Regulated expression of proenkephalin A during ontogenic development of mesenchymal derivative tissues.

Proenkephalin A (PEA), a neuropeptide-encoding gene, is widely expressed in the nervous and endocrine systems. Recently, we demonstrated that in addition to its abundance in fetal brain tissue; PEA is markedly expressed in nondifferentiated mesodermal cells of developing fetuses. To evaluate the implication of these findings for the normal development of tissues of mesodermal origin, we examined the expression of PEA in rat mesenchymal tissues during pre- and postnatal development. Using in situ hybridization analysis combined with RNA blots and a Met-enkephalin-specific radioimmunoassay, we showed that (i) PEA mRNA levels in embryonic and newborn mesenchymal derivative tissues were as high as in the developing brain, (ii) PEA mRNA concentrations in these tissues dropped to undetectable levels shortly after birth, and (iii) this mRNA was translated and processed differentially among different mesenchymal tissues, yielding a tissue-specific pattern of PEA-derived peptides. Our results demonstrate multilevel regulation of PEA gene expression during ontogenic development of mesenchymal derivative tissues. The transient expression and the correlation between PEA mRNA and tissue maturation support the notion that peptides encoded by PEA play a significant role in normal development of these tissues. These findings provide a framework for examination of the mechanisms and roles of PEA gene expression during mesenchymal ontogeny.