The origins of diversity and specificity in g protein-coupled receptor signaling

Heterologous regulation of Mu-opioid (MOP) receptor mobility in the membrane of SH-SY5Y cells

The dynamic organization of G protein-coupled receptors in the plasma membrane is suspected of playing a role in their function. The regulation of the diffusion mode of the mu-opioid (MOP) receptor was previously shown to be agonist-specific. Here we investigate the regulation of MOP receptor diffusion by heterologous activation of other G protein-coupled receptors and characterize the dynamic properties of the MOP receptor within the heterodimer MOP/neuropeptide FF (NPFF2) receptor. The data show that the dynamics and signaling of the MOP receptor in SH-SY5Y cells are modified by the activation of α2-adrenergic and NPFF2 receptors, but not by the activation of receptors not described to interact with the opioid receptor. By combining, for the first time, fluorescence recovery after photobleaching at variable radius experiments with bimolecular fluorescence complementation, we show that the MOP/NPFF2 heterodimer adopts a specific diffusion behavior that corresponds to a mix of the dynamic properties of both MOP and NPFF2 receptors. Altogether, the data suggest that heterologous regulation is accompanied by a specific organization of receptors in the membrane.

Low-dose levodopa protects nerve cells from oxidative stress and up-regulates expression of pCREB and CD39

Objective

This study aimed to investigate the influence of low-dose levodopa (L-DOPA) on neuronal cell death under oxidative stress.

Methods

PC12 cells were treated with L-DOPA at different concentrations. We detected the L-DOPA induced reactive oxygen species (ROS). Meanwhile, MTT and LDH assay were performed to determine the proliferation and growth of PC12 cells with or without ROS scavenger. In addition, after pretreatment with L-DOPA at different concentrations alone or in combination with CD39 inhibitor, PC12 cells were incubated with hydrogen peroxide (H₂O₂) and the cell viability was evaluated by MTT and LDH assay. In addition, the expression of pCREB and CD39 was detected by immunofluorescence staining and Western blot assay in both cells and rat’s brain after L-DOPA treatment.

Results

After treatment with L-DOPA for 3 days, the cell proliferation and growth were promoted when the L-DOPA concentration was <30 µM, while cell proliferation was comparable to that in control group when the L-DOPA concentration was >30 µM. Low dose L-DOPA could protect the PC12 cells from H₂O₂ induced oxidative stress, which was compromised by CD39 inhibitor. In addition, the expression of CD39 and pCREB increased in both PC12 cells and rats’ brain after L-DOPA treatment.

Conclusions

L-DOPA at different concentrations has distinct influence on proliferation and growth of PC12 cells, and low dose (<30 µM) L-DOPA protects PC12 cells against oxidative stress which might be related to the up-regulation of CD39 and pCREB expression.

BAY60-6583 acts as a partial agonist at adenosine A2B receptors

BAY60-6583 [2-({6-amino-3,5-dicyano-4-[4-(cyclopropylmethoxy)phenyl]pyridin-2-yl}sulfanyl)acetamide] is the most potent and selective adenosine A2B receptor (A2B AR) agonist known to date. Therefore, it has been widely used for in vitro and in vivo experiments. In the present study, we investigated the binding and functional properties of BAY60-6583 in various native and recombinant cell lines with different A2B AR expression levels. In cAMP accumulation and calcium mobilization assays, BAY60-6583 was found to be significantly less efficacious than adenosine or the adenosine derivative NECA. When it was tested in human embryonic kidney (HEK)293 cells, its efficacy correlated with the A2B expression level of the cells. In Jurkat T cells, BAY60-6583 antagonized the agonistic effect of NECA and adenosine as determined in cAMP accumulation assays. On the basis of these results, we conclude that BAY60-6583 acts as a partial agonist at adenosine A2B receptors. At high levels of the physiologic agonist adenosine, BAY60-6583 may act as an antagonist and block the effects of adenosine at A2B receptors. This has to be considered when applying the A2B-selective "agonist" BAY60-6583 in pharmacological studies, and previous research results may have to be reinterpreted.

Diversity and Bias through Receptor-Receptor Interactions in GPCR Heteroreceptor Complexes. Focus on Examples from Dopamine D2 Receptor Heteromerization

Allosteric receptor-receptor interactions in GPCR heteromers appeared to introduce an intermolecular allosteric mechanism contributing to the diversity and bias in the protomers. Examples of dopamine D2R heteromerization are given to show how such allosteric mechanisms significantly change the receptor protomer repertoire leading to diversity and biased recognition and signaling. In 1980s and 1990s, it was shown that neurotensin (NT) through selective antagonistic NTR-D2 like receptor interactions increased the diversity of DA signaling by reducing D2R-mediated dopamine signaling over D1R-mediated dopamine signaling. Furthermore, D2R protomer appeared to bias the specificity of the NTR orthosteric binding site toward neuromedin N vs. NT in the heteroreceptor complex. Complex CCK2R-D1R-D2R interactions in possible heteroreceptor complexes were also demonstrated further increasing receptor diversity. In D2R-5-HT2AR heteroreceptor complexes, the hallucinogenic 5-HT2AR agonists LSD and DOI were recently found to exert a biased agonist action on the orthosteric site of the 5-HT2AR protomer leading to the development of an active conformational state different from the one produced by 5-HT. Furthermore, as recently demonstrated allosteric A2A-D2R receptor-receptor interaction brought about not only a reduced affinity of the D2R agonist binding site but also a biased modulation of the D2R protomer signaling in A2A-D2R heteroreceptor complexes. A conformational state of the D2R was induced, which moved away from Gi/o signaling and instead favored β-arrestin2-mediated signaling. These examples on allosteric receptor-receptor interactions obtained over several decades serve to illustrate the significant increase in diversity and biased recognition and signaling that develop through such mechanisms.

Arrestin-dependent activation of ERK and Src family kinases

The four members of the mammalian arrestin family, two visual and two nonvisual, share the property of stimulus-dependent docking to G protein-coupled receptors. This conformational selectivity permits them to function in receptor desensitization, as arrestin binding sterically inhibits G protein coupling. The two nonvisual arrestins further act as adapter proteins, linking receptors to the clathrin-dependent endocytic machinery and regulating receptor sequestration, intracellular trafficking, recycling, and degradation. Arrestins also function as ligand-regulated scaffolds, recruiting catalytically active proteins into receptor-based multiprotein "signalsome" complexes. Arrestin binding thus marks the transition from a transient G protein-coupled state on the plasma membrane to a persistent arrestin-coupled state that continues to signal as the receptor internalizes. Two of the earliest discovered and most studied arrestin-dependent signaling pathways involve regulation of Src family nonreceptor tyrosine kinases and the ERK1/2 mitogen-activated kinase cascade. In each case, arrestin scaffolding imposes constraints on kinase activity that dictate signal duration and substrate specificity. Evidence suggests that arrestin-bound ERK1/2 and Src not only play regulatory roles in receptor desensitization and trafficking but also mediate longer term effects on cell growth, migration, proliferation, and survival.

