RIPK1 is dispensable for cell death regulation in β-cells during hyperglycemia

OBJECTIVE

Receptor-interacting protein kinase 1 (RIPK1) orchestrates the decision between cell survival and cell death in response to tumor necrosis factor (TNF) and other cytokines. Whereas the scaffolding function of RIPK1 is crucial to prevent TNF-induced apoptosis and necroptosis, its kinase activity is required for necroptosis and partially for apoptosis. Although TNF is a proinflammatory cytokine associated with β-cell loss in diabetes, the mechanism by which TNF induces β-cell demise remains unclear.

METHODS

Here, we dissected the contribution of RIPK1 scaffold versus kinase functions to β-cell death regulation using mice lacking RIPK1 specifically in β-cells (Ripk1β-KO mice) or expressing a kinase-dead version of RIPK1 (Ripk1D138N mice), respectively. These mice were challenged with streptozotocin, a model of autoimmune diabetes. Moreover, Ripk1β-KO mice were further challenged with a high-fat diet to induce hyperglycemia. For mechanistic studies, pancreatic islets were subjected to various killing and sensitising agents.

RESULTS

Inhibition of RIPK1 kinase activity (Ripk1D138N mice) did not affect the onset and progression of hyperglycemia in a type 1 diabetes model. Moreover, the absence of RIPK1 expression in β-cells did not affect normoglycemia under basal conditions or hyperglycemia under diabetic challenges. Ex vivo, primary pancreatic islets are not sensitised to TNF-induced apoptosis and necroptosis in the absence of RIPK1. Intriguingly, we found that pancreatic islets display high levels of the antiapoptotic cellular FLICE-inhibitory protein (cFLIP) and low levels of apoptosis (Caspase-8) and necroptosis (RIPK3) components. Cycloheximide treatment, which led to a reduction in cFLIP levels, rendered primary islets sensitive to TNF-induced cell death which was fully blocked by caspase inhibition.

CONCLUSIONS

Unlike in many other cell types (e.g., epithelial, and immune), RIPK1 is not required for cell death regulation in β-cells under physiological conditions or diabetic challenges. Moreover, in vivo and in vitro evidence suggest that pancreatic β-cells do not undergo necroptosis but mainly caspase-dependent death in response to TNF. Last, our results show that β-cells have a distinct mode of regulation of TNF-cytotoxicity that is independent of RIPK1 and that may be highly dependent on cFLIP.

The RIPK1 death domain restrains ZBP1- and TRIF-mediated cell death and inflammation

RIPK1 is a multi-functional kinase that regulates cell death and inflammation and has been implicated in the pathogenesis of inflammatory diseases. RIPK1 acts in a kinase-dependent and kinase-independent manner to promote or suppress apoptosis and necroptosis, but the underlying mechanisms remain poorly understood. Here, we show that a mutation (R588E) disrupting the RIPK1 death domain (DD) caused perinatal lethality induced by ZBP1-mediated necroptosis. Additionally, these mice developed postnatal inflammatory pathology, which was mediated by necroptosis-independent TNFR1, TRADD, and TRIF signaling, partially requiring RIPK3. Our biochemical mechanistic studies revealed that ZBP1- and TRIF-mediated activation of RIPK3 required RIPK1 kinase activity in wild-type cells but not in Ripk1R588E/R588E cells, suggesting that DD-dependent oligomerization of RIPK1 and its interaction with FADD determine the mechanisms of RIPK3 activation by ZBP1 and TRIF. Collectively, these findings revealed a critical physiological role of DD-dependent RIPK1 signaling that is important for the regulation of tissue homeostasis and inflammation.

