RhoGDI-3 regulates RhoG and targets this protein to the Golgi complex through its unique N-terminal domain

The distinct localization of CDC42 isoforms is responsible for their specific functions during migration

The small G-protein CDC42 is an evolutionary conserved polarity protein and a key regulator of polarized cell functions, including directed cell migration. In vertebrates, alternative splicing gives rise to two CDC42 proteins: the ubiquitously expressed isoform (CDC42u) and the brain isoform (CDC42b), which only differ in their carboxy-terminal sequence, including the CAAX motif essential for their association with membranes. We show that these divergent sequences do not directly affect the range of CDC42's potential binding partners but indirectly influence CDC42-driven signaling by controlling the subcellular localization of the two isoforms. In astrocytes and neural precursors, which naturally express both variants, CDC42u associates with the leading-edge plasma membrane of migrating cells, where it recruits the Par6-PKCζ complex to fulfill its polarity function. In contrast, CDC42b mainly localizes to intracellular membrane compartments, where it regulates N-WASP-mediated endocytosis. Both CDC42 isoforms contribute their specific functions to promote the chemotaxis of neural precursors, demonstrating that their expression pattern is decisive for tissue-specific cell behavior.

PLK1 phosphorylates RhoGDI1 and promotes cancer cell migration and invasion

BACKGROUND

Rho guanine nucleotide dissociation inhibitor 1 (RhoGDI1) plays an important role in diverse cellular processes by regulating Rho guanosine triphosphate (GTP)ases activity. RhoGDI1 phosphorylation regulates the spatiotemporal activation of Rho GTPases during cell migration. In this study, we identified polo-like kinase 1 (PLK1) as a novel kinase of RhoGDI1 and investigated the molecular mechanism by which the interaction between RhoGDI1 and PLK1 regulates cancer cell migration.

METHODS

Immunoprecipitation, GST pull-down assay, and proximity ligation assay (PLA) were performed to analyze the interaction between RhoGDI1 and PLK1. In vitro kinase assay and immunoprecipitation were performed with Phospho-(Ser/Thr) antibody. We evaluated RhoA activation using RhoGTPases activity assay. Cell migration and invasion were analyzed by transwell assays.

RESULTS

GST pull-down assays and PLA showed that PLK1 directly interacted with RhoGDI1 in vitro and in vivo. Truncation mutagenesis revealed that aa 90-111 of RhoGDI1 are critical for interacting with PLK1. We also showed that PLK1 phosphorylated RhoGDI1 at Thr7 and Thr91, which induces cell motility. Overexpression of the GFP-tagged RhoGDI1 truncated mutant (aa 90-111) inhibited the interaction of PLK1 with RhoGDI1 and attenuated RhoA activation by PLK1. Furthermore, the overexpression of the RhoGDI1 truncated mutant reduced cancer cell migration and invasion in vitro and suppressed lung metastasis in vivo.

CONCLUSIONS

Collectively, we demonstrate that the phosphorylation of RhoGDI1 by PLK1 promotes cancer cell migration and invasion through RhoA activation. This study connects the interaction between PLK1 and RhoGDI1 to the promotion of cancer cell behavior associated with malignant progression, thereby providing opportunities for cancer therapeutic interventions.

Hyperglycemic Stress Induces Expression, Degradation, and Nuclear Association of Rho GDP Dissociation Inhibitor 2 (RhoGDIβ) in Pancreatic β-Cells

Small G proteins (e.g., Rac1) play critical regulatory roles in islet β-cell function in health (physiological insulin secretion) and in metabolic stress (cell dysfunction and demise). Multiple regulatory factors for these G proteins, such as GDP dissociation inhibitors (GDIs), have been implicated in the functional regulation of these G proteins. The current set of investigations is aimed at understanding impact of chronic hyperglycemic stress on the expression and subcellular distribution of three known isoforms of RhoGDIs (RhoGDIα, RhoGDIβ, and RhoGDIγ) in insulin-secreting β-cells. The data accrued in these studies revealed that the expression of RhoGDIβ, but not RhoGDIα or RhoGDIγ, is increased in INS-1 832/13 cells, rat islets, and human islets. Hyperglycemic stress also promoted the cleavage of RhoGDIβ, leading to its translocation to the nuclear compartment. We also report that RhoGDIα, but not RhoGDIγ, is associated with the nuclear compartment. However, unlike RhoGDIβ, hyperglycemic conditions exerted no effects on RhoGDIα's association with nuclear fraction. Based on these observations, and our earlier findings of the translocation of Rac1 to the nuclear compartment under the duress of metabolic stress, we conclude that the RhoGDIβ-Rac1 signaling module promotes signals from the cytosolic to the nucleus, culminating in accelerated β-cell dysfunction under metabolic stress.

RhoGDI3 at the trans-Golgi network participates in NLRP3 inflammasome activation, VSMC phenotypic modulation, and neointima formation

BACKGROUND AND AIMS

The pathological roles and mechanisms of Rho-specific guanine nucleotide dissociation inhibitor 3 (RhoGDI3) in vascular smooth muscle cell (VSMC) phenotypic modulation and neointima formation are currently unknown. This study aimed to investigate how RhoGDI3 regulates the Nod-like receptor protein 3 (NLRP3) inflammasome in platelet-derived growth factor-BB (PDGF-BB)-induced neointima formation.

