Physiological regulation of G protein-linked signaling

Gastrointestinal neuroendocrinology

There exists individual enteroendocrine cells spread throughout the gastrointestinal mucosa that release specific peptide, as well as nonpeptide, hormones to have various endocrine action on target cells bearing cell surface receptors selectively sensitive to these regulatory substances. Following receptor activation, a series of events is set into motion that serves to transduce the information imparted to the target cell. Such transduction mechanisms are numerous, and may be excitatory or inhibitory to the cell depending upon which G-protein subunits the receptor is coupled.

Search for the target genes involved in the suppression of antibody production by TCDD in C57BL/6 mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) suppresses antibody production through activation of a transcription factor, the aryl hydrocarbon receptor (AhR). To explore the genes that are involved in the suppression of antibody production by TCDD, we investigated TCDD-induced changes in gene expression in the CD4 T cells and B cells of C57BL/6 mice immunized with ovalbumin (OVA) plus alum as an adjuvant. Changes in gene expression were analyzed with Affymetrix oligonucleotide microarrays. The results showed that OVA-immunization alone up-regulated expression levels of many genes in the CD4 T cells as early as 3 h after immunization, with 55 up-regulated and 5 down-regulated. At 24 h, 42 genes were found to be up-regulated and 30 down-regulated. Fewer genes were affected in the B cells than in the CD4 T cells. In contrast to the up-regulation of genes induced by immunization in the CD4 T cells, administration of TCDD to mice 3 h prior to the immunization mainly caused down-regulation of genes in the CD4 T cells when compared with immunization alone, with 1 being up-regulated and 4 down-regulated at 3 h after immunization and 3 up-regulated and 34 down-regulated at 24 h. In particular, at 3 and 24 h, TCDD suppressed expression of three and seven genes, respectively, that were up-regulated by immunization. Another characteristic of the TCDD-induced changes in gene expression was the suppression of many genes encoding proteins that are involved in GTP-binding protein-linked signaling in CD4 T cells. These results suggest that the inhibition of immunization-induced gene expression and modulation of G-protein-linked signaling in CD4 T cells are responsible for the TCDD-induced suppression of antibody production.

YM-254890 analogues, novel cyclic depsipeptides with Galpha(q/11) inhibitory activity from Chromobacterium sp. QS3666

The structure elucidation and biological activity of novel YM-254890 (1) analogues and semi-synthetic derivatives are described. Three natural analogues, YM-254891 (2), YM-254892 (3), and YM-280193 (4), were isolated from the fermentation broth of Chromobacterium sp. QS3666, and two hydrogenated derivatives, YM-385780 (5) and YM-385781 (6), were synthesized from YM-254890. Their structures were determined by one- and two-dimensional NMR studies and mass spectrometry. Among these compounds, two natural analogues 2-3 which possessed acyl groups at beta-HyLeu-1 and one derivative 6 whose conformation was similar to that of 1 showed comparable Galpha(q/11) inhibitory activity to that of 1. This indicates that the acyl beta-HyLeu residue plays an important role in activity and also that the alpha,beta-unsaturated carbonyl group of the N-MeDha residue is not critical to activity. The other hydrogenated derivative 5 had significantly less activity, which could be attributed to conformational differences.

Plants: the latest model system for G-protein research

In humans, heterotrimeric G proteins couple stimulus perception by G-protein-coupled receptors (GPCRs) with numerous downstream effectors. By contrast, despite great complexity in their signal-transduction attributes, plants have a simpler repertoire of G-signalling components. Nonetheless, recent studies on Arabidopsis thaliana have shown the importance of plant G-protein signalling in such fundamental processes as cell proliferation, hormone perception and ion-channel regulation.

PGE2 increases the tetrodotoxin-resistant Nav1.9 sodium current in mouse DRG neurons via G-proteins

Inflammation caused by tissue damage results in pain, reflecting an increase in excitability of the primary afferent neurons innervating the area. There is some evidence to suggest that altered function of voltage-gated sodium channels is responsible for the hyperexcitability produced by inflammatory agents, possibly acting through G-proteins, but the role of different channel subtypes has not been fully explored. The tetrodotoxin-resistant (TTX-R) sodium channel Na(v)1.9 is expressed selectively in C- and A-fibre nociceptive-type units and is upregulated by G-protein activation. In this study, we examined the effects of the inflammatory agent prostaglandin-E(2) (PGE(2)) on Na(v)1.9 current in both Na(v)1.8-null and wild-type (WT) mice and explored the role of specific G-proteins in modulation. PGE(2) caused a twofold increase in Na(v)1.9 current (p<0.05) in both systems. Steady-state activation was shifted in a hyperpolarizing direction by 6-8 mV and availability of channels by 12 mV. No differences in the activation and inactivation kinetics could be detected. The increase in current was blocked by pertussis toxin (PTX) but not cholera toxin (CTX), showing involvement of G(i/o) but not G(s) subunits. Our data indicate that Na(v)1.9 current can be increased during inflammation via a G-protein dependent mechanism and suggest that this could contribute to the regulation of electrogenesis in dorsal root ganglia (DRG) neurons.

Desensitization of T lymphocyte function by CXCR3 ligands in human hepatocellular carcinoma

AIM: Despite the presence of lymphocyte infiltration, human hepatocellular carcinoma (HCC) is typically a rapidly progressive disease. The mechanism of regulation of lymphocyte migration is poorly understood. In this study, we investigated various factors regulating T cell migration in HCC patients. We examined serum CXC chemokine levels in HCC patients and demonstrated the production of CXC chemokines by HCC cell lines. We determined the effect of both HCC patient serum and tumor cell conditioned supernatant upon lymphocyte expression of chemokine receptor CXCR3 as well as lymphocyte migration. Lastly, we examined the chemotactic responses of lymphocytes derived from HCC patients.

METHODS: The serum chemokines IP-10 (CXCL10) and Mig (CXCL9) levels were measured by cytometric bead array (CBA) and the tumor tissue IP-10 concentration was measured by ELISA. The surface expression of CXCR3 on lymphocytes was determined by flow cytometry. The migratory function of lymphocytes to the corresponding chemokines was assessed using an in vitro chemotactic assay. Phosphorylation of extracellular signal-regulated kinase (ERK) was determined by Western blot analysis.

RESULTS: Increased levels of IP-10 and Mig were detected in HCC patient serum and culture supernatants of HCC cell lines. The IP-10 concentration in the tumor was significantly higher than that in the non-involved adjacent liver tissues. HCC cell lines secreted functional chemokines that induced a CXCR3-specific chemotactic response of lymphocytes. Furthermore, tumor-cell-derived chemokines induced initial rapid phosphorylation of lymphocyte ERK followed by later inhibition of ERK phosphorylation. The culture of normal lymphocytes with HCC cell line supernatants or medium containing serum from HCC patients resulted in a significant reduction in the proportion of lymphocytes exhibiting surface expression of CXCR3. The reduction in T cell expression of CXCR3 resulted in reduced migration toward the ligand IP-10, and both CD⁴⁺ and CD⁸⁺ T cells from HCC patients exhibited diminished chemotactic responses to IP-10 in vitro compared to T cells from healthy control subjects.

CONCLUSION: This study demonstrates functional desensitization of the chemokine receptor CXCR3 in lymphocytes from HCC patients by CXCR3 ligands secreted by tumor cells. This may cause lymphocyte dysfunction and subsequently impaired immune defense against the tumor.

