In Depth
Rebuilding bone
Pages 1 • 2
A whole new skeleton
Mesenchymal stem cells also hold promise for repairing joint cartilage damaged by arthritis, Niyibizi says. With numerous clinical trials underway around the world, this kind of localized repair may become available as a standard treatment option in just a few years, he says. Meanwhile he and other researchers are embarked on a more ambitious quest: to develop stem-cell therapies for the correction of genetic disorders.
A composite full-body image shows the fluorescent stems cells concentrated in the mouse's lungs (top) and pelvic girdle. Courtesy Christopher Niyibizi
His current focus is brittle bone disease. Also known as osteogenesis imperfecta, this condition results from a defect in the genes that produce type I collagen, the main component in bone. "The mineral doesn't deposit correctly and as a result the bones that are formed break very easily," Niyibizi explains. In severe cases, infants with the disease may die before or shortly after birth. Even in its milder forms, it can cause frequent fractures and stunted growth. "Surgeons try to fix these bones with metal rods, but there's really no effective treatment," he says.
A cell-therapy approach would involve transplanting mesenchymal cells from a normal-bone donor into the brittle-bone patient, with growth-factor "instructions" to regenerate the entire skeleton. Already researchers have tried this in a handful of severely afflicted children, as an adjunct to standard bone-marrow transplant, and seen significant improvement in bone growth.
Again, to better understand the cell biology, Niyibizi turned to his model, injecting stem cells tagged with green fluorescent protein directly into newborn mice with brittle bone disease. He followed the fate of these cells by imaging their locations at various points during the early weeks of development.
"The cells do go to the bones," he reports, summoning a slide that shows green-glowing shapes dimly recognizable as mammalian ribs and pelvis. "But they also go to the liver, the lung—they go all over the place." In fact, as the next slide makes clear, after 25 days most of the stem cells end up in the lungs. What this confirms, Niyibizi says, is the need to find a way to target cells specifically to bone.
One hope, he suggests, lies in searching carefully through the variety of stem cell sub-types that are present in bone marrow. "Some of these types may have a higher affinity for bone. If so, then maybe we can find out why." Already, Feng Li, a postdoctoral fellow in Niyibizi's lab, has identified a specific population of stem cells that will migrate, engraft, and synthesize normal type I collagen in the bones of brittle-bone mice. "This is quite exciting," Niyibizi says. "Now we are looking closely at the nature of this population."
Newborn mice with brittle bone disease. Stem cells marked with green fluorescent protein are injected through a vein on the side of the head.Courtesy Christopher Niyibizi
"It's also possible," he adds, "that we could actually condition stem cells to want to go to bone." In another of his mouse experiments he tried exactly that: Stem cells were injected into bone, allowed to remain for a period of two weeks— "not long enough to differentiate"—then re-drawn. This process was repeated three times before the cells were finally injected into the mouse's bloodstream. "What we found," he says, "is that these 'trained' cells will go back to where they were. Something changes. They know that bone is where they belong."
Getting past rejection
Even if stem cells can be targeted specifically, transplantation still raises the possibility of rejection. The drastic measures required to limit the body's response to foreign tissue "are in the long run not acceptable," Niyibizi says simply. That's why he is also exploring the use of gene therapy for skeletal regeneration.
The process involved is very similar to what he uses to get fluorescence into his test cells, Niyibizi explains. "We take the gene that the mouse is missing and we engineer it into a virus." The mouse-patient is infected, and the virus ferries the normal gene into the abnormal cells. "Once it goes in, the infected cells start spitting out the proteins you engineered into the virus.
"We have been able to demonstrate that this is do-able," Niyibizi says. "But putting a virus into a person"—even a gutted virus, from which the disease-producing genes have been removed—"is very scary. Right now, many researchers are trying to find other vehicles. Lipids are one possibility. The problem is they don't express the gene as well."
Assuming that the right vehicle can be found, he acknowledges, there remain many other obstacles to successful gene therapy. But if he and his colleagues can one day overcome these hurdles, defective stem cells could be removed, corrected, and returned to the same patient to produce healthy new bone.
"If it can be done, and done safely," Niyibizi says, "this would be the best approach of all."
Pages 1 • 2
Christopher Niyibizi, Ph.D., is associate professor of orthopaedics and rehabilitation and biochemistry and molecular biology at the Penn State College of Medicine in Hershey. He can be reached at cniyibizi@psu.edu.
Niyibizi presented a talk on the work described here at CrossOver 2005, a symposium held at University Park in October 2005 to explore the interface of life sciences and materials at Penn State. His work is supported by grants from the National Institutes of Health and the Children's Brittle Bone Foundation.