Recognition of cytoplasmic RNA results in cathepsin-dependent inflammasome activation and apoptosis in human macrophages

Contribution of podocyte inflammatory exosome release to glomerular inflammation and sclerosis during hyperhomocysteinemia

The nucleotide-binding oligomerization domain-like receptor containing pyrin domain 3 (NLRP3) inflammasome has been implicated in podocyte injury and glomerular sclerosis in response to hyperhomocysteinemia (hHcy). However, it remains unknown how the products of NLRP3 inflammasome in cytoplasm are secreted out of podocytes. In the present study, we tested whether exosome release serves as a critical mechanism to mediate the action of NLRP3 inflammasome activation in hHcy-induced glomerular injury. By various approaches, we found that hHcy induced NLRP3 inflammasome activation and neutrophil infiltration in glomeruli of WT/WT mice. Lysosome-MVB interaction in glomeruli remarkably decreased in WT/WT mice fed with FF diet, leading to elevation of urinary exosome excretion of these mice. Podocyte-derived exosomes containing pro-inflammatory cytokines increased in urine of WT/WT mice in response to hHcy. The release of inflammatory exosomes from podocytes was prevented by Smpd1 gene deletion but enhanced by podocyte-specific Smpd1 gene overexpression (Smpd1 encodes Asm in mice). Pathologically, hHcy-induced podocyte injury and glomerular sclerosis were blocked by Smpd1 gene knockout but amplified by podocyte-specific Smpd1 gene overexpression. Taken together, our results suggest that Asm-ceramide signaling pathway contributes to NLRP3 inflammasome activation and robust release of inflammatory exosomes in podocytes during hHcy, which together trigger local glomerular inflammation and sclerosis.

BAFF-driven NLRP3 inflammasome activation in B cells

BAFF supports B-cell survival and homeostasis by activating the NF-κB pathway. While NF-κB is also involved in the priming signal of NLRP3 inflammasome, the role of BAFF in NLRP3 inflammasome regulation is unknown. Here we report BAFF engagement to BAFF receptor elicited both priming and activating signals for NLRP3 inflammasomes in primary B cells and B lymphoma cell lines. This induction of NLRP3 inflammasomes by BAFF led to increased NLRP3 and IL-1β expression, caspase-1 activation, IL-1β secretion, and pyroptosis. Mechanistically, BAFF activated NLRP3 inflammasomes by promoting the association of cIAP-TRAF2 with components of NLRP3 inflammasomes, and by inducing Src activity-dependent ROS production and potassium ion efflux. B-cell receptor (BCR) stimulation on the Lyn signaling pathway inhibited BAFF-induced Src activities and attenuated BAFF-induced NLRP3 inflammasome activation. These findings reveal an additional function of BAFF in B-cell homeostasis that is associated with BCR activities.

An emerging interplay between extracellular vesicles and cytokines

Extracellular vesicles (EVs) are small membrane-bound particles that are naturally released from cells. They are recognized as potent vehicles of intercellular communication both in prokaryotes and eukaryotes. Because of their capacity to carry biological macromolecules such as proteins, lipids and nucleic acids, EVs influence different physiological and pathological functions of both parental and recipient cells. Although multiple pathways have been proposed for cytokine secretion beyond the classical ER/Golgi route, EVs have recently recognized as an alternative secretory mechanism. Interestingly, cytokines/chemokines exploit these vesicles to be released into the extracellular milieu, and also appear to modulate their release, trafficking and/or content. In this review, we provide an overview of the cytokines/chemokines that are known to be associated with EVs or their regulation with a focus on TNFα, IL-1β and IFNs.

A STING to inflammation and autoimmunity

Various intracellular pattern recognition receptors (PRRs) recognize cytosolic pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs). Cyclic GMP-AMP synthase (cGAS), a cytosolic PRR, recognizes cytosolic nucleic acids including dsDNAs. The recognition of dsDNA by cGAS generates cyclic GMP-AMP (GAMP). The cGAMP is then recognized by STING generating type 1 IFNs and NF-κB-mediated generation of pro-inflammatory cytokines and molecules. Thus, cGAS-STING signaling mediated recognition of cytosolic dsDNA causing the induction of type 1 IFNs plays a crucial role in innate immunity against cytosolic pathogens, PAMPs, and DAMPs. The overactivation of this system may lead to the development of autoinflammation and autoimmune diseases. The article opens with the introduction of different PRRs involved in the intracellular recognition of dsDNA and gives a brief introduction of cGAS-STING signaling. The second section briefly describes cGAS as intracellular PRR required to recognize intracellular nucleic acids (dsDNA and CDNs) and the formation of cGAMP. The cGAMP acts as a second messenger to activate STING- and TANK-binding kinase 1-mediated generation of type 1 IFNs and the activation of NF-κB. The third section of the article describes the role of cGAS-STING signaling in the induction of autoinflammation and various autoimmune diseases. The subsequent fourth section describes both chemical compounds developed and the endogenous negative regulators of cGAS-STING signaling required for its regulation. Therapeutic targeting of cGAS-STING signaling could offer new ways to treat inflammatory and autoimmune diseases.

