BMP signaling stimulates chondrocyte maturation and the expression of Indian hedgehog

Mutant BMP receptors were transfected into cultured embryonic upper sternal chrondrocytes using retroviral vectors to determine if BMP signaling is re quired for chondrocyte maturation and the expression of a key regulatory mo lecule, indian hedgedog (Ihh). Chondrocytes infected with replication compe tent avian retroviruses (RCAS) viruses carrying constitutive active :CA BMP R-IA and BMPR-IB had enhanced expression of type X collagen and Ihh mRNA. A ddition of PTHrP, a known inhibitor of chondrocyte maturation, abolished th e expression of types X collagen, BMP-6, and Ihh mRh'As in control cells, I n contrast, PTHrP treated cultures infected with of CA BMPR-IA or CA BMPR-I B had low levels of BMP-6 and type X collagen. but high levels of Ihh expre ssion. Although dominant negative (DN) BMPR-IA had no effect, DN BMPR-IB in hibited the expression of type X collagen and BMP-6, and decreased alkaline phosphatase activity, even in the presence of exogenously added BMP-2 and BMP-6. DN BMPR-IB also completely blocked Ihh expression. Overall, the effe ct of DN BMPR-1B mimicked the effects of PTHrP. To determine if there is an autocrine role for the BMPs in chondrocyte maturation, the cultures were t reated with noggin and follistatin, molecules that bind BMP-2/-4 and BMP-6/ -7. respectively. While noggin and follistatin inhibited the effects of rec ombinant BMP-2 and BMP-6. respectively. they had only minimal effects on th e spontaneous maturation of chondrocytes in culture, suggesting that more t han one subgroup of BMPs regulates chondrocyte maturation. The results demo nstrate that: ii) BMP signaling stimulates chondrocyte maturation: iii) BMP signaling increases Ihh expression independent of maturational effects; an d (iii) BMP signaling can partially overcome the inhibitory effects of PTHr P on maturation. (C) 2001 Orthopaedic Research Society. Published by Elsevi er Science Ltd. All rights reserved.