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. 2009 Feb 12;61(3):397-411.
doi: 10.1016/j.neuron.2008.12.024.

Two pathways of synaptic vesicle retrieval revealed by single-vesicle imaging

Yongling Zhu  1 , Jian Xu, Stephen F Heinemann
Affiliations

Two pathways of synaptic vesicle retrieval revealed by single-vesicle imaging

Yongling Zhu et al. Neuron. .

Abstract

Synaptic vesicle recycling is essential for maintaining efficient synaptic transmission. Detailed dissection of single-vesicle recycling still remains a major challenge. We have developed a fluorescent pH reporter that permits us to follow the fate of individual vesicles at hippocampal synapses after exocytosis. Here we show that, during low-frequency stimulation, single-vesicle fusion leads to two distinct vesicle internalizations, instead of one, as in general perception: one by a fast endocytosis pathway ( approximately 3 s), the other by a slow endocytosis pathway (after 10 s). The exocytosed vesicular proteins are preferentially recaptured in both pathways. RNAi knockdown of clathrin inhibits both pathways. As stimulation frequency increases, the number of endocytosed vesicles begins to match antecedent exocytosis. Meanwhile, the slow endocytosis is accelerated and becomes the predominant pathway. These results reveal that two pathways of endocytosis are orchestrated during neuronal activity, establishing a highly efficient endocytosis at central synapses.

