A generic approach for the purification of signaling complexes that specifically interact with the carboxyl-terminal domain of G protein-coupled receptors

Identification of TSPAN4 as Novel Histamine H4 Receptor Interactor

The histamine H4 receptor (H4R) is a G protein-coupled receptor that is predominantly expressed on immune cells and considered to be an important drug target for various inflammatory disorders. Like most GPCRs, the H4R activates G proteins and recruits β-arrestins upon phosphorylation by GPCR kinases to induce cellular signaling in response to agonist stimulation. However, in the last decade, novel GPCR-interacting proteins have been identified that may regulate GPCR functioning. In this study, a split-ubiquitin membrane yeast two-hybrid assay was used to identify H4R interactors in a Jurkat T cell line cDNA library. Forty-three novel H4R interactors were identified, of which 17 have also been previously observed in MYTH screens to interact with other GPCR subtypes. The interaction of H4R with the tetraspanin TSPAN4 was confirmed in transfected cells using bioluminescence resonance energy transfer, bimolecular fluorescence complementation, and co-immunoprecipitation. Histamine stimulation reduced the interaction between H4R and TSPAN4, but TSPAN4 did not affect H4R-mediated G protein signaling. Nonetheless, the identification of novel GPCR interactors by MYTH is a starting point to further investigate the regulation of GPCR signaling.

Melatonin receptor structures shed new light on melatonin research

The tryptophan derivative melatonin is an evolutionary old molecule that is involved in a pleiotropy of physiological functions. In humans, age-related decline of circulating melatonin levels and/or dysregulation of its circadian synthesis pattern have been associated with several disorders and disease states. Several molecular targets have been proposed for melatonin since its discovery, in 1959. Among them, melatonin MT₁ and MT₂ receptors are the best characterized melatonin targets, mediating melatonin effects in a variety of tissues. They belong to the superfamily of G protein-coupled receptors. Two back-to-back articles published in the "Nature" Journal earlier this year present the first crystal structures of the human MT₁ and MT₂ in its inactive states. Here, we will briefly outline the discovery path of melatonin receptors until their structural elucidation and discuss how these new findings will guide future research toward a better understanding of their function and rational drug design.

Melatonin receptors: molecular pharmacology and signalling in the context of system bias

Melatonin, N-acetyl-5-methoxytryptamine, an evolutionally old molecule, is produced by the pineal gland in vertebrates, and it binds with high affinity to melatonin receptors, which are members of the GPCR family. Among the multiple effects attributed to melatonin, we will focus here on those that are dependent on the activation of the two mammalian MT1 and MT2 melatonin receptors. We briefly summarize the latest developments on synthetic melatonin receptor ligands, including multi-target-directed ligands, and the characterization of signalling-biased ligands. We discuss signalling pathways activated by melatonin receptors that appear to be highly cell- and tissue-dependent, emphasizing the impact of system bias on the functional outcome. Different proteins have been demonstrated to interact with melatonin receptors, and thus, we postulate that part of this system bias has its molecular basis in differences of the expression of receptor-associated proteins including heterodimerization partners. Finally, bias at the level of the receptor, by the expression of genetic receptor variants, will be discussed to show how a modified receptor function can have an effect on the risk for common diseases like type 2 diabetes in humans. LINKED ARTICLES: This article is part of a themed section on Recent Developments in Research of Melatonin and its Potential Therapeutic Applications. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.16/issuetoc.

The G protein-coupled receptors deorphanization landscape

G protein-coupled receptors (GPCRs) are usually highlighted as being both the largest family of membrane proteins and the most productive source of drug targets. However, most of the GPCRs are understudied and hence cannot be used immediately for innovative therapeutic strategies. Besides, there are still around 100 orphan receptors, with no described endogenous ligand and no clearly defined function. The race to discover new ligands for these elusive receptors seems to be less intense than before. Here, we present an update of the various strategies employed to assign a function to these receptors and to discover new ligands. We focus on the recent advances in the identification of endogenous ligands with a detailed description of newly deorphanized receptors. Replication being a key parameter in these endeavors, we also discuss the latest controversies about problematic ligand-receptor pairings. In this context, we propose several recommendations in order to strengthen the reporting of new ligand-receptor pairs.

