Regulated necrosis, a proinflammatory cell death, potentially counteracts pathogenic infections

Erythronecroptosis: an overview of necroptosis or programmed necrosis in red blood cells

Necroptosis is considered a programmed necrosis that requires receptor-interacting protein kinase 1 (RIPK1), receptor-interacting protein kinase 3 (RIPK3), and pore-forming mixed lineage kinase domain-like protein (MLKL) to trigger a regulated cell membrane lysis. Membrane rupture in necroptosis has been shown to fuel innate immune response due to release of damage-associated molecular patterns (DAMPs). Recently published studies indicate that mature erythrocytes can undergo necroptosis as well. In this review, we provide an outline of multiple cell death modes occurring in erythrocytes, discuss possible immunological aspects of diverse erythrocyte cell deaths, summarize available evidence related to the ability of erythrocytes to undergo necroptosis, outline key involved molecular mechanisms, and discuss the potential implication of erythrocyte necroptosis in the physiology and pathophysiology. Furthermore, we aim to highlight the interplay between necroptosis and eryptosis signaling in erythrocytes, emphasizing specific characteristics of these pathways distinct from their counterparts in nucleated cells. Thus, our review provides a comprehensive summary of the current knowledge of necroptosis in erythrocytes. To reflect critical differences between necroptosis of nucleated cells and necroptosis of erythrocytes, we suggest a term erythronecroptosis for necroptosis of enucleated cells.

Purified Astragalus Polysaccharide Combined with Inactivated Vaccine Markedly Prevents Infectious Haematopoietic Necrosis Virus Infection in Rainbow Trout (Oncorhynchus mykiss)

Rainbow trout (Oncorhynchus mykiss) is experiencing a catastrophic pandemic. In recent years, infectious hematopoietic necrosis virus (IHNV) has spread nationwide, resulting in significant mortality. Currently, there are no available treatments or vaccines for IHNV in China. Here, the Astragalus extract was purified and characterized. Then, we developed an inactivated IHNV vaccine with purified Astragalus polysaccharide (P-APS) as an adjuvant. Safety assays showed that IHNV was successfully inactivated. After a serious IHNV challenge, the cumulative mortality rates were 76.0, 38.0, and 22.1% in control, vaccine, and P-APS + vaccine groups, respectively. P-APS + vaccine was effective at reducing head kidney damage and apoptosis after IHNV challenge by histopathological and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) analyses. The P-APS + vaccine group showed better results in enhancing specific antibodies (IgM) and immune enzyme activities (C3, LZM, GOT, and GPT). RNA-seq revealed that many immune-related pathways were significantly enriched. TLR2, TLR7, C3, IFN-γ, IgM, MHC1, MHC2, MX1, and VIG1 were identified as core genes based on RNA-seq and PPI networks. Mechanistic investigations showed that P-APS + vaccine activates the immune pathway by upregulating the expression of these genes. P-ASP+vaccine induced effective innate and adaptive immune responses that were stronger than single vaccines after vaccination and IHNV challenged. Our findings will provide a promising vaccine candidate against IHNV.

Neuroprotective and anti-inflammatory effects of the RIPK3 inhibitor GSK872 in an MPTP-induced mouse model of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder triggered by the loss of dopaminergic neurons in the substantia nigra (SN). Recent studies have demonstrated that necroptosis is involved in dopaminergic neuronal cell death and the resulting neuroinflammation. During the process of necroptosis, a necrosome complex is formed consisting of the proteins receptor-interacting protein kinase 1 (RIPK1), RIPK3, and mixed lineage kinase domain-like protein (MLKL). Although the neuroprotective effects of the RIPK1-specific inhibitor necrostatin-1, as well as RIPK3 and MLKL knockout in mice, have been described, the effects of RIPK3 pharmacological inhibitors have not yet been reported in animal models of PD. In the present study, we investigated the neuroprotective effects of GSK872, a specific RIPK3 inhibitor, in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced mouse model of PD. GSK872 rescued MPTP-induced motor impairment and inhibited tyrosine hydroxylase-positive dopaminergic cell death in the SN and striatum. Additionally, GSK872 inhibited the MPTP-induced increase in the expression of p-RIPK3 and p-MLKL in both the dopaminergic neurons and microglia, as assessed by biochemical and histological analyses. GSK872 further inhibited microglial activation and the expression of inflammatory mediators including NLRP3, interleukin (IL)-1β, IL-6, tumor necrosis factor-alpha, and inducible nitric oxide synthase in the SN region of MPTP mice. Using in vitro experiments, we validated the effects of GSK872 on necroptosis in SH-SY5Y neuronal and BV2 microglial cells. Overall, our results suggest that GSK872 exerts neuroprotective and anti-inflammatory effects, and may thus have therapeutic potential for PD.

