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

Microglia Biology: One Century of Evolving Concepts

Microglia were first recognized as a distinct cell population in the CNS one century ago. For a long time, they were primarily considered to be phagocytes responsible for removing debris during CNS development and disease. More recently, advances in imaging and genetics and the advent of single-cell technologies provided new insights into the much more complex and fascinating biology of microglia. The ontogeny of microglia was identified, and their functions in health and disease were better defined. Although many questions about microglia and their roles in human diseases remain unanswered, the prospect of targeting microglia for the treatment of neurological and psychiatric disorders is tantalizing.

Tumor suppressor death-associated protein kinase 1 inhibits necroptosis by p38 MAPK activation

Death-associated protein kinase 1 (DAPK1, DAPk, DAPK) is known for its involvement in apoptosis and autophagy-associated cell death. Here, we identified an unexpected function of DAPK1 in suppressing necroptosis. DAPK1-deficiency renders macrophages and dendritic cells susceptible to necroptotic death. We also observed an inhibitory role for DAPK1 in necroptosis in HT-29 cells, since knockdown or knockout of DAPK1 in such cells increased their sensitivity to necroptosis. Increased necroptosis was associated with enhanced formation of the RIPK1-RIPK3-MLKL complex in these DAPK1-deficient cells. We further found that DAPK1-deficiency led to decreased MAPK activated kinase 2 (MK2) activation and reduced RIPK1 S321 phosphorylation, with this latter representing a critical step controlling necrosome formation. Most TNF signaling pathways, including ERK, JNK, and AKT, were not regulated by DAPK. In contrast, DAPK bound p38 MAPK and selectively promoted p38 MAPK activation, resulting in enhanced MK2 phosphorylation. Our results reveal a novel role for DAPK1 in inhibiting necroptosis and illustrate an unexpected selectivity for DAPK1 in promoting p38 MAPK-MK2 activation. Importantly, our study suggests that modulation of necroptosis and p38/MK2-mediated inflammation may be achieved by targeting DAPK1.

ALS-FTLD-linked mutations of SQSTM1/p62 disrupt selective autophagy and NFE2L2/NRF2 anti-oxidative stress pathway

Macroautophagy (autophagy) is a key catabolic pathway for the maintenance of proteostasis through constant digestion of selective cargoes. The selectivity of autophagy is mediated by autophagy receptors that recognize and recruit cargoes to autophagosomes. SQSTM1/p62 is a prototype autophagy receptor, which is commonly found in protein aggregates associated with major neurodegenerative diseases. While accumulation of SQSTM1 implicates a disturbance of selective autophagy pathway, the pathogenic mechanism that contributes to impaired autophagy degradation remains poorly characterized. Herein we show that amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD)-linked mutations of TBK1 and SQSTM1 disrupt selective autophagy and cause neurotoxicity. Our data demonstrates that proteotoxic stress activates serine/threonine kinase TBK1, which coordinates with autophagy kinase ULK1 to promote concerted phosphorylation of autophagy receptor SQSTM1 at the UBA domain and activation of selective autophagy. In contrast, ALS-FTLD-linked mutations of TBK1 or SQSTM1 reduce SQSTM1 phosphorylation and compromise ubiquitinated cargo binding and clearance. Moreover, disease mutation SQSTM1G427R abolishes phosphorylation of Ser351 and impairs KEAP1-SQSTM1 interaction, thus diminishing NFE2L2/Nrf2-targeted gene expression and increasing TARDBP/TDP-43 associated stress granule formation under oxidative stress. Furthermore, expression of SQSTM1G427R in neurons impairs dendrite morphology and KEAP1-NFE2L2 signaling. Therefore, our results reveal a mechanism whereby pathogenic SQSTM1 mutants inhibit selective autophagy and disrupt NFE2L2 anti-oxidative stress response underlying the neurotoxicity in ALS-FTLD.Abbreviations: ALS: amyotrophic lateral sclerosis; FTLD: frontotemporal lobar degeneration; G3BP1: GTPase-activating protein (SH3 domain) binding protein 1; GSTM1: glutathione S-transferase, mu 1; HMOX/HO-1: Heme oxygenase 1; IP: immunoprecipitation; KEAP1: kelch-like ECH associated protein 1; KI: kinase inactive; KIR: KEAP1 interaction region; KO: knockout; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MBP: maltose binding protein; NBR1: NBR1, autophagy cargo receptor; NFE2L2/Nrf2: nuclear factor, erythroid derived 2, like 2; NQO1: NAD(P)H quinone dehydrogenase 1; SQSTM1/p62: sequestosome 1; SOD1: superoxide dismutase 1, soluble; S.S.: serum starvation; TARDBP/TDP-43: TAR DNA binding protein; TBK1: TANK binding kinase 1; UBA: ubiquitin association; ULK1: unc-51 like autophagy activating kinase 1; WT: wild type.

A20 and Cell Death-driven Inflammation

A20 is a potent anti-inflammatory molecule, and mutations in TNFAIP3, the gene encoding A20, are associated with a wide panel of inflammatory pathologies, both in human and mouse. The anti-inflammatory properties of A20 are commonly attributed to its ability to suppress inflammatory NF-κB signaling by functioning as a ubiquitin-editing enzyme. However, A20 also protects cells from death, independently of NF-κB regulation, and recent work has demonstrated that cell death may drive some of the inflammatory conditions caused by A20 deficiency. Adding to the fact that the protective role of A20 does not primarily rely on its catalytic activities, these findings shed new light on A20 biology.

Stress Management: Death Receptor Signalling and Cross-Talks with the Unfolded Protein Response in Cancer

Throughout tumour progression, tumour cells are exposed to various intense cellular stress conditions owing to intrinsic and extrinsic cues, to which some cells are remarkably able to adapt. Death Receptor (DR) signalling and the Unfolded Protein Response (UPR) are two stress responses that both regulate a plethora of outcomes, ranging from proliferation, differentiation, migration, cytokine production to the induction of cell death. Both signallings are major modulators of physiological tissue homeostasis and their dysregulation is involved in tumorigenesis and the metastastic process. The molecular determinants of the control between the different cellular outcomes induced by DR signalling and the UPR in tumour cells and their stroma and their consequences on tumorigenesis are starting to be unravelled. Herein, I summarize the main steps of DR signalling in relation to its cellular and pathophysiological roles in cancer. I then highlight how the UPR and DR signalling control common cellular outcomes and also cross-talk, providing potential opportunities to further understand the development of malignancies.

