Agonist-independent regulation of constitutively active G-protein-coupled receptors

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.

Allosteric Regulation of G-Protein-Coupled Receptors: From Diversity of Molecular Mechanisms to Multiple Allosteric Sites and Their Ligands

Allosteric regulation is critical for the functioning of G protein-coupled receptors (GPCRs) and their signaling pathways. Endogenous allosteric regulators of GPCRs are simple ions, various biomolecules, and protein components of GPCR signaling (G proteins and β-arrestins). The stability and functional activity of GPCR complexes is also due to multicenter allosteric interactions between protomers. The complexity of allosteric effects caused by numerous regulators differing in structure, availability, and mechanisms of action predetermines the multiplicity and different topology of allosteric sites in GPCRs. These sites can be localized in extracellular loops; inside the transmembrane tunnel and in its upper and lower vestibules; in cytoplasmic loops; and on the outer, membrane-contacting surface of the transmembrane domain. They are involved in the regulation of basal and orthosteric agonist-stimulated receptor activity, biased agonism, GPCR-complex formation, and endocytosis. They are targets for a large number of synthetic allosteric regulators and modulators, including those constructed using molecular docking. The review is devoted to the principles and mechanisms of GPCRs allosteric regulation, the multiplicity of allosteric sites and their topology, and the endogenous and synthetic allosteric regulators, including autoantibodies and pepducins. The allosteric regulation of chemokine receptors, proteinase-activated receptors, thyroid-stimulating and luteinizing hormone receptors, and beta-adrenergic receptors are described in more detail.

The mechanism for ligand activation of the GPCR-G protein complex

G protein–coupled receptors (GPCRs) activate cellular responses ranging from odorants to neurotransmitters. Binding an agonist leads to activation of a heterotrimeric G protein (GP) that stimulates external signaling. Unfortunately, the mechanism remains unknown. We show for 15 class A GPCRs, including opioids, adrenergics, adenosines, chemokines, muscarinics, cannabinoids, serotonins, and dopamines, that interaction of an inactive GP, including Gs, Gi, Go, G11, and Gq, to the inactive GPCR, containing the intracellular ionic lock between transmembrane (TM) helices 3 and 6, evolves exothermically to form a precoupled GPCR-GP complex with an opened TM3-TM6 and the GP-α5 helix partially inserted into the GPCR but not activated. We show that binding of agonist to this precoupled GPCR-GP complex causes the Gα protein to open into its active form, with the guanosine diphosphate exposed for signaling. This GP-first paradigm provides a strategy for developing selective agonists for GPCRs since it is the pharmacophore for the precoupled GPCR-GP complex that should be used to design drugs.

Ciliary and extraciliary Gpr161 pools repress hedgehog signaling in a tissue-specific manner

The role of compartmentalized signaling in primary cilia during tissue morphogenesis is not well understood. The cilia localized G protein-coupled receptor, Gpr161, represses hedgehog pathway via cAMP signaling. We engineered a knock-in at the Gpr161 locus in mice to generate a variant (Gpr161^mut1), which was ciliary localization defective but cAMP signaling competent. Tissue phenotypes from hedgehog signaling depend on downstream bifunctional Gli transcriptional factors functioning as activators or repressors. Compared to knockout (ko), Gpr161^mut1/ko had delayed embryonic lethality, moderately increased hedgehog targets, and partially down-regulated Gli3 repressor. Unlike ko, the Gpr161^mut1/ko neural tube did not show Gli2 activator-dependent expansion of ventral-most progenitors. Instead, the intermediate neural tube showed progenitor expansion that depends on loss of Gli3 repressor. Increased extraciliary receptor levels in Gpr161^mut1/mut1 prevented ventralization. Morphogenesis in limb buds and midface requires Gli repressor; these tissues in Gpr161^mut1/mut1 manifested hedgehog hyperactivation phenotypes—polydactyly and midfacial widening. Thus, ciliary and extraciliary Gpr161 pools likely establish tissue-specific Gli repressor thresholds in determining morpho-phenotypic outcomes.

Synthesis and Preclinical Validation of Novel Indole Derivatives as a GPR17 Agonist for Glioblastoma Treatment

The discovery of a potential ligand-targeting G protein-coupled receptor 17 (GPR17) is important for developing chemotherapeutic agents against glioblastoma multiforme (GBM). We used the integration of ligand- and structure-based cheminformatics and experimental approaches for identifying the potential GPR17 ligand for GBM treatment. Here, we identified a novel indoline-derived phenolic Mannich base as an activator of GPR17 using molecular docking of over 6000 indoline derivatives. One of the top 10 hit molecules, CHBC, with a glide score of -8.390 was synthesized through a multicomponent Petasis borono-Mannich reaction. The CHBC-GPR17 interaction leads to a rapid decrease of cAMP and Ca²⁺. CHBC exhibits the cytotoxicity effect on GBM cells in a dose-dependent manner with an IC₅₀ of 85 μM, whereas the known agonist MDL29,951 showed a negligible effect. Our findings suggest that the phenolic Mannich base could be a better GPR17 agonist than MDL29,951, and further uncovering their pharmacological properties could potentiate an inventive GBM treatment.

Viewing rare conformations of the β2 adrenergic receptor with pressure-resolved DEER spectroscopy

The β₂ adrenergic receptor (β₂AR) is an archetypal G protein coupled receptor (GPCR). One structural signature of GPCR activation is a large-scale movement (ca. 6 to 14 Å) of transmembrane helix 6 (TM6) to a conformation which binds and activates a cognate G protein. The β₂AR exhibits a low level of agonist-independent G protein activation. The structural origin of this basal activity and its suppression by inverse agonists is unknown but could involve a unique receptor conformation that promotes G protein activation. Alternatively, a conformational selection model proposes that a minor population of the canonical active receptor conformation exists in equilibrium with inactive forms, thus giving rise to basal activity of the ligand-free receptor. Previous spin-labeling and fluorescence resonance energy transfer experiments designed to monitor the positional distribution of TM6 did not detect the presence of the active conformation of ligand-free β₂AR. Here we employ spin-labeling and pressure-resolved double electron-electron resonance spectroscopy to reveal the presence of a minor population of unliganded receptor, with the signature outward TM6 displacement, in equilibrium with inactive conformations. Binding of inverse agonists suppresses this population. These results provide direct structural evidence in favor of a conformational selection model for basal activity in β₂AR and provide a mechanism for inverse agonism. In addition, they emphasize 1) the importance of minor populations in GPCR catalytic function; 2) the use of spin-labeling and variable-pressure electron paramagnetic resonance to reveal them in a membrane protein; and 3) the quantitative evaluation of their thermodynamic properties relative to the inactive forms, including free energy, partial molar volume, and compressibility.

Endogenous analgesia, dependence, and latent pain sensitization

Endogenous activation of µ-opioid receptors (MORs) provides relief from acute pain. Recent studies have established that tissue inflammation produces latent pain sensitization (LS) that is masked by spinal MOR signaling for months, even after complete recovery from injury and re-establishment of normal pain thresholds. Disruption with MOR inverse agonists reinstates pain and precipitates cellular, somatic, and aversive signs of physical withdrawal; this phenomenon requires N-methyl-D-aspartate receptor-mediated activation of calcium-sensitive adenylyl cyclase type 1 (AC1). In this review, we present a new conceptual model of the transition from acute to chronic pain, based on the delicate balance between LS and endogenous analgesia that develops after painful tissue injury. First, injury activates pain pathways. Second, the spinal cord establishes MOR constitutive activity (MORCA) as it attempts to control pain. Third, over time, the body becomes dependent on MORCA, which paradoxically sensitizes pain pathways. Stress or injury escalates opposing inhibitory and excitatory influences on nociceptive processing as a pathological consequence of increased endogenous opioid tone. Pain begets MORCA begets pain vulnerability in a vicious cycle. The final result is a silent insidious state characterized by the escalation of two opposing excitatory and inhibitory influences on pain transmission: LS mediated by AC1 (which maintains the accelerator) and pain inhibition mediated by MORCA (which maintains the brake). This raises the prospect that opposing homeostatic interactions between MORCA analgesia and latent NMDAR-AC1-mediated pain sensitization creates a lasting vulnerability to develop chronic pain. Thus, chronic pain syndromes may result from a failure in constitutive signaling of spinal MORs and a loss of endogenous analgesic control. An overarching long-term therapeutic goal of future research is to alleviate chronic pain by either (a) facilitating endogenous opioid analgesia, thus restricting LS within a state of remission, or (b) extinguishing LS altogether.

