Rho Exchange Research Articles

{{rho }} exchange contribution to neutrinoless double beta decay

We consider rho meson contributions to neutrinoless double beta decay amplitude stemming from the hadronization of the short distance quark-electron currents. These contributions are evaluated within vacuum dominance approximation. The one and two rho exchange contributions affect the Fermi transition nuclear matrix element in a way that lead to near cancellations in the same chirality, left–left and right–right, short range amplitudes when these new contributions are combined with the conventional short range amplitudes, while the left–right amplitude almost triples. A rough estimate comparing this contribution to the long range pion exchange contribution indicates that rho exchange contribution can easily be mathcal {O}(10%) of the pion contribution. When considering precision predictions, rho exchange amplitudes should be included in any phenomenological study, like in left–right theories.

The Rho guanosine nucleotide exchange factors Vav2 and Vav3 modulate epidermal stem cell function

It is known that Rho GTPases control different aspects of the biology of skin stem cells (SSCs). However, little information is available on the role of their upstream regulators under normal and tumorigenic conditions in this process. To address this issue, we have used here mouse models in which the activity of guanosine nucleotide exchange factors of the Vav subfamily has been manipulated using both gain- and loss-of-function strategies. These experiments indicate that Vav2 and Vav3 regulate the number, functional status, and responsiveness of hair follicle bulge stem cells. This is linked to gene expression programs related to the reinforcement of the identity and the quiescent state of normal SSCs. By contrast, in the case of cancer stem cells, they promote transcriptomal programs associated with the identity, activation state, and cytoskeletal remodeling. These results underscore the role of these Rho exchange factors in the regulation of normal and tumor epidermal stem cells.

Vav2 pharmaco-mimetic mice reveal the therapeutic value and caveats of the catalytic inactivation of a Rho exchange factor.

The current paradigm holds that the inhibition of Rho guanosine nucleotide exchange factors (GEFs), the enzymes that stimulate Rho GTPases, can be a valuable therapeutic strategy to treat Rho-dependent tumors. However, formal validation of this idea using in vivo models is still missing. In this context, it is worth remembering that many Rho GEFs can mediate both catalysis-dependent and independent responses, thus raising the possibility that the inhibition of their catalytic activities might not be sufficient per se to block tumorigenic processes. On the other hand, the inhibition of these enzymes can trigger collateral side effects that could preclude the practical implementation of anti-GEF therapies. To address those issues, we have generated mouse models to mimic the effect of the systemic application of an inhibitor for the catalytic activity of the Rho GEF Vav2 at the organismal level. Our results indicate that lowering the catalytic activity of Vav2 below specific thresholds is sufficient to block skin tumor initiation, promotion, and progression. They also reveal that the negative side effects typically induced by the loss of Vav2 can be bypassed depending on the overall level of Vav2 inhibition achieved in vivo. These data underscore the pros and cons of anti-Rho GEF therapies for cancer treatment. They also support the idea that Vav2 could represent a viable drug target.

Extracellular Signal-Regulated Kinase Regulates RhoA Activation and Tumor Cell Plasticity by Inhibiting Guanine Exchange Factor H1 Activity

In certain Ras mutant cell lines, the inhibition of extracellular signal-regulated kinase (ERK) signaling increases RhoA activity and inhibits cell motility, which was attributed to a decrease in Fra-1 levels. Here we report a Fra-1-independent augmentation of RhoA signaling during short-term inhibition of ERK signaling. Using mass spectrometry-based proteomics, we identified guanine exchange factor H1 (GEF-H1) as mediating this effect. ERK binds to the Rho exchange factor GEF-H1 and phosphorylates it on S959, causing inhibition of GEF-H1 activity and a consequent decrease in RhoA activity. Knockdown experiments and expression of a nonphosphorylatable S959A GEF-H1 mutant showed that this site is crucial in regulating cell motility and invasiveness. Thus, we identified GEF-H1 as a critical ERK effector that regulates motility, cell morphology, and invasiveness.

Vav Proteins' Role in Skin Cancer

Vav Proteins' Role in Skin Cancer

The Rho exchange factors Vav2 and Vav3 favor skin tumor initiation and promotion by engaging extracellular signaling loops.

