SYNAPTOBREVIN IS ESSENTIAL FOR SECRETION BUT NOT FOR THE DEVELOPMENT OF SYNAPTIC PROCESSES

The formation of small synaptic vesicles represents a hallmark during synaptogenesis. The small synaptic vesicle protein synaptophysin is co nsidered as a marker protein for synapses during neuronal development. Another small synaptic vesicle protein, synaptobrevin, is now well ac cepted to play an important role for the function of synapses in being a key component of exocytosis. its role during synaptogenesis is not known. Tetanus toxin which exclusively proteolyses synaptobrevin there by inhibiting secretion from all types of neurons was used to investig ate consequences of inactivating synaptobrevin for the formation of sm all synaptic vesicles and synaptic contacts, In primary cultures of mo use hypothalamic and cerebellar neurons cultivated for 3 to 4 days, sy naptobrevin appears earlier on small synaptic vesicles and in synaptic contacts than synaptophysin. Upon longer cultivation up to 12 to 14 d ays in vitro both proteins associated equally with small synaptic vesi cles, Interestingly, GABA secretion stimulated by 50 mM potassium or 5 00 pM alpha-latrotoxin, did not vary during cultivation time. Tetanus toxin added to neuronal cultures at day 2 in vitro cleaved synaptobrev in and inhibited regulated GABA secretion during the whole cultivation time. Despite the impaired function of synaptobrevin other synaptic p roteins such as synaptophysin, synaptotagmin, rab 3A, protein SV2, SNA P-25 and syntaxin were found in processes and synaptic contacts compar able to untreated cultures. The expression of various synaptic protein s was also followed in vivo. In mouse brains taken at different embryo nic days, synaptobrevin, synaptotagmin, rab 6 and the membrane protein SNAP-25 were expressed earlier than synaptophysin and protein SV2. We conclude that synaptobrevin represents a marker for early events in s ynaptogenesis. Its proteolysis by tetanus toxin, however, does not int erfere with the formation of synaptic contacts and neuronal differenti ation.