Analysis of human TAAR8 and murine Taar8b mediated signaling pathways and expression profile

The dark sides of the GPCR tree - research progress on understudied GPCRs

A large portion of the human GPCRome is still in the dark and understudied, consisting even of entire subfamilies of GPCRs such as odorant receptors, class A and C orphans, adhesion GPCRs, Frizzleds and taste receptors. However, it is undeniable that these GPCRs bring an untapped therapeutic potential that should be explored further. Open questions on these GPCRs span diverse topics such as deorphanisation, the development of tool compounds and tools for studying these GPCRs, as well as understanding basic signalling mechanisms. This review gives an overview of the current state of knowledge for each of the diverse subfamilies of understudied receptors regarding their physiological relevance, molecular mechanisms, endogenous ligands and pharmacological tools. Furthermore, it identifies some of the largest knowledge gaps that should be addressed in the foreseeable future and lists some general strategies that might be helpful in this process.

Class A and C GPCR Dimers in Neurodegenerative Diseases

Neurodegenerative diseases affect over 30 million people worldwide with an ascending trend. Most individuals suffering from these irreversible brain damages belong to the elderly population, with onset between 50 and 60 years. Although the pathophysiology of such diseases is partially known, it remains unclear upon which point a disease turns degenerative. Moreover, current therapeutics can treat some of the symptoms but often have severe side effects and become less effective in long-term treatment. For many neurodegenerative diseases, the involvement of G proteincoupled receptors (GPCRs), which are key players of neuronal transmission and plasticity, has become clearer and holds great promise in elucidating their biological mechanism. With this review, we introduce and summarize class A and class C GPCRs, known to form heterodimers or oligomers to increase their signalling repertoire. Additionally, the examples discussed here were shown to display relevant alterations in brain signalling and had already been associated with the pathophysiology of certain neurodegenerative diseases. Lastly, we classified the heterodimers into two categories of crosstalk, positive or negative, for which there is known evidence.

Trace Amine-Associated Receptor 1 Trafficking to Cilia of Thyroid Epithelial Cells

Trace amine-associated receptor 1 (rodent Taar1/human TAAR1) is a G protein-coupled receptor that is mainly recognized for its functions in neuromodulation. Previous in vitro studies suggested that Taar1 may signal from intracellular compartments. However, we have shown Taar1 to localize apically and on ciliary extensions in rodent thyrocytes, suggesting that at least in the thyroid, Taar1 may signal from the cilia at the apical plasma membrane domain of thyrocytes in situ, where it is exposed to the content of the follicle lumen containing putative Taar1 ligands. This study was designed to explore mouse Taar1 (mTaar1) trafficking, heterologously expressed in human and rat thyroid cell lines in order to establish an in vitro system in which Taar1 signaling from the cell surface can be studied in future. The results showed that chimeric mTaar1-EGFP traffics to the apical cell surface and localizes particularly to spherical structures of polarized thyroid cells, procilia, and primary cilia upon serum-starvation. Moreover, mTaar1-EGFP appears to form high molecular mass forms, possibly homodimers and tetramers, in stably expressing human thyroid cell lines. However, only monomeric mTaar1-EGFP was cell surface biotinylated in polarized human thyrocytes. In polarized rat thyrocytes, mTaar1-EGFP is retained in the endoplasmic reticulum, while cilia were reached by mTaar1-EGFP transiently co-expressed in combination with an HA-tagged construct of the related mTaar5. We conclude that Taar1 trafficking to cilia depends on their integrity. The results further suggest that an in vitro cell model was established that recapitulates Taar1 trafficking in thyrocytes in situ, in principle, and will enable studying Taar1 signaling in future, thus extending our general understanding of its potential significance for thyroid autoregulation.