ZBP1 causes inflammation by inducing RIPK3-mediated necroptosis and RIPK1 kinase activity-independent apoptosis

Z-DNA binding protein 1 (ZBP1) has important functions in anti-viral immunity and in the regulation of inflammatory responses. ZBP1 induces necroptosis by directly engaging and activating RIPK3, however, the mechanisms by which ZBP1 induces inflammation and in particular the role of RIPK1 and the contribution of cell death-independent signaling remain elusive. Here we show that ZBP1 causes skin inflammation by inducing RIPK3-mediated necroptosis and RIPK1-caspase-8-mediated apoptosis in keratinocytes. ZBP1 induced TNFR1-independent skin inflammation in mice with epidermis-specific ablation of FADD by triggering keratinocyte necroptosis. Moreover, transgenic expression of C-terminally truncated constitutively active ZBP1 (ZBP1ca) in mouse epidermis caused skin inflammation that was only partially inhibited by abrogation of RIPK3-MLKL-dependent necroptosis and fully prevented by combined deficiency in MLKL and caspase-8. Importantly, ZBP1ca induced caspase-8-mediated skin inflammation by RHIM-dependent but kinase activity-independent RIPK1 signaling. Furthermore, ZBP1ca-induced inflammatory cytokine production in the skin was completely prevented by combined inhibition of apoptosis and necroptosis arguing against a cell death-independent pro-inflammatory function of ZBP1. Collectively, these results showed that ZBP1 induces inflammation by activating necroptosis and RIPK1 kinase activity-independent apoptosis.

IKKε and TBK1 prevent RIPK1 dependent and independent inflammation

TBK1 and IKKε regulate multiple cellular processes including anti-viral type-I interferon responses, metabolism and TNF receptor signaling. However, the relative contributions and potentially redundant functions of IKKε and TBK1 in cell death, inflammation and tissue homeostasis remain poorly understood. Here we show that IKKε compensates for the loss of TBK1 kinase activity to prevent RIPK1-dependent and -independent inflammation in mice. Combined inhibition of IKKε and TBK1 kinase activities caused embryonic lethality that was rescued by heterozygous expression of kinase-inactive RIPK1. Adult mice expressing kinase-inactive versions of IKKε and TBK1 developed systemic inflammation that was induced by both RIPK1-dependent and -independent mechanisms. Combined inhibition of IKKε and TBK1 kinase activities in myeloid cells induced RIPK1-dependent cell death and systemic inflammation mediated by IL-1 family cytokines. Tissue-specific studies showed that IKKε and TBK1 were required to prevent cell death and inflammation in the intestine but were dispensable for liver and skin homeostasis. Together, these findings revealed that IKKε and TBK1 exhibit tissue-specific functions that are important to prevent cell death and inflammation and maintain tissue homeostasis.

FADD- and RIPK3-Mediated Cell Death Ensures Clearance of Ly6Chigh Wound Macrophages from Damaged Tissue

Cells of the monocyte/macrophage lineage are an integral component of the body's innate ability to restore tissue function after injury. In parallel to mounting an inflammatory response, clearance of monocytes/macrophages from the wound site is critical to re-establish tissue functionality and integrity during the course of healing. The role of regulated cell death in macrophage clearance from damaged tissue and its implications for the outcome of the healing response is little understood. In this study, we explored the role of macrophage-specific FADD-mediated cell death on Ripk3-/- background in a mechanical skin injury model in mice. We found that combined inhibition of RIPK3-mediated necroptosis and FADD-caspase-8-mediated apoptosis in macrophages profoundly delayed wound healing. Importantly, RIPK3 deficiency alone did not considerably alter the wound healing process and macrophage population dynamics, arguing that inhibition of FADD-caspase-8-dependent death of macrophages is primarily responsible for delayed wound closure. Notably, TNF blockade reversed the accumulation of Ly6Chigh macrophages induced by combined deficiency of FADD and RIPK3, indicating a critical dual role of TNF-mediated prosurvival and cell death signaling, particularly in this highly proinflammatory macrophage subset. Our findings reveal a previously uncharacterized cross-talk of inflammatory and cell death signaling in macrophages in regulating repair processes in the skin.