Cracking Lysine Crotonylation (Kcr): Enlightening a Promising Post-Translational Modification

Lysine crotonylation (Kcr) is a recently discovered post-translational modification (PTM). Both histone and non-histone Kcr-proteins have been associated with numerous diseases including cancer, acute kidney injury, HIV latency, and cardiovascular disease. Histone Kcr enhances gene expression to a larger extend than the extensively studied lysine acetylation (Kac), suggesting Kcr as a novel potential therapeutic target. Although numerous scientific reports on crotonylation were published in the last years, relevant knowledge gaps concerning this PTM and its regulation still remain. To date, only few selective Kcr-interacting proteins have been identified and selective methods for the enrichment of Kcr-proteins in chemical proteomics analysis are still lacking. The development of new techniques to study this underexplored PTM could then clarify its function in health and disease and hopefully accelerate the development of new therapeutics for Kcr-related disease. Herein we briefly review what is known about the regulation mechanisms of Kcr and the current methods used to identify Kcr-proteins and their interacting partners. This report aims to highlight the significant potential of Kcr as a therapeutic target and to identify the existing scientific gaps that new research must address.

Cytosolic serpins act in a cytoprotective feedback loop that limits ESX-1-dependent death of Mycobacterium marinum-infected macrophages

Serine protease inhibitors (serpins) constitute the largest family of protease inhibitors expressed in humans, but their role in infection remains largely unexplored. In infected macrophages, the mycobacterial ESX-1 type VII secretion system permeabilizes internal host membranes and causes leakage into the cytosol of host DNA, which induces type I interferon (IFN) production via the cyclic GMP-AMP synthase (cGAS) and stimulator of IFN genes (STING) surveillance pathway, and promotes infection in vivo. Using the Mycobacterium marinum infection model, we show that ESX-1-mediated type I IFN signaling in macrophages selectively induces the expression of serpina3f and serpina3g, two cytosolic serpins of the clade A3. The membranolytic activity of ESX-1 also caused leakage of cathepsin B into the cytosol where it promoted cell death, suggesting that the induction of type I IFN comes at the cost of lysosomal rupture and toxicity. However, the production of cytosolic serpins suppressed the protease activity of cathepsin B in this compartment and thus limited cell death, a function that was associated with increased bacterial growth in infected mice. These results suggest that cytosolic serpins act in a type I IFN-dependent cytoprotective feedback loop to counteract the inevitable toxic effect of ESX-1-mediated host membrane rupture.

IMPORTANCE

The ESX-1 type VII secretion system is a key virulence determinant of pathogenic mycobacteria. The ability to permeabilize host cell membranes is critical for several ESX-1-dependent virulence traits, including phagosomal escape and induction of the type I interferon (IFN) response. We find that it comes at the cost of lysosomal leakage and subsequent host cell death. However, our results suggest that ESX-1-mediated type I IFN signaling selectively upregulates serpina3f and serpina3g and that these cytosolic serpins limit cell death caused by cathepsin B that has leaked into the cytosol, a function that is associated with increased bacterial growth in vivo. The ability to rupture host membranes is widespread among bacterial pathogens, and it will be of interest to evaluate the role of cytosolic serpins and this type I IFN-dependent cytoprotective feedback loop in the context of human infection.

