Characterization of TCL, a new GTPase of the rho family related to TC10 andCcdc42

RhoJ: an emerging biomarker and target in cancer research and treatment

RhoJ is a Rho GTPase that belongs to the Cdc42 subfamily and has a molecular weight of approximately 21 kDa. It can activate the p21-activated kinase family either directly or indirectly, influencing the activity of various downstream effectors and playing a role in regulating the cytoskeleton, cell movement, and cell cycle. RhoJ's expression and activity are controlled by multiple upstream factors at different levels, including expression, subcellular localization, and activation. High RhoJ expression is generally associated with a poor prognosis for cancer patients and is mainly due to an increased number of tumor blood vessels and abnormal expression in malignant cells. RhoJ promotes tumor progression through several pathways, particularly in tumor angiogenesis and drug resistance. Clinical data also indicates that high RhoJ expression is closely linked to the pathological features of tumor malignancy. There are various cancer treatment methods that target RhoJ signaling, such as direct binding to inhibit the RhoJ effector pocket, inhibiting RhoJ expression, blocking RhoJ upstream and downstream signals, and indirectly inhibiting RhoJ's effect. RhoJ is an emerging cancer biomarker and a significant target for future cancer clinical research and drug development.

RHOJ as a novel mechanosensitive modulator of endothelial inflammation

Physiological high shear stress (HSS), a frictional force generated by flowing blood, is essential for endothelial homeostasis under normal physiological conditions. HSS suppresses atherosclerosis by inhibiting endothelial inflammation. However, the molecular mechanisms underlying this process have not been fully elucidated. Here, we report that HSS downregulated the mRNA and protein levels of ras homolog family member J (RHOJ) in endothelial cells (ECs). Silencing endogenous RHOJ expression decreased the mRNA and protein levels of proinflammatory vascular cell adhesion molecule 1 (VCAM-1) and intercellular cell adhesion molecule 1 (ICAM-1) in ECs, leading to a reduction in monocyte adhesion to ECs. Conversely, the overexpression of RHOJ had the opposite effect. RNA-sequencing analysis uncovered several differentially expressed genes (such as yes-associated protein 1 (YAP1),heme oxygenase-1 (HO1), and monocyte chemoattractant protein-1 (MCP1)) and pathways (such as nuclear factor-kappa B (NF-κB), fluid shear stress and atherosclerosis, and cell adhesion pathways) as RHOJ targets. Additionally, HSS was observed to alleviate endothelial inflammation by inhibiting RHOJ expression. Finally, methylated RNA immunoprecipitation sequencing (MeRIP-seq) illustrated that fluid shear stress regulates RHOJ expression in an N6-methyladenosine (m6A)-dependent manner. Mechanistically, the RNA m6A writer, methyltransferase 3 (METTL3), and the RNA m6A readers, YTH N6-methyladenosine RNA-binding protein F 3 (YTHDF3) and YTH N6-methyladenosine RNA-binding protein C 1/2 (YTHDC1/2), are involved in this process. Taken together, our data demonstrate that HSS-induced downregulation of RHOJ contributes to endothelial homeostasis by suppressing endothelial inflammation and that RHOJ inhibition in ECs is a promising therapeutic strategy for endothelial dysfunction.

RHOJ controls EMT-associated resistance to chemotherapy

The resistance of cancer cells to therapy is responsible for the death of most patients with cancer1. Epithelial-to-mesenchymal transition (EMT) has been associated with resistance to therapy in different cancer cells2,3. However, the mechanisms by which EMT mediates resistance to therapy remain poorly understood. Here, using a mouse model of skin squamous cell carcinoma undergoing spontaneous EMT during tumorigenesis, we found that EMT tumour cells are highly resistant to a wide range of anti-cancer therapies both in vivo and in vitro. Using gain and loss of function studies in vitro and in vivo, we found that RHOJ-a small GTPase that is preferentially expressed in EMT cancer cells-controls resistance to therapy. Using genome-wide transcriptomic and proteomic profiling, we found that RHOJ regulates EMT-associated resistance to chemotherapy by enhancing the response to replicative stress and activating the DNA-damage response, enabling tumour cells to rapidly repair DNA lesions induced by chemotherapy. RHOJ interacts with proteins that regulate nuclear actin, and inhibition of actin polymerization sensitizes EMT tumour cells to chemotherapy-induced cell death in a RHOJ-dependent manner. Together, our study uncovers the role and the mechanisms through which RHOJ acts as a key regulator of EMT-associated resistance to chemotherapy.

Knockdown of RhoQ, a member of Rho GTPase, accelerates TGF-β-induced EMT in human lung adenocarcinoma

Lung cancer is the leading cause of cancer-related deaths worldwide, and the most common subtype of lung cancer is adenocarcinoma. RhoQ is a Rho family GTPase with primary sequence and structural similarities to Cdc42 and RhoJ. RhoQ is involved in neurite outgrowth via membrane trafficking and is essential for insulin-stimulated glucose uptake in mature adipocytes. However, the function of RhoQ in lung adenocarcinoma (LUAD) remains unclear. In this study, RhoQ siRNAs were introduced into A549 and PC-9 cells. Expression level of EMT-related genes and invasion ability were investigated using Western blot and transwell assay. To examine the relationship between RhoQ expression and prognosis of LUAD, Kaplan–Meier plotter was used. We discovered that suppressing RhoQ expression promoted TGF-β-mediated EMT and invasion in LUAD cell lines. Furthermore, RhoQ knockdown increased Smad3 phosphorylation and Snail expression, indicating that RhoQ was involved in TGF/Smad signaling during the EMT process. Moreover, Kaplan–Meier plotter analysis revealed that low RhoQ levels were associated with poor overall survival in patients with LUAD. In conclusion, these findings shed light on RhoQ's role as a negative regulator of TGF-β-mediated EMT in LUAD.

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Knockdown of RhoQ expression promoted TGF-β-mediated EMT and invasion in human lung adenocarcinoma cells.

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RhoQ knockdown increased Smad3 phosphorylation and Snail expression during the EMT process.

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Low RhoQ levels were associated with poor overall survival in patients with lung adenocarcinoma.

Structure-based design of CDC42 effector interaction inhibitors for the treatment of cancer

CDC42 family GTPases (RHOJ, RHOQ, CDC42) are upregulated but rarely mutated in cancer and control both the ability of tumor cells to invade surrounding tissues and the ability of endothelial cells to vascularize tumors. Here, we use computer-aided drug design to discover a chemical entity (ARN22089) that has broad activity against a panel of cancer cell lines, inhibits S6 phosphorylation and MAPK activation, activates pro-inflammatory and apoptotic signaling, and blocks tumor growth and angiogenesis in 3D vascularized microtumor models (VMT) in vitro. Additionally, ARN22089 has a favorable pharmacokinetic profile and can inhibit the growth of BRAF mutant mouse melanomas and patient-derived xenografts in vivo. ARN22089 selectively blocks CDC42 effector interactions without affecting the binding between closely related GTPases and their downstream effectors. Taken together, we identify a class of therapeutic agents that influence tumor growth by modulating CDC42 signaling in both the tumor cell and its microenvironment.

