Quantitative single cell analysis uncovers the life/death decision in CD95 network

CD95/Fas/APO-1 is a member of the death receptor family that triggers apoptotic and anti-apoptotic responses in particular, NF-κB. These responses are characterized by a strong heterogeneity within a population of cells. To determine how the cell decides between life and death we developed a computational model supported by imaging flow cytometry analysis of CD95 signaling. Here we show that CD95 stimulation leads to the induction of caspase and NF-κB pathways simultaneously in one cell. The related life/death decision strictly depends on cell-to-cell variability in the formation of the death-inducing complex (DISC) on one side (extrinsic noise) vs. stochastic gene expression of the NF-κB pathway on the other side (intrinsic noise). Moreover, our analysis has uncovered that the stochasticity in apoptosis and NF-kB pathways leads not only to survival or death of a cell, but also causes a third type of response to CD95 stimulation that we termed ambivalent response. Cells in the ambivalent state can undergo cell death or survive which was subsequently validated by experiments. Taken together, we have uncovered how these two competing pathways control the fate of a cell, which in turn plays an important role for development of anti-cancer therapies.CD95/Fas/APO-1 作为死亡受体家族成员，可介导细胞凋亡与抗凋亡应答，其中尤以核因子κB（NF-κB）通路的激活为代表。这类应答在细胞群体中呈现显著的异质性。为阐明细胞如何做出生死抉择，我们构建了一套基于CD95信号通路成像流式细胞术（imaging flow cytometry）分析的计算模型。本研究发现，CD95刺激可同时在单个细胞内激活半胱天冬酶（caspase）与核因子κB通路。相关的生死决策严格取决于两类细胞间差异：一方面是死亡诱导信号复合物（death-inducing complex, DISC）形成过程中的细胞间变异（外在噪声，extrinsic noise），另一方面则是核因子κB通路的随机基因表达（内在噪声，intrinsic noise）。此外，我们的分析还揭示，凋亡与核因子κB通路的随机性不仅会导致细胞存活或死亡，还会引发第三种CD95刺激应答类型——我们将其命名为双向应答（ambivalent response）。处于双向应答状态的细胞既可能发生凋亡也可存活，该结论后续已通过实验验证。综上，我们阐明了这两条相互竞争的通路如何调控细胞命运，而细胞命运的调控对于抗癌疗法的开发具有重要意义。