Genetically encoded proteins create MRI contrast
Superparamagnetic particles, such as iron oxides, are useful contrast agents, but when they are used to follow gene expression in rapidly dividing cells, the particles become so diffuse that they cease to be effective. A newly developed genetically encoded label can increase contrast for MRI without using superparamagnetic metals to accomplish this. The label is based instead on a lysine-rich protein reporter that is genetically engineered into tumor cells.
Researchers from Johns Hopkins University School of Medicine in Baltimore developed the genetically encoded label. Assaf A. Gilad, Michael T. McMahon, Peter C.M. van Zijl and Jeff W.M.Bulte sought to create a biodegradable contrast material detectable in the micromolar range that can distinguish viable from nonviable cells and that enables a constant endogenous level of expression even after multiple cell divisions.
The label is based on a method of enhancing contrast that involves using artificial proteins designed to alter contrast by reducing the signal from available water molecules. Called chemical-exchange saturation transfer, the contrast can be switched on and off by selectively irradiating a sample with particular radio frequencies.
The researchers developed oligonucleotides that encode the lysine-rich reporter protein and then transfected into rat cells a vector that expresses the reporter in tandem with enhanced GFP. For in vivo tests, the vector was tested in mouse brains. Each brain was inoculated in one hemisphere with the vector and in the other with a control vector that contained GFP only. The results demonstrated that the vector successfully encoded the reporter protein and that the protein provided effective contrast when compared with the control tumor.
According to Bulte, the reporter could eventually be useful for clinical MRI studies. “We expect the sensitivity of the technique to be slightly reduced at the field strengths that are used clinically,” Bulte explained. “On the other hand, we are improving the sensitivity of our reporter genes.”
Using a new method for genetically engineering an MRI contrast-enhancing protein called a lysine-rich reporter protein, researchers used the contrast enhancement in rat glioma to distinguish between the experimental tumor containing the protein (a) and the control tumor (b), which had only enhanced GFP. Reprinted with permission of Nature Biotechnology.
He added that, because the method involves a protein, it may be simpler to gain regulatory approval for clinical studies. “There are no toxic metals or chemicals involved,” he said. “We have not seen any toxicity so far; we believe the only concern for clinical application is the use of recombinant DNA.”
The researchers are working to develop slightly different reporters that have distinguishable magnetic resonance labeling frequencies. The specific frequencies can be processed to show up as artificial colors in the MRI image, similar to the way green and red fluorescent proteins can be used in multiple labeling of cells. The group also noted that the protein is strongly sensitive to the pH of the cellular environment, which may allow it to be used to detect ischemia or cell death.
Nature Biotechnology, February 2007, pp. 217-219.
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