The Role of Zinc in Axon Formation via the mTORC1 Pathway

Zinc is an essential micronutrient required for proper function during neuronal development because it can modulate neuronal function and structure. A fully functional description of zinc in axonal processing in the central nervous system remains elusive. Here, we define the role of intracellular zinc in axon formation and elongation, involving the mammalian target of rapamycin complex 1 (mTORC1). To investigate the involvement of zinc in axon growth, we performed an ex vivo culture of mouse hippocampal neurons and administrated ZnCl₂ as a media supplement. At 2 days in vitro, the administration of zinc induced the formation of multiple and elongated axons in the ex vivo culture system. A similar outcome was witnessed in callosal projection neurons in a developing mouse brain. Treatment with extracellular zinc activated the mTORC1 signaling pathway in mouse hippocampal neuronal cultures. The zinc-dependent enhancement of neuronal processing was inhibited either by the deactivation of mTORC1 with RAPTOR shRNA or by mTOR-insensitive 4EBP1 mutants. Additionally, zinc-dependent mTORC1 activation enhanced the axonal translation of TC10 and Par3 may be responsible for axonal growth. We identified a promising role of zinc in controlling axonogenesis in the developing brain, which, in turn, may indicate a novel structural role of zinc in the cytoskeleton and developing neurons.

Small GTPases all over invadosomes

Cell invasion is associated with numerous patho-physiologic states including cell development and metastatic dissemination. This process couples the activation of cell motility with the capacity to degrade the extracellular matrix, thereby permitting cells to pass through basal membranes. Invasion is sustained by the actions of invadosomes, an ensemble of subcellular structures with high functional homology. Invadosomes are 3D acto-adhesive structures that can also mediate local extracellular matrix degradation through the controlled delivery of proteases. Intracellular RHO GTPases play a central role in the regulation of invadosomes where their complex interplay regulates multiple invadosome functions. This review aims to provide an overview of the synergistic activities of the small GTPases in invadosome biology. This broad-based review also reinforces the importance of the spatiotemporal regulation of small GTPases and the impact of this process on invadosome dynamics.

ARHGEF26 enhances Salmonella invasion and inflammation in cells and mice

Salmonella hijack host machinery in order to invade cells and establish infection. While considerable work has described the role of host proteins in invasion, much less is known regarding how natural variation in these invasion-associated host proteins affects Salmonella pathogenesis. Here we leveraged a candidate cellular GWAS screen to identify natural genetic variation in the ARHGEF26 (Rho Guanine Nucleotide Exchange Factor 26) gene that renders lymphoblastoid cells susceptible to Salmonella Typhi and Typhimurium invasion. Experimental follow-up redefined ARHGEF26’s role in Salmonella epithelial cell infection. Specifically, we identified complex serovar-by-host interactions whereby ARHGEF26 stimulation of S. Typhi and S. Typhimurium invasion into host cells varied in magnitude and effector-dependence based on host cell type. While ARHGEF26 regulated SopB- and SopE-mediated S. Typhi (but not S. Typhimurium) infection of HeLa cells, the largest effect of ARHGEF26 was observed with S. Typhimurium in polarized MDCK cells through a SopB- and SopE2-independent mechanism. In both cell types, knockdown of the ARHGEF26-associated protein DLG1 resulted in a similar phenotype and serovar specificity. Importantly, we show that ARHGEF26 plays a critical role in S. Typhimurium pathogenesis by contributing to bacterial burden in the enteric fever murine model, as well as inflammation in the colitis infection model. In the enteric fever model, SopB and SopE2 are required for the effects of Arhgef26 deletion on bacterial burden, and the impact of sopB and sopE2 deletion in turn required ARHGEF26. In contrast, SopB and SopE2 were not required for the impacts of Arhgef26 deletion on colitis. A role for ARHGEF26 on inflammation was also seen in cells, as knockdown reduced IL-8 production in HeLa cells. Together, these data reveal pleiotropic roles for ARHGEF26 during infection and highlight that many of the interactions that occur during infection that are thought to be well understood likely have underappreciated complexity.

During infection, Salmonella manipulates host cells into engulfing the bacteria and establishing an intracellular niche. While many studies have identified genes involved in different stages of this Salmonella invasion process, few studies have examined how differences between human hosts contribute to infection susceptibility. Here we leveraged a candidate genetic screen to identify natural genetic variation in the human ARHGEF26 gene that correlates with Salmonella invasion. Springboarding from this result, we experimentally tested and redefined ARHGEF26’s role in Salmonella invasion, discovered a new role for ARHGEF26 in regulating inflammation during Salmonella disease, and demonstrated the relevance of these findings in mouse models. Building on how ARHGEF26 functions in other contexts, we implicated two ARHGEF26-interacting host proteins as contributors to Salmonella pathobiology. Collectively, these results identify a potential source of inter-person diversity in susceptibility to Salmonella disease and expand our molecular understanding of Salmonella infection to include a multifaceted role for ARHGEF26. They further identify important future directions in understanding how Salmonella recruit and manipulate ARHGEF26 as well as how ARHGEF26 is able to drive Salmonella-beneficial processes.

Activation of TC10-Like Transcription by Lysine Demethylase KDM4B in Colorectal Cancer Cells

Malignant colorectal cancers (CRCs) are characterized by enhanced migration and invasion thus acquiring the ability to metastasize. We have previously shown that the small GTPase TC10-like (TCL) contributes to aggressive migration and invasion in malignant CRC cells. TCL expression is differentially expressed in CRC cells and can be upregulated by hypoxia although the underlying epigenetic mechanism is not fully appreciated. Here, we report that differential TCL expression in CRC cells appeared to be associated with histone H3K9 methylation. RNAi screening revealed that the lysine demethylase KDM4B was essential for TCL transcription in CRC cells. KDM4B interacted with and was recruited by the sequence-specific transcription factor ETS-related gene 1 (ERG1) to the TCL promoter to activate transcription. Mechanistically, KDM4B mediated H3K9 demethylase facilitated the assembly of pre-initiation complex (PIC) on the TCL promoter. KDM4B knockdown attenuated migration and invasion of CRC cells. Importantly, KDM4B expression was upregulated in human CRC specimens of advanced stages compared to those of lower grades and associated with poor prognosis. Together, these data uncover a novel epigenetic mechanism underlying malignant transformation of CRC cells and suggest that KDM4B may be considered as a therapeutic target in CRC intervention.

Repression of RhoJ expression promotes TGF-β-mediated EMT in human non-small-cell lung cancer A549cells

Non-small-cell lung cancer (NSCLC) accounts for most cancer-related deaths because of its strong metastatic ability. It is important to understand NSCLC's molecular mechanisms of metastasis. RhoJ, a protein that belongs to the Rho family of small GTPases, regulates endothelial motility, angiogenesis, and adipogenesis. Recently, bioinformatics analysis showed that NSCLC patients with lower RhoJ expression had a worse survival outcome than those with high RhoJ expression. However, little is known about RhoJ's role in NSCLC. In the present study, we demonstrated that RhoJ knockdown accelerated TGF-βmediated epithelial-to-mesenchymal transition (EMT), an important cancer metastasis process, in A549 and PC-9 cells. Furthermore, using Matrigel-coated transwell chambers, we showed that RhoJ knockdown enhanced the invasion capacity of A549 cells that had undergone EMT. Also, reduced RhoJ expression increased Smad3 phosphorylation and Snail expression during the EMT process. Our results provide the first evidence of a potential novel role for RhoJ in the inhibition of EMT via modulation of the TGF-β-Smad signaling pathway, and shed new light on the mechanisms underlying EMT in NSCLC.

