Dynamics of Gαq-protein-p63RhoGEF interaction and its regulation by RGS2

Regulator of G protein signaling 2 inhibits Gαq-dependent uveal melanoma cell growth

Activating mutations in Gα_q/11 are a major driver of uveal melanoma (UM), the most common intraocular cancer in adults. While progress has recently been made in targeting Gα_q/11 for UM therapy, the crucial role for these proteins in normal physiology and their high structural similarity with many other important GTPase proteins renders this approach challenging. The aim of the current study was to validate whether a key regulator of Gq signaling, regulator of G protein signaling 2 (RGS2), can inhibit Gα_q-mediated UM cell growth. We used two UM cell lines, 92.1 and Mel-202, which both contain the most common activating mutation Gα_q^Q209L and developed stable cell lines with doxycycline-inducible RGS2 protein expression. Using cell viability assays, we showed that RGS2 could inhibit cell growth in both of these UM cell lines. We also found that this effect was independent of the canonical GTPase-activating protein activity of RGS2 but was dependent on the association between RGS2 and Gα_q. Furthermore, RGS2 induction resulted in only partial reduction in cell growth as compared to siRNA-mediated Gαq knockdown, perhaps because RGS2 was only able to reduce mitogen-activated protein kinase signaling downstream of phospholipase Cβ, while leaving activation of the Hippo signaling mediators yes-associated protein 1/TAZ, the other major pathway downstream of Gα_q, unaffected. Taken together, our data indicate that RGS2 can inhibit UM cancer cell growth by associating with Gα_q^Q209L as a partial effector antagonist.

RGS2: A multifunctional signaling hub that balances brown adipose tissue function and differentiation

Objective

Recruitment of brown adipose tissue (BAT) is a potential new strategy for increasing energy expenditure (EE) to treat obesity. G protein–coupled receptors (GPCRs) represent promising targets to activate BAT, as they are the major regulators of BAT biological function. To identify new regulators of GPCR signaling in BAT, we studied the role of Regulator of G protein Signaling 2 (RGS2) in brown adipocytes and BAT.

Methods

We combined pharmacological and genetic tools to investigate the role of RGS2 in BAT in vitro and in vivo. Adipocyte progenitors were isolated from wild-type (WT) and RGS2 knockout (RGS2−/−) BAT and differentiated to brown adipocytes. This approach was complemented with knockdown of RGS2 using lentiviral shRNAs (shRGS2). Adipogenesis was analyzed by Oil Red O staining and by determining the expression of adipogenic and thermogenic markers. Pharmacological modulators and fluorescence staining of F-acting stress fibers were employed to identify the underlying signaling pathways. In vivo, the activity of BAT was assessed by ex vivo lipolysis and by measuring whole-body EE by indirect calorimetry in metabolic cages.

Results

RGS2 is highly expressed in BAT, and treatment with cGMP—an important enhancer of brown adipocyte differentiation—further increased RGS2 expression. Loss of RGS2 strongly suppressed adipogenesis and the expression of thermogenic genes in brown adipocytes. Mechanistically, we found increased Gq/Rho/Rho kinase (ROCK) signaling in the absence of RGS2. Surprisingly, in vivo analysis revealed elevated BAT activity in RGS2-deficient mice that was caused by enhanced Gs/cAMP signaling.

Conclusion

Overall, RGS2 regulates two major signaling pathways in BAT: Gq and Gs. On the one hand, RGS2 promotes brown adipogenesis by counteracting the inhibitory action of Gq/Rho/ROCK signaling. On the other hand, RGS2 decreases the activity of BAT through the inhibition of Gs signaling and cAMP production. Thus, RGS2 might represent a stress modulator that protects BAT from overstimulation.

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RGS2 regulates brown adipose tissue (BAT) by inhibiting two major G protein-coupled receptor (GPCR) pathways – Gq and Gs.

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Deletion of RGS2 impairs the differentiation of murine brown adipocytes due to elevated Gq/Rho/ROCK signaling.

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In vivo, RGS2 knock-out mice show an increase in BAT lipolysis and whole-body energy expenditure.

G-Alpha Subunit Abundance and Activity Differentially Regulate β-Catenin Signaling

Heterotrimeric G proteins are signal transduction proteins involved in regulating numerous signaling events. In particular, previous studies have demonstrated a role for G-proteins in regulating β-catenin signaling. However, the link between G-proteins and β-catenin signaling is controversial and appears to depend on G-protein specificity. We describe a detailed analysis of a link between specific G-alpha subunits and β-catenin using G-alpha subunit genetic knockout and knockdown approaches. The Pasteurella multocida toxin was utilized as a unique tool to activate G-proteins, with LiCl treatment serving as a β-catenin signaling agonist. The results show that Pasteurella multocida toxin (PMT) significantly enhanced LiCl-induced active β-catenin levels in HEK293T cells and mouse embryo fibroblasts. Evaluation of the effect of specific G-alpha proteins on the regulation of β-catenin showed that G_q/11 and G_12/13 knockout cells had significantly higher levels of active and total β-catenin than wild-type cells. The stimulation of active β-catenin by PMT and LiCl was lost upon both constitutive and transient knockdown of G₁₂ and G₁₃ but not G_q Based on our results, we conclude that endogenous G-alpha proteins are negative regulators of active β-catenin; however, PMT-activated G-alpha subunits positively regulate LiCl-induced β-catenin expression in a G_12/13-dependent manner. Hence, G-alpha subunit regulation of β-catenin is context dependent.

