Exogenous seeding of cerebral β-amyloid deposition in βAPP-transgenic rats

Abstract

Deposition of the amyloid-β (Aβ) peptide in senile plaques and cerebral Aβ angiopathy (CAA) can be stimulated in Aβ-precursor protein (APP)-transgenic mice by the intracerebral injection of dilute brain extracts containing aggregated Aβ seeds. Growing evidence implicates a prion-like mechanism of corruptive protein templating in this phenomenon, in which aggregated Aβ itself is the seed. Unlike prion disease, which can be induced de novo in animals that are unlikely to spontaneously develop the disease, previous experiments with Aβ seeding have employed animal models that, as they age, eventually will generate Aβ lesions in the absence of seeding. In the present study, we first established that a transgenic rat model expressing human APP (APP21 line) does not manifest endogenous deposits of Aβ within the course of its median lifespan (30 months). Next, we injected 3-month-old APP21 rats intrahippocampally with dilute Alzheimer brain extracts containing aggregated Aβ. After a 9-month incubation period, these rats had developed senile plaques and CAA in the injected hippocampus, whereas control rats remained free of such lesions. These findings underscore the co-dependence of agent and host in governing seeded protein aggregation, and show that cerebral Aβ-amyloidosis can be induced even in animals that are relatively refractory to the spontaneous origination of parenchymal and vascular deposits of Aβ.

Fig. 1

Analysis of senile plaques, CAA and APP in unseeded APP21 rats. A: Low magnification overview of a parasagittal section from an unseeded 30-month-old APP21 rat; B: Higher magnification of the hippocampal formation and dentate gyrus. Antibody 6E10 detected light immunoreactivity of transgenic (human-sequence) Aβ/APP in neuronal somata throughout the brain, but no extracellular deposition of Aβ was seen in any unseeded APP21 rat. Bar in panel B = 200μm. C: Western blot of cortical homogenates from APP21 and wild-type control rats at three different ages, immunostained with mouse monoclonal antibody 6E10. A preparation of aggregated, synthetic Aβ42 (10ng) is in the far left lane as a positive control. In the transgenic rats only, 6E10-immunoreactive bands corresponding to APP (~100kDa) were detected in similar quantities at all 3 ages. Aβ in all rats was below detection level. The bands at ~17kDa (arrowhead) are nonspecific cross-reactive material.

Figure 2

Seeded deposition of Aβ in the hippocampus proper and dentate gyrus (A), and the subiculum (B) of a representative, 12-month old APP21 transgenic rat that had received an intracerebral infusion of dilute AD cortical extract at 3 months of age. Diffuse Aβ deposits occurred in the walls of blood vessels (black arrow), as sheet-like formations (arrowheads mark Aβ immunoreactivity in the granule cell layer), and as parenchymal, plaque-like deposits (gray arrow, middle). A mean of 2.3 ± 0.8% of the area of the dorsal hippocampus was occupied by Aβ deposits in the 9-month seeded rats. C: Light, perivascular Aβ deposition (arrow) in the hippocampal formation of an APP21 transgenic rat 3 months following infusion of cortical extract. Non-transgenic rats similarly injected (D) did not have immunoreactive Aβ in the hippocampus after 9 months. Antibodies 4G8 (A) and 6E10 (B-D); Bars = 200 μm.