Maurice Darding

Two independent pathways of regulated necrosis mediate ischemia–reperfusion injury

Regulated necrosis (RN) may result from cyclophilin (Cyp)D-mediated mitochondrial permeability transition (MPT) and receptor-interacting protein kinase (RIPK)1-mediated necroptosis, but it is currently unclear whether there is one common pathway in which CypD and RIPK1 act in or whether separate RN pathways exist. Here, we demonstrate that necroptosis in ischemia–reperfusion injury (IRI) in mice occurs as primary organ damage, independent of the immune system, and that mice deficient for RIPK3, the essential downstream partner of RIPK1 in necroptosis, are protected from IRI. Protection of RIPK3-knockout mice was significantly stronger than of CypD-deficient mice. Mechanistically, in vivo analysis of cisplatin-induced acute kidney injury and hyperacute TNF-shock models in mice suggested the distinctness of CypD-mediated MPT from RIPK1/RIPK3-mediated necroptosis. We, therefore, generated CypD-RIPK3 double-deficient mice that are viable and fertile without an overt phenotype and that survived prolonged IRI, which was lethal to each single knockout. Combined application of the RIPK1 inhibitor necrostatin-1 and the MPT inhibitor sanglifehrin A confirmed the results with mutant mice. The data demonstrate the pathophysiological coexistence and corelevance of two separate pathways of RN in IRI and suggest that combination therapy targeting distinct RN pathways can be beneficial in the treatment of ischemic injury.

TNFR1-dependent cell death drives inflammation in Sharpin-deficient mice

SHARPIN regulates immune signaling and contributes to full transcriptional activity and prevention of cell death in response to TNF in vitro. The inactivating mouse Sharpin cpdm mutation causes TNF-dependent multi-organ inflammation, characterized by dermatitis, liver inflammation, splenomegaly, and loss of Peyers patches. TNF-dependent cell death has been proposed to cause the inflammatory phenotype and consistent with this we show Tnfr1, but not Tnfr2, deficiency suppresses the phenotype (and it does so more efficiently than Il1r1 loss). TNFR1-induced apoptosis can proceed through caspase-8 and BID, but reduction in or loss of these players generally did not suppress inflammation, although Casp8 heterozygosity significantly delayed dermatitis. Ripk3 or Mlkl deficiency partially ameliorated the multi-organ phenotype, and combined Ripk3 deletion and Casp8 heterozygosity almost completely suppressed it, even restoring Peyers patches. Unexpectedly, Sharpin, Ripk3 and Casp8 triple deficiency caused perinatal lethality. These results provide unexpected insights into the developmental importance of SHARPIN. DOI: http://dx.doi.org/10.7554/eLife.03464.001

HOIP Deficiency Causes Embryonic Lethality by Aberrant TNFR1-Mediated Endothelial Cell Death

Linear ubiquitination is crucial for innate and adaptive immunity. The linear ubiquitin chain assembly complex (LUBAC), consisting of HOIL-1, HOIP, and SHARPIN, is the only known ubiquitin ligase that generates linear ubiquitin linkages. HOIP is the catalytically active LUBAC component. Here, we show that both constitutive and Tie2-Cre-driven HOIP deletion lead to aberrant endothelial cell death, resulting in defective vascularization and embryonic lethality at midgestation. Ablation of tumor necrosis factor receptor 1 (TNFR1) prevents cell death, vascularization defects, and death at midgestation. HOIP-deficient cells are more sensitive to death induction by both tumor necrosis factor (TNF) and lymphotoxin-α (LT-α), and aberrant complex-II formation is responsible for sensitization to TNFR1-mediated cell death in the absence of HOIP. Finally, we show that HOIPs catalytic activity is necessary for preventing TNF-induced cell death. Hence, LUBAC and its linear-ubiquitin-forming activity are required for maintaining vascular integrity during embryogenesis by preventing TNFR1-mediated endothelial cell death.

Holding RIPK1 on the Ubiquitin Leash in TNFR1 Signaling

The kinase RIPK1 is an essential signaling node in various innate immune signaling pathways being most extensively studied in the TNFR1 signaling pathway. TNF signaling can result in different biological outcomes including gene activation and cell death induction in the form of apoptosis or necroptosis. RIPK1 is believed to be crucial for regulating the balance between these opposing outcomes. It is therefore not surprising that RIPK1 is highly regulated, most notably by phosphorylation, ubiquitination, and their respective reversals. In this review, we discuss the biological functions of RIPK1 within the context of TNFR1 signaling. Finally, we discuss recent advances in the knowledge on three ubiquitin E3 ligases that exert regulatory functions on RIPK1 signaling: cIAP1, cIAP2, and LUBAC.

