The identification and characterization of a GDP-dissociation inhibitor (GDI) for the CDC42Hs protein

Atypical RhoUV GTPases in development and disease

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

RhoA Signaling in Neurodegenerative Diseases

Ras homolog gene family member A (RhoA) is a small GTPase of the Rho family involved in regulating multiple signal transduction pathways that influence a diverse range of cellular functions. RhoA and many of its downstream effector proteins are highly expressed in the nervous system, implying an important role for RhoA signaling in neurons and glial cells. Indeed, emerging evidence points toward a role of aberrant RhoA signaling in neurodegenerative diseases such as Parkinson’s disease, Alzheimer’s disease, Huntington’s disease, and amyotrophic lateral sclerosis. In this review, we summarize the current knowledge of RhoA regulation and downstream cellular functions with an emphasis on the role of RhoA signaling in neurodegenerative diseases and the therapeutic potential of RhoA inhibition in neurodegeneration.

RhoA: a dubious molecule in cardiac pathophysiology

The Ras homolog gene family member A (RhoA) is the founding member of Rho GTPase superfamily originally studied in cancer cells where it was found to stimulate cell cycle progression and migration. RhoA acts as a master switch control of actin dynamics essential for maintaining cytoarchitecture of a cell. In the last two decades, however, RhoA has been coined and increasingly investigated as an essential molecule involved in signal transduction and regulation of gene transcription thereby affecting physiological functions such as cell division, survival, proliferation and migration. RhoA has been shown to play an important role in cardiac remodeling and cardiomyopathies; underlying mechanisms are however still poorly understood since the results derived from in vitro and in vivo experiments are still inconclusive. Interestingly its role in the development of cardiomyopathies or heart failure remains largely unclear due to anomalies in the current data available that indicate both cardioprotective and deleterious effects. In this review, we aimed to outline the molecular mechanisms of RhoA activation, to give an overview of its regulators, and the probable mechanisms of signal transduction leading to RhoA activation and induction of downstream effector pathways and corresponding cellular responses in cardiac (patho)physiology. Furthermore, we discuss the existing studies assessing the presented results and shedding light on the often-ambiguous data. Overall, we provide an update of the molecular, physiological and pathological functions of RhoA in the heart and its potential in cardiac therapeutics.

Cullin 3/KCTD5 Promotes the Ubiqutination of Rho Guanine Nucleotide Dissociation Inhibitor 1 and Regulates Its Stability

Rho guanine nucleotide dissociation inhibitor 1 (RhoGDI1) plays important roles in numerous cellular processes, including cell motility, adhesion, and proliferation, by regulating the activity of Rho GTPases. Its expression is altered in various human cancers and is associated with malignant progression. Here, we show that RhoGDI1 interacts with Cullin 3 (CUL3), a scaffold protein for E3 ubiquitin ligase complexes. Ectopic expression of CUL3 increases the ubiquitination of RhoGDI1. Furthermore, potassium channel tetramerization domain containing 5 (KCTD5) also binds to RhoGDI1 and increases its interaction with CUL3. Ectopic expression of KCTD5 increases the ubiquitination of RhoGDI1, whereas its knockdown by RNA interference has the opposite effect. Depletion of KCTD5 or expression of dominant-negative CUL3 (DN-CUL3) enhances the stability of RhoGDI1. Our findings reveal a previously unknown mechanism for controlling RhoGDI1 degradation that involves a CUL3/KCTD5 ubiquitin ligase complex.

RAS and RHO family GTPase mutations in cancer: twin sons of different mothers?

The RAS and RHO family comprise two major branches of the RAS superfamily of small GTPases. These proteins function as regulated molecular switches and control cytoplasmic signaling networks that regulate a diversity of cellular processes, including cell proliferation and cell migration. In the early 1980s, mutationally activated RAS genes encoding KRAS, HRAS and NRAS were discovered in human cancer and now comprise the most frequently mutated oncogene family in cancer. Only recently, exome sequencing studies identified cancer-associated alterations in two RHO family GTPases, RAC1 and RHOA. RAS and RHO proteins share significant identity in their amino acid sequences, protein structure and biochemistry. Cancer-associated RAS mutant proteins harbor missense mutations that are found primarily at one of three mutational hotspots (G12, G13 and Q61) and have been identified as gain-of-function oncogenic alterations. Although these residues are conserved in RHO family proteins, the gain-of-function mutations found in RAC1 are found primarily at a distinct hotspot. Unexpectedly, the cancer-associated mutations found with RHOA are located at different hotspots than those found with RAS. Furthermore, since the RHOA mutations suggested a loss-of-function phenotype, it has been unclear whether RHOA functions as an oncogene or tumor suppressor in cancer development. Finally, whereas RAS mutations are found in a broad spectrum of cancer types, RHOA and RAC1 mutations occur in a highly restricted range of cancer types. In this review, we focus on RHOA missense mutations found in cancer and their role in driving tumorigenesis, with comparisons to cancer-associated mutations in RAC1 and RAS GTPases.

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.

Extraction of active RhoGTPases by RhoGDI regulates spatiotemporal patterning of RhoGTPases

The RhoGTPases are characterized as membrane-associated molecular switches that cycle between active, GTP-bound and inactive, GDP-bound states. However, 90-95% of RhoGTPases are maintained in a soluble form by RhoGDI, which is generally viewed as a passive shuttle for inactive RhoGTPases. Our current understanding of RhoGTPase:RhoGDI dynamics has been limited by two experimental challenges: direct visualization of the RhoGTPases in vivo and reconstitution of the cycle in vitro. We developed methods to directly image vertebrate RhoGTPases in vivo or on lipid bilayers in vitro. Using these methods, we identified pools of active and inactive RhoGTPase associated with the membrane, found that RhoGDI can extract both inactive and active RhoGTPases, and found that extraction of active RhoGTPase contributes to their spatial regulation around cell wounds. These results indicate that RhoGDI directly contributes to the spatiotemporal patterning of RhoGTPases by removing active RhoGTPases from the plasma membrane.

Regulation of Rho GTPases by RhoGDIs in Human Cancers

Rho GDP dissociation inhibitors (RhoGDIs) play important roles in various cellular processes, including cell migration, adhesion, and proliferation, by regulating the functions of the Rho GTPase family. Dissociation of Rho GTPases from RhoGDIs is necessary for their spatiotemporal activation and is dynamically regulated by several mechanisms, such as phosphorylation, sumoylation, and protein interaction. The expression of RhoGDIs has changed in many human cancers and become associated with the malignant phenotype, including migration, invasion, metastasis, and resistance to anticancer agents. Here, we review how RhoGDIs control the function of Rho GTPases by regulating their spatiotemporal activity and describe the regulatory mechanisms of the dissociation of Rho GTPases from RhoGDIs. We also discuss the role of RhoGDIs in cancer progression and their potential uses for therapeutic intervention.