G-protein-coupled receptors, Hedgehog signaling and primary cilia

The Hedgehog (Hh) pathway has become an important model to study the cell biology of primary cilia, and reciprocally, the study of ciliary processes provides an opportunity to solve longstanding mysteries in the mechanism of vertebrate Hh signal transduction. The cilium is emerging as an unique compartment for G-protein-coupled receptor (GPCR) signaling in many systems. Two members of the GPCR family, Smoothened and Gpr161, play important roles in the Hh pathway. We review the current understanding of how these proteins may function to regulate Hh signaling and also highlight some of the critical unanswered questions being tackled by the field. Uncovering GPCR-regulated mechanisms important in Hh signaling may provide therapeutic strategies against the Hh pathway that plays important roles in development, regeneration and cancer.

Mechanotransduction

Physical forces are central players in development and morphogenesis, provide an ever-present backdrop influencing physiological functions, and contribute to a variety of pathologies. Mechanotransduction encompasses the rich variety of ways in which cells and tissues convert cues from their physical environment into biochemical signals. These cues include tensile, compressive and shear stresses, and the stiffness or elastic modulus of the tissues in which cells reside. This article focuses on the proximal events that lead directly from a change in physical state to a change in cell-signaling state. A large body of evidence demonstrates a prominent role for the extracellular matrix, the intracellular cytoskeleton, and the cell matrix adhesions that link these networks in transduction of the mechanical environment. Recent work emphasizes the important role of physical unfolding or conformational changes in proteins induced by mechanical loading, with examples identified both within the focal adhesion complex at the cell-matrix interface and in extracellular matrix proteins themselves. Beyond these adhesion and matrix-based mechanisms, classical and new mechanisms of mechanotransduction reside in stretch-activated ion channels, the coupling of physical forces to interstitial autocrine and paracrine signaling, force-induced activation of extracellular proteins, and physical effects directly transmitted to the cell's nucleus. Rapid progress is leading to detailed delineation of molecular mechanisms by which the physical environment shapes cellular signaling events, opening up avenues for exploring how mechanotransduction pathways are integrated into physiological and pathophysiological cellular and tissue processes.

Regulation of Constitutive GPR3 Signaling and Surface Localization by GRK2 and β-arrestin-2 Overexpression in HEK293 Cells

G protein-coupled receptor 3 (GPR3) is a constitutively active receptor that maintains high 3′-5′-cyclic adenosine monophosphate (cAMP) levels required for meiotic arrest in oocytes and CNS function. Ligand-activated G protein-coupled receptors (GPCRs) signal at the cell surface and are silenced by phosphorylation and β-arrestin recruitment upon endocytosis. Some GPCRs can also signal from endosomes following internalization. Little is known about the localization, signaling, and regulation of constitutively active GPCRs. We demonstrate herein that exogenously-expressed GPR3 localizes to the cell membrane and undergoes internalization in HEK293 cells. Inhibition of endocytosis increased cell surface-localized GPR3 and cAMP levels while overexpression of GPCR-Kinase 2 (GRK2) and β-arrestin-2 decreased cell surface-localized GPR3 and cAMP levels. GRK2 by itself is sufficient to decrease cAMP production but both GRK2 and β-arrestin-2 are required to decrease cell surface GPR3. GRK2 regulates GPR3 independently of its kinase activity since a kinase inactive GRK2-K220R mutant significantly decreased cAMP levels. However, GRK2-K220R and β-arrestin-2 do not diminish cell surface GPR3, suggesting that phosphorylation is required to induce GPR3 internalization. To understand which residues are targeted for desensitization, we mutated potential phosphorylation sites in the third intracellular loop and C-terminus and examined the effect on cAMP and receptor surface localization. Mutation of residues in the third intracellular loop dramatically increased cAMP levels whereas mutation of residues in the C-terminus produced cAMP levels comparable to GPR3 wild type. Interestingly, both mutations significantly reduced cell surface expression of GPR3. These results demonstrate that GPR3 signals at the plasma membrane and can be silenced by GRK2/β-arrestin overexpression. These results also strongly implicate the serine and/or threonine residues in the third intracellular loop in the regulation of GPR3 activity.

The ciliary G-protein-coupled receptor Gpr161 negatively regulates the Sonic hedgehog pathway via cAMP signaling

The primary cilium is required for Sonic hedgehog (Shh) signaling in vertebrates. In contrast to mutants affecting ciliary assembly, mutations in the intraflagellar transport complex A (IFT-A) paradoxically cause increased Shh signaling. We previously showed that the IFT-A complex, in addition to its canonical role in retrograde IFT, binds to the tubby-like protein, Tulp3, and recruits it to cilia. Here, we describe a conserved vertebrate G-protein-coupled receptor, Gpr161, which localizes to primary cilia in a Tulp3/IFT-A-dependent manner. Complete loss of Gpr161 in mouse causes midgestation lethality and increased Shh signaling in the neural tube, phenocopying Tulp3/IFT-A mutants. Constitutive Gpr161 activity increases cAMP levels and represses Shh signaling by determining the processing of Gli3 to its repressor form. Conversely, Shh signaling directs Gpr161 to be internalized from cilia, preventing its activity. Thus, Gpr161 defines a morphogenetic pathway coupling protein kinase A activation to Shh signaling during neural tube development.

Cannabinoid receptors: nomenclature and pharmacological principles

The CB1 and CB2 cannabinoid receptors are members of the G protein-coupled receptor (GPCR) family that are pharmacologically well defined. However, the discovery of additional sites of action for endocannabinoids as well as synthetic cannabinoid compounds suggests the existence of additional cannabinoid receptors. Here we review this evidence, as well as the current nomenclature for classifying a target as a cannabinoid receptor. Basic pharmacological definitions, principles and experimental conditions are discussed in order to place in context the mechanisms underlying cannabinoid receptor activation. Constitutive (agonist-independent) activity is observed with the overexpression of many GPCRs, including cannabinoid receptors. Allosteric modulators can alter the pharmacological responses of cannabinoid receptors. The complex molecular architecture of each of the cannabinoid receptors allows for a single receptor to recognize multiple classes of compounds and produce an array of distinct downstream effects. Natural polymorphisms and alternative splice variants may also contribute to their pharmacological diversity. As our knowledge of the distinct differences grows, we may be able to target select receptor conformations and their corresponding pharmacological responses. Importantly, the basic biology of the endocannabinoid system will continue to be revealed by ongoing investigations.

