Abstract
Mesenchymal stem cells (MSCs) are known to differentiate into connective tissue lineages but intracellular signaling pathways that maintain cells in an undifferentiated state remain largely unexplored. Previously, we reported that fibroblast growth factor 2 (Fgf2) reversibly inhibited multilineage differentiation of primary mouse MSCs and now identify a unique compliment of signaling proteins that are dynamically regulated by this mitogen and whose expression levels are strongly correlated with inhibition of cell differentiation. Fgf2 selectively induced expression of Twist2 and Sprouty4 (Spry4) and repressed expression of soluble frizzled related receptor 2 (Sfrp2), runt-related transcription factor 2 (Runx2), and peroxisome proliferation activated receptor gamma (Pparg). In contrast, Wnt3a induced expression of Twist but not Twist2 or Spry4 and bone morphogenetic protein 2 (Bmp2) failed to alter expression of all three genes. Moreover, pretreatment of MSCs with Fgf2 delayed extracellular regulated kinase 1 (Erk1) and Erk2 phosphorylation and repressed bone-specific gene expression during an osteoinduction time course. Alternatively, pretreatment with Wnt3a had no effect, whereas Bmp2 pretreatment augmented Erk1/2 activation and bone-specific gene expression. Fgf2 also induced expression of Fgf receptor 1 (Fgfr1) and Fgfr4 and repressed Fgfr2 and Fgfr3 expression in MSCs, whereas Wnt3a and Bmp2 had the opposite effect. Finally, immunostaining revealed that Twist and Spry4 were coexpressed in MSCs and that Fgf2 treatment altered their subcellular distribution in a manner consistent with their mode of action. Collectively, these studies demonstrate that inhibition of mouse MSC differentiation by Fgf2 is strongly correlated with upregulation of Twist2 and Spry4 and suppression of Erk1/2 activation.