Phosphorylation-mediated regulation of GEFs for RhoA

Annexin A1 protects epidermal stem cells against ultraviolet-B irradiation-induced mitochondrial dysfunction

Ultraviolet-B (UV-B) radiation overexposure causes function impairment of epidermal stem cells (ESCs). We explored the mechanism of Annexin A1 (ANXA1) ameliorating UV-B-induced ESC mitochondrial dysfunction/cell injury. ESCs were cultured in vitro and irradiated with different doses of UV-B. Cell viability/ANXA1 protein level were assessed. After oe-ANXA1 transfection, ESCs were treated with oe-ANXA1/UV-B irradiation/CCCP/CCG-1423/3-methyladenine for 12 h. Cell viability/death, and adenosine triphosphate (ATP)/reactive oxygen species (ROS) levels were determined. Mitochondrial membrane potential (MMP) changes/DNA (mtDNA) content/oxygen consumption and RhoA activation were assessed. ROCK1/p-MYPT1/MYPT1/(LC3BII/I)/Beclin-1/p62 protein levels were determined. Mitochondrial morphology was observed. Mito-Tracker Green (MTG) and LC3B levels were determined. UV-B irradiation decreased cell viability/ANXA1 expression in a dose-dependent manner. UV-B-treated ESCs exhibited reduced cell viability/ATP content/MMP level/mitochondrial respiratory control ratio/mtDNA number/RhoA activity/MYPT1 phosphorylation/MTG+LC3B+ cells/(LC3BII/I) and Beclin-1 proteins, increased cell death/ROS/p62/IL-1β/IL-6/TNF-α expression, contracted mitochondrial, disappeared mitochondrial cristae, and increased vacuolar mitochondria, which were averted by ANXA1 overexpression, suggesting that UV-B induced ESC mitochondrial dysfunction/cell injury/inflammation by repressing mitophagy, but ANXA1 promoted mitophagy by activating the RhoA/ROCK1 pathway, thus repressing UV-B's effects. Mitophagy activation ameliorated UV-B-caused ESC mitochondrial dysfunction/cell injury/inflammation. Mitophagy inhibition partly diminished ANXA1-ameliorated UV-B's effects. Conjointly, ANXA1 promoted mitophagy by activating the RhoA/ROCK1 pathway, thereby improving UV-B-induced ESC mitochondrial dysfunction/cell injury.

Lfc subcellular localization and activity is controlled by αv-class integrin

Fibronectin (FN)-binding integrins control a variety of cellular responses through Rho GTPases. The FN-binding integrins, αvβ3 and α5β1, are known to induce different effects on cell morphology and motility. Here, we report that FN-bound αvβ3 integrin, but not FN-bound α5β1 integrin, triggers the dissociation of the RhoA GEF Lfc (also known as GEF-H1 and ARHGEF2 in humans) from microtubules (MTs), leading to the activation of RhoA, formation of stress fibres and maturation of focal adhesions (FAs). Conversely, loss of Lfc expression decreases RhoA activity, stress fibre formation and FA size, suggesting that Lfc is the major GEF downstream of FN-bound αvβ3 that controls RhoA activity. Mechanistically, FN-engaged αvβ3 integrin activates a kinase cascade involving MARK2 and MARK3, which in turn leads to phosphorylation of several phospho-sites on Lfc. In particular, S151 was identified as the main site involved in the regulation of Lfc localization and activity. Our findings indicate that activation of Lfc and RhoA is orchestrated in FN-adherent cells in an integrin-specific manner.

PI Kinase-EhGEF2-EhRho5 axis contributes to LPA stimulated macropinocytosis in Entamoeba histolytica

