The actions of endogenous opioids and nociceptin/orphanin FQ are mediated by four homologous G protein-coupled receptors that constitute the opioid receptor family. However, little is known about opioid systems in cyclostomes (living jawless fish) and how opioid systems might have evolved from invertebrates. Here, we leveraged de novo transcriptome and low-coverage whole-genome assembly in the Pacific hagfish (Eptatretus stoutii) to identify and characterize the first full-length coding sequence for a functional opioid receptor in a cyclostome. Additionally, we define two novel endogenous opioid precursors in this species that predict several novel opioid peptides. Bioinformatic analysis shows no closely related opioid receptor genes in invertebrates with regard either to the genomic organization or to conserved opioid receptor-specific sequences that are common in all vertebrates. Furthermore, no proteins analogous to vertebrate opioid precursors could be identified by genomic searches despite previous claims of protein or RNA-derived sequences in several invertebrate species. The presence of an expressed orthologous receptor and opioid precursors in the Pacific hagfish confirms that a functional opioid system was likely present in the common ancestor of all extant vertebrates some 550 million years ago, earlier than all previous authenticated accounts. We discuss the premise that the cyclostome and vertebrate opioid systems evolved from invertebrate systems concerned with antimicrobial defense and speculate that the high concentrations of opioid precursors in tissues such as the testes, gut, and activated immune cells are key remnants of this evolutionary role.

中文翻译：

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太平洋盲鳗阿片系统的遗传和功能分析

内源性阿片类药物和伤害感受素/孤儿啡肽 FQ 的作用由构成阿片类受体家族的四种同源 G 蛋白偶联受体介导。然而，关于环口鱼（活的无颌鱼）中的阿片类药物系统以及阿片类药物系统是如何从无脊椎动物进化而来的知之甚少。在这里，我们利用了太平洋盲鳗 ( Eptatretus stoutii ) 中的从头转录组和低覆盖全基因组组装。) 以识别和表征 cyclostome 中功能性阿片受体的第一个全长编码序列。此外，我们在该物种中定义了两种新的内源性阿片样物质前体，可预测几种新的阿片样肽。生物信息学分析显示无脊椎动物中的阿片受体基因与基因组组织或所有脊椎动物中常见的保守阿片受体特异性序列没有密切相关。此外，尽管先前声称在几种无脊椎动物物种中存在蛋白质或 RNA 衍生序列，但基因组搜索无法鉴定出类似于脊椎动物阿片样物质前体的蛋白质。太平洋盲鳗中表达的直系同源受体和阿片样物质前体的存在证实，大约 5.5 亿年前，功能性阿片样物质系统可能存在于所有现存脊椎动物的共同祖先中，比以前所有经过验证的帐户都早。我们讨论了环状体和脊椎动物阿片系统从与抗菌防御有关的无脊椎动物系统进化而来的前提，并推测睾丸、肠道和激活的免疫细胞等组织中高浓度的阿片前体是这种进化作用的关键残余。