TRIM11 Suppresses AIM2 Inflammasome by Degrading AIM2 via p62-Dependent Selective Autophagy

The AIM2 inflammasome is a key cytosolic signaling complex that is activated by double-stranded DNA, leading to the maturation of proinflammatory cytokines such as interleukin-1β (IL-1β) and IL-18. Dysregulated AIM2 inflammasome activity is associated with human inflammatory diseases and cancers, suggesting that its activity must be tightly regulated. However, the precise molecular mechanisms that control AIM2 levels and activity are still poorly understood. Here, we report tripartite motif 11 (TRIM11) as a key negative regulator of the AIM2 inflammasome. Upon DNA virus infection, TRIM11 binds to AIM2 via its PS domain and undergoes auto-polyubiquitination at K458 to promote an association between TRIM11 and the autophagic cargo receptor p62 to mediate AIM2 degradation via selective autophagy. These findings identify a role for TRIMs in AIM2 inflammasome activation where TRIM11 acts as a secondary receptor to deliver AIM2 to the autophagosomes for degradation in a p62-dependent manner.

Pyroptotic neuronal cell death mediated by the AIM2 inflammasome

The central nervous system (CNS) is an active participant in the innate immune response to infection and injury. In these studies, we show embryonic cortical neurons express a functional, deoxyribonucleic acid (DNA)-responsive, absent in melanoma 2 (AIM2) inflammasome that activates caspase-1. Neurons undergo pyroptosis, a proinflammatory cell death mechanism characterized by the following: (a) oligomerization of apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC); (b) caspase-1 dependency; (c) formation of discrete pores in the plasma membrane; and (d) release of the inflammatory cytokine interleukin-1β (IL-1β). Probenecid and Brilliant Blue FCF, inhibitors of the pannexin1 channel, prevent AIM2 inflammasome-mediated cell death, identifying pannexin1 as a cell death effector during pyroptosis and probenecid as a novel pyroptosis inhibitor. Furthermore, we show activation of the AIM2 inflammasome in neurons by cerebrospinal fluid (CSF) from traumatic brain injury (TBI) patients and oligomerization of ASC. These findings suggest neuronal pyroptosis is an important cell death mechanism during CNS infection and injury that may be attenuated by probenecid.

Increased nucleic Acid receptor expression in chronic periodontitis

BACKGROUND

Nucleic acid sensing has emerged as one of the important components of the immune system triggering inflammation. The aim of this study is to determine the expression of bacterial DNA sensors, including Toll-like receptor 9 (TLR-9), DNA-dependent activator of interferon-regulatory factors (DAI), and absent in melanoma 2 (AIM2) in chronic periodontitis (CP versus healthy) (H) tissues.

METHODS

Thirty-five CP and 27 H gingival biopsies were included. Real-time quantitative polymerase chain reaction was performed to determine mRNA levels of AIM2, DAI, and TLRs (TLR-1 through TLR-9). The difference in gene expression for each sensor between CP and H tissues was calculated using analysis of covariance. The Spearman test was used to determine correlations among innate receptors. The expression of TLR-9, AIM2, and DAI in gingival tissues was further confirmed using immunohistochemistry.

RESULTS

The present results reveal statistically significant upregulation of TLR-9 (P <0.006), DAI (P <0.001), and TLR-8 (P <0.01) in CP tissues compared to H sites. Although mRNA expression was not changed significantly between groups for other receptors, the present results reveal significant correlations between receptors (P <0.05), suggesting that cooperation between multiple components of the host immune system may influence the overall response. Immunohistochemistry further confirmed expression of TLR-9, AIM2, and DAI in gingival tissues.

CONCLUSIONS

This study highlights a possible role for nucleic acid receptors in periodontal inflammation. Future investigations will determine whether cytoplasmic receptors and their ligands can be targeted to improve clinical outcomes in periodontitis.