RhoGDIβ affects HeLa cell spindle orientation following UVC irradiation

The molecular signals that regulate mitotic spindle orientation to determine the proper division axis play a critical role in the development and maintenance of tissue homeostasis. However, deregulation of signaling events can result in spindle misorientation, which in turn can trigger developmental defects and cancer progression. Little is known about the cellular signaling pathway involved in the misorientation of proliferating cells that evade apoptosis after DNA damage. In this study, we found that perturbations to spindle orientation were induced in ultraviolet C (UVC)-irradiated surviving cells. N-terminal truncated Rho GDP-dissociation inhibitor β (RhoGDIβ), which is produced by UVC irradiation, distorted the spindle orientation of HeLa cells cultured on Matrigel. The short hairpin RNA-mediated knockdown of RhoGDIβ significantly attenuated UVC-induced misorientation. Subsequent expression of wild-type RhoGDIβ, but not a noncleavable mutant, RhoGDIβ (D19A), again led to a relative increase in spindle misorientation in response to UVC. Our findings revealed that RhoGDIβ impacts spindle orientation in response to DNA damage.

Cell Polarity in Yeast

A conserved molecular machinery centered on the Cdc42 GTPase regulates cell polarity in diverse organisms. Here we review findings from budding and fission yeasts that reveal both a conserved core polarity circuit and several adaptations that each organism exploits to fulfill the needs of its lifestyle. The core circuit involves positive feedback by local activation of Cdc42 to generate a cluster of concentrated GTP-Cdc42 at the membrane. Species-specific pathways regulate the timing of polarization during the cell cycle, as well as the location and number of polarity sites.

Rho GDP-dissociation inhibitor α is a potential prognostic biomarker and controls telomere regulation in colorectal cancer

Rho GDP‐dissociation inhibitor α (RhoGDIα) is an essential regulator for Rho GTPases. Although RhoGDIα may serve as an oncogene in colorectal cancer (CRC), the underlying mechanism is still unclear. We investigated the function, mechanism, and clinical significance of RhoGDIα in CRC progression. We founded that downregulation of RhoGDIα repressed CRC cell proliferation, motility, and invasion. Overexpression of RhoGDIα increased DNA damage response signals at telomeres, and led to telomere shortening in CRC cells, also being validated in 26 pairs of CRC tissues. Mechanistic studies revealed that RhoGDIα could promote telomeric repeat factor 1 (TRF1) expression through the phosphatidylinositol 3‐kinase–protein kinase B signal pathway. Moreover, RhoGDIα protein levels were strongly correlated with TRF1 in CRC tissues. A cohort of 297 CRC samples validated the positive relationship between RhoGDIα and TRF1, and revealed that RhoGDIα and TRF1 levels were negatively associated with CRC patients' survival. Taken together, our results suggest that RhoGDIα regulate TRF1 and telomere length and may be novel prognostic biomarkers in colorectal cancer.

A Two-Tiered Mechanism Enables Localized Cdc42 Signaling during Enterocyte Polarization

Signaling by the small GTPase Cdc42 governs a diverse set of cellular processes that contribute to tissue morphogenesis. Since these processes often require highly localized signaling, Cdc42 activity must be clustered in order to prevent ectopic signaling. During cell polarization, apical Cdc42 signaling directs the positioning of the nascent apical membrane. However, the molecular mechanisms that drive Cdc42 clustering during polarity establishment are largely unknown. Here, we demonstrate that during cell polarization localized Cdc42 signaling is enabled via activity-dependent control of Cdc42 mobility. By performing photoconversion experiments, we show that inactive Cdc42-GDP is 30-fold more mobile than active Cdc42-GTP. This switch in apical mobility originates from a dual mechanism involving RhoGDI-mediated membrane dissociation of Cdc42-GDP and Tuba-mediated immobilization of Cdc42-GTP. Interference with either mechanism affects Cdc42 clustering and as a consequence impairs Cdc42-mediated apical membrane clustering. We therefore identify a molecular network, comprised of Cdc42, the guanine nucleotide exchange factor (GEF) Tuba, and RhoGDI, that enables differential diffusion of inactive and active Cdc42 and is required to establish localized Cdc42 signaling during enterocyte polarization.

Guanine nucleotide induced conformational change of Cdc42 revealed by hydrogen/deuterium exchange mass spectrometry

Cdc42 regulates pathways related to cell division. Dysregulation of Cdc42 can lead to cancer, cardiovascular diseases and neurodegenerative diseases. GTP induced activation mechanism plays an important role in the activity and biological functions of Cdc42. P-loop, Switch I and Switch II are critical regions modulating the enzymatic activity of Cdc42. We applied amide hydrogen/deuterium exchange coupled with liquid chromatography mass spectrometry (HDXMS) to investigate the dynamic changes of apo-Cdc42 after GDP, GTP and GMP-PCP binding. The natural substrate GTP induced significant decreases of deuteration in P-loop and Switch II, moderate changes of deuteration in Switch I and significant changes of deuteration in the α7 helix, a region far away from the active site. GTP binding induced similar effects on H/D exchange to its non-hydrolysable analog, GMP-PCP. HDXMS results indicate that GTP binding blocked the solvent accessibility in the active site leading to the decrease of H/D exchange rate surrounding the active site, and further triggered a conformational change resulting in the drastic decrease of H/D exchange rate at the remote α7 helix. Comparing the deuteration levels in three activation states of apo-Cdc42, Cdc42-GDP and Cdc42-GMP-PCP, the apo-Cdc42 has the most flexible structure, which can be stabilized by guanine nucleotide binding. The rates of H/D exchange of Cdc42-GDP are between the GMP-PCP-bound and the apo form, but more closely to the GMP-PCP-bound form. Our results show that the activation of Cdc42 is a process of conformational changes involved with P-loop, Switch II and α7 helix for structural stabilization.

α2β1 integrin, GPVI receptor, and common FcRγ chain on mouse platelets mediate distinct responses to collagen in models of thrombosis

Objective

Platelets express the α2β1 integrin and the glycoprotein VI (GPVI)/FcRγ complex, both collagen receptors. Understanding platelet-collagen receptor function has been enhanced through use of genetically modified mouse models. Previous studies of GPVI/FcRγ-mediated collagen-induced platelet activation were perfomed with mice in which the FcRγ subunit was genetically deleted (FcRγ^−/−) or the complex was depleted. The development of α2β1^−/− and GPVI^−/− mice permits side-by-side comparison to address contributions of these collagen receptors in vivo and in vitro.

