The small GTPase RhoV is an essential regulator of neural crest induction in Xenopus

Atypical RhoUV GTPases in development and disease

RhoU and RhoV are members of the Rho family of small GTPases that comprise their own subfamily. RhoUV GTPases are classified as atypical due to the kinetics of their GTP/GDP binding cycles. They also possess unique N- and C-termini that regulate their subcellular localization and activity. RhoU and RhoV have been linked to cytoskeletal regulation, cell adhesion, and cell migration. They each exhibit distinct expression patterns during embryonic development and diseases such as cancer metastasis, suggesting they have specialized functions. In this review, we will discuss the known functions of RhoU and RhoV, with a focus on their roles in early development, organogenesis, and disease.

Prognostic significance of the rho GTPase RHOV and its role in tumor immune cell infiltration: a comprehensive pan-cancer analysis

Ras homolog gene family member V (RHOV) is an atypical Rho GTPase that participates in various important cellular processes. Although RHOV has been identified to play an oncogenic role in lung cancer and triple-negative breast cancer, its role in other types of tumors remains unknown. In this study, we investigated the expression of RHOV in pan-cancer analysis using The Cancer Genome Atlas (TCGA) and Gene-Tissue Expression datasets. RHOV mRNA levels were dysregulated in several types of tumors. RHOV expression was identified as an independent prognostic factor in 7 of 33 types of tumors; however, the relationship varied according to tumor type. Higher RHOV expression was associated with a favorable prognosis in kidney renal cell carcinoma and prostate adenocarcinoma, for which RHOV expression was downregulated, whereas RHOV expression was associated with a poor prognosis for patients with adenoid cystic carcinoma, lung adenocarcinoma, pancreatic ductal adenocarcinoma, skin cutaneous melanoma, and uveal melanoma with upregulated RHOV expression. Furthermore, RHOV expression was associated with various clinicopathological parameters in these tumors. RHOV expression showed varied associations with different types of tumor-infiltrating immune cells and demonstrated a potential impact on the response to immunotherapy depending on the cancer type. Additionally, functional enrichment analysis of RHOV-related genes demonstrated a role in a wide range of developmental and immune-related processes. This study provides valuable insights into the role of RHOV in pan-cancer development, indicating its role as a tumor suppressor or oncogene according to the cancer type and tumor microenvironment.

Neural crest cells development and neuroblastoma progression: Role of Wnt signaling

Neuroblastoma (NB) is one of the most common heterogeneous extracranial cancers in infancy that arises from neural crest (NC) cells of the sympathetic nervous system. The Wnt signaling pathway, both canonical and noncanonical pathway, is a highly conserved signaling pathway that regulates the development and differentiation of the NC cells during embryogenesis. Reports suggest that aberrant activation of Wnt ligands/receptors in Wnt signaling pathways promote progression and relapse of NB. Wnt signaling pathways regulate NC induction and migration in a similar manner; it regulates proliferation and metastasis of NB. Inhibiting the Wnt signaling pathway or its ligands/receptors induces apoptosis and abrogates proliferation and tumorigenicity in all major types of NB cells. Here, we comprehensively discuss the Wnt signaling pathway and its mechanisms in regulating the development of NC and NB pathogenesis. This review highlights the implications of aberrant Wnt signaling in the context of etiology, progression, and relapse of NB. We have also described emerging strategies for Wnt-based therapies against the progression of NB that will provide new insights into the development of Wnt-based therapeutic strategies for NB.

Single-cell RNA sequencing analysis to explore immune cell heterogeneity and novel biomarkers for the prognosis of lung adenocarcinoma

Background: Single-cell RNA sequencing is necessary to understand tumor heterogeneity, and the cell type heterogeneity of lung adenocarcinoma (LUAD) has not been fully studied.

Method: We first reduced the dimensionality of the GSE149655 single-cell data. Then, we statistically analysed the subpopulations obtained by cell annotation to find the subpopulations highly enriched in tumor tissues. Monocle was used to predict the development trajectory of five subpopulations; beam was used to find the regulatory genes of five branches; qval was used to screen the key genes; and cellchart was used to analyse cell communication. Next, we used the differentially expressed genes of TCGA-LUAD to screen for overlapping genes and established a prognostic risk model through univariate and multivariate analyses. To identify the independence of the model in clinical application, univariate and multivariate Cox regression were used to analyse the relevant HR, 95% CI of HR and p value. Finally, the novel biomarker genes were verified by qPCR and immunohistochemistry.

Results: The single-cell dataset GSE149655 was subjected to quality control, filtration and dimensionality reduction. Finally, 23 subpopulations were screened, and 11-cell subgroups were annotated in 23 subpopulations. Through the statistical analysis of 11 subgroups, five important subgroups were selected, including lung epithelial cells, macrophages, neuroendocrine cells, secret cells and T cells. From the analysis of cell trajectory and cell communication, it is found that the interaction of five subpopulations is very complex and that the communication between them is dense. We believe that these five subpopulations play a very important role in the occurrence and development of LUAD. Downloading the TCGA data, we screened the marker genes of these five subpopulations, which are also the differentially expressed genes in tumorigenesis, with a total of 462 genes, and constructed 10 gene prognostic risk models based on related genes. The 10-gene signature has strong robustness and can achieve stable prediction efficiency in datasets from different platforms. Two new molecular markers related to LUAD, HLA-DRB5 and CCDC50, were verified by qPCR and immunohistochemistry. The results showed that HLA-DRB5 expression was negatively correlated with the risk of LUAD, and CCDC50 expression was positively correlated with the risk of LUAD.