Loss of mitochondrial ClpP, Lonp1, and Tfam triggers transcriptional induction of Rnf213, a susceptibility factor for moyamoya disease

Human RNF213, which encodes the protein mysterin, is a known susceptibility gene for moyamoya disease (MMD), a cerebrovascular condition with occlusive lesions and compensatory angiogenesis. Mysterin mutations, together with exposure to environmental trigger factors, lead to an elevated stroke risk since childhood. Mysterin is induced during cell stress, to function as cytosolic AAA+ ATPase and ubiquitylation enzyme. Little knowledge exists, in which context mysterin is needed. Here, we found that genetic ablation of several mitochondrial matrix factors, such as the peptidase ClpP, the transcription factor Tfam, as well as the peptidase and AAA+ ATPase Lonp1, potently induces Rnf213 transcript expression in various organs, in parallel with other components of the innate immune system. Mostly in mouse fibroblasts and human endothelial cells, the Rnf213 levels showed prominent upregulation upon Poly(I:C)-triggered TLR3-mediated responses to dsRNA toxicity, as well as upon interferon gamma treatment. Only partial suppression of Rnf213 induction was achieved by C16 as an antagonist of PKR (dsRNA-dependent protein kinase). Since dysfunctional mitochondria were recently reported to release immune-stimulatory dsRNA into the cytosol, our results suggest that mysterin becomes relevant when mitochondrial dysfunction or infections have triggered RNA-dependent inflammation. Thus, MMD has similarities with vasculopathies that involve altered nucleotide processing, such as Aicardi-Goutières syndrome or systemic lupus erythematosus. Furthermore, in MMD, the low penetrance of RNF213 mutations might be modified by dysfunctions in mitochondria or the TLR3 pathway.

Autophosphorylation at serine 166 regulates RIP kinase 1-mediated cell death and inflammation

Receptor interacting protein kinase 1 (RIPK1) regulates cell death and inflammatory responses downstream of TNFR1 and other receptors, and has been implicated in the pathogenesis of inflammatory and degenerative diseases. RIPK1 kinase activity induces apoptosis and necroptosis, however the mechanisms and phosphorylation events regulating RIPK1-dependent cell death signaling remain poorly understood. Here we show that RIPK1 autophosphorylation at serine 166 plays a critical role for the activation of RIPK1 kinase-dependent apoptosis and necroptosis. Moreover, we show that S166 phosphorylation is required for RIPK1 kinase-dependent pathogenesis of inflammatory pathologies in vivo in four relevant mouse models. Mechanistically, we provide evidence that trans autophosphorylation at S166 modulates RIPK1 kinase activation but is not by itself sufficient to induce cell death. These results show that S166 autophosphorylation licenses RIPK1 kinase activity to induce downstream cell death signaling and inflammation, suggesting that S166 phosphorylation can serve as a reliable biomarker for RIPK1 kinase-dependent pathologies.

Receptor interacting protein kinase 1 (RIPK1) regulates cell death and inflammatory responses. Here the authors show that autophosphorylation at Ser166 is required for RIPK1-mediated cell death and inflammation in mouse models of inflammatory pathologies, making Ser166 phosphorylation a possible biomarker for RIPK1-mediated inflammatory diseases.

TBK1 and IKKε Act Redundantly to Mediate STING-Induced NF-κB Responses in Myeloid Cells

Stimulator of Interferon Genes (STING) is a critical component of host innate immune defense but can contribute to chronic autoimmune or autoinflammatory disease. Once activated, the cyclic guanosine monophosphate (GMP)-adenosine monophosphate (AMP) (cGAMP) synthase (cGAS)-STING pathway induces both type I interferon (IFN) expression and nuclear factor-κB (NF-κB)-mediated cytokine production. Currently, these two signaling arms are thought to be mediated by a single upstream kinase, TANK-binding kinase 1 (TBK1). Here, using genetic and pharmacological approaches, we show that TBK1 alone is dispensable for STING-induced NF-κB responses in human and mouse immune cells, as well as in vivo. We further demonstrate that TBK1 acts redundantly with IκB kinase ε (IKKε) to drive NF-κB upon STING activation. Interestingly, we show that activation of IFN regulatory factor 3 (IRF3) is highly dependent on TBK1 kinase activity, whereas NF-κB is significantly less sensitive to TBK1/IKKε kinase inhibition. Our work redefines signaling events downstream of cGAS-STING. Our findings further suggest that cGAS-STING will need to be targeted directly to effectively ameliorate the inflammation underpinning disorders associated with STING hyperactivity.

Ubiquitin and Receptor-Dependent Mitophagy Pathways and Their Implication in Neurodegeneration

Selective autophagy of mitochondria, or mitophagy, refers to the specific removal and degradation of damaged or surplus mitochondria via targeting to the lysosome for destruction. Disruptions in this homeostatic process may contribute to disease. The identification of diverse mitophagic pathways and how selectivity for each of these pathways is conferred is just beginning to be understood. The removal of both damaged and healthy mitochondria under disease and physiological conditions is controlled by either ubiquitin-dependent or receptor-dependent mechanisms. In this review, we will discuss the known types of mitophagy observed in mammals, recent findings related to PINK1/Parkin-mediated mitophagy (which is the most well-studied form of mitophagy), the implications of defective mitophagy to neurodegenerative processes, and unanswered questions inspiring future research that would enhance our understanding of mitochondrial quality control.

The Loss of TBK1 Kinase Activity in Motor Neurons or in All Cell Types Differentially Impacts ALS Disease Progression in SOD1 Mice

DNA sequence variants in the TBK1 gene associate with or cause sporadic or familial amyotrophic lateral sclerosis (ALS). Here we show that mice bearing human ALS-associated TBK1 missense loss-of-function mutations, or mice in which the Tbk1 gene is selectively deleted in motor neurons, do not display a neurodegenerative disease phenotype. However, loss of TBK1 function in motor neurons of the SOD1^G93A mouse model of ALS impairs autophagy, increases SOD1 aggregation, and accelerates early disease onset without affecting lifespan. By contrast, point mutations that decrease TBK1 kinase activity in all cells also accelerate disease onset but extend the lifespan of SOD1 mice. This difference correlates with the failure to activate high levels of expression of interferon-inducible genes in glia. We conclude that loss of TBK1 kinase activity impacts ALS disease progression through distinct pathways in different spinal cord cell types and further implicate the importance of glia in neurodegeneration.

New insights into the regulation of apoptosis, necroptosis, and pyroptosis by receptor interacting protein kinase 1 and caspase-8

Necroptosis and pyroptosis are inflammatory forms of regulated necrotic cell death as opposed to apoptosis that is generally considered immunologically silent. Recent studies revealed unexpected links in the pathways regulating and executing cell death in response to activation of signaling cascades inducing apoptosis, necroptosis, and pyroptosis. Emerging evidence suggests that receptor interacting protein kinase 1 and caspase-8 control the cross-talk between apoptosis, necroptosis, and pyroptosis and determine the type of cell death induced in response to activation of cell death signaling.

