Previous studies have shown that brain-derived Aβ aggregates can induce Aβ lesions in the brains of susceptible mice [11
]. In the present study, we used the genomic-based R1.40 APP-tg model as host. These animals exhibit endogenous Aβ deposition at approximately 15 months of age [13
], thus allowing for the analysis of extended incubation periods in Aβ extract-inoculated mice. Our results reveal that intracerebral injections of Aβ-containing brain extract also induce Aβ deposition in this model, although the Aβ induction after a 6-month incubation period was less than that seen in APP23 mice after 4 months of incubation [16
]. This observation is consistent with the 3-fold overexpression of human APP in R1.40 mice compared to 7-fold overexpression in APP23 animals [26
], and supports the view that the induction of Aβ deposition is dependent on the concentration of Aβ in the inoculum as well its production by the host [8
We found that a 6-month incubation period induced similar levels of Aβ aggregation in the brains of young (3 months) and older (9 months) R1.40 APP-tg mice, indicating that the aged brain does not provide a more favorable environment for the induction of Aβ deposition once Aβ seeds are present. Our results, while surprising in light of the view that age is a prominent risk factor for amyloid lesions [7
], may be attributed to the fact that the brain levels of Aβ are constant in pre-depositing R1.40 mice between 3 months and 15 months of age [17
]. Unchanging Aβ levels prior to Aβ pathology also have been reported in other AD mouse models [12
]. These findings suggest that age may increase the risk of AD by impairing the ability of cells to dispose pathogenic seeds [20
]. In addition, it cannot be excluded that a similar experiment in which seeding is compared in a young host vs. a host at an age closer to the end of its mean life span would yield different results.
The most striking finding to emerge from the inoculations in R1.40 mice was the presence of Aβ deposits in much of the forebrain after the 12 month incubation period. Previous work has shown that the focal infusion of Aβ seeds into the hippocampus is sufficient to induce Aβ deposition throughout the entire hippocampus and even in connected brain areas; however, induction of lesions in widespread regions of the forebrain has not been previously observed [5
]. The mechanism of this extensive spreading of Aβ deposition is not clear, but implies that seeds not only migrate within brain structures and along defined neuronal pathways, but may also disseminate along perivascular fluid drainage channels, by vascular transport, or by simple diffusion through the brain parenchyma [10
]. The recent finding that small, soluble Aβ species are particularly potent inducers of β-amyloidosis [16
], similar to the strong infectivity of small, non-fibrillar prion particles [24
], underscores potential mechanistic similarities in the induction and spread of Aβ and PrP aggregates in the brain.
Birefringence under crossed-polarizing filters is indicative of the amyloid-like nature of proteinaceous deposits after staining with the dye Congo red [30
]. In seeded R1.40 APP-tg mice, the duration of the incubation period influenced the degree to which the resulting Aβ deposits were congophilic. Whereas the 6-month incubation period, starting at either 3 or 9 months of age, did not result in the induction of congophilic lesions, Congo red-positive plaques were observed after the 12-month incubation period. However, congophilic deposits were only observed near the injection site, indicating that diffuse pathology occurs first and eventually develops into Congo red-positive (i.e., amyloid) plaques. Because the cytological changes associated with Aβ deposition are most strongly associated with amyloid per se
] rather than with diffuse deposits, the lack of congophilic amyloid induction after the 6-month incubation period did not allow us to determine whether the amyloid-associated cytopathology (neuritic dystrophy and glial activation) differs between young (9 months) and older (15 months) hosts [7
From a translational perspective, the present findings support the widely held notion that therapies directed at mitigating the formation of Aβ pathology should begin early in the pathogenic cascade that leads to AD [19
]. The exposure to Aβ seeds profoundly influences the onset and degree of β-amyloid pathology in APP-tg mouse models. While there is not yet direct evidence that exogenous seeds are involved in the initiation of AD, it is likely that endogenously generated seeds become increasingly prevalent with age due to the decline of proteostatic processes [20
]. The neutralization or removal of endogenous Aβ seeds thus remains a promising but elusive therapeutic goal for interfering with the pathogenesis of AD.