Evolution of the vertebrate pth2 (tip39) gene family and the regulation of PTH type 2 receptor (pth2r) and its endogenous ligand pth2 by hedgehog signaling in zebrafish development

Abstract

In mammals, parathyroid hormone (PTH), secreted by parathyroid glands, increases calcium levels in the blood from reservoirs in bone. While mammals have two PTH receptor genes,PTH1RandPTH2R, zebrafish has three receptors,pth1r,pth2r, andpth3r. PTH can activate all three zebrafish Pthrs while PTH2 (aliastuberoinfundibular peptide 39, TIP39) preferentially activates zebrafish and mammalian PTH2Rs. We know little about the roles of the PTH2/PTH2R system in the development of any animal. To determine the roles of PTH2 and PTH2R during vertebrate development, we evaluated their expression patterns in developing zebrafish, observed their phylogenetic and conserved synteny relationships with humans, and described the genomic organization ofpth2,pth2r, andpth2rsplice variants. Expression studies showed thatpth2is expressed in cells adjacent to the ventral part of the posterior tuberculum in the diencephalon, whereaspth2ris robustly expressed throughout the central nervous system. Otic vesicles express bothpth2andpth2r, but heart expresses onlypth2. Analysis of mutants showed that hedgehog (Hh) signaling regulates the expression ofpth2transcripts more than that of nearbygnrh2-expressing cells. Genomic analysis showed that a lizard, chicken, and zebra finch lack aPTH2gene, which is associated with an inversion breakpoint. Likewise, chickens lack PTH2R, while humans lack PTH3R, a case of reciprocally missing ohnologs (paralogs derived from a genome duplication). The considerable evolutionary conservation in genomic structure, synteny relationships, and expression of zebrafishpth2andpth2rprovides a foundation for exploring the endocrine roles of this system in developing vertebrate embryos.