RhoG GTPase controls a pathway that independently activates Rac1 and Cdc42Hs

The evolution of mammalian Rem2: unraveling the impact of purifying selection and coevolution on protein function, and implications for human disorders

Rad And Gem-Like GTP-Binding Protein 2 (Rem2), a member of the RGK family of Ras-like GTPases, is implicated in Huntington's disease and Long QT Syndrome and is highly expressed in the brain and endocrine cells. We examine the evolutionary history of Rem2 identified in various mammalian species, focusing on the role of purifying selection and coevolution in shaping its sequence and protein structural constraints. Our analysis of Rem2 sequences across 175 mammalian species found evidence for strong purifying selection in 70% of non-invariant codon sites which is characteristic of essential proteins that play critical roles in biological processes and is consistent with Rem2's role in the regulation of neuronal development and function. We inferred epistatic effects in 50 pairs of codon sites in Rem2, some of which are predicted to have deleterious effects on human health. Additionally, we reconstructed the ancestral evolutionary history of mammalian Rem2 using protein structure prediction of extinct and extant sequences which revealed the dynamics of how substitutions that change the gene sequence of Rem2 can impact protein structure in variable regions while maintaining core functional mechanisms. By understanding the selective pressures, protein- and gene - interactions that have shaped the sequence and structure of the Rem2 protein, we gain a stronger understanding of its biological and functional constraints.

Role of RhoG as a regulator of cellular functions: integrating insights on immune cell activation, migration, and functions

BACKGROUND

RhoG is a multifaceted member of the Rho family of small GTPases, sharing the highest sequence identity with the Rac subfamily members. It acts as a molecular switch, when activated, plays a central role in regulating the fundamental processes in immune cells, such as actin-cytoskeleton dynamics, transendothelial migration, survival, and proliferation, including immunological functions (e.g., phagocytosis and trogocytosis) during inflammatory responses.

METHOD

We have performed a literature review based on published original and review articles encompassing the significant effect of RhoG on immune cell functions from central databases, including PubMed and Google Scholar.

RESULTS AND CONCLUSIONS

Recently published data shows that the dynamic expression of different transcription factors, non-coding RNAs, and the spatiotemporal coordination of different GEFs with their downstream effector molecules regulates the cascade of Rho signaling in immune cells. Additionally, alterations in RhoG-specific signaling can lead to physiological, pathological, and developmental adversities. Several mutations and RhoG-modulating factors are also known to pre-dispose the downstream signaling with abnormal gene expression linked to multiple diseases. This review focuses on the cellular functions of RhoG, interconnecting different signaling pathways, and speculates the importance of this small GTPase as a prospective target against several pathological conditions.

RhoG's Role in T Cell Activation and Function

The role of RhoG in T cell development is redundant with other Racs subfamily members, and this redundancy may be attributed to redundant signal transduction pathways. However, the absence of RhoG increases TCR signalling and proliferation, implying that RhoG activity is critical during late T cell activation following antigen–receptor interaction. Moreover, RhoG is required to halt signal transduction and prevent hyper-activated T cells. Despite increase in TCR signalling, cell proliferation is inhibited, implying that RhoG induces T cell anergy by promoting the activities of transcription factors, including nuclear factor of activated T cell (NFAT)/AP-1. The role of NFAT plays in T cell anergy is inducing the transcription of anergy-associated genes, such as IL-2, IL-5, and IFN-γ. Although information about RhoG in T cell-related diseases is limited, mutant forms of RhoG, Ala151Ser and Glu171Lys have been observed in thymoma and hemophagocytic lymphohistiocytosis (HLH), respectively. Current information only focuses on these two diseases, and thus the role of RhoG in normal and pathological circumstances should be further investigated. This approach is necessary because RhoG and its associated proteins represent prospective targets for attack particularly in the therapy of cancer and immune-mediated illnesses.

Rac GTPase Signaling in Immune-Mediated Mechanisms of Atherosclerosis

Coronary artery disease caused by atherosclerosis is a major cause of morbidity and mortality around the world. Data from preclinical and clinical studies support the belief that atherosclerosis is an inflammatory disease that is mediated by innate and adaptive immune signaling mechanisms. This review sought to highlight the role of Rac-mediated inflammatory signaling in the mechanisms driving atherosclerotic calcification. In addition, current clinical treatment strategies that are related to targeting hypercholesterolemia as a critical risk factor for atherosclerotic vascular disease are addressed in relation to the effects on Rac immune signaling and the implications for the future of targeting immune responses in the treatment of calcific atherosclerosis.

Role of RhoC in cancer cell migration

Migration is one of the five major behaviors of cells. Although RhoC-a classic member of the Rho gene family-was first identified in 1985, functional RhoC data have only been widely reported in recent years. Cell migration involves highly complex signaling mechanisms, in which RhoC plays an essential role. Cell migration regulated by RhoC-of which the most well-known function is its role in cancer metastasis-has been widely reported in breast, gastric, colon, bladder, prostate, lung, pancreatic, liver, and other cancers. Our review describes the role of RhoC in various types of cell migration. The classic two-dimensional cell migration cycle constitutes cell polarization, adhesion regulation, cell contraction and tail retraction, most of which are modulated by RhoC. In the three-dimensional cell migration model, amoeboid migration is the most classic and well-studied model. Here, RhoC modulates the formation of membrane vesicles by regulating myosin II, thereby affecting the rate and persistence of amoeba-like migration. To the best of our knowledge, this review is the first to describe the role of RhoC in all cell migration processes. We believe that understanding the detail of RhoC-regulated migration processes will help us better comprehend the mechanism of cancer metastasis. This will contribute to the study of anti-metastatic treatment approaches, aiding in the identification of new intervention targets for therapeutic or genetic transformational purposes.

Progress in the therapeutic inhibition of Cdc42 signalling

Cdc42 is a member of the Rho family of small GTPases and a key regulator of the actin cytoskeleton, controlling cell motility, polarity and cell cycle progression. It signals downstream of the master regulator Ras and is essential for cell transformation by this potent oncogene. Overexpression of Cdc42 is observed in several cancers, where it is linked to poor prognosis. As a regulator of both cell architecture and motility, deregulation of Cdc42 is also linked to tumour metastasis. Like Ras, Cdc42 and other components of the signalling pathways it controls represent important potential targets for cancer therapeutics. In this review, we consider the progress that has been made targeting Cdc42, its regulators and effectors, including new modalities and new approaches to inhibition. Strategies under consideration include inhibition of lipid modification, modulation of Cdc42-GEF, Cdc42-GDI and Cdc42-effector interactions, and direct inhibition of downstream effectors.

RhoG deficiency abrogates cytotoxicity of human lymphocytes and causes hemophagocytic lymphohistiocytosis

Exocytosis of cytotoxic granules (CG) by lymphocytes is required for the elimination of infected and malignant cells. Impairments in this process underly a group of diseases with dramatic hyperferritinemic inflammation termed hemophagocytic lymphohistiocytosis (HLH). Although genetic and functional studies of HLH have identified proteins controlling distinct steps of CG exocytosis, the molecular mechanisms that spatiotemporally coordinate CG release remain partially elusive. We studied a patient exhibiting characteristic clinical features of HLH associated with markedly impaired cytotoxic T lymphocyte (CTL) and natural killer (NK) cell exocytosis functions, who beared biallelic deleterious mutations in the gene encoding the small GTPase RhoG. Experimental ablation of RHOG in a model cell line and primary CTLs from healthy individuals uncovered a hitherto unappreciated role of RhoG in retaining CGs in the vicinity of the plasma membrane (PM), a fundamental prerequisite for CG exocytotic release. We discovered that RhoG engages in a protein-protein interaction with Munc13-4, an exocytosis protein essential for CG fusion with the PM. We show that this interaction is critical for docking of Munc13-4+ CGs to the PM and subsequent membrane fusion and release of CG content. Thus, our study illuminates RhoG as a novel essential regulator of human lymphocyte cytotoxicity and provides the molecular pathomechanism behind the identified here and previously unreported genetically determined form of HLH.

