Strike a pose: Gαq complexes at the membrane

Hepatic GRK2 is dispensable for glucose homeostasis and other key metabolic parameters in mice

OBJECTIVE

G-protein-coupled receptor (GPCR) kinases (GRKs) abrogate GPCR signaling by promoting receptor desensitization and internalization. Accumulating evidence suggests that GRK2 represents an important regulator of GPCR-mediated effects on systemic glucose metabolism, obesity, and insulin resistance. Despite the key role of the liver in maintaining euglycemia, the potential metabolic relevance of hepatic GRK2 has yet to be examined. Thus, the goal of this study was to explore the potential role of hepatic GRK2 in maintaining glucose homeostasis and other key metabolic functions.

METHODS

To address this question, we generated mice that showed a ∼90% reduction in GRK2 protein expression selectively in hepatocytes (Hep-GRK2-KO mice) and subjected these mice, together with their control littermates, to systematic metabolic phenotyping studies.

RESULTS

We found that Hep-GRK2-KO mice maintained on regular chow did not differ significantly from their control littermates in glycemia, glucose tolerance, insulin sensitivity, in vivo gluconeogenesis, and glucagon-induced hyperglycemia. We obtained similar findings when we analyzed Hep-GRK2-KO mice and control littermates consuming an obesogenic high-fat diet. Likewise, plasma levels of insulin, glucagon, free fatty acids, and ketone bodies remained unaffected by the lack of hepatocyte GRK2. The same was true when we examined the expression levels of key genes regulating hepatic glucose and fatty acid metabolism.

CONCLUSION

In summary, our data suggest that hepatocyte GRK2 is dispensable for systemic glucose homeostasis and other key metabolic functions in both lean and obese mice. This finding suggests that drug development efforts aimed at inhibiting GRK2 to improve impaired glucose homeostasis and insulin sensitivity need to focus on other metabolically important tissues.

A regulatory node involving Gαq, PLCβ, and RGS proteins modulates platelet reactivity to critical agonists

BACKGROUND

Most platelet agonists work through G protein-coupled receptors, activating pathways that involve members of the Gq, Gi, and G12/G13 families of heterotrimeric G proteins. Gq signaling has been shown to be critical for efficient platelet activation. Growing evidence suggests that regulatory mechanisms converge on G protein-coupled receptors and Gq to prevent overly robust platelet reactivity.

OBJECTIVES

To identify and characterize mechanisms by which Gq signaling is regulated in platelets.

METHODS

Based on our prior experience with a Gαi2 variant that escapes regulation by regulator of G protein signaling (RGS) proteins, a Gαq variant was designed with glycine 188 replaced with serine (G188S) and then incorporated into a mouse line so that its effects on platelet activation and thrombus formation could be studied in vitro and in vivo.

RESULTS AND CONCLUSIONS

As predicted, the G188S substitution in Gαq disrupted its interaction with RGS18. Unexpectedly, it also uncoupled PLCβ-3 from activation by platelet agonists as evidenced by a loss rather than a gain of platelet function in vitro and in vivo. Binding studies showed that in addition to preventing the binding of RGS18 to Gαq, the G188S substitution also prevented the binding of PLCβ-3 to Gαq. Structural analysis revealed that G188 resides in the region that is also important for Gαq binding to PLCβ-3 in platelets. We conclude that the Gαq signaling node is more complex than that has been previously understood, suggesting that there is cross-talk between RGS proteins and PLCβ-3 in the context of Gαq signaling.