Conclusion: Therefore, we identified a prognostic risk model including CCL20, CP, HLA-DRB5, RHOV, CYP4B1, BASP1, ACSL4, GNG7, CCDC50 and SPATS2 as risk biomarkers and verified their predictive value for the prognosis of LUAD, which could serve as a new therapeutic target.

RHOV promotes lung adenocarcinoma cell growth and metastasis through JNK/c-Jun pathway

Lung adenocarcinoma (LUAD) is a common type of lung cancer with high frequent metastasis and a high death rate. However, genes responsible for LUAD metastasis are still largely unknown. Here, we identify an important role of ras homolog family member V (RHOV) in LUAD metastasis using a combination of bioinformatic analysis and functional experiments. Bioinformatic analysis shows five hub LUAD metastasis driver genes (RHOV, ZIC5, CYP4B1, GPR18 and TCP10L2), among which RHOV is the most significant gene associated with LUAD metastasis. High RHOV expression predicted shorter overall survival in LUAD patients. RHOV overexpression promotes proliferation, migration, and invasion of LUAD cells, whereas RHOV knockdown inhibits these biological behaviors. Moreover, knockdown of RHOV suppresses LUAD tumor growth and metastasis in nude mice. Mechanistically, RHOV activates Jun N-terminal Kinase (JNK)/c-Jun signalling pathway, an important pathway in lung cancer development and progression, and regulates the expression of markers of epithelial-to-mesenchymal transition, a process involved in cancer cell migration, invasion and metastasis. RHOV-induced malignant biological behaviors are inhibited by pyrazolanthrone, a JNK inhibitor. Our findings indicate a critical role of RHOV in LUAD metastasis and may provide a biomarker for prognostic prediction and a target for LUAD therapy.

Overexpression of RhoV Promotes the Progression and EGFR-TKI Resistance of Lung Adenocarcinoma

Background

The Rho GTPase family with ~20 member genes play central roles in a wide variety of cellular processes and tumor cell migration and metastasis. Different Rho GTPase may play different roles in the progression of lung adenocarcinoma.

Methods

We comprehensively examined the expression of all Rho GTPase family member genes in a panel of lung adenocarcinoma patient’s tumors and matched normal tissues. We next investigated the critical role of RhoV in different lung adenocarcinoma cells and animal models.

Results

RhoV was identified as one of the most significantly overexpressed Rho GTPases in lung adenocarcinoma and associated with patients’ survival. Silencing RhoV expression inhibits proliferation, migration and invasion, and tumorigenicity capacities of lung adenocarcinoma cells. Moreover, knockdown RhoV promoted the sensitivity of EGFR-TKI in the gefitinib resistant PC9 cells (PC9-GR) and aggravated gefitinib-induced lung cancer cell apoptosis both in PC9 and PC9-GR cells. Our data also indicated that RhoV induced progression and EGFR-TKI resistance of lung adenocarcinoma may be related to the activation of the AKT/ERK pathway.

Conclusion

Overexpression of RhoV in lung adenocarcinoma promotes the progression and EGFR-TKI resistance, suggesting RhoV is a promising prognosis and therapeutic target of lung adenocarcinoma.

The heart of the neural crest: cardiac neural crest cells in development and regeneration

Cardiac neural crest cells (cNCCs) are a migratory cell population that stem from the cranial portion of the neural tube. They undergo epithelial-to-mesenchymal transition and migrate through the developing embryo to give rise to portions of the outflow tract, the valves and the arteries of the heart. Recent lineage-tracing experiments in chick and zebrafish embryos have shown that cNCCs can also give rise to mature cardiomyocytes. These cNCC-derived cardiomyocytes appear to be required for the successful repair and regeneration of injured zebrafish hearts. In addition, recent work examining the response to cardiac injury in the mammalian heart has suggested that cNCC-derived cardiomyocytes are involved in the repair/regeneration mechanism. However, the molecular signature of the adult cardiomyocytes involved in this repair is unclear. In this Review, we examine the origin, migration and fates of cNCCs. We also review the contribution of cNCCs to mature cardiomyocytes in fish, chick and mice, as well as their role in the regeneration of the adult heart.

Regulation and functions of RhoU and RhoV

Rho GTPases play central roles in a wide variety of cellular processes, including cytoskeletal dynamics, cell adhesion and cell polarity. RhoU and RhoV are Rho GTPases that have some atypical properties compared with classical Rho family members, such as the presence of N- and C-terminal extension regions, unusual GDP/GTP cycling and post-translational modification by palmitoylation but not prenylation. Their activity and localization is regulated by the N-terminal and C-terminal regions, and so far no GEFs or GAPs have been identified for them. Similar to Rac and Cdc42, they interact with PAK serine/threonine kinases, and in the case of PAK4, this interaction leads to RhoU protein stabilization. In cells, RhoU and RhoV alter cell shape and cell adhesion, which probably underlies some of the phenotypes reported for these proteins in vivo, for example in heart development and epithelial morphogenesis. However, the molecular basis for these functions of RhoU and RhoV remains to be characterized.

Wnt Signaling in Neural Crest Ontogenesis and Oncogenesis

Neural crest (NC) cells are a temporary population of multipotent stem cells that generate a diverse array of cell types, including craniofacial bone and cartilage, smooth muscle cells, melanocytes, and peripheral neurons and glia during embryonic development. Defective neural crest development can cause severe and common structural birth defects, such as craniofacial anomalies and congenital heart disease. In the early vertebrate embryos, NC cells emerge from the dorsal edge of the neural tube during neurulation and then migrate extensively throughout the anterior-posterior body axis to generate numerous derivatives. Wnt signaling plays essential roles in embryonic development and cancer. This review summarizes current understanding of Wnt signaling in NC cell induction, delamination, migration, multipotency, and fate determination, as well as in NC-derived cancers.