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Figures

Figure 1
Figure 1. Detection of Single Vesicle Release by SypHluorins
(A) Images of neurons expressing SypHluorin 4x (sypH 4x) or SynaptopHluorin (spH). (B) Schematic representations of SynaptopHluorin and SypHluorins with 1,2, and 4 phluorin reporters. (C) Fluorescence response as a function of time for the constructs during stimulation of 40 action potentials at 20Hz. The change in fluorescence intensity (ΔF) was normalized to the fluorescence intensity before stimulation (F0). (D) Loss of FM4-64 from preloaded control synapses and synapses expressing SypHluorin 4x as a function of time during 10 Hz stimulation. (E) Bouton fluorescence (arbitrary units) as a function of time during 0.2 Hz stimulation. (F) Histogram of bouton fluorescence (arbitrary units) for 505 single stimulus presentations. Smooth curve is one expected for quantal releases (see text, P = 0.99, χ2 test, d.f. = 15). Error bars represent SEM
Figure 2
Figure 2. Time Course of Single Vesicle Internalization
(A) Example traces of fluorescence intensity (arbitrary units) as a function of times (s) for single vesicles. Action Potential was delivered at 0s, as indicated by the arrow. (B) Average of 86 single vesicle responses. (C) An example fit (black line) of a trace with two distinct phases of exponential decay. (D) Histogram of dwell time (s) of the initial state (see text) for 86 vesicles. Smooth curve is the fit with Gaussian distribution, yielding a mean of 2.6s with a standard deviation of 1.2s (E) Dwell time histogram from fusion pore opening to the end of the plateau state. Gaussian fit (shown as smooth curve) yield a mean of 18.2s with a standard deviation of 5.2s. (F) Distribution of times from fusion pore opening to the end of both states by combining D and E. Smooth curve is the fit with two-component Gaussian distribution, yielding a mean of 2.9s with a standard deviation of 1.4s for the first component, and a mean of 19.3s with a standard deviation of 6.8s for the second component. (G) Histogram of time constant of the first phase of decay, the mean value is 2.6 ± 0.2s. (H) Histogram of time constant of the second phase of decay, the mean value is 3.2 ± 0.3s. Data in (G) and (H) are presented as mean ± SEM
Figure 3
Figure 3. Both Phases of Fluorescence Decay Reflect Internalization and Subsequent Acidification of Reporters from Surface Membrane
(A) Time course of normalized fluorescence intensity change in small region (red, 1.6μm diameter, n = 41 traces) and larger region (green, 3.5μm diameter, n = 41 traces) after single vesicle fusion. (B) Averaged single vesicle fluorescence intensity as a function of time in the presence of bafilomycin (filled black circle, n = 92 traces), as well as in the absence of bafilomycin (open gray circle, n= 68 traces). (C) Fluorescence intensity of synaptic boutons before (pH=7.3) and after (pH=5.6) extracellular acidification (right panel, n = 155 traces), with synapses expressing a new pH sensor with phluorin fused to the cytoplasmic C-terminal of Synaptophysin (left panel). (D) Fluorescence intensity as a function of time with extracellular pH alternating between 7.3 and 5.6 before and after exocytosis of a population of vesicles (right panel, n = 45 traces), when SypHluorin 4x (left panel) are expressed in synapses. Error bars represent SEM
Figure 4
Figure 4. Two Distinct Vesicles Internalized After One Vesicle Fusion
(A) Estimation of fluorescence intensity of a single vesicle labeled with FM 1-43. The solid and dashed lines are the overall and individual fits to multiple Gaussians (P = 0.99, χ2 test, d.f. = 40). The distribution of fluorescence has peaks at integral multiples of quantized fluorescence intensity. The average distance between the peaks was 451 units and is taken to correspond to the fluorescence of a single vesicle. (B) Protocol used to measure the FM 1-43 uptake at individual synapse during a designated time of dye exposure (t) after single action potential. The fluorescence loss between images 1 and 2 provided the measurement of FM uptake by single action potential at individual synapses. The subtraction image of images 3 and 4 provided the identification of functional synapses. The positions of functional synapses were retrospectively projected onto the subtraction image of images 1 and 2, and the total fluorescence intensity of each functional synapse was measured. The stimulation frequency was 10Hz for 1200 action potentials, and 20Hz for 100 action potentials. (C) Distribution of fluorescence intensity at individual synapses after labeling the internalized vesicles by the end of the ‘initial state’. Insert presents the time of FM 1-43 exposure corresponding to SypHluorin response. The solid and dashed lines are the overall and individual fits to multiple Gaussians, showing peaks at 11 and 435 (P = 0.99, χ2 test, d.f. = 30). (D) Distribution of fluorescence intensity at individual synapses after labeling the internalized vesicles by the end the ‘plateau state’. Insert presents the time of FM 1-43 exposure corresponding to SypHluorin response. The solid and dashed lines are the overall and individual fits to multiple Gaussians, showing peaks at 0 and 850 (P = 0.96, χ2 test, d.f. = 39). (E) A plot of endocytosed vesicle number (estimated by FM 1-43 uptake during 35s) as a function of exocytosed vesicle number (estimated by SypHluorin response) under the stimulation of 2, 5, 10, 20, and 30 action potentials at 20Hz. The vesicle number was calculated by: Ftotal/Fsingle vesicle, where Ftotal is the total fluorescence intensity, and Fsingle vesicle is the fluorescence intensity of single vesicle. Error bars represent SEM
