Alzheimer's disease neuropathology is characterized by three major lesions: intracellular neurofibrillary tangles and extracellular deposits of amyloid in senile plaques and blood vessels. Both intraneuronal and extracellular lesions appear very early in the disease process and are likely to form as a result of an aberrant proteolytic degradation, suggesting that they have an important role in the pathogenesis of the disease. Specifically, the neurofibrillary tangles were shown to contain ubiquitin, a molecule involved in the ATP-dependent degradation of abnormal and/or denatured cellular proteins and the amyloid was shown by us to contain the serine protease inhibitor alpha1-antichymotrypsin (ACT). The major component of the amyloid fibrils is the beta-protein, a proteolytic fragment of a larger precursor, the beta-amyloid protein precursor. Further studies will be conducted to identify the brain serine proteases that are the substrates for inhibition by alpha1-antichymotrypsin and the potential involvement of such proteases in the post-translational processing of the beta-amyloid protein precursor, since it is known that the N-terminal cleavage site of the beta-protein is a chymotrypsin- cathepsin G site (the proteases inhibited best by ACT). This goal may be achieved by fractionating brain homogenates and testing their activity on synthetic peptides flanking the cleavage sites of the beta-protein. Alternatively, we will take advantage of ACT's ability to form SDS- insoluble equimolar complexes with its target protease in order to purify such complexes and identify the protease. It is currently believed that it is the aberrant proteolytic processing of the precursor that results in the formation of the beta-protein, a very hydrophobic peptide which easily assumes a beta-pleated sheet conformation and precipitates as amyloid fibrils. Studies will also be directed to answer the question whether ACT is also involved in the intracellular degradation of abnormal proteins via the ubiquitin system or via the newly described organelle: the proteasome. Antibodies to the proteasomes will first be tested on Alzheimer's disease neurofibrillary tangles. If positive, ACT's ability to inhibit one or more of the proteolytic activities of the proteasome will be tested. What triggers the specific and premature neuronal cell death in Alzheimer's disease is still a mystery, but understanding the formation of plaques and tangles may provide the best clue we have today.
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