Germline stem cells are the pool of stem cells capable of becoming eggs or sperm. They divide asymmetrically, such that one of the cells resulting from a division is another stem cell and the other is a differentiated cell, which has progressed one step further down the path towards becoming an egg or sperm.
生殖細胞系幹細胞是能成為卵子或精子的幹細胞庫。它們不對稱分裂，因此分裂產生的細胞，一個又是幹細胞，另一個是已經更進一步，朝成為卵子或精子之道路進展的分化細胞。
Researchers have thought that this asymmetrical division served to replenish the pool of stem cells—making sperm or eggs, but also making more stem cells to produce future sperm or eggs.
研究人員們一直認為，這種不對稱分裂有助於補充，產生精子或卵子的幹細胞庫，同時也產生更多的幹細胞，來產生未來的精子或卵子。
However, the germline has another way to replenish itself: cells that have differentiated only one or two steps down the path to becoming eggs or sperm are capable of reverting into stem cells. Why, then, do stem cells divide asymmetrically?
不過，此生殖細胞系有另一種自行補充的方式：在成為卵子或精子的過程中，已經朝成為卵子或精子之僅分化了一或兩步驟的細胞，能恢復為幹細胞。那麼，為何幹細胞不對稱分裂呢？
New research from Whitehead Institute Member Yukiko Yamashita, who is also a professor of biology at the Massachusetts Institute of Technology and an HHMI Investigator, and former postdoc in her lab Jonathan Nelson shows that asymmetrical division in germline stem cells serves a different but equally important purpose in male fruit flies (Drosophila melanogaster), a common model animal for germline research.
來自美國懷特海德生物醫學研究所成員，也是麻省理工學院生物學教授及美國霍華德休斯醫學研究所(HHMI：Howard Hughes Medical Institute)研究員的Yukiko Yamashita，與其實驗室前博士後研究員Jonathan Nelson的新研究顯示，生殖細胞系幹細胞不對稱分裂，在雄性果蠅(Drosophila melanogaster，生殖系研究常見的動物模型)中，起一種不同卻同樣重要目的的作用。
The work, published in the journal Proceedings of the National Academy of Sciences (PNAS) on November 13, suggests that in flies, germline stem cells divide asymmetrically in order to unequally split a certain kind of DNA, called ribosomal DNA (rDNA), between the two dividing cells and then keep the cell with more rDNA in the stem cell pool.
該項於(2023年)11月13日，發表於《美國國家科學院院刊》(PNAS)的研究暗示，於果蠅中，生殖細胞系幹細胞不對稱分裂，以便在兩個分裂中的細胞間，不均勻地分裂。被稱為核醣體DNA(rDNA)的一種DNA，然後將具有更多rDNA的細胞，保留於幹細胞庫中。
This is necessary in order to keep the germline viable over generations of cell divisions, and so to keep individual flies fertile and capable of reproduction. The researchers show that only germline stem cells, and not other types of germ cells, drive this process, and explain why stem cells’ asymmetric divisions make them uniquely suited to maintaining rDNA.
為了保持此生殖細胞系，於細胞分裂的世代中存活，及保持個體果蠅的可生育與能繁殖，這是必要的。研究人員證實，只有生殖細胞系的幹細胞，而非其他類型的生殖細胞，驅動此過程。因此，解釋了為何幹細胞的不對稱分裂，使其特別適合維持rDNA。
Ribosomal DNA is critical to maintain in the germline because it contains the instructions for making a major part of ribosomes, the cellular machines that build proteins from genetic instructions. Proteins are the main workhorses of the cell, and so cells need to make many ribosomes in order to build all of the proteins that they need.
於生殖細胞系中，維持核醣體DNA至關重要。因為，它具有製造核醣體主要部分的指令。核醣體是根據遺傳指令，建構蛋白質的細胞機器。
Consequently, rDNA exists as many copies repeated in a row of the code for components of the ribosome. All of these repeats make it easy for the cell to mass produce ribosomes, but they also come with a risk: repetitive DNA is prone to losing repeats during cell division.
因此，rDNA存在如同，於核醣體成分之一行遺傳密碼中，重複的多份複製。所有此些重複部分，使細胞較容易大量產生核醣體，不過它們也伴隨著風險：因為，在細胞分裂期間，重複的DNA易於喪失複製部分。
When the cell’s rDNA is copied, it’s easy for a few of the many identical repeats to get cut out, so that the resulting copy of the genome has fewer rDNA repeats than the original.