The atypical RhoU/Wrch1 Rho GTPase controls cell proliferation and apoptosis in the gut epithelium

BACKGROUND

The mammalian gut epithelium displays among the highest rates of self-renewal, with a turnover time of less than 5 days. Renewal involves concerted proliferation at the bottom of the crypt, migration and differentiation along the crypt-villus axis and anoïkis/shedding in the luminal epithelium. Renewal is controlled by interplay between signalling pathways, among which canonical and non-canonical Wnt signals play prominent roles. Overall 92% of colon tumours show increased canonical Wnt signalling resulting from mutations, established as major driver steps towards carcinogenesis.

RESULTS

Here, we examined the physiological role of RhoU/Wrch1 in gut homeostasis. RhoU is an atypical Rho GTPase related to Cdc42/Rac1 and identified as a transcriptional target of non-canonical Wnt signalling. We found that RHOU expression is reduced in human colorectal tumour samples. We show that RhoU is mainly expressed in the differentiated compartment of the gut epithelium. Rhou specific invalidation in the mouse gut elicits cell hyperplasia and is associated in the colon with a highly disorganized luminal epithelium. Hyperplasia affects all cell types in the small intestine and colon and has a higher impact on goblet cells. Hyperplasia is associated with a reduction of apoptosis and an increased proliferation. RhoU knockdown in human DLD-1 colon cancer cells also elicits a higher growth index and reduces cell apoptosis. Last, loss of RhoU function in the mouse gut epithelium or in DLD-1 cells increases RhoA activity and the level of phosphorylated Myosin Light Chain-2, which may functionally link RhoU activity to apoptosis.

CONCLUSION

RhoU is mostly expressed in the differentiated compartment of the gut. It plays a role in homeostasis as its specific invalidation elicits hyperplasia of all cell types. This mainly results from a reduction of apoptosis, through actomyosin-dependent mechanisms.

SIGNIFICANCE

RhoU negatively controls cell growth in the intestinal epithelium. Since its expression is sensitive to non-canonical Wnt signals and is reduced in colorectal tumours, downregulating RhoU may thus have an instrumental role in tumour progression.

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.

The neural border: Induction, specification and maturation of the territory that generates neural crest cells

The neural crest is induced at the edge between the neural plate and the nonneural ectoderm, in an area called the neural (plate) border, during gastrulation and neurulation. In recent years, many studies have explored how this domain is patterned, and how the neural crest is induced within this territory, that also participates to the prospective dorsal neural tube, the dorsalmost nonneural ectoderm, as well as placode derivatives in the anterior area. This review highlights the tissue interactions, the cell-cell signaling and the molecular mechanisms involved in this dynamic spatiotemporal patterning, resulting in the induction of the premigratory neural crest. Collectively, these studies allow building a complex neural border and early neural crest gene regulatory network, mostly composed by transcriptional regulations but also, more recently, including novel signaling interactions.

Ketamine Modulates Zic5 Expression via the Notch Signaling Pathway in Neural Crest Induction

Ketamine is a potent dissociative anesthetic and the most commonly used illicit drug. Many addicts are women at childbearing age. Although ketamine has been extensively studied as a clinical anesthetic, its effects on embryonic development are poorly understood. Here, we applied the Xenopus model to study the effects of ketamine on development. We found that exposure to ketamine from pre-gastrulation (stage 7) to early neural plate (stage 13.5) resulted in disruption of neural crest (NC) derivatives. Ketamine exposure did not affect mesoderm development as indicated by the normal expression of Chordin, Xbra, Wnt8, and Fgf8. However, ketamine treatment significantly inhibited Zic5 and Slug expression at early neural plate stage. Overexpression of Zic5 rescued ketamine-induced Slug inhibition, suggesting the blockage of NC induction was mediated by Zic5. Furthermore, we found Notch signaling was altered by ketamine. Ketamine inhibited the expression of Notch targeted genes including Hes5.2a, Hes5.2b, and ESR1 and ketamine-treated embryos exhibited Notch-deficient somite phenotypes. A 15 bp core binding element upstream of Zic5 was induced by Notch signaling and caused transcriptional activation. These results demonstrated that Zic5 works as a downstream target gene of Notch signaling in Xenopus NC induction. Our study provides a novel teratogenic mechanism whereby ketamine disrupts NC induction via targeting a Notch-Zic5 signaling pathway.

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.

Neural crest specification by inhibition of the ROCK/Myosin II pathway

Neural crest is a population of multipotent progenitor cells that form at the border of neural and non-neural ectoderm in vertebrate embryos, and undergo epithelial-mesenchymal transition and migration. According to the traditional view, the neural crest is specified in early embryos by signaling molecules including BMP, FGF, and Wnt proteins. Here, we identify a novel signaling pathway leading to neural crest specification, which involves Rho-associated kinase (ROCK) and its downstream target nonmuscle Myosin II. We show that ROCK inhibitors promote differentiation of human embryonic stem cells (hESCs) into neural crest-like progenitors (NCPs) that are characterized by specific molecular markers and ability to differentiate into multiple cell types, including neurons, chondrocytes, osteocytes, and smooth muscle cells. Moreover, inhibition of Myosin II was sufficient for generating NCPs at high efficiency. Whereas Myosin II has been previously implicated in the self-renewal and survival of hESCs, we demonstrate its role in neural crest development during ESC differentiation. Inhibition of this pathway in Xenopus embryos expanded neural crest in vivo, further indicating that neural crest specification is controlled by ROCK-dependent Myosin II activity. We propose that changes in cell morphology in response to ROCK and Myosin II inhibition initiate mechanical signaling leading to neural crest fates.

