Reversible translocation of p115-RhoGEF by G(12/13)-coupled receptors

Ogt Deficiency Induces Abnormal Cerebellar Function and Behavioral Deficits of Adult Mice through Modulating RhoA/ROCK Signaling

Previous studies have shown the essential roles of O-GlcNAc transferase (Ogt) and O-GlcNAcylation in neuronal development, function and neurologic diseases. However, the function of Ogt and O-GlcNAcylation in the adult cerebellum has not been well elucidated. Here, we have found that cerebellum has the highest level of O-GlcNAcylation relative to cortex and hippocampus of adult male mice. Specific deletion of Ogt in granule neuron precursors (GNPs) induces abnormal morphology and decreased size of the cerebellum in adult male Ogt deficient [conditional knock-out (cKO)] mice. Adult male cKO mice show the reduced density and aberrant distribution of cerebellar granule cells (CGCs), the disrupted arrangement of Bergman glia (BG) and Purkinje cells. In addition, adult male cKO mice exhibit aberrant synaptic connection, impaired motor coordination, and learning and memory abilities. Mechanistically, we have identified G-protein subunit α12 (Gα12) is modified by Ogt-mediated O-GlcNAcylation. O-GlcNAcylation of Gα12 facilitates its binding to Rho guanine nucleotide exchange factor 12 (Arhgef12) and consequently activates RhoA/ROCK signaling. RhoA/ROCK pathway activator LPA can rescue the developmental deficits of Ogt deficient CGCs. Therefore, our study has revealed the critical function and related mechanisms of Ogt and O-GlcNAcylation in the cerebellum of adult male mice.SIGNIFICANCE STATEMENT Cerebellar function are regulated by diverse mechanisms. To unveil novel mechanisms is critical for understanding the cerebellar function and the clinical therapy of cerebellum-related diseases. In the present study, we have shown that O-GlcNAc transferase gene (Ogt) deletion induces abnormal cerebellar morphology, synaptic connection, and behavioral deficits of adult male mice. Mechanistically, Ogt catalyzes O-GlcNAcylation of Gα12, which promotes the binding to Arhgef12, and regulates RhoA/ROCK signaling pathway. Our study has uncovered the important roles of Ogt and O-GlcNAcylation in regulating cerebellar function and cerebellum-related behavior. Our results suggest that Ogt and O-GlcNAcylation could be potential targets for some cerebellum-related diseases.

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.

RhoA Signaling in Neurodegenerative Diseases

Ras homolog gene family member A (RhoA) is a small GTPase of the Rho family involved in regulating multiple signal transduction pathways that influence a diverse range of cellular functions. RhoA and many of its downstream effector proteins are highly expressed in the nervous system, implying an important role for RhoA signaling in neurons and glial cells. Indeed, emerging evidence points toward a role of aberrant RhoA signaling in neurodegenerative diseases such as Parkinson’s disease, Alzheimer’s disease, Huntington’s disease, and amyotrophic lateral sclerosis. In this review, we summarize the current knowledge of RhoA regulation and downstream cellular functions with an emphasis on the role of RhoA signaling in neurodegenerative diseases and the therapeutic potential of RhoA inhibition in neurodegeneration.

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.

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.

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.

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Dp114RhoGEF activates junctional Rho1 and is involved in cell intercalation

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

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Gβγ subunits control planar polarity of junctional Rho1 signaling via Dp114RhoGEF

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Tissue-specific RhoGEFs could diversify morphogenesis in different tissues

Role of age, Rho-kinase 2 expression, and G protein-mediated signaling in the myogenic response in mouse small mesenteric arteries

The myogenic response (MR) and myogenic tone (MT) in resistance vessels is crucial for maintaining peripheral vascular resistance and blood flow autoregulation. Development of MT involves G protein-coupled receptors, and may be affected by aging.