Author SummaryGDP/GTP exchange factors (GEFs) involved in Rho GTPase activation have been traditionally considered as potential anticancer drug targets. However, little is known about the best GEFs to inhibit in different tumor types, the pro-tumorigenic programs that they regulate, and the collateral effects that inactivation may induce in healthy tissues. Here, we investigate this issue in HRas-dependent skin tumors using genetic techniques. Despite the large number of Rho GEFs present in both normal and tumoral epidermis, we demonstrate that the co-expression of the exchange factors Vav2 and Vav3 is critical for the development of this tumor type. We also identify a previously unknown Vav-dependent autocrine/paracrine program that favors keratinocyte survival/proliferation and the formation of an inflammatory state during the initiation and promotion phases of this tumor. Finally, our results indicate that inactivation of Vav proteins is innocuous for the homeostasis of normal epidermis. Taken together, these results imply that Vav protein-based therapies may be of interest for skin tumor prevention and/or treatment.

The Rho Exchange Factors Vav2 and Vav3 Control a Lung Metastasis–Specific Transcriptional Program in Breast Cancer Cells

The guanosine triphosphatases of the Rho and Rac subfamilies regulate protumorigenic pathways and are activated by guanine nucleotide exchange factors (Rho GEFs), which could be potential targets for anticancer therapies. We report that two Rho GEFs, Vav2 and Vav3, play synergistic roles in breast cancer by sustaining tumor growth, neoangiogenesis, and many of the steps involved in lung-specific metastasis. The involvement of Vav proteins in these processes did not correlate with Rac1 and RhoA activity or cell migration, implying the presence of additional biological programs. Microarray analyses revealed that Vav2 and Vav3 controlled a vast transcriptional program in breast cancer cells through mechanisms that were shared between the two proteins, isoform-specific or synergistic. Furthermore, the abundance of Vav-regulated transcripts was modulated by Rac1-dependent and Rac1-independent pathways. This transcriptome encoded therapeutically targetable proteins that played nonredundant roles in primary tumorigenesis and lung-specific metastasis, such as integrin-linked kinase (Ilk), the transforming growth factor-β family ligand inhibin βA, cyclooxygenase-2, and the epithelial cell adhesion molecule Tacstd2. It also contained gene signatures that predicted disease outcome in breast cancer patients. These results identify possible targets for treating breast cancer and lung metastases and provide a potential diagnostic tool for clinical use.

Microtubules regulate GEF-H1 in response to extracellular matrix stiffness

Breast epithelial cells sense the stiffness of the extracellular matrix through Rho-mediated contractility. In turn, matrix stiffness regulates RhoA activity. However, the upstream signaling mechanisms are poorly defined. Here we demonstrate that the Rho exchange factor GEF-H1 mediates RhoA activation in response to extracellular matrix stiffness. We demonstrate the novel finding that microtubule stability is diminished by a stiff three-dimensional (3D) extracellular matrix, which leads to the activation of GEF-H1. Surprisingly, activation of the mitogen-activated protein kinase kinase/extracellular signal-regulated kinase pathway did not contribute to stiffness-induced GEF-H1 activation. Loss of GEF-H1 decreases cell contraction of and invasion through 3D matrices. These data support a model in which matrix stiffness regulates RhoA through microtubule destabilization and the subsequent release and activation of GEF-H1.

RhoA guanine exchange factor expression profile in arteries: evidence for a Rho kinase-dependent negative feedback in angiotensin II-dependent hypertension

