5.6. Analysis of cytosolic parameters in dying cells by immunoblotting
To distinguish between apoptosis and necroptosis, several biochemical
cytosolic events can be analyzed, such as activation of
caspases, proteolytic cleavage of Bid, PARP or ICAD, release of certain
mitochondrial proteins, or use of a new biochemical marker
such as RIPK1/3 phosphorylation. Typically, activation of executioner
caspases and cleavage of their substrates (such as PARP or
ICAD) is analyzed first. If caspase activation is observed, one can
subsequently analyze the mitochondrial contribution by checking
Bid proteolytic activation, tBid translocation, and release of mitochondrial
proteins such as cytochrome c, AIF, HTRA2/Omi and
Endonuclease G. To detect the release of mitochondrial proteins
in the cytosol, the organelle and cytosolic fractions should be
separated without causing release of cytochrome c from the mitochondria
during organelle preparation. It is advisable to use a mild
detergent, digitonin, which permeabilizes the plasma membrane
but leaves the mitochondria and lysosomes intact.