Serum Cystatin C Levels in COPD: Potential Diagnostic Value and Relation between Respiratory Functions

BACKGROUND

The aim of this study was to determine the level of serum cystatin C (CysC) in patients with Chronic Obstructive Pulmonary Disease (COPD) during exacerbation and stable periods and to investigate its potential diagnostic value and the relationship between CysC levels and the pulmonary function test (PFT).

METHODS

One hundred twenty-six patients with COPD (68 in stable periods, 58 during exacerbation periods) and 50 healthy subjects were included in the study. PFT, body mass index (BMI), white blood cell counts, C-reactive protein (CRP), serum urea and creatinine levels were evaluated in both groups of patients. CysC levels were measured in all participants.

RESULTS

Serum CysC levels were statistically higher in both COPD groups than the control group (p<0.001 for both) although there was no statistically significant difference between COPD groups (p>0.05). CysC levels showed negative correlation with forced expiratory volume in 1 second (FEV1) and a positive correlation with C-reactive protein (CRP) levels in patients with stable COPD. There was a positive correlation between serum CysC levels and serum urea, creatinine, CRP levels in patients with COPD exacerbation (r=0.333, p=0.011; r=0.260, p=0.049; r=0.414, p<0.01 respectively). When stable COPD and control groups were evaluated, serum CysC had an area under the curve (AUC) in the receiver operating characteristic (ROC) curve of 0.951 (0.909-0.994 95% CI: p<0.001).

CONCLUSIONS

Our results showed that CysC levels increased in both COPD groups. Increased CysC levels may be related with lung function decline and inflammation in COPD patients. In addition, CysC levels may be a potential indicator for the diagnosis of COPD.

From Inflammasome to Exosome-Does Extracellular Vesicle Secretion Constitute an Inflammasome-Dependent Immune Response?

Inflammasomes are intracellular protein complexes of pattern recognition receptors and caspase-1, with essential functions in regulating inflammatory responses of macrophages and dendritic cells. The primary role of inflammasomes is to catalyze processing and secretion of pro-inflammatory cytokines IL-1β and IL-18. Recently, intracellular non-canonical inflammasome activation by caspases-4/5, which are also regulators of pyroptosis via processing gasdermin D, has been elucidated. Caspase-1, the effector protease of inflammasome complex, is also known to modulate secretion of large number of other proteins. Thereby, besides its known role in processing pro-inflammatory cytokines, the inflammasome turns into a universal regulator of protein secretion, which allows the danger-exposed cells to release various proteins in order to alert and guide neighboring cells. Majority of these proteins are not secreted through the conventional ER-Golgi secretory pathway. Instead, they are segregated in membrane-enclosed compartment and secreted in nanosized extracellular vesicles, which protect their cargo and guide it for delivery. Growing evidence indicates that inflammasome activity correlates with enhanced secretion of extracellular vesicles and modulation of their protein cargo. This inflammasome-driven unconventional, vesicle-mediated secretion of multitude of immunoregulatory proteins may constitute a novel paradigm in inflammatory responses. In this mini review we discuss the current knowledge and highlight unsolved questions about metabolic processes, signals, and mechanisms linking inflammasome activity with regulated extracellular vesicle secretion of proteins. Further investigations on this relationship may in the future help understanding the significance of extracellular vesicle secretion in inflammatory diseases such as atherosclerosis, gouty arthritis, asthma, Alzheimer's and many others.

The role of mitochondria in NLRP3 inflammasome activation

The NLRP3 inflammasome is a multiprotein platform which is activated upon cellular infection or stress. Its activation leads to caspase-1-dependent secretion of proinflammatory cytokines like interleukin-1β (IL-1β) and IL-18, and an inflammatory form of cell death termed as pyroptosis. Recent studies have unveiled the pivotal roles of mitochondria in initiation and regulation of the NLRP3 (nucleotide-binding domain, leucine-rich-repeat containing family, pyrin domain-containing 3) inflammasome. NLRP3 activators induce mitochondrial destabilization, NLRP3 deubiquitination, linear ubiquitination of ASC, and externalization or release of mitochondria-derived molecules such as cardiolipin and mitochondrial DNA. These molecules bind to NLRP3 that is translocated on mitochondria and activate the NLRP3 inflammasome. Here we review recently described mechanisms by which mitochondria regulate NLRP3 inflammasome activation.

