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The homeobox gene Msx1 encodes a transcription factor that is highly expressed during embryogenesis and postnatal development in bone. Mutations of the MSX1 gene in humans are associated with cleft palate and (or) tooth agenesis. A similar phenotype is observed in newborn mice invalidated for the Msx1 gene. However, little is known about Msx1 function in osteoblast differentiation and bone mineralization in vivo. In the present study, we aimed to explore the variations of individualized bone shape in a subtle way avoiding the often severe consequences associated with gene mutations. We established transgenic mice that specifically express Msx1 in mineral-matrix-secreting cells under the control of the mouse 2.3kb collagen 1 alpha 1 (Col1α1) promoter, which enabled us to investigate Msx1 function in bone in vivo. Adult transgenic mice (Msx1-Tg) presented altered skull shape and mineralization resulting from increased Msx1 expression during bone development. Serial section analysis of the mandibles showed a high amount of bone matrix in these mice. In addition, osteoblast number, cell proliferation and apoptosis were higher in Msx1-Tg mice than in controls with regional differences that could account for alterations of bone shape. However, Von Kossa staining and μCT analysis showed that bone mineralization was lower in Msx1-Tg mice than in controls due to alteration of osteoblastic differentiation. Msx1 appears to act as a modeling factor for membranous bone; it stimulates trabecular bone metabolism but limits cortical bone growth by promoting apoptosis, and concomitantly controls the collagen-based mineralization process., (Copyright © 2014 Elsevier Inc. All rights reserved.)

The present study was designed to compare the expression pattern of Dlx3 in four different mineralized tissues because of: 1-its role in skeleton patterning, 2-its expression in dental epithelium and mesenchyme during morphogenesis, 3-the membranous and endochondral bone and tooth phenotype of tricho-dento-osseous syndrome related to Dlx3 gene mutation and 4-recently emerging knowledge on Dlx family members in the bone field. Ameloblasts, odontoblasts, osteoblasts and chondrocytes were analyzed in vitro and in vivo. Dlx3 transcripts were detected by RT-PCR in established model systems (microdissected dental epithelium and mesenchyme; primary cultures of rat chondrocytes), as recently performed in osteoblasts in vitro. A human 414-bp Dlx3 probe was generated. A 4.5-kb human Dlx3 sense RNA was identified in maxillo-facial samples by Northern blotting. Immunolabeling and in situ hybridization were performed in mice from Theiler stage E 14.5 until birth. In teeth, although Dlx3 was still expressed in differentiated ameloblasts, it was down regulated during odontoblast polarization. During endochondral bone formation, Dlx3 protein was detected in chondrocytes and was most strongly expressed in the prehypertrophic cartilage zone and in differentiating and differentiated osteoblasts of metaphyseal periosteum. In vitro, real-time PCR studies supported this upregulation in prehypertrophic chondrocytes, closely correlated with Ihh variations. In membranous bone, Dlx3 was present in preosteoblasts, osteoblasts and osteoid-osteocytes. The present data on Dlx3 and recently published functional studies show that this transcription factor may be instrumental during growth in the control of matrix deposition and biomineralization in the entire skeleton.