The isolation of purified neurosecretory granules from bovine pituitary posterior lobes. Comparison of granule protein constituents with those of neurophysin

Microvesicles of the neurohypophysis are biochemically related to small synaptic vesicles of presynaptic nerve terminals

Nerve endings of the posterior pituitary are densely populated by dense-core neurosecretory granules which are the storage sites for peptide neurohormones. In addition, they contain numerous clear microvesicles which are the same size as small synaptic vesicles of typical presynaptic nerve terminals. Several of the major proteins of small synaptic vesicles of presynaptic nerve terminals are present at high concentration in the posterior pituitary. We have now investigated the subcellular localization of such proteins. By immunogold electron microscopy carried out on bovine neurohypophysis we have found that three of these proteins, synapsin I, Protein III, and synaptophysin (protein p38) were concentrated on microvesicles but were not detectable in the membranes of neurosecretory granules. In addition, we have studied the distribution of the same proteins and of the synaptic vesicle protein p65 in subcellular fractions of bovine posterior pituitaries obtained by sucrose density centrifugation. We have found that the intrinsic membrane proteins synaptophysin and p65 had an identical distribution and were restricted to low density fractions of the gradient which contained numerous clear microvesicles with a size range the same as that of small synaptic vesicles. The peripheral membrane proteins synapsin I and Protein III exhibited a broader distribution extending into the denser part of the gradient. However, the amount of these proteins clearly declined in the fractions preceding the peak of neurosecretory granules. Our results suggest that microvesicles of the neurohypophysis are biochemically related to small synaptic vesicles of all other nerve terminals and argue against the hypothesis that such vesicles represent an endocytic byproduct of exocytosis of neurosecretory granules.

Isolation of neurosecretory granules containing vasotocin, mesotocin, MSEL- and VLDV-neurophysins from goose neurohypophysis

Neurosecretory granules have been isolated from goose posterior pituitaries and their contents have been analyzed by reverse-phase high-pressure liquid chromatography and polyacrylamide gel electrophoresis. Vasotocin and mesotocin have been identified by their biological activities and their retention times compared with those of synthetic peptides. MSEL- and VLDV-neurophysins have been characterized by their N-terminal sequences, their electrophoretic migrations and their retention times, compared with those of purified goose neurophysins. In contrast to the two-step processing of mammalian provasopressin, processing of the vasotocin - MSEL-neurophysin precursor appears to involve only one cleavage giving the hormone and a "big" MSEL-neurophysin homologous to mammalian MSEL-neurophysin extended by copeptin.

Enzymatic protein carboxyl methylation in rat posterior pituitary: neurophysins in rapid-turnover pool determine methyl accepting capacity

In vitro stimulation of intact rat posterior pituitary by either veratridine or K+ depolarization results in the concomitant release of neurophysins and in a decrease (70-80%) in their carboxyl methylation as measured either with L-[methyl-3H]methionine in the intact lobes after stimulation or in their homogenates with [methyl-3H]S-adenosyl-L-methionine and purified protein carboxyl methyltransferase. A similar reduction in neurophysin methylation (60%) was observed when the arrival of newly synthesized neurophysins at the posterior pituitary was blocked by colchicine. Experimental data indicate that the reduction in neurophysin content of the lobes after 12 h of colchicine treatment (less than 7%) or after in vitro stimulation (about 10%) cannot account for the marked reduction in neurophysin methylation. The results suggest that the granule pool characterized by rapid turnover of neurophysins probably represents the major source of methyl acceptor proteins in the lobe. In spite of the marked reduction in neurophysin methyl accepting capacity observed after stimulation, there was no parallel increase in methyl accepting capacity of the released neurophysins. We propose that a neurophysin subfraction that might be associated with the membrane of releasable granules participates in the methylation reaction in situ.

