Membrane-induced Allosteric Control of Phospholipase C-β Isozymes

All peripheral membrane proteins must negotiate unique constraints intrinsic to the biological interface of lipid bilayers and the cytosol. Phospholipase C-β (PLC-β) isozymes hydrolyze the membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP₂) to propagate diverse intracellular responses that underlie the physiological action of many hormones, neurotransmitters, and growth factors. PLC-β isozymes are autoinhibited, and several proteins, including Gα_q, Gβγ, and Rac1, directly engage distinct regions of these phospholipases to release autoinhibition. To understand this process, we used a novel, soluble analog of PIP₂ that increases in fluorescence upon cleavage to monitor phospholipase activity in real time in the absence of membranes or detergents. High concentrations of Gα_q or Gβ₁γ₂ did not activate purified PLC-β3 under these conditions despite their robust capacity to activate PLC-β3 at membranes. In addition, mutants of PLC-β3 with crippled autoinhibition dramatically accelerated the hydrolysis of PIP₂ in membranes without an equivalent acceleration in the hydrolysis of the soluble analog. Our results illustrate that membranes are integral for the activation of PLC-β isozymes by diverse modulators, and we propose a model describing membrane-mediated allosterism within PLC-β isozymes.     

Identification and Characterization of a G Protein-binding Cluster in α7 Nicotinic Acetylcholine Receptors

α7 nicotinic acetylcholine receptors (nAChRs) play an important role in synaptic transmission and inflammation. In response to ligands, this receptor channel opens to conduct cations into the cell but desensitizes rapidly. In recent studies we show that α7 nAChRs bind signaling proteins such as heterotrimeric GTP-binding proteins (G proteins). Here, we demonstrate that direct coupling of α7 nAChRs to G proteins enables a downstream calcium signaling response that can persist beyond the expected time course of channel activation. This process depends on a G protein-binding cluster (GPBC) in the M3-M4 loop of the receptor. A mutation of the GPBC in the α7 nAChR (α7_345–348A) abolishes interaction with Gα_q as well as Gβγ while having no effect on receptor synthesis, cell-surface trafficking, or α-bungarotoxin binding. Expression of α7_345–348A, however, did significantly attenuate the α7 nAChR-induced Gα_q calcium signaling response as evidenced by a decrease in PLC-β activation and IP₃R-mediated calcium store release in the presence of the α7 selective agonist choline. Taken together, the data provides new evidence for the existence of a GPBC in nAChRs serving to promote intracellular signaling.     

Structure of the Regulator of G Protein Signaling 8 (RGS8)-Gαq Complex: MOLECULAR BASIS FOR Gα SELECTIVITY*

Regulator of G protein signaling (RGS) proteins interact with activated Gα subunits via their RGS domains and accelerate the hydrolysis of GTP. Although the R4 subfamily of RGS proteins generally accepts both Gα_i/o and Gα_q/11 subunits as substrates, the R7 and R12 subfamilies select against Gα_q/11. In contrast, only one RGS protein, RGS2, is known to be selective for Gα_q/11. The molecular basis for this selectivity is not clear. Previously, the crystal structure of RGS2 in complex with Gα_q revealed a non-canonical interaction that could be due to interfacial differences imposed by RGS2, the Gα subunit, or both. To resolve this ambiguity, the 2.6 Å crystal structure of RGS8, an R4 subfamily member, was determined in complex with Gα_q. RGS8 adopts the same pose on Gα_q as it does when bound to Gα_i3, indicating that the non-canonical interaction of RGS2 with Gα_q is due to unique features of RGS2. Based on the RGS8-Gα_q structure, residues in RGS8 that contact a unique α-helical domain loop of Gα_q were converted to those typically found in R12 subfamily members, and the reverse substitutions were introduced into RGS10, an R12 subfamily member. Although these substitutions perturbed their ability to stimulate GTP hydrolysis, they did not reverse selectivity. Instead, selectivity for Gα_q seems more likely determined by whether strong contacts can be maintained between α6 of the RGS domain and Switch III of Gα_q, regions of high sequence and conformational diversity in both protein families.     

