Key hepatic metabolic pathways are altered in germ-free mice during pregnancy

Pregnancy is associated with metabolic changes to accommodate the mother and her growing fetus. The microbiome has been shown to modulate host metabolism of endogenous and exogenous substances. However, the combined effects of pregnancy and the microbiome on host metabolism have not been investigated. The objective of this study was to investigate how the microbiome affects overall hepatic metabolic processes during pregnancy. We assessed these changes within 4 groups of C57BL/6 mice: conventional non-pregnant, conventional pregnant, germ-free non-pregnant, and germ-free pregnant mice. We performed RNA-seq analysis on liver tissues and LC-MS/MS analysis of the plasma to assess the effects of pregnancy and the microbiome on hepatic transcriptome and untargeted plasma metabolome to describe metabolic changes as results of both pregnancy and lack of microbiome. By integrating transcriptomics and metabolomics data, we identified eight metabolic pathways that were significantly enriched for differentially expressed genes associated with pregnancy in both conventional and germ-free mice. Notably, of the eight pathways, 4 pathways (retinol metabolism, arachidonic acid metabolism, linoleic acid metabolism, and steroid hormone biosynthesis) which are all critical for normal pregnancy and fetal development were affected by the germ-free status in pregnant mice, but not at all in non-pregnant mice, indicating that the alterations in these four pathways caused by the lack of microbiome are unique for pregnancy. These results provide novel insight into the role of the microbiome in modulating host metabolic processes critical for maternal health and fetal development during pregnancy.