Lysosomal Cathepsin Release Is Required for NLRP3-Inflammasome Activation by Mycobacterium tuberculosis in Infected Macrophages

Lysosomal cathepsin B (CTSB) has been proposed to play a role in the induction of acute inflammation. We hypothesised that the presence of active CTSB in the cytosol is crucial for NLRP3-inflammasome assembly and, consequently, for mature IL-1β generation after mycobacterial infection in vitro. Elevated levels of CTSB was observed in the lungs of mice and rabbits following infection with Mycobacterium tuberculosis (Mtb) H37Rv as well as in plasma from acute tuberculosis patients. H37Rv-infected murine bone marrow-derived macrophages (BMDMs) displayed both lysosomal leakage, with release of CTSB into the cytosol, as well as increased levels of mature IL-1β. These responses were diminished in BMDM infected with a mutant H37Rv deficient in ESAT-6 expression. Pharmacological inhibition of cathepsin activity with CA074-Me resulted in a substantial reduction of both mature IL-1β production and caspase-1 activation in infected macrophages. Moreover, cathepsin inhibition abolished the interaction between NLRP3 and ASC, measured by immunofluorescence imaging in H37Rv-infected macrophages, demonstrating a critical role of the enzyme in NLRP3-inflammasome activation. These observations suggest that during Mtb infection, lysosomal release of activated CTSB and possibly other cathepsins inhibitable by CA07-Me is critical for the induction of inflammasome-mediated IL-1β processing by regulating NLRP3-inflammasome assembly in the cytosol.

Novel therapeutic roles for surfactant-inositols and -phosphatidylglycerols in a neonatal piglet ARDS model: a translational study

The biological and immune-protective properties of surfactant-derived phospholipids and phospholipid subfractions in the context of neonatal inflammatory lung disease are widely unknown. Using a porcine neonatal triple-hit acute respiratory distress syndrome (ARDS) model (repeated airway lavage, overventilation, and LPS instillation into airways), we assessed whether the supplementation of surfactant (S; poractant alfa) with inositol derivatives [inositol 1,2,6-trisphosphate (IP3) or phosphatidylinositol 3,5-bisphosphate (PIP2)] or phosphatidylglycerol subfractions [16:0/18:1-palmitoyloleoyl-phosphatidylglycerol (POPG) or 18:1/18:1-dioleoyl-phosphatidylglycerol (DOPG)] would result in improved clinical parameters and sought to characterize changes in key inflammatory pathways behind these improvements. Within 72 h of mechanical ventilation, the oxygenation index (S+IP3, S+PIP2, and S+POPG), the ventilation efficiency index (S+IP3 and S+POPG), the compliance (S+IP3 and S+POPG) and resistance (S+POPG) of the respiratory system, and the extravascular lung water index (S+IP3 and S+POPG) significantly improved compared with S treatment alone. The inositol derivatives (mainly S+IP3) exerted their actions by suppressing acid sphingomyelinase activity and dependent ceramide production, linked with the suppression of the inflammasome nucleotide-binding domain, leucine-rich repeat-containing protein-3 (NLRP3)-apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC)-caspase-1 complex, and the profibrotic response represented by the cytokines transforming growth factor-β1 and IFN-γ, matrix metalloproteinase (MMP)-1/8, and elastin. In addition, IκB kinase activity was significantly reduced. S+POPG and S+DOPG treatment inhibited polymorphonuclear leukocyte activity (MMP-8 and myeloperoxidase) and the production of interleukin-6, maintained alveolar-capillary barrier functions, and reduced alveolar epithelial cell apoptosis, all of which resulted in reduced pulmonary edema. S+DOPG also limited the profibrotic response. We conclude that highly concentrated inositol derivatives and phosphatidylglycerol subfractions in surfactant preparations mitigate key inflammatory pathways in inflammatory lung disease and that their clinical application may be of interest for future treatment of the acute exudative phase of neonatal ARDS.

Proteomics to study macrophage response to viral infection

Viral infections are a major burden to human and animal health. Immune response against viruses consists of innate and adaptive immunity which are both critical for the eradication of the viral infection. The innate immune system is the first line of defense against viral infections. Proper innate immune response is required for the activation of adaptive, humoral and cell-mediated immunity. Macrophages are innate immune cells which have a central role in detecting viral infections including influenza A and human immunodeficiency viruses. Macrophages and other host cells respond to viral infection by modulating their protein expression levels, proteins' posttranslational modifications, as well as proteins' intracellular localization and secretion. Therefore the detailed characterization how viruses dynamically manipulate host proteome is needed for understanding the molecular mechanisms of viral infection. It is critical to identify cellular host factors which are exploited by different viruses, and which are less prone for mutations and could serve as potential targets for novel antiviral compounds. Here, we review how proteomics studies have enhanced our understanding of macrophage response to viral infection with special focus on Influenza A and Human immunodeficiency viruses, and virus infections of swine.

