The propagation of prion-like protein inclusions in neurodegenerative diseases  Michel Goedert, Florence Clavaguera, Markus Tolnay  Trends in Neurosciences  Volume 33, Issue 7, Pages 317-325 (July 2010) DOI: 10.1016/j.tins.2010.04.003 Copyright © 2010 Elsevier Ltd Terms and Conditions

Figure 1 Temporospatial spreading of tau-positive neurofibrillary lesions in the process of Alzheimer's disease (left, green) and α-synuclein-positive lesions (Lewy bodies and neurites) in the process of Parkinson's disease and dementia with Lewy bodies (right, red). (Left) According to Braak and Braak [18], six stages (I–VI) of tau pathology can be distinguished. Stages I–II show alterations that are largely confined to the upper layers of the transentorhinal cortex (transentorhinal stages). Stages III–IV are characterized by a severe involvement of the transentorhinal and entorhinal regions, with a less severe involvement of the hippocampus and several subcortical nuclei (limbic stages). Stages V–VI show the massive development of neurofibrillary pathology in neocortical association areas (isocortical stages) and a further increase in pathology in the brain regions affected during stages I–IV. The shading intensities of the areas colored in green are proportional to the severity of tau pathology. Adapted, with permission, from Ref. [18]. (Right) According to Braak et al. [19,83], six stages (I–VI) of α-synuclein pathology can be distinguished. The first lesions appear in the olfactory bulb, the anterior olfactory nucleus and the dorsal motor nucleus of the vagus and glossopharyngeal nerves in the medulla oblongata (stages I and II). From the brainstem, the inclusions take an ascending path to the lower raphe nuclei, the gigantocellular reticular nucleus and the locus coeruleus (indicated by white arrows). In stages III and IV, they reach the amygdala, the cholinergic nuclei of the basal forebrain and the substantia nigra. The cerebral cortex also becomes affected starting with the anteromedial temporal mesocortex. In stages V and VI, the inclusions spread to the higher order sensory association and prefrontal areas, the first order sensory association areas, the premotor area and the primary sensory and motor fields. The shading intensities of the areas colored in red are proportional to the severity of α-synuclein pathology. Adapted, with permission, from Ref. [83]. Trends in Neurosciences 2010 33, 317-325DOI: (10.1016/j.tins.2010.04.003) Copyright © 2010 Elsevier Ltd Terms and Conditions

Figure 2 Schematic representation of the human tau gene and the six tau isoforms expressed in adult brain. The human tau gene consists of 16 exons (E). Alternative splicing of E2 (red), E3 (green) and E10 (yellow) gives rise to the six tau isoforms (352–441 amino acids). The constitutively spliced exons (E1, E4, E5, E7, E9, E11, E12, E13) are indicated in blue. E0, which is part of the promoter, and E14 are non-coding (white). E6 and E8 (violet) are not transcribed in human brain. E4a (orange) is only expressed in the peripheral nervous system. Black bars indicate the microtubule-binding repeats of tau, with three isoforms having four repeats each (4R hTau) and three isoforms having three repeats each (3R hTau). Each repeat is 31 or 32 amino acids in length. Similar levels of 4R and 3R tau isoforms are expressed in normal human brain. The exons and introns are not drawn to scale. Trends in Neurosciences 2010 33, 317-325DOI: (10.1016/j.tins.2010.04.003) Copyright © 2010 Elsevier Ltd Terms and Conditions

Figure 3 Induction of filamentous tau pathology in the brain of transgenic ALZ17 mice expressing human wild-type tau following the injection of brain extract from mice transgenic for human mutant P301S tau. (a) Mice expressing the 383 amino acid four-repeat isoform of human tau (4R hTau) with the P301S mutation under the control of the murine Thy1 promoter develop abundant Gallyas–Braak silver-positive filamentous tau inclusions and widespread nerve cell loss, including in the brainstem, the brain region used for preparation of the extract injected in parts (c) and (d). The silver-positive tau inclusions are immunoreactive with antibody AT8, a marker for hyperphosphorylated tau. In humans, the P301S mutation causes an aggressive form of FTDP-17T [109]. (b) In contrast, mice expressing the 441 amino acid 4R hTau isoform under the control of the murine Thy1 promoter (line ALZ17) do not develop Gallyas–Braak silver-positive inclusions (right inset) or nerve cell loss, even though human tau is hyperphosphorylated at the AT8 epitope (left inset), as shown for the hippocampus. Hyperphosphorylation (detected by AT8) precedes the assembly of tau into filaments (detected by Gallyas–Braak silver). The mechanistic connections between hyperphosphorylation and aggregation of tau, two invariant features of human tauopathies, remain to be fully elucidated. (c) The injection of brain extract from P301S tau transgenic mice into the hippocampus and the cerebral cortex of ALZ17 mice induces the formation of Gallyas–Braak silver-positive inclusions made of filamentous, hyperphosphorylated wild-type human tau [29]. Hippocampal dentate gyrus from an ALZ17 mouse is shown 15 months after the injection of brainstem extract from a 6-month-old P301S mouse. Silver-positive neurofibrillary tangles, neuropil threads and oligodendroglial coiled bodies are in evidence. (d) Injection of the same extract as in part (c), but immunodepleted of tau, shows no Gallyas–Braak silver-positive inclusions 15 months later. Scale bar, 50μm. Trends in Neurosciences 2010 33, 317-325DOI: (10.1016/j.tins.2010.04.003) Copyright © 2010 Elsevier Ltd Terms and Conditions

Figure 4 Host-to-graft spreading of Lewy body pathology in a patient with Parkinson's disease. This patient received a transplant of fetal human mesencephalic dopaminergic neurons into the putamen 16 years previously. Immunohistochemistry for α-synuclein visualizes Lewy bodies and Lewy neurites in (a) the host substantia nigra and (b, c) the transplant. Scale bars, 40μm. Adapted, with permission, from Ref. [86]. Trends in Neurosciences 2010 33, 317-325DOI: (10.1016/j.tins.2010.04.003) Copyright © 2010 Elsevier Ltd Terms and Conditions

Figure 5 Potential mechanisms underlying the intercellular transfer of misfolded proteins. (a) Abnormal protein inclusions (such as neurofibrillary tangles and Lewy bodies) form in the cytoplasm of donor cells. They are engulfed by multivesicular bodies and released from the cells as exosomes, following fusion of the multivesicular bodies with the plasma membrane. Exosomes can then be internalized by neighboring acceptor cells through endocytosis or fusion with the plasma membrane. The contents of the endosomes enter the cytoplasm of acceptor cells by mechanisms that remain largely unknown, but which could involve fusion with the endocytic and plasma membranes [shown in the acceptor cell of part (a)] or diffusion of protein aggregates across endosomal membranes [shown in donor and acceptor cells of part (b)]. Upon their release, the protein inclusions nucleate the polymerization of more inclusions within acceptor cells. (b) Extracellular protein inclusions (such as ghost tangles and Lewy bodies) are endocytosed by donor cells. Membrane-bound inclusions can also be taken up, as shown in part (a). Following their diffusion across endosomal membranes into the cytoplasm of donor cells, the protein inclusions nucleate the polymerization of more inclusions. Alternatively, the inclusions can travel in endosomes to acceptor cells that are connected to the donor cells by tunneling nanotubes. Upon their diffusion across endosomal membranes into the cytoplasm of acceptor cells, the protein inclusions nucleate the polymerization of more inclusions. Trends in Neurosciences 2010 33, 317-325DOI: (10.1016/j.tins.2010.04.003) Copyright © 2010 Elsevier Ltd Terms and Conditions