GPCR deorphanizations: the novel, the known and the unexpected transmitters

Orphan GPCRs in Neurodegenerative Disorders: Integrating Structural Biology and Drug Discovery Approaches

Neurodegenerative disorders, particularly Alzheimer's and Parkinson's diseases, continue to challenge modern medicine despite therapeutic advances. Orphan G-protein-coupled receptors (GPCRs) have emerged as promising targets in the central nervous system, offering new avenues for drug development. This review focuses on the structural biology of orphan GPCRs implicated in these disorders, providing a comprehensive analysis of their molecular architecture and functional mechanisms. We examine recent breakthroughs in structural determination techniques, such as cryo-electron microscopy and X-ray crystallography, which have elucidated the intricate conformations of these receptors. The review highlights how structural insights inform our understanding of orphan GPCR activation, ligand binding and signaling pathways. By integrating structural data with molecular pharmacology, we explore the potential of structure-guided approaches in developing targeted therapeutics toward orphan GPCRs. This structural-biology-centered perspective aims to deepen our comprehension of orphan GPCRs and guide future drug discovery efforts in neurodegenerative disorders.

Exploring the constitutive activation mechanism of the class A orphan GPR20

GPR20, an orphan G protein-coupled receptor (GPCR), shows significant expression in intestinal tissue and represents a potential therapeutic target to treat gastrointestinal stromal tumors. GPR20 performs high constitutive activity when coupling with Gi. Despite the pharmacological importance of GPCR constitutive activation, determining the mechanism has long remained unclear. In this study, we explored the constitutive activation mechanism of GPR20 through large-scale unbiased molecular dynamics simulations. Our results unveil the allosteric nature of constitutively activated GPCR signal transduction involving extracellular and intracellular domains. Moreover, the constitutively active state of the GPR20 requires both the N-terminal cap and Gi protein. The N-terminal cap of GPR20 functions like an agonist and mediates long-range activated conformational shift. Together with the previous study, this study enhances our knowledge of the self-activation mechanism of the orphan receptor, facilitates the drug discovery efforts that target GPR20.

Review: The evolution of peptidergic signaling in Cnidaria and Placozoa, including a comparison with Bilateria

Bilateria have bilateral symmetry and are subdivided into Deuterostomia (animals like vertebrates) and Protostomia (animals like insects and mollusks). Neuropeptides occur in both Proto- and Deuterostomia and they are frequently structurally related across these two lineages. For example, peptides belonging to the oxytocin/vasopressin family exist in both clades. The same is true for the G protein-coupled receptors (GPCRs) of these peptides. These observations suggest that these neuropeptides and their GPCRs were already present in the common ancestor of Proto- and Deuterostomia, which lived about 700 million years ago (MYA). Furthermore, neuropeptides and their GPCRs occur in two early-branching phyla that diverged before the emergence of Bilateria: Cnidaria (animals like corals and sea anemones), and Placozoa (small disk-like animals, feeding on algae). The sequences of these neuropeptides and their GPCRs, however, are not closely related to those from Bilateria. In addition, cnidarian neuropeptides and their receptors are not closely related to those from Placozoa. We propose that the divergence times between Cnidaria, Placozoa, and Bilateria might be too long for recognizing sequence identities. Leucine-rich repeats-containing GPCRs (LGRs) are a special class of GPCRs that are characterized by a long N-terminus containing 10-20 leucine-rich domains, which are used for ligand binding. Among the ligands for LGRs are dimeric glycoprotein hormones, and insulin-like peptides, such as relaxin. LGRs have been found not only in Proto- and Deuterostomia, but also in early emerging phyla, such as Cnidaria and Placozoa. Humans have eight LGRs. In our current review, we have revisited the annotations of LGRs from the sea anemone Nematostella vectensis and the placozoan Trichoplax adhaerens. We identified 13 sea anemone LGRs and no less than 46 LGRs from T. adhaerens. All eight human LGRs appear to have orthologues in sea anemones and placozoans. LGRs and their ligands, therefore, have a long evolutionary history, going back to the common ancestor of Cnidaria and Placozoa.

Mass Spectrometry Approaches Empowering Neuropeptide Discovery and Therapeutics

The discovery of insulin in the early 1900s ushered in the era of research related to peptides acting as hormones and neuromodulators, among other regulatory roles. These essential gene products are found in all organisms, from the most primitive to the most evolved, and carry important biologic information that coordinates complex physiology and behavior; their misregulation has been implicated in a variety of diseases. The evolutionary origins of at least 30 neuropeptide signaling systems have been traced to the common ancestor of protostomes and deuterostomes. With the use of relevant animal models and modern technologies, we can gain mechanistic insight into orthologous and paralogous endogenous peptides and translate that knowledge into medically relevant insights and new treatments. Groundbreaking advances in medicine and basic science influence how signaling peptides are defined today. The precise mechanistic pathways for over 100 endogenous peptides in mammals are now known and have laid the foundation for multiple drug development pipelines. Peptide biologics have become valuable drugs due to their unique specificity and biologic activity, lack of toxic metabolites, and minimal undesirable interactions. This review outlines modern technologies that enable neuropeptide discovery and characterization, and highlights lessons from nature made possible by neuropeptide research in relevant animal models that is being adopted by the pharmaceutical industry. We conclude with a brief overview of approaches/strategies for effective development of peptides as drugs. SIGNIFICANCE STATEMENT: Neuropeptides, an important class of cell-cell signaling molecules, are involved in maintaining a range of physiological functions. Since the discovery of insulin's activity, over 100 bioactive peptides and peptide analogs have been used as therapeutics. Because these are complex molecules not easily predicted from a genome and their activity can change with subtle chemical modifications, mass spectrometry (MS) has significantly empowered peptide discovery and characterization. This review highlights contributions of MS-based research towards the development of therapeutic peptides.

The immunomodulatory effect of docosahexaenoic acid (DHA) on the RAW264.7 cells by modification of the membrane structure and function

DHA can regulate various physiological functions of cells. Our group has clarified the immunomodulatory activity and molecular mechanism of DHA on RAW264.7 cells.

