Intra- and Interdimeric Caspase-8 Self-Cleavage Controls Strength and Timing of CD95-Induced Apoptosis

Abstract

Apoptosis in response to the ligand CD95L (also known as Fas ligand) is initiated by caspase-8, which is activated by dimerization and self-cleavage at death-inducing signaling complexes (DISCs). Previous work indicated that the degree of substrate cleavage by caspase-8 determines whether a cell dies or survives in response to a death stimulus. To determine how a death ligand stimulus is effectively translated into caspase-8 activity, we assessed this activity over time in single cells with compartmentalized probes that are cleaved by caspase-8, and used multiscale modeling to simultaneously describe single-cell and population data with an ensemble of single-cell models. We derived and experimentally validated a minimal model in which cleavage of caspase-8 in the enzymatic domain occurs in an interdimeric manner through interaction between DISCs, whereas prodomain cleavage sites are cleaved in an intradimeric manner within DISCs. Modeling indicated that sustained membrane-bound caspase-8 activity is followed by transient cytosolic activity, which can be interpreted as a molecular timer mechanism reflected by a limited lifetime of active caspase-8. The activation of caspase-8 by combined intra- and interdimeric cleavage ensures weak signaling at low concentrations of CD95L and strongly accelerated activation at higher ligand concentrations, thereby contributing to precise control of apoptosis.

The Inflammasome Adaptor ASC Induces Procaspase-8 Death Effector Domain Filaments

“…The importance of procaspase-8 dimerization for activation of its proteolytic activity has been established, and roles for intradimer and interdimer cleavage for procaspase-8 activation have been proposed (53,54), but it is not clear how procaspase-8 is recruited and activated at apoptosis-inducing signaling complexes. Procaspase-8 recruitment and activation have mostly been studied at the death receptor-mediated death-inducing signaling complex (DISC).…”

Caspase‐8: not so silently deadly

“…Although cFLIP s acts as a direct inhibitor of caspase-8, cFLIP L is incorporated into the DISC thereby preventing pro-caspase-8 interdomain processing but facilitating non-apoptotic caspase-8 cleavage of a limited number of substrates around the DISC. 24, 25 …”

Surviving apoptosis: life–death signaling in single cells

“…Ubiquitination of caspase-8 is required both for its activation and for subsequent dampening of its activity through degradation 73,74 , and the extent of death effector domain (DED)-mediated caspase-8 chain formation at the DISC can influence the degree of apoptosis induction and potentially affect the activation of alternative pathways 75,76 . Finally, it has been proposed that caspase-8 dimerization at the DISC, in the absence of proteolytic cleavage, can lead to survival signaling by processing a limited repertoire of substrates in the absence of cell death; intra- vs. inter-dimeric caspase-8 cleavage may also regulate this process 77,78 . Thus, the decision between life and death is mediated by fine-tuning the extent of both cell-death pathway activation and survival pathway induction.…”