Non-canonical signaling and localizations of heterotrimeric G proteins

Fungal alkaloid malbrancheamide reorients the lipid binding domain of GRK5

G protein-coupled receptors (GPCRs) are the largest group of receptors involved in various types of signaling. GPCR signaling is regulated via receptor phosphorylation by G protein-coupled receptor kinases 5 (GRK5). Calmodulin (CaM), a universal Ca2+ sensor, inhibits receptor phosphorylation by binding to GRK5. However, the inhibitor malbrancheamide (MBC), which binds at CaM C-lobe, allows for receptor phosphorylation. To understand the phosphorylation mechanism by GRK5, we carried out a MD simulation of the CaM/GRK5 complex in the presence and absence of the MBC inhibitor. The lipid binding domain (LBD) of GRK5 adopted different positions in the presence and absence of inhibitor. Furthermore, the inhibitor MBC restricted the movement of the N-lobe tether (NLT) loop, probably blocking the autophosphorylation of GRK5.Communicated by Ramaswamy H. Sarma.

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 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 results provide a detailed 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.

Non-canonical G protein signaling

The original paradigm of classical - also referred to as canonical - cellular signal transduction of heterotrimeric G proteins (G protein) is defined by a hierarchical, orthograde interaction of three players: the agonist-activated G protein-coupled receptor (GPCR), which activates the transducing G protein, that in turn regulates its intracellular effectors. This receptor-transducer-effector concept was extended by the identification of regulators and adapters such as the regulators of G protein signaling (RGS), receptor kinases like βARK, or GPCR-interacting arrestin adapters that are integrated into this canonical signaling process at different levels to enable fine-tuning. Finally, the identification of atypical signaling mechanisms of classical regulators, together with the discovery of novel modulators, added a new and fascinating dimension to the cellular G protein signal transduction. This heterogeneous group of accessory G protein modulators was coined "activators of G protein signaling" (AGS) proteins and plays distinct roles in canonical and non-canonical G protein signaling pathways. AGS proteins contribute to the control of essential cellular functions such as cell development and division, intracellular transport processes, secretion, autophagy or cell movements. As such, they are involved in numerous biological processes that are crucial for diseases, like diabetes mellitus, cancer, and stroke, which represent major health burdens. Although the identification of a large number of non-canonical G protein signaling pathways has broadened the spectrum of this cellular communication system, their underlying mechanisms, functions, and biological effects are poorly understood. In this review, we highlight and discuss atypical G protein-dependent signaling mechanisms with a focus on inhibitory G proteins (Gi) involved in canonical and non-canonical signal transduction, review recent developments and open questions, address the potential of new approaches for targeted pharmacological interventions.

Arabidopsis G-protein β subunit AGB1 represses abscisic acid signaling via attenuation of the MPK3-VIP1 phosphorylation cascade

Heterotrimeric G proteins play key roles in cellular processes. Although phenotypic analyses of Arabidopsis Gβ (AGB1) mutants have implicated G proteins in abscisic acid (ABA) signaling, the AGB1-mediated modules involved in ABA responses remain unclear. We found that a partial AGB1 protein was localized to the nucleus where it interacted with ABA-activated VirE2-interacting protein 1 (VIP1) and mitogen-activated protein kinase 3 (MPK3). AGB1 acts as an upstream negative regulator of VIP1 activity by initiating responses to ABA and drought stress, and VIP1 regulates the ABA signaling pathway in an MPK3-dependent manner in Arabidopsis. AGB1 outcompeted VIP1 for interaction with the C-terminus of MPK3, and prevented phosphorylation of VIP1 by MPK3. Importantly, ABA treatment reduced AGB1 expression in the wild type, but increased in vip1 and mpk3 mutants. VIP1 associates with ABA response elements present in the AGB1 promoter, forming a negative feedback regulatory loop. Thus, our study defines a new mechanism for fine-tuning ABA signaling through the interplay between AGB1 and MPK3-VIP1. Furthermore, it suggests a common G protein mechanism to receive and transduce signals from the external environment.

ACKR3 Proximity Labeling Identifies Novel G protein- and β-arrestin-independent GPCR Interacting Proteins

The canonical paradigm of GPCR signaling recognizes G proteins and β-arrestins as the two primary transducers that promote GPCR signaling. Recent evidence suggests the atypical chemokine receptor 3 (ACKR3) does not couple to G proteins, and β-arrestins are dispensable for some of its functions. Here, we employed proximity labeling to identify proteins that interact with ACKR3 in cells devoid of β-arrestin. We identified proteins involved in the endocytic machinery and evaluated a subset of proteins conserved across several GPCR-based proximity labeling experiments. We discovered that the bone morphogenic protein 2-inducible kinase (BMP2K) interacts with many different GPCRs with varying dependency on β-arrestin. Together, our work highlights the existence of modulators that can act independently of G proteins and β-arrestins to regulate GPCR signaling and provides important evidence for other targets that may regulate GPCR signaling.

The G protein alpha chaperone and guanine-nucleotide exchange factor RIC-8 regulates cilia morphogenesis in Caenorhabditis elegans sensory neurons

Heterotrimeric G (αβγ) proteins are canonical transducers of G-protein-coupled receptor (GPCR) signaling and play critical roles in communication between cells and their environment. Many GPCRs and heterotrimeric G proteins localize to primary cilia and modulate cilia morphology via mechanisms that are not well understood. Here, we show that RIC-8, a cytosolic guanine nucleotide exchange factor (GEF) and chaperone for Gα protein subunits, shapes cilia membrane morphology in a subset of Caenorhabditis elegans sensory neurons. Consistent with its role in ciliogenesis, C. elegans RIC-8 localizes to cilia in different sensory neuron types. Using domain mutagenesis, we demonstrate that while the GEF function alone is not sufficient, both the GEF and Gα-interacting chaperone motifs of RIC-8 are required for its role in cilia morphogenesis. We identify ODR-3 as the RIC-8 Gα client and demonstrate that RIC-8 functions in the same genetic pathway with another component of the non-canonical G protein signaling AGS-3 to shape cilia morphology. Notably, despite defects in AWC cilia morphology, ags-3 null mutants exhibit normal chemotaxis toward benzaldehyde unlike odr-3 mutant animals. Collectively, our findings describe a novel function for the evolutionarily conserved protein RIC-8 and non-canonical RIC-8-AGS-3-ODR-3 signaling in cilia morphogenesis and uncouple Gα ODR-3 functions in ciliogenesis and olfaction.

Beyond the G protein α subunit: investigating the functional impact of other components of the Gαi3 heterotrimers

BACKGROUND

Specific interactions between G protein-coupled receptors (GPCRs) and G proteins play a key role in mediating signaling events. While there is little doubt regarding receptor preference for Gα subunits, the preferences for specific Gβ and Gγ subunits and the effects of different Gβγ dimer compositions on GPCR signaling are poorly understood. In this study, we aimed to investigate the subcellular localization and functional response of Gαi3-based heterotrimers with different combinations of Gβ and Gγ subunits.

