Mutant BMP receptors were transfected into cultured embryonic upper sternal 
chrondrocytes using retroviral vectors to determine if BMP signaling is required 
for chondrocyte maturation and the expression of a key regulatory molecule, 
Indian hedgehog (Ihh). Chondrocytes infected with replication competent avian 
retroviruses (RCAS) viruses carrying constitutive active (CA) BMPR-IA and 
BMPR-IB had enhanced expression of type X collagen and Ihh mRNA. Addition of 
PTHrP, a known inhibitor of chondrocyte maturation, abolished the expression of 
type X collagen, BMP-6, and Ihh mRNAs in control cells. In contrast, PTHrP 
treated cultures infected with of CA BMPR-IA or CA BMPR-IB had low levels of 
BMP-6 and type X collagen, but high levels of Ihh expression. Although dominant 
negative (DN) BMPR-IA had no effect, DN BMPR-IB inhibited 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 effect of DN BMPR-IB mimicked the effects 
of PTHrP. To determine if there is an autocrine role for the BMPs in chondrocyte 
maturation, the cultures were treated with noggin and follistatin, molecules 
that bind BMP-2/-4 and BMP-6/-7, respectively. While noggin and follistatin 
inhibited the effects of recombinant BMP-2 and BMP-6, respectively, they had 
only minimal effects on the spontaneous maturation of chondrocytes in culture, 
suggesting that more than one subgroup of BMPs regulates chondrocyte maturation. 
The results demonstrate that: (i) BMP signaling stimulates chondrocyte 
maturation; (ii) BMP signaling increases Ihh expression independent of 
maturational effects; and (iii) BMP signaling can partially overcome the 
inhibitory effects of PTHrP on maturation.