Four functionally distinct C-type natriuretic peptides found in fish reveal evolutionary history of the natriuretic peptide system

Comparative physiology of the piscine natriuretic peptide system

The natriuretic peptide (NP) family is a seemingly ubiquitous sodium and volume reducing endocrine system of predominantly cardiac origin. Members of the NP system include ANP, BNP, CNP, VNP, their guanylate cyclase (GC)-linked receptors (NPR-A and NPR-B), and clearance receptor (NPR-C). Through the activation of their membrane-bound GC receptors, these small peptides modulate cellular functions that affect both salt and water balance. The elucidation of piscine NP sequences, structure, and functions has steadily advanced over the past 15 years spearheaded by research from Dr. Yoshio Takei's laboratory. The development of these homologous NPs has led to extensive research into both the evolutionary and physiological significance of NPs in fishes. One outcome has been the development of two seemingly disparate hypotheses of NP function; a role in salt excretion, the osmoregulatory hypothesis, versus a role in protecting the heart, the cardioprotective hypotheses. In the osmoregulatory hypothesis NPs are released in response to elevated ambient salinity and inhibit drinking and intestinal uptake of salt, thereby effectively reducing plasma sodium levels. In contrast, the cardioprotective theory depicts NPs acting to prevent debilitating cardiodilation from an excess of either venous or arterial pressure through vasodilation and a reduction of blood volume. These seemingly distinct hypotheses may be elements of a more general regulatory system and certainly require further investigation. Undoubtedly their resolution will not only give us a better perspective of the evolutionary basis of the NP system but will provide us with a greater appreciation of salt and water homeostasis in vertebrates.

Exploring novel hormones essential for seawater adaptation in teleost fish

Marine fish are dehydrated in hyperosmotic seawater (SW), but maintain water balance by drinking surrounding SW if they are capable of excreting the excess ions, particularly Na(+) and Cl(-), absorbed with water by the intestine. An integrative approach is essential for understanding the mechanisms for SW adaptation, in which hormones play pivotal roles. Comparative genomic analyses have shown that hormones that have Na(+)-extruding and vasodepressor properties are greatly diversified in teleost fish. Physiological studies at molecular to organismal levels have revealed that these diversified hormones are much more potent and efficacious in teleost fish than in mammals and are important for survival in SW and for maintenance of low arterial pressure in a gravity-free aquatic environment. This is typified by the natriuretic peptide (NP) family, which is diversified into seven members (ANP, BNP, VNP and CNP1, 2, 3 and 4) and exerts potent hyponatremic and vasodepressor actions in marine fish. Another example is the guanylin family, which consists of three paralogs (guanylin, uroguanylin and renoguanylin), and stimulates Cl(-) secretion into the intestinal lumen and activates the absorptive-type Na-K-2Cl cotransporter by local luminocrine actions. The most recent addition is the adrenomedullin (AM) family, which has five members (AM1, 2, 3, 4 and 5), with AM2 and AM5 showing the most potent or efficacious vasodepressor and osmoregulatory effects among known hormones in teleost fish. Accumulating evidence strongly indicates that members of these diversified hormone families play essential roles in SW adaptation in teleost fish. In this short review, the author has attempted to propose a novel approach for identification of new hormones that are important for SW adaptation using comparative genomic and functional studies. The author has also suggested potential hormone families that are diversified in teleost fish and appear to be involved in SW adaptation through their ion-extruding actions.

Potent cardiovascular actions of homologous adrenomedullins in eels

Adrenomedullin (AM), known as a multifunctional hormone in mammals, forms a unique family of five paralogous peptides in teleost fish. To examine their cardiovascular effects using homologous AMs in eels, we isolated cDNAs encoding four eel AMs, and named AM1 (ortholog of mammalian AM), AM2, AM3 (paralog of AM2 generated only in teleost lineage), and AM5 according to the known teleost AM sequences. Unlike pufferfish, not only AM1 but AM2/3 and AM5 were expressed ubiquitously in various eel tissues. Synthetic mature AM1, AM2, and AM5 exhibited vasodepressor effects after intra-arterial injections, and the effects were more potent at dorsal aorta than at ventral aorta. This indicates that AMs preferentially act on peripheral resistance vessels rather than on branchial arterioles. The potency was in the order of AM2 = AM5 ≫ AM1 in both freshwater (FW) and seawater (SW) eels, which is different from the result of mammals in which AM1 is as potent as, or more potent than, AM2 when injected peripherally. The minimum effective dose of AM2 and AM5 in eels was 1/10 that of AM1 in mammals. The hypotension reached 50% at 1.0 nmol/kg of AM2 and AM5, which is much greater than atrial natriuretic peptide (20%), another potent vasodepressor hormone. Even with such hypotension, AMs did not change heart rate in eels. In addition, AM1 increased blood pressure at ventral aorta and dorsal aorta immediately after an initial hypotension at 5.0 nmol/kg, but not with AM2 and AM5. These data strongly suggest that specific receptors for AM2 and AM5 exist in eels, which differ from the AM1 receptors identified in mammals.

Expression of natriuretic peptide receptor mRNA and functional response to atrial natriuretic peptide and C-type natriuretic peptide in rainbow trout (Oncorhynchus mykiss) head kidney leucocytes

The stimulatory effect of vasomodulatory natriuretic peptide hormones on macrophages and peripheral blood leucocytes in mammals is well-established. However, the relationship in lower vertebrates has not been characterised. Expression of atrial natriuretic peptide, ventricular natriuretic peptide and C-type natriuretic peptide-1, and the guanylyl cyclase-linked (GC) natriuretic peptide receptor-A and -B-type receptors (NPR-A and NPR-B, respectively) was determined by PCR from the mRNA of rainbow trout head kidney leucocytes yielding gene fragments with 100% homology to the same respective natriuretic peptide and NPR-A and -B sequences obtained from other rainbow trout tissues. A mixed population of isolated rainbow trout head kidney leucocytes was stimulated in vitro with trout atrial natriuretic peptide (specific NPR-A agonist) and trout C-type natriuretic peptide (NPR-A and -B agonist) as well as the cGMP agonist 8-bromo-cGMP or the GC inhibitor 8-bromo-phenyl-eutheno-cGMP. Respiratory burst was stimulated by trout atrial natriuretic peptide, trout C-type natriuretic peptide-1 and 8-bromo-cGMP in a dose dependant manner with the highest activity as a result of stimulation with trout C-type natriuretic peptide-1 in excess of that achieved by phorbol myristate acetate (PMA). Equimolar concentrations of the inhibitor, inhibited the respiratory burst caused by the natriuretic peptides and 8-bromo-cGMP. The natriuretic peptide receptors on rainbow trout head kidney leucocytes appear to have a stimulatory function with regard to respiratory burst that is activated through a cGMP second messenger pathway and the natriuretic peptides expressed in the head kidney leucocytes may well act in a paracrine/autocrine manner.

