Structure and function of heterotrimeric G protein-regulated Rho guanine nucleotide exchange factors

Apigenin inhibits tumor angiogenesis by hindering microvesicle biogenesis via ARHGEF1

Extracellular vesicles are essential for intercellular communication and are involved in tumor progression. Inhibiting the direct release of extracellular vesicles seems to be an effective strategy in inhibiting tumor progression, but lacks of investigation. Here, we report a natural flavonoid compound, apigenin, could significantly inhibit the growth of hepatocellular carcinoma by preventing microvesicle secretion. Mechanistically, apigenin primarily targets the guanine nucleotide exchange factor ARHGEF1, inhibiting the activity of small G protein Cdc42, which is essential in regulating the release of microvesicles from tumor cells. In turn, this inhibits tumor angiogenesis related to VEGF90K transported on microvesicles, ultimately impeding tumor progression. Collectively, these findings highlight the therapeutic potential of apigenin and shed light on its anticancer mechanisms through inhibiting microvesicle biogenesis, providing a solid foundation for the refinement and practical application of apigenin.

Roles of G proteins and their GTPase-activating proteins in platelets

Platelets are small anucleate blood cells supporting vascular function. They circulate in a quiescent state monitoring the vasculature for injuries. Platelets adhere to injury sites and can be rapidly activated to secrete granules and to form platelet/platelet aggregates. These responses are controlled by signalling networks that include G proteins and their regulatory guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Recent proteomics studies have revealed the complete spectrum of G proteins, GEFs, and GAPs present in platelets. Some of these proteins are specific for platelets and very few have been characterised in detail. GEFs and GAPs play a major role in setting local levels of active GTP-bound G proteins in response to activating and inhibitory signals encountered by platelets. Thus, GEFs and GAPs are highly regulated themselves and appear to integrate G protein regulation with other cellular processes. This review focuses on GAPs of small G proteins of the Arf, Rab, Ras, and Rho families, as well as of heterotrimeric G proteins found in platelets.

Target-based drug discovery: Applications of fluorescence techniques in high throughput and fragment-based screening

Target-based discovery of first-in-class therapeutics demands an in-depth understanding of the molecular mechanisms underlying human diseases. Precise measurements of cellular and biochemical activities are critical to gain mechanistic knowledge of biomolecules and their altered function in disease conditions. Such measurements enable the development of intervention strategies for preventing or treating diseases by modulation of desired molecular processes. Fluorescence-based techniques are routinely employed for accurate and robust measurements of in-vitro activity of molecular targets and for discovering novel chemical molecules that modulate the activity of molecular targets. In the current review, the authors focus on the applications of fluorescence-based high throughput screening (HTS) and fragment-based ligand discovery (FBLD) techniques such as fluorescence polarization (FP), Förster resonance energy transfer (FRET), fluorescence thermal shift assay (FTSA) and microscale thermophoresis (MST) for the discovery of chemical probe to exploring target's role in disease biology and ultimately, serve as a foundation for drug discovery. Some recent advancements in these techniques for compound library screening against important classes of drug targets, such as G-protein-coupled receptors (GPCRs) and GTPases, as well as phosphorylation- and acetylation-mediated protein-protein interactions, are discussed. Overall, this review presents a landscape of how these techniques paved the way for the discovery of small-molecule modulators and biologics against these targets for therapeutic benefits.

Dynamic encounters with red blood cells trigger splenic marginal zone B cell retention and function

Spleen marginal zone (MZ) B cells are important for antibody responses against blood-borne antigens. The signals they use to detect exposure to blood are not well defined. Here, using intravital two-photon microscopy in mice, we observe transient contacts between MZ B cells and red blood cells that are in flow. We show that MZ B cells use adhesion G-protein-coupled receptor ADGRE5 (CD97) for retention in the spleen. CD97 function in MZ B cells depends on its ability to undergo autoproteolytic cleavage and signaling via Gα13 and ARHGEF1. Red blood cell expression of the CD97 ligand CD55 is required for MZ B cell homeostasis. Applying a pulling force on CD97-transfected cells using an optical C-trap and CD55+ beads leads to accumulation of active RhoA and membrane retraction. Finally, we show that CD97 deficiency leads to a reduced T cell-independent IgM response. Thus, our studies provide evidence that MZ B cells use mechanosensing to position in a manner that enhances antibody responses against blood-borne antigens.

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.

Intracellular signaling pathways of muscarinic acetylcholine receptor-mediated detrusor muscle contractions

Acetylcholine plays an essential role in the regulation of detrusor muscle contractions, and antimuscarinics are widely used in the management of overactive bladder syndrome. However, several adverse effects limit their application and patients' compliance. Thus, this study aimed to further analyze the signal transduction of M2 and M3 receptors in the murine urinary bladder to eventually find more specific therapeutic targets. Experiments were performed on adult male wild-type, M2, M3, M2/M3, or Gαq/11 knockout (KO), and pertussis toxin (PTX)-treated mice. Contraction force and RhoA activity were measured in the urinary bladder smooth muscle (UBSM). Our results indicate that carbamoylcholine (CCh)-induced contractions were associated with increased activity of RhoA and were reduced in the presence of the Rho-associated kinase (ROCK) inhibitor Y-27632 in UBSM. CCh-evoked contractile responses and RhoA activation were markedly reduced in detrusor strips lacking either M2 or M3 receptors and abolished in M2/M3 KO mice. Inhibition of Gαi-coupled signaling by PTX treatment shifted the concentration-response curve of CCh to the right and diminished RhoA activation. CCh-induced contractile responses were markedly decreased in Gαq/11 KO mice; however, RhoA activation was unaffected. In conclusion, cholinergic detrusor contraction and RhoA activation are mediated by both M2 and M3 receptors. Furthermore, whereas both Gαi and Gαq/11 proteins mediate UBSM contraction, the activation at the RhoA-ROCK pathway appears to be linked specifically to Gαi. These findings may aid the identification of more specific therapeutic targets for bladder dysfunctions.NEW & NOTEWORTHY Muscarinic acetylcholine receptors are of utmost importance in physiological regulation of micturition and also in the development of voiding disorders. We demonstrate that the RhoA-Rho-associated kinase (ROCK) pathway plays a crucial role in contractions induced by cholinergic stimulation in detrusor muscle. Activation of RhoA is mediated by both M2 and M3 receptors as well as by Gi but not Gq/11 proteins. The Gi-RhoA-ROCK pathway may provide a novel therapeutic target for overactive voiding disorders.

Oncogenic Gαq activates RhoJ through PDZ-RhoGEF

Oncogenic Gαq causes uveal melanoma via non-canonical signaling pathways. This constitutively active mutant GTPase is also found in cutaneous melanoma, lung adenocarcinoma, and seminoma, as well as in benign vascular tumors, such as congenital hemangiomas. We recently described that PDZ-RhoGEF (also known as ARHGEF11), a canonical Gα12/13 effector, is enabled by Gαs Q227L to activate CdcIn addition, and we demonstrated that constitutively active Gαq interacts with the PDZ-RhoGEF DH-PH catalytic module, but does not affect its binding to RhoA or Cdc. This suggests that it guides this RhoGEF to gain affinity for other GTPases. Since RhoJ, a small GTPase of the Cdc42 subfamily, has been involved in tumor-induced angiogenesis and the metastatic dissemination of cancer cells, we hypothesized that it might be a target of oncogenic Gαq signaling via PDZ-RhoGEF. Consistent with this possibility, we found that Gαq Q209L drives full-length PDZ-RhoGEF and a DH-PH construct to interact with nucleotide-free RhoJ-G33A, a mutant with affinity for active RhoJ-GEFs. Gαq Q209L binding to PDZ-RhoGEF was mapped to the PH domain, which, as an isolated construct, attenuated the interaction of this mutant GTPase with PDZ-RhoGEF's catalytic module (DH-PH domains). Expression of these catalytic domains caused contraction of endothelial cells and generated fine cell sprouts that were inhibited by co-expression of dominant negative RhoJ. Using relational data mining of uveal melanoma patient TCGA datasets, we got an insight into the signaling landscape that accompanies the Gαq/PDZ-RhoGEF/RhoJ axis. We identified three transcriptional signatures statistically linked with shorter patient survival, including GPCRs and signaling effectors that are recognized as vulnerabilities in cancer cell synthetic lethality datasets. In conclusion, we demonstrated that an oncogenic Gαq mutant enables the PDZ-RhoGEF DH-PH module to recognize RhoJ, suggesting an allosteric mechanism by which this constitutively active GTPase stimulates RhoJ via PDZ-RhoGEF. These findings highlight PDZ-RhoGEF and RhoJ as potential targets in tumors driven by mutant Gαq.

