Synaptobrevin2-expressing vesicles in rat astrocytes : insights into molecular characterization, dynamics and exocytosis

The SNARE-dependent exocytosis of glutamate-containing vesicles in astrocytes is increasingly viewed as an important signal at the basis of the astrocyte-to-neurone communication system in the brain. Here we provide further insights into the molecular features and dynamics of vesicles in cultured astrocytes. We found that immunoisolated synaptobrevin2 vesicles are clear vesicles quite heterogenous in size and contain the vesicular glutamate transporter v-Glut-2. Moreover, they are immunopositive for synaptotagmin IV, for AMPA receptor subunits GluR2,3 and, to a lesser extent, for GluR1. We also provide direct evidence for the functional expression of v-Glut-2 in astrocytes and demonstrate that synaptobrevin2-positive vesicles can specifically take up (3H)L-glutamate via a bafilomycin-sensitive mechanism. Finally, by time lapse confocal microscopy, we show that a subpopulation of vesicles (tagged with a synaptobrevin2-EGFP chimera) is highly mobile and can fuse with the plasma membrane, preferentially at the level of the astrocyte processes, in a Ca2+-dependent manner. These latter observations, together with the evidence reported here for the expression of functional v-Glut-2 in synaptobrevin2-positive vesicles, provide a molecular basis for regulated exocytosis in astrocyte.