Self-activating G protein α subunits engage seven-transmembrane regulator of G protein signaling (RGS) proteins and a Rho guanine nucleotide exchange factor effector in the amoeba Naegleria fowleri

The free-living amoeba Naegleria fowleri is a causative agent of primary amoebic meningoencephalitis and is highly resistant to current therapies, resulting in mortality rates >97%. As many therapeutics target G protein–centered signal transduction pathways, further understanding the functional significance of G protein signaling within N. fowleri should aid future drug discovery against this pathogen. Here, we report that the N. fowleri genome encodes numerous transcribed G protein signaling components, including G protein–coupled receptors, heterotrimeric G protein subunits, regulator of G protein signaling (RGS) proteins, and candidate Gα effector proteins. We found N. fowleri Gα subunits have diverse nucleotide cycling kinetics; Nf Gα5 and Gα7 exhibit more rapid nucleotide exchange than GTP hydrolysis (i.e., “self-activating” behavior). A crystal structure of Nf Gα7 highlights the stability of its nucleotide-free state, consistent with its rapid nucleotide exchange. Variations in the phosphate binding loop also contribute to nucleotide cycling differences among Gα subunits. Similar to plant G protein signaling pathways, N. fowleri Gα subunits selectively engage members of a large seven-transmembrane RGS protein family, resulting in acceleration of GTP hydrolysis. We show Nf Gα2 and Gα3 directly interact with a candidate Gα effector protein, RGS-RhoGEF, similar to mammalian Gα_12/13 signaling pathways. We demonstrate Nf Gα2 and Gα3 each engage RGS-RhoGEF through a canonical Gα/RGS domain interface, suggesting a shared evolutionary origin with G protein signaling in the enteric pathogen Entamoeba histolytica. These findings further illuminate the evolution of G protein signaling and identify potential targets of pharmacological manipulation in N. fowleri.     

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.     

Modulation of distinct isoforms of L-type calcium channels by Gq-coupled receptors in Xenopus oocytes: Antagonistic effects of Gβγ and protein kinase C

L-type voltage dependent Ca²⁺ channels (L-VDCCs; Ca_v1.2) are crucial in cardiovascular physiology. In heart and smooth muscle, hormones and transmitters operating via G_q enhance L-VDCC currents via essential protein kinase C (PKC) involvement. Heterologous reconstitution studies in Xenopus oocytes suggested that PKC and G_q-coupled receptors increased L-VDCC currents only in cardiac long N-terminus (NT) isoforms of α_1C, whereas known smooth muscle short-NT isoforms were inhibited by PKC and G_q activators. We report a novel regulation of the long-NT α_1C isoform by Gβγ. Gβγ inhibited whereas a Gβγ scavenger protein augmented the G_q- but not phorbol ester-mediated enhancement of channel activity, suggesting that Gβγ acts upstream from PKC. In vitro binding experiments reveal binding of both Gβγ and PKC to α_1C-NT. However, PKC modulation was not altered by mutations of multiple potential phosphorylation sites in the NT, and was attenuated by a mutation of C-terminally located serine S1928. The insertion of exon 9a in intracellular loop 1 rendered the short-NT α_1C sensitive to PKC stimulation and to Gβγ scavenging. Our results suggest a complex antagonistic interplay between G_q-activated PKC and Gβγ in regulation of L-VDCC, in which multiple cytosolic segments of α_1C are involved.     

