Signaling efficiency of Gαq through its effectors p63RhoGEF and GEFT depends on their subcellular location

Opto-RhoGEFs, an optimized optogenetic toolbox to reversibly control Rho GTPase activity on a global to subcellular scale, enabling precise control over vascular endothelial barrier strength

The inner layer of blood vessels consists of endothelial cells, which form the physical barrier between blood and tissue. This vascular barrier is tightly regulated and is defined by cell-cell contacts through adherens and tight junctions. To investigate the signaling that regulates vascular barrier strength, we focused on Rho GTPases, regulators of the actin cytoskeleton and known to control junction integrity. To manipulate Rho GTPase signaling in a temporal and spatial manner we applied optogenetics. Guanine-nucleotide exchange factor (GEF) domains from ITSN1, TIAM1, and p63RhoGEF, activating Cdc42, Rac, and Rho, respectively, were integrated into the optogenetic recruitment tool improved light-induced dimer (iLID). This tool allows for Rho GTPase activation at the subcellular level in a reversible and non-invasive manner by recruiting a GEF to a specific area at the plasma membrane, The membrane tag of iLID was optimized and a HaloTag was applied to gain more flexibility for multiplex imaging. The resulting optogenetically recruitable RhoGEFs (Opto-RhoGEFs) were tested in an endothelial cell monolayer and demonstrated precise temporal control of vascular barrier strength by a cell-cell overlap-dependent, VE-cadherin-independent, mechanism. Furthermore, Opto-RhoGEFs enabled precise optogenetic control in endothelial cells over morphological features such as cell size, cell roundness, local extension, and cell contraction. In conclusion, we have optimized and applied the optogenetic iLID GEF recruitment tool, that is Opto-RhoGEFs, to study the role of Rho GTPases in the vascular barrier of the endothelium and found that membrane protrusions at the junction region can rapidly increase barrier integrity independent of VE-cadherin.

Direct targeting of Gαq and Gα11 oncoproteins in cancer cells

Somatic gain-of-function mutations of GNAQ and GNA11, which encode α subunits of heterotrimeric Gαq/11 proteins, occur in about 85% of cases of uveal melanoma (UM), the most common cancer of the adult eye. Molecular therapies to directly target these oncoproteins are lacking, and current treatment options rely on radiation, surgery, or inhibition of effector molecules downstream of these G proteins. A hallmark feature of oncogenic Gαq/11 proteins is their reduced intrinsic rate of hydrolysis of guanosine triphosphate (GTP), which results in their accumulation in the GTP-bound, active state. Here, we report that the cyclic depsipeptide FR900359 (FR) directly interacted with GTPase-deficient Gαq/11 proteins and preferentially inhibited mitogenic ERK signaling rather than canonical phospholipase Cβ (PLCβ) signaling driven by these oncogenes. Thereby, FR suppressed the proliferation of melanoma cells in culture and inhibited the growth of Gαq-driven UM mouse xenografts in vivo. In contrast, FR did not affect tumor growth when xenografts carried mutated B-RafV600E as the oncogenic driver. Because FR enabled suppression of malignant traits in cancer cells that are driven by activating mutations at codon 209 in Gαq/11 proteins, we envision that similar approaches could be taken to blunt the signaling of non-Gαq/11 G proteins.

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.

The role of a conserved membrane proximal cysteine in altering αPS2CβPS integrin diffusion

Cysteine residues (Cys) in the membrane proximal region are common post-translational modification (PTM) sites in transmembrane proteins. Herein, the effects of a highly conserved membrane proximal α-subunit Cys¹³⁶⁸ on the diffusion properties of αPS2CβPS integrins are reported. Sequence alignment shows that this cysteine is palmitoylated in human α3 and α6 integrin subunits. Replacing Cys¹³⁶⁸ in wild-type integrins with valine (Val¹³⁶⁸) putatively blocks a PTM site and alters integrins' ligand binding and diffusion characteristics. Both fluorescence recovery after photobleaching (FRAP) and single particle tracking (SPT) diffusion measurements show Val¹³⁶⁸ integrins are more mobile compared to wild-type integrins. Approximately 33% and 8% more Val¹³⁶⁸ integrins are mobile as measured by FRAP and SPT, respectively. The mobile Val¹³⁶⁸ integrins also exhibit less time-dependent diffusion, as measured by FRAP. Tandem mass spectrometry data suggest that Cys¹³⁶⁸ contains a redox or palmitoylation PTM in αPS2CβPS integrins. This membrane proximal Cys may play an important role in the diffusion of other alpha subunits that contain this conserved residue.

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.

