Caspase proteases have a fundamental role in the process of apoptosis. Two general types of caspases have distinct biochemical properties—while both function as dimers, the executioner caspases are activated by cleavage of the inactive dimeric zymogen, while initiator and inflammatory caspases pre-exist as inactive monomers and are activated by assembly on caspase-activation platforms. The formation of these platforms define apoptotic pathways.
Caspase-8 has two opposing biological functions - it promotes cell death by triggering the extrinsic pathway of apoptosis, but also has a survival activity, as it is required for embryonic development, T lymphocyte activation, and resistance to necrosis induced by Tumor Necrosis Factor-a (TNF) and related family ligands. This “programmed necrosis” is mediated by Receptor Interacting Protein Kinase-1 (RIPK1) and Receptor Interacting Protein Kinase-3 (RIPK3). We have found that development of caspase-8-deficient mice is completely rescued by ablation of RIPK3. Adult animals lacking both caspase-8 and RIPK3 display a progressive lymphoaccumulative disease resembling that seen with defects in CD95 or CD95-ligand, and resist the lethal effects of CD95 ligation in vivo. We have found that caspase-8 prevents RIPK3-dependent necrosis without inducing apoptosis by functioning in a proteolytically active complex with FLICE-Like Inhibitory Protein Long (FLIPL), and this complex is required for the protective function. We have now obtained genetic support for the idea that FADD-caspase-8-FLIP function in development only as an inhibitory complex for the control of RIPK-dependent lethal events (most likely necrosis). In addition, these studies involve an effect that has not been previously seen to our knowledge—two lethal genotypes (one KO, one DKO) when combined into a TKO, result in normal development. We think that’s pretty cool.
We have further explored how RIPK3 may actually trigger necroptosis. Several studies have implicated mitochondria as the final effectors of this form of death, perhaps via ROS production. We’ll present evidence that fairly unambiguously shows that while apoptosis depends on mitochondria, necroptosis induced by RIPK3 does not. We’ve done that by producing cells that completely lack mitochondria. We think this is pretty cool, too.