AIMS

(1) to estimate the mesenteric blood flow in myogenically active small mesenteric arteries; (2) to investigate the signaling from Gαq/11 and/or Gα12 activation to MT development; (3) to investigate the role of Rho-kinase 2 and aging on MT in mesenteric resistance arteries.

METHODS

we used pressure myography, quantitative real-time PCR, and immunolocalization to study small (<200 μm) mesenteric arteries (SMA) from young, mature adult, and middle aged mice.

RESULTS

Poiseuille flow calculations indicated autoregulation of blood flow at 60-120 mm Hg arterial pressure. Gαq/11 and Gα12 were abundantly expressed at the mRNA and protein levels in SMA. The Gαq/11 inhibitor YM-254890 suppressed MT development, and the Phosholipase C inhibitors U73122 and ET-18-OCH3 robustly inhibited it. We found an age-dependent increase in ROCK2 mRNA expression, and in basal MT. The specific ROCK2 inhibitor KD025 robustly inhibited MT in SMAs in all mice with an age-dependent variation in KD025 sensitivity. The inhibitory effect of KD025 was not prevented by the L-type Ca2+ channel activator BayK 8644. KD025 reversibly inhibited MT and endothelin-1 vasoconstriction in small pial arteries from Göttingen minipigs.

CONCLUSIONS

MT development in SMAs occurs through a Gαq/11 /PLC/Ca2+ -dependent pathway, and is maintained via ROCK2-mediated Ca2+ sensitization. Increased MT at mature adulthood can be explained by increased ROCK2 expression/activity.

A RhoA-YAP-c-Myc signaling axis promotes the development of polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is an inherited disorder caused by mutations in PKD1 or PKD2 and affects one in 500-1000 humans. Limited treatment is currently available for ADPKD. Here we identify the Hippo signaling effector YAP and its transcriptional target, c-Myc, as promoters of cystic kidney pathogenesis. While transgenic overexpression of YAP promotes proliferation and tubule dilation in mouse kidneys, loss of YAP/TAZ or c-Myc suppresses cystogenesis in a mouse ADPKD model resulting from Pkd1 deficiency. Through a comprehensive kinase inhibitor screen based on a novel three-dimensional (3D) culture of Pkd1 mutant mouse kidney cells, we identified a signaling pathway involving the RhoGEF (guanine nucleotide exchange factor) LARG, the small GTPase RhoA, and the RhoA effector Rho-associated kinase (ROCK) as a critical signaling module between PKD1 and YAP. Further corroborating its physiological importance, inhibition of RhoA signaling suppresses cystogenesis in 3D culture of Pkd1 mutant kidney cells as well as Pkd1 mutant mouse kidneys in vivo. Taken together, our findings implicate the RhoA-YAP-c-Myc signaling axis as a critical mediator and potential drug target in ADPKD.

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.

Transforming growth factor-β enhances Rho-kinase activity and contraction in airway smooth muscle via the nucleotide exchange factor ARHGEF1

Key points

Transforming growth‐factor‐β (TGF‐β) and RhoA/Rho‐kinase are independently implicated in the airway hyper‐responsiveness associated with asthma, but how these proteins interact is not fully understood.

We examined the effects of pre‐treatment with TGF‐β on expression and activity of RhoA, Rho‐kinase and ARHGEF1, an activator of RhoA, as well as on bradykinin‐induced contraction, in airway smooth muscle.

TGF‐β enhanced bradykinin‐induced RhoA translocation, Rho‐kinase‐dependent phosphorylation and contraction, but partially suppressed bradykinin‐induced RhoA activity (RhoA‐GTP content).

TGF‐β enhanced the expression of ARHGEF1, while a small interfering RNA against ARHGEF1 and a RhoGEF inhibitor prevented the effects of TGF‐β on RhoA and Rho‐kinase activity and contraction, respectively.

ARHGEF1 expression was also enhanced in airway smooth muscle from asthmatic patients and ovalbumin‐sensitized mice.