Sustained overactivation of RhoA is a common component for the pathogenesis of several cardiovascular disorders, including hypertension. Although activity of Rho proteins depends on Rho exchange factors (Rho-GEFs), the identity of Rho-GEFs expressed in vascular smooth muscle cells (VSMC) and participating in the control of Rho protein activity and Rho-dependent functions remains unknown. To address this question, we analyzed by quantitative RT-PCR the expression profile of 28 RhoA-GEFs in arteries of normotensive (saline-treated) and hypertensive (ANG II-treated) rats. Sixteen RhoA-GEFs were downregulated in mesenteric arteries of hypertensive rats, among which nine are also downregulated in cultured VSMC stimulated by ANG II (100 nM, 48 h), suggesting a direct effect of ANG II. Inhibition of type 1 ANG II receptors (losartan, 1 μM) or Rho kinase (fasudil, 10 μM) prevented ANG II-induced RhoA-GEF downregulation. Functionally, ANG II-induced downregulation of RhoA-GEFs is associated with decreased Rho kinase activation in response to endothelin-1, norepinephrine, and U-46619. This work thus identifies a group of RhoA-GEFs that controls RhoA and RhoA-dependent functions in VSMC, and a negative feedback of RhoA/Rho kinase activity on the expression of these RhoA-GEFs that may play an adaptative role to limit RhoA/Rho kinase activation.

Mechanistic insights into specificity, activity and regulation of Rho exchange factors

The multimodular guanine nucleotide exchange factors (GEFs) of the Dbl family mostly share a tandem Dbl homology (DH) and pleckstrin homology (PH) domain organization. The role of this and other domains in the DH‐mediated regulation of the GDP/GTP exchange reaction of the Rho proteins is not well understood. This study presents detailed kinetic data on specificity, activity and regulation of the catalytic DH domains of four GEFs, namely p115, p190, PRG and LARG. We demonstrate that (i) these GEFs are specific exchange factors for the Rho isoforms (RhoA, B, C) and inactive towards other member of the Rho family including Rac1, Cdc42 and TC10; (ii) the DH domain of LARG exhibits the highest catalytic activity, reported for a Dbl protein till now, with a maximal acceleration of 107‐fold (iii) an N‐terminal region of the DH domain is involved in its association with GDP‐bound RhoA monitored by a fluorescently labeled RhoA; (iv) the tandem PH domains of p115 and PRG efficiently contribute to the DH‐mediated nucleotide exchange reaction; (v) in contrast to the isolated DH or DH‐PH domains, a p115 fragment encompassing both the regulator of G‐protein signaling (RGS) and the DH domains, revealed a significantly reduced GEF activity supporting a proposed model of the RGS‐mediated intramolecular autoinhibitory mechanism for p115‐like RhoGEFs. This work was supported in part by the DFG and Forschungskommission der Medizinischen.

RhoA Phosphorylation Induces Rac1 Release from Guanine Dissociation Inhibitor α and Stimulation of Vascular Smooth Muscle Cell Migration

Although overactivation of RhoA is recognized as a common component of vascular disorders, the molecular mechanisms regulating RhoA activity in vascular smooth muscle cells (VSMC) are still unclear. We have previously shown that in VSMC, RhoA is phosphorylated on Ser188 by nitric oxide (NO)-stimulated cGMP-dependent kinase (PKG), which leads to RhoA-Rho kinase pathway inhibition. In this study, we showed that expression of phosphoresistant RhoA mutants prevented the stimulation of VSMC migration and adhesion induced by NO-PKG pathway activation. In contrast, under basal conditions, phosphomimetic RhoA mutants stimulated VSMC adhesion and migration through a signaling pathway requiring Rac1 and the Rho exchange factor Vav3. RhoA phosphorylation or phosphomimetic RhoA mutants induced Rac1 activation but did not activate Vav3. Indeed, phosphorylated RhoA or phosphomimetic mutants trapped guanine dissociation inhibitor α (GDIα), leading to the release of Rac1 and its translocation to the membrane, where it was then activated by the basal Vav3 nucleotide exchange activity. In vivo, RhoA phosphorylation induced by PKG activation in the aortas of rats treated with sildenafil induced dissociation of Rac1 from GDIα and activation of the Rac1 signaling pathway. These results suggest that the phosphorylation of RhoA represents a novel potent and physiological GDIα displacement factor that leads to Rac1 activation and regulation of Rac1-dependent VSMC functions.

Cellular responses to extracellular guidance cues

Extracellular guidance cues have a key role in orchestrating cell behaviour. They can take many forms, including soluble and cell-bound ligands (proteins, lipids, peptides or small molecules) and insoluble matrix substrates, but to act as guidance cues, they must be presented to the cell in a spatially restricted manner. Cells that recognize such cues respond by activating intracellular signal transduction pathways in a spatially restricted manner and convert the extracellular information into intracellular polarity. Although extracellular cues influence a broad range of cell polarity decisions, such as mitotic spindle orientation during asymmetric cell division, or the establishment of apical-basal polarity in epithelia, this review will focus specifically on guidance cues that promote cell migration (chemotaxis), or localized cell shape changes (chemotropism).