METHODS

For in vitro assays, human aortic VSMCs (HA-VSMCs) were transfected with pcDNA3.1-GDI3 and RhoGDI3 siRNA to overexpress and knockdown RhoGDI3, respectively. HA-VSMCs were also treated with an NLRP3 inhibitor (CY-09) or agonist (NSS). Protein transcription and expression, cell proliferation and migration, Golgi morphology, and protein binding and colocalization were measured. For the in vivo assays, balloon injury (BI) rats were injected with recombinant adenovirus carrying RhoGDI3 shRNA. Carotid arterial morphology, protein expression and colocalization, and activation of the NLRP3 inflammasome were measured.

RESULTS

PDGF-BB treatment induced transcription and expression of RhoGDI3 through PDGF receptor αβ (PDGFRαβ) rather than PDGFRαα or PDGFRββ in HA-VSMCs. RhoGDI3 suppression blocked PDGF-BB-induced VSMC phenotypic transformation. In contrast, RhoGDI3 overexpression further promoted PDGF-BB-induced VSMC dedifferentiation. The in vivo results also confirmed that RhoGDI3 expressed in VSMCs participated in neointima formation and muscle fiber and collagen deposition caused by balloon injury. In addition, PDGF-BB increased binding of RhoGDI3 to NLRP3 and apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) at the trans-Golgi membrane, which depended on the normal Golgi network. However, recruitment of NLRP3 and ASC to the trans-Golgi network after PDGF-BB treatment was independent of RhoGDI3. Moreover, RhoGDI3 knockdown significantly inhibited ASC expression and NLRP3 inflammasome assembly and activation and reduced NLRP3 protein stability in PDGF-BB-treated HA-VSMCs. Inhibiting NLRP3 effectively prevented PDGF-BB-induced VSMC phenotypic modulation, and an NLRP3 agonist reversed the decline in VSMC phenotypic transformation caused by RhoGDI3 knockdown. Furthermore, RhoGDI3 suppression reduced the protein levels and assembly of NLRP3 and ASC, and the activation of the NLRP3 inflammasome in VSMCs in a rat balloon injury model.

CONCLUSIONS

The results of this study reveal a novel mechanism through which RhoGDI3 regulates VSMC phenotypic modulation and neointima formation by activating the NLRP3 inflammasome.

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.

Rho-GDP-dissociation inhibitor-γ negatively regulates NF-κB signaling by promoting the degradation of TAK1 in miiuy croaker (Miichthys miiuy)

Transforming growth factor-beta activated kinase 1 (TAK1) is an adaptor molecular in TLR-mediated NF-κB signaling pathway and plays indispensable roles in innate immunity. As the most typical innate immune pathway, the strict regulation of NF-κB signaling pathway is particularly important. Rho-GDP-dissociation inhibitor-γ (Rho-GDIγ) is a member of the Rho protein family that regulates many important physiological processes. In this study, we demonstrated the mechanism of suppressing TAK1 expression in the teleost and found that Rho-GDIγ negatively regulated the NF-κB signaling pathway mediated by TAK1. We determined that TAK1 could directly interact with Rho-GDIγ. It is interesting that Rho-GDIγ promotes TAK1 degradation through the ubiquitin proteasome pathway. This study brings a new experimental basis to the teleost fish innate immune signaling pathway. Moreover, this discovery may provide new insights into innate immune regulation mechanism in mammals.

Electrostatic Forces Mediate the Specificity of RHO GTPase-GDI Interactions

Three decades of research have documented the spatiotemporal dynamics of RHO family GTPase membrane extraction regulated by guanine nucleotide dissociation inhibitors (GDIs), but the interplay of the kinetic mechanism and structural specificity of these interactions is as yet unresolved. To address this, we reconstituted the GDI-controlled spatial segregation of geranylgeranylated RHO protein RAC1 in vitro. Various biochemical and biophysical measurements provided unprecedented mechanistic details for GDI function with respect to RHO protein dynamics. We determined that membrane extraction of RHO GTPases by GDI occurs via a 3-step mechanism: (1) GDI non-specifically associates with the switch regions of the RHO GTPases; (2) an electrostatic switch determines the interaction specificity between the C-terminal polybasic region of RHO GTPases and two distinct negatively-charged clusters of GDI1; (3) a non-specific displacement of geranylgeranyl moiety from the membrane sequesters it into a hydrophobic cleft, effectively shielding it from the aqueous milieu. This study substantially extends the model for the mechanism of GDI-regulated RHO GTPase extraction from the membrane, and could have implications for clinical studies and drug development.

The RHO Family GTPases: Mechanisms of Regulation and Signaling

Much progress has been made toward deciphering RHO GTPase functions, and many studies have convincingly demonstrated that altered signal transduction through RHO GTPases is a recurring theme in the progression of human malignancies. It seems that 20 canonical RHO GTPases are likely regulated by three GDIs, 85 GEFs, and 66 GAPs, and eventually interact with >70 downstream effectors. A recurring theme is the challenge in understanding the molecular determinants of the specificity of these four classes of interacting proteins that, irrespective of their functions, bind to common sites on the surface of RHO GTPases. Identified and structurally verified hotspots as functional determinants specific to RHO GTPase regulation by GDIs, GEFs, and GAPs as well as signaling through effectors are presented, and challenges and future perspectives are discussed.

Molecular subversion of Cdc42 signalling in cancer

Cdc42 is a member of the Rho family of small GTPases and a master regulator of the actin cytoskeleton, controlling cell motility, polarity and cell cycle progression. This small G protein and its regulators have been the subject of many years of fruitful investigation and the advent of functional genomics and proteomics has opened up new avenues of exploration including how it functions at specific locations in the cell. This has coincided with the introduction of new structural techniques with the ability to study small GTPases in the context of the membrane. The role of Cdc42 in cancer is well established but the molecular details of its action are still being uncovered. Here we review alterations found to Cdc42 itself and to key components of the signal transduction pathways it controls in cancer. Given the challenges encountered with targeting small G proteins directly therapeutically, it is arguably the regulators of Cdc42 and the effector signalling pathways downstream of the small G protein which will be the most tractable targets for therapeutic intervention. These will require interrogation in order to fully understand the global signalling contribution of Cdc42, unlock the potential for mapping new signalling axes and ultimately produce inhibitors of Cdc42 driven signalling.

