Identification of a novel actin-binding domain within the Rho guanine nucleotide exchange factor TEM4

ARHGEF17/TEM4 regulates the cell cycle through control of G1 progression

The Ras homolog (Rho) small GTPases coordinate diverse cellular functions including cell morphology, adhesion and motility, cell cycle progression, survival, and apoptosis via their role in regulating the actin cytoskeleton. The upstream regulators for many of these functions are unknown. ARHGEF17 (also known as TEM4) is a Rho family guanine nucleotide exchange factor (GEF) implicated in cell migration, cell-cell junction formation, and the mitotic checkpoint. In this study, we characterize the regulation of the cell cycle by TEM4. We demonstrate that TEM4-depleted cells exhibit multiple defects in mitotic entry and duration, spindle morphology, and spindle orientation. In addition, TEM4 insufficiency leads to excessive cortical actin polymerization and cell rounding defects. Mechanistically, we demonstrate that TEM4-depleted cells delay in G1 as a consequence of decreased expression of the proproliferative transcriptional co-activator YAP. TEM4-depleted cells that progress through to mitosis do so with decreased levels of cyclin B as a result of attenuated expression of CCNB1. Importantly, cyclin B overexpression in TEM4-depleted cells largely rescues mitotic progression and chromosome segregation defects in anaphase. Our study thus illustrates the consequences of Rho signaling imbalance on cell cycle progression and identifies TEM4 as the first GEF governing Rho GTPase-mediated regulation of G1/S.

Rhogef17: A novel target for endothelial barrier function

ARHGEF17 encodes the protein RhoGEF17, which is highly expressed in vascular endothelial cells. It is a guanine nucleotide exchange factor (GEF) that accelerates the exchange of GDP with GTP on many small GTPases through its Dbl homology (DH) domain, enabling the activation of Rho-GTPases such as RhoA, RhoB, and RhoC. Rho GTPase-regulated changes in the actin cytoskeleton and cell adhesion kinetics are the main mechanisms mediating many endothelial cell (EC) alterations, including cell morphology, migration, and division changes, which profoundly affect EC barrier function. This review focuses on ARHGEF17 expression, activation and biological functions in ECs, linking its regulation of cellular morphology, migration, mitosis and other cellular behaviors to disease onset and progression. Understanding ARHGEF17 mechanisms of action will contribute to the design of therapeutic approaches targeting RhoGEF17, a potential drug target for the treatment of various endothelium-related diseases, Such as vascular inflammation, carcinogenesis and transendothelial metastasis of tumors.

Guanine nucleotide exchange factors for Rho GTPases (RhoGEFs) as oncogenic effectors and strategic therapeutic targets in metastatic cancer

Metastatic cancer cells dynamically adjust their shape to adhere, invade, migrate, and expand to generate secondary tumors. Inherent to these processes is the constant assembly and disassembly of cytoskeletal supramolecular structures. The subcellular places where cytoskeletal polymers are built and reorganized are defined by the activation of Rho GTPases. These molecular switches directly respond to signaling cascades integrated by Rho guanine nucleotide exchange factors (RhoGEFs), which are sophisticated multidomain proteins that control morphological behavior of cancer and stromal cells in response to cell-cell interactions, tumor-secreted factors and actions of oncogenic proteins within the tumor microenvironment. Stromal cells, including fibroblasts, immune and endothelial cells, and even projections of neuronal cells, adjust their shapes and move into growing tumoral masses, building tumor-induced structures that eventually serve as metastatic routes. Here we review the role of RhoGEFs in metastatic cancer. They are highly diverse proteins with common catalytic modules that select among a variety of homologous Rho GTPases enabling them to load GTP, acquiring an active conformation that stimulates effectors controlling actin cytoskeleton remodeling. Therefore, due to their strategic position in oncogenic signaling cascades, and their structural diversity flanking common catalytic modules, RhoGEFs possess unique characteristics that make them conceptual targets of antimetastatic precision therapies. Preclinical proof of concept, demonstrating the antimetastatic effect of inhibiting either expression or activity of βPix (ARHGEF7), P-Rex1, Vav1, ARHGEF17, and Dock1, among others, is emerging.