Biology of the natriuretic peptides

The biology of the natriuretic peptide (NP) system is complex, yet highly phylogenetically preserved. It regulates salt and water handling, promotes vasodilatation, and exerts favorable effects on the heart in the context of processes such as heart failure. Prior assumptions about the production of B-type NP (BNP) and its amino-terminal precursor fragment (NT-proBNP) have recently been refuted. It is now recognized that rather than a 1:1 secretion of these 2 NPs, a mixture of cleaved and uncleaved NPs is released by the cardiomyocyte. It is also recognized that BNP is rapidly modified into a mixture of various fragments. Commercial assays for the detection of BNP and NT-proBNP measure a mixture of cleaved and uncleaved NPs as well as varying amounts of degraded BNP. BNP and NT-proBNP are cleared differentially: BNP is actively removed from the bloodstream and also has passive clearance mechanisms, including renal clearance; NT-proBNP is cleared more passively by organs with high rates of blood flow, including the kidney.

Global gene expression profiling of androgen disruption in Qurt strain medaka

Androgen disrupting chemicals (ADCs) are endocrine disrupting chemicals (EDCs) that mimic or antagonize the effect of physiological androgens. Microarray-based detection of altered gene expression can be used as a biomarker of EDC exposure. Therefore, the purpose of this study was to identify and compare gene expression profiles of the androgen 11-ketotestosterone (11-KT), the antiandrogen flutamide (FLU), and the antiandrogenic fungicide vinclozolin (VIN), on Qurt medaka (Oryzias latipes). Biologically effective concentrations for 11-KT (100 microg/L), VIN (100 microg/L), and FLU (1000 microg/L) determined in range-finding studies were used for exposures. The oligonucleotide microarray included 9379 probes for EDC-affected genes, medaka cDNAs, sequences from the medaka genome project, and the UniGene database. We found that treatment with FLU, VIN, and 11-KT caused significant (false discovery rate = 0.01) differential expression of at least 87, 82, and 578 genes, respectively. Two sets of responsive genes are associated to vertebrate sex differentiation and growth, and 50 genes were useful in discriminating between ADC classes. The discriminating capacity was confirmed by a remarkable similarity of the antiandrogenic expression profiles of VIN and FLU, which were distinct from the androgenic profile of 11-KT. Gene expression profiles characterized in this study allow for reliable screening of ADC activity.

Genomic analyses and cloning of novel chicken natriuretic peptide genes reveal new insights into natriuretic peptide evolution

The natriuretic peptide (NP) family consists of multiple subtypes in teleosts, including atrial, B-type, ventricular, and C-type NPs (ANP, BNP, VNP, CNP-1-4, respectively), but only ANP, BNP, CNP-3, and CNP-4 have been identified in tetrapods. As part of understanding the molecular evolution of NPs in the tetrapod lineage, we identified NP genes in the chicken genome. Previously, only BNP and CNP-3 have been identified in birds, but we characterized two new chicken NP genes by cDNA cloning, synteny and phylogenetic analyses. One gene is an orthologue of CNP-1, which has only ever been reported in teleostei and bichir. The second gene could not be assigned to a particular NP subtype because of high sequence divergence and was named renal NP (RNP) due to its predominant expression in the kidney. CNP-1 mRNA was only detected in brain, while CNP-3 mRNA was expressed in kidney, heart, and brain. In the developing embryo, BNP and RNP transcripts were most abundant 24h post-fertilization, while CNP mRNA increased in a stage-dependent manner. Synthetic chicken RNP stimulated an increase in cGMP production above basal level in chicken kidney membrane preparations and caused a potent dose-dependent vasodilation of pre-constricted dorsal aortic rings. From conserved chromosomal synteny, we propose that the CNP-4 and ANP genes have been lost in chicken, and that RNP may have evolved from a VNP-like gene. Furthermore, we have demonstrated for the first time that CNP-1 is retained in the tetrapod lineage.

Natriuretic peptides affect Pseudomonas aeruginosa and specifically modify lipopolysaccharide biosynthesis

Natriuretic peptides of various forms are present in animals and plants, and display structural similarities to cyclic antibacterial peptides. Pretreatment of Pseudomonas aeruginosa PAO1 with brain natriuretic peptide (BNP) or C-type natriuretic peptide (CNP) increases bacterium-induced glial cell necrosis. In eukaryotes, natriuretic peptides act through receptors coupled to cyclases. We observed that stable analogs of cAMP (dibutyryl cAMP) and cGMP (8-bromo-cGMP) mimicked the effect of brain natriuretic peptide and CNP on bacteria. Further evidence for the involvement of bacterial cyclases in the regulation of P. aeruginosa PAO1 cytotoxicity by natriuretic peptides is provided by the observed doubling of intrabacterial cAMP concentration after exposure to CNP. Lipopolysaccharide (LPS) extracted from P. aeruginosa PAO1 treated with both dibutyryl cAMP and 8-bromo-cGMP induces higher levels of necrosis than LPS extracted from untreated bacteria. Capillary electrophoresis and MALDI-TOF MS analysis have shown that differences in LPS toxicity are due to specific differences in the structure of the macromolecule. Using a strain deleted in the vfr gene, we showed that the Vfr protein is essential for the effect of natriuretic peptides on P. aeruginosa PAO1 virulence. These data support the hypothesis that P. aeruginosa has a cyclic nucleotide-dependent natriuretic peptide sensor system that may affect virulence by activating the expression of Vfr and LPS biosynthesis.