Entamoeba histolytica is a protozoan responsible for several pathologies in humans. Trophozoites breach the intestinal site to enter the bloodstream and thus traverse to a secondary site. Macropinocytosis and phagocytosis, collectively accounting for heterophagy, are the two major processes responsible for sustenance of Entamoeba histolytica within the host. Both of these processes require significant rearrangements in the structure to entrap the target. Rho GTPases play an indispensable role in mustering proteins that regulate cytoskeletal remodelling. Unlike phagocytosis which has been studied in extensive detail, information on machinery of macropinocytosis in E. histolytica is still limited. In the current study, using site directed mutagenesis and RNAi based silencing, coupled with functional studies, we have demonstrated the involvement of EhRho5 in constitutive and LPA stimulated macropinocytosis. We also report that LPA, a bioactive phospholipid present in the bloodstream of the host, activates EhRho5 and translocates it from cytosol to plasma membrane and endomembrane compartments. Using biochemical and FRAP studies, we established that a PI Kinase acts upstream of EhRho5 in LPA mediated signalling. We further identified EhGEF2 as a guanine nucleotide exchange factor of EhRho5. In the amoebic trophozoites, EhGEF2 depletion leads to reduced macropinocytic efficiency of trophozoites, thus phenocopying its substrate. Upon LPA stimulation, EhGEF2 is found to sequester near the plasma membrane in a wortmannin sensitive fashion, explaining a possible mode for activation of EhRho5 in the amoebic trophozoites. Collectively, we propose that LPA stimulated macropinocytosis in E. histolytica is driven by the PI Kinase-EhGEF2-EhRho5 axis.

β-Arrestin2 Is Critically Involved in the Differential Regulation of Phosphosignaling Pathways by Thyrotropin-Releasing Hormone and Taltirelin

In recent years, thyrotropin-releasing hormone (TRH) and its analogs, including taltirelin (TAL), have demonstrated a range of effects on the central nervous system that represent potential therapeutic agents for the treatment of various neurological disorders, including neurodegenerative diseases. However, the molecular mechanisms of their actions remain poorly understood. In this study, we investigated phosphosignaling dynamics in pituitary GH1 cells affected by TRH and TAL and the putative role of β-arrestin2 in mediating these effects. Our results revealed widespread alterations in many phosphosignaling pathways involving signal transduction via small GTPases, MAP kinases, Ser/Thr- and Tyr-protein kinases, Wnt/β-catenin, and members of the Hippo pathway. The differential TRH- or TAL-induced phosphorylation of numerous proteins suggests that these ligands exhibit some degree of biased agonism at the TRH receptor. The different phosphorylation patterns induced by TRH or TAL in β-arrestin2-deficient cells suggest that the β-arrestin2 scaffold is a key factor determining phosphorylation events after TRH receptor activation. Our results suggest that compounds that modulate kinase and phosphatase activity can be considered as additional adjuvants to enhance the potential therapeutic value of TRH or TAL.

Angiopoietin-2-induced lymphatic endothelial cell migration drives lymphangiogenesis via the β1 integrin-RhoA-formin axis

Lymphangiogenesis is an essential physiological process but also a determining factor in vascular-related pathological conditions. Angiopoietin-2 (Ang2) plays an important role in lymphatic vascular development and function and its upregulation has been reported in several vascular-related diseases, including cancer. Given the established role of the small GTPase RhoA on cytoskeleton-dependent endothelial functions, we investigated the relationship between RhoA and Ang2-induced cellular activities. This study shows that Ang2-driven human dermal lymphatic endothelial cell migration depends on RhoA. We demonstrate that Ang2-induced migration is independent of the Tie receptors, but dependent on β1 integrin-mediated RhoA activation with knockdown, pharmacological approaches, and protein sequencing experiments. Although the key proteins downstream of RhoA, Rho kinase (ROCK) and myosin light chain, were activated, blockade of ROCK did not abrogate the Ang2-driven migratory effect. However, formins, an alternative target of RhoA, were identified as key players, and especially FHOD1. The Ang2-RhoA relationship was explored in vivo, where lymphatic endothelial RhoA deficiency blocked Ang2-induced lymphangiogenesis, highlighting RhoA as an important target for anti-lymphangiogenic treatments.

Regulation of Leukaemia Associated Rho GEF (LARG/ARHGEF12)

The Ras homologous (Rho) protein family of GTPases (RhoA, RhoB and RhoC) are the members of the Ras superfamily and regulate cellular processes such as cell migration, proliferation, polarization, adhesion, gene transcription and cytoskeletal structure. Rho GTPases function as molecular switches that cycle between GTP-bound (active state) and GDP-bound (inactive state) forms. Leukaemia-associated RhoGEF (LARG) is a guanine nucleotide exchange factor (GEF) that activates RhoA subfamily GTPases by promoting the exchange of GDP for GTP. LARG is selective for RhoA subfamily GTPases and is an essential regulator of cell migration and invasion. Here, we describe the mechanisms by which LARG is regulated to facilitate the understanding of how LARG mediates functions like cell motility and to provide insight for better therapeutic targeting of these functions.