Dying for a cause: The pathogenic manipulation of cell death and efferocytic pathways

Cell death is a natural consequence of infection. However, although the induction of cell death was solely thought to benefit the pathogen, compelling data now show that the activation of cell death pathways serves as a nuanced antimicrobial strategy that couples pathogen elimination with the generation of inflammatory cytokines and the priming of innate and adaptive cellular immunity. Following cell death, the phagocytic uptake of the infected dead cell by antigen-presenting cells and the subsequent lysosomal fusion of the apoptotic body containing the pathogen serve as an important antimicrobial mechanism that furthers the development of downstream adaptive immune responses. Despite the complexity of regulated cell death pathways, pathogens are highly adept at evading them. Here, we provide an overview of the remarkable diversity of cell death and efferocytic pathways and discuss illustrative examples of virulence strategies employed by pathogens, including oral pathogens, to counter their activation and persist within the host.

Cell death induced by acute renal injury: a perspective on the contributions of accidental and programmed cell death

The involvement of cell death in acute kidney injury (AKI) is linked to multiple factors including energy depletion, electrolyte imbalance, reactive oxygen species, inflammation, mitochondrial dysfunction, and activation of several cell death pathway components. Since our review in 2003, discussing the relative contributions of apoptosis and necrosis, several other forms of cell death have been identified and are shown to contribute to AKI. Currently, these various forms of cell death can be fundamentally divided into accidental cell death and regulated or programmed cell death based on functional aspects. Several death initiator and effector molecules switch molecules that may act as signaling components triggering either death or protective mechanisms or alternate cell death pathways have been identified as part of the machinery. Intriguingly, several of these cell death pathways share components and signaling pathways suggesting complementary or compensatory functions. Thus, defining the cross talk between distinct cell death pathways and identifying the unique molecular effectors for each type of cell death may be required to develop novel strategies to prevent cell death. Furthermore, depending on the multiple forms of cell death simultaneously induced in different AKI settings, strategies for combination therapies that block multiple cell death pathways need to be developed to completely prevent injury, cell death, and renal function. This review highlights the various cell death pathways, cross talk, and interactions between different cell death modalities in AKI.

Glutathione‑degrading enzymes in the complex landscape of tumors (Review)

Glutathione (GSH)‑degrading enzymes are essential for starting the first stages of GSH degradation. These enzymes include extracellular γ‑glutamyl transpeptidase (GGT) and intracellular GSH‑specific γ‑glutamylcyclotransferase 1 (ChaC1) and 2. These enzymes are essential for cellular activities, such as immune response, differentiation, proliferation, homeostasis regulation and programmed cell death. Tumor tissue frequently exhibits abnormal expression of GSH‑degrading enzymes, which has a key impact on the development and spread of malignancies. The present review summarizes gene and protein structure, catalytic activity and regulation of GSH‑degrading enzymes, their vital roles in tumor development (including regulation of oxidative and endoplasmic reticulum stress, control of programmed cell death, promotion of inflammation and tumorigenesis and modulation of drug resistance in tumor cells) and potential role as diagnostic biomarkers and therapeutic targets.

The crystal structure of human ferroptosis suppressive protein 1 in complex with flavin adenine dinucleotide and nicotinamide adenine nucleotide

Ferroptosis is a recently discovered form of regulated cell death characterized by its distinct dependence on iron and the peroxidation of lipids within cellular membranes. Ferroptosis plays a crucial role in physiological and pathological situations and has attracted the attention of numerous scientists. Ferroptosis suppressive protein 1 (FSP1) is one of the main regulators that negatively regulates ferroptosis through the GPX4-independent FSP1-CoQ10-NAD(P)H axis and is a potential therapeutic target for ferroptosis-related diseases. However, the crystal structure of FSP1 has not been resolved, which hinders the development of therapeutic strategies targeting FSP1. To unravel this puzzle, we purified the human FSP1 (hFSP1) protein using the baculovirus eukaryotic cell expression system and solved its crystal structure at a resolution of 1.75 Å. Furthermore, we evaluated the oxidoreductase activity of hFSP1 with NADH as the substrate and identified E156 as the key amino acid in maintaining hFSP1 activity. Interestingly, our results indicated that hFSP1 exists and functions in a monomeric state. Mutagenesis analysis revealed the critical role of the C-terminal domain in the binding of substrate. These findings significantly enhance our understanding of the functional mechanism of FSP1 and provide a precise model for further drug development.

