Co-occurrence of Alzheimer's disease ß-amyloid and τ pathologies at synapses

Abstract

Although beta-amyloid (Abeta) plaques and tau neurofibrillary tangles are hallmarks of Alzheimer's disease (AD) neuropathology, loss of synapses is considered the best correlate of cognitive decline in AD, rather than plaques or tangles. How pathological Abeta and tau aggregation relate to each other and to alterations in synapses remains unclear. Since aberrant tau phosphorylation occurs in amyloid precursor protein (APP) Swedish mutant transgenic mice, and since neurofibrillary tangles develop in triple transgenic mice harboring mutations in APP, tau and presenilin 1, we utilized these well-characterized mouse models to explore the relation between Abeta and tau pathologies. We now report that pathological accumulation of Abeta and hyperphosphorylation of tau develop concomitantly within synaptic terminals.

Figure 1. Aβ42 accumulation and hyperphosphorylated tau localize to dystrophic neurites around plaques in Tg2576 mouse and human brain

(A) Dystrophic neurites around amyloid plaques contained both Aβ42 peptides (antibody AB5078P) and hyperphosphorylated tau (antibody AT8). Thin arrows show examples of immunofluorescence co-localization. The core of the amyloid plaque was not labeled by AT8, and dystrophic neurites with Aβ42 labeling but no AT8 labeling were also present. (B) Dual immuno-EM with AT8 (immuno-peroxidase) and Aβ42 (gold particles) antibodies revealed AT8 labeling as filamentous structures (thin arrows) and clusters of peroxidase (thick arrows) in a distal dendrite/postsynaptic compartment accumulating Aβ42 peptides (arrowheads) in the brain of a 26-month-old Tg2576 mouse. A postsynaptic density is indicated by a white arrow. (C) Clusters of peroxidase markedly labeled by AT8 (thick arrows) in an Aβ42 peptide (arrowheads) accumulating synaptic compartment. (D) Localization of hyperphosphorylated tau (antibody AT8; immuno-peroxidase; thin arrows) near Aβ42 (immuno-gold; arrowheads) is evident in a neurite in human AD brain. Abbreviation: mt, mitochondrion. Scale bars, 25 µm (A); 1 µm (B); 250 µm (C): 500 nm (D).

Figure 2. Aβ42 accumulation and tau hyperphosphorylation co-occur in the SLM of the CA1 region of hippocampus in 3xTg-AD mouse brain

(A) Early Aβ42 accumulation and tau hyperphosphorylation in 5-month-old 3xTg-AD mouse brain prior to plaques and tangles. In the CA1 region of the hippocampus, Aβ42 is prominent in the pyramidal cell layer (asterisk) and in the SLM layer (thin arrows). AT8 antibody labeling of hyperphosphorylated tau is most pronounced in the SLM layer (thin arrows). A few pyramidal neurons with AT8 labeling (arrowheads) are evident in the subiculum at this age. (B) Section through the rostral hippocampus from a 13-month-old 3xTg-AD mouse labeled with Aβ42 and AT8 antibodies by immunofluorescence. Nuclei were labeled with Hoechst dye 33342 to distinguish the subiculum from the hippocampal CA1 region, where the pyramidal cell nuclei are clearly lined up. Amyloid plaques were apparent within the subiculum of the hippocampus (boxed area), but were also evident in the SLM of CA1. Both Aβ42 peptides and hyperphosphorylated tau were markedly increased in the SLM (plus signs) of CA1. Hyperphosphorylated tau (AT8) also labeled a few cell bodies within CA1 (thick arrows). (C) Higher magnification image of the subiculum from the squared area in (B) with prominent plaque pathology. Aβ42 and hyperphosphorylated tau co-localized in several pyramidal cell bodies (asterisks). Hyperphosphorylated tau and Aβ42 peptide co-localization was prominent in dystrophic neurites around a large amyloid plaque (thin arrows). Punctate co-localization of Aβ42 and hyperphosphorylated tau was also evident in the SLM (plus signs). Abbreviation: S, subiculum, SLM, stratum lacunosum-moleculare. Scale bars: 250 µm (A, B); 100 µm (C).

Figure 3. Aβ42 accumulation and tau hyperphosphorylation localize near to each other within neurons of the hippocampus CA1 region in 3xTg-AD mouse brain using dual-labeling EM

(A) Aβ42 gold particles (arrowheads) localized to a diffusely AT8-labeled postsynaptic compartment (thick black arrow) in the SLM of the CA1 region of hippocampus in a 13-month-old 3xTg-AD mouse. The postsynaptic compartment is recognizable by the postsynaptic density (thin arrow). (B) A dendrite/post-synaptic compartment with Aβ42 accumulation (arrowheads) revealed more localized tau hyperphosphorylation, including AT8-positive clusters (white thick arrows). Note also the synaptic compartments and distal neurites in A and B devoid of Aβ42 and AT8 labeling. Scale bars: 500 nm (A, B). (C) Image of a markedly AT8-positive apical dendrite (bottom, left inset). Higher magnification of the squared area in the inset demonstrated thick filaments immuno-labeled by antibody AT8 (thin arrows). Nearby Aβ42 peptide gold particles were associated with endosomal/smaller vesicles in the dendrite (arrowheads). AT8-labeled thick filaments were tangled (top, right inset). (D) Inset: Lower magnification image of pyramidal neuron cell somata in the CA1 region labeled with Aβ42 and AT8 antibodies. The higher magnification image of the squared area in the inset revealed Aβ42 immuno-gold particles especially in the outer membranes of multivesicular bodies (thick arrows). In contrast, AT8-labeled thick filaments (thin arrows) are present along microtubules throughout the cell body cytoplasm of the CA1 pyramidal neuron. Abbreviation: mt, mitochondrion, N, nucleus. Scale bars: 500 nm (A, B, C, D); 250 µm (inset C).