Figure 5
Figure 5. Newly Exocytosed SypHluorins were Preferentially Retrieved in Both Endocytosis
(A) Estimation of the amount of SypHluorins on the bouton surface at rest. A pulse of an acidic solution (pH 5.6) was applied before the single vesicle fusion being evoked at 0s. Averaging over 47 responses shows that the amount of SypHluorins on surface is about 93% of that inside a single vesicle. (B) Example traces of single vesicle response before (left panel) and after photobleaching (right panel). (C) Comparison of averaged fluorescence recovery after single vesicle fusion before (filled circle, n = 92 traces) and after photobleaching (open circle, n = 96 traces). (D) Individual traces in C were realigned with the initial time of the fast endocytosis (left panel) or slow endocytosis (right panel), then averaged. (E) Photobleaching surface SypHluorins reduces fluorescence intensity change in both the fast and slow endocytosis (See text). ***p < 0.001, paired t test. Error bars represent SEM
Figure 6
Figure 6. Clathrin Knockdown by RNAi Inhibits Both Endocytosis
(A) Example traces of single vesicle response with clathrin RNAi knockdown (upper 3 panels), and control (the bottom panel). The responses with clathrin RNAi knockdown can be classified into three groups. (i). Both the fast and slow endocytosis were detectable (accounting for ~20% of the total recordings), as shown in the top panel; (ii). Only the fast endocytosis was detected (~38% of the total recordings), as shown in the second panel; (iii). Neither fast nor slow endocytosis was detected (~42% of the total recordings), as shown in the third panel. Bottom panel presents an example trace of single vesicle response from a neuron transfected with scrambled RNAi (control). (B and C) Clathrin RNAi knockdown inhibits both the fast (B) and slow (C) endocytosis. Frequency distribution of fractional fluorescence decrease during each endocytosis in control neurons (n=120 traces) is shown in the left panels, and in RNAi knockdown neurons (n =192 traces) is shown in the middle panels. The traces without detectable endocytosis were grouped as a bar at 0. The detectable endocytosis events were further fitted with Gaussian distribution (smooth curves). Right panels are cumulative histograms for events in control neurons and detectable events in RNAi knockdown neurons. Gaussian fits of the fast endocytosis in (B) yielded an estimation of 0.50 ± 0.11 (mean ± SD, n = 192 traces) for control and 0.40 ± 0.12 (mean ± SD, n = 104 traces) for the detectable events in CHC-RNAi knockdown. Two populations are significantly different, ***p < 0.001, t test. Gaussian fits of the slow endocytosis in (C) yielded estimations of 0.47 ± 0.14 (mean ± SD, n = 192 traces) for control and 0.47 ± 0.11 (mean ± SD, n = 43 traces) for the detectable events in RNAi knockdown. No significant difference was observed, p > 0.05, t test. (D and E) Clathrin knockdown delays both endocytosis. Left panels in (D) and (E) present frequency dwell time distributions of the fast (D) and slow (E) endocytosis detected in RNAi knockdown neurons (filled gray bar, n = 104 traces for the fast endocytosis, n = 43 traces for the slow endocytosis) and control neurons (striped bar, n = 120 for both endocytosis). Right panels are the cumulative dwell time distributions of the fast (D) and slow (E) endocytosis. For both endocytosis, clathrin knockdown significantly prolonged the dwell time (see text), ***p < 0.001, t test compared to control.
Figure 7
Figure 7. Spontaneous Endocytosis Detected by SypHluorins and FM 1-43
(A) Example traces of spontaneous vesicle retrieval observed from single bouton expressing SypHluorins in the absence of stimulation (upper three panels). The bottom panel shows the average trace over 146 boutons, including 53 boutons with retrieval events and 93 boutons without retrieval events. (B) Distribution of the initial time for 53 retrieval events observed in A. The recording started from −5 sec and lasted for 30 sec. (C) Example traces of vesicle retrieval observed from single bouton expressing SypHluorins with single stimulus, but without evoked exocytosis events (upper three panels). The bottom panel shows the average trace over 104 boutons, including 46 boutons with retrieval events and 58 boutons without retrieval events. (D) Distribution of the initial time for 46 retrieval events observed in C. The recording started from −5 sec, the stimulus was delivered at 0 sec. (E) Distribution of fluorescence intensity of FM 1-43 at individual synapses after exposing the neurons in 10 μM FM 1-43 for 6 sec without stimulation. (F) As the same as E, but prolonging the dye exposure time to 25 sec. The solid and dashed lines in E and F are the overall and individual fits to single (E) or multiple Gaussians (F), showing single peak at 3 in E and multiple peaks at 2 and 475 in F.
