Ottawa Hospital Research Institute, Ottawa, ON, Canada.
Department of Cellular and Molecular Medicine, University of Ottawa Faculty of Medicine, Ottawa, ON, Canada.
Biol Reprod. 2024 Jul 12;111(1):63-75. doi: 10.1093/biolre/ioae053.
Betaine has important roles in preimplantation mouse embryos, including as an organic osmolyte that functions in cell volume regulation in the early preimplantation stages and as a donor to the methyl pool in blastocysts. The origin of betaine in oocytes and embryos was largely unknown. Here, we found that betaine was present from the earliest stage of growing oocytes. Neither growing oocytes nor early preantral follicles could take up betaine, but antral follicles were able to transport betaine and supply the enclosed oocyte. Betaine is synthesized by choline dehydrogenase, and female mice lacking Chdh did not have detectable betaine in their oocytes or early embryos. Supplementing betaine in their drinking water restored betaine in the oocyte only when supplied during the final stages of antral follicle development but not earlier in folliculogenesis. Together with the transport results, this implies that betaine can only be exogenously supplied during the final stages of oocyte growth. Previous work showed that the amount of betaine in the oocyte increases sharply during meiotic maturation due to upregulated activity of choline dehydrogenase within the oocyte. This betaine present in mature eggs was retained after fertilization until the morula stage. There was no apparent role for betaine uptake via the SIT1 (SLC6A20) betaine transporter that is active at the 1- and 2-cell stages. Instead, betaine was apparently retained because its major route of efflux, the volume-sensitive organic osmolyte - anion channel, remained inactive, even though it is expressed and capable of being activated by a cell volume increase.
甜菜碱在小鼠胚胎植入前有重要作用，包括作为一种有机渗透物，在早期植入前阶段调节细胞体积，以及作为囊胚甲基池的供体。卵母细胞和胚胎中甜菜碱的来源在很大程度上是未知的。在这里，我们发现甜菜碱存在于生长卵母细胞的最早阶段。生长的卵母细胞和早期的原始卵泡都不能摄取甜菜碱，但腔卵泡能够运输甜菜碱并为其内部的卵母细胞提供。甜菜碱由胆碱脱氢酶合成，缺乏 Chdh 的雌性小鼠的卵母细胞或早期胚胎中没有检测到甜菜碱。在饮用水中补充甜菜碱只能在腔卵泡发育的最后阶段供应时恢复卵母细胞中的甜菜碱，但在卵泡发生的早期阶段则不能。结合运输结果，这意味着甜菜碱只能在卵母细胞生长的最后阶段从外部供应。以前的工作表明，由于卵母细胞内胆碱脱氢酶活性的上调，卵母细胞减数分裂成熟过程中甜菜碱的含量急剧增加。成熟卵中存在的这种甜菜碱在受精后一直保留到桑葚胚阶段。通过在 1 细胞和 2 细胞阶段活跃的 SIT1（SLC6A20）甜菜碱转运蛋白摄取甜菜碱似乎没有明显作用。相反，甜菜碱显然被保留下来，因为其主要的流出途径，即体积敏感的有机渗透物-阴离子通道，仍然不活跃，尽管它被表达并且能够通过细胞体积增加被激活。
