Mapping Cell Membrane Fluctuations Reveals Their Active Regulation and Transient Heterogeneities

Abstract

Shape fluctuations of the plasma membrane occur in all cells, are incessant, and are proposed to affect membrane functioning. Although studies show how membrane fluctuations are affected by cellular activity in adherent cells, their spatial regulation and the corresponding change in membrane mechanics remain unclear. In this article, we study how ATP-driven activities and actomyosin cytoskeleton impact basal membrane fluctuations in adherent cells. Using interference imaging, we map height fluctuations within single cells and compare the temporal spectra with existing theoretical models to gain insights about the underlying membrane mechanics. We find that ATP-dependent activities enhance the nanoscale z fluctuations but stretch out the membrane laterally. Although actin polymerization or myosin-II activity individually enhances fluctuations, the cortex in unperturbed cells stretches out the membrane and dampens fluctuations. Fitting with models suggest this dampening to be due to confinement by the cortex. However, reduced fluctuations on mitosis or on ATP-depletion/stabilization of cortex correlate with increased tension. Both maps of fluctuations and local temporal autocorrelation functions reveal ATP-dependent transient short-range (<2 μm) heterogeneities. Together, our results show how various ATP-driven processes differently affect membrane mechanics and hence fluctuations, while creating distinct local environments whose functional role needs future investigation.

Figure 1

Details of calibration of IRM. (a) Given here is an IRM setup schematic showing illumination and detection path. Zoomed-in view shows reflections at interface 1 (coverslip-medium, blue arrows) and 2 (medium-cell, green arrows), which interfere. (b) (Top) Shown here is an IRM image of a bead with a typical radial line in yellow. (Bottom) Shown here is an averaged intensity-versus-height profile (N = 10) with linear fit of the first branch in red. (c) (Top) Shown here is a plot of I_max, I_min and S/2 from profiles of same beads is imaged at different exposure times (n = 5 beads, four line scans per bead) for a particular day. (Bottom) Given here is ΔI/Δh versus exposure times for beads. (d) Shown here is a typical IRM image of a HeLa cell. White ROIs are used to calculate S/2 (cell). (e) Given here is a comparison of I_max, I_min and S/2 between beads and cells for different days (N = 20). Gray region covers y = x ± 0.1 x. (f) Shown here is the SD_(time) measured and the S/2 (cell) used for different days versus the conversion (ΔI/Δh) used. (g) Shown here are minima (red) and maxima (green) projections of an HeLa cell with FBRs overlaid in cyan. (h) Shown here, FBRs are overlaid on the corresponding IRM image. (i) Shown here are relative height maps at any given time-point of six FBRs in an HeLa cell. Scale bars represent 1 μm. Scale bars for (a–h) = 10 μm. See also Fig. S1; Supporting Discussion. To see this figure in color, go online.

Figure 2

Quantification of temporal fluctuations and spatial undulations. (a) (i) Given here is a relative height map of a membrane patch (FBR) at two time-points. (ii) Shown here is a distribution of relative heights across the FBR at any particular time-point. (iii) Representative spatial ACF is measured at a long FBR. Solid line shows fit to three-term multiexponential function. (iv) Given here is a time-series of relative heights of a pixel in red in (i). (v) Distribution of relative heights is given for a time-series in (iv). (vi) Shown here is a representative temporal ACF of a small FBR. Solid line shows fit to a three-term multiexponential function. Black arrow points out a bump. (vii) Given here is a PSD at an FBR, and the parameters extracted from it. (viii) Fitting of a background-subtracted PSD was made to Eq. 1. (b) Shown here are SD_(time) maps of an HeLa cell (non-FBRs blocked by white; scale bars, 10 μm) and two FBRs. Scale bars, 1 μm. To see this figure in color, go online.

Figure 3

Effect of prevention of cellular activity on membrane fluctuations. (a) Shown here are IRM images (scale bars, 10 μm) and SD_(time) maps (scale bars, 1 μm) of FBRs in control, ATP-depleted, staurosporine-treated (Stauro., 5 μM, 60 min), and fixed HeLa cells. (b) Given here are PSDs of FBRs in treated cells and their controls (solid lines) with their backgrounds (dashed lines); inset shows the value f. (Top) Shown here are N = 50 cells each, n_control = 1083 FBRs, and n_{ATP depletion} = 880 FBRs. (Bottom) Shown here are N = 10 cells each, n_control = 412 FBRs, n_{staurosporine} = 336 FBRs, and n_fixed = 329 FBRs. (c and d) Given here are temporal and spatial parameters for (b). ^∗p < 0.05, ^∗∗p < 0.001, one-way ANOVA. (e) Shown here are averaged spatial ACFs (and their log-log plots, top inset) for control and ATP-depleted cells (N = 80 cells, n_control = 2024 FBRs, and n_{ATP depletion} = 1244 FBRs). (Bottom inset) Given here are correlation lengths. (f) Weighted distribution of correlation timescales was obtained from temporal ACFs (inset: solid line shows fits) (N = 9 cells, n_control = 3765 FBRs, and n_{ATP depletion} = 1987 FBRs). (g) Given here are IRM images (scale bars, 20 μm), with FBRs overlaid in yellow and their corresponding p-value maps (Kolmogorov-Smirnov hypothesis testing). Scale bars, 1 μm. (Right) Given here is the p value for FBRs in control versus ATP-depleted cells. Shown here are N = 30 cells, n_control = 41,760 pixels, and n_{ATP depletion} = 25,766 pixels. Asterisks in (e) and (g) indicate ^∗∗p < 0.001, Mann-Whitney U test. See also Figs. S5 and S7; Tables S2 and S3. To see this figure in color, go online.