Endothelial Focal Adhesions Are Functional Obstacles for Leukocytes During Basolateral Crawling

An inflammatory response requires leukocytes to migrate from the circulation across the vascular lining into the tissue to clear the invading pathogen. Whereas a lot of attention is focused on how leukocytes make their way through the endothelial monolayer, it is less clear how leukocytes migrate underneath the endothelium before they enter the tissue. Upon finalization of the diapedesis step, leukocytes reside in the subendothelial space and encounter endothelial focal adhesions. Using TIRF microscopy, we show that neutrophils navigate around these focal adhesions. Neutrophils recognize focal adhesions as physical obstacles and deform to get around them. Increasing the number of focal adhesions by silencing the small GTPase RhoJ slows down basolateral crawling of neutrophils. However, apical crawling and diapedesis itself are not affected by RhoJ depletion. Increasing the number of focal adhesions drastically by expressing the Rac1 GEF Tiam1 make neutrophils to avoid migrating underneath these Tiam1-expressing endothelial cells. Together, our results show that focal adhesions mark the basolateral migration path of neutrophils.

Rho GTPases in Retinal Vascular Diseases

The Rho family of small GTPases (Rho GTPases) act as molecular switches that transduce extrinsic stimuli into cytoskeletal rearrangements. In vascular endothelial cells (ECs), Cdc42, Rac1, and RhoA control cell migration and cell–cell junctions downstream of angiogenic and inflammatory cytokines, thereby regulating vascular formation and permeability. While these Rho GTPases are broadly expressed in various types of cells, RhoJ is enriched in angiogenic ECs. Semaphorin 3E (Sema3E) releases RhoJ from the intracellular domain of PlexinD1, by which RhoJ induces actin depolymerization through competition with Cdc42 for their common effector proteins. RhoJ further mediates the Sema3E-induced association of PlexinD1 with vascular endothelial growth factor receptor (VEGFR) 2 and the activation of p38. Upon stimulation with VEGF-A, RhoJ facilitates the formation of a holoreceptor complex comprising VEGFR2, PlexinD1, and neuropilin-1, leading to the prevention of VEGFR2 degradation and the maintenance of intracellular signal transduction. These pleiotropic roles of RhoJ are required for directional EC migration in retinal angiogenesis. This review highlights the latest insights regarding Rho GTPases in the field of vascular biology, as it will be informative to consider their potential as targets for the treatment of aberrant angiogenesis and hyperpermeability in retinal vascular diseases.

Rho GTPases as Key Molecular Players within Intestinal Mucosa and GI Diseases

Rho proteins operate as key regulators of the cytoskeleton, cell morphology and trafficking. Acting as molecular switches, the function of Rho GTPases is determined by guanosine triphosphate (GTP)/guanosine diphosphate (GDP) exchange and their lipidation via prenylation, allowing their binding to cellular membranes and the interaction with downstream effector proteins in close proximity to the membrane. A plethora of in vitro studies demonstrate the indispensable function of Rho proteins for cytoskeleton dynamics within different cell types. However, only in the last decades we have got access to genetically modified mouse models to decipher the intricate regulation between members of the Rho family within specific cell types in the complex in vivo situation. Translationally, alterations of the expression and/or function of Rho GTPases have been associated with several pathological conditions, such as inflammation and cancer. In the context of the GI tract, the continuous crosstalk between the host and the intestinal microbiota requires a tight regulation of the complex interaction between cellular components within the intestinal tissue. Recent studies demonstrate that Rho GTPases play important roles for the maintenance of tissue homeostasis in the gut. We will summarize the current knowledge on Rho protein function within individual cell types in the intestinal mucosa in vivo, with special focus on intestinal epithelial cells and T cells.

Epigenetic activation of the small GTPase TCL contributes to colorectal cancer cell migration and invasion

TC10-like (TCL) is a small GTPase that has been implicated in carcinogenesis. Elevated TCL expression has been observed in many different types of cancers although the underlying epigenetic mechanism is poorly understood. Here we report that TCL up-regulation was associated with high malignancy in both human colorectal cancer biopsy specimens and in cultured colorectal cancer cells. Hypoxia, a pro-metastatic stimulus, up-regulated TCL expression in HT-29 cells. Further studies revealed that myocardin-related transcription factor A (MRTF-A) promoted migration and invasion of HT-29 cells in a TCL-dependent manner. MRTF-A directly bound to the proximal TCL promoter in response to hypoxia to activate TCL transcription. Chromatin immunoprecipitation (ChIP) assay showed that hypoxia stimulation specifically enhanced acetylation of histone H4K16 surrounding the TCL promoter, which was abolished by MRTF-A depletion or inhibition. Mechanistically, MRTF-A interacted with and recruited the H4K16 acetyltransferase hMOF to the TCL promoter to cooperatively regulate TCL transcription. hMOF depletion or inhibition attenuated hypoxia-induced TCL expression and migration/invasion of HT-29 cells. In conclusion, our data identify a novel MRTF-A-hMOF-TCL axis that contributes to colorectal cancer metastasis.

A functional antagonism between RhoJ and Cdc42 regulates fibronectin remodelling during angiogenesis

Angiogenesis is the formation of new blood vessels from pre-existing ones. Angiogenesis requires endothelial cells to change shape and polarity, as well as acquire the ability to directionally migrate ‒ processes that are classically regulated by the Rho family of GTPases. RhoJ (previously TCL) is an endothelium enriched Rho GTPase with a 78% amino acid similarity to the ubiquitously expressed Cdc42. In our recent publication, we demonstrate that α5β1 integrin co-traffics with RhoJ. RhoJ specifically represses the internalization of the active α5β1 conformer, leading to a reduced ability of endothelial cells to form fibronectin fibrils. Surprisingly, this function of RhoJ is in opposition to the role of Cdc42, a known driver of fibrillogenesis. Intriguingly, we discovered that the competition for limiting amounts of the shared effector, PAK3, could explain the ability of these two Rho GTPases to regulate fibrillogenesis in opposing directions. Consequently, RhoJ null mice show excessive fibronectin deposition around retinal vessels, possibly due to the unopposed action of Cdc42. Our work suggests that the functional antagonism between RhoJ and Cdc42 could restrict fibronectin remodelling to sites of active angiogenesis to form a provisional matrix for vessel growth. One correlate of our findings is that RhoJ dependent repression of fibronectin remodelling could be atheroprotective in quiescent vessels.

