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Differential roles for snapin and synaptotagmin in the synaptic vesicle cycle.

Yu SC, Klosterman SM, Martin AA, Gracheva EO, Richmond JE - PLoS ONE (2013)

Bottom Line: The kinetics of synaptic transmission were unaffected at snpn-1 mutant neuromuscular junctions (NMJs), but the number of docked, fusion competent vesicles was significantly reduced.However, analyses of snt-1 and snt-1;snpn-1 double mutants suggest that the docking role of SNPN-1 is independent of Synaptotagmin.Based on these results we propose that the primary role of Snapin in C. elegans is to promote vesicle priming, consistent with the stabilization of SNARE complex formation through established interactions with SNAP-25 upstream of the actions of Synaptotagmin in calcium-sensing and endocytosis.

View Article: PubMed Central - PubMed

Affiliation: Biological Sciences, University of Illinois at Chicago, Chicago, Illinois, United States of America.

ABSTRACT
Evoked synaptic transmission is dependent on interactions between the calcium sensor Synaptotagmin I and the SNARE complex, comprised of Syntaxin, SNAP-25, and Synaptobrevin. Recent evidence suggests that Snapin may be an important intermediate in this process, through simultaneous interactions of Snapin dimers with SNAP-25 and Synaptotagmin. In support of this model, cultured neurons derived from embryonically lethal Snapin mutant mice exhibit desynchronized release and a reduced readily releasable vesicle pool. Based on evidence that a dimerization-defective Snapin mutation specifically disrupts priming, Snapin is hypothesized to stabilize primed vesicles by structurally coupling Synaptotagmin and SNAP-25. To explore this model in vivo we examined synaptic transmission in viable, adult C. elegans Snapin (snpn-1) mutants. The kinetics of synaptic transmission were unaffected at snpn-1 mutant neuromuscular junctions (NMJs), but the number of docked, fusion competent vesicles was significantly reduced. However, analyses of snt-1 and snt-1;snpn-1 double mutants suggest that the docking role of SNPN-1 is independent of Synaptotagmin. Based on these results we propose that the primary role of Snapin in C. elegans is to promote vesicle priming, consistent with the stabilization of SNARE complex formation through established interactions with SNAP-25 upstream of the actions of Synaptotagmin in calcium-sensing and endocytosis.

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PD proximal vesicle docking deficits in snt-1 and snt-1;snpn-1 mutants correlate with their release defects.(A) The distance from docked synaptic vesicle (SV) membrane to the closest PD referred to as dPD, is used to plot the distribution of docked vesicles relative to the PD, in graphs (B-D). The extent of the vesicle docking defect (highlighted by the horizontal dashed line) within 90 nm of the PD (depicted as a vertical rectangular box on graphs) is similar in snt-1 (C) and snt-1;snpn-1 double mutants (D). All statistically significant values for dPD, plotted as mean and SEM for mutants when compared to the wild type are shown.
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pone-0057842-g005: PD proximal vesicle docking deficits in snt-1 and snt-1;snpn-1 mutants correlate with their release defects.(A) The distance from docked synaptic vesicle (SV) membrane to the closest PD referred to as dPD, is used to plot the distribution of docked vesicles relative to the PD, in graphs (B-D). The extent of the vesicle docking defect (highlighted by the horizontal dashed line) within 90 nm of the PD (depicted as a vertical rectangular box on graphs) is similar in snt-1 (C) and snt-1;snpn-1 double mutants (D). All statistically significant values for dPD, plotted as mean and SEM for mutants when compared to the wild type are shown.

Mentions: There is an apparent disparity between the additivity of the docking defect in the snt-1;snpn-1 double mutant relative to the single snt-1 mutant (p = 0.0001) (Fig. 4D), and the lack of additivity of the EJC deficit in the double mutant when compared to the snt-1 mutant alone (EJC amplitude p = 1.0, charge integral p = 0.55) (Fig. 2G,H). To address this issue, the distribution of docked vesicles relative to the presynaptic density (Fig. 5A), the presumptive Ca2+ entry and release site of the NMJ, was examined in the snpn-1 and snt-1 single and double mutants. This analysis demonstrated that the number of vesicles docked near the presynaptic density was reduced to similar levels in snt-1 and snt-1;snpn-1 double mutants (Fig. 5C,D), both of which were more severe than snpn-1 alone (Fig. 5B). Thus, the similar extent of the electrophysiological deficits observed in snt-1 single and snt-1;snpn-1 double mutants may be a reflection of the similar degree to which releasable docked vesicles adjacent to the presynaptic density are reduced.


