Functional analysis of class I odorant receptors and their ligands

脊椎动物的嗅觉系统可以识别数百万种环境中的化学物质，并将它们视为气味。嗅觉感觉神经元是嗅觉系统中的主要感觉神经元，通过检测气味并与气味受体（ORs）相互作用来启动感知过程。ORs属于迄今为止所分析的任何基因组中最大的基因超家族。OR基因被分为两类。I类感受器，类鱼，在系统发育上更古老的感受器与水气味分子结合。另一种类型被称为II类，是陆生动物特有的，似乎与挥发性气味分子结合在一起。虽然陆生哺乳动物的I类逆转录酶一直被认为是鱼类向陆生动物进化的遗迹，但最近的研究表明，陆生哺乳动物的I类逆转录酶仍保留着完整的功能基因，这表明I类逆转录酶可能具有重要的生理功能。在本研究中，我们研究了陆生哺乳动物中I类Ors识别水相气味分子的功能。我们把重点放在羊水（AF）作为水气味剂的来源。房颤是子宫内保护胚胎的母体液体，这是陆地哺乳动物唯一的水生环境。我们通过小鼠行为分析、组织化学和生理研究证明AF含有一些水相气味分子。此外，我们建立了能够可视化I类OR转基因表达和OSN轴突投影的转基因系，为I类OR基因的顺式作用元件提供了第一个实验证据。该转基因小鼠不仅可以作为筛选I类OR配体的有效工具，也可以作为研究I类OR基因调控的有效工具。

The vertebrate olfactory system recognizes millions of environmental chemicals and perceives them as odors. Olfactory sensory neurons, the primary sensory neurons in the olfactory system, initiate the perception process by detecting odorants, which interact with odorant receptors (ORs). ORs belong to the largest gene superfamily present in any genome analyzed so far. OR genes are classified into two classes. Class I ORs, which are fish-like and phylogenetically more ancient receptors bind aqueous odorant molecules. The other type is called class II ORs, which are specific to terrestrial animals seem to bind volatile odorant molecules. Although class I ORs in terrestrial mammals had been considered as a relic of evolution from fish to terrestrial, recent studies have demonstrated that intact and functional genes still remain, suggesting that class I Ors in terrestrial mammals might have important physiological functions. In this study, we investigated function of class I Ors in terrestrial mammals to identify aqueous odorous molecules. We focused on amniotic fluid (AF) as source for aqueous odorants. AF is maternal fluid in uterus protecting embryo that is the only aquatic environment for terrestrial mammals. We demonstrated that AF contains some aqueous odor molecules through mouse behavior analyses, histochemical and physiological studies. In addition, we established transgenic lines which enable to visualize class I OR transgene expression and OSN axonal projections, providing the first experimental evidence of a cis-acting element for a class I OR gene. This transgenic mice can be useful tool not only for screening ligands of class I OR but also for study on class I OR gene regulation.