Roles of G protein subunits in transmembrane signalling

Protein Kinase A in neurological disorders

Cyclic adenosine 3', 5' monophosphate (cAMP)-dependent Protein Kinase A (PKA) is a multi-functional serine/threonine kinase that regulates a wide variety of physiological processes including gene transcription, metabolism, and synaptic plasticity. Genomic sequencing studies have identified both germline and somatic variants of the catalytic and regulatory subunits of PKA in patients with metabolic and neurodevelopmental disorders. In this review we discuss the classical cAMP/PKA signaling pathway and the disease phenotypes that result from PKA variants. This review highlights distinct isoform-specific cognitive deficits that occur in both PKA catalytic and regulatory subunits, and how tissue-specific distribution of these isoforms may contribute to neurodevelopmental disorders in comparison to more generalized endocrine dysfunction.

Ligand-Dependent Modulation of the Dynamics of Intracellular Loops Dictates Functional Selectivity of 5-HT2AR

The serotonin 2A receptor (5-HT_2AR) subtype of the G protein-coupled receptor (GPCR) family is involved in a plethora of neuromodulatory functions (e.g., neurogenesis, sleep, and cognitive processes). 5-HT_2AR is the target of pharmacologically distinct classes of ligands, binding of which either activate or inactivate the receptor. Although high-resolution structures of 5-HT_2AR as well as several other 5-HT GPCRs provided snapshots of both active and inactive conformational states, these structures, representing a truncated form of the receptor, cannot fully explain the mechanism of conformational transitions during their function. Importantly, biochemical studies have suggested the importance of intracellular loops in receptor functions. In our previous study, a model of the ligand-free form of 5-HT_2AR with the third intracellular loop (ICL3) has been meticulously built. Here, we have investigated the functional regulation of 5-HT_2AR with intact intracellular loops in ligand-free and five distinct ligand-bound configurations using unbiased atomistic molecular dynamics (MD) simulations. The selected ligands belong to either of the full, partial, or inverse agonist classes, which exert distinct pharmacological responses. We have observed significant structural, dynamic, and thermodynamic differences within ligand-bound complexes. Our results revealed, for the first time, that either activation or inactivation of the receptor upon specific ligand binding is primarily achieved through conformational transitions of its second and third intracellular loops (ICL2 and ICL3). A remarkable allosteric cross-talk between the ligand-binding site and the distal intracellular parts of the receptor, where binding of a specific ligand thermodynamically controls (either stabilizes or destabilizes) the intracellular region, consisting of crucial dynamic elements ICL2 and ICL3, and differential conformational transitions of these loops determine ligand-dependent functional selectivity.

Current Methods for Detecting Cell Membrane Transient Interactions

Short-lived cell membrane complexes play a key role in regulating cell signaling and communication. Many of these complexes are formed based on low-affinity and transient interactions among various lipids and proteins. New techniques have emerged to study these previously overlooked membrane transient interactions. Exciting functions of these transient interactions have been discovered in cellular events such as immune signaling, host-pathogen interactions, and diseases such as cancer. In this review, we have summarized current experimental methods that allow us to detect and analyze short-lived cell membrane protein-protein, lipid-protein, and lipid-lipid interactions. These methods can provide useful information about the strengths, kinetics, and/or spatial patterns of membrane transient interactions. However, each method also has its own limitations. We hope this review can be used as a guideline to help the audience to choose proper approaches for studying membrane transient interactions in different membrane trafficking and cell signaling events.

Structural insights into emergent signaling modes of G protein-coupled receptors

G protein-coupled receptors (GPCRs) represent the largest family of cell membrane proteins, with >800 GPCRs in humans alone, and recognize highly diverse ligands, ranging from photons to large protein molecules. Very important to human medicine, GPCRs are targeted by about 35% of prescription drugs. GPCRs are characterized by a seven-transmembrane α-helical structure, transmitting extracellular signals into cells to regulate major physiological processes via heterotrimeric G proteins and β-arrestins. Initially viewed as receptors whose signaling via G proteins is delimited to the plasma membrane, it is now recognized that GPCRs signal also at various intracellular locations, and the mechanisms and (patho)physiological relevance of such signaling modes are actively investigated. The propensity of GPCRs to adopt different signaling modes is largely encoded in the structural plasticity of the receptors themselves and of their signaling complexes. Here, we review emerging modes of GPCR signaling via endosomal membranes and the physiological implications of such signaling modes. We further summarize recent structural insights into mechanisms of GPCR activation and signaling. We particularly emphasize the structural mechanisms governing the continued GPCR signaling from endosomes and the structural aspects of the GPCR resensitization mechanism and discuss the recently uncovered and important roles of lipids in these processes.

Covalently modified carboxyl side chains on cell surface leads to a novel method toward topology analysis of transmembrane proteins

The research on transmembrane proteins (TMPs) is quite widespread due to their biological importance. Unfortunately, only a little amount of structural data is available of TMPs. Since technical difficulties arise during their high-resolution structure determination, bioinformatics and other experimental approaches are widely used to characterize their low-resolution structure, namely topology. Experimental and computational methods alone are still limited to determine TMP topology, but their combination becomes significant for the production of reliable structural data. By applying amino acid specific membrane-impermeable labelling agents, it is possible to identify the accessible surface of TMPs. Depending on the residue-specific modifications, new extracellular topology data is gathered, allowing the identification of more extracellular segments for TMPs. A new method has been developed for the experimental analysis of TMPs: covalent modification of the carboxyl groups on the accessible cell surface, followed by the isolation and digestion of these proteins. The labelled peptide fragments and their exact modification sites are identified by nanoLC-MS/MS. The determined peptides are mapped to the primary sequences of TMPs and the labelled sites are utilised as extracellular constraints in topology predictions that contribute to the refined low-resolution structure data of these proteins.

Ca2+ -depended signaling pathways regulate self-renewal and pluripotency of stem cells

Ca²⁺ -mediated signaling is widely spread in nature and plays critical role in the individual development of various organisms ranging from microorganisms to mammals. In vertebrates, Ca²⁺ is involved in important developmental events: fertilization, body plan establishment, and organogenesis. The two later events are defined by embryonic stem cells (ESCs). ESCs are capable of self-renewal and are pluripotent by nature, that is, can give rise to all types of cells that make up the body. Given the paramount importance of Ca²⁺ signalization in the development, it is therefore not surprising this process also plays role in the biology of stem cells. In this review, we scrutinize the published experimental data on the role of Ca²⁺ ions in embryonic stem cells self-renewal and pluripotency. In line with this, we also discuss possible mechanisms of p53 inhibition as a major hindrance to self-renewal of ESCs. Finally, we argue about the role of G-protein-coupled receptors (GPCRs), the largest family of heteromeric transmembrane receptors, and GPCR-mediated signalization in stem cells, and propose the role for the GPCR-G-protein-PLC-Ca²⁺ -downstream signaling pathway in the regulation of pluripotency of both mouse and human ESCs.

Identification of 15 candidate structured noncoding RNA motifs in fungi by comparative genomics

BACKGROUND

With the development of rapid and inexpensive DNA sequencing, the genome sequences of more than 100 fungal species have been made available. This dataset provides an excellent resource for comparative genomics analyses, which can be used to discover genetic elements, including noncoding RNAs (ncRNAs). Bioinformatics tools similar to those used to uncover novel ncRNAs in bacteria, likewise, should be useful for searching fungal genomic sequences, and the relative ease of genetic experiments with some model fungal species could facilitate experimental validation studies.

RESULTS

We have adapted a bioinformatics pipeline for discovering bacterial ncRNAs to systematically analyze many fungal genomes. This comparative genomics pipeline integrates information on conserved RNA sequence and structural features with alternative splicing information to reveal fungal RNA motifs that are candidate regulatory domains, or that might have other possible functions. A total of 15 prominent classes of structured ncRNA candidates were identified, including variant HDV self-cleaving ribozyme representatives, atypical snoRNA candidates, and possible structured antisense RNA motifs. Candidate regulatory motifs were also found associated with genes for ribosomal proteins, S-adenosylmethionine decarboxylase (SDC), amidase, and HexA protein involved in Woronin body formation. We experimentally confirm that the variant HDV ribozymes undergo rapid self-cleavage, and we demonstrate that the SDC RNA motif reduces the expression of SAM decarboxylase by translational repression. Furthermore, we provide evidence that several other motifs discovered in this study are likely to be functional ncRNA elements.