A Complete Survey of RhoGDI Targets Reveals Novel Interactions with Atypical Small GTPases

There are three RhoGDIs in mammalian cells, which were initially defined as negative regulators of Rho family small GTPases. However, it is now accepted that RhoGDIs not only maintain small GTPases in their inactive GDP-bound form but also act as chaperones for small GTPases, targeting them to specific intracellular membranes and protecting them from degradation. Studies to date with RhoGDIs have usually focused on the interactions between the "typical" or "classical" small GTPases, such as the Rho, Rac, and Cdc42 subfamily members, and either the widely expressed RhoGDI-1 or the hematopoietic-specific RhoGDI-2. Less is known about the third member of the family, RhoGDI-3 and its interacting partners. RhoGDI-3 has a unique N-terminal extension and is found to localize in both the cytoplasm and the Golgi. RhoGDI-3 has been shown to target RhoB and RhoG to endomembranes. In order to facilitate a more thorough understanding of RhoGDI function, we undertook a systematic study to determine all possible Rho family small GTPases that interact with the RhoGDIs. RhoGDI-1 and RhoGDI-2 were found to have relatively restricted activity, mainly binding members of the Rho and Rac subfamilies. RhoGDI-3 displayed wider specificity, interacting with the members of Rho, Rac, and Cdc42 subfamilies but also forming complexes with "atypical" small Rho GTPases such as Wrch2/RhoV, Rnd2, Miro2, and RhoH. Levels of RhoA, RhoB, RhoC, Rac1, RhoH, and Wrch2/RhoV bound to GTP were found to decrease following coexpression with RhoGDI-3, confirming its role as a negative regulator of these small Rho GTPases.

Transcriptomic analysis of the trade-off between endurance and burst-performance in the frog Xenopus allofraseri

BACKGROUND

Variation in locomotor capacity among animals often reflects adaptations to different environments. Despite evidence that physical performance is heritable, the molecular basis of locomotor performance and performance trade-offs remains poorly understood. In this study we identify the genes, signaling pathways, and regulatory processes possibly responsible for the trade-off between burst performance and endurance observed in Xenopus allofraseri, using a transcriptomic approach.

RESULTS

We obtained a total of about 121 million paired-end reads from Illumina RNA sequencing and analyzed 218,541 transcripts obtained from a de novo assembly. We identified 109 transcripts with a significant differential expression between endurant and burst performant individuals (FDR ≤ 0.05 and logFC ≥2), and blast searches resulted in 103 protein-coding genes. We found major differences between endurant and burst-performant individuals in the expression of genes involved in the polymerization and ATPase activity of actin filaments, cellular trafficking, proteoglycans and extracellular proteins secreted, lipid metabolism, mitochondrial activity and regulators of signaling cascades. Remarkably, we revealed transcript isoforms of key genes with functions in metabolism, apoptosis, nuclear export and as a transcriptional corepressor, expressed in either burst-performant or endurant individuals. Lastly, we find two up-regulated transcripts in burst-performant individuals that correspond to the expression of myosin-binding protein C fast-type (mybpc2). This suggests the presence of mybpc2 homoeologs and may have been favored by selection to permit fast and powerful locomotion.

CONCLUSION

These results suggest that the differential expression of genes belonging to the pathways of calcium signaling, endoplasmic reticulum stress responses and striated muscle contraction, in addition to the use of alternative splicing and effectors of cellular activity underlie locomotor performance trade-offs. Ultimately, our transcriptomic analysis offers new perspectives for future analyses of the role of single nucleotide variants, homoeology and alternative splicing in the evolution of locomotor performance trade-offs.

MicroRNA Regulation of the Small Rho GTPase Regulators-Complexities and Opportunities in Targeting Cancer Metastasis

The small Rho GTPases regulate important cellular processes that affect cancer metastasis, such as cell survival and proliferation, actin dynamics, adhesion, migration, invasion and transcriptional activation. The Rho GTPases function as molecular switches cycling between an active GTP-bound and inactive guanosine diphosphate (GDP)-bound conformation. It is known that Rho GTPase activities are mainly regulated by guanine nucleotide exchange factors (RhoGEFs), GTPase-activating proteins (RhoGAPs), GDP dissociation inhibitors (RhoGDIs) and guanine nucleotide exchange modifiers (GEMs). These Rho GTPase regulators are often dysregulated in cancer; however, the underlying mechanisms are not well understood. MicroRNAs (miRNAs), a large family of small non-coding RNAs that negatively regulate protein-coding gene expression, have been shown to play important roles in cancer metastasis. Recent studies showed that miRNAs are capable of directly targeting RhoGAPs, RhoGEFs, and RhoGDIs, and regulate the activities of Rho GTPases. This not only provides new evidence for the critical role of miRNA dysregulation in cancer metastasis, it also reveals novel mechanisms for Rho GTPase regulation. This review summarizes recent exciting findings showing that miRNAs play important roles in regulating Rho GTPase regulators (RhoGEFs, RhoGAPs, RhoGDIs), thus affecting Rho GTPase activities and cancer metastasis. The potential opportunities and challenges for targeting miRNAs and Rho GTPase regulators in treating cancer metastasis are also discussed. A comprehensive list of the currently validated miRNA-targeting of small Rho GTPase regulators is presented as a reference resource.