In vivo imaging of C. elegans ASH neurons: cellular response and adaptation to chemical repellents

ASH sensory neurons are required in Caenorhabditis elegans for a wide range of avoidance behaviors in response to chemical repellents, high osmotic solutions and nose touch. The ASH neurons are therefore hypothesized to be polymodal nociceptive neurons. To understand the nature of polymodal sensory response and adaptation at the cellular level, we expressed the calcium indicator protein cameleon in ASH and analyzed intracellular Ca(2+) responses following stimulation with chemical repellents, osmotic shock and nose touch. We found that a variety of noxious stimuli evoked strong responses in ASH including quinine, denatonium, detergents, heavy metals, both hyper- and hypo-osmotic shock and nose touch. We observed that repeated chemical stimulation led to a reversible reduction in the magnitude of the sensory response, indicating that adaptation occurs within the ASH sensory neuron. A key component of ASH adaptation is GPC-1, a G-protein gamma-subunit expressed specifically in chemosensory neurons. We hypothesize that G-protein gamma-subunit heterogeneity provides a mechanism for repellent-specific adaptation, which could facilitate discrimination of a variety of repellents by these polymodal sensory neurons.

The 15-amino acid motif of the C terminus of the beta2-adrenergic receptor is sufficient to confer insulin-stimulated counterregulation to the beta1-adrenergic receptor

Insulin counterregulates catecholamine action in part by inducing the sequestration of beta2-adrenergic receptors. Although similar to agonist-induced sequestration, insulin-induced internalization of beta2-adrenergic receptors operates through a distinct and better-understood cellular pathway. The effects of insulin treatment on the function and trafficking of both beta1- and beta2-adrenergic receptors were tested. The beta2-adrenergic receptors were counterregulated and internalized in response to insulin. The beta1-adrenergic receptors, in sharp contrast, are shown to be resistant to the ability of insulin to counterregulate function and induce receptor internalization. Using chimeric receptors composed of beta1-/beta2-adrenergic receptors in tandem with mutagenesis, we explored the role of the C-terminal cytoplasmic tail of the beta2-adrenergic receptors for insulin-induced counterregulation. Substitution of the C-terminal cytoplasmic tail of the beta2-adrenergic receptor on the beta1-adrenergic receptor enabled the chimeric G protein-coupled receptor to be functionally and spatially regulated by insulin. Truncation of the beta2-adrenergic receptor C-terminal cytoplasmic tail to a 15-amino acid motif harboring a potential Src homology 2-binding domain at Y350 and an Akt phosphorylation site at S345,346 was sufficient to enable receptor regulation by insulin.

Regulation of secondary metabolism in filamentous fungi

Fungal secondary metabolites are of intense interest to humankind due to their pharmaceutical (antibiotics) and/or toxic (mycotoxins) properties. In the past decade, tremendous progress has been made in understanding the genes that are associated with production of various fungal secondary metabolites. Moreover, the regulatory mechanisms controlling biosynthesis of diverse groups of secondary metabolites have been unveiled. In this review, we present the current understanding of the genetic regulation of secondary metabolism from clustering of biosynthetic genes to global regulators balancing growth, sporulation, and secondary metabolite production in selected fungi with emphasis on regulation of metabolites of agricultural concern. Particularly, the roles of G protein signaling components and developmental regulators in the mycotoxin sterigmatocystin biosynthesis in the model fungus Aspergillus nidulans are discussed in depth.

IRS-1 and Vascular Complications in Diabetes Mellitus

The expected explosive increase in the number of patients with diabetes mellitus will increase the stress on health care. Treatment is focused on preventing vascular complications associated with the disorder. In order to develop better treatment regimens, the field of research has made a great effort in understanding this disorder. This chapter summarizes the current views on the insulin signaling pathway with emphasis on intracellular signaling events associated with insulin resistance, which lead to the prothrombotic condition in the vasculature of patience with diabetes mellitus.

Thyroid hormone regulates Galphai1 gene expression in the rat cerebellar cortex during post-natal development

Thyroid hormone regulates the expression of G protein in tissues such as fat and heart. In the brain, very little information is available relative to the regulation by thyroid hormone of G proteins. Here, we show that the expression of the Galphai1 gene is induced by thyroid hormones in the rat cerebellum during development. Hence, the levels of Galphai1 transcripts and protein were decreased in the cerebellum of hypothyroid neonates. In situ hybridization studies showed that the neurons of the cerebellar cortex, particularly Purkinje cells, were affected. Surprisingly, and in contrast with the in vivo stimulatory effect described above, thyroid hormone repressed the activity of the rat Galphai1 promoter in vitro, suggesting that the effect of this hormone in the cerebellum is indirect. In this regard, we present data suggesting that the transcription factor C/EBPbeta could be implicated. First, there are active CEBP binding sites in the Galphai1 promoter. Second, we have found a diminished DNA binding activity of hypothyroid nuclear proteins to a Galphai1 promoter sequence containing a C/EBP binding site. Third, this complex is likely to contain C/EBPbeta protein as it is displaced by specific anti-C/EBPbeta antibodies. Finally, there is a significant decrease in the C/EBPbeta protein content in the hypothyroid cerebellar cortex.

Plant G proteins, phytohormones, and plasticity: three questions and a speculation

Heterotrimeric guanine nucleotide-binding proteins (G proteins) composed of Galpha, Gbeta, and Ggamma subunits are important transducers of hormonal signals in organisms as evolutionarily distant as plants and humans. The genomes of diploid angiosperms, such as that of the model species Arabidopsis thaliana, encode only single canonical Galpha and Gbeta subunits, only two identified Ggamma subunits, and just one regulator of G protein signaling (RGS) protein. However, a wide range of processes-including seed germination, shoot and root growth, and stomatal regulation-are altered in Arabidopsis and rice plants with mutations in G protein components. Such mutants exhibit altered responsiveness to a number of plant hormones, including gibberellins, brassinosteroids, abscisic acid, and auxin. This review describes possible mechanisms by which such pleiotropic effects are generated and considers possible explanations for why G protein component mutations in plants fail to be lethal. A possible role of G protein signaling in the control of phenotypic plasticity, a hallmark of plant growth, is also discussed.

G protein-coupled receptor 30 down-regulates cofactor expression and interferes with the transcriptional activity of glucocorticoid

G protein-coupled receptor 30 (GPR30) has previously been described to be important in steroid-mediated growth and to inhibit cell proliferation. Here we investigated whether the effect of GPR30 on cell growth is dependent on steroid hormone receptors. We stably introduced GPR30 in immortalized normal mammary epithelial (HME) cells using retroviruses for gene delivery. GPR30 inhibited the growth and proliferation of the cells. They expressed glucocorticoid receptor, but not estrogen or progesterone receptor. GPR30 down-regulated the expression of cofactor transcription intermediary factor 2 (TIF2) analyzed using quantitative RT-PCR analysis, and also diminished the expression of TIF2 at protein level analyzed by Western blotting using nuclear extracts from mammary epithelial cells. When HME cells were transiently transfected with the glucocorticoid response element MMTV-luc reporter plasmid, stable expression of GPR30 resulted in the abolition of ligand-induced transactivation of the promoter. In COS cells, transient transfection of GPR30 with glucocorticoid receptor alpha resulted in an abrogation of the MMTV-luc and GRE-luc reporter activities induced by dexamethasone. The results suggest a novel mechanism by which membrane-initiated signaling interferes with steroid signaling.

