Claire de La Serre - US grants

ABSTRACT There is substantial evidence linking the gastrointestinal (GI) microbiota and obesity. Germ free (GF) mice do not gain weight when fed a high fat (HF) diet. Microbiota composition rapidly changes in response to HF feeding and colonization of GF animals with an ?obese? microbiota results in recapitulation of the donor phenotype. This data suggest that an unfavorable microbiota is sufficient to cause obesity, however the mechanisms and pathways remain unclear. Excessive energy intake is the main cause for obesity. Food intake is regulated by homeostatic and hedonic cues. Hedonic eating refers to consumption of food ?for pleasure?, in the absence of or beyond energy needs and is linked to reward signaling in the forebrain. Food consumption, particularly HF food, leads to the release of dopamine in the brain, hyper- and hyposensitivity of this system have been linked to abnormal weight gain. The microbiota has previously been shown to alter neural development and gene expression in the brain. However the potential influence of the microbiota on reward signaling and appetitive eating has yet to be studied. Using GF animals colonized with microbiota from lean or obese donors we will test the hypothesis that the microbiota reduces dopamine release in the nucleus accumbens to increase motivation and preference for fat. In order to aid in the development of microbiota- based therapies, it is necessary to understand the route by which the microbiota communicates to the brain. There is evidence that microbiota to brain signaling is relayed by vagal afferents innervating the GI tract but our understanding of the pathway is limited, partially because traditional techniques, such as vagotomy and capsaicin, lack specificity and can indirectly alter microbiota composition. In the second aim of this proposal, we will use a ribosome inactivating protein to ablate vagal afferent signaling in colonized animals. We aim to demonstrate that microbiota to brain communication is vagally mediated. Knowledge from this proposal will support the development of microbiota-based therapies aimed at food addiction and weight loss. Microbiota and vagal signaling could be more easily manipulated with fewer side effects than central targets.

PROJECT SUMMARY Obesity is one of the defining public health problems of our time. At its root, increases in fat storage is caused by an imbalance in energy homeostasis, favoring energy intake over expenditure. Physiological mechanisms are in place to prevent excess caloric intake, yet these defense mechanisms fail in the face a modern food environment that promotes food intake. This is underscored by the lack of efficacy of non-invasive strategies, such as caloric restriction or medications, to sustain long-term weight loss. Thus, there is a critical need to understand the pathophysiology leading to food overconsumption and develop novel strategies to promote weight loss. The vagus nerve provides direct communication about nutrient intake from the gut to the brain. Removing of the vagus in lean animals results in significant overeating when presented with palatable calorie dense diets, suggesting a protective role of the vagus nerve to prevent overconsumption of calories. In obesity, vagal communication of gut metabolic cues to the brain is impaired, and preventing vagal signaling results in weight loss in animals fed high fat diet. The mechanisms for the switch from protection against, towards promoting obesity are unclear, but we have recently demonstrated that chronic consumption of high fat diet results in anatomical restructuring of vagal fibers in the brain. Therefore, we propose a new hypothesis that vagal gut-brain axis is reprogramed in response to high fat diets to drive obesity. We use a combination of molecular and genetic approaches to deconstruct the sensory vagus into cellular components based on their site of innervation to fully elucidate the role of high fat feeding on vagal remodeling. In aim 1 we assess the impact of diet on vagal fiber anatomy, synaptic function, and the behavioral consequences, including meal termination and motivation for food. In aims 2 and 3 we consider the mechanisms by which diet causes vagal remodeling. We hypothesize that a gut microbiota-driven immune response triggers the rewiring of the gut-brain axis. This is supported by our previous work and preliminary data showing abnormal microbiota composition is necessary and sufficient to alter vagal innervation in the NTS. In aim 2, we will use germ free rats and microbiota transplant to determine 1) if microbiota dysbiosis is sufficient for vagal remodeling, and 2) if restoring a symbiotic microbiota in obesity can normalize vagal signaling, feeding behavior and body weight. In aim 3 we will combine genetic and molecular tools to investigate the recruitment of immune cells with the vagal afferent pathway as mediators of diet-driven vagal maladaptation. Completion of these studies will identify vagal rewiring as a novel pathway in the etiology of obesity, and establish microbiota and microglia as potential tools for the development of weight loss strategies. .