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The impact of osteoclast activity on dental development has been previously analyzed but in the context of severe osteopetrosis. The present study sought to investigate the effects of osteoclast hypofunction,present in Msx2 gene knockin mutant mice (Msx2-/-), and hyperfunction, in transgenic mice driving RANK over-expression in osteoclast precursors (RANK(Tg)), on tooth development. In Msx2-/- mice, moderate osteopetrosis was observed, occurring exclusively in the periodontal region. Microradiographical and histological analyses revealed an abnormal dental epithelium histogenesis that gave rise to odontogenic tumor-like structures. This led to impaired tooth eruption, especially of the third mandibular molars. In RANK(Tg) mice, root histogenesis showed site-specific upregulation of dental cell proliferation and differentiation rates. This culminated in roots with a reduced diameter and pulp size albeit of normal length. These two reverse experimental systems will enable the investigation of distinctive dental cell and osteoclast communication in normal growth and tumorigenesis., (Copyright © 2011 S. Karger AG, Basel.)

Craniofacial development involves a large number of genes involved in a complex time- and site-specific cascade of cellular crosstalk. Msx homeobox genes are expressed very early and have been implicated in multiple signaling processes. However, little is known about their role in postnatal growth and at adult stages. The aim of this study was to compare the patterns of expression of Msx1 and Msx2 during postnatal growth and homeostasis. We used transgenic mice with a knock-in for Msx1 or Msx2. Msx expression was analyzed on whole-mount experiments on heterozygous mice. The results were confirmed by quantitative RT-PCR on mandible and tibia samples. Steady-state levels of Msx2 mRNA were determined at 2 ages, at postnatal day 14 and after 3 months, corresponding to phases of growth and homeostasis, respectively. Consistent with previous findings, the expression profiles of Msx1 and Msx2 overlapped during embryonic development. By contrast, marked differences in the patterns of expression of these 2 genes were observed during the growth phase. Msx1 was found to be expressed in basal bone during postnatal growth. Msx1 was not expressed in alveolar bone, whereas Msx2 was strongly and continually expressed. Msx2 was present in all growth plate cartilages, as previously shown for Msx1. Autopods displayed different patterns of expression during the mouse life cycle, with continuous expression of Msx1 only. Interestingly, both secretory cells (osteoblasts) and cells involved in bone resorption (osteoclasts) were found to be involved in Msx molecular pathways, their precise involvement depending on the anatomical site. The observed patterns correspond to specific sites during growth and constitute landmarks in our understanding of growth-related oral facial dysmorphologies., (Copyright 2008 S. Karger AG, Basel.)

The physiological function of the transcription factor Msx2 in tooth and alveolar bone was analysed using a knock-in transgenic mouse line. In this mouse line, the beta-galactosidase gene was used to disrupt Msx2: thus, beta-galactosidase expression was driven by the Msx2 promoter, but Msx2 was not produced. This allowed to monitor Msx2 expression using a beta-galactosidase assay. Msx2 transgenic mice ubiquitously and continuously expressed the mutated Msx2-nlacZ gene in cells of the complex formed by tooth and alveolar bone. Msx2 -/- homozygous mice displayed a wide spectrum of alterations in tooth eruption and morphology as well as dental and periodontal defects from the first post-natal weeks up to 6 months. These defects culminated with the formation of an odontogenic tumour at the mandibular third molar site. This study suggests that bone resorption is a functional target of Msx2 in the alveolar compartment, since Msx2 was expressed in osteoclasts, with the highest expression levels found in the active sites of bone modelling associated with tooth eruption and root elongation. The RANK osteoclast differentiation pathway was affected in microdissected Msx2 -/- mouse alveolar bone (as inferred by RANK ligand mRNA levels) compared to basal bone and wild-type controls. Decreased alveolar osteoclast activity was observed in Msx2 -/- mice, similar to that seen in osteopetrosis, another condition in which osteoclast activity is impaired and odontogenic tumours form. These data suggest a pleiotropic role for Msx2 in oral bone growth from birth until adult homeostasis. RANK pathway appeared to be modulated by Msx2, in addition to the previously reported modulations of BMP4 and laminin5alpha3 in early tooth development. Non-overlapping Msx1 and Msx2 expression patterns suggested that these two homeogenes play non-redundant roles in skeletal growth, with Msx1 targeting basal bone and Msx2 targeting alveolar bone. This study provides a detailed analysis of the phenotype resulting from the Msx2 null mutation and identifies the impact of Msx1 and Msx2 on post-natal oral bone growth.

Molecular and structural biodiversity characterises dental mineral tissues. Groups of matrix proteins belong specifically to each tissue; amelogenins to enamel, DSPP to dentine and CAP to cementum. A wide group of proteins is also shares with other mineralized tissues such as calcium (calbindins) and phosphate (alkaline phosphatase) handling proteins. Dental tissues organisation is also based on specific cellular programs of morpho-differentiation (polarity) and on expression patterns of proteins implicated in mineralisation. The regulation of gene expression in tooth has been analysed regarding various hormones such as vitamin D in a first step and recently transcription factors (Osf-2/Cbfa1/Aml3). Other molecular families encoded by divergent homeobox genes (Msx and Dlx) are implicated in the determinism of this gene regulation and of early development. Genetic and hormonal abnormalities of dental mineralized tissues should now be interpreted thanks to the recent availability of cellular models and of odontogenic protein promoter structure.