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Clean Up after Yourself
Molecular Cell
Previews Clean Up after Yourself Alice Lepelley1 and Sankar Ghosh1,* 1Department of Microbiology & Immunology, Columbia University, College of Physicians & Surgeons, New York, NY 10032, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.molcel.2016.02.021
NLRP3 inflammasome activation is accompanied by induction of mitochondrial damage. In the current issue of Cell, Zhong et al. describe an intracellular mechanism orchestrated by NF-kB to remove inflammasomeactivating damaged mitochondria and prevent pathologic inflammation. Optimally calibrated responses against infection and tissue damage are necessary to maintain homeostasis and health. Tight control of the inflammatory response is required to achieve efficient immune activation, pathogen clearance, and tissue repair, while maintaining tissue integrity. Numerous chronic diseases such as autoinflammatory syndromes, obesity, atherosclerosis, type 2 diabetes, and many more, result from a failure to control or resolve inflammatory processes (Broderick et al., 2015). The wellstudied NF-kB signaling pathway is a key regulator of physiological inflammatory responses and hence tightly regulating its activity is likely critical to maintain homeostasis (Oeckinghaus et al., 2011). In a recent paper in Cell, Zhong et al. (2016) describe a new NF-kB-p62mitophagy pathway that prevents prolonged inflammatory responses. Specialized cells of the immune system like macrophages initiate inflammatory responses upon detection of pathogen- and damage-associated molecular patterns (PAMPs and DAMPs) by Toll-like receptors (TLRs) and NOD-like receptors (NLRs). The NOD-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome is a multiprotein complex that is essential for responses to infection or cellular stress. Its activation involves a two-step process with priming upon PAMPs recognition by TLRs, along with detection of cytoplasmic danger signals by NLRP3, leading to processing of the inflammatory cytokines IL1b and IL18 (Figure 1) (Elliott and Sutterwala, 2015). The NLRP3 inflammasome has been shown to be involved in several acquired and inherited inflammatory diseases (Broderick et al., 2015). The mechanism by which the NLRP3 inflammasome is activated has been diffi-
cult to pin down mainly because of the diversity of activating stimuli. It has been hypothesized that inflammasome activators trigger a common danger signature detected by NLRP3, like mitochondrial damage, which translates into release of mtDNA, increase in mitochondrial reactive oxygen species (mROS) and cytoplasmic exposure of cardiolipins. NLRP3 inflammasome activation has been suggested to induce and require mitochondrial damage, although no consensus on the latter has been reached so far (Elliott and Sutterwala, 2015). In their report, Zhong et al. (2016) confirm that mitochondrial dysfunction acts upstream of NLRP3 inflammasome activation in their system, and add another role for mitochondrial damage in NLRP3 inflammasome regulation. Little is known about how the NLRP3 inflammasome is controlled to prevent overt inflammation. Selective autophagy enables the specific elimination of cellular danger signals such as protein aggregates, damaged mitochondria, and intracellular pathogens upon recognition by autophagy receptors, and autophagy has been suggested to act as a brake for the NLRP3 inflammasome (Elliott and Sutterwala, 2015; Saitoh et al., 2008). But such work emphasized requirement for autophagy blockade for optimal inflammasome activation. Inhibition of mitophagy, autophagic removal of damaged mitochondria, increased mitochondrial damage to enhance inflammasome activation (Elliott and Sutterwala, 2015; Zhou et al., 2011). Zhong et al. (2016) now hypothesize that autophagy might also prevent NLRP3 inflammasome activation from going awry. They propose the existence of a negative feedback loop orchestrated by the NF-kB pathway, which is required for