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.

EphrinB1 promotes cancer cell migration and invasion through the interaction with RhoGDI1

Eph receptors and their corresponding ephrin ligands have been associated with regulating cell–cell adhesion and motility, and thus have a critical role in various biological processes including tissue morphogenesis and homeostasis, as well as pathogenesis of several diseases. Aberrant regulation of Eph/ephrin signaling pathways is implicated in tumor progression of various human cancers. Here, we show that a Rho family GTPase regulator, Rho guanine nucleotide dissociation inhibitor 1 (RhoGDI1), can interact with ephrinB1, and this interaction is enhanced upon binding the extracellular domain of the cognate EphB2 receptor. Deletion mutagenesis revealed that amino acids 327–334 of the ephrinB1 intracellular domain are critical for the interaction with RhoGDI1. Stimulation with an EphB2 extracellular domain-Fc fusion protein (EphB2-Fc) induces RhoA activation and enhances the motility as well as invasiveness of wild-type ephrinB1-expressing cells. These Eph-Fc-induced effects were markedly diminished in cells expressing the mutant ephrinB1 construct (Δ327–334) that is ineffective at interacting with RhoGDI1. Furthermore, ephrinB1 depletion by siRNA suppresses EphB2-Fc-induced RhoA activation, and reduces motility and invasiveness of the SW480 and Hs578T human cancer cell lines. Our study connects the interaction between RhoGDI1 and ephrinB1 to the promotion of cancer cell behavior associated with tumor progression. This interaction may represent a therapeutic target in cancers that express ephrinB1.

Immunological and Functional Characterization of RhoGDI3 and Its Molecular Targets RhoG and RhoB in Human Pancreatic Cancerous and Normal Cells

RhoGDI proteins have been implicated in several human cancers; changes in their expression levels have shown pro- or anti-tumorigenic effects. Pancreatic Ductal Adenocarcinoma (PDAC) is a complex pathology, with poor prognosis, and most patients die shortly after diagnosis. Efforts have been focused on understanding the role of RhoGDI's in PDAC, specially, RhoGDI1 and RhoGDI2. However, the role of RhoGDI3 has not been studied in relation to cancer or to PDAC. Here, we characterized the expression and functionality of RhoGDI3 and its target GTPases, RhoG and RhoB in pancreatic cell lines from both normal pancreatic tissue and tissue in late stages of PDAC, and compared them to human biopsies. Through immunofluorescences, pulldown assays and subcellular fractionation, we found a reduction in RhoGDI3 expression in the late stages of PDAC, and this reduction correlates with tumor progression and aggressiveness. Despite the reduction in the expression of RhoGDI3 in PDAC, we found that RhoB was underexpressed while RhoG was overexpressed, suggesting that cancerous cells preserve their capacity to activate this pathway, thus these cells may be more eager to response to the stimuli needed to proliferate and become invasive unlike normal cells. Surprisingly, we found nuclear localization of RhoGDI3 in non-cancerous pancreatic cell line and normal pancreatic tissue biopsies, which could open the possibility of novel nuclear functions for this protein, impacting gene expression regulation and cellular homeostasis.

Structural and Mechanistic Insights into the Regulation of the Fundamental Rho Regulator RhoGDIα by Lysine Acetylation

Rho proteins are small GTP/GDP-binding proteins primarily involved in cytoskeleton regulation. Their GTP/GDP cycle is often tightly connected to a membrane/cytosol cycle regulated by the Rho guanine nucleotide dissociation inhibitor α (RhoGDIα). RhoGDIα has been regarded as a housekeeping regulator essential to control homeostasis of Rho proteins. Recent proteomic screens showed that RhoGDIα is extensively lysine-acetylated. Here, we present the first comprehensive structural and mechanistic study to show how RhoGDIα function is regulated by lysine acetylation. We discover that lysine acetylation impairs Rho protein binding and increases guanine nucleotide exchange factor-catalyzed nucleotide exchange on RhoA, these two functions being prerequisites to constitute a bona fide GDI displacement factor. RhoGDIα acetylation interferes with Rho signaling, resulting in alteration of cellular filamentous actin. Finally, we discover that RhoGDIα is endogenously acetylated in mammalian cells, and we identify CBP, p300, and pCAF as RhoGDIα-acetyltransferases and Sirt2 and HDAC6 as specific deacetylases, showing the biological significance of this post-translational modification.

RhoGDI2 expression in astrocytes after an excitotoxic lesion in the mouse hippocampus

The Rho GDP-dissociation inhibitor (RhoGDI) originally downregulates Rho family GTPases by preventing nucleotide exchange and membrane association. Although RhoGDI2 functions as a metastasis regulator, little is known in glial cells under neuropathological conditions. We monitored RhoGDI2 expression in the mouse brain after administering a kainic acid(KA)-induced excitotoxic lesion. In control, RhoGDI2 immunoreactivity (IR) was evident in the neuronal layer of the hippocampus. However, RhoGDI2 IR was increased in astrocytes markedly throughout the hippocampus at day 3 post-treatment with KA. To further investigate the molecular mechanism of RhoGDI2-induced cellular migration, primary astrocytes were transfected with the flag-tagged RhoGDI2 cDNA. Cell migration assay revealed that RhoGDI2 cDNA transfection inhibits astrocyte migration. Overexpression of RhoGDI2 leads to inhibit protein kinase B (PKB) activation and cdc42 and cAMP-responsive element-binding protein (CREB) phosphorylation. In conclusion, our results suggested for the first time that RhoGDI2 is required for PKB and CREB activation and cdc42 expression in astrocyte migration after KA-mediated excitotoxic lesion in mouse brain.