GPCRs that Rhoar the Guanine nucleotide exchange factors

Cell migration, a crucial step in numerous biological processes, is tightly regulated in space and time. Cells employ Rho GTPases, primarily Rho, Rac, and Cdc42, to regulate their motility. Like other small G proteins, Rho GTPases function as biomolecular switches in regulating cell migration by operating between GDP bound 'OFF' and GTP bound 'ON' states. Guanine nucleotide exchange factors (GEFs) catalyse the shuttling of GTPases from OFF to ON state. G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors that are involved in many signalling phenomena including cell survival and cell migration events. In this review, we summarize signalling mechanisms, involving GPCRs, leading to the activation of RhoGEFs. GPCRs exhibit diverse GEF activation modes that include the interaction of heterotrimeric G protein subunits with different domains of GEFs, phosphorylation, protein-protein interaction, protein-lipid interaction, and/or a combination of these processes.

Proteomic Analysis Unveils Expressional Changes in Cytoskeleton- and Synaptic Plasticity-Associated Proteins in Rat Brain Six Months after Withdrawal from Morphine

Drug withdrawal is associated with abstinence symptoms including deficits in cognitive functions that may persist even after prolonged discontinuation of drug intake. Cognitive deficits are, at least partially, caused by alterations in synaptic plasticity but the precise molecular mechanisms have not yet been fully identified. In the present study, changes in proteomic and phosphoproteomic profiles of selected brain regions (cortex, hippocampus, striatum, and cerebellum) from rats abstaining for six months after cessation of chronic treatment with morphine were determined by label-free quantitative (LFQ) proteomic analysis. Interestingly, prolonged morphine withdrawal was found to be associated especially with alterations in protein phosphorylation and to a lesser extent in protein expression. Gene ontology (GO) term analysis revealed enrichment in biological processes related to synaptic plasticity, cytoskeleton organization, and GTPase activity. More specifically, significant changes were observed in proteins localized in synaptic vesicles (e.g., synapsin-1, SV2a, Rab3a), in the active zone of the presynaptic nerve terminal (e.g., Bassoon, Piccolo, Rims1), and in the postsynaptic density (e.g., cadherin 13, catenins, Arhgap35, Shank3, Arhgef7). Other differentially phosphorylated proteins were associated with microtubule dynamics (microtubule-associated proteins, Tppp, collapsin response mediator proteins) and the actin–spectrin network (e.g., spectrins, adducins, band 4.1-like protein 1). Taken together, a six-month morphine withdrawal was manifested by significant alterations in the phosphorylation of synaptic proteins. The altered phosphorylation patterns modulating the function of synaptic proteins may contribute to long-term neuroadaptations induced by drug use and withdrawal.

Protein tyrosine phosphatases in cell adhesion

Adhesive structures between cells and with the surrounding matrix are essential for the development of multicellular organisms. In addition to providing mechanical integrity, they are key signalling centres providing feedback on the extracellular environment to the cell interior, and vice versa. During development, mitosis and repair, cell adhesions must undergo extensive remodelling. Post-translational modifications of proteins within these complexes serve as switches for activity. Tyrosine phosphorylation is an important modification in cell adhesion that is dynamically regulated by the protein tyrosine phosphatases (PTPs) and protein tyrosine kinases. Several PTPs are implicated in the assembly and maintenance of cell adhesions, however, their signalling functions remain poorly defined. The PTPs can act by directly dephosphorylating adhesive complex components or function as scaffolds. In this review, we will focus on human PTPs and discuss their individual roles in major adhesion complexes, as well as Hippo signalling. We have collated PTP interactome and cell adhesome datasets, which reveal extensive connections between PTPs and cell adhesions that are relatively unexplored. Finally, we reflect on the dysregulation of PTPs and cell adhesions in disease.

