Hormonal interactions at the molecular level. A study of oxytocin and vasopressin binding to bovine neurophysins

Protein/ Hormone Based Nanoparticles as Carriers for Drugs Targeting Protein-Protein Interactions

BACKGROUND

In this review, protein-protein interactions (PPIs) were defined, and their behaviors in normal in disease conditions are discussed. Their status at nuclear, molecular and cellular level was underscored, as for their interference in many diseases. Finally, the use of protein nanoscale structures as possible carriers for drugs targeting PPIs was highlighted.

OBJECTIVE

The objective of this review is to suggest a novel approach for targeting PPIs. By using protein nanospheres and nanocapsules, a promising field of study can be emerged.

METHODS

To solidify this argument, PPIs and their biological significance was discussed, same as their role in hormone signaling.

RESULTS

We shed the light on the drugs that targets PPI and we suggested the use of nanovectors to encapsulate these drugs to possibly achieve better results.

CONCLUSION

Protein based nanoparticles, due to their advantages, can be suitable carriers for drugs targeting PPIs. This can open a new opportunity in the emerging field of multifunctional therapeutics.

Expression, folding, and thermodynamic properties of the bovine oxytocin-neurophysin precursor: relationships to the intermolecular oxytocin-neurophysin complex

Earlier thermodynamic studies of the intermolecular interactions between mature oxytocin and neurophysin, and of the effects of these interactions on neurophysin folding, raised questions about the intramolecular interactions of oxytocin with neurophysin within their common precursor. To address this issue, the disulfide-rich precursor of oxytocin-associated bovine neurophysin was expressed in Escherichia coli and folded in vitro to yield milligram quantities of purified protein; evidence of significant impediments to yield resulting from damage to Cys residues is presented. The inefficiency associated with the refolding of reduced mature neurophysin in the presence of oxytocin was found not to be alleviated in the precursor. Consistent with this, the effects of pH on the spectroscopic properties of the precursor and on the relative stabilities of the precursor and mature neurophysin to guanidine denaturation indicated that noncovalent intramolecular bonding between oxytocin and neurophysin in the precursor had only a small thermodynamic advantage over the corresponding bonding in the intermolecular complex. Loss of the principal interactions between hormone and protein, and of the enhanced stability of the precursor relative to that of the mature unliganded protein, occurred reversibly upon increasing the pH, with a midpoint at pH 10. Correlation of these results with evidence from NMR studies of structural differences between the precursor and the intermolecular complex, which persist beyond the pH 10 transition, suggests that the covalent attachment of the hormone in the precursor necessitates a conformational change in its neurophysin segment and leads to properties of the system that are distinct from those of either the liganded or unliganded mature protein.

Structural basis of neurophysin hormone specificity: Geometry, polarity, and polarizability in aromatic ring interactions

The structural origins of the specificity of the neurophysin hormone-binding site for an aromatic residue in peptide position 2 were explored by analyzing the binding of a series of peptides in the context of the crystal structure of liganded neurophysin. A new modeling method for describing the van der Waals surface of binding sites assisted in the analysis. Particular attention was paid to the unusually large (5 kcal/mol) difference in binding free energy between Phe and Leu in position 2, a value representing more than three times the maximum expected based on hydrophobicity alone, and additionally remarkable since modeling indicated that the Leu side chain was readily accommodated by the binding pocket. Although evidence was obtained of a weak thermodynamic linkage between the binding interactions of the residue 2 side chain and of the peptide alpha-amino group, two factors are considered central. (1) The bound Leu side chain can establish only one-third of the van der Waals contacts available to a Phe side chain. (2) The bound Phe side chain appears to be additionally stabilized relative to Leu by more favorable dipole and induced dipole interactions with nonaromatic polar and sulfur ligands in the binding pocket, as evidenced by examination of its interactions in the pocket, analysis of the detailed energetics of transfer of Phe and Leu side chains from water to other phases, and comparison with thermodynamic and structural data for the binding of residue 1 side chains in this system. While such polar interactions of aromatic rings have been previously observed, the present results suggest their potential for significant thermodynamic contributions to protein structure and ligand recognition.

Structure-function relationships of the vasopressin prohormone domains

1. In this review the structure-function relationships of the different vasopressin prohormone domains are dated and discussed, with special reference to the neurophysin and glycopeptide domains. 2. The primary structures of the currently known neurophysins and glycopeptide sequences are compared and discussed. 3. The hormone-binding and aggregational properties of neurophysin are reviewed and related to a possible function within the regulated secretory pathway. 4. It is proposed, based on the properties reviewed here as well as our own data shown here, that the sorting of the vasopressin prohormone is initiated by hormone binding, which triggers aggregation of the prohormone into the characteristic dense cores of the regulated secretory pathway. 5. This may suggest that prohormone sorting into the regulated secretory pathway is, in general, determined by noncovalent, intramolecular interactions that promote aggregation.