當細胞的rDNA被複製時，多個相同複製部分的少數，是容易被刪除的。因此，從而產生的基因體複製部分，具有比原來之基因體較少的rDNA複製部分。
Most cells can afford to lose a few rDNA repeats without too many negative effects, but the germline cannot. Whereas other cells die with the body they are in, germ cells produce eggs and sperm that will form a new body, which produces new germ cells, and so on.
大多數細胞能經得起，喪去少數rDNA重複部分，沒有太多負面影響。不過，生殖細胞系不能。儘管其他細胞隨其所在的主體死亡，生殖細胞產生能形成，產生新生殖細胞之主體的卵子及精子，等等。
The germ cell lineage is effectively immortal. Over the course of its endless cycle of cell division, the loss of rDNA repeats would add up until the cells became dysfunctional and then died. This would make the individual bearing those germ cells infertile, and so cause their lineage to go extinct.
實際上，生殖細胞譜系是長生的。在其細胞分裂的無限周而復始中，rDNA重複部分的喪失，會累積直到細胞出現功能障礙，然後死亡。這會使得懷有那些生殖細胞的個體無生殖力，因此導致其譜系走向滅絕。
Researchers have known that germ cells have some way to regain rDNA repeats when the number gets too low—if germ cells couldn’t do this, none of us would exist—but the details of how cells achieve this have been largely mysterious. One proposed model was that when a germ cell divides, sometimes it might divide up its rDNA unequally between the two resulting cells, so that one cell would gain rDNA repeats.
研究人員們已經知曉，當rDNA重複部分過低時，生殖細胞有某種，重新獲得rDNA重複部分的方法(倘若生殖細胞不能做到這一點，我們無一能存在)。不過，生殖細胞如何達成的細節，大部分一直是費解的。一種被提出的模型是，當生殖細胞分裂時，有時候它可能在兩個從而產生的細胞之間，不均勻分配其rDNA。因此，一個細胞會獲得諸多rDNA重複部分。
Yamashita and Nelson have previously found evidence that this model is correct, and they discovered some of the specific mechanisms that enable it to happen. In In a 2023 PNAS paper, the researchers showed that a retrotransposon, a “selfish” genetic element whose function is to make more copies of itself, actually helps germ cells maintain rDNA.
先前，Yamashita及Nelson已經發現，該模型是正確的證據，且他們發現了一些使其能發生的具體機制。在2023年，發表於《美國國家科學院院刊》的一篇論文中，此些研究人員證實了逆轉錄轉座子，一種功能是複製更多自身之“自私的”遺傳因子，實際上有助於生殖細胞維持rDNA。
During cell division, the retrotransposon R2 slices open one copy of the chromosome containing rDNA in its quest to insert extra copies of itself into the genome.
於細胞分裂期間，在其尋求將自身的額外複製嵌入該基因體中上，逆轉錄轉座子R2切開，具有rDNA之染色體的一份複製。
The cell tries to repair the break using the copy on the other intact chromosome, but the tricky nature of repetitive DNA can cause the cell to lose its place, so that it stitches a stretch of rDNA repeats from one copy of the chromosome into the other copy instead.
該細胞嘗試使用，另一條完整染色體上的複製，來修復此斷裂。不過，重複之DNA的棘手性質，可能會導致細胞失去其空間。因此，它反而將一段來自該染色體的rDNA重複部分，縫合到另一個複製中。
Through this process, the germline can boost the level of rDNA in a cell—but only by as much as another cell loses. How does this win-lose exchange lead to an overall increase in rDNA levels across the germline cell population to compensate for lost rDNA?
透過此過程，於一個細胞中，該生殖細胞系能提高rDNA的水平。不過，僅達與另一細胞喪失的一樣多。此種獲得與喪失的調換，如何導致於整個生殖系細胞群中，rDNA水平整體增加，來補償喪失的rDNA？
In this latest work, Yamashita and Nelson show through mathematical modeling that in cells that divide symmetrically, it would not. Gains and losses in rDNA through this form of exchange would occur essentially at random and cancel each other out over time.