SIGNIFICANCE

Influenza A viruses (IAVs) and human immunodeficiency viruses (HIV) infect annually millions of people worldwide and they form a severe threat to human health. Both IAVs and HIV-1 can efficiently antagonize host response and develop drug-resistant variants. Most current antiviral drugs are directed against viral proteins, and there is a constant need to develop new next-generation drugs targeting host proteins that are essential for viral replication. Porcine reproductive and respiratory syndrome virus (PRRSV) and porcine circovirus type 2 (PCV2) are economically important swine pathogens. Both PRRSV and PCV2 cause severe respiratory tract illnesses in swine. IAVs, HIV-1, and swine viruses infect macrophages activating antiviral response against these viruses. Macrophages also have a central role in the replication and spread of these viruses. However, macrophage response to these viruses is incompletely understood. Current proteomics methods can provide a global view of host-response to viral infection which is needed for in-depth understanding the molecular mechanisms of viral infection. Here we review the current proteomics studies on macrophage response to viral infection and provide insight into the global host proteome changes upon viral infection.

Frontline Science: Multiple cathepsins promote inflammasome-independent, particle-induced cell death during NLRP3-dependent IL-1β activation

Sterile particles cause several chronic, inflammatory diseases, characterized by repeating cycles of particle phagocytosis and inflammatory cell death. Recent studies have proposed that these processes are driven by the NLRP3 inflammasome, a platform activated by phagocytosed particles, which controls both caspase-1-dependent cell death (pyroptosis) and mature IL-1β secretion. After phagocytosis, particles can disrupt lysosomes, and inhibitor studies have suggested that the resulting release of a lysosomal protease-cathepsin B-into the cytosol somehow activates NLRP3. However, using primary murine macrophages, we found that particle-induced cell death occurs independent of NLRP3/caspase-1 and depends instead on multiple, redundant cathepsins. In contrast, nigericin, a soluble activator of NLRP3 inflammasomes, induced cell death that was dependent on the NLRP3. Interestingly, nigericin-induced cell death depended partly on a single cathepsin, cathepsin X. By inhibiting or silencing multiple cathepsins in macrophages, several key proinflammatory events induced by sterile particles are blocked, including cell death, pro-IL-1β production, and IL-1β secretion. These data suggest that cathepsins might be potential therapeutic targets in particulate-mediated inflammatory disease. In support of this concept, we find that a broad-spectrum cathepsin inhibitor can suppress particle-induced IL-1-dependent peritonitis.

Extracellular cathepsin S and intracellular caspase 1 activation are surrogate biomarkers of particulate-induced lysosomal disruption in macrophages

Background

Particulate matter has been shown to stimulate the innate immune system and induce acute inflammation. Therefore, while nanotechnology has the potential to provide therapeutic formulations with improved efficacy, there are concerns such pharmaceutical preparations could induce unwanted inflammatory side effects. Accordingly, we aim to examine the utility of using the proteolytic activity signatures of cysteine proteases, caspase 1 and cathepsin S (CTSS), as biomarkers to assess particulate-induced inflammation.

Methods

Primary peritoneal macrophages and bone marrow-derived macrophages from C57BL/6 mice and ctss^−/− mice were exposed to micro- and nanoparticulates and also the lysosomotropic agent, L-leucyl-L-leucine methyl ester (LLOME). ELISA and immunoblot analyses were used to measure the IL-1β response in cells, generated by lysosomal rupture. Affinity-binding probes (ABPs), which irreversibly bind to the active site thiol of cysteine proteases, were then used to detect active caspase 1 and CTSS following lysosomal rupture. Reporter substrates were also used to quantify the proteolytic activity of these enzymes, as measured by substrate turnover.

Results

We demonstrate that exposure to silica, alum and polystyrene particulates induces IL-1β release from macrophages, through lysosomal destabilization. IL-1β secretion positively correlated with an increase in the proteolytic activity signatures of intracellular caspase 1 and extracellular CTSS, which were detected using ABPs and reporter substrates. Interestingly IL-1β release was significantly reduced in primary macrophages from ctss^−/− mice.

Conclusions

This study supports the emerging significance of CTSS as a regulator of the innate immune response, highlighting its role in regulating IL-1β release. Crucially, the results demonstrate the utility of intracellular caspase 1 and extracellular CTSS proteolytic activities as surrogate biomarkers of lysosomal rupture and acute inflammation. In the future, activity-based detection of these enzymes may prove useful for the real-time assessment of particle-induced inflammation and toxicity assessment during the development of nanotherapeutics.

