Trace amine-associated receptors form structurally and functionally distinct subfamilies of novel G protein-coupled receptors

Genomic drift and copy number variation of chemosensory receptor genes in humans and mice

Recent studies about the structural variation of genomic sequences have shown that there is a large amount of copy number variations (CNVs) of genes within species. Analyzing Redon et al.'s (2006) crude data on copy number variable regions (CNVRs), we previously showed that CNVs are particularly high for chemosensory receptor genes in human populations. In this paper, we reanalyzed the CNVs of these genes using more refined data by Perry et al. (2008). The results showed that the extent of CNVs is somewhat lower in this dataset than in the previous one, but that the extent is still substantial for olfactory receptor (OR), vomeronasal receptor (VR), and taste receptor (TR) genes. We also studied the CNVs for chemosensory receptor genes in mice, using CNVR data obtained from inbred strains. It was found that the extent of CNVs is quite substantial but is lower than that for human populations. However, because the mouse data came from inbred strains and might be biased, this conclusion should be regarded as tentative. Despite this reservation, the distribution of gene copy number for the OR gene family was approximately normal in both humans and mice, suggesting that genomic drift caused by random duplication and deletion of genes plays important roles in determining the evolutionary change of chemosensation.

Modulation of cardiac ionic homeostasis by 3-iodothyronamine

3-iodothyronamine (T₁AM) is a novel endogenous relative of thyroid hormone, able to interact with trace amine-associated receptors, a class of plasma membrane G protein-coupled receptors, and to produce a negative inotropic and chronotropic effect. In the isolated rat heart 20–25 μM T₁AM decreased cardiac contractility, but oxygen consumption and glucose uptake were either unchanged or disproportionately high when compared to mechanical work. In adult rat cardiomyocytes acute exposure to 20 μM T₁AM decreased the amplitude and duration of the calcium transient. In patch clamped cardiomyocytes sarcolemmal calcium current density was unchanged while current facilitation by membrane depolarization was abolished consistent with reduced sarcoplasmic reticulum (SR) calcium release. In addition, T₁AM decreased transient outward current (I_to) and I_K1 background current. SR studies involving 20 μM T₁AM revealed a significant decrease in ryanodine binding due to reduced B_max, no significant change in the rate constant of calcium-induced calcium release, a significant increase in calcium leak measured under conditions promoting channel closure, and no effect on oxalate-supported calcium uptake. Based on these observations we conclude T₁AM affects calcium and potassium homeostasis and suggest its negative inotropic action is due to a diminished pool of SR calcium as a result of increased diastolic leak through the ryanodine receptor, while increased action potential duration is accounted for by inhibition of I_to and I_K1 currents.

Positive Darwinian selection and the birth of an olfactory receptor clade in teleosts

Trace amine-associated receptors (TAARs) in mammals recently have been shown to function as olfactory receptors. We have delineated the taar gene family in jawless, cartilaginous, and bony fish (zero, 2, and >100 genes, respectively). We conclude that taar genes are evolutionary much younger than the related OR and ORA/V1R olfactory receptor families, which are present already in lamprey, a jawless vertebrate. The 2 cartilaginous fish genes appear to be ancestral for 2 taar classes, each with mammalian and bony fish (teleost) representatives. Unexpectedly, a whole new clade, class III, of taar genes originated even later, within the teleost lineage. Taar genes from all 3 classes are expressed in subsets of zebrafish olfactory receptor neurons, supporting their function as olfactory receptors. The highly conserved TAAR1 (shark, mammalian, and teleost orthologs) is not expressed in the olfactory epithelium and may constitute the sole remnant of a primordial, nonolfactory function of this family. Class III comprises three-fourths of all teleost taar genes and is characterized by the complete loss of the aminergic ligand-binding motif, stringently conserved in the other 2 classes. Two independent intron gains in class III taar genes represent extraordinary evolutionary dynamics, considering the virtual absence of intron gains during vertebrate evolution. The d(N)/d(S) analysis suggests both minimal global negative selection and an unparalleled degree of local positive selection as another hallmark of class III genes. The accelerated evolution of class III teleost taar genes conceivably might mark the birth of another olfactory receptor gene family.

The hallucinogen N,N-dimethyltryptamine (DMT) is an endogenous sigma-1 receptor regulator

The sigma-1 receptor is widely distributed in the central nervous system and periphery. Originally mischaracterized as an opioid receptor, the sigma-1 receptor binds a vast number of synthetic compounds but does not bind opioid peptides; it is currently considered an orphan receptor. The sigma-1 receptor pharmacophore includes an alkylamine core, also found in the endogenous compound N,N-dimethyltryptamine (DMT). DMT acts as a hallucinogen, but its receptor target has been unclear. DMT bound to sigma-1 receptors and inhibited voltage-gated sodium ion (Na+) channels in both native cardiac myocytes and heterologous cells that express sigma-1 receptors. DMT induced hypermobility in wild-type mice but not in sigma-1 receptor knockout mice. These biochemical, physiological, and behavioral experiments indicate that DMT is an endogenous agonist for the sigma-1 receptor.

The evolution of animal chemosensory receptor gene repertoires: roles of chance and necessity

Chemosensory receptors are essential for the survival of organisms that range from bacteria to mammals. Recent studies have shown that the numbers of functional chemosensory receptor genes and pseudogenes vary enormously among the genomes of different animal species. Although much of the variation can be explained by the adaptation of organisms to different environments, it has become clear that a substantial portion is generated by genomic drift, a random process of gene duplication and deletion. Genomic drift also generates a substantial amount of copy-number variation in chemosensory receptor genes within species. It seems that mutation by gene duplication and inactivation has important roles in both the adaptive and non-adaptive evolution of chemosensation.