RhoGDI3 and RhoG: Vesicular trafficking and interactions with the Sec3 Exocyst subunit

RhoGDIs are negative regulators of small GTP-binding proteins of the Rho family, which have essential cellular functions in most aspects of actin-based morphology and motility processes. They extract Rho proteins from membranes, keep them in inactive rhoGDI/Rho complexes and eventually deliver them again to specific membranes in response to cellular signals. RhoGDI3, the most divergent member of the rhoGDI family, is well suited to document the underlying molecular mechanisms, since the active and inactive forms of its cellular target, RhoG, have well-separated subcellular localizations. In this study, we investigate trafficking structures and molecular interactions involved in rhoGDI3-mediated shuttling of RhoG between the Golgi and the plasma membrane.Bimolecular fluorescence complementation and acceptor-photobleaching FRET experiments suggest that rhoGDI3 and RhoG form complexes on Golgi and vesicular structures in mammalian cells. 4D-videomicroscopy confirms this localization, and show that RhoG/rhoGDI3-labelled structures are less dynamic than RhoG and rhoGDI3-labeled vesicles, consistent with the inhibitory function of rhoGDI3. Next, we identify the Exocyst subunit Sec3 as a candidate rhoGDI3 partner in cells. RhoGDI3 relocates a subcomplex of the Exocyst (Sec3 and Sec8) from the cytoplasm to the Golgi, while Sec6 is unaffected. Remarkably, Sec3 increases the level of GTP-bound endogenous RhoG, the RhoG-dependent induction of membrane ruffles, and the formation of intercellular tunneling nanotube-like protrusions.Altogether, our study identifies a novel link between vesicular traffic and the regulation of Rho proteins by rhoGDIs. It also suggests that components of the Exocyst machinery may be involved in RhoG functions, possibly regulated by rhoGDI3.

Negative charges in the flexible N-terminal domain of Rho GDP-dissociation inhibitors (RhoGDIs) regulate the targeting of the RhoGDI-Rac1 complex to membranes

In its resting state, Rho GDP-dissociation inhibitor (RhoGDI) α forms a soluble cytoplasmic heterodimer with the GDP-bound form of Rac. Upon stimulation, the dissociation of RhoGDIα from the RhoGDIα-Rac complex is a mandatory step for Rac activation; however, this mechanism is poorly understood. In this study, we examined how the cytoplasm/membrane cycles of the RhoGDI-Rac complex are regulated, as well as where RhoGDI dissociates from the RhoGDI-Rac complex, during FcγR-mediated phagocytosis. The negatively charged and flexible N terminus (25 residues) of RhoGDIα, particularly its second negative amino acid cluster possessing five negatively charged amino acids, was a pivotal regulator in the cytoplasm/membrane cycles of the RhoGDI-Rac complex. We also found that RhoGDIα translocated to the phagosomes as a RhoGDIα-Rac1 complex, and this translocation was mediated by an interaction between the polybasic motif in the C terminus of Rac1 and anionic phospholipids produced on phagosomes, such as phosphatidic acid, that is, by a phagosome-targeting mechanism of Rac1. Thus, we demonstrated that the targeting/accumulation of the RhoGDIα-Rac1 complex to phagosomes is regulated by a balance between three factors: 1) the negatively charged and flexible N-terminal of RhoGDIα, 2) the binding affinity of RhoGDIα for Rac1, and 3) anionic phospholipids produced on phagosomes. Moreover, we demonstrated that the mechanism of targeting/accumulation of the RhoGDIα-Rac1 complex is also applicable for the RhoGDIβ-Rac1 complex.

Cholecystokinin-mediated RhoGDI phosphorylation via PKCα promotes both RhoA and Rac1 signaling

RhoA and Rac1 have been implicated in the mechanism of CCK-induced amylase secretion from pancreatic acini. In all cell types studied to date, inactive Rho GTPases are present in the cytosol bound to the guanine nucleotide dissociation inhibitor RhoGDI. Here, we identified the switch mechanism regulating RhoGDI1-Rho GTPase dissociation and RhoA translocation upon CCK stimulation in pancreatic acini. We found that both Gα13 and PKC, independently, regulate CCK-induced RhoA translocation and that the PKC isoform involved is PKCα. Both RhoGDI1 and RhoGDI3, but not RhoGDI2, are expressed in pancreatic acini. Cytosolic RhoA and Rac1 are associated with RhoGDI1, and CCK-stimulated PKCα activation releases the complex. Overexpression of RhoGDI1, by binding RhoA, inhibits its activation, and thereby, CCK-induced apical amylase secretion. RhoA translocation is also inhibited by RhoGDI1. Inactive Rac1 influences CCK-induced RhoA activation by preventing RhoGDI1 from binding RhoA. By mutational analysis we found that CCK-induced PKCα phosphorylation on RhoGDI1 at Ser96 releases RhoA and Rac1 from RhoGDI1 to facilitate Rho GTPases signaling.

The faces and friends of RhoGDI2

RhoGDI2 is a guanine nucleotide dissociation inhibitor (GDI) specific for the Rho family of small GTPases that plays dual opposite roles in tumor progression, being both a promoter in tissues such as breast and a metastasis suppressor in tissues such as the bladder. Despite a clear role for this protein in modulating the invasive and metastatic process, the mechanisms through which RhoGDI2 executes these functions remain unclear. This review will highlight the current state of our knowledge regarding how RhoGDI2 functions in metastasis with a focus on bladder cancer and will also seek to highlight other potential underappreciated avenues through which this protein may affect cancer cell behavior.