Fatty Acid Signaling Mechanisms in Neural Cells: Fatty Acid Receptors

Fatty acids (FAs) are typically associated with structural and metabolic roles, as they can be stored as triglycerides, degraded by β-oxidation or used in phospholipids’ synthesis, the main components of biological membranes. It has been shown that these lipids exhibit also regulatory functions in different cell types. FAs can serve as secondary messengers, as well as modulators of enzymatic activities and substrates for cytokines synthesis. More recently, it has been documented a direct activity of free FAs as ligands of membrane, cytosolic, and nuclear receptors, and cumulative evidence has emerged, demonstrating its participation in a wide range of physiological and pathological conditions. It has been long known that the central nervous system is enriched with poly-unsaturated FAs, such as arachidonic (C20:4ω-6) or docosohexaenoic (C22:6ω-3) acids. These lipids participate in the regulation of membrane fluidity, axonal growth, development, memory, and inflammatory response. Furthermore, a whole family of low molecular weight compounds derived from FAs has also gained special attention as the natural ligands for cannabinoid receptors or key cytokines involved in inflammation, largely expanding the role of FAs as precursors of signaling molecules. Nutritional deficiencies, and alterations in lipid metabolism and lipid signaling have been associated with developmental and cognitive problems, as well as with neurodegenerative diseases. The molecular mechanism behind these effects still remains elusive. But in the last two decades, different families of proteins have been characterized as receptors mediating FAs signaling. This review focuses on different receptors sensing and transducing free FAs signals in neural cells: (1) membrane receptors of the family of G Protein Coupled Receptors known as Free Fatty Acid Receptors (FFARs); (2) cytosolic transport Fatty Acid-Binding Proteins (FABPs); and (3) transcription factors Peroxisome Proliferator-Activated Receptors (PPARs). We discuss how these proteins modulate and mediate direct regulatory functions of free FAs in neural cells. Finally, we briefly discuss the advantages of evaluating them as potential targets for drug design in order to manipulate lipid signaling. A thorough characterization of lipid receptors of the nervous system could provide a framework for a better understanding of their roles in neurophysiology and, potentially, help for the development of novel drugs against aging and neurodegenerative processes.

Ferritinophagy activation and sideroflexin1-dependent mitochondria iron overload is involved in apelin-13-induced cardiomyocytes hypertrophy

Excess iron accumulation and cardiac oxidative stress have been shown as important mediators of cardiac hypertrophy, whereas it remains largely elusive about the occurrence of mitochondrial iron overload and its significance during cardiac hypertrophy. In the present study, we aim to investigate the role of NCOA4-mediated ferritinophagy and SFXN1-dependent mitochondria iron overload in apelin-13-induced cardiomyocytes hypertrophy. Apelin-13 significantly promotes ferric citrate (FAC)-induced total cellular and mitochondria ion production, as well as mitochondria ROS contents. Mechanistically, apelin-13 effectively induces the expression of SFXN1, a mitochondria iron transporting protein and NCOA4, a cargo receptor of ferritinophagy in dose and time-dependent manner. Conversely, blockade of APJ by F13A abolishes these stimulatory effects. In addition, apelin-13-triggered mitochondria iron overload is reversed by the genetic inhibition of SFXN1 and NCOA4. NCOA4 deficiency via its silencing also interferes with the enhanced expression of SFXN1 evoked by apelin-13. In apelin-13-treated H9c2 cells, the promotion in cell diameter, volume as well as protein contents are obviously suppressed by the knockdown of NCOA4 and SFXN1 with their corresponding siRNAs. Remarkably, the human and murine hypertrophic hearts models, as well as apelin-13-injected mice models, present evident cardiac mitochondrial iron deposition and raised expressions of NCOA4 and SFXN1. Taken together, these results provide experimental evidences that NCOA4-mediated ferritinophagy might be defined as an essential mechanism leading to apelin-13-cardiomyocytes hypertrophy in SFXN1-dependent mitochondria iron overload manners.

GPR17 receptor modulators and their therapeutic implications: review of recent patents

INTRODUCTION

The GPR17 receptor, phylogenetically related to both purinergic P2Y and CysLT receptors, is mainly expressed in the CNS and, in general, in organs that can typically undergo ischemic damage. This receptor is involved in various pathologies including stroke, brain and spinal cord trauma, multiple sclerosis and in all diseases characterized by neuronal and myelin dysfunction. Therefore, there is a strong needed to identify molecules capable of binding specifically to GPR17 receptors.

AREAS COVERED

The review provides a summary of patents, published between 2009 and 2018, on chemicals and biologics and their clinical use. In this work, information is reported about the representative structures and biological activity of recently developed GPR17 receptor ligands.

EXPERT OPINION

The GPR17 receptor is an enigmatic receptor and an interesting therapeutic target in a variety of brain disorders and demyelinating diseases such as multiple sclerosis, stroke, schizophrenia, and depression. The modulation of this receptor could also be potentially useful in obesity treatment. Unfortunately, so far, there are no compounds under investigation in clinical trials but many researchers and companies are investing in the discovery of future potential GPR17 receptor drugs.

The immunomodulatory activity and mechanism of docosahexenoic acid (DHA) on immunosuppressive mice models

In this study, the immunomodulatory activity of docosahexaenoic acid (DHA) on the immunosuppressive BALB/c mice model and its molecular mechanism are elucidated. It was found that the weight indexes of the spleen and thymus were significantly increased by DHA (44.0 mg kg-1 and 88.0 mg kg-1) treatment in the prevention or cure groups. The result of macrophages showed that DHA (44.0 mg kg-1 and 88.0 mg kg-1) could promote the proliferation and phagocytosis activity of macrophages in the prevention or cure groups. In addition, DHA could activate macrophages by the G-protein coupled cell membrane receptor GPR120- Mitogen-Activated Protein Kinases (MAPKs)-nuclear factor κB (NF-κB) p65 pathway in vivo. The result of the spleen showed that DHA (44.0 mg kg-1 and 88.0 mg kg-1) could promote the proliferation of spleen cells and the natural killer (NK) cells activity in vivo. In the prevention or cure groups, the quantitative real-time polymerase chain reaction (qRT-PCR) results revealed that DHA (44.0 mg kg-1 and 88.0 mg kg-1) could enhance the production of cytokines IL-1β, IL-2, TNF-α and IFN-γ in the spleen of immunosuppressive mice. The HE (hematoxylin and eosin) stained histopathological images showed that DHA could repair the damage induced by CTX in the spleen cells of the prevention or cure groups. These results suggested that DHA has a remarkable immunomodulatory activity on the immunosuppressive mice model in the prevention or cure groups.

