Modulation of adult stem cell activity by microbial indole production in the host-microbiome interaction context

• Main Author: Xing, Yuli
• Other Authors: Scott Rice
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Science::Biological sciences::Molecular biology
• Online Access: https://hdl.handle.net/10356/152812
• Institution: Nanyang Technological University

The recognition of the role of the gut microbiome-host interactions in regulating mammalian physiology is a major paradigm shift in 20th century medicine. This intimate interplay is assumed to be mediated through the repertoire of molecules produced by both parties. Microbe-derived metabolites have been reported to impact on cell biology and organ function in varied host organisms. Indole is a metabolite of microbial tryptophan metabolism that has been found to modulate a spectrum of physiological activities in mammals and plants. While previous reports have described indole’s beneficial effects on animal physiology and function, there remains a lack of detailed studies on the ways in which indole modulates host cellular activities and metabolic homeostasis. This thesis thus focuses on the modulatory roles of indoles on stem cell activities, organ functions, and metabolism, using multiple animal models and ex vivo organoid systems. This thesis first presents a general and in-depth review of the literature relevant to the host-gut microbiome interactions and host physiology (Part 1). The following result chapter (Chapter 4.1) presents experimental data from specific pathogen-free and germ-free mice treated with exogenous indole. The results show the in vivo effect of indole on intestinal stem cell activities and hence intestinal epithelial renewal. In particular, while long-term (5 weeks) indole treatment elevated proliferating cell number in mouse small intestines, short-term indole (10 days) treatment also enlarged the mucus-producing goblet cell population. This suggests that exposure of the intestines to indole could alter the intestinal epithelial cell composition by modulating stem cell fate determination and thus cell differentiation. Experimental data obtained using organoids cultured from mouse small intestine are also included, which supplements the in vivo experiments. By studying the effects of exogenous indole supply on mice, the first result chapter serves as a prologue to the next. The second result chapter (Chapter 4.2), presented as a manuscript for publication, aims to study the roles of gut microbial indole production on the systemic and organ-specific functions in the host animals. Germ-free mice were mono-colonized with indole-producing (wild type) or indole-non-producing (tryptophanase-encoding tnaA gene-knockout mutant) Escherichia coli and subjected to various functional, physiological, and histological analyses. The indole-non-producing E. coli-colonized mice displayed reduced body weight and sizes of multiple metabolically relevant organs, including liver, skeletal muscles, and adipose tissues. Evidence of reduced glucose metabolism and lipogenesis in liver and muscle tissues, as well as increased lipolysis and glyceroneogenesis in adipose tissues was also found, suggestive of metabolic stress in indole-deficient mice. Metabolomic analysis suggested that reduction in the production of serotonin, a neurotransmitter, might be one mechanism responsible for the metabolic stress observed in the mice with indole-non-producing E. coli. Correspondingly, histological study of the intestines revealed less serotonin-producing enterochromaffin cells in the colon. As gut-derived serotonin regulates gastrointestinal motility and hence intestinal digestion and absorption, reduced serotonin level and enterochromaffin cells might contribute to the metabolic stress observed in these mice. These results suggested indole’s role in modulating intestinal stem cell activities, particularly in the secretory cell lineage differentiation, which in turn affects the metabolic homeostasis of the animal hosts. Result Chapter 4.3 presents data that I generated in a collaborative project investigating the effect of indole on adult hippocampal neurogenesis in mice. A published journal paper from this project and a review article I authored on the effects of gut microbial metabolites on intestinal stem cell activities are included in the Appendices. The results presented in this thesis demonstrate the role of the microbial metabolite indole in modulating adult stem cell activities in different host compartments, and thus tissue renewal and functions. This illustrates the importance of host-microbiome interactions in animal physiology, providing insights for the design of the next generation therapeutic interventions to support human health and extend healthspan.