RhoJ Regulates α5β1 Integrin Trafficking to Control Fibronectin Remodeling during Angiogenesis

Rho guanosine triphosphatases (GTPases) are master regulators of cell shape and cell movement [1]. The archetypal family members RhoA, Rac1, and Cdc42 arose early in eukaryotic evolution and coordinate a diverse range of cell morphologies and migrations. Evolution of the vertebrates was paralleled by expansion of this family through gene duplication. Emergence of an adaptive immune system and more complex neural systems presented new roles for Rho GTPases, filled by new family members. Cdc42 underwent gene duplication to produce two related proteins-RhoQ and RhoJ [2]. RhoQ is active in neural dynamics; however, RhoJ is highly expressed in endothelial cells under control of the endothelial-specific promoter ERG [3, 4]. RhoJ is required for angiogenesis [5, 6] and has multiple roles in this process [7, 8]. We recently demonstrated that RhoJ regulates the endosomal trafficking of podocalyxin during angiogenesis to control lumen formation [9]. Here, we use vesicle purification and proteomic analysis to identify the endothelial targets of RhoJ-mediated trafficking. We identify α5β1 integrin as a major RhoJ cargo and show that RhoJ regulates the intracellular trafficking of active α5β1 integrin in endothelial cells to repress fibronectin fibrillogenesis. Accordingly, mice lacking RhoJ show deregulated deposition of fibronectin around vessels during developmental angiogenesis. Intriguingly, we show that RhoJ acts in opposition to Cdc42 in this process through competition for a shared partner, PAK3. These studies identify a critical role for RhoJ in matrix remodeling during blood vessel formation and demonstrate a functional interrelationship between RhoJ and its evolutionary parent.

RhoJ integrates attractive and repulsive cues in directional migration of endothelial cells

During angiogenesis, VEGF acts as an attractive cue for endothelial cells (ECs), while Sema3E mediates repulsive cues. Here, we show that the small GTPase RhoJ integrates these opposing signals in directional EC migration. In the GTP-bound state, RhoJ interacts with the cytoplasmic domain of PlexinD1. Upon Sema3E stimulation, RhoJ released from PlexinD1 induces cell contraction. PlexinD1-bound RhoJ further facilitates Sema3E-induced PlexinD1-VEGFR2 association, VEGFR2 transphosphorylation at Y1214, and p38 MAPK activation, leading to reverse EC migration. Upon VEGF stimulation, RhoJ is required for the formation of the holoreceptor complex comprising VEGFR2, PlexinD1, and neuropilin-1, thereby preventing degradation of internalized VEGFR2, prolonging downstream signal transductions via PLCγ, Erk, and Akt, and promoting forward EC migration. After conversion to the GDP-bound state, RhoJ shifts from PlexinD1 to VEGFR2, which then terminates the VEGFR2 signals. RhoJ deficiency in ECs efficiently suppressed aberrant angiogenesis in ischemic retina. These findings suggest that distinct Rho GTPases may act as context-dependent integrators of chemotactic cues in directional cell migration and may serve as candidate therapeutic targets to manipulate cell motility in disease or tissue regeneration.

Endothelial cell sprouting driven by RhoJ directly activated by a membrane-anchored Intersectin 1 (ITSN1) RhoGEF module

Endothelial cell sprouting is a critical event in tumor-induced angiogenesis. In melanoma and lung cancer murine models, targeting RhoJ prevents endothelial sprouting, tumor growth and metastasis and enhances the effects of conventional anti-neoplastic therapy. Aiming to understand how RhoJ is activated, we used a gain of function approach to identify constitutively active Rho guanine nucleotide exchange factors (RhoGEFs) able to promote RhoJ-dependent actin-driven membrane protrusions. We demonstrate that a membrane-anchored Intersectin 1 (ITSN1) DH-PH construct promotes endothelial cell sprouting via RhoJ. Mechanistically, this is controlled by direct interaction between the catalytic ITSN1 DH-PH module and RhoJ, it is sensitive to phosphorylation by focal adhesion kinase (FAK) and to endosomal trapping of the ITSN1 construct by dominant negative RhoJ. This ITSN1/RhoJ signaling axis is independent of Cdc42, a previously characterized ITSN1 target and a RhoJ close homologue. In conclusion, our results elucidate an ITSN1/RhoJ molecular link able to promote endothelial cell sprouting and set the basis to explore this signaling pathway in the context of tumor-induced angiogenesis.

The Atypical Rho GTPase CHW-1 Works with SAX-3/Robo To Mediate Axon Guidance in Caenorhabditis elegans

During development, neuronal cells extend an axon toward their target destination in response to a cue to form a properly functioning nervous system. Rho proteins, Ras-related small GTPases that regulate cytoskeletal organization and dynamics, cell adhesion, and motility, are known to regulate axon guidance. Despite extensive knowledge about canonical Rho proteins (RhoA/Rac1/Cdc42), little is known about the Caenorhabditis elegans (C. elegans) atypical Cdc42-like family members CHW-1 and CRP-1 in regards to axon pathfinding and neuronal migration. chw-1(Chp/Wrch) encodes a protein that resembles human Chp (Wrch-2/RhoV) and Wrch-1 (RhoU), and crp-1 encodes for a protein that resembles TC10 and TCL. Here, we show that chw-1 works redundantly with crp-1 and cdc-42 in axon guidance. Furthermore, proper levels of chw-1 expression and activity are required for proper axon guidance. When examining CHW-1 GTPase mutants, we found that the native CHW-1 protein is likely partially activated, and mutations at a conserved residue (position 12 using Ras numbering, position 18 in CHW-1) alter axon guidance and neural migration. Additionally, we showed that chw-1 genetically interacts with the guidance receptor sax-3 in PDE neurons. Finally, in VD/DD motor neurons, chw-1 works downstream of sax-3 to control axon guidance. In summary, this is the first study implicating the atypical Rho GTPases chw-1 and crp-1 in axon guidance. Furthermore, this is the first evidence of genetic interaction between chw-1 and the guidance receptor sax-3 These data suggest that chw-1 is likely acting downstream and/or in parallel to sax-3 in axon guidance.

Paving the Rho in cancer metastasis: Rho GTPases and beyond

Malignant carcinomas are often characterized by metastasis, the movement of carcinoma cells from a primary site to colonize distant organs. For metastasis to occur, carcinoma cells first must adopt a pro-migratory phenotype and move through the surrounding stroma towards a blood or lymphatic vessel. Currently, there are very limited possibilities to target these processes therapeutically. The family of Rho GTPases is an ubiquitously expressed division of GTP-binding proteins involved in the regulation of cytoskeletal dynamics and intracellular signaling. The best characterized members of the Rho family GTPases are RhoA, Rac1 and Cdc42. Abnormalities in Rho GTPase function have major consequences for cancer progression. Rho GTPase activation is driven by cell surface receptors that activate GTP exchange factors (GEFs) and GTPase-activating proteins (GAPs). In this review, we summarize our current knowledge on Rho GTPase function in the regulation of metastasis. We will focus on key discoveries in the regulation of epithelial-mesenchymal-transition (EMT), cell-cell junctions, formation of membrane protrusions, plasticity of cell migration and adaptation to a hypoxic environment. In addition, we will emphasize on crosstalk between Rho GTPase family members and other important oncogenic pathways, such as cyclic AMP-mediated signaling, canonical Wnt/β-catenin, Yes-associated protein (YAP) and hypoxia inducible factor 1α (Hif1α) and provide an overview of the advancements and challenges in developing pharmacological tools to target Rho GTPase and the aforementioned crosstalk in the context of cancer therapeutics.

