Gut microbiota and hepatic PPARS in the regulation of liver circadian rhythm

• Main Author: Oh, Penny Hui Yun
• Other Authors: Tan Nguan Soon, Andrew
• Format: Theses and Dissertations
• Language: English
• Published: 2019
• Subjects: DRNTU::Science::Medicine
• Online Access: https://hdl.handle.net/10356/90199; http://hdl.handle.net/10220/47322
• Institution: Nanyang Technological University

Feeding regimen, which has an impact on oscillating gut microbiota composition and gut microbiota derived metabolites, influences circadian gene expression in the peripheral organs. The gut microbiota has been recently proposed, in association with diet, to regulate the intestinal peripheral clock, but little is known for the liver clock, although, metabolites produced by the gut microbiota influence many aspects of liver metabolic functions. Interestingly, studies have shown that diet not only influences the oscillating composition of gut microbiota and microbiota-derived metabolites but also peroxisome proliferator-activated receptors (PPARs). PPARs are a group of ligand activated transcription factors known to play an important role in metabolic homeostasis and circadian core clock regulation. PPARs can be activated by microbiota-derived ligands such as gut microbiota derived-short chain fatty chain (SCFAs). Therefore, PPARs are closely linked to the gut microbiota and circadian rhythm. In this thesis, I worked on two mouse models to understand how liver circadian rhythm can be impacted by gut microbiota and PPARs. In the first project, we looked into the liver circadian rhythm upon differential depletion of the different spectra of the gut microbiota in C57BL/6 mice. As for the second project, we examined if circadian rhythm would be affected in hepatocyte-specific triple Ppar deletions in mice (C57BL/6 background). In our antibiotic-induced selective depletion of gut microbiota, elimination of gram- positive bacteria by antibiotic treatment perturbed liver circadian rhythm as shown by the altered hepatic core clock gene expression and changes in proteins and metabolites under circadian regulation. We also present data showing the potential role of gut microbiota in protecting against pharmacological-induced circadian rhythm disruption by the antibiotic metronidazole. In addition, our results reveal a possible link between liver cell proliferation and gram-positive bacteria whose manipulation could be tested as a potential therapeutic intervention to suppress liver cancer. In brief, this study provides important new information for researchers who are addressing the issue on how the microbiota can impact peripheral organs in terms of circadian rhythm, physiological functions, and possibly cancer development. In our second model, we have generated hepatocyte-specific triple Ppar-null mice and validated the deletion of Pparα, Pparβ and Pparγ at both the deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) levels. Our quantitative polymerase chain reaction (q-PCR) and RNAseq (RNA sequencing) data indicate that mice with deleted PPARs have no significant alteration in their liver circadian core clock genes. Supporting this, our RNAseq data indicated no significant alteration in circadian target genes as well.