Homogentisic Acid Disrupts the Blood–Brain Barrier and Promotes Aβ Aggregation in Alzheimer’s Disease

Objective Alzheimer’s disease (AD) is a progressive neurodegenerative disease associated with metabolic dysregulation. This study aimed to investigate the role of homogentisic acid (HGA), a tyrosine metabolite, in AD pathogenesis and explore its potential as a noninvasive diagnostic biomarker. Methods Human saliva samples from AD patients and controls were analyzed. In vivo experiments were conducted using APP/PS1 (Aβ-driven) and P301S (tauopathy-focused) mouse models, which received exogenous HGA via gavage. Key techniques included behavioral tests (Morris water maze, novel object recognition, fear conditioning), Western blot, immunofluorescence, real-time PCR, and mass spectrometry to assess cognitive function, blood–brain barrier (BBB) integrity, Aβ aggregation, synaptic protein expression, and HGA metabolism. In vitro experiments were performed on HT22, SY5Y cells, and primary brain microvascular endothelial cells (BMECs) to verify HGA’s direct effects. Results Salivary HGA levels were higher in AD patients than in controls, correlating with BBB impairment. Exogenous HGA significantly exacerbated cognitive deficits, BBB leakage, Aβ deposition, and loss of synaptic proteins (PSD93, synaptophysin) in mice, with effects more pronounced in the APP/PS1 than in the P301S model. In vitro, HGA exerted dose-dependent neurotoxicity, promoted Aβ aggregation, and downregulated tight junction proteins (claudin-5, occludin, ZO-1) in BMECs. Mechanistically, AD patients showed reduced expression of HGA-metabolizing enzymes (homogentisate 1,2-dioxygenase, maleylacetoacetate isomerase) and downstream metabolites, indicating impaired HGA catabolism. These findings confirm HGA promotes AD progression via two mutually reinforcing pathways: (1) accelerating Aβ aggregation and synaptic dysfunction; (2) disrupting BBB integrity through downregulating tight junction proteins. Conclusion This study identifies salivary HGA as a potential noninvasive biomarker and highlights targeting HGA metabolism or BBB protection as promising strategies for early AD intervention.