) of its receptor, 2) Understand the mechanism of the unusual ectodomain phosphorylation of betaAPP and the role of phosphorylation/dephosphorylation in both the functions of the surface and secreted isoforms and the proteolytic processing of betaAPP. This work postulates that betaAPP is one of a very few known surface proteins whose multiple extracellular functions are modulated by their state of ectodomain phosphorylation (cf, osteopontin; CD36). 3) Analyze a potential mechanism for the clearance of secreted Abeta from extracellular fluid: examine physiologically in living cells the hypothesis that apolipoprotein E mediates the clearance of Abeta from extracellular fluids via LRP or another receptor. 4) Characterize the formation and fate of natural multimers of endogenously secreted Abeta which have been recently detected in cell culture. Growing evidence suggests that Abeta polymerization is a critical determinant of its cytotoxicity, but prior studies have used high doses of synthetic Abeta in non-physiological conditions. Taken together, these Aims build on extensive published and unpublished studies of betaAPP form this lab and others to address new questions about this complexly processed, multi-functional molecule. The results should provide insights into the normal biology of this class of receptor/secretory protein and the role of betaAPP in AD.
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