G alpha COOH-terminal minigene vectors dissect heterotrimeric G protein signaling

Coincident Regulation of PLCβ Signaling by Gq-Coupled and μ-Opioid Receptors Opposes Opioid-Mediated Antinociception

Pain management is an important problem worldwide. The current frontline approach for pain management is the use of opioid analgesics. The primary analgesic target of opioids is the μ-opioid receptor (MOR). Deletion of phospholipase Cβ3 (PLCβ3) or selective inhibition of Gβγ regulation of PLCβ3 enhances the potency of the antinociceptive effects of morphine suggesting a novel strategy for achieving opioid-sparing effects. Here we investigated a potential mechanism for regulation of PLC signaling downstream of MOR in human embryonic kidney 293 cells and found that MOR alone could not stimulate PLC but rather required a coincident signal from a Gq-coupled receptor. Knockout of PLCβ3 or pharmacological inhibition of its upstream regulators, Gβγ or Gq, ex vivo in periaqueductal gray slices increased the potency of the selective MOR agonist [D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin acetate salt in inhibiting presynaptic GABA release. Finally, inhibition of Gq- G protein-coupled receptor coupling in mice enhanced the antinociceptive effects of morphine. These data support a model where Gq and Gβγ-dependent signaling cooperatively regulate PLC activation to decrease MOR-dependent antinociceptive potency. Ultimately, this could lead to identification of new non-MOR targets that would allow for lower-dose utilization of opioid analgesics. SIGNIFICANCE STATEMENT: Previous work demonstrated that deletion of phospholipase Cβ3 (PLCβ3) in mice potentiates μ-opioid receptor (MOR)-dependent antinociception. How PLCβ3 is regulated downstream of MOR had not been clearly defined. We show that PLC-dependent diacylglycerol generation is cooperatively regulated by MOR-Gβγ and Gq-coupled receptor signaling through PLCβ3 and that blockade of either Gq-signaling or Gβγ signaling enhances the potency of opioids in ex vivo brain slices and in vivo. These results reveal potential novel strategies for improving opioid analgesic potency and safety.

Selective activation of Gαob by an adenosine A1 receptor agonist elicits analgesia without cardiorespiratory depression

The development of therapeutic agonists for G protein-coupled receptors (GPCRs) is hampered by the propensity of GPCRs to couple to multiple intracellular signalling pathways. This promiscuous coupling leads to numerous downstream cellular effects, some of which are therapeutically undesirable. This is especially the case for adenosine A1 receptors (A1Rs) whose clinical potential is undermined by the sedation and cardiorespiratory depression caused by conventional agonists. We have discovered that the A1R-selective agonist, benzyloxy-cyclopentyladenosine (BnOCPA), is a potent and powerful analgesic but does not cause sedation, bradycardia, hypotension or respiratory depression. This unprecedented discrimination between native A1Rs arises from BnOCPA's unique and exquisitely selective activation of Gob among the six Gαi/o subtypes, and in the absence of β-arrestin recruitment. BnOCPA thus demonstrates a highly-specific Gα-selective activation of the native A1R, sheds new light on GPCR signalling, and reveals new possibilities for the development of novel therapeutics based on the far-reaching concept of selective Gα agonism.

Mice Expressing Regulators of G protein Signaling-insensitive Gαo Define Roles of μ Opioid Receptor Gαo and Gαi Subunit Coupling in Inhibition of Presynaptic GABA Release

Regulators of G protein signaling (RGS) proteins modulate signaling by G protein-coupled receptors. Using a knock-in transgenic mouse model with a mutation in Gαo that does not bind RGS proteins (RGS-insensitive), we determined the effect of RGS proteins on presynaptic μ opioid receptor (MOR)-mediated inhibition of GABA release in the ventrolateral periaqueductal gray (vlPAG). The MOR agonists [d-Ala², N-MePhe⁴, Gly-ol]-enkephalin (DAMGO) and met-enkephalin (ME) inhibited evoked inhibitory postsynaptic currents (eIPSCs) in the RGS-insensitive mice compared with wild-type (WT) littermates, respectively. Fentanyl inhibited eIPSCs similarly in both WT and RGS-insensitive mice. There were no differences in opioid agonist inhibition of spontaneous GABA release between the genotypes. To further probe the mechanism underlying these differences between opioid inhibition of evoked and spontaneous GABA release, specific myristoylated Gα peptide inhibitors for Gαo₁ and Gαi_1-3 that block receptor-G protein interactions were used to test the preference of agonists for MOR-Gα complexes. The Gαo₁ inhibitor reduced DAMGO inhibition of eIPSCs, but Gαi_1-3 inhibitors had no effect. Both Gαo₁ and Gαi_1-3 inhibitors separately reduced fentanyl inhibition of eIPSCs but had no effects on ME inhibition. Gαi_1-3 inhibitors blocked the inhibitory effects of ME and fentanyl on miniature postsynaptic current (mIPSC) frequency, but both Gαo₁ and Gαi_1-3 inhibitors were needed to block the effects of DAMGO. Finally, baclofen-mediated inhibition of GABA release is unaffected in the RGS-insensitive mice and in the presence of Gαo₁ and Gαi_1-3 inhibitor peptides, suggesting that GABA_B receptor coupling to G proteins in vlPAG presynaptic terminals is different than MOR coupling. SIGNIFICANCE STATEMENT: Presynaptic μ opioid receptors (MORs) in the ventrolateral periaqueductal gray are critical for opioid analgesia and are negatively regulated by RGS proteins. These data in RGS-insensitive mice provide evidence that MOR agonists differ in preference for Gαo versus Gαi and regulation by RGS proteins in presynaptic terminals, providing a mechanism for functional selectivity between agonists. The results further define important differences in MOR and GABA_B receptor coupling to G proteins that could be exploited for new pain therapies.

