Repairing bones with stem cells

When you break a bone, you get a cast, and in a few weeks or months, your fracture heals. But what happens when the injury is so severe that the bone can’t knit itself back together? Or when the patient has osteoporosis, diabetes or another condition that impairs bones’ healing ability? If you are lucky, you might have a metal rod implanted that helps hold everything together, but in severe cases, people can lose limbs.

Researchers at Texas A&M are trying to find a better way.

Carl Gregory, Ph.D., an associate professor in the Institute for Regenerative Medicine (IRM) at the Texas A&M Health Science Center College of Medicine, is working on using mesenchymal stem cells (MSCs) to heal bones faster and more effectively.

MSCs are not the same as the embryonic stem cells that are the source of so much controversy. Instead, MSCs primarily come from adult bone marrow and fat tissue. They form a number of types of connective tissue, and when they are injected at the site of a bone injury, they can (in theory, at least) change, or differentiate, into the cells that repair bones.

However, the studies testing this in live animals haven’t worked as well as researchers had hoped.

“It’s taken us quite a while to discover why these cells do so well in the dish, but they don’t do well in animal models,” said Gregory, who joined the Texas A&M Health Science Center in 2008. Although about two teaspoons of adult bone marrow can be cultured into millions of cells, the process is very complicated: the cells have to be grown correctly and primed in such a way that they know they are going to become a bone-producing cell. At the same time, if a cell is primed too much, it dies rapidly after it is put into the body.

Even when all of this is done correctly, the cells tend to disappear after a few weeks at the site of the bone fracture. “The body will only invest a certain amount of time to bridge a bone injury with new tissue, usually around two weeks for a small animal, and a bit longer for a person. If this process fails, the injury will not heal.” Gregory said. To extend this time, he and his biomedical and electrical engineering colleagues in the Texas A&M Dwight Look College of Engineering are developing a way to inject the cells, along with an accompanying “biomatrix,” to the injured area. This biomatrix is essentially a mix of proteins that make up young bone tissue, and research done by Gregory and his team and supported, in part, by the National Institutes of Health (NIH), has shown that the matrix can help the cells stay put and create a living, bone-like construct. The engineers are now working on better ways, including nanotechnologies, to deliver the cells and the matrix.

“The innovation here is the understanding of the developmental process that occurs when bone heals and our ability to basically replicate it with a treatment,” Gregory said. He became interested in the regeneration of damaged tissue while getting his Ph.D. in Biochemistry & Molecular Biology from the University of Manchester in the United Kingdom. “I was always interested in structural biology,” he said, but I didn’t really start to focus on MSCs until I came to Tulane University and started working with Dr. Darwin Prockop, one the leading authorities on the subject.” Prockop is now the director of the IRM, a state-of-the-art institute of about 50 members working in many areas of regenerative medicine including diabetes, cancer, transplant rejection, heart disease, and traumatic brain injury, in addition to bone regeneration.

Putting MSCs to the test

Collaborating with researchers in the Texas A&M College of Veterinary Medicine & Biomedical Sciences (CVM), Gregory and team hope to utilize a similar approach in dogs with naturally occurring injuries.

Brian Saunders, DVM, Ph.D., diplomate ACVS, is the director of the Canine Comparative Orthopedics & Cellular Therapeutics Lab at the CVM. His team has been working to more rigorously characterize adult MSCs from several canine tissues and to partner with Gregory to use his team’s approach to create a method to enhance the bone healing capacity of canine MSCs.

“We have a lot to learn about dog stem cells,” Gregory said. “Dog physiology is not as similar to human physiology as you might think.” While a modest number of publications exist documenting that canine MSCs have similar characteristics of human ones, little is known about the ability of canine MSCs to differentiate into bone, or to serve as cell-therapy agents for use in treating challenging, non-healing fractures.

“Our team has recently completed a large, rigorous, donor-matched study in which we have compared the properties of canine MSCs from several tissues,” Saunders said. “We found that there is tremendous variability in the cell’s properties based on not only the source of the canine cells (what tissue they were isolated from), but also the donor that provided those cells. Furthermore, the canine cells behave quite differently in classic assays used to assess human cells. Our goal is to use Dr. Gregory’s approach to improve the ability of canine MSCs to differentiate into bone, and to use those cells and bone-friendly scaffolds to heal challenging fractures that might result in limb amputation or euthanasia in client-owned animals.”

Human (or canine) MSCs taken from adult donors in their physical prime are purified and allowed to divide. The donor cells can only make a certain amount of cells before they stop working and die, which makes them difficult to research but is actually a good thing when the cells are to be injected into an adult animal, because unchecked cell division causes tumors.

“We would like to extend our canine studies, not just as a test model, but because there are dogs out there that require the kind of therapy that we can offer,” Gregory said. “If these approaches succeed, we will go into human trials.”

Although physicians can take cells from patients, concentrate them, clean them up and put them back without going through the Food and Drug Administration (FDA) approval process, to transplant cultured cells from one person to another and to use the biomatrix, they do need the regulators to give permission.

“We are frustratingly close to FDA approval,” Gregory said. “One of the great things about the Institute for Regenerative Medicine is that we have facilities and expertise to generate cells that, in the near future, will meet the stringent requirements of the FDA.” The institute ships these cells to researchers around the world.

“Given that nearly all chronic diseases have an inflammatory component,” Gregory said, “MSCs might be a widely used clinical tool of the future.”

One major challenge with getting FDA approval for MSC treatment is that cells from different donors have different efficacies. “The solution is induced pluripotent cells (iPS cells),” Gregory said, which are another type of stem cell similar to embryonic stem cells, but that originate from adult tissues. They divide without limit, differentiate into virtually every cell in the body, and can be made from simple skin biopsy or blood sample. Investigators at the IRM can generate MSCs from iPS cells without the need for bone marrow donors. Best of all? “They make the same product every single time,” he said.

This research could eventually help create treatments for not only bone injuries, but also a number of autoimmune diseases—Type 1 diabetes, Crohn’s disease, arthritis and more—because MSCs also differentiate into a tissue called stroma, which is found all over the body and modulates the immune system. Some research has shown that MSCs can migrate to the site of autoimmune damage, support survival of the cells at risk and inhibit the inflammatory and autoimmune effects. “Given that nearly all chronic diseases have an inflammatory component,” Gregory said, “MSCs might be a widely used clinical tool of the future.”

“It’s exciting when we can leverage Texas A&M’s unparalleled strengths in veterinary medicine and engineering on this one-of-a-kind collaborative, multidisciplinary research,” said Paul Ogden, M.D., interim executive vice president of the Texas A&M Health Science Center. “By working hand-in-hand, together, we will discover new treatments and therapies more efficiently than ever before.”