Corbett Hannah E, Dubé Chantal D, Slow Sandy, Lever Michael, Trasler Jacquetta M, Baltz Jay M
Ottawa Hospital Research Institute, Ottawa, Ontario, Canada.
Biol Reprod. 2014 Apr 17;90(4):81. doi: 10.1095/biolreprod.113.116939. Print 2014 Apr.
Betaine (N,N,N-trimethylglycine) has previously been shown to function in cell volume homeostasis in early mouse embryos and also to be a key donor to the methyl pool in the blastocyst. A betaine transporter (SLC6A20A or SIT1) has been shown to be activated after fertilization, but there is no saturable betaine uptake in mouse oocytes or eggs. Unexpectedly, the same high level of betaine is present in mature metaphase II (MII) eggs as is found in one-cell embryos despite the lack of transport in oocytes or eggs. Significant saturable betaine transport is, however, present in intact cumulus-oocyte complexes (COCs). This transport system has an affinity for betaine of ∼227 μM. The inhibition profile indicates that betaine transport by COCs could be completely blocked by methionine, proline, leucine, lysine, and arginine, and transport is dependent on Na(+) but not Cl(-). This is consistent with transport by a y+L-type amino acid transport system. Both transcripts and protein of one y+L isoform, SLC7A6 (y+LAT2), are present in COCs, with little or no expression in isolated germinal vesicle (GV)-stage oocytes, MII eggs, or one-cell embryos. Betaine accumulated by COCs is transferred into the enclosed GV oocyte, which requires functional gap junctions. Thus, at least a portion of the endogenous betaine in MII eggs could be derived from transport into cumulus cells and subsequent transfer into the enclosed oocyte before gap junction closure during meiotic maturation. The oocyte-derived betaine then could be regulated and supplemented by the SIT1 transporter that arises in the embryo after fertilization.
甜菜碱（N，N，N-三甲基甘氨酸）先前已被证明在小鼠早期胚胎的细胞体积稳态中发挥作用，并且也是囊胚中甲基池的关键供体。一种甜菜碱转运蛋白（SLC6A20A或SIT1）已被证明在受精后被激活，但在小鼠卵母细胞或卵子中不存在可饱和的甜菜碱摄取。出乎意料的是，尽管卵母细胞或卵子中缺乏转运，但成熟的中期II（MII）期卵子中甜菜碱的水平与单细胞胚胎中的水平相同。然而，完整的卵丘-卵母细胞复合体（COC）中存在显著的可饱和甜菜碱转运。该转运系统对甜菜碱的亲和力约为227μM。抑制谱表明，COC对甜菜碱的转运可被蛋氨酸、脯氨酸、亮氨酸、赖氨酸和精氨酸完全阻断，且转运依赖于Na(+)而非Cl(-)。这与y+L型氨基酸转运系统的转运一致。一种y+L亚型SLC7A6（y+LAT2）的转录本和蛋白在COC中均有表达，而在分离的生发泡（GV）期卵母细胞、MII期卵子或单细胞胚胎中几乎没有或没有表达。COC积累的甜菜碱被转运到封闭的GV卵母细胞中，这需要功能性的间隙连接。因此，MII期卵子中至少一部分内源性甜菜碱可能来自于在减数分裂成熟过程中间隙连接关闭之前转运到卵丘细胞中并随后转移到封闭的卵母细胞中。然后，卵母细胞衍生的甜菜碱可以由受精后胚胎中出现的SIT1转运蛋白进行调节和补充。