Figure 4

Effect of the actomyosin cortex on membrane fluctuations. (a) Given here are IRM images (scale bars, 10 μm) and SD_(time) maps (scale bars, 1 μm) of HeLa cells under mentioned conditions. (b) Shown here are PSDs for cells (solid lines) and their backgrounds (dashed lines), with the inset showing the value f for all conditions. (Left) Shown here are N = 30 cells each, n_control = 741 FBRs, n_{Cyto D} = 564 FBRs, n_{Lat B} = 426 FBRs, and n_Jas = 259 FBRs. (Middle) Shown here are N = 10 cells each, n_control = 141 FBRs, n_{ATP depletion} = 158 FBRs, n_{ATP depletion + Cyto D} = 254 FBRs, and n_{Cyto D + ATP depletion} = 162 FBRs. (Right) Shown here are N = 20 cells each, n_control = 402 FBRs, n_blebbistatin = 333 FBRs, and n_{blebbistatin + Cyto D} = 193 FBRs. (c and d) The parameters of temporal fluctuations and spatial undulations of cells are given under different conditions. ^∗p < 0.05, ^∗∗p < 0.001, one-way ANOVA. (e) Given here are averaged spatial ACFs (and their log-log plots, top inset) and correlations lengths (n_control = 1085 FBRs, n_{Cyto D} = 494 FBRs, N = 40 cells each; n_control = 884 FBRs, n_{Lat B} = 495 FBRs, N = 30 cells; n_control = 300 FBRs, n_Jas = 102 FBRs, N = 20 cells; and n_control = 811 FBRs, n_blebbistatin = 729 FBRs, N = 30 cells). ^∗p < 0.05, ^∗∗p < 0.001, Mann-Whitney U test. (f) Given here is the normalized fraction of cortex clearance (N = 10 cells each). See also Figs. S8, S9, S10, S11, and S12; Tables S2 and S3. To see this figure in color, go online.

Figure 5

Membrane fluctuations in mitotic HeLa cells and in PMSs. (a) Mitotic HeLa cell is observed in fluorescence (top left) (DNA stained with Hoechst 33342), IRM (bottom left), and differential interference contrast (top right) modes along with the SD_(time) map (bottom right). Scale bars, 10 μm. (b) Shown here is the linearized and color-coded cortex (60 μm × 9 μm) of actin-labeled interphase and mitotic cells in normal and Cyto D-treated conditions. (c) Given here are representative images of IRM (left) and SD_(time) maps (right) of CHO-derived PMS (cell-free PMS (top), cell-attached PMS (bottom)). Scale bars, 5 μm. (d) Given here are PSDs for cells, PMSs (solid lines), and their backgrounds (dashed lines) with the insets showing the value f; (left) N = 30 cells each, n_interphase = 639 FBRs, and n_mitotic = 352 FBRs; (middle) N = 15 cells each, n_mitotic = 105 FBRs, n_{mitotic + Cyto D} = 85 FBRs, n_{mitotic + Lat B} = 182 FBRs, and n_{mitotic + blebbistatin} = 215 FBRs. (Right) Shown here is N = 10 cells/PMSs each, n_cell = 70 FBRs, and n_PMS = 85 FBRs. (e and f) The parameters of temporal fluctuations and spatial undulations of cells/PMSs are given in the mentioned conditions. ^∗p < 0.05, ^∗∗p < 0.001, one-way ANOVA. (g) Shown here are the averaged spatial ACFs (and their log-log plots, top inset) and correlation lengths (n_interphase = 447 FBRs, n_mitotic = 199 FBRs, N = 50 cells each; n_cells = 130 FBRs, n_PMS = 39 FBRs, and N = 15 cells/PMS). ^∗p < 0.05, ^∗∗p < 0.001, Mann-Whitney U test. See also Figs. S13 and S14; Tables S2 and S3. To see this figure in color, go online.

Figure 6

Characteristics of transient heterogeneities in fluctuations. (a) Given here is the SD_(time) and mean relative height map of an FBR. Scale bars, 1 μm. (b) Shown here is a line scan of SD_(time) across the feature (white arrow) at the mentioned time-points. The numbers mentioned are the SD of the Gaussian fits to profiles. (c) Variation of peak values of SD_(time) are given at different time-points, exponential fits, and extracted timescales. (d) Given here are typical maps of SD_(time) for a pair of FBRs. Scale bars, 1 μm. (e) Shown here is SD(SD_(time)) for different conditions (N = 30 cells each). ^∗p < 0.05 and ^∗∗p < 0.001, one-way ANOVA. See also Fig. S15; Tables S2 and S3. To see this figure in color, go online.

Figure 7

Local heterogeneities and underlying membrane mechanics. (a) Shown here are zoomed-in temporal ACFs (blue) and slopes (green) of smoothened ACFs (red). Horizontal lines denote the slope threshold (Th) set in each given to detect features (arrow). The vertical lines denote the crossovers. (b) Given here is the fraction of ACF curves with features obtained using different thresholds (N = 9 cells, n_control = 3765 FBRs, n_{ATP depletion} = 1987 FBRs; N = 10 PMSs, and n = 352 FBRs). (c) Membrane mechanical parameters A, η_eff, κ, μ, σ, and γ are obtained from fitting PSDs to a theoretical model (n_control = 1238 FBRs, n_{ATP depletion} = 811 FBRs, N = 80 cells each; n_control = 595 FBRs, n_{Cyto D} = 378 FBRs, N = 40 cells each): n_control = 488 FBRs, n_{Lat B} = 300 FBRs, N = 30 cells each; n_control = 199 FBRs, n_Jas = 106 FBRs, N = 20 cells each; and n_interphase = 329 FBRs, n_mitotic = 101 FBRs, N = 30 cells each). ^∗p < 0.05, ^∗∗p < 0.001, Mann-Whitney U test. To see this figure in color, go online.