Very-early-onset Inflammatory Bowel Disease in an Infant with a Partial RIPK1 Deletion

The monogenic causes of very-early-onset inflammatory bowel disease (VEO-IBD) have been defined by genetic studies, which were usually related to primary immunodeficiencies. Receptor-interacting serine/threonine-protein kinase-1 (RIPK1) protein is an important signalling molecule in inflammation and cell death pathways. Its deficiency may lead to various clinical features linked to immunodeficiency and/or inflammation, including IBD. Here, we discuss an infant with malnutrition, VEO-IBD, recurrent infections and polyathritis who has a homozygous partial deletion in RIPK1 gene.

ADAR1 mutation causes ZBP1-dependent immunopathology

The RNA-editing enzyme ADAR1 is essential for the suppression of innate immune activation and pathology caused by aberrant recognition of self-RNA, a role it carries out by disrupting the duplex structure of endogenous double-stranded RNA species1,2. A point mutation in the sequence encoding the Z-DNA-binding domain (ZBD) of ADAR1 is associated with severe autoinflammatory disease3-5. ZBP1 is the only other ZBD-containing mammalian protein6, and its activation can trigger both cell death and transcriptional responses through the kinases RIPK1 and RIPK3, and the protease caspase 8 (refs. 7-9). Here we show that the pathology caused by alteration of the ZBD of ADAR1 is driven by activation of ZBP1. We found that ablation of ZBP1 fully rescued the overt pathology caused by ADAR1 alteration, without fully reversing the underlying inflammatory program caused by this alteration. Whereas loss of RIPK3 partially phenocopied the protective effects of ZBP1 ablation, combined deletion of caspase 8 and RIPK3, or of caspase 8 and MLKL, unexpectedly exacerbated the pathogenic effects of ADAR1 alteration. These findings indicate that ADAR1 is a negative regulator of sterile ZBP1 activation, and that ZBP1-dependent signalling underlies the autoinflammatory pathology caused by alteration of ADAR1.

RIPK1 and TRADD Regulate TNF-Induced Signaling and Ripoptosome Formation

TNF is a proinflammatory cytokine that is critical for the coordination of tissue homeostasis. RIPK1 and TRADD are the main participants in the transduction of TNF signaling. However, data on the cell fate-controlling functions of both molecules are quite controversial. Here, we address the functions of RIPK1 and TRADD in TNF signaling by generating RIPK1- or TRADD-deficient human cell lines. We demonstrate that RIPK1 is relevant for TNF-induced apoptosis and necroptosis in conditions with depleted IAPs. In addition, TRADD is dispensable for necroptosis but required for apoptosis. We reveal a new possible function of TRADD as a negative regulator of NIK stabilization and subsequent ripoptosome formation. Furthermore, we show that RIPK1 and TRADD do not appear to be essential for the activation of MAPK signaling. Moreover, partially repressing NF-κB activation in both RIPK1 and TRADD KO cells does not result in sensitization to TNF alone due to the absence of NIK stabilization. Importantly, we demonstrate that RIPK1 is essential for preventing TRADD from undergoing TNF-induced ubiquitination and degradation. Taken together, our findings provide further insights into the specific functions of RIPK1 and TRADD in the regulation of TNF-dependent signaling, which controls the balance between cell death and survival.

Cyclophilin D Regulates the Nuclear Translocation of AIF, Cardiac Endothelial Cell Necroptosis and Murine Cardiac Transplant Injury

Ischemia-reperfusion injury (IRI) is an inevitable consequence of organ transplant procedure and associated with acute and chronic organ rejection in transplantation. IRI leads to various forms of programmed cell death, which worsens tissue damage and accelerates transplant rejection. We recently demonstrated that necroptosis participates in murine cardiac microvascular endothelial cell (MVEC) death and murine cardiac transplant rejection. However, MVEC death under a more complex IRI model has not been studied. In this study, we found that simulating IRI conditions in vitro by hypoxia, reoxygenation and treatment with inflammatory cytokines induced necroptosis in MVECs. Interestingly, the apoptosis-inducing factor (AIF) translocated to the nucleus during MVEC necroptosis, which is regulated by the mitochondrial permeability molecule cyclophilin D (CypD). Furthermore, CypD deficiency in donor cardiac grafts inhibited AIF translocation and mitigated graft IRI and rejection (n = 7; p = 0.002). Our studies indicate that CypD and AIF play significant roles in MVEC necroptosis and cardiac transplant rejection following IRI. Targeting CypD and its downstream AIF may be a plausible approach to inhibit IRI-caused cardiac damage and improve transplant survival.