CONCLUSIONS

Systematic screening of fungal genomes using a computational discovery pipeline has revealed the existence of a variety of novel structured ncRNAs. Genome contexts and similarities to known ncRNA motifs provide strong evidence for the biological and biochemical functions of some newly found ncRNA motifs. Although initial examinations of several motifs provide evidence for their likely functions, other motifs will require more in-depth analysis to reveal their functions.

Structural Insights into GIRK Channel Function

G protein-gated inwardly rectifying potassium (GIRK; Kir3) channels, which are members of the large family of inwardly rectifying potassium channels (Kir1-Kir7), regulate excitability in the heart and brain. GIRK channels are activated following stimulation of G protein-coupled receptors that couple to the G(i/o) (pertussis toxin-sensitive) G proteins. GIRK channels, like all other Kir channels, possess an extrinsic mechanism of inward rectification involving intracellular Mg(2+) and polyamines that occlude the conduction pathway at membrane potentials positive to E(K). In the past 17 years, more than 20 high-resolution atomic structures containing GIRK channel cytoplasmic domains and transmembrane domains have been solved. These structures have provided valuable insights into the structural determinants of many of the properties common to all inward rectifiers, such as permeation and rectification, as well as revealing the structural bases for GIRK channel gating. In this chapter, we describe advances in our understanding of GIRK channel function based on recent high-resolution atomic structures of inwardly rectifying K(+) channels discussed in the context of classical structure-function experiments.

Structure, Function, Pharmacology, and Therapeutic Potential of the G Protein, Gα/q,11

G protein coupled receptors (GPCRs) are one of the major classes of cell surface receptors and are associated with a group of G proteins consisting of three subunits termed alpha, beta, and gamma. G proteins are classified into four families according to their α subunit; Gα_i, Gα_s, Gα_12/13, and Gα_q. There are several downstream pathways of Gα_q of which the best known is upon activation via guanosine triphosphate (GTP), Gα_q activates phospholipase Cβ, hydrolyzing phosphatidylinositol 4,5-biphosphate into diacylglycerol and inositol triphosphate and activating protein kinase C and increasing calcium efflux from the endoplasmic reticulum. Although G proteins, in particular, the Gα_q/11 are central elements in GPCR signaling, their actual roles have not yet been thoroughly investigated. The lack of research of the role on Gα_q/11 in cell biology is partially due to the obscure nature of the available pharmacological agents. YM-254890 is the most useful Gα_q-selective inhibitor with antiplatelet, antithrombotic, and thrombolytic effects. YM-254890 inhibits Gα_q signaling pathways by preventing the exchange of guanosine diphosphate for GTP. UBO-QIC is a structurally similar compound to YM-254890, which can inhibit platelet aggregation and cause vasorelaxation in rats. Many agents are available for the study of signaling downstream of Gα_q/11. The role of G proteins could potentially represent a novel therapeutic target. This review will explore the range of pharmacological and molecular tools available for the study of the role of Gα_q/11 in GPCR signaling.

Regulation of myocardial contractility: Insights from transgenic mice

Myocardial contraction occurs when calcium (Ca(2+)) is released from the sarcoplasmic reticulum, binding troponin C and allowing actin and myosin to cross link. Ca(2+) release and uptake is closely regulated by G protein-coupled β-adrenergic receptors through the action of the second messenger cAMP. An increase in cAMP level leads to phosphorylation of key regulatory proteins affecting intracellular Ca(2+) homeostasis. The β-adrenergic receptors themselves are regulated by a set of specific kinases, termed the G-protein-coupled receptor kinases (GRKs). The study of this complex system in vivo has recently been advanced by the development of transgenic and gene-targeted (knockout) mouse models. Combining transgenic technology with sophisticated physiological measurements of cardiac hemodynamics is an extremely powerful approach to the study of myocardial contractility and its regulation. This review focuses on several recent transgenic mouse models that have increased our understanding of the regulation of cardiac contractility.

Inhibition and Termination of Physiological Responses by GTPase Activating Proteins

Physiological processes are strictly organized in space and time. However, in cell physiology research, more attention is given to the question of space rather than to time. To function as a signal, environmental changes must be restricted in time; they need not only be initiated but also terminated. In this review, we concentrate on the role of one specific protein family involved in biological signal termination. GTPase activating proteins (GAPs) accelerate the endogenously low GTP hydrolysis rate of monomeric guanine nucleotide-binding proteins (GNBPs), limiting thereby their prevalence in the active, GTP-bound form. We discuss cases where defective or excessive GAP activity of specific proteins causes significant alteration in the function of the nervous, endocrine, and hemopoietic systems, or contributes to development of infections and tumors. Biochemical and genetic data as well as observations from human pathology support the notion that GAPs represent vital elements in the spatiotemporal fine tuning of physiological processes.

AtIRE1A/AtIRE1B and AGB1 independently control two essential unfolded protein response pathways in Arabidopsis

The endoplasmic reticulum (ER) has the ability to maintain the balance between demand for and synthesis of secretory proteins. To ensure protein-folding homeostasis in the ER, cells invoke signaling pathways known as the unfolded protein response (UPR). To initiate UPR, yeasts largely rely on a conserved sensor, IRE1. In metazoans, there are at least three independent UPR signalling pathways. Some UPR transducers have been identified in plants, but no genetic interaction among them has yet been examined. The Arabidopsis genome encodes two IRE1 sequence homologs, AtIRE1A and AtIRE1B. Here we provide evidence that AtIRE1A and AtIRE1B have overlapping functions that are essential for the plant UPR. A double mutant of AtIRE1A and AtIRE1B, atire1a atire1b, showed reduced ER stress tolerance and a compromised UPR activation phenotype. We have also established that Arabidopsis AGB1, a subunit of the ubiquitous heterotrimeric GTP-binding protein family, and AtIRE1A/AtIRE1B independently control two essential plant UPR pathways. By demonstrating that atire1a atire1b has a short root phenotype that is enhanced by an agb1 loss-of-function mutation, we have identified a role for UPR transducers in organ growth regulation.

Bothrops jararaca peptide with anti-hypertensive action normalizes endothelium dysfunction involved in physiopathology of preeclampsia

Preeclampsia, a pregnancy-specific syndrome characterized by hypertension, proteinuria and edema, is a major cause of fetal and maternal morbidity and mortality especially in developing countries. Bj-PRO-10c, a proline-rich peptide isolated from Bothrops jararaca venom, has been attributed with potent anti-hypertensive effects. Recently, we have shown that Bj-PRO-10c-induced anti-hypertensive actions involved NO production in spontaneous hypertensive rats. Using in vitro studies we now show that Bj-PRO-10c was able to increase NO production in human umbilical vein endothelial cells from hypertensive pregnant women (HUVEC-PE) to levels observed in HUVEC of normotensive women. Moreover, in the presence of the peptide, eNOS expression as well as argininosuccinate synthase activity, the key rate-limiting enzyme of the citrulline-NO cycle, were enhanced. In addition, excessive superoxide production due to NO deficiency, one of the major deleterious effects of the disease, was inhibited by Bj-PRO-10c. Bj-PRO-10c induced intracellular calcium fluxes in both, HUVEC-PE and HUVEC, which, however, led to activation of eNOS expression only in HUVEC-PE. Since Bj-PRO-10c promoted biological effects in HUVEC from patients suffering from the disorder and not in normotensive pregnant women, we hypothesize that Bj-PRO-10c induces its anti-hypertensive effect in mothers with preeclampsia. Such properties may initiate the development of novel therapeutics for treating preeclampsia.

