Hierarchical cell-type-specific functions of caspase-11 in LPS shock and antibacterial host defense

Caspase-11 drives macrophage hyperinflammation in models of Polg-related mitochondrial disease

Mitochondrial diseases (MtD) represent a significant public health challenge due to their heterogenous clinical presentation, often severe and progressive symptoms, and lack of effective therapies. Environmental exposures, such bacterial and viral infection, can further compromise mitochondrial function and exacerbate the progression of MtD. However, the underlying immune alterations that enhance immunopathology in MtD remain unclear. Here we employ in vitro and in vivo approaches to clarify the molecular and cellular basis for innate immune hyperactivity in models of polymerase gamma (Polg)-related MtD. We reveal that type I interferon (IFN-I)-mediated upregulation of caspase-11 and guanylate-binding proteins (GBP) increase macrophage sensing of the opportunistic microbe Pseudomonas aeruginosa (PA) in Polg mutant mice. Furthermore, we show that excessive cytokine secretion and activation of pyroptotic cell death pathways contribute to lung inflammation and morbidity after infection with PA. Our work provides a mechanistic framework for understanding innate immune dysregulation in MtD and reveals potential targets for limiting infection- and inflammation-related complications in Polg-related MtD.

The immune response to Francisella tularensis

Francisella tularensis (Ft) is a Gram negative intracellular bacterial pathogen, commonly transmitted via arthropod bites, but is most lethal when contracted via inhalation. The nature of a Gram-negative intracellular pathogen presents unique challenges to the mammalian immune response, unlike more common viral pathogens and extracellular bacterial pathogens. The current literature on Ft involves numerous variables, including the use of differing research strains and variation in animal models. This review aims to consolidate much of the recent literature on Ft to suggest promising research to better understand the complex immune response to this bacterium.

Caspase-11 drives macrophage hyperinflammation in models of Polg-related mitochondrial disease

Mitochondrial diseases (MtD) represent a significant public health challenge due to their heterogenous clinical presentation, often severe and progressive symptoms, and lack of effective therapies. Environmental exposures, such bacterial and viral infection, can further compromise mitochondrial function and exacerbate the progression of MtD. Infections in MtD patients more frequently progress to sepsis, pneumonia, and other detrimental inflammatory endpoints. However, the underlying immune alterations that enhance immunopathology in MtD remain unclear, constituting a key gap in knowledge that complicates treatment and increases mortality in this vulnerable population. Here we employ in vitro and in vivo approaches to clarify the molecular and cellular basis for innate immune hyperactivity in models of polymerase gamma (Polg)-related MtD. We reveal that type I interferon (IFN-I)-mediated upregulation of caspase-11 and guanylate-binding proteins (GBPs) increase macrophage sensing of the opportunistic microbe Pseudomonas aeruginosa (PA) in Polg mutant mice. Furthermore, we show that excessive cytokine secretion and activation of pyroptotic cell death pathways contribute to lung inflammation and morbidity after infection with PA. Our work sheds new light on innate immune dysregulation in MtD and reveals potential targets for limiting infection- and inflammation-related complications in Polg-related MtD.

The intricate interactions between inflammasomes and bacterial pathogens: Roles, mechanisms, and therapeutic potentials

Inflammasomes are intracellular multiprotein complexes that consist of a sensor, an adaptor, and a caspase enzyme to cleave interleukin (IL)-1β and IL-18 into their mature forms. In addition, caspase-1 and -11 activation results in the cleavage of gasdermin D to form pores, thereby inducing pyroptosis. Activation of the inflammasome and pyroptosis promotes host defense against pathogens, whereas dysregulation of the inflammasome can result in various pathologies. Inflammasomes exhibit versatile microbial signal detection, directly or indirectly, through cellular processes, such as ion fluctuations, reactive oxygen species generation, and the disruption of intracellular organelle function; however, bacteria have adaptive strategies to manipulate the inflammasome by altering microbe-associated molecular patterns, intercepting innate pathways with secreted effectors, and attenuating inflammatory and cell death responses. In this review, we summarize recent advances in the diverse roles of the inflammasome during bacterial infections and discuss how bacteria exploit inflammasome pathways to establish infections or persistence. In addition, we highlight the therapeutic potential of harnessing bacterial immune subversion strategies against acute and chronic bacterial infections. A more comprehensive understanding of the significance of inflammasomes in immunity and their intricate roles in the battle between bacterial pathogens and hosts will lead to the development of innovative strategies to address emerging threats posed by the expansion of drug-resistant bacterial infections.

