Detection of Caspases Activation In Situ by Fluorochrome-Labeled Inhibitors of Caspases (FLICA)

Caspases are cy steine-a spartic a cid s pecific p roteases that are activated in response to different cell death inducing stimuli (1 –3 ). Their activation initiates specific cleavage of the respective target proteins and therefore is considered to be a marker of the irreversible steps that lead to cell demise (reviews 4 ,5 ). Caspases specifically recognize a four-amino acid sequence on their substrate proteins; the carboxyl end of aspartic acid within this sequence is the target for cleavage. Several approaches have been developed to detect the process of caspases activation. Because the activation involves the transcatalytic cleavage of the zymogen pro-caspases (reviews 6 –8 ) the cleavage products having lower molecular weight than the zymogen can be revealed electrophoretically and identified in Western blots using caspase-specific antibodies. Another approach utilizes the fluorogenic (or chromogenic) substrates of caspases. Peptide substrates were developed which are colorless or nonfluorescent, but upon cleavage, generate colored or fluorescing products (9 –11 ). Utility of these two approaches, however, was limited to the measurement of caspases activation in cell extracts thereby providing no information on the in situ caspases activation. This would allow one to study individual cells, assess heterogeneity of cell populations, or reveal correlation with other cell attributes.