Gα13 Signals via p115RhoGEF Cascades Regulating JNK1 and Primitive Endoderm Formation

The heterotrimeric G-protein G(13) mediates the formation of primitive endoderm from mouse P19 embryonal carcinoma cells in response to retinoic acid, signaling to the level of activation of c-Jun N-terminal kinase. The signal linkage map from MEKK1/MEKK4 to MEK1/MKK4 to JNK is obligate in this G alpha(13)-mediated pathway, whereas that between G alpha(13) and MEKKs is not known. The overall pathway to primitive endoderm formation was shown to be inhibited by treatment with Clostridium botulinum C3 exotoxin, a specific inactivator of RhoA family members. Constitutively active G alpha(13) was found to activate RhoA as well as Cdc42 and Rac1 in these cells. Although constitutively active Cdc42, Rac1, and RhoA all can activate JNK1, only the RhoA mutant was able to promote formation of primitive endoderm, mimicking expression of the constitutively activated G alpha(13). Expression of the constitutively active mutant form of p115RhoGEF (guanine nucleotide exchange factor) was found to activate RhoA and JNK1 activities. Expression of the dominant negative p115RhoGEF was able to inhibit activation of both RhoA and JNK1 in response to either retinoic acid or the expression of a constitutively activated mutant of G alpha(13). Expression of the dominant negative mutants of RhoA as well as those of either Cdc42 or Rac1, but not Ras, attenuated G alpha(13)-stimulated as well as retinoic acid-stimulated activation of all three of these small molecular weight GTPases, suggesting complex interrelationships among the three GTPases in this pathway. The formation of primitive endoderm in response to retinoic acid also could be blocked by expression of dominant negative mutants of RhoA, Cdc42, or Rac1. Thus, the signal propagated from G alpha(13) to JNK requires activation of p115RhoGEF cascades, including p115RhoGEF itself, RhoA, Cdc42, and Rac1. In a concerted effort, RhoA in tandem with Cdc42 and Rac1 activates the MEKK1/4, MEK1/MKK4, and JNK cascade, thereby stimulating formation of primitive endoderm.

Lipid raft proteins and their identification in T lymphocytes

This review focuses on how membrane lipid rafts have been detected and isolated, mostly from lymphocytes, and their associated proteins identified. These proteins include transmembrane antigens/receptors, GPI-anchored proteins, cytoskeletal proteins, Src-family protein kinases, G-proteins, and other proteins involved in signal transduction. To further understand the biology of lipid rafts, new methodological approaches are needed to help characterize the raft protein component, and changes that occur in this component as a result of cell perturbation. We describe the application of new proteomic approaches to the identification and quantification of raft proteins in T-lymphocytes. Similar approaches, applied to other model cell systems, will provide valuable new insights into both cellular signal transduction and lipid raft biology.

The Gβγ Dimer Drives the Interaction of Heterotrimeric Gi Proteins with Nonlamellar Membrane Structures

Heterotrimeric G proteins are peripheral membrane proteins that propagate signals from membrane receptors to regulatory proteins localized in distinct cellular compartments. To facilitate signal amplification, G proteins are in molar excess with respect to G protein-coupled receptors. Because G proteins are capable of translocating from membrane to cytosol, protein-lipid interactions play a crucial role in signal transduction. Here, we studied the binding of heterotrimeric G proteins (Galphabetagamma) to model membranes (liposomes) and that of the entities formed upon receptor-mediated activation (Galpha and Gbetagamma). The model membranes used were composed of defined membrane lipids capable of organizing into either lamellar or nonlamellar (hexagonal H(II)) membrane structures. We demonstrated that although heterotrimeric G(i) proteins and Gbetagamma dimers can bind to lipid bilayers of phosphatidylcholine, their binding to membranes was markedly and significantly enhanced by the presence of nonlamellar phases of phosphatidylethanolamine. Conversely, activated G protein alpha subunits showed an opposite membrane binding behavior with a marked preference for lamellar membranes. These results have important consequences in cell signaling. First, the binding characteristics of the Gbetagamma dimer account for the lipid binding behavior and the cellular localization of heterotrimeric G proteins. Second, the distinct protein-lipid interactions of heterotrimeric G proteins, Gbetagamma dimers, and Galpha subunits with membrane lipids explain, in part, their different cellular mobilizations during signaling upon receptor activation. Finally, their differential interactions with lipids suggest an active role of the membrane lipid secondary structure in the propagation of signals through G protein-coupled receptors.

AKAPs (A-kinase anchoring proteins) and molecules that compose their G-protein-coupled receptor signalling complexes

Cell signalling mediated via GPCRs (G-protein-coupled receptors) is a major paradigm in biology, involving the assembly of receptors, G-proteins, effectors and downstream elements into complexes that approach in design 'solid-state' signalling devices. Scaffold molecules, such as the AKAPs (A-kinase anchoring proteins), were discovered more than a decade ago and represent dynamic platforms, enabling multivalent signalling. AKAP79 and AKAP250 were the first to be shown to bind to membrane-embedded GPCRs, orchestrating the interactions of various protein kinases (including tyrosine kinases), protein phosphatases (e.g. calcineurin) and cytoskeletal elements with at least one member of the superfamily of GPCRs, the prototypical beta2-adrenergic receptor. In this review, the multivalent interactions of AKAP250 with the cell membrane, receptor, cytoskeleton and constituent components are detailed, providing a working model for AKAP-based GPCR signalling complexes. Dynamic regulation of the AKAP-receptor complex is mediated by ordered protein phosphorylation.

Kinin receptors in cultured rat microglia

Kinins are produced and act at the site of injury and inflammation in various tissues. They are likely to initiate a particular cascade of inflammatory events, which evokes physiological and pathophysiological responses including an increase in blood flow and plasma leakage. In the central nervous system (CNS), kinins are potent stimulators of the production and release of pro-inflammatory mediators represented by prostanoids and cytotoxins. They are known to induce neural tissue damage. Many of the cytotoxins such as cytokines and free radicals and prostanoids are released from glial cells. Among glial cells, astrocytes and oligodendrocytes are known to possess bradykinin (BK) B(2) receptors that phosphoinositide (PI) turnover and raise intracellular Ca(2+) concentration. The presence of bradykinin receptors in microglia has been of great significance. We recently showed that rat primary microglia express kinin receptors. In resting microglia, B(2) receptors but not B(1) receptors are expressed. When the microglia are activated by bradykinin, B(1) receptors are up-regulated, while B(2) receptors are down-regulated. As observed in other glial cells, electrophysiological measurements suggest that B(2) receptors in phosphoinositide turnover and intracellular Ca(2+) concentration in microglia. Release of cytotoxins is likely consequent upon the activation of BK receptors. Our study provides the first evidence that microglia express functional kinin receptors and suggests that microglia play an important role in CNS inflammatory responses.

Probing receptor structure/function with chimeric G-protein-coupled receptors

Owing its name to an image borrowed from Greek mythology, a chimera is seen to represent a new entity created as a composite from existing creatures or, in this case, molecules. Making use of various combinations of three basic domains of the receptors (i.e., exofacial, transmembrane, and cytoplasmic segments) that couple agonist binding into activation of effectors through heterotrimeric G-proteins, molecular pharmacology has probed the basic organization, structure/function relationships of this superfamily of heptahelical receptors. Chimeric G-protein-coupled receptors obviate the need for a particular agonist ligand when the ligand is resistant to purification or, in the case of orphan receptors, is not known. Chimeric receptors created from distant members of the heptahelical receptors enable new strategies in understanding how these receptors transduce agonist binding into receptor activation and may be able to offer insights into the evolution of G-protein-coupled receptors from yeast to humans.