Atypical RhoV and RhoU GTPases control development of the neural crest

This review addresses the developmental roles of 2 GTPases of the Rho family, RhoV/Chp and RhoU/Wrch. These two GTPases form a distinct subfamily related to Rac and Cdc42 proteins and were detected in a screen for Rho members that are particularly expressed in the neural crest, an embryonic tissue peculiar to vertebrates. The neural crest represents a physiological model of normal epithelial to mesenchymal transition (EMT), in which epithelial cells at the border of neural and non-neural ectoderm differentiate, lose their intercellular connections and migrate throughout the embryo. We showed that RhoV, transiently induced by the canonical Wnt pathway, is required for the full differentiation of neural crest cells, while RhoU, induced later by the non-canonical Wnt pathway, is necessary for the migration process. These two GTPases, which are highly conserved across vertebrates, are thus tightly functionally linked to Wnt signaling, whose implication in embryonic development and cancer progression is well established. In the light of the recent literature, we discuss how RhoV and RhoU may achieve their physiological functions.

Establishing neural crest identity: a gene regulatory recipe

The neural crest is a stem/progenitor cell population that contributes to a wide variety of derivatives, including sensory and autonomic ganglia, cartilage and bone of the face and pigment cells of the skin. Unique to vertebrate embryos, it has served as an excellent model system for the study of cell behavior and identity owing to its multipotency, motility and ability to form a broad array of cell types. Neural crest development is thought to be controlled by a suite of transcriptional and epigenetic inputs arranged hierarchically in a gene regulatory network. Here, we examine neural crest development from a gene regulatory perspective and discuss how the underlying genetic circuitry results in the features that define this unique cell population.

Atypical RhoV and RhoU GTPases control development of the neural crest

This review addresses the developmental roles of two GTPases of the Rho family, RhoV/Chp and RhoU/Wrch. These two GTPases form a distinct subfamily related to Rac and Cdc42 proteins and were detected in a screen for Rho members that are particularly expressed in the neural crest, an embryonic tissue peculiar to vertebrates. The neural crest represents a physiological model of normal epithelial to mesenchymal transition (EMT), in which epithelial cells at the border of neural and non-neural ectoderm differentiate, lose their intercellular connections and migrate throughout the embryo. We showed that RhoV, transiently induced by the canonical Wnt pathway, is required for the full differentiation of neural crest cells, while RhoU, induced later by the non-canonical Wnt pathway, is necessary for the migration process. These two GTPases, which are highly conserved across vertebrates, are thus tightly functionally linked to Wnt signaling, whose implication in embryonic development and cancer progression is well established. In the light of the recent literature, we discuss how RhoV and RhoU may achieve their physiological functions.

Rho family GTPase Chp/RhoV induces PC12 apoptotic cell death via JNK activation

Rho GTPases regulate numerous cellular processes including apoptosis. Chp/RhoV is an atypical Rho GTPase which functions are poorly understood. Here we investigated the role of Chp in regulation of cell viability using PC12 cells with inducible expression of Chp as a model. We found that expression of Chp results in apoptosis in PC12 cells. Chp-induced apoptosis was accompanied by activation of JNK signaling and both death receptor-mediated and mitochondrial apoptotic pathways as justified by caspase-8 and caspase-9 activation, respectively. Moreover, inhibition of JNK by SP600125 rescued PC12 cells from Chp-triggered cell death and attenuated activation of caspases-9 and -3/7 suggesting that activation of JNK mediates pro-apoptotic effect of Chp. Expression of Chp resulted in increased phosphorylation of c-Jun in PC12 cells, and Chp expression in HE K293 cells upregulated AP-1-dependent transcription in a JNK-dependent manner. Together results of our study reveal the role of Chp GTPase as a putative regulator of JNK-dependent apoptotic death in PC12 cells, similarly to previously described pro-apoptotic activity of the related Cdc42 and Rac1 GTPases.

Kruppel-like factor-9 (KLF9) inhibits glioblastoma stemness through global transcription repression and integrin α6 inhibition

It is increasingly important to understand the molecular basis for the plasticity of neoplastic cells and their capacity to transition between differentiated and stemlike phenotypes. Kruppel-like factor-9 (KLF9), a member of the large KLF transcription factor family, has emerged as a regulator of oncogenesis, cell differentiation, and neural development; however, the molecular basis for the diverse contextual functions of KLF9 remains unclear. This study focused on the functions of KLF9 in human glioblastoma stemlike cells. We established for the first time a genome-wide map of KLF9-regulated targets in human glioblastoma stemlike cells and show that KLF9 functions as a transcriptional repressor and thereby regulates multiple signaling pathways involved in oncogenesis and stem cell regulation. A detailed analysis of one such pathway, integrin signaling, showed that the capacity of KLF9 to inhibit glioblastoma cell stemness and tumorigenicity requires ITGA6 repression. These findings enhance our understanding of the transcriptional networks underlying cancer cell stemness and differentiation and identify KLF9-regulated molecular targets applicable to cancer therapeutics.

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.

