Department of Biosciences and Nutrition, Karolinska Institutet, NEO Building, 141 83, Huddinge, Sweden.
Department of Natural and Life Sciences, The Open University of Israel, Ra'anana, Israel.
Epigenetics Chromatin. 2019 Jul 17;12(1):45. doi: 10.1186/s13072-019-0292-7.
Cellular quiescence is a reversible differentiation state during which cells modify their gene expression program to inhibit metabolic functions and adapt to a new cellular environment. The epigenetic changes accompanying these alterations are not well understood. We used fission yeast cells as a model to study the regulation of quiescence. When these cells are starved for nitrogen, the cell cycle is arrested in G1, and the cells enter quiescence (G0). A gene regulatory program is initiated, including downregulation of thousands of genes-for example, those related to cell proliferation-and upregulation of specific genes-for example, autophagy genes-needed to adapt to the physiological challenge. These changes in gene expression are accompanied by a marked alteration of nuclear organization and chromatin structure.
Here, we investigated the role of Leo1, a subunit of the conserved RNA polymerase-associated factor 1 (Paf1) complex, in the quiescence process using fission yeast as the model organism. Heterochromatic regions became very dynamic in fission yeast in G0 during nitrogen starvation. The reduction of heterochromatin in early G0 was correlated with reduced target of rapamycin complex 2 (TORC2) signaling. We demonstrated that cells lacking Leo1 show reduced survival in G0. In these cells, heterochromatic regions, including subtelomeres, were stabilized, and the expression of many genes, including membrane transport genes, was abrogated. TOR inhibition mimics the effect of nitrogen starvation, leading to the expression of subtelomeric genes, and this effect was suppressed by genetic deletion of leo1.
We identified a protein, Leo1, necessary for survival during quiescence. Leo1 is part of a conserved protein complex, Paf1C, linked to RNA polymerase II. We showed that Leo1, acting downstream of TOR, is crucial for the dynamic reorganization of chromosomes and the regulation of gene expression during cellular quiescence. Genes encoding membrane transporters are not expressed in quiescent leo1 mutant cells, and cells die after 2 weeks of nitrogen starvation. Taken together, our results suggest that Leo1 is essential for the dynamic regulation of heterochromatin and gene expression during cellular quiescence.
细胞静止是一种可逆的分化状态，在此过程中，细胞改变其基因表达程序以抑制代谢功能并适应新的细胞环境。伴随这些变化的表观遗传变化尚不清楚。我们使用裂殖酵母细胞作为模型来研究静止的调节。当这些细胞被氮饥饿时，细胞周期在 G1 期被阻断，细胞进入静止（G0）。启动了一个基因调控程序，包括下调数千个基因，例如与细胞增殖相关的基因，以及上调特定基因，例如适应生理挑战所需的自噬基因。这些基因表达的变化伴随着核组织和染色质结构的明显改变。
在这里，我们使用裂殖酵母作为模型生物，研究了 RNA 聚合酶相关因子 1（Paf1）复合物的亚基 Leo1 在静止过程中的作用。在氮饥饿期间的 G0 中，裂殖酵母的异染色质区域变得非常动态。早期 G0 中异染色质的减少与雷帕霉素靶蛋白复合物 2（TORC2）信号的减少有关。我们证明，缺乏 Leo1 的细胞在 G0 中存活能力降低。在这些细胞中，异染色质区域，包括端粒，稳定下来，许多基因的表达，包括膜转运基因，被阻断。TOR 抑制模拟氮饥饿的效果，导致端粒基因的表达，而这种效果被 leo1 的遗传缺失所抑制。
我们鉴定出一种在静止期间存活所必需的蛋白质 Leo1。Leo1 是与 RNA 聚合酶 II 相关的保守蛋白复合物 Paf1C 的一部分。我们表明，作为 TOR 的下游因子的 Leo1，对于染色体的动态重排和细胞静止期间基因表达的调节至关重要。在静止的 leo1 突变细胞中，编码膜转运蛋白的基因不表达，并且在氮饥饿 2 周后细胞死亡。总之，我们的结果表明，Leo1 对于细胞静止期间异染色质和基因表达的动态调节是必不可少的。