Mitochondrial dysfunction abrogates dietary lipid processing in enterocytes

Digested dietary fats are taken up by enterocytes where they are assembled into pre-chylomicrons in the endoplasmic reticulum followed by transport to the Golgi for maturation and subsequent secretion to the circulation1. The role of mitochondria in dietary lipid processing is unclear. Here we show that mitochondrial dysfunction in enterocytes inhibits chylomicron production and the transport of dietary lipids to peripheral organs. Mice with specific ablation of the mitochondrial aspartyl-tRNA synthetase DARS2 (ref. 2), the respiratory chain subunit SDHA3 or the assembly factor COX10 (ref. 4) in intestinal epithelial cells showed accumulation of large lipid droplets (LDs) in enterocytes of the proximal small intestine and failed to thrive. Feeding a fat-free diet suppressed the build-up of LDs in DARS2-deficient enterocytes, which shows that the accumulating lipids derive mostly from digested fat. Furthermore, metabolic tracing studies revealed an impaired transport of dietary lipids to peripheral organs in mice lacking DARS2 in intestinal epithelial cells. DARS2 deficiency caused a distinct lack of mature chylomicrons concomitant with a progressive dispersal of the Golgi apparatus in proximal enterocytes. This finding suggests that mitochondrial dysfunction results in impaired trafficking of chylomicrons from the endoplasmic reticulum to the Golgi, which in turn leads to storage of dietary lipids in large cytoplasmic LDs. Taken together, these results reveal a role for mitochondria in dietary lipid transport in enterocytes, which might be relevant for understanding the intestinal defects observed in patients with mitochondrial disorders5.

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.

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.

RIPK1 blocks T cell senescence mediated by RIPK3 and caspase-8

Receptor-interacting protein kinase 1 (RIPK1) regulates cell death and inflammation. Here, we show that T cell-specific RIPK1 deficiency in mice leads to the premature senescence of T cells and induces various age-related diseases, resulting in premature death. RIPK1 deficiency causes higher basal activation of mTORC1 (mechanistic target of rapamycin complex 1) that drives enhanced cytokine production, induction of senescence-related genes, and increased activation of caspase-3/7, which are restored by inhibition of mTORC1. Critically, normal aged T cells exhibit similar phenotypes and responses. Mechanistically, a combined deficiency of RIPK3 and caspase-8 inhibition restores the impaired proliferative responses; the elevated activation of Akt, mTORC1, extracellular signal-regulated kinase, and caspase-3/7; and the increased expression of senescence-related genes in RIPK1-deficient CD4 T cells. Last, we revealed that the senescent phenotype of RIPK1-deficient and aged CD4 T cells is restored in the normal tissue environment. Thus, we have clarified the function of RIPK3 and caspase-8 in inducing CD4 T cell senescence, which is modulated by environmental signals.

NEMO- and RelA-dependent NF-κB signaling promotes small cell lung cancer

Small cell lung cancer (SCLC) is an aggressive type of lung cancer driven by combined loss of the tumor suppressors RB1 and TP53. SCLC is highly metastatic and despite good initial response to chemotherapy patients usually relapse, resulting in poor survival. Therefore, better understanding of the mechanisms driving SCLC pathogenesis is required to identify new therapeutic targets. Here we identified a critical role of the IKK/NF-κB signaling pathway in SCLC development. Using a relevant mouse model of SCLC, we found that ablation of NEMO/IKKγ, the regulatory subunit of the IKK complex that is essential for activation of canonical NF-κB signaling, strongly delayed the onset and growth of SCLC resulting in considerably prolonged survival. In addition, ablation of the main NF-κB family member p65/RelA also delayed the onset and growth of SCLC and prolonged survival, albeit to a lesser extent than NEMO. Interestingly, constitutive activation of IKK/NF-κB signaling within the tumor cells did not exacerbate the pathogenesis of SCLC, suggesting that endogenous NF-κB levels are sufficient to fully support tumor development. Moreover, TNFR1 deficiency did not affect the development of SCLC, showing that TNF signaling does not play an important role in this tumor type. Taken together, our results revealed that IKK/NF-κB signaling plays an important role in promoting SCLC, identifying the IKK/NF-κB pathway as a promising therapeutic target.

Smooth muscle cell specific NEMO deficiency inhibits atherosclerosis in ApoE−/− mice

