The caleosin CLO7 and its role in the heterotrimeric G-protein signalling network

The investigation of the caleosin CLO7 in relation to heterotrimeric G-protein signalling in Arabidopsis showed that the gene plays a role in seed germination and embryo viability. The caleosin CLO7 belongs to a multi-gene family of calcium-binding proteins which are characterized by single EF-hand motifs. Other members of the caleosin gene family have been shown to affect transpiration and seed germination as well as play a role in both abiotic and biotic stress responses. The proteins are associated with lipid droplets/oil bodies and some members of the gene family have been shown to have peroxygenase activity. Members of the gene family have also been shown to interact with the α subunit of the heterotrimeric G protein complex. In this study, we further expand on the diversity of physiological responses in which members of this gene family play regulatory roles. Utilizing BiFC and Y2H protein-protein interaction assays, CLO7 is identified as an interactor of the heterotrimeric G protein α subunit, GPA1. The full-length CLO7 is shown to interact with both the wild-type GPA1 and its constitutively active form, GPA1^QL, at the plasma membrane. Point mutations to critical amino acids for calcium binding in the EF-hand of CLO7 indicate that the interaction with GPA1 is calcium-dependent and that the interaction with GPA1^QL is enhanced by calcium. Protein-protein interaction assays also show that CLO7 interacts with Pirin1, a member of the cupin gene superfamily and a known downstream effector of GPA1, and this interaction is calcium-dependent. The N-terminal portion of CLO7 is responsible for these interactions. GFP-tagged CLO7 protein localizes to the endoplasmic reticulum (ER) and to lipid bodies. Characterization of the clo7 mutant line has shown that CLO7 is implicated in the abscisic acid (ABA) and mannitol-mediated inhibition of seed germination, with the clo7 mutant displaying higher germination rates in response to osmotic stress and ABA hormone treatment. These results provide insight into the role of CLO7 in seed germination in response to abiotic stress as well as its interaction with GPA1 and Pirin1. CLO7 also plays a role in embryo viability with the clo7gpa1 double mutant displaying embryo lethality, and therefore the double mutant cannot be recovered.

The G-protein alpha subunit CgGa1 mediates growth, sporulation, penetration and pathogenicity in Colletotrichum gloeosporioides

Colletotrichum gloeosporioides is the main pathogen causing rubber anthracnose, which brings huge economic loss to the natural rubber industry. Heterotrimeric G proteins play a vital role in signal transduction in filamentous fungi, and G alpha subunits are the major component of G proteins. In this study, we characterize a group I Gα subunit CgGa1 in C. gloeosporioides as a homolog of MagB in Pyricularia oryzae. CgGa1 encodes a 353-amino acid protein and has a G_alpha domain. Deletion of CgGa1 results in reduced vegetative growth and conidia yield, and the mutant cannot produce a fruiting body. The CgGa1 deletion mutant also exhibits decreased conidial germination and appressorium formation significantly. Moreover, the mutant has an obvious deficiency in penetration and loses its virulence completely. Transcriptome analysis showed that CgGa1 could affect the expression of many genes related to carbohydrate metabolism, amino acid metabolism and signal transduction, etc. In conclusion, CgGa1 regulates growth, asexual and sexual sporulation, appressorium formation, penetration and pathogenicity of C. gloeosporioides.

Characterization of a G protein α subunit encoded gene from the dimorphic fungus-Tremella fuciformis

Tremella fuciformis is a dimorphic fungus which can undertake the reversible transition between yeast and pseudohypha forms. G protein α subunit (Gα) carries different signals to regulate a variety of biological processes in eukaryotes, including fungal dimorphism. In this study, a novel Gα subunit encoded gene, TrGpa1, was firstly cloned from T. fuciformis. The TrGpa1 open reading frame has 1059 nucleotides, and encodes a protein which belongs to the group I of Gα_i superfamily. Furthermore, the role of TrGpa1 in the T. fuciformis dimorphism was analysed by gene overexpression and knockdown. Stable integration of the target gene into the genome was confirmed by PCR and Southern blot hybridization. Transformants with the highest and lowest TrGpa1 expression levels were selected via quantitative real-time PCR analysis and Western blot. Each transformant was compared with the wild-type strain about the morphological change under different environmental factors, including pH values, temperature, cultivation time, inoculum size, and quorum-sensing molecules (farnesol and tyrosol). Comparing with the wild-type strain, the overexpression transformant always had higher ratios of pseudohyphae, while the knockdown transformant had less proportions of pseudohyphae. Therefore, the TrGpa1 is involved in the dimorphism of T. fuciformis and plays a positive role in promoting pseudohyphal growth.

A prominent role of the flagellin receptor FLAGELLIN-SENSING2 in mediating stomatal response to Pseudomonas syringae pv tomato DC3000 in Arabidopsis

The FLAGELLIN-SENSING2 (FLS2) receptor kinase recognizes bacterial flagellin and initiates a battery of downstream defense responses to reduce bacterial invasion through stomata in the epidermis and bacterial multiplication in the apoplast of infected plants. Recent studies have shown that during Pseudomonas syringae pv tomato (Pst) DC3000 infection of Arabidopsis (Arabidopsis thaliana), FLS2-mediated immunity is actively suppressed by effector proteins (such as AvrPto and AvrPtoB) secreted through the bacterial type III secretion system (T3SS). We provide evidence here that T3SS effector-based suppression does not appear to be sufficient to overcome FLS2-based immunity during Pst DC3000 infection, but that the phytotoxin coronatine (COR) produced by Pst DC3000 also plays a critical role. COR-deficient mutants of Pst DC3000 are severely reduced in virulence when inoculated onto the leaf surface of wild-type Columbia-0 plants, but this defect was rescued almost fully in fls2 mutant plants. Although bacteria are thought to carry multiple microbe-associated molecular patterns, stomata of fls2 plants are completely unresponsive to COR-deficient mutant Pst DC3000 bacteria. The responses of fls2 plants were similar to those of the Arabidopsis G-protein alpha subunit1-3 mutant, which is defective in abscisic acid-regulated stomatal closure, but were distinct from those of the Arabidopsis non-expressor of PR genes1 mutant, which is defective in salicylic acid-dependent stomatal closure and apoplast defense. Epistasis analyses show that salicylic acid signaling acts upstream of abscisic acid signaling in bacterium-triggered stomatal closure. Taken together, these results suggest a particularly important role of FLS2-mediated resistance to COR-deficient mutant Pst DC3000 bacteria, and nonredundant roles of COR and T3SS effector proteins in the suppression of FLS2-mediated resistance in the Arabidopsis-Pst DC3000 interaction.

Migratory dermal dendritic cells act as rapid sensors of protozoan parasites

Dendritic cells (DC), including those of the skin, act as sentinels for intruding microorganisms. In the epidermis, DC (termed Langerhans cells, LC) are sessile and screen their microenvironment through occasional movements of their dendrites. The spatio-temporal orchestration of antigen encounter by dermal DC (DDC) is not known. Since these cells are thought to be instrumental in the initiation of immune responses during infection, we investigated their behavior directly within their natural microenvironment using intravital two-photon microscopy. Surprisingly, we found that, under homeostatic conditions, DDC were highly motile, continuously crawling through the interstitial space in a Galpha(i) protein-coupled receptor-dependent manner. However, within minutes after intradermal delivery of the protozoan parasite Leishmania major, DDC became immobile and incorporated multiple parasites into cytosolic vacuoles. Parasite uptake occurred through the extension of long, highly dynamic pseudopods capable of tracking and engulfing parasites. This was then followed by rapid dendrite retraction towards the cell body. DDC were proficient at discriminating between parasites and inert particles, and parasite uptake was independent of the presence of neutrophils. Together, our study has visualized the dynamics and microenvironmental context of parasite encounter by an innate immune cell subset during the initiation of the immune response. Our results uncover a unique migratory tissue surveillance program of DDC that ensures the rapid detection of pathogens.

