BID-dependent release of mitochondrial SMAC dampens XIAP-mediated immunity against Shigella

Mitochondria and cell death

Mitochondria are cellular factories for energy production, calcium homeostasis and iron metabolism, but they also have an unequivocal and central role in intrinsic apoptosis through the release of cytochrome c. While the subsequent activation of proteolytic caspases ensures that cell death proceeds in the absence of collateral inflammation, other phlogistic cell death pathways have been implicated in using, or engaging, mitochondria. Here we discuss the emerging complexities of intrinsic apoptosis controlled by the BCL-2 family of proteins. We highlight the emerging theory that non-lethal mitochondrial apoptotic signalling has diverse biological roles that impact cancer, innate immunity and ageing. Finally, we delineate the role of mitochondria in other forms of cell death, such as pyroptosis, ferroptosis and necroptosis, and discuss mitochondria as central hubs for the intersection and coordination of cell death signalling pathways, underscoring their potential for therapeutic manipulation.

The regulation of the apoptotic pore-An immunological tightrope walk

Apoptotic pore formation in mitochondria is the pivotal point for cell death during mitochondrial apoptosis. It is regulated by BCL-2 family proteins in response to various cellular stress triggers and mediates mitochondrial outer membrane permeabilization (MOMP). This allows the release of mitochondrial contents into the cytosol, which triggers rapid cell death and clearance through the activation of caspases. However, under conditions of low caspase activity, the mitochondrial contents released into the cytosol through apoptotic pores serve as inflammatory signals and activate various inflammatory responses. In this chapter, we discuss how the formation of the apoptotic pore is regulated by BCL-2 proteins as well as other cellular or mitochondrial proteins and membrane lipids. Moreover, we highlight the importance of sublethal MOMP in the regulation of mitochondrial-activated inflammation and discuss its physiological consequences in the context of pathogen infection and disease and how it can potentially be exploited therapeutically, for example to improve cancer treatment.

Murine Models of Familial Cytokine Storm Syndromes

Hemophagocytic lymphohistiocytosis (HLH) is a hyperinflammatory disease caused by mutations in effectors and regulators of cytotoxicity in cytotoxic T cells (CTL) and natural killer (NK) cells. The complexity of the immune system means that in vivo models are needed to efficiently study diseases like HLH. Mice with defects in the genes known to cause primary HLH (pHLH) are available. However, these mice only develop the characteristic features of HLH after the induction of an immune response (typically through infection with lymphocytic choriomeningitis virus). Nevertheless, murine models have been invaluable for understanding the mechanisms that lead to HLH. For example, the cytotoxic machinery (e.g., the transport of cytotoxic vesicles and the release of granzymes and perforin after membrane fusion) was first characterized in the mouse. Experiments in murine models of pHLH have emphasized the importance of cytotoxic cells, antigen-presenting cells (APC), and cytokines in hyperinflammatory positive feedback loops (e.g., cytokine storms). This knowledge has facilitated the development of treatments for human HLH, some of which are now being tested in the clinic.

cIAPs control RIPK1 kinase activity-dependent and -independent cell death and tissue inflammation

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.

Expanding roles of BCL-2 proteins in apoptosis execution and beyond

The proteins of the BCL-2 family are known as key regulators of apoptosis, with interactions between family members determining permeabilisation of the mitochondrial outer membrane (MOM) and subsequent cell death. However, the exact mechanism through which they form the apoptotic pore responsible for MOM permeabilisation (MOMP), the structure and specific components of this pore, and what roles BCL-2 proteins play outside of directly regulating MOMP are incompletely understood. Owing to the link between apoptosis dysregulation and disease, the BCL-2 proteins are important targets for drug development. With the development and clinical use of drugs targeting BCL-2 proteins showing success in multiple haematological malignancies, enhancing the efficacy of these drugs, or indeed developing novel drugs targeting BCL-2 proteins is of great interest to treat cancer patients who have developed resistance or who suffer other disease types. Here, we review our current understanding of the molecular mechanism of MOMP, with a particular focus on recently discovered roles of BCL-2 proteins in apoptosis and beyond, and discuss what implications these functions might have in both healthy tissues and disease.

Mechanisms of BCL-2 family proteins in mitochondrial apoptosis

The proteins of the BCL-2 family are key regulators of mitochondrial apoptosis, acting as either promoters or inhibitors of cell death. The functional interplay and balance between the opposing BCL-2 family members control permeabilization of the outer mitochondrial membrane, leading to the release of activators of the caspase cascade into the cytosol and ultimately resulting in cell death. Despite considerable research, our knowledge about the mechanisms of the BCL-2 family of proteins remains insufficient, which complicates cell fate predictions and does not allow us to fully exploit these proteins as targets for drug discovery. Detailed understanding of the formation and molecular architecture of the apoptotic pore in the outer mitochondrial membrane remains a holy grail in the field, but new studies allow us to begin constructing a structural model of its arrangement. Recent literature has also revealed unexpected activities for several BCL-2 family members that challenge established concepts of how they regulate mitochondrial permeabilization. In this Review, we revisit the most important advances in the field and integrate them into a new structure-function-based classification of the BCL-2 family members that intends to provide a comprehensive model for BCL-2 action in apoptosis. We close this Review by discussing the potential of drugging the BCL-2 family in diseases characterized by aberrant apoptosis.

X-linked inhibitor of apoptosis protein is a prognostic marker for a favorable outcome in three identified subsets in resectable adenocarcinoma of the pancreas

PURPOSE

Pancreatic ductal adenocarcinoma (PDAC) is currently one of the leading causes of cancer death worldwide. Therefore, building further subgroups as well as enabling individual patient therapy and diagnostics are needed. X-linked inhibitor of apoptosis protein (XIAP) is known to modulate apoptotic and inflammatory pathways. Its expression was found to correlate with patients' survival in other tumor entities. This study aims to examine the role of XIAP in patients with PDAC in relation to the inflammatory microenvironment.

METHODS

The PANCALYZE multicenter study group included 257 patients with PDAC. Paraffin-embedded tumor samples were stained immunohistochemically for CD3, CD20, CD38, CD56, CD66b, CD117, and CD163 and XIAP. These stainings were further analyzed digitally with QuPath and survival analyses were done.