The behavior of the active site salt bridge of bovine neurophysins as monitored by 15N NMR spectroscopy and chemical substitution. Relationship to biochemical properties

The active site of liganded neurophysin contains a salt bridge that involves the side chains of Arg-8 and Glu-47 of the protein and the alpha-amino group of bound hormone or related peptide. The extent to which the Arg-8-Glu-47 salt bridge persists in the absence of peptide, or to which the environment of Arg-8 in the unliganded state differs in monomers and dimers, is relevant to an understanding of allosteric mechanism in this system. In the present study, the behavior of the salt bridge was investigated by 15N NMR and chemical replacement of Arg-8. Bovine neurophysin-I was converted to its des 1-8 derivative, and Arg-8 was replaced by 15N-substituted Arg or by other residues using chemical semisynthesis. The relative abilities of different amino acids to restore peptide affinity to the des 1-8 protein were in good accord with the view of the salt bridge in the liganded state obtained from crystals of bovine neurophysin-II complexes. In the unliganded state, comparison of the 15N and proton NMR signals from Arg-8 with those in smaller arginine systems suggested the absence of significant interactions between the guanidinium of Arg-8 and Glu-47 or between the amino terminal region of Arg-8 and other elements of the protein. No evidence of a difference in Arg-8 environment between unliganded monomers and dimers was found. Marked spectral changes accompanying the binding of oxytocin indicated changes in the environment of both the side chain and amino terminal region of Arg-8. The NMR results were in good agreement with a recently emerging comparison of bovine neurophysin-II derivatives in the liganded and unliganded states, with the notable exception of the extent of salt bridge formation in the unliganded state. The results are shown to be consistent with, and to help explain, significant differences between the two bovine neurophysins in the susceptibility to tryptic cleavage at Arg-8 in the unliganded state and in the pH dependence of peptide binding and additionally constrain potential allosteric mechanisms underlying neurophysin ligand-facilitated dimerization.

Crystal structure of the neurophysin-oxytocin complex

The first crystal structure of the pituitary hormone oxytocin complexed with its carrier protein neurophysin has been determined and refined to 3.0 A resolution. The hormone-binding site is located at the end of a 3(10)-helix and involves residues from both domains of each monomer. Hormone residues Tyr 2, which is buried deep in the binding pocket, and Cys 1 have been confirmed as the key residues involved in neurophysin-hormone recognition. We have compared the bound oxytocin observed in the neurophysin-oxytocin complex, the X-ray structures of unbound oxytocin analogues and the NMR-derived structure for bound oxytocin. We find that while our structure is in agreement with the previous crystallographic findings, it differs from the NMR result with regard to how Tyr 2 of the hormone is recognized by neurophysin.

Neurophysin-neurohypophyseal hormone interactions: studies using a dansylated vasotocin analogue

We have synthesized a neurohypophyseal hormone analogue containing an extrinsic fluorescence probe by linking a dansyl (DNS) group to the epsilon-amino group of the lysine at residue 8 of vasotocin. The fluorescence properties of this analogue have been characterized by steady-state and time-resolved spectroscopic methods and compared with those of epsilon-DNS-lysine and the dansylated carboxyl terminal tripeptide Pro-Lys(DNS)-GlyNH2. The binding of this hormone analogue to purified isoforms of bovine neurophysins, the natural carrier proteins of the neurohypophyseal hormones, results in changes in several fluorescence parameters of the dansyl probe. These changes include an increase in intensity and average lifetime, a shift of the emission band to higher energies, and an increase in the emission anisotropy. Anisotropy changes have been used to determine dissociation constants for binding to these neurophysin isoforms. Based on the changes in the fluorescence properties of the dansyl probe, the dansyl group itself interacts with the protein. The degree of the dansyl-neurophysin interaction, however, appears to be different for the full sequence isoform of neurophysin I and the Val89 isoform of neurophysin II.

Neurophysins, rather than Receptors, are Involved in [H]Oxytocin and [H]Vasopressin Binding Detected by Autoradiography in the Hypothalamo-Neurohypophyseal System

Abstract The goal of the present experiments was to analyse the binding of oxytocin (OT) and vasopressin (VP) in the hypothalamo-neurohypophyseal system to determine whether [(3)H]OT and [(3)H]VP binding in this system involved interaction with receptor sites or with neurophysins. Using quantitative autoradiography, several experiments were performed to compare [(3)H]OT- and [(3)H]VP-binding characteristics in this system and in brain areas containing identified receptor sites. Saturation experiments indicated much lower affinity of [(3)H]OT and [(3)H]VP binding in the magnocellular nuclei and neural lobe than on brain receptors. Competition experiments using selective ligands indicated interaction with neurophysins rather than with receptors in the hypothalamo-neurohypophyseal system. This system was never labelled in the presence of a [(125)I]OT antagonist, a selective OT receptor ligand. In contrast with receptors elsewhere in the brain, the magnocellular nuclei were labelled by [(3)H]OT and [(3)H]VP in the absence of MgCI(2). In the pituitary neural lobe, density of binding sites was moreover obviously related to the amount of neurosecretory granules, as seen in acutely dehydrated rats. Taken together, these data strongly suggest that in the hypothalamo-neurohypophyseal system [(3)H]OT and [(3)H]VP bind to neurophysins rather than to specific receptors.