Figure 8
Figure 8. Increased Activity Selectively Accelerates the Slow Endocytosis, but Not the Fast Endocytosis
(A) Example traces of fluorescence intensity as a function of time following 1, 2, 3, 4, and 5 vesicle fusion events. Action Potentials (1Hz, 5Hz, 10Hz, 20Hz and 30Hz with duration of 0.5s) were delivered at 0s, as indicated by the bars. (B) Comparisons of the dwell time distributions of endocytosis after different number of exocytosis events. Each histogram shows the dwell time distribution of the fast (filled bar) and slow (open bar) endocytosis after the release of one vesicle (n=86), two vesicles (n=64), three vesicles (n=64), four vesicles (n=51) and five vesicles (n=51). Further kinetic analyses of these traces are presented in C–F. (C) Fractional retrieval of SypHluorins by the fast (filled circle) and slow (open circle) endocytosis as a function of the number of exocytosed vesicles. With the increased number of exocytosed vesicles, a larger fraction of SypHluorins was retrieved by the slow endocytosis. (D) Averaged dwell time of the fast (filled circle) and slow (open circle) endocytosis as a function of the number of exocytosed vesicles. Increasing vesicle number shortened the dwell time of the slow endocytosis, but had no significant effect on the fast endocytosis. (E) Averaged decay time constant of the fast (filled circle) and slow (open circle) endocytosis as a function of the number of exocytosed vesicles. Increasing vesicle number prolonged the decay time constant of the slow endocytosis, but not the fast endocytosis. (F) Normalized average traces of fluorescence responses after multiple vesicle fusions. Single exponential fit provided a relatively good account of the fluorescence decay except for one vesicle, yielding time constant (τ) of 19.6 ± 0.7 s, 17.7 ± 1.4 s, 16.3 ± 0.9 s, 15.6 ± 1.0s and 14.4 ± 0.7 s for 1, 2, 3, 4, and 5 vesicles, respectively. No significant differences were observed between 2–5 vesicles (Turkey’s multiple comparison test, p>0.05) (G) Example traces of fluorescence intensity as a function of time for single vesicle fusion in 1mM and 3.5mM external calcium. Action Potential was delivered at 0s, as indicated by the arrow. (H) Fractional retrieval of SypHluorins by the fast (filled circle) and slow (open circle) endocytosis as a function of [Ca2+] ex, after single vesicle fusion. The number of single vesicle fusion events for different [Ca2+] ex are: n = 49 for 1mM, n = 44 for 2mM, n = 51 for 3.5mM, and n = 43 for 5mM. No significant differences were observed for both the fast and slow endocytosis, with [Ca2+] ex increased from 1mM to 5mM (Turkey’s multiple comparison test, p>0.05). Further kinetic analyses of these traces are presented in I and J. (I) Averaged dwell time of the fast (filled circle) and slow (open circle) endocytosis as a function of [Ca2+] ex, after single vesicle fusion. Increasing [Ca2+] ex shortened the dwell time of the slow endocytosis, but had no effect on the fast endocytosis. (Turkey’s multiple comparison test, at the level of 0.05). (J) Averaged decay time constant of the fast (filled circle) and slow (open circle) endocytosis as a function of [Ca2+] ex, after single vesicle fusion. No significant differences were observed for 1mM to 5mM for both endocytosis (Turkey’s multiple comparison test, p>0.05). Error bars represent SEM.
Figure 9
Figure 9. Two-phase of Endocytosis after Single Vesicle Fusion Detected by vGlut1-pHluorin
(A) Histogram of bouton fluorescence for 533 single stimulus presentations from 65 boutons. (P = 0.99, χ2 test, d.f. = 21). Smooth solid curve is the sum of three Gaussian distributions expected for quantal releases, yielding a baseline noise standard deviation of 37.9 fluorescence units, and mean single vesicle fluorescence of 181.4 units with a standard deviation of 51.4 units. Thus, the signal to noise ratio for single vesicle detection is about 4.8 and the variability in expression of the reporter among vesicles is at most 28%. (B) Example traces of fluorescence intensity as a function of time for single vesicles. Single action potential was delivered at 0s, as indicated by the arrow. (C) An example fit (black line) of a trace with two distinct phases of exponential decay. (D) Histogram of dwell time distribution of the fast endocytosis for 95 vesicles. Smooth curve is the fit with Gaussian distribution, yielding a mean of 2.5s with a standard deviation of 1.8s. (E) Dwell time histogram of the slow endocytosis. Gaussian fit yielded a mean of 14.4s with a standard deviation of 3.8s. (F) Distribution of combined dwell times of the fast and slow endocytosis. Smooth curve is the fit with two-component Gaussian distribution, yielding a mean of 2.5s with a standard deviation of 1.8s for the first component, and a mean of 14.7s with a standard deviation of 4.5s for the second component. (G) Histogram of the reacidification time constant for the fast endocytosis, with a mean of 1.5 ± 0.2s. (H) Histogram of the reacidification time constant for the slow endocytosis, with a mean of 2.2 ± 0.2s. Data in (G) and (H) are presented as mean ± SEM
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