The development of atherosclerotic plaques is the result of a chronic inflammatory response coordinated by stromal and immune cellular components of the vascular wall. While endothelial cells and leukocytes are well-recognised mediators of inflammation in atherosclerosis, the role of smooth muscle cells (SMCs) remains incompletely understood. Here we aimed to address the role of canonical NF-κB signalling in SMCs in the development of atherosclerosis. We investigated the role of NF-κB signalling in SMCs in atherosclerosis by employing SMC-specific ablation of NEMO, an IKK complex subunit that is essential for canonical NF-κB activation, in ApoE^−/− mice. We show that SMC-specific ablation of NEMO (NEMO^SMCiKO) inhibited high fat diet induced atherosclerosis in ApoE^−/− mice. NEMO^SMCiKO/ApoE^−/− mice developed less and smaller atherosclerotic plaques, which contained fewer macrophages, decreased numbers of apoptotic cells and smaller necrotic areas and showed reduced inflammation compared to the plaques of ApoE^−/− mice. In addition, the plaques of NEMO^SMCiKO/ApoE^−/− mice showed higher expression of α-SMA and lower expression of the transcriptional factor KLF4 compared to those of ApoE^−/− mice. Consistently, in vitro, NEMO-deficient SMCs exhibited reduced proliferation and migration, as well as decreased KLF4 expression and lower production of IL-6 and MCP-1 upon inflammatory stimulus (TNF or LPS) compared to NEMO-expressing SMCs. In conclusion, NEMO-dependent activation of NF-κB signalling in SMCs critically contributes to the pathogenesis of atherosclerosis by regulating SMC proliferation, migration and phenotype switching in response to inflammatory stimuli.

NIK/MAP3K14 in hepatocytes orchestrates NASH to hepatocellular carcinoma progression via JAK2/STAT5 inhibition

OBJECTIVE

Nonalcoholic fatty liver disease (NAFLD) ranges from steatosis to nonalcoholic steatohepatitis (NASH), which often progresses to hepatocellular carcinoma (HCC) through a largely undefined mechanism. NASH and HCC depend on inflammatory signaling, whose master regulator is the NFκB transcription factor family, activated by canonical and non-canonical pathways.

METHODS

Here, we investigated non-canonical NFκB-inducing kinase (NIK/MAP3K14) in metabolic NASH, NASH to HCC transition, and DEN-induced HCC. To this end, we performed dietary and chemical interventions in mice that were analyzed via single nucleus sequencing, gene expression and histochemical methods. Ultimately, we verified our mouse results in human patient samples.

RESULTS

We revealed that hepatocyte-specific NIK deficiency (NIKLKO) ameliorated metabolic NASH complications and reduced hepatocarcinogenesis, independent of its role in the NFκB pathway. Instead, hepatic NIK attenuated hepatoprotective JAK2/STAT5 signaling that is a prerequisite for NASH and NASH to HCC progression in mice and humans.

CONCLUSIONS

Our data suggest NIK-mediated inhibitory JAK2 phosphorylation at serine 633 that might be amenable for future therapeutic interventions in patients.

Leishmania guyanensis suppressed inducible nitric oxide synthase provoked by its viral endosymbiont

Inducible nitric oxide synthase (iNOS) is essential to the production of nitric oxide (NO), an efficient effector molecule against intracellular human pathogens such as Leishmania protozoan parasites. Some strains of Leishmania are known to bear a viral endosymbiont termed Leishmania RNA virus 1 (LRV1). Recognition of LRV1 by the innate immune sensor Toll-like receptor-3 (TLR3) leads to conditions worsening the disease severity in mice. This process is governed by type I interferon (type I IFNs) arising downstream of TLR3 stimulation and favoring the formation of secondary metastatic lesions. The formation of these lesions is mediated by the inflammatory cytokine IL-17A and occurs in the absence, or low level of, protective cytokine IFN-γ. Here, we described that the presence of LRV1 led to the initial expression of iNOS and low production of NO that failed to control infection. We subsequently showed that LRV1-triggered type I IFN was essential but insufficient to induce robust iNOS induction, which requires strong activation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). Leishmania guyanensis carrying LRV1 (LgyLRV1+) parasites mitigated strong iNOS production by limiting NF-kB activation via the induction of tumor necrosis factor-alpha-induced protein 3 (TNFAIP3), also known as A20. Moreover, our data suggested that production of LRV1-induced iNOS could be correlated with parasite dissemination and metastasis via elevated secretion of IL-17A in the draining lymph nodes. Our findings support an additional strategy by which LRV1-bearing Leishmania guyanensis evaded killing by nitric oxide and suggest that low levels of LRV1-induced NO might contribute to parasite metastasis.