Effects of dietary amines on the gut and its vasculature

Trace amines, including tyramine and β-phenylethylamine (β-PEA), are constituents of many foods including chocolate, cheeses and wines and are generated by so-called ‘friendly’ bacteria such as Lactobacillus, Lactococcus and Enterococcus species, which are found in probiotics. We therefore examined whether these dietary amines could exert pharmacological effects on the gut and its vasculature. In the present study we examined the effects of tyramine and β-PEA on the contractile activity of guinea-pig and rat ileum and upon the isolated mesenteric vasculature and other blood vessels. Traditionally, these amines are regarded as sympathomimetic amines, exerting effects through the release of noradrenaline from sympathetic nerve endings, which should relax the gut. A secondary aim was therefore to confirm this mechanism of action. However, contractile effects were observed in the gut and these were independent of noradrenaline, acetylcholine, histamine and serotonin receptors. They were therefore probably due to the recently described trace amine-associated receptors. These amines relaxed the mesenteric vasculature. In contrast, the aorta and coronary arteries were constricted, a response that was also independent of a sympathomimetic action. From these results, we propose that after ingestion, trace amines could stimulate the gut and improve intestinal blood flow. Restriction of blood flow elsewhere diverts blood to the gut to aid digestion. Thus, trace amines in the diet may promote the digestive process through stimulation of the gut and improved gastrointestinal circulation.

Beta-phenylethylamine inhibits K+ currents in neocortical neurons of the rat: a possible mechanism of beta-phenylethylamine-induced seizures

beta-Phenylethylamine (beta-PEA), an endogenous amine synthesized in the brain, serves as a neuromodulator and is involved in the pathophysiology of various neurological disorders such as depression, schizophrenia, and attention-deficit hyperactivity disorder. beta-PEA fully exerts the physiological effects within the nanomolar concentration range via the trace amine receptors, but beta-PEA also causes convulsions at much higher concentrations via an as yet unknown mechanism. To investigate the electrophysiological mechanism by which beta-PEA induces convulsions, we examined the effect of beta-PEA on ionic currents passing through the cell membrane of dissociated rat cerebral cortical neurons, using a patch-clamp technique. The external application of beta-PEA suppressed ionic currents which continuously flowed when the membrane potential was held at -25 mV. The suppression was in a concentration-dependent manner and a half-maximal effective concentration was 540 muM. These currents suppressed by beta-PEA consisted of two K(+) currents: a time- and voltage-dependent K(+) current (M-current) and a leakage K(+) current. The suppression of the M-current reduces the efficacy of the current in limiting excessive neuronal firing, and the suppression of the leakage K(+) current can cause membrane depolarization and thus promote neuronal excitation. Reducing both of these currents in concert may produce neuronal seizing activity, which could conceivably underlie the convulsions induced by high-dose beta-PEA.

Human olfactory receptor families and their odorants

The human nose detects volatile chemical stimuli by at least three different receptor families: odorant receptors, trace amine-associated receptors, and vomeronasal type-1 receptors. As G protein-coupled receptors, all of the few functionally characterized olfactory receptors share major functional features: when expressed in heterologous cell systems, they 1) respond to odorants of certain chemical groups, e.g., amines, aliphatic carboxylic acids or aldehydes, floral or fruity odorants, including certain key-food odorants, and putative pheromones, and 2) transduce their signals to intracellular cAMP signaling. However, little is known yet about specific differences in the functional designation of the three olfactory receptor families. Recently, two heterologous cell systems expressing olfactory signaling molecules have been developed. Different screening strategies will shed light on the yet sparsely available odorant specificity profiles and structure-function relationships of olfactory receptors, as well as the structure-activity relationships of their odorants.

Dietary trace amine-dependent vasoconstriction in porcine coronary artery

BACKGROUND AND PURPOSE

The dietary trace amines tyramine and beta-phenylethylamine (beta-PEA) can increase blood pressure. However, the mechanisms involved in the vascular effect of trace amines have not been fully established. The purpose of this study was to evaluate whether trace amine-dependent vasoconstriction was brought about by tyramine and beta-PEA acting as indirect sympathomimetic agents, as previously assumed, or whether trace amine-dependent vasoconstriction could be mediated by recently discovered trace amine-associated (TAA) receptors.

EXPERIMENTAL APPROACH

The responses to p-tyramine and beta-PEA were investigated in vitro in rings of the left anterior descending coronary arteries of pigs.

KEY RESULTS

p-Tyramine induced a concentration-dependent (0.1-3 mM) vasoconstriction. The maximum response and pD(2) value for p-tyramine was unaffected by endothelium removal or pre-treatment with antagonists for adrenoceptors, histamine, dopamine or 5-HT receptors. beta-PEA also produced a concentration-dependent (0.3-10 mM) vasoconstriction which was unaffected by endothelium removal, beta-adrenoceptor or 5-HT receptor antagonists. A substantial, but reduced, response to beta-PEA was obtained in the presence of prazosin (alpha(1)-adrenoceptor antagonist), haloperidol (D(2)/D(3) dopamine receptor antagonist) or mepyramine (H(1) histamine receptor antagonist). The pD(2) value for beta-PEA was unaffected by any of the antagonists tested.

CONCLUSIONS AND IMPLICATIONS

Vasoconstriction induced by p-tyramine does not involve an indirect sympathomimetic effect, although vasoconstriction caused by beta-PEA may occur, in part, by this mechanism. We therefore propose that trace amine-dependent vasoconstriction is mediated by phenylethylamine-specific receptors, which are closely related to or identical to TAA receptors. These receptors could provide a target for new antihypertensive therapies.

Toward deciphering the code to aminergic G protein-coupled receptor drug design

The trace amine-associated receptor 1 (TAAR(1)) is a biogenic amine G protein-coupled receptor (GPCR) that is potently activated by 3-iodothyronamine (1, T(1)AM) in vitro. Compound 1 is an endogenous derivative of the thyroid hormone thyroxine which rapidly induces hypothermia, anergia, and bradycardia when administered to mice. To explore the role of TAAR(1) in mediating the effects of 1, we rationally designed and synthesized rat TAAR(1) superagonists and lead antagonists using the rotamer toggle switch model of aminergic GPCR activation. The functional activity of a ligand is proposed to be correlated to its probable interactions with the rotamer switch residues; agonists allow the rotamer switch residues to toggle to their active conformation, whereas antagonists interfere with this conformational transition. These agonist and antagonist design principles provide a conceptual model for understanding the relationship between the molecular structure of a drug and its pharmacological properties.