Extracellular matrix modulates beta2-adrenergic receptor signaling in human airway smooth muscle cells

The airways of patients with chronic asthma commonly develop an element of fixed airway obstruction, which fails to reverse with inhaled beta2-adrenoceptor agonists. Airway remodeling refers to the structural changes of the bronchi in longstanding asthma and is characterized by increased deposition and altered ratios of extracellular matrix (ECM) proteins. We therefore assessed whether ECM proteins alter beta2-adrenoceptor signaling in human airway smooth muscle cells. We report that a fibronectin environment increases responses to beta2-adrenoceptor stimulation, whereas exposure to collagen V or laminin decreases accumulation of the second messenger cyclic AMP when compared with collagens I or IV. These differences are likely to be physiologically significant as they translate into altered phosphorylation of the downstream target VASP. The altered cAMP levels are due to differences in adenylyl cyclase activity, although expression of the relevant isoforms of enzyme appears unaltered. However, inhibition of Galphai abrogates the differences in beta2-adrenoceptor-mediated cAMP accumulation in cells exposed to different matrix factors. The difference in Galphai signaling is not due to altered Galphai expression. We conclude therefore that ECM modulates Galphai activity in human airway smooth muscle cells, and propose that these changes could contribute to the fixed airway obstruction seen in patients with chronic asthma.

Characterization of imido [8-(3)H] guanosine 5'-triphosphate binding sites to rat brain membranes

Besides their well-defined intracellular roles in transmembrane signals transduction, guanine derivatives play important roles by acting from the outside of neural cell membranes. These roles are mediated by two different pool sites in cell membranes: G proteins, which bind to specific (GDP and GTP) intracellular guanine derivatives, and sites that bind to extracellular guanine derivatives. In this study we investigated some methodological characteristics of both guanine derivatives binding sites (intracellular and extracellular) in rat brain neural membranes. By investigating the binding of a poorly hydrolyzed GTP analogue and the adenylate cyclase activity in neural membranes, we observed some distinctiveness of guanine derivatives binding sites: stability to washing procedures (extracellular) and modulation of adenylate cyclase activity (intracellular). These results allow dealing with each site separately, which could be useful for discriminating the roles of extracellular and intracellular guanine derivatives in the central nervous system.

The molecular mechanisms of cellular tolerance to delta-opioid agonists. A minireview

Chronic treatment with deltaopioid agonists, similar to other agonist drugs, causes tolerance. Tolerance is a complex adaptation process that consists of multiple, cellular and neural-system adaptations. Cellular tolerance to delta-opioid agonists involves feedback-regulation of the function, concentration, and localization of the delta-opioid receptors (receptor desensitization) as well as of intracellular effectors (functional desensitization). We are using a recombinant Chinese hamster ovary cell line expressing the human delta-opioid receptors (hDOR/CHO) to investigate the molecular mechanisms of cellular tolerance. We found that the structurally distinct delta-opioid agonists mediate receptor down-regulation by different mechanisms. Thus, truncation of the last 35 C-terminal amino acids of the hDOR completely abolished DPDPE, but not SNC 80-mediated receptor down-regulation. In addition, down-regulation of the wild type-, and the truncated hDORs exhibited different inhibitor sensitivity-profile. Chronic delta-opioid agonist treatment also causes functional desensitization of forskolin-stimulated cAMP formation and cAMP overshoot in the hDOR/CHO cells. We have demonstrated that chronic SNC 80 treatment also causes concurrent phosphorylation of the adenylyl cyclase (AC) VI isoenzyme hDOR/CHO cells. Both AC superactivation and AC VI phosphorylation were SNC 80 dose-dependent, naltrindole-sensitive, and exhibited similar time course-, and protein kinase inhibitor-sensitivity profile. We hypothesize that phosphorylation of AC VI plays an important role in delta-opioid agonist-mediated AC superactivation in hDOR/CHO cells.

Voltage-gated ion channels in nociceptors: modulation by cGMP

In tissue or nerve injury, proinflammatory mediators are released that can modulate a variety of ion channels found in nociceptors. The changes in channel activity, which primarily occurs through changes in intracellular pathways, may lead to the pathological states of hyperalgesia and allodynia. To understand further the regulatory mechanisms underlying the changes in channel activity, we used whole cell patch-clamp recordings from capsaicin-sensitive nociceptive neurons in rat trigeminal ganglion neurons to examine how the cGMP-dependent pathways may regulate ion channel function. Addition of the 8-(4-chlorophenylthio)-3',5' (CPT)-cGMP, a membrane permeant modulator of ion channels, decreased the number of evoked action potentials by 36% and inhibited the tetrodotoxin-resistant (TTX-R) sodium currents and IA potassium currents by 37 and 32%, respectively. Delayed rectifier potassium (IK) currents were unaffected, suggesting that the effects of CPT-cGMP are unlikely to arise from a nonspecific effect on channel activity as a consequence of the adsorption of amphipathic CPT-cGMP molecules to the membrane's bilayer component. This conclusion was reinforced by the lack of changes in gramicidin A channel function in the presence of CTP-cGMP. In summary, the activation of the cGMP-dependent pathways reduces nociceptor excitability, in part, by decreasing the activity of voltage-gated TTX-R sodium channels. This pathway may be a target for efforts to produce selective analgesics.

Characterization of CHO Cells Stably Expressing a Gα16/zChimera for High Throughput Screening of GPCRs

G protein-coupled receptors (GPCRs) are important therapeutic targets for drug discovery. The identification and characterization of new ligands ideally requires the use of high throughput assays that are applicable to all GPCR subtypes. To circumvent the problem of different GPCRs coupling to distinct intracellular second messenger pathways, we describe a new method that uses the chimeric Galpha protein 16z25 to facilitate this process. Stably expressed in Chinese hamster ovary cells, 16z25 allows G(i/o)- and G(s)-coupled receptors to mobilize intracellular Ca(2+) upon agonist stimulation. We have generated nine cell lines each stably expressing 16z25 and a GPCR. All cell lines respond to appropriate agonist stimulation in fluorometric imaging plate reader (FLIPR) assays with robust and potent Ca(2+) mobilization. Several of these lines have been pharmacologically characterized using agonists and antagonists. We also demonstrate that the coexpression of GPCR and 16z25 does not interfere with the receptors' ability to activate endogenous signaling pathways. The ability of 16z25 to functionally mediate the agonist stimulation of a broad spectrum of GPCRs indicates that the use of cell lines stably coexpressing this chimera and GPCRs will simplify the drug screening process and aid in the deorphanization of new receptors.

Involvement of G protein betagamma-subunits in diverse signaling induced by G(i/o)-coupled receptors: study using the Xenopus oocyte expression system

We studied the functions of betagamma-subunits of G(i/o) protein using the Xenopus oocyte expression system. Isoproterenol (ISO) elicited cAMP production and slowly activating Cl(-) currents in oocytes expressing beta(2)-adrenoceptor and the protein kinase A-dependent Cl(-) channel encoded by the cystic fibrosis transmembrane conductance regulator (CFTR) gene. 5-Hydroxytryptamine (5-HT), [d-Ala(2), d-Leu(5)]-enkephalin (DADLE), and baclofen enhanced ISO-induced cAMP levels and CFTR currents in oocytes expressing beta(2)-adrenoceptor-CFTR and 5-HT(1A) receptor (5-HT(1A)R), delta-opioid receptor, or GABA(B) receptor, respectively. 5-HT also enhanced pituitary adenylate cyclase activating peptide (PACAP) 38-induced cAMP levels and CFTR currents in oocytes expressing PACAP receptor, CFTR and 5-HT(1A)R. The 5-HT-induced enhancement of G(s)-coupled receptor-mediated currents was abrogated by pretreatment with pertussis toxin (PTX) and coexpression of G transducin alpha (G(t)alpha). The 5-HT-induced enhancement was further augmented by coexpression of the Gbetagamma-activated form of adenylate cyclase (AC) type II but not AC type III. Thus betagamma-subunits of G(i/o) protein contribute to the enhancement of G(s)-coupled receptor-mediated responses. 5-HT and DADLE did not elicit any currents in oocytes expressing 5-HT(1A)R or delta-opioid receptor alone. They elicited Ca(2+)-activated Cl(-) currents in oocytes coexpressing these receptors with the Gbetagamma-activated form of phospholipase C (PLC)-beta2 but not with PLC-beta1. These currents were inhibited by pretreatment with PTX and coexpression of G(t)alpha, suggesting that betagamma-subunits of G(i/o) protein activate PLC-beta2 and then cause intracellular Ca(2+) mobilization. Our results indicate that betagamma-subunits of G(i/o) protein participate in diverse intracellular signals, enhancement of G(s)-coupled receptor-mediated responses, and intracellular Ca(2+) mobilization.

