Section for Cell and Neurobiology, Department of Biology, University of Copenhagen, Universitetsparken 15, DK-2100, Copenhagen, Denmark.
BMC Genomics. 2020 Sep 29;21(1):666. doi: 10.1186/s12864-020-06945-9.
Nervous systems originated before the split of Proto- and Deuterostomia, more than 600 million years ago. Four animal phyla (Cnidaria, Placozoa, Ctenophora, Porifera) diverged before this split and studying these phyla could give us important information on the evolution of the nervous system. Here, we have annotated the neuropeptide preprohormone genes of twenty species belonging to the subclass Hexacorallia or Ceriantharia (Anthozoa: Cnidaria), using thirty-seven publicly accessible genome or transcriptome databases. Studying hexacorals is important, because they are versatile laboratory models for development (e.g., Nematostella vectensis) and symbiosis (e.g., Exaiptasia diaphana) and also are prominent reef-builders.
We found that each hexacoral or ceriantharian species contains five to ten neuropeptide preprohormone genes. Many of these preprohormones contain multiple copies of immature neuropeptides, which can be up to 50 copies of identical or similar neuropeptide sequences. We also discovered preprohormones that only contained one neuropeptide sequence positioned directly after the signal sequence. Examples of them are neuropeptides that terminate with the sequence RWamide (the Antho-RWamides). Most neuropeptide sequences are N-terminally protected by pyroglutamyl (pQ) or one or more prolyl residues, while they are C-terminally protected by an amide group. Previously, we isolated and sequenced small neuropeptides from hexacorals that were N-terminally protected by an unusual L-3-phenyllactyl group. In our current analysis, we found that these N-phenyllactyl-peptides are derived from N-phenylalanyl-peptides located directly after the signal sequence of the preprohormone. The N-phenyllactyl- peptides appear to be confined to the hexacorallian order Actiniaria and do not occur in other cnidarians. On the other hand, (1) the neuropeptide Antho-RFamide (pQGRFamide); (2) peptides with the C-terminal sequence GLWamide; and (3) tetrapeptides with the XPRXamide consensus sequence (most frequently GPRGamide) are ubiquitous in Hexacorallia.
We found GRFamide, GLWamide, and XPRXamide peptides in all tested Hexacorallia. Previously, we discovered these three neuropeptide classes also in Cubozoa, Scyphozoa, and Staurozoa, indicating that these neuropeptides originated in the common cnidarian ancestor and are evolutionarily ancient. In addition to these ubiquitous neuropeptides, other neuropeptides appear to be confined to specific cnidarian orders or subclasses.
神经系统起源于原口动物和后口动物分裂之前，距今超过 6 亿年。有四个动物门（刺胞动物门、扁盘动物门、栉水母动物门、多孔动物门）在这一分化之前就已经分化，研究这些门可以为我们提供有关神经系统进化的重要信息。在这里，我们使用三十七个公开可访问的基因组或转录组数据库，对属于六放珊瑚亚纲或钙质珊瑚纲（珊瑚：刺胞动物门）的二十个物种的神经肽前激素基因进行了注释。研究六放珊瑚很重要，因为它们是发育（例如，海葵 Nematostella vectensis）和共生（例如，Exaiptasia diaphana）的多功能实验室模型，也是突出的造礁者。
我们发现每个六放珊瑚或钙质珊瑚物种都含有五到十个神经肽前激素基因。其中许多前激素都包含多个不成熟的神经肽副本，这些副本可以多达 50 个相同或相似的神经肽序列。我们还发现了只包含一个神经肽序列的前激素，该序列直接位于信号序列之后。它们的例子是末端为 RWamide 序列的神经肽（Antho-RWamides）。大多数神经肽序列在 N 端被焦谷氨酸（pQ）或一个或多个脯氨酸残基保护，而在 C 端被酰胺基保护。以前，我们从六放珊瑚中分离并测序了小神经肽，这些小神经肽的 N 端被不寻常的 L-3-苯乙酰基保护。在我们目前的分析中，我们发现这些 N-苯乙酰肽是由直接位于前激素信号序列后的 N-苯丙氨酰肽衍生而来。N-苯乙酰肽似乎仅限于六放珊瑚目 Actiniaria，而不在其他刺胞动物中出现。另一方面，（1）神经肽 Antho-RFamide（pQGRFamide）；（2）具有 C 末端序列 GLWamide 的肽；和（3）具有 XPRXamide 共识序列（最常见的是 GPRGamide）的四肽在六放珊瑚中普遍存在。
我们在所有测试的六放珊瑚中都发现了 GRFamide、GLWamide 和 XPRXamide 肽。以前，我们在立方水母纲、钵水母纲和十字水母纲中也发现了这三种神经肽类，表明这些神经肽起源于共同的刺胞动物祖先，是古老的进化产物。除了这些普遍存在的神经肽外，其他神经肽似乎仅限于特定的刺胞动物目或亚纲。
