Characterizing molecular recognition principles of specific odorant molecule interactions with an olfactory receptor and a nuclear hormone receptor

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Olfaction, the detection of odorous chemicals in the environment, is one of the oldest mammalian sensory systems and involves large number (up to 1000) of distinct G protein coupled olfactory receptors (OR). The chemical interaction of volatile molecules with specific ORs marks the primary step in odor perception and is of critical importance for elucidating the molecular basis of detecting and discriminating thousands of odorous chemicals. Despite the fact that a number of putative ORs have been cloned, only limited progress has been made in assigning odorant molecules to specific receptors. Furthermore, the analysis of OR–ligand interactions at a cellular level is complicated by combinatorial recognition principles and thus requires large scale odorant and receptor screening to establish receptor-specific ligand profiles. Described herein are efforts towards the development of reliable and efficient assays for detecting and quantifying the responses of heterologously expressed ORs to odorants. Three strategies were employed for the optical or electrophysiological detection of ORs responses with functional assays in HEK293 cells. In two approaches, OR activation was monitored via the endogenous cAMP pathway, which was coupled either to the cAMP response element-mediated expression of different reporter genes, or to a cyclic nucleotide gated ion channel. In a third approach, OR activation was measured using Ca2+ imaging in the presence of a promiscuous Gαq protein. ORs of different species were then functionally analyzed using the different assays. A library of 250 odorant compounds representative of the chemical functional groups relevant in perfumery was screened; 16 novel specific ligands for the mouse eugenol odorant receptor (EGOR) were identified. Ten of these compounds are structurally unrelated to previously identified ligands and thus provide an extended molecular basis for describing the ligand-specificity and selectivity of EGOR in new detail. Interestingly, some ORs exhibited odorant responses in the cAMP-based assays, but not in Ca2+ imaging assays and vice-versa, which indicates a possible preferential OR coupling to specific G proteins. Such preferential coupling might be dependent on the cell type or influenced by specific odorant molecules and could potentially increase the complexity in odorantmediated signaling. A second part of this thesis describes investigations into other potential activities of odorants apart from their role in olfaction. An odorant compound library was screened to determine possible xenoestrogenic activity on human ERα expressed in cultured mammalian cells. An image-based assay to observe the nuclear redistribution processes of a functional YFP-ERα fluorescent chimera allowed the identification of seven odorant molecules that interact with the receptor. As the optical detection of YFP-ERα redistribution processes did not distinguish between agonistic and antagonistic effects, we tested the active substances in a reporter assay using luciferase under transcription control of two estrogen response elements (ERE). Two active odorants identified in the screening exhibited agonist activity as indicated by a dose-dependent activation of ERα mediated transcription of the luciferase reporter gene.