Inhibiting Necroptosis of Spermatogonial Stem Cell as a Novel Strategy for Male Fertility Preservation

Fertility preservation is a common concern for male cancer survivors of reproductive age. However, except for testicular tissue cryopreservation, which is not very effective, there is no feasible and precise therapy capable of protecting spermatogenesis for prepubertal boys before or during gonadotoxic treatment. This study aims to investigate the effects of inhibiting necroptosis of spermatogonial stem cell (SSC) in fertility preservation. Male mice 12 weeks of age were used to establish gonadotoxicity with two intraperitoneal injections of busulfan at a total dose of 40 mg kg^-1. The mouse model and the primary cultured mouse SSCs were used to characterize the relationship between necroptosis of SSC and gonadotoxicity. Meanwhile, the effects of an inhibitor of necroptosis pathway, RIPA-56, were observed on day 36 in the mouse model of busulfan-induced gonadotoxicity. We found that the number of SSCs was decreased, but the level of necroptosis was upregulated on day 18 after busulfan treatment in testes from gonadotoxic mice. Further experiments in primary cultured cells showed that the necroptosis caused cell death in busulfan-treated SSCs and could be inhibited by RIPA-56. After suppressing the necroptosis of SSCs, the busulfan-induced mice had a decreased loss of spermatogenic cells as shown by histology and an increased Johnsen's score. Moreover, the quantities of SSCs and epididymal spermatozoa were restored after intervention with RIPA-56, indicating a series of beneficial effects by targeting the necroptosis of SSCs in mice undergoing busulfan treatment. In conclusion, our findings reveal that the necroptosis of SSCs plays a critical role in busulfan-induced gonadotoxicity and may be a potential target for male fertility preservation.

Multitasking Kinase RIPK1 Regulates Cell Death and Inflammation

Receptor-interacting serine threonine kinase 1 (RIPK1) is a widely expressed kinase that is essential for limiting inflammation in both mice and humans. Mice lacking RIPK1 die at birth from multiorgan inflammation and aberrant cell death, whereas humans lacking RIPK1 are immunodeficient and develop very early-onset inflammatory bowel disease. In contrast to complete loss of RIPK1, inhibiting the kinase activity of RIPK1 genetically or pharmacologically prevents cell death and inflammation in several mouse disease models. Indeed, small molecule inhibitors of RIPK1 are in phase I clinical trials for amyotrophic lateral sclerosis, and phase II clinical trials for psoriasis, rheumatoid arthritis, and ulcerative colitis. This review focuses on which signaling pathways use RIPK1, how activation of RIPK1 is regulated, and when activation of RIPK1 appears to be an important driver of inflammation.

To catch a thief: regulated RIPK1 post-translational modifications as a fail-safe system to detect and overcome pathogen subversion of immune signaling

Any pathogen worth its salt has mechanisms to evade, subvert, or antagonize host innate immune responses induced by pattern recognition receptors. Resistance against such pathogens therefore requires alternative means to activate protective immune responses. Intriguingly, the receptors that regulate antimicrobial gene expression are coupled to cell death pathways that are activated by blockade of NF-κB and MAPK signaling. In this review, we discuss the regulation of apoptosis in response to pathogen disruption of immune signaling and the role of this cell death response in protection against such pathogens. Stanley often observed that bacterial pathogens are excellent cell biologists and immunologists, and he noted that studying pathogen-host interactions could pave the way to new insights about host biology. Indeed, how Yersinia and other pathogens disrupt innate immune signaling has provided new insight into these pathways and revealed new ways to think about immunogenic properties of apoptosis during bacterial infection.

Sequential activation of necroptosis and apoptosis cooperates to mediate vascular and neural pathology in stroke

Apoptosis and necroptosis are two regulated cell death mechanisms; however, the interaction between these cell death pathways in vivo is unclear. Here we used cerebral ischemia/reperfusion as a model to investigate the interaction between apoptosis and necroptosis. We show that the activation of RIPK1 sequentially promotes necroptosis followed by apoptosis in a temporally specific manner. Cerebral ischemia/reperfusion insult rapidly activates necroptosis to promote cerebral hemorrhage and neuroinflammation. Ripk3 deficiency reduces cerebral hemorrhage and delays the onset of neural damage mediated by inflammation. Reduced cerebral perfusion resulting from arterial occlusion promotes the degradation of TAK1, a suppressor of RIPK1, and the transition from necroptosis to apoptosis. Conditional knockout of TAK1 in microglial/infiltrated macrophages and neuronal lineages sensitizes to ischemic infarction by promoting apoptosis. Taken together, our results demonstrate the critical role of necroptosis in mediating neurovascular damage and hypoperfusion-induced TAK1 loss, which subsequently promotes apoptosis and cerebral pathology in stroke and neurodegeneration.

Implications of Selective Autophagy Dysfunction for ALS Pathology

Amyotrophic lateral sclerosis (ALS) is a lethal neurodegenerative disorder that progressively affects motor neurons in the brain and spinal cord. Due to the biological complexity of the disease, its etiology remains unknown. Several cellular mechanisms involved in the neurodegenerative process in ALS have been found, including the loss of RNA and protein homeostasis, as well as mitochondrial dysfunction. Insoluble protein aggregates, damaged mitochondria, and stress granules, which contain RNA and protein components, are recognized and degraded by the autophagy machinery in a process known as selective autophagy. Autophagy is a highly dynamic process whose dysregulation has now been associated with neurodegenerative diseases, including ALS, by numerous studies. In ALS, the autophagy process has been found deregulated in both familial and sporadic cases of the disease. Likewise, mutations in genes coding for proteins involved in the autophagy machinery have been reported in ALS patients, including selective autophagy receptors. In this review, we focus on the role of selective autophagy in ALS pathology.

Reversal of CYLD phosphorylation as a novel therapeutic approach for adult T-cell leukemia/lymphoma (ATLL)

Adult T-cell leukemia/lymphoma (ATLL) is a malignancy of mature T cells associated with chronic infection by human T-cell lymphotropic virus type-1 (HTLV-1). ATLL patients with aggressive subtypes have dismal outcomes. We demonstrate that ATLL cells co-opt an early checkpoint within the tumor necrosis factor receptor 1 (TNFR1) pathway, resulting in survival advantage. This early checkpoint revolves around an interaction between the deubiquitinase CYLD and its target RIPK1. The status of RIPK1 K63-ubiquitination determines cell fate by creating either a prosurvival signal (ubiquitinated RIPK1) or a death signal (deubiquitinated RIPK1). In primary ATLL samples and in cell line models, an increased baseline level of CYLD phosphorylation was observed. We therefore tested the hypothesis that this modification of CYLD, which has been reported to inhibit its deubiquitinating function, leads to increased RIPK1 ubiquitination and thus provides a prosurvival signal to ATLL cells. CYLD phosphorylation can be pharmacologically reversed by IKK inhibitors, specifically by TBK1/IKKε and IKKβ inhibitors (MRT67307 and TPCA). Both of the IKK sub-families can phosphorylate CYLD, and the combination of MRT67307 and TPCA have a marked effect in reducing CYLD phosphorylation and triggering cell death. ATLL cells overexpressing a kinase-inactive TBK1 (TBK1-K38A) demonstrate lower CYLD phosphorylation and subsequently reduced proliferation. IKK blockade reactivates CYLD, as evidenced by the reduction in RIPK1 ubiquitination, which leads to the association of RIPK1 with the death-inducing signaling complex (DISC) to trigger cell death. In the absence of CYLD, RIPK1 ubiquitination remains elevated following IKK blockade and it does not associate with the DISC. SMAC mimetics can similarly disrupt CYLD phosphorylation and lead to ATLL cell death through reduction of RIPK1 ubiquitination, which is CYLD dependent. These results identify CYLD as a crucial regulator of ATLL survival and point to its role as a potential novel target for pharmacologic modification in this disease.

