In another study led by Michel et al,36 we used both multiparameter and superresolution imaging to verify how the protein tau propagates in a cell model of tau aggregation. In this study, a short fragment of the human form of the protein tau, K18Tau, covalently labeled with a synthetic fluorophore, was used as a monomeric solution to incubate neuron-like cells (see Fig. 6). Confocal time-correlated single photon counting (TCSPC) microscopy reveals that the protein is readily taken up by cells and is compartmentalized in endocytotic vesicles, small acidic compartments that quite apparently enhance the aggregation rate of the ingested protein. The fluorescence lifetime of the reporter fluorophore drops dramatically upon ingestion into the cells indicating a strong degree of tau aggregation is taking place. This led to the proposal that acidic conditions favor tau aggregation, which was verified in vitro, again via multiparametric imaging (see panels b and c in Fig. 6). Indeed, this finding suggests that uptake of tau via endocytosis is potentially a detrimental process; acidification and crowding upon ingestion into transport vesicles appear to provide conditions by which tau can aggregate. The last panel in Fig. 6(d), shows a two-color superresolution image of fibrillar species found in the extracellular medium, after extended periods of K18Tau incubation of the cells. Intriguingly, the cell expels fibrillar aggregates that contain both the ingested K18Tau, shown in red, and endogenous wild-type tau, already present in the cell, shown in green. The image proves beyond doubt that the extracellular K18Tau, which aggregates on ingestion into neuron-like cells, acts as a template, which “seeds” the coaggregation of endogenous, “healthy” tau. This has major pathological implications, as the model suggests that ingested tau could potentially become toxic and lead to the formation of aggregation nuclei, from which coaggregation of healthy protein species can proceed and propagate from cell to cell. The study furthermore suggests that the mere presence of tau in the extracellular space, e.g., following a brain trauma that causes local cell death and thus release of tau, can have potentially devastating effects; subsequent ingestion by adjacent, healthy cells triggers its aggregation and the consecutive recruitment and coaggregation of endogenous tau, starting a vicious circle, which mimics the disease cascade that may go on in humans. More generally, the study complements our other study on uptake and aggregation26 in suggesting that vesicular compartmentalization and acidification may be quite a general principle by which amyloid aggregation may proceed in cells.