Evaluating the ligand specificity of zebrafish parathyroid hormone (PTH) receptors: Comparison of PTH, PTH-related protein, and tuberoinfundibular peptide of 39 residues

Homologs of mammalian PTH1 and PTH2 receptors, and a novel PTH3 receptor ha ve been identified in zebrafish (zPTH1, zPTH2, and zPTH3). zPTH1 receptor l igand specificity is similar to that of mammalian PTH1 receptors. The zPTH2 receptor is selective for PTH over PTH-related protein (PTHrP); however, P TH produces only modest cAMP accumulation. A PTH2 receptor-selective peptid e, tuberoinfundibular peptide of 39 residues (TIP39), has recently been pur ified from bovine hypothalamus. The effect of TIP39 has not previously been examined on zebrafish receptors. The zPTH3 receptor was initially describe d as PTHrP selective based on comparison with the effects of human PTH. We have now examined the Ligand specificity of the zebrafish PTH-recognizing r eceptors expressed in COS-7 cells using a wide range of ligands. TIP39 is a potent agonist for stimulation of cAMP accumulation at two putative splice variants of the zPTH2 receptor (EC50, 2.6 and 5.2 nM); in comparison, PTH is a partial agonist [maximal effect (E-max) of PTH peptides ranges from 28 -49% of the TIP39 E-max]. As TIP39 is much more efficacious than any known PTH-like peptide, a homolog of TIP39 may be the zPTH2 receptor's endogenous ligand. At the zPTH3 receptor, rat PTH-(1-34) and rat PTH-(1-84) (EC50, 0. 22 and 0.45 nM) are more potent than PTHrP (EC50, 1.5 nM), and rPTH-(1-34) binds with high affinity (3.2 nM). PTH has not been isolated from fish. PTH rP-like peptides, which have been identified in fish, may be the natural li gands for zPTH1 and zPTH3 receptors.