Urotensin II (UII) is a piscine neuropeptide originally isolated from the teleost urophysis. The existence of UII in mammals has been demonstrated by cloning of the mammalian orthologs of UII precursor protein genes. While rat and mouse orthologs have been reported, only the tentative structures of UII peptides of these animals have been demonstrated, since prepro-UII proteins lack the typical processing sites in the amino-terminal region of the mature peptides. A novel peptide, UII-related peptide (URP), was discovered by monitoring UII-immunoreactivity in the rat brain, and its amino acid sequence was determined to be ACFWKYCV. cDNAs encoding rat, mouse, and human precursor proteins for URP were cloned and showed that the sequences of mouse and human URP peptides are identical to that for rat URP. URP was found to bind and activate the human or rat urotensin II receptors GPR14, UT receptor (UTR) and showed a hypotensive effect when administrated to anesthetized rats. The prepro-URP gene is expressed in several rat tissues, although with lower levels than the prepro-UII gene and, in the human, is expressed comparably to prepro-UII in several tissues except the spinal cord. These results suggest that URP is the endogenous and functional ligand for urotensin II receptor in the rat and mouse, and possibly in the human.
Urotensin II (UII) was first isolated from an extract of the urophysis of goby, Gillichthys mirabilis, by monitoring the contraction activity against the trout hind gut, and has been reported to be involved in several important regulatory functions, such as cardiovascular regulation, osmoregulation for seawater adaptation, and the regulation of lipid metabolism in fish. UII peptides from other teleost fish or chondrostean and elasmobranch fish have also been isolated and their structures determined.
In the carp, frog, pig, and human, the mature form of UII peptides can be easily predicted from their precursor proteins since their prepro-forms possess the typical processing sites (Arg–Lys–Arg or Lys–Lys–Arg) for processing enzymes which precede the mature UII peptides located at the carboxy-termini. In fact, the structures of the UII peptides of the carp, frog, and pig, as predicted from their precursor proteins, were coincident with those of the UIIs isolated from tissue extracts.
Based on the amino acid sequence of URP, we searched the databases for candidate genes encoding the precursor proteins of URP, and we used this information to clone the cDNAs of the human, rat, and mouse prepro-URP genes. The human, rat, and mouse precursor proteins of URP consist of 119, 118, and 113 amino acids, respectively. They show adequate amino acid sequence homology as follows: 54.2% for human and rat, 47.5% for human and mouse, and 73.7% for rat and mouse. The mature URP peptides are located at the carboxy-termini of the precursor proteins and are flanked by the typical processing sites, Arg–Lys–Arg.
URP has a high potency for intracellular Ca 2+ mobilization activity (EC 50 values; 4.8 nM (human) and 0.55 nM (rat)) and high affinity binding (Kd values; 170 pM (human) and 91 pM (rat)) to CHO cells expressing the human or rat receptor for UII (GPR14 (SENR) or UTR), and the effective values are comparable to those for human UII. Intravenously administrated URP in the anesthetized rat causes a long-lasting hypotensive effect, which is similar to that of the tentative 14-residue rat UII.
A tissue distribution analysis using TaqMan PCR showed that the prepro-URP gene is expressed in several peripheral or central tissues in the rat, such as those from the thymus, spleen, testis, and spinal cord, but that the expression level is relatively lower when compared with the tentative rat prepro-UII gene, even in the brain. However, any immunoreactivity other than URP could not be detected in the rat brain extract, indicating that URP, but not the potential peptides processed from prepro-UII, is the endogenous ligand for UTR in the rat brain.
URP was discovered in the rat brain as a UII-immunoreactive substance and is considered to be the functional ligand for UTR in rat tissues, and possibly in mouse tissues. These results indicate that the pathophysiological significance of the UII-like molecule in the rat or mouse models of human diseases must be interpreted by the behavior of this novel peptide or its precursor protein gene. The gene encoding prepro-URP is also expressed in human tissues, which suggests that several pathophysiological functions of UII in humans may be mediated by URP.