Differential roles for snapin and synaptotagmin in the synaptic vesicle cycle.

Yu SC, Klosterman SM, Martin AA, Gracheva EO, Richmond JE - PLoS ONE (2013)

PD proximal vesicle docking deficits in snt-1 and snt-1;snpn-1 mutants correlate with their release defects.(A) The distance from docked synaptic vesicle (SV) membrane to the closest PD referred to as dPD, is used to plot the distribution of docked vesicles relative to the PD, in graphs (B-D). The extent of the vesicle docking defect (highlighted by the horizontal dashed line) within 90 nm of the PD (depicted as a vertical rectangular box on graphs) is similar in snt-1 (C) and snt-1;snpn-1 double mutants (D). All statistically significant values for dPD, plotted as mean and SEM for mutants when compared to the wild type are shown.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3585204&req=5

pone-0057842-g005: PD proximal vesicle docking deficits in snt-1 and snt-1;snpn-1 mutants correlate with their release defects.(A) The distance from docked synaptic vesicle (SV) membrane to the closest PD referred to as dPD, is used to plot the distribution of docked vesicles relative to the PD, in graphs (B-D). The extent of the vesicle docking defect (highlighted by the horizontal dashed line) within 90 nm of the PD (depicted as a vertical rectangular box on graphs) is similar in snt-1 (C) and snt-1;snpn-1 double mutants (D). All statistically significant values for dPD, plotted as mean and SEM for mutants when compared to the wild type are shown.
Mentions: There is an apparent disparity between the additivity of the docking defect in the snt-1;snpn-1 double mutant relative to the single snt-1 mutant (p = 0.0001) (Fig. 4D), and the lack of additivity of the EJC deficit in the double mutant when compared to the snt-1 mutant alone (EJC amplitude p = 1.0, charge integral p = 0.55) (Fig. 2G,H). To address this issue, the distribution of docked vesicles relative to the presynaptic density (Fig. 5A), the presumptive Ca2+ entry and release site of the NMJ, was examined in the snpn-1 and snt-1 single and double mutants. This analysis demonstrated that the number of vesicles docked near the presynaptic density was reduced to similar levels in snt-1 and snt-1;snpn-1 double mutants (Fig. 5C,D), both of which were more severe than snpn-1 alone (Fig. 5B). Thus, the similar extent of the electrophysiological deficits observed in snt-1 single and snt-1;snpn-1 double mutants may be a reflection of the similar degree to which releasable docked vesicles adjacent to the presynaptic density are reduced.

Bottom Line: The kinetics of synaptic transmission were unaffected at snpn-1 mutant neuromuscular junctions (NMJs), but the number of docked, fusion competent vesicles was significantly reduced.However, analyses of snt-1 and snt-1;snpn-1 double mutants suggest that the docking role of SNPN-1 is independent of Synaptotagmin.Based on these results we propose that the primary role of Snapin in C. elegans is to promote vesicle priming, consistent with the stabilization of SNARE complex formation through established interactions with SNAP-25 upstream of the actions of Synaptotagmin in calcium-sensing and endocytosis.

View Article: PubMed Central - PubMed

Affiliation: Biological Sciences, University of Illinois at Chicago, Chicago, Illinois, United States of America.

ABSTRACT
Evoked synaptic transmission is dependent on interactions between the calcium sensor Synaptotagmin I and the SNARE complex, comprised of Syntaxin, SNAP-25, and Synaptobrevin. Recent evidence suggests that Snapin may be an important intermediate in this process, through simultaneous interactions of Snapin dimers with SNAP-25 and Synaptotagmin. In support of this model, cultured neurons derived from embryonically lethal Snapin mutant mice exhibit desynchronized release and a reduced readily releasable vesicle pool. Based on evidence that a dimerization-defective Snapin mutation specifically disrupts priming, Snapin is hypothesized to stabilize primed vesicles by structurally coupling Synaptotagmin and SNAP-25. To explore this model in vivo we examined synaptic transmission in viable, adult C. elegans Snapin (snpn-1) mutants. The kinetics of synaptic transmission were unaffected at snpn-1 mutant neuromuscular junctions (NMJs), but the number of docked, fusion competent vesicles was significantly reduced. However, analyses of snt-1 and snt-1;snpn-1 double mutants suggest that the docking role of SNPN-1 is independent of Synaptotagmin. Based on these results we propose that the primary role of Snapin in C. elegans is to promote vesicle priming, consistent with the stabilization of SNARE complex formation through established interactions with SNAP-25 upstream of the actions of Synaptotagmin in calcium-sensing and endocytosis.

Show MeSH
Related in: MedlinePlus