Melatonin and Hippo Pathway: Is There Existing Cross-Talk?

Melatonin is an indolic hormone that regulates a plethora of functions ranging from the regulation of circadian rhythms and antioxidant properties to the induction and maintenance of tumor suppressor pathways. It binds to specific receptors as well as to some cytosolic proteins, leading to several cellular signaling cascades. Recently, the involvement of melatonin in cancer insurgence and progression has clearly been demonstrated. In this review, we will first describe the structure and functions of melatonin and its receptors, and then discuss both molecular and epidemiological evidence on melatonin anticancer effects. Finally, we will shed light on potential cross-talk between melatonin signaling and the Hippo signaling pathway, along with the possible implications for cancer therapy.

Systematic protein-protein interaction mapping for clinically relevant human GPCRs

G‐protein‐coupled receptors (GPCRs) are the largest family of integral membrane receptors with key roles in regulating signaling pathways targeted by therapeutics, but are difficult to study using existing proteomics technologies due to their complex biochemical features. To obtain a global view of GPCR‐mediated signaling and to identify novel components of their pathways, we used a modified membrane yeast two‐hybrid (MYTH) approach and identified interacting partners for 48 selected full‐length human ligand‐unoccupied GPCRs in their native membrane environment. The resulting GPCR interactome connects 686 proteins by 987 unique interactions, including 299 membrane proteins involved in a diverse range of cellular functions. To demonstrate the biological relevance of the GPCR interactome, we validated novel interactions of the GPR37, serotonin 5‐HT4d, and adenosine ADORA2A receptors. Our data represent the first large‐scale interactome mapping for human GPCRs and provide a valuable resource for the analysis of signaling pathways involving this druggable family of integral membrane proteins.

Update on melatonin receptors: IUPHAR Review 20

Melatonin receptors are seven transmembrane-spanning proteins belonging to the GPCR superfamily. In mammals, two melatonin receptor subtypes exist - MT1 and MT2 - encoded by the MTNR1A and MTNR1B genes respectively. The current review provides an update on melatonin receptors by the corresponding subcommittee of the International Union of Basic and Clinical Pharmacology. We will highlight recent developments of melatonin receptor ligands, including radioligands, and give an update on the latest phenotyping results of melatonin receptor knockout mice. The current status and perspectives of the structure of melatonin receptor will be summarized. The physiological importance of melatonin receptor dimers and biologically important and type 2 diabetes-associated genetic variants of melatonin receptors will be discussed. The role of melatonin receptors in physiology and disease will be further exemplified by their functions in the immune system and the CNS. Finally, antioxidant and free radical scavenger properties of melatonin and its relation to melatonin receptors will be critically addressed.

Protein interactome mining defines melatonin MT1 receptors as integral component of presynaptic protein complexes of neurons

In mammals, the hormone melatonin is mainly produced by the pineal gland with nocturnal peak levels. Its peripheral and central actions rely either on its intrinsic antioxidant properties or on binding to melatonin MT1 and MT2 receptors, belonging to the G protein-coupled receptor (GPCR) super-family. Melatonin has been reported to be involved in many functions of the central nervous system such as circadian rhythm regulation, neurotransmission, synaptic plasticity, memory, sleep, and also in Alzheimer's disease and depression. However, little is known about the subcellular localization of melatonin receptors and the molecular aspects involved in neuronal functions of melatonin. Identification of protein complexes associated with GPCRs has been shown to be a valid approach to improve our understanding of their function. By combining proteomic and genomic approaches we built an interactome of MT1 and MT2 receptors, which comprises 378 individual proteins. Among the proteins interacting with MT1 , but not with MT2 , we identified several presynaptic proteins, suggesting a potential role of MT1 in neurotransmission. Presynaptic localization of MT1 receptors in the hypothalamus, striatum, and cortex was confirmed by subcellular fractionation experiments and immunofluorescence microscopy. MT1 physically interacts with the voltage-gated calcium channel Cav 2.2 and inhibits Cav 2.2-promoted Ca(2+) entry in an agonist-independent manner. In conclusion, we show that MT1 is part of the presynaptic protein network and negatively regulates Cav 2.2 activity, providing a first hint for potential synaptic functions of MT1.