Interaction of RIPK1 and A20 modulates MAPK signaling in murine acetaminophen toxicity

Acetaminophen (APAP)-induced liver necrosis is a form of regulated cell death (RCD) in which APAP activates the mitogen-activated protein kinases (MAPKs) and specifically the c-Jun-N-terminal kinase (JNK) pathway, leading to necrotic cell death. Previously, we have shown that receptor interacting protein kinase-1 (RIPK1) knockdown is also protective against APAP RCD upstream of JNK. However, whether the kinase or platform function of RIPK1 is involved in APAP RCD is not known. To answer this question, we used genetic mouse models of targeted hepatocyte RIPK1 knockout (RIPK1^HepCKO) or kinase dead knock-in (RIPK1^D138N) and adult hepatocyte specific knockout of the cytoprotective protein A20 (A20^HepCKO), known to interact with RIPK1, to study its potential involvement in MAPK signaling. We observed no difference in injury between WT and RIPK^1D138N mice post APAP. However, RIPK1^HepCKO was protective. We found that RIPK1^HepCKO mice had attenuated pJNK activation, while A20 was simultaneously upregulated. Conversely, A20^HepCKO markedly worsened liver injury from APAP. Mechanistically, we observed a significant upregulation of apoptosis signal-regulating kinase 1 (ASK1) and increased JNK activation in A20^HepCKO mice compared with littermate controls. We also demonstrated that A20 coimmunoprecipitated (co-IP) with both RIPK1 and ASK1, and that in the presence of RIPK1, there was less A20-ASK1 association than in its absence. We conclude that the kinase-independent platform function of RIPK1 is involved in APAP toxicity. Adult RIPK1^HepCKO mice are protected against APAP by upregulating A20 and attenuating JNK signaling through ASK1, conversely, A20^HepCKO worsens injury from APAP.

Flexible Usage and Interconnectivity of Diverse Cell Death Pathways Protect against Intracellular Infection

Programmed cell death contributes to host defense against pathogens. To investigate the relative importance of pyroptosis, necroptosis, and apoptosis during Salmonella infection, we infected mice and macrophages deficient for diverse combinations of caspases-1, -11, -12, and -8 and receptor interacting serine/threonine kinase 3 (RIPK3). Loss of pyroptosis, caspase-8-driven apoptosis, or necroptosis had minor impact on Salmonella control. However, combined deficiency of these cell death pathways caused loss of bacterial control in mice and their macrophages, demonstrating that host defense can employ varying components of several cell death pathways to limit intracellular infections. This flexible use of distinct cell death pathways involved extensive cross-talk between initiators and effectors of pyroptosis and apoptosis, where initiator caspases-1 and -8 also functioned as executioners when all known effectors of cell death were absent. These findings uncover a highly coordinated and flexible cell death system with in-built fail-safe processes that protect the host from intracellular infections.

Loss of RIP3 initiates annihilation of high-fat diet initialized nonalcoholic hepatosteatosis: A mechanism involving Toll-like receptor 4 and oxidative stress

Non-alcoholic fatty liver disease (NAFLD) is a prevalent and complex disease that confers a high risk of severe liver disorders. Although such public and clinical health importance, very few effective therapies are presently available for NAFLD. Here, we showed that receptor-interacting kinase-3 (RIP3) was up-regulated in liver of mouse with hepatic steatosis induced by high fat diet (HFD). After 16 weeks on a HFD, obesity, insulin resistance, metabolic syndrome, hepatic steatosis, inflammatory response and oxidative stress were significantly alleviated in liver of mice with the loss of RIP3. We provided mechanistic evidence that RIP3 knockdown attenuated hepatic dyslipidemia through preventing the expression of lipogenesis-associated genes. Furthermore, in the absence of RIP3, the transcription factor of nuclear factor-κB (NF-κB) signaling pathway activated by HFD was blocked, accompanied with the inhibition of NLRP3 inflammasome. We also found that RIP3 knockdown-induced activation of nuclear factor-erythroid 2 related factor 2/heme oxygenase-1 (Nrf-2/HO-1) led to the inhibition of oxidative stress. The detrimental effects of RIP3 on hepatic steatosis and related pathologies were confirmed in palmitate (PAL)-treated mouse liver cells. Of note, lipopolysaccharide (LPS)- or PAL-activated TLR-4 resulted in the up-regulation of RIP3 that was accompanied by the elevated inflammation and lipid deposition, and these effects were reversed in TLR-4 knockdown cells. Furthermore, promoting Nrf-2 pathway activation effectively reduced reactive oxygen species (ROS) generation and RIP3 expression in PAL-stimulated cells, consequently leading to the suppression of cellular inflammation and lipid accumulation. In contrast, blocking Nrf-2/HO-1 signaling abrogated RIP3 knockdown-reduced reactive oxygen species (ROS), inflammatory response and lipid deposition in PAL-stimulated cells. Taken together, the present study helped to elucidate how HFD-induced hepatic steatosis was regulated by RIP3, via the TLR-4/NF-κB and Nrf-2/HO-1 signaling pathways.