Na/H Exchanger Regulatory Factors Control Parathyroid Hormone Receptor Signaling by Facilitating Differential Activation of Gα Protein Subunits

The Na/H exchanger regulatory factors, NHERF1 and NHERF2, are adapter proteins involved in targeting and assembly of protein complexes. The parathyroid hormone receptor (PTHR) interacts with both NHERF1 and NHERF2. The NHERF proteins toggle PTHR signaling from predominantly activation of adenylyl cyclase in the absence of NHERF to principally stimulation of phospholipase C when the NHERF proteins are expressed. We hypothesized that this signaling switch occurs at the level of the G protein. We measured G protein activation by [³⁵S]GTPγS binding and Gα subtype-specific immunoprecipitation using three different cellular models of PTHR signaling. These studies revealed that PTHR interactions with NHERF1 enhance receptor-mediated stimulation of Gα_q but have no effect on stimulation of Gα_i or Gα_s. In contrast, PTHR associations with NHERF2 enhance receptor-mediated stimulation of both Gα_q and Gα_i but decrease stimulation of Gα_s. Consistent with these functional data, NHERF2 formed cellular complexes with both Gα_q and Gα_i, whereas NHERF1 was found to interact only with Gα_q. These findings demonstrate that NHERF interactions regulate PTHR signaling at the level of G proteins and that NHERF1 and NHERF2 exhibit isotype-specific effects on G protein activation.     

Assembly and Function of the Regulator of G protein Signaling 14 (RGS14)·H-Ras Signaling Complex in Live Cells Are Regulated by Gαi1 and Gαi-linked G Protein-coupled Receptors

Regulator of G protein signaling 14 (RGS14) is a multifunctional scaffolding protein that integrates heterotrimeric G protein and H-Ras signaling pathways. RGS14 possesses an RGS domain that binds active Gα_i/o-GTP subunits to promote GTP hydrolysis and a G protein regulatory (GPR) motif that selectively binds inactive Gα_i1/3-GDP subunits to form a stable heterodimer at cellular membranes. RGS14 also contains two tandem Ras/Rap binding domains (RBDs) that bind H-Ras. Here we show that RGS14 preferentially binds activated H-Ras-GTP in live cells to enhance H-Ras cellular actions and that this interaction is regulated by inactive Gα_i1-GDP and G protein-coupled receptors (GPCRs). Using bioluminescence resonance energy transfer (BRET) in live cells, we show that RGS14-Luciferase and active H-Ras(G/V)-Venus exhibit a robust BRET signal at the plasma membrane that is markedly enhanced in the presence of inactive Gα_i1-GDP but not active Gα_i1-GTP. Active H-Ras(G/V) interacts with a native RGS14·Gα_i1 complex in brain lysates, and co-expression of RGS14 and Gα_i1 in PC12 cells greatly enhances H-Ras(G/V) stimulatory effects on neurite outgrowth. Stimulation of the Gα_i-linked α_2A-adrenergic receptor induces a conformational change in the Gα_i1·RGS14·H-Ras(G/V) complex that may allow subsequent regulation of the complex by other binding partners. Together, these findings indicate that inactive Gα_i1-GDP enhances the affinity of RGS14 for H-Ras-GTP in live cells, resulting in a ternary signaling complex that is further regulated by GPCRs.     

Cannabinoid receptor interacting protein 1a interacts with myristoylated Gαi N terminus via a unique gapped β-barrel structure

Cannabinoid receptor interacting protein 1a (CRIP1a) modulates CB₁ cannabinoid receptor G-protein coupling in part by altering the selectivity for Gα_i subtype activation, but the molecular basis for this function of CRIP1a is not known. We report herein the first structure of CRIP1a at a resolution of 1.55 Å. CRIP1a exhibits a 10-stranded and antiparallel β-barrel with an interior comprised of conserved hydrophobic residues and loops at the bottom and a short helical cap at the top to exclude solvent. The β-barrel has a gap between strands β8 and β10, which deviates from β-sandwich fatty acid–binding proteins that carry endocannabinoid compounds and the Rho-guanine nucleotide dissociation inhibitor predicted by computational threading algorithms. The structural homology search program DALI identified CRIP1a as homologous to a family of lipidated-protein carriers that includes phosphodiesterase 6 delta subunit and Unc119. Comparison with these proteins suggests that CRIP1a may carry two possible types of cargo: either (i) like phosphodiesterase 6 delta subunit, cargo with a farnesyl moiety that enters from the top of the β-barrel to occupy the hydrophobic interior or (ii) like Unc119, cargo with a palmitoyl or a myristoyl moiety that enters from the side where the missing β-strand creates an opening to the hydrophobic pocket. Fluorescence polarization analysis demonstrated CRIP1a binding of an N-terminally myristoylated 9-mer peptide mimicking the Gα_i N terminus. However, CRIP1a could not bind the nonmyristolyated Gα_i peptide or cargo of homologs. Thus, binding of CRIP1a to Gαi proteins represents a novel mechanism to regulate cell signaling initiated by the CB₁ receptor.     