Ascorbate-induced oxidative stress mediates TRP channel activation and cytotoxicity in human etoposide-sensitive and -resistant retinoblastoma cells

There are indications that pharmacological doses of ascorbate (Asc) used as an adjuvant improve the chemotherapeutic management of cancer. This favorable outcome stems from its cytotoxic effects due to prooxidative mechanisms. Since regulation of intracellular Ca2+ levels contributes to the maintenance of cell viability, we hypothesized that one of the effects of Asc includes disrupting regulation of intracellular Ca2+ homeostasis. Accordingly, we determined if Asc induced intracellular Ca2+ influx through activation of pertussis sensitive Gi/o-coupled GPCR which in turn activated transient receptor potential (TRP) channels in both etoposide-resistant and -sensitive retinoblastoma (WERI-Rb1) tumor cells. Ca2+ imaging, whole-cell patch-clamping, and quantitative real-time PCR (qRT-PCR) were performed in parallel with measurements of RB cell survival using Trypan Blue cell dye exclusion. TRPM7 gene expression levels were similar in both cell lines whereas TRPV1, TRPM2, TRPA1, TRPC5, TRPV4, and TRPM8 gene expression levels were downregulated in the etoposide-resistant WERI-Rb1 cells. In the presence of extracellular Ca2+, 1 mM Asc induced larger intracellular Ca2+ transients in the etoposide-resistant WERI-Rb1 than in their etoposide-sensitive counterpart. With either 100 µM CPZ, 500 µM La3+, 10 mM NAC, or 100 µM 2-APB, these Ca2+ transients were markedly diminished. These inhibitors also had corresponding inhibitory effects on Asc-induced rises in whole-cell currents. Pertussis toxin (PTX) preincubation blocked rises in Ca2+ influx. Microscopic analyses showed that after 4 days of exposure to 1 mM Asc cell viability fell by nearly 100% in both RB cell lines. Taken together, one of the effects underlying oxidative mediated Asc-induced WERI-Rb1 cytotoxicity stems from its promotion of Gi/o coupled GPCR mediated increases in intracellular Ca2+ influx through TRP channels. Therefore, designing drugs targeting TRP channel modulation may be a viable approach to increase the efficacy of chemotherapeutic treatment of RB. Furthermore, Asc may be indicated as a possible supportive agent in anti-cancer therapies.

TAAR Agonists

Trace amine-associated receptors (TAARs) are a family of G protein-coupled receptors (GPCRs) that are evolutionarily conserved in vertebrates. The first discovered TAAR1 is mainly expressed in the brain, and is able to detect low abundant trace amines. TAAR1 is also activated by several synthetic compounds and psychostimulant drugs like amphetamine. Activation of TAAR1 by specific agonists can regulate the classical monoaminergic systems in the brain. Further studies have revealed that other TAAR family members are highly expressed in the olfactory system which are termed olfactory TAARs. In vertebrates, olfactory TAARs can specifically recognize volatile or water-soluble amines. Some of these TAAR agonists are produced by decarboxylation of amino acids. In addition, some TAAR agonists are ethological odors that mediate animal innate behaviors. In this study, we provide a comprehensive review of TAAR agonists, including their structures, biosynthesis pathways, and functions.

3-Iodothyronamine Induces Diverse Signaling Effects at Different Aminergic and Non-Aminergic G-Protein Coupled Receptors

The thyroid hormone metabolite 3-iodothyronamine (3-T1AM) exerts diverse physiological reactions such as a decrease of body temperature, and negative inotropic and chronotropic effects. This observed pleomorphic effect in physiology can be barely explained by interaction with only one target protein such as the trace-amine receptor 1 (TAAR1), a class A G-protein coupled receptor (GPCR). Moreover, Taar1 knock-out mice still react to 3-T1AM through physiological responses with a rapid decrease in body temperature. These facts propelled our group and others to search for further targets for this molecule.The group of TAARs evolved early in evolution and, according to sequence similarities, they are closely related to adrenoceptors and other aminergic receptors. Therefore, several of these receptors were characterized by their potential to interplay with 3-T1AM. Indeed, 3-T1AM acts as a positive allosteric modulator on the beta2-adrenoceptor (ADRB2) and as a biased agonist on the serotonin receptor 1B (5HT1b) and the alpha2-adrenoceptor (ADRA2A). In addition, 3-T1AM was reported to be a weak antagonist at a non-aminergic muscarinic receptor (M3).These findings impressively reflect that such trace amines can unselectively and simultaneously function at different receptors expressed by one cell or at different tissues. In conclusion, the role of 3-T1AM is hypothesized to concert the fine-tuning of specific cell reactions by the accentuation of certain pathways dependent on distinct receptors. 3-T1AM acts as a regulator of signals by blocking, modulating, or inducing simultaneously distinct intracellular signaling cascades via different GPCRs.

