Tartia Alina P, Rudraraju Nirmala, Richards Tiffany, Hammer Mary-Anne, Talbot Prudence, Baltz Jay M
Ottawa Health Research Institute, Ottawa, Ontario, Canada K1Y4E9.
Development. 2009 Jul;136(13):2247-54. doi: 10.1242/dev.036756.
Fertilized mouse eggs regulate their size principally by accumulating glycine as an intracellular osmolyte using the GLYT1 (SLC6A9) transporter, a mechanism of cell volume homeostasis apparently unique to early embryos before the morula stage. However, nothing was known of cell volume regulation in oocytes before fertilization. We show here that GLYT1 is quiescent in mouse germinal-vesicle-stage oocytes but becomes fully activated within hours after ovulation is triggered. This initiates accumulation of substantial amounts of intracellular glycine in oocytes during meiotic progression, reaching a maximal level in mature eggs. Measurements of endogenous free glycine showed that there were nearly undetectable levels in ovarian germinal-vesicle-stage oocytes, but high levels were present in mature ovulated eggs and in preimplantation embryos through the two-cell stage, but not in morulae. Furthermore, intracellular glycine was regulated in response to changes in external tonicity in eggs and embryos through the two-cell stage, but not in oocytes or embryos after the two-cell stage. Before activation of GLYT1, oocytes were unable to independently regulate their volume. As GLYT1 became active, however, oocyte volume decreased substantially and oocytes gained the ability to regulate their size, which required GLYT1 activity. Before ovulation, oocyte size was instead determined by a strong adhesion to the rigid extracellular matrix of the oocyte, the zona pellucida, which was released coincident with GLYT1 activation. The ability to acutely regulate cell size is thus acquired by the oocyte only after ovulation, when it first develops glycine-dependent cell volume regulation.
受精的小鼠卵主要通过利用GLYT1（SLC6A9）转运蛋白积累甘氨酸作为细胞内渗透剂来调节其大小，这是一种在桑椹胚阶段之前早期胚胎所特有的细胞体积稳态机制。然而，受精前卵母细胞的细胞体积调节情况尚不清楚。我们在此表明，GLYT1在小鼠生发泡期卵母细胞中处于静止状态，但在排卵触发后数小时内会完全激活。这在减数分裂进程中启动了卵母细胞内大量甘氨酸的积累，在成熟卵中达到最高水平。内源性游离甘氨酸的测量表明，卵巢生发泡期卵母细胞中的水平几乎检测不到，但在成熟排卵卵和直到二细胞阶段的植入前胚胎中水平很高，而在桑椹胚中则没有。此外，通过二细胞阶段的卵和胚胎中，细胞内甘氨酸会根据外部张力的变化进行调节，但在二细胞阶段后的卵母细胞或胚胎中则不会。在GLYT1激活之前，卵母细胞无法独立调节其体积。然而，随着GLYT1变得活跃，卵母细胞体积大幅减小，卵母细胞获得了调节其大小的能力，这需要GLYT1的活性。在排卵前，卵母细胞的大小反而由其与卵母细胞坚硬的细胞外基质透明带的强粘附力决定，透明带在GLYT1激活时会释放。因此，卵母细胞只有在排卵后才获得急性调节细胞大小的能力，此时它首次发展出依赖甘氨酸的细胞体积调节。