The mechanism of Gαq regulation of PLCβ3-catalyzed PIP2 hydrolysis

PLCβ (Phospholipase Cβ) enzymes cleave phosphatidylinositol 4,5-bisphosphate (PIP2) producing IP3 and DAG (diacylglycerol). PIP2 modulates the function of many ion channels, while IP3 and DAG regulate intracellular Ca2+ levels and protein phosphorylation by protein kinase C, respectively. PLCβ enzymes are under the control of G protein coupled receptor signaling through direct interactions with G proteins Gβγ and Gαq and have been shown to be coincidence detectors for dual stimulation of Gαq and Gαi-coupled receptors. PLCβs are aqueous-soluble cytoplasmic enzymes but partition onto the membrane surface to access their lipid substrate, complicating their functional and structural characterization. Using newly developed methods, we recently showed that Gβγ activates PLCβ3 by recruiting it to the membrane. Using these same methods, here we show that Gαq increases the catalytic rate constant, kcat, of PLCβ3. Since stimulation of PLCβ3 by Gαq depends on an autoinhibitory element (the X-Y linker), we propose that Gαq produces partial relief of the X-Y linker autoinhibition through an allosteric mechanism. We also determined membrane-bound structures of the PLCβ3·Gαq and PLCβ3·Gβγ(2)·Gαq complexes, which show that these G proteins can bind simultaneously and independently of each other to regulate PLCβ3 activity. The structures rationalize a finding in the enzyme assay, that costimulation by both G proteins follows a product rule of each independent stimulus. We conclude that baseline activity of PLCβ3 is strongly suppressed, but the effect of G proteins, especially acting together, provides a robust stimulus upon G protein stimulation.

The mechanism of Gα q regulation of PLCβ3 -catalyzed PIP2 hydrolysis

PLCβ enzymes cleave PIP2 producing IP3 and DAG. PIP2 modulates the function of many ion channels, while IP3 and DAG regulate intracellular Ca 2+ levels and protein phosphorylation by protein kinase C, respectively. PLCβ enzymes are under the control of GPCR signaling through direct interactions with G proteins Gβγ and Gα q and have been shown to be coincidence detectors for dual stimulation of Gα q and G α i coupled receptors. PLCβs are aqueous-soluble cytoplasmic enzymes, but partition onto the membrane surface to access their lipid substrate, complicating their functional and structural characterization. Using newly developed methods, we recently showed that Gβγ activates PLCβ3 by recruiting it to the membrane. Using these same methods, here we show that Gα q increases the catalytic rate constant, k cat , of PLCβ3 . Since stimulation of PLCβ3 by Gα q depends on an autoinhibitory element (the X-Y linker), we propose that Gα q produces partial relief of the X-Y linker autoinhibition through an allosteric mechanism. We also determined membrane-bound structures of the PLCβ3-Gα q , and PLCβ3-Gβγ(2)-Gα q complexes, which show that these G proteins can bind simultaneously and independently of each other to regulate PLCβ3 activity. The structures rationalize a finding in the enzyme assay, that co-stimulation by both G proteins follows a product rule of each independent stimulus. We conclude that baseline activity of PLCβ3 is strongly suppressed, but the effect of G proteins, especially acting together, provides a robust stimulus upon G protein stimulation.

Significance Statement

For certain cellular signaling processes, the background activity of signaling enzymes must be minimal and stimulus-dependent activation robust. Nowhere is this truer than in signaling by PLCβ3 , whose activity regulates intracellular Ca 2+ , phosphorylation by Protein Kinase C, and the activity of numerous ion channels and membrane receptors. In this study we show how PLCβ3 enzymes are regulated by two kinds of G proteins, Gβγ and Gα q . Enzyme activity studies and structures on membranes show how these G proteins act by separate, independent mechanisms, leading to a product rule of co-stimulation when they act together. The findings explain how cells achieve robust stimulation of PLCβ3 in the setting of very low background activity, properties essential to cell health and survival.

P2Y1R and P2Y2R: potential molecular triggers in muscle regeneration

Muscle regeneration is indispensable for skeletal muscle health and daily life when injury, muscular disease, and aging occur. Among the muscle regeneration, muscle stem cells' (MuSCs) activation, proliferation, and differentiation play a key role in muscle regeneration. Purines bind to its specific receptors during muscle development, which transmit environmental stimuli and play a crucial role of modulator of muscle regeneration. Evidences proved P2R expression during development and regeneration of skeletal muscle, both in human and mouse. In contrast to P2XR, which have been extensively investigated in skeletal muscles, the knowledge of P2YR in this tissue is less comprehensive. This review summarized muscle regeneration via P2Y1R and P2Y2R and speculated that P2Y1R and P2Y2R might be potential molecular triggers for MuSCs' activation and proliferation via the p-ERK1/2 and PLC pathways, explored their cascade effects on skeletal muscle, and proposed P2Y1/2 receptors as potential pharmacological targets in muscle regeneration, to advance the purinergic signaling within muscle and provide promising strategies for alleviating muscular disease.

