Functional expression of heteromeric calcitonin gene-related peptide and adrenomedullin receptors in yeast

Yeast-based screening platforms to understand and improve human health

Detailed molecular understanding of the human organism is essential to develop effective therapies. Saccharomyces cerevisiae has been used extensively for acquiring insights into important aspects of human health, such as studying genetics and cell-cell communication, elucidating protein-protein interaction (PPI) networks, and investigating human G protein-coupled receptor (hGPCR) signaling. We highlight recent advances and opportunities of yeast-based technologies for cost-efficient chemical library screening on hGPCRs, accelerated deciphering of PPI networks with mating-based screening and selection, and accurate cell-cell communication with human immune cells. Overall, yeast-based technologies constitute an important platform to support basic understanding and innovative applications towards improving human health.

Engineering G protein-coupled receptor signalling in yeast for biotechnological and medical purposes

G protein-coupled receptors (GPCRs) comprise the largest class of membrane proteins in the human genome, with a common denominator of seven-transmembrane domains largely conserved among eukaryotes. Yeast is naturally armoured with three different GPCRs for pheromone and sugar sensing, with the pheromone pathway being extensively hijacked for characterising heterologous GPCR signalling in a model eukaryote. This review focusses on functional GPCR studies performed in yeast and on the elucidated hotspots for engineering, and discusses both endogenous and heterologous GPCR signalling. Key emphasis will be devoted to studies describing important engineering parameters to consider for successful coupling of GPCRs to the yeast mating pathway. We also review the various means of applying yeast for studying GPCRs, including the use of yeast armed with heterologous GPCRs as a platform for (i) deorphanisation of orphan receptors, (ii) metabolic engineering of yeast for production of bioactive products and (iii) medical applications related to pathogen detection and drug discovery. Finally, this review summarises the current challenges related to expression of functional membrane-bound GPCRs in yeast and discusses the opportunities to continue capitalising on yeast as a model chassis for functional GPCR signalling studies.

G protein-coupled receptors expressed and studied in yeast. The adenosine receptor as a prime example

G protein-coupled receptors (GPCRs) are the largest class of membrane proteins with around 800 members in the human genome/proteome. Extracellular signals such as hormones and neurotransmitters regulate various biological processes via GPCRs, with GPCRs being the bodily target of 30-40% of current drugs on the market. Complete identification and understanding of GPCR functionality will provide opportunities for novel drug discovery. Yeast expresses three different endogenous GPCRs regulating pheromone and sugar sensing, with the pheromone pathway offering perspectives for the characterization of heterologous GPCR signaling. Moreover, yeast offers a ''null" background for studies on mammalian GPCRs, including GPCR activation and signaling, ligand identification, and characterization of disease-related mutations. This review focuses on modifications of the yeast pheromone signaling pathway for functional GPCR studies, and on opportunities and usage of the yeast system as a platform for human GPCR studies. Finally, this review discusses in some further detail studies of adenosine receptors heterologously expressed in yeast, and what Geoff Burnstock thought of this approach.

Engineering a Model Cell for Rational Tuning of GPCR Signaling

G protein-coupled receptor (GPCR) signaling is the primary method eukaryotes use to respond to specific cues in their environment. However, the relationship between stimulus and response for each GPCR is difficult to predict due to diversity in natural signal transduction architecture and expression. Using genome engineering in yeast, we constructed an insulated, modular GPCR signal transduction system to study how the response to stimuli can be predictably tuned using synthetic tools. We delineated the contributions of a minimal set of key components via computational and experimental refactoring, identifying simple design principles for rationally tuning the dose response. Using five different GPCRs, we demonstrate how this enables cells and consortia to be engineered to respond to desired concentrations of peptides, metabolites, and hormones relevant to human health. This work enables rational tuning of cell sensing while providing a framework to guide reprogramming of GPCR-based signaling in other systems.

New approaches in bioprocess-control: Consortium guidance by synthetic cell-cell communication based on fungal pheromones

Bioconversions in industrial processes are currently dominated by single-strain approaches. With the growing complexity of tasks to be carried out, microbial consortia become increasingly advantageous and eventually may outperform single-strain fermentations. Consortium approaches benefit from the combined metabolic capabilities of highly specialized strains and species, and the inherent division of labor reduces the metabolic burden for each strain while increasing product yields and reaction specificities. However, consortium-based designs still suffer from a lack of available tools to control the behavior and performance of the individual subpopulations and of the entire consortium. Here, we propose to implement novel control elements for microbial consortia based on artificial cell-cell communication via fungal mating pheromones. Coupling to the desired output is mediated by pheromone-responsive gene expression, thereby creating pheromone-dependent communication channels between different subpopulations of the consortia. We highlight the benefits of artificial communication to specifically target individual subpopulations of microbial consortia and to control e.g. their metabolic profile or proliferation rate in a predefined and customized manner. Due to the steadily increasing knowledge of sexual cycles of industrially relevant fungi, a growing number of strains and species can be integrated into pheromone-controlled sensor-actor systems, exploiting their unique metabolic properties for microbial consortia approaches.

