Toxic neuronal apoptosis and modifications of tau and APP gene and protein expressions

The causes and the mechanisms of neuronal death in Alzheimer's disease are not elucidated, although some new insights have been proposed over the past years, including free-radical toxicity, beta-amyloid toxicity, excitotoxicity, and disturbed cellular calcium metabolism. Some authors have also pointed out that apoptosis could play a role in neuronal degeneration, but it is still largely debated. Here, we review some recent data linking the induction of experimental neuronal apoptosis in vitro and the molecular pathology of the tau protein and amyloid precursor protein (APP). In cultures exposed to mild glutamate toxicity, tau mRNA expression, not beta-actin, is enhanced in stressed neurons. The Guam cycad toxin metabolite methylazoxymethanol also produces an increase of tau gene transcription that exacerbates changes induced by glutamate. In serum-deprived cultures or glutamate-exposed cultures, neurons committed to apoptosis have a reduced tau gene expression, whereas resistant neurons display a stable or even augmented tau mRNA expression accompanied by a persistent tau phosphorylation near serine 202. In the same conditions, stressed neurons produce membrane blebbings strongly immunopositive for APP and putative amyloidogenic fragments that are subsequently released in the extracellular space. Experimental apoptosis in neurons can recapitulate tau and APP modifications that could be associated with a selective vulnerability and a progression of cellular degeneration along the neuronal network.