Background:In individual cells, EGF stimulation results in sporadic pulses of ERK activity. Results: ERK pulses are disrupted upon stimulation by alternate receptors or inhibition of EGFR. Conclusion: ERK activity pulses are generated at the level of EGFR. Significance: The dynamics of ERK activity, which control cellular proliferation and gene expression, are inconsistent with models that rely on downstream feedback.
We formulated a computational model for a MAPK signaling cascade downstream of the EGF receptor to investigate how interlinked positive and negative feedback loops process EGF signals into ERK pulses of constant amplitude but dose-dependent duration and frequency. A positive feedback loop involving RAS and SOS, which leads to bistability and allows for switch-like responses to inputs, is nested within a negative feedback loop that encompasses RAS and RAF, MEK, and ERK that inhibits SOS via phosphorylation. This negative feedback, operating on a longer time scale, changes switch-like behavior into oscillations having a period of 1 hour or longer. Two auxiliary negative feedback loops, from ERK to MEK and RAF, placed downstream of the positive feedback, shape the temporal ERK activity profile but are dispensable for oscillations. Thus, the positive feedback introduces a hierarchy among negative feedback loops, such that the effect of a negative feedback depends on its position with respect to the positive feedback loop. Furthermore, a combination of the fast positive feedback involving slow-diffusing membrane components with slower negative feedbacks involving faster diffusing cytoplasmic components leads to local excitation/global inhibition dynamics, which allows the MAPK cascade to transmit paracrine EGF signals into spatially non-uniform ERK activity pulses.
mTORC1 senses nutrient and growth factor status and phosphorylates downstream targets, including the transcription factor TFEB, to coordinate metabolic supply and demand. The molecular mechanisms of mTORC1 activation are thought to enforce a strict requirement for simultaneous amino acid and growth factor stimuli, but this model has not been evaluated with quantitative or single-cell methods. Here, we develop a series of fluorescent protein-TFEB fusions and investigate how combinations of stimuli jointly regulate signaling from mTORC1 to TFEB at the single-cell level. Live-cell imaging of individual cells revealed that mTORC1-TFEB signaling responds with graded changes to individual amino acid and growth factor inputs, rather than behaving as a logical AND gate. We find that mTORC1 inputs can be sequentially sensed, with responses that vary between mTORC1 substrates and are amplified by input from other kinases, including GSK3β. In physiologically relevant concentrations of amino acids, we observe fluctuations in mTORC1-TFEB signaling that indicate continuous responsiveness to nutrient availability. Our results clarify how the molecular regulation of mTORC1 enables homeostatic processes at the cellular level and provide a more precise understanding of its behavior as an integrator of multiple inputs.
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