A G protein-coupled receptor from zebrafish is activated by human parathyroid hormone and not by human or teleost parathyroid hormone-related peptide- Implications for the evolutionary conservation of calcium-regulating peptide hormones

Genomic and cDNA clones encoding portions of a putative catfish parathyroid hormone (PTH) 2 receptor (PTH2R) led to the isolation of a cDNA encoding a full-length zebrafish PTH2R (zPTH2R), The zPTH2R shared 63 and 60% amino a cid sequence identity with human and rat PTH2Rs, respectively, 47-52% ident ity with mammalian and frog PTH/PTHrP receptors (PTH1R), and less than 37% with other members of this family of G protein-coupled receptors, COS-7 cel ls expressing zPTH2R(43), a 5' splice variant that lacked 17 amino acids in the amino-terminal extracellular domain, showed cAMP accumulation when cha llenged with [Tyr(34)]hPTH(1-34)-amide (hPTH) (EC50, 1.64 +/- 0.95 nM) and [Ile(5),Trp(23),Tyr(36)]hPTHrP-(1-36)-amide ([Ile5, Trp23]hPTKrP) (EC50, 46 .8 +/- 12.1 nM) but not when stimulated with [Tyr(36)]hPTHrP-(1-36)-amide ( hPTHrP), [Trp(23),Tyr(36)]hPTHrP-(1-36)-amide ([Trp(23)]hPTHrP), or [Ala(29 ),Glu(30), Ala(34), Glu(35), Tyr(36)]fugufish PTHrP-(1-36)-amide (fuguPTHrP ), FuguPTHrP also failed to activate the human PTH2R but had similar effici ency and efficacy as hPTH and hPTHrP when tested with cells expressing the human PTH1R. Agonist-dependent activation of zPTH2R was less efficient than that of zPTH2R(43), and both receptor variants showed no cAMP accumulation when stimulated with either secretin, growth hormone-releasing hormone, or calcitonin. The zPTH2R thus has ligand specificity similar to that of the human homolog, which raises the possibility that a PTH-like molecule exists in zebrafish, species which lack parathyroid glands.