The RhoJ-BAD signaling network: An Achilles' heel for BRAF mutant melanomas

Genes and pathways that allow cells to cope with oncogene-induced stress represent selective cancer therapeutic targets that remain largely undiscovered. In this study, we identify a RhoJ signaling pathway that is a selective therapeutic target for BRAF mutant cells. RhoJ deletion in BRAF mutant melanocytes modulates the expression of the pro-apoptotic protein BAD as well as genes involved in cellular metabolism, impairing nevus formation, cellular transformation, and metastasis. Short-term treatment of nascent melanoma tumors with PAK inhibitors that block RhoJ signaling halts the growth of BRAF mutant melanoma tumors in vivo and induces apoptosis in melanoma cells in vitro via a BAD-dependent mechanism. As up to 50% of BRAF mutant human melanomas express high levels of RhoJ, these studies nominate the RhoJ-BAD signaling network as a therapeutic vulnerability for fledgling BRAF mutant human tumors.

BRAF^V600E is the most common mutation in human melanomas, and kinase inhibitors that block BRAF^V600E signaling can rapidly induce tumor regression but only modestly improve melanoma long-term survival. In this study, we identify a novel therapeutic vulnerability for BRAF mutant melanoma tumors. Targeting RhoJ signaling with existing PAK inhibitors was more efficient at blocking the progression of BRAF mutant tumors in vivo when compared to kinase inhibitors currently used to treat melanoma. Approximately half of all human BRAF mutant melanoma tumors express high levels of RhoJ, identifying the RhoJ-BAD pathway as a novel target for melanoma.

The Small Rho GTPase TC10 Modulates B Cell Immune Responses

Rho family GTPases regulate diverse cellular events, such as cell motility, polarity, and vesicle traffic. Although a wealth of data exists on the canonical Rho GTPases RhoA, Rac1, and Cdc42, several other family members remain poorly studied. In B cells, we recently demonstrated a critical role for Cdc42 in plasma cell differentiation. In this study, we focus on a close homolog of Cdc42, TC10 (also known as RhoQ), and investigate its physiological role in B cells. By generating a TC10-deficient mouse model, we show that despite reduced total B cell numbers, B cell development in these mice occurs normally through distinct developmental stages. Upon immunization, IgM levels were reduced and, upon viral infection, germinal center responses were defective in TC10-deficient mice. BCR signaling was mildly affected, whereas cell migration remained normal in TC10-deficient B cells. Furthermore, by generating a TC10/Cdc42 double knockout mouse model, we found that TC10 can compensate for the lack of Cdc42 in TLR-induced cell activation and proliferation, so the two proteins play partly redundant roles. Taken together, by combining in vivo and in vitro analysis using TC10-deficient mice, we define the poorly studied Rho GTPase TC10 as an immunomodulatory molecule playing a role in physiological B cell responses.

TCL/RhoJ Plasma Membrane Localization and Nucleotide Exchange Is Coordinately Regulated by Amino Acids within the N Terminus and a Distal Loop Region

TCL/RhoJ is a Cdc42-related Rho GTPase with reported activities in endothelial cell biology and angiogenesis, metastatic melanoma, and corneal epithelial cells; however, less is known about how it is inherently regulated in comparison to its closest homologues TC10 and Cdc42. TCL has an N-terminal extension of 18 amino acids in comparison to Cdc42, but the function of this amino acid sequence has not been elucidated. A truncation mutant lacking the N terminus (ΔN) was found to alter TCL plasma membrane localization and nucleotide binding, and additional truncation and point mutants mapped the alterations of TCL biochemistry to amino acids 17-20. Interestingly, whereas the TCL ΔN mutant clearly influenced nucleotide exchange, deletion of the N terminus from its closest homologue, TC10, did not have a similar effect. Chimeras of TCL and TC10 revealed amino acids 121-129 of TCL contributed to the differences in nucleotide loading. Together, these results identify amino acids within the N terminus and a loop region distal to the nucleotide binding pocket of TCL capable of allosterically regulating nucleotide exchange and thus influence membrane association of the protein.

The Role of RhoJ in Endothelial Cell Biology and Tumor Pathology

Background. RhoJ, an endothelially expressed member of Cdc42 (cell division cycle 42) subfamily of Rho GTPase, plays an important role in endocytic pathway, adipocyte differentiation, endothelial motility, tube formation, and focal adhesion. RhoJ is a selective and effective therapeutic target in tumor tissues or retinopathy. Methods. A systematic review was related to “small Rho GTPase” or “RhoJ” with “endothelial motility, tube formation and focal adhesion” and “tumor therapy”. This led to many cross-references involving RhoJ and these data have been incorporated into the following study. Results. We have grouped the role of RhoJ according to three main effects: RhoJ regulates endocytic pathway and adipocyte differentiation in early studies, and RhoJ shows an important role in endothelial cell biology; furthermore, RhoJ blockade serves as a target in tumor vasculature and enhances the effects of anticancer drug. Conclusions. More research is necessary to understand the role of RhoJ in many aspects, on the basis of current knowledge of the role of RhoJ blockade in tumor vessels, there are opportunities for the therapy of tumor, and RhoJ is expressed outside tumour vasculature and is involved in wound healing. Taking advantage of the opportunities could result in a development in tumor therapy.

Rho GTPase RhoJ is Associated with Gastric Cancer Progression and Metastasis

Rho GTPases play a pivotal role in tumor progression by regulating tumor cell migration and invasion. However, the role of Rho GTPases in gastric cancer (GC) remains unexplored. This study aimed to investigate the clinical implications of RhoJ, which is an uncharted member of Rho family. RhoJ expression in human GC cell lines and surgical specimens from GC patients were analyzed. Moreover, in vitro gain-of-function analysis was performed to evaluate the malignant phenotypes of RhoJ-overexpressing GC cells. The extent of RhoJ expression varied among GC cell lines and GC patients. YCC-9 cell line displayed the strongest expression, while YCC-10, -11, and -16 showed scant expressions. Of the 70 GC patients, 34 (48.6%) had RhoJ expression in their GC tissue, and patients with high RhoJ expression had more diffuse type GC (73.5% vs. 41.7%), were at more advanced stages (stage III, IV: 85.3% vs. 58.4%), and had more frequent metastasis (47.1% vs. 11.1%), denoting that RhoJ has a potential role in GC progression and metastasis. High RhoJ expression significantly correlated with poor overall survival and recurrence-free survival after surgical resection of gastric cancer. Finally, In vitro gain-of-function experiments showed 41.3% enhanced motility and 60.4% enhanced invasiveness in RhoJ-overexpressing GC cells compared to control, with negligible difference in cell proliferation. Collectively, high RhoJ expression is an independent negative prognostic factor for the survival outcome of GC and correlated with the increased cell motility and invasiveness.

