Effects of teleost urotensins on intestinal absorption of water and Nacl in tilapia, sarotherodon mossambicus, adapted to fresh water or seawater

[The caudal neurosecretory system, the other "neurohypophysial" system in fish]

The caudal neurosecretory system (CNSS) is a neuroendocrine complex whose existence is specific to fishes. Structurally, it has many similarities with the hypothalamic-neurohypophyseal complex of other vertebrates. However, it differs regarding its position at the caudal end of the spinal cord and the nature of the hormones it secretes, the most important being urotensins. The CNSS was first described more than 60 years ago, but its embryological origin is totally unknown and its role is still poorly understood. Paradoxically, it is almost no longer studied today. Recent developments in imaging and genome editing could make it possible to resume investigations on CNSS in order to solve the mysteries that still surround it.

International Union of Basic and Clinical Pharmacology. XCII. Urotensin II, urotensin II-related peptide, and their receptor: from structure to function

Urotensin II (UII) is a cyclic neuropeptide that was first isolated from the urophysis of teleost fish on the basis of its ability to contract the hindgut. Subsequently, UII was characterized in tetrapods including humans. Phylogenetic studies and synteny analysis indicate that UII and its paralogous peptide urotensin II-related peptide (URP) belong to the somatostatin/cortistatin superfamily. In mammals, the UII and URP genes are primarily expressed in cholinergic neurons of the brainstem and spinal cord. UII and URP mRNAs are also present in various organs notably in the cardiovascular, renal, and endocrine systems. UII and URP activate a common G protein-coupled receptor, called UT, that exhibits relatively high sequence identity with somatostatin, opioid, and galanin receptors. The UT gene is widely expressed in the central nervous system (CNS) and in peripheral tissues including the retina, heart, vascular bed, lung, kidney, adrenal medulla, and skeletal muscle. Structure-activity relationship studies and NMR conformational analysis have led to the rational design of a number of peptidic and nonpeptidic UT agonists and antagonists. Consistent with the wide distribution of UT, UII has now been shown to exert a large array of biologic activities, in particular in the CNS, the cardiovascular system, and the kidney. Here, we review the current knowledge concerning the pleiotropic actions of UII and discusses the possible use of antagonists for future therapeutic applications.

Urotensin II, from fish to human

The cyclic peptide urotensin II (UII) was originally isolated from the urophysis of teleost fish on the basis of its ability to contract intestinal smooth muscle. The UII peptide has subsequently been isolated from frog brain and, later on, the pre-proUII cDNA has been characterized in mammals, including humans. A UII paralog called urotensin II-related peptide (URP) has been identified in the rat brain. The UII and URP genes originate from the same ancestral gene as the somatostatin and cortistatin genes. In the central nervous system (CNS) of tetrapods, UII is expressed primarily in motoneurons of the brainstem and spinal cord. The biological actions of UII and URP are mediated through a G protein-coupled receptor, termed UT, that exhibits high sequence similarity with the somatostatin receptors. The UT gene is widely expressed in the CNS and in peripheral organs. Consistent with the broad distribution of UT, UII and URP exert a large array of behavioral effects and regulate endocrine, cardiovascular, renal, and immune functions.

Urotensin II and renal function in the rat

Urotensin II (UII) is a potent vasoactive hormone in mammals. However, despite its well-known effects on epithelial sodium transport in fish, little is known about its actions on the mammalian kidney. The aim of this study was to determine the effects of UII on renal function in the rat. Using standard clearance methods, the effects of rUII and the rat UII receptor (UT) antagonist, urantide, were studied. UII was measured in plasma and urine by radioimmunoassay. UII and UT were localized in the kidney by immunohistochemistry and mRNA expression quantified. Rat urinary [UII] was 1,650-fold higher than that in plasma. Immunoreactive-UII was localized to the proximal tubules, outer and inner medullary collecting ducts (IMCD); UT receptor was identified in glomerular arterioles, thin ascending limbs, and IMCD. UII and UT mRNA expression was greater in the medulla; expression was higher still in spontaneously hypertensive rats (SHRs) associated with raised plasma (UII). Injection of rUII induced reductions in glomerular filtration rate (GFR), urine flow, and sodium excretion. Urantide infusion resulted in increases in these variables. Endogenous UII appears to contribute to the regulation of GFR and renal sodium and water handling in the rat. While hemodynamic changes predominate, we cannot rule out the possibility of a direct tubular action of UII. Increased expression of UII and UT in the SHR suggests that UII plays a role in the pathophysiology of cardiovascular disease.

