Expression of cartilage-specific genes during neochondrogenesis in periosteal explants

The expression of several cartilage-specific genes (collagen alpha 1 (II) and aggrecan), collagen alpha 1 (I) and endogenous TCF-beta 1 was characterized during in vitro chondrogenesis in rabbit periosteal explants and correlated with the histological changes. Explants were cultured in agarose for 0, 3, 7, 14, 21, or 28 days with 10 ng/mL TGF-beta 1 added for the first 2 days and RNA levels were quantitated by dot blot analysis. The first observed effect of exogenous TGF-beta 1 was to briefly increase the expression of endogenous TGF-beta 1, which then declined between days 3-7. Aggrecan gene expression increased within 3 days and reached a maximal (approximate to 6-fold) increase at 21 days. Procollagen alpha 1 (II) RNA showed a larger increase from approximately 11 fold at 7 days to over 200 fold after 21 days. The change in procollagen alpha 1 (II) gene expression was clearly dependent on TGF-beta 1 as explants cultured without added TGF-beta 1 showed a much smaller increase in RNA (procollagen alpha 1 (II)) after 2 weeks. Collagen alpha 1 (I) RNA levels decreased markedly (80%) at the same time that collagen alpha 1 (II) RNA levels were increasing, and this response was also TGF-beta 1 dependent. This study demonstrates that TGF-beta 1 significantly enhances in vitro chondrogenesis in rabbit periosteal explants and acts, at least in part, by inducing expression of cartilage-specific genes and inhibiting expression of collagen alpha 1 (I). Its effects are most likely mediated by the initiation of a cascade of events.