A 'reverse' phylogenetic approach for identification of novel osmoregulatory and cardiovascular hormones in vertebrates

Vertebrates expanded their habitats from aquatic to terrestrial environments during the course of evolution. In parallel, osmoregulatory and cardiovascular systems evolved to counter the problems of desiccation and gravity on land. In our physiological studies on body fluid and blood pressure regulation in various vertebrate species, we found that osmoregulatory and cardiovascular hormones have changed their structure and function during the transition from aquatic to terrestrial life. In fact, Na(+)-regulating and vasodepressor hormones play essential roles in fishes, while water-regulating and vasopressor hormones are dominant in tetrapods. Accordingly, Na(+)-regulating and vasodepressor hormones, such as natriuretic peptide (NP) and adrenomedullin (AM), are much diversified in teleost fishes compared with mammals. Based on this finding, new NPs and AMs were identified in mammals and other tetrapods. These hormones have only minor roles in the maintenance of normal blood volume and pressure in mammals, but their importance seems to increase when homeostasis is disrupted. Therefore, such hormones can be used for diagnosis and treatment of body fluid and cardiovascular disorders such as cardiac/renal failure and hypertension. In this review, we introduce a new approach for identification of novel Na(+)-regulating and vasodepressor hormones in mammals based on fish studies. Until recently, new hormones were first discovered in mammals, and then identified and applied in fishes. However, chances are increasing in recent years to identify new hormones first in fishes then in mammals, based on the difference in the regulatory systems between fishes and tetrapods. As the direction is opposite from the traditional phylogenetic approach, we added 'reverse' to its name. The 'reverse' phylogenetic approach offers a typical example of how comparative fish studies can contribute to the general and clinical endocrinology.

Natriuretic peptide receptor B signaling in the cardiovascular system: protection from cardiac hypertrophy

Natriuretic peptides (NP) represent a family of structurally homologous but genetically distinct peptide hormones involved in regulation of fluid and electrolyte balance, blood pressure, fat metabolism, cell proliferation, and long bone growth. Recent work suggests a role for natriuretic peptide receptor B (NPR-B) signaling in regulation of cardiac growth by either a direct effect on cardiomyocytes or by modulation of other signaling pathways including the autonomic nervous system. The research links NPR-B for the first time to a cardiac phenotype in vivo and underlines the importance of the NP in the cardiovascular system. This manuscript will focus on the role of NPR-B and its ligand C-type natriuretic peptide in cardiovascular physiology and disease and will evaluate these new findings in the context of the known function of this receptor, with a perspective on how future research might further elucidate NPR-B function.

In vitro effects of homologous natriuretic peptides on growth hormone and prolactin release in the tilapia, Oreochromis mossambicus

C-type natriuretic peptide (CNP) cDNA was cloned from the tilapia brain and its inferred mature sequence was chemically synthesized together with previously cloned tilapia A-type and B-type natriuretic peptides (ANP and BNP). The cloned CNP belongs to the CNP-1 type of teleosts. Reverse-transcription polymerase chain reaction showed that the ANP and BNP genes were hardly expressed in the tilapia brain and pituitary, whereas the CNP gene was expressed strongly in the brain and slightly in the pituitary. Effects of homologous natriuretic peptides (100 nM each) on growth hormone (GH) and prolactin (PRL) release were examined using dispersed tilapia pituitary cells. Tilapia ANP and BNP stimulated GH and PRL release during 4-8, and 8-24 h of incubation. BNP appeared to be more potent than ANP, also stimulating GH and PRL release during 0-4 h of incubation. CNP stimulated GH release only during 4-8 h of incubation; CNP was without effect on PRL release. All three NPs stimulated GH and PRL mRNA expression in dispersed pituitary cells following 24 h of incubation. ANP and BNP significantly elevated intracellular cGMP accumulation in dispersed pituitary cells after 15 min of exposure, whereas no effect of CNP was observed. These results indicate a long-lasting stimulation of GH and PRL release by ANP and BNP that is mediated, at least in part, by the guanylyl cyclase-linked NP receptor.

Evolutionary history of the calcitonin gene-related peptide family in vertebrates revealed by comparative genomic analyses

The calcitonin gene-related peptide (CGRP) family is composed of CGRP, amylin and adrenomedullin (AM) in mammals. In teleost fish, AM forms an independent subfamily of five members (AM1-5), which inspired us to trace the evolutionary history of the CGRP family throughout vertebrates by comparative genomic approach. Linkage mapping and synteny analyses of the CGRP family genes in medaka, Oryzias latipes, revealed that AM1/CGRP, AM2/amylin, and AM5 genes were located on respective proto-chromosomes before the divergence of teleost lineage. In teleost fish, additional whole genome duplication generated AM1/4, CGRP1/2, AM2/3, but one of the duplicated amylin and AM5 genes was silenced. In mammals, the amylin or AM2 gene was translocated to different chromosomes, while the CGRP gene was multiplied in tandem to generate CGRP-alpha,beta, and recently identified calcitonin receptor-stimulating peptide genes. Based on these data, we identified a novel AM5 gene in several mammalian species as we previously did for AM2.

