Aquaglyceroporin 9 in brain pathologies.

Aquaglyceroporins belong to the aquaporin family and are permeable to water and also to small solutes such as glycerol and urea. In this review, we will compare the expression of aquaporin 9 (AQP9), an aquaglyceroporin, with that of AQP4, a pure water channel, in pathological conditions. In astrocytes, AQP4 is mainly involved in water and ionic homeostasis. Its expression is highly modified in several brain disorders and it plays a key role in cerebral edema formation. AQP9 is expressed in astrocytes and in catecholaminergic neurons. The level of expression of brain AQP9 is under the control of blood insulin concentrations, and its expression is increased in diabetes, suggesting that AQP9 could be involved in brain energy metabolism. The induction of AQP9 in astrocytes is observed over time after stroke onset, suggesting participation in the clearance of excess lactate in the extracellular space. In some models, AQP9 is also induced in non-catecholaminergic neurons after global ischemia and in the periphery of gliomas, however functional roles are still unclear. The review of literature underlies that each AQP has several distinctive roles which depend on the AQP and cell types.