RIPK1 and Caspase-8 Ensure Chromosome Stability Independently of Their Role in Cell Death and Inflammation

Receptor-interacting protein kinase (RIPK) 1 functions as a key mediator of tissue homeostasis via formation of Caspase-8 activating ripoptosome complexes, positively and negatively regulating apoptosis, necroptosis, and inflammation. Here, we report an unanticipated cell-death- and inflammation-independent function of RIPK1 and Caspase-8, promoting faithful chromosome alignment in mitosis and thereby ensuring genome stability. We find that ripoptosome complexes progressively form as cells enter mitosis, peaking at metaphase and disassembling as cells exit mitosis. Genetic deletion and mitosis-specific inhibition of Ripk1 or Caspase-8 results in chromosome alignment defects independently of MLKL. We found that Polo-like kinase 1 (PLK1) is recruited into mitotic ripoptosomes, where PLK1's activity is controlled via RIPK1-dependent recruitment and Caspase-8-mediated cleavage. A fine balance of ripoptosome assembly is required as deregulated ripoptosome activity modulates PLK1-dependent phosphorylation of downstream effectors, such as BUBR1. Our data suggest that ripoptosome-mediated regulation of PLK1 contributes to faithful chromosome segregation during mitosis.

Cyclophilin D-mediated regulation of the permeability transition pore is altered in mice lacking the mitochondrial calcium uniporter

Aims

Knockout (KO) of the mitochondrial Ca2+ uniporter (MCU) in mice abrogates mitochondrial Ca2+ uptake and permeability transition pore (PTP) opening. However, hearts from global MCU-KO mice are not protected from ischaemic injury. We aimed to investigate whether adaptive alterations occur in cell death signalling pathways in the hearts of global MCU-KO mice.

Methods and results

First, we examined whether cell death may occur via an upregulation in necroptosis in MCU-KO mice. However, our results show that neither RIP1 inhibition nor RIP3 knockout afford protection against ischaemia-reperfusion injury in MCU-KO as in wildtype (WT) hearts, indicating that the lack of protection cannot be explained by upregulation of necroptosis. Instead, we have identified alterations in cyclophilin D (CypD) signalling in MCU-KO hearts. In the presence of a calcium ionophore, MCU-KO mitochondria take up calcium and do undergo PTP opening. Furthermore, PTP opening in MCU-KO mitochondria has a lower calcium retention capacity (CRC), suggesting that the calcium sensitivity of PTP is higher. Phosphoproteomics identified an increase in phosphorylation of CypD-S42 in MCU-KO. We investigated the interaction of CypD with the putative PTP component ATP synthase and identified an approximately 50% increase in this interaction in MCU-KO cardiac mitochondria. Mutation of the novel CypD phosphorylation site S42 to a phosphomimic reduced CRC, increased CypD-ATP synthase interaction by approximately 50%, and increased cell death in comparison to a phospho-resistant mutant.

Conclusion

Taken together these data suggest that MCU-KO mitochondria exhibit an increase in phosphorylation of CypD-S42 which decreases PTP calcium sensitivity thus allowing activation of PTP in the absence of an MCU-mediated increase in matrix calcium.

