Bacterial detection by NAIP/NLRC4 elicits prompt contractions of intestinal epithelial cell layers Pilar Samperio Ventayola, Petra Geisera, Maria Letizia Di Martinoa, Alexandra Florbranta, Stefan A. Fattingera,b, Naemi Walderb, Eduardo Simac, Feng Shaod, Nelson O. Gekarae, Magnus Sundbomc, Wolf-Dietrich Hardtb, Dominic-Luc Webbf, Per M. Hellströmf, Jens Erikssona, and Mikael E. Sellina,1 aScience for Life Laboratory, Department of Medical Biochemistry and Microbiology, Uppsala University, 75123 Uppsala, Sweden; bInstitute of Microbiology, Department of Biology, ETH Zürich, 8093 Zürich, Switzerland; cDepartment of Surgical Sciences, Uppsala University, 75185 Uppsala, Sweden; dNational Institute of Biological Sciences, 102206 Beijing, China; eDepartment of Molecular Biosciences, The Wenner-Gren Institute, Stockholm University, Stockholm 10691, Sweden; and fDepartment of Medical Sciences, Gastroenterology and Hepatology Unit, Uppsala University, 75185 Uppsala, Sweden Edited by Ralph R. Isberg, Tufts University School of Medicine, Boston, MA, and approved February 24, 2021 (received for review July 3, 2020) The gut epithelium serves to maximize the surface for nutrient and form inflammasomes in the cytosol] (7–9). Epithelial recognition fluid uptake, but at the same time must provide a tight barrier to of pathogens through PRRs elicits a panel of countermeasures, pathogens and remove damaged intestinal epithelial cells (IECs) including production of proinflammatory cytokines, chemokines, without jeopardizing barrier integrity. How the epithelium coor- lipids (10–13), secretion of antimicrobial peptides (11, 14), and dinates these tasks remains a question of significant interest. We the death and expulsion of infected IECs into the lumen (12, 15, used imaging and an optical flow analysis pipeline to study the 16). An intricate cross-talk exists between IECs and immune cells dynamicity of untransformed murine and human intestinal epi- residing in the underlying lamina propria (17). Epithelial defense thelia, cultured atop flexible hydrogel supports. Infection with the pathogen Salmonella Typhimurium (S.Tm) within minutes elicited signaling has in some cases also been shown to engage bystander – focal contractions with inward movements of up to ∼1,000 IECs. epithelial cells surrounding a pathogen-infected IEC (18 20). Still, Genetics approaches and chimeric epithelial monolayers revealed it remains poorly explored how PRR recognition of invading contractions to be triggered by the NAIP/NLRC4 inflammasome, pathogens can instruct tissue-scale epithelial responses. which sensed type-III secretion system and flagellar ligands upon Intestinal epithelial organoids (denoted “enteroids” when estab- bacterial invasion, converting the local tissue into a contraction lished from small intestinal crypts) provide a powerful experimental MICROBIOLOGY epicenter. Execution of the response required swift sublytic Gas- system to assess the behavior of untransformed epithelia in the dermin D pore formation, ion fluxes, and the propagation of a absence of other mucosal cell types (21). Organoids can be grown as myosin contraction pulse across the tissue. Importantly, focal con- three-dimensional (3D) miniature organs within matrix domes (22, tractions preceded, and could be uncoupled from, the death and 23) or be disrupted to produce two-dimensional (2D) monolayers expulsion of infected IECs. In both two-dimensional monolayers atop coated surfaces (24–27). By contrast to tumor cell lines, and three-dimensional enteroids, multiple infection-elicited con- organoids maintain untransformed properties over time (28) and tractions coalesced to produce shrinkage of the epithelium as a show reliable PRR expression patterns that mimic the intact gut 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 Significance integrity to a markedly higher extent than wild-type counterparts. We propose that prompt NAIP/NLRC4/Caspase-1/Gasdermin Contractile movements in the mammalian intestine typically D/myosin–dependent contractions allow the epithelium to densify rely on dedicated muscle cells. Here, we however show that its cell packing in infected regions, thereby preventing tissue dis- untransformed intestinal epithelial cell layers initiate immedi- integration due to the subsequent IEC death and expulsion ate focal contractions, affecting hundreds to thousands of ep- process. ithelial cells, in response to bacterial infection. This epithelial contraction response occurs in the absence of other mucosal bacterial infection | epithelium | organoid | inflammasome | contraction cell types. Instead, the epithelium itself senses the pathogen intrusion through a pattern recognition