Is the membrane estrogen receptor, GPER1, a promiscuous receptor that modulates nuclear estrogen receptor-mediated functions in the brain?

Contribution to Special Issue on Fast effects of steroids. Estrogen signals both slowly to regulate transcription and rapidly to activate kinases and regulate calcium levels. Both rapid, non-genomic signaling as well as genomic transcriptional signaling via intracellular estrogen receptors (ER)s can change behavior. Rapid non-genomic signaling is initiated from the plasma membrane by a G-protein coupled receptor called GPER1 that binds 17β-estradiol. GPER1 or GPR30 is one of the candidates for a membrane ER (mER) that is not only highly expressed in pathology i.e. cancers but also in several behaviorally-relevant brain regions. In the brain, GPER1 signaling, in response to estrogen, facilitates neuroprotection, social behaviors and cognition. In this review, we describe several notable characteristics of GPER1 such as the ability of several endogenous steroids as well as artificially synthesized molecules to bind the GPER1. In addition, GPER1 is localized to the plasma membrane in breast cancer cell lines but may be present in the endoplasmic reticulum or the Golgi apparatus in the hippocampus. Unusually, GPER1 can also translocate to the perinuclear space from the plasma membrane. We explore the idea that subcellular localization and ligand promiscuity may determine the varied downstream signaling cascades of the activated GPER1. Lastly, we suggest that GPER1 can act as a modulator of ERα-mediated action on a convergent target, spinogenesis, in neurons that in turn drives female social behaviors such as lordosis and social learning.

Immunomodulatory Activity of Docosahexenoic Acid on RAW264.7 Cells Activation through GPR120-Mediated Signaling Pathway

In this study, we elucidated the immunomodulatory activity of docosahexaenoic acid (DHA) on protein expression in RAW264.7 cells and its molecular mechanism. The results showed that the proliferation index of RAW264.7 cells at 48 h was about 173.03 ± 7.82% after the treatment of 2.4 μM DHA. DHA could activate RAW264.7 cells by the G-protein coupled cell membrane receptor GPR120-C-Raf- mitogen-activated protein kinases (MAPKs)-nuclear factor κB (NF-κB) p65 pathway. In addition, 2.4 μM of DHA could significantly increase (P < 0.01) the mRNA and protein expression of inducible nitric oxide synthase (iNOS), which is consistent with the result of the NO release. ELISA results revealed that DHA could enhance the protein expression of cytokines IL-1β, IL-6, IL-10, IL-12, TNF-α, IFN-γ, and TGF-β. These results indicated that the immunomodulatory mechanism of RAW264.7 cells by DHA was associated with the release of NO and cytokines by stimulating the GPR120, C-Raf, and MAPKs to the NF-κB p65 pathway.

Modulating and evaluating receptor promiscuity through directed evolution and modeling

The promiscuity of G-protein-coupled receptors (GPCRs) has broad implications in disease, pharmacology and biosensing. Promiscuity is a particularly crucial consideration for protein engineering, where the ability to modulate and model promiscuity is essential for developing desirable proteins. Here, we present methodologies for (i) modifying GPCR promiscuity using directed evolution and (ii) predicting receptor response and identifying important peptide features using quantitative structure-activity relationship models and grouping-exhaustive feature selection. We apply these methodologies to the yeast pheromone receptor Ste2 and its native ligand α-factor. Using directed evolution, we created Ste2 mutants with altered specificity toward a library of α-factor variants. We then used the  Vectors of Hydrophobic, Steric, and Electronic properties and partial least squares regression to characterize receptor-ligand interactions, identify important ligand positions and properties, and predict receptor response to novel ligands. Together, directed evolution and computational analysis enable the control and evaluation of GPCR promiscuity. These approaches should be broadly useful for the study and engineering of GPCRs and other protein-small molecule interactions.

Possible abilities of dietary factors to prevent and treat diabetes via the stimulation of glucagon-like peptide-1 secretion

There is a pressing need for countermeasures against diabetes, which has increased in incidence, becoming a global issue. Glucagon-like peptide-1 (GLP-1), a molecule secreted in enteroendocrine L cells in the lower small and large intestines, is thought to be one of the most important molecular targets for the prevention and treatment of diabetes. There has been increasing interest in the possible ability of dietary factors to treat diabetes via modulating GLP-1 secretion. There is thought to be a close relationship between incretin and diet, and the purported best approach for using dietary factors to increase GLP-1 activity is promotion of secretion of endogenous GLP-1. It have been reported that nutrients as well as various non-nutrient dietary factors can function as GLP-1 secretogogues. Here, we present our findings on the GLP-1 secretion-stimulating functions of two dietary factors, curcumin and extract of edible sweet potato leaves, which contain caffeoylquinic acid derivatives. However, it is necessary to reveal in greater detail the stimulation of GLP-1 secretion by dietary factors for preventing and treating diabetes. It is desirable to clarify the exact GLP-1 secretory pathway, the effect of metabolites derived from dietary factors in gut lumen, and the relationship between incretin and meal.

