Kawauchi Hiroshi, Sower Stacia A
Laboratory of Molecular Endocrinology, School of Fisheries Sciences, Kitasato University, Sanriku, Iwate 022-0101, Japan.
Gen Comp Endocrinol. 2006 Aug;148(1):3-14. doi: 10.1016/j.ygcen.2005.10.011. Epub 2005 Dec 13.
The adenohypophysial hormones have been believed to have evolved from several ancestral genes by duplication followed by evolutionary divergence. To understand the origin and evolution of the endocrine systems in vertebrates, we have characterized adenohypophysial hormones in an agnathan, the sea lamprey Petromyzon marinus. In gnathostomes, adrenocorticotropin (ACTH) and melanotropin (MSH) together with beta-endorphins (beta-END) are encoded in a single gene, designated as proopiomelanocortin (POMC), however in sea lamprey, ACTH and MSH are encoded in two distinct genes, proopoicortin (POC) gene and proopiomelanotropin (POM) gene, respectively. The POC and POM genes are expressed specifically in the rostral pars distalis (RPD) and the pars intermedia (PI), respectively. Consequently, the final products from both tissues are the same in all vertebrates, i.e., ACTH from the PD and MSH from the PI. The POMC gene might have been established in the early stages of invertebrate evolution by internal gene duplication of the MSH domains. The ancestral gene might be then inherited in lobe-finned fish and tetrapods, while internal duplication and deletion of MSH domains as well as duplication of whole POMC gene took place in lamprey and gnathostome fish. Sea lamprey growth hormone (GH) is expressed in the cells of the dorsal half of the proximal pars distalis (PPD) and stimulates the expression of an insulin-like growth factor (IGF) gene in the liver as in other vertebrates. Its gene consists of 5 exons and 4 introns spanning 13.6 kb, which is the largest gene among known GH genes. GH appears to be the only member of the GH family in the sea lamprey, which suggests that GH is the ancestral hormone of the GH family that originated first in the molecular evolution of the GH family in vertebrates and later, probably during the early evolution of gnathostomes. The other member of the gene family, PRL and SL, appeared by gene duplication. A beta-chain cDNA belonging to the gonadotropin (GTH) and thyrotropin (TSH) family was cloned. It is expressed in cells of the ventral half of PPD. Since the expression of this gene is stimulated by lamprey gonadotropin-releasing hormone, it was assigned to be a GTHbeta. This GTHbeta is far removed from beta-subunits of LH, FSH, and TSH in an unrooted tree derived from phylogenetic analysis, and takes a position as an out group, suggesting that lampreys have a single GTH gene, which duplicated after the agnathans and prior to the evolution of gnathostomes to give rise to LH and FSH.
腺垂体激素被认为是由几个祖先基因通过复制，随后发生进化分歧而演化而来。为了了解脊椎动物内分泌系统的起源和进化，我们对一种无颌类动物——海七鳃鳗（Petromyzon marinus）的腺垂体激素进行了特征描述。在有颌类动物中，促肾上腺皮质激素（ACTH）、促黑素（MSH）以及β-内啡肽（β-END）由一个单一基因编码，该基因被命名为阿黑皮素原（POMC），然而在海七鳃鳗中，ACTH和MSH分别由两个不同的基因——促阿皮质素（POC）基因和促阿黑皮素（POM）基因编码。POC基因和POM基因分别在吻部远侧部（RPD）和中间部（PI）特异性表达。因此，在所有脊椎动物中，这两个组织的最终产物是相同的，即来自RPD的ACTH和来自PI的MSH。POMC基因可能是在无脊椎动物进化的早期阶段通过MSH结构域的内部基因复制而形成的。然后，这个祖先基因可能在肉鳍鱼类和四足动物中得以继承，而MSH结构域的内部复制和缺失以及整个POMC基因的复制则发生在七鳃鳗和有颌类鱼类中。海七鳃鳗生长激素（GH）在近端远侧部（PPD）背侧一半的细胞中表达，并像在其他脊椎动物中一样刺激肝脏中胰岛素样生长因子（IGF）基因的表达。其基因由5个外显子和4个内含子组成，跨度为13.6 kb，这是已知GH基因中最大的基因。GH似乎是海七鳃鳗中GH家族的唯一成员，这表明GH是GH家族的祖先激素，它最早起源于脊椎动物GH家族的分子进化过程中，后来可能在有颌类动物的早期进化阶段出现。该基因家族的其他成员催乳素（PRL）和促甲状腺素（SL）是通过基因复制出现的。克隆了一个属于促性腺激素（GTH）和促甲状腺激素（TSH）家族的β链cDNA。它在PPD腹侧一半的细胞中表达。由于该基因的表达受到七鳃鳗促性腺激素释放激素的刺激，因此被确定为GTHβ。在系统发育分析得到的数据构建的无根树中，这个GTHβ与促黄体生成素（LH）、促卵泡激素（FSH）和促甲状腺激素（TSH）的β亚基相距甚远，并处于外类群的位置，这表明七鳃鳗有一个单一的GTH基因，该基因在无颌类动物之后、有颌类动物进化之前发生了复制，从而产生了LH和FSH。
