[Water homeostasis in the living: molecular organization, osmoregulatory reflexes and evolution]. / L'homéostase hydrique dans le vivant: organisation moléculaire, réflexes osmorégulateurs et évolution.

Human body weight is about 70% water; 55% of the water are in cells and 45% in extracellular compartments, mainly body fluids. Each compartment has its own osmoregulatory system. The mechanisms of intracellular osmoregulation likely appeared with the cell itself, i.e. in primitive prokaryotes, some 3.8 billions years ago. Osmotic stress responses observed in present-day bacteria, yeast, plant and animals cells in culture are very similar. Variations in the cell volume entail an extension or retraction of plasma membrane that activates mechanically-gated ion channels or mechanoreceptors. Some of them have been cloned from E. coli, the nematode C. elegans, the drosophila. Osmotic stress determines an intracellular cascade of transactivations, the last transcription factor binding to a Tonicity response element (TonE) of osmoprotective genes. These genes encode enzymes synthesizing compatible osmolytes that reequilibrate the osmotic pressure. Volume and osmolality of the biological fluids (le milieu intérieur) are regulated by neuroendocrine reflexes involving an afferent neural limb from baro- and osmo-receptors to hypothalamus and an efferent endocrine limb from neurosecretory cells to target cells, the hydroosmotic cells localized in osmoregulatory organs. Afferent signals trigger the biosynthesis and the processing of neurohypophyseal preprohormones in magnocellular neurons of the supraoptic and paraventricular nuclei of hypothalamus. Vasopressin in mammais and vasotocin in other vertebrates, endowed with antidiuretic and antinatriuretic properties, act on hydroosmotic cells localized in the nephron collecting duct. A specific vasopressin receptor, the V2 type receptor, located in the basolateral membrane of the principal cells, is coupled with an heterotridimeric protein Gs that activates adenylate cyclase. The cAMP product, in turn, stimulates the protein kinase A (PKA). The latter mobilizes 5 effectors located in the apical membrane: 1) the water channel aquaporin 2; 2) the urea transporter UT1; 3) the amiloride-sensitive sodium channel ENaC; 4) the inward-rectifying potassium channel ROM-K1; 5) the chloride channel CFTR. Modulation of each effector is ensured by an A kinase anchoring protein (AKAP). The effectors are transported from cytosol to apical membrane through transport vesicles equipped with membrane fusion proteins (such as VAMP2) that recognize specific apical membrane proteins (such as syntaxins). Water flows into principal cells from collecting duct lumen through AQP2 and flows out to interstitium through AQP3 and AQP4 and finally reachs blood circulation through AQP1 capillaries. Vasopressin orchestrates the live effectors so as to keep water and solutes reabsorptions in balance with volaemia and osmolality set points.

Unique evolution of neurohypophysial hormones in cartilaginous fishes: possible implications for urea-based osmoregulation.

Most bony vertebrate species display a great evolutionary stability of their two neurohypophysial hormones, so that two molecular lineages, isotocin-mesotocin-oxytocin and vasotocin-vasopressin, have been traced from bony fishes to mammals. Chondrichthyes, in contrast, show a striking diversity of their oxytocin-like hormones, yet show a substantial decrease in vasotocin stored in neurohypophysis when compared to nonmammalian bony vertebrates. In the rays, glumitocin ([Ser(4),Gln(8)]-oxytocin) has been identified. In the spiny dogfish, aspargtocin ([Asn4]-oxytocin) and valitocin ([Val(8)]-oxytocin) have been characterized whereas in the spotted dogfish, asvatocin ([Asn(4),Val(8)]-oxytocin) and phasvatocin ([Phe(3),Asn(4),Val(8)]-oxytocin) have been found. Finally, in the holocephalian Pacific ratfish, oxytocin, the typical peptide of placental mammals, has been discovered. The duplication of the oxytocin-like hormone gene found in dogfishes has been observed only in some Australian and American marsupials. Cartilaginous fishes have developed an original urea-based osmoregulation involving a glutamine-dependent urea synthesis and blood urea retention through renal urea transporters. Furthermore, marine species use a rectal salt gland for sodium chloride excretion. Although vasopressin, in mammals, and vasotocin, in nonmammalian tetrapods, are clearly implied in water and salt homeostasis, the hormones involved in the blood osmotic pressure regulation of elasmobranchs are still largely unknown. It is suggested that the great diversity of oxytocin-like hormones in elasmobranchs expresses a release from an evolutionary receptor-binding constraint, so that amino-acid substitutions reflect neutral evolution. In contrast, the preservation of vasotocin suggests a selective pressure, which may be related to the regulation of renal urea transporter-recruitment mechanisms, as it has been shown for vasopressin in mammals. J. Exp. Zool. 284:475-484, 1999.