Recruitment of dendritic cell progenitors to foci of influenza A virus infection sustains immunity

Protection from infection with respiratory viruses such as influenza A virus (IAV) requires T cell–mediated immune responses initiated by conventional dendritic cells (cDCs) that reside in the respiratory tract. Here, we show that effective induction of T cell responses against IAV in mice requires reinforcement of the resident lung cDC network by cDC progenitors. We found that CCR2-binding chemokines produced during IAV infection recruit pre-cDCs from blood and direct them to foci of infection, increasing the number of progeny cDCs next to sites of viral replication. Ablation of CCR2 in the cDC lineage prevented this increase and resulted in a deficit in IAV-specific T cell responses and diminished resistance to reinfection. These data suggest that the homeostatic network of cDCs in tissues is insufficient for immunity and reveal a chemokine-driven mechanism of expansion of lung cDC numbers that amplifies T cell responses against respiratory viruses.

The SARS-CoV-2 main protease Mpro causes microvascular brain pathology by cleaving NEMO in brain endothelial cells

Coronavirus disease 2019 (COVID-19) can damage cerebral small vessels and cause neurological symptoms. Here we describe structural changes in cerebral small vessels of patients with COVID-19 and elucidate potential mechanisms underlying the vascular pathology. In brains of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-infected individuals and animal models, we found an increased number of empty basement membrane tubes, so-called string vessels representing remnants of lost capillaries. We obtained evidence that brain endothelial cells are infected and that the main protease of SARS-CoV-2 (M^pro) cleaves NEMO, the essential modulator of nuclear factor-κB. By ablating NEMO, M^pro induces the death of human brain endothelial cells and the occurrence of string vessels in mice. Deletion of receptor-interacting protein kinase (RIPK) 3, a mediator of regulated cell death, blocks the vessel rarefaction and disruption of the blood-brain barrier due to NEMO ablation. Importantly, a pharmacological inhibitor of RIPK signaling prevented the M^pro-induced microvascular pathology. Our data suggest RIPK as a potential therapeutic target to treat the neuropathology of COVID-19.

Intercrypt sentinel macrophages tune antibacterial NF-κB responses in gut epithelial cells via TNF

Intestinal epithelial cell (IEC) NF-κB signaling regulates the balance between mucosal homeostasis and inflammation. It is not fully understood which signals tune this balance and how bacterial exposure elicits the process. Pure LPS induces epithelial NF-κB activation in vivo. However, we found that in mice, IECs do not respond directly to LPS. Instead, tissue-resident lamina propria intercrypt macrophages sense LPS via TLR4 and rapidly secrete TNF to elicit epithelial NF-κB signaling in their immediate neighborhood. This response pattern is relevant also during oral enteropathogen infection. The macrophage-TNF-IEC axis avoids responses to luminal microbiota LPS but enables crypt- or tissue-scale epithelial NF-κB responses in proportion to the microbial threat. Thereby, intercrypt macrophages fulfill important sentinel functions as first responders to Gram-negative microbes breaching the epithelial barrier. The tunability of this crypt response allows the induction of defense mechanisms at an appropriate scale according to the localization and intensity of microbial triggers.

Gasdermin D mediates host cell death but not interleukin-1β secretion in Mycobacterium tuberculosis-infected macrophages

Necrotic cell death represents a major pathogenic mechanism of Mycobacterium tuberculosis (Mtb) infection. It is increasingly evident that Mtb induces several types of regulated necrosis but how these are interconnected and linked to the release of pro-inflammatory cytokines remains unknown. Exploiting a clinical cohort of tuberculosis patients, we show here that the number and size of necrotic lesions correlates with IL-1β plasma levels as a strong indicator of inflammasome activation. Our mechanistic studies reveal that Mtb triggers mitochondrial permeability transition (mPT) and subsequently extensive macrophage necrosis, which requires activation of the NLRP3 inflammasome. NLRP3-driven mitochondrial damage is dependent on proteolytic activation of the pore-forming effector protein gasdermin D (GSDMD), which links two distinct cell death machineries. Intriguingly, GSDMD, but not the membranolytic mycobacterial ESX-1 secretion system, is dispensable for IL-1β secretion from Mtb-infected macrophages. Thus, our study dissects a novel mechanism of pathogen-induced regulated necrosis by identifying mitochondria as central regulatory hubs capable of delineating cytokine secretion and lytic cell death.