Deletion of Ripk3 Prevents Motor Neuron Death In Vitro but not In Vivo

Increasing evidence suggests that necroptosis, a form of programmed cell death (PCD), contributes to neurodegeneration in several disorders, including ALS. Supporting this view, investigations in both in vitro and in vivo models of ALS have implicated key molecular determinants of necroptosis in the death of spinal motor neurons (MNs). Consistent with a pathogenic role of necroptosis in ALS, we showed increased mRNA levels for the three main necroptosis effectors Ripk1, Ripk3, and Mlkl in the spinal cord of mutant superoxide dismutase-1 (SOD1G93A) transgenic mice (Tg), an established model of ALS. In addition, protein levels of receptor-interacting protein kinase 1 (RIPK1; but not of RIPK3, MLKL or activated MLKL) were elevated in spinal cord extracts from these Tg SOD1G93A mice. In postmortem motor cortex samples from sporadic and familial ALS patients, no change in protein levels of RIPK1 were detected. Silencing of Ripk3 in cultured MNs protected them from toxicity associated with SOD1G93A astrocytes. However, constitutive deletion of Ripk3 in Tg SOD1G93A mice failed to provide behavioral or neuropathological improvement, demonstrating no similar benefit of Ripk3 silencing in vivo. Lastly, we detected no genotype-specific myelin decompaction, proposed to be a proxy of necroptosis in ALS, in either Tg SOD1G93A or Optineurin knock-out mice, another ALS mouse model. These findings argue against a role for RIPK3 in Tg SOD1G93A-induced neurodegeneration and call for further preclinical investigations to determine if necroptosis plays a critical role in the pathogenesis of ALS.

TBK1 Suppresses RIPK1-Driven Apoptosis and Inflammation during Development and in Aging

Aging is a major risk factor for both genetic and sporadic neurodegenerative disorders. However, it is unclear how aging interacts with genetic predispositions to promote neurodegeneration. Here, we investigate how partial loss of function of TBK1, a major genetic cause for amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) comorbidity, leads to age-dependent neurodegeneration. We show that TBK1 is an endogenous inhibitor of RIPK1 and the embryonic lethality of Tbk1-/- mice is dependent on RIPK1 kinase activity. In aging human brains, another endogenous RIPK1 inhibitor, TAK1, exhibits a marked decrease in expression. We show that in Tbk1+/- mice, the reduced myeloid TAK1 expression promotes all the key hallmarks of ALS/FTD, including neuroinflammation, TDP-43 aggregation, axonal degeneration, neuronal loss, and behavior deficits, which are blocked upon inhibition of RIPK1. Thus, aging facilitates RIPK1 activation by reducing TAK1 expression, which cooperates with genetic risk factors to promote the onset of ALS/FTD.

Does necroptosis have a crucial role in hepatic ischemia-reperfusion injury?

Background

Previous studies have demonstrated protective effects of anti-receptor interacting protein kinase 1 (RIP1), a key necroptosis molecule. However, it is uncertain whether necroptosis has a crucial role in hepatic IR injury. Therefore, we evaluated the role of necroptosis in hepatic IR injury.

Method

The IR mice underwent 70% segmental IR injury induced by the clamping of the hepatic artery and portal vein for 1 hr followed by reperfusion for 4 hr. The key necroptosis molecules (RIP1, RIP3, and MLKL) and other key molecules of regulated necrosis (PGAM5 and caspase-1) were evaluated in the warm IR injury model. A RIP1 inhibitor (necrostain-1s) and/or an anti-mitochondrial permeability transition (MPT)-mediated necrosis mediator (cyclosporine A, CyA) were administered before clamping. Necrotic injury was quantified using Suzuki’s scoring system. qRT-PCR and western blot were performed to evaluate RIP1, RIP3, MLKL and PGAM5 expressions.

Results

RIP1, RIP3, MLKL and PGAM5 expression did not change in the hepatic IR injury model. Moreover, Nec1s pretreatment did not improve histology or biochemical markers. The overall Suzuki score (cytoplasmic vacuolization, sinusoidal congestion and hepatocytes necrosis) was increased in the RIP3^(-/-) mice compared to the IR group (3.5 vs. 5, p = 0.026). CyA pretreatment and/or RIP3^(-/-) mice decreased Bax/Bcl2 expression; however, it did lead to an overall change in the levels of AST, ALT and LDH or necrotic injury. The Bax/Bcl2 ratio and the expression of caspase-1 and caspase-3 did not increase in our hepatic IR injury model.

Conclusion

Key necroptosis molecules did not increase in the necrosis-dominant hepatic IR injury model. Anti-necroptosis and/or cyclosporine-A treatment did not have an overall protective effect on necrosis-dominant hepatic IR injury.

