Abstract
In addition to the canonical olfactory receptors, TAARs were currently suggested to be a second class of chemosensory receptors in the olfactory epithelium of vertebrates. In contrast to several deorphanized murine TAARs, agonists for the intact human TAAR genes 2, 5, 6, 8 and 9 that are potentially expressed in the human olfactory epithelium have not been determined so far. Moreover, the physiological relevance of TAARs still remains elusive. We present the first successful functional expression of a human TAAR and agonists of human TAAR5. We performed a ligand screening using recombinantly expressed human TAAR5 in HANA3A cells and Xenopus laevis oocytes. In order to measure receptor activity, we used a cAMP-dependent reporter gene assay and two-electrode voltage clamp technique. As a result, human TAAR5 can be activated in a concentration-dependent manner by trimethylamine and with less efficacy by dimethylethylamine. It could neither be activated by any other of the tested single amines with a related chemical structure (42 in total), nor by any of the tested odorant mixtures. The hypothesis that Single Nucleotide Polymorphisms (SNP) within the reading frame of an olfactory receptor gene can cause a specific anosmia, formed the basis for clarifying the question, if anosmia for trimethylamine is caused by a SNP in a TAAR coding sequence. All functional human TAAR gene reading frames of subjects with specific anosmia for trimethylamine were amplified and products analyzed regarding SNP distribution. We demonstrated that the observed specific anosmia for trimethylamine is not correlated with a SNP in the coding sequence of one of the putatively functional human TAAR genes.
Highlights
Trace amine-associated receptors (TAAR) belong to the family of G-protein coupled receptors (GPCR) whose first deorphanized member TAAR1, responds to biogenic trace amines like ßphenylethylamine, p-tyramine or octopamine
Presence of the expressed receptor protein was tested by immunocytochemical detection of the extracellular Nterminal rho-epitope tag in fixed HANA3A cells (Fig. 1) and by live-cell staining on the surface of HANA3A cells (Figure S1)
Tested repertoire (Materials and methods) first based on the volatile amines known as agonist for the murine ‘‘olfactory TAARs’’ [2] and trace amine ligands for h/mTAAR1 [1,15]
Summary
Trace amine-associated receptors (TAAR) belong to the family of G-protein coupled receptors (GPCR) whose first deorphanized member TAAR1, responds to biogenic trace amines like ßphenylethylamine, p-tyramine or octopamine. Human and murine TAAR1 (h/mTAAR1) are expressed in a variety of tissues including brain, stomach, kidney, lung and intestine, but not in the olfactory epithelium (OE) [1]. TAARs have been identified as olfactory receptors (ORs) in vertebrates, because recombinantly expressed ‘‘olfactory TAARs’’ respond to volatile amines, amongst others N-methylpiperidine (mTAAR7f), trimethylamine (TMA) (mTAAR5) and isoamylamine (mTAAR3) [2,3,4]. The mTAAR agonists TMA and isoamylamine are enriched in male mouse urine and may act as murine pheromones [7,8]. The mTAAR4 agonist ß-phenylethylamine acts as kairomone in the chemical detection of carnivore odor by prey and is related to stress response in both rodents and humans [5,9,10]. TAARs have been suggested to be involved in the detection of social cues [2,4,5]
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