Systems-Level G Protein-Coupled Receptor Therapy Across a Neurodegenerative Continuum by the GLP-1 Receptor System

With our increasing appreciation of the true complexity of diseases and pathophysiologies, it is clear that this knowledge needs to inform the future development of pharmacotherapeutics. For many disorders, the disease mechanism itself is a complex process spanning multiple signaling networks, tissues, and organ systems. Identifying the precise nature and locations of the pathophysiology is crucial for the creation of systemically effective drugs. Diseases once considered constrained to a limited range of organ systems, e.g., central neurodegenerative disorders such as Alzheimer's disease (AD), Parkinson's disease (PD), and Huntington' disease (HD), the role of multiple central and peripheral organ systems in the etiology of such diseases is now widely accepted. With this knowledge, it is increasingly clear that these seemingly distinct neurodegenerative disorders (AD, PD, and HD) possess multiple pathophysiological similarities thereby demonstrating an inter-related continuum of disease-related molecular alterations. With this systems-level appreciation of neurodegenerative diseases, it is now imperative to consider that pharmacotherapeutics should be developed specifically to address the systemic imbalances that create the disorders. Identification of potential systems-level signaling axes may facilitate the generation of therapeutic agents with synergistic remedial activity across multiple tissues, organ systems, and even diseases. Here, we discuss the potentially therapeutic systems-level interaction of the glucagon-like peptide 1 (GLP-1) ligand-receptor axis with multiple aspects of the AD, PD, and HD neurodegenerative continuum.

Targeting G protein coupled receptor-related pathways as emerging molecular therapies

G protein coupled receptors (GPCRs) represent the most important targets in modern pharmacology because of the different functions they mediate, especially within brain and peripheral nervous system, and also because of their functional and stereochemical properties. In this paper, we illustrate, via a variety of examples, novel advances about the GPCR-related molecules that have been shown to play diverse roles in GPCR pathways and in pathophysiological phenomena. We have exemplified how those GPCRs’ pathways are, or might constitute, potential targets for different drugs either to stimulate, modify, regulate or inhibit the cellular mechanisms that are hypothesized to govern some pathologic, physiologic, biologic and cellular or molecular aspects both in vivo and in vitro. Therefore, influencing such pathways will, undoubtedly, lead to different therapeutical applications based on the related pharmacological implications. Furthermore, such new properties can be applied in different fields. In addition to offering fruitful directions for future researches, we hope the reviewed data, together with the elements found within the cited references, will inspire clinicians and researchers devoted to the studies on GPCR’s properties.

Long-term artificial sweetener acesulfame potassium treatment alters neurometabolic functions in C57BL/6J mice

With the prevalence of obesity, artificial, non-nutritive sweeteners have been widely used as dietary supplements that provide sweet taste without excessive caloric load. In order to better understand the overall actions of artificial sweeteners, especially when they are chronically used, we investigated the peripheral and central nervous system effects of protracted exposure to a widely used artificial sweetener, acesulfame K (ACK). We found that extended ACK exposure (40 weeks) in normal C57BL/6J mice demonstrated a moderate and limited influence on metabolic homeostasis, including altering fasting insulin and leptin levels, pancreatic islet size and lipid levels, without affecting insulin sensitivity and bodyweight. Interestingly, impaired cognitive memory functions (evaluated by Morris Water Maze and Novel Objective Preference tests) were found in ACK-treated C57BL/6J mice, while no differences in motor function and anxiety levels were detected. The generation of an ACK-induced neurological phenotype was associated with metabolic dysregulation (glycolysis inhibition and functional ATP depletion) and neurosynaptic abnormalities (dysregulation of TrkB-mediated BDNF and Akt/Erk-mediated cell growth/survival pathway) in hippocampal neurons. Our data suggest that chronic use of ACK could affect cognitive functions, potentially via altering neuro-metabolic functions in male C57BL/6J mice.

Plurigon: three dimensional visualization and classification of high-dimensionality data

High-dimensionality data is rapidly becoming the norm for biomedical sciences and many other analytical disciplines. Not only is the collection and processing time for such data becoming problematic, but it has become increasingly difficult to form a comprehensive appreciation of high-dimensionality data. Though data analysis methods for coping with multivariate data are well-documented in technical fields such as computer science, little effort is currently being expended to condense data vectors that exist beyond the realm of physical space into an easily interpretable and aesthetic form. To address this important need, we have developed Plurigon, a data visualization and classification tool for the integration of high-dimensionality visualization algorithms with a user-friendly, interactive graphical interface. Unlike existing data visualization methods, which are focused on an ensemble of data points, Plurigon places a strong emphasis upon the visualization of a single data point and its determining characteristics. Multivariate data vectors are represented in the form of a deformed sphere with a distinct topology of hills, valleys, plateaus, peaks, and crevices. The gestalt structure of the resultant Plurigon object generates an easily-appreciable model. User interaction with the Plurigon is extensive; zoom, rotation, axial and vector display, feature extraction, and anaglyph stereoscopy are currently supported. With Plurigon and its ability to analyze high-complexity data, we hope to see a unification of biomedical and computational sciences as well as practical applications in a wide array of scientific disciplines. Increased accessibility to the analysis of high-dimensionality data may increase the number of new discoveries and breakthroughs, ranging from drug screening to disease diagnosis to medical literature mining.

Non-traditional roles of G protein-coupled receptors in basic cell biology

G protein-coupled receptors (GPCRs) are key signaling proteins that regulate how cells interact with their environment. Traditional signaling cascades involving GPCRs have been well described and are well established and very important clinical targets. With the development of more recent technologies, hints about the involvement of GPCRs in fundamental cell biological processes are beginning to emerge. In this review, we give a basic introduction to GPCR signaling and highlight some less well described roles of GPCRs, including in cell division and membrane trafficking, which may occur through canonical and non-canonical signaling pathways.

