Therapeutic Potential of Microbiota Modulation in Alzheimer’s Disease: A Review of Preclinical Studies

Abstract

Alzheimer’s disease (AD) is the most common neurodegenerative disease, yet it currently lacks effective treatment due to its complex etiology. The pathological changes in AD have been linked to the neurotoxic immune responses following aggregation of Aβ and phosphorylated tau. The gut microbiota (GM) is increasingly studied for modulating neuroinflammation in neurodegenerative diseases and in vivo studies emerge for AD. This critical review selected 7 empirical preclinical studies from 2019 onwards assessing therapy approaches targeting GM modulating microglia neuroinflammation in AD mouse models. Results from probiotics, fecal microbiota transplantation, and drugs were compared and contrasted, including for cognition, neuroinflammation, and toxic aggregation of proteins. Studies consistently reported significant amelioration or prevention of cognitive deficits, decrease in microglial activation, and lower levels of pro-inflammatory cytokines, compared to AD mouse models. However, there were differences across papers for the brain regions affected, and changes in astrocytes were inconsistent. Aβ plaques deposition significantly decreased in all papers, apart from Byur dMar Nyer lNga Ril Bu (BdNlRB) treatment. Tau phosphorylation significantly declined in 5 studies. Effects in microbial diversity following treatment varied across studies. Findings are encouraging regarding the efficacy of study but information on the effect size is limited. Potentially, GM reverses GM derived abnormalities, decreasing neuroinflammation, which reduces AD toxic aggregations of proteins in the brain, resulting in cognitive improvements. Results support the hypothesis of AD being a multifactorial disease and the potential synergies through multi-target approaches. The use of AD mice models limits conclusions around effectiveness, as human translation is challenging.

Oral Microbiome and Alzheimer’s Disease

“…Gut dysbiosis, which disrupts the balance of bacteria in the gut, is known to impact cognition, likely through the gut-brain axis [7][8][9]. Notably, gut dysbiosis has been observed in patients with AD [10][11][12][13][14][15][16]. Beyond the gut, the oral cavity hosts a diverse community of bacteria that interact with each other and with the host.…”

Causal relationship between gut microflora and dementia: a Mendelian randomization study

“…In the present study, we found with MR analysis that the family/order Desulfovibrionaceae was associated with an increased risk of developing dementia and Alzheimer’s disease. A growing body of research has been able to demonstrate that altering GMs can attenuate microglia-mediated neuroinflammation and reduce Aβ deposition in the brain, thereby improving cognition ( Abraham et al, 2019 ; Sun et al, 2020 ; Benichou Haziot and Birak, 2023 ). Supporting the above findings, we propose that the effect of family/order Desulfovibrionaceae on patients of dementia or Alzheimer’s disease might be related to brain Aβ deposition.…”