The promotive effect of Caspase-11 overexpression in a rat model of chronic kidney disease and the therapeutic efficacy of exosome-delivered siRNA in inhibiting Caspase-11

This study investigates the role of Caspase-11 in Chronic Kidney Disease (CKD) and examines the therapeutic potential of inhibiting Caspase-11 using exosome-mediated siRNA. We established a CKD rat model and analyzed the expression of Caspase-11 through immunohistochemistry. The study involved overexpressing Caspase-11 using an adeno-associated virus (AAV) and constructing exosomes loaded with siRNA targeting Caspase-11 (exo-si-Caspase-11). Renal tissue damage and fibrosis were assessed using H&E staining, Masson's trichrome, TUNEL assay, and Sirius Red staining. Additionally, urinary protein and blood urea nitrogen (BUN) levels were measured, alongside analyses of serum calcium and phosphorus levels. H&E staining was performed to evaluate the effects of exo-si-Caspase-11 on damage to the heart, liver, spleen, and lungs. The results showed that the CKD model group experienced significant weight loss, increased blood pressure, and elevated Caspase-11 expression. AAV-mediated Caspase-11 overexpression led to substantial renal fibrosis, increased apoptosis, and elevated urinary protein and BUN levels. Additionally, the group with Caspase-11 overexpression exhibited elevated serum calcium and phosphorus levels. Conversely, treatment with exo-si-Caspase-11 reduced these pathological changes in renal tissue without causing damage to other major organs. These findings suggest that exosome-mediated siRNA delivery targeting Caspase-11 is an effective therapeutic strategy for CKD.

Cecal necroptosis triggers lethal cardiac dysfunction in TNF-induced severe SIRS

Tumor necrosis factor (TNF) induces systemic inflammatory response syndrome (SIRS), and severe SIRS can serve as a model for studying animal death caused by organ failure. Through strategic cecectomy, we demonstrate that necroptosis in the cecum initiates the death process in TNF-treated mice, but it is not the direct cause of death. Instead, we show that it is the cardiac dysfunction downstream of cecum damage that ultimately leads to the death of TNF-treated mice. By in vivo and ex vivo physiological analyses, we reveal that TNF and the damage-associated molecular patterns (DAMPs) released from necroptotic cecal cells jointly target cardiac endothelial cells, triggering caspase-8 activation and subsequent cardiac endothelial damage. Cardiac endothelial damage is a primary cause of the deterioration of diastolic function in the heart of TNF-treated mice. Our research provides insights into the pathophysiological process of TNF-induced lethality.

Dysregulated dendritic cells in sepsis: functional impairment and regulated cell death

Sepsis is defined as life-threatening organ dysfunction caused by a dysregulated host response to infection. Studies have indicated that immune dysfunction plays a central role in the pathogenesis of sepsis. Dendritic cells (DCs) play a crucial role in the emergence of immune dysfunction in sepsis. The major manifestations of DCs in the septic state are abnormal functions and depletion in numbers, which are linked to higher mortality and vulnerability to secondary infections in sepsis. Apoptosis is the most widely studied pathway of number reduction in DCs. In the past few years, there has been a surge in studies focusing on regulated cell death (RCD). This emerging field encompasses various forms of cell death, such as necroptosis, pyroptosis, ferroptosis, and autophagy-dependent cell death (ADCD). Regulation of DC's RCD can serve as a possible therapeutic focus for the treatment of sepsis. Throughout time, numerous tactics have been devised and effectively implemented to improve abnormal immune response during sepsis progression, including modifying the functions of DCs and inhibiting DC cell death. In this review, we provide an overview of the functional impairment and RCD of DCs in septic states. Also, we highlight recent advances in targeting DCs to regulate host immune response following septic challenge.