Selective optogenetic control of Gq signaling using human Neuropsin

G_q proteins are universally important for signal transduction in mammalian cells. The underlying kinetics and transformation from extracellular stimuli into intracellular signaling, however could not be investigated in detail so far. Here we present the human Neuropsin (hOPN5) for specific and repetitive manipulation of G_q signaling in vitro and in vivo with high spatio-temporal resolution. Properties and G protein specificity of hOPN5 are characterized by UV light induced IP₃ generation, Ca²⁺ transients and inhibition of G_IRK channel activity in HEK cells. In adult hearts from a transgenic animal model, light increases the spontaneous beating rate. In addition, we demonstrate light induced contractions in the small intestine, which are not detectable after pharmacological G_q protein block. All-optical high-throughput screening for TRPC6 inhibitors is more specific and sensitive than conventional pharmacological screening. Thus, we demonstrate specific G_q signaling of hOPN5 and unveil its potential for optogenetic applications.

BRET-based effector membrane translocation assay monitors GPCR-promoted and endocytosis-mediated Gq activation at early endosomes

G protein-coupled receptors (GPCRs) are gatekeepers of cellular homeostasis and the targets of a large proportion of drugs. In addition to their signaling activity at the plasma membrane, it has been proposed that their actions may result from translocation and activation of G proteins at endomembranes-namely endosomes. This could have a significant impact on our understanding of how signals from GPCR-targeting drugs are propagated within the cell. However, little is known about the mechanisms that drive G protein movement and activation in subcellular compartments. Using bioluminescence resonance energy transfer (BRET)-based effector membrane translocation assays, we dissected the mechanisms underlying endosomal G_q trafficking and activity following activation of G_q-coupled receptors, including the angiotensin II type 1, bradykinin B₂, oxytocin, thromboxane A₂ alpha isoform, and muscarinic acetylcholine M₃ receptors. Our data reveal that GPCR-promoted activation of G_q at the plasma membrane induces its translocation to endosomes independently of β-arrestin engagement and receptor endocytosis. In contrast, G_q activity at endosomes was found to rely on both receptor endocytosis-dependent and -independent mechanisms. In addition to shedding light on the molecular processes controlling subcellular G_q signaling, our study provides a set of tools that will be generally applicable to the study of G protein translocation and activation at endosomes and other subcellular organelles, as well as the contribution of signal propagation to drug action.

Dissociation of the G protein βγ from the Gq-PLCβ complex partially attenuates PIP2 hydrolysis

Phospholipase C β (PLCβ), which is activated by the Gq family of heterotrimeric G proteins, hydrolyzes the inner membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP2), generating diacylglycerol and inositol 1,4,5-triphosphate (IP3). Because Gq and PLCβ regulate many crucial cellular processes and have been identified as major disease drivers, activation and termination of PLCβ signaling by the Gαq subunit have been extensively studied. Gq-coupled receptor activation induces intense and transient PIP2 hydrolysis, which subsequently recovers to a low-intensity steady-state equilibrium. However, the molecular underpinnings of this equilibrium remain unclear. Here, we explored the influence of signaling crosstalk between Gq and Gi/o pathways on PIP2 metabolism in living cells using single-cell and optogenetic approaches to spatially and temporally constrain signaling. Our data suggest that the Gβγ complex is a component of the highly efficient lipase Gαq_GTP-PLCβ-Gβγ. We found that over time, Gβγ dissociates from this lipase complex, leaving the less-efficient Gαq_GTP-PLCβ lipase complex and allowing the significant partial recovery of PIP2 levels. Our findings also indicate that the subtype of the Gγ subunit in Gβγ fine-tunes the lipase activity of Gq-PLCβ, in which cells expressing Gγ with higher plasma membrane interaction show lower PIP2 recovery. Given that Gγ shows cell- and tissue-specific subtype expression, our findings suggest the existence of tissue-specific distinct Gq-PLCβ signaling paradigms. Furthermore, these results also outline a molecular process that likely safeguards cells from excessive Gq signaling.