Phosducin regulates the expression of transducin betagamma subunits in rod photoreceptors and does not contribute to phototransduction adaptation

For over a decade, phosducin's interaction with the betagamma subunits of the G protein, transducin, has been thought to contribute to light adaptation by dynamically controlling the amount of transducin heterotrimer available for activation by photoexcited rhodopsin. In this study we directly tested this hypothesis by characterizing the dark- and light-adapted response properties of phosducin knockout (Pd- / -) rods. Pd- / - rods were notably less sensitive to light than wild-type (WT) rods. The gain of transduction, as measured by the amplification constant using the Lamb-Pugh model of activation, was 32% lower in Pd- / - rods than in WT rods. This reduced amplification correlated with a 36% reduction in the level of transducin betagamma-subunit expression, and thus available heterotrimer in Pd- / - rods. However, commonly studied forms of light adaptation were normal in the absence of phosducin. Thus, phosducin does not appear to contribute to adaptation mechanisms of the outer segment by dynamically controlling heterotrimer availability, but rather is necessary for maintaining normal transducin expression and therefore normal flash sensitivity in rods.

Lentiviral gene transfer of RPE65 rescues survival and function of cones in a mouse model of Leber congenital amaurosis

BACKGROUND

RPE65 is specifically expressed in the retinal pigment epithelium and is essential for the recycling of 11-cis-retinal, the chromophore of rod and cone opsins. In humans, mutations in RPE65 lead to Leber congenital amaurosis or early-onset retinal dystrophy, a severe form of retinitis pigmentosa. The proof of feasibility of gene therapy for RPE65 deficiency has already been established in a dog model of Leber congenital amaurosis, but rescue of the cone function, although crucial for human high-acuity vision, has never been strictly proven. In Rpe65 knockout mice, photoreceptors show a drastically reduced light sensitivity and are subject to degeneration, the cone photoreceptors being lost at early stages of the disease. In the present study, we address the question of whether application of a lentiviral vector expressing the Rpe65 mouse cDNA prevents cone degeneration and restores cone function in Rpe65 knockout mice.

METHODS AND FINDINGS

Subretinal injection of the vector in Rpe65-deficient mice led to sustained expression of Rpe65 in the retinal pigment epithelium. Electroretinogram recordings showed that Rpe65 gene transfer restored retinal function to a near-normal pattern. We performed histological analyses using cone-specific markers and demonstrated that Rpe65 gene transfer completely prevented cone degeneration until at least four months, an age at which almost all cones have degenerated in the untreated Rpe65-deficient mouse. We established an algorithm that allows prediction of the cone-rescue area as a function of transgene expression, which should be a useful tool for future clinical trials. Finally, in mice deficient for both RPE65 and rod transducin, Rpe65 gene transfer restored cone function when applied at an early stage of the disease.

CONCLUSIONS

By demonstrating that lentivirus-mediated Rpe65 gene transfer protects and restores the function of cones in the Rpe65(-/-) mouse, this study reinforces the therapeutic value of gene therapy for RPE65 deficiencies, suggests a cone-preserving treatment for the retina, and evaluates a potentially effective viral vector for this purpose.

Downregulation of cone-specific gene expression and degeneration of cone photoreceptors in the Rpe65-/- mouse at early ages

PURPOSE

RPE65 is essential for the generation of 11-cis retinal. Rod photoreceptors in the RPE65-knockout (Rpe65(-/-)) mouse are known to degenerate slowly with age. This study was designed to examine cone photoreceptors and the expression of cone-specific genes in the Rpe65(-/-) mouse.

METHODS

Gene expression changes were identified by microarray and confirmed by real-time RT-PCR. Cone photoreceptors were stained by peanut agglutinin (PNA) lectin in the flatmounted retina. The 9- or 11-cis retinal was supplied by intraperitoneal injections.

RESULTS

The short-wavelength (SWL) cone opsin mRNA was markedly decreased at 2 weeks of age, whereas the decrease in the middle-wavelength (MWL) cone opsin mRNA occurred relatively later in age. In contrast, the rhodopsin mRNA level did not show any significant change at all the ages analyzed. Consistent with the cone opsin changes, the cone transducin alpha-subunit mRNA decreased at both 4 and 8 weeks of age, whereas again the rod transducin alpha-subunit did not show any significant change. Rpe65(-/-) mice showed significant cone loss in both the central and ventral retina between 2 and 3 weeks of age. Administration of 9- or 11-cis retinal to Rpe65(-/-) mice 2 weeks of age increased cone density by twofold in these areas.

CONCLUSIONS

In the Rpe65(-/-) mouse, the expression of cone-specific genes is downregulated and is accompanied by cone degeneration at early ages. Early administration of 9- or 11-cis retinal can partially prevent cone loss, suggesting that the absence of 11-cis chromophore may be responsible for the early cone degeneration.

Mammalian Pins is a conformational switch that links NuMA to heterotrimeric G proteins

During asymmetric cell divisions, mitotic spindles align along the axis of polarization. In invertebrates, spindle positioning requires Pins or related proteins and a G protein alpha subunit. A mammalian Pins, called LGN, binds Galphai and also interacts through an N-terminal domain with the microtubule binding protein NuMA. During mitosis, LGN recruits NuMA to the cell cortex, while cortical association of LGN itself requires the C-terminal Galpha binding domain. Using a FRET biosensor, we find that LGN behaves as a conformational switch: in its closed state, the N and C termini interact, but NuMA or Galphai can disrupt this association, allowing LGN to interact simultaneously with both proteins, resulting in their cortical localization. Overexpression of Galphai or YFP-LGN causes a pronounced oscillation of metaphase spindles, and NuMA binding to LGN is required for these spindle movements. We propose that a related switch mechanism might operate in asymmetric cell divisions in the fly and nematode.

Purification and in vitro functional analysis of the Arabidopsis thaliana regulator of G-protein signaling-1

The model organism Arabidopsis thaliana contains a restricted set of heterotrimeric G-protein subunits, with only one canonical Galpha subunit (AtGPA1), one Gbeta subunit (AtAGB1), and two Ggamma subunits (AtAGG1 and AtGG2) identified. We have identified a novel additional component of heterotrimeric G-protein signaling in the A. thaliana genome, regulator of G-protein signaling-1 (AtRGS1). This protein has the predicted topology and structure of a G-protein-coupled receptor in that it contains seven transmembrane domains, but AtRGS1 also contains a unique C-terminal extension, namely a regulator of G-protein signaling domain (RGS box). This article describes methods for the purification and in vitro functional analysis of the RGS box of AtRGS1.

Expression of olfactory receptors in the cribriform mesenchyme during prenatal development

Olfactory receptors (ORs) are expressed in sensory neurons of the nasal epithelium, where they are supposed to be involved in the recognition of suitable odorous compounds and in the guidance of outgrowing axons towards the appropriate glomeruli in the olfactory bulb. During development, some olfactory receptor subtypes have also been found in non-sensory tissues, including the cribriform mesenchyme between the prospective olfactory epithelium and the developing telencephalon, but it is elusive if this is a typical phenomenon for ORs. Monitoring the onset and time course of expression for several receptor subtypes revealed that 'extraepithelial' expression of ORs occurs very early and transiently, in particular between embryonic stages E10.25 and E14.0. In later stages, a progressive loss of receptor expressing cells was observed. Molecular phenotyping demonstrated that the receptor expressing cells in the cribriform mesenchyme co-express key elements, including Galpha(olf), ACIII and OMP, characteristic for olfactory neurons in the nasal epithelium. Studies on transgenic OMP/GFP-mice showed that 'extraepithelial' OMP/GFP-positive cells are located in close vicinity to axon bundles projecting from the nasal epithelium to the presumptive olfactory bulb. Moreover, these cells are primarily located where axons fasciculate and change direction towards the anterior part of the forebrain.

Photoreceptor-specific nuclear receptor NR2E3 functions as a transcriptional activator in rod photoreceptors

NR2E3, a photoreceptor-specific orphan nuclear receptor, is believed to play a pivotal role in the differentiation of photoreceptors. Mutations in the human NR2E3 gene and its mouse ortholog are associated with enhanced S-cones and retinal degeneration. In order to gain insights into the NR2E3 function, we performed temporal and spatial expression analysis, yeast two-hybrid screening, promoter activity assays and co-immunoprecipitation studies. The Nr2e3 expression was localized preferentially to the rod, and not to the cone, photoreceptor nuclei in rodent retina. The yeast two-hybrid screening of a retinal cDNA library, using NR2E3 as the bait, identified another orphan nuclear receptor NR1D1 (Rev-erbalpha). The interaction of NR2E3 with NR1D1 was confirmed by glutathione S-transferase pulldown and co-immunoprecipitation experiments. In transient transfection studies using HEK 293 cells, both NR2E3 and NR1D1 activated the promoters of rod phototransduction genes synergistically with neural retina leucine zipper (NRL) and cone-rod homeobox (CRX). All four proteins, NR2E3, NR1D1, NRL and CRX, could be co-immunoprecipitated from the bovine retinal nuclear extract, suggesting their existence in a multi-protein transcriptional regulatory complex in vivo. Our results demonstrate that NR2E3 is involved in regulating the expression of rod photoreceptor-specific genes and support its proposed role in transcriptional regulatory network(s) during rod differentiation.

