Live cell imaging of Gs and the beta2-adrenergic receptor demonstrates that both alphas and beta1gamma7 internalize upon stimulation and exhibit similar trafficking patterns that differ from that of the beta2-adrenergic receptor

Visualization of endogenous G proteins on endosomes and other organelles

Classical G-protein-coupled receptor (GPCR) signaling takes place in response to extracellular stimuli and involves receptors and heterotrimeric G proteins located at the plasma membrane. It has recently been established that GPCR signaling can also take place from intracellular membrane compartments, including endosomes that contain internalized receptors and ligands. While the mechanisms of GPCR endocytosis are well understood, it is not clear how well internalized receptors are supplied with G proteins. To address this gap, we use gene editing, confocal microscopy, and bioluminescence resonance energy transfer to study the distribution and trafficking of endogenous G proteins. We show here that constitutive endocytosis is sufficient to supply newly internalized endocytic vesicles with 20-30% of the G protein density found at the plasma membrane. We find that G proteins are present on early, late, and recycling endosomes, are abundant on lysosomes, but are virtually undetectable on the endoplasmic reticulum, mitochondria, and the medial-trans Golgi apparatus. Receptor activation does not change heterotrimer abundance on endosomes. Our findings provide a subcellular map of endogenous G protein distribution, suggest that G proteins may be partially excluded from nascent endocytic vesicles, and are likely to have implications for GPCR signaling from endosomes and other intracellular compartments.

Cell swelling enhances ligand-driven β-adrenergic signaling

G protein-coupled receptors' conformational landscape can be affected by their local, microscopic interactions within the cell plasma membrane. We employ here a pleiotropic stimulus, namely osmotic swelling, to alter the cortical environment within intact cells and monitor the response in terms of receptor function and downstream signaling. We observe that in osmotically swollen cells the β2-adrenergic receptor, a prototypical GPCR, favors an active conformation, resulting in cAMP transient responses to adrenergic stimulation that have increased amplitude. The results are validated in primary cell types such as adult cardiomyocytes, a model system where swelling occurs upon ischemia-reperfusion injury. Our results suggest that receptors' function is finely modulated by their biophysical context, and specifically that osmotic swelling acts as a potentiator of downstream signaling, not only for the β2-adrenergic receptor, but also for other receptors, hinting at a more general regulatory mechanism.

Visualization of endogenous G proteins on endosomes and other organelles

Classical G protein-coupled receptor (GPCR) signaling takes place in response to extracellular stimuli and involves receptors and heterotrimeric G proteins located at the plasma membrane. It has recently been established that GPCR signaling can also take place from intracellular membrane compartments, including endosomes that contain internalized receptors and ligands. While the mechanisms of GPCR endocytosis are well understood, it is not clear how well internalized receptors are supplied with G proteins. To address this gap we use gene editing, confocal microscopy, and bioluminescence resonance energy transfer to study the distribution and trafficking of endogenous G proteins. We show here that constitutive endocytosis is sufficient to supply newly internalized endocytic vesicles with 20-30% of the G protein density found at the plasma membrane. We find that G proteins are present on early, late, and recycling endosomes, are abundant on lysosomes, but are virtually undetectable on the endoplasmic reticulum, mitochondria, and the medial Golgi apparatus. Receptor activation does not change heterotrimer abundance on endosomes. Our findings provide a subcellular map of endogenous G protein distribution, suggest that G proteins may be partially excluded from nascent endocytic vesicles, and are likely to have implications for GPCR signaling from endosomes and other intracellular compartments.

Optical control of cell-surface and endomembrane-exclusive β-adrenergic receptor signaling

Beta-adrenergic receptors (βARs) are G protein-coupled receptors (GPCRs) that mediate catecholamine hormone-induced stress responses, such as elevation of heart rate. Besides those that are plasma membrane-bound, endomembrane βARs are also signaling competent. Dysregulation of βAR pathways underlies severe pathological conditions. Emerging evidence indicates pathological molecular signatures in deeper endomembrane βARs signaling, likely contributing to conditions such as cardiomyocyte hypertrophy and apoptosis. However, the lack of approaches to control endomembrane β1ARs has impeded linking signaling with pathology. Informed by the β1AR-catecholamine interactions, we engineered an efficient photolabile proligand (OptoIso) to trigger βAR signaling exclusively in endomembrane regions using blue light stimulation. Not only does OptoIso undergo blue light deprotection in seconds, but also efficiently enters cells and allows examination of G protein heterotrimer activation exclusively at endomembranes. OptoIso also allows optical activation of plasma membrane βAR signaling in selected single cells with native fidelity, which can be reversed by terminating blue light. Thus, OptoIso will be a valuable experimental tool to elicit spatial and temporal control of βAR signaling in user-defined endomembrane or plasma membrane regions in unmodified cells with native fidelity.

Optical Control of Cell-Surface and Endomembrane-Exclusive β-Adrenergic Receptor Signaling

Beta-adrenergic receptors (βARs) are G protein-coupled receptors (GPCRs) that mediate catecholamine-induced stress responses, such as heart rate increase and bronchodilation. In addition to signals from the cell surface, βARs also broadcast non-canonical signaling activities from the cell interior membranes (endomembranes). Dysregulation of these receptor pathways underlies severe pathological conditions. Excessive βAR stimulation is linked to cardiac hypertrophy, leading to heart failure, while impaired stimulation causes compromised fight or flight stress responses and homeostasis. In addition to plasma membrane βAR, emerging evidence indicates potential pathological implications of deeper endomembrane βARs, such as inducing cardiomyocyte hypertrophy and apoptosis, underlying heart failure. However, the lack of approaches to control their signaling in subcellular compartments exclusively has impeded linking endomembrane βAR signaling with pathology. Informed by the β1AR-catecholamine interactions, we engineered an efficiently photo-labile, protected hydroxy β1AR pro-ligand (OptoIso) to trigger βAR signaling at the cell surface, as well as exclusive endomembrane regions upon blue light stimulation. Not only does OptoIso undergo blue light deprotection in seconds, but it also efficiently enters cells and allows examination of G protein heterotrimer activation exclusively at endomembranes. In addition to its application in the optical interrogation of βARs in unmodified cells, given its ability to control deep organelle βAR signaling, OptoIso will be a valuable experimental tool.

Genetically encoded fluorescent biosensors for GPCR research

G protein-coupled receptors (GPCRs) regulate a wide range of physiological and pathophysiological cellular processes, thus it is important to understand how GPCRs are activated and function in various cellular contexts. In particular, the activation process of GPCRs is dynamically regulated upon various extracellular stimuli, and emerging evidence suggests the subcellular functions of GPCRs at endosomes and other organelles. Therefore, precise monitoring of the GPCR activation process with high spatiotemporal resolution is required to investigate the underlying molecular mechanisms of GPCR functions. In this review, we will introduce genetically encoded fluorescent biosensors that can precisely monitor the real-time GPCR activation process in live cells. The process includes the binding of extracellular GPCR ligands, conformational change of GPCR, recruitment of G proteins or β-arrestin, GPCR internalization and trafficking, and the GPCR-related downstream signaling events. We will introduce fluorescent GPCR biosensors based on a variety of strategies such as fluorescent resonance energy transfer (FRET), bioluminescence resonance energy transfer (BRET), circular permuted fluorescent protein (cpFP), and nanobody. We will discuss the pros and cons of these GPCR biosensors as well as their applications in GPCR research.

