Cold stress inhibits colonic motility and increases gut permeability in mice
A total of 15 mice (8 in control group and 7 in cold water group) were able to complete the experiment, as one mouse died from choking during intragastric administration. In the cold stress group, lower fecal output (t = 2.493, P = 0.027) (Figure 1A), lower fecal moisture percentage (t = 4.815, P < 0.001) (Figure 1B) and lower Bristol scores (t = 3.015, P = 0.010) (Figure 1C) were observed. These results suggest that cold stress induce a decreased gut motility, which are consistent with previous studies(22). Altered fecal properties, bloody stools and weight loss are the most important characteristics in IBD, then we used the DAI scores to evaluate the severity of the colitis. We found that the DAI scores in the cold water group were significantly higher than in the control group (t = -4.861, P < 0.001) (Figure 1D). Correspondingly, hematoxylin-eosin staining showed sparse intestinal villus, and edema in the intestinal villi of the jejunum (Figure 1E), a significant reduction in goblet cells, an enhanced inflammation response, and an elevated histological score in the colon (t = -2.193, P = 0.047) (Figure 1F). As to the gut barrier, we found the expression of ZO-1 and ZO-2 were lower by western blot (Figure 1G), compared to that in the control group. The immunofluorescence results confirmed the lower expression of ZO-2 in the cold water group (Figure 1H).
Cold stress exacerbates the inflammatory response of the intestinal tissue in mice
To further identify the changes in gene expression in the intestinal tissue after cold stress, we selected out 6 mice from the two groups randomly and investigated the gene expression profiles of the intestinal tissue using RNA sequencing (Figure 2A). A total of 432 genes with differential expression between groups were identified, with 330 genes upregulated and 102 genes downregulated in the cold water group when compared with the control group (Figure 2B). Gene Ontology analysis showed that all the genes most strongly enriched in immune function processes, including innate immune response and adaptive immune response, such as the B cell activation and B cell receptor complex, immunoglobulin production, immunoglobulin complex, complement activation, antigen binding and defense response to a bacterium (Figure 2C). In accordance with these findings, KEGG pathway analysis also showed that the genes were enriched in immune activation in the colon, including the primary immunodeficiency, B cell receptor signaling, intestinal immune network for IgA production, NK cell-mediated cytotoxicity pathways, leukocyte transendothelial migration, Th1, and Th2 cell differentiation, Th17 cell differentiation, cytokine–cytokine receptor interaction, chemokine and Fc gamma R-mediated phagocytosis, all of which are closely related to the immune response in the colon (Figure 2D).
To further reveal the effects of cold water on the colon, we used gene set enrichment analysis to analyze the genes which revealed substantial upregulation of genes involved in the B and T cell receptor signaling pathway, leukocyte transendothelial migration, FC epsilon RI signaling pathway, chemokine signaling pathway, and cytokine–cytokine receptor interaction (Figure 2E). These findings suggested that cold water triggers an IBD-like phenotype in mice.
Cold stress leads to low bacterial diversity and a fragile ecological network in the gut microbiota
In general, IBD is considered to occur when the immune system overreacts to the resident gut microbiota, inducing a chain of inflammatory events that can destroy the gut barrier (23). These findings prompted us to further investigate whether changes in microbiota or bacterial metabolites from the feces under cold water stimulation may regulate the gut barrier and gut–brain interactions.
Eight individual fecal samples from the control group and seven individual fecal samples from the cold stress group were collected and sequenced. The principal component plots with unweighted UniFrac distances showed a clear separation between the cold stress and control groups (Figure 3A), which suggested that cold stress led to a significant alteration in the gut microbiota composition. The Shannon index also showed a low bacterial diversity in the cold stress group (Figure 3B). At the operational taxonomic unit (OTU) level, the microbiota were significantly differed (Figure 3C). The abundances of OTU198 (Lachnospiraceae_unclassified), OTU337 (Clostridiales_Incertae_unclassified), OTU156 (Muribaculaceae_unclassified), OTU254 (Erysipelotrichaceae_unclassified), OTU88 (Duncaniella), OTU334 (Muribaculaceae_unclassified), OTU13 (Paramuribaculum), OTU433 (Proteobacteria_unclassified), OTU215 (Lachnospiraceae_unclassified), OTU258 (Firmicutes_unclassified), OTU115 (Paramuribaculum), OTU272 (Mailhella), and OTU34 (Akkermansia) were significantly increased in the cold stress group. Additionally, the abundances of OTU246 (Anaerotruncus), OTU15 (Alistipes), and OTU209 (Clostridiales_unclassified) decreased under cold stress (Figure 3D).
Linear discriminant analysis of effect size further showed that the bacteria with increased abundance in the cold stress group mainly belong to Proteobacteria (Supplementary Figure 1A). To determine the pattern of bacteria, we constructed their networks in the two group respectively, we found that the network of the cold water-treated mice had a simpler property (nodes/edges = 81/176) than the control group (nodes/edges = 80/234), indicating that cold water may induce vulnerability to environmental stress in the gut microbiota (Figure 3E).
Cold water downregulates metabolites of the dopamine-related pathway in the intestinal flora
As a microbe–host bridge, some metabolites of the intestinal flora can affect host physiology by entering the bloodstream. Therefore, we analyzed fecal metabolites using LC-MS. We found that the metabolic data clusters of the control and cold stress groups were separated from each other in both positive- and negative-ion modes by partial least-squares discriminant analysis (Figure 4A–B). The heatmap also showed that cold stress led to significant alterations in fecal metabolite levels (Figure 4C); 1179 metabolites were upregulated, and 1896 metabolites were downregulated with significant changes (Figure 4D). The most strongly impacted metabolic pathways included cocaine addiction, dopaminergic synapse, amphetamine addiction, and alcoholism addiction (Figure 4E), all of which were accompanied by a significant reduction in levels of dopamine, l-dopamine, l-tyrosine, and homovanillic acid (Figure 4F). As one of the most important neurotransmitters, decreased dopamine levels may contribute to anxiety-like and depression-like behaviors in the cold stress mice; Additionally, patients with IBD present some similar symptoms to patients with IBS, including anxiety and depression. Therefore, these behaviors were further evaluated.
Cold water increases depression-like behaviors in mice
In the tail suspended test, the struggling time was significantly decreased in the cold stress group (t = 2.618, P = 0.021) (Figure 5A), suggesting an increase in depression-like behaviors in mice exposed to cold stress. Furthermore, in the open field test, the center time (t = 2.195, P = 0.047) were reduced in the cold stress group (Figure 5C), implying a tendency of decreased exploratory behavior and increased anxiety-like behavior in the cold stress group.
Correlations of gut microbiota and metabolic changes
Finally, to explore the functional significance of the metabolite perturbations in the gut microbiota of the cold water-treated group, the 97 annotated metabolites with significant differences were selected, and their Spearman correlation coefficients with different bacteria were calculated. Significant correlations were observed between the gut microbiota and metabolites (Supplementary Figure 2A), and also observed between metabolites and gut function (Supplementary Figure 2B).