Vascular and molecular pharmacology of the metabolically stable CGRP analogue, SAX

The main purpose of this study was to compare in vitro pharmacological properties of human αCGRP (CGRP) and a recently discovered metabolically stable CGRP analogue, SAX, in isolated rat and human artery segments. In rat, CGRP and SAX induced similar vasodilatory responses in isolated mesenteric artery with the potency of SAX being lower than that of CGRP (vasodilatory pEC₅₀ 8.2 ± 0.12 and 9.0 ± 0.11, respectively). A corresponding difference in receptor binding affinity of SAX and CGRP was determined in rat cerebral membranes (pKi 8.3 ± 0.19 and 9.3 ± 0.14, respectively). CGRP and SAX-induced vasodilation was antagonised with similar potencies by the CGRP receptor antagonist BIBN4096BS supporting a uniform receptor population for the agonists. In human tissue, SAX and CGRP induced similar pharmacological responses with different potencies in subcutaneous artery (vasodilatory pEC₅₀ 8.8 ± 0.18 and 9.5 ± 0.13, respectively) and human recombinant receptors (cAMP signalling pEC₅₀ 9.1 ± 0.16 and 10.2 ± 0.19). Like in the rat mesenteric artery, both SAX and CGRP-responses were inhibited by the CGRP receptor antagonist BIBN4096BS with similar antagonistic potencies. In conclusion, all pharmacological characteristics of SAX and CGRP in human and rat sources points towards action via a uniform BIBN4096BS sensitive receptor population with the potency of SAX being 5-10 fold lower than that of CGRP.

Calcitonin gene-related peptide (CGRP): A novel target for Alzheimer's disease

Alzheimer's disease (AD) is leading cause of death among older characterized by neurofibrillary tangles, oxidative stress, progressive neuronal deficits, and increased levels of amyloid-β (Aβ) peptides. Cholinergic treatment could be the best suitable physiological therapy for AD. Calcitonin gene-related peptide (CGRP) is a thirty-seven-amino acid regulatory neuropeptide resulting from different merging of the CGRP gene, which also includes adrenomedullin, amylin, calcitonin, intermedin, and calcitonin receptor-stimulating peptide. It is a proof for a CGRP receptor within nucleus accumbens of brain that is different from either the CGRP1 or CGRP2 receptor in which it demonstrates similar high-affinity binding for salmon calcitonin, CGRP, and amylin, a possession which is not shared by any extra CGRP receptors. Binding of CGRP to its receptor increases activated cAMP-dependent pkA and PI3 kinase, resulting in N-terminal fragments that are shown to exert complex inhibitory as well facilitator actions on nAChRs. Fragments such as CGRP1-4, CGRP1-5, and CGRP1-6 rapidly as well as reversibly improve agonist sensitivity of nAChRs without straight stimulating those receptors and produce the Ca2+ -induced intracellular Ca2+ mobilization. Renin-angiotensin-aldosterone system (RAAS)-activated angiotensin-type (AT4) receptor is also beneficial in AD. It has been suggested that exogenous administration of CGRP inhibits infiltration of macrophages and expression of various inflammatory mediators such as NFkB, IL-1b, TNF-α, iNOS, matrix metalloproteinase (MMP)-9, and cell adhesion molecules like intercellular adhesion molecule (ICAM)-1 which attenuates consequence of inflammation in AD. Donepezil, a ChEI, inhibits acetylcholinesterase and produces angiogenesis and neurogenesis, in the dentate gyrus of the hippocampus of WT mice after donepezil administration. However, none of the results discovered in CGRP-knockout mice and WT mice exposed to practical denervation. Therefore, selective agonists of CGRP receptors may become the potential candidates for treatment of AD.

Receptor Activity-modifying Protein-directed G Protein Signaling Specificity for the Calcitonin Gene-related Peptide Family of Receptors

The calcitonin gene-related peptide (CGRP) family of G protein-coupled receptors (GPCRs) is formed through the association of the calcitonin receptor-like receptor (CLR) and one of three receptor activity-modifying proteins (RAMPs). Binding of one of the three peptide ligands, CGRP, adrenomedullin (AM), and intermedin/adrenomedullin 2 (AM2), is well known to result in a Gα_s-mediated increase in cAMP. Here we used modified yeast strains that couple receptor activation to cell growth, via chimeric yeast/Gα subunits, and HEK-293 cells to characterize the effect of different RAMP and ligand combinations on this pathway. We not only demonstrate functional couplings to both Gα_s and Gα_q but also identify a Gα_i component to CLR signaling in both yeast and HEK-293 cells, which is absent in HEK-293S cells. We show that the CGRP family of receptors displays both ligand- and RAMP-dependent signaling bias among the Gα_s, Gα_i, and Gα_q/11 pathways. The results are discussed in the context of RAMP interactions probed through molecular modeling and molecular dynamics simulations of the RAMP-GPCR-G protein complexes. This study further highlights the importance of RAMPs to CLR pharmacology and to bias in general, as well as identifying the importance of choosing an appropriate model system for the study of GPCR pharmacology.

Involvement of Receptor Activity-Modifying Protein 3 (RAMP3) in the Vascular Actions of Adrenomedullin in Rat Mesenteric Artery Smooth Muscle Cells