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.

Protein Networks Associated with Native Metabotropic Glutamate 1 Receptors (mGlu1) in the Mouse Cerebellum

The metabotropic glutamate receptor 1 (mGlu1) plays a pivotal role in synaptic transmission and neuronal plasticity. Despite the fact that several interacting proteins involved in the mGlu1 subcellular trafficking and intracellular transduction mechanisms have been identified, the protein network associated with this receptor in specific brain areas remains largely unknown. To identify novel mGlu1-associated protein complexes in the mouse cerebellum, we used an unbiased tissue-specific proteomic approach, namely co-immunoprecipitation followed by liquid chromatography/tandem mass spectrometry analysis. Many well-known protein complexes as well as novel interactors were identified, including G-proteins, Homer, δ2 glutamate receptor, 14-3-3 proteins, and Na/K-ATPases. A novel putative interactor, KCTD12, was further investigated. Reverse co-immunoprecipitation with anti-KCTD12 antibodies revealed mGlu1 in wild-type but not in KCTD12-knock-out homogenates. Freeze-fracture replica immunogold labeling co-localization experiments showed that KCTD12 and mGlu1 are present in the same nanodomain in Purkinje cell spines, although at a distance that suggests that this interaction is mediated through interposed proteins. Consistently, mGlu1 could not be co-immunoprecipitated with KCTD12 from a recombinant mammalian cell line co-expressing the two proteins. The possibility that this interaction was mediated via GABAB receptors was excluded by showing that mGlu1 and KCTD12 still co-immunoprecipitated from GABAB receptor knock-out tissue. In conclusion, this study identifies tissue-specific mGlu1-associated protein clusters including KCTD12 at Purkinje cell synapses.

Interaction kinetics between p115-RhoGEF and Gα13 are determined by unique molecular interactions affecting agonist sensitivity

The three RH-RhoGEFs (Guanine nucleotide exchange factors) p115-RhoGEF, LARG (leukemia-associated RhoGEF) and PDZ-RhoGEF link G-protein coupled receptors (GPCRs) with RhoA signaling through activation of Gα_12/13. In order to find functional differences in signaling between the different RH-RhoGEFs we examined their interaction with Gα₁₃ in high spatial and temporal resolution, utilizing a FRET-based single cell assay. We found that p115-RhoGEF interacts significantly shorter with Gα₁₃ than LARG and PDZ-RhoGEF, while narrowing the structural basis for these differences down to a single amino acid in the rgRGS domain of p115-RhoGEF. The mutation of this amino acid led to an increased interaction time with Gα₁₃ and an enhanced agonist sensitivity, comparable to LARG, while mutating the corresponding amino acid in Gα₁₃ the same effect could be achieved. While the rgRGS domains of RH-RhoGEFs showed GAP (GTPase-activating protein) activity towards Gα₁₃ in vitro, our approach suggests higher GAP activity of p115-RhoGEF in intact cells.

Structural/functional studies of Trio provide insights into its configuration and show that conserved linker elements enhance its activity for Rac1

Trio is a large and highly conserved metazoan signaling scaffold that contains two Dbl family guanine nucleotide exchange factor (GEF) modules, TrioN and TrioC, selective for Rac and RhoA GTPases, respectively. The GEF activities of TrioN and TrioC are implicated in several cancers, especially uveal melanoma. However, little is known about how these modules operate in the context of larger fragments of Trio. Here we show via negative stain electron microscopy that the N-terminal region of Trio is extended and could thus serve as a rigid spacer between the N-terminal putative lipid-binding domain and TrioN, whereas the C-terminal half of Trio seems globular. We found that regions C-terminal to TrioN enhance its Rac1 GEF activity and thus could play a regulatory role. We went on to characterize a minimal, well-behaved Trio fragment with enhanced activity, Trio1284-1959, in complex with Rac1 using cryo-electron microscopy and hydrogen-deuterium exchange mass spectrometry and found that the region conferring enhanced activity is disordered. Deletion of two different strongly conserved motifs in this region eliminated this enhancement, suggesting that they form transient intramolecular interactions that promote GEF activity. Because Dbl family RhoGEF modules have been challenging to directly target with small molecules, characterization of accessory Trio domains such as these may provide alternate routes for the development of therapeutics that inhibit Trio activity in human cancer.

Effector membrane translocation biosensors reveal G protein and βarrestin coupling profiles of 100 therapeutically relevant GPCRs

The recognition that individual GPCRs can activate multiple signaling pathways has raised the possibility of developing drugs selectively targeting therapeutically relevant ones. This requires tools to determine which G proteins and βarrestins are activated by a given receptor. Here, we present a set of BRET sensors monitoring the activation of the 12 G protein subtypes based on the translocation of their effectors to the plasma membrane (EMTA). Unlike most of the existing detection systems, EMTA does not require modification of receptors or G proteins (except for G_s). EMTA was found to be suitable for the detection of constitutive activity, inverse agonism, biased signaling and polypharmacology. Profiling of 100 therapeutically relevant human GPCRs resulted in 1500 pathway-specific concentration-response curves and revealed a great diversity of coupling profiles ranging from exquisite selectivity to broad promiscuity. Overall, this work describes unique resources for studying the complexities underlying GPCR signaling and pharmacology.

Chemo- and mechanosensing by dendritic cells facilitate antigen surveillance in the spleen

Spleen dendritic cells (DC) are critical for initiation of adaptive immune responses against blood-borne invaders. Key to DC function is their positioning at sites of pathogen entry, and their abilities to selectively capture foreign antigens and promptly engage T cells. Focusing on conventional DC2 (cDC2), we discuss the contribution of chemoattractant receptors (EBI2 or GPR183, S1PR1, and CCR7) and integrins to cDC2 positioning and function. We give particular attention to a newly identified role in cDC2 for adhesion G-protein coupled receptor E5 (Adgre5 or CD97) and its ligand CD55, detailing how this mechanosensing system contributes to splenic cDC2 positioning and homeostasis. Additional roles of CD97 in the immune system are reviewed. The ability of cDC2 to be activated by circulating missing self-CD47 cells and to integrate multiple red blood cell (RBC)-derived inputs is discussed. Finally, we describe the process of activated cDC2 migration to engage and prime helper T cells. Throughout the review, we consider the insights into cDC function in the spleen that have emerged from imaging studies.

GPCRs that Rhoar the Guanine nucleotide exchange factors

Cell migration, a crucial step in numerous biological processes, is tightly regulated in space and time. Cells employ Rho GTPases, primarily Rho, Rac, and Cdc42, to regulate their motility. Like other small G proteins, Rho GTPases function as biomolecular switches in regulating cell migration by operating between GDP bound 'OFF' and GTP bound 'ON' states. Guanine nucleotide exchange factors (GEFs) catalyse the shuttling of GTPases from OFF to ON state. G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors that are involved in many signalling phenomena including cell survival and cell migration events. In this review, we summarize signalling mechanisms, involving GPCRs, leading to the activation of RhoGEFs. GPCRs exhibit diverse GEF activation modes that include the interaction of heterotrimeric G protein subunits with different domains of GEFs, phosphorylation, protein-protein interaction, protein-lipid interaction, and/or a combination of these processes.

Insight Into Rho Kinase Isoforms in Obesity and Energy Homeostasis

Obesity and associated complications increasingly jeopardize global health and contribute to the rapidly rising prevalence of type 2 diabetes mellitus and obesity-related diseases. Developing novel methods for the prevention and treatment of excess body adipose tissue expansion can make a significant contribution to public health. Rho kinase is a Rho-associated coiled-coil-containing protein kinase (Rho kinase or ROCK). The ROCK family including ROCK1 and ROCK2 has recently emerged as a potential therapeutic target for the treatment of metabolic disorders. Up-regulated ROCK activity has been involved in the pathogenesis of all aspects of metabolic syndrome including obesity, insulin resistance, dyslipidemia and hypertension. The RhoA/ROCK-mediated actin cytoskeleton dynamics have been implicated in both white and beige adipogenesis. Studies using ROCK pan-inhibitors in animal models of obesity, diabetes, and associated complications have demonstrated beneficial outcomes. Studies via genetically modified animal models further established isoform-specific roles of ROCK in the pathogenesis of metabolic disorders including obesity. However, most reported studies have been focused on ROCK1 activity during the past decade. Due to the progress in developing ROCK2-selective inhibitors in recent years, a growing body of evidence indicates more attention should be devoted towards understanding ROCK2 isoform function in metabolism. Hence, studying individual ROCK isoforms to reveal their specific roles and principal mechanisms in white and beige adipogenesis, insulin sensitivity, energy balancing regulation, and obesity development will facilitate significant breakthroughs for systemic treatment with isoform-selective inhibitors. In this review, we give an overview of ROCK functions in the pathogenesis of obesity and insulin resistance with a particular focus on the current understanding of ROCK isoform signaling in white and beige adipogenesis, obesity and thermogenesis in adipose tissue and other major metabolic organs involved in energy homeostasis regulation.

