Antibodies against cysteine string proteins inhibit evoked neurotransmitter release at Xenopus neuromuscular junctions

Cysteine string proteins (CSPs) are evolutionarily conserved proteins that are associated with synaptic vesicles and other regulated secretory organel les. To investigate the role of CSPs in vertebrate neuromuscular transmissi on, we introduced anti-CSP antibodies into the cell bodies of Xenopus spina l motor neurons that form synapses with embryonic muscle cells in culture. These antibodies produced a rapid (within 3-6 min), and in most cases compl ete, inhibition of stimulus-dependent neurotransmitter secretion. However, spontaneous neurotransmitter release was stable (both in frequency and ampl itude) throughout the period of antibody exposure. Several control experime nts validated the specificity of the anti-CSP antibody effects. First, the anti-CSP antibody actions were not mimicked either by antibodies against an other synaptic vesicle protein SV,, or by nonspecific immunoglobins. Second , heat treatment of the anti-CSP antibodies eliminated their effect on evok ed secretion. Third, immunoblot experiments showed that the anti-CSP and an ti-SV, antibodies were highly selective for their respective antigens in th ese Xenopus cultures. We conclude from these results that CSPs are vital co nstituents of the pathway for regulated neurotransmitter release in vertebr ates. Moreover, the selective inhibition of evoked, but nor spontaneous tra nsmitter release by anti-CSP antibodies indicates that there is a fundament al difference in the machinery that mediates these secretory processes.