CALCB, ADM, and ADM2 are potent vasodilators that share a seven-transmembrane GPCR, calcitonin receptor-like receptor (CALCRL), whose ligand specificity is dictated by the presence of one of the three receptor activity-modifying proteins (RAMPs). We assessed the relative pharmacologic potency of these peptides in mesenteric artery smooth muscle cells (VSMCs) and the specific RAMP that mediates the effect of ADM in VSMCs. VSMCs, with or without RAMP knockdown, were treated with CALCB, ADM, or ADM2 in the presence or absence of their antagonists, CALCB8-37, ADM22-52, and ADM217-47, respectively, to assess the relative effect of peptides on cAMP production and their pharmacologic potency. Proximity ligation assay was used to assess the specific RAMP that associates with CALCRL to mediate the actions of ADM in VSMCs. All three peptides induced cAMP generation in VSMCs and the order of their potency is CALCB > ADM > ADM2. Effects of CALCB were blocked by CALCB8-37, ADM effects were blocked by CALCB8-37 and ADM217-47 but not ADM22-52, and ADM2 effects were blocked by all three antagonists. Knockdown of RAMP2 was ineffective, whereas knockdown of RAMP3 inhibited ADM-induced cAMP production in VSMCs, suggesting involvement of RAMP3 with CALCRL to mediate ADM effects. Absence of both RAMP2 and RAMP3 further increased CALCB-induced cAMP synthesis compared to control (P < 0.05). ADM increased CALCRL and RAMP3 association and RAMP3 knockdown inhibited the interaction of ADM with CALCRL.

Investigating G protein signalling bias at the glucagon-like peptide-1 receptor in yeast

BACKGROUND AND PURPOSE

The glucagon-like peptide 1 (GLP-1) receptor performs an important role in glycaemic control, stimulating the release of insulin. It is an attractive target for treating type 2 diabetes. Recently, several reports of adverse side effects following prolonged use of GLP-1 receptor therapies have emerged: most likely due to an incomplete understanding of signalling complexities.

EXPERIMENTAL APPROACH

We describe the expression of the GLP-1 receptor in a panel of modified yeast strains that couple receptor activation to cell growth via single Gα/yeast chimeras. This assay enables the study of individual ligand-receptor G protein coupling preferences and the quantification of the effect of GLP-1 receptor ligands on G protein selectivity.

KEY RESULTS

The GLP-1 receptor functionally coupled to the chimeras representing the human Gαs, Gαi and Gαq subunits. Calculation of the dissociation constant for a receptor antagonist, exendin-3 revealed no significant difference between the two systems. We obtained previously unobserved differences in G protein signalling bias for clinically relevant therapeutic agents, liraglutide and exenatide; the latter displaying significant bias for the Gαi pathway. We extended the use of the system to investigate small-molecule allosteric compounds and the closely related glucagon receptor.

CONCLUSIONS AND IMPLICATIONS

These results provide a better understanding of the molecular events involved in GLP-1 receptor pleiotropic signalling and establish the yeast platform as a robust tool to screen for more selective, efficacious compounds acting at this important class of receptors in the future.

Adrenomedullin promotes rat trophoblast stem cell differentiation

Accumulating data suggest that adrenomedullin (ADM) regulates the trophoblast cell growth, migration, and invasion. However, the effect of ADM on trophoblast differentiation is poorly understood. In this study, we hypothesized that ADM promotes the differentiation of trophoblast stem cells (TSCs) into trophoblast giant cells (TGCs). Using rat TSCs, Rcho-1 cells, we investigated the effect of ADM on TSC differentiation into TGCs in differentiation or stem cell media, respectively, and explored the effect of ADM on the mechanistic target of rapamycin (MTOR) signaling in trophoblast cell differentiation. The results include: 1) in the presence of differentiation medium, 10⁻⁷ M ADM, but not lower doses, elevated (P < 0.05) Prl3b1/Esrrb (i.e., the ratio of mRNA levels) by 1.7-fold compared to that in control; 2) the supplementation of ADM antagonist, regardless of the concentration of ADM, reduced (P < 0.05) Prl3b1/Esrrb by 2-fold, compared to control group, while the supplementation of CGRP antagonist, regardless of the concentration of ADM, did not change Prl3b1/Esrrb; 3) in the presence of stem cell medium, ADM did not alter the expression of TSC and TGC marker genes, however, the ratio of Prl3b1/Esrrb was reduced (P < 0.05) by ADM antagonist compared to that in control; and 4) ADM increased (P < 0.05) phosphorylated MTOR proteins and the ratio of phosphorylated to total MTOR proteins by 2.0- and 1.7-fold, respectively. The results indicate that ADM promotes but does not induce the differentiation of TSCs to TGCs in a dose-dependent manner and MTOR signaling may play a role in this process.

High-throughput de novo screening of receptor agonists with an automated single-cell analysis and isolation system

Reconstitution of signaling pathways involving single mammalian transmembrane receptors has not been accomplished in yeast cells. In this study, intact EGF receptor (EGFR) and a cell wall-anchored form of EGF were co-expressed on the yeast cell surface, which led to autophosphorylation of the EGFR in an EGF-dependent autocrine manner. After changing from EGF to a conformationally constrained peptide library, cells were fluorescently labeled with an anti-phospho-EGFR antibody. Each cell was subjected to an automated single-cell analysis and isolation system that analyzed the fluorescent intensity of each cell and automatically retrieved each cell with the highest fluorescence. In ~3.2 × 10⁶ peptide library, we isolated six novel peptides with agonistic activity of the EGFR in human squamous carcinoma A431 cells. The combination of yeast cells expressing mammalian receptors, a cell wall-anchored peptide library, and an automated single-cell analysis and isolation system might facilitate a rational approach for de novo drug screening.

