Bistability in the Rac1, PAK, and RhoA Signaling Network Drives Actin Cytoskeleton Dynamics and Cell Motility Switches

Abstract

Dynamic interactions between RhoA and Rac1, members of the Rho small GTPase family, play a vital role in the control of cell migration. Using predictive mathematical modeling, mass spectrometry-based quantitation of network components, and experimental validation in MDA-MB-231 mesenchymal breast cancer cells, we show that a network containing Rac1, RhoA, and PAK family kinases can produce bistable, switch-like responses to a graded PAK inhibition. Using a small chemical inhibitor of PAK, we demonstrate that cellular RhoA and Rac1 activation levels respond in a history-dependent, bistable manner to PAK inhibition. Consequently, we show that downstream signaling, actin dynamics, and cell migration also behave in a bistable fashion, displaying switches and hysteresis in response to PAK inhibition. Our results demonstrate that PAK is a critical component in the Rac1-RhoA inhibitory crosstalk that governs bistable GTPase activity, cell morphology, and cell migration switches.

Keywords: PAK inhibition; Rac1; RhoA; bistable switches; cell motility; mathematical modeling.

Graphical abstract

Figure 1

Reaction Scheme and Schematic Interactions of the Rac1-RhoA Network Model (A) Reaction scheme of the kinetic Rac1-RhoA model showing three layers of interconnected regulation of Rac1 and RhoA via PAK (see main text and the Supplemental Information for more details). iPAK and pPAK indicate inactive and active PAK, respectively; we denote PAK Inh as a general but selective PAK inhibitor; and Rac1-GTP, and RhoA-GTP indicate the GTP-bound, active forms of Rac1 and RhoA, respectively. (B) An abstract-level, schematic diagram of the Rac1-RhoA network shows the flow of signaling and highlights the double-negative feedback regulation between Rac1 and RhoA.

Figure 2

The Rac1-RhoA Double-Negative Feedback Loop Can Result in Bistable Behavior (A) Active RhoA-GTP responses to increasing Rac1 abundance in a bistable manner, resulting in abrupt switches of activity level. SS, Steady State. (B) Two-parameter (2D) bifurcation diagrams showing dependence of bistability on the abundance of Rac1 and RhoA. (C) A parallel coordinate plot showing the bistable parameter sets (purple) obtained by assessing the dynamics of 100,000 sets with randomly sampled Rac1, RhoA, PAK, GEF-H1, and 14-3-3 totals within the indicated ranges (in nanomolars). The values are normalized between 0 and 1. A detailed description of the multi-dimensional dynamic analysis and parallel coordinate representation is given in Nguyen et al. (2015) and Supplemental Experimental Procedures. (D) An intuitive simplified illustration of the bistable region in the corresponding 5D parameter space. (E and F) Simulated bistability and hysteresis for active Rac1 and RhoA in response to increasing the PAK inhibitor level. All simulations were carried out with parameter values given in Table S4.

Figure 3

Experimental Validation of RhoA and Rac1 Bistable Switches in MDA-MB-231 Cells (A–D) PAK was inhibited in MDA-MB-231 cells by adding specific inhibitor IPA-3 at the indicated concentrations for 40 min (blue) or by incubating MDA-MB-231 cells with 15 μM IPA-3 for 20 min. The inhibitor was subsequently washed out, and the cells were incubated for an additional 20 min with IPA-3 at the indicated concentrations (red). (A) RhoA-GTP was precipitated with GST-Rhotekin beads and western blotted. (B) Densitometric analysis of three biological replicates. Error bars represent SD. (C) Rac1-GTP was precipitated with GST-PAK-CRIB beads and western blotted. (D) Densitometric analysis of three biological replicates. Error bars represent SD. (E) MDA-MB-231 cells seeded on collagen were treated for 80 min with the indicated concentrations of IPA-3, fixed and stained with an anti-pS19 MLC2 antibody. 20× image. (F) MDA-MB-231 cells seeded on collagen were treated for 80 min with the indicated concentrations of IPA-3, fixed and stained with an anti-pS19 MLC2 antibody. Histogram of single-cell, averaged lamellipodial intensity. (G) MDA-MB-231 cells seeded on collagen were treated for 20 min with 7.5 μM IPA-3. The cells were subsequently washed and treated for an additional 60 min with the indicated concentrations of IPA-3, fixed and stained with an anti-pS19 MLC2 antibody. Histogram of single-cell, averaged lamellipodial intensity. (H) MDA-MB-231 cells seeded on collagen were treated for 80 min with the indicated concentrations of IPA-3, fixed and stained with fluorescent phalloidin. 20× image. (I) MDA-MB-231 cells seeded on collagen were treated for 80 min with the indicated concentrations of IPA-3, fixed and stained with fluorescent phalloidin. Histogram of single-cell, averaged cytoplasmatic intensity. (J) MDA-MB-231 cells seeded on collagen were treated for 20 min with 7.5 μM IPA-2. The cells were subsequently washed and treated for an additional 60 min with the indicated concentrations of IPA-3, fixed and stained with fluorescent phalloidin. Histogram of single-cell, averaged cytoplasmatic intensity.

