Chondromodulin-I derived from the inner meniscus prevents endothelial cell proliferation

The meniscus is a fibrocartilaginous tissue that plays an important role in controlling complex biomechanics of the knee. A perimeniscal capillary plexus supplies the outer meniscus, whereas the inner meniscus is composed of avascular tissue. Anti-angiogenic molecules, such as chondromodulin-I (ChM-I) and endostatin, have pivotal roles in preserving the avascularity of cartilage. However, the anti-angiogenic role of ChM-I is unclear in the meniscus. We hypothesized that the inner meniscus might maintain its avascular feature by expressing ChM-I. Immunohistochemical analyses revealed that ChM-I was mainly detected in the inner and superficial zones of the meniscus. On the other hand, endostatin distribution was similar between the inner and outer meniscus. In Western blot, ChM-I was detected only in the inner meniscus, whereas endostatin was equally observed in both inner and outer menisci. In addition, ChM-I concentration of the inner meniscus-derived conditioned medium was higher than that of the outer meniscus-derived medium. ChM-I removal from the inner meniscus-derived medium and functional blocking of ChM-I significantly increased endothelial cell proliferation. In this study, we demonstrated that the inner meniscus contained larger amounts of ChM-I, and that the inner meniscus-derived ChM-I inhibited endothelial cell proliferation. Our results suggest that ChM-I may be a key anti-angiogenic factor for maintaining the avascularity of the inner meniscus.