ARHGEF1 is a key TGF‐β target gene, an important regulator of Rho‐kinase activity and therefore a potential therapeutic target for the treatment of asthmatic airway hyper‐responsiveness.

Abstract

Transforming growth factor‐β (TGF‐β), RhoA/Rho‐kinase and Src‐family kinases (SrcFK) have independently been implicated in airway hyper‐responsiveness, but how they interact to regulate airway smooth muscle contractility is not fully understood. We found that TGF‐β pre‐treatment enhanced acute contractile responses to bradykinin (BK) in isolated rat bronchioles, and inhibitors of RhoGEFs (Y16) and Rho‐kinase (Y27632), but not the SrcFK inhibitor PP2, prevented this enhancement. In cultured human airway smooth muscle cells (hASMCs), TGF‐β pre‐treatment enhanced the protein expression of the Rho guanine nucleotide exchange factor ARHGEF1, MLC₂₀, MYPT‐1 and the actin‐severing protein cofilin, but not of RhoA, ROCK2 or c‐Src. In hASMCs, acute treatment with BK triggered subcellular translocation of ARHGEF1 and RhoA and enhanced auto‐phosphorylation of SrcFK and phosphorylation of MYPT1 and MLC₂₀, but induced de‐phosphorylation of cofilin. TGF‐β pre‐treatment amplified the effects of BK on RhoA translocation and MYPT1/MLC₂₀ phosphorylation, but suppressed the effects of BK on RhoA‐GTP content, SrcFK auto‐phosphorylation and cofilin de‐phosphorylation. In hASMCs, an ARHGEF1 small interfering RNA suppressed the effects of BK and TGF‐β on RhoA‐GTP content, RhoA translocation and MYPT1 and MLC₂₀ phosphorylation, but minimally influenced the effects of TGF‐β on cofilin expression and phosphorylation. ARHGEF1 expression was also enhanced in ASMCs of asthmatic patients and in lungs of ovalbumin‐sensitized mice. Our data indicate that TGF‐β enhances BK‐induced contraction, RhoA translocation and Rho‐kinase activity in airway smooth muscle largely via ARHGEF1, but independently of SrcFK and total RhoA‐GTP content. A role for smooth muscle ARHGEF1 in asthmatic airway hyper‐responsiveness is worthy of further investigation.

Transforming growth‐factor‐β (TGF‐β) and RhoA/Rho‐kinase are independently implicated in the airway hyper‐responsiveness associated with asthma, but how these proteins interact is not fully understood.

We examined the effects of pre‐treatment with TGF‐β on expression and activity of RhoA, Rho‐kinase and ARHGEF1, an activator of RhoA, as well as on bradykinin‐induced contraction, in airway smooth muscle.

TGF‐β enhanced bradykinin‐induced RhoA translocation, Rho‐kinase‐dependent phosphorylation and contraction, but partially suppressed bradykinin‐induced RhoA activity (RhoA‐GTP content).

TGF‐β enhanced the expression of ARHGEF1, while a small interfering RNA against ARHGEF1 and a RhoGEF inhibitor prevented the effects of TGF‐β on RhoA and Rho‐kinase activity and contraction, respectively.

ARHGEF1 expression was also enhanced in airway smooth muscle from asthmatic patients and ovalbumin‐sensitized mice.

ARHGEF1 is a key TGF‐β target gene, an important regulator of Rho‐kinase activity and therefore a potential therapeutic target for the treatment of asthmatic airway hyper‐responsiveness.

