Trace Amine-Associated Receptor 1 Localization at the Apical Plasma Membrane Domain of Fisher Rat Thyroid Epithelial Cells Is Confined to Cilia

Abstract

Background: The trace amine-associated receptor 1 (Taar1) is one member of the Taar family of G-protein-coupled receptors (GPCR) accepting various biogenic amines as ligands. It has been proposed that Taar1 mediates rapid, membrane-initiated effects of thyronamines, the endogenous decarboxylated and deiodinated relatives of the classical thyroid hormones T₄ and T₃. Objectives: Although the physiological actions of thyronamines in general and 3-iodothyronamine (T₁AM) in particular are incompletely understood, studies published to date suggest that synthetic T₁AM-activated Taar1 signaling antagonizes thyromimetic effects exerted by T₃. However, the location of Taar1 is currently unknown. Methods: To fill this gap in our knowledge we employed immunofluorescence microscopy and a polyclonal antibody to detect Taar1 protein expression in thyroid tissue from Fisher rats, wild-type and taar1-deficient mice, and in the polarized FRT cells. Results: With this approach we found that Taar1 is expressed in the membranes of subcellular compartments of the secretory pathway and on the apical plasma membrane of FRT cells. Three-dimensional analyses further revealed Taar1 immunoreactivity in cilial extensions of postconfluent FRT cell cultures that had formed follicle-like structures. Conclusions: The results suggest Taar1 transport along the secretory pathway and its accumulation in the primary cilium of thyrocytes. These findings are of significance considering the increasing interest in the role of cilia in harboring functional GPCR. We hypothesize that thyronamines can reach and activate Taar1 in thyroid follicular epithelia by acting from within the thyroid follicle lumen, their potential site of synthesis, as part of a nonclassical mechanism of thyroid autoregulation.

Trace Amines and Their Receptors

“…TAAR1 presence in thyroid gland epithelial cells has also been suggested (Szumska et al, 2015), although, again, validation of these findings is required. In contrast to earlier reports of TAAR1 presence in adipose tissue, blood vessels, heart, kidney, liver, lung, and testes (Borowsky et al, 2001;Chiellini et al, 2007Chiellini et al, , 2012Regard et al, 2008;Fehler et al, 2010), more recent studies with more robust protocols and reagents have failed to provide replication of the presence of TAAR1 (Revel et al, 2013;Raab et al, 2015).…”

“…Somewhat surprisingly for a GPCR, when TAAR1 protein has been identified, it has almost always been reported to exhibit a predominantly intracellular localization (Bunzow et al, 2001;Miller et al, 2005;Revel et al, 2013;Raab et al, 2015;Pei et al, 2016), possibly due to the absence of one or more N-terminal glycosylation sites (Barak et al, 2008). Intracellular TAAR1 may be associated with membrane structures (Xie et al, 2008a;Szumska et al, 2015), although the identity of these is unknown. Although reagents appropriate for ultrastructural studies await development, there is good evidence that TAAR1 is present both pre- (Bradaia et al, 2009;Revel et al, 2011;Leo et al, 2014) and postsynaptically within neurons (Espinoza et al, 2015a).…”

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

“…In fact, Chiellini et al (2005), reported findings which argued against the involvement of TAAR1 in T1AM-induced reduction of body temperature. However, the relationship between TAAR1 and T1AM would merit to be further explored in light of the findings from Szumska et al (2015). These authors reported TAAR1 expression on thyroid follicles, a localization supporting their role, and of T1AM, its endogenous ligand, in thyroid regulation.…”

“…Muscarinic receptors are expressed at the central nervous system, where they have a main role in memory and pain circuits, and in the periphery were they are involved in smooth muscle contraction, in exocrine and endocrine gland secretion, including the thyroid (Szumska et al, 2015) and the pancreas (Nakajima et al, 2013). The recognition of muscarinic receptors by T1AM might rationally sustain some pharmacological effects of the amine and open new perspectives for investigating unexplored T1AM functions governed by muscarinic signaling, including the control of visceral smooth muscle tone.…”

“TAARgeting Addiction”—The Alamo Bears Witness to Another Revolution

“…Functional assays, immunocytochemical detections and biotinylation experiments all suggest that the majority of the receptor protein remains associated with membranes in the intracellular milieu (Borowsky et al, 2001; Xie et al, 2007, 2008; Szumska et al, 2015). The biological significance of this cellular distribution remains enigmatic.…”