Brain endothelial GSDMD activation mediates inflammatory BBB breakdown

The blood-brain barrier (BBB) protects the central nervous system from infections or harmful substances1; its impairment can lead to or exacerbate various diseases of the central nervous system2-4. However, the mechanisms of BBB disruption during infection and inflammatory conditions5,6 remain poorly defined. Here we find that activation of the pore-forming protein GSDMD by the cytosolic lipopolysaccharide (LPS) sensor caspase-11 (refs. 7-9), but not by TLR4-induced cytokines, mediates BBB breakdown in response to circulating LPS or during LPS-induced sepsis. Mice deficient in the LBP-CD14 LPS transfer and internalization pathway10-12 resist BBB disruption. Single-cell RNA-sequencing analysis reveals that brain endothelial cells (bECs), which express high levels of GSDMD, have a prominent response to circulating LPS. LPS acting on bECs primes Casp11 and Cd14 expression and induces GSDMD-mediated plasma membrane permeabilization and pyroptosis in vitro and in mice. Electron microscopy shows that this features ultrastructural changes in the disrupted BBB, including pyroptotic endothelia, abnormal appearance of tight junctions and vasculature detachment from the basement membrane. Comprehensive mouse genetic analyses, combined with a bEC-targeting adeno-associated virus system, establish that GSDMD activation in bECs underlies BBB disruption by LPS. Delivery of active GSDMD into bECs bypasses LPS stimulation and opens the BBB. In CASP4-humanized mice, Gram-negative Klebsiella pneumoniae infection disrupts the BBB; this is blocked by expression of a GSDMD-neutralizing nanobody in bECs. Our findings outline a mechanism for inflammatory BBB breakdown, and suggest potential therapies for diseases of the central nervous system associated with BBB impairment.

Caspase-5: Structure, Pro-Inflammatory Activity and Evolution

Caspase-5 is a protease that induces inflammation in response to lipopolysaccharide (LPS), a component of the cell envelope of Gram-negative bacteria. The expression level of the CASP5 gene is very low in the basal state, but strongly increases in the presence of LPS. Intracellular LPS binds to the caspase activation and recruitment domain (CARD) of caspase-5, leading to the formation of a non-canonical inflammasome. Subsequently, the catalytic domain of caspase-5 cleaves gasdermin D and thereby facilitates the formation of cell membrane pores through which pro-inflammatory cytokines of the interleukin-1 family are released. Caspase-4 is also able to form a non-canonical inflammasome upon binding to LPS, but its expression is less dependent on LPS than the expression of caspase-5. Caspase-4 and caspase-5 have evolved via the duplication of a single ancestral gene in a subclade of primates, including humans. Notably, the main biomedical model species, the mouse, has only one ortholog, namely caspase-11. Here, we review the structural features and the mechanisms of regulation that are important for the pro-inflammatory roles of caspase-5. We summarize the interspecies differences and the evolution of pro-inflammatory caspases in mammals and discuss the potential roles of caspase-5 in the defense against Gram-negative bacteria and in sepsis.

Host extracellular vesicles confer cytosolic access to systemic LPS licensing non-canonical inflammasome sensing and pyroptosis

Intracellular surveillance for systemic microbial components during homeostasis and infections governs host physiology and immunity. However, a long-standing question is how circulating microbial ligands become accessible to intracellular receptors. Here we show a role for host-derived extracellular vesicles (EVs) in this process; human and murine plasma-derived and cell culture-derived EVs have an intrinsic capacity to bind bacterial lipopolysaccharide (LPS). Remarkably, circulating host EVs capture blood-borne LPS in vivo, and the LPS-laden EVs confer cytosolic access for LPS, triggering non-canonical inflammasome activation of gasdermin D and pyroptosis. Mechanistically, the interaction between the lipid bilayer of EVs and the lipid A of LPS underlies EV capture of LPS, and the intracellular transfer of LPS by EVs is mediated by CD14. Overall, this study demonstrates that EVs capture and escort systemic LPS to the cytosol licensing inflammasome responses, uncovering EVs as a previously unrecognized link between systemic microbial ligands and intracellular surveillance.

