title: Shigella type III secretion protein MxiI is recognized by Naip2 to induce Nlrc4 inflammasome activation independently of Pkcδ.
abstract: Recognition of intracellular pathogenic bacteria by members of the nucleotide-binding domain and leucine-rich repeat containing (NLR) family triggers immune responses against bacterial infection. A major response induced by several Gram-negative bacteria is the activation of caspase-1 via the Nlrc4 inflammasome. Upon activation, caspase-1 regulates the processing of proIL-1β and proIL-18 leading to the release of mature IL-1β and IL-18, and induction of pyroptosis. The activation of the Nlrc4 inflammasome requires the presence of an intact type III or IV secretion system that mediates the translocation of small amounts of flagellin or PrgJ-like rod proteins into the host cytosol to induce Nlrc4 activation. Using the Salmonella system, it was shown that Naip2 and Naip5 link flagellin and the rod protein PrgJ, respectively, to Nlrc4. Furthermore, phosphorylation of Nlrc4 at Ser533 by Pkcδ was found to be critical for the activation of the Nlrc4 inflammasome. Here, we show that Naip2 recognizes the Shigella T3SS inner rod protein MxiI and induces Nlrc4 inflammasome activation. The expression of MxiI in primary macrophages was sufficient to induce pyroptosis and IL-1β release, which were prevented in macrophages deficient in Nlrc4. In the presence of MxiI or Shigella infection, MxiI associated with Naip2, and Naip2 interacted with Nlrc4. siRNA-mediated knockdown of Naip2, but not Naip5, inhibited Shigella-induced caspase-1 activation, IL-1β maturation and Asc pyroptosome formation. Notably, the Pkcδ kinase was dispensable for caspase-1 activation and secretion of IL-1β induced by Shigella or Salmonella infection. These results indicate that activation of caspase-1 by Shigella is triggered by the rod protein MxiI that interacts with Naip2 to induce activation of the Nlrc4 inflammasome independently of the Pkcδ kinase.
authors: Department of Pathology and Comprehensive Cancer Center, University of Michigan Medical School, Ann Arbor, Michigan, United States of America ; Division of Bacterial Infection Biology, Institute of Medical Science, University of Tokyo, Tokyo, Japan.
authors: Shiho
authors: S
authors: Suzuki
authors: Department of Pathology and Comprehensive Cancer Center, University of Michigan Medical School, Ann Arbor, Michigan, United States of America ; Lycera Corp., Ann Arbor, Michigan, United States of America.
authors: Luigi
authors: L
authors: Franchi
authors: Department of Pathology and Comprehensive Cancer Center, University of Michigan Medical School, Ann Arbor, Michigan, United States of America.
authors: Yuan
authors: Y
authors: He
authors: Department of Pathology and Comprehensive Cancer Center, University of Michigan Medical School, Ann Arbor, Michigan, United States of America.
authors: Raul
authors: R
authors: Muñoz-Planillo
authors: Division of Bacteriology, Department of Infectious Diseases Control, International Research Center for Infectious Diseases, Institute of Medical Science, University of Tokyo, Tokyo, Japan.
authors: Hitomi
authors: H
authors: Mimuro
authors: Department of Molecular Bacteriology and Immunology, Graduate School of Medicine, University of the Ryukyus, Okinawa, Japan.
authors: Toshihiko
authors: T
authors: Suzuki
authors: Division of Bacterial Infection Biology, Institute of Medical Science, University of Tokyo, Tokyo, Japan ; Nippon Institute for Biological Science, Tokyo, Japan.
authors: Chihiro
authors: C
authors: Sasakawa
authors: Department of Pathology and Comprehensive Cancer Center, University of Michigan Medical School, Ann Arbor, Michigan, United States of America.
authors: Gabriel
authors: G
authors: Núñez
mesh_terms: D000818:Animals; D051017:Apoptosis Regulatory Proteins / Q000378:metabolism*; D001426:Bacterial Proteins / Q000378:metabolism*; D002135:Calcium-Binding Proteins / Q000378:metabolism*; D020170:Caspase 1 / Q000378:metabolism; D006790:Host-Parasite Interactions / Q000276:immunology*; D047468:Immunoprecipitation; D058847:Inflammasomes / Q000378:metabolism*; D051379:Mice; D008810:Mice, Inbred C57BL; D008817:Mice, Mutant Strains; D051616:Neuronal Apoptosis-Inhibitory Protein / Q000378:metabolism*; D051745:Protein Kinase C-delta / Q000378:metabolism*; D014162:Transfection
chemical_list: D051017:Apoptosis Regulatory Proteins; D001426:Bacterial Proteins; D002135:Calcium-Binding Proteins; D058847:Inflammasomes; C512299:Ipaf protein, mouse; C459782:MxiI protein, Shigella flexneri; C496802:Naip2 protein, mouse; D051616:Neuronal Apoptosis-Inhibitory Protein; C497259:Prkcd protein, mouse; D051745:Protein Kinase C-delta; D020170:Caspase 1