Case report: Expanding the phenotype of ARHGEF17 mutations from increased intracranial aneurysm risk to a neurodevelopmental disease

RhoGTPase regulators play a key role in the development of the nervous system, and their dysfunction can result in brain malformation and associated disorders. Several guanine nucleotide exchange factors (GEF) have been linked to neurodevelopmental disorders. In line with this, ARHGEF17 has been recently linked as a risk gene to intracranial aneurysms. Here we report siblings of a consanguineous Pakistani family with biallelic variants in the ARHGEF17 gene associated with a neurodevelopmental disorder with intellectual disability, speech delay and motor dysfunction but not aneurysms. Cranial MRI performed in one patient revealed generalized brain atrophy with an enlarged ventricular system, thin corpus callosum and microcephaly. Whole exome sequencing followed by Sanger sequencing in two of the affected individuals revealed a homozygous missense variant (g.11:73021307, c.1624C>T (NM_014786.4), p.R542W) in the ARHGEF17 gene. This variant is in a highly conserved DCLK1 phosphorylation consensus site (I/L/V/F/M]RRXX[pS/pT][I/L/M/V/F) of the protein. Our report expands the phenotypic spectrum of ARHGEF17 variants from increased intracranial aneurysm risk to neurodevelopmental disease and thereby add ARHGEF17 to the list of GEF genes involved in neurodevelopmental disorders.

A commercial ARHGEF17/TEM4 antibody cross-reacts with Nuclear Mitotic Apparatus protein 1 (NuMA)

The Rho family Guanine nucleotide exchange factor (GEF) ARHGEF17 (also known as TEM4) is a large protein with only 3 annotated regions: an N-terminal actin-binding domain, a Rho-specific dbl homology (DH)- pleckstrin homology (PH) type GEF domain and a seven bladed β propeller fold at the C-terminus with unknown function. TEM4 has been implicated in numerous activities that rely on regulation of the cytoskeleton including cell migration, cell-cell junction formation and the spindle assembly checkpoint during mitosis. Here we have assessed the specificity of a TEM4 polyclonal antibody that has been commonly used as a Western blotting and immunocytochemistry probe for TEM4 in mammalian cells. We find that this antibody, in addition to its intended target, cross-reacts with the Nuclear Mitotic Apparatus Protein 1 (NuMA) in Western blotting and immunoprecipitation, and detects NuMA preferentially in immunocytochemistry. This cross-reactivity, with an abundant chromatin- and mitotic spindle-associated factor, is likely to affect the interpretation of experiments that make use of this antibody probe, in particular by immunocytochemistry and immunoprecipitation.

Gβγ mediates activation of Rho guanine nucleotide exchange factor ARHGEF17 that promotes metastatic lung cancer progression

Metastatic lung cancer is a major cause of death worldwide. Dissemination of cancer cells can be facilitated by various agonists within the tumor microenvironment, including by lysophosphatidic acid (LPA). We postulate that Rho guanine nucleotide exchange factors (RhoGEFs), which integrate signaling cues driving cell migration, are critical effectors in metastatic cancer. Specifically, we addressed the hypothetical role of ARHGEF17, a RhoGEF, as a potential effector of Gβγ in metastatic lung cancer cells responding to LPA. Here, we show that ARHGEF17, originally identified as a tumor endothelial marker, is involved in tumor growth and metastatic dissemination of lung cancer cells in an immunocompetent murine model. Gene expression–based analysis of lung cancer datasets showed that increased levels of ARHGEF17 correlated with reduced survival of patients with advanced-stage tumors. Cellular assays also revealed that this RhoGEF participates in the invasive and migratory responses elicited by Gi protein–coupled LPA receptors via the Gβγ subunit complex. We demonstrate that this signaling heterodimer promoted ARHGEF17 recruitment to the cell periphery and actin fibers. Moreover, Gβγ allosterically activates ARHGEF17 by the removal of inhibitory intramolecular restrictions. Taken together, our results indicate that ARHGEF17 may be a valid potential target in the treatment of metastatic lung cancer.

