Zebrafish express the common parathyroid hormone/parathyroid hormone-related peptide receptor (PTH1R) and a novel receptor (PTH3R) that is preferentially activated by mammalian and fugufish parathyroid hormone-related peptide

To further explore the evolution of receptors for parathyroid hormone (PTH) and PTH-related peptide (PTHrP), we searched for zebrafish (z) homologs of the PTH/PTHrP receptor (PTH1R). In mammalian genes encoding this receptor, exons M6/7 and M7 are highly conserved and separated by 81-84 intronic nucleotides. Genomic polymerase chain reaction using degenerate primers based on these exons led to two distinct DNA fragments comprising portions of genes encoding the zPTH1R and the novel zPTH3R. Sequence comparison of both full-length teleost receptors revealed 69% similarity (61% identity), but less homology with zPTH2R, When compared with hPTH1R, zPTH1R showed 76% and zPTH3R 67% amino acid sequence similarity; similarity with hPTH2R was only 59% for both teleost receptors. When expressed in COS-7 cells, zPTH1R bound [TyrS4]hPTH-(1-34)-amide (hPTH), [Tyr(36)]hPTHrP(1-36)-amide (hPTHrP), and [Ala(29),Glu(30),Ala(34),Glu(35), Tyr(36)]fugufish PTHrP-(1-36)-amide (fuguPTHrP) with a high apparent affinity (IC50: 1.2-3.5 nM), and was efficiently activated by all three peptides (EC50: 1.1-1.7 nM). In contrast, zPTH3R showed higher affinity for fuguPTHrP and hPTHrP (IC50: 2.1-11.1 nM) than for hPTH (IC50: 118.2-127.0 nM); cAMP accumulation was more efficiently stimulated by fugufish and human PTHrP (EC50: 0.47 +/- 0.27 and 0.45 +/- 0.16, respectively) than by hPTH (EC50: 9.95 +/- 1.5 nM). Agonist-stimulated total inositol phosphate accumulation was observed with zPTH1R, but not zPTH3R.