Restriction of sonic hedgehog signalling during early tooth development

Abstract

The signalling peptide encoded by the sonic hedgehog gene is restricted to localised thickenings of oral epithelium, which mark the first morphological evidence of tooth development, and is known to play a crucial role during the initiation of odontogenesis. We show that at these stages in the murine mandibular arch in the absence of epithelium, the Shh targets Ptc1and Gli1 are upregulated in diastema mesenchyme, an edentulous region between the sites of molar and incisor tooth formation. This ectopic expression is not associated with Shh transcription but with Shh protein, undetectable in the presence of epithelium. These findings suggest that, in diastema mesenchyme, restriction of Shh activity is dependent upon the overlying epithelium. This inhibitory activity was demonstrated by the ability of transplanted diastema epithelium to downregulate Ptc1 in tooth explants, and for isolated diastema mesenchyme to express Ptc1. A candidate inhibitor in diastema mesenchyme is the glycosylphosphatidylinositol-linked membrane glycoprotein Gas1. Gas1is normally expressed throughout mandibular arch mesenchyme; however, in the absence of epithelium this expression was downregulated specifically in the diastema where ectopic Shh protein was identified. Although Shh signalling has no effect upon Gas1 expression in mandibular arch mesenchyme,overexpression of Gas1 results in downregulation of ectopic Ptc1. Therefore, control of the position of tooth initiation in the mandibular arch involves a combination of Shh signalling at sites where teeth are required and antagonism in regions destined to remain edentulous.

Making a tooth: growth factors, transcription factors, and stem cells

“…Although Shh is exclusively expressed in the epithelial compartment during early tooth development, accumulation of Shh protein is found in the mesenchyme of distema region 18 h after removal of oral epithelium. This observation implies that exclusion of Shh activity from distema mesenchyme may be required for correct tooth initiation at right position in the oral cavity [77]. After the bud stage, Shh expression is confined to the enamel knot at the cap stage, and spreads to the surrounding inner enamel epithelium and the stratum intermedium cells during the following stages [40,75,78].…”

Revisiting the supernumerary: the epidemiological and molecular basis of extra teeth

“…29 Further evidence that upregulation of Shh activity in the diastema mesenchyme can produce ectopic tooth formation is found in mice mutant for another molecule called Gas1, which encodes a Shh proteinantagonist in this region. 27 Gas1 mutants also have ectopic diastema teeth in both the mandible and maxilla, which resemble premolars (Fig. 3) and are accompanied by increased Shh signal activity within this region.…”

“…25,26 Appropriate restriction of Shh activity along the axis of the developing jaws is therefore important to ensure the correct number of teeth form in the right positions. 26,27 However, there is now evidence from a number of mouse mutants to suggest that inappropriate regulation of Shh activity might also play a key role in the formation of supernumerary teeth.…”

The Role of Gas1 in Embryonic Development and its Implications for Human Disease

“…30,31 Lastly, overexpression of Gas1 showed that it could inhibit SHH signaling in two embryonic explant culture systems. 27,32 It was therefore proposed that GAS1 acts as a sink to sequester excessive SHH and prevent its action at an unwanted distance. Again lacking, however, was in vivo evidence to demonstrate such a role.…”