Electronic supplementary material

The online version of this article (doi:10.1186/s12989-016-0129-5) contains supplementary material, which is available to authorized users.

Pathophysiological role of different tubular epithelial cell death modes in acute kidney injury

The histological substrate of many forms of intrinsic acute kidney injury (AKI) has been classically attributed to tubular necrosis. However, more recent studies indicate that necrosis is not the main form of cell death in AKI and that other forms such as apoptosis, regulated necrosis (i.e. necroptosis and parthanatos), autophagic cell death and mitotic catastrophe, also participate in AKI and that their contribution depends on the cause and stage of AKI. Herein, we briefly summarize the main characteristics of the major types of cell death and we also critically review the existing evidence on the occurrence of different types of cell death reported in the most common experimental models of AKI and human specimens. We also discuss the pathophysiological mechanisms linking tubule epithelial cell death with reduced glomerular filtration, azotaemia and hydroelectrolytic imbalance. For instance, special relevance is given to the analysis of the inflammatory component of some forms of cell death over that of others, as an important and differential pathophysiological determinant. Finally, known molecular mechanisms and signalling pathways involved in each cell death type pose appropriate targets to specifically prevent or reverse AKI, provided that further knowledge of their participation and repercussion in each AKI syndrome is progressively increased in the near future.

Multiple Cathepsins Promote Pro-IL-1β Synthesis and NLRP3-Mediated IL-1β Activation

Sterile particles induce robust inflammatory responses that underlie the pathogenesis of diseases like silicosis, gout, and atherosclerosis. A key cytokine mediating this response is IL-1β. The generation of bioactive IL-1β by sterile particles is mediated by the NOD-like receptor containing a pyrin domain 3 (NLRP3) inflammasome, although exactly how this occurs is incompletely resolved. Prior studies have found that the cathepsin B inhibitor, Ca074Me, suppresses this response, supporting a model whereby ingested particles disrupt lysosomes and release cathepsin B into the cytosol, somehow activating NLRP3. However, reports that cathepsin B-deficient macrophages have no defect in particle-induced IL-1β generation have questioned cathepsin B's involvement. In this study, we examine the hypothesis that multiple redundant cathepsins (not just cathepsin B) mediate this process by evaluating IL-1β generation in murine macrophages, singly or multiply deficient in cathepsins B, L, C, S and X. Using an activity-based probe, we measure specific cathepsin activity in living cells, documenting compensatory changes in cathepsin-deficient cells, and Ca074Me's dose-dependent cathepsin inhibition profile is analyzed in parallel with its suppression of particle-induced IL-1β secretion. Also, we evaluate endogenous cathepsin inhibitors cystatins C and B. Surprisingly, we find that multiple redundant cathepsins, inhibited by Ca074Me and cystatins, promote pro-IL-1β synthesis, and to our knowledge, we provide the first evidence that cathepsin X plays a nonredundant role in nonparticulate NLRP3 activation. Finally, we find cathepsin inhibitors selectively block particle-induced NLRP3 activation, independently of suppressing pro-IL-1β synthesis. Altogether, we demonstrate that both small molecule and endogenous cathepsin inhibitors suppress particle-induced IL-1β secretion, implicating roles for multiple cathepsins in both pro-IL-1β synthesis and NLRP3 activation.

Differential Analysis of the Secretome of WRL68 Cells Infected with the Chikungunya Virus

The Chikungunya virus (CHIKV) is an arthropod borne virus. In the last 50 years, it has been the cause of numerous outbreaks in tropical and temperate regions, worldwide. There is limited understanding regarding the underlying molecular mechanisms involved in CHIKV replication and how the virus interacts with its host. In the present study, comparative proteomics was used to identify secreted host proteins that changed in abundance in response to early CHIKV infection. Two-dimensional gel electrophoresis was used to analyse and compare the secretome profiles of WRL-68 cells infected with CHIKV against mock control WRL-68 cells. The analysis identified 25 regulated proteins in CHIKV infected cells. STRING network analysis was then used to predict biological processes that may be affected by these proteins. The processes predicted to be affected include signal transduction, cellular component and extracellular matrix (ECM) organization, regulation of cytokine stimulus and immune response. These results provide an initial view of CHIKV may affect the secretome of infected cells during early infection. The results presented here will compliment earlier results from the study of late host response. However, functional characterization will be necessary to further enhance our understanding of the roles played by these proteins in the early stages of CHIKV infection in humans.