644 Molecular Cell 61, March 3, 2016 ª2016 Elsevier Inc.
NLRP3 inflammasome function through priming downstream of TLRs. Recent observations in mice and humans have suggested that NF-kB pathway could exert an inhibitory effect on NLRP3 inflammasome (Greten et al., 2007). One possible mechanism involves the ability of NF-kB to activate autophagy (Baldwin, 2012). Zhong et al. (2016) observed that NF-kB-dependent priming of macrophages induced prolonged upregulation of the autophagy receptor p62/SQSTM1, implicated in the elimination of polyubiquitinated proteins and damaged mitochondria (Komatsu et al., 2012). Fittingly, lack of p62 caused enhanced inflammasome activation both in vivo and in vitro in the Zhong et al. (2016) study. This effect was specific to the NLRP3 inflammasome, and the involvement of the NF-kB pathway was demonstrated by inhibition of its key upstream activator, IKKb (Zhong et al., 2016). As many NLRP3 inflammasome stimuli trigger mitochondrial damage causing sustained inflammasome activation, p62 could be promoting the removal of dysfunctional mitochondria. Mitophagy involves Parkin, a protein implicated in Parkinson’s disease, for the recruitment of the autophagy machinery, controlled by ATG7 (Eiyama and Okamoto, 2015). Zhong et al. (2016) show by diverse methods that NLRP3 inflammasome activation initiates a mitophagic response in macrophages. They observed Parkindependent accumulation of polyubiquitin chains on mitochondria and lack of Parkin or ATG7 led to increase in the pool of dysfunctional mitochondria, along with the potential NLRP3 triggers, mROS and cytosolic mtDNA. Accordingly, inflammasome activation was enhanced in Parkin- and ATG7-deficient cells. Interestingly, lack of p62 phenocopied Parkin
Molecular Cell
Previews disruption, suggesting that inflammation) (Greten et al., both molecules participate 2007) has complicated efforts in mitophagy induction to to target this pathway in limit accumulation of clinical settings. The work damaged mitochondria and described in Zhong et al. activation of NLRP3 inflam(2016) further emphasizes masome. Furthermore, scavthe care that should be taken enging mROS inhibited IL1b when attempting to inhibit release in wild-type and NF-kB pathway. Interestingly, p62-deficient macrophages. defects in mitochondrial qualBesides reinforcing the ity control have been implicated in neurodegenerative role of mitochondrial disrupdiseases, suggesting that mition in NLRP3 inflammasome tophagy is a very versatile activation, the study by actor in homeostasis mainteZhong et al. (2016) sheds nance, whose modulation light on a safeguard mechacould be significant for many nism orchestrated by NF-kB diseases. to help resolve inflammation and maintain immune homeostasis. This feedback ACKNOWLEDGMENTS loop implicates the repair of Research in the authors laboratory is a mitochondrial damage Figure 1. Mitochondrial Feedback Loop during NLRP3 supported by a fellowship from state in macrophages by the Inflammasome Activation BrightFocus Foundation (A.L.) and mitophagy machinery. A In macrophages, recognition of PAMPs, like lipopolysaccharide (LPS), by grants from NIH (S.G.). pathogen recognition receptors, like TLRs, leads to NF-kB activation and innumber of exciting questions flammasome priming by upregulation of NLRP3 inflammasome components are also raised from these REFERENCES and cytokine precursors. NLRP3 inflammasome stimuli triggering mitochonfindings. For example, the drial damage contribute to NLRP3 inflammasome complex assembly and role of p62 in mitophagy and Baldwin, A.S. (2012). Immunol. Rev. activation leading to Caspase 1 activation and maturation of pro-inflammatory 246, 327–345. cytokine IL1b and IL18. Inhibition of mitophagy has been suggested to in inflammation has been contribute to accumulation of dysfunctional mitochondria for optimal insomewhat controversial. A Broderick, L., De Nardo, D., Franklin, flammasome activity. In parallel, NF-kB activation induces prolonged p62 B.S., Hoffman, H.M., and Latz, E. recent systematic study of expression, which contributes to recruitment of the mitophagy machinery to (2015). Annu. Rev. Pathol. 10, dysfunctional mitochondria, limitation of mitochondrial damage, and control of mitophagy in HeLa cells has 395–424. inflammasome activation for optimal pathogen clearance and tissue integrity questioned the requirement maintenance, preventing overt inflammation. Eiyama, A., and Okamoto, K. (2015). for p62 (Lazarou et al., Curr. Opin. Cell Biol. 33, 95–101. 