MAGED2 Is Required under Hypoxia for cAMP Signaling by Inhibiting MDM2-Dependent Endocytosis of G-Alpha-S

Mutations in MAGED2 cause transient Bartter syndrome characterized by severe renal salt wasting in fetuses and infants, which leads to massive polyhydramnios causing preterm labor, extreme prematurity and perinatal death. Notably, this condition resolves spontaneously in parallel with developmental increase in renal oxygenation. MAGED2 interacts with G-alpha-S (Gαs). Given the role of Gαs in activating adenylyl cyclase at the plasma membrane and consequently generating cAMP to promote renal salt reabsorption via protein kinase A (PKA), we hypothesized that MAGED2 is required for this signaling pathway under hypoxic conditions such as in fetuses. Consistent with that, under both physical and chemical hypoxia, knockdown of MAGED2 in renal (HEK293) and cancer (HeLa) cell culture models caused internalization of Gαs, which was fully reversible upon reoxygenation. In contrast to Gαs, cell surface expression of the β2-adrenergic receptor, which is coupled to Gαs, was not affected by MAGED2 depletion, demonstrating specific regulation of Gαs by MAGED2. Importantly, the internalization of Gαs due to MAGED2 deficiency significantly reduced cAMP generation and PKA activity. Interestingly, the internalization of Gαs was blocked by preventing its endocytosis with dynasore. Given the role of E3 ubiquitin ligases, which can be regulated by MAGE-proteins, in regulating endocytosis, we assessed the potential role of MDM2-dependent ubiquitination in MAGED2 deficiency-induced internalization of Gαs under hypoxia. Remarkably, MDM2 depletion or its chemical inhibition fully abolished Gαs-endocytosis following MAGED2 knockdown. Moreover, endocytosis of Gαs was also blocked by mutation of ubiquitin acceptor sites in Gαs. Thus, we reveal that MAGED2 is essential for the cAMP/PKA pathway under hypoxia to specifically regulate Gαs endocytosis by blocking MDM2-dependent ubiquitination of Gαs. This may explain, at least in part, the transient nature of Bartter syndrome caused by MAGED2 mutations and opens new avenues for therapy in these patients.

A Novel Classification of Glioma Subgroup, Which Is Highly Correlated With the Clinical Characteristics and Tumor Tissue Characteristics, Based on the Expression Levels of Gβ and Gγ Genes

Purpose

Glioma is a classical type of primary brain tumors that is most common seen in adults, and its high heterogeneity used to be a reference standard for subgroup classification. Glioma has been diagnosed based on histopathology, grade, and molecular markers including IDH mutation, chromosome 1p/19q loss, and H3K27M mutation. This subgroup classification cannot fully meet the current needs of clinicians and researchers. We, therefore, present a new subgroup classification for glioma based on the expression levels of Gβ and Gγ genes to complement studies on glioma and Gβγ subunits, and to support clinicians to assess a patient’s tumor status.

Methods

Glioma samples retrieved from the CGGA database and the TCGA database. We clustered the gliomas into different groups by using expression values of Gβ and Gγ genes extracted from RNA sequencing data. The Kaplan–Meier method with a two-sided log-rank test was adopted to compare the OS of the patients between GNB2 group and non-GNB2 group. Univariate Cox regression analysis was referred to in order to investigate the prognostic role of each Gβ and Gγ genes. KEGG and ssGSEA analysis were applied to identify highly activated pathways. The “estimate” package, “GSVA” package, and the online analytical tools CIBERSORTx were employed to evaluate immune cell infiltration in glioma samples.

Results

Three subgroups were identified. Each subgroup had its own specific pathway activation pattern and other biological characteristics. High M2 cell infiltration was observed in the GNB2 subgroup. Different subgroups displayed different sensitivities to chemotherapeutics. GNB2 subgroup predicted poor survival in patients with gliomas, especially in patients with LGG with mutation IDH and non-codeleted 1p19q.

Conclusion

The subgroup classification we proposed has great application value. It can be used to select chemotherapeutic drugs and the prognosis of patients with target gliomas. The unique relationships between subgroups and tumor-related pathways are worthy of further investigation to identify therapeutic Gβγ heterodimer targets.

Subtype-dependent regulation of Gβγ signalling

G protein-coupled receptors (GPCRs) transmit information to the cell interior by transducing external signals to heterotrimeric G protein subunits, Gα and Gβγ subunits, localized on the inner leaflet of the plasma membrane. Though the initial focus was mainly on Gα-mediated events, Gβγ subunits were later identified as major contributors to GPCR-G protein signalling. A broad functional array of Gβγ signalling has recently been attributed to Gβ and Gγ subtype diversity, comprising 5 Gβ and 12 Gγ subtypes, respectively. In addition to displaying selectivity towards each other to form the Gβγ dimer, numerous studies have identified preferences of distinct Gβγ combinations for specific GPCRs, Gα subtypes and effector molecules. Importantly, Gβ and Gγ subtype-dependent regulation of downstream effectors, representing a diverse range of signalling pathways and physiological functions have been found. Here, we review the literature on the repercussions of Gβ and Gγ subtype diversity on direct and indirect regulation of GPCR/G protein signalling events and their physiological outcomes. Our discussion additionally provides perspective in understanding the intricacies underlying molecular regulation of subtype-specific roles of Gβγ signalling and associated diseases.

Optimized iLID Membrane Anchors for Local Optogenetic Protein Recruitment

Optogenetic protein dimerization systems are powerful tools to investigate the biochemical networks that cells use to make decisions and coordinate their activities. These tools, including the improved Light-Inducible Dimer (iLID) system, offer the ability to selectively recruit components to subcellular locations, such as micron-scale regions of the plasma membrane. In this way, the role of individual proteins within signaling networks can be examined with high spatiotemporal resolution. Currently, consistent recruitment is limited by heterogeneous optogenetic component expression, and spatial precision is diminished by protein diffusion, especially over long time scales. Here, we address these challenges within the iLID system with alternative membrane anchoring domains and fusion configurations. Using live cell imaging and mathematical modeling, we demonstrate that the anchoring strategy affects both component expression and diffusion, which in turn impact recruitment strength, kinetics, and spatial dynamics. Compared to the commonly used C-terminal iLID fusion, fusion proteins with large N-terminal anchors show stronger local recruitment, slower diffusion of recruited components, efficient recruitment over wider gene expression ranges, and improved spatial control over signaling outputs. We also define guidelines for component expression regimes for optimal recruitment for both cell-wide and subcellular recruitment strategies. Our findings highlight key sources of imprecision within light-inducible dimer systems and provide tools that allow greater control of subcellular protein localization across diverse cell biological applications.

Endogenous Cyclin D1 Promotes the Rate of Onset and Magnitude of Mitogenic Signaling via Akt1 Ser473 Phosphorylation

Cyclin D1 encodes the regulatory subunit of a holoenzyme that phosphorylates RB and functions as a collaborative nuclear oncogene. The serine threonine kinase Akt plays a pivotal role in the control of cellular metabolism, survival, and mitogenic signaling. Herein, Akt1-mediated phosphorylation of downstream substrates in the mammary gland is reduced by cyclin D1 genetic deletion and is induced by mammary-gland-targeted cyclin D1 overexpression. Cyclin D1 is associated with Akt1 and augments the rate of onset and maximal cellular Akt1 activity induced by mitogens. Cyclin D1 is identified in a cytoplasmic-membrane-associated pool, and cytoplasmic-membrane-localized cyclin D1—but not nuclear-localized cyclin D1—recapitulates Akt1 transcriptional function. These studies identify a novel extranuclear function of cyclin D1 to enhance proliferative functions via augmenting Akt1 phosphorylation at Ser473.

Chen et al. show that the rate of onset and maximal cellular Akt1 activity induced by mitogens was augmented by cyclin D1. Cyclin D1 bound and phosphorylated Akt1 at Ser473. These studies identify a novel extranuclear function of cyclin D1 to enhance proliferative functions via augmenting Akt1 phosphorylation at Ser473.