Trio family proteins as regulators of cell migration and morphogenesis in development and disease - mechanisms and cellular contexts

The well-studied members of the Trio family of proteins are Trio and kalirin in vertebrates, UNC-73 in Caenorhabditis elegans and Trio in Drosophila Trio proteins are key regulators of cell morphogenesis and migration, tissue organization, and secretion and protein trafficking in many biological contexts. Recent discoveries have linked Trio and kalirin to human disease, including neurological disorders and cancer. The genes for Trio family proteins encode a series of large multidomain proteins with up to three catalytic activities and multiple scaffolding and protein-protein interaction domains. As such, Trio family proteins engage a wide array of cell surface receptors, substrates and interaction partners to coordinate changes in cytoskeletal regulatory and protein trafficking pathways. We provide a comprehensive review of the specific mechanisms by which Trio family proteins carry out their functions in cells, highlight the biological and cellular contexts in which they occur, and relate how alterations in these functions contribute to human disease.

Challenge of Bovine Foot Skin Fibroblasts With Digital Dermatitis Treponemes Identifies Distinct Pathogenic Mechanisms

Bovine digital dermatitis (BDD) is a common infectious disease of digital skin in cattle and an important cause of lameness worldwide, with limited treatment options. It is of increasing global concern for both animal welfare and food security, imposing a large economic burden on cattle farming industries each year. A polytreponemal etiology has been consistently identified, with three key phylogroups implicated globally: Treponema medium, Treponema phagedenis, and Treponema pedis. Pathogenic mechanisms which might enable targeted treatment/therapeutic development are poorly defined. This study used RNA sequencing to determine global differential mRNA expression in primary bovine foot skin fibroblasts following challenge with three representative BDD treponemes and a commensal treponeme, Treponema ruminis. A pro-inflammatory response was elicited by the BDD treponemes, mediated through IL-8/IL-17 signaling. Unexpectedly, the three BDD treponemes elicited distinct mechanisms of pathogenesis. T. phagedenis and T. pedis increased abundance of mRNA transcripts associated with apoptosis, while T. medium and T. pedis increased transcripts involved in actin rearrangement and loss of cell adhesion, likely promoting tissue invasion. The upregulation of antimicrobial peptide precursor, DEFB123, by T. phagedenis spirochaetes may present a microbial ecological advantage to all treponemes within BDD infected tissue, explaining their dominance within lesions. A commensal, T. ruminis, significantly dysregulated over three times the number of host mRNA transcripts compared to BDD treponemes, implying BDD treponemes, akin to the syphilis pathogen (Treponema pallidum), have evolved as "stealth pathogens" which avoid triggering substantial host immune/inflammatory responses to enable persistence and tissue invasion. Immunohistochemistry demonstrated increased IL-6, IL-8, RND1, and CFB protein expression in BDD lesions, confirming in vitro fibroblast observations and highlighting the system's value in modeling BDD pathogenesis. Several unique shared gene targets were identified, particularly RGS16, GRO1, MAFF, and ZC3H12A. The three key BDD Treponema phylogroups elicited both distinct and shared pathogenic mechanisms in bovine foot skin; upregulating inflammation whilst simultaneously suppressing adaptive immunity. The novel gene targets identified here should enable future vaccine/therapeutic approaches.

SGEF forms a complex with Scribble and Dlg1 and regulates epithelial junctions and contractility

The canonical Scribble polarity complex is implicated in regulation of epithelial junctions and apical polarity. Here, we show that SGEF, a RhoG-specific GEF, forms a ternary complex with Scribble and Dlg1, two members of the Scribble complex. SGEF targets to apical junctions in a Scribble-dependent fashion and functions in the regulation of actomyosin-based contractility and barrier function at tight junctions as well as E-cadherin-mediated formation of adherens junctions. Surprisingly, SGEF does not control the establishment of polarity. However, in 3D cysts, SGEF regulates the formation of a single open lumen. Interestingly, SGEF's nucleotide exchange activity regulates the formation and maintenance of adherens junctions, and in cysts the number of lumens formed, whereas SGEF's scaffolding activity is critical for regulation of actomyosin contractility and lumen opening. We propose that SGEF plays a key role in coordinating junctional assembly and actomyosin contractility by bringing together Scribble and Dlg1 and targeting RhoG activation to cell-cell junctions.

The small GTPase RhoG regulates microtubule-mediated focal adhesion disassembly

Focal adhesions (FA) are a complex network of proteins that allow the cell to form physical contacts with the extracellular matrix (ECM). FA assemble and disassemble in a dynamic process, orchestrated by a variety of cellular components. However, the underlying mechanisms that regulate adhesion turnover remain poorly understood. Here we show that RhoG, a Rho GTPase related to Rac, modulates FA dynamics. When RhoG expression is silenced, FA are more stable and live longer, resulting in an increase in the number and size of adhesions, which are also more mature and fibrillar-like. Silencing RhoG also increases the number and thickness of stress fibers, which are sensitive to blebbistatin, suggesting contractility is increased. The molecular mechanism by which RhoG regulates adhesion turnover is yet to be characterized, but our results demonstrate that RhoG plays a role in the regulation of microtubule-mediated FA disassembly.

Congenital Defects in Actin Dynamics of Germinal Center B Cells

The germinal center (GC) is a transient anatomical structure formed during the adaptive immune response that leads to antibody affinity maturation and serological memory. Recent works using two-photon microscopy reveals that the GC is a highly dynamic structure and GC B cells are highly motile. An efficient selection of high affinity B cells clones within the GC crucially relies on the interplay of proliferation, genome editing, cell-cell interaction, and migration. All these processes require actin cytoskeleton rearrangement to be well-coordinated. Dysregulated actin dynamics may impede on multiple stages during B cell affinity maturation, which could lead to aberrant GC response and result in autoimmunity and B cell malignancy. This review mainly focuses on the recent works that investigate the role of actin regulators during the GC response.

RHOG Activates RAC1 through CDC42 Leading to Tube Formation in Vascular Endothelial Cells

Angiogenesis is a hallmark of cancer cell malignancy. The role of the RHO family GTPase RHOG in angiogenesis in vascular endothelial cells has recently been elucidated. However, the regulation of RHOG during this process, as well as its cross-talk with other RHO GTPases, have yet to be fully examined. In this study, we found that siRNA-mediated depletion of RHOG strongly inhibits tube formation in vascular endothelial cells (ECV cells), an effect reversed by transfecting dominant active constructs of CDC42 or RAC1 in the RHOG-depleted cells. We also found CDC42 to be upstream from RAC1 in these cells. Inhibiting either Phosphatidyl inositol (3) kinase (PI3K) with Wortmannin or the mitogen-activated protein kinase extracellular-regulated kinase (MAPK ERK) with U0126 leads to the inhibition of tube formation. While knocking down either RHO, GTPase did not affect p-AKT levels, and p-ERK decreased in response to the knocking down of RHOG, CDC42 or RAC1. Recovering active RHO GTPases in U0126-treated cells also did not reverse the inhibition of tube formation, placing ERK downstream from PI3K-RHOG-CDC42-RAC1 in vascular endothelial cells. Finally, RHOA and the Rho activated protein kinases ROCK1 and ROCK2 positively regulated tube formation independently of ERK, while RHOC seemed to inhibit the process. Collectively, our data confirmed the essential role of RHOG in angiogenesis, shedding light on a potential new therapeutic target for cancer malignancy and metastasis.

