Aquaporin-4 deletion ameliorates hypoglycemia-induced BBB permeability by inhibiting inflammatory responses

Background: Severe hypoglycemia induces brain edema by upregulating aquaporin-4 (AQP4) expression and by degrading tight junctions. Acute severe hypoglycemia induces a proinflammatory environment that may contribute to a disruption in the epithelial barrier by decreasing tight junction protein expression. Interestingly, the altered AQP4 expression has been considered to play a critical role in neuroinflammation during acute brain injury. It has been shown that AQP4 deletion reduces brain inflammation in AQP4-null mice after intracerebral LPS injection. However, the effect of AQP4 deletion regarding protection against hypoglycemia-induced blood-brain barrier (BBB) breakdown is unknown. Methods: An acute severe hypoglycemic stress model was established via injection of 4 unit/kg body weight of insulin. Evans blue (EB) staining and water measurement were used to assess BBB permeability. Western blot, reverse transcription polymerase chain reaction, and immunofluorescence were used to detect the expression of related proteins. The production of cytokines was assessed via enzyme-linked immunosorbent assay. Results: Hypoglycemia-induced brain edema and BBB leakage were reduced in AQP4(-/-) mice. AQP4 deletion upregulated PPAR-gamma and inhibited proinflammatory responses. Moreover, knockdown of aquaporin-4 by small interfering RNA in astrocytes co-cultured with endothelial cells effectively reduced transendothelial permeability and degradation of tight junctions. Treatment with PPAR-gamma inhibitors showed that upregulation of PPAR-gamma was responsible for the protective effect of AQP4 deletion under hypoglycemic conditions. Conclusions: Our data suggest that AQP4 deletion protects BBB integrity by reducing inflammatory responses due to the upregulation of PPAR-gamma expression and attenuation of proinflammatory cytokine release. Reduction in AQP4 may be protective in acute severe hypoglycemia.