Biosynthesis of pairs of peptides related to melanotropin, corticotropin and endorphin in the pars intermedia of the amphibian pituitary gland

α-TC1.9 cells--a model system for analyzing the endoproteolytic processing of POMC

The mouse α-TC1.9 endocrine cell line was used to analyze the amino acid requirements for endoproteolytic processing at the paired basic amino acid cleavage site, K(141)R(142) that is N-terminal to the ACTH sequence in the POMC proprotein of the anuran amphibian, Silurana tropicalis. Real-Time PCR analysis of non-transfected α-TC1.9 cells indicated that these cells endogenously express the pc2 (proprotein convertase 2) gene, but do not express the pc1/3 (proprotein convertase 1/3) gene or the pomc gene. In addition, immunocytochemical analysis and RIA analysis of non-transfected α-TC1.9 cells did not detect the presence of POMC-related products in these cells. For this study the open reading frame of a S. tropicalis POMC cDNA (wild-type) was placed into an expression vector and transiently transfected into α-TC1.9 cells. Two days after transfection the steady-state levels of α-MSH-related and β-endorphin-related end-products were nearly the same as the steady-state levels of these POMC-related end-products in extracts of the S. tropicalis intermediate pituitary. Transient transfection of either the R(142)/A(142)pomc construct or the K(141)/A(141)pomc construct completely blocked cleavage at this site and yielded a 6K immunoreactive product that had the ACTH(1-13)NH(2) sequence at the C-terminal end of the fusion protein. However, substitution of an alanine residue at R(137), Q(138), E(139), and N(140) had no effect on cleavage at the K(141)R(142) cleavage site. Collectively, these results indicate that secondary structure N-terminal to the K(141)R(142) does not appear to influence cleavage at this site. However, both K(141) and R(142) are required for the integrity of this cleavage site. Finally, this study indicates that α-TC1.9 cells should be useful for studying the amino acid requirements for the other endoproteolytic cleavage sites in the S. tropicalis POMC proprotein.

Analysis of the melanotrope cell neuroendocrine interface in two amphibian species, Rana ridibunda and Xenopus laevis: a celebration of 35 years of collaborative research

This review gives an overview of the functioning of the hypothalamo-hypophyseal neuroendocrine interface in the pituitary neurointermediate lobe, as it relates to melanotrope cell function in two amphibian species, Rana ridibunda and Xenopus laevis. It primarily but not exclusively concerns the work of two collaborating laboratories, the Laboratory for Molecular and Cellular Neuroendocrinology (University of Rouen, France) and the Department of Cellular Animal Physiology (Radboud University Nijmegen, The Netherlands). In the course of this review it will become apparent that Rana and Xenopus have, for the most part, developed the same or similar strategies to regulate the release of α-melanophore-stimulating hormone (α-MSH). The review concludes by highlighting the molecular and cellular mechanisms utilized by thyrotropin-releasing hormone (TRH) to activate Rana melanotrope cells and the function of autocrine brain-derived neurotrophic factor (BDNF) in the regulation of Xenopus melanotrope cell function.

Incomplete posttranslational prohormone modifications in hyperactive neuroendocrine cells

BACKGROUND

In black-background-adapted Xenopus laevis, the intermediate pituitary melanotrope cells are hyperactive, producing large amounts of their major secretory cargo proopiomelanocortin (POMC, representing ~80% of all newly synthesised proteins), whereas in white-adapted frogs these cells are only basally active. Here we explored in the hyperactive and basally active melanotrope cells the capacity for posttranslational POMC processing events in the secretory pathway.

RESULTS

We found that the hyperactive cells produced mainly non-complex N-glycosylated POMC, whereas in the basally active cells POMC was mostly complex N-glycosylated. Furthermore, the relative level of POMC sulphation was ~5.5-fold lower in the hyperactive than in the basally active cells. When the cargo load in the secretory pathway of the hyperactive cells was pharmacologically reduced, the relative amount of complex glycosylated POMC markedly increased.

