This assay makes use of the dye Acridine Orange (AO) to determine the stability of lysosomes in living cells upon exposure to a confocal microscope laser.

AO is a lipophilic amine that readily diffuses into cells (Figure 1). Inside the cell it enters the acidic lysosomal compartment where it is protonated and sequestered, shifting its emission spectrum towards a longer wavelength (i.e. red). Once inside the lysosomes, the metachromatic AO sensitizes the lysosomal membrane to photo-oxidation by blue light (Brunk et al., 1997). Upon light-induced loss of the lysosomal pH gradient and subsequent leakage of AO into the cytosol, the emission spectrum of AO shifts from red to green (Figure 2). Hence, loss of lysosomal integrity can be measured as a ‘loss of red dots’ or as a quantitative rise in green fluorescence (Petersen et al., 2010; Kirkegaard et al., 2010; Petersen et al., 2013).


Figure 1. Acridine Orange


Figure 2. Snapshots visualizing the U2OS cells at various steps of the recording procedure (Petersen et al., 2010)