Developmental lineage priming in Dictyostelium by heterogeneous Ras activation.
Developmental lineage priming in Dictyostelium by heterogeneous Ras activation.
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DOI：
10.7554/elife.01067
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发表时间：
2013-11-26
期刊：
影响因子：
7.7
通讯作者：
Thompson CR
中科院分区：
文献类型：
--
作者：
Chattwood A;Nagayama K;Bolourani P;Harkin L;Kamjoo M;Weeks G;Thompson CR
在细胞培养中，遗传上相同的细胞通常表现出异质性行为，只有“谱系引发”的细胞对分化诱导信号做出反应。最近有人提出，这种异质性存在于正常的胚胎发育过程中，以允许基于“盐和胡椒”分化和分选的位置独立模式。然而，谱系引发的分子基础以及它如何导致可重复的细胞类型比例还知之甚少。为了解决这个问题，我们采用了一种新的正向遗传方法在模式生物盘基网柄藻。这些研究表明，Ras-GTdR调节剂gefE是正常谱系启动和盐和胡椒分化所必需的。这是因为Ras-GTdR活性设定了对谱系特异性分化信号的内在响应阈值。重要的是，我们表明，虽然gefE的表达是统一的，其目标，rasD的转录，是异质性和动态的，从而提供了一种新的机制，异质性的产生和位置无关的分化。http://dx.doi.org/10.7554/eLife.01067.001基因相同的细胞如何发育成不同的细胞类型是生物学中的基本问题之一。已知某些分子作为信号，告诉祖细胞它们应该成为什么类型的细胞。细胞在胚胎中的位置可以决定它暴露于这些信号中的哪一个，从而影响它的命运。然而，也有可能一组细胞暴露于相同的信号，但只有少数细胞做出反应。这就产生了“盐和胡椒”分化细胞以明显随机的方式分化，产生不同细胞类型的混合物，但这种现象的分子基础尚不清楚。一种被称为盘基网柄菌（Dictyosteelium discoideum）的生物，通常被称为黏菌，经常被用来研究这些过程。Dictyostelium有一个不寻常的生命周期;当它的细菌食物来源充足时，它以单个细胞存在，当食物稀缺时，细胞聚集在一起，形成一个可以四处移动的多细胞蛞蝓。蛞蝓体内的细胞变成孢子或柄细胞，柄细胞将孢子抬出地面，使它们能够分散。在适当的条件下，每个孢子都会孵化出一个细胞;一旦找到新的食物来源，这个细胞就开始分裂，从而使生命周期重新开始开始。柄细胞和孢子细胞的形成呈“盐和胡椒”模式。一种称为DIF的化学信使触发细胞成为柄细胞，而不管它们在细胞聚集体中的位置如何。现在，Chatwood等人已经表明，这一过程取决于两种蛋白质的活性：GefE及其底物RasD。令人惊讶的是，这两种蛋白质在细胞分化前几个小时就表达了，当时细胞仍然处于良好的喂养和分裂状态。虽然GefE在这些细胞中均匀表达，但其底物--一种称为RasD的蛋白质--仅在一部分细胞中表达，正是这些细胞后来对DIF产生反应并最终成为茎细胞。RasD的可变表达解释了在将明显相同的细胞均匀暴露于DIF后如何出现“盐和胡椒”图案。类似的机制很可能在高等生物中得到了保护，因此这些发现可能有助于更好地理解祖细胞如何在多细胞植物和动物中发育成特定的细胞类型。DOI：http://dx.doi.org/10.7554/eLife.01067.002
In cell culture, genetically identical cells often exhibit heterogeneous behavior, with only ‘lineage primed’ cells responding to differentiation inducing signals. It has recently been proposed that such heterogeneity exists during normal embryonic development to allow position independent patterning based on ‘salt and pepper’ differentiation and sorting out. However, the molecular basis of lineage priming and how it leads to reproducible cell type proportioning are poorly understood. To address this, we employed a novel forward genetic approach in the model organism Dictyostelium discoideum. These studies reveal that the Ras-GTPase regulator gefE is required for normal lineage priming and salt and pepper differentiation. This is because Ras-GTPase activity sets the intrinsic response threshold to lineage specific differentiation signals. Importantly, we show that although gefE expression is uniform, transcription of its target, rasD, is both heterogeneous and dynamic, thus providing a novel mechanism for heterogeneity generation and position-independent differentiation. DOI: http://dx.doi.org/10.7554/eLife.01067.001 How genetically identical cells develop into distinct cell types is one of the fundamental questions in biology. Certain molecules are known to act as signals that tell progenitor cells what type of cell they should become. The position of a cell within an embryo can determine which of these signals it is exposed to and thus influence its fate. However, it is also possible for a group of cells to be exposed to the same signal, but for only a few to respond. This gives rise to ‘salt and pepper’ differentiation—in which the cells differentiate in an apparently random manner to produce a mixture of different cell types—but the molecular basis of this phenomenon is unclear. An organism called Dictyostelium discoideum, commonly known as slime mould, is often used to study these processes. Dictyostelium has an unusual life cycle; existing as individual cells when its bacterial food source is plentiful, with the cells coming together when food is scarce to form a multicellular slug that can move around. Cells within the slug turn into spores or into stalk cells, which lift the spores above the ground so that they can disperse. Under the right conditions, a single cell hatches from each spore; upon finding a new food source, this cell begins dividing thus allowing the life cycle to begin again. The formation of stalk and spore cells occurs in a ‘salt and pepper’ pattern. A chemical messenger called DIF triggers cells to become stalk cells irrespective of their position within the aggregated mass of cells. Now, Chattwood et al. have shown that this process depends on the activity of two proteins; GefE and its substrate RasD. Surprisingly, both proteins are expressed many hours before cells differentiate, when cells are still well fed and dividing. Although GefE is uniformly expressed in these cells, its substrate—a protein called RasD—is expressed in only a subset of cells, and it is these cells that will later respond to DIF and ultimately become stalk cells. The variable expression of RasD explains how ‘salt and pepper’ patterning arises following uniform exposure of apparently identical cells to DIF. It is likely that similar mechanisms have been conserved in higher organisms, so these findings could lead to a better understanding of how progenitor cells develop into specific cell types in multicellular plants and animals. DOI: http://dx.doi.org/10.7554/eLife.01067.002