receptor complex— — pithelial cells make up barriers that shield the interior of the NAIP/NLRC4 and initiates actomyosin contractions that prop- Ebody from the environment. In the homeostatic intestine, the agate across the epithelial layer. Within minutes, this response surface of the epithelium is maximized to facilitate uptake of densifies the cell packing at infection sites and may prevent ingested nutrients, water, and electrolytes (1, 2). This large sur- tissue disintegration during the subsequent stage of epithelial cell death and expulsion. Our results highlight a previously face, however, makes the gut epithelium vulnerable to attack by unappreciated dynamic behavior of intestinal epithelia. pathogenic microorganisms. Pathogen onslaught and the ensuing inflammation causes death and loss of intestinal epithelial cells Author contributions: P.S.V. and M.E.S. conceived the study; P.S.V., P.G., M.L.D.M., A.F., (IECs) (3, 4). This can be beneficial as damaged cells and intra- S.A.F., E.S., M.S., W.-D.H., D.-L.W., P.M.H., and M.E.S. designed research; P.S.V., P.G., cellular pathogens are cleared from the mucosa. At the same time, M.L.D.M., A.F., S.A.F., N.W., E.S., and J.E. performed research; M.L.D.M., S.A.F., F.S., N.O.G., W.-D.H., and J.E. contributed new reagents/analytic tools; P.S.V., P.G., and A.F. cell loss may jeopardize epithelial integrity when insufficient IEC analyzed data; P.S.V. and M.E.S. wrote the paper; and all authors reviewed and edited numbers remain to uphold the barrier. One way to prevent such an the paper. outcome would be to compact the epithelial layer in affected re- The authors declare no competing interest. gions. It is well known that smooth muscle contraction results in This article is a PNAS Direct Submission. shrinkage and compaction of the intestinal wall during inflamma- This open access article is distributed under Creative Commons Attribution-NonCommercial- tion (5), but whether and how the epithelium itself can alter its IEC NoDerivatives License 4.0 (CC BY-NC-ND). packing upon infection appears less clear. 1To whom correspondence may be addressed. Email: [email protected]. As a system to sense mucosal intrusions, IECs express pattern This article contains supporting information online at https://www.pnas.org/lookup/suppl/ recognition receptors (PRRs) [e.g., Toll-like receptors associated doi:10.1073/pnas.2013963118/-/DCSupplemental. with cell membranes (6), and Nod-like receptors (NLRs) that Published April 12, 2021. PNAS 2021 Vol. 118 No. 16 e2013963118 https://doi.org/10.1073/pnas.2013963118 | 1of11 Downloaded by guest on October 1, 2021 epithelium (6, 9). Organoid models have for these reasons become S.Tm TTSS-1 Docking and Invasion Elicits Focal Epithelial Contractions. attractive tools for physiological studies of gut infection (25, 29–32). S.Tm employs flagella and TTSS-1 to reach and invade the ep- In this work, we used time-lapse imaging to follow the tissue ithelium (36–38). Using fluorescence-labeled bacteria, we noted dynamicity of enteroid-derived mouse and human intestinal ep- that contraction foci emerged in areas with S.Tm bound to the ithelia, placed atop pliable matrix supports. Upon infection with epithelial surface (Fig. 2A). This led us to postulate that S.Tm the prototypical enteropathogen Salmonella enterica serovar docking or invasion elicits focal contractions. To test this hy- pothesis, we infected murine monolayers with S.Tmwt or Typhimurium (S.Tm), we observed prompt and large epithelial Δ S.Tm invG (lacks a structural component of TTSS-1). As expec- contraction foci which preceded and could be uncoupled from wt ΔinvG IEC death and expulsion. Bacterial type-three secretion system ted, S.Tm invaded IECs, while S.Tm failed to do so (SI (TTSS) and/or cytosolic flagellin triggered the epithelial NAIP/ Appendix, Fig. S1 E and F). Live DIC imaging over a range of NLRC4 inflammasome, sublytic Gasdermin D pore formation, MOIs (0.2 to 20) showed that epithelial contractions increased in frequency and intensity with S.Tmwt dose (Fig. 2B). By sharp ion fluxes, and myosin-dependent contractions spreading from ΔinvG sensing events at focal epicentres. We show that this swift re- contrast, S.Tm infection did not elicit epithelial contrac- sponse allows the epithelium to increase its IEC packing at sites tions, not even at the highest MOI (Fig. 2B). This points to a central role for TTSS-1 in triggering S.Tm-induced epithelial of infection, which may minimize the disruptive effects of sub- contractions. sequent cell death and expulsion. TTSS-1 docks to the host cell membrane via a SipBC trans- Results locon, resulting in the transfer of TTSS-1-effectors into the host cell (39). Among