Inverse Agonistic Action of 3-Iodothyronamine at the Human Trace Amine-Associated Receptor 5

Abstract

Objective Application of 3-iodothyronamine (3-T1AM) results in decreased body temperature and body weight in rodents. The trace amine-associated receptor (TAAR) 1, a family A G protein-coupled receptor, is a target of 3-T1AM. However, 3-T1AM effects still persist in mTaar1 knockout mice, which suggest so far unknown further receptor targets that are of physiological relevance. TAAR5 is a highly conserved TAAR subtype among mammals and we here tested TAAR5 as a potential 3-T1AM target. First, we investigated mouse Taar5 (mTaar5) expression in several brain regions of the mouse in comparison to mTaar1. Secondly, to unravel the full spectrum of signaling capacities, we examined the distinct Gs-, Gi/o-, G12/13-, Gq/11- and MAP kinase-mediated signaling pathways of mouse and human TAAR5 under ligand-independent conditions and after application of 3-T1AM. We found overlapping localization of mTaar1 and mTaar5 in the amygdala and ventromedial hypothalamus of the mouse brain. Second, the murine and human TAAR5 (hTAAR5) display significant basal activity in the Gq/11 pathway but show differences in the basal activity in Gs and MAP kinase signaling. In contrast to mTaar5, 3-T1AM application at hTAAR5 resulted in significant reduction in basal IP3 formation and MAP kinase signaling. In conclusion, our data suggest that the human TAAR5 is a target for 3-T1AM, exhibiting inhibitory effects on IP3 formation and MAP kinase signaling pathways, but does not mediate Gs signaling effects as observed for TAAR1. This study also indicates differences between TAAR5 orthologs with respect to their signaling profile. In consequence, 3-T1AM-mediated effects may differ between rodents and humans.

Trace Amines and Their Receptors

“…This has subsequently been verified by numerous laboratories (Mühlhaus et al, 2014;Cöster et al, 2015;Chiellini et al, 2017), with 3IT also shown to act as an agonist at TAAR2 (Babusyte et al, 2013;Cichero and Tonelli, 2017) and as an inverse agonist at TAAR5 (Dinter et al, 2015c). 3IT is promiscuous, however, and also interacts with high affinity at a 2 -adrenoceptors (Regard et al, 2007;Dinter et al, 2015b), b-adrenergic receptors (Meyer and Hesch, 1983;Kleinau et al, 2011;Dinter et al, 2015a), muscarinic acetylcholine receptors (Laurino et al, 2016), transient receptor potential cation channel subfamily M member 8 ion channels (Khajavi et al, 2015;Lucius et al, 2016), various monoamine and organic anion transporters (Snead et al, 2007;Panas et al, 2010), and molecular target(s) within mitochondria, including the F 1 -F 0 ATP synthase (Cumero et al, 2012) and possibly complex III (Venditti et al, 2011).…”

“…In the olfactory system, TAAR5 appears to be coexpressed with G olf and stimulates cAMP accumulation (Liberles and Buck, 2006). It has also been shown that human TAAR5 can couple to the G s cascade (Wallrabenstein et al, 2013), the G q/11 cascade (Dinter et al, 2015c), and G 12/13 -dependent mitogen-activated protein kinase pathways (Dinter et al, 2015c), suggesting that in different cell groups TAAR5 might demonstrate functional selectivity by coupling to different signaling modalities. Whether such an effect is due to receptor heterodimerization, as seen with other TAAR isoforms, with different partners present in different cell types, or due to ligand bias, requires systematic study.…”

“…3) and these compounds are often associated with metabolic routes that are distinct from those of the compounds traditionally called trace amines. Such compounds include the endogenous thyroid hormone metabolite 3-iodothyronamine (3IT) (Scanlan et al, 2004;Dinter et al, 2015c), the catecholamine neurotransmitter metabolites 3-methoxytyramine (3-MT) and normetanephrine (Bunzow et al, 2001;Sotnikova et al, 2010), trimethylamine (Ferrero et al, 2012;Wallrabenstein et al, 2013;Li et al, 2015), isoamylamine (Liberles and Buck, 2006;Ferrero et al, 2012), the polyamines putrescine and cadaverine (Hussain et al, 2013), and possibly agmatine, spermine, and spermidine (Saraiva et al, 2016). In addition, the N-methylated metabolites of both PEA and TYR, Nmethylphenylethylamine and N-methyltyramine, are also TAAR agonists , as is the N-methyl metabolite of TRP N,N-dimethyltryptamine (DMT); although in this latter instance, this shows a strong species dependence (Simmler et al, 2016).…”

3-iodothyronamine (T1AM), a novel antagonist of muscarinic receptors

“…Scanlan et al (2004) described T1AM as an high affinity ligand (in the nanomolar range of affinity) for the trace amine-associated receptor type 1 (TAAR1). Later, by virtue of its -phenylethyl skeleton, in-vitro studies indicated T1AM recognized, in the micromolar range of concentrations, monoaminergic targets including beta 2 and alpha 2 receptors (Regards et al, 2007) (Dinter et al, 2015), working as an agonist and as an inverse agonist respectively (Dinter et al, 2015a ) (Dinter et al, 2015b). More recently, Dinter et al (2015) reported T1AM as an inverse agonist of TAAR type 5 (TAAR5) (Dinter et al, 2015c).…”

“…Later, by virtue of its -phenylethyl skeleton, in-vitro studies indicated T1AM recognized, in the micromolar range of concentrations, monoaminergic targets including beta 2 and alpha 2 receptors (Regards et al, 2007) (Dinter et al, 2015), working as an agonist and as an inverse agonist respectively (Dinter et al, 2015a ) (Dinter et al, 2015b). More recently, Dinter et al (2015) reported T1AM as an inverse agonist of TAAR type 5 (TAAR5) (Dinter et al, 2015c). Further, recent electrophysiological evidence indicated T1AM (in the range of 0.1-10 M) as an activator of the TRPM8 cold channel (Lucius et al, 2016).…”