The atypical Rho GTPase, RhoU, regulates cell-adhesion molecules during cardiac morphogenesis

The vertebrate heart undergoes early complex morphologic events in order to develop key cardiac structures that regulate its overall function (Fahed et al., 2013). Although many genetic factors that participate in patterning the heart have been elucidated (Tu and Chi, 2012), the cellular events that drive cardiac morphogenesis have been less clear. From a chemical genetic screen to identify cellular pathways that control cardiac morphogenesis in zebrafish, we observed that inhibition of the Rho signaling pathways resulted in failure to form the atrioventricular canal and loop the linear heart tube. To identify specific Rho proteins that may regulate this process, we analyzed cardiac expression profiling data and discovered that RhoU was expressed at the atrioventricular canal during the time when it forms. Loss of RhoU function recapitulated the atrioventricular canal and cardiac looping defects observed in the ROCK inhibitor treated zebrafish. Similar to its family member RhoV/Chp (Tay et al., 2010), we discovered that RhoU regulates the cell junctions between cardiomyocytes through the Arhgef7b/Pak kinase pathway in order to guide atrioventricular canal development and cardiac looping. Inhibition of this pathway resulted in similar underlying cardiac defects and conversely, overexpression of a PAK kinase was able to rescue the loss of RhoU cardiac defect. Finally, we found that Wnt signaling, which has been implicated in atrioventricular canal development (Verhoeven et al., 2011), may regulate the expression of RhoU at the atrioventricular canal. Overall, these findings reveal a cardiac developmental pathway involving RhoU/Arhgef7b/Pak signaling, which helps coordinate cell junction formation between atrioventricular cardiomyocytes to promote cell adhesiveness and cell shapes during cardiac morphogenesis. Failure to properly form these cell adhesions during cardiac development may lead to structural heart defects and mechanistically account for the cellular events that occur in certain human congenital heart diseases.

The RHOV gene is overexpressed in human non-small cell lung cancer

Rho family GTPases act as molecular switches to regulate numerous cellular processes, including malignant transformation. Commonly, overexpression of Rho GTPases contributes to tumorigenesis. Elevated expression of several Rho GTPases has been reported in lung cancer and is associated with poor prognosis. The RHOV gene encodes the atypical Rho family GTPase Chp/RhoV, which is capable of transforming fibroblasts, although other functions of Chp remain largely elusive. RHOV is expressed in normal lung tissue in rats, but not in humans. RHOV expression was found in several human cancer cell lines, including non-small-cell lung cancer (NSCLC) cell line A549, but expression of RHOV in NSCLC tumors has never been investigated. Here we studied the expression of the RHOV gene in lung cancer cell lines and in 29 matched pairs of NSCLC tumors and adjacent nontumorous tissues. We found that RHOV is expressed in lung cancer cell lines and is upregulated in the majority of studied lung tumors. Analysis of the Oncomine database revealed correlation between elevated RHOV level and poor patient survival. We propose that the RHOV gene could be validated as a diagnostic or prognostic marker for NSCLC, and that observed overexpression of RHOV might contribute to tumorigenesis.

Rho activation is apically restricted by Arhgap1 in neural crest cells and drives epithelial-to-mesenchymal transition

Epithelial-to-mesenchymal transitions (EMTs) are crucial for morphogenesis and carcinoma metastasis, yet mechanisms controlling the underlying cell behaviors are poorly understood. RhoGTPase signaling has been implicated in EMT; however, previous studies have yielded conflicting results regarding Rho function, and its role in EMT remains poorly understood. Elucidation of precise Rho functions has been challenging because Rho signaling is highly context dependent and its activity is tightly regulated spatiotemporally within the cell. To date, few studies have examined how Rho affects cell motility in intact organisms, and the pattern of Rho activity during motile cell behaviors of EMT has not been determined in any system. Here, we image endogenous active Rho during EMT in vivo, and analyze effects of Rho and Rho-kinase (ROCK) manipulation on cell motility in vivo. We show that Rho is activated in a discrete apical region of premigratory neural crest cells during EMT, and Rho-ROCK signaling is essential for apical detachment and generation of motility within the neuroepithelium, a process that has been poorly understood. Furthermore, we find that Arhgap1 restricts Rho activation to apical areas, and this restriction is necessary for detachment. Our results provide new insight into mechanisms controlling local Rho activation and how it affects dynamic cell behaviors and actomyosin contraction during key steps of EMT in an intact living organism.

Pak6 protein kinase is a novel effector of an atypical Rho family GTPase Chp/RhoV

Chp/RhoV is an atypical Rho GTPase whose functions are far from being fully understood. To date several effector proteins of Chp have been identified, including p21-activated kinases Pak1, Pak2, and Pak4. Using a yeast two-hybrid system and co-immunoprecipitation, here we show that another p21-activated kinase, Pak6, is a novel Chp-binding protein. Interaction between Chp and Pak6 depends on the activation state of the GTPase, suggesting that Pak6 is an effector protein for Chp. Point mutations in the effector domain of Chp or in the CRIB motif of Pak6 significantly impair the interaction between Chp and Pak6 upon co-immunoprecipitation, suggesting that the binding interface involves the effector domain of Chp and the CRIB motif in Pak6. We found that Chp does not affect the phosphorylation status of the S560 residue in the catalytic domain of Pak6 when Chp and Pak6 are co-expressed in HEK293 cells. Therefore, similarly to Cdc42, Chp is not likely to activate Pak6. In NCI-H1299 cells, Chp co-localizes with Pak6 on vesicular structures in activation state-dependent manner. Taking the data together, we report here the identification of p21-activated kinase Pak6 as a novel effector of the atypical Rho GTPase Chp. Our data suggest further directions in elucidating biological functions of these proteins.

