Selective inhibition of the NLRP3 inflammasome by targeting to promyelocytic leukemia protein in mouse and human

Inflammasome activity is controlled by ZBTB16-dependent SUMOylation of ASC

Inflammasome activity is important for the immune response and is instrumental in numerous clinical conditions. Here we identify a mechanism that modulates the central Caspase-1 and NLR (Nod-like receptor) adaptor protein ASC (apoptosis-associated speck-like protein containing a CARD). We show that the function of ASC in assembling the inflammasome is controlled by its modification with SUMO (small ubiquitin-like modifier) and identify that the nuclear ZBTB16 (zinc-finger and BTB domain-containing protein 16) promotes this SUMOylation. The physiological significance of this activity is demonstrated through the reduction of acute inflammatory pathogenesis caused by a constitutive hyperactive inflammasome by ablating ZBTB16 in a mouse model of Muckle-Wells syndrome. Together our findings identify an further mechanism by which ZBTB16-dependent control of ASC SUMOylation assembles the inflammasome to promote this pro-inflammatory response.

Human neurotropic polyomavirus, JC virus, late coding region encodes a novel nuclear protein, ORF4, which targets the promyelocytic leukemia nuclear bodies (PML-NBs) and modulates their reorganization

We previously reported the discovery and characterization of two novel proteins (ORF1 and ORF2) generated by the alternative splicing of the JC virus (JCV) late coding region. Here, we report the discovery and partial characterization of three additional novel ORFs from the same coding region, ORF3, ORF4 and ORF5, which potentially encode 70, 173 and 265 amino acid long proteins respectively. While ORF3 protein exhibits a uniform distribution pattern throughout the cells, we were unable to detect ORF5 expression. Surprisingly, ORF4 protein was determined to be the only JCV protein specifically targeting the promyelocytic leukemia nuclear bodies (PML-NBs) and inducing their reorganization in nucleus. Although ORF4 protein has a modest effect on JCV replication, it is implicated to play major roles during the JCV life cycle, perhaps by regulating the antiviral response of PML-NBs against JCV infections and thus facilitating the progression of the JCV-induced disease in infected individuals.

Mitochondrial DNA Release in Innate Immune Signaling

According to the endosymbiotic theory, most of the DNA of the original bacterial endosymbiont has been lost or transferred to the nucleus, leaving a much smaller (∼16 kb in mammals), circular molecule that is the present-day mitochondrial DNA (mtDNA). The ability of mtDNA to escape mitochondria and integrate into the nuclear genome was discovered in budding yeast, along with genes that regulate this process. Mitochondria have emerged as key regulators of innate immunity, and it is now recognized that mtDNA released into the cytoplasm, outside of the cell, or into circulation activates multiple innate immune signaling pathways. Here, we first review the mechanisms through which mtDNA is released into the cytoplasm, including several inducible mitochondrial pores and defective mitophagy or autophagy. Next, we cover how the different forms of released mtDNA activate specific innate immune nucleic acid sensors and inflammasomes. Finally, we discuss how intracellular and extracellular mtDNA release, including circulating cell-free mtDNA that promotes systemic inflammation, are implicated in human diseases, bacterial and viral infections, senescence and aging.

Pinellia ternata (Thunb.) Breit. Attenuates the allergic airway inflammation of cold asthma via inhibiting the activation of TLR4-medicated NF-kB and NLRP3 signaling pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Pinellia ternata (Thunb.) Breit. (PT) has been demonstrated to be effective against the allergic airway inflammation (AAI) in clinical practices, especially in cold asthma (CA). Until now, the active ingredients, protective effect, and possible mechanism of PT against CA remain unknown.

AIM OF THE STUDY

The aim of this investigation was to examine the therapeutic impact and elucidate the underlying mechanism of PT on the AAI of CA.

METHODS

The compositions of PT water extract were determined via the UPLC-Q-TOF-MS/MS. The ovalbumin (OVA) and cold-water baths were used to induce CA in female mice. Morphological characteristic observations, expectorant effect, bronchial hyperreactivity (BHR), excessive mucus secretion, and inflammatory factors were used to uncover the treatment effect of PT water extract. In addition, the mucin 5AC (MUC5AC) mRNA and protein levels and the aquaporin 5 (AQP5) mRNA and protein levels were detected via qRT-PCR, immunohistochemistry (IHC), and western blotting. Moreover, the protein expressions associated with the TLR4, NF-κB, and NLRP3 signaling pathway were monitored by western blot analysis.

RESULTS

Thirty-eight compounds were identified from PT water extract. PT showed significant therapeutic effects on mice with cold asthma in terms of expectorant activity, histopathological changes, airway inflammation, mucus secretion, and hyperreactivity. PT exhibited good anti-inflammatory effects in vitro and in vivo. The expression levels of MUC5AC mRNA and protein decreased significantly, while AQP5 expression levels increased significantly in the lung tissues of mice after administration with PT as compared to mice induced by CA. Furthermore, the protein expressions of TLR4, p-iκB, p-p65, IL-1β, IL-18, NLRP3, cleaved caspase-1, and ASC were markedly reduced following PT treatment.

CONCLUSIONS

PT attenuated the AAI of CA by modulating Th1- and Th2-type cytokines. PT could inhibit the TLR4-medicated NF-kB signaling pathway and activate the NLRP3 inflammasome to reduce CA. This study provides an alternative therapeutic agent of the AAI of CA after administration with PT.

LDL Affects the Immunomodulatory Response of Endothelial Cells by Modulation of the Promyelocytic Leukemia Protein (PML) Expression via PKC

In addition to its function as an intravascular lipid transporter, LDL also triggers signal transduction in endothelial cells (ECs), which, among other things, trigger immunomodulatory cascades, e.g., IL-6 upregulation. However, the molecular mechanisms of how these LDL-triggered immunological responses in ECs are realized are not fully understood. Since promyelocytic leukemia protein (PML) plays a role in promoting inflammatory processes, we examined the relationship between LDL, PML, and IL-6 in human ECs (HUVECs and EA.hy926 cells). RT-qPCR, immunoblotting, and immunofluorescence analyses showed that LDL but not HDL induced higher PML expression and higher numbers of PML-nuclear bodies (PML-NBs). Transfection of the ECs with a PML gene-encoding vector or PML-specific siRNAs demonstrated PML-regulated IL-6 and IL-8 expression and secretion after LDL exposure. Moreover, incubation with the PKC inhibitor sc-3088 or the PKC activator PMA showed that LDL-induced PKC activity leads to the upregulation of PML mRNA and PML protein. In summary, our experimental data suggest that high LDL concentrations trigger PKC activity in ECs to upregulate PML expression, which then increases production and secretion of IL-6 and IL-8. This molecular cascade represents a novel cellular signaling pathway with immunomodulatory effects in ECs in response to LDL exposure.

