Feedback regulation through myosin II confers robustness on RhoA signalling at E-cadherin junctions

Coronin 1B supports RhoA signaling at cell-cell junctions through Myosin II

Non-muscle myosin II (NMII) motor proteins are responsible for generating contractile forces inside eukaryotic cells. There is also a growing interest in the capacity for these motor proteins to influence cell signaling through scaffolding, especially in the context of RhoA GTPase signaling. We previously showed that NMIIA accumulation and stability within specific regions of the cell cortex, such as the zonula adherens (ZA), allows the formation of a stable RhoA signaling zone. Now we demonstrate a key role for Coronin 1B in maintaining this junctional pool of NMIIA, as depletion of Coronin 1B significantly compromised myosin accumulation and stability at junctions. The loss of junctional NMIIA, upon Coronin 1B knockdown, perturbed RhoA signaling due to enhanced junctional recruitment of the RhoA antagonist, p190B Rho GAP. This effect was blocked by the expression of phosphomimetic MRLC-DD, thus reinforcing the central role of NMII in regulating RhoA signaling.

Cycling Rho for tissue contraction

Cell contractility, driven by the RhoA GTPase, is a fundamental determinant of tissue morphogenesis. In this issue, Mason et al. (2016. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201603077) reveal that cyclic inactivation of RhoA, mediated by its antagonist, C-GAP, is essential for effective contractility to occur.

RhoA GTPase inhibition organizes contraction during epithelial morphogenesis

Mason et al. show that RhoA activity is regulated in space and time by a GEF/GAP module that tunes cell behavior and is required for proper tissue folding and shape during Drosophila morphogenesis.

During morphogenesis, contraction of the actomyosin cytoskeleton within individual cells drives cell shape changes that fold tissues. Coordination of cytoskeletal contractility is mediated by regulating RhoA GTPase activity. Guanine nucleotide exchange factors (GEFs) activate and GTPase-activating proteins (GAPs) inhibit RhoA activity. Most studies of tissue folding, including apical constriction, have focused on how RhoA is activated by GEFs to promote cell contractility, with little investigation as to how GAPs may be important. Here, we identify a critical role for a RhoA GAP, Cumberland GAP (C-GAP), which coordinates with a RhoA GEF, RhoGEF2, to organize spatiotemporal contractility during Drosophila melanogaster apical constriction. C-GAP spatially restricts RhoA pathway activity to a central position in the apical cortex. RhoGEF2 pulses precede myosin, and C-GAP is required for pulsation, suggesting that contractile pulses result from RhoA activity cycling. Finally, C-GAP expression level influences the transition from reversible to irreversible cell shape change, which defines the onset of tissue shape change. Our data demonstrate that RhoA activity cycling and modulating the ratio of RhoGEF2 to C-GAP are required for tissue folding.

The polarity protein Scribble positions DLC3 at adherens junctions to regulate Rho signaling

The spatial regulation of cellular Rho signaling by GAP proteins is still poorly understood. By performing mass spectrometry, we here identify the polarity protein Scribble as a scaffold for the RhoGAP protein DLC3 (also known as StarD8) at cell-cell adhesions. This mutually dependent interaction is mediated by the PDZ domains of Scribble and a PDZ ligand (PDZL) motif in DLC3. Both Scribble depletion and PDZL deletion abrogated DLC3 junctional localization. Using a RhoA biosensor and a targeted GAP domain, we demonstrate that DLC3 activity locally regulates RhoA-ROCK signaling at and Scribble localization to adherens junctions, and is required for their functional integrity. In a 3D model of cyst development, we furthermore show that DLC3 depletion impairs polarized morphogenesis, phenocopying the effects observed upon Scribble knockdown. We thus propose a new function for Scribble in Rho regulation that entails positioning of DLC3 GAP activity at cell junctions in polarized epithelial cells.

Tension on JAM-A activates RhoA via GEF-H1 and p115 RhoGEF

Forces on JAM-A activate RhoA to increase cell stiffness. Activation of RhoA requires GEF-H1 and p115 RhoGEF activation downstream of FAK/ERK and Src family kinases, respectively.

