A new study published this month by the Athanasiou lab in the Biomedical Engineering Department at UC Davis has shown for the first time that depriving cartilage cells of oxygen at just the right time increases collagen crosslinks and doubles the tensile properties of engineered cartilage.
Healthy cartilage allows joints to move freely, but injured cartilage cannot heal itself and leads to debilitating conditions. Cartilage is composed largely of interlinked collagen fibers that give it strength and resilience. To restore damaged cartilage in humans, suitable tissue engineered cartilage implants should also possess organized and mature extracellular matrix.
Lately, emphasis has been placed on enhancing collagen type and quantity to improve the mechanical properties of engineered cartilage. However, not much attention has been placed on promoting collagen crosslinking in the engineered tissues, either in cartilage or in other, collagen-rich tissues whose primary function is biomechanical.
The new study by Eleftherios A. Makris, Jerry C. Hu, and Kyriacos A. Athanasiou shows that collagen tensile properties can be improved without affecting collagen type or quantity by enhancing crosslinks through hypoxia that is applied at the right time. They showed that this occurs through hypoxia’s upregulation of an enzyme called LOX, which is responsible for crosslinking collagen. By just reducing the cell’s oxygen exposure to 4%, the LOX gene became ~20-times more active, leading to greater crosslink concentration and significant increases in engineered cartilage’s mechanical properties. These benefits were only seen when hypoxia was applied during the third and fourth weeks of neotissue development.
“Since collagen is produced mostly during the early phases of neocartilage development, applying hypoxia too early disrupts its formation. We realized that, to improve the tensile properties of engineered cartilage, hypoxia should be applied after collagen already exists and is ready to be crosslinked,” explained Dr. Makris, the lead author of this study.
The study suggests that previous work using hypoxia has seen mixed success because it had not been clear that hypoxia can be both beneficial and detrimental, depending on when it is applied. The researchers believe that hypoxia during the early collagen synthesis phase activates the maturation process in the tissue, promoting collagen crosslinking but also blocking the synthetic process of the cells, resulting in low collagen. This, in turn, leads to poor biomechanical properties. By applying hypoxia after the correct types of collagen are already present, crosslinks were induced without compromising collagen content. Since most of the body’s tissues contain collagen, this work might inform researchers interested in engineering other tissues.
Hypoxia-induced collagen crosslinking as a mechanism for enhancing mechanical properties of engineered articular cartilage. Eleftherios A. Makris, MD, PhD Jerry C. Hu, PhD Kyriacos A. Athanasiou, PhD. “Osteoarthritis and Cartilage” http://dx.doi.org/10.1016/j.joca.2013.01.007