Amyloid deposition precedes tangle formation in a triple transgenic model of Alzheimer’s disease
Introduction
Alzheimer’s disease (AD) is an age-dependent and irreversible neurodegenerative disorder that causes a progressive deterioration of cognitive functions, including a profound loss of memory. The hallmark neuropathological lesions of AD include amyloid deposits, in the form of either diffuse or neuritic plaques, and neurofibrillary tangles, which consist of hyperphosphorylated tau aggregates [9]. The occurrence of both of these signature lesions is necessary to neuropathologically confirm a diagnosis of AD.
AD can manifest either sporadically or be transmitted in an autosomal dominant fashion. Three genetic loci have been found to underlie autosomal dominant, early onset AD: βAPP on chromosome 21, PS1 on chromosome 14, and PS2 on chromosome 1 [10]. Clinical mutations in each of these genes alter the metabolism of APP processing, leading to either increased levels of total Aβ or a selective augmentation of the longer more amyloidogenic Aβ42 species [7]. It is predominantly this genetic evidence that has provided the strongest support for the amyloid cascade hypothesis, which predicts that Aβ is the trigger for all cases of AD [3]. In contrast, mutations in the tau gene do not lead to AD, but rather to another form of dementia called frontotemporal dementia with parkinsonism-17, which is marked by neurofibrillary pathology similar to that in AD, although without any amyloid deposition.
Gene-targeted and transgenic mice have proven to be invaluable for studying the pathogenesis of AD, although no transgenic model recapitulates its complete neuropathological spectrum [12]. For example, the overexpression of mutant isoforms of human βAPP in transgenic mice leads to amyloid deposition in the murine brain, but is insufficient for triggering the full spectrum of AD neuropathology, including the development of neurofibrillary pathology. This outcome is surprising because human genetic data indicate that overproduction of βAPP, such as in Down syndrome, is sufficient to trigger the complete spectrum of AD neuropathology. Therefore, the concomitant manifestation of both plaques and tangles in a mouse requires aggressive biotechnical strategies, such as introducing multiple transgenes into a mouse or through alternative means such as the microinjection of pathological proteins into the brains of genetically modified mice.
We recently reported the derivation of a triple transgenic model of AD [6]. The triple transgenic mice (3xTg-AD) harbor three mutant transgenes: PS1M146V, APPSwe, and tauP301L. The 3xTg-AD mice develop an age-dependent and progress neuropathology that includes plaque and tangle pathology. Here we show that despite equivalent overexpression of human APP and tau, Aβ pathology precedes typical indications of tau pathology such as conformational or hyperphosphorylation changes in the tau protein; these results are consistent with the amyloid cascade hypothesis which predicts that Aβ deposition is the earliest pathological trigger of AD.
Section snippets
Generation of triple transgenic mice
Human βAPP cDNA harboring the Swedish double mutation and human tauP301L cDNA were individually subcloned into the Thy1.2 expression cassette. Both fragments were isolated from the cloning vector by digestion with EcoRI and PvuI and purified by sucrose gradient fractionation. After overnight dialysis in injection buffer (10 mM Tris, pH 7.5, 0.25 mM EDTA), the two constructs were microinjected into the pronuclei of single-cell embryos from PS1M146V knockin mice at the Transgenic Mouse Facility at
Triple transgenic model
In an attempt to better phenocopy the major neuropathological features of AD, we generated a novel triple transgenic model, harboring three mutant human transgenes: PS1M146V, βAPPSwe, and tauP301L. Details about the generation of this model have been described elsewhere [6]. Briefly, we co-microinjected two independent transgenes encoding human βAPPSwe and human tauP301L, both under control of the mouse Thy1.2 regulatory element, into single-cell embryos harvested from homozygous mutant PS1M146V
Discussion
In this study, we characterized the regional and temporal profile of Aβ and tau aggregates in the 3xTg-AD hemizygous and homozygous mice. We find that the anatomical and temporal pattern of both signature lesions develop analogously to that observed in the AD brain [5]. For Aβ deposits, we observed both diffuse and fibrillar Aβ aggregates, each of which was reactive with an anti-Aβ42-specific antibody. Cortical areas were initially more affected than subcortical regions such as the hippocampus.
Acknowledgements
This work was supported by a grant from the Alzheimer’s Association and by the National Institutes of Health Grants AG17968 and AG0212982.
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