Synthesis and processing of pro-ocytocin in bovine corpus luteum and granulosa cells

Bovine corpus luteum is the site of intense production of pro-ocytocin-neurophysin mRNA at day 1 after estrus (Ivell et al. (1985) FEBS Lett. 190, 263-267) which is followed by apparent delayed production of ocytocin. Therefore it is a good model to study both the translational and post-translational production of this neuropeptide in non-hypothalamic tissues and its regulation. In order to assess if this mRNA is translated during the lag period we have analyzed the neurophysin-like species produced in this organ. As early as day 2 after estrus one neurophysin species (pI approximately 4.7) could be detected and was unequivocally identified as pro-ocytocin-neurophysin. In primary cultures of luteinizing granulosa cells, biosynthetic intermediates were characterized, i.e. ocytocin-Gly, ocytocin-Gly-Lys and ocytocin-Gly-Lys-Arg, whereas amidated, fully mature, ocytocin was undetectable. We conclude that translation of pro-ocytocin-neurophysin mRNA takes place soon after transcription and we propose that incomplete processing could be responsible for the low level of ocytocin in the early bovine corpus luteum.

A radioiodinated linear vasopressin antagonist: a ligand with high affinity and specificity for V1a receptors

A linear vasopressin antagonist, Phaa-D-Tyr(Me)-Phe-Gln-Asn-Arg-Pro-Arg-Tyr-NH2 (Linear AVP Antag) (Phaa = Phenylacetyl), was monoiodinated at the phenyl moiety of the tyrosylamide residue at position 9. This antagonist appeared to be a highly potent anti-vasopressor peptide with a pA2 value in vivo of 8.94. It was demonstrated to bind to rat liver membrane preparations with a very high affinity (Kd = 0.06 nM). The affinity for the rat uterus oxytocin receptor was lower (Ki = 2.1 nM), and affinities for the rat kidney- and adenohypophysis-vasopressin receptors were much lower (Ki = 47 nM and 92 nM, respectively), resulting in a highly specific vasopressin V1a receptor ligand. Autoradiographical studies using rat brain slices showed that this ligand is a good tool for studies on vasopressin receptor localization and characterization.

Labelling of vasopressin and oxytocin receptors from the human uterus

Four labelled ligands, [3H]arginine vasopressin ([3H]AVP), [3H]oxytocin ([3H]OT), [3H]d(CH2)5[Tyr(Me)2]AVP ([3H]VPA), and [125I]d(CH2)5[Tyr(Me)2-Thr4-Orn8-Tyr(NH2)9]OT([125I]OTA] and nine unlabelled analogues exhibiting enhanced selectivity for rat oxytocin (OT) and vasopressin (VP) receptors were used to characterize OT and VP receptors on myometrial membranes from non-pregnant and pregnant human uteri. On membranes from non-pregnant uteri, [3H]AVP, [3H]VPA, and [125I]OTA labelled with high affinity (Kd values: 3.2, 2 and 0.8 nM, respectively) a major and apparently homogeneous population of sites, the ligand selectivity of which resembled that of rat V1a VP receptors. On membranes from pregnant and non-pregnant uteri, [3H]OT labelled a single population of high-affinity sites that could be distinguished from VP receptors on the basis of ligand selectivity. Several analogues (in particular [125I]OTA) that are highly selective for rat OT receptors exhibited a much less pronounced selectivity for human OT receptors. Experiments with [3H]VPA allowed detection of VP receptors on myometrical membranes from pregnant uteri and confirmed that only OT but not VP receptors increase during pregnancy in humans.

Vasopressin in the brain of the golden hamster: the distribution of vasopressin binding sites and of immunoreactivity to the vasopressin-related glycopeptide

Using in vitro light microscopic autoradiography and immunocytochemistry, the distribution of vasopressin binding sites and that of the vasopressin-related glycopeptide are described in the brain of golden hamster (Mesocricetus auratus). Vasopressin binding sites and immunoreactive axons were observed in the suprachiasmatic nucleus, in the anterior hypothalamus/median preoptic area, in the medial preoptic nucleus, in the bed nucleus of the stria terminalis, in the habenular complex, in the thalamic paraventricular nucleus, and in the nucleus of the solitary tract. In addition we observed binding sites in regions where no immunoreactivity could be evidenced: the lateral septal nucleus, the central amygdaloid nucleus, the subiculum, the dentate gyrus, the anterodorsal and anteroventral thalamic nuclei, the superior colliculus, the vestibular nuclei, and in the prepositus hypoglossal nucleus. In the golden hamster, exogenous vasopressin excites single neurones located in the suprachiasmatic nucleus and induces flank-marking behavior when microinjected into the preoptic area. Our results provide a morphological basis for similar effects exerted by endogenous vasopressin. A comparison of the present data with those previously described in the rat reveals marked species differences in the brain distribution of vasopressin and of its binding sites.

Quantitative autoradiographic mapping of neurohypophysial hormone binding sites in the rat forebrain and pituitary gland--II. Comparative study on the Long-Evans and Brattleboro strains