Identification and expression of natriuretic peptide receptor type-A and -B mRNA in freshwater and seawater rainbow trout

Natriuretic peptide receptors mediate the physiological response of natriuretic peptide hormones. One of the natriuretic peptide receptor types is the particulate guanylyl cyclase receptors, of which there are two identified: NPR-A and NPR-B. In fishes, these have been sequenced and characterized in eels, medaka, and dogfish shark (NPR-B only). The euryhaline rainbow trout provides an opportunity to further pursue examination of the system in teleosts. In this study, partial rainbow trout NPR-A-like and NPR-B-like mRNA sequences were identified via PCR and cloning. The sequence information was used in real-time PCR to examine mRNA expression in a variety of tissues of freshwater rainbow trout and rainbow trout acclimated to 35 parts per thousand seawater for a period of 10 days. In the excretory kidney and posterior intestine, real-time PCR analysis showed greater expression of NPR-B in freshwater fish than in those adapted to seawater; otherwise, there was no difference in the expression of the individual receptors in fresh water or seawater. In general, the expression of the NPR-A and NPR-B type receptors was quite low. These findings indicate that NPR-A and NPR-B mRNA expression is minimally altered under the experimental regime used in this study.

Contribution of comparative fish studies to general endocrinology: structure and function of some osmoregulatory hormones

Fish endocrinologists are commonly motivated to pursue their research driven by their own interests in these aquatic animals. However, the data obtained in fish studies not only satisfy their own interests but often contribute more generally to the studies of other vertebrates, including mammals. The life of fishes is characterized by the aquatic habitat, which demands many physiological adjustments distinct from the terrestrial life. Among them, body fluid regulation is of particular importance as the body fluids are exposed to media of varying salinities only across the thin respiratory epithelia of the gills. Endocrine systems play pivotal roles in the homeostatic control of body fluid balance. Judging from the habitat-dependent control mechanisms, some osmoregulatory hormones of fish should have undergone functional and molecular evolution during the ecological transition to the terrestrial life. In fact, water-regulating hormones such as vasopressin are essential for survival on the land, whereas ion-regulating hormones such as natriuretic peptides, guanylins and adrenomedullins are diversified and exhibit more critical functions in aquatic species. In this short review, we introduce some examples illustrating how comparative fish studies contribute to general endocrinology by taking advantage of such differences between fishes and tetrapods. In a functional context, fish studies often afford a deeper understanding of the essential actions of a hormone across vertebrate taxa. Using the natriuretic peptide family as an example, we suggest that more functional studies on fishes will bring similar rewards of understanding. At the molecular level, recent establishment of genome databases in fishes and mammals brings clues to the evolutionary history of hormone molecules via a comparative genomic approach. Because of the functional and molecular diversification of ion-regulating hormones in fishes, this approach sometimes leads to the discovery of new hormones in tetrapods as exemplified by adrenomedullin 2.

Extremely high conservation in the untranslated region as well as the coding region of CNP mRNAs throughout elasmobranch species

C-type natriuretic peptide (CNP) is a crucial osmoregulatory hormone in elasmobranchs, participating in salt secretion and drinking. In contrast to teleosts and tetrapods in which the NP family is composed of a group of structurally related peptides, we have shown that CNP is the sole NP in sharks. In the present study, CNP cDNAs were cloned from four species of batoids, another group of elasmobranchs. The cloned batoid CNP precursors contained a plausible mature peptide of 22 amino acid residues that is identical to most shark CNP-22s, but five successive amino acids were consistently deleted in the prosegment compared with shark precursors, supporting the diphyletic classification of sharks and rays. In addition, molecular phylogenetic trees of CNP precursors were consistent with a diphyletic interpretation. Except for the deletion, the nucleotide and deduced amino acid sequences of the CNP cDNAs are extremely well-conserved among all elasmobranch species, even between sharks and rays. Surprisingly, high conservation is evident not only for the coding region, but also for the untranslated regions. It is most likely that the high conservation is due to the low nucleotide substitution rate in the elasmobranch genome, and high selection pressure. The 3'-untranslated region of the elasmobranch CNP cDNAs contained three to six repeats of the ATTTA motif that is associated with the regulation of mRNA stability and translation efficiency. Alternative polyadenylation sites were also found; the long 3'-untranslated region contains a core of ATTTA motifs while the short form has only one or no ATTTA motif, indicating that the post-transcriptional modification of mRNA is important for regulation of CNP synthesis. These characteristics in the 3'-untranslated region were conserved among all elasmobranch CNP cDNAs. Since CNP has been implicated as a fast-acting hormone to facilitate salt secretion from the rectal gland, the conserved 3'-untranslated region most likely contributes to rapid regulation of CNP synthesis in elasmobranchs in response to acute changes in internal and external environments.

Identification of a natriuretic peptide (NP) in cyclostomes (lamprey and hagfish): CNP-4 is the ancestral gene of the NP family

In bony fishes, natriuretic peptides (NPs) comprise a hormone family that is composed of seven subtypes; ANP, BNP, VNP that have an intramolecular ring and N- and C-terminal extensions, and four CNPs (CNP-1 to -4) that lack the C-terminal extension. To assess the ancestral molecule of the NP family, we determined the NP sequences in several species of two extant cyclosotome groups, lampreys and hagfishes. A cDNA encoding CNP was cloned from the heart and brain of three phylogenetically distant species of lampreys, Geotria australis, Lampetra japonica, and Petromyzon marinus. In the deduced prohormone sequence of each species, two potential processing signals, lysine-lysine (KK) that is commonly present in CNP precursors, and arginine-X-X-arginine (RXXR) for furin-like proprotein convertase (PC) that is typical for CNP-4 were present. The deduced mature peptides that are released at each signal were highly conserved among three species; 100% cleaved at KK and >92% processed at RXXR. In L. japonica, the CNP gene was expressed almost exclusively in the heart and brain. Meanwhile, a cDNA encoding NP with a C-terminal tail sequence was cloned from the heart and brain of three hagfish species in different genera, Myxine glutinosa, Eptatretus cirrhatus, and Paramyxine atami. The precursor sequences including the prosegment had >80% identity among the three hagfish species. A processing signal, RXXR, is also conserved in the prosegment of all hagfish NPs. The molecular phylogenetic analyses inferred that the lamprey CNP and hagfish NP belong to the CNP-4 group, even though the hagfish NP has a C-terminal sequence extended from the intramolecular ring. The presence of a processing signal, RXXR, in the prosegment of cyclostome NPs supports the above classification. Based on the current findings, we suggest that the ancestral gene of the NP family is CNP-4.

