Parathyroid hormone-related protein in lower vertebrates

The genes for parathyroid hormone-related protein (PTHrP) have been cloned in two teleost fishes, cDNA of sea bream (Sparus aurata) and genomic DNA of puffer fish (Fugu rubripes). The gene sequences show that there is significant conservation of amino acid identity, with specific domains most highly conserved. The N-terminus, responsible for bone matrix lysis in mammals and chickens, is present in the fish genes with 52% sequence identity to higher vertebrate PTHrP peptides; the nuclear transporter region shares 73% identity, and the RNA-binding sequence is 65% identical. However, the peptides are shorter then mammalian PTHrP, lacking the C-terminus responsible for inhibition of osteoclast lytic activity, but they have an additional inserted sequence between amino acids 38 and 54 that is not present in higher vertebrate PTHrPs. The N-terminus 1-38 Fugu PTHrP proved to be hypercalcaemic in larval Sparus, suggesting that it may be a physiological regulator of calcium homeostasis in fish. Using homologous nucleotide probes for in situ hybridisation and reverse-transcription polymerase chain reaction (RT-PCR) of extracted RNA, PTHrP gene expression has been widely found in both developing and adult fish. Antiserum to the fish insert sequence demonstrated transcription of PTHrP in all stages of Sparus development, and also detected the same epitope in tissues of developing frog (Rana temporaria), indicating that this has been retained during evolution of the amphibia.