Microbiota-gut-brain axis in models of small intestine neuroinflammation: role of serotonergic neurotransmission and Toll-like receptor 4 signaling

The different cell populations of the ENS (neurons, glial cells, smooth muscle cells, interstitial cells of Cajal) directly interact with the luminal environment, which is populated by trillions of nonpathogenic microbes, constituting the commensal microbiota. Moreover, the mammalian gut is sufficiently permeable to permit nutrients absorption, efficiently preventing the penetration of gut microbes and consequent crossing of the mucosal barrier. Our knowledge regarding the influence of the gut microbiota on host development and function has increased exponentially in the last decade, but we are still a long way from understanding which molecular mechanisms are involved. Gut microbes influence host physiology directly through its metabolic products, or indirectly by interacting with microbe-associated molecular pattern receptors, such as TLRs. These receptors play a key role in the microflora-host homeostatic relationship and are essential for activating innate immune responses against pathogenic microorganisms. In particular, TLR4 is expressed in enteric neurons, glia and immune cells, regulating structural and functional integrity of ENS and in controlling GI motility. Furthermore, TLRs are involved in the onset of intestinal inflammation and in immunological alterations found in IBD patients. Indeed, polymorphisms in TLR4 encoding gene have been associated with functional abnormalities and inflammatory features both in the small intestine and colon of IBD patients and related murine models. Moreover, gut microbiota seems to be directly implicated in modulating the development and function of CNS and ENS, suggesting that changes in commensal microbial composition can perturb neuronal circuitries integrity and activity. In recent years, the microbiota-gut-brain axis has emerged as an important communication pathway, also involving vagal and sympathetic extrinsic pathways, the intestinal epithelial barrier, and immune cells. In addition, the gut microbiota is involved in signaling pathways and neurotransmitters content, due to its capacity to synthetize neurotransmitters and neuromodulators, such as tryptophan and 5-HT. The relationship between commensal gut microbes and their host can move from commensalism to pathogenicity in several diseases affecting both the CNS and ENS, such multiple sclerosis and IBD. Therefore, several recent studies have been focused on understanding the "top-down" or "bottom-up" impact of enteric and central neuroinflammation. The primary aim of the present PhD research thesis is to characterize the role of microbiota-gut-brain axis in the control of CNS and ENS homeostasis, and its interaction with TLR4 activity by addressing the following objectives: ➢ characterize TLR4 signaling in tuning structural and functional integrity of ENS in presence of DNBS-induced ileitis in order to identify the neuronal pathways involved in the neuroimmune crosstalk; ➢ define the impact of the demyelinating process on microbiota-gut-brain axis in a murine cuprizone model, a toxin-induced demyelination model; ➢ determine the relationship between intestinal dysbiosis and the serotoninergic pathway in presence of enteric and central neuroinflammation, to identify potential therapeutic targets.