NLRP3 inflammasome plays a redundant role with caspase 8 to promote IL-1β-mediated osteomyelitis

Caspases: structural and molecular mechanisms and functions in cell death, innate immunity, and disease

Caspases are critical regulators of cell death, development, innate immunity, host defense, and disease. Upon detection of pathogens, damage-associated molecular patterns, cytokines, or other homeostatic disruptions, innate immune sensors, such as NLRs, activate caspases to initiate distinct regulated cell death pathways, including non-lytic (apoptosis) and innate immune lytic (pyroptosis and PANoptosis) pathways. These cell death pathways are driven by specific caspases and distinguished by their unique molecular mechanisms, supramolecular complexes, and enzymatic properties. Traditionally, caspases are classified as either apoptotic (caspase-2, -3, -6, -7, -8, -9, and -10) or inflammatory (caspase-1, -4, -5, and -11). However, extensive data from the past decades have shown that apoptotic caspases can also drive lytic inflammatory cell death downstream of innate immune sensing and inflammatory responses, such as in the case of caspase-3, -6, -7, and -8. Therefore, more inclusive classification systems based on function, substrate specificity, or the presence of pro-domains have been proposed to better reflect the multifaceted roles of caspases. In this review, we categorize caspases into CARD-, DED-, and short/no pro-domain-containing groups and examine their critical functions in innate immunity and cell death, along with their structural and molecular mechanisms, including active site/exosite properties and substrates. Additionally, we highlight the emerging roles of caspases in cellular homeostasis and therapeutic targeting. Given the clinical relevance of caspases across multiple diseases, improved understanding of these proteins and their structure-function relationships is critical for developing effective treatment strategies.

Evolutionary and Functional Analysis of Caspase-8 and ASC Interactions to Drive Lytic Cell Death, PANoptosis

Caspases are evolutionarily conserved proteins essential for driving cell death in development and host defense. Caspase-8, a key member of the caspase family, is implicated in nonlytic apoptosis, as well as lytic forms of cell death. Recently, caspase-8 has been identified as an integral component of PANoptosomes, multiprotein complexes formed in response to innate immune sensor activation. Several innate immune sensors can nucleate caspase-8-containing PANoptosome complexes to drive inflammatory lytic cell death, PANoptosis. However, how the evolutionarily conserved and diverse functions of caspase-8 drive PANoptosis remains unclear. To address this, we performed evolutionary, sequence, structural, and functional analyses to decode caspase-8's complex-forming abilities and its interaction with the PANoptosome adaptor ASC. Our study distinguished distinct subgroups within the death domain superfamily based on their evolutionary and functional relationships, identified homotypic traits among subfamily members, and captured key events in caspase evolution. We also identified critical residues defining the heterotypic interaction between caspase-8's death effector domain and ASC's pyrin domain, validated through cross-species analyses, dynamic simulations, and in vitro experiments. Overall, our study elucidated recent evolutionary adaptations of caspase-8 that allowed it to interact with ASC, improving our understanding of critical molecular associations in PANoptosome complex formation and the underlying PANoptotic responses in host defense and inflammation. These findings have implications for understanding mammalian immune responses and developing new therapeutic strategies for inflammatory diseases.

PTGS2 Silencing Inhibits Ferroptosis in Staphylococcus Aureus-induced Osteomyelitis By Blocking the IL-17A Signaling Pathway

OBJECTIVE

Osteomyelitis caused by Staphylococcus aureus (S. aureus) infection is an inflammatory bone disease characterized by continuous bone destruction, which is difficult to treat. This research aimed to explore the molecular mechanisms of S. aureus-induced osteomyelitis.

METHODS

Using the GSE166522 and GSE227521 datasets, hub differentially expressed genes (DEGs) were screened by bioinformatics analysis. Hub gene expression levels were validated in S. aureus-induced mouse models. An inhibitor of PTGS2, etoricoxib, was used to assess the role of PTGS2 in the osteomyelitis mouse model. PTGS2 was silenced in an LPS-induced MC3T3-E1 cell model to study its effect on cell function.

RESULTS

Six hub genes were screened, including ARG1, TIMP1, NOS2, PTGS2, SOCS3, and IL1B, highly expressed in the S. aureus-induced osteomyelitis model. Etoricoxib treatment attenuated the inflammatory infiltration of tibial tissue in mice with osteomyelitis. In vivo and in vitro, etoricoxib treatment and PTGS2 silencing reduced inflammatory factor (TNF-α, IL-1β, and IL-6) levels. PTGS2 silencing promoted LPS-induced MC3T3-E1 cell viability and inhibited apoptosis and ferroptosis. GPX4 and SLC7A11 protein levels were significantly increased after PTGS2 silencing. Mechanistically, IL-17A intervention significantly counteracted the impact of PTGS2 silencing on cell behaviors and secukinumab combined with PTGS2 silencing more effectively suppressed inflammation and ferroptosis, indicating that PTGS2 impeded the osteomyelitis progression by inhibiting the IL-17A pathway.

CONCLUSION

Silencing PTGS2 reduces ferroptosis in S. aureus-induced osteomyelitis by obstructing the IL-17A pathway, which suggests a new approach for the treatment of osteomyelitis.

Innate immune sensor NLRP3 drives PANoptosome formation and PANoptosis

Inflammasomes are multiprotein innate immune complexes formed in response to infections, tissue damage, or cellular stress that promote the maturation and release of IL-1β/IL-18 and are implicated in lytic cell death. The NLRP3 inflammasome is canonically activated by an initial priming event followed by an activation stimulus, leading to rapid cell death that occurs through caspase-1 (CASP1) and gasdermin D (GSDMD) activation, called pyroptosis. CASP1- and GSDMD-deficient cells are protected from the rapid LPS plus ATP-induced pyroptosis. However, innate immune responses physiologically occur over time, extending beyond minutes to hours and days. Therefore, in this study, we assessed lytic cell death beyond the early timepoints. While cells lacking the innate immune sensor NLRP3 were protected from cell death induced by the canonical NLRP3 trigger, LPS priming and ATP stimulation (LPS plus ATP), for extended time, CASP1- and GSDMD-deficient cells started to lyse in a time-dependent manner after 2 h. Nevertheless, robust IL-1β and IL-18 release was still dependent on CASP1 activation. These data suggested that NLRP3 engages an additional innate immune, lytic cell death pathway. Indeed, LPS plus ATP induced the activation of caspases and RIPKs associated with PANoptosis in WT cells, and cells deficient in PANoptosis machinery were protected from cell death for extended times. A PANoptosome complex containing NLRP3, ASC, CASP8, and RIPK3 was observed by microscopy in WT, as well as CASP1- or GSDMD-deficient, cells by 30 min post-stimulation. Overall, these findings highlight the central role of NLRP3 as a PANoptosome sensor. Given the physiological role of innate immune cell death, PANoptosis, in health and disease, our study emphasizes the importance of a comprehensive understanding of PANoptosomes, and their components, as therapeutic targets.

Machine learning-based characterization of PANoptosis-related biomarkers and immune infiltration in ulcerative colitis: A comprehensive bioinformatics analysis and experimental validation

Ulcerative colitis (UC) is a heterogeneous autoimmune condition. PANoptosis, a new form of programmed cell death, plays a role in inflammatory diseases. This study aimed to identify differentially expressed PANoptosis-related genes (PRGs) involved in immune dysregulation in UC. Three key PRGs-BIRC3, MAGED1, and PSME2 were found using weighted gene co-expression network analysis (WGCNA) and machine learning. Immune infiltration analysis revealed that these key PRGs were associated with neutrophils, CD8+ T cells, activated CD4 T cells, and NK cells. Moreover, these key PRGs were significantly enriched in pathways related to inflammatory bowel disease, the IL-17 signaling pathway, and NOD-like receptor signaling pathway. The expression levels of the key PRGs were validated in various datasets, animal models, and UC intestinal tissue samples. Our findings confirmed the involvement of PANoptosis in UC and predict hub genes and immune characteristics, providing new insights for further investigations into UC pathogenic mechanisms and therapeutic strategies.

TLR4/TRIF/Caspase-8/Caspase-1 Pathway in Choroidal Endothelial Cells Promotes Choroidal Neovascularization

PURPOSE

The purpose of this study was to investigate the role and mechanism of caspase-8 in the development of choroidal neovascularization induced by age-related macular degeneration, with the aim of identifying a potential therapeutic target for neovascular age-related macular degeneration.

METHODS

Mouse models of laser photocoagulation-induced choroidal neovascularization and hypoxic human choroidal endothelial cells were utilized to examine the involvement of caspase-8 in choroidal neovascularization development. The toll-like receptor 4/TIR domain-containing adaptor molecule 1/caspase-8 pathway was explored in hypoxic human choroidal endothelial cells to elucidate its contribution to pathological angiogenesis. Various experimental techniques, including inhibition assays and immunoblotting analysis, were employed to assess the effects and mechanisms of caspase-8 activation.