Drosophila GoLoco-Protein Pins Is a Target of Gαo-mediated G Protein–coupled Receptor Signaling

G protein–coupled receptors (GPCRs) transduce their signals through trimeric G proteins, inducing guanine nucleotide exchange on their Gα-subunits; the resulting Gα-GTP transmits the signal further inside the cell. GoLoco domains present in many proteins play important roles in multiple trimeric G protein–dependent activities, physically binding Gα-subunits of the Gα_i/o class. In most cases GoLoco binds exclusively to the GDP-loaded form of the Gα-subunits. Here we demonstrate that the poly-GoLoco–containing protein Pins of Drosophila can bind to both GDP- and GTP-forms of Drosophila Gα_o. We identify Pins GoLoco domain 1 as necessary and sufficient for this unusual interaction with Gα_o-GTP. We further pinpoint a lysine residue located centrally in this domain as necessary for the interaction. Our studies thus identify Drosophila Pins as a target of Gα_o-mediated GPCR receptor signaling, e.g., in the context of the nervous system development, where Gα_o acts downstream from Frizzled and redundantly with Gα_i to control the asymmetry of cell divisions.     

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 Gα_s (Gα_sQ227L), which enabled endogenous PRG to gain affinity for Cdc42. A chemogenetic approach revealed that signaling by G_s-coupled receptors, but not by those coupled to G_i or G_q, enabled PRG to bind Cdc42. This receptor-dependent effect, as well as CREB phosphorylation, was blocked by a construct derived from the PRG:Gα_s-binding region, PRG-linker. Active Gα_s interacted with isolated PRG DH and PH domains and their linker. In addition, this construct interfered with Gα_sQ227L''s ability to guide PRG''s interaction with Cdc42. Endogenous G_s-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 Gα_s can recognize PRG as a novel effector directing its DH/PH catalytic module to gain affinity for Cdc42.     

Different Biochemical Properties Explain Why Two Equivalent Gα Subunit Mutants Cause Unrelated Diseases

There is an increasing number of disease-associated Gα mutations identified from genome-wide sequencing campaigns or targeted efforts. Albright''s Hereditary Osteodystrophy (AHO) was the first inherited disease associated with loss-of-function mutations in a G protein (Gαs) and other studies revealed gain-of-function Gα mutations in cancer. Here we attempted to solve the apparent quandary posed by the fact that the same mutation in two different G proteins appeared associated with both AHO and cancer. We first confirmed the presence of an inherited Gαs-R265H mutation from a previously described clinical case report of AHO. This mutation is structurally analogous to Gαo-R243H, an oncogenic mutant with increased activity in vitro and in cells due to rapid nucleotide exchange. We found that, contrary to Gαo-R243H, Gαs-R265H activity is compromised due to greatly impaired nucleotide binding in vitro and in cells. We obtained equivalent results when comparing another AHO mutation in Gαs (D173N) with a counterpart cancer mutation in Gαo (D151N). Gαo-R243H binds nucleotides efficiently under steady-state conditions but releases GDP much faster than the WT protein, suggesting diminished affinity for the nucleotide. These results indicate that the same disease-linked mutation in two different G proteins affects a common biochemical feature (nucleotide affinity) but to a different grade depending on the G protein (mild decrease for Gαo and severe for Gαs). We conclude that Gαs-R265H has dramatically impaired nucleotide affinity leading to the loss-of-function in AHO whereas Gαo-R243H has a mild decrease in nucleotide affinity that causes rapid nucleotide turnover and subsequent hyperactivity in cancer.     

