5.8. Cell death typing using reporter assays
Alternatively, several transgenic cell lines have been engineered
to express Förster resonance energy transfer (FRET)-based caspase
reporter cassettes [58] or intermembrane space proteins fused to
fluorescent proteins [59]. Typically, these FRET-based fluorescent
protein partners are linked by a short stretch of amino acids containing
a caspase cleavage site. Upon proteolytic cleavage, the FRET
efficiency is decreased, thus quenching the emission of the FRET
acceptor [60,61].
However, the limited release of mitochondrial intermembrane
space proteins has also been shown to occur in settings unrelated
to cell death [62]. Mitochondrial dysfunction is often paralleled by
changes in mitochondrial morphology [63], which can be assessed
by the expression of mitochondrial matrix-targeted fluorescent
proteins or by transmembrane-insensitive mitochondriophylic
dyes [64]. Importantly, it should be kept in mind that changes in
mitochondrial morphology can also be caused by interference in
the mitochondrial fission and fusion machineries, independently
of cell death.