Expression and localization of activin receptors during endochondral bone development

The expression and localization of activins (dimeric protein of inhibin beta subunit) and activin receptors in skeletal tissue were examined. RT-PCR revealed that cultured chondrocytes expressed mRNAs of inhibin/activin betaA and four activin receptors (two type I (ActRI and ActRIB) and two type II (ActRII and ActRIIB)). Immunohistochemical analyses showed that activin betaA, ActRI and ActRII were localized in proliferating chondrocytes and osteoblasts in tibiae of neonatal rats, and in implants of demineralized bone matrix, a well-established model of ectopic bone formation. The immunoreactivities of osteoblasts were decreased with aging in the tibiae and with progressing endochondral bone development in the implants. The strong expression of ActRI was also detected in hypertrophic chondrocytes both in the tibial growth plate and in the implants, whereas immunoreactive ActRII was lower in hypertrophic chondrocytes. Western blot analysis also showed that immunoreactive ActRI, migrating at 52 kDa, was detected only in the implants on days 9 and 11, the period of conversion from cartilage to bone. In view of the sharing of type II receptors between activins and bone morphogenetic proteins (BMPs), our findings suggest that activin/BMP activity involves in bone modeling, especially during active chondro- and osteogenesis and during the conversion from cartilage to bone.