The RHO Family GTPases: Mechanisms of Regulation and Signaling

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

GTP binding by Arabidopsis extra-large G protein 2 is not essential for its functions

The extra-large guanosine-5'-triphosphate (GTP)-binding protein 2, XLG2, is an unconventional Gα subunit of the Arabidopsis (Arabidopsis thaliana) heterotrimeric GTP-binding protein complex with a major role in plant defense. In vitro biochemical analyses and molecular dynamic simulations show that affinity of XLG2 for GTP is two orders of magnitude lower than that of the conventional Gα, AtGPA1. Here we tested the physiological relevance of GTP binding by XLG2. We generated an XLG2(T476N) variant with abolished GTP binding, as confirmed by in vitro GTPγS binding assay. Yeast three-hybrid, bimolecular fluorescence complementation, and split firefly-luciferase complementation assays revealed that the nucleotide-depleted XLG2(T476N) retained wild-type XLG2-like interactions with the Gβγ dimer and defense-related receptor-like kinases. Both wild-type and nucleotide-depleted XLG2(T476N) restored the defense responses against Fusarium oxysporum and Pseudomonas syringae compromised in the xlg2 xlg3 double mutant. Additionally, XLG2(T476N) was fully functional restoring stomatal density, root growth, and sensitivity to NaCl, but failed to complement impaired germination and vernalization-induced flowering. We conclude that XLG2 is able to function in a GTP-independent manner and discuss its possible mechanisms of action.

RhoA: a dubious molecule in cardiac pathophysiology

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

Modifications of Plasma Membrane Organization in Cancer Cells for Targeted Therapy

Modifications of the composition or organization of the cancer cell membrane seem to be a promising targeted therapy. This approach can significantly enhance drug uptake or intensify the response of cancer cells to chemotherapeutics. There are several methods enabling lipid bilayer modifications, e.g., pharmacological, physical, and mechanical. It is crucial to keep in mind the significance of drug resistance phenomenon, ion channel and specific receptor impact, and lipid bilayer organization in planning the cell membrane-targeted treatment. In this review, strategies based on cell membrane modulation or reorganization are presented as an alternative tool for future therapeutic protocols.

A Gs-RhoGEF interaction: An old G protein finds a new job

The heterotrimeric G proteins are known to have a variety of downstream effectors, but G_s was long thought to be specifically coupled to adenylyl cyclases. A new study indicates that activated G_s can also directly interact with a guanine nucleotide exchange factor for Rho family small GTPases, PDZ-RhoGEF. This novel interaction mediates activation of the small G protein Cdc42 by G_s-coupled GPCRs, inducing cytoskeletal rearrangements and formation of filopodia-like structures. Furthermore, overexpression of a minimal PDZ-RhoGEF fragment can down-regulate cAMP signaling, suggesting that this effector competes with canonical signaling. This first demonstration that the Gα_s subfamily regulates activity of Rho GTPases extends our understanding of Gα_s activity and establishes RhoGEF coupling as a universal Gα function.

Gαs directly drives PDZ-RhoGEF signaling to Cdc42

Gα proteins promote dynamic adjustments of cell shape directed by actin-cytoskeleton reorganization via their respective RhoGEF effectors. For example, Gα13 binding to the RGS-homology (RH) domains of several RH-RhoGEFs allosterically activates these proteins, causing them to expose their catalytic Dbl-homology (DH)/pleckstrin-homology (PH) regions, which triggers downstream signals. However, whether additional Gα proteins might directly regulate the RH-RhoGEFs was not known. To explore this question, we first examined the morphological effects of expressing shortened RH-RhoGEF DH/PH constructs of p115RhoGEF/ARHGEF1, PDZ-RhoGEF (PRG)/ARHGEF11, and LARG/ARHGEF12. As expected, the three constructs promoted cell contraction and activated RhoA, known to be downstream of Gα13 Intriguingly, PRG DH/PH also induced filopodia-like cell protrusions and activated Cdc42. This pathway was stimulated by constitutively active Gαs (GαsQ227L), which enabled endogenous PRG to gain affinity for Cdc42. A chemogenetic approach revealed that signaling by Gs-coupled receptors, but not by those coupled to Gi or Gq, enabled PRG to bind Cdc42. This receptor-dependent effect, as well as CREB phosphorylation, was blocked by a construct derived from the PRG:Gαs-binding region, PRG-linker. Active Gαs interacted with isolated PRG DH and PH domains and their linker. In addition, this construct interfered with GαsQ227L's ability to guide PRG's interaction with Cdc42. Endogenous Gs-coupled prostaglandin receptors stimulated PRG binding to membrane fractions and activated signaling to PKA, and this canonical endogenous pathway was attenuated by PRG-linker. Altogether, our results demonstrate that active Gαs can recognize PRG as a novel effector directing its DH/PH catalytic module to gain affinity for Cdc42.

Naturally occurring hotspot cancer mutations in Gα13 promote oncogenic signaling

Heterotrimeric G-proteins are signaling switches broadly divided into four families based on the sequence and functional similarity of their Gα subunits: Gs, Gi/o, Gq/11, and G12/13 Artificial mutations that activate Gα subunits of each of these families have long been known to induce oncogenic transformation in experimental systems. With the advent of next-generation sequencing, activating hotspot mutations in Gs, Gi/o, or Gq/11 proteins have also been identified in patient tumor samples. In contrast, patient tumor-associated G12/13 mutations characterized to date lead to inactivation rather than activation. By using bioinformatic pathway analysis and signaling assays, here we identified cancer-associated hotspot mutations in Arg-200 of Gα13 (encoded by GNA13) as potent activators of oncogenic signaling. First, we found that components of a G12/13-dependent signaling cascade that culminates in activation of the Hippo pathway effectors YAP and TAZ is frequently altered in bladder cancer. Up-regulation of this signaling cascade correlates with increased YAP/TAZ activation transcriptional signatures in this cancer type. Among the G12/13 pathway alterations were mutations in Arg-200 of Gα13, which we validated to promote YAP/TAZ-dependent (TEAD) and MRTF-A/B-dependent (SRE.L) transcriptional activity. We further showed that this mechanism relies on the same RhoGEF-RhoGTPase cascade components that are up-regulated in bladder cancers. Moreover, Gα13 Arg-200 mutants induced oncogenic transformation in vitro as determined by focus formation assays. In summary, our findings on Gα13 mutants establish that naturally occurring hotspot mutations in Gα subunits of any of the four families of heterotrimeric G-proteins are putative cancer drivers.

Identification and signaling characterization of four urotensin II receptor subtypes in the western clawed frog, Xenopus tropicalis

Urotensin II (UII) is involved, via the UII receptor (UTR), in many physiological and pathological processes, including vasoconstriction, locomotion, osmoregulation, immune response, and metabolic syndrome. In silico studies have revealed the presence of four or five distinct UTR (UTR1-UTR5) gene sequences in nonmammalian vertebrates. However, the functionality of these receptor subtypes and their associations to signaling pathways are unclear. In this study, full-length cDNAs encoding four distinct UTR subtypes (UTR1, UTR3, UTR4, and UTR5) were isolated from the western clawed frog (Xenopus tropicalis). In functional analyses, homologous Xenopus UII stimulation of cells expressing UTR1 or UTR5 induced intracellular calcoum mobilization and phosphorylation of extracellular signal-regulated kinase 1/2. Cells expressing UTR3 or UTR4 did not show this response. Furthermore, UII induced the phosphorylation of cyclic adenosine monophosphate (cAMP) response element binding protein (CREB) through the UII-UTR1/5 system. However, intracellular cAMP accumulation was not observed, suggesting that UII-induced CREB phosphorylation is caused by a signaling pathway different from that involving Gs protein. In contrast, the administration of UII to cells increased the phosphorylation of guanine nucleotide exchange factor-H1 (GEF-H1) and myosin light chain 2 (MLC2) in all UTR subtypes. These results define four distinct UTR functional subtypes and are consistent with the molecular evolution of UTR subtypes in vertebrates. Further understanding of signaling properties associated with UTR subtypes may help in clarifying the functional roles associated with UII-UTR interactions in nonmammalian vertebrates.