Serological proteome analysis reveals new specific biases in the IgM and IgG autoantibody repertoires in autoimmune polyendocrine syndrome type 1

OBJECTIVE

Autoimmune polyendocrine syndrome type 1 (APS 1) is caused by mutations in the AIRE gene that induce intrathymic T-cell tolerance breakdown, which results in tissue-specific autoimmune diseases.

DESIGN

To evaluate the effect of a well-defined T-cell repertoire impairment on humoral self-reactive fingerprints, comparative serum self-IgG and self-IgM reactivities were analyzed using both one- and two-dimensional western blotting approaches against a broad spectrum of peripheral tissue antigens.

METHODS

Autoantibody patterns of APS 1 patients were compared with those of subjects affected by other autoimmune endocrinopathies (OAE) and healthy controls.

RESULTS

Using a Chi-square test, significant changes in the Ab repertoire were found when intergroup patterns were compared. A singular distortion of both serum self-IgG and self-IgM repertoires was noted in APS 1 patients. The molecular characterization of these antigenic targets was conducted using a proteomic approach. In this context, autoantibodies recognized more significantly either tissue-specific antigens, such as pancreatic amylase, pancreatic triacylglycerol lipase and pancreatic regenerating protein 1α, or widely distributed antigens, such as peroxiredoxin-2, heat shock cognate 71-kDa protein and aldose reductase. As expected, a well-defined self-reactive T-cell repertoire impairment, as described in APS 1 patients, affected the tissue-specific self-IgG repertoire. Interestingly, discriminant IgM reactivities targeting both tissue-specific and more widely expressed antigens were also specifically observed in APS 1 patients. Using recombinant targets, we observed that post translational modifications of these specific antigens impacted upon their recognition.

CONCLUSIONS

The data suggest that T-cell-dependent but also T-cell-independent mechanisms are involved in the dynamic evolution of autoimmunity in APS 1.

Melatonin or ramelteon therapy differentially affects hepatic gene expression profiles after haemorrhagic shock in rat--A microarray analysis

BACKGROUND & AIMS

Melatonin has been demonstrated to reduce liver damage in different models of stress. However, there is only limited information on the impact of this hormone on hepatic gene expression. The aim of this study was, to investigate the influence of melatonin or the melatonergic agonist ramelteon on hepatic gene expression profiles after haemorrhagic shock using a whole genome microarray analysis.

METHODS

Male Sprague-Dawley rats (200-300 g, n=4/group) underwent haemorrhagic shock (mean arterial pressure 35±5 mmHg). After 90 min of shock, animals were resuscitated with shed blood and Ringer's and treated with vehicle (5% dimethyl sulfoxide), melatonin or ramelteon (each 1.0 mg/kg intravenously). Sham-operated animals were treated likewise but did not undergo haemorrhage. After 2 h of reperfusion, the liver was harvested, and a whole genome microarray analysis was performed. Functional gene expression profiles were determined using the Panther® classification system; promising candidate genes were evaluated by quantitative polymerase chain reaction (PCR).

RESULTS

Microarray and PCR data showed a good correlation (r(2)=0.84). A strong influence of melatonin on receptor mediated signal transduction was revealed using the functional gene expression profile analysis, whereas ramelteon mainly influenced transcription factors. Shock-induced upregulation of three candidate genes with relevant functions for hepatocytes (ppp1r15a, dusp5, rhoB) was significantly reduced by melatonin (p<0.05 vs. shock/vehicle), but not by ramelteon. Two genes previously known as haemorrhage-induced (il1b, s100a8) were transcriptionally repressed by both drugs.

CONCLUSIONS

Melatonin and ramelteon appear to induce specific hepatic gene expression profiles after haemorrhagic shock in rats. The observed differences between both substances are likely to be attributable to a distinct mechanism of action in these agents.

Structural features of the G-protein/GPCR interactions

BACKGROUND

The details of the functional interaction between G proteins and the G protein coupled receptors (GPCRs) have long been subjected to extensive investigations with structural and functional assays and a large number of computational studies.

SCOPE OF REVIEW

The nature and sites of interaction in the G-protein/GPCR complexes, and the specificities of these interactions selecting coupling partners among the large number of families of GPCRs and G protein forms, are still poorly defined.