Protein charge and mass contribute to the spatio-temporal dynamics of protein-protein interactions in a minimal proteome

We constructed and simulated a "minimal proteome" model using Langevin dynamics. It contains 206 essential protein types that were compiled from the literature. For comparison, we generated six proteomes with randomized concentrations. We found that the net charges and molecular weights of the proteins in the minimal genome are not random. The net charge of a protein decreases linearly with molecular weight, with small proteins being mostly positively charged and large proteins negatively charged. The protein copy numbers in the minimal genome have the tendency to maximize the number of protein-protein interactions in the network. Negatively charged proteins that tend to have larger sizes can provide a large collision cross-section allowing them to interact with other proteins; on the other hand, the smaller positively charged proteins could have higher diffusion speed and are more likely to collide with other proteins. Proteomes with random charge/mass populations form less stable clusters than those with experimental protein copy numbers. Our study suggests that "proper" populations of negatively and positively charged proteins are important for maintaining a protein-protein interaction network in a proteome. It is interesting to note that the minimal genome model based on the charge and mass of Escherichia coli may have a larger protein-protein interaction network than that based on the lower organism Mycoplasma pneumoniae.

Emergent biological properties of arrestin pathway-selective biased agonism

Our growing appreciation of the pluridimensionality of G protein-coupled receptor (GPCR) signaling, combined with the phenomenon of orthosteric ligand "bias", has created the possibility of drugs that selectively modulate different aspects of GPCR function for therapeutic benefit. When viewed from the short-term perspective, e.g. changes in receptor conformation, effector coupling or second messenger generation, biased ligands appear to activate a subset of the response profile produced by a conventional agonist. Yet when examined in vivo, the limited data available suggest that biased ligand effects can diverge from their conventional counterparts in ways that cannot be predicted from their in vitro efficacy profile. What is currently missing, at least with respect to G protein and arrestin pathway-selective ligands, is a rational framework for relating the in vitro efficacy of a "biased" agonist to its in vivo actions that will enable drug screening programs to identify ligands with the desired biological effects.

Neuron-type specific cannabinoid-mediated G protein signalling in mouse hippocampus

Type 1 cannabinoid receptor (CB1) is expressed in different neuronal populations in the mammalian brain. In particular, CB1 on GABAergic or glutamatergic neurons exerts different functions and display different pharmacological properties in vivo. This suggests the existence of neuron-type specific signalling pathways activated by different subpopulations of CB1. In this study, we analysed CB1 expression, binding and signalling in the hippocampus of conditional mutant mice, bearing CB1 deletion in GABAergic (GABA-CB1-KO mice) or cortical glutamatergic neurons (Glu-CB1-KO mice). Compared to their wild-type littermates, Glu-CB1-KO displayed a small decrease of CB1 mRNA amount, immunoreactivity and [³H]CP55,940 binding. Conversely, GABA-CB1-KO mice showed a drastic reduction of these parameters, confirming that CB1 is present at much higher density on hippocampal GABAergic interneurons than glutamatergic neurons. Surprisingly, however, saturation analysis of HU210-stimulated [(35) S]GTPγS binding demonstrated that 'glutamatergic' CB1 is more efficiently coupled to G protein signalling than 'GABAergic' CB1. Thus, the minority of CB1 on glutamatergic neurons is paradoxically several fold more strongly coupled to G protein signalling than 'GABAergic' CB1. This selective signalling mechanism raises the possibility of designing novel cannabinoid ligands that differentially activate only a subset of physiological effects of CB1 stimulation, thereby optimizing therapeutic action.

New concepts in pharmacological efficacy at 7TM receptors: IUPHAR review 2

The present-day concept of drug efficacy has changed completely from its original description as the property of agonists that causes tissue activation. The ability to visualize the multiple behaviours of seven transmembrane receptors has shown that drugs can have many efficacies and also that the transduction of drug stimulus to various cellular stimulus-response cascades can be biased towards some but not all pathways. This latter effect leads to agonist 'functional selectivity', which can be favourable for the improvement of agonist therapeutics. However, in addition, biased agonist potency becomes cell type dependent with the loss of the monotonic behaviour of stimulus-response mechanisms, leading to potential problems in agonist quantification. This has an extremely important effect on the discovery process for new agonists since it now cannot be assumed that a given screening or lead optimization assay will correctly predict therapeutic behaviour. This review discusses these ideas and how new approaches to quantifying agonist effect may be used to circumvent the cell type dependence of agonism. This article, written by a corresponding member of the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC-IUPHAR), reviews our current understanding of the interaction of ligands with seven transmembrane receptors. Further information on these pharmacological concepts is being incorporated into the IUPHAR/BPS database GuideToPharmacology.org.

β-arrestin-selective G protein-coupled receptor agonists engender unique biological efficacy in vivo

Biased G protein-coupled receptor agonists are orthosteric ligands that possess pathway-selective efficacy, activating or inhibiting only a subset of the signaling repertoire of their cognate receptors. In vitro, D-Trp(12),Tyr(34)-bPTH(7-34) [bPTH(7-34)], a biased agonist for the type 1 PTH receptor, antagonizes receptor-G protein coupling but activates arrestin-dependent signaling. In vivo, both bPTH(7-34) and the conventional agonist hPTH(1-34) stimulate anabolic bone formation. To understand how two PTH receptor ligands with markedly different in vitro efficacy could elicit similar in vivo responses, we analyzed transcriptional profiles from calvarial bone of mice treated for 8 wk with vehicle, bPTH(7-34) or hPTH(1-34). Treatment of wild-type mice with bPTH(7-34) primarily affected pathways that promote expansion of the osteoblast pool, notably cell cycle regulation, cell survival, and migration. These responses were absent in β-arrestin2-null mice, identifying them as downstream targets of β-arrestin2-mediated signaling. In contrast, hPTH(1-34) primarily affected pathways classically associated with enhanced bone formation, including collagen synthesis and matrix mineralization. hPTH(1-34) actions were less dependent on β-arrestin2, as might be expected of a ligand capable of G protein activation. In vitro, bPTH(7-34) slowed the rate of preosteoblast proliferation, enhanced osteoblast survival when exposed to an apoptotic stimulus, and stimulated cell migration in wild-type, but not β-arrestin2-null, calvarial osteoblasts. These results suggest that bPTH(7-34) and hPTH(1-34) affect bone mass in vivo through predominantly separate genomic mechanisms created by largely distinct receptor-signaling networks and demonstrate that functional selectivity can be exploited to change the quality of G protein-coupled receptor efficacy.