Dissecting Gq/11-Mediated Plasma Membrane Translocation of Sphingosine Kinase-1

Diverse extracellular signals induce plasma membrane translocation of sphingosine kinase-1 (SphK1), thereby enabling inside-out signaling of sphingosine-1-phosphate. We have shown before that G_q-coupled receptors and constitutively active Gα_q/11 specifically induced a rapid and long-lasting SphK1 translocation, independently of canonical G_q/phospholipase C (PLC) signaling. Here, we further characterized G_q/11 regulation of SphK1. SphK1 translocation by the M₃ receptor in HEK-293 cells was delayed by expression of catalytically inactive G-protein-coupled receptor kinase-2, p63Rho guanine nucleotide exchange factor (p63RhoGEF), and catalytically inactive PLCβ₃, but accelerated by wild-type PLCβ₃ and the PLCδ PH domain. Both wild-type SphK1 and catalytically inactive SphK1-G82D reduced M₃ receptor-stimulated inositol phosphate production, suggesting competition at Gα_q. Embryonic fibroblasts from Gα_q/11 double-deficient mice were used to show that amino acids W263 and T257 of Gα_q, which interact directly with PLCβ₃ and p63RhoGEF, were important for bradykinin B₂ receptor-induced SphK1 translocation. Finally, an AIXXPL motif was identified in vertebrate SphK1 (positions 100–105 in human SphK1a), which resembles the Gα_q binding motif, ALXXPI, in PLCβ and p63RhoGEF. After M₃ receptor stimulation, SphK1-A100E-I101E and SphK1-P104A-L105A translocated in only 25% and 56% of cells, respectively, and translocation efficiency was significantly reduced. The data suggest that both the AIXXPL motif and currently unknown consequences of PLCβ/PLCδ(PH) expression are important for regulation of SphK1 by G_q/11.

Targeting Opsin4/Melanopsin with a Novel Small Molecule Suppresses PKC/RAF/MEK/ERK Signaling and Inhibits Lung Adenocarcinoma Progression

The identification of oncogenic biomolecules as drug targets is an unmet need for the development of clinically effective novel anticancer therapies. In this study, we report for the first time that opsin 4/melanopsin (OPN4) plays a critical role in the pathogenesis of non-small cell lung cancer (NSCLC) and is a potential drug target. Our study has revealed that OPN4 is overexpressed in human lung cancer tissues and cells, and is inversely correlated with patient survival probability. Knocking down expression of OPN4 suppressed cells growth and induced apoptosis in lung cancer cells. We have also found that OPN4, a G protein-coupled receptor, interacted with Gα11 and triggered the PKC/BRAF/MEK/ERKs signaling pathway in lung adenocarcinoma cells. Genetic ablation of OPN4 attenuated the multiplicity and the volume of urethane-induced lung tumors in mice. Importantly, our study provides the first report of AE 51310 (1-[(2,5-dichloro-4-methoxyphenyl)sulfonyl]-3-methylpiperidine) as a small-molecule inhibitor of OPN4, suppressed the anchorage-independent growth of lung cancer cells and the growth of patient-derived xenograft tumors in mice. IMPLICATIONS: Overall, this study unveils the role of OPN4 in NSCLC and suggests that targeting OPN4 with small molecules, such as AE 51310 would be interesting to develop novel anticancer therapies for lung adenocarcinoma.

Dampening of neurotransmitter action: molecular similarity within the melatonin structure

OBJECTIVES

Melatonin initiates physiologic and therapeutic responses in various tissues through binding to poorly defined MT receptors regulated by G-proteins and purine nucleotides. Melatonin's interaction with other G-protein regulated receptors, including those of serotonin, is unclear. This study explores the potential for the interaction of melatonin with nucleotide and receptor ligand structures.

METHODS

The study uses a computational program to investigate relative molecular similarity by the comparative superimposition and quantitative fitting of molecular structures to adenine and guanine nucleotide templates.