G protein-coupled receptors: what a difference a 'partner' makes

G protein-coupled receptors (GPCRs) are important cell signaling mediators, involved in essential physiological processes. GPCRs respond to a wide variety of ligands from light to large macromolecules, including hormones and small peptides. Unfortunately, mutations and dysregulation of GPCRs that induce a loss of function or alter expression can lead to disorders that are sometimes lethal. Therefore, the expression, trafficking, signaling and desensitization of GPCRs must be tightly regulated by different cellular systems to prevent disease. Although there is substantial knowledge regarding the mechanisms that regulate the desensitization and down-regulation of GPCRs, less is known about the mechanisms that regulate the trafficking and cell-surface expression of newly synthesized GPCRs. More recently, there is accumulating evidence that suggests certain GPCRs are able to interact with specific proteins that can completely change their fate and function. These interactions add on another level of regulation and flexibility between different tissue/cell-types. Here, we review some of the main interacting proteins of GPCRs. A greater understanding of the mechanisms regulating their interactions may lead to the discovery of new drug targets for therapy.

Calcitonin gene related family peptides: importance in normal placental and fetal development

Synchronized molecular and cellular events occur between the uterus and the implanting embryo to facilitate successful pregnancy outcome. Nevertheless, the molecular signaling network that coordinates strategies for successful decidualization, placentation and fetal growth are not well understood. The discovery of calcitonin/calcitonin gene-related peptides (CT/CGRP) highlighted new signaling mediators in various physiological processes, including reproduction. It is known that CGRP family peptides including CGRP, adrenomedulin and intermedin play regulatory functions during implantation, trophoblast proliferation and invasion, and fetal organogenesis. In addition, all the CGRP family peptides and their receptor components are found to be expressed in decidual, placental and fetal tissues. Additionally, plasma levels of peptides of the CGRP family were found to fluctuate during normal gestation and to induce placental cellular differentiation, proliferation, and critical hormone signaling. Moreover, aberrant signaling of these CGRP family peptides during gestation has been associated with pregnancy disorders. It indicates the existence of a possible regulatory role for these molecules during decidualization and placentation processes, which are known to be particularly vulnerable. In this review, the influence of the CGRP family peptides in these critical processes is explored and discussed.

Frizzled receptors signal through G proteins

Frizzled receptors have long been thought to couple to G proteins but biochemical evidence supporting such an interaction has been lacking. Here we expressed mammalian Wnt-Frizzled fusion proteins in Saccharomyces cerevisiae and tested the receptors' ability to activate the yeast mitogen-activated protein kinase (MAPK) pathway via heterotrimeric G proteins. Our results show that Frizzled receptors can interact with Gαi, Gαq, and Gαs proteins, thus confirming that Frizzled functions as a G protein coupled receptor (GPCR). However, the activity level of Frizzled-mediated G protein signaling was much lower than that of a typical GPCR and, surprisingly, was highest when coupled to Gαs. The Frizzled/Gαs interaction was further established in vivo as Drosophila expressing a loss-of-function Gαs allele rescued the photoreceptor differentiation phenotype of Frizzled mutant flies. Together, these data point to an important role for Frizzled as a nontraditional GPCR that preferentially couples to Gαs heterotrimeric G proteins.

Shared and separate functions of the RAMP-based adrenomedullin receptors

Adrenomedullin (AM) is a novel hypotensive peptide that exerts a variety of strongly protective effects against multiorgan damage. AM-specific receptors were first identified as heterodimers composed of calcitonin-receptor-like receptor (CLR), a G protein coupled receptor, and one of two receptor activity-modifying proteins (RAMP2 or RAMP3), which are accessory proteins containing a single transmembrane domain. RAMPs are required for the surface delivery of CLR and the determination of its phenotype. CLR/RAMP2 (AM₁ receptor) is more highly AM-specific than CLR/RAMP3 (AM₂ receptor). Although there have been no reports showing differences in intracellular signaling via the two AM receptors, in vitro studies have shed light on their distinct trafficking and functionality. In addition, the tissue distributions of RAMP2 and RAMP3 differ, and their gene expression is differentially altered under pathophysiological conditions, which is suggestive of the separate roles played by AM₁ and AM₂ receptors in vivo. Both AM and the AM₁ receptor, but not the AM₂ receptor, are crucial for the development of the fetal cardiovascular system and are able to effectively protect against various vascular diseases. However, AM₂ receptors reportedly play an important role in maintaining a normal body weight in old age and may be involved in immune function. In this review article, we focus on the shared and separate functions of the AM receptor subtypes and also discuss the potential for related drug discovery. In addition, we mention their possible function as receptors for AM2 (or intermedin), an AM-related peptide whose biological functions are similar to those of AM.

Expression of human melanocortin 4 receptor in Saccharomyces cerevisiae

The melanocortin 4 receptor (MC4R) is involved in the regulation of energy homeostasis and is known as one of the major hypothalamic regulators of food intake. Several studies have shown that replacement of aspartic acid at position 126 of the MC4R abolishes the ligand binding. We used the modified yeast Saccharomyces cerevisiae strain MMY28 to functionally express the MC4R and characterise the importance of this amino acid for ligand based activation of the receptor. The efficiency of the functional expression system was estimated by activation with αMSH, ACTH and THIQ and compared with cAMP response in mammalian cells. We generated the library of MC4R mutants randomised at the amino acid position 126. Recombinant MC4R clones were screened for the αMSH induced activity in yeast. From 9 different amino acids obtained only the natural aspartic acid displayed the ligand dependent activity of MC4R. The MC4R variants with glutamic acid and leucine at position 126, however, displayed higher background activity than other amino acid substitutions. The results suggest that the yeast expression system is suitable for screening of the MC4R receptor ligands and that the substitution of aspartic acid at position 126 of MC4R by different amino acids functionally inactivates the receptor.