Inhibitors Targeting RIPK1/RIPK3: Old and New Drugs

The scaffolding function of receptor-interacting protein kinase 1 (RIPK1) regulates prosurvival signaling and inflammatory gene expression, while its kinase activity mediates both apoptosis and necroptosis; the latter involving RIPK3 kinase activity. The mutual transition between the scaffold and kinase functions of RIPK1 is regulated by (de)ubiquitylation and (de)phosphorylation. RIPK1-mediated cell death leads to disruption of epithelial barriers and/or release of damage-associated molecular patterns (DAMPs), cytokines, and chemokines, propagating inflammatory and degenerative diseases. Many drug development programs have pursued targeting RIPK1, and to a lesser extent RIPK3 kinase activity. In this review, we classify existing and novel small-molecule drugs based on their pharmacodynamic (PD) type I, II, and III binding mode. Finally, we discuss their applicability and therapeutic potential in inflammatory and degenerative experimental disease models.

Metformin mediates cardioprotection against aging-induced ischemic necroptosis

Necroptosis is crucially involved in severe cardiac pathological conditions. However, whether necroptosis contributes to age‐related intolerance to ischemia/reperfusion (I/R) injury remains elusive. In addition, metformin as a potential anti‐aging related injury drug, how it interacts with myocardial necroptosis is not yet clear. Male C57BL/6 mice at 3–4‐ (young) and 22–24 months of age (aged) and RIPK3‐deficient (Ripk3^−/−) mice were used to investigate aging‐related I/R injury in vivo. Metformin (125 μg/kg, i.p.), necrostatin‐1 (3.5 mg/kg), and adenovirus vector encoding p62‐shRNAs (Ad‐sh‐p62) were used to treat aging mice. I/R‐induced myocardial necroptosis was exaggerated in aged mice, which correlated with autophagy defects characterized by p62 accumulation in aged hearts or aged human myocardium. Functionally, blocking autophagic flux promoted H/R‐evoked cardiomyocyte necroptosis in vitro. We further revealed that p62 forms a complex with RIP1‐RIP3 (necrosome) and promotes the binding of RIP1 and RIP3. In mice, necrostatin‐1 treatment (a RIP1 inhibitor), RIP3 deficiency, and cardiac p62 knockdown in vivo demonstrated that p62‐RIP1‐RIP3‐dependent myocardial necroptosis contributes to aging‐related myocardial vulnerability to I/R injury. Notably, metformin treatment disrupted p62‐RIP1‐RIP3 complexes and effectively repressed I/R‐induced necroptosis in aged hearts, ultimately reducing mortality in this model. These findings highlight previously unknown mechanisms of aging‐related myocardial ischemic vulnerability: p62‐necrosome‐dependent necroptosis. Metformin acts as a cardioprotective agent that inhibits this unfavorable chain mechanism of aging‐related I/R susceptibility.

RIPK1 Kinase-Dependent Death: A Symphony of Phosphorylation Events

The serine/threonine kinase RIPK1 has emerged as a crucial component of the inflammatory response activated downstream of several immune receptors, where it paradoxically functions as a scaffold to protect the cell from death or instead as an active kinase to promote the killing of the cell. While RIPK1 kinase-dependent cell death has revealed its physiological importance in the context of microbial infection, aberrant activation of RIPK1 is also demonstrated to promote cell death-driven inflammatory pathologies, highlighting the importance of fundamentally understanding proper RIPK1 regulation. Recent advances in the field demonstrated the crucial role of phosphorylation in the fine-tuning of RIPK1 activation and, additionally, question the exact mechanism by which RIPK1 enzymatic activity transmits the death signal.

Linear Ubiquitin Chains: Cellular Functions and Strategies for Detection and Quantification

Ubiquitination of proteins is a sophisticated post-translational modification implicated in the regulation of an ever-growing abundance of cellular processes. Recent insights into different layers of complexity have shaped the concept of the ubiquitin code. Key players in determining this code are the number of ubiquitin moieties attached to a substrate, the architecture of polyubiquitin chains, and post-translational modifications of ubiquitin itself. Ubiquitination can induce conformational changes of substrates and alter their interactive profile, resulting in the formation of signaling complexes. Here we focus on a distinct type of ubiquitination that is characterized by an inter-ubiquitin linkage through the N-terminal methionine, called M1-linked or linear ubiquitination. Formation, recognition, and disassembly of linear ubiquitin chains are highly specific processes that are implicated in immune signaling, cell death regulation and protein quality control. Consistent with their role in influencing signaling events, linear ubiquitin chains are formed in a transient and spatially regulated manner, making their detection and quantification challenging.

Immunological signatures in frontotemporal lobar degeneration

PURPOSE OF REVIEW

Over the last year, research into the immunological and inflammatory signatures of frontotemporal lobar degeneration (FTLD) has accelerated greatly. Herein, we highlight recently proposed roles of brain-resident microglia as well as peripheral myeloid cells in frontotemporal dementia (FTD)-spectrum disorders.

RECENT FINDINGS

Recent unbiased genetic, transcriptomic, and proteomic surveys using human data confirm significantly altered immune-function genes as well as transcript and protein modules associated with inflammatory and immune function. Beyond human studies, novel animal models indicate important roles for both microglia and monocytes, and central involvement of genes such as Trem2, Apoe, and Tbk1.

SUMMARY

The importance of neuroinflammatory activity in FTD pathophysiology is unambiguous, but whether this activity is primarily beneficial or detrimental remains unclear, with variable results reported for distinct disease paradigms. Going forward, it will be crucial to determine which types of microglial and peripheral myeloid responses are favorable, in response to which specific proteinopathies, and at which point in disease course.

Fragile X mental retardation protein protects against tumour necrosis factor-mediated cell death and liver injury

OBJECTIVE

The Fragile X mental retardation (FMR) syndrome is a frequently inherited intellectual disability caused by decreased or absent expression of the FMR protein (FMRP). Lack of FMRP is associated with neuronal degradation and cognitive dysfunction but its role outside the central nervous system is insufficiently studied. Here, we identify a role of FMRP in liver disease.

DESIGN

Mice lacking Fmr1 gene expression were used to study the role of FMRP during tumour necrosis factor (TNF)-induced liver damage in disease model systems. Liver damage and mechanistic studies were performed using real-time PCR, Western Blot, staining of tissue sections and clinical chemistry.

RESULTS

Fmr1^null mice exhibited increased liver damage during virus-mediated hepatitis following infection with the lymphocytic choriomeningitis virus. Exposure to TNF resulted in severe liver damage due to increased hepatocyte cell death. Consistently, we found increased caspase-8 and caspase-3 activation following TNF stimulation. Furthermore, we demonstrate FMRP to be critically important for regulating key molecules in TNF receptor 1 (TNFR1)-dependent apoptosis and necroptosis including CYLD, c-FLIP_S and JNK, which contribute to prolonged RIPK1 expression. Accordingly, the RIPK1 inhibitor Necrostatin-1s could reduce liver cell death and alleviate liver damage in Fmr1^null mice following TNF exposure. Consistently, FMRP-deficient mice developed increased pathology during acute cholestasis following bile duct ligation, which coincided with increased hepatic expression of RIPK1, RIPK3 and phosphorylation of MLKL.