Diallyl trisulfide inhibits monosodium urate-induced NLRP3 inflammasome activation via NOX3/4-dependent mitochondrial oxidative stress in RAW 264.7 and bone marrow-derived macrophages

BACKGROUND

Monosodium urate (MSU) crystals are associated with gouty inflammatory diseases. MSU-associated inflammation is majorly triggered by NOD-like receptor protein 3 (NLRP3) inflammasome that promotes interleukin (IL)-1β secretion. Although diallyl trisulfide (DATS) is well-known polysulfide garlic compounds with anti-inflammatory effects, its action in MSU-induced inflammasome activation has not been known yet.

PURPOSE

The objective of the current study was to investigate anti-inflammasome effects and mechanisms of DATS in RAW 264.7 and bone marrow-derived macrophages (BMDM).

METHODS

The concentrations of IL-1β were analyzed with enzyme-linked immunosorbent assay. The MSU-induced mitochondrial damage and reactive oxygen species (ROS) production were detected by fluorescence microscope and flow cytometry. The protein expressions of NLRP3 signaling molecules, NADPH oxidase (NOX) 3/4 were assessed with Western blotting.

RESULTS

DATS suppressed MSU-induced IL-1β and caspase-1 accompanied by decreased inflammasome complex formation in RAW 264.7 and BMDM. In addition, DATS restored mitochondrial damage. DATS downregulated NOX 3/4 that were upregulated by MSU as predicted by gene microarray and confirmed by Western blotting.

CONCLUSION

This study first reports mechanistic finding that DATS alleviates MSU-induced NLRP3 inflammasome by mediating NOX3/4-dependent mitochondrial ROS production in macrophages in vitro and ex vivo, suggesting DATS could be effective therapeutic candidate for gouty inflammatory condition.

PML at mitochondria-associated membranes governs a trimeric complex with NLRP3 and P2X7R that modulates the tumor immune microenvironment

Uncontrolled inflammatory response arising from the tumor microenvironment (TME) significantly contributes to cancer progression, prompting an investigation and careful evaluation of counter-regulatory mechanisms. We identified a trimeric complex at the mitochondria-associated membranes (MAMs), in which the purinergic P2X7 receptor - NLRP3 inflammasome liaison is fine-tuned by the tumor suppressor PML. PML downregulation drives an exacerbated immune response due to a loss of P2X7R-NLRP3 restraint that boosts tumor growth. PML mislocalization from MAMs elicits an uncontrolled NLRP3 activation, and consequent cytokines blast fueling cancer and worsening the tumor prognosis in different human cancers. New mechanistic insights are provided for the PML-P2X7R-NLRP3 axis to govern the TME in human carcinogenesis, fostering new targeted therapeutic approaches.

Reconstitution System of NLRP3 Inflammasome in HEK293T Cells

NLRP3 (NOD-, LRR-, and pyrin domain-containing protein 3) inflammasome is a cytosolic multimeric protein complex that plays key roles in the host innate immune response to both pathogenic and sterile insults. Here we describe a comprehensive guide to study NLRP3 inflammasome activation in HEK293T cell reconstitution system, which could provide direct biochemical evidence in protein interaction and posttranslational modification of the complex.

PARP-1 regulates inflammasome activity by poly-ADP-ribosylation of NLRP3 and interaction with TXNIP in primary macrophages

Poly(ADP-ribose) polymerase-1 (PARP-1) plays an essential role in DNA repair by catalyzing the polymerization of ADP-ribose unit to target proteins. Several studies have shown that PARP-1 can regulate inflammatory responses in various disease models. The intracellular Nod-like receptor NLRP3 has emerged as the most crucial innate immune receptor because of its broad specificity in mediating immune response to pathogen invasion and danger signals associated with cellular damage. In our study, we found NLRP3 stimuli-induced caspase-1 maturation and IL-1β production were impaired by PARP-1 knockout or PARP-1 inhibition in bone marrow-derived macrophages (BMDM). The step 1 signal of NLRP3 inflammasome activation was not affected by PARP-1 deficiency. Moreover, ATP-induced cytosolic ROS production was lower in Parp-1-/- BMDM, resulting in the decreased inflammasome complex assembly. PARP-1 can translocate to cytosol upon ATP stimulation and trigger the PARylation modification on NLRP3, leading to NLRP3 inflammasome assembly. PARP-1 was also a bridge between NLRP3 and thioredoxin-interacting protein (TXNIP) and participated in NLRP3/TXNIP complex formation for inflammasome activation. Overall, PARP-1 positively regulates NLRP3 inflammasome activation via increasing ROS production and interaction with TXNIP and NLRP3, leading to PARylation of NLRP3. Our data demonstrate a novel regulatory mechanism for NLRP3 inflammasome activation by PARP-1. Therefore, PARP-1 can serve as a potential target in the treatment of IL-1β associated inflammatory diseases.

