Articular cartilage is a specialized avascular connective tissue found at the contact regions of diarthrodial joints. Cartilage has few cells (< 5% of the volume), though these cells can maintain the balance of turnover in healthy tissue, when the tissue is damaged, they are not able to repair the defects [1–3]. Extra cellular matrix (ECM) in cartilage comprises water, collagen (principally type II), proteoglycans and noncollagenous proteins. The type II collagen network, which is the dominant structural protein in cartilage ECM, constrains the expansion of the resident PGs and is generally held in mechanical tension. In osteoarthritis (OA), the balance of cartilage tissue production/degradation is thought to be affected by abnormal mechanical stimuli leading to net matrix resorption through production of excess degradative enzymes (e.g. matrix metalloproteinases (MMP) and aggrecanases) [4–8]. In OA tissue the amount of MMP-13 is thought to be increased relative to healthy tissue. OA typically occurs in older adults where, as cartilage ages, there is a marked decrease in the fixed charge density (FCD), the hydration and, consequently, mechanical tension on the collagen type II network [9–11]. We have hypothesized that loss of tension on the collagen network accelerates degradation by MMP. Detection of the effect of MMP on loaded, native cartilage could lead to insight about cartilage degradation kinetics in OA. However, it is quite difficult to controllably deliver MMP to cartilage, to activate the MMP during detensioning of the collagen network and to detect the effect on the cartilage mechanics (because cost limits the amount of MMP used). We have developed a transpirational enzyme loading method which is capable of precisely dosing bovine cartilage explants with a small, known quantity of MMP-13. Following enzyme insertion, we are able to detect the activity of the MMP on osmotically compressed cartilage (i.e. cartilage with a detensioned collagen network) via a simple hydration measurement.
Inactivation of one allele of the type II collagen gene alters the collagen network in murine articular cartilage and makes cartilage softer