Urotensin II: Ancient Hormone with New Functions in Vertebrate Body Fluid Regulation

Urotensin II (UII), described in many fish species, is secreted by the caudal neurosecretory system, a unique fish neuroendocrine structure. We have examined UII secretion and its control in euryhaline fish, supporting a proposed role in osmoregulation. However, it is now apparent that UII is present in other vertebrates, including mammals. The 12-amino-acid peptide has been highly conserved and the key cyclic region is common from fish to humans. Our UII radioimmunoassay for flounder, directed to this cyclic region, has shown circulating UII levels in humans and rats comparable with those in fish. In mammals, UII cardiovascular effects vary between species, with vasoconstriction only evident in specific vascular beds. The kidney expresses UII receptors and responds to UII administration by a reduction in glomerular filtration rate, urine flow, and excretion of the major ions. Interestingly, plasma levels of UII are chronically elevated in rat models of hypertension. These observations imply an unforeseen role for this ancient fish hormone in the physiological and perhaps pathophysiological regulation of body fluids in higher vertebrates, including humans.

Ancient evolution of stress-regulating peptides in vertebrates

Recent studies on genomic sequences have led to the discovery of novel corticotropin-releasing factor (CRF) type 2 receptor-selective agonists, stresscopin (SCP)/urocortin III (UcnIII), and stresscopin-related peptide (SRP)/urocortin II (UcnII). In addition, analyses of vertebrate genomes showed that the CRF peptide family includes four distinct genes, CRF, urocortin/urotensin I, SCP/UcnIII, and SRP/UcnII. Each of these four genes is highly conserved during evolution and the identity between mammalian and teleost orthologs ranges from >96% for CRF to >55% for SCP. Phylogenetic studies showed that the origin of each of these peptides predates the evolution of tetrapods and teleosts, and that this family of peptide hormones evolved from an ancestor gene that developed the CRF/urocortin and SCP/SRP branches through an early gene duplication event. These two ancestral branches then gave rise to additional paralogs through a second round of gene duplication. Consequently, each of these peptides participates in the regulation of stress responses over the 550 million years of vertebrate evolution. The study also suggested that the fight-or-flight and stress-coping responses mediated mainly by CRF types 1 and 2 receptors evolved early in chordate evolution. In addition, we hypothesize that the CRF/CRF receptor signaling evolved from the same ancestors that also gave rise to the diuretic hormone/diuretic hormone receptors in insects. Thus, a complete inventory of CRF family ligands and their receptors in the genomes of different organisms provides an opportunity to reveal an integrated view of the physiology and pathophysiology of the CRF/SCP family peptides, and offers new insights into the evolution of stress regulation in vertebrates.

Singular contributions of fish neuroendocrinology to mammalian regulatory peptide research