Structure and function of adenosine receptor heteromers

Adenosine is one of the most ancient signaling molecules and has receptors in both animals and plants. In mammals there are four specific receptors, A1, A2A, A2B, and A3, which belong to the superfamily of G-protein-coupled receptors (GPCRs). Evidence accumulated in the last 20 years indicates that GPCRs are often expressed as oligomeric complexes formed by a number of equal (homomers) or different (heteromers) receptors. This review presents the data showing the occurrence of heteromers formed by A1 and A2A, A2A and A2B, and A2A and A3 receptors highlighting (i) their tetrameric structural arrangements, and (ii) the functional diversity that those heteromers provide to adenosinergic signaling.

Allosteric modulation of adenosine A1 and cannabinoid 1 receptor signaling by G-peptides

While allosteric modulation of GPCR signaling has gained prominence to address the need for receptor specificity, efforts have mainly focused on allosteric sites adjacent to the orthosteric ligand-binding pocket and lipophilic molecules that target transmembrane helices. In this study, we examined the allosteric influence of native peptides derived from the C-terminus of the Gα subunit (G-peptides) on signaling from two Gi-coupled receptors, adenosine A1 receptor (A1 R) and cannabinoid receptor 1 (CB1 ). We expressed A1 R and CB1 fusions with G-peptides derived from Gαs, Gαi, and Gαq in HEK 293 cells using systematic protein affinity strength modulation (SPASM) and monitored the impact on downstream signaling in the cell compared to a construct lacking G-peptides. We used agonists N6 -Cyclopentyladenosine (CPA) and 5'-N-Ethylcarboxamidoadenosine (NECA) for A1 R and 2-Arachidonoylglycerol (2-AG) and WIN 55,212-2 mesylate (WN) for CB1 . G-peptides derived from Gαi and Gαq enhance agonist-dependent cAMP inhibition, demonstrating their effect as positive allosteric modulators of Gi-coupled signaling. In contrast, both G-peptides suppress agonist-dependent IP1 levels suggesting that they differentially function as negative allosteric modulators of Gq-coupled signaling. Taken together with our previous studies on Gs-coupled receptors, this study provides an extended model for the allosteric effects of G-peptides on GPCR signaling, and highlights their potential as probe molecules to enhance receptor specificity.

Voltage-dependent activation in EAG channels follows a ligand-receptor rather than a mechanical-lever mechanism

Ether-a-go-go family (EAG) channels play a major role in many physiological processes in humans, including cardiac repolarization and cell proliferation. Cryo-EM structures of two of them, K_V10.1 and human ether-a-go-go-related gene (hERG or K_V11.1), have revealed an original nondomain-swapped structure, suggesting that the mechanism of voltage-dependent gating of these two channels is quite different from the classical mechanical-lever model. Molecular aspects of hERG voltage-gating have been extensively studied, indicating that the S4-S5 linker (S4-S5_L) acts as a ligand binding to the S6 gate (S6 C-terminal part, S6_T) and stabilizes it in a closed state. Moreover, the N-terminal extremity of the channel, called N-Cap, has been suggested to interact with S4-S5_L to modulate channel voltage-dependent gating, as N-Cap deletion drastically accelerates hERG channel deactivation. In this study, using COS-7 cells, site-directed mutagenesis, electrophysiological measurements, and immunofluorescence confocal microscopy, we addressed whether these two major mechanisms of voltage-dependent gating are conserved in K_V10.2 channels. Using cysteine bridges and S4-S5_L-mimicking peptides, we show that the ligand/receptor model is conserved in K_V10.2, suggesting that this model is a hallmark of EAG channels. Truncation of the N-Cap domain, Per-Arnt-Sim (PAS) domain, or both in K_V10.2 abolished the current and altered channel trafficking to the membrane, unlike for the hERG channel in which N-Cap and PAS domain truncations mainly affected channel deactivation. Our results suggest that EAG channels function via a conserved ligand/receptor model of voltage gating, but that the N-Cap and PAS domains have different roles in these channels.

Regulators of G-Protein Signaling (RGS) Proteins Promote Receptor Coupling to G-Protein-Coupled Inwardly Rectifying Potassium (GIRK) Channels

Regulators of G-protein signaling (RGS) proteins negatively modulate presynaptic μ-opioid receptor inhibition of GABA release in the ventrolateral periaqueductal gray (vlPAG). Paradoxically, we find that G-protein-coupled receptor (GPCR) activation of G-protein-gated inwardly rectifying K⁺ channels (GIRKs) in the vlPAG is reduced in an agonist- and receptor-dependent manner in transgenic knock-in mice of either sex expressing mutant RGS-insensitive Gαo proteins. μ-Opioid receptor agonist activation of GIRK currents was reduced for DAMGO and fentanyl but not for [Met⁵]-enkephalin acetate salt hydrate (ME) in the RGS-insensitive heterozygous (Het) mice compared with wild-type mice. The GABA_B agonist baclofen-induced GIRK currents were also reduced in the Het mice. We confirmed the role of Gαo proteins in μ-opioid receptor and GABA_B receptor signaling pathways in wild-type mice using myristoylated peptide inhibitors of Gαo₁ and Gαi_1-3 The results using these inhibitors indicate that receptor activation of GIRK channels is dependent on the preference of the agonist-stimulated receptor for Gαo versus that for Gαi. DAMGO and fentanyl-mediated GIRK currents were reduced in the presence of the Gαo₁ inhibitor, but not the Gαi_1-3 inhibitors. In contrast, the Gαo₁ peptide inhibitor did not affect ME activation of GIRK currents, which is consistent with results in the Het mice, but the Gαi_1-3 inhibitors significantly reduced ME-mediated GIRK currents. Finally, the reduction in GIRK activation in the Het mice plays a role in opioid- and baclofen-mediated spinal antinociception, but not supraspinal antinociception. Thus, our studies indicate that RGS proteins have multiple mechanisms of modulating GPCR signaling that produce negative and positive regulation of signaling depending on the effector.SIGNIFICANCE STATEMENT Regulators of G-protein signaling (RGS) proteins positively modulate GPCR coupling to GIRKs, and this coupling is critical for opioid- and baclofen-mediated spinal antinociception, whereas μ-opioid receptor-mediated supraspinal antinociception depends on presynaptic inhibition that is negatively regulated by RGS proteins. The identification of these opposite roles for RGS proteins has implications for signaling via other GPCRs.