Uveal melanoma: progress in molecular biology and therapeutics

Uveal melanoma (UM) is the most common intraocular malignancy in adults. So far, no systemic therapy or standard treatment exists to reduce the risk of metastasis and improve overall survival of patients. With the increased knowledge regarding the molecular pathways that underlie the oncogenesis of UM, it is expected that novel therapeutic approaches will be available to conquer this disease. This review provides a summary of the current knowledge of, and progress made in understanding, the pathogenesis, genetic mutations, epigenetics, and immunology of UM. With the advent of the omics era, multi-dimensional big data are publicly available, providing an innovation platform to develop effective targeted and personalized therapeutics for UM patients. Indeed, recently, a great number of therapies have been reported specifically for UM caused by oncogenic mutations, as well as other etiologies. In this review, special attention is directed to advancements in targeted therapies. In particular, we discuss the possibilities of targeting: GNAQ/GNA11, PLCβ, and CYSLTR2 mutants; regulators of G-protein signaling; the secondary messenger adenosine diphosphate (ADP)-ribosylation factor 6 (ARF6); downstream pathways, such as those involving mitogen-activated protein kinase/MEK/extracellular signal-related kinase, protein kinase C (PKC), phosphoinositide 3-kinase/Akt/mammalian target of rapamycin (mTOR), Trio/Rho/Rac/Yes-associated protein, and inactivated BAP1; and immune-checkpoint proteins cytotoxic T-lymphocyte antigen 4 and programmed cell-death protein 1/programmed cell-death ligand 1. Furthermore, we conducted a survey of completed and ongoing clinical trials applying targeted and immune therapies for UM. Although drug combination therapy based on the signaling pathways involved in UM has made great progress, targeted therapy is still an unmet medical need.

Histamine-induced biphasic activation of RhoA allows for persistent RhoA signaling

The small GTPase RhoA is a central signaling enzyme that is involved in various cellular processes such as cytoskeletal dynamics, transcription, and cell cycle progression. Many signal transduction pathways activate RhoA—for instance, Gα_q-coupled Histamine 1 Receptor signaling via Gα_q-dependent activation of RhoGEFs such as p63. Although multiple upstream regulators of RhoA have been identified, the temporal regulation of RhoA and the coordination of different upstream components in its regulation have not been well characterized. In this study, live-cell measurement of RhoA activation revealed a biphasic increase of RhoA activity upon histamine stimulation. We showed that the first and second phase of RhoA activity are dependent on p63 and Ca²⁺/PKC, respectively, and further identified phosphorylation of serine 240 on p115 RhoGEF by PKC to be the mechanistic link between PKC and RhoA. Combined approaches of computational modeling and quantitative measurement revealed that the second phase of RhoA activation is insensitive to rapid turning off of the receptor and is required for maintaining RhoA-mediated transcription after the termination of the receptor signaling. Thus, two divergent pathways enable both rapid activation and persistent signaling in receptor-mediated RhoA signaling via intricate temporal regulation.

The small GTPase RhoA is a central signaling enzyme that is involved in various cellular processes such as cytoskeletal dynamics, transcription, and cell cycle progression. This study shows that histamine induces biphasic activation of RhoA via two divergent signaling pathways, allowing for intricate regulation of cellular processes.

Leukemia-Associated Rho Guanine Nucleotide Exchange Factor and Ras Homolog Family Member C Play a Role in Glioblastoma Cell Invasion and Resistance

Glioblastoma (GBM) is the most common primary malignant brain cancer in adults. A hallmark of GBM is aggressive invasion of tumor cells into the surrounding normal brain. Both the current standard of care and targeted therapies have largely failed to specifically address this issue. Therefore, identifying key regulators of GBM cell migration and invasion is important. The leukemia-associated Rho guanine nucleotide exchange factor (LARG) has previously been implicated in cell invasion in other tumor types; however, its role in GBM pathobiology remains undefined. Herein, we report that the expression levels of LARG and ras homolog family members C (RhoC), and A (RhoA) increase with glial tumor grade and are highest in GBM. LARG and RhoC protein expression is more prominent in invading cells, whereas RhoA expression is largely restricted to cells in the tumor core. Knockdown of LARG by siRNA inhibits GBM cell migration in vitro and invasion ex vivo in organotypic brain slices. Moreover, siRNA-mediated silencing of RhoC suppresses GBM cell migration in vitro and invasion ex vivo, whereas depletion of RhoA enhances GBM cell migration and invasion, supporting a role for LARG and RhoC in GBM cell migration and invasion. Depletion of LARG increases the sensitivity of GBM cells to temozolomide treatment. Collectively, these results suggest that LARG and RhoC may represent unappreciated targets to inhibit glioma invasion.

Divergent C-terminal motifs in Gα12 and Gα13 provide distinct mechanisms of effector binding and SRF activation

The G12/13 subfamily of heterotrimeric guanine nucleotide binding proteins comprises the α subunits Gα12 and Gα13, which transduce signals for cell growth, cytoskeletal rearrangements, and oncogenic transformation. In an increasing range of cancers, overexpressed Gα12 or Gα13 are implicated in aberrant cell proliferation and/or metastatic invasion. Although Gα12 and Gα13 bind non-redundant sets of effector proteins and participate in unique signalling pathways, the structural features responsible for functional differences between these α subunits are largely unknown. Invertebrates encode a single G12/13 homolog that participates in cytoskeletal changes yet appears to lack signalling to SRF (serum response factor), a transcriptional activator stimulated by mammalian Gα12 and Gα13 to promote growth and tumorigenesis. Our previous studies identified an evolutionarily divergent region in Gα12 for which replacement by homologous sequence from Drosophila melanogaster abolished SRF signalling, whereas the same invertebrate substitution was fully tolerated in Gα13 [Montgomery et al. (2014) Mol. Pharmacol. 85: 586]. These findings prompted our current approach of evolution-guided mutagenesis to identify fine structural features of Gα12 and Gα13 that underlie their respective SRF activation mechanisms. Our results identified two motifs flanking the α4 helix that play a key role in Gα12 signalling to SRF. We found the region encompassing these motifs to provide an interacting surface for multiple Gα12-specific target proteins that fail to bind Gα13. Adjacent to this divergent region, a highly-conserved domain was vital for SRF activation by both Gα12 and Gα13. However, dissection of this domain using invertebrate substitutions revealed different signalling mechanisms in these α subunits and identified Gα13-specific determinants of binding Rho-specific guanine nucleotide exchange factors. Furthermore, invertebrate substitutions in the C-terminal, α5 helical region were selectively disruptive to Gα12 signalling. Taken together, our results identify key structural features near the C-terminus that evolved after the divergence of Gα12 and Gα13, and should aid the development of agents to selectively manipulate signalling by individual α subunits of the G12/13 subfamily.

ILK supports RhoA/ROCK-mediated contractility of human intestinal epithelial crypt cells by inducing the fibrillogenesis of endogenous soluble fibronectin during the spreading process

BACKGROUND

Fibronectin (FN) assembly into an insoluble fibrillar matrix is a crucial step in many cell responses to extracellular matrix (ECM) properties, especially with regards to the integrin-related mechanosensitive signaling pathway. We have previously reported that the silencing of expression of integrin-linked kinase (ILK) in human intestinal epithelial crypt (HIEC) cells causes significant reductions in proliferation and spreading through concomitantly acquired impairment of soluble FN deposition. These defects in ILK-depleted cells are rescued by growth on exogenous FN. In the present study we investigated the contribution of ILK in the fibrillogenesis of FN and its relation to integrin-actin axis signaling and organization.