PAK4 promotes kinase-independent stabilization of RhoU to modulate cell adhesion

PAK4, via a novel kinase-independent mechanism, protects RhoU from a Rab40A/Cullin 5 ubiquitin ligase complex–driven K48 ubiquitination to regulate breast cancer cell adhesion.

P21-activated kinase 4 (PAK4) is a Cdc42 effector protein thought to regulate cell adhesion disassembly in a kinase-dependent manner. We found that PAK4 expression is significantly higher in high-grade human breast cancer patient samples, whereas depletion of PAK4 modifies cell adhesion dynamics of breast cancer cells. Surprisingly, systematic analysis of PAK4 functionality revealed that PAK4-driven adhesion turnover is neither dependent on Cdc42 binding nor kinase activity. Rather, reduced expression of PAK4 leads to a concomitant loss of RhoU expression. We report that RhoU is targeted for ubiquitination by the Rab40A–Cullin 5 complex and demonstrate that PAK4 protects RhoU from ubiquitination in a kinase-independent manner. Overexpression of RhoU rescues the PAK4 depletion phenotype, whereas loss of RhoU expression reduces cell adhesion turnover and migration. These data support a new kinase-independent mechanism for PAK4 function, where an important role of PAK4 in cellular adhesions is to stabilize RhoU protein levels. Thus, PAK4 and RhoU cooperate to drive adhesion turnover and promote cell migration.

Rho GTPase signalling in cell migration

Cells migrate in multiple different ways depending on their environment, which includes the extracellular matrix composition, interactions with other cells, and chemical stimuli. For all types of cell migration, Rho GTPases play a central role, although the relative contribution of each Rho GTPase depends on the environment and cell type. Here, I review recent advances in our understanding of how Rho GTPases contribute to different types of migration, comparing lamellipodium-driven versus bleb-driven migration modes. I also describe how cells migrate across the endothelium. In addition to Rho, Rac and Cdc42, which are well known to regulate migration, I discuss the roles of other less-well characterized members of the Rho family.

Rho GTPase signalling in cell migration

Cells migrate in multiple different ways depending on their environment, which includes the extracellular matrix composition, interactions with other cells, and chemical stimuli. For all types of cell migration, Rho GTPases play a central role, although the relative contribution of each Rho GTPase depends on the environment and cell type. Here, I review recent advances in our understanding of how Rho GTPases contribute to different types of migration, comparing lamellipodium-driven versus bleb-driven migration modes. I also describe how cells migrate across the endothelium. In addition to Rho, Rac and Cdc42, which are well known to regulate migration, I discuss the roles of other less-well characterized members of the Rho family.

Rho GTPase isoforms in cell motility: Don't fret, we have FRET

The Rho-family of p21 small GTPases are directly linked to the regulation of actin-based motile machinery and play a key role in the control of cell migration. Aside from the original and most well-characterized canonical Rho GTPases RhoA, Rac1, and Cdc42, numerous isoforms of these key proteins have been identified and shown to have specific roles in regulating various cellular motility processes. The major difficulty in addressing these isoform-specific effects is that isoforms typically contain highly similar primary amino acid sequences and thus are able to interact with the same upstream regulators and the downstream effector targets. Here, we will introduce the major members of each GTPase subfamily and discuss recent advances in the design and application of fluorescent resonance energy transfer-based probes, which are at the forefront of the technologies available to directly probe the differential, spatiotemporal activation dynamics of these proteins in live single cells. Currently, it is possible to specifically detect the activation status of RhoA vs. RhoC isoforms, as well as Cdc42 vs. TC-10 isoforms in living cells. Clearly, additional efforts are still required to produce biosensor systems capable of detecting other isoforms of Rho GTPases including RhoB, Rac2/3, RhoG, etc. Through such efforts, we will uncover the isoform-specific roles of these near-identical proteins in living cells, clearly an important area of the Rho GTPase biology that is not yet fully appreciated.

Regulation of cerebral cortex development by Rho GTPases: insights from in vivo studies

The cerebral cortex is the site of higher human cognitive and motor functions. Histologically, it is organized into six horizontal layers, each containing unique populations of molecularly and functionally distinct excitatory projection neurons and inhibitory interneurons. The stereotyped cellular distribution of cortical neurons is crucial for the formation of functional neural circuits and it is predominantly established during embryonic development. Cortical neuron development is a multiphasic process characterized by sequential steps of neural progenitor proliferation, cell cycle exit, neuroblast migration and neuronal differentiation. This series of events requires an extensive and dynamic remodeling of the cell cytoskeleton at each step of the process. As major regulators of the cytoskeleton, the family of small Rho GTPases has been shown to play essential functions in cerebral cortex development. Here we review in vivo findings that support the contribution of Rho GTPases to cortical projection neuron development and we address their involvement in the etiology of cerebral cortex malformations.

RhoJ interacts with the GIT-PIX complex and regulates focal adhesion disassembly

RhoJ is a Rho GTPase expressed in endothelial cells and tumour cells, which regulates cell motility, invasion, endothelial tube formation and focal adhesion numbers. This study aimed to further delineate the molecular function of RhoJ. Using timelapse microscopy RhoJ was found to regulate focal adhesion disassembly; small interfering RNA (siRNA)-mediated knockdown of RhoJ increased focal adhesion disassembly time, whereas expression of an active mutant (daRhoJ) decreased it. Furthermore, daRhoJ co-precipitated with the GIT–PIX complex, a regulator of focal adhesion disassembly. An interaction between daRhoJ and GIT1 was confirmed using yeast two-hybrid experiments, and this depended on the Spa homology domain of GIT1. GIT1, GIT2, β-PIX (also known as ARHGEF7) and RhoJ all colocalised in focal adhesions and depended on each other for their recruitment to focal adhesions. Functionally, the GIT–PIX complex regulated endothelial tube formation, with knockdown of both GIT1 and GIT2, or β-PIX phenocopying RhoJ knockdown. RhoJ-knockout mice showed reduced tumour growth and diminished tumour vessel density, identifying a role for RhoJ in mediating tumour angiogenesis. These studies give new insight into the molecular function of RhoJ in regulating cell motility and tumour vessel formation.

PleiotRHOpic: Rho pathways are essential for all stages of Neural Crest development

Neural Crest (NC) cells are a multipotent migratory stem cell population unique to vertebrates, which contributes extensively to the formation of a wide array of neural and non-neural structures in the embryo. NC cells originate in the ectoderm at the border of the neural tube, undergo an epithelial-mesenchymal transition and acquire outstanding individual and collective migratory properties that allow them to disseminate and differentiate to different parts of the body. This exquisite capacity to switch from an epithelium to motile cells represents both a puzzling biological issue and an attractive model to address the basic mechanisms of cell migration and their alteration during cancer progression. Here we review how signaling pathways controlled by Rho GTPases, key players in cell adhesion, contraction, migration and polarity, contribute to the control the different phases of NC development.

