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Cellular inhibitor of apoptosis proteins (cIAPs) are RING‐containing E3 ubiquitin ligases that ubiquitylate receptor‐interacting protein kinase 1 (RIPK1) to regulate TNF signalling. Here, we established mice simultaneously expressing enzymatically inactive cIAP1/2 variants, bearing mutations in the RING domains of cIAP1/2 (cIAP1/2 mutant RING, cIAP1/2MutR). cIap1/2MutR/MutR mice died during embryonic development due to RIPK1‐mediated apoptosis. While expression of kinase‐inactive RIPK1D138N rescued embryonic development, Ripk1D138N/D138N/cIap1/2MutR/MutR mice developed systemic inflammation and died postweaning. Cells expressing cIAP1/2MutR and RIPK1D138N were still susceptible to TNF‐induced apoptosis and necroptosis, implying additional kinase‐independent RIPK1 activities in regulating TNF signalling. Although further ablation of Ripk3 did not lead to any phenotypic improvement, Tnfr1 gene knock‐out prevented early onset of systemic inflammation and premature mortality, indicating that cIAPs control TNFR1‐mediated toxicity independent of RIPK1 and RIPK3. Beyond providing novel molecular insights into TNF‐signalling, the mouse model established in this study can serve as a useful tool to further evaluate ongoing therapeutic protocols using inhibitors of TNF, cIAPs and RIPK1. Synopsis: cIAPs control RIPK1‐dependent and ‐independent TNF toxicity in mouse. This study finds ubiquitylation and inhibition of RIPK1 by cIAPs to be required for mouse embryonic development, while RIPK1‐independent cIAP inhibition of TNF‐induced cell death regulates tissue inflammation in adult mice. cIAP1/2 E3 ubiquitin ligase activity is required for mouse embryonic development.Catalytic activity of cIAPs regulates RIPK1 kinase activity to promote embryonic survival.TNF induces apoptosis and necroptosis in cells lacking the cIAPs‐RIPK1 regulatory axis.cIAPs control non‐overlapping signalling processes downstream of RIPK1 and TNFR1. [ABSTRACT FROM AUTHOR]

Genetic lesions in X‐linked inhibitor of apoptosis (XIAP) pre‐dispose humans to cell death–associated inflammatory diseases, although the underlying mechanisms remain unclear. Here, we report that two patients with XIAP deficiency–associated inflammatory bowel disease display increased inflammatory IL‐1β maturation as well as cell death–associated caspase‐8 and Gasdermin D (GSDMD) processing in diseased tissue, which is reduced upon patient treatment. Loss of XIAP leads to caspase‐8‐driven cell death and bioactive IL‐1β release that is only abrogated by combined deletion of the apoptotic and pyroptotic cell death machinery. Namely, extrinsic apoptotic caspase‐8 promotes pyroptotic GSDMD processing that kills macrophages lacking both inflammasome and apoptosis signalling components (caspase‐1, ‐3, ‐7, ‐11 and BID), while caspase‐8 can still cause cell death in the absence of both GSDMD and GSDME when caspase‐3 and caspase‐7 are present. Neither caspase‐3 and caspase‐7‐mediated activation of the pannexin‐1 channel, or GSDMD loss, prevented NLRP3 inflammasome assembly and consequent caspase‐1 and IL‐1β maturation downstream of XIAP inhibition and caspase‐8 activation, even though the pannexin‐1 channel was required for NLRP3 triggering upon mitochondrial apoptosis. These findings uncouple the mechanisms of cell death and NLRP3 activation resulting from extrinsic and intrinsic apoptosis signalling, reveal how XIAP loss can co‐opt dual cell death programs, and uncover strategies for targeting the cell death and inflammatory pathways that result from XIAP deficiency. Synopsis: XIAP deficiency causes hemophagocytic lymphohistiocytosis and Crohn's disease. Analysis of XIAP‐deficient patients and cellular studies show that apoptotic caspases and GSDMD act redundantly downstream of caspase‐8 to cause excess cell death and IL‐1β release. XIAP deficiency increases cleaved caspase‐8 and GSDMD in mucosal tissue and cellsCrosstalk in apoptosis and pyroptosis sensitises cells to caspase‐8‐driven death and IL‐1β activation upon XIAP inhibitionPannexin‐1 and GSDMD are dispensable for NLRP3 inflammasome formation downstream of caspase‐8Pannexin‐1 cleavage by caspase‐3 and caspase‐7 is required for efficient NLRP3 activation following intrinsic apoptosis [ABSTRACT FROM AUTHOR]

