The Cause of Cartilage Degeneration
Researchers have long tried to determine what causes cartilage degeneration associated with osteoarthritis. A breakthrough by scientists points to the loss of a specific protein from the surface layer of cartilage in joints. That protein, referred to as HMGB2, appears to play a key role in cartilage degeneration.
Here's What Happens
Osteoarthritis starts when there is a disruption at the surface layer of cartilage -- also referred to as the superficial zone. The surface layer is the most important of the four layers of cartilage in a joint, in terms of proper joint motion. Normal joints have a smooth surface layer of cartilage that allows joints to glide over one another. Cartilage also stabilizes joints and absorbs force. When the surface layer begins to deteriorate, though, osteoarthritis starts to develop and an irreversible process is initiated that ultimately destroys underlying layers of cartilage until the end-stage occurs: bone is rubbing on bone in the joint.
Researchers have known that the beginning phase of osteoarthritis was associated with deterioration of cartilage in the surface layer. What researchers now know is that even before the destruction in the surface layer occurs, there is loss of the DNA-binding protein, HMGB2.
More About HMGB2
On the surface layer of cartilage in joints, HMGB2 supports chondrocyte survival. Chondrocytes are the only cells found in cartilage -- they actually produce cartilage. Simply put, the loss of HMGB2 is associated with aging and with chondrocytes being either reduced or eliminated in the surface layer of cartilage.
What This Breakthrough Means for the Future
What's the significance of the finding, which came from a collaboration among researchers from Scripps Research Institute in La Jolla, California; San Raffaele University in Milan, Italy; and Kogoshima University in Japan? Potentially -- if molecules that would stop the loss of HMGB2 could be discovered -- or the reverse, if production of HMGB2 could be stimulated -- osteoarthritis conceivably could someday be either prevented or reversed. The discovery of the role of HMGB2 in osteoarthritis may also impact how stem cells are used in tissue regeneration in the future.
Aging-related loss of the chromatin protein HMGB2 in articular cartilage is linked to reduced cellularity and osteoarthritis. Tanaguchi N et al. PNAS. January 12, 2009 (online).