Promyelocytic leukemia protein targets MK2 to promote cytotoxicity

Promyelocytic leukemia protein (PML) is a tumor suppressor possessing multiple modes of action, including induction of apoptosis. We unexpectedly find that PML promotes necroptosis in addition to apoptosis, with Pml^-/- macrophages being more resistant to TNF-mediated necroptosis than wild-type counterparts and PML-deficient mice displaying resistance to TNF-induced systemic inflammatory response syndrome. Reduced necroptosis in PML-deficient cells is associated with attenuated receptor-interacting protein kinase 1 (RIPK1) activation, as revealed by reduced RIPK1[S166] phosphorylation, and attenuated RIPK1-RIPK3-MLKL necrosome complex formation. We show that PML deficiency leads to enhanced TNF-induced MAPK-activated kinase 2 (MK2) activation and elevated RIPK1[S321] phosphorylation, which suppresses necrosome formation. MK2 inhibitor treatment or MK2 knockout abrogates resistance to cell death induction in PML-null cells and mice. PML binds MK2 and p38 MAPK, thereby inhibiting p38-MK2 interaction and MK2 activation. Moreover, PML participates in autocrine production of TNF induced by cellular inhibitors of apoptosis 1 (cIAP1)/cIAP2 degradation, since PML-knockout attenuates autocrine TNF. Thus, by targeting MK2 activation and autocrine TNF, PML promotes necroptosis and apoptosis, representing a novel tumor-suppressive activity for PML.

A Tale of Usurpation and Subversion: SUMO-Dependent Integrity of Promyelocytic Leukemia Nuclear Bodies at the Crossroad of Infection and Immunity

Promyelocytic leukemia nuclear bodies (PML NBs) are multi-protein assemblies representing distinct sub-nuclear structures. As phase-separated molecular condensates, PML NBs exhibit liquid droplet-like consistency. A key organizer of the assembly and dynamics of PML NBs is the ubiquitin-like SUMO modification system. SUMO is covalently attached to PML and other core components of PML NBs thereby exhibiting a glue-like function by providing multivalent interactions with proteins containing SUMO interacting motifs (SIMs). PML NBs serve as the catalytic center for nuclear SUMOylation and SUMO-SIM interactions are essential for protein assembly within these structures. Importantly, however, formation of SUMO chains on PML and other PML NB-associated proteins triggers ubiquitylation and proteasomal degradation which coincide with disruption of these nuclear condensates. To date, a plethora of nuclear activities such as transcriptional and post-transcriptional regulation of gene expression, apoptosis, senescence, cell cycle control, DNA damage response, and DNA replication have been associated with PML NBs. Not surprisingly, therefore, SUMO-dependent PML NB integrity has been implicated in regulating many physiological processes including tumor suppression, metabolism, drug-resistance, development, cellular stemness, and anti-pathogen immune response. The interplay between PML NBs and viral infection is multifaceted. As a part of the cellular antiviral defense strategy, PML NB components are crucial restriction factors for many viruses and a mutual positive correlation has been found to exist between PML NBs and the interferon response. Viruses, in turn, have developed counterstrategies for disarming PML NB associated immune defense measures. On the other end of the spectrum, certain viruses are known to usurp specific PML NB components for successful replication and disruption of these sub-nuclear foci has recently been linked to the stimulation rather than curtailment of antiviral gene repertoire. Importantly, the ability of invading virions to manipulate the host SUMO modification machinery is essential for this interplay between PML NB integrity and viruses. Moreover, compelling evidence is emerging in favor of bacterial pathogens to negotiate with the SUMO system thereby modulating PML NB-directed intrinsic and innate immunity. In the current context, we will present an updated account of the dynamic intricacies between cellular PML NBs as the nuclear SUMO modification hotspots and immune regulatory mechanisms in response to viral and bacterial pathogens.

Promise of the NLRP3 Inflammasome Inhibitors in In Vivo Disease Models

Nucleotide-binding oligomerization domain NOD-like receptors (NLRs) are conserved cytosolic pattern recognition receptors (PRRs) that track the intracellular milieu for the existence of infection, disease-causing microbes, as well as metabolic distresses. The NLRP3 inflammasome agglomerates are consequent to sensing a wide spectrum of pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs). Certain members of the NLR family have been documented to lump into multimolecular conglomerates called inflammasomes, which are inherently linked to stimulation of the cysteine protease caspase-1. Following activation, caspase-1 severs the proinflammatory cytokines interleukin (IL)-1β and IL-18 to their biologically active forms, with consequent commencement of caspase-1-associated pyroptosis. This type of cell death by pyroptosis epitomizes a leading pathway of inflammation. Accumulating scientific documentation has recorded overstimulation of NLRP3 (NOD-like receptor protein 3) inflammasome involvement in a wide array of inflammatory conditions. IL-1β is an archetypic inflammatory cytokine implicated in multiple types of inflammatory maladies. Approaches to impede IL-1β's actions are possible, and their therapeutic effects have been clinically demonstrated; nevertheless, such strategies are associated with certain constraints. For instance, treatments that focus on systemically negating IL-1β (i.e., anakinra, rilonacept, and canakinumab) have been reported to result in an escalated peril of infections. Therefore, given the therapeutic promise of an NLRP3 inhibitor, the concerted escalated venture of the scientific sorority in the advancement of small molecules focusing on direct NLRP3 inflammasome inhibition is quite predictable.

Ubiquitin-Modified Proteome of SARS-CoV-2-Infected Host Cells Reveals Insights into Virus-Host Interaction and Pathogenesis

The outbreak of coronavirus disease 2019 (COVID-19), which is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has posed a serious threat to global public health. The mechanism of pathogenesis and the host immune response to SARS-CoV-2 infection are largely unknown. In the present study, we applied a quantitative proteomic technology to identify and quantify the ubiquitination changes that occur in both the virus and the Vero E6 cells during SARS-CoV-2 infection. By applying label-free, quantitative liquid chromatography with tandem mass spectrometry proteomics, 8943 lysine ubiquitination sites on 3086 proteins were identified, of which 138 sites on 104 proteins were quantified as significantly upregulated, while 828 sites on 447 proteins were downregulated at 72 h post-infection. Bioinformatics analysis suggested that SARS-CoV-2 infection might modulate host immune responses through the ubiquitination of important proteins, including USP5, IQGAP1, TRIM28, and Hsp90. Ubiquitination modification was also observed on 11 SAR-CoV-2 proteins, including proteins involved in virus replication and inhibition of the host innate immune response. Our study provides new insights into the interaction between SARS-CoV-2 and the host as well as potential targets for the prevention and treatment of COVID-19.