The 'invisible hand': regulation of RHO GTPases by RHOGDIs

The 'invisible hand' is a term originally coined by Adam Smith in The Theory of Moral Sentiments to describe the forces of self-interest, competition and supply and demand that regulate the resources in society. This metaphor continues to be used by economists to describe the self-regulating nature of a market economy. The same metaphor can be used to describe the RHO-specific guanine nucleotide dissociation inhibitor (RHOGDI) family, which operates in the background, as an invisible hand, using similar forces to regulate the RHO GTPase cycle.

An essential role for phospholipase D in the recruitment of vesicle amine transport protein-1 to membranes in human neutrophils

Although phosphatidic acid (PA) regulates a wide variety of physiological processes, its targets remain poorly characterized in human neutrophils. By co-sedimentation with PA-containing vesicles we identified several PA-binding proteins including vesicle amine transport protein-1 (VAT-1), Annexin A3 (ANXA3), Rac2, Cdc42 and RhoG in neutrophil cytosol. Except for ANXA3, protein binding to PA-containing liposomes was calcium-independent. Cdc42 and RhoG preferentially interacted with PA whereas VAT-1 bound to PA or phosphatidylserine with the same affinity. VAT-1 translocated to neutrophil membranes upon N-formyl-methionyl-leucyl-phenylalanine (fMLF) stimulation. Inhibition of fMLF-induced PLD activity with the Src kinase inhibitor PP2, the selective inhibitor of PLD FIPI, or of PA formation with primary alcohols reduced VAT-1 translocation. In contrast, inhibition of PA hydrolysis with propranolol enhanced fMLF-mediated VAT-1 recruitment to membranes. PMA also redistributed VAT-1 to membranes in a PKC- and PLD-dependent manner. Though fMLF and PMA increased VAT-1 phosphorylation, different kinases appear to be involved. Cell fractionation revealed that a pool of VAT-1 was co-localized with primary, secondary and tertiary granules and plasma membrane markers in resting neutrophils. Stimulation with fMLF enhanced VAT-1 co-localization with CD32a, a plasma membrane marker. Confocal microscopy revealed that VAT-1 decorates granular structures at the cell periphery and double labeling with VAT-1/lactoferrin antibodies showed a partial co-localization with secondary granules in control and fMLF-stimulated cells. Characterization of these putative PA-binding proteins constitutes another step forward for a better understanding of the role of PLD-derived PA in neutrophil physiology.

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.

Suppression of RhoG activity is mediated by a syndecan 4-synectin-RhoGDI1 complex and is reversed by PKCalpha in a Rac1 activation pathway

Fibroblast growth factor 2 (FGF2) is a major regulator of developmental, pathological, and therapeutic angiogenesis. Its activity is partially mediated by binding to syndecan 4 (S4), a proteoglycan receptor. Angiogenesis requires polarized activation of the small guanosine triphosphatase Rac1, which involves localized dissociation from RhoGDI1 and association with the plasma membrane. Previous work has shown that genetic deletion of S4 or its adapter, synectin, leads to depolarized Rac activation, decreased endothelial migration, and other physiological defects. In this study, we show that Rac1 activation downstream of S4 is mediated by the RhoG activation pathway. RhoG is maintained in an inactive state by RhoGDI1, which is found in a ternary complex with synectin and S4. Binding of S4 to synectin increases the latter's binding to RhoGDI1, which in turn enhances RhoGDI1's affinity for RhoG. S4 clustering activates PKCalpha, which phosphorylates RhoGDI1 at Ser(96). This phosphorylation triggers release of RhoG, leading to polarized activation of Rac1. Thus, FGF2-induced Rac1 activation depends on the suppression of RhoG by a previously uncharacterized ternary S4-synectin-RhoGDI1 protein complex and activation via PKCalpha.

Role of the amphipathic peptide of Semliki forest virus replicase protein nsP1 in membrane association and virus replication

Semliki Forest virus RNA replication takes place in association with specific cytoplasmic vacuoles, derived from the endosomal apparatus. Of the four virus-encoded replicase proteins, nsP1 serves as the membrane anchor of the replication complex. An amphipathic peptide segment, G245STLYTESRKLLRSWHLPSV264, has been implicated in the membrane binding of nsP1. nsP1 variants with changes within the peptide were studied after protein expression and in the context of virus infection. Proteins with mutations R253E and W259A accumulated in the cytoplasm and were very poorly palmitoylated. The same mutations also drastically affected the localization of the precursor polyprotein P123, and they were lethal when introduced into the virus genome. Mutations R253A and L255A+L256A partially changed the localization of nsP1, and the respective viruses acquired compensatory changes. L255A+L256A only yielded virus encoding L255A+L256V, indicating the importance of a hydrophobic residue in the central 256 position. When fused to green fluorescent protein, the peptide was required in at least two tandem copies to effect a change in localization, but even then the fusion protein was associated with membranes in a nonspecific manner. Thus, the amphipathic peptide is a crucial element for the membrane association of nsP1 and the replication complex. It provides essential affinity for membranes, and other regions of nsP1 also appear to contribute to the localization of the protein.

Rho GTPases and actin dynamics in membrane protrusions and vesicle trafficking

Rho GTPases are well known to regulate actin dynamics. They activate two types of actin nucleators, WASP/WAVE proteins and Diaphanous-related formins (DRFs), which induce different types of actin organization. Their ability to interact with membranes allows them to target actin polymerization to discrete sites on the plasma membrane and to intracellular membrane compartments and thereby induce membrane protrusions or regulate vesicle movement. Most studies have concentrated on just three of the 22 mammalian Rho proteins, RhoA, Rac1 and Cdc42. However, recent research indicates that several other members of the Rho family, including Rif, RhoD, TC10 and Wrch1, and also related Rho-of-plants proteins (ROPs) in plants, stimulate actin polymerization and affect plasma membrane protrusion and/or vesicular traffic.

