Institute of Genetics, University of Cologne, 50674 Cologne, Germany.
Proc Natl Acad Sci U S A. 2013 May 7;110(19):7714-9. doi: 10.1073/pnas.1302088110. Epub 2013 Apr 23.
Mammalian olfactory receptor families are segregated into different olfactory organs, with type 2 vomeronasal receptor (v2r) genes expressed in a basal layer of the vomeronasal epithelium. In contrast, teleost fish v2r genes are intermingled with all other olfactory receptor genes in a single sensory surface. We report here that, strikingly different from both lineages, the v2r gene family of the amphibian Xenopus laevis is expressed in the main olfactory as well as the vomeronasal epithelium. Interestingly, late diverging v2r genes are expressed exclusively in the vomeronasal epithelium, whereas "ancestral" v2r genes, including the single member of v2r family C, are restricted to the main olfactory epithelium. Moreover, within the main olfactory epithelium, v2r genes are expressed in a basal zone, partially overlapping, but clearly distinct from an apical zone of olfactory marker protein and odorant receptor-expressing cells. These zones are also apparent in the spatial distribution of odor responses, enabling a tentative assignment of odor responses to olfactory receptor gene families. Responses to alcohols, aldehydes, and ketones show an apical localization, consistent with being mediated by odorant receptors, whereas amino acid responses overlap extensively with the basal v2r-expressing zone. The unique bimodal v2r expression pattern in main and accessory olfactory system of amphibians presents an excellent opportunity to study the transition of v2r gene expression during evolution of higher vertebrates.
哺乳动物嗅觉受体家族分为不同的嗅觉器官，其中 2 型犁鼻器受体 (v2r) 基因在犁鼻器上皮的基底层表达。相比之下，硬骨鱼的 v2r 基因与所有其他嗅觉受体基因混合在单个感觉表面中。我们在这里报告的是，与这两个谱系截然不同的是，两栖动物非洲爪蟾的 v2r 基因家族在主嗅觉上皮和犁鼻器上皮中都有表达。有趣的是，晚期分化的 v2r 基因仅在犁鼻器上皮中表达，而“祖先”的 v2r 基因，包括 v2r 家族 C 的单一成员，则局限于主嗅觉上皮。此外，在主嗅觉上皮中，v2r 基因在基底区域表达，与嗅觉标记蛋白和气味受体表达细胞的顶端区域部分重叠，但明显不同。这些区域在嗅觉反应的空间分布中也很明显，使得可以尝试将嗅觉反应分配给嗅觉受体基因家族。对醇、醛和酮的反应呈顶端定位，与气味受体介导一致，而氨基酸反应与基底 v2r 表达区域广泛重叠。两栖动物主嗅觉和辅助嗅觉系统中独特的双模态 v2r 表达模式为研究高等脊椎动物进化过程中 v2r 基因表达的转变提供了极好的机会。
