运动细胞自适应性过程中双相性现象的分子机制及动态数值模拟

盘基网柄菌细胞(Dictyostelium)和白细胞(leukocyte)等真核运动细胞受到外界信号刺激时,在最初的1～2 min内,胞内信号转导的首要成员PI(3,4,5)P3的浓度随时间变化呈现"双相性"(biphasic adaptation),即先后出现一大一小两个峰值,然后平息。为解释这一现象,特别是第二个峰值产生的原因,根据已有实验资料,分析了有关分子机制,建立了相应的数学模型。其中,PI(3,4,5)P3及其激活酶和抑制酶的浓度变化由一组耦合的非稳态反应-扩散方程描述,外界刺激及效应因子(如Rac和Scar/WAVE)的相互激励包含在源项中,并由蒙特-卡诺(Monte-Carlo)法处理,数值模拟结果与已有实验一致。研究发现,质膜上处于激活态的效应因子Scar/WAVE是影响PI(3,4,5)P3第二个峰值的关键,起正反馈作用。在受到胞外信号刺激后的前期,Scar/WAVE的激活态浓度受到小G蛋白Rac活性的抑制,后期反过来受到PI(3,4,5)P3的抑制,从而始终处于较低水平,这使得第二个峰值较小;当Scar/WAVE的总浓度低于0.005μmol/L后,PI(3,4,5)P3不会出现第二个峰值。由于Scar/WAVE是肌动蛋白结合蛋白,可以推测:许多经肌动蛋白合成抑制剂处理过的盘基网柄菌细胞在实验中仍然出现"双相性",应与此时的细胞骨架活性未被完全抑制有关。

Molecular Mechanisms and Dynamic Simulations of Biphasic Adaptation of Chemotatic Cells

Some eukaryotic cells,such as Dictyostelium and leukocyte,are chemotactic.As a response of the cell to extracellular attractants in the initial 1～2 minutes,a curial regulator PI(3,4,5)P3 in plasma membrane appears in two peaks of concentration,one large and one small,then in silence,i.e.in a biphasic adaptation.To interpret this biphasic phenomenon,especially the origins of the second peak,a mathematical model was constructed based on the principle of "local excitation and global inhibitation" as well as the mechanism of positive feedback in signal transduction.The evolution of PI(3,4,5)P3 along with the activation and inhibitation of enzymes(i.e.PI3K,PTEN) was described by a set of coupled transient diffusion-reaction equations.The source terms include the stochastic effects of stimulation of extracellular attractants(i.e.cAMP) and interaction of intracellular effectors(i.e.Rac,Scar/WAVE),which were treated by a Monte-Carlo method.And the numerical simulations agree well with the existing experiments.It shows that the level of activated Scar/WAVE in plasma membrane is the key to prompt the second peak of PI(3,4,5)P3,and the minimum required concentration of Scar/WAVE is 0.005 μmol/L.Furthermore,as a actin binding protein,Scar/WAVE is controlled initially by Rac and lately by PI(3,4,5)P3 inversely,and is always in lower level,therefore the second peak is much smaller than the first one.The results suggest that the biphasic is closely related to cytoskeleton remodeling.That latrunculin-treated Dictyostelium cells still appear in biphasic adaptation should be attributed to the incomplete inhibitation of polymerization of cytoskeleton F-actins.