Soluble osteogenic molecular signals and the induction of bone formation

Abstract

The induction of bone formation starts by erecting scaffolds of smart biomimetic matrices acting as insoluble signals affecting the release of soluble osteogenic molecular signals. The cascade of bone differentiation by induction develops as a mosaic structure singly initiated by the osteogenic proteins of the transforming growth factor-beta (TGF-beta) supergene family. The osteogenic signals when combined with an insoluble signal or substratum initiate de novo bone formation by induction and are deployed singly, synergistically and synchronously to sculpt the architecture of the mineralized bone/bone marrow organ. The osteogenic proteins of the TGF-beta superfamily are the common molecular initiators deployed for embryonic development and the induction of bone in postnatal osteogenesis, whereby molecules exploited in embryonic development are re-deployed in postnatal tissue morphogenesis as a recapitulation of embryonic development. The pleiotropy of the osteogenic proteins of the TGF-beta superfamily is highlighted by the apparent redundancy of molecular signals initiating bone formation by induction including the TGF-beta isoforms per se, powerful inducers of endochondral bone but in the primate only. Bone induction by the TGF-beta isoforms in the primate is site and tissue specific with substantial endochondral bone induction in heterotopic sites but with absent osteoinductivity in orthotopic calvarial sites on day 30 and only limited osteogenesis pericranially on day 90. Ebaf/Lefty-A, a novel member of the TGF-beta superfamily, induces chondrogenesis in calvarial defects of Papio ursinus and bone regeneration across the defect on day 30 and 90, respectively. The strikingly pleiotropic effects of the bone morphogenetic and osteogenic proteins (BMPs/OPs) spring from amino acid sequence variations in the carboxy-terminal domain and in the transduction of distinct signalling pathways by individual Smad proteins after transmembrane serine/threonine kinase complexes of type I and II receptors. Predictable bone regeneration in clinical contexts requires information concerning the expression and cross regulation of gene products of the TGF-beta superfamily. OP-1, BMP-3, TGF-beta1 and type IV collagen mRNAs expression correlates to the morphological induction and maintenance of engineered ossicles by the hOP-1 osteogenic devices in the non-human primate P. ursinus. Amino-acid sequence variations amongst BMPs/OPs in the carboxy terminal domain confer the structure/activity profile responsible for the pleiotropic activity that controls tissue induction and morphogenesis of a variety of tissues and organs by different BMPs/OPs which are helping to engineer skeletal tissue regeneration in molecular terms.