Regulation of AQP4 surface expression via vesicle mobility in astrocytes.

Aquaporin 4 (AQP4) is the predominant water channel in the brain, expressed mainly in astrocytes and involved in water transport in physiologic and pathologic conditions. Besides the classical isoforms M1 (a) and M23 (c), additional ones may be present at the plasma membrane, such as the recently described AQP4b, d, e, and f. Water permeability regulation by AQP4 isoforms may involve several processes, such as channel conformational changes, the extent and arrangement of channels at the plasma membrane, and the dynamics of channel trafficking to/from the plasma membrane. To test whether vesicular trafficking affects the abundance of AQP4 channel at the plasma membrane, we studied the subcellular localization of AQP4 in correlation with vesicle mobility of AQP4e, one of the newly discovered AQP4 isoforms. In cultured rat astrocytes, recombinant AQP4e acquired plasma membrane localization, which resembled that of the antibody labeled endogenous AQP4 localization. Under conditions mimicking reactivation of astrocytes (increase in cytosolic cAMP) and brain edema, an increase in the AQP4 plasma membrane localization was observed. The cytoskeleton remained unaffected with the exception of rearranged actin filaments in the model of reactive astrocytes and vimentin meshwork depolymerization in hypoosmotic conditions. AQP4e vesicle mobility correlated with changes in the plasma membrane localization of AQP4 in all stimulated conditions. Hypoosmotic stimulation triggered a transient reduction in AQP4e vesicle mobility mirrored by the transient changes in AQP4 plasma membrane localization. We suggest that regulation of AQP4 surface expression in pathologic conditions is associated with the mobility of AQP4-carrying vesicles.