Deletion of guanine nucleotide binding protein alpha z subunit in mice induces a gene dose dependent tolerance to morphine

The mechanism underlying the development of tolerance to morphine is still incompletely understood. Morphine binds to opioid receptors, which in turn activates downstream second messenger cascades through heterotrimeric guanine nucleotide binding proteins (G proteins). In this paper, we show that G(z), a member of the inhibitory G protein family, plays an important role in mediating the analgesic and lethality effects of morphine after tolerance development. We blocked signaling through the G(z) second messenger cascade by genetic ablation of the alpha subunit of the G protein in mice. The Galpha(z) knockout mouse develops significantly increased tolerance to morphine, which depends on Galpha(z) gene dosage. Further experiments demonstrate that the enhanced morphine tolerance is not caused by pharmacokinetic and behavioural learning mechanisms. The results suggest that G(z) signaling pathways are involved in transducing the analgesic and lethality effects of morphine following chronic morphine treatment.

RGS17/RGSZ2, a Novel Regulator of Gi/o, Gz, and Gq Signaling

To identify novel regulators of Galpha(o), the most abundant G-protein in brain, we used yeast two-hybrid screening with constitutively active Galpha(o) as bait and identified a new regulator of G-protein signaling (RGS) protein, RGS17 (RGSZ2), as a novel human member of the RZ (or A) subfamily of RGS proteins. RGS17 contains an amino-terminal cysteine-rich motif and a carboxyl-terminal RGS domain with highest homology to hRGSZ1- and hRGS-Galpha-interacting protein. RGS17 RNA was strongly expressed as multiple species in cerebellum and other brain regions. The interactions between hRGS17 and active forms of Galpha(i1-3), Galpha(o), Galpha(z), or Galpha(q) but not Galpha(s) were detected by yeast two-hybrid assay, in vitro pull-down assay, and co-immunoprecipitation studies. Recombinant RGS17 acted as a GTPase-activating protein (GAP) on free Galpha(i2) and Galpha(o) under pre-steady-state conditions, and on M2-muscarinic receptor-activated Galpha(i1), Galpha(i2), Galpha(i3), Galpha(z), and Galpha(o) in steady-state GTPase assays in vitro. Unlike RGSZ1, which is highly selective for G(z), RGS17 exhibited limited selectivity for G(o) among G(i)/G(o) proteins. All RZ family members reduced dopamine-D2/Galpha(i)-mediated inhibition of cAMP formation and abolished thyrotropin-releasing hormone receptor/Galpha(q)-mediated calcium mobilization. RGS17 is a new RZ member that preferentially inhibits receptor signaling via G(i/o), G(z), and G(q) over G(s) to enhance cAMP-dependent signaling and inhibit calcium signaling. Differences observed between in vitro GAP assays and whole-cell signaling suggest additional determinants of the G-protein specificity of RGS GAP effects that could include receptors and effectors.

Glomerular territories in the olfactory bulb from the larval stage of the sea lamprey Petromyzon marinus

The goal of this study was to investigate the spatial organization of olfactory glomeruli and of substances relevant to olfactory sensory neuron activity in the developing agnathan, the sea lamprey Petromyzon marinus. A 45-kD protein immunoreactive to G(olf), a cAMP-dependent olfactory G protein, was present in the ciliary fraction of sea lamprey olfactory epithelium and in olfactory sensory neurons of larval and adult sea lampreys. This result implies that G(olf) expression was present during early vertebrate evolution or evolved in parallel in gnathostome and agnathostome vertebrates. Serial sectioning of the olfactory bulb revealed a consistent pattern of olfactory glomeruli stained by GS1B(4) lectin and by anterograde labeling with fluorescent dextran. These glomerular territories included the dorsal cluster, dorsal ring, anterior plexus, lateral chain, medial glomeruli, ventral ring, and ventral cluster. The dorsal, anterior, lateral, and ventral glomeruli contained olfactory sensory axon terminals that were G(olf)-immunoreactive. However, a specific subset, the medial glomeruli, did not display this immunoreactivity. Olfactory glomeruli in the dorsal hemisphere of the olfactory bulb, the dorsal cluster, dorsal ring, anterior plexus, lateral chain, and medial glomeruli, were seen adjacent to 5HT-immunoreactive fibers. However, glomeruli in the ventral hemisphere, the ventral ring, and ventral cluster did not display this association. The presence of specific glomerular territories and discrete glomerular subsets with substances relevant to olfactory sensory neuron activity suggest a spatial organization of information flow in the lamprey olfactory pathway.

Comparative expression of the human β2 and β3 adrenergic receptors in Saccharomyces cerevisiae

The beta(3) adrenergic receptor (beta(3)AR) is the predominant beta subtype in human brown adipocytes and is essential for regulating thermogenic lipolysis. To establish a novel experimental system for the biochemical analysis of this protein, we engineered several yeast strains. We show that the sterol background of the host strain greatly modulates the beta(3)AR expression but not in the same way as it modulates the beta(2) adrenergic receptor (beta(2)AR), the other main studied adipocyte subtype. The human beta(3)AR expressed in yeast is N-glycosylated but not phosphorylated. This latter characteristic distinguishes it from the beta(2)AR. We showed that both beta(2)AR and beta(3)AR follow the secretory pathway to the yeast plasma membrane (PM) and are degraded in the vacuole. In the yeast strains used in this work, the two receptors also share a common mechanism of direct signal transduction through the yeast G(alpha) protein, Gpa1p. These strains thus appear to be useful for biochemical and structural studies of the human beta(3)AR in an in vivo reconstitution system.

A Seven-Transmembrane RGS Protein That Modulates Plant Cell Proliferation

G protein-coupled receptors (GPCRs) at the cell surface activate heterotrimeric G proteins by inducing the G protein alpha (Galpha) subunit to exchange guanosine diphosphate for guanosine triphosphate. Regulators of G protein signaling (RGS) proteins accelerate the deactivation of Galpha subunits to reduce GPCR signaling. Here we identified an RGS protein (AtRGS1) in Arabidopsis that has a predicted structure similar to a GPCR as well as an RGS box with GTPase accelerating activity. Expression of AtRGS1 complemented the pheromone supersensitivity phenotype of a yeast RGS mutant, sst2Delta. Loss of AtRGS1 increased the activity of the Arabidopsis Galpha subunit, resulting in increased cell elongation in hypocotyls in darkness and increased cell production in roots grown in light. These findings suggest that AtRGS1 is a critical modulator of plant cell proliferation.

Gonadotropin-releasing hormone signaling pathways in an experimental ovarian tumor

Previous results showed that GnRH signaling is altered in cells from rat luteinized ovarian tumors (tumor group) because it did not activate the phospholipase C pathway, in contrast to control ovarian cells from superovulated prepubertal rats (SPO). In the present work, alternate GnRH-induced second messengers such as phospholipase A(2) and phospholipase D activation, cAMP production, ERK1/2 phosphorylation, and the presence of G proteins were evaluated to determine GnRH mechanism of action in tumor cells. G proteins examined were present in both cell types. Buserelin, a GnRH agonist, (1, 10, and 100 ng/ml) increased phosphatidylethanol in SPO, indicating phospholipase D activation. Only 100 ng/ml buserelin induced a significant response in the tumor group. Buserelin (100 ng/ml) increased (3)H-arachidonic acid in culture media in SPO, indicating phospholipase A(2) activation; no effect was observed in the tumor group. Buserelin (100 and 1000 ng/ml) induced pertussis toxin-insensitive cAMP increases in both cell types, with similar potencies. In the tumor group, buserelin (100 ng/ml) inhibited human chorionic gonadotropin-induced cAMP and progesterone; this effect was protein kinase C (PKC) dependent (inhibited by GF109203X, a PKC inhibitor). Buserelin (100 and 1000 ng/ml) induced ERK1/2 phosphorylation in both cell kinds. Buserelin-induced ERK1/2 activation was G(i/0) independent and PKC dependent. Only in the tumor group, buserelin-induced ERK1/2 activation was cAMP dependent (abolished by SQ 22536, the adenylyl cyclase inhibitor). Furthermore, dibutyryl cAMP-induced ERK1/2 activation in the tumor group was PKC dependent (inhibited by GF109203X). In conclusion, activation of phospholipases in tumor cells does not seem to mediate GnRH effects. GnRH signaling seems to involve adenylyl cyclase activation, PKC stimulation, and ERK1/2 phosphorylation.

