Nuclear and cytoskeletal translocation and localization of heterotrimeric G-proteins

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.

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.

GPCRs, G Proteins, and Their Impact on β-cell Function

Glucose-induced (physiological) insulin secretion from the islet β-cell involves interplay between cationic (i.e., changes in intracellular calcium) and metabolic (i.e., generation of hydrophobic and hydrophilic second messengers) events. A large body of evidence affirms support for novel regulation, by G proteins, of specific intracellular signaling events, including actin cytoskeletal remodeling, transport of insulin-containing granules to the plasma membrane for fusion, and secretion of insulin into the circulation. This article highlights the following aspects of GPCR-G protein biology of the islet. First, it overviews our current understanding of the identity of a wide variety of G protein regulators and their modulatory roles in GPCR-G protein-effector coupling, which is requisite for optimal β-cell function under physiological conditions. Second, it describes evidence in support of novel, noncanonical, GPCR-independent mechanisms of activation of G proteins in the islet. Third, it highlights the evidence indicating that abnormalities in G protein function lead to islet β-cell dysregulation and demise under the duress of metabolic stress and diabetes. Fourth, it summarizes observations of potential beneficial effects of GPCR agonists in preventing/halting metabolic defects in the islet β-cell under various pathological conditions (e.g., metabolic stress and inflammation). Lastly, it identifies knowledge gaps and potential avenues for future research in this evolving field of translational islet biology. Published 2020. Compr Physiol 10:453-490, 2020.

BIOPYDB: A Dynamic Human Cell Specific Biochemical Pathway Database with Advanced Computational Analyses Platform

BIOPYDB: BIOchemical PathwaY DataBase is developed as a manually curated, readily updatable, dynamic resource of human cell specific pathway information along with integrated computational platform to perform various pathway analyses. Presently, it comprises of 46 pathways, 3189 molecules, 5742 reactions and 6897 different types of diseases linked with pathway proteins, which are referred by 520 literatures and 17 other pathway databases. With its repertoire of biochemical pathway data, and computational tools for performing Topological, Logical and Dynamic analyses, BIOPYDB offers both the experimental and computational biologists to acquire a comprehensive understanding of signaling cascades in the cells. Automated pathway image reconstruction, cross referencing of pathway molecules and interactions with other databases and literature sources, complex search operations to extract information from other similar resources, integrated platform for pathway data sharing and computation, etc. are the novel and useful features included in this database to make it more acceptable and attractive to the users of pathway research communities. The RESTful API service is also made available to the advanced users and developers for accessing this database more conveniently through their own computer programmes.

Biochemical, Biophysical and Cellular Techniques to Study the Guanine Nucleotide Exchange Factor, GIV/Girdin

Canonical signal transduction via heterotrimeric G proteins is spatiotemporally restricted, i.e., triggered exclusively at the plasma membrane, only by agonist activation of G protein-coupled receptors via a finite process that is terminated within a few hundred milliseconds. Recently, a rapidly emerging paradigm has revealed a noncanonical pathway for activation of heterotrimeric G proteins via the nonreceptor guanidine-nucleotide exchange factor, GIV/Girdin. Biochemical, biophysical, and functional studies evaluating this pathway have unraveled its unique properties and distinctive spatiotemporal features. As in the case of any new pathway/paradigm, these studies first required an in-depth optimization of tools/techniques and protocols, governed by rationale and fundamentals unique to the pathway, and more specifically to the large multimodular GIV protein. Here we provide the most up-to-date overview of protocols that have generated most of what we know today about noncanonical G protein activation by GIV and its relevance in health and disease. © 2016 by John Wiley & Sons, Inc.

Nuclear localization of Formyl-Peptide Receptor 2 in human cancer cells

Current models of G protein-coupled receptors (GPCRs) signaling describe binding of external agonists to cell surface receptors which, in turn, trigger several biological responses. New paradigms indicate that GPCRs localize to and signal at the nucleus, thus regulating distinct signaling cascades. The formyl-peptide receptor FPR2 belongs to the GPCR super-family and is coupled to PTX-sensitive Gi proteins. We show by western blot analysis, immunofluorescence experiments and radioligand binding assays that FPR2 is expressed at nuclear level in CaLu-6 and AGS cells. Nuclear FPR2 is a functional receptor, since it participates in intra-nuclear signaling, as assessed by decreased G protein-FPR2 association and enhanced ERK2, c-Jun and c-Myc phosphorylation upon stimulation of intact nuclei with the FPR2 agonist, WKYMVm. We analyzed FPR2 sequence for the search of a nuclear localization sequence (NLS) and we found a stretch of basic aminoacids (227-KIHKK-231) in the third cytoplasmic loop of the receptor. We performed single (K230A) and multiple (H229A/K230A/K231A) mutagenesis of NLS. The constructs were individually overexpressed in HEK293 cells and immunofluorescence and western blot analysis showed that nuclear localization or translocation of FPR2 depends on the integrity of the H(229) and K(231) residues within the NLS.

Nuclear G protein signaling: new tricks for old dogs

According to the standard model of G protein-coupled receptor (GPCR) signaling, GPCRs are localized to the cell membrane where they respond to extracellular signals. Stimulation of GPCRs leads to the activation of heterotrimeric G proteins and their intracellular signaling pathways. However, this model fails to accommodate GPCRs, G proteins, and their downstream effectors that are found on the nuclear membrane or in the nucleus. Evidence from isolated nuclei indicates the presence of GPCRs on the nuclear membrane that can activate similar G protein-dependent signaling pathways in the nucleus as at the cell surface. These pathways also include activation of cyclic adenosine monophosphate, calcium and nitric oxide synthase signaling in cardiomyocytes. In addition, a number of distinct heterotrimeric and monomeric G proteins have been found in the nucleus of various cell types. This review will focus on understanding the function of nuclear G proteins with a focus on cardiac signaling where applicable.

Changes in the nuclear proteome of developing wheat (Triticum aestivum L.) grain

Wheat grain end-use value is determined by complex molecular interactions that occur during grain development, including those in the cell nucleus. However, our knowledge of how the nuclear proteome changes during grain development is limited. Here, we analyzed nuclear proteins of developing wheat grains collected during the cellularization, effective grain-filling, and maturation phases of development, respectively. Nuclear proteins were extracted and separated by two-dimensional gel electrophoresis. Image analysis revealed 371 and 299 reproducible spots in gels with first dimension separation along pH 4-7 and pH 6-11 isoelectric gradients, respectively. The relative abundance of 464 (67%) protein spots changed during grain development. Abundance profiles of these proteins clustered in six groups associated with the major phases and phase transitions of grain development. Using nano liquid chromatography-tandem mass spectrometry to analyse 387 variant and non-variant protein spots, 114 different proteins were identified that were classified into 16 functional classes. We noted that some proteins involved in the regulation of transcription, like HMG1/2-like protein and histone deacetylase HDAC2, were most abundant before the phase transition from cellularization to grain-filling, suggesting that major transcriptional changes occur during this key developmental phase. The maturation period was characterized by high relative abundance of proteins involved in ribosome biogenesis. Data are available via ProteomeXchange with identifier PXD002999.