Integration of G Protein α (Gα) Signaling by the Regulator of G Protein Signaling 14 (RGS14)

RGS14 contains distinct binding sites for both active (GTP-bound) and inactive (GDP-bound) forms of Gα subunits. The N-terminal regulator of G protein signaling (RGS) domain binds active Gα_i/o-GTP, whereas the C-terminal G protein regulatory (GPR) motif binds inactive Gα_i1/3-GDP. The molecular basis for how RGS14 binds different activation states of Gα proteins to integrate G protein signaling is unknown. Here we explored the intramolecular communication between the GPR motif and the RGS domain upon G protein binding and examined whether RGS14 can functionally interact with two distinct forms of Gα subunits simultaneously. Using complementary cellular and biochemical approaches, we demonstrate that RGS14 forms a stable complex with inactive Gα_i1-GDP at the plasma membrane and that free cytosolic RGS14 is recruited to the plasma membrane by activated Gα_o-AlF₄⁻. Bioluminescence resonance energy transfer studies showed that RGS14 adopts different conformations in live cells when bound to Gα in different activation states. Hydrogen/deuterium exchange mass spectrometry revealed that RGS14 is a very dynamic protein that undergoes allosteric conformational changes when inactive Gα_i1-GDP binds the GPR motif. Pure RGS14 forms a ternary complex with Gα_o-AlF₄⁻ and an AlF₄⁻-insensitive mutant (G42R) of Gα_i1-GDP, as observed by size exclusion chromatography and differential hydrogen/deuterium exchange. Finally, a preformed RGS14·Gα_i1-GDP complex exhibits full capacity to stimulate the GTPase activity of Gα_o-GTP, demonstrating that RGS14 can functionally engage two distinct forms of Gα subunits simultaneously. Based on these findings, we propose a working model for how RGS14 integrates multiple G protein signals in host CA2 hippocampal neurons to modulate synaptic plasticity.     

Activation Biosensor for G Protein-Coupled Receptors: A FRET-Based m1 Muscarinic Activation Sensor That Regulates Gq

We describe the design, construction and validation of a fluorescence sensor to measure activation by agonist of the m1 muscarinic cholinergic receptor, a prototypical class I G_q-coupled receptor. The sensor uses an established general design in which Förster resonance energy transfer (FRET) from a circularly permuted CFP mutant to FlAsH, a selectively reactive fluorescein, is decreased 15–20% upon binding of a full agonist. Notably, the sensor displays essentially wild-type capacity to catalyze activation of Gα_q, and the purified and reconstituted sensor displays appropriate regulation of affinity for agonists by G_q. We describe the strategies used to increase the agonist-driven change in FRET while simultaneously maintaining regulatory interactions with Gα_q, in the context of the known structures of Class I G protein-coupled receptors. The approach should be generally applicable to other Class I receptors which include numerous important drug targets.     

Crystal Structures of the Scaffolding Protein LGN Reveal the General Mechanism by Which GoLoco Binding Motifs Inhibit the Release of GDP from Gαi

GoLoco (GL) motif-containing proteins regulate G protein signaling by binding to Gα subunit and acting as guanine nucleotide dissociation inhibitors. GLs of LGN are also known to bind the GDP form of Gα_i/o during asymmetric cell division. Here, we show that the C-terminal GL domain of LGN binds four molecules of Gα_i·GDP. The crystal structures of Gα_i·GDP in complex with LGN GL3 and GL4, respectively, reveal distinct GL/Gα_i interaction features when compared with the only high resolution structure known with GL/Gα_i interaction between RGS14 and Gα_i1. Only a few residues C-terminal to the conserved GL sequence are required for LGN GLs to bind to Gα_i·GDP. A highly conserved “double Arg finger” sequence (RΨ(D/E)(D/E)QR) is responsible for LGN GL to bind to GDP bound to Gα_i. Together with the sequence alignment, we suggest that the LGN GL/Gα_i interaction represents a general binding mode between GL motifs and Gα_i. We also show that LGN GLs are potent guanine nucleotide dissociation inhibitors.     