ROS-dependent activation of RhoA/Rho-kinase in pulmonary artery: Role of Src-family kinases and ARHGEF1

The role of reactive oxygen species (ROS) in smooth muscle contraction is poorly understood. We hypothesised that G-protein coupled receptor (GPCR) activation and hypoxia induce Rho-kinase activity and contraction in rat intra-pulmonary artery (IPA) via stimulation of ROS production and subsequent Src-family kinase (SrcFK) activation. The T-type prostanoid receptor agonist U46619 induced ROS production in pulmonary artery smooth muscle cells (PASMC). U46619 also induced c-Src cysteine oxidation, SrcFK auto-phosphorylation, MYPT-1 and MLC₂₀ phosphorylation and contraction in IPA, and all these responses were inhibited by antioxidants (ebselen, Tempol). Contraction and SrcFK/MYPT-1/MLC₂₀ phosphorylations were also inhibited by combined superoxide dismutase and catalase, or by the SrcFK antagonist PP2, while contraction and MYPT-1/MLC₂₀ phosphorylations were inhibited by the Rho guanine nucleotide exchange factor (RhoGEF) inhibitor Y16. H₂O₂ and the superoxide-generating quinoledione LY83583 both induced c-Src oxidation, SrcFK auto-phosphorylation and contraction in IPA. LY83583 and H₂O₂-induced contractions were inhibited by PP2, while LY83583-induced contraction was also inhibited by antioxidants and Y16. SrcFK auto-phosphorylation and MYPT-1/MLC₂₀ phosphorylation was also induced by hypoxia in IPA and this was blocked by mitochondrial inhibitors rotenone and myxothiazol. In live PASMC, sub-cellular translocation of RhoA and the RhoGEF ARHGEF1 was triggered by both U46619 and LY83583 and this translocation was blocked by antioxidants and PP2. RhoA translocation was also inhibited by an ARHGEF1 siRNA. U46619 enhanced ROS-dependent co-immunoprecipitation of ARHGEF1 with c-Src. Our results demonstrate a link between GPCR-induced cytosolic ROS or hypoxia-induced mitochondrial ROS and SrcFK activity, Rho-kinase activity and contraction. ROS and SrcFK activate RhoA via ARHGEF1.

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.

WNT Stimulation Dissociates a Frizzled 4 Inactive-State Complex with Gα12/13

Frizzleds (FZDs) are unconventional G protein-coupled receptors that belong to the class Frizzled. They are bound and activated by the Wingless/Int-1 lipoglycoprotein (WNT) family of secreted lipoglycoproteins. To date, mechanisms of signal initiation and FZD-G protein coupling remain poorly understood. Previously, we showed that FZD6 assembles with Gαi1/Gαq (but not with Gαs, Gαo and Ga12/13), and that these inactive-state complexes are dissociated by WNTs and regulated by the phosphoprotein Dishevelled (DVL). Here, we investigated the inactive-state assembly of heterotrimeric G proteins with FZD4, a receptor important in retinal vascular development and frequently mutated in Norrie disease or familial exudative vitreoretinopathy. Live-cell imaging experiments using fluorescence recovery after photobleaching show that human FZD4 assembles-in a DVL-independent manner-with Gα12/13 but not representatives of other heterotrimeric G protein subfamilies, such as Gαi1, Gαo, Gαs, and Gαq The FZD4-G protein complex dissociates upon stimulation with WNT-3A, WNT-5A, WNT-7A, and WNT-10B. In addition, WNT-induced dynamic mass redistribution changes in untransfected and, even more so, in FZD4 green fluorescent protein-transfected cells depend on Gα12/13 Furthermore, expression of FZD4 and Gα12 or Gα13 in human embryonic kidney 293 cells induces WNT-dependent membrane recruitment of p115-RHOGEF (RHO guanine nucleotide exchange factor, molecular weight 115 kDa), a direct target of Gα12/13 signaling, underlining the functionality of an FZD4-Gα12/13-RHO signaling axis. In summary, Gα12/13-mediated WNT/FZD4 signaling through p115-RHOGEF offers an intriguing and previously unappreciated mechanistic link of FZD4 signaling to cytoskeletal rearrangements and RHO signaling with implications for the regulation of angiogenesis during embryonic and tumor development.

