A practical method for monitoring FRET-based biosensors in living animals using two-photon microscopy

• Published in: American Journal of Physiology: Cell Physiology Vol. 309; no. 11; pp. C724 - C735
• Main Authors: Tao, Wen, Rubart, Michael, Ryan, Jennifer, Xiao, Xiao, Qiao, Chunping, Hato, Takashi, Davidson, Michael W, Dunn, Kenneth W, Day, Richard N
• Format: Journal Article
• Language: English
• Published: United States American Physiological Society 01.12.2015
• Series: Cell Signaling: Proteins, Pathways and Mechanisms
• Subjects: Animals; Biology; Biosensing Techniques - methods; Biosensors; Call for Papers; Cells; Cells, Cultured; Fluorescence Resonance Energy Transfer - methods; Fluorescent Dyes - analysis; HEK293 Cells; Humans; Luminescent Proteins - analysis; Mice; Microscopy; Microscopy, Confocal - methods; Microscopy, Fluorescence, Multiphoton - methods; Myocytes, Cardiac - chemistry; Rodents; Förster resonance energy transfer; fluorescent proteins; two-photon excitation; PKA activity; intravital microscopy; biosensor probe
• ISSN: 0363-6143; 1522-1563
• DOI: 10.1152/ajpcell.00182.2015

The commercial availability of multiphoton microscope systems has nurtured the growth of intravital microscopy as a powerful technique for evaluating cell biology in the relevant context of living animals. In parallel, new fluorescent protein (FP) biosensors have become available that enable studies of the function of a wide range of proteins in living cells. Biosensor probes that exploit Förster resonance energy transfer (FRET) are among the most sensitive indicators of an array of cellular processes. However, differences between one-photon and two-photon excitation (2PE) microscopy are such that measuring FRET by 2PE in the intravital setting remains challenging. Here, we describe an approach that simplifies the use of FRET-based biosensors in intravital 2PE microscopy. Based on a systematic comparison of many different FPs, we identified the monomeric (m) FPs mTurquoise and mVenus as particularly well suited for intravital 2PE FRET studies, enabling the ratiometric measurements from linked FRET probes using a pair of experimental images collected simultaneously. The behavior of the FPs is validated by fluorescence lifetime and sensitized emission measurements of a set of FRET standards. The approach is demonstrated using a modified version of the AKAR protein kinase A biosensor, first in cells in culture, and then in hepatocytes in the liver of living mice. The approach is compatible with the most common 2PE microscope configurations and should be applicable to a variety of different FRET probes.