Stem cells are the brightest lights in osteoarthritis (OA) research. Because they have unique regenerative abilities and can differentiate into different types of tissue, they have the potential to repair worn-out cartilage — even generate new joints. Stem cells also provide an ideal platform for studying the genetics of OA, possibly leading to new patient-specific drugs.  

Researchers at Duke University in Durham, N.C., have spent years exploring how stem cells might benefit people with OA. A decade ago, Farshid Guilak, PhD, professor of orthopaedic surgery and an Arthritis Foundation-funded researcher, was the first to grow cartilage from fat-derived stem cells. Now, he and his team have engineered mouse cartilage using induced pluripotent stem cells, or iPSCs. Their research was recently published online in Proceedings of the National Academy of Sciences.

Cells With Benefits
iPSCs are differentiated adult stem cells reprogrammed to have the properties of stem cells isolated from embryos. They are created by injecting adult cells with four gene-regulating proteins – a technique that earned Shinya Yamanaka of Kyoto University the 2012 Nobel Prize in medicine.

Like embryonic stem cells, iPSCs can transform into a vast array of cell types and tissues, but without the ethical concerns associated with embryonic research. The cells in Dr. Guilak's study were derived from adult mouse fibroblasts. As important, iPSCs culture far more easily and abundantly than adult stem cells do.

"One of the limitations of [adult] stem cells is that after they have divided many times, they stop being stem cells, so it's difficult to make enough for repairing cartilage or screening for arthritis drugs," Dr. Guilak says. "What this study shows in a mouse model is that you can grow an unlimited supply of stem cells that can form cartilage."

But the proliferative nature of pluripotent cells has a downside. If iPSCs take a wrong turn on the way to becoming cartilage cells, they can form unusual tumors called teratomas. To prevent this, Brian Diekman, PhD, a postdoctoral researcher in the Guilak lab, tested different growth factors until he found some that encouraged cartilage production. Adding them to the culture medium nudged iPSCs in the right developmental direction. The iPSCs were also tailored to glow with a green fluorescent protein when they successfully transformed into cartilage cells to distinguish them from cells that didn't form cartilage.

Ultimately, about 10 percent of the iPSCs were usable. They were treated with more growth factors, and in three weeks, Dr. Guilak says, "they formed beautiful, cartilaginous pellets, rich in proteoglycans and collagen, which indicates they would work well repairing cartilage damage in the body." The iPSC cartilage was of higher quality than cartilage derived from adult bone marrow or fat tissue.

The next step will be to use human iPSCs from skin or fat tissue to test the cartilage-growing technique. If all goes well, human safety trials could start in five or six years.
 

Dr. Guilak notes, "There are so many people in their 50s and 60s who have severe osteoarthritis, and no options other than joint replacement. But you don't want joint replacement when you're 55 because revisions can be a problem. So the goal is to use technology to put off joint replacement – not just by patching defects but by resurfacing the whole joint.

Whether human hips and knees can be grown in the laboratory remains to be seen. It's also unclear how long such tissue might last. "It could be a few years or 10 or 15 years," Dr. Guilak says. "There is such a mechanical demand on joints, we won't know until it's actually being done."

Drug Screening
In addition to therapeutic applications, Dr. Guilak says iPSC technology can provide patient-specific tissue models to screen for potential arthritis medications.

"There are no drugs that moderate disease severity [in OA]," he points out. "To under better understand how different drugs might work, you need a large source of cartilage from a controlled genetic background. With iPSCs, you can make genetically specific cartilage for different patients. So if you know one person is susceptible to arthritis and another isn't, you can grow cartilage with those genetic backgrounds from just a few skin cells."

Other Ideas
Eben Alsberg, PhD, associate professor of biomedical engineering and orthopaedic surgery at Case Western Reserve University in Cleveland, says Duke scientists have successfully tackled one of the most vexing problems in cartilage tissue engineering: identifying the best cell source for research.

"Dr. Guilak's group has presented a very elegant approach to identify and purify a more uniform population of iPSC-derived cells with improved cartilage-forming potential, which is tremendously exciting for regenerative medicine applications and for creating models to study cartilage growth, repair and disease," explains Dr. Alsberg, who is also working on laboratory-grown cartilage. Eventually, he hopes to use a patient's own cells to produce cartilage for joint repair.

Warren Grayson, PhD, assistant professor of biomedical engineering at John Hopkins University in Baltimore, Md., is also enthusiastic about the Duke research.

"It's really fascinating," he says, "because it speaks to the idea of getting specific patient's cells to treat osteoarthritis, and I think that has been one of the major challenges – how do you get enough stem cells for any particular application? What they have shown is you can use a few skin cells to create iPSCs abundantly and without any morbidity and transform them in such a way that they can be used for specific therapies. That's a really huge advantage."

Dr. Grayson's lab is using stem cells to develop patient-specific, anatomically correct bone grafts for use in reconstructions of congenital defects and traumatic injuries.