Ottawa Hospital Research Institute, Ottawa, Ontario, Canada.
J Cell Physiol. 2013 Oct;228(10):2042-53. doi: 10.1002/jcp.24370.
The coupled action of the Na(+)/H(+) exchanger NHE1 and the HCO3(-)/Cl(-) exchanger AE2 constitutes the principal mechanism for acute correction of decreased cell volume in mammalian somatic cells, while, when acting separately, they regulate intracellular pH. It was previously found that AE2 becomes inactivated during meiosis in mouse oocytes. Similarly, NHE1 activity stimulated by intracellular acidosis was present in preovulatory germinal vesicle stage (GV) mouse oocytes and then decreased during meiotic maturation. In contrast, NHE1 activity stimulated by decreased cell volume was low in GV oocytes but became active during meiotic maturation as the oocyte detached from the zona pellucida. It then decreased again in mature eggs similar to activity stimulated by acidosis. The subcellular localization of NHE1 was investigated with YFP-tagged NHE1. Exogenous NHE1 expressed in GV oocytes localized to the plasma membrane and resulted in increased Na(+)/H(+) exchanger activity, but only when co-expressed with calcineurin homologous protein 1 (CHP1). When oocytes expressing functional NHE1 were matured to eggs, however, membrane localization of NHE1 and Na(+)/H(+) exchanger activity were lost. It was unknown why NHE1 and AE2 activities are suppressed during meiotic maturation. Maintenance of cell volume in preimplantation embryos requires glycine accumulation via the GLYT1 transporter, a process unique to eggs and early embryos that is initiated during meiotic maturation. When NHE1 and AE2 activities were maintained in GV oocytes by exogenous expression, glycine accumulation was inhibited. We propose that NHE1-mediated acute cell volume regulation is inactivated during meiotic maturation to allow preferential accumulation of glycine in eggs.
钠氢交换蛋白 NHE1 和碳酸酐酶 AE2 的偶联作用构成了哺乳动物体细胞体积减少时的主要快速校正机制，而当它们单独作用时，则调节细胞内 pH 值。先前发现，AE2 在小鼠卵母细胞减数分裂过程中失活。同样，在促排卵的生发泡期（GV）卵母细胞中存在由细胞内酸中毒刺激的 NHE1 活性，然后在减数分裂成熟过程中减少。相比之下，GV 卵母细胞中的 NHE1 活性因细胞体积减少而受到刺激，但在卵母细胞从透明带脱离时变得活跃。然后，它在成熟卵中再次下降，与酸中毒刺激的活性相似。用 YFP 标记的 NHE1 研究了 NHE1 的亚细胞定位。在 GV 卵母细胞中表达的外源性 NHE1 定位于质膜，并导致 Na+/H+交换活性增加，但仅在与钙调磷酸酶同源蛋白 1（CHP1）共表达时才会增加。然而，当表达功能性 NHE1 的卵母细胞成熟为卵子时，NHE1 的膜定位和 Na+/H+交换活性丧失。尚不清楚为什么 NHE1 和 AE2 活性在减数分裂成熟过程中受到抑制。植入前胚胎的细胞体积维持需要通过 GLYT1 转运体积累甘氨酸，这是卵母细胞和早期胚胎特有的过程，始于减数分裂成熟。当 NHE1 和 AE2 活性通过外源表达在 GV 卵母细胞中维持时，甘氨酸积累被抑制。我们提出，NHE1 介导的急性细胞体积调节在减数分裂成熟过程中失活，以允许甘氨酸在卵子中优先积累。