Regulation of prostaglandin EP1 and EP4 receptor signaling by carrier-mediated ligand reuptake

After synthesis and release from cells, prostaglandin E₂ (PGE₂) undergoes reuptake by the prostaglandin transporter (PGT), followed by cytoplasmic oxidation. Although genetic inactivation of PGT in mice and humans results in distinctive phenotypes, and although experiments in localized environments show that manipulating PGT alters downstream cellular events, a direct mechanistic link between PGT activity and PGE₂ (EP) receptor activation has not been made. Toward this end, we created two reconstituted systems to examine the effect of PGT expression on PGE₂ signaling via two of its receptors (EP₁ and EP₄). In human embryonic kidney cells engineered to express the EP₁ receptor, exogenous PGE₂ induced a dose-dependent increase in cytoplasmic Ca²⁺. When PGT was expressed at the plasma membrane, the PGE₂ dose–response curve was right-shifted, consistent with reduction in cell surface PGE₂ availability; a potent PGT inhibitor acutely reversed this shift. When bradykinin was used to induce endogenous PGE₂ release, PGT expression similarly induced a reduction in Ca²⁺ responses. In separate experiments using Madin–Darby Canine Kidney cells engineered to express the PGE₂ receptor EP₄, bradykinin again induced autocrine PGE₂ signaling, as judged by an abrupt increase in intracellular cAMP. As in the EP₁ experiments, expression of PGT at the plasma membrane caused a reduction in bradykinin-induced cAMP accumulation. Pharmacological concentrations of exogenous PGE₂ induced EP₄ receptor desensitization, an effect that was mitigated by PGT. Thus, at an autocrine/paracrine level, plasma membrane PGT regulates PGE₂ signaling by decreasing ligand availability at cell surface receptors.

The effects of propionate and valerate on insulin responsiveness for glucose uptake in 3T3-L1 adipocytes and C2C12 myotubes via G protein-coupled receptor 41

Since insulin resistance can lead to hyperglycemia, improving glucose uptake into target tissues is critical for regulating blood glucose levels. Among the free fatty acid receptor (FFAR) family of G protein-coupled receptors, GPR41 is known to be the Gα_i/o-coupled receptor for short-chain fatty acids (SCFAs) such as propionic acid (C3) and valeric acid (C5). This study aimed to investigate the role of GPR41 in modulating basal and insulin-stimulated glucose uptake in insulin-sensitive cells including adipocytes and skeletal muscle cells. Expression of GPR41 mRNA and protein was increased with maximal expression at differentiation day 8 for 3T3-L1 adipocytes and day 6 for C2C12 myotubes. GPR41 protein was also expressed in adipose tissues and skeletal muscle. After analyzing dose-response relationship, 300 µM propionic acid or 500 µM valeric acid for 30 min incubation was used for the measurement of glucose uptake. Both propionic acid and valeric acid increased insulin-stimulated glucose uptake in 3T3-L1 adipocyte, which did not occur in cells transfected with siRNA for GPR41 (siGPR41). In C2C12 myotubes, these SCFAs increased basal glucose uptake, but did not potentiate insulin-stimulated glucose uptake, and siGPR41 treatment reduced valerate-stimulated basal glucose uptake. Therefore, these findings indicate that GPR41 plays a role in insulin responsiveness enhanced by both propionic and valeric acids on glucose uptake in 3T3-L1 adipocytes and C2C12 myotubes, and in valerate-induced increase in basal glucose uptake in C2C12 myotubes.

Identification of a cDNA encoding a novel small secretory protein, neurosecretory protein GL, in the chicken hypothalamic infundibulum

To find novel neuropeptide and/or peptide hormone precursors in the avian brain, we performed a cDNA subtractive screen of the chicken hypothalamic infundibulum, which contains one of the feeding and neuroendocrine centers. After sequencing 596 clones, we identified a novel cDNA encoding a previously unknown protein. The deduced precursor protein consisted of 182 amino acid residues, including one putative small secretory protein of 80 amino acid residues. This small protein was flanked at the N-terminus by a signal peptide and at the C-terminus by a glycine amidation signal and a dibasic amino acid cleavage site. Because the predicted C-terminal amino acids of the small protein were Gly-Leu-NH2, the small protein was named neurosecretory protein GL (NPGL). Quantitative RT-PCR analysis demonstrated specific expression of the NPGL precursor mRNA in the hypothalamic infundibulum. Furthermore, the mRNA levels in the hypothalamic infundibulum increased during post-hatching development. In situ hybridization analysis showed that the cells containing the NPGL precursor mRNA were localized in the medial mammillary nucleus and infundibular nucleus within the hypothalamic infundibulum of 8- and 15-day-old chicks. Subcutaneous infusion of NPGL in chicks increased body weight gain without affecting food intake. To our knowledge, this is the first report to describe the identification and localization of the NPGL precursor mRNA and the function of its translated product in animals. Our findings indicate that NPGL may participate in the growth process in chicks.

Activation of GPR40 as a therapeutic target for the treatment of type 2 diabetes

The stimulation of insulin secretion by glucose can be modulated by multiple nutritive, hormonal, and pharmacological inputs. Fatty acids potentiate insulin secretion through the generation of intracellular signaling molecules and through the activation of cell surface receptors. The G-protein-coupled receptor 40 (GPR40), also known as free fatty acid receptor 1 (we will use GPR40 in this review), has emerged as an important component in the fatty acid augmentation of insulin secretion. By signaling predominantly through Gαq/11, GPR40 increases intracellular calcium and activates phospholipases to generate diacylglycerols resulting in increased insulin secretion. Synthetic small-molecule agonists of GPR40 enhance insulin secretion in a glucose-dependent manner in vitro and in vivo with a mechanism similar to that found with fatty acids. GPR40 agonists have shown efficacy in increasing insulin secretion and lowering blood glucose in rodent models of type 2 diabetes. Recent phase I and phase II clinical trials in humans have shown that the GPR40 agonist TAK-875 reduces fasting and postprandial blood glucose and lowers HbA1c with efficacy equal to that of the sulfonylurea glimepiride without inducing hypoglycemia or evidence of tachyphylaxis. These data suggest that targeting the GPR40 receptor can be a viable therapeutic option for the treatment of type 2 diabetes.

