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Aquaporins (AQP) are water channel proteins that play important roles in the regulation of water homeostasis in physiological and pathological conditions. AQP4 and AQP9, the main aquaporin subtypes in the brain, are expressed in the adult forebrain subventricular zone (SVZ), where neural stem cells (NSCs) reside, but little is known about their expression and role in the NSC population, either in vivo or in vitro. Also, no reports are available on the presence of these proteins in human NSCs. We performed a detailed molecular and phenotypical characterization of different AQPs, and particularly AQP4 and AQP9, in murine and human NSC cultures at predetermined stages of differentiation. We demonstrated that AQP4 and AQP9 are expressed in adult murine SVZ-derived NSCs (ANSCs) and that their levels of expression and cellular localization are differentially regulated upon ANSC differentiation into neurons and glia. AQP4 (but not AQP9) is expressed in human NSCs and their progeny. The presence of AQP4 and AQP9 in different subsets of ANSC-derived glial cells and in different cellular compartments suggests different roles of the two proteins in these cells, indicating that ANSC-derived astrocytes might maintain in vitro the heterogeneity that characterize the astrocyte-like cell populations in the SVZ in vivo. The development of therapeutic strategies based on modulation of AQP function relies on a better knowledge of the functional role of these channels in brain cells. We provide a reliable and standardized in vitro experimental model to perform functional studies as well as toxicological and pharmacological screenings., (2005 Wiley-Liss, Inc.)