RESULTS

XIAP-positive patients with T-cell, respectively, neutrophil enriched tumors survived significantly longer compared to XIAP-negative patients (CD3: 37.6 vs. 24.6 months, p = 0.028; CD66b: 34.1 vs. 14.9 months, p = 0.027). Additionally, XIAP-positive patients showed better survival in the lymph node-negative population (48.4 vs. 24.2 months, p = 0.019). Regarding the total population, our findings did not show a correlation between XIAP expression and survival. In multivariate cox regression analyzes XIAP proves to be an independent factor for better survival in the identified subgroups (CD3: p = 0.043; CD66b: p = 0.012, N0: p = 0.040).

CONCLUSION

We found XIAP-positive subgroups with significantly better survival in patients with PDAC in T-cell-rich, neutrophil-rich, or lymph node-negative cohorts. This could lead to further individualized cancer treatment with less aggressive therapy protocols for XIAP-positive tumors or more intensive follow-up for XIAP-negative tumors.

Apoptotic cell death in disease-Current understanding of the NCCD 2023

Apoptosis is a form of regulated cell death (RCD) that involves proteases of the caspase family. Pharmacological and genetic strategies that experimentally inhibit or delay apoptosis in mammalian systems have elucidated the key contribution of this process not only to (post-)embryonic development and adult tissue homeostasis, but also to the etiology of multiple human disorders. Consistent with this notion, while defects in the molecular machinery for apoptotic cell death impair organismal development and promote oncogenesis, the unwarranted activation of apoptosis promotes cell loss and tissue damage in the context of various neurological, cardiovascular, renal, hepatic, infectious, neoplastic and inflammatory conditions. Here, the Nomenclature Committee on Cell Death (NCCD) gathered to critically summarize an abundant pre-clinical literature mechanistically linking the core apoptotic apparatus to organismal homeostasis in the context of disease.

Cathepsin B in programmed cell death machinery: mechanisms of execution and regulatory pathways

Cathepsin B (CatB), a cysteine protease, is primarily localized within subcellular endosomal and lysosomal compartments. It is involved in the turnover of intracellular and extracellular proteins. Interest is growing in CatB due to its diverse roles in physiological and pathological processes. In functional defective tissues, programmed cell death (PCD) is one of the regulable fundamental mechanisms mediated by CatB, including apoptosis, pyroptosis, ferroptosis, necroptosis, and autophagic cell death. However, CatB-mediated PCD is responsible for disease progression under pathological conditions. In this review, we provide an overview of the critical roles and regulatory pathways of CatB in different types of PCD, and discuss the possibility of CatB as an attractive target in multiple diseases. We also summarize current gaps in the understanding of the involvement of CatB in PCD to highlight future avenues for research.

Mitochondrial pores at the crossroad between cell death and inflammatory signaling

Mitochondria are cellular organelles with a major role in many cellular processes, including not only energy production, metabolism, and calcium homeostasis but also regulated cell death and innate immunity. Their proteobacterial origin makes them a rich source of potent immune agonists, normally hidden within the mitochondrial membrane barriers. Alteration of mitochondrial permeability through mitochondrial pores thus provides efficient mechanisms not only to communicate mitochondrial stress to the cell but also as a key event in the integration of cellular responses. In this regard, eukaryotic cells have developed diverse signaling networks that sense and respond to the release of mitochondrial components into the cytosol and play a key role in controlling cell death and inflammatory pathways. Modulating pore formation at mitochondria through direct or indirect mechanisms may thus open new opportunities for therapy. In this review, we discuss the current understanding of the structure and molecular mechanisms of mitochondrial pores and how they function at the interface between cell death and inflammatory signaling to regulate cellular outcomes.

Role of mitochondrial outer membrane permeabilization during bacterial infection

Beyond the initial 'powerhouse' view, mitochondria have numerous functions in their mammalian cell and contribute to many physiological processes, and many of these we understand only partially. The control of apoptosis by mitochondria is firmly established. Many questions remain however how this function is embedded into physiology, and how other signaling pathways regulate mitochondrial apoptosis; the interplay of bacteria with the mitochondrial apoptosis pathway is one such example. The outer mitochondrial membrane regulates both import into mitochondria and the release of intermembrane, and in some situations also matrix components from mitochondria, and these mitochondrial components can have signaling function in the cytosol. One function is the induction of apoptotic cell death. An exciting, more recently discovered function is the regulation of inflammation. Mitochondrial molecules, both proteins and nucleic acids, have inflammatory activity when released from mitochondria, an activity whose regulation is intertwined with the activation of apoptotic caspases. Bacterial infection can have more general effects on mitochondrial apoptosis-regulation, through effects on host transcription and other pathways, such as signals controlled by pattern recognition. Some specialized bacteria have products that more specifically regulate signaling to the outer mitochondrial membrane, and to apoptosis; both pro- and anti-apoptotic mechanisms have been reported. Among the intriguing recent findings in this area are signaling contributions of porins and the sub-lethal release of intermembrane constituents. We will here review the literature and place the new developments into the established context of mitochondrial signaling during the contact of bacterial pathogens with human cells.

Apoptotic mitochondrial poration by a growing list of pore-forming BCL-2 family proteins

The pore-forming BCL-2 family proteins are effectors of mitochondrial poration in apoptosis initiation. Two atypical effectors-BOK and truncated BID (tBID)-join the canonical effectors BAK and BAX. Gene knockout revealed developmental phenotypes in the absence the effectors, supporting their roles in vivo. During apoptosis effectors are activated and change shape from dormant monomers to dynamic oligomers that associate with and permeabilize mitochondria. BID is activated by proteolysis, BOK accumulates on inhibition of its degradation by the E3 ligase gp78, while BAK and BAX undergo direct activation by BH3-only initiators, autoactivation, and crossactivation. Except tBID, effector oligomers on the mitochondria appear as arcs and rings in super-resolution microscopy images. The BH3-in-groove dimers of BAK and BAX, the tBID monomers, and uncharacterized BOK species are the putative building blocks of apoptotic pores. Effectors interact with lipids and bilayers but the mechanism of membrane poration remains elusive. I discuss effector-mediated mitochondrial poration.

Sub-lethal signals in the mitochondrial apoptosis apparatus: pernicious by-product or physiological event?