The pathogenesis and potential therapeutic targets in sepsis

Sepsis is defined as "a life-threatening organ dysfunction caused by dysregulated host systemic inflammatory and immune response to infection." At present, sepsis continues to pose a grave healthcare concern worldwide. Despite the use of supportive measures in treating traditional sepsis, such as intravenous fluids, vasoactive substances, and oxygen plus antibiotics to eradicate harmful pathogens, there is an ongoing increase in both the morbidity and mortality associated with sepsis during clinical interventions. Therefore, it is urgent to design specific pharmacologic agents for the treatment of sepsis and convert them into a novel targeted treatment strategy. Herein, we provide an overview of the molecular mechanisms that may be involved in sepsis, such as the inflammatory response, immune dysfunction, complement deactivation, mitochondrial damage, and endoplasmic reticulum stress. Additionally, we highlight important targets involved in sepsis-related regulatory mechanisms, including GSDMD, HMGB1, STING, and SQSTM1, among others. We summarize the latest advancements in potential therapeutic drugs that specifically target these signaling pathways and paramount targets, covering both preclinical studies and clinical trials. In addition, this review provides a detailed description of the crosstalk and function between signaling pathways and vital targets, which provides more opportunities for the clinical development of new treatments for sepsis.

Pyroptosis-induced inflammation and tissue damage

Pyroptosis is a programmed necrotic cell death executed by gasdermins, a family of pore-forming proteins. The cleavage of gasdermins by specific proteases enables their pore-forming activity. The activation of the prototype member of the gasdermin family, gasdermin D (GSDMD), is linked to innate immune monitoring by inflammasomes. Additional gasdermins such as GSDMA, GSDMB, GSDMC, and GSDME are activated by inflammasome-independent mechanisms. Pyroptosis is emerging as a key host defense strategy against pathogens. However, excessive pyroptosis causes cytokine storm and detrimental inflammation leading to tissue damage and organ dysfunction. Consequently, dysregulated pyroptotic responses contribute to the pathogenesis of various diseases, including sepsis, atherosclerosis, acute respiratory distress syndrome, and neurodegenerative disorders. This review will discuss the inflammatory consequences of pyroptosis and the mechanisms of pyroptosis-induced tissue damage and disease pathogenesis.

Gasdermin D inhibition ameliorates neutrophil mediated brain damage in acute ischemic stroke

Acute ischemic stroke (AIS) induces high level of neutrophils, which correlates inversely with patient survival. Pyroptosis induced by gasdermin D (GSDMD) has been shown to have an important role in the pathophysiology of several inflammatory disorders. The role of GSDMD in the high level of neutrophils after AIS is unknown. Using a middle cerebral artery occlusion (MCAO) mouse model, we identified activation of pyroptosis signal, including expression of caspase-1/11, GSDMD, and interleukin-1β/18 (IL-1β/18), in the brain and spleen at early ischemic injury. Knockout of GSDMD in mice reduced infarct size, improved neurological function, and increased survival after MCAO. GSDMD deficiency decreased the overall degree of inflammation and the proportion of neutrophils in the brain after MCAO. Quantitative studies of neutrophils at several time intervals and organs demonstrated that early inflammatory leucocyte production and supplement (1 day after MCAO) was GSDMD-dependent. A series of bone marrow transplantation experiments, neutrophil depletion experiments, and RNA sequencing results demonstrated that neutrophil specific GSDMD is essential for the production and supply of neutrophil in bone marrow to blood. Moreover, pharmacological suppression of GSDMD decreased pathological abnormalities, infarct volume, and ameliorated neurological function. These results provided a new viewpoint on the immunological modulation of neutrophils after MCAO and suggest that suppression of GSDMD may relieve the neuroinflammatory load, thereby providing a potential treatment strategy for stroke. The absence of GSDMD reduces the high level of neutrophils in the brain, the production of neutrophils in bone marrow, and the supply of blood and spleen, while simultaneously the neutrophil-specific GSDMD signal deficiency restrains leukocytosis to improve the pathological outcome of AIS.

