Olfactory Receptor Antagonism in the OR-I7 Aldehyde Receptor and the Mammalian Olfactory System
The detection of smell is initiated when an odorant binds and activates olfactory receptors (ORs) within the nose. Chapter 1 studies the structural requirements for activation of the OR-I7 aldehyde receptor. Octanal is an agonist of the OR-I7 receptor. 4-Ethylcyclohexylacetaldehyde (1.1), with a cyclohexyl ring that locks the C4-C5 bond of octanal in the gauche conformation, was previously found to have higher potency than octanal, suggesting that the OR-I7 receptor prefers the gauche conformation of octanal during activation. However, 1.1 had only been previously tested as a mixture of cis- and trans-isomers, and therefore, it was not possible to determine whether the response of 1.1 was stimulated by either or both of the isomers. In order to distinguish the response of each isomer, the cis and trans compounds were individually synthesized and tested on olfactory sensory neurons (OSNs) that co-express the OR-I7 receptor via calcium imaging. Through this study, we gained insight into the preferred conformation of the C6-C7 bond of octanal for activation of the OR-I7 receptor. Removing the ethyl group of 4-ethylcyclohexylacetaldehyde (1.1) to form cyclohexylacetaldehyde (2.2), transforms the molecule from an activator to an inhibitor of OR-I7. Chapter 2 focuses on antagonism of the rodent OR-I7 aldehyde receptor. Besides 2.2, the response of octanal can be antagonized by several short aliphatic linear and cyclic aldehydes at micromolar concentrations. It was proposed that these antagonists compete with octanal for the binding site via their aldehyde moiety, but fail to activate the receptor due to their shorter chain lengths. The strength of antagonism for this receptor appeared to correlate with the number of aliphatic carbons within 7 Å of the aldehyde when it was modeled in the most extended conformation, but the data set was too small to firmly support this. To further test the possibility, a structure-activity relationship (SAR) study was carried out in which additional carbons were systematically substituted onto the structures of OR-I7 antagonists without exceeding 7 Å. The synthesized analogues were tested for their ability to inhibit activation by octanal in OR-I7 receptors expressed in HEK293 cells using a cyclic AMP detection assay. This work led to more precise information on what makes a good OR-I7 antagonist. Currently, there is limited data on the determinants of antagonism, but several olfactory receptors, such as OR-I7 and mOR-EG, have been reported to be antagonized by odorants which are structurally similar to the agonist(s) of the receptor. Structural similarities may point to similar, orthosteric binding sites within the receptor, as opposed to allosteric binding sites. Chapter 3 looks at determinants of antagonism in the mammalian olfactory system as a whole. For the OR-I7 receptor, the main difference between the structure of the agonist, octanal, and the antagonists is their chain lengths, whereas the agonist of the mOR-EG receptor, eugenol, differs from the antagonists by the functional group on the phenyl ring and the position of the double bond. For these two receptors, these structural variants may be directing the shift from agonist to antagonist. A study was devised to look at the extent of OR antagonism that can be stimulated by pairs of structurally similar odorants, differing only in their carbon chain length or their oxygen containing functional group. The response of endogenous ORs to these odorants was acquired in live olfactory sensory neurons via calcium imaging. The response pattern of over 11,000 cells was analyzed. Based on this study, both the chain type and functional group type can be the determinant of OR antagonism, but the carboxyl group inhibited the highest percentage of cells out of the three functional groups tested, and cells activated by aldehydes were the most susceptible to antagonism.
Chemistry, Organic Chemistry, OR-I7, aldehyde, odorant, olfactory receptor, antagonism
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