Increased misfolding and truncation of tau in APP/PS1/tau transgenic mice compared to mutant tau mice
Introduction
Sporadic and familial forms of Alzheimer's disease (AD) due to mutations in the presenilins (PS) and the amyloid precursor protein (APP) genes are neuropathologically characterized by the presence of both neurofibrillary tangles (NFT) and senile plaques. NFT are made of bundles of paired helical filaments (PHF) composed of the microtubule-associated protein tau (Brion et al., 1985). PHF are composed of modified tau (PHF-tau) proteins that are hyperphosphorylated and abnormally phosphorylated (Grundke-Iqbal et al., 1986), conformationally altered, and truncated (Buée et al., 2000). Senile plaques are complex structures made of an extracellular core of amyloid surrounded by dystrophic neurites and reactive glial cells. The amyloid core is mainly composed of the Aβ peptide derived from proteolytic cleavage of APP. According to the amyloid cascade hypothesis, the formation of the Aβ peptide is an upstream event inducing tau phosphorylation, tau aggregation and NFT formation (Hardy and Selkoe, 2002). However, transgenic mice overexpressing mutated APP and/or PS1 gene(s) do not develop NFT, even when they also express a human wild-type tau protein (Boutajangout et al., 2004). Conversely, transgenic mice expressing an FTD mutant tau develop NFT but not amyloid plaques. Several studies in animal models have nevertheless indicated that Aβ can increase tau pathology in transgenic mice expressing a FTD mutant tau. Injection of Aβ (Gotz et al., 2001) or of Aβ-containing brain extract from APP mice (Bolmont et al., 2007) was reported to increase NFT formation in mutant P301L tau mice. Double transgenic mice expressing mutants APP and tau proteins develop an enhanced tau pathology (Bolmont et al., 2007, Hurtado et al., 2010, Lewis et al., 2001, Paulson et al., 2008, Perez et al., 2005, Seino et al., 2010, Terwel et al., 2008). The enhancement of tauopathy by Aβ accumulation in APP/tau models has been described in some models as slow, developing with aging, and relatively modest (Paulson et al., 2008, Perez et al., 2005, Seino et al., 2010, Terwel et al., 2008). The mechanisms of worsened tau pathology in these APP/tau models are however not well understood, and potential additional misprocessing of tau in these APP/tau models compared to tau in FTD tau models has not been clarified. This link between Aβ-mediated toxicity and tau pathology is relevant for the understanding of AD since tau pathology by itself can cause neurodegeneration (Ballatore et al., 2007).
The potential involvement of PS1 in modulation of tauopathy has also not been extensively investigated, except in the 3xTg-AD transgenic model in which expression of mutants APP, PS1 and P301L tau resulted in formation of amyloid deposits and accumulation of phosphorylated tau, although the potential enhancement of neurofibrillary degeneration compared to single P301L tau transgenic mice was not analyzed (Oddo et al., 2003).
To further investigate the misprocessing of tau and the modulation of tauopathy in experimental models developing an Aβ pathology, we compared tau pathology and Aβ load in FTD mutant tau transgenic mice (Tg30), mutant APP/PS1 line transgenic mice (5xFAD) and in a new model resulting from their crossing. These APP/PS1 × Tg30 mice (named in the text 5xFAD × Tg30) showed a severe enhancement of neurofibrillary degeneration associated to additional conformational, phosphorylation and cleavage changes in tau mimicking more closely than in Tg30 mice tau the modifications observed in PHF-tau in AD, and to an increased recruitment of endogenous wild-type mouse tau in NFT. These results suggest that mutant APP/PS1 or amyloid pathology was driving additional misprocessing of tau responsible for the acceleration and enhancement of tau pathology, a mechanism that might play a role in the development of tau pathology and neuronal loss in AD.
Section snippets
Generation of 5xFAD × Tg30 mice
The transgenic mouse line Tg30 tau express a mutated tau transgene (1N4R human tau isoform bearing G272V and P301S mutations), under transcriptional control of the neuron-specific mouse thy-1 promoter (Schindowski et al., 2006) and has been described previously (Leroy et al., 2007). The Tg30 mice were maintained on C57Bl/6J genetic background and only heterozygous transgenic mice were used for this study. The 5xFAD double transgenic mice co-express and co-inherit the 695 amino acids isoform of
Expression of mutant tau, APP and PS1 in 5xFAD × Tg30 mice
The expression of human tau, human APP and human PS1 transgenic proteins in the brain and spinal cord was analyzed by western blotting with human specific antibodies. Human tau proteins were expressed only in Tg30 and 5xFAD × Tg30 mice whereas human mutants PS1 and APP were expressed only in 5xFAD and in 5xFAD × Tg30 mice (Fig. 1A). At 3 months (Fig. 1B) of age, the levels of human APP, human tau and human PS1 (full length or N-terminal fragment (NTF PS1)), were not significantly different in the
Discussion
To analyze and adequately compare misprocessing of tau and generation of NFT in a mutant tau mouse without and with concomitant development of Aβ pathology, we crossed 5xFAD mice, an AD amyloid model co-expressing mutants human APP and PS1 with Tg30 mice, a mouse model of tauopathy overexpressing mutant human tau. Crossing of 5xFAD mice with Tg30 mice allowed adequate comparison of behavioral deficits, NFT, tau misprocessing and Aβ levels between 5xFAD, Tg30 and 5xFAD × Tg30 mice, an analysis
Acknowledgments
This study was supported by grants from the Diane program (Walloon region) (816856) and from the Fonds de la Recherche Scientifique Médicale (3.4504.10) and performed in the frameship of the Interuniversity Attraction Poles program (P7/16) of the Belgian Federal Science Policy Office. We thank Dr R. Vassar (Northwestern University, Chicago) for providing the Tg6799 APP/PS1 mice and Dr Luc Buée (INSERM, U837, Lille) for providing the Tg30 line.
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