TC10 regulates breast cancer invasion and metastasis by controlling membrane type-1 matrix metalloproteinase at invadopodia

During breast cancer metastasis, cancer cell invasion is driven by actin-rich protrusions called invadopodia, which mediate the extracellular matrix degradation required for the success of the invasive cascade. In this study, we demonstrate that TC10, a member of a Cdc42 subfamily of p21 small GTPases, regulates the membrane type 1 matrix metalloproteinase (MT1-MMP)-driven extracellular matrix degradation at invadopodia. We show that TC10 is required for the plasma membrane surface exposure of MT1-MMP at these structures. By utilizing our Förster resonance energy transfer (FRET) biosensor, we demonstrate the p190RhoGAP-dependent regulation of spatiotemporal TC10 activity at invadopodia. We identified a pathway that regulates invadopodia-associated TC10 activity and function through the activation of p190RhoGAP and the downstream interacting effector Exo70. Our findings reveal the role of a previously unknown regulator of vesicular fusion at invadopodia, TC10 GTPase, in breast cancer invasion and metastasis.

RhoGEF17-An Essential Regulator of Endothelial Cell Death and Growth

The Rho guanine nucleotide exchange factor RhoGEF17 was described to reside in adherens junctions (AJ) in endothelial cells (EC) and to play a critical role in the regulation of cell adhesion and barrier function. The purpose of this study was to analyze signal cascades and processes occurring subsequent to AJ disruption induced by RhoGEF17 knockdown. Primary human and immortalized rat EC were used to demonstrate that an adenoviral-mediated knockdown of RhoGEF17 resulted in cell rounding and an impairment in spheroid formation due to an enhanced proteasomal degradation of AJ components. In contrast, β-catenin degradation was impaired, which resulted in an induction of the β-catenin-target genes cyclin D1 and survivin. RhoGEF17 depletion additionally inhibited cell adhesion and sheet migration. The RhoGEF17 knockdown prevented the cells with impeded cell–cell and cell–matrix contacts from apoptosis, which was in line with a reduction in pro-caspase 3 expression and an increase in Akt phosphorylation. Nevertheless, the cells were not able to proliferate as a cell cycle block occurred. In summary, we demonstrate that a loss of RhoGEF17 disturbs cell–cell and cell–substrate interaction in EC. Moreover, it prevents the EC from cell death and blocks cell proliferation. Non-canonical β-catenin signaling and Akt activation could be identified as a potential mechanism.

Yap activation in irradiated parotid salivary glands is regulated by ROCK activity

Radiotherapy plays a major role in the curative treatment of head and neck cancer, either as a single modality therapy, or in combination with surgery or chemotherapy, or both. Despite advances to limit radiation-induced side-effects, the major salivary glands are often affected. This frequently leads to hyposalivation which causes an increased risk for xerostomia, dental caries, mucositis, and malnutrition culminating in a significant impact on patients' quality of life. Previous research demonstrated that loss of salivary function is associated with a decrease in polarity regulators and an increase in nuclear Yap localization in a putative stem and progenitor cell (SPC) population. Yap activation has been shown to be essential for regeneration in intestinal injury models; however, the highest levels of nuclear Yap are observed in irradiated salivary SPCs that do not regenerate the gland. Thus, elucidating the inputs that regulate nuclear Yap localization and determining the role that Yap plays within the entire tissue following radiation damage and during regeneration is critical. In this study, we demonstrate that radiation treatment increases nuclear Yap localization in acinar cells and Yap-regulated genes in parotid salivary tissues. Conversely, administration of insulin-like growth factor 1 (IGF1), known to restore salivary function in mouse models, reduces nuclear Yap localization and Yap transcriptional targets to levels similar to untreated tissues. Activation of Rho-associated protein kinase (ROCK) using calpeptin results in increased Yap-regulated genes in primary acinar cells while inhibition of ROCK activity (Y-27632) leads to decreased Yap transcriptional targets. These results suggest that Yap activity is dependent on ROCK activity and provides new mechanistic insights into the regulation of radiation-induced hyposalivation.