Structure-activity correlations for beta-phenethylamines at human trace amine receptor 1

A cell line in which RD-HGA16 cells were stably transfected with the hTAAR 1 receptor was created and utilized to carry out a systematic evaluation of a series of beta-phenethylamines. Fair agreement was observed with data obtained for aryl and ethylene chain substituted analogs in an AV12-664 cell line in which hemagglutinin-tagged hTAAR 1 was stably co-expressed with rat G alpha(s). Analogs with multiple substituents as well as analogs with bulky groups were found to be partial agonists. Analogs in which the primary amino group was converted to a secondary or a tertiary amino group by N-methylation were also partial agonists. Comparative Molecular Field Analysis (CoMFA) using the potency data yielded a regression coefficient r(2) of 0.824. The steric field contribution to the model was 61% with the balance (39%) contributed by the electrostatic field. The collective results suggest that increasing steric bulk both at the amino nitrogen, particularly by N-dimethylation, and at the 4-position of the aromatic ring leads to low efficacy ligands.

Distinct evolutionary patterns between chemoreceptors of 2 vertebrate olfactory systems and the differential tuning hypothesis

Most tetrapod vertebrates have 2 olfactory systems, the main olfactory system (MOS) and the vomeronasal system (VNS). According to the dual olfactory hypothesis, the MOS detects environmental odorants, whereas the VNS recognizes intraspecific pheromonal cues. However, this strict functional distinction has been blurred by recent reports that both systems can perceive both types of signals. Studies of a limited number of receptors suggest that MOS receptors are broadly tuned generalists, whereas VNS receptors are narrowly tuned specialists. However, whether this distinction applies to all MOS and VNS receptors remains unknown. The differential tuning hypothesis predicts that generalist MOS receptors detect an overlapping set of ligands and thus are more likely to be conserved over evolutionary time than specialist VNS receptors, which would evolve in a more lineage-specific manner. Here we test this prediction for all olfactory chemoreceptors by examining the evolutionary patterns of MOS-expressed odorant receptors (ORs) and trace amine-associated receptors (TAARs) and VNS-expressed vomeronasal type 1 receptors (V1Rs) and vomeronasal type 2 receptors (V2Rs) in 7 tetrapods (mouse, rat, dog, opossum, platypus, chicken, and frog). The phylogenies of V1Rs and V2Rs show abundant lineage-specific gene gains/losses and virtually no one-to-one orthologs between species. Opposite patterns are found for ORs and TAARs. Analysis of functional data and ligand-binding sites of ORs confirms that paralogous chemoreceptors are more likely than orthologs to have different ligands and that functional divergence between paralogous chemoreceptors is established relatively quickly following gene duplication. Together, these results strongly suggest that the functional profile of the VNS chemoreceptor repertoire evolves much faster than that of the MOS chemoreceptor repertoire and that the differential tuning hypothesis applies to the majority, if not all, of MOS and VNS receptors.

Tyramine and octopamine independently inhibit serotonin-stimulated aversive behaviors in Caenorhabditis elegans through two novel amine receptors

Biogenic amines modulate key behaviors in both vertebrates and invertebrates. In Caenorhabditis elegans, tyramine (TA) and octopamine (OA) inhibit aversive responses to 100%, but not dilute (30%) octanol. TA and OA also abolish food- and serotonin-dependent increases in responses to dilute octanol in wild-type but not tyra-3(ok325) and f14d12.6(ok371) null animals, respectively, suggesting that TA and OA modulated responses to dilute octanol are mediated by separate, previously uncharacterized, G-protein-coupled receptors. TA and OA are high-affinity ligands for TYRA-3 and F14D12.6, respectively, based on their pharmacological characterization after heterologous expression. f14d12.6::gfp is expressed in the ASHs, the neurons responsible for sensitivity to dilute octanol, and the sra-6-dependent expression of F14D12.6 in the ASHs is sufficient to rescue OA sensitivity in f14d12.6(ok371) null animals. In contrast, tyra-3::gfp appears not to be expressed in the ASHs, but instead in other neurons, including the dopaminergic CEP/ADEs. However, although dopamine (DA) also inhibits 5-HT-dependent responses to dilute octanol, TA still inhibits in dop-2; dop-1; dop-3 animals that do not respond to DA and cat-2(tm346) and Pdat-1::ICE animals that lack significant dopaminergic signaling, suggesting that DA is not an intermediate in TA inhibition. Finally, responses to TA and OA selectively desensitize after preexposure to the amines. Our data suggest that although tyraminergic and octopaminergic signaling yield identical phenotypes in these olfactory assays, they act independently through distinct receptors to modulate the ASH-mediated locomotory circuit and that C. elegans is a useful model to study the aminergic modulation of sensory-mediated locomotory behaviors.

Trace amine-associated receptor 1-Family archetype or iconoclast?

Interest has recently been rekindled in receptors that are activated by low molecular weight, noncatecholic, biogenic amines that are typically found as trace constituents of various vertebrate and invertebrate tissues and fluids. The timing of this resurgent focus on receptors activated by the "trace amines" (TA) beta-phenylethylamine (PEA), tyramine (TYR), octopamine (OCT), synephrine (SYN), and tryptamine (TRYP) is the direct result of 2 publications that appeared in 2001 describing the cloning of a novel G protein-coupled receptor (GPCR) referred to by their discoverers Borowsky et al. as TA1 and Bunzow et al. as TA receptor 1 (TAR1). When heterologously expressed in Xenopus laevis oocytes and various eukaryotic cell lines, recombinant rodent and human TAR dose-dependently couple to the stimulation of adenosine 3',5'-monophosphate (cAMP) production. Structure-activity profiling based on this functional response has revealed that in addition to the TA, other biologically active compounds containing a 2-carbon aliphatic side chain linking an amino group to at least 1 benzene ring are potent and efficacious TA receptor agonists with amphetamine (AMPH), methamphetamine, 3-iodothyronamine, thyronamine, and dopamine (DA) among the most notable. Almost 100 years after the search for TAR began, numerous TA1/TAR1-related sequences, now called TA-associated receptors (TAAR), have been identified in the genome of every species of vertebrate examined to date. Consequently, even though heterologously expressed TAAR1 fits the pharmacological criteria established for a bona fide TAR, a major challenge for those working in the field is to discern the in vivo pharmacology and physiology of each purported member of this extended family of GPCR. Only then will it be possible to establish whether TAAR1 is the family archetype or an iconoclast.