The endogenous ligand Stunted of the GPCR Methuselah extends lifespan in Drosophila

Many extracellular signals are transmitted to the interior of the cell by receptors with seven membrane-spanning helices that trigger their effects by means of heterotrimeric guanine-nucleotide-binding regulatory proteins (G proteins). These G-protein-coupled receptors (GPCRs) control various physiological functions in evolution from pheromone-induced mating in yeast to cognition in humans. The potential role of the G-protein signalling system in the control of animal ageing has been highlighted by the genetic revelation that mutation of a GPCR encoded by methuselah extends the lifespan of adult Drosophila flies. How methuselah functions in controlling ageing is not clear. A first essential step towards the understanding of methuselah function is to determine the ligands of Methuselah. Here we report the identification and characterization of two endogenous peptide ligands of Methuselah, designated Stunted A and B. Flies with mutations in the gene encoding these ligands show an increase in lifespan and resistance to oxidative stress. We conclude that the Stunted-Methuselah system is involved in the control of animal ageing.

Novel metabotropic glutamate receptor negatively coupled to adenylyl cyclase in cultured rat cerebellar astrocytes

Several excitatory amino acid ligands were found potently to inhibit forskolin-stimulated cAMP accumulation in rat cultured cerebellar astrocytes: L-cysteine sulfinic acid (L-CSA) = L-aspartate > L-glutamate >/= the glutamate uptake inhibitor, L-PDC. This property did not reflect activation of conventional glutamate receptors, since the selective ionotropic glutamate receptor agonists NMDA, AMPA, and kainate, as well as several mGlu receptor agonists [(1S,3R)-ACPD, (S)-DHPG, DCG-IV, L-AP4, L-quisqualate, and L-CCG-I], were without activity. In addition, the mGlu receptor antagonists, L-AP3, (S)-4CPG, Eglu, LY341495, (RS)-CPPG, and (S)-MCPG failed to reverse 30 microM glutamate-mediated inhibitory responses. L-PDC-mediated inhibition was abolished by the addition of the enzyme glutamate-pyruvate transaminase. This finding suggests that the effect of L-PDC is indirect and that it is mediated through endogenously released L-glutamate. Interestingly, L-glutamate-mediated inhibitory responses were resistant to pertussis toxin, suggesting that G(i)/G(o) type G proteins were not involved. However, inhibition of protein kinase C (PKC, either via the selective PKC inhibitor GF109203X or chronic PMA treatment) augmented glutamate-mediated inhibitory responses. Although mGlu3 receptors (which are negatively coupled to adenylyl cyclase) are expressed in astrocyte populations, in our study Western blot analysis indicated that this receptor type was not expressed in cerebellar astrocytes. We therefore suggest that cerebellar astrocytes express a novel mGlu receptor, which is negatively coupled to adenylyl cyclase, and possesses an atypical pharmacological profile.

Clathrin-mediated endocytosis of m3 muscarinic receptors. Roles for Gbetagamma and tubulin

Receptors as well as some G protein subunits internalize after agonist stimulation. It is not clear whether Galpha(q) or Gbetagamma undergo such regulated translocation. Recent studies demonstrate that m3 muscarinic receptor activation in SK-N-SH neuroblastoma cells causes recruitment of tubulin to the plasma membrane. This subsequently transactivates Galpha(q) and activates phospholipase Cbeta1. Interaction of tubulin-GDP with Gbetagamma at the offset of phospholipase Cbeta1 signaling appears involved in translocation of tubulin and Gbetagamma to vesicle-like structures in the cytosol (Popova, J. S., and Rasenick, M. M. (2003) J. Biol. Chem. 278, 34299-34308). The relationship of this internalization to the clathrin-mediated endocytosis of the activated m3 muscarinic receptors or Galpha(q) involvement in this process has not been clarified. To test this, SK-N-SH cells were treated with carbachol, and localization of Galpha(q), Gbetagamma, tubulin, clathrin, and m3 receptors were analyzed by both cellular imaging and biochemical techniques. Upon agonist stimulation both tubulin and clathrin translocated to the plasma membrane and co-localized with receptors, Galpha(q) and Gbetagamma. Fifteen minutes later receptors, Gbetagamma and tubulin, but not Galpha(q), internalized with the clathrin-coated vesicles. Coimmunoprecipitation of m3 receptors with Gbetagamma, tubulin, and clathrin from the cytosol of carbachol-treated cells was readily observed. These data suggested that Gbetagamma subunits might organize the formation of a multiprotein complex linking m3 receptors to tubulin since they interacted with both proteins. Such protein assemblies might explain the dynamin-dependent but beta-arrestin-independent endocytosis of m3 muscarinic receptors since tubulin interaction with dynamin might guide or insert the complex into clathrin-coated pits. This novel mechanism of internalization might prove important for other beta-arrestin-independent endocytic pathways. It also suggests cross-regulation between G protein-mediated signaling and the dynamics of the microtubule cytoskeleton.

Guanine derivatives modulate L-glutamate uptake into rat brain synaptic vesicles

Glutamate uptake into synaptic vesicles is driven by a proton electrochemical gradient generated by a vacuolar H(+)-ATPase and stimulated by physiological concentrations of chloride. This uptake plays an important role in glutamatergic transmission. We show here that vesicular glutamate uptake is selectively inhibited by guanine derivatives, in a time- and concentration-dependent manner. Guanosine, GMP, GDP, guanosine-5'-O-2-thiodiphosphate, GTP, or 5'-guanylylimidodiphosphate (GppNHp) inhibited glutamate uptake in 1.5 and 3 min incubations, however, when incubating for 10 min, only GTP or GppNHp displayed such inhibition. By increasing ATP concentrations, the inhibitory effect of GTP was no longer observed, but GppNHp still inhibited glutamate uptake. In the absence of ATP, vesicular ATPase can hydrolyze GTP in order to drive glutamate uptake. However, 5mM GppNHp inhibited ATP hydrolysis by synaptic vesicle preparations. GTP or GppNHp decreased the proton electrochemical gradient, whereas the other guanine derivatives did not. Glutamate saturation curves were assayed in order to evaluate the specificity of inhibition of the vesicular glutamate carrier by the guanine derivatives. The maximum velocity of the initial rate of glutamate uptake was decreased by all guanine derivatives. These results indicate that, although GppNHp can inhibit ATPase activity, guanine derivatives are more likely to be acting through interaction with vesicular glutamate carrier.

