Inhwa Hwang

The Mitochondrial Antiviral Protein MAVS Associates with NLRP3 and Regulates Its Inflammasome Activity

NLRP3 assembles an inflammasome complex that activates caspase-1 upon sensing various danger signals derived from pathogenic infection, tissue damage, and environmental toxins. How NLRP3 senses these various stimuli is still poorly understood, but mitochondria and mitochondrial reactive oxygen species have been proposed to play a critical role in NLRP3 activation. In this article, we provide evidence that the mitochondrial antiviral signaling protein MAVS associates with NLRP3 and facilitates its oligomerization leading to caspase-1 activation. In reconstituted 293T cells, full-length MAVS promoted NLRP3-dependent caspase-1 activation, whereas a C-terminal transmembrane domain–truncated mutant of MAVS (MAVS-ΔTM) did not. MAVS, but not MAVS-ΔTM, interacted with NLRP3 and triggered the oligomerization of NLRP3, suggesting that mitochondrial localization of MAVS and intact MAVS signaling are essential for activating the NLRP3 inflammasome. Supporting this, activation of MAVS signaling by Sendai virus infection promoted NLRP3-dependent caspase-1 activation, whereas knocking down MAVS expression clearly attenuated the activation of NLRP3 inflammasome by Sendai virus in THP-1 and mouse macrophages. Taken together, our results suggest that MAVS facilitates the recruitment of NLRP3 to the mitochondria and may enhance its oligomerization and activation by bringing it in close proximity to mitochondrial reactive oxygen species.

Defective mitochondrial fission augments NLRP3 inflammasome activation.

Despite the fact that deregulated NLRP3 inflammasome activation contributes to the pathogenesis of chronic inflammatory or metabolic disorders, the underlying mechanism by which NLRP3 inflammasome signaling is initiated or potentiated remains poorly understood. Much attention is being paid to mitochondria as a regulator of NLRP3 inflammasome activation, but little is known about the role of mitochondrial dynamics for the inflammasome pathway. Here, we present evidence that aberrant mitochondrial elongation caused by the knockdown of dynamin-related protein 1 (Drp1) lead to a marked increase in NLRP3-dependent caspase-1 activation and interleukin-1-beta secretion in mouse bone marrow-derived macrophages. Conversely, carbonyl cyanide m-chlorophenyl hydrazone, a chemical inducer of mitochondrial fission, clearly attenuated NLRP3 inflammasome assembly and activation. Augmented activation of NLRP3 inflammasome by mitochondrial elongation is not resulted from the increased mitochondrial damages of Drp1-knockdown cells. Notably, enhanced extracellular signal-regulated kinase (ERK) signaling in Drp1-knockdown macrophages is implicated in the potentiation of NLRP3 inflammasome activation, possibly via mediating mitochondrial localization of NLRP3 to facilitate the assembly of NLRP3 inflammasome. Taken together, our results provide a molecular insight into the importance of mitochondrial dynamics in potentiating NLRP3 inflammasome activation, leading to aberrant inflammation.

Ribotoxic stress through p38 mitogen-activated protein kinase activates in vitro the human pyrin inflammasome

Background: Human pyrin is an important regulator of inflammation and is involved in the pathogenesis of familial Mediterranean fever (FMF). Results: Ribotoxic stress activates the human pyrin inflammasome. Conclusion: p38 MAPK signaling is required for assembly of the human pyrin inflammasome. Significance: Understanding the mechanism of activation of the pyrin inflammasome is crucial for the development of therapeutics to treat FMF. Human pyrin with gain-of-function mutations in its B30.2/SPRY domain causes the autoinflammatory disease familial Mediterranean fever by assembling an ASC-dependent inflammasome that activates caspase-1. Wild-type human pyrin can also form an inflammasome complex with ASC after engagement by autoinflammatory PSTPIP1 mutants. How the pyrin inflammasome is activated in the absence of disease-associated mutations is not yet known. We report here that ribotoxic stress triggers the assembly of the human pyrin inflammasome, leading to ASC oligomerization and caspase-1 activation in THP-1 macrophages and in a 293T cell line stably reconstituted with components of the pyrin inflammasome. Knockdown of pyrin and selective inhibition of p38 MAPK greatly attenuated caspase-1 activation by ribotoxic stress, whereas expression of the conditional mutant ΔMEKK3:ER* allowed the activation of caspase-1 without ribotoxic stress. Disruption of microtubules by colchicine also inhibited pyrin inflammasome activation by ribotoxic stress. Together, our results indicate that ribotoxic stress activates the human pyrin inflammasome through a mechanism that requires p38 MAPK signaling and microtubule stability.

25-hydroxycholesterol contributes to cerebral inflammation of X-linked adrenoleukodystrophy through activation of the NLRP3 inflammasome.