One of the tasks of mitochondria is the rule over life and death: when the outer membrane is permeabilized, the release of intermembrane space proteins causes cell death by apoptosis. For a long time, this mitochondrial outer membrane permeabilization (MOMP) has been accepted as the famous step from which no cell returns. Recent results have however shown that this quite plainly does not have to be the case. A cell can also undergo only a little MOMP, and it can efficiently repair damage it has incurred in the process. There is no doubt now that such low-scale permeabilization occurs. A major unclarified issue is the biological relevance. Is small-scale mitochondrial permeabilization an accident, a leakiness of the apoptosis apparatus, perhaps during restructuring of the mitochondrial network? Is it attempted suicide, where cell death by apoptosis is the real goal but the stimulus failed to reach the threshold? Or, more boldly, is there a true biological meaning behind the event of the release of low amounts of mitochondrial components? We will here explore this last possibility, which we believe is on one hand appealing, on the other hand plausible and supported by some evidence. Recent data are consistent with the view that sub-lethal signals in the mitochondrial apoptosis pathway can drive inflammation, the first step of an immune reaction. The apoptosis apparatus is almost notoriously easy to trigger. Sub-lethal signals may be even easier to set off. We suggest that the apoptosis apparatus is used in this way to sound the call when the first human cell is infected by a pathogen.

When cell death goes wrong: inflammatory outcomes of failed apoptosis and mitotic cell death

Apoptosis is a regulated cellular pathway that ensures that a cell dies in a structured fashion to prevent negative consequences for the tissue or the organism. Dysfunctional apoptosis is a hallmark of numerous pathologies, and treatments for various diseases are successful based on the induction of apoptosis. Under homeostatic conditions, apoptosis is a non-inflammatory event, as the activation of caspases ensures that inflammatory pathways are disabled. However, there is an increasing understanding that under specific conditions, such as caspase inhibition, apoptosis and the apoptotic machinery can be re-wired into a process which is inflammatory. In this review we discuss how the death receptor and mitochondrial pathways of apoptosis can activate inflammation. Furthermore, we will highlight how cell death due to mitotic stress might be a special case when it comes to cell death and the induction of inflammation.

Secreted glucose regulated protein78 ameliorates DSS-induced mouse colitis

The secreted form of 78-kDa glucose-regulated protein (sGRP78) has been widely reported for its property in aiding resolution of inflammatory. However, little is known on its potential in the treatment of colitis. To investigate the expression pattern and functional outcome of GRP78 in ulcerative colitis, its expression was measured in human and murine colitis samples. It was found that GRP78 was spontaneously secreted to a high level in gut, which is a physiological site of immune tolerance. During the active phase of DSS-induced colitis, the sGRP78 level was significantly reduced but rebounded quickly during resolving phase, making it a potential candidate for the treatment of colitis. In the following experiments, the administration of sGRP78 was proved to decrease susceptibility to experimental colitis, as indicated by an overall improvement of intestinal symptoms, restoration of TJ integrity, decreased infiltration of immune cells and impaired production of inflammatory cytokines. And specific cleavage of endogenous sGRP78 could aggravate DSS colitis. Adoptive transfer of sGRP78-conditioned BMDMs reduced inflammation in the gut. We linked sGRP78 treatment with altered macrophage biology and skewed macrophage polarization by inhibiting the TLR4-dependent MAP-kinases and NF-κB pathways. Based on these studies, as a naturally occurring immunomodulatory molecule, sGRP78 might be an attractive novel therapeutic agent for acute intestinal inflammation.

BCL-2-family protein tBID can act as a BAX-like effector of apoptosis

During apoptosis, the BCL-2-family protein tBID promotes mitochondrial permeabilization by activating BAX and BAK and by blocking anti-apoptotic BCL-2 members. Here, we report that tBID can also mediate mitochondrial permeabilization by itself, resulting in release of cytochrome c and mitochondrial DNA, caspase activation and apoptosis even in absence of BAX and BAK. This previously unrecognized activity of tBID depends on helix 6, homologous to the pore-forming regions of BAX and BAK, and can be blocked by pro-survival BCL-2 proteins. Importantly, tBID-mediated mitochondrial permeabilization independent of BAX and BAK is physiologically relevant for SMAC release in the immune response against Shigella infection. Furthermore, it can be exploited to kill leukaemia cells with acquired venetoclax resistance due to lack of active BAX and BAK. Our findings define tBID as an effector of mitochondrial permeabilization in apoptosis and provide a new paradigm for BCL-2 proteins, with implications for anti-bacterial immunity and cancer therapy.

Mitochondria supply sub-lethal signals for cytokine secretion and DNA-damage in H. pylori infection

The bacterium Helicobacter pylori induces gastric inflammation and predisposes to cancer. H. pylori-infected epithelial cells secrete cytokines and chemokines and undergo DNA-damage. We show that the host cell's mitochondrial apoptosis system contributes to cytokine secretion and DNA-damage in the absence of cell death. H. pylori induced secretion of cytokines/chemokines from epithelial cells, dependent on the mitochondrial apoptosis machinery. A signalling step was identified in the release of mitochondrial Smac/DIABLO, which was required for alternative NF-κB-activation and contributed to chemokine secretion. The bacterial cag-pathogenicity island and bacterial muropeptide triggered mitochondrial host cell signals through the pattern recognition receptor NOD1. H. pylori-induced DNA-damage depended on mitochondrial apoptosis signals and the caspase-activated DNAse. In biopsies from H. pylori-positive patients, we observed a correlation of Smac-levels and inflammation. Non-apoptotic cells in these samples showed evidence of caspase-3-activation, correlating with phosphorylation of the DNA-damage response kinase ATM. Thus, H. pylori activates the mitochondrial apoptosis pathway to a sub-lethal level. During infection, Smac has a cytosolic, pro-inflammatory role in the absence of apoptosis. Further, DNA-damage through sub-lethal mitochondrial signals is likely to contribute to mutagenesis and cancer development.

Bacterial subversion of NLR-mediated immune responses

Members of the mammalian Nod-like receptor (NLR) protein family are important intracellular sensors for bacteria. Bacteria have evolved under the pressure of detection by host immune sensing systems, leading to adaptive subversion strategies to dampen immune responses for their benefits. These include modification of microbe-associated molecular patterns (MAMPs), interception of innate immune pathways by secreted effector proteins and sophisticated instruction of anti-inflammatory adaptive immune responses. Here, we summarise our current understanding of subversion strategies used by bacterial pathogens to manipulate NLR-mediated responses, focusing on the well-studied members NOD1/2, and the inflammasome forming NLRs NLRC4, and NLRP3. We discuss how bacterial pathogens and their products activate these NLRs to promote inflammation and disease and the range of mechanisms used by bacterial pathogens to evade detection by NLRs and to block or dampen NLR activation to ultimately interfere with the generation of host immunity. Moreover, we discuss how bacteria utilise NLRs to facilitate immunotolerance and persistence in the host and outline how various mechanisms used to attenuate innate immune responses towards bacterial pathogens can also aid the host by reducing immunopathologies. Finally, we describe the therapeutic potential of harnessing immune subversion strategies used by bacteria to treat chronic inflammatory conditions.