Identification of novel drug targets for the treatment of diabetic retinopathy

Vision loss in diabetic retinopathy (DR) is attributable to retinal vascular disorders that result in macular edema and neoangiogenesis. In addition to laser photocoagulation therapy, intraocular injections of antivascular endothelial growth factor drugs have contributed to the treatment of these disease conditions. Nonetheless, the clinical feasibility of intraocular drug administration has raised an increasing demand to develop alternative drugs that can fundamentally ameliorate the retinal vascular dysfunctions in DR. For this purpose, experimental animal models that reproduce human DR would be of clinical benefit. Despite the unavailability of DR models in rats or mice, pharmacological and genetic manipulations without hyperglycemia have successfully recapitulated retinal edema and neoangiogenesis in postnatal mouse retinas, thereby enabling the understanding of the pathophysiology underlying DR. This article highlights the utility of experimental mouse models of retinal vascular abnormalities and discusses cellular and molecular mechanisms responsible for the onset and progression of DR. These approaches will lead to the identification of novel drug targets for the restoration of vascular integrity and regeneration of functional capillaries in DR.

RhoJ modulates melanoma invasion by altering actin cytoskeletal dynamics

Rho family GTPases regulate diverse processes in human melanoma ranging from tumor formation to metastasis and chemoresistance. In this study, a combination of in vitro and in vivo approaches was utilized to determine whether RHOJ, a CDC42 homologue that regulates melanoma chemoresistance, also controls melanoma migration. Depletion or overexpression of RHOJ altered cellular morphology, implicating a role for RHOJ in modulating actin cytoskeletal dynamics. RHOJ depletion inhibited melanoma cell migration and invasion in vitro and melanoma tumor growth and lymphatic spread in mice. Molecular studies revealed that RHOJ alters actin cytoskeletal dynamics by inducing the phosphorylation of LIMK, cofilin, and p41-ARC (ARP2/3 complex subunit) in a PAK1-dependent manner in vitro and in tumor xenografts. Taken together, these observations identify RHOJ as a melanoma linchpin determinant that regulates both actin cytoskeletal dynamics and chemoresistance by activating PAK1.

Arhgef15 promotes retinal angiogenesis by mediating VEGF-induced Cdc42 activation and potentiating RhoJ inactivation in endothelial cells

BACKGROUND

Drugs inhibiting vascular endothelial growth factor (VEGF) signaling are globally administered to suppress deregulated angiogenesis in a variety of eye diseases. However, anti-VEGF therapy potentially affects the normal functions of retinal neurons and glias which constitutively express VEGF receptor 2. Thus, it is desirable to identify novel drug targets which are exclusively expressed in endothelial cells (ECs). Here we attempted to identify an EC-specific Rho guanine nucleotide exchange factor (GEF) and evaluate its role in retinal angiogenesis.

METHODOLOGY/PRINCIPAL FINDINGS

By exploiting fluorescence-activated cell sorting and microarray analyses in conjunction with in silico bioinformatics analyses, we comprehensively identified endothelial genes in angiogenic retinal vessels of postnatal mice. Of 9 RhoGEFs which were highly expressed in retinal ECs, we show that Arhgef15 acted as an EC-specific GEF to mediate VEGF-induced Cdc42 activation and potentiated RhoJ inactivation, thereby promoting actin polymerization and cell motility. Disruption of the Arhgef15 gene led to delayed extension of vascular networks and subsequent reduction of total vessel areas in postnatal mouse retinas.

CONCLUSIONS/SIGNIFICANCE

Our study provides information useful to the development of new means of selectively manipulating angiogenesis without affecting homeostasis in un-targeted tissues; not only in eyes but also in various disease settings such as cancer.

RhoJ regulates melanoma chemoresistance by suppressing pathways that sense DNA damage

Melanomas resist conventional chemotherapeutics, in part, through intrinsic disrespect of apoptotic checkpoint activation. In this study, using an unbiased genome-wide RNA interference screen, we identified RhoJ and its effector PAK1, as key modulators of melanoma cell sensitivity to DNA damage. We find that RhoJ activates PAK1 in response to drug-induced DNA damage, which then uncouples ATR from its downstream effectors, ultimately resulting in a blunted DNA damage response (DDR). In addition, ATR suppression leads to the decreased phosphorylation of ATF2 and consequent increased expression of the melanocyte survival gene Sox10 resulting in a higher DDR threshold required to engage melanoma cell death. In the setting of normal melanocyte behavior, this regulatory relationship may facilitate appropriate epidermal melanization in response to UV-induced DNA damage. However, pathologic pathway activation during oncogenic transformation produces a tumor that is intrinsically resistant to chemotherapy and has the propensity to accumulate additional mutations. These findings identify DNA damage agents and pharmacologic inhibitors of RhoJ/PAK1 as novel synergistic agents that can be used to treat melanomas that are resistant to conventional chemotherapies.

The role of small GTPases in neuronal morphogenesis and polarity

The highly dynamic remodeling and cross talk of the microtubule and actin cytoskeleton support neuronal morphogenesis. Small RhoGTPases family members have emerged as crucial regulators of cytoskeletal dynamics. In this review we will comprehensively analyze findings that support the participation of RhoA, Rac, Cdc42, and TC10 in different neuronal morphogenetic events ranging from migration to synaptic plasticity. We will specifically address the contribution of these GTPases to support neuronal polarity and axonal elongation.

The role of RhoJ in endothelial cell biology and angiogenesis

RhoJ is an endothelially expressed member of the Cdc42 (cell division cycle 42) subfamily of small Rho GTPases. It is expressed in both the developing mammalian vasculature and the vascular beds of a number of adult tissues, with its expression regulated by the endothelial transcription factor ERG (ETS-related gene). RhoJ has been shown to regulate endothelial motility, tubulogenesis and lumen formation in vitro, and modulates the vascularization of Matrigel plugs in vivo. Both vascular endothelial growth factor and semaphorin 3E have been found to affect its activation. RhoJ has been shown to be a focal-adhesion-localized Rho GTPase which can modulate focal adhesion number, actomyosin contractility and activity of Cdc42 and Rac1. The present review discusses the biology of RhoJ with a focus on recent reports of its role in endothelial cells and angiogenesis.

RhoJ is an endothelial cell-restricted Rho GTPase that mediates vascular morphogenesis and is regulated by the transcription factor ERG

ERG is a member of the ETS transcription factor family that is highly enriched in endothelial cells (ECs). To further define the role of ERG in regulating EC function, we evaluated the effect of ERG knock-down on EC lumen formation in 3D collagen matrices. Blockade of ERG using siRNA completely interferes with EC lumen formation. Quantitative PCR (QPCR) was used to identify potential downstream gene targets of ERG. In particular, we identified RhoJ as the Rho GTPase family member that is closely related to Cdc42 as a target of ERG. Knockdown of ERG expression in ECs led to a 75% reduction in the expression of RhoJ. Chromatin immunoprecipitation and transactivation studies demonstrated that ERG could bind to functional sites in the proximal promoter of the RhoJ gene. Knock-down of RhoJ similarly resulted in a marked reduction in the ability of ECs to form lumens. Suppression of either ERG or RhoJ during EC lumen formation was associated with a marked increase in RhoA activation and a decrease in Rac1 and Cdc42 activation and their downstream effectors. Finally, in contrast to other Rho GTPases, RhoJ exhibits a highly EC-restricted expression pattern in several different tissues, including the brain, heart, lung, and liver.

