The molecular and cellular mechanism regulating the developmental expression of three neuropeptide genes coding for the pro-opiomelanocortin (POMC), thyrotropin releasing hormone (TRH) and the pro-[met]enkephalin family of peptides, were investigated using the frog (Xenopus laevis) model system. The temporal and spatial expression of POMC, TRH and [met]enkephalin mRNA were mapped by in situ hybridization histochemistry. Embryonic POMC cells were found to be at first restricted to forebrain (the posterior diencephalon), before morphogenesis is complete. This suggests that the POMC gene is locally induced. Embryonic TRH cells also developed in groups as neuronal subsets before discrete brain nuclei form, again indicating that cell fate is dictated by local induction or cell lineage and not by final position. Retinoic acid, a morphogen during embryogenesis was found to delete the X. laevis anterior-posterior (a-P) CNS. At 10-7 M concentration, the anterior brain resembled enlarged hindbrain and POMC cell expression in pituitary and CNS was eliminated. Since POMC is the anterior-most embryonic marker of gene transcription, it would appear that retinoic acid dramatically affects the A-P patterning of the embryonic X. laevis CNS. Analysis of the promoter of the X. laevis pro-enkephalin gene has indicated the existence of a 928 bp (-2465 to -1537 bp) element that suppress basal and forskolin induced promoter activity. This suppressor element may play a role in tissue-specific expression of this gene. Analysis of the X. laevis POMC gene promoter has revealed several cAMP regulatory elements (CRE) within 473 bases 5' upstream of the initiation site. These CREs responded to forskolin stimulation in a tissue-specific manner and may be involved in the tissue-specific expression of the POMC gene.