The anatomical distribution and pharmacological characteristics of the different types of neurohypophysial hormone binding sites were compared in the forebrains and pituitary glands of Long-Evans rats and its mutant Brattleboro strain, genetically deficient in vasopressin. Quantitative autoradiography on sections incubated in the presence of 5 nM of either [3H]oxytocin or [3H]vasopressin revealed the presence of the same types of sites in the brains of both strains but noticeable variations in their densities were found in several areas. In the forebrain, oxytocin/vasopressin sites, which bind both peptides with similar high nanomolar affinities, had the same locations and densities in the ventral subiculum, in several nuclei of the amygdala, the bed nucleus of the stria terminalis and the olfactory tubercle. The density of such sites was, in contrast, lower in the ventromedial hypothalamic nucleus of the Brattleboro rat. Selective vasopressin sites which bind [3H]vasopressin with a nanomolar-range affinity and [3H]oxytocin with a much lower affinity showed more variations. They were not found in the Brattleboro rat thalamus but were highly concentrated in several thalamic nuclei in the Long-Evans rat. Conversely, their densities were higher in the dopaminergic A13 cell group of the zona incerta and the suprachiasmatic nucleus of the Brattleboro rat. Their densities were similar in the lateral septal nucleus and in the fundus striati of both strains. In the hypothalamo-neurohypophysial system, [3H]oxytocin and [3H]vasopressin binding occurred in the Long-Evans rat with characteristics different from those found in other brain areas. In the Brattleboro rat, no [3H]vasopressin binding and only low [3H]oxytocin binding, restricted to the magnocellular nuclei, were found.(ABSTRACT TRUNCATED AT 250 WORDS)

Quantitative autoradiographic mapping of neurohypophysial hormone binding sites in the rat forebrain and pituitary gland--I. Characterization of different types of binding sites and their distribution in the Long-Evans strain

Oxytocin and vasopressin binding sites were localized and characterized by quantitative autoradiography on consecutive sections of Long-Evans rat forebrains and pituitary glands, incubated in the presence of 5 nM [3H]oxytocin or 5 nM [3H]vasopressin. In the forebrain, two types of neurohypophysial hormone binding sites were thus defined. (1) Oxytocin/vasopressin sites with similar nanomolar-range affinities for [3H]oxytocin and [3H]vasopressin; both tritiated peptides were displaced from these sites in the presence of 10 microM of either oxytocin or vasopressin. The main areas bearing such sites were the ventral subiculum, several nuclei of the amygdala, the ventromedial hypothalamic nucleus, the bed nucleus of the stria terminalis and the olfactory tubercle. (2) Selective vasopressin sites, binding [3H]vasopressin with nanomolar-range affinity and [3H]oxytocin with a much lower affinity; these sites were not labelled in the presence of 5 nM [3H]oxytocin, and 10 microM oxytocin displaced [3H]vasopressin binding by 80%. Such sites occurred in several thalamic nuclei, in the dopaminergic A13 cell group of the zona incerta, the suprachiasmatic nucleus, the fundus striati and the lateral septal nucleus. No selective oxytocin sites were detected. Different oxytocin and vasopressin binding characteristics were found in the hypothalamo-neurohypophysial system. In the paraventricular and supraoptic nuclei and in the pituitary neural lobe the [3H]vasopressin binding density was twice that of [3H]oxytocin; vasopressin was always more potent than oxytocin in displacing both [3H]vasopressin and [3H]oxytocin binding from those sites. Interaction of the tritiated peptides with neurophysins cannot be completely ruled out in these locations. The present data are discussed in correlation with the functional roles of the neurohypophysial peptides in the brain and the pharmacological characteristics of their receptors.

Synthetic peptide substrates as models to study a pro-ocytocin/neurophysin converting enzyme

The selectivity and mechanism of processing at paired basic amino acids in hormone precursors was studied on several analogues of the (1-20)-aminoterminal domain of the ocytocin/neurophysin precursor in a cleavage assay by an endoprotease partially purified from bovine pituitary secretory granules. Peptide analogues with amino acid substitutions in, and around, the basic doublet were synthesized and used as substrates. The data obtained demonstrate the strict requirement of the processing enzyme for basic amino acids in tandem within a possibly preferred conformation which may be highly conserved in the aminoterminal domain of this hormone precursor.

125I-d(CH2)5[Tyr(Me)2, Tyr(NH2)9]AVP: iodination and binding characteristics of a vasopressin receptor ligand

A radioiodinated vasopressin antagonist, d(CH2)5[Tyr(NH2)9]AVP has been prepared. Iodination was carried out at the phenyl moiety of the tyrosylamide residue at position 9, followed by HPLC purification. Non-radiolabelled monoiodinated antagonist was used as a reference for identification. 125I-d(CH2)5[Tyr(Me)2, Tyr(NH2)9]AVP binding appeared to take place with a dissociation constant of 0.28 +/- 0.09 nM (Kd +/- SD) to V1 vasopressin receptors on rat liver membranes.

125I-labelled d(CH2)5[Tyr(Me)2,Thr4,Tyr-NH2(9)]OVT: a selective oxytocin receptor ligand

An oxytocic antagonist, [1-(beta-mercapto-beta, beta-cyclopentamethylenepropionic acid,2-O-methyltyrosine,4-threonine, 8-ornithine,9-tyrosylamide]vasotocin (d(CH2)5[Tyr(Me)2, Thr4,Tyr-NH2(9)]OVT [corrected], was monoiodinated at the phenyl moiety of the tyrosylamide residue at position 9. 125I-labelling was performed with 1,3,4,6-tetrachloro-3 alpha,6 alpha-diphenyl-glycoluril. Iodination resulted in an increased affinity for rat uterine oxytocin receptors. A considerably lower affinity for rat vascular V1- and renal V2-receptors was found, resulting in a highly specific oxytocin receptor ligand. 125I-labelled d(CH2)5[Tyr(Me)2,Thr4,Tyr-NH2(9)]OVT [corrected] was demonstrated to bind selectively to one population of binding sites in rat uterus and ventral hippocampal membrane preparations. Dissociation constants ranged between 0.03 and 0.06 nM. After 3 days of exposure autoradiography revealed binding in regions known to contain oxytocin receptors as well as labelling in some new regions, while no binding was found in the lateral septum, a structure containing mainly [8-arginine]vasopressin receptors. The high specific radioactivity of 125I-labelling allowed important reductions in membrane protein amount, gain in precision of binding analysis as well as considerably lower exposure times for autoradiography.

