Casp8 is involved in pathogenesis of different human diseases including lymphatic disorders and recent research efforts have successfully evaluated its therapeutic potential (Emricasan in phase II-III clinical trials).
By using novel transgenic mice mimicking the therapeutic inhibition of Casp8 enzymatic activity this project will provide substantial knowledge about the role of Casp8 in lymphocyte homeostasis and its therapeutic potential in B cell lymphoma.
Extrinsic apoptosis is a central regulatory mechanism of the B cell compartment and its dysfunction causes B cell diseases such as autoimmunity and lymphoma. Caspase-8 (Casp8) is the initiator caspase of the extrinsic apoptotic pathway and was initially described to induce this non-lytic programmed cell death (apoptosis). Casp8 was further characterized as the key inhibitor of necroptosis (lytic regulated cell death) and was shown to be indispensible for embryonic development.
Recently, we discovered an additional function of Casp8 demonstrating that enzymatic inactive Casp8 acts as a scaffold and induces pyroptosis (lytic inflammatory regulated cell death) and inflammation. Our studies explored Casp8 as a central molecular switch controlling apoptosis, necroptosis and pyroptosis. Based on this striking observation, we aim to investigate the role of Casp8 scaffold function versus its enzymatic activity in B cell homeostasis.
In order to investigate the physiologic role of Casp8 enzymatic activity we recently generated a knock-in mouse line (using CRISPR/Cas9 gene-editing) expressing enzymatically inactive Casp8C362S Fritsch et al., Nature 2019). Similar to Casp8-/-, lack of Casp8 enzymatic activity caused lethality in Casp8C362S/C362S embryos.
Our data revealed that the inhibition of necroptosis by MLKL deletion (Mlkl-/-) rescued the embryonic development in Casp8-/- mice but unexpectedly failed to prevent embryonic lethality of Casp8C362S/C362S mice, indicating that the expression of enzymatically inactive Casp8C362S causes necroptosis-independent death during embryonic development. Further extensive analysis using tissue-specific expression of Casp8C362S discovered a novel function for Casp8 demonstrating that enzymatically inactive Casp8C362S acts as a scaffold and induces pyroptosis.
We could show that Casp8C362S induces the nucleation of the inflammasome adaptor ASC, promotes the activation of the inflammatory caspase-1 (Casp1) and results in pyroptosis as well as secretion of inflammatory cytokines IL-1β and IL-18. Accordingly, only the combined deletion of MLKL together with ASC (Casp8C362S/C362S/Mlkl-/-/Asc-/-) or Casp1 (Casp8C362S/C362S/Mlkl-/-/Casp1-/-) rescued embryonic lethality of Casp8C362S/C362S mice.
Our studies explored a novel role of Casp8 in cellular death and inflammatory cascades and represent Casp8 as a central molecular switch controlling apoptosis, necroptosis and pyroptosis (Fritsch et al., Nature 2019).
Institute for Molecular Immunology | CECAD Research Center
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