Editors' ChoiceHost-Pathogen Interactions

Shigella Disables Defenses

Sci. Signal.  14 Oct 2014:
Vol. 7, Issue 347, pp. ec283
DOI: 10.1126/scisignal.aaa0468

Inflammasomes are protein signaling complexes in the myeloid immune cells of host organisms that respond to pathogenic signals to elicit production of proinflammatory cytokines and programmed cell death of the host cell, which can limit the spread of infection. However, host cell lysis can also promote dissemination of microbial pathogens and thereby contribute to infection. Inflammasome composition is variable but typically includes the protease caspase 1, the adaptor protein ASC, and receptors that bind to damage- or pathogen-associated molecular patterns, such as NLRP3 and NRL4C, which are members of the nucleotide-binding oligomerization domain (NOD)-like receptor family. Suzuki et al. found that the infectious bacteria Shigella flexneri delivers an E3 ubiquitin ligase (IpaH7.8) into the host cell cytoplasm that promotes inflammasome activation and cell lysis. Deletion of the gene encoding IpaH7.8 (ΔipaH7.8) reduced the ability of Shigella to induce caspase 1 activation, cytokine production, and cell death of cultured primary and immortalized macrophages. These effects were reversed by reconstituting ΔipaH7.8 Shigella with IpaH7.8, but not with a Cys-Ala mutant form of IpaH7.8 that lacked E3 ubiquitin ligase activity. Pharmacological or genetic inhibition of caspase 1, but not caspase 3, reduced the ability of Shigella to induce macrophage cell death. Moreover, macrophages from Nlrc4-knockout mice or those with knockdown of NLRC4 or NLRP3 produced fewer cytokines in response to Shigella infection. Expression of IpaH7.8, but not the Cys-Ala mutant IpaH7.8, stimulated cytokine production in cultured human embryonic kidney 293T cells heterologously expressing the inflammasome components. IpaH7.8 interacted with glomulin, a protein that inhibits Cullin ring type E3 ligases. Macrophages infected with Shigella with intact IpaH7.8 showed increased glomulin turnover. IpaH7.8 promoted degradation of glomulin in 293T cells and the ubiquitylation of glomulin in vitro. Glomulin colocalized with inflammasome components in macrophages stimulated with lipopolysaccharide (LPS) and ATP. Overexpression of glomulin reduced cytokine production and macrophage cell death in response to LPS and ATP stimulation or Shigella infection. In 293T cells with heterologous expression of inflammasome components, knockdown of glomulin stimulated cytokine production in the absence of infection. In mice, intranasal inoculation with Shigella, or ΔipaH7.8 Shigella reconstituted with IpaH7.8, produced more inflammation and a larger bacterial load in the lung compared with ΔipaH7.8 Shigella or ΔipaH7.8 Shigella expressing the Cys-Ala IpaH7.8 mutant. Mice with heterozygous knockout of the gene encoding glomulin (Glmn +/- mice) produced a stronger inflammatory response and had higher bacterial loads in the lung than mice with intact glomulin (Glmn +/+ mice). Cultured macrophages from Glmn+/- mice showed increased cytotoxicity and cytokine production compared to those from Glmn+/+ mice when stimulated with LPS and ATP or infected with Shigella. Thus, Shigella inject an E3 ubiquitin ligase into host macrophages to degrade a negative regulator of inflammasome activation leading to host cell lysis and increased infection.

S. Suzuki, H. Mimuro, M. Kim, M. Ogawa, H. Ashida, T. Toyotome, L. Franchi, M. Suzuki, T. Sanada, T. Suzuki, H. Tsutsui, G. Núñez, C. Sasakawa, Shigella IpaH7.8 E3 ubiquitin ligase targets glomulin and activates inflammasomes to demolish macrophages. Proc. Natl. Acad. Sci. U.S.A. 111, E4254–E4263 (2014). [Abstract] [Full Text]

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