Adaptive evolution of water homeostasis regulation in amphibians: vasotocin and hydrins.

Volemia and osmolality homeostasis is ensured in vertebrates through neuroendocrine reflexes, involving an afferent neural branch from baro- and osmo-receptors to hypothalamus and an efferent endocrine branch from secretory neurons to target hydroosmotic cells equipped with receptors and effectors. Whereas the osmoregulatory system in the tadpole comprises three organs, namely gut, kidney and gills, as in freshwater fishes, the adult displays a quaternary strategy with gut, kidney, urinary bladder and skin. In particular, the cutaneous permeability entails a great evaporative water loss when the animal is in the open air, loss that must be compensated by water reabsorption through the nephron and the urinary bladder and mainly by water uptake through the skin. Adaptation occurred at the level of these organs by regulation of their permeability through neurohypophysial hormones. Aside from vasotocin, active on the three organs, all anuran Amphibia possess hydrin 2 (vasotocinyl-Gly), a peptide resulting from a down-regulation of provasotocin processing. Exceptionally Xenopus laevis, a permanent aquatic toad, has hydrin 1 (vasotocinyl-Gly-Lys-Arg) instead of hydrin 2. Hydrins are somewhat more active than vasotocin on water permeation of skin and bladder but are devoid of antidiuretic activity. Adaptive evolution has created, along with the vasotocin-nephron system, preserved in all terrestrial non-mammalian tetrapods, additional functions such as the hydrin-skin and hydrin-bladder rehydration mechanisms. Specific hydrin receptors might exist in the skin and the bladder, different from those of vasotocin in the kidney. It is assumed that the water channel recruitment mechanism, found for vasopressin acting on the collecting duct principal cells in mammals, is also involved when vasotocin and hydrins stimulate their hydroosmotic target cells and that hormone-regulated aquaporin 2-like proteins could be identified in the three osmoregulatory organs of amphibians.

Molecular evolution of fish neurohypophysial hormones: neutral and selective evolutionary mechanisms.

Chemical identification of neurohypophysial hormones from about 80 vertebrate species reveals that two evolutionary lineages can be traced in bony vertebrates, a vasopressin-like hormone line and an oxytocin-like hormone line, which were derived from the duplication of an ancestral gene that may have been present in agnathans. All of the 13 neurohypophysial hormones are built in the same structural pattern, namely, a nonapeptide with a disulfide bridge linking half-cystines in positions 1 and 6. There is a striking evolutionary stability in bony vertebrates since virtually all species belonging to a given class are endowed with the same peptides. In contrast, in cartilaginous fishes, the oxytocin-like hormone displays a great diversity. Six distinct peptides are characterized in this group. The proposed hypothesis is that the stability in primary structure in bony vertebrates is due to selective pressure. This selective pressure is associated with an ion-based osmoregulation, whereas in Chondrichthyes the occurrence of an urea-based osmoregulation has relieved the hormones from the control of ionic homeostasis. Variations in primary structures in cartilaginous fishes are regarded as relevant to the neutral evolution as defined by Kimura. According to this concept, oxytocin of placental mammals results from selective evolution, whereas the same molecule found in ratfish proceeds from random genetic drift.

Distinct hydro-osmotic receptors for the neurohypophysial peptides vasotocin and hydrins in the frog Rana esculenta.

