Department of Neuroscience, Functional Pharmacology, Uppsala University, BMC, Box 593, 751 24 Uppsala, Sweden.
Gene. 2013 Sep 10;526(2):122-33. doi: 10.1016/j.gene.2013.05.005. Epub 2013 May 15.
Saccoglossus kowalevskii (the acorn worm) is a hemichordate belonging to the superphylum of deuterostome bilateral animals. Hemichordates are sister group to echinoderms, and closely related to chordates. S. kowalevskii has chordate like morphological traits and serves as an important model organism, helping developmental biologists to understand the evolution of the central nervous system (CNS). Despite being such an important model organism, the signalling system repertoire of the largest family of integral transmembrane receptor proteins, G protein-coupled receptors (GPCRs) is largely unknown in S. kowalevskii. Here, we identified 260 unique GPCRs and classified as many as 257 of them into five main mammalian GPCR families; Glutamate (23), Rhodopsin (212), Adhesion (18), Frizzled (3) and Secretin (1). Despite having a diffuse nervous system, the acorn worm contains well conserved orthologues for human Adhesion and Glutamate family members, with a similar N-terminal domain architecture. This is particularly true for genes involved in CNS development and regulation in vertebrates. The average sequence identity between the GPCR orthologues in human and S. kowalevskii is around 47%, and this is same as observed in couple of the closest vertebrate relatives, Ciona intestinalis (41%) and Branchiostoma floridae (~47%). The Rhodopsin family has fewer members than vertebrates and lacks clear homologues for 6 of the 13 subgroups, including olfactory, chemokine, prostaglandin, purine, melanocyte concentrating hormone receptors and MAS-related receptors. However, the peptide and somatostatin binding receptors have expanded locally in the acorn worm. Overall, this study is the first large scale analysis of a major signalling gene superfamily in the hemichordate lineage. The establishment of orthologue relationships with genes involved in neurotransmission and development of the CNS in vertebrates provides a foundation for understanding the evolution of signal transduction and allows for further investigation of the hemichordate neurobiology.
海鞘（acorn worm）是半索动物门的后口动物，属于脊索动物超门的两侧对称动物。半索动物与棘皮动物是姐妹群，与脊索动物关系密切。海鞘具有脊索动物样的形态特征，是一种重要的模式生物，有助于发育生物学家理解中枢神经系统（CNS）的进化。尽管是如此重要的模式生物，但在海鞘中，G 蛋白偶联受体（GPCR）这一最大的整合跨膜受体蛋白家族的信号系统谱在很大程度上仍是未知的。在这里，我们鉴定了 260 个独特的 GPCR，并将多达 257 个分类为五个主要的哺乳动物 GPCR 家族；谷氨酸（23）、视紫红质（212）、黏附（18）、卷曲（3）和分泌素（1）。尽管海鞘具有弥散性神经系统，但它包含了与人类黏附素和谷氨酸家族成员高度保守的同源物，具有相似的 N 端结构域架构。这在涉及脊椎动物 CNS 发育和调节的基因中尤其如此。人类和海鞘 GPCR 同源物之间的平均序列同一性约为 47%，与最接近的两种脊椎动物近亲（文昌鱼和佛罗里达柱头虫）观察到的相同。视紫红质家族的成员比脊椎动物少，并且缺乏 13 个亚组中的 6 个明显同源物，包括嗅觉、趋化因子、前列腺素、嘌呤、黑素细胞浓缩激素受体和 MAS 相关受体。然而，肽和生长抑素结合受体在海鞘中局部扩张。总的来说，这项研究是对半索动物谱系中主要信号基因超家族的首次大规模分析。与脊椎动物 CNS 神经传递和发育相关基因建立同源关系，为理解信号转导的进化提供了基础，并允许进一步研究半索动物神经生物学。
