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doi: 10.1242/10.1242/dev.00100
1 Department of Oral Biochemistry, Sahlgrenska Academy at Göteborg
University, SE-405 30 Göteborg, Sweden
2 Division of Genetics, Brigham and Women's Hospital, Harvard Medical School, 20
Shattuck Street, Boston, MA 02115, USA
3 Curis Inc., 45 Moulton Street, Cambridge, MA 02138, USA
4 Department of Molecular and Cellular Biology, Harvard University, 16 Divinity
Avenue, Cambridge, MA 02135, USA
* Authors for correspondence (e-mail: amcmahon{at}mcb.harvard.edu amel{at}odontologi.gu.se)
Accepted 12 August 2002
Sonic hedgehog (Shh), a member of the mammalian Hedgehog (Hh) family, plays a key role during embryogenesis and organogenesis. Tooth development, odontogenesis, is governed by sequential and reciprocal epithelial-mesenchymal interactions. Genetic removal of Shh activity from the dental epithelium, the sole source of Shh during tooth development, alters tooth growth and cytological organization within both the dental epithelium and mesenchyme of the tooth. In this model it is not clear which aspects of the phenotype are the result of the direct action of Shh on a target tissue and which are indirect effects due to deficiencies in reciprocal signalings between the epithelial and mesenchymal components. To distinguish between these two alternatives and extend our understanding of Shh's actions in odontogenesis, we have used the Cre-loxP system to remove Smoothened (Smo) activity in the dental epithelium. Smo, a seven-pass membrane protein is essential for the transduction of all Hh signals. Hence, removal of Smo activity from the dental epithelium should block Shh signaling within dental epithelial derivatives while preserving normal mesenchymal signaling. Here we show that Shh-dependent interactions occur within the dental epithelium itself. The dental mesenchyme develops normally up until birth. In contrast, dental epithelial derivatives show altered proliferation, growth, differentiation and polarization. Our approach uncovers roles for Shh in controlling epithelial cell size, organelle development and polarization. Furthermore, we provide evidence that Shh signaling between ameloblasts and the overlying stratum intermedium may involve subcellular localization of Patched 2 and Gli1 mRNAs, both of which are targets of Shh signaling in these cells.
Key words: Sonic hedgehog, Smoothened, Patched2, mRNA subcellular localization, CyclinD1, Cell polarity, Cell size, ZO-1, Molar fusion, Tooth, Mouse
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