METHODS

Live-cell imaging microscopy and colocalization analysis were used to investigate the subcellular localization of Gαi3 in combination with Gβ1 or Gβ2 heterotrimers, along with representative Gγ subunits. Furthermore, fluorescence lifetime imaging microscopy (FLIM-FRET) was used to investigate the nanoscale distribution of Gαi3-based heterotrimers in the plasma membrane, specifically with the dopamine D2 receptor (D2R). In addition, the functional response of the system was assessed by monitoring intracellular cAMP levels and conducting bioinformatics analysis to further characterize the heterotrimer complexes.

RESULTS

Our results show that Gαi3 heterotrimers mainly localize to the plasma membrane, although the degree of colocalization is influenced by the accompanying Gβ and Gγ subunits. Heterotrimers containing Gβ2 showed slightly lower membrane localization compared to those containing Gβ1, but certain combinations, such as Gαi3β2γ8 and Gαi3β2γ10, deviated from this trend. Examination of the spatial arrangement of Gαi3 in relation to D2R and of changes in intracellular cAMP level showed that the strongest functional response is observed for those trimers for which the distance between the receptor and the Gα subunit is smallest, i.e. complexes containing Gβ1 and Gγ8 or Gγ10 subunit. Deprivation of Gαi3 lipid modifications resulted in a significant decrease in the amount of protein present in the cell membrane, but did not always affect intracellular cAMP levels.

CONCLUSION

Our studies show that the composition of G protein heterotrimers has a significant impact on the strength and specificity of GPCR-mediated signaling. Different heterotrimers may exhibit different conformations, which further affects the interactions of heterotrimers and GPCRs, as well as their interactions with membrane lipids. This study contributes to the understanding of the complex signaling mechanisms underlying GPCR-G-protein interactions and highlights the importance of the diversity of Gβ and Gγ subunits in G-protein signaling pathways. Video Abstract.

The G protein alpha Chaperone and Guanine-Nucleotide Exchange Factor RIC-8 Regulates Cilia Morphogenesis in Caenorhabditis elegans Sensory Neurons

Heterotrimeric G (αβγ) proteins are canonical transducers of G-protein-coupled receptor (GPCR) signaling and play critical roles in communication between cells and their environment. Many GPCRs and heterotrimeric G proteins localize to primary cilia and modulate cilia morphology via mechanisms that are not well understood. Here, we show that RIC-8, a cytosolic guanine nucleotide exchange factor (GEF) and chaperone for Gα protein subunits, shapes cilia membrane morphology in a subset of Caenorhabditis elegans sensory neurons. Consistent with its role in ciliogenesis, C. elegans RIC-8 localizes to cilia in different sensory neuron types. Using domain mutagenesis, we demonstrate that while the GEF function alone is not sufficient, both the GEF and Gα-interacting chaperone motifs of RIC-8 are required for its role in cilia morphogenesis. We identify ODR-3 as the RIC-8 Gα client and demonstrate that RIC-8 functions in the same genetic pathway with another component of the non-canonical G protein signaling AGS-3 to shape cilia morphology. Notably, despite severe defects in AWC cilia morphology, ags-3 null mutants exhibit normal chemotaxis toward benzaldehyde unlike odr-3 mutant animals. Collectively, our findings describe a novel function for the evolutionarily conserved protein RIC-8 and non-canonical RIC-8-AGS-3-ODR-3 signaling in cilia morphogenesis and uncouple Gα ODR-3 functions in ciliogenesis and olfaction.

Surfaceome Profiling Identifies Basigin-Chaperoned Protein Clients

The surface proteome or "surfaceome" is a critical mediator of cellular biology, facilitating cell-to-cell interactions and communication with extracellular biomolecules. Constituents of the surfaceome can serve as biomarkers for changing cell states and as targets for pharmacological intervention. While some pathways of cell surface trafficking are well characterized to allow prediction of surface localization, some non-canonical trafficking mechanisms do not. Basigin (Bsg), a cell surface glycoprotein, has been shown to chaperone protein clients to the cell surface. However, understanding which proteins are served by Bsg is not always straightforward. To accelerate such identification, we applied a surfaceome proximity labeling method that is integrated with quantitative mass spectrometry-based proteomics to discern changes in the surfaceome of hepatic stellate cells that occur in response to the genetic loss of Bsg. Using this strategy, we observed that the loss of Bsg leads to corresponding reductions in the cell surface expression of monocarboxylate transporters MCT1 and MCT4. We also found that these relationships were unique to Bsg and not found in neuroplastin (Nptn), a related family member. These results establish the utility of the surfaceome proximity labeling method to determine clients of cell surface chaperone proteins.

Gβγ signaling regulates microtubule-dependent control of Golgi integrity

Gβγ subunits regulate several non-canonical functions at distinct intracellular organelles. Previous studies have shown that Gβγ signaling at the Golgi is necessary to mediate vesicular protein transport function and to regulate mitotic Golgi fragmentation. Disruption of Golgi structure also occurs in response to microtubule depolymerizing agents, such as nocodazole. In this study, we use siRNA against Gβ1/2 or specific Gγ subunits to deplete their expression, and show that their knockdown causes a significant reduction in nocodazole-induced Golgi fragmentation. We establish that knockdown of Gβγ or inhibition of Gβγ with gallein resulted in decreased activation of protein kinase D (PKD) in response to nocodazole treatment. We demonstrate that restricting the amount of free Gβγ available for signaling by either inhibiting Gαi activation using pertussis toxin or by knockdown of the non-GPCR GEF, Girdin/GIV protein, results in a substantial decrease in nocodazole-induced Golgi fragmentation and PKD phosphorylation. Our results also indicate that depletion of Gβγ or inhibition with gallein or pertussis toxin significantly reduces the microtubule disruption-dependent Golgi fragmentation phenotype observed in cells transfected with mutant SOD1, a major causative protein in familial amyotrophic lateral sclerosis (ALS). These results provide compelling evidence that Gβγ signaling is critical for the regulation of Golgi integrity.

Odorant Binding Causes Cytoskeletal Rearrangement, Leading to Detectable Changes in Endothelial and Epithelial Barrier Function and Micromotion

Non-olfactory cells have excellent biosensor potential because they express functional olfactory receptors (ORs) and are non-neuronal cells that are easy to culture. ORs are G-protein coupled receptors (GPCRs), and there is a well-established link between different classes of G-proteins and cytoskeletal structure changes affecting cellular morphology that has been unexplored for odorant sensing. Thus, the present study was conducted to determine if odorant binding in non-olfactory cells causes cytoskeletal changes that will lead to cell changes detectable by electric cell-substrate impedance sensing (ECIS). To this end, we used the human umbilical vein endothelial cells (HUVECs), which express OR10J5, and the human keratinocyte (HaCaT) cells, which express OR2AT4. Using these two different cell barriers, we showed that odorant addition, lyral and Sandalore, respectively, caused an increase in cAMP, changes in the organization of the cytoskeleton, and a decrease in the integrity of the junctions between the cells, causing a decrease in cellular electrical resistance. In addition, the random cellular movement of the monolayers (micromotion) was significantly decreased after odorant exposure. Collectively, these data demonstrate a new physiological role of olfactory receptor signaling in endothelial and epithelial cell barriers and represent a new label-free method to detect odorant binding.

