Gomer R H, Ammann R R
Department of Biochemistry and Cell Biology, Rice University, Houston, Texas 77251-1892, USA.
Dev Biol. 1996 Feb 25;174(1):82-91. doi: 10.1006/dbio.1996.0053.
Upon starvation, cells of the simple eukaryote Dictyostelium discoideum aggregate and differentiate into several cell types. Two main cell types are prestalk and prespore, which later usually become stalk and spore cells. The differentiation is plastic, and several factors can alter cell-type ratios. Two mechanisms have been proposed to regulate the initial cell type. We and others have proposed that cell type is initially determined by cell-cycle phase at the time of starvation: prestalk cells are derived from cells which, are the time of starvation, happen to be in a roughly 2-hr-long sector of the cell cycle which overlaps S and early G2 and that certain extracellular factors are then used to maintain the proper prestalk:prespore ratio and to control later stages of development such as the prestalk-to-stalk conversion. To examine the relationship between initial cell-type choice and the cell cycle, and how this 2-hr-long sector is generated, we increased the length of S phase by mild treatments of cells with DNA-synthesis inhibitors. When the fraction of the cell cycle occupied by S phase is increased and the cells are then starved, the prestalk:prespore ratio increases. This increase was observed using two markers for prestalk cells, CP2 and ecmA::lacZ. In addition, there is a close correlation between the fraction of the cell cycle occupied by S phase and the prestalk:prespore ratio, irrespective of total cell-cycle length. These results validate the hypothesis that the initial choice of cell type is determined by cell-cycle phase at the time of starvation, and indicate that the cell-type choice mechanism monitors the cell cycle rather than using an independent 2-hr-long timer started at the beginning of S phase.
在饥饿状态下，简单真核生物盘基网柄菌的细胞会聚集并分化为几种细胞类型。两种主要的细胞类型是前柄细胞和前孢子细胞，它们随后通常会分别变成柄细胞和孢子细胞。这种分化具有可塑性，多种因素可以改变细胞类型的比例。目前已经提出了两种机制来调节初始细胞类型。我们和其他人提出，细胞类型最初是由饥饿时的细胞周期阶段决定的：前柄细胞源自饥饿时恰好处于细胞周期中一个大约2小时长的时间段内的细胞，该时间段与S期和早期G2期重叠，然后某些细胞外因子被用来维持适当的前柄细胞与前孢子细胞的比例，并控制发育的后期阶段，如前柄细胞向柄细胞的转变。为了研究初始细胞类型选择与细胞周期之间的关系，以及这个2小时长的时间段是如何产生的，我们通过用DNA合成抑制剂对细胞进行温和处理来延长S期的长度。当S期在细胞周期中所占的比例增加，然后使细胞饥饿时，前柄细胞与前孢子细胞的比例会增加。使用前柄细胞的两种标记物CP2和ecmA::lacZ观察到了这种增加。此外，无论细胞周期的总长度如何，S期在细胞周期中所占的比例与前柄细胞与前孢子细胞的比例之间都存在密切的相关性。这些结果验证了以下假设：细胞类型的初始选择是由饥饿时的细胞周期阶段决定的，并表明细胞类型选择机制监测细胞周期而非使用一个从S期开始时启动的独立的2小时长的定时器。