The biological properties of vasotocin, hydrin 1 (vasotocinyl-Gly-Lys-Arg) and hydrin 2 (vasotocinyl-Gly), in particular the hydro-osmotic activities on the frog skin, the frog urinary bladder and the frog kidney, have been compared. Hydrins are as active or more active than vasotocin on the first two organs but they are virtually devoid of antidiuretic activity in the rat and the frog, in contrast to vasotocin. It appears that where the oxytocin ring (residues 1-6), present in the three peptides, is necessary for the action on the three organs, the C-terminal amidated group of vasotocin is necessary for the renal receptor but not for the skin and bladder receptors. It is known that amphibians have two types of vasotocin receptors, V1 and V2, homologous to the vascular/hepatic V1 and the renal V2 vasopressin receptors of mammals, respectively. We suggest that adaptation has led to specialization of (at least) two subtypes of hydro-osmotic V2 receptors, the renal subtype on which vasotocin is mainly active for the reabsorption of tubular water, and the skin/bladder subtype on which hydrin 2 is specifically involved in ensuring the rehydration of the animal. Cooperative evolution might have created in anuran Amphibia, on the one hand, two hydro-osmotic peptides, vasotocin and hydrin 2, derived from a single precursor through differential processing; and on the other hand, two corresponding receptors in kidney and skin for internal and external water recovery.

The neurohypophysial endocrine regulatory cascade: precursors, mediators, receptors, and effectors.

The neurohypophysial endocrine regulatory cascade has been described as a molecular model of neuroendocrine control of organismal functions. Any physiological function can be analyzed in molecular terms as a succession of interactions occurring either in a solution or in a membrane system. The key mechanism in the ordering of the cascade is the conformational recognition of the two partners at each step. Each interaction results in a change of conformation of a recognized protein that in turn becomes a recognizer for the following molecule. The cascade starts within the secretory cell by the processing of the expressed precursor along the secretory pathway until the storage of the mature mediator in vesicles and its subsequent exocytic secretion in blood. The circulating mediator recognizes the target cell through specific membrane receptors that transduce the message within this target cell. A second intracellular cascade leads to activation of the effector, the protein fulfilling the physiological function. The complexity of the messages is, in part, due to the duplication propensity of the genomic DNA, the frequent occurrence of multiple copies for precursors, mediators, receptors, and effectors, and therefore, a combinatorial diversity that increases during the course of evolution. Vertebrate neurohypophysial hormones can be ordered in two main evolutionary lineages, culminating in oxytocin and vasopressin in placental mammals. In this field, diversification of the messages was made by differential processing of the precursors, secondary gene duplications, the emergence of several types of receptors for each hormone, and a variety of effectors triggered by the second messengers within differentiated target cells. This review is an attempt to integrate neurohypophysial functions at the molecular, cellular, and organismal levels.

A new neurohypophysial peptide, seritocin ([Ser5,Ile8]-oxytocin), identified in a dryness-resistant African toad, Bufo regularis.

From the pituitary neurointermediate lobe of the African toad Bufo regularis, vasotocin, hydrin 2 (vasotocinyl-Gly) and a mesotocin-like peptide have been isolated by HPLC and characterized by mass spectrometry, amino acid sequence and chromatographic coelution with synthetic peptides. The mesotocin-like peptide has been identified as [Ser5,Ile8]-oxytocin in place of mesotocin ([Ile8]-oxytocin) found in all other amphibians investigated to date. The name seritocin is suggested. The molecule is virtually devoid of oxytocic activity on rat uterus in contrast to mesotocin. On the other hand, the molar ratio of hydrin 2 to vasotocin in the pituitary reaches 2, whereas it is about 1 in toads and frogs from temperate regions. B. regularis is an anuran species able to withstand a hot and dry season by burrowing. The possible relationship between occurrence of seritocin and adaptation to arid environment remains to be demonstrated.

Man and the chimaera. Selective versus neutral oxytocin evolution.