Promyelocytic leukemia protein promotes the phenotypic switch of smooth muscle cells in atherosclerotic plaques of human coronary arteries

Promyelocytic leukemia protein (PML) is a constitutive component of PML nuclear bodies (PML-NBs), which function as stress-regulated SUMOylation factories. Since PML can also act as a regulator of the inflammatory and fibroproliferative responses characteristic of atherosclerosis, we investigated whether PML is implicated in this disease. Immunoblotting, ELISA and immunohistochemistry showed a stronger expression of PML in segments of human atherosclerotic coronary arteries and sections compared with non-atherosclerotic ones. In particular, PML was concentrated in PML-NBs from α-smooth muscle actin (α-SMA)-immunoreactive cells in plaque areas. To identify possible functional consequences of PML-accumulation in this cell type, differentiated human coronary artery smooth muscle cells (dHCASMCs) were transfected with a vector containing the intact PML-gene. These PML-transfected dHCASMCs showed higher levels of small ubiquitin-like modifier (SUMO)-1-dependent SUMOylated proteins, but lower levels of markers for smooth muscle cell (SMC) differentiation and revealed more proliferation and migration activities than dHCASMCs transfected with the vector lacking a specific gene insert or with the vector containing a mutated PML-gene coding for a PML-form without SUMOylation activity. When dHCASMCs were incubated with different cytokines, higher PML-levels were observed only after interferon γ (IFN-γ) stimulation, while the expression of differentiation markers was lower. However, these phenotypic changes were not observed in dHCASMCs treated with small interfering RNA (siRNA) suppressing PML-expression prior to IFN-γ stimulation. Taken together, our results imply that PML is a previously unknown functional factor in the molecular cascades associated with the pathogenesis of atherosclerosis and is positioned in vascular SMCs (VSMCs) between upstream IFN-γ activation and downstream SUMOylation.

New Anti SARS-Cov-2 Targets for Quinoline Derivatives Chloroquine and Hydroxychloroquine

The rapid spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has created a severe global health crisis. In this paper, we used docking and simulation methods to identify potential targets and the mechanism of action of chloroquine (CQ) and hydroxychloroquine (HCQ) against SARS-CoV-2. Our results showed that both CQ and HCQ influenced the functionality of the envelope (E) protein, necessary in the maturation processes of the virus, due to interactions that modify the flexibility of the protein structure. Furthermore, CQ and HCQ also influenced the proofreading and capping of viral RNA in SARS-CoV-2, performed by nsp10/nsp14 and nsp10/nsp16. In particular, HCQ demonstrated a better energy binding with the examined targets compared to CQ, probably due to the hydrogen bonding of the hydroxyl group of HCQ with polar amino acid residues.

Revisiting promyelocytic leukemia protein targeting by human cytomegalovirus immediate-early protein 1

Promyelocytic leukemia (PML) bodies are nuclear organelles implicated in intrinsic and innate antiviral defense. The eponymous PML proteins, central to the self-organization of PML bodies, and other restriction factors found in these organelles are common targets of viral antagonism. The 72-kDa immediate-early protein 1 (IE1) is the principal antagonist of PML bodies encoded by the human cytomegalovirus (hCMV). IE1 is believed to disrupt PML bodies by inhibiting PML SUMOylation, while PML was proposed to act as an E3 ligase for IE1 SUMOylation. PML targeting by IE1 is considered to be crucial for hCMV replication at low multiplicities of infection, in part via counteracting antiviral gene induction linked to the cellular interferon (IFN) response. However, current concepts of IE1-PML interaction are largely derived from mutant IE1 proteins known or predicted to be metabolically unstable and globally misfolded. We performed systematic clustered charge-to-alanine scanning mutagenesis and identified a stable IE1 mutant protein (IE1cc172-176) with wild-type characteristics except for neither interacting with PML proteins nor inhibiting PML SUMOylation. Consequently, IE1cc172-176 does not associate with PML bodies and is selectively impaired for disrupting these organelles. Surprisingly, functional analysis of IE1cc172-176 revealed that the protein is hypermodified by mixed SUMO chains and that IE1 SUMOylation depends on nucleosome rather than PML binding. Furthermore, a mutant hCMV expressing IE1cc172-176 was only slightly attenuated compared to an IE1-null virus even at low multiplicities of infection. Finally, hCMV-induced expression of cytokine and IFN-stimulated genes turned out to be reduced rather than increased in the presence of IE1cc172-176 relative to wild-type IE1. Our findings challenge present views on the relationship of IE1 with PML and the role of PML in hCMV replication. This study also provides initial evidence for the idea that disruption of PML bodies upon viral infection is linked to activation rather than inhibition of innate immunity.

Old dog, new trick: Trivalent arsenic as an immunomodulatory drug

Trivalent arsenic (As(III)) is recently found to be an immunomodulatory agent. As(III) has therapeutic potential in several autoimmune and inflammatory diseases in vivo. In vitro, it selectively induces apoptosis of immune cells due to different sensitivity. At a non-toxic level, As(III) shows its multifaceted nature by inducing either pro- or anti-inflammatory functions of immune subsets. These effects are exerted by either As(III)-protein interactions or as a consequence of As(III)-induced homeostasis imbalance. The immunomodulatory properties also show synergistic effects of As(III) with cancer immunotherapy. In this review, we summarize the immunomodulatory effects of As(III), focusing on the effects of As(III) on immune subsets in vitro, on mouse models of immune-related diseases, and the role of As(III) in cancer immunotherapy. Updates of the mechanisms of action, the pioneer clinical trials, dosing, and adverse events of therapeutic As(III) are also provided.

NLRP3 Inflammasome-A Key Player in Antiviral Responses

The NACHT, LRR, and PYD domains-containing protein 3 (NLRP3) inflammasome is an oligomeric complex comprised of the NOD-like receptor NLRP3, the adaptor ASC, and caspase-1. This complex is crucial to the host's defense against microbes as it promotes IL-1β and IL-18 secretion and induces pyroptosis. NLRP3 recognizes variety of pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs) generated during viral replication that triggers the NLRP3 inflammasome-dependent antiviral immune responses and facilitates viral eradication. Meanwhile, several viruses have evolved elaborate strategies to evade the immune system by targeting the NLRP3 inflammasome. In this review, we will focus on the crosstalk between the NLRP3 inflammasome and viruses, provide an overview of viral infection-induced NLRP3 inflammasome activation, and the immune escape strategies of viruses through their modulation of the NLRP3 inflammasome activity.