Uncoupled pyroptosis and IL-1β secretion downstream of inflammasome signaling

Inflammasomes are supramolecular platforms that organize in response to various damage-associated molecular patterns and pathogen-associated molecular patterns. Upon activation, inflammasome sensors (with or without the help of ASC) activate caspase-1 and other inflammatory caspases that cleave gasdermin D and pro-IL-1β/pro-IL-18, leading to pyroptosis and mature cytokine secretion. Pyroptosis enables intracellular pathogen niche disruption and intracellular content release at the cost of cell death, inducing pro-inflammatory responses in the neighboring cells. IL-1β is a potent pro-inflammatory regulator for neutrophil recruitment, macrophage activation, and T-cell expansion. Thus, pyroptosis and cytokine secretion are the two main mechanisms that occur downstream of inflammasome signaling; they maintain homeostasis, drive the innate immune response, and shape adaptive immunity. This review aims to discuss the possible mechanisms, timing, consequences, and significance of the two uncoupling preferences downstream of inflammasome signaling. While pyroptosis and cytokine secretion may be usually coupled, pyroptosis-predominant and cytokine-predominant uncoupling are also observed in a stimulus-, cell type-, or context-dependent manner, contributing to the pathogenesis and development of numerous pathological conditions such as cryopyrin-associated periodic syndromes, LPS-induced sepsis, and Salmonella enterica serovar Typhimurium infection. Hyperactive cells consistently release IL-1β without LDH leakage and pyroptotic death, thereby leading to prolonged inflammation, expanding the lifespans of pyroptosis-resistant neutrophils, and hyperactivating stimuli-challenged macrophages, dendritic cells, monocytes, and specific nonimmune cells. Death inflammasome activation also induces GSDMD-mediated pyroptosis with no IL-1β secretion, which may increase lethality in vivo. The sublytic GSDMD pore formation associated with lower expressions of pyroptotic components, GSDMD-mediated extracellular vesicles, or other GSDMD-independent pathways that involve unconventional secretion could contribute to the cytokine-predominant uncoupling; the regulation of caspase-1 dynamics, which may generate various active species with different activities in terms of GSDMD or pro-IL-1β, could lead to pyroptosis-predominant uncoupling. These uncoupling preferences enable precise reactions to different stimuli of different intensities under specific conditions at the single-cell level, promoting cooperative cell and host fate decisions and participating in the pathogen "game". Appropriate decisions in terms of coupling and uncoupling are required to heal tissues and eliminate threats, and further studies exploring the inflammasome tilt toward pyroptosis or cytokine secretion may be helpful.

Dysregulation of neutrophil death in sepsis

Sepsis is a prevalent disease that has alarmingly high mortality rates and, for several survivors, long-term morbidity. The modern definition of sepsis is an aberrant host response to infection followed by a life-threatening organ dysfunction. Sepsis has a complicated pathophysiology and involves multiple immune and non-immune mediators. It is now believed that in the initial stages of sepsis, excessive immune system activation and cascading inflammation are usually accompanied by immunosuppression. During the pathophysiology of severe sepsis, neutrophils are crucial. Recent researches have demonstrated a clear link between the process of neutrophil cell death and the emergence of organ dysfunction in sepsis. During sepsis, spontaneous apoptosis of neutrophils is inhibited and neutrophils may undergo some other types of cell death. In this review, we describe various types of neutrophil cell death, including necrosis, apoptosis, necroptosis, pyroptosis, NETosis, and autophagy, to reveal their known effects in the development and progression of sepsis. However, the exact role and mechanisms of neutrophil cell death in sepsis have not been fully elucidated, and this remains a major challenge for future neutrophil research. We hope that this review will provide hints for researches regarding neutrophil cell death in sepsis and provide insights for clinical practitioners.

