Alzheimer's disease (AD) is an idiopathic progressive dementia affecting a large proportion of our increasingly aged population. Although the pathogenesis of AD is not completely understood, present data suggest that it probably involves the abnormal proteolytic cleavage of a large precursor protein to yield about 42 residue protein, referred to as amyloid Beta protein or A4, which subsequently polymerizes and forms the amyloid deposits characteristic of this disease. The precursor of the amyloid Beta-protein (i.e., amyloid Beta-protein precursor; APP) is encoded by at least three mRNAs that arise through alternative splicing of two exons. The two most abundantly expressed forms contain a 56 residue insert, which has significant homology with the Kunitz-type protease inhibitors (KPI). Platelet coagulation Factor Xla-inhibitor was recently identified to be a form of APP that contains the KPI domain. Presently, little information exists on the origin of APP in the platelet or its processing during the maturation of megakaryocytes/platelets. The long-term objective of the work described in this application is to provide a detailed understanding of the production and processing of APP associated with megakaryocytes/platelets. Immunologic probes will be developed against specific regions of the APP molecule and utilized in immunofluorescent and immunoelectron microscopic techniques to define the cellular distribution of APP in (i) intact platelets, (ii) platelets during adhesion/aggregation reactions, (iii) megakaryocytic cell lines, and (iv) freshly isolated and cultured bone marrow megakaryocytes as they mature. The forms of APP present in intact platelets, as well as the processing of APP that occurs during platelet adhesion/aggregation reactions, will be analyzed utilizing immunoblotting protocols and radioimmunoassays. These studies will form the framework for analyzing the location and forms of APP associated with abnormal platelets that are present in the blood of AD patients. The origin of platelet APP will then be defined by utilizing immunologic assays and metabolic labeling/immunoprecipitation experiments to analyze the production of APP during the differentiation of megakaryocytic cell lines and bone marrow-derived cells in vitro. These studies will be complemented by characterizing the protease activity(ies) responsible for the processing of megakaryocyte/platelet APP. Furthermore, the structural domain(s) on APP that are critical for its targeting into platelet alpha-granules will be defined by the use of expression vector/APP constructs in cell lines that exhibit a storage secretory pathway. These constructs will include deletion mutants of APP, site-directed mutants of APP, and hybrid APP molecules. Because platelets have been proposed as a peripheral model for neurons, it is expected that information on the mechanisms by which megakaryocytes/platelets process APP will likely provide an insight into the expression and cleavage of this protein by neurons.
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