The NLRP3 Inflammasome Renders Cell Death Pro-inflammatory

NLRP3 is the most studied inflammasome sensor due to its crucial involvement in sterile and infection-triggered inflammation. Although its molecular mode of activation remains to be defined, it is well established that low intracellular potassium concentrations result in its activation. This functionality allows the classical NLRP3 pathway to serve as a highly sensitive, but non-specific surveillance mechanism responding to any type of perturbation that breaches plasma membrane integrity and the associated potassium gradient across the membrane. Here, we review our current knowledge on potassium efflux-dependent NLRP3 activation, with a special focus on how major cell death programs are rendered pro-inflammatory by secondary NLRP3 activation. Apart from the “alternative inflammasome” as the major exception to the rule, this connection explains the fundamental importance of NLRP3 in cell death-associated inflammation and firmly establishes NLRP3 as a principal surveillance mechanism of cellular integrity.     

Mechanisms of Gasdermin Family Members in Inflammasome Signaling and Cell Death

The Gasdermin (GSDM) family consists of Gasdermin A (GSDMA), Gasdermin B (GSDMB), Gasdermin C (GSDMC), Gasdermin D (GSDMD), Gasdermin E (GSDME) and Pejvakin (PJVK). GSDMD is activated by inflammasome-associated inflammatory caspases. Cleavage of GSDMD by human or mouse caspase-1, human caspase-4, human caspase-5, and mouse caspase-11 liberates the N-terminal effector domain from the C-terminal inhibitory domain. The N-terminal domain oligomerizes in the cell membrane and forms a pore of 10–16?nm in diameter, through which substrates of a smaller diameter, such as interleukin-1β and interleukin-18, are secreted. The increasing abundance of membrane pores ultimately leads to membrane rupture and pyroptosis, releasing the entire cellular content. Other than GSDMD, the N-terminal domain of all GSDMs, with the exception of PJVK, have the ability to form pores. There is evidence to suggest that GSDMB and GSDME are cleaved by apoptotic caspases. Here, we review the mechanistic functions of GSDM proteins with respect to their expression and signaling profile in the cell, with more focused discussions on inflammasome activation and cell death.     

Genetic variations of mitochondrial antiviral signaling gene (MAVS) in domestic chickens

Mitochondrial antiviral signaling (MAVS) gene plays a key role in antiviral regulation in mammals potentially by activating IRF3/7 and NF-κB and leading to the induction of type I interferon (IFN)-mediated antiviral and inflammatory responses. In this study, we screened genetic polymorphisms of the MAVS gene in various Chinese domestic chicken breeds/populations and evaluated its potential effect on gene expression. Among the sequenced fragment (4678 bp), a total of 75 single nucleotide polymorphisms (SNPs) were identified in 46 chickens from 10 breeds/populations, including 30 coding SNPs and 45 non-coding SNPs. Extremely high haplotype diversity (37 nucleotide haplotypes, 18 amino acid haplotypes) was observed in the coding region (CDS), and a similar pattern of high polymorphisms was also observed for the 3′-untranslated region (3′-UTR). Luciferase assays of two representative 3′-UTR haplotypes were performed in both HEK293 cells and DF-1 chicken fibroblast cells, and we found that they were differentially associated with different abilities on regulating mRNA expression level (P < 0.05). Collectively, we observed a considerably high genetic variability of the MAVS gene, and the 3′-UTR variants had an ability to regulate mRNA expression. These results would cast some clues on understanding the potential role of MAVS on viral resistance in chicken.     

Mechanisms of NLRP1-Mediated Autoinflammatory Disease in Humans and Mice

NLRP1 was the first NOD-like receptor described to form an inflammasome, recruiting ASC to activate caspase-1, which processes interleukin-1β and interleukin-18 to their active form. A wealth of new genetic information has now redefined our understanding of this innate immune sensor. Specifically, rare loss-of-function variants in the N-terminal pyrin domain indicate that this part of NLRP1 is autoinhibitory and normally acts to prevent a familial autoinflammatory skin disease associated with cancer. In the absence of a ligand to trigger human NLRP1, these mutations have now confirmed the requirement of NLRP1 autolytic cleavage within the FIIND domain, which had previously been implicated in NLRP1 activation. Autolytic cleavage generates a C-terminal fragment of NLRP1 containing the CARD domain which then forms an ASC-dependent inflammasome. The CARD domain as an inflammasome linker is consistent with the observation that under some conditions, particularly for mouse NLRP1, caspase-1 can be engaged directly, and although it is no longer processed, it is still capable of producing mature IL-1β. Additional rare variants in a linker region between the LRR and FIIND domains of NLRP1 also cause autoinflammatory disease in both humans and mice. This new genetic information is likely to provide for more mechanistic insight in the years to come, contributing to our understanding of how NLRP1 functions as an innate immune sensor of infection and predisposes to autoimmune or autoinflammatory diseases.     