Cross-communication between Gi and Gs in a G-protein-coupled receptor heterotetramer guided by a receptor C-terminal domain

Background

G-protein-coupled receptor (GPCR) heteromeric complexes have distinct properties from homomeric GPCRs, giving rise to new receptor functionalities. Adenosine receptors (A₁R or A_2AR) can form A₁R-A_2AR heteromers (A₁-A_2AHet), and their activation leads to canonical G-protein-dependent (adenylate cyclase mediated) and -independent (β-arrestin mediated) signaling. Adenosine has different affinities for A₁R and A_2AR, allowing the heteromeric receptor to detect its concentration by integrating the downstream G_i- and G_s-dependent signals. cAMP accumulation and β-arrestin recruitment assays have shown that, within the complex, activation of A_2AR impedes signaling via A₁R.

Results

We examined the mechanism by which A₁-A_2AHet integrates G_i- and G_s-dependent signals. A₁R blockade by A_2AR in the A₁-A_2AHet is not observed in the absence of A_2AR activation by agonists, in the absence of the C-terminal domain of A_2AR, or in the presence of synthetic peptides that disrupt the heteromer interface of A₁-A_2AHet, indicating that signaling mediated by A₁R and A_2AR is controlled by both G_i and G_s proteins.

Conclusions

We identified a new mechanism of signal transduction that implies a cross-communication between G_i and G_s proteins guided by the C-terminal tail of the A_2AR. This mechanism provides the molecular basis for the operation of the A₁-A_2AHet as an adenosine concentration-sensing device that modulates the signals originating at both A₁R and A_2AR.

Electronic supplementary material

The online version of this article (10.1186/s12915-018-0491-x) contains supplementary material, which is available to authorized users.

hERG S4-S5 linker acts as a voltage-dependent ligand that binds to the activation gate and locks it in a closed state

Delayed-rectifier potassium channels (hERG and KCNQ1) play a major role in cardiac repolarization. These channels are formed by a tetrameric pore (S5–S6) surrounded by four voltage sensor domains (S1-S4). Coupling between voltage sensor domains and the pore activation gate is critical for channel voltage-dependence. However, molecular mechanisms remain elusive. Herein, we demonstrate that covalently binding, through a disulfide bridge, a peptide mimicking the S4-S5 linker (S4-S5_L) to the channel S6 C-terminus (S6_T) completely inhibits hERG. This shows that channel S4-S5_L is sufficient to stabilize the pore activation gate in its closed state. Conversely, covalently binding a peptide mimicking S6_T to the channel S4-S5_L prevents its inhibiting effect and renders the channel almost completely voltage-independent. This shows that the channel S4-S5_L is necessary to stabilize the activation gate in its closed state. Altogether, our results provide chemical evidence that S4-S5_L acts as a voltage-controlled ligand that binds S6_T to lock the channel in a closed state, elucidating the coupling between voltage sensors and the gate in delayed rectifier potassium channels and potentially other voltage-gated channels.

Quaternary structure of a G-protein-coupled receptor heterotetramer in complex with Gi and Gs

Background

G-protein-coupled receptors (GPCRs), in the form of monomers or homodimers that bind heterotrimeric G proteins, are fundamental in the transfer of extracellular stimuli to intracellular signaling pathways. Different GPCRs may also interact to form heteromers that are novel signaling units. Despite the exponential growth in the number of solved GPCR crystal structures, the structural properties of heteromers remain unknown.

Results

We used single-particle tracking experiments in cells expressing functional adenosine A₁-A_2A receptors fused to fluorescent proteins to show the loss of Brownian movement of the A₁ receptor in the presence of the A_2A receptor, and a preponderance of cell surface 2:2 receptor heteromers (dimer of dimers). Using computer modeling, aided by bioluminescence resonance energy transfer assays to monitor receptor homomerization and heteromerization and G-protein coupling, we predict the interacting interfaces and propose a quaternary structure of the GPCR tetramer in complex with two G proteins.

Conclusions

The combination of results points to a molecular architecture formed by a rhombus-shaped heterotetramer, which is bound to two different interacting heterotrimeric G proteins (G_i and G_s). These novel results constitute an important advance in understanding the molecular intricacies involved in GPCR function.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0247-4) contains supplementary material, which is available to authorized users.

The experimental power of FR900359 to study Gq-regulated biological processes

Despite the discovery of heterotrimeric αβγ G proteins ∼25 years ago, their selective perturbation by cell-permeable inhibitors remains a fundamental challenge. Here we report that the plant-derived depsipeptide FR900359 (FR) is ideally suited to this task. Using a multifaceted approach we systematically characterize FR as a selective inhibitor of Gq/11/14 over all other mammalian Gα isoforms and elaborate its molecular mechanism of action. We also use FR to investigate whether inhibition of Gq proteins is an effective post-receptor strategy to target oncogenic signalling, using melanoma as a model system. FR suppresses many of the hallmark features that are central to the malignancy of melanoma cells, thereby providing new opportunities for therapeutic intervention. Just as pertussis toxin is used extensively to probe and inhibit the signalling of Gi/o proteins, we anticipate that FR will at least be its equivalent for investigating the biological relevance of Gq.