在該項最新研究中，Yamashita及Nelson透過數學模型製作證實，在對稱分裂的細胞中，它不會。透過這種形式的調換，於rDNA的獲得及喪失，基本上會隨機發生，且隨著時間推移相互抵消。
Now consider an asymmetric division. After a germline stem cell divides, the cell that differentiates will go through a few more divisions and ultimately create a specific number of sperm cells–the number happens to be sixty-four.
目前，認為是一種不對稱分裂。在生殖細胞系幹細胞分裂後，分化的細胞會經歷多幾次的分裂。結果，最終產生特定數量的精子細胞。此數量剛好是64。
If this daughter cell gets the chromosome with more rDNA repeats, then that would lead to sixty-four sperm with more rDNA repeats—but that would be it, as the sperm have exited the pool of replicating germline stem cells.
倘若此子細胞獲得具有更多rDNA重複部分的染色體，那麼那會導致64個具有更多rDNA重複部分的精子。不過，僅此而已。因為，精子已經退出，複製生殖細胞系幹細胞庫。
However, the daughter cell that remains a germline stem cell will divide again to create a differentiated cell (which will become sixty-four sperm) and another stem cell, which will divide again, leading to another sixty-four sperm and another stem cell—and so on.
不過，仍然是生殖細胞系幹細胞的子細胞會再度分裂，來產生一個分化的細胞(會成為64個精子)及會再度分裂的另一個幹細胞，而導致另外64個精子及另一個幹細胞，等等。
All of these cells, including many sperm, would inherit the higher number of rDNA repeats. Furthermore, at each division, there would be an opportunity for another unequal split of rDNA.
所有此些細胞，包括諸多精子，會繼承較多的rDNA重複部分。此外，在每一分裂中，會有另一次rDNA不均等分裂的機會。
As long as the stem cell always gets the boost in rDNA, then the cumulative number of rDNA repeats would keep growing in the overall population over time—and Yamashita and colleagues’ past work shows that the germline can ensure this.
在rDNA方面，只要幹細胞始終獲得增加。那麼，隨著時間推移，在整個細胞群中，rDNA重複部分的累積數量，會持續增多。因此，Yamashita及其同僚們，過去的研究證實了，該生殖細胞系能確保這一點。
A 2022 Science Advances paper from Yamashita and then-postdoc in her lab George Watase showed that when a germline stem cell divides, the DNA strand with more rDNA repeats is tagged with a protein that the researchers named Indra, which helps mark it to stay in the daughter cell that will become another stem cell.
於2022年，來自Yamashita及當時在其實驗室的博士後研究員George Watase，發表於由美國科學促進會出版之《科學進展》期刊的一篇論文證實，當生殖細胞系幹細胞分裂時，具有更多rDNA重複部分的DNA股，被以一種此些研究人員取名為Indra的蛋白質所標記。這有助於標記其留在，會成為另一幹細胞的子細胞中。
Yamashita and Nelson’s new paper includes mathematical modeling by second author Tomohiro Kumon, a postdoc in Yamashita’s lab, that proves that this is not only sufficient to restore the level of rDNA repeats over time, but that it is the most effective and efficient way for the germline to do so.
Yamashita及Nelson之新論文包括了，由第二撰文人，在Yamashita實驗室的博士後研究員Tomohiro Kumon，證實這不僅足以隨著時間推移，恢復rDNA重複部分的水平，而且是該生殖細胞系，最有效且最有效率之方法的數學模型製作。
“There was this problem with the unequal exchange model of rescuing rDNA, because every cell that gained rDNA did so at the expense of another that was losing it,”Nelson says. “What we show here is that the reason why there's a bias towards gain in the germline is because this process is happening within these asymmetrically dividing germline stem cells that can gain and gain and gain, while the cells that lose rDNA exit the cycle and so have a limited effect.”
Nelson宣稱：「有關拯救rDNA的不均等調換模式，存在此問題。因為，獲得rDNA的每一細胞，以犧牲失去它的另一細胞，做到那樣。在這裡，我們證實的是，在生殖細胞系中，有一種朝增益趨勢的原因，是因為此過程正發生於，此些能不斷增益之不對稱分裂的生殖細胞系幹細胞內。不過，失去rDNA的細胞退出該周而復始，因此具一種有限的影響。」
The researchers complemented their mathematical modeling with evidence that the process to increase rDNA repeats occurs primarily or solely in germline stem cells. They found that when the number of rDNA repeats got low enough, then expression of R2 and the presence of double-stranded DNA breaks both increased in germline stem cells, but not significantly in other germ cell types.
