Cardiovascular Research Institute and Department of Biochemistry and Biophysics, University of California, San Francisco, San Francisco, CA 94158.
Mol Biol Cell. 2019 Jun 15;30(13):1610-1620. doi: 10.1091/mbc.E18-09-0560. Epub 2019 May 1.
The Ras-Map kinase (MAPK) cascade underlies functional decisions in a wide range of cell types and organisms. In B-cells, positive feedback-driven Ras activation is the proposed source of the digital (all or none) MAPK responses following antigen stimulation. However, an inability to measure endogenous Ras activity in living cells has hampered our ability to test this model directly. Here we leverage biosensors of endogenous Ras and ERK activity to revisit this question. We find that B-cell receptor (BCR) ligation drives switch-like Ras activation and that lower BCR signaling output is required for the maintenance versus the initiation of Ras activation. Surprisingly, digital ERK responses persist in the absence of positive feedback-mediated Ras activation, and digital ERK is observed at a threshold level of Ras activation. These data suggest an independent analogue-to-digital switch downstream of Ras activation and reveal that multiple sources of signal amplification exist within the Ras-ERK module of the BCR pathway.
Ras-MAP 激酶（MAPK）级联反应是多种细胞类型和生物体功能决策的基础。在 B 细胞中，抗原刺激后，正反馈驱动的 Ras 激活被认为是 MAPK 产生数字（全有或全无）反应的来源。然而，由于无法在活细胞中测量内源性 Ras 活性，我们无法直接测试该模型。在这里，我们利用内源性 Ras 和 ERK 活性的生物传感器来重新研究这个问题。我们发现 B 细胞受体（BCR）的交联驱动 Ras 的开关式激活，并且较低的 BCR 信号输出对于 Ras 激活的维持比起始更为重要。令人惊讶的是，即使没有正反馈介导的 Ras 激活，数字 ERK 反应仍然存在，并且在 Ras 激活的阈值水平观察到数字 ERK。这些数据表明，Ras 激活下游存在独立的模拟到数字的开关，并且揭示了 BCR 途径中的 Ras-ERK 模块内存在多种信号放大源。
