Monitoring actin cortex thickness in live cells

Abstract

Animal cell shape is controlled primarily by the actomyosin cortex, a thin cytoskeletal network that lies directly beneath the plasma membrane. The cortex regulates cell morphology by controlling cellular mechanical properties, which are determined by network structure and geometry. In particular, cortex thickness is expected to influence cell mechanics. However, cortex thickness is near the resolution limit of the light microscope, making studies relating cortex thickness and cell shape challenging. To overcome this, we developed an assay to measure cortex thickness in live cells, combining confocal imaging and subresolution image analysis. We labeled the actin cortex and plasma membrane with chromatically different fluorophores and measured the distance between the resulting intensity peaks. Using a theoretical description of cortex geometry and microscopic imaging, we extracted an average cortex thickness of ∼190 nm in mitotic HeLa cells and tested the validity of our assay using cell images generated in silico. We found that thickness increased after experimental treatments preventing F-actin disassembly. Finally, we monitored physiological changes in cortex thickness in real-time during actin cortex regrowth in cellular blebs. Our investigation paves the way to understanding how molecular processes modulate cortex structure, which in turn drives cell morphogenesis.

Figure 1

Relative localization of the actin cortex and plasma membrane. (A) Schematic representation of the experimental approach. The actin cortex and plasma membrane are labeled with chromatically different fluorophores (here, GFP-Actin and mCherry-CAAX, respectively; top). From the fluorescence intensity linescans across the cell border (bottom), the distance between the fluorescence intensity peaks of the two probes, Δ, can be measured and related to cortex thickness, h. (B) A representative HeLa cell stably expressing GFP-Actin and transfected with mCherry-CAAX blocked in prometaphase and imaged by confocal microscopy. (Inset) High-zoom image of area in box. (C) Linescans orthogonal to the cell border obtained based on segmentation of the cell shown in panel B. Scale bars for panels B and C = 10 μm. Higher zoom images for regions with many (a) or few (b) microvilli are shown in panel C′, with actin, PM, and merge images in the same order as in panel C. Scale bars for panel C′ = 2 μm. (D) Average linescans of actin and the PM, calculated by averaging individual linescans from panel C. Fluorescence peak position and intensity were determined by fitting several points around the pixel with maximum intensity (blue lines).

Figure 2

A theoretical description of cortex geometry accounts for the dependence of peak separation on background intensities. (A) Images of a single HeLa cell expressing GFP-Actin and mCherry-CAAX, blocked in prometaphase and in culture medium alone (left panel) or in the presence of increasing concentrations of extracellular Alexa488-Dextran (three right-most panels). Scale bar = 10 μm. Note: Even for the case where the background ratio, i_out/i_in, was as high as 2.0 (right panel), the peak could be fit; in this case, the ratio of peak intensity to extracellular background, i_p/i_out = 1.37. (B) Plot of the separation between the actin and PM intensity peaks, Δ, as a function of i_out/i_in. (Points) Experimental data. The leftmost point (i_out/i_in = 0.4) corresponds to no extracellular Alexa488-Dextran. (Dotted line) Δ values calculated from Eqs. 1 and 2 varying i_out and fixing h (200 nm), i_c (225.5 AU), and i_in (43.66 AU; see the Supporting Material for details). (C) Plot of Δ as a function of background ratio i_out/i_in for a population of mitotic HeLa cells in the presence of differing amounts of Alexa488-Dextran. Pearson correlation coefficient, r = −0.674 (p < 0.001). (D) A linescan reflecting a simplified model of cortex geometry (blue) and the resulting linescan after convolution to mimic the imaging process (green). The peak intensity of the convolved linescan, i_p, is lower than i_c, and the position of the peak, x_c, is shifted toward the side of higher background intensity (here, the cytoplasm) with respect to the center of the cortex, X_c. (E) Cortex thickness, h, extracted from the population of cells in panel C. The extracted h values were not significantly correlated with the background intensity ratio. r = −0.117 (p = 0.554).

Figure 3

Thickness extraction is robust to exchange of fluorescent probes. (A) Prometaphase HeLa cells expressing the F-actin-binding probe Lifeact and the PM probe CAAX. Scale bar = 10 μm. (B) Average linescans of GFP-Lifeact and mCherry-CAAX signal for cell in A (top panels). (C) Box plot comparing actin-PM intensity peak separation, Δ, between cells expressing GFP-Actin/mCherry-CAAX and GFP-Lifeact/mCherry-CAAX. ^∗∗∗p < 0.001 by Welch t-test. (D) Box plot comparing extracted cortex thickness, h, between cells expressing GFP-Actin/mCherry-CAAX (Actin/PM), EGFP-Lifeact/mCherry-CAAX (Lifeact/PM) and EGFP-CAAX/mCherry-Lifeact (PM/Lifeact [color swap]). p > 0.05 for all categories by Welch t-test.

Figure 4

Evaluating cortex thickness measurements by model convolution. (A, left) Multiple magnifications of a synthetic cell image capturing main cortex features, drawn with a pixel size of 4 nm. (Middle) Intracellular background intensity is added and the image is convolved with a Gaussian with σ = 170 nm, comparable to the PSF of the microscope. (Right) The image is downsampled to a pixel size corresponding to experimental imaging parameters (69 nm). Gaussian noise is added to mimic experimental noise. (Insets are denoted by boxes.) Scale bar = 10 μm. (B) Cortex-PM peak separation, Δ, and cortex thickness, h, for synthetic images plotted as a function of background intensity ratio, i_out/i_in. (Dotted lines) Linear fits to the data. (C) Plot comparing h extracted from linescan analysis to the cortex thickness values specified during synthetic image generation (h imposed) for synthetic cell images with i_out/i_in = 0.25. (Points) Mean ± SD for n ≥ 25 synthetic cells for each imposed thickness. The identity function (gray line) is plotted for comparison.

Figure 5

Cortex thickness increases after treatments reducing actin disassembly. (A) Prometaphase HeLa cells expressing GFP-Actin and mCherry-CAAX treated with DMSO or Jasplakinolide (Jas.). Scale bar = 10 μm. (B) GFP-Actin and mCherry-CAAX-expressing HeLa cells after transfection of scrambled (Scr.) siRNA or siRNA targeted against cofilin 1 (cofilin 1 knock-down, CFL1 KD). Scale bar = 10 μm. (C) Box plot comparing cortex thickness, h, between untreated control cells and cells treated with DMSO or Jasplakinolide or transfected with scrambled or anti-CFL1 siRNA. ^∗∗p < 0.01, ^∗∗∗p < 0.001 by Welch t-test. (D) Western blot confirming knock-down of CFL1 after transfection of anti-CFL1 siRNA. GAPDH was used as a loading control.

Figure 6

Monitoring cortex thickness in blebs. (A) Montage from a timelapse image series of bleb growth and retraction after laser ablation of the cortex in a prometaphase HeLa cell expressing GFP-Actin and mCherry-CAAX. Scale bar = 5 μm. See also Movie S1. (B) Average linescans across the bleb border at different time points from panel A. (C) Plot of cortex thickness during bleb retraction shown in panel A. (D) A time course from another bleb experiment in which bleb cortex thickness exceeded preablation thickness and subsequently relaxed back to the initial value. For panels A–D, time 0 corresponds to ablation time. For panels C and D, bleb cortex thickness data (points) were smoothed with a moving average of window size 11 points (lines).