Figure 1
Sarkosyl insoluble tau (SI-tau) is internalized and degraded by microglia.
(a) The SI fraction isolated from postmortem AD brain, but not from non-AD (control) brain, is enriched in hyperphosphorylated PHF-tau species as shown as three major bands by western blots using tau10 and AT8 antibodies. (b and c) Primary microglia cultured from postnatal day 1–3 wild-type C57BL/6 mice were incubated with 1 μg/ml SI-tau for the indicated times. The levels of total tau and AT8 positive-tau in the media (b) and cells (c) were determined by tau specific ELISAs. Cells were subjected to 5 min trypsin digestion to deplete surface bound SI-tau before lysis. Statistical analyses were performed by student t-test (unpaired two tails) by comparing samples incubated with microglia to that incubated with media alone in (b) and by comparing different time points to 0 h time point (mean ± s.e.m.) (c). **p < 0.01 ***p < 0.005 ****p < 0.001. Note the dramatic reduction in total and p-tau in the media following incubation with microglia compared to medium alone (b). Note the early increase in internalized tau and subsequent decrease over time (c). (d) Internalization of SI-tau by microglia is shown by confocal microscopy using indirect immunofluorescence. Primary murine microglia were incubated with 1 μg/ml SI-tau for 120 min and imaged using confocal microscopy after being immunostained with AT8 or MC1 antibody. Plasma membrane was stained by red-fluorescent Alexa Fluor594 wheat germ agglutinin (WGA) and nuclei were stained by blue Hoechst 33342 dye. Note the intracellular localization of AT8 or MC1-positive tau puncta (arrows) in microglia. Large tau aggregates attached to microglial surface were also observed (arrowheads). Scale bar represents 20 μm.