2015). Moreover, prevention of inflammation in p62-deficient animals mitochondrial damage (Lazarou et al., Elliott, E.I., and Sutterwala, F.S. (2015). Immunol. (Komatsu et al., 2012) is in contrast with 2015). Hence, it is possible that the extent Rev. 265, 35–52. this work and others focusing on NLRP3 of mitochondrial damage induced by Greten, F.R., Arkan, M.C., Bollrath, J., Hsu, L.C., inflammasome activation in macro- NLRP3 inflammasome is subtler than pre- Goode, J., Miething, C., Go¨ktuna, S.I., Neuenhahn, M., Fierer, J., Paxian, S., et al. (2007). Cell 130, phages. Interestingly, p62 has also been viously thought, and damaged mitochon- 918–931. implicated in direct removal of inflamma- dria easily replaced. Further studies are some components (Elliott and Sutterwala, necessary to determine the extent of mito- Komatsu, M., Kageyama, S., and Ichimura, Y. (2012). Pharmacol. Res. 66, 457–462. 2015). Discrepancies in choices of cell chondrial disruption required for NLRP3 types, stimuli, read-outs, and not to activation. How is mitophagy balanced Lazarou, M., Sliter, D.A., Kane, L.A., Sarraf, S.A., Wang, C., Burman, J.L., Sideris, D.P., Fogel, A.I., mention means of gene disruption, might to achieve initial mitochondrial damage and Youle, R.J. (2015). Nature 524, 309–314. explain many of the recent controversies. accumulation and ultimate elimination? A., Hayden, M.S., and Ghosh, S. Even if p62 is redundant in mitophagy, an The prolonged upregulation of p62 by Oeckinghaus, (2011). Nat. Immunol. 12, 695–708. important question is what makes p62 the NF-kB pathway offers a possible lead. so essential in mitochondrial damage Considering the numerous roles of Saitoh, T., Fujita, N., Jang, M.H., Uematsu, S., Yang, B.G., Satoh, T., Omori, H., Noda, T., Yamaresponse in NLRP3 inflammasome acti- NF-kB in tumorigenesis and chronic moto, N., Komatsu, M., et al. (2008). Nature 456, vation in macrophages. Therefore, could inflammation, inhibition of this pathway 264–268. p62 have additional functions triggered has garnered significant interest through Zhong, Z., Umemura, A., Sanchez-Lopez, E., by the NF-kB pathway to control NLRP3 the years as a potential therapeutic Liang, S., Shalapour, S., Wong, J., He, F., Boassa, inflammasome? approach (Baldwin, 2012; Oeckinghaus D., Perkins, G., Ali, S.R., McGeough, M.G., Ellisman, M.H., Seki, E., Gustafsson, A.B., Hoffman, Another surprising observation is the et al., 2011). However, the surprising role H.M., Diaz-Meco, M.T., Moscat, J., and Karin, M. lack of evidence of mitochondrial clear- of NF-kB inhibition in increasing inflam- (2016). Cell 164, this issue, 667–676. ance upon inflammasome activation, as mation (e.g., IKKb antagonism-depen- Zhou, R., Yazdi, A.S., Menu, P., and Tschopp, J. can be observed in other cell types upon dent increase in IL1b leading to excessive (2011). Nature 469, 221–225. Molecular Cell 61, March 3, 2016 ª2016 Elsevier Inc. 645
Previews Clean Up after Yourself Alice Lepelley1 and Sankar Ghosh1,* 1Department of Microbiology & Immunology, Columbia University, College of Physicians & Surgeons, New York, NY 10032, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.molcel.2016.02.021
NLRP3 inflammasome activation is accompanied by induction of mitochondrial damage. In the current issue of Cell, Zhong et al. describe an intracellular mechanism orchestrated by NF-kB to remove inflammasomeactivating damaged mitochondria and prevent pathologic inflammation. Optimally calibrated responses against infection and tissue damage are necessary to maintain homeostasis and health. Tight control of the inflammatory response is required to achieve efficient immune activation, pathogen clearance, and tissue repair, while maintaining tissue integrity. Numerous chronic diseases such as autoinflammatory syndromes, obesity, atherosclerosis, type 2 diabetes, and many more, result from a failure to control or resolve inflammatory processes (Broderick et al., 2015). The wellstudied NF-kB signaling pathway is a key regulator of physiological inflammatory responses and hence tightly regulating its activity is likely critical to maintain homeostasis (Oeckinghaus et al., 2011). In a recent paper in Cell, Zhong et al. (2016) describe a new NF-kB-p62mitophagy pathway that prevents prolonged inflammatory responses. Specialized cells of the immune system like macrophages initiate inflammatory responses upon detection of pathogen- and damage-associated molecular patterns (PAMPs and DAMPs) by Toll-like receptors (TLRs) and NOD-like receptors (NLRs). The NOD-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome is a multiprotein complex that is essential for responses to infection or cellular stress. Its activation involves a two-step process with priming upon PAMPs recognition by TLRs, along with detection of cytoplasmic danger signals by NLRP3, leading to processing of the inflammatory cytokines IL1b and IL18 (Figure 1) (Elliott and Sutterwala, 2015). The NLRP3 inflammasome has been shown to be involved in several acquired and inherited inflammatory diseases (Broderick et al., 2015). The mechanism by which the NLRP3 inflammasome is activated has been diffi-
