Quantifying forms and functions of intestinal bile acid pools in mice

Bile acids (BAs) are gastrointestinal metabolites that serve dual functions in lipid absorption and cell signaling. BAs circulate actively between the liver and distal small intestine (i.e., ileum), yet the dynamics through which complex BA pools are absorbed in the ileum and interact with intestinal cells in vivo remain ill-defined. Through multi-site sampling of nearly 100 BA species in individual wild type mice, as well as mice lacking the ileal BA transporter, Asbt/Slc10a2, we calculate the ileal BA pool in fasting C57BL/6J mice to be ~0.3 mmoles/g. Asbt-mediated transport accounts for ~80% of this pool and amplifies size, whereas passive absorption explains the remaining ~20%, and generates diversity. Accordingly, ileal BA pools in mice lacking Asbt are ~5-fold smaller than in wild type controls, enriched in secondary BA species normally found in the colon, and elicit unique transcriptional responses in cultured ileal explants. This work quantitatively defines ileal BA pools in mice and reveals how BA dysmetabolism can impinge directly on intestinal physiology. To investigate gene-regulatory functions of endogenous bile acids, ileal fragments from male 4-hours-starved C57BL/6J mice were treated with indicated additives for 8 hours. We then analyzed differential expressed genes using 3' mRNA-seq.