Alkylation of sulfhydryl groups on Galpha(s/olf) subunits by N-ethylmaleimide: regulation by guanine nucleotides

In rat striatum A(2A) adenosine receptors activate adenylyl cyclase through coupling to G(s)-like proteins, mainly G(olf) that is expressed at high levels in this brain region. In this study we report that the sulfhydryl alkylating reagent, N-ethylmaleimide (NEM), causes a concentration- and time-dependent inhibition of [3H] 2-p-(2-carboxyethyl)phenylethylamino)-5'-N-ethylcarboxamido adenosine ([3H]CGS21680) binding to rat striatal membranes. Membrane treatment with [14C]N-ethylmaleimide ([14C]NEM) labels numerous proteins while addition of 5'-guanylylimidodiphosphate (Gpp(NH)p) reduces labeling of only three protein bands that migrate in SDS-polyacrylamide gel electrophoresis with apparent molecular masses of approximately 52, 45 and 39 kDa, respectively. The 52- and 45-kDa labeled bands show electrophoretic motilities as Galpha(s)-long and Galpha(s)-short/Galpha(olf) subunits. An anti-Galpha(s/olf) antiserum immunoprecipitates two 14C labeled bands of 44 and 39 kDa. The band density decreases by 21-26% when membranes are treated with NEM in the presence of Gpp(NH)p. An anti-A(2A) receptor antibody also immunoprecipitates two 14C labeled bands of 40 and 38 kDa, respectively. However, such protein bands do not show any decrease of their density upon membrane treatment with NEM plus Gpp(NH)p. These results indicate that in rat striatal membranes NEM alkylates sulfhydryl groups of both Galpha(s/olf) subunits and A(2A) adenosine receptors. In addition, cysteine residues of Galpha(s/olf) are easily accessible to modification when the subunit is in the GDP-bound form. The 39- and 38-kDa labeled proteins may represent proteolytic fragments of Galpha(s/olf) and A(2A) adenosine receptor, respectively.

Analysis of polymorphisms in the alpha-subunit of the olfactory G-protein Golf in lithium-treated bipolar patients

OBJECTIVE

This study examines the alpha-subunit of the olfactory G-protein (G(olf)) as a possible candidate gene for bipolar disorder. The alpha-subunit of the G(olf) gene maps to a region on chromosome 18p that has been implicated in several linkage studies as a potential site of a bipolar disorder susceptibility loci.

METHODS

We investigated whether two polymorphisms in the alpha-subunit of the G(olf) gene (A-->G in intron 3 and T-->G in intron 10) are associated with bipolar disorder in a sample of 149 bipolar patients under lithium treatment compared with 139 healthy controls using haplotype analysis.

RESULTS

There was no evidence for an association between the investigated polymorphisms in the G(olf) gene and bipolar disorders, as well as to response to lithium treatment or common side effects, like hand tremor, weight gain and cognitive dysfunction.

CONCLUSION

The results of the present study do not support the hypothesis that the G(olf) gene is a major susceptibility factor for bipolar disorders.

Endoplasmic reticulum retention, degradation, and aggregation of olfactory G-protein coupled receptors

The mammalian olfactory G-protein coupled receptor family is comprised of hundreds of proteins that mediate odorant binding and initiate signal transduction cascades leading to the sensation of smell. However, efforts to functionally express olfactory receptors and identify specific odorant ligand-olfactory receptor interactions have been severely impeded by poor olfactory receptor surface expression in heterologous systems. Therefore, experiments were performed to elucidate the cellular mechanism(s) responsible for inefficient olfactory receptor cell surface expression. We determined that the mouse odorant receptors mI7 and mOREG are not selected for export from the ER and therefore are not detectable at the Golgi apparatus or plasma membrane. Specifically, olfactory receptors interact with the ER chaperone calnexin, are excluded from ER export sites, do not accumulate in ER-Golgi transport intermediates at 15 degrees C, and contain endoglycosidase H-sensitive oligosaccharides, consistent with olfactory receptor exclusion from post-ER compartments. A labile pool of ER-retained olfactory receptors are post-translationally modified by polyubiquitination and targeted for degradation by the proteasome. In addition, olfactory receptors are sequestered into ER aggregates that are degraded by autophagy. Collectively, these data demonstrate that poor surface expression of olfactory receptors in heterologous cells is attributable to a combination of ER retention due to inefficient folding and poor coupling to ER export machinery, aggregation, and degradation via both proteasomal and autophagic pathways.

Mechanosensory inputs influence Caenorhabditis elegans pharyngeal activity via ivermectin sensitivity genes

Mechanical stimulation induces opposite behavioral responses in the adult and dauer pharynx. Tail tap of adults inhibits pharyngeal pumping via a pathway involving the innexin gene unc-7 and components of the glutamatergic pathway encoded by the genes avr-14 and avr-15. Tail tap of dauers stimulates pumping through a mechanism involving G alpha o and G alpha q. The nematocidal drug ivermectin is believed to kill worms by opening a glutamate-gated chloride channel (AVR-15) on pharyngeal muscle, causing complete pumping inhibition. However, ivermectin can also inhibit pumping in the absence of this channel. We propose that one of the ways ivermectin could prevent pumping, in the absence of the AVR-15 ivermectin-binding channel on pharynx muscle, is to target AVR-14 and AVR-15, which are expressed in the inhibitory pathway linking mechanosensation and pumping activity.

A reevaluation of the role of the heterotrimeric G protein in coupling light responses in Arabidopsis

Previous studies implicated the involvement of a heterotrimeric G protein in red (R) and far-red (FR) light signal transduction, but these studies utilized pharmacological or gain-of-function approaches and, therefore, are indirect tests. Here, we reexamine the role of the single canonical heterotrimeric G protein in R and FR control of hypocotyl growth using a loss-of-function approach. Single- and double-null mutants for the GPA1, AGB1 genes encoding the alpha and beta subunit of the heterotrimeric G protein, respectively, have wild-type sensitivity to R and FR. Ectopic overexpression of wild type and a constitutive active form of the alpha subunit and of the wild-type beta subunit had no effect that can be unequivocally attributed to altered R and FR responsiveness. These results preclude a direct role for the heterotrimeric G complex in R and FR transduction in Arabidopsis leading to growth control in the hypocotyl.

Structure-function studies of the C3a-receptor: C-terminal serine and threonine residues which influence receptor internalization and signaling

The anaphylatoxic peptide C3a is a pro-inflammatory mediator generated during complement activation, whose specific G protein coupled receptor is expressed on granulocytes, monocytes, mast cells, activated lymphocytes, and in the nervous tissue. We have generated RBL-2H3 cell clones stably expressing mutants of the human C3a-receptor (C3aR) with combined alanine (Ala) substitutions of ten C-terminal serine (Ser) or threonine (Thr) residues, which may represent putative phosphorylation sites to characterize their role in ligand-induced C3aR internalization and signaling. Ser475/479 and Thr480/481 as well as Ser449 seemed not to be involved in ligand-induced receptor internalization. Either directly or by a conformational change they even "inhibit" C3aR internalization. In contrast, mutants with Ala substitutions at Ser465/470 and Thr463/466 were poorly internalized, and Thr463 seemed to be the most important C-terminal Thr or Ser residue directly effecting receptor internalization. However, it is likely that other C3aR regions additionally participate in this negative feed-back mechanism since even mutants with multiple Ala substitutions still internalized to a limited degree. Interestingly, in a mutant with a single exchange of Ser449 to Ala, the signal transduction assessed by a Ca(2+) assay and [(35)S]GTP gamma S-binding on HEK cells transiently co-transfected with G-alpha 16 or G-alpha O, respectively, was severely impaired, indicating that this residue of C3aR is involved in G protein coupling.