Impact of 5-HT6 Receptor Subcellular Localization on Its Signaling and Its Pathophysiological Roles

The serotonin (5-HT)₆ receptor still raises particular interest given its unique spatio-temporal pattern of expression among the serotonin receptor subtypes. It is the only serotonin receptor specifically expressed in the central nervous system, where it is detected very early in embryonic life and modulates key neurodevelopmental processes, from neuronal migration to brain circuit refinement. Its predominant localization in the primary cilium of neurons and astrocytes is also unique among the serotonin receptor subtypes. Consistent with the high expression levels of the 5-HT₆ receptor in brain regions involved in the control of cognitive processes, it is now well-established that the pharmacological inhibition of the receptor induces pro-cognitive effects in several paradigms of cognitive impairment in rodents, including models of neurodevelopmental psychiatric disorders and neurodegenerative diseases. The 5-HT₆ receptor can engage several signaling pathways in addition to the canonical Gs signaling, but there is still uncertainty surrounding the signaling pathways that underly its modulation of cognition, as well as how the receptor's coupling is dependent on its cellular compartmentation. Here, we describe recent findings showing how the proper subcellular localization of the receptor is achieved, how this peculiar localization determines signaling pathways engaged by the receptor, and their pathophysiological influence.

The role of Hedgehog and Notch signaling pathway in cancer

Notch and Hedgehog signaling are involved in cancer biology and pathology, including the maintenance of tumor cell proliferation, cancer stem-like cells, and the tumor microenvironment. Given the complexity of Notch signaling in tumors, its role as both a tumor promoter and suppressor, and the crosstalk between pathways, the goal of developing clinically safe, effective, tumor-specific Notch-targeted drugs has remained intractable. Drugs developed against the Hedgehog signaling pathway have affirmed definitive therapeutic effects in basal cell carcinoma; however, in some contexts, the challenges of tumor resistance and recurrence leap to the forefront. The efficacy is very limited for other tumor types. In recent years, we have witnessed an exponential increase in the investigation and recognition of the critical roles of the Notch and Hedgehog signaling pathways in cancers, and the crosstalk between these pathways has vast space and value to explore. A series of clinical trials targeting signaling have been launched continually. In this review, we introduce current advances in the understanding of Notch and Hedgehog signaling and the crosstalk between pathways in specific tumor cell populations and microenvironments. Moreover, we also discuss the potential of targeting Notch and Hedgehog for cancer therapy, intending to promote the leap from bench to bedside.

G protein gamma subunit, a hidden master regulator of GPCR signaling

Heterotrimeric G proteins (αβγ subunits) that are activated by G protein-coupled receptors (GPCRs) mediate the biological responses of eukaryotic cells to extracellular signals. The α subunits and the tightly bound βγ subunit complex of G proteins have been extensively studied and shown to control the activity of effector molecules. In contrast, the potential roles of the large family of γ subunits have been less studied. In this review, we focus on present knowledge about these proteins. Induced loss of individual γ subunit types in animal and plant models result in strikingly distinct phenotypes indicating that γ subtypes play important and specific roles. Consistent with these findings, downregulation or upregulation of particular γ subunit types result in various types of cancers. Clues about the mechanistic basis of γ subunit function have emerged from imaging the dynamic behavior of G protein subunits in living cells. This shows that in the basal state, G proteins are not constrained to the plasma membrane but shuttle between membranes and on receptor activation βγ complexes translocate reversibly to internal membranes. The translocation kinetics of βγ complexes varies widely and is determined by the membrane affinity of the associated γ subtype. On translocating, some βγ complexes act on effectors in internal membranes. The variation in translocation kinetics determines differential sensitivity and adaptation of cells to external signals. Membrane affinity of γ subunits is thus a parsimonious and elegant mechanism that controls information flow to internal cell membranes while modulating signaling responses.

The complex web of canonical and non-canonical Hedgehog signaling

Hedgehog (Hh) signaling is a widely studied signaling pathway because of its critical roles during development and in cell homeostasis. Vertebrate canonical and non-canonical Hh signaling are typically assumed to be distinct and occur in different cellular compartments. While research has primarily focused on the canonical form of Hh signaling and its dependency on primary cilia - microtubule-based signaling hubs - an extensive list of crucial functions mediated by non-canonical Hh signaling has emerged. Moreover, amounting evidence indicates that canonical and non-canonical modes of Hh signaling are interlinked, and that they can overlap spatially, and in many cases interact functionally. Here, we discuss some of the many cellular effects of non-canonical signaling and discuss new evidence indicating inter-relationships with canonical signaling. We discuss how Smoothened (Smo), a key component of the Hh pathway, might coordinate such diverse downstream effects. Collectively, pursuit of questions such as those proposed here will aid in elucidating the full extent of Smo function in development and advance its use as a target for cancer therapeutics.

Assessment of Intracellular GTP Levels Using Genetically Encoded Fluorescent Sensors

Changes in intracellular GTP levels, even incremental ones, profoundly affect the activity of several GTP-binding proteins ultimately resulting in alteration of several basal cellular phenotypes including cell motility, invasion, and tumorigenesis. However, until recently, no tools were available for GTP quantification in live cells. Therefore, in the current chapter, we describe the methodology for the quantitative assessment of spatiotemporal changes in GTP levels in the cells using genetically encoded fluorescent ratiometric GTP sensors termed GEVALs for GTP evaluators.

Constitutive activation of S1P receptors at the trans-Golgi network is required for surface transport carrier formation

The importance of the G-protein βγ subunits in the regulation of cargo transport from the trans-Golgi network (TGN) to the plasma membrane (PM) is well accepted; however, the molecular mechanism underlying the G-protein activation at the TGN remains unclear. We show here that sphingosine 1-phosphate (S1P) receptors at the PM were trafficked to the TGN in response to a surface transport cargo, temperature-sensitive vesicular stomatitis virus glycoprotein tagged with green fluorescent protein accumulation in the Golgi. The receptor internalization occurred in an S1P-independent manner but required phosphorylation by G-protein receptor kinase 2 and β-arrestin association before internalization. Continuously activated S1P receptors in a manner dependent on S1P at the TGN kept transmitting G-protein signals including the βγ subunits supply necessary for transport carrier formation at the TGN destined for the PM.

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S1P receptors traffic from the PM to Golgi in a surface cargo-dependent manner

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S1PR trafficking follows GRK2-dependent phosphorylation and β-arrestin binding

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S1PRs at the Golgi are continuously activated by S1P while sending G-protein signals

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S1PR/Gβγ signals at the Golgi are indispensable for surface transport carrier formation

A short C-terminal peptide in Gγ regulates Gβγ signaling efficacy

G protein beta-gamma (Gβγ) subunits anchor to the plasma membrane (PM) through the carboxy-terminal (CT) prenyl group in Gγ. This interaction is crucial for the PM localization and functioning of Gβγ, allowing GPCR-G protein signaling to proceed. The diverse Gγ family has 12 members, and we have recently shown that the signaling efficacies of major Gβγ effectors are Gγ-type dependent. This dependency is due to the distinct series of membrane-interacting abilities of Gγ. However, the molecular process allowing for Gβγ subunits to exhibit a discrete and diverse range of Gγ-type-dependent membrane affinities is unclear and cannot be explained using only the type of prenylation. The present work explores the unique designs of membrane-interacting CT residues in Gγ as a major source for this Gγ-type-dependent Gβγ signaling. Despite the type of prenylation, the results show signaling efficacy at the PM, and associated cell behaviors of Gβγ are governed by crucially located specific amino acids in the five to six residue preprenylation region of Gγ. The provided molecular picture of Gγ-membrane interactions may explain how cells gain Gγ-type-dependent G protein-GPCR signaling as well as how Gβγ elicits selective signaling at various subcellular compartments.

