Hydrogen gas alleviates blood-brain barrier impairment and cognitive dysfunction of septic mice in an Nrf2-dependent pathway

Sepsis-associated encephalopathy (SAE) is a cognitive impairment caused by sepsis and is related to increased morbidity and mortality. Damage to the blood-brain barrier (BBB) has been proved to be one of the important causes of SAE. Molecular hydrogen (H₂) is a promising method for the treatment of SAE, yet the underlying mechanism is not clear. This study was designed to demonstrate whether H₂ can alleviate SAE by protecting the BBB, and whether it is protected by Nuclear factor erythroid-2-related factor 2 (Nrf2) and its downstream signaling pathways. Either a sham or a cecal ligation and puncture (CLP) procedure was applied to female wild-type (WT) and Nrf2-knock-out (Nrf2^-/-) C57BL/6J mice. H₂ (2%) was given for 60 min starting at 1 h and 6 h after the sham or CLP procedure. In addition, bEnd.3 cells cultured with medium which contained LPS, Saline, DMSO or ML385 (a Nrf2 inhibitor) were also used in the research. The 7-day survival rates were recorded. The Morris water maze was used to determine cognitive function. Pro-inflammatory and anti-inflammatory cytokines [tumor necrosis factor-alpha (TNF-α), interleukin 6 (IL-6), HMGB1, and IL-10), antioxidant enzymes, and oxidation products [superoxide dismutase (SOD), chloramphenicol acetyltransferase (CAT), malondialdehyde (MDA), and (8-iso-PGF2α)] were determined by enzyme-linked immunosorbent assay (ELISA). Brain water content, Dextran tracer, and Evans blue extravasation were used to detect the damage of the BBB. Western blot analysis was used to detect β-catenin, phosphorylated β-catenin, adhesion-linked protein VE-cadherin, and associated tight junction protein ZO-1. We found that H₂ can improve survival in septic mice, decrease escape latency and platform crossing times, decrease pro-inflammatory cytokines and oxidative product levels in the mouse cortex, and increase the expression of anti-inflammatory factors in WT, but not Nrf2^-/-, mice. Moreover, H₂ can also decrease brain water content, extravascular dextran, extravascular Evans blue dye, and β-catenin level, and increase ZO-1 and VE-cadherin expressions in WT mice, but not in Nrf2^-/- mice. Our result shows that H₂ can protect the BBB by decreasing its permeability, thereby reducing SAE and improving cognitive function, which is mediated through Nrf2 and its downstream signaling pathways.