The characteristic deposits of beta-amyloid protein (beta-A4) in the brain and vasculature of Alzheimer's Disease (AD) patients were described many years ago. There is a strong indication that the beta-A4 originates from the amyloid precursor protein (beta-APP) which is present in many cells including megakaryocytes and the alpha-granules of platelets. Platelet activation by thrombin leads to secretion of granule contents, including beta-APP (known to inhibit Factor XIa), which participate in normal thrombosis and hemostasis. The mechanism of precursor release from platelets from AD, non-AD dementias and Downs Syndrome (DS) patients may differ from that of normal controls and has yet to be examined. Alternatively, the activation process and the beta-APP release may be normal in platelets from these patients but proteolysis of the precursor into the beta-A4 may be abnormal. It has already been shown that beta-APP is identical in non-familial AD and in normal individuals but differences in its secretion from platelets from AD and other dementias patients are only now being investigated. Furthermore, the proteolytic processing to form the amyloid peptide, and possibly the targeting of the peptide to specific tissues, may be different in dementias. Additionally, abnormalities in platelet function and morphology have been reported in AD, Parkinsons Disease (PSD) and DS which suggest further that disease-specific differences in the brain could be linked to platelet function. We hypothesize that beta-APP from platelets from patients with varying dementias may be characterizable by an abnormality in either the localization of, the form of, or the proteolytic products of beta-APP. Based on our extensive experience in the study of early platelet responses to agonists, including degranulation, we propose to investigate the quantity of, the form of, the release of, and the processing of beta-APP from AD, non-AD dementias, and DS platelets, in comparison to those individuals of comparable age. The proposed research aims are (1) to use a human megakaryocyte leukemic cell line (Dami) to examine the production, processing, and release characteristics of beta-APP from these cells; (2) to investigate differences in the size and quantity of the beta-amyloid precursor protein (beta-APP) released upon thrombin stimulation from platelets from patients with AD, DS, and other non-Alzheimer dementias and (3) to localize the beta-APP within the platelet by examining subcellular platelet fractions from patients with AD, DS, and other non-Alzheimer's dementias. We hope that our studies will help to delineate differences in the released beta-APP from AD, non-AD dementias and DS platelets and will lead to major insights into the role of platelet-contained beta-APP not only in dementias marked by amyloid deposits but also normal platelets, and hence in thrombosis and hemostasis.