Chronic treatment with the opioid antagonist naltrexone favours the coupling of spinal cord μ-opioid receptors to Gαz protein subunits

Sustained administration of opioid antagonists to rodents results in an enhanced antinociceptive response to agonists. We investigated the changes in spinal μ-opioid receptor signalling underlying this phenomenon. Rats received naltrexone (120 μg/h; 7 days) via osmotic minipumps. The antinociceptive response to the μ-agonist sufentanil was tested 24 h after naltrexone withdrawal. In spinal cord samples, we determined the interaction of μ-receptors with Gα proteins (agonist-stimulated [(35)S]GTPγS binding and immunoprecipitation of [(35)S]GTPγS-labelled Gα subunits) as well as μ-opioid receptor-dependent inhibition of the adenylyl cyclase (AC) activity. Chronic naltrexone treatment augmented DAMGO-stimulated [(35)S]GTPγS binding, potentiated the inhibitory effect of DAMGO on the AC/cAMP pathway, and increased the inverse agonist effect of naltrexone on cAMP accumulation. In control rats, the inhibitory effect of DAMGO on cAMP production was antagonized by pertussis toxin (PTX) whereas, after chronic naltrexone, the effect became resistant to the toxin, suggesting a coupling of μ-receptors to PTX-insensitive Gα(z) subunits. Immunoprecipitation assays confirmed the transduction switch from Gα(i/o) to Gα(z) proteins. The consequence was an enhancement of the antinociceptive response to sufentanil that, in consonance with the neurochemical data, was prevented by Gα(z)-antisense oligodeoxyribonucleotides but not by PTX. Such changes in opioid receptor signalling can be a double-edged sword. On the one hand, they may have potential applicability to the optimisation of the analgesic effects of opioid drugs for the control of pain. On the other hand, they represent an important homeostatic dysregulation of the endogenous opioid system that might account for undesirable effects in patients chronically treated with opioid antagonists. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Measurement of inverse agonism in β-adrenoceptors

Increasing numbers of compounds, previously classified as antagonists, were shown to inhibit this spontaneous or constitutive receptor activity, instead of leave it unaffected as expected for a formal antagonist. In addition, some other antagonists did not have any effect by themselves, but prevented the inhibition of constitutive activity induced by thought-to-be antagonists. These thought-to-be antagonists with negative efficacy are now known as "inverse agonists." Inverse agonism at βAR has been evidenced for both subtypes in wild-type GPCRs systems and in engineered systems with high constitutive activity. It is important to mention that native systems are of particular importance for analyzing the in vivo relevance of constitutive activity because these systems have physiological expression levels of target receptors. Studies of inverse agonism of β blockers in physiological setting have also evidenced that pathophysiological conditions can affect pharmacodynamic properties of these ligands. To date, hundreds of clinically well-known drugs have been tested and classified for this property. Prominent examples include the beta-blockers propranolol, alprenolol, pindolol, and timolol used for treating hypertension, angina pectoris, and arrhythmia that act on the β₂ARs, metoprolol, and bisoprolol used for treating hypertension, coronary heart disease, and arrhythmias by acting on β₁ARs. Inverse agonists seem to be useful in the treatment of chronic disease characterized by harmful effects resulting from β₁AR and β₂AR overactivation, such as heart failure and asthma, respectively.

Ghrelin receptor: high constitutive activity and methods for developing inverse agonists

The ghrelin receptor is a G protein-coupled receptor (GPCR) mainly distributed in the brain, and also expressed in peripheral tissues. Remarkably, the ghrelin receptor possesses a naturally high constitutive activity representing 50% of its maximal activity. Its endogenous ligand ghrelin is the only known orexigenic gastrointestinal peptide and plays a central role in the regulation of appetite, food intake, and energy homeostasis. Reducing the constitutive activity of the ghrelin receptor by inverse agonists is the strategy adopted by our group to develop anti-obesity drugs. Therefore, short peptides were synthesized and showed high inverse agonist potency toward the ghrelin receptor. This review describes the methods used to synthesize the peptides and to evaluate their biological activity. Peptide synthesis was performed on solid phase using a Fmoc/tBu-strategy. Peptide potency was measured with a signal transduction assay, the inositol trisphosphate turnover assay, adapted to a receptor expressing constitutive activity.

Screening for novel constitutively active CXCR2 mutants and their cellular effects

Chemokines play an important role in inflammatory, developmental, and homeostatic processes. Deregulation of this system results in various diseases including tumorigenesis and cancer metastasis. Deregulation can occur when constitutively active mutant (CAM) chemokine receptors are locked in the "on" position. This can lead to cellular transformation/tumorigenesis. The CXC chemokine receptor 2 (CXCR2) is a G-protein-coupled receptor (GPCR) expressed on neutrophils, some monocytes, endothelial cells, and some epithelial cells. CXCR2 activation with CXC chemokines induces leukocyte migration, trafficking, leukocyte degranulation, cellular differentiation, and angiogenesis. Activation of CXCR2 can lead to cellular transformation. We hypothesized that CAM CXCR2s may play a role in cancer development. In order to identify CXCR2 CAMs, potential mutant CXCR2 receptors were screened using a modified Saccharomyces cerevisiae high-throughput system. S. cerevisiae has been used successfully to identify GPCR/G-protein interactions and autocrine selection for peptide agonists. The CXCR2 CAMs identified from this screen were characterized in mammalian cells. Their ability to transform cells in vitro was shown using foci formation, soft-agar growth, impedance measurement assays, and in vivo tumor growth following hind flank inoculation into mice. Signaling pathways contributing to cellular transformation were identified using luciferase reporter assays. Studying constitutively active GPCRs is an approach to "capturing" pluridimensional GPCRs in a "locked" activation state. In order to address the residues necessary for CXCR2 activation, we used S. cerevisiae for screening novel CAMs and characterized them using mammalian reporter assays.

High ambient glucose induces angiotensin-independent AT-1 receptor activation, leading to increases in proliferation and extracellular matrix accumulation in MES-13 mesangial cells

Diabetic nephropathy is associated with mesangial ECM (extracellular matrix) accumulation. We have shown that AT-1R [Ang II (angiotensin II) type I receptor] signalling induces ECM proteins via transactivation of PI3K (phosphoinositide 3-kinase) in mesangial cells. In the present study, we examined the mechanisms underlying the effect of high ambient glucose on cell proliferation and ECM expansion in a mesangial context. High glucose induced increases in PI3K activity, proliferation and ECM accumulation in mesangial cells. These effects were abrogated by losartan, an AT-1R antagonist, but not by [Sar1,Thr8]-Ang II (Sar is sarcosine), an inactive analogue of Ang II, or by a neutralizing antibody against Ang I/II. Overexpression of a constitutively active PI3Kα or AT-1R alone was sufficient to induce similar changes by high glucose. In contrast, overexpression of an inactive AT-1R lowered the basal levels and rendered the cells non-responsive to high glucose. Moreover, cells overexpressing wild-type AT-1R had enhanced sensitivity to acute Ang II stimulation. These cells, however, did not respond to conditioned medium obtained from mesangial cells cultured in high glucose. We further demonstrated that iAng (intracellular Ang II) can be induced by high glucose but only under certain conditions. Efficient suppression of iAng by short hairpin RNA against angiotensinogen, however, did not affect high glucose-induced effects on MES-13 cells. These results suggest that high ambient glucose induces activation of AT-1R in an Ang II-independent manner to transactivate PI3K, resulting in proliferation and ECM accumulation in mesangial cells.

Mechanisms and functions of agonist-independent activation in the angiotensin II type 1 receptor

The angiotensin II (AngII) type 1 (AT(1)) receptor is a seven-transmembrane G protein-coupled receptor, and is involved in regulating the physiological and pathological process of the cardiovascular system. Systemically and locally generated AngII has agonistic action on AT(1) receptor, but recent studies have demonstrated that AT(1) receptor inherently shows spontaneous activity even in the absence of AngII. Furthermore, mechanical stress can activate AT(1) receptor by inducing conformational switch without the involvement of AngII, and induce cardiac hypertrophy in vivo. These agonist-independent activities of AT(1) receptor can be inhibited by inverse agonists, but not by neutral antagonists. Considerable attention has been directed to molecular mechanisms and clinical implications of agonist-independent AT(1) receptor activation, and inverse agonist activity emerges as an important pharmacological parameter for AT(1) receptor blockers that will improve efficacy and expand therapeutic potentials in cardiovascular medicine.