From atoms to physiology: what it takes to really understand inflammasomes

Rapid inflammatory responses to cytosolic threats are mediated by inflammasomes - large macromolecular signalling complexes that control the activation of the pro-inflammatory cytokines interleukin (IL)-1β and IL-18, as well as cell death by pyroptosis. Different inflammasome sensors are activated by diverse direct and indirect signals, and subsequently nucleate the polymerization of the adaptor molecule ASC to form signalling platforms macroscopically observed as ASC specks. Caspase-1 is autocatalytically activated at these sites and subsequently matures pro-inflammatory cytokines and the pore-forming effector molecule gasdermin D. While most molecules and basic assembly principles have been deduced from reductionist experimental systems, we still lack fundamental information on the structure and regulation of these complexes in their physiological environment and in the interplay with other signalling pathways. In this review, novel experimental approaches are proposed, including some that rely on nanobodies and single domain antibodies, to understand inflammasome assembly and regulation in the context of the relevant tissues or cells.

NLRP3 negatively regulates Treg differentiation through Kpna2-mediated nuclear translocation

Naïve CD4⁺ T cells in the periphery differentiate into regulatory T cells (Tregs) in which Foxp3 is expressed for their suppressive function. NLRP3, a pro-inflammatory molecule, is known to be involved in inflammasome activation associated with several diseases. Recently, the expression of NLRP3 in CD4⁺ T cells, as well as in myeloid cells, has been described; however, a role of T cell-intrinsic NLRP3 in Treg differentiation remains unknown. Here, we report that NLRP3 impeded the expression of Foxp3 independent of inflammasome activation in Tregs. NLRP3-deficient mice elevate Treg generation in various organs in the de novo pathway. NLRP3 deficiency increased the amount and suppressive activity of Treg populations, whereas NLRP3 overexpression reduced Foxp3 expression and Treg abundance. Importantly, NLRP3 interacted with Kpna2 and translocated to the nucleus from the cytoplasm under Treg-polarizing conditions. Taken together, our results identify a novel role for NLRP3 as a new negative regulator of Treg differentiation, mediated via its interaction with Kpna2 for nuclear translocation.

Cichorium intybus L. Extract Suppresses Experimental Gout by Inhibiting the NF-κB and NLRP3 Signaling Pathways

BACKGROUND

The production and maturation of interleukin (IL)-1β, regulated by the NF-κB and NLRP3 signaling pathways, lie at the core of gout. This study aimed to evaluate the antigout effect of Cichorium intybus L. (also known as chicory) in vivo and in vitro.

METHODS

A gout animal model was established with monosodium urate (MSU) crystal injections. Rats were orally administered with chicory extract or colchicine. Levels of ankle edema, inflammatory activity, and IL-1β release were observed. Several essential targets of the NF-κB and NLRP3 signaling pathways were detected. Primary macrophages were isolated to verify the antigout mechanism of chicory extract as well as chicoric acid in vitro.

RESULTS

Improvements of swelling degree, inflammatory activity, and histopathological lesion in MSU-injected ankles were observed in the treatment with chicory extract. Further, the chicory extract significantly decreased IL-1β release by suppressing the NF-κB and NLRP3 signaling pathways in gout rats. Similar to the in vivo results, IL-1β release was also inhibited by chicory extract and chicoric acid, a specific effective compound in chicory, through the NF-κB and NLRP3 signaling pathways.

CONCLUSION

This study suggests that chicory extract and chicoric acid may be used as promising therapeutic agents against gout by inhibiting the NF-κB and NLRP3 signaling pathways.

Protein kinase D mediates inflammatory responses of human placental macrophages to Group B Streptococcus

PROBLEM

During pregnancy, Group B Streptococcus (GBS) can infect fetal membranes to cause chorioamnionitis, resulting in adverse pregnancy outcomes. Macrophages are the primary resident phagocyte in extraplacental membranes. Protein kinase D (PKD) was recently implicated in mediating pro-inflammatory macrophage responses to GBS outside of the reproductive system. This work aimed to characterize the human placental macrophage inflammatory response to GBS and address the extent to which PKD mediates such effects.

METHOD

Primary human placental macrophages were infected with GBS in the presence or absence of a specific, small molecule PKD inhibitor, CRT 0066101. Macrophage phenotypes were characterized by evaluating gene expression, cytokine release, assembly of the NLRP3 inflammasome, and NFκB activation.

RESULTS

GBS evoked a strong inflammatory phenotype characterized by the release of inflammatory cytokines (TNFα, IL-1β, IL-6 (P ≤ 0.05), NLRP3 inflammasome assembly (P ≤ 0.0005), and NFκB activation (P ≤ 0.05). Pharmacological inhibition of PKD suppressed these responses, newly implicating a role for PKD in mediating immune responses of primary human placental macrophages to GBS.

CONCLUSION

PKD plays a critical role in mediating placental macrophage inflammatory activation in response to GBS infection.

Mercury and arsenic attenuate canonical and non-canonical NLRP3 inflammasome activation

Exposure to heavy metals can cause several diseases associated with the immune system. Although the effects of heavy metals on production of inflammatory cytokines have been previously studied, the role of heavy metals in inflammasome activation remains poorly studied. The inflammasome is an intracellular multi-protein complex that detects intracellular danger signals, resulting in inflammatory responses such as cytokine maturation and pyroptosis. In this study, we elucidated the effects of four heavy metals, including cadmium (Cd), mercury (Hg), arsenic (As), and lead (Pb), on the activation of NLRP3, NLRC4, and AIM2 inflammasomes. In our results, mercury and arsenic inhibited interleukin (IL)-1β and IL-18 secretion resulting from canonical and non-canonical NLRP3 inflammasome activation in macrophages and attenuated elevation of serum IL-1β in response to LPS treatment in mice. In the mechanical studies, mercury interrupted production of mitochondrial reactive oxygen species, release of mitochondrial DNA, and activity of recombinant caspase-1, whereas arsenic down-regulated expression of promyelocytic leukemia protein. Both mercury and arsenic inhibited Asc pyroptosome formation and gasdermin D cleavage. Thus, we suggest that exposure to mercury and/or arsenic could disrupt inflammasome-mediated inflammatory responses, which might cause unexpected side effects.