Periodic propagating waves coordinate RhoGTPase network dynamics at the leading and trailing edges during cell migration

Migrating cells need to coordinate distinct leading and trailing edge dynamics but the underlying mechanisms are unclear. Here, we combine experiments and mathematical modeling to elaborate the minimal autonomous biochemical machinery necessary and sufficient for this dynamic coordination and cell movement. RhoA activates Rac1 via DIA and inhibits Rac1 via ROCK, while Rac1 inhibits RhoA through PAK. Our data suggest that in motile, polarized cells, RhoA–ROCK interactions prevail at the rear, whereas RhoA-DIA interactions dominate at the front where Rac1/Rho oscillations drive protrusions and retractions. At the rear, high RhoA and low Rac1 activities are maintained until a wave of oscillatory GTPase activities from the cell front reaches the rear, inducing transient GTPase oscillations and RhoA activity spikes. After the rear retracts, the initial GTPase pattern resumes. Our findings show how periodic, propagating GTPase waves coordinate distinct GTPase patterns at the leading and trailing edge dynamics in moving cells.

Extracellular biophysical environment: Guilty of being a modulator of drug sensitivity in ovarian cancer cells

The roles of the extracellular biophysical environment in cancer are barely studied. This study considers the possibility that cell-like topography of a cancer cell environment may influence chemo-responses. Here, a novel bioimprinting technique was employed to produce cell-like topography on the polystyrene substrates used for cell culture. In this work, we have shown that extracellular biophysical cues generated from the topography alter the chemosensitivity of ovarian cancer cells. The three-dimensionality of the bioimprinted surface altered the cell-surface interaction, which consequently modulated intracellular signalling and chemoresponses. Sensitivity to platinum was altered more than that to paclitaxel. The effect was largely mediated through the integrin/focal adhesion system and the Rho/ROCK pathway. Moreover, the results provided evidence that biophysical cues also modulate MAPK signalling associated with chemo-resistance in ovarian cancer. Therefore, the novel findings from of this study revealed for the first time that the effects of the biophysical environment, such as substrate topography, influences ovarian cancer cell responses to clinical drugs. These observations suggest that a full clinical understanding of ovarian cancer will include biophysical aspects of tumour microenvironment.

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.

cAMP guided his way: a life for G protein-mediated signal transduction and molecular pharmacology-tribute to Karl H. Jakobs

Karl H. Jakobs, former editor-in-chief of Naunyn-Schmiedeberg's Archives of Pharmacology and renowned molecular pharmacologist, passed away in April 2018. In this article, his scientific achievements regarding G protein-mediated signal transduction and regulation of canonical pathways are summarized. Particularly, the discovery of inhibitory G proteins for adenylyl cyclase, methods for the analysis of receptor-G protein interactions, GTP supply by nucleoside diphosphate kinases, mechanisms in phospholipase C and phospholipase D activity regulation, as well as the development of the concept of sphingosine-1-phosphate as extra- and intracellular messenger will presented. His seminal scientific and methodological contributions are put in a general and timely perspective to display and honor his outstanding input to the current knowledge in molecular pharmacology.

Rho Guanine Nucleotide Exchange Factor ARHGEF17 Is a Risk Gene for Intracranial Aneurysms

BACKGROUND

Intracranial aneurysm (IA) is usually a late-onset disease, affecting 1% to 3% of the general population and leading to life-threatening subarachnoid hemorrhage. Genetic susceptibility has been implicated in IAs, but the causative genes remain elusive.

METHODS

We performed next-generation sequencing in a discovery cohort of 20 Chinese IA patients. Bioinformatics filters were exploited to search for candidate deleterious variants with rare and low allele frequency. We further examined the candidate variants in a multiethnic sample collection of 86 whole exome sequenced unsolved familial IA cases from 3 previously published studies.