In vivo dynamics of Rac-membrane interactions

The small GTPase Rac cycles between the membrane and the cytosol as it is activated by nucleotide exchange factors (GEFs) and inactivated by GTPase-activating proteins (GAPs). Solubility in the cytosol is conferred by binding of Rac to guanine-nucleotide dissociation inhibitors (GDIs). To analyze the in vivo dynamics of Rac, we developed a photobleaching method to measure the dissociation rate constant (k(off)) of membrane-bound GFP-Rac. We find that k(off) is 0.048 s(-1) for wtRac and approximately 10-fold less (0.004 s(-1)) for G12VRac. Thus, the major route for dissociation is conversion of membrane-bound GTP-Rac to GDP-Rac; however, dissociation of GTP-Rac occurs at a detectable rate. Overexpression of the GEF Tiam1 unexpectedly decreased k(off) for wtRac, most likely by converting membrane-bound GDP-Rac back to GTP-Rac. Both overexpression and small hairpin RNA-mediated suppression of RhoGDI strongly affected the amount of membrane-bound Rac but surprisingly had only slight effects on k(off). These results indicate that RhoGDI controls Rac function mainly through effects on activation and/or membrane association.

Dual specificity of the interfacial inhibitor brefeldin a for arf proteins and sec7 domains

Guanine nucleotide exchange factors (GEFs), which activate small GTP-binding proteins (SMG) by stimulating their GDP/GTP exchange, are emerging as candidate targets for the inhibition of cellular pathways involved in diseases. However, their specific inhibition by competitive inhibitors is challenging, because GEF and SMG families comprise highly similar members. Nature shows us an alternative strategy called interfacial inhibition, exemplified by Brefeldin A (BFA). BFA inhibits the activation of Arf1 by its GEFs in vivo by stabilizing an abortive complex between Arf-GDP and the catalytic Sec7 domain of some of its GEFs. Here we characterize the specificity of BFA toward wild-type (ARNO and BIG1) and mutant Sec7 domains and toward class I, II, and III Arfs. We find that BFA sensitivity of the exchange reaction depends on the nature of both the Sec7 domain and the Arf protein. A single Phe/Tyr substitution is sufficient to achieve BFA sensitivity of the Sec7 domain, which is supported by our characterization of brefeldin C (BFC), a BFA analog that cannot interact with the Tyr residue, and by free energy computations. We further show that Arf1 and Arf5, but not Arf6, are BFA-sensitive, despite their having every BFA-interacting residue in common. Analysis of Arf6 mutants points to the dynamics of the interswitch, which is involved in membrane-to-nucleotide signal propagation, as contributing to, although not sufficient for, BFA sensitivity. Altogether, our results reveal the Tyr/Phe substitution as a novel tool for monitoring BFA sensitivity of cellular ArfGEFs and document the exquisite and dual specificity that can be achieved by an interfacial inhibitor.

RhoGDI: multiple functions in the regulation of Rho family GTPase activities

RhoGDI (Rho GDP-dissociation inhibitor) was identified as a down-regulator of Rho family GTPases typified by its ability to prevent nucleotide exchange and membrane association. Structural studies on GTPase-RhoGDI complexes, in combination with biochemical and cell biological results, have provided insight as to how RhoGDI exerts its effects on nucleotide binding, the membrane association-dissociation cycling of the GTPase and how these activities are controlled. Despite the initial negative roles attributed to RhoGDI, recent evidence has come to suggest that it may also act as a positive regulator necessary for the correct targeting and regulation of Rho activities by conferring cues for spatial restriction, guidance and availability to effectors. These potential functions are discussed in the context of RhoGDI-associated multimolecular complexes, the newly emerged shuttling capability and the importance of the particular membrane microenvironment that represents the site of action for GTPases. All these results point to a wider role for RhoGDI than initially perceived, making it a binding partner that can tightly control Rho GTPases, but which also allows them to reach their full spectrum of activities.

RhoGDIs Revisited: Novel Roles in Rho Regulation

Small GTP-binding proteins of the Rho/Rac/Cdc42 family combine their GDP/GTP cycle, regulated by guanine nucleotide-exchange factors and GTPase-activating proteins, to a cytosol/membrane cycle, regulated by guanine nucleotide dissociation inhibitors (rhoGDIs). RhoGDIs are endowed with dual functions in the cytosol where they form soluble complexes with geranylgeranylated GDP-bound Rho proteins and at membrane interfaces where they monitor the delivery and extraction of Rho proteins to/from their site of action. They have little diversity compared with other Rho protein regulators and therefore have been regarded mostly as housekeeping regulators that distribute Rho proteins equally to any membranes. Recently, acquired data show that rhoGDIs, by interacting with candidate receptors/displacement factors or by phosphorylation, may in fact have active contributions to targeting Rho proteins to specific subcellular membranes and signaling pathways. In addition, the GDP/GTP and membrane/cytosol cycles can be uncoupled in certain cases, with Rho proteins either escaping the membrane/cytosol cycle or being regulated by rhoGDIs in their GTP-bound form. Here, we survey recent structure-function relationships and cellular studies on rhoGDIs and revisit their classical housekeeping role into novel and more specific functions. We also review their involvement in diseases.