Plasma membrane restricted RhoGEF activity is sufficient for RhoA-mediated actin polymerization

The small GTPase RhoA is involved in cell morphology and migration. RhoA activity is tightly regulated in time and space and depends on guanine exchange factors (GEFs). However, the kinetics and subcellular localization of GEF activity towards RhoA are poorly defined. To study the mechanism underlying the spatiotemporal control of RhoA activity by GEFs, we performed single cell imaging with an improved FRET sensor reporting on the nucleotide loading state of RhoA. By employing the FRET sensor we show that a plasma membrane located RhoGEF, p63RhoGEF, can rapidly activate RhoA through endogenous GPCRs and that localized RhoA activity at the cell periphery correlates with actin polymerization. Moreover, synthetic recruitment of the catalytic domain derived from p63RhoGEF to the plasma membrane, but not to the Golgi apparatus, is sufficient to activate RhoA. The synthetic system enables local activation of endogenous RhoA and effectively induces actin polymerization and changes in cellular morphology. Together, our data demonstrate that GEF activity at the plasma membrane is sufficient for actin polymerization via local RhoA signaling.

Gαq signalling: the new and the old

In the last few years the interactome of Gαq has expanded considerably, contributing to improve our understanding of the cellular and physiological events controlled by this G alpha subunit. The availability of high-resolution crystal structures has led the identification of an effector-binding region within the surface of Gαq that is able to recognise a variety of effector proteins. Consequently, it has been possible to ascribe different Gαq functions to specific cellular players and to identify important processes that are triggered independently of the canonical activation of phospholipase Cβ (PLCβ), the first identified Gαq effector. Novel effectors include p63RhoGEF, that provides a link between G protein-coupled receptors and RhoA activation, phosphatidylinositol 3-kinase (PI3K), implicated in the regulation of the Akt pathway, or the cold-activated TRPM8 channel, which is directly inhibited upon Gαq binding. Recently, the activation of ERK5 MAPK by Gq-coupled receptors has also been described as a novel PLCβ-independent signalling axis that relies upon the interaction between this G protein and two novel effectors (PKCζ and MEK5). Additionally, the association of Gαq with different regulatory proteins can modulate its effector coupling ability and, therefore, its signalling potential. Regulators include accessory proteins that facilitate effector activation or, alternatively, inhibitory proteins that downregulate effector binding or promote signal termination. Moreover, Gαq is known to interact with several components of the cytoskeleton as well as with important organisers of membrane microdomains, which suggests that efficient signalling complexes might be confined to specific subcellular environments. Overall, the complex interaction network of Gαq underlies an ever-expanding functional diversity that puts forward this G alpha subunit as a major player in the control of physiological functions and in the development of different pathological situations.

Altered reactivity of resistance vasculature contributes to hypertension in elastin insufficiency

Elastin (Eln) insufficiency in mice and humans is associated with hypertension and altered structure and mechanical properties of large arteries. However, it is not known to what extent functional or structural changes in resistance arteries contribute to the elevated blood pressure that is characteristic of Eln insufficiency. Here, we investigated how Eln insufficiency affects the structure and function of the resistance vasculature. A functional profile of resistance vasculature in Eln(+/-) mice was generated by assessing small mesenteric artery (MA) contractile and vasodilatory responses to vasoactive agents. We found that Eln haploinsufficiency had a modest effect on phenylephrine-induced vasoconstriction, whereas ANG II-evoked vasoconstriction was markedly increased. Blockade of ANG II type 2 receptors with PD-123319 or modulation of Rho kinase activity with the inhibitor Y-27632 attenuated the augmented vasoconstriction, whereas acute Y-27632 administration normalized blood pressure in Eln(+/-) mice. Sodium nitroprusside- and isoproterenol-induced vasodilatation were normal, whereas ACh-induced vasodilatation was severely impaired in Eln(+/-) MAs. Histologically, the number of smooth muscle layers did not change in Eln(+/-) MAs; however, an additional discontinuous layer of Eln appeared between the smooth muscle layers that was absent in wild-type arteries. We conclude that high blood pressure arising from Eln insufficiency is due partly to permanent changes in vascular tone as a result of increased sensitivity of the resistance vasculature to circulating ANG II and to impaired vasodilatory mechanisms arising from endothelial dysfunction characterized by impaired endothelium-dependent vasodilatation. Eln insufficiency causes augmented ANG II-induced vasoconstriction in part through a novel mechanism that facilitates contraction evoked by ANG II type 2 receptors and altered G protein signaling.

Strike a pose: Gαq complexes at the membrane

The heterotrimeric G protein Gαq is a central player in signal transduction, relaying signals from activated G-protein-coupled receptors (GPCRs) to effectors and other proteins to elicit changes in intracellular Ca(2+), the actin cytoskeleton, and gene transcription. Gαq functions at the intracellular surface of the plasma membrane, as do its best-characterized targets, phospholipase C-β, p63RhoGEF, and GPCR kinase 2 (GRK2). Recent insights into the structure and function of these signaling complexes reveal several recurring themes, including complex multivalent interactions between Gαq, its protein target, and the membrane, that are likely essential for allosteric control and maximum efficiency in signal transduction. Thus, the plasma membrane is not only a source of substrates but also a key player in the scaffolding of Gαq-dependent signaling pathways.