RESULTS

We identified that the low-frequency variant c.4394C>A_p.Ala1465Asp (rs2298808) of ARHGEF17 was significantly associated with IA in our Chinese discovery cohort (P=7.3×10-4; odds ratio=7.34). It was subsequently replicated in Japanese familial IA patients (P=0.039; odds ratio=4.00; 95% confidence interval=0.832-14.8) and was associated with IA in the large Chinese sample collection comprising 832 sporadic IA-affected and 599 control individuals (P=0.041; odds ratio=1.51; 95% confidence interval=1.02-Inf). When combining the sequencing data of all familial IA patients from 4 different ethnicities (ie, Chinese, Japanese, European American, and French-Canadian), we identified a significantly increased mutation burden for ARHGEF17 (21/106 versus 11/306; P=8.1×10-7; odds ratio=6.6; 95% confidence interval=2.9-15.8) in cases as compared with controls. In zebrafish, arhgef17 was highly expressed in the brain blood vessel. arhgef17 knockdown caused blood extravasation in the brain region. Endothelial lesions were identified exclusively on cerebral blood vessels in the arhgef17-deficient zebrafish.

CONCLUSIONS

Our results provide compelling evidence that ARHGEF17 is a risk gene for IA.

Biomarkers Discovery for Colorectal Cancer: A Review on Tumor Endothelial Markers as Perspective Candidates

Colorectal cancer (CRC) is the third most common cancer in the world. The early detection of CRC, during the promotion/progression stages, is an enormous challenge for a successful outcome and remains a fundamental problem in clinical approach. Despite the continuous advancement in diagnostic and therapeutic methods, there is a need for discovery of sensitive and specific, noninvasive biomarkers. Tumor endothelial markers (TEMs) are associated with tumor-specific angiogenesis and are potentially useful to discriminate between tumor and normal endothelium. The most promising TEMs for oncogenic signaling in CRC appeared to be the TEM1, TEM5, TEM7, and TEM8. Overexpression of TEMs especially TEM1, TEM7, and TEM8 in colorectal tumor tissue compared to healthy tissue suggests their role in tumor blood vessels formation. Thus TEMs appear to be perspective candidates for early detection, monitoring, and treatment of CRC patients. This review provides an update on recent data on tumor endothelial markers and their possible use as biomarkers for screening, diagnosis, and therapy of colorectal cancer patients.

PLEKHG3 enhances polarized cell migration by activating actin filaments at the cell front

Cells migrate by directing Ras-related C3 botulinum toxin substrate 1 (Rac1) and cell division control protein 42 (Cdc42) activities and by polymerizing actin toward the leading edge of the cell. Previous studies have proposed that this polarization process requires a local positive feedback in the leading edge involving Rac small GTPase and actin polymerization with PI3K likely playing a coordinating role. Here, we show that the pleckstrin homology and RhoGEF domain containing G3 (PLEKHG3) is a PI3K-regulated Rho guanine nucleotide exchange factor (RhoGEF) for Rac1 and Cdc42 that selectively binds to newly polymerized actin at the leading edge of migrating fibroblasts. Optogenetic inactivation of PLEKHG3 showed that PLEKHG3 is indispensable both for inducing and for maintaining cell polarity. By selectively binding to newly polymerized actin, PLEKHG3 promotes local Rac1/Cdc42 activation to induce more local actin polymerization, which in turn promotes the recruitment of more PLEKHG3 to induce and maintain cell front. Thus, autocatalytic reinforcement of PLEKHG3 localization to the leading edge of the cell provides a molecular basis for the proposed positive feedback loop that is required for cell polarization and directed migration.

Flat cells come full sphere: Are mutant cytoskeletal-related proteins oncoprotein-monsters or useful immunogens?