RhoGDI-3, a promising system to investigate the regulatory function of rhoGDIs: uncoupling of inhibitory and shuttling functions of rhoGDIs

rhoGDIs (Rho GDP dissociation inhibitors) are postulated to regulate the activity and the localization of small G-proteins of the Rho family by a shuttling process involving extraction of Rho from donor membranes, formation of inhibitory cytosolic rhoGDI/Rho complexes, and delivery of Rho to target membranes. However, the role of rhoGDIs in site-specific membrane targeting or extraction of Rho is still poorly understood. We investigated here the in vivo functions of two mammalian rhoGDIs: the specific rhoGDI-3 and the well-studied rhoGDI-1 (rhoGDI) after structure-based mutagenesis. We identified two sites in rhoGDIs, forming conserved interactions with their Rho target, whose mutation results in the uncoupling of inhibitory and shuttling functions of rhoGDIs in vivo. Remarkably, these rhoGDI mutants were detected at Rho-induced membrane ruffles or protrusions, where they co-localized with RhoG or Cdc42, probably identifying for the first time the site of extraction of a Rho protein by a rhoGDI in vivo. We propose that these mutations act by modifying the steady-state kinetics of the shuttling process regulated by rhoGDIs, such that transient steps at the cell membranes now become detectable. They should provide valuable tools for future investigations of the dynamics of membrane extraction or delivery of Rho proteins and their regulation by cellular partners.

Uncoupling of Inhibitory and Shuttling Functions of Rho GDP Dissociation Inhibitors

Rho GDP dissociation inhibitors (rhoGDIs) are postulated to regulate the activity of small G proteins of the Rho family by a shuttling process involving the extraction of Rho from donor membranes, the formation of the inhibitory cytosolic Rho/rhoGDI complexes, and delivery of Rho to target membranes. However, the role of rhoGDIs in site-specific membrane targeting or extraction of Rho is still poorly understood. Here we investigated the molecular functions of two rhoGDIs, the specific rhoGDI-3 and the less specific but well studied rhoGDI-1, in HeLa cells using structure-based mutagenesis of the rhoGDI protein. We identified two sites in rhoGDI, which form conserved interactions with their Rho target, whose mutation results in the uncoupling of inhibitory and shuttling functions of rhoGDIs: D66GDI-3 (equivalent to D45GDI-1), a conserved residue in the helix-loop-helixGDI/switch 1Rho interface, and D206GDI-3 (equivalent to D185GDI-1) in the beta-sandwichGDI/switch 2Rho interface. Mutations of both sites result in the loss of rhoGDI-3 or rhoGDI-1 inhibitory activity but not of their ability to form cytosolic complexes with RhoG or Cdc42 in vivo. Remarkably, the mutants were detected at Rho-induced membrane ruffles or protrusions where they co-localized with RhoG or Cdc42, likely identifying for the first time the site of extraction of a Rho protein by a rhoGDI in vivo. We propose that these mutations act by modifying the steady-state kinetics of the shuttling process regulated by rhoGDIs, such that transient steps at the cell membranes now become detectable. They should provide valuable tools for future investigations of the dynamics of membrane extraction or delivery of Rho proteins and their regulation by cellular partners.

Rho-family GTPases: it's not only Rac and Rho (and I like it)

The Rho-family proteins make up a major branch of the Ras superfamily of small GTPases. To date, 22 human genes encoding at least 25 proteins have been described. The best known 'classical' members are RhoA, Rac1 and Cdc42. Highly related isoforms of these three proteins have not been studied as intensively, in part because it has been assumed that they are functionally identical to their better-studied counterparts. This now appears not to be the case. Variations in C-terminal-signaled modifications and subcellular targeting cause otherwise highly biochemically related isoforms (e.g. RhoA, RhoB and RhoC) to exhibit surprisingly divergent biological activities. Whereas the classical Rho GTPases are regulated by GDP/GTP cycling, other Rho GTPases are also regulated by other mechanisms, particularly by transcriptional regulation. Newer members of the family possess additional sequence elements beyond the GTPase domain, which suggests they exhibit yet other mechanisms of regulation.

Activation of Rho GTPases by synaptic transmission in the hippocampus

Ras-related GTPases of the Rho family, such as RhoA and RhoB, are well-characterised mediators of morphological change in peripheral tissues via their effects on the actin cytoskeleton. We tested the hypothesis that Rho family GTPases are involved in synaptic transmission in the CA1 region of the hippocampus. We show that GTPases are activated by synaptic transmission. RhoA and RhoB were activated by low frequency stimulation, while the induction of long-term potentiation (LTP) by high frequency stimulation was associated with specific activation of RhoB via NMDA receptor stimulation. This illustrates that these GTPases are potential mediators of synaptic transmission in the hippocampus, and raises the possibility that RhoB may play a role in plasticity at hippocampal synapses during LTP.

RhoE binds to ROCK I and inhibits downstream signaling

RhoE belongs to the Rho GTPase family, the members of which control actin cytoskeletal dynamics. RhoE induces stress fiber disassembly in a variety of cell types, whereas RhoA stimulates stress fiber assembly. The similarity of RhoE and RhoA sequences suggested that RhoE might compete with RhoA for interaction with its targets. Here, we show that RhoE binds ROCK I but none of the other RhoA targets tested. The interaction of RhoE with ROCK I was confirmed by coimmunoprecipitation of the endogenous proteins, and the two proteins colocalized on the trans-Golgi network in COS-7 cells. Although RhoE and RhoA were not able to bind ROCK I simultaneously, RhoE bound to the amino-terminal region of ROCK I encompassing the kinase domain, at a site distant from the carboxy-terminal RhoA-binding site. Overexpression of RhoE inhibited ROCK I-induced stress fiber formation and phosphorylation of the ROCK I target myosin light chain phosphatase. These data suggest that RhoE induces stress fiber disassembly by directly binding ROCK I and inhibiting it from phosphorylating downstream targets.