Artificial membrane-like environments for in vitro studies of purified G-protein coupled receptors

Functional reconstitution of transmembrane proteins remains a significant barrier to their biochemical, biophysical, and structural characterization. Studies of seven-transmembrane G-protein coupled receptors (GPCRs) in vitro are particularly challenging because, ideally, they require access to the receptor on both sides of the membrane as well as within the plane of the membrane. However, understanding the structure and function of these receptors at the molecular level within a native-like environment will have a large impact both on basic knowledge of cell signaling and on pharmacological research. The goal of this article is to review the main classes of membrane mimics that have been, or could be, used for functional reconstitution of GPCRs. These include the use of micelles, bicelles, lipid vesicles, nanodiscs, lipidic cubic phases, and planar lipid membranes. Each of these approaches is evaluated with respect to its fundamental advantages and limitations and its applications in the field of GPCR research. This article is part of a Special Issue entitled: Membrane protein structure and function.

Recent insights into Pasteurella multocida toxin and other G-protein-modulating bacterial toxins

Over the past few decades, our understanding of the bacterial protein toxins that modulate G proteins has advanced tremendously through extensive biochemical and structural analyses. This article provides an updated survey of the various toxins that target G proteins, ending with a focus on recent mechanistic insights in our understanding of the deamidating toxin family. The dermonecrotic toxin from Pasteurella multocida (PMT) was recently added to the list of toxins that disrupt G-protein signal transduction through selective deamidation of their targets. The C3 deamidase domain of PMT has no sequence similarity to the deamidase domains of the dermonecrotic toxins from Escherichia coli (cytotoxic necrotizing factor [CNF]1-3), Yersinia (CNFY) and Bordetella (dermonecrotic toxin). The structure of PMT-C3 belongs to a family of transglutaminase-like proteins, with active site Cys-His-Asp catalytic triads distinct from E. coli CNF1.

Molecular organization of the complex between the muscarinic M3 receptor and the regulator of G protein signaling, Gbeta(5)-RGS7

The complex of the regulator of G protein signaling (RGS), Gbeta(5)-RGS7, can inhibit signal transduction via the M3 muscarinic acetylcholine receptor (M3R). RGS7 consists of three distinct structural entities: the DEP domain and its extension DHEX, the Ggamma-like (GGL) domain, which is permanently bound to Gbeta subunit Gbeta(5), and the RGS domain responsible for the interaction with Galpha subunits. Inhibition of the M3R by Gbeta(5)-RGS7 is independent of the RGS domain but requires binding of the DEP domain to the third intracellular loop of the receptor. Recent studies identified the dynamic intramolecular interaction between the Gbeta(5) and DEP domains, which suggested that the Gbeta(5)-RGS7 dimer could alternate between the "open" and "closed" conformations. Here, we identified point mutations that weaken DEP-Gbeta(5) binding, presumably stabilizing the open state, and tested their effects on the interaction of Gbeta(5)-RGS7 with the M3R. We found that these mutations facilitated binding of Gbeta(5)-RGS7 to the recombinant third intracellular loop of the M3R but did not enhance its ability to inhibit M3R-mediated Ca(2+) mobilization. This led us to the idea that the M3R can effectively induce the Gbeta(5)-RGS7 dimer to open; such a mechanism would require a region of the receptor distinct from the third loop. Indeed, we found that the C-terminus of M3R interacts with Gbeta(5)-RGS7. Truncation of the C-terminus rendered the M3R insensitive to inhibition by wild-type Gbeta(5)-RGS7; however, the open mutant of Gbeta(5)-RGS7 was able to inhibit signaling by the truncated M3R. The GST fusion of the M3R C-tail could not bind to wild-type Gbeta(5)-RGS7 but could associate with its open mutant as well as with the separated recombinant DEP domain or Gbeta(5). Taken together, our data are consistent with the following model: interaction of the M3R with Gbeta(5)-RGS7 causes the DEP domain and Gbeta(5) to dissociate from each other and bind to the C-tail, and the DEP domain also binds to the third loop, thereby inhibiting M3R-mediated signaling.

The Galpha4 G protein subunit interacts with the MAP kinase ERK2 using a D-motif that regulates developmental morphogenesis in Dictyostelium

G protein Galpha subunits contribute to the specificity of different signal transduction pathways in Dictyostelium discoideum but Galpha subunit-effector interactions have not been previously identified. The requirement of the Dictyostelium Galpha4 subunit for MAP kinase (MAPK) activation and the identification of a putative MAPK docking site (D-motif) in this subunit suggested a possible interaction between the Galpha4 subunit and MAPKs. In vivo association of the Galpha4 subunit and ERK2 was demonstrated by pull-down and co-immunoprecipitation assays. Alteration of the D-motif reduced Galpha4 subunit-ERK2 interactions but only slightly altered MAPK activation in response to folate. Expression of the Galpha4 subunit with the altered D-motif in galpha4(-)cells allowed for slug formation but not the morphogenesis associated with culmination. Expression of this mutant Galpha4 subunit was sufficient to rescue chemotactic movement to folate. Alteration of the D-motif also reduced the aggregation defect associated with constitutively active Galpha4 subunits. These results suggest Galpha4 subunit-MAPK interactions are necessary for developmental morphogenesis but not for chemotaxis to folate.

The rapid activation of N-Ras by alpha-thrombin in fibroblasts is mediated by the specific G-protein Galphai2-Gbeta1-Ggamma5 and occurs in lipid rafts

alpha-thrombin is a potent mitogen for fibroblasts and initiates a rapid signal transduction pathway leading to the activation of Ras and the stimulation of cell cycle progression. While the signaling events downstream of Ras have been studied in significant detail and appear well conserved across many species and cell types, the precise molecular events beginning with thrombin receptor activation and leading to the activation of Ras are not as well understood. In this study, we examined the immediate events in the rapid response to alpha-thrombin, in a single cell type, and found that an unexpected degree of specificity exists in the pathway linking alpha-thrombin to Ras activation. Specifically, although IIC9 cells express all three Ras isoforms, only N-Ras is rapidly activated by alpha-thrombin. Further, although several Galpha subunits associate with PAR1 and are released following stimulation, only Galpha(i2) couples to the rapid activation of Ras. Similarly, although IIC9 cells express many Gbeta and Ggamma subunits, only a subset associates with Galpha(i2), and of those, only a single Gbetagamma dimer, Gbeta(1)gamma(5), participates in the rapid activation of N-Ras. We then hypothesized that co-localization into membrane microdomains called lipid rafts, or caveolae, is at least partially responsible for this degree of specificity. Accordingly, we found that all components localize to lipid rafts and that disruption of caveolae abolishes the rapid activation of N-Ras by alpha-thrombin. We thus report the molecular elucidation of an extremely specific and rapid signal transduction pathway linking alpha-thrombin stimulation to the activation of Ras.

Understanding the ligand-receptor-G protein ternary complex for GPCR drug discovery

Understanding the ternary complex between G protein-coupled receptors (GPCRs), cognate G proteins, and their ligands is an important landmark for drug discovery. Yet, little is known about the specific interactions between GPCRs and G proteins. For a better perspective on the ternary complex dynamics, we adapted a beta(2)-adrenergic receptor(beta(2)AR)-tetGs(alpha) reconstitution system and found evidence that for efficient coupling of the beta(2)AR to Gs does not require specific interactions between the betagamma-subunits and the beta(2)AR. Our results demonstrate that specific interactions between betagamma and the beta(2)AR are not required for G protein activation but likely serve to anchor Gs(alpha) to the plasma membrane. Our results also suggests that the advantages of analysis of G protein activation by using beta(2)AR receptor-tetGs(alpha) system in vitro at the close proximity of the receptor may constitute a simple screening system that avoids false positives and potentially adapted to screen drugs for other GPCRs.