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.

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.

Dynamics of G protein effector interactions and their impact on timing and sensitivity of G protein-mediated signal transduction

G protein coupled receptors regulate numerous cellular functions primarily via coupling to heterotrimeric G proteins and subsequent regulation of effector proteins such as ion channels, enzymes or GTP exchange factors for small G proteins. The dynamics of interactions between G protein subunits and effectors have been difficult to study particularly in a cellular context. The introduction of Förster resonance energy transfer methods into the field of GPCR research led to interesting insights into the temporal patterns of interactions between G protein subunits and their effectors. In this review we specifically focus on the interaction of Gαi subunits with adenylyl cyclases and of Gαq subunits with p63RhoGEF or G protein coupled receptor kinases type 2. Comparing the dynamics of these interactions revealed remarkable differences between different G protein effectors regarding the ability to be modulated by members of the regulator of G protein signalling protein family as well as the sensitivity towards receptor activation.

Adenylyl cyclases 5 and 6 underlie PIP3-dependent regulation

Many different neurotransmitters and hormones control intracellular signaling by regulating the production of the second messenger cAMP. The function of the broadly expressed adenylyl cyclases (ACs) 5 and 6 is regulated by either stimulatory or inhibitory G proteins. By analyzing a well-known rebound stimulation phenomenon after withdrawal of Gi protein in atrial myocytes, we discovered that AC5 and -6 are tightly regulated by the second messenger PIP3. By monitoring cAMP levels in real time by means of Förster resonance energy transfer (FRET)-based biosensors, we reproduced the rebound stimulation in a heterologous expression system specifically for AC5 or -6. Strikingly, this cAMP rebound stimulation was completely blocked by the PI3K inhibitor wortmannin, both in atrial myocytes and in transfected human embryonic kidney cells. Similar effects were observed by heterologous expression of the PIP3 phosphatase and tensin homolog (PTEN). However, general kinase inhibitors or inhibitors of Akt had no effect, suggesting a PIP3-dependent mechanism. These findings demonstrate the existence of a novel general pathway for regulation of AC5 and -6 activity via PIP3 that leads to pronounced alterations of cytosolic cAMP levels.

Diacylglycerol mediates regulation of TASK potassium channels by Gq-coupled receptors

The two-pore domain potassium (K2P) channels TASK-1 (KCNK3) and TASK-3 (KCNK9) are important determinants of background K(+) conductance and membrane potential. TASK-1/3 activity is regulated by hormones and transmitters that act through G protein-coupled receptors (GPCR) signalling via G proteins of the Gαq/11 subclass. How the receptors inhibit channel activity has remained unclear. Here, we show that TASK-1 and -3 channels are gated by diacylglycerol (DAG). Receptor-initiated inhibition of TASK required the activity of phospholipase C, but neither depletion of the PLC substrate PI(4,5)P2 nor release of the downstream messengers IP3 and Ca(2+). Attenuation of cellular DAG transients by DAG kinase or lipase suppressed receptor-dependent inhibition, showing that the increase in cellular DAG-but not in downstream lipid metabolites-mediates channel inhibition. The findings identify DAG as the signal regulating TASK channels downstream of GPCRs and define a novel role for DAG that directly links cellular DAG dynamics to excitability.

Engineered hyperphosphorylation of the β2-adrenoceptor prolongs arrestin-3 binding and induces arrestin internalization

G protein-coupled receptor phosphorylation plays a major role in receptor desensitization and arrestin binding. It is, however, unclear how distinct receptor phosphorylation patterns may influence arrestin binding and subsequent trafficking. Here we engineer phosphorylation sites into the C-terminal tail of the β2-adrenoceptor (β2AR) and demonstrate that this mutant, termed β2AR(SSS), showed increased isoprenaline-stimulated phosphorylation and differences in arrestin-3 affinity and trafficking. By measuring arrestin-3 recruitment and the stability of arrestin-3 receptor complexes in real time using fluorescence resonance energy transfer and fluorescence recovery after photobleaching, we demonstrate that arrestin-3 dissociated quickly and almost completely from the β2AR, whereas the interaction with β2AR(SSS) was 2- to 4-fold prolonged. In contrast, arrestin-3 interaction with a β2-adrenoceptor fused to the carboxyl-terminal tail of the vasopressin type 2 receptor was nearly irreversible. Further analysis of arrestin-3 localization revealed that by engineering phosphorylation sites into the β2-adrenoceptor the receptor showed prolonged interaction with arrestin-3 and colocalization with arrestin in endosomes after internalization. This is in contrast to the wild-type receptor that interacts transiently with arrestin-3 at the plasma membrane. Furthermore, β2AR(SSS) internalized more efficiently than the wild-type receptor, whereas recycling was very similar for both receptors. Thus, we show how the interaction between arrestins and receptors can be increased with minimal receptor modification and that relatively modest increases in receptor-arrestin affinity are sufficient to alter arrestin trafficking.