Loss of cartilage structure, stiffness, and frictional properties in mice lacking PRG4.

PRG4 is expressed by synoviocytes and superficial zone chondrocytes as the glycoproteins lubricin and superficial zone protein (1–3), which contribute to boundary lubrication of loaded articular cartilage (4–6). PRG4 is also expressed in meniscus (7), tendon (8–10), ligament, heart, lung, liver, and other tissues (11,12). We use the term lubricin to collectively refer to these proteins, recognizing that variations exist in messenger RNA splicing, posttranslational modification, and cellular origins. The N- and C-terminal regions of lubricin are globular and are separated by a long, heavily glycosylated mucin-like domain (13). In addition to reducing friction, lubricin inhibits synovial cell overgrowth, reduces cell adhesion (14), and prevents cartilage–cartilage integration (15). Osteoarthritis (OA) is a multifactorial disease with complex etiologies involving hereditary, developmental, metabolic, mechanical, and other factors. The disease progression involves a combination of abnormal mechanical stresses and biochemical imbalances that lead to a loss of proteoglycans, disruption of the collagen network, and alterations in other joint tissues such as the synovium and subchondral bone (16,17). Matrix degeneration generally begins in superficial cartilage and progresses to deeper regions of the tissue (18). Lubricin down-regulation has been seen in animal models of OA (19,20), and a low lubricin concentration has been observed in human patients following anterior cruciate ligament injury, a significant risk factor for OA (21). In recent studies, direct intraarticular injections of recombinant lubricin were shown to slow cartilage degeneration in a rat model of OA, suggesting that lubricin can play a chondroprotective role in the joint (22). A better understanding of the role of lubricin in preventing cartilage damage and degeneration may lead to improved treatment of joint diseases. Truncating mutations in Prg4 are responsible for the rare, autosomal-recessive camptodactyly-arthropathy–coxa vara–pericarditis syndrome (CACP) (23). Patients with this disorder have flexion contractures of phalangeal joints at an early age and develop a noninflammatory arthropathy that leads to premature joint failure (24). In addition, synovial fluid from patients with CACP shows no ability to lower friction in a latex-on-glass bearing, which can be effectively lubricated by normal synovial fluid (25). Since cartilage from CACP patients has not been available for study, cartilage from Prg4−/− mice provides a novel experimental model for the absence of lubricin in humans. Furthermore, this cartilage also provides a convenient model substrate to investigate boundary lubrication by lubricin using atomic force microscopy (AFM). Initial studies have shown that mice lacking PRG4 exhibit changes similar to those in CACP patients, including an abnormal cartilage surface, synovial hyperplasia, and eventual joint failure (14). Cartilage surfaces of these mice were irregular, with abnormal protein deposits and a loss of superficial zone chondrocytes (14), and whole joint friction was higher than in the joints of Prg4+/−mice (25). Previous studies have not directly characterized the frictional or mechanical properties of the cartilage of Prg4−/− mice due to the small size of mouse joints. In this study, we measured the frictional properties, stiffness, and topography of cartilage of Prg4−/− and wild-type (WT) mice ages 2–16 weeks by AFM and related these measurements to histologic appearance. The low coefficients of friction which have often been measured on cartilage are due in large part to load sharing by pressurization of the interstitial fluid, which makes up the majority of the tissue (26). Boundary lubrication can further reduce friction, and possibly wear, in regions of joint contact, acting independently of interstitial fluid pressure (27). Lubrication by interstitial fluid pressurization depends on the biphasic time constant of the tissue (28), which becomes very small for small contact areas. For microscale contacts, the time constant is on the order of milliseconds or less. Thus, AFM allows measurement of cartilage friction in near or complete absence of interstitial fluid pressurization (29,30), making it an excellent tool for the focused study of boundary lubrication properties.