Characterization of presynaptic proteins involved in synaptic vesicle exocytosis in the nervous system of Torpedo marmorata

Calcium-dependent acetylcholine release from Xenopus oocytes: simultaneous ionic currents and acetylcholine release recordings

The fusion of synaptic vesicles with presynaptic membranes is controlled by a complex network of protein-protein and protein-lipid interactions. SNAP-25, syntaxin and synaptobrevin (SNARE complex) are thought to participate in the formation of the core of the membrane fusion machine but the molecular basis of SNARE interactions is not completely understood. Thus, it would be interesting to design experiments to test those relationships in a new model. Xenopus laevis oocytes are valuable tools for studying the molecular structure and function of ionic channels and neurotransmitter receptors. Here we show that SNARE proteins are present in native Xenopus oocytes and that those oocytes injected with acetylcholine and presynaptic plasma membranes extracted from the electric organ of Torpedo marmorata assume some of the functions of a cholinergic nerve terminal. Neurotransmitter release and macroscopic currents were recorded and analysed simultaneously in a single oocyte electrically depolarized: acetylcholine release was detected using a chemiluminiscent method and calcium entry was measured by exploiting the endogenous Ca2+-activated chloride current of the oocyte with a two-electrode voltage-clamp system. Neurotransmitter release was calcium- and voltage-dependent and partially reduced in the presence of several calcium channel blockers. Clostridial neurotoxins, both holotoxin and injected light-chain forms, also inhibited acetylcholine release. We also studied the role of the SNARE complex in synaptic transmission and membrane currents by using monoclonal antibodies against SNAP-25, syntaxin or VAMP/synaptobrevin. The use of antibodies against VAMP/synaptobrevin, SNAP-25 and syntaxin inhibited acetylcholine release, as did clostridial toxins. However, macroscopic currents were only modified either by syntaxin antibody or by Botulinium-C1 neurotoxin. This model constitutes a new approach for understanding the vesicle exocytosis processes.

Cloning of two loci for synapse protein Snap25 in zebrafish: comparison of paralogous linkage groups suggests loss of one locus in the mammalian lineage

Synaptosome-associated protein of 25 kDa (Snap25) is an intracellular protein that is defined as a target receptor for synapse vesicles prior to neurotransmitter release. Snap25 is highly conserved, with 61% identity between human and Drosophila melanogaster. Whereas mammals and chicken have a single locus for Snap25, the tetraploid goldfish has at least three loci. We report that the zebrafish has two loci with 91% amino acid identity to each other. The alternative splicing of exon 5 arose before the gene duplication. The expression patterns of the two loci are virtually identical in adult zebrafish. The two zebrafish snap25 loci are located in paralogous linkage groups that seem to correspond to human chromosome 20, which harbors the SNAP locus, and human chromosome 14. Because no additional Snap25 homologue has been reported for any mammal or chicken, snap25.2 may have been lost in the amniote or even tetrapod lineage.

Molecular cloning of synaphins/complexins, cytosolic proteins involved in transmitter release, in the electric organ of an electric ray (Narke japonica)

Synaphins/complexins are cytosolic proteins associated with the docking/fusion complex crucial to transmitter release. The electric organ of the electric ray Narke japonica contained at least two kinds of synaphins as revealed by immunoblotting. cDNAs for three synaphins were cloned from a cDNA library prepared from the electric lobe where cell bodies of electromotor nerves innervating the electric organ exist. The proteins encoded by these cDNAs were named Nj-synaphins 1a, 1b and 2 on the basis of their high homologies (83-93%) to mammalian synaphins 1 and 2. Nj-Synaphins were immunoprecipitated by an anti-syntaxin monoclonal antibody, together with syntaxin, SNAP-25 and VAMP (synaptobrevin), suggesting the presence of a docking/fusion complex similar to that in the mammalian brain.