GDF-5 induces epidermal stem cell migration via RhoA-MMP9 signalling

The migration of epidermal stem cells (EpSCs) is critical for wound re-epithelization and wound healing. Recently, growth/differentiation factor-5 (GDF-5) was discovered to have multiple biological effects on wound healing; however, its role in EpSCs remains unclear. In this work, recombinant mouse GDF-5 (rmGDF-5) was found via live imaging in vitro to facilitate the migration of mouse EpSCs in a wound-scratch model. Western blot and real-time PCR assays demonstrated that the expression levels of RhoA and matrix metalloproteinase-9 (MMP9) were correlated with rmGDF-5 concentration. Furthermore, we found that rmGDF-5 stimulated mouse EpSC migration in vitro by regulating MMP9 expression at the mRNA and protein levels through the RhoA signalling pathway. Moreover, in a deep partial-thickness scald mouse model in vivo, GDF-5 was confirmed to promote EpSC migration and MMP9 expression via RhoA, as evidenced by the tracking of cells labelled with 5-bromo-2-deoxyuridine (BrdU). The current study showed that rmGDF-5 can promote mouse EpSC migration in vitro and in vivo and that GDF-5 can trigger the migration of EpSCs via RhoA-MMP9 signalling.

RhoA as a Key Regulator of Innate and Adaptive Immunity

RhoA is a ubiquitously expressed cytoplasmic protein that belongs to the family of small GTPases. RhoA acts as a molecular switch that is activated in response to binding of chemokines, cytokines, and growth factors, and via mDia and the ROCK signaling cascade regulates the activation of cytoskeletal proteins, and other factors. This review aims to summarize our current knowledge on the role of RhoA as a general key regulator of immune cell differentiation and function. The contribution of RhoA for the primary functions of innate immune cell types, namely neutrophils, macrophages, and conventional dendritic cells (DC) to (i) get activated by pathogen-derived and endogenous danger signals, (ii) migrate to sites of infection and inflammation, and (iii) internalize pathogens has been fairly established. In activated DC, which constitute the most potent antigen-presenting cells of the immune system, RhoA is also important for the presentation of pathogen-derived antigen and the formation of an immunological synapse between DC and antigen-specific T cells as a prerequisite to induce adaptive T cell responses. In T cells and B cells as the effector cells of the adaptive immune system Rho signaling is pivotal for activation and migration. More recently, mutations of Rho and Rho-modulating factors have been identified to predispose for autoimmune diseases and as causative for hematopoietic malignancies.

NDR2 kinase contributes to cell invasion and cytokinesis defects induced by the inactivation of RASSF1A tumor-suppressor gene in lung cancer cells

Background

RASSF1A, a tumor suppressor gene, is frequently inactivated in lung cancer leading to a YAP-dependent epithelial-mesenchymal transition (EMT). Such effects are partly due to the inactivation of the anti-migratory RhoB GTPase via the inhibitory phosphorylation of GEF-H1, the GDP/GTP exchange factor for RhoB. However, the kinase responsible for RhoB/GEF-H1 inactivation in RASSF1A-depleted cells remained unknown.

Methods

NDR1/2 inactivation by siRNA or shRNA effects on epithelial-mesenchymal transition, invasion, xenograft formation and growth in SCID−/− Beige mice, apoptosis, proliferation, cytokinesis, YAP/TAZ activation were investigated upon RASSF1A loss in human bronchial epithelial cells (HBEC).

Results

We demonstrate here that depletion of the YAP-kinases NDR1/2 reverts migration and metastatic properties upon RASSF1A loss in HBEC. We show that NDR2 interacts directly with GEF-H1 (which contains the NDR phosphorylation consensus motif HXRXXS/T), leading to GEF-H1 phosphorylation. We further report that the RASSF1A/NDR2/GEF-H1/RhoB/YAP axis is involved in proper cytokinesis in human bronchial cells, since chromosome proper segregation are NDR-dependent upon RASSF1A or GEF-H1 loss in HBEC.

Conclusion

To summarize, our data support a model in which, upon RASSF1A silencing, NDR2 gets activated, phosphorylates and inactivates GEF-H1, leading to RhoB inactivation. This cascade induced by RASSF1A loss in bronchial cells is responsible for metastasis properties, YAP activation and cytokinesis defects.

Electronic supplementary material

The online version of this article (10.1186/s13046-019-1145-8) contains supplementary material, which is available to authorized users.

Emerging regulators of vascular smooth muscle cell migration

Vascular smooth muscle cells (VSMCs) are the predominant cell type in the blood vessel wall and normally adopt a quiescent, contractile phenotype. VSMC migration is tightly controlled, however, disease associated changes in the soluble and insoluble environment promote VSMC migration. Classically, studies investigating VSMC migration have described the influence of soluble factors. Emerging data has highlighted the importance of insoluble factors, including extracellular matrix stiffness and porosity. In this review, we will recap on the important signalling pathways that regulate VSMC migration and reflect on the potential importance of emerging regulators of VSMC function.

