Single-molecule discrimination of discrete perisynaptic and distributed sites of actin filament assembly within dendritic spines

Abstract

Within dendritic spines, actin is presumed to anchor receptors in the postsynaptic density and play numerous roles regulating synaptic transmission. However, the submicron dimensions of spines have hindered examination of actin dynamics within them and prevented live-cell discrimination of perisynaptic actin filaments. Using photoactivated localization microscopy, we measured movement of individual actin molecules within living spines. Velocity of single actin molecules along filaments, an index of filament polymerization rate, was highly heterogeneous within individual spines. Most strikingly, molecular velocity was elevated in discrete, well-separated foci occurring not principally at the spine tip, but in subdomains throughout the spine, including the neck. Whereas actin velocity on filaments at the synapse was substantially elevated, at the endocytic zone there was no enhanced polymerization activity. We conclude that actin subserves spatially diverse, independently regulated processes throughout spines. Perisynaptic actin forms a uniquely dynamic structure well suited for direct, active regulation of the synapse.

Figure 1. Heterogeneous actin polymerization rates within single spines

A. PA-actin fluorescence immediately after (green) or 32 s after (red) photoactivation at the spine tip. Intensity was thresholded to reveal the peak 25% of the fluorescence at each time point. PA-actin distribution is shown superimposed on cell morphology obtained from tdTomato image (blue). Scale bar, 0.5 μm in A and C. B. PA-actin intensity measured within tip (red) or center (black) regions, following photoactivation in spine tip. Intensities are normalized within-region. n = 8 spines, separate neurons. C. As in A, except photoactivation was targeted at the spine center. D. As in B, except photoactivation was targeted at the spine center. E. Optical monomer incorporation assay to identify sites of ongoing polymerization. Actin in single dendritic spines was entirely photoactivated and then photobleached. After a pause to allow additional, unactivated monomers to enter the spine, spines were photoactivated again. Newly incorporated actin moved inward from the spine membrane, with restricted regions of high and low flow along the membrane (arrows). F. Analysis of example shown in E. Fractional PA-Actin intensity was normalized to the intensity after the first activation step within ROIs at the spine tip (red) and center (black). G. Group data from monomer incorporation assay (n = 9 spines, separate neurons), showing higher fractional intensity in the spine tip following subsequent photoactivation steps. The grey line represents the expected time course of new monomer incorporation within whole spines, estimated from the time course (measured in Fig. 1b) of actin loss from whole spines following photoactivation. *, p=0.014.

Figure 2. Single actin molecules tracked with PALM within dendritic spines

A. Time series from sequential images, showing a molecule which appears at time = 0 s and persists for 6 frames before disappearing. Scale bar, 200 nm. B. Overlays showing the molecule when it first appeared (red) and in each of the ensuing frames (green) before its disappearance. The final image shows the position of the molecule at time = 0 s (red) and 10 s (green) superimposed over the mean intensity image of all 1531 frames (blue). C. Mean intensity projection of all 1531 frames provides a diffraction-limited image of the neuronal dendrite. Scale bar, C&E, 5 μm. D. High magnification of boxed region in D. Dashed line represents the border of the image in E. Scale bar, D&F, 250 nm. E. The local density of polymerized actin calculated as the total number of molecules which appeared within a radius of 111 nm (1 camera pixel) and that were tracked over two or more frames (green) or localized in only a single frame (red). Output plotted in 27.8 nm bins (1/4 the search radius). F. High magnification view of the highlighted region in E. The dashed line in D, F, and G represents the outline of this image. G. The first localized position (red) and last localized position (green) of all molecules tracked for ≥3 frames and over a distance greater than 100 nm are displayed. Tracks are concentrated in the spine head, and point predominantly away from the spine membrane. Scale bar, 250 nm. H. Net distance traveled by molecules in a representative cell plotted as a function of the time since their appearance. 150 ms exposures were collected at 0.5 Hz (triangles) or 3.3 Hz (circles). 1978 frames were collected at 3.3 Hz, in which 7608 molecules were localized in only a single frame, and 3278 were tracked over 2 frames or more. 1025 frames were collected at 0.5 Hz, in which 12708 molecules were localized for a single frame and 3662 were tracked for at least 2 frames. Of these, 54.9% were tracked for 2 frames, and only 3.1% persisted >5 frames. Following jasplakinolide (open circles), 704 frames were collected at 0.5 Hz, in which 2599 molecules were localized in a single frame and 1921 were tracked for at least 2 frames. Of these tracked molecules, 44.0% persisted at least 2 frames, and 12.8% persisted >5 frames. Fiducial beads showed little motion (gray symbols).

