Periodic patterns of actin turnover in lamellipodia and lamellae of migrating epithelial cells analyzed by quantitative Fluorescent Speckle Microscopy

Abstract

We measured actin turnover in lamellipodia and lamellae of migrating cells, using quantitative Fluorescent Speckle Microscopy. Lamellae disassembled at low rates from the front to the back. However, the dominant feature in their turnover was a spatially random pattern of periodic polymerization and depolymerization moving with the retrograde flow. Power spectra contained frequencies between 0.5 and 1 cycle/min. The spectra remained unchanged when applying Latrunculin A and Jasplakinolide in low doses, except that additional frequencies occurred beyond 1 cycle/min. Whereas Latrunculin did not change the rate of mean disassembly, Jasplakinolide halted it completely, indicating that the steady state and the dynamics of actin turnover are differentially affected by pharmacological agents. Lamellipodia assembled in recurring bursts at the leading edge and disassembled approximately 2.5 microm behind. Events of polymerization correlated spatially and temporally with transient formation of Arp2/3 clusters. In lamellae, Arp2/3 accumulation and polymerization correlated only spatially, suggesting an Arp2/3-independent mechanism for filament nucleation. To acquire these data we had to enhance the resolution of quantitative Fluorescent Speckle Microscopy to the submicron level. Several algorithmic advances in speckle image processing are described enabling the analysis of kinetic and kinematic activities of polymer networks at the level of single speckles.

FIGURE 1

Iterative speckle extraction and effects on mapping of F-actin turnover. (a) Raw FSM image. (b) Speckle extraction in four consecutive time-points. (Red, first-order speckles; yellow, second-order speckles; and cyan, third-order speckles.) Numbers identify the speckles in different time-points. F-actin turnover mapped from primary speckles only (c) and speckles up to order-three (d). The color representation of polymerization and depolymerization agrees with illustrations in Ponti et al. (14), allowing better comparison with previous studies. Arrowheads and arrows highlight regions where the additional information generated by high-order speckles yields significant shifts in the location and magnitude of polymerization and depolymerization peaks, respectively.

FIGURE 2

Tracking of speckle trajectories probing the F-actin retrograde flow in a newt lung epithelial cell. (a) Speckle displacements at the leading edge tracked by global nearest-neighbor assignment. (b) Speckle displacements for the same region tracked by generalized nearest-neighbor (GNN) assignment. More links are extracted which reflect a directionally more coherent flow field. The asterisk in the inset highlights a pair of speckles, which is wrongly tracked in forward (toward the edge) direction in a, but is captured correctly in b. (c) Speckle trajectories initiated in the first 30 frames of the movie. Colors encode the frame number (dark blue, early time-point; light green, late time-points). The perimeters of the lamellipodium (Lp), lamella (La), and convergence zone (CZ), as defined by the ensemble kinematics and kinetics of the speckles in these regions in Ponti et al. (11), are overlaid.

FIGURE 3

F-actin turnover in migrating and nonmigrating cells. (a) Steady-state map of assembly (red) and disassembly (green) in a migrating cell. Scale bar is 5 μm. Animated maps are available in Movie 3, Supplementary Materials and Methods. (b) Net assembly (= assembly – disassembly) along three profiles indicated in a. (c) Map of retrograde flow. Vectors show the flow velocity. Colors encode the vector magnitude, i.e., the flow speed. Animated speed/velocity maps are available in Movie 4, Supplementary Materials and Methods. White boxes display the end position of 1.2 μm × 1.2 μm probing windows, which move with the retrograde flow. For visual simplicity, their trajectories over 60 frames are approximated by the white line segments. (d1) Time evolution of turnover in three arbitrarily selected probing windows. (d2) Autocorrelation of the three curves, revealing a periodicity in turnover of 85–100 s. (e) Steady-state map of assembly (red) and disassembly (green) in a nonmigrating cell. Animated maps are available in Movie 5 in Supplementary Materials and Methods. Scale bar is 5 μm. (f1) Time evolution of turnover rate for three randomly selected probing windows for the cell in e. (f2) Autocorrelation, revealing a periodicity in turnover of 90–110 s. (g) Simulated FRAP curves using the primary periodicity extracted in d and f. They project a half-time of fluorescence recovery after complete photobleaching of 20–23 s.

FIGURE 4

Power spectra of La turnover. (a–d) Comparison of spectra from four cells with different mean retrograde flow (). Arrows, open arrows, asterisks, and circle indicate frequencies above the noise floor and their harmonic relationships. (e and f) Spectra before and after perfusion of the cell with Calyculin A (CA).

FIGURE 5

Power spectra of La turnover in presence of known inhibitors of F-actin dynamics. (a and b) Spectra before and after perfusion of the cell with Jasplakinolide (Jasp), an inhibitor of actin depolymerization. (c and d) Spectra before and after perfusion of the cell with Latrunculin A (LatA).

FIGURE 6

Correlation of F-actin assembly in Lp and La with dynamic clustering of Arp2/3 visualized by the GFP-p34 subcomplex. (a) Time-averaged map of F-actin assembly (top) and Arp2/3 accumulation (bottom). Arrowheads indicate positions where high assembly visually correlates with maxima in the Arp2/3 signal. (b) Cross-correlation analysis between Lp Arp2/3 signal and polymerization (solid line); La Arp2/3 signal and polymerization (dashed line); and La Arp2/3 signal and depolymerization (shaded dashed line). (Inset) Cross-correlation between a random field of polymerization scores and Arp2/3 signal.

FIGURE 7

Correlation of periodic F-actin assembly at the leading edge and disassembly at the Lp base. (a) Segmentation of the Lp region as described in Ponti et al. (11); compare to Movie 6 in Supplementary Materials and Methods. Scale bar is 5 μm. (b1) Turnover at the Lp front and at the Lp base for sector S1. Continuously positive values at the front indicate continuous net assembly. Continuously negative values at the base indicate continuous net disassembly. (b2) Autocorrelation of assembly and disassembly indicating a weak periodicity. (b3) Cross-correlation between assembly and disassembly, indicating that high rates of assembly at the Lp front is concurrent with a reduced disassembly at the Lp base. (c1–c3) The same analysis for sector S2.