CONCLUSIONS

We show that FMRP plays a central role in the inhibition of TNF-mediated cell death during infection and liver disease.

Necroptosis and RIPK1-mediated neuroinflammation in CNS diseases

Apoptosis is crucial for the normal development of the nervous system, whereas neurons in the adult CNS are relatively resistant to this form of cell death. However, under pathological conditions, upregulation of death receptor family ligands, such as tumour necrosis factor (TNF), can sensitize cells in the CNS to apoptosis and a form of regulated necrotic cell death known as necroptosis that is mediated by receptor-interacting protein kinase 1 (RIPK1), RIPK3 and mixed lineage kinase domain-like protein (MLKL). Necroptosis promotes further cell death and neuroinflammation in the pathogenesis of several neurodegenerative diseases, including multiple sclerosis, amyotrophic lateral sclerosis, Parkinson disease and Alzheimer disease. In this Review, we outline the evidence implicating necroptosis in these neurological diseases and suggest that targeting RIPK1 might help to inhibit multiple cell death pathways and ameliorate neuroinflammation.

A dominant autoinflammatory disease caused by non-cleavable variants of RIPK1

Activation of RIPK1 controls TNF-mediated apoptosis, necroptosis and inflammatory pathways¹. Cleavage of human and mouse RIPK1 after residues D324 and D325, respectively, by caspase-8 separates the RIPK1 kinase domain from the intermediate and death domains. The D325A mutation in mouse RIPK1 leads to embryonic lethality during mouse development^2,3. However, the functional importance of blocking caspase-8-mediated cleavage of RIPK1 on RIPK1 activation in humans is unknown. Here we identify two families with variants in RIPK1 (D324V and D324H) that lead to distinct symptoms of recurrent fevers and lymphadenopathy in an autosomal-dominant manner. Impaired cleavage of RIPK1 D324 variants by caspase-8 sensitized patients' peripheral blood mononuclear cells to RIPK1 activation, apoptosis and necroptosis induced by TNF. The patients showed strong RIPK1-dependent activation of inflammatory signalling pathways and overproduction of inflammatory cytokines and chemokines compared with unaffected controls. Furthermore, we show that expression of the RIPK1 mutants D325V or D325H in mouse embryonic fibroblasts confers not only increased sensitivity to RIPK1 activation-mediated apoptosis and necroptosis, but also induction of pro-inflammatory cytokines such as IL-6 and TNF. By contrast, patient-derived fibroblasts showed reduced expression of RIPK1 and downregulated production of reactive oxygen species, resulting in resistance to necroptosis and ferroptosis. Together, these data suggest that human non-cleavable RIPK1 variants promote activation of RIPK1, and lead to an autoinflammatory disease characterized by hypersensitivity to apoptosis and necroptosis and increased inflammatory response in peripheral blood mononuclear cells, as well as a compensatory mechanism to protect against several pro-death stimuli in fibroblasts.

Casein kinase-1γ1 and 3 stimulate tumor necrosis factor-induced necroptosis through RIPK3

Upon necroptosis activation, receptor interacting serine/threonine kinase (RIPK)1 and RIPK3 form a necrosome complex with pseudokinase mixed lineage kinase-like (MLKL). Although protein phosphorylation is a key event for RIPK1 and RIPK3 activation in response to a necroptosis signal, relatively little is known about other factors that might regulate the activity of these kinases or necrosome formation. Through a gain-of-function screen with 546 kinases and 127 phosphatases, we identified casein kinase 1 gamma (CK1γ) as a candidate necroptosis-promoting factor. Here, we show that the decreased activity or amounts of CK1γ1 and CK1γ3, either by treatment with a chemical inhibitor or knockdown in cells, reduced TNFα-induced necroptosis. Conversely, ectopic expression of CK1γ1 or CK1γ3 exacerbated necroptosis, but not apoptosis. Similar to RIPK1 and RIPK3, CK1γ1 was also cleaved at Asp³⁴³ by caspase-8 during apoptosis. CK1γ1 and CK1γ3 formed a protein complex and were recruited to the necrosome harboring RIPK1, RIPK3 and MLKL. In particular, an autophosphorylated form of CK1γ3 at Ser^344/345 was detected in the necrosome and was required to mediate the necroptosis. In addition, in vitro assays with purified proteins showed that CK1γ phosphorylated RIPK3, affecting its activity, and in vivo assays showed that the CK1γ-specific inhibitor Gi prevented abrupt death in mice with hypothermia in a model of TNFα-induced systemic inflammatory response syndrome. Collectively, these data suggest that CK1γ1 and CK1γ3 are required for TNFα-induced necroptosis likely by regulating RIPK3.

Effects of ALS-associated TANK binding kinase 1 mutations on protein-protein interactions and kinase activity

Exonic DNA sequence variants in the Tbk1 gene associate with both sporadic and familial amyotrophic lateral sclerosis (ALS). Here, we examine functional defects in 25 missense TBK1 mutations, focusing on kinase activity and protein-protein interactions. We identified kinase domain (KD) mutations that abolish kinase activity or display substrate-specific defects in specific pathways, such as innate immunity and autophagy. By contrast, mutations in the scaffold dimerization domain (SDD) of TBK1 can cause the loss of kinase activity due to structural disruption, despite an intact KD. Familial ALS mutations in ubiquitin-like domain (ULD) or SDD display defects in dimerization; however, a subset retains kinase activity. These observations indicate that TBK1 dimerization is not required for kinase activation. Rather, dimerization seems to increase protein stability and enables efficient kinase-substrate interactions. Our study revealed many aspects of TBK1 activities affected by ALS mutations, highlighting the complexity of disease pathogenicity and providing insights into TBK1 activation mechanism.

ZBP1 and TAK1: Master Regulators of NLRP3 Inflammasome/Pyroptosis, Apoptosis, and Necroptosis (PAN-optosis)

Cell death is central to development, organismal homeostasis, and immune responses. The cell death field has experienced tremendous progress by delineating the molecular programs specific to each of the apoptotic and inflammatory cell death pathways. Moreover, the discovery of the inflammasomes and pyroptosis and necroptosis pathway regulators have provided the genetic basis for the programmed inflammatory cell death pathways. Earlier research highlighted the unique regulation of each of these pathways, but emerging studies discovered co-regulation and crosstalk between these seemingly different cell death complexes. The progress in this area has led to an idea that master regulators play central roles in orchestrating multiple cell death pathways. Here, we provide a brief review of the master regulators, the innate immune sensor ZBP1 and the essential cell survival kinase TAK1, that play vital roles in the regulation of RIPK1/RIPK3-FADD-caspase-8 cell death complex assembly and its versatility in executing Pyroptosis, Apoptosis, and Necroptosis, which we dubbed here as PAN-optosis. Furthermore, we discuss the implications and therapeutic potential of targeting these master regulators in health and disease.

One Sentence Summary

ZBP1 and TAK1 regulate PAN-optosis.

