Magnetic Tag Lets Scientists Track Stem Cells In Living Rat

In the December issue of the journal Nature Biotechnology, the team said the neuronal stem cells take up and hold onto the spheres, which contain a compound of iron and oxygen. The iron-laden cells create a magnetic black hole easily spotted by magnetic resonance imaging, or MRI, they report.

"Until now, tissue had to be removed from an animal to see where stem cells were going, so this gives us an important tool," says author Jeff Bulte, Ph.D., associate professor of radiology at the Johns Hopkins School of Medicine. "Tracking stem cells non-invasively will likely be required as research advances, although human studies are still some time away."

Scientists at the University of Wisconsin School of Veterinary Medicine mixed the magnetic spheres, made by Trevor Douglas at Temple University, with stem cells that make the white matter, or neuronal covering, of the brain. Then they injected the iron-laden cells into the brains of rats that lack that covering. Using MRI scanners at the National Institutes of Health, Bulte watched the cells travel away from the injection site. The research was funded by the National Science Foundation, the Oscar Rennebohm Foundation and the Keck Foundation.

The rusty spheres, known as magnetic dendrimers, represent an important improvement over other magnetic tags, Bulte says. And even though the amount of iron used to label the cells is tiny compared to the total amount of iron in the body, the labeled cells stand out from other cells, magnetically speaking.

"During scanning, these labeled cells disturb the magnetic field created by the MRI machine, causing water molecules that pass by to get 'out of phase,'" he explains. "When this happens, the imaging scanner loses the signal, and the area looks black on the image."

Other researchers have used dendrimers containing gadolinium, which is also useful as a contrast medium for MRI, but which is toxic if it stays in the body for a prolonged time. But animal cells have a process to deal with iron and a storage mechanism for the metal, making the iron-based dendrimers inherently safer, says Bulte. For instance, iron is a key part of the transporter for oxygen and carbon monoxide found in red blood cells.

He adds that while it was not easy to develop the way to make magnetic dendrimers, it is easy to label cells with them. In essence, the dendrimer and the cell do that work themselves. Dendrimers stick to cells because they are charged -- kind of like static electricity. Cells then suck them inside and lock them away in the cellular equivalent of a garbage can -- a tiny holding spot called an endosome.

Other magnetic tags have used antibodies or other molecules that recognize and bind to certain features on cells, says Bulte. Unlike those tags, the magnetic dendrimers are universal; the scientists showed that different cell types will take in dendrimers just by mixing the spheres and the cells together, without affecting the cells' behavior.

Bulte's research with magnetic dendrimers is aligned with the Johns Hopkins Institute of Cell Engineering, created in early 2001 to advance research into the biology and potential application of pluripotent stem cells (primitive cells that become any type of cell in the body) and multipotent or adult stem cells (precursor cells that are naturally limited to becoming a specific tissue's cell types).

A next step with magnetic dendrimers, Bulte says, is watching the cells' distribution when they are injected into the circulatory system instead of the brain. Bulte also wants to study white blood cells in diseases of the central nervous system, such as multiple sclerosis, as well as the behavior of embryonic stem cells and stem cells from bone marrow. (Stem cells from bone marrow and blood have been used for decades in cancer treatments and more recently for some inherited metabolic disorders.)

Other co-authors of the report are Ian Duncan, Brian Witwer and Su-Chun Zhang of the University of Wisconsin School of Veterinary Medicine; Erica Strable of Temple University; Joseph Frank, Bobbi Lewis, Holly Zywicke, Brad Miller and Peter van Gelderen of the NIH; and Bruce Moskowitz of the Institute for Rock Magnetism at the University of Minnesota, Minneapolis. Bulte (NIH) and Douglas (Temple) have applied for a patent on these magnetic dendrimers.

On the Web: http://biotech.nature.com

Image available at: http://www.hopkinsmedicine.org/press/2001/DECEMBER/011214.htm