The Galphai and Galphaq proteins mediate the effects of melatonin on steroid/thyroid hormone receptor transcriptional activity and breast cancer cell proliferation

Melatonin, via its MT1 receptor, but not the MT2 receptor, can modulate the transcriptional activity of various nuclear receptors - estrogen receptor alpha (ERalpha) and retinoic acid receptor alpha (RARalpha), but not ERbeta- in MCF-7, T47D, and ZR-75-1 human breast cancer cell lines. The anti-proliferative and nuclear receptor modulatory actions of melatonin are mediated via the MT1 G protein-coupled receptor expressed in human breast cancer cells. However, the specific G proteins and associated pathways involved in the nuclear receptor transcriptional regulation by melatonin are not yet clear. Upon activation, the MT1 receptor specifically couples to the G(alphai2), G(alphai3), G(alphaq), and G(alphall) proteins, and via activation of G(alphai2) proteins, melatonin suppresses forskolin-induced 3',5'-cyclic adenosine monophosphate production, while melatonin activation of G(alphaq), is able to inhibit phospholipid hydrolysis and ATP's induction of inositol triphosphate production in MCF-7 breast cancer cells. Employing dominant-negative and dominant-positive) forms of these G proteins, we demonstrate that G(alphai2) proteins mediate the suppression of estrogen-induced ERalpha transcriptional activity by melatonin, while the G(q) protein mediates the enhancement of retinoid-induced RARalpha transcriptional activity by melatonin. However, the growth-inhibitory actions of melatonin are mediated via both G(alphai2) and G(alphaq) proteins.

EBV infection renders B cells resistant to growth inhibition via adenylyl cyclase

Cyclic AMP (cAMP) is an important physiological growth inhibitor of lymphoid cells, and the cAMP/protein kinase A (PKA) pathway is disrupted in several immunological disorders and cancers. Epstein Barr virus (EBV) infection of B lymphocytes is responsible for the development of lymphoproliferative disease as well as certain B-lymphoid malignancies. Here we hypothesized that EBV infection might render B lymphocytes resistant to cAMP/PKA-mediated growth inhibition. To test this, we assessed the growth-inhibitory response of cAMP-elevating compounds such as forskolin and isoproterenol, as well as the PKA activator 8-CPT-cAMP in normal B lymphocytes, EBV-infected B cells and in the EBV-negative B lymphoid cell line Reh. We could demonstrate that EBV infection indeed abolished cAMP-mediated growth inhibition of B cells. The defect was pinpointed to defective adenylyl cyclase (AC) activation by forskolin and isoproterenol, resulting in reduced formation of cAMP and lack of PKA activation and CREB phosphorylation. In contrast, 8-CPT-cAMP which directly activates PKA was able to inhibit EBV-infected B cell growth. The physiological implications of these results were underlined by the observation that the ability of forskolin to inhibit camptothecin-induced apoptosis was abolished in EBV-infected B cells. We conclude that EBV infection of B cells abrogates the activation of AC and thereby cAMP formation, and that this dysfunction renders the cells resistant to growth inhibition via the cAMP/PKA pathway.

The anti-secretory and anti-ulcer activities of esomeprazole in comparison with omeprazole in the stomach of rats and rabbits

Proton pump inhibitors (PPIs) are widely used to treat hyperacid secretion and stomach ulcers. The study investigated the anti-secretory and anti-ulcer effects of esomeprazole, the S-isomer of omeprazole on dimaprit, histamine and dibutyryl adenosine 3, 5 cyclic monophosphate (dbcAMP)-evoked gastric acid secretion, acidified ethanol (AE) and indomethacin (INDO)-induced haemorrhagic lesions and on prostaglandin E2 (PGE2) level in the rat in vivo and rabbit in vitro preparations. The effect of omeprazole was also investigated for comparison. Dimaprit-induced acid secretion was significantly (P < 0.05) inhibited by both PPIs in a dose-dependent manner. In the isolated rabbit gastric glands, both PPIs elicited marked reductions in histamine- and dbcAMP-evoked acid secretion with similar potency. The lesions induced by either AE or INDO were significantly (P < 0.05) reduced in the presence of either esomeprazole or omeprazole compared to control values. Increasing doses of esomeprazole before AE treatment resulted in a marked degree of cytoprotection and an elevation in the concentration of bound PGE2 in the stomach tissue homogenate. The results show that esomeprazole and omeprazole were equally effective against gastric haemorrhagic lesions induced by either AE or INDO and in inhibiting dimaprit-, dbcAMP- and histamine-induced gastric acid secretion in the rat and rabbit stomach both in vivo and in vitro. The gastro-protective effect of esomeprazole was found to be proportional to the bound PGE2 levels in the glandular area of the stomach.

Identification of a protein hydrolysate responsive G protein-coupled receptor in enterocytes

G protein-coupled receptors (GPCRs) have the potential to play a role as molecular sensors responsive to luminal dietary contents. Although such a role for GPCRs has been implicated in the intestinal response to protein hydrolysate, no GPCR directly involved in this process has been previously identified. In the present study, for the first time, we identified GPR93 expression in enterocytes and demonstrated its activation in these cells by protein hydrolysate with EC50 of 10.6 mg/ml as determined by the induction of intracellular free Ca2+. In enterocytes, GPR93 was synergistically activated by protein hydrolysate in combination with an agonist, oleoyl-l-alpha-lysophosphatidic acid (LPA), which activated the receptor in these enterocytes with EC50 of 7.9 nM. The increased intracellular Ca2+ by GPR93 activation was observed without the addition of a promiscuous Galpha protein and was pertussis toxin sensitive, which suggests Galpha(q)- and Galpha(i)-mediated pathways. Activated GPR93 also induced pertussis toxin-sensitive ERK1/2 phosphorylation. Both nuclear factor of activated T cells and 12-O-tetradecanoylphorbol 13-acetate responsive elements reporter activities were induced by protein hydrolysate in cells exogenously expressing GPR93. The peptidomimetic cefaclor by itself did not activate GPR93 but potentiated the protein hydrolysate response and further amplified the synergistic enhancement of GPR93 activation by protein hydrolysate and LPA. These data suggest that, physiologically, the composition of stimuli might determine GPR93 activity or its sensitivity toward a given activator and suggest a new mechanism of the regulation of mucosal cell proliferation and differentiation and hormonal secretion by dietary products in the lumen.

G proteins in development

The focus of developmental biologists has expanded from the analysis of gene expression to include the analysis of cell signalling. Heterotrimeric G proteins (G proteins) mediate signalling from a superfamily of heptahelical receptors (G-protein-coupled receptors) to a smaller number of effector units that include adenylyl cyclases, phospholipase C and various ion channels. The convergence of developmental biology with cell signalling has now revealed overlaps in which G proteins mediate complex pathways in embryonic development.

Isoproterenol induces actin depolymerization in human airway smooth muscle cells via activation of an Src kinase and GS

In a previous study, we showed that isoproterenol induced actin depolymerization in human airway smooth muscle cells by both protein kinase A (PKA)-dependent and -independent signaling pathways. We now investigate the signaling pathway of PKA-independent actin depolymerization induced by isoproterenol in these cells. Cells were briefly exposed to isoproterenol or PGE1 in the presence and absence of specific inhibitors of Src-family tyrosine kinases, phosphatidylinositol-3-kinase (PI3 kinase), or MAP kinase, and actin depolymerization was measured by concomitant staining of filamentous actin with FITC-phalloidin and globular actin with Texas red DNase I. Isoproterenol, cholera toxin, and PGE1 induced actin depolymerization, indicated by a decrease in the intensity of filamentous/globular fluorescent staining. Pretreatment with the Src kinase inhibitors 4-amino-5-(4-chlorophenyl)-7-( t-butyl)pyrazolo[3,4-d]pyriimidine (PP2) or geldanamycin or the PKA inhibitor Rp-cAMPS only partly inhibited isoproterenol- or PGE1-induced actin depolymerization. In contrast, PP2 and geldanamycin did not inhibit forskolin-induced actin depolymerization, and AG-213 (an EGF receptor tyrosine kinase inhibitor) did not inhibit isoproterenol- or PGE1-induced actin depolymerization. PI3 kinase or MAP kinase inhibition did not inhibit isoproterenol-induced actin depolymerization. Moreover, isoproterenol but not forskolin induced tyrosine phosphorylation of an Src family member at position 416. These results further confirm that both PKA-dependent and PKA-independent pathways mediate actin depolymerization in human airway smooth muscle cells and that the PKA-independent pathway by which isoproterenol induces actin depolymerization in human airway smooth muscle cells involves Src protein tyrosine kinases and the Gs protein.