Regulated localization is sufficient for hormonal control of regulator of G protein signaling homology Rho guanine nucleotide exchange factors (RH-RhoGEFs)

The regulator of G protein signaling homology (RH) Rho guanine nucleotide exchange factors (RhoGEFs) (p115RhoGEF, leukemia-associated RhoGEF, and PDZ-RhoGEF) contain an RH domain and are specific GEFs for the monomeric GTPase RhoA. The RH domains interact specifically with the α subunits of G12 heterotrimeric GTPases. Activated Gα13 modestly stimulates the exchange activity of both p115RhoGEF and leukemia-associated RhoGEF but not PDZ-RhoGEF. Because all three RH-RhoGEFs can localize to the plasma membrane upon expression of activated Gα13, cellular localization of these RhoGEFs has been proposed as a mechanism for controlling their activity. We use a small molecule-regulated heterodimerization system to rapidly control the localization of RH-RhoGEFs. Acute localization of the proteins to the plasma membrane activates RhoA within minutes and to levels that are comparable with activation of RhoA by hormonal stimulation of G protein-coupled receptors. The catalytic activity of membrane-localized RhoGEFs is not dependent on activated Gα13. We further show that the conserved RH domains can rewire two different RacGEFs to activate Rac1 in response to a traditional activator of RhoA. Thus, RH domains act as independent detectors for activated Gα13 and are sufficient to modulate the activity of RhoGEFs by hormones via mediating their localization to substrate, membrane-associated RhoA.

Modification of p115RhoGEF Ser(330) regulates its RhoGEF activity

p115RhoGEF is a member of a family of Rho-specific guanine nucleotide exchange factors that also contains a regulator of G protein signaling homology domain (RH-RhoGEFs) that serves as a link between Gα13 signaling and RhoA activation. While the mechanism of regulation of p115RhoGEF by Gα13 is becoming well-known, the role of other regulatory mechanisms, such as post-translational modification or autoinhibition, in mediating p115RhoGEF activity is less well-characterized. Here, putative phosphorylation sites on p115RhoGEF are identified and characterized. Mutation of Ser(330) leads to a decrease in serum response element-mediated transcription as well as decreased activation by Gα13 in vitro. Additionally, this study provides the first report of the binding kinetics between full-length p115RhoGEF and RhoA in its various nucleotide states and examines the binding kinetics of phospho-mutant p115RhoGEF to RhoA. These data, together with other recent reports on regulatory mechanisms of p115RhoGEF, suggest that this putative phosphorylation site serves as a means for initiation or relief of autoinhibition of p115RhoGEF, providing further insight into the regulation of its activity.

A finer tuning of G-protein signaling through regulated control of RGS proteins

Regulators of G-protein signaling (RGS) proteins are GTPase-activating proteins (GAP) for various Gα subunits of heterotrimeric G proteins. Through this mechanism, RGS proteins regulate the magnitude and duration of G-protein-coupled receptor signaling and are often referred to as fine tuners of G-protein signaling. Increasing evidence suggests that RGS proteins themselves are regulated through multiple mechanisms, which may provide an even finer tuning of G-protein signaling and crosstalk between G-protein-coupled receptors and other signaling pathways. This review summarizes the current data on the control of RGS function through regulated expression, intracellular localization, and covalent modification of RGS proteins, as related to cell function and the pathogenesis of diseases.

Plasma membrane association of p63 Rho guanine nucleotide exchange factor (p63RhoGEF) is mediated by palmitoylation and is required for basal activity in cells