Poly-ubiquitination in TNFR1-mediated necroptosis

Tumor necrosis factor (TNF) is a master pro-inflammatory cytokine, and inappropriate TNF signaling is implicated in the pathology of many inflammatory diseases. Ligation of TNF to its receptor TNFR1 induces the transient formation of a primary membrane-bound signaling complex, known as complex I, that drives expression of pro-survival genes. Defective complex I activation results in induction of cell death, in the form of apoptosis or necroptosis. This switch occurs via internalization of complex I components and assembly and activation of secondary cytoplasmic death complexes, respectively known as complex II and necrosome. In this review, we discuss the crucial regulatory functions of ubiquitination—a post-translational protein modification consisting of the covalent attachment of ubiquitin, and multiples thereof, to target proteins—to the various steps of TNFR1 signaling leading to necroptosis.

LUBAC deficiency perturbs TLR3 signaling to cause immunodeficiency and autoinflammation

LUBAC components interact with the TLR3 signaling cascade at different levels, thereby tightly controlling TLR3-mediated innate immunity.

Linear ubiquitin chain assembly complex coordinates late thymic T-cell differentiation and regulatory T-cell homeostasis

The linear ubiquitin chain assembly complex (LUBAC) is essential for innate immunity in mice and humans, yet its role in adaptive immunity is unclear. Here we show that the LUBAC components HOIP, HOIL-1 and SHARPIN have essential roles in late thymocyte differentiation, FOXP3+ regulatory T (Treg)-cell development and Treg cell homeostasis. LUBAC activity is not required to prevent TNF-induced apoptosis or necroptosis but is necessary for the transcriptional programme of the penultimate stage of thymocyte differentiation. Treg cell-specific ablation of HOIP causes severe Treg cell deficiency and lethal immune pathology, revealing an ongoing requirement of LUBAC activity for Treg cell homeostasis. These data reveal stage-specific requirements for LUBAC in coordinating the signals required for T-cell differentiation.

LUBAC prevents lethal dermatitis by inhibiting cell death induced by TNF, TRAIL and CD95L

The linear ubiquitin chain assembly complex (LUBAC), composed of HOIP, HOIL-1 and SHARPIN, is required for optimal TNF-mediated gene activation and to prevent cell death induced by TNF. Here, we demonstrate that keratinocyte-specific deletion of HOIP or HOIL-1 (E-KO) results in severe dermatitis causing postnatal lethality. We provide genetic and pharmacological evidence that the postnatal lethal dermatitis in HoipE-KO and Hoil-1E-KO mice is caused by TNFR1-induced, caspase-8-mediated apoptosis that occurs independently of the kinase activity of RIPK1. In the absence of TNFR1, however, dermatitis develops in adulthood, triggered by RIPK1-kinase-activity-dependent apoptosis and necroptosis. Strikingly, TRAIL or CD95L can redundantly induce this disease-causing cell death, as combined loss of their respective receptors is required to prevent TNFR1-independent dermatitis. These findings may have implications for the treatment of patients with mutations that perturb linear ubiquitination and potentially also for patients with inflammation-associated disorders that are refractory to inhibition of TNF alone.TNF mediated inflammation is critical in autoimmune mediated pathology, however many patients are refractory to current anti-TNF therapeutics. Here the authors show induction of several death ligands, in addition to TNF is sufficient to cause fatal dermatitis in a LUBAC deficient murine model of disease.

Regulation of Death Receptor-Induced Necroptosis by Ubiquitination

Death ligands, a subset of the tumour necrosis factor (TNF) super family, play an important role in innate immunity, inflammatory disorders and cancer and have been the focus of intense study over the past two decades. Depending on the cellular context, the binding of these ligands to their cognate receptors (death receptors) can mediate different cell outcomes: gene activation and cell death. Whilst death receptor-mediated apoptosis has received most of the attention over the years, only recently TNF-induced necrosis has entered the spotlight. Although necrosis was long regarded as an unregulated form of cell death, death ligand-induced necrosis—also referred to as necroptosis—has recently been demonstrated to be dependent on the activity of the kinase activities of the receptor-interacting proteins 1 and 3 (RIP1 and RIP3) and, hence, to be a regulated process. This discovery revealed the opportunity of pharmacological interference with this form of cell death which appears to be involved in a number of pathological conditions. Signalling downstream of TNF is heavily dependent on degradative and non-degradative ubiquitination events. Here, we discuss the current understanding of how the necroptosis pathway is regulated and how the ubiquitin system is thought to control the balance between gene induction, apoptosis and necroptosis.