Molecular mechanism of promotive regulation of vitellogenin synthesis and spermatogenesis by GnRH neuron in scallop

1.用BrdU掺入法定量检测了性成熟过程中扇贝生殖细胞的有丝分裂活性。结果表明，性腺细胞的增殖模式可分为两个阶段：阶段I；卵原细胞和精原细胞在生长期缓慢增殖：阶段II；卵原细胞发育成卵母细胞，精原细胞在成熟期和产卵期开始快速增殖。在不同的内分泌控制下，性腺细胞的增殖可能处于I期和II期。在II期，精原细胞的增殖受到mGnRH样肽的促进。mGnRH样肽是两种GnRH中的一种，存在于扇贝中枢神经系统的神经元中，并通过血细胞输送到性腺。GnRH诱导的增殖是通过GnRH受体介导的，随后通过性腺中雌激素细胞的雌激素合成而介导。因此，GnRH似乎刺激了性腺中雌激素的合成和释放，导致精原细胞有丝分裂的激活。卵母细胞在卵黄蛋白原(VTG)合成和积累的基础上，在第二时相的生长逐渐受到中枢GnRH神经元的控制。与男性相同类型的GnRH刺激卵巢中的雌激素细胞，合成的雌激素促进辅助细胞中VTG基因的转录，类似于卵泡细胞。另一方面，另一类GnRH样神经肽，新近被鉴定为VPF(卵黄发生促进因子)，可促进VTG蛋白从VTG mRNA翻译过来。VTG基因表达通过雌激素受体(ER)介导。在包括扇贝在内的3种双壳类中发现了ER基因，并且这些基因受各自的配体雌激素的诱导，这表明雌激素信号可能是通过自我调节机制放大并有效地传导到辅助细胞中，实现卵黄蛋白原基因的高表达，导致卵黄蛋白原蛋白的合成和卵母细胞的生长。

1. Mitotic activity of gonial cell of scallop was quantitatively detected using BrdU incorporation through sexual maturation. It was suggested that the pattern of proliferation of gonial cells can be divided into two phases: phase I; oogonia and spermatogonia slowly proliferate through the growing stage: phase II; oogonia develop into oocytes and spermatogonia begin to proliferate rapidly through the mature and spawning stages. The proliferation of gonial cells is likely under the different endocine controls in phase I and II. The spermatogonial proliferation at phase II was promoted by a mGnRH-like peptide, one of two types of GnRH, which were identified in the neuron of CNS of scallop and transported to the gonad via hemocytes. The GnRH-induced proliferation was mediated through GnRH receptor and subsequently through estrogen synthesis in the estrogenic cell in the gonad. Therefore, it seems that GnRH stimulates estrogen synthesis and release in the gonad, resulting in activation of mitosis of spermatogonia.2. Oocyte growth based on vitellogenin (Vtg) synthesis and accumulation at phase II was progressively controlled by GnRH neuron in CNS. The same type of GnRH as in male stimulated estrogenic cell in the ovary and the synthesized estrogen promoted Vtg gene transcription in the auxiliary cell which is similar to a follicle cell. On the other hand, another type of GnRH-like neuro peptide, whish has been newly identified as VPF (vitellogenesis promoting factor), promoted Vtg protein translation from Vtg mRNA. Vtg gene expression mediated through an estrogen receptor (ER). ER cDNAs were identified in three kinds of bivalves including scallop and the genes were induced by their own ligand, estrogen, suggesting that estrogen signaling is likely amplified by auto-regulation mechanism and effectively transduced into the auxiliary cell to achieve a high vitellogenin gene expression, resulting in vitellogenin protein synthesis and oocyte growth.