Characterization of an enzyme that is capable of processing pro-gonadotropin-releasing hormone protein

A new membrane bound protease has been identified in bovine hypothalamic neurosecretory granules using synthetic substrates that we prepared based on the sequence in pro-gonadotropin-releasing hormone protein that overlaps gonadotropin-releasing hormone and gonadotropin-associated peptide (thought to be prolactin-releasing hormone-inhibiting hormone). The enzyme was solubilized from neurosecretory granules using the detergent Triton X-100 and was further purified by high-performance gel permeation liquid chromatography. The enzyme hydrolyzes the Arg-2-naphthylamide (NA) bond of benzoyl(Bz)-Gly-Leu-Arg-Pro-Gly-Gly-Lys-Arg-2-NA which contains two likely processing sites, Arg-Pro and Lys-Arg. On the basis of the ratio of Vmax to Km as a measure of substrate specificity, Bz-Gly-Leu-Arg-Pro-Gly-Gly-Lys-Arg-2-NA is about 50-fold better than Bz-Gly-Gly-Lys-Arg-2-NA. Bz-Leu-Arg-2-NA and Bz-Gly-Leu-Arg-Pro-Gly-Gly are not hydrolyzed. The pH optimum for hydrolysis is 7.2 (Bz-Gly-Gly-Lys-Arg-2-NA substrate). As determined by gel permeation chromatography, the apparent molecular weight of the enzyme depends on the chromatography conditions; in the absence of NaCl, the Mr is approximately equal to 160,000 but is approximately equal to 80,000 if NaCl is included in the eluting buffer. After high-performance gel permeation liquid chromatography, the peak fraction containing the enzyme was lyophilized and then subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis; silver staining revealed a single protein band, Mr approximately equal to 70,000.

Purification of bovine neurosecretory granule membranes by density gradient centrifugation

A procedure for the subfractionation of neurosecretory granules into membrane and content components is described. The procedure involves the hypotonic lysis of the secretory granule fraction and further purification of the membranes by centrifugation through a discontinuous sucrose gradient. The neurosecretory granule membranes represented 5.2% of the total proteins of the neurosecretory granule fraction and were highly enriched in cytochrome b561. Electron microscopic analysis of the purified membranes showed vesicles devoid of electrodense content.

Mercuric chloride inhibition of vasopressin release from the isolated neurointermediate lobe of the rat pituitary

The effects of HgCl2 and ouabain on vasopressin release and Ca2+ uptake and distribution was examined in the neurointermediate lobe of the rat pituitary. HgCl2 (0.5 mM) inhibited vasopressin release by approx. 90% in both basal and potassium depolarized states. With 0.1 mM HgCl2 vasopressin release was inhibited by 50% in the depolarized state, but release was not effected in basal state. On the other hand, ouabain (0.5 mM) caused a 3-fold stimulation of vasopressin release in the depolarized state. Both HgCl2 (0.5 mM) and ouabain (0.5 mM) increased net 45Ca+2 uptake by about 80% in groups of neurointermediate lobes. Following 45Ca+2 uptake, HgCl2 (0.5 mM), which is absorbed by the neurointermediate lobe, produced an increase in cytosolic 45Ca+2 content and a decrease in mitochondrial 45Ca+2 content compared to control. In comparison, ouabain (0.5 mM), which does not penetrate the neurointermediate lobe, gave no change in cytosolic 45Ca+2, but an increase in mitochondrial 45Ca+2. These results suggest that HgCl2 inhibits vasopressin release from the neurointermediate lobe of the rat pituitary at a point distal to Ca+2 uptake by the gland.

Identification of calmodulin-binding proteins on membranes of secretory granules isolated from bovine neurohypophyses

Membrane proteins from isolated, purified ox neurohypophyseal secretory granules were separated by sodium dodecylsulphate (SDS) polyacrylamide gel electrophoresis (PAGE). Using a gel overlay technique, after renaturation procedures, it was demonstrated that 125J calmodulin bound in a Ca2+-dependent way to two protein bands with molecular weights (MW) of 58,000 and 52,000. Binding of small amounts of calmodulin to other protein bands was independent of calcium. No calmodulin binding to granule content proteins could be detected. Treatment of the granules with trypsin prior to separation of membrane proteins removed the Ca2+-dependent binding proteins from the granule membrane. On incubation of granules with [gamma-32P]ATP, protein bands with MW of 52,000 and 45,000 showed a marked phosphorylation activity. The 52,000 band had the same electrophoretic mobility as one of the calmodulin-binding bands. However, no effect of calmodulin on phosphorylation of this band could be demonstrated.