C-terminally extended ocytocin and pro-ocytocin: neurophysin peptide converting enzyme in bovine corpus luteum

Purified secretory granules from the corpus luteum of super ovulated and fecundated cows, at day 7-8 after the heat period, were used as a source of pro-ocytocin/neurophysin (pro OT/Np) processing enzymes. An endoprotease comparable to the previously described pituitary enzyme both by its catalytic properties and sensitivity to various inhibitors was characterized. This protease cleaves pro OT/Np (1-20) after the basic Lys11Arg12 doublet to release OT Gly10 Lys11 Arg12. Moreover C-terminally extended ocytocin, i.e. OTGlyLys and OTGlyLysArg together with ocytocin were identified in extracts from the corpus luteum. Together these data argue strongly in favor of pro OT/Np processing pathways in which cleavage of the precursor at the Arg12-Ala13 peptide bond is the primary event.

Light microscopic autoradiographic localization of [3H]oxytocin binding sites in the rat brain, pituitary and mammary gland

An autoradiographical oxytocin (OXT) labeling procedure using frozen, unfixed tissue sections resulted in very dense labeling of the mammary gland. Binding sites for OXT were also found in various forebrain areas, including the hippocampus, especially the ventral subiculum and taenia tecta, central amygdala, posterior part of the anterior olfactory nucleus, claustrum, nucleus accumbens, bed nucleus of the stria terminalis, ventromedial hypothalamic nucleus, and the posterior pituitary. The ependyma of the lateral ventricle and/or the chorioid plexus near the lateral septum was labeled as well. These data support the hypothesis that OXT plays a role in a number of centrally regulated processes.

Isolation and characterization of a neurophysin from ostrich neurohypophyses

A neurophysin has been isolated from ostrich neurohypophyses using acid acetone extraction, salt fractionation and Sephadex G-75 chromatography. The crude neurophysin eluting from the Sephadex G-75 column was subjected to a) reverse-phase HPLC followed by Sephadex G-75 chromatography, b) DEAE-Sephadex A-50 chromatography or c) isoelectric focusing. The different homogeneous ostrich neurophysin fractions so obtained were compared i.t.o. amino acid composition, spectral properties, N-terminal amino acid residues and PAGE. They all revealed a single N-terminal Ala residue and displayed spectral properties (A280/A260 less than 1) which are typical of mammalian neurophysin-like polypeptides. Ultracentrifugation studies on purified ostrich neurophysin over a range of concentrations revealed a reversible concentration dependent association behaviour characterized by the presence of dimeric complexes at higher concentrations. Partial sequencing from the N-terminus revealed the molecule to be VLDV-like. The purified molecule was also submitted to CNBr fragmentation.

Salt-dependent structural changes of neurohormones: lithium ions induce conformational rearrangements of ocytocin to a vasopressin-like structure

The preferred average conformation and structural subdomain interactions of the nonapeptide hormones vasopressin and ocytocin have been analyzed through the determination of their hydrodynamic volume and the thermal coefficient of the frictional resistance to rotation of their tyrosine residue. A spherical gross shape and an ellipsoidal gross shape were assessed respectively for ocytocin and vasopressin by fluorescence polarization analysis. Investigation of the thermal coefficient of viscosity and the critical temperature of both hormones and analogues indicated that strong interactions hold together the two structural subdomains of ocytocin (the flexible six-membered ring and the COOH-terminal tripeptide tail). An opposite situation was found in the case of vasopressin where such interactions could not be detected between the rigid ring and the flexible COOH-terminal tail. Lithium ions were shown to promote ocytocin binding to specific neurophysin sites restricted, under standard conditions, to vasopressin. In the presence of lithium, the gross conformational shape of ocytocin becomes similar to that of vasopressin but in the absence of salt. In addition, the ocytocin ring becomes more rigid in the presence of lithium while decreasing interactions between the ring and the COOH-terminal tail were detected. It is proposed that lithium ions induce specific conformational rearrangements of ocytocin toward a vasopressin-like structure, allowing recognition of this hormonal ligand by a specific vasopressin binding domain of neurophysins.

Reduction in hepatic but not in renal and vascular vasopressin receptor number in hyperinsulinemic mice and rats

Hepatic plasma membranes of female obese mice C57 BL-6 orl ob/ob (ob/ob mice) completely lack vasopressin (VP) receptors of the V1 type whereas kidney VP receptors are normally expressed and functionally coupled to adenylate cyclase. To discover if these alterations are linked to a genetic defect of the V1 receptor, we have studied the binding of VP on liver and kidney membranes of two other models, female diabetic mice C57 BL-6 orl db/db (db/db mice) and female Zucker rats Fatty/orl fa/fa (fa/fa rats), which exhibit different temporal pattern of obesity, hyperinsulinemia and insulin resistance. In addition, since VP is known to exert its vascular response through stimulation of V1 receptors, we have studied the reactivity of VP of isolated tail artery in the three different models, ob/ob and db/db mice and fa/fa rats, and in their respective controls. In all cases, VP kidney receptors and VP vascular reactivity are normal. db/db mice exhibit a marked decrease in hepatic VP receptors whereas a 50% decrease was observed in 32 week fa/fa rats. Angiotensin II and prazosin binding sites are still present as well as the adenylate cyclase response to glucagon. These results suggest that the specific alteration in liver VP receptors is not related to a defect in V1 receptor genetic expression but is specific for liver and appears to parallel the level of hyperinsulinemia and/or insulin resistance.

