Background: Chronic stress is a major risk factor for cognitive decline and blood–brain barrier (BBB) disruption, yet the underlying molecular mechanisms remain elusive. This study aimed to investigate the specific role of the metabolic intermediate homocysteine (Hcy) in chronic stress-induced BBB dysfunction and cognitive impairment. Methods: We utilized a male Sprague-Dawley rat model of chronic unpredictable mild stress (CUMS) and administered vitamin B complex to lower Hcy levels in vivo. Regional Hcy accumulation, BBB permeability, and cognitive behaviors were assessed. In vitro, primary rat brain microvascular endothelial cells (BMECs) were exposed to Hcy to evaluate barrier-forming function, transcriptomic alterations, DNA methylation patterns, Cav1.2 expression, and reactive oxygen species (ROS) production. Results: CUMS selectively induced BBB hyperpermeability and significant Hcy accumulation predominantly within the rat hippocampus, which correlated intimately with cognitive deficits. Lowering Hcy levels via vitamin B supplementation successfully restored hippocampal BBB integrity and alleviated cognitive impairment. In addition, elevated Hcy severely impaired the barrier function of BMECs. Mechanistically, Hcy reduced global DNA methylation in BMECs and specifically induced targeted DNA hypomethylation at the intro region of Cacna1c. This epigenetic shift caused the transcriptional derepression and overexpression of the Cav1.2 calcium channel. Upregulated Cav1.2 subsequently triggered a robust ROS burst, leading to tight junction degradation. Conclusions: Our findings unveil a novel metabolic–epigenetic axis where Hcy-driven Cacna1c hypomethylation directly disrupts BMECs function to dismantle the hippocampal BBB. Lowering Hcy or targeting this Hcy-Cav1.2 pathway establishes a promising therapeutic strategy for mitigating stress-related neurovascular damage and cognitive disorders.
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