"Take a knee." "On bended knee." A "knee-slapper." "The bee's knees." The knee certainly carries a lot of weight in the English language, just as it does in the body. As the largest and one of the most complicated joints, the knee is also one of the most easily injured. Anatomically, it's no wonder – the knee connects the two longest levers in the body: the thigh and lower leg.
The meniscus – the cartilage cushion between the meeting of the thighbone and the shinbone – can be torn during all kinds of activities that put pressure on or rotate the knee joint, such as shoveling snow or running. However, the dense extracellular matrix in deep tissues makes it difficult for injuries occurring in cartilage, tendons, and ligaments to heal themselves. Torn menisci are so common and so severe that the surgery required to repair the injury is the most common orthopedic procedure, and cases are on the rise. However, meniscus surgery may not be fully effective, since it involves removing only the torn part of a meniscus, leaving behind less of it to bear the same load of body weight as before.
Researchers at Penn are working to uncover ways to encourage the cells in and around the meniscus to repair themselves, hopefully leading to less invasive procedures. Scientists within the McKay Orthopaedic Research Laboratory are taking a tissue-engineering approach to improve many types of healing related to the skeletal system, says Lou Soslowsky, PhD, the vice chair for research in the department of Orthopaedic Surgery. "We blend biology with engineering to understand normal and pathologic processes to ultimately replace or regenerate musculoskeletal tissues."
The cellular matrix of the meniscus tissue consists of proteins and sugars that fill the spaces between cells, binding tissues together. In dense tissues, there's so much of the matrix that it's difficult for stem cells to move to the site of injury and start the repair process.
Using an animal meniscus as a model system, Robert Mauck, PhD, a professor of Orthpaedic Surgery, and Jason Burdick, PhD, a professor of Bioengineering, are testing the idea of an implantable platform for wound healing. This microscopic scaffold contains a growth factor to attract stem cells and is laced with an enzyme to loosen the matrix. When they placed it in meniscus tissue from a cow, the matrix loosened, allowing bone stem cells to migrate to the scaffold and start the repair process.
"The immediate next step is to test this system in a large animal model to see if it can repair a meniscus tear," Mauck said. "This approach could possibly be in human trials in about five years."
Joel Boerckel, PhD, an assistant professor of Orthopaedic Surgery, studies a different angle of the matrix. His lab, including doctoral student Christopher Kegelman, concentrates on "brother/sister" proteins YAP and TAZ and their role in the development of bone stem cells and the matrix. "They are very similar to each other in many ways, but depending on the cell type, they can have either equivalent or opposite functions," Boerckel said. Over the last decade, studies have been highly contradictory, with some finding that the proteins behave similarly, and some the opposite.
To resolve this conflict, Boerckel studied their individual and combined functions in bone development in mice engineered to have either YAP or TAZ or both deleted in skeletal stem cells. "We found that mice without YAP, TAZ or both did not have complete stem cell differentiation and were not able to properly mobilize to sites of injury," he said.
Similar to Mauck's scaffold one day helping build back new meniscus tissue, Boerkel says YAP and TAZ are critical for bone cells' ability to produce the right amount of matrix its mechanical properties.
Although we will have to wait a while for these efforts to provide new treatment options, one of the striking findings of Boerckel's work is the more immediate relevance to the biology of a rare disease – a case of research to benefit a large number of people also providing new clues for a small group with profound needs. When both YAP and TAZ were deleted, mice showed characteristics similar to the brittle bone disease osteogenesis imperfecta (OI).
"These mice sustain spontaneous bone fractures and have defective bone matrix quality just like human OI patients," he said. "YAP and TAZ regulate the expression of genes whose mutation in people is known to cause OI. We are now exploring whether and how this signaling pathway is involved in patients with OI."