The research team from the Worcester Polytechnic Institute (WPI) and CellThera, a private company located in WPI's Life Sciences and Bioengineering Center, applied a novel technique to manipulate mature human muscle cells into a stem cell-like state and then grew the reprogrammed cells on biopolymer microthreads. The researchers removed a large section of leg muscle from a mouse, placing the threads into the wound and found that with time, the threads and cells restored the muscle to almost its normal function.
To their surprise, the researchers discovered that the microthreads, which were simply used as a scaffold to support the reprogrammed human cells, actually appeared to speed up the generation process by recruiting progenitor mouse muscle cells. This indicates that these microthreads by itself could potentially serve as a therapeutic tool to treat major muscle trauma.
Author Raymond Page, assistant professor of biomedical engineering at WPI, and chief scientific officer at CellThera, announced: "We are pleased with the progress of this work, and frankly we were surprised by the level of muscle regeneration that was achieved."
The current study is part of a multi-year program that is partly funded by grants from the National Institutes of Health. The advanced research program of the U.S Department of Defense (DARPA), who also supports the study, encourages the development of new technologies and treatments for individuals sustaining serious wounds as well as limb loss.
In mammals, skeletal muscles are able to repair small injuries caused by excessive exertion or minor trauma, as they have the ability to repair themselves by recruiting muscle progenitor cells (i.e. cells that have not fully developed into muscle fibers), to the injury site to restore the muscle. The body's first priority in major injuries however is to stem the bleeding, which results in a rapid formation of scar tissue at the wound site that overrides any muscle repair.
In the current study, the WPI/CellThera team tried to prevent the formation of scars and prompt muscle re-growth by using a combination of two unique technologies.
The first method had been developed earlier by the team to reprogram mature human skin cells without using viruses or extra genes (Cloning, Stem Cells. 2009 Jul 21). Although the reprogrammed cells express stem cell genes and multiply in great numbers, they do not differentiate into specific tissues. The second method applied by the scientists was using biopolymer microthreads as a scaffold to support the cells. The microthreads, approximately the thickness of a hair, are made of fibrin, which is a protein that helps blood clot and were developed by George Pins, an associate professor of biomedical engineering at WPI.
The researchers removed a portion of the tibialis anterior leg muscle in several mice, as injury to this muscle does not prevent the mice from walking but it affects the foot's range of motion and left the injuries to heal on their own in some of the animals. The wounds of the other mice were filled with bundles of microthreads that contained reprogrammed human muscle cells.
They observed substantial scarring at the injury site with no restoration of muscle function in the untreated mice, whilst those that were implanted with the reprogrammed cells grew new muscle fibers and developed very little scarring. An examination at week 10 following the implantation demonstrated, the regenerated tibialis anterior muscle functioned with almost as much strength as an uninjured muscle.
Since the implanted cells were taken from human muscle, the scientists assumed the majority of the regenerated muscle also to be composed of human cells, however, to their surprise they discovered that most of the new muscle fiber was made of mouse cells. They hypothesized that the fibrin microthreads, which are similar to muscle fibers in composition and shape, may stimulate resident mouse progenitor cells to migrate into the wound and start restoring the tissue. The researchers believe that they may also prevent the natural inflammatory response that leads to scarring following a major injury.
This unsuspected result indicates that just fibrin microthreads alone could be used for the treatment of major muscle trauma. Research on enhancing regeneration with reprogrammed human cells continues.
The authors conclude:
"The contribution of the fibrin microthreads alone to wound healing should not be understated. While this clearly points to room for improving cell delivery techniques, it suggests that fibrin microthreads alone have tremendous potential for reducing fibrosis and remodeling large muscle injuries. Future studies will address, more completely, the capability of microthreads alone and determine, at what point, a combinational cell therapy is required for full functional tissue restoration."