Abstract
Transient versus sustained ERK MAP kinase (MAPK) activation dynamics induce proliferation versus differentiation in response to epidermal (EGF) or nerve (NGF) growth factors in PC-12 cells. Duration of ERK activation has therefore been proposed to specify cell fate decisions. Using a biosensor to measure ERK activation dynamics in single living cells reveals that sustained EGF/NGF application leads to a heterogeneous mix of transient and sustained ERK activation dynamics in distinct cells of the population, different than the population average. EGF biases toward transient, while NGF biases toward sustained ERK activation responses. In contrast, pulsed growth factor application can repeatedly and homogeneously trigger ERK activity transients across the cell population. These datasets enable mathematical modeling to reveal salient features inherent to the MAPK network. Ultimately, this predicts pulsed growth factor stimulation regimes that can bypass the typical feedback activation to rewire the system toward cell differentiation irrespective of growth factor identity.
Highlights
Complex signaling networks allow cells to translate external stimuli into specific cell fates
By virtue of a nuclear export sequence, EKAR2G localizes to, and measures extracellular-regulated kinase (ERK) activity in the cytosol (Fig 1B). This does not seem to be true for all cell types (Ahmed et al, 2014), we assumed that cytosolic and nuclear pools of ERK activity are in equilibrium, since, at least for EGF-stimulated PC-12 cells, there is no apparent time lag between nuclear and cytosolic ERK activation dynamics (Herbst et al, 2011)
Given the strong similarity between the Western blot, immunofluorescence, and EKAR2G datasets, we assumed that FRET ratio measurements could be used directly as a proxy for ERK activity. These results indicate that, while EKAR2G expression has a slight impact on a specific phase of ERK activation dynamics, it remains a faithful indicator of ERK signaling
Summary
Complex signaling networks allow cells to translate external stimuli into specific cell fates. Stimulation with EGF or NGF leads to transient or sustained ERK activation dynamics, respectively, triggering proliferation or differentiation (Marshall, 1995; Avraham & Yarden, 2011) These different ERK activation dynamics involve activation of different Ras isoforms (Sasagawa et al, 2005), as well as GF-dependent control of the MAPK network topology (Santos et al, 2007), with negative or positive feedbacks producing adaptive or bistable outputs (Xiong & Ferrell, 2003; Santos et al, 2007; Avraham & Yarden, 2011). We dynamically probe the ERK signaling flux through application of GF pulses, which homogenizes ERK activation
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