Loss of G protein gamma 7 alters behavior and reduces striatal alpha(olf) level and cAMP production

The G protein beta gamma-dimer is required for receptor interaction and effector regulation. However, previous approaches have not identified the physiologic roles of individual subtypes in these processes. We used a gene knockout approach to demonstrate a unique role for the G protein gamma(7)-subunit in mice. Notably, deletion of Gng7 caused behavioral changes that were associated with reductions in the alpha(olf)-subunit content and adenylyl cyclase activity of the striatum. These data demonstrate that an individual gamma-subunit contributes to the specificity of a given signaling pathway and controls the formation or stability of a particular G protein heterotrimer.

Krh1p and Krh2p act downstream of the Gpa2p G(alpha) subunit to negatively regulate haploid invasive growth

The yeast G(alpha) subunit Gpa2p and its coupled receptor Gpr1p function in a signaling pathway that is required for the transition to pseudohyphal and invasive growth. A two-hybrid screen using a constitutively active allele of GPA2 identified the KRH1 gene as encoding a potential binding partner of Gpa2p. Strains containing deletions of KRH1 and its homolog KRH2 were hyper-invasive and displayed a high level of expression of FLO11, a gene involved in pseudohyphal and invasive growth. Therefore, KRH1 and KRH2 encode negative regulators of the invasive growth pathway. Cells containing krh1Delta krh2Delta mutations also displayed increased sensitivity to heat shock and decreased sporulation efficiency, indicating that Krh1p and Krh2p regulate multiple processes controlled by the cAMP/PKA pathway. The krh1Delta krh2Delta mutations suppressed the effect of a gpa2Delta mutation on FLO11 expression and eliminated the effect of a constitutively active GPA2 allele on induction of FLO11 and heat shock sensitivity, suggesting that Krh1p and Krh2p act downstream of Gpa2p. The Sch9p kinase was not required for the signal generated by deletion of KRH1 and KRH2; however, the cAMP-dependent kinase Tpk2p was required for generation of this signal. These results support a model in which activation of Gpa2p relieves the inhibition exerted by Krh1p and Krh2p on components of the cAMP/PKA signaling pathway.

Effect of the pheromone-responsive G(alpha) and phosphatase proteins of Saccharomyces cerevisiae on the subcellular localization of the Fus3 mitogen-activated protein kinase

The mating-specific G(alpha) protein of Saccharomyces cerevisiae, Gpa1, stimulates adaptation to pheromone by a mechanism independent of G(beta gamma) sequestration. Genetic evidence suggests that Gpa1 targets the Fus3 mitogen-activated protein kinase, and it has recently been shown that the two proteins interact in cells responding to pheromone. To test the possibility that Gpa1 downregulates the mating signal by affecting the localization of Fus3, we created a Fus3-green fluorescent protein (GFP) fusion protein. In vegetative cells, Fus3-GFP was found in both the cytoplasm and the nucleus. Pheromone stimulated a measurable increase in the ratio of nuclear to cytoplasmic Fus3-GFP. In contrast, the relative level of nuclear Fus3-GFP decreased as cells recovered from pheromone arrest and did not increase when cells adapted to chronic stimulus were challenged again. Accumulation of Fus3-GFP in the nuclei of stimulated cells was also inhibited by overexpression of either wild-type Gpa1, the E364K hyperadaptive mutant form of Gpa1, or the Msg5 dually specific phosphatase. The effects of Gpa1 and Msg5 on Fus3 are partially interdependent. In a genetic screen for adaptive defective mutants, a nonsense allele of the nucleocytoplasmic transport receptor, Kap104, was identified. Truncation of the Kap104 cargo-binding domain blocked the effect of both Gpa1(E364K) and Msg5 on Fus3-GFP localization. Based on these results, we propose that Gpa1 and Msg5 work in concert to downregulate the mating signal and that they do so by inhibiting the pheromone-induced increase of Fus3 in the nucleus. Kap104 is required for the G(alpha)/phosphatase-mediated effect on Fus3 localization.

The beta-subunit of the Arabidopsis G protein negatively regulates auxin-induced cell division and affects multiple developmental processes

Plant cells respond to low concentrations of auxin by cell expansion, and at a slightly higher concentration, these cells divide. Previous work revealed that null mutants of the alpha-subunit of a putative heterotrimeric G protein (GPA1) have reduced cell division. Here, we show that this prototypical G protein complex acts mechanistically by controlling auxin sensitivity toward cell division. Loss-of-function G protein mutants have altered auxin-mediated cell division throughout development, especially during the auxin-induced formation of lateral and adventitious root primordia. Ectopic expression of the wild-type Galpha-subunit phenocopies the Gbeta mutants (auxin hypersensitivity), probably by sequestering the Gbetagamma-subunits, whereas overexpression of Gbeta reduces auxin sensitivity and a constitutively active (Q222L) mutant Galpha behaves like the wild type. These data are consistent with a model in which Gbetagamma acts as a negative regulator of auxin-induced cell division. Accordingly, basal repression of approximately one-third of the identified auxin-regulated genes (47 of 150 upregulated genes among 8300 quantitated) is lost in the Gbeta transcript-null mutant. Included among these are genes that encode proteins proposed to control cell division in root primordia formation as well as several novel genes. These results suggest that although auxin-regulated cell division is not coupled directly by a G protein, the Gbeta-subunit attenuates this auxin pathway upstream of the control of mRNA steady state levels.

Changes in PKA activity and Gs? and Golf? levels after amphetamine- and cocaine-induced behavioral sensitization

Neuroadaptations in the cAMP signal transduction system have been proposed to play a critical role in psychostimulant-induced behavioral sensitization. Studies relevant to this hypothesis have provided conflicting evidence, however, because repeated cocaine and amphetamine treatment has been alternately reported to either increase or decrease protein kinase A (PKA) activity in the nucleus accumbens and dorsal striatum. In an attempt to reconcile these disparate findings, PKA activity and the stimulatory G-protein subunits, G(salpha) and G(olfalpha), were measured after rats had received a sensitization-inducing regimen of amphetamine or cocaine. Results showed that seven consecutive daily injections of amphetamine or cocaine produced locomotor sensitization and caused a reduction in accumbal PKA activity. A challenge injection of amphetamine on the test day also reduced dorsal striatal PKA activity. G(salpha) levels were unaffected by psychostimulant exposure, while G(olfalpha) levels in the nucleus accumbens declined significantly after amphetamine or cocaine pretreatment. Because repeated amphetamine and cocaine treatment both induced behavioral sensitization and decreased accumbal PKA activity, it appears that psychostimulant-induced increases in PKA activity are not necessary for the ultimate expression of behavioral sensitization.

Association and colocalization of G protein alpha subunits and Purkinje cell protein 2 (Pcp2) in mammalian cerebellum

Previously, we have demonstrated a novel interaction between Galpha(o) protein and Purkinje cell protein-2 (Pcp2, also known as L7) in vitro and in transfected cells (Luo and Denker [1999] J. Biol. Chem. 274:10685-10688). Pcp2 is uniquely expressed in cerebellar Purkinje cells and in retinal bipolar neurons, and it may function as a cell-type specific modulator for G protein-mediated cell signaling. This interaction has been further evaluated in the present studies. Coimmunoprecipitation experiments reveal that Pcp2 associates with Galpha(o) in vivo in mouse cerebellum and eye extract. Pcp2 also associate with Galpha(i2) in the cerebellum. No detectable associations of Pcp2 with Galpha(z) and Galpha(q) subunits are observed. The association of Galpha(o) and Pcp2 is detected at postnatal day 1 (P1), and the association remains stable from day 3 (P3) until adulthood. Further, immunofluorescent double labeling and confocal microscopy suggest that Pcp2 and Galpha(o) are colocalized in the distal processes of cerebellar Purkinje cells including axonal endings and dendritic spines. Taken together, these findings indicate colocalization and association of Galpha(o) and Pcp2 in cerebellum and suggest a functional role in regions of synaptic activity.

