Cleavage of syntaxin prevents G-protein regulation of presynaptic calcium channels

Abstract

Neurotransmitter release into the synapse is stimulated by calcium influx through ion channels that are closely associated with the transmitter release sites. This link may involve the membrane protein syntaxin, which is known to be associated with the release sites and to bind to the calcium channels. There is evidence that presynaptic calcium channels are downregulated by second messenger pathways involving G proteins. Here we use the patch-clamp technique to test whether calcium current is regulated by G proteins in a vertebrate presynaptic nerve terminal, and whether this regulation is affected by the linkage to syntaxin. The calcium current in the nerve terminal showed typical G-protein-mediated changes in amplitude and activation kinetics which were reversed by a preceding depolarization. These effects of the G protein were virtually eliminated if syntaxin was first cleaved with botulinum toxin C1. Our findings indicate that this sensitivity of the current to modulation by G proteins requires the association of the presynaptic calcium channel with elements of the transmitter release site, which may ensure that channels tethered at release sites are preferentially regulated by the G-protein second messenger pathway.

Alteration of Ca2+Dependence of Neurotransmitter Release by Disruption of Ca2+Channel/Syntaxin Interaction

“…It has been reported that coexpression of syntaxin with L-, Q-and N-type calcium channels in Xenopus oocytes resulted in a decrease of calcium influx through these channels because of stabilization of the inactivated state (Bezprozvanny et al, 1995;Wiser et al, 1996), but this effect was not observed in isolated chick ciliary ganglion nerve terminals (Stanley and Mirotznik, 1997). If syntaxin inhibits Ca 2ϩ channel function in our preparation, disruption of the native syntaxin/channel-interaction by the synprint peptide would therefore lead to an increase in Ca 2ϩ influx and consequently to an underestimation of the physiological effect.…”

Synaptic transmission at retinal ribbon synapses

“…Syntaxin 1 also interacts at the molecular level with N-type calcium channels (Sheng et al, 1994). The latter is thought to assist in localizing these calcium channels to the sites of vesicle fusion (Sheng et al, 1998) and to modulate calcium channel gating (Stanley and Mirotznik, 1997;Stanley, 2003). Interestingly, syntaxin 1, the isoform present at conventional synapses, appears to be absent from rodent retinal ribbon synapses (Brandstatter et al, 1996b).…”

Old proteins, developing roles: The regulation of calcium channels by synaptic proteins

“…Unlike the effect of syntaxin 1A on channel availability, this modulation persists in the presence of SNAP-25. Evidence for the functional significance of this effect in neurons is still scant, although it is known that G protein regulation of N-type channels in chick calyces is altered upon treatment with botulinum toxin C1, 73 and Gβγ subunits have been shown to regulate neurotransmission with their interactions with SNARE complexes. 74 A third regulatory modality linked to the synprint motif is subcellular targeting of the channel.…”