Compartmentalized GPCR Signaling from Intracellular Membranes

G protein-coupled receptors (GPCRs) are integral membrane proteins that transduce a wide array of inputs including light, ions, hormones, and neurotransmitters into intracellular signaling responses which underlie complex processes ranging from vision to learning and memory. Although traditionally thought to signal primarily from the cell surface, GPCRs are increasingly being recognized as capable of signaling from intracellular membrane compartments, including endosomes, the Golgi apparatus, and nuclear membranes. Remarkably, GPCR signaling from these membranes produces functional effects that are distinct from signaling from the plasma membrane, even though often the same G protein effectors and second messengers are activated. In this review, we will discuss the emerging idea of a "spatial bias" in signaling. We will present the evidence for GPCR signaling through G protein effectors from intracellular membranes, and the ways in which this signaling differs from canonical plasma membrane signaling with important implications for physiology and pharmacology. We also highlight the potential mechanisms underlying spatial bias of GPCR signaling, including how intracellular membranes and their associated lipids and proteins affect GPCR activity and signaling.

G protein-regulated endocytic trafficking of adenylyl cyclase type 9

GPCRs are increasingly recognized to initiate signaling via heterotrimeric G proteins as they move through the endocytic network, but little is known about how relevant G protein effectors are localized. Here we report selective trafficking of adenylyl cyclase type 9 (AC9) from the plasma membrane to endosomes while adenylyl cyclase type 1 (AC1) remains in the plasma membrane, and stimulation of AC9 trafficking by ligand-induced activation of Gs-coupled GPCRs. AC9 transits a similar, dynamin-dependent early endocytic pathway as ligand-activated GPCRs. However, unlike GPCR traffic control which requires β-arrestin but not Gs, AC9 traffic control requires Gs but not β-arrestin. We also show that AC9, but not AC1, mediates cAMP production stimulated by endogenous receptor activation in endosomes. These results reveal dynamic and isoform-specific trafficking of adenylyl cyclase in the endocytic network, and a discrete role of a heterotrimeric G protein in regulating the subcellular distribution of a relevant effector.

Optical approaches for single-cell and subcellular analysis of GPCR-G protein signaling

G protein-coupled receptors (GPCRs), G proteins, and their signaling associates are major signal transducers that control the majority of cellular signaling and regulate key biological functions including immune, neurological, cardiovascular, and metabolic processes. These pathways are targeted by over one-third of drugs on the market; however, the current understanding of their function is limited and primarily derived from cell-destructive approaches providing an ensemble of static, multi-cell information about the status and composition of molecules. Spatiotemporal behavior of molecules involved is crucial to understanding in vivo cell behaviors both in health and disease, and the advent of genetically encoded fluorescence proteins and small fluorophore-based biosensors has facilitated the mapping of dynamic signaling in cells with subcellular acuity. Since we and others have developed optogenetic methods to regulate GPCR-G protein signaling in single cells and subcellular regions using dedicated wavelengths, the desire to develop and adopt optogenetically amenable assays to measure signaling has motivated us to take a broader look at the available optical tools and approaches compatible with measuring single-cell and subcellular GPCR-G protein signaling. Here we review such key optical approaches enabling the examination of GPCR, G protein, secondary messenger, and downstream molecules such as kinase and lipid signaling in living cells. The methods reviewed employ both fluorescence and bioluminescence detection. We not only further elaborate the underlying principles of these sensors but also discuss the experimental criteria and limitations to be considered during their use in single-cell and subcellular signal mapping.

Smallest near-infrared fluorescent protein evolved from cyanobacteriochrome as versatile tag for spectral multiplexing

From a single domain of cyanobacteriochrome (CBCR) we developed a near-infrared (NIR) fluorescent protein (FP), termed miRFP670nano, with excitation at 645 nm and emission at 670 nm. This is the first CBCR-derived NIR FP evolved to efficiently bind endogenous biliverdin chromophore and brightly fluoresce in mammalian cells. miRFP670nano is a monomer with molecular weight of 17 kDa that is 2-fold smaller than bacterial phytochrome (BphP)-based NIR FPs and 1.6-fold smaller than GFP-like FPs. Crystal structure of the CBCR-based NIR FP with biliverdin reveals a molecular basis of its spectral and biochemical properties. Unlike BphP-derived NIR FPs, miRFP670nano is highly stable to denaturation and degradation and can be used as an internal protein tag. miRFP670nano is an effective FRET donor for red-shifted NIR FPs, enabling engineering NIR FRET biosensors spectrally compatible with GFP-like FPs and blue-green optogenetic tools. miRFP670nano unlocks a new source of diverse CBCR templates for NIR FPs.

High-Affinity Functional Fluorescent Ligands for Human β-Adrenoceptors

Visualization of the G-protein coupled receptor (GPCR) is of great importance for studying its function in a native cell. We have synthesized a series of red-emitting fluorescent probes targeting β-adrenergic receptor (βAR) that are compatible with confocal and Stimulated Emission Depletion (STED) microscopy as well as with Time-Resolved Fluorescence Resonance Energy Transfer (TR-FRET) binding assay in living cells. The probe based on the agonist BI-167107 and fluorescent dye KK114 demonstrates nanomolar binding affinity and up to nine-fold β₂AR selectivity over β₁AR. Carazolol-derived probes are fluorogenic and allow no-wash imaging experiments. STED microscopy of β₂ARs stained at the native expression level on pancreatic CAPAN cells provides two-fold improvement in lateral optical resolution over confocal mode and reveals the formation of receptor microdomains. These probes retain their functional (agonist or antagonist) properties, allowing simultaneous modulation of cyclic adenosine monophosphate (cAMP) levels and receptor internalization as well as imaging receptor localization.

Super-resolution Visualization of Caveola Deformation in Response to Osmotic Stress

Caveolae are protein-dense plasma membrane domains structurally composed of caveolin-1 or -3 along with other proteins. Our previous studies have shown that caveolae enhance calcium signals generated through the Gα_q/phospholipase Cβ signaling pathway and that subjecting cells to hypo-osmotic stress reverses this enhancement. In this study, we have used super-resolution fluorescence microscopy supplemented by fluorescence correlation studies to determine the structural factors that underlie this behavior. We find similar and significant population of Gα_q and one of its receptors, bradykinin type 2 receptor (B2R), as well as a significant population of Gα_i and its coupled β2-adrenergic receptor (βAR), are localized to caveola domains. Although mild osmotic stress deforms caveolae and alters interactions between the caveolae and these proteins, the general structure and the localization of caveola components remain largely unchanged. This deformation eliminates the ability of caveolae to stabilize calcium signals mediated through Gα_q-B2R, but does not affect cAMP signals mediated through Gα_i and βAR. Structurally, we find that mild osmotic stress corresponding roughly to a pressure of 3.82 newtons/m² increases the domain diameter by ∼30% and increases the fluorescence intensity in the center of the domain mouth suggesting a flattening of the invagination. Approximate calculations show that caveolae in muscle tissue have the strength to handle the stress of muscle movement.

Activated G Protein Gαs Samples Multiple Endomembrane Compartments

Heterotrimeric G proteins are localized to the plasma membrane where they transduce extracellular signals to intracellular effectors. G proteins also act at intracellular locations, and can translocate between cellular compartments. For example, Gαs can leave the plasma membrane and move to the cell interior after activation. However, the mechanism of Gαs translocation and its intracellular destination are not known. Here we use bioluminescence resonance energy transfer (BRET) to show that after activation, Gαs rapidly associates with the endoplasmic reticulum, mitochondria, and endosomes, consistent with indiscriminate sampling of intracellular membranes from the cytosol rather than transport via a specific vesicular pathway. The primary source of Gαs for endosomal compartments is constitutive endocytosis rather than activity-dependent internalization. Recycling of Gαs to the plasma membrane is complete 25 min after stimulation is discontinued. We also show that an acylation-deacylation cycle is important for the steady-state localization of Gαs at the plasma membrane, but our results do not support a role for deacylation in activity-dependent Gαs internalization.