OTULIN inhibits RIPK1-mediated keratinocyte necroptosis to prevent skin inflammation in mice

Linear ubiquitination regulates inflammatory and cell death signalling. Deficiency of the linear ubiquitin chain-specific deubiquitinase, OTULIN, causes OTULIN-related autoinflammatory syndrome (ORAS), a systemic inflammatory pathology affecting multiple organs including the skin. Here we show that mice with epidermis-specific OTULIN deficiency (OTULINE-KO) develop inflammatory skin lesions that are driven by TNFR1 signalling in keratinocytes and require RIPK1 kinase activity. OTULINE-KO mice lacking RIPK3 or MLKL have only very mild skin inflammation, implicating necroptosis as an important etiological mediator. Moreover, combined loss of RIPK3 and FADD fully prevents skin lesion development, showing that apoptosis also contributes to skin inflammation in a redundant function with necroptosis. Finally, MyD88 deficiency suppresses skin lesion development in OTULINE-KO mice, suggesting that toll-like receptor and/or IL-1 signalling are involved in mediating skin inflammation. Thus, OTULIN maintains homeostasis and prevents inflammation in the skin by inhibiting TNFR1-mediated, RIPK1 kinase activity-dependent keratinocyte death and primarily necroptosis.

NF-κB inhibition in keratinocytes causes RIPK1-mediated necroptosis and skin inflammation

TNFR1 was found to cause skin inflammation in mice with epidermal keratinocyte-specific ablation of IKK2 or of both the NF-kB subunits RelA and cRel by triggering RIPK1-dependent, RIPK3-MLKL–mediated necroptosis of keratinocytes.

Tumor necrosis factor receptor 1 (TNFR1) activates NF-κB–dependent pro-inflammatory gene expression, but also induces cell death by triggering apoptosis and necroptosis. Inhibition of inhibitor of NF-κB kinase (IKK)/NF-κB signaling in keratinocytes paradoxically unleashed spontaneous TNFR1-mediated skin inflammation in mice, but the underlying mechanisms remain poorly understood. Here, we show that TNFR1 causes skin inflammation in mice with epidermis-specific knockout of IKK2 by inducing receptor interacting protein kinase 1 (RIPK1)–dependent necroptosis, and to a lesser extent also apoptosis, of keratinocytes. Combined epidermis-specific ablation of the NF-κB subunits RelA and c-Rel also caused skin inflammation by inducing TNFR1-mediated keratinocyte necroptosis. Contrary to the currently established model that inhibition of NF-κB–dependent gene transcription causes RIPK1-independent cell death, keratinocyte necroptosis, and skin inflammation in mice with epidermis-specific RelA and c-Rel deficiency also depended on RIPK1 kinase activity. These results advance our understanding of the mechanisms regulating TNFR1-induced cell death and identify RIPK1-mediated necroptosis as a potent driver of skin inflammation.

OTULIN Prevents Liver Inflammation and Hepatocellular Carcinoma by Inhibiting FADD- and RIPK1 Kinase-Mediated Hepatocyte Apoptosis

Inflammatory signaling pathways are tightly regulated to avoid chronic inflammation and the development of disease. OTULIN is a deubiquitinating enzyme that controls inflammation by cleaving linear ubiquitin chains generated by the linear ubiquitin chain assembly complex. Here, we show that ablation of OTULIN in liver parenchymal cells in mice causes severe liver disease which is characterized by liver inflammation, hepatocyte apoptosis, and compensatory hepatocyte proliferation, leading to steatohepatitis, fibrosis, and hepatocellular carcinoma (HCC). Genetic ablation of Fas-associated death domain (FADD) completely rescues and knockin expression of kinase inactive receptor-interacting protein kinase 1 (RIPK1) significantly protects mice from developing liver disease, demonstrating that apoptosis of OTULIN-deficient hepatocytes triggers disease pathogenesis in this model. Finally, we demonstrate that type I interferons contribute to disease in hepatocyte-specific OTULIN-deficient mice. Our study reveals the critical importance of OTULIN in protecting hepatocytes from death, thereby preventing the development of chronic liver inflammation and HCC.

