There are several techniques used by researchers and physicians to image the internal organs of people and animals, but each of these techniques have their shortcomings. X-rays and computed tomography (CT) scanning, for instance, involve exposing the subject to radiation. Magnetic resonance imaging (MRI) is safer, although subjects must sometimes ingest a contrasting agent in order to obtain more distinct images. The use of injected colored fluorescent proteins is another approach, but has been limited by the fact that hemoglobin in the subject's blood absorbs much of the wavelength of the light used for imaging. Now, however, scientists from New York's Albert Einstein College of Medicine of Yeshiva University have engineered a new fluorescent protein that sidesteps this limitation.
The protein was created in the lab of associate professor of anatomy and structural biology, Vladislav Verkhusha. Named iRFP, it is based on a bacterial phytochrome, which is a pigment that certain bacteria use to detect light. Unlike other fluorescent proteins, which respond to visible light wavelengths such as green, blue and red, iRFP both absorbs and emits near-infrared light. Mammalian tissues (and hemoglobin) are nearly transparent in this wavelength, so they don't obscure images of cells tagged with the protein.
To test the technology, researchers injected mice with adenovirus particles containing the gene for iRFP. Once those particles reached and infected the liver, the infected cells expressed that gene, producing the protein. The mice were then exposed to near-infrared light and placed on a whole-body imaging device, at which point their fluorescent livers were easily imaged. While images were first discernible two days after infection, the fluorescence was at its highest at the five-day mark.
Subsequent tests indicated that iRFP is nontoxic.
"Our study found that iRFP was far superior to the other fluorescent proteins that reportedly help in visualizing the livers of live animals," said Grigory Filonov, Ph.D., a postdoctoral fellow in Dr. Verkhusha''s laboratory. "iRFP not only produced a far brighter image, with higher contrast than the other fluorescent proteins, but was also very stable over time. We believe it will significantly broaden the potential uses for noninvasive whole-body imaging."