Expression of nuclear retinoic acid receptors during mouse odontogenesis

Abstract The developmental expression of retinoic acid (RA) nuclear receptors RAR( α , β , γ ) and RXR( α , β , γ ) was analysed during mouse odontogenesis by in situ hybridization on frozen sections and compared with the expression patterns of the cellular retinoic acid binding proteins CRABPI and II. The transcripts distribution of each RAR and RXR was basically similar in developing molars and incisors. RAR α and RXR α were preferentially expressed in dental epithelia, whereas RAR γ and RXR γ were transcribed in the dental mesenchyme. RAR β , RAR γ and RXR β displayed both epithelial and mesenchymal expression. RAR β expression was initiated during bell stage. RXR γ transcripts were observed only at day 19.5 post coitum in the mitogenic mesenchyme facing the epithelial loops. Odontoblasts expressed RAR β and RAR γ , RXR α and RXR β . Preameloblasts expressed RXR α and RXR β and ameloblasts RXR γ , RXR α and RXR β . RAR α 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 are necessary to activate the transcriptions of target genes. CRABPI and CRABPII demonstrated graded variation of expression during odontogenesis in the mesenchyme and in the inner dental epithelium respectively. The pattern of CRABPI transcripts overlapped at least partially with expressions of all the studied nuclear receptors whereas CRABPII epithelial expression was superimposed with the transcription of RAR α , RXR α and RXR β . These cytoplasmic proteins might participate in the storage and/or metabolism of RA and then distribute RA to colocalized nuclear receptors. Thus, we show here that the developing dentition, which expresses the entire network of RA nuclear receptors and cytoplasmic binding proteins, is a potentially RA dependent system.