Abstract

The fluorescence lifetime of a fluorophore is affected by its local environment, but is relatively independent of its concentration. Changes in viscosity, temperature, ion or oxygen concentration, as well as the presence of other fluorophores (FRET) can be accurately measured. In this seminar, I will show how we have developed FLIM assays in live cells to measure protein aggregation, nuclear compaction, and to improve the detection of low protein concentrations. These assays have been crucial to two projects:

In Alzheimer’s Disease, Tau aggregates spread through the brain, but it is not yet clear what triggers Tau aggregation nor how these aggregates are transferred between neurons. We have found that extracellular monomeric Tau can be taken up by neurons, and the low pH of the lysosomal compartment is necessary and sufficient to trigger its aggregation. This aggregated Tau in endosomes and lysosomes can be transferred between neurons via two different mechanisms – via axonal connections, or secreted into the media. We propose that aggregated Tau in lysosomes constitutes a decision point between its degradation, its secretion into the extracellular space, or its anterograde transmission via synapses. Treatments that affect lysosome physiology may increase the safe disposal of aggregated Tau and reduce its prion-like transmission.

Current methods to study genome compaction in live cells by fluorescence microscopy require the expression of fluorescent proteins. Using a new fluorescence lifetime assay, we could confirm increased nuclear compaction in Nanog-/- embryonic stem (ES) cells. We have found that naïve ES cells have a partially compacted genome, supporting the hypothesis that ES cells must transit through a ‘primed’ de-compacted state before committing to differentiation.