cult to pin down mainly because of the diversity of activating stimuli. It has been hypothesized that inflammasome activators trigger a common danger signature detected by NLRP3, like mitochondrial damage, which translates into release of mtDNA, increase in mitochondrial reactive oxygen species (mROS) and cytoplasmic exposure of cardiolipins. NLRP3 inflammasome activation has been suggested to induce and require mitochondrial damage, although no consensus on the latter has been reached so far (Elliott and Sutterwala, 2015). In their report, Zhong et al. (2016) confirm that mitochondrial dysfunction acts upstream of NLRP3 inflammasome activation in their system, and add another role for mitochondrial damage in NLRP3 inflammasome regulation. Little is known about how the NLRP3 inflammasome is controlled to prevent overt inflammation. Selective autophagy enables the specific elimination of cellular danger signals such as protein aggregates, damaged mitochondria, and intracellular pathogens upon recognition by autophagy receptors, and autophagy has been suggested to act as a brake for the NLRP3 inflammasome (Elliott and Sutterwala, 2015; Saitoh et al., 2008). But such work emphasized requirement for autophagy blockade for optimal inflammasome activation. Inhibition of mitophagy, autophagic removal of damaged mitochondria, increased mitochondrial damage to enhance inflammasome activation (Elliott and Sutterwala, 2015; Zhou et al., 2011). Zhong et al. (2016) now hypothesize that autophagy might also prevent NLRP3 inflammasome activation from going awry. They propose the existence of a negative feedback loop orchestrated by the NF-kB pathway, which is required for
644 Molecular Cell 61, March 3, 2016 ª2016 Elsevier Inc.
NLRP3 inflammasome function through priming downstream of TLRs. Recent observations in mice and humans have suggested that NF-kB pathway could exert an inhibitory effect on NLRP3 inflammasome (Greten et al., 2007). One possible mechanism involves the ability of NF-kB to activate autophagy (Baldwin, 2012). Zhong et al. (2016) observed that NF-kB-dependent priming of macrophages induced prolonged upregulation of the autophagy receptor p62/SQSTM1, implicated in the elimination of polyubiquitinated proteins and damaged mitochondria (Komatsu et al., 2012). Fittingly, lack of p62 caused enhanced inflammasome activation both in vivo and in vitro in the Zhong et al. (2016) study. This effect was specific to the NLRP3 inflammasome, and the involvement of the NF-kB pathway was demonstrated by inhibition of its key upstream activator, IKKb (Zhong et al., 2016). As many NLRP3 inflammasome stimuli trigger mitochondrial damage causing sustained inflammasome activation, p62 could be promoting the removal of dysfunctional mitochondria. Mitophagy involves Parkin, a protein implicated in Parkinson’s disease, for the recruitment of the autophagy machinery, controlled by ATG7 (Eiyama and Okamoto, 2015). Zhong et al. (2016) show by diverse methods that NLRP3 inflammasome activation initiates a mitophagic response in macrophages. They observed Parkindependent accumulation of polyubiquitin chains on mitochondria and lack of Parkin or ATG7 led to increase in the pool of dysfunctional mitochondria, along with the potential NLRP3 triggers, mROS and cytosolic mtDNA. Accordingly, inflammasome activation was enhanced in Parkin- and ATG7-deficient cells. Interestingly, lack of p62 phenocopied Parkin
Molecular Cell
Previews disruption, suggesting that inflammation) (Greten et al., both molecules participate 2007) has complicated efforts in mitophagy induction to to target this pathway in limit accumulation of clinical settings. The work damaged mitochondria and described in Zhong et al. activation of NLRP3 inflam(2016) further emphasizes masome. Furthermore, scavthe care that should be taken enging mROS inhibited IL1b when attempting to inhibit release in wild-type and NF-kB pathway. Interestingly, p62-deficient macrophages. defects in mitochondrial qualBesides reinforcing the ity control have been implicated in neurodegenerative role of mitochondrial disrupdiseases, suggesting that mition in NLRP3 inflammasome tophagy is a very versatile activation, the study by actor in homeostasis mainteZhong et al. (2016) sheds nance, whose modulation light on a safeguard mechacould be significant for many nism orchestrated by NF-kB diseases. to help resolve inflammation and maintain immune