Alzheimer's Disease (AD) is a major neurological disorder of the elderly affecting as much as 50% of the population over 80. No cure for this disease is known and its etiology is obscure. However, several genetic loci have been identified in subsets of AD patients causing early onset forms of the disorder or increasing the risk of a late onset form. One of these mutations is in the gene coding for the amyloid precursor protein (APP). Normal processing of this protein generates a peptide fragment, termed Ab, which is a major component of the senile plaques in brains of AD patients. These observations suggest an important role of APP in the causation of this disorder. Our work has focused on establishing a model system in which mutant APP genes are over-expressed in several types of cultured cells thought to be important in AD. We previously showed that neuronal-like PC-12 cells and endothelial cells over-expressing mutant APPs undergo morphological changes and shift the processing of APP towards production of larger carboxyl terminal amyloidogenic fragments within the cells. We have now determined by a combination of TUNEL staining and FACS analysis that as many as 20% of PC-12 cells expressing these mutant APPs undergo an apoptotic cell death. Conditioned media from these cells, containing slightly increased amounts of Ab peptide did not induce apoptosis in untransfected cells, suggesting an intracellular site for this effect. Scanning electron microscopic analysis of these cells revealed cell body compaction and membrane blebbing while transmission EM showed nuclear chromatin condensation at the periphery of the nucleus, both phenomenon characteristic of apoptosis. Over-expression of mutant APPs may cause apoptosis by inducing oxidative damage to cells since an anti-oxidant compound, L-cysteine prevents DNA laddering and cell death. We are currently examining other possible mechanisms of apoptosis in these cells due to mutant APP over-expression. Our emphasis is on examining intracellular calcium levels, pH changes, cell adhesion properties, and mitochondrial oxidative damage.
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