Differential onset of expression of mRNAs encoding proopiomelanocortin, prohormone convertases 1 and 2, and granin family members during Xenopus laevis development

The production of peptide hormones through proteolytic cleavage of prohormo nes, e.g., proopiomelanocortin (POMC), involves a number of regulated secre tory proteins, such as prohormone convertase PC1, PC2 and granin family mem bers, that are co-expressed with the prohormone. Although the expression of these proteins has been well-studied in adult animals, data on their expre ssion during development are limited. We used whole-mount in situ hybridiza tion to visualize POMC mRNA expression in the intermediate and anterior pit uitary of Xenopus tadpoles. A more sensitive analysis, namely semi-quantita tive reverse-transcription polymerase chain reaction (RT-PCR) on total RNA isolated from Xenopus developmental stages, revealed that the expression of POMC, PC1 and PC2 mRNA commenced at stages 13 (neural plate stage), 15 (ne ural fold stage) and 19 (neural tube stage), respectively, with a gradual i ncrease in their expression levels during further development. Surprisingly , and in contrast to what holds for POMC and the convertases, mRNAs for sec retogranin II and III (SgII, SgIII) and 7B2 were not only expressed during neural development, but could already be detected in unfertilized mature oo cytes, the first cleavage stages and in blastula-stage embryos. These grani ns are thus maternally present in Xenopus embryos suggesting that they may have a role during oogenesis and/or early embryonic development. (C) 2000 E lsevier Science B.V. All rights reserved.