CdGAP, a novel proline-rich GTPase-activating protein for Cdc42 and Rac

Mouse scalp development requires Rac1 and SRF for the maintenance of mechanosensing mesenchyme

Regulation of essential cellular responses like proliferation, migration, and differentiation is crucial for normal development. Rac1, a ubiquitously expressed small GTPase, executes these responses under the regulation of guanine nucleotide exchange factors (GEFs) and GTPase activating proteins (GTPases). Mutations in specific GEFs (DOCK6) and GTPases (AHGAP31) that regulate Rac1 are associated with Adams-Oliver syndrome (AOS), a developmental syndrome characterized by congenital scalp defects and limb truncations. Genetic ablation of Rac1 in the mouse embryonic limb ectoderm results in limb truncation. However, the etiology of Rac1-associated cranial defects is unknown. To investigate the origin and nature of cranial defects, we used a mesenchymal Cre line ( Pdgfra-Cre ) to delete Rac1 in cranial mesenchyme. Rac1 -KO mice died perinatally and lacked the apical portion of the calvarium and overlying dermis, resembling cranial defects seen in severe cases of AOS. In control embryos, α-smooth muscle actin (αSMA) expression was spatially restricted to the apical mesenchyme, suggesting a mechanical interaction between the growing brain and the overlying mesenchyme. In Rac1 -KO embryos there was reduced proliferation of apical mesenchyme, and reduced expression of αSMA and its regulator, serum response factor (SRF). Remarkably, Srf -KO mice generated with Pdgfra-Cre recapitulated the cranial phenotype observed in Rac1- KO mice. Together, these data suggest a model where Rac1 and SRF are critical to maintaining apical fibroblasts in a mechano-sensitive and proliferative state needed to complete cranial development.

Translation efficiency covariation across cell types is a conserved organizing principle of mammalian transcriptomes

Characterization of shared patterns of RNA expression between genes across conditions has led to the discovery of regulatory networks and novel biological functions. However, it is unclear if such coordination extends to translation, a critical step in gene expression. Here, we uniformly analyzed 3,819 ribosome profiling datasets from 117 human and 94 mouse tissues and cell lines. We introduce the concept of Translation Efficiency Covariation (TEC), identifying coordinated translation patterns across cell types. We nominate potential mechanisms driving shared patterns of translation regulation. TEC is conserved across human and mouse cells and helps uncover gene functions. Moreover, our observations indicate that proteins that physically interact are highly enriched for positive covariation at both translational and transcriptional levels. Our findings establish translational covariation as a conserved organizing principle of mammalian transcriptomes.

Actin networks modulate heterogeneous NF-κB dynamics in response to TNFα

The canonical NF-κB transcription factor RELA is a master regulator of immune and stress responses and is upregulated in pancreatic ductal adenocardinoma (PDAC) tumours. In this study, we characterised previously unexplored endogenous RELA-GFP dynamics in PDAC cell lines through live single-cell imaging. Our observations revealed that TNFα stimulation induces rapid, sustained, and non-oscillatory nuclear translocation of RELA. Through Bayesian analysis of single-cell datasets with variation in nuclear RELA, we predicted that RELA heterogeneity in PDAC cell lines is dependent on F-actin dynamics. RNA-seq analysis identified distinct clusters of RELA-regulated gene expression in PDAC cells, including TNFα-induced RELA upregulation of the actin regulators NUAK2 and ARHGAP31. Further, siRNA-mediated depletion of ARHGAP31 and NUAK2 altered TNFα-stimulated nuclear RELA dynamics in PDAC cells, establishing a novel negative feedback loop that regulates RELA activation by TNFα. Additionally, we characterised the NF-κB pathway in PDAC cells, identifying how NF-κB/IκB proteins genetically and physically interact with RELA in the absence or presence of TNFα. Taken together, we provide computational and experimental support for interdependence between the F-actin network and the NF-κB pathway with RELA translocation dynamics in PDAC.

Intersectin - many facets of a scaffold protein

Intersectin (ITSN) is a multi-domain scaffold protein with a diverse array of functions including regulation of endocytosis, vesicle transport, and activation of various signal transduction pathways. There are two ITSN genes located on chromosomes 21 and 2 encoding for proteins ITSN1 and ITSN2, respectively. Each ITSN gene encodes two major isoforms, ITSN-Long (ITSN-L) and ITSN-Short (ITSN-S), due to alternative splicing. ITSN1 and 2, collectively referred to as ITSN, are implicated in many physiological and pathological processes, such as neuronal maintenance, actin cytoskeletal rearrangement, and tumor progression. ITSN is mis-regulated in many tumors, such as breast, lung, neuroblastomas, and gliomas. Altered expression of ITSN is also found in several neurodegenerative diseases, such as Down Syndrome and Alzheimer's disease. This review summarizes recent studies on ITSN and provides an overview of the function of this important family of scaffold proteins in various biological processes.

CdGAP is a talin-binding protein and a target of TGF-β signaling that promotes HER2-positive breast cancer growth and metastasis

Epithelial-to-mesenchymal transition (EMT) plays a crucial role in metastasis, which is the leading cause of death in breast cancer patients. Here, we show that Cdc42 GTPase-activating protein (CdGAP) promotes tumor formation and metastasis to lungs in the HER2-positive (HER2+) murine breast cancer model. CdGAP facilitates intravasation, extravasation, and growth at metastatic sites. CdGAP depletion in HER2+ murine primary tumors mediates crosstalk with a Dlc1-RhoA pathway and is associated with a transforming growth factor β (TGF-β)-induced EMT transcriptional signature. CdGAP is positively regulated by TGF-β signaling during EMT and interacts with the adaptor talin to modulate focal adhesion dynamics and integrin activation. Moreover, HER2+ breast cancer patients with high CdGAP mRNA expression combined with a high TGF-β-EMT signature are more likely to present lymph node invasion. Our results suggest CdGAP as a candidate therapeutic target for HER2+ metastatic breast cancer by inhibiting TGF-β and integrin/talin signaling pathways.

A DLG1-ARHGAP31-CDC42 axis is essential for the intestinal stem cell response to fluctuating niche Wnt signaling

A central factor in the maintenance of tissue integrity is the response of stem cells to variations in the levels of niche signals. In the gut, intestinal stem cells (ISCs) depend on Wnt ligands for self-renewal and proliferation. Transient increases in Wnt signaling promote regeneration after injury or in inflammatory bowel diseases, whereas constitutive activation of this pathway leads to colorectal cancer. Here, we report that Discs large 1 (Dlg1), although dispensable for polarity and cellular turnover during intestinal homeostasis, is required for ISC survival in the context of increased Wnt signaling. RNA sequencing (RNA-seq) and genetic mouse models demonstrated that DLG1 regulates the cellular response to increased canonical Wnt ligands. This occurs via the transcriptional regulation of Arhgap31, a GTPase-activating protein that deactivates CDC42, an effector of the non-canonical Wnt pathway. These findings reveal a DLG1-ARHGAP31-CDC42 axis that is essential for the ISC response to increased niche Wnt signaling.