Approach and Results

To understand the different roles played by the α2β1 integrin, the GPVI receptor or FcRγ subunit in collagen-stimulated hemostasis and thrombosis, we compared α2β1^−/−, FcRγ^−/−, and GPVI^−/− mice in models of endothelial injury and intravascular thrombosis in vivo and their platelets in collagen-stimulated activation in vitro. We demonstrate that both the α2β1 integrin and the GPVI receptor, but not the FcRγ subunit influence carotid artery occlusion in vivo. In contrast, the GPVI receptor and the FcRγ chain, but not the α2β1 integrin, play similar roles in intravascular thrombosis in response to soluble Type I collagen. FcRγ^−/− platelets showed less attenuation of tyrosine phosphorylation of several proteins including RhoGDI when compared to GPVI^−/− and wild type platelets. The difference between FcRγ^−/− and GPVI^−/− platelet phosphotyrosine levels correlated with the in vivo thrombosis findings.

Conclusion

Our data demonstrate that genetic deletion of GPVI receptor, FcRγ chain, or the α2β1 integrin changes the thrombotic potentials of these platelets to collagen dependent on the stimulus mechanism. The data suggest that the FcRγ chain may provide a dominant negative effect through modulating signaling pathways in platelets involving several tyrosine phosphorylated proteins such as RhoGDI. In addition, these findings suggest a more complex signaling network downstream of the platelet collagen receptors than previously appreciated.

Liposome reconstitution and modulation of recombinant prenylated human Rac1 by GEFs, GDI1 and Pak1

Small Rho GTPases are well known to regulate a variety of cellular processes by acting as molecular switches. The regulatory function of Rho GTPases is critically dependent on their posttranslational modification at the carboxyl terminus by isoprenylation and association with proper cellular membranes. Despite numerous studies, the mechanisms of recycling and functional integration of Rho GTPases at the biological membranes are largely unclear. In this study, prenylated human Rac1, a prominent member of the Rho family, was purified in large amount from baculovirus-infected Spodoptera frugiperda insect cells using a systematic detergent screening. In contrast to non-prenylated human Rac1 purified from Escherichia coli, prenylated Rac1 from insect cells was able to associate with synthetic liposomes and to bind Rho-specific guanine nucleotide dissociation inhibitor 1 (GDI1). Subsequent liposome reconstitution experiments revealed that GDI1 efficiently extracts Rac1 from liposomes preferentially in the inactive GDP-bound state. The extraction was prevented when Rac1 was activated to its GTP-bound state by Rac-specific guanine nucleotide exchange factors (GEFs), such as Vav2, Dbl, Tiam1, P-Rex1 and TrioN, and bound by the downstream effector Pak1. We found that dissociation of Rac1-GDP from its complex with GDI1 strongly correlated with two distinct activities of especially Dbl and Tiam1, including liposome association and the GDP/GTP exchange. Taken together, our results provided first detailed insights into the advantages of the in vitro liposome-based reconstitution system to study both the integration of the signal transducing protein complexes and the mechanisms of regulation and signaling of small GTPases at biological membranes.

Loss of ARHGDIA expression is associated with poor prognosis in HCC and promotes invasion and metastasis of HCC cells

Rho GTPases control a wide range of cellular processes and contribute to tumor invasion and metastasis. As a regulator of Rho activity, ARHGDIA is aberrantly expressed in several types of tumors and plays different roles in the tumor process. To elucidate the role of ARHGDIA in HCC, we investigated the patterns of its expression, prognosis and clinical profiles in HCC. Functional assays were performed to investigate whether the alteration of ARHGDIA has an effect on cell growth, migration and invasion, as well as the status of Rho GTPases. We found that ARHGDIA was frequently downregulated in HCC and associated with tumor invasion and metastasis. Moreover, ARHGDIA was significantly associated with OS and TTR of HCC patients. Low level of ARHGDIA exhibited a decreased postoperative OS and a shorter TTR compared those with high levels. Functional assays showed that loss of ARHGDIA promoted HCC cell migration and invasion in vitro and lung metastasis formation in vivo. We found that loss of ARHGDIA significantly induced Rac1 and RhoA activation which may contribute to invasion and metastasis of HCC. In conclusion, the present study has identified loss of ARHGDIA contributed to the processes of hepatic tumorigenesis, in particular invasion and metastasis which may provide a potential therapeutic target for HCC.

Small Rho GTPases in the control of cell shape and mobility

Rho GTPases are a class of evolutionarily conserved proteins comprising 20 members, which are predominantly known for their role in regulating the actin cytoskeleton. They are primarily regulated by binding of GTP/GDP, which is again controlled by regulators like GEFs, GAPs, and RhoGDIs. Rho GTPases are thus far well known for their role in the regulation of actin cytoskeleton and migration. Here we present an overview on the role of Rho GTPases in regulating cell shape and plasticity of cell migration. Finally, we discuss the emerging roles of ubiquitination and sumoylation in regulating Rho GTPases and cell migration.

VEGF-C mediates RhoGDI2-induced gastric cancer cell metastasis and cisplatin resistance

Rho GDP dissociation inhibitor 2 (RhoGDI2) expression is correlated with tumor growth, metastasis and chemoresistance in gastric cancer. However, the mechanisms by which RhoGDI2 promotes tumor cell survival and metastasis remain unclear. In this study, we clearly demonstrate that RhoGDI2 upregulates VEGF-C expression and RhoGDI2 expression is positively correlated with VEGF-C expression in human gastric tumor tissues as well as parental gastric cancer cell lines. VEGF-C depletion suppressed RhoGDI2-induced gastric cancer metastasis and sensitized RhoGDI2-overexpressing cells to cisplatin-induced apoptosis in vitro and in vivo. Secreted VEGF-C enhanced gastric cancer cell invasion and conferred cisplatin resistance to RhoGDI2-overexpressing cells. We also show that RhoGDI2 positively regulates Rac1 activity in gastric cancer cells. Inhibition of Rac1 expression suppressed RhoGDI2-induced VEGF-C expression, and this inhibition was associated with decreased invasiveness and increased sensitivity to cisplatin in RhoGDI2-overexpressing cells. Our results indicate that RhoGDI2 might be a potential therapeutic target for simultaneously reducing metastasis risk and enhancing chemotherapy efficacy in gastric cancer.