Current translational potential and underlying molecular mechanisms of necroptosis

Cell death has a fundamental impact on the evolution of degenerative disorders, autoimmune processes, inflammatory diseases, tumor formation and immune surveillance. Over the past couple of decades extensive studies have uncovered novel cell death pathways, which are independent of apoptosis. Among these is necroptosis, a tightly regulated, inflammatory form of cell death. Necroptosis contribute to the pathogenesis of many diseases and in this review, we will focus exclusively on necroptosis in humans. Necroptosis is considered a backup mechanism of apoptosis, but the in vivo appearance of necroptosis indicates that both caspase-mediated and caspase-independent mechanisms control necroptosis. Necroptosis is regulated on multiple levels, from the transcription, to the stability and posttranslational modifications of the necrosome components, to the availability of molecular interaction partners and the localization of receptor-interacting serine/threonine-protein kinase 1 (RIPK1), receptor-interacting serine/threonine-protein kinase 3 (RIPK3) and mixed lineage kinase domain-like protein (MLKL). Accordingly, we classified the role of more than seventy molecules in necroptotic signaling based on consistent in vitro or in vivo evidence to understand the molecular background of necroptosis and to find opportunities where regulating the intensity and the modality of cell death could be exploited in clinical interventions. Necroptosis specific inhibitors are under development, but >20 drugs, already used in the treatment of various diseases, have the potential to regulate necroptosis. By listing necroptosis-modulated human diseases and cataloging the currently available drug-repertoire to modify necroptosis intensity, we hope to kick-start approaches with immediate translational potential. We also indicate where necroptosis regulating capacity should be considered in the current applications of these drugs.

Fundamental Mechanisms of Regulated Cell Death and Implications for Heart Disease

Twelve regulated cell death programs have been described. We review in detail the basic biology of nine including death receptor-mediated apoptosis, death receptor-mediated necrosis (necroptosis), mitochondrial-mediated apoptosis, mitochondrial-mediated necrosis, autophagy-dependent cell death, ferroptosis, pyroptosis, parthanatos, and immunogenic cell death. This is followed by a dissection of the roles of these cell death programs in the major cardiac syndromes: myocardial infarction and heart failure. The most important conclusion relevant to heart disease is that regulated forms of cardiomyocyte death play important roles in both myocardial infarction with reperfusion (ischemia/reperfusion) and heart failure. While a role for apoptosis in ischemia/reperfusion cannot be excluded, regulated forms of necrosis, through both death receptor and mitochondrial pathways, are critical. Ferroptosis and parthanatos are also likely important in ischemia/reperfusion, although it is unclear if these entities are functioning as independent death programs or as amplification mechanisms for necrotic cell death. Pyroptosis may also contribute to ischemia/reperfusion injury, but potentially through effects in non-cardiomyocytes. Cardiomyocyte loss through apoptosis and necrosis is also an important component in the pathogenesis of heart failure and is mediated by both death receptor and mitochondrial signaling. Roles for immunogenic cell death in cardiac disease remain to be defined but merit study in this era of immune checkpoint cancer therapy. Biology-based approaches to inhibit cell death in the various cardiac syndromes are also discussed.

Exquisite sensitivity of adrenocortical carcinomas to induction of ferroptosis

Adrenocortical carcinomas (ACCs) are rare and highly malignant cancers associated with poor survival of patients. Currently, mitotane, a nonspecific derivative of the pesticide DDT (1,1-(dichlorobiphenyl)-2,2-dichloroethane), is used as the standard treatment, but its mechanism of action in ACCs remains elusive. Here we demonstrate that the human ACC NCI-H295R cell line is remarkably sensitive to induction of ferroptosis, while mitotane does not induce this iron-dependent mode of regulated necrosis. Supplementation with insulin, transferrin, and selenium (ITS) is commonly used to keep NCI-H295R cells in cell culture. We show that this supplementation prevents spontaneous ferroptosis, especially when it contains polyunsaturated fatty acids (PUFAs), such as linoleic acid. Inhibitors of apoptosis (zVAD, emricasan) do not prevent the mitotane-induced cell death but morphologically prevent membrane blebbing. The expression of glutathione peroxidase 4 (GPX4) in H295R cells, however, is significantly higher when compared to HT1080 fibrosarcoma cells, suggesting a role for ferroptosis. Direct inhibition of GPX4 in H295R cells led to high necrotic populations compared to control, while cotreatment with ferrostatin-1 (Fer-1) completely reverted ferroptosis. Interestingly, the analysis of public databases revealed that several key players of the ferroptosis pathway are hypermethylated and/or mutated in human ACCs. Finally, we also detected that growth hormone-releasing hormone (GHRH) antagonists, such as MIA602, kill H295R cells in a nonapoptotic manner. In summary, we found elevated expression of GPX4 and higher sensitivity to ferroptosis in ACCs. We hypothesize that instead of treatment with mitotane, human adrenocortical carcinomas may be much more sensitive to induction of ferroptosis.

A20 protects cells from TNF-induced apoptosis through linear ubiquitin-dependent and -independent mechanisms

The cytokine TNF promotes inflammation either directly by activating the MAPK and NF-κB signaling pathways, or indirectly by triggering cell death. A20 is a potent anti-inflammatory molecule, and mutations in the gene encoding A20 are associated with a wide panel of inflammatory pathologies, both in human and in the mouse. Binding of TNF to TNFR1 triggers the NF-κB-dependent expression of A20 as part of a negative feedback mechanism preventing sustained NF-κB activation. Apart from acting as an NF-κB inhibitor, A20 is also well-known for its ability to counteract the cytotoxic potential of TNF. However, the mechanism by which A20 mediates this function and the exact cell death modality that it represses have remained incompletely understood. In the present study, we provide in vitro and in vivo evidences that deletion of A20 induces RIPK1 kinase-dependent and -independent apoptosis upon single TNF stimulation. We show that constitutively expressed A20 is recruited to TNFR1 signaling complex (Complex I) via its seventh zinc finger (ZF7) domain, in a cIAP1/2-dependent manner, within minutes after TNF sensing. We demonstrate that Complex I-recruited A20 protects cells from apoptosis by stabilizing the linear (M1) ubiquitin network associated to Complex I, a process independent of its E3 ubiquitin ligase and deubiquitylase (DUB) activities and which is counteracted by the DUB CYLD, both in vitro and in vivo. In absence of linear ubiquitylation, A20 is still recruited to Complex I via its ZF4 and ZF7 domains, but this time protects the cells from death by deploying its DUB activity. Together, our results therefore demonstrate two distinct molecular mechanisms by which constitutively expressed A20 protect cells from TNF-induced apoptosis.