Control systems of membrane transport at the interface between the endoplasmic reticulum and the Golgi

A fundamental property of cellular processes is to maintain homeostasis despite varying internal and external conditions. Within the membrane transport apparatus, variations in membrane fluxes from the endoplasmic reticulum (ER) to the Golgi complex are balanced by opposite fluxes from the Golgi to the ER to maintain homeostasis between the two organelles. Here we describe a molecular device that balances transport fluxes by integrating transduction cascades with the transport machinery. Specifically, ER-to-Golgi transport activates the KDEL receptor at the Golgi, which triggers a cascade that involves Gs and adenylyl cyclase and phosphodiesterase isoforms and then PKA activation and results in the phosphorylation of transport machinery proteins. This induces retrograde traffic to the ER and balances transport fluxes between the ER and Golgi. Moreover, the KDEL receptor activates CREB1 and other transcription factors that upregulate transport-related genes. Thus, a Golgi-based control system maintains transport homeostasis through both signaling and transcriptional networks.

The G protein-coupled receptor GPRC5B contributes to neurogenesis in the developing mouse neocortex

Neural progenitor cells in the developing brain give rise to neurons and glia. Multiple extrinsic signalling molecules and their cognate membrane receptors have been identified to control neural progenitor fate. However, a role for G protein-coupled receptors in cell fate decisions in the brain remains largely putative. Here we show that GPRC5B, which encodes an orphan G protein-coupled receptor, is present in the ventricular surface of cortical progenitors in the mouse developing neocortex and is required for their neuronal differentiation. GPRC5B-depleted progenitors fail to adopt a neuronal fate and ultimately become astrocytes. Furthermore, GPRC5B-mediated signalling is associated with the proper regulation of β-catenin signalling, a pathway crucial for progenitor fate decision. Our study uncovers G protein-coupled receptor signalling in the neuronal fate determination of cortical progenitors.

cAMP-induced expression of neuropilin1 promotes retinal axon crossing in the zebrafish optic chiasm

Growing axons navigate a complex environment as they respond to attractive and repellent guidance cues. Axons can modulate their responses to cues through a G-protein-coupled, cAMP-dependent signaling pathway. To examine the role of G-protein signaling in axon guidance in vivo, we used the GAL4/UAS system to drive expression of dominant-negative heterotrimeric G-proteins (DNG) in retinal ganglion cells (RGCs) of embryonic zebrafish. Retinal axons normally cross at the ventral midline and project to the contralateral tectum. Expression of DNGα(S) in RGCs causes retinal axons to misproject to the ipsilateral tectum. These errors resemble misprojections in adcy1, adcy8, nrp1a, sema3D, or sema3E morphant embryos, as well as in sema3D mutant embryos. nrp1a is expressed in RGCs as their axons extend toward and across the midline. sema3D and sema3E are expressed adjacent to the chiasm, suggesting that they facilitate retinal midline crossing. We demonstrate synergistic induction of ipsilateral misprojections between adcy8 knockdown and transgenic DNGα(S) expression, adcy8 and nrp1a morphants, or nrp1a morphants and transgenic DNGα(S) expression. Using qPCR analysis, we show that either transgenic DNGα(S)-expressing embryos or adcy8 morphant embryos have decreased levels of nrp1a and nrp1b mRNA. Ipsilateral misprojections in adcy8 morphants are corrected by the expression of an nrp1a rescue construct expressed in RGCs. These findings are consistent with the idea that elevated cAMP levels promote Neuropilin1a expression in RGCs, increasing the sensitivity of retinal axons to Sema3D, Sema3E, or other neuropilin ligands at the midline, and consequently facilitate retinal axon crossing in the chiasm.

Molecular targeting of Gα and Gβγ subunits: a potential approach for cancer therapeutics

G-Protein-coupled receptors (GPCRs) signal through G protein α and βγ subunit families to regulate a wide range of physiological and pathophysiological processes. As such, GPCRs are major targets for therapeutic drugs. Downstream targets of GPCRs have also gained interest as a therapeutic approach to complex pathologies involving multiple GPCRs. One such approach involves targeting of the G proteins themselves. Several small molecule Gα and Gβγ modulators have been developed and been tested in various animal models of disease. Here we will discuss the requirements for targeting Gα and Gβγ subunits, the mechanisms of action of currently identified inhibitors, and focus on the potential utility of Gα and Gβγ inhibitors in the treatment of various cancers.

Linking receptor activation to changes in Sw I and II of Gα proteins

G-protein coupled receptors catalyze nucleotide exchange on G proteins, which results in subunit dissociation and effector activation. In the recent β2AR-Gs structure, portions of Switch I and II of Gα are not fully elucidated. We paired fluorescence studies of receptor-Gαi interactions with the β2AR-Gs and other Gi structures to investigate changes in Switch I and II during receptor activation and GTP binding. The β2/β3 loop containing Leu194 of Gαi is located between Switches I and II, in close proximity to IC2 of the receptor and the C-terminus of Gα, thus providing an allosteric connection between these Switches and receptor activation. We compared the environment of residues in myristoylated Gαi proteins in the heterotrimer to that upon receptor activation and subsequent GTP binding. Upon receptor activation, residues in both Switch regions are less solvent-exposed, as compared to the heterotrimer. Upon GTPγS binding, the environment of several residues in Switch I resemble the receptor-bound state, while Switch II residues display effects on their environment which are consistent with their role in GTP binding and Gβγ dissociation. The ability to merge available crystal structures with solution studies is a powerful tool to gain insight into conformational changes associated with receptor-mediated Gi protein activation.

