Pulsed feedback defers cellular differentiation.
Pulsed feedback defers cellular differentiation.
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DOI：
10.1371/journal.pbio.1001252
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发表时间：
2012-01
期刊：
影响因子：
9.8
通讯作者：
Elowitz MB
中科院分区：
文献类型：
--
作者：
Levine JH;Fontes ME;Dworkin J;Elowitz MB
为应对突发性环境应激，B。枯草芽孢杆菌细胞可以使用脉冲正反馈环延迟孢子形成多个细胞周期。环境信号诱导不同的细胞分化程序。在某些系统中，细胞在信号出现后延迟分化很长一段时间，在进入新的命运之前通过多轮细胞分裂增殖。细胞如何设定比细胞周期长得多的延迟时间？在这里，我们研究枯草芽孢杆菌细胞，响应突然的营养限制与多轮的生长和分裂，然后分化成孢子。一个良好表征的遗传回路控制着主调节因子Spo0A的浓度和磷酸化，Spo0A上升到临界浓度以启动孢子形成。然而，目前还不清楚这种电路如何使细胞推迟多个细胞周期的孢子形成。使用定量延时荧光显微镜Spo0A在单个细胞中的动态，我们观察到Spo0A磷酸化脉冲在一个特征性的细胞周期阶段。脉冲幅度增长系统和细胞自主多个细胞周期导致孢子形成。这种脉冲生长需要一个涉及孢子形成激酶的关键正反馈回路，没有它，孢子形成的延迟对激酶表达变得超敏感。因此，延迟由脉冲正反馈回路控制，其中激酶表达由Spo0A磷酸化脉冲激活。这种脉冲正反馈结构提供了比组成型激酶表达更稳健的机制来设置延迟时间。最后，使用数学建模，我们展示了脉冲和时间延迟如何一起实现“多相”正反馈，其中反馈回路的不同部分在不同时间处于活动状态。多相反馈可以实现长延迟时间的更准确的调谐。总之，这些结果表明，枯草芽孢杆菌使用脉冲正反馈回路来实现在比细胞周期长得多的时间尺度上操作的“计时器”。一个细胞需要等待多长时间才能对环境变化做出反应？虽然许多途径，如影响趋化性的途径，对环境信号的反应很快，但在其他情况下，细胞可能希望推迟其反应，直到信号出现很长时间后，有时等待多个细胞周期。细胞如何创造“定时器”来调节这些长时间的延迟？我们在枯草芽孢杆菌中研究了这个问题，枯草芽孢杆菌通过转化为休眠孢子来应对压力。我们证明了B。枯草芽孢杆菌可以通过首先经历多轮生长和增殖，然后才形成孢子，从而将孢子形成延迟延长的时间。这种延迟的计时器是一个脉冲正反馈回路，它在多个细胞周期内将孢子形成主调节剂Spo0A的浓度逐步提高到临界水平。最后，使用数学建模，我们说明了如何一种新的动态反馈机制，“多相正反馈”，让细胞推迟孢子形成比其他电路策略更强大。开发能够以更高的时间分辨率访问脉冲和时间延迟动态的技术将使我们能够确定这种多相策略是否为其他系统中看到的多细胞周期延迟时间的调节提供了一般设计原则。
In response to sudden environmental stress, B. subtilis cells can defer sporulation for multiple cell cycles using a pulsed positive feedback loop. Environmental signals induce diverse cellular differentiation programs. In certain systems, cells defer differentiation for extended time periods after the signal appears, proliferating through multiple rounds of cell division before committing to a new fate. How can cells set a deferral time much longer than the cell cycle? Here we study Bacillus subtilis cells that respond to sudden nutrient limitation with multiple rounds of growth and division before differentiating into spores. A well-characterized genetic circuit controls the concentration and phosphorylation of the master regulator Spo0A, which rises to a critical concentration to initiate sporulation. However, it remains unclear how this circuit enables cells to defer sporulation for multiple cell cycles. Using quantitative time-lapse fluorescence microscopy of Spo0A dynamics in individual cells, we observed pulses of Spo0A phosphorylation at a characteristic cell cycle phase. Pulse amplitudes grew systematically and cell-autonomously over multiple cell cycles leading up to sporulation. This pulse growth required a key positive feedback loop involving the sporulation kinases, without which the deferral of sporulation became ultrasensitive to kinase expression. Thus, deferral is controlled by a pulsed positive feedback loop in which kinase expression is activated by pulses of Spo0A phosphorylation. This pulsed positive feedback architecture provides a more robust mechanism for setting deferral times than constitutive kinase expression. Finally, using mathematical modeling, we show how pulsing and time delays together enable “polyphasic” positive feedback, in which different parts of a feedback loop are active at different times. Polyphasic feedback can enable more accurate tuning of long deferral times. Together, these results suggest that Bacillus subtilis uses a pulsed positive feedback loop to implement a “timer” that operates over timescales much longer than a cell cycle. How long should a cell wait to respond to an environmental change? While many pathways such as those affecting chemotaxis respond to environmental signals quickly, in other contexts a cell may want to defer its response until long after the signal's onset—sometimes waiting multiple cell cycles. How can cells create “timers” to regulate these long deferrals? We study this question in the bacterium Bacillus subtilis, which responds to stress by transforming into a dormant spore. We show that B. subtilis can defer sporulation for extended time periods by first undergoing multiple rounds of growth and proliferation, and only then sporulating. The timer for this deferral is a pulsed positive feedback loop, which ratchets up the concentration of the sporulation master-regulator Spo0A to a critical level over multiple cell cycles. Finally, using mathematical modeling, we illustrate how a novel dynamic feedback mechanism, “polyphasic positive feedback,” lets cells defer sporulation more robustly than with other circuit strategies. Developing techniques that can access pulsing and time-delay dynamics with higher time resolution will enable us to determine if this polyphasic strategy provides a general design principle for the regulation of multi-cell-cycle deferral times seen in other systems.
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影响因子：
64.5
作者：
BURBULYS, D;TRACH, KA;HOCH, JA
通讯作者：
HOCH, JA
DOI：
10.1073/pnas.1002499107
发表时间：
2010-05-04
影响因子：
11.1
作者：
Chastanet, Arnaud;Vitkup, Dennis;Losick, Richard M.
通讯作者：
Losick, Richard M.
影响因子：
3.6
作者：
Kearns, DB;Chu, F;Losick, R
通讯作者：
Losick, R
影响因子：
64.8
作者：
Cai, Long;Dalal, Chiraj K.;Elowitz, Michael B.
通讯作者：
Elowitz, Michael B.
影响因子：
3.2
作者：
Eswaramoorthy, Prahathees;Duan, Daniel;Fujita, Masaya
通讯作者：
Fujita, Masaya