RESULTS

A minimum energy melatonin conformer replicates the nucleotide fits of ligand structures that regulate Gαi and Gαq proteins via serotonin, dopamine, opioid, α-adrenoceptor, and muscarinic receptor classes. The same conformer also replicates the nucleotide fits of ligand structures regulating K+ and Ca2+ ion channels. The acyl-methoxy distance within the melatonin conformer matches a carbonyl-hydroxyl distance in guanine nucleotide.

CONCLUSION

Molecular similarity within the melatonin and ligand structures relates to the established effects of melatonin on cell receptors regulated by purine nucleotides in cell signal transduction processes. Pharmacologic receptor promiscuity may contribute to the widespread effects of melatonin.

Cell signal transduction: hormones, neurotransmitters and therapeutic drugs relate to purine nucleotide structure

Purine nucleotides transduce cell membrane receptor responses and modulate ion channel activity. This is accomplished through conformational change in the structure of nucleotides and cell membrane associated proteins. The aim of this study is to enhance our understanding of nucleotide dependence in regard to signal transduction events, drug action and pharmacological promiscuity. Nucleotides and ligand structures regulating Gα protein subunits, voltage- and ligand-gated ion channels are investigated for molecular similarity using a computational program. Results differentiate agonist and antagonist structures, identify molecular similarity within nucleotide and ligand structures and demonstrate the potential of ligands to regulate nucleotide conformational change. Relative molecular similarity within nucleotides and the ligands of the major receptor classes provides insight into mechanisms of receptor and ion channel regulation. The nucleotide template model has some merit as an initial screening tool in the study and comparison of drug and hormone structures.

Phosphatidic acid binding inhibits RGS1 activity to affect specific signaling pathways in Arabidopsis

Modulation of the active versus inactive forms of the Gα protein is critical for the signaling processes mediated by the heterotrimeric G-protein complex. We have recently established that in Arabidopsis, the regulator of G-protein signaling (RGS1) protein and a lipid-hydrolyzing enzyme, phospholipase Dα1 (PLDα1), both act as GTPase-activity accelerating proteins (GAPs) for the Gα protein to attenuate its activity. RGS1 and PLDα1 interact with each other, and RGS1 inhibits the activity of PLDα1 during regulation of a subset of responses. In this study, we present evidence that this regulation is bidirectional. Phosphatidic acid (PA), a second messenger typically derived from the lipid-hydrolyzing activity of PLDα1, is a molecular target of RGS1. PA binds and inhibits the GAP activity of RGS1. A conserved lysine residue in RGS1 (Lys²⁵⁹ ) is directly involved in RGS1-PA binding. Introduction of this RGS1 protein variant in the rgs1 mutant background makes plants hypersensitive to a subset of abscisic acid-mediated responses. Our data point to the existence of negative feedback loops between these two regulatory proteins that precisely modulate the level of active Gα, consequently generating a highly controlled signal-response output.

ORP4L Facilitates Macrophage Survival via G-Protein-Coupled Signaling: ORP4L-/- Mice Display a Reduction of Atherosclerosis

RATIONALE

Macrophage survival within the arterial wall is a central factor contributing to atherogenesis. Oxysterols, major components of oxidized low-density lipoprotein, exert cytotoxic effects on macrophages.

OBJECTIVE

To determine whether oxysterol-binding protein-related protein 4 L (ORP4L), an oxysterol-binding protein, affects macrophage survival and the pathogenesis of atherosclerosis.

METHODS AND RESULTS

By hiring cell biological approaches and ORP4L^-/- mice, we show that ORP4L coexpresses with and forms a complex with Gα_q/11 and phospholipase C (PLC)-β3 in macrophages. ORP4L facilitates G-protein-coupled ligand-induced PLCβ3 activation, IP₃ production, and Ca²⁺ release from the endoplasmic reticulum. Through this mechanism, ORP4L sustains antiapoptotic Bcl-XL expression through Ca²⁺-mediated c-AMP responsive element binding protein transcriptional regulation and thus protects macrophages from apoptosis. Excessive stimulation with the oxysterol 25-hydroxycholesterol disassembles the ORP4L/Gα_q/11/PLCβ3 complexes, resulting in reduced PLCβ3 activity, IP₃ production, and Ca²⁺ release, as well as decreased Bcl-XL expression and increased apoptosis. Overexpression of ORP4L counteracts these oxysterol-induced defects. Mice lacking ORP4L exhibit increased apoptosis of macrophages in atherosclerotic lesions and a reduced lesion size.