Figure 3. Single molecule motion describes the behavior of polymerized actin monomers

A. A neuron expressing actin-mEos2 was imaged sequentially at 3.3 Hz and 0.5 Hz. Following the addition of jasplakinolide, the neuron was again imaged at 3.3 Hz and 0.5 Hz. Each black point represents the distance traveled by a tracked molecule. Red circles represent the mean distance traveled of 100 consecutive tracked molecules. Note that the time-dependent increase in molecule motion is abrogated by the addition of jasplakinolide. B. Local mean net distance traveled by localized molecules within cells which were consecutively imaged at 3.3 Hz and 0.5 Hz before and after jasplakinolide (n = 7) as in A, and separate cells which were fixed in 4% paraformaldehyde before imaging at 0.5 Hz (n = 7 cells from 4 cultures). Two-way ANOVA; ** p < 0.001. C. Molecule velocity map of the same neuron before (left) and after (right) jasplakinolide. Molecule velocity decreased in all spines following treatment. Inset: High magnification of a representative spine from indicated region. Scale bar: 5 μm, inset 1 μm. D. Local mean velocity was calculated by averaging the velocity of each track originating within a 111 nm radius of each 55.5 nm output pixel (as in A). Local mean velocity was decreased in 14/14 cells (8 cultures) following jasplakinolide (averaged per pixel). **, p < 0.001. E. Representative example showing average time over which molecules were tracked (in seconds; tracked at 0.5 Hz; same cell as S3A, C) before and after addition of jasplakinolide. Each point represents the average track length of 100 tracked molecules. F. The mean time over which molecules were tracked increased after the addition of jasplakinolide in 14/14 cells (8 cultures). Paired t-test; ** p < .0001. G. Analysis of all tracked molecules before and after the addition of jasplakinolide (14 neurons, 8 cultures). Total molecules tracked before jasplakinolide was 172,879. Total molecules tracked after the addition of jasplakinolide was 96,240. The fraction of tracks persisting over the designated number of frames is plotted.

Figure 4. Spatially restricted distribution of polymerized actin visualized with PALM near synapses

A. Two-color PALM image of actin-tdEos and GKAP-dronpa in a fixed neuron. Molecules localized with error less than 30 nm (Actin-tdEos, red) or 100 nm (GKAP-Dronpa, green) are shown mapped with a pixel size of 5 nm. Scale bar, 1 μm. B. Closer view of synapses on a spine (top) and shaft (bottom), showing lines along which measurements were made in C. Scale bar, 500 nm. C. Relative molecular density probability of actin-tdEos (green) and GKAP-Dronpa (red) along 75-nm wide lines (30 pixels) shown in B. Dotted lines highlight the width of the profile at a probability of 0.5 (top, 121 nm; bottom, 87 nm). D. The local density of polymerized actin calculated as the number of tracked actin-mEos2 molecules within a 111 nm radius. Local density (green) was plotted at ¼ this scale (27.8 nm bins), and overlaid on the diffraction-limited image of PSD-95-cerulean (red). Scale bar, 5 μm. E. The distribution of tracked actin-mEos2 molecules as a function of distance from the center of PSDs identified by localization of PSD-95-cerulean (n = 4 neurons, 148 PSDs). F–H. Three synapses from D, revealing close association of polymerizing actin with the PSD. Scale bars, 500 nm. I. Actin distribution at a shaft synapse showing actin (left) encircling the PSD (middle). Scale bar, 500 nm. Green color bar represents local density of polymerized actin in A–E.