Phosphoproteomics of cAMP signaling of Bordetella adenylate cyclase toxin in mouse dendritic cells

The adenylate cyclase toxin (CyaA) of the whooping cough agent Bordetella pertussis subverts immune functions of host myeloid cells expressing the αMβ2 integrin (CD11b/CD18, CR3 or Mac-1). CyaA delivers into cytosol of cells an extremely catalytically active adenylyl cyclase enzyme, which disrupts the innate and adaptive immune functions of phagocytes through unregulated production of the key signaling molecule cAMP. We have used phosphoproteomics to analyze cAMP signaling of CyaA in murine bone marrow-derived dendritic cells. CyaA action resulted in alterations of phosphorylation state of a number of proteins that regulate actin cytoskeleton homeostasis, including Mena, Talin-1 and VASP. CyaA action repressed mTOR signaling through activation of mTORC1 inhibitors TSC2 and PRAS40 and altered phosphorylation of multiple chromatin remodelers, including the class II histone deacetylase HDAC5. CyaA toxin action further elicited inhibitory phosphorylation of SIK family kinases involved in modulation of immune response and provoked dephosphorylation of the transcriptional coactivator CRTC3, indicating that CyaA-promoted nuclear translocation of CRTC3 may account for CyaA-induced IL-10 production. These findings document the complexity of subversive physiological manipulation of myeloid phagocytes by the CyaA toxin, serving in immune evasion of the pertussis agent.

Tyr42 phosphorylation of RhoA GTPase promotes tumorigenesis through nuclear factor (NF)-κB

Dysregulation of reactive oxygen species (ROS) levels is implicated in the pathogenesis of several diseases, including cancer. However, the molecular mechanisms for ROS in tumorigenesis have not been well established. In this study, hydrogen peroxide activated nuclear factor-κB (NF-κB) and RhoA GTPase. In particular, we found that hydrogen peroxide lead to phosphorylation of RhoA at Tyr42 via tyrosine kinase Src. Phospho-Tyr42 (p-Tyr42) residue of RhoA is a binding site for Vav2, a guanine nucleotide exchange factor (GEF), which then activates p-Tyr42 form of RhoA. P-Tyr42 RhoA then binds to IκB kinase γ (IKKγ), leading to IKKβ activation. Furthermore, RhoA WT and phospho-mimic RhoA, RhoA Y42E, both promoted tumorigenesis, whereas the dephospho-mimic RhoA, RhoA Y42F suppressed it. In addition, hydrogen peroxide induced NF-κB activation and cell proliferation, along with expression of c-Myc and cyclin D1 in the presence of RhoA WT and RhoA Y42E, but not RhoA Y42F. Indeed, levels of p-Tyr42 Rho, p-Src, and p-65 are significantly increased in human breast cancer tissues and show correlations between each of the two components. Conclusively, the posttranslational modification of as RhoA p-Tyr42 may be essential for promoting tumorigenesis in response to generation of ROS.

Modelling GTPase dynamics to understand RhoA-driven cancer cell invasion

Metastasis, initially driven by cells migrating and invading through the local environment, leads to most cancer-associated deaths. Cells can use a variety of modes to move in vitro, all of which depend on Rho GTPases at some level. While traditionally it was thought that Rac1 activity drives protrusive lamellipodia at the leading edge of a polarised cell while RhoA drives rear retraction, more recent work in 3D microenvironments has revealed a much more complicated picture of GTPase dynamics. In particular, RhoA activity can dominate the leading edge polymerisation of actin to form filopodial actin-spike protrusions that drive more invasive cell migration. We recently described a potential mechanism to abrogate this pro-invasive localised leading edge Rac1 to RhoA switch via manipulation of a negative feedback loop that was revealed by adopting a logical modelling approach. Both challenging dogma and taking a formal, mathematical approach to understanding signalling involved in motility may be vital to harnessing harmful cell migration and preventing metastasis in future research.

RhoA promotes epidermal stem cell proliferation via PKN1-cyclin D1 signaling

OBJECTIVE

Epidermal stem cells (ESCs) play a critical role in wound healing, but the mechanism underlying ESC proliferation is not well defined. Here, we explore the effects of RhoA on ESC proliferation and the possible underlying mechanism.

