Cells sense and respond to their environment by activating distinct intracellular signaling pathways, however signal transmission in individual cells is not well understood. To assess the accuracy of signal transmission in individual cells, we developed an optimized genetically encoded sensor for IGF-I signaling. We stably expressed this sensor in HeLa cells and used live-cell imaging to monitor dynamic responses to IGF-I in individual cells. Across the population, signaling responses overlapped between different IGF-I doses, suggesting limited transmission accuracy. However, analysis of individual cell traces revealed relatively constant responses over time. An information theoretic approach to calculate the channel capacity using variance of the single cell time course data--rather than population data--predicted that cells were capable of discriminating multiple growth factor doses. We validated these predictions by tracking individual cell responses to multiple IGF-I doses and found that cells can accurately distinguish at least four different IGF-I concentrations, and also that the input-output relation varies across the population of individual cells. Furthermore, by monitoring responses to the PI3K inhibitor alpelisib we found a similar discriminatory ability to pathway inhibition. Our studies indicate that heterogeneous responses to IGF-I arise from cells encoding the growth factor input into different signaling outputs and that individual cells can accurately sense and respond to a range of stimuli of varying strengths. These observations reveal the importance of viewing each cell as having its own communication channel and underscore the importance of understanding responses at the single cell level.

High resolution AKT signaling in individual cells