Volume Changes During Active Shape Fluctuations in Cells

Cells modify their volume in response to changes in osmotic pressure but it is usually assumed that other active shape variations do not involve significant volume fluctuations. Here we report experiments demonstrating that water transport in and out of the cell is needed for the formation of blebs, commonly observed protrusions in the plasma membrane driven by cortex contraction. We develop and simulate a model of fluid-mediated membrane-cortex deformations and show that a permeable membrane is necessary for bleb formation which is otherwise impaired. Taken together, our experimental and theoretical results emphasize the subtle balance between hydrodynamics and elasticity in actively driven cell morphological changes.

Figure 1

Volume and surface fluctuations during blebbing are controlled by AQPs. (a) Representative phenotypes for bleb formation in PGCs under different conditions: wild type (WT), cells expressing DN-ROK mutant which impairs contractility by interfering with actomyosin contraction (DN-ROK), cells where AQP1a and AQP3a are suppressed ($\mathrm{AQP}-$), cells with overexpression of AQP1a and AQP3a ($\mathrm{AQP}+$). The scale bar corresponds to $10\mu \mathrm{m}$. The cumulative distribution of relative volume (b) and surface (c) fluctuations for the four conditions illustrated in (a), together with a Gaussian fit (dashed lines). The distributions are sampled over $N$ different time frames corresponding to $n$ different cells. (WT: $n=15$ cells and $N=800$ time-frames; DN-ROK: $n=2$ and $N=155$; $\mathrm{AQP}-$: $n=4$ and $N=180$; and $\mathrm{AQP}+$: $n=7$ and $N=183$.)

Figure 2

Bleb formation is directly related to volume changes. (a) Time lapse of a single $\mathrm{AQP}+$ cell. Two-dimensional sections are shown in the upper panels and the corresponding reconstructed three-dimensional meshes in the lower ones. Arrows indicate blebs. The scale bar corresponds to $1\mu \mathrm{m}$. (b) Evolution of the volume for the same cell. Time points corresponding to the panels in (a) are denoted in yellow. An increase in volume is observed in correspondence with each newly formed bleb.

Figure 3

Numerical simulations show that membrane porosity is needed for blebbing. (a) A series of snapshots of the numerical simulations of bleb formation and retraction. The color represents the local fluid pressure, red for positive and blue for negative pressures while the cell membrane is green. (a) Results obtained with physiological permeability: $\alpha =2\times 10^{-14}{\mathrm{m}}^2\mathrm{s}/\mathrm{kg}$. (b) For $\alpha =0$, bleb formation is impaired. (c) The cumulative distribution of relative area fluctuations for different permeabilities. (d) The standard deviation of the distribution of relative cell areas as a function of permeability. The physiological range is depicted in gray.

Figure 4

(a) Spatio-temporal evolution of the pressure jump across the membrane computed from the normal stress jump condition in numerical simulations with $\alpha =2\times 10^{-14}{\mathrm{m}}^2\mathrm{s}/\mathrm{kg}$. The pressure drop is strongly enhanced in localized regions corresponding to blebs. (b) The corresponding distribution of pressure jumps displays long tails.