A perspective on the role of natriuretic peptides in amphibian osmoregulation

The natriuretic peptide (NP) system is a complex family of peptides and receptors that is primarily linked to the maintenance of osmotic and cardiovascular homeostasis. In amphibians, the potential role(s) of NPs is complicated by the range of osmoregulatory strategies found in amphibians, and the different tissues that participate in osmoregulation. Atrial NP, brain NP, and C-type NP have been isolated or cloned from a number of species, which has enabled physiological studies to be performed with homologous peptides. In addition, three types of NP receptors have been cloned and partially characterised. Natriuretic peptides are always potent vasodilators in amphibian blood vessels, and ANP has been shown to increase the permeability of the microcirculation. In the perfused kidney, ANP causes vasodilation, diuresis and natriuresis that are caused by an increased GFR rather than effects in the renal tubules. These data are supported by the presence of ANP receptors only on the glomeruli and renal blood vessels. In the bladder and skin, the function of NPs is enigmatic because physiological analysis of the effects of ANP on bladder and skin function has yielded conflicting data with no clear role for NPs being revealed. Overall, NPs often have no direct effect, but in some studies they have been shown to inhibit the function of AVT. In addition, there is evidence that ANP can inhibit salt retention in amphibians since it can inhibit the ability of adrenocorticotrophic hormone or angiotensin II to stimulate corticosteroid secretion. It is proposed that an important role for cardiac NPs could be in the control of hypervolaemia during periods of rapid rehydration, which occurs in terrestrial amphibians.

The effects of freshwater to seawater transfer on circulating levels of angiotensin II, C-type natriuretic peptide and arginine vasotocin in the euryhaline elasmobranch, Carcharhinus leucas

This study examined the effect of transfer to increased environmental salinity on the circulating levels of angiotensin II (ANG II), C-type natriuretic peptide (CNP), and arginine vasotocin (AVT) in the euryhaline elasmobranch, Carcharhinus leucas. Plasma levels of ANG II and CNP were significantly increased in C. leucas chronically acclimated to seawater (SW) in comparison to freshwater (FW) acclimated fish. There was no difference in plasma AVT levels. Acute transfer of FW fish to 75% SW induced an increase in plasma ANG II levels within 12 h, and subsequent transfer from 75 to 100% SW further increased plasma ANG II levels at both 24 and 72 h. No change in plasma CNP was observed during acute transfer to increased salinity. However, a significant increase in plasma AVT levels was observed following 96 h in 75% SW and 24 h in 100% SW. In chronically SW acclimated C. leucas plasma osmolality, sodium, chloride, and urea were all significantly higher than FW acclimated fish but there was no difference in haematocrit. Acute transfer of C. leucas to 75% SW induced a significant increase in plasma osmolality, sodium and urea concentrations within 96 h of transfer. Subsequent transfer from 75 to 100% SW induced a further increase in these variables within 24 h in addition to a significant increase in plasma chloride above control levels. Haematocrit did not differ between the experimental and control groups throughout the acute study. Circulating levels of ANG II were significantly correlated to plasma, sodium, chloride, and urea concentrations during acclimation to SW. Conversely, circulating levels of CNP and AVT did not correlate to plasma osmolytes, however, CNP was significantly correlated to haematocrit during acclimation to seawater.

Multiple natriuretic peptides coexist in the most primitive extant ray-finned fish, bichir Polypterus endlicheri

Natriuretic peptides (NPs) have diversified from a single NP in cyclostomes and elasmobranchs to multiple NPs in ray-finned fishes where ANP, BNP, VNP, and/or up to four CNPs (CNP-1, 2, 3, and/or 4) have been identified. To trace the evolutionary diversification of NPs in fishes, we analyzed the bichir (Polypterus endlicheri), believed to be the most primitive extant ray-finned fish, for the presence of any NPs by a PCR-based method using primers that amplify all NP cDNAs identified to date. We have cloned cDNAs encoding ANP, BNP, VNP from the heart and three CNPs (CNP-1, 3, and 4) from the brain. An extensive search for CNP-2 from the brain was not successful. The C-terminus of bichir ANP presented an amidation signal as in ray-finned fish ANP. The bichir BNP mRNA had AUUUA repeats in the 3'-untranslated region as observed in all BNP cDNAs of vertebrates. The bichir VNP had a long C-terminal 'tail' sequence extending from the intramolecular ring as does teleost VNP. The three bichir CNPs are structurally similar to each teleost counterpart and are grouped after molecular phylogenetic analyses. ANP was most abundantly expressed in the atrium, BNP in the ventricle, and VNP was expressed in both atrium and ventricle. The three CNPs are most abundantly expressed in the brain, and CNP-4 transcripts were found in small amounts in the ventricle and kidney. Taken together, it is clear that all major NPs exist prior to the whole genome duplication that occurred in the teleost lineage. Furthermore, this is the first observation that CNP-3, ANP, BNP, and VNP, whose genes are colocalized in the same chromosome, coexist in a single fish species including teleosts, thereby confirming that CNP-3 is not an ortholog of VNP, and that ANP, BNP, and VNP genes were generated by tandem duplication from the CNP-3 gene.

Synergistic activation of salmon cardiac function by endothelin and beta-adrenergic stimulation

The aim was to find out the effects of endothelin-1 (ET-1) in salmon (Salmo salar) cardiac contractile and endocrine function and its possible interaction with beta-adrenergic regulation. We found that ET-1 has a positive inotropic effect in salmon heart. ET-1 (30 nM) increased the contraction amplitude 17+/-4.7% compared with the basal level. beta-Adrenergic activation (isoprenaline, 100 nM) increased contraction amplitude 30+/-13.1%, but it did not affect the contractile response to ET-1. ET-1 (10 nM) stimulated the secretion of salmon cardiac natriuretic peptide (sCP) from isolated salmon ventricle (3.3+/-0.14-fold compared with control) but did not have any effect on ventricular sCP mRNA. Isoprenaline alone (0.1-1,000 nM) did not stimulate sCP release, but ET-1 (10 nM) together with isoprenaline (0.1 nM) caused a significantly greater increase of sCP release than ET-1 alone (5.4+/-0.07 vs. 3.3+/-0.14 times increase compared with control). The effects on the contractile and secretory function could be inhibited by a selective ETA-receptor antagonist BQ-610 (1 microM), whereas ETB-receptor blockage (by 100 nM BQ-788) enhanced the secretory response. Thus ET-1 is a phylogenetically conserved regulator of cardiac function, which has synergistic action with beta-adrenergic stimulation. The modulatory effects of ET-1 may therefore be especially important in situations with high beta-adrenergic tone.