Figure 4

Actin Dynamics Behave in a Bistable Manner in Response to PAK Inhibition Sparsely seeded MDA-MB-231 cells expressing a LifeAct-mCherry probe were treated with the PAK inhibitor IPA-3 (A) at the indicated concentrations for 60 min (blue) or (B) by incubating cells with 7.5 μM IPA-3 for 20 min. The inhibitor was subsequently washed out, and the cells were incubated for an additional 60 min with IPA-3 at the indicated concentrations (red). Images were taken every 15 s. Montage images represent changes in actin dynamics during the cells’ migration, showing three images (0, 30, and 60 min) over the 1 hr period. Scale bar, 20 μM.

Figure 5

Migration of MDA-MB-231 Cells Is Regulated in a Bistable Manner by PAK (A) Undirected cell migration. Sparsely seeded MDA-MB-231 cells were treated with the PAK inhibitor IPA-3 at the indicated concentrations for 40 min (blue) or by incubating MDA-MB-231 cells with 7.5 μM IPA-3 for 20 min. The inhibitor was subsequently washed out, and the cells were incubated for an additional 20 min with IPA-3 at the indicated concentrations (red). Images were taken every 20 min, and random cell migration of 25 cells was subsequently manually tracked using ImageJ. Each line represents a single cell’s migration in one field of view over 12 hr. The line color shows the distance migrated from 0–3 hr (red), 3–6 hr (orange), 6–9 hr (green), and 9–12 hr (blue). Line origins have been artificially placed at (0,0) for display purposes, but lengths remain unchanged. The width and height of each plot is 200 μm. (B) The mean speed ± SD of three single-cell tracking biological replicates over 12 hr, a representative example of which is found in Figure 5A. The differences between pretreated and non-pretreated 1.185, 2.5, and 3.75 μM IPA-3 were all statistically significant, with p = 0.017, 0.015, and 0.0001, respectively, measured using a two-tailed, non-paired t test. Error bars represent SEM. (C) Directed cell migration. Confluent MDA-MB-231 cells were scratched with a plastic tip. IPA-3 was added at the indicated concentrations for the duration of the experiment (blue) or by incubating MDA-MB-231 cells with 15 μM IPA-3 for 20 min. The inhibitor was subsequently washed out, and the cells were incubated for the remainder of the experiment with IPA-3 at the indicated concentrations (red). Images of the wound were taken at 0 and 18 hr. (D) Graph representing the percentage of wound closure of three biological replicates. Error bars represent SD. (E) MDA-MB-231 cells were sparsely seeded on a thick collagen layer. After 24 hr, they were treated with the PAK inhibitor IPA-3 at the indicated concentrations (non-treated) or by incubating MDA-MB-231 cells with 7.5 μM IPA-3 for 20 min. The inhibitor was subsequently washed out, and the cells were incubated with IPA-3 at the indicated concentrations (pretreated). Images were taken 24 hr later. Histogram represents cell number over cell roundness, with 0 as an infinite line and 1 as a perfect circle.