Gβγ regulates mitotic Golgi fragmentation and G2/M cell cycle progression

Heterotrimeric G proteins (αβγ) function at the cytoplasmic surface of a cell’s plasma membrane to transduce extracellular signals into cellular responses. However, numerous studies indicate that G proteins also play noncanonical roles at unique intracellular locations. Previous work has established that G protein βγ subunits (Gβγ) regulate a signaling pathway on the cytoplasmic surface of Golgi membranes that controls the exit of select protein cargo. Now, we demonstrate a novel role for Gβγ in regulating mitotic Golgi fragmentation, a key checkpoint of the cell cycle that occurs in the late G2 phase. We show that small interfering RNA–mediated depletion of Gβ1 and Gβ2 in synchronized cells causes a decrease in the number of cells with fragmented Golgi in late G2 and a delay of entry into mitosis and progression through G2/M. We also demonstrate that during G2/M Gβγ acts upstream of protein kinase D and regulates the phosphorylation of the Golgi structural protein GRASP55. Expression of Golgi-targeted GRK2ct, a Gβγ-sequestering protein used to inhibit Gβγ signaling, also causes a decrease in Golgi fragmentation and a delay in mitotic progression. These results highlight a novel role for Gβγ in regulation of Golgi structure.

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.

cAMP-Independent Activation of the Unfolded Protein Response by Cholera Toxin

Cholera toxin (CT) is an AB₅ protein toxin that activates the stimulatory alpha subunit of the heterotrimeric G protein (Gsα) through ADP-ribosylation. Activation of Gsα produces a cytopathic effect by stimulating adenylate cyclase and the production of cAMP. To reach its cytosolic Gsα target, CT binds to the plasma membrane of a host cell and travels by vesicle carriers to the endoplasmic reticulum (ER). The catalytic CTA1 subunit then exploits the quality control mechanism of ER-associated degradation to move from the ER to the cytosol. ER-associated degradation is functionally linked to another quality control system, the unfolded protein response (UPR). However, the role of the UPR in cholera intoxication is unclear. We report here that CT triggers the UPR after 4 h of toxin exposure. A functional toxin was required to induce the UPR, but, surprisingly, activation of the adenylate cyclase signaling pathway was not sufficient to trigger the process. Toxin-induced activation of the UPR coincided with increased toxin accumulation in the cytosol. Chemical activation of the heterotrimeric G protein or the UPR also enhanced the onset of CTA1 delivery to the cytosol, thus producing a toxin-sensitive phenotype. These results indicate there is a cAMP-independent response to CT that activates the UPR and thereby enhances the efficiency of intoxication.

GNG5 Controls the Number of Apical and Basal Progenitors and Alters Neuronal Migration During Cortical Development

Cortical development is a very complex process in which any temporal or spatial alterations can give rise to a wide range of cortical malformations. Among those malformations, periventricular heterotopia (PH) is characterized by clusters of neurons that do not migrate to the correct place. Cerebral organoids derived from patients with mutations in DCHS1 and FAT4, which have been associated with PH, exhibit higher levels of GNG5 expression in a patient-specific cluster of neurons. Here we investigate the role of GNG5 during the development of the cerebral cortex in mice and human cerebral organoids. GNG5, highly expressed in progenitors and downregulated in neurons, is critical for controlling the number of apical and basal progenitors and neuronal migration. Moreover, forced expression of GNG5 recapitulates some of the alterations observed upon downregulation of Dchs1 and Fat4 in mice and human cerebral organoids derived from DCHS1 and FAT4 patients, suggesting a critical role of GNG5 in cortical development.

RGS4 controls Gαi3-mediated regulation of Bcl-2 phosphorylation on TGN38-containing intracellular membranes

Intracellular pools of the heterotrimeric G-protein α-subunit Gαi3 (encoded by GNAI3) have been shown to promote growth factor signaling, while at the same time inhibiting the activation of JNK and autophagic signaling following nutrient starvation. The precise molecular mechanisms linking Gαi3 to both stress and growth factor signaling remain poorly understood. Importantly, JNK-mediated phosphorylation of Bcl-2 was previously found to activate autophagic signaling following nutrient deprivation. Our data shows that activated Gαi3 decreases Bcl-2 phosphorylation, whereas inhibitors of Gαi3, such as RGS4 and AGS3 (also known as GPSM1), markedly increase the levels of phosphorylated Bcl-2. Manipulation of the palmitoylation status and intracellular localization of RGS4 suggests that Gαi3 modulates phosphorylated Bcl-2 levels and autophagic signaling from discreet TGN38 (also known as TGOLN2)-labeled vesicle pools. Consistent with an important role for these molecules in normal tissue responses to nutrient deprivation, increased Gαi signaling within nutrient-starved adrenal glands from RGS4-knockout mice resulted in a dramatic abrogation of autophagic flux, compared to wild-type tissues. Together, these data suggest that the activity of Gαi3 and RGS4 from discreet TGN38-labeled vesicle pools are critical regulators of autophagic signaling that act via their ability to modulate phosphorylation of Bcl-2.

Divergent C-terminal motifs in Gα12 and Gα13 provide distinct mechanisms of effector binding and SRF activation

The G12/13 subfamily of heterotrimeric guanine nucleotide binding proteins comprises the α subunits Gα12 and Gα13, which transduce signals for cell growth, cytoskeletal rearrangements, and oncogenic transformation. In an increasing range of cancers, overexpressed Gα12 or Gα13 are implicated in aberrant cell proliferation and/or metastatic invasion. Although Gα12 and Gα13 bind non-redundant sets of effector proteins and participate in unique signalling pathways, the structural features responsible for functional differences between these α subunits are largely unknown. Invertebrates encode a single G12/13 homolog that participates in cytoskeletal changes yet appears to lack signalling to SRF (serum response factor), a transcriptional activator stimulated by mammalian Gα12 and Gα13 to promote growth and tumorigenesis. Our previous studies identified an evolutionarily divergent region in Gα12 for which replacement by homologous sequence from Drosophila melanogaster abolished SRF signalling, whereas the same invertebrate substitution was fully tolerated in Gα13 [Montgomery et al. (2014) Mol. Pharmacol. 85: 586]. These findings prompted our current approach of evolution-guided mutagenesis to identify fine structural features of Gα12 and Gα13 that underlie their respective SRF activation mechanisms. Our results identified two motifs flanking the α4 helix that play a key role in Gα12 signalling to SRF. We found the region encompassing these motifs to provide an interacting surface for multiple Gα12-specific target proteins that fail to bind Gα13. Adjacent to this divergent region, a highly-conserved domain was vital for SRF activation by both Gα12 and Gα13. However, dissection of this domain using invertebrate substitutions revealed different signalling mechanisms in these α subunits and identified Gα13-specific determinants of binding Rho-specific guanine nucleotide exchange factors. Furthermore, invertebrate substitutions in the C-terminal, α5 helical region were selectively disruptive to Gα12 signalling. Taken together, our results identify key structural features near the C-terminus that evolved after the divergence of Gα12 and Gα13, and should aid the development of agents to selectively manipulate signalling by individual α subunits of the G12/13 subfamily.