CONCLUSIONS

ORP4L is crucial for macrophage survival. It counteracts the cytotoxicity of oxysterols/oxidized low-density lipoprotein to protect macrophage from apoptosis, thus playing an important role in the development of atherosclerosis.

Phospholipase Cβ1 induces membrane tubulation and is involved in caveolae formation

Lipid membrane curvature plays important roles in various physiological phenomena. Curvature-regulated dynamic membrane remodeling is achieved by the interaction between lipids and proteins. So far, several membrane sensing/sculpting proteins, such as Bin/amphiphysin/Rvs (BAR) proteins, are reported, but there remains the possibility of the existence of unidentified membrane-deforming proteins that have not been uncovered by sequence homology. To identify new lipid membrane deformation proteins, we applied liposome-based microscopic screening, using unbiased-darkfield microscopy. Using this method, we identified phospholipase Cβ1 (PLCβ1) as a new candidate. PLCβ1 is well characterized as an enzyme catalyzing the hydrolysis of phosphatidylinositol-4,5-bisphosphate (PIP2). In addition to lipase activity, our results indicate that PLCβ1 possessed the ability of membrane tubulation. Lipase domains and inositol phospholipids binding the pleckstrin homology (PH) domain of PLCβ1 were not involved, but the C-terminal sequence was responsible for this tubulation activity. Computational modeling revealed that the C terminus displays the structural homology to the BAR domains, which is well known as a membrane sensing/sculpting domain. Overexpression of PLCβ1 caused plasma membrane tubulation, whereas knockdown of the protein reduced the number of caveolae and induced the evagination of caveolin-rich membrane domains. Taken together, our results suggest a new function of PLCβ1: plasma membrane remodeling, and in particular, caveolae formation.

A peptide of the RGS domain of GRK2 binds and inhibits Gα(q) to suppress pathological cardiac hypertrophy and dysfunction

G protein-coupled receptor (GPCR) kinases (GRKs) play a critical role in cardiac function by regulating GPCR activity. GRK2 suppresses GPCR signaling by phosphorylating and desensitizing active GPCRs, and through protein-protein interactions that uncouple GPCRs from their downstream effectors. Several GRK2 interacting partners, including Gα(q), promote maladaptive cardiac hypertrophy, which leads to heart failure, a leading cause of mortality worldwide. The regulator of G protein signaling (RGS) domain of GRK2 interacts with and inhibits Gα(q) in vitro. We generated TgβARKrgs mice with cardiac-specific expression of the RGS domain of GRK2 and subjected these mice to pressure overload to trigger adaptive changes that lead to heart failure. Unlike their nontransgenic littermate controls, the TgβARKrgs mice exhibited less hypertrophy as indicated by reduced left ventricular wall thickness, decreased expression of genes linked to cardiac hypertrophy, and less adverse structural remodeling. The βARKrgs peptide, but not endogenous GRK2, interacted with Gα(q) and interfered with signaling through this G protein. These data support the development of GRK2-based therapeutic approaches to prevent hypertrophy and heart failure.

