Functional analysis of class I odorant receptors and their ligands

脊椎动物的嗅觉系统识别数百万种环境化学物质，并将它们感知为气味。嗅觉感觉神经元是嗅觉系统中的主要感觉神经元，通过检测气味而启动感知过程，并与气味受体(OR)相互作用。ORs属于迄今为止所分析的所有基因组中存在的最大的基因超家族。或者基因被分成两类。I类受体与鱼类相似，在系统发育上更古老，它与水性气味分子结合在一起。另一种类型被称为第二类ORs，它是陆地动物特有的，似乎能结合挥发性气味分子。虽然陆生哺乳动物的I类ORS被认为是鱼类进化到陆地的遗迹，但最近的研究表明，完整的和功能基因仍然存在，这表明陆地哺乳动物的I类ORS可能具有重要的生理功能。在这项研究中，我们研究了陆生哺乳动物中I类ORS识别含水气味分子的功能。我们把重点放在羊水(AF)作为房水气味的来源。AF是保护胚胎的母体液，是陆生哺乳动物唯一的水生环境。我们通过对小鼠的行为分析、组织化学和生理研究，证明了房颤中含有一些含水的气味分子。此外，我们建立了能够可视化I类OR转基因表达和OSN轴突投射的转基因系，为I类OR基因具有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用于筛选I类OR配体，而且可用于I类OR基因调控的研究。

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.