Wnt signaling is essential for many developmental processes, including skeletogenesis. To investigate the effects of Wnt signaling during skeletogenesis we studied the effects of Wnt on cultured chondrocytic cells and differentiating limb-bud mesenchyme. We showed that Wnt3a strongly repressed chondrogenesis and chondrocyte gene expression. Canonical Wnt signaling was responsible for the repression of differentiation, as evidenced by results showing that inhibition of glycogen synthase kinase 3 or expression of ␤-catenin caused similar repression of differentiation. Significantly, we showed that the transcription repressor Twist1 is induced by canonical Wnt signaling. Expression of Twist1 strongly inhibited chondrocyte gene expression and short hairpin RNA knockdown of Twist1 transcript levels caused increased expression of the chondrocyte-specific genes aggrecan and type II collagen. Interestingly, Twist1 interfered with BMP2-induced expression of aggrecan and type II collagen expression and knockdown of Twist1 augmented BMP2-induced aggrecan and type II collagen expression. These data support the conclusions that Twist1 contributes to the repression of chondrogenesis and chondrocyte gene expression resulting from canonical Wnt signaling and that Twist1 interferes with BMP-dependent signaling.Wnt signaling is divided into canonical and non-canonical pathways. Canonical Wnt signaling regulates the protein levels of ␤-catenin (␤cat).2 The binding of Wnt ligands to cell-surface receptors leads to the activation of the intracellular protein Dishevelled. When activated, Dishevelled is released from the receptor complex at the cell surface and interacts with the multiprotein complex that controls ␤cat levels. This complex includes axin, the adenomatous polyposis coli gene product, glycogen synthase kinase 3 (GSK3), ␤cat, and other proteins (1). Within the complex, phosphorylation of ␤cat by GSK3 leads to its ubiquitination and degradation by the proteasome. GSK3 is inhibited by activated Dishevelled. This in turn stabilizes ␤cat, thereby increasing the amount of ␤cat, which accumulates in the nucleus and regulates gene expression in conjunction with the TCF/Lef family of transcription factors. Thus the canonical Wnt signaling pathway is in part a transcription control pathway that regulates the levels of the transcription co-activator ␤cat. Non-canonical Wnt signaling pathways are subdivided into the Wntcalcium and planar cell polarity pathways (2-4). The Wnt-calcium pathway increases intracellular calcium levels in response to Wnt signaling. Like canonical Wnt signaling, this pathway also requires Dishevelled (5). The release of calcium stimulates calcium-dependent kinases and transcription factors, like the nuclear factor of activated T-cells family of transcription factors (6, 7). The planar cell polarity pathway, also linked to Dishevelled activation, requires the Rho family GTPases and Jun kinase. Through this pathway Wnt signaling regulates polarized cell movements.A hierarchy of Wnt signaling regulates diverse...

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Glycogen synthase kinase 3 (GSK3) inhibits signaling pathways that are essential for bone development. To study the requirement for GSK activity during endochondral bone development, we inhibited GSK3 in cultured metatarsal bones with pharmacological antagonists. Interestingly, we find that inhibition of GSK3 strongly repressed chondrocyte and perichondrial osteoblast differentiation. Moreover, chondrocyte proliferation was inhibited, whereas perichondrial cell proliferation was stimulated. These results mirror the effects of fibroblast growth factor signaling (FGF), suggesting the FGF expression is induced. Indeed, we showed that (1) FGF18 expression is stimulated following inhibition of GSK3 and (2) GSK3 regulates FGF18 expression through the control of beta-catenin levels. Stimulation of cultured metatarsal with FGF18 had similar effects on the differentiation and proliferation of chondrocytes and perichondrial cells as GSK3 repression. This suggests that the regulation of FGF18 expression is a major function of GSK3 during endochondral bone development. Consistent with this, we showed that the effect of GSK3 inhibition on chondrocyte proliferation is repressed in tissues lacking a receptor for FGF18, FGF receptor 3.