Lysophosphatidic acid increases proximal tubule cell secretion of profibrotic cytokines PDGF-B and CTGF through LPA2- and Gαq-mediated Rho and αvβ6 integrin-dependent activation of TGF-β

After ischemia-reperfusion injury (IRI), kidney tubules show activated transforming growth factor β (TGF-β) signaling and increased expression of profibrotic peptides, platelet-derived growth factor-B (PDGF-B) and connective tissue growth factor (CTGF). If tubule repair after IRI is incomplete, sustained paracrine activity of these peptides can activate interstitial fibroblast progenitors and cause fibrosis. We show that lysophosphatidic acid (LPA), a ubiquitous phospholipid that is increased at sites of injury and inflammation, signals through LPA2 receptors and Gαq proteins of cultured proximal tubule cells to transactivate latent TGF-β in a Rho/Rho-kinase and αvβ6 integrin-dependent manner. Active TGF-β peptide then initiates signaling to increase the production and secretion of PDGF-B and CTGF. In a rat model of IRI, increased TGF-β signaling that was initiated early during reperfusion did not subside during recovery, but progressively increased, causing tubulointerstitial fibrosis. This was accompanied by correspondingly increased LPA2 and β6 integrin proteins and elevated tubule expression of TGF-β1, together with PDGF-B and CTGF. Treatment with a pharmacological TGF-β type I receptor antagonist suppressed TGF-β signaling, decreased the expression of β6 integrin, PDGF-B, and CTGF, and ameliorated fibrosis. We suggest that LPA-initiated autocrine signaling is a potentially important mechanism that gives rise to paracrine profibrotic signaling in injured kidney tubule cells.

Probing heterotrimeric G protein activation: applications to biased ligands

Cell surface G protein-coupled receptors (GPCRs) drive numerous signaling pathways involved in the regulation of a broad range of physiologic processes. Today, they represent the largest target for modern drugs development with potential application in all clinical fields. Recently, the concept of "ligand-directed trafficking" has led to a conceptual revolution in pharmacological theory, thus opening new avenues for drug discovery. Accordingly, GPCRs do not function as simple on-off switch but rather as filters capable of selecting the activation of specific signals and thus generating texture responses to ligands, a phenomenon often referred to as ligand-biased signaling. Also, one challenging task today remains optimization of pharmacological assays with increased sensitivity so to better appreciate the inherent texture of ligands. However, considering that a single receptor has pleiotropic signaling properties and that each signal can crosstalk at different levels, biased activity remains thus difficult to evaluate. One strategy to overcome these limitations would be examining the initial steps following receptor activation. Even, if some G protein independent functions have been recently described, heterotrimeric G protein activation remains a general hallmark for all GPCRs families and the first cellular event subsequent to agonist binding to the receptor. Herein, we review the different methodologies classically used or recently developed to monitor G protein activation and discussed them in the context of G protein biased-ligands.

The KDEL receptor couples to Gαq/11 to activate Src kinases and regulate transport through the Golgi

Membrane trafficking involves large fluxes of cargo and membrane across separate compartments. These fluxes must be regulated by control systems to maintain homoeostasis. While control systems for other key functions such as protein folding or the cell cycle are well known, the mechanisms that control secretory transport are poorly understood. We have previously described a signalling circuit operating at the Golgi complex that regulates intra-Golgi trafficking and is initiated by the KDEL receptor (KDEL-R), a protein previously known to mediate protein recycling from the Golgi to the endoplasmic reticulum (ER). Here, we investigated the KDEL-R signalling mechanism. We show that the KDEL-R is predicted to fold like a G-protein-coupled receptor (GPCR), and that it binds and activates the heterotrimeric signalling G-protein Gα(q/11) which, in turn, regulates transport through the Golgi complex. These findings reveal an unexpected GPCR-like mode of action of the KDEL-R and shed light on a core molecular control mechanism of intra-Golgi traffic.

Antagonist minigenes identify genes regulated by parathyroid hormone through G protein-selective and G protein co-regulated mechanisms in osteoblastic cells

Parathyroid hormone (PTH) is the major hormone regulating bone remodeling. Binding of PTH to the PTH1 receptor (PTH1R), a heterotrimeric G protein coupled receptor (GPCR), can potentially trigger multiple signal transduction pathways mediated through several different G proteins. In this study, we employed G protein antagonist minigenes inhibiting Gα(s), Gα(q) or Gα₁₂ to selectively dissect out which of these G proteins were responsible for effects of PTH(1-34) in targeted signaling and osteogenesis arrays consisting of 159 genes. Among the 32 genes significantly regulated by 24h PTH treatment in UMR-106 osteoblastic cells, 9 genes were exclusively regulated through G(s), 6 genes were solely mediated through G(q), and 3 genes were only controlled through G₁₂. Such findings support the concept that there is some absolute specificity in downstream responses initiated at the G protein level following binding of PTH to the PTH1R. On the other hand, 6 PTH-regulated genes were regulated by both G(s) and G(q), 3 genes were regulated by both G(s) and G₁₂, and 3 genes were controlled by G(s), G(q) and G₁₂. These findings indicate potential overlapping or sequential interactions among different G protein-mediated pathways. In addition, two PTH-regulated genes were not regulated through any of the G proteins examined, suggesting that additional signaling mechanisms may be involved. Selectivity was largely maintained over a 2-48-hour time period. The minigene effects were mimicked by downstream inhibitors. The dissection of the differential effects of multiple G protein pathways on gene regulation provides a more complete understanding of PTH signaling in osteoblastic cells.