RESULTS

Inhibition of caspase-8 demonstrated attenuated choroidal neovascularization development in mice subjected to laser photocoagulation. Activation of the toll-like receptor 4/TIR domain-containing adaptor molecule 1/caspase-8 pathway was observed in hypoxic human choroidal endothelial cells. Upon activation by the toll-like receptor 4/TIR domain-containing adaptor molecule 1 axis, caspase-8 directly cleaved caspase-1, leading to the cleavage of interleukin-1β and interleukin-18 by caspase-1. Consequently, activation of interleukin-1β and interleukin-18 through the toll-like receptor 4/TIR domain-containing adaptor molecule 1/caspase-8/caspase-1 pathway promoted the proliferative, migratory, and tube-forming abilities of hypoxic human choroidal endothelial cells.

CONCLUSION

The findings of this study indicate that caspase-8 plays a crucial role in promoting choroidal neovascularization by activating interleukin-1β and interleukin-18 through the toll-like receptor 4/TIR domain-containing adaptor molecule 1/caspase-8/caspase-1 pathway in choroidal endothelial cells. Therefore, targeting caspase-8 may hold promise as a therapeutic approach for neovascular age-related macular degeneration.

A novel approach to the prevention and management of chemotherapy-induced cardiotoxicity: PANoptosis

As a fundamental component of antitumor therapy, chemotherapy-induced cardiotoxicity (CIC) has emerged as a leading cause of long-term mortality in patients with malignant tumors. Unfortunately, there are currently no effective therapeutic preventive or treatment strategies, and the underlying pathophysiological mechanisms of CIC remain inadequately understood. A growing number of studies have shown that different mechanisms of cell death, such as apoptosis, pyroptosis, and necroptosis, are essential for facilitating the cardiotoxic effects of chemotherapy. The PANoptosis mode represents a highly synchronized and dynamically balanced programmed cell death (PCD) process that integrates the principal molecular characteristics of necroptosis, apoptosis, and pyroptosis. Recent research has revealed a significant correlation between PANoptosis and the apoptosis of tumor cells. Chemotherapy drugs can activate PANoptosis, which is involved in the development of cardiovascular diseases. These findings suggest that PANoptosis marks the point where the effectiveness of chemotherapy against tumors overlaps with the onset and development of cardiovascular diseases. Furthermore, previous studies have demonstrated that CIC can simultaneously induce pyrodeath, apoptosis, and necrotic apoptosis. Therefore, PANoptosis may represent a potential mechanism and target for the prevention of CIC. This study explored the interactions among the three main mechanisms of PCD, pyroptosis, apoptosis, and necroptosis in CICs and analyzed the relevant literature on PANoptosis and CICs. The purpose of this work is to serve as a reference for future investigations on the role of PANoptosis in the development and mitigation of cardiotoxicity associated with chemotherapy.

Genetic background affects neutrophil activity and determines the severity of autoinflammatory osteomyelitis in mice

The knowledge about the contribution of the innate immune system to health and disease is expanding. However, to obtain reliable results, it is critical to select appropriate mouse models for in vivo studies. Data on genetic and phenotypic changes associated with different mouse strains can assist in this task. Such data can also facilitate our understanding of how specific polymorphisms and genetic alterations affect gene function, phenotypes, and disease outcomes. Extensive information is available on genetic changes in all major mouse strains. However, comparatively little is known about their impact on immune response and, in particular, on innate immunity. Here, we analyzed a mouse model of chronic multifocal osteomyelitis, an autoinflammatory disease driven exclusively by the innate immune system, which is caused by an inactivating mutation in the Pstpip2 gene. We investigated how the genetic background of BALB/c, C57BL/6J, and C57BL/6NCrl strains alters the molecular mechanisms controlling disease progression. While all mice developed the disease, symptoms were significantly milder in BALB/c and partially also in C57BL/6J when compared to C57BL/6NCrl. Disease severity correlated with the number of infiltrating neutrophils and monocytes and with the production of chemokines attracting these cells to the site of inflammation. It also correlated with increased expression of genes associated with autoinflammation, rheumatoid arthritis, neutrophil activation, and degranulation, resulting in altered neutrophil activation in vivo. Together, our data demonstrate striking effects of genetic background on multiple parameters of neutrophil function and activity influencing the onset and course of chronic multifocal osteomyelitis.

Advances of the multifaceted functions of PSTPIP2 in inflammatory diseases

The complex interaction between the immune system and autoinflammatory disorders highlights the centrality of autoimmune mechanisms in the pathogenesis of autoinflammatory diseases. With the exploration of PSTPIP2, it has been discovered to play an inhibitory role in immune diseases, suggesting its potential utility in the research and treatment of rheumatic diseases. This review outlines the mechanisms of PSTPIP2 in chronic multifocal osteomyelitis (CMO), rheumatoid arthritis (RA), synovitis-acne-pustulosis-hyperostosis-osteitis (SAPHO) syndrome, liver diseases, renal diseases, pressure ulcer sepsis and diabetic obesity. The mechanisms include inhibiting the IL-1β inflammatory responses, NF-κB, ERK phosphorylation etc., promoting Erβ, and modulating the polarization of macrophage to prevent the inflammatory diseases. This review summarized current findings and offered perspectives on future research directions, laying a foundation for applying of PSTPIP2 in inflammatory diseases.

PANoptosis in autoimmune diseases interplay between apoptosis, necrosis, and pyroptosis

PANoptosis is a newly identified inflammatory programmed cell death (PCD) that involves the interplay of apoptosis, necrosis, and pyroptosis. However, its overall biological effects cannot be attributed to any one type of PCD alone. PANoptosis is regulated by a signaling cascade triggered by the recognition of pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) by various sensors. This triggers the assembly of the PANoptosome, which integrates key components from other PCD pathways via adapters and ultimately activates downstream execution molecules, resulting in cell death with necrotic, apoptotic, and pyroptotic features. Autoimmune diseases are characterized by reduced immune tolerance to self-antigens, leading to abnormal immune responses, often accompanied by systemic chronic inflammation. Consequently, PANoptosis, as a unique innate immune-inflammatory PCD pathway, has significant pathophysiological relevance to inflammation and autoimmunity. However, most previous research on PANoptosis has focused on tumors and infectious diseases, leaving its activation and role in autoimmune diseases unclear. This review briefly outlines the characteristics of PANoptosis and summarizes several newly identified PANoptosome complexes, their activation mechanisms, and key components. We also explored the dual role of PANoptosis in diseases and potential therapeutic approaches targeting PANoptosis. Additionally, we review the existing evidence for PANoptosis in several autoimmune diseases and explore the potential regulatory mechanisms involved.

Caspase-8 in inflammatory diseases: a potential therapeutic target

Caspase-8, a renowned cysteine-aspartic protease within its enzyme family, initially garnered attention for its regulatory role in extrinsic apoptosis. With advancing research, a growing body of evidence has substantiated its involvement in other cell death processes, such as pyroptosis and necroptosis, as well as its modulatory effects on inflammasomes and proinflammatory cytokines. PANoptosis, an emerging concept of cell death, encompasses pyroptosis, apoptosis, and necroptosis, providing insight into the often overlapping cellular mortality observed during disease progression. The activation or deficiency of caspase-8 enzymatic activity is closely linked to PANoptosis, positioning caspase-8 as a key regulator of cell survival or death across various physiological and pathological processes. Aberrant expression of caspase-8 is closely associated with the development and progression of a range of inflammatory diseases, including immune system disorders, neurodegenerative diseases (NDDs), sepsis, and cancer. This paper delves into the regulatory role and impact of caspase-8 in these conditions, aiming to elucidate potential therapeutic strategies for the future intervention.

Classical apoptotic stimulus, staurosporine, induces lytic inflammatory cell death, PANoptosis

Innate immunity is the body's first line of defense against disease, and regulated cell death is a central component of this response that balances pathogen clearance and inflammation. Cell death pathways are generally categorized as non-lytic and lytic. While non-lytic apoptosis has been extensively studied in health and disease, lytic cell death pathways are also increasingly implicated in infectious and inflammatory diseases and cancers. Staurosporine (STS) is a well-known inducer of non-lytic apoptosis. However, in this study, we observed that STS also induces lytic cell death at later timepoints. Using biochemical assessments with genetic knockouts, pharmacological inhibitors, and gene silencing, we identified that STS triggered PANoptosis via the caspase-8/RIPK3 axis, which was mediated by RIPK1. PANoptosis is a lytic, innate immune cell death pathway initiated by innate immune sensors and driven by caspases and RIPKs through PANoptosome complexes. Deletion of caspase-8 and RIPK3, core components of the PANoptosome complex, protected against STS-induced lytic cell death. Overall, our study identifies STS as a time-dependent inducer of lytic cell death, PANoptosis. These findings emphasize the importance of understanding trigger- and time-specific activation of distinct cell death pathways to advance our understanding of the molecular mechanisms of innate immunity and cell death for clinical translation.

