Site‐Specific Opening of the Blood‐Brain Barrier by Extracellular Histones

Increased extracellular histones in the blood are known as a biomarker for vascular dysfunction observed in patients with severe trauma or sepsis. While many previous studies have been focused on histone‐associated coagulopathy and organ (lung, kidney, liver) failure, limited information is available regarding the effects of histones on the brain or cerebral vasculature. Brain endothelial cells and cell‐cell junctions constitute the blood‐brain barrier (BBB) providing a critical paracellular seal that restricts the transfer of harmful substances from the circulation into the brain. It remains unclear whether circulating histones affect BBB structure or permeability function. To address these gaps, we applied a combination of in vitro functional analyses to evaluate the direct effects of histones (10–100 micrograms/mL) on brain microvascular endothelial cells by measuring transendothelial electrical resistance and solute flux. We also used in vivo approaches to obtain evidence of their influence on BBB integrity by quantifying brain tissue accumulation of intravenously injected tracers of different sizes: sodium fluorescein (376 Da), Alexa fluor 555‐cadaverine (1 kDa) and 3 kDa Texas‐Red dextran, in mice receiving a sublethal dose of histones (45 mg/Kg, iv). In mouse brain endothelial cells, treatment with 100 micrograms/mL histones resulted in increased paracellular permeability to sodium fluorescein and reduced barrier resistance, accompanied by discontinuous staining of the tight junction proteins zona ocludens‐1 (ZO‐1) and claudin‐5. The histone effect did not seem to result from cell death or toxicity, as indicated by negative propidium iodide staining. In mice infused with histones, the BBB permeability to small solutes of <1 kDa was increased, whereas tracers larger than 3 kDa remained impermeable across brain microvessels. Further analysis of coronal sections in different brain regions showed that histone‐induced tracer leakage mainly occurred in the hippocampus, but not in the cerebral cortex. Moreover, BBB dysfunction was associated with a reduced expression of several junction proteins in the hippocampal region. Together, these data suggest for the first time that circulating histones cause a region‐specific disruption of the BBB which may be responsible for impaired cognitive function observed in sepsis survivors. Support or Funding Information This work is supported by NIH grants: GM097270, HL070752, HL126646 and HL070752 to SYY.