Inactivation of Galpha(z) causes disassembly of the Golgi apparatus

We showed previously that overexpression of the alpha subunit of G(z) or G(i2) suppresses nordihydroguaiaretic acid-induced Golgi disassembly. To determine whether the active form of Galpha is required to maintain the structure of the Golgi apparatus, we examined the effects of a series of Galpha GAPs, regulators of G protein signaling (RGS) proteins, on the Golgi structure. Expression of RGSZ1 or RGSZ2, both of which exhibit high selectivity for Galpha(z), markedly induced dispersal of the Golgi apparatus, whereas expression of RGS proteins that are rather selective for Galpha(q) or other Galpha(i) species did not. A mutated RGSZ1, which is deficient in the interaction with Galpha(z), did not induce Golgi disassembly. These results suggest that the active form of Galpha(z), but not Galpha(i2), is crucial for maintenance of the structure of the Golgi apparatus. Consistent with this idea, Golgi disruption also took place in cells transfected with a dominant-negative Galpha(z) mutant. Although previous studies showed that the expression of Galpha(z) is confined to neuronal cells and platelets, immunofluorescence and mRNA expression analyses revealed that it is also expressed, albeit at low levels, in non-neuronal cells, and is located in the Golgi apparatus. These results taken together suggest a general regulatory role for Galpha(z) in the control of the Golgi structure.

Gpa2, a G-protein alpha subunit required for hyphal development in Candida albicans

Candida albicans is able to respond to environmental changes by inducing a distinct morphological program, which is related to the ability to infect mammalian hosts. Although some of the signal transduction pathways involved in this response are known, it is not clear how the environmental signals are sensed and transmitted to these transduction cascades. In this work, we have studied the function of GPA2, a new gene from C. albicans, which encodes a G-protein alpha-subunit homologue. We demonstrate that Gpa2 plays an important role in the yeast-hypha dimorphic transition in the response of C. albicans to some environmental inducers. Deletion of both alleles of the GPA2 gene causes in vitro defects in morphological transitions in Spider medium and SLAD medium and in embedded conditions but not in medium containing serum. These defects cannot be reversed by exogenous addition of cyclic AMP. However, overexpression of HST7, which encodes a component of the filament-inducing mitogen-activated protein kinase (MAPK) cascade, bypasses the Gpa2 requirement. We have obtained different gain-of-function and loss-of-function mutant alleles of the GPA2 gene, which we have introduced in several C. albicans genetic backgrounds. Our results indicate that, in response to environmental cues, Gpa2 is required for the regulation of a MAPK signaling pathway.

Activation of Gz attenuates Rap1-mediated differentiation of PC12 cells

We previously identified a specific activation-dependent interaction between the alpha subunit of the heterotrimeric G protein, G(z), and a regulator of Rap1 signaling, Rap1GAP (Meng, J., Glick, J. L., Polakis, P., and Casey, P. J. (1999) J. Biol. Chem. 274, 36663-36669). We now demonstrate that activated forms of Galpha(z) are able to recruit Rap1GAP from a cytosolic location to the membrane. Using PC12 cells as a model for neuronal differentiation, the influence of G(z) activation on Rap1-mediated cell differentiation was examined. Introduction of constitutively-activated Galpha(z) into PC12 cells markedly attenuated the differentiation process of these cells induced by a cAMP analogue. Treatment of PC12 cells expressing wild type Galpha(z) with a specific agonist to the alpha(2A)-adrenergic receptor also attenuated cAMP-induced PC12 cell differentiation, demonstrating that receptor-mediated activation of G(z) was also effective in this regard. Furthermore, activation of G(z) decreased the ability of the cAMP analogue to trigger both Rap1 and extracellular-regulated kinase (ERK) activation. Differentiation of PC12 cells induced by nerve growth factor (NGF) is also thought to be a Rap1-mediated process, and G(z) activation was found to attenuate this process as well. Rap1 activation, ERK phosphorylation, and PC12 cell differentation induced by NGF treatment were all significantly attenuated by either transfection of constitutively activated Galpha(z) or receptor-mediated G(z) activation. Based on these findings, a model is proposed in which activation of G(z) results in recruitment of Rap1GAP to the membrane where it can effectively down-regulate Rap1 signaling. The implications of these findings in regard to a possible role for G(z) in neuronal development are discussed.

Identity of adenylyl cyclase isoform determines the G protein mediating chronic opioid-induced adenylyl cyclase supersensitivity

To determine the intracellular signal transduction pathway responsible for the development of tolerance/dependence, the ability of Gzalpha to substitute for pertussis toxin (PTX)-sensitive G proteins in mediating adenylyl cyclase (AC) supersensitivity was examined in the presence of defined AC isoforms. In transiently micro-opioid receptor (OR) transfected COS-7 cells (endogenous inhibitory G proteins: Gialpha2, Gialpha3 and Gzalpha), neither acute (1 micro mol/L) nor chronic morphine treatment (1 micromol/L; 18 h) influenced intracellular cAMP production. Coexpression of the micro -OR together with AC type V and VI fully restored the ability of morphine to acutely inhibit cAMP generation. Chronic morphine treatment further resulted in the development of tolerance/dependence, as assessed by desensitization of the acute inhibitory opioid effect (tolerance) as well as the induction of AC supersensitivity after drug withdrawal (dependence). Specific direction of micro -OR signalling via Gzalpha by both PTX treatment and Gzalpha over-expression had no effect on chronic morphine regulation of AC type V, but completely abolished the development of tolerance/dependence with AC type VI. Similar results were obtained in stably micro -OR-expressing HEK293 cells transiently cotransfected with Gzalpha and either AC type V or VI. Coprecipitation studies further verified that Gzalpha specifically binds to AC type V but not type VI. Taken together, these results demonstrate that in principle each of the OR-activated G proteins per se is able to mediate AC supersensitivity. However, they also indicate that it is the molecular nature of AC isoform that selects and determines the OR-activated G protein mediating tolerance/dependence.

A Drosophila dopamine 2-like receptor: Molecular characterization and identification of multiple alternatively spliced variants

Dopamine is an important neurotransmitter in the central nervous system of both Drosophila and mammals. Despite the evolutionary distance, functional parallels exist between the fly and mammalian dopaminergic systems, with both playing roles in modulating locomotor activity, sexual function, and the response to drugs of abuse. In mammals, dopamine exerts its effects through either dopamine 1-like (D1-like) or D2-like G protein-coupled receptors. Although pharmacologic data suggest the presence of both receptor subtypes in insects, only cDNAs encoding D1-like proteins have been isolated previously. Here we report the cloning and characterization of a newly discovered Drosophila dopamine receptor. Sequence analysis reveals that this putative protein shares highest homology with known mammalian dopamine 2-like receptors. Eight isoforms of the Drosophila D2-like receptor (DD2R) transcript have been identified, each the result of alternative splicing. The encoded heptahelical receptors range in size from 461 to 606 aa, with variability in the length and sequence of the third intracellular loop. Pharmacologic assessment of three DD2R isoforms, DD2R-606, DD2R-506, and DD2R-461, revealed that among the endogenous biogenic amines, dopamine is most potent at each receptor. As established for mammalian D2-like receptors, stimulation of the Drosophila homologs with dopamine triggers pertussis toxin-sensitive Gi/o-mediated signaling. The D2-like receptor agonist, bromocriptine, has nanomolar potency at DD2R-606, -506, and -461, whereas multiple D2-like receptor antagonists (as established with mammalian receptors) have markedly reduced if any affinity when assessed at the fly receptor isoforms. The isolation of cDNAs encoding Drosophila D2-like receptors extends the range of apparent parallels between the dopaminergic system in flies and mammals. Pharmacologic and genetic manipulation of the DD2Rs will provide the opportunity to better define the physiologic role of these proteins in vivo and further explore the utility of invertebrates as a model system for understanding dopaminergic function in higher organisms.

mRNA and protein expression of selective alpha subunits of G proteins are abnormal in prefrontal cortex of suicide victims