Reduced expression of Rho GDP dissociation inhibitor 2 mRNA is associated with lymph node metastasis in gastric carcinoma

Small GTPase proteins, including RhoA, RhoB, RhoC, Rac1 and cdc42, are molecules that have significant roles in linking cell shape and cell cycle progression in cytoskeletal arrangements and mitogenic signaling. Rho GDP dissociation inhibitor 2 (RhoGDI2) has recently been identified as a metastasis suppressor gene in models of bladder cancer. RhoGDI2 has also been identified as a potential regulator of tumorigenesis and cancer progression. The present study aimed to clarify the significance of RhoGDI2 gene expression in gastric carcinoma and to evaluate the outcome of affected patients. A total of 46 pairs of normal mucosa and cancer specimens were obtained from patients who had undergone a gastrectomy for primary gastric carcinoma and were subjected to semi-quantitative reverse transcription polymerase chain reaction (RT-PCR) for RhoGDI2. The expression of RhoGDI2 mRNA was significantly higher in early-stage gastric cancer specimens compared with the normal gastric epithelium samples. By contrast, the depth of the tumor was negatively correlated with RhoGDI2 mRNA expression. In addition, a reduced expression of RhoGDI2 mRNA was associated with venous system invasion and lymph node metastasis. RhoGDI2 mRNA was more frequently expressed in differentiated adenocarcinoma compared with poorly-differentiated adenocarcinoma. Although the statistical significance was not established, RhoGDI2-positive patients tended to have a superior oncological outcome compared with RhoGDI2-negative patients. The reduced expression of RhoGDI2 mRNA in gastric carcinoma is associated with venous system invasion and lymph node metastasis.

Rho GTPases in platelet function

The Rho family of GTP binding proteins, also commonly referred to as the Rho GTPases, are master regulators of the platelet cytoskeleton and platelet function. These low-molecular-weight or 'small' GTPases act as signaling switches in the spatial and temporal transduction, and amplification of signals from platelet cell surface receptors to the intracellular signaling pathways that drive platelet function. The Rho GTPase family members RhoA, Cdc42 and Rac1 have emerged as key regulators in the dynamics of the actin cytoskeleton in platelets and play key roles in platelet aggregation, secretion, spreading and thrombus formation. Rho GTPase regulators, including GEFs and GAPs and downstream effectors, such as the WASPs, formins and PAKs, may also regulate platelet activation and function. In this review, we provide an overview of Rho GTPase signaling in platelet physiology. Previous studies of Rho GTPases and platelets have had a shared history, as platelets have served as an ideal, non-transformed cellular model to characterize Rho function. Likewise, recent studies of the cell biology of Rho GTPase family members have helped to build an understanding of the molecular regulation of platelet function and will continue to do so through the further characterization of Rho GTPases as well as Rho GAPs, GEFs, RhoGDIs and Rho effectors in actin reorganization and other Rho-driven cellular processes.

Identification and characterisation of the RalA-ERp57 interaction: evidence for GDI activity of ERp57

RalA is a membrane-associated small GTPase that regulates vesicle trafficking. Here we identify a specific interaction between RalA and ERp57, an oxidoreductase and signalling protein. ERp57 bound specifically to the GDP-bound form of RalA, but not the GTP-bound form, and inhibited the dissociation of GDP from RalA in vitro. These activities were inhibited by reducing agents, but no disulphide bonds were detected between RalA and ERp57. Mutation of all four of ERp57’s active site cysteine residues blocked sensitivity to reducing agents, suggesting that redox-dependent conformational changes in ERp57 affect binding to RalA. Mutations in the switch II region of the GTPase domain of RalA specifically reduced or abolished binding to ERp57, but did not block GTP-specific binding to known RalA effectors, the exocyst and RalBP1. Oxidative treatment of A431 cells with H₂O₂ inhibited cellular RalA activity, and the effect was exacerbated by expression of recombinant ERp57. The oxidative treatment significantly increased the amount of RalA localised to the cytosol. These findings suggest that ERp57 regulates RalA signalling by acting as a redox-sensitive guanine-nucleotide dissociation inhibitor (RalGDI).

Cellular function of RhoGDI-α mediates the cycling of Rac1 and the regulation of pancreatic beta cell death

Mycophenolic acid (MPA) is an immunosuppressive agent that is widely used in clinical therapy, including pancreas and islet transplantation. Previously, we showed that MPA induces significant apoptosis of insulin-secreting cells by downregulating RhoGDI-α and increasing JNK expression. In this study, we investigated Rac1 directly associated with RhoGDI-α during MPA-induced apoptosis in INS-1E cells (an insulin-secreting cell line). Cells were treated with MPA for 24 and 36 hours. Immunoprecipitation was used to examine physical interactions between RhoGDI-α and Rac1. Activation and immunoprecipitation assays showed expressions of Rac1 and RhoGDI-α to be directly correlated. Rac1 binding to RhoGDI-α decreased after MPA treatment, and Rac1 was induced and subsequently activated by MPA. We concluded that this novel RhoGDI-α/Rac1/JNK pathway induced apoptosis of transplanted islet cells after MPA treatment.

Cell polarization and cytokinesis in budding yeast

Asymmetric cell division, which includes cell polarization and cytokinesis, is essential for generating cell diversity during development. The budding yeast Saccharomyces cerevisiae reproduces by asymmetric cell division, and has thus served as an attractive model for unraveling the general principles of eukaryotic cell polarization and cytokinesis. Polarity development requires G-protein signaling, cytoskeletal polarization, and exocytosis, whereas cytokinesis requires concerted actions of a contractile actomyosin ring and targeted membrane deposition. In this chapter, we discuss the mechanics and spatial control of polarity development and cytokinesis, emphasizing the key concepts, mechanisms, and emerging questions in the field.

Proteomics-based strategy to delineate the molecular mechanisms of RhoGDI2-induced metastasis and drug resistance in gastric cancer

Rho GDP dissociation inhibitor 2 (RhoGDI2) was initially identified as a regulator of the Rho family of GTPases. Our recent works suggest that RhoGDI2 promotes tumor growth and malignant progression, as well as enhances chemoresistance in gastric cancer. Here, we delineate the mechanism by which RhoGDI2 promotes gastric cancer cell invasion and chemoresistance using two-dimensional gel electrophoresis (2-DE) on proteins derived from a RhoGDI2-overexpressing SNU-484 human gastric cancer cell line and control cells. Differentially expressed proteins were identified using matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOF-MS). In total, 47 differential protein spots were identified; 33 were upregulated, and 14 were downregulated by RhoGDI2 overexpression. Upregulation of SAE1, Cathepsin D, Cofilin1, CIAPIN1, and PAK2 proteins was validated by Western blot analysis. Loss-of-function analysis using small interference RNA (siRNA) directed against candidate genes reveals the need for CIAPIN1 and PAK2 in RhoGDI2-induced cancer cell invasion and Cathepsin D and PAK2 in RhoGDI2-mediated chemoresistance in gastric cancer cells. These data extend our understanding of the genes that act downstream of RhoGDI2 during the progression of gastric cancer and the acquisition of chemoresistance.

