Fig 1.
The distribution of RhoA and actomyosin at the epithelial zonula adherens.
i. Scheme of coherent epithelial cells and the distribution of the active RhoA zone (green) at the apical adherens junctions (zonula adherens, ZA). ii. Diagram of the actomyosin cortex located on the cytoplasmic side of a cell. The ZA corresponds to a cortical zone enriched in NMIIA and RhoA. Note that RhoA is activated at the ZA, but this lipid-anchored molecule can potentially diffuse horizontally onto the apical surface of the cells as well as vertically down into the lateral cell-cell junctions. iii. Scheme of the top view of the RhoA zone from a group of cells (left) and its comparison with a spinning disk confocal image taken at the apical region of the cells expressing the GTP-RhoA reporter, GFP-AHPH. The inset in the right image corresponds to the same field of view seen by differential interference contrast microscopy (DIC) and shows that the analyzed field belongs to a confluent epithelial monolayer. Note also that the optical (X-Y) plane of the imaging captures the apical surface of the epithelium.
Fig 2.
Phase-portrait of the bistable spatially uniform system described by Eq (1).
The steady states are at the intersections of the nullclines (red and yellow curves), the light green/blue curves show the stable/unstable manifolds of the unstable steady state, and the arrows show the direction and rate of change at different locations on the RhoA-NMIIA phase-plane. The parameters are: α = 1, κ1 = κ2 = 0.4, n = 4. For this choice of parameters the location of the unstable saddle point is closer to the low fixed point at origin.
Fig 3.
Space-time plots of the concentration field RhoA(x,t) and flow field v(x,t): top row: Pe = 10, the front propagates with constant speed; bottom row: Pe = 12, stationary RhoA zone; the other parameters are: α = 1, κ1 = κ2 = 0.2, K = 1
Fig 4.
The width of the contractile zone (with higher concentrations of RhoA and NMIIA), defined as the integral of the RhoA concentration over the whole domain, as a function of time, starting from a localized initial condition for a range of different contraction strengths (Pe = 2 to 16).
Note, that i) the slope (i.e. propagation speed) decreases as Pe is increased and there is a transition to a stationary contractile zone when Pe > 10. (α = 1, κ1 = κ2 = 0.2, K = 1); ii) the curves corresponding to Pe = 0–10 propagate with constant speed until they reach the end of the computational domain.
Fig 5.
Propagation speed of the front (A), width of the stationary active zone (B) as a function of the Peclet number for α = 1, 4 and 10. (C) Example profile of stationary active zone and the corresponding flow profile (Pe = 15, α = 1). (D) Example of a stationary active zone in simulations in two dimensions (Pe = 15, α = 1).
Fig 6.
Numerical results for the system in the case when only the force-generating component (NMIIA) is transported by the flow.
(A) Stationary active zone (Pe = 35, α = 1). (B) The critical Peclet number (i.e. the value at which Pe trends when c→0, see also Fig 5A) as a function of the strength of the bistable signaling (parameter α), in the case when both components are affected by the flow (continuous line) and when only the contractile component (NMIIA) is advected (dashed line).
Fig 7.
Response of the GTP-RhoA zone at the zonula adherens to changes in actomyosin contractility.
Spinning disk time lapse imaging of confluent MCF-7 epithelial cell monolayers expressing the GTP-RhoA reporter GFP-AHPH in control cells (Control, see also S1 Movie); before and after Y-27632 addition (60 μM, time of addition = 3 min, +Y-27632, see also S2 Movie) and before and after Y-27632 washout (S3 Movie). In the last case cells were pre-treated with 60 μM Y-27632 for 1 hour. A-C. Snapshot of the apical region of cells showing the distribution of GTP-RhoA at 0 and 120 min of image acquisition (A), before and after 40 min of Y-27632 treatment (B) and before and 150 min after removal of Y-27632 (C). D. Kymograph analysis of the GTP-RhoA zone for the conditions in A-C. Panels correspond to the analysis of the cell-cell junction indicated in A-C and S1, S2 and S4 Movies. E and F. Quantitation of the temporal changes (normalized to time t = 0) of the width (E) and mean fluorescence intensity (F) of the GTP-RhoA zone at the epithelial zonula adherens. Plots are mean ± SEM from 9 junctions from 3 independent movies.
Fig 8.
Space-time plot of the response to an external perturbation at t = 40, that decreases the stability of NMIIA by a 2-fold increase of its decay rate and at the same time the contractility of NMIIA is switched off (i.e. Pe = 0).
The initial parameters are: Pe = 8, α = 0.02, κ1 = κ2 = 0.2, K = 1.
Fig 9.
Space-time plots in the case when only the force-generating component (NMIIA) is transported by the flow.
showing that for intermediate Pe there is a clustering instability of the contractile component inside the expanding active zone (Pe = 20, α = 1).