MAJOR CONCLUSIONS

Many of the contact sites between the two proteins in specific complexes have been identified, but the three dimensional molecular architecture of a receptor-Gα interface is only known for one pair. Consequently, many fundamental questions regarding this macromolecular assembly and its mechanism remain unanswered.

GENERAL SIGNIFICANCE

In the context of current structural data we review the structural details of the interfaces and recognition sites in complexes of sub-family A GPCRs with cognate G-proteins, with special emphasis on the consequences of activation on GPCR structure, the prevalence of preassembled GPCR/G-protein complexes, the key structural determinants for selective coupling and the possible involvement of GPCR oligomerization in this process.

Melatonin and pancreatic islets: interrelationships between melatonin, insulin and glucagon

The pineal hormone melatonin exerts its influence in the periphery through activation of two specific trans-membrane receptors: MT1 and MT2. Both isoforms are expressed in the islet of Langerhans and are involved in the modulation of insulin secretion from β-cells and in glucagon secretion from α-cells. De-synchrony of receptor signaling may lead to the development of type 2 diabetes. This notion has recently been supported by genome-wide association studies identifying particularly the MT2 as a risk factor for this rapidly spreading metabolic disturbance. Since melatonin is secreted in a clearly diurnal fashion, it is safe to assume that it also has a diurnal impact on the blood-glucose-regulating function of the islet. This factor has hitherto been underestimated; the disruption of diurnal signaling within the islet may be one of the most important mechanisms leading to metabolic disturbances. The study of melatonin–insulin interactions in diabetic rat models has revealed an inverse relationship: an increase in melatonin levels leads to a down-regulation of insulin secretion and vice versa. Elucidation of the possible inverse interrelationship in man may open new avenues in the therapy of diabetes.

Identification of GPCR-interacting cytosolic proteins using HDL particles and mass spectrometry-based proteomic approach

G protein-coupled receptors (GPCRs) have critical roles in various physiological and pathophysiological processes, and more than 40% of marketed drugs target GPCRs. Although the canonical downstream target of an agonist-activated GPCR is a G protein heterotrimer; there is a growing body of evidence suggesting that other signaling molecules interact, directly or indirectly, with GPCRs. However, due to the low abundance in the intact cell system and poor solubility of GPCRs, identification of these GPCR-interacting molecules remains challenging. Here, we establish a strategy to overcome these difficulties by using high-density lipoprotein (HDL) particles. We used the β₂-adrenergic receptor (β₂AR), a GPCR involved in regulating cardiovascular physiology, as a model system. We reconstituted purified β₂AR in HDL particles, to mimic the plasma membrane environment, and used the reconstituted receptor as bait to pull-down binding partners from rat heart cytosol. A total of 293 proteins were identified in the full agonist-activated β₂AR pull-down, 242 proteins in the inverse agonist-activated β₂AR pull-down, and 210 proteins were commonly identified in both pull-downs. A small subset of the β₂AR-interacting proteins isolated was confirmed by Western blot; three known β₂AR-interacting proteins (Gsα, NHERF-2, and Grb2) and 3 newly identified known β₂AR-interacting proteins (AMPKα, acetyl-CoA carboxylase, and UBC-13). Profiling of the identified proteins showed a clear bias toward intracellular signal transduction pathways, which is consistent with the role of β₂AR as a cell signaling molecule. This study suggests that HDL particle-reconstituted GPCRs can provide an effective platform method for the identification of GPCR binding partners coupled with a mass spectrometry-based proteomic analysis.

Tandem affinity purification and identification of GPCR-associated protein complexes

The first tandem affinity purification (TAP) protocol was described in 1999. Originally designed for the purification of protein complexes in yeast RNA splicing, its application rapidly expanded towards whole proteome analysis in yeast and mammalian cells. More recently, TAP has been applied to the purification of G protein-coupled receptor (GPCR)-associated protein complexes (GAPCs). This approach is particularly attractive for GPCRs, as the native, seven transmembrane structure is used as bait to purify GAPCs from mammalian cells expressing receptors at physiological levels. Here, a detailed protocol of the TAP method applied to GPCRs is presented.

