IR fluorescent proteins shown to enable whole-body imaging in small animals

MAY 11, 2009--Green fluorescent protein (GFP) has been critical to bioscience; its discovery and development netted the 2008 Nobel Prize in Chemistry for Roger Tsien and two other scientists. But its wavelengths are not long enough to allow a living animal's inner cells to glow. Now a University of California, San Diego research team--led by Tsien, professor of pharmacology, chemistry and biochemistry--reports that bacterial proteins called phytochromes can be engineered into infrared-fluorescent proteins (IFPs) whose wavelength can penetrate tissue. Thus, the proteins are suitable for whole-body imaging in small animals.

"The development of IFPs may be important for future studies in animals--to find out how cancers develop, how infections grow or diminish in mice, or perhaps how neurons are firing in flies," said Tsien.

First author Xiaokun Shu, PhD, of the UC San Diego School of Medicine's Department of Pharmacology and the Howard Hughes Medical Institute, coerced the phytochrome from the bacteria Deinococcus radiodurans to fluoresce--the first protein to glow in infrared and work in mouse models. A phytochrome is a photoreceptor--a pigment that plants and bacteria use to detect light--which is sensitive to light in the red and far-red region of the visible spectrum.

"IFPs express well in mammalian cells and spontaneously incorporate biliverdin, a green pigment that is present in humans and other mammals," said Tsien. Biliverdin is the substance responsible for the yellowish-green color of a bruise as it fades, for example. Biliverdin normally has negligible fluorescence. However, Shu was able to coax the biliverdin-containing protein to fluoresce by cutting off the parts of the phytochrome that divert the energy of the light.

"We hoped that by doing so, the light's energy wouldn't go anywhere else but would instead go out and become fluorescent," Shu said, adding that the protein is "moderately fluorescent, but we still have a long way to go."

Tsien stated that while this work is promising for future studies in animal models, he doesn't think it will be applied directly to imaging in humans for several reasons.

"First, all fluorescent proteins derived from corals, jellyfish, and now bacteria are powerful in basic research because they are encoded by a gene," said Tsien. "Introducing such genes into people would pose big scientific and ethical problems."

He explained that, secondly, humans are still too thick and opaque for the infrared fluorescence to get deep inside our bodies, although scientists can now see faintly through a mouse with infrared, because mice are so much smaller.

The Tsien lab is working on a different project to develop a technique without these limitations, one that can be used for imaging in humans. His hope is that, one day, people will be able to go in for their annual check ups and know if they have cancer because tumors will light up by magnetic resonance imaging of diagnostic molecules.

But for now, Tsien, Shu and their colleagues at UC San Diego hope that the prototype they have developed can be used to make other, improved fluorescent bacterial proteins from among the huge numbers harnessed from other organisms--IFPs that can be used in important animal studies.

The team's findings appear in the May 8 edition of the journal Science. This technology (SD2008-303) and related technologies are available for licensing and commercial development through the UCSD Technology Transfer Office.

For more information see the paper, Mammalian Expression of Infrared Fluorescent Proteins Engineered from a Bacterial Phytochrome in Science. See also the UCSD Technology Transfer Office site.

Posted by Barbara G. Goode, barbarag@pennwell.com, for BioOptics World.

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