Decoding Gαq signaling

Gαq signals with phospholipase C-β (PLC-β) to modify behavior in response to an agonist-bound GPCR. While the fundamental steps which prime Gαq to interact with PLC-β have been identified, questions remain concerning signal strength with PLC-β and other effectors. Gαq is generally viewed to function as a simple ON and OFF switch for its effector, dependent on the binding of GTP or GDP. However, Gαq does not have a single effector, Gαq has many different effectors. Furthermore, select effectors also regulate Gαq activity. PLC-β is a lipase and a GTPase activating protein (GAP) selective for Gαq. The contribution of G protein regulating activity to signal amplitude remains unclear. The unique PLC-β coiled-coil domain is essential for maximum Gαq response, both lipase and GAP. Nonetheless, coiled-coil domain associations necessary to maximum response have not been revealed by the structural approach. This review discusses progress towards understanding the basis for signal strength with PLC-β and other effectors. Shared and effector-specific interactions have been identified. Finally, the evidence for allosteric regulation of lipase stimulation by protein kinase C, the membrane, phosphatidic acid, phosphatidylinositol-4, 5-bisphosphate and GPCR is explored. Endogenous allosteric regulators can suppress or enhance maximum lipase stimulation dependent on the PLC-β coiled-coil domain. A better understanding of allosteric modulation may therefore identify a wealth of new targets to regulate signal strength and behavior.

Gq-Coupled Receptors in Autoimmunity

Heterotrimeric G proteins can be divided into Gi, Gs, Gq/11, and G12/13 subfamilies according to their α subunits. The main function of G proteins is transducing signals from G protein coupled receptors (GPCRs), a family of seven transmembrane receptors. In recent years, studies have demonstrated that GPCRs interact with Gq, a member of the Gq/11 subfamily of G proteins. This interaction facilitates the vital role of this family of proteins in immune regulation and autoimmunity, particularly for Gαq, which is considered the functional α subunit of Gq protein. Therefore, understanding the mechanisms through which Gq-coupled receptors control autoreactive lymphocytes is critical and may provide insights into the treatment of autoimmune disorders. In this review, we summarize recent advances in studies of the role of Gq-coupled receptors in autoimmunity, with a focus on their pathologic role and downstream signaling.

Stable G protein-effector complexes in striatal neurons: mechanism of assembly and role in neurotransmitter signaling

In the striatum, signaling via G protein-coupled neurotransmitter receptors is essential for motor control. Critical to this process is the effector enzyme adenylyl cyclase type 5 (AC5) that produces second messenger cAMP upon receptor-mediated activation by G protein Golf. However, the molecular organization of the Golf-AC5 signaling axis is not well understood. In this study, we report that in the striatum AC5 exists in a stable pre-coupled complex with subunits of Golf heterotrimer. We use genetic mouse models with disruption in individual components of the complex to reveal hierarchical order of interactions required for AC5-Golf stability. We further identify that the assembly of AC5-Golf complex is mediated by PhLP1 chaperone that plays central role in neurotransmitter receptor coupling to cAMP production motor learning. These findings provide evidence for the existence of stable G protein-effector signaling complexes and identify a new component essential for their assembly.

Association of activated Gαq to the tumor suppressor Fhit is enhanced by phospholipase Cβ

BACKGROUND

G proteins are known to modulate various growth signals and are implicated in the regulation of tumorigenesis. The tumor suppressor Fhit is a newly identified interaction partner of Gq proteins that typically stimulate the phospholipase C pathway. Activated Gαq subunits have been shown to interact directly with Fhit, up-regulate Fhit expression and enhance its suppressive effect on cell growth and migration. Other signaling molecules may be involved in modulating Gαq/Fhit interaction.

METHODS

To test the relationship of PLCβ with the interaction between Gαq and Fhit, co-immunoprecipication assay was performed on HEK293 cells co-transfected with different combinations of Flag-Fhit, Gα16, Gα16QL, pcDNA3 vector, and PLCβ isoforms. Possible associations of Fhit with other effectors of Gαq were also demonstrated by co-immunoprecipitation. The regions of Gαq for Fhit interaction and PLCβ stimulation were further evaluated by inositol phosphates accumulation assay using a series of Gα16/z chimeras with discrete regions of Gα16 replaced by those of Gαz.

RESULTS

PLCβ1, 2 and 3 interacted with Fhit regardless of the expression of Gαq. Expression of PLCβ increased the affinities of Fhit for both wild-type and activated Gαq. Swapping of the Fhit-interacting α2-β4 region of Gαq with Gαi eliminated the association of Gαq with Fhit without affecting the ability of the mutant to stimulate PLCβ. Other effectors of Gαq including RGS2 and p63RhoGEF were unable to interact with Fhit.