RESULTS

We show that de novo fibrillogenesis of endogenous soluble FN is ILK-dependent. This function seemingly induces the assembly of an ECM that supports increased cytoskeletal tension and the development of a fully spread contractile cell phenotype. We observed that HIEC cell adhesion to exogenous FN or collagen-I (Col-I) is sufficient to restore fibrillogenesis of endogenous FN in ILK-depleted cells. We also found that optimal engagement of the Ras homolog gene family member A (RhoA) GTPase/Rho-associated kinase (ROCK-1, ROCK-2)/myosin light chain (MLC) pathway, actin ventral stress fiber formation, and integrin adhesion complex (IAC) maturation rely primarily upon the cell's capacity to execute FN fibrillogenesis, independent of any significant ILK input. Lastly, we confirm the integrin α5β1 as the main integrin responsible for FN assembly, although in ILK-depleted cells αV-class integrins expression is needed to allow the rescue of FN fibrillogenesis on exogenous substrate.

CONCLUSION

Our study demonstrates that ILK specifically induces the initiation of FN fibrillogenesis during cell spreading, which promotes RhoA/ROCK-dependent cell contractility and maturation of the integrin-actin axis structures. However, the fibrillogenesis process and its downstream effect on RhoA signaling, cell contractility and spreading are ILK-independent in human intestinal epithelial crypt cells.

Transcriptional and posttranscriptional regulation of the SMC-selective blood pressure-associated gene, ARHGAP42

We previously showed that ARHGAP42 is a smooth muscle cell (SMC)-selective, RhoA-specific GTPase activating protein that regulates blood pressure and that a minor allele single nucleotide variation within a DNAse hypersensitive regulatory element in intron1 (Int1DHS) increased ARHGAP42 expression by promoting serum response factor binding. The goal of the current study was to identify additional transcriptional and posttranscriptional mechanisms that control ARHGAP42 expression. Using deletion/mutation, gel shift, and chromatin immunoprecipitation experiments, we showed that recombination signal binding protein for immunoglobulin κ-J region (RBPJ) and TEA domain family member 1 (TEAD1) binding to a conserved core region was required for full IntDHS transcriptional activity. Importantly, overexpression of the notch intracellular domain (NICD) or plating SMCs on recombinant jagged-1 increased IntDHS activity and endogenous ARHGAP42 expression while siRNA-mediated knockdown of TEAD1 inhibited ARHGAP42 mRNA levels. Re-chromatin immunoprecipitation experiments indicated that RBPJ and TEAD1 were bound to the Int1DHS enhancer at the same time, and coimmunoprecipitation assays indicated that these factors interacted physically. Our results also suggest TEAD1 and RBPJ bound cooperatively to the Int1DHS and that the presence of TEAD1 promoted the recruitment of NICD by RBPJ. Finally, we showed that ARHGAP42 expression was inhibited by micro-RNA 505 (miR505) which interacted with the ARHGAP42 3'-untranslated region (UTR) to facilitate its degradation and by AK124326, a long noncoding RNA that overlaps with the ARHGAP42 transcription start site on the opposite DNA strand. Since siRNA-mediated depletion of AK124326 was associated with increased H3K9 acetylation and RNA Pol-II binding at the ARHGAP42 gene, it is likely that AK124326 inhibits ARHGAP42 transcription.NEW & NOTEWORTHY First, RBPJ and TEAD1 converge at an intronic enhancer to regulate ARHGAP42 expression in SMCs. Second, TEAD1 and RBPJ interact physically and bind cooperatively to the ARHGAP42 enhancer. Third, miR505 interacts with the ARHGAP42 3'-UTR to facilitate its degradation. Finally, LncRNA, AK124326, inhibits ARHGAP42 transcription.

Nucleotide exchange-dependent and nucleotide exchange-independent functions of plant heterotrimeric GTP-binding proteins

Heterotrimeric guanine nucleotide-binding proteins (G proteins), which are composed of α, β, and γ subunits, are versatile, guanine nucleotide-dependent, molecular on-off switches. In animals and fungi, the exchange of GDP for GTP on Gα controls G protein activation and is crucial for normal cellular responses to diverse extracellular signals. The model plant Arabidopsis thaliana has a single canonical Gα subunit, AtGPA1. We found that, in planta, the constitutively active, GTP-bound AtGPA1(Q222L) mutant and the nucleotide-free AtGPA1(S52C) mutant interacted with Gβγ1 and Gβγ2 dimers with similar affinities, suggesting that G protein heterotrimer formation occurred independently of nucleotide exchange. In contrast, AtGPA1(Q222L) had a greater affinity than that of AtGPA1(S52C) for Gβγ3, suggesting that the GTP-bound conformation of AtGPA1(Q222L) is distinct and tightly associated with Gβγ3. Functional analysis of transgenic lines expressing either AtGPA1(S52C) or AtGPA1(Q222L) in the gpa1-null mutant background revealed various mutant phenotypes that were complemented by either AtGPA1(S52C) or AtGPA1(Q222L). We conclude that, in addition to the canonical GDP-GTP exchange-dependent mechanism, plant G proteins can function independently of nucleotide exchange.

Interconnecting Flexibility, Structural Communication, and Function in RhoGEF Oncoproteins

Dbl family Rho guanine nucleotide exchange factors (RhoGEFs) play a central role in cell biology by catalyzing the exchange of guanosine 5'-triphosphate for guanosine 5'-diphosphate (GDP) on RhoA. Insights into the oncogenic constitutive activity of the Lbc RhoGEF were gained by analyzing the structure and dynamics of the protein in different functional states and in comparison with a close homologue, leukemia-associated RhoGEF. Higher intrinsic flexibility, less dense and extended structure network, and less stable allosteric communication pathways in Lbc, compared to the nonconstitutively active homologue, emerged as major determinants of the constitutive activity. Independent of the state, the essential dynamics of the two RhoGEFs is contributed by the last 10 amino acids of Dbl homology (DH) and the whole pleckstrin homology (PH) domains and tends to be equalized by the presence of RhoA. The catalytic activity of the RhoGEF relies on the scaffolding action of the DH domain that primarily turns the switch I (SWI) of RhoA on itself through highly conserved amino acids participating in the stability core and essential for function. Changes in the conformation of SWI and disorganization of the RhoA regions deputed to nucleotide binding are among the major RhoGEF effects leading to GDP release. Binding of RhoA reorganizes the allosteric communication on RhoGEF, strengthening the communication among the canonical RhoA binding site on DH, a secondary RhoA binding site on PH, and the binding site for heterotrimeric G proteins, suggesting dual roles for RhoA as a catalysis substrate and as a regulatory protein. The structure network-based analysis tool employed in this study proved to be useful for predicting potentially druggable regulatory sites in protein structures.

Distinct RhoGEFs Activate Apical and Junctional Contractility under Control of G Proteins during Epithelial Morphogenesis

Small RhoGTPases direct cell shape changes and movements during tissue morphogenesis. Their activities are tightly regulated in space and time to specify the desired pattern of actomyosin contractility that supports tissue morphogenesis. This is expected to stem from polarized surface stimuli and from polarized signaling processing inside cells. We examined this general problem in the context of cell intercalation that drives extension of the Drosophila ectoderm. In the ectoderm, G protein-coupled receptors (GPCRs) and their downstream heterotrimeric G proteins (Gα and Gβγ) activate Rho1 both medial-apically, where it exhibits pulsed dynamics, and at junctions, where its activity is planar polarized. However, the mechanisms responsible for polarizing Rho1 activity are unclear. We report that distinct guanine exchange factors (GEFs) activate Rho1 in these two cellular compartments. RhoGEF2 acts uniquely to activate medial-apical Rho1 but is recruited both medial-apically and at junctions by Gα_12/13-GTP, also called Concertina (Cta) in Drosophila. On the other hand, Dp114RhoGEF (Dp114), a newly characterized RhoGEF, is required for cell intercalation in the extending ectoderm, where it activates Rho1 specifically at junctions. Its localization is restricted to adherens junctions and is under Gβ13F/Gγ1 control. Furthermore, Gβ13F/Gγ1 activates junctional Rho1 and exerts quantitative control over planar polarization of Rho1. Finally, we found that Dp114RhoGEF is absent in the mesoderm, arguing for a tissue-specific control over junctional Rho1 activity. These results clarify the mechanisms of polarization of Rho1 activity in different cellular compartments and reveal that distinct GEFs are sensitive tuning parameters of cell contractility in remodeling epithelia.