Proteolytic disassembly of viral outer capsid proteins is crucial for reovirus-mediated type-I interferon induction in both reovirus-susceptible and reovirus-refractory tumor cells

Oncolytic reovirus induces innate immune responses, which contribute to the antitumor activity of reovirus, following in vivo application. Reovirus-induced innate immune responses have been relatively well characterized in immune cells and mouse embryonic fibroblasts cells; however, the mechanisms and profiles of reovirus-induced innate immune responses in human tumor cells have not been well understood. In particular, differences in reovirus-induced innate immune responses between reovirus-susceptible and reovirus-refractory tumor cells remain unknown, although the intracellular trafficking of reovirus differs between these tumor cells. In this study, we examined reovirus-induced upregulation of interferon- (IFN-) β and of the proapoptotic gene, Noxa, in reovirus-susceptible and -refractory tumor cells. IFN-β and Noxa were significantly induced by reovirus via the IFN-β promoter stimulator-1 (IPS-1) signaling in both types of tumor cells. Inhibition of cathepsins B and L, which are important for disassembly of reovirus outer capsid proteins and escape into cytoplasm, largely suppressed reovirus-induced upregulation of IFN-β and Noxa expression in not only reovirus-susceptible but also reovirus-refractory tumor cells. These results indicated that in both reovirus-susceptible and reovirus-refractory tumor cells, disassembly of the outer capsid proteins by cathepsins and the escape into the cytoplasm were crucial steps for reovirus-induced innate immunity.

Sensing cytosolic RpsL by macrophages induces lysosomal cell death and termination of bacterial infection

The intracellular bacterial pathogen Legionella pneumophila provokes strong host responses and has proven to be a valuable model for the discovery of novel immunosurveillance pathways. Our previous work revealed that an environmental isolate of L. pneumophila induces a noncanonical form of cell death, leading to restriction of bacterial replication in primary mouse macrophages. Here we show that such restriction also occurs in infections with wild type clinical isolates. Importantly, we found that a lysine to arginine mutation at residue 88 (K88R) in the ribosome protein RpsL that not only confers bacterial resistance to streptomycin, but more importantly, severely attenuated the induction of host cell death and enabled L. pneumophila to replicate in primary mouse macrophages. Although conferring similar resistance to streptomycin, a K43N mutation in RpsL does not allow productive intracellular bacterial replication. Further analysis indicated that RpsL is capable of effectively inducing macrophage death via a pathway involved in lysosomal membrane permeabilization; the K88R mutant elicits similar responses but is less potent. Moreover, cathepsin B, a lysosomal protease that causes cell death after being released into the cytosol upon the loss of membrane integrity, is required for efficient RpsL-induced macrophage death. Furthermore, despite the critical role of cathepsin B in delaying RpsL-induced cell death, macrophages lacking cathepsin B do not support productive intracellular replication of L. pneumophila harboring wild type RpsL. This suggests the involvement of other yet unidentified components in the restriction of bacterial replication. Our results identified RpsL as a regulator in the interactions between bacteria such as L. pneumophila and primary mouse macrophages by triggering unique cellular pathways that restrict intracellular bacterial replication.

The death of the host cell during infection can be triggered by one or more microbial molecules; this “live or die” selection provides effective means for the dissection of immune recognition mechanisms as well as for the identification of the microbial molecules responsible for such responses. We found that infection of primary mouse macrophages by Legionella pneumophila strains harboring wild type RpsL, the S12 component of the bacterial ribosome, causes macrophage death by a mechanism independent of the three inflammatory caspases, caspase 1, 7 and 11. Importantly, although both confer resistance to streptomycin at indistinguishable effectiveness, the K88R, but not the K43N mutation in RpsL enables L. pneumophila to replicate in macrophages. Purified RpsL and RpsL_K43N physically delivered into macrophages cause cell death by inducing damage to lysosomal membranes and the release of cathepsins. We also found that the lysosomal protease cathepsin B is required for efficient RpsL-induced cell death but its absence is not sufficient for macrophages to support intracellular bacterial replication. Thus, RpsL functions as an immune induction molecule to trigger one or more signaling cascades that leads to lysosomal cell death as well as the termination of bacterial replication.

The expanding role of NLRs in antiviral immunity

Nucleotide oligomerization and binding domain (NOD)-like receptors (NLRs) are a major constituent of the cytosolic innate immune-sensing machinery and participate in a wide array of pathways including nuclear factor κB (NF-κB), mitogen-activated protein kinase (MAPK), inflammasome, and type I interferon (IFN) signaling. NLRs have known roles in autoimmune, autoinflammatory, and infectious diseases. With respect to virus infection, NLRP3 is the most extensively studied NLR, including mechanisms of activation and inhibition. Furthermore, the importance of NLRP3 in both innate and adaptive immunity has been demonstrated. In comparison to NLRP3, the roles of other NLRs during virus infection are only just emerging. NLRC2 is an important activator of innate antiviral signaling and was recently found to mitigate inflammation during virus infection through autophagy. Finally, functions for NLRX1 in immune modulation and reactive oxygen species production require further examination and the importance of NLRC5 as a transactivator of major histocompatibility complex (MHC) class I and antigen presentation is currently developing. In this review, we discuss current knowledge pertaining to viruses and NLRs as well as areas of potential research, which will help advance the study of NLR biology during virus infection.

