Neurofibrillary tangles and tau phosphorylation

Neurofibrillary tangles (NFTs) are a characteristic neuropathological lesion of Alzheimer's disease (AD). They are composed of a highly-phosphorylated form of the microtubule-associated protein tau. We are investigating the relationship between NFTs and microtubule stability and how tau phosphorylation and function is affected in transgenic models and by co-expression with ϐ-amyloid precursor protein and presenilins. In most NFT-bearing neurons, we observed a strong reduction in acetylated α-tubulin immunoreactivity (a marker of stable microtubules) and a reduction of the in situ hybridization signal for tubulin mRNA. In transfected cells, mutated tau forms (corresponding to tau mutations identified in familial forms of frontotemporal dementias linked to chromosome 17) were less efficient in their ability to sustain microtubule growth. These observations are consistent with the hypothesis that destabilization of the microtubule network is an important mechanism of cell dysfunction in Alzheimer's disease. The glycogen synthase kinase-3 ϐ (GSK-3ϐ) generates many phosphorylated sites on tau. We performed a neuroanatomical study of GSK-3ϐ distribution showing that developmental evolution of GSK-3ϐ compartmentalization in neurons paralleled that of phosphorylated tau. Studies on transfected cells and on cultured neurons showed that GSK-3 ϐ activity controls tau phosphorylation and tau functional interaction with microtubules. Tau phosphorylation was not affected in neurons overexpressing ϐ-amyloid precursor protein. Transgenic mice expressing a human tau isoform and double transgenic animals for tau and mutated presenilin 1 have been generated; a somatodendritic accumulation of phosphorylated transgenic tau proteins, as observed in the pretangle stage in AD, has been observed but NFTs were not found, suggesting that additional factors might be necessary to induce their formation.