The oxytocin/vasopressin superfamily encompasses vertebrate and invertebrate peptides and therefore the ancestral gene encoding the precursor protein antedates the divergence between the two groups, about 700 million years ago. The preserved nonapeptide pattern indicates that both the precursor structures and the processing enzymatic machinery were greatly conserved to ensure the building of a specific conformation. Substitutions, which may be neutral or selective, occurred in precise positions. Virtually all vertebrate species possess an oxytocin-like and a vasopressin-like peptide so that two evolutionary lineages can be traced. Because a single peptide, vasotocin ([Ile3]-vasopressin or [Arg8]-oxytocin) has been found in the most primitive Cyclostomata, a primordial gene duplication and subsequent mutations are assumed to have given rise to the two lineages. They started with vasotocin and isotocin ([Ser4,Ile8]-oxytocin) in bony fishes and culminated with vasopressin and oxytocin in placental mammals. Mesotocin ([Ile3]-oxytocin), found in lungfishes, amphibians, reptiles, birds and marsupials, appears as an evolutionary intermediate. The change from isotocin ([Ser4,Ile8]-oxytocin) into mesotocin ([Ile8]-oxytocin), can be observed in African and Australian lungfishes, species making the transition from bony fishes to land vertebrates. On the other hand the replacement of mesotocin by oxytocin can be detected in marsupials, particularly in the North-American opossum and the Australian Northern bandicoot that have both mesotocin and oxytocin whereas placental mammals possess only oxytocin. The invariability of this peptide in placentals can be explained by receptor-fitting selective pressure. In contrast to bony vertebrates in which neurohypophysial hormones revealed a remarkable structural stability, cartilaginous fishes displayed an unique oxytocin-like hormone evolution with variability and duality. Aside from vasotocin, in the subclass Selachii, rays have glumitocin ([Gln8-oxytocin]) and sharks possess two peptides: aspargtocin ([Asn4-oxytocin]) and valitocin ([Val8-oxytocin]) for the spiny dogfish, asvatocin ([Asn4,Val8]-oxytocin) and phasvatocin ([Phe3,Asn4,Val8]-oxytocin) for the spotted dogfish. In the other subclass Holocephali, the chimaera (ratfish) has oxytocin, the typical hormone of placental mammals. Cartilaginous fishes used urea rather than salts for their osmoregulation and oxytocin-like hormones could have been relieved from osmoregulatory functions and able to accept many neutral variations.

[Water homeostasis in amphibia: vasotocin and hydrins]. / L'homéostase hydrique chez les amphibiens: vasotocine et hydrines.

Water homeostasis is ensured in vertebrates through neuroendocrine reflexes involving hormones and specialized exchange organs depending whether the habitat is freshwater, sea or land. Amphibians have a development recapitulating two adaptive programs, first a freshwater fish program as a tadpole, then a land vertebrate program as an adult. The second, however, is imperfect because of the great evaporative water loss through the skin when the animal is in the open air. This loss must be compensated by water reabsorption through the nephron, the urinary bladder and mainly by water uptake through the skin. Water reabsorption by the nephron is not as efficient as in higher vertebrates but water uptake through the skin is crucial because adult amphibians, like tadpoles, do not drink. Adaptation occurred at the level of three organs, nephron, urinary bladder and skin whose permeability is under control of specific hormones. Aside from vasotocin, active on these three organs, differential maturation of provasotocin led to processing-arrested intermediates, namely vasotocinyl-Gly in virtually all anuran amphibians and vasotocinyl-Gly-Lys-Arg in Xenopus laevis. These intermediates result from a down regulation of the alpha-amidating enzyme or carboxypeptidase E, respectively. They have been termed hydrin 2 and hydrin 1 because they are endowed with hydroosmotic properties equal or superior to those of vasotocin on the skin and the bladder. However, in contrast to vasotocin, they are devoid of antidiuretic activity. Adaptive evolution has created, aside from the osmoregulatory vasotocin-nephron system that was preserved in strictly terrestrial nonmammalian tetrapods, additional functions such as the hydrin-skin and the hydrin-bladder rehydrations with specific messengers and organs. The adjuvant systems disappeared in true land vertebrates because the vasotocin-nephron system became more efficient.

Special evolution of neurohypophysial hormones in cartilaginous fishes: asvatocin and phasvatocin, two oxytocin-like peptides isolated from the spotted dogfish (Scyliorhinus caniculus).