CdGAP maintains podocyte function and modulates focal adhesions in a Src kinase-dependent manner

Rho GTPases are regulators of the actin cytoskeleton and their activity is modulated by GTPase-activating proteins (GAPs) and guanine nucleotide exchanging factors (GEFs). Glomerular podocytes have numerous actin-based projections called foot processes and their alteration is characteristic of proteinuric kidney diseases. We reported previously that Rac1 hyperactivation in podocytes causes proteinuria and glomerulosclerosis in mice. However, which GAP and GEF modulate Rac1 activity in podocytes remains unknown. Here, using a proximity-based ligation assay, we identified CdGAP (ARHGAP31) and β-PIX (ARHGEF7) as the major regulatory proteins interacting with Rac1 in human podocytes. CdGAP interacted with β-PIX through its basic region, and upon EGF stimulation, they both translocated to the plasma membrane in podocytes. CdGAP-depleted podocytes had altered cell motility and increased basal Rac1 and Cdc42 activities. When stimulated with EGF, CdGAP-depleted podocytes showed impaired β-PIX membrane-translocation and tyrosine phosphorylation, and reduced activities of Src kinase, focal adhesion kinase, and paxillin. Systemic and podocyte-specific CdGAP-knockout mice developed mild but significant proteinuria, which was exacerbated by Adriamycin. Collectively, these findings show that CdGAP contributes to maintain podocyte function and protect them from injury.

A screening-compatible live cell fluorescence resonance energy transfer-based assay for modulation of Rho GTPase activity

Rho family GTPases are central regulators of cytoskeletal dynamics controlled by guanine nucleotide exchange factors (RhoGEFs) and GTPase-activating proteins (RhoGAPs). This protocol presents a workflow for a robust high-throughput compatible biosensor assay to analyze changes in Rho GTPase activity by these proteins in the native cellular environment. The procedure can be used for semi-quantitative comparison of GEF/GAP function and extended for analysis of additional modulators. The experimental design is applicable also to other monomolecular ratiometric FRET sensors. For complete details on the use and execution of this protocol, please refer to Müller et al. (2020).

CdGAP promotes prostate cancer metastasis by regulating epithelial-to-mesenchymal transition, cell cycle progression, and apoptosis

High mortality of prostate cancer patients is primarily due to metastasis. Understanding the mechanisms controlling metastatic processes remains essential to develop novel therapies designed to prevent the progression from localized disease to metastasis. CdGAP plays important roles in the control of cell adhesion, migration, and proliferation, which are central to cancer progression. Here we show that elevated CdGAP expression is associated with early biochemical recurrence and bone metastasis in prostate cancer patients. Knockdown of CdGAP in metastatic castration-resistant prostate cancer (CRPC) PC-3 and 22Rv1 cells reduces cell motility, invasion, and proliferation while inducing apoptosis in CdGAP-depleted PC-3 cells. Conversely, overexpression of CdGAP in DU-145, 22Rv1, and LNCaP cells increases cell migration and invasion. Using global gene expression approaches, we found that CdGAP regulates the expression of genes involved in epithelial-to-mesenchymal transition, apoptosis and cell cycle progression. Subcutaneous injection of CdGAP-depleted PC-3 cells into mice shows a delayed tumor initiation and attenuated tumor growth. Orthotopic injection of CdGAP-depleted PC-3 cells reduces distant metastasic burden. Collectively, these findings support a pro-oncogenic role of CdGAP in prostate tumorigenesis and unveil CdGAP as a potential biomarker and target for prostate cancer treatments.

Mehra et al. investigate the role of CdGAP in early biochemical recurrence and bone metastasis in prostate cancer. The authors find that knocking down CdGAP leads to reduced cell motility, invasion and proliferation in PC-3 and 22Rv1 cells while orthotopic injection of CdGAP-depleted PC-3 cells reduces distant metastatic burden.

The Global Phosphorylation Landscape of SARS-CoV-2 Infection

The causative agent of the coronavirus disease 2019 (COVID-19) pandemic, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has infected millions and killed hundreds of thousands of people worldwide, highlighting an urgent need to develop antiviral therapies. Here we present a quantitative mass spectrometry-based phosphoproteomics survey of SARS-CoV-2 infection in Vero E6 cells, revealing dramatic rewiring of phosphorylation on host and viral proteins. SARS-CoV-2 infection promoted casein kinase II (CK2) and p38 MAPK activation, production of diverse cytokines, and shutdown of mitotic kinases, resulting in cell cycle arrest. Infection also stimulated a marked induction of CK2-containing filopodial protrusions possessing budding viral particles. Eighty-seven drugs and compounds were identified by mapping global phosphorylation profiles to dysregulated kinases and pathways. We found pharmacologic inhibition of the p38, CK2, CDK, AXL, and PIKFYVE kinases to possess antiviral efficacy, representing potential COVID-19 therapies.

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.

Integrated analysis of dysregulated long non-coding RNAs/microRNAs/mRNAs in metastasis of lung adenocarcinoma

BACKGROUND

Lung adenocarcinoma (LUAD), largely remains a primary cause of cancer-related death worldwide. The molecular mechanisms in LUAD metastasis have not been completely uncovered.

METHODS

In this study, we identified differentially expressed genes (DEGs), miRNAs (DEMs) and lncRNAs (DELs) underlying metastasis of LUAD from The Cancer Genome Atlas database. Intersection mRNAs were used to perform gene ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway and co-expression network analysis. In addition, survival analyses of intersection mRNAs were conducted. Finally, intersection mRNAs, miRNAs and lncRNAs were subjected to construct miRNA-mRNA-lncRNA network.

RESULTS

A total of 1015 DEGs, 54 DEMs and 22 DELs were identified in LUAD metastasis and non-metastasis samples. GO and KEGG pathway analysis had proven that the functions of intersection mRNAs were closely related with many important processes in cancer pathogenesis. Among the co-expression interactions network, 22 genes in the co-expression network were over the degree 20. These genes imply that they have connections with many other gene nodes. In addition, 14 target genes (ARHGAP11A, ASPM, HELLS, PRC1, TMPO, ARHGAP30, CD52, IL16, IRF8, P2RY13, PRKCB, PTPRC, SASH3 and TRAF3IP3) were found to be associated with survival in patients with LUAD significantly (log-rank P < 0.05). Two lncRNAs (LOC96610 and ADAM6) acting as ceRNAs were identified based on the miRNA-mRNA-lncRNA network.

CONCLUSIONS

Taken together, the results may provide a novel perspective to develop a multiple gene diagnostic tool for LUAD prognosis, which might also provide potential biomarkers or therapeutic targets for LUAD.

CdGAP/ARHGAP31 is regulated by RSK phosphorylation and binding to 14-3-3β adaptor protein

Cdc42 GTPase-activating protein (CdGAP, also named ARHGAP31) is a negative regulator of the GTPases Rac1 and Cdc42. Associated with the rare developmental disorder Adams-Oliver Syndrome (AOS), CdGAP is critical for embryonic vascular development and VEGF-mediated angiogenesis. Moreover, CdGAP is an essential component in the synergistic interaction between TGFβ and ErbB-2 signaling pathways during breast cancer cell migration and invasion, and is a novel E-cadherin transcriptional co-repressor with Zeb2 in breast cancer. CdGAP is highly phosphorylated on serine and threonine residues in response to growth factors and is a substrate of ERK1/2 and GSK-3. Here, we identified Ser1093 and Ser1163 in the C-terminal region of CdGAP, which are phosphorylated by RSK in response to phorbol ester. These phospho-residues create docking sites for binding to 14-3-3 adaptor proteins. The interaction between CdGAP and 14-3-3 proteins inhibits the GAP activity of CdGAP and sequesters CdGAP into the cytoplasm. Consequently, the nucleocytoplasmic shuttling of CdGAP is inhibited and CdGAP-induced cell rounding is abolished. In addition, 14-3-3β inhibits the ability of CdGAP to repress the E-cadherin promoter and to induce cell migration. Finally, we show that 14-3-3β is unable to regulate the activity and subcellular localization of the AOS-related mutant proteins lacking these phospho-residues. Altogether, we provide a novel mechanism of regulation of CdGAP activity and localization, which impacts directly on a better understanding of the role of CdGAP as a promoter of breast cancer and in the molecular causes of AOS.