BRET Biosensor Analysis of Receptor Tyrosine Kinase Functionality

Bioluminescence resonance energy transfer (BRET) is an improved version of earlier resonance energy transfer technologies used for the analysis of biomolecular protein interaction. BRET analysis can be applied to many transmembrane receptor classes, however the majority of the early published literature on BRET has focused on G protein-coupled receptor (GPCR) research. In contrast, there is limited scientific literature using BRET to investigate receptor tyrosine kinase (RTK) activity. This limited investigation is surprising as RTKs often employ dimerization as a key factor in their activation, as well as being important therapeutic targets in medicine, especially in the cases of cancer, diabetes, neurodegenerative, and respiratory conditions. In this review, we consider an array of studies pertinent to RTKs and other non-GPCR receptor protein-protein signaling interactions; more specifically we discuss receptor-protein interactions involved in the transmission of signaling communication. We have provided an overview of functional BRET studies associated with the RTK superfamily involving: neurotrophic receptors [e.g., tropomyosin-related kinase (Trk) and p75 neurotrophin receptor (p75NTR)]; insulinotropic receptors [e.g., insulin receptor (IR) and insulin-like growth factor receptor (IGFR)] and growth factor receptors [e.g., ErbB receptors including the EGFR, the fibroblast growth factor receptor (FGFR), the vascular endothelial growth factor receptor (VEGFR) and the c-kit and platelet-derived growth factor receptor (PDGFR)]. In addition, we review BRET-mediated studies of other tyrosine kinase-associated receptors including cytokine receptors, i.e., leptin receptor (OB-R) and the growth hormone receptor (GHR). It is clear even from the relatively sparse experimental RTK BRET evidence that there is tremendous potential for this technological application for the functional investigation of RTK biology.

Arrestins as regulators of kinases and phosphatases

The discovery that, in addition to mediating G protein-coupled receptor (GPCR) desensitization and endocytosis, arrestins bind to diverse catalytically active nonreceptor proteins and act as ligand-regulated signaling scaffolds led to a paradigm shift in the study of GPCR signal transduction. Research over the past decade has solidified the concept that arrestins confer novel GPCR-signaling capacity by recruiting protein and lipid kinase, phosphatase, phosphodiesterase, and ubiquitin ligase activity into receptor-based multiprotein "signalsome" complexes. Signalsomes regulate downstream pathways controlled by Src family nonreceptor tyrosine kinases, mitogen-activated protein kinases, protein kinase B (AKT), glycogen synthase kinase 3, protein phosphatase 2A, nuclear factor-κB, and several others, imposing spatial and temporal control on their function. While many arrestin-bound kinases and phosphatases are involved in the control of cytoskeletal rearrangement, vesicle endocytosis, exocytosis, and cell migration, other signals reach into the nucleus, affecting cell proliferation, apoptosis, and survival. Indeed, the kinase/phosphatase network regulated by arrestins may be fully as diverse as that regulated by heterotrimeric G proteins.

The role of Thyrotropin Releasing Hormone in aging and neurodegenerative diseases

Thyrotropin releasing hormone (TRH) is primarily known as the central regulator of the hypothalamic-pituitary-thyroid (HPT) axis. However, TRH also exerts a variety of central nervous system effects independent from its activity in the HPT axis. With advancing age, decreases in TRH synthesis, expression, and activity have been demonstrated. Associated with this emerging evidence suggests that TRH is implicated in neurodegenerative diseases of aging, including Alzheimer's disease and Parkinson's disease. TRH and its synthetic analogs have been recognized as trophic factors in neurons of the diencephalon and spinal cord, and as neuroprotectants against oxidative stress, glutamate toxicity, caspase-induced cell death, DNA fragmentation, and inflammation. In this review, we will provide an overview of some of the roles of TRH, outside of the HPT axis, associated with pathological aging and neurodegeneration and we shall discuss the potential of TRH and TRH analogs for the treatment of neurodegenerative diseases.

Structured crowding and its effects on enzyme catalysis

Macromolecular crowding decreases the diffusion rate, shifts the equilibrium of protein-protein and protein-substrate interactions, and changes protein conformational dynamics. Collectively, these effects contribute to enzyme catalysis. Here we describe how crowding may bias the conformational change and dynamics of enzyme populations and in this way affect catalysis. Crowding effects have been studied using artificial crowding agents and in vivo-like environments. These studies revealed a correlation between protein dynamics and function in the crowded environment. We suggest that crowded environments be classified into uniform crowding and structured crowding. Uniform crowding represents random crowding conditions created by synthetic particles with a narrow size distribution. Structured crowding refers to the highly coordinated cellular environment, where proteins and other macromolecules are clustered and organized. In structured crowded environments the perturbation of protein thermal stability may be lower; however, it may still be able to modulate functions effectively and dynamically. Dynamic, allosteric enzymes could be more sensitive to cellular perturbations if their free energy landscape is flatter around the native state; on the other hand, if their free energy landscape is rougher, with high kinetic barriers separating deep minima, they could be more robust. Above all, cells are structured; and this holds both for the cytosol and for the membrane environment. The crowded environment is organized, which limits the search, and the crowders are not necessarily inert. More likely, they too transmit allosteric effects, and as such play important functional roles. Overall, structured cellular crowding may lead to higher enzyme efficiency and specificity.

ATP inhibits Ins(1,4,5)P3-evoked Ca2+ release in smooth muscle via P2Y1 receptors

Adenosine 5′-triphosphate (ATP) mediates a variety of biological functions following nerve-evoked release, via activation of either G-protein-coupled P2Y- or ligand-gated P2X receptors. In smooth muscle, ATP, acting via P2Y receptors (P2YR), may act as an inhibitory neurotransmitter. The underlying mechanism(s) remain unclear, but have been proposed to involve the production of inositol 1,4,5-trisphosphate [Ins(1,4,5)P₃] by phospholipase C (PLC), to evoke Ca²⁺ release from the internal store and stimulation of Ca²⁺-activated potassium (K_Ca) channels to cause membrane hyperpolarization. This mechanism requires Ca²⁺ release from the store. However, in the present study, ATP evoked transient Ca²⁺ increases in only ~10% of voltage-clamped single smooth muscle cells. These results do not support activation of K_Ca as the major mechanism underlying inhibition of smooth muscle activity. Interestingly, ATP inhibited Ins(1,4,5)P₃-evoked Ca²⁺ release in cells that did not show a Ca²⁺ rise in response to purinergic activation. The reduction in Ins(1,4,5)P₃-evoked Ca²⁺ release was not mimicked by adenosine and therefore, cannot be explained by hydrolysis of ATP to adenosine. The reduction in Ins(1,4,5)P₃-evoked Ca²⁺ release was, however, also observed with its primary metabolite, ADP, and blocked by the P2Y₁R antagonist, MRS2179, and the G protein inhibitor, GDPβS, but not by PLC inhibition. The present study demonstrates a novel inhibitory effect of P2Y₁R activation on Ins(1,4,5)P₃-evoked Ca²⁺ release, such that purinergic stimulation acts to prevent Ins(1,4,5)P₃-mediated increases in excitability in smooth muscle and promote relaxation.

