Dynamic visualization of signal transduction in living cells: from second messengers to kinases

The study of signal transduction, or the highly regulated series of biochemical events which allow a cell to convert a given stimulus into a functional response, has seen a paradigm shift with a recent explosion in the number of genetically encoded FRET-based biosensors capable of detecting spatial and temporal regulation of various signaling events in living cells. The two classes of biosensors discussed, namely kinase activity and second messenger biosensors, utilize two fluorescent proteins (FP) suitable for FRET and convert a signaling event of interest into a conformational change in the biosensor that can be measured as a change in FRET between the two FPs. Individually, these biosensors have been used to elucidate many complex signal transduction mechanisms in various biological systems. However, it has become increasingly clear that it is often more desirable to study multiple signaling events simultaneously, allowing for precise correlation of the temporal profiles of multiple signaling molecules without the complication of cell to cell variability. With the design of spectrally distinct biosensors and new coimaging strategies, simultaneous imaging of multiple signaling events is not only possible, but has aided in mapping the intricate network of cellular signal transduction cascades. Furthermore, as aberrant signal transduction involving second messengers and kinases is implicated in numerous disease states, it is hopeful that these FRET-based biosensors and coimaging strategies can help to unravel the molecular links between altered signal transduction and certain disease states.