Magnificent seven: roles of G protein-coupled receptors in extracellular sensing in fungi

G protein-coupled receptors (GPCRs) represent the largest family of transmembrane receptors and are responsible for transducing extracellular signals into intracellular responses that involve complex intracellular-signaling networks. This review highlights recent research advances in fungal GPCRs, including classification, extracellular sensing, and G protein-signaling regulation. The involvement of GPCRs in pheromone and nutrient sensing has been studied extensively over the past decade. Following recent advances in fungal genome sequencing projects, a panoply of GPCR candidates has been revealed and some have been documented to play key roles sensing diverse extracellular signals, such as pheromones, sugars, amino acids, nitrogen sources, and even photons. Identification and deorphanization of additional putative GPCRs may require the development of new research tools. Here, we compare research on GPCRs in fungi with information derived from mammalian systems to provide a useful road map on how to better understand ligand-GPCR-G protein interactions in general. We also emphasize the utility of yeast as a discovery tool for systemic studies of GPCRs from other organisms.

PSNCBAM-1, a novel allosteric antagonist at cannabinoid CB1 receptors with hypophagic effects in rats

BACKGROUND AND PURPOSE

Rimonabant (Acomplia, SR141716A), a cannabinoid CB1 receptor inverse agonist, has recently been approved for the treatment of obesity. There are, however, concerns regarding its side effect profile. Developing a CB1 antagonist with a different pharmacological mechanism may lead to a safer alternative. To this end we have screened a proprietary small molecule library and have discovered a novel class of allosteric antagonist at CB1 receptors. Herein, we have characterized an optimized prototypical molecule, PSNCBAM-1, and its hypophagic effects in vivo.

EXPERIMENTAL APPROACH

A CB1 yeast reporter assay was used as a primary screen. PSNCBAM-1 was additionally characterized in [35S]-GTPgammaS, cAMP and radioligand binding assays. An acute rat feeding model was used to evaluate its effects on food intake and body weight in vivo.

KEY RESULTS

In CB1 receptor yeast reporter assays, PSNCBAM-1 blocked the effects induced by agonists such as CP55,940, WIN55212-2, anandamide (AEA) or 2-arachidonoyl glycerol (2-AG). The antagonist characteristics of PSNCBAM-1 were confirmed in [35S]-GTPgammaS binding and cAMP assays and was shown to be non-competitive by Schild analyses. PSNCBAM-1 did not affect CB2 receptors. In radioligand binding assays, PSNCBAM-1 increased the binding of [3H]CP55,940 despite its antagonist effects. In an acute rat feeding model, PSNCBAM-1 decreased food intake and body weight.

CONCLUSIONS AND IMPLICATIONS

PSNCBAM-1 exerted its effects through selective allosteric modulation of the CB1 receptor. The acute effects on food intake and body weight induced in rats provide a first report of in vivo activity for an allosteric CB1 receptor antagonist.

Chemical sensing of DNT by engineered olfactory yeast strain

With the increasing threat of environmental toxicants including biological and chemical warfare agents, fabricating innovative biomimetic systems to detect these harmful agents is critically important. With the broad objective of developing such a biosensor, here we report the construction of a Saccharomyces cerevisiae strain containing the primary components of the mammalian olfactory signaling pathway. In this engineered yeast strain, WIF-1alpha, olfactory receptor signaling is coupled to green fluorescent protein expression. Using this 'olfactory yeast', we screened for olfactory receptors that could report the presence of the odorant 2,4-dinitrotoluene, an explosive residue mimic. With this approach, we have identified the novel rat olfactory receptor Olfr226, which is closely related to the mouse olfactory receptors Olfr2 and MOR226-1, as a 2,4-dinitrotoluene-responsive receptor.

Ventral Tegmental Transcriptome Response to Intermittent Nicotine Treatment and Withdrawal in BALB/cJ, C57BL/6ByJ, and Quasi-Congenic RQI Mice

The aim of this study was to identify neurochemical pathways and candidate genes involved in adaptation to nicotine treatment and withdrawal. Locomotor sensitization was assessed in a nicotine challenge test after exposure to intermittent nicotine treatment and withdrawal. About 24 h after the challenge test the ventral tegmentum of the mesencephaion was dissected and processed using oligonucleotide microarrays with 22,690 probe sets (Affymetrix 430A 2.0). Quasi-congenic RQI, and donor BALB/cJ mice developed significant locomotor sensitization, while sensitization was not significant in the background partner, C57BL/6By. Comparing saline treated controls of C57BL/6ByJ and BALB/cJ by a rigorous statistical microarray analysis method we identified 238 differentially expressed transcripts. Quasi-congenic strains B6.Cb4i5-alpha4/Vad and B6.Ib5i7-beta25A/Vad significantly differed from the background strain in 11 and 11 transcripts, respectively. Identification of several cis- and trans-regulated genes indicates that further work with quasi-congenic strains can quickly lead to mapping of Quantitative Trait Loci for nicotine susceptibility because donor chromosome regions have been mapped in quasi-congenic strains. Nicotine treatment significantly altered the abundance of 41, 29, 54, and 14 ventral tegmental transcripts in strains C57BL/6ByJ, BALB/cJ, B6.Cb4i5-alpha4/Vad, and B6.Ib5i7-beta25A/Vad, respectively. Although transcript sets overlapped to some extent, each strain showed a distinct profile of nicotine sensitive genes, indicating genetic effects on nicotine-induced gene expression. Nicotine-responsive genes were related to processes including regulation of signal transduction, intracellular protein transport, proteasomal ubiquitin-dependent protein catabolism, and neuropeptide signaling pathway. Our results suggest that while there are common regulatory mechanisms across inbred strains, even relatively small differences in genetic constitution can significantly affect transcriptome response to nicotine.