C-terminal di-arginine motif of Cdc42 protein is essential for binding to phosphatidylinositol 4,5-bisphosphate-containing membranes and inducing cellular transformation

Rho GTPases regulate a diverse range of processes that are dependent on their proper cellular localization. The membrane localization of these GTPases is due in large part to their carboxyl-terminal geranylgeranyl moiety. In addition, most of the Rho family members contain a cluster of positively charged residues (i.e. a "polybasic domain"), directly preceding their geranylgeranyl moiety, and it has been suggested that this domain serves to fine-tune their localization among different cellular membrane sites. Here, we have taken a closer look at the role of the polybasic domain of Cdc42 in its ability to bind to membranes and induce the transformation of fibroblasts. A FRET assay for the binding of Cdc42 to liposomes of defined composition showed that Cdc42 associates more strongly with liposomes containing phosphatidylinositol 4,5-bisphosphate (PIP(2)) when compared either with uncharged control membranes or with liposomes containing a charge-equivalent amount of phosphatidylserine. The carboxyl-terminal di-arginine motif (Arg-186 and Arg-187) was shown to play an essential role in the binding of Cdc42 to PIP(2)-containing membranes. We further showed that substitutions for the di-arginine motif, when introduced within a constitutively active ("fast cycling") Cdc42(F28L) background, had little effect on the ability of the activated Cdc42 mutant to induce microspikes/filopodia in NIH 3T3 cells, whereas they eliminated its ability to transform fibroblasts. Taken together, these findings suggest that the di-arginine motif within the carboxyl terminus of Cdc42 is necessary for this GTPase to bind at membrane sites containing PIP(2), where it can initiate signaling activities that are essential for the oncogenic transformation of cells.

Analysis of the role of RhoGDI1 and isoprenylation in the degradation of RhoGTPases

RhoGDI1 is one of the three major regulators of the Rho switch along with RhoGEFs and RhoGAPs. RhoGDI1 extracts prenylated Rho proteins from lipid membranes, sequesters them in the cytosol, and prevents nucleotide exchange or hydrolysis. In addition, RhoGDI1 protects prenylated Rho proteins from degradation. Here, we describe techniques to monitor Rho proteins degradation upon depletion of RhoGDI1 and their dependence upon prenylation for degradation.

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.

A switch I mutant of Cdc42 exhibits less conformational freedom

Cdc42 is a Ras-related small G-protein and functions as a molecular switch in signal transduction pathways linked with cell growth and differentiation. It is controlled by cycling between GTP-bound (active) and GDP-bound (inactive) forms. Nucleotide binding and hydrolysis are modulated by interactions with effectors and/or regulatory proteins. These interactions are centralized in two relatively flexible "Switch" regions as characterized by internal dynamics on multiple time scales [Loh, A. P., et al. (2001) Biochemistry 40, 4590-4600], and this flexibility may be essential for protein interactions. In the Switch I region, Thr(35) seems to be critical for function, as it is completely invariant in Ras-related proteins. To investigate the importance of conformational flexibility in Switch I of Cdc42, we mutated threonine to alanine, determined the solution structure, and characterized the backbone dynamics of the single-point mutant protein, Cdc42(T35A). Backbone dynamics data suggest that the mutation changes the time scale of the internal motions of several residues, with several resonances not being discernible in wild-type Cdc42 [Adams, P. D., and Oswald, R. E. (2007) Biomol. NMR Assignments 1, 225-227]. The mutation does not appear to affect the thermal stability of Cdc42, and chymotrypsin digestion data further suggest that changes in the conformational flexibility of Switch I slow proteolytic cleavage relative to that of the wild type. In vitro binding assays show less binding of Cdc42(T35A), relative to that of wild type, to a GTPase binding protein that inhibits GTP hydrolysis in Cdc42. These results suggest that the mutation of T(35) leads to the loss of conformational freedom in Switch I that could affect effector-regulatory protein interactions.

Concordance and interaction of guanine nucleotide dissociation inhibitor (RhoGDI) with RhoA in oogenesis and early development of the sea urchin

Rho GTPases are Ras-related GTPases that regulate a variety of cellular processes. In the sea urchin Strongylocentrotus purpuratus, RhoA in the oocyte associates with the membrane of the cortical granules and directs their movement from the cytoplasm to the cell cortex during maturation to an egg. RhoA also plays an important role regulating the Na(+) -H(+) exchanger activity, which determines the internal pH of the cell during the first minutes of embryogenesis. We investigated how this activity may be regulated by a guanine-nucleotide dissociation inhibitor (RhoGDI). The sequence of this RhoA regulatory protein was identified in the genome on the basis of its similarity to other RhoGDI species, especially for key segments in the formation of the isoprenyl-binding pocket and in interactions with the Rho GTPase. We examined the expression and the subcellular localization of RhoGDI during oogenesis and in different developmental stages. We found that RhoGDI mRNA levels were high in eggs and during cleavage divisions until blastula, when it disappeared, only to reappear in gastrula stage. RhoGDI localization overlaps the presence of RhoA during oogenesis and in embryonic development, reinforcing the regulatory premise of the interaction. By use of recombinant protein interactions in vitro, we also find that these two proteins selectively interact. These results support the hypothesis of a functional relationship in vivo and now enable mechanistic insight for the cellular and organelle rearrangements that occur during oogenesis and embryonic development.

RhoA phosphorylation induces Rac1 release from guanine dissociation inhibitor alpha and stimulation of vascular smooth muscle cell migration

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

RhoGDI2 as a therapeutic target in cancer

IMPORTANCE OF THE FIELD

Rho GDP dissociation inhibitor 2 (RhoGDI2) has been identified as a regulator of Rho GTPases that play important roles in the development of numerous aspects of the malignant phenotype, including cell cycle progression, resistance to apoptotic stimuli, neovascularization, tumor cell motility, invasiveness, and metastasis. Although RhoGDI2 has been known to be expressed only in hematopoietic tissues, recent studies suggest that this protein is also aberrantly expressed in several human cancers and contributes to aggressive phenotypes, such as invasion and metastasis. Hence, RhoGDI2 appears to be a target of interest for therapeutic manipulation.

