Applications of Fluorescent Protein-Based Sensors in Bioimaging

In the last two decades, there have been enormous developments in the area of reporter gene imaging for various bioimaging applications, especially to track cellular events that are occurring in intact cells and cells within living animals. As part of this process, there has been a significant interest in identifying new reporters or developing new substrates that can allow us to image multiple cellular events simultaneously without any signal overlap between the targets. Even though chemical dyes are useful for some of these applications, reporter proteins which mimic biological properties of proteins when tagged directly with the target proteins are very useful. Although molecular imaging has significantly advanced through use of different imaging probes (radiolabeled ligands, MR contrast agents, CT contrast agents, fluorescent dyes, fluorescent and bioluminescent proteins) and techniques (PET, SPECT, MRI, CT, optical, ultrasound, and photoacoustic imaging), optical imaging, such as fluorescence and bioluminescence imaging, has shown promising applications in various preclinical settings, especially in imaging cellular pathways and studies involving drug development. This is mainly owing to its simple and easy nature in performing the assay and also its high-throughput and cost-effective applications. In this chapter, we review the evolution of optical imaging with specific emphasis on fluorescent proteins, as well as an introduction regarding the general approach of optical imaging in in vitro and in vivo applications. We explain this by briefly introducing different optical imaging methods and fluorescent assays developed based on fluorescent dyes and fluorescent proteins followed by a detailed review of different fluorescent proteins currently used for various assay developments and applications.