Asymmetry of GPCR oligomers supports their functional relevance

G protein-coupled receptors (GPCRs) can exist as dimers or as larger oligomeric clusters that enable intercommunication between different receptor protomers within the same complex. This phenomenon is observed at three distinct levels: (i) at the level of ligand binding where the activation of one protomer can allosterically inhibit or facilitate ligand binding to the second protomer; (ii) at the level of ligand-induced conformational switches, which occur between transmembrane domains of the two protomers; and (iii) within GPCR-associated protein complexes, either directly at the level of GPCR-interacting proteins or at further downstream levels of the complex. Intercommunication at these different levels introduces asymmetry within GPCR dimers wherein each protomer fulfills its specific task. In this review, we discuss how the asymmetric behavior of GPCRs highlights the advantage of oligomeric receptor organization and supports the functional relevance of GPCR dimerization.

Peptide affinity purification for the isolation and identification of GPCR-associated protein complexes

Protein networks and their dynamic regulation play a fundamental role in biological systems. Seven transmembrane-spanning G protein-coupled receptors (GPCRs) constitute the largest family of membrane receptors controlling the flow of information from the extracellular environment into cells by inducing intracellular signaling pathways. Several GPCR-associated protein complexes (GAPCs), particularly those binding to the intracellular carboxyl-terminus (C-terminus), have been identified over the last 20 years. Recent optimizations in purification protocols and advances in mass spectrometry-based protein identification techniques have considerably accelerated the identification of GAPCs. We will concentrate here on a description of the latest version of the peptide affinity purification approach dedicated to the purification of GAPCs interacting with GPCR C-termini or any other soluble receptor subdomain.

Ligand-based peptide design and combinatorial peptide libraries to target G protein-coupled receptors

G protein-coupled receptors (GPCRs) are considered to represent the most promising drug targets; it has been repeatedly said that a large fraction of the currently marketed drugs elicit their actions by binding to GPCRs (with cited numbers varying from 30-50%). Closer scrutiny, however, shows that only a modest fraction of (≈60) GPCRs are, in fact, exploited as drug targets, only ≈20 of which are peptide-binding receptors. The vast majority of receptors in the humane genome have not yet been explored as sites of action for drugs. Given the drugability of this receptor class, it appears that opportunities for drug discovery abound. In addition, GPCRs provide for binding sites other than the ligand binding sites (referred to as the "orthosteric site"). These additional sites include (i) binding sites for ligands (referred to as "allosteric ligands") that modulate the affinity and efficacy of orthosteric ligands, (ii) the interaction surface that recruits G proteins and arrestins, (iii) the interaction sites of additional proteins (GIPs, GPCR interacting proteins that regulate G protein signaling or give rise to G protein-independent signals). These sites can also be targeted by peptides. Combinatorial and natural peptide libraries are therefore likely to play a major role in identifying new GPCR ligands at each of these sites. In particular the diverse natural peptide libraries such as the venom peptides from marine cone-snails and plant cyclotides have been established as a rich source of drug leads. High-throughput screening and combinatorial chemistry approaches allow for progressing from these starting points to potential drug candidates. This will be illustrated by focusing on the ligand-based drug design of oxytocin (OT) and vasopressin (AVP) receptor ligands using natural peptide leads as starting points.

Expanding the Concept of G Protein-Coupled Receptor (GPCR) Dimer Asymmetry towards GPCR-Interacting Proteins

G protein-coupled receptors (GPCRs), major targets of drug discovery, are organized in dimeric and/or oligomeric clusters. The minimal oligomeric unit, the dimer, is composed of two protomers, which can behave differently within the dimer. Several examples of GPCR asymmetry within dimers at the level of ligand binding, ligand-promoted conformational changes, conformational changes within transmembrane domains, G protein coupling, and most recently GPCR-interacting proteins (GIPs), have been reported in the literature. Asymmetric organization of GPCR dimers has important implications on GPCR function and drug design. Indeed, the extension of the “asymmetry concept” to GIPs adds a new level of specific therapeutic intervention.

