Tgat, a Rho-specific guanine nucleotide exchange factor, activates NF-kappaB via physical association with IkappaB kinase complexes

The RhoGEF Trio: A Protein with a Wide Range of Functions in the Vascular Endothelium

Many cellular processes are controlled by small GTPases, which can be activated by guanine nucleotide exchange factors (GEFs). The RhoGEF Trio contains two GEF domains that differentially activate the small GTPases such as Rac1/RhoG and RhoA. These small RhoGTPases are mainly involved in the remodeling of the actin cytoskeleton. In the endothelium, they regulate junctional stabilization and play a crucial role in angiogenesis and endothelial barrier integrity. Multiple extracellular signals originating from different vascular processes can influence the activity of Trio and thereby the regulation of the forementioned small GTPases and actin cytoskeleton. This review elucidates how various signals regulate Trio in a distinct manner, resulting in different functional outcomes that are crucial for endothelial cell function in response to inflammation.

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.

The C-terminus of the oncoprotein TGAT is necessary for plasma membrane association and efficient RhoA-mediated signaling

Background

Rho guanine exchange factors (RhoGEFs) control cellular processes such as migration, adhesion and proliferation. Alternative splicing of the RhoGEF Trio produces TGAT. The RhoGEF TGAT is an oncoprotein with constitutive RhoGEF activity. We investigated whether the subcellular location of TGAT is critical for its RhoGEF activity.

Methods

Since plasma membrane associated RhoGEFs are particularly effective at activating RhoA, plasma membrane localization of TGAT was examined. To this end, we developed a highly sensitive image analysis method to quantitatively measure plasma membrane association. The method requires a cytoplasmic marker and a plasma membrane marker, which are co-imaged with the tagged protein of interest. Linear unmixing is performed to determine the plasma membrane and cytoplasmic component in the fluorescence signal of protein of interest.

Results

The analysis revealed that wild-type TGAT is partially co-localized with the plasma membrane. Strikingly, cysteine TGAT-mutants lacking one or more putative palmitoylation sites in the C-tail, still showed membrane association. In contrast, a truncated variant, lacking the last 15 amino acids, TGAT^Δ15, lost membrane association. We show that membrane localization of TGAT was responsible for high RhoGEF activity by using a RhoA FRET-sensor and by determining F-actin levels. Mutants of TGAT that still maintained membrane association showed similar activity as wild-type TGAT. In contrast, the activity was abrogated for the cytoplasmic TGAT^Δ15 variant. Synthetic recruitment of TGAT^Δ15 to membranes confirmed that TGAT effectively activates RhoA at the plasma membrane.

Conclusion

Together, these results show that membrane association of TGAT is critical for its activity.

Electronic supplementary material

The online version of this article (10.1186/s12860-018-0155-2) contains supplementary material, which is available to authorized users.

Function and regulation of the Rho guanine nucleotide exchange factor Trio

Rho GTPases oscillate between an inactive GDP-bound state and an active GTP-bound state. They are activated by Rho Guanine nucleotide Exchange Factors (GEF), which accelerate the GDP to GTP exchange. RhoGEFs fall into two different classes: the Dbl family and the DOCK family of proteins. In this review, we focus on the function and regulation of the Dbl family RhoGEF Trio. Trio and its paralog Kalirin are unique within this family in that they display two GEF domains of distinct specificity. Trio is a major regulator of neuronal development, and its function is conserved through evolution. Moreover, Trio plays an important role in cell adhesion and in signaling pathways elicited by Gαq protein-coupled receptors. Combined, these observations suggest that Trio has a major role in cellular physiology. Of note, Trio is an essential gene for mouse development, with a prominent role in the development of the nervous system. Finally, Trio expression is significantly increased in different types of tumors and it has been proposed that it could participate in oncogenesis.

Aptamer-derived peptide inhibitors of Rho guanine nucleotide exchange factors

Small G proteins of the Rho family and their activators the guanine nucleotide exchange factors (RhoGEFs) regulate essential cellular functions and their deregulation has been associated with an amazing variety of human disorders, including cancer, inflammation, vascular diseases, and mental retardation. Rho GTPases and RhoGEFs therefore represent important targets for inhibition, not only in basic research but also for therapeutic purposes, and strategies to inhibit their function are actively being sought. Our lab has been very active in this field and has used the peptide aptamer technology to develop the first RhoGEF inhibitor, using the RhoGEF Trio as a model. Trio function has been described mainly in cell motility and axon growth in the nervous system via Rac1 GTPase activation, but recent findings suggest it to play also a role in the aggressive phenotype of various cancers, making it an attractive target for drug discovery. The object of this chapter is to demonstrate that targeting a RhoGEF using the peptide aptamer technology represents a valid and efficient approach to inhibit cellular processes in which Rho GTPase activity is upregulated. This is illustrated here by the first description of a peptide inhibitor of the oncogenic RhoGEF Tgat, TRIP(E32G), which is functional in vivo. On a long-term perspective, these peptide inhibitors can also serve as therapeutic tools or as guides for the discovery of small-molecule drugs, using an aptamer displacement screen.

The many faces of the guanine-nucleotide exchange factor trio

Small Rho-GTPases are enzymes that are bound to GDP or GTP, which determines their inactive or active state, respectively. The exchange of GDP for GTP is catalyzed by so-called Rho-guanine nucleotide exchange factors (GEFs). Rho-GEFs are characterized by a Dbl-homology (DH) and adjacent Pleckstrin-homology (PH) domain that serves as enzymatic unit for the GDP/GTP exchange. Rho-GEFs show different GTPase specificities, meaning that a particular GEF can activate either multiple GTPases or only one specific GTPase. We recently reported that the Rho-GEF Trio, known to be able to exchange GTP on Rac1, RhoG and RhoA, regulates lamellipodia formation to mediate cell spreading and migration in a Rac1-dependent manner. In this commentary, we review the current knowledge of Trio in several aspects of cell biology.

Aptamer-derived peptides as potent inhibitors of the oncogenic RhoGEF Tgat

Guanine nucleotide exchange factors (GEFs) activate the Rho GTPases by accelerating their GDP/GTP exchange rate. Some RhoGEFs have been isolated based on their oncogenic potency, and strategies to inhibit their activity are therefore actively being sought. In this study we devise a peptide inhibitor screening strategy to target the GEF activity of Tgat, an oncogenic isoform of the RhoGEF Trio, based on random mutations of the Trio inhibitor TRIP alpha, which we previously isolated using a peptide aptamer screen. This identifies one peptide, TRIP(E32G), which specifically inhibits Tgat GEF activity in vitro and significantly reduces Tgat-induced RhoA activation and foci formation. Furthermore, subcutaneous injection of cells expressing Tgat and TRIP(E32G) into nude mice reduces the formation of Tgat-induced tumors. Our approach thus demonstrates that peptide aptamers are potent inhibitors that can be used to interfere with RhoGEF functions in vivo.