Heterotrimeric Gα12/13 proteins in kidney injury and disease

Gα₁₂ and Gα₁₃ are ubiquitous members of the heterotrimeric guanine nucleotide-binding protein (G protein) family that play central and integrative roles in the regulation of signal transduction cascades within various cell types in the kidney. Gα₁₂/Gα₁₃ proteins enable the kidney to adapt to an ever-changing environment by transducing stimuli from cell surface receptors and accessory proteins to effector systems. Therefore, perturbations in Gα₁₂/Gα₁₃ levels or their activity can contribute to the pathogenesis of various renal diseases, including renal cancer. This review will highlight and discuss the complex and expanding roles of Gα₁₂/Gα₁₃ proteins on distinct renal pathologies, with emphasis on more recently reported findings. Deciphering how the different Gα₁₂/Gα₁₃ interaction networks participate in the onset and development of renal diseases may lead to the discovery of new therapeutic strategies.

Biased signaling of G protein coupled receptors (GPCRs): Molecular determinants of GPCR/transducer selectivity and therapeutic potential

G protein coupled receptors (GPCRs) convey signals across membranes via interaction with G proteins. Originally, an individual GPCR was thought to signal through one G protein family, comprising cognate G proteins that mediate canonical receptor signaling. However, several deviations from canonical signaling pathways for GPCRs have been described. It is now clear that GPCRs can engage with multiple G proteins and the line between cognate and non-cognate signaling is increasingly blurred. Furthermore, GPCRs couple to non-G protein transducers, including β-arrestins or other scaffold proteins, to initiate additional signaling cascades. Receptor/transducer selectivity is dictated by agonist-induced receptor conformations as well as by collateral factors. In particular, ligands stabilize distinct receptor conformations to preferentially activate certain pathways, designated 'biased signaling'. In this regard, receptor sequence alignment and mutagenesis have helped to identify key receptor domains for receptor/transducer specificity. Furthermore, molecular structures of GPCRs bound to different ligands or transducers have provided detailed insights into mechanisms of coupling selectivity. However, receptor dimerization, compartmentalization, and trafficking, receptor-transducer-effector stoichiometry, and ligand residence and exposure times can each affect GPCR coupling. Extrinsic factors including cell type or assay conditions can also influence receptor signaling. Understanding these factors may lead to the development of improved biased ligands with the potential to enhance therapeutic benefit, while minimizing adverse effects. In this review, evidence for ligand-specific GPCR signaling toward different transducers or pathways is elaborated. Furthermore, molecular determinants of biased signaling toward these pathways and relevant examples of the potential clinical benefits and pitfalls of biased ligands are discussed.

GRK5 - A Functional Bridge Between Cardiovascular and Neurodegenerative Disorders

Complex aging-triggered disorders are multifactorial programs that comprise a myriad of alterations in interconnected protein networks over a broad range of tissues. It is evident that rather than being randomly organized events, pathophysiologies that possess a strong aging component such as cardiovascular diseases (hypertensions, atherosclerosis, and vascular stiffening) and neurodegenerative conditions (dementia, Alzheimer's disease, mild cognitive impairment, Parkinson's disease), in essence represent a subtly modified version of the intricate molecular programs already in place for normal aging. To control such multidimensional activities there are layers of trophic protein control across these networks mediated by so-called "keystone" proteins. We propose that these "keystones" coordinate and interconnect multiple signaling pathways to control whole somatic activities such as aging-related disease etiology. Given its ability to control multiple receptor sensitivities and its broad protein-protein interactomic nature, we propose that G protein coupled receptor kinase 5 (GRK5) represents one of these key network controllers. Considerable data has emerged, suggesting that GRK5 acts as a bridging factor, allowing signaling regulation in pathophysiological settings to control the connectivity between both the cardiovascular and neurophysiological complications of aging.

Gβγ signaling to the chemotactic effector P-REX1 and mammalian cell migration is directly regulated by Gαq and Gα13 proteins

G protein-coupled receptors stimulate Rho guanine nucleotide exchange factors that promote mammalian cell migration. Rac and Rho GTPases exert opposing effects on cell morphology and are stimulated downstream of Gβγ and Gα_12/13 or Gα_q, respectively. These Gα subunits might in turn favor Rho pathways by preventing Gβγ signaling to Rac. Here, we investigated whether Gβγ signaling to phosphatidylinositol 3,4,5-trisphosphate-dependent Rac exchange factor 1 (P-REX1), a key Gβγ chemotactic effector, is directly controlled by Rho-activating Gα subunits. We show that pharmacological inhibition of Gα_q makes P-REX1 activation by G_q/G_i-coupled lysophosphatidic acid receptors more effective. Moreover, chemogenetic control of G_i and G_q by designer receptors exclusively activated by designer drugs (DREADDs) confirmed that G_i differentially activates P-REX1. GTPase-deficient Gα_qQL and Gα₁₃QL variants formed stable complexes with Gβγ, impairing its interaction with P-REX1. The N-terminal regions of these variants were essential for stable interaction with Gβγ. Pulldown assays revealed that chimeric Gα_13-i2QL interacts with Gβγ unlike to Gα_i2-13QL, the reciprocal chimera, which similarly to Gα_i2QL could not interact with Gβγ. Moreover, Gβγ was part of tetrameric Gβγ-Gα_qQL-RGS2 and Gβγ-Gα_13-i2QL-RGS4 complexes, whereas Gα₁₃QL dissociated from Gβγ to interact with the PDZ-RhoGEF-RGS domain. Consistent with an integrated response, Gβγ and AKT kinase were associated with active SDF-1/CXCL12-stimulated P-REX1. This pathway was inhibited by Gα_qQL and Gα₁₃QL, which also prevented CXCR4-dependent cell migration. We conclude that a coordinated mechanism prioritizes Gα_q- and Gα₁₃-mediated signaling to Rho over a Gβγ-dependent Rac pathway, attributed to heterotrimeric G_i proteins.

Signaling mechanisms and physiological functions of G-protein Gα13 in blood vessel formation, bone homeostasis, and cancer

Heterotrimeric G-proteins are cellular signal transducers. They mainly relay signals from G-protein-coupled receptors (GPCRs). GPCRs function as guanine nucleotide-exchange factors to active these G-proteins. Based on the sequence and functional similarities, these G-proteins are grouped into four subfamilies: G_s , G_i , G_q , and G_12/13 . The G_12/13 subfamily consists of two members: G₁₂ and G₁₃ . G_12/13 -mediated signaling pathways play pivotal roles in a variety of physiological processes, while aberrant regulation of this pathway has been identified in various human diseases. Here we summarize the signaling mechanisms and physiological functions of Gα₁₃ in blood vessel formation and bone homeostasis. We further discuss the expanding roles of Gα₁₃ in cancers, serving as oncogenes as well as tumor suppressors.