Leukotriene D4 activates distinct G-proteins in intestinal epithelial cells to regulate stress fibre formation and to generate intracellular Ca2+ mobilisation and ERK1/2 activation

We have shown that the pro-inflammatory mediator LTD4, via its G-protein-coupled receptor CysLT1, signals through both pertussis-toxin-sensitive and -insensitive G-proteins to induce various cellular responses. To further characterise the initial step of the different signalling pathways emanating from the CysLT1 receptor, we transfected intestinal epithelial cells, Int 407, with different mini vectors that each express a specific inhibitory peptide directed against a unique alpha subunit of a specific heterotrimeric G-protein. Our results revealed that LTD4-induced stress fibre formation is inhibited approximately 80% by a vector expressing an inhibitory peptide against the pertussis-toxin-insensitive Galpha12-protein in intestinal epithelial Int 407 cells. Control experiments revealed that the LPA-induced stress fibre formation, mediated via the Galpha12-protein in other cell types, was blocked by the same peptide in intestinal Int 407 cells. Furthermore, the CysLT1-receptor-mediated calcium signal and activation of the proliferative ERK1/2 kinase are blocked in cells transfected with a vector expressing an inhibitory peptide against the Galphai3-protein, whereas in cells transfected with an empty ECFP-vector or vectors expressing inhibitory peptides against the Galphai1-2-, Galpha12-, GalphaR-proteins these signals are not significantly affected. Consequently, the CysLT1 receptor has the capacity to activate at least two distinctly different heterotrimeric G-proteins that transduce activation of unique downstream cellular events.

Volatile anesthetics inhibit calcitonin gene-related peptide receptor-mediated responses in pithed rats and human neuroblastoma cells

Calcitonin gene-related peptide (CGRP) has a potent vasodilatory effect that is mediated by specific receptors predominantly coupled to the activation of adenylate cyclase. The effects of volatile anesthetics on CGRP-induced vasodilation are unclear. We studied the effects of sevoflurane and isoflurane on CGRP-induced vasodilation in pithed rats and CGRP receptor-mediated responses in SK-N-MC cells, which are used as a model system to study the CGRP receptor and its downstream pathways. Male Wistar rats were pithed by inserting a stainless steel rod into the spinal cord. Mean arterial pressure (MAP) and cardiac output were maintained at approximately 100 mmHg and 50 ml.min(-1), respectively, with continuous infusion of noradrenaline. After 30 min of inhalation of anesthetics, CGRP (0.1, 0.3, 1.0, and 3.0 microg/kg) was administered intravenously. In SK-N-MC cells, CGRP-, forskolin-, or cholera toxin-induced cAMP production was measured with or without anesthetics using radioimmunoassays. CGRP receptor binding density and affinity for the agonist were determined with (2-[125I]iodohystidyl10) CGRP with or without the anesthetics. Sevoflurane (4%) and isoflurane (2%) significantly inhibited the decrease in MAP and systemic vascular resistance. Furthermore, both anesthetics significantly inhibited CGRP- but not forskolin-induced cAMP production. Sevoflurane (4%) and isoflurane (4%) significantly inhibited cholera toxin-induced cAMP production. Both anesthetics did not affect ligand binding. These data suggest that sevoflurane and isoflurane inhibit CGRP-induced vasodilation at the site between the CGRP receptor and adenylate cyclase activation. The inhibitory site of volatile anesthetics on the CGRP receptor-mediated response involves Gs protein.

Critical determinants of the G protein gamma subunits in the Gbetagamma stimulation of G protein-activated inwardly rectifying potassium (GIRK) channel activity

The betagamma subunits of G proteins modulate inwardly rectifying potassium (GIRK) channels through direct interactions. Although GIRK currents are stimulated by mammalian Gbetagamma subunits, we show that they were inhibited by the yeast Gbetagamma (Ste4/Ste18) subunits. A chimera between the yeast and the mammalian Gbeta1 subunits (ymbeta) stimulated or inhibited GIRK currents, depending on whether it was co-expressed with mammalian or yeast Ggamma subunits, respectively. This result underscores the critical functional influence of the Ggamma subunits on the effectiveness of the Gbetagamma complex. A series of chimeras between Ggamma2 and the yeast Ggamma revealed that the C-terminal half of the Ggamma2 subunit is required for channel activation by the Gbetagamma complex. Point mutations of Ggamma2 to the corresponding yeast Ggamma residues identified several amino acids that reduced significantly the ability of Gbetagamma to stimulate channel activity, an effect that was not due to improper association with Gbeta. Most of the identified critical Ggamma residues clustered together, forming an intricate network of interactions with the Gbeta subunit, defining an interaction surface of the Gbetagamma complex with GIRK channels. These results show for the first time a functional role for Ggamma in the effector role of Gbetagamma.

Toll-like receptor-mediated tumor necrosis factor and interleukin-10 production differ during systemic inflammation

Major trauma is associated with a decreased capacity of patients' leukocytes to produce proinflammatory cytokines on in vitro stimulation. We studied leukocytes from 48 patients with trauma and showed that this hyporeactivity was restricted to gram-negative bacteria, Escherichia coli endotoxin, and unmethylated bacterial DNA, whereas Leptospira interrogans endotoxin-induced tumor necrosis factor production was similar to that observed with healthy donors. As well, tumor necrosis factor and interleukin-6 production in response to gram-positive bacteria was not altered. The expression of toll-like receptor (TLR) 2 was not reduced on patients' monocytes as compared with healthy control subjects, whereas that of TLR4 was reduced. However, the hyporeactivity to gram-negative bacteria and E. coli endotoxin cannot be fully explained by the downregulation of TLR4. Indeed, unlike proinflammatory cytokines, after stimulation with these microbial products the release of antiinflammatory cytokines was increased as compared with healthy control subjects. The increased interleukin-10 production was analyzed in terms of intracellular signaling in peripheral blood mononuclear cells from patients with trauma: our results suggest the involvement of p38 mitogen-activated protein kinase, Sp-1 transcription factor, heterotrimeric Gi protein, and phosphatidylinositol-3'-kinase. In conclusion, the immunodysregulation described for patients with trauma is not a generalized phenomenon but depends on the stimulus and the signaling pathway.

Involvement of G(q/11) in signal transduction in the mammalian vomeronasal organ

Social behaviors of most mammals are profoundly affected by pheromones. Pheromones are detected by G-protein coupled receptors in the vomeronasal organ (VNO). To investigate the role of G alpha(q/11) in vomeronasal signal transduction pathways, microvillar membranes from murine VNO were prepared. Incubation of such membranes from prepubertal females with adult male urine results in an increase in production of inositol-(1,4,5)-trisphosphate (IP(3)). This stimulation is mimicked by GTP gamma S, blocked by GDP beta S and is tissue specific. Furthermore, use of bacterial toxins such as pertussis that lead to ADP-ribosylation of the G-protein alpha subunits of G(o) and G(i2) do not block the increase in IP(3) levels but U-73122, a PLC inhibitor, blocks the production of IP(3). Studies with monospecific antibodies revealed the presence of three G-proteins, G alpha(o), G alpha(i2) and G alpha(q/11)-related protein, in vomeronasal neurons, concentrated on their microvilli. Our observations indicate that pheromones in male urine act on vomeronasal neurons in the female VNO via a receptor-mediated, G alpha(q/11)-protein-dependent increase in IP(3) levels.

