Muscle Builders

Zebrafish and stem cells enable fast drug discovery for muscle disorders

Newly created muscle progenitor cells (green) and muscle fibers (red) in a zebrafish embryo. Image: Zon Lab

Scientists have had little success growing skeletal muscle for people with muscular dystrophy and other disorders that degrade and weaken muscle. Undertaking experiments in zebrafish, mice and human cells, researchers have now identified a way to do that, creating muscle cells that the team hopes to see tested in patients in the next several years.

What really excites principal investigator Leonard Zon, professor of stem cell and regenerative biology and Grousbeck Professor of Pediatrics at Harvard Medical School and Boston Children’s Hospital, is the power of the chemical screening platform he and his colleagues used. Described Nov. 7 in the journal Cell, in a matter of weeks it revealed a cocktail of three compounds that induced human muscle cells to grow. Zon believes it could fast-track drug discovery for multiple disorders.

The Zon Lab has already made a name for itself with its zebrafish drug-screening platform, using it to identify a drug that boosts production of blood stem cells. Originally developed to treat stomach ulcers, that drug is now in a Phase II clinical trial to help patients receiving cord blood transplants recover their immune function more quickly.

Zebrafish chemical screening on steroids

Zon’s original technique involved loading tiny, fast-growing zebrafish into 96-well plates, adding a different chemical to each well and then examining the fish in the lab to see which ones displayed the desired effect.

However, the process involved laborious and time-consuming biochemistry. Zon and colleagues wanted something faster. So, in their first step, they placed tiny zebrafish blastomeres—cells from very early-stage embryos—into 384-well culture dishes. They also genetically modified the blastomeres so any newly created muscle progenitor cells would show up in fluorescent green and muscle fibers in red.

“Automated imagers can take a picture to assay all the 384 wells within a matter of a week, compared with several months with our earlier technique,” Zon explained.

The colors are readily visible in the developing embryos, which, unlike adult zebrafish, are see-through.

Within a week, this rapid culture system enabled them to narrow 2,400 different chemicals down to just six.

Validation in mice and human cells

For the second step, the Zon Lab and collaborators at the Joslin Diabetes Center tested the six compounds in a mouse model of Duchenne muscular dystrophy. One compound, forskolin, increased engraftment of progenitor cells into the muscles of the mice, thereby increasing muscle mass. Forskolin has been used since ancient times for a variety of disorders and is sold as a dietary supplement.

In the final step, the team took skin cells from patients with Duchenne muscular dystrophy and converted them into induced pluripotent stem cells (iPS cells), a technique increasingly used to model disease in a dish.

“Many muscle diseases could not be adequately modeled in iPS cells in the past,” noted Zon. “No one’s been able to get iPS cells to form skeletal muscle.”

It turned out that a cocktail of forskolin and two other small molecules could reprogram iPS cells into muscle cells, and that these cells could successfully engraft in mice.

Zon’s next goal is to transplant them into patients in a clinical trial he expects to be developed in several years. The first trial might focus on increasing muscle mass in a specific anatomic location, but for a condition like Duchenne muscular dystrophy, Zon envisions a series of local injections in different parts of the body.

Broader applications

Zon believes that the rapid zebrafish screening platform—for which he’s filed a patent—will greatly speed up therapeutic development when combined with patient-specific iPS cells, allowing scientists to quickly vet large numbers of compounds and possible culture conditions.

“We’ve created a very fast system to find what chemicals you need,” he said. “Because it’s so fast, we don’t need a hypothesis to do an experiment—you can just do the screen, find the result and see what warrants pursuing.”

Funding for this study included the National Institutes of Health (5P30 DK49216-19, 5R01CA103846-10, DP2OD004345, UO1HL100402 and DK31036), the Harvard Stem Cell Institute and the Howard Hughes Medical Institute. Zon is a founder and stockholder of Fate Therapeutics Inc., which is sponsoring the immune function clinical trial.

Adapted from a Boston Children’s news article.