Although the characteristic deposits of beta-amyloid protein in the brain of Alzheimer's Disease (AD) patients were described many years ago, the discovery that such patients also have beta-amyloid deposits in the vasculature is more recent. There is a strong indication that this proteins's origin is the amyloid precursor protein which has recently been discovered to be present, in relatively large quantities, in the platelet alpha-granules. The mechanism of platelet activation, and subsequent degranulation, as well as the ways in which this precursor's release in AD patients differ from those of normal controls, remain unelucidated, as does the alternative possibility that the platelet activation process and the precursor protein release are normal but that the processing of the precursor into the amyloid protein is abnormal. The origin of this processing enzyme(s), also remains controversial. Since the amyloid deposits are found in the vasculature, the possibility that AD platelets interact differently with the endothelial cells of the vascular wall must be considered. Thus, since deposits of the abnormal degradation product of APP are found in AD but not in normal patients of comparable age, there may be a difference in the platelets' APP content, stimulus response, processing of released products and/or interactions with or in the presence of endothelial cells. Based on our extensive experience in the study of early platelet responses to agonists, we have already begun to investigate the contents, stimulus response, signal transduction and degranulation of AD and normal age- matched platelets, with young adults as controls. In preliminary studies to date we have been able to demonstrate that AD platelets exhibit normal resting membrane potentials and cytoplasmic Ca++ concentrations, but low cytoplasmic pH. Stimulation with thrombin appears to lead to abnormally high levels of granule product release, in conjunction with normal membrane potential and [Ca++] in changes. The granule release appears to be greater in AD than in normal platelets in the presence of endothelial cells as well, and appear to penetrate through that layer when confluent human umbilical vein endothelial cells are involved; cells with tighter junctions, better models of the blood brain barrier, are being prepared. We plan to examine this finding in greater detail, and to use our multiparameter flow kinetic technique which will permit determination of several of these platelet-related events simultaneously, in all the cells or in subpopulations, on very small samples (<1 million cells). We already have permission to study a 75 inpatient, 50 out-patient AD population at the V.A. hospital in Bedford, MA and are getting such permission for the Boston City Hospital's AD hospice which has more female and minority patients. We hope that our studies will delineate differences in AD platelets' interactions with and/or stimulation responses in the presence of endothelial cells. Differences in platelet stimulus responses, per se, may allow the detection of AD, possibly at a very early stage, in living individuals. Differences in their interaction with or processing in the presence of endothelial cells may explain the presence of vascular plaques.