Analysis of self-association of bovine neurophysins by gel chromatography

Analytical gel chromatography has been used to examine self-association of bovine neurophysins I and II under several sets of conditions. The data provide no evidence for associated species larger than the dimer. Association constants and Stokes radii of both monomer and dimer are very similar for both proteins in both 0.1 M KOAc, 0.16 M KCl and 0.1 M KPO4, 0.16 M KCl at pH 5.6 and 25 degrees C. The average values derived for the Stokes radii of the monomer and dimer under these conditions are 14.5 +/- 0.7 and 23.0 +/- 0.4 A, respectively. These results confirm the conclusion of Rholam and Nicolas [(1981) Biochemistry 20, 5837-5843] that the monomer and, to a lesser extent, the dimer are highly assymmetric. The Stokes radius of the monomer calculated by Rholam and Nicolas (op cit.) is approximately 30% larger than the value derived here. This discrepancy is probably the result of end-on penetration of the gel by elongated molecules [Y. Nozaki, N. M. Schechter, J. A. Reynolds, and C. Tanford (1976) Biochemistry 15, 3884-3890]. In contrast to Tellam and Winzor [(1980) Arch. Biochem. Biophys. 201, 20-24], it was found that neurophysin II does not exist solely as the dimer in 0.1 M KPO4, pH 5.6, although substitution of 0.1 M KPO4 for 0.1 M KOAc does increase the association constant by a factor of seven. Addition of 1.4 M LiCl at pH 8.1 also increases the association constant sevenfold, as well as increasing the Stokes radius of the monomer approximately 20%. The effects of ionic strength are consistent with the conclusion of Nicolas et al. [(1978) J. Biol. Chem 253, 2633-2639] that formation of the dimer depends upon hydrophobic bonding.

Plasma antidiuretic hormone levels and kidney responsiveness to vasopressin in the jerboa, Jaculus orientalis

The plasma antidiuretic hormone (ADH) concentration and the kidney medulla responsiveness to vasopressin were measured in adult jerboas ( Jaculus orientalis) in different states of hydration. In 15 jerboas adapted to 30 degrees and fed a dry diet, the average ADH concentration in blood plasma was 479 +/- 59 pg/ml, as measured by a radioimmunoassay. About 6 hr after receiving a 5% body wt water load by gavage, the plasma ADH concentration fell to 130 +/- 30 pg/ml in the 5 jerboas still producing hypertonic urine (1022 +/- 267 mosmol/liter) and to 41.5 +/- 8.4 pg/ml in the 6 jerboas producing hypoosmotic urine (157 +/- 6 mosmol/liter). In vitro biochemical experiments were performed on the kidney medullas from two groups of 5 jerboas fed a dry diet (group I) or a water-enriched diet (group II), respectively, for 4 to 7 weeks. Compared to group II, group I animals exhibited (a) higher plasma ADH values, 372 +/- 86 versus 76 +/- 25 pg/ml; (b) higher urine osmolarities (3817 +/- 638 versus 647 +/- 90 mosmol/liter); (c) some decrease in [3H]lysine-vasopressin (LVP) binding capacity to kidney membrane fractions (maximal binding: 0.4 versus 0.6 pmol [3H]LVP bound/mg protein); d) decreased adenylate cyclase responses to arginine-vasopressin, lysine-vasopressin, and oxytocin in kidney membrane fractions; and (e) weaker adenylate cyclase responses to arginine-vasopressin in microdissected pieces of the medullary thick ascending limb of Henle's loop. The values found for (a) the dissociation constant of [3H]lysine-vasopressin binding to membranes (KD); (b) adenylate cyclase sensitivity to the three neurohypophyseal hormones (KA); and (c) adenylate cyclase sensitivity to arginine-vasopressin (KA) in medullary collecting tubules and medullary thick ascending limbs are similar in the two groups of jerboas and roughly comparable to those previously reported for the rat kidney medulla. The reduced maximal adenylate cyclase responses to vasopressin in the jerboas fed a dry diet might indicate some physiological "down regulation" of the number of vasopressin-specific receptors in the kidney as a result of the huge ADH concentration present in blood plasma under these conditions. However, this desensitization is not sufficient to account for the production of hypoosmotic urine in spite of the relatively high ADH plasma levels which persisted after acute overhydration.

