Osteoblasts Directly Control Lineage Commitment of Mesenchymal Progenitor Cells through Wnt Signaling

Lineage commitment of mesenchymal progenitor cells is still poorly understood. Here we demonstrate that Wnt signaling by osteoblasts is essential for mesenchymal progenitor cells to differentiate away from a default adipogenic into an osteoblastic lineage. Dominant adipogenesis and reduced osteoblastogenesis were observed in calvarial cell cultures from transgenic mice characterized by osteoblast-targeted disruption of glucocorticoid signaling. This phenotypic shift in mesenchymal progenitor cell commitment was associated with reciprocal regulation of early adipogenic and osteoblastogenic transcription factors and with a reduction in Wnt7b and Wnt10b mRNA and beta-catenin protein levels in transgenic versus non-transgenic cultures. Transwell co-culture of transgenic mesenchymal progenitor cells with wild type osteoblasts restored commitment to the osteoblast lineage. This effect was blocked by adding sFRP1, a Wnt inhibitor, to the co-culture. Treatment of transgenic cultures with Wnt3a resulted in stimulation of osteoblastogenesis and suppression of adipogenesis. Our findings suggest a novel cellular mechanism in bone cell biology in which osteoblasts exert direct control over the lineage commitment of their mesenchymal progenitor through Wnt signaling. This glucocorticoid-dependent forward control function indicates a central role for osteoblasts in the regulation of early osteoblastogenesis.