Pyroptosis in host defence against bacterial infection

Pyroptosis, a regulated form of pro-inflammatory cell death, is characterised by cell lysis and by the release of cytokines, damage- and pathogen-associated molecular patterns. It plays an important role during bacterial infection, where it can promote an inflammatory response and eliminate the replicative niche of intracellular pathogens. Recent work, using a variety of bacterial pathogens, has illuminated the versatility of pyroptosis, revealing unexpected and important concepts underlying host defence. In this Review, we overview the molecular mechanisms underlying pyroptosis and discuss their role in host defence, from the single cell to the whole organism. We focus on recent studies using three cellular microbiology paradigms - Mycobacterium tuberculosis, Salmonella Typhimurium and Shigella flexneri - that have transformed the field of pyroptosis. We compare insights discovered in tissue culture, zebrafish and mouse models, highlighting the advantages and disadvantages of using these complementary infection models to investigate pyroptosis and for modelling human infection. Moving forward, we propose that in-depth knowledge of pyroptosis obtained from complementary infection models can better inform future studies using higher vertebrates, including humans, and help develop innovative host-directed therapies to combat bacterial infection.

The "Self-Sacrifice" of ImmuneCells in Sepsis

Sepsis is a life-threatening organ dysfunction caused by the host’s malfunctioning response to infection. Due to its high mortality rate and medical cost, sepsis remains one of the world’s most intractable diseases. In the early stage of sepsis, the over-activated immune system and a cascade of inflammation are usually accompanied by immunosuppression. The core pathogenesis of sepsis is the maladjustment of the host’s innate and adaptive immune response. Many immune cells are involved in this process, including neutrophils, mononuclear/macrophages and lymphocytes. The immune cells recognize pathogens, devour pathogens and release cytokines to recruit or activate other cells in direct or indirect manner. Pyroptosis, immune cell-extracellular traps formation and autophagy are several novel forms of cell death that are different from apoptosis, which play essential roles in the progress of sepsis. Immune cells can initiate “self-sacrifice” through the above three forms of cell death to protect or kill pathogens. However, the exact roles and mechanisms of the self-sacrifice in the immune cells in sepsis are not fully elucidated. This paper mainly analyzes the self-sacrifice of several representative immune cells in the forms of pyroptosis, immune cell-extracellular traps formation and autophagy to reveal the specific roles they play in the occurrence and progression of sepsis, also to provide inspiration and references for further investigation of the roles and mechanisms of self-sacrifice of immune cells in the sepsis in the future, meanwhile, through this work, we hope to bring inspiration to clinical work.

A TLR4-independent critical role for CD14 in intracellular LPS sensing

Intracellular lipopolysaccharide (LPS) sensing by the noncanonical inflammasome comprising caspase-4 or −11 governs antibacterial host defense. How LPS gains intracellular access in vivo is largely unknown. Here, we show that CD14—an LPS-binding protein with a well-documented role in TLR4 activation—plays a vital role in intracellular LPS sensing in vivo. By generating Cd14^−/− and Casp11^−/− mice strains on a Tlr4^−/− background, we dissociate CD14’s known role in TLR4 signaling from its role in caspase-11 activation and show a TLR4-independent role for CD14 in GSDMD activation, pyroptosis, alarmin release, and the lethality driven by cytosolic LPS. Mechanistically, CD14 enables caspase-11 activation by mediating cytosolic localization of LPS in a TLR4-independent manner. Overall, our findings attribute a critical role for CD14 in noncanonical inflammasome sensing of LPS in vivo and establish—together with previous literature—CD14 as an essential proximal component of both TLR4-based extracellular and caspase-11-based intracellular LPS surveillance.

