The multilineage differentiation potential of adult tissue-derived mesenchymal stem cells (MSC), such as those from bone marrow and trabecular bones, makes them a useful model to investigate mechanisms regulating tissue development and regeneration, such as cartilage. The authors examined the mechanisms of transforming growth factor-beta (TGF-beta)-mediated MSC chondrogenesis, specifically the involvement of MAP kinase and Wnt signalling cascades. Therefore MSC were nucleofected with different expression plasmids encoding: a fragment of the aggrecan promoter, TCF (T-cell factor) binding site, a fragment of human collagen type II a1 procollagen. The results demonstrate the involvement of p38, ERK-1 and JNK MAP kinase cascade in the positive regulation of mesenchymal chondrogenesis induced by TGF-beta1. In addition, the authors demonstrate the functional role of TGF-beta1-stimulated N-cadherin expression in the chondrogenic differentiation of MSC.
The multilineage differentiation potential of adult tissue-derived mesenchymal progenitor cells (MPCs), such as those from bone marrow and trabecular bone, makes them a useful model to investigate mechanisms regulating tissue development and regeneration, such as cartilage. Treatment with transforming growth factor-beta (TGF-beta) superfamily members is a key requirement for the in vitro chondrogenic differentiation of MPCs. Intracellular signaling cascades, particularly those involving the mitogen-activated protein (MAP) kinases, p38, ERK-1, and JNK, have been shown to be activated by TGF-betas in promoting cartilage-specific gene expression. MPC chondrogenesis in vitro also requires high cell seeding density, reminiscent of the cellular condensation requirements for embryonic mesenchymal chondrogenesis, suggesting common chondro-regulatory mechanisms. Prompted by recent findings of the crucial role of the cell adhesion protein, N-cadherin, and Wnt signaling in condensation and chondrogenesis, we have examined here their involvement, as well as MAP kinase signaling, in TGF-beta1-induced chondrogenesis of trabecular bone-derived MPCs. Our results showed that TGF-beta1 treatment initiates and maintains chondrogenesis of MPCs through the differential chondro-stimulatory activities of p38, ERK-1, and to a lesser extent, JNK. This regulation of MPC chondrogenic differentiation by the MAP kinases involves the modulation of N-cadherin expression levels, thereby likely controlling condensation-like cell-cell interaction and progression to chondrogenic differentiation, by the sequential up-regulation and progressive down-regulation of N-cadherin. TGF-beta1-mediated MAP kinase activation also controls WNT-7A gene expression and Wnt-mediated signaling through the intracellular beta-catenin-TCF pathway, which likely regulates N-cadherin expression and subsequent N-cadherin-mediated cell-adhesion complexes during the early steps of MPC chondrogenesis.