Stress molecules in sepsis and systemic inflammatory response syndrome

During sepsis, microbial derived products ("pathogen-associated molecular patterns", PAMPs) are recognized as exogenous danger signals by specific sensors of the host ("pattern recognitions receptors", PRRs). This interaction leads to the release of numerous stress proteins that are a prerequisite to fight infection, though their overzealous production can contribute to tissue damage, organ dysfunction and eventually death. In critically ill patients, translocation of PAMPs can occur from the gut, and injured tissues and cells release endogenous danger signals called "alarmins" (e.g. High mobility group box-1); that share some properties with PAMPs. Thus, numerous similarities occur during infectious and non-infectious systemic inflammation.     

New functions of mitochondria associated membranes in cellular signaling

In all eukaryotic cells, the endoplasmic reticulum (ER) and the mitochondria establish a tight interplay, which is structurally and functionally modulated through a proteinaceous tether formed at specific subdomains of the ER membrane, designated mitochondria-associated membranes or MAMs. The tethering function of the MAMs allows the regulation of lipid synthesis and rapid transmission of calcium (Ca^2 +) signals between the ER and mitochondria, which is crucial to shape intracellular Ca^2 + signaling and regulate mitochondrial bioenergetics. Research on the molecular characterization and function of MAMs has boomed in the last few years and the list of signaling and structural proteins dynamically associated with the ER–mitochondria contact sites in physiological and pathological conditions, is rapidly increasing along with the realization of an unprecedented complexity underlying the functional role of MAMs. Besides their established role as a signaling hub for Ca^2 + and lipid transfer between ER and mitochondria, MAMs have been recently shown to regulate mitochondrial shape and motility, energy metabolism and redox status and to be central to the modulation of various key processes like ER stress, autophagy and inflammasome signaling. In this review we will discuss some emerging cell-autonomous and cell non-autonomous roles of the MAMs in mammalian cells and their relevance for important human diseases. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.     

Mass Spectrometry-based Structural Analysis and Systems Immunoproteomics Strategies for Deciphering the Host Response to Endotoxin

One cause of sepsis is systemic maladaptive immune response of the host to bacteria and specifically, to Gram-negative bacterial outer-membrane glycolipid lipopolysaccharide (LPS). On the host myeloid cell surface, proinflammatory LPS activates the innate immune system via Toll-like receptor-4/myeloid differentiation factor-2 complex. Intracellularly, LPS is also sensed by the noncanonical inflammasome through caspase-11 in mice and 4/5 in humans. The minimal functional determinant for innate immune activation is the membrane anchor of LPS called lipid A. Even subtle modifications to the lipid A scaffold can enable, diminish, or abolish immune activation. Bacteria are known to modify their LPS structure during environmental stress and infection of hosts to alter cellular immune phenotypes. In this review, we describe how mass spectrometry-based structural analysis of endotoxin helped uncover major determinations of molecular pathogenesis. Through characterization of LPS modifications, we now better understand resistance to antibiotics and cationic antimicrobial peptides, as well as how the environment impacts overall endotoxin structure. In addition, mass spectrometry-based systems immunoproteomics approaches can assist in elucidating the immune response against LPS. Many regulatory proteins have been characterized through proteomics and global/targeted analysis of protein modifications, enabling the discovery and characterization of novel endotoxin-mediated protein translational modifications.     

How Mitochondrial Metabolism Contributes to Macrophage Phenotype and Functions

Metabolic reprogramming of cells from the innate immune system is one of the most noteworthy topics in immunological research nowadays. Upon infection or tissue damage, innate immune cells, such as macrophages, mobilize various immune and metabolic signals to mount a response best suited to eradicate the threat. Current data indicate that both the immune and metabolic responses are closely interconnected. On account of its peculiar position in regulating both of these processes, the mitochondrion has emerged as a critical organelle that orchestrates the coordinated metabolic and immune adaptations in macrophages. Significant effort is now underway to understand how metabolic features of differentiated macrophages regulate their immune specificities with the eventual goal to manipulate cellular metabolism to control immunity. In this review, we highlight some of the recent work that place cellular and mitochondrial metabolism in a central position in the macrophage differentiation program.     