Inflammasomes at the Foundation of Inflammatory Cell Death Complexes

This Pillars of Immunology article is a commentary on “The Inflammasome: A Molecular Platform Triggering Activation of Inflammatory Caspases and Processing of proIL-β,” a pivotal article written by F. Martinon, K. Burns, and J. Tschopp, published in Molecular Cell in 2002. https://doi.org/10.1016/S1097-2765(02)00599-3.

PANoptosis: a novel target for cardiovascular diseases

PANoptosis is a unique innate immune inflammatory lytic cell death pathway initiated by an innate immune sensor and driven by caspases and RIPKs. As a distinct pathway, the execution of PANoptosis cannot be hindered by targeting other cell death pathways, such as pyroptosis, apoptosis, or necroptosis. Instead, targeting key PANoptosome components can serve as a strategy to prevent this form of cell death. Given the physiological relevance in several diseases, PANoptosis is a pivotal therapeutic target. Notably, previous research has primarily focused on the role of PANoptosis in cancer and infectious and inflammatory diseases. By contrast, its role in cardiovascular diseases has not been comprehensively discussed. Here, we review the available evidence on PANoptosis in cardiovascular diseases, including cardiomyopathy, atherosclerosis, myocardial infarction, myocarditis, and aortic aneurysm and dissection, and explore a variety of agents that target PANoptosis, with the overarching goal of providing a novel complementary approach to combatting cardiovascular diseases.

NLRC5 senses NAD+ depletion, forming a PANoptosome and driving PANoptosis and inflammation

NLRs constitute a large, highly conserved family of cytosolic pattern recognition receptors that are central to health and disease, making them key therapeutic targets. NLRC5 is an enigmatic NLR with mutations associated with inflammatory and infectious diseases, but little is known about its function as an innate immune sensor and cell death regulator. Therefore, we screened for NLRC5's role in response to infections, PAMPs, DAMPs, and cytokines. We identified that NLRC5 acts as an innate immune sensor to drive inflammatory cell death, PANoptosis, in response to specific ligands, including PAMP/heme and heme/cytokine combinations. NLRC5 interacted with NLRP12 and PANoptosome components to form a cell death complex, suggesting an NLR network forms similar to those in plants. Mechanistically, TLR signaling and NAD+ levels regulated NLRC5 expression and ROS production to control cell death. Furthermore, NLRC5-deficient mice were protected in hemolytic and inflammatory models, suggesting that NLRC5 could be a potential therapeutic target.

Defining PANoptosis: Biochemical and Mechanistic Evaluation of Innate Immune Cell Death Activation

The innate immune system is the first line of host defense. Innate immune activation utilizes pattern recognition receptors to detect pathogens, pathogen-associated and damage-associated molecular patterns (PAMPs and DAMPs), and homeostatic alterations and drives inflammatory signaling pathways and regulated cell death. Cell death activation is critical to eliminate pathogens and aberrant or damaged cells, while excess activation can be linked to inflammation, tissue damage, and disease. Therefore, there is increasing interest in studying cell death mechanisms to understand the underlying biology and identify therapeutic strategies. However, there are significant technical challenges, as many cell death pathways share key molecules with each other, and genetic models where these cell death molecules are deleted remain the gold standard for evaluation. Furthermore, extensive crosstalk has been identified between the cell death pathways pyroptosis, apoptosis, necroptosis, and the more recently characterized PANoptosis, which is defined as a prominent, unique innate immune, lytic, and inflammatory cell death pathway initiated by innate immune sensors and driven by caspases and RIPKs through PANoptosomes. PANoptosomes are multi-protein complexes assembled by innate immune sensor(s) in response to pathogens, PAMPs, DAMPs, cytokines, and homeostatic changes that drive PANoptosis. In this article, we provide methods for molecularly defining distinct cell death pathways, including PANoptosis, using both genetic and chemical approaches through western blot, LDH assay, and microscopy readouts. This procedure allows for the assessment of cell death on the cell population and single-cell levels even without access to genetic models. Having this comprehensive workflow that is more accessible to all labs will improve our ability as a scientific community to accelerate discovery. Using these protocols will help identify new innate immune sensors that drive PANoptosis and define the molecular mechanisms and regulators involved to establish new targets for clinical translation. © 2024 The Author(s). Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Induction and quantification of cell death using live cell imaging Alternate Protocol 1: Quantification of cell death using LDH Alternate Protocol 2: Assessment of cell death complexes in single cells using immunofluorescence staining Basic Protocol 2: Analysis of cell death mechanisms by immunoblots (western blots).

Implications of inflammatory cell death-PANoptosis in health and disease

Regulated cell death (RCD) pathways, such as pyroptosis, apoptosis, and necroptosis, are essential for maintaining the body's balance, defending against pathogens, and eliminating abnormal cells that could lead to diseases like cancer. Although these pathways operate through distinct mechanisms, recent genetic and pharmacological studies have shown that they can interact and influence each other. The concept of "PANoptosis" has emerged, highlighting the interplay between pyroptosis, apoptosis, and necroptosis, especially during cellular responses to infections. This article provides a concise overview of PANoptosis and its molecular mechanisms, exploring its implications in various diseases. The review focuses on the extensive interactions among different RCD pathways, emphasizing the role of PANoptosis in infections, cytokine storms, inflammatory diseases, and cancer. Understanding PANoptosis is crucial for developing novel treatments for conditions involving infections, sterile inflammations, and cancer.

Systematic analysis of PANoptosis-related genes identifies XIAP as a functional oncogene in breast cancer

BACKGROUND

Breast cancer (BC) is the most prevalent malignant disease affecting women globally. PANoptosis, a novel form of cell death combining features of pyroptosis, apoptosis, and necroptosis, has recently gained attention. However, its precise function in BC and the predictive values of PANoptosis-related genes remain unclear.

METHODS

We used the expression data and clinical information of BC tissues or normal breast tissues from public databases, and then successfully developed and verified a BC PANoptosis-related risk model through a combination of univariate Cox regression, least absolute shrinkage and selection operator (LASSO) regression, and Kaplan-Meier (KM) analysis. A nomogram was constructed to estimate survival probability, and its accuracy was assessed using calibration curves.

RESULTS

Among 37 PANoptosis-related genes, we identified 4 differentially expressed genes related to overall survival (OS). Next, a risk model incorporating these four PANoptosis-related genes was established. Patients were stratified into low/high-risk groups based on the median risk score, with the low-risk group showing better prognoses and higher levels of immune infiltration. Utilizing the risk score and clinical features, we developed a nomogram to predict 1-, 3- and 5-year survival probability. X-linked inhibitor of apoptosis protein (XIAP) emerged as a potentially risky factor with the highest hazard ratio. In vitro experiments demonstrated that XIAP inhibition enhances the antitumor effect of doxorubicin through the PANoptosis pathway.

CONCLUSION

PANoptosis holds an important role in BC prognosis and treatment.

Innate Immunity in Protection and Pathogenesis During Coronavirus Infections and COVID-19

The COVID-19 pandemic was caused by the recently emerged β-coronavirus SARS-CoV-2. SARS-CoV-2 has had a catastrophic impact, resulting in nearly 7 million fatalities worldwide to date. The innate immune system is the first line of defense against infections, including the detection and response to SARS-CoV-2. Here, we discuss the innate immune mechanisms that sense coronaviruses, with a focus on SARS-CoV-2 infection and how these protective responses can become detrimental in severe cases of COVID-19, contributing to cytokine storm, inflammation, long-COVID, and other complications. We also highlight the complex cross talk among cytokines and the cellular components of the innate immune system, which can aid in viral clearance but also contribute to inflammatory cell death, cytokine storm, and organ damage in severe COVID-19 pathogenesis. Furthermore, we discuss how SARS-CoV-2 evades key protective innate immune mechanisms to enhance its virulence and pathogenicity, as well as how innate immunity can be therapeutically targeted as part of the vaccination and treatment strategy. Overall, we highlight how a comprehensive understanding of innate immune mechanisms has been crucial in the fight against SARS-CoV-2 infections and the development of novel host-directed immunotherapeutic strategies for various diseases.

Histone deacetylase-mediated silencing of PSTPIP2 expression contributes to aristolochic acid nephropathy-induced PANoptosis

BACKGROUND AND PURPOSE

Aristolochic acid nephropathy (AAN) is a progressive kidney disease caused by using herbal medicines. Currently, no therapies are available to treat or prevent aristolochic acid nephropathy. Histone deacetylase (HDAC) plays a crucial role in the development and progression of renal disease. We tested whether HDAC inhibitors could prevent aristolochic acid nephropathy and determined the underlying mechanism.

EXPERIMENTAL APPROACH

HDACs expression in the aristolochic acid nephropathy model was examined. The activation of PANoptosis of mouse kidney and renal tubular epithelial cell were assessed after exposure to HDAC1 and HDAC2 blockade. Kidney-specific knock-in of proline-serine-threonine-phosphatase-interacting protein 2 (PSTPIP2) mice were used to investigate whether PSTPIP2 affected the production of PANoptosome.