Regulatory Roles of Flavonoids on Inflammasome Activation during Inflammatory Responses

Inflammation is an innate immune response to noxious stimuli to protect the body from pathogens. Inflammatory responses consist of two main steps: priming and triggering. In priming, inflammatory cells increase expressions of inflammatory molecules, while in triggering, inflammasomes are activated, resulting in cell death and pro-inflammatory cytokine secretion. Inflammasomes are protein complexes comprising intracellular pattern recognition receptors (PRRs) (e.g., nucleotide-binding oligomerization domain-like receptors (NLRs), absent in melanoma 2 (AIM2), and caspases-4/5/11) and pro-caspase-1 with or without a bipartite adaptor molecule ASC. Inflammasome activation induces pyroptosis, inflammatory cell death, and stimulates caspase-1-mediated secretion of interleukin (IL)-1b and IL-18. Flavonoids are secondary metabolites found in various plants and are considered as critical ingredients promoting health and ameliorating various disease symptoms. Anti-inflammatory activity of flavonoids and underlying mechanisms have been widely studied. This review introduces current knowledge on different types of inflammasomes and their activation during inflammatory responses and discusses recent studies regarding anti-inflammatory roles of flavonoids as suppressors of inflammasomes in inflammatory conditions. Understanding the regulatory effects of flavonoids on inflammasome activation will increase our knowledge of flavonoid-mediated anti-inflammatory activity and provide new insights into the development of flavonoid preparations to prevent and treat human inflammatory diseases.

The Human CMV IE1 Protein: An Offender of PML Nuclear Bodies

PML nuclear bodies (PML-NBs) are SUMOylation-dependent, highly complex protein assemblies that accumulate in the interchromosomal territories of the cell nucleus. Research of the last two decades revealed that many viruses have evolved effector proteins that modify PML-NBs. This correlates with antagonization of individual PML-NB components which act as host cell restriction factors. The multifunctional immediate-early protein IE1 of human cytomegalovirus directly interacts with the PML protein resulting in a disruption of the dot-like structure of PML-NBs. This review summarizes recent advances on the functional consequences of PML-NB modification by IE1. In particular, we describe that PML exerts a novel co-regulatory role during the interferon response which is abrogated by IE1. Via binding to PML, IE1 is able to compromise both intrinsic antiviral defense mechanisms and classical innate immune responses. These interactions of IE1 with innate host defenses are crucial for the onset of lytic replication and, consequently, may represent promising targets for antiviral strategies.

Innate responses to gene knockouts impact overlapping gene networks and vary with respect to resistance to viral infection

Analyses of the levels of mRNAs encoding IFIT1, IFI16, RIG-1, MDA5, CXCL10, LGP2, PUM1, LSD1, STING, and IFNβ in cell lines from which the gene encoding LGP2, LSD1, PML, HDAC4, IFI16, PUM1, STING, MDA5, IRF3, or HDAC 1 had been knocked out, as well as the ability of these cell lines to support the replication of HSV-1, revealed the following: (i) Cell lines lacking the gene encoding LGP2, PML, or HDAC4 (cluster 1) exhibited increased levels of expression of partially overlapping gene networks. Concurrently, these cell lines produced from 5 fold to 12 fold lower yields of HSV-1 than the parental cells. (ii) Cell lines lacking the genes encoding STING, LSD1, MDA5, IRF3, or HDAC 1 (cluster 2) exhibited decreased levels of mRNAs of partially overlapping gene networks. Concurrently, these cell lines produced virus yields that did not differ from those produced by the parental cell line. The genes up-regulated in cell lines forming cluster 1, overlapped in part with genes down-regulated in cluster 2. The key conclusions are that gene knockouts and subsequent selection for growth causes changes in expression of multiple genes, and hence the phenotype of the cell lines cannot be ascribed to a single gene; the patterns of gene expression may be shared by multiple knockouts; and the enhanced immunity to viral replication by cluster 1 knockout cell lines but not by cluster 2 cell lines suggests that in parental cells, the expression of innate resistance to infection is specifically repressed.

PML: Regulation and multifaceted function beyond tumor suppression

Promyelocytic leukemia protein (PML) was originally identified as a fusion partner of retinoic acid receptor alpha in acute promyelocytic leukemia patients with the (15;17) chromosomal translocation, giving rise to PML–RARα and RARα–PML fusion proteins. A body of evidence indicated that PML possesses tumor suppressing activity by regulating apoptosis, cell cycle, senescence and DNA damage responses. PML is enriched in discrete nuclear substructures in mammalian cells with 0.2–1 μm diameter in size, referred to as alternately Kremer bodies, nuclear domain 10, PML oncogenic domains or PML nuclear bodies (NBs). Dysregulation of PML NB formation results in altered transcriptional regulation, protein modification, apoptosis and cellular senescence. In addition to PML NBs, PML is also present in nucleoplasm and cytoplasmic compartments, including the endoplasmic reticulum and mitochondria-associated membranes. The role of PML in tumor suppression has been extensively studied but increasing evidence indicates that PML also plays versatile roles in stem cell renewal, metabolism, inflammatory responses, neural function, mammary development and angiogenesis. In this review, we will briefly describe the known PML regulation and function and include new findings.

Effects of Mollugo pentaphylla extract on monosodium urate crystal-induced gouty arthritis in mice

Background

Gout is an inflammatory condition induced by the deposition of monosodium urate (MSU) crystals in joints and soft tissues, and it can lead to acute or chronic arthritis. MSU are pro-inflammatory stimuli that can initiate, amplify and sustain an intense inflammatory response. In this study, we evaluated the anti-inflammatory effect of an extract of Mollugo pentaphylla (MPE) on MSU-induced gouty arthritis in a mouse model.

Method

An MSU crystal suspension (4 mg/50 μL) was injected intradermally into the right paw. The mice were orally administered MPE (150 mg/kg or 300 mg/kg) or the positive control drug colchicine (1 mg/kg) 1 h before the MSU crystals were injected and then once daily for 3 days. The effects of MPE included inflammatory paw edema and pain upon weight-bearing activity, and we evaluated the inflammatory cytokine expression and paw tissue inflammation-related gene expression.

