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      The NLRP6 inflammasome

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          Abstract

          In this review, we summarize recent understandings on the assembly, activation and regulation patterns of the NLRP6 inflammasome, as well as its diverse impacts on tissue homeostasis and disease in multiple organs.

          Summary

          The NOD‐like receptor family pyrin domain containing 6 (NLRP6), a member of the NOD‐like receptor (NLR) family, acts as a cytosolic innate immune sensor that recognizes microbe‐associated molecular patterns. In some circumstances upon activation, NLRP6 recruits the adaptor apoptosis‐associated speck‐like protein (ASC) and the inflammatory caspase‐1 or caspase‐11 to form an inflammasome, which mediates the maturation and secretion of the pro‐inflammatory cytokines IL‐18 and IL‐1β. In other contexts, NLRP6 can exert its function in an inflammasome‐independent manner. Tight regulation of the NLRP6 inflammasome is critical in maintaining tissue homeostasis, while improper inflammasome activation may contribute to the development of multiple diseases. In intestinal epithelial cells, the NLRP6 inflammasome is suggested to play a role in regulating gut microbiome composition, goblet cell function and related susceptibility to gastrointestinal inflammatory, infectious and neoplastic diseases. Additionally, NLRP6 may regulate extra‐intestinal diseases. In this review, we summarize current knowledge on the NLRP6 inflammasome and its activation and regulation patterns, as well as its effector functions contributing to disease modulation. We discuss current challenges in NLRP6 research and future prospects in harnessing its function into potential human interventions.

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          Most cited references63

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          Inflammasome-activated gasdermin D causes pyroptosis by forming membrane pores.

          Inflammatory caspases (caspases 1, 4, 5 and 11) are activated in response to microbial infection and danger signals. When activated, they cleave mouse and human gasdermin D (GSDMD) after Asp276 and Asp275, respectively, to generate an N-terminal cleavage product (GSDMD-NT) that triggers inflammatory death (pyroptosis) and release of inflammatory cytokines such as interleukin-1β. Cleavage removes the C-terminal fragment (GSDMD-CT), which is thought to fold back on GSDMD-NT to inhibit its activation. However, how GSDMD-NT causes cell death is unknown. Here we show that GSDMD-NT oligomerizes in membranes to form pores that are visible by electron microscopy. GSDMD-NT binds to phosphatidylinositol phosphates and phosphatidylserine (restricted to the cell membrane inner leaflet) and cardiolipin (present in the inner and outer leaflets of bacterial membranes). Mutation of four evolutionarily conserved basic residues blocks GSDMD-NT oligomerization, membrane binding, pore formation and pyroptosis. Because of its lipid-binding preferences, GSDMD-NT kills from within the cell, but does not harm neighbouring mammalian cells when it is released during pyroptosis. GSDMD-NT also kills cell-free bacteria in vitro and may have a direct bactericidal effect within the cytosol of host cells, but the importance of direct bacterial killing in controlling in vivo infection remains to be determined.
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            Pore-forming activity and structural autoinhibition of the gasdermin family.

            Inflammatory caspases cleave the gasdermin D (GSDMD) protein to trigger pyroptosis, a lytic form of cell death that is crucial for immune defences and diseases. GSDMD contains a functionally important gasdermin-N domain that is shared in the gasdermin family. The functional mechanism of action of gasdermin proteins is unknown. Here we show that the gasdermin-N domains of the gasdermin proteins GSDMD, GSDMA3 and GSDMA can bind membrane lipids, phosphoinositides and cardiolipin, and exhibit membrane-disrupting cytotoxicity in mammalian cells and artificially transformed bacteria. Gasdermin-N moved to the plasma membrane during pyroptosis. Purified gasdermin-N efficiently lysed phosphoinositide/cardiolipin-containing liposomes and formed pores on membranes made of artificial or natural phospholipid mixtures. Most gasdermin pores had an inner diameter of 10–14 nm and contained 16 symmetric protomers. The crystal structure of GSDMA3 showed an autoinhibited two-domain architecture that is conserved in the gasdermin family. Structure-guided mutagenesis demonstrated that the liposome-leakage and pore-forming activities of the gasdermin-N domain are required for pyroptosis. These findings reveal the mechanism for pyroptosis and provide insights into the roles of the gasdermin family in necrosis, immunity and diseases.
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              The inflammasome: a molecular platform triggering activation of inflammatory caspases and processing of proIL-beta.

              Generation of Interleukin (IL)-1beta via cleavage of its proform requires the activity of caspase-1 (and caspase-11 in mice), but the mechanism involved in the activation of the proinflammatory caspases remains elusive. Here we report the identification of a caspase-activating complex that we call the inflammasome. The inflammasome comprises caspase-1, caspase-5, Pycard/Asc, and NALP1, a Pyrin domain-containing protein sharing structural homology with NODs. Using a cell-free system, we show that proinflammatory caspase activation and proIL-1beta processing is lost upon prior immunodepletion of Pycard. Moreover, expression of a dominant-negative form of Pycard in differentiated THP-1 cells blocks proIL-1beta maturation and activation of inflammatory caspases induced by LPS in vivo. Thus, the inflammasome constitutes an important arm of the innate immunity.
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                Author and article information

                Contributors
                eran.elinav@weizmann.ac.il , e.elinav@dkfz-heidelberg.de
                Journal
                Immunology
                Immunology
                10.1111/(ISSN)1365-2567
                IMM
                Immunology
                John Wiley and Sons Inc. (Hoboken )
                0019-2805
                1365-2567
                27 December 2020
                March 2021
                27 December 2020
                : 162
                : 3 ( doiID: 10.1111/imm.v162.3 )
                : 281-289
                Affiliations
                [ 1 ] Immunology Department Weizmann Institute of Science Rehovot Israel
                [ 2 ] Department of Gastroenterology, The First Affiliated Hospital Sun Yat‐sen University Guangzhou Guangdong China
                [ 3 ] Cancer‐Microbiome Division Deutsches Krebsforschungszentrum (DKFZ) Heidelberg Germany
                Author notes
                [*] [* ] Correspondence

                Eran Elinav, Immunology Department, Weizmann Institute of Science, 234 Herzl Street, Rehovot 7610001, Israel.

                Emails: eran.elinav@ 123456weizmann.ac.il ; e.elinav@ 123456dkfz-heidelberg.de

                Senior author: Eran Elinav.

                Author information
                https://orcid.org/0000-0002-5775-2110
                Article
                IMM13293
                10.1111/imm.13293
                7884648
                33314083
                e16953e4-2bd8-4d2b-912d-b147ed6650cf
                © 2020 The Authors. Immunology published by John Wiley & Sons Ltd.

                This is an open access article under the terms of the http://creativecommons.org/licenses/by-nc/4.0/ License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

                History
                : 29 July 2020
                : 29 October 2020
                : 10 November 2020
                Page count
                Figures: 4, Tables: 0, Pages: 9, Words: 6696
                Categories
                Review Article
                Enigmatic Inflammasomes: Lesser‐Studied NLRs Series
                Series Editor: Kathy Triantafilou
                Review Articles
                Custom metadata
                2.0
                March 2021
                Converter:WILEY_ML3GV2_TO_JATSPMC version:5.9.7 mode:remove_FC converted:15.02.2021

                Immunology
                caspase,inflammasome,innate immune receptors,microbiome,nlrp6,pattern recognition receptors
                Immunology
                caspase, inflammasome, innate immune receptors, microbiome, nlrp6, pattern recognition receptors

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