Functional signaling biases in G protein-coupled receptors: Game Theory and receptor dynamics

Pharmacotherapeutic targeting of G protein-coupled receptors (GPCRs) is perhaps the most important field of drug design, as agents designed to control these receptors constitute more than half of the pharmacopeia. Initially GPCRs were considered to be unitary entities, possessing all of their potential functionality in their characteristic heptahelical core. Early models of the functional activity of GPCRs considered them to possess just a simple 'on' or 'off' status. Recent research however has allowed us to realize that GPCR functionality is dependent upon many other proteins outside of the heptahelical core, on the site of GPCR expression in a tissue or a microdomain in a cell, and, most importantly, on the formation of differential 'active' states preferentially coupled to specific signal transduction structures. The recognition of such signaling diversity has facilitated the ability to appreciate and identify ligands for GPCRs that demonstrate a bias towards one signaling form of a receptor to another. However while potentially increasing our ability for selective signal targeting, our approach to understanding the physiological ramifications of systemic signaling manipulation is underdeveloped. This explosion in the complexity of GPCR signaling is now becoming familiar territory to receptor biologists, yet the application of this knowledge to drug design is relatively limited. This review will attempt to outline potential pitfalls and unseen benefits of using signaling bias in therapeutic design as well as highlighting new applications such as Game Theory for uncovering new therapeutic applications for biased agonists.

VENNTURE--a novel Venn diagram investigational tool for multiple pharmacological dataset analysis

As pharmacological data sets become increasingly large and complex, new visual analysis and filtering programs are needed to aid their appreciation. One of the most commonly used methods for visualizing biological data is the Venn diagram. Currently used Venn analysis software often presents multiple problems to biological scientists, in that only a limited number of simultaneous data sets can be analyzed. An improved appreciation of the connectivity between multiple, highly-complex datasets is crucial for the next generation of data analysis of genomic and proteomic data streams. We describe the development of VENNTURE, a program that facilitates visualization of up to six datasets in a user-friendly manner. This program includes versatile output features, where grouped data points can be easily exported into a spreadsheet. To demonstrate its unique experimental utility we applied VENNTURE to a highly complex parallel paradigm, i.e. comparison of multiple G protein-coupled receptor drug dose phosphoproteomic data, in multiple cellular physiological contexts. VENNTURE was able to reliably and simply dissect six complex data sets into easily identifiable groups for straightforward analysis and data output. Applied to complex pharmacological datasets, VENNTURE's improved features and ease of analysis are much improved over currently available Venn diagram programs. VENNTURE enabled the delineation of highly complex patterns of dose-dependent G protein-coupled receptor activity and its dependence on physiological cellular contexts. This study highlights the potential for such a program in fields such as pharmacology, genomics, and bioinformatics.

Effective correction of experimental errors in quantitative proteomics using stable isotope labeling by amino acids in cell culture (SILAC)

Accurate and reliable quantitative proteomics in cell culture has been considerably facilitated by the introduction of the stable isotope labeling by amino acids in cell culture (SILAC), combined with high resolution mass spectrometry. There are however several major sources of quantification errors that commonly occur with SILAC techniques, i.e. incomplete incorporation of isotopic amino acids, arginine-to-proline conversion, and experimental errors in final sample mixing. Dataset normalization is a widely adopted solution to such errors, however this may not completely prevent introducing incorrect expression ratios. Here we demonstrate that a label-swap replication of SILAC experiments was able to effectively correct experimental errors by averaging ratios measured in individual replicates using quantitative proteomics and phosphoproteomics of ligand treatment of neural cell cultures. Furthermore, this strategy was successfully applied to a SILAC triplet experiment, which presents a much more complicated experimental matrix, affected by both incomplete labeling and arginine-to-proline conversion. Based on our results, we suggest that SILAC experiments should be designed to incorporate label-swap replications for enhanced reliability in expression ratios.

G protein-coupled receptors in the hypothalamic paraventricular and supraoptic nuclei--serpentine gateways to neuroendocrine homeostasis

G protein-coupled receptors (GPCRs) are the largest family of transmembrane receptors in the mammalian genome. They are activated by a multitude of different ligands that elicit rapid intracellular responses to regulate cell function. Unsurprisingly, a large proportion of therapeutic agents target these receptors. The paraventricular nucleus (PVN) and supraoptic nucleus (SON) of the hypothalamus are important mediators in homeostatic control. Many modulators of PVN/SON activity, including neurotransmitters and hormones act via GPCRs--in fact over 100 non-chemosensory GPCRs have been detected in either the PVN or SON. This review provides a comprehensive summary of the expression of GPCRs within the PVN/SON, including data from recent transcriptomic studies that potentially expand the repertoire of GPCRs that may have functional roles in these hypothalamic nuclei. We also present some aspects of the regulation and known roles of GPCRs in PVN/SON, which are likely complemented by the activity of 'orphan' GPCRs.

Bioinformatic approaches to metabolic pathways analysis

The growth and development in the last decade of accurate and reliable mass data collection techniques has greatly enhanced our comprehension of cell signaling networks and pathways. At the same time however, these technological advances have also increased the difficulty of satisfactorily analyzing and interpreting these ever-expanding datasets. At the present time, multiple diverse scientific communities including molecular biological, genetic, proteomic, bioinformatic, and cell biological, are converging upon a common endpoint, that is, the measurement, interpretation, and potential prediction of signal transduction cascade activity from mass datasets. Our ever increasing appreciation of the complexity of cellular or receptor signaling output and the structural coordination of intracellular signaling cascades has to some extent necessitated the generation of a new branch of informatics that more closely associates functional signaling effects to biological actions and even whole-animal phenotypes. The ability to untangle and hopefully generate theoretical models of signal transduction information flow from transmembrane receptor systems to physiological and pharmacological actions may be one of the greatest advances in cell signaling science. In this overview, we shall attempt to assist the navigation into this new field of cell signaling and highlight several methodologies and technologies to appreciate this exciting new age of signal transduction.

'Biasing' the parathyroid hormone receptor: a novel anabolic approach to increasing bone mass?

