Single cell transcriptomics unveiled that early life BDE-99 exposure reprogrammed the gut-liver axis to promote a pro-inflammatory metabolic signature in male mice at late adulthood

Polybrominated diphenyl ethers (PBDEs) are a class of legacy flame retardants that bioaccumulate in the environment, raising global health concerns. The gut microbiome is an important regulator of liver including xenobiotic biotransformation, nutrient homeostasis, and immune regulation. Using bulk RNA-Seq, we recently showed that neonatal exposure to BDE-99, a human breast milk-enriched PBDE congener, up-regulated pro-inflammation- and down-regulated drug metabolism-related genes predominantly in males in young adulthood. However, it remains unknown whether such dysregulation persists into late adulthood, how various cell types in the liver contribute to the hepatotoxicity, and to what extent gut microbiome is involved in BDE-99 mediated developmental reprogramming of the liver. To address these knowledge gaps, male C57BL/6 mouse pups were orally exposed to corn oil (10 ml/kg) or BDE-99 (57 mg/kg) once daily from postnatal days 2-4. At 15 months of age, single cell transcriptomics (scRNA-seq) in liver showed that neonatal BDE-99 exposure down-regulated key xenobiotic- and fatty acid metabolizing enzymes and up-regulated genes involved in microbial influx in hepatocytes. Neonatal BDE-99 exposure also led to a persistent increase in the hepatic proportion of neutrophils, a predicted increase of macrophage migration inhibitory factor (MIF) signaling, which activates macrophage populations, as well as histopathological abnormalities of the liver in 15 months of age. The BDE-99 mediated hepatic reprogramming is associated with decreased intestinal tight junction protein (Tjp) transcripts, persistent dysbiosis of the gut microbiome, and dysregulation of inflammation-related fatty acid metabolites. ScRNA-seq in germ-free (GF) mice demonstrated the necessity of a normal gut microbiome in maintaining hepatic immunotolerance. Fecal microbiome transplant to GF mice using large intestinal microbiome from adults that were neonatally exposed to BDE-99 down-regulated Tjp transcripts and up-regulated several cytokines in the large intestine. In conclusion, neonatal BDE-99 exposure reprogrammed the cell type-specific gene expression and cell-cell communication networks in liver towards a pro-inflammation with compromised metabolic functions at late adulthood. Importantly, gut microbiome is necessary in promoting immunotolerance in the liver, and BDE-99-mediated pro-inflammatory signaling may be partly due to the dysregulated gut environment. ### Competing Interest Statement The authors have declared no competing interest.