F‐spondin regulates chondrocyte terminal differentiation and endochondral bone formation

This study examines the role of F‐spondin, an extracellular matrix protein of osteoarthritic cartilage, during chondrocyte maturation in embryonic growth plate cartilage. In chick tibia, F‐spondin expression localized to the hypertrophic and calcified zones of the growth plate. Functional studies using tibial organ cultures indicated that F‐spondin inhibited (～35%, p = 0.02), and antibodies to F‐spondin increased (～30%, p < 0.1) longitudinal limb growth relative to untreated controls. In cell cultures, induction of chondrocyte maturation, by retinoic acid (RA) or transforming growth factor (TGF)‐β treatment led to a significant upregulation of F‐spondin (p < 0.05). F‐spondin transfection increased mineral deposition, alkaline phosphatase (AP) and matrix metalloproteinase (MMP)‐13 mRNA levels (p < 0.05), and AP activity following RA stimulation, compared to mock transfected controls. Using AP as a differentiation marker we then investigated the mechanism of F‐spondin promaturation effects. Blocking endogenous F‐spondin via its thrombospondin (TSR) domain inhibited RA induced AP activity 40% compared to controls (p < 0.05). The stimulatory effect of F‐spondin on AP expression was also inhibited following depletion of TGF‐β from culture supernatants. Our findings indicate that F‐spondin is expressed in embryonic cartilage, where it has the capacity to enhance chondrocyte terminal differentiation and mineralization via interactions in its TSR domain and TGF‐β dependent pathways. Published by Wiley Periodicals, Inc. J Orthop Res 28:1323–1329, 2010