Investigating the metabolic and immunological alterations of the liver from early life environmental toxicant exposure and the modulation by the gut microbiome

Abstract

The liver is an important organ for xenobiotic biotransformation and nutrient homeostasis, as well as local immunological regulation. The function of the liver is a result of unique combinations of cell types that perform specialized tasks and are further modulated by a spatial gradient that forms distinct zones, as well as external variables, such as diet. In addition, the gut microbiome, the collection of microorganisms in the gut, closely communicates with the liver through the gut-liver axis. Polychlorinated biphenyls (PCBs), which were used as industrial insulators and coolants, and polybrominated diphenyl ethers (PBDEs), which were popular flame retardants, are persistent organic pollutants (POPs) that bioaccumulate in the food chain and are readily detected in human biospecimens. Tetrabromobisphenol A (TBBPA) is globally the most widely used brominated flame retardant that replaced PBDEs. Exposure to these environmental toxicants have been closely linked to oxidative stress and inflammation, gut microbiome toxicity, and metabolic injury in the liver. PCBs, PBDEs, and TBBPA cross the placenta and are detected in breast milk, indicating that early life is a critical window for exposure to these environmental toxicants. Increasing evidence suggests that early life exposure can alter disease susceptibility or severity later in life. However, little is known regarding the cell type-specific response to early life environmental toxicant exposure and the involvement of the gut microbiome in modulating hepatic metabolic and immunological processes. Therefore, the goal of my dissertation is to strategically investigate key hepatic cell type-specific processes at both the basal level and from early life environmental toxicant exposure, as well as the contribution of the gut microbiome and diet in modulating hepatic processes. My central hypothesis is that early life environmental toxicant exposure dysregulates the metabolic and immunological landscape of the liver, further modulated by the gut microbiome and diet in vivo. I used single cell RNA sequencing (scRNA-seq) and spatial transcriptomics data of the mouse liver to systematically profile the baseline mRNA enrichment of genes that encode drug metabolizing enzymes and transporters (i.e., drug processing genes, DPGs), as well as their regulatory transcription factors in different liver cell types and zones. Furthermore, I showed that developmental PCB exposure during pregnancy and lactation resulted in cell type-specific metabolic dysregulation related to endoplasmic reticulum (ER) stress and insulin sensitivity in the livers of the offspring. From a 3 day neonatal exposure to BDE-99, a human breast milk-enriched PBDE congener, I showed that the proportion of neutrophils increased in the liver in late adulthood in mice, accompanied by a predicted increase of the macrophage inhibitory factor signaling and upregulated proinflammatory cytokine mRNA expression. Concordantly, the gut environment was altered in late adulthood from the early life BDE-99 exposure, which the changes included decreased large intestinal tight junction protein (Tjp) transcripts, altered gut microbiome, and dysregulation of inflammation-related metabolites. Microbiota transplant to germ-free (GF) mice using large intestinal content from adults neonatally exposed to BDE-99 downregulated Tjp transcripts and upregulated proinflammatory cytokines in the large intestine. I further demonstrated that the existence of a normal gut microbiome is necessary for maintaining hepatic immune tolerance. Using genetically modified mice that contain human transcripts involved in xenobiotic biotransformation and comparing various colonized microbiomes, I showed that hepatic processes are regulated by the gut microbiome, and such regulation of hepatic processes are further regulated by hepatic zones. Lastly, I showed that western high-fat diet-mediated metabolic inflammation signatures in the liver are exacerbated by developmental exposure to TBBPA, through the placenta and breast milk. Taken together, the studies in my dissertation showed that early life environmental toxicant exposure dysregulates hepatic cell type-specific responses linked to metabolic inflammation later in life, and that the gut microbiome is a modulator of metabolic and immunological properties of the liver. The studies outlined in my dissertation contribute to our understanding how early life toxicant exposures dysregulates metabolic processes in the liver and the gut environment. The mechanistic insights from the findings can further help identify novel therapeutic strategies to mitigate metabolic alterations and decrease the internal burden and effects of early life environmental toxicant exposures.