Mechanical stimulus alters conformation of type 1 parathyroid hormone receptor in bone cells

The molecular mechanisms by which bone cells transduce mechanical stimuli into intracellular biochemical responses have yet to be established. There is evidence that mechanical stimulation acts synergistically with parathyroid hormone PTH(1-34) in mediating bone growth. Using picosecond time-resolved fluorescence microscopy and G protein-coupled receptor conformation-sensitive fluorescence resonance energy transfer (FRET), we investigated conformational transitions in parathyroid hormone type 1 receptor (PTH1R). 1) A genetically engineered PTH1R sensor containing an intramolecular FRET pair was constructed that enabled detection of conformational activity of PTH1R in single cells. 2) The nature of ligand-dependent conformational change of PTH1R depends on the type of ligand: stimulation with the PTH(1-34) leads to conformational transitions characterized by decrease in FRET efficiency while NH(2)-terminal truncated ligand PTH(3-34) stimulates conformational transitions characterized by higher FRET efficiencies. 3) Stimulation of murine preosteoblastic cells (MC3T3-E1) with fluid shear stress (FSS) leads to significant changes in conformational equilibrium of the PTH1R in MC3T3-E1 cells, suggesting that mechanical perturbation of the plasma membrane leads to ligand-independent response of the PTH1R. Conformational transitions induced by mechanical stress were characterized by an increase in FRET efficiency, similar to those induced by the NH(2)-terminal truncated ligand PTH(3-34). The response to the FSS stimulation was inhibited in the presence of PTH(1-34) in the flow medium. These results indicate that the FSS can modulate the action of the PTH(1-34) ligand. 4) Plasma membrane fluidization using benzyl alcohol or cholesterol extraction also leads to conformational transitions characterized by increased FRET levels. We therefore suggest that PTH1R is involved in mediating primary mechanochemical signal transduction in MC3T3-E1 cells.

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.

Current status of inverse agonism at serotonin2A (5-HT2A) and 5-HT2C receptors

Contemporary receptor theory was developed to account for the existence of constitutive activity, as defined by the presence of receptor signaling in the absence of any ligand. Thus, ligands acting at a constitutively active receptor, can act as agonists, antagonists, and inverse agonists. In vitro studies have also revealed the complexity of ligand/receptor interactions including agonist-directed stimulus trafficking, a finding that has led to multi-active state models of receptor function. Studies with a variety of cell types have established that the serotonin 5-HT(2A) and 5-HT(2C) receptors also demonstrate constitutive activity and inverse agonism. However, until recently, there has been no evidence to suggest that these receptors also demonstrate constitutive activity and hence reveal inverse agonist properties of ligands in vivo. This paper describes our current knowledge of constitutive activity in vitro and then examines the evidence for constitutive activity in vivo. Both the serotonin 5-HT(2A) and 5-HT(2C) receptors are involved in a number of physiological and behavioral functions and are the targets for treatment of schizophrenia, anxiety, weight control, Parkinsonism, and other disorders. The existence of constitutive activity at these receptors in vivo, along with the possibility of inverse agonism, provides new avenues for drug development.

The cannabinoid receptors are required for ultraviolet-induced inflammation and skin cancer development

Solar UV irradiation is an important carcinogen that leads to the development of skin cancer, which is the most common human cancer. However, the receptors that mediate UV-induced skin carcinogenesis have not yet been unequivocally identified. Here we showed that UV irradiation directly activates cannabinoid receptors 1 and 2 (CB1/2). Notably, our data indicated that the absence of the CB1/2 receptors in mice results in a dramatic resistance to UVB-induced inflammation and a marked decrease in UVB-induced skin carcinogenesis. A marked attenuation of UVB-induced activation of mitogen-activated protein kinases and nuclear factor- kappaB was associated with CB1/2 deficiency. These data provide direct evidence indicating that the CB1/2 receptors play a key role in UV-induced inflammation and skin cancer development.

Inverse agonist exposure enhances ligand binding and G protein activation of the human MT1 melatonin receptor, but leads to receptor down-regulation

Melatonin binds and activates G protein-coupled melatonin receptors. The density and affinity of the endogenous melatonin receptors change throughout the 24-hr day, and the exposure of recombinant melatonin receptors to melatonin often results in desensitization of the receptors. Receptor density, G protein activation and expression level were analyzed in CHO cell lines stably expressing the human MT1 receptors after 1 or 72 hr of exposure to melatonin (agonist, 10 nm) and luzindole (antagonist/inverse agonist, 10 microm). The 72-hr exposure to luzindole significantly increased the apparent receptor density in cell lines with both high and low MT1 receptor expression levels (MT1(high) and MT1(low) cells, respectively). In the constitutively active MT1(high) cells, luzindole pretreatment also stimulated the functional response to melatonin in [(35)S]GTPgammaS binding assays, whereas melatonin pretreatment attenuated the functional response at both time points. Receptor ELISA was used to analyze the cell membrane and total expression level of the MT1 receptor in intact and permeabilized cells, respectively. Luzindole pretreatment decreased the total cellular level of MT1 receptor in the MT1(high) cells at both time points but increased the cell surface expression of MT1 receptor at 72 hr. Melatonin significantly decreased MT1 receptor cell surface expression only in MT1(high) cells after a 1-hr treatment. These results indicate that melatonin treatment desensitizes MT1 receptors, whereas luzindole increases ligand binding and G-protein activation. Luzindole also stimulates downregulation of the MT1 receptor protein, interfering with the synthesis and/or degradation of the receptor.

Inverse agonist-induced signaling and down-regulation of the platelet-activating factor receptor

Platelet-activating factor (PAF) is a potent phospholipid mediator involved in several diseases such as allergic asthma, atherosclerosis and psoriasis. The human PAF receptor (PAFR) is a member of the G-protein-coupled receptor family. Following stimulation, PAFR becomes rapidly desensitized; this refractory state is dependent on PAFR phosphorylation, internalization and down-regulation. In this report, we show that the PAFR inverse agonist, WEB2086, can induce phosphorylation and down-regulation of PAFR. Using selective inhibitors, we determined that the agonist, PAF, and WEB2086 could induce phosphorylation of PAFR by PKC. Moreover, dominant-negative (DN) mutant of PKC isoforms beta inhibited WEB2086-stimulated PAFR phosphorylation, whereas PAF-stimulated phosphorylation was inhibited by DN PKCalpha and delta. WEB2086 also induced PAFR down-regulation which could be blocked by PKC inhibitors and by DN PKCbeta. WEB2086-induced down-regulation was dynamin-dependent but arrestin-independent. Unlike PAF, WEB2086-stimulated intracellular trafficking of PAFR was independent of Rab5. Specific inhibitors of lysosomal proteases and of proteasomes were both effective in reducing WEB2086-induced PAFR down-regulation, indicating the importance of receptor targeting to both lysosomes and proteasomes in long-term cell desensitization to WEB2086. These results indicate that although both agonists and inverse agonists induce receptor PAFR down-regulation, this may be accomplished through different signal transduction and trafficking pathways.

Pharmacogenomic and Structural Analysis of Constitutive G Protein–Coupled Receptor Activity

G protein-coupled receptors (GPCRs) respond to a chemically diverse plethora of signal transduction molecules. The notion that GPCRs also signal without an external chemical trigger, i.e., in a constitutive or spontaneous manner, resulted in a paradigm shift in the field of GPCR pharmacology. The discovery of constitutive GPCR activity and the fact that GPCR binding and signaling can be strongly affected by a single point mutation drew attention to the evolving area of GPCR pharmacogenomics. For a variety of GPCRs, point mutations have been convincingly linked to human disease. Mutations within conserved motifs, known to be involved in GPCR activation, might explain the properties of some naturally occurring, constitutively active GPCR variants linked to disease. In this review, we provide a brief historical introduction to the concept of constitutive receptor activity and the pharmacogenomic and structural aspects of constitutive receptor activity.