A Strategy Combining Differential Low-Throughput Screening and Virtual Screening (DLS-VS) Accelerating the Discovery of new Modulators for the Orphan GPR34 Receptor

The DLS-VS strategy was developed as an integrated method for identifying chemical modulators for orphan GPCRs. It combines differential low-throughput screening (DLS) and virtual screening (VS). The two cascaded techniques offer complementary advantages and allow the experimental testing of a minimal number of compounds. First, DLS identifies modulators specific for the considered receptor among a set of receptors, through the screening of a small library with diverse chemical compounds. Then, an active molecular model of the receptor is built by homology to a validated template, and it is progressively refined by rotamers modification for key side-chains, by VS of the already screened library, and by iterative selection of the model generating the best enrichment. The refined active model is finally used for the VS of a large chemical library and the selection of a small set of compounds for experimental testing. Applied to the orphan receptor GPR34, the DLS-VS strategy combined the experimental screening of 20 000 compounds and the virtual screening of 1 250 000 compounds. It identified one agonist and eight inverse agonists, showing a high chemical diversity. We describe the method. The strategy can be applied to other GPCRs.

Engineering vascularized tissues using natural and synthetic small molecules

Vascular growth and remodeling are complex processes that depend on the proper spatial and temporal regulation of many different signaling molecules to form functional vascular networks. The ability to understand and regulate these signals is an important clinical need with the potential to treat a wide variety of disease pathologies. Current approaches have focused largely on the delivery of proteins to promote neovascularization of ischemic tissues, most notably VEGF and FGF. Although great progress has been made in this area, results from clinical trials are disappointing and safer and more effective approaches are required. To this end, biological agents used for therapeutic neovascularization must be explored beyond the current well-investigated classes. This review focuses on potential pathways for novel drug discovery, utilizing small molecule approaches to induce and enhance neovascularization. Specifically, four classes of new and existing molecules are discussed, including transcriptional activators, receptor selective agonists and antagonists, natural product-derived small molecules, and novel synthetic small molecules.

APJ acts as a dual receptor in cardiac hypertrophy

Cardiac hypertrophy is initiated as an adaptive response to sustained overload but progresses pathologically as heart failure ensues. Here we report that genetic loss of APJ, a G-protein-coupled receptor, confers resistance to chronic pressure overload by markedly reducing myocardial hypertrophy and heart failure. In contrast, mice lacking apelin (the endogenous APJ ligand) remain sensitive, suggesting an apelin-independent function of APJ. Freshly isolated APJ-null cardiomyocytes exhibit an attenuated response to stretch, indicating that APJ is a mechanosensor. Activation of APJ by stretch increases cardiomyocyte cell size and induces molecular markers of hypertrophy. Whereas apelin stimulates APJ to activate Gαi and elicits a protective response, stretch signals in an APJ-dependent, G-protein-independent fashion to induce hypertrophy. Stretch-mediated hypertrophy is prevented by knockdown of β-arrestins or by pharmacological doses of apelin acting through Gαi. Taken together, our data indicate that APJ is a bifunctional receptor for both mechanical stretch and the endogenous peptide apelin. By sensing the balance between these stimuli, APJ occupies a pivotal point linking sustained overload to cardiomyocyte hypertrophy.

Mechanism of N-terminal modulation of activity at the melanocortin-4 receptor GPCR

Most of our understanding of G protein-coupled receptor (GPCR) activation has been focused on the direct interaction between diffusible ligands and their seven-transmembrane domains. However, a number of these receptors depend on their extracellular N-terminal domain for ligand recognition and activation. To dissect the molecular interactions underlying both modes of activation at a single receptor, we used the unique properties of the melanocortin-4 receptor (MC4R), a GPCR that shows constitutive activity maintained by its N-terminal domain and is physiologically activated by the peptide α-melanocyte stimulating hormone (αMSH). We find that activation by the N-terminal domain and αMSH relies on different key residues in the transmembrane region. We also demonstrate that agouti-related protein, a physiological antagonist of MC4R, acts as an inverse agonist by inhibiting N terminus-mediated activation, leading to the speculation that a number of constitutively active orphan GPCRs could have physiological inverse agonists as sole regulators.

Low affinity GPCRs for metabolic intermediates: challenges for pharmacologists

The discovery that a number of metabolites and metabolic intermediates can act through G protein-coupled receptors has attracted great interest in the field and has led to new therapeutic targets for diseases such as hypertension, type 2 diabetes, inflammation, and metabolic syndrome. However, the low apparent affinity of these ligands for their cognate receptors poses a number of challenges for pharmacologists interested in investigating receptor structure, function or physiology. Furthermore, the endogenous ligands matched to their receptors have other, well established metabolic roles and thus selectivity is difficult to achieve. This review discusses some of the issues researchers face when working with these receptors and highlights the ways in which a number of these obstacles have been overcome.

Investigating endogenous peptides and peptidases using peptidomics

Rather than simply being protein degradation products, peptides have proven to be important bioactive molecules. Bioactive peptides act as hormones, neurotransmitters, and antimicrobial agents in vivo. The dysregulation of bioactive peptide signaling is also known to be involved in disease, and targeting peptide hormone pathways has been a successful strategy in the development of novel therapeutics. The importance of bioactive peptides in biology has spurred research to elucidate the function and regulation of these molecules. Classical methods for peptide analysis have relied on targeted immunoassays, but certain scientific questions necessitated a broader and more detailed view of the peptidome--all the peptides in a cell, tissue, or organism. In this review we discuss how peptidomics has emerged to fill this need through the application of advanced liquid chromatography--tandem mass spectrometry (LC-MS/MS) methods that provide unique insights into peptide activity and regulation.