KEY RESULTS

Aristolochic acid upregulated the expression of HDAC1 and HDAC2 in the kidneys. Notably, the HDAC1 and HDAC2 specific inhibitor, romidepsin (FK228, depsipeptide), suppressed aristolochic acid-induced kidney injury, epithelial cell pyroptosis, apoptosis and necroptosis (PANoptosis). Moreover, romidepsin upregulated PSTPIP2 in renal tubular epithelial cells, which was enhanced by aristolochic acid treatment. Conditional knock-in of PSTPIP2 in the kidney protected against aristolochic acid nephropathy. In contrast, the knockdown of PSTPIP2 expression in PSTPIP2-knock-in mice restored kidney damage and PANoptosis. PSTPIP2 function was determined in vitro using PSTPIP2 knockdown or overexpression in mouse renal tubular epithelial cells (mTECs). Additionally, PSTPIP2 was found to regulate caspase 8 in aristolochic acid nephropathy.

CONCLUSION AND IMPLICATIONS

HDAC-mediated silencing of PSTPIP2 may contribute to aristolochic acid nephropathy. Hence, HDAC1 and HDAC2 specific inhibitors or PSTPIP2 could be valuable therapeutic agents for preventing aristolochic acid nephropathy.

The NLR family of innate immune and cell death sensors

Nucleotide-binding oligomerization domain (NOD)-like receptors, also known as nucleotide-binding leucine-rich repeat receptors (NLRs), are a family of cytosolic pattern recognition receptors that detect a wide variety of pathogenic and sterile triggers. Activation of specific NLRs initiates pro- or anti-inflammatory signaling cascades and the formation of inflammasomes-multi-protein complexes that induce caspase-1 activation to drive inflammatory cytokine maturation and lytic cell death, pyroptosis. Certain NLRs and inflammasomes act as integral components of larger cell death complexes-PANoptosomes-driving another form of lytic cell death, PANoptosis. Here, we review the current understanding of the evolution, structure, and function of NLRs in health and disease. We discuss the concept of NLR networks and their roles in driving cell death and immunity. An improved mechanistic understanding of NLRs may provide therapeutic strategies applicable across infectious and inflammatory diseases and in cancer.

A comparative study of apoptosis, pyroptosis, necroptosis, and PANoptosis components in mouse and human cells

Regulated cell death is a key component of the innate immune response, which provides the first line of defense against infection and homeostatic perturbations. However, cell death can also drive pathogenesis. The most well-defined cell death pathways can be categorized as nonlytic (apoptosis) and lytic (pyroptosis, necroptosis, and PANoptosis). While specific triggers are known to induce each of these cell death pathways, it is unclear whether all cell types express the cell death proteins required to activate these pathways. Here, we assessed the protein expression and compared the responses of immune and non-immune cells of human and mouse origin to canonical pyroptotic (LPS plus ATP), apoptotic (staurosporine), necroptotic (TNF-α plus z-VAD), and PANoptotic (influenza A virus infection) stimuli. When compared to fibroblasts, both mouse and human innate immune cells, macrophages, expressed higher levels of cell death proteins and activated cell death effectors more robustly, including caspase-1, gasdermins, caspase-8, and RIPKs, in response to specific stimuli. Our findings highlight the importance of considering the cell type when examining the mechanisms regulating inflammation and cell death. Improved understanding of the cell types that contain the machinery to execute different forms of cell death and their link to innate immune responses is critical to identify new strategies to target these pathways in specific cellular populations for the treatment of infectious diseases, inflammatory disorders, and cancer.

NINJ1 mediates inflammatory cell death, PANoptosis, and lethality during infection conditions and heat stress

Innate immunity provides the first line of defense through multiple mechanisms, including pyrogen production and cell death. While elevated body temperature during infection is beneficial to clear pathogens, heat stress (HS) can lead to inflammation and pathology. Links between pathogen exposure, HS, cytokine release, and inflammation have been observed, but fundamental innate immune mechanisms driving pathology during pathogen exposure and HS remain unclear. Here, we use multiple genetic approaches to elucidate innate immune pathways in infection or LPS and HS models. Our results show that bacteria and LPS robustly increase inflammatory cell death during HS that is dependent on caspase-1, caspase-11, caspase-8, and RIPK3 through the PANoptosis pathway. Caspase-7 also contributes to PANoptosis in this context. Furthermore, NINJ1 is an important executioner of this cell death to release inflammatory molecules, independent of other pore-forming executioner proteins, gasdermin D, gasdermin E, and MLKL. In an in vivo HS model, mortality is reduced by deleting NINJ1 and fully rescued by deleting key PANoptosis molecules. Our findings suggest that therapeutic strategies blocking NINJ1 or its upstream regulators to prevent PANoptosis may reduce the release of inflammatory mediators and benefit patients.

MicroRNA-18a regulates the Pyroptosis, Apoptosis, and Necroptosis (PANoptosis) of osteoblasts induced by tumor necrosis factor-α via hypoxia-inducible factor-1α

BACKGROUND

Tumor necrosis factor-α (TNF-α) is involved in inflammatory responses and promotes cell death and the inhibition of osteogenic differentiation. MicroRNA (miRNA) plays a crucial role in the infected bone diseases, however, the biological role of miRNAs in inflammation-induced impaired osteogenic differentiation remains unclear. This study aimed to explore the role of miRNA-18a-5p (miR-18a) in regulating PANoptosis and osteogenic differentiation in an inflammatory environment via hypoxia-inducible factor-1α (HIF1-α).

METHODS

The expression of miR-18a in MC3T3-E1 cells was analyzed using quantitative reverse transcription-polymerase chain reaction in an inflammatory environment induced by TNF-α. The expression of HIF1-α and NLRP3 in LV-miR-18a or sh-miR-18a cells was analyzed using western blotting. Fluorescence imaging for cell death, flow cytometry, and alkaline phosphatase activity analysis were used to analyze the role of miR-18a in TNF-α-induced PANoptosis and the inhibition of osteogenic differentiation. An animal model of infectious bone defect was established to validate the regulatory role of miR-18a in an inflammatory environment.

RESULTS

The expression of miRNA-18a in the MC3T3-E1 cell line was significantly lower under TNF-α stimulation than in the normal environment. miR-18a significantly inhibited the expression of HIF1-α and NLRP3, and inhibition of HIF1-α expression further inhibited NLRP3 expression. Furthermore, inhibition of miR-18a expression promoted the TNF-α-induced PANoptosis and inhibition of osteogenic differentiation, whereas miR-18a overexpression and the inhibition of both HIF1-α and NLRP3 reduced the effects of TNF-α. These findings are consistent with those of the animal experiments.

CONCLUSION

miRNA-18a negatively affects HIF1-α/NLRP3 expression, inhibits inflammation-induced PANoptosis, and impairs osteogenic differentiation. Thus, it is a potential therapeutic candidate for developing anti-inflammatory strategies for infected bone diseases.

PSTPIP2 ameliorates aristolochic acid nephropathy by suppressing interleukin-19-mediated neutrophil extracellular trap formation

Aristolochic acid nephropathy (AAN) is a progressive kidney disease caused by herbal medicines. Proline-serine-threonine phosphatase-interacting protein 2 (PSTPIP2) and neutrophil extracellular traps (NETs) play important roles in kidney injury and immune defense, respectively, but the mechanism underlying AAN regulation by PSTPIP2 and NETs remains unclear. We found that renal tubular epithelial cell (RTEC) apoptosis, neutrophil infiltration, inflammatory factor, and NET production were increased in a mouse model of AAN, while PSTPIP2 expression was low. Conditional knock-in of Pstpip2 in mouse kidneys inhibited cell apoptosis, reduced neutrophil infiltration, suppressed the production of inflammatory factors and NETs, and ameliorated renal dysfunction. Conversely, downregulation of Pstpip2 expression promoted kidney injury. In vivo, the use of Ly6G-neutralizing antibody to remove neutrophils and peptidyl arginine deiminase 4 (PAD4) inhibitors to prevent NET formation reduced apoptosis, alleviating kidney injury. In vitro, damaged RTECs released interleukin-19 (IL-19) via the PSTPIP2/nuclear factor (NF)-κB pathway and induced NET formation via the IL-20Rβ receptor. Concurrently, NETs promoted apoptosis of damaged RTECs. PSTPIP2 affected NET formation by regulating IL-19 expression via inhibition of NF-κB pathway activation in RTECs, inhibiting RTEC apoptosis, and reducing kidney damage. Our findings indicated that neutrophils and NETs play a key role in AAN and therapeutic targeting of PSTPIP2/NF-κB/IL-19/IL-20Rβ might extend novel strategies to minimize Aristolochic acid I-mediated acute kidney injury and apoptosis.