The RhoA Activator GEF-H1/Lfc Is a Transforming Growth Factor-β Target Gene and Effector That Regulates α-Smooth Muscle Actin Expression and Cell Migration

Maintenance of the epithelial phenotype is crucial for tissue homeostasis. In the retina, dedifferentiation and loss of integrity of the retinal pigment epithelium (RPE) leads to retinal dysfunction and fibrosis. Transforming growth factor (TGF)-β critically contributes to RPE dedifferentiation and induces various responses, including increased Rho signaling, up-regulation of α-smooth muscle actin (SMA), and cell migration and dedifferentiation. Cellular TGF-β responses are stimulated by different signal transduction pathways: some are Smad dependent and others Smad independent. Alterations in Rho signaling are crucial to both types of TGF-β signaling, but how TGF-β-stimulates Rho signaling is poorly understood. Here, we show that primary RPE cells up-regulated GEF-H1 in response to TGF-β. GEF-H1 was the only detectable Rho exchange factor increased by TGF-β1 in a genome-wide expression analysis. GEF-H1 induction was Smad4-dependant and led to Rho activation. GEF-H1 inhibition counteracted α-SMA up-regulation and cell migration. In patients with retinal detachments and fibrosis, migratory RPE cells exhibited increased GEF-H1 expression, indicating that induction occurs in diseased RPE in vivo. Our data indicate that GEF-H1 is a target and functional effector of TGF-β by orchestrating Rho signaling to regulate gene expression and cell migration, suggesting that it represents a new marker and possible therapeutic target for degenerative and fibrotic diseases.

The Rho exchange factor Arhgef1 mediates the effects of angiotensin II on vascular tone and blood pressure

Hypertension is one of the most frequent pathologies in the industrialized world. Although recognized to be dependent on a combination of genetic and environmental factors, its molecular basis remains elusive. Increased activity of the monomeric G protein RhoA in arteries is a common feature of hypertension. However, how RhoA is activated and whether it has a causative role in hypertension remains unclear. Here we provide evidence that Arhgef1 is the RhoA guanine exchange factor specifically responsible for angiotensin II-induced activation of RhoA signaling in arterial smooth muscle cells. We found that angiotensin II activates Arhgef1 through a previously undescribed mechanism in which Jak2 phosphorylates Tyr738 of Arhgef1. Arhgef1 inactivation in smooth muscle induced resistance to angiotensin II-dependent hypertension in mice, but did not affect normal blood pressure regulation. Our results show that control of RhoA signaling through Arhgef1 is central to the development of angiotensin II-dependent hypertension and identify Arhgef1 as a potential target for the treatment of hypertension.

Abstract 5074: The Rho Exchange Factor Arhgef1 in Vascular Smooth Muscle Cells is Essential for Angiotensin II-induced Hypertension

