Compared with wild-type (WT) mice, biglycan/fibromodulin double-deficient mice develop severe knee osteoarthritis. We undertook this study to compare type II collagen catabolism in the 2 genotypes and to compare the usefulness of 3 biomarkers of collagen degradation (C2C [also known as Col2-3/4C(long mono)] as well as the peptide Coll2-1 and its nitrated form, Coll2-1NO2) for evaluating collagen catabolism in vivo.
In 15 WT mice and 15 biglycan/fibromodulin double-deficient mice, we determined serum levels of C2C at ages 66 and 141 days, and we determined serum levels of Coll2-1 and Coll2-1NO2 at ages 49, 81, 95, and 141 days. Expression of the biomarkers in knee sections was examined using immunohistochemistry.
The mean concentrations of C2C and Coll2-1 were higher in biglycan/fibromodulin double-deficient mice at all time points. For C2C and Coll2-1, the ratio of the serum concentration in biglycan/fibromodulin double-deficient mice to that in WT mice (the double-deficient:WT ratio) was constant over time and was approximately 1.63 and approximately 1.15, respectively. In contrast, the double-deficient:WT ratio for Coll2-1NO2 varied and, depending on age, was >1 or <1. No significant correlation was found between the expression of the different biomarkers, except for a weak, negative correlation between Coll2-1NO2 and C2C. In both genotypes, antibodies to each biomarker labeled some fibroblasts in the tendons and menisci as well as chondrocytes above the tidemark in articular cartilage. Growth plates were unstained. For each biomarker, extracellular staining was limited to fibrocartilage areas in the tendons and menisci in all mice and was limited to some focal lesions of the cartilage in biglycan/fibromodulin double-deficient mice.
The different double-deficient:WT ratios observed with C2C, Coll2-1, and Coll2-1NO2 in the absence of any correlation between the expression of the 3 biomarkers indicate that these biomarkers give complementary, rather than redundant, information about in vivo type II collagen catabolism.