Induction of cerebral beta-amyloidosis: intracerebral versus systemic Abeta inoculation

Abstract

Despite the importance of the aberrant polymerization of Abeta in the early pathogenic cascade of Alzheimer's disease, little is known about the induction of Abeta aggregation in vivo. Here we show that induction of cerebral beta-amyloidosis can be achieved in many different brain areas of APP23 transgenic mice through the injection of dilute Abeta-containing brain extracts. Once the amyloidogenic process has been exogenously induced, the nature of the induced Abeta-deposition is determined by the brain region of the host. Because these observations are reminiscent of a prion-like mechanism, we then investigated whether cerebral beta-amyloidosis also can be induced by peripheral and systemic inoculations or by the intracerebral implantation of stainless steel wires previously coated with minute amounts of Abeta-containing brain extract. Results reveal that oral, intravenous, intraocular, and intranasal inoculations yielded no detectable induction of cerebral beta-amyloidosis in APP23 transgenic mice. In contrast, transmission of cerebral beta-amyloidosis through the Abeta-contaminated steel wires was demonstrated. Notably, plasma sterilization, but not boiling of the wires before implantation, prevented the induction of beta-amyloidosis. Our results suggest that minute amounts of Abeta-containing brain material in direct contact with the CNS can induce cerebral beta-amyloidosis, but that systemic cellular mechanisms of prion uptake and transport to the CNS may not apply to Abeta.

Fig. 1.

Induction of Aβ deposition in different brain regions of young APP23 mice. β-amyloid-containing brain extract from aged APP23 transgenic mice (Tg extract) or extract from age-matched nontransgenic (wild-type) control mice (WT extract) was injected into the hippocampus, entorhinal cortex, parietal cortex, striatum, or olfactory bulb of 2- to 5-month-old APP23 mice. The mice were then analyzed 3 to 6 months postinjection. Aβ immunoreactivity was detected in all brain regions injected with the Tg brain extract at 3 months (data not shown) and 6 months (A, C, E, G, I, and K), but not in regions injected with the WT extract (B, D, F, H, J, and L). The postsurgery interval for the olfactory bulb was 4 months. The induction of Aβ deposits was most robust in proximity to the injection site, but was observed throughout the injected brain region. Some of the induced amyloid deposits in the hippocampus, neocortex, and entorhinal cortex were congophilic, while only diffuse Aβ deposits were found in the striatum (inserts in A, C, G, I, and K). Aβ deposition was not induced in the APP23 mice after PBS injections or when Tg extract was injected into nontransgenic littermate control mice (data not shown). The number of mice was n = 3 for each region and time point and type of extract. (M) Brain extracts were analyzed by Western blot. Aβ bands (monomer and oligomer bands denoted by asterisks) were detected in the Tg extract, but not in the WT extract, using the human Aβ-specific antibody 6E10. A mixture of synthetic Aβ1–40 and Aβ1–42 (syn Aβ) was used as a positive control. (Scale bar: 200 μm; inserts, 20 μm.)

Fig. 2.

Stainless steel wire coated with Aβ rich brain extract induced cerebral β-amyloidosis. Stainless steel wires were immersed for 16 h at room temperature with brain extract from aged APP23 transgenic mice (Tg extract) (A and C) or aged wild-type control mice (WT extract) (B). After air-drying, wires were implanted unilaterally into the hippocampus and overlying cortex of APP23 hosts (stereotaxic coordinates: AP, −2.5 mm; L, +2.0 mm; DV, −1.8 mm). Immunohistochemical analysis with anti-Aβ antibody revealed strong local induction of Aβ deposits in the vicinity of the Tg extract-coated wire (arrowhead in A). The number of mice used was n = 8. Higher magnification of the dentate gyrus region of adjacent sections double-stained with anti-Aβ antibody and Congo red (C) revealed spreading of Aβ deposition along the hippocampal fissure and dentate gyrus molecular layers (distance between sections shown, 600 μm). No Aβ deposits were observed when the wire was coated with WT extract (B). The number of mice used was n = 4. Bicine-tris urea SDS/PAGE of coated wires and subsequent Western blot detection with 6E10 antibody (D) revealed detectable amounts of Aβ on wire (WTg; lane 2), corresponding to approximately 0.02–0.05 μL of 10% seeding extract (reflecting 0.2–0.5 × 10⁻⁵ g of total brain) (lane 1). Similar amounts of Aβ were detected after the wires were heated in PBS for 10 min at 95 °C (WTg 95 °C; lane 3). After plasma sterilization (WTg pl; lane 4), no Aβ40 or Aβ42 was left on the wire, but a weak higher-molecular-weight band appeared that likely represents Aβ-specific signal, because plasma-sterilized wires coated with WT extract exhibited no bands (WWT pl; lane 5). (Scale bar: 1,000 μm in A and B; 200 μm in C.)