3-Iodothyronamine-A Thyroid Hormone Metabolite With Distinct Target Profiles and Mode of Action

The rediscovery of the group of thyronamines (TAMs), especially the first detailed description of their most prominent congener 3-iodothyronamine (3T1AM) 14 years ago, boosted research on this thyroid hormone metabolite tremendously. TAMs exert actions partly opposite to and distinct from known functions of thyroid hormones. These fascinating metabolic, anapyrexic, cytoprotective, and brain effects quickly evoked the hope to use hormone-derived TAMs as a therapeutic option. The G protein-coupled receptor (GPCR) TAAR1, a member of the trace amine-associated receptor (TAAR) family, was identified as the first target and effector of TAM action. The initial enthusiasm on pharmacological actions of exogenous TAMs elicited many questions, such as sites of biosynthesis, analytics, modes of action, inactivation, and role of TAMs in (patho)physiology. Meanwhile, it became clear that TAMs not only interact with TAAR1 or other TAAR family members but also with several aminergic receptors and non-GPCR targets such as transient receptor potential channels, mitochondrial proteins, and the serum TAM-binding protein apolipoprotein B100, thus classifying 3T1AM as a multitarget ligand. The physiological mode of action of TAMs is still controversial because regulation of endogenous TAM production and the sites of its biosynthesis are not fully elucidated. Methods for 3T1AM analytics need further validation, as they revealed different blood and tissue concentrations depending on detection principles used such as monoclonal antibody-based immunoassay vs liquid chromatography- matrix-assisted laser desorption/ionization mass spectrometry or time-of-flight mass spectrometry. In this review, we comprehensively summarize and critically evaluate current basic, translational, and clinical knowledge on 3T1AM and its main metabolite 3-iodothyroacetic acid, focusing on endocrine-relevant aspects and open but highly challenging issues.

Trace Amines and Their Receptors

Trace amines are endogenous compounds classically regarded as comprising β-phenylethyalmine, p-tyramine, tryptamine, p-octopamine, and some of their metabolites. They are also abundant in common foodstuffs and can be produced and degraded by the constitutive microbiota. The ability to use trace amines has arisen at least twice during evolution, with distinct receptor families present in invertebrates and vertebrates. The term "trace amine" was coined to reflect the low tissue levels in mammals; however, invertebrates have relatively high levels where they function like mammalian adrenergic systems, involved in "fight-or-flight" responses. Vertebrates express a family of receptors termed trace amine-associated receptors (TAARs). Humans possess six functional isoforms (TAAR1, TAAR2, TAAR5, TAAR6, TAAR8, and TAAR9), whereas some fish species express over 100. With the exception of TAAR1, TAARs are expressed in olfactory epithelium neurons, where they detect diverse ethological signals including predators, spoiled food, migratory cues, and pheromones. Outside the olfactory system, TAAR1 is the most thoroughly studied and has both central and peripheral roles. In the brain, TAAR1 acts as a rheostat of dopaminergic, glutamatergic, and serotonergic neurotransmission and has been identified as a novel therapeutic target for schizophrenia, depression, and addiction. In the periphery, TAAR1 regulates nutrient-induced hormone secretion, suggesting its potential as a novel therapeutic target for diabetes and obesity. TAAR1 may also regulate immune responses by regulating leukocyte differentiation and activation. This article provides a comprehensive review of the current state of knowledge of the evolution, physiologic functions, pharmacology, molecular mechanisms, and therapeutic potential of trace amines and their receptors in vertebrates and invertebrates.