Molecular basis of melanocortin-4 receptor for AGRP inverse agonism

We have investigated receptor structural components of the melanocortin-4 receptor (MC4R) responsible for ligand-dependent inverse agonism. We utilized agouti-related protein (AGRP), an inverse agonist which reduces MC4R basal cAMP production, as a tool to determine the molecular mechanism. We tested a series of chimeric receptors and utilized MC4R and MC1R as templates, in which AGRP is an inverse agonist for MC4R but not for MC1R. Our results indicate that replacements of the extracellular loops 1, 2 and 3 of MC4R with the corresponding regions of MC1R did not affect AGRP inverse agonist activity. However, replacement of the N terminus of MC4R with the same region of MC1R decreases AGRP inverse agonism. Replacement of transmembrane domains 3, 4, 5 and 6 of MC4R with the corresponding regions of MC1R did not affect AGRP inverse agonist activity but mutation of D90A in transmembrane 2 (TM2) and D298A in TM7 abolished AGRP inverse activity. Deletion of the distal MC4R C terminus fails to maintain AGRP mediated reduction in basal cAMP production although it maintains NDP-MSH mediated cAMP production. In conclusion, our results indicate that the N terminus and the distal C terminus of MC4R do appear to play important roles in AGRP inverse agonism but not NDP-MSH mediated receptor activation. Our results also indicate that the residues D90 in TM2 and D298 in TM7 of hMC4R are involved in not only NDP-MSH mediated receptor activation but also AGRP mediated inverse agonism.

A Chemical Switch for the Modulation of the Functional Activity of Higher Homologues of Histamine on the Human Histamine H3 Receptor: Effect of Various Substitutions at the Primary Amino Function

In an effort to establish the structural requirements for agonism, neutral antagonism, and inverse agonism at the human histamine H(3) receptor (H(3)R) we have prepared a series of higher homologues of histamine in which the terminal nitrogen of the side chain has been either mono- or disubstituted with several aliphatic, alicyclic, and aromatic moieties or incorporated in cyclic systems. The novel ligands have been pharmacologically investigated in vitro for their affinities on the human H(3)R and H(4)R subtypes by radioligand displacement experiments and for their intrinsic H(3)R activities via a CRE-mediated beta-galactosidase reporter gene assay. Subtle changes of the substitution pattern at the side chain nitrogen alter enormously the pharmacological activity of the ligands, resulting in a series of compounds with a wide spectrum of pharmacological activities. Among the several neutral H(3)R antagonists identified within this series, compounds 2b and 2h display an H(3)R affinity in the low nanomolar concentration range (pK(i) values of 8.1 and 8.4, respectively). A very potent and selective H(3)R agonist (1l, pEC(50) = 8.9, alpha = 0.94) and a very potent, though not highly selective, H(3)R inverse agonist (2k, pIC(50) = 8.9, alpha = -0.97) have been identified as well.

Role of cholesterol in the function and organization of G-protein coupled receptors

Cholesterol is an essential component of eukaryotic membranes and plays a crucial role in membrane organization, dynamics and function. The modulatory role of cholesterol in the function of a number of membrane proteins is well established. This effect has been proposed to occur either due to a specific molecular interaction between cholesterol and membrane proteins or due to alterations in the membrane physical properties induced by the presence of cholesterol. The contemporary view regarding heterogeneity in cholesterol distribution in membrane domains that sequester certain types of membrane proteins while excluding others has further contributed to its significance in membrane protein function. The seven transmembrane domain G-protein coupled receptors (GPCRs) are among the largest protein families in mammals and represent approximately 2% of the total proteins coded by the human genome. Signal transduction events mediated by this class of proteins are the primary means by which cells communicate with and respond to their external environment. GPCRs therefore represent major targets for the development of novel drug candidates in all clinical areas. In view of their importance in cellular signaling, the interaction of cholesterol with such receptors represents an important determinant in functional studies of such receptors. This review focuses on the effect of cholesterol on the membrane organization and function of GPCRs from a variety of sources, with an emphasis on the more contemporary role of cholesterol in maintaining a domain-like organization of such receptors on the cell surface. Importantly, the recently reported role of cholesterol in the function and organization of the neuronal serotonin(1A) receptor, a representative of the GPCR family which is present endogenously in the hippocampal region of the brain, will be highlighted.

Genetic analysis of the first and third extracellular loops of the C5a receptor reveals an essential WXFG motif in the first loop

The extracellular loops of G protein-coupled receptors (GPCRs) frequently contain binding sites for peptide ligands. However, the mechanism of receptor activation following ligand binding and the influence of the extracellular loops in other aspects of receptor function are poorly understood. Here we report a structure-function analysis of the first and third extracellular loops of the human C5a receptor, a GPCR that binds a 74-amino acid peptide ligand. Amino acid substitutions were randomly incorporated into each loop, and functional receptors were identified in yeast. The first extracellular loop contains a large number of positions that cannot tolerate amino acid substitutions, especially residues within the WXFG motif found in many rhodopsin-like GPCRs, yet disruption of these residues does not alter C5a binding affinity. These results demonstrate an unanticipated role for the first extracellular loop, and the WXFG motif in particular, in ligand-mediated activation of the C5a receptor. This motif likely serves a similar role in other GPCRs. The third extracellular loop, in contrast, contains far fewer preserved residues and appears to play a less essential role in receptor activation.

Inverse Agonist Activity of Sarpogrelate, a Selective 5-HT2A-Receptor Antagonist, at the Constitutively Active Human 5-HT2A Receptor

Mutations producing constitutively active G-protein coupled receptors have been found in the pathophysiology of several diseases, implying that inverse agonists at the constitutively active receptors may have preferred therapeutic applications. Because of the involvement of 5-HT(2A) receptors in mediating many cardiovascular diseases, constitutively active mutants of the 5-HT(2A) receptor may be responsible for the disease states. Thus, the purpose of the present study was to investigate the inverse agonist activity of sarpogrelate, a selective 5-HT(2A)-receptor antagonist, and its active metabolite, M-1; and we compared their activities with those of other 5-HT(2A)-receptor antagonists such as ritanserin, ketanserin, and cyproheptadine. Using a constitutively active mutant (C322K) of the human 5-HT(2A) receptor, we demonstrated that like other 5-HT(2A)-receptor antagonists, sarpogrelate acts as a potent inverse agonist by significantly reducing basal inositol phosphate levels. However, there were no significant differences between sarpogrelate and other 5-HT(2A)-receptor antagonists for their inverse agonist activity. Compared with the wild type receptor, mutant receptor displayed significantly higher affinity for 5-HT and lower affinity for sarpogrelate. These results indicate that stabilization of the inactive conformation of the 5-HT(2A) receptor may be a key component of the mechanism of action of sarpogrelate.

Molecular biology of cannabinoid receptors

To date, two cannabinoid receptors have been isolated by molecular cloning. The CB1 and CB2 cannabinoid receptors are members of the G protein-coupled receptor family. There is also evidence for additional cannabinoid receptor subtypes. The CB1 and CB2 receptors recognize endogenous and exogenous cannabinoid compounds, which fall into five structurally diverse classes. Mutagenesis and molecular modeling studies have identified several key amino acid residues involved in the selective recognition of these ligands. Numerous residues involved in receptor activation have been elucidated. Regions of the CB1 receptor mediating desensitization and internalization have also been discovered. The known genetic structures of the CB1 and CB2 receptors indicate polymorphisms and multiple exons that maybe involved in tissue and species-specific regulation of these genes. The cannabinoid receptors are regulated during chronic agonist exposure, and gene expression is altered in disease states. There is a complex molecular architecture of the cannabinoid receptors that allows a single receptor to recognize multiple classes of compounds and produce an array of distinct downstream effects.

