According to the Muscular Dystrophy Association, about 250,000 people in the United States have some form of the disease. The most well known, Duchenne muscular dystrophy, is caused by a genetic mutation that disrupts the formation of dystrophin, an important protein involved in the formation of muscle cells. In the absence of dystrophin, muscles are unable to regenerate, and they gradually weaken and waste away. Eventually, the deteriorated area is taken over by fat and connective tissue.
Rita Perlingeiro, assistant professor of developmental biology at UT Southwestern, says that embryonic stem cells may be the key to reversing muscular dystrophy's debilitating effects. The advantage lies in the cells' pluripotency--the ability to transform into any mature cell, be it bone, muscle, or cartilage. However, many researchers have found it difficult to direct every stem cell in a culture to form a specific type of cell. In lab experiments, scientists often end up with a mixture of cells that, when injected into an animal, form large clusters resembling a tumor.
So Perlingeiro and her team set two main goals: to find the right set of cues to convert embryonic stem cells into muscle cells, and to look for ways to isolate muscle cells from the rest of the culture medium, in order to inject a dose of pure muscle cells into a mouse model.
In normal embryologic development, stem cells turn into various tissue and bone, depending on a combination of molecular and genetic signals. In the case of muscle cells, past research has shown that the gene Pax-3 is essential in pointing stem cells down the path of muscle formation. With this knowledge, Perlingeiro and her team grew mouse-derived embryonic stem cells in a culture dish, then genetically manipulated the solution to overexpress Pax-3. They found that, compared with mixtures without Pax-3, a significant number of stem cells exposed to the activated gene formed muscle cells.
However, not all of the cells turned into muscle, and when the team injected the solution into a mouse with a mild form of muscular dystrophy, the mixture caused tumors to form. The team then focused on developing an identification process that would make muscle cells stand out from the rest of the solution. Once again, Perlingeiro looked to basic developmental research and found that, during normal muscle formation in the embryo, stem cells that become very early versions of muscle cells display certain surface molecules, or markers. The team repeated the first phase of its experiment, exposing embryonic stem cells to Pax-3, and looked for the telltale markers indicating muscle cells. The researchers then isolated these cells, creating a solution that consisted solely of muscle cells.
In preparation for injecting the new solution into a mouse model, the team first injected cardiotoxin into the mouse's leg. The effect inhibited the production of dystrophin, causing a weakening of the muscle--a condition resembling muscular dystrophy. Perlingeiro and her colleagues then injected the mouse with the muscle-cell solution. The team then took muscle biopsies and, after immuno-staining, found that, compared with mice that did not receive the solution, treated mice exhibited more dystrophin, indicating healthy muscle regeneration.
To confirm their results, the researchers ran both groups of mice on a treadmill; they found that the mice that received the solution outlasted the group that did not. Perlingeiro went a step further: after sacrificing both animal groups, she and her colleagues extracted every leg muscle, treated or untreated. They then placed each muscle in a bath and tested its strength by exposing it to an electrical impulse. The team found that the stronger contractions came from the muscles treated with the stem-cell-derived solution.
Perlingeiro says that the study's results are encouraging, as she envisions one day providing stem-cell-based therapy for people with muscular dystrophy and other muscle-related diseases. However, there will have to be more follow-up studies before the technique can be applied to humans.
"I have a long to-do list," says Perlingeiro. "We'd like to use the same technique on human embryonic stem cells."
Recently, researchers were able to turn human skin cells into embryonic stem cells, a technique that bypasses the thorny issues currently surrounding use of embryonic stem cells. Perlingeiro says that combining this technique with her muscle-deriving method may one day yield effective, efficient treatment of diseases such as muscular dystrophy.
"If we can reprogram skin cells to become pluripotent, and use Pax-3 to make muscle, then we would be able to make cells from the patient, and we wouldn't face ethical issues or problems of rejection," says Perlingeiro.
Paul Muhlrad, a research program coordinator for the Muscular Dystrophy Association, says that the study's results are a promising step toward effective treatment for muscle-related diseases. "These researchers present a nice proof of principle that embryonic stem cells can be turned into muscle-producing cells in the laboratory and used to deliver healthy muscle to people with Duchenne muscular dystrophy," says Muhlrad. "Of course, these experiments were done with mice. We've yet to see whether they will work in humans, but this study offers us much hope."
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