Free fatty acid receptors FFAR1 and GPR120 as novel therapeutic targets for metabolic disorders

Free fatty acids (FFAs) are not only essential nutritional components, but they also act as signaling molecules in various physiological processes. Recently, a G-protein-coupled receptor deorphanizing strategy has successfully identified a family of receptors that are activated by FFAs. FFA receptors (FFARs) are proposed to play critical roles in a variety of physiological and pathophysiological processes, especially in metabolic disorders. Among the FFARs, FFAR1 (GPR40) and GPR120 are activated by medium- and long-chain FFAs. FFAR1 facilitates glucose-stimulated insulin secretion from pancreatic β-cells, whereas GPR120 regulates the secretion of glucagon-like peptide-1 in the intestine, as well as insulin sensitivity in macrophages. Because these receptors are potential therapeutic targets for metabolic disorders such as type 2 diabetes, selective ligands have been developed. In this review, we discuss recent advances in the identification of ligands, structure activity relationships, and pharmacological characterization of FFAR1 and GPR120, and we present a summary of recent progress in understanding their physiological roles and their potential as drug targets.

Design, synthesis, and biological activity of potent and orally available G protein-coupled receptor 40 agonists

G protein-coupled receptor 40 (GPR40) is being recently considered to be a new potential drug target for the treatment of type 2 diabetes because of its role in the enhancement of free fatty acid-regulated glucose-stimulated insulin secretion in pancreatic β-cells. We initially identified benzyloxyphenylpropanoic acid (1b) (EC(50) = 510 nM), which was designed based on the structure of free fatty acids, as a promising lead compound with GPR40 agonist activity. Chemical modification of compound 1b led to the discovery of 3-{4-[(2',6'-dimethylbiphenyl-3-yl)methoxy]-2-fluorophenyl}propanoic acid (4p) as a potent GPR40 agonist (EC(50) = 5.7 nM). Compound 4p exhibited acceptable pharmacokinetic profiles and significant glucose-lowering effects during an oral glucose tolerance test in diabetic rats. Moreover, no hypoglycemic event was observed even after administration of a high dose of compound 4p to normal fasted rats. These pharmacological results suggest that GPR40 agonists might be novel glucose-dependent insulin secretagogues with little or no risk of hypoglycemia.

Novel neuropeptides as ligands of orphan G protein-coupled receptors

Neuropeptides control a wide spectrum of physiological functions. They are central to our understanding of brain functions. They exert their actions by interacting with specific G protein-coupled receptors. We however have not found all the neuropeptides that exist in organisms. The search for novel neuropeptides is thus of great interest as it will lead to a better understanding of brain function and disorders. In this review, we will discuss the historical as well as the current approaches to neuropeptide discovery, with a particular emphasis on the orphan GPCR-based strategies. We will also discuss two novel peptides, neuropeptide S and neuromedin S, as examples of the impact of neuropeptide discovery on our understanding of brain functions. Finally, the challenges facing neuropeptide discovery will be discussed.

New frontiers in gut nutrient sensor research: free fatty acid sensing in the gastrointestinal tract

Utilizing the human genome database, the recently developed G-protein-coupled receptors (GPCRs) deorphanizing strategy successfully identified multiple receptors of free fatty acids (FFAs). FFAs have been demonstrated to act as ligands of several GPCRs (FFAR1, FFAR2, FFAR3, and GPR120). These fatty acid receptors are proposed to play critical roles in various types of physiological homeostases. FFAR1 and GPR120 are activated by medium- and long-chain FFAs. In contrast, FFAR2 and FFAR3 are activated by short-chain FFAs. It has been elucidated that these four receptors are expressed in the gastrointestinal tract and have many essential roles as sensors of FFA. In this review, we summarize the physiological and pharmacological function of the receptors in the gastrointestinal tract.

Deorphanization of novel peptides and their receptors

Peptide hormones and neuropeptides play important roles in endocrine and neural signaling, often using G protein-coupled receptor (GPCR)-mediated signaling pathways. However, the rate of novel peptide discovery has slowed dramatically in recent years. Genomic sequencing efforts have yielded a large number of cDNA sequences that potentially encode novel candidate peptide precursors, as well as hundreds of orphan GPCRs with no known cognate ligands. The complexity of peptide signaling is further highlighted by the requirement for specific posttranslational processing steps, and these must be accomplished in vitro prior to testing newly discovered peptide precursor candidates in receptor assays. In this review, we present historic as well as current approaches to peptide discovery and GPCR deorphanization. We conclude that parallel and combinatorial discovery methods are likely to represent the most fruitful avenues for both peptide discovery as well as for matching the remaining GPCRs with their peptide ligands.

Heterotrimeric G proteins regulate reproductive trait plasticity in response to water availability

Phenotypic plasticity is the ability of one genotype to display different phenotypes under different environmental conditions. Although variation for phenotypic plasticity has been documented in numerous species, little is known about the genetic mechanisms underlying phenotypic plasticity. Given their widespread roles in hormonal and environmental signaling, we examined whether genes which encode heterotrimeric G proteins are plasticity genes. We grew multiple alleles of heterotrimeric G-protein mutants, together with wild-type Arabidopsis thaliana, under different watering regimes to determine the contributions of G-protein genes to phenotypic plasticity for a number of developmental and reproduction-related traits. G-protein mutations did not affect significantly the amount of phenotypic variation within an environment for any trait, but did affect significantly the amount of phenotypic plasticity for certain traits. AGB1, which encodes the beta subunit of the heterotrimeric G protein in Arabidopsis, is a plasticity gene and regulates reproductive trait plasticity in response to water availability, resulting in increased fitness (defined as seed production) under drought stress.

Heterodimerization of the GABAB receptor-implications for GPCR signaling and drug discovery

The identification of the molecular nature of the GABA(B) receptor and the demonstration of its heterodimeric structure has led to extensive studies investigating the mechanism of activation and signaling. Phylogenetic studies suggest that the formation of the heterodimer is a relatively recent event arising in conjunction with the evolution of the central nervous system. Heterodimerization has now been demonstrated for many other G-protein-coupled receptors (GPCRs) and plays a role in signaling and trafficking. This presents both challenges and opportunities for GPCR drug discovery. In the case of the GABA(B) receptor the best hope for the development of new drugs directed at this receptor is from allosteric modulators. This chapter summarizes our current understanding of the molecular function of the GABA(B) receptor and recent developments in the identification of allosteric modulators. The broader implication of heterodimerization on GPCR function and drug discovery is also discussed.