Caveolin-1α regulates primary cilium length by controlling RhoA GTPase activity

The primary cilium is a single non-motile protrusion of the plasma membrane of most types of mammalian cell. The structure, length and function of the primary cilium must be tightly controlled because their dysfunction is associated with disease. Caveolin 1 (Cav1), which is best known as a component of membrane invaginations called caveolae, is also present in non-caveolar membrane domains whose function is beginning to be understood. We show that silencing of α and β Cav1 isoforms in different cell lines increases ciliary length regardless of the route of primary ciliogenesis. The sole expression of Cav1α, which is distributed at the apical membrane, restores normal cilium size in Cav1 KO MDCK cells. Cells KO for only Cav1α, which also show long cilia, have a disrupted actin cytoskeleton and reduced RhoA GTPase activity at the apical membrane, and a greater accumulation of Rab11 vesicles at the centrosome. Subsequent experiments showed that DIA1 and ROCK help regulate ciliary length. Since MDCK cells lack apical caveolae, our results imply that non-caveolar apical Cav1α is an important regulator of ciliary length, exerting its effect via RhoA and its effectors, ROCK and DIA1.

Human serum albumin and oxidative stress in preeclamptic women and the mechanism of albumin for stress reduction

AIMS

The present study to address one of the mechanisms in preeclampsia, examined whether levels of oxidative stress, human serum albumin, and endothelial function correlate in pregnant women and whether human serum albumin reduces levels of superoxide produced by NADPH oxidase activation in the human vascular smooth muscle cells.

MATERIALS AND METHODS

Pregnant women with (Preeclampsia group, n = 33) and without preeclampsia (Normal group, n = 37) were recruited to determine levels of reactive oxygen species (serum diacron-reactive oxygen metabolite [d-ROM]), and the flow-mediated dilation (FMD). Human coronary arterial smooth muscle cells or omental arteries were subjected to evaluate isometric force recordings, levels of superoxide, western immunoblotting, and immunohistochemistry. The superoxide scavenging assay was also performed in a cell-free system.

KEY FINDINGS

Women in the preeclampsia group demonstrated lower FMD and higher serum d-ROM values than those in the normal group. There were the inverse correlations between serum levels of d-ROM and the degree of FMD and between serum levels of albumin and those of d-ROM. D-glucose reduced the levcromakalim-induced dilation of human omental arteries, and it increased levels of superoxide and the recruitment of the NADPH oxidase subunit p47phox in human coronary arterial smooth muscle cells. Human serum albumin (0.05 to 0.5 g/dL) prevented these alterations whereas it exerted no superoxide scavenging effect.

SIGNIFICANCE

Serum albumin relates to oxidative stress inversely, but to the endothelial function positively, in pregnant women. Human serum albumin appears to reduce oxidative stress via NADPH oxidase inhibition in the human vascular smooth muscle, indicating that the serum level may be a critical determinant of vascular oxidative stress in some human diseases.

Regulation of circular dorsal ruffles, macropinocytosis, and cell migration by RhoG and its exchange factor, Trio

The small GTPase RhoG and its exchange factor, Trio, regulate the formation and size of circular dorsal ruffles and associated functions, including macropinocytosis and cell migration.

Circular dorsal ruffles (CDRs) are actin-rich structures that form on the dorsal surface of many mammalian cells in response to growth factor stimulation. CDRs represent a unique type of structure that forms transiently and only once upon stimulation. The formation of CDRs involves a drastic rearrangement of the cytoskeleton, which is regulated by the Rho family of GTPases. So far, only Rac1 has been consistently associated with CDR formation, whereas the role of other GTPases in this process is either lacking or inconclusive. Here we show that RhoG and its exchange factor, Trio, play a role in the regulation of CDR dynamics, particularly by modulating their size. RhoG is activated by Trio downstream of PDGF in a PI3K- and Src-dependent manner. Silencing RhoG expression decreases the number of cells that form CDRs, as well as the area of the CDRs. The regulation of CDR area by RhoG is independent of Rac1 function. In addition, our results show the RhoG plays a role in the cellular functions associated with CDR formation, including macropinocytosis, receptor internalization, and cell migration. Taken together, our results reveal a novel role for RhoG in the regulation of CDRs and the cellular processes associated with their formation.

A RhoG-mediated signaling pathway that modulates invadopodia dynamics in breast cancer cells

One of the hallmarks of cancer is the ability of tumor cells to invade surrounding tissues and metastasize. During metastasis, cancer cells degrade the extracellular matrix, which acts as a physical barrier, by developing specialized actin-rich membrane protrusion structures called invadopodia. The formation of invadopodia is regulated by Rho GTPases, a family of proteins that regulates the actin cytoskeleton. Here, we describe a novel role for RhoG in the regulation of invadopodia disassembly in human breast cancer cells. Our results show that RhoG and Rac1 have independent and opposite roles in the regulation of invadopodia dynamics. We also show that SGEF (also known as ARHGEF26) is the exchange factor responsible for the activation of RhoG during invadopodia disassembly. When the expression of either RhoG or SGEF is silenced, invadopodia are more stable and have a longer lifetime than in control cells. Our findings also demonstrate that RhoG and SGEF modulate the phosphorylation of paxillin, which plays a key role during invadopodia disassembly. In summary, we have identified a novel signaling pathway involving SGEF, RhoG and paxillin phosphorylation, which functions in the regulation of invadopodia disassembly in breast cancer cells.

Apoptotic cell recognition receptors and scavenger receptors

Phosphatidylserine recognition receptors are a highly diverse set of receptors grouped by their ability to recognize the 'eat-me' signal phosphatidylserine on apoptotic cells. Most of the phosphatidylserine recognition receptors dampen inflammation by inducing the production of anti-inflammatory mediators during the phagocytosis of apoptotic corpses. However, many phosphatidylserine receptors are also capable of recognizing other ligands, with some receptors being categorized as scavenger receptors. It is now appreciated that these receptors can elicit different downstream events for particular ligands. Therefore, how phosphatidylserine recognition receptors mediate specific signals during recognition of apoptotic cells versus other ligands, and how this might help regulate the inflammatory state of a tissue is an important question that is not fully understood. Here, we revisit the work on signaling downstream of the phosphatidylserine recognition receptor BAI1, and evaluate how these and other signaling modules mediate signaling downstream from other receptors, including Stabilin-2, MerTK, and αvβ5. We also propose the concept that phosphatidylserine recognition receptors could be viewed as a subset of scavenger receptors that are capable of eliciting anti-inflammatory responses to apoptotic cells.