CONCLUSION

Collectively, our data show that the secretory pathway in hyperactive neuroendocrine secretory cells lacks the capacity to fully comply with the high demands for complex glycosylation and sulphation of the overload of secretory cargo. Thus, a hyperactive secretory cell may run short in providing an output of correctly modified biological signals.

The coding sequence of amyloid-beta precursor protein APP contains a neural-specific promoter element

The amyloid-beta precursor protein APP is generally accepted to be involved in the pathology of Alzheimer's disease. Since its physiological role is still unclear, we decided to study the function of APP via stable transgenesis in the amphibian Xenopus laevis. However, the application of constructs encoding (mutant) APP fused to the C-terminus of the green fluorescent protein GFP (GFP-APP), and harboring a tissue-specific or an inducible gene promoter did not result in transgene expression of APP in neuronal and neuroendocrine cells. Surprisingly, a construct encoding either Xenopus or human APP fused to the N-terminus of GFP (APP-GFP) gave fluorescence throughout the whole brain of the tadpole, despite the fact that a proopiomelanocortin gene promoter was used to target transgene expression specifically to the intermediate pituitary cells. Detailed analysis with deletion mutants revealed the presence of a neural-specific, transcriptionally active DNA element within the 3'-end of the APP-coding sequence that gave rise to an aberrant transcript and protein in the APP-GFP transgenic animals. The DNA element appears to prevent proper APP transgene expression in Xenopus neuronal and neuroendocrine cells. Thus, the coding sequences of Xenopus and human APP contain a neural-specific promoter element, the physiological significance of which is at present unclear.

Neuronal, Neurohormonal, and Autocrine Control ofXenopusMelanotrope Cell Activity

Amphibian pituitary melanotropes are used to investigate principles of neuroendocrine translation of neural input into hormonal output. Here, the steps in this translation process are outlined for the melanotrope cell of Xenopus laevis, with attention to external stimuli, neurochemical messengers, receptor dynamics, second-messenger pathways, and control of the melanotrope secretory process. Emphasis is on the pathways that neurochemical messengers follow to reach the melanotrope. The inhibitory messengers, dopamine, gamma-aminobutyric acid, and neuropeptide Y, act on the cells by synaptic input from the suprachiasmatic nucleus, whereas the locus coeruleus and raphe nucleus synaptically stimulate the cells via noradrenaline and serotonin, respectively. Autoexcitatory actions are exerted by acetylcholine, brain-derived neurotrophic factor (BDNF), and the calcium-sensing receptor. At least six messengers released from the pituitary neural lobe stimulate melanotropes in a neurohormonal way: corticotropin-releasing hormone, thyrotropin-releasing hormone, BDNF, urocortin, mesotocin, and vasotocin. They all are produced by the magnocellular nucleus and coexist in various combinations in two types of neurohemal axon terminal. Most of the relevant receptors of the melanotropes have been elucidated. Apparently, the neural lobe has a dominant role in activating melanotrope secretory activity. The intracellular mechanisms translating the various inputs into cellular activities like biosynthesis and secretion constitute the adenylyl cyclase-cAMP pathway and Ca(2+) in the form of periodic changes of the intracellular Ca(2+) concentration, known as Ca(2+) oscillations. It is proposed that the pattern of these oscillations encodes specific regulatory information and that it is set by first messengers that control, for example, via G proteins and cAMP-related events, specific ion channel-mediated events in the membrane of the melanotrope cell.