•

Dp114RhoGEF activates junctional Rho1 and is involved in cell intercalation

•

Gα/Cta and Gβγ subunits tune, respectively, RhoGEF2 and Dp114RhoGEF membrane levels

•

Gβγ subunits control planar polarity of junctional Rho1 signaling via Dp114RhoGEF

•

Tissue-specific RhoGEFs could diversify morphogenesis in different tissues

Identification of guanine nucleotide exchange factors that increase Cdc42 activity in primary human endothelial cells

The Rho GTPase family is involved in actin dynamics and regulates the barrier function of the endothelium. One of the main barrier-promoting Rho GTPases is Cdc42, also known as cell division control protein 42 homolog. Currently, regulation of Cdc42-based signalling networks in endothelial cells (ECs) lack molecular details. To examine these, we focused on a subset of 15 Rho guanine nucleotide exchange factors (GEFs), which are expressed in the endothelium. By performing single cell FRET measurements with Rho GTPase biosensors in primary human ECs, we monitored GEF efficiency towards Cdc42 and Rac1. A new, single cell-based analysis was developed and used to enable the quantitative comparison of cellular activities of the overexpressed full-length GEFs. Our data reveal GEF dependent activation of Cdc42, with the most efficient Cdc42 activation induced by PLEKHG2, FGD1, PLEKHG1 and PREX1 and the highest selectivity for FGD1. Additionally, we generated truncated GEF constructs that comprise only the catalytic dbl homology (DH) domain or together with the adjacent pleckstrin homology domain (DHPH). The DH domain by itself did not activate Cdc42, whereas the DHPH domain of ITSN1, ITSN2 and PLEKHG1 showed activity towards Cdc42. Together, our study characterized endothelial GEFs that may directly or indirectly activate Cdc42, which will be of great value for the field of vascular biology.

Structural design principles that underlie the multi-specific interactions of Gαq with dissimilar partners

Gα_q is a ubiquitous molecular switch that activates the effectors phospholipase-C-β3 (PLC-β3) and Rho guanine-nucleotide exchange factors. Gα_q is inactivated by regulators of G protein signaling proteins, as well as by PLC-β3. Gα_q further interacts with G protein-coupled receptor kinase 2 (GRK2), although the functional role of this interaction is debated. While X-ray structures of Gα_q bound to representatives of these partners have revealed details of their interactions, the mechanistic basis for differential Gα_q interactions with multiple partners (i.e., Gα_q multi-specificity) has not been elucidated at the individual residue resolution. Here, we map the structural determinants of Gα_q multi-specificity using structure-based energy calculations. We delineate regions that specifically interact with GTPase Activating Proteins (GAPs) and residues that exclusively contribute to effector interactions, showing that only the Gα_q “Switch II” region interacts with all partners. Our analysis further suggests that Gα_q-GRK2 interactions are consistent with GRK2 functioning as an effector, rather than a GAP. Our multi-specificity analysis pinpoints Gα_q residues that uniquely contribute to interactions with particular partners, enabling precise manipulation of these cascades. As such, we dissect the molecular basis of Gα_q function as a central signaling hub, which can be used to target Gα_q-mediated signaling in therapeutic interventions.

GPCR-independent activation of G proteins promotes apical cell constriction in vivo

Heterotrimeric G proteins are signaling switches that control organismal morphogenesis across metazoans. In invertebrates, specific GPCRs instruct G proteins to promote collective apical cell constriction in the context of epithelial tissue morphogenesis. In contrast, tissue-specific factors that instruct G proteins during analogous processes in vertebrates are largely unknown. Here, we show that DAPLE, a non-GPCR protein linked to human neurodevelopmental disorders, is expressed specifically in the neural plate of Xenopus laevis embryos to trigger a G protein signaling pathway that promotes apical cell constriction during neurulation. DAPLE localizes to apical cell-cell junctions in the neuroepithelium, where it activates G protein signaling to drive actomyosin-dependent apical constriction and subsequent bending of the neural plate. This function is mediated by a Gα-binding-and-activating (GBA) motif that was acquired by DAPLE in vertebrates during evolution. These findings reveal that regulation of tissue remodeling during vertebrate development can be driven by an unconventional mechanism of heterotrimeric G protein activation that operates in lieu of GPCRs.

Structure of the C-terminal guanine nucleotide exchange factor module of Trio in an autoinhibited conformation reveals its oncogenic potential

The C-terminal guanine nucleotide exchange factor (GEF) module of Trio (TrioC) transfers signals from the Gα_q/11 subfamily of heterotrimeric G proteins to the small guanosine triphosphatase (GTPase) RhoA, enabling Gα_q/11-coupled G protein-coupled receptors (GPCRs) to control downstream events, such as cell motility and gene transcription. This conserved signal transduction axis is crucial for tumor growth in uveal melanoma. Previous studies indicate that the GEF activity of the TrioC module is autoinhibited, with release of autoinhibition upon Gα_q/11 binding. Here, we determined the crystal structure of TrioC in its basal state and found that the pleckstrin homology (PH) domain interacts with the Dbl homology (DH) domain in a manner that occludes the Rho GTPase binding site, thereby suggesting the molecular basis of TrioC autoinhibition. Biochemical and biophysical assays revealed that disruption of the autoinhibited conformation destabilized and activated the TrioC module in vitro. Last, mutations in the DH-PH interface found in patients with cancer activated TrioC and, in the context of full-length Trio, led to increased abundance of guanosine triphosphate-bound RhoA (RhoA·GTP) in human cells. These mutations increase mitogenic signaling through the RhoA axis and, therefore, may represent cancer drivers operating in a Gα_q/11-independent manner.

Structural and biochemical characterization of the pleckstrin homology domain of the RhoGEF P-Rex2 and its regulation by PIP3

P-Rex family Rho guanine-nucleotide exchange factors are important regulators of cell motility through their activation of a subset of small GTPases. Both P-Rex1 and P-Rex2 have also been implicated in the progression of certain cancers, including breast cancer and melanoma. Although these molecules display a high level of homology, differences exist in tissue distribution, physiological function, and regulation at the molecular level. Here, we sought to compare the P-Rex2 pleckstrin homology (PH) domain structure and ability to interact with PIP₃ with those of P-Rex1. The 1.9 Å crystal structure of the P-Rex2 PH domain reveals conformational differences in the loop regions, yet biochemical studies indicate that the interaction of the P-Rex2 PH domain with PIP₃ is very similar to that of P-Rex1. Binding of the PH domain to PIP₃ is critical for P-Rex2 activity but not membrane localization, as previously demonstrated for P-Rex1. These studies serve as a starting point in the identification of P-Rex structural features that are divergent between isoforms and could be exploited for the design of P-Rex selective compounds.

Gβγ signaling to the chemotactic effector P-REX1 and mammalian cell migration is directly regulated by Gαq and Gα13 proteins

G protein-coupled receptors stimulate Rho guanine nucleotide exchange factors that promote mammalian cell migration. Rac and Rho GTPases exert opposing effects on cell morphology and are stimulated downstream of Gβγ and Gα_12/13 or Gα_q, respectively. These Gα subunits might in turn favor Rho pathways by preventing Gβγ signaling to Rac. Here, we investigated whether Gβγ signaling to phosphatidylinositol 3,4,5-trisphosphate-dependent Rac exchange factor 1 (P-REX1), a key Gβγ chemotactic effector, is directly controlled by Rho-activating Gα subunits. We show that pharmacological inhibition of Gα_q makes P-REX1 activation by G_q/G_i-coupled lysophosphatidic acid receptors more effective. Moreover, chemogenetic control of G_i and G_q by designer receptors exclusively activated by designer drugs (DREADDs) confirmed that G_i differentially activates P-REX1. GTPase-deficient Gα_qQL and Gα₁₃QL variants formed stable complexes with Gβγ, impairing its interaction with P-REX1. The N-terminal regions of these variants were essential for stable interaction with Gβγ. Pulldown assays revealed that chimeric Gα_13-i2QL interacts with Gβγ unlike to Gα_i2-13QL, the reciprocal chimera, which similarly to Gα_i2QL could not interact with Gβγ. Moreover, Gβγ was part of tetrameric Gβγ-Gα_qQL-RGS2 and Gβγ-Gα_13-i2QL-RGS4 complexes, whereas Gα₁₃QL dissociated from Gβγ to interact with the PDZ-RhoGEF-RGS domain. Consistent with an integrated response, Gβγ and AKT kinase were associated with active SDF-1/CXCL12-stimulated P-REX1. This pathway was inhibited by Gα_qQL and Gα₁₃QL, which also prevented CXCR4-dependent cell migration. We conclude that a coordinated mechanism prioritizes Gα_q- and Gα₁₃-mediated signaling to Rho over a Gβγ-dependent Rac pathway, attributed to heterotrimeric G_i proteins.