Membrane proteome analysis of glioblastoma cell invasion

Glioblastoma multiforme (GBM) tumor invasion is facilitated by cell migration and degradation of the extracellular matrix. Invadopodia are actin-rich structures that protrude from the plasma membrane in direct contact with the extracellular matrix and are proposed to participate in epithelial-mesenchymal transition. We characterized the invasiveness of 9 established GBM cell lines using an invadopodia assay and performed quantitative mass spectrometry-based proteomic analyses on enriched membrane fractions. All GBM cells produced invadopodia, with a 65% difference between the most invasive cell line (U87MG) and the least invasive cell line (LN229) (p = 0.0001). Overall, 1,141 proteins were identified in the GBM membrane proteome; the levels of 49 proteins correlated with cell invasiveness. Ingenuity Pathway Analysis predicted activation "cell movement" (z-score = 2.608, p = 3.94E(-04)) in more invasive cells and generated a network of invasion-associated proteins with direct links to key regulators of invadopodia formation. Gene expression data relating to the invasion-associated proteins ITGA5 (integrin α5), CD97, and ANXA1 (annexin A1) showed prognostic significance in independent GBM cohorts. Fluorescence microscopy demonstrated ITGA5, CD97, and ANXA1 localization in invadopodia assays, and small interfering RNA knockdown of ITGA5 reduced invadopodia formation in U87MG cells. Thus, invasion-associated proteins, including ITGA5, may prove to be useful anti-invasive targets; volociximab, a therapeutic antibody against integrin α5β1, may be useful for treatment of patients with GBM.

Platelet Rho GTPases-a focus on novel players, roles and relationships

Rho GTPases are critical for platelet function. Although the roles of RhoA, Rac and Cdc42 are characterized, platelets express other Rho GTPases, whose activities are less well understood. This review summarizes our understanding of the roles of platelet Rho GTPases and focuses particularly on the functions of Rif and RhoG. In human platelets, Rif interacts with cytoskeleton regulators including formins mDia1 and mDia3, whereas RhoG binds SNARE-complex proteins and cytoskeletal regulators ELMO and DOCK1. Knockout mouse studies suggest that Rif plays no critical functions in platelets, likely due to functional overlap with other Rho GTPases. In contrast, RhoG is essential for normal granule secretion downstream of the collagen receptor GPVI. The central defect in RhoG-/- platelets is reduced dense granule secretion, which impedes integrin activation and aggregation and limits platelet recruitment to growing thrombi under shear, translating into reduced thrombus formation in vivo. Potential avenues for future work on Rho GTPases in platelets are also highlighted, including identification of the key regulator for platelet filopodia formation and investigation of the role of the many Rho GTPase regulators in platelet function in both health and disease.

p104 binds to Rac1 and reduces its activity during myotube differentiation of C2C12 cell

The p104 protein inhibits cellular proliferation when overexpressed in NIH3T3 cells and has been shown to associate with p85α, Grb2, and PLCγ1. In order to isolate other proteins that interact with p104, yeast two-hybrid screening was performed. Rac1 was identified as a binding partner of p104 and the interaction between p104 and Rac1 was confirmed by immunoprecipitation. Using a glutathione S-transferase (GST) pull-down assay with various p104 fragments, the 814–848 amino acid residue at the carboxyl-terminal region of p104 was identified as the key component to interact with Rac1. The CrkII which is involved in the Rac1-mediated cellular response was also found to interact with p104 protein. NIH3T3 cells which overexpressed p104 showed a decrease of Rac1 activity. However, neither the proline-rich domain mutant, which is unable to interact with CrkII, nor the carboxy-terminal deletion mutant could attenuate Rac1 activity. During the differentiation of myoblasts, the amount of p104 protein as well as transcript level was increased. The overexpression of p104 enhanced myotube differentiation, whereas siRNA of p104 reversed this process. In this process, more Rac1 and CrkII were bound to increased p104. Based on these results, we conclude that p104 is involved in muscle cell differentiation by modulating the Rac1 activity.

Requirements for and consequences of Rac-dependent protrusion

Small GTPases of the Rac subfamily exert multiple functions, the most prominent of which includes stimulation of dynamic actin rearrangements at the cell periphery. Frequently, these actin reorganizations cause the protrusion of leaflets of plasma membrane, so-called lamellipodia, which remain anchored at flat surfaces during forward protrusion of migrating cells, or develop into ruffles when lifting up- and backwards. Ruffling membranes are also engaged in fluid and particle uptake during pino- and phagocytosis, respectively. In recent work, we sought to clarify the precise role of Rac GTPases in actin-based protrusion, using a gene disruption approach. Furthermore, we aimed at dissecting the function of its downstream target Arp2/3 complex employing its instantaneous inhibition during simultaneous Rac activation. These complementary approaches allow comparison of the consequences of Rac versus Arp2/3 complex loss of function at the cell periphery, and help to formulate a working hypothesis for how the actin network in lamellipodia is initiated and maintained.

RhoG protein regulates platelet granule secretion and thrombus formation in mice

Rho GTPases such as Rac, RhoA, and Cdc42 are vital for normal platelet function, but the role of RhoG in platelets has not been studied. In other cells, RhoG orchestrates processes integral to platelet function, including actin cytoskeletal rearrangement and membrane trafficking. We therefore hypothesized that RhoG would play a critical role in platelets. Here, we show that RhoG is expressed in human and mouse platelets and is activated by both collagen-related peptide (CRP) and thrombin stimulation. We used RhoG(-/-) mice to study the function of RhoG in platelets. Integrin activation and aggregation were reduced in RhoG(-/-) platelets stimulated by CRP, but responses to thrombin were normal. The central defect in RhoG(-/-) platelets was reduced secretion from α-granules, dense granules, and lysosomes following CRP stimulation. The integrin activation and aggregation defects could be rescued by ADP co-stimulation, indicating that they are a consequence of diminished dense granule secretion. Defective dense granule secretion in RhoG(-/-) platelets limited recruitment of additional platelets to growing thrombi in flowing blood in vitro and translated into reduced thrombus formation in vivo. Interestingly, tail bleeding times were normal in RhoG(-/-) mice, suggesting that the functions of RhoG in platelets are particularly relevant to thrombotic disorders.

RhoG protein regulates glycoprotein VI-Fc receptor γ-chain complex-mediated platelet activation and thrombus formation

We investigated the mechanism of activation and functional role of a hitherto uncharacterized signaling molecule, RhoG, in platelets. We demonstrate for the first time the expression and activation of RhoG in platelets. Platelet aggregation, integrin αIIbβ3 activation, and α-granule and dense granule secretion in response to the glycoprotein VI (GPVI) agonists collagen-related peptide (CRP) and convulxin were significantly inhibited in RhoG-deficient platelets. In contrast, 2-MeSADP- and AYPGKF-induced platelet aggregation and secretion were minimally affected in RhoG-deficient platelets, indicating that the function of RhoG in platelets is GPVI-specific. CRP-induced phosphorylation of Syk, Akt, and ERK, but not SFK (Src family kinase), was significantly reduced in RhoG-deficient platelets. CRP-induced RhoG activation was consistently abolished by a pan-SFK inhibitor but not by Syk or PI3K inhibitors. Interestingly, unlike CRP, platelet aggregation and Syk phosphorylation induced by fucoidan, a CLEC-2 agonist, were unaffected in RhoG-deficient platelets. Finally, RhoG(-/-) mice had a significant delay in time to thrombotic occlusion in cremaster arterioles compared with wild-type littermates, indicating the important in vivo functional role of RhoG in platelets. Our data demonstrate that RhoG is expressed and activated in platelets, plays an important role in GPVI-Fc receptor γ-chain complex-mediated platelet activation, and is critical for thrombus formation in vivo.