Multiple lineage specific expansions within the guanylyl cyclase gene family

BACKGROUND

Guanylyl cyclases (GCs) are responsible for the production of the secondary messenger cyclic guanosine monophosphate, which plays important roles in a variety of physiological responses such as vision, olfaction, muscle contraction, homeostatic regulation, cardiovascular and nervous function. There are two types of GCs in animals, soluble (sGCs) which are found ubiquitously in cell cytoplasm, and receptor (rGC) forms which span cell membranes. The complete genomes of several vertebrate and invertebrate species are now available. These data provide a platform to investigate the evolution of GCs across a diverse range of animal phyla.

RESULTS

In this analysis we located GC genes from a broad spectrum of vertebrate and invertebrate animals and reconstructed molecular phylogenies for both sGC and rGC proteins. The most notable features of the resulting phylogenies are the number of lineage specific rGC and sGC expansions that have occurred during metazoan evolution. Among these expansions is a large nematode specific rGC clade comprising 21 genes in C. elegans alone; a vertebrate specific expansion in the natriuretic receptors GC-A and GC-B; a vertebrate specific expansion in the guanylyl GC-C receptors, an echinoderm specific expansion in the sperm rGC genes and a nematode specific sGC clade. Our phylogenetic reconstruction also shows the existence of a basal group of nitric oxide (NO) insensitive insect and nematode sGCs which are regulated by O2. This suggests that the primordial eukaryotes probably utilized sGC as an O2 sensor, with the ligand specificity of sGC later switching to NO which provides a very effective local cell-to-cell signalling system. Phylogenetic analysis of the sGC and bacterial heme nitric oxide/oxygen binding protein domain supports the hypothesis that this domain originated from a cyanobacterial source.

CONCLUSION

The most salient feature of our phylogenies is the number of lineage specific expansions, which have occurred within the GC gene family during metazoan evolution. Our phylogenetic analyses reveal that the rGC and sGC multi-domain proteins evolved early in eumetazoan evolution. Subsequent gene duplications, tissue specific expression patterns and lineage specific expansions resulted in the evolution of new networks of interaction and new biological functions associated with the maintenance of organismal complexity and homeostasis.

Haemoconcentration via diuresis in short-term hypoxia: a possible role for cardiac natriuretic peptide in rainbow trout

Rainbow trout, exposed to acute hypoxia (decrease of oxygen level from full to 30% air saturation for 1 h, stable 30% air saturation for 2 h), showed more than twofold increase in urine flow rate. Hypoxic diuresis was associated with a sustained increase in dorsal aortic cardiac peptide (sCP) level, and the diuresis could be completely inhibited by a bolus injection of sCP antiserum. These results suggest that hypoxic haemoconcentration, which is partially achieved via increased urine flow rate in vertebrates, is caused by cardiac peptides. The results further suggest that cardiac peptide receptors in hypoxic fish gills modulate the postbranchial systemic level of sCP.

Natriuretic peptides, their receptors, and cyclic guanosine monophosphate-dependent signaling functions

Natriuretic peptides are a family of structurally related but genetically distinct hormones/paracrine factors that regulate blood volume, blood pressure, ventricular hypertrophy, pulmonary hypertension, fat metabolism, and long bone growth. The mammalian members are atrial natriuretic peptide, B-type natriuretic peptide, C-type natriuretic peptide, and possibly osteocrin/musclin. Three single membrane-spanning natriuretic peptide receptors (NPRs) have been identified. Two, NPR-A/GC-A/NPR1 and NPR-B/GC-B/NPR2, are transmembrane guanylyl cyclases, enzymes that catalyze the synthesis of cGMP. One, NPR-C/NPR3, lacks intrinsic enzymatic activity and controls the local concentrations of natriuretic peptides through constitutive receptor-mediated internalization and degradation. Single allele-inactivating mutations in the promoter of human NPR-A are associated with hypertension and heart failure, whereas homozygous inactivating mutations in human NPR-B cause a form of short-limbed dwarfism known as acromesomelic dysplasia type Maroteaux. The physiological effects of natriuretic peptides are elicited through three classes of cGMP binding proteins: cGMP-dependent protein kinases, cGMP-regulated phosphodiesterases, and cyclic nucleotide-gated ion channels. In this comprehensive review, the structure, function, regulation, and biological consequences of natriuretic peptides and their associated signaling proteins are described.

Utility of brain natriuretic peptide (BNP) measurement in cardiovascular disease

Cardiac failure is a prevalent and costly condition in Western society. The ageing of the population, together with current medical options which improve, rather than eradicate heart failure, lead to the projection that this problem will increase substantially in the foreseeable future. The availability of a simple test to assist the diagnosis and effective management of heart failure would greatly assist the clinical approach to this problem. This review examines the physiological basis for the measurement of natriuretic peptides as markers of the presence or risk of heart failure. It considers its use in the hospital and non-hospital setting and examines the cost-effectiveness of current assays. It is possible that in future natriuretic peptides may offer a form of treatment for heart failure, but this is beyond the scope of this review. Nevertheless, the review highlights the potential benefits of this group of tests in the management of heart failure.