Multiple control and dynamic response of the Xenopus melanotrope cell

Some amphibian brain-melanotrope cell systems are used to study how neuronal and (neuro)endocrine mechanisms convert environmental signals into physiological responses. Pituitary melanotropes release alpha-melanophore-stimulating hormone (alpha-MSH), which controls skin color in response to background light stimuli. Xenopus laevis suprachiasmatic neurons receive optic input and inhibit melanotrope activity by releasing neuropeptide Y (NPY), dopamine (DA) and gamma-aminobutyric acid (GABA) when animals are placed on a light background. Under this condition, they strengthen their synaptic contacts with the melanotropes and enhance their secretory machinery by upregulating exocytosis-related proteins (e.g. SNAP-25). The inhibitory transmitters converge on the adenylyl cyclase system, regulating Ca(2+) channel activity. Other messengers like thyrotropin-releasing hormone (TRH) and corticotropin-releasing hormone (CRH, from the magnocellular nucleus), noradrenalin (from the locus coeruleus), serotonin (from the raphe nucleus) and acetylcholine (from the melanotropes themselves) stimulate melanotrope activity. Ca(2+) enters the cell and the resulting Ca(2+) oscillations trigger alpha-MSH secretion. These intracellular Ca(2+) dynamics can be described by a mathematical model. The oscillations travel as a wave through the cytoplasm and enter the nucleus where they may induce the expression of genes involved in biosynthesis and processing (7B2, PC2) of pro-opiomelanocortin (POMC) and release (SNAP-25, munc18) of its end-products. We propose that various environmental factors (e.g. light and temperature) act via distinct brain centers in order to release various neuronal messengers that act on the melanotrope to control distinct subcellular events (e.g. hormone biosynthesis, processing and release) by specifically shaping the pattern of melanotrope Ca(2+) oscillations.

Melanotrope cell plasticity: a key mechanism for the physiological adaptation to background color changes

The intermediate lobe of the pituitary secretes the melanotropic hormone alpha-MSH, which in amphibians plays a crucial role in skin color adaptation. It has been previously demonstrated that, in the frog Rana ridibunda, the intermediate lobe is composed of two distinct subpopulations of melanotrope cells that can be separated in vitro by using Percoll density gradients. These two melanotrope cell subsets, referred to as high-density (HD) and low-density (LD) cells, differ in their ultrastructural characteristics as well as in their biosynthetic and secretory activity. However, the specific, physiological role of the heterogeneity displayed by melanotrope cells remains elusive. In the present study, we investigated the effects of background color adaptation on melanotrope cell subpopulations. We found that adaptation of frogs to dark or white environment did not modify either the overall number of cells per intermediate lobe or the apoptotic and proliferation rates of melanotrope cells. On the other hand, adaptation of the animals to a white background significantly increased the proportion of hormone-storage HD cells and caused a concomitant decrease in that of LD cells (which exhibit higher levels of alpha-MSH release and POMC messenger RNA than HD cells). Conversely, after black-background adaptation the proportion of LD cells was markedly increased, suggesting that interconversion of HD cells to LD cells occurs during physiological activation of the intermediate lobe. In addition, black-background adaptation also enhanced alpha-MSH release by both cell subpopulations and increased inositol phosphate production in LD cells. These data indicate that, in frog, the proportions of the two melanotrope cell subsets undergo marked modifications during skin color adaptation, likely reflecting the occurrence of a secretory cell cycle whose dynamics are highly correlated to the hormonal demand imposed by the environment.

Physiological control of Xunc18 expression in neuroendocrine melanotrope cells of Xenopus laevis