Multiomics blueprint of PANoptosis in deciphering immune characteristics and prognosis stratification of glioma patients

BACKGROUND

As the most prevalent primary brain tumor in adults, glioma accounts for the majority of all central nervous system malignant tumors. The concept of PANoptosis is a relatively new, underlining the interconnection and synergy among three distinct pathways: pyroptosis, apoptosis and necroptosis.

METHODS

We performed single-cell annotations of glioma cells and determined crucial signaling pathways through cell chat analysis. Using least absolute shrinkage and selection operator (LASSO) and Cox analyses, we identified a gene set with prognostic values. Our model was validated using independent external cohort. In addition, we employed single-sample gene set enrichment analysis and xCell analyses to describe the detailed profile of infiltrated immune cells and depicted the gene mutation landscape in the two groups.

RESULTS

We identified seven distinct cell clusters in glioma samples, including oligodendrocyte precursor cells (OPCs), myeloid cells, tumor cells, oligodendrocytes, astrocytes, vascular cells and neuronal cells. We found that myeloid cells showed the highest PANoptosis activity. An intense mutual cell communication pattern between the tumor cells and OPCs and oligodendrocytes was observed. Differentially expressed genes between the high-PANoptosis and low-PANoptosis cell groups were obtained, which were enriched to actin cytoskeleton, cell adhesion molecules and gamma R-mediated phagocytosis pathways. We determined a set of five genes of prognostic significance: SAA1, SLPI, DCX, S100A8 and TNR. The prognostic differences between the two groups in the internal and external sets were found to be statistically significant. We found a marked correlation between S100A8 and activated dendritic cell, macrophage, mast cell, myeloid derived suppressor cell and Treg infiltration. Moreover, we have observed a significant increase of PTEN mutation in the high risk (HR) group of glioma patients.

CONCLUSIONS

In the present study, we have constructed a prognostic model that is based on the PANoptosis, and we have demonstrated its significant efficacy in stratifying patients with glioma. This innovative prognostic model offers novel insights into precision immune treatments that could be used to combat this disease and improve patient outcomes, thereby providing a new avenue for personalized treatment options.

Therapeutic potential of PANoptosis: innate sensors, inflammasomes, and RIPKs in PANoptosomes

The innate immune system initiates cell death pathways in response to pathogens and cellular stress. Cell death can be either non-lytic (apoptosis) or lytic (PANoptosis, pyroptosis, and necroptosis). PANoptosis has been identified as an inflammatory, lytic cell death pathway driven by caspases and RIPKs that is regulated by PANoptosome complexes, making it distinct from other cell death pathways. Several PANoptosome complexes (including ZBP1-, AIM2-, RIPK1-, and NLRP12-PANoptosomes) have been characterized to date. Furthermore, PANoptosis is implicated in infectious and inflammatory diseases, cancers, and homeostatic perturbations. Therefore, targeting its molecular components offers significant potential for therapeutic development. This review covers PANoptosomes and their assembly, PANoptosome-mediated cell death mechanisms, and ongoing progress in developing therapeutics that target PANoptosis.

The role of caspase-8 in inflammatory signalling and pyroptotic cell death

The programmed cell death machinery exhibits surprising flexibility, capable of crosstalk and non-apoptotic roles. Much of this complexity arises from the diverse functions of caspase-8, a cysteine-aspartic acid protease typically associated with activating caspase-3 and - 7 to induce apoptosis. However, recent research has revealed that caspase-8 also plays a role in regulating the lytic gasdermin cell death machinery, contributing to pyroptosis and immune responses in contexts such as infection, autoinflammation, and T-cell signalling. In mice, loss of caspase-8 results in embryonic lethality from unrestrained necroptotic killing, while in humans caspase-8 deficiency can lead to an autoimmune lymphoproliferative syndrome, immunodeficiency, inflammatory bowel disease or, when it can't cleave its substrate RIPK1, early onset periodic fevers. This review focuses on non-canonical caspase-8 signalling that drives immune responses, including its regulation of inflammatory gene transcription, activation within inflammasome complexes, and roles in pyroptotic cell death. Ultimately, a deeper understanding of caspase-8 function will aid in determining whether, and when, targeting caspase-8 pathways could be therapeutically beneficial in human diseases.

Innate immune inflammatory cell death: PANoptosis and PANoptosomes in host defense and disease

Regulated cell death (RCD) triggered by innate immune activation is an important strategy for host survival during pathogen invasion and perturbations of cellular homeostasis. There are two main categories of RCD, including nonlytic and lytic pathways. Apoptosis is the most well-characterized nonlytic RCD, and the inflammatory pyroptosis and necroptosis pathways are among the best known lytic forms. While these were historically viewed as independent RCD pathways, extensive evidence of cross-talk among their molecular components created a knowledge gap in our mechanistic understanding of RCD and innate immune pathway components, which led to the identification of PANoptosis. PANoptosis is a unique innate immune inflammatory RCD pathway that is regulated by PANoptosome complexes upon sensing pathogens, pathogen-associated molecular patterns (PAMPs), damage-associated molecular patterns (DAMPs) or the cytokines produced downstream. Cytosolic innate immune sensors and regulators, such as ZBP1, AIM2 and RIPK1, promote the assembly of PANoptosomes to drive PANoptosis. In this review, we discuss the molecular components of the known PANoptosomes and highlight the mechanisms of PANoptosome assembly, activation and regulation identified to date. We also discuss how PANoptosomes and mutations in PANoptosome components are linked to diseases. Given the impact of RCD, and PANoptosis specifically, across the disease spectrum, improved understanding of PANoptosomes and their regulation will be critical for identifying new therapeutic targets and strategies.

Amygdalin Reverses Macrophage PANoptosis Induced by Drug-Resistant Escherichia coli

Infectious diseases caused by drug-resistant Escherichia coli (E. coli) pose a critical concern for medical institutions as they can lead to high morbidity and mortality rates. In this study, amygdalin exhibited anti-inflammatory and antioxidant activities, as well as other potentials. However, whether it could influence the drug-resistant E. coli-infected cells remained unanswered. Amygdalin was therefore tested in a cellular model in which human macrophages were exposed to resistant E. coli. Apoptosis was measured by flow cytometry and the lactate dehydrogenase (LDH) assay. Western immunoblotting and quantitative reverse-transcription polymerase chain reaction (qRT-PCR) were used to quantify interleukin-18 (IL-18), interleukin-1β (IL-1β), and interleukin-6 (IL-6). The production of reactive oxygen species (ROS) in macrophages was detected by ROS kit. The expression of panapoptotic proteins in macrophages was measured by qRT-PCR and Western immunoblotting. Drug-Resistant E. coli inhibited cell viability and enhanced apoptosis in the cellular model. In cells treated with amygdalin, this compound can inhibit cell apoptosis and reduce the expression of pro - inflammatory cytokines such as IL-1β, IL-18 and IL-6. Additionally, it decreases the production of PANoptosis proteins, Furthermore, amygdalin lowered the levels of reactive oxygen species induced by drug-resistant E. coli, in cells, demonstrating its antioxidant effects. Amygdalin, a drug with a protective role, alleviated cell damage caused by drug-resistant E. coli in human macrophages by inhibiting the PANoptosis signaling pathway.

Immune regulator IRF1 contributes to ZBP1-, AIM2-, RIPK1-, and NLRP12-PANoptosome activation and inflammatory cell death (PANoptosis)

The innate immune system provides the first line of defense against pathogens and cellular insults and is activated by pattern recognition receptors sensing pathogen- or damage-associated molecular patterns. This activation can result in inflammation via cytokine release as well as the induction of lytic regulated cell death (RCD). Innate immune signaling can also induce the expression of interferon regulatory factor 1 (IRF1), an important molecule in regulating downstream inflammation and cell death. While IRF1 has been shown to modulate some RCD pathways, a comprehensive evaluation of its role in inflammatory cell death pathways is lacking. Here, we examined the role of IRF1 in cell death during inflammasome and PANoptosome activation using live cell imaging, Western blotting, and ELISA in primary murine macrophages. IRF1 contributed to the induction of ZBP1- (Z-DNA binding protein 1), AIM2- (absent in melanoma-2), RIPK1- (receptor interacting protein kinase 1), and NLRP12 (NOD-like receptor family, pyrin domain-containing 12)-PANoptosome activation and PANoptosis. Furthermore, IRF1 regulated the cell death under conditions where inflammasomes, along with caspase-8 and RIPK3, act as integral components of PANoptosomes to drive PANoptosis. However, it was dispensable for other inflammasomes that form independent of the PANoptosome to drive pyroptosis. Overall, these findings define IRF1 as an upstream regulator of PANoptosis and suggest that modulating the activation of molecules in the IRF1 pathway could be used as a strategy to treat inflammatory and infectious diseases associated with aberrant inflammatory cell death.

