Neuropeptide delivery to synapses by long-range vesicle circulation and sporadic capture

Abstract

Neurotransmission requires anterograde axonal transport of dense core vesicles (DCVs) containing neuropeptides and active zone components from the soma to nerve terminals. However, it is puzzling how one-way traffic could uniformly supply sequential release sites called en passant boutons. Here, Drosophila neuropeptide-containing DCVs are tracked in vivo for minutes with a new method called simultaneous photobleaching and imaging (SPAIM). Surprisingly, anterograde DCVs typically bypass proximal boutons to accumulate initially in the most distal bouton. Then, excess distal DCVs undergo dynactin-dependent retrograde transport back through proximal boutons into the axon. Just before re-entering the soma, DCVs again reverse for another round of anterograde axonal transport. While circulating over long distances, both anterograde and retrograde DCVs are captured sporadically in en passant boutons. Therefore, vesicle circulation, which includes long-range retrograde transport and inefficient bidirectional capture, overcomes the limitations of one-way anterograde transport to uniformly supply release sites with DCVs.

Figure 1. Neuropeptide accumulates initially in the most distal bouton

A. Nerve terminal Anf-GFP fluorescence images after RU486 induction for 5 and 100 hours. TRITC-conjugated HRP labeling localized the neuronal membrane. En passant boutons are numbered from the most distal (#1) to proximal (Scale bar, 5 μm). B. Quantification of fluorescence relative to the most distal bouton (%F) after RU486 (5 h, n=11; 20 h, n=10; 100 h, n=12). Note that data are displayed from the most distal bouton #1 to proximal bouton #4. C. Nerve terminal Dilp2-GFP images after heat shock induction for 5 or 24 hours. Scale bar, 5 μm. D. Quantification of Dilp2-GFP fluorescence after heat shock induction for 5 h (n=8) and 24 h, n=5). Note that data are displayed from the most distal bouton #1 to proximal bouton #4. E. Time-lapse images before and after photobleaching (Scale bar, 2.5 μm). Contrast was adjusted to visualize DCV puncta. F. FRAP quantified in the most distal bouton (#1) and a proximal bouton (#4) (n=12). In all figures, error bars are standard error of the mean. See also Movie S1.

Figure 2. Complex anterograde DCV traffic in the terminal

A. SPAIM experimental design. Blue boxes, photobleaching regions; Red arrow, possible trajectory of the newcomer DCV. At least 4 boutons were examined in each experiment. B. Anterograde DCV images (upper and middle) and trajectory (bottom). Red, anterograde; Blue, retrograde. Colored numbers indicate pause durations in seconds. Scale bar, 5 μm. C. Frequency distribution of visits >5 s (n= 138 from 62 tracked DCVs in 17 animals, 30 s binning) at en passant boutons, which comprise 55% of total events. Dashed line indicates events that were interrupted by the end of time-lapse data acquisition. D. Probability of DCVs to bypass (duration 0–5 s), transiently visit (duration 5–120 s) or be captured (duration >120 s) at boutons #1 and #4 (n=86 events from 14 DCVs in 14 animals). E. Anterograde DCV flux between boutons in 5 minutes after photobleaching (n=15). See also Movie S2.

Figure 3. Retrograde transport of excess DCVs out of the distal bouton

A. SPAIM strategies for detecting DCV efflux from distal (Top) and proximal (bottom) boutons in 5 minutes after photobleaching large blue rectangles. The green boutons were spared from photobleaching and then anterograde newcomers were photobleached by the beam positioned at the location of the small blue squares. Flux was measured at the position of the arrows (Red, anterograde; Blue, retrograde). B. Quantification of DCV flux from proximal (n=10) and distal (n=13) boutons. C. Pseudo-color images showing distal Anf-GFP accumulation after inhibiting dynactin with Gl^Δ96 or Dmn. Scale bar, 5 μm. Warmer colors represent higher fluorescence. D. Anf-GFP intensity relative to the wild type control (WT) distal bouton (%F) (WT, n=18; Gl^Δ96, n=20; Dmn, n=7). See also Figure S1 and Movies S3 and S4.

Figure 4. Retrograde DCV traffic and capture in the terminal

A. Top two panels, SPAIM strategy for detecting retrograde DCVs leaving the most distal bouton. Third panel, image of boutons prior to photobleaching. Bottom three panels, sample retrograde trajectories. The first shows transient visits and an anterograde reversal. The second shows capture. The third shows bypasses to exit region of interest. Blue, retrograde; Red, anterograde; Overlapping anterograde and retrograde, black; Scale bar, 5 μm. B. Frequency distribution of retrograde visits >5 s (n=106 events in 63 tracked DCVs in 20 animals), which comprise 57% of total events. Dashed line indicates events that were interrupted by the end of time-lapse data acquisition. C. Retrograde flux of DCVs from bouton #1 in 5 minutes after photobleaching (n=7).

Figure 5. Uncaptured retrograde DCVs leave the terminal to enter the axon

A. Projection stack showing Anf-GFP in a muscle 4 terminal with photobleached branch point indicated with a box. Scale bar, 10 μm. B. Single image of the photobleached region showing DCVs and possible retrograde trajectories (colored arrows). Scale bar, 2.5 μm. C. SPAIM strategy for viewing retrograde DCVs entering branch point. Following photobleaching of the branch point (large blue box), anterograde newcomers are photobleached (small blue box) while simultaneously imaging retrograde DCVs. D. Quantification of retrograde DCV flux at the branch point (n=7). Note that the preferential traffic into the axon was also seen at muscle 6/7 terminals. See also Movie S5.

Figure 6. Accumulation of retrograde DCVs in the proximal axon

A. Anf-GFP (Top) expressed in a lateral td neuron. Neuronal membrane was labeled with TRITC-HRP antibody. Scale bar, 10 μm. B. Localization of the Golgi marker (Man II-GFP, green) and the microtubule associated protein futsch detected by immunofluorescence (blue). Scale bar, 5 μm. C. Representative images of the lateral td neuron soma and proximal axon before and after photobleaching. Scale bar, 5 μm. D. Quantification of retrograde DCV flux into the proximal axon and soma (n=10). See also Movie S6.

Figure 7. Retrograde DCVs reverse in the proximal axon to reenter the axon

A. Anterograde flux from the proximal axon and soma in 5 minutes after photobleaching. +SPAIM indicates that retrograde newcomers were photobleached. B. SPAIM experimental design to prevent retrograde newcomers from the axon entering the large photobleached region so flux from nascent DCVs moving out of the soma (red arrow) can be quantified. C. SPAIM experimental design to determine trajectories of retrograde DCVs entering the proximal axon. Note that photobleaching of retrograde newcomers began only after a one DCV entered the proximal axon from the axon. D. 5 minute trajectories of retrograde DCVs after returning to the proximal axon. Left images describe trajectory classes. Right images show representative data. Percentage of tracked DCVs (n=54) for each class is presented on the right. Blue lines, retrograde; Red lines, anterograde. E. Vesicle circulation model for synaptic neuropeptide delivery. Anterograde DCVs are routed from the soma toward the most distal bouton in each branch. After reversing, dynactin-mediated retrograde transport routes DCVs past branch points into the axon. Once arriving in the proximal axon, many retrograde DCVs reverse to journey toward release sites again, thus avoiding degradation in the soma. While undergoing long-distance circulation, there is a low probability of being captured at each proximal bouton. This produces initial accumulation in the most distal bouton. However, because capture is slow compared to vesicle flux, neuropeptide stores in en passant boutons eventually fill equally despite differences in distance from the soma. See also Movies S6 and S7.