ARF6 Is an Actionable Node that Orchestrates Oncogenic GNAQ Signaling in Uveal Melanoma

Activating mutations in Gαq proteins, which form the α subunit of certain heterotrimeric G proteins, drive uveal melanoma oncogenesis by triggering multiple downstream signaling pathways, including PLC/PKC, Rho/Rac, and YAP. Here we show that the small GTPase ARF6 acts as a proximal node of oncogenic Gαq signaling to induce all of these downstream pathways as well as β-catenin signaling. ARF6 activates these diverse pathways through a common mechanism: the trafficking of GNAQ and β-catenin from the plasma membrane to cytoplasmic vesicles and the nucleus, respectively. Blocking ARF6 with a small-molecule inhibitor reduces uveal melanoma cell proliferation and tumorigenesis in a mouse model, confirming the functional relevance of this pathway and suggesting a therapeutic strategy for Gα-mediated diseases.

PAQR3 regulates Golgi vesicle fission and transport via the Gβγ-PKD signaling pathway

Heterotrimeric G proteins function at diverse subcellular locations, in addition to canonical signaling at the plasma membrane (PM). Gβγ signals at the Golgi, via protein kinase D (PKD), to regulate fission of PM-destined vesicles. However, the mechanism by which Gβγ is regulated at the Golgi in this process remains elusive. Recent studies have revealed that PAQR3 (Progestin and AdipoQ Receptor 3), also called RKTG (Raf Kinase Trapping to the Golgi), interacts with the Gβ subunit and localizes Gβ to the Golgi thereby inhibiting Gβγ signaling at the PM. Herein we show that, in contrast to this inhibition of canonical Gβγ signaling at the PM, PAQR3 promotes Gβγ signaling at the Golgi. Expression of PAQR3 causes fragmentation of the Golgi, while a Gβ binding-deficient mutant of PAQR3 does not cause Golgi fragmentation. Also, a C-terminal fragment of GRK2 (GRK2ct), which interacts with Gβγ and inhibits Gβγ signaling, and gallein, a small molecule inhibitor of Gβγ, are both able to inhibit PAQR3-mediated Golgi fragmentation. Furthermore, a dominant negative form of PKD (PKD-DN) and a pharmacological inhibitor of PKD, Gö6976, also inhibit PAQR3-mediated fragmentation of the Golgi. Importantly, expression of the Gβ binding-deficient mutant of PAQR3 inhibits the constitutive transport of the model cargo protein VSV-G from the Golgi to the PM, indicating the involvement of PAQR3 in Golgi-to PM vesicle transport and a dominant negative role for this mutant. Collectively, these results reveal a novel role for the newly characterized, Golgi-localized PAQR3 in regulating Gβγ at the non-canonical subcellular location of the Golgi and thus for controlling Golgi-to-PM protein transport via the Gβγ-PKD signaling pathway.

Inhibition of Gαs/cAMP Signaling Decreases TCR-Stimulated IL-2 transcription in CD4(+) T Helper Cells

BACKGROUND

The role of cAMP in regulating T cell activation and function has been controversial. cAMP is generally known as an immunosuppressant, but it is also required for generating optimal immune responses. As the effect of cAMP is likely to depend on its cellular context, the current study investigated whether the mechanism of activation of Gαs and adenylyl cyclase influences their effect on T cell receptor (TCR)-stimulated interleukin-2 (IL-2) mRNA levels.

METHODS

The effect of blocking Gs-coupled receptor (GsPCR)-mediated Gs activation on TCR-stimulated IL-2 mRNA levels in CD4(+) T cells was compared with that of knocking down Gαs expression or inhibiting adenylyl cyclase activity. The effect of knocking down Gαs expression on TCR-stimulated cAMP accumulation was compared with that of blocking GsPCR signaling.

RESULTS

ZM-241385, an antagonist to the Gs-coupled A2A adenosine receptor (A2AR), enhanced TCR-stimulated IL-2 mRNA levels in primary human CD4(+) T helper cells and in Jurkat T cells. A dominant negative Gαs construct, GαsDN3, also enhanced TCR-stimulated IL-2 mRNA levels. Similar to GsPCR antagonists, GαsDN3 blocked GsPCR-dependent activation of both Gαs and Gβγ. In contrast, Gαs siRNA and 2',5'-dideoxyadenosine (ddA), an adenylyl cyclase inhibitor, decreased TCR-stimulated IL-2 mRNA levels. Gαs siRNA, but not GαsDN3, decreased TCR-stimulated cAMP synthesis. Potentiation of IL-2 mRNA levels by ZM-241385 required at least two days of TCR stimulation, and addition of ddA after three days of TCR stimulation enhanced IL-2 mRNA levels.

CONCLUSIONS

GsPCRs play an inhibitory role in the regulation of TCR-stimulated IL-2 mRNA levels whereas Gαs and cAMP can play a stimulatory one. Additionally, TCR-dependent activation of Gαs does not appear to involve GsPCRs. These results suggest that the context of Gαs/cAMP activation and the stage of T cell activation and differentiation determine the effect on TCR-stimulated IL-2 mRNA levels.

Heterotrimeric G protein-mediated signaling and its non-canonical regulation in the heart

Heterotrimeric guanine nucleotide-binding proteins (G proteins) regulate a multitude of signaling pathways in mammalian cells by transducing signals from G protein-coupled receptors (GPCRs) to effectors, which in turn regulate cellular function. In the myocardium, G protein signaling occurs in all cardiac cell types and is centrally involved in the regulation of heart rate, pump function, and vascular tone and in the response to hemodynamic stress and injury. Perturbations in G protein-mediated signaling are well known to contribute to cardiac hypertrophy, failure, and arrhythmias. Most of the currently used drugs for cardiac and other diseases target GPCR signaling. In the canonical G protein signaling paradigm, G proteins that are located at the cytoplasmic surface of the plasma membrane become activated after an agonist-induced conformational change of GPCRs, which then allows GTP-bound Gα and free Gβγ subunits to activate or inhibit effector proteins. Research over the past two decades has markedly broadened the original paradigm with a GPCR-G protein-effector at the cell surface at its core by revealing novel binding partners and additional subcellular localizations for heterotrimeric G proteins that facilitate many previously unrecognized functional effects. In this review, we focus on non-canonical and epigenetic-related mechanisms that regulate heterotrimeric G protein expression, activation, and localization and discuss functional consequences using cardiac examples where possible. Mechanisms reviewed involve microRNAs, histone deacetylases, chaperones, alternative modes of G protein activation, and posttranslational modifications. Some of these newly characterized mechanisms may be further developed into novel strategies for the treatment of cardiac disease and beyond.

Redistribution of Gαs in mouse salivary glands following β-adrenergic stimulation

OBJECTIVE

Signalling via β-adrenergic receptors activates heterotrimeric G-proteins, which dissociate into α and βγ subunits. In salivary glands, the α subunit of Gs stimulates adenylate cyclase, increasing cyclic AMP levels and promoting exocytosis. The goals of this study were to determine Gαs localization in salivary glands and whether it undergoes redistribution upon activation.

METHODS

Mouse parotid and submandibular (SMG) glands were fixed with paraformaldehyde and prepared for immunofluorescence labelling with anti-Gαs.

RESULTS

In unstimulated parotid and SMG acinar cells, Gαs was localized mainly to basolateral membranes. Some parotid acinar cells also exhibited cytoplasmic fluorescence. Isoproterenol (IPR) stimulation resulted in decreased membrane fluorescence and increased cytoplasmic fluorescence, which appeared relatively uniform by 30 min. Beginning about 2 h after IPR, cytoplasmic fluorescence decreased and membrane fluorescence increased, approaching unstimulated levels in SMG acini by 4 h. Some parotid acini exhibited cytoplasmic fluorescence up to 8 h after IPR. The IPR-induced redistribution of Gαs was prevented (SMG) or reduced (parotid) by prior injection of propranolol. Striated duct cells of unstimulated mice exhibited general cytoplasmic fluorescence, which was unchanged after IPR.

CONCLUSIONS

Gαs is localized to basolateral membranes of unstimulated salivary acinar cells. Activation of Gαs causes its release from the cell membrane and movement into the cytoplasm. Reassociation of Gαs with the membrane begins about 2 h after stimulation in the SMG, but complete reassociation takes several hours in the parotid gland. The presence of Gαs in striated duct cells suggests a role in signal transduction of secretion and/or electrolyte transport processes.