Regulated cell death pathways and their roles in homeostasis, infection, inflammation, and tumorigenesis

Pyroptosis, apoptosis, necroptosis, and ferroptosis, which are the most well-studied regulated cell death (RCD) pathways, contribute to the clearance of infected or potentially neoplastic cells, highlighting their importance in homeostasis, host defense against pathogens, cancer, and a wide range of other pathologies. Although these four RCD pathways employ distinct molecular and cellular processes, emerging genetic and biochemical studies have suggested remarkable flexibility and crosstalk among them. The crosstalk among pyroptosis, apoptosis and necroptosis pathways is more evident in cellular responses to infection, which has led to the conceptualization of PANoptosis. In this review, we provide a brief overview of the molecular mechanisms of pyroptosis, apoptosis, necroptosis, and ferroptosis and their importance in maintaining homeostasis. We discuss the intricate crosstalk among these RCD pathways and the current evidence supporting PANoptosis, focusing on infectious diseases and cancer. Understanding the fundamental processes of various cell death pathways is crucial to inform the development of new therapeutics against many diseases, including infection, sterile inflammation, and cancer.

A Review on Pathophysiology, and Molecular Mechanisms of Bacterial Chondronecrosis and Osteomyelitis in Commercial Broilers

Modern day broilers have a great genetic potential to gain heavy bodyweights with a huge metabolic demand prior to their fully mature ages. Moreover, this made the broilers prone to opportunistic pathogens which may enter the locomotory organs under stress causing bacterial chondronecrosis and osteomyelitis (BCO). Such pathogenic colonization is further accelerated by microfractures and clefts that are formed in the bones due to rapid growth rate of the broilers along with ischemia of blood vessels. Furthermore, there are several pathways which alter bone homeostasis like acute phase response, and intrinsic and extrinsic cell death pathways. In contrast, all the affected birds may not exhibit clinical lameness even with the presence of lameness associated factors causing infection. Although Staphylococcus, E. coli, and Enterococcus are considered as common bacterial pathogens involved in BCO, but there exist several other non-culturable bacteria. Any deviation from maintaining a homeostatic environment in the gut might lead to bacterial translocation through blood followed by proliferation of pathogenic bacteria in respective organs including bones. It is important to alleviate dysbiosis of the blood which is analogous to dysbiosis in the gut. This can be achieved by supplementing pro, pre, and synbiotics which helps in providing a eubiotic environment abating the bacterial translocation that was studied to the incidence of BCO. This review focused on potential and novel biomarkers, pathophysiological mechanism, the economic significance of BCO, immune mechanisms, and miscellaneous factors causing BCO. In addition, the role of gut microbiomes along with their diversity and cell culture models from compact bones of chicken in better understanding of BCO were explored.

Recent advances in ZBP1-derived PANoptosis against viral infections

Innate immunity is an important first line of defense against pathogens, including viruses. These pathogen- and damage-associated molecular patterns (PAMPs and DAMPs, respectively), resulting in the induction of inflammatory cell death, are detected by specific innate immune sensors. Recently, Z-DNA binding protein 1 (ZBP1), also called the DNA-dependent activator of IFN regulatory factor (DAI) or DLM1, is reported to regulate inflammatory cell death as a central mediator during viral infection. ZBP1 is an interferon (IFN)-inducible gene that contains two Z-form nucleic acid-binding domains (Zα1 and Zα2) in the N-terminus and two receptor-interacting protein homotypic interaction motifs (RHIM1 and RHIM2) in the middle, which interact with other proteins with the RHIM domain. By sensing the entry of viral RNA, ZBP1 induces PANoptosis, which protects host cells against viral infections, such as influenza A virus (IAV) and herpes simplex virus (HSV1). However, some viruses, particularly coronaviruses (CoVs), induce PANoptosis to hyperactivate the immune system, leading to cytokine storm, organ failure, tissue damage, and even death. In this review, we discuss the molecular mechanism of ZBP1-derived PANoptosis and pro-inflammatory cytokines that influence the double-edged sword of results in the host cell. Understanding the ZBP1-derived PANoptosis mechanism may be critical for improving therapeutic strategies.

Advances in mechanism and regulation of PANoptosis: Prospects in disease treatment

PANoptosis, a new research hotspot at the moment, is a cell death pattern in which pyroptosis, apoptosis, and necroptosis all occur in the same cell population. In essence, PANoptosis is a highly coordinated and dynamically balanced programmed inflammatory cell death pathway that combines the main features of pyroptosis, apoptosis, and necroptosis. Many variables, such as infection, injury, or self-defect, may be involved in the occurrence of PANoptosis, with the assembly and activation of the PANoptosome being the most critical. PANoptosis has been linked to the development of multiple systemic diseases in the human body, including infectious diseases, cancer, neurodegenerative diseases, and inflammatory diseases. Therefore, it is necessary to clarify the process of occurrence, the regulatory mechanism of PANoptosis, and its relation to diseases. In this paper, we summarized the differences and relations between PANoptosis and the three types of programmed cell death, and emphatically expounded molecular mechanism and regulatory patterns of PANoptosis, with the expectation of facilitating the application of PANoptosis regulation in disease treatment.

Role of the caspase-8/RIPK3 axis in Alzheimer's disease pathogenesis and Aβ-induced NLRP3 inflammasome activation

The molecular mediators of cell death and inflammation in Alzheimer's disease (AD) have yet to be fully elucidated. Caspase-8 is a critical regulator of several cell death and inflammatory pathways; however, its role in AD pathogenesis has not yet been examined in detail. In the absence of caspase-8, mice are embryonic lethal due to excessive receptor interacting protein kinase 3-dependent (RIPK3-dependent) necroptosis. Compound RIPK3 and caspase-8 mutants rescue embryonic lethality, which we leveraged to examine the roles of these pathways in an amyloid β-mediated (Aβ-mediated) mouse model of AD. We found that combined deletion of caspase-8 and RIPK3, but not RIPK3 alone, led to diminished Aβ deposition and microgliosis in the mouse model of AD carrying human presenilin 1 and amyloid precursor protein with 5 familial AD mutations (5xFAD). Despite its well-known role in cell death, caspase-8 did not appear to affect cell loss in the 5xFAD model. In contrast, we found that caspase-8 was a critical regulator of Aβ-driven inflammasome gene expression and IL-1β release. Interestingly, loss of RIPK3 had only a modest effect on disease progression, suggesting that inhibition of necroptosis or RIPK3-mediated cytokine pathways is not critical during midstages of Aβ amyloidosis. These findings suggest that therapeutics targeting caspase-8 may represent a novel strategy to limit Aβ amyloidosis and neuroinflammation in AD.

Inflammasome signaling in colorectal cancer

Colorectal cancer (CRC) is one of the leading causes of cancer-related deaths in the world. Inflammation is often an underlying risk factor for developing CRC. Maintaining gut homeostasis and balancing inflammation is therefore critical to prevent CRC development. One key class of molecular complexes that impact gut homeostasis are inflammasomes, cytosolic multiprotein immune complexes that assemble upon sensing various intracellular alterations. Inflammasomes regulate inflammation, cell death, cytokine release, signaling cascades, and other cellular processes. Roles for inflammasomes in colitis and colitis-associated CRC have been shown in multiple animal models. The activation of inflammasomes leads to the release of the bioactive forms of interleukin (IL)-1β and IL-18, the inflammasome effector cytokines. These cytokines ensure an optimal inflammatory immune response during colitis and colitis-associated CRC. The activation of some inflammasome sensors, including NLRP3, NLRP1, NLRP6, and Pyrin, provides protection from colitis-associated CRC via effector cytokine-dependent mechanisms. Additionally, activation of other inflammasome sensors, such as AIM2, NLRC4, and NAIPs, provides mostly effector cytokine-independent protection. Inflammasomes can also act as integral components of PANoptosomes, which are multifaceted complexes that integrate components from other cell death pathways and regulate a unique form of innate immune inflammatory cell death called PANoptosis. Furthermore, IRF1, a key regulator of some inflammasomes and PANoptosomes, has been implicated in CRC. It is therefore critical to consider the role of inflammasomes in effector cytokine-dependent and -independent protection as well as their role in PANoptosis to modulate CRC for therapeutic targeting. Here, we discuss the mechanisms of inflammasome activation, the functions of inflammasomes in CRC, and current obstacles and future perspectives in inflammasome and CRC research.