Activation of G protein-coupled receptors at the cell surface leads to the activation or inhibition of intracellular effector enzymes, which include various Rho guanine nucleotide exchange factors (RhoGEFs). RhoGEFs activate small molecular weight GTPases at the plasma membrane (PM). Many of the known G protein-coupled receptor-regulated RhoGEFs are found in the cytoplasm of unstimulated cells, and PM recruitment is a critical aspect of their regulation. In contrast, p63RhoGEF, a Gα(q)-regulated RhoGEF, appears to be constitutively localized to the PM. The objective of this study was to determine the molecular basis for the localization of p63RhoGEF and the impact of its subcellular localization on its regulation by Gα(q). Herein, we show that the pleckstrin homology domain of p63RhoGEF is not involved in its PM targeting. Instead, a conserved string of cysteines (Cys-23/25/26) at the N terminus of the enzyme is palmitoylated and required for membrane localization and full basal activity in cells. Conversion of these residues to serine relocates p63RhoGEF from the PM to the cytoplasm, diminishes its basal activity, and eliminates palmitoylation. The activity of palmitoylation-deficient p63RhoGEF can be rescued by targeting to the PM by fusion with tandem phospholipase C-δ1 pleckstrin homology domains or by co-expression with wild-type Gα(q) but not with palmitoylation-deficient Gα(q). Our data suggest that p63RhoGEF is regulated chiefly through allosteric control by Gα(q), as opposed to other known Gα-regulated RhoGEFs, which are instead sequestered in the cytoplasm, perhaps because of their high basal activity.

Angiotensin II-induced vascular endothelial dysfunction through RhoA/Rho kinase/p38 mitogen-activated protein kinase/arginase pathway

Enhanced vascular arginase activity impairs endothelium-dependent vasorelaxation by decreasing l-arginine availability to endothelial nitric oxide (NO) synthase, thereby reducing NO production. Elevated angiotensin II (ANG II) is a key component of endothelial dysfunction in many cardiovascular diseases and has been linked to elevated arginase activity. We determined signaling mechanisms by which ANG II increases endothelial arginase function. Results show that ANG II (0.1 μM, 24 h) elevates arginase activity and arginase I expression in bovine aortic endothelial cells (BAECs) and decreases NO production. These effects are prevented by the arginase inhibitor BEC (100 μM). Blockade of ANG II AT(1) receptors or transfection with small interfering RNA (siRNA) for Gα12 and Gα13 also prevents ANG II-induced elevation of arginase activity, but siRNA for Gαq does not. ANG II also elevates active RhoA levels and induces phosphorylation of p38 MAPK. Inhibitors of RhoA activation (simvastatin, 0.1 μM) or Rho kinase (ROCK) (Y-27632, 10 μM; H1152, 0.5 μM) block both ANG II-induced elevation of arginase activity and phosphorylation of p38 MAPK. Furthermore, pretreatment of BAECs with p38 inhibitor SB-202190 (2 μM) or transfection with p38 MAPK siRNA prevents ANG II-induced increased arginase activity/expression and maintains NO production. Additionally, inhibitors of p38 MAPK (SB-203580, 5 μg·kg(-1)·day(-1)) or arginase (ABH, 8 mg·kg(-1)·day(-1)) or arginase gene knockout in mice prevents ANG II-induced vascular endothelial dysfunction and associated enhancement of arginase. These results indicate that ANG II increases endothelial arginase activity/expression through Gα12/13 G proteins coupled to AT(1) receptors and subsequent activation of RhoA/ROCK/p38 MAPK pathways leading to endothelial dysfunction.

The Lbc Rho guanine nucleotide exchange factor α-catulin axis functions in serotonin-induced vascular smooth muscle cell mitogenesis and RhoA/ROCK activation

