A Surgical Method for Continuous Intra-Portal Infusion of Gut Microbial Metabolites in Mice

Gut-derived microbial metabolites have been shown to play key roles in human physiology and disease. However, establishing mechanistic links between gut microbial metabolites and disease pathogenesis in animal models presents many challenges. The major route of absorption for microbe-derived small molecules is venous drainage via the portal vein to the liver. In the event of extensive liver first pass- or presystemic hepatic metabolism, the route of administration of these metabolites becomes critical. Here we describe a novel portal vein cannulation technique using a subcutaneously implanted osmotic pump to achieve continuous portal vein infusion in mice. First, the microbial metabolite trimethylamine (TMA) was administered over 4 weeks and compared to a vehicle control. Using a liquid chromatography-tandem mass spectrometry (LC-MS/MS), an increase in peripheral plasma levels of TMA and its host liver-derived co-metabolite trimethylamine-N-oxide (TMAO) were observed in a sexually-dimorphic manner. Next, 4-hydroxyphenylacetic acid (4-HPAA), a structurally-distinct microbial metabolite that undergoes extensive hepatic first pass metabolism, was administered portally to examine its effects on hepatic gene expression. Peripheral plasma levels showed no difference in free or total 4-HPAA. However, while liver tissue demonstrated no difference in free 4-HPAA, total levels were increased when compared to the control group. More importantly, significant changes were observed in hepatic gene expression using an unbiased RNA sequencing approach. Collectively, this work describes a novel method for administering gut microbe-derived metabolites via the portal vein, mimicking their physiologic delivery in vivo. Overall design: Comparison of liver mRNA transcription profiles after intraportal administration of either normal saline or 4-hydroxyphenylacetic acid (4-HPAA). N=4 per group.