Nitrate-mediated luminal expansion of Salmonella Typhimurium is dependent on the ER stress protein CHOP

Abstract

Salmonella Typhimurium is an enteric pathogen that employs a variety of mechanisms to exploit inflammation resulting in expansion in the intestinal tract, but host factors that contribute to or counteract the luminal expansion are not well-defined. Endoplasmic reticulum (ER) stress induces inflammation and plays an important role in the pathogenesis of infectious diseases. However, little is known about the contribution of ER stress-induced inflammation during Salmonella pathogenesis. Here, we demonstrate that the ER stress markers Hspa5 and Xbp1 are induced in the colon of S. Typhimurium infected mice, but the pro-apoptotic transcription factor Ddit3, that encodes for the protein CHOP, is significantly downregulated. S. Typhimurium-infected mice deficient for CHOP displayed a significant decrease in inflammation, colonization, dissemination, and pathology compared to littermate control mice. Preceding the differences in S. Typhimurium colonization, a significant decrease in Nos2 gene and iNOS protein expression was observed. Deletion of Chop decreased the bioavailability of nitrate in the colon leading to reduced fitness advantage of wild type S. Typhimurium over a napA narZ narG mutant strain (deficient in nitrate respiration). CD11b+ myeloid cells, but not intestinal epithelial cells, produced iNOS resulting in nitrate bioavailability for S. Typhimurium to expand in the intestinal tract in a CHOP-dependent manner. Altogether our work demonstrates that the host protein CHOP facilitates iNOS expression in CD11b+ cells thereby contributing to luminal expansion of S. Typhimurium via nitrate respiration. Author SummarySalmonella Typhimurium is a gastroenteric bacterium that replicates to large numbers within the gastrointestinal (GI) tract allowing for efficient host-to-host transmission. One strategy that allows Salmonella to expand in the GI tract is via nitrate respiration that is generated during Salmonella infections. Our results presented here provide more insight into the role of the host protein CHOP in the production of nitrate and the subsequent growth of Salmonella in the GI tract. CHOP expression is regulated within the unfolded protein response (UPR), an adaptive response pathway that is activated when cells are undergoing endoplasmic reticulum (ER) stress. ER stress has been implicated in several infectious and inflammatory diseases; however, little is known about the contribution of ER stress and the UPR during Salmonella infections. Here, we demonstrate that Chop is downregulated in mice infected with S. Typhimurium, and that mice deficient for CHOP have reduced bacterial numbers in the colon, suggesting that downregulation of Chop is a host response to resist intestinal colonization by Salmonella. Our results further show that CHOP contributes to increased expression of iNOS, responsible for nitrate production, thereby increasing the bioavailability of nitrate that allows for Salmonella growth. Altogether, our research provides a better understanding of the contribution of the ER stress protein CHOP in intestinal health and disease.