Gut Microbiota Dysbiosis Drives Early Alzheimer's Pathogenesis via Microglial TREM2/SYK/NF-κB Signaling Axis

Gut microbiota dysbiosis is implicated in Alzheimer's disease (AD), but causal evidence and mechanisms linking it to microglial dysfunction remain unclear. This study aimed to determine whether gut microbiota drives neuroinflammation and cognitive impairment via the microglial TREM2/SYK signaling axis in early AD. Using six-month-old APP/PS1 mice, fecal microbiota transplantation (FMT) was performed between AD and wild-type mice. Cognitive function, gut microbiota composition (16S rRNA sequencing), serum metabolites, hippocampal neuroinflammation, microglial polarization, and TREM2/SYK/NF-κB pathway activity were assessed. BV2 microglial cells were treated with Aβ oligomers, a TREM2 agonist, or a SYK inhibitor for mechanistic validation. AD mice exhibited cognitive decline, reduced microbial diversity (e.g., decreased Bacteroidetes and Lactobacillus), and altered circulating metabolites, including decreased butyrate and elevated LPS. Their hippocampi exhibited heightened glial activation, elevated pro-inflammatory cytokines (TNF-α, IL-1β, IL-6), and a shift toward pro-inflammatory activation markers (M1-associated). At the molecular level, TREM2 expression was downregulated, whereas SYK phosphorylation and NF-κB activation were enhanced, concomitant with synaptic protein loss. Critically, FMT from healthy donors reversed these abnormalities and improved cognition, whereas AD microbiota induced mild pathology in wild-type mice. In vitro, TREM2 activation or SYK inhibition attenuated Aβ-induced M1 polarization and cytokine release in microglia. Gut microbiota dysbiosis promotes early AD pathogenesis by dysregulating the microglial TREM2/SYK/NF-κB pathway, thereby driving neuroinflammation and synaptic dysfunction. Targeting this microbiota-signaling axis may offer novel therapeutic strategies.