The scaffold protein Ajuba suppresses CdGAP activity in epithelia to maintain stable cell-cell contacts

Levels of active Rac1 at epithelial junctions are partially modulated via interaction with Ajuba, an actin binding and scaffolding protein. Here we demonstrate that Ajuba interacts with the Cdc42 GTPase activating protein CdGAP, a GAP for Rac1 and Cdc42, at cell-cell contacts. CdGAP recruitment to junctions does not require Ajuba; rather Ajuba seems to control CdGAP residence at sites of cell-cell adhesion. CdGAP expression potently perturbs junctions and Ajuba binding inhibits CdGAP activity. Ajuba interacts with Rac1 and CdGAP via distinct domains and can potentially bring them in close proximity at junctions to facilitate activity regulation. Functionally, CdGAP-Ajuba interaction maintains junctional integrity in homeostasis and diseases: (i) gain-of-function CdGAP mutants found in Adams-Oliver Syndrome patients strongly destabilize cell-cell contacts and (ii) CdGAP mRNA levels are inversely correlated with E-cadherin protein expression in different cancers. We present conceptual insights on how Ajuba can integrate CdGAP binding and inactivation with the spatio-temporal regulation of Rac1 activity at junctions. Ajuba provides a novel mechanism due to its ability to bind to CdGAP and Rac1 via distinct domains and influence the activation status of both proteins. This functional interplay may contribute towards conserving the epithelial tissue architecture at steady-state and in different pathologies.

The Cdc42/Rac1 regulator CdGAP is a novel E-cadherin transcriptional co-repressor with Zeb2 in breast cancer

The loss of E-cadherin causes dysfunction of the cell-cell junction machinery, which is an initial step in epithelial-to-mesenchymal transition (EMT), facilitating cancer cell invasion and the formation of metastases. A set of transcriptional repressors of E-cadherin (CDH1) gene expression, including Snail1, Snail2 and Zeb2 mediate E-cadherin downregulation in breast cancer. However, the molecular mechanisms underlying the control of E-cadherin expression in breast cancer progression remain largely unknown. Here, by using global gene expression approaches, we uncover a novel function for Cdc42 GTPase-activating protein (CdGAP) in the regulation of expression of genes involved in EMT. We found that CdGAP used its proline-rich domain to form a functional complex with Zeb2 to mediate the repression of E-cadherin expression in ErbB2-transformed breast cancer cells. Conversely, knockdown of CdGAP expression led to a decrease of the transcriptional repressors Snail1 and Zeb2, and this correlated with an increase in E-cadherin levels, restoration of cell-cell junctions, and epithelial-like morphological changes. In vivo, loss of CdGAP in ErbB2-transformed breast cancer cells impaired tumor growth and suppressed metastasis to lungs. Finally, CdGAP was highly expressed in basal-type breast cancer cells, and its strong expression correlated with poor prognosis in breast cancer patients. Together, these data support a previously unknown nuclear function for CdGAP where it cooperates in a GAP-independent manner with transcriptional repressors to function as a critical modulator of breast cancer through repression of E-cadherin transcription. Targeting Zeb2-CdGAP interactions may represent novel therapeutic opportunities for breast cancer treatment.

Quantitative GTPase Affinity Purification Identifies Rho Family Protein Interaction Partners

Although Rho GTPases are essential molecular switches involved in many cellular processes, an unbiased experimental comparison of their interaction partners was not yet performed. Here, we develop quantitative GTPase affinity purification (qGAP) to systematically identify interaction partners of six Rho GTPases (Cdc42, Rac1, RhoA, RhoB, RhoC, and RhoD), depending on their nucleotide loading state. The method works with cell line or tissue-derived protein lysates in combination with SILAC-based or label-free quantification, respectively. We demonstrate that qGAP identifies known and novel binding partners that can be validated in an independent assay. Our interaction network for six Rho GTPases contains many novel binding partners, reveals highly promiscuous interaction of several effectors, and mirrors evolutionary relationships among Rho GTPases.

CdGAP/ARHGAP31, a Cdc42/Rac1 GTPase regulator, is critical for vascular development and VEGF-mediated angiogenesis

Mutations in the CdGAP/ARHGAP31 gene, which encodes a GTPase-activating protein for Rac1 and Cdc42, have been reported causative in the Adams-Oliver developmental syndrome often associated with vascular defects. However, despite its abundant expression in endothelial cells, CdGAP function in the vasculature remains unknown. Here, we show that vascular development is impaired in CdGAP-deficient mouse embryos at E15.5. This is associated with superficial vessel defects and subcutaneous edema, resulting in 44% embryonic/perinatal lethality. VEGF-driven angiogenesis is defective in CdGAP(-/-) mice, showing reduced capillary sprouting from aortic ring explants. Similarly, VEGF-dependent endothelial cell migration and capillary formation are inhibited upon CdGAP knockdown. Mechanistically, CdGAP associates with VEGF receptor-2 and controls VEGF-dependent signaling. Consequently, CdGAP depletion results in impaired VEGF-mediated Rac1 activation and reduced phosphorylation of critical intracellular mediators including Gab1, Akt, PLCγ and SHP2. These findings are the first to demonstrate the importance of CdGAP in embryonic vascular development and VEGF-induced signaling, and highlight CdGAP as a potential therapeutic target to treat pathological angiogenesis and vascular dysfunction.

Cdc42 and k-Ras Control Endothelial Tubulogenesis through Apical Membrane and Cytoskeletal Polarization: Novel Stimulatory Roles for GTPase Effectors, the Small GTPases, Rac2 and Rap1b, and Inhibitory Influence of Arhgap31 and Rasa1

A critical and understudied property of endothelial cells is their ability to form lumens and tube networks. Although considerable information has been obtained concerning these issues, including the role of Cdc42 and Rac1 and their effectors such as Pak2, Pak4, Par6b, and co-regulators such as integrins, MT1-MMP and Par3; many key questions remain that are necessary to elucidate molecular and signaling requirements for this fundamental process. In this work, we identify new small GTPase regulators of EC tubulogenesis including k-Ras, Rac2 and Rap1b that act in conjunction with Cdc42 as well as the key downstream effectors, IQGAP1, MRCKβ, beta-Pix, GIT1, and Rasip1 (which can assemble into multiprotein complexes with key regulators including α2β1 integrin and MT1-MMP). In addition, we identify the negative regulators, Arhgap31 (by inactivating Cdc42 and Rac) and Rasa1 (by inactivating k-Ras) and the positive regulator, Arhgap29 (by inactivating RhoA) which play a major functional role during the EC tubulogenic process. Human EC siRNA suppression or mouse knockout of Rasip1 leads to identical phenotypes where ECs form extensive cord networks, but cannot generate lumens or tubes. Essential roles for these molecules during EC tubulogenesis include; i) establishment of asymmetric EC cytoskeletal polarization (subapical distribution of acetylated tubulin and basal membrane distribution of F-actin); and ii) directed membrane trafficking of pinocytic vacuoles or other intracellular vesicles along acetylated tubulin tracks to the developing apical membrane surface. Cdc42 co-localizes subapically with acetylated tubulin, while Rac1 and k-Ras strongly label vacuole/ vesicle membranes which accumulate and fuse together in a polarized, perinuclear manner. We observe polarized apical membrane and subapical accumulation of key GTPases and effectors regulating EC lumen formation including Cdc42, Rac1, Rac2, k-Ras, Rap1b, activated c-Raf and Rasip1 to control EC tube network assembly. Overall, this work defines novel key regulators and their functional roles during human EC tubulogenesis.