Junctional adhesion molecule A (JAM-A) is a broadly expressed adhesion molecule that regulates cell–cell contacts and facilitates leukocyte transendothelial migration. The latter occurs through interactions with the integrin LFA-1. Although we understand much about JAM-A, little is known regarding the protein’s role in mechanotransduction or as a modulator of RhoA signaling. We found that tension imposed on JAM-A activates RhoA, which leads to increased cell stiffness. Activation of RhoA in this system depends on PI3K-mediated activation of GEF-H1 and p115 RhoGEF. These two GEFs are further regulated by FAK/ERK and Src family kinases, respectively. Finally, we show that phosphorylation of JAM-A at Ser-284 is required for RhoA activation in response to tension. These data demonstrate a direct role of JAM-A in mechanosignaling and control of RhoA and implicate Src family kinases in the regulation of p115 RhoGEF.

Protective effect of Ac-SDKP on alveolar epithelial cells through inhibition of EMT via TGF-β1/ROCK1 pathway in silicosis in rat

The epithelial-mesenchymal transition (EMT) is a critical stage during the development of silicosis fibrosis. In the current study, we hypothesized that the anti-fibrotic tetrapeptide, N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) may exert its anti-fibrotic effects via activation of the TGF-β1/ROCK1 pathway, leading to inhibition of EMT. To address this hypothesis, we first examined the effect of Ac-SDKP upon EMT using an in vivo rat silicosis model, as well as in an in vitro model of TGF-β1-induced EMT. Confocal laser scanning microscopy was used to examine colocalization of surfactant protein A (SP-A), fibroblast specific protein-1 (FSP-1) and α-smooth muscle actin (α-SMA) in vivo. Western blot analysis was used to examine for changes in the protein levels of E-cadherin (E-cad) and SP-A (epithelial cell markers), vimentin (mesenchymal cell marker), α-SMA (active myofibroblast marker), and collagen I and III in both in vivo and in vitro experiments. Secondly, we utilized Western blot analysis and confocal laser scanning microscopy to examine the protein expression of TGF-β1 and ROCK1 in in vivo and in vitro studies. The results revealed that Ac-SDKP treatment prevented increases in the expression of mesenchymal markers as well as TGF-β1, ROCK1, collagen I and III. Furthermore, Ac-SDKP treatment prevented decreases in the expression of epithelial cell markers in both in vivo and in vitro experiments. Based on the results, we conclude that Ac-SDKP inhibits the transition of epithelial cell-myofibroblast in silicosis via activation of the TGF-β1/ROCK1 signaling pathway, which may serve as a novel mechanism by which it exerts its anti-fibrosis properties.

Measurement of Mechanical Tension at Cell-cell Junctions Using Two-photon Laser Ablation

The cortical actomyosin cytoskeleton is found in all non-muscle cells where a key function is to control mechanical force (Salbreux et al., 2012). When coupled to E-cadherin cell-cell adhesion, cortical actomyosin generates junctional tension that influences many aspects of tissue function, organization and morphogenesis (Lecuit and Yap, 2015). Uncovering the molecular mechanisms underlying the generation of junctional tension requires tools for measuring it in live cells with a high spatio-temporal resolution. For this, we have set up a technique of laser ablation, in which we use the high power output of a two-photon laser to physically cut the actin cortex at the sites of cell-cell adhesion labeled with E-cadherin-GFP. Tension, thus is visualized as the outwards recoil of the vertices that define a junction after this was ablated/cut. Analysis of recoil versus time allows extracting parameters related to the amount of contractile force that is applied to the junction before ablation (initial recoil) and the ratio between elasticity of the junction and viscosity of the media (cytoplasm) in which the junctional cortex is immersed. Using this approach we have discovered how Src protein-tyrosine kinase (Gomez et al., 2015); actin-binding proteins such as tropomyosins (Caldwell et al., 2014) and N-WASP (Wu et al., 2014); Myosin II (Priya et al., 2015) and coronin-1B (Michael et al., 2016) contribute to the molecular apparatus responsible for generating tension at the cell-cell junctions. This protocol describes the experimental procedure for setting up laser ablation experiments and how to optimize ablation and acquisition conditions for optimal measurements of junctional tension. It also provides a full description, step by step, of the post-acquisition analysis required to evaluate changes in contractile force as well as cell elasticity and/or cytoplasm viscosity.