KCNQ1 channels voltage dependence through a voltage-dependent binding of the S4-S5 linker to the pore domain

Voltage-dependent potassium (Kv) channels are tetramers of six transmembrane domain (S1-S6) proteins. Crystallographic data demonstrate that the tetrameric pore (S5-S6) is surrounded by four voltage sensor domains (S1-S4). One key question remains: how do voltage sensors (S4) regulate pore gating? Previous mutagenesis data obtained on the Kv channel KCNQ1 highlighted the critical role of specific residues in both the S4-S5 linker (S4S5(L)) and S6 C terminus (S6(T)). From these data, we hypothesized that S4S5(L) behaves like a ligand specifically interacting with S6(T) and stabilizing the closed state. To test this hypothesis, we designed plasmid-encoded peptides corresponding to portions of S4S5(L) and S6(T) of the voltage-gated potassium channel KCNQ1 and evaluated their effects on the channel activity in the presence and absence of the ancillary subunit KCNE1. We showed that S4S5(L) peptides inhibit KCNQ1, in a reversible and state-dependent manner. S4S5(L) peptides also inhibited a voltage-independent KCNQ1 mutant. This inhibition was competitively prevented by a peptide mimicking S6(T), consistent with S4S5(L) binding to S6(T). Val(254) in S4S5(L) is known to contact Leu(353) in S6(T) when the channel is closed, and mutations of these residues alter the coupling between the two regions. The same mutations introduced in peptides altered their effects, further confirming S4S5(L) binding to S6(T). Our results suggest a mechanistic model in which S4S5(L) acts as a voltage-dependent ligand bound to its receptor on S6 at rest. This interaction locks the channel in a closed state. Upon plasma membrane depolarization, S4 pulls S4S5(L) away from S6(T), allowing channel opening.

Hormonal inhibition of endocytosis: novel roles for noradrenaline and G protein G(z)

The modulation of endocytosis following exocytosis by noradrenaline (NA), a physiological inhibitor of insulin secretion, was investigated in INS 832/13 cells using patch-clamp capacitance measurements. Endocytosis was inhibited by NA in a pertussis toxin-insensitive manner. Dialysing a synthetic peptide mimicking the C-terminus of the α-subunit of G(z) into the cells blocked the inhibition of endocytosis by NA. Cell-attached capacitance measurements indicated that inhibition by NA was due to a decreased number of endocytotic events without a change in vesicle size. Analysis of fission pore closure kinetics revealed two distinct fission modes, with NA selectively inhibiting the rapid fission pore closure events. Comparison of the actions of NA and deltamethrin, a calcineurin antagonist and potent inhibitor of endocytosis, demonstrated that they inhibit endocytosis by different mechanisms. These findings establish novel actions for NA and G(z) in insulin-secreting cells and possibly other cell types.

Noradrenaline inhibits exocytosis via the G protein βγ subunit and refilling of the readily releasable granule pool via the α(i1/2) subunit

The molecular mechanisms responsible for the 'distal' effect by which noradrenaline (NA) blocks exocytosis in the β-cell were examined by whole-cell and cell-attached patch clamp capacitance measurements in INS 832/13 β-cells. NA inhibited Ca(2+)-evoked exocytosis by reducing the number of exocytotic events, without modifying vesicle size. Fusion pore properties also were unaffected. NA-induced inhibition of exocytosis was abolished by a high level of Ca(2+) influx, by intracellular application of antibodies against the G protein subunit Gβ and was mimicked by the myristoylated βγ-binding/activating peptide mSIRK. NA-induced inhibition was also abolished by treatment with BoNT/A, which cleaves the C-terminal nine amino acids of SNAP-25, and also by a SNAP-25 C-terminal-blocking peptide containing the BoNT/A cleavage site. These data indicate that inhibition of exocytosis by NA is downstream of increased [Ca(2+)](i) and is mediated by an interaction between Gβγ and the C-terminus of SNAP-25, as is the case for inhibition of neurotransmitter release. Remarkably, in the course of this work, a novel effect of NA was discovered. NA induced a marked retardation of the rate of refilling of the readily releasable pool (RRP) of secretory granules. This retardation was specifically abolished by a Gα(i1/2) blocking peptide demonstrating that the effect is mediated via activation of Gα(i1) and/or Gα(i2).

Role of G-proteins in odor-sensing and CO2-sensing neurons in Drosophila

A central question in insect chemoreception is whether signaling occurs via G-proteins. Two families of seven-transmembrane-domain chemoreceptors, the odor (Or) and gustatory receptor (Gr) families, have been identified in Drosophila (Clyne et al., 1999, 2000; Vosshall et al., 1999). Ors mediate odor responses, whereas two Grs, Gr21a and Gr63a, mediate CO2 response (Hallem et al., 2004; Jones et al., 2007; Kwon et al., 2007). Using single-sensillum recordings, we systematically investigate the role of Galpha proteins in vivo, initially with RNA interference constructs, competitive peptides, and constitutively active Galpha proteins. The results do not support a role for Galpha proteins in odor sensitivity. In parallel experiments, manipulations of Galpha(q), but not other Galpha proteins, affected CO2 response. Transient, conditional, and ectopic expression analyses consistently supported a role for Galpha(q) in the response of CO2-sensing neurons, but not odor-sensing neurons. Genetic mosaic analysis confirmed that odor responses are normal in the absence of Galpha(q). Ggamma30A is also required for normal CO2 response. The simplest interpretation of these results is that Galpha(q) and Ggamma30A play a role in the response of CO2-sensing neurons, but are not required for Or-mediated odor signaling.