Suppression of Slit2/Robo1 mediated HUVEC migration by Robo4

Slit proteins and their receptors, the Roundabout (Robo) family, are known to have a pivotal role in the vascular system. Slit2/Robo1 regulates the migration of human umbilical vein endothelial cells (HUVECs) and tumor-associated endothelial cells. Robo4, the endothelial-specific Robo, is also considered to be involved in vascular cell migration. However, the Slit/Robo signaling pathway is still unclear. Using a Boyden chamber assay, we found that Slit2 induces the migration of HUVECs under a Robo4 knockdown condition. This effect disappeared in Robo1 knockdown cells. The co-existence of the N-terminal extracellular portion of Robo1 blocked the Slit2-evoked migration of HUVECs, while that of Robo4 caused no effect. These results show that the Slit2 signal is transduced through Robo1, while the negative regulation of Robo4 is an intracellular event. Targeted proteomics using an anti-Robo1 monoclonal antibody identified CdGAP, an adhesion-localized Rac1-and Cdc42-specific GTPase activating protein, as a candidate for Slit2/Robo1 signaling. Robo1 and CdGAP were co-immunoprecipitated from CHO cells co-transfected with Robo1 and CdGAP genes. These results suggest that Slit2/Robo1 binding exerts an effect on cell migration, which is negatively regulated by Robo4, and Robo1 may function by interacting with CdGAP in HUVECs.

SrGAP2-Dependent Integration of Membrane Geometry and Slit-Robo-Repulsive Cues Regulates Fibroblast Contact Inhibition of Locomotion

Migrating fibroblasts undergo contact inhibition of locomotion (CIL), a process that was discovered five decades ago and still is not fully understood at the molecular level. We identify the Slit2-Robo4-srGAP2 signaling network as a key regulator of CIL in fibroblasts. CIL involves highly dynamic contact protrusions with a specialized actin cytoskeleton that stochastically explore cell-cell overlaps between colliding fibroblasts. A membrane curvature-sensing F-BAR domain pre-localizes srGAP2 to protruding edges and terminates their extension phase in response to cell collision. A FRET-based biosensor reveals that Rac1 activity is focused in a band at the tip of contact protrusions, in contrast to the broad activation gradient in contact-free protrusions. SrGAP2 specifically controls the duration of Rac1 activity in contact protrusions, but not in contact-free protrusions. We propose that srGAP2 integrates cell edge curvature and Slit-Robo-mediated repulsive cues to fine-tune Rac1 activation dynamics in contact protrusions to spatiotemporally coordinate CIL.

Rho GAPs and GEFs: controling switches in endothelial cell adhesion

Within blood vessels, endothelial cell–cell and cell–matrix adhesions are crucial to preserve barrier function, and these adhesions are tightly controlled during vascular development, angiogenesis, and transendothelial migration of inflammatory cells. Endothelial cellular signaling that occurs via the family of Rho GTPases coordinates these cell adhesion structures through cytoskeletal remodelling. In turn, Rho GTPases are regulated by GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs). To understand how endothelial cells initiate changes in the activity of Rho GTPases, and thereby regulate cell adhesion, we will discuss the role of Rho GAPs and GEFs in vascular biology. Many potentially important Rho regulators have not been studied in detail in endothelial cells. We therefore will first overview which GAPs and GEFs are highly expressed in endothelium, based on comparative gene expression analysis of human endothelial cells compared with other tissue cell types. Subsequently, we discuss the relevance of Rho GAPs and GEFs for endothelial cell adhesion in vascular homeostasis and disease.

Crossroads of PI3K and Rac pathways

Rac and PI3Ks are intracellular signal transducers able to regulate multiple signaling pathways fundamental for cell behavior. PI3Ks are lipid kinases that produce phosphorylated lipids which, in turn, transduce extracellular cues within the cell, while Rac is a small G protein that impacts on actin organization. Compelling evidence indicates that in multiple circumstances the 2 signaling pathways appear intermingled. For instance, phosphorylated lipids produced by PI3Ks recruit and activate GEF and GAP proteins, key modulators of Rac function. Conversely, PI3Ks interact with activated Rac, leading to Rac signaling amplification. This review summarizes the molecular mechanisms underlying the cross-talk between Rac and PI3K signaling in 2 different processes, cell migration and ROS production.

CdGAP regulates cell migration and adhesion dynamics in two-and three-dimensional matrix environments

CdGAP is a Rac1/Cdc42 specific GTPase activating protein (GAP) that localizes to cell-matrix adhesions through an interaction with the adhesion scaffold α-parvin/actopaxin to regulate lamellipodia formation and cell spreading. Herein, we demonstrate, using a combination of siRNA-mediated silencing and overexpression, that cdGAP negatively regulates directed and random migration by controlling adhesion maturation and dynamics through the regulation of both adhesion assembly and disassembly. Interestingly, cdGAP was also localized to adhesions formed in three-dimensional (3D) matrix environments and cdGAP depletion promoted cancer cell migration and invasion through 3D matrices. These findings highlight the importance of GAP proteins in the regulation of Rho family GTPases and the coordination of the cell migration machinery..

Ostrinia furnacalis integrin β1 may be involved in polymerization of actin to modulate spreading and encapsulation of plasmatocytes

Insect hemocytes must change their state from non-adhesive to adhesive when they spread on or encapsulate foreign invaders. Although integrin β has been reported to play an important role in hemocyte spreading and encapsulation in several insects, how it is involved in the encapsulation process is still unclear. Here we report that integrin β1 of Ostrinia furnacalis (Ofint β1) may modulate plasmatocyte spreading by regulating polymerization of F-actin and further affecting formation of capsules. In the Sephadex A-25 bead-injected larvae, hemocytes forming capsules expressed approximately ten times more Ofint β1 than hemocytes that are free in circulation in hemolymph. When the expression of Ofint β1 in hemocytes was inhibited by dsRNA of Ofint β1 (dsINT), polymerization of F-actin in hemocytes, especially in plasmatocytes, was significantly decreased, spreading of plasmatocytes was inhibited, and encapsulation rate of Sephadex beads was also significantly decreased. Furthermore, hemocytes formed individual aggregates on beads in the dsINT injected larvae, while hemocytes formed complete capsules surrounding the beads in the control larvae; and most of the hemocytes on the beads in the dsINT-injected larvae assumed round forms rather than spread forms. Based on these results, we speculate that integrins on cellular membranes may modulate hemocyte spreading by regulating polymerization of F-actin and further affecting encapsulation of foreign objects.