Isolation of neurosecretory granules from the neurohaemal areas of peptidergic systems of Lymnaea stagnalis, with special reference to the ovulation hormone-producing caudodorsal cells

Neurosecretory granules (NSG) were isolated by density-gradient centrifugation from homogenates of the intercerebral commissure (COM) and the median lip nerves (LN) of the freshwater snail Lymnaea stagnalis. COM and LN are rich in axon terminals of the ovulation hormone-producing caudodorsal cells (CDC) and of the growth hormone-producing light green cells (LGC), respectively. Electron microscopy of COM isolates showed that a single fraction contained large numbers of NSG, which were identical to NSG of CDC axon terminals of the intact COM. High ovulation hormone activity was associated with this fraction. LN NSG were also isolated in large numbers in a single fraction. The isolated LN NSG were identical to NSG of intact LGC axon terminals. They appeared to have a higher specific gravity than CDC NSG.

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.

Fusion of neurohypophyseal membranes in vitro

Freeze cleaving electron microscopy has shown that fusion of isolated secretory vesicles from bovine neurohypophyses was induced by Ca2+ in micromolar concentrations. Mg2+ and Sr2+ were ineffective. Mg2+ inhibited Ca2+-induced fusion. In suspensions containing secretory vesicles as well as sheets of cell membrane, release of vasopressin parallel to intervesicular fusion and fusion of secretory vesicles with sheets of cell membrane was observed after exposure to Ca2+. Mg2+ and Sr2+ were ineffective in replacing Ca2+ as trigger for fusion or vasopressin release. Intervesicular fusion and exocytotic profiles were observed when isolated neurohypophyses or neurosecretosomes were exposed to cold.

Clearance from the circulation of the rat and whole-body autoradiography in the mouse of 125I-labelled neurophysins

1. The rate of clearance of 125I-labelled porcine neurophysins I and II from the circulation of the rat has an initial fast component followed by a slower component. 2. In the initial phase of clearance the half-life of neurophysin I was 1.50 min (s.e.m. = 0.03) and for neurophysin II was 1.74 min (s.e.m. = 0.05). In the slower phase of clearance the half-life of neurophysin I was 22.6 min (s.e.m. = 2.2) and for neurophysin II was 27.3 min (s.e.m. = 5.8). 3. The first component had a volume of distribution similar to the blood volume and the second component had a volume of distribution similar to the volume of extracellular fuid of the rat. 4. The metabolic clearance rates per 200 g of body weight were 1.94 ml/min (s.e.m. = 0.12) for neurophysin I and 1.29 ml/min (s.e.m. = 0.15) for neurophysin II. 5. Using whole-body autoradiography, the kidney was shown to be the major site of uptake of radioactivity in both virgin female and lactating mice.

Isolation and protein composition of membranes of neurosecretory vesicles and plasma membranes from the neural lobe of the bovine pituitary gland

1. Homogenates of neural lobes of bovine pituitary glands were fractionated by differential ultracentrifugation. 2. Neurosecretory vesicles were isolated by sucrose-gradient ultracentrifugation and membranes were obtained after hypo-osmotic lysis of the particles. 3. A method is described for the isolation of a preparation of purified neuronal plasma membranes by using a fraction enriched in nerve endings as a starting material. 4. The purity of the subcellular fractions was estimated by enzyme assays and by examination with the electron microscope. 5. On the basis of the results it was estimated that neuronal plasma membranes constitute more than 30% of the protein of the nerve endings and neurosecretory vesicles more than 45% of the total amount of protein in the homogenate. 6. The proteins of membranes of neurosecretory vesicles and of plasma membranes were solubilized by means of sodium dodecyl sulphate. Polyacrylamide-gel electrophoresis of such preparations showed that both membranes contained a large number of proteins, including three glycoproteins.

Movement of neurosecretory product through the anatomical compartments of the neural lobe of the pituitary gland. An electron microscopic autoradiographic study

Electron-microscope autoradiographs have been prepared from the neural lobes of the pituitary glands of rats which had received intracisternal injections of [35S] cysteine at various times before gland removal. The rate of appearance and disappearance of autoradiographically demonstrable radioactivity in the neural lobe closely paralleled that previously determined, biochemically, for radioactive hormones and neurophysins. Radioactivity was appreciably associated with the undilated parts of neurosecretory axons only during the first few hours after injection of the label. The axonal dilations were subdivided into those in which small vesicles could be seen ("endings") and those in which no small vesicles could be seen ("swellings"). Radioactivity appeared first in "endings" and then in progressively larger and larger profiles of "swellings". It appeared that newly arrived granules were found close to the limiting membrane of the nerve swelling and that as time progressed they moved deeper and deeper into the swelling. On the basis of the results, suggestions were made for an anatomical explanation of the readily-releasable pool of hormone which has been demonstrated pharmacologically.