Investigation of Naturally Occurring Single-Nucleotide Variants in Human TAAR1

Activation of trace amine-associated receptor 1 (TAAR1) in endocrine pancreas is involved in weight regulation and glucose homeostasis. The purpose of this study was the identification and characterization of potential TAAR1 variants in patients with overweight/obesity and disturbed glucose homeostasis. Screening for TAAR1 variants was performed in 314 obese or overweight patients with impaired insulin secretion. The detected variants were functionally characterized concerning TAAR1 cell surface expression and signaling properties and their allele frequencies were determined in the population-based Study of Health in Pomerania (SHIP). Three heterozygous carriers of the single nucleotide missense variants p.Arg23Cys (R23C, rs8192618), p.Ser49Leu (S49L, rs140960896), and p.Ille171Leu (I171L, rs200795344) were detected in the patient cohort. While p.Ser49Leu and p.Ille171Leu were found in obese/overweight patients with slightly impaired glucose homeostasis, p.Arg23Cys was identified in a patient with a complete loss of insulin production. Functional in vitro characterization revealed a like wild-type function for I171L, partial loss of function for S49L and a complete loss of function for R23C. The frequency of the R23C variant in 2018 non-diabetic control individuals aged 60 years and older in the general population-based SHIP cohort was lower than in the analyzed patient sample. Both variants are rare in the general population indicating a recent origin in the general gene pool and/or the consequence of pronounced purifying selection, in line with the obvious detrimental effect of the mutations. In conclusion, our study provides hints for the existence of naturally occurring TAAR1 variants with potential relevance for weight regulation and glucose homeostasis.

The impact of scopolamine pretreatment on 3-iodothyronamine (T1AM) effects on memory and pain in mice

We previously demonstrated that 3-iodothyronamine (T1AM), a by-product of thyroid hormone metabolism, pharmacologically administered to mice acutely stimulated learning and memory acquisition and provided hyperalgesia with a mechanism which remains to be defined. We now aimed to investigate whether the T1AM effect on memory and pain was maintained in mice pre-treated with scopolamine, a non-selective muscarinic antagonist expected to induce amnesia and, possibly, hyperalgesia. Mice were pre-treated with scopolamine and, after 20min, injected intracerebroventricularly (i.c.v.) with T1AM (0.13, 0.4, 1.32μg/kg). 15min after T1AM injection, the mice learning capacity or their pain threshold were evaluated by the light/dark box and by the hot plate test (51.5°C) respectively. Experiments in the light/dark box were repeated in mice receiving clorgyline (2.5mg/kg, i.p.), a monoamine oxidase (MAO) inhibitor administered 10min before scopolamine (0.3mg/kg). Our results demonstrated that 0.3mg/kg scopolamine induced amnesia without modifying the murine pain threshold. T1AM fully reversed scopolamine-induced amnesia and produced hyperalgesia at a dose as low as 0.13μg/kg. The T1AM anti-amnestic effect was lost in mice pre-treated with clorgyline. We report that the removal of muscarinic signalling increases T1AM pro learning and hyperalgesic effectiveness suggesting T1AM as a potential treatment as a "pro-drug" for memory dysfunction in neurodegenerative diseases.

Thyronamines and Derivatives: Physiological Relevance, Pharmacological Actions, and Future Research Directions

