The Trace Amine-Associated Receptor 1 Agonist 3-Iodothyronamine Induces Biased Signaling at the Serotonin 1b Receptor.

Julia Bräunig,Michal Szczepek,Gunnar Kleinau,Sarah Paisdzior,Carolin S Höfig,Jens Mittag,Heike Biebermann,Juliane Dinter,Patrick Scheerer,Mark Rosowski

doi:10.3389/fphar.2018.00222

Abstract

Trace amine-associated receptors (TAARs) belong to the class A G-protein-coupled receptors (GPCR) and are evolutionary related to aminergic receptors. TAARs have been identified to mediate effects of trace amines. TAAR1 signaling is mainly mediated via activation of the Gs/adenylyl cyclase pathway. In addition to classical trace amines, TAAR1 can also be activated by the thyroid hormone derivative 3-iodothyronamine (3-T1AM). Pharmacological doses of 3-T1AM induced metabolic and anapyrexic effects, which might be centrally mediated in the hypothalamus in rodents. However, the observed anapyrexic effect of 3-T1AM persists in Taar1 knock-out mice which raises the question whether further GPCRs are potential targets for 3-T1AM and mediate the observed physiological effect. Anapyrexia has been observed to be related to action on aminergic receptors such as the serotonin receptor 1b (5-HT1b). This receptor primarily activates the Gi/o mediated pathway and PLC signaling through the Gβγ of Gi/o. Since the expression profiles of TAAR1 and 5-HT1b overlap, we questioned whether 3-T1AM may activate 5-HT1b. Finally, we also evaluated heteromerization between these two GPCRs and tested signaling under co-expressed conditions. In this study, we showed, that 3-T1AM can induce Gi/o signaling through 5-HT1b in a concentration of 10 μM. Strikingly, at 5-HT1b the ligand 3-T1AM only activates the Gi/o mediated reduction of cAMP accumulation, but not PLC activation. Co-stimulation of 5-HT1b by both ligands did not lead to additive or synergistic signaling effects. In addition, we confirmed the capacity for heteromerization between TAAR1 and 5-HT1b. Under co-expression of TAAR1 and HTR1b, 3-T1AM action is only mediated via TAAR1 and activation of 5-HT1b is abrogated. In conclusion, we found evidence for 5-HT1b as a new receptor target for 3-T1AM, albeit with a different signaling effect than the endogenous ligand. Altogether, this indicates a complex interrelation of signaling effects between the investigated GPCRs and respective ligands.

Highlights

The trace amine associated receptor 1 (TAAR1) belongs to the class A G-protein-coupled receptors (GPCR) and is evolutionary related to the aminergic receptors (Krautwurst, 2008)
To test whether 5-HT acts as a potential antagonist on TAAR1, costimulation of 3-T1AM and 5-HT in equimolar concentrations was carried out, which resulted in signaling identical to 3-T1AM stimulation alone, making an antagonistic activity of 5-HT at TAAR1 unlikely (Figure 1A)
Our aim in this study was to elucidate whether 5-HT1b is a potential 3-T1AM target

Summary

Introduction

The trace amine associated receptor 1 (TAAR1) belongs to the class A G-protein-coupled receptors (GPCR) and is evolutionary related to the aminergic receptors (Krautwurst, 2008). In addition to trace amines and synthetic ligands, the thyroid hormone derivative 3-iodothyronamine (3-T1AM) is another agonistic ligand for TAAR1 (Scanlan et al, 2004; Kleinau et al, 2011). 3-T1AM is a promiscuous ligand as it binds to other GPCRs such as TAAR5 (Dinter et al, 2015b), the β2adrenoreceptor (ADRB2) (Dinter et al, 2015a) and the α2Aadrenoreceptor (ADRA2A) (Dinter et al, 2015a). It should be of note that TAAR1 arose from gene duplication of serotonin receptors (Lindemann et al, 2005) and, as a consequence, comparing it with other aminergic receptors showed strong similarities in the ligand binding pocket (Kleinau et al, 2011)

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