Granulin epithelin precursor (GEP) has been implicated in development, tissue regeneration, tumorigenesis, and inflammation. Herein we report that GEP stimulates chondrocyte differentiation from mesenchymal stem cells in vitro and endochondral ossification ex vivo, and GEP-knockdown mice display skeleton defects. Similar to bone morphogenic protein (BMP) 2, application of the recombinant GEP accelerates rabbit cartilage repair in vivo. GEP is a key downstream molecule of BMP2, and it is required for BMP2-mediated chondrocyte differentiation. We also show that GEP activates chondrocyte differentiation through Erk1/2 signaling and that JunB transcription factor is one of key downstream molecules of GEP in chondrocyte differentiation. Collectively, these findings reveal a novel critical role of GEP growth factor in chondrocyte differentiation and the molecular events both in vivo and in vitro.

ADAMTS-7, a metalloproteinase that belongs to ADAMTS family, is important for the degradation of cartilage extracellular matrix proteins in arthritis. Herein we report that ADAMTS-7 is upregulated during chondrocyte differentiation and demonstrates the temporal and spatial expression pattern during skeletal development. ADAMTS-7 potently inhibits chondrocyte differentiation and endochondral bone formation, and this inhibition depends on its proteolytic activity. The cysteine-rich domain of ADAMTS-7 is required for its interaction with the extracellular matrix, and the C-terminal four-thrombospondin motifs are necessary for its full proteolytic activity and inhibition of chondrocyte differentiation. ADAMTS-7 is an important target of canonical PTHrP signaling, since (i) PTHrP induces ADAMTS-7, (ii) ADAMTS-7 is downregulated in PTHrP null mutant (PTHrP؊/؊) growth plate chondrocytes, and (iii) blockage of ADAMTS-7 almost abolishes PTHrP-mediated inhibition of chondrocyte hypertrophy and endochondral bone growth. ADAMTS-7 associates with granulin-epithelin precursor (GEP), an autocrine growth factor that has been implicated in tissue regeneration, tumorigenesis, and inflammation. In addition, ADAMTS-7 acts as a new GEP convertase and neutralizes GEP-stimulated endochondral bone formation.Collectively, these findings demonstrate that ADAMTS-7, a direct target of PTHrP signaling, negatively regulates endochondral bone formation by associating with and inactivating GEP chondrogenic growth factor.The ADAMTS family consists of secreted zinc metalloproteinases with a precisely ordered modular organization that includes at least one thrombospondin type I repeat (51, 53). Important functions have been established for several members of the ADAMTS family. ADAMTS-1, ADAMTS-4, ADAMTS-5, ADAMTS-8, ADAMTS-9, ADAMTS-16, and ADAMTS-18 degrade the cartilage aggrecan (1,19,36,61,84,88), and ADAMTS-5 plays a primary role in aggrecan loss in arthritis (36, 84). ADAMTS-2, ADAMTS-3, and ADAMTS-14 are procollagen N-propeptidases (18, 30). ADAMTS-2 mutations cause dermatosparaxis, an inherited disorder characterized by severe skin fragility (17). ADAMTS-13 is a von Willebrand factor-cleaving protease, and its mutations lead to heritable life-threatening thrombocytopenic purpura (65). ADAMTS-12 and ADAMTS-7 share the same domain organization and structure and form a subgroup within the ADAMTS family (13, 83). These two enzymes have been found to associate with alpha-2-macroglobulin (13,70,83), and ADAMTS-12 also degrades aggrecan (68). Studies from our group demonstrated that ADAMTS-7 and ADAMTS-12 directly associate with and degrade COMP, a prominent noncollagenous component of cartilage (66,67). COMP is a 524-kDa, pentameric, disulfide-bonded, multidomain glycoprotein composed of approximately equal subunits (ϳ110 kDa each) (43,75). Although the function of COMP is not completely understood, it appears to mediate chondrocyte attachment by an integrin receptor (15,29), and accumulating evidence suggests that COMP may function to stabilize...

ADAMTS-12, a metalloproteinase that belongs to ADAMTS family, is strongly upregulated during chondrogenesis and demonstrates prominent expression in the growth plate chondrocytes. ADAMTS-12 potently inhibits chondrocyte differentiation, as revealed by altered expression of both early and later genes critical for chondrogenesis. In addition, ADAMTS-12-mediated inhibition of chondrogenesis depends on its enzymatic activity, since its point mutant lacking enzymatic activity completely loses this activity. Furthermore, the C-terminal four thrombospondin motifs known to bind COMP substrate is necessary for its full proteolytic activity and inhibition of chondrocyte differentiation. Mechanism studies demonstrate that ADAMTS-12 induces PTHrP, whereas it inhibits IHH during chondrogenesis. Furthermore, PTHrP induces ADAMTS-12 and ADAMTS-12 is hardly detectable in PTHrP-/-growth plate chondrocytes. Importantly, knocking down ADAMTS-12 mRNA levels or blocking ADAMTS-12 activity almost abolishes the PTHrP-mediated inhibition of type X collagen expression. Collectively, these findings demonstrate that ADAMTS-12, a downstream molecule of PTHrP signaling, is a novel regulator of chondrogenesis.