XIAP promotes melanoma growth by inducing tumour neutrophil infiltration

Elevated expression of the X‐linked inhibitor of apoptosis protein (XIAP) has been frequently reported in malignant melanoma suggesting that XIAP renders apoptosis resistance and thereby supports melanoma progression. Independent of its anti‐apoptotic function, XIAP mediates cellular inflammatory signalling and promotes immunity against bacterial infection. The pro‐inflammatory function of XIAP has not yet been considered in cancer. By providing detailed in vitro analyses, utilising two independent mouse melanoma models and including human melanoma samples, we show here that XIAP is an important mediator of melanoma neutrophil infiltration. Neutrophils represent a major driver of melanoma progression and are increasingly considered as a valuable therapeutic target in solid cancer. Our data reveal that XIAP ubiquitylates RIPK2, involve TAB1/RIPK2 complex and induce the transcriptional up‐regulation and secretion of chemokines such as IL8, that are responsible for intra‐tumour neutrophil accumulation. Alteration of the XIAP‐RIPK2‐TAB1 inflammatory axis or the depletion of neutrophils in mice reduced melanoma growth. Our data shed new light on how XIAP contributes to tumour growth and provides important insights for novel XIAP targeting strategies in cancer.

Calpain as a therapeutic target in cancer

INTRODUCTION

Calpain-1 and calpain-2 are prototypical classical isoforms of the calpain family of calcium-activated cysteine proteases. Their substrate proteins participate in a wide range of cellular processes, including transcription, survival, proliferation, apoptosis, migration, and invasion. Dysregulated calpain activity has been implicated in tumorigenesis, suggesting that calpains may be promising therapeutic targets.

AREAS COVERED

This review covers clinical and basic research studies implicating calpain-1 and calpain-2 expression and activity in tumorigenesis and metastasis. We highlight isoform specific functions and provide an overview of substrates and cancer-related signalling pathways affected by calpain-mediated proteolytic cleavage. We also discuss efforts to develop clinically relevant calpain specific inhibitors and spotlight the challenges facing inhibitor development.

EXPERT OPINION

Rationale for targeting calpain-1 and calpain-2 in cancer is supported by pre-clinical and clinical studies demonstrating that calpain inhibition has the potential to attenuate carcinogenesis and block metastasis of aggressive tumors. The wide range of substrates and cleavage products, paired with inconsistencies in model systems, underscores the need for more complete understanding of physiological substrates and how calpain cleavage alters their function in cellular processes. The development of isoform specific calpain inhibitors remains an important goal with therapeutic potential in cancer and other diseases.

The evolution of regulated cell death pathways in animals and their evasion by pathogens

The coevolution of host-pathogen interactions underlies many human physiological traits associated with protection from or susceptibility to infections. Among the mechanisms that animals utilize to control infections are the regulated cell death pathways of pyroptosis, apoptosis, and necroptosis. Over the course of evolution these pathways have become intricate and complex, coevolving with microbes that infect animal hosts. Microbes, in turn, have evolved strategies to interfere with the pathways of regulated cell death to avoid eradication by the host. Here, we present an overview of the mechanisms of regulated cell death in Animalia and the strategies devised by pathogens to interfere with these processes. We review the molecular pathways of regulated cell death, their roles in infection, and how they are perturbed by viruses and bacteria, providing insights into the coevolution of host-pathogen interactions and cell death pathways.

14-3-3 and erlin proteins differentially interact with RIPK2 complexes

The receptor interacting serine/threonine kinase 2 (RIPK2) is essential for signal transduction induced by the pattern recognition receptors NOD1 and NOD2 (referred to collectively as NOD1/2). Upon NOD1/2 activation, RIPK2 forms complexes in the cytoplasm of human cells. Here, we identified the molecular composition of these complexes. Infection with Shigella flexneri to activate NOD1-RIPK2 revealed that RIPK2 formed dynamic interactions with several cellular proteins, including A20 (also known as TNFAIP3), erlin-1, erlin-2 and 14-3-3. Whereas interaction of RIPK2 with 14-3-3 proteins was strongly reduced upon infection with Shigella, erlin-1 and erlin-2 (erlin-1/2) specifically bound to RIPK2 complexes. The interaction of these proteins with RIPK2 was validated using protein binding assays and immunofluorescence staining. Beside bacterial activation of NOD1/2, depletion of the E3 ubiquitin ligase XIAP and treatment with RIPK2 inhibitors also led to the formation of RIPK2 cytosolic complexes. Although erlin-1/2 were recruited to RIPK2 complexes following XIAP inhibition, these proteins did not associate with RIPK2 structures induced by RIPK2 inhibitors. While the specific recruitment of erlin-1/2 to RIPK2 suggests a role in innate immune signaling, the biological response regulated by the erlin-1/2-RIPK2 association remains to be determined.

XIAP Knockdown in Alcohol-Associated Liver Disease Models Exhibits Divergent in vitro and in vivo Phenotypes Owing to a Potential Zonal Inhibitory Role of SMAC

Alcohol-associated liver disease (ALD) has been recognized as the most common cause of advanced liver disease worldwide, though mechanisms of pathogenesis remain incompletely understood. The X-linked inhibitor of apoptosis (XIAP) protein was originally described as an anti-apoptotic protein that directly binds and inhibits caspases-3, 7, and 9. Here, we investigated the function of XIAP in hepatocytes in vitro using gain and loss-of-function approaches. We noted an XIAP-dependent increase in caspase activation as well as increased inflammatory markers and pro-inflammatory EV release from hepatocytes in vitro. Primary hepatocytes (PMH) from Xiap Alb.Cre and Xiap loxP mice exhibited higher cell death but surprisingly, lower expression of inflammation markers. Conditioned media from these isolated Xiap deleted PMH further decrease inflammation in bone marrow-derived macrophages. Also, interestingly, when administered an ethanol plus Fas-agonist-Jo2 model and an ethanol plus CCl4 model, these animals failed to develop an exacerbated disease phenotype in vivo. Of note, neither Xiap Alb . Cre nor Xiap AAV8.Cre mice presented with aggravated liver injury, hepatocyte apoptosis, liver steatosis, or fibrosis. Since therapeutics targeting XIAP are currently in clinical trials and caspase-induced death is very important for development of ALD, we sought to explore the potential basis of this unexpected lack of effect. We utilized scRNA-seq and spatially reconstructed hepatocyte transcriptome data from human liver tissue and observed that XIAP was significantly zonated, along with its endogenous inhibitor second mitochondria-derived activator of caspases (SMAC) in periportal region. This contrasted with pericentral zonation of other IAPs including cIAP1 and Apollon as well as caspases 3, 7, and 9. Thus providing a potential explanation for compensation of the effect of Xiap deletion by other IAPs. In conclusion, our findings implicate a potential zonallydependent role for SMAC that prevented development of a phenotype in XIAP knockout mice in ALD models. Targeting SMAC may also be important in addition to current efforts of targeting XIAP in treatment of ALD.

