Yamazaki Wataru, Badescu Dunarel, Tan Seang Lin, Ragoussis Jiannis, Taketo Teruko
Department of Surgery, McGill University, Montreal, QC, Canada.
Research Institute of McGill University Health Centre, Montreal, QC, Canada.
Front Genet. 2021 Dec 20;12:792604. doi: 10.3389/fgene.2021.792604. eCollection 2021.
The sex chromosome complement, XX or XY, determines sexual differentiation of the gonadal primordium into a testis or an ovary, which in turn directs differentiation of the germ cells into sperm and oocytes, respectively, in eutherian mammals. When the X monosomy or XY sex reversal occurs, XO and XY females exhibit subfertility and infertility in the mouse on the C57BL/6J genetic background, suggesting that functional germ cell differentiation requires the proper sex chromosome complement. Using these mouse models, we asked how the sex chromosome complement affects gene transcription in the oocytes during follicular growth. An oocyte accumulates cytoplasmic components such as mRNAs and proteins during follicular growth to support subsequent meiotic progression, fertilization, and early embryonic development without transcription. However, how gene transcription is regulated during oocyte growth is not well understood. Our results revealed that XY oocytes became abnormal in chromatin configuration, mitochondria distribution, and transcription compared to XX or XO oocytes near the end of growth phase. Therefore, we compared transcriptomes by RNA-sequencing among the XX, XO, and XY oocytes of 50-60 µm in diameter, which were still morphologically comparable. The results showed that the X chromosome dosage limited the X-linked and autosomal gene transcript levels in XO oocytes whereas many genes were transcribed from the Y chromosome and made the transcriptome in XY oocytes closer to that in XX oocytes. We then compared the transcript levels of 3 X-linked, 3 Y-linked and 2 autosomal genes in the XX, XO, and XY oocytes during the entire growth phase as well as at the end of growth phase using quantitative RT-PCR. The results indicated that the transcript levels of most genes increased with oocyte growth while largely maintaining the X chromosome dosage dependence. Near the end of growth phase, however, transcript levels of some X-linked genes did not increase in XY oocytes as much as XX or XO oocytes, rendering their levels much lower than those in XX oocytes. Thus, XY oocytes established a distinct transcriptome at the end of growth phase, which may be associated with abnormal chromatin configuration and mitochondria distribution.
在真兽亚纲哺乳动物中，性染色体组成（XX或XY）决定了性腺原基向睾丸或卵巢的性分化，进而分别指导生殖细胞分化为精子和卵母细胞。当发生X单体或XY性反转时，在C57BL/6J遗传背景下的小鼠中，XO和XY雌性表现出亚生育力和不育，这表明功能性生殖细胞分化需要合适的性染色体组成。利用这些小鼠模型，我们研究了性染色体组成如何在卵泡生长过程中影响卵母细胞中的基因转录。卵母细胞在卵泡生长过程中积累细胞质成分，如mRNA和蛋白质，以支持随后的减数分裂进程、受精和早期胚胎发育，而无需转录。然而，卵母细胞生长过程中基因转录是如何调控的，目前还不太清楚。我们的结果显示，与生长阶段末期的XX或XO卵母细胞相比，XY卵母细胞在染色质构型、线粒体分布和转录方面出现异常。因此，我们通过RNA测序比较了直径为50 - 60 µm的XX、XO和XY卵母细胞的转录组，这些卵母细胞在形态上仍然具有可比性。结果表明，X染色体剂量限制了XO卵母细胞中X连锁和常染色体基因的转录水平，而许多基因从Y染色体转录，使XY卵母细胞的转录组更接近XX卵母细胞的转录组。然后，我们使用定量RT-PCR比较了XX、XO和XY卵母细胞在整个生长阶段以及生长阶段末期3个X连锁、3个Y连锁和2个常染色体基因的转录水平。结果表明，大多数基因的转录水平随着卵母细胞生长而增加，同时在很大程度上保持X染色体剂量依赖性。然而，在生长阶段末期，XY卵母细胞中一些X连锁基因的转录水平没有像XX或XO卵母细胞那样增加，使其水平远低于XX卵母细胞。因此，XY卵母细胞在生长阶段末期建立了一个独特的转录组，这可能与异常的染色质构型和线粒体分布有关。