Trace amines differentially regulate adult locomotor activity, cocaine sensitivity, and female fertility in Drosophila melanogaster

The trace biogenic amines tyramine and octopamine are found in the nervous systems of animals ranging in complexity from nematodes to mammals. In insects such as Drosophila melanogaster, the trace amine octopamine is a well-established neuromodulator that mediates a diverse range of physiological processes, but an independent role for tyramine is less clear. Tyramine is synthesized from tyrosine by the enzyme tyrosine decarboxylase (TDC). We previously reported the identification of two Tdc genes in Drosophila: the peripherally-expressed Tdc1 and the neurally-expressed Tdc2. To further clarify the neural functions of the trace amines in Drosophila, we examined normal and cocaine-induced locomotor activity in flies that lack both neural tyramine and octopamine because of mutation in Tdc2 (Tdc2(RO54)). Tdc2(RO54) flies have dramatically reduced basal locomotor activity levels and are hypersensitive to an initial dose of cocaine. Tdc2-targeted expression of the constitutively active inward rectifying potassium channel Kir2.1 replicates these phenotypes, and Tdc2-driven expression of Tdc1 rescues the phenotypes. However, flies that contain no measurable neural octopamine and an excess of tyramine due to a null mutation in the tyramine beta-hydroxylase gene (TbetaH(nM18)) exhibit normal locomotor activity and cocaine responses in spite of showing female sterility due to loss of octopamine. The ability of elevated levels of neural tyramine in TbetaH(nM18) flies to supplant the role of octopamine in adult locomotor and cocaine-induced behaviors, but not in functions related to female fertility, indicates mechanistic differences in the roles of trace amines in these processes.

Expression of neuronal trace amine-associated receptor (Taar) mRNAs in leukocytes

Trace amines such as tyramine, octopamine and beta-phenylethylamine bind with high affinity to the mammalian trace amine-associated receptor 1 (Taar1), potentially activating G-proteins in the synaptic membranes of target neurons. Recently there has been significant interest in Taar1, since this receptor can bind certain psychoactive drugs of abuse such as Ecstasy (3,4-methylenedioxymethamphetamine). Surprisingly, Ecstasy has been shown to alter responses of immune cells, and we questioned whether Taar receptors might be responsible for this effect. Using sensitive and quantitative RT-PCR assays, we found no detectable expression of Taar mRNA in bone marrow, or in primary cultures of mouse macrophages and dendritic cells whether quiescent or activated by exposure to lipopolysaccharide or the mouse gamma herpesvirus-68 (gammaHV-68). Mouse B cells and NK cells isolated from spleen, however, showed expression of several Taar mRNA species. Taar mRNA expression was also upregulated in human peripheral blood lymphocytes following in vitro stimulation with PHA. These studies represent the first to define expression of the mRNAs encoding these trace amine receptors in leukocytes.

The sense of smell: molecular basis of odorant recognition

Most animal species rely on odorant compounds to locate food, predators, or toxins. The sense of smell is also involved in animal communication, and revealing the underlying mechanisms will therefore facilitate a deeper understanding of animal behaviour. Since the 1940s different theories have speculated on the fundamental basis of olfaction. It was assumed that odorant molecules were recognized by selective protein receptors in the nose, triggering a nervous signal processed by the brain. The discovery of these receptors in the early 1990s allowed great progress in understanding the physiological and biochemical principles of olfaction. An overview of the different mechanisms involved in the coding of odour character as well as odour intensity is presented here, focusing on the biochemical basis of odorant recognition. Despite the enormous progress achieved in recent years, details of odorant-receptor interaction at the molecular level and the mechanisms of olfactory receptor activation are poorly understood. The likely role of metal ions in odorant recognition is discussed, and also the perireceptor events involved in odorant transport and biotransformation, with a view to providing a comprehensive overview of mammalian olfaction to guide future computational structural models and the design of functional experiments. Recent studies have analysed the olfactory genome of several species, providing information about the evolution of olfaction. The role of the olfactory system in animal communication is also described.

The G protein-coupled receptor subset of the rat genome

BACKGROUND

The superfamily of G protein-coupled receptors (GPCRs) is one of the largest within most mammals. GPCRs are important targets for pharmaceuticals and the rat is one of the most widely used model organisms in biological research. Accurate comparisons of protein families in rat, mice and human are thus important for interpretation of many physiological and pharmacological studies. However, current automated protein predictions and annotations are limited and error prone.

RESULTS

We searched the rat genome for GPCRs and obtained 1867 full-length genes and 739 pseudogenes. We identified 1277 new full-length rat GPCRs, whereof 1235 belong to the large group of olfactory receptors. Moreover, we updated the datasets of GPCRs from the human and mouse genomes with 1 and 43 new genes, respectively. The total numbers of full-length genes (and pseudogenes) identified were 799 (583) for human and 1783 (702) for mouse. The rat, human and mouse GPCRs were classified into 7 families named the Glutamate, Rhodopsin, Adhesion, Frizzled, Secretin, Taste2 and Vomeronasal1 families. We performed comprehensive phylogenetic analyses of these families and provide detailed information about orthologues and species-specific receptors. We found that 65 human Rhodopsin family GPCRs are orphans and 56 of these have an orthologue in rat.

CONCLUSION

Interestingly, we found that the proportion of one-to-one GPCR orthologues was only 58% between rats and humans and only 70% between the rat and mouse, which is much lower than stated for the entire set of all genes. This is in mainly related to the sensory GPCRs. The average protein sequence identities of the GPCR orthologue pairs is also lower than for the whole genomes. We found these to be 80% for the rat and human pairs and 90% for the rat and mouse pairs. However, the proportions of orthologous and species-specific genes vary significantly between the different GPCR families. The largest diversification is seen for GPCRs that respond to exogenous stimuli indicating that the variation in their repertoires reflects to a large extent the adaptation of the species to their environment. This report provides the first overall roadmap of the GPCR repertoire in rat and detailed comparisons with the mouse and human repertoires.