Pore-forming toxin-mediated ion dysregulation leads to death receptor-independent necroptosis of lung epithelial cells during bacterial pneumonia

We report that pore-forming toxins (PFTs) induce respiratory epithelial cell necroptosis independently of death receptor signaling during bacterial pneumonia. Instead, necroptosis was activated as a result of ion dysregulation arising from membrane permeabilization. PFT-induced necroptosis required RIP1, RIP3 and MLKL, and could be induced in the absence or inhibition of TNFR1, TNFR2 and TLR4 signaling. We detected activated MLKL in the lungs from mice and nonhuman primates experiencing Serratia marcescens and Streptococcus pneumoniae pneumonia, respectively. We subsequently identified calcium influx and potassium efflux as the key initiating signals responsible for necroptosis; also that mitochondrial damage was not required for necroptosis activation but was exacerbated by MLKL activation. PFT-induced necroptosis in respiratory epithelial cells did not involve CamKII or reactive oxygen species. KO mice deficient in MLKL or RIP3 had increased survival and reduced pulmonary injury during S. marcescens pneumonia. Our results establish necroptosis as a major cell death pathway active during bacterial pneumonia and that necroptosis can occur without death receptor signaling.

Targeting macrophage necroptosis for therapeutic and diagnostic interventions in atherosclerosis

Atherosclerosis results from maladaptive inflammation driven primarily by macrophages, whose recruitment and proliferation drive plaque progression. In advanced plaques, macrophage death contributes centrally to the formation of plaque necrosis, which underlies the instability that promotes plaque rupture and myocardial infarction. Hence, targeting macrophage cell death pathways may offer promise for the stabilization of vulnerable plaques. Necroptosis is a recently discovered pathway of programmed cell necrosis regulated by RIP3 and MLKL kinases that, in contrast to apoptosis, induces a proinflammatory state. We show herein that necroptotic cell death is activated in human advanced atherosclerotic plaques and can be targeted in experimental atherosclerosis for both therapeutic and diagnostic interventions. In humans with unstable carotid atherosclerosis, expression of RIP3 and MLKL is increased, and MLKL phosphorylation, a key step in the commitment to necroptosis, is detected in advanced atheromas. Investigation of the molecular mechanisms underlying necroptosis showed that atherogenic forms of low-density lipoprotein increase RIP3 and MLKL transcription and phosphorylation-two critical steps in the execution of necroptosis. Using a radiotracer developed with the necroptosis inhibitor necrostatin-1 (Nec-1), we show that (123)I-Nec-1 localizes specifically to atherosclerotic plaques in Apoe (-/-) mice, and its uptake is tightly correlated to lesion areas by ex vivo nuclear imaging. Furthermore, treatment of Apoe (-/-) mice with established atherosclerosis with Nec-1 reduced lesion size and markers of plaque instability, including necrotic core formation. Collectively, our findings offer molecular insight into the mechanisms of macrophage cell death that drive necrotic core formation in atherosclerosis and suggest that this pathway can be used as both a diagnostic and therapeutic tool for the treatment of unstable atherosclerosis.

Molecular Cell Biology of Apoptosis and Necroptosis in Cancer

Cell death is a major mechanism to eliminate cells in which DNA is damaged, organelles are stressed, or oncogenes are overexpressed, all events that would otherwise predispose cells to oncogenic transformation. The pathways that initiate and execute cell death are complex, genetically encoded, and subject to significant regulation. Consequently, while these pathways are often mutated in malignancy, there is considerable interest in inducing cell death in tumor cells as therapy. This chapter addresses our current understanding of molecular mechanisms contributing to two cell death pathways, apoptotic cell death and necroptosis, a regulated form of necrotic cell death. Apoptosis can be induced by a wide variety of signals, leading to protease activation that dismantles the cell. We discuss the physiological importance of each apoptosis pathway and summarize their known roles in cancer suppression and the current efforts at targeting each pathway therapeutically. The intricate mechanistic link between death receptor-mediated apoptosis and necroptosis is described, as well as the potential opportunities for utilizing necroptosis in the treatment of malignancy.

