Bacterial detection by NAIP/NLRC4 elicits prompt contractions of intestinal epithelial cell layers

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Abstract
The gut epithelium serves to maximize the surface for nutrient and fluid uptake, but at the same time must provide a tight barrier to pathogens and remove damaged intestinal epithelial cells (IECs) without jeopardizing barrier integrity. How the epithelium coordinates these tasks remains a question of significant interest. We used imaging and an optical flow analysis pipeline to study the dynamicity of untransformed murine and human intestinal epithelia, cultured atop flexible hydrogel supports. Infection with the pathogen Salmonella Typhimurium (S.Tm) within minutes elicited focal contractions with inward movements of up to ∼1,000 IECs. Genetics approaches and chimeric epithelial monolayers revealed contractions to be triggered by the NAIP/NLRC4 inflammasome, which sensed type-III secretion system and flagellar ligands upon bacterial invasion, converting the local tissue into a contraction epicenter. Execution of the response required swift sublytic Gasdermin D pore formation, ion fluxes, and the propagation of a myosin contraction pulse across the tissue. Importantly, focal contractions preceded, and could be uncoupled from, the death and expulsion of infected IECs. In both two-dimensional monolayers and three-dimensional enteroids, multiple infection-elicited contractions coalesced to produce shrinkage of the epithelium as a whole. Monolayers deficient for Caspase-1(-11) or Gasdermin D failed to elicit focal contractions but were still capable of infected IEC death and expulsion. Strikingly, these monolayers lost their integrity to a markedly higher extent than wild-type counterparts. We propose that prompt NAIP/NLRC4/Caspase-1/Gasdermin D/myosin–dependent contractions allow the epithelium to densify its cell packing in infected regions, thereby preventing tissue disintegration due to the subsequent IEC death and expulsion process. Significance Contractile movements in the mammalian intestine typically rely on dedicated muscle cells. Here, we however show that untransformed intestinal epithelial cell layers initiate immediate focal contractions, affecting hundreds to thousands of epithelial cells, in response to bacterial infection. This epithelial contraction response occurs in the absence of other mucosal cell types. Instead, the epithelium itself senses the pathogen intrusion through a pattern recognition receptor complex—NAIP/NLRC4—and initiates actomyosin contractions that propagate across the epithelial layer. Within minutes, this response densifies the cell packing at infection sites and may prevent tissue disintegration during the subsequent stage of epithelial cell death and expulsion. Our results highlight a previously unappreciated dynamic behavior of intestinal epithelia.
Funding Information
  • Vetenskapsrådet (2018-02223)
  • Stiftelsen för Strategisk Forskning (ICA16-0031)
  • Stiftelsen för Strategisk Forskning (FFL18-0165)
  • Knut och Alice Wallenbergs Stiftelse (2016.0063)
  • Malin och Lennart Philipsons stiftelse (Lennart Philipson Award)
  • Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung (310030_192567)

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