Egenhard Link

Tetanus Toxin Action: Inhibition of Neurotransmitter Release Linked to Synaptobrevin Proteolysis

Tetanus toxin is a potent neurotoxin that inhibits the release of neurotransmitters from presynaptic nerve endings. The mature toxin is composed of a heavy and a light chain that are linked via a disulfide bridge. After entry of tetanus toxin into the cytoplasm, the released light chain causes block of neurotransmitter release. Recent evidence suggests that the L-chain may act as a metalloendoprotease. Here we demonstrate that blockade of neurotransmission by tetanus toxin in isolated nerve terminals is associated with a selective proteolysis of synaptobrevin, an integral membrane protein of synaptic vesicles. No other proteins appear to be affected by tetanus toxin. In addition, recombinant light chain selectively cleaves synaptobrevin when incubated with purified synaptic vesicles. Our data suggest that cleavage of synaptobrevin is the molecular mechanism of tetanus toxin action.

Distribution of synaptic vesicle proteins in the mammalian retina identifies obligatory and facultative components of ribbon synapses

The mammalian retina contains two synaptic layers. The outer plexiform layer (OPL) is primarily composed of ribbon synapses while the inner plexiform layer (IPL) comprises largely conventional synapses. In presynaptic terminals of ribbon synapses, electron‐dense projections called ribbons are present at the synaptic plasma membranes. Ribbons bind synaptic vesicles and guide them to the synaptic membrane for fusion. In this manner, ribbons are thought to accelerate the delivery of vesicles for continuous exocytosis. In recent years, a large number of synaptic proteins has been described but it is not known if these protein colocalize in the same types of synapses. In previous studies, several proteins essential for synaptic function were not detected in ribbon synapses, suggesting that the mechanism of synaptic vesicle exocytosis may be very different in ribbon and conventional synapses. Using confocal laser scanning microscopy, we have now systematically investigated the protein composition of ribbon synapses. Our results show that, of the 19 synaptic proteins investigated, all except synapsin and rabphilin are obligatorily present in ribbon synapses. For example, rab3 which was reported to be absent from ribbon synapses, was found in bovine, rat and mouse ribbon synapses using multiple independent antibodies. In addition, we found staining in these synapses for PSD‐95 and NMDA receptors, which suggested a similar design for the postsynaptic component in ribbon and conventional synapses. Our data show that ribbon synapses are more conventional in composition than reported, that most synaptic proteins are colocalized to the same type of synapse, and that synapsin and rabphilin are likely to be dispensible for basic synaptic functions.

Inhibition of neurotransmitter release by clostridial neurotoxins correlates with specific proteolysis of synaptosomal proteins

Rat brain synaptosomes were used to study the effect of several clostridial neurotoxins on the neurotransmitter release. In this system the blockade of transmitter release correlated with the proteolytic activity of the toxins. Blockade of glutamate release was linked to selective proteolysis of one of the following synaptic proteins: synaptobrevin (BoNT/D, BoNT/F); SNAP-25 (BoNT/A, BoNT/E), or HPC-1/syntaxin (BoNT/C1). All the toxins used had an inhibitory effect on synaptosomes with the exception of BoNT/F. BoNT/F cleaved synaptobrevin in permeabilized synaptosomes but failed to produce the same effect on intact synaptosomes.