Results

MPE suppressed inflammatory paw edema and pain in the MSU-induced mice. MPE showed anti-inflammatory activity by inhibiting the production of TNF-α, interleukin (IL)-1β, NLRP3 inflammasome and NF-κB.

Conclusion

These results suggest that MPE has potent anti-inflammatory activities and may be useful as a therapeutic agent against gouty arthritis.

Autoimmune Regulator (AIRE) Is Expressed in Spermatogenic Cells, and It Altered the Expression of Several Nucleic-Acid-Binding and Cytoskeletal Proteins in Germ Cell 1 Spermatogonial (GC1-spg) Cells

Autoimmune regulator (AIRE) is a gene associated with autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED). AIRE is expressed heavily in the thymic epithelial cells and is involved in maintaining self-tolerance through regulating the expression of tissue-specific antigens. The testes are the most predominant extrathymic location where a heavy expression of AIRE is reported. Homozygous Aire-deficient male mice were infertile, possibly due to impaired spermatogenesis, deregulated germ cell apoptosis, or autoimmunity. We report that AIRE is expressed in the testes of neonatal, adolescent, and adult mice. AIRE expression was detected in glial cell derived neurotrophic factor receptor alpha (GFRα)(+) (spermatogonia), GFRα(-)/synaptonemal complex protein (SCP3)(+) (meiotic), and GFRα(-)/Phosphoglycerate kinase 2 (PGK2)(+) (postmeiotic) germ cells in mouse testes. GC1-spg, a germ-cell-derived cell line, did not express AIRE. Retinoic acid induced AIRE expression in GC1-spg cells. Ectopic expression of AIRE in GC1-spg cells using label-free LC-MS/MS identified a total of 371 proteins that were differentially expressed. 100 proteins were up-regulated, and 271 proteins were down-regulated. Data are available via ProteomeXchange with identifier PXD002511. Functional analysis of the differentially expressed proteins showed increased levels of various nucleic-acid-binding proteins and transcription factors and a decreased level of various cytoskeletal and structural proteins in the AIRE overexpressing cells as compared with the empty vector-transfected controls. The transcripts of a select set of the up-regulated proteins were also elevated. However, there was no corresponding decrease in the mRNA levels of the down-regulated set of proteins. Molecular function network analysis indicated that AIRE influenced gene expression in GC1-spg cells by acting at multiple levels, including transcription, translation, RNA processing, protein transport, protein localization, and protein degradation, thus setting the foundation in understanding the functional role of AIRE in germ cell biology.

PML regulates neuroprotective innate immunity and neuroblast commitment in a hypoxic-ischemic encephalopathy model

Regulation of innate immune responses and activation of tissue regenerative processes are key elements in the pathophysiology of brain injuries. The promyelocytic leukemia (PML) gene was originally identified on a breakpoint of chromosomal translocation t(15;17) associated with acute PML. We have studied the role of PML protein during acute and regenerative phases after hypoxia-ischemia (HI) in brains of neonatal mice. We found that PML prevents tissue loss and apoptotic cell death selectively in subcortical regions of the brain at early stages after damage. In accordance with this, we revealed that PML is important for microglia activation and production of key inflammatory cytokines such as IL1α, IL1β, IL1RN, CXCL10, CCL12 and TNFα. During the regenerative phase, PML-depleted mice were found to have impaired transformation of transit-amplifying precursors into migratory progenitors. This was accompanied by increased ratios of symmetric versus asymmetric neural progenitor cell divisions during tissue repair and a specific defect in tissue restoration within the striatum 42 days after HI. The data demonstrate a dual role of PML in protection and recovery after brain injury.

Emerging Role of PML Nuclear Bodies in Innate Immune Signaling

Research in the last 2 decades has demonstrated that a specific organelle of the cell nucleus, termed PML nuclear body (PML-NB) or nuclear domain 10 (ND10), is frequently modified during viral infection. This correlates with antagonization of a direct repressive function of individual PML-NB components, such as the PML, hDaxx, Sp100, or ATRX protein, that are able to act as cellular restriction factors. Recent studies now reveal an emerging role of PML-NBs as coregulatory structures of both type I and type II interferon responses. This emphasizes that targeting of PML-NBs by viral regulatory proteins has evolved as a strategy to compromise intrinsic antiviral defense and innate immune responses.

Negative regulation of the inflammasome: keeping inflammation under control

In addition to its roles in controlling infection and tissue repair, inflammation plays a critical role in diverse and distinct chronic diseases, such as cancer, metabolic syndrome, and neurodegenerative disorders, underscoring the harmful effect of an uncontrolled inflammatory response. Regardless of the nature of the stimulus, initiation of the inflammatory response is mediated by assembly of a multimolecular protein complex called the inflammasome, which is responsible for the production of inflammatory cytokines, such as interleukin-1β (IL-1β) and IL-18. The different stimuli and mechanisms that control inflammasome activation are fairly well understood, but the mechanisms underlying the control of undesired inflammasome activation and its inactivation remain largely unknown. Here, we review recent advances in our understanding of the molecular mechanisms that negatively regulate inflammasome activation to prevent unwanted activation in the resting state, as well as those involved in terminating the inflammatory response after a specific insult to maintain homeostasis.

Measuring NLR Oligomerization V: In Situ Proximity Ligation Assay

The NLRP3 inflammasome is assembled in macrophages and monocytes in response to inflammatory and danger stimuli. The atypical nature of the NLRP3 complex impedes detection of NLRP3 inflammasome formation by conventional biochemical and cell biology methods. In situ proximity ligation assay (PLA) provides an alternative method of detection, localization, and quantification of protein-protein interactions in tissue and cell samples. Two primary antibodies raised in different species detect the two proteins of interest. When the proteins are in close proximity, secondary antibodies conjugated with specific DNA probes hybridize with linking oligonucleotides to form a DNA bridge between the two proteins. Amplification of the DNA bridge then facilitates detection by microscopy using fluorescence probes. Here, we describe application of in situ PLA to detect NLRP3 inflammasome assembly in mouse bone marrow-derived macrophages and human monocyte cell line THP1.