Evaluation of Caspase Activation to Assess Innate Immune Cell Death

Innate immunity provides the critical first line of defense in response to pathogens and sterile insults. A key mechanistic component of this response is the initiation of innate immune programmed cell death (PCD) to eliminate infected or damaged cells and propagate immune responses. However, excess PCD is associated with inflammation and pathology. Therefore, understanding the activation and regulation of PCD is a central aspect of characterizing innate immune responses and identifying new therapeutic targets across the disease spectrum. This protocol provides methods for characterizing innate immune PCD activation by monitoring caspases, a family of cysteine-dependent proteases that are often associated with diverse PCD pathways, including apoptosis, pyroptosis, necroptosis, and PANoptosis. Initial reports characterized caspase-2, caspase-8, caspase-9, and caspase-10 as initiator caspases and caspase-3, caspase-6, and caspase-7 as effector caspases in apoptosis, while later studies found the inflammatory caspases, caspase-1, caspase-4, caspase-5, and caspase-11, drive pyroptosis. It is now known that there is extensive crosstalk between the caspases and other innate immune and cell death molecules across the previously defined PCD pathways, identifying a key knowledge gap in the mechanistic understanding of innate immunity and PCD and leading to the characterization of PANoptosis. PANoptosis is a unique innate immune inflammatory PCD pathway regulated by PANoptosome complexes, which integrate components, including caspases, from other cell death pathways. Here, methods for assessing the activation of caspases in response to various stimuli are provided. These methods allow for the characterization of PCD pathways both in vitro and in vivo, as activated caspases undergo proteolytic cleavage that can be visualized by western blotting using optimal antibodies and blotting conditions. A protocol and western blotting workflow have been established that allow for the assessment of the activation of multiple caspases from the same cellular population, providing a comprehensive characterization of the PCD processes. This method can be applied across research areas in development, homeostasis, infection, inflammation, and cancer to evaluate PCD pathways throughout cellular processes in health and disease.

Molecular interactions of adaptor protein PSTPIP2 control neutrophil-mediated responses leading to autoinflammation

Introduction

Autoinflammatory diseases are characterized by dysregulation of innate immune system leading to spontaneous sterile inflammation. One of the well-established animal models of this group of disorders is the mouse strain Pstpip2^cmo . In this strain, the loss of adaptor protein PSTPIP2 leads to the autoinflammatory disease chronic multifocal osteomyelitis. It is manifested by sterile inflammation of the bones and surrounding soft tissues of the hind limbs and tail. The disease development is propelled by elevated production of IL-1β and reactive oxygen species by neutrophil granulocytes. However, the molecular mechanisms linking PSTPIP2 and these pathways have not been established. Candidate proteins potentially involved in these mechanisms include PSTPIP2 binding partners, PEST family phosphatases (PEST-PTPs) and phosphoinositide phosphatase SHIP1.

Methods

To address the role of these proteins in PSTPIP2-mediated control of inflammation, we have generated mouse strains in which PEST-PTP or SHIP1 binding sites in PSTPIP2 have been disrupted. In these mouse strains, we followed disease symptoms and various inflammation markers.

Results

Our data show that mutation of the PEST-PTP binding site causes symptomatic disease, whereas mice lacking the SHIP1 interaction site remain asymptomatic. Importantly, both binding partners of PSTPIP2 contribute equally to the control of IL-1β production, while PEST-PTPs have a dominant role in the regulation of reactive oxygen species. In addition, the interaction of PEST-PTPs with PSTPIP2 regulates the production of the chemokine CXCL2 by neutrophils. Its secretion likely creates a positive feedback loop that drives neutrophil recruitment to the affected tissues.

Conclusions

We demonstrate that PSTPIP2-bound PEST-PTPs and SHIP1 together control the IL-1β pathway. In addition, PEST-PTPs have unique roles in the control of reactive oxygen species and chemokine production, which in the absence of PEST-PTP binding to PSTPIP2 shift the balance towards symptomatic disease.

Determining distinct roles of IL-1α through generation of an IL-1α knockout mouse with no defect in IL-1β expression

Interleukin 1α (IL-1α) and IL-1β are the founding members of the IL-1 cytokine family, and these innate immune inflammatory mediators are critically important in health and disease. Early studies on these molecules suggested that their expression was interdependent, with an initial genetic model of IL-1α depletion, the IL-1α KO mouse (Il1a-KOline1), showing reduced IL-1β expression. However, studies using this line in models of infection and inflammation resulted in contrasting observations. To overcome the limitations of this genetic model, we have generated and characterized a new line of IL-1α KO mice (Il1a-KOline2) using CRISPR-Cas9 technology. In contrast to cells from Il1a-KOline1, where IL-1β expression was drastically reduced, bone marrow-derived macrophages (BMDMs) from Il1a-KOline2 mice showed normal induction and activation of IL-1β. Additionally, Il1a-KOline2 BMDMs showed normal inflammasome activation and IL-1β expression in response to multiple innate immune triggers, including both pathogen-associated molecular patterns and pathogens. Moreover, using Il1a-KOline2 cells, we confirmed that IL-1α, independent of IL-1β, is critical for the expression of the neutrophil chemoattractant KC/CXCL1. Overall, we report the generation of a new line of IL-1α KO mice and confirm functions for IL-1α independent of IL-1β. Future studies on the unique functions of IL-1α and IL-1β using these mice will be critical to identify new roles for these molecules in health and disease and develop therapeutic strategies.

PANoptosis: A Unique Innate Immune Inflammatory Cell Death Modality

Innate immunity is the first response to protect against pathogens and cellular insults. Pattern recognition receptors sense pathogen- and damage-associated molecular patterns and induce an innate immune response characterized by inflammation and programmed cell death (PCD). In-depth characterization of innate immune PCD pathways has highlighted significant cross-talk. Recent advances led to the identification of a unique inflammatory PCD modality called PANoptosis, which is regulated by multifaceted PANoptosome complexes that are assembled by integrating components from other PCD pathways. The totality of biological effects observed in PANoptosis cannot be accounted for by any other PCD pathway alone. In this review, we briefly describe mechanisms of innate immune cell death, including molecular mechanisms of PANoptosis activation and regulation. We also highlight the PANoptosomes identified to date and provide an overview of the implications of PANoptosis in disease and therapeutic targeting. Improved understanding of innate immune-mediated cell death, PANoptosis, is critical to inform the next generation of treatment strategies.

ZBP1-dependent inflammatory cell death, PANoptosis, and cytokine storm disrupt IFN therapeutic efficacy during coronavirus infection

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the virus responsible for coronavirus disease 2019 (COVID-19), continues to cause substantial morbidity and mortality in the ongoing global pandemic. Understanding the fundamental mechanisms that govern innate immune and inflammatory responses during SARS-CoV-2 infection is critical for developing effective therapeutic strategies. Whereas interferon (IFN)-based therapies are generally expected to be beneficial during viral infection, clinical trials in COVID-19 have shown limited efficacy and potential detrimental effects of IFN treatment during SARS-CoV-2 infection. However, the underlying mechanisms responsible for this failure remain unknown. In this study, we found that IFN induced Z-DNA-binding protein 1 (ZBP1)-mediated inflammatory cell death, PANoptosis, in human and murine macrophages and in the lungs of mice infected with β-coronaviruses, including SARS-CoV-2 and mouse hepatitis virus (MHV). In patients with COVID-19, expression of the innate immune sensor ZBP1 was increased in immune cells from those who succumbed to the disease compared with those who recovered, further suggesting a link between ZBP1 and pathology. In mice, IFN-β treatment after β-coronavirus infection increased lethality, and genetic deletion of Zbp1 or its Zα domain suppressed cell death and protected the mice from IFN-mediated lethality during β-coronavirus infection. Overall, our results identify that ZBP1 induced during coronavirus infection limits the efficacy of IFN therapy by driving inflammatory cell death and lethality. Therefore, inhibiting ZBP1 activity may improve the efficacy of IFN therapy, paving the way for the development of new and critically needed therapeutics for COVID-19 as well as other infections and inflammatory conditions where IFN-mediated cell death and pathology occur.

Do pyroptosis, apoptosis, and necroptosis (PANoptosis) exist in cerebral ischemia? Evidence from cell and rodent studies

Some scholars have recently developed the concept of PANoptosis in the study of infectious diseases where pyroptosis, apoptosis and necroptosis act in consort in a multimeric protein complex, PANoptosome. This allows all the components of PANoptosis to be regulated simultaneously. PANoptosis provides a new way to study the regulation of cell death, in that different types of cell death may be regulated at the same time. To test whether PANoptosis exists in diseases other than infectious diseases, we chose cerebral ischemia/reperfusion injury as the research model, collected articles researching cerebral ischemia/reperfusion from three major databases, obtained the original research data from these articles by bibliometrics, data mining and other methods, then integrated and analyzed these data. We selected papers that investigated at least two of the components of PANoptosis to check its occurrence in ischemia/reperfusion. In the cell model simulating ischemic brain injury, pyroptosis, apoptosis and necroptosis occur together and this phenomenon exists widely in different passage cell lines or primary neurons. Pyroptosis, apoptosis and necroptosis also occurred in rat and mouse models of ischemia/reperfusion injury. This confirms that PANoptosis is observed in ischemic brain injury and indicates that PANoptosis can be a target in the regulation of various central nervous system diseases.