The role of mitochondria in sepsis-induced cardiomyopathy

Sepsis is defined as a life-threatening organ dysfunction caused by a dysregulated host response to infection. Myocardial dysfunction, often termed sepsis-induced cardiomyopathy, is a frequent complication and is associated with worse outcomes. Numerous mechanisms contribute to sepsis-induced cardiomyopathy and a growing body of evidence suggests that bioenergetic and metabolic derangements play a central role in its development; however, there are significant discrepancies in the literature, perhaps reflecting variability in the experimental models employed or in the host response to sepsis. The condition is characterised by lack of significant cell death, normal tissue oxygen levels and, in survivors, reversibility of organ dysfunction. The functional changes observed in cardiac tissue may represent an adaptive response to prolonged stress that limits cell death, improving the potential for recovery. In this review, we describe our current understanding of the pathophysiology underlying myocardial dysfunction in sepsis, with a focus on disrupted mitochondrial processes.     

Role of Autophagy Pathway in Parkinson’s Disease and Related Genetic Neurological Disorders

The elucidation of the function of the PINK1 protein kinase and Parkin ubiquitin E3 ligase in the elimination of damaged mitochondria by autophagy (mitophagy) has provided unprecedented understanding of the mechanistic pathways underlying Parkinson’s disease (PD). We provide a comprehensive overview of the general importance of autophagy in Parkinson’s disease and related disorders of the central nervous system. This reveals a critical link between autophagy and neurodegenerative and neurodevelopmental disorders and suggests that strategies to modulate mitophagy may have greater relevance in the CNS beyond PD.     

Lipid Replacement Therapy: A natural medicine approach to replacing damaged lipids in cellular membranes and organelles and restoring function

Lipid Replacement Therapy, the use of functional oral supplements containing cell membrane phospholipids and antioxidants, has been used to replace damaged, usually oxidized, membrane glycerophospholipids that accumulate during aging and in various clinical conditions in order to restore cellular function. This approach differs from other dietary and intravenous phospholipid interventions in the composition of phospholipids and their defense against oxidation during storage, ingestion, digestion and uptake as well as the use of protective molecules that noncovalently complex with phospholipid micelles and prevent their enzymatic and bile disruption. Once the phospholipids have been taken in by transport processes, they are protected by several natural mechanisms involving lipid receptors, transport and carrier molecules and circulating cells and lipoproteins until their delivery to tissues and cells where they can again be transferred to intracellular membranes by specific and nonspecific transport systems. Once delivered to membrane sites, they naturally replace and stimulate removal of damaged membrane lipids. Various chronic clinical conditions are characterized by membrane damage, mainly oxidative but also enzymatic, resulting in loss of cellular function. This is readily apparent in mitochondrial inner membranes where oxidative damage to phospholipids like cardiolipin and other molecules results in loss of trans-membrane potential, electron transport function and generation of high-energy molecules. Recent clinical trials have shown the benefits of Lipid Replacement Therapy in restoring mitochondrial function and reducing fatigue in aged subjects and patients with a variety of clinical diagnoses that are characterized by loss of mitochondrial function and include fatigue as a major symptom. This Article is Part of a Special Issue Entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.     

Pathophysiologic implications of innate immunity and autoinflammation in the biliary epithelium

The most studied physiological function of biliary epithelial cells (cholangiocytes) is to regulate bile flow and composition, in particular the hydration and alkalinity of the primary bile secreted by hepatocytes. After almost three decades of studies it is now become clear that cholangiocytes are also involved in epithelial innate immunity, in inflammation, and in the reparative processes in response to liver damage. An increasing number of evidence highlights the ability of cholangiocyte to undergo changes in phenotype and function in response to liver damage. By participating actively to the immune and inflammatory responses, cholangiocytes represent a first defense line against liver injury from different causes. Indeed, cholangiocytes express a number of receptors able to recognize pathogen- or damage-associated molecular patterns (PAMPs/DAMPs), such as Toll-like receptors (TLR), which modulate their pro-inflammatory behavior. Cholangiocytes can be both the targets and the initiators of the inflammatory process. Derangements of the signals controlling these mechanisms are at the basis of the pathogenesis of different cholangiopathies, both hereditary and acquired, such as cystic fibrosis-related liver disease and sclerosing cholangitis. This article is part of a Special Issue entitled: Cholangiocytes in Health and Diseaseedited by Jesus Banales, Marco Marzioni, Nicholas LaRusso and Peter Jansen.     