AREAS COVERED IN THIS REVIEW

Here, we summarize the role of RhoGDI2 in human cancers, specifically metastasis-related processes, and discuss its potential as a therapeutic target.

WHAT THE READER WILL GAIN

RhoGDI2 modulates the invasiveness and metastatic ability of cancer cells through regulation of Rac1 activity.

TAKE HOME MESSAGE

RhoGDI2 may be a useful marker for tumor progression in human cancers, and interruption of the RhoGDI2-mediated cancer cell invasion and metastasis by an interfacial inhibitor may be a powerful therapeutic approach to cancer.

Protein thiol oxidation in murine airway epithelial cells in response to naphthalene or diethyl maleate

Naphthalene (NA) is a semivolatile aromatic hydrocarbon to which humans are exposed from a variety of sources. NA results in acute cytotoxicity to respiratory epithelium in rodents. Cytochrome P450-dependent metabolic activation to form reactive intermediates and loss of soluble cellular thiols (glutathione) are critical steps in NA toxicity, but the precise mechanisms by which this chemical results in cellular injury remain unclear. Protein thiols are likely targets of reactive NA metabolites. Loss of these, through adduction or thiol oxidation mechanisms, may be important underlying mechanisms for NA toxicity. To address the hypothesis that loss of thiols on specific cellular proteins is critical to NA-induced cytotoxicity, we compared reduced to oxidized thiol ratios in airway epithelial cell proteins isolated from lungs of mice treated with NA or the nontoxic glutathione depletor, diethyl maleate (DEM). At 300 mg/kg doses, NA administration resulted in a greater than 85% loss of glutathione levels in the airway epithelium, which is similar to the loss observed after DEM treatment. Using differential fluorescent maleimide labeling followed by 2DE separation of proteins, we identified more than 35 unique proteins that have treatment-specific differential sulfhydryl oxidation. At doses of NA and DEM that produce similar levels of glutathione depletion, Cy3/Cy5 labeling ratios were statistically different for 16 nonredundant proteins in airway epithelium. Proteins identified include a zinc finger protein, several aldehyde dehydrogenase variants, beta-actin, and several other structural proteins. These studies show distinct patterns of protein thiol alterations with the noncytotoxic DEM and the cytotoxic NA.

Morphological and proliferative abnormalities in renal mesangial cells lacking RhoGDI

The regulation of Rho GTPase activities and expression is critical in the development and function of the kidney. Rho GTPase activities and cytosol-membrane cycling are regulated by Rho GDP Dissociation Inhibitor (RhoGDI), and RhoGDI knockout mice develop defects in kidney structure and function that lead to death due to renal failure. It is therefore important to understand the changes in RhoGDI-regulated Rho GTPase activities and cell morphology that lead to kidney failure in RhoGDI (-/-) mice. Here, we characterize a renal mesangial cell line derived from the RhoGDI (-/-) mouse in which we verify the absence of GDI proteins. In the absence of RhoGDI, we show an increase in the specific activity of Rac1, and to a lesser extent, RhoA and Cdc42 GTPases in these cells. This is accompanied by a compensatory decrease in the steady-state protein levels of Rho GTPases. Morphological analysis of RhoGDI (-/-) mesangial cells reveals a decrease in cell spreading and in focal contacts compared to wild-type cells. Finally, RhoGDI (-/-) mesangial cells show a decreased ability to proliferate and survive. These functional and structural changes are likely to contribute to the defects in renal architecture and function observed in the RhoGDI (-/-) mouse.

New insights into how the Rho guanine nucleotide dissociation inhibitor regulates the interaction of Cdc42 with membranes

The subcellular localization of the Rho family GTPases is of fundamental importance to their proper functioning in cells. The Rho guanine nucleotide dissociation inhibitor (RhoGDI) plays a key regulatory role by influencing the cellular localization of Rho GTPases and is essential for the transforming activity of oncogenic forms of Cdc42. However, the mechanism by which RhoGDI helps Cdc42 to undergo the transition between a membrane-associated protein and a soluble (cytosolic) species has been poorly understood. Here, we examine how RhoGDI influences the binding of Cdc42 to lipid bilayers. Despite having similar affinities for the signaling-inactive (GDP-bound) and signaling-active (GTP-bound) forms of Cdc42 in solution, we show that when RhoGDI interacts with Cdc42 along the membrane surface, it has a much higher affinity for GDP-bound Cdc42 compared with its GTP-bound counterpart. Interestingly, the rate for the dissociation of Cdc42.RhoGDI complexes from membranes is unaffected by the nucleotide-bound state of Cdc42. Moreover, the membrane release of Cdc42.RhoGDI complexes occurs at a similar rate as the release of Cdc42 alone, with the major effect of RhoGDI being to impede the re-association of Cdc42 with membranes. These findings lead us to propose a new model for how RhoGDI influences the ability of Cdc42 to move between membranes and the cytosol, which highlights the role of the membrane in helping RhoGDI to distinguish between the GDP- and GTP-bound forms of Cdc42 and holds important implications for how it functions as a key regulator of the cellular localization and signaling activities of this GTPase.

Expression of Rho GDIalpha in rat osteoblasts intermittently exposed to parathyroid hormone in vitro and in vivo

AIM

To investigate the mechanism of the bone-forming effects of intermittent parathyroid hormone (PTH) administration and to search for novel molecules of bone anabolism via the PTH signaling pathway.

METHODS

Primary cultures of rat osteoblasts (ROBs) were divided into an intermittent PTH-treated group (Itm) and a control group (Ctr). Imitating the pharmacokinetics of intermittent PTH administration in vivo, the ROBs in the Itm group were exposed to PTH for 6 h in a 24-h incubation cycle, and the ROBs in the Ctr group were exposed to vehicle for the entire incubation cycle. The cells were collected at 6 h and 24 h of the final cycle, and the proteins in the Itm and Ctr groups were analyzed by two-dimensional electrophoresis (2-DE) coupled with peptide mass fingerprinting and matrix assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) to detect proteins that were differentially expressed. The proteins with the most significant changes in vitro were validated by immunohistochemistry (IHC) in a rat model.

RESULTS

The proteomics analysis indicated that a total of 26 proteins were up- or down-regulated in the Itm group compared with the Ctr group at 6 h and 24 h; among these, 15 proteins were successfully identified. These proteins mainly belong to the cytoskeleton and molecular chaperone protein families, and most of these have anti-apoptotic effects in various cells. Rho GDP-dissociation inhibitor alpha (RhoGDIalpha) and vimentin were the most significantly changed proteins. Further studies by IHC showed that the expression of RhoGDIalpha in ROBs was significantly higher in PTH-treated sham-operated rats than in vehicle-treated sham-operated rats, but the difference was not significant between PTH-treated and vehicle-treated OVX rats. Vimentin expression was not changed in either PTH-treated sham-operated rats or PTH-treated OVX rats.