Pharmacological characterization of a selective agonist for bombesin receptor subtype-3

Bombesin receptor subtype-3 (BRS-3) is an orphan G protein-coupled receptor in the bombesin receptor family that still awaits identification of its natural ligand. BRS-3 deficient mice develop a mild late-onset obesity with metabolic defects, implicating BRS-3 plays a role in feeding and metabolism. We describe here the pharmacological characterization of a synthetic compound, 16a, which serves as a potent agonist for BRS-3. This compound is selective for BRS-3 as it does not activate neuromedin B or gastrin-releasing peptide receptors, two most closely related bombesin receptors, as well as a series of other GPCRs. We assessed the receptor trafficking of BRS-3 and found that compound 16a promoted beta-arrestin translocation to the cell membrane. Neither central nor peripheral administration of compound 16a affects locomotor activity in mice. Therefore compound 16a is a potential tool to study the function of the BRS-3 system in vitro and possibly in vivo.

Identification of surrogate agonists and antagonists for orphan G-protein-coupled receptor GPR139

GPR139 is an orphan G-protein-coupled receptor (GPCR) that is expressed nearly exclusively in the central nervous system and may play a role in the control of locomotor activity. The signal transduction pathway and pharmacological function of GPR139, however, are still controversial due to the lack of natural or synthetic ligands. The authors report the characterization of human GPR139 signaling pathway and identification of surrogate agonists and antagonists. In both transient and stable transfections of HEK293F cells, overexpression of GPR139 increased basal intracellular cAMP concentrations compared to control cells. Furthermore, forskolin and isoproterenol-stimulated cAMP responses were enhanced in GPR139-expressing cells, suggesting that GPR139 is predominantly coupled to Galpha(s). The authors screened a large library of small molecules for compounds that increase cAMP levels in GPR139-expressing cells and identified a compound with GPR139 agonist activity. This compound increased cAMP production specifically in cells expressing GPR139 but not in cells expressing its highly homologous receptor GPR142. Furthermore, this compound did not induce calcium mobilization in GPR139 cells, indicating no Galpha(q)-mediated response. In addition, antagonist screening with the identified agonist yielded 2 classes of compounds as antagonists. The identification of surrogate agonists and antagonists of human GPR139 provides important tools for further study of this orphan GPCR.

RHYTHM--a server to predict the orientation of transmembrane helices in channels and membrane-coils

RHYTHM is a web server that predicts buried versus exposed residues of helical membrane proteins. Starting from a given protein sequence, secondary and tertiary structure information is calculated by RHYTHM within only a few seconds. The prediction applies structural information from a growing data base of precalculated packing files and evolutionary information from sequence patterns conserved in a representative dataset of membrane proteins ('Pfam-domains'). The program uses two types of position specific matrices to account for the different geometries of packing in channels and transporters ('channels') or other membrane proteins ('membrane-coils'). The output provides information on the secondary structure and topology of the protein and specifically on the contact type of each residue and its conservation. This information can be downloaded as a graphical file for illustration, a text file for analysis and statistics and a PyMOL file for modeling purposes. The server can be freely accessed at: URL: http://proteinformatics.de/rhythm.

Free fatty acid receptors act as nutrient sensors to regulate energy homeostasis

Free fatty acids (FFAs) have been demonstrated to act as ligands of several G-protein-coupled receptors (GPCRs) (FFAR1, FFAR2, FFAR3, GPR84, and GPR120). These fatty acid receptors are proposed to play critical roles in a variety of types of physiological homeostasis. FFAR1 and GPR120 are activated by medium- and long-chain FFAs. GPR84 is activated by medium-chain, but not long-chain, FFAs. In contrast, FFAR2 and FFAR3 are activated by short-chain FFAs. FFAR1 is expressed mainly in pancreatic beta-cells and mediates insulin secretion, whereas GPR120 is expressed abundantly in the intestine and promotes the secretion of glucagon-like peptide-1 (GLP-1). FFAR3 is expressed in enteroendocrine cells and regulates host energy balance through effects that are dependent upon the gut microbiota. In this review, we summarize the identification, structure, and pharmacology of these receptors and present an essential overview of the current understanding of their physiological roles.

Islet G protein-coupled receptors as potential targets for treatment of type 2 diabetes

Islet dysfunction - characterized by a combination of defective insulin secretion, inappropriately high glucagon secretion and reduced beta-cell mass - has a central role in the pathophysiology of type 2 diabetes. Several G protein-coupled receptors (GPCRs) expressed in islet beta-cells are known to be involved in the regulation of islet function, and therefore are potential therapeutic targets. This is evident from the recent success of glucagon-like peptide 1 (GLP1) mimetics and dipeptidyl peptidase 4 (DPP4) inhibitors, which promote activation of the GLP1 receptor to stimulate insulin secretion and inhibit glucagon secretion, and also have the potential to increase beta-cell mass. Other islet beta-cell GPCRs that are involved in the regulation of islet function include the glucose-dependent insulinotropic peptide (GIP) receptor, lipid GPCRs, pleiotropic peptide GPCRs and islet biogenic amine GPCRs. This Review summarizes islet GPCR expression, signalling and function, and highlights their potential as targets for the treatment of type 2 diabetes.

A constitutively active GPCR governs morphogenic transitions in Cryptococcus neoformans

Sex in fungi is driven by peptide pheromones sensed through seven-transmembrane pheromone receptors. In Cryptococcus neoformans, sexual reproduction occurs through an outcrossing/heterothallic a- sexual cycle or an inbreeding/homothallic - unisexual mating process. Pheromone receptors encoded by the mating-type locus (MAT) mediate reciprocal pheromone sensing during opposite-sex mating and contribute to but are not essential for unisexual mating. A pheromone receptor-like gene, CPR2, was discovered that is not encoded by MAT and whose expression is induced during a- mating. cpr2 mutants are fertile but have a fusion defect and produce abnormal hyphal structures, whereas CPR2 overexpression elicits unisexual reproduction. When heterologously expressed in Saccharomyces cerevisiae, Cpr2 activates pheromone responses in the absence of any ligand. This constitutive activity results from an unconventional residue, Leu(222), in place of a conserved proline in transmembrane domain six; a Cpr2(L222P) mutant is no longer constitutively active. Cpr2 engages the same G-protein activated signalling cascade as the Ste3a/alpha pheromone receptors, and thereby competes for pathway activation. This study established a new paradigm in which a naturally occurring constitutively active G protein-coupled receptor governs morphogenesis in fungi.