During the past 20 years, several bioactive peptides have been identified in teleost fishes that subsequently have been shown to play important regulatory roles in mammalian physiology. The urophysis, corpuscles of Stannius and Brockmann body are anatomical structures particular to fish that have no obvious counterpart in mammals. Extracts and/or cDNA libraries prepared from these tissues have been used to identify for the first time urotensin II (U-II), urotensin-I (U-I), stanniocalcin and glucagon-like peptide-1 (GLP-1). Although U-II and U-I were originally regarded as exclusively the products of the teleost urophysis, the peptides have a wide phylogenetic distribution across the vertebrate lineage, including mammals. U-II is localized to motor neurones in the human spinal cord and is a potent vasoconstrictor that may be implicated in the pathogenesis of heart failure. The human ortholog of urotensin-I is urocortin which is synthesized in selected regions of the brain and is the endogenous ligand for the CRF type 2 receptor. Urocortin is believed to important in mediating the effects of stress on appetite. Stanniocalcin is involved in maintaining calcium and phosphate homeostasis in teleost fish. An ortholog of stanniocalcin has a widespread distribution in mammalian tissues and is postulated to regulate renal phosphate excretion and to protect neurons against damage during cerebral ischemia. The biological actions and therapeutic potential of GLP-1 in humans are now fully appreciated but the peptide was first identified as a domain in a preproglucagon cDNA prepared from anglerfish Brockmann bodies. In contrast to mammalian preproglucagons, GLP-1 is present in anglerfish preproglucagon as the bioactive, truncated sequence [corresponding to human GLP-1(7-37)] rather than the inactive, N-terminally extended form [corresponding to GLP-1(1-37)]. Failure to appreciate the significance of this fact retarded progress in the field for several years.

Evolution and physiology of the corticotropin-releasing factor (CRF) family of neuropeptides in vertebrates

Corticotropin-releasing factor (CRF), urotensin-I, urocortin and sauvagine belong to a family of related neuropeptides found throughout chordate taxa and likely stem from an ancestral peptide precursor early in metazoan ancestry. In vertebrates, current evidence suggests that CRF on one hand, and urotensin-I, urocortin and sauvagine, on the other, form paralogous lineages. Urocortin and sauvagine appear to represent tetrapod orthologues of fish urotensin-I. Sauvagine's unique structure may reflect the distinctly derived evolutionary history of the anura and the amphibia in general. The physiological actions of these peptides are mediated by at least two receptor subtypes and a soluble binding protein. Although the earliest functions of these peptides may have been associated with osmoregulation and diuresis, a constellation of physiological effects associated with stress and anxiety, vasoregulation, thermoregulation, growth and metabolism, metamorphosis and reproduction have been identified in various vertebrate species. The elaboration of neural circuitry for each of the two paralogous neuropeptide systems appears to have followed distinct pathways in the actinopterygian and sarcopterygian lineages of vertebrates. A comparision of the functional differences between these two lineages predicts additional functions of these peptides.

Adrenergic receptor activation hyperpolarizes the caudal neurosecretory cells of the flounder, Platichthys flesus

The physiological factors that govern activity of the caudal neurosecretory system in teleost fish are poorly understood. Immunocytochemical evidence indicates that the neurosecretory Dahlgren cells are innervated by descending monoaminergic fibres. Using intracellular recording techniques in an isolated preparation of the posterior spinal cord of the flounder (Platichthys flesus) we have demonstrated that superfusion of adrenaline or noradrenaline (10(-7) - 10(-3) M) causes hyperpolarization of Dahlgren cells (up to -30 mV). This hyperpolarization is likely to reflect an inhibitory effect of noradrenergic nerves on the neurosecretory system in vivo, reducing the rate of hormone release. Fluctuations in the input resistance and membrane time constant suggest involvement of a multiplicity of cellular mechanisms, including the opening and closing of populations of ion-selective channels. Superfusion with dopamine (10(-7) - 10(-3) M) had no effect. Superfusion with the beta-adrenoreceptor agonist, isoprenaline, caused hyperpolarization but to a markedly lesser extent than the maximum effect of adrenaline or noradrenaline, suggesting that their effects are mediated, only in part, by a beta-adrenoreceptor subtype. Superfusion of the preparation with a membrane permeable, non-hydrolysable cyclic AMP analogue (8-[4-chlorophenylthio]-cAMP) resulted in a slight hyperpolarization which was accompanied by a small, but significant, increase in input resistance. These data are consistent with at least part of the beta-adrenoreceptor mediated effect involving closure of cAMP-sensitive ion channels. Superfusion with the alpha 1-adrenoreceptor agonist, phenylephrine, had no effect on any electrophysiological parameter studied. However, the alpha 2-adrenoreceptor agonist, clonidine, caused hyperpolarization which again failed to reach the maximum level produced by adrenaline or noradrenaline. Together, these data suggest that the adrenergic inhibition of Dahlgren cell activity is mediated by both alpha 2- and beta-adrenoreceptor subtypes.