K63-linked ubiquitination regulates RIPK1 kinase activity to prevent cell death during embryogenesis and inflammation

Receptor-interacting protein kinase 1 (RIPK1) is a critical regulator of cell death through its kinase activity. However, how its kinase activity is regulated remains poorly understood. Here, we generate Ripk1^K376R/K376R knock-in mice in which the Lys(K)63-linked ubiquitination of RIPK1 is impaired. The knock-in mice display an early embryonic lethality due to massive cell death that is resulted from reduced TAK1-mediated suppression on RIPK1 kinase activity and forming more TNFR1 complex II in Ripk1^K376R/K376R cells in response to TNFα. Although TNFR1 deficiency delays the lethality, concomitant deletion of RIPK3 and Caspase8 fully prevents embryonic lethality of Ripk1^K376R/K376R mice. Notably, Ripk1^K376R/- mice are viable but develop severe systemic inflammation that is mainly driven by RIPK3-dependent signaling pathway, indicating that K63-linked ubiquitination on Lys376 residue of RIPK1 also contributes to inflammation process. Together, our study reveals the mechanism by which K63-linked ubiquitination on K376 regulates RIPK1 kinase activity to control cell death programs.

Receptor-interacting protein kinase 1 (RIPK1) is a critical regulator of cell death but its regulation is unclear. Here, the authors report that RIPK1 kinase activity is regulated by ubiquitination.

Innate immunity in allergy

Innate immune system quickly responds to invasion of microbes and foreign substances through the extracellular and intracellular sensing receptors, which recognize distinctive molecular and structural patterns. The recognition of innate immune receptors leads to the induction of inflammatory and adaptive immune responses by activating downstream signaling pathways. Allergy is an immune-related disease and results from a hypersensitive immune response to harmless substances in the environment. However, less is known about the activation of innate immunity during exposure to allergens. New insights into the innate immune system by sensors and their signaling cascades provide us with more important clues and a framework for understanding allergy disorders. In this review, we will focus on recent advances in the innate immune sensing system.

Sorafenib attenuated the function of natural killer cells infiltrated in HCC through inhibiting ERK1/2

Sorafenib has been systemically utilized to therapy the advanced hepatocellular carcinoma (HCC). Natural killer cells (NKs) are important cytotoxic innate lymphocytes, which can exert effector functions especially in liver and tumor. However, how Sorafenib affects the function of NKs remains to be elucidated. Here, we utilized the subcutaneous and in situ tumor bearing mice with mouse hepatoma cell line hepa 1-6. At the endpoint, the number and function of NKs in blood, liver, TDLN, and tumor were explored using FACS, ELISA, WB, etc. To confirm the direct effects of Sorafenib on NKs, the NKs were sorted using FACS, which were then stimulated with Sorafenib to detect the functions and the relevant mechanisms using qPCR, western blot, and FACS in vitro. Finally, we found that Sorafenib led to a significant block of tumor progression, but reduced the number of NK cells through suppressing the proliferation of NK cells. This phenotype made us study the terminal function of NK cells, revealing that Sorafenib could decrease the production of effector molecules and cytokines, such as perforin, granzyme B, TNF-α, IFN-γ, etc. Besides, p-ERK1/2 in NK cells was inhibited after treatment with Sorafenib, and a similar tendency of NK cells could be achieved using ERK1/2 inhibitor. Collectively, our data suggested that Sorafenib functioned as a critical inhibitor that controlled the number and function of NK cells through inhibiting ERK1/2.

Ripoptocide - A Spark for Inflammation

The clinical success of biologics that inhibit TNF (Tumor Necrosis Factor) in inflammatory bowel diseases (IBD), psoriasis and rheumatoid arthritis (RA) has clearly established a pathogenic role for this cytokine in these inflammatory disorders. TNF binding to its receptors activates NFκB and MAPK signaling, inducing the expression of downstream pro-inflammatory genes. This is thought to be the primary mechanism by which TNF elicits inflammation. TNF is also a well-known trigger of caspase-dependent apoptosis or caspase-independent necroptosis. Whether cell death has any role in TNF-mediated inflammation has been less clear. Emerging data from animal models now suggest that cellular demise caused by TNF may indeed provoke inflammation. The default response of most cells to TNF stimulation is survival, rather than death, due to the presence of two sequential cell death checkpoints. The early checkpoint is transcription-independent involving the non-degradative ubiquitination of RIPK1 to prevent RIPK1 from becoming a death-signaling molecule. The later checkpoint requires the induction of pro-survival genes by NFκB-mediated transcription. When the early checkpoint is disrupted, RIPK1 initiates cell death and we suggest the term ripoptocide to describe this manner of death (encompassing both apoptosis and necroptosis). The sensitivity of a cell to ripoptocide is determined by the balance between regulatory molecules that enforce and those that disassemble the early checkpoint. As there is evidence suggesting that ripoptocide is inflammatory, individuals may develop inflammation due to ripoptocide brought about by genetic, epigenetic or post-translational alteration of these checkpoint regulators. For these individuals, drugs that reinforce the early checkpoint and inhibit ripoptocide could be useful in ameliorating inflammation.

ULK1/2 Restricts the Formation of Inducible SINT-Speckles, Membraneless Organelles Controlling the Threshold of TBK1 Activation

Membraneless organelles (MLOs) are liquid-like subcellular compartments providing spatiotemporal control to biological processes. This study reveals that cellular stress leads to the incorporation of the adaptor protein SINTBAD (TBKBP1) into membraneless, cytosolic speckles. Determination of the interactome identified >100 proteins forming constitutive and stress-inducible members of an MLO that we termed SINT-speckles. SINT-speckles partially colocalize with activated TBK1, and deletion of SINTBAD and the SINT-speckle component AZI2 leads to impaired TBK1 phosphorylation. Dynamic formation of SINT-speckles is positively controlled by the acetyltransferase KAT2A (GCN5) and antagonized by heat shock protein-mediated chaperone activity. SINT-speckle formation is also inhibited by the autophagy-initiating kinases ULK1/2, and knockdown of these kinases prevented focal TBK1 phosphorylation in a pathway-specific manner. The phlebovirus-encoded non-structural protein S enhances ULK1-mediated TBK1 phosphorylation and shows a stress-induced translocation to SINT-speckles, raising the possibility that viruses can also target this signaling hub to manipulate host cell functions.

miR-425 deficiency promotes necroptosis and dopaminergic neurodegeneration in Parkinson's disease

A major hallmark of Parkinson’s disease (PD) is the degeneration of dopaminergic neurons in the substantia nigra, and the causative mechanism is thought to be the activation of programmed neuronal death. Necroptosis is a regulated process of cell death triggered by RIPK1. Although the pathophysiology of PD has been studied extensively, the cellular mechanism underlying dopaminergic neuron death remains unclear. In this study, we detected a specific miRNA, miR-425, in response to MPTP toxicity and dopaminergic degeneration. In MPTP-treated mice, we observed necroptosis activation and miR-425 deficiency in the substantia nigra, which is correlated with dopaminergic neuron loss. This miRNA targeted RIPK1 transcripts and promoted the phosphorylation of MLKL and necroptosis. Similarly, in the brains of PD patients, miR-425 deficiency and necroptosis activation were also confirmed in dopaminergic neuron. Furthermore, we found that genetic knockdown of miR-425 aggravated MPTP-induced motor deficits and dopaminergic neurodegeneration via early upregulation of necroptotic genes. Intracerebral miR-425 mimics (AgomiR-425) treatment attenuated necroptosis activation and dopaminergic neuron loss, and improved locomotor behaviors. In conclusion, our study suggests that miR-425 deficiency triggers necroptosis of dopaminergic neurons, and targeting miR-425 in MPTP-treated mice restored dysfunctional dopaminergic neurodegeneration and ameliorated behavioral deficits. These findings identify brain delivery of miR-425 as a potential therapeutic approach for the treatment of PD.