Identification of Necroptosis-related Molecular Subtypes and Construction of Necroptosis-related Gene Signature for Glioblastoma Multiforme

BACKGROUND

Necroptosis is a highly regulated and genetically controlled process, and therefore, attention has been paid to the exact effects of this disorder on a variety of diseases, including cancer. An in-depth understanding of the key regulatory factors and molecular events that trigger necroptosis can not only identify patients at risk of cancer development but can also help to develop new treatment strategies.

AIMS

This study aimed to increase understanding of the complex role of necroptosis in glioblastoma multiforme (GBM) and provide a new perspective and reference for accurate prediction of clinical outcomes and gene-targeted therapy in patients with GBM. The objective of this study was to analyze the gene expression profile of necroptosis regulatory factors in glioblastoma multiforme (GBM) and establish a necroptosis regulatory factor-based GBM classification and prognostic gene signature to recognize the multifaceted impact of necroptosis on GBM.

METHODS

The necroptosis score of the glioblastoma multiforme (GBM) sample in TCGA was calculated by ssGSEA, and the correlation between each gene and the necroptosis score was calculated. Based on necroptosis score-related genes, unsupervised consensus clustering was employed to classify patients. The prognosis, tumor microenvironment (TME), genomic changes, biological signal pathways and gene expression differences among clusters were analyzed. The gene signature of GBM was constructed by Cox and LASSO regression analysis of differentially expressed genes (DEGs).

RESULT

Based on 34 necroptosis score-related genes, GBM was divided into two clusters with different overall survival (OS) and TME. A necroptosis-related gene signature (NRGS) containing 8 genes was developed, which could stratify the risk of GBM in both the training set and verification set and had good prognostic value. NRGS and age were both independent prognostic indicators of GBM, and a nomogram developed by the integration of both of them showed a better predictive effect than traditional clinical features.

CONCLUSION

In this study, patients from public data sets were divided into two clusters and the unique TME and molecular characteristics of each cluster were described. Furthermore, an NRGS was constructed to effectively and independently predict the survival outcome of GBM, which provides some insights for the implementation of personalized precision medicine in clinical practice.

The Intersection between Bacterial Metabolism and Innate Immunity

BACKGROUND

The innate immune system is the first line of defense against microbial pathogens and is essential for maintaining good health. If pathogens breach innate barriers, the likelihood of infection is significantly increased. Many bacterial pathogens pose a threat to human health on account of their ability to evade innate immunity and survive in growth-restricted environments. These pathogens have evolved sophisticated strategies to obtain nutrients as well as manipulate innate immune responses, resulting in disease or chronic infection.

SUMMARY

The relationship between bacterial metabolism and innate immunity is complex. Although aspects of bacterial metabolism can be beneficial to the host, particularly those related to the microbiota and barrier integrity, others can be harmful. Several bacterial pathogens harness metabolism to evade immune responses and persist during infection. The study of these adaptive traits provides insight into the roles of microbial metabolism in pathogenesis that extend beyond energy balance. This review considers recent studies on bacterial metabolic pathways that promote infection by circumventing several facets of the innate immune system. We also discuss relationships between innate immunity and antibiotics and highlight future directions for research in this field.

KEY MESSAGES

Pathogenic bacteria have a remarkable capacity to harness metabolism to manipulate immune responses and promote pathogenesis. While we are beginning to understand the multifaceted and complex metabolic adaptations that occur during infection, there is still much to uncover with future research.

Macrophage PTEN controls STING-induced inflammation and necroptosis through NICD/NRF2 signaling in APAP-induced liver injury

BACKGROUND

Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) signaling has been known to play a critical role in maintaining cellular and tissue homeostasis, which also has an essential role in the inflammatory response. However, it remains unidentified whether and how the macrophage PTEN may govern the innate immune signaling stimulator of interferon genes (STING) mediated inflammation and hepatocyte necroptosis in APAP-induced liver injury (AILI).

METHODS

Myeloid-specific PTEN knockout (PTEN^M-KO) and floxed PTEN (PTEN^FL/FL) mice were treated with APAP (400 mg/kg) or PBS. In a parallel in vitro study, bone marrow-derived macrophages (BMMs) were isolated from these conditional knockout mice and transfected with CRISPR/Cas9-mediated Notch1 knockout (KO) or CRISPR/Cas9-mediated STING activation vector followed by LPS (100 ng/ml) stimulation.