Molecular evolution of the natriuretic peptide system as revealed by comparative genomics

The natriuretic peptide (NP) family is a group of peptides involved in cardiovascular and body fluid regulation in vertebrates. While only C-type NP (CNP) has been found in elasmobranchs, atrial NP (ANP), B-type NP (BNP) and CNP have been found in mammals, and ventricular NP (VNP) instead of BNP in teleosts. Thus, it was once hypothesized that CNP is the ancestral NP, from which ANP and BNP/VNP were generated. However, the discovery of hfNP in the hagfish, and CNP in the lamprey suggested that the ancestral NP had characteristics common to these two peptides. Genomic studies in ray-finned fish revealed multiplication processes of NP genes: The ancestral gene was duplicated into four CNP genes before the divergence of elasmobranchs, and ANP, BNP and VNP genes were generated from one of the four CNP genes by tandem duplications. From up to seven NP genes thus generated, tetrapods are supposed to have lost some of them. Concerning NP receptors, teleosts also have more subtypes (three guanylyl cyclase-coupled receptors and two clearance receptors) than mammals. It is of interest to examine how the complicated NP system in teleosts compared with tetrapods, is involved in the adaptation to a wide variety of osmotic environments.

Sequence, circulating levels, and expression of C-type natriuretic peptide in a euryhaline elasmobranch, Carcharhinus leucas

The present study has examined expression and circulating levels of C-type natriuretic peptide (CNP) in the euryhaline bull shark, Carcharhinus leucas. Complementary DNA and deduced amino acid sequence for CNP in C. leucas were determined by RACE methods. Homology of CNP amino acid sequence in C. leucas was high both for proCNP and for mature CNP when compared with previously identified elasmobranch CNPs. Mature CNP sequence in C. leucas was identical to that in Triakis scyllia and Scyliorhinus canicula. Levels of expression of CNP mRNA were significantly decreased in the atrium but did not change in either the brain or ventricle following acclimation to a SW environment. However, circulating levels of CNP significantly increased from 86.0+/-7.9 fmol ml(-1) in FW to 144.9+/-19.5 fmol ml(-1) in SW. The results presented demonstrate that changes in environmental salinity influences both synthesis of CNP from the heart and also circulating levels in C. leucas. Potential stimulus for release and modes of action are discussed.

Identification of novel bradykinin-potentiating peptides and C-type natriuretic peptide from Lachesis muta venom

The generation of expressed sequence tags (ESTs) from the pit-viper snake Lachesis muta venom glands allowed us to identify two cDNA isoforms which encode the precursors for bradykinin-potentiating peptides (BPPs) and a C-type natriuretic peptide (CNP). The sequence data derived from these cDNAs combined with the venom peptides identification using MALDI-TOF mass spectrometry analysis predicted that these molecules are the precursor protein isoforms that are further processed to produce five novel BPPs and a CNP. They were identified directly in crude venom using MALDI-TOF. The BPPs sequences were further confirmed by MALDI-TOF/TOF de novo sequencing, and an unusual BPP with a residue of tryptophan at the N-terminus (usually it is pyroglutamate) was identified. The putative processing steps required to form the mature BPPs and CNP seem to be similar to those proposed for the ones found in the venom of Bothrops jararaca and Glodyus blomhoffi.

Diverse Forms of Guanylyl Cyclases in Medaka Fish – Their Genomic Structure and Phylogenetic Relationships to those in Vertebrates and Invertebrates

Fish species such as medaka fish, fugu, and zebrafish contain more guanylyl cyclases (GCs) than do mammals. These GCs can be divided into two types: soluble GCs and membrane GCs. The latter are further divided into four subfamilies: (i) natriuretic peptide receptors, (ii) STa/guanylin receptors, (iii) sensory-organ-specific membrane GCs, and (iv) orphan receptors. Phylogenetic analyses of medaka fish GCs, along with those of fugu and zebrafish, suggest that medaka fish is a much closer relative to fugu than to zebrafish. Analyses of nucleotide data available on a web site (http://www.ncbi. nlm.nih.gov/) of GCs from a range of organisms from bacteria to vertebrates suggest that gene duplication, and possibly chromosomal duplication, play important roles in the divergence of GCs. In particular, the membrane GC genes were generated by chromosomal duplication before the divergence of tetrapods and teleosts.

Molecular regulation of the brain natriuretic peptide gene

After brain natriuretic peptide (BNP) was isolated in 1988, rapid progress was made in cloning its cDNA and gene, facilitating studies of tissue-specific expression and molecular regulation of gene expression. This review focuses on the molecular determinants of regulation of the rat and human BNP genes, including signaling pathways that impact on changes in gene expression and cis regulatory elements responsive to these signaling pathways. For both rat and human genes, elements in the proximal promoter (-124 to -80), including GATA, MCAT, and AP-1-like, have been shown to contribute to basal and inducible regulation. More distal elements in the human BNP gene respond to calcium signals (an NF-AT site at -927), thyroid hormone (a thyroid-responsive element at -1000), and mechanical stretch (shear stress-responsive elements at -652 and -162). Understanding how BNP is regulated by signaling molecules that are activated in the hypertrophied and ischemic heart should be useful in understanding the underlying pathology. This may lead to therapeutic strategies that prevent hypertrophy while allowing for the beneficial effects of BNP production.

Endothelium-derived C-type natriuretic peptide: more than just a hyperpolarizing factor

The perceived importance of C-type natriuretic peptide (CNP) in the mammalian vasculature has been raised by its recent identification as an endothelium-derived hyperpolarizing factor (EDHF). This aspect of its biological activity is likely to be significant in the regulation of vascular tone, local blood flow and systemic blood pressure. However, the importance of CNP to cardiovascular homeostasis is likely to extend beyond that of a "hyperpolarizing factor" ; indeed, there is evidence that CNP has a key role in preventing smooth muscle proliferation, leukocyte recruitment and platelet reactivity. As such, endothelium-derived CNP is likely to exert a strong anti-atherogenic influence on blood vessel walls and represent a new therapeutic target in the fight against inflammatory cardiovascular disorders. Moreover, this profile of activity defines a new paradigm for the biological significance of EDHF.