A stretch of polybasic residues mediates Cdc42 GTPase-activating protein (CdGAP) binding to phosphatidylinositol 3,4,5-trisphosphate and regulates its GAP activity

The Rho family of small GTPases are membrane-associated molecular switches involved in the control of a wide range of cellular activities, including cell migration, adhesion, and proliferation. Cdc42 GTPase-activating protein (CdGAP) is a phosphoprotein showing GAP activity toward Rac1 and Cdc42. CdGAP activity is regulated in an adhesion-dependent manner and more recently, we have identified CdGAP as a novel molecular target in signaling and an essential component in the synergistic interaction between TGFβ and Neu/ErbB-2 signaling pathways in breast cancer cells. In this study, we identified a small polybasic region (PBR) preceding the RhoGAP domain that mediates specific binding to negatively charged phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3). In vitro reconstitution of membrane vesicles loaded with prenylated Rac1 demonstrates that the PBR is required for full activation of CdGAP in the presence of PI(3,4,5)P3. In fibroblast cells, the expression of CdGAP protein mutants lacking an intact PBR shows a significant reduced ability of the protein mutants to induce cell rounding or to mediate negative effects on cell spreading. Furthermore, an intact PBR is required for CdGAP to inactivate Rac1 signaling into cells, whereas it is not essential in an in vitro context. Altogether, these studies reveal that specific interaction between negatively charged phospholipid PI(3,4,5)P3 and the stretch of polybasic residues preceding the RhoGAP domain regulates CdGAP activity in vivo and is required for its cellular functions.

Identification of vascular and hematopoietic genes downstream of etsrp by deep sequencing in zebrafish

The transcription factor etsrp/Er71/Etv2 is a master control gene for vasculogenesis in all species studied to date. It is also required for hematopoiesis in zebrafish and mice. Several novel genes expressed in vasculature have been identified through transcriptional profiling of zebrafish embryos overexpressing etsrp by microarrays. Here we re-examined this transcriptional profile by Illumina RNA-sequencing technology, revealing a substantially increased number of candidate genes regulated by etsrp. Expression studies of 50 selected candidate genes from this dataset resulted in the identification of 39 new genes that are expressed in vascular cells. Regulation of these genes by etsrp was confirmed by their ectopic induction in etsrp overexpressing and decreased expression in etsrp deficient embryos. Our studies demonstrate the effectiveness of the RNA-sequencing technology to identify biologically relevant genes in zebrfish and produced a comprehensive profile of genes previously unexplored in vascular endothelial cell biology.

Detection of Rho GEF and GAP activity through a sensitive split luciferase assay system

Rho GTPases regulate the assembly of cellular actin structures and are activated by GEFs (guanine-nucleotide-exchange factors) and rendered inactive by GAPs (GTPase-activating proteins). Using the Rho GTPases Cdc42, Rac1 and RhoA, and the GTPase-binding portions of the effector proteins p21-activated kinase and Rhophilin1, we have developed split luciferase assays for detecting both GEF and GAP regulation of these GTPases. The system relies on purifying split luciferase fusion proteins of the GTPases and effectors from bacteria, and our results show that the assays replicate GEF and GAP specificities at nanomolar concentrations for several previously characterized Rho family GEFs (Dbl, Vav2, Trio and Asef) and GAPs [p190, Cdc42 GAP and PTPL1-associated RhoGAP]. The assay detected activities associated with purified recombinant GEFs and GAPs, cell lysates expressing exogenous proteins, and immunoprecipitates of endogenous Vav1 and p190. The results demonstrate that the split luciferase system provides an effective sensitive alternative to radioactivity-based assays for detecting GTPase regulatory protein activities and is adaptable to a variety of assay conditions.

CdGAP is required for transforming growth factor β- and Neu/ErbB-2-induced breast cancer cell motility and invasion

RhoA, Rac1 and Cdc42, the best-characterized members of the Rho family of small GTPases, are critical regulators of many cellular activities. Cdc42 GTPase-activating protein (CdGAP) is a serine- and proline-rich RhoGAP protein showing GAP activity against both Cdc42 and Rac1 but not RhoA. CdGAP is phosphorylated downstream of the MEK-ERK (extracellular signal-regulated kinase) pathway in response to serum and is required for normal cell spreading and polarized lamellipodia formation. In this study, we found that CdGAP protein and mRNA levels are highly increased in mammary tumor explants expressing an activated Neu/ErbB-2 (Neu-NT) receptor. In response to transforming growth factor-β (TGFβ) stimulation, Neu-NT-expressing mammary tumor explants demonstrate a clear induction in cell motility and invasion. We show that downregulation of CdGAP expression by small interfering RNA abrogates the ability of TGFβ to induce cell motility and invasion of Neu-NT-expressing mammary tumor explants. However, it has no effect on TGFβ-mediated cell adhesion on type 1 collagen and fibronectin. Interestingly, protein expression of E-Cadherin is highly increased in Neu-NT-expressing mammary tumor explants depleted of CdGAP. In addition, complete loss of E-Cadherin expression is not observed in CdGAP-depleted cells during TGFβ-mediated epithelial to mesenchymal transition. Downregulation of the CdGAP expression also decreases cell proliferation of Neu-NT-expressing mammary tumor explants independently of TGFβ. Rescue analysis using re-expression of various CdGAP deletion-mutant proteins revealed that the proline-rich domain (PRD) but not the GAP domain of CdGAP is essential to mediate TGFβ-induced cell motility and invasion. Finally, we found that TGFβ induces the expression and phosphorylation of CdGAP in mammary epithelial NMuMG cells. Taken together, these studies identify CdGAP as a novel molecular target in TGFβ signaling and implicate CdGAP as an essential component in the synergistic interaction between TGFβ and Neu/ErbB-2 signaling pathways in breast cancer cells.

Current knowledge of the large RhoGAP family of proteins

The Rho GTPases are implicated in almost every fundamental cellular process. They act as molecular switches that cycle between an active GTP-bound and an inactive GDP-bound state. Their slow intrinsic GTPase activity is greatly enhanced by RhoGAPs (Rho GTPase-activating proteins), thus causing their inactivation. To date, more than 70 RhoGAPs have been identified in eukaryotes, ranging from yeast to human, and based on sequence homology of their RhoGAP domain, we have grouped them into subfamilies. In the present Review, we discuss their regulation, biological functions and implication in human diseases.

Focal adhesion kinase controls morphogenesis of the Drosophilaoptic stalk

Photoreceptor cell axons (R axons) innervate optic ganglia in the Drosophila brain through the tubular optic stalk. This structure consists of surface glia (SG) and forms independently of R axon projection. In a screen for genes involved in optic stalk formation, we identified Fak56D encoding a Drosophila homolog of mammalian focal adhesion kinase (FAK). FAK is a main component of the focal adhesion signaling that regulates various cellular events, including cell migration and morphology. We show that Fak56D mutation causes severe disruption of the optic stalk structure. These phenotypes were completely rescued by Fak56D transgene expression in the SG cells but not in photoreceptor cells. Moreover, Fak56D genetically interacts with myospheroid, which encodes an integrin β subunit. In addition,we found that CdGAPr is also required for optic stalk formation and genetically interacts with Fak56D. CdGAPr encodes a GTPase-activating domain that is homologous to that of mammalian CdGAP, which functions in focal adhesion signaling. Hence the optic stalk is a simple monolayered structure that can serve as an ideal system for studying glial cell morphogenesis and the developmental role(s) of focal adhesion signaling.

