Tendon, the cord-like tissue that connects muscle to bone, contains a small subset of previously unknown adult stem cells, scientists at the National Institute of Dental and Craniofacial Research (NIDCR) part of the National Institutes of Health, and their colleagues have discovered.
The finding, published online in the journal Nature Medicine, points to a natural source of tendon-producing cells in adults and raises the possibility that, with further research, these cells one day could help to mend torn or degenerating tendons that are slow to heal.
Marian Young, Ph.D., an NIDCR scientist and the senior author on the study, said the findings also bring to light an unexpected biochemical habitat, or niche, that harbors stem cells. The cells are embedded between layers of extracellular matrix (ECM), the chain-like coils of protein that give tendon its elasticity and contain relatively few cells or blood vessels. To date, most known adult stem cells occupy cell-rich environments with a ready source of nutrients.
"We read a lot about the promise of stem cells, but sometimes overlooked is the importance of the niches that surround them," said Young. "Each individual niche in the body helps to carefully regulate the activities of a given stem cell. What?s nice is we have begun to characterize both in tandem, and that gives the field a head start in learning to meld an understanding of both and hopefully one day to re-engineer damaged tendon."
According to Young, the stem cells, which her group named tendon stem/progenitor cells, or TSPCs, would have never been discovered had it not been for their studies with mice — and good fortune. Young?s laboratory for several years had been knocking out, or inactivating, specific genes in developing mice that likely were involved in forming skeleton and its associated tissues. Among these genes were those that encoded the structural proteins biglycan and fibromodulin, major components of the ECM.
Having knocked out the genes for biglycan and fibromodulin in a new litter of mouse pups, they noticed the mice developed an unusual gait and had difficulty flexing their limbs at two months old. Subsequent X-rays provided the reason: Without biglycan and fibromodulin, the mice were abnormally forming bones within their tendons.
Young and her colleagues theorized that the tendons in these mice might contain stem cells that normally form tendon and, when their niche is altered, misguidedly create bone. If so, they theorized the ECM might house the stem cells, and biglycan and fibromodulin likely played a key role in regulating their normal activity. To test this theory, Young said she sought the help of her colleague Songtao Shi, DDS, Ph.D., now a scientist at the University of Southern California, who had previously discovered stem cells in adult dental tissues.
Their speculations turned out to be correct. Yanming Bi, Ph.D., a cell biologist in Young?s laboratory, isolated various cells from mouse and human tendon and cultured them in the laboratory. "Cells must meet very specific criteria to be termed stem cells," said Bi, the lead author on the Nature Medicine paper. "They must produce copies, or clones, of themselves. They must be self renewing, or proliferate at a rapid rate. Finally, they must display certain proteins on their cell surface that indicate a capacity to differentiate into other tissues, such as bone and cartilage."
Bi said the cell culture studies produced a small population of mouse and human cells that ultimately met all of the criteria. The obvious next question was whether the TSPCs could actually form new tendon. In follow-up mouse experiments, they showed both human and mouse cells formed tissues that resembled tendon and, characteristic of the tendon, attached to bone.
"These results and the abnormal bones in our earlier mice strongly suggested that the ECM might serve as the niche for the stem cells," said Young. "It was a novel idea, and we wanted to know more about how the ECM might regulate their ability to produce tendon."
Young said the group first localized the stem cells within the ECM, confirming their presence in tendon. Thereafter, returning to experiments with mice that lack the genes for biglycan and fibromodulin, they showed the loss of these molecules led to the production of abnormally disorganized structural proteins called collagen, which form the majority of the ECM. With the change in collagen and thus the normal stem cell niche, the TSPCs produced bone in these mice instead of tendon.
"The lesson here is: Follow the phenotype," said Young, using a term in genetics that means the physical manifestation that arises when a gene is inactivated. "Without the original mouse model, we could have never predicted that the stem cells were present in the extracellular matrix. The phenotype pointed us in the right direction and got us thinking in the right way."
Young said her group is now attempting to determine whether other adult stem cells could be prompted in similar niches to form tendon. If so, they could create a more plentiful supply of tendon-producing stem cells, which would better enable additional work to characterize their responses to conditions in the ECM.
"Our understanding of tendon biology is very much a work in progress," said Young. "The TSPCs give us a needed early entry point to better understand its developmental and regenerative dynamics."
"It?s also reasonable to say that the TSPCs one day could have an important role therapeutically," she continued. "As millions of Americans know first hand, torn tendons are much slower to heal than bone injuries, in part because tendons are so blood vessel poor and that inhibits the regenerative process. Using stem cells to create new tendon gets around that problem. But to reach that point, there?s a lot of biology and uncertainty that will need to be worked out."
The National Institute of Dental and Craniofacial Research (NIDCR) is the nation's leading funder of research on oral, dental, and craniofacial health.