Keeping Cell Death in Check: Ubiquitylation-Dependent Control of TNFR1 and TLR Signaling

Pro-inflammatory signaling pathways, induced by pathogens, tissue damage or cytokines, depend on the ubiquitylation of various subunits of receptor signaling complexes, controlled by ubiquitin ligases and deubiquitinases. Ubiquitylation sets the stage for the activation of kinases within these receptor complexes, which ultimately regulate pro-inflammatory gene expression. The receptors, which transduce pro-inflammatory signals, can often induce cell death, which is controlled by ubiquitylation as well. In this review, we discuss the key role of ubiquitylation in pro-inflammatory signaling by TNFR1 and TLRs and its role in setting the threshold for cell death induced by these pro-inflammatory triggers.

Microglia-derived TNF-α mediates endothelial necroptosis aggravating blood brain-barrier disruption after ischemic stroke

Endothelium (EC) is a key component of blood–brain barrier (BBB), and has an important position in the neurovascular unit. Its dysfunction and death after cerebral ischemic/reperfusion (I/R) injury not only promote evolution of neuroinflammation and brain edema, but also increase the risk of intracerebral hemorrhage of thrombolytic therapies. However, the mechanism and specific interventions of EC death after I/R injury are poorly understood. Here we showed that necroptosis was a mechanism underlying EC death, which promoted BBB breakdown after I/R injury. Treatment of rats with receptor interacting protein kinase 1 (RIPK1)-inhibitor, necrostatin-1 reduced endothelial necroptosis and BBB leakage. We furthermore showed that perivascular M1-like microglia-induced endothelial necroptosis leading to BBB disruption requires tumor necrosis factor-α (TNF-α) secreted by M1 type microglia and its receptor, TNF receptor 1 (TNFR1), on endothelium as the primary mediators of these effects. More importantly, anti-TNFα (infliximab, a potent clinically used drug) treatment significantly ameliorate endothelial necroptosis, BBB destruction and improve stroke outcomes. Our data identify a previously unexplored role for endothelial necroptosis in BBB disruption and suggest infliximab might serve as a potential drug for stroke therapy.

Cell death-mediated cytokine release and its therapeutic implications

Targeting apoptosis to treat diseases has seen tremendous success over the past decades. More recently, alternative forms of regulated cell death, including pyroptosis and necroptosis, have been described. Understanding the molecular cascades regulating both pyroptosis and necroptosis will yield even more targets to treat diseases. These lytic forms of cell death are distinct from apoptosis due to their characteristic lysis and release of cellular components that promote disease or direct a beneficial immune response. In this review, we focus on how pyroptosis and necroptosis, which release potent immune cytokines such as IL-1 and IL-18, contribute to various diseases. We also consider the important role that the executioners of these cell death pathways, GSDMD and MLKL, play in the progression of inflammatory diseases. Crosstalk between the different cell death pathways likely plays a major role physiologically. New therapeutic strategies targeting these specific molecules hold enormous potential for managing inflammatory diseases.

Targeting RIPK1 for the treatment of human diseases

RIPK1 kinase has emerged as a promising therapeutic target for the treatment of a wide range of human neurodegenerative, autoimmune, and inflammatory diseases. This was supported by extensive studies which demonstrated that RIPK1 is a key mediator of apoptotic and necrotic cell death as well as inflammatory pathways. Furthermore, human genetic evidence has linked the dysregulation of RIPK1 to the pathogenesis of ALS as well as other inflammatory and neurodegenerative diseases. Importantly, unique allosteric small-molecule inhibitors of RIPK1 that offer high selectivity have been developed. These molecules can penetrate the blood-brain barrier, thus offering the possibility to target neuroinflammation and cell death which drive various neurologic conditions including Alzheimer's disease, ALS, and multiple sclerosis as well as acute neurological diseases such as stroke and traumatic brain injuries. We discuss the current understanding of RIPK1 regulatory mechanisms and emerging evidence for the pathological roles of RIPK1 in human diseases, especially in the context of the central nervous systems.

Supramolecular Complexes in Cell Death and Inflammation and Their Regulation by Autophagy

Signaling activation is a tightly regulated process involving myriad posttranslational modifications such as phosphorylation/dephosphorylation, ubiquitylation/deubiquitylation, proteolytical cleavage events as well as translocation of proteins to new compartments within the cell. In addition to each of these events potentially regulating individual proteins, the assembly of very large supramolecular complexes has emerged as a common theme in signal transduction and is now known to regulate many signaling events. This is particularly evident in pathways regulating both inflammation and cell death/survival. Regulation of the assembly and silencing of these complexes plays important roles in immune signaling and inflammation and the fate of cells to either die or survive. Here we will give a summary of some of the better studied supramolecular complexes involved in inflammation and cell death, particularly with a focus on diseases caused by their autoactivation and the role autophagy either plays or may be playing in their regulation.

The molecular machinery of regulated cell death

Cells may die from accidental cell death (ACD) or regulated cell death (RCD). ACD is a biologically uncontrolled process, whereas RCD involves tightly structured signaling cascades and molecularly defined effector mechanisms. A growing number of novel non-apoptotic forms of RCD have been identified and are increasingly being implicated in various human pathologies. Here, we critically review the current state of the art regarding non-apoptotic types of RCD, including necroptosis, pyroptosis, ferroptosis, entotic cell death, netotic cell death, parthanatos, lysosome-dependent cell death, autophagy-dependent cell death, alkaliptosis and oxeiptosis. The in-depth comprehension of each of these lethal subroutines and their intercellular consequences may uncover novel therapeutic targets for the avoidance of pathogenic cell loss.

Serine 25 phosphorylation inhibits RIPK1 kinase-dependent cell death in models of infection and inflammation

RIPK1 regulates cell death and inflammation through kinase-dependent and -independent mechanisms. As a scaffold, RIPK1 inhibits caspase-8-dependent apoptosis and RIPK3/MLKL-dependent necroptosis. As a kinase, RIPK1 paradoxically induces these cell death modalities. The molecular switch between RIPK1 pro-survival and pro-death functions remains poorly understood. We identify phosphorylation of RIPK1 on Ser25 by IKKs as a key mechanism directly inhibiting RIPK1 kinase activity and preventing TNF-mediated RIPK1-dependent cell death. Mimicking Ser25 phosphorylation (S > D mutation) protects cells and mice from the cytotoxic effect of TNF in conditions of IKK inhibition. In line with their roles in IKK activation, TNF-induced Ser25 phosphorylation of RIPK1 is defective in TAK1- or SHARPIN-deficient cells and restoring phosphorylation protects these cells from TNF-induced death. Importantly, mimicking Ser25 phosphorylation compromises the in vivo cell death-dependent immune control of Yersinia infection, a physiological model of TAK1/IKK inhibition, and rescues the cell death-induced multi-organ inflammatory phenotype of the SHARPIN-deficient mice.