Spatial and temporal relationships between actin-filament nucleation, capping, and disassembly

Abstract

Background: The leading actin network in motile cells is composed of two compartments, the lamellipod and the lamellum. Construction of the lamellipod requires a set of conserved proteins that form a biochemical cycle. The timing of this cycle and the roles of its components in determining actin network architecture in vivo, however, are not well understood.

Results: We performed fluorescent speckle microscopy on spreading Drosophila S2 cells by using labeled derivatives of actin, the Arp2/3 complex, capping protein, and tropomyosin. We find that capping protein and the Arp2/3 complex both incorporate at the cell edge but that capping protein dissociates after covering less than half the width of the lamellipod, whereas the Arp2/3 complex dissociates after crossing two thirds of the lamellipod. The lamellipodial actin network itself persists long after the loss of the Arp2/3 complex. Depletion of capping protein by RNAi results in the displacement of the Arp2/3 complex and disappearance of the lamellipod. In contrast, depletion of cofilin, slingshot, twinfilin, and tropomyosin, all factors that control the stability of actin filaments, dramatically expanded the lamellipod at the expense of the lamellum.

Conclusions: The Arp2/3 complex is incorporated into the lamellipodial network at the cell edge but debranches well before the lamellipodial network itself is disassembled. Capping protein is required for the formation of a lamellipodial network but dissociates from the network precisely when filament disassembly is first detected. Cofilin, twinfilin, and tropomyosin appear to play no role in lamellipodial network assembly but function to limit its size.

Figure 1. GFP-Tagged Actin and Actin-Regulatory Proteins Occupy Unique Compartments at the Leading Edge

(A) S2 cells expressing GFP-actin, GFP-p16 (Arp2/3 subunit), GFP-capping protein α (CPA), and GFP-cytoskeletal tropomyosin (cTm) were imaged by confocal microscopy. Images are a single frame from time-lapse movies of live S2 cells plated on concanavilin A for approximately 1 hr. The scale bar represents 10 µm. (B) Kymographs of GFP speckles reveal dynamic compartmentalization of actin and actin-binding proteins. Left, maximum-intensity projections of time-lapse movies of S2 cells expressing GFP-actin (top), GFP-p16, GFP-CPA, and GFP-cTm. The scale bar represents 10µm. The white line at the 9 o’clock position indicates the cell area from which kymographs (right) were constructed. Right panel: The x axis scale bar represents 1 µm; the y axis scale bar represents 30 s.

Figure 2. Quantitative Analysis of GFP-actin, GFP-p16, GFP-CPA, and GFP cTm Speckle Localization and Dynamics

(A) Normalized average-fluorescence-intensity line scan of GFP fluorescence taken from a representative movie of an S2 cell expressing GFP-actin, GFP-p16, GFP-CPA, or GFP-cTm and displayed as a function of distance from the cell edge, as described in the Experimental Procedures. (B–D) Individual speckle trajectories from at least ten cells were analyzed via kymograph analyses. (B) Line histogram showing the distance traveled by GFP-actin, GFP-p16, and GFP-CPA speckles originating in the LP. Trajectories used in this histogram are shown as green circles in each of the scatter plots in (C). (C) Scatter plots of the distance from the cell edge versus velocity. From top to bottom: actin, cTm, p16, and CPA. Speckle trajectories were labeled as originating in the lamellipod (LP, green circles), lamellum (LM, blue circles), or cell body (CB, red circles) based on their starting distance from the cell edge. Vertical lines indicate the mean velocity for each population, and shaded areas indicate the standard deviation. (D) Probability density function of the velocities of all GFP speckles of (top to bottom) actin, cTm, p16, and CPA. The probability density function is overlayed with the best-fit line (black line) derived from cumulative density-function-curve fitting (data not shown). Vertical lines indicate means, and shaded areas indicate the standard deviation for each population.