A region of 35 kb containing the trace amine associate receptor 6 (TAAR6) gene is associated with schizophrenia in the Irish study of high-density schizophrenia families

The TAAR6 gene has been previously associated with schizophrenia in 192 pedigrees of European and African ancestry. To replicate these findings we performed an association study of TAAR6 in 265 pedigrees of the Irish Study of High-Density Schizophrenia Families (ISHDSF). Of the 24 genotyped single-nucleotide polymorphisms only rs12189813 and rs9389011 provided single-marker evidence for association (0.0094<or=P<or=0.03). Two-marker haplotypes (rs7772821 and rs12189813; 0.0071<or=P<or=0.0023) and four-marker haplotypes (rs8192622, rs7772821, rs12189813 and rs9389011; 0.0047<or=P<or=0.018) gave strongest evidence for association. The associated high-risk (HR) haplotype in the ISHDSF is defined by the major alleles at rs7772821 and rs12189813 (0.00097<or=P<or=0.023). The associated HR remains positive in a case only test of association by Operational Criteria score analysis in which significant association was observed only with the highest threshold for delusions (P<0.009). When analysis was restricted to affected individuals under the core schizophrenia (D2) diagnosis, the observed associations for HR were most significant in the highest threshold for delusions (P<0.004) and hallucinations (P<0.0004), supporting the family-based association with schizophrenia.

Evolution of Trace Amine–Associated Receptor (TAAR) Gene Family in Vertebrates: Lineage-Specific Expansions and Degradations of a Second Class of Vertebrate Chemosensory Receptors Expressed in the Olfactory Epithelium

The trace amine-associated receptors (TAARs) form a specific family of G protein-coupled receptors in vertebrates. TAARs were initially considered neurotransmitter receptors, but recent study showed that mouse TAARs function as chemosensory receptors in the olfactory epithelium. To clarify the evolutionary dynamics of the TAAR gene family in vertebrates, near-complete repertoires of TAAR genes and pseudogenes were identified from the genomic assemblies of 4 teleost fishes (zebrafish, fugu, stickleback, and medaka), western clawed frogs, chickens, 3 mammals (humans, mice, and opossum), and sea lampreys. Database searches revealed that fishes had many putatively functional TAAR genes (13-109 genes), whereas relatively small numbers of TAAR genes (3-22 genes) were identified in tetrapods. Phylogenetic analysis of these genes indicated that the TAAR gene family was subdivided into 5 subfamilies that diverged before the divergence of ray-finned fishes and tetrapods. In tetrapods, virtually all TAAR genes were located in 1 specific region of their genomes as a gene cluster; however, in fishes, TAAR genes were scattered throughout more than 2 genomic locations. This possibly reflects a whole-genome duplication that occurred in the common ancestor of ray-finned fishes. Expression analysis of zebrafish and stickleback TAAR genes revealed that many TAARs in these fishes were expressed in the olfactory organ, suggesting the relatively high importance of TAARs as chemosensory receptors in fishes. A possible evolutionary history of the vertebrate TAAR gene family was inferred from the phylogenetic and comparative genomic analyses.

Exploring the structure-activity relationship of the ethylamine portion of 3-iodothyronamine for rat and mouse trace amine-associated receptor 1

3-iodothyronamine (1, T1AM) is a naturally occurring derivative of thyroid hormone that can potently activate the orphan G protein-coupled receptor (GPCR) known as the trace amine-associated receptor 1 (TAAR1). We have previously found that modifying the outer ring of the phenoxyphenethylamine core scaffold of 1 can improve potency and provide potent agonists. In this study, we explored the tolerance of rat and mouse TAAR1 (rTAAR1 and mTAAR1) for structural modifications in the ethylamine portion of 1. We found that incorporating unsaturated hydrocarbon substituents and polar, hydrogen-bond-accepting groups were beneficial for rTAAR1 and mTAAR1, respectively, providing compounds that were equipotent or more potent than 1. Additionally, we have discovered that a naphthyl group is an excellent isosteric replacement for the iodophenyl ring of 1.

Cardiac effects of 3-iodothyronamine: a new aminergic system modulating cardiac function

3-Iodothyronamine T1AM is a novel endogenous thyroid hormone derivative that activates the G protein-coupled receptor known as trace anime-associated receptor 1 (TAAR1). In the isolated working rat heart and in rat cardiomyocytes, T1AM produced a reversible, dose-dependent negative inotropic effect (e.g., 27+/-5, 51+/-3, and 65+/-2% decrease in cardiac output at 19, 25, and 38 microM concentration, respectively). An independent negative chronotropic effect was also observed. The hemodynamic effects of T1AM were remarkably increased in the presence of the tyrosine kinase inhibitor genistein, whereas they were attenuated in the presence of the tyrosine phosphatase inhibitor vanadate. No effect was produced by inhibitors of protein kinase A, protein kinase C, calcium-calmodulin kinase II, phosphatidylinositol-3-kinase, or MAP kinases. Tissue cAMP levels were unchanged. In rat ventricular tissue, Western blot experiments with antiphosphotyrosine antibodies showed reduced phosphorylation of microsomal and cytosolic proteins after perfusion with synthetic T1AM; reverse transcriptase-polymerase chain reaction experiments revealed the presence of transcripts for at least 5 TAAR subtypes; specific and saturable binding of [125I]T1AM was observed, with a dissociation constant in the low micromolar range (5 microM); and endogenous T1AM was detectable by tandem mass spectrometry. In conclusion, our findings provide evidence for the existence of a novel aminergic system modulating cardiac function.