Thyronamines (3-T₁AM, T₀AM) are endogenous compounds probably derived from L-thyroxine or its intermediate metabolites. Combined activities of intestinal deiodinases and ornithine decarboxylase generate 3-T₁AM in vitro. Alternatively, 3-T₁AM might be formed by the thyroid gland and secreted into the blood. 3-T₁AM and T₀AM concentrations have been determined by liquid chromatography-tandem mass spectrometry analysis (LC-MS/MS) in tissues, serum, and cell lines. However, large variations of 3-T₁AM concentrations in human serum were reported by LC-MS/MS compared with a monoclonal antibody-based immunoassay. These differences might be caused by strong binding of the highly hydrophobic 3-T₁AM to apolipoprotein B100. Pharmacological administration of 3-T₁AM results in dose-dependent reversible effects on body temperature, cardiac function, energy metabolism, and neurological functions. The physiological relevance of these actions is unclear, but may occur at tissue concentrations close to the estimated endogenous concentrations of 3-T₁AM or its metabolites T₀AM or thyroacetic acid (TA₁). A number of putative receptors, binding sites, and cellular target molecules mediating actions of the multi-target ligand 3-T₁AM have been proposed. Among those are members of the trace amine associated receptor family, the adrenergic receptor ADRα2a, and the thermosensitive transient receptor potential melastatin 8 channel. Preclinical studies employing various animal experimental models are in progress, and more stable receptor-selective agonistic and antagonistic analogues of 3-T₁AM are now available for testing. The potent endogenous thyroid hormone-derived biogenic amine 3-T₁AM exerts marked cryogenic, metabolic, cardiac and central actions and represents a valuable lead compound linking endocrine, metabolic, and neuroscience research to advance development of new drugs.

The emerging roles of human trace amines and human trace amine-associated receptors (hTAARs) in central nervous system

Human trace amines (TAs) are endogenous compounds, previously almost ignored in human pathology for many reasons (difficulty of their measurement in biological fluids, unknown receptors for elusive amines), are now considered to play a significant role in synaptic transmission within the central nervous system (CNS) acting as neuromodulators. 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. Human trace amine receptors (including TAAR1 TAAR2 TAAR5 TAAR6 TAAR8 TAAR9) are expressed in the brain and play significant physiological and neuropathological roles by activation of trace amines. We herein discuss the recent findings that provide insights into the functional roles of human trace amines (including P-Octopamine, β phenylethylamine, Tryptamine, Tyramine, Synephrine, 3-Iodothyronamine, 3-Methoxytyramine, N-Methyltyramine, N-Methylphenethylamine) in brain. Furthermore, we discuss the known functions of human trace amine receptors in brain.

The Multitarget Ligand 3-Iodothyronamine Modulates β-Adrenergic Receptor 2 Signaling

BACKGROUND

3-Iodothyronamine (3-T1AM), a signaling molecule with structural similarities to thyroid hormones, induces numerous physiological responses including reversible body temperature decline. One target of 3-T1AM is the trace amine-associated receptor 1 (TAAR1), which is a member of the rhodopsin-like family of G protein-coupled receptors (GPCRs). Interestingly, the effects of 3-T1AM remain detectable in TAAR1 knockout mice, suggesting further targets for 3-T1AM such as adrenergic receptors. Therefore, we evaluated whether β-adrenergic receptor 1 (ADRB1) and 2 (ADRB2) signaling is affected by 3-T1AM in HEK293 cells and in human conjunctival epithelial cells (IOBA-NHC), where these receptors are highly expressed endogenously.

METHODS

A label-free EPIC system for prescreening the 3-T1AM-induced effects on ADRB1 and ADRB2 in transfected HEK293 cells was used. In addition, ADRB1 and ADRB2 activation was analyzed using a cyclic AMP assay and a MAPK reporter gene assay. Finally, fluorescence Ca(2+) imaging was utilized to delineate 3-T1AM-induced Ca(2+) signaling.

RESULTS

3-T1AM (10(-5)-10(-10)M) enhanced isoprenaline-induced ADRB2-mediated Gs signaling but not that of ADRB1-mediated signaling. MAPK signaling remained unaffected for both receptors. In IOBA-NHC cells, norepinephrine-induced Ca(2+) influxes were blocked by the nonselective ADRB blocker timolol (10 µM), indicating that ADRBs are most likely linked with Ca(2+) channels. Notably, timolol was also found to block 3-T1AM (10(-5)M)-induced Ca(2+) influx.

CONCLUSIONS

The presented data support that 3-T1AM directly modulates β-adrenergic receptor signaling. The relationship between 3-T1AM and β-adrenergic signaling also reveals a potential therapeutic value for suppressing Ca(2+) channel-mediated inflammation processes, occurring in eye diseases such as conjunctivitis.