Tumor Suppressor Protein p53 and Inhibitor of Apoptosis Proteins in Colorectal Cancer-A Promising Signaling Network for Therapeutic Interventions

Simple Summary

Tumor suppressor 53 (p53) is a multifunctional protein that regulates cell cycle, DNA repair, apoptosis and metabolic pathways. In colorectal cancer (CRC), mutations of the gene occur in 60% of patients and are associated with a more aggressive tumor phenotype and resistance to anti-cancer therapy. In addition, inhibitor of apoptosis (IAP) proteins are distinguished biomarkers overexpressed in CRC that impact on a diverse set of signaling pathways associated with the regulation of apoptosis/autophagy, cell migration, cell cycle and DNA damage response. As these mechanisms are further firmly controlled by p53, a transcriptional and post-translational regulation of IAPs by p53 is expected to occur in cancer cells. Here, we aim to review the molecular regulatory mechanisms between IAPs and p53 and discuss the therapeutic potential of targeting their interrelationship by multimodal treatment options.

Abstract

Despite recent advances in the treatment of colorectal cancer (CRC), patient’s individual response and clinical follow-up vary considerably with tumor intrinsic factors to contribute to an enhanced malignancy and therapy resistance. Among these markers, upregulation of members of the inhibitor of apoptosis protein (IAP) family effects on tumorigenesis and radiation- and chemo-resistance by multiple pathways, covering a hampered induction of apoptosis/autophagy, regulation of cell cycle progression and DNA damage response. These mechanisms are tightly controlled by the tumor suppressor p53 and thus transcriptional and post-translational regulation of IAPs by p53 is expected to occur in malignant cells. By this, cellular IAP1/2, X-linked IAP, Survivin, BRUCE and LIVIN expression/activity, as well as their intracellular localization is controlled by p53 in a direct or indirect manner via modulating a multitude of mechanisms. These cover, among others, transcriptional repression and the signal transducer and activator of transcription (STAT)3 pathway. In addition, p53 mutations contribute to deregulated IAP expression and resistance to therapy. This review aims at highlighting the mechanistic and clinical importance of IAP regulation by p53 in CRC and describing potential therapeutic strategies based on this interrelationship.

Cytosolic Gram-negative bacteria prevent apoptosis by inhibition of effector caspases through lipopolysaccharide

The cytosolic appearance and propagation of bacteria cause overwhelming cellular stress responses that induce apoptosis under normal conditions. Therefore, successful bacterial colonization depends on the ability of intracellular pathogens to block apoptosis and to safeguard bacterial replicative niches. Here, we show that the cytosolic Gram-negative bacterium Shigella flexneri stalls apoptosis by inhibiting effector caspase activity. Our data identified lipopolysaccharide (LPS) as a bona fide effector caspase inhibitor that directly binds caspases by involving its O-antigen (O Ag) moiety. Bacterial strains that lacked the O Ag or failed to replicate within the cytosol were incapable of blocking apoptosis and exhibited reduced virulence in a murine model of bacterial infection. Our findings demonstrate how Shigella inhibits pro-apoptotic caspase activity, effectively delays coordinated host-cell demise and supports its intracellular propagation. Next to the recently discovered pro-inflammatory role of cytosolic LPS, our data reveal a distinct mode of LPS action that, through the disruption of the early coordinated non-lytic cell death response, ultimately supports the inflammatory breakdown of infected cells at later time points.

Caspase-8 is the molecular switch for apoptosis, necroptosis and pyroptosis

Caspase-8 is the initiator caspase of extrinsic apoptosis^1,2 and inhibits necroptosis mediated by RIPK3 and MLKL. Accordingly, caspase-8 deficiency in mice causes embryonic lethality³, which can be rescued by deletion of either Ripk3 or Mlkl^4-6. Here we show that the expression of enzymatically inactive CASP8(C362S) causes embryonic lethality in mice by inducing necroptosis and pyroptosis. Similar to Casp8^-/- mice^3,7, Casp8^C362S/C362S mouse embryos died after endothelial cell necroptosis leading to cardiovascular defects. MLKL deficiency rescued the cardiovascular phenotype but unexpectedly caused perinatal lethality in Casp8^C362S/C362S mice, indicating that CASP8(C362S) causes necroptosis-independent death at later stages of embryonic development. Specific loss of the catalytic activity of caspase-8 in intestinal epithelial cells induced intestinal inflammation similar to intestinal epithelial cell-specific Casp8 knockout mice⁸. Inhibition of necroptosis by additional deletion of Mlkl severely aggravated intestinal inflammation and caused premature lethality in Mlkl knockout mice with specific loss of caspase-8 catalytic activity in intestinal epithelial cells. Expression of CASP8(C362S) triggered the formation of ASC specks, activation of caspase-1 and secretion of IL-1β. Both embryonic lethality and premature death were completely rescued in Casp8^C362S/C362SMlkl^-/-Asc^-/- or Casp8^C362S/C362SMlkl^-/-Casp1^-/- mice, indicating that the activation of the inflammasome promotes CASP8(C362S)-mediated tissue pathology when necroptosis is blocked. Therefore, caspase-8 represents the molecular switch that controls apoptosis, necroptosis and pyroptosis, and prevents tissue damage during embryonic development and adulthood.

Guardians of the Cell: Effector-Triggered Immunity Steers Mammalian Immune Defense

The mammalian innate immune system deals with invading pathogens and stress by activating pattern-recognition receptors (PRRs) in the host. Initially proposed to be triggered by the discrimination of defined molecular signatures from pathogens rather than from self, it is now clear that PRRs can also be activated by endogenous ligands, bacterial metabolites and, following pathogen-induced alterations of cellular processes, changes in the F-actin cytoskeleton. These processes are collectively referred to as effector-triggered immunity (ETI). Here, we summarize the molecular and conceptual advances in our understanding of cell autonomous innate immune responses against bacterial pathogens, and discuss how classical activation of PRRs and ETI interplay to drive inflammatory responses.