Activation of microtubule dynamics increases neuronal growth via the nerve growth factor (NGF)- and Gαs-mediated signaling pathways

Signals that activate the G protein Gαs and promote neuronal differentiation evoke Gαs internalization in rat pheochromocytoma (PC12) cells. These agents also significantly increase Gαs association with microtubules, resulting in an increase in microtubule dynamics because of the activation of tubulin GTPase by Gαs. To determine the function of Gαs/microtubule association in neuronal development, we used real-time trafficking of a GFP-Gαs fusion protein. GFP-Gαs concentrates at the distal end of the neurites in differentiated living PC12 cells as well as in cultured hippocampal neurons. Gαs translocates to specialized membrane compartments at tips of growing neurites. A dominant-negative Gα chimera that interferes with Gαs binding to tubulin and activation of tubulin GTPase attenuates neurite elongation and neurite number both in PC12 cells and primary hippocampal neurons. This effect is greatest on differentiation induced by activated Gαs. Together, these data suggest that activated Gαs translocates from the plasma membrane and, through interaction with tubulin/microtubules in the cytosol, is important for neurite formation, development, and outgrowth. Characterization of neuronal G protein dynamics and their contribution to microtubule dynamics is important for understanding the molecular mechanisms by which G protein-coupled receptor signaling orchestrates neuronal growth and differentiation.

Inhibition of G-protein βγ signaling enhances T cell receptor-stimulated interleukin 2 transcription in CD4+ T helper cells

G-protein-coupled receptor (GPCR) signaling modulates the expression of cytokines that are drug targets for immune disorders. However, although GPCRs are common targets for other diseases, there are few GPCR-based pharmaceuticals for inflammation. The purpose of this study was to determine whether targeting G-protein βγ (Gβγ) complexes could provide a useful new approach for modulating interleukin 2 (IL-2) levels in CD4⁺ T helper cells. Gallein, a small molecule inhibitor of Gβγ, increased levels of T cell receptor (TCR)-stimulated IL-2 mRNA in primary human naïve and memory CD4⁺ T helper cells and in Jurkat human CD4⁺ leukemia T cells. Gβ₁ and Gβ₂ mRNA accounted for >99% of Gβ mRNA, and small interfering RNA (siRNA)-mediated silencing of Gβ₁ but not Gβ₂ enhanced TCR-stimulated IL-2 mRNA increases. Blocking Gβγ enhanced TCR-stimulated increases in IL-2 transcription without affecting IL-2 mRNA stability. Blocking Gβγ also enhanced TCR-stimulated increases in nuclear localization of nuclear factor of activated T cells 1 (NFAT1), NFAT transcriptional activity, and levels of intracellular Ca²⁺. Potentiation of IL-2 transcription required continuous Gβγ inhibition during at least two days of TCR stimulation, suggesting that induction or repression of additional signaling proteins during T cell activation and differentiation might be involved. The potentiation of TCR-stimulated IL-2 transcription that results from blocking Gβγ in CD4⁺ T helper cells could have applications for autoimmune diseases.

Gαs regulates the post-endocytic sorting of G protein-coupled receptors

The role of Gαs in G protein-coupled receptor (GPCR) signalling at the cell surface is well established. Recent evidence has revealed the presence of Gαs on endosomes and its capacity to elicit GPCR-promoted signalling from this intracellular compartment. Here, we report an unconventional role for Gαs in the endocytic sorting of GPCRs to lysosomes. Cellular depletion of Gαs specifically delays the lysosomal degradation of GPCRs by disrupting the transfer of GPCRs into the intraluminal vesicles (ILVs) of multivesicular bodies. We show that Gαs interacts with GPCR-associated binding protein-1 (GASP1) and dysbindin, two key proteins that serve as linkers between GPCRs and the endosomal-sorting complex required for transport (ESCRT) machinery involved in receptor sorting into ILVs. Our findings reveal that Gαs plays a role in both GPCR signalling and trafficking pathways, providing another piece in the intertwining molecular network between these processes.

Gαz regulates BDNF-induction of axon growth in cortical neurons

The disruption of neurotransmitter and neurotrophic factor signaling in the central nervous system (CNS) is implicated as the root cause of neuropsychiatric disorders, including schizophrenia, epilepsy, chronic pain, and depression. Therefore, identifying the underlying molecular mechanisms by which neurotransmitter and neurotrophic factor signaling regulates neuronal survival or growth may facilitate identification of more effective therapies for these disorders. Previously, our lab found that the heterotrimeric G protein, Gz, mediates crosstalk between G protein-coupled receptors and neurotrophin signaling in the neural cell line PC12. These data, combined with Gαz expression profiles--predominantly in neuronal cells with higher expression levels corresponding to developmental times of target tissue innervation--suggested that Gαz may play an important role in neurotrophin signaling and neuronal development. Here, we provide evidence in cortical neurons, both manipulated ex vivo and those cultured from Gz knockout mice, that Gαz is localized to axonal growth cones and plays a significant role in the development of axons of cortical neurons in the CNS. Our findings indicate that Gαz inhibits BDNF-stimulated axon growth in cortical neurons, establishing an endogenous role for Gαz in regulating neurotrophin signaling in the CNS.

The expanding roles of Gβγ subunits in G protein-coupled receptor signaling and drug action

Gβγ subunits from heterotrimeric G proteins perform a vast array of functions in cells with respect to signaling, often independently as well as in concert with Gα subunits. However, the eponymous term "Gβγ" does not do justice to the fact that 5 Gβ and 12 Gγ isoforms have evolved in mammals to serve much broader roles beyond their canonical roles in cellular signaling. We explore the phylogenetic diversity of Gβγ subunits with a view toward understanding these expanded roles in different cellular organelles. We suggest that the particular content of distinct Gβγ subunits regulates cellular activity, and that the granularity of individual Gβ and Gγ action is only beginning to be understood. Given the therapeutic potential of targeting Gβγ action, this larger view serves as a prelude to more specific development of drugs aimed at individual isoforms.

The hydrophobic amino acid cluster at the cytoplasmic end of transmembrane helix III modulates the coupling of the β(1)-adrenergic receptor to G(s)

Abstract A cluster of hydrophobic amino acids at the cytoplasmic end of trans-membranal helix III (TM-III) is a common feature among class-A of G protein-coupled receptors (GPCR). We mutagenized alanine 159(3.53) to glutamic acid and isoleucine160(3.54) to arginine (A159E/I160R) in TM-III of the human β(1)-adrenergic receptor (β(1)-AR) to disrupt the function of the hydrophobic cluster. Structurally, the combined mutations of A159E/I160R caused an almost 90° tilt in the rotation of Arg156(3.50) in the E/DRY motif of TM-III and displaced Tyr166(3.60) in intracellular loop 2. The A159E/I160R β(1)-AR was uncoupled from G(s) as determined by cyclic AMP/adenylyl cyclase assays and by FRET-based proximity measurements between the β(1)-AR and G(s)α. Isoproterenol induced β-arrestin trafficking in cells expressing both the wild-type β(1)-AR and the A159E/I160R β(1)-AR. Isoproterenol markedly increased the phosphorylation of ERK1/2 in cells expressing the WT β(1)-AR and this effect was dependent on the activation of the G(s)-cyclic AMP-dependent protein kinase → Rap → B-raf axis. However, in cells bearing the A159E/I160R β(1)-AR, isoproterenol failed to increase the phosphorylation of ERK(1/2). These results indicate that mutations in the G(s)α-binding pocket of the GPCR interfered with receptor coupling to G(s) and with its downstream signaling cascades.

Bimolecular fluorescence complementation analysis of G protein-coupled receptor dimerization in living cells

Emerging evidence indicates that G protein-coupled receptor (GPCR) signaling is mediated by receptor-receptor interactions at multiple levels. Thus, understanding the biochemistry and pharmacology of those receptor complexes is an important part of delineating the fundamental processes associated with GPCR-mediated signaling in human disease. A variety of experimental approaches have been used to explore these complexes, including bimolecular fluorescence complementation (BiFC) and multicolor BiFC (mBiFC). BiFC approaches have recently been used to explore the composition, cellular localization, and drug modulation of GPCR complexes. The basic methods for applying BiFC and mBiFC to study GPCRs in living cells are the subject of the present chapter.

