Relationship between AQP4 expression and structural damage to the blood-brain barrier at early stages of traumatic brain injury in rats

BACKGROUND: Although some studies have reported that aquaporin-4 (AQP4) plays an important role in the brain edema after traumatic brain injury (TBI), little is known about the AQP4 expression in the early stage of TBI, or about the correlation between the structural damage to the blood-brain barrier (BBB) and angioedema. The aim of this project was to investigate the relationship between AQP4 expression and damage to the BBB at early stages of TBI. METHODS: One hundred and twenty healthy adult Wistar rats were randomly divided into two groups: sham operation group (SO) and TBI group. The TBI group was divided into five sub-groups according to the different time intervals: 1, 3, 6, 12, and 24 hours. The brains of the animals were taken out at different time points after TBI to measure brain water content. The cerebral edema and BBB changes in structure were examined with an optical microscopy (OM) and transmission electron microscopy (TEM), and the IgG content and AQP4 protein expression in traumatic brain tissue were determined by means of immunohistochemistry and Western blotting. The data were analyzed with SPSS 13.0 statistical software. RESULTS: In the SO group, tissue was negative for IgG, and there were no abnormalities in brain water content or AQP4 expression. In the TBI group, brain water content significantly increased at 6 hours and peaked at 24 hours following injury. IgG expression significantly increased from 1 to 6 hours following injury, and remained at a high level at 24 hours. Pathological observation revealed BBB damage at 1 hour following injury. Angioedema appeared at 1 hour, was gradually aggravated, and became obvious at 6 hours. Intracellular edema occurred at 3 hours, with the presence of large glial cell bodies and mitochondrial swelling. These phenomena were aggravated with time and became obvious at 12 hours. In addition, microglial proliferation was visible at 24 hours. AQP4 protein expression were reduced at 1 hour, lowest at 6 hours, and began to increase at 12 hours, showing a V-shaped curve. CONCLUSIONS: The angioedema characterized by BBB damage was the primary type of early traumatic brain edema. It was followed by mixed cerebral edema that consisted of angioedema and cellular edema and was aggravated with time. AQP4 expression was down-regulated during the angioedema attack, but AQP4 expression was upregulated during intracellular edema.