Flotillin microdomains stabilize cadherins at cell-cell junctions

Cadherins are essential in many fundamental processes and assemble at regions of cell-cell contact in large macromolecular complexes named adherens junctions. We have identified flotillin 1 and 2 as new partners of the cadherin complexes. We show that flotillins are localised at cell-cell junctions (CCJs) in a cadherin-dependent manner. Flotillins and cadherins are constitutively associated at the plasma membrane and their colocalisation at CCJ increases with CCJ maturation. Using three-dimensional structured illumination super-resolution microscopy, we found that cadherin and flotillin complexes are associated with F-actin bundles at CCJs. The knockdown of flotillins dramatically affected N- and E-cadherin recruitment at CCJs in mesenchymal and epithelial cell types and perturbed CCJ integrity and functionality. Moreover, we determined that flotillins are required for cadherin association with GM1-containing plasma membrane microdomains. This allows p120 catenin binding to the cadherin complex and its stabilization at CCJs. Altogether, these data demonstrate that flotillin microdomains are required for cadherin stabilization at CCJs and for the formation of functional CCJs.

The Src homology 3 domain-containing guanine nucleotide exchange factor is overexpressed in high-grade gliomas and promotes tumor necrosis factor-like weak inducer of apoptosis-fibroblast growth factor-inducible 14-induced cell migration and invasion via tumor necrosis factor receptor-associated factor 2

Glioblastoma (GB) is the highest grade of primary adult brain tumors, characterized by a poorly defined and highly invasive cell population. Importantly, these invading cells are attributed with having a decreased sensitivity to radiation and chemotherapy. TNF-like weak inducer of apoptosis (TWEAK)-Fn14 ligand-receptor signaling is one mechanism in GB that promotes cell invasiveness and survival and is dependent upon the activity of multiple Rho GTPases, including Rac1. Here we report that Src homology 3 domain-containing guanine nucleotide exchange factor (SGEF), a RhoG-specific guanine nucleotide exchange factor, is overexpressed in GB tumors and promotes TWEAK-Fn14-mediated glioma invasion. Importantly, levels of SGEF expression in GB tumors inversely correlate with patient survival. SGEF mRNA expression is increased in GB cells at the invasive rim relative to those in the tumor core, and knockdown of SGEF expression by shRNA decreases glioma cell migration in vitro and invasion ex vivo. Furthermore, we showed that, upon TWEAK stimulation, SGEF is recruited to the Fn14 cytoplasmic tail via TRAF2. Mutation of the Fn14-TRAF domain site or depletion of TNF receptor-associated factor 2 (TRAF2) expression by siRNA oligonucleotides blocked SGEF recruitment to Fn14 and inhibited SGEF activity and subsequent GB cell migration. We also showed that knockdown of either SGEF or RhoG diminished TWEAK activation of Rac1 and subsequent lamellipodia formation. Together, these results indicate that SGEF-RhoG is an important downstream regulator of TWEAK-Fn14-driven GB cell migration and invasion.

miR-124-regulated RhoG: A conductor of neuronal process complexity

RhoG is a member of the Rho family of small GTPases sharing highest sequence similarity with Rac and Cdc42. Mig-2 and Mtl represent the functional equivalents of RhoG in Caenorhabditis elegans and Drosophila, respectively. RhoG has attracted great interest because it plays a central role in the regulation of cytoskeletal reorganization in various physiological and pathophysiological situations. For example, it is fundamental to phagocytotic processes, is able to regulate gene expression, cell survival and proliferation, and is involved in cell migration and in the invasion of pathogenic bacteria. The activation of Rac1 via an ELMO/Dock180 module has been elaborated to be important for RhoG signaling. Although a stimulatory role for neurite outgrowth in the pheochromocytoma PC12 cell line has been assigned to RhoG, the exact function of this GTPase for the development of the processes of primary neurons remains to be clarified. In this view, we discuss the impact of RhoG on axonal and dendritic differentiation, its role as a conductor of Rac1 and Cdc42 activity and the functional regulation of RhoG expression by the microRNA miR-124.

The small GTPase RhoG mediates glioblastoma cell invasion

Background

The invasion of glioblastoma cells into regions of the normal brain is a critical factor that limits current therapies for malignant astrocytomas. Previous work has identified roles for the Rho family guanine nucleotide exchange factors Trio and Vav3 in glioblastoma invasion. Both Trio and Vav3 act on the small GTPase RhoG. We therefore examined the role of RhoG in the invasive behavior of glioblastoma cells.

Results

We found that siRNA-mediated depletion of RhoG strongly inhibits invasion of glioblastoma cells through brain slices ex vivo. In addition, depletion of RhoG has a marginal effect on glioblastoma cell proliferation, but significantly inhibits glioblastoma cell survival in colony formation assays. We also observed that RhoG is activated by both HGF and EGF, two factors that are thought to be clinically relevant drivers of glioblastoma invasive behavior, and that RhoG is overexpressed in human glioblastoma tumors versus non-neoplastic brain. In search of a mechanism for the contribution of RhoG to the malignant behavior of glioblastoma cells, we found that depletion of RhoG strongly inhibits activation of the Rac1 GTPase by both HGF and EGF. In line with this observation, we also show that RhoG contributes to the formation of lamellipodia and invadopodia, two functions that have been shown to be Rac1-dependent.

Conclusions

Our functional analysis of RhoG in the context of glioblastoma revealed a critical role for RhoG in tumor cell invasion and survival. These results suggest that targeting RhoG-mediated signaling presents a novel avenue for glioblastoma therapy.

miR-124-regulated RhoG reduces neuronal process complexity via ELMO/Dock180/Rac1 and Cdc42 signalling

The small GTPase RhoG plays a central role in actin remodelling during diverse biological processes such as neurite outgrowth, cell migration, phagocytosis of apoptotic cells, and the invasion of pathogenic bacteria. Although it is known that RhoG stimulates neurite outgrowth in the rat pheochromocytoma PC12 cell line, neither the physiological function nor the regulation of this GTPase in neuronal differentiation is clear. Here, we identify RhoG as an inhibitor of neuronal process complexity, which is regulated by the microRNA miR-124. We find that RhoG inhibits dendritic branching in hippocampal neurons in vitro and in vivo. RhoG also inhibits axonal branching, acting via an ELMO/Dock180/Rac1 signalling pathway. However, RhoG inhibits dendritic branching dependent on the small GTPase Cdc42. Finally, we show that the expression of RhoG in neurons is suppressed by the CNS-specific microRNA miR-124 and connect the regulation of RhoG expression by miR-124 to the stimulation of neuronal process complexity. Thus, RhoG emerges as a cellular conductor of Rac1 and Cdc42 activity, in turn regulated by miR-124 to control axonal and dendritic branching.

Identification of novel neuronal isoforms of the Rho-GEF Trio

BACKGROUND INFORMATION

The large family of GEFs (guanine nucleotide-exchange factors) for Rho GTPases activate the GTPases by accelerating their GDP/GTP exchange. The multidomain protein Trio is the founding member of an intriguing subfamily of Rho-GEFs exhibiting two Rho-GEF and numerous additional domains. The members of the Trio family play an important role in neuronal physiology, and their structural organization is very well conserved through evolution. It has previously been shown that all the members, except mammalian Trio, display several isoforms, the functions of which have been well established.