Historical review: Negative efficacy and the constitutive activity of G-protein-coupled receptors

The idea that a receptor can produce signalling without agonist intervention and that several antagonists can be 'active' in repressing such spontaneous activity is contained in the concept of ligand-induced conformational changes. Yet, this idea was neglected by pharmacologists for many years. In this article, we review the events that brought inverse agonism and constitutive activity to general attention and made this phenomenon a topic of current research. We also suggest a classification of antagonists based on the cooperativity that links their primary site of interaction with other functional domains of the receptor.

Different pathways leading to activation of extracellular signal-regulated kinase and p38 MAP kinase by formyl-methionyl-leucyl-phenylalanine or platelet activating factor in human neutrophils

The signaling pathways leading to extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase (MAPK) activation by N-formyl-Met-Leu-Phe (fMLP) or platelet activating factor (PAF) in human neutrophils were examined. Previously, we found that changes of intracellular Ca2+ ([Ca2+]i) stimulated by PAF and fMLP were due to Ca2+ influx and internal Ca2+ release, respectively. To further determine the mechanism of MAPK activation and its relation with Ca2+ influx, blood from healthy human volunteers was taken by venous puncture. Human polymorphonuclear cells (PMNs) were isolated and incubated with protein kinase C (PKC) inhibitor Calphostin C, PKC-gamma isoform inhibitor GF109203X, phosphatidylinositol 3-kinase (PI3K) inhibitors wortmannin and LY294002, phospholipase C (PLC) inhibitor U73122, phospholipase A2 (PLA2) inhibitor Aristolochic acid, store-operated calcium (SOC) channel inhibitor SKF96365, or extracellular calcium chelator EGTA followed by fMLP or PAF treatment. Phosphorylation of ERK p38 was determined by immunoblotting analysis. Our data indicate that neutrophil MAPK signaling pathways mediated by fMLP and PAF are different. PAF-induced ERK phosphorylation is mediated by PI3K, PKC, PLA2, PLC, and extracellular calcium, whereas fMLP-induced ERK phosphorylation does not involve the PKC-gamma isoform and extracellular calcium. PAF-induced p38 phosphorylation involves PLA2, whereas fMLP-induced p38 activation is PLC dependent.

A "locked-on," constitutively active mutant of the adenosine A1 receptor

We studied the wild-type human adenosine A1 receptor and three mutant receptors, in which the glycine at position 14 had been changed into an alanine, a leucine, or a threonine residue. All receptors were characterized in radioligand binding experiments, the wild-type and the Gly14Thr mutant receptor in greater detail. Both receptors were allosterically modulated by sodium ions and PD81,723 (2-amino-4,5-dimethyl-3-thienyl-[3(trifluoromethyl)-phenyl]methanone), although in a different way. All mutant receptors appeared to be spontaneously or "constitutively" active in a [35S]GTPgammaS binding assay, the first demonstration of the existence of such CAM (constitutively active mutant) receptors for the adenosine A1 receptor. The Gly14Thr mutant receptor was also constitutively active in another functional assay, i.e., the inhibition of forskolin-induced cAMP production in intact cells. Importantly, this mutant displayed a peculiar "locked-on" phenotype, i.e., neither agonist nor inverse agonist was capable of modulating the basal activity in both the GTPgammaS and the cAMP assay, unlike the wild-type and the two other mutant receptors.

Single Mutations at Asn295and Leu305in the Cytoplasmic Half of Transmembrane α-Helix Domain 7 of the AT1Receptor Induce Promiscuous Agonist Specificity for Angiotensin II Fragments: A Pseudo-Constitutive Activity

The most striking feature of a G protein-coupled receptor (GPCR) is its highly exclusive agonist specificity. This feature guarantees that a GPCR recognizes only its specific native agonist(s). In this study, we showed that two point mutations of N295S and L305Q enabled the AT(1) receptors to recognize multiple Ang II fragments. Similar to the well established constitutively active AT(1) mutant receptor N111G, the mutations of N295S and L305Q induced an increased production of basal inositol 1,4,5-phosphates in the absence of exogenous Ang II when expressed in HEK293 cells. Distinct from the N111G, however, is the fact that the increased basal activity disappeared in COS-7 cells because of the lack of endogenous Ang II fragments produced by the cells-a pseudo-constitutive activity. It is surprising that the Ang II analog [Sar(1),Ile(4),Ile(8)]Ang II and the native angiotensin II fragments Ang 1-7, Ang IV, and Ang 5-8, which are inactive in activating the wild-type receptor, activated N295S and L305Q. Results generated by lowering the Na(+) concentration suggest that the mutant N295S and L305Q may be trapped in neutral conformational states (R(N)). These data allow us to identify for the first time a novel pattern of GPCR mutations with a broad spectrum of agonist specificity, suggesting possible existence of functional GPCRs in nature that are activated through conformational "selection" rather than "induction" mechanisms.

The histamine H3 receptor: from gene cloning to H3 receptor drugs

Since the cloning of the histamine H(3) receptor cDNA in 1999 by Lovenberg and co-workers, this histamine receptor has gained the interest of many pharmaceutical companies as a potential drug target for the treatment of various important disorders, including obesity, attention-deficit hyperactivity disorder, Alzheimer's disease, schizophrenia, as well as for myocardial ischaemia, migraine and inflammatory diseases. Here, we discuss relevant information on this target protein and describe the development of various H(3) receptor agonists and antagonists, and their effects in preclinical animal models.

Neuronal nitric oxide modulates morphine antinociceptive tolerance by enhancing constitutive activity of the mu-opioid receptor

NO is a key mediator of morphine antinociceptive tolerance. This work was conducted to evaluate the specific effects of NO on mu-opioid receptor activity. To investigate the effects of morphine- and L-arginine (the NO precursor)-induced increases in NO, five groups of rats were treated with saline, l-arginine (100-, 300-, or 500-mg/kg/h), or morphine 3-mg/kg/h for 8h on Day 1; brain tissue was collected on Day 2. To evaluate the effects of additional increases in NO on morphine-induced alterations of the mu-opioid receptor, six groups of rats were treated with 8-h intravenous infusions for two consecutive days as per the following scheme (Day 1:Day 2): saline:saline (control); saline:morphine 3-mg/kg/h (tolerant); L-arginine 500-mg/kg/h:saline (NO control); L-arginine 100-mg/kg/h:morphine 3-mg/kg/h; L-arginine 300-mg/kg/h:morphine 3-mg/kg/h; and L-arginine 500-mg/kg/h:morphine 3-mg/kg/h (supertolerant). Brain tissue was collected at the end of Day 2. The time course of effects on morphine-induced receptor alterations due to increased NO also was evaluated. Brain tissue was analyzed for changes in radioligand (agonist and antagonist) binding and [(35)S]GTPgammaS binding (agonist and antagonist). In the absence of agonist exposure, NO produced an alteration in the mu-opioid receptor that increased receptor activity. In the presence of agonist, NO increased constitutive activation of the mu-opioid receptor and reduced the ability of a selective mu-opioid agonist to activate the mu-opioid G-protein-coupled receptor; these molecular effects occurred in a time course consistent with the development of antinociceptive tolerance. This work establishes important NO-induced alterations in mu-opioid receptor functionality, which directly lead to the development of opioid antinociceptive tolerance.