New evidence for a role of melatonin in glucose regulation

Glucose triggers insulin secretion of the pancreatic β-cells. The pineal hormone melatonin interferes in this process by inhibiting secretion and transmitting circadian timing information to the islets. Circadian insulin secretion is adapted to day/night changes through melatonin-dependent synchronization. In rats and mice, melatonin levels are high during the dark period, which is their active feeding period, while, in humans, melatonin levels are high during the overnight fasting and sleeping period. This implies a different read-out of melatonin signaling in day-active species, including man. Dysregulation of circadian secretion may be a key to the increase of type 2 diabetes (T2D). This review discusses the impact of melatonin on insulin secretion transmitted through both the pertussis-toxin-sensitive membrane receptors MT1 (MTNR1a) and MT2 (MTNR1b) and the second messengers cAMP, cGMP and IP3. This is an important topic since, in several genetic association studies, single nucleotide polymorphisms of the human MT2-receptor have been described as being causally linked with an elevated risk of developing T2D. This article summarizes interrelationships between melatonin and insulin in type 1 diabetic (T1D) and type 2 diabetic (T2D) rats and humans.

Molecular organization and dynamics of the melatonin MT₁ receptor/RGS20/G(i) protein complex reveal asymmetry of receptor dimers for RGS and G(i) coupling

Functional asymmetry of G-protein-coupled receptor (GPCR) dimers has been reported for an increasing number of cases, but the molecular architecture of signalling units associated to these dimers remains unclear. Here, we characterized the molecular complex of the melatonin MT₁ receptor, which directly and constitutively couples to G(i) proteins and the regulator of G-protein signalling (RGS) 20. The molecular organization of the ternary MT₁/G(i)/RGS20 complex was monitored in its basal and activated state by bioluminescence resonance energy transfer between probes inserted at multiple sites of the complex. On the basis of the reported crystal structures of G(i) and the RGS domain, we propose a model wherein one G(i) and one RGS20 protein bind to separate protomers of MT₁ dimers in a pre-associated complex that rearranges upon agonist activation. This model was further validated with MT₁/MT₂ heterodimers. Collectively, our data extend the concept of asymmetry within GPCR dimers, reinforce the notion of receptor specificity for RGS proteins and highlight the advantage of GPCRs organized as dimers in which each protomer fulfils its specific task by binding to different GPCR-interacting proteins.

Recent methodological advances in the discovery of GPCR-associated protein complexes

Protein-interaction networks have important roles in cellular homeostasis and the generation of complexity in biological systems. G-protein-coupled receptors (GPCRs), the largest family of membrane receptors and important drug targets, are integral parts of these networks. Ligand stimulation and the dynamic interaction with GPCR-associated protein complexes (GAPCs) constitute two important regulatory mechanisms of GPCR function. Several genomic and proteomic approaches have been developed to identify GAPCs in the past. However, this task turned out to be particularly demanding owing to difficulties in preserving the complex three-dimensional GPCR structure during receptor solubilization and to inherent limitations in the use of isolated receptor domains as bait. Newly emerging methods have the potential to overcome these limitations and will certainly boost the identification of functionally relevant GAPCs to finally increase our knowledge of the regulation of GPCRs and provide novel drug targets. Here, we focus on the comparison of two complementary GAPC purification strategies, which are based on soluble GPCR subdomains and entire GPCRs.

Melatonin receptors, heterodimerization, signal transduction and binding sites: what's new?

Melatonin is a neurohormone that has been claimed to be involved in a wide range of physiological functions. Nevertheless, for most of its effects, the mechanism of action is not really known. In mammals, two melatonin receptors, MT1 and MT2, have been cloned. They belong to the G-protein-coupled receptor (GPCR) superfamily. They share some specific short amino-acid sequences, which suggest that they represent a specific subfamily. Another receptor from the same subfamily, the melatonin-related receptor has been cloned in different species including humans. This orphan receptor also named GPR50 does not bind melatonin and its endogenous ligand is still unknown. Nevertheless, this receptor has been shown to behave as an antagonist of the MT1 receptor, which opens new pharmacological perspectives for GPR50 despite the lack of endogenous or synthetic ligands. Moreover, MT1 and MT2 interact together through the formation of heterodimers at least in cells transfected with the cDNA of these two receptors. Lastly, signalling complexes associated with MT1 and MT2 receptors are starting to be deciphered. A third melatonin-binding site has been purified and characterized as the enzyme quinone reductase 2 (QR2). Inhibition of QR2 by melatonin may explain melatonin's protective effect that has been reported in different animal models and that is generally associated with its well-documented antioxidant properties.