A highly insoluble state of Aβ similar to that of Alzheimer's disease brain is found in Arctic APP transgenic mice
Introduction
Alzheimer's disease (AD) is, according to the amyloid cascade hypothesis, initiated by aggregates of amyloid-β (Aβ) peptides which directly or indirectly cause neurodegeneration and multiple cognitive deficits. The strongest support for this theory comes from genetic discoveries of mutations in the β-amyloid precursor protein (APP) and presenilin genes that are linked to early onset AD. These mutations favor production of β-amyloid peptide 42 (Aβ42) and its deposition in the brain parenchyma (Hardy and Selkoe, 2002). Great efforts are devoted to develop drugs that clear Aβ aggregates or reduce Aβ production, e.g., by immunotherapy or by targeting regulatory enzymatic pathways. APP transgenic mice allow such disease-modifying strategies to be tested in vivo before being launched in clinical trials (Games et al., 1995, Hsiao et al., 1996). Surprisingly, a great variety of therapeutic strategies that are not well linked to Aβ biology have also been shown to reduce Aβ burden in transgenic mice. Thus, the predictive validity of the animal models seems somewhat questionable (Blennow et al., 2006). Aβ in transgenic models is largely soluble in SDS-containing buffers (Kuo et al., 2001), and therefore, likely more easy to clear, than Aβ in AD brain which does not dissolve unless formic acid (FA) or other harsh chemicals are used (Masters et al., 1985). We and others have recently generated transgenic mouse models, tgAPP-ArcSwe, producing high levels of Arctic Aβ by combining the Swedish (KM670/671NL) and Arctic (E693G) APP mutations (Lord et al., 2006, Knobloch et al., 2007). The Swedish and Arctic APP mutations both lead to early onset AD (Lannfelt et al., 1995, Basun et al., in press). The Swedish mutation is situated outside the Aβ domain and leads to overproduction of Aβ (Citron et al., 1992), while the Arctic mutation is located inside the Aβ domain and favors intracellular Aβ production (Crowther et al., 2005, Sahlin et al., 2007) and Aβ protofibril formation (Nilsberth et al., 2001, Stenh et al., 2005, Johansson et al., 2006). Interestingly, tgAPP-ArcSwe mice show granular intraneuronal Aβ immunostaining that is resistant to pretreatment with FA, and present with high levels of Aβ protofibrils, compared to tgAPP-Swe (Lord et al., 2006, Englund et al., 2007).
Here, we have compared the biochemical solubility of Aβ and investigated the neuropathology at the light microscopic and ultrastructural level in aged tgAPP-ArcSwe, tgAPP-Swe animals and in AD brain. We found that Aβ fibrils in plaques of tgAPP-ArcSwe mouse brain were densely packed and mainly SDS insoluble, and thus in some aspects more similar to those of AD brain. The characteristic structure of Aβ deposits in tgAPP-ArcSwe was also associated with an increased resistance to a therapeutic intervention.
Section snippets
Transgenic mice and human Alzheimer's disease brain
APP transgenic mice with both the Arctic (E693G) and the Swedish (KM670/671NL) APP mutations, tgAPP-ArcSwe (Lord et al., 2006) were analyzed at six (n = 6), 10 (n = 8), 14 (n = 10) and 16 (n = 13) months and mice with the Swedish APP mutation alone (tgAPP-Swe) at 12 (n = 4) and 16 (n = 11) months. The mice were genotyped, perfused with 0.9% NaCl and frozen brain tissues were prepared as described (Lord et al., 2006). Biochemical experiments were done on the mid-portion of one hemisphere between bregma
Earlier and more abundant Aβ plaque deposition in tgAPP-ArcSwe
TgAPP-ArcSwe showed threefold and tgAPP-Swe sevenfold expression of human APP in brain as compared to murine APP. Senile plaque formation began at ∼6 months of age in tgAPP-ArcSwe mice and at ∼12 months of age in tgAPP-Swe mice. The Aβ deposition markedly increased with age in both models (Fig. 1A). Aβ burden was quantified in groups of tgAPP-ArcSwe mice at 10, 14 and 16 months and tgAPP-Swe mice at 12 and 16 months of age, using C-terminal Aβ40 and Aβ42 specific antibodies. At 16 months of
Discussion
By comparing the neuropathology of tgAPP-ArcSwe and tgAPP-Swe mice we showed that the inclusion of the Arctic mutation resulted in a more rapid and abundant formation of extracellular Aβ deposits, with a variant morphological feature. Aβ deposits in tgAPP-ArcSwe mouse brain appeared 6 months earlier than in tgAPP-Swe. Amyloid cores in tgAPP-Swe brain varied greatly in size and large amyloid cores with radiating fibrils were frequently seen. In tgAPP-ArcSwe mouse brain, patches of tightly packed
Disclosure
There are no actual or potential conflicts of interest in this study. Appropriate procedures have been pursued according to guidelines for ethical conduct in science and approved by the committee of ethical conduct in research on animals at the Uppsala University (permits C242/5 and C258/6, approved 2005-10-28 and 2006-11-24, respectively).
Acknowledgements
This work was funded by grants from Uppsala University, Landstinget i Uppsala län, The Swedish Brain Fund, Bertil Hållstens Forskningsstiftelse, Alzheimerfonden (L.L.), the Sahlgrenska University Hospital (K.B.), Gamla Tjänarinnor, Gun och Bertil Stohnes Stiftelse (L.N., O.P.), Magnus Bergvall, Åhlénsstiftelsen, Lars Hierta, Lundströms Minne, Frimurarstiftelsen, Svenska Läkarsällskapet (L.N.), the Swedish Research Council [(#2006-2822) L.L., (#2006-2818) L.N., (#2006-6326; #2006-3464) M.I.,
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