Tsuchiya Dai, Yang Yang, Lacefield Soni
Department of Biology, Indiana University, Bloomington, Indiana, United States of America.
PLoS Genet. 2014 Jun 5;10(6):e1004398. doi: 10.1371/journal.pgen.1004398. eCollection 2014 Jun.
In budding yeast, meiotic commitment is the irreversible continuation of the developmental path of meiosis. After reaching meiotic commitment, cells finish meiosis and gametogenesis, even in the absence of the meiosis-inducing signal. In contrast, if the meiosis-inducing signal is removed and the mitosis-inducing signal is provided prior to reaching meiotic commitment, cells exit meiosis and return to mitosis. Previous work has shown that cells commit to meiosis after prophase I but before entering the meiotic divisions. Since the Ndt80 transcription factor induces expression of middle meiosis genes necessary for the meiotic divisions, we examined the role of the NDT80 transcriptional network in meiotic commitment. Using a microfluidic approach to analyze single cells, we found that cells commit to meiosis in prometaphase I, after the induction of the Ndt80-dependent genes. Our results showed that high-level expression of NDT80 is important for the timing and irreversibility of meiotic commitment. A modest reduction in NDT80 levels delayed meiotic commitment based on meiotic stages, although the timing of each meiotic stage was similar to that of wildtype cells. A further reduction of NDT80 resulted in the surprising finding of inappropriately uncommitted cells: withdrawal of the meiosis-inducing signal and addition of the mitosis-inducing signal to cells at stages beyond metaphase I caused return to mitosis, leading to multi-nucleate cells. Since Ndt80 enhances its own transcription through positive feedback, we tested whether positive feedback ensured the irreversibility of meiotic commitment. Ablating positive feedback in NDT80 expression resulted in a complete loss of meiotic commitment. These findings suggest that irreversibility of meiotic commitment is a consequence of the NDT80 transcriptional positive feedback loop, which provides the high-level of Ndt80 required for the developmental switch of meiotic commitment. These results also illustrate the importance of irreversible meiotic commitment for maintaining genome integrity by preventing formation of multi-nucleate cells.
在出芽酵母中，减数分裂承诺是减数分裂发育途径的不可逆延续。达到减数分裂承诺后，细胞完成减数分裂和配子发生，即使在没有减数分裂诱导信号的情况下也是如此。相反，如果在达到减数分裂承诺之前去除减数分裂诱导信号并提供有丝分裂诱导信号，细胞会退出减数分裂并恢复到有丝分裂。先前的研究表明，细胞在减数第一次分裂前期但在进入减数分裂之前就承诺进行减数分裂。由于Ndt80转录因子诱导减数分裂所必需的减数分裂中期基因的表达，我们研究了NDT80转录网络在减数分裂承诺中的作用。使用微流控方法分析单细胞，我们发现细胞在Ndt80依赖性基因诱导后的减数第一次分裂前中期承诺进行减数分裂。我们的结果表明，NDT80的高水平表达对于减数分裂承诺的时间和不可逆性很重要。基于减数分裂阶段，NDT80水平的适度降低会延迟减数分裂承诺，尽管每个减数分裂阶段的时间与野生型细胞相似。NDT80的进一步降低导致了一个惊人的发现，即出现了不适当的未承诺细胞：在减数第一次分裂中期之后的阶段，去除减数分裂诱导信号并向细胞添加有丝分裂诱导信号会导致恢复到有丝分裂，从而产生多核细胞。由于Ndt80通过正反馈增强自身转录，我们测试了正反馈是否确保了减数分裂承诺的不可逆性。消除NDT80表达中的正反馈会导致减数分裂承诺完全丧失。这些发现表明，减数分裂承诺的不可逆性是NDT80转录正反馈环的结果，该反馈环提供了减数分裂承诺发育转换所需的高水平Ndt80。这些结果还说明了不可逆的减数分裂承诺对于通过防止多核细胞形成来维持基因组完整性的重要性。
