Chapter 22. Expression Patterns of Olfactory Receptors
Introduction
Analyze the Data 22-1: Expression Patterns of Olfactory Receptors
Olfaction occurs when volatile compounds bind to specific odorant receptors. In mammals, each olfactory receptor neuron in the olfactory nasal epithelium expresses a single type of odorant receptor. These odorant receptors constitute a large multigene family (>1000 members) of related proteins. Binding of odorant induces a signaling cascade that is mediated via a G protein, Gαolf. Recent studies suggest that there are a small number of olfactory sensory neurons in the nasal epithelium that express members of the trace-amine–associated receptor (TAAR) family, chemoreceptors that are G protein–coupled receptors (GPCRs) but are unrelated to classical odorant receptors (see Liberles and Buck, 2006, Nature 442:645–650). The mouse genome encodes 15 TAAR genes while the human genome encodes 6.
a. In order to examine the expression pattern of different TAARs in the olfactory nasal epithelium, researchers localized TAAR RNA by in situ hybridization in pairwise combinations. All possible pairwise combinations of the 15 mouse TAARs were examined. A typical example of the results obtained is shown in the top set of panels in the figure below, in which TAAR6 and TAAR7 have been localized with fluorescent probes in mouse nasal epithelium. The TAAR6 probe was labeled with a green fluor, the TAAR7 probe with a red fluor. The lower set of panels shows localization of mouse odorant receptor 28 (MOR28; green), a classical odorant receptor, and TAAR6 (red). Each stained patch in the images is the staining pattern of an individual olfactory neuron. The “merge” panels show the two other images superimposed. What do these data suggest about expression patterns of the TAARs?
TAAR6 and TAAR7 are expressed in different neurons. If this expression pattern is observed with each TAAR member, then each olfactory neuron produces only a single member of the TAAR family, just as each olfactory neuron produces only one of the classical odorant receptors. In addition, the bottom panel of images in the figure above suggests that the cells expressing TAAR are not the same cells that express classical odorant receptors. Thus, neurons in the nasal epithelium express a single type of olfactory receptor, be it an odorant receptor or a TAAR.
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