In vivo optical imaging of deep tissues is most feasible between 650 and 900 nm because hemoglobin, water, and lipids minimally absorb these wavelenghts and tissue light-scattering is also minimal. Roger Tsien, last year's Nobel Prize in chemistry for his research on fluorescent proteins, describes now in a Science report a modified version of the Deinococcus radiodurans phytochrome engineered to be a Infrared Fluorescent Protein (IFP). The group transduced IFP into the mouse liver through an adenovirus-vector, and observed with a Maestro spectral imager that this infrared fluorescence protein performed better than a red fluorescent protein (mKate). For more background info read Brainwindows.
Actually, this advance makes me a little gloomy, since I spoke about mKate in my first post. It was September 2007, does it take only two years for a fluorescent protein to be outperformed? It is dramatic: it takes roughly two years to make a transgenic fluorescent mice, and another two-three years to get data with that model! Once you start, you know that your model will be surpassed at half of your journey.
The infrared war continues: two years later, IFP1.4 is outdated and we probably have a brighter infrared fluorescent protein with emission maxima at 713 nm. The new IFP, called iRFP comes from the laboratory of Vladislav Verkhusha and does not require exogeneous substrates like biliverdin. Luckily, the generation of transgenic mice promises to be more convenient thanks to zinc finger nuclease transgenesis approaches .
Shu, X., Royant, A., Lin, M., Aguilera, T., Lev-Ram, V., Steinbach, P., & Tsien, R. (2009). Mammalian Expression of Infrared Fluorescent Proteins Engineered from a Bacterial Phytochrome Science, 324 (5928), 804-807 DOI: 10.1126/science.1168683
Filonov GS, Piatkevich KD, Ting LM, Zhang Z, Kim K & Verkhusha VV (2011).
Bright and stable near-infrared fluorescent protein for in vivo imaging
Nature Biotechnology, 29, 757–761 DOI: 10.1038/nbt.1918