XIAP controls RIPK2 signaling by preventing its deposition in speck-like structures

This study provides evidence that the NOD1/2-associated kinase RIPK2 localizes to detergent insoluble cytosolic complexes upon activation and suggests novel regulatory mechanisms for RIPK2 signaling.

The receptor interacting serine/threonine kinase 2 (RIPK2) is essential for linking activation of the pattern recognition receptors NOD1 and NOD2 to cellular signaling events. Recently, it was shown that RIPK2 can form higher order molecular structures in vitro. Here, we demonstrate that RIPK2 forms detergent insoluble complexes in the cytosol of host cells upon infection with invasive enteropathogenic bacteria. Formation of these structures occurred after NF-κB activation and depended on the caspase activation and recruitment domain of NOD1 or NOD2. Complex formation upon activation required RIPK2 autophosphorylation at Y474 and was influenced by phosphorylation at S176. We found that the E3 ligase X-linked inhibitor of apoptosis (XIAP) counteracts complex formation of RIPK2, accordingly mutation of the XIAP ubiquitylation sites in RIPK2 enhanced complex formation. Taken together, our work reveals novel roles of XIAP in the regulation of RIPK2 and expands our knowledge on the function of RIPK2 posttranslational modifications in NOD1/2 signaling.

Elevated X-linked inhibitor of apoptosis protein (XIAP) expression uncovers detrimental prognosis in subgroups of neoadjuvant treated and T-cell rich esophageal adenocarcinoma

Background

Molecular markers predicting survival in esophageal adenocarcinoma (EAC) are rare. Specifically, in favorable oncologic situations, e.g. nodal negativity or major neoadjuvant therapy response, there is a lack of additional risk factors that serve to predict patients’ outcome more precisely. This study evaluated X-linked inhibitor of apoptosis protein (XIAP) as a potential marker improving outcome prediction.

Methods

Tissue microarrays from 362 patients that were diagnosed with resectable EAC were included in the study. XIAP was stained by immunohistochemistry and correlated to clinical outcome, molecular markers and markers of the cellular tumor microenvironment.

Results

XIAP did not impact on overall survival (OS) in the whole study collective. Subgroup analyses stratifying for common genetic markers (TP53, ERBB2, ARID1A/SWI/SNF) did not disclose any impact of XIAP expression on survival. Detailed subgroup analyses of [1] nodal negative patients, [2] highly T-cell infiltrated tumors and [3] therapy responders to neoadjuvant treatment revealed a significant inverse role of high XIAP expression in these specific oncologic situations; elevated XIAP expression detrimentally affected patients’ outcome in these subgroups. [1]: OS XIAP low: 202 months (m) vs. XIAP high: 38 m; [2]: OS 116 m vs. 28.2 m; [3]: OS 31 m vs. 4 m).

Conclusions

Our data suggest XIAP expression in EAC as a worthy tool to improve outcome prediction in specific oncologic settings that might directly impact on clinical diagnosis and treatment of EAC in the future.

Electronic supplementary material

The online version of this article (10.1186/s12885-019-5722-1) contains supplementary material, which is available to authorized users.

The X-linked inhibitor of apoptosis protein (XIAP) is involved in melanoma invasion by regulating cell migration and survival

BACKGROUND

The X-linked inhibitor of apoptosis (XIAP) is a potent cellular inhibitor of apoptosis, based on its unique capability to bind and to inhibit caspases. However, XIAP is also involved in a number of additional cellular activities independent of its caspase inhibitory function. The aim of this study was to investigate whether modulation of XIAP expression affects apoptosis-independent functions of XIAP in melanoma cells, restores their sensitivity to apoptosis and/or affects their invasive and metastatic capacities.

METHODS

XIAP protein levels were analyzed by immunohistochemical staining of human tissues and by Western blotting of melanoma cell lysates. The effects of pharmacological inhibition or of XIAP down-regulation were investigated using ex-vivo and transwell invasion assays. The biological effects of XIAP down-regulation on melanoma cells were analyzed in vitro using BrdU/PI, nucleosome quantification, adhesion and migration assays. In addition, new XIAP binding partners were identified by co-immunoprecipitation followed by mass spectrometry.

RESULTS

Here we found that the expression of XIAP is increased in metastatic melanomas and in invasive melanoma-derived cell lines. We also found that the bivalent IAP antagonist birinapant significantly reduced the invasive capability of melanoma cells. This reduction could be reproduced by downregulating XIAP in melanoma cells. Furthermore, we found that the migration of melanoma cells and the formation of focal adhesions at cellular borders on fibronectin-coated surfaces were significantly reduced upon XIAP knockdown. This reduction may depend on an altered vimentin-XIAP association, since we identified vimentin as a new binding partner of XIAP. As a corollary of these molecular alterations, we found that XIAP down-regulation in melanoma cells led to a significant decrease in invasion of dermal skin equivalents.

CONCLUSION

From our data we conclude that XIAP acts as a multifunctional pro-metastatic protein in skin melanomas and, as a consequence, that XIAP may serve as a therapeutic target for these melanomas.

Corticosteroids inhibit Mycobacterium tuberculosis-induced necrotic host cell death by abrogating mitochondrial membrane permeability transition

Corticosteroids are host-directed drugs with proven beneficial effect on survival of tuberculosis (TB) patients, but their precise mechanisms of action in this disease remain largely unknown. Here we show that corticosteroids such as dexamethasone inhibit necrotic cell death of cells infected with Mycobacterium tuberculosis (Mtb) by facilitating mitogen-activated protein kinase phosphatase 1 (MKP-1)-dependent dephosphorylation of p38 MAPK. Characterization of infected mixed lineage kinase domain-like (MLKL) and tumor necrosis factor receptor 1 (TNFR1) knockout cells show that the underlying mechanism is independent from TNFα-signaling and necroptosis. Our results link corticosteroid function and p38 MAPK inhibition to abrogation of necrotic cell death mediated by mitochondrial membrane permeability transition, and open new avenues for research on novel host-directed therapies (HDT).

Corticosteroids are host-directed drugs that enhance survival of tuberculosis patients through unclear mechanisms. Here, Gräb et al. show that corticosteroids inhibit necrotic death of cells infected with Mycobacterium tuberculosis by facilitating MKP-1-dependent dephosphorylation of p38 MAPK.

