Page:Human wild-type full-length tau accumulation disrupts mitochondrial dynamics and the functions via increasing mitofusins.pdf/6

https://www.nature.com/srep/

'''Figure 4. Expression of htau increases fusion proteins and reduces ubiquitination of Mfn2. (a,b)''' The increased fusion proteins (Mfn1, Mfn2 and OPA1) in hippocampal extracts of htau transgenic mice (htau) compared with the age-matched wild-type littermates (wt) measured by Western blotting. (Mfn1, p = 0.0025; Mfn2, p = 0.0136; OPA1, p = 0.0142; Fis1, p = 0.798). (c) The perinuclear accumulation of mitochondria in hippocampal neurons measured by electron microscopy. (d,f) Mitochondrial dysfunction represented by the reduced levels of ATP (p = 0.0265), ratio of ATP/ADP (p = 0.0377), and complex I activity (p = 0.0452) in hippocampal extracts of the htau mice. Data were expressed as mean ± SD. *,p < 0.05, **,p < 0.01, ***,p < 0.001 vs wt.

deficits identical to loss of Mfn2, therefore silence Miro/Milton may also rescue the htau-induced mitochondrial fusion. Additionally, we have previously proposed that tau may play a dual role in leading the cell escape acute apoptosis and triggering the chronic neurodegeneration, , , based on our systemic studies , ,. We believe that the perinuclear accumulation of mitochondria observed in the current study may represent an early compensatory response of tau to preserve the cell bodies from the acute apoptosis, while the chronic neurodegeneration is the unavoidable fate of the cells with increasing tau accumulation and depriving mitochondria from the axon terminals.

Mitochondrial function is regulated by dynamic fusion and fission. Previous studies indicate that mitochondrial fusion is neuron protective, leading to the exchange of mitochondrial DNA, reorganization of mitochondrial cristae, and protect cells from apoptosis, whereas mitochondrial fission seems a sign of apoptosis,. Here, we show that expression of htau promotes mitochondrial fusion with increased cell viability at 48 h after tau transfection, which is consistent with our recent reports showing that expression of htau renders the cells more resistant to the acute cell apoptosis induced by exogenous apoptotic inducers. We also observed that mitochondria elongation and their perinuclear cumulating appeared at 24 h, when the neurite density and the morphology were not changed much; while degeneration of the neuronal processes and loss of neurites were seen at 48 h after overexpression of htau. The sequential appearance of mitochondrial abnormalities and the axon degeneration upon expression of htau were also detected by live cell time-lapse imaging, which strongly supports that the abnormal mitochondria dynamics is upstream of neurodegeneration. Together with our previous studies, we propose that tau accumulation ameliorates apoptosis at early stage while it causes neurodegenerasis (degenerative cell death) at later stage, because the mitochondrial dysfunction leads to the deficits of basic energy supply for cells to survival. Therefore, the current study provides evidence supporting the dual role of tau in AD neurodegeneration.

A previous study in drosophila and fibroblast showed that overexpression of human R406W mutant tau could elongate mitochondria with the mechanisms involving reduced fission. Here, we also detected mitochondrial elongation with an enhanced fusion but without change of fission after overexpression of wild-type htau. Another study reported that levels of the mitofusins decreased in the hippocampus of the AD patients, and β-amyloid treatment reduced the mitofusin level in primary hippocampal neurons. In the current study, we found a remarkable elevation of the mitofusins, which was positively correlated with abnormal tau accumulation in vitro and in htau transgenic mice. These data indicate that β-amyloid and different types of tau proteins may impair mitochondrial dynamics and induce mitochondrial dysfunction with different mechanisms. Additionally, a previous study indicated that overexpression of wild type tau in primary cortical neurons suppressed mitochondrial motility and fusion and induced mitochondrial fragmentation. The exact reason for the discrepancy is currently not clear, but we think different sources of the primary neurons, i.e., the hippocampus versus cerebral cortex, may be one of them.

How the intracellular htau accumulation may cause elevation of the mitofusins? Since we did not see any significant change of the mRNA levels of the mitofusins, we speculate that the htau accumulation may affect the proteolytic turnover of the fusion proteins. This point was supported by the significantly reduced ubiquitination of Mfn2, a crucial step for the proteolysis, in the htau-expressing cells. The mitofusin ubiquitination is regulated by PTEN-induced kinase 1 (PINK1)-parkin pathway   and the mitochondrial residing of PINK1/parkin is essential for the proteasome-dependent proteolysis of the mitofusins. It is conceivable that tau accumulation may reduce the recruitment of PINK1/parkin into the injured mitochondria and thus decrease the ubiquitination Scientific Reports