METHODS

Human ESCs were enriched by rapid adhesion to collagen IV. RhoA(+/+)(G14V), RhoA(-/-)(T19N) and pGFP control plasmids were transfected into human ESCs. The effect of RhoA on cell proliferation was detected by cell proliferation and DNA synthesis assays. Induction of PKN1 activity by RhoA was determined by immunoblot analysis, and the effects of PKN1 on RhoA in terms of inducing cell proliferation and cyclin D1 expression were detected using specific siRNA targeting PKN1. The effects of U-46619 (a RhoA agonist) and C3 transferase (a RhoA antagonist) on ESC proliferation were observed in vivo.

RESULTS

RhoA had a positive effect on ESC proliferation, and PKN1 activity was up-regulated by the active RhoA mutant (G14V) and suppressed by RhoA T19N. Moreover, the ability of RhoA to promote ESC proliferation and DNA synthesis was interrupted by PKN1 siRNA. Additionally, cyclin D1 protein and mRNA expression levels were up-regulated by RhoA G14V, and these effects were inhibited by siRNA-mediated knock-down of PKN1. RhoA also promoted ESC proliferation via PKN in vivo.

CONCLUSION

This study shows that the effect of RhoA on ESC proliferation is mediated by activation of the PKN1-cyclin D1 pathway in vitro, suggesting that RhoA may serve as a new therapeutic target for wound healing.

Inhibition of Uterine Contractility by Thalidomide Analogs via Phosphodiesterase-4 Inhibition and Calcium Entry Blockade

Uterine relaxation is crucial during preterm labor. Phosphodiesterase-4 (PDE-4) inhibitors have been proposed as tocolytics. Some thalidomide analogs are PDE-4 inhibitors. The aim of this study was to assess the uterus-relaxant properties of two thalidomide analogs, methyl 3-(4-nitrophthalimido)-3-(3,4-dimethoxyphenyl)-propanoate (4NO2PDPMe) and methyl 3-(4-aminophthalimido)-3-(3,4-dimethoxyphenyl)-propanoate (4APDPMe) and were compared to rolipram in functional studies of spontaneous phasic, K⁺-induced tonic, and Ca²⁺-induced contractions in isolated pregnant human myometrial tissues. The accumulation of cAMP was quantified in HeLa cells. The presence of PDE-4B2 and phosphorylated myosin light-chain (pMLC), in addition to the effect of thalidomide analogs on oxytocin-induced pMLC, were assessed in human uterine myometrial cells (UtSMCs). Thalidomide analogs had concentration-dependent inhibitory effects on spontaneous and tonic contractions and inhibited Ca²⁺-induced responses. Tonic contraction was equipotently inhibited by 4APDPMe and rolipram (IC₅₀ = 125 ± 13.72 and 98.45 ± 8.86 µM, respectively). Rolipram and the thalidomide analogs inhibited spontaneous and tonic contractions equieffectively. Both analogs increased cAMP accumulation in a concentration-dependent manner (p < 0.05) and induced changes in the subcellular localization of oxytocin-induced pMLC in UtSMCs. The inhibitory effects of thalidomide analogs on the contractions of pregnant human myometrium tissue may be due to their PDE-4 inhibitory effect and novel mechanism as calcium-channel blockers.

Rationalizing Rac1 and RhoA GTPase signaling: A mathematical approach

Precise spatiotemporal dynamics of Rho GTPases are essential for efficient cell migration. Manipulating Rac1 and RhoA signaling is thus a potential intervention strategy to abrogate harmful cell invasion and subsequent metastasis; however GTPase signaling can be extremely complicated due to crosstalk and the multitude of upstream regulators and downstream effectors. Studying Rho GTPase networks in a formal mathematical setting can therefore be of great use. We recently built a predictive model based on Boolean logic which identified a negative feedback loop critical for RhoA and Rac1 activity. Here, we discuss the value and potential pitfalls of different mathematical approaches which have been used to study Rho GTPase dynamics, and highlight the importance of choosing the correct approach given the data available and outputs desired. Overall, a mathematical approach, particularly when combined iteratively with in vitro experiments, can be of great use in deriving new biological insight to further harness the activity of Rho GTPases.

