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

Abstract

Objective: To assess the role of the glycoprotein PRG4 in joint lubrication and chondroprotection by measuring friction, stiffness, surface topography, and subsurface histology of the hip joints of Prg4(-/-) and wild-type (WT) mice.

Methods: Friction and elastic modulus were measured in cartilage from the femoral heads of Prg4(-/-) and WT mice ages 2, 4, 10, and 16 weeks using atomic force microscopy, and the surface microstructure was imaged. Histologic sections of each femoral head were stained and graded.

Results: Histologic analysis of the joints of Prg4(-/-) mice showed an enlarged, fragmented surface layer of variable thickness with Safranin O-positive formations sometimes present, a roughened underlying articular cartilage surface, and a progressive loss of pericellular proteoglycans. Friction was significantly higher on cartilage of Prg4(-/-) mice at age 16 weeks, but statistically significant differences in friction were not detected at younger ages. The elastic modulus of the cartilage was similar between cartilage surfaces of Prg4(-/-) and WT mice at young ages, but cartilage of WT mice showed increasing stiffness with age, with significantly higher moduli than cartilage of Prg4(-/-) mice at older ages.

Conclusion: Deletion of the gene Prg4 results in significant structural and biomechanical changes in the articular cartilage with age, some of which are consistent with osteoarthritic degeneration. These findings suggest that PRG4 plays a significant role in preserving normal joint structure and function.

Figure 1

Sample micrographs of histologically stained cross-sections of cartilage from the femoral heads of wild-type and Prg4^−/− mice ages 2, 4, 10, and 16 weeks. Arrowheads indicate delamination of the surface layer (i), a Safranin O–positive feature in the surface layer (ii), a cell in the surface layer (iii), pericellular loss of Safranin O staining (iv), and absence of the surface layer, leaving the underlying cartilage exposed to damage (v). Bars = 50 μm.

Figure 2

Graphs depicting qualitative and quantitative histology scores in wild-type (WT) and Prg4^−/− mice. Seven mice of each genotype were studied per age group. A, Total degeneration score, an aggregate score of articular cartilage structure, surface layer morphology, and loss of Safranin O staining, at each age tested. B, Thickness of uncalcified cartilage at ages 10 and 16 weeks. Values in A and B are the mean and SD. ∗ = P < 0.05. C, Percentages of joints of Prg4^−/− mice (joints of 7 mice at each age; only 1 joint was tested per mouse) exhibiting distinct characteristics. None of these features were observed in joints of WT mice. † = P < 0.05.

Figure 3

Representative atomic force microscopy images of cartilage from the anterior femoral head of wild-type and Prg4^−/− mice ages 2, 4, 10, and 16 weeks. Bars = 10 μm.

Figure 4

Cartilage surface properties determined by atomic force microscopy (AFM). Seven mice of each genotype were studied per age group. A, Root mean square (RMS) roughness measured from AFM images. B, Coefficient of friction (COF) between the cartilage and the functionalized probe. C, Effective elastic modulus of superficial cartilage. Values are the mean and SD. ∗ = P < 0.05.