Molecular regulators in cartilage and bone formation
Abstract: Cartilage functions as a scaffold for bone formation during development. This process, endochondral ossification, is regulated by systemic hormones and local growth factors. Members of the transforming growth factor-beta (TGF-β) superfamily are essential mediators in cell proliferation and differentiation and play a regulatory role in cartilage and bone formation. The TGF-β superfamily consists of three major subgroups: TGF-β1-3, bone morphogenetic proteins (BMPs), and activins. This thesis presents functional data on TGF-β1 in cartilage formation, BMP2 and BMP3 in bone formation, and a potential role of the BMP-regulatory protein noggin in BMP2 processing. TGF-β1 is found in articular cartilage, stabilizing the chondrocytic phenotype and inhibiting terminal differentiation of chondrocytes. The effect of retrovirally gene-manipulated chondrocytes overexpressing TGF-β1 was investigated. Eight weeks after infection the transgene TGF-β1 was still expressed and the transduced chondrocytes displayed enhanced expression of cartilage-associated matrix molecules. BMP2 possesses the capacity to induce differentiation of osteoprogenitor cells into osteocytes. The gene therapy concept was utilized to assess the osteogenic capacity of osteoprogenitor cells overexpressing BMP2. These cells were capable of differentiating into osteocytes in vitro and formed ectopic bone in vivo. The effect of BMP3 was investigated in cell culture where it counteracted the effects of BMP2. BMP3 was shown to exert its effect through the activin pathway. The effect of BMP3 on bone formation was further demonstrated in mutant mice with targeted deletion of the BMP3 gene. Radiographic and histomorphometric analyses on femurs revealed a higher trabecular bone density in these animals as compared to that in matched wild-type controls. Noggin antagonizes the effects of BMP2 and BMP4 by direct binding. In co-transfection experiments noggin was shown to promote secretion of BMP2. Cells co-expressing BMP2 and noggin displayed an enhanced bone inductive capacity in vitro and in vivo. Noggin may participate in the processing and secretion of BMP2 in addition to inhibiting the mature secreted BMP2 molecule.
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