Sema3E-PlexinD1 signaling selectively suppresses disoriented angiogenesis in ischemic retinopathy in mice

During development, the retinal vasculature grows toward hypoxic areas in an organized fashion. By contrast, in ischemic retinopathies, new blood vessels grow out of the retinal surfaces without ameliorating retinal hypoxia. Restoration of proper angiogenic directionality would be of great benefit to reoxygenize the ischemic retina and resolve disease pathogenesis. Here, we show that binding of the semaphorin 3E (Sema3E) ligand to the transmembrane PlexinD1 receptor initiates a signaling pathway that normalizes angiogenic directionality in both developing retinas and ischemic retinopathy. In developing mouse retinas, inhibition of VEGF signaling resulted in downregulation of endothelial PlexinD1 expression, suggesting that astrocyte-derived VEGF normally promotes PlexinD1 expression in growing blood vessels. Neuron-derived Sema3E signaled to PlexinD1 and activated the small GTPase RhoJ in ECs, thereby counteracting VEGF-induced filopodia projections and defining the retinal vascular pathfinding. In a mouse model of ischemic retinopathy, enhanced expression of PlexinD1 and RhoJ in extraretinal vessels prevented VEGF-induced disoriented projections of the endothelial filopodia. Remarkably, intravitreal administration of Sema3E protein selectively suppressed extraretinal vascular outgrowth without affecting the desired regeneration of the retinal vasculature. Our study suggests a new paradigm for vascular regeneration therapy that guides angiogenesis precisely toward the ischemic retina.

Chromatin structure predicts epigenetic therapy responsiveness in sarcoma

To formally explore the potential therapeutic effect of histone deacetylase inhibitors (HDACI) and DNA-methyltransferase inhibitors (DNA-MI) on sarcomas, we treated a large sarcoma cell line panel with five different HDACIs in the absence and presence of the DNA-MI decitabine. We observed that the IC(50) value of each HDACI was consistent for all cell lines whereas decitabine as a single agent showed no growth inhibition at standard doses. Combination HDACI/DNA-MI therapy showed a preferential synergism for specific sarcoma cell lines. Subsequently, we identified and validated (in vitro and in vivo) a two-gene set signature (high CUGBP2; low RHOJ) that associated with the synergistic phenotype. We further uncover that the epigenetic synergism leading to specific upregulation of CDKI p21 in specific cell lines is a function of the differences in the degree of baseline chromatin modification. Finally, we show that these chromatin and gene expression patterns are similarly present in the majority of high-grade primary sarcomas. Our results provide the first demonstration of a gene set that can predict responsiveness to HDACI/DNA-MI and links this responsiveness mechanistically to the baseline chromatin structure.

Activity of the RhoU/Wrch1 GTPase is critical for cranial neural crest cell migration

The neural crest (NC) is a stem cell-like population that arises at the border of neural and non-neural ectoderm. During development, NC undergoes an epithelio-mesenchymal transition (EMT), i.e. loss of epithelial junctions and acquisition of pro-migratory properties, invades the entire embryo and differentiates into a wide diversity of terminal tissues. We have studied the implication of Rho pathways in NC development and previously showed that RhoV is required for cranial neural crest (CNC) cell specification. We show here that the non-canonical Wnt response rhoU/wrch1 gene, closely related to rhoV, is also expressed in CNC cells but at later stages. Using both gain- and loss-of-function experiments, we demonstrate that the level of RhoU expression is critical for CNC cell migration and subsequent differentiation into craniofacial cartilages. In in vitro cultures, RhoU activates pathways that cooperate with PAK1 and Rac1 in epithelial adhesion, cell spreading and directional cell migration. These data support the conclusion that RhoU is an essential regulator of CNC cell migration.

RhoJ/TCL regulates endothelial motility and tube formation and modulates actomyosin contractility and focal adhesion numbers

OBJECTIVE

RhoJ/TCL was identified by our group as an endothelial-expressed Rho GTPase. The aim of this study was to determine its tissue distribution, subcellular localization, and function in endothelial migration and tube formation.

METHODS AND RESULTS

Using in situ hybridization, RhoJ was localized to endothelial cells in a set of normal and cancerous tissues and in the vasculature of mouse embryos; endogenous RhoJ was localized to focal adhesions by immunofluorescence. The proangiogenic factor vascular endothelial growth factor activated RhoJ in endothelial cells. Using either small interfering (si)RNA-mediated knockdown of RhoJ expression or overexpression of constitutively active RhoJ (daRhoJ), RhoJ was found to positively regulate endothelial motility and tubule formation. Downregulating RhoJ expression increased focal adhesions and stress fibers in migrating cells, whereas daRhoJ overexpression resulted in the converse. RhoJ downregulation resulted in increased contraction of a collagen gel and increased phospho-myosin light chain, indicative of increased actomyosin contractility. Pharmacological inhibition of Rho-kinase (which phosphorylates myosin light chain) or nonmuscle myosin II reversed the defective tube formation and migration of RhoJ knockdown cells.

CONCLUSIONS

RhoJ is endothelial-expressed in vivo, activated by vascular endothelial growth factor, localizes to focal adhesions, regulates endothelial cell migration and tube formation, and modulates actomyosin contractility and focal adhesion numbers.

Podosome regulation by Rho GTPases in myeloid cells

Myeloid cells form a first line of defense against infections. They migrate from the circulation to the infected tissues by adhering to and subsequently crossing the vascular wall. This process requires precise control and proper regulation of these interactions with the environment is therefore crucial. Podosomes are the most prominent adhesion structures in myeloid cells. Podosomes control both the adhesive and migratory properties of myeloid cells and the regulation of podosomes is key to the proper functioning of these cells. Here we discuss the regulation of podosomes by Rho GTPases, well known regulators of adhesion and migration, focusing on myeloid cells. In addition, the regulation of podosomes by GTPase regulators such as GEFs and GAPs, as well as the effects of some Rho GTPase effector pathways, will be discussed.

PAK as a therapeutic target in gastric cancer

IMPORTANCE OF THE FIELD

Gastric cancer is one of the most common causes of cancer death worldwide. P21-activated kinases (PAKs), regulators of cancer-cell signalling networks, play fundamental roles in a range of cellular processes through their binding partners or kinase substrates.

AREAS COVERED IN THIS REVIEW

The complex regulation of PAKs through their upstream or downstream effectors in human cancers, especially in gastric cancer, are described and the identified inhibitors of PAKs are summarized.

WHAT THE READERS WILL GAIN

The structural differences and activation mechanisms between two subgroups of PAK are described. Both groups of PAKs play complicated and important roles in human gastric cancer, which indicated a possible way for us to identify the specific inhibitors targeting PAKs for gastric cancer.

TAKE HOME MESSAGE

PAKs play important roles in progression of many cancer types, the full mechanisms of PAKs in gastric cancer are still unclear. It seems there are different roles for two groups of PAKs in cancers. Group I PAKs play their functions mostly through their specific substrates, however, many binding partners that are independent of phosphorylation by group II PAKs were identified. Finding specific inhibitors of PAKs will help us discover the roles of PAKs and target these kinases in human gastric cancer.