Development of Generic Calcium Imaging Assay for Monitoring Gi-Coupled Receptors and G-Protein Interaction

G-protein-coupled receptors (GPCRs) are important therapeutic targets for many areas of drug research and development. Although chimeric Gα16 proteins are valuable tools for detecting the activation of Gαi/o-coupled receptors, the details of the activation process remain unclear. The authors introduce a series of chimeras that combine both Gα16 and Gα i/o (Gα16/o, Gα16/i2, and Gα 16/i3) into a well-established transient expression system to examine the ability of these chimeras to interact with D2 long-form (D 2L) dopamine and 5-HT1A serotonin receptors. The pEC 50 data obtained for known agonists were similar to results from previous studies that used other cell-based assays, thus indicating sufficient sensitivity for the assay. Moreover, quinpirole exhibited similar intrinsic activity to dopamine at the D2L receptor, whereas S-(—)-3-PPP displayed partial activity of dopamine and quinpirole in the presence of the Gα16/o chimera. The potency of dopamine for D2L receptors was similar among Gα16/o, Gα16/i2, and Gα 16/i3. In contrast, the 5-HT1A receptor exhibited a significantly preferential coupling for Gα16/i3 compared with Gα 16/i2 when serotonin was used as a ligand. This finding was in close agreement with the results of previous reports. The present system could therefore be used as a rapid functional assay for high-throughput screening and deorphanization. ( Journal of Biomolecular Screening 2009:781-788)

Regulation of heterotrimeric G protein signaling in airway smooth muscle

Heterotrimeric G proteins transduce signals from G protein-coupled receptors to regulate numerous signaling events and functions in airway smooth muscle (ASM). In this article, we detail the function and regulation of heterotrimeric G protein signaling in ASM. We further discuss recent advances in the development of experimental tools in the study of G protein signaling, and speculate how these tools might be used in therapeutic strategies that seek to mitigate bronchospasm and airway remodeling that occur in obstructive airway disease.

Heterotrimeric G-proteins: a short history

Some 865 genes in man encode G-protein-coupled receptors (GPCRs). The heterotrimeric guanine nucleotide-binding proteins (G-proteins) function to transduce signals from this vast panoply of receptors to effector systems including ion channels and enzymes that alter the rate of production, release or degradation of intracellular second messengers. However, it was not until the 1970s that the existence of such transducing proteins was even seriously suggested. Combinations of bacterial toxins that mediate their effects via covalent modification of the alpha-subunit of certain G-proteins and mutant cell lines that fail to generate cyclic AMP in response to agonists because they either fail to express or express a malfunctional G-protein allowed their identification and purification. Subsequent to initial cloning efforts, cloning by homology has defined the human G-proteins to derive from 35 genes, 16 encoding alpha-subunits, five beta and 14 gamma. All function as guanine nucleotide exchange on-off switches and are mechanistically similar to other proteins that are enzymic GTPases. Although not readily accepted initially, it is now well established that beta/gamma complexes mediate as least as many functions as the alpha-subunits. The generation of chimeras between different alpha-subunits defined the role of different sections of the primary/secondary sequence and crystal structures and cocrystals with interacting proteins have given detailed understanding of their molecular structure and basis of function. Finally, further modifications of such chimeras have generated a range of G-protein alpha-subunits with greater promiscuity to interact across GPCR classes and initiated the use of such modified G-proteins in drug discovery programmes.

Mechanistic pathways and biological roles for receptor-independent activators of G-protein signaling

Signal processing via heterotrimeric G-proteins in response to cell surface receptors is a central and much investigated aspect of how cells integrate cellular stimuli to produce coordinated biological responses. The system is a target of numerous therapeutic agents and plays an important role in adaptive processes of organs; aberrant processing of signals through these transducing systems is a component of various disease states. In addition to G-protein coupled receptor (GPCR)-mediated activation of G-protein signaling, nature has evolved creative ways to manipulate and utilize the Galphabetagamma heterotrimer or Galpha and Gbetagamma subunits independent of the cell surface receptor stimuli. In such situations, the G-protein subunits (Galpha and Gbetagamma) may actually be complexed with alternative binding partners independent of the typical heterotrimeric Galphabetagamma. Such regulatory accessory proteins include the family of regulator of G-protein signaling (RGS) proteins that accelerate the GTPase activity of Galpha and various entities that influence nucleotide binding properties and/or subunit interaction. The latter group of proteins includes receptor-independent activators of G-protein signaling (AGS) proteins that play surprising roles in signal processing. This review provides an overview of our current knowledge regarding AGS proteins. AGS proteins are indicative of a growing number of accessory proteins that influence signal propagation, facilitate cross talk between various types of signaling pathways, and provide a platform for diverse functions of both the heterotrimeric Galphabetagamma and the individual Galpha and Gbetagamma subunits.

GPR92 as a new G12/13- and Gq-coupled lysophosphatidic acid receptor that increases cAMP, LPA5

The signaling effects of lysophospholipids such as lysophosphatidic acid (LPA) are mediated by G protein-coupled receptors (GPCRs). There are currently four LPA receptors known as LPA(1-4). Genetic deletion studies have identified essential biological functions for LPA receptors in mice. However, these studies have also revealed phenotypes consistent with the existence of as yet unidentified receptors. Toward identifying new LPA receptors, we have screened collections of GPCR cDNAs using reverse transfection and cell-based assays. Here we report an interim result of one screen to identify receptors that produced LPA-dependent changes in cell shape: the orphan receptor GPR92 has properties of a new LPA receptor. Sequence analyses of human GPR92 and its mouse homolog have approximately 35% amino acid identity with LPA4/GPR23. The same cell-based approaches that were used to identify and/or characterize LPA(1-4), particularly heterologous expression in B103 cells or RH7777 cells, were utilized and compared with known LPA receptors. Retroviral-mediated expression of epitope-tagged receptors was further combined with G protein minigenes and pharmacological intervention, along with calcium imaging and whole-cell patch clamp electrophysiology. LPA-dependent receptor internalization following exposure to LPA but not related lysophospholipids was observed. Furthermore, LPA induced concentration-dependent activation of G(12/13) and G(q) and increased cAMP levels. Specific [3H]LPA binding was detected in cell membranes heterologously expressing GPR92 but not control membranes. Northern blot and reverse transcriptase-PCR studies indicated a broad low level of expression in many tissues including embryonic brain and enrichment in small intestine and sensory dorsal root ganglia, as well as embryonic stem cells. These results support GPR92 as a fifth LPA receptor, LPA5, which likely has distinct physiological functions in view of its expression pattern.