C-type natriuretic peptide in vascular physiology and disease

Natriuretic peptides play a critical role in coordination of fluid/electrolyte balance and vascular tone. The renal effects of circulating atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are distinct from the paracrine effects of vascular C-type natriuretic peptide (CNP). CNP is widely expressed throughout the vasculature and is found in particularly high concentrations in the endothelium. Recent studies demonstrate that CNP is a novel endothelium-derived hyperpolarising factor (EDHF) that complements the actions of other endothelial vasorelaxant mediators such as nitric oxide (NO) and prostacyclin. Since several cardiovascular disorders are associated with dysfunction of natriuretic peptide activity, selective modulation of the natriuretic peptide pathways represents an important therapeutic target; whilst this has been exploited to some degree in terms of ANP/BNP, the therapeutic potential of CNP has yet to be tapped. This review focuses on recent findings on the actions and mechanism of locally produced endothelial-derived CNP in the cardiovascular system and highlights many potential avenues for therapeutic intervention, via modulation of CNP-signalling, in cardiovascular disease.

Comparative analysis of the natriuretic peptide precursor gene cluster in vertebrates reveals loss of ANF and retention of CNP-3 in chicken

We identified and characterized the chicken natriuretic peptide precursor gene cluster and found its organization to be highly conserved compared with the mammalian Nppb-Nppa cluster. However, phylogenetic analysis indicated that the putative chicken natriuretic peptide precursor genes are the homologues of CNP-3 and Nppb, respectively. Comparative expression analysis revealed that, in human, mouse, and rat hearts, Nppb is a novel marker for the differentiating working myocardium. Its expression pattern is strikingly similar to that of Nppa before birth, and diverges only after birth. In contrast, whereas the chicken Nppb gene expression profile resembled that of mammalian Nppb, the CNP-3 gene showed very limited expression in the heart, not resembling the pattern of either Nppa or Nppb. These results show that, in chicken, the Nppa gene has been lost from the natriuretic peptide precursor gene cluster, whereas the CNP-3 gene has been retained.

The Multifunctional Fish Gill: Dominant Site of Gas Exchange, Osmoregulation, Acid-Base Regulation, and Excretion of Nitrogenous Waste

The fish gill is a multipurpose organ that, in addition to providing for aquatic gas exchange, plays dominant roles in osmotic and ionic regulation, acid-base regulation, and excretion of nitrogenous wastes. Thus, despite the fact that all fish groups have functional kidneys, the gill epithelium is the site of many processes that are mediated by renal epithelia in terrestrial vertebrates. Indeed, many of the pathways that mediate these processes in mammalian renal epithelial are expressed in the gill, and many of the extrinsic and intrinsic modulators of these processes are also found in fish endocrine tissues and the gill itself. The basic patterns of gill physiology were outlined over a half century ago, but modern immunological and molecular techniques are bringing new insights into this complicated system. Nevertheless, substantial questions about the evolution of these mechanisms and control remain.

Comparative aspects of natriuretic peptide physiology in non-mammalian vertebrates: a review

The natriuretic peptide system is a complex family of peptides and receptors that is primarily linked to the maintenance of osmotic and cardiovascular homeostasis. A natriuretic peptide system is present in each vertebrate class but there are varying degrees of complexity in the system. In agnathans and chondrichthyians, only one natriuretic peptide has been identified, while new data has revealed that multiple types of natriuretic peptides are present in bony fish. However, it seems in tetrapods that there has been a reduction in the number of natriuretic peptide genes, such that only three natriuretic peptides are present in mammals. The peptides act via a family of guanylyl cyclase receptors to generate the second messenger cGMP, which mediates a range of physiological effects at key targets such as the gills, kidney and the cardiovascular system. This review summarises the current knowledge of the natriuretic peptide system in non-mammalian vertebrates and discusses the physiological actions of the peptides.

Asian medaka fishes offer new models for studying mechanisms of seawater adaptation

Japanese medaka (Oryzias latipes) is a freshwater (FW) teleost that is popular throughout the world for laboratory use. In this paper, we discuss the utility of Japanese medaka and related species for studying mechanisms of seawater (SW) adaptation. In addition to general advantages as an experimental animal such as their daily spawning activity, transparency of embryos, short generation time and established transgenic techniques, Japanese medaka have some adaptability to SW unlike the strictly stenohaline zebrafish (Danio rerio). Since other species in the genus Oryzias exhibit different degrees of adaptability to SW, comparative studies between Japanese medaka, where molecular-biological and genetic information is abundant, and other Oryzias species are expected to present varying approaches to solving the problems of SW adaptation. We introduce some examples of interspecies comparison for SW adaptabilities both in adult fish and in embryos. Oryzias species are good models for evolutionary, ecological and zoogeographical studies and a relationship between SW adaptability and geographic distribution has been suggested. Medaka fishes may thus deliver new insights into our understanding of how fish have expanded their distribution to a wide variety of osmotic environments.

Identification of a novel adrenomedullin gene family in teleost fish

Adrenomedullin (AM) is a multifunctional peptide known to form a hormone family with calcitonin gene-related peptide (CGRP) and amylin. We have cloned five distinct AM cDNAs from the pufferfish, Takifugu rubripes, and named them TrAM-1, -2, -3, -4, and -5. Judging from the deduced precursor sequences and processing pattern of the C-terminal mature peptides, TrAMs may be divided into at least two groups; AM-2 and -3, and AM-1, -4, and possibly -5. Phylogenetic analysis of the mature peptides, exon-intron structure of their genes, and tissue distribution of their mRNA also support this classification. TrAM-1 and -4 were ubiquitously expressed in various tissues including the kidney and interrenal (adrenal homolog) as in the case of mammalian AM, while TrAM-2 and -3 were expressed most abundantly in the brain followed by the vascular tissues. Synteny of the genes around AM gene showed that TrAM-1 is the ortholog of mammalian AM. The presence of a PAMP-like sequence in the prosegment of TrAM-1 also supports this notion. Multiple AMs were also detected in another pufferfish, Tetraodon nigroviridis, and in zebrafish, Danio rerio. The present study shows for the first time the presence of a novel AM family in teleost fish that is independent from CGRP and amylin, which further suggests the possible existence of multiple AMs in mammals.