Wnt Signaling Behaves as a “Master Regulator” in the Osteogenic and Adipogenic Commitment of Human Amniotic Fluid Mesenchymal Stem Cells

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Abstract
Human amniotic fluid mesenchymal stem cells (huAFMSCs) are emerging as a promising therapeutic option in regenerative medicine. Here, we characterized huAFMSC phenotype and multipotentiality. When cultured in osteogenic medium, huAFMSC displayed a significant increase in: Alkaline Phosphatase (ALP) activity and mRNA expression, Alizarin Red S staining and Runx2 mRNA expression; whereas maintaining these cells in an adipogenic culture medium gave a time-dependent increase in PPARγ and FABP4 mRNA expression, glycerol-3-phosphate dehydrogenase (GPDH) activity and positivity to Oil Red Oil staining. These results confirm that huAFMSCs can differentiate toward osteogenic and adipogenic phenotypes. The canonical Wnt/ßcatenin signaling pathway appears to trigger huAFMSC osteoblastogenesis, since during early phases of osteogenic differentiation, the expression of Dishevelled-2 (Dvl-2), of the non-phosphorylated form of ß-catenin, and the phosphorylation of glycogen synthase kinase-3ß (GSK3ß) at serine 9 were upregulated. On the contrary, during adipogenic differentiation Dvl-2 expression decreased, whereas that of ß-catenin remained unchanged. This was associated with a late increase in GSK3ß phosphorylation. Consistent with this scenario, huAFMSCs exposure to Dickkopf-1, a selective inhibitor of the Wnt signaling, abolished Runx2 and ALP mRNA upregulation during huAFMSC osteogenic differentiation, whereas it enhanced FABP4 expression in adipocyte-differentiating cells. Taken together, these results unravel novel molecular determinants of huAFMSC commitment towards osteoblastogenesis, which may represent potential targets for directing the differentiation of these cells and improving their use in regenerative medicine. Schematic representation of Wnt pathway involved in the osteogenic and adipogenic differentiation of huAFMSCs. Our paper demonstrates that osteogenic commitment of these cells is linked to the stimulation of Wnt signal leading to the final transcriptional activation of early osteogenic markers such as RUNX-2 and ALP, mediated by β-catenin. DKK1 is a secreted Wnt antagonist that may be used as a drug to inhibit Wnt signal. In contrast, adipogenic commitment involves early inhibition of Wnt pathway leading to ubiquitination/degradation of β-catenin. This results in the transcription of PPARγ and FABP4, considered as the main initiators of adipogenesis. APC, adenomatous polyposis coli; βcat, β-catenin; CK1, casein kinase 1; DKK1, dickkopf 1; Dvl, Dishevelled; GSK3β, glycogen synthase kinase 3β; LRP5/6, low density lipoprotein receptor-related protein 5/6

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