Inverse agonist properties of atypical antipsychotic drugs

Mechanisms of action of several atypical antipsychotic drugs have been examined at the D(2) dopamine receptor expressed in CHO cells. The drugs tested were found to exhibit inverse agonist activity at the D(2) dopamine receptor based on their effects to potentiate forskolin-stimulated cyclic AMP (cAMP) accumulation. Each of the antipsychotic drugs tested (clozapine, olanzapine, quetiapine and risperidone) increased cAMP accumulation to the same extent. The increase in cAMP was also similar to that seen with typical antipsychotic drugs. Inverse agonism at the D(2) dopamine receptor seems, therefore, to be a property common to all classes of antipsychotic drugs. The effect of sodium ions on the binding of the drugs to the receptor was also assessed. Each of the atypical antipsychotic drugs tested here bound with higher affinity in the absence of sodium ions. Previous studies have shown that some antipsychotic drugs are insensitive to sodium ions and some bind with higher affinity in the presence of sodium ions. Given that all of these antipsychotic drugs are inverse agonists, it may be concluded that this sodium ion sensitivity is unrelated to mechanisms of inverse agonism.

The CXC chemokine receptor encoded by herpesvirus saimiri, ECRF3, shows ligand-regulated signaling through Gi, Gq, and G12/13 proteins but constitutive signaling only through Gi and G12/13 proteins

Open reading frame 74 (ORF74) of many gamma(2)-herpesviruses encodes a CXC chemokine receptor. The molecular pharmacological profile of ORF74 from herpesvirus saimiri, ECRF3, is characterized here and compared with that of the well known ORF74 from human herpesvirus 8 (HHV8). The ECRF3 receptor bound the so-called ELR (Glu-Leu-Arg) CXC chemokines (125)I-CXCL1/GRO alpha, (125)I-CXCL6/GCP-2, and (125)I-CXCL8/interleukin-8 with high affinity; but in contrast to ORF74 from HHV8, it did not bind the non-ELR CXC chemokine (125)I-CXCL10/IP10. Interestingly, the B(max) value for CXCL6/GCP-2 was 3-fold higher than the capacity for maximal binding of CXCL1/GRO alpha to ECRF3 and 85-fold higher than that of CXCL8/interleukin-8, despite similar affinities. Like ORF74 from HHV8, ECRF3 activated a broad range of pathways (G(q), G(i), and G(12/13) as well as the cAMP response element-binding protein, NF-kappa B, NFAT, and serum response element transcription factors) in a ligand-regulated manner, with CXCL6/GCP-2 being the most potent and efficacious agonist. ECRF3 signaled constitutively through G(i) and G(12/13), but surprisingly not through G(q). At the level of transcription factor activation, the serum response element was activated constitutively by ECRF3, whereas cAMP response element-binding protein, NFAT, and NF-kappa B were only ligand-regulated. The maximal signaling capacities were similar for the two receptors; however, the ligand-regulated signaling was responsible for the major part of the total ECRF3 signaling and only for a minor part of the total HHV8 ORF74 signaling. The activation pattern of ECRF3 with constitutive activation of some (but not all) of the employed pathways has not been seen before in endogenous or virus-encoded chemokine receptors. The results suggest that the unique ligand selectivity of ECRF3 among ORF74 receptors could reflect differences in the cellular tropism of the gamma(2)-herpesviruses.

Mechanical stress activates angiotensin II type 1 receptor without the involvement of angiotensin II

The angiotensin II type 1 (AT1) receptor has a crucial role in load-induced cardiac hypertrophy. Here we show that the AT1 receptor can be activated by mechanical stress through an angiotensin-II-independent mechanism. Without the involvement of angiotensin II, mechanical stress not only activates extracellular-signal-regulated kinases and increases phosphoinositide production in vitro, but also induces cardiac hypertrophy in vivo. Mechanical stretch induces association of the AT1 receptor with Janus kinase 2, and translocation of G proteins into the cytosol. All of these events are inhibited by the AT1 receptor blocker candesartan. Thus, mechanical stress activates AT1 receptor independently of angiotensin II, and this activation can be inhibited by an inverse agonist of the AT1 receptor.

The RAIG Family Member, GPRC5D, Is Associated with Hard-Keratinized Structures

Retinoic acid-inducible gene-1 was originally identified as an orphan G-protein coupled receptor induced by retinoic acid. Three highly homologous oGPCR (GPRC5B, GPRC5C, and GPRC5D) have since been classified into the RAIG1 family. We describe here, the unique tissue distribution of GPRC5D and its mechanism of expression. Hybridization in situ has shown that GPRC5D is expressed in differentiating cells that produce hard keratin, including cortical cells of the hair shaft, the keratogenous zone of the nail, and in a central region of the filiform papillae of the tongue. The GPRC5D transcript is expressed in hair follicles during mid- and late anagen, and catagen but not at telogen and early anagen phases. The differentiation-inducer, all-trans retinoic acid, induces GPRC5D expression in cultured hair bulb cells. Because the tissue distribution of GPRC5D indicates a relationship with hard keratins that constitute the major structural proteins of hard epithelial tissues, we investigated the effect of GPRC5D on acid hard keratins. Analyses of cultured cells showed that transient overexpression resulted in suppression of Ha3 and stimulation of Ha4 hair keratin gene expression. The expression was maintained in the hair follicles of whn-deficient (nude) mice, suggesting that this gene is regulated by a signal pathway different from that of hair keratin synthesis. Collectively, these data provide a framework for understanding the molecular mechanisms of GPRC5D function in hard keratinization.

Dopamine modulates the plasticity of mechanosensory responses in Caenorhabditis elegans

Dopamine-modulated behaviors, including information processing and reward, are subject to behavioral plasticity. Disruption of these behaviors is thought to support drug addictions and psychoses. The plasticity of dopamine-mediated behaviors, for example, habituation and sensitization, are not well understood at the molecular level. We show that in the nematode Caenorhabditis elegans, a D1-like dopamine receptor gene (dop-1) modulates the plasticity of mechanosensory behaviors in which dopamine had not been implicated previously. A mutant of dop-1 displayed faster habituation to nonlocalized mechanical stimulation. This phenotype was rescued by the introduction of a wild-type copy of the gene. The dop-1 gene is expressed in mechanosensory neurons, particularly the ALM and PLM neurons. Selective expression of the dop-1 gene in mechanosensory neurons using the mec-7 promoter rescues the mechanosensory deficit in dop-1 mutant animals. The tyrosine hydroxylase-deficient C. elegans mutant (cat-2) also displays these specific behavioral deficits. These observations provide genetic evidence that dopamine signaling modulates behavioral plasticity in C. elegans.

Inverse agonist activity of selected ligands of the cysteinyl-leukotriene receptor 1

Cysteinyl leukotrienes (CysLTs) are associated with several inflammatory processes, including asthma. Due to this association, considerable effort has been invested in the development of antagonists to the CysLT receptors (CysLT(1)R). Many of these molecules have been shown to specifically interact with CysLT(1)R, but little is known about their impact on the conformation of the receptor and its activity. We were especially interested in possible inverse agonist activity of the antagonists. Using a constitutively active mutant (N106A) of the human CysLT(1)R and the wild-type (WT) receptor coexpressed with the G(alphaq) subunit of the trimeric G protein, we were able to address this issue with ligands commonly used in therapy. We demonstrated that some of these molecules are inverse agonists, whereas others act as partial agonists. In cells expressing the CysLT(1)R mutant N106A exposed to Montelukast, Zafirlukast, or 3-[[3-[2-(7-chloroquinolin-2-yl)vinyl]phenyl]-(2-dimethylcarbamoylethylsulfanyl)methylsulfanyl] propionic acid (MK571), the basal inositol phosphate production was reduced by 53 +/- 6, 44 +/- 3, and 54 +/- 4%, respectively. On the other hand, 6(R)-(4-carboxyphenylthio)-5(S)-hydroxy-7(E),9(E),11(Z),14(Z)-eicosatetraenoic acid (BayU9773) and 1-[2-hydroxy-3-propyl-4-[4-(1H-tetrazole-5-YL)-butoxy]-phenyl ethanone] (LY171883) acted as partial agonists and alpha-pentyl-3-[2-quinolinylmethoxy] benzyl alcohol (REV 5901) as a neutral antagonist. However, in cells expressing CysLT(1)R and G(alphaq), all antagonists used had inverse agonist activity. The decrease in basal inositol phosphate production by ligands with inverse agonist activity could be inhibited by a more neutral antagonist, confirming the specificity of the reaction. We demonstrate here that Montelukast, MK571, and Zafirlukast can act as inverse agonists on the human CysLT(1) receptor.

