bio sphere
A new spin on MRI N
themselves with iron. Transduction had ow that transgenics, or the inserno toxic effects and did not impair protion of foreign genes, has come of liferation. Incubating the cells with an age, a method for monitoring transgene iron salt more than doubled the spinexpression over time in living organisms spin NMR relaxation rate, producing a is sorely needed. Eric Ahrens and cocorresponding increase in image contrast workers at Carnegie Mellon University compared with uninfected controls. and the University of Pittsburgh School of Medicine have recently developed a new MRI monitoring method that brings researchers one step closer to achieving that goal (Nat. Med. 2005, 11, 450– 454). In the new method, the researchers force brain cells in mice to acquire their own contrast agent by overexpressing the iron-storage protein ferritin. The ferritin-expressing cells load themselves with iron and appear darker than neighboring cells MRI detects iron stored by the expressed ferritin on MRI images. only at the injection site. (Courtesy of Nicole ReadCurrently, researchers can sequening and Eric Ahrens, Carnegie Mellon University.) tially monitor gene expression in culNext, Ahrens and co-workers injecttured cells, but this task is much more ed the vector and ferritin transgenes challenging in living animals. Typically, into the brains of living mice. MRI imonly a single point in time is assayed ages obtained with a microimaging sysafter an animal is sacrificed. The opacity tem five days later revealed clearly deof biological tissues and poor resolution limit the use of fluorescence microscopy fined and very localized regions of high or positron emission tomography for se- contrast in the injected area of each brain. The same vector was also used to rial monitoring in intact animals. Some deliver a control gene into the other pioneering papers suggested the use of side of the brain, where no increased MRI for monitoring gene expression. But Ahrens says, “We were looking for a contrast was seen. Histological analysis of mice sacriway to use MRI without having to supficed 1–5 days later showed that expresply any exogenous metal-complexed sion of both light and heavy ferritin agent.” That would be advantageous subunits occurred with the same time because contrast agents penetrate poorprogression and in the same location ly into many tissues. as the development of MRI contrast. The researchers incorporated the Thus, the contrast appeared to result genes for the heavy and light subunits from iron uptake by the recombinant of human ferritin into a vector. When ferritin. “So our main conclusion was expressed, these subunits form a shell that it is feasible to use only a genetiaround a core of ferrihydrite. When cally encoded reporter to impart extrinAhrens and co-workers transduced a sic MRI contrast to cells in intact anitumor cell line with the transgenes, the mals,” Ahrens says. cells overexpressed ferritin and loaded 288 A
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In a study reported two months before the Ahrens paper (Neoplasia 2005, 7, 109–117), a group led by Michal Neeman at the Weizmann Institute of Science (Israel) used ferritin as an endogenous MRI reporter. Instead of directly introducing the transgenes into living animals, these researchers inoculated rat glioma cells containing the reporter gene into the hind limbs of mice and then used MRI to detect transgene expression at various stages of tumor growth. “The general limitations of Ahrens’s approach compared with ours are related to sensitivity and specificity,” Neeman says. Ahrens envisions two main applications for the MRI reporter gene: monitoring of transgene expression over time in living animals and preclinical studies of gene therapy. For example, the reporter gene could be used to determine dosage and delivery parameters. Jeff Bulte at the Johns Hopkins University School of Medicine thinks the technology might be useful for longterm tracking of cells because current labels, such as magnetic iron-oxide particles, become diluted when a cell divides. “A reporter gene like this would be permanent because it would be replicated,” he points out. Scott Fraser at the California Institute of Technology says, “These are still early days. But with this sort of approach, it might be possible to do the bookkeeping on how stem cells interact with a host. What fraction of the cells goes to the right place and does the right thing? And what happens to those cells that don’t?” Meanwhile, Bulte applauds Ahrens’s novel contribution. “He got the gene, transfected it, did an in vivo experiment, and has nice confirming histology. He did a great job,” says Bulte. a —Linda Sage © 2005 AMERICAN CHEMICAL SOCIETY