The present investigation was undertaken to examine whether there is an abnormality in the expression of alpha and beta gamma subunits of G proteins both at the transcriptional and translational level in postmortem brain of adult and teenage suicide subjects and whether these abnormalities are related to mental disorders or suicide per se. In addition, an attempt has been made to investigate whether these abnormalities are similar or dissimilar in teenage and adult suicide, because the etiology of teenage suicide may be different than that of adults.A significant decrease in both mRNA and protein levels of G(i2)alpha and G(O)alpha and a significant increase in levels of G(s)alpha(-S) were observed in prefrontal cortex of suicide subjects (n = 43) compared with non-psychiatric control subjects (n = 38). When subjects were grouped according to age, a significantly decreased expression of G(i2)alpha and G(O)alpha and significantly increased expression of G(s)alpha(-S) were observed in adult suicide subjects (age > or = 20 yrs; n = 20) as compared with age-matched controls (n = 27). These changes were present in all adult suicide subjects regardless of psychiatric diagnosis. On the other hand, although there were no significant differences in any alpha or beta gamma subunits in teenage suicide subjects (age < or = 19 yrs; n = 16) when compared with matched control subjects (n = 18); however, mRNA and protein levels of G(i2)alpha and G(O)alpha were significantly decreased and of G(s)alpha(-S) were significantly increased only in those teenage suicide subjects who had a history of mental illness (n = 11). Our results suggest that there are defects in the expression of selective G protein alpha subunits in prefrontal cortex of adult and teenage suicide subjects, which appear to be related to mental disorders.

The dopamine D1 receptor is a critical mediator for cocaine-induced gene expression

The dopamine D1 receptor plays a major role in mediating behavioral responses to cocaine administration. The time course for the acquisition and the relative stability for the expression of behavioral responses suggest the involvement of enduring neuroadaptations in response to repeated cocaine exposure. Changes in gene expression through the D1 receptors may accompany and mediate the development of such neuroadaptations to repeated cocaine stimulation. To test this possibility, we systematically compared the expression of the fos and Jun family immediate early genes in the nucleus accumbens and caudoputamen in D1 receptor mutant and wild-type control mice after acute and repeated cocaine exposure. Moreover, we compared the expression of three molecules that have been implicated in mediating the actions of cocaine, Galphaolf, beta-catenin and brain-derived neurotrophic factor, in the two groups of mice before and after cocaine administration. We found that there is a lack of induction of c-Fos, FosB, Fra-2 and JunB by acute cocaine exposure, and of DeltaFosB by repeated cocaine administration in both the NAc and CPu of D1 receptor mutant mice compared with wild-type control mice. Moreover, the D1 receptor is differentially required for mediating Galphaolf, beta-catenin and BDNF expression in the NAc and CPu upon cocaine exposure. These results suggest that the D1 receptor is a critical mediator for cocaine-induced expression of these genes.

Distinct interactions of G(salpha-long), G(salpha-short), and G(alphaolf) with GTP, ITP, and XTP

The G(s)-proteins G(salpha-short) (G(salphaS)) and G(salpha-long) (G(salphaL)), and the olfactory G(s) protein (G(alphaolf)) mediate activation of adenylyl cyclase by the beta(2)-adrenoceptor (beta(2)AR). Early studies showed that the purine nucleotides GTP, ITP, and XTP differentially support receptor-mediated adenylyl cyclase activation in various native membrane systems, but those findings have remained unexplained thus far. We systematically analyzed the effects of GTP, ITP, and XTP on the coupling of the beta(2)AR to G(salphaS), G(salphaL), and G(alphaolf), respectively, using fusion proteins expressed in Sf9 insect cells. Fusion proteins ensure defined receptor/G-protein stoichiometry and efficient coupling. At all three fusion proteins, GTP, ITP, and XTP exhibited unique profiles with respect to their potency and efficacy at disrupting high-affinity agonist binding and supporting adenylyl cyclase activation by partial and full agonists. Our data can be interpreted in two ways: (i) GTP, ITP, and XTP may stabilize different active conformations in various G(s)-proteins, or (ii) GTP, ITP, and XTP may differ from one another in the kinetics of interaction with various G(s)-proteins. Regardless of which of the two explanations is correct, our present data demonstrate that GTP, ITP, and XTP are highly efficient regulators of signal transduction mediated through a specific G-protein. Also discussed is the possibility that G-protein activation by ITP and XTP may be of relevance in Lesch-Nyhan syndrome, a defect of the purine salvage pathway associated with abnormalities in various neurotransmitter systems.

Genomic structure and homoeologous relationship of the two alpha-subunit genes of a heterotrimeric GTP-binding protein in tobacco

A heterotrimeric GTP-binding protein (G protein) plays a number of important roles in the signal-transduction pathways of eukaryotic cells. The allotetraploid tobacco genome has two alpha-subunit genes, NtGA1 and NtGA2, of the heterotrimeric G protein. In this study, we determined the nucleotide sequences and the exon-intron structures of the NtGA loci in tobacco and its ancestral diploid species. The genomic sequences of the NtGA loci were interrupted by 13 introns. The sizes of most exons (12 of 14) were completely conserved among the NtGA genes and the Arabidopsis alpha-subunit gene (GPA1), but most introns (11 of 13) in the NtGA genes were longer than those in GPA1. In comparison with the genomic sequences of the NtGA orthologues of ancestral Nicotiana sylvestris and Nicotiana tomentosiformis, the tobacco NtGA1 and NtGA2 were concluded to be homoeologous and assigned to the S and T genomes, respectively. More than 300 mutations including insertions-deletions (indels) and nucleotide substitutions were found in the intron regions between the NtGA1 and NtGA2 loci, whereas the exon sequences were highly conserved among these and GPA1. The structural comparison revealed larger divergence at the NtGA2 locus than at NtGA1.

The Galpha protein Gpa2 controls yeast differentiation by interacting with kelch repeat proteins that mimic Gbeta subunits

G protein coupled receptors (GPCR) sense diverse ligands and signal via heterotrimeric G proteins. The Saccharomyces cerevisiae GPCR Gpr1 senses glucose and controls filamentous growth via an unusual Galpha protein, Gpa2, which lacks any known Gbetagamma subunits. Our genetic and biochemical studies identify Gpa2 interaction partners (Gpb1/2, Gpg1) and provide evidence that these proteins function as G protein subunit mimics and signaling effectors. Gpb1 and Gpb2 lack the seven WD-40 repeats found in Gbeta subunits and instead contain seven kelch repeats implicated in protein-protein interactions. Gbeta subunits and the kelch repeat protein galactose oxidase fold into strikingly similar seven-bladed beta propellers. Our studies demonstrate that Gpa2 signals in conjunction with Gbeta structural mimics and that homologous G protein subunits or effectors may be conserved in multicellular eukaryotes.

Impaired olfactory behavior in mice deficient in the alpha subunit of G(o)

The ability to respond to chemical signals is essential for the survival and reproduction of most organisms. Olfactory signaling involves odorant receptor-mediated activation of G(olf), a homologue of G(s), on the dendrites of olfactory neurons. Olfactory receptor cells, however, also express Galpha(i2) and Galpha(o) on their axons, with all neurons expressing G(o) and a subset G(i2). Despite their abundance, possible contributions of G(o) and G(i2) to chemoreception remain unexplored. We investigated whether homologous recombinant mice deficient in the alpha subunit of G(o) are able to respond to odorants, whether possible olfactory impairments are dependent on genetic background, and whether formation of glomeruli in their olfactory bulbs is compromised. In an olfactory habituation/dishabituation test, G(o)-/- mice were unresponsive when exposed to odorants. Analysis of variance shows that performance of G(o)+/- mice crossed into the CD-1 background is also diminished in this test compared to their G(o)+/+ counterparts. Following food deprivation, G(o)-/- mice in the 129 Sv-ter/C57BL/6 genetic background were unable to locate a buried food pellet until they were approximately 10 weeks of age after which they performed as well as their litter mate controls. However, CD-1 G(o)-/- mice could locate a buried food pellet even when tested immediately after weaning. Despite their compromised olfactory responsiveness, histological examination did not reveal gross alterations in the olfactory bulbs of G(o)-/- mice. Thus, Galpha(o) is necessary for the expression of olfactory behavior under normal conditions and dependent on genetic background, but is not essential for the formation and maintenance of glomeruli.