CONCLUSIONS

PLCβ may participate in the regulation of Fhit by Gq in a unique way. PLCβ interacts with Fhit and increases the interaction between Gαq and Fhit. The Gαq/PLCβ/Fhit complex formation points to a novel signaling pathway that may negatively regulate tumor cell growth.

PLCβ3 mediates cortactin interaction with WAVE2 in MCP1-induced actin polymerization and cell migration

Monocyte chemotactic protein 1 (MCP1) stimulates vascular smooth muscle cell (VSMC) migration in vascular wall remodeling. However, the mechanisms underlying MCP1-induced VSMC migration have not been understood. Here we identify the signaling pathway associated with MCP1-induced human aortic smooth muscle cell (HASMC) migration. MCP1, a G protein-coupled receptor agonist, activates phosphorylation of cortactin on S405 and S418 residues in a time-dependent manner, and inhibition of its phosphorylation attenuates MCP1-induced HASMC G-actin polymerization, F-actin stress fiber formation, and migration. Cortactin phosphorylation on S405/S418 is found to be critical for its interaction with WAVE2, a member of the WASP family of cytoskeletal regulatory proteins required for cell migration. In addition, the MCP1-induced cortactin phosphorylation is dependent on PLCβ3-mediated PKCδ activation, and siRNA-mediated down-regulation of either of these molecules prevents cortactin interaction with WAVE2, affecting G-actin polymerization, F-actin stress fiber formation, and HASMC migration. Upstream, MCP1 activates CCR2 and Gαq/11 in a time-dependent manner, and down-regulation of their levels attenuates MCP1-induced PLCβ3 and PKCδ activation, cortactin phosphorylation, cortactin-WAVE2 interaction, G-actin polymerization, F-actin stress fiber formation, and HASMC migration. Together these findings demonstrate that phosphorylation of cortactin on S405 and S418 residues is required for its interaction with WAVE2 in MCP1-induced cytoskeleton remodeling, facilitating HASMC migration.

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.

Dysfunction of phospholipase Cγ in immune disorders and cancer

The surge in genetic and genomic investigations over the past 5 years has resulted in many discoveries of causative variants relevant to disease pathophysiology. Although phospholipase C (PLC) enzymes have long been recognized as important components in intracellular signal transmission, it is only recently that this approach highlighted their role in disease development through gain-of-function mutations. In this review we describe the new findings that link the PLCγ family to immune disorders and cancer, and illustrate further efforts to elucidate the molecular mechanisms that underpin their dysfunction.

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 Gαq, 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 Gαq 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.

G protein-coupled receptor kinase 2: a link between myocardial contractile function and cardiac metabolism

Heart failure (HF) causes a tremendous burden on the worldwide healthcare system, affecting >23 million people. There are many cardiovascular disorders that contribute to the development of HF and multiple risk factors that accelerate its occurrence, but regardless of its underlying cause, HF is characterized by a marked decrease in myocardial contractility and loss of pump function. One biomarker molecule consistently shown to be upregulated in human HF and several animal models is G protein-coupled receptor kinase-2 (GRK2), a kinase originally discovered to be involved in G protein-coupled receptor desensitization, especially β-adrenergic receptors. Higher levels of GRK2 can impair β-adrenergic receptor-mediated inotropic reserve and its inhibition, or molecular reduction has shown to improve pump function in several animal models including a preclinical pig model of HF. Recently, nonclassical roles for GRK2 in cardiovascular disease have been described, including negative regulation of insulin signaling, a role in myocyte cell survival and apoptotic signaling, and it has been shown to be localized in/on mitochondria. These new roles of GRK2 suggest that GRK2 may be a nodal link in the myocyte, influencing both cardiac contractile function and cell metabolism and survival and contributing to HF independent of its canonical role in G protein-coupled receptor desensitization. In this review, classical and nonclassical roles for GRK2 will be discussed, focusing on recently discovered roles for GRK2 in cardiomyocyte metabolism and the effects that these roles may have on myocardial contractile function and HF development.