Regulation of mitochondrial transport and inter-microtubule spacing by tau phosphorylation at the sites hyperphosphorylated in Alzheimer's disease

The microtubule-associated protein Tau is a major component of the neurofibrillary tangles that serve as a neuropathological hallmark of Alzheimer's disease. Tau is a substrate for protein phosphorylation at multiple sites and occurs in tangles in a hyperphosphorylated state. However, the physiological functions of Tau phosphorylation or how it may contribute mechanistically to Alzheimer's pathophysiology are not completely understood. Here, we examined the function of human Tau phosphorylation at three sites, Ser199, Ser202, and Thr205, which together comprise the AT8 sites that mark abnormal phosphorylation in Alzheimer's disease. Overexpression of wild-type Tau or mutated forms in which these sites had been changed to either unphosphorylatable alanines or phosphomimetic aspartates inhibited mitochondrial movement in the neurite processes of PC12 cells as well as the axons of mouse brain cortical neurons. However, the greatest effects on mitochondrial translocation were induced by phosphomimetic mutations. These mutations also caused expansion of the space between microtubules in cultured cells when membrane tension was reduced by disrupting actin filaments. Thus, Tau phosphorylation at the AT8 sites may have meaningful effects on mitochondrial movement, likely by controlling microtubule spacing. Hyperphosphorylation of the AT8 sites may contribute to axonal degeneration by disrupting mitochondrial transport in Alzheimer's disease.