Glycogen synthase kinase-3 phosphorylates CdGAP at a consensus ERK 1 regulatory site

Rho GTPases regulate a multitude of cellular processes from cytoskeletal reorganization to gene transcription and are negatively regulated by GTPase-activating proteins (GAPs). Cdc42 GTPase-activating protein (CdGAP) is a ubiquitously expressed GAP for Rac1 and Cdc42. In this study, we set out to identify CdGAP-binding partners and, using a yeast two-hybrid approach, glycogen synthase kinase 3alpha (GSK-3alpha) was identified as a partner for CdGAP. GSK-3 exists in two isoforms, alpha and beta, and is involved in regulating many cellular functions from insulin response to tumorigenesis. We show that GSK-3alpha and -beta interact with CdGAP in mammalian cells. We also demonstrate that GSK-3 phosphorylates CdGAP both in vitro and in vivo on Thr-776, which we have previously shown to be an ERK 1/2 phosphorylation site involved in CdGAP regulation. We report that the mRNA and protein levels of CdGAP are increased upon serum stimulation and that GSK-3 activity is necessary for the up-regulation of the protein levels of CdGAP but not for the increase in mRNA. We conclude that GSK-3 is an important regulator of CdGAP and that regulation of CdGAP protein levels by serum presents a novel mechanism for cells to control Cdc42/Rac1 GTPase signaling pathways.

The human orthologue of CdGAP is a phosphoprotein and a GTPase-activating protein for Cdc42 and Rac1 but not RhoA

BACKGROUND INFORMATION

Rho GTPases regulate a wide range of cellular functions affecting both cell proliferation and cytoskeletal dynamics. They cycle between inactive GDP- and active GTP-bound states. This cycle is tightly regulated by GEFs (guanine nucleotide-exchange factors) and GAPs (GTPase-activating proteins). Mouse CdGAP (mCdc42 GTPase-activating protein) has been previously identified and characterized as a specific GAP for Rac1 and Cdc42, but not for RhoA. It consists of an N-terminal RhoGAP domain and a C-terminal proline-rich region. In addition, CdGAP-related genes are present in both vertebrates and invertebrates. We have recently reported that two predominant isoforms of CdGAP (250 and 90 kDa) exist in specific mouse tissues.

RESULTS

In the present study, we have identified and characterized human CdGAP (KIAA1204) which shares 76% sequence identity to the long isoform of mCdGAP (mCdGAP-l). Similar to mCdGAP, it is active in vitro and in vivo on both Cdc42 and Rac1, but not RhoA, and is phosphorylated in vivo on serine and threonine residues. In contrast with mCdGAP-l, human CdGAP interacts with ERK1/2 (extracellular-signal-regulated kinase 1/2) through a region that does not involve a DEF (docking site for ERK Phe-Xaa-Phe-Pro) domain. Also, the tissue distribution of CdGAP proteins appears to be different between human and mouse species. Interestingly, we found that CdGAP proteins cause membrane blebbing in COS-7 cells.

CONCLUSIONS

Our results suggest that CdGAP properties are well conserved between human and mouse species, and that CdGAP may play an unexpected role in apoptosis.

CdGAP associates with actopaxin to regulate integrin-dependent changes in cell morphology and motility

BACKGROUND

Integrin signaling, stimulated by cell adhesion to the extracellular matrix, plays a critical role in coordinating changes in cell morphology and migration. The requisite remodeling of the cytoskeleton is controlled by the Rho family of small GTPases, which are, in turn, regulated via activation by guanine-nucleotide exchange factors (GEFs) and inactivation by GTPase-activating proteins (GAPs). However, the mechanisms contributing to the precise spatial and temporal regulation of these Rho GTPase modulators remain poorly understood.

RESULTS

The Cdc42/Rac GAP CdGAP has previously been implicated as an inhibitor of growth-factor-induced lamellipodia formation. Herein, CdGAP is shown to localize to focal adhesions, potentially through its direct association with the amino terminus of actopaxin, a paxillin and actin binding protein. CdGAP activity is regulated in an adhesion-dependent manner and, through the overexpression of wild-type CdGAP and a GAP-deficient mutant, as well as RNA interference, is shown to be required for normal cell spreading, polarized lamellipodia formation, and cell migration. Introduction of an actopaxin mutant defective for CdGAP binding, or reduction of actopaxin by using RNAi, significantly attenuated these effects.

CONCLUSIONS

We have established that CdGAP is an important regulator of integrin-induced Rho family signaling to the cytoskeleton and that its interaction with the focal-adhesion protein actopaxin is critical for the correct spatial and/or temporal regulation of CdGAP function. A complete understanding of the coordination of signaling events downstream of integrin engagement with the extracellular matrix will provide valuable insight into the regulation of cell migration during processes such as wound repair, development, and tumor cell metastasis.

MEGAP impedes cell migration via regulating actin and microtubule dynamics and focal complex formation

Over the past several years, it has become clear that the Rho family of GTPases plays an important role in various aspects of neuronal development including cytoskeleton dynamics and cell adhesion processes. We have analysed the role of MEGAP, a GTPase-activating protein that acts towards Rac1 and Cdc42 in vitro and in vivo, with respect to its putative regulation of cytoskeleton dynamics and cell migration. To investigate the effects of MEGAP on these cellular processes, we have established an inducible cell culture model consisting of a stably transfected neuroblastoma SHSY-5Y cell line that endogenously expresses MEGAP albeit at low levels. We can show that the induced expression of MEGAP leads to the loss of filopodia and lamellipodia protrusions, whereas constitutively activated Rac1 and Cdc42 can rescue the formation of these structures. We have also established quantitative assays for evaluating actin dynamics and cellular migration. By time-lapse microscopy, we show that induced MEGAP expression reduces cell migration by 3.8-fold and protrusion formation by 9-fold. MEGAP expressing cells also showed impeded microtubule dynamics as demonstrated in the TC-7 3x-GFP epithelial kidney cells. In contrast to the wild type, overexpression of MEGAP harbouring an artificially introduced missense mutation R542I within the functionally important GAP domain did not exert a visible effect on actin and microtubule cytoskeleton remodelling. These data suggest that MEGAP negatively regulates cell migration by perturbing the actin and microtubule cytoskeleton and by hindering the formation of focal complexes.

Rac3-induced neuritogenesis requires binding to Neurabin I

Rac3, a neuronal GTP-binding protein of the Rho family, induces neuritogenesis in primary neurons. Using yeast two-hybrid analysis, we show that Neurabin I, the neuronal F-actin binding protein, is a direct Rac3-interacting molecule. Biochemical and light microscopy studies indicate that Neurabin I copartitions and colocalizes with Rac3 at the growth cones of neurites, inducing Neurabin I association to the cytoskeleton. Moreover, Neurabin I antisense oligonucleotides abolish Rac3-induced neuritogenesis, which in turn is rescued by exogenous Neurabin I but not by Neurabin I mutant lacking the Rac3-binding domain. These results show that Neurabin I mediates Rac3-induced neuritogenesis, possibly by anchoring Rac3 to growth cone F-actin.

