Hirano Bodies Protect against Cell Death Figure 11. Both 352PHPtau and c31 co-localize with model Hirano bodies. H4 cells were transfected with 352PHPtau or c31, and either CT- GFP to induce model Hirano bodies or GFP as a control. In control cells, tau is predominantly found in the cytoplasm while c31 is evenly diffused throughout the cell. In cells expressing CT-GFP, c31 is primarily found co-localizing with model Hirano bodies. Tau also co-localizes with model Hirano bodies, but its overall distribution is not altered. Scale bar 15 mm. doi:10.1371/journal.pone.0044996.g011 patients [87]. These mice also exhibit hyperphosphorylated tau, increased levels of insoluble tau, and neurodegeneration also found in Alzheimer’s patients [78]. Finally, mice with a mutation in the AICD PDAPP(D664A) which cannot be cleaved by caspases and overexpression of Fe65 have reduced levels of seizures to that approaching wild type mice [84], suggesting a role for further proteolytic processing of AICD to c31 in neurodegeneration. Interaction of Tau and c-terminal Fragments of APP The interaction of tau with APP and its proteolytic fragments in the progression of Alzheimer’s disease has been shown in numerous studies (reviewed [14,62]). However, a direct link between tau and AICD in cell death has not been documented. We have shown for the first time that both AICDc58 as well as APP/c31-induced cell death significantly increases in the presence of 352PHPtau, a mutant form of tau that mimics hyperpho- sphorylation [54,55]. The amount of exogenous APP required to induce cell death in the presence of both c31 and 352PHPtau is lower than for APP/c31 alone. Furthermore, the amount of cell death induced by exogenous APP/c31/352PHPtau is greater than that induced by either APP/c31 or 352PHPtau alone, indicating a relationship between APP, c31, and tau in causing cell death. The presence of model Hirano bodies significantly decreased the level of cell death. In contrast, 352WTtau, unlike 352PHPtau did not significantly increase AICDc58-induced cell death. The mecha- nism underlying the ability of 352PHPtau to significantly increase AICDc58-induced cell death compared to 352WTtau is not clear. It has been shown that phosphorylation of specific residues on tau is required to prime tau for additional phosphorylation [88,89,90,91]. It has also been shown that 352PHPtau becomes phosphorylated in cell cultures [92]. It is possible that the additional phosphorylation of 352PHPtau contributes to the enhanced cell death observed in the presence of either AICDc58 or c31 but the exact mechanism is unknown. Numerous prior studies support the interaction of APP and tau in progression of Alzheimer’s disease. A link between APP and tau has been supported in whole animal models [28,29,93]. Others have shown that cognitive decline in transgenic mice, due to expression of familial disease mutant forms of APP, are ameliorated by deletion of tau [28,29,94]. It has recently been shown that amyloid-beta can cause tau hyperacetylation in primary neurons through an unknown mechanism [93,95]. Acetylation of tau prevents its degradation by competing with ubiquitination, which targets tau to the proteasome. Phosphory- lated forms of tau begin to accumulate leading to hyperpho- sphorylated states [93]. We suggest tau enhances the level of APP/ AICDc58 and APP/c31-induced cell death, and could contribute to progression of Alzheimer’s disease. PLOS ONE | www.plosone.org 7 September 2012 | Volume 7 | Issue 9 | e44996