Serotonin (5-hydroxytryptamine, 5-HT) is mitogenic for several cell types including pulmonary arterial smooth muscle cells (PASMC), and is associated with the abnormal vascular smooth muscle remodeling that occurs in pulmonary arterial hypertension. RhoA/Rho kinase (ROCK) function is required for 5-HT-induced PASMC mitogenesis, and 5-HT activates RhoA; however, the signaling steps are poorly defined. Rho guanine nucleotide exchange factors (Rho GEFs) transduce extracellular signals to Rho, and we found that 5-HT treatment of PASMC led to increased membrane-associated Lbc Rho GEF, suggesting modulation by 5-HT. Lbc knockdown by siRNA attenuated 5-HT-induced thymidine uptake in PASMC, indicating a role in PASMC mitogenesis. 5-HT triggered Rho-dependent serum response factor-mediated reporter activation in PASMC, and this was reduced by Lbc depletion. Lbc knockdown reduced 5-HT-induced RhoA/ROCK activation, but not p42/44 ERK MAP kinase activation, suggesting that Lbc is an intermediary between 5-HT and RhoA/ROCK, but not ERK. 5-HT stimulation of PASMC led to increased association between Lbc, RhoA, and the α-catulin scaffold. Furthermore, α-catulin knockdown attenuated 5-HT-induced PASMC thymidine uptake. 5-HT-induced PASMC mitogenesis was reduced by dominant-negative G(q) protein, suggesting cooperation with Lbc/α-catulin. These results for the first time define a Rho GEF involved in vascular smooth muscle cell growth and serotonin signaling, and suggest that Lbc Rho GEF family members play distinct roles. Thus, the Lbc/α-catulin axis participates in 5-HT-induced PASMC mitogenesis and RhoA/ROCK signaling, and may be an interventional target in diseases involving vascular smooth muscle remodeling.

Galpha q allosterically activates and relieves autoinhibition of p63RhoGEF

Galpha(q) directly activates p63RhoGEF and closely related catalytic domains found in Trio and Kalirin, thereby linking G(q)-coupled receptors to the activation of RhoA. Although the crystal structure of G alpha(q) in complex with the catalytic domains of p63RhoGEF is available, the molecular mechanism of activation has not yet been defined. In this study, we show that membrane translocation does not appear to play a role in G alpha(q)-mediated activation of p63RhoGEF, as it does in some other RhoGEFs. G alpha(q) instead must act allosterically. We next identify specific structural elements in the PH domain that inhibit basal nucleotide exchange activity, and provide evidence that G alpha(q) overcomes this inhibition by altering the conformation of the alpha 6-alpha N linker that joins the DH and PH domains, a region that forms direct contacts with RhoA. We also identify residues in G alpha(q) that are important for the activation of p63RhoGEF and that contribute to G alpha subfamily selectivity, including a critical residue in the G alpha(q) C-terminal helix, and demonstrate the importance of these residues for RhoA activation in living cells.

Structure and function of regulator of G protein signaling homology domains

All regulator of G protein signaling (RGS) proteins contain a conserved domain of approximately 130 amino acids that binds to activated heterotrimeric G protein α subunits (Gα) and accelerates their rate of GTP hydrolysis. Homologous domains are found in at least six other protein families, including a family of Rho guanine nucleotide exchange factors (RhoGEFs) and the G protein-coupled receptor kinases (GRKs). Although some of the RhoGEF and GRK RGS-like domains can also bind to activated Gα subunits, they do so in distinct ways and with much lower levels of GTPase activation. In other protein families, the domains have as of yet no obvious relationship to heterotrimeric G protein signaling. These RGS homology (RH) domains are now recognized as mediators of extraordinarily diverse protein-protein interactions. Through these interactions, they play roles that range from enzyme to molecular scaffold to signal transducing module. In this review, the atomic structures of RH domains from RGS proteins, Axins, RhoGEFs, and GRKs are compared in light of what is currently known about their functional roles.

A conserved hydrophobic surface of the LARG pleckstrin homology domain is critical for RhoA activation in cells

Leukemia associated Rho guanine nucleotide exchange factor (LARG) activates RhoA in response to signals received by specific classes of cell surface receptors. The catalytic core of LARG is a Dbl homology (DH) domain whose activity is modulated by an adjacent pleckstrin homology (PH) domain. In this study, we used a transcriptional assay and confocal microscopy to examine the roles of several novel structural features of the LARG DH/PH domains, including a conserved and exposed hydrophobic patch on the PH domain that mediates protein-protein interactions in crystal structures of LARG and its close homolog PDZ-RhoGEF. Mutation of the hydrophobic patch has no effect on nucleotide exchange activity in vitro, but abolished the ability of LARG to activate RhoA and to induce stress fiber formation in cultured cells. The activity of these mutants could be rescued by fusion with exogenous membrane-targeting domains. However, because membrane recruitment by activated G alpha(13) subunits was not sufficient to rescue activity of a hydrophobic patch mutant, the LARG PH domain cannot solely contribute to membrane targeting. Instead, it seems likely that the domain is involved in regulatory interactions with other proteins near the membrane surface. We also show that the hydrophobic patch of the PH domain is likely important for the activity of all Lbc subfamily RhoGEFs.

