In normal aging of humans and, to a much greater extent, in Alzheimer's disease and Down's syndrome, extracellular deposits of amyloid accumulate in senile plaques and blood vessel walls. Since the amyloid is among the first markers of aging in humans, we hypothesize here that the abnormal deposition of amyloid is detrimental to its surroundings, causes the neuritic response,and possible, the tangle formation, synaptic and neuronal cell loss and finally cognitive deficits. Nonhuman primates exhibit similar neuroanatomical changes in their brains and in their cognitive behavior, providing the best animal model to study the temporal and spacial modifications occurring as a function of age. One of the amyloid components is the beta-protein, a 4Kd fragment of a much larger protein precursor. We believe that an abnormal proteolytic degradation of the beta-protein precursor (beta-PP) must occur in order to generate very hydrophobic fragments which can aggregate and precipitate as amyloid. A hint for an abnormal processing of the beta-PP comes from the second component found very tightly associated with the beta-protein, the serine protease inhibitor, a-1-antichymotrypsin (ACT). Finding ACT in the amyloid determined us to search for proteases which may be involved in the proteolytic degradation of the beta-PP and may serve substrates for inhibition by ACT. Using synthetic peptides made according to known sequences of the beta-PP, we identified a protease fraction able to cleave at the N-terminus of the beta-protein. We propose to study the appearance of amyloid in the monkey brain, as a function of age and to correlate its formation with the presence of our protease. In order to accomplish this goal, we need to purify the protease to homogeneity, to sequence it and to produce monoclonal and polyclonal antibodies to it. The antibodies will aid in determining the distribution of the enzyme in brain and to clone its gene by screening an expression cDNA library. The cloned cDNA will then be used to identify cells producing the enzyme by in situ hybridization and to localize the gene on the human chromosome. If the gene resides on chromosomes 21 or 19, the two possible locations for an familial Alzheimer's disease gene (FAD), we will perform linkage analysis using the protease cDNA to determine if the gene coding for our protease is the FAD gene. A time-course analysis of the appearance in the monkey brain of amyloid and our protease, neuritic degeneration, synaptic, receptor and neuronal cell loss and cognitive changes, should shed light on the amyloid role in all those alterations.
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