Induction of Endochondral Bone Formation by Recombinant Human Transforming Growth Factor-β2 in the Baboon (Papio ursinus)

Abstract

Members of the transforming growth factor-β(TGF-β) superfamily, the bone morphogenetic and osteogenic proteins (BMPs/OPs) but not the TGF-β proteins themselves, induce endochondral bone formation in vivo, when implanted in extraskeletal heterotopic sites of rodents. Here we show that recombinant human TGF-β2 (hTGF-β2) induces endochondral bone formation 30 days after implantation in heterotopic intramuscular sites of the baboon (Papio ursinus) at doses of 1,5 and 25 μg per 100 mg of guanidinium-inactivated collagenous bone matrix as carrier. On day 90 there was generation of large radiopaque and corticalized intramuscular ossicles. Five and 25 ug hTGF-β2 induced large ossicles in the rectus abdominis of the primate as evaluated by key parameters of bone formation, including generated tissue area, mineralized bone and osteoid volumes, and tissue alkaline phosphatase activity. On day 30 and 90 after healing, hTGF-β2 also induced bone formation when implanted in the rectus abdominis in conjunction with a sintered porous hydroxyapatite as carrier. mRNA expression in tissues from heterotopic specimens showed OP-1 (BMP-7) and BMP-3 transcripts in low abundance and with a linear dose-dependent increase both in collagenous matrix and hydroxyapatite samples. Type IV collagen mRNA expression, a marker of angiogenesis, was stronger in collagenous than hydroxyapatite samples. Growth and differentiation factor-10 (GDF-10) mRNA transcripts were expressed in ossicles with a distinctly chondrogenic phase, but its expression was greater in ossicles generated in porous hydroxyapatites, in which bone formation is not via a chondrogenic phase, but is rather intramembranous, without expression of type II collagen mRNA. In the same animals, however, 10 and 100 μg of the recombinant morphogen delivered by identical carriers (collagenous matrix and sintered hydroxyapatite) failed to heal calvarial defects. Thus in the primate, TGF-βs themselves are inducers of endochondral bone formation, although the present data strongly indicate that the bone inductive activity of hTGF-β2 is site and tissue specific, since a single application of hTGF-β, or hTGF-β1 in previously published experiments, did not induce bone in calvarial defects, but did induce endochondral bone differentiation in heterotopic sites.