Transmission and spreading of tauopathy in transgenic mouse brain

Abstract

Hyperphosphorylated tau makes up the filamentous intracellular inclusions of several neurodegenerative diseases, including Alzheimer's disease. In the disease process, neuronal tau inclusions first appear in the transentorhinal cortex from where they seem to spread to the hippocampal formation and neocortex. Cognitive impairment becomes manifest when inclusions reach the hippocampus, with abundant neocortical tau inclusions and extracellular beta-amyloid deposits being the defining pathological hallmarks of Alzheimer's disease. An abundance of tau inclusions, in the absence of beta-amyloid deposits, defines Pick's disease, progressive supranuclear palsy, corticobasal degeneration and other diseases. Tau mutations cause familial forms of frontotemporal dementia, establishing that tau protein dysfunction is sufficient to cause neurodegeneration and dementia. Thus, transgenic mice expressing mutant (for example, P301S) human tau in nerve cells show the essential features of tauopathies, including neurodegeneration and abundant filaments made of hyperphosphorylated tau protein. By contrast, mouse lines expressing single isoforms of wild-type human tau do not produce tau filaments or show neurodegeneration. Here we have used tau-expressing lines to investigate whether experimental tauopathy can be transmitted. We show that injection of brain extract from mutant P301S tau-expressing mice into the brain of transgenic wild-type tau-expressing animals induces assembly of wild-type human tau into filaments and spreading of pathology from the site of injection to neighbouring brain regions.

Figure 1. Induction of filamentous tau pathology in ALZ17 mice injected with brain extract from mice transgenic for human P301S tau

(a) Staining of the hippocampal CA3 region from 18 month-old ALZ17 mice with anti-tau antibody AT8, Gallyas-Braak silver or anti-tau antibody AT100. Non-injected (left), 15 months after injection with brain extract from non-transgenic control mice (middle) and 15 months after injection with brain extract from 6 month-old mice transgenic for human P301S tau protein (right). The sections were counterstained with haematoxylin. Scale bar, 50 μm (same magnification in all panels). (b) Immunoelectron microscopy of filaments extracted from the brain of an ALZ17 mouse 15 months after the injection of brain extract from mice transgenic for human P301S tau. Labelling with anti-tau sera 134, 189 and 304, and antibody AT100. Scale bar, 100 nm. (c) No filamentous tau pathology in ALZ17 mice injected with tau-immunodepleted human P301S tau brain extract. Top left: Western blot with anti-tau antibody HT7 of P301S brain extract before (lane 1) and after (lane 2) immunodepletion. Clockwise: Gallyas-Braak staining of dentate gyrus, subiculum, and fimbria of ALZ17 mice 6 months after injection with tau-immunodepleted P301S brain extract. The sections were counterstained with haematoxylin. Scale bar, 50 μm (same magnification in all panels). The full scan of the Western blot data is available in the Supplementary Information, Fig. S7.

Figure 2. Temporal increase in the number of Gallyas-Braak-positive structures at the injection sites (− 2.5 mm from bregma) in ALZ17 mice

(a) A statistically significant increase in silver-positive structures was observed in hippocampus between 6 and 12 months and between 12 and 15 months. In cerebral cortex, a significant increase was present between 12 and 15 months. The results are expressed as means ± S.E.M. (n=5). *p<0.05. (b) In hippocampus, significant increases in neuropil threads and coiled bodies were observed between 6 and 12 months, and between 12 and 15 months. For neurofibrillary tangles, a significant increase was observed between 12 and 15 months. The results are expressed as means ± S.E.M. (n=5). *p<0.05; **p<0.001; ***p<0.0001. (c) Gallyas staining of the hippocampal region (CA1, stratum radiatum and subiculum) of ALZ17 mice injected with human P301S tau brain extract 6 (top), 12 (middle) and 15 (bottom) months after injection. The sections were counterstained with haematoxylin. Scale bar, 50 μm (same magnification in all panels).

Figure 3. Spreading of filamentous tau pathology in ALZ17 mice injected with brain extract from mice transgenic for human P301S tau

Gallyas-Braak silver staining of brain regions at a distance from the injection sites 15 months post-injection. The sections were counterstained with haematoxylin. Scale bar, 50 μm (same magnification in all panels).

Figure 4. Induction of filamentous tau pathology in non-transgenic C57BL/6 mice injected with brain extract from mice transgenic for human P301S tau

Gallyas-Braak silver staining and immunostaining with phosphorylation-dependent anti-tau antibody AT8, murine tau-specific antibody MT1 and human tau-specific antibody T14 in the fimbria of a non-transgenic mouse 6 months after the injection with the human P301S tau brain extract (left column). Gallyas-Braak-, AT8- and MT1-positive, T14-negative, tau deposits were observed in neurites (arrows) and in oligodendrocytes (arrowheads). No tau pathology was observed in age-matched sham-lesioned animals (right column). The sections were counterstained with haematoxylin. Scale bar, 50 μm (same magnification in all panels).