The mechanism of MI arrest of starfish oocytes regulated by intracellular pH and MAP kinase

减数分裂中MI停滞的机制知之甚少，尽管它是无脊椎动物中广泛观察到的现象。成熟海星卵母细胞在减数分裂I前期的阻滞被激素1-甲基腺嘌呤释放。研究表明，海星卵母细胞的减数分裂即使在没有受精的情况下也能完全进行，没有MI或MII阻滞。在这项研究中，我们表明MI逮捕海星卵母细胞发生在卵巢后，生殖囊泡破裂。这种停滞是由Mos/MEK/MAP激酶途径和产卵前卵巢细胞内pH值增加的阻断维持的。产卵后，卵母细胞内的Na^+/H^+逆向转运蛋白立即诱导细胞内pH（pHi）从7.0升高到7.3，减数分裂重新开始。蛋白酶体的内源性底物多泛素化细胞周期蛋白B在pH 7.0下稳定，而在pH 7.3下降解。当MAP激酶途径被MEK抑制剂U 0126阻断时，即使在pH 7.0下也发生多泛素化细胞周期蛋白B的降解，而蛋白酶体的肽酶活性不增加。这些结果表明，在pH 7.0的蛋白酶体活性是足够的多泛素化的细胞周期蛋白B的降解和MAP激酶途径阻断多泛素化的细胞周期蛋白B在卵巢中的成熟卵母细胞的降解。产卵后，Na^+/H^+反向转运蛋白介导的pHi升高立即抵消了MAP激酶途径的抑制作用，导致多聚泛素化细胞周期蛋白B降解，并解除了阻滞。因此，在海星卵母细胞中MI停滞的关键步骤发生在细胞周期蛋白B的多泛素化之后，但在细胞周期蛋白B被蛋白酶体蛋白水解之前。

The mechanism of MI arrest in meiosis is poorly understood, although it is a widely observed phenomenon in invertebrates. The blockage of fully grown starfish oocytes in prophase of meiosis I is released by the hormone 1-methyladenine. It has been believed that meiosis of starfish oocytes proceeds completely without MI or MII arrest, even when fertilization dose not occur. In this study, we show that MI arrest of starfish oocytes occurs in the ovary after germinal vesicle breakdown. This arrest is maintained both by the Mos/MEK/MAP kinase pathway and the blockage of an increase of intracellular pH in the ovary before spawning. Immediately after spawning, an increase of intracellular pH (pHi) from〜7.0 to〜7.3 is induced by Na^+/H^+ antiporter in oocytes, and meiosis reinitiation occurs. An endogenous substrate of the proteasome, polyubiquitinated cyclin B, is stable at pH 7.0, while it is degraded at pH 7.3. When the MAP kinase pathway is blocked by MEK inhibitor U0126,degradation of polyubiquitinated cyclin B occurs even at pH 7.0 without an increase of the peptidase activity of the proteasome. These results indicate that the proteasome activity at pH 7.0 is sufficient for degradation of polyubiquitinated cyclin B and that the MAP kinase pathway blocks the degradation of polyubiquitinated cyclin B in the maturing oocytes in the ovary. Immediately after spawning, the increase in pHi mediated by Na^+/H^+ antiporter cancels the inhibitory effects of the MAP kinase pathway, resulting in the degradation of polyubiquitinated cyclin B and the release of the arrest. Thus, the key step of MI arrest in starfish oocytes occurs after the polyubiqutination of cyclin B but before cyclin B proteolysis by the proteasome.