Figure S2.

Cortical Contractility Fluctuations Drive Stable-Bleb Cell Transformation, Related to Figure 3

(A) Illustration of contractility-driven cortical instability in a non-polarized cell. In an element of cortex (shown as zoom in), a gradient in actin filament density (filaments are represented by red sticks) results in a net contractile force to the right; this results in a net flow, which amplifies the accumulation of actin on the right. The instability is opposed by actin turnover (given by polymerization and depolymerization rates, k_p and k_d), osmotic effects (i.e., steric repulsion between filaments), friction (with coefficient ξ), and actin viscosity. During cortical instability and in the polarized state of the cell, the cortical tension is inhomogeneous, and the tension component parallel to the polarization axis (T_∥) is generally different from the perpendicular component (T_⊥).

(B) Myl12.1-eGFP (myosin II) localization in progenitor cells cultured in DMEM-F12 medium alone (ctrl, top) and in the presence of 1 μM LPA (middle) or 10 μM LPA (bottom). Corresponding fluorescence intensity profiles were measured across the cell body (red lines) and bleb protrusions (blue lines), indicating a differential contractility between cell cortex and bleb cortex that increases for higher LPA concentrations.

(C) Relative bleb sizes given as the ratio of bleb contour length L_Bleb to cell contour length L_Cell obtained from fluorescence cross-sectional images of progenitor cells as shown in (B) at varying amounts of LPA (n = 60 for all conditions).

(D) Growth rate of cortical instability versus density fluctuation mode number l (∼1/wavelength). The growth rate, λ_l in units $R^4\xi /{\rho }_0^2\gamma$, is shown for three values of cortical contractility, given by the parameter $\tilde{\zeta }=R^2/{\rho }_0\gamma \left(\zeta -\beta {\rho }_0^3\right)$: $\tilde{\zeta }=1$, $\tilde{\zeta }=3$, $\tilde{\zeta }=5$. Other parameter values are η/ξR² = 0.1, ${\zeta }^{\prime }{\beta }_2/\xi R^2\chi =0,$ and $\left(R^4\xi /{\rho }_0^2\gamma \right)k_d=1$. The curves indicate that, due to the competing effects of contractility and density gradient energy penalties, there is, in general, only a particular window of fluctuation wavelengths that are unstable.

(E) Bright-field (top) and fluorescence time-lapse images of Myl12.1-eGFP (myosin II) localization during progenitor cell polarization in a local 100 μM LPA diffusion gradient set by a micropipette (red arrow).

(F) Table of pharmacological inhibitors summarizing their molecular targets and effects on stable-bleb cell polarization. Myl12.1-eGFP expressing cells in (B) and (E) were obtained from Tg(actβ1:myl12.1eGFP) transgenic embryos at sphere stage (4 hpf). Number of cells is given by n. All scale bars, 10 μm.