Abstract
Olfactory receptor (OR)-associated events are mediated by well-conserved components in the olfactory epithelium, including olfactory G-protein (Golf), adenylate cyclase III (ACIII), and olfactory marker protein (OMP). The expression of ORs has recently been observed in non-olfactory tissues where they are involved in monitoring extracellular chemical cues. The large number of OR genes and their sequence similarities illustrate the need to find an effective and simple way to detect non-olfactory OR-associated events. In addition, expression profiles and physiological functions of ORs in non-olfactory tissues are largely unknown. To overcome limitations associated with using OR as a target protein, this study used OMP with Golf and ACIII as targets to screen for potential OR-mediated sensing systems in non-olfactory tissues. Here, we show using western blotting, real-time PCR, and single as well as double immunoassays that ORs and OR-associated proteins are co-expressed in diverse tissues. The results of immunohistochemical analyses showed OMP (+) cells in mouse heart and in the following cells using the corresponding marker proteins c-kit, keratin 14, calcitonin, and GFAP in mouse tissues: interstitial cells of Cajal of the bladder, medullary thymic epithelial cells of the thymus, parafollicular cells of the thyroid, and Leydig cells of the testis. The expression of ORs in OMP (+) tissues was analyzed using a refined microarray analysis and validated with RT-PCR and real-time PCR. Three ORs (olfr544, olfr558, and olfr1386) were expressed in the OMP (+) cells of the bladder and thyroid as shown using a co-immunostaining method. Together, these results suggest that OMP is involved in the OR-mediated signal transduction cascade with olfactory canonical signaling components between the nervous and endocrine systems. The results further demonstrate that OMP immunohistochemical analysis is a useful tool for identifying expression of ORs, suggesting OMP expression is an indicator of potential OR-mediated chemoreception in non-olfactory systems.
Highlights
Chemoreception is an ancient and evolutionarily pivotal physiological system that deciphers both the identity and intensity of distinct environmental stimuli
The question of whether olfactory marker protein (OMP) is expressed with olfactory receptor (OR) in non-olfactory tissues is interesting because ORs are expressed in many tissues, and OMP is widely accepted as a marker of mature Olfactory receptor neurons (ORNs) and associated with an OR signal transduction cascade
We performed immunological experiments using a double immunoassay technique to support the results of the mRNA studies and determine whether the OMP is expressed at the protein level
Summary
Chemoreception is an ancient and evolutionarily pivotal physiological system that deciphers both the identity and intensity of distinct environmental stimuli. This system has evolved to maximize the sensitivity and discriminatory capabilities of an organism. Olfactory chemoreception depends on a large multigene family of olfactory specific G-protein coupled receptors (GPCRs) that were initially identified in the rat and are referred to as the OR family [12]. Approximately 400 OR genes in human and more than 1000 genes in rodents have been identified, with extensive sequence diversity within their transmembrane domains (homology of 40–90%) [12,13,14]
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