Osteogenesis imperfecta is inherited as a dominant disease because if one allele is mutated, it contributes a mutant, destructive subunit polypeptide to collagen, which requires many subunits to form normal, polymeric, collagenous structures. Recent cancer genome atlas (TCGA) data indicate that cytoskeletal-related proteins are among the most commonly mutated proteins in human cancers, in distinct mutation frequency groups, i.e., including low mutation frequency groups. Part of the explanation for this observation is likely to be the fact that many of the coding regions for these proteins are very large, and indeed, it is likely these coding regions are mutated in many cells that never become cancerous. However, it would not be surprising if mutations in cytoskeletal proteins, when combined with oncoprotein or tumor suppressor protein mutations, had significant impacts on cancer development, for a number of reasons, including results obtained almost 5 decades ago indicating that well-spread cells in tissue culture, with well-formed cytoskeletons, were less tumorigenic than spherical cells with disrupted cytoskeletons. This raises the question, are mutant cytoskeletal proteins, which would likely interfere with polymer formation, a new class of oncoproteins, in particular, dominant negative oncoproteins? If these proteins are so commonly mutant, could they be the bases for common cancer vaccines?

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.

RhoGEFs in cell motility: novel links between Rgnef and focal adhesion kinase

Rho guanine exchange factors (GEFs) are a large, diverse family of proteins defined by their ability to catalyze the exchange of GDP for GTP on small GTPase proteins such as Rho family members. GEFs act as integrators from varied intra- and extracellular sources to promote spatiotemporal activity of Rho GTPases that control signaling pathways regulating cell proliferation and movement. Here we review recent studies elucidating roles of RhoGEF proteins in cell motility. Emphasis is placed on Dbl-family GEFs and connections to development, integrin signaling to Rho GTPases regulating cell adhesion and movement, and how these signals may enhance tumor progression. Moreover, RhoGEFs have additional domains that confer distinctive functions or specificity. We will focus on a unique interaction between Rgnef (also termed Arhgef28 or p190RhoGEF) and focal adhesion kinase (FAK), a non-receptor tyrosine kinase that controls migration properties of normal and tumor cells. This Rgnef-FAK interaction activates canonical GEF-dependent RhoA GTPase activity to govern contractility and also functions as a scaffold in a GEF-independent manner to enhance FAK activation. Recent studies have also brought to light the importance of specific regions within the Rgnef pleckstrin homology (PH) domain for targeting the membrane. As revealed by ongoing Rgnef-FAK investigations, exploring GEF roles in cancer will yield fundamental new information on the molecular mechanisms promoting tumor spread and metastasis.

Phosphorylation of Cdc42 effector protein-4 (CEP4) by protein kinase C promotes motility of human breast cells

Cdc42 effector protein-4 (CEP4) was recently identified by our laboratory to be a substrate of multiple PKC isoforms in non-transformed MCF-10A human breast cells. The significance of phosphorylated CEP4 to PKC-stimulated motility of MCF-10A cells was evaluated. Single site mutants at Ser residues embedded in potential PKC consensus sites (Ser(18), Ser(77), Ser(80), and Ser(86)) were individually replaced with Asp residues to simulate phosphorylation. Following expression in weakly motile MCF-10A cells, the S18D and S80D mutants each promoted increased motility, and the double mutant (S18D/S80D) produced a stronger effect. MS/MS analysis verified that Ser(18) and Ser(80) were directly phosphorylated by PKCα in vitro. Phosphorylation of CEP4 severely diminished its affinity for Cdc42 while promoting Rac activation and formation of filopodia (microspikes). In contrast, the phosphorylation-resistant double mutant S18A/S80A-CEP4 blocked CEP4 phosphorylation and inhibited motility of MCF-10A cells that had been stimulated with PKC activator diacylglycerol lactone. In view of the dissociation of phospho-CEP4 from Cdc42, intracellular binding partners were explored by expressing each CEP4 double mutant from a tandem affinity purification vector followed by affinity chromatography, SDS-PAGE, and identification of protein bands evident only with S18D/S80D-CEP4. One binding partner was identified as tumor endothelial marker-4 (TEM4; ARHGEF17), a guanine nucleotide exchange factor that is involved in migration. In motile cells expressing S18D/S80D-CEP4, knockdown of TEM4 inhibited both Rac activation and motility. These findings support a model in which PKC-mediated phosphorylation of CEP4 at Ser(18) and Ser(80) causes its dissociation from Cdc42, thereby increasing its affinity for TEM4 and producing Rac activation, filopodium formation, and cell motility.

