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Abstract
Synaptic plasticity is a fundamental feature of the nervous system that allows adaptation to changing behavioral environments. Most studies of synaptic plasticity have examined the regulated trafficking of postsynaptic glutamate receptors that generates alterations in synaptic transmission. Whether and how changes in the presynaptic release machinery contribute to neuronal plasticity is less clear. The SNARE complex mediates neurotransmitter release in response to presynaptic Ca(2+) entry. Here we show that the SNARE fusion clamp Complexin undergoes activity-dependent phosphorylation that alters the basic properties of neurotransmission in Drosophila. Retrograde signaling following stimulation activates PKA-dependent phosphorylation of the Complexin C terminus that selectively and transiently enhances spontaneous release. Enhanced spontaneous release is required for activity-dependent synaptic growth. These data indicate that SNARE-dependent fusion mechanisms can be regulated in an activity-dependent manner and highlight the key role of spontaneous neurotransmitter release as a mediator of functional and structural plasticity.
Figure 1 Cpx knockdown larvae exhibit increased spontaneous release and enhanced synaptic growth. (A) Expression of Cpx protein was reduced in Cpx RNAi knockdown lines (w1118; c164-GAL4+; CpxRBAi+) compared to control (w1118; c164-GAL4+) using a motor neuron driver (c164-GAL4) to express Cpx-specific RNAi. Larvae were co-stained with anti-HRP and anti-Cpx specific antisera. Images were obtained from muscle 6/7 NMJs of 3rd instar larvae (B) Sample traces of spontaneous release from control and Cpx RNAi knockdown larvae. (C) Mean mini frequency (Hz ± SEM) recorded for each line. (D) Representative images of NMJs from control and Cpx RNAi knockdown larvae. Larvae were stained with anti-HRP (magenta) and anti-GluRIII (green) specific antisera to identify synaptic boutons. (E) Mean synaptic bouton number for control and Cpx RNAi knockdowns. Bouton number was normalized using muscle 6/7 area. Indicated comparisons were made using Student’s T-tests. Mean ± SEM is indicated. Scale bar = 20 μm.
Figure 2
Synaptic growth associated with enhanced…
Figure 2
Synaptic growth associated with enhanced minis is suppressed by synaptotagmin 4 ( syt…
Figure 2 Synaptic growth associated with enhanced minis is suppressed by synaptotagmin 4 (syt 4BA1) and BMP receptor (wit) mutants. (A) Representative traces of spontaneous release recorded from syntaxin 1A mutant (w1118;; syx3-69) and control (w1118) larval muscle 6 NMJs. (B) Representative images of muscle 6/7 NMJs of control (w1118), syx3-69 mutant (w1118;; syx3-69), syt 4BA1 null (w1118;; syt 4BA1), and syx3-69, syt 4BA1 double mutant (w1118;; syx3-69, syt 4BA1). (C) Mean synaptic bouton number measured for indicated genotypes. (D) Representative images of muscle 6/7 NMJs of the indicated genotypes. (E) Mean synaptic bouton number measured for control (w1118;; CpxPE), cpxSH1 null (w1118;; cpxSH1), control Syt 4PE (w1118;; Syt 4PE), syt 4BA1 null (w1118;; syt 4BA1), and syx3-69, syt 4BA1 double mutant (w1118;; syt 4BA1, cpxSH1) (F) Representative images of muscle 6/7 NMJs from control (w1118;; CpxPE), cpxSH1 null, wit null (w1118;; witA12witB11), wit, cpxSH1 double mutant (w1118;; cpxSH1,witA12cpxSH1,witB11), and wit heterozygotes in cpx null background (w1118;; cpxSH1,witA12cpxSH1). (G) Mean synaptic bouton number measured for the indicated genotypes. Indicated comparisons were made using ANOVA with post hoc Tukey analysis. Mean ± SEM is indicated. Scale bars = 10 μm.