G protein beta3 gene variant, vascular function, and insulin sensitivity in type 2 diabetes

A common polymorphism (825 C/T) in exon 10 of the GNB3 gene, that encodes for the beta-3 subunit, has been associated with different degrees of activation of heterotrimeric guanine nucleotide binding proteins (G proteins). Many hormones and neurotransmitters use specific receptors that interact noncovalently with G proteins in the transmembrane signaling process. Among them, insulin uses an inhibitory G protein-sensitive mechanism that is involved in metabolic and vascular events, leading to enhanced glucose transport and vasodilation. We hypothesized differences in peripheral and vascular insulin sensitivity according to GNB3 gene polymorphism in type 2 diabetic patients. To address this issue, we used an intervention-optimization protocol to examine whether diabetic patients with the variant show a different response in terms of insulin-sensitivity. Interindividual differences in baseline insulin sensitivity and vascular dysfunction (vasodilatory response to glyceryl trinitrate) were not attributable to this polymorphism of the GNB3 gene. However, in contrast to normal homozygotes, insulin sensitivity (S(I)) significantly improved (P=0.01) in carriers of the 825T variant. Parallel to these findings, stimulated C-peptide tended to decrease, and the response to glyceryl trinitrate significantly improved (P=0.004) among 825T carriers. Body mass index, systolic and diastolic blood pressure, heart rate, or serum lipid levels did not significantly change in either group. Our findings suggest an effect of GNB3 gene polymorphism on important phenotypic variations in type 2 diabetes mellitus. The GNB3 gene polymorphism might be an example of pharmacogenetics, with the underlying etiological genetic defect altering the response to treatment.

Trichoderma atroviride G-protein alpha-subunit gene tga1 is involved in mycoparasitic coiling and conidiation

The soil fungus Trichoderma atroviride, a mycoparasite, responds to a number of external stimuli. In the presence of a fungal host, T. atroviride produces hydrolytic enzymes and coils around the host hyphae. In response to light or nutrient depletion, asexual sporulation is induced. In a biomimetic assay, different lectins induce coiling around nylon fibers; coiling in the absence of lectins can be induced by applying cyclic AMP (cAMP) or the heterotrimeric G-protein activator mastoparan. We isolated a T. atroviride G-protein alpha-subunit (Galpha) gene (tgal) belonging to the fungal subfamily with the highest similarity to the Galpha1 class. Generated transgenic lines that overexpress Galpha show very delayed sporulation and coil at a higher frequency. Furthermore, transgenic lines that express an activated mutant protein with no GTPase activity do not sporulate and coil at a higher frequency. Lines that express an antisense version of the gene are hypersporulating and coil at a much lower frequency in the biomimetic assay. The loss of Tgal in these mutants correlates with the loss of GTPase activity stimulated by the peptide toxin Mas-7. The application of Mas-7 to growing mycelial colonies raises intracellular cAMP levels, suggesting that Tgal can activate adenylyl cyclase. In contrast, cAMP levels and cAMP-dependent protein kinase activity drop when diffusible host signals are encountered and the mycoparasitism-related genes ech42 and prb1 are highly expressed. Mycoparasitic signaling is unlikely to be a linear pathway from host signals to increased cAMP levels. Our results demonstrate that the product of the tga1 gene is involved in both coiling and conidiation.

The Neural Cell Adhesion Molecule (N-CAM) Modulates K+ Channels in Cultured Glial Precursor Cells

Application of antibodies against the neural cell adhesion molecule (N-CAM) to O4-positive murine glial precursor cells in vitro results in a reduction of two distinct K+ currents measured using the whole cell patch clamp technique. Both the A-type and delayed rectifier K+ currents are reduced in amplitude within a few minutes of the application of poly- or monoclonal antibodies against N-CAM. This effect is not due to the binding of any antibody to the surface of the glial precursor cells because monoclonal antibody directed against the O4 surface antigen, or polyclonal antibodies directed against liver cell membranes (which also bind to the surface of glial precursor cells), do not affect membrane currents. Activators of protein kinase C, such as phorbol esters or diacylglycerol, also induce changes in potassium currents that appear, both in magnitude and kinetics, to be similar to those induced by antibodies against N-CAM. In contrast, activation of G proteins upregulates K+ currents. Glial precursor cells thus respond to triggering of N-CAM by altering channel properties. These observations suggest that adhesive events between neural cells can influence the intracellular ionic milieu.

The Identification and Localization of the Guanine Nucleotide Binding Protein G0 in the Auditory System

The identification of guanine nucleotide binding proteins (G proteins) in guinea-pig tissues was assessed by the adenosine diphosphate-ribosylation of the alpha subunit by Bordetella pertussis toxin using [alpha32P]nicotinamide adenine dinucleotide as the substrate followed by sodium dodecyl sulphate - polyacrylamide gel electrophoresis and autoradiography. Three tissues (inferior colliculus, neuroblastoma cells, and the organ of Corti) contained G0alpha (39 kD), as well as Gi2alpha (40 kD) and Gi1alpha and/or Gi3alpha (41 kD). The stria vascularis and the VIIIth nerve contained mainly Gi2alpha, Gi1alpha and/or Gi3alpha, but G0alpha was barely detectable. A purified preparation of outer hair cells from the organ of Corti contained all three pertussis toxin substrates including G0alpha, with the Gi2alpha (40 kD) subunit being the most prominent. The immunocytochemical localization of the G0alpha subunit was determined by light microscopy after incubating isolated outer hair cells, Hensen cells and the stria vascularis with affinity-purified anti-G0alpha antibodies. In hair cells a positive reaction was observed along the plasma membrane and around the perimeter of the cuticular plate (zona adherens). Positive reaction was also observed within the infracuticular network extending from the cuticular plate towards the nucleus in outer hair cells. Finally, the base of the outer hair cells also contained G0alpha. However, it is likely that the G0alpha that is present in this cell region is not within the hair cell itself, but rather in nerve terminals which remained attached during dissection.

p21ras Oncogene Protein Selectively Increases Low-voltage-activated Ca2+ Current Density in Embryonic Chick Dorsal Root Ganglion Neurons

p21ras protein resembles the alpha subunit of trimeric G-proteins, which regulate ion channel function. We now report a modulation of Ca2+ channels in vertebrate sensory neurons by p21ras in addition to its role in cell growth and differentiation. Quantitative microinjection of oncogenic p21-H-ras into embryonic chick dorsal root ganglion neurons was performed. After 4 h the current density of the low-voltage-activated (LVA; T-type) Ca2+ channels was increased. However, in contrast to trimeric G-proteins, which inhibit high-voltage-activated (HVA) Ca2+ channels in chick dorsal root ganglion neurons, p21ras did not significantly affect HVA Ca2+ currents. To study the time course of p21ras action, guanosine triphosphate-preloaded p21ras was added to the patch pipette. Full-length ras was effective only after a delay of 20 - 30 min. C-terminal modification by cellular enzymes is required to activate full-length ras, and can account for the observed delay. Unexpectedly, C-terminal-truncated p21ras, which was found to be inactive in biological assays, enhanced LVA Ca2+ currents within minutes. This suggests a G-protein-like modulation of the LVA Ca2+ channel by p21ras. In an early phase of neuronal differentiation, dorsal root ganglion neurons express only LVA Ca2+ currents. The regulatory role of p21ras on LVA channels may therefore be particularly important during differentiation.

Comparison of the antisecretory and antiulcer activity of epidermal growth factor, urogastrone and transforming growth factor alpha and its derivative in rodents in vivo