Detection of cannabinoid CB1, adenosine A1, muscarinic acetylcholine, and GABA(B) receptor-dependent G protein activity in transducin-deactivated membranes and autoradiography sections of rat retina

1. Several G-protein-coupled receptors (GPCRs) have been localized to various layers of the vertebrate retina, using autoradiographic and immunohistochemical techniques, but the functional data concerning G protein activation are limited. Here, we establish optimized assay conditions to detect receptor-dependent G protein activity in membranes and tissue sections of the rat retina. 2. Agonist-stimulated [35S]GTPgammaS-binding responses were characterized for the Gi/o-linked adenosine A1, cannabinoid CB1, m2/m4 muscarinic acetylcholine, and GABA(B) receptors. Initial assumption was that G protein activity under "basal conditions" is high due to enrichment and activity of rhodopsin and transducin in this tissue. 3. We found that pretreatment of retina membranes with hydroxylamine (10 mM), a rhodopsin-inactivating drug, substantially (up to 60%) reduced basal G protein activity, thereby improving signal-to-noise ratio to detect agonist-stimulated G protein activation for all studied receptors. [35S]GTPgammaS autoradiography revealed that hydroxylamine specifically reduced basal binding in the transducin-enriched photoreceptor layer. In contrast, hydroxylamine did not affect GPCR signaling in brain membranes, indicating specific action on retinal transducin. 4. For all studied receptors, [35S]GTPgammaS autoradiography allowed localization of G protein activity to different retinal layers, with the bulk of signal detected in the ganglion cell layer. Strongest responses were observed for adenosine and muscarinic receptor agonists. Additional G protein activity was detected in the inner plexiform layer. 5. Responses to all tested agonists were reversed in the presence of appropriate receptor-selective antagonists, indicating receptor-mediated G protein activation.

Protein kinase A regulates AKAP250 (gravin) scaffold binding to the beta2-adrenergic receptor

A-kinase-anchoring protein 250 (AKAP250; gravin) acts as a scaffold that binds protein kinase A (PKA), protein kinase C and protein phosphatases, associating reversibly with the beta(2)-adrenergic receptor. The receptor-binding domain of the scaffold and the regulation of the receptor-scaffold association was revealed through mutagenesis and biochemical analyses. The AKAP domain found in other members of this superfamily is essential for the scaffold-receptor interactions. Gravin constructs lacking the AKAP domain displayed no binding to the receptor. Metabolic labeling studies in vivo demonstrate agonist-stimulated phosphorylation of gravin and enhanced gravin-receptor association. Analysis of the AKAP domain revealed two canonical PKA sites phosphorylated in response to elevated cAMP, blocked by PKA inhibitor, and essential for scaffold-receptor association and for resensitization of the receptor. The AKAP appears to provide the catalytic PKA activity responsible for phosphorylation of the scaffold in response to agonist activation of the receptor as well as for the association of the scaffold with the receptor, a step critical to receptor resensitization.

Oral administration of guanosine impairs inhibitory avoidance performance in rats and mice

Extracellular guanine-based purines, mainly the nucleoside guanosine, have recently been shown to exert neuroprotective effects, which seem to be related to antagonism of the glutamatergic system. In this study, we investigated the effects of acute oral administration of guanosine on inhibitory avoidance task in rats and mice. We also studied its effects on locomotor activity, anxiety-related behaviors and mechanisms of action involving the purinergic system. Guanosine (2.0 and 7.5mg/kg, per os), administered 75min pretraining, dose-dependently impaired retention of the inhibitory avoidance task in rats and mice, an effect not prevented by the adenosine receptor antagonist caffeine. Guanosine presented no effects on locomotor activity and anxiety-related behaviors. This amnesic effect of guanosine may be compatible with inhibition of glutamatergic system and seems to be not mediated by adenosine.

Distinct roles for Src tyrosine kinase in beta2-adrenergic receptor signaling to MAPK and in receptor internalization

G protein-coupled receptors form the largest family of membrane receptors and transmit diverse ligand signals to modulate various cellular responses. After activation by their ligands, some of these G protein-coupled receptors are desensitized, internalized (endocytosed), and down-regulated (degraded). In HEK 293 cells, the G(s)-coupled beta2-adrenergic receptor was postulated to initiate a second wave of signaling, such as the activation of the mitogen-activated protein kinase (MAPK) pathway after the receptor is internalized. The tyrosine kinase c-Src plays a critical role in these events. Here we used mouse embryonic fibroblast (MEF) cells deficient in Src family tyrosine kinases to examine the role of Src in beta2-adrenergic receptor signaling to the MAPK pathway and in receptor internalization. We found that in Src-deficient cells the beta2-adrenergic receptor could activate the MAPK pathway. However, the internalization of beta2-adrenergic receptors was blocked in Src-deficient MEF cells. Furthermore, we observed that in MEF cells deficient in beta-arrestin 2 the internalization of the beta2-adrenergic receptor was impaired, whereas the activation of the MAPK pathway by the beta2-adrenergic receptor was normal. Our data demonstrate that although Src and beta-arrestin 2 play essential roles in beta2-adrenergic receptor internalization, they are not required for the activation of the MAPK pathway by the beta2-adrenergic receptor. In other words, our finding suggests that receptor internalization is not required for beta2-adrenergic receptor signaling to the MAPK pathway in MEF cells.

Trafficking of β2-adrenergic receptors: insulin and β-agonists regulate internalization by distinct cytoskeletal pathways

Insulin and β-adrenergic agonists stimulate a rapid phosphorylation and sequestration of the β2-adrenergic receptors (β2ARs). Although the expectation was that a common pathway would be involved in the trafficking of the β2AR in response to either hormone, studies reported herein show the existence of unique cytoskeletal requirements for internalization/recycling of G-protein-coupled receptors, such as the β2AR. Treatment of human epidermoid carcinoma A431 cells with nocodazole, which binds tubulin monomer in vivo and catalyzes the depolymerization of microtubules, effectively blocks β-adrenergic agonist-induced, but not insulin-induced, sequestration of β2ARs. Treatment with latrunculin-A, an agent that sequesters actin monomer and leads to loss of actin filaments, had no effect on the ability of β-adrenergic agonists to stimulate internalization of β2ARs, but blocked the ability of insulin to stimulate counterregulation of β2ARs via internalization. Although nocodazole had no effect on insulin-stimulated sequestration of β2ARs, the recycling of the internalized receptors to the cell membrane was sensitive to depolymerization of microtubules by this agent. Latrunculin-A, by contrast, blocks the recycling of β2ARs internalized in response to β-agonist, while attenuating recycling of receptors internalized in response to insulin stimulation. These data show the existence of unique cytoskeletal requirements for G-protein-coupled-receptor trafficking in response to agonist compared with a counterregulatory hormone, and for sequestration versus recycling of the receptors to the cell membrane.

N,N′,N′-triacetylglucosamine, an inhibitor of lysozyme, prevents myocardial depression in Escherichia coli sepsis in dogs*

OBJECTIVE

Reversible myocardial depression in sepsis has been ascribed to the release of inflammatory mediators. We recently found that lysozyme c (Lzm-S), consistent with that originating from the spleen, was a mediator of myocardial depression in an Escherichia coli model of septic shock in dogs. We further showed in a right ventricular trabecular (RVT) preparation that Lzm-S's depressant activity could be blocked by N,N',N" triacetylglucosamine (TAC), a competitive inhibitor of Lzm-S. We hypothesized that Lzm-S binds to or cleaves a cardiac membrane glycoprotein, thereby interfering with myocardial contraction in sepsis. In the present study, we examined whether TAC could prevent myocardial depression in an in vivo preparation and whether other related N-acetylglucosamine (NAG) structures could also inhibit Lzm-S's effect in RVT.