Trace amine-associated receptor 1 displays species-dependent stereoselectivity for isomers of methamphetamine, amphetamine, and para-hydroxyamphetamine

The synthetic amines methamphetamine (METH), amphetamine (AMPH), and their metabolite para-hydroxyamphetamine (POHA) are chemically and structurally related to the catecholamine neurotransmitters and a small group of endogenous biogenic amines collectively referred to as the trace amines (TAs). Recently, it was reported that METH, AMPH, POHA, and the TAs para-tyramine (TYR) and beta-phenylethylamine (PEA) stimulate cAMP production in human embryonic kidney (HEK)-293 cells expressing rat trace amine-associated receptor 1 (rTAAR1). The discovery that METH and AMPH activate the rTAAR1 motivated us to study the effect of these drugs on the mouse TAAR1 (mTAAR1) and a human-rat chimera (hrChTAAR1). Furthermore, because S-(+)-isomers of METH and AMPH are reported to be more potent and efficacious in vivo than R-(-), we determined the enantiomeric selectivity of all three species of TAAR1. In response to METH, AMPH, or POHA exposure, the accumulation of cAMP by HEK-293 cells stably expressing different species of TAAR1 was concentration- and isomer-dependent. EC50 values for S-(+)-METH were 0.89, 0.92, and 4.44 microM for rTAAR1, mTAAR1, and h-rChTAAR1, respectively. PEA was a potent and full agonist at each species of TAAR1, whereas TYR was a full agonist for the rodent TAAR1s but was a partial agonist at h-rChTAAR1. Interestingly, both isomers of METH were full agonists at mTAAR1 and h-rChTAAR1, whereas both were partial agonists at rTAAR1. Taken together, these in vitro results suggest that, in vivo, TAAR1 could be a novel mediator of the effects of these drugs.

The Trace Amine 1 receptor knockout mouse: an animal model with relevance to schizophrenia

Trace amines have been implicated in a number of neuropsychiatric disorders including depression and schizophrenia. Although long known to modulate neurotransmission indirectly through the release of catecholamines, the identification of the Trace Amine 1 receptor (TA1) offers a mechanism by which trace amines can influence synaptic activity directly. TA1 binds and is activated by trace amines such as beta-phenylethylamine and tyramine. Our pharmacological characterization of mouse TA1 showed that, as in rat and primate, amphetamine is an agonist at this receptor but with surprisingly high potency. Without selective ligands for TA1 that do not also possess catecholamine-releasing properties, however, it has not been possible to study its physiological role in the central nervous system. To that end, a line of mice lacking the TA1 receptor was generated to characterize its contribution to the regulation of behavior. Compared with wild-type littermates, TA1 knockout (KO) mice displayed a deficit in prepulse inhibition. Knockout animals, in which the TA1-agonist influence of amphetamine was absent, showed enhanced sensitivity to the psychomotor-stimulating effect of this drug, which was temporally correlated with significantly larger increases in the release of both dopamine and norepinephrine in the dorsal striatum and associated with a 262% increase in the proportion of striatal high-affinity D2 receptors. TA1 therefore appears to play a modulatory role in catecholaminergic function and represents a potentially novel mechanism for the treatment of neuropsychiatric disorders. Furthermore, the TA1 KO mouse may provide a useful model for the development of treatments for some positive symptoms of schizophrenia.

Trace amine-associated receptors and their ligands

Classical biogenic amines (adrenaline, noradrenaline, dopamine, serotonin and histamine) interact with specific families of G protein-coupled receptors (GPCRs). The term 'trace amines' is used when referring to p-tyramine, beta-phenylethylamine, tryptamine and octopamine, compounds that are present in mammalian tissues at very low (nanomolar) concentrations. The pharmacological effects of trace amines are usually attributed to their interference with the aminergic pathways, but in 2001 a new gene was identified, that codes for a GPCR responding to p-tyramine and beta-phenylethylamine but not to classical biogenic amines. Several closely related genes were subsequently identified and designated as the trace amine-associated receptors (TAARs). Pharmacological investigations in vitro show that many TAAR subtypes may not respond to p-tyramine, beta-phenylethylamine, tryptamine or octopamine, suggesting the existence of additional endogenous ligands. A novel endogenous thyroid hormone derivative, 3-iodothyronamine, has been found to interact with TAAR1 and possibly other TAAR subtypes. In vivo, micromolar concentrations of 3-iodothyronamine determine functional effects which are opposite to those produced on a longer time scale by thyroid hormones, including reduction in body temperature and decrease in cardiac contractility. Expression of all TAAR subtypes except TAAR1 has been reported in mouse olfactory epithelium, and several volatile amines were shown to interact with specific TAAR subtypes. In addition, there is evidence that TAAR1 is targeted by amphetamines and other psychotropic agents, while genetic linkage studies show a significant association between the TAAR gene family locus and susceptibility to schizophrenia or bipolar affective disorder.

Pharmacologic characterization of the cloned human trace amine-associated receptor1 (TAAR1) and evidence for species differences with the rat TAAR1

The hemagglutinin-tagged human trace amine-associated receptor1 (TAAR1) was stably coexpressed with rat Galpha(s) in the AV12-664 cell line, and receptor activation was measured as the stimulation of cAMP formation. After blockade of endogenously expressed alpha2- and beta-adrenoceptors with 2-[2-(2-methoxy-1,4-benzodioxanyl)]-imidazoline hydrochloride (2-methoxyidazoxan, RX821002) and alprenolol, respectively, the resulting pharmacology was consistent with that of a unique receptor subtype. beta-Phenylethylamine (beta-PEA), the putative endogenous ligand, gave an EC50 of 106 +/- 5 nM in the assay. For a series of beta-PEA analogs used to explore the pharmacophore, small substituents at ring positions 3 and/or 4 generally resulted in compounds having lower potency than beta-PEA, although several were as potent as beta-PEA. However, small substituents at ring position 2 resulted in a number of compounds having potencies as good as or better than beta-PEA. A number of nonselective antagonists known to share affinity for multiple monoaminergic receptors were evaluated for their ability to inhibit beta-PEA stimulation of the human TAAR1. None had an IC50 <10 microM. For comparison, the rat TAAR1 receptor was expressed in the AV12-664 cell line. A number of agonist compounds had significantly different relative potencies between the rat and human TAAR1, demonstrating a significant species difference between the rat and human TAAR1. The TAAR1 receptor exhibits a pharmacologic profile uniquely different from those of classic monoaminergic receptors, consistent with the structural information that places them in a distinct family of receptors. This unique pharmacologic profile suggests the potential for development of TAAR-selective agonists and antagonists to study their physiologic roles.