GPR3 receptor, a novel actor in the emotional-like responses

GPR3 is an orphan G protein-coupled receptor endowed with constitutive Gs signaling activity, which is expressed broadly in the central nervous system, with maximal expression in the habenula. We investigated the consequences of its genetic deletion in several behavioral paradigms and on neurotransmission. Compared to wild-type, hippocampal neurons from Gpr3(-/-) mice displayed lower basal intracellular cAMP levels, consistent with the strong constitutive activity of GPR3 in transiently transfected cells. Behavioral analyses revealed that Gpr3(-/-) mice exhibited a high level of avoidance of novel and unfamiliar environment, associated with increased stress reactivity in behavioral despair paradigms and aggressive behavior in the resident-intruder test. On the contrary, no deficit was found in the learning ability to avoid an aversive event in active avoidance task. The reduced ability of Gpr3(-/-) mice to cope with stress was unrelated to dysfunction of the hypothalamic-pituitary-adrenal axis, with Gpr3(-/-) mice showing normal corticosterone production under basal or stressful conditions. In contrast, dramatic alterations of monoamine contents were found in hippocampus, hypothalamus and frontal cortex of Gpr3(-/-) mice. Our results establish a link between tonic stimulation of the cAMP signaling pathway by GPR3 and control of neurotransmission by monoamines throughout the forebrain. GPR3 qualifies as a new player in the modulation of behavioral responses to stress and constitutes a novel promising pharmacological target for treatment of emotional disorders.

Organizing bioactive compound discovery in target families

The sequencing of genomes gave access to the complete set of building blocks for organisms of various species. A plethora of "-omics"-technologies has been developed to investigate the dynamic interactions of the building blocks in order to understand the functioning of living organisms. This has given rise to the clustering of proteins into target families based on the phylogenetic and structural similarities. In this chapter we will discuss how the concept of target families enables to investigate and modulate biochemical function in the quest to chart Chemical and Biological Spaces.

Free fatty acid receptors and drug discovery

Utilizing the human genome database, the recently developed G-protein-coupled receptor (GPCR) deorphanizing strategy successfully identified multiple receptors of free fatty acids (FFAs) and is proposed to play a critical role in a variety of physiologic homeostasis mechanisms. GPR40 and GPR120 are activated by medium- and long-chain FFAs, whereas GPR41 and GPR43 are activated by short-chain FFAs. GPR40, which is preferentially expressed in pancreatic beta-cells, mediates insulin secretion. On the other hand, GPR120, which is abundantly expressed in the intestine, functions as a receptor for unsaturated long-chain FFAs and promotes the secretion of glucagon-like peptide-1 (GLP-1). In this review, we summarize the identification, structure, and pharmacology of the receptors and speculate on the respective physiologic roles that FFA receptor family members may play.

IUPHAR-DB: the IUPHAR database of G protein-coupled receptors and ion channels

The IUPHAR database (IUPHAR-DB) integrates peer-reviewed pharmacological, chemical, genetic, functional and anatomical information on the 354 nonsensory G protein-coupled receptors (GPCRs), 71 ligand-gated ion channel subunits and 141 voltage-gated-like ion channel subunits encoded by the human, rat and mouse genomes. These genes represent the targets of approximately one-third of currently approved drugs and are a major focus of drug discovery and development programs in the pharmaceutical industry. IUPHAR-DB provides a comprehensive description of the genes and their functions, with information on protein structure and interactions, ligands, expression patterns, signaling mechanisms, functional assays and biologically important receptor variants (e.g. single nucleotide polymorphisms and splice variants). In addition, the phenotypes resulting from altered gene expression (e.g. in genetically altered animals or in human genetic disorders) are described. The content of the database is peer reviewed by members of the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification (NC-IUPHAR); the data are provided through manual curation of the primary literature by a network of over 60 subcommittees of NC-IUPHAR. Links to other bioinformatics resources, such as NCBI, Uniprot, HGNC and the rat and mouse genome databases are provided. IUPHAR-DB is freely available at http://www.iuphar-db.org.

Topology of class A G protein-coupled receptors: insights gained from crystal structures of rhodopsins, adrenergic and adenosine receptors

Biological membranes are densely packed with membrane proteins that occupy approximately half of their volume. In almost all cases, membrane proteins in the native state lack the higher-order symmetry required for their direct study by diffraction methods. Despite many technical difficulties, numerous crystal structures of detergent solubilized membrane proteins have been determined that illustrate their internal organization. Among such proteins, class A G protein-coupled receptors have become amenable to crystallization and high resolution X-ray diffraction analyses. The derived structures of native and engineered receptors not only provide insights into their molecular arrangements but also furnish a framework for designing and testing potential models of transformation from inactive to active receptor signaling states and for initiating rational drug design.

Discovery and validation of novel peptide agonists for G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) represent an important group of targets for pharmaceutical therapeutics. The completion of the human genome revealed a large number of putative GPCRs. However, the identification of their natural ligands, and especially peptides, suffers from low discovery rates, thus impeding development of therapeutics based on these potential drug targets. We describe the discovery of novel GPCR ligands encrypted in the human proteome. Hundreds of potential peptide ligands were predicted by machine learning algorithms. In vitro screening of selected 33 peptides on a set of 152 GPCRs, including a group of designated orphan receptors, was conducted by intracellular calcium measurements and cAMP assays. The screening revealed eight novel peptides as potential agonists that specifically activated six different receptors in a dose-dependent manner. Most of the peptides showed distinct stimulatory patterns targeted at designated and orphan GPCRs. Further analysis demonstrated a significant in vivo effect for one of the peptides in a mouse inflammation model.