RESULTS

Here, we report that myeloid-specific PTEN knockout (PTEN^M-KO) mice were resistant to oxidative stress-induced hepatocellular injury with reduced macrophage/neutrophil accumulation and proinflammatory mediators in AILI. PTEN^M-KO increased the interaction of nuclear Notch intracellular domain (NICD) and nuclear factor (erythroid-derived 2)-like 2 (NRF2) in the macrophage nucleus, reducing reactive oxygen species (ROS) generation. Mechanistically, it is worth noting that macrophage NICD and NRF2 co-localize within the nucleus under inflammatory conditions. Additionally, Notch1 promotes the interaction of immunoglobulin kappa J region (RBPjκ) with NRF2. Disruption of the Notch1 signal in PTEN deletion macrophages, reduced RBPjκ and NRF2 binding, and activated STING signaling. Moreover, PTEN^M-KO macrophages with STING activated led to ROS generation and TNF-α release, resulting in hepatocyte necroptosis upon co-culture with primary hepatocytes.

CONCLUSIONS

Our findings demonstrate that the macrophage PTEN-NICD/NRF2-STING axis is critical to regulating oxidative stress-induced liver inflammation and necroptosis in AILI and implies the therapeutic potential for managing sterile liver inflammation. Video Abstract.

A positive feedback cycle between the alarmin S100A8/A9 and NLRP3 inflammasome-GSDMD signalling reinforces the innate immune response in Candida albicans keratitis

OBJECTIVE

Fungal keratitis is a severe sight-threatening ocular infection, without effective treatment strategies available now. Calprotectin S100A8/A9 has recently attracted great attention as a critical alarmin modulating the innate immune response against microbial challenges. However, the unique role of S100A8/A9 in fungal keratitis is poorly understood.

METHODS

Experimental fungal keratitis was established in wild-type and gene knockout (TLR4^-/- and GSDMD^-/-) mice by infecting mouse corneas with Candida albicans. The degree of mouse cornea injuries was evaluated by clinical scoring. To interrogate the molecular mechanism in vitro, macrophage RAW264.7 cell line was challenged with Candida albicans or recombinant S100A8/A9 protein. Label-free quantitative proteomics, quantitative real-time PCR, Western blotting, and immunohistochemistry were conducted in this research.

RESULTS

Herein, we characterized the proteome of mouse corneas infected with Candida albicans and found that S100A8/A9 was robustly expressed at the early stage of the disease. S100A8/A9 significantly enhanced disease progression by promoting NLRP3 inflammasome activation and Caspase-1 maturation, accompanied by increased accumulation of macrophages in infected corneas. In response to Candida albicans infection, toll-like receptor 4 (TLR4) sensed extracellular S100A8/A9 and acted as a bridge between S100A8/A9 and NLRP3 inflammasome activation in mouse corneas. Furthermore, the deletion of TLR4 resulted in noticeable improvement in fungal keratitis. Remarkably, NLRP3/GSDMD-mediated macrophage pyroptosis in turn facilitates S100A8/A9 secretion during Candida albicans keratitis, thus forming a positive feedback cycle that amplifies the proinflammatory response in corneas.

CONCLUSIONS

The present study is the first to reveal the critical roles of the alarmin S100A8/A9 in the immunopathology of Candida albicans keratitis, highlighting a promising approach for therapeutic intervention in the future.

Virulence Factors of Mycobacterium tuberculosis as Modulators of Cell Death Mechanisms

Mycobacterium tuberculosis (Mtb) modulates diverse cell death pathways to escape the host immune responses and favor its dissemination, a complex process of interest in pathogenesis-related studies. The main virulence factors of Mtb that alter cell death pathways are classified according to their origin as either non-protein (for instance, lipomannan) or protein (such as the PE family and ESX secretion system). The 38 kDa lipoprotein, ESAT-6 (early antigen-secreted protein 6 kDa), and another secreted protein, tuberculosis necrotizing toxin (TNT), induces necroptosis, thereby allowing mycobacteria to survive inside the cell. The inhibition of pyroptosis by blocking inflammasome activation by Zmp1 and PknF is another pathway that aids the intracellular replication of Mtb. Autophagy inhibition is another mechanism that allows Mtb to escape the immune response. The enhanced intracellular survival (Eis) protein, other proteins, such as ESX-1, SecA2, SapM, PE6, and certain microRNAs, also facilitate Mtb host immune escape process. In summary, Mtb affects the microenvironment of cell death to avoid an effective immune response and facilitate its spread. A thorough study of these pathways would help identify therapeutic targets to prevent the survival of mycobacteria in the host.