RESULTS

In this study, we have identified, by a combination of different approaches, novel Trio isoforms that have been generated by alternative splicing, giving rise to proteins that exhibit one or two Rho-GEF domains (GEFDs). These isoforms are specifically expressed in the nervous system, at a higher level than the full-length Trio, which is ubiquitously expressed. In addition, we show that all the GEFD1-containing isoforms induce neurite outgrowth in neuroblastoma cells.

CONCLUSIONS

We have identified neuronal specific isoforms of Trio which could be essential for Trio function in neuronal morphology.

Rac1 and RhoA GTPases have antagonistic functions during N-cadherin-dependent cell-cell contact formation in C2C12 myoblasts

BACKGROUND INFORMATION

N-cadherin, a member of the Ca(2+)-dependent cell-cell adhesion molecule family, plays an essential role in the induction of the skeletal muscle differentiation programme. However, the molecular mechanisms which govern the formation of N-cadherin-dependent cell-cell contacts in myoblasts remain unexplored.

RESULTS

In the present study, we show that N-cadherin-dependent cell contact formation in myoblasts is defined by two stages. In the first phase, N-cadherin is highly mobile in the lamellipodia extensions between the contacting cells. The second stage corresponds to the formation of mature N-cadherin-dependent cell contacts, characterized by the immobilization of a pool of N-cadherin which appears to be clustered in the interdigitated membrane structures that are also membrane attachment sites for F-actin filaments. We also demonstrated that the formation of N-cadherin-dependent cell-cell contacts requires a co-ordinated and sequential activity of Rac1 and RhoA. Rac1 is involved in the first stage and facilitates N-cadherin-dependent cell-cell contact formation, but it is not absolutely required. Conversely, RhoA is necessary for N-cadherin-dependent cell contact formation, since, via ROCK (Rho-associated kinase) signalling and myosin 2 activation, it allows the stabilization of N-cadherin at the cell-cell contact sites.

CONCLUSIONS

We have shown that Rac1 and RhoA have opposite effects on N-cadherin-dependent cell-cell contact formation in C2C12 myoblasts and act sequentially to allow its formation.

Normal levels of Rac1 are important for dendritic but not axonal development in hippocampal neurons

BACKGROUND INFORMATION

Rho family GTPases are required for cytoskeletal reorganization and are considered important for the maturation of neurons. Among these proteins, Rac1 is known to play a crucial role in the regulation of actin dynamics, and a number of studies indicate the involvement of this protein in different steps of vertebrate neuronal maturation. There are two distinct Rac proteins expressed in neurons, namely the ubiquitous Rac1 and the neuron-specific Rac3. The specific functions of each of these GTPases during early neuronal development are largely unknown.

RESULTS

The combination of the knockout of Rac3 with Rac1 down-regulation by siRNA (small interfering RNA) has been used to show that down-regulation of Rac1 affects dendritic development in mouse hippocampal neurons, without affecting axons. F-actin levels are strongly decreased in neuronal growth cones following down-regulation of Rac1, and time-lapse analysis indicated that the reduction of Rac1 levels decreases growth-cone dynamics.

CONCLUSIONS

These results show that normal levels of endogenous Rac1 activity are critical for early dendritic development, whereas dendritic outgrowth is not affected in hippocampal neurons from Rac3-null mice. On the other hand, early axonal development appears normal after Rac1 down-regulation. Our findings also suggest that the initial establishment of neuronal polarity is not affected by Rac1 down-regulation.

Rho protein crosstalk: another social network?

Many fundamental processes in cell biology are regulated by Rho GTPases, including cell adhesion, migration and differentiation. While regulating cellular functions, members of the Rho protein family cooperate or antagonize each other. The resulting molecular network exhibits many levels of interaction dynamically regulated in time and space. In the first part of this review we describe the main mechanisms of this crosstalk, which can occur at three different levels of the pathway: (i) through regulation of activity, (ii) through regulation of protein expression and stability, and (iii) through regulation of downstream signaling pathways. In the second part we illustrate the importance of Rho protein crosstalk with two examples: integrin-based adhesion and cell migration.

Identification of a novel virulence factor in Clostridium difficile that modulates toxin sensitivity of cultured epithelial cells

Two glucosylating toxins named toxins A and B play a role in the pathogenesis of Clostridium Difficile infection. The interaction of the toxins with host cell factors proceeds to downstream stages of cytotoxic effects in cells, in which involvement of other C. difficile factors remains unknown. We utilized culture filtrate of C. difficile with a low dilution to characterize the influence of putative minor proteins on the organization of the actin cytoskeleton in cultured epithelial cells and found a previously uncharacterized F-actin aggregated structure, termed "actin aggregate," at the juxtanuclear region. We reasoned that formation of actin aggregate was due to an additional factor(s) in the culture filtrate rather than the glucosylating toxins, because treatment of purified toxins rarely caused actin aggregate in cells. We focused on a previously uncharacterized hypothetical protein harboring a KDEL-like sequence as a candidate. The product of the candidate gene was detected in culture filtrate of C. difficile ATCC 9689 and was renamed Srl. Purified glutathione S-transferase-tagged Srl triggered formation of actin aggregate in the cells in the presence of either toxin A or B and enhanced cytotoxicity of each of the two toxins, including decreases in both cell viability and transepithelial resistance of cultured epithelial monolayer, although the recombinant Srl alone did not show detectable cytotoxicity. Srl-neutralized culture filtrate partially inhibited morphological changes of the cells in parallel with decreased actin aggregate formation in the cells. Thus, Srl might contribute to the modulation of toxin sensitivity of intestinal epithelial cells by enhancing cytotoxicity of C. difficile toxins.

Activation of AMP-activated protein kinase is essential for lysophosphatidic acid-induced cell migration in ovarian cancer cells

Lysophosphatidic acid (LPA) is a bioactive phospholipid that affects various biological functions, such as cell proliferation, migration, and survival, through LPA receptors. Among them, the motility of cancer cells is an especially important activity for invasion and metastasis. Recently, AMP-activated protein kinase (AMPK), an energy-sensing kinase, was shown to regulate cell migration. However, the specific role of AMPK in cancer cell migration is unknown. The present study investigated whether LPA could induce AMPK activation and whether this process was associated with cell migration in ovarian cancer cells. We found that LPA led to a striking increase in AMPK phosphorylation in pathways involving the phospholipase C-β3 (PLC-β3) and calcium/calmodulin-dependent protein kinase kinase β (CaMKKβ) in SKOV3 ovarian cancer cells. siRNA-mediated knockdown of AMPKα1, PLC-β3, or (CaMKKβ) impaired the stimulatory effects of LPA on cell migration. Furthermore, we found that knockdown of AMPKα1 abrogated LPA-induced activation of the small GTPase RhoA and ezrin/radixin/moesin proteins regulating membrane dynamics as membrane-cytoskeleton linkers. In ovarian cancer xenograft models, knockdown of AMPK significantly decreased peritoneal dissemination and lung metastasis. Taken together, our results suggest that activation of AMPK by LPA induces cell migration through the signaling pathway to cytoskeletal dynamics and increases tumor metastasis in ovarian cancer.