Recent advances in understanding inhibitor of apoptosis proteins

The inhibitor of apoptosis proteins (IAPs) are a family of proteins that were chiefly known for their ability to inhibit apoptosis by blocking caspase activation or activity. Recent research has shown that cellular IAP1 (cIAP1), cIAP2, and X-linked IAP (XIAP) also regulate signaling by receptors of the innate immune system by ubiquitylating their substrates. These IAPs thereby act at the intersection of pathways leading to cell death and inflammation. Mutation of IAP genes can impair tissue homeostasis and is linked to several human diseases. Small-molecule IAP antagonists have been developed to treat certain malignant, infectious, and inflammatory diseases. Here, we will discuss recent advances in our understanding of the functions of cIAP1, cIAP2, and XIAP; the consequences of their mutation or dysregulation; and the therapeutic potential of IAP antagonist drugs.

Regulatory role of calpain in neuronal death

Calpains are a group of calcium-dependent proteases that are over activated by increased intracellular calcium levels under pathological conditions. A wide range of substrates that regulate necrotic, apoptotic and autophagic pathways are affected by calpain. Calpain plays a very important role in neuronal death and various neurological disorders. This review introduces recent research progress related to the regulatory mechanisms of calpain in neuronal death. Various neuronal programmed death pathways including apoptosis, autophagy and regulated necrosis can be divided into receptor interacting protein-dependent necroptosis, mitochondrial permeability transition-dependent necrosis, pyroptosis and poly (ADP-ribose) polymerase 1-mediated parthanatos. Calpains cleave series of key substrates that may lead to cell death or participate in cell death. Regarding the investigation of calpain-mediated programed cell death, it is necessary to identify specific inhibitors that inhibit calpain mediated neuronal death and nervous system diseases.

Calpain research for drug discovery: challenges and potential

Calpains are a family of proteases that were scientifically recognized earlier than proteasomes and caspases, but remain enigmatic. However, they are known to participate in a multitude of physiological and pathological processes, performing 'limited proteolysis' whereby they do not destroy but rather modulate the functions of their substrates. Calpains are therefore referred to as 'modulator proteases'. Multidisciplinary research on calpains has begun to elucidate their involvement in pathophysiological mechanisms. Therapeutic strategies targeting malfunctions of calpains have been developed, driven primarily by improvements in the specificity and bioavailability of calpain inhibitors. Here, we review the calpain superfamily and calpain-related disorders, and discuss emerging calpain-targeted therapeutic strategies.

Loss of XIAP facilitates switch to TNFα-induced necroptosis in mouse neutrophils

Neutrophils are essential players in the first-line defense against invading bacteria and fungi. Besides its antiapoptotic role, the inhibitor of apoptosis protein (IAP) family member X-linked IAP (XIAP) has been shown to regulate innate immune signaling. Whereas the role of XIAP in innate signaling pathways is derived mostly from work in macrophages and dendritic cells, it is not known if and how XIAP contributes to these pathways in neutrophils. Here we show that in response to bacterial lipopolysaccharides (LPS), mouse neutrophils secreted considerable amounts of tumor necrosis factor-α (TNFα) and interleukin-1β (IL-1β) and, in accordance with earlier reports, XIAP prevented LPS-induced hypersecretion of IL-1β also in neutrophils. Interestingly, and in contrast to macrophages or dendritic cells, Xiap-deficient neutrophils were insensitive to LPS-induced cell death. However, combined loss of function of XIAP and cIAP1/-2 resulted in rapid neutrophil cell death in response to LPS. This cell death occurred by classical apoptosis initiated by a TNFα- and RIPK1-dependent, but RIPK3- and MLKL-independent, pathway. Inhibition of caspases under the same experimental conditions caused a shift to RIPK3-dependent cell death. Accordingly, we demonstrate that treatment of neutrophils with high concentrations of TNFα induced apoptotic cell death, which was fully blockable by pancaspase inhibition in wild-type neutrophils. However, in the absence of XIAP, caspase inhibition resulted in a shift from apoptosis to RIPK3- and MLKL-dependent necroptosis. Loss of XIAP further sensitized granulocyte–macrophage colony-stimulating factor (GM-CSF)-primed neutrophils to TNFα-induced killing. These data suggest that XIAP antagonizes the switch from TNFα-induced apoptosis to necroptosis in mouse neutrophils. Moreover, our data may implicate an important role of neutrophils in the development of hyperinflammation and disease progression of patients diagnosed with X-linked lymphoproliferative syndrome type 2, which are deficient in XIAP.

Cellular Aspects of Shigella Pathogenesis: Focus on the Manipulation of Host Cell Processes

Shigella is a Gram-negative bacterium that is responsible for shigellosis. Over the years, the study of Shigella has provided a greater understanding of how the host responds to bacterial infection, and how bacteria have evolved to effectively counter the host defenses. In this review, we provide an update on some of the most recent advances in our understanding of pivotal processes associated with Shigella infection, including the invasion into host cells, the metabolic changes that occur within the bacterium and the infected cell, cell-to-cell spread mechanisms, autophagy and membrane trafficking, inflammatory signaling and cell death. This recent progress sheds a new light into the mechanisms underlying Shigella pathogenesis, and also more generally provides deeper understanding of the complex interplay between host cells and bacterial pathogens in general.

How Shigella Utilizes Ca(2+) Jagged Edge Signals during Invasion of Epithelial Cells

Shigella, the causative agent of bacillary dysentery invades intestinal epithelial cells using a type III secretion system (T3SS). Through the injection of type III effectors, Shigella manipulates the actin cytoskeleton to induce its internalization in epithelial cells. At early invasion stages, Shigella induces atypical Ca(2+) responses confined at entry sites allowing local cytoskeletal remodeling for bacteria engulfment. Global Ca(2+) increase in the cell triggers the opening of connexin hemichannels at the plasma membrane that releases ATP in the extracellular milieu, favoring Shigella invasion and spreading through purinergic receptor signaling. During intracellular replication, Shigella regulates inflammatory and death pathways to disseminate within the epithelium. At later stages of infection, Shigella downregulates hemichannel opening and the release of extracellular ATP to dampen inflammatory signals. To avoid premature cell death, Shigella activates cell survival by upregulating the PI3K/Akt pathway and downregulating the levels of p53. Furthermore, Shigella interferes with pro-apoptotic caspases, and orients infected cells toward a slow necrotic cell death linked to mitochondrial Ca(2+) overload. In this review, we will focus on the role of Ca(2+) responses and their regulation by Shigella during the different stages of bacterial infection.