X-linked adrenoleukodystrophy (X-ALD), caused by an ABCD1 mutation, is a progressive neurodegenerative disorder associated with the accumulation of very long-chain fatty acids (VLCFA). Cerebral inflammatory demyelination is the major feature of childhood cerebral ALD (CCALD), the most severe form of ALD, but its underlying mechanism remains poorly understood. Here, we identify the aberrant production of cholesterol 25-hydroxylase (CH25H) and 25-hydroxycholesterol (25-HC) in the cellular context of CCALD based on the analysis of ALD patient-derived induced pluripotent stem cells and ex vivo fibroblasts. Intriguingly, 25-HC, but not VLCFA, promotes robust NLRP3 inflammasome assembly and activation via potassium efflux-, mitochondrial reactive oxygen species (ROS)- and liver X receptor (LXR)-mediated pathways. Furthermore, stereotaxic injection of 25-HC into the corpus callosum of mouse brains induces microglial recruitment, interleukin-1β production, and oligodendrocyte cell death in an NLRP3 inflammasome-dependent manner. Collectively, our results indicate that 25-HC mediates the neuroinflammation of X-ALD via activation of the NLRP3 inflammasome.

Dysbiosis-induced IL-33 contributes to impaired antiviral immunity in the genital mucosa

Significance Protective mechanisms of commensal bacteria against viral infection are limited to how immune inductive signals are provided by commensal bacteria for enhancing immunity. Whether, or how, commensal bacteria might influence the effector arm of immune responses remains unknown. Here, we demonstrate that dysbiosis within the vaginal microbiota results in severe impairment of antiviral protection against herpes simplex virus type 2 infection. IL-33 released into the vaginal tract after antibiotic treatment blocks the ability of effector T cells to migrate into the vaginal tissue and secrete the antiviral cytokine, IFN-γ. Thus, our findings suggest a previously unstudied role of commensal bacteria in the effector phase of the antiviral immune response against genital herpes. Commensal microbiota are well known to play an important role in antiviral immunity by providing immune inductive signals; however, the consequence of dysbiosis on antiviral immunity remains unclear. We demonstrate that dysbiosis caused by oral antibiotic treatment directly impairs antiviral immunity following viral infection of the vaginal mucosa. Antibiotic-treated mice succumbed to mucosal herpes simplex virus type 2 infection more rapidly than water-fed mice, and also showed delayed viral clearance at the site of infection. However, innate immune responses, including type I IFN and proinflammatory cytokine production at infection sites, as well as induction of virus-specific CD4 and CD8 T-cell responses in draining lymph nodes, were not impaired in antibiotic-treated mice. By screening the factors controlling antiviral immunity, we found that IL-33, an alarmin released in response to tissue damage, was secreted from vaginal epithelium after the depletion of commensal microbiota. This cytokine suppresses local antiviral immunity by blocking the migration of effector T cells to the vaginal tissue, thereby inhibiting the production of IFN-γ, a critical cytokine for antiviral defense, at local infection sites. These findings provide insight into the mechanisms of homeostasis maintained by commensal bacteria, and reveal a deleterious consequence of dysbiosis in antiviral immune defense.

Non-transcriptional regulation of NLRP3 inflammasome signaling by IL-4

Th2 cytokine IL‐4 has been previously shown to suppress the production of proinflammatory cytokines in monocytes. However, the underlying molecular mechanism by which IL‐4 signaling antagonizes proinflammatory responses is poorly characterized. In particular, whether IL‐4 can modulate inflammasome signaling remains unknown. Here, we provide evidence that IL‐4 suppresses NLRP3‐dependent caspase‐1 activation and the subsequent IL‐1β secretion but does not inhibit absent in melanoma 2 (AIM2)‐ or NLRC4 (NOD‐like receptor family, CARD domain‐containing 4)‐dependent caspase‐1 activation in THP‐1 and mouse bone marrow‐derived macrophages. Upon lipopolysaccharide (LPS) or LPS/ATP stimulation, IL‐4 markedly inhibited the assembly of NLRP3 inflammasome, including NLRP3‐dependent ASC (apoptosis‐associated speck‐like protein containing a caspase recruitment domain) oligomerization, NLRP3‐ASC interaction and NLRP3 speck‐like oligomeric structure formation. The negative regulation of NLRP3 inflammasome by IL‐4 was not due to the impaired mRNA or protein production of NLRP3 and proinflammatory cytokines. Supporting this observation, IL‐4 attenuated NLRP3 inflammasome activation even in reconstituted NLRP3‐expressing macrophages in which NLRP3 expression is not transcriptionally regulated by TLR‐NF‐κB signaling. Furthermore, the IL‐4‐mediated suppression of NLRP3 inflammasome was independent of STAT6‐dependent transcription and mitochondrial reactive oxygen species (ROS). Instead, IL‐4 inhibited subcellular redistribution of NLRP3 into mitochondria and microtubule polymerization upon NLRP3‐activating stimulation. Our results collectively suggest that IL‐4 could suppress NLRP3 inflammasome activation in a transcription‐independent manner, thus providing an endogenous regulatory machinery to prevent excessive inflammasome activation.

