Impact of knee joint loading on fragmentation of serum cartilage oligomeric matrix protein.

The aim of the study was to examine the effect of mechanical knee joint loading on the fragmentation pattern of serum cartilage oligomeric matrix protein (COMP). Ten healthy men ran with knee orthoses that were passive or active (+30.9 N·m external flexion moments) on a treadmill (30 minute; v = 2.2 m/s). Lower-limb mechanics, serum COMP levels, and fragmentation patterns (baseline; 0, 0.5, 1, 2 hours postrunning) were analyzed. Running with active orthoses enhanced knee flexion moments, ankle dorsiflexion, and knee flexion angles (P < .05). There was an increase in serum COMP (+25%; pre: 8.9 ± 2.4 U/l; post: 10.7 ± 1.9 U/l, P = .001), COMP pentamer/tetramer (+88%; 1.88 ± 0.81, P = .007), trimer (+209%; 3.09 ± 2.65, P = .005), and monomer (+78%; 1.78 ± 0.85, P = .007) after running with passive orthoses and in serum COMP (+41%; pre: 8.5 ± 2.7 U/l; post: 11.3 ± 2.1 U/l, P < .001), COMP pentamer/tetramer (+57%; 1.57 ± 0.39, P = .007), trimer (+86%; 1.86 ± 0.47, P = .005), and monomer (+19%; 1.19 ± 0.34, P = .114) after running with active orthoses. Increased fragmentation might indicate COMP release from cartilage while running. Interestingly, 0.5 h up to 2 hours after running with passive orthoses, trimer (0.5 hour: 2.73 ± 3.40, P = .029; 2 hours: 2.33 ± 2.88, P = .037), and monomer (0.5 hour: 2.23 ± 2.33, P = .007; 1 hour: 2.55 ± 1.96, P = .012; 2 hours: 2.65 ± 2.50, P = .009) increased while after running with active orthoses, pentamer/tetramer (1 hour: 0.79 ± 0.28, P = .029), and trimer (1 hour: 0.63 ± 0.14, P = .005; 2 hours: 0.68 ± 0.34, P = .047) decreased. It seems that COMP degradation and clearance vary depending on joint loading characteristics.

Effect of increased mechanical knee joint loading during running on the serum concentration of cartilage oligomeric matrix protein (COMP).

The purpose of the study was to investigate the effect of an increase in mechanical knee joint loading during running on the serum COMP level. On two different test days, 20 healthy men ran with knee orthoses for 30 min on a treadmill (v = 2.2 m/s). On day 1, the orthoses were passive, whereas on day 2 they were pneumatically driven (active) and thus increased the external knee flexion moments (+30.9 Nm) during stance phase. Lower-limb mechanics and serum COMP levels (baseline; 0, 0.5, 1, 2 h post running) were analyzed. COMP levels increased immediately after running with passive (+35%; pre: 7.5 U/l, 95%CI: 6.4, 8.7, post: 9.8 U/l, 95%CI: 8.8, 10.8, p < 0.001) and active orthoses (+45%; pre: 7.6 U/l; 95%CI: 6.4, 8.8, post: 10.3 U/l, 95%CI: 9.2, 11.5, p < 0.001), but they did not differ between interventions. While running with active orthoses, greater ankle dorsiflexion angles, knee flexion angles, and moments occurred (p < 0.05). Comparing both interventions, the Δ COMP pre-post, meaning the difference (Δ) between running with active and passive orthoses in pre to post COMP level change (=level after (post) running minus level before (pre) running), correlated negatively with Δ COMP baseline (difference between the baseline COMP level before running with active and passive orthoses, r = -0.616; p = 0.004), and with a positive tendence with the Δ maximum knee flexion (r = 0.388; p = 0.091). Therefore, changes in COMP concentration after physical activity seem to be highly influenced by the COMP baseline level. In addition, correlation analysis indicates that modifications in knee joint kinematics have a greater effect on cartilage metabolism than an increase in joint moments. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1937-1946, 2018.

Extracellular Distribution of Collagen II and Perifibrillar Adapter Proteins in Healthy and Osteoarthritic Human Knee Joint Cartilage.

Perifibrillar adapter proteins, interconnecting collagen fibrils, and linking the collagen network with the aggrecan matrix seem to play a crucial role in the pathogenesis of osteoarthritis (OA). Therefore, we examined immunohistochemically the extracellular distribution of collagen II and the main perifibrillar adapter proteins-collagen IX, decorin, cartilage oligomeric matrix protein (COMP), and matrilin-3-in human samples of healthy (n=4) and OA (n=42) knee joint cartilage. Histopathology assessment was performed using an OA score. Staining patterns were evaluated in relation to the disease stage. The perifibrillar adapter proteins were uniformly distributed in the upper zones of healthy cartilage. In moderate OA (n=8; score 14.3 ± 4.7), all proteins analyzed were locally absent in the fibrillated area or the superficial and upper mid zone. In advanced OA (n=20; score 18.9 ± 5.3), they were uniformly distributed in these zones and accumulated pericellularly. Perifibrillar adapter proteins are important for the stabilization of the collagen network in the upper zones of healthy cartilage. Their degradation might be a critical event in early OA. In advanced OA, there are indications for an increased synthesis in an attempt to regenerate the lost tissue and to protect the remaining cartilage from further destruction.

Moderate cyclic tensile strain alters the assembly of cartilage extracellular matrix proteins in vitro.

Mechanical loading influences the structural and mechanical properties of articular cartilage. The cartilage matrix protein collagen II essentially determines the tensile properties of the tissue and is adapted in response to loading. The collagen II network is stabilized by the collagen II-binding cartilage oligomeric matrix protein (COMP), collagen IX, and matrilin-3. However, the effect of mechanical loading on these extracellular matrix proteins is not yet understood. Therefore, the aim of this study was to investigate if and how chondrocytes assemble the extracellular matrix proteins collagen II, COMP, collagen IX, and matrilin-3 in response to mechanical loading. Primary murine chondrocytes were applied to cyclic tensile strain (6%, 0.5 Hz, 30 min per day at three consecutive days). The localization of collagen II, COMP, collagen IX, and matrilin-3 in loaded and unloaded cells was determined by immunofluorescence staining. The messenger ribo nucleic acid (mRNA) expression levels and synthesis of the proteins were analyzed using reverse transcription-polymerase chain reaction (RT-PCR) and western blots. Immunofluorescence staining demonstrated that the pattern of collagen II distribution was altered by loading. In loaded chondrocytes, collagen II containing fibrils appeared thicker and strongly co-stained for COMP and collagen IX, whereas the collagen network from unloaded cells was more diffuse and showed minor costaining. Further, the applied load led to a higher amount of COMP in the matrix, determined by western blot analysis. Our results show that moderate cyclic tensile strain altered the assembly of the extracellular collagen network. However, changes in protein amount were only observed for COMP, but not for collagen II, collagen IX, or matrilin-3. The data suggest that the adaptation to mechanical loading is not always the result of changes in RNA and/or protein expression but might also be the result of changes in matrix assembly and structure.