EXPRESSION OF NUCLEAR RETINOIC ACID RECEPTORS DURING MOUSE ODONTOGENESIS

The developmental expression of retinoic acid (RA) nuclear receptors R AR(alpha, beta, gamma) and RXR(alpha, beta, gamma) was analysed during mouse odontogenesis by in situ hybridization on frozen sections and c ompared with the expression patterns of the cellular retinoic acid bin ding proteins CRABPI and II. The transcripts distribution of each RAR and RXR was basically similar in developing molars and incisors. RAR a lpha and RXR alpha were preferentially expressed in dental epithelia, whereas RAR gamma and RXR gamma were transcribed in the dental mesench yme. RAR beta, RAR gamma and RXR beta displayed both epithelial and me senchymal expression. RAR beta expression was initiated during bell st age. RXR gamma transcripts were observed only at day 19.5 post coitum in the mitogenic mesenchyme facing the epithelial loops. Odontoblasts expressed RAR beta and RAR gamma, RXR alpha and RXR beta. Preameloblas ts expressed RXR alpha and RXR beta and ameloblasts RXR gamma, RXR alp ha and RXR beta RAR alpha transcription in the incisor preameloblasts and ameloblasts was not observed in the first molar. The coexpression between RARs and RXRs might be important to form RAR/RXR heterodimers which art necessary to activate the transcriptions of target genes. CR ABPI and CRABPII demonstrated graded variation of expression during od ontogenesis in the mesenchyme and in the inner dental epithelium respe ctively. The pattern of CRABPI transcripts overlapped at least partial ly with expressions of all the studied nuclear receptors whereas CRABP II epithelial expression was superimposed with the transcription of RA R alpha, RXR alpha and RXR beta. These cytoplasmic proteins might part icipate in the storage and/or metabolism of RA and then distribute RA to colocalized nuclear receptors. Thus, we show here that the developi ng dentition, which expresses the entire network of RA nuclear recepto rs and cytoplasmic binding proteins, is a potentially RA dependent sys tem.