The functional impact of gut microbiota on CNS regulation of local and systemic homeostasis

Abstract

The “gut microbiota” is widely accepted as an integral part of the gut homeostasis, and is thought to contribute to the establishment of intestinal barrier. Growing body of research suggest that the influence of gut microbiota on host development and physiology reaches beyond the gastrointestinal tract, and the brain is not an exception. The brain plays a critical role in regulating systemic homeostasis through continuous monitoring of body energy state and integration of the peripheral signals. Evidences of microbiota impact on brain at different levels including development, neurobiology, and even behavior have been documented. This thesis places two aspects of central regulation of homeostasis under the spotlight, and explores the potential impact of gut microbes in this context: (i) Brain regulation of local homeostasis through the function of the blood-brain barrier (BBB). The BBB is a specialized barrier that segregates the neural tissue from the circulation and controls the provision of nutrients to the brain. An intact BBB is critical for maintaining a homeostatic environment for normal function of the brain cells. (ii) Brain regulation of systemic homeostasis in relevance to anorexia nervosa. Anorexia nervosa is a serious eating disorder with altered homeostatic function. Dysbiosis in the gut microbiota have been reported in anorectic patients.

By taking advantage of germ-free mouse model, we showed that in the absence of gut microbiota, the integrity and function of the BBB is impaired during the intrauterine period, suggesting that maternal gut microbiota mediates the development and maturation of the BBB. Impaired BBB integrity persisted into adulthood and was associated with decreased expression of endothelial tight junction proteins including occludin, claudin-5 and zona occludens-1. The alterations in structure and permeability of the BBB were restored by introducing normal gut flora into the germ-free mice, reinforcing the role of gut microbiome for the integrity of the BBB. Furthermore, we showed that short-chain fatty acids (SCFAs, bacterial metabolites of dietary fiber fermentation) improve BBB integrity in line with previous observations that SCFAs enhance the integrity of intestinal epithelial barrier. Germfree mice monocolonized with Clostridium tyrobutyricum that mainly produces butyrate or with Bacteroides thetaiotaomicron which produces acetate and propionate exhibited decreased BBB permeability. Treatment with butyrate salt mimicked the effects. The influence of SCFAs might be mediated by epigenetic mechanisms as monocolonized and SCFA-treated germ-free mice displayed enhanced levels of histone acetylation in brain lysates.

Preterm birth is associated with impaired development and vascular fragility in the brain. During the critical early postnatal period, normal brain growth and maturation may be negatively affected by the prematurity-related factors such as nutrient deprivation or a serious infection. We used a preterm porcine model to study the effects of gestational age, early feeding, and infection on brain barriers following preterm birth. Preterm pigs spontaneously develop diet- and microbiota-related diseases including necrotizing enterocolitis. We showed that preterm piglets have impaired BBB-associated protein expression, decreased endothelial integrity, and enhanced blood-CSF barrier permeability in comparison to term counterparts. The observed impairment in endothelial integrity measured as astroglial perivascular coverage persisted into postnatal day five independent of enteral or parenteral feeding. Next, to investigate brain barrier function in preterm piglets under inflammation, we fed a group of animals with formula which is known to increase the risk of necrotizing enterocolitis. Our results indicate that severe necrotizing enterocolitis following five day formula treatment is associated with increased systemic inflammation, impaired blood-CSF barrier, enhanced neuronal death and elevated IL-6 levels in the hippocampus.

As an example of systemic homeostasis, we hypothesized that gut microbiota has a functional relevance in anorexia nervosa, a disease with altered homeostatic function. We transplanted fecal microbiota from a female individual with anorexia nervosa and a sex-matched healthy control into female germ-free mice. Following fecal microbiota transplantation, some of the phenotypic aspects of anorexia were replicated in the recipient mice, including reduced weight gain, elevated serum corticosterone levels, and increased anxiety-like behavior measured by open-field test. This hypothesis was further reinforced by the fact that mice subjected to these transplantations display significant changes in gene-expression in the nucleus accumbens (but not in the hippocampus), a region implicated in reward and affected in patients with anorexia.

In summary, the findings in this thesis reinforce the proposed impact of gut microbiota on host homeostasis. Specifically, on local level, we showed the influences on BBB development and function, and on systemic level, we demonstrated the effects on genes involved in energy homeostasis in nucleus accumbens. Future studies will uncover the exact mechanisms underlying the impact of gut microbiota on the brain.