Severe acute respiratory syndrome coronavirus E protein transports calcium ions and activates the NLRP3 inflammasome

Severe acute respiratory syndrome coronavirus (SARS-CoV) envelope (E) protein is a viroporin involved in virulence. E protein ion channel (IC) activity is specifically correlated with enhanced pulmonary damage, edema accumulation and death. IL-1β driven proinflammation is associated with those pathological signatures, however its link to IC activity remains unknown. In this report, we demonstrate that SARS-CoV E protein forms protein–lipid channels in ERGIC/Golgi membranes that are permeable to calcium ions, a highly relevant feature never reported before. Calcium ions together with pH modulated E protein pore charge and selectivity. Interestingly, E protein IC activity boosted the activation of the NLRP3 inflammasome, leading to IL-1β overproduction. Calcium transport through the E protein IC was the main trigger of this process. These findings strikingly link SARS-CoV E protein IC induced ionic disturbances at the cell level to immunopathological consequences and disease worsening in the infected organism.

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SARS-CoV E protein forms calcium ion channels, a novel highly relevant function.

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Transport of calcium ions through E protein channel stimulates the inflammasome.

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Inflammasome derived exacerbated proinflammation causes SARS worsening.

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E protein ion channel and its driven proinflammation may be targets to treat SARS.

NOD-like receptors: versatile cytosolic sentinels

Nucleotide binding oligomerization domain (NOD)-like receptors are cytoplasmic pattern-recognition receptors that together with RIG-I-like receptor (retinoic acid-inducible gene 1), Toll-like receptor (TLR), and C-type lectin families make up the innate pathogen pattern recognition system. There are 22 members of NLRs in humans, 34 in mice, and even a larger number in some invertebrates like sea urchins, which contain more than 200 receptors. Although initially described to respond to intracellular pathogens, NLRs have been shown to play important roles in distinct biological processes ranging from regulation of antigen presentation, sensing metabolic changes in the cell, modulation of inflammation, embryo development, cell death, and differentiation of the adaptive immune response. The diversity among NLR receptors is derived from ligand specificity conferred by the leucine-rich repeats and an NH2-terminal effector domain that triggers the activation of different biological pathways. Here, we describe NLR genes associated with different biological processes and the molecular mechanisms underlying their function. Furthermore, we discuss mutations in NLR genes that have been associated with human diseases.

Advances in Nod-like receptors (NLR) biology

The innate immune system is composed of a wide repertoire of conserved pattern recognition receptors (PRRs) able to trigger inflammation and host defense mechanisms in response to endogenous or exogenous pathogenic insults. Among these, nucleotide-binding and oligomerization domain (NOD)-like receptors (NLRs) are intracellular sentinels of cytosolic sanctity capable of orchestrating innate immunity and inflammatory responses following the perception of noxious signals within the cell. In this review, we elaborate on recent advances in the signaling mechanisms of NLRs, operating within inflammasomes or through alternative inflammatory pathways, and discuss the spectrum of their effector functions in innate immunity. We describe the progressive characterization of each NLR with associated controversies and cutting edge discoveries.

PML control of cytokine signaling

The promyelocytic leukemia (PML) protein is a tumor suppressor acting as the organizer of nuclear matrix-associated structures named nuclear bodies (NBs). The involvement of PML in various cell processes, including cell death, senescence or antiviral defense underlines the multiple functions of PML due to its ability to interact with various partners either in the cytoplasm or in the nucleus. The importance of paracrine signaling in the regulation of PML expression is well established. More recently, a growing body of evidence also supports PML as a key regulator of cytokine signaling. These findings shed light on unsuspected biological functions of PML such as immune response, inflammation and cytokine-induced apoptosis. Here we review the current understanding of the pleiotropic activities of PML on cytokine-induced signaling.

Arsenic trioxide and other arsenical compounds inhibit the NLRP1, NLRP3, and NAIP5/NLRC4 inflammasomes

Inflammasomes are large cytoplasmic multiprotein complexes that activate caspase-1 in response to diverse intracellular danger signals. Inflammasome components termed nucleotide-binding oligomerization domain-like receptor (NLR) proteins act as sensors for pathogen-associated molecular patterns, stress, or danger stimuli. We discovered that arsenicals, including arsenic trioxide and sodium arsenite, inhibited activation of the NLRP1, NLRP3, and NAIP5/NLRC4 inflammasomes by their respective activating signals, anthrax lethal toxin, nigericin, and flagellin. These compounds prevented the autoproteolytic activation of caspase-1 and the processing and secretion of IL-1β from macrophages. Inhibition was independent of protein synthesis induction, proteasome-mediated protein breakdown, or kinase signaling pathways. Arsenic trioxide and sodium arsenite did not directly modify or inhibit the activity of preactivated recombinant caspase-1. Rather, they induced a cellular state inhibitory to both the autoproteolytic and substrate cleavage activities of caspase-1, which was reversed by the reactive oxygen species scavenger N-acetylcysteine but not by reducing agents or NO pathway inhibitors. Arsenicals provided protection against NLRP1-dependent anthrax lethal toxin-mediated cell death and prevented NLRP3-dependent neutrophil recruitment in a monosodium urate crystal inflammatory murine peritonitis model. These findings suggest a novel role in inhibition of the innate immune response for arsenical compounds that have been used as therapeutics for a few hundred years.

Promyelocytic leukemia protein interacts with the apoptosis-associated speck-like protein to limit inflammasome activation

The apoptosis-associated speck-like protein containing a caspase-activating recruitment domain (ASC) is an essential component of several inflammasomes, multiprotein complexes that regulate caspase-1 activation and inflammation. We report here an interaction between promyelocytic leukemia protein (PML) and ASC. We observed enhanced formation of ASC dimers in PML-deficient macrophages. These macrophages also display enhanced levels of ASC in the cytosol. Furthermore, IL-1β production was markedly enhanced in these macrophages in response to both NLRP3 and AIM2 inflammasome activation and following bone marrow-derived macrophage infection with herpes simplex virus-1 (HSV-1) and Salmonella typhimurium. Collectively, our data indicate that PML limits ASC function, retaining ASC in the nucleus.