This study investigates the effects of epidermal growth factor (EGF), urogastrone (UG) and transforming growth factor-alpha (TGFalpha) and its derivative on dimaprit- and pentagastrin-induced gastric acid secretion and on acidified ethanol (AE)-evoked ulcer formation in anaesthetized rats. EGF, TGFalpha and UG administered subcutaneously (s.c.) 30 min before dimaprit inhibited gastric acid secretion. Against pentagastrin-stimulated secretion, TGFalpha inhibited, while EGF and UG potentiated, acid secretion dose-dependently. Intraduodenal (i.d.) administration of TGFalpha and UG had no effect, while EGF potentiated, both secretagogue-induced acid secretion in the same dosage schedule. Administration of either EGF, UG or TGFalpha i.v. bolus, in response to continuous infusion of dimaprit resulted in a significant (p < 0.05-p < 0.001) inhibition of acid secretion which was transient and returned to normal within 30-45 min for UG while it slowly returned to normal for EGF and TGFalpha. The truncated form of TGFa (amino acids 34-43) did not show any antisecretory effect when administered parenterally. Acidified ethanol produced gastric haemorrhagic lesions in the rat 1 h after oral administration. The gastric mucosal protective effects of TGFalpha, EGF and UG administered either orally or s.c. 30 min before the administration of AE were dose-dependent against this model of ulcer induction. Indomethacin (Indo), administered 15 min before AE to inhibit prostanoids biosynthesis, significantly (p < 0.001) reduced the cytoprotective effects of TGFalpha, EGF and UG and aggravated the ulcer index when administered s.c. The results show that PGs may be involved in mediating the protective effects of the three growth factors. Administration of NG-nitro-L argininemethylester (L-NAME) 15 min prior to TGFa, EGF and UG s.c. or orally, significantly (p < 0.001) decreased the degree of ulcer indices and was able to reduce the protective effects of TGFalpha, EGF and UG, thus including the role of NO in mediating the protective effects of these growth factors. In conclusion, these results have demonstrated that EGF, UG and TGFalpha have a short and reversible inhibitory effect on dimaprit-stimulated gastric acid secretion and each is effective parenterally but not orally. UG and EGF potentiated, while, TGFa inhibited pentagastrin-stimulated acid secretion. In addition, TGFalpha seems to lose its activity when it is truncated from the C terminus. The present study also suggests that EGF, UG and TGFalpha are equally effective against AE-induced gastric ulcer and bring about their cytoprotective action through their reduction of acid secretion and through PG and NO pathways.

G protein pathways

The heterotrimeric guanine nucleotide-binding proteins (G proteins) are signal transducers that communicate signals from many hormones, neurotransmitters, chemokines, and autocrine and paracrine factors. The extracellular signals are received by members of a large superfamily of receptors with seven membrane-spanning regions that activate the G proteins, which route the signals to several distinct intracellular signaling pathways. These pathways interact with one another to form a network that regulates metabolic enzymes, ion channels, transporters, and other components of the cellular machinery controlling a broad range of cellular processes, including transcription, motility, contractility, and secretion. These cellular processes in turn regulate systemic functions such as embryonic development, gonadal development, learning and memory, and organismal homeostasis.

Drug efficacy at G protein-coupled receptors

Efficacy has been defined in receptor pharmacology as a proportionality factor denoting the amount of physiological response a given ligand imparts to a biological system for a given amount of receptor occupancy. While first defined in terms of response, the concept can be expanded to a wide variety of G protein-coupled receptor (GPCR) behaviors, which includes pleiotropic interaction with multiple G proteins, internalization, oligomerization, desensitization, and interaction with membrane auxilliary proteins. Thus, there can be numerous types of efficacy, and different ligands can have a range of efficacies for different receptor behaviors. This review discusses the use of the efficacy concept in GPCR models based on the thermodynamic linkage theory and also in terms of the protein ensemble theory, in which macroaffinity of ligands for an ensemble of receptor microstates produces a new ligand-bound ensemble. The pharmacological characteristics of the ligand emerge from the intersection of the ligand-bound ensemble with the various ensembles defining pharmacological receptor behaviors. Receptor behaviors discussed are activation of G proteins; ability to be phosphorylated, desensitized, and internalized; formation of dimers and oligomers; and the interaction with auxiliary membrane and cytosolic proteins. The concepts of ligand-specific receptor conformation and conditional efficacy are also discussed in the context of ligand control of physiological response.

The association of heterotrimeric GTP-binding protein (Go) with microtubules

The heterotrimeric GTP-binding regulatory proteins (G proteins) play an important role in the regulation of membrane signal transduction. Recently, we identified the association of Go protein with mitotic spindles. Here we have investigated the relationship between Go protein and microtubules. We used temperature-dependent reversible assembly and taxol methods to purify microtubules from bovine brains. Goalpha and Gbeta proteins were identified in the microtubular fraction by both methods. The Goalpha subunit in the microtubular fraction could be ADP ribosylated by pertussis toxin. Co-immunoprecipitation data also revealed that Go protein can interact with microtubules. Exogenous Go protein could be incorporated into the assembled microtubular fraction, and 5 microg/ml (60 nM) of Go protein inhibited 40% of microtubule assembly. Western blot analysis of Goalpha-1 and Goalpha-2 in microtubular fractions showed that only Goalpha-1 is associated with microtubules. We conclude that the Goalpha-1betagamma proteins are associated with microtubules and may play some role in regulating the assembly and disassembly of microtubules.

Diabetes reduces right atrial β-adrenergic signaling but not agonist stimulation of heart rate in swine

This study assessed the effects of streptozotocin diabetes in swine on the heart rate response to β-adrenergic stimulation the adenylyl cyclase signal transduction pathway. Diabetic animals (n = 9) were hyperglycemic compared to the control group (n = 10) (12.6 ± 1.0 vs. 3.53 ± 0.29 mM). There were no significant differences between the diabetic and nondiabetic groups in the heart rate response to isoproterenol, however, there was a significant reduction (14%) in β-adrenergic receptor density in the right atrium in the diabetic (61 ± 3 fmol/mg protein) versus the nondiabetic group (71 ± 3) (P < 0.05). The content of guanosine triphosphate binding regulatory proteins (Gs and Gi) in the right atrium was not affected by diabetes, nor was adenylyl cyclase activity under unstimulated conditions or with receptor-dependent stimulation with isoproterenol. On the other hand, adenylyl cyclase activity was 34% lower when directly stimulated with forskolin, and it was reduced by 23% when stimulated through Gs with Gpp(NH)p. In conclusion, beta-adrenergic stimulation of heart rate with isoproteronol and the receptor-dependent signal transduction pathway remained intact in the right atrium of diabetic swine despite reduced beta-adrenergic receptor density, G-protein content, and direct stimulation of adenylyl cyclase activity.Key words: diabetes, G-proteins, heart rate, receptors, signal transduction.

cAMP-independent phosphorylation activation of CFTR by G proteins in native human sweat duct

It is generally believed that cAMP-dependent phosphorylation is the principle mechanism for activating cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channels. However, we showed that activating G proteins in the sweat duct stimulated CFTR Cl(-) conductance (G(Cl)) in the presence of ATP alone without cAMP. The objective of this study was to test whether the G protein stimulation of CFTR G(Cl) is independent of protein kinase A. We activated G proteins and monitored CFTR G(Cl) in basolaterally permeabilized sweat duct. Activating G proteins with guanosine 5'-O-(3-thiotriphosphate) (10-100 microM) stimulated CFTR G(Cl) in the presence of 5 mM ATP alone without cAMP. G protein activation of CFTR G(Cl) required Mg(2+) and ATP hydrolysis (5'-adenylylimidodiphosphate could not substitute for ATP). G protein activation of CFTR G(Cl) was 1) sensitive to inhibition by the kinase inhibitor staurosporine (1 microM), indicating that the activation process requires phosphorylation; 2) insensitive to the adenylate cyclase (AC) inhibitors 2',5'-dideoxyadenosine (1 mM) and SQ-22536 (100 microM); and 3) independent of Ca(2+), suggesting that Ca(2+)-dependent protein kinase C and Ca(2+)/calmodulin-dependent kinase(s) are not involved in the activation process. Activating AC with 10(-6) M forskolin plus 10(-6) M IBMX (in the presence of 5 mM ATP) did not activate CFTR, indicating that cAMP cannot accumulate sufficiently to activate CFTR in permeabilized cells. We concluded that heterotrimeric G proteins activate CFTR G(Cl) endogenously via a cAMP-independent pathway in this native absorptive epithelium.

Calcium currents and arrhythmias: insights from molecular biology

Calcium channels are critical to normal cardiac function. They are involved in the generation and conduction of the action potential and in contraction. Three surface membrane channels have been identified. The L-type Ca channel is most abundant and is responsible for Ca entry into the cell that triggers contraction. T-type Ca channels are most prevalent in the conduction system and are probably involved in automaticity. A newly described TTX-sensitive calcium current may be important in "boosting" or enhancing conduction and contraction. The main intracellular Ca channel resides in the sarcoplasmic reticulum and is responsible for the release of the Ca that activates contraction. Oscillatory behavior of this channel influences the sarcolemmal membrane, causing delayed aftercontractions and arrhythmias such as those seen in digoxin toxicity. The on-going molecular characterization of these channels will enhance our knowledge of their normal function and dysfunction in disease states, leading to the development of new therapeutic agents to treat arrhythmias and contractile dysfunction.