此些研究人員，以此過程增加rDNA重複部分，主要或僅發生於生殖細胞系幹細胞中的證據，補充了他們的數學模型製作。他們發現，當rDNA重複部分的數量獲得夠低時，在生殖細胞系幹細胞中，逆轉錄轉座子R2的表現及雙股DNA 斷裂的存在皆增加。不過，在其他生殖細胞類型中，不顯著。
Yamashita and Nelson propose that the different cell types in the germline take on different functions to create a pipeline for maximizing the health of future sperm.
Yamashita及Nelson認為，於生殖細胞系中，不同細胞類型具有不同的功能，來為未來精子之健康，產生一條達最大限度的管道。
Germ cells that are one or two steps down the path of differentiation from stem cells are essentially identical to them, to the point that they can be difficult to tell apart in testing, but they divide symmetrically. They are also much more sensitive to DNA damage; the researchers found that R2 exposure kills these cells.
從幹細胞邁向分化之途，一或兩步的生殖細胞，與幹細胞本質上是相同的，達在測試中很難區分它們的程度，不過它們對稱分裂。它們對DNA損傷也較為敏感得多。此些研究人員發現，R2暴露殺死這些細胞。
Germline stem cells, with their asymmetrical division and ability to tolerate R2 expression, serve to restore rDNA levels when they get too low. Then the differentiated germ cells serve to weed out mutations—including those introduced during R2 expression in the earlier stem cell stage—by killing off cells with DNA damage.
具有不對稱分裂及耐受R2表現能力的生殖細胞系幹細胞，當rDNA水平過低時，有助於恢復其水平。之後，分化的生殖細胞藉由殺死具DNA損傷的細胞，有助於清除包括，於較初期之幹細胞階段，那些在R2表現期間引入的突變。
The different strengths of the different types of germ cells creates an effective pipeline to produce the largest number of sperm cells with high rDNA repeat number and low DNA damage.
不同類型之生殖細胞的不同優勢，創造了一條有效的管道來產生，具有高rDNA重複部分及低DNA損傷之最大數量的精子細胞。
Eventually, this new understanding of the details of how cells maintain their rDNA could lead to medical therapies. For example, cancer cells are, like germ cells, an essentially immortal cell line, and so must have a way to maintain their rDNA. If researchers could someday find a way to prevent them from doing so, that could be a good treatment strategy.
最終，有關細胞如何維持其rDNA之細節的新瞭解，可能引領出諸多醫學療法。譬如，癌細胞與生殖細胞一樣，本質上是一種長生的細胞系。因此，諒必有一種方法來維持其rDNA。倘若研究人員們有朝一日能找到一種方法，來阻止他們這樣做，那可能是一種很好的治療策略。
The work also may have implications for research on aging, as rDNA decreases with age in other cell types. In the meantime, Yamashita and Nelson are excited to have solved several long-standing mysteries in their field, including how germ cells can restore rDNA at a population level when each division creates an equal loss and gain of rDNA, and why germline stem cells divide asymmetrically.
就針對老化的研究而言，這項研究也可能具有諸多意涵。因為，在其他細胞類型中，rDNA隨著年齡遞減。在此同時，Yamashita及Nelson感到振奮，因為解決了在其領域中，包括當每一分裂產生相同的rDNA喪失及增益時，生殖細胞如何在總體水平上恢復rDNA，及為什麼生殖細胞系幹細胞不對稱分裂等，幾個長期存在的謎團。
“Typically, when you publish a paper, you feel like you’ve fit two puzzle pieces together, but in this case, I feel like we fit a bunch of puzzle pieces together,” Yamashita says. “It’s been immensely satisfying to find answers to multiple questions and see how they all fit together to explain the mechanisms of this process that’s necessary for germline immortality.”
Yamashita宣稱：「通常，當發表一篇論文時，感覺如同已經將兩塊拼圖，拼湊在一起。不過，在此事例中，我感覺如同我們將一堆拼圖，拼湊在一起。找到多個問題的答案，且瞭解它們如何全拼湊在一起，來解釋生殖細胞系長生所需之此過程的機制，已經是非常令人滿意。」
網址：https://wi.mit.edu/news/maintaining-fertility-requires-uneven-division-dna
翻譯：許東榮
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