Signaling Mechanisms Regulating Endothelial Permeability

The microvascular endothelial cell monolayer localized at the critical interface between the blood and vessel wall has the vital functions of regulating tissue fluid balance and supplying the essential nutrients needed for the survival of the organism. The endothelial cell is an exquisite “sensor” that responds to diverse signals generated in the blood, subendothelium, and interacting cells. The endothelial cell is able to dynamically regulate its paracellular and transcellular pathways for transport of plasma proteins, solutes, and liquid. The semipermeable characteristic of the endothelium (which distinguishes it from the epithelium) is crucial for establishing the transendothelial protein gradient (the colloid osmotic gradient) required for tissue fluid homeostasis. Interendothelial junctions comprise a complex array of proteins in series with the extracellular matrix constituents and serve to limit the transport of albumin and other plasma proteins by the paracellular pathway. This pathway is highly regulated by the activation of specific extrinsic and intrinsic signaling pathways. Recent evidence has also highlighted the importance of the heretofore enigmatic transcellular pathway in mediating albumin transport via transcytosis. Caveolae, the vesicular carriers filled with receptor-bound and unbound free solutes, have been shown to shuttle between the vascular and extravascular spaces depositing their contents outside the cell. This review summarizes and analyzes the recent data from genetic, physiological, cellular, and morphological studies that have addressed the signaling mechanisms involved in the regulation of both the paracellular and transcellular transport pathways.

Extracellular signal-regulated kinase 1 interacts with and phosphorylates CdGAP at an important regulatory site

Rho GTPases regulate multiple cellular processes affecting both cell proliferation and cytoskeletal dynamics. Their cycling between inactive GDP- and active GTP-bound states is tightly regulated by guanine nucleotide exchange factors and GTPase-activating proteins (GAPs). We have previously identified CdGAP (for Cdc42 GTPase-activating protein) as a specific GAP for Rac1 and Cdc42. CdGAP consists of an N-terminal RhoGAP domain and a C-terminal proline-rich region. In addition, CdGAP is a member of the impressively large number of mammalian RhoGAP proteins that is well conserved among both vertebrates and invertebrates. In mice, we find two predominant isoforms of CdGAP differentially expressed in specific tissues. We report here that CdGAP is highly phosphorylated in vivo on serine and threonine residues. We find that CdGAP is phosphorylated downstream of the MEK-extracellular signal-regulated kinase (ERK) pathway in response to serum or platelet-derived growth factor stimulation. Furthermore, CdGAP interacts with and is phosphorylated by ERK-1 and RSK-1 in vitro. A putative DEF (docking for ERK FXFP) domain located in the proline-rich region of CdGAP is required for efficient binding and phosphorylation by ERK1/2. We identify Thr776 as an in vivo target site of ERK1/2 and as an important regulatory site of CdGAP activity. Together, these data suggest that CdGAP is a novel substrate of ERK1/2 and mediates cross talk between the Ras/mitogen-activated protein kinase pathway and regulation of Rac1 activity.

Regulation of membrane trafficking by a novel Cdc42-related protein in Caenorhabditis elegans epithelial cells

Rho GTPases are mainly known for their implication in cytoskeleton remodeling. They have also been recently shown to regulate various aspects of membrane trafficking. Here, we report the identification and the characterization of a novel Caenorhabditis elegans Cdc42-related protein, CRP-1, that shows atypical enzymatic characteristics in vitro. Expression in mouse fibroblasts revealed that, in contrast with CDC-42, CRP-1 was unable to reorganize the actin cytoskeleton and mainly localized to trans-Golgi network and recycling endosomes. This subcellular localization, as well as its expression profile restricted to a subset of epithelial-like cells in C. elegans, suggested a potential function for this protein in polarized membrane trafficking. Consistent with this hypothesis, alteration of CRP-1 expression affected the apical trafficking of CHE-14 in vulval and rectal epithelial cells and sphingolipids (C(6)-NBD-ceramide) uptake and/or trafficking in intestinal cells. However, it did not affect basolateral trafficking of myotactin in the pharynx and the targeting of IFB-2 and AJM-1, two cytosolic apical markers of intestine epithelial cells. Hence, our data demonstrate a function for CRP-1 in the regulation of membrane trafficking in a subset of cells with epithelial characteristics.

A novel testicular RhoGAP-domain protein induces apoptosis

The GTPase-activating proteins (GAPs) accelerate the hydrolysis of GTP to GDP by small GTPases. The GTPases play diverse roles in many cellular processes, including proliferation, cell motility, endocytosis, nuclear import/export, and nuclear membrane formation. Little is known about GAP-domain proteins in spermatogenesis. We isolated a novel RhoGAP domain-containing tGAP1 protein from male germ cells that exhibits unusual properties. The tGAP1 is expressed at low levels in early spermatogonia. Robust transcription initiates in midpachytene spermatocytes and continues after meiosis. The 175-kDa tGAP1 protein localizes to the cytoplasm of spermatocytes and to the cytoplasm and nucleus in spermatids. The protein contains four GAP domain-related sequences, in contrast to all other GAP proteins that harbor one such domain. No activity toward RhoA, Rac1, or Cdc42 could be detected. Results of transfection studies in various somatic cells indicated that low-level tGAP1 expression significantly slows down the cell cycle. Expression of higher levels of tGAP1 by infection of somatic cells with recombinant adenoviruses demonstrated that tGAP1 efficiently induces apoptosis, which to our knowledge is the first such demonstration for a RhoGAP protein. Based on its subcellular location in spermatids and its activity, tGAP1 may play a role in nuclear import/export.

Coactivation of Rac1 and Cdc42 at lamellipodia and membrane ruffles induced by epidermal growth factor

A major function of Rho-family GTPases is to regulate the organization of the actin cytoskeleton; filopodia, lamellipodia, and stress fiber are regarded as typical phenotypes of the activated Cdc42, Rac, and Rho, respectively. Using probes based on fluorescent resonance energy transfer, we report on the spatiotemporal regulation of Rac1 and Cdc42 at lamellipodia and membrane ruffles. In epidermal growth factor (EGF)-stimulated Cos1 and A431 cells, both Rac1 and Cdc42 were activated diffusely at the plasma membrane, followed by lamellipodial protrusion and membrane ruffling. Although Rac1 activity subsided rapidly, Cdc42 activity was sustained at lamellipodia. A critical role of Cdc42 in these EGF-induced morphological changes was demonstrated as follows. First, phorbol 12-myristate 13-acetate, which activated Rac1 but not Cdc42, could not induce full-grown lamellipodia in Cos1 cells. Second, a GTPase-activating protein for Cdc42, KIAA1204/CdGAP, inhibited lamellipodial protrusion and membrane ruffling without interfering with Rac1 activation. Third, expression of the Cdc42-binding domain of N-WASP inhibited the EGF-induced morphological changes. Therefore, Rac1 and Cdc42 seem to synergistically induce lamellipodia and membrane ruffles in EGF-stimulated Cos1 cells and A431 cells.