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.

Identification and subcellular localization of molecular complexes of Gq/11α protein in HEK293 cells

Heterotrimeric G-proteins localized in the plasma membrane convey the signals from G-protein-coupled receptors (GPCRs) to different effectors. At least some types of G-protein α subunits have been shown to be partly released from plasma membranes and to move into the cytosol after receptor activation by the agonists. However, the mechanism underlying subcellular redistribution of trimeric G-proteins is not well understood and no definitive conclusions have been reached regarding the translocation of Gα subunits between membranes and cytosol. Here we used subcellular fractionation and clear-native polyacrylamide gel electrophoresis to identify molecular complexes of G(q/11)α protein and to determine their localization in isolated fractions and stability in naïve and thyrotropin-releasing hormone (TRH)-treated HEK293 cells expressing high levels of TRH receptor and G(11)α protein. We identified two high-molecular-weight complexes of 300 and 140 kDa in size comprising the G(q/11) protein, which were found to be membrane-bound. Both of these complexes dissociated after prolonged treatment with TRH. Still other G(q/11)α protein complexes of lower molecular weight were determined in the cytosol. These 70 kDa protein complexes were barely detectable under control conditions but their levels markedly increased after prolonged (4-16 h) hormone treatment. These results support the notion that a portion of G(q/11)α can undergo translocation from the membrane fraction into soluble fraction after a long-term activation of TRH receptor. At the same time, these findings indicate that the redistribution of G(q/11)α is brought about by the dissociation of high-molecular-weight complexes and concomitant formation of low-molecular-weight complexes containing the G(q/11)α protein.

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.

Dopamine D(2) Receptor-Mediated Heterologous Sensitization of AC5 Requires Signalosome Assembly

Chronic dopamine receptor activation is implicated in several central nervous system disorders. Although acute activation of Gα_i-coupled D₂ dopamine receptors inhibits adenylyl cyclase, persistent activation enhances adenylyl cyclase activity, a phenomenon called heterologous sensitization. Previous work revealed a requirement for Gα_s in D₂-induced heterologous sensitization of AC5. To elucidate the mechanism of Gα_s dependency, we expressed Gα_s mutants in Gα_s-deficient Gnas^E2−/E2− cells. Neither Gα_s-palmitoylation nor Gα_s-Gβγ interactions were required for sensitization of AC5. Moreover, we found that coexpressing βARKct-CD8 or Sar1(H79G) blocked heterologous sensitization. These studies are consistent with a role for Gα_s-AC5 interactions in sensitization however, Gβγ appears to have an indirect role in heterologous sensitization of AC5, possibly by promoting proper signalosome assembly.

Non-canonical signaling and localizations of heterotrimeric G proteins

Heterotrimeric G proteins typically transduce signals from G protein-coupled receptors (GPCRs) to effector proteins. In the conventional G protein signaling paradigm, the G protein is located at the cytoplasmic surface of the plasma membrane, where, after activation by an agonist-bound GPCR, the GTP-bound Gα and free Gβγ bind to and regulate a number of well-studied effectors, including adenylyl cyclase, phospholipase Cβ, RhoGEFs and ion channels. However, research over the past decade or more has established that G proteins serve non-canonical roles in the cell, whereby they regulate novel effectors, undergo activation independently of a GPCR, and/or function at subcellular locations other than the plasma membrane. This review will highlight some of these non-canonical aspects of G protein signaling, focusing on direct interactions of G protein subunits with cytoskeletal and cell adhesion proteins, the role of G proteins in cell division, and G protein signaling at diverse organelles.

G protein trafficking

The classical view of heterotrimeric G protein signaling places G -proteins at the cytoplasmic surface of the cell's plasma membrane where they are activated by an appropriate G protein-coupled receptor. Once activated, the GTP-bound Gα and the free Gβγ are able to regulate plasma membrane-localized effectors, such as adenylyl cyclase, phospholipase C-β, RhoGEFs and ion channels. Hydrolysis of GTP by the Gα subunit returns the G protein to the inactive Gαβγ heterotrimer. Although all of these events in the G protein cycle can be restricted to the cytoplasmic surface of the plasma membrane, G protein localization is dynamic. Thus, it has become increasingly clear that G proteins are able to move to diverse subcellular locations where they perform non-canonical signaling functions. This chapter will highlight our current understanding of trafficking pathways that target newly synthesized G proteins to the plasma membrane, activation-induced and reversible translocation of G proteins from the plasma membrane to intracellular locations, and constitutive trafficking of G proteins.

Subtype differences in pre-coupling of muscarinic acetylcholine receptors

Based on the kinetics of interaction between a receptor and G-protein, a myriad of possibilities may result. Two extreme cases are represented by: 1/Collision coupling, where an agonist binds to the free receptor and then the agonist-receptor complex “collides” with the free G-protein. 2/Pre-coupling, where stable receptor/G-protein complexes exist in the absence of agonist. Pre-coupling plays an important role in the kinetics of signal transduction. Odd-numbered muscarinic acetylcholine receptors preferentially couple to G_q/11, while even-numbered receptors prefer coupling to G_i/o. We analyzed the coupling status of the various subtypes of muscarinic receptors with preferential and non-preferential G-proteins. The magnitude of receptor-G-protein coupling was determined by the proportion of receptors existing in the agonist high-affinity binding conformation. Antibodies directed against the C-terminus of the α-subunits of the individual G-proteins were used to interfere with receptor-G-protein coupling. Effects of mutations and expression level on receptor-G-protein coupling were also investigated. Tested agonists displayed biphasic competition curves with the antagonist [³H]-N-methylscopolamine. Antibodies directed against the C-terminus of the α-subunits of the preferential G-protein decreased the proportion of high-affinity sites, and mutations at the receptor-G-protein interface abolished agonist high-affinity binding. In contrast, mutations that prevent receptor activation had no effect. Expression level of preferential G-proteins had no effect on pre-coupling to non-preferential G-proteins. Our data show that all subtypes of muscarinic receptors pre-couple with their preferential classes of G-proteins, but only M₁ and M₃ receptors also pre-couple with non-preferential G_i/o G-proteins. Pre-coupling is not dependent on agonist efficacy nor on receptor activation. The ultimate mode of coupling is therefore dictated by a combination of the receptor subtype and the class of G-protein.

G protein-coupled receptors and resistance to inhibitors of cholinesterase-8A (Ric-8A) both regulate the regulator of g protein signaling 14 RGS14·Gαi1 complex in live cells

Regulator of G protein Signaling 14 (RGS14) is a multifunctional scaffolding protein that integrates both conventional and unconventional G protein signaling pathways. Like other RGS (regulator of G protein signaling) proteins, RGS14 acts as a GTPase accelerating protein to terminate conventional Gα(i/o) signaling. However, unlike other RGS proteins, RGS14 also contains a G protein regulatory/GoLoco motif that specifically binds Gα(i1/3)-GDP in cells and in vitro. The non-receptor guanine nucleotide exchange factor Ric-8A can bind and act on the RGS14·Gα(i1)-GDP complex to play a role in unconventional G protein signaling independent of G protein-coupled receptors (GPCRs). Here we demonstrate that RGS14 forms a Gα(i/o)-dependent complex with a G(i)-linked GPCR and that this complex is regulated by receptor agonist and Ric-8A (resistance to inhibitors of cholinesterase-8A). Using live cell bioluminescence resonance energy transfer, we show that RGS14 functionally associates with the α(2A)-adrenergic receptor (α(2A)-AR) in a Gα(i/o)-dependent manner. This interaction is markedly disrupted after receptor stimulation by the specific agonist UK14304, suggesting complex dissociation or rearrangement. Agonist-mediated dissociation of the RGS14·α(2A)-AR complex occurs in the presence of Gα(i/o) but not Gα(s) or Gα(q). Unexpectedly, RGS14 does not dissociate from Gα(i1) in the presence of stimulated α(2A)-AR, suggesting preservation of RGS14·Gα(i1) complexes after receptor activation. However, Ric-8A facilitates dissociation of both the RGS14·Gα(i1) complex and the Gα(i1)-dependent RGS14·α(2A)-AR complex after receptor activation. Together, these findings indicate that RGS14 can form complexes with GPCRs in cells that are dependent on Gα(i/o) and that these RGS14·Gα(i1)·GPCR complexes may be substrates for other signaling partners such as Ric-8A.