Receptor-Interacting Protein Kinase 3 Deficiency Delays Cutaneous Wound Healing

Wound healing consists of a complex, dynamic and overlapping process involving inflammation, proliferation and tissue remodeling. A better understanding of wound healing process at the molecular level is needed for the development of novel therapeutic strategies. Receptor-interacting protein kinase 3 (RIPK3) controls programmed necrosis in response to TNF-α during inflammation and has been shown to be highly induced during cutaneous wound repair. However, its role in wound healing remains to be demonstrated. To study this, we created dorsal cutaneous wounds on male wild-type (WT) and RIPK3-deficient (Ripk3^-/-) mice. Wound area was measured daily until day 14 post-wound and skin tissues were collected from wound sites at various days for analysis. The wound healing rate in Ripk3^-/- mice was slower than the WT mice over the 14-day course; especially, at day 7, the wound size in Ripk3^-/- mice was 53% larger than that of WT mice. H&E and Masson-Trichrome staining analysis showed impaired quality of wound closure in Ripk3^-/- wounds with delayed re-epithelialization and angiogenesis and defected granulation tissue formation and collagen deposition compared to WT. The neutrophil infiltration pattern was altered in Ripk3^-/- wounds with less neutrophils at day 1 and more neutrophils at day 3. This altered pattern was also reflected in the differential expression of IL-6, KC, IL-1β and TNF-α between WT and Ripk3^-/- wounds. MMP-9 protein expression was decreased with increased Timp-1 mRNA in the Ripk3^-/- wounds compared to WT. The microvascular density along with the intensity and timing of induction of proangiogenic growth factors VEGF and TGF-β1 were also decreased or delayed in the Ripk3^-/- wounds. Furthermore, mouse embryonic fibroblasts (MEFs) from Ripk3^-/- mice migrated less towards chemoattractants TGF-β1 and PDGF than MEFs from WT mice. These results clearly demonstrate that RIPK3 is an essential molecule to maintain the temporal manner of the normal progression of wound closure.

DSTYK kinase domain ablation impaired the mice capabilities of learning and memory in water maze test

DSTYK (Dual serine/threonine and tyrosine protein kinase) is a putative dual Ser/Thr and Tyr protein kinase with unique structural features. It is proposed that DSTYK may play important roles in brain because of its high expression in most brain areas. In the present study, a DSTYK knockout (KO) mouse line with the ablation of C-terminal of DSTYK including the kinase domain was generated to study the physiological function of DSTYK. The DSTYK KO mice are fertile and have no significant morphological defects revealed by Nissl staining compared with wildtype mice. Open field test and rotarod test showed there is no obvious difference in basic motor and balance capacity between the DSTYK homozygous KO mice and DSTYK heterozygous KO mice. In water maze test, however, the DSTYK homozygous KO mice show impaired capabilities of learning and memory compared with the DSTYK heterozygous KO mice.

Type-I interferon signaling through ISGF3 complex is required for sustained Rip3 activation and necroptosis in macrophages

Myeloid cells play a critical role in perpetuating inflammation during various chronic diseases. Recently the death of macrophages through programmed necrosis (necroptosis) has emerged as an important mechanism in inflammation and pathology. We evaluated the mechanisms that lead to the induction of necrotic cell death in macrophages. Our results indicate that type I IFN (IFN-I) signaling is a predominant mechanism of necroptosis, because macrophages deficient in IFN-α receptor type I (IFNAR1) are highly resistant to necroptosis after stimulation with LPS, polyinosinic-polycytidylic acid, TNF-α, or IFN-β in the presence of caspase inhibitors. IFN-I-induced necroptosis occurred through both mechanisms dependent on and independent of Toll/IL-1 receptor domain-containing adaptor inducing IFN-β (TRIF) and led to persistent phosphorylation of receptor-interacting protein 3 (Rip3) kinase, which resulted in potent necroptosis. Although various IFN-regulatory factors (IRFs) facilitated the induction of necroptosis in response to IFN-β, IRF-9-STAT1- or -STAT2-deficient macrophages were highly resistant to necroptosis. Our results indicate that IFN-β-induced necroptosis of macrophages proceeds through tonic IFN-stimulated gene factor 3 (ISGF3) signaling, which leads to persistent expression of STAT1, STAT2, and IRF9. Induction of IFNAR1/Rip3-dependent necroptosis also resulted in potent inflammatory pathology in vivo. These results reveal how IFN-I mediates acute inflammation through macrophage necroptosis.