The mysterious trace amines: protean neuromodulators of synaptic transmission in mammalian brain

The trace amines are a structurally related group of amines and their isomers synthesized in mammalian brain and peripheral nervous tissues. They are closely associated metabolically with the dopamine, noradrenaline and serotonin neurotransmitter systems in mammalian brain. Like dopamine, noradrenaline and serotonin the trace amines have been implicated in a vast array of human disorders of affect and cognition. The trace amines are unique as they are present in trace concentrations, exhibit high rates of metabolism and are distributed heterogeneously in mammalian brain. While some are synthesized in their parent amine neurotransmitter systems, there is also evidence to suggest other trace amines may comprise their own independent neurotransmitter systems. A substantial body of evidence suggests that the trace amines may play very significant roles in the coordination of biogenic amine-based synaptic physiology. At high concentrations, they have well-characterized presynaptic "amphetamine-like" effects on catecholamine and indolamine release, reuptake and biosynthesis; at lower concentrations, they possess postsynaptic modulatory effects that potentiate the activity of other neurotransmitters, particularly dopamine and serotonin. The trace amines also possess electrophysiological effects that are in opposition to these neurotransmitters, indicating to some researchers the existence of receptors specific for the trace amines. While binding sites or receptors for a few of the trace amines have been advanced, the absence of cloned receptor protein has impeded significant development of their detailed mechanistic roles in the coordination of catecholamine and indolamine synaptic physiology. The recent discovery and characterization of a family of mammalian G protein-coupled receptors responsive to trace amines such as beta-phenylethylamine, tyramine, and octopamine, including socially ingested psychotropic drugs such as amphetamine, 3,4-methylenedioxymethamphetamine, N,N-dimethyltryptamine, and lysergic acid diethylamide, have revitalized the field of scientific studies investigating trace amine synaptic physiology, and its association with major human disorders of affect and cognition.

A Rapid Functional Assay for the Human Trace Amine-Associated Receptor 1 Based on the Mobilization of Internal Calcium

The molecular targets for trace amines (TAs) such as p-tyramine and β-phenylethylamine have been recently discovered and have been shown to comprise a family of G-protein-coupled receptors based on DNA sequence homologies. These have been termed trace amine-associated receptors (TAARs) because TAs do not activate all of the identified receptors. Because TA may be involved in modulating a variety of behaviors including mood, cognition, and addiction, it is of interest to discover novel ligands for TAARs to probe the role TAs play in brain function. Pharmacophore development for the Gs-coupled human TAAR1 ( hTAAR1) would be aided by a rapid functional assay amenable to screening libraries of compounds. Accordingly, the authors used RD-HGA16 CHO-1 cells from Molecular Devices, which stably express the promiscuous Gq, Gα16, to create a cell line stably expressing hTAAR1, thereby coupling receptor activation to the mobilization of internal calcium. They used this cell line to develop a homogenous fluorometric imaging plate reader-based assay using the Calcium 3 fluorescent dye. The EC50 and Emax data obtained for known TAs are in close agreement with previous work using transient hTAAR1 expression systems or a chimeric receptor. These data indicate that the hTAAR1 retains its reported pharmacological characteristics when coupled to Gα16.

Comprehensive repertoire and phylogenetic analysis of the G protein-coupled receptors in human and mouse

Understanding differences in the repertoire of orthologous gene pairs is vital for interpretation of pharmacological and physiological experiments if conclusions are conveyed between species. Here we present a comprehensive dataset for G protein-coupled receptors (GPCRs) in both human and mouse with a phylogenetic road map. We performed systematic searches applying several search tools such as BLAST, BLAT, and Hidden Markov models and searches in literature data. We aimed to gather a full-length version of each human or mouse GPCR in only one copy referring to a single chromosomal position. Moreover, we performed detailed phylogenetic analysis of the transmembrane regions of the receptors to establish accurate orthologous pairs. The results show the identity of 495 mouse and 400 human functional nonolfactory GPCRs. Overall, 329 of the receptors are found in one-to-one orthologous pairs, while 119 mouse and 31 human receptors originate from species-specific expansions or deletions. The average percentage similarity of the orthologue pairs is 85%, while it varies between the main GRAFS families from an average of 59 to 94%. The orthologous pairs for the lipid-binding GPCRs had the lowest levels of conservation, while the biogenic amines had highest levels of conservation. Moreover, we searched for expressed sequence tags (ESTs) and identified more than 17,000 ESTs matching GPCRs in mouse and human, providing information about their expression patterns. On the whole, this is the most comprehensive study of the gene repertoire that codes for human and mouse GPCRs. The datasets are available for downloading.

An improved method for the separation and detection of biogenic amines in adult Drosophila brain extracts by high performance liquid chromatography

Biogenic amines are critically important neuromodulators in both vertebrates and invertebrates. Quantification of these amines can be difficult, particularly in neural extracts of Drosophila melanogaster that contain interfering electroactive compounds. We have developed a method for the reliable separation and quantification of the biogenic amines dopamine, serotonin, tyramine, and octopamine in Drosophila brain extracts using high performance liquid chromatography with electrochemical detection. Our method obviates the need for complex preparatory procedures or instrumentation, and can reproducibly detect picogram quantities of these amines. By optimizing the composition of the mobile phase and the electrode potential, and by examining common complications in the analysis of biological samples, we have developed a reliable technique for monitoring levels of biogenic amines in the Drosophila brain.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Trace amine-associated receptor agonists: synthesis and evaluation of thyronamines and related analogues