Trichothecene mycotoxins activate NLRP3 inflammasome through a P2X7 receptor and Src tyrosine kinase dependent pathway

Inflammasome is an intracellular molecular platform of the innate immunity that is a key mediator of inflammation. The inflammasome complex detects pathogens and different danger signals, and triggers cysteine protease caspase-1-dependent processing of pro-inflammatory cytokines IL-1β, and IL-18 in dendritic cells and macrophages. Previously, we have shown that water-damaged building associated trichothecene mycotoxins, including roridin A, trigger IL-1β and IL-18 secretion in human macrophages. However, the molecular basis as well as mechanism behind this trichothecene-induced cytokine secretion has remained uncharacterized. Here, we show that the trichothecene-induced IL-1β secretion is dependent on NLRP3 inflammasome in human primary macrophages. Pharmacological inhibition and small interfering RNA approach showed that the trichothecene-induced NLRP3 inflammasome activation is mediated through ATP-gated P2X7 receptor. Moreover, we show that trichothecene-triggered NLRP3 inflammasome activation is dependent on Src tyrosine kinase activity. In addition, gene silencing of c-Cbl, a negative autophagy-related regulator of c-Src, resulted in enhanced secretion of IL-1β and IL-18 in response to trichothecene mycotoxin stimulation in human macrophages. In conclusion, our results suggest that roridin A, a fungal trichothecene mycotoxin, acts as microbial danger signals that trigger activation of NLRP3 inflammasome through P2X7R and Src tyrosine kinase signaling dependent pathway in human primary macrophages.

Glial uptake of amyloid beta induces NLRP3 inflammasome formation via cathepsin-dependent degradation of NLRP10

The NLRP3 inflammasome forms in response to a diverse range of stimuli and is responsible for the processing and release of interleukin-1β (IL-1β) from the immunocompetent cells of the brain. The pathological peptide of Alzheimer's disease, amyloid beta (Aβ), induces formation of the NLRP3 inflammasome in a manner dependent on the family of proteases, cathepsins; however, the pathway by which cathepsins induce formation of the inflammasome has not yet been elucidated. In this study, we show that Aβ treatment of primary rat glial cultures increases cathepsin activation in the cytosol, formation of the NLRP3 inflammasome, caspase 1 activation and IL-1β release. We also show that a second NOD-like protein, NLRP10, is found bound to apoptosis-associated speck-like protein under resting conditions; however, with Aβ treatment, both in vitro and in vivo, NLRP10 is decreased. Further to these data, we show that cathepsins are capable of degrading NLRP10 and that treatment of glial cultures with recombinant NLRP10 reduces Aβ-induced caspase 1 activation and IL-1β release. We propose that Aβ-induced cathepsin released into the cytosol degrades NLRP10, thus allowing dissociation of NLRP3 and formation of the inflammasome.

Human RNAi pathway: crosstalk with organelles and cells

Understanding gene regulation mechanisms has been a serious challenge in biology. As a novel mechanism, small non-coding RNAs are an alternative means of gene regulation in a specific and efficient manner. There are growing reports on regulatory roles of these RNAs including transcriptional gene silencing/activation and post-transcriptional gene silencing events. Also, there are several known small non-coding RNAs which all work through RNA interference pathway. Interestingly, these small RNAs are secreted from cells toward targeted cells presenting new communication approach in cell-cell or cell-organ signal transduction. In fact, understanding cellular and molecular basis of these pathways will strongly improve developing targeted therapies and potent and specific regulatory tools. This study will review some of the most recent findings in this subject and will introduce a super-pathway RNA interference-based small RNA silencing network.

Poly IC triggers a cathepsin D- and IPS-1-dependent pathway to enhance cytokine production and mediate dendritic cell necroptosis

RIG-I-like receptors (RLRs) sense virus-derived RNA or polyinosinic-polycytidylic acid (poly IC) to exert antiviral immune responses. Here, we examine the mechanisms underlying the adjuvant effects of poly IC. Poly IC was taken up by dendritic cells (DCs), and it induced lysosomal destabilization, which, in turn, activated an RLR-dependent signaling pathway. Upon poly IC stimulation, cathepsin D was released into the cytoplasm from the lysosome to interact with IPS-1, an adaptor molecule for RLRs. This interaction facilitated cathepsin D cleavage of caspase 8 and the activation of the transcription factor NF-κB, resulting in enhanced cytokine production. Further recruitment of the kinase RIP-1 to this complex initiated the necroptosis of a small number of DCs. HMGB1 released by dying cells enhanced IFN-β production in concert with poly IC. Collectively, these findings suggest that cathepsin D-triggered, IPS-1-dependent necroptosis is a mechanism that propagates the adjuvant efficacy of poly IC.