Atypical activation of the G protein Gαq by the oncogenic mutation Q209P

The causative role of G protein-coupled receptor (GPCR) pathway mutations in uveal melanoma (UM) has been well-established. Nearly all UMs bear an activating mutation in a GPCR pathway mediated by G proteins of the G_q/11 family, driving tumor initiation and possibly metastatic progression. Thus, targeting this pathway holds therapeutic promise for managing UM. However, direct targeting of oncogenic Gα_q/11 mutants, present in ∼90% of UMs, is complicated by the belief that these mutants structurally resemble active Gα_q/11 WT. This notion is solidly founded on previous studies characterizing Gα mutants in which a conserved catalytic glutamine (Gln-209 in Gα_q) is replaced by leucine, which leads to GTPase function deficiency and constitutive activation. Whereas Q209L accounts for approximately half of GNAQ mutations in UM, Q209P is as frequent as Q209L and also promotes oncogenesis, but has not been characterized at the molecular level. Here, we characterized the biochemical and signaling properties of Gα_q Q209P and found that it is also GTPase-deficient and activates downstream signaling as efficiently as Gα_q Q209L. However, Gα_q Q209P had distinct molecular and functional features, including in the switch II region of Gα_q Q209P, which adopted a conformation different from that of Gα_q Q209L or active WT Gα_q, resulting in altered binding to effectors, Gβγ, and regulators of G-protein signaling (RGS) proteins. Our findings reveal that the molecular properties of Gα_q Q209P are fundamentally different from those in other active Gα_q proteins and could be leveraged as a specific vulnerability for the ∼20% of UMs bearing this mutation.

Targeting G protein-coupled receptor signalling by blocking G proteins

G protein-coupled receptors (GPCRs) are the largest class of drug targets, largely owing to their druggability, diversity and physiological efficacy. Many drugs selectively target specific subtypes of GPCRs, but high specificity for individual GPCRs may not be desirable in complex multifactorial disease states in which multiple receptors may be involved. One approach is to target G protein subunits rather than the GPCRs directly. This approach has the potential to achieve broad efficacy by blocking pathways shared by multiple GPCRs. Additionally, because many GPCRs couple to multiple G protein signalling pathways, blocking specific G protein subunits can 'bias' GPCR signals by inhibiting only a subset of these signals. Molecules that target G protein α or βγ-subunits have been developed and show strong efficacy in multiple preclinical disease models and biased inhibition of G protein signalling. In this Review, we discuss the development and characterization of G protein α and βγ-subunit ligands and the preclinical evidence that this exciting new approach has potential for therapeutic efficacy in a number of indications, such as pain, thrombosis, asthma and heart failure.

A FRET-based biosensor for measuring Gα13 activation in single cells

Förster Resonance Energy Transfer (FRET) provides a way to directly observe the activation of heterotrimeric G-proteins by G-protein coupled receptors (GPCRs). To this end, FRET based biosensors are made, employing heterotrimeric G-protein subunits tagged with fluorescent proteins. These FRET based biosensors complement existing, indirect, ways to observe GPCR activation. Here we report on the insertion of mTurquoise2 at several sites in the human Gα13 subunit, aiming to develop a FRET-based Gα13 activation biosensor. Three fluorescently tagged Gα13 variants were found to be functional based on i) plasma membrane localization and ii) ability to recruit p115-RhoGEF upon activation of the LPA2 receptor. The tagged Gα13 subunits were used as FRET donor and combined with cp173Venus fused to the Gγ2 subunit, as the acceptor. We constructed Gα13 biosensors by generating a single plasmid that produces Gα13-mTurquoise2, Gβ1 and cp173Venus-Gγ2. The Gα13 activation biosensors showed a rapid and robust response when used in primary human endothelial cells that were exposed to thrombin, triggering endogenous protease activated receptors (PARs). This response was efficiently inhibited by the RGS domain of p115-RhoGEF and from the biosensor data we inferred that this is due to GAP activity. Finally, we demonstrated that the Gα13 sensor can be used to dissect heterotrimeric G-protein coupling efficiency in single living cells. We conclude that the Gα13 biosensor is a valuable tool for live-cell measurements that probe spatiotemporal aspects of Gα13 activation.

Rho GTPase signaling complexes in cell migration and invasion

Cell migration is dependent on the dynamic formation and disassembly of actin filament-based structures, including lamellipodia, filopodia, invadopodia, and membrane blebs, as well as on cell-cell and cell-extracellular matrix adhesions. These processes all involve Rho family small guanosine triphosphatases (GTPases), which are regulated by the opposing actions of guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Rho GTPase activity needs to be precisely tuned at distinct cellular locations to enable cells to move in response to different environments and stimuli. In this review, we focus on the ability of RhoGEFs and RhoGAPs to form complexes with diverse binding partners, and describe how this influences their ability to control localized GTPase activity in the context of migration and invasion.

An AKAP-Lbc-RhoA interaction inhibitor promotes the translocation of aquaporin-2 to the plasma membrane of renal collecting duct principal cells

Stimulation of renal collecting duct principal cells with antidiuretic hormone (arginine-vasopressin, AVP) results in inhibition of the small GTPase RhoA and the enrichment of the water channel aquaporin-2 (AQP2) in the plasma membrane. The membrane insertion facilitates water reabsorption from primary urine and fine-tuning of body water homeostasis. Rho guanine nucleotide exchange factors (GEFs) interact with RhoA, catalyze the exchange of GDP for GTP and thereby activate the GTPase. However, GEFs involved in the control of AQP2 in renal principal cells are unknown. The A-kinase anchoring protein, AKAP-Lbc, possesses GEF activity, specifically activates RhoA, and is expressed in primary renal inner medullary collecting duct principal (IMCD) cells. Through screening of 18,431 small molecules and synthesis of a focused library around one of the hits, we identified an inhibitor of the interaction of AKAP-Lbc and RhoA. This molecule, Scaff10-8, bound to RhoA, inhibited the AKAP-Lbc-mediated RhoA activation but did not interfere with RhoA activation through other GEFs or activities of other members of the Rho family of small GTPases, Rac1 and Cdc42. Scaff10-8 promoted the redistribution of AQP2 from intracellular vesicles to the periphery of IMCD cells. Thus, our data demonstrate an involvement of AKAP-Lbc-mediated RhoA activation in the control of AQP2 trafficking.

Paving the Rho in cancer metastasis: Rho GTPases and beyond

Malignant carcinomas are often characterized by metastasis, the movement of carcinoma cells from a primary site to colonize distant organs. For metastasis to occur, carcinoma cells first must adopt a pro-migratory phenotype and move through the surrounding stroma towards a blood or lymphatic vessel. Currently, there are very limited possibilities to target these processes therapeutically. The family of Rho GTPases is an ubiquitously expressed division of GTP-binding proteins involved in the regulation of cytoskeletal dynamics and intracellular signaling. The best characterized members of the Rho family GTPases are RhoA, Rac1 and Cdc42. Abnormalities in Rho GTPase function have major consequences for cancer progression. Rho GTPase activation is driven by cell surface receptors that activate GTP exchange factors (GEFs) and GTPase-activating proteins (GAPs). In this review, we summarize our current knowledge on Rho GTPase function in the regulation of metastasis. We will focus on key discoveries in the regulation of epithelial-mesenchymal-transition (EMT), cell-cell junctions, formation of membrane protrusions, plasticity of cell migration and adaptation to a hypoxic environment. In addition, we will emphasize on crosstalk between Rho GTPase family members and other important oncogenic pathways, such as cyclic AMP-mediated signaling, canonical Wnt/β-catenin, Yes-associated protein (YAP) and hypoxia inducible factor 1α (Hif1α) and provide an overview of the advancements and challenges in developing pharmacological tools to target Rho GTPase and the aforementioned crosstalk in the context of cancer therapeutics.