Sensing adenovirus infection: activation of interferon regulatory factor 3 in RAW 264.7 cells

We have used the RAW 264.7 murine macrophage-like cell line as a platform to characterize the recognition and early signaling response to recombinant adenoviral vectors (rAdV). Infection of RAW 264.7 cells triggers an early response (2 to 6 h postinfection) that includes phosphorylation of the interferon (IFN) response factor 3 (IRF3) transcription factor, upregulation of IRF3 primary response genes (interferon-stimulated gene 56 [ISG56], beta IFN [IFN-β]), and subsequent type I IFN secondary signaling (STAT1/2 phosphorylation). Using short hairpin RNA (shRNA) lentiviral vectors, we show an essential role for Tank binding kinase 1 (TBK1) in this pathway. Data also support a role for STING (MITA) as an adaptor functioning in response to rAdV infection. Using UV/psoralen (Ps)-inactivated virus to block viral transcription, Ps-inactivated virus stimulated primary (IRF3) and secondary (STAT1/2) activation events to the same degree as untreated virus. IRF3 phosphorylation was not blocked in RAW 264.7 cells pretreated with the RNA polymerase III inhibitor ML60218. However, they were compromised in the type I IFN-dependent secondary response (phosphorylation of STAT1/STAT2). At 24 h postinfection, ML60218-treated cells were compromised in the overall antiviral response. Therefore, initial sensing of rAdV or viral DNA (vDNA) does not depend on viral template transcription, but ML60218 treatment influences cellular cascades required for an antiviral response to rAdV. Using overexpression or knockdown assays, we examined how four DNA sensors influence the antiviral response. Knockdown of DNA Activator of Interferon (DAI) and p204, the murine ortholog to IFI16, had minimal influence on IRF3 phosphorylation. However, knockdown of absent in melanoma 2 (AIM2) and the helicase DDX41 resulted in diminished levels of (pser388)IRF3 following rAdV infection. Based on these data, multiple DNA sensors contribute to an antiviral DNA recognition response, leading to TBK1-dependent IRF3 phosphorylation in RAW 264.7 cells.

Interferon-inducible p200-family proteins as novel sensors of cytoplasmic DNA: role in inflammation and autoimmunity

Deregulated innate immune responses that result in increased levels of type I interferons (IFNs) and stimulation of IFN-inducible genes are thought to contribute to chronic inflammation and autoimmunity. One family of IFN-inducible genes is the Ifi200 family, which includes the murine (eg, Ifi202a, Ifi202b, Ifi203, Ifi204, Mndal, and Aim2) and human (eg, IFI16, MNDA, IFIX, and AIM2) genes. Genes in the family encode structurally related proteins (the p200-family proteins), which share at least one partially conserved repeat of 200-amino acid (200-AA) residues. Consistent with the presence of 2 consecutive oligonucleotide/oligosaccharide-binding folds in the repeat, the p200-family proteins can bind to DNA. Additionally, these proteins (except the p202 proteins) also contain a pyrin (PYD) domain in the N-terminus. Increased expression of p202 proteins in certain strains of female mice is associated with lupus-like disease. Interestingly, only the Aim2 protein is conserved between the mouse and humans. Several recent studies have provided evidence that the Aim2 and p202 proteins can recognize DNA in cytoplasm and the Aim2 protein upon sensing DNA can form a caspase-1-activating inflammasome. In this review, we discuss how the ability of p200-family proteins to sense cytoplasmic DNA may contribute to the development of chronic inflammation and associated diseases.

Critical functions of priming and lysosomal damage for NLRP3 activation

Inflammasomes are cytosolic multi-protein complexes that form in response to infectious or injurious challenges. Inflammasomes control the activity of caspase-1, which is essential for the maturation and release of IL-1beta family cytokines. The NLRP1, IPAF and AIM2 inflammasomes recognize specific substances, while the NLRP3 inflammasome responds to many structurally and chemically diverse triggers. Here, we discuss the critical roles of priming and lysosomal damage in NLRP3 inflammasome activation.

Cutting edge: critical role for PYCARD/ASC in the development of experimental autoimmune encephalomyelitis

Multiple sclerosis is an autoimmune disease in which self-reactive T cells attack oligodendrocytes that myelinate axons in the CNS. Experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis, is dependent on caspase-1; however, the role of Nod-like receptors upstream of caspase-1 is unknown. Danger- and pathogen-associated molecular patterns activate Nod-like receptor 3, which activates caspase-1 through the adaptor protein, apoptosis-associated speck-like protein containing CARD (ASC). We report that the progression of EAE is dependent on ASC and caspase-1 but not Nod-like receptor 3. ASC(-/-) mice were even more protected from the progression of EAE than were caspase-1(-/-) mice, suggesting that an inflammasome-independent function of ASC contributes to the progression of EAE. We found that CD4(+) T cells deficient in ASC exhibited impaired survival; accordingly, ASC(-/-) mice had fewer myelin oligodendrocyte glycoprotein-specific T cells in the draining lymph nodes and CNS.