CONCLUSION

Our research suggests that intermittent PTH treatment induces changes in expression of many proteins in ROBs in vitro, and it results in RhoGDIalpha up-regulation in ROBs both in vitro and in vivo when estrogen is present. This up-regulation of RhoGDIalpha may be one of the mechanisms underlying the synergistic bone-forming effect of PTH and estrogen.Acta Pharmacologica Sinica (2009) 30: 1001-1007; doi: 10.1038/aps.2009.60.

Effector proteins exert an important influence on the signaling-active state of the small GTPase Cdc42

GTP-binding (G) proteins regulate the flow of information in cellular signaling pathways by alternating between a GTP-bound "active" state and a GDP-bound "inactive" state. Cdc42, a member of the Rho family of Ras-related small G-proteins, plays key roles in the regulation of cell shape, motility, and growth. Here we describe the high resolution x-ray crystal structure for Cdc42 bound to the GTP analog guanylyl beta,gamma-methylene-diphosphonate (GMP-PCP) (i.e. the presumed signaling-active state) and show that it is virtually identical to the structures for the signaling-inactive, GDP-bound form of the protein, contrary to what has been reported for Ras and other G-proteins. Especially surprising was that the GMP-PCP- and GDP-bound forms of Cdc42 did not show detectable differences in their Switch I and Switch II loops. Fluorescence studies using a Cdc42 mutant in which a tryptophan residue was introduced at position 32 of Switch I also showed that there was little difference in the Switch I conformation between the GDP- and GMP-PCP-bound states (i.e. <10%), which again differed from Ras where much larger changes in Trp-32 fluorescence were observed when comparing these two nucleotide-bound states (>30%). However, the binding of an effector protein induced significant changes in the Trp-32 emission specifically from GMP-PCP-bound Cdc42, as well as in the phosphate resonances for GTP bound to this G-protein as indicated in NMR studies. An examination of the available structures for Cdc42 complexed to different effector proteins, versus the x-ray crystal structure for GMP-PCP-bound Cdc42, provides a possible explanation for how effectors can distinguish between the GTP- and GDP-bound forms of this G-protein and ensure that the necessary conformational changes for signal propagation occur.

Downregulation of Rho-GDI gamma promotes differentiation of neural stem cells

Rho-GDIgamma belongs to the Rho-GDI protein family, which was observed to have high level expression in the entire brain. Although it exists in neuronal population, its physiological function is poorly understood. This study shows that Rho-GDIgamma is a key factor in the G13 signaling pathway based on an analysis of global gene expression. By using RNAi technology to downregulate expression of Rho-GDIgamma we found distinct morphological changes in neural stem cell line C17.2. More important, RT-PCR confirmed that RNAi-mediated downregulation of Rho-GDIgamma decreased expression of Rho-GDIgamma-regulated genes RhoA, Cdc42, Limk2, and N-WASP and slightly increased expression of Rac1. Further, immunochemical staining indicated that downregulation of Rho-GDIgamma increased the tendency of C17.2 cells to differentiate. These data strongly suggest that Rho-GDIgamma plays a key role in the differentiation of neural stem cells.

Activity of the Bcr GTPase-activating domain is regulated through direct protein/protein interaction with the Rho guanine nucleotide dissociation inhibitor

The cycling of Rac GTPases, alternating between an active GTP- and an inactive GDP-bound state, is controlled by guanine nucleotide exchange factors, GTPase-activating proteins (GAPs), and guanine nucleotide dissociation inhibitors (GDIs). Little is known about how these controlling activities are coordinated. Studies using null mutant mice have demonstrated that Bcr and Abr are two physiologically important GAPs for Rac. Here, we report that in the presence of RhoGDIalpha, Bcr is unable to convert Rac-GTP to Rac-GDP because RhoGDI forms a direct protein complex with Bcr. Interestingly, RhoGDIalpha binds to the GAP domain in Bcr and Abr, a domain that also binds to Rac-GTP and catalyzes conversion of the bound GTP to GDP on Rac. The presence of activated Rac diminished the Bcr/RhoGDIalpha interaction. Moreover, a Bcr mutant that lacks the ability to promote hydrolysis of Rac-GTP bound to its GAP domain did not bind to RhoGDIalpha in cell lysates, indicating that binding of RhoGDIalpha and Rac-GTP to the Bcr GAP domain is mutually exclusive. Our results provide the first identification of a protein that regulates BcrGAP activity.

NMR assignment of Cdc42(T35A), an active Switch I mutant of Cdc42

Cdc42(T35A) is an active construct of Cdc42, a Ras GTPase involved in signal transduction, containing a single-point mutation in an important effector-binding region. We determined the backbone and side chain resonance assignments of (13)C,(15)N-labelled Cdc42(T35A) from E. coli.

Regulators of small GTPases

Small GTPases are converted from the GDP-bound inactive form to the GTP-bound active form by a GDP/GTP exchange reaction which is regulated by GDP/GTP exchange proteins (GEPs). We have found both stimulatory and inhibitory GEPs, which we have named GDP dissociation stimulators (GDSs) and GDP dissociation inhibitors (GDIs) respectively. We have isolated Smg GDS, Rho GDI and Rab GDI, cloned them, and determined their primary structures. These GEPs are active on a group of small GTPases: Smg GDS on at least K-Ras, Rap1/Smg21, Rho and Rac; Rho GDI on at least Rho, Rac and Cdc42; Rab GDI on most of the Rab family members. These GEPs have an additional function, regulating the translocation of their substrate small GTPases between the membrane and the cytosol. The GEPs interact only with the post-translationally modified form of their substrate small GTPases.

Proteome analysis of cultivated vascular smooth muscle cells from a CADASIL patient

Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is a vascular dementing disease caused by mutations in the NOTCH3 gene, most which are missense mutations leading to an uneven number of cysteine residues in epidermal growth factor-like repeats in the extracellular domain of Notch3 receptor (N3ECD). CADASIL is characterized by degeneration of vascular smooth muscle cells (VSMC) and accumulation of N3ECD on the VSMCs of small and middle-sized arteries. Recent studies have demonstrated that impairment of Notch3 signaling is not the primary cause of the disease. In the present study we used proteomic analysis to characterize the protein expression pattern of a unique material of genetically genuine cultured human CADASIL VSMCs. We identified 11 differentially expressed proteins, which are involved in protein degradation and folding, contraction of VSMCs, and cellular stress. Our findings indicate that misfolding of Notch3 may cause endoplasmic reticulum stress and activation of unfolded protein response, leading to increased reactive oxygen species and inhibition of cell proliferation. In addition, upregulation of contractile proteins suggests an alteration in the signaling system of VSMC contraction. The accumulation of N3ECD on the cell surface possibly upregulates the angiotensin II regulatory feedback loop and thereby enhances the readiness of the cells to respond to angiotensin II stimulation.