'Functional selectivity' refers to the ability of a ligand to activate and/or inhibit only a subset of the signals capable of emanating from its cognate G-protein-coupled receptor (GPCR). Whereas conventional GPCR agonism and antagonism can be viewed as modulating the quantity of efficacy, functionally selective or 'biased' ligands qualitatively change the nature of information flow across the plasma membrane, raising the prospect of drugs with improved therapeutic efficacy or reduced side effects. Nonetheless, there is little experimental evidence that biased ligands offer advantages over conventional agonists/antagonists in vivo. Recent work with the type I parathyroid hormone receptor (PTH(1) R) suggests that biased ligands that selectively activate G-protein-independent arrestin-mediated signalling pathways may hold promise in the treatment of osteoporosis. Parathyroid hormone (PTH) is a principle regulator of bone and calcium metabolism. In bone, PTH exerts complex effects; promoting new bone formation through direct actions on osteoblasts while simultaneously stimulating bone loss through indirect activation of osteoclastic bone resorption. Although the conventional PTH(1) R agonist teriparatide, PTH(1-34), is effective in the treatment of osteoporosis, its utility is limited by its bone-resorptive effects and propensity to promote hypercalcaemia/hypercalcuria. In contrast, d-Trp(12) ,Tyr(34) -bPTH(7-34) (PTH-βarr), an arrestin pathway-selective agonist for the PTH(1) R, induces anabolic bone formation independent of classic G-protein-coupled signalling mechanisms. Unlike PTH(1-34), PTH-βarr appears to 'uncouple' the anabolic effects of PTH(1) R activation from its catabolic and calcitropic effects. Such findings offer evidence that arrestin pathway-selective GPCR agonists can elicit potentially beneficial effects in vivo that cannot be achieved using conventional agonist or antagonist ligands.

ss-TEA: Entropy based identification of receptor specific ligand binding residues from a multiple sequence alignment of class A GPCRs

BACKGROUND

G-protein coupled receptors (GPCRs) are involved in many different physiological processes and their function can be modulated by small molecules which bind in the transmembrane (TM) domain. Because of their structural and sequence conservation, the TM domains are often used in bioinformatics approaches to first create a multiple sequence alignment (MSA) and subsequently identify ligand binding positions. So far methods have been developed to predict the common ligand binding residue positions for class A GPCRs.

RESULTS

Here we present 1) ss-TEA, a method to identify specific ligand binding residue positions for any receptor, predicated on high quality sequence information. 2) The largest MSA of class A non olfactory GPCRs in the public domain consisting of 13324 sequences covering most of the species homologues of the human set of GPCRs. A set of ligand binding residue positions extracted from literature of 10 different receptors shows that our method has the best ligand binding residue prediction for 9 of these 10 receptors compared to another state-of-the-art method.

CONCLUSIONS

The combination of the large multi species alignment and the newly introduced residue selection method ss-TEA can be used to rapidly identify subfamily specific ligand binding residues. This approach can aid the design of site directed mutagenesis experiments, explain receptor function and improve modelling. The method is also available online via GPCRDB at http://www.gpcr.org/7tm/.

Identifying the immunodeficiency gateway proteins in humans and their involvement in microRNA regulation

Very little is known to date about the regulation protocol between transcription factors (TFs), genes and microRNAs (miRNAs) associated with diseases in various organisms. In this paper, we focus on finding the activity of miRNAs through the HIV-1 regulatory pathway in humans at the systems level. For this, we integrate and study the characteristics of the interaction information between HIV-1 and human proteins obtained from literature and prediction analysis. This information, realized in the form of a bipartite network, is subsequently mined with an exhaustive graph search technique to identify the strong significant biclusters, which are effectively the bicliques. They are unified further to form the core bipartite subnetwork. Many of the known HIV-1 associated kinase proteins (including LCK) are found in this core module. From this, the secondary significant proteins are identified by mapping these gateway proteins to the human protein-protein interaction network. Finally, these proteins are mapped onto the TF-to-miRNA and miRNA-to-gene regulatory networks derived from a couple of current studies to obtain a global view of the HIV-1 mediated TF-gene-miRNA inter-regulatory network. Interestingly, a few miRNAs participating in this pathway at the secondary level are found to have oncogenic involvement.

Terbutaline causes immobilization of single β2-adrenergic receptor-ligand complexes in the plasma membrane of living A549 cells as revealed by single-molecule microscopy

G-protein-coupled receptors are important targets for various drugs. After signal transduction, regulatory processes, such as receptor desensitization and internalization, change the lateral receptor mobility. In order to study the lateral diffusion of β(2)-adrenergic receptors (β(2)AR) complexed with fluorescently labeled noradrenaline (Alexa-NA) in plasma membranes of A549 cells, trajectories of single receptor-ligand complexes were monitored using single-particle tracking. We found that a fraction of 18% of all β(2)ARs are constitutively immobile. About 2/3 of the β(2)ARs moved with a diffusion constant of D(2) = 0.03 ± 0.001 μm(2)/s and about 17% were diffusing five-fold faster (D(3) = 0.15 ± 0.02 μm(2)/s). The mobile receptors moved within restricted domains and also showed a discontinuous diffusion behavior. Analysis of the trajectory lengths revealed two different binding durations with τ(1) = 77 ± 1 ms and τ(2) = 388 ± 11 ms. Agonistic stimulation of the β(2)AR-Alexa-NA complexes with 1 μM terbutaline caused immobilization of almost 50% of the receptors within 35 min. Simultaneously, the mean area covered by the mobile receptors decreased significantly. Thus, we demonstrated that agonistic stimulation followed by cell regulatory processes results in a change in β(2)AR mobility suggesting that different receptor dynamics characterize different receptor states.

Allosteric modulation of family C G-protein-coupled receptors: from molecular insights to therapeutic perspectives

Allosteric receptor modulation is an attractive concept in drug targeting because it offers important potential advantages over conventional orthosteric agonism or antagonism. Allosteric ligands modulate receptor function by binding to a site distinct from the recognition site for the endogenous agonist. They often have no effect on their own and therefore act only in conjunction with physiological receptor activation. This article reviews the current status of allosteric modulation at family C G-protein coupled receptors in the light of their specific structural features on the one hand and current concepts in receptor theory on the other hand. Family C G-protein-coupled receptors are characterized by a large extracellular domain containing the orthosteric agonist binding site known as the "venus flytrap module" because of its bilobal structure and the dynamics of its activation mechanism. Mutational analysis and chimeric constructs have revealed that allosteric modulators of the calcium-sensing, metabotropic glutamate and GABA(B) receptors bind to the seven transmembrane domain, through which they modify signal transduction after receptor activation. This is in contrast to taste-enhancing molecules, which bind to different parts of sweet and umami receptors. The complexity of interactions between orthosteric and allosteric ligands is revealed by a number of adequate biochemical and electrophysiological assay systems. Many allosteric family C GPCR modulators show in vivo efficacy in behavioral models for a variety of clinical indications. The positive allosteric calcium sensing receptor modulator cinacalcet is the first drug of this type to enter the market and therefore provides proof of principle in humans.