Histone deacetylase 6 negatively regulates NLRP3 inflammasome activation.

Emerging reports demonstrate that deregulated NLRP3 inflammasome activation is implicated in a variety of inflammatory and metabolic disorders, but the molecular mechanism underlying NLRP3 inflammasome regulation remains uncertain. Here, we present evidence that histone deacetylase 6 (HDAC6) inhibits the activation of NLRP3 inflammasome through its direct association with NLRP3. ShRNA-mediated knockdown of HDAC6 in bone marrow-derived macrophages (BMDMs) showed a significant increase in caspase-1 activation and interleukin-1 beta (IL-1β) secretion in response to NLRP3-activating stimulations, but not to absent in melanoma 2 (AIM2)-activating stimulation. In addition, knockdown of HDAC6 in BMDMs enhanced the oligomerization of ASC upon LPS/nigericin stimulation. The augmented NLRP3 inflammasome activation seen in HDAC6-knockdown BMDMs is independent of the deacetylase activity of HDAC6. Instead, HDAC6 directly associates with NLRP3 through its ubiquitin-binding domain. Moreover, PR619 treatment (deubiquitinase inhibitor) resulted in the elevation in the interaction of NLRP3 with HDAC6 and the decrease in NLRP3-dependent caspase-1 activation. Taken together, our results indicate that HDAC6 negatively regulates NLRP3 inflammasome activation through its interaction to ubiquitinated NLRP3.

Restoration of ASC expression sensitizes colorectal cancer cells to genotoxic stress-induced caspase-independent cell death

Apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC), an essential component of the inflammasome complex, is frequently silenced by epigenetic methylation in many tumor cells. Here, we demonstrate that restoration of ASC expression in human colorectal cancer DLD-1 cells, in which ASC is silenced by aberrant methylation, potentiated cell death mediated by DNA damaging agent. Contrarily, ASC knockdown in HT-29 cells rendered cells less susceptible to etoposide toxicity. The increased susceptibility of ASC-expressing DLD-1 cells to genotoxic stress was independent of inflammasome or caspase activation, but partially dependent on mitochondrial ROS production and JNK activation. Thus, our data suggest that ASC expression in cancer cells is an important factor in determining their susceptibility to chemotherapy.

Cobalt Chloride-induced Hypoxia Ameliorates NLRP3-Mediated Caspase-1 Activation in Mixed Glial Cultures.

Hypoxia has been shown to promote inflammation, including the release of proinflammatory cytokines, but it is poorly investigated how hypoxia directly affects inflammasome signaling pathways. To explore whether hypoxic stress modulates inflammasome activity, we examined the effect of cobalt chloride (CoCl2)-induced hypoxia on caspase-1 activation in primary mixed glial cultures of the neonatal mouse brain. Unexpectedly, hypoxia induced by oxygen-glucose deprivation or CoCl2 treatment failed to activate caspase-1 in microglial BV-2 cells and primary mixed glial cultures. Of particular interest, CoCl2-induced hypoxic condition considerably inhibited NLRP3-dependent caspase-1 activation in mixed glial cells, but not in bone marrow-derived macrophages. CoCl2-mediated inhibition of NLRP3 inflammasome activity was also observed in the isolated brain microglial cells, but CoCl2 did not affect poly dA:dT-triggered AIM2 inflammasome activity in mixed glial cells. Our results collectively demonstrate that CoCl2-induced hypoxia may negatively regulate NLRP3 inflammasome signaling in brain glial cells, but its physiological significance remains to be determined.

Salmonella Promotes ASC Oligomerization-dependent Caspase-1 Activation

Innate immune cells sense and respond to the cytoplasmic infection of bacterial pathogens through NLRP3, NLRC4 or AIM2 inflammasome depending on the unique molecular pattern of invading pathogens. The infection of flagellin- or type III secretion system (T3SS)-containing Gram-negative bacteria such as Salmonella enterica serovar Typhimurium (S. typhimurium) or Pseudomonas aeruginosa (P. aeruginosa) triggers NLRC4-dependent caspase-1 activation leading to the secretion of proinflammatory cytokines such as interleukin-1-beta (IL-1β) and IL-18. Previous studies have shown that apoptosis-associated speck-like protein containing a CARD (ASC) is also required for Salmonella-induced caspase-1 activation, but it is still unclear how ASC contributes to the activation of NLRC4 inflammasome in response to S. typhimurium infection. In this study, we demonstrate that S. typhimurium triggers the formation of ASC oligomer in a potassium depletion-independent manner as determined by in vitro crosslinking and in situ fluorescence imaging. Remarkably, inhibition of potassium efflux failed to block Salmonella-promoted caspase-1 activation and macrophage cell death. These results collectively suggest that ASC is substantially oligomerized to facilitate the activation of caspase-1 in response to S. typhimurium infection. Contrary to NLRP3 inflammasome, intracellular potassium depletion is not critical for NLRC4 inflammasome signaling by S. typhimurium.