Role of Rip2 in development of tumor-infiltrating MDSCs and bladder cancer metastasis

Tumor invasion and metastases represent a complex series of molecular events that portends a poor prognosis. The contribution of inflammatory pathways mediating this process is not well understood. Nod-like receptors (NLRs) of innate immunity function as intracellular sensors of pathogen motifs and danger molecules. We propose a role of NLRs in tumor surveillance and in programming tumor-infiltrating lymphocytes (TILs). In this study, we examined the downstream serine/threonine and tyrosine kinase Rip2 in a murine model of bladder cancer. In Rip2-deficient C57Bl6 mice, larger orthotopic MB49 tumors developed with more numerous and higher incidence of metastases compared to wild-type controls. As such, increased tumor infiltration of CD11b⁺Gr1^hi myeloid-derived suppressor cells (MDSCs) with concomitant decrease in T cells and NK cells were observed in Rip2-deficient tumor bearing animals using orthotopic and subcutaneous tumor models. Rip2-deficient tumors showed enhanced epithelial-to-mesenchymal transition, with elevated expression of zeb1, zeb2, twist, and snail in the tumor microenvironment. We found that the absence of Rip2 plays an intrinsic role in fostering the development of granulocytic MDSCs by an autocrine and paracrine effect of granulocytic colony stimulating factor (G-CSF) expression. Our findings suggest that NLR pathways may be a novel modality to program TILs and influence tumor metastases.

RIPK3 as a potential therapeutic target for Gaucher's disease

Gaucher's disease (GD), an inherited metabolic disorder caused by mutations in the glucocerebrosidase gene (GBA), is the most common lysosomal storage disease. Heterozygous mutations in GBA are a major risk factor for Parkinson's disease. GD is divided into three clinical subtypes based on the absence (type 1) or presence (types 2 and 3) of neurological signs. Type 1 GD was the first lysosomal storage disease (LSD) for which enzyme therapy became available, and although infusions of recombinant glucocerebrosidase (GCase) ameliorate the systemic effects of GD, the lack of efficacy for the neurological manifestations, along with the considerable expense and inconvenience of enzyme therapy for patients, renders the search for alternative or complementary therapies paramount. Glucosylceramide and glucosylsphingosine accumulation in the brain leads to massive neuronal loss in patients with neuronopathic GD (nGD) and in nGD mouse models. However, the mode of neuronal death is not known. Here, we show that modulating the receptor-interacting protein kinase-3 (Ripk3) pathway markedly improves neurological and systemic disease in a mouse model of GD. Notably, Ripk3 deficiency substantially improved the clinical course of GD mice, with increased survival and motor coordination and salutary effects on cerebral as well as hepatic injury.

Receptor interacting protein kinase 3 is a critical early mediator of acetaminophen-induced hepatocyte necrosis in mice

Acetaminophen (APAP) overdose is a major cause of hepatotoxicity and acute liver failure in the U.S., but the pathophysiology is incompletely understood. Despite evidence for apoptotic signaling, hepatic cell death after APAP is generally considered necrotic in mice and in humans. Recent findings suggest that the receptor interacting protein kinase 3 (RIP3) acts as a switch from apoptosis to necrosis (programmed necrosis). Thus, the aim of the current investigation was to determine if RIP3 is involved in APAP-induced liver cell death. APAP (200-300 mg/kg) caused glutathione depletion and protein adduct formation, oxidant stress, mitochondrial release of apoptosis inducing factor, and nuclear DNA fragmentation resulting in centrilobular necrosis in C57Bl/6J mice. Inhibiting RIP3 protein induction with antisense morpholinos in wild-type animals or using RIP3-deficient mice had no effect on protein adduct formation but attenuated all other parameters, including necrotic cell death, at 6 hours after APAP. In addition, cultured hepatocytes from RIP3-deficient mice showed reduced injury compared to wild-type cells after 24 hours. Interestingly, APAP-induced mitochondrial translocation of dynamin-related protein 1 (Drp1), the initiator of mitochondrial fission, was inhibited by reduced RIP3 protein expression and the Drp1 inhibitor MDIVI reduced APAP-induced cell death at 24 hours. All of these protective effects were lost after 24 hours in vivo or 48 hours in vitro.

CONCLUSION

RIP3 is an early mediator of APAP hepatotoxicity, involving modulation of mitochondrial dysfunction and oxidant stress. Controlling RIP3 expression could be a promising new approach to reduce APAP-induced liver injury, but requires complementary strategies to control mitochondrial dysfunction for long-term protection.