Role of the serotonin 5-HT(2A) receptor in learning

This study reviews the role of the serotonin 5-HT2A receptor in learning as measured by the acquisition of the rabbit's classically conditioning nictitating membrane response, a component of the eyeblink response. Agonists at the 5-HT2A receptor including LSD (d-lysergic acid diethylamide) enhanced associative learning at doses that produce cognitive effects in humans. Some antagonists such as BOL (d-bromolysergic acid diethylamide), LY53,857, and ketanserin acted as neutral antagonists in that they had no effect on learning, whereas others (MDL11,939, ritanserin, and mianserin) acted as inverse agonists in that they retarded learning through an action at the 5-HT2A receptor. These results were placed in the context of what is known concerning the anatomical distribution and electrophysiological effects of 5-HT2A receptor activation in frontal cortex and hippocampus, as well as the role of cortical 5-HT2A receptors in schizophrenia. It was concluded that the 5-HT2A receptor demonstrates constitutive activity, and that variations in this activity can produce profound alterations in cognitive states.

Enhanced spontaneous activity of the mu opioid receptor by cysteine mutations: characterization of a tool for inverse agonist screening

Background

The concept of spontaneous- or constitutive-activity has become widely accepted and verified for numerous G protein-coupled receptors and this ligand-independent activity is also acknowledged to play a role in some pathologies. Constitutive activity has been reported for the mu opioid receptor. In some cases the increase in receptor basal activity was induced by chronic morphine administration suggesting that constitutive activity may contribute to the development of drug tolerance and dependence. Constitutively active mutants represent excellent tools for gathering information about the mechanisms of receptor activation and the possible physiological relevance of spontaneous receptor activity. The high basal level of activity of these mutants also allows for easier identification of inverse agonists, defined as ligands able to suppress spontaneous receptor activity, and leads to a better comprehension of their modulatory effects as well as possible in vivo use.

Results

Cysteines 348 and 353 of the human mu opioid receptor (hMOR) were mutated into alanines and Ala^348,353 hMOR was stably expressed in HEK 293 cells. [³⁵S] GTPγS binding experiments revealed that Ala^348,353 hMOR basal activity was significantly higher when compared to hMOR, suggesting that the mutant receptor is constitutively active. [³⁵S] GTPγS binding was decreased by cyprodime or CTOP indicating that both ligands have inverse agonist properties. All tested agonists exhibited binding affinities higher for Ala^348,353 hMOR than for hMOR, with the exception of endogenous opioid peptides. Antagonist affinity remained virtually unchanged except for CTOP and cyprodime that bound the double mutant with higher affinities. The agonists DAMGO and morphine showed enhanced potency for the Ala^348,353 hMOR receptor in [³⁵S] GTPγS experiments. Finally, pretreatment with the antagonists naloxone, cyprodime or CTOP significantly increased Ala^348,353 hMOR expression.

Conclusion

Taken together our data indicate that the double C348/353A mutation results in a constitutively active conformation of hMOR that is still activated by agonists. This is the first report of a stable CAM of hMOR with the potential to screen for inverse agonists.

Ligand Specific Up-Regulation of aRenilla reniformisLuciferase-Tagged, Structurally Unstable Muscarinic M3Chimeric G Protein-Coupled Receptor

The rat muscarinic acetylcholine receptor subtype 3 was modified by swapping the third intracellular loop with the corresponding region of a constitutively active mutant human beta2-adrenergic receptor and attaching Renilla reniformis luciferase to its C terminus. The chimeric fusion receptor displayed constitutive Gq- and Gs-coupled activity as demonstrated in nuclear factor of activated T cell and cAMP response element reporter gene assays. The chimeric receptor displayed a pharmacological binding profile comparable with that of the wild-type receptor for agonists, antagonists, and inverse agonists but showed a large decrease in expression in both human embryonic kidney 293 and COS-7 cells. Long-term treatment of cells expressing the chimeric receptor with agonists, antagonists, and inverse agonists resulted in a concentration-dependent up-regulation in the steady-state levels that was not observed for the wild-type receptor. The EC50 of neutral antagonists and inverse agonists was significantly correlated to their binding affinities at the wild-type receptor, whereas agonists demonstrated greater EC50 values for the chimeric receptor. To validate the approach as a means of discovering novel receptor modulators, a cell-based, high-throughput screening assay was developed and used to screen a small molecule compound collection against the chimeric fusion receptor. Several novel hits were identified and confirmed by ligand binding assay and functional assays using the wild-type rat muscarinic acetylcholine receptor subtype 3.

Synthesis and Pharmacological Identification of Neutral Histamine H1-Receptor Antagonists

In the present study we searched for neutral antagonists for the human histamine H(1)-receptor (H(1)R) by screening newly synthesized ligands that are structurally related to H(1)R agonists for their affinity using radioligand displacement studies and by assessing their functional activity via performing a NF-kappaB driven reporter-gene assay that allows for the detection of both agonistic and inverse agonistic responses. Starting from the endogenous agonist for the H(1)R, histamine, we synthesized and tested various analogues and ultimately identified several compounds with partial inverse agonistic properties and two neutral H(1)-receptor antagonists, namely 2-[2-(4,4-diphenylbutyl)-1H-imidazol-4-yl]ethylamine (histabudifen, 18d) (pK(i) = 5.8, alpha = 0.02) and 2-[2-(5,5-diphenylpentyl)-1H-imidazol-4-yl]ethylamine (histapendifen, 18e) (pK(i) = 5.9, alpha = -0.09).

Constitutively active Gq/11-coupled receptors enable signaling by co-expressed G(i/o)-coupled receptors

Co-expression of guanine nucleotide-binding regulatory (G) protein-coupled receptors (GPCRs), such as the G(i/o)-coupled human 5-hydroxytryptamine receptor 1B (5-HT(1B)R), with the G(q/11)-coupled human histamine 1 receptor (H1R) results in an overall increase in agonist-independent signaling, which can be augmented by 5-HT(1B)R agonists and inhibited by a selective inverse 5-HT(1B)R agonist. Interestingly, inverse H1R agonists inhibit constitutively H1R-mediated as well as 5-HT(1B)R agonist-induced signaling in cells co-expressing both receptors. This phenomenon is not solely characteristic of 5-HT(1B)R; it is also evident with muscarinic M2 and adenosine A1 receptors and is mimicked by mastoparan-7, an activator of G(i/o) proteins, or by over-expression of Gbetagamma subunits. Likewise, expression of the G(q/11)-coupled human cytomegalovirus (HCMV)-encoded chemokine receptor US28 unmasks a functional coupling of G(i/o)-coupled CCR1 receptors that is mediated via the constitutive activity of receptor US28. Consequently, constitutively active G(q/11)-coupled receptors, such as the H1R and HCMV-encoded chemokine receptor US28, constitute a regulatory switch for signal transduction by G(i/o)-coupled receptors, which may have profound implications in understanding the role of both constitutive GPCR activity and GPCR cross-talk in physiology as well as in the observed pathophysiology upon HCMV infection.