Gβ2 Regulates the Multipolar-Bipolar Transition of Newborn Neurons in the Developing Neocortex

Proper neuronal migration is critical for the formation of the six-layered neocortex in the mammalian brain. However, the precise control of neuronal migration is not well understood. Heterotrimeric guanine nucleotide binding proteins (G proteins), composed of Gα and Gβγ, transduce signals from G protein-coupled receptors to downstream effectors and play crucial roles in brain development. However, the functions of individual subunits of G proteins in prenatal brain development remain unclear. Here, we report that Gβ2 is expressed in the embryonic neocortex, with abundant expression in the intermediate zone, and is significantly upregulated in differentiated neurons. Perturbation of Gβ2 expression impairs the morphogenetic transformation of migrating neurons from multipolar to bipolar and subsequently delays neuronal migration. Moreover, Gβ2 acts as a scaffold protein to organize the MP1-MEK1-ERK1/2 complex and mediates the phosphorylation of ERK1/2. Importantly, expression of a constitutively active variant of MEK1 rescues the migration defects that are caused by the loss of Gβ2. In conclusion, our findings reveal that Gβ2 regulates proper neuronal migration during neocortex development by activating the ERK1/2 signaling pathway.

Gαs protein binds ubiquitin to regulate epidermal growth factor receptor endosomal sorting

The Gα_s subunit is classically involved in the signal transduction of G protein-coupled receptors (GPCRs) at the plasma membrane. Recent evidence has revealed noncanonical roles for Gα_s in endosomal sorting of receptors to lysosomes. However, the mechanism of action of Gα_s in this sorting step is still poorly characterized. Here, we report that Gα_s interacts with ubiquitin to regulate the endosomal sorting of receptors for lysosomal degradation. We reveal that the N-terminal extremity of Gα_s contains a ubiquitin-interacting motif (UIM), a sorting element usually found in the endosomal sorting complex required for transport (ESCRT) machinery responsible for sorting ubiquitinated receptors into intraluminal vesicles (ILVs) of multivesicular bodies (MVBs). Mutation of the UIM in Gα_s confirmed the importance of ubiquitin interaction for the sorting of epidermal growth factor receptor (EGFR) into ILVs for lysosomal degradation. These findings demonstrate a role for Gα_s as an integral component of the ubiquitin-dependent endosomal sorting machinery and highlight the dual role of Gα_s in receptor trafficking and signaling for the fine-tuning of the cellular response.

Molecular Mechanisms of GPCR Signaling: A Structural Perspective

G protein-coupled receptors (GPCRs) are cell surface receptors that respond to a wide variety of stimuli, from light, odorants, hormones, and neurotransmitters to proteins and extracellular calcium. GPCRs represent the largest family of signaling proteins targeted by many clinically used drugs. Recent studies shed light on the conformational changes that accompany GPCR activation and the structural state of the receptor necessary for the interactions with the three classes of proteins that preferentially bind active GPCRs, G proteins, G protein-coupled receptor kinases (GRKs), and arrestins. Importantly, structural and biophysical studies also revealed activation-related conformational changes in these three types of signal transducers. Here, we summarize what is already known and point out questions that still need to be answered. Clear understanding of the structural basis of signaling by GPCRs and their interaction partners would pave the way to designing signaling-biased proteins with scientific and therapeutic potential.

GPCR-Gα protein precoupling: Interaction between Ste2p, a yeast GPCR, and Gpa1p, its Gα protein, is formed before ligand binding via the Ste2p C-terminal domain and the Gpa1p N-terminal domain

G protein coupled receptors bind ligands that initiate intracellular signaling cascades via heterotrimeric G proteins. In this study, involvement of the N-terminal residues of yeast G-alpha (Gpa1p) with the C-terminal residues of a full-length or C-terminally truncated Ste2p were investigated using bioluminescence resonance energy transfer (BRET), a non-radiative energy transfer phenomenon where protein-protein interactions can be quantified between a donor bioluminescent molecule and a suitable acceptor fluorophore. Constitutive and position-dependent BRET signal was observed in the absence of agonist (α-factor). Upon the activation of the receptors with α-factor, no significant change in BRET signal was observed. The location of Ste2p-Gpa1p heterodimer was investigated using confocal fluorescence microscopy and bimolecular fluorescence complementation (BiFC) assay, a technique where two non-fluorescent fragments of a fluorescent protein reassemble in vivo to restore fluorescence property thereby directly reporting a protein-protein interaction. BiFC experiments resulted in a dimerization signal intracellularly during biosynthesis on the endoplasmic reticulum (ER) and on the plasma membrane (PM). The constitutive BRET and BiFC signals observed on ER between Ste2p and Gpa1p in their quiescent and activated states are indicative of pre-coupling between these two proteins. This study is the first to show that the extreme N-terminus of yeast G protein alpha subunit is in close proximity to its receptor. The data suggests a pre-coupled heterodimer prior to receptor activation. The images presented in this study are the first direct in vivo evidence showing the localization of receptor - G protein heterodimers during biosynthesis and before reaching the plasma membrane.

Endogenous RGS14 is a cytoplasmic-nuclear shuttling protein that localizes to juxtanuclear membranes and chromatin-rich regions of the nucleus

Regulator of G protein signaling 14 (RGS14) is a multifunctional scaffolding protein that integrates G protein and H-Ras/MAPkinase signaling pathways to regulate synaptic plasticity important for hippocampal learning and memory. However, to date, little is known about the subcellular distribution and roles of endogenous RGS14 in a neuronal cell line. Most of what is known about RGS14 cellular behavior is based on studies of tagged, recombinant RGS14 ectopically overexpressed in unnatural host cells. Here, we report for the first time a comprehensive assessment of the subcellular distribution and dynamic localization of endogenous RGS14 in rat B35 neuroblastoma cells. Using confocal imaging and 3D-structured illumination microscopy, we find that endogenous RGS14 localizes to subcellular compartments not previously recognized in studies of recombinant RGS14. RGS14 localization was observed most notably at juxtanuclear membranes encircling the nucleus, at nuclear pore complexes (NPC) on both sides of the nuclear envelope and within intranuclear membrane channels, and within both chromatin-poor and chromatin-rich regions of the nucleus in a cell cycle-dependent manner. In addition, a subset of nuclear RGS14 localized adjacent to active RNA polymerase II. Endogenous RGS14 was absent from the plasma membrane in resting cells; however, the protein could be trafficked to the plasma membrane from juxtanuclear membranes in endosomes derived from ER/Golgi, following constitutive activation of endogenous RGS14 G protein binding partners using AlF4¯. Finally, our findings show that endogenous RGS14 behaves as a cytoplasmic-nuclear shuttling protein confirming what has been shown previously for recombinant RGS14. Taken together, the findings highlight possible cellular roles for RGS14 not previously recognized that are distinct from the regulation of conventional GPCR-G protein signaling, in particular undefined roles for RGS14 in the nucleus.