Mixed lineage kinase domain‐like (MLKL) is the executioner in the caspase‐independent form of programmed cell death called necroptosis. Receptor‐interacting serine/threonine protein kinase 3 (RIPK3) phosphorylates MLKL, triggering MLKL oligomerization, membrane translocation and membrane disruption. MLKL also undergoes ubiquitylation during necroptosis, yet neither the mechanism nor the significance of this event has been demonstrated. Here, we show that necroptosis‐specific multi‐mono‐ubiquitylation of MLKL occurs following its activation and oligomerization. Ubiquitylated MLKL accumulates in a digitonin‐insoluble cell fraction comprising organellar and plasma membranes and protein aggregates. Appearance of this ubiquitylated MLKL form can be reduced by expression of a plasma membrane‐located deubiquitylating enzyme. Oligomerization‐induced MLKL ubiquitylation occurs on at least four separate lysine residues and correlates with its proteasome‐ and lysosome‐dependent turnover. Using a MLKL‐DUB fusion strategy, we show that constitutive removal of ubiquitin from MLKL licences MLKL auto‐activation independent of necroptosis signalling in mouse and human cells. Therefore, in addition to the role of ubiquitylation in the kinetic regulation of MLKL‐induced death following an exogenous necroptotic stimulus, it also contributes to restraining basal levels of activated MLKL to avoid unwanted cell death. SYNOPSIS: RIPK3 phosphorylates the necroptotic effector molecule MLKL leading to its oligomerization and translocation to the plasma membrane to kill cells. MLKL becomes multi mono‐ubiquitylated in an oligomerization dependent manner. Forced de‐ubiquitylation of MLKL increases MLKL's cytotoxic potential and confers RIPK3 and necroptotic stimulus independent activation and cell death. UbiCRest analysis shows that MLKL becomes multi mono‐ubiquitylated contemporaneously with activating phosphorylation and translocation to membranes.Analysis of gain and loss of function MLKL mutants indicates that MLKL oligomerization is required for necroptosis induced ubiquitylation.MLKL‐deubiquitylating enzyme (DUB) fusions kill cells more rapidly than MLKL‐catalytically dead DUB fusions and can induce necroptosis without a necroptotic stimulus, suggesting that ubiquitylation serves as a brake on MLKL's cytotoxic potential. [ABSTRACT FROM AUTHOR]

In this issue of The EMBO Journal, a multi-layered global proteome resource, which comprehensively covers TNF-induced phosphorylation and ubiquitination events as well as receptor interactions, has led to identification of SPATA2 as a novel player that contributes to TNF signalling by interacting with the LUBAC ubiquitin ligase and the CYLD deubiquitylase. Loss of SPATA2 augments transcriptional activation of NF-κB and limits TNF-induced necroptosis. A separate screen published in EMBO Reports corroborates these findings by showing that SPATA2 deficiency regulates CYLD activity, TNF-induced NF-κB signalling and cell death. [ABSTRACT FROM AUTHOR]

Inhibitor of apoptosis (IAP) proteins cIAP1, cIAP2, and XIAP (X-linked IAP) regulate apoptosis and cytokine receptor signalling, but their overlapping functions make it difficult to distinguish their individual roles. To do so, we deleted the genes for IAPs separately and in combination. While lack of any one of the IAPs produced no overt phenotype in mice, deletion of cIap1 with cIap2 or Xiap resulted in mid-embryonic lethality. In contrast, Xiap−/−cIap2−/− mice were viable. The death of cIap2−/−cIap1−/− double mutants was rescued to birth by deletion of tumour necrosis factor (TNF) receptor 1, but not TNFR2 genes. Remarkably, hemizygosity for receptor-interacting protein kinase 1 (Ripk1) allowed Xiap−/−cIap1−/− double mutants to survive past birth, and prolonged cIap2−/−cIap1−/− embryonic survival. Similarly, deletion of Ripk3 was able to rescue the mid-gestation defect of cIap2−/−cIap1−/− embryos, as these embryos survived to E15.5. cIAPs are therefore required during development to limit activity of RIP kinases in the TNF receptor 1 signalling pathway. [ABSTRACT FROM AUTHOR]