Rho exchange factors (Rho-GEFs) are responsible for Rho protein activation by catalyzing the exchange of GDP for GTP. Although over-activation of RhoA and its effector Rho kinase is recognized as a critical component of the pathogenesis of hypertension in experimental models and in human, the molecular mechanisms and the RhoA-GEF(s) involved in this activation of RhoA signaling in vascular smooth muscle cells (VSMCs) are unknown. Among the Rho-GEFs family, Arhgef1 (p115) constitutes with Arhgef11 (PDZ-Rho GEF) and Arhgef12 (Larg) the subfamily of RGS-containing Rho-GEFs that have been suggested to couple G-protein associated receptors to RhoA activation. To establish the role of Arhgef1 in the regulation of vascular tone and hypertension in vivo , we generated mice lacking Arhgef1 specifically in SMCs (SM-Arhgef1-KO). We used Arhgef1 lox/lox mice, which were mated to SMMHC-CreER T2 . SM-Arhgef1-KO mice were then obtained by treatment with tamoxifen (1 mg/day, IP, 5 days). In VSMCs from SM-Arhgef1-KO mice, stimulation with angiotensin II (Ang II, 0.1 μ M) was unable to induce RhoA/Rho kinase activation whereas that produced by U46619, phenylephrine (PhE), and endothelin-1 (ET1) was similar to that recorded in control mice. Ex vivo analysis of the contractile properties showed that, in aortic segments from SM-Arhgef1-KO mice, KCl-, PhE-, U46619- and ET1-induced contractions were similar to those obtained in control mice. In contrast, Ang II-induced contractions were severely reduced. In vivo, acute pressor responses to U46619 (3–100 μ g/kg), ET1 (0.03–1 μ g/kg) and PhE (0.1–100 μ g/kg) were similar in control and SM-Arhgef1-KO mice, but responses to Ang II (0.1–30 μ g/kg) were abolished. Basal blood pressure levels were similar in SM-Arhgef1-KO and control mice. In control mice, chronic infusion of Ang II (1 mg/kg/day) increased blood pressure whereas SM-Arhgef1-KO mice were resistant to the development of Ang II-induced hypertension. We therefore conclude that Arhgef1, as the molecular link between Ang II, RhoA activation and vascular tone regulation, provides an alternative target to reducing elevated vascular tone under hypertensive conditions without affecting normal blood pressure regulation by other vasoactive factors.

Abstract 5278: Essential Role of the Rho Exchange Factor Arhgef1 for Angiotensin II-induced RhoA/Rho Kinase Activation and Arterial Tone Regulation

Rho exchange factors (Rho-GEFs) are signaling molecules responsible for Rho protein activation by catalyzing the exchange of GDP for GTP. Although over-activation of the small G protein RhoA and its effector Rho kinase is recognized as a critical component of the pathogenesis of hypertension in experimental models and in human, the molecular mechanisms and the RhoA-GEF(s) involved in this activation of RhoA signaling in vascular smooth muscle cells (VSMCs) are still unknown. Nevertheless, angiotensin II (Ang II) has been shown to play a key role in the over-activation of RhoA in hypertension. Here we identify the RhoA-GEF and the molecular mechanism responsible for the activation of RhoA and Rho kinase in response to Ang II type 1 receptor activation (AT1R) in rat aortic VSMCs. Systematic functional analysis by siRNA-mediated silencing of the 28 RhoA GEFs indicated that only Arhgef1 (p115, lsc) is involved in Ang II (0.1 μ M)-mediated RhoA/Rho kinase activation. Knock-down of Arhgef1 had no significant effect on the activation of RhoA/Rho kinase induced by the thromboxan analog U46619, endothelin-1 and noradrenaline. The potency and efficacy of noradrenaline, phenyl-ephrine, U46619 and endothelin-1 to elicit contraction of rat aortic rings were not modified by Arhgef1 silencing whereas the contractile effect of Ang II was abolished. Ang II-induced Arhgef1 activation was further confirmed by in vitro guanine nucleotide exchange assay on RhoA, and we demonstrated by the transfection of Arhgef1 mutants that Ang II-induced Arhgef1 activation was due to its phosphorylation on Tyr738 by the tyrosine kinase Jak2. To confirm the functional role of Arhgef1 in Ang II-mediated regulation of vascular tone and hypertension, we then determined the activity of Arhgef1 in arteries from Ang II treated rats by pull-down assay. Chronic infusion of Ang II (250 ng/kg/min, 14 days) through an osmotic minipump induced an increase in arterial pressure associated with enhanced RhoA/Rho kinase activity and activation of Arhgef1 in arteries. Our results thus show that Arhgef1 activation and its downstream RhoA signaling are essential for Ang II-induced contraction and identify Arhgef1 as a potential specific target for the treatment of Ang II-dependent hypertension.

