Cell Progression Through Meiosis: A Signal from Recombination to the First Division

减数分裂的细胞进展：从重组到第一次分裂的信号

• 批准号: 0083816
• 负责人: Robert Malone
• 金额: $ 39万
• 依托单位: University of Iowa
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2001
• 资助国家: 美国
• 起止时间: 2001-05-15 至 2004-04-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0083816&HistoricalAwards=false
• 关键词: Cell; Progression; Through; Meiosis; Signal

Meiosis, the special division process in eukaryotes wherein chromosome number is reduced from diploid to haploid during the production of gametes, is complex and highly conserved. It is possible to think of the various steps of meiosis somewhat like an intracellular developmental pathway. Mitotic cells get a signal to enter meiosis and must go through premeiotic DNA synthesis, recombination and synapsis, reductional division, equational division, and packaging of the haploid nuclei in the proper order. An interesting and important question is how the various steps of meiosis communicate with each other to ensure that they occur at the right time and in the right sequence. In the budding yeast, Saccharomyces cerevisiae, answers to some of the questions are known. A key feature ensuring that events occur properly is ordered transcriptional regulation. Entry into meiosis involves the loss of a mitotic repressor (RME1) and expression of a meiotic activator (IME1). This results in the transcription of the "Early Meiotic Genes", which include the genes necessary for meiotic recombination. These early events activate a second transcriptional regulator, NDT80, which is required for expression of the "Middle Meiotic Genes" and the first and second meiotic division. NDT80 activation also results in the expression of "Late Meiotic Genes", though it is not yet completely clear whether this is direct or/and a consequence of NDT80 activation of the Middle genes. A second layer of regulation in meiosis is meiotic checkpoints. For example, a failure to perform premeiotic replication results in cell arrest. Recent interest has been stimulated by the realization that cells also have a checkpoint that assesses the state of meiotic recombination. In certain recombination mutants (e.g., dmc1) blocked at intermediate points in the meiotic recombination pathway, the cell arrests before the first division. This arrest requires several genes (e.g., RAD17, MEC1 ) known to be involved in mitotic DNA damage recognition checkpoints. It has been proposed that this checkpoint assessing the state of recombination is a key feature of normal progression through meiosis and that the "dmc1 checkpoint" also occurs in wildtype cells, as the intermediate is made normally during recombination. It makes good sense that the high frequency of breaking and rejoining chromosomes during recombination is a process which the cell should be able to sense. Attempts to segregate chromosomes before recombination was finished would be disastrous. There is yet another mode whereby meiotic recombination communicates with the first division, and that this communication occurs as recombination starts in wild type cells. It has been shown that meiotic cells are capable of recognizing that recombination has been started; the response is to delay the first division for a time equivalent to the time necessary to accomplish recombination. Null mutations in four genes required to initiate recombination ("EE" genes) result in a earlier first division. This is intuitively pleasing; it seems eminently reasonable that starting the complex process of recombination should signal the next meiotic step, the first division. It appears that this signaling process is complex since null mutations in different EE genes can have somewhat different effects on the timing of the first division. It has been shown that the signal is not the formation of double strand breaks (the first easily observed DNA intermediate in recombination initiation). The importance of this initiation signal is indicated by noting that, in its absence, the first division occurs at the time when homologs would normally be recombining. This indicates that the first division segregation apparatus can be ready and functional considerably earlier than it normally acts. Data from this laboratory indicates that the start of recombination prevents this premature division. This project asks how this novel signal from recombination to the first division works. Does it require the majority of the initiation genes, consistent with the idea that the signal is the formation of an initiation complex? What is the role of the synaptonemal complex and its component parts in sending the signal? Is the normal signal from recombination initiation recognized and communicated by the checkpoint genes that also respond to the dmc1 mutant block which occurs at later stages? Does the signal work by affecting activation of the central meiotic regulator NDT80? Finally, what are the other genes involved in this signal from recombination to the first division? This work will define how this intracellular signaling process, crucial for the proper progression through meiosis, functions to ensure that two critical steps in meiosis happen at the proper times.

减数分裂是真核生物中一种特殊的分裂过程，在配子产生过程中，染色体数目从二倍体减少到单倍体，是一个复杂且高度保守的过程。可以认为减数分裂的各个步骤有点像细胞内的发育途径。有丝分裂细胞获得进入减数分裂的信号，必须按正确的顺序经历减数分裂前DNA合成、重组和联会、还原分裂、等式分裂和单倍体细胞核的包装。一个有趣而重要的问题是，减数分裂的各个步骤如何相互沟通，以确保它们在正确的时间和正确的顺序发生。在萌芽中的酵母--酿酒酵母中，一些问题的答案是已知的。确保事件正确发生的一个关键特征是有序的转录调控。进入减数分裂涉及有丝分裂抑制因子(RME1)的丢失和减数分裂激活因子(IME1)的表达。这导致了“早期减数分裂基因”的转录，其中包括减数分裂重组所必需的基因。这些早期事件激活了第二个转录调控因子NDT80，它是“中期减数分裂基因”表达以及第一次和第二次减数分裂所必需的。NDT80的激活也会导致“晚期减数分裂基因”的表达，尽管目前还不完全清楚这是直接的还是/和NDT80激活中基因的结果。减数分裂的第二层调控是减数分裂检查点。例如，未能进行减数分裂前复制会导致细胞停滞。最近，人们意识到细胞也有一个检查点来评估减数分裂重组的状态，这激发了人们的兴趣。在某些被阻断在减数分裂重组途径中间点的重组突变体(如dmc1)中，细胞在第一次分裂之前就停止了。这种抑制需要几个已知参与有丝分裂DNA损伤识别检查点的基因(如RAD17，Mec1)。有人提出，评估重组状态的这个检查点是减数分裂正常进行的一个关键特征，而且野生型细胞中也存在“dmc1检查点”，因为中间产物在重组过程中正常产生。重组过程中高频率的染色体断裂和重新连接是细胞应该能够感觉到的过程，这是很有道理的。在重组完成之前分离染色体的尝试将是灾难性的。还有另一种方式，减数分裂重组与第一次分裂通信，这种通信发生在野生型细胞中重组开始时。已经证明，减数分裂细胞能够识别重组已经开始；其反应是将第一次分裂推迟相当于完成重组所需的时间。启动重组所需的四个基因(“EE”基因)的零突变会导致较早的第一次分裂。这在直觉上是令人愉悦的；启动复杂的重组过程应该预示着减数分裂的下一步，即第一次分裂，这似乎是非常合理的。这一信号传递过程似乎很复杂，因为不同EE基因的零突变对第一次分裂的时间可能有一些不同的影响。已经证明，信号不是形成双链断裂(重组启动的第一个容易观察到的DNA中间体)。这个起始信号的重要性是通过注意，在没有它的情况下，第一次分裂发生在同源基因正常重组的时候。这表明第一分区分离装置可以比其正常工作早得多地准备就绪并起作用。来自该实验室的数据表明，重组的开始防止了这种过早的分裂。这个项目询问从重组到第一分裂的这个新信号是如何工作的。它是否需要大多数启动基因，这与信号是启动复合体的形成的想法一致？联会复合体及其组成部分在发送信号中的作用是什么？来自重组启动的正常信号是否由检查点基因识别和传递，这些检查点基因也对发生在后期的dmc1突变块做出反应？该信号是否通过影响中央减数分裂调节因子NDT80的激活而起作用？最后，从重组到第一次分裂，与这个信号有关的其他基因是什么？这项工作将确定这一细胞内信号过程是如何发挥作用的，以确保减数分裂中的两个关键步骤在适当的时间发生。