Deorphanization of a G protein-coupled receptor for oleoylethanolamide and its use in the discovery of small-molecule hypophagic agents

The endogenous lipid signaling agent oleoylethanolamide (OEA) has recently been described as a peripherally acting agent that reduces food intake and body weight gain in rat feeding models. This paper presents evidence that OEA is an endogenous ligand of the orphan receptor GPR119, a G protein-coupled receptor (GPCR) expressed predominantly in the human and rodent pancreas and gastrointestinal tract and also in rodent brain, suggesting that the reported effects of OEA on food intake may be mediated, at least in part, via the GPR119 receptor. Furthermore, we have used the recombinant receptor to discover novel selective small-molecule GPR119 agonists, typified by PSN632408, which suppress food intake in rats and reduce body weight gain and white adipose tissue deposition upon subchronic oral administration to high-fat-fed rats. GPR119 therefore represents a novel and attractive potential target for the therapy of obesity and related metabolic disorders.

Functional analysis of heterologous GPCR signalling pathways in yeast

G protein-coupled receptors (GPCRs) regulate diverse biological processes in eukaryotes and such conservation allows an almost unrestricted interchange of signalling components between different cell types. Yeasts are attractive hosts in which to study GPCRs--they are amenable to both genetic and biochemical manipulation and their robustness, low cost and our ability to create strains that lack endogenous GPCRs make them ideal starting points for the development of assays suitable for high-throughput screening. Here we introduce readers to the possibilities of using yeast to analyse GPCRs describing the endogenous signalling pathways, the development of assays for heterologous GPCRs and the technology to elucidate GPCR structure and activity, focusing on the budding yeast Saccharomyces cerevisiae and recent developments using the fission yeast Schizosaccharomyces pombe.

GPCR modulation by RAMPs

Our conceptual understanding of the molecular architecture of G-protein coupled receptors (GPCRs) has transformed over the last decade. Once considered as largely independent functional units (aside from their interaction with the G-protein itself), it is now clear that a single GPCR is but part of a multifaceted signaling complex, each component providing an additional layer of sophistication. Receptor activity-modifying proteins (RAMPs) provide a notable example of proteins that interact with GPCRs to modify their function. They act as pharmacological switches, modifying GPCR pharmacology for a particular subset of receptors. However, there is accumulating evidence that these ubiquitous proteins have a broader role, regulating signaling and receptor trafficking. This article aims to provide the reader with a comprehensive appraisal of RAMP literature and perhaps some insight into the impact that their discovery has had on those who study GPCRs.

Development of angiotensin II-induced hypertension: role of CGRP and its receptor

OBJECTIVE

To determine the role of endogenous calcitonin gene-related peptide (CGRP) and its receptor in development of angiotensin II (Ang II)-induced hypertension.

DESIGN AND METHODS

Seven-week-old male Wistar rats were given either Ang II (100 ng/kg per min) or saline via mini osmotic pumps with or without minoxidil (9 mg/kg per day) in their drinking water for 10 days. Mean arterial pressure (MAP) and its response to alpha-CGRP (1 microg/kg, iv) and its receptor antagonist, CGRP(8-37) (1 mg/kg, iv), were determined in conscious and unrestrained rats. Radioimmunoassay and Western blotting were employed, respectively, to determine CGRP levels in plasma and dorsal root ganglia (DRG) and CGRP receptor protein content in mesenteric arteries.

RESULTS

After the 10-day treatment, MAP was higher in the Ang II group compared to control (Con), control plus minoxidil (Con-Min), and Ang II plus minoxidil (Ang II-Min) (P < 0.01). CGRP decreased MAP in the Ang II group compared to Con, Con-Min and Ang II-Min rats (P < 0.01). In contrast, CGRP8-37 increased MAP in Con-Min and Ang II-Min groups compared with Con and Ang II groups (P < 0.01). Radioimmunoassay showed that CGRP levels in plasma and DRG were not different among the four groups. Western blots showed that calcitonin receptor-like receptor (CRLR) and receptor activity-modifying protein 1 (RAMP1), which constitute a CGRP receptor, were significantly upregulated in mesenteric arteries in the Ang II group compared to the other three groups (P < 0.05).

CONCLUSION

These data indicate that long-term Ang II infusion is accompanied by an increase in CGRP receptor expression in mesenteric arteries but not in CGRP levels in plasma and DRG. The increase in mesenteric CGRP receptor expression appears to be pressure dependent and to enhance the blood pressure response to CGRP. Minoxidil enhances the hypertensive effect of CGRP8-37 to the same degree in control and Ang II-treated rats, indicating that this KATP channel activator sensitizes the blood pressure response regardless of the baseline pressure while CGRP receptors are blocked.