A novel strategy for specifically down-regulating individual Rho GTPase activity in tumor cells

The Rho family GTPases RhoA, RhoB, and RhoC regulate the actin cytoskeleton, cell movement, and cell growth. Unlike Ras, up-regulation or overexpression of these GDP/GTP binding molecular switches, but not activating point mutations, has been associated with human cancer. Although they share over 85% sequence identity, RhoA, RhoB, and RhoC appear to play distinct roles in cell transformation and metastasis. In NIH 3T3 cells, RhoA or RhoB overexpression causes transformation whereas RhoC increases the cell migration rate. To specifically target RhoA, RhoB, or RhoC function, we have generated a set of chimeric molecules by fusing the RhoGAP domain of p190, a GTPase-activating protein that accelerates the intrinsic GTPase activity of all three Rho GTPases, with the C-terminal hypervariable sequences of RhoA, RhoB, or RhoC. The p190-Rho chimeras were active as GTPase-activating proteins toward RhoA in vitro, co-localized with the respective active Rho proteins, and specifically down-regulated Rho protein activities in cells depending on which Rho GTPase sequences were included in the chimeras. In particular, the p190-RhoA-C chimera specifically inhibited RhoA-induced transformation whereas p190-RhoC-C specifically reversed the migration phenotype induced by the active RhoC. In human mammary epithelial-RhoC breast cancer cells, p190-RhoC-C, but not p190-RhoA-C or p190-RhoB-C, reversed the anchorage-independent growth and invasion phenotypes caused by RhoC overexpression. In the highly metastatic A375-M human melanoma cells, p190-RhoC-C specifically reversed migration, and invasion phenotypes attributed to RhoC up-regulation. Thus, we have developed a novel strategy utilizing RhoGAP-Rho chimeras to specifically down-regulate individual Rho activity and demonstrate that this approach may be applied to multiple human tumor cells to reverse the growth and/or invasion phenotypes associated with disregulation of a distinct subtype of Rho GTPase.

Spatio-temporal regulation of Rac1 and Cdc42 activity during nerve growth factor-induced neurite outgrowth in PC12 cells

Neurite outgrowth is an important process in the formation of neuronal networks. It is widely accepted that Rac1 and Cdc42, members of the Rho GTPase family, positively regulate neurite extension through reorganization of the actin cytoskeleton; however, it remains largely unknown when and where Rac1 and Cdc42 are activated during neuritogenesis. This study visualized the spatio-temporal regulation of Rac1 and Cdc42 activities during nerve growth factor (NGF)-induced neurite outgrowth in living PC12 cells by using probes based on the principle of fluorescence resonance energy transfer (FRET). Immediately after the addition of NGF, Rac1 and Cdc42 were transiently activated in broad areas of the cell periphery; a repetitive activation and inactivation cycle was then observed at the motile tips of protrusions. This localized activation, which was more evident in PC12 cells treated with NGF for more than 24 h, might facilitate neurite extension, because the expression of constitutively active mutants of Rac1 and Cdc42 abrogated NGF-induced neurite outgrowth. FRET imaging also delineated a difference between the localization of activated Rac1 and that of Cdc42 within the neurite tips. Experiments with dominant-negative mutants suggested that Rac1 and Cdc42 were activated by a common guanine nucleotide exchange factor(s) in an early stage of the activation phase. Therefore, the difference between Rac1- and Cdc42-activated areas possibly came from the differential localization between Rac1-specific GTPase-activation proteins (GAPs) and Cdc42-specific GAPs. It was concluded that the localized activation of Rac1 and Cdc42 was caused by both guanine nucleotide exchange factors and GAPs, and was important for neurite extension.

GC-GAP, a Rho family GTPase-activating protein that interacts with signaling adapters Gab1 and Gab2

Gab1 and Gab2 are scaffolding proteins acting downstream of cell surface receptors and interact with a variety of cytoplasmic signaling proteins such as Grb2, Shp-2, phosphatidylinositol 3-kinase, Shc, and Crk. To identify new binding partners for GAB proteins and better understand their functions, we performed a yeast two-hybrid screening with hGab2-(120-587) as bait. This work led to identification of a novel GTPase-activating protein (GAP) for Rho family GTPases. The GAP domain shows high similarity to the recently cloned CdGAP and displays activity toward RhoA, Rac1, and Cdc42 in vitro. The protein was named GC-GAP for its ability to interact with GAB proteins and its activity toward Rac and Cdc42. GC-GAP is predominantly expressed in the brain with low levels detected in other tissues. Antibodies directed against GC-GAP recognized a protein of approximately 200 kDa. Expression of GC-GAP in 293T cells led to a reduction in active Rac1 and Cdc42 levels but not RhoA. Suppression of GC-GAP expression by siRNA inhibited proliferation of C6 astroglioma cells. In addition, GC-GAP contains several classic proline-rich motifs, and it interacts with the first SH3 domain of Crk and full-length Nck in vitro. We propose that Gab1 and Gab2 in cooperation with other adapter molecules might regulate the cellular localization of GC-GAP under specific stimuli, acting to regulate precisely Rac and Cdc42 activities. Given that GC-GAP is specifically expressed in the nervous system and that it is localized to the dendritic processes of cultured neurons, GC-GAP may play a role in dendritic morphogenesis and also possibly in neural/glial cell proliferation.

A role for epsin N-terminal homology/AP180 N-terminal homology (ENTH/ANTH) domains in tubulin binding

The epsin N-terminal homology (ENTH) domain is a protein module of approximately 150 amino acids found at the N terminus of a variety of proteins identified in yeast, plants, nematode, frog, and mammals. ENTH domains comprise multiple alpha-helices folded upon each other to form a compact globular structure that has been implicated in interactions with lipids and proteins. In characterizing this evolutionarily conserved domain, we isolated and identified tubulin as an ENTH domain-binding partner. The interaction, which is direct and has a dissociation constant of approximately 1 microm, was observed with ENTH domains of proteins present in various species. Tubulin is co-immunoprecipitated from rat brain extracts with the ENTH domain-containing proteins, epsins 1 and 2, and punctate epsin staining is observed along the microtubule cytoskeleton of dissociated cortical neurons. Consistent with a role in microtubule processes, the over-expression of epsin ENTH domain in PC12 cells stimulates neurite outgrowth. These data demonstrate an evolutionarily conserved property of ENTH domains to interact with tubulin and microtubules.

p250GAP, a novel brain-enriched GTPase-activating protein for Rho family GTPases, is involved in the N-methyl-d-aspartate receptor signaling

N-methyl-d-aspartate (NMDA) receptors regulate structural plasticity by modulating actin organization within dendritic spines. Herein, we report identification and characterization of p250GAP, a novel GTPase-activating protein for Rho family proteins that interacts with the GluRepsilon2 (NR2B) subunit of NMDA receptors in vivo. The p250GAP mRNA was enriched in brain, with high expression in cortex, corpus striatum, hippocampus, and thalamus. Within neurons, p250GAP was highly concentrated in the postsynaptic density and colocalized with the GluRepsilon2 (NR2B) subunit of NMDA receptors and with postsynaptic density-95. p250GAP promoted GTP hydrolysis of Cdc42 and RhoA in vitro and in vivo. When overexpressed in neuroblastoma cells, p250GAP suppressed the activities of Rho family proteins, which resulted in alteration of neurite outgrowth. Finally, NMDA receptor stimulation led to dephosphorylation and redistribution of p250GAP in hippocampal slices. Together, p250GAP is likely to be involved in NMDA receptor activity-dependent actin reorganization in dendritic spines.