Smad-independent TGF-β2 signaling pathways in human trabecular meshwork cells

Aberrant expression and signaling of Transforming Growth Factor (TGF)-β is strongly associated with development of elevated intraocular pressure (IOP) and primary open-angle glaucoma (POAG). In cells of the trabecular meshwork, a key component of the conventional outflow pathway, TGF-β is well-known to promote expression of multiple ocular hypertensive mediators, including genes associated with fibrosis as well as cellular contractility. These effects are mediated by induction of canonical (Smad) as well as non-canonical (MAPK, Rho GTPase) signaling cascades. In the present review, we will highlight the non-canonical, Smad-independent signaling pathways activated by TGF-β2 in human TM cells, as well as the genes known to be induced by non-canonical TGF-β2 signaling.

A MAPK-Driven Feedback Loop Suppresses Rac Activity to Promote RhoA-Driven Cancer Cell Invasion

Cell migration in 3D microenvironments is fundamental to development, homeostasis and the pathobiology of diseases such as cancer. Rab-coupling protein (RCP) dependent co-trafficking of α5β1 and EGFR1 promotes cancer cell invasion into fibronectin (FN) containing extracellular matrix (ECM), by potentiating EGFR1 signalling at the front of invasive cells. This promotes a switch in RhoGTPase signalling to inhibit Rac1 and activate a RhoA-ROCK-Formin homology domain-containing 3 (FHOD3) pathway and generate filopodial actin-spike protrusions which drive invasion. To further understand the signalling network that drives RCP-driven invasive migration, we generated a Boolean logical model based on existing network pathways/models, where each node can be interrogated by computational simulation. The model predicted an unanticipated feedback loop, whereby Raf/MEK/ERK signalling maintains suppression of Rac1 by inhibiting the Rac-activating Sos1-Eps8-Abi1 complex, allowing RhoA activity to predominate in invasive protrusions. MEK inhibition was sufficient to promote lamellipodia formation and oppose filopodial actin-spike formation, and led to activation of Rac and inactivation of RhoA at the leading edge of cells moving in 3D matrix. Furthermore, MEK inhibition abrogated RCP/α5β1/EGFR1-driven invasive migration. However, upon knockdown of Eps8 (to suppress the Sos1-Abi1-Eps8 complex), MEK inhibition had no effect on RhoGTPase activity and did not oppose invasive migration, suggesting that MEK-ERK signalling suppresses the Rac-activating Sos1-Abi1-Eps8 complex to maintain RhoA activity and promote filopodial actin-spike formation and invasive migration. Our study highlights the predictive potential of mathematical modelling approaches, and demonstrates that a simple intervention (MEK-inhibition) could be of therapeutic benefit in preventing invasive migration and metastasis.

The majority of cancer-related fatalities are caused by the movement of cancer cells away from the primary site to form metastases, making understanding the signalling mechanisms which underpin cell migration and invasion through their local environment of paramount importance. Much has been discovered about key events leading to invasive cell migration. Here, we have taken this prior knowledge to build a powerful predictive model based on simple ON/OFF relations and logic to determine potential intervention targets to reduce harmful invasive migration. Interrogating our model, we have identified a negative feedback loop important to the signalling that determines invasive migration, the breaking of which reverts cells to a slower, less invasive phenotype. We have supported this feedback loop prediction using an array of in vitro experiments performed in cells within 2-D and physiologically relevant 3-D environments. Our findings demonstrate the predictive power of such modelling techniques, and could form the basis for clinical intervention to prevent metastasis in certain cancers.

Rho protein GTPases and their interactions with NFκB: crossroads of inflammation and matrix biology

The RhoGTPases, with RhoA, Cdc42 and Rac being major members, are a group of key ubiquitous proteins present in all eukaryotic organisms that subserve such important functions as cell migration, adhesion and differentiation. The NFκB (nuclear factor κB) is a family of constitutive and inducible transcription factors that through their diverse target genes, play a major role in processes such as cytokine expression, stress regulation, cell division and transformation. Research over the past decade has uncovered new molecular links between the RhoGTPases and the NFκB pathway, with the RhoGTPases playing a positive or negative regulatory role on NFκB activation depending on the context. The RhoA–NFκB interaction has been shown to be important in cytokine-activated NFκB processes, such as those induced by TNFα (tumour necrosis factor α). On the other hand, Rac is important for activating the NFκB response downstream of integrin activation, such as after phagocytosis. Specific residues of Rac1 are important for triggering NFκB activation, and mutations do obliterate this response. Other upstream triggers of the RhoGTPase–NFκB interactions include the suppressive p120 catenin, with implications for skin inflammation. The networks described here are not only important areas for further research, but are also significant for discovery of targets for translational medicine.