Figure 5. Heterogeneous actin dynamics within individual spines

A. Map of actin molecule velocity across the inner extent of a dendritic spine. The mean velocity of molecules within 111 nm was plotted at each 55.5 nm pixel for which there were ≥3 molecules within this radius. Restricted areas of high velocity are clearly visible. Scale bar, 250 nm B. Actin velocity map of fixed spines, showing that error stemming from finite molecular localization precision is minimal and spatially unmodulated. Scale bar, 500 nm. C–D. Representative spines showing inward orientation of actin flow. Arrow length represents relative velocity. Gray scale represents tracked molecular density. Scale bar, 200 nm; vector, 100 nm/s. E. Actin velocity map and (F) locally averaged molecular movement vector plotted superimposed on local tracked molecule density (green) and the deconvolved widefield image of PSD-95-cerulean (red). Some but not all foci of high velocity motion are closely associated with the synapse. Scale bar, 250 nm; vector, 200 nm/s. Color bar represents tracked molecular density for F.

Figure 6. Widespread distribution of focal points of high velocity flow within spines

A–C. Definition of polymerization foci. (A) Spatially averaged velocity maps were thresholded to their mean + 1.5 times their standard deviation, to produce binary maps of pixels with high average velocity (B). These were band pass filtered to exclude and combine single pixels. (C) Foci (red) superimposed on an image map showing density of both tracked (green) and untracked molecules (blue). F. Average number of foci per spine before and after 5 min treatment with jasplakinolide, and then after recalculation of the velocity map threshold following jasplakinolide (** p < .01, *** p < .0001, repeated measures ANOVA; n = 12 spines from 5 neurons, 5 cultures). G. Distribution of foci positions within spine subregions delineated as sketched, (n = 68 foci, 30 spines from 5 neurons, 5 cultures). The majority were at the tip and lateral regions. H. The distribution of focal regions from E, with respect to the plasma membrane. I. The average velocity of all pixels within individual spines (n = 32 spines, 6 neurons, 6 cultures). Solid line shows the linear regression. J. Number of foci per spine as a function of spine area (n = 32 spines, 6 neurons, 6 cultures). Solid line shows the linear regression.

Figure 7. The PSD and the endocytic zone show distinct patterns of polymerization dynamics

A. The distribution of tracked actin-mEos2 molecules as a function of distance from the center of PSDs (black) or EZs (red). (n = 148 PSDs, 4 neurons, 4 cultures; 354 EZ, 4 neurons, 2 cultures) B. Frequent colocalization in spines of PSD-95-cerulean (blue) with focal areas of high velocity actin motion (red). The density of tracked actin-mEos2 is shown (green). Arrowheads indicate PSDs. B1 and B2 show spines from two different neurons. Scale bars, 500 nm. C. Infrequent and less direct colocalization of clathrin-cerulean3 (blue) with actin polymerization foci (red). The density of tracked actin-mEos2 is shown (green). Arrowheads indicate EZs. C1 and C2 show spines from two different neurons. Scale bars, 500 nm in C1, 1 μm in C2. D. Histogram of the distribution of distances from localized PSDs (black) or EZs (red) to the nearest focal point of high-velocity actin motion. A significantly higher fraction of PSDs had associated foci within the closest 250 nm (p < .05, t-test). The average PSD was closer to a focus of high velocity particle motion than the average EZ (median 420.0 ± 14.4 vs 565.5 ± 42.3 nm, p < .05 t-test). Data from 4 cells from 4 cultures. E. Cumulative distribution plot of individual PSDs (n=148) or EZs (n=354) from the cells in D. These curves were different (p<0.01, Mann-Whitney U test). F. Percent of PSDs or EZs that colocalized (overlapped pixels with) actin polymerization foci (*, p < .05, t test; n = 100 PSDs, 4 neurons, 4 cultures; 148 EZs, 4 neurons, 2 cultures). G. Average velocity of tracked particles originating within thresholded PSDs, within 300 nm of the PSD, or elsewhere in each of 10 cells. Tracked molecules originating at or near the PSD moved significantly faster than molecules originating elsewhere in the cell (p < .05; paired t-test with Bonferroni correction). H. Average velocity of tracked particles originating within thresholded EZs, within 300 nm of the EZ, or elsewhere in each of 13 cells. Tracked molecules originating at or near the EZ did not move significantly differently than molecules originating elsewhere in the cell (p > .05, paired t-test with Bonferroni correction). I. Average velocity of tracked particles at the PSD or EZ, within a 300 nm radius, or elsewhere in the cell. The velocity of tracked actin molecules originating at the PSD was significantly higher than that of tracked actin molecules originating at the EZ (p < .01, unpaired t-test with Bonferroni correction).