Mechanotransduction: from the cell surface to the nucleus via RhoA

Abstract

Cells respond and adapt to their physical environments and to the mechanical forces that they experience. The translation of physical forces into biochemical signalling pathways is known as mechanotransduction. In this review, we focus on two aspects of mechanotransduction. First, we consider how forces exerted on cell adhesion molecules at the cell surface regulate the RhoA signalling pathway by controlling the activities of guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GAPs). In the second part of the review, we discuss how the nucleus contributes to mechanotransduction as a physical structure connected to the cytoskeleton. We focus on recent studies that have either severed the connections between the nucleus and the cytoskeleton, or that have entirely removed the nucleus from cells. These actions reduce the levels of active RhoA, thereby altering the mechanical properties of cells and decreasing their ability to generate tension and respond to external mechanical forces. This article is part of a discussion meeting issue 'Forces in cancer: interdisciplinary approaches in tumour mechanobiology'.

Keywords: RhoA; cell adhesion molecules; cytoskeleton; fibrosis; mechanotransduction; nucleus.

Figure 1.

RhoA regulation. RhoA bound to GDP is inactive and can be sequestered by Rho guanine nucleotide dissociation inhibitor (RhoGDI). GDP exchange for GTP is promoted by guanine nucleotide exchange factors (GEFs), whereas GTP hydrolysis is stimulated by GTPase activating proteins (GAPs). In the active, GTP-bound state, RhoA activates the kinase ROCK which promotes contractility and bundling of actin filaments by activating myosin via phosphorylation of the regulatory myosin light chain (MLC). RhoA also binds and activates the formin mDia, stimulating actin polymerization.

Figure 2.

Pathways by which tension on integrins may increase RhoA activity. In (A) the tyrosine kinase Fyn is activated in an unresolved manner. It phosphorylates and activates the Rho GEF LARG. In (B), FAK is activated and via the Ras/Erk pathway activates GEF-H1. In (C), downstream from FAK, PI3 kinase (PI3 K) is activated triggering the activation of AKT, which phosphorylates the Rho GAP DLC1, thereby decreasing its activity. In (D), DLC1 is active when bound to talin but tension transmitted from integrins to talin, stretches talin, releasing DLC1 allowing it to adopt an inactive conformation. Some of these pathways may act in parallel and synergize to stimulate RhoA activity in response to tension exerted on integrins.

Figure 3.

RhoA signalling in fibrosis. A positive feed-forward cycle is illustrated. Transforming growth factor-beta (TGFβ) is released in response to tissue wounding. TGFβ represses synthesis of Rnd3 causing a decrease in p190RhoGAP activity and a consequent increase in RhoA activity. The increase in active RhoA stimulates both myosin-mediated contractility and increased expression of ECM genes. In turn, these both promote increased matrix assembly resulting in a stiffer matrix. The stiff matrix further stimulates elevated RhoA activity. The high levels of contractility and the stiff matrix promote release of more active TGFβ from its inactive matrix-bound state, continuing the cycle.

Figure 4.

Comparison of migration velocities of intact cells and cytoplasts on substrata of different stiffness. Intact REF52 fibroblasts or REF52 cells that had been enucleated to generate cytoplasts were plated on substrata of different rigidity. Cells were plated on polyacrylamide hydrogels of different compliance (Matrigen™) coated with 10 µg ml⁻¹ fibronectin. After allowing cells and cytoplasts to spread for 3 h, individual cells were tracked using an Olympus VivaView microscope and the velocities calculated. Reproduced from Graham et al. [99], originally published in the Journal of Cell Biology, doi:10.1083/jcb.201706097. (Online version in colour.)

Figure 5.

Cytoplasts have decreased RhoA activity compared with intact cells. The levels of active RhoA were measured using the G-Lisa technique (Cytoskeleton™) from parallel cultures of REF52 cells and their corresponding cytoplasts. The experiment was performed in triplicate. Graphical data is represented as a mean with data bars representing s.e.m. *p < 0.05 as determined by t-test.