CCR2 deficiency in monocytes impairs angiogenesis and functional recovery after ischemic stroke in mice

Inflammatory Ly6C^hiCCR2⁺ monocytes infiltrate the brain after stroke but their functions are not entirely clear. We report that CCR2⁺ monocytes and CCR2⁺ lymphocytes infiltrate the brain after permanent ischemia. To underscore the role of CCR2⁺ monocytes, we generated mice with selective CCR2 deletion in monocytes. One day post-ischemia, these mice showed less infiltrating monocytes and reduced expression of pro-inflammatory cytokines, markers of alternatively macrophage activation, and angiogenesis. Accordingly, Ly6C^hi monocytes sorted from the brain of wild type mice 24 h post-ischemia expressed pro-inflammatory genes, M2 genes, and pro-angiogenic genes. Flow cytometry showed heterogeneous phenotypes within the infiltrating Ly6C^hiCCR2⁺ monocytes, including a subgroup of Arginase-1⁺ cells. Mice with CCR2-deficient monocytes displayed a delayed inflammatory rebound 15 days post-ischemia that was not found in wild type mice. Furthermore, they showed reduced angiogenesis and worse behavioral performance. Administration of CCR2^+/+ bone-marrow monocytes to mice with CCR2-deficient monocytes did not improve the behavioral performance suggesting that immature bone-marrow monocytes lack pro-reparative functions. The results show that CCR2⁺ monocytes contribute to acute post-ischemic inflammation and participate in functional recovery. The study unravels heterogeneity in the population of CCR2⁺ monocytes infiltrating the ischemic brain and suggests that pro-reparative monocyte subsets promote functional recovery after ischemic stroke.

FADD and Caspase-8 Regulate Gut Homeostasis and Inflammation by Controlling MLKL- and GSDMD-Mediated Death of Intestinal Epithelial Cells

Pathways controlling intestinal epithelial cell (IEC) death regulate gut immune homeostasis and contribute to the pathogenesis of inflammatory bowel diseases. Here, we show that caspase-8 and its adapter FADD act in IECs to regulate intestinal inflammation downstream of Z-DNA binding protein 1 (ZBP1)- and tumor necrosis factor receptor-1 (TNFR1)-mediated receptor interacting protein kinase 1 (RIPK1) and RIPK3 signaling. Mice with IEC-specific FADD or caspase-8 deficiency developed colitis dependent on mixed lineage kinase-like (MLKL)-mediated epithelial cell necroptosis. However, MLKL deficiency fully prevented ileitis caused by epithelial caspase-8 ablation, but only partially ameliorated ileitis in mice lacking FADD in IECs. Our genetic studies revealed that caspase-8 and gasdermin-D (GSDMD) were both required for the development of MLKL-independent ileitis in mice with epithelial FADD deficiency. Therefore, FADD prevents intestinal inflammation downstream of ZBP1 and TNFR1 by inhibiting both MLKL-induced necroptosis and caspase-8-GSDMD-dependent pyroptosis-like death of epithelial cells.

Innate Sensing through Mesenchymal TLR4/MyD88 Signals Promotes Spontaneous Intestinal Tumorigenesis

MyD88, an adaptor molecule downstream of innate pathways, plays a significant tumor-promoting role in sporadic intestinal carcinogenesis of the Apc^min/+ model, which carries a mutation in the Apc gene. Here, we show that deletion of MyD88 in intestinal mesenchymal cells (IMCs) significantly reduces tumorigenesis in this model. This phenotype is associated with decreased epithelial cell proliferation, altered inflammatory and tumorigenic immune cell infiltration, and modified gene expression similar to complete MyD88 knockout mice. Genetic deletion of TLR4, but not interleukin-1 receptor (IL-1R), in IMCs led to altered molecular profiles and reduction of intestinal tumors similar to the MyD88 deficiency. Ex vivo analysis in IMCs indicated that these effects could be mediated through downstream signals involving growth factors and inflammatory and extracellular matrix (ECM)-regulating genes, also found in human cancer-associated fibroblasts (CAFs). Our results provide direct evidence that during tumorigenesis, IMCs and CAFs are activated by innate TLR4/MyD88-mediated signals and promote carcinogenesis in the intestine.