Functional implications of caspase-mediated RhoGDI2 processing during apoptosis of HL60 and K562 leukemia cells

RhoGDI2, a cytosolic regulator of Rho GTPase, is cleaved during apoptosis in a caspase-3 dependent fashion. By using 2D-gel electrophoresis, mass spectrometry and Western blotting we investigate in this paper the functional consequences of RhoGDI2 processing. We can show that loss of the N-terminal 19 amino acids results in a shift of the isoelectric point of the truncated RhoGDI2 (NDelta19) to a more basic value due to the removal of 9 acidic amino acids from the N-terminus, which may be responsible for enhanced retention of the N-terminally truncated protein within the nuclear compartment. Fusion of the p53 nuclear export signaling sequence MFRELNEALELK to NDelta19 (NDelta19NES) abolished its apoptosis promoting properties, while overexpression of NDelta19 significantly increased the susceptibility to apoptosis induction by the proteasome inhibitor PSI and by staurosporine. These results suggest that cleavage of RhoGDI2 by caspase-3 is not a functionally irrelevant bystander effect of caspase activation during apoptosis, but rather expedites progression of the apoptotic process.

Central roles of small GTPases in the development of cell polarity in yeast and beyond

SUMMARY

The establishment of cell polarity is critical for the development of many organisms and for the function of many cell types. A large number of studies of diverse organisms from yeast to humans indicate that the conserved, small-molecular-weight GTPases function as key signaling proteins involved in cell polarization. The budding yeast Saccharomyces cerevisiae is a particularly attractive model because it displays pronounced cell polarity in response to intracellular and extracellular cues. Cells of S. cerevisiae undergo polarized growth during various phases of their life cycle, such as during vegetative growth, mating between haploid cells of opposite mating types, and filamentous growth upon deprivation of nutrition such as nitrogen. Substantial progress has been made in deciphering the molecular basis of cell polarity in budding yeast. In particular, it becomes increasingly clear how small GTPases regulate polarized cytoskeletal organization, cell wall assembly, and exocytosis at the molecular level and how these GTPases are regulated. In this review, we discuss the key signaling pathways that regulate cell polarization during the mitotic cell cycle and during mating.

Use of bimolecular fluorescence complementation to study in vivo interactions between Cdc42p and Rdi1p of Saccharomyces cerevisiae

Saccharomyces cerevisiae Cdc42p functions as a GTPase molecular switch, activating multiple signaling pathways required to regulate cell cycle progression and the actin cytoskeleton. Regulatory proteins control its GTP binding and hydrolysis and its subcellular localization, ensuring that Cdc42p is appropriately activated and localized at sites of polarized growth during the cell cycle. One of these, the Rdi1p guanine nucleotide dissociation inhibitor, negatively regulates Cdc42p by extracting it from cellular membranes. In this study, the technique of bimolecular fluorescence complementation (BiFC) was used to study the dynamic in vivo interactions between Cdc42p and Rdi1p. The BiFC data indicated that Cdc42p and Rdi1p interacted in the cytoplasm and around the periphery of the cell at the plasma membrane and that this interaction was enhanced at sites of polarized cell growth during the cell cycle, i.e., incipient bud sites, tips and sides of small- and medium-sized buds, and the mother-bud neck region. In addition, a ring-like structure containing the Cdc42p-Rdi1p complex transiently appeared following release from G1-phase cell cycle arrest. A homology model of the Cdc42p-Rdi1p complex was used to introduce mutations that were predicted to affect complex formation. These mutations resulted in altered BiFC interactions, restricting the complex exclusively to either the plasma membrane or the cytoplasm. Data from these studies have facilitated the temporal and spatial modeling of Rdi1p-dependent extraction of Cdc42p from the plasma membrane during the cell cycle.

Solution structure of an oncogenic mutant of Cdc42Hs

Cdc42Hs(F28L) is a single-point mutant of Cdc42Hs, a member of the Ras superfamily of GTP-binding proteins, that facilitates cellular transformation brought about by an increased rate of cycling between GTP and GDP [Lin, R., et al. (1997) Curr. Biol. 7, 794-797]. Dynamics studies of Cdc42Hs(F28L)-GDP have shown increased flexibility for several residues at the nucleotide-binding site [Adams, P. D., et al. (2004) Biochemistry 43, 9968-9977]. The solution structure of Cdc42Hs-GDP (wild type) has previously been determined by NMR spectroscopy [Feltham, J. L., et al. (1997) Biochemistry 36, 8755-8766]. Here, we describe the solution structure of Cdc42Hs(F28L)-GDP, which provides insight into the structural basis for the change in affinity for GDP. Heteronuclear NMR experiments were performed to assign resonances in the protein, and distance, hydrogen bonding, residual dipolar coupling, and dihedral angle constraints were used to calculate a set of low-energy structures using distance geometry and simulated annealing refinement protocols. The overall structure of Cdc42Hs(F28L)-GDP is very similar to that of wild-type Cdc42Hs, consisting of a centrally located six-stranded beta-sheet structure surrounding the C-terminal alpha-helix [Feltham, J. L., et al. (1997) Biochemistry 36, 8755-8766]. In addition, the same three regions in wild-type Cdc42Hs that show structural disorder (Switch I, Switch II, and the Insert region) are disordered in F28L as well. Although the structure of Cdc42Hs(F28L)-GDP is very similar to that of the wild type, interactions with the nucleotide and hydrogen bonding within the nucleotide binding site are altered, and the region surrounding L28 is substantially more disordered.

GDIs: central regulatory molecules in Rho GTPase activation

The GDP dissociation inhibitors (GDIs) are pivotal regulators of Rho GTPase function. GDIs control the access of Rho GTPases to regulatory guanine nucleotide exchange factors and GTPase-activating proteins, to effector targets and to membranes where such effectors reside. We discuss here our current understanding of how Rho GTPase-GDI complexes are regulated by various proteins, lipids and enzymes that exert GDI displacement activity. We propose that phosphorylation mediated by diverse kinases might provide a means of controlling and coordinating Rho GTPase activation.