ABSTRACT: Overall hypothesis: RNAseq can be used to examine and compare the vitamin A (VA) effect, Infection effect caused by Citrobacter rodentium (C. rodentium), and the Interaction effect (VA status × C. rodentium infection) in mouse small intestine (SI) and colon, on the scale of global transcriptome. Methods: Vitamin A deficient (VAD) mice were generated by feeding VAD diet to the pregnant C57/BL6 dams and their post-weaning offspring. Vitamin A sufficient (VAS) mice were fed with purified diet containing normal amount of VA. At weaning, mice were maintained on their respective diets and assigned to one of the four treatment groups (non-infected VAD, infected VAD, non-infected VAS and infected VAS) until the end of the experiments. For the infected groups, age-matched VAD or VAS mice were orally inoculated with C. rodentium. Mice during early (SI study) or peak (colon study) C. rodentium infection were euthanized in two separate studies. The SI study focused on SI and analyses were performed on samples collected 5 days post-infection. The colon study analyzed samples on post-infection day 10, the peak of C. rodentium infection. Total mRNA extracted from SI and colon were sequenced using Illumina HiSeq 2500 platform. Three to six biological replicates were sequenced in each group. Differentially Expressed Gene (DEG), Gene Ontology (GO) enrichment, and Weighted Gene Co-expression Network Analysis (WGCNA) were performed to characterize expression patterns and co-expression patterns. Results: a) differentially expression genes (DEGs) corresponding to VA effect were present in both the SI and colon. In SI, DEGs altered by VA were mainly involved in the retinoid metabolic pathway and immunity-related pathways. Novel target genes (e.g. Mbl2, Mmp9, Cxcl14, and Nr0b2) and cell types (based on Tuft cell and mast cell markers) under the regulation of VA were suggested by differential expression analysis coupled with the co-expression network analysis. In regard to the colon, VA demonstrated an overall suppressive effect on the cell division pathway. Finally, the comparison of co-expression modules between SI and colon indicated distinct regulatory networks in these two organs. b) In the SI, no Infection effect was detected during early infection, whereas in the colon, during peak of C. rodentium infection: 1) It has been shown that infection upregulated innate and adaptive immune functions, epithelial proliferation, apoptosis, endoplasmic reticulum stress, and reactive oxygen species production, as well as downregulated ion and water absorption. This helped to confirm the findings in previous studies, consolidating the knowledge-base about the host response to C. rodentium, 2) Pathways (e.g. neuron activity, smooth muscle contraction, vascular constriction, and developmental regulation) and additional ion transporters previously not known to be influenced by C. rodentium were suggested as potential mechanisms responsible for the diarrhea pathology and host response. 3) Retinoic acid receptor (RAR) and vitamin D receptor (VDR) signaling pathways appeared to be downregulated by infection, indicating that the malabsorption of micronutrients might be a danger signal for the host to switch from homeostasis to an anti-infection mode. c) In the SI, no Interaction (VA status × C. rodentium infection) effect was detected during early infection. In colon, 1329 genes corresponded to the Interaction effect. During the peak of C. rodentium infection, the sufficient stimulation of cell division, protein catabolism, apoptosis, transcription regulation, MHC class I, interferons, and cytokine signaling might be of key importance for the resistance of VAS mice. In addition, the proper maintenance of the proliferation, differentiation, migration, and activation of T cells and leukocytes during peak infection may also confer the VAS hosts with higher survival and clearance rate. On the other hand, our data suggested that the higher rate of lethal dehydration observed in infected VAD mice, may be attributed to the disrupted of HCO3- metabolism and an improperly high level of Cl- secretion. Conclusions: VA alters the gene expression profile in both SI and colon, whereas the Infection effect and the Interaction effect were only observed in colon during peak of C. rodentium infection, but not in the SI during early infection. Our studies suggest novel target genes and cell types under the regulation of VA in the SI, and indicate that the resistance of the VAS hosts may be attributed to the concordant control over ion/water absorption, immune functions, as well as the balance between epithelial hyperplasia and apoptosis.