Inhibition of the de-myelinating properties of Aicardi-Goutières syndrome lymphocytes by cathepsin D silencing

Molecular mechanisms relating interferon-alpha (IFN-alpha) to brain damage have recently been identified in a microarray analysis of cerebrospinal fluid lymphocytes from patients with Aicardi-Goutières Syndrome (AGS). These findings demonstrate that the inhibition of angiogenesis and the activation of neurotoxic lymphocytes are the major pathogenic mechanisms involved in the brain damage consequent to elevated interferon-alpha levels. Our previous study demonstrated that cathepsin D, a lysosomal aspartyl endopeptidase, is the primary mediator of the neurotoxicity exerted by AGS lymphocytes. Cathepsin D is a potent pro-apoptotic, neurotoxic, and demyelinating protease if it is not properly inhibited by the activities of leukocystatins. In central nervous system white matter, demyelination results from cathepsin over-expression when not balanced by the expression of its inhibitors. In the present study, we used RNA interference to inhibit cathepsin D expression in AGS lymphocytes with the aim of decreasing the neurotoxicity of these cells. Peripheral blood lymphocytes collected from an AGS patient were immortalized and co-cultured with astrocytes in the presence of interferon alpha with or without cathepsin D RNA interference probes. Cathepsin D expression was measured by qPCR, and neurotoxicity was evaluated by microscopy. RNA interference inhibited cathepsin D over-production by 2.6-fold (P<0.01) in AGS lymphocytes cultured in the presence of interferon alpha. AGS lymphocytes treated using RNA interference exhibited a decreased ability to induce neurotoxicity in astrocytes. Such neurotoxicity results in the inhibition of astrocyte growth and the inhibition of the ability of astrocytes to construct web-like aggregates. These results suggest a new strategy for repairing AGS lymphocytes in vitro by inhibiting their ability to induce astrocyte damage and leukodystrophy.

Global secretome characterization of herpes simplex virus 1-infected human primary macrophages

Herpes simplex virus 1 (HSV-1) is a common pathogen infecting the majority of people worldwide at some stage in their lives. The early host response to viral infection is initiated by the cells of the innate immune response, including macrophages. Here, we have characterized the secretome of HSV-1-infected human primary macrophages using high-throughput quantitative proteomics. We identified and quantified 516 distinct human proteins with high confidence from the macrophage secretome upon HSV-1 infection, and the secretion of 411 proteins was >2-fold increased upon beta interferon (IFN-β) priming and/or HSV-1 infection. Bioinformatics analysis of the secretome data revealed that most of the secreted proteins were intracellular, and almost 80% of the proteins whose secretion increased more than 2-fold were known exosomal proteins. This strongly suggests that nonclassical, vesicle-mediated protein secretion is activated in IFN-β-primed and HSV-1-infected macrophages. Proteins related to immune and inflammatory responses, interferon-induced proteins, and endogenous danger signal proteins were efficiently secreted upon IFN-β priming and HSV-1 infection. The secreted IFN-induced proteins include interferon-induced tetratricopeptide protein 2 (IFIT2), IFIT3, signal transducer and activator of transcription 1 (STAT1), and myxovirus resistance protein A (MxA), implicating that these proteins also have important extracellular antiviral functions. Proinflammatory cytokine interleukin-1β was not released by HSV-1-infected macrophages, demonstrating that HSV-1 can antagonize inflammasome function. In conclusion, our results provide a global view of the secretome of HSV-1-infected macrophages, revealing host factors possibly having a role in antiviral defense.

Molecular definitions of cell death subroutines: recommendations of the Nomenclature Committee on Cell Death 2012

In 2009, the Nomenclature Committee on Cell Death (NCCD) proposed a set of recommendations for the definition of distinct cell death morphologies and for the appropriate use of cell death-related terminology, including 'apoptosis', 'necrosis' and 'mitotic catastrophe'. In view of the substantial progress in the biochemical and genetic exploration of cell death, time has come to switch from morphological to molecular definitions of cell death modalities. Here we propose a functional classification of cell death subroutines that applies to both in vitro and in vivo settings and includes extrinsic apoptosis, caspase-dependent or -independent intrinsic apoptosis, regulated necrosis, autophagic cell death and mitotic catastrophe. Moreover, we discuss the utility of expressions indicating additional cell death modalities. On the basis of the new, revised NCCD classification, cell death subroutines are defined by a series of precise, measurable biochemical features.