Molecular mechanisms of onset of puberty in salmonids : regulation of GnRH receptor genes

脊椎动物青春期的发生是由下丘脑-垂体系统控制的。然而，神经内分泌调节青春期的分子机制尚不清楚，特别是在低等脊椎动物如鱼类中。鲑鱼是研究青春期神经内分泌机制的一个有趣的模型，因为它们的性成熟随着季节的变化而缓慢。我们重点研究了促性腺激素释放激素受体（GnRH- r）编码基因的表达调控，这是对GnRH反应的关键分子。在鲑科鱼类中，一种单一的I型GnRH-R最初是在虹鳟的大脑中发现的。我们在马苏鲑鱼中证明了5种不同的GnRH-R基因，分别为msGnRH-R1、R2、R3、R4和R5，它们在大脑、垂体和其他外周组织中以不同的模式表达。msGnRH-R1的剪接变体也在这些组织中表达。所有这些受体都是I型受体，但又分为两种亚型，一种亚型（Ia型）包括R1、R2和R3，另一种亚型（Ib型）包括R4和R5。R1、R2和R3的核苷酸序列同源性为96-99%，R4和R5的核苷酸序列同源性为81%。两个亚型之间的同源性下降到59-71%。GnRH对促性腺激素（GTH）分泌的影响可能部分归因于GnRH- rs表达的变化。因此，我们通过实时PCR检测了生殖周期中垂体中5个msGnRH-R基因表达的季节性变化。所有5种msGnRH-R基因亚型均在垂体中表达，但R4 mRNA占主导地位。有趣的是，五种msGnRH-R基因的表达模式在不同亚型中存在差异。其中，R4 mRNA仅在产卵前升高，此时GnRH刺激促黄体生成素（LH）基因的表达。这些结果表明R4亚型参与了gnrh诱导的LH合成。其他亚型mrna在不同时期增加，可能与垂体中GnRH的其他作用有关，如GnRH诱导的催乳素基因表达。为了更准确地了解多种GnRH-R亚型的功能，需要确定GnRH-R在垂体中的细胞类型特异性分布。少

The onset of puberty is controlled by hypothalamus-pituitary system in vertebrates. However, molecular mechanisms of neuroendocrine regulation of puberty are unknown, particularly in lower vertebrates like fish. Salmonids are an interesting model for investigating the neuroendocrine mechanisms of puberty, because their sexual maturation progress slowly with seasons. We focused on regulation of expression of genes encoding gonadotropin-releasing hormone receptor(GnRH-R), which is a key molecule of responsiveness to GnRH. In salmonids, a single type I GnRH-R was originally identified in the brain of rainbow trout. We demonstrated in masu salmon that five different GnRH-R genes, termed msGnRH-R1,R2,R3,R4,and R5,are expressed in the brain, pituitary and other peripheral tissues with different pattern. A splicing variant of msGnRH-R1 is also expressed in these tissues. All these receptors are type I receptors, but are divided into two subtypes, one subtype (type Ia) includes R1,R2, and R3 a … More nd the other (type Ib) includes R4 and R5. The identity of nucleotide sequences among R1,R2 and R3 is 96-99%, while that of R4 and R5 is 81%. The identity between the two subtypes decreases to 59-71%. The effect of GnRH on gonadotropin (GTH) secretion may be attributed in part to changes in expression of GnRH-Rs. We therefore examined seasonal variations in expression of the five msGnRH-R genes in the pituitary during the reproductive cycle by real-time PCR. All five subtypes of msGnRH-R genes were expressed in the pituitary, although R4 mRNA was dominant. Interestingly, the expression patterns of five msGnRH-R genes differed among different subtypes. Among them, R4 mRNA increased only in the pre-spawning period, when GnRH stimulates expression of luteinizing hormone(LH) genes. These results suggest that R4 subtype is involved in the GnRH-induced LH synthesis. Other subtype mRNAs increased in different periods and may be related to other GnRH action in the pituitary such as GnRH-induced prolactin gene expression. To obtain more accurate information on the function of the multiple GnRH-R subtypes, cell type specific distribution of the GnRH-Rs in the pituitary needs to be determined. Less