DIFFERENTIAL EXPRESSION OF THE FULL-LENGTH AND SECRETED TRUNCATED FORMS OF EGF RECEPTOR DURING FORMATION OF DENTAL-TISSUES

The developmental regulation of various receptor forms may be a key-el ement in the local fine tuning of growth factor effects. The present s tudy focuses on the tissue- and stage-specificity of the alternative s plicing of EGF receptor transcripts in the rat incisor. In situ hybrid ization, as well as light- and electron-microscopic immunolocalization were performed with a set of tools which enabled us to discriminate t he full-length and secreted truncated forms of EGF receptor. Our data show that, apart from a transient expression in differentiating odonto blasts, EGF receptor expression was predominantly observed in the dent al epithelium. In the crown, the expression of the full-length EGF rec eptor was maximal during preameloblast proliferation and differentiati on, decreased in differentiated ameloblasts, and remained low througho ut enamel secretion. On the other hand, maturation stage ameloblasts, which regulate the final mineralization of enamel, express high levels of the full-length EGF receptor. In contrast with ameloblasts, epithe lial supra-ameloblastic cells, which are not directly involved in the deposition of enamel matrix, showed an alternating predominance of the secreted truncated form during the secretion stage, and the full-leng th form during the maturation stage. The presence of the secreted trun cated EGF receptor form was supported by the electron microscopic dete ction of extracellular aggregates of immunoreactive EGF receptor. Fina lly, Northern-blotting of enamel organ samples confirmed the presence of transcripts corresponding to mRNAs of both EGF receptor forms. Duri ng root formation, a decreasing gradient of full-length EGF receptor f orm expression was observed from the apical loop to the disrupting zon e in root epithelium. The secreted truncated EGF receptor form was ess entially detected in epithelial cells of the disrupting zone of root e pithelium. During crown formation. the secreted truncated EGF receptor form, which appears to be synthesized by epithelial supra-ameloblasti c cells and secreted toward ameloblasts, may competitively bind EGF re ceptor ligands and modify activation of the full-length EGF receptor.