The Many Faces of MLKL, the Executor of Necroptosis

Necroptosis is a recently discovered form of regulated cell death characterized by the disruption of plasma membrane integrity and the release of intracellular content. Mixed lineage kinase domain-like (MLKL) protein is the main player of this cell death pathway as it mediates the final step of plasma membrane permeabilization. Despite the significant progress in our knowledge of the necroptotic pathway and MLKL biology, the precise mechanism of how MLKL functions remain unclear. To understand in what way MLKL executes necroptosis, it is crucial to decipher how the molecular machinery of regulated cell death is activated in response to different stimuli or stressors. It is also indispensable to unveiling the structural elements of MLKL and the cellular players that are required for its regulation. In this review, we discuss the key steps that lead to MLKL activation, possible models that explain how it becomes the death executor in necroptosis, and its emerging alternative functions. We also summarize the current knowledge about the role of MLKL in human disease and provide an overview of existing strategies aimed at developing new inhibitors that target MLKL for necroptosis intervention.

Short-Chain Fatty-Acid-Producing Bacteria: Key Components of the Human Gut Microbiota

Short-chain fatty acids (SCFAs) play a key role in health and disease, as they regulate gut homeostasis and their deficiency is involved in the pathogenesis of several disorders, including inflammatory bowel diseases, colorectal cancer, and cardiometabolic disorders. SCFAs are metabolites of specific bacterial taxa of the human gut microbiota, and their production is influenced by specific foods or food supplements, mainly prebiotics, by the direct fostering of these taxa. This Review provides an overview of SCFAs' roles and functions, and of SCFA-producing bacteria, from their microbiological characteristics and taxonomy to the biochemical process that lead to the release of SCFAs. Moreover, we will describe the potential therapeutic approaches to boost the levels of SCFAs in the human gut and treat different related diseases.

Progress on innate immune evasion and live attenuated vaccine of pseudorabies virus

Pseudorabies virus (PRV) is a highly infectious disease that can infect most mammals, with pigs as the only natural host, has caused considerable economic losses to the pig husbandry of the world. Innate immunity is the first defense line of the host against the attack of pathogens and is essential for the proper establishment of adaptive immunity. The host uses the innate immune response to against the invasion of PRV; however PRV makes use of various strategies to inhibit the innate immunity to promote the virus replication. Currently, live attenuated vaccine is used to prevent pig from infection with the PRV worldwide, such as Bartha K61. However, a growing number of data indicates that these vaccines do not provide complete protection against new PRV variants that have emerged since late 2011. Here we summarized the interactions between PRV and host innate immunity and the current status of live attenuated PRV vaccines to promote the development of novel and more effective PRV vaccines.

Resolution Potential of Necrotic Cell Death Pathways

During tissue damage caused by infection or sterile inflammation, not only damage-associated molecular patterns (DAMPs), but also resolution-associated molecular patterns (RAMPs) can be activated. These dying cell-associated factors stimulate immune cells localized in the tissue environment and induce the production of inflammatory mediators or specialized proresolving mediators (SPMs). Within the current prospect of science, apoptotic cell death is considered the main initiator of resolution. However, more RAMPs are likely to be released during necrotic cell death than during apoptosis, similar to what has been observed for DAMPs. The inflammatory potential of many regulated forms of necrotic cell death modalities, such as pyroptosis, necroptosis, ferroptosis, netosis, and parthanatos, have been widely studied in necroinflammation, but their possible role in resolution is less considered. In this review, we aim to summarize the relationship between necrotic cell death and resolution, as well as present the current available data regarding the involvement of certain forms of regulated necrotic cell death in necroresolution.