Lack of vasopressin receptors in liver, but not in kidney, of ob/ob mice

The activity of phosphorylase a was measured in isolated hepatocytes from fed lean and ob/ob mice after addition of vasopressin, angiotensin, phenylephrine and glucagon. The binding of these hormones to purified liver plasma membranes was also determined. In hepatocytes of ob/ob mice, no increase in phosphorylase a was measured after addition of vasopressin, whereas the other hormones promoted an increase in the activity of the enzyme. No specific vasopressin receptors could be measured on purified liver plasma membrane of ob/ob mice. A decrease in the number of receptors for angiotensin and glucagon, without modification of the affinity, was also observed. No restoration of the number of vasopressin receptors was observed in liver of ob/ob mice starved for 3 days or in younger (5-6 weeks) animals. Vasopressin receptors and vasopressin-stimulated adenylate cyclase, measured on purified kidney medulla membranes, were similar in both lean and ob/ob mice. The data indicate a selective lack of vasopressin receptors and metabolic response in liver of the ob/ob mouse.

Light microscopic autoradiographic localization of [3H]arginine-vasopressin binding sites in the rat brain and kidney

Specific binding sites for arginine-vasopressin (VP) were demonstrated in various major target areas of VP in the rat brain and kidney by light microscopic autoradiography. In the kidney moderate and intense labelling was found in the cortical and medullar areas, respectively. Within the brain intense labelling was shown in the lateral septum, which lends further support to the hypothesis that VP acts as a neurotransmitter. In the hypophysis moderate and heavy labelling was found in the anterior and neural lobes, respectively, which is in agreement with the idea that VP influences hypophyseal functioning.

[3H]vasopressin binding to rat hippocampal synaptic plasma membrane. Kinetic and pharmacological characterization

Characterization of specific vasopressin binding sites to rat hippocampal membranes has been assayed using tritiated lysine-vasopressin labelled on the tyrosyl residue. At 30 degrees C specific [3H]vasopressin binding was saturable. The estimated equilibrium dissociation constant was 7.1 nM, the mean maximal binding capacity was 78 fmol/mg protein. Arginine-vasopressin has a high affinity (Kd = 2.8 nM) and dDAVP has a low affinity (Kd = 249 nM) for hippocampal synaptic membranes. (OH)AVP and Phe2Orn8VT are at least as active as AVP in inhibiting [3H]vasopressin binding. Adenylate cyclase was activated by VIP and inhibited by PIA, but not affected by lysine-vasopressin.

Identification of neurophysin-related proteins in bovine neurosecretory granules

A highly purified fraction of neurosecretory granules isolated from the bovine posterior hypophysis was obtained using a continuous isotonic Percoll gradient. The analysis of their protein content indicates that the main products consist of 10000-Mr neurophysin (95%) and about 5% of high-molecular-weight forms of this protein. Species ranging from Mr 80000-14000 were detected by radioimmunoassay and immunoprecipitated by anti-neurophysin antibodies after 125I labeling of the neurosecretory granule lysates. The chemical relationship with neurophysin, of one of the immunoreactive species (Mr 19000), was ascertained by tryptic map analysis of the radioiodinated material. It is concluded that species both immunologically and chemically related to neurophysin represent a large majority of the protein content of the granules. They compare with the higher molecular weight forms of these neurosecretory components detected in the hypothalamo-neurohypophyseal tract and proposed as putative precursors of both neurophysin and vasopressin.

Binding of neurohypophyseal peptides to neurophysin dimer promotes formation of compact and spherical complexes

Previous hydrodynamic studies [Rholam, M., & Nicolas, P. (1981) Biochemistry 20, 5837-5843] have demonstrated that the dimerization of a neurophysin monomer (prolate ellipsoid with an axial ratio, due to asymmetry, of 5.2) results in a decreased asymmetry (axial ratio, due to asymmetry, of 3.6) as the consequence of a side-by-side association process. By a combination of hydrodynamic measurements, including the use of sedimentation velocity, viscometry, and fluorescence polarization spectroscopy, the influence of hormone binding on the shape and asymmetry properties of the neurophysin dimer was evaluated. The binding of ocytocin, vasopressin, and the tripeptide analogue of the N-terminal sequence of ocytocin, Cys(S-Me)-Tyr-Ile-NH2, results in an increase of S020,W and a decrease in both the reduced viscosity and rotational relaxation time of the bis-liganded dimeric species vs. the nonliganded form. The axial ratio (a/b) due to asymmetry of the ligand-bound dimers was found in each case to be equal to, or slightly greater than, 1.0, indicating a compact spherical shape (Stokes radius 21 A). The profound alteration on molecular dimensions observed upon ligand binding is shown to be the consequence of a ligand-induced conformational change and might explain the intradimeric binding sites positive cooperativity. It is tentatively proposed that the pseudospherical shape of the neurophysin-hormone complexes may enhance the stability of neurophysin and contribute to the prevention of leakage of neuropeptides through the membrane of neurosecretory granules. The data provide a remarkable example of a small protein with a high content in disulfide links and that undergoes conspicuous changes in conformation under the influence of nonapeptide, or tripeptide, ligands.

