Deorphanization and characterization of sea lamprey olfactory trace amine-associated receptors

The sense of smell plays an important role in mediating diverse behaviors in the animal kingdom. Odor detection in the sea lamprey is mediated by a limited number of odorant receptors (ORs) and trace amine-associated receptors (TAARs). Upon binding with odorants, the receptors are activated and subsequently activate the downstream neuronal signaling cascade that transforms the chemical information into electrophysiological signals. Odorous biogenic amines, when enriched in biological excretions, stimulate TAARs of the main olfactory epithelium and evoke innate behaviors in animals. I hypothesized that these biogenic amines are potent ligands for lamprey TAARs, and characterized the structural basis for amine recognition in these receptors. Chapter 1 describes discovery that spermine, an odorous polyamine in semen, serves as a sex pheromone in sea lamprey. Spermine potently stimulates the lamprey olfactory system, activates TAAR348 receptor, and attracts ovulated females. A novel antagonist to this receptor inhibits olfactory and female behavioral responses to spermine. This discovery elucidates a mechanism that male animals recruit mates through the release of chemical cues in ejaculates. In chapter 2, I demonstrated that two clades of independently evolved TAARs, represented by sea lamprey TAAR365 (sTAAR365) and mouse TAAR9 (mTAAR9), share a similar response profile. The results suggest a conserved mechanism whereby independently evolved TAAR receptors utilize convergent structural bases to detect various biogenic polyamines. In chapter 3, I found that a cadaverine-responsive sea lamprey TAAR receptor, TAAR346a, exhibits high basal activity when heterologously expressed in HEK293T cells. Triethylamine serves as an inverse agonist for TAAR346a that can specifically attenuate its high basal activity. These data support a model in which the inverse agonist recognizes only one of the two orthosteric sites used by the agonist as it elicits its inhibitory effect on the basal activity of the receptor. Further evidence was provided to highlight the importance of interhelical interactions in modulating ligand-independent activation of TAAR346a. Thus, this thesis contributes to a better understanding of sea lamprey olfaction and the structural basis of TAARs for amine recognition in vertebrate animals.