In contrast to most vertebrate species that possess one oxytocin-like hormone and one vasopressin-like hormone, a few groups, such as marsupials or cartilaginous fishes, are endowed with two peptides of either or both types, suggesting possible gene duplications. We have now isolated two oxytocin-like hormones from the pituitary of the spotted dogfish Scyliorhinus caniculus (suborder Galeoidei). Microsequencing as well as chromatographic and pharmacological comparisons with synthetic peptides show that these peptides are [Asn4,Val8]oxytocin (asvatocin) and [Phe3,Asn4,Val8]-oxytocin (phasvatocin). Asvatocin and phasvatocin display oxytocic activity on rat uterus, about 80 and 5 milliunits per nmol, respectively, and virtually no pressor activity on anesthetized rats. They occur in roughly equal molar amounts in the gland; vasotocin is also present in a proportional amount that is lower by about a factor of 20. In addition to the duality, conservative amino acid substitutions are observed in the two oxytocic peptides in positions 4 (Gln-4-->Asn) and 8 (Leu-8-->Val), when compared with oxytocin. Furthermore, replacement of the isoleucine residue found in position 3 of all other oxytocin-like hormones by phenylalanine in phasvatocin is exceptional; it determines a dramatic decrease of the oxytocic activity. Preservation of the C-terminal-amidated nonapeptide pattern in the 12 vertebrate neurohypophysial hormones known to date suggests that both precursors and processing enzymes have coevolved tightly. On the other hand, whereas the great evolutionary stability of the mature hormones (generally observed in vertebrates) suggests a strict messenger-receptor coevolution, the exceptional diversity found in cartilaginous fishes (six oxytocin-like peptides identified out of eight known) might be due to a looseness of selective constraints, perhaps in relationship with their specific urea osmoregulation.

Action of neurohypophysial granule Lys-Arg endopeptidase on synthetic polypeptides comprising the processing sequence of provasopressin-neurophysin.

Neurohypophysial granule Ca(2+)-dependent endopeptidases have been allowed to act on synthetic polypeptides derived from the N-terminal sequence of bovine provasopressin-neurophysin, namely vasopressinyl-glycyl-lysyl-arginyl-alanylamide and vasopressinyl-glycyl-lysyl-arginyl-alanyl-methionyl-serinamide+ ++. Membrane-bound enzymes have been used at pH 5.5 for 16 hr at 37 degrees C. Products have been identified by high-pressure liquid chromatography (HPLC) and by mass spectrometry performed on substances isolated by HPLC. With both substrates, vasopressinyl-Gly-Lys-Arg(OH) has been identified as a product confirming the Lys-Arg specificity previously observed on small peptide fluorogenic substrates. Cleavage yields, however, appear low suggesting that some factors are missing, for example a targeting action of the precursor neurophysin domain to the granule membrane.

Bony fish neurophysins. Identification of MSEL- and VLDV-neurophysins of the pollack (Pollachius virens).

The two types of neurophysins known in vertebrate species, namely MSEL-neurophysin (vasopressin-like hormone-associated neurophysin) and VLDV-neurophysin (oxytocin-like hormone-associated neurophysin) have been purified from the pollack (Pollachius virens) pituitary through a combination of molecular sieving and high-pressure liquid chromatography (HPLC). Homogeneity has been checked by gel electrophoresis and return in HPLC. The apparent molecular masses measured by SDS-electrophoresis are near 12 kDa, significantly higher than those found for their mammalian homologues (10 kDa). The two types of neurophysins have been recognized through their N-terminal amino acid sequences. The primary structure of MSEL-neurophysin has been partially determined using automated Edman degradation applied on native and reduced-alkylated protein, as well as peptides derived by trypsin or staphylococcal proteinase hydrolyses. Comparison of pollack MSEL-neurophysin with ox, goose and frog counterparts reveals that particular positions in the polypeptide chain are subjected to substitutions and that the numbers of substitutions do not seem closely related to the paleontological times of divergence between the different vertebrate classes.

Chemical identification of the mammalian oxytocin in a holocephalian fish, the ratfish (Hydrolagus colliei).

The neurohypophysial hormones of the ratfish (Hydrolagus colliei), a species belonging to the subclass Holocephali of cartilaginous fishes, have been investigated. An oxytocin-like hormone has been isolated from acetone-desiccated pituitary glands by using successively molecular sieving and high-pressure liquid chromatography. The peptide has been identified as oxytocin by coelution with synthetic oxytocin in HPLC, amino acid sequencing, mass spectrometry, and C-terminal sequencing through carboxypeptidase Y. Vasotocin may be present in a very small amount. Cartilaginous fishes appear to display a great diversity in their oxytocin-like hormones since five different peptides have been identified in rays and sharks that belong to the second subclass Selachii.

