Day, Richard N.Tao, WenDunn, Kenneth W.2017-07-132017-07-132016-11Day, R. N., Tao, W., & Dunn, K. W. (2016). A simple approach for measuring FRET in fluorescent biosensors using two-photon microscopy. Nature Protocols, 11(11), 2066-2080. http://doi.org/10.1038/nprot.2016.121https://hdl.handle.net/1805/13444Genetically encoded fluorescent protein (FP)-based biosensor probes are useful tools for monitoring cellular events in living cells and tissues. Because these probes were developed for one-photon excitation approaches, their broad two-photon excitation (2PE) and poorly understood photobleaching characteristics have made their implementation in studies using two-photon laser-scanning microscopy (TPLSM) challenging. Here we describe a protocol that simplifies the use of Förster resonance energy transfer (FRET)-based biosensors in TPLSM. First, the TPLSM system is evaluated and optimized using FRET standards expressed in living cells, which enables the determination of spectral bleed-through (SBT) and the confirmation of FRET measurements from the known standards. Next, we describe how to apply the approach experimentally using a modified version of the A kinase activity reporter (AKAR) protein kinase A (PKA) biosensor as an example—first in cells in culture and then in hepatocytes in the liver of living mice. The microscopic imaging can be accomplished in a day in laboratories that routinely use TPLSM.enPublisher Policytwo photon laser-scanning microscopyTPLSMFörster resonance energy transferArticle