Gastrin-stimulated Gα13 Activation of Rgnef Protein (ArhGEF28) in DLD-1 Colon Carcinoma Cells

The guanine nucleotide exchange factor Rgnef (also known as ArhGEF28 or p190RhoGEF) promotes colon carcinoma cell motility and tumor progression via interaction with focal adhesion kinase (FAK). Mechanisms of Rgnef activation downstream of integrin or G protein-coupled receptors remain undefined. In the absence of a recognized G protein signaling homology domain in Rgnef, no proximal linkage to G proteins was known. Utilizing multiple methods, we have identified Rgnef as a new effector for Gα₁₃ downstream of gastrin and the type 2 cholecystokinin receptor. In DLD-1 colon carcinoma cells depleted of Gα₁₃, gastrin-induced FAK Tyr(P)-397 and paxillin Tyr(P)-31 phosphorylation were reduced. RhoA GTP binding and promoter activity were increased by Rgnef in combination with active Gα₁₃. Rgnef co-immunoprecipitated with activated Gα₁₃Q226L but not Gα₁₂Q229L. The Rgnef C-terminal (CT, 1279–1582) region was sufficient for co-immunoprecipitation, and Rgnef-CT exogenous expression prevented Gα₁₃-stimulated SRE activity. A domain at the C terminus of the protein close to the FAK binding domain is necessary to bind to Gα₁₃. Point mutations of Rgnef-CT residues disrupt association with active Gα₁₃ but not Gα_q. These results show that Rgnef functions as an effector of Gα₁₃ signaling and that this linkage may mediate FAK activation in DLD-1 colon carcinoma cells.     

Stargazin Modulates Neuronal Voltage-dependent Ca2+ Channel Cav2.2 by a Gβγ-dependent Mechanism

Loss of neuronal protein stargazin (γ₂) is associated with recurrent epileptic seizures and ataxia in mice. Initially, due to homology to the skeletal muscle calcium channel γ₁ subunit, stargazin and other family members (γ_3–8) were classified as γ subunits of neuronal voltage-gated calcium channels (such as Ca_V2.1-Ca_V2.3). Here, we report that stargazin interferes with G protein modulation of Ca_V2.2 (N-type) channels expressed in Xenopus oocytes. Stargazin counteracted the Gβγ-induced inhibition of Ca_V2.2 channel currents, caused either by coexpression of the Gβγ dimer or by activation of a G protein-coupled receptor. Expression of high doses of Gβγ overcame the effects of stargazin. High affinity Gβγ scavenger proteins m-cβARK and m-phosducin produced effects similar to stargazin. The effects of stargazin and m-cβARK were not additive, suggesting a common mechanism of action, and generally independent of the presence of the Ca_Vβ₃ subunit. However, in some cases, coexpression of Ca_Vβ₃ blunted the modulation by stargazin. Finally, the Gβγ-opposing action of stargazin was not unique to Ca_V2.2, as stargazin also inhibited the Gβγ-mediated activation of the G protein-activated K⁺ channel. Purified cytosolic C-terminal part of stargazin bound Gβγ in vitro. Our results suggest that the regulation by stargazin of biophysical properties of Ca_V2.2 are not exerted by direct modulation of the channel but via a Gβγ-dependent mechanism.     

Osmotic Stress Reduces Ca2+ Signals through Deformation of Caveolae

Caveolae are membrane invaginations that can sequester various signaling proteins. Caveolae have been shown to provide mechanical strength to cells by flattening to accommodate increased volume when cells are subjected to hypo-osmotic stress. We have previously found that caveolin, the main structural component of caveolae, specifically binds Gα_q and stabilizes its activation state resulting in an enhanced Ca²⁺ signal upon activation. Here, we show that osmotic stress caused by decreasing the osmolarity in half reversibly changes the configuration of caveolae without releasing a significant portion of caveolin molecules. This change in configuration due to flattening leads to a loss in Cav1-Gα_q association. This loss in Gα_q/Cav1 association due to osmotic stress results in a significant reduction of Gα_q/phospholipase Cβ-mediated Ca²⁺ signals. This reduced Ca²⁺ response is also seen when caveolae are reduced by treatment with siRNA(Cav1) or by dissolving them by methyl-β-cyclodextran. No change in Ca²⁺ release with osmotic swelling can be seen when growth factor pathways are activated. Taken together, these results connect the mechanical deformation of caveolae to Gα_q-mediated Ca²⁺ signals.     

