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  • It is well described that Tau is a

    2019-07-05

    It is well described that Tau is a MAP and that its phosphorylation state regulates Tau binding to MTs, which correlates with Tau aggregation and MT damage in AD [50,51]. In AD brains it was reported that Tau phosphorylation at ser396 is significantly increased when compared with other phosphorylation sites, and occurs prior to the appearance of NFTs [52]. Tau P301L mutation is characterized by the accumulation of phosphorylated Tau (ser396) and is widely used to study tauopathies. As expected, we found that in SH-SY5Y Ezatiostat ic50 transfected with Tau constructs that carry P301L mutation, phosphorylated Tau protein levels are increased relatively to the parental SH cells. Interestingly, this was also observed in hybrid cells harboring sAD patient mitochondria. Compelling evidence suggests a preponderant involvement of MTs derived-axonal transport failure in AD and PD [53]. Acetylation of MT-α-tubulin plays a role in the maintenance of stable populations of MTs [14]. In line with this, we observed in Tau and sAD cells, as well as, in ASYN cells that acetylated-tubulin levels were decreased indicating MT instability and axonal transport defects. Recently, studies have shown that Tau is also post-translationally modified by lysine acetylation being a disease-specific modification in AD, likely representing a major regulatory tau modification [21,42,54]. However, data from literature on this topic is contradictory. It was described that Tau is acetylated and that this acetylation prevents the degradation of phosphorylated Tau [21]. On the other hand, it was reported that the acetylation of Tau on KXGS motifs (in the MT-binding domain) inhibits phosphorylation and also prevents Tau aggregation [45]. In this study we show that Tau phosphorylation increase is positively correlated with Tau acetylation in sAD and fAD models which underlines MT instability characterized by a decrease in α-tubulin acetylation leading to a reduction in the autophagic flux [55]. We demonstrated that tubulin deacetylation by SIRT2 or HDAC6 activation in the cytosol leads to MT loss of stability and depolymerization, which facilitates Tau dissociation and consequent phosphorylation in AD models. In fact, in 3xTg-AD mice SIRT2 inhibition was shown to increase tubulin acetylation [56]. Surprisingly, p300 inhibition did not change tubulin acetylation in any AD models. Although there is no described evidence in the literature in respect to tubulin as a p300 substrate, we observed a significant decrease in acetylated tubulin in the treated C646 WT SH-SY5Y and CT cybrid cells. Until now the exact mechanism by which C646 decreases acetylated tubulin is not clear, but it was described that p300 can modulate the function of HDAC6 and SIRT2. It was reported that p300 interacts with and acetylates HDAC6 resulting in the down-regulation of HDAC6 deacetylase activity [57]. Similarly p300 was reported to mediate SIRT2 deacetylase activity [57]. These results show that p300 can regulate the acetylation status of tubulin indirectly through the action of SIRT2 and HDAC6. Moreover, p300 appears to work directly by decreasing Tau acetylation levels and indirectly as a modulator of Tau phosphorylation. Corroborating this data, Tau acetylation by p300 was seen in the early stages of AD and hyperacetylation impairs Tau degradation, promoting the accumulation of abnormally phosphorylated Tau [21,58]. We found that in fAD and sAD models both SIRT2 and HDAC6 inhibition improved tubulin acetylation but only C646 decreased Tau acetylation. Additionally, we found that the inhibition of p300, HDAC6 and SIRT2 contributed to the reduction of phosphorylated Tau in AD probably due to a decrease in acetylated Tau or increase in tubulin acetylation favoring autophagic cargo removal in both models. This result is interesting taking into account that HDAC6 tubulin deacetylase activity has been shown to be required for autophagy progression and clearance. However, others have demonstrated that inhibition of this enzyme increases the acetylation of MTs and contributes to an improvement of autophagy. On the other hand, SIRT2 over-activation has been associated with reduction of autophagy efficiency [59].