Urotensin II in the central nervous system of the frog Rana ridibunda: immunohistochemical localization and biochemical characterization

Urotensin II (UII) is traditionally regarded as a product of the neurosecretory cells in the caudal portion of the spinal cord of jawed fishes. A peptide related to UII has been recently isolated from the frog brain, thereby providing the first evidence that UII is also present in the central nervous system of a tetrapod. In the present study, we have investigated the distribution of UII-immunoreactive elements in the brain and spinal cord of the frog Rana ridibunda by immunofluorescence using an antiserum directed against the conserved cyclic region of the peptide. Two distinct populations of UII-immunoreactive perikarya were visualized. The first group of positive neurons was found in the nucleus hypoglossus of the medulla oblongata, which controls two striated muscles of the tongue. The second population of immunoreactive cell bodies was represented by a subset of motoneurons that were particularly abundant in the caudal region of the cord (34% of the motoneuron population). The telencephalon, diencephalon, mesencephalon, and metencephalon were totally devoid of UII-containing cell bodies but displayed dense networks of UII-immunoreactive fibers, notably in the thalamus, the tectum, the tegmentum, and the granular layer of the cerebellum. In addition, a dense bundle of long varicose processes projecting rostrocaudally was observed coursing along the ventral surface of the brain from the midtelencephalon to the medulla oblongata. Reversed-phase high-performance liquid chromatography analysis of frog brain, medulla oblongata, and spinal cord extracts revealed that, in all three regions, UII-immunoreactive material eluted as a single peak which exhibited the same retention time as synthetic frog UII. Taken together, these data indicate that UII, in addition to its neuroendocrine functions in fish, is a potential regulatory peptide in the central nervous system of amphibians.

Recognition by goby intestine of a somatostatin analog, SMS 201-995

The caudal neurosecretory peptide urotensin II and its partial structural analog the hypothalamic and gastroenteropancreatic peptide somatostatin can be distinguished by the goby posterior intestine; urotensin II stimulates Na+ and Cl- absorption whereas somatostatin is without effect. Sandostatin (Sandoz compound SMS 201-995) is a partial structural analog of somatostatin, possessing the active core common to both urotensin II and somatostatin but lacking the C- and N-terminal tails which apparently confer recognition in species specificity. SMS 201-995 mimicked the effects of urotensin II, namely, increased net Cl- absorption and decreased radiochloride backflux consistent with the observed increase in transepithelial resistance and the cellular depolarization typical of transport stimulation in this tissue. Speculation on the role of the tails in species-specific recognition is supported by the nature of amino acid residue substitutions in urotensin II, somatostatin, and SMS 201-995 and suggests the existence of multiple receptor types for this group of peptides.

Specificity of response of intestinal ion transport systems to a pair of natural peptide hormone analogs: somatostatin and urotensin II

Specificity of two intestinal ion transport systems toward the natural peptide hormone analogs somatostatin and urotensin II has been demonstrated by electrophysiological and radiotracer studies in vitro. Somatostatin inhibits active C1 secretion across the theophylline-treated rat colon but urotensin II, a dodecapeptide somatostatin analog from the teleost caudal neurosecretory system, is without effect. Conversely, urotensin II stimulates active Na and C1 absorption across the posterior intestine of the 5% seawater-adapted goby, Gillichthys mirabilis, but somatostatin is ineffective. From these and others' studies, it appears that in both systems, increased net absorption results from increased mucosal-to-serosal unidirectional ion fluxes. Based on structure-activity relationships in these and other systems, it is speculated that the difference in amino acid residues at position 4 (somatostatin-Lys, urotensin II-Ala) may contribute to the observed specificity.