Galpha13 mediates a signal that is essential for proliferation and survival of thymocyte progenitors

G protein signaling via the Galpha12 family (Galpha12 and Galpha13) has not been well studied in T cells. To investigate whether Galpha12 and Galpha13 are involved in thymopoiesis, we expressed the regulator of G protein signaling domain of p115RhoGEF to inhibit Galpha12 and Galpha13 during thymopoiesis. Fetal thymus organ cultures seeded with p115DeltaDH-expressing progenitor cells showed impaired thymopoiesis with a block at the CD4-CD8-CD44-CD25+ (DN3) stage. Using Galpha13 or Galpha12 minigenes, we demonstrated that Galpha13, but not Galpha12, is required for thymopoiesis. T progenitor cells expressing p115DeltaDH showed reduced proliferation and increased cell death. T cell receptor stimulation of the fetal thymus organ cultures did not rescue the block. Overexpression of the antiapoptotic gene Bcl2 rescued the defect in DN3 cells and partially rescued T cell development. Therefore, Galpha13-mediated signaling is necessary in early thymocyte proliferation and survival.

Selective inhibition of calcineurin-NFAT signaling by blocking protein-protein interaction with small organic molecules

Transient or reversible protein-protein interactions are commonly used to ensure efficient targeting of signaling enzymes to their cellular substrates. These interactions include direct binding to substrate, interaction with an accessory or scaffold protein, and positioning at subcellular locations in proximity to substrates. The existence of specialized targeting mechanisms raises the possibility of designing inhibitors that do not block enzyme activity per se, but rather interfere with targeting of the enzyme to one or more of its substrates within the cell. Here, we identify small organic molecules that specifically block targeting of the protein phosphatase calcineurin to its substrate nuclear factor of activated T cells (NFAT, also termed NFATc) and show that they are effective inhibitors of calcineurin-NFAT signaling.

The GTP-binding protein RhoA mediates Na,K-ATPase exocytosis in alveolar epithelial cells

The purpose of this study was to define the role of the Rho family of small GTPases in the beta-adrenergic regulation of the Na,K-ATPase in alveolar epithelial cells (AEC). The beta-adrenergic receptor agonist isoproterenol (ISO) increased the Na,K-ATPase protein abundance at the plasma membrane and activated RhoA in a time-dependent manner. AEC pretreated with mevastatin, a specific inhibitor of prenylation, or transfected with the dominant negative RhoAN19, prevented ISO-mediated Na,K-ATPase exocytosis to the plasma membrane. The ISO-mediated activation of RhoA in AEC occurred via beta2-adrenergic receptors and involved Gs-PKA as demonstrated by incubation with the protein kinase A (PKA)-specific inhibitors H89 and PKI (peptide specific inhibitor), and Gi, as incubation with pertussis toxin or cells transfected with a minigene vector for Gi inhibited the ISO-mediated RhoA activation. However, cells transfected with minigene vectors for G12 and G13 did not prevent RhoA activation by ISO. Finally, the ISO-mediated Na,K-ATPase exocytosis was regulated by the Rho-associated kinase (ROCK), as preincubation with the specific inhibitor Y-27632 or transfection with dominant negative ROCK, prevented the increase in Na,K-ATPase at the plasma membrane. Accordingly, ISO regulates Na,K-ATPase exocytosis in AEC via the activation of beta2-adrenergic receptor, Gs, PKA, Gi, RhoA, and ROCK.

Functional interaction between T2R taste receptors and G-protein alpha subunits expressed in taste receptor cells

Bitter taste perception is a conserved chemical sense against the ingestion of poisonous substances in mammals. A multigene family of G-protein-coupled receptors, T2R (so-called TAS2R or TRB) receptors and a G-protein alpha subunit (Galpha), gustducin, are believed to be key molecules for its perception, but little is known about the molecular basis for its interaction. Here, we use a heterologous expression system to determine a specific domain of gustducin necessary for T2R coupling. Two chimeric Galpha16 proteins harboring 37 and 44 gustducin-specific sequences at their C termini (G16/gust37 and G16/gust44) responded to different T2R receptors with known ligands, but G16/gust 23, G16/gust11, and G16/gust5 did not. The former two chimeras contained a predicted beta6 sheet, an alpha5 helix, and an extreme C terminus of gustducin, and all the domains were indispensable to the expression of T2R activity. We also expressed G16 protein chimeras with the corresponding domain from other Galpha(i) proteins, cone-transducin (Galpha(t2)), Galpha(i2), and Galpha(z) (G16/t2, G16/i2, and G16/z). As a result, G16/t2 and G16/i2 produced specific responses of T2Rs, but G16/z did not. Because Galpha(t2) and Galpha(i2) are expressed in the taste receptor cells, these G-protein alpha(i) subunits may also be involved in bitter taste perception via T2R receptors. The present Galpha16-based chimeras could be useful tools to analyze the functions of many orphan G-protein-coupled taste receptors.