Shh signaling within the dental epithelium is necessary for cell proliferation, growth and polarization

Abstract

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.

Signaling Networks Regulating Tooth Organogenesis and Regeneration, and the Specification of Dental Mesenchymal and Epithelial Cell Lineages

“…The Fgfs also signal to dental mesenchyme and induce e.g., Runx2, and Fgf3, which signals back to epithelium illustrating the bidirectional Fgf signaling between epithelium and mesenchyme regulating tooth morphogenesis (Klein et al 2006). Shh from the enamel knot stimulates epithelial morphogenesis indirectly via the mesenchyme (Gritli-Linde et al 2002).…”

“…4) (Coin et al 1999;Wang et al 2004). In addition, Shh from the epithelial stratum intermedium cells is required to support ameloblast differentiation and maturation (Dassule et al 2000;Gritli-Linde et al 2002). Other epithelial growth factors regulating ameloblasts are TFGb1, Wnt3, Eda, and Follistatin (Bei 2009a).…”

Disruption of Fgf10/Fgfr2b-coordinated epithelial-mesenchymal interactions causes cleft palate

“…Using Cre-recombinant technology we generated mice deficient in either Shh or Smo under the Keratin 14 (K-14) promoter (20,21). In these transgenic mice, Shh and Smo expression is inactivated only in the epithelium (20,22).…”

“…The generation and genotyping of these mice has been described previously (16,46). The generation of K-14Cre;Shh c/n and K-14Cre;Smo c/n mice has been described previously (20,21). Whole heads of mice aged between E11 and NB were dissected free under a stereomicroscope.…”

Constitutive Activation of Smoothened Leads to Impaired Developments of Postnatal Bone in Mice

“…For example, null embryos specific for Shh (Chiang et al, 1996) or Smo (Zhang et al, 2001) died at birth or around E9, respectively. Both Shh (Dassule et al, 2000) and Smo conditional knockout mice (Gritli-Linde et al, 2002) died within 1 day after birth. The construction of a SmoM2 mutant led to great advantages for the functional analysis of Smo signaling in postnatal development and to understand the role of Smo-mediated signaling.…”