Shigella manipulates host immune responses by delivering effector proteins with specific roles

The intestinal epithelium deploys multiple defense systems against microbial infection to sense bacterial components and danger alarms, as well as to induce intracellular signal transduction cascades that trigger both the innate and the adaptive immune systems, which are pivotal for bacterial elimination. However, many enteric bacterial pathogens, including Shigella, deliver a subset of virulence proteins (effectors) via the type III secretion system (T3SS) that enable bacterial evasion from host immune systems; consequently, these pathogens are able to efficiently colonize the intestinal epithelium. In this review, we present and select recently discovered examples of interactions between Shigella and host immune responses, with particular emphasis on strategies that bacteria use to manipulate inflammatory outputs of host-cell responses such as cell death, membrane trafficking, and innate and adaptive immune responses.

PRR-signaling pathways: Learning from microbial tactics

Recognition of bacterial pathogens by the mammalian host relies on the induction of early innate immune responses initiated by the activation of pattern-recognition receptors (PRRs) upon sensing of their cognate microbe-associated-patterns (MAMPs). Successful pathogens have evolved to intercept PRR activation and signaling at multiple steps. The molecular dissection of the underlying mechanisms revealed many of the basic mechanisms used by the immune system. Here we provide an overview of the different strategies used by bacterial pathogens and commensals to subvert and reprogram PPR-mediated innate immune responses. A particular attention is given to recent discoveries highlighting novel molecular details of the host inflammatory response in mammalian cells and current advances in our understanding of the interaction of commensals with PRR-mediated responses.

IAP antagonization promotes inflammatory destruction of vascular endothelium

In this study, we show for the first time that the therapeutic antagonization of inhibitor of apoptosis proteins (IAPs) inhibits B16 melanoma growth by disrupting tumor vasculature. Specifically, the treatment of mice bearing B16 melanoma with an IAP antagonist compound A (Comp A) inhibits tumor growth not by inducing direct cytotoxicity against B16 cells but rather by a hitherto unrecognized antiangiogenic activity against tumor vessels. Our detailed analysis showed that Comp A treatment induces NF-κB activity in B16 tumor cells and facilitates the production of TNF. In the presence of Comp A, endothelial cells (ECs) become highly susceptible to TNF and undergo apoptotic cell death. Accordingly, the antiangiogenic and growth-attenuating effects of Comp A treatment were completely abolished in TNF-R knockout mice. This novel targeting approach could be of clinical value in controlling pathological neoangiogenesis under inflammatory condition while sparing blood vessels under normal condition.

Cytochrome c oxidase deficiency accelerates mitochondrial apoptosis by activating ceramide synthase 6

Although numerous pathogenic changes within the mitochondrial respiratory chain (RC) have been associated with an elevated occurrence of apoptosis within the affected tissues, the mechanistic insight into how mitochondrial dysfunction initiates apoptotic cell death is still unknown. In this study, we show that the specific alteration of the cytochrome c oxidase (COX), representing a common defect found in mitochondrial diseases, facilitates mitochondrial apoptosis in response to oxidative stress. Our data identified an increased ceramide synthase 6 (CerS6) activity as an important pro-apoptotic response to COX dysfunction induced either by chemical or genetic approaches. The elevated CerS6 activity resulted in accumulation of the pro-apoptotic C_16 : 0 ceramide, which facilitates the mitochondrial apoptosis in response to oxidative stress. Accordingly, inhibition of CerS6 or its specific knockdown diminished the increased susceptibility of COX-deficient cells to oxidative stress. Our results provide new insights into how mitochondrial RC dysfunction mechanistically interferes with the apoptotic machinery. On the basis of its pivotal role in regulating cell death upon COX dysfunction, CerS6 might potentially represent a novel target for therapeutic intervention in mitochondrial diseases caused by COX dysfunction.

Inhibitors of apoptosis (IAPs) regulate intestinal immunity and inflammatory bowel disease (IBD) inflammation

The inhibitor of apoptosis (IAP) family members, notably cIAP1, cIAP2, and XIAP, are critical and universal regulators of tumor necrosis factor (TNF) mediated survival, inflammatory, and death signaling pathways. Furthermore, IAPs mediate the signaling of nucleotide-binding oligomerization domain (NOD)1/NOD2 and other intracellular NOD-like receptors in response to bacterial pathogens. These pathways are important to the pathogenesis and treatment of inflammatory bowel disease (IBD). Inactivating mutations in the X-chromosome-linked IAP (XIAP) gene causes an immunodeficiency syndrome, X-linked lymphoproliferative disease type 2 (XLP2), in which 20% of patients develop severe intestinal inflammation. In addition, 4% of males with early-onset IBD also have inactivating mutations in XIAP. Therefore, the IAPs play a greater role in gut homeostasis, immunity and IBD development than previously suspected, and may have therapeutic potential.

The cofilin phosphatase slingshot homolog 1 (SSH1) links NOD1 signaling to actin remodeling

NOD1 is an intracellular pathogen recognition receptor that contributes to anti-bacterial innate immune responses, adaptive immunity and tissue homeostasis. NOD1-induced signaling relies on actin remodeling, however, the details of the connection of NOD1 and the actin cytoskeleton remained elusive. Here, we identified in a druggable-genome wide siRNA screen the cofilin phosphatase SSH1 as a specific and essential component of the NOD1 pathway. We show that depletion of SSH1 impaired pathogen induced NOD1 signaling evident from diminished NF-κB activation and cytokine release. Chemical inhibition of actin polymerization using cytochalasin D rescued the loss of SSH1. We further demonstrate that NOD1 directly interacted with SSH1 at F-actin rich sites. Finally, we show that enhanced cofilin activity is intimately linked to NOD1 signaling. Our data thus provide evidence that NOD1 requires the SSH1/cofilin network for signaling and to detect bacterial induced changes in actin dynamics leading to NF-κB activation and innate immune responses.

A near death experience: Shigella manipulates host death machinery to silence innate immunity

Release of mitochondrial contents often triggers inflammation and cell death, and modulating this process can be advantageous to invading pathogens. In this issue of The EMBO Journal, Andree and colleagues reveal new findings that an intracellular bacterial pathogen exploits apoptotic machinery to suppress host immune signaling, yet avoids cell death. This study emphasizes the need to expand our understanding of the roles played by pro‐apoptotic proteins in non‐death scenarios.