G-protein alpha(olf) subunit promotes cellular invasion, survival, and neuroendocrine differentiation in digestive and urogenital epithelial cells

The heterotrimeric G-protein subunits Galpha and Gbetagamma are involved in cellular transformation and tumor development. Here, we report the expression of Galpha(olf) in human digestive and urogenital epithelial cells using RT-PCR and Western blot. When the constitutively activated form of Galpha(olf)Q214L (AGalpha(olf)) was stably transfected in canine kidney MDCKts.src and human colonic HCT-8/S11 epithelial cells, it induced cellular invasion in collagen gels. AGalpha(olf)-mediated invasion was abrogated by agonists of platelet activating factor receptors (PAF-R) and protease-activated receptors -1 (PAR-1), pharmacological inhibitors of PI3'-Kinase (wortmannin), protein kinase C (Gö6976 and GF109203X), Rho GTPase (C3T exoenzyme), but was independent of protein kinase A. Accordingly, the invasive phenotype induced by AGalpha(olf) in HCT-8/S11 cells was reversed by the RhoA antagonist RhoD (G26V). Although AGalpha(olf) protected MDCKts.src cells against serum starvation-mediated apoptosis via a Rho-independent pathway, both AGalpha(olf) and Rho inhibition by C3T induced neuroendocrine-like differentiation linked to extensive neurite outgrowth and parathyroid hormone-related protein expression in human prostatic LNCaP-AGalpha(olf) cells. Since prostate tumors with a larger neuroendocrine cell population display increased invasiveness, persistent activation of the G-protein alpha(olf) may exert convergent adverse effects on cellular invasion and survival in solid tumors during the neoplastic progression towards metastasis. doi:10.1038/sj.onc.1205498

Phospholipase C is required for glucose-induced calcium influx in budding yeast

Intracellular calcium is a second messenger involved in several processes in yeast, such as mating, nutrient sensing, stress response and cell cycle events. It was reported that glucose addition stimulates a rapid increase in free calcium level in yeast. To investigate the calcium level variations induced by different stimuli we used a reporter system based on the photoprotein aequorin. Glucose addition (50 mM) to nutrient-starved cells induced an increase in free intracellular calcium concentration, mainly due to an influx from external medium. The increase of calcium reached its maximum 100-120 s after the stimulus. A concentration of about 20 mM glucose was required for a 50% increase in intracellular calcium. This response was completely abolished in strain plc1 Delta and in the isogenic wild-type strain treated with 3-nitrocoumarin, a phosphatidylinositol-specific phospholipase C inhibitor, suggesting that Plc1p is essential for glucose-induced calcium increase. This suggests that Plc1p should have a significant role in transducing glucose signal. The calcium influx induced by addition of high glucose on cells previously stimulated with low glucose levels was inhibited in strains with a deletion in the GPR1 or GPA2 genes, which suggests that glucose would be detected through the Gpr1p/Gpa2p receptor/G protein-coupled (GPCR) complex. Moreover, the signal was completely abolished in a strain unable to phosphorylate glucose, which is consistent with the reported requirement of glucose phosphorylation for GPCR complex activation.

Role of a heterotrimeric G protein in regulation of Arabidopsis seed germination

Seed germination is regulated by many signals. We investigated the possible involvement of a heterotrimeric G protein complex in this signal regulation. Seeds that carry a protein null mutation in the gene encoding the alpha subunit of the G protein in Arabidopsis (GPA1) are 100-fold less responsive to gibberellic acid (GA), have increased sensitivity to high levels of Glc, and have a near-wild-type germination response to abscisic acid and ethylene, indicating that GPA1 does not directly couple these signals in germination control. Seeds ectopically expressing GPA1 are at least a million-fold more responsive to GA, yet still require GA for germination. We conclude that the GPA1 indirectly operates on the GA pathway to control germination by potentiation. We propose that this potentiation is directly mediated by brassinosteroids (BR) because the BR response and synthesis mutants, bri1-5 and det2-1, respectively, share the same GA sensitivity as gpa1 seeds. Furthermore, gpa1 seeds are completely insensitive to brassinolide rescue of germination when the level of GA in seeds is reduced. A lack of BR responsiveness is also apparent in gpa1 roots and hypocotyls suggesting that BR signal transduction is likely coupled by a heterotrimeric G protein at various points in plant development.

Regulation of MAPK function by direct interaction with the mating-specific Galpha in yeast

The mating response of the budding yeast Saccharomyces cerevisiae is mediated by a prototypical heterotrimeric GTP-binding protein (G protein) and mitogen-activated protein kinase (MAPK) cascade. Although signal transmission by such pathways has been modeled in detail, postreceptor down-regulation is less well understood. The pheromone-responsive G protein alpha subunit (Galpha) of yeast down-regulates the mating signal, but its targets are unknown. We have found that Galpha binds directly to the mating-specific MAPK in yeast cells responding to pheromone. This interaction contributes both to modulation of the mating signal and to the chemotropic response, and it demonstrates direct communication between the top and bottom of a Galpha-MAPK pathway.

Direct identification of a G protein ubiquitination site by mass spectrometry

Covalent attachment of ubiquitin is well-known to target proteins for degradation. Here, mass spectrometry was used to identify the site of ubiquitination in Gpa1, the G protein alpha subunit in yeast Saccharomyces cerevisiae. The modified residue is located at Lys165 within the alpha-helical domain of Galpha, a region of unknown function. Substitution of Lys165 with Arg (Gpa1(K165R)) results in a substantial decrease in ubiquitination. In addition, yeast expressing the Gpa1(K165R) mutant are moderately resistant to pheromone in growth inhibition assays-a phenotype consistent with enhanced Galpha signaling activity. These findings indicate that the alpha-helical domain may serve to regulate the turnover of Gpa1.

Analysis of polymorphisms in the olfactory G-protein Golf in major depression

It is well established that G-proteins represent essential regulatory components in transmembrane signaling. The alpha subunit of the olfactory G-protein Golf (GNAL) maps to a region on chromosome 18 where linkage to affective disorders has been reported, as well as a parent-of-origin effect in affective disorders with some markers near the locus for the alpha subunit of the Golf gene. We investigated whether two polymorphisms in the alpha subunit of the Golf gene (A-->G in intron 3, and T-->G in intron 10) are associated with major depression in 176 major depressive patients compared with 145 healthy control subjects, and additionally tested for a parent-of-origin effect in separated gender groups. In the control group, we found a significant increase in the G-allele frequency of the intron 3 polymorphism in females (P=0.0036, odds ratio=2.13, 95% confidence interval=1.29-3.54, Fisher's Exact Test). In patients, we found a similar tendency for higher G-allele frequencies in females. Concerning the intron 10 polymorphism, no differences in the genotype or allele frequencies were detectable for any of the separated gender groups. Also, the total patient and control groups showed no differences in allele or genotype frequencies for any of the investigated polymorphisms. The results of this study agree with the reported parent-of-origin effects on chromosome 18, but do not support the hypothesis that the Golf gene is a major susceptibility factor for major depression.

Activation of adenylyl cyclases, regulation of insulin status, and cell survival by G(alpha)olf in pancreatic beta-cells

Because we recently identified the G(alpha)olf subunit in rat pancreatic beta-cells, we investigated the downstream effectors and the biological functions of this G protein in HEK-293T cells and the insulin-secreting mouse betaTC-3 cell line. With the use of transient transfection of HEK-293T cells with constitutively activated G(alpha)olf (G(alpha)olfQ214L, i.e., AG(alpha)olf), together with expression vectors encoding the adenylyl cyclase (AC) isoforms (AC-I to -VIII and soluble AC), compared with cotransfections using AG(alphas) (G(alphas)R201C), we observed that AG(alpha)olf preferentially activates AC-I and -VIII, which are also expressed in beta-cells. Stable overexpression of wild-type or AG(alpha)olf in betaTC-3 cells resulted in partial attenuation of insulin secretion and biosynthesis, suggesting that chronic activation of the G(alpha)olf-signaling pathway is associated with beta-cell desensitization. In agreement, transfected betaTC-3 cells present a decreased insulin content with respect to parental cells, whereas the proinsulin convertases PC-1 and PC-2 were unaffected. Furthermore, betaTC-3-AG(alpha)olf cells are resistant to serum starvation-induced apoptosis. Our findings suggest that G(alpha)olf is involved in insulin status, cell survival, and regeneration of the insulin-secreting beta-cells during development and diabetes.