Lungfish neurohypophysial hormones: chemical identification of mesotocin in the neurointermediate pituitary of the Australian lungfish Neoceratodus forsteri.

The neurohypophysial hormones contained in the neurointermediate lobe of the pituitary of the Australian lungfish Neoceratodus forsteri have been investigated. Mesotocin was identified by coelution with the synthetic peptide in high-pressure liquid chromatography, by Edman amino acid sequencing, by mass spectrometry, by C-terminal sequencing through carboxypeptidase Y, and cleavage with prolyl endopeptidase. Vasotocin, if present, would be in very small amounts and hydrins were not detected. Oxytocin appears absent. Although Neoceratodus and Protopterus have different habitats, the former being permanently aquatic, the latter burrowing during estivation, the proportions of neurohypophysial hormones stored in neurohypophysis were roughly similar in the two species and not apparently affected by environmental conditions.

Differential processing of provasotocin: relative increase of hydrin 2 (vasotocinyl-Gly) in amphibians able to adapt to an arid environment.

Hydrin 2 (vasotocinyl-Gly) is an intermediate in pro-vasotocin processing found, along with vasotocin, only in the neurohypophysis of anuran amphibians. It increases cutaneous water permeability in the frog and is likely involved in neuroendocrine control of osmoregulation. The relative amounts of vasotocin and hydrin 2 stored in neurohypophysis have been measured on the one hand in amphibian species known not to adapt in dry areas, on the other hand in two species, Bufo regularis (Africa) and Bufo viridis (Near-East) able to survive in an arid environment. In the first group, the proportions of the two peptides are approximately equal whereas in the two toads the molar ratio hydrin 2 to vasotocin reaches 2. The ratio does not appear to vary significantly when these toads are either submitted to dehydration or placed in saline solutions. Predominance of hydrin 2 suggests an adaptive decrease of the activity of the alpha-amidating enzymatic system involved in the conversion of vasotocinyl-Gly into mature amidated vasotocin.

Vasotocin and hydrin 2 (vasotocinyl-Gly) in the African toad Bufo regularis: study under various environmental conditions.

1. The neurohypophysial osmoregulatory hormones of the African toad Bufo regularis, a species adapted to estivate under dry and hot conditions, have been investigated. Vasotocin and hydrin 2 (vasotocinyl-Gly) have been identified by their retention times in high-pressure reverse-phase liquid chromatography and coelution with synthetic peptides, their pharmacological properties (vasotocin) and microsequencing. 2. Vasotocin-associated neurophysin (MSEL-neurophysin type) has been characterized by its N-terminal amino acid sequence. 3. In toads subjected to dehydration by evaporation (20% weight loss) or to osmotic stress by immersion in 2% NaCl for 3 hr (6% weight loss), the molar ratio hydrin 2/vasotocin (about 2:1) remained similar to the one observed in control animals. 4. In toads exposed to saline solution, there was a large decrease (roughly 30%) in the amounts of both hormones in the neuro-hypophysis. Environmental conditions for distinct secretions of vasotocin and hydrin 2 remain to be found.

Processing endopeptidase deficiency in neurohypophysial secretory granules of the diabetes insipidus (Brattleboro) rat.

The homozygote Brattleboro rat exhibits a hereditary diabetes insipidus due to a deficiency of vasopressin, the antidiuretic hormone. It has previously been shown that in this animal a single nucleotide deletion in the provasopressin gene leads to a mutant precursor with a C-terminal amino acid sequence different from that of the wild-type. However the N-terminal region including the hormone moiety, the processing signal as well as the first two-thirds of the neurophysin is entirely preserved and absence of maturation has to be explained by an additional cause. We show here that the neurohypophysis of the homozygote Brattleboro rat, in contrast to the adenohypophysis, displays a significant decrease in the Lys-Arg processing endopeptidase activity when compared to the heterozygote or the wild-type Wistar. It is suggested that hypothalamic vasopressinergic neurons of the homozygote Brattleboro rat display a deficiency in the processing enzyme in contrast to the oxytocinergic neurons in which processing of prooxytocin is normal.