Aquaporin‐4 gene deletion in mice increases focal edema associated with staphylococcal brain abscess

Brain abscess is associated with local vasogenic edema, which leads to increased intracranial pressure and significant morbidity. Aquaporin‐4 (AQP4) is a water channel expressed in astroglia at the blood–brain and brain–CSF barriers. To investigate the role of AQP4 in brain abscess‐associated edema, live Staphylococcus aureus (105 colony‐forming units) was injected into the striatum to create a focal abscess. Wild‐type and AQP4‐deficient mice had comparable immune responses as measured by brain abscess volume (∼ 3.7 mm3 at 3 days), bacterial count and cytokine levels in brain homogenates. Blood–brain barrier permeability was increased comparably in both groups as assessed by extravasation of Evans blue dye. However, at 3 days the AQP4 null mice had significantly higher intracranial pressure (mean ± SEM 27 ± 2 vs. 17 ± 2 mmHg; p < 0.001) and brain water content (81.0 ± 0.3 vs. 79.3 ± 0.5 % water by weight in the abscess‐containing hemisphere; p < 0.01) than wild‐type mice. Reactive astrogliosis was found throughout the abscess‐containing hemisphere; however, only a subset of astrocytes in the peri‐abscess region of wild‐type mice had increased AQP4 immunoreactivity. Our findings demonstrate a protective effect of AQP4 on brain swelling in bacterial abscess, suggesting that AQP4 induction may reduce vasogenic edema associated with cerebral infection.