Salmonella - at home in the host cell

The Gram-negative bacterium Salmonella enterica has developed an array of sophisticated tools to manipulate the host cell and establish an intracellular niche, for successful propagation as a facultative intracellular pathogen. While Salmonella exerts diverse effects on its host cell, only the cell biology of the classic “trigger”-mediated invasion process and the subsequent development of the Salmonella-containing vacuole have been investigated extensively. These processes are dependent on cohorts of effector proteins translocated into host cells by two type III secretion systems (T3SS), although T3SS-independent mechanisms of entry may be important for invasion of certain host cell types. Recent studies into the intracellular lifestyle of Salmonella have provided new insights into the mechanisms used by this pathogen to modulate its intracellular environment. Here we discuss current knowledge of Salmonella-host interactions including invasion and establishment of an intracellular niche within the host.

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.

Kidins220/ARMS regulates Rac1-dependent neurite outgrowth by direct interaction with the RhoGEF Trio

Neurite extension depends on extracellular signals that lead to changes in gene expression and rearrangement of the actin cytoskeleton. A factor that might orchestrate these signalling pathways with cytoskeletal elements is the integral membrane protein Kidins220/ARMS, a downstream target of neurotrophins. Here, we identified Trio, a RhoGEF for Rac1, RhoG and RhoA, which is involved in neurite outgrowth and axon guidance, as a binding partner of Kidins220. This interaction is direct and occurs between the N-terminus of Trio and the ankyrin repeats of Kidins220. Trio and Kidins220 colocalise at the tips of neurites in NGF-differentiated PC12 cells, where F-actin and Rac1 also accumulate. Expression of the ankyrin repeats of Kidins220 in PC12 cells inhibits NGF-dependent and Trio-induced neurite outgrowth. Similar results are seen in primary hippocampal neurons. Our data indicate that Kidins220 might localise Trio to specific membrane sites and regulate its activity, leading to Rac1 activation and neurite outgrowth.

Bayesian integrated modeling of expression data: a case study on RhoG

BACKGROUND

DNA microarrays provide an efficient method for measuring activity of genes in parallel and even covering all the known transcripts of an organism on a single array. This has to be balanced against that analyzing data emerging from microarrays involves several consecutive steps, and each of them is a potential source of errors. Errors tend to accumulate when moving from the lower level towards the higher level analyses because of the sequential nature. Eliminating such errors does not seem feasible without completely changing the technologies, but one should nevertheless try to meet the goal of being able to realistically assess degree of the uncertainties that are involved when drawing the final conclusions from such analyses.

RESULTS

We present a Bayesian hierarchical model for finding differentially expressed genes between two experimental conditions, proposing an integrated statistical approach where correcting signal saturation, systematic array effects, dye effects, and finding differentially expressed genes, are all modeled jointly. The integration allows all these components, and also the associated errors, to be considered simultaneously. The inference is based on full posterior distribution of gene expression indices and on quantities derived from them rather than on point estimates. The model was applied and tested on two different datasets.

CONCLUSIONS

The method presents a way of integrating various steps of microarray analysis into a single joint analysis, and thereby enables extracting information on differential expression in a manner, which properly accounts for various sources of potential error in the process.

How and why does the endoplasmic reticulum move?

The ER (endoplasmic reticulum) is a fascinating organelle that is highly dynamic, undergoing constant movement and reorganization. It has many key roles, including protein synthesis, folding and trafficking, calcium homoeostasis and lipid synthesis. It can expand in size when needed, and the balance between tubular and lamellar regions can be altered. The distribution and organization of the ER depends on both motile and static interactions with microtubules and the actin cytoskeleton. In the present paper, we review how the ER moves, and consider why this movement may be important for ER and cellular function.

RhoG promotes neural progenitor cell proliferation in mouse cerebral cortex

In early cortical development, neural progenitor cells (NPCs) expand their population in the ventricular zone (VZ), and produce neurons. Although a series of studies have revealed the process of neurogenesis, the molecular mechanisms regulating NPC proliferation are still largely unknown. Here we found that RhoG, a member of Rho family GTPases, was expressed in the VZ at early stages of cortical development. Expression of constitutively active RhoG promoted NPC proliferation and incorporation of bromodeoxyuridine (BrdU) in vitro, and the proportion of Ki67-positive cells in vivo. In contrast, knockdown of RhoG by RNA interference suppressed the proliferation, BrdU incorporation, and the proportion of Ki67-positive cells in NPCs. However, knockdown of RhoG did not affect differentiation and survival of NPC. The RhoG-induced promotion of BrdU incorporation required phosphatidylinositol 3-kinase (PI3K) activity but not the interaction with ELMO. Taken together, these results indicate that RhoG promotes NPC proliferation through PI3K in cortical development.

A cell active chemical GEF inhibitor selectively targets the Trio/RhoG/Rac1 signaling pathway

RhoGEFs (guanine nucleotide exchange factors of the Rho GTPase family) are upstream regulators of cell adhesion and migration pathways, thus representing attractive yet relatively unexplored targets for the development of anti-invasive drugs. We screened for chemical inhibitors of TrioN, the N-terminal GEF domain of the multidomain Trio protein, and identified ITX3 as a nontoxic inhibitor. In transfected mammalian cells, ITX3 blocked TrioN-mediated dorsal membrane ruffling and Rac1 activation while having no effect on GEF337-, Tiam1-, or Vav2-mediated RhoA or Rac1 activation. ITX3 specifically inhibited endogenous TrioN activity, as evidenced by its ability to inhibit neurite outgrowth in nerve growth factor (NGF)-stimulated PC12 cells or C2C12 differentiation into myotubes. This study introduces a selective cell active inhibitor of the Trio/RhoG/Rac1 pathway and validates RhoGEFs as druggable targets.

Yersinia pseudotuberculosis virulence determinants invasin, YopE, and YopT modulate RhoG activity and localization

The Yersinia pseudotuberculosis surface protein invasin binds to multiple beta1 integrins with high affinity, leading to misregulation of Rac1 activity. Upon host cell binding, alteration of Rho GTPase activity results from the action of several Yersinia outer proteins (Yops) that are translocated into the cytoplasm. We report here that three virulence determinants encoded by Y. pseudotuberculosis manipulate the Rho GTPase RhoG. Y. pseudotuberculosis binding to cells caused robust recruitment of RhoG to the site of attachment, which required high-affinity invasin-beta1 integrin association. Furthermore, inactivation of RhoG significantly reduced the efficiency of invasin-mediated bacterial internalization. To investigate the activation state of RhoG, a fluorescence resonance energy transfer-based activation biosensor was developed and used to show distinct spatial activation of RhoG at the site of bacterial attachment. The biosensor was also used to show efficient RhoG inactivation by Y. pseudotuberculosis YopE, a potent Rho GTPase activating protein. Additionally, RhoG mislocalization by the prenylcysteine endoprotease YopT was demonstrated by two independent assays. Functional bacterial uptake experiments demonstrated that RhoG activation can bypass a deficit in Rac1 activity. Interestingly, increasing the size of the particle gave results more consistent with a linear pathway, in which RhoG acts as an upstream activator of Rac1, indicating that increased surface area introduces constraints on the signaling pathways required for efficient internalization. Taken together, these data demonstrate the misregulation of RhoG by multiple Y. pseudotuberculosis virulence determinants. Since RhoG is imperative for proper neutrophil function, this misregulation may represent a unique mechanism by which Yersinia species dampen the immune response.