Refining efficacy: allosterism and bias in G protein-coupled receptor signaling

Receptors on the surface of cells function as conduits for information flowing between the external environment and the cell interior. Since signal transduction is based on the physical interaction of receptors with both extracellular ligands and intracellular effectors, ligand binding must produce conformational changes in the receptor that can be transmitted to the intracellular domains accessible to G proteins and other effectors. Classical models of G protein-coupled receptor (GPCR) signaling envision receptor conformations as highly constrained, wherein receptors exist in equilibrium between single "off" and "on" states distinguished by their ability to activate effectors, and ligands act by perturbing this equilibrium. In such models, ligands can be classified based upon two simple parameters; affinity and efficacy, and ligand activity is independent of the assay used to detect the response. However, it is clear that GPCRs assume multiple conformations, any number of which may be capable of interacting with a discrete subset of possible effectors. Both orthosteric ligands, molecules that occupy the natural ligand-binding pocket, and allosteric modulators, small molecules or proteins that contact receptors distant from the site of ligand binding, have the ability to alter the conformational equilibrium of a receptor in ways that affect its signaling output both qualitatively and quantitatively. In this context, efficacy becomes pluridimensional and ligand classification becomes assay dependent. A more complete description of ligand-receptor interaction requires the use of multiplexed assays of receptor activation and screening assays may need to be tailored to detect specific efficacy profiles.

Refining efficacy: exploiting functional selectivity for drug discovery

Early models of G protein-coupled receptor (GPCR) activation envisioned the receptor in equilibrium between unique "off" and "on" states, wherein ligand binding affected signaling by increasing or decreasing the fraction of receptors in the active conformation. It is now apparent that GPCRs spontaneously sample multiple conformations, any number of which may couple to one or more downstream effectors. Such "multistate" models imply that the receptor-ligand complex, not the receptor alone, defines which active receptor conformations predominate. "Functional selectivity" refers to the ability of a ligand to activate only a subset of its receptor's signaling repertoire. There are now numerous examples of ligands that "bias" receptor coupling between different G protein pools and non-G protein effectors such as arrestins. The type 1 parathyroid hormone receptor (PTH(1)R) is a particularly informative example, not only because of the range of biased effects that have been produced, but also because the actions of many of these ligands have been characterized in vivo. Biased PTH(1)R ligands can selectively couple the PTH(1)R to G(s) or G(q/11) pathways, with or without activating arrestin-dependent receptor desensitization and signaling. These reagents have provided insight into the contribution of different signaling pathways to PTH action in vivo and suggest it may be possible to exploit ligand bias to uncouple the anabolic effects of PTH(1)R from its catabolic and calcitropic effects. Whereas conventional agonists and antagonists only modulate the quantity of efficacy, functionally selective ligands qualitatively change GPCR signaling, offering the prospect of drugs with improved therapeutic efficacy or reduced side effects.

Repetitive peroxide exposure reveals pleiotropic mitogen-activated protein kinase signaling mechanisms

Oxidative stressors such as hydrogen peroxide control the activation of many interconnected signaling systems and are implicated in neurodegenerative disease etiology. Application of hydrogen peroxide to PC12 cells activated multiple tyrosine kinases (c-Src, epidermal growth factor receptor (EGFR), and Pyk2) and the serine-threonine kinase ERK1/2. Peroxide-induced ERK1/2 activation was sensitive to intracellular calcium chelation and EGFR and c-Src kinase inhibition. Acute application and removal of peroxide allowed ERK1/2 activity levels to rapidly subside to basal serum-deprived levels. Using this protocol, we demonstrated that ERK1/2 activation tachyphylaxis developed upon repeated peroxide exposures. This tachyphylaxis was independent of c-Src/Pyk2 tyrosine phosphorylation but was associated with a progressive reduction of peroxide-induced EGFR tyrosine phosphorylation, EGFR interaction with growth factor receptor binding protein 2, and a redistribution of EGFR from the plasma membrane to the cytoplasm. Our data indicates that components of peroxide-induced ERK1/2 cascades are differentially affected by repeated exposures, indicating that oxidative signaling may be contextually variable.

Complex and multidimensional lipid raft alterations in a murine model of Alzheimer's disease

Various animal models of Alzheimer's disease (AD) have been created to assist our appreciation of AD pathophysiology, as well as aid development of novel therapeutic strategies. Despite the discovery of mutated proteins that predict the development of AD, there are likely to be many other proteins also involved in this disorder. Complex physiological processes are mediated by coherent interactions of clusters of functionally related proteins. Synaptic dysfunction is one of the hallmarks of AD. Synaptic proteins are organized into multiprotein complexes in high-density membrane structures, known as lipid rafts. These microdomains enable coherent clustering of synergistic signaling proteins. We have used mass analytical techniques and multiple bioinformatic approaches to better appreciate the intricate interactions of these multifunctional proteins in the 3xTgAD murine model of AD. Our results show that there are significant alterations in numerous receptor/cell signaling proteins in cortical lipid rafts isolated from 3xTgAD mice.

Gonadotropin-releasing hormone receptor system: modulatory role in aging and neurodegeneration

Receptors for hormones of the hypothalamic-pituitary-gonadal axis are expressed throughout the brain. Age-related decline in gonadal reproductive hormones cause imbalances of this axis and many hormones in this axis have been functionally linked to neurodegenerative pathophysiology. Gonadotropin-releasing hormone (GnRH) plays a vital role in both central and peripheral reproductive regulation. GnRH has historically been known as a pituitary hormone; however, in the past few years, interest has been raised in GnRH actions at non-pituitary peripheral targets. GnRH ligands and receptors are found throughout the brain where they may act to control multiple higher functions such as learning and memory function and feeding behavior. The actions of GnRH in mammals are mediated by the activation of a unique rhodopsin-like G protein-coupled receptor that does not possess a cytoplasmic carboxyl terminal sequence. Activation of this receptor appears to mediate a wide variety of signaling mechanisms that show diversity in different tissues. Epidemiological support for a role of GnRH in central functions is evidenced by a reduction in neurodegenerative disease after GnRH agonist therapy. It has previously been considered that these effects were not via direct GnRH action in the brain, however recent data has pointed to a direct central action of these ligands outside the pituitary. We have therefore summarized the evidence supporting a central direct role of GnRH ligands and receptors in controlling central nervous physiology and pathophysiology.

