Parathyroid hormone without parathyroid glands.

In mammals, PTH is a component in a complex signaling system involving three structurally related ligands and two G protein-coupled receptors, which share significant amino acid sequence homology (Fig. 1). PTH is a peptide hormone comprising 84 amino acids that is made by the parathyroid glands (and to a much smaller extend by the thymus) and acts through the PTH/PTH-related peptide (PTHrP) receptor to regulate the serum calcium concentration. PTHrP is made in virtually all tissues, but exploration of its biological roles has been best studied in the growth plate and the mammary gland. Due to alternative splicing, there are three different forms of PTHrP in humans comprising 139, 141, and 173 amino acids. PTHrP also acts through the PTH/PTHrP receptor, but instead of functioning as a circulating hormone, it is primarily a paracrine/autocrine factor that regulates cellular growth and differentiation (only during lactation and in patients with the humoral hypercalcemia of malignancy syndrome does it function as a hormone). PTH also activates a second receptor, the PTH-2 receptor. However, only the human PTH-2 receptor is efficiently activated by PTH, whereas the receptor homologs from other species are activated poorly, if at all. It was therefore not too surprising that the primary ligand for the PTH-2 receptor was determined to be a distinct peptide, tuberoinfundibular peptide (TIP39), which is only distantly related PTH and PTHrP (1). The biological roles of this novel peptide are largely unknown, but it seems to be involved in nociception and perhaps in the regulation of some pituitary hormones (2–4). However, intact TIP39 and amino-terminally truncated TIP39 analogs can bind to the PTH/PTHrP receptor, making it possible that this peptide could also modulate actions mediated by PTH or PTHrP. Fish do not have anatomical structures corresponding to parathyroid glands, and these animals were therefore thought not to produce PTH. However, there was early evidence that certain fish tissues contain PTH-like immunoreactivity (5, 6), and a partial trout peptide with significant amino acid sequence homology to mammalian PTH had been deduced from trout genomic DNA sequences (7). Furthermore, fish have been shown to produce a PTHrP molecule (8), which was not too surprising given the presence of cartilagenous and/or osseous structures in most fish species. Fish PTHrP signals through a receptor that is homologous to the mammalian PTH/PTHrP receptor as shown in zebrafish (zPTH1R) (9). Zebrafish also express proteins homologous to mammalian TIP39 (10) and the PTH-2 receptor (11), and their expression patterns suggest that both proteins have biological roles which may be related to those in humans. Lastly, zebrafish have a third receptor, the PTH3 receptor (zPTH3R), which appears to be activated less efficiently by human PTH than by human PTHrP, and it thus appeared to be a PTHrPselective receptor (9). The recent discovery of PTH molecules in fish has provided further insights into this complex system involving multiple receptors and multiple ligands (12, 13). In contrast to mammals, fish appear to have two distinct PTH molecules, PTH1 and PTH2, and both ligands efficiently activate the zPTH1R and the zPTH3R (12, 13). PTH2 is considerably less potent at the zPTH1R than is PTH1, whereas both ligands have similar potency at the zPTH3R. Phylogenetic analyses suggest that PTH1 and PTH2 evolved from a common precursor in fish, likely through a gene duplication event, and a similar mechanism may have led to the development of two PTH/PTHrP receptors, the zPTH1R and the zPTH3R. The appearance of two PTH ligands (13, 14), and possibly even more PTHrP ligands as suggested by the presence of several homologs in fish genomic databases, appears to have occurred after the development of distinct PTH, PTHrP, and TIP39 molecules. Although there is some indication to suggest that TIP39 may be the basal group from which PTH and PTHrP were derived (10), sequence analysis of the homologs of these peptides from additional species is required to confirm this prediction. Although cDNA cloning from total fish embryos confirmed that both PTH ligands are indeed expressed, the cellular source of these mRNAs and their biological roles remained elusive. Hogan et al. (14) have now shown interesting expression patterns for PTH1 and PTH2 in zebrafish. For their detailed study, both PTH ligands were cloned from cDNA, expression was verified by in situ hybridization, and the presence of the PTH1 protein product was verified by immunohistochemical studies. Although both ligands were found to be expressed transiently early in development along the forming lateral line, only PTH1 was expressed in brain and neuronal tissue. The lateral line in fish is histologically closely related to the mammalian vestibular apparatus and inner ear, with hair cells embedded in a gelatinous mass (cupula) (15, 16). Expression of the zebrafish homologs of the calcium-sensing receptor and of the transcription factor glial cells missing 2 (gcm-2) (17), important in mammals for the regulation of PTH secretion and for parathyroid gland development, respectively, was not investigated. It is therefore not certain whether the PTH-expressing cells of the lateral line and the identified neuronal structures may represent parathyroid cell equivalents. However, based on their anatomical locations, it would seem unlikely that these sources for fish PTH expression would be related to the regulation of calcium homeostasis. At least for the lateral line, where PTH1 and PTH2 are expressed only transiently, it is more likely that both peptides have as yet unknown developmental roles, Abbreviations: PTHrP, PTH-related peptide; TIP39, tuberoinfundibular peptide; zPTH1R, zebrafish PTH/PTHrP receptor; zPTH3R, zebrafish PTH3 receptor.