Peripheral inflammatory injury alters the relative abundance of Gα subunits in the dorsal horn of the spinal cord and in the rostral ventromedial medulla of male rats

Abstract

A diverse array of G protein-coupled receptors (GPCRs) is implicated in the modulation of nociception. The efficacy and potency of several GPCR agonists change as a consequence of peripheral inflammatory injury. Whether these changes reflect alterations in expression of the G proteins themselves is not known. This study examined the expression of transcripts and proteins for the α subunits of three classes of heteromeric G proteins in the dorsal horn of the spinal cord and the rostral ventromedial medulla (RVM) of male rats four days and two weeks after intraplantar injection of complete Freund’s adjuvant (CFA) or saline. Levels of Gα transcript in the dorsal horn or RVM were unchanged by CFA treatment. However, in the dorsal horn, Gαi protein decreased in cytosolic and membrane fractions four days after CFA treatment. Levels of Gαz protein decreased in the membrane fraction. Levels of the other Gα subunits did not differ. Levels of the Gα subunits were unchanged two weeks after CFA treatment. In the RVM, Gαz protein levels decreased in the cytosolic fraction four days after CFA treatment. No other differences were observed. Two weeks after CFA, the levels for all Gα subunits trended higher in the RVM. These data indicate that peripheral inflammatory injury induces subtle changes in the abundance of Gα subunits that is specific with respect to class, subcellular compartment, tissue, and time after injury. These changes have the potential to alter the balance of the different subcellular signaling pathways through which GPCR agonists act to modulate nociception.

G Protein-Coupled Receptor Kinases in the Inflammatory Response and Signaling

G protein-coupled receptor kinases (GRKs) are serine/threonine kinases that regulate a large and diverse class of G protein-coupled receptors (GPCRs). Through GRK phosphorylation and β-arrestin recruitment, GPCRs are desensitized and their signal terminated. Recent work on these kinases has expanded their role from canonical GPCR regulation to include noncanonical regulation of non-GPCR and nonreceptor substrates through phosphorylation as well as via scaffolding functions. Owing to these and other regulatory roles, GRKs have been shown to play a critical role in the outcome of a variety of physiological and pathophysiological processes including chemotaxis, signaling, migration, inflammatory gene expression, etc. This diverse set of functions for these proteins makes them popular targets for therapeutics. Role for these kinases in inflammation and inflammatory disease is an evolving area of research currently pursued in many laboratories. In this review, we describe the current state of knowledge on various GRKs pertaining to their role in inflammation and inflammatory diseases.

T1R3 homomeric sweet taste receptor regulates adipogenesis through Gαs-mediated microtubules disassembly and Rho activation in 3T3-L1 cells

We previously reported that 3T3-L1 cells express a functional sweet taste receptor possibly as a T1R3 homomer that is coupled to Gs and negatively regulates adipogenesis by a Gαs-mediated but cAMP-independent mechanism. Here, we show that stimulation of this receptor with sucralose or saccharin induced disassembly of the microtubules in 3T3-L1 preadipocytes, which was attenuated by overexpression of the dominant-negative mutant of Gαs (Gαs-G226A). In contrast, overexpression of the constitutively active mutant of Gαs (Gαs-Q227L) as well as treatment with cholera toxin or isoproterenol but not with forskolin caused disassembly of the microtubules. Sweetener-induced microtubule disassembly was accompanied by activation of RhoA and Rho-associated kinase (ROCK). This was attenuated with by knockdown of GEF-H1, a microtubule-localized guanine nucleotide exchange factor for Rho GTPase. Furthermore, overexpression of the dominant-negative mutant of RhoA (RhoA-T19N) blocked sweetener-induced dephosphorylation of Akt and repression of PPARγ and C/EBPα in the early phase of adipogenic differentiation. These results suggest that the T1R3 homomeric sweet taste receptor negatively regulates adipogenesis through Gαs-mediated microtubule disassembly and consequent activation of the Rho/ROCK pathway.

Inducible Inhibition of Gβγ Reveals Localization-dependent Functions at the Plasma Membrane and Golgi

Heterotrimeric G proteins signal at a variety of endomembrane locations, in addition to their canonical function at the cytoplasmic surface of the plasma membrane (PM), where they are activated by cell surface G protein-coupled receptors. Here we focus on βγ signaling at the Golgi, where βγ activates a signaling cascade, ultimately resulting in vesicle fission from the trans-Golgi network (TGN). To develop a novel molecular tool for inhibiting endogenous βγ in a spatial-temporal manner, we take advantage of a lipid association mutant of the widely used βγ inhibitor GRK2ct (GRK2ct-KERE) and the FRB/FKBP heterodimerization system. We show that GRK2ct-KERE cannot inhibit βγ function when expressed in cells, but recruitment to a specific membrane location recovers the ability of GRK2ct-KERE to inhibit βγ signaling. PM-recruited GRK2ct-KERE inhibits lysophosphatidic acid-induced phosphorylation of Akt, whereas Golgi-recruited GRK2ct-KERE inhibits cargo transport from the TGN to the PM. Moreover, we show that Golgi-recruited GRK2ct-KERE inhibits model basolaterally targeted but not apically targeted cargo delivery, for both PM-destined and secretory cargo, providing the first evidence of selectivity in terms of cargo transport regulated by βγ. Last, we show that Golgi fragmentation induced by ilimaquinone and nocodazole is blocked by βγ inhibition, demonstrating that βγ is a key regulator of multiple pathways that impact Golgi morphology. Thus, we have developed a new molecular tool, recruitable GRK2ct-KERE, to modulate βγ signaling at specific subcellular locations, and we demonstrate novel cargo selectivity for βγ regulation of TGN to PM transport and a novel role for βγ in mediating Golgi fragmentation.

Genetic Variations in the Human G Protein-coupled Receptor Class C, Group 6, Member A (GPRC6A) Control Cell Surface Expression and Function

GPRC6A is a G protein-coupled receptor activated by l-amino acids, which, based on analyses of knock-out mice, has been suggested to have physiological functions in metabolism and testicular function. The human ortholog is, however, mostly retained intracellularly in contrast to the cell surface-expressed murine and goldfish orthologs. The latter orthologs are G_q-coupled and lead to intracellular accumulation of inositol phosphates and calcium release. In the present study we cloned the bonobo chimpanzee GPRC6A receptor, which is 99% identical to the human receptor, and show that it is cell surface-expressed and functional. By analyses of chimeric human/mouse and human/bonobo receptors, bonobo receptor mutants, and the single nucleotide polymorphism database at NCBI, we identify an insertion/deletion variation in the third intracellular loop responsible for the intracellular retention and lack of function of the human ortholog. Genetic analyses of the 1000 genome database and the Inter99 cohort of 6,000 Danes establish the distribution of genotypes among ethnic groups, showing that the cell surface-expressed and functional variant is much more prevalent in the African population than in European and Asian populations and that this variant is partly linked with a stop codon early in the receptor sequence (rs6907580, amino acid position 57). In conclusion, our data solve a more than decade-old question of why the cloned human GPRC6A receptor is not cell surface-expressed and functional and provide a genetic framework to study human phenotypic traits in large genome sequencing projects linked with physiological measurement and biomarkers.

Modulation of Calcium Entry by the Endo-lysosomal System

Endo-lysosomes are acidic organelles that besides the role in macromolecules degradation, act as intracellular Ca(2+) stores. Nicotinic acid adenine dinucleotide phosphate (NAADP), the most potent Ca(2+)-mobilizing second messenger, produced in response to agonist stimulation, activates Ca(2+)-releasing channels on endo-lysosomes and modulates a variety of cellular functions. NAADP-evoked signals are amplified by Ca(2+) release from endoplasmic reticulum, via the recruitment of inositol 1,4,5-trisphosphate and/or ryanodine receptors through a Ca(2+)-induced Ca(2+)- release (CICR) mechanism. The endo-lysosomal Ca(2+) channels activated by NAADP were recently identified as the two-pore channels (TPCs). In addition to TPCs, endo-lysosomes express another distinct family of Ca(2+)- permeable channels, namely the transient receptor potential mucolipin (TRPML) channels, functionally distinct from TPCs. TPCs belong to the voltage-gated channels, resembling voltage-gated Na(+) and Ca(2+) channels. TPCs have important roles in vesicular fusion and trafficking, in triggering a global Ca(2+) signal and in modulation of the membrane excitability. Depletion of acidic Ca(2+) stores has been shown to activate store-operated Ca(2+) entry in human platelets and mouse pancreatic β-cells. In human platelets, Ca(2+) influx in response to acidic stores depletion is facilitated by the tubulin-cytoskeleton and occurs through non-selective cation channels and transient receptor potential canonical (TRPC) channels. Emerging evidence indicates that activation of intracellular receptors, situated on endo-lysosomes, elicits canonical and non-canonical signaling mechanisms that involve CICR and activation of non-selective cation channels in plasma membrane. The ability of endo-lysosomal Ca(2+) stores to modulate the Ca(2+) release from other organelles and the Ca(2+) entry increases the diversity and complexity of cellular signaling mechanisms.

A Gα12-specific Binding Domain in AKAP-Lbc and p114RhoGEF

AKAP-Lbc is a Rho-activating guanine nucleotide exchange factor (RhoGEF) important in heart development and pro-fibrotic signaling in cardiomyocytes. Heterotrimeric G proteins of the G12/13 subfamily, comprising Gα12 and Gα13, are well characterized as stimulating a specialized group of RhoGEFs through interaction with their RGS-homology (RH) domain. Despite lacking an RH domain, AKAP-Lbc is bound by Gα12 through an unknown mechanism to activate Rho signaling. We identified a Gα12-binding region near the C-terminus of AKAP-Lbc, closely homologous to a region of p114RhoGEF that we also discovered to interact with Gα12. This binding mechanism is distinct from the well-studied interface between RH-RhoGEFs and G12/13 α subunits, as demonstrated by Gα12 mutants selectively impaired in binding either this AKAP-Lbc/p114RhoGEF region or RH-RhoGEFs. AKAP-Lbc and p114RhoGEF showed high specificity for binding Gα12 in comparison to Gα13, and experiments using chimeric G12/13 α subunits mapped determinants of this selectivity to the N-terminal region of Gα12. In cultured cells expressing constitutively GDP-bound Gα12 or Gα13, the Gα12 construct was more potent in exerting a dominant-negative effect on serum-mediated signaling to p114RhoGEF, demonstrating coupling of these signaling proteins in a cellular pathway. In addition, charge-reversal of conserved residues in AKAP-Lbc and p114RhoGEF disrupted Gα12 binding for both proteins, suggesting they harbor a common structural mechanism for interaction with this α subunit. Our results provide the first evidence of p114RhoGEF as a Gα12 signaling effector, and define a novel region conserved between AKAP-Lbc and p114RhoGEF that allows Gα12 signaling input to these non-RH RhoGEFs.

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.

Gβγ subunits-Different spaces, different faces

Gβγ subunits play key roles in modulation of canonical effectors in G protein-coupled receptor (GPCR)-dependent signalling at the cell surface. However, a number of recent studies of Gβγ function have revealed that they regulate a large number of molecules at distinct subcellular sites. These novel, non-canonical Gβγ roles have reshaped our understanding of how important Gβγ signalling is compared to our original notion of Gβγ subunits as simple negative regulators of Gα subunits. Gβγ dimers have now been identified as regulators of transcription, anterograde and retrograde trafficking and modulators of second messenger molecule generation in intracellular organelles. Here, we review some recent advances in our understanding of these novel non-canonical roles of Gβγ.

G protein γ subunit 7 induces autophagy and inhibits cell division

GNG7 (G protein γ subunit 7), a subunit of heterotrimeric G protein, is ubiquitously expressed in multiple tissues but is down-regulated in various cancers. Its expression could reduce tumor volume in mice but the mechanism was not clear. Here we show that GNG7 overexpression inhibits cell proliferation and increases cell death. GNG7 level is cell cycle-dependent and it regulates actin cytoskeleton and cell division. In addition, GNG7 is an autophagy inducer, which is the first reported Gγ protein involved in autophagy. GNG7 knockdown reduces Rapamycin and starvation-induced autophagy. Further analysis reveals that GNG7 inhibits MTOR in cells, a central regulator for autophagy and cell proliferation. In conclusion, GNG7 inhibits MTOR pathway to induce autophagy and cell death, inhibits cell division by regulating actin cytoskeleton. These combined effects lead to the antitumor capacity of GNG7.

The impact of RGS and other G-protein regulatory proteins on Gαi-mediated signaling in immunity

Leukocyte chemoattractant receptors are members of the G-protein coupled receptor (GPCR) family. Signaling downstream of these receptors directs the localization, positioning and homeostatic trafficking of leukocytes; as well as their recruitment to, and their retention at, inflammatory sites. Ligand induced changes in the molecular conformation of chemoattractant receptors results in the engagement of heterotrimeric G-proteins, which promotes α subunits to undergo GTP/GDP exchange. This results in the functional release of βγ subunits from the heterotrimers, thereby activating downstream effector molecules, which initiate leukocyte polarization, gradient sensing, and directional migration. Pertussis toxin ADP ribosylates Gαi subunits and prevents chemoattractant receptors from triggering Gαi nucleotide exchange. The use of pertussis toxin revealed the essential importance of Gαi subunit nucleotide exchange for chemoattractant receptor signaling. More recent studies have identified a range of regulatory mechanisms that target these receptors and their associated heterotrimeric G-proteins, thereby helping to control the magnitude, kinetics, and duration of signaling. A failure in these regulatory pathways can lead to impaired receptor signaling and immunopathology. The analysis of mice with targeted deletions of Gαi isoforms as well as some of these G-protein regulatory proteins is providing insights into their roles in chemoattractant receptor signaling.

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.

Human Nek7-interactor RGS2 is required for mitotic spindle organization

The mitotic spindle apparatus is composed of microtubule (MT) networks attached to kinetochores organized from 2 centrosomes (a.k.a. spindle poles). In addition to this central spindle apparatus, astral MTs assemble at the mitotic spindle pole and attach to the cell cortex to ensure appropriate spindle orientation. We propose that cell cycle-related kinase, Nek7, and its novel interacting protein RGS2, are involved in mitosis regulation and spindle formation. We found that RGS2 localizes to the mitotic spindle in a Nek7-dependent manner, and along with Nek7 contributes to spindle morphology and mitotic spindle pole integrity. RGS2-depletion leads to a mitotic-delay and severe defects in the chromosomes alignment and congression. Importantly, RGS2 or Nek7 depletion or even overexpression of wild-type or kinase-dead Nek7, reduced γ-tubulin from the mitotic spindle poles. In addition to causing a mitotic delay, RGS2 depletion induced mitotic spindle misorientation coinciding with astral MT-reduction. We propose that these phenotypes directly contribute to a failure in mitotic spindle alignment to the substratum. In conclusion, we suggest a molecular mechanism whereupon Nek7 and RGS2 may act cooperatively to ensure proper mitotic spindle organization.