CARD3 deficiency exacerbates diet-induced obesity, hepatosteatosis, and insulin resistance in male mice

Caspase activation and recruitment domain 3 (CARD3) is a 61-kDa protein kinase with an N-terminal serine/threonine kinase domain and a C-terminal CARD. Previous research on the function of CARD3 has focused on its role in the immune response and inflammatory diseases. Obesity is now a worldwide health problem and is generally recognized as an inflammatory disease. Unexpectedly, we found that CARD3 expression was lower during obesity. In this study, we explored the biological and genetic bases of obesity using CARD3-knockout (KO) and wild-type (WT) mice fed a high-fat diet (HFD) for 24 weeks. We demonstrate that KO mice were more obese than their WT littermates, and KO mice exhibited obvious visceral fat accumulation and liver weight gains after 24 weeks of HFD feeding. We also observed more severe hepatosteatosis in KO mice compared with the WT controls. Hepatic steatosis in the HFD-fed KO mice was linked to a significant increase in the expression of key lipogenic and cholesterol synthesis enzymes, whereas the expression of the enzymes involves in β-oxidation was dramatically reduced. Furthermore, we confirmed the repression of AMP-activated protein kinase signaling and activation of the endoplasmic reticulum stress response. Fatty liver impaired the global glucose and lipid metabolism, which further exacerbated the insulin resistance associated with the repression of Akt signaling and up-regulated systemic inflammation through the M1/M2 (pro- and anti-inflammation) type switch and the activation of the nuclear factor-κB pathway. Our studies demonstrate the crucial role of CARD3 in metabolism and indicate that CARD3 deficiency promotes the diet-induced phenotype of type 2 diabetes.

Induction of microRNA-155 is TLR- and type IV secretion system-dependent in macrophages and inhibits DNA-damage induced apoptosis

Helicobacter pylori is a gastric pathogen responsible for a high disease burden worldwide. Deregulated inflammatory responses, possibly involving macrophages, are implicated in H. pylori-induced pathology, and microRNAs, such as miR-155, have recently emerged as crucial regulators of innate immunity and inflammatory responses. miR-155 is regulated by Toll-like receptor (TLR) ligands in monocyte-derived cells and has been shown to be induced in macrophages during H. pylori infection. Here, we investigated the regulation of miR-155 expression in primary murine bone marrow-derived macrophages (BMMs) during H. pylori infection and examined the downstream mRNA targets of this microRNA using microarray analysis. We report TLR2/4- and NOD1/2-independent up-regulation of miR-155, which was found to be dependent on the major H. pylori pathogenicity determinant, the type IV secretion system (T4SS). miR-155 expression was dependent on NF-κB signaling but was independent of CagA. Microarray analysis identified known gene targets of miR-155 in BMMs during H. pylori infection that are proapoptotic. We also identified and validated miR-155 binding sites in the 3' UTRs of the targets, Tspan14, Lpin1, and Pmaip1. We observed that H. pylori-infected miR-155(-/-) BMMs were significantly more susceptible to cisplatin DNA damage-induced apoptosis than were wild-type BMMs. Thus, our data suggest a function for the prototypical H. pylori pathogenicity factor, the T4SS, in the up-regulation of miR-155 in BMMs. We propose the antiapoptotic effects of miR-155 could enhance macrophage resistance to apoptosis induced by DNA damage during H. pylori infection.

Effective T-cell immune responses in the absence of the serine/threonine kinase RIP2

The serine/threonine kinase RIP2 has been reported to be essential for Nod1 and Nod2 mediated cell activation, and has been suggested to play a role in the signaling cascade downstream of the T-cell receptor. We sought to ascertain the exact role of RIP2 in T-helper cell differentiation and CD8+ T-cell effector function in vivo and in vitro. In contrast to previous reports, we found that RIP2-deficient T cells did not exhibit impaired proliferation upon TCR engagement in vitro, and differentiation to cytokine producing Th1 or Th2 cells was normal in the absence of RIP2. These results were confirmed in vivo, as wild-type and RIP2-deficient virus-specific CD8+ T cells expanded comparably in mice after LCMV infection. Wild-type and RIP2-deficient CD4+ and CD8+ T cells from infected mice also showed similar proliferation and cytokine production when restimulated with full or partial agonist peptides ex vivo. Furthermore, no significant difference in adaptive T-cell responses could be observed between wild-type and RIP2-deficient mice after Listeria monocytogenes infection. Thus contrary to early reports, our data show that RIP2 is not an essential component of the TCR signaling machinery.