XIAP is a mammalian inhibitor of apoptosis protein (lAP). To determine residues within the second baculoviral lAP repeat (BIR2) required for inhibition of caspase 3, we screened a library of BIR2 mutants for toss of the ability to inhibit caspase 3 toxicity in the yeast Schizosaccharomyces pombe. Four of the mutations, not predicted to affect the structure of the BIR fold, clustered together on the N-terminal region that flanks BIR2, suggesting that this is a site of interaction with caspase 3. Introduction of these mutations into full-length XIAP reduced caspase 3 inhibitory activity up to 500-fold, but did not affect its ability to inhibit caspase 9 or interact with the lAP antagonist IMABLO. Furthermore, these mutants retained full ability to inhibit apoptosis in transfected cells, demonstrating that although XIAP is able to inhibit caspase 3, this activity is dispensable for inhibition of apoptosis by XIAP in vivo. [ABSTRACT FROM AUTHOR]

We have isolated the recently identified Drosophila caspase DRONC through its interaction with the effector caspase drICE. Ectopic expression of DRONC induces cell death in Schizosaccharomyces pombe, mammalian fibroblasts and the developing Drosophila eye. The caspase inhibitor p35 fails to rescue DRONC-induced cell death in vivo and is not cleaved by DRONC in vitro, making DRONC the first identified p35-resistant caspase. The DRONC pro-domain interacts with Drosphila inhibitor of apoptosis protein 1 (DIAP1), and co-expression of DIAP1 in the developing Drosophila eye completely reverts the eye ablation phenotype induced by pro-DRONC expression. In contrast, DIAP1 fails to rescue eye ablation induced by DRONC lacking the pro-domain, indicating that interaction of DIAP1 with the pro-domain of DRONC is required for suppression of DRONC-mediated cell death. Heterozygosity at the diap1 locus enhances the pro-DRONC eye phenotype, consistent with a role for endogenous DIAP1 in suppression of DRONC activation. Both heterozygosity at the dronc locus and expression of dominant-negative DRONC mutants suppress the eye phenotype caused by reaper (RPR) and head involution defective (HID), consistent with the idea that DRONC functions in the RPR and HID pathway. [ABSTRACT FROM AUTHOR]

To study the role of various caspases during apoptosis, we have designed a series of caspase inhibitors based on the cowpox virus cytokine response modifier A (crmA) protein. Wild-type crmA inhibits caspases 1 and 8 and thereby protects cells from apoptosis triggered by ligation of CD95 or tumour necrosis factor (TNF) receptors, but it does not protect against death mediated by other caspases. By replacing the tetrapeptide pseudosubstrate region of crmA (LVAD) with tetrapeptides that are optimal substrates for the different families of caspases, or with the four residues from the cleavage site of the baculovirus protein p35 (DQMD), we have generated a family of caspase inhibitors that show altered ability to protect against cell death. Although DEVD is the optimal substrate for caspase 3, crmA DEVD was degraded rapidly and was a weaker inhibitor than crmA DQMD, which was not degraded. Unlike wild-type crmA and crmA DEVD, crmA DQMD was able to inhibit apoptosis caused by direct activation of caspase 3 and protected lymphoid cells from death induced by radiation and dexamethasone. Significantly, the protected cells were capable of sustained growth. [ABSTRACT FROM AUTHOR]

In vertebrates, transcriptionally active promoters are undermethylated. Since the transcription factor Sp1, and more recently NF-κB, have been implicated in the demethylation process, we examined the effect of transcription factors on demethylation by injecting in vitro methylated plasmid DNA into Xenopus fertilized eggs. We found that various transactivation domains, including a strong acidic activation domain from the viral protein VP16, can enhance demethylation of a promoter region when fused to a DNA binding domain which recognizes the promoter. Furthermore, demethylation occurs only after the midblastula transition, when the general transcription machinery of the host embryo becomes available. Nevertheless, transcription factor binding need not be followed by actual transcription, since demethylation is not blocked by α-amanitin treatment. Finally, replication of the target DNA is a prerequisite for efficient demethylation since only plasmids that carry the bovine papilloma virus sequences which support plasmid replication after the midblastula transition are demethylated. No demethylation is detectable in the oocyte system where DNA is not replicated. These results suggest that, in the Xenopus embryo, promoters for which transcription factors are available are demethylated by a replicationdependent, possibly passive mechanism. [ABSTRACT FROM AUTHOR]