Retinoic acid leads to cytoskeletal rearrangement through AMPK-Rac1 and stimulates glucose uptake through AMPK-p38 MAPK in skeletal muscle cells

Retinoic acid (RA) is one of the major components of vitamin A. In the present study, we found that retinoic acid activated AMP-activated protein kinase (AMPK). RA induced Rac1-GTP formation and phosphorylation of its downstream target, p21-activated kinase (PAK), whereas the inhibition of AMPK blocked RA-induced Rac1 activation. Moreover, cofilin, an actin polymerization regulator, was activated when incubated with RA. We then showed that inhibition of AMPK by compound C, a selective inhibitor of AMPK, or small interfering RNA of AMPK alpha1 blocked RA-induced cofilin phosphorylation. Additionally, we found that retinoic acid-stimulated glucose uptake in differentiated C2C12 myoblast cells and activated p38 mitogen-activated protein kinase (MAPK). Finally, the inhibition of AMPK and p38 MAPK blocked retinoic acid-induced glucose uptake. In summary, our results suggest that retinoic acid may have cytoskeletal roles in skeletal muscle cells via stimulation of the AMPK-Rac1-PAK-cofillin pathway and may also have beneficial roles in glucose metabolism via stimulation of the AMPK-p38 MAPK pathway.

Endogenous RhoG is dispensable for integrin-mediated cell spreading but contributes to Rac-independent migration

Rac activation by integrins is essential for cell spreading, migration, growth and survival. Based mainly on overexpression of dominant-negative mutants, RhoG has been proposed to mediate integrin-dependent Rac activation upstream of ELMO and Dock180. RhoG-knockout mice, however, display no significant developmental or functional abnormalities. To clarify the role of RhoG in integrin-mediated signaling, we developed a RhoG-specific antibody, which, together with shRNA-mediated knockdown, allowed analysis of the endogenous protein. Despite dramatic effects of dominant-negative constructs, nearly complete RhoG depletion did not substantially inhibit cell adhesion, spreading, migration or Rac activation. Additionally, RhoG was not detectably activated by adhesion to fibronectin. Using Rac1(-/-) cells, we found that constitutively active RhoG induced membrane ruffling via both Rac-dependent and -independent pathways. Additionally, endogenous RhoG was important for Rac-independent cell migration. However, RhoG did not significantly contribute to cell spreading even in these cells. These data therefore clarify the role of RhoG in integrin signaling and cell motility.

Functions of Rac GTPases during neuronal development

The small GTPases of the Rho family are important regulators of the actin cytoskeleton and are critical for several aspects of neuronal development including the establishment of neuronal polarity, extension of axon and dendrites, neurite branching, axonal navigation and synapse formation. The aim of this review is to present evidence supporting the function of Rac and Rac-related proteins in different aspects of neuronal maturation, based on work performed with organisms including nematodes, Drosophila, Xenopus and mice, and with primary cultures of developing neurons. Three of the 4 vertebrate Rac-related genes, namely Rac1, Rac3 and RhoG, are expressed in the nervous system, and several data support an essential role of all 3 GTPases in distinct aspects of neuronal development and function. Two important points emerge from the analysis presented: highly homologous Rac-related proteins may perform different functions in the developing nervous system; on the other hand, the data also indicate that similar GTPases may perform redundant functions in vivo.

Interaction of ezrin with the novel guanine nucleotide exchange factor PLEKHG6 promotes RhoG-dependent apical cytoskeleton rearrangements in epithelial cells

The mechanisms underlying functional interactions between ERM (ezrin, radixin, moesin) proteins and Rho GTPases are not well understood. Here we characterized the interaction between ezrin and a novel Rho guanine nucleotide exchange factor, PLEKHG6. We show that ezrin recruits PLEKHG6 to the apical pole of epithelial cells where PLEKHG6 induces the formation of microvilli and membrane ruffles. These morphological changes are inhibited by dominant negative forms of RhoG. Indeed, we found that PLEKHG6 activates RhoG and to a much lesser extent Rac1. In addition we show that ezrin forms a complex with PLEKHG6 and RhoG. Furthermore, we detected a ternary complex between ezrin, PLEKHG6, and the RhoG effector ELMO. We demonstrate that PLEKHG6 and ezrin are both required in macropinocytosis. After down-regulation of either PLEKHG6 or ezrin expression, we observed an inhibition of dextran uptake in EGF-stimulated A431 cells. Altogether, our data indicate that ezrin allows the local activation of RhoG at the apical pole of epithelial cells by recruiting upstream and downstream regulators of RhoG and that both PLEKHG6 and ezrin are required for efficient macropinocytosis.

Trio controls the mature organization of neuronal clusters in the hindbrain

During the embryonic development of the hindbrain, movements of neuronal clusters allow the formation of mature "pools", in particular for inferior olivary (ION) and facial motor (fMN) nuclei. The cellular mechanisms of neuron clustering remain uncharacterized. We report that the absence of the Rho-guanine exchange factor Trio, which can activate both RhoG and Rac1 in vivo, prevents the proper formation of ION and fMN subnuclei. Rac1, but not RhoG, appears to be a downstream actor in Trio-induced lamellation. In addition, we report that Cadherin-11 is expressed by a subset of neurons through the overall period of ION and fMN parcellations, and defects observed in trio mutant mice are located specifically in Cadherin-11-expressing regions. Moreover, endogenous Cadherin-11 is found in a complex with Trio when lamellation occurs. Altogether, those results establish a link between Trio activity, the subsequent Rac1 activation, and neuronal clusters organization, as well as a possible recruitment of the Cadherin-11 adhesive receptor to form a complex with Trio.

Angiopoietin 1 directly induces destruction of the rheumatoid joint by cooperative, but independent, signaling via ERK/MAPK and phosphatidylinositol 3-kinase/Akt

OBJECTIVE

To determine whether angiopoietin 1 (Ang-1) potentiates overgrowth of the synovium and joint degradation in rheumatoid arthritis (RA), and to clarify the cell-signaling mechanisms of Ang-1 in the rheumatoid joint.

METHODS

Expression of Ang-1, TIE-2 (a receptor for Ang-1), and matrix metalloproteinase 3 (MMP-3) was studied by immunohistochemistry. Activation of the ERK/MAPK and phosphatidylinositol (PI) 3-kinase/Akt pathways and of NF-kappaB was determined by Western blotting and an NF-kappaB p65 DNA binding activity assay, respectively. Induction of apoptosis was evaluated by nuclear staining, cell viability assay, and Western blotting of caspases. Synovial cell migration was evaluated by actin polymerization, Western blotting of Rho family proteins, and affinity purification with Rhotekin-Rho and p21-activated kinase 1. Matrix degradation was examined by induction of proMMP-3 secretion from synovial cells followed by in vitro cartilaginous matrix degradation assay.

RESULTS

Ang-1 stimulated the ERK/MAPK and PI 3-kinase/Akt pathways in a cooperative but independent manner, which enhanced rheumatoid synovium overgrowth and joint destruction. In addition, Ang-1 activated NF-kappaB via Akt to promote cell growth, but also inhibited cell apoptosis via ERK and Akt. Ang-1 directly potentiated the extension of synovial cells in an ERK- and Akt-dependent manner by up-regulating Rho family proteins, which attenuated Rac signaling and led to membrane ruffling. Ang-1 induced proMMP-3 secretion from synovial cells, which resulted in direct degradation of the cartilaginous matrix.

CONCLUSION

Ang-1 stimulates the ERK/MAPK and PI 3-kinase/Akt pathways cooperatively, but in a manner independent of each other, to directly potentiate synovium overgrowth and joint destruction in RA. In addition to inflammatory cytokines, Ang-1/TIE-2 signaling appears to be an independent factor that contributes to the destruction of the rheumatoid joint.