How LPS attains cytosolic access in vivo is unclear. Vasudevan et al. define a TLR4-independent role for CD14 in the cytosolic localization of LPS, triggering noncanonical inflammasome activation and pyroptosis in vivo. This finding positions CD14 as an integral component of both extracellular and intracellular LPS surveillance pathways.

Activation and manipulation of inflammasomes and pyroptosis during bacterial infections

Following detection of pathogen infection and disrupted cellular homeostasis, cells can activate a range of cell death pathways, such as apoptosis, necroptosis and pyroptosis, as part of their defence strategy. The initiation of pro-inflammatory, lytic pyroptosis is controlled by inflammasomes, which respond to a range of cellular perturbations. As is true for many host defence pathways, pathogens have evolved multiple mechanisms to subvert this pathway, many of which have only recently been described. Herein, we will discuss the mechanisms by which inflammasomes sense pathogen invasion and initiate pyroptosis and the effector mechanisms used by pathogens to suppress this pathway and preserve their niche.

Inflammasome and gasdermin signalling in neutrophils

Inflammasomes and gasdermins mount potent host defence pathways against invading microbial pathogens, however, dysregulation in these pathways can drive a variety of inflammatory disorders. Neutrophils, historically regarded as effector phagocytes that drive host defence via microbial killing, are now emerging as critical drivers of immunity in vivo. Here, we summarise the latest advancement in inflammasome, gasdermin and cell death signalling in neutrophils. We discuss the mechanisms by which neutrophils resist caspase-1-dependent pyroptosis, thsse lytic function of gasdermin D and E during NETosis and Yersinia infection, and the contribution of neutrophil inflammasomes to inflammatory disorders.

Mechanisms and Consequences of Noncanonical Inflammasome-Mediated Pyroptosis

The noncanonical inflammasome, comprising inflammatory caspases 4, 5, or 11, monitors the cytosol for bacterial lipopolysaccharide (LPS). Intracellular LPS-elicited autoproteolysis of these inflammatory caspases leads to the cleavage of the pore-forming protein gasdermin D (GSDMD). GSDMD pore formation induces a lytic form of cell death known as pyroptosis and the release of inflammatory cytokines and DAMPs, thereby promoting inflammation. The noncanonical inflammasome-dependent innate sensing of cytosolic LPS plays important roles in bacterial infections and sepsis pathogenesis. Exciting studies in the recent past have significantly furthered our understanding of the biochemical and structural basis of the caspase-4/11 activation of GSDMD, caspase-4/11's substrate specificity, and the biological consequences of noncanonical inflammasome activation of GSDMD. This review will discuss these recent advances and highlight the remaining gaps in our understanding of the noncanonical inflammasome and pyroptosis.

The Programmed Cell Death of Macrophages, Endothelial Cells, and Tubular Epithelial Cells in Sepsis-AKI

Sepsis is a systemic inflammatory response syndrome caused by infection, following with acute injury to multiple organs. Sepsis-induced acute kidney injury (AKI) is currently recognized as one of the most severe complications related to sepsis. The pathophysiology of sepsis-AKI involves multiple cell types, including macrophages, vascular endothelial cells (ECs) and renal tubular epithelial cells (TECs), etc. More significantly, programmed cell death including apoptosis, necroptosis and pyroptosis could be triggered by sepsis in these types of cells, which enhances AKI progress. Moreover, the cross-talk and connections between these cells and cell death are critical for better understanding the pathophysiological basis of sepsis-AKI. Mitochondria dysfunction and oxidative stress are traditionally considered as the leading triggers of programmed cell death. Recent findings also highlight that autophagy, mitochondria quality control and epigenetic modification, which interact with programmed cell death, participate in the damage process in sepsis-AKI. The insightful understanding of the programmed cell death in sepsis-AKI could facilitate the development of effective treatment, as well as preventive methods.

