F‐spondin regulates chondrocyte terminal differentiation and endochondral bone formation

Abstract

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.

F-Spondin Deficient Mice Have a High Bone Mass Phenotype

“…In wild type mice (WT), RT-PCR revealed expression of F-spondin mRNA in brain, lung, bone, liver and hypertrophic cartilage from rib, consistent with our previous observations (Fig 1C) [7]. Among isolated cell cultures of growth plate chondrocytes, calvaria osteoblasts, and in vitro differentiated osteoclasts, Spon1 expression was detected only in chondrocytes (data not shown), in agreement with our previous analysis of chick and mouse cartilage [7]. Importantly, no Spon1 PCR products were detected in any tissues or cell cultures isolated from knockout (KO) mice (Fig 1C).…”

“…We have previously reported that F-spondin increases active TGF-β1 levels in OA and hypertrophic cartilage [7], [8]. The current study showed further evidence of TGF-β regulation by Spon1 and provides a possible mechanism for regulation of bone mass.…”

“…This finding was unexpected given that we have previously shown F-spondin expression in hypertrophic cartilage and its regulation of chondrocyte terminal differentiation when added exogenously to cell and tibia explant cultures [7]. Moreover, chondrocyte cell cultures also exhibited reduced TGF-β levels and elevation of P-SMAD1/5, both of which have been shown to affect terminal differentiation [25].…”

Spondin 1 promotes metastatic progression through Fak and Src dependent pathway in human osteosarcoma

“…Initially, treatment with recombinant SPON1 protein promotes neural cell adhesion and neuronal outgrowth, which indicates the functions of SPON1 in the modulation of axonal growth in the embryonic central nervous system [9]. Apart from neuronal tissues, SPON1 expression has recently been detected in several other tissues including ovary [10], embryonic growth plate cartilage [11], periodontal tissue [12] and osteoarthritic cartilage [13]. However, whether SPON1 is differentially expressed in tumors, especially in osteosarcoma, remains largely unexplored.…”

F‐Spondin Inhibits Migration and Differentiation of Osteoclastic Precursors

“…In periodontal tissue, F‐spondin reportedly promotes cementoblastic differentiation of periodontal ligament cells 6 . Furthermore, it regulates chondrocytic terminal differentiation via the transforming growth factor‐β pathway 9 . However, its effects on osteoclasts and their precursors have not been reported.…”