Guanine Nucleotide Exchange Factor-H1 Regulates Cell Migration via Localized Activation of RhoA at the Leading Edge

Cell migration involves the cooperative reorganization of the actin and microtubule cytoskeletons, as well as the turnover of cell-substrate adhesions, under the control of Rho family GTPases. RhoA is activated at the leading edge of motile cells by unknown mechanisms to control actin stress fiber assembly, contractility, and focal adhesion dynamics. The microtubule-associated guanine nucleotide exchange factor (GEF)-H1 activates RhoA when released from microtubules to initiate a RhoA/Rho kinase/myosin light chain signaling pathway that regulates cellular contractility. However, the contributions of activated GEF-H1 to coordination of cytoskeletal dynamics during cell migration are unknown. We show that small interfering RNA-induced GEF-H1 depletion leads to decreased HeLa cell directional migration due to the loss of the Rho exchange activity of GEF-H1. Analysis of RhoA activity by using a live cell biosensor revealed that GEF-H1 controls localized activation of RhoA at the leading edge. The loss of GEF-H1 is associated with altered leading edge actin dynamics, as well as increased focal adhesion lifetimes. Tyrosine phosphorylation of focal adhesion kinase and paxillin at residues critical for the regulation of focal adhesion dynamics was diminished in the absence of GEF-H1/RhoA signaling. This study establishes GEF-H1 as a critical organizer of key structural and signaling components of cell migration through the localized regulation of RhoA activity at the cell leading edge.

Relativistic chiral Hartree-Fock description of nuclear matter with constraints from nucleon structure and confinement

We present a relativistic chiral effective theory for symmetric and asymmetric nuclear matter taken in the Hartree-Fock scheme. The nuclear binding is ensured by a background chiral invariant scalar field associated with the radial fluctuations of the chiral quark condensate. Nuclear matter saturation is obtained once the scalar response of the nucleon-generating three-body repulsive forces is incorporated. For those parameters related to the scalar sector and quark confinement mechanism inside the nucleon we make use of an analysis of lattice results on nucleon mass evolution with quark mass. The other parameters are constrained as much as possible by standard hadron and nuclear phenomenology. Special attention is paid to the treatment of the propagation of the scalar fluctuations. The rearrangement terms associated with in-medium modified mass and coupling constants are explicitly included to satisfy the Hugenholtz-Van Hove theorem. We point out the important role of the tensor piece of the rho exchange Fock term to reproduce the asymmetry energy of nuclear matter. We also discuss the isospin dependence of the Landau nucleon effective mass.

Rho exchange factors in the cardiovascular system

Increasing evidence has accumulated to implicate overactivation of Rho protein as a common component for the pathogenesis of several cardiovascular disorders including hypertension, coronary and cerebral vasospasm, atherosclerosis, and diabetes. Recent advances in Rho protein signaling research indicate that the Rho exchange factors (Rho GEFs) which activate Rho proteins by catalyzing the exchange of GDP for GTP are major regulators of Rho protein activity. In addition, linkage analysis and association studies have recently identified Rho GEFs as susceptibility genes for cardiovascular diseases. All of these data are converging to suggest that as upstream activators of Rho proteins, Rho GEFs expressed in cardiovascular cells are good candidate targets for the treatment of cardiovascular disorders.

GEF-H1 Modulates Localized RhoA Activation during Cytokinesis under the Control of Mitotic Kinases

Formation of the mitotic cleavage furrow is dependent upon both microtubules and activity of the small GTPase RhoA. GEF-H1 is a microtubule-regulated exchange factor that couples microtubule dynamics to RhoA activation. GEF-H1 localized to the mitotic apparatus in HeLa cells, particularly at the tips of cortical microtubules and the midbody, and perturbation of GEF-H1 function induced mitotic aberrations, including asymmetric furrowing, membrane blebbing, and impaired cytokinesis. The mitotic kinases Aurora A/B and Cdk1/Cyclin B phosphorylate GEF-H1, thereby inhibiting GEF-H1 catalytic activity. Dephosphorylation of GEF-H1 occurs just prior to cytokinesis, accompanied by GEF-H1-dependent GTP loading on RhoA. Using a live cell biosensor, we demonstrate distinct roles for GEF-H1 and Ect2 in regulating Rho activity in the cleavage furrow, with GEF-H1 catalyzing Rho activation in response to Ect2-dependent localization and initiation of cell cleavage. Our results identify a GEF-H1-dependent mechanism to modulate localized RhoA activation during cytokinesis under the control of mitotic kinases.