Adenosine triphosphate dependent calcium uptake by subcellular fractions from bovine neurohypophyses

Bovine neurohypophyses were fractionated by differential and density gradient ultracentrifugation and the Ca-2+ uptake and ATPase activities in the microsomal, mitochondrial and secretory granule fractions were studied. The microsomal and mitochondrial fractions accumulated Ca-2+ in the presence of ATP. The accumulation by the latter per mg protein was at least twice as large as by the former. This Ca2+ accumulation was accompanied by liberation of inorganic phosphate (Pi). In the presence of sodium azide (2 mM) Ca-2+ uptake and Pi liberation were inhibited in the mitochondrial, but not in the microsomal fraction. Further studies of the microsomal fractions revealed that the ATP-dependent Ca-2+ uptake and Pi liberation activities were temperature and pH-dependent and required Mg-2+. Both activities were stimulated by very low concentrations of Ca-2+ (1-10 muM) and were inhibited by EGTA (2 mM). N-ethylmaleimide (2 mM) inhibited both the Ca-2+ uptake and ATPase activities of the microsomal fraction. These results suggest the presence of a membrane ATPase that is stimulated by both Ca-2+ and Mg-2+. It is suggested that the observed Ca-2+ uptake activities are involved in maintaining a low axoplasmic free Ca-2+ concentration, thus playing an important role in the release mechanism of vasopressin by the neurosecretory terminals.

Observations on the lipolytic and melanotropic properties of neurophysin proteins

Previous work indicated that brain contains 3 types of lipolytic-melanotropic peptide: (1) in adenohypophysis: ACTH, alpha-MSH, beta-MSH, peptide I, peptide L', beta-lipotropin and gamma-lipotropin; (2) in neurohypophysis: peptide 7D6, also termed neurophysin I, peptide II or Wuu-Saffran peptide; (3) in extrahypophyseal regions: peptide IIF. Bovine and human neurophysin I prepared by R. Walter has now been found devoid of lipolytic and melanotropic activities. Porcine and bovine peptide 7D6, closely similar or identical to bovine neurophysin I in electrophoretic mobility and amino acid composition, were therefore reexamined to determine whether their lipolytic-melanotropic property resided in a contaminating factor. When peptide 7D6 was analyzed in 100 transfer counter current distribution (1 butanol/0.1M NH4 HCO3), the neurophysin was recovered in tubes 1-9 (7D6-alpha) representing 95% of 7D6. 7D6-alpha was inactive in lipolytic and melanotropic assays. The biologic activities of 7D6 were recovered instead in tubes 50-70 (labeled 7D6-beta), representing 5% of 7D6. 7D6-beta proved to be a peptide with MW 1000-3000, closely similar to peptide IIF in amino acid composition, MW, and Rf values in 4 systems of paper chromatography.

Adenosine triphosphatase activity in the neural lobe of the bovine pituitary gland

1. Homogenates of neural lobes of bovine pituitary glands were fractionated by differential and density-gradient ultracentrifugation and the distribution of adenosine triphosphatase (ATPase) activity was studied. It was shown that all the activity was membrane-bound. 2. On the basis of ionic requirements the ATPase activity was grouped into three categories: (a) Mg(2+)-dependent, (b) Ca(2+)-dependent and (c) Mg(2+)+Na(+)+K(+)-dependent (ouabain-sensitive) ATPases. The activity in the absence of bivalent cations was negligible. The ratio between the activities of the three ATPases varied between the different subcellular fractions. 3. Preincubation of the subcellular fractions with deoxycholate increased the activity of the Mg(2+)+Na(+)+K(+)-dependent enzyme, whereas the Mg(2+)- and Ca(2+)-activated ATPases were either unaffected or slightly inhibited. Triton X-100 solubilized the Mg(2+)- and Ca(2+)-ATPases; however, the activity of the Mg(2+)+Na(+)+K(+)-ATPase was abolished by the concentration of Triton X-100 used. 4. All the subfractions displayed unspecific nucleotide triphosphatase activity towards GTP, ITP and UTP. These substrates inhibited the hydrolysis of ATP by all three ATPases. ADP also inhibited the ATPases. 5. Polyacrylamide-gel electrophoresis of extracts containing the Mg(2+)- and Ca(2+)-dependent ATPase activity solubilized by Triton X-100 revealed the presence of two enzymes; one activated by either Mg(2+) or Ca(2+) and the other activated only by Ca(2+). 6. In sucrose density gradients the distribution of vasopressin was different from that of all three types of ATPases. It is therefore suggested that the neurosecretory granules do not possess ATPase activity.