Figure 3. Visualization of Network Assembly in S2 cells

(A and B) Time-averaged turnover map of F-actin (A) and p16 (B) calculated from speckle-tracking analysis of an S2 cell expressing GFP-actin or GFP-p16. Speckles were tracked and analyzed with fsmCenter, created by the Danuser group (Scripps Research Institute), with algorithms previously described [14]. In the left panels, red marks the locations of rapid actin (A) and Arp2/3 (B) speckle appearance. Green marks rapid actin (A) and Arp2/3 (B) speckle disappearance. In the middle and right panels, the rates of actin (A) and Arp2/3 (B) speckle birth (middle) and death (right) are plotted as grayscale values. The scale bar represents 10 µm. (C) Net actin and Arp2/3 speckle turnover rates are plotted versus distance from the cell edge. Data are from single S2 cells expressing either GFP-actin (A) or GFP-p16 (B). The thick green line indicates net actin filament assembly/disassembly calculated from the cell in (A). The thick blue line indicates net Arp2/3 association/dissociation to the actin network; this was calculated from the cell in (B). Mean (vertical line) and standard deviation (shaded rectangle) of the distance traveled by capping protein (red), Arp2/3 (blue) and actin (green) were calculated from kymograph analyses. The figure was created with cytoProbe software from the Danuser group (Scripps). (D) A turnover map of F-actin was calculated from speckle-tracking analysis of an S2 cell depleted of cTm and expressing GFP-actin. Actin assembly is shown in red (center), and actin disassembly is shown in green (right). The scale bar represents 10 µm.

Figure 4. Tropomyosin RNAi Causes Expansion of the Lamellipod at the Expense of the Lamellum

(A) Normalized average-fluorescence-intensity line scan of GFP fluorescence taken from a representative movie of a cTm-depleted S2 cell expressing GFP-actin, GFP-p16, or GFP-CPA and displayed as a function of distance from the cell edge. (B) Line histogram showing distance traveled by GFP-actin, GFP-p16, and GFP-CPA speckles originating in the LP. Speckles used in this histogram are shown as green circles in each of the scatter plots in (C). (C) Scatter plots were created as described in Figure 2C. Tropomyosin-depleted cells expressing (from top to bottom) GFP-actin, GFP-p16, and GFP-CPA were analyzed. (D) Probability density functions were created as described in Figure 2D. Tropomyosin-depleted cells expressing (from top to bottom) actin, p16, or CPA were analyzed.

Figure 5. Cofilin RNAi Causes Decreased Velocities and Lamellipodial Expansion

(A) Line histogram showing the distance traveled by GFP-actin, GFP-p16, and GFP-CPA speckles originating in the LP in cofilin-depleted cells. Speckles used in this histogram are shown as green circles in each of the scatter plots in (B). (B) Scatter plots were created as described in Figure 2C. Cofilin-depleted cells expressing (from top to bottom) GFP-actin,GFP-p16, or GFP-CPA were analyzed. (C) Probability density functions were created as described in Figure 2D. Cofilin-depleted cells expressing (from top to bottom) actin, p16, or CPA were analyzed.

Figure 6. Depletion of Capping Protein Abolishes the Lamellipod but has Little Effect on Lamellar Dynamics

(A) Line histogram showing the distance traveled by GFP-actin and GFP-p16 speckles originating in the LP in CPA-depleted cells. Speckles used in this histogram are shown as green circles in each of the scatter plots in (C). (B) S2 cells were fixed and F-actin and visualized with Alexa488 phalloidin. Left: untreated cell. Right: CPA-depleted cell. (C) Scatter plots were created as described in Figure 2C. Capping-protein-depleted cells expressing (from top to bottom) GFP-actin, GFP-p16, and GFP-cTm were analyzed. (D) Probability density functions (PDF) were created as described in Figure 2D. Capping protein depleted cells expressing (from top to bottom) GFP-actin, GFP-p16 or GFP-cTm were analyzed.

Figure 7. Schematic Model of Actin and Actin-Regulatory Proteins at the Leading Edge

Arp2/3 (green circles), capping protein (red circles), cofilin (yellow triangles), and actin (white lines) build lamellipodial actin networks (green area), whereas tropomyosin (blue “S” shapes) is associated with lamellar actin networks (blue area). A capping decrease due to depletion of capping protein leads to the collapse of the lamellipod and expansion of the lamellum. A disassembly decrease due to cofilin, slingshot, twinfilin, or tropomyosin depletion leads to the expansion of the lamellipod at the expense of the lamellum.