Discontinuous pH gradient-mediated separation of TiO2-enriched phosphopeptides

Global profiling of phosphoproteomes has proven to be a great challenge due to the relatively low stoichiometry of protein phosphorylation and poor ionization efficiency in mass spectrometers. Effective, physiologically relevant, phosphoproteome research relies on the efficient phosphopeptide enrichment from complex samples. Immobilized metal affinity chromatography and titanium dioxide chromatography can greatly assist selective phosphopeptide enrichment. However, the complexity of resultant enriched samples is often still high, suggesting that further separation of enriched phosphopeptides is required. We have developed a pH gradient elution technique for enhanced phosphopeptide identification in conjunction with titanium dioxide chromatography. Using this process, we demonstrated its superiority to the traditional "one-pot" strategies for differential protein identification. Our technique generated a highly specific separation of phosphopeptides by an applied pH gradient between 9.2 and 11.3. The most efficient elution range for high-resolution phosphopeptide separation was between pHs 9.2 and 9.4. High-resolution separation of multiply phosphorylated peptides was primarily achieved using elution ranges greater than pH 9.4. Investigation of phosphopeptide sequences identified in each pH fraction indicated that phosphopeptides with phosphorylated residues proximal to acidic residues, including glutamic acid, aspartic acid, and other phosphorylated residues, were preferentially eluted at higher pH values.

An evolutionarily conserved arginine is essential for Tre1 G protein-coupled receptor function during germ cell migration in Drosophila melanogaster

BACKGROUND

G protein-coupled receptors (GPCRs) play central roles in mediating cellular responses to environmental signals leading to changes in cell physiology and behaviors, including cell migration. Numerous clinical pathologies including metastasis, an invasive form of cell migration, have been linked to abnormal GPCR signaling. While the structures of some GPCRs have been defined, the in vivo roles of conserved amino acid residues and their relationships to receptor function are not fully understood. Trapped in endoderm 1 (Tre1) is an orphan receptor of the rhodopsin class that is necessary for primordial germ cell migration in Drosophila melanogaster embryos. In this study, we employ molecular genetic approaches to identify residues in Tre1 that are critical to its functions in germ cell migration.

METHODOLOGY/PRINCIPAL FINDINGS

First, we show that the previously reported scattershot mutation is an allele of tre1. The scattershot allele results in an in-frame deletion of 8 amino acids at the junction of the third transmembrane domain and the second intracellular loop of Tre1 that dramatically impairs the function of this GPCR in germ cell migration. To further refine the molecular basis for this phenotype, we assayed the effects of single amino acid substitutions in transgenic animals and determined that the arginine within the evolutionarily conserved E/N/DRY motif is critical for receptor function in mediating germ cell migration within an intact developing embryo.

CONCLUSIONS/SIGNIFICANCE

These structure-function studies of GPCR signaling in native contexts will inform future studies into the basic biology of this large and clinically important family of receptors.

Beyond desensitization: physiological relevance of arrestin-dependent signaling

Heptahelical G protein-coupled receptors are the most diverse and therapeutically important family of receptors in the human genome. Ligand binding activates heterotrimeric G proteins that transmit intracellular signals by regulating effector enzymes or ion channels. G protein signaling is terminated, in large part, by arrestin binding, which uncouples the receptor and G protein and targets the receptor for internalization. It is clear, however, that heptahelical receptor signaling does not end with desensitization. Arrestins bind a host of catalytically active proteins and serve as ligand-regulated scaffolds that recruit protein and lipid kinase, phosphatase, phosphodiesterase, and ubiquitin ligase activity into the receptor-arrestin complex. Although many of these arrestin-bound effectors serve to modulate G protein signaling, degrading second messengers and regulating endocytosis and trafficking, other signals seem to extend beyond the receptor-arrestin complex to regulate such processes as protein translation and gene transcription. Although these findings have led to a re-envisioning of heptahelical receptor signaling, little is known about the physiological roles of arrestin-dependent signaling. In vivo, the duality of arrestin function makes it difficult to dissociate the consequences of arrestin-dependent desensitization from those that might be ascribed to arrestin-mediated signaling. Nonetheless, recent evidence generated using arrestin knockouts, G protein-uncoupled receptor mutants, and arrestin pathway-selective "biased agonists" is beginning to reveal that arrestin signaling plays important roles in the retina, central nervous system, cardiovascular system, bone remodeling, immune system, and cancer. Understanding the signaling roles of arrestins may foster the development of pathway-selective drugs that exploit these pathways for therapeutic benefit.

Adenosine mediated desensitization of cAMP signaling enhances T-cell responses

Adenosine has long been regarded as a crucial anti-inflammatory agent that protects the host from excessive damage. It has been reported to play an important role in suppressing immune activation, particularly that of T cells. However, it is a general observation that induction of T-cell activation is an efficient event despite the high adenosine levels that are often present in the affected host due to injury or stress. We report here that prior to antigenic stimulation via TCR/CD3, exposure of T cells to adenosine desensitizes adenosine receptors, so as to create a window of time where the T cells are insensitive to this ubiquitous suppressor. T cells from mice that were pre-exposed to this manipulation showed stronger responses to antigenic stimulation; therefore, the P1 adenosine receptor desensitization demonstrated an adjuvant-like effect. Our results suggest that adenosine receptor desensitization may be a mechanism for T cells to escape the general suppression during early points of T-cell activation and may emerge as a potential alternative for vaccine adjuvants.

Lack of effect of the alpha2C-adrenoceptor Del322-325 polymorphism on inhibition of cyclic AMP production in HEK293 cells

BACKGROUND AND PURPOSE

The alpha(2C)-adrenoceptor has multiple functions, including inhibiting release of noradrenaline from presynaptic nerve terminals. A human alpha(2C) polymorphism, Del322-325, a potential risk factor for heart failure, has been reported to exhibit reduced signalling in CHO cells. To further understand the role of the Del322-325 polymorphism on receptor signalling, we attempted to replicate and further study the reduced signalling in HEK293 cells.

EXPERIMENTAL APPROACH

Human alpha(2C) wild-type (WT) and Del322-325 adrenoceptors were stably transfected into HEK293 cells. Radioligand binding was performed to determine affinities for both receptors. In intact cells, inhibition of forskolin-stimulated cyclic AMP production by WT and Del322-325 clones with a range of receptor densities (200-2320 fmol.mg(-1) protein) was measured following agonist treatment.

KEY RESULTS

Noradrenaline, brimonidine and clonidine exhibited similar binding affinities for WT and Del322-325. Brimonidine and clonidine also had similar efficacies and potencies for both receptors for the inhibition of cyclic AMP production at all receptor densities tested. A linear regression analysis comparing efficacy and potency with receptor expression levels showed no differences in slopes between WT and Del322-325.

CONCLUSIONS AND IMPLICATIONS

The alpha(2C) WT and Del322-325 adrenoceptors exhibited similar binding properties. Additionally, inhibition of cyclic AMP production by Del322-325 was similar to that of WT over a range of receptor densities. Therefore, in intact HEK293 cells, the alpha(2C)-Del322-325 polymorphism does not exhibit reduced signalling to adenylyl cyclase and may not represent a clinically important phenotype.