Gut microbiota modulation in Alzheimer's disease.

Alzheimer’s disease (AD) is a progressive neurodegenerative disease and the main cause of dementia in the elderly. It is a complex multifactorial disease involving cognitive dysfunctions, neuroinflammation, oxidative stress, impaired metabolic and proteolytic pathways. Recently, probiotics have emerged as a promising and safe strategy to manipulate gut microbiota composition affecting the gut-brain axis and increasing the host health status through a multi-level mechanism that is currently under investigation. The present thesis describes specific mechanisms through which probiotics chronic supplementation exerted neuroprotective and anti-inflammatory effects in a murine model of AD, focusing on both cerebral hypoxia and lipid metabolism and contributes to unravel the neuroprotective properties of gut microbial metabolites in a cellular model of AD and in immortalized hippocampal AD astrocytes, focusing on proteolysis. Specifically, upon SLAB51 multi-strain probiotic formulation, cholesterol biosynthesis was inhibited in 3xTg-AD mice and alternative pathways of bile acid synthesis were influenced. The plasmatic increase of arachidonic acid in treated AD mice reflected dynamic interactions among several actors of a complex inflammatory response, in which polyunsaturated fatty acids can compete each other and simultaneously co- operate in the resolution of inflammation. In addition, chronic supplementation with SLAB51 enhanced cerebral expression of hypoxia inducible factor 1α and counteracted the increase of inducible nitric oxide synthase brain expression and nitric oxide (NO) plasma levels in AD mice, supporting probiotics anti-inflammatory and antioxidant properties. Considering that (poly)phenols and their derivatives/metabolites have been recognized as promising candidatesfor the prevention of AD due to their multiple beneficial effects, the ability of a selection of gut microbiota-derived metabolites of flavan-3-ols to modulate the functionality of cellular proteolytic pathways in a cellular model of AD has been investigated. In vitro and in silico studies demonstrated that the phenyl-γ- valerolactones modulated cellular proteolysis via proteasome inhibition and compensatory autophagy upregulation, consequently reducing the amount of intra- and extracellular amyloid-beta (1-42) peptides in SH-SY5Y neuroblastoma cells stably transfected with the 717 valine-to-glycine amyloid precursor protein mutated gene. Furthermore, proteolytic pathways have been deeply studied and characterized in immortalized hippocampal astrocytes obtained from 3xTg-AD and wild-type mice (3Tg-iAstro and WT-iAstro, respectively), considering that astrocytes exert protective roles in the central nervous system and are implicated in the pathogenesis of neurodegenerative diseases such as AD with not completely unraveled mechanisms. Impaired proteostasis in AD astrocytes has been observed, with proteasome inhibition and autophagic compensatory activation. In addition, 4-phenylbutyric acid, a neuroprotective aromatic short chain fatty acid, restored proteolysis in 3Tg-iAstro cells highlighting a new mechanism involved in the beneficial effect of this FDA approved compound in neurodegenerative disorders. Collectively, in silico, in vitro and in vivo studies here reported contribute to deeply explain the molecular pathways affected by probiotics and/or their metabolites in AD, supporting their inclusion in future AD therapeutic/preventative protocols.