Caspase proteases are the key executioners of apoptosis. Under normal conditions, these proteases exist as inactive proforms in the cytosol. But as soon as the permeability of the outer mitochondria membrane increases, cytochrome c enters the cytosol and the proteolytic activity of caspases is unleashed.
Human HeLa cells express a FRET reporter that indicates caspase activity at the plasma membrane. They are also stained with TMRM to detect changes in mitochondrial membrane potential (left). The color changes of the FRET reporter (pseudocolored)-from yellow/green to blue/green-demonstrates the rapid onset of caspase activity occurring immediately after mitochondria start to lose their TMRM fluorescence (right). Time lapse imaging was performed on a Zeiss LSM 780 confocal microscope with time intervals of 1 min.
Video courtesy of Heiko Duessmann and Jochen Prehn, Royal College of Surgeons in Ireland.
See more cell death images in the Cell Picture Show: https://www.cell.com/pictureshow/cell-death