Cardiac Overexpression of Constitutively Active Galpha q Causes Angiotensin II Type1 Receptor Activation,Leading to Progressive Heart Failure and Ventricular Arrhythmias in Transgenic Mice

Background

Transgenic mice with transient cardiac expression of constitutively active Galpha q (Gα_q-TG) exhibt progressive heart failure and ventricular arrhythmias after the initiating stimulus of transfected constitutively active Gα_q becomes undetectable. However, the mechanisms are still unknown. We examined the effects of chronic administration of olmesartan on heart failure and ventricular arrhythmia in Gα_q-TG mice.

Methodology/Principal Findings

Olmesartan (1 mg/kg/day) or vehicle was chronically administered to Gα_q-TG from 6 to 32 weeks of age, and all experiments were performed in mice at the age of 32 weeks. Chronic olmesartan administration prevented the severe reduction of left ventricular fractional shortening, and inhibited ventricular interstitial fibrosis and ventricular myocyte hypertrophy in Gα_q-TG. Electrocardiogram demonstrated that premature ventricular contraction (PVC) was frequently (more than 20 beats/min) observed in 9 of 10 vehicle-treated Gα_q-TG but in none of 10 olmesartan-treated Gα_q-TG. The collected QT interval and monophasic action potential duration in the left ventricle were significantly shorter in olmesartan-treated Gα_q-TG than in vehicle-treated Gα_q-TG. CTGF, collagen type 1, ANP, BNP, and β-MHC gene expression was increased and olmesartan significantly decreased the expression of these genes in Gα_q-TG mouse ventricles. The expression of canonical transient receptor potential (TRPC) 3 and 6 channel and angiotensin converting enzyme (ACE) proteins but not angiotensin II type 1 (AT₁) receptor was increased in Gα_q-TG ventricles compared with NTG mouse ventricles. Olmesartan significantly decreased TRPC6 and tended to decrease ACE expressions in Gα_q-TG. Moreover, it increased AT₁ receptor in Gα_q-TG.

Conclusions/Significance

These findings suggest that angiotensin II type 1 receptor activation plays an important role in the development of heart failure and ventricular arrhythmia in Gα_q-TG mouse model of heart failure.     

A naturally occurring membrane-anchored Gαs variant,XLαs,activates phospholipase Cβ4

Extra-large stimulatory Gα (XLα_s) is a large variant of G protein α_s subunit (Gα_s) that uses an alternative promoter and thus differs from Gα_s at the first exon. XLα_s activation by G protein–coupled receptors mediates cAMP generation, similarly to Gα_s; however, Gα_s and XLα_s have been shown to have distinct cellular and physiological functions. For example, previous work suggests that XLα_s can stimulate inositol phosphate production in renal proximal tubules and thereby regulate serum phosphate levels. In this study, we show that XLα_s directly and specifically stimulates a specific isoform of phospholipase Cβ (PLCβ), PLCβ4, both in transfected cells and with purified protein components. We demonstrate that neither the ability of XLα_s to activate cAMP generation nor the canonical G protein switch II regions are required for PLCβ stimulation. Furthermore, this activation is nucleotide independent but is inhibited by Gβγ, suggesting a mechanism of activation that relies on Gβγ subunit dissociation. Surprisingly, our results indicate that enhanced membrane targeting of XLα_s relative to Gα_s confers the ability to activate PLCβ4. We also show that PLCβ4 is required for isoproterenol-induced inositol phosphate accumulation in osteocyte-like Ocy454 cells. Taken together, we demonstrate a novel mechanism for activation of phosphoinositide turnover downstream of G_s-coupled receptors that may have a critical role in endocrine physiology.