We have previously shown that several thyronamines, decarboxylated and deiodinated metabolites of the thyroid hormone, potently activate an orphan G protein-coupled receptor in vitro (TAAR1) and induced hypothermia in vivo on a rapid time scale [Scanlan, T. S.; Suchland, K. L.; Hart, M. E.; Chiellini, G.; Huang, Y.; Kruzich, P. J.; Frascarelli, S.; Crossley, D. A.; Bunzow, J. R.; Ronca-Testoni, S.; Lin, E. T.; Hatton, D.; Zucchi, R.; Grandy, D. K. 3-Iodothyronamine is an endogenous and rapid-acting derivative of thyroid hormone. Nat. Med. 2004, 10 (6), 638-642]. Herein, we report the synthesis of these thyronamines. Additionally, a large number of thyroamine derivatives were synthesized in an effort to understand the molecular basis of TAAR1 activation and hypothermia induction. Several derivatives were found to potently activate both rTAAR1 and mTAAR1 in vitro (compounds 77, 85, 91, and 92). When administered to mice at a 50 mg/kg dose, these derivatives all induced significant hypothermia within 60 min and exhibited a hypothermic induction profile analogous to 3-iodothyronamine (1, T(1)AM) except 91, which proved to be more efficacious. On the basis of this result, a dose-dependent profile for 91 was generated and an ED(50) of 30 mumol/kg was calculated. Compound 91 proved to be more potent than T(1)AM for TAAR1 activation and exhibits increased potency and efficacy for hypothermia induction. These data further strengthen the pharmacological correlation linking TAAR1 activation by thyronamines and hypothermia induction in mice.

Receptors of mammalian trace amines

The discovery of a family of G-protein coupled receptors, some of which bind and are activated by biogenic trace amines, has prompted speculation as to the physiological role of these receptors. Observations associated with the distribution of these trace amine associated receptors (TAARs) suggest that they may be involved in depression, attention-deficit hyperactivity disorder, eating disorders, migraine headaches, and Parkinson's disease. Preliminary in vitro data, obtained using cloned receptors, also suggest a role for TAARs in the function of hallucinogens.

Cellular and molecular biology of orphan G protein-coupled receptors

The superfamily of G protein-coupled receptors (GPCRs) is the largest and most diverse group of membrane-spanning proteins. It plays a variety of roles in pathophysiological processes by transmitting extracellular signals to cells via heterotrimeric G proteins. Completion of the human genome project revealed the presence of approximately 168 genes encoding established nonsensory GPCRs, as well as 207 genes predicted to encode novel GPCRs for which the natural ligands remained to be identified, the so-called orphan GPCRs. Eighty-six of these orphans have now been paired to novel or previously known molecules, and 121 remain to be deorphaned. A better understanding of the GPCR structures and classification; knowledge of the receptor activation mechanism, either dependent on or independent of an agonist; increased understanding of the control of GPCR-mediated signal transduction; and development of appropriate ligand screening systems may improve the probability of discovering novel ligands for the remaining orphan GPCRs.

An automated system for the analysis of G protein-coupled receptor transmembrane binding pockets: alignment, receptor-based pharmacophores, and their application

G protein-coupled receptors (GPCRs) share a common architecture consisting of seven transmembrane (TM) domains. Various lines of evidence suggest that this fold provides a generic binding pocket within the TM region for hosting agonists, antagonists, and allosteric modulators. Here, a comprehensive and automated method allowing fast analysis and comparison of these putative binding pockets across the entire GPCR family is presented. The method relies on a robust alignment algorithm based on conservation indices, focusing on pharmacophore-like relationships between amino acids. Analysis of conservation patterns across the GPCR family and alignment to the rhodopsin X-ray structure allows the extraction of the amino acids lining the TM binding pocket in a so-called ligand binding pocket vector (LPV). In a second step, LPVs are translated to simple 3D receptor pharmacophore models, where each amino acid is represented by a single spherical pharmacophore feature and all atomic detail is omitted. Applications of the method include the assessment of selectivity issues, support of mutagenesis studies, and the derivation of rules for focused screening to identify chemical starting points in early drug discovery projects. Because of the coarseness of this 3D receptor pharmacophore model, however, meaningful scoring and ranking procedures of large sets of molecules are not justified. The LPV analysis of the trace amine-associated receptor family and its experimental validation is discussed as an example. The value of the 3D receptor model is demonstrated for a class C GPCR family, the metabotropic glutamate receptors.

Genome wide survey of G protein-coupled receptors in Tetraodon nigroviridis

Background

The G-protein-coupled receptors (GPCRs) constitute one of the largest and most ancient superfamilies of membrane proteins. They play a central role in physiological processes affecting almost all aspects of the life cycle of an organism. Availability of the complete sets of putative members of a family from diverse species provides the basis for cross genome comparative studies.

Results

We have defined the repertoire of GPCR superfamily of Tetraodon complement with the availability of complete sequence of the freshwater puffer fish Tetraodon nigroviridis. Almost all 466 Tetraodon GPCRs (Tnig-GPCRs) identified had a clear human homologue. 189 putative human and Tetraodon GPCR orthologous pairs could be identified. Tetraodon GPCRs are classified into five GRAFS families, by phylogenetic analysis, concurrent with human GPCR classification.

Conclusion

Direct comparison of GPCRs in Tetraodon and human genomes displays a high level of orthology and supports large-scale gene duplications in Tetraodon. Examples of lineage specific gene expansions were also observed in opsin and odorant receptors. The human and Tetraodon GPCR sequences are analogous in terms of GPCR subfamilies but display disproportionate numbers of receptors at the subfamily level. The teleost genome with its expanded set of GPCRs provides additional and interesting comparators to study both evolution and function of these receptors.

A renaissance in trace amines inspired by a novel GPCR family

Trace amines (TAs) are endogenous compounds that are related to biogenic amine neurotransmitters and are present in the mammalian nervous system in trace amounts. Although their pronounced pharmacological effects and tight link to major human disorders such as depression and schizophrenia have been studied for decades, the understanding of their molecular mode of action remained incomplete because of the apparent absence of specialized receptors. However, the recent discovery of a novel family of G-protein-coupled receptors (GPCRs) that includes individual members that are highly specific for TAs indicates a potential role for TAs as vertebrate neurotransmitters or neuromodulators, although the majority of these GPCRs so far have not been demonstrated to be activated by TAs. The unique pharmacology and expression pattern of these receptors make them prime candidates for targets in drug development in the context of several neurological diseases. Current research focuses on dissecting their molecular pharmacology and on the identification of endogenous ligands for the apparently TA-insensitive members of this receptor family.