RECIPROCAL TEMPOROSPATIAL PATTERNS OF MSX2 AND OSTEOCALCIN GENE-EXPRESSION DURING MURINE ODONTOGENESIS

Msx2 is a homeodomain transcription factor that regulates craniofacial development in vivo and osteocalcin (Osc) promoter activity in vitro. Msx2 is expressed in many craniofacial structures prior to embryonic day (E) E14 but is expressed at later stages in a restricted pattern, primarily in developing teeth and the calvarium. We examine Osc expres sion by in situ hybridization during murine development, detailing tem porospatial relationships with Msx2 expression during preappositional and appositional odontogenesis and calvarial osteogenesis. Osc express ion at E14-14.5 is very low, limited to a few perichondrial osteoblast s in the dorsal aspect of developing ribs. At E16.5 and E18.5, Osc exp ression is much higher, widely expressed in skeletal osteoblasts, incl uding calvarial osteoblasts that do not express Msx2. No Osc is detect ed in early preappositional teeth that express Msx2. In incisors studi ed at an early appositional phase, Msx2 is widely expressed in the too th, primarily in ovoid preodontoblasts and subjacent dental papilla ce lls. Osc is detected only in a small number of maturing odontoblasts t hat also express alpha(1)(l) collagen (Col1a1) and that are postprolif erative (do not express histone H4). Msx2 expression greatly overlaps both histone H4 and Col1a1 expression in ovoid preodontoblasts and den tal papilla cells. By the late appositional phases of E18.5 and neonat al teeth, Osc mRNA is highly expressed in mature columnar odontoblasts adjacent to accumulating dentin. In appositional bell-stage molars, r eciprocal patterns of Msx2 and Osc are observed in adjacent preodontob lasts and odontoblasts within the same tooth. Osc is expressed in matu re columnar odontoblasts, while Msx2 is expressed in adjacent immature ovoid preodontoblasts. In less mature teeth populated only by immatur e ovoid preodontoblasts, only Msx2 is expressed-no Osc is detected. Th us, Msx2 and Osc are expressed in reciprocal patterns during craniofac ial development in vivo, and Msx2 expression in preodontoblasts clearl y preceeds Osc expression in odontoblasts. In functional studies using MC3T3-E1 calvarial osteoblasts, Msx2 suppresses endogeneous Osc, but not osteopontin, mRNA accumulation. In tote, these data suggest that M sx2 suppresses Osc expression in the craniofacial skeleton at stages i mmediately preceeding odontoblast and osteoblast terminal differentiat ion.