Cell-type specific targeting of the alpha 2c-adrenoceptor. Evidence for the organization of receptor microdomains during neuronal differentiation of PC12 cells

We have previously shown differences in the intracellular targeting of alpha2a (alpha(2A))- and alpha2c (alpha(2C))-adrenoreceptors expressed in the same cell line (von Zastrow, M., Link, R., Daunt, D. , Barsh, G., and Kobilka, B. (1993) J. Biol. Chem. 268, 763-766; Daunt, D. A., Hurt, C., Hein, L., Kallio, J., Feng, F., and Kobilka, B. K. (1997) Mol. Pharmacol. 51, 711-720). alpha(2A)-Adrenoreceptors reside primarily in the plasma membrane in HEK 293 cells, while co-expressed alpha(2C)-adrenoreceptors are found mainly in an intracellular compartment. Since alpha(2c)-adrenoreceptors are expressed primarily in the brain, we compared the intracellular targeting of alpha(2C)-adrenoreceptors in two neuroendocrine cell lines with the targeting in three epithelial cell lines and one fibroblast cell line. In transiently transfected COS7 cells, and in stably transfected normal rat kidney cells, Madin-Darby canine kidney cells, and Rat1 fibroblasts, a significant proportion of alpha(2C)-adrenoreceptor detected by immunocytochemistry co-localized with markers for both the endoplasmic reticulum and the cis/medial Golgi compartments. In contrast, both PC12 cells and AtT20 cells efficiently targeted alpha(2C)-adrenoreceptors to the plasma membrane. Ligand binding and Western blot analyses indicate that intracellular receptor in normal rat kidney cells is functional and undergoes normal post-translational processing. In PC12 cells the expressed alpha(2C)-adrenoreceptors become concentrated in neurite outgrowths in discrete regions of the plasma membrane having a high density of F-actin following treatment with nerve growth factor. These findings provide evidence for cell-type specific factors that facilitate the targeting of the G protein-coupled receptors to the plasma membrane.

Modulation of neutrophil chemokine receptors by Staphylococcus aureus supernate

In a previous study, we showed that Staphylococcus aureus supernate (SaS) is a potent agonist for both neutrophils and mononuclear cells. To further investigate the immunomodulating effects of SaS, the effect on different neutrophil receptors was studied. Expression of various neutrophil receptors, before and after treatment with SaS, was quantified by flow cytometry. We found that SaS treatment of neutrophils resulted in a specific and total downregulation of the C5a and the fMLP receptor, both serpentine receptors, while other receptors were totally unaffected. Since these two receptors are both involved in chemotaxis, we tested the effect of SaS in calcium flux and chemotaxis assays. We showed that preincubation with SaS abrogated the rise in intracellular calcium concentration upon triggering with fMLP and C5a. We also showed that SaS is a potent inhibitor of neutrophil chemotaxis towards fMLP and C5a, but does not interfere with chemotaxis towards interleukin-8. These findings indicate that S. aureus produces a virulence factor extracellularly, which impairs chemotaxis towards the infected site.

Vascular β-adrenoceptor function in hypertension and in ageing

Functional β-adrenoceptors (β-AR) have been identified and characterized in blood vessels under in vivo conditions as well as in vascular smooth muscle cells (SMC) grown in culture. Agonist occupancy of β-AR activates adenylyl cyclase (AC) via the stimulatory guanine nucleotide-binding protein (Gs) and leads to elevations in intracellular adenosine 3',5'-cyclic monophosphate levels (cAMP). Increased cAMP activates the cAMP-dependent protein kinase (PKA), with subsequent phosphorylation of various target proteins. This β-AR pathway interacts with several other intracellular signalling pathways via cross-talk, so that activation by β-AR agonists may also modulate other second messengers and protein kinases. SMC β-AR play an important role in SMC function. In intact blood vessels they mediate SMC relaxation by various intracellular mechanisms, ultimately causing a decrease in intracellular Ca2+ levels. In cultured SMC, activation of the β-AR pathway results in inhibition of cellular proliferation, the development of SMC polyploidy, and SMC apoptosis. Blood vessels from hypertensive animals are characterized by an increase in SMC cell mass, a greater incidence of SMC polyploidy in the aorta, and an impairment in the β-agonist-mediated SMC relaxation. Some of these changes may result from an attenuation of β-AR function due to agonist-induced receptor desensitization caused by the uncoupling of receptors from the Gs-AC system. The phosphorylated β-AR may in turn trigger new signals and activate different intracellular pathways. However, the details of these mechanisms are still unresolved. Since functional β-AR play such a prominent and multi-faceted role in SMC function, it is important to understand how these diverse physiological effects are mediated by this receptor system, and how they contribute to the development of hypertension. With ageing, a decrease in β-AR-Gs-AC coupling is observed, and this is implicated in the reduced responsiveness of SMC. The similarities in SMC β-AR functional changes in hypertension and in ageing suggest that the underlying mechanisms are also analogous.Key words: smooth muscle, β-adrenoceptors, cyclic AMP, protein kinase A, cell proliferation, polyploidy, relaxation, apoptosis, hypertension, ageing.

The effects of the dynamic state of the cytoskeleton on neuronal plasticity

The effects of degrading and stabilizing microtubules and microfilaments on the formation of plastic reactions were studied in isolated nerve cells from the mollusk Lymnaea stagnalis. Degradation of the cytoskeleton affected the performance, retention, and repeated acquisition of plastic reactions. Stabilization of microtubules led to the appearance of a relationship between the dynamics of the development and retention of plastic reactions and the series of stimulation. Stabilization of microfilaments led to transient plastic reaction, along with long-term reactions. These results show that rearrangements of the cytoskeleton have a key role in the processes of neuronal plasticity.

Ontogeny of G-protein expression: control by beta-adrenoceptors

Cardiac cell homeostasis is maintained in the face of excessive beta-adrenoceptor stimulation through the process of desensitization. Desensitization is not an inherent property of these cells but rather is acquired during development; neonates given beta-agonists actually show heterologous sensitization, involving changes in the expression and catalytic activity of adenylyl cyclase (AC) as well as an increased receptor/G-protein coupling. The current study examines the role of specific G-protein components, G(s)alpha and G(i)alpha, in the ontogeny of beta-adrenoceptor responses and in the transition from agonist-induced sensitization to desensitization. Between postnatal days (PN) 6 and 15 there was a significant decrease in the 52 kDa isoform of G(s)alpha with no accompanying change of the 45 kDa form; over the same period, G(i)alpha3 also declined substantially. In contrast, the 45 kDa isoform of G(s)alpha and G(i)alpha1,2 remained fairly constant over the same period and fluoride-stimulated AC activity increased. Treatment with isoproterenol on PN2-5 did not result in any significant changes in G(s)alpha expression but robustly decreased G(i)alpha1,2. These changes were accompanied by heterologous sensitization of AC activity at the level of AC itself, evidenced by equivalent increases in the enzymatic response to fluoride and forskolin-Mn2+. Isoproterenol given to older animals (PN11-14) also caused specific loss of G(i) protein, in this case targeting G(i)alpha3, whereas G(s)alpha again was unchanged; in contrast to the younger group, the older animals displayed heterologous desensitization of AC at the level of G-protein function (specific loss of the fluoride response). These results indicate that the normal ontogenetic increase of cardiac beta-adrenoceptor coupling to AC is not dependent on the absolute amount of G-proteins, nor on the relative balance of stimulatory (G(s)) and inhibitory (G(i)) subunits. However, the ability of receptor stimulation to downregulate G(i)alpha1,2, an event which is specific to immature cardiac cells, is likely to be an important component of the resistance of the fetal/neonatal heart to agonist-induced desensitization and hypertrophy. The maintenance of cardiac beta-adrenoceptor signaling in the face of intense stimulation is likely to play an important role in the physiologic adaptations necessary to the perinatal transition.