Subcellular organization of neurophysins, oxytocin, (8-lysine)-vasopressin and adenosine triphosphatase in porcine posterior pituitary lobes

Posterior pituitary lobes from young pigs were fractionated by differential and sucrose-density-gradient centrifugation. The distributions of oxytocin and [8-lysine]-vasopressin were measured by bioassay and the distributions of neurophysin-I and -II by radioimmunoassays specific for each of these two proteins. Most of the hormone and neurophysin applied to the density gradient was localized in particles with the density expected of neurosecretory granules. However, the neurosecretory granules were separated into two bands (D and E). A close statistical correlation between the distributions of [8-lysine]-vasopressin and neurophysin-I, and of oxytocin and neurophysin-II on the gradients, suggested that in vivo porcine neurophysin-I binds [8-lysine]-vasopressin within one population of granules and porcine neurophysin-II binds oxytocin within another type of granule. However, there was no significant separation of oxytocin and vasopressin in fractions D and E. The molar ratios of hormones and neurophysins indicated that there was insufficient of either neurophysin to bind the [8-lysine]-vasopressin in the granule fractions or in the whole gland. Polyacrylamide-gel electrophoresis showed that only bands corresponding in mobility to porcine neurophysins-I, -II and -III were present in large amounts in the whole gland and in the granule fractions. The component with the mobility of neurophysin-III was, however, relatively enriched in whole young glands and granule fractions compared with adult gland extracts. It is suggested that the vasopressin that cannot be assigned to neurophysin-I may occur in (a) vesicles containing vasopressin but no neurophysin, (b) vesicles containing vasopressin and a protein that cannot be quantified by the radioimmunoassays used, such as porcine neurophysin-III, or (c) normal vasopressin-neurophysin granules which have accumulated extra vasopressin. Band E of the gradient was rich in adenosine triphosphatase activity, whereas band D possessed very little of this enzyme.

Neurophysins and posterior pituitary hormones in the suiformes

The neurophysins are proteins that bind the posterior pituitary hormones within the neurosecretory vesicles of the vertebrate neurohypophysis. In the pig, immunofluorescence localization of porcine neurophysin II suggests that this protein is stored with [8-lysine]-vasopressin (LVP). Gel electrophoresis and immunodiffusion of neurohypophysial proteins from two other Suiformes species, the hippopotamus and warthog, hitherto thought to contain LVP, reveal that the warthog has a protein electrophoretically and immunologically identical with porcine neurophysin II, whereas the neurohypophysial proteins of the hippopotamus do not cross-react with a rabbit antiserum specific to this protein. The vasopressins of the hippopotamus and warthog have been recharacterized by gel filtration and by thinlayer chromatography in two solvent systems not hitherto used for this purpose. The presence of LVP in the warthog has been confirmed, but only [8-arginine]-vasopressin (AVP) was detected in the hippopotamus. This suggests that the hippopotamus possesses only AVP, or that the occurrence of LVP in this species is sufficiently rare to make it undetectable by the above techniques. The two species of the Family Suidae studied, i. e. the pig and the warthog, thus seem to be closely related to the structure of their vasopressins and neurophysins, whereas the hippopotamus (Family Hippopotamidae) is less closely related, and seems in this respect to be more similar to other Artiodactyls. This contrasts with the earlier identification of the hippopotamus vasopressin as LVP, which gave biochemical support for the inclusion of the Hippopotamidae within the Suiformes. The weight of the biochemical evidence now favours a contrary view.