Partial digestion of neurophysins with proteolytic enzymes: unusual interactions between bovine neurophysin II and chymotrypsin

Bovine neurophysin II was partially digested by chymotrypsin and by chymotrypsin followed by carboxy-peptidase B to produce large fragments collectively representing deletions of residues 1-5 and 91-95. All such fragments were capable of binding peptides to the principal hormone-binding site of neurophysin with normal or near-normal affinity, indicating that residues 1-5 and 91-95 do not directly participate in binding. In addition, preliminary results with thermolysin-derived fragments suggested that residue 6 does not participate in peptide binding. During the course of chymotrypsin studies, it was demonstrated that bovine neurophysin II behaves as a transient competitive inhibitor of chymotrypsin; for neurophysin-peptide complexes, Ki congruent to 8 x 10(-6) M. This inhibition is dependent on neurophysin conformation and is relieved by the anomalous preferential splitting by chymotrypsin of Arg-Arg and Phe-Pro bonds near the carboxyl terminus of neurophysin II. It is suggested that this phenomenon might reflect the interaction of neurophysin II with a chymotrypsin-related enzyme in the pituitary. One approach used in the study of binding properties of proteolytically modified neurophysin was affinity chromatography; the preparation and properties of a conveniently prepared affinity column for neurophysin are described.

Synthesis of peptide spin-labels that bind to neurophysin and their application to distance measurements within neurophysin complexes

The synthesis of two spin-labels capable of binding to the hormone-binding site(s) of neurophysin is described. The two spin-labels are 4-(glycyl-L-phenylalanylamido)-2,2,6,6-tetramethylpiperidinyl-1-oxy and S-[[[3-(2,2,5,5-tetramethylpyrrolidine-1-oxy)amino]carbonyl]methyl]-L-cysteinyl -L-tyrosine amide; synthesis of the former is achieved by a novel route to circumvent problems associated with nitroxide instability under standard conditions of peptide deblocking. NMR studies of the effects of binding these spin-labels on relaxation rates of individual proton resonances of neurophysin were used to calculate correlation times and distances between the bound nitroxides and the observed protons. The results indicate that residue 3 of peptides bound to the strong site of neurophysin is greater than or equal to 14 A from Tyr-49 and argue against a distance of < 5 A between the ortho ring protons of Tyr-49 and those of residue 2 of peptides bound to the strong site. Alternatively, the data suggest that the previously observed nuclear Overhauser effect between these protons reflects spin diffusion at the strong site and a contribution of uncertain magnitude from a second but very weak binding site; this second site is close to Tyr-49 and is detected by the increased relaxation rate of Tyr-49 ring protons when 4-(glycyl-L-phenylalanylamido)-2,2,6,6-tetramethylpiperidinyl-1-oxy is displaced from the strong site by competing diamagnetic peptide. Additionally, the data indicate that residue 3 of bound peptides at the strong site is distant from His-80 but approximately 12 A from the amino terminus. The extended side chain of residue 1 of peptides at the strong site is calculated as less than or equal to 10 A from Tyr-49.

N.M.R. and equilibrium dialysis studies of the interaction of bovine neurophysin-1 with vasopressin and small peptides

The binding to bovine neurophysin of lysine-vasopressin and of lysine-vasopressin selectively deuterated at the protons ortho to the tyrosine hydroxyl was studied by proton n.m.r. and equilibrium dialysis. The principal object of these studies was to investigate reports that, at standard salt concentrations, neurophysin contained a second site specific for vasopressin. At pH 6, the effects of neurophysin-I on the line-width, longitudinal relaxation rate and nuclear Overhauser properties of the lysine-vasopressin tyrosine ring protons were interpretable in terms of a slow-exchange 1:1 interaction between lysine-vasopressin and neurophysin. Additionally, n.m.r. competition studies between lysine-vasopressin and L-phenylalanyl-L tyrosinamide suggested 1:1 competition for a single binding site on neurophysin. No evidence pointing to a significant second lysine-vasopressin-binding site was obtained from the n.m.r. studies. The lack of a moderately strong second binding site for lysine-vasopressin at neutral pH was also indicated by equilibrium dialysis studies at relatively high free hormone concentrations. These studies demonstrated only a single thermodynamically significant site for either oxytocin or vasopressin and failed to confirm a reported effect of LiCl on the number of sites available to oxytocin. It is suggested that secondary sites for the hormones are probably markedly weaker and less specific than reported elsewhere.

(3H)-vasopressin binding to isolated rat hepatocytes and liver membranes: regulation by GTP and relation to glycogen phosphorylase activation

Specific vasopressin binding to rat hepatocytes and rat liver membranes was measured using biologically active (3H)-Tyr2-Lys8-vasopressin (8.5 Ci/mM). In both systems, vasopressin binding was found to be time-dependent, reversible, and saturable. The kinetic parameters for vasopressin binding were: apparent dissociation constants (Kd): 4.9 nM and 15 nM; maximal binding capacities: 0.83 pmoles/mg protein and 2.10(5) sites/Cell for purified membranes and intact cells respectively. The relative affinities of 19 vasopressin structural analogues were deduced from competition experiments and compared to the previously determined glycogenolytic (or antiglycogenolytic) potencies of these analogues. For both agonists and antagonists, a highly significant correlation was demonstrated between pKd and pKa (or pKi) values, suggesting that the detected binding sites are the physiological receptors involved in the glycogenolytic action of vasopressin on the rat liver. The affinity of antagonists for binding to these receptors is the same for both membranes and cells. In contrast, agonists which bind to vasopressin receptor sites have a higher affinity for purified membranes than for intact cells (Kd membranes/Kd cells = 8 +/- 1). GTP (0.1mM) reduced the affinity of agonists but not of antagonists for binding to membranes and abolished the differences between Kd values for binding to hepatocytes and membranes.