Gasdermin D and Beyond - Gasdermin-mediated Pyroptosis in Bacterial Infections

The discovery of pyroptosis and its subsequent implications in infection and immunity has uncovered a new angle of host-defence against pathogen assault. At its most simple, gasdermin-mediated pyroptosis in bacterial infection would be expected to remove pathogens from the relative safety of the cytosol or pathogen containing vacuole/phagosome whilst inducing a rapid and effective immune response. Differences in gasdermin-mediated pyroptosis between cell types, stimulation conditions, pathogen and even animal species, however, make things more complex. The excessive inflammation associated with the pathogen-induced gasdermin-mediated pyroptosis contributes to a downward spiral in sepsis. With no currently approved effective treatment options for sepsis understanding how gasdermin-mediated pyroptotic pathways are regulated provides an opportunity to identify novel therapeutic candidates against this complex disease. In this review we cover recent advances in the field of gasdermin-mediated pyroptosis with a focus on bacterial infection and sepsis models in the context of humans and other animal species. Importantly we also consider why there is considerable redundancy set into these ancient immune pathways.

[Antibacterial mechanism of Brevinin-2GHk, an antimicrobial peptide from Fejervarya limnocharis skin]

OBJECTIVE

To investigate the secondary structure, physicochemical properties and antibacterial activity of Brevinin- 2GHk (BR2GK), an antimicrobial peptide from frog skin, and explore its antibacterial mechanism.

METHODS

BR2GK was synthesized, purified with high performance liquid chromatography (HPLC) and identified using mass spectrometry. Circular dichroism was used to determine the secondary structure and physicochemical properties of BR2GK. Two-fold dilution method was used to determine the antibacterial activity of BR2GK, and its antibacterial mechanism was explored using laser scanning confocal microscopy (LSCM) and scanning electron microscopy (SEM). The hemolytic activity of BR2GK was analyzed in red blood cells. Isothermal titration calorimetry (ITC) and surface plasmon resonance imaging (SPRi) were employed to detect the binding of BR2GK to lipopolysaccharide (LPS), and the antioxidant activity of BR2GK was evaluated using biochemical kits.

RESULTS

The synthesized BR2GK, with a purity exceeding 95% after purification, had the basic characteristics of cationic antimicrobial peptides. BR2GK consisted mainly of α-helical structure in SDS solution and exhibited a broad-spectrum antibacterial activity. Antibacterial activity assay showed that BR2GK had inhibitory and killing activity against a variety of strains with a minimum inhibitory concentration (MIC) of 2.76 μmol/L against Staphylococcus aureus. Observation with LSCM and SEM showed that BR2GK at an active concentration caused bacterial cell membrane damage, cell swelling, contraction, deformation, and massive exudation of intracellular contents without causing hemolysis. ITC showed that the binding affinity K_D of BR2GK to LPS was 18.2±0.8 μmol/L. The antioxidant test showed that BR2GK was capable of effectively scavenging NO, ABTS and DPPH.

CONCLUSION

BR2GK, as a broad-spectrum antibacterial peptide with also a strong antioxidant capacity, does not cause hemolysis and is capable of binding to LPS. BR2GK has an important value for future design and synthesis of antimicrobial peptides with stronger antibacterial activity and lower cytotoxicity.

Approaching Neutrophil Pyroptosis

All cells must die at some point, and the dogma is that they do it either silently via apoptosis or via pro-inflammatory, lytic forms of death. Amongst these lytic cell death pathways, pyroptosis is one of the best characterized. Pyroptosis depends on inflammatory caspases which activate members of the gasdermin family of proteins, and it is associated with the release of the pro-inflammatory cytokines interleukin (IL)-1β and IL-18. Pyroptosis is an essential component of innate immunity, it initiates and amplifies inflammation and it removes the replication niche for intracellular pathogens. Most of the literature on pyroptosis focuses on monocytes and macrophages. However, the most abundant phagocytes in humans are neutrophils. This review addresses whether neutrophils undergo pyroptosis and the underlying mechanisms. Furthermore, I discuss how and why neutrophils might be able to resist pyroptosis.