DESIGN

Randomized experimental study.

SETTING

University laboratory.

SUBJECTS

Anesthetized, mechanically ventilated dogs.

INTERVENTIONS

We produced sepsis by infusion of E. coli over an approximately 6-hr period.

MEASUREMENTS AND MAIN RESULTS

We examined the effect of TAC on stroke work, our primary index of myocardial function, when treatment was administered before sepsis (pretreatment) and after 1.5 hrs (early treatment study) and 3.5 hrs of sepsis (late treatment study; LTS). In the pretreatment study and early treatment study, myocardial depression would have not yet occurred but would have already been present in the late treatment study. In RVT, we assessed the effect of other NAG oligosaccharides and variants to the NAG structure on Lzm-S's depressant activity. In pretreatment and the early treatment study, TAC prevented the reduction in stroke work observed in nontreated septic groups but did not reverse the reduction found in the late treatment study. In RVT, of the compounds tested, only N,N'-diacetylglucosamine showed an inhibitory effect.

CONCLUSIONS

We found that TAC, a competitive inhibitor of Lzm-S, prevented myocardial depression in experimental sepsis. Only specific NAG structures are inhibitory to Lzm-S's depressant activity. TAC may be useful in attenuating cardiovascular collapse in sepsis.

The Ca2+-sensing receptor couples to Galpha12/13 to activate phospholipase D in Madin-Darby canine kidney cells

The Ca2+-sensing receptor (CaR) couples to multiple G proteins involved in distinct signaling pathways: Galphai to inhibit the activity of adenylyl cyclase and activate ERK, Galphaq to stimulate phospholipase C and phospholipase A2, and Gbetagamma to stimulate phosphatidylinositol 3-kinase. To determine whether the receptor also couples to Galpha12/13, we investigated the signaling pathway by which the CaR regulates phospholipase D (PLD), a known Galpha12/13 target. We established Madin-Darby canine kidney (MDCK) cell lines that stably overexpress the wild-type CaR (CaRWT) or the nonfunctional mutant CaRR796W as a negative control, prelabeled these cells with [3H]palmitic acid, and measured CaR-stimulated PLD activity as the formation of [3H]phosphatidylethanol (PEt). The formation of [3H]PEt increased in a time-dependent manner in the cells that overexpress the CaRWT but not the CaRR796W. Treatment of the cells with C3 exoenzyme inhibited PLD activity, which indicates that the CaR activates the Rho family of small G proteins, targets of Galpha12/13. To determine which G protein(s) the CaR couples to in order to activate Rho and PLD, we pretreated the cells with pertussis toxin to inactivate Galphai or coexpressed regulators of G protein-signaling (RGS) proteins to attenuate G protein signaling (RGS4 for Galphai and Galphaq, and a p115RhoGEF construct containing the RGS domain for Galpha12/13). Overexpression of p115RhoGEF-RGS in the MDCK cells that overexpress CaRWT inhibited extracellular Ca2+-stimulated PLD activity, but pretreatment of cells with pertussis toxin and overexpression of RGS4 were without effect. The involvement of other signaling components such as protein kinase C, ADP-ribosylation factor, and phosphatidylinositol biphosphate was excluded. These findings demonstrate that the CaR couples to Galpha12/13 to regulate PLD via a Rho-dependent mechanism and does so independently of Galphai and Galphaq. This suggests that the CaR may regulate cytoskeleton via Galpha12/13, Rho, and PLD.

Modulation of bone marrow-derived neutrophil signaling by H2O2: disparate effects on kinases, NF-kappaB, and cytokine expression

Reactive oxygen species (ROS), including hydrogen peroxide (H2O2), are generated in increased amounts in pathological, biological processes and can play a role in signal transduction. Neutrophils often accumulate in acute inflammatory reactions, at sites where elevated concentrations of ROS are present. ROS have been demonstrated to participate in the activation of intracellular signaling pathways, including those involved in modulating nuclear accumulation and transcriptional activity of NF-kappaB. However, the role of ROS in affecting such events in neutrophils has not been examined. Using exposure of murine bone marrow neutrophils to H2O2 as a model of oxidative stress, we found both strong and persistent activation of ERK1/2, p38, JNK, and PKB, but not the p21-activated kinase. Stimulating the bone marrow-derived neutrophils with H2O2 did not affect nuclear translocation of NF-kappaB. However, production and secretion of the proinflammatory cytokine TNF-alpha in LPS-stimulated neutrophils were inhibited by H2O2. Exposure of LPS- or TNF-alpha-stimulated neutrophils to H2O2 decreased nuclear translocation of NF-kappaB. LPS-induced activation of the transcriptional factor AP-1 was also inhibited by H2O2. This inhibition of nuclear accumulation of NF-kappaB by H2O2 was not caused by an impaired capacity of LPS to stimulate the IKK pathway or to direct oxidative effects on NF-kappaB but rather reflected diminished degradation of IkappaB-alpha. These results indicate that oxidative stress, despite being able to selectively activate intracellular kinases in bone marrow-derived neutrophils, also inhibits NF-kappaB activation and associated TNF-alpha expression. Such inhibitory effects on neutrophil activation may limit tissue damage produced by oxidative stress.

Melatonin prevents apoptosis and enhances HSP27 mRNA expression induced by heat shock in HL-60 cells: possible involvement of the MT2 receptor

Previous studies have reported that melatonin protects cells and tissues against stressful stimuli. In the present study using HL-60 cells, we show that cells acquire increased resistance to apoptosis normally induced by heat shock when they are incubated with melatonin. This effect of melatonin is saturable at nanomolar concentrations and appears to be mediated by the MT2 subtype melatonin receptor. The high affinity melatonin receptor agonist, 2-iodomelatonin, reproduced the melatonin effect while it was fully blocked by the selective MT2 antagonist 4-phenyl-2-propionamidotetraline. The melatonin response to heat shock-induced apoptosis was pertussis toxin sensitive and, interestingly, the non-selective MT1/MT2 melatonin receptor ligand luzindole was found to display agonistic activity. Furthermore, we provide evidence that melatonin enhanced HSP27 mRNA expression as a result of heat shock - HSP27, is known to play an important role in the defense of cells against apoptosis induced by stressful agents. Together, these results demonstrate that melatonin, likely via receptor mechanisms, interferes with the apoptotic pathway activated by heat shock.

Evidence of altered secondary lymphoid-tissue chemokine responsiveness in Balb/c mice infected with Schistosoma mansoni

To determine the extent to which splenic T cells were affected by Schistosoma mansoni infection, we investigated the ability of the T cells to produce interferon (IFN)-gamma, as well as their chemotactic ability 7 wk PI. In this study, we report that splenic T cells from Balb/c mice with S. mansoni infections were capable of producing levels of IFN-gamma comparable with splenic T cells from naive mice. However, the T cells exhibited altered chemotactic activity, as evidenced by an inability to respond to secondary lymphoid-tissue chemokine (SLC/CCL21). Although no difference in chemokine expression was found between the spleens of infected versus control mice, chemokine production was greater in the livers of infected versus control mice. Collectively, these data indicate that Balb/c mice with 7-wk S. mansoni infection possess splenic T cells with altered chemotactic activity and that the alterations may be a consequence of the granulomatous response in the liver.