The RhoGEF TEM4 Regulates Endothelial Cell Migration by Suppressing Actomyosin Contractility

Persistent cellular migration requires efficient protrusion of the front of the cell, the leading edge where the actin cytoskeleton and cell-substrate adhesions undergo constant rearrangement. Rho family GTPases are essential regulators of the actin cytoskeleton and cell adhesion dynamics. Here, we examined the role of the RhoGEF TEM4, an activator of Rho family GTPases, in regulating cellular migration of endothelial cells. We found that TEM4 promotes the persistence of cellular migration by regulating the architecture of actin stress fibers and cell-substrate adhesions in protruding membranes. Furthermore, we determined that TEM4 regulates cellular migration by signaling to RhoC as suppression of RhoC expression recapitulated the loss-of-TEM4 phenotypes, and RhoC activation was impaired in TEM4-depleted cells. Finally, we showed that TEM4 and RhoC antagonize myosin II-dependent cellular contractility and the suppression of myosin II activity rescued the persistence of cellular migration of TEM4-depleted cells. Our data implicate TEM4 as an essential regulator of the actin cytoskeleton that ensures proper membrane protrusion at the leading edge of migrating cells and efficient cellular migration via suppression of actomyosin contractility.

TEM4 is a junctional Rho GEF required for cell-cell adhesion, monolayer integrity and barrier function

Signaling events mediated by Rho family GTPases orchestrate cytoskeletal dynamics and cell junction formation. The activation of Rho GTPases is tightly regulated by guanine-nucleotide-exchange factors (GEFs). In this study, we identified a novel Rho-specific GEF called TEM4 (tumor endothelial marker 4) that associates with multiple members of the cadherin-catenin complex and with several cytoskeleton-associated proteins. Depending on confluence, TEM4 localized to either actin stress fibers or areas of cell-cell contact. The junctional localization of TEM4 was independent of actin binding. Depletion of endogenous TEM4 by shRNAs impaired Madin-Darby canine kidney (MDCK) and human umbilical vein endothelial cell (HUVEC) cell junctions, disrupted MDCK acini formation in 3D culture and negatively affected endothelial barrier function. Taken together, our findings implicate TEM4 as a novel and crucial junctional Rho GEF that regulates cell junction integrity and epithelial and endothelial cell function.

RhoGEF17, a Rho-specific guanine nucleotide exchange factor activated by phosphorylation via cyclic GMP-dependent kinase Iα

RhoGEF17, the product of the ARHGEF17 gene, is a Rho-specific guanine nucleotide exchange factor (GEF) with an unusual structure and so far unknown function. In order to get insights in its regulation, we studied a variety of signaling pathways for activation of recombinantly expressed RhoGEF17. We found that in the presence of stable cGMP analogs RhoGEF17 associates with and is phosphorylated by co-expressed cGKIα at distinct phosphorylation sites leading to a cooperative activation of RhoA, the Rho dependent kinases (ROCK) and serum response factor-induced gene transcription. Activation of protein kinase A did not induce phosphorylation of RhoGEF17 nor altered its activity. Furthermore, we obtained evidence for a ROCK-driven positive feedback mechanism involving serine/threonine protein phosphatases, which further enhanced cGMP/cGKIα-induced RhoGEF17 activation. By using mutants of RhoA which are phosphorylation resistant to cGK or mimic phosphorylation at serine 188, we could show that RhoGEF17 is able to activate RhoA independently of its phosphorylation state. Together with the ROCK-enforced activation of RhoGEF17 by cGMP/cGKIα, this might explain why expression of RhoGEF17 switches the inhibitory effect of cGMP/cGKIα on serum-induced RhoA activation into a stimulatory one. We conclude that RhoGEF17, depending on its expression profile and level, might drastically alter the effect of cGMP/cGK involving signaling pathways on RhoA-activated downstream effectors.