What is bad in cancer is good in the embryo: importance of EMT in neural crest development

Although the epithelial to mesenchymal transition (EMT) is famous for its role in cancer metastasis, it also is a normal developmental event in which epithelial cells are converted into migratory mesenchymal cells. A prime example of EMT during development occurs when neural crest (NC) cells emigrate from the neural tube thus providing an excellent model to study the principles of EMT in a nonmalignant environment. NC cells start life as neuroepithelial cells intermixed with precursors of the central nervous system. After EMT, they delaminate and begin migrating, often to distant sites in the embryo. While proliferating and maintaining multipotency and cell survival the transitioning neural crest cells lose apicobasal polarity and the basement membrane is broken down. This review discusses how these events are coordinated and regulated, by series of events involving signaling factors, gene regulatory interactions, as well as epigenetic and post-transcriptional modifications. Even though the series of events involved in NC EMT are well known, the sequence in which these steps take place remains a subject of debate, raising the intriguing possibility that, rather than being a single event, neural crest EMT may involve multiple parallel mechanisms.

Neural crest specification by noncanonical Wnt signaling and PAR-1

Neural crest (NC) cells are multipotent progenitors that form at the neural plate border, undergo epithelial-mesenchymal transition and migrate to diverse locations in vertebrate embryos to give rise to many cell types. Multiple signaling factors, including Wnt proteins, operate during early embryonic development to induce the NC cell fate. Whereas the requirement for the Wnt/β-catenin pathway in NC specification has been well established, a similar role for Wnt proteins that do not stabilize β-catenin has remained unclear. Our gain- and loss-of-function experiments implicate Wnt11-like proteins in NC specification in Xenopus embryos. In support of this conclusion, modulation of β-catenin-independent signaling through Dishevelled and Ror2 causes predictable changes in premigratory NC. Morpholino-mediated depletion experiments suggest that Wnt11R, a Wnt protein that is expressed in neuroectoderm adjacent to the NC territory, is required for NC formation. Wnt11-like signals might specify NC by altering the localization and activity of the serine/threonine polarity kinase PAR-1 (also known as microtubule-associated regulatory kinase or MARK), which itself plays an essential role in NC formation. Consistent with this model, PAR-1 RNA rescues NC markers in embryos in which noncanonical Wnt signaling has been blocked. These experiments identify novel roles for Wnt11R and PAR-1 in NC specification and reveal an unexpected connection between morphogenesis and cell fate.

RNA profiling and chromatin immunoprecipitation-sequencing reveal that PTF1a stabilizes pancreas progenitor identity via the control of MNX1/HLXB9 and a network of other transcription factors

Pancreas development is initiated by the specification and expansion of a small group of endodermal cells. Several transcription factors are crucial for progenitor maintenance and expansion, but their interactions and the downstream targets mediating their activity are poorly understood. Among those factors, PTF1a, a basic helix-loop-helix (bHLH) transcription factor which controls pancreas exocrine cell differentiation, maintenance, and functionality, is also needed for the early specification of pancreas progenitors. We used RNA profiling and chromatin immunoprecipitation (ChIP) sequencing to identify a set of targets in pancreas progenitors. We demonstrate that Mnx1, a gene that is absolutely required in pancreas progenitors, is a major direct target of PTF1a and is regulated by a distant enhancer element. Pdx1, Nkx6.1, and Onecut1 are also direct PTF1a targets whose expression is promoted by PTF1a. These proteins, most of which were previously shown to be necessary for pancreas bud maintenance or formation, form a transcription factor network that allows the maintenance of pancreas progenitors. In addition, we identify Bmp7, Nr5a2, RhoV, and P2rx1 as new targets of PTF1a in pancreas progenitors.

Normal prostate-derived stromal cells stimulate prostate cancer development

Stromal cells play a decisive role in regulating tumor progression. In this study, we assessed the significance of normal prostate-derived stromal cells (PSCs) in prostate cancer development. An in vivo s.c. tumor model was established as follows: Group 1, DU145 cells alone; Group 2, DU145 + PSCs; Group 3, DU145 cells alone injected into pre-castrated mice; and Group 4, DU145 + PSCs injected into pre-castrated mice. Following injection, tumors were only detectable in the first two groups, with more aggressive growth in Group 2 than in Group 1 (P < 0.05). Immunohistochemical analysis revealed significantly higher proliferation (P < 0.05), but not apoptosis or altered expression of androgen receptor in Group 2, as compared with Group 1. In vitro, DU145 cells isolated from Group 1 tumors showed lower viability and migratory capability than those from Group 2. cDNA microarray on isolated DU145 cells from Groups 1 and 2 revealed the differential expression of genes regulating cell cycle progression and cell mobility, including GADD45A, RHOV, KLK11, and PCK1. Our results suggest that stromal cells derived from normal prostate potentiate the development of tumor growth in vivo, which is achieved at least in part through the regulation of cell-cycle- and migration-related gene expression within the tumor cells.

Evolutionary plasticity of segmentation clock networks

The vertebral column is a conserved anatomical structure that defines the vertebrate phylum. The periodic or segmental pattern of the vertebral column is established early in development when the vertebral precursors, the somites, are rhythmically produced from presomitic mesoderm (PSM). This rhythmic activity is controlled by a segmentation clock that is associated with the periodic transcription of cyclic genes in the PSM. Comparison of the mouse, chicken and zebrafish PSM oscillatory transcriptomes revealed networks of 40 to 100 cyclic genes mostly involved in Notch, Wnt and FGF signaling pathways. However, despite this conserved signaling oscillation, the identity of individual cyclic genes mostly differed between the three species, indicating a surprising evolutionary plasticity of the segmentation networks.