homeostasis. This feedback ACKNOWLEDGMENTS loop implicates the repair of Research in the authors laboratory is a mitochondrial damage Figure 1. Mitochondrial Feedback Loop during NLRP3 supported by a fellowship from state in macrophages by the Inflammasome Activation BrightFocus Foundation (A.L.) and mitophagy machinery. A In macrophages, recognition of PAMPs, like lipopolysaccharide (LPS), by grants from NIH (S.G.). pathogen recognition receptors, like TLRs, leads to NF-kB activation and innumber of exciting questions flammasome priming by upregulation of NLRP3 inflammasome components are also raised from these REFERENCES and cytokine precursors. NLRP3 inflammasome stimuli triggering mitochonfindings. For example, the drial damage contribute to NLRP3 inflammasome complex assembly and role of p62 in mitophagy and Baldwin, A.S. (2012). Immunol. Rev. activation leading to Caspase 1 activation and maturation of pro-inflammatory 246, 327–345. cytokine IL1b and IL18. Inhibition of mitophagy has been suggested to in inflammation has been contribute to accumulation of dysfunctional mitochondria for optimal insomewhat controversial. A Broderick, L., De Nardo, D., Franklin, flammasome activity. In parallel, NF-kB activation induces prolonged p62 B.S., Hoffman, H.M., and Latz, E. recent systematic study of expression, which contributes to recruitment of the mitophagy machinery to (2015). Annu. Rev. Pathol. 10, dysfunctional mitochondria, limitation of mitochondrial damage, and control of mitophagy in HeLa cells has 395–424. inflammasome activation for optimal pathogen clearance and tissue integrity questioned the requirement maintenance, preventing overt inflammation. Eiyama, A., and Okamoto, K. (2015). for p62 (Lazarou et al., Curr. Opin. Cell Biol. 33, 95–101. 2015). Moreover, prevention of inflammation in p62-deficient animals mitochondrial damage (Lazarou et al., Elliott, E.I., and Sutterwala, F.S. (2015). Immunol. (Komatsu et al., 2012) is in contrast with 2015). Hence, it is possible that the extent Rev. 265, 35–52. this work and others focusing on NLRP3 of mitochondrial damage induced by Greten, F.R., Arkan, M.C., Bollrath, J., Hsu, L.C., inflammasome activation in macro- NLRP3 inflammasome is subtler than pre- Goode, J., Miething, C., Go¨ktuna, S.I., Neuenhahn, M., Fierer, J., Paxian, S., et al. (2007). Cell 130, phages. Interestingly, p62 has also been viously thought, and damaged mitochon- 918–931. implicated in direct removal of inflamma- dria easily replaced. Further studies are some components (Elliott and Sutterwala, necessary to determine the extent of mito- Komatsu, M., Kageyama, S., and Ichimura, Y. (2012). Pharmacol. Res. 66, 457–462. 2015). Discrepancies in choices of cell chondrial disruption required for NLRP3 types, stimuli, read-outs, and not to activation. How is mitophagy balanced Lazarou, M., Sliter, D.A., Kane, L.A., Sarraf, S.A., Wang, C., Burman, J.L., Sideris, D.P., Fogel, A.I., mention means of gene disruption, might to achieve initial mitochondrial damage and Youle, R.J. (2015). Nature 524, 309–314. explain many of the recent controversies. accumulation and ultimate elimination? A., Hayden, M.S., and Ghosh, S. Even if p62 is redundant in mitophagy, an The prolonged upregulation of p62 by Oeckinghaus, (2011). Nat. Immunol. 12, 695–708. important question is what makes p62 the NF-kB pathway offers a possible lead. so essential in mitochondrial damage Considering the numerous roles of Saitoh, T., Fujita, N., Jang, M.H., Uematsu, S., Yang, B.G., Satoh, T., Omori, H., Noda, T., Yamaresponse in NLRP3 inflammasome acti- NF-kB in tumorigenesis and chronic moto, N., Komatsu, M., et al. (2008). Nature 456, vation in macrophages. Therefore, could inflammation, inhibition of this pathway 264–268. p62 have additional functions triggered has garnered significant interest through Zhong, Z., Umemura, A., Sanchez-Lopez, E., by the NF-kB pathway to control NLRP3 the years as a potential therapeutic Liang, S., Shalapour, S., Wong, J., He, F., Boassa, inflammasome? approach (Baldwin, 2012; Oeckinghaus D., Perkins, G., Ali, S.R., McGeough, M.G., Ellisman, M.H., Seki, E., Gustafsson, A.B., Hoffman, Another surprising observation is the et al., 2011). However, the surprising role H.M., Diaz-Meco, M.T., Moscat, J., and Karin, M. lack of evidence of mitochondrial clear- of NF-kB inhibition in increasing inflam- (2016). Cell 164, this issue, 667–676. ance upon inflammasome activation, as mation (e.g., IKKb antagonism-depen- Zhou, R., Yazdi, A.S., Menu, P., and Tschopp, J. can be observed in other cell types upon dent increase in IL1b leading to excessive (2011). Nature 469, 221–225. Molecular Cell 61, March 3, 2016 ª2016 Elsevier Inc. 645