Role of the N-terminal domain of the calcitonin receptor-like receptor in ligand binding

Calcitonin receptor-like receptor (CRLR) is a seven-transmembrane (7-TM) domain class B G protein-coupled receptor (GPCR) which requires coexpression of different receptor activity modifying proteins (RAMP) to become a functional calcitonin gene-related peptide (CGRP) receptor or an adrenomedullin (AM) receptor. The N-terminal (Nt) extracellular region of class B GPCRs in ligand binding has been reported for receptors such as glucagon and parathyroid hormone. We hypothesize that the Nt-domain of CRLR (Nt-CRLR) is an autonomously folded unit possessing a well-defined structure and is involved in ligand binding and specificity. To obtain structural and functional information on the Nt-CRLR, we cloned and expressed the Nt-CRLR as a fusion protein in Escherichia coli. Overexpressed protein formed an inclusion body, which was refolded and purified, resulting in a soluble monomeric protein. Far-UV CD and fluorescence spectra of Nt-CRLR showed characteristics of a folded protein. The ability of Nt-CRLR to bind CGRP and AM independent of RAMPs was determined by studying inhibition of (125)I-CGRP and (125)I-AM binding to pregnant rat uterine membrane in the presence of Nt-CRLR protein. We observe that Nt-CRLR inhibits (125)I-CGRP and (125)I-AM binding to rat uterus in a dose-dependent fashion (IC(50) = 0.25 and 0.29 muM, respectively). Taken together, our data provide evidence that Nt-CRLR is structured and further that a significant part of the binding affinity comes from binding to the Nt-domain.

Functional expression of olfactory receptors in yeast and development of a bioassay for odorant screening

The functional expression of olfactory receptors (ORs) is a primary requirement to examine the molecular mechanisms of odorant perception and coding. Functional expression of the rat I7 OR and its trafficking to the plasma membrane was achieved under optimized experimental conditions in the budding yeast Saccharomyces cerevisiae. The membrane expression of the receptor was shown by Western blotting and immunolocalization methods. Moreover, we took advantage of the functional similarities between signal transduction cascades of G protein-coupled receptor in mammalian cells and the pheromone response pathway in yeast to develop a novel biosensor for odorant screening using luciferase as a functional reporter. Yeasts were engineered to coexpress I7 OR and mammalian G(alpha) subunit, to compensate for the lack of endogenous Gpa1 subunit, so that stimulation of the receptor by its ligands activates a MAP kinase signaling pathway and induces luciferase synthesis. The sensitivity of the bioassay was significantly enhanced using mammalian G(olf) compared to the G(alpha15) subunit, resulting in dose-dependent responses of the system. The biosensor was probed with an array of odorants to demonstrate that the yeast-borne I7 OR retains its specificity and selectivity towards ligands. The results are confirmed by functional expression and bioluminescence response of human OR17-40 to its specific ligand, helional. Based on these findings, the bioassay using the luciferase reporter should be amenable to simple, rapid and inexpensive odorant screening of hundreds of ORs to provide insight into olfactory coding mechanisms.

Decrease in arterial pressure induced by adrenomedullin in the hypothalamic paraventricular nucleus is mediated by nitric oxide and GABA

We tested the hypothesis that the decrease in arterial pressure induced by adrenomedullin (ADM) in the hypothalamic paraventricular nucleus (PVN) is mediated by nitric oxide (NO) and/or GABA. Unilateral microinjections of ADM into the PVN of anesthetized rats caused a significant decrease in mean arterial pressure (MAP). The ADM-induced decrease in MAP was significantly attenuated by pretreatment with N(psi)-nitro-L-arginine methyl ester (L-NAME, a non-selective NOS inhibitor), 7-nitroindazole sodium salt (7-NiNa, a selective neuronal NOS inhibitor), N5-(1-Iminoethyl)-L-ornithine (L-NIO, a selective endothelial NOS inhibitor) or bicuculline methiodide, but pretreatment with S-methylisothiourea (SMIT, a selective inducible NOS inhibitor) had no effect on this ADM-induced effect. In addition, coronal sections of rat brains were processed for combined NADPH-diaphorase (a marker of neuronal NOS-containing neurons) histochemistry and in situ hybridization for the receptor-activity-modifying protein 2 (a specific ADM receptor component). Double-labeled neurons were found in both parvocellular and magnocellular subdivisions of the PVN, confirming that NO-producing neurons in the PVN are capable of mediating ADM's effects. Thus, our data provide evidence that the ADM-induced decrease in MAP in the PVN is mediated by NO from neuronal and endothelial NOS, and by GABA.

Adrenomedullin receptors: pharmacological features and possible pathophysiological roles

Three receptor activity modifying proteins (RAMPs) chaperone calcitonin-like receptor (CLR) to the cell surface. RAMP2 enables CLR to form an adrenomedullin (AM)-specific receptor that is sensitive to AM-(22-52) (AM(1) receptor). RAMP3 enables CLR to form an AM receptor sensitive to both calcitonin gene-related peptide (CGRP)-(8-37) and AM-(22-52) (AM(2) receptor), though rat and mouse AM(2) receptors show a clear preference for CGRP alpha-(8-37) over AM-(22-52). RAMP1 enables CRL to form the CGRP-(8-37)-sensitive CGRP(1) receptor, which can also be activated by higher concentrations of AM. Here we review the available information on the pharmacological features and possible pathophysiological roles of the aforementioned AM receptors.