Extra-long Gαs variant XLαs protein escapes activation-induced subcellular redistribution and is able to provide sustained signaling

Murine models indicate that Gαs and its extra-long variant XLαs, both of which are derived from GNAS, markedly differ regarding their cellular actions, but these differences are unknown. Here we investigated activation-induced trafficking of Gαs and XLαs, using immunofluorescence microscopy, cell fractionation, and total internal reflection fluorescence microscopy. In transfected cells, XLαs remained localized to the plasma membrane, whereas Gαs redistributed to the cytosol after activation by GTPase-inhibiting mutations, cholera toxin treatment, or G protein-coupled receptor agonists (isoproterenol or parathyroid hormone (PTH)(1-34)). Cholera toxin treatment or agonist (isoproterenol or pituitary adenylate cyclase activating peptide-27) stimulation of PC12 cells expressing Gαs and XLαs endogenously led to an increased abundance of Gαs, but not XLαs, in the soluble fraction. Mutational analyses revealed two conserved cysteines and the highly charged domain as being critically involved in the plasma membrane anchoring of XLαs. The cAMP response induced by M-PTH(1-14), a parathyroid hormone analog, terminated quickly in HEK293 cells stably expressing the type 1 PTH/PTH-related peptide receptor, whereas the response remained maximal for at least 6 min in cells that co-expressed the PTH receptor and XLαs. Although isoproterenol-induced cAMP response was not prolonged by XLαs expression, a GTPase-deficient XLαs mutant found in certain tumors and patients with fibrous dysplasia of bone and McCune-Albright syndrome generated more basal cAMP accumulation in HEK293 cells and caused more severe impairment of osteoblastic differentiation of MC3T3-E1 cells than the cognate Gαs mutant (gsp oncogene). Thus, activated XLαs and Gαs traffic differently, and this may form the basis for the differences in their cellular actions.

Polarity mediates asymmetric trafficking of the Gbeta heterotrimeric G-protein subunit GPB-1 in C. elegans embryos

Asymmetric cell division is an evolutionarily conserved process that gives rise to daughter cells with different fates. In one-cell stage C. elegans embryos, this process is accompanied by asymmetric spindle positioning, which is regulated by anterior-posterior (A-P) polarity cues and driven by force generators located at the cell membrane. These force generators comprise two Gα proteins, the coiled-coil protein LIN-5 and the GoLoco protein GPR-1/2. The distribution of GPR-1/2 at the cell membrane is asymmetric during mitosis, with more protein present on the posterior side, an asymmetry that is thought to be crucial for asymmetric spindle positioning. The mechanisms by which the distribution of components such as GPR-1/2 is regulated in time and space are incompletely understood. Here, we report that the distribution of the Gβ subunit GPB-1, a negative regulator of force generators, varies across the cell cycle, with levels at the cell membrane being lowest during mitosis. Furthermore, we uncover that GPB-1 trafficks through the endosomal network in a dynamin- and RAB-5-dependent manner, which is most apparent during mitosis. We find that GPB-1 trafficking is more pronounced on the anterior side and that this asymmetry is regulated by A-P polarity cues. In addition, we demonstrate that GPB-1 depletion results in the loss of GPR-1/2 asymmetry during mitosis. Overall, our results lead us to propose that modulation of Gβ trafficking plays a crucial role during the asymmetric division of one-cell stage C. elegans embryos.

Identification of a specific assembly of the g protein golf as a critical and regulated module of dopamine and adenosine-activated cAMP pathways in the striatum

In the principal neurons of striatum (medium spiny neurons, MSNs), cAMP pathway is primarily activated through the stimulation of dopamine D1 and adenosine A_2A receptors, these receptors being mainly expressed in striatonigral and striatopallidal MSNs, respectively. Since cAMP signaling pathway could be altered in various physiological and pathological circumstances, including drug addiction and Parkinson’s disease, it is of crucial importance to identify the molecular components involved in the activation of this pathway. In MSNs, cAMP pathway activation is not dependent on the classical Gs GTP-binding protein but requires a specific G protein subunit heterotrimer containing Gαolf/β2/γ7 in particular association with adenylyl cyclase type 5. This assembly forms an authentic functional signaling unit since loss of one of its members leads to defects of cAMP pathway activation in response to D1 or A_2A receptor stimulation, inducing dramatic impairments of behavioral responses dependent on these receptors. Interestingly, D1 receptor (D1R)-dependent cAMP signaling is modulated by the neuronal levels of Gαolf, indicating that Gαolf represents the rate-limiting step in this signaling cascade and could constitute a critical element for regulation of D1R responses. In both Parkinsonian patients and several animal models of Parkinson’s disease, the lesion of dopamine neurons produces a prolonged elevation of Gαolf levels. This observation gives an explanation for the cAMP pathway hypersensitivity to D1R stimulation, occurring despite an unaltered D1R density. In conclusion, alterations in the highly specialized assembly of Gαolf/β2/γ7 subunits can happen in pathological conditions, such as Parkinson’s disease, and it could have important functional consequences in relation to changes in D1R signaling in the striatum.

Differential response to morphine of the oligomeric state of μ-opioid in the presence of δ-opioid receptors

Prolonged morphine treatment induces extensive desensitization of the μ-opioid receptor (μOR) which is the G-protein-coupled receptor that primarily mediates the cellular response to morphine. To date, the molecular mechanism underlying this process is unknown. Here, we have used live cell fluorescence imaging to investigate whether prolonged morphine treatment affects the physical environment of μOR, or its coupling with G-proteins, in two neuronal cell lines. We find that chronic morphine treatment does not change the amount of enhanced yellow fluorescence protein (eYFP)-tagged μOR on the plasma membrane, and only slightly decreases its association with G-protein subunits. Additionally, morphine treatment does not have a detectable effect on the diffusion coefficient of eYFP-μOR. However, in the presence of another family member, the δ-opioid receptor (δOR), prolonged morphine exposure results in a significant increase in the diffusion rate of μOR. Number and brightness measurements suggest that μOR exists primarily as a dimer that will oligomerize with δOR into tetramers, and morphine promotes the dissociation of these tetramers. To provide a plausible structural context to these data, we used homology modeling techniques to generate putative configurations of μOR-δOR tetramers. Overall, our studies provide a possible rationale for morphine sensitivity.

Fluorescent protein complementation assays: new tools to study G protein-coupled receptor oligomerization and GPCR-mediated signaling

G protein-coupled receptor (GPCR) signaling is mediated by protein-protein interactions at multiple levels. The characterization of the corresponding protein complexes is therefore paramount to the basic understanding of GPCR-mediated signal transduction. The number of documented interactions involving GPCRs is rapidly growing, and appreciating the functional significance of these complexes is clearly the next challenge. New experimental approaches including protein complementation assays (PCAs) have recently been used to examine the composition, plasma membrane targeting, and desensitization of protein complexes involved in GPCR signaling. These methods also hold promise for better understanding of drug-induced effects on GPCR interactions. This review focuses on the application of fluorescent PCAs for the study of GPCR signaling. Potential applications of PCAs in high-content screens are also presented. Non-fluorescent PCA techniques as well as combined assays for the detection of ternary and quaternary protein complexes are briefly discussed.