Innate Immune Cell Death in Neuroinflammation and Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder molecularly characterized by the formation of amyloid β (Aβ) plaques and type 2 microtubule-associated protein (Tau) abnormalities. Multiple studies have shown that many of the brain's immunological cells, specifically microglia and astrocytes, are involved in AD pathogenesis. Cells of the innate immune system play an essential role in eliminating pathogens but also regulate brain homeostasis and AD. When activated, innate immune cells can cause programmed cell death through multiple pathways, including pyroptosis, apoptosis, necroptosis, and PANoptosis. The cell death often results in the release of proinflammatory cytokines that propagate the innate immune response and can eliminate Aβ plaques and aggregated Tau proteins. However, chronic neuroinflammation, which can result from cell death, has been linked to neurodegenerative diseases and can worsen AD. Therefore, the innate immune response must be tightly balanced to appropriately clear these AD-related structural abnormalities without inducing chronic neuroinflammation. In this review, we discuss neuroinflammation, innate immune responses, inflammatory cell death pathways, and cytokine secretion as they relate to AD. Therapeutic strategies targeting these innate immune cell death mechanisms will be critical to consider for future preventive or palliative treatments for AD.

DEAD/H-Box Helicases in Immunity, Inflammation, Cell Differentiation, and Cell Death and Disease

DEAD/H-box proteins are the largest family of RNA helicases in mammalian genomes, and they are present in all kingdoms of life. Since their discovery in the late 1980s, DEAD/H-box family proteins have been a major focus of study. They have been found to play central roles in RNA metabolism, gene expression, signal transduction, programmed cell death, and the immune response to bacterial and viral infections. Aberrant functions of DEAD/H-box proteins have been implicated in a wide range of human diseases that include cancer, neurodegeneration, and inherited genetic disorders. In this review, we provide a historical context and discuss the molecular functions of DEAD/H-box proteins, highlighting the recent discoveries linking their dysregulation to human diseases. We will also discuss the state of knowledge regarding two specific DEAD/H-box proteins that have critical roles in immune responses and programmed cell death, DDX3X and DDX58, also known as RIG-I. Given their importance in homeostasis and disease, an improved understanding of DEAD/H-box protein biology and protein-protein interactions will be critical for informing strategies to counteract the pathogenesis associated with several human diseases.

It’s All in the PAN: Crosstalk, Plasticity, Redundancies, Switches, and Interconnectedness Encompassed by PANoptosis Underlying the Totality of Cell Death-Associated Biological Effects

The innate immune system provides the first line of defense against cellular perturbations. Innate immune activation elicits inflammatory programmed cell death in response to microbial infections or alterations in cellular homeostasis. Among the most well-characterized programmed cell death pathways are pyroptosis, apoptosis, and necroptosis. While these pathways have historically been defined as segregated and independent processes, mounting evidence shows significant crosstalk among them. These molecular interactions have been described as ‘crosstalk’, ‘plasticity’, ‘redundancies’, ‘molecular switches’, and more. Here, we discuss the key components of cell death pathways and note several examples of crosstalk. We then explain how the diverse descriptions of crosstalk throughout the literature can be interpreted through the lens of an integrated inflammatory cell death concept, PANoptosis. The totality of biological effects in PANoptosis cannot be individually accounted for by pyroptosis, apoptosis, or necroptosis alone. We also discuss PANoptosomes, which are multifaceted macromolecular complexes that regulate PANoptosis. We consider the evidence for PANoptosis, which has been mechanistically characterized during influenza A virus, herpes simplex virus 1, Francisella novicida, and Yersinia infections, as well as in response to altered cellular homeostasis, in inflammatory diseases, and in cancers. We further discuss the role of IRF1 as an upstream regulator of PANoptosis and conclude by reexamining historical studies which lend credence to the PANoptosis concept. Cell death has been shown to play a critical role in infections, inflammatory diseases, neurodegenerative diseases, cancers, and more; therefore, having a holistic understanding of cell death is important for identifying new therapeutic strategies.

Programming inflammatory cell death for therapy

Programmed cell death (PCD) is an essential part of organismal development and plays fundamental roles in host defense against pathogens and the maintenance of homeostasis. However, excess activation of PCD pathways has proven to be detrimental and can drive disease. Additionally, resistance to PCD can also contribute to disease development. Modulation of PCD, therefore, has great therapeutic potential in a wide range of diseases, including infectious, neurodegenerative, autoinflammatory, and metabolic diseases and cancer. Nevertheless, manipulation of cell death and inflammation for therapeutic intervention is a delicate process, highly specific to the context of the disease of interest, making the selection of the appropriate target molecule crucially important. Several PCD pathways are associated with innate immunity, including pyroptosis, apoptosis, necroptosis, and PANoptosis, which is defined as an inflammatory PCD pathway with key features of pyroptosis, apoptosis, and/or necroptosis that cannot be accounted for by any of these three PCD pathways alone. All of these PCD pathways are regulated by upstream sensors and signaling cascades that assemble multimeric complexes to serve as activation platforms for downstream molecules; these sensors and signaling molecules provide attractive target points for therapeutic intervention. Here, we discuss the molecular mechanisms of innate immune-mediated cell death in health and disease, with a particular focus on the molecules putatively involved in the formation of the PANoptosome and the induction of inflammatory cell death. Further, we discuss the implications and feasibility of targeting these molecules to improve disease outcomes, as well as current clinical approaches.

PANoptosis in Viral Infection: The Missing Puzzle Piece in the Cell Death Field

In the past decade, emerging viral outbreaks like SARS-CoV-2, Zika and Ebola have presented major challenges to the global health system. Viruses are unique pathogens in that they fully rely on the host cell to complete their lifecycle and potentiate disease. Therefore, programmed cell death (PCD), a key component of the host innate immune response, is an effective strategy for the host cell to curb viral spread. The most well-established PCD pathways, pyroptosis, apoptosis and necroptosis, can be activated in response to viruses. Recently, extensive crosstalk between PCD pathways has been identified, and there is evidence that molecules from all three PCD pathways can be activated during virus infection. These findings have led to the emergence of the concept of PANoptosis, defined as an inflammatory PCD pathway regulated by the PANoptosome complex with key features of pyroptosis, apoptosis, and/or necroptosis that cannot be accounted for by any of these three PCD pathways alone. While PCD is important to eliminate infected cells, many viruses are equipped to hijack host PCD pathways to benefit their own propagation and subvert host defense, and PCD can also lead to the production of inflammatory cytokines and inflammation. Therefore, PANoptosis induced by viral infection contributes to either host defense or viral pathogenesis in context-specific ways. In this review, we will discuss the multi-faceted roles of PCD pathways in controlling viral infections.

The Transcription Factor IRF9 Promotes Colorectal Cancer via Modulating the IL-6/STAT3 Signaling Axis

Colorectal cancer (CRC) is a leading cause of cancer-related deaths worldwide, and innate immune responses and inflammation are known to affect the course of disease. Interferon (IFN) signaling in particular is critical for modulating inflammation-associated diseases including CRC. While the effects of IFN signaling in CRC have been studied, results have been conflicting. Furthermore, individual molecules in the IFN pathway that could be therapeutically targeted have distinct functions, with many of their diverse roles in CRC remaining unclear. Here, we found that IRF9 had an oncogenic effect in CRC; loss of IRF9 reduced tumorigenesis in both azoxymethane (AOM)/dextran sodium sulfate (DSS)-induced and spontaneous CRC models. IRF9 also reduced DSS-induced colitis and inflammation in the colon, but it had no effect on the NF-κB and MAPK signaling activation. Instead, IRF9 enhanced the transcription and production of the inflammatory cytokine IL-6. By promoting IL-6 release, IRF9 drove the activation of pro-oncogenic STAT3 signaling in the colon. Overall, our study found that IRF9 promoted the development of CRC via modulation of the IL-6/STAT3 signaling axis, identifying multiple potential targets and suggesting new therapeutic strategies for the treatment of CRC.

The IFN-inducible GTPase IRGB10 regulates viral replication and inflammasome activation during influenza A virus infection in mice

The upregulation of interferon (IFN)-inducible GTPases in response to pathogenic insults is vital to host defense against many bacterial, fungal, and viral pathogens. Several IFN-inducible GTPases play key roles in mediating inflammasome activation and providing host protection after bacterial or fungal infections, though their role in inflammasome activation after viral infection is less clear. Among the IFN-inducible GTPases, the expression of immunity-related GTPases (IRGs) varies widely across species for unknown reasons. Here, we report that IRGB10, but not IRGM1, IRGM2, or IRGM3, is required for NLRP3 inflammasome activation in response to influenza A virus (IAV) infection in mice. While IRGB10 functions to release inflammasome ligands in the context of bacterial and fungal infections, we found that IRGB10 facilitates endosomal maturation and nuclear translocation of IAV, thereby regulating viral replication. Corresponding with our in vitro results, we found that Irgb10^-/- mice were more resistant to IAV-induced mortality than WT mice. The results of our study demonstrate a detrimental role of IRGB10 in host immunity in response to IAV and a novel function of IRGB10, but not IRGMs, in promoting viral translocation into the nucleus.