Adrenomedullin in the rostral ventrolateral medulla increases arterial pressure and heart rate: roles of glutamate and nitric oxide

This study was done to investigate the effects of microinjections of adrenomedullin (ADM), a vasoactive neuropeptide, in the rostral ventrolateral medulla (RVLM) on mean arterial pressure (MAP) and heart rate (HR) in urethane-anesthetized rats, and to assess the potential roles of glutamate and nitric oxide (NO) in these effects. Unilateral injections of ADM (0.01 or 0.1 pmol) into the RVLM significantly increased MAP and HR in a dose-dependent manner, whereas ADM at 0.001 pmol was ineffective. Microinjections of ADM (0.01 pmol) outside the RVLM had no effects on MAP or HR. Coinjections of a putative ADM receptor antagonist, ADM(22-52) (0.01 pmol), abolished the increases in MAP and HR evoked by ADM (0.01 pmol). The vasopressor effects of ADM (0.01 pmol) in the RVLM were abolished by coinjections of either dizocilpine hydrogen maleate (a selective NMDA glutamate receptor antagonist, 500 pmol) or 6-cyano-7-nitroquinoxaline-2,3-dione (a selective non-NMDA glutamate receptor antagonist, 50 pmol). The ADM-induced vasopressor effects were also abolished by coadministration of either 7-nitroindazole sodium salt (a selective neuronal NO synthase inhibitor, 0.05 pmol) or methylene blue (a soluble guanylyl cyclase inhibitor, 100 pmol). These results suggest that ADM in the RVLM stimulates increases in MAP and HR through ADM receptor-mediated mechanisms. These effects are mediated by glutamate via both NMDA and non-NMDA receptors. NO, derived from neuronal NO synthase, also contributes to the ADM-induced vasopressor effects via a soluble guanylyl cyclase-associated signaling pathway.

Tumor necrosis factor-alpha downregulates adrenomedullin receptors in human coronary artery smooth muscle cells

We examined the effects of tumor necrosis factor (TNF)-alpha on the expression and functionality of adrenomedullin (AM) receptors in cultured human coronary artery smooth muscle cells. Analysis of real-time quantitative polymerase chain reactions showed that these cells abundantly express two AM receptors comprised of calcitonin receptor-like receptor (CRLR) and receptor activity-modifying protein 1 (RAMP1) or RAMP2. TNF-alpha induced time- and dose-dependent decreases in the expression of CRLR and RAMP1/2 mRNAs, thereby diminishing AM-evoked cAMP production. The suppression of these three mRNAs was unaffected by inhibiting NOS, protein kinase G, protein kinase A, superoxide formation or NF-kappaB activation.

Cell and molecular biology of the multifunctional peptide, adrenomedullin

Adrenomedullin (AM) is a recently discovered regulatory peptide involved in many functions including vasodilatation, electrolyte balance, neurotransmission, growth, and hormone secretion regulation, among others. This 52-amino acid peptide is expressed by specific cell types in many organs throughout the body. A complex receptor system has been described for AM; it requires at least the presence of a seven-transmembrane-domain G-protein-coupled receptor, a single-transmembrane-domain receptor activity modifying protein, and a receptor component protein needed to establish the connection with the downstream signal transduction pathway, which usually involves cyclicAMP. In addition, a serum-binding protein regulates the biological actions of AM, frequently by increasing AM functional attributes. Changes in levels of circulating AM correlate with several critical diseases, including cardiovascular and renal disorders, sepsis, cancer, and diabetes. Whether AM is a causal agent, a protective reaction, or just a marker for these diseases is currently under investigation. New technologies seeking to elevate and/or reduce AM levels are being investigated as potential therapeutic avenues.

Regulation of pancreatic physiology by adrenomedullin and its binding protein

Adrenomedullin (AM) is a 52 amino acid, multifunctional hormone. It is expressed in many tissues of the human body including the pancreas, where it is mainly localized to the periphery of the islets of Langerhans and specifically to the pancreatic polypeptide-expressing cells. The AM receptor, a complex formed by calcitonin receptor-like receptor (CRLR) and receptor activity-modifying proteins (RAMPs), and the recently discovered AM-binding protein, complement factor H (fH), are expressed in the insulin-producing beta-cells. The colocalization of these key elements of the AM system in the endocrine portion of the pancreas implicates AM in the control of both normal and altered pancreatic physiologies. AM inhibits insulin secretion both in vitro (isolated rat islets) and in vivo (oral glucose tolerance test in rats) in a dose-dependent manner. The addition of fH to isolated rat islets produces a further reduction of insulin secretion in the presence of AM. Furthermore, AM is elevated in plasma from patients with pancreatic dysfunctions such as type 1 or type 2 diabetes and insulinoma. Using a diabetic model in rats, we have shown that AM increases circulating glucose levels whereas a blocking monoclonal antibody against AM has the opposite effect and improves postprandial recovery. Such experimental evidence implicates AM as a fundamental factor in maintaining insulin homeostasis and normoglycemia, and suggests the implication of AM as a possible causal agent in diabetes. Further investigation focused on the development of blocking agents for AM could result in new treatments for pancreatic AM-related disorders.

Adrenomedullin selectivity of calcitonin-like receptor/receptor activity modifying proteins

Co-expression of an initially orphan calcitonin receptor-like (CL)1 receptor with individual receptor-activity-modifying proteins (RAMP)1, -2 and -3 results in CL receptor/RAMP1, -2 and -3 proteins at the cell surface. The RAMP define the selectivity of the CL receptor for the vasodilatory peptides adrenomedullin (AM) and calcitonin gene-related peptide (CGRP). The selectivity for AM and CGRP agonists and antagonists of human, rat, porcine and bovine CL receptors, co-expressed with RAMP2 and -3, has been studied in different cell types. This revealed CL receptor/RAMP2 and CL receptor/RAMP3 as AM1 and AM2 receptor subtypes, respectively. The AM1 receptor crossreacts with CGRP at high and the AM2 receptor at lower concentrations. Here the pharmacological properties of the cloned AM receptors are compared to those revealed in tissues and cell lines. According to nomenclature recommendation of the IUPHAR (International Union of Pharmacology) subcommittee XXXII, the former CRLR is now the CL receptor (1).