The effect of membrane domains on the G protein-phospholipase Cbeta signaling pathway

The plasma membrane serves as a barrier to limit the exit and entry of components into and out of the cell, offering protection from the external environment. Communication between the cell and the external environment is mediated by multiple signaling pathways. While the plasma membrane was historically viewed as a lipid bilayer with freely diffusing proteins, the last decade has shown that the lipids and proteins in the plasma membrane are organized in a non-random manner, and that this organization can direct and modify various signaling pathways in the cell. In this review, we qualitatively discuss the ways that membrane domains can affect cell signaling. We then focus on how membrane domains can affect a specific signaling pathway--the G protein-phospholipase Cbeta pathway and show how membrane domains can play an active role in directing or redirecting G protein signals.

The small G protein Rac1 activates phospholipase Cdelta1 through phospholipase Cbeta2

Rac1, which is associated with cytoskeletal pathways, can activate phospholipase Cbeta2 (PLCbeta2) to increase intracellular Ca(2+) levels. This increased Ca(2+) can in turn activate the very robust PLCdelta1 to synergize Ca(2+) signals. We have previously found that PLCbeta2 will bind to and inhibit PLCdelta1 in solution by an unknown mechanism and that PLCbeta2.PLCdelta1 complexes can be disrupted by Gbetagamma subunits. However, because the major populations of PLCbeta2 and PLCdelta1 are cytosolic, their regulation by Gbetagamma subunits is not clear. Here, we have found that the pleckstrin homology (PH) domains of PLCbeta2 and PLCbeta3 are the regions that result in PLCdelta1 binding and inhibition. In cells, PLCbeta2.PLCdelta1 form complexes as seen by Förster resonance energy transfer and co-immunoprecipitation, and microinjection of PHbeta2 dissociates the complex. Using PHbeta2 as a tool to assess the contribution of PLCbeta inhibition of PLCdelta1 to Ca(2+) release, we found that, although PHbeta2 only results in a 25% inhibition of PLCdelta1 in solution, in cells the presence of PHbeta2 appears to eliminates Ca(2+) release suggesting a large threshold effect. We found that the small plasma membrane population of PLCbeta2.PLCdelta1 is disrupted by activation of heterotrimeric G proteins, and that the major cytosolic population of the complexes are disrupted by Rac1 activation. Thus, the activity of PLCdelta1 is controlled by the amount of bound PLCbeta2 that changes with displacement of the enzyme by heterotrimeric or small G proteins. Through PLCbeta2, PLCdelta1 activation is linked to surface receptors as well as signals that mediate cytoskeletal pathways.

Pituitary adenylate cyclase-activating polypeptide (PACAP)/PAC1HOP1 receptor activation coordinates multiple neurotrophic signaling pathways: Akt activation through phosphatidylinositol 3-kinase gamma and vesicle endocytosis for neuronal survival

MAPK and Akt pathways are predominant mediators of trophic signaling for many neuronal systems. Among the vasoactive intestinal peptide/secretin/glucagon family of related peptides, pituitary adenylate cyclase-activating polypeptide (PACAP) binding to specific PAC(1) receptor isoforms can engage multiple signaling pathways and promote neuroprotection through mechanisms that are not well understood. Using a primary sympathetic neuronal system, the current studies demonstrate that PACAP activation of PAC(1)HOP1 receptors engages both MAPK and Akt neurotrophic pathways in an integrated program to facilitate neuronal survival after growth factor withdrawal. PACAP not only stimulated prosurvival ERK1/2 and ERK5 activation but also abrogated SAPK/JNK and p38 MAPK signaling in parallel. In contrast to the potent and rapid effects of PACAP in ERK1/2 phosphorylation, PACAP stimulated Akt phosphorylation in a late phase of PAC(1)HOP1 receptor signaling. From inhibitor and immunoprecipitation analyses, the PACAP/PAC(1)HOP1 receptor-mediated Akt responses did not represent transactivation mechanisms but appeared to depend on G alpha(q)/phosphatidylinositol 3-kinase gamma activity and vesicular internalization pathways. Phosphatidylinositol 3-kinase gamma-selective inhibitors blocked PACAP-stimulated Akt phosphorylation in primary neuronal cultures and in PAC(1)HOP1-overexpressing cell lines; RNA interference-mediated knockdown of the receptor effectors attenuated PACAP-mediated Akt activation. Similarly, perturbation of endocytic pathways also blocked Akt phosphorylation. Between ERK and Akt pathways, PACAP-stimulated Akt signaling was the primary cascade that attenuated cultured neuron apoptosis after growth factor withdrawal. The partitioning of PACAP-mediated Akt signaling in endosomes may be a key mechanism contributing to the high spatial and temporal specificity in signal transduction necessary for survival pathways.

Bimolecular fluorescence complementation: lighting up seven transmembrane domain receptor signalling networks

There is increasing complexity in the organization of seven transmembrane domain (7TM) receptor signalling pathways, and in the ability of their ligands to modulate and direct this signalling. Underlying these events is a network of protein interactions between the 7TM receptors themselves and associated effectors, such as G proteins and beta-arrestins. Bimolecular fluorescence complementation, or BiFC, is a technique capable of detecting these protein-protein events essential for 7TM receptor function. Fluorescent proteins, such as those from Aequorea victoria, are split into two non-fluorescent halves, which then tag the proteins under study. On association, these fragments refold and regenerate a mature fluorescent protein, producing a BiFC signal indicative of complex formation. Here, we review the experimental criteria for successful application of BiFC, considered in the context of 7TM receptor signalling events such as receptor dimerization, G protein and beta-arrestin signalling. The advantages and limitations of BiFC imaging are compared with alternative resonance energy transfer techniques. We show that the essential simplicity of the fluorescent BiFC measurement allows high-content and advanced imaging applications, and that it can probe more complex multi-protein interactions alone or in combination with resonance energy transfer. These capabilities suggest that BiFC techniques will become ever more useful in the analysis of ligand and 7TM receptor pharmacology at the molecular level of protein-protein interactions.

Caveolin-1 and lipid microdomains regulate Gs trafficking and attenuate Gs/adenylyl cyclase signaling

Lipid rafts and caveolae are specialized membrane microdomains implicated in regulating G protein-coupled receptor signaling cascades. Previous studies have suggested that rafts/caveolae may regulate beta-adrenergic receptor/Galpha(s) signaling, but underlying molecular mechanisms are largely undefined. Using a simplified model system in C6 glioma cells, this study disrupts rafts/caveolae using both pharmacological and genetic approaches to test whether caveolin-1 and lipid microdomains regulate G(s) trafficking and signaling. Lipid rafts/caveolae were disrupted in C6 cells by either short-term cholesterol chelation using methyl-beta-cyclodextrin or by stable knockdown of caveolin-1 and -2 by RNA interference. In imaging studies examining Galpha(s)-GFP during signaling, stimulation with the betaAR agonist isoproterenol resulted in internalization of Galpha(s)-GFP; however, this trafficking was blocked by methyl-beta-cyclodextrin or by caveolin knockdown. Caveolin knockdown significantly decreased Galpha(s) localization in detergent insoluble lipid raft/caveolae membrane fractions, suggesting that caveolin localizes a portion of Galpha(s) to these membrane microdomains. Methyl-beta-cyclodextrin or caveolin knockdown significantly increased isoproterenol or thyrotropin-stimulated cAMP accumulation. Furthermore, forskolin- and aluminum tetrafluoride-stimulated adenylyl cyclase activity was significantly increased by caveolin knockdown in cells or in brain membranes obtained from caveolin-1 knockout mice, indicating that caveolin attenuates signaling at the level of Galpha(s)/adenylyl cyclase and distal to GPCRs. Taken together, these results demonstrate that caveolin-1 and lipid microdomains exert a major effect on Galpha(s) trafficking and signaling. It is suggested that lipid rafts/caveolae are sites that remove Galpha(s) from membrane signaling cascades and caveolins might dampen globally Galpha(s)/adenylyl cyclase/cAMP signaling.