Figure 3
Activity-dependent enhancement of minis requires…
Figure 3
Activity-dependent enhancement of minis requires Syt 4 and PKA activity. (A) Representative traces…
Figure 3 Activity-dependent enhancement of minis requires Syt 4 and PKA activity. (A) Representative traces of minis before (top trace) and after (bottom trace) tetanic stimulation recorded from control (w1118;; CpxPE and w1118;; Syt 4PE) and syt 4BA1 nulls (w1118;; syt 4BA1). Representative traces of minis are also shown for control animals pre-treated with the PKA-specific inhibitor H89. (B) Mean mini frequency plotted as a function of time. Tetanic stimulation occurs at the 0 sec time point (Red Arrow). Mini frequency is normalized to average baseline mini frequency 60 seconds prior to stimulation for each preparation. (C) Average fold enhancement of mini frequency compared to baseline mini frequency 100 seconds after tetanic stimulation. Statistically significant differences in fold enhancement of minis were observed in control animals compared to syt 4BA1 nulls and H89 treated controls at each indicated time point starting at 100 seconds following HFMR stimulation up to 300 seconds, when minis started to return to pre-stimulus baseline mini frequency. (D) Pseudo-color activity heatmaps showing the distribution of spontaneous fusion rates for individual release sites at the same NMJ of muscle 4 prior (left panel) and after (right panel) tetanus stimulation. Each circle is a single region of interest (ROI) that represents the detection of a release event indicated by postsynaptic myrGCaMP6s fluorescence. Color-coding in the heatmaps indicate the number of mini events detected over a period of 2 minutes before and after HFMR stimulation. (E) Number of ROIs exhibiting indicated event frequency before (black) and after (magenta) HFMR stimulation (F) Representative images of Ca++ dynamics detected by presynaptic myrGCAMP6s expression before, during, and after stimulation. (G) Mean fluorescence change as a function of time during tetanus is shown for one representative NMJ stimulation trial. Red arrows indicate each 100 Hz stimulation event. Mean ± SEM is indicated. Indicated comparisons were made using ANOVA with post hoc Tukey analysis.
Figure 4
Cpx is a substrate for…
Figure 4
Cpx is a substrate for PKA phosphorylation. (A) Proposed retrograde Syt 4-dependent signaling…
Figure 4 Cpx is a substrate for PKA phosphorylation. (A) Proposed retrograde Syt 4-dependent signaling pathway that is activated by strong stimulation. Postsynaptic Ca++ elevation following stimulation drives release of a retrograde signal that activates the presynaptic PKA pathway. PKA is hypothesized to phosphorylate Cpx, altering its clamping properties. In addition, PKA may phosphorylate additional targets to facilitate structural growth. (B) Structure of Drosophila (Dm) Cpx and the location of the predicted PKA phosphorylation site (S126) in the C-terminus. The N-terminus and C-terminus flank the indicated accessory and central helix domains. The alignment of C-terminal regions of Dm Cpx, mouse (Mm) Cpx I, and C. elegans (Ce) Cpx is shown below. Predicted amphipathic helix domains are highlighted in yellow, and predicted phosphorylation sites indicated in bold. S115 of Mm Cpx I is phosphorylated by protein kinase CK2 (Shata et al., 2007). T119 of Mm Cpx I is a predicted substrate for PKC, and T129 of Ce Cpx is a predicted substrate for AKT (Wong et al., 2007). (C) Helical wheel diagrams of the amphipathic region for Dm Cpx, Mm Cpx I, and Ce Cpx. Hydrophilic resides are indicated in magenta, hydrophobic residues in green, and nonpolar and uncharged residues in grey. Predicted phosphorylation sites are indicated by *. (D) CpxS126A exhibited greatly decreased [32P] incorporation compared to WT Cpx (CpxS126A 32P incorporation was reduced to 35.2 ± 10.6 % relative band intensity of WT Cpx 32P incorporation (n=4)). Recombinant control Cpx (WT) and phosphoincompetent CpxS126A were incubated with [32P]ATP and recombinant active PKA. Phosphorylation was visualized by autoradiography after proteins were separated by SDS-PAGE (bottom panel). Gels were also stained with Coomassie blue to assay protein loading (top panel).