The effects of hydrocortisone on rat heart muscarinic and adrenergic alpha 1, beta 1 and beta 2 receptors, propranolol-resistant binding sites and on some subsequent steps in intracellular signalling

Glucocorticoids affect the expression and density of neurotransmitter receptors in many tissues but data concerning the heart are contradictory and incomplete. We injected rats with hydrocortisone for 1-12 days and measured the densities of cardiac muscarinic receptors, alpha(1)-, beta(1)- and beta(2)-adrenoceptors and propranolol-resistant binding sites (formerly assumed to be the putative beta(4)-adrenoceptor). Some aspects of intracellular signalling were also evaluated: we measured adenylyl cyclase activity (basal, isoprenaline- and forskolin-stimulated and carbachol-inhibited), the coupling between muscarinic receptors and G proteins and basal and isoprenaline-stimulated heart rate. The density of cardiac muscarinic receptors increased (in both the atria and the ventricles). The density of beta(1)-adrenoceptors increased in the atria and was little changed in the ventricles. The density of beta(2)-adrenoceptors increased in both the atria and the ventricles. The number of alpha(1)-adrenoceptors decreased initially, followed by a transient increase in the atria and did not change in the ventricles. The density of propranolol-resistant binding sites first increased and then diminished in the atria and did not change in the ventricles. Although there were noticeable changes in receptor densities, the stimulatory and inhibitory effects on adenylyl cyclase, basal and isoprenaline-stimulated heart rate and the coupling between muscarinic receptors and G proteins were not significantly altered. This may indicate that changes in receptor densities might be one of the mechanisms maintaining stable functional output.

G beta gamma mediates the interplay between tubulin dimers and microtubules in the modulation of Gq signaling

Agonist stimulation causes tubulin association with the plasma membrane and activation of PLC beta 1 through direct interaction with, and transactivation of, G alpha q. Here we demonstrate that G beta gamma interaction with tubulin down-regulates this signaling pathway. Purified G beta gamma, alone or with phosphatidylinositol 4,5-bisphosphate (PIP2), inhibited carbachol-evoked membrane recruitment of tubulin and G alpha q transactivation by tubulin. Polymerization of microtubules elicited by G beta gamma overrode tubulin translocation to the membrane in response to carbachol stimulation. G beta gamma sequestration of tubulin reduced the inhibition of PLC beta 1 observed at high tubulin concentration. G beta 1 gamma 2 interacted preferentially with tubulin-GDP, whereas G alpha q was transactivated by tubulin-GTP. Prenylation of the gamma 2 polypeptide was required for G beta gamma/tubulin interaction. Both confocal microscopy and coimmunoprecipitation studies revealed the spatiotemporal pattern of G beta gamma/tubulin interaction during carbachol stimulation of neuroblastoma SK-N-SH cells. In resting cells G beta gamma localized predominantly at the cell membrane, whereas tubulin was found in well defined microtubules in the cytosol. Within 2 min of agonist exposure, a subset of tubulin translocated to the plasma membrane and colocalized with G beta. Fifteen min post-carbachol addition, tubulin and G beta colocalized in vesicle-like structures in the cytosol. G beta/tubulin colocalization increased after pretreatment of cells with the microtubule-depolymerizing agent, colchicine, and was inhibited by taxol. Taxol also inhibited carbachol-induced PIP2 hydrolysis. It is suggested that G beta gamma/tubulin interaction mediates internalization of membrane-associated tubulin at the offset of PLC beta 1 signaling. Newly cytosolic G beta gamma/tubulin complexes might promote microtubule polymerization attenuating further tubulin association with the plasma membrane. Thus G protein-coupled receptors might evoke G alpha and G beta gamma to orchestrate regulation of phospholipase signaling by tubulin dimers and control of cell shape by microtubules.

Proper targeting and activity of a nonfunctioning thyroid-stimulating hormone receptor (TSHr) combining an inactivating and activating TSHr mutation in one receptor

Activating mutations of the thyroid-stimulating hormone receptor (TSHr) have been identified as a cause of toxic adenomas. Germline-inactivating TSHr mutations have been described as a cause of congenital hypothyroidism. The effects of combining activating and inactivating mutations within a single receptor was studied. The double mutant T477I/P639S contained an activating TSHr mutation (P639S) together with an inactivating one (T477I). The other one (I486M/P639S) contained two activating mutations. Constructs were expressed in COS-7 cells and basal and TSH-stimulated cyclic AMP (cAMP) accumulation and inositol phosphate (IP) production were determined. The expression at the cell surface was studied both with binding and fluorescence-activated cell scanning analysis. Our results show that the effect of combining the two activating mutations is an increase in the constitutive activity only for the cAMP pathway and not for the IP pathway suggesting that different mutations result in receptor conformations with different relative abilities to couple to Gs-alpha or Gq-alpha. Surprisingly the double mutant containing the T477I behaves as an activating receptor with constitutive activity both for the cAMP and IP pathways. These data show that an inactive form of the TSHr which is trapped inside a cell after transfection is able to gain the membrane surface when combined with an activated form of the receptor.

Genes and erectile function

PURPOSE OF REVIEW

Few human studies have been performed with specific genetic endpoints coupled with erectile function or dysfunction. Most knowledge of gene expression and the function thereof on penile erection has been acquired in experimental models. The purpose of the present review is to give an overview of the available information obtained in studies of genes or genetic products versus erectile function or dysfunction.

RECENT FINDINGS

The association of, for example, systemic vascular disease with diminished erectile function has brought attention to investigations of the distribution, in men with erectile dysfunction, of some genotype variants proposed to be involved in cardiovascular disease. Altered expression or activities of some smooth muscle regulatory components of the ischaemic, diabetic, or ageing penis have been reported.

SUMMARY

Although penile erection can be considered a polygenic trait, some key effectors for normal erectile function within, for example, the nitric oxide/cyclic guanosine monophosphate pathway may be identified. Findings in future population-based studies may disclose the presence of a particular mutation of a gene or gene variants that may predispose to the development of erectile dysfunction. The exact molecular pathogenesis of erectile dysfunction is not known, and may vary between different forms of erectile dysfunction. With integrated approaches in genetic, molecular, and functional investigations, we can learn more of the impact of a particular genotype on erectile function, and also identify targets for preventive, pharmacological, or molecular measures.

Pharmacological rationale for the use of somatostatin and analogues in portal hypertension

Somatostatin and its analogue octreotide have been used for two decades to treat oesophageal variceal haemorrhage. The drug was introduced because of its capacity to decrease portal venous pressure without major side effects. In clinical trials assessing the efficacy of somatostatin and long-acting analogues in arresting variceal haemorrhage, conflicting results have been obtained. Furthermore, in haemodynamic studies evaluating the effects of somatostatin and analogues in patients with cirrhosis, divergent effects were observed. The main reason for these differences is probably related to different affinities of the drugs for different somatostatin receptor subtypes. The effects of somatostatin and analogues are mediated via five different G-protein coupled receptors (somatostatin receptor subtypes 1-5), which regulate the activity of ion channels (Ca2+, K+, Na+ and Cl-) and enzymes (adenyl cyclase, phospholipase C, phospholipase A2, phosphoinositide 3-kinase and guanylate cyclase) responsible for the synthesis or degradation of intracellular second messengers including cyclic AMP, inositol 1,4,5-trisphosphate, diacylglycerol and cyclic GMP. Despite universal use of somatostatin, the cellular and biochemical mechanisms of its effects in portal hypertension are relatively poorly studied and remain incompletely understood. In this review, we summarize relevant signal transduction of somatostatin and analogues, the haemodynamic effects of the drugs and the possible mechanisms by which these effects are mediated.