New labeling proteins that fluoresce in the infrared spectrum allow scientists to see deep inside the body of living mammals without lifting a scalpel, according to a study published in Science tomorrow (May 8).
Researchers in the lab of Roger Tsien, who received the 2008 Nobel Prize in Chemistry for his role in the development of green fluorescent protein (GFP) to label tissue, have now come up with a new marker, called infrared-fluorescent protein (IFP), specifically designed for in vivo studies of whole animals. They used the molecule, derived from a mutated bacterial protein, to visualize the livers of living mice.
The technique "is a tremendous advantage," said Gary Borisy, the director and chief executive of the Marine Biological Laboratory in Woods Hole, Massachusetts. "It's possible to look deeper inside tissue, look deeper inside our bodies, or look deeper inside organs with infrared."
The application of GFP to cellular imaging 15 years ago allowed scientists to observe cellular processes in unprecedented detail. "Because it is genetically encoded, you just fuse GFP DNA to the protein you're studying," said Xiaokun Shu, a postdoc in Tsien's lab at the University of California, San Diego, and the study's lead author. GFP and a variety of color variants, however, emit visible light, which can only penetrate the skin. "Visible light is significantly absorbed by the brain tissue and scattered by bone," said Shu. Infrared light, on the other hand, can shine all the way through a cat's head, he said.
The visible light spectrum spans from around 400 nanometers (nm) to 700nm in wavelength. Infrared light is just above this range, with slightly longer wavelengths than visible red light. In this study, the researchers synthesized a bacterial pigment known to fluoresce at 622nm, the low end of the red spectrum, when bound to a naturally occurring molecule called biliverdin. (Biliverdin is formed during the breakdown of heme and found in all aerobic organisms.) They subjected this molecule to several rounds of mutagenesis to engineer a protein that fluoresced at 710nm, just outside the visible spectrum. They then fused the IFP protein to an adenoviral vector and injected the construct into the tail vein of mice to deliver it to the liver.
Within five days of injection, the IFP caused the liver to fluoresce weak infrared light, binding to the biliverdin naturally produced in the mice. Giving an injection of exogenous biliverdin enabled the researchers to boost the signal fivefold and view whole livers through the shaved skin of the mice.
The IFP signal is still inefficient relative to that of visibly fluorescing proteins. While GFP emits around 80% of the light it absorbs, the IFPs developed in this study only emit 7%. Even so, said Borisy, "it's an excellent start on which to build further development."