The nuclear localization signal is required for nuclear GPER translocation and function in breast Cancer-Associated Fibroblasts (CAFs)

Cancer associated fibroblasts (CAFs) actively contribute to the growth and invasion of cancer cells. In recent years, the G protein estrogen receptor (GPER) has been largely involved in the estrogenic signals in diverse types of normal and tumor cells. In CAFs, GPER was localized into the nucleus, however the molecular mechanisms which regulate its nuclear shuttle remain to be clarified. In the present study, we demonstrate that in breast CAFs GPER translocates into the nucleus through an importin-dependent mechanism. Moreover, we show that a nuclear localization signal is involved in the nuclear import of GPER, in the up-regulation of its target genes c-fos and CTGF and in the migration of CAFs induced by estrogens. Our data provide novel insights into the nuclear localization and function of GPER in CAFs toward a better understanding of the estrogen action elicited through these key players of the tumor microenvironment.

Regulation of cardiac nitric oxide signaling by nuclear β-adrenergic and endothelin receptors

At the cell surface, βARs and endothelin receptors can regulate nitric oxide (NO) production. β-adrenergic receptors (βARs) and type B endothelin receptors (ETB) are present in cardiac nuclear membranes and regulate transcription. The present study investigated the role of the NO pathway in the regulation of gene transcription by these nuclear G protein-coupled receptors. Nitric oxide production and transcription initiation were measured in nuclei isolated from the adult rat heart. The cell-permeable fluorescent dye 4,5-diaminofluorescein diacetate (DAF2 DA) was used to provide a direct assessment of nitric oxide release. Both isoproterenol and endothelin increased NO production in isolated nuclei. Furthermore, a β3AR-selective agonist, BRL 37344, increased NO synthesis whereas the β1AR-selective agonist xamoterol did not. Isoproterenol increased, whereas ET-1 reduced, de novo transcription. The NO synthase inhibitor l-NAME prevented isoproterenol from increasing either NO production or de novo transcription. l-NAME also blocked ET-1-induced NO-production but did not alter the suppression of transcription initiation by ET-1. Inhibition of the cGMP-dependent protein kinase (PKG) using KT5823 also blocked the ability of isoproterenol to increase transcription initiation. Furthermore, immunoblotting revealed eNOS, but not nNOS, in isolated nuclei. Finally, caged, cell-permeable isoproterenol and endothelin-1 analogs were used to selectively activate intracellular β-adrenergic and endothelin receptors in intact adult cardiomyocytes. Intracellular release of caged ET-1 or isoproterenol analogs increased NO production in intact adult cardiomyocytes. Hence, activation of the NO synthase/guanylyl cyclase/PKG pathway is necessary for nuclear β3ARs to increase de novo transcription. Furthermore, we have demonstrated the potential utility of caged receptor ligands in selectively modulating signaling via endogenous intracellular G protein-coupled receptors.

Distinct phospholipase C-β isozymes mediate lysophosphatidic acid receptor 1 effects on intestinal epithelial homeostasis and wound closure

Maintenance of the epithelial barrier in the intestinal tract is necessary to protect the host from the hostile luminal environment. Phospholipase C-β (PLC-β) has been implicated to control myriad signaling cascades. However, the biological effects of selective PLC-β isozymes are poorly understood. We describe novel findings that lysophosphatidic acid (LPA) regulates PLC-β1 and PLC-β2 via two distinct pathways to enhance intestinal epithelial cell (IEC) proliferation and migration that facilitate wound closure and recovery of the intestinal epithelial barrier. LPA acting on the LPA1 receptor promotes IEC migration by facilitating the interaction of Gαq with PLC-β2. LPA-induced cell proliferation is PLC-β1 dependent and involves translocation of Gαq to the nucleus, where it interacts with PLC-β1 to induce cell cycle progression. An in vivo study using LPA1-deficient mice (Lpar1(-/-)) shows a decreased number of proliferating IECs and migration along the crypt-luminal axis. Additionally, LPA enhances migration and proliferation of IECs in an LPA1-dependent manner, and Lpar1(-/-) mice display defective mucosal wound repair that requires cell proliferation and migration. These findings delineate novel LPA1-dependent lipid signaling that facilitates mucosal wound repair via spatial targeting of distinct PLC-βs within the cell.

Association between WDR21A polymorphisms and airway responsiveness to inhaled corticosteroids in asthmatic patients

OBJECTIVE

Genetic polymorphism is partially responsible for the wide variation in the response of moderate-to-severe asthmatic patients to inhaled corticosteroids. The goal of the study was to examine polymorphisms in WDR21A, which encodes a putative glucocorticoid receptor (GR)-interacting protein, for their possible association with corticosteroid responsiveness.

METHODS

The change in forced expiratory volume in 1 s [FEV(1) (ΔFEV(1))] induced by 4 weeks of inhaled treatment with fluticasone propionate (1000 µg daily) was measured in 230 asthmatic patients. Fifteen single nucleotide polymorphisms (SNPs) of WDR21A were genotyped using a TaqMan assay, and 11 SNPs were used for further analysis. WDR21A transcripts were analyzed for variant splicing using reverse transcriptase-PCR. The WDR21A protein structure was predicted using a template-based modeling method and docked to a GR using Zdock.

RESULTS

Of the 11 SNPs and three haplotypes of WDR21A analyzed, only the intronic SNP -10073G>C appeared to affect ΔFEV(1). The ΔFEV(1) of the -10073C/C homozygous genotype was twice that of the -10073G/G and -10073C/G genotypes (P(corr)=0.04 in recessive model). No splicing variant of WDR21A was observed, regardless of genotype. The predicted WDR21A protein structure was similar to the Gβ(1) protein structure (template modeling-score=0.93).

CONCLUSION

The minor allele -10073C of WDR21A may induce a good response to inhaled corticosteroids possibly through competition with the Gβ(1) proteins for binding to GRs.

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.

Nuclear GPCRs in cardiomyocytes: an insider's view of β-adrenergic receptor signaling

In recent years, we have come to appreciate the complexity of G protein-coupled receptor signaling in general and β-adrenergic receptor (β-AR) signaling in particular. Starting originally from three β-AR subtypes expressed in cardiomyocytes with relatively simple, linear signaling cascades, it is now clear that there are large receptor-based networks which provide a rich and diverse set of responses depending on their complement of signaling partners and the physiological state. More recently, it has become clear that subcellular localization of these signaling complexes also enriches the diversity of phenotypic outcomes. Here, we review our understanding of the signaling repertoire controlled by nuclear β-AR subtypes as well our understanding of the novel roles for G proteins themselves in the nucleus, with a special focus, where possible, on their effects in cardiomyocytes. Finally, we discuss the potential pathological implications of alterations in nuclear β-AR signaling.

