Synaptobrevin isoforms in secretory granules and synaptic-like microvesicles in anterior pituitary cells

A set of synaptic proteins have been shown to be essential for the life cycle and exocytosis of synaptic vesicles at the nerve terminal. Recently, these proteins have also been identified in certain endocrine cells. Here we analysed the presence and location of some of these synaptic proteins in anterior pituitary cells. Immunoblotting data demonstrated that Rab3a, synaptotagmin, cellubrevin, synaptobrevin 2, syntaxin 1, SNAP-25 and synaptophysin were well represented in anterior pituitary cells as well as in the corticotroph cell line AtT-20. Cellubrevin was the most abundant synaptobrevin isoform present in pituitary cells. Moreover, both cellubrevin and synaptobrevin 2 took part of a protein complex involved in the fusion process in adenohypophyseal cells. Immunocytochemical and subcellular fractionation showed that cellubrevin, synaptobrevin 2, Rab3a and synaptotagmin were located in both secretory granules and synaptic-like microvesicles fractions. In contrast, SNAP-25 and syntaxin 1 were mainly associated with plasma membrane fractions. Therefore, these results suggest similar secretory mechanisms for synaptic vesicles and secretory organelles in both neuronal and endocrine cells.