Studies of mice

Cerebral water accumulation—clinically denoted as brain edema—is a potentially life threatening complication of almost every intracranial neuropathological state. The molecular membrane water channel aquaporin-4 (AQP4) has been shown to be present at the blood–brain barrier (BBB) where it plays pivotal role in the transport of water between the tissue water compartments of the brain. Accumulating evidence indicates that the blockade of AQP4 function at the BBB would be a new therapeutic approach to the treatment and prevention of brain swelling. The cytoskeletal protein dystrophin has been shown to be involved in the maintenance of the polarized expression of AQP4 at the BBB. In order to further elucidate the mechanisms responsible for the highly polarized AQP4 expression, we studied brain tissue water accumulation during induction of brain edema in dystrophin-null transgenic mice (mdx-bgeo) and control mice. Immunofluorescence and immunoelectron microscopic analyses of dystrophin-null brains revealed a dramatic reduction of AQP4 in astroglial end-feet surrounding capillaries (BBB) and at the glia limitans (cerebrospinal fluid–brain interface). The AQP4 protein is mislocalized, because immunoblotting showed that the total AQP4 protein abundance was unaltered. Brain edema was induced by i.p. injection of distilled water and 8-deamino-arginine vasopressin. Changes in cerebral water compartments were assessed by diffusion-weighted MRI (DWI) with determination of the apparent diffusion coefficient (ADC). In dystrophin-null mice and control mice, ADC gradually decreased by 5–6% from baseline levels during the first 35 min, indicating the initial phase of intracellular water accumulation is similar in the two groups. At this point, the control mice sustained an abrupt, rapid decline in ADC to 58%±2.2% of the baseline at 52.5 min, and all of the animals were dead by 56 min. After a consistent delay, the dystrophin-null mice sustained a similar decline in ADC to 55%±3.4% at 66.5 min, when all of the mice were dead. These results demonstrate that dystrophin is necessary for polarized distribution of AQP4 protein in brain where facilitated movements of water occur across the BBB and cerebrospinal fluid–brain interface. Moreover, these results predict that interference with the subcellular localization of AQP4 may have therapeutic potential for delaying the onset of impending brain edema.