Differences in Galpha12- and Galpha13-mediated plasma membrane recruitment of p115-RhoGEF

Regulator of G protein signaling domain-containing Rho guanine-nucleotide exchange factors (RGS-RhoGEFs) directly links activated forms of the G12 family of heterotrimeric G protein alpha subunits to the small GTPase Rho. Stimulation of G(12/13)-coupled GPCRs or expression of constitutively activated forms of alpha(12) and alpha(13) has been shown to induce the translocation of the RGS-RhoGEF, p115-RhoGEF, from the cytoplasm to the plasma membrane (PM). However, little is known regarding the functional importance and mechanisms of this regulated PM recruitment, and thus PM recruitment of p115-RhoGEF is the focus of this report. A constitutively PM-localized mutant of p115-RhoGEF shows a much greater activity compared to wild type p115-RhoGEF in promoting Rho-dependent neurite retraction of NGF-differentiated PC12 cells, providing the first evidence that PM localization can activate p115-RhoGEF signaling. Next, we uncovered the unexpected finding that Rho is required for alpha(13)-induced PM translocation of p115-RhoGEF. However, inhibition of Rho did not prevent alpha(12)-induced PM translocation of p115-RhoGEF. Additional differences between alpha(13) and alpha(12) in promoting PM recruitment of p115-RhoGEF were revealed by analyzing RGS domain mutants of p115-RhoGEF. Activated alpha(12) effectively recruits the isolated RGS domain of p115-RhoGEF to the PM, whereas alpha(13) only weakly does. On the other hand, alpha(13) strongly recruits to the PM a p115-RhoGEF mutant containing amino acid substitutions in an acidic region at the N-terminus of the RGS domain; however, alpha(12) is unable to recruit this p115-RhoGEF mutant to the PM. These studies provide new insight into the function and mechanisms of alpha(12/13)-mediated PM recruitment of p115-RhoGEF.

Regulation of RhoGEF proteins by G12/13-coupled receptors

G protein-coupled receptors (GPCRs) represent a large family of seven transmembrane receptors, which communicate extracellular signals into the cellular lumen. The human genome contains 720-800 GPCRs, and their diverse signal characteristics are determined by their specific tissue and subcellular expression profiles, as well as their coupling profile to the various G protein families (G(s), G(i), G(q), G(12)). The G protein coupling pattern links GPCR activation to the specific downstream effector pathways. G(12/13) signalling of GPCRs has been studied only recently in more detail, and involves activation of RhoGTPase nucleotide exchange factors (RhoGEFs). Four mammalian RhoGEFs regulated by G(12/13) proteins are known: p115-RhoGEF, PSD-95/Disc-large/ZO-1 homology-RhoGEF, leukemia-associated RhoGEF and lymphoid blast crisis-RhoGEF. These link GPCRs to activation of the small monomeric GTPase RhoA, and other downstream effectors. Misregulated G(12/13) signalling is involved in multiple pathophysiological conditions such as cancer, cardiovascular diseases, arterial and pulmonary hypertension, and bronchial asthma. Specific targeting of G(12/13) signalling-related diseases of GPCRs hence provides novel therapeutic approaches. Assays to quantitatively measure GPCR-mediated activation of G(12/13) are only emerging, and are required to understand the G(12/13)-linked pharmacology. The review gives an overview of G(12/13) signalling of GPCRs with a focus on RhoGEF proteins as the immediate mediators of G(12/13) activation.