In mammals, the brain-specific protein munc18-1 regulates synaptic vesicle exocytosis at the synaptic junction, in a step before vesicle fusion. We hypothesize that the rate of biosynthesis of munc18-1 messenger RNA (mRNA) and the amount of munc18-1 present in neurons and neuroendocrine cells are related to the physiologically controlled state of activity. To test this hypothesis, the homolog of munc18-1 in the clawed toad Xenopus laevis, xunc18, was studied in the brain and in the neuroendocrine melanotrope cells in the intermediate lobe of the pituitary gland, at both the mRNA and the protein level. In toads adapted to a black background, the melanotropes release the peptide alpha-melanophore-stimulating hormone (alpha-MSH), which induces darkening of the skin, whereas in animals adapted to a white background the cells hardly release but store alpha-MSH, making the animal's skin look pale. The intermediate pituitary lobe of black-adapted animals revealed a strong hybridization reaction with the xunc18 mRNA probe, whereas a much weaker hybridization was observed in the intermediate lobe of white-adapted animals (optical density black: 3.4 +/- 0.2 vs. white: 0.8 +/- 0.1; P < 0.02). Immunocytochemically, Xenopus munc18-like protein has been detected throughout the brain, in identified neuronal perikarya as well as in axon tracts. Western blot analysis and immunocytochemistry further demonstrated the presence of xunc18 in the neural, intermediate and distal lobe of the pituitary gland. Xunc18 protein was furthermore determined in immunoblots of homogenates of melanotropes dissociated from the pituitary gland. In melanotropes of toads adapted to a black background, the integrated optical density of the xunc18 immunosignal was 2.7 +/- 0.5 times higher than in cells of white-adapted toads (P < 0.0001). It is concluded that, in Xenopus melanotrope cells, the amounts of both xunc18 mRNA and xunc18 protein are up-regulated in conjunction with the induction of exocytosis of alpha-MSH as a result of a physiological stimulation (environmental light condition). We propose that xunc18 is involved in physiologically controlled exocytotic secretion of neuroendocrine messengers.

Differential effects of dopamine on two frog melanotrope cell subpopulations

The frog intermediate lobe consists of a single endocrine cell type, the melanotrope cells, which are under the tonic inhibitory control of dopamine. Separation of dispersed pars intermedia cells in a Percoll density gradient has revealed the existence of two melanotrope cell subpopulations, referred to as high-density (HD) and low-density (LD) cells. The aim of the present study was to investigate the effects of dopamine on each of these melanotrope cell subsets. Increasing doses of dopamine, ranging from 10(-9)-10(-6) M, inhibited the release of alpha-melanocyte-stimulating hormone (alpha-MSH) in LD (but not in HD) melanotrope cells. In addition, dopamine provoked a significant reduction of the rate of acetylation of alpha-MSH in LD cells but not in HD cells. Similarly, dopamine significantly decreased the accumulation of POMC messenger RNA in LD cells, whereas it did not affect POMC gene expression in the HD melanotrope subset. On the other hand, microfluorimetric studies revealed that dopamine induced a significant reduction of KCl-stimulated cytosolic free calcium concentration in both LD and HD cells. The present study provides additional evidence for functional heterogeneity of melanotrope cells in the frog pars intermedia. Because dopamine plays a pivotal role in the regulation of alpha-MSH secretion, these data suggest the involvement of cell heterogeneity in the physiological process of background color adaptation in amphibians.

The secretory granule and pro-opiomelanocortin processing in Xenopus melanotrope cells during background adaptation

In this immunocytochemical study, we used light and electron microscopic observations in combination with morphometry to analyze the processing of pro-opiomelanocortin (POMC) in melanotrope cells of the intermediate pituitary of Xenopus laevis adapted to either a white or a black background. An antiserum was raised against a synthetic peptide including the cleavage site between ACTH and beta-lipotropic hormone in Xenopus. Western blotting revealed that this antiserum recognizes only a 38-kD protein, the POMC prohormone, from extracts of Xenopus neurointermediate pituitary. Light immunocytochemistry showed differential immunostaining for anti-POMC compared to anti-alpha-MSH. Anti-POMC was predominantly found in the perinuclear region, whereas anti-alpha-MSH yielded staining throughout the cytoplasm. Immunogold double labeling revealed that electron-dense secretory granules (DGs) show high immunoreactivity for anti-POMC and low immunoreactivity for anti-alpha-MSH. Electron-lucent granules (LGs) are immunoreactive to anti-alpha-MSH only. Moderately electron-dense granules (MGs) revealed intermediate reactivity compared to DGs and LGs. Background light intensity has significant effects on the morphology and the