IDENTIFICATION FUNCTIONAL-CHARACTERIZATION, AND DEVELOPMENTAL EXPRESSION OF 2 NONALLELIC PARATHYROID-HORMONE (PTH) PTH-RELATED PEPTIDE RECEPTOR ISOFORMS IN XENOPUS-LAEVIS (DAUDIN)/

Complementary DNAs encoding two nonallelic PTH/PTH-related peptide (PT HrP) receptor (PPR) isoforms, xPPR-A and xPPR-B, were isolated from a kidney complementary DNA library of the tetraploid African clawed frog Xenopus laevis. Both isoforms differ in their coding region by 19 ami no acids, and lack the region corresponding to the mammalian exon E2. When expressed in mammalian COS-7 cells, both receptor isoforms bound radiolabeled PTH-(1-34) and PTHrP-(1-36) analogs with comparable affin ity, and both unlabeled peptides equivalently stimulated the accumulat ion of cAMP. xPPR-A also mediated inositol phosphate turnover in COS c ells and stimulated channel-mediated current changes in voltage clamp experiments after injection into oocytes. Using ribonuclease protectio n analysis, significant xPPR-A messenger RNA expression was first dete cted in neurula stage embryos, which subsequently increased approximat ely 30-fold during tadpole development. Expression reached a maximum a t the metamorphotic climax, when isoform B also became detectable at s ignificant levels, and subsequently declined in postmetamorphotic frog lets. In the adult frog, xPPR-A was prominently expressed in lung, bra in, small bowel, and skin, whereas isoform B was highest in lung, hear t, and brain. Using an xPPR-A antisense riboprobe for in situ hybridiz ation, expression appeared during metamorphosis at all sites of chondr ogenesis, specifically in the maturing zone of the amphibian growth pl ate. xPPR-A expression was also seen in a subpopulation of mononuclear cells, possibly representing osteoblasts that line perichondral bone and diaphyseal bone trabeculae. Our findings suggest that xPPRs serve a prominent role in am phibian skeletal development and possibly other functions during embryonal and early larval development.