EF_Kuo,Alfred_withtotalhippatientRay,Gerald.gif
Dr. Kuo (left) discusses postoperative rehabilitation with Jerry Ray, a patient recovering from a total hip replacement at the San Francisco Veterans Affairs Medical Center.
Courtesy of Alfred Kuo, MD, PhD

AAOS Now

Published 7/1/2011
|
Jay D. Lenn

Developing cell-based therapies for articular cartilage injury

OREF grant recipient looks for keys to chondrogenesis

Articular cartilage has a limited capacity to repair itself once it is damaged. The cartilage has no direct blood supply, and its chondrocytes are sequestered in tiny cavities in a matrix that prevents the migration of cells to the point of injury.

Although surgical treatments can repair or replace damaged cartilage, these procedures do not restore its normal structure, composition, and function. Therefore, researchers have been investigating several cell-based strategies for tissue regeneration.

“One of the issues facing the field is how to turn progenitor cells into exactly the tissue we want them to be,” explained Alfred Kuo, MD, PhD, assistant professor in residence of orthopaedic surgery at the University of California, San Francisco.

Some of these efforts have resulted in tissues that more closely resemble hypertrophic cartilage or fibrocartilage, rather than articular cartilage. “A number of factors influence cellular differentiation and development. Unfortunately, we’re not yet able to take stem cells or chondrocytes that have been grown in the laboratory and turn them into tissue that resembles our own articular cartilage,” said Dr. Kuo.

Harnessing self-repair potential
Although the specific triggers that drive articular chondrogenesis are unknown, researchers have found that articular cartilage itself can stimulate the process. In 2010, Dr. Kuo was awarded an Orthopaedic Research and Education Foundation (OREF)/Journal of Bone and Joint Surgery Clinician Scientist Award to investigate the mechanisms by which components of articular cartilage direct stem cells to develop into articular chondrocytes.

Dr. Kuo stated, “We’re trying to figure out how this process occurs, and hopefully at some point, we’ll be able to harness this process to make better repair cartilage for patients.”

Studies by Dr. Kuo’s group and other investigators have demonstrated that articular cartilage–stimulated chondrogenesis from progenitor cells can be induced by three forms of stimuli:

  • Direct cell-to-cell communication between chondrocytes and stem cells
  • Diffusible proteins (paracrine factors) released by cells—in this case, chondrocytes—that can induce changes in nearby cells
  • Interactions between devitalized articular cartilage matrix and cells

Dr. Kuo observed that the mechanisms by which each of these stimuli induce chondrogenesis may have notable variations. Each stimulus may have different qualitative and quantitative effects on the development of chondrocytes.

Defining the process
One aim of Dr. Kuo’s team is to compare the relative ability of each of these three stimuli to induce articular chondrogenesis in mesenchymal stem cells, the progenitors of chondrocytes, osteoblasts, and other cells. To this end, the researchers have developed a model system of articular cartilage–stimulated chondrogenesis using human mesenchymal stem cells and bovine cartilage.

The researchers will assess the potency of chondrocytes, chondrocyte-derived paracrine factors, and devitalized cartilage matrix to induce articular chondrogenesis. They will measure the outcomes using several endpoints, including tissue structure analysis, gene expression patterns, and the concentration of various protein markers of chondrogenesis.

The signaling molecules involved in articular cartilage–stimulated chondrogenesis are also unknown. In a series of experiments with the same model system and three different stimuli, Dr. Kuo and his research team will examine the potential role of members of the transforming growth factor beta (TGF-β) superfamily, which are known to play key roles in other forms of chondrogenesis. They will assess newly generated chondrocytes for molecular “footprints” that would indicate the activation of TGF-β superfamily members. Also, to determine if TGF-β superfamily signaling is necessary for articular cartilage–stimulated chondrogenesis, the researchers will assess the outcome of selectively blocking certain TGF-β isoforms.

A final round of experiments with their model will determine the role of mitogen-activated protein kinases (MAPKs), intracellular signal transducers that are involved in fundamental cellular processes, such as differentiation, proliferation, and apoptosis. Kinases modify other proteins by chemically adding phosphate groups to them. The kinase signaling—or chain of multiple phosphorylation events—that regulates a cellular process may be identified by a particular sequential pattern of phosphorylation. The researchers expect that each of the three cartilage-derived stimuli will lead to specific patterns of MAPK phosphorylation that regulate chondrogenesis.

Laying the foundation for therapies
Identifying the roles of key players in articular cartilage–stimulated chondrogenesis is an essential foundation for developing cell-based therapies. Dr. Kuo stated, “If we know how things work, if we can identify the signaling pathways in this process, eventually we may be able to enhance articular chondrogenesis, thereby improving the quality and quantity of repair tissues. You might imagine that if we had the proper mixture of factors, we could improve the results of current surgical procedures to treat cartilage injury.”

This investigation is a perfect blend of Dr. Kuos’ roles as both clinician and scientist. He explained, “Orthopaedics is a field where we can do a lot for people, and there’s definitely that satisfaction of seeing a patient whose symptoms have improved, whose function has improved. But at the same time, you realize the limitations. We don’t have good solutions for all of our patients. That’s what motivates me to do research.”

The one-year Clinician Scientist Award provided Dr. Kuo an annual stipend to compensate for the loss of income that will result from devoting less time to clinical practice and more time to research. In return, he will serve as a role model for orthopaedic residents, interns, and medical students in the science and practice of musculoskeletal surgery; organize and participate in conferences; and work with students, interns, and residents in the operating room.

Jay D. Lenn is a contributing writer for OREF. He can be reached at communications@oref.org.