Figure 5
Mimicking phosphorylation of Cpx at…
Figure 5
Mimicking phosphorylation of Cpx at S126 selectively modulates Cpx function as a fusion…
Figure 5 Mimicking phosphorylation of Cpx at S126 selectively modulates Cpx function as a fusion clamp and regulates synaptic growth. (A) Averaged traces of EJPs from control (w1118;; CpxPE), cpx null (w1118;; cpxSH1), WT Cpx rescues (C155;; WT Cpx, cpxSH1), and CpxS126A and CpxS126D mutant rescues (C155;; CpxS126A, cpxSH1 and C155;; CpxS126D, cpxSH1, respectively) recorded over a range of external Ca++ concentrations (0.1, 0.15, 0.2, 0.3, and 0.4 mM). Numbers in parentheses (n) represent individual muscle recordings at the indicated [Ca++]. (B) Summary of mean EJP amplitude (mV ± SEM) plotted at the indicated [Ca++]. (C) Representative traces of spontaneous release events from muscle 6 of control, cpxSH1 null, and WT Cpx, CpxS126A and CpxS126D rescue lines. (D) Cumulative probability plot of inter-event intervals (seconds) between mini events recorded indicated genotypes. (E) Summary of mean mini frequency (Hz ± SEM) for indicated genotypes. (F) Representative images of muscle 6/7 NMJs of control, cpxSH1 null, and WT Cpx, CpxS126A, CpxS126D rescue lines. Larvae were stained with anti-HRP (magenta) and anti GluRIII (green) specific antisera to identify synaptic boutons. Scale bar = 20 μm (G) Summary of mean synaptic bouton number normalized to muscle area for the indicated genotypes. Indicated comparisons were made using ANOVA with post hoc Tukey analysis. Mean ± SEM is indicated.
Figure 6
Cpx PKA phosphorylation site S126…
Figure 6
Cpx PKA phosphorylation site S126 is required for HFMR expression. (A) Representative traces…
Figure 6 Cpx PKA phosphorylation site S126 is required for HFMR expression. (A) Representative traces of minis before (top) and after (bottom) tetanic stimulation recorded from cpxSH1 rescue lines neuronally expressing WT Cpx rescues (C155;; WT Cpx, cpxSH1) with or without PKA inhibitor H89, and CpxS126A and CpxS126D mutant rescues (C155;; CpxS126A, cpxSH1 and C155;; CpxS126D, cpxSH1, respectively). (B) Mean mini frequency plotted as a function of time. Tetanic stimulation occurs at the 0 sec time point (Red Arrow). Mini frequency is normalized to average baseline mini frequency prior to stimulation for each preparation. (C) Average fold enhancement of mini frequency compared to baseline 100 seconds after tetanic stimulation. Indicated comparisons were made using ANOVA with post hoc Tukey analysis. Mean ± SEM is indicated.
Figure 7
Cpx PKA phosphorylation site S126…
Figure 7
Cpx PKA phosphorylation site S126 is required for PKA and activity-dependent synaptic growth.…
Figure 7 Cpx PKA phosphorylation site S126 is required for PKA and activity-dependent synaptic growth. (A) Representative images of muscle 6/7 NMJs from control (C155 elav-GAL4), and lines neuronally overexpressing constitutively active PKA alone (C155; CA-PKA+ ) or in the cpxSH1 null (C155; CA-PKA+; cpxSH1), WT Cpx rescue (C155; CA-PKA+; WTCpx,cpxSH1cpxSH1), CpxS126A rescue (C155; CA-PKA+; CpxS126A,cpxSH1cpxSH1), and CpxS126D rescue (C155; CA-PKA+; CpxS126D,cpxSH1cpxSH1) backgrounds. (B) Summary of mean synaptic bouton number normalized to muscle area for the indicated genetic backgrounds. Indicated comparisons were made using ANOVA with post hoc Tukey analysis. (C) Representative images of control (w1118;; CpxPE) and WT Cpx rescues (C155;; WT Cpx, cpxSH1), CpxS126A rescues (C155;; CpxS126A, cpxSH1), and CpxS126D rescues (C155;; CpxS126D, cpxSH1) reared at 25° or 29°. (D) Summary of % change in synaptic bouton number normalized to muscle area for the indicated genetic backgrounds and rearing temperatures. Synaptic bouton number was normalized to bouton number of larvae reared at 25° for each respective genotype to reflect % change. Indicated comparisons were made using Student’s T-test. Scale bars = 20 μm. Mean ± SEM is indicated in all figures. All figures (7) See this image and copyright information in PMC
References
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