Prohibitin1 acts as a neural crest specifier in Xenopus development by repressing the transcription factor E2F1

Prohibitin 1 (phb1), which was initially described as an inhibitor of cell proliferation, is a highly conserved protein found in multiple cellular compartments. In the nucleus it interacts with the transcriptional regulators Rb and E2F1 and controls cell proliferation and apoptosis. Here we unravel an unexpected novel function for phb1 in Xenopus cranial neural crest (CNC) development. Xphb1 is maternally expressed; zygotically expressed neurula stage transcripts accumulate in the CNC and the neural tube. Knockdown of Xphb1 by antisense morpholino injection results in the loss of foxD3, snail2 and twist expression, whereas expression of c-myc, AP-2 and snail1 remains unaffected. Xphb2, its closest relative, cannot substitute for Xphb1, underlining the specificity of Xphb1 function. Epistatic analyses place Xphb1 downstream of c-myc and upstream of foxD3, snail2 and twist. To elucidate which subdomain in Xphb1 is required for neural crest gene regulation we generated deletion mutants and tested their rescue ability in Xphb1 morphants. The E2F1-binding domain was found to be necessary for Xphb1 function in neural crest development. Gain- and loss-of-function experiments reveal that Xphb1 represses E2F1 activity; suppression of E2F1 through Xphb1 is required for twist, snail2 and foxD3 expression in the CNC. With the Xphb1 dependency of a subset of CNC specifiers downstream of c-myc, we have identified a new branching point in the neural crest gene regulatory network.

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.

Diversity in the molecular and cellular strategies of epithelium-to-mesenchyme transitions: Insights from the neural crest

Although epithelial to mesenchymal transitions (EMT) are often viewed as a unique event, they are characterized by a great diversity of cellular processes resulting in strikingly different outcomes. They may be complete or partial, massive or progressive, and lead to the complete disruption of the epithelium or leave it intact. Although the molecular and cellular mechanisms of EMT are being elucidated owing chiefly from studies on transformed epithelial cell lines cultured in vitro or from cancer cells, the basis of the diversity of EMT processes remains poorly understood. Clues can be collected from EMT occuring during embryonic development and which affect equally tissues of ectodermal, endodermal or mesodermal origins. Here, based on our current knowledge of the diversity of processes underlying EMT of neural crest cells in the vertebrate embryo, we propose that the time course and extent of EMT do not depend merely on the identity of the EMT transcriptional regulators and their cellular effectors but rather on the combination of molecular players recruited and on the possible coordination of EMT with other cellular processes.

A vertebrate-specific Chp-PAK-PIX pathway maintains E-cadherin at adherens junctions during zebrafish epiboly

Background

In early vertebrate development, embryonic tissues modulate cell adhesiveness and acto-myosin contractility to correctly orchestrate the complex processes of gastrulation. E-cadherin (E-cadh) is the earliest expressed cadherin and is needed in the mesendodermal progenitors for efficient migration [1], [2]. Regulatory mechanisms involving directed E-cadh trafficking have been invoked downstream of Wnt11/5 signaling [3]. This non-canonical Wnt pathway regulates RhoA-ROK/DAAM1 to control the acto-myosin network. However, in this context nothing is known of the intracellular signals that participate in the correct localization of E-cadh, other than a need for Rab5c signaling [3].

Methodology/Principal Findings

By studying loss of Chp induced by morpholino-oligonucleotide injection in zebrafish, we find that the vertebrate atypical Rho-GTPase Chp is essential for the proper disposition of cells in the early embryo. The underlying defect is not leading edge F-actin assembly (prominent in the cells of the envelope layer), but rather the failure to localize E-cadh and β-catenin at the adherens junctions. Loss of Chp results in delayed epiboly that can be rescued by mRNA co-injection, and phenocopies zebrafish E-cadh mutants [4], [5]. This new signaling pathway involves activation of an effector kinase PAK, and involvement of the adaptor PAK-interacting exchange factor PIX. Loss of signaling by any of the three components results in similar underlying defects, which is most prominent in the epithelial-like envelope layer.

Conclusions/Significance

Our current study uncovers a developmental pathway involving Chp/PAK/PIX signaling, which helps co-ordinate E-cadh disposition to promote proper cell adhesiveness, and coordinate movements of the three major cell layers in epiboly. Our data shows that without Chp signaling, E-cadh shifts to intracellular vesicles rather than the adhesive contacts needed for directed cell movement. These events may mirror the requirement for PAK2 signaling essential for the proper formation of the blood-brain barrier [6], [7].

Xenopus Rnd1 and Rnd3 GTP-binding proteins are expressed under the control of segmentation clock and required for somite formation

The process of segmentation in vertebrates is described by a clock and wavefront model consisting of a Notch signal and an fibroblast growth factor-8 (FGF8) gradient, respectively. To further investigate the segmentation process, we screened gene expression profiles for downstream targets of the segmentation clock. The Rnd1 and Rnd3 GTP-binding proteins comprise a subgroup of the Rho GTPase family that show a specific expression pattern similar to the Notch signal component ESR5, suggesting an association between Rnd1/3 and the segmentation clock. Rnd1/3 expression patterns are disrupted by overexpression of dominant-negative or active forms of Notch signaling genes, and responds to the FGF inhibitor SU5402 by a posterior shift analogous to other segmentation-related genes, suggesting that Rnd1/3 expressions are regulated by the segmentation clock machinery. We also show that antisense morpholino oligonucleotides to Rnd1/3 inhibit somite segmentation and differentiation in Xenopus embryos. These results suggest that Rnd1/3 are required for Xenopus somitogenesis.

Epithelial-mesenchymal transitions in development and disease

The epithelial to mesenchymal transition (EMT) plays crucial roles in the formation of the body plan and in the differentiation of multiple tissues and organs. EMT also contributes to tissue repair, but it can adversely cause organ fibrosis and promote carcinoma progression through a variety of mechanisms. EMT endows cells with migratory and invasive properties, induces stem cell properties, prevents apoptosis and senescence, and contributes to immunosuppression. Thus, the mesenchymal state is associated with the capacity of cells to migrate to distant organs and maintain stemness, allowing their subsequent differentiation into multiple cell types during development and the initiation of metastasis.