Emerging role of NF-κB signaling in the induction of senescence-associated secretory phenotype (SASP)

The major hallmark of cellular senescence is an irreversible cell cycle arrest and thus it is a potent tumor suppressor mechanism. Genotoxic insults, e.g. oxidative stress, are important inducers of the senescent phenotype which is characterized by an accumulation of senescence-associated heterochromatic foci (SAHF) and DNA segments with chromatin alterations reinforcing senescence (DNA-SCARS). Interestingly, senescent cells secrete pro-inflammatory factors and thus the condition has been called the senescence-associated secretory phenotype (SASP). Emerging data has revealed that NF-κB signaling is the major signaling pathway which stimulates the appearance of SASP. It is known that DNA damage provokes NF-κB signaling via a variety of signaling complexes containing NEMO protein, an NF-κB essential modifier, as well as via the activation of signaling pathways of p38MAPK and RIG-1, retinoic acid inducible gene-1. Genomic instability evoked by cellular stress triggers epigenetic changes, e.g. release of HMGB1 proteins which are also potent enhancers of inflammatory responses. Moreover, environmental stress and chronic inflammation can stimulate p38MAPK and ceramide signaling and induce cellular senescence with pro-inflammatory responses. On the other hand, two cyclin-dependent kinase inhibitors, p16INK4a and p14ARF, are effective inhibitors of NF-κB signaling. We will review in detail the signaling pathways which activate NF-κB signaling and trigger SASP in senescent cells.     

Dual character of Toll-like receptor signaling: Pro-tumorigenic effects and anti-tumor functions

As a major class of pattern-recognition receptors, Toll-like receptors (TLRs) play a critical role in defense against invading pathogens. Increasing evidence demonstrates that, in addition to infection, TLRs are involved in other important pathological processes, such as tumorigenesis. Activation of TLRs results in opposing outcomes, pro-tumorigenic effects and anti-tumor functions. TLR signaling can inhibit apoptosis and promote chronic inflammation-induced tumorigenesis. TLR activation in tumor cells and immune cells can induce production of cytokines, increase tumor cell proliferation and apoptosis resistance, promote invasion and metastasis, and inhibit immune cell activity resulting in tumor immune escape. In contrast, the engagement of other TLRs directly induces growth inhibition and apoptosis of tumor cells and triggers activation of immune cells enhancing anti-tumor immune responses. Thus, the interpretation of the precise function of each TLR in tumors is very important for targeting TLRs and using TLR agonists in tumor therapy. We review the role of TLR signaling in tumors and discuss the factors that affect outcomes of TLR activation.     

Pyroptosis,and its Role in Central Nervous System Disease

Pyroptosis is an inflammatory form of cell death executed by transmembrane pore-forming proteins known as gasdermins and can be activated in an inflammasome-dependent or -independent manner. Inflammasome-dependent pyroptosis is triggered in response to pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMPs) and has emerged as an important player in the pathogenesis of multiple inflammatory diseases, mainly by releasing inflammatory contents. More recently, numerous studies have revealed the intricate mechanisms of pyroptosis and its role in the development of neuroinflammation in central nervous system (CNS) diseases. In this review, we summarize current understandings of the molecular and regulatory mechanisms of pyroptosis. In addition, we discuss how pyroptosis can drive different forms of neurological diseases and new promising therapeutic strategies targeting pyroptosis that can be leveraged to treat neuroinflammation.     

Modeling Innate Antiviral Immunity in Physiological Context

An effective innate antiviral response is critical for the mitigation of severe disease and host survival following infection. In vivo, the innate antiviral response is triggered by cells that detect the invading pathogen and then communicate through autocrine and paracrine signaling to stimulate the expression of genes that inhibit viral replication, curtail cell proliferation, or modulate the immune response. In other words, the innate antiviral response is complex and dynamic. Notably, in the laboratory, culturing viruses and assaying viral life cycles frequently utilizes cells that are derived from tissues other than those that support viral replication during natural infection, while the study of viral pathogenesis often employs animal models. In recapitulating the human antiviral response, it is important to consider that variation in the expression and function of innate immune sensors and antiviral effectors exists across species, cell types, and cell differentiation states, as well as when cells are placed in different contexts. Thus, to gain novel insight into the dynamics of the host response and how specific sensors and effectors impact infection kinetics by a particular virus, the model system must be selected carefully. In this review, we briefly introduce key signaling pathways involved in the innate antiviral response and highlight how these differ between systems. We then review the application of tissue-engineered or 3D models for studying the antiviral response, and suggest how these in vitro culture systems could be further utilized to assay physiologically-relevant host responses and reveal novel insight into virus-host interactions.