Cell adhesion controlled by adhesion G protein-coupled receptor GPR124/ADGRA2 is mediated by a protein complex comprising intersectins and Elmo-Dock

Developmental angiogenesis and the maintenance of the blood-brain barrier involve endothelial cell adhesion, which is linked to cytoskeletal dynamics. GPR124 (also known as TEM5/ADGRA2) is an adhesion G protein-coupled receptor family member that plays a pivotal role in brain angiogenesis and in ensuring a tight blood-brain barrier. However, the signaling properties of GPR124 remain poorly defined. Here, we show that ectopic expression of GPR124 promotes cell adhesion, additive to extracellular matrix-dependent effect, coupled with filopodia and lamellipodia formation and an enrichment of a pool of the G protein-coupled receptor at actin-rich cellular protrusions containing VASP, a filopodial marker. Accordingly, GPR124-expressing cells also displayed increased activation of both Rac and Cdc42 GTPases. Mechanistically, we uncover novel direct interactions between endogenous GPR124 and the Rho guanine nucleotide exchange factors Elmo/Dock and intersectin (ITSN). Small fragments of either Elmo or ITSN1 that bind GPR124 blocked GPR124-induced cell adhesion. In addition, Gβγ interacts with the C-terminal tail of GPR124 and promotes the formation of a GPR124-Elmo complex. Furthermore, GPR124 also promotes the activation of the Elmo-Dock complex, as measured by Elmo phosphorylation on a conserved C-terminal tyrosine residue. Interestingly, Elmo and ITSN1 also interact with each other independently of their GPR124-recognition regions. Moreover, endogenous phospho-Elmo and ITSN1 co-localize with GPR124 at lamellipodia of adhering endothelial cells, where GPR124 expression contributes to polarity acquisition during wound healing. Collectively, our results indicate that GPR124 promotes cell adhesion via Elmo-Dock and ITSN. This constitutes a previously unrecognized complex formed of atypical and conventional Rho guanine nucleotide exchange factors for Rac and Cdc42 that is putatively involved in GPR124-dependent angiogenic responses.

Potentiation of receptor responses induced by prolonged binding of Gα13 and leukemia-associated RhoGEF

Diverse cellular functions are controlled by RhoA-GTPases, which are activated by trimeric G proteins via RhoGEFs, among others. In this study, we focused on the signaling from GPCRs to RhoA via Gα₁₃ and leukemia-associated RhoGEF (LARG). The activation of Gα₁₃ was elucidated in living cells with high temporal and spatial resolution by means of FRET. The inactivation after agonist withdrawal occurred in the same range (t_1/2 = 25.3 ± 2.2 s; mean ± sem; n = 22) as described for other Gα proteins. The interaction of Gα₁₃ and LARG and the thereby-induced LARG translocation to the plasma membrane were at least 1 order of magnitude more stable after agonist withdrawal, exceeding Gα₁₃ deactivation in the absence of LARG several fold. Consequently, we observed an almost 100-fold higher agonist sensitivity of the Gα₁₃ LARG interaction compared to the Gα₁₃ activation in the absence of LARG.-Bodmann, E.-L., Krett, A.-L., Bünemann, M. Potentiation of receptor responses induced by prolonged binding of Gα₁₃ and leukemia-associated RhoGEF.

Heterotrimeric G protein Gαs subunit attenuates PLEKHG2, a Rho family-specific guanine nucleotide exchange factor, by direct interaction

PLEKHG2 is a Gβγ-dependent guanine nucleotide exchange factor (GEF) for the small GTPases Rac and Cdc42, and has been shown to mediate signalling pathways such as actin cytoskeleton reorganization and serum response element (SRE)-dependent gene transcription. Here we show that the constitutively active mutant of the Gαs subunit significantly attenuated PLEKHG2-induced SRE-mediated gene transcription. Strikingly, we observed that the constitutive activation of endogenous Gαs by treatment with CTx caused a similar inhibitory effect on PLEKHG2-induced activation of SRE. However, both the enforced expression of the catalytic subunit β of protein kinase A and the treatment with dibutyl-cyclic AMP failed to mimic the inhibitory effect of Gαs on PLEKHG2. Furthermore, the dominant negative mutant of protein kinase A had no effect on PLEKHG2-mediated SRE activation. Performing immunoprecipitation and an in vitro pulldown assay, we found that PLEKHG2 directly interacted with the active form of the Gαs subunit in cells. The interaction between PLEKHG2 and Gαs required the N-terminal region of PLEKHG2, which includes the DH domain, a functional domain of GEF, suggesting that Gαs directly masks the DH domain of PLEKHG2. In a previous study, we reported that Gβγ accelerates PLEKHG2-mediated SRE-dependent gene transcription. Interestingly, Gαs also inhibited the hyperactivation of SRE induced by the co-expression of Gβγ and PLEKHG2; however, Gαs and Gβγ bind to different regions of PLEKHG2. This is the first report to show that PLEKHG2 is a novel effector of Gαs, and is negatively regulated by the Gαs subunit through direct interaction.

Kinetics of recruitment and allosteric activation of ARHGEF25 isoforms by the heterotrimeric G-protein Gαq

Rho GTPases are master regulators of the eukaryotic cytoskeleton. The activation of Rho GTPases is governed by Rho guanine nucleotide exchange factors (GEFs). Three RhoGEF isoforms are produced by the gene ARHGEF25; p63RhoGEF⁵⁸⁰, GEFT and a recently discovered longer isoform of 619 amino acids (p63RhoGEF⁶¹⁹). The subcellular distribution of p63RhoGEF⁵⁸⁰ and p63RhoGEF⁶¹⁹ is strikingly different in unstimulated cells, p63RhoGEF⁵⁸⁰ is located at the plasma membrane and p63RhoGEF⁶¹⁹ is confined to the cytoplasm. Interestingly, we find that both P63RhoGEF⁵⁸⁰ and p63RhoGEF⁶¹⁹ activate RhoGTPases to a similar extent after stimulation of Gαq coupled GPCRs. Furthermore, we show that p63RhoGEF⁶¹⁹ relocates to the plasma membrane upon activation of Gαq coupled GPCRs, resembling the well-known activation mechanism of RhoGEFs activated by Gα_12/13. Synthetic recruitment of p63RhoGEF⁶¹⁹ to the plasma membrane increases RhoGEF activity towards RhoA, but full activation requires allosteric activation via Gαq. Together, these findings reveal a dual role for Gαq in RhoGEF activation, as it both recruits and allosterically activates cytosolic ARHGEF25 isoforms.

Potent and Selective Peptide-based Inhibition of the G Protein Gαq

In contrast to G protein-coupled receptors, for which chemical and peptidic inhibitors have been extensively explored, few compounds are available that directly modulate heterotrimeric G proteins. Active Gα_q binds its two major classes of effectors, the phospholipase C (PLC)-β isozymes and Rho guanine nucleotide exchange factors (RhoGEFs) related to Trio, in a strikingly similar fashion: a continuous helix-turn-helix of the effectors engages Gα_q within its canonical binding site consisting of a groove formed between switch II and helix α3. This information was exploited to synthesize peptides that bound active Gα_q in vitro with affinities similar to full-length effectors and directly competed with effectors for engagement of Gα_q A representative peptide was specific for active Gα_q because it did not bind inactive Gα_q or other classes of active Gα subunits and did not inhibit the activation of PLC-β3 by Gβ₁γ₂ In contrast, the peptide robustly prevented activation of PLC-β3 or p63RhoGEF by Gα_q; it also prevented G protein-coupled receptor-promoted neuronal depolarization downstream of Gα_q in the mouse prefrontal cortex. Moreover, a genetically encoded form of this peptide flanked by fluorescent proteins inhibited Gα_q-dependent activation of PLC-β3 at least as effectively as a dominant-negative form of full-length PLC-β3. These attributes suggest that related, cell-penetrating peptides should effectively inhibit active Gα_q in cells and that these and genetically encoded sequences may find application as molecular probes, drug leads, and biosensors to monitor the spatiotemporal activation of Gα_q in cells.

Small-Molecule Protein-Protein Interaction Inhibitor of Oncogenic Rho Signaling

Uncontrolled activation of Rho signaling by RhoGEFs, in particular AKAP13 (Lbc) and its close homologs, is implicated in a number of human tumors with poor prognosis and resistance to therapy. Structure predictions and alanine scanning mutagenesis of Lbc identified a circumscribed hot region for RhoA recognition and activation. Virtual screening targeting that region led to the discovery of an inhibitor of Lbc-RhoA interaction inside cells. By interacting with the DH domain, the compound inhibits the catalytic activity of Lbc, halts cellular responses to activation of oncogenic Lbc pathways, and reverses a number of prostate cancer cell phenotypes such as proliferation, migration, and invasiveness. This study provides insights into the structural determinants of Lbc-RhoA recognition. This is a successful example of structure-based discovery of a small protein-protein interaction inhibitor able to halt oncogenic Rho signaling in cancer cells with therapeutic implications.