Researchers at Johns Hopkins have devised a way to detect whether cells
previously transplanted into a living animal are alive or dead, an innovation
they say is likely to speed the development of cell replacement therapies for
conditions such as liver failure and type 1 diabetes. As reported in the March issue of Nature
Materials, the study used nanoscale pH sensors and magnetic resonance
imaging (MRI) machines to tell if liver cells injected
into mice survived over time.
"This technology has the potential to turn
the human body into less of a black box and tell us if transplanted cells are
still alive," says Mike McMahon, Ph.D., an associate professor of radiology at the Johns Hopkins University School
of Medicine who oversaw the study. "That information will be invaluable in
fine-tuning therapies."
Regenerative medicine advances depend on
reliable means of replacing damaged or missing cells, such as injecting
pancreatic cells in people with diabetes whose own cells don't make enough
insulin. To protect the transplanted cells from the immune system, while
allowing the free flow of nutrients and insulin between the cells and the body,
they can be encased in squishy hydrogel membranes before transplantation. But,
explains McMahon, "once you put the cells in, you really have no idea how long
they survive." Such transplanted cells eventually stop working in most patients,
who must resume taking insulin. At that point, physicians can only assume that
cells have died, but they don't know when or why, says McMahon.
With
that problem in mind, McMahon's group, which specializes in methods of detecting
chemical changes, collaborated with the research group headed by Jeff Bulte,
Ph.D., the director of cellular imaging at Hopkins' Institute for Cell
Engineering. Bulte's group devises ways of tracking implanted cells through the
body using MRI. Led by research fellow Kannie Chan, Ph.D., the team devised an
extremely tiny, or nanoscale, sensor filled with L-arginine, a nutritional
supplement that responds chemically to small changes in acidity (pH) caused by
the death of nearby cells. Changes in the acidity would in turn set off changes
in sensor molecules embedded in the thin layer of fat that makes up the outside
of the nanoparticle, giving off a signal that could be detected by MRI.
To test how these nanosensors would work in a living body, the team
loaded them into hydrogel spheres along with liver cells - a potential therapy
for patients with liver failure - and another sensor that gives off
bioluminescent light only while the cells are alive. The spheres were injected
just under the skin of mice. As confirmed by the light signal, the MRI
accurately detected where the cells were in the body and what proportion were
still alive. (Such light indicators cannot be used to track cells in humans
because our bodies are too large for visible signals to get through, but these
indicators allowed the team to check whether the MRI nanosensors were working
properly in the mice.)
"It was exciting to see that this works so well
in a living body," Chan says. The team hopes that because the components of the
system - hydrogel membrane, fat molecules, and L-arginine - are safe for humans,
adapting their discovery for clinical use will prove relatively straightforward.
"This should take a lot of the guesswork out of cell transplantation by letting
doctors see whether the cells survive, and if not, when they die," Chan says.
"That way they may be able to figure out what's killing the cells, and how to
prevent it."
Potential applications of the sensors are not limited to
cells inside hydrogel capsules, Bulte notes. "These nanoparticles would work
outside capsules, and they could be paired with many different kinds of cells.
For example, they may be used to see whether tumor cells are dying in response to chemotherapy," he says.
Other authors on the paper were Guanshu Liu, Xiaolei Song, Heechul Kim, Tao Yu,
Dian R. Arifin, Assaf A. Gilad, Justin Hanes, Piotr Walczak and Peter C. M. van
Zijl, all of the Johns Hopkins University School of Medicine.
The study was
funded by the National Institute of Biomedical Imaging and Bioengineering (grant
numbers R01 EB012590, EB015031, EB015032 and EB007825).
The paper can be
found here: http://www.nature.com/nmat/journal/vaop/ncurrent/abs/nmat3525.html.
Johns Hopkins
Medicine
Source - Medical News Today
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