Stage-specific control of neural crest stem cell proliferation by the small rho GTPases Cdc42 and Rac1

The neural crest (NC) generates a variety of neural and non-neural tissues during vertebrate development. Both migratory NC cells and their target structures contain cells with stem cell features. Here we show that these populations of neural crest-derived stem cells (NCSCs) are differentially regulated by small Rho GTPases. Deletion of either Cdc42 or Rac1 in the NC results in size reduction of multiple NC target structures because of increased cell-cycle exit, while NC cells emigrating from the neural tube are not affected. Consistently, Cdc42 or Rac1 inactivation reduces self-renewal and proliferation of later stage, but not early migratory NCSCs. This stage-specific requirement for small Rho GTPases is due to changes in NCSCs that, during development, acquire responsiveness to mitogenic EGF acting upstream of both Cdc42 and Rac1. Thus, our data reveal distinct mechanisms for growth control of NCSCs from different developmental stages.

A role for Syndecan-4 in neural induction involving ERK- and PKC-dependent pathways

Syndecan-4 (Syn4) is a heparan sulphate proteoglycan that is able to bind to some growth factors, including FGF, and can control cell migration. Here we describe a new role for Syn4 in neural induction in Xenopus. Syn4 is expressed in dorsal ectoderm and becomes restricted to the neural plate. Knockdown with antisense morpholino oligonucleotides reveals that Syn4 is required for the expression of neural markers in the neural plate and in neuralised animal caps. Injection of Syn4 mRNA induces the cell-autonomous expression of neural, but not mesodermal, markers. We show that two parallel pathways are involved in the neuralising activity of Syn4: FGF/ERK, which is sensitive to dominant-negative FGF receptor and to the inhibitors SU5402 and U0126, and a PKC pathway, which is dependent on the intracellular domain of Syn4. Neural induction by Syn4 through the PKC pathway requires inhibition of PKCdelta and activation of PKCalpha. We show that PKCalpha inhibits Rac GTPase and that c-Jun is a target of Rac. These findings might account for previous reports implicating PKC in neural induction and allow us to propose a link between FGF and PKC signalling pathways during neural induction.

A negative modulatory role for rho and rho-associated kinase signaling in delamination of neural crest cells

Background

Neural crest progenitors arise as epithelial cells and then undergo a process of epithelial to mesenchymal transition that precedes the generation of cellular motility and subsequent migration. We aim at understanding the underlying molecular network. Along this line, possible roles of Rho GTPases that act as molecular switches to control a variety of signal transduction pathways remain virtually unexplored, as are putative interactions between Rho proteins and additional known components of this cascade.

Results

We investigated the role of Rho/Rock signaling in neural crest delamination. Active RhoA and RhoB are expressed in the membrane of epithelial progenitors and are downregulated upon delamination. In vivo loss-of-function of RhoA or RhoB or of overall Rho signaling by C3 transferase enhanced and/or triggered premature crest delamination yet had no effect on cell specification. Consistently, treatment of explanted neural primordia with membrane-permeable C3 or with the Rock inhibitor Y27632 both accelerated and enhanced crest emigration without affecting cell proliferation. These treatments altered neural crest morphology by reducing stress fibers, focal adhesions and downregulating membrane-bound N-cadherin. Reciprocally, activation of endogenous Rho by lysophosphatidic acid inhibited emigration while enhancing the above. Since delamination is triggered by BMP and requires G1/S transition, we examined their relationship with Rho. Blocking Rho/Rock function rescued crest emigration upon treatment with noggin or with the G1/S inhibitor mimosine. In the latter condition, cells emigrated while arrested at G1. Conversely, BMP4 was unable to rescue cell emigration when endogenous Rho activity was enhanced by lysophosphatidic acid.

Conclusion

Rho-GTPases, through Rock, act downstream of BMP and of G1/S transition to negatively regulate crest delamination by modifying cytoskeleton assembly and intercellular adhesion.

Dynamic expression patterns of RhoV/Chp and RhoU/Wrch during chicken embryonic development

Rho GTPases play central roles in the control of cell adhesion and migration, cell cycle progression, growth, and differentiation. However, although most of our knowledge of Rho GTPase function comes from the study of the three classic Rho GTPases RhoA, Rac1, and Cdc42, recent studies have begun to explore the expression, regulation, and function of some of the lesser-known members of the Rho GTPase family. In the present study, we cloned the avian orthologues of RhoV (or Chp for Cdc42 homologous protein) and RhoU (or Wrch-1 for Wnt-regulated Cdc42 homolog-1) and examined their expression patterns by in situ hybridization analysis both during early chick embryogenesis and later on, during gastrointestinal tract development. Our data show that both GTPases are detected in the primitive streak, the somites, the neural crest cells, and the gastrointestinal tract with distinct territories and/or temporal expression windows. Although both proteins are 90% identical, our results indicate that cRhoV and cRhoU are distinctly expressed during chicken embryonic development.

A gene regulatory network orchestrates neural crest formation

The neural crest is a multipotent, migratory cell population that is unique to vertebrate embryos and gives rise to many derivatives, ranging from the peripheral nervous system to the craniofacial skeleton and pigment cells. A multimodule gene regulatory network mediates the complex process of neural crest formation, which involves the early induction and maintenance of the precursor pool, emigration of the neural crest progenitors from the neural tube via an epithelial to mesenchymal transition, migration of progenitor cells along distinct pathways and overt differentiation into diverse cell types. Here, we review our current understanding of these processes and discuss the molecular players that are involved in the neural crest gene regulatory network.