Neuroprotective Effect of Gut Microbiota Modulation in Alzheimer's Disease: a Preclinical Study

Alzheimer’s disease (AD) is the most common neurodegenerative disorder and the main cause of dementia in the elderly with intricate pathobiology. It is a complex multifactorial disease involving neuroinflammation, oxidative stress, impaired metabolic and proteolytic pathways and cognitive dysfunctions. Recently, the role of gut-brain axis in the onset and progression of the disorder has been elucidated. In this study, we used different approaches to demonstrate how a proper gut microbiota modulation can positively influence the progression of AD. In addition, considering that AD is characterized by extensive protein aggregation and that autophagy deeply regulates protein homeostasis, we also explored the mechanistic insight into regulation of this proteolytic pathway under the condition of Golgi fragmentation due to impaired tethering complex. In details, 3xTg-AD mice were used as a model of AD and the effects of the probiotic formulation SLAB51, of p62-engineered Lactobacilli and of pasteurized beer on gut microbiota and brain, functions were evaluated. Upon wild-type and transgenic animals treatment with SLAB51, markers of glucose metabolism including brain expression levels of key glucose transporters, insulin-like growth factor receptor β, and glycated hemoglobin plasma levels were evaluated. For the p62-LAB study, neuronal proteolysis, oxidative stress, neuroinflammation and load of Aβ deposition in AD brain were monitored. Gut microbiota composition and gut-brain axis components were also studied. Regarding the pasteurized beer treatment, upon mice sacrifice, amyloid-beta (Aβ) load and neuro-inflammatory markers were evaluated in the brain and plasma of treated animals. Finally, gut hormones as well as a possible shift in gut bacterial and fungal compositions were evaluated. Cog3 knockout cell line was used to evaluate the effect of Golgi fragmentation in autophagy. To identify cellular trafficking or sorting defects, subcellular fractionation was performed in sucrose density gradient. Then, colocalization of autophagy proteins with autophagosomal markers and their trafficking sites were studied under basal and starvation conditions. Our results demonstrated that SLAB51 treatment, through gut microbiota modulation, positively influenced metabolic homeostasis in the brain, contributing to a reduction in neuroinflammation. The treatment restored the major neuronal glucose transporters GLUT3 and GLUT1 and significantly decreased p-Tau levels in AD brain. The oral administration of p62-enginereed LAB improved neuronal proteolysis reduced the load of Aβ deposition in AD brain and the amount of inflammatory and oxidative markers. However, the treatment did not have a pronounced effect on the gut microbiota composition or on hormones of the gut-brain axis. Beer-enriched with brewing yeasts ameliorate the cognitive functions of transgenic mice by reducing neuro-inflammatory markers and the levels of the Aβ(1–42) peptide, in the PFC and hippocampus of AD mice. In addition, treatments with beer and beer-yeast also increase the richness in gut bacterial population of AD mice. Finally, the analysis of the autophagic process in Cog3 knockout cell line demonstrated that the cargo receptor p62 colocalized with the autophagosomal fraction without any glycosylation defect and observed functional for selective autophagy under basal condition. Our studies deeply dissected the role of gut-brain axis in AD, evaluating the effect of different strategies for gut microbiota modulation. The tested strategies positively regulated neuronal processes in AD mice, restoring proteolysis and glucose metabolism, counteracting inflammation and oxidation and decreasing the load of amyloid aggregates. These results therefore suggest new approaches and opportunities for the development of therapies against neurological disorders.