Reduced Oral Nutrition Contributes to Gastrointestinal Toxicity of Total Body Irradiation Via Changes to the Gut Microbiome (2019)

Reduced oral nutrition (RON) is common after cytotoxic therapy for hematologic malignancies. We examined the interplay between nutrition, intestinal bacteria and intestinal barrier function in mouse models of total body irradiation (TBI).By 3 days following 9 Gy of a single dose of TBI, C57BL/6 mice had an approximately 50% reduction in oral intake (2g/day, data not shown) and 3 days later displayed marked changes in the microbiome by 16S deep sequencing, including an expansion of mucolytic bacteria Akkermansia muciniphila and Bacteroides thetaiotaomicron (Fig 1). These mice lost roughly 20% of their body weight and showed compromise of intestinal barrier function assayed by oral challenge with FITC-dextran (data not shown). To ask if RON could be driving microbiome changes, we limited the access of unirradiated mice to 2g/day of chow for 7 days. The microbiome was perturbed in these mice with changes reminiscent of that following TBI, including expansion of Akkermansia muciniphila and Bacteroides thetaiotaomicron (Fig 1).The colonic mucus layer was thinned in these mice (Fig 2), though intestinal barrier function remained intact (data not shown). To explore how RON could favor mucolytic bacteria, we characterized metabolic activity of colonic bacteria during RON. Bomb calorimetry demonstrated reduced substrates in the cecum during RON (Fig 2), and colonic luminal pH showed reduced acidity (Fig 2). IC-MS quantified lower concentrations of acetate, propionate, butyrate, and lactate, and elevated succinate (Fig 2). RNA sequencing of stool samples revealed broad downregulation by several intestinal bacterial species of phosphoenolpyruvate carboxykinase, an enzyme known to be associated with production of propionate from succinate (data not shown).Finally, we evaluated in our RON model the impact of administration of oral sugars by supplementing a well-absorbed sugar (glucose), and a poorly-absorbed prebiotic sugar in the drinking water of mice. We found that sugar supplementation with the prebiotic acidified the colonic lumen (data not shown) and prevented thinning of the mucus layer in mice undergoing RON (Fig 3). Additionally, in mice following 9 Gy of TBI, daily dosing of the prebiotic started 24 hours later was highly effective in preventing weight loss (Fig 3), nearly completely abrogated compromise of intestinal barrier function (Fig 3), and improved overall survival (Fig 3).In summary, reduced oral intake following total body irradiation contributes to thinning of the colonic mucus layer, an effect that appears to be independent of the cytotoxic effects on the host and is rather mediated by changes in the intestinal microbiome, including reduced metabolism of organic acids. Strategies to restore colonic acidity with prebiotic sugars may successfully target this phenomenon with potential clinical benefits, particularly in preventing disruption to the intestinal barrier. Reduced oral nutrition (RON) is common after cytotoxic therapy for hematologic malignancies. We examined the interplay between nutrition, intestinal bacteria and intestinal barrier function in mouse models of total body irradiation (TBI). By 3 days following 9 Gy of a single dose of TBI, C57BL/6 mice had an approximately 50% reduction in oral intake (2g/day, data not shown) and 3 days later displayed marked changes in the microbiome by 16S deep sequencing, including an expansion of mucolytic bacteria Akkermansia muciniphila and Bacteroides thetaiotaomicron (Fig 1). These mice lost roughly 20% of their body weight and showed compromise of intestinal barrier function assayed by oral challenge with FITC-dextran (data not shown). To ask if RON could be driving microbiome changes, we limited the access of unirradiated mice to 2g/day of chow for 7 days. The microbiome was perturbed in these mice with changes reminiscent of that following TBI, including expansion of Akkermansia muciniphila and Bacteroides thetaiotaomicron (Fig 1). The colonic mucus layer was thinned in these mice (Fig 2), though intestinal barrier function remained intact (data not shown). To explore how RON could favor mucolytic bacteria, we characterized metabolic activity of colonic bacteria during RON. Bomb calorimetry demonstrated reduced substrates in the cecum during RON (Fig 2), and colonic luminal pH showed reduced acidity (Fig 2). IC-MS quantified lower concentrations of acetate, propionate, butyrate, and lactate, and elevated succinate (Fig 2). RNA sequencing of stool samples revealed broad downregulation by several intestinal bacterial species of phosphoenolpyruvate carboxykinase, an enzyme known to be associated with production of propionate from succinate (data not shown). Finally, we evaluated in our RON model the impact of administration of oral sugars by supplementing a well-absorbed sugar (glucose), and a poorly-absorbed prebiotic sugar in the drinking water of mice. We found that sugar supplementation with the prebiotic acidified the colonic lumen (data not shown) and prevented thinning of the mucus layer in mice undergoing RON (Fig 3). Additionally, in mice following 9 Gy of TBI, daily dosing of the prebiotic started 24 hours later was highly effective in preventing weight loss (Fig 3), nearly completely abrogated compromise of intestinal barrier function (Fig 3), and improved overall survival (Fig 3). In summary, reduced oral intake following total body irradiation contributes to thinning of the colonic mucus layer, an effect that appears to be independent of the cytotoxic effects on the host and is rather mediated by changes in the intestinal microbiome, including reduced metabolism of organic acids. Strategies to restore colonic acidity with prebiotic sugars may successfully target this phenomenon with potential clinical benefits, particularly in preventing disruption to the intestinal barrier. Figure 1Figure 2Figure 3Figure 2One week of caloric restriction (2 g/day) in unirradiated mice leads to thinning of the colonic mucus layer (top left). This is associated with reduced nutritional substrates in the cecum (top right), reduced acidity in the colonic lumen (bottom left), and changes in concentrations of bacterial metabolites in the colonic lumen (bottom right).View Large Image Figure ViewerDownload Hi-res image Download (PPT)Figure 3Comparison of oral supplementation with glucose or a prebiotic (poorly absorbed by the small bowel) in the setting of caloric restriction shows that prebiotic supplementation is superior for preventing thinning of the colonic mucus layer (top left). In the setting of total body irradiation, probiotic Supplementation started 24 hours later prevented weight loss in mica (top right), impairment of intestinal barrier function (bottom left), and improved overall survival (bottom right).View Large Image Figure ViewerDownload Hi-res image Download (PPT)