Nuclear localization of vasoactive intestinal peptide (VIP) receptors in human breast cancer

Vasoactive intestinal peptide (VIP) and its receptors (VPACs) are involved in proliferation, survival, and differentiation in human breast cancer cells. Its mechanism of action is traditionally thought to be through specific plasma membrane receptors. There is compelling evidence for a novel intracrine mode of genomic regulation by G-protein-coupled receptors (GPCRs) that implies both endocytosis and nuclear translocation of peripheral GPCR and/or the activation of nuclear-located GPCRs by endogenously-produced, non-secreted ligands. Regarding to VPAC receptors, which are GPCRs, there is only a report suggesting them as a dynamic system for signaling from plasma membrane and nuclear membrane complex. In this study, we show that VPAC(1) receptor is localized in cell nuclear fraction whereas VPAC(2) receptor presents an extranuclear localization and its protein expression is lower than that of VPAC(1) receptor in human breast tissue samples. Both receptors as well as VIP are overexpressed in breast cancer as compared to non-tumor tissue. Moreover, we report the markedly nuclear localization of VPAC(1) receptors in estrogen-dependent (T47D) and independent (MDA-MB-468) human breast cancer cell lines. VPAC(1) receptors are functional in plasma membrane and nucleus as shown by VIP stimulation of cAMP production in both cell lines. In addition, VIP increases its own intracellular and extracellular levels, and could be involved in the regulation of VPAC(1)-receptor traffic from the plasma membrane to the nucleus. These results support new concepts on function and regulation of nuclear GPCRs which could have an impact on development of new therapeutic drugs.

Nuclear β-adrenergic receptors modulate gene expression in adult rat heart

Both β(1)- and β(3)-adrenergic receptors (β(1)ARs and β(3)ARs) are present on nuclear membranes in adult ventricular myocytes. These nuclear-localized receptors are functional with respect to ligand binding and effector activation. In isolated cardiac nuclei, the non-selective βAR agonist isoproterenol stimulated de novo RNA synthesis measured using assays of transcription initiation (Boivin et al., 2006 Cardiovasc Res. 71:69-78). In contrast, stimulation of endothelin receptors, another G protein-coupled receptor (GPCR) that localizes to the nuclear membrane, resulted in decreased RNA synthesis. To investigate the signalling pathway(s) involved in GPCR-mediated regulation of RNA synthesis, nuclei were isolated from intact adult rat hearts and treated with receptor agonists in the presence or absence of inhibitors of different mitogen-activated protein kinase (MAPK) and PI3K/PKB pathways. Components of p38, JNK, and ERK1/2 MAP kinase cascades as well as PKB were detected in nuclear preparations. Inhibition of PKB with triciribine, in the presence of isoproterenol, converted the activation of the βAR from stimulatory to inhibitory with regards to RNA synthesis, while ERK1/2, JNK and p38 inhibition reduced both basal and isoproterenol-stimulated activity. Analysis by qPCR indicated an increase in the expression of 18S rRNA following isoproterenol treatment and a decrease in NFκB mRNA. Further qPCR experiments revealed that isoproterenol treatment also reduced the expression of several other genes involved in the activation of NFκB, while ERK1/2 and PKB inhibition substantially reversed this effect. Our results suggest that GPCRs on the nuclear membrane regulate nuclear functions such as gene expression and this process is modulated by activation/inhibition of downstream protein kinases within the nucleus.

Nuclear-delimited angiotensin receptor-mediated signaling regulates cardiomyocyte gene expression

Angiotensin-II (Ang-II) from extracardiac sources and intracardiac synthesis regulates cardiac homeostasis, with mitogenic and growth-promoting effects largely due to altered gene expression. Here, we assessed the possibility that angiotensin-1 (AT1R) or angiotensin-2 (AT2R) receptors on the nuclear envelope mediate effects on cardiomyocyte gene expression. Immunoblots of nucleus-enriched fractions from isolated cardiomyocytes indicated the presence of AT1R and AT2R proteins that copurified with the nuclear membrane marker nucleoporin-62 and histone-3, but not markers of plasma (calpactin-I), Golgi (GRP-78), or endoplasmic reticulum (GM130) membranes. Confocal microscopy revealed AT1R and AT2R proteins on nuclear membranes. Microinjected Ang-II preferentially bound to nuclear sites of isolated cardiomyocytes. AT1R and AT2R ligands enhanced de novo RNA synthesis in isolated cardiomyocyte nuclei incubated with [alpha-(32)P]UTP (e.g. 36.0 +/- 6.0 cpm/ng of DNA control versus 246.4 +/- 15.4 cpm/ng of DNA Ang-II, 390.1 +/- 15.5 cpm/ng of DNA L-162313 (AT1), 180.9 +/- 7.2 cpm/ng of DNA CGP42112A (AT2), p < 0.001). Ang-II application to cardiomyocyte nuclei enhanced NFkappaB mRNA expression, a response that was suppressed by co-administration of AT1R (valsartan) and/or AT2R (PD123177) blockers. Dose-response experiments with Ang-II applied to purified cardiomyocyte nuclei versus intact cardiomyocytes showed greater increases in NFkappaB mRNA levels at saturating concentrations with approximately 2-fold greater affinity upon nuclear application, suggesting preferential nuclear signaling. AT1R, but not AT2R, stimulation increased [Ca(2+)] in isolated cardiomyocyte nuclei. Inositol 1,4,5-trisphosphate receptor blockade by 2-aminoethoxydiphenyl borate prevented AT1R-mediated Ca(2+) release and attenuated AT1R-mediated transcription initiation responses. We conclude that cardiomyocyte nuclear membranes possess angiotensin receptors that couple to nuclear signaling pathways and regulate transcription. Signaling within the nuclear envelope (e.g. from intracellularly synthesized Ang-II) may play a role in Ang-II-mediated changes in cardiac gene expression, with potentially important mechanistic and therapeutic implications.

Multi-component signaling complexes of the delta-opioid receptor with STAT5B and G proteins

Besides mediating opioid responses in the nervous system and the peripheral tissues, opioid receptors are implicated in signaling mechanisms shared by cytokine receptors. Recent observations have shown that the Signal Transducer and Activator of Transcription 5A (STAT5A) interacts with the mu-opioid receptor (mu-OR) and is phosphorylated upon mu-OR stimulation (Mazarakou and Georgoussi, 2005). In the present study we demonstrate that another member of the STAT family, STAT5B, associates constitutively with the C-terminal tail of the delta-opioid receptor (delta-CT). [D-Ser(2), Leu(5), Thr(6)]-enkephalin-exposure of HEK293 cells, expressing stably the delta-opioid receptor (delta-OR), leads to receptor-dependent STAT5B tyrosine phosphorylation and transcriptional activation. This phosphorylation occurs in a G protein-dependent manner and is carried out by a c-Src kinase. Co-immunoprecipitation studies indicate that STAT5B forms pairs with selective Galpha and Gbetagamma subunits of G proteins and activated c-Src kinase in HEK293 cells. These interactions are formed either constitutively, or upon receptor stimulation. We also demonstrate that the delta-CT serves as a platform for the formation of a multi-component signaling complex (signalosome), consisting of STAT5B, c-Src and selective G protein members. We can thus conclude that STAT5B signaling can be modulated by its coupling with a specific subset of G protein subunits, revealing a novel signaling mechanism for the transcriptional regulation of STAT5B-dependent genes.

Submembraneous microtubule cytoskeleton: regulation of microtubule assembly by heterotrimeric Gproteins

Heterotrimeric Gproteins participate in signal transduction by transferring signals from cell surface receptors to intracellular effector molecules. Gproteins also interact with microtubules and participate in microtubule-dependent centrosome/chromosome movement during cell division, as well as neuronal differentiation. In recent years, significant progress has been made in our understanding of the biochemical/functional interactions between Gprotein subunits (alpha and betagamma) and microtubules, and the molecular details emerging from these studies suggest that alpha and betagamma subunits of Gproteins interact with tubulin/microtubules to regulate the assembly/dynamics of microtubules, providing a novel mechanism for hormone- or neurotransmitter-induced rapid remodeling of cytoskeleton, regulation of the mitotic spindle for centrosome/chromosome movements in cell division, and neuronal differentiation in which structural plasticity mediated by microtubules is important for appropriate synaptic connections and signal transmission.

G protein-coupled receptors in and on the cell nucleus: a new signaling paradigm?

Signaling from internalizing and endosomal receptors has almost become a classic GPCR paradigm in the last several years. However, it has become clear in recent years that GPCRs also elicit signals when resident at other subcellular sites including the endoplasmic reticulum, Golgi apparatus, and the nucleus. In this review we discuss the nature, function, and trafficking of nuclear GPCR signaling complexes, as well as potential sources of endogenous and exogenous ligands. Finally, we pose a series of questions that will need to be answered in the coming years to confirm and extend this as a new paradigm for GPCR signaling.

Expression of endogenous nuclear bradykinin B2 receptors mediating signaling in immediate early gene activation

Bradykinin (BK) represents a pro-inflammatory mediator that partakes in many inflammatory diseases. The mechanism of action of BK is thought to be primarily mediated by specific cell surface membrane B2 receptors (B2Rs). Some evidence has suggested, however, the existence of an intracellular/nuclear B2R population. Whether these receptors are functional and contribute to BK signaling remains to be determined. In this study, by mean of Western blotting, 3D-confocal microscopy, receptor autoradiography and radioligand binding analysis, we showed that plasma membrane and highly purified nuclei from isolated rat hepatocytes contain specific B2R that bind BK. The results depicting B2R nuclear expression in isolated nuclear organelles were reproduced in situ on hepatic sections by immunogold labeling and transmission electron microscopy. Functional tests on single nuclei, by means of confocal microscopy and the calcium-sensitive probe fluo-4AM, showed that BK induces concentration-dependent transitory mobilization of nucleoplasmic calcium; these responses were blocked by B2R antagonist HOE 140, not by the B1R antagonist R954 and, were also found in wild-type C57/Bl6 mice, but not in B2R-KO mice. In isolated nuclei, BK elicited activation/phosphorylation of Akt, acetylation of histone H3 and ensuing pro-inflammatory iNOS gene induction as determined by Western blot and RT-PCR. ChIP assay confirmed binding of acetylated-histone H3 complexes, but not B2R, to promoter region of iNOS gene suggesting that B2R-mediated gene expression is bridged with accessory downstream effectors. This study discloses a previously undescribed mechanism in BK-induced transcriptional events, via intracrine B2R-mediated signaling, occurring in rat autologous hepatic cells.

Activated nuclear metabotropic glutamate receptor mGlu5 couples to nuclear Gq/11 proteins to generate inositol 1,4,5-trisphosphate-mediated nuclear Ca2+ release

Recently we have shown that the metabotropic glutamate 5 (mGlu5) receptor can be expressed on nuclear membranes of heterologous cells or endogenously on striatal neurons where it can mediate nuclear Ca2+ changes. Here, pharmacological, optical, and genetic techniques were used to show that upon activation, nuclear mGlu5 receptors generate nuclear inositol 1,4,5-trisphosphate (IP3) in situ. Specifically, expression of an mGlu5 F767S mutant in HEK293 cells that blocks Gq/11 coupling or introduction of a dominant negative Galphaq construct in striatal neurons prevented nuclear Ca2+ changes following receptor activation. These data indicate that nuclear mGlu5 receptors couple to Gq/11 to mobilize nuclear Ca2+. Nuclear mGlu5-mediated Ca2+ responses could also be blocked by the phospholipase C (PLC) inhibitor, U73122, the phosphatidylinositol (PI) PLC inhibitor 1-O-octadecyl-2-O-methyl-sn-glycero-3-phosphorylcholine (ET-18-OCH3), or by using small interfering RNA targeted against PLCbeta1 demonstrating that PI-PLC is involved. Direct assessment of inositol phosphate production using a PIP2/IP3 "biosensor" revealed for the first time that IP3 can be generated in the nucleus following activation of nuclear mGlu5 receptors. Finally, both IP3 and ryanodine receptor blockers prevented nuclear mGlu5-mediated increases in intranuclear Ca2+. Collectively, this study shows that like plasma membrane receptors, activated nuclear mGlu5 receptors couple to Gq/11 and PLC to generate IP3-mediated release of Ca2+ from Ca2+-release channels in the nucleus. Thus the nucleus can function as an autonomous organelle independent of signals originating in the cytoplasm, and nuclear mGlu5 receptors play a dynamic role in mobilizing Ca2+ in a specific, localized fashion.

Expression analysis and subcellular localization of the Arabidopsis thaliana G-protein beta-subunit AGB1

Heterotrimeric GTP-binding proteins (G-proteins), consisting of Galpha, Gbeta, and Ggamma subunits, function as molecular switches in many eukaryotic signal transduction pathways. In contrast to many eukaryotes, plants contain very few G-protein subunit isoforms that mediate a diverse array of signalling functions. We investigated the contribution of cell type-specific expression and subcellular localization to this multifunctional signalling capacity for the Arabidopsis thaliana Gbeta subunit, AGB1. Analysis of AGB1 promoter::beta-glucuronidase (GUS) fusions in germinating seeds, seedlings, and flowering plants revealed that AGB1 is widely expressed throughout development in a complex manner. As well as demonstrating similarities to existing Arabidopsis G-protein subunit expression data, several features of the AGB1 expression pattern align closely with known or proposed G-protein functions. A C-terminal AGB1-green fluorescent protein (GFP) fusion was localized at the plasma membrane and in the nucleus of leaf epidermal cells, trichomes and root cells, supporting previous evidence that plant G-protein functionality relies on subcellular compartmentalization.

Interaction of the melanocortin 2 receptor with nucleoporin 50: evidence for a novel pathway between a G-protein-coupled receptor and the nucleus

The adrenocorticotropin (ACTH) receptor (melanocortin 2 receptor, or MC2R) is the smallest G-protein-coupled receptor that, when activated by the peptide hormone ACTH, stimulates cAMP production and adrenal steroidogenesis. Receptor expression is dependent on a specific membrane trafficking process involving an accessory protein (melanocortin 2 receptor accessory protein, or MRAP) and other unidentified components. In an attempt to discover novel receptor interacting proteins, the C-terminal tail of the MC2R was used to screen a mouse adrenal Y6 cell cDNA library using the bacterial two-hybrid system. This identified the nucleoporin Nup 50 (Npap60) as the major full-length interacting protein. Interaction was confirmed by a GST pulldown assay and by coimmunoprecipitation in human H295R cells (which express both proteins endogenously). Deletion analysis identified the region between residues 143 and 466 in Nup50 as being required for interaction with the MC2R. Stimulation of H295R cells with ACTH (10(-6) M) was followed by a gradual translocation of the Nup50-MC2R complex from the membrane to the nucleus after 30 min. This time course is most consistent with MC2R internalization dynamics and may suggest a novel role for Nup50.

Plasma membrane and nuclear localization of G protein coupled receptor kinase 6A

G protein-coupled receptor (GPCR) kinases (GRKs) specifically phosphorylate agonist-occupied GPCRs at the inner surface of the plasma membrane (PM), leading to receptor desensitization. Here we show that the C-terminal 30 amino acids of GRK6A contain multiple elements that either promote or inhibit PM localization. Disruption of palmitoylation by individual mutation of cysteine 561, 562, or 565 or treatment of cells with 2-bromopalmitate shifts GRK6A from the PM to both the cytoplasm and nucleus. Likewise, disruption of the hydrophobic nature of a predicted amphipathic helix by mutation of two leucines to alanines at positions 551 and 552 causes a loss of PM localization. Moreover, acidic amino acids in the C-terminus appear to negatively regulate PM localization; mutational replacement of several acidic residues with neutral or basic residues rescues PM localization of a palmitoylation-defective GRK6A. Last, we characterize the novel nuclear localization, showing that nuclear export of nonpalmitoylated GRK6A is sensitive to leptomycin B and that GRK6A contains a potential nuclear localization signal. Our results suggest that the C-terminus of GRK6A contains a novel electrostatic palmitoyl switch in which acidic residues weaken the membrane-binding strength of the amphipathic helix, thus allowing changes in palmitoylation to regulate PM versus cytoplasmic/nuclear localization.

Constitutive and ligand-induced nuclear localization of oxytocin receptor

Oxytocin receptor (OTR) is a membrane protein known to mediate oxytocin (OT) effects, in both normal and neoplastic cells. We report here that human osteosarcoma (U2OS, MG63, OS15 and SaOS2), breast cancer (MCF7), and primary human fibroblastic cells (HFF) all exhibit OTR not only on the cell membrane, but also in the various nuclear compartments including the nucleolus. Both an OTR-GFP fusion protein and the native OTR appear to be localized to the nucleus as detected by transfection and/or confocal immunofluorescence, respectively. Treatment with oxytocin causes internalization of OTR and the resulting vesicles accumulate in the vicinity of the nucleus and some of the perinuclear OTR enters the nucleus. Western blots indicate that OTR in the nucleus and on the plasma membrane are likely to be the same biochemical and immunological entities. It appears that OTR is first visible in the nucleoli and subsequently disperses within the nucleus into 4–20 spots while some of the OTR diffuses throughout the nucleoplasm.The behaviour and kinetics of OTR-GFP and OTR are different, indicating interference by GFP in both OTR entrance into the nucleus and subsequent relocalization of OTR within the nucleus. There are important differences among the tested cells, such as the requirement of a ligand for transfer of OTR in nuclei. A constitutive internalization of OTR was found only in osteosarcoma cells, while the nuclear localization in all other tested cells was dependent on ligand binding. The amount of OTR-positive material within and in the vicinity of the nucleus increased following a treatment with oxytocin in both constitutive and ligand-dependent type of cells. The evidence of OTR compartmentalization at the cell nucleus (either ligand-dependent or constitutive) in different cell types suggests still unknown biological functions of this protein or its ligand and adds this G-protein-coupled receptor to other heptahelical receptors displaying this atypical and unexpected nuclear localization.

Roles of G-protein-coupled receptor signaling in cancer biology and gene transcription

G-protein-coupled receptors (GPCRs) are ubiquitous mediators of signal transduction across mammalian cell membranes. Among other roles, GPCRs are known to regulate cellular motility, growth and differentiation, and gene transcription, three factors central to the biology of cancer. Because GPCRs are tractable drug targets, mechanisms by which receptors and their associated proteins impact cellular transformation and metastasis might lead to novel cancer therapies. Recent work has elucidated mechanisms explaining correlations between cancer progression and the expression of GPCRs, such as a protease-activated receptor (PAR1), and G-proteins, such as Galpha(12/13). Of special interest, the discovery of novel nuclear roles for heterotrimeric G-proteins expands the direct impact of G-protein signaling on processes fundamental to the pathology of cancer.

Nuclear localization of functional metabotropic glutamate receptor mGlu1 in HEK293 cells and cortical neurons: role in nuclear calcium mobilization and development

The Group I metabotropic glutamate receptor (mGlu1) plays an important role in neuromodulation, development, and synaptic plasticity. Using immunocytochemistry, subcellular fractionation, and western blot analysis, the present study shows that mGlu1a receptors are present on nuclear membranes in stably transfected human embryonic kidney 293 (HEK293) cells as well as being endogenously expressed on rat cortical nuclei. Both glutamate and the group I agonist, quisqualate, directly activate nuclear mGlu1 receptors leading to a characteristic oscillatory pattern of calcium flux in isolated HEK nuclei and a slow rise to plateau in isolated cortical nuclei. In either case calcium responses could be terminated upon application of the mGlu1-selective antagonist, 7-(hydroxyamino)cyclopropa[b]chromen-1a-carboxylate ethyl ester. Responses could also be blocked by ryanodine and inositol 1,4,5-triphosphate receptor inhibitors, demonstrating the involvement of these calcium channels. Agonist activation of intracellular receptors was driven by Na(+)-dependent and -independent processes in nuclei isolated from either HEK or cortical neurons. Finally, mGlu1 nuclear receptors were dramatically up-regulated in the course of post-natal development. Therefore, like the other Group I receptor, mGlu5, mGlu1 can function as an intracellular receptor, suggesting a more encompassing role for nuclear G protein-coupled receptors and downstream signaling elements in the regulation of nuclear events.

G protein βγ subunits interact with αβ- and γ-tubulin and play a role in microtubule assembly in PC12 cells

The betagamma subunit of G proteins (Gbetagamma) is known to transfer signals from cell surface receptors to intracellular effector molecules. Recent results suggest that Gbetagamma also interacts with microtubules and is involved in the regulation of the mitotic spindle. In the current study, the anti-microtubular drug nocodazole was employed to investigate the mechanism by which Gbetagamma interacts with tubulin and its possible implications in microtubule assembly in cultured PC12 cells. Nocodazole-induced depolymerization of microtubules drastically inhibited the interaction between Gbetagamma and tubulin. Gbetagamma was preferentially bound to microtubules and treatment with nocodazole suggested that the dissociation of Gbetagamma from microtubules is an early step in the depolymerization process. When microtubules were allowed to recover after removal of nocodazole, the tubulin-Gbetagamma interaction was restored. Unlike Gbetagamma, however, the interaction between tubulin and the alpha subunit of the Gs protein (Gsalpha) was not inhibited by nocodazole, indicating that the inhibition of tubulin-Gbetagamma interactions during microtubule depolymerization is selective. We found that Gbetagamma also interacts with gamma-tubulin, colocalizes with gamma-tubulin in centrosomes, and co-sediments in centrosomal fractions. The interaction between Gbetagamma and gamma-tubulin was unaffected by nocodazole, suggesting that the Gbetagamma-gamma-tubulin interaction is not dependent on assembled microtubules. Taken together, our results suggest that Gbetagamma may play an important and definitive role in microtubule assembly and/or stability. We propose that betagamma-microtubule interaction is an important step for G protein-mediated cell activation. These results may also provide new insights into the mechanism of action of anti-microtubule drugs.

Biosynthesis and trafficking of seven transmembrane receptor signalling complexes

Recent studies have shown that 7-transmembrane receptors (7TM-Rs), their associated signalling molecules and scaffolding proteins are often constitutively associated under basal conditions. These studies highlight that receptor ontogeny and trafficking are likely to play key roles in the determination of both signalling specificity and efficacy. This review highlights information about how 7TM-Rs and their associated signalling molecules are trafficked to the cell surface as well as other intracellular destinations.

G-protein-coupled receptors signalling at the cell nucleus: an emerging paradigm

G-protein-coupled receptors (GPCRs) comprise a wide family of monomeric heptahelical glycoproteins that recognize a broad array of extracellular mediators including cationic amines, lipids, peptides, proteins, and sensory agents. Thus far, much attention has been given towards the comprehension of intracellular signaling mechanisms activated by cell membrane GPCRs, which convert extracellular hormonal stimuli into acute, non-genomic (e.g., hormone secretion, muscle contraction, and cell metabolism) and delayed, genomic biological responses (e.g., cell division, proliferation, and apoptosis). However, with respect to the latter response, there is compelling evidence for a novel intracrine mode of genomic regulation by GPCRs that implies either the endocytosis and nuclear translocation of peripheral-liganded GPCR and (or) the activation of nuclearly located GPCR by endogenously produced, nonsecreted ligands. A noteworthy example of the last scenario is given by heptahelical receptors that are activated by bioactive lipoids (e.g., PGE(2) and PAF), many of which may be formed from bilayer membranes including those of the nucleus. The experimental evidence for the nuclear localization and signalling of GPCRs will be reviewed. We will also discuss possible molecular mechanisms responsible for the atypical compartmentalization of GPCRs at the cell nucleus, along with their role in gene expression.

Nuclear Ptdlns(3,4,5)P3 signaling: an ongoing story

Phosphatidylinositol 3,4,5-trisphosphate (Ptdlns(3,4,5)P(3)) is linked to a variety of cellular functions, such as growth, cell survival, and differentiation. Ptdlns(3,4,5)P(3) is primarily synthesized by class I phosphoinositide 3-kinases and its hydrolysis by two 3-phosphoinositide 3-phosphatases, PTEN and SHIP proteins, leads to the production of two other second messengers, Ptdlns(4,5)P(2) and Ptdlns(3,4)P(2), respectively. Evidence accumulated over the last years strongly suggest that Ptdlns(3,4,5)P(3) is an important component of signaling pathway operating within the nucleus. Moreover, recent advances indicated that nuclear translocation of cell surface receptors could activate nuclear phosphoinositide 3-kinase suggesting a new mode of signal transduction. The aim of this review is intended to summarize the state of our knowledge on nuclear Ptdlns(3,4,5)P(3) and its metabolizing enzymes, and to highlight the emerging roles for intranuclear Ptdlns(3,4,5)P(3).

Cellular stress increases RGS2 mRNA and decreases RGS4 mRNA levels in SH-SY5Y cells

Modulation of the expression of regulator of G-protein signaling (RGS) proteins is a major mechanism used to modulate their actions. Besides control by second messengers, the expression of RGS proteins, particularly RGS2, can be regulated by cell stress. Because RGS2 and RGS4 expression can be regulated by the cell cycle, we examined if cell cycle signals are involved in their regulation following stress. Treatment of SH-SY5Y cells with camptothecin increased RGS2 mRNA and decreased RGS4 mRNA levels. This effect on RGS2 mRNA was blocked by the cyclin-dependent kinase-2 (cdk2) inhibitors roscovitine and purvalanol. Cell cycle arrest was further implicated in regulating RGS mRNA levels because geldanamycin, which causes cell cycle arrest by inhibiting the actions of heat shock protein 90, caused changes in the mRNA levels of RGS2 and RGS4 similar to, and additive with, the effects of camptothecin. Overall, these results indicate that cell cycle arrest regulates the expression of RGS2 and RGS4, and that the expression of these two RGS family members is oppositely regulated by stress that causes cell cycle arrest.

Nuclear localization of SP, CGRP, and NK1R in a subpopulation of dorsal root ganglia subpopulation cells in rats

Signals generated by renal pelvic afferent nerves in response to stimulation are transmitted from peripheral processes of dorsal root ganglia neurons to their central terminals in the dorsal horn of the spinal cord to cause the release of neuropeptides, including SP and CGRP. All of the cellular activities of SP are considered to be mediated through interaction with NK(1)R located on the cell surface. We have investigated the colocalization and subcellular distribution of NK(1)R, SP, and CGRP in different subpopulations of neurons that innervate renal tissue. Our findings therefore provide the first evidence for the presence of NK(1)R, SP, and CGRP in the nuclei of DGR neural cells. The physiological significance of this localization remains unknown. One possibility is that pelvic sensory neurons may regulate their responses to different stimuli by modulating the ratio of CGRP and SP release and/or nuclear NK(1)R expression.

Transcriptional analysis of avian embryonic tissues following infection with avian infectious bronchitis virus

Avian infectious bronchitis virus (IBV) infection is one of the major viral respiratory diseases of chickens. Better understanding of the molecular basis of viral pathogenesis should contribute significantly towards the development of improved prophylactic, therapeutic and diagnostic reagents to control infections. In the present investigation, transcriptional profiles were analyzed by using RNA recovered from the lung tissue of IBV infected 18-day-old chicken embryos at 6, 24, 48 and 72 h post IBV infection. This microarray analysis was completed using avian cDNA arrays comprised of fragments of 1191 unique chicken and turkey gene transcripts. These arrays were generated from normalized cDNA subtraction libraries that were derived from avian pneumovirus (APV) infected chicken embryo fibroblast (CEF) cultures and tissues obtained from APV infected turkeys subtracted with their respective uninfected cultures and tissues. Of the 1191 unique genes represented on the array, the expression of a total of 327 genes (27% of total) were altered by two-fold or more from 6 through 72 h post-infection. A comparative analysis of IBV regulated genes with genes previously reported to change in expression following infection with other avian respiratory viruses revealed both conserved and unique changes. Real-time qRT-PCR was used to confirm the regulated expression of genes related to several functional classes including kinases, interferon induced genes, chemokines and adhesion molecules, vesicular trafficking and fusion protein genes, extracellular matrix protein genes, cell cycle, metabolism, cell physiology and development, translation, RNA binding, lysosomal, protein degradation and ubiquitination related genes. Microarray analysis served as an efficient tool in facilitating a comparative analysis of avian respiratory viral infections and provided insight into host transcriptional changes that were conserved as well as those which were unique to individual pathogens.

G-protein beta subunit of Cochliobolus heterostrophus involved in virulence, asexual and sexual reproductive ability, and morphogenesis

Previous work established that mutations in mitogen-activated protein (MAP) kinase (CHK1) and heterotrimeric G-protein alpha (Galpha) subunit (CGA1) genes affect the development of several stages of the life cycle of the maize pathogen Cochliobolus heterostrophus. The effects of mutating a third signal transduction pathway gene, CGB1, encoding the Gbeta subunit, are reported here. CGB1 is the sole Gbeta subunit-encoding gene in the genome of this organism. cgb1 mutants are nearly wild type in vegetative growth rate; however, Cgb1 is required for appressorium formation, female fertility, conidiation, regulation of hyphal pigmentation, and wild-type virulence on maize. Young hyphae of cgb1 mutants grow in a straight path, in contrast to those of the wild type, which grow in a wavy pattern. Some of the phenotypes conferred by mutations in CGA1 are found in cgb1 mutants, suggesting that Cgb1 functions in a heterotrimeric G protein; however, there are also differences. In contrast to the deletion of CGA1, the loss of CGB1 is not lethal for ascospores, evidence that there is a Gbeta subunit-independent signaling role for Cga1 in mating. Furthermore, not all of the phenotypes conferred by mutations in the MAP kinase CHK1 gene are found in cgb1 mutants, implying that the Gbeta heterodimer is not the only conduit for signals to the MAP kinase CHK1 module. The additional phenotypes of cgb1 mutants, including severe loss of virulence on maize and of the ability to produce conidia, are consistent with CGB1 being unique in the genome. Fluorescent DNA staining showed that there is often nuclear degradation in mature hyphae of cgb1 mutants, while comparable wild-type cells have intact nuclei. These data may be genetic evidence for a novel cell death-related function of the Gbeta subunit in filamentous fungi.

Functional endothelin ET B receptors are selectively expressed in human oligodendrogliomas

Endothelin-1 (ET-1), a vasoactive and mitogenic peptide mainly produced by vascular endothelial cells, may be involved in the progression of several human tumors. Here, we present an immunohistochemical analysis of the expression pattern of ET-1 receptor subtypes (ET(A)-R and ET(B)-R) and a functional study of their potential role in human oligodendrogliomas and oligoastrocytomas. By comparison, we assessed the corresponding expression patterns of glioblastomas. Interestingly, a nuclear localization of ET-1 receptor subtypes (associated or not with a cytoplasmic labeling) was constantly observed in tumor cells from all three glioma types. Moreover, we noted a distinct receptor distribution in the different gliomas: a nuclear expression of ET(B)-R by tumor cells was found to be restricted to oligodendrogliomas and oligoastrocytomas, while a nuclear expression of ET(A)-R was only detected in tumor cells from some glioblastomas. Using primary cultures of oligodendroglial tumor cells, we confirmed the selective expression of nuclear ET(B)-R, together with a plasma membrane expression, and further demonstrated that this receptor was functionally coupled to intracellular signaling pathways known to be involved in cell survival and/or proliferation: extracellular signal-regulated kinase and focal adhesion kinase activation, actin cytoskeleton reorganization. In addition, impairment of ET(B)-R activation in these cells by in vitro treatment with an ET(B)-R-specific antagonist induced cell death. These data point to ET-1 as a possible survival factor for oligodendrogliomas via ET(B)-R activation and suggest that ET(B)-R-specific antagonists might constitute a potential therapeutic alternative for oligodendrogliomas.

Return of the GDI: the GoLoco motif in cell division

The GoLoco motif is a 19-amino-acid sequence with guanine nucleotide dissociation inhibitor activity against G-alpha subunits of the adenylyl-cyclase-inhibitory subclass. The GoLoco motif is present as an independent element within multidomain signaling regulators, such as Loco, RGS12, RGS14, and Rap1GAP, as well as in tandem arrays in proteins, such as AGS3, G18, LGN, Pcp-2/L7, and Partner of Inscuteable (Pins/Rapsynoid). Here we discuss the biochemical mechanisms of GoLoco motif action on G-alpha subunits in light of the recent crystal structure of G-alpha-i1 bound to the RGS14 GoLoco motif. Currently, there is sparse evidence for GoLoco motif regulation of canonical G-protein-coupled receptor signaling. Rather, studies of asymmetric cell division in Drosophila and Caenorhabditis elegans, as well as mammalian mitosis, implicate GoLoco proteins, such as Pins, GPR-1/GPR-2, LGN, and RGS14, in mitotic spindle organization and force generation. We discuss potential mechanisms by which GoLoco/Galpha complexes might modulate spindle dynamics.

Agonist-independent nuclear localization of the Apelin, angiotensin AT1, and bradykinin B2 receptors

Signaling of the apelin, angiotensin, and bradykinin peptides is mediated by G protein-coupled receptors related through structure and similarities of physiological function. We report nuclear expression as a characteristic of these receptors, including a nuclear localization for the apelin receptor in brain and cerebellum-derived D283 Med cells and the AT(1) and bradykinin B(2) receptors in HEK-293T cells. Immunocytochemical analyses revealed the apelin receptor with localization in neuronal nuclei in cerebellum and hypothalamus, exhibiting expression in neuronal cytoplasm or in both nuclei and cytoplasm. Confocal microscopy of HEK-293T cells revealed the majority of transfected cells displayed constitutive nuclear localization of AT(1) and B(2) receptors, whereas apelin receptors did not show nuclear localization in these cells. The majority of apelin receptor-transfected cerebellum D283 Med cells showed receptor nuclear expression. Immunoblot analyses of subcellular-fractionated D283 Med cells demonstrated endogenous apelin receptor species in nuclear fractions. In addition, an identified nuclear localization signal motif in the third intracellular loop of the apelin receptor was disrupted by a substituted glutamine in place of lysine. This apelin receptor (K242Q) did not exhibit nuclear localization in D283 Med cells. These results demonstrate the following: (i) the apelin receptor exhibits nuclear localization in human brain; (ii) distinct cell-dependent mechanisms for the nuclear transport of apelin, AT(1), and B(2) receptors; and (iii) the disruption of a nuclear localization signal sequence disrupts the nuclear translocation of the apelin receptor. This discovery of apelin, AT(1), and B(2) receptors with agonist-independent nuclear translocation suggests major unanticipated roles for these receptors in cell signaling and function.

In through the out door: nuclear localization of the regulators of G protein signaling

The regulators of G protein signaling (RGS) are an extraordinary class of diverse multifunctional signaling proteins best known for their potent capacity to down-regulate the activity of Galpha subunits at the plasma membrane. In certain circumstances, some RGS proteins undergo translocation to the nucleus or plasma membrane from the cytoplasm. Translocation demonstrates a potentially dynamic alternative mechanism for Galpha subunit or effector regulation. The nuclear localization of the regulators of G protein signaling proteins further suggests these proteins possess even greater functional heterogeneity than that envisioned previously, as regulators of transcription and cell cycle control.

Functional endothelin receptors are present on nuclei in cardiac ventricular myocytes

Endothelins are thought to act through two specific, plasmalemmal G protein-coupled receptor subtypes, ETAR and ETBR. However, in subfractionated cardiac membranes, ETAR immunoreactivity was detected only in the plasma membrane whereas ETBR immunoreactivity was detected predominantly in membranes of intracellular origin. Confocal microscopy demonstrated the presence of intracellular ETAR and ETBR in ventricular myocytes. ETAR were primarily on plasma membrane (surface membranes and transverse-tubules) and to a lesser extent on the nucleus while ETBR localized primarily to the nuclei. Western blot analysis of nuclei isolated from the heart indicated the presence of endothelin receptors: both ETAR and ETBR copurified with nucleoporin 62, whereas markers of endoplasmic reticulum and Golgi membranes were depleted. Radioligand binding studies revealed that isolated nuclei contain specific [125I]ET-1 binding sites. Specific [125I]ET-1 binding was reduced by 70-80% using the ETAR-selective antagonist BQ610 and 20-30% using the ETBR-specific antagonist BQ788. IRL-1620, an ETBR-specific agonist, also reduced [125I]ET-1 binding. Furthermore, ET-1 and IRL-1620 altered the incorporation of 32P into nuclear proteins and caused a transient increase in nuclear Ca2+ concentration. Hence, cardiac nuclei possess both ETAR and ETBR subtypes, which are functional with respect to ligand binding and are coupled to signaling mechanisms within the nuclear membrane.

Activation of metabotropic glutamate receptor mGlu5 on nuclear membranes mediates intranuclear Ca2+ changes in heterologous cell types and neurons

Nuclear Ca2+ plays a critical role in many cellular functions although its mode (s) of regulation is unclear. This study shows that the metabotropic glutamate receptor, mGlu5, mobilizes nuclear Ca2+ independent of cytosolic Ca2+ regulation. Immunocytochemical, ultrastructural, and subcellular fractionation techniques revealed that the metabotropic glutamate receptor, mGlu5, can be localized to nuclear membranes in heterologous cells as well as midbrain and cortical neurons. Nuclear mGlu5 receptors derived from HEK cells or cortical cell types bound [3H]quisqualate. When loaded with Oregon Green BAPTA, nuclei isolated from mGlu5-expressing HEK cells responded to the addition of glutamate with rapid, oscillatory [Ca2+] elevations that were blocked by antagonist or EGTA. In contrast, carbachol-activation of endogenous muscarinic receptors led to cytoplasmic but not nuclear Ca2+ responses. Similarly, activation of mGlu5 receptors expressed on neuronal nuclei led to sustained Ca2+ oscillatory responses. These results suggest mGlu5 may mediate intranuclear signaling pathways.

Colchicine down-regulates cytochrome P450 2B6, 2C8, 2C9, and 3A4 in human hepatocytes by affecting their glucocorticoid receptor-mediated regulation

The xenobiotic-mediated induction of three major human liver cytochrome P450 genes, CYP2B6, CYP2C9, and CYP3A4, is known to be regulated by the constitutive androstane receptor (CAR) and the pregnane X receptor (PXR). CAR and PXR are regulated, at least in part, by the glucocorticoid receptor (GR) and the hypothesis of a signal transduction cascade GR-[CAR/PXR]-P450 has been proposed. This study was aimed at testing this hypothesis in primary human hepatocytes by using the tubulin network disrupting agent colchicine. Colchicine (COL) decreased both basal and rifampicin- and phenobarbital-inducible expression of CYP2B6, CYP2C8/9, and CYP3A4. A parallel down-regulation of mRNA expression of CAR, PXR, and tyrosine aminotransferase, a prototypic gene directly regulated by GR, was observed. COL affected neither the level of GR mRNA nor ligand binding to GR. To evaluate the effect of colchicine on GR-mediated gene transactivation, HeLa cells stably or transiently transfected with a GR-responsive element-dependent luciferase reporter gene were used. COL decreased the dexamethasone-induced luciferase expression in stably transfected cell line by 50%, whereas GR transactivation in transiently transfected cells was not affected by COL. In contrast, ligand-dependent GR translocation in the human embryonic kidney 293 cell line transiently transfected with GFP-GR was inhibited by COL. We conclude that alteration of the signal transduction mediated through the GR-[CAR/PXR]-P450 cascade by colchicine is responsible for the down-regulation of CYP2C9 and CYP3A4, implicating cytoskeleton as necessary for correct functioning of this cascade under physiological conditions.

A specific domain of Gialpha required for the transactivation of Gialpha by tubulin is implicated in the organization of cellular microtubules

G(s)alpha, G(i)alpha(1), and G(q)alpha subunits bind tubulin with high affinity, whereas transducin (G(t)alpha) does not. The interaction between tubulin and Galpha, which also involves the direct transfer of GTP from tubulin to Galpha (transactivation), is not yet fully understood. This study, using chimeras of G(i)alpha and G(t)alpha, showed that the G(i)alpha (215-295) segment converted G(t)alpha to bind to tubulin and this chimera (chimera 1) could be transactivated by tubulin. Insertion of G(t)alpha (237-270) into chimera 1 to form chimera 2 resulted in a protein that, like G(t)alpha, did not bind tubulin. Thus, it was thought that the G(i)alpha (237-270) domain was essential to modulate the binding of G(i)alpha(1) to tubulin. Surprisingly, when domain (237-270) of G(i)alpha was replaced by G(t)alpha (237-270) to form chimera 3, the chimera bound to tubulin with a similar affinity (K(D) congruent with 120 nm) as wild-type G(i)alpha(1). However, even though chimera 3 displayed normal GTP binding, it was not transactivated by GTP-tubulin. Furthermore, when these chimeras were expressed in COS-1 cells, cellular processes in cells overexpressing G(i)alpha(1) or chimera 1 were more abundant and longer than those in native cells. Galpha was seen throughout the length of the process. Morphology of cells expressing chimera 2 was identical to controls. Consistent with the role of Chimera 3 as a "dominant negative" Galpha, cells transfected with chimera 3 had only few truncated processes. This study demonstrates that although G(i)alpha (237-270) is not obligatory for the binding of G(i)alpha to tubulin, it is crucial for the transactivation of Galpha by tubulin. These results also suggest that the transactivation of Galpha by tubulin may play an important role in modulating microtubule organization and cell morphology.

Structure and function of heterotrimeric G proteins in plants

Heterotrimeric G proteins are mediators that transmit the external signals via receptor molecules to effector molecules. The G proteins consist of three different subunits: alpha, beta, and gamma subunits. The cDNAs or genes for all the alpha, beta, and gamma subunits have been isolated from many plant species, which has contributed to great progress in the study of the structure and function of the G proteins in plants. In addition, rice plants lacking the alpha subunit were generated by the antisense method and a rice mutant, Daikoku d1, was found to have mutation in the alpha-subunit gene. Both plants show abnormal morphology such as dwarfism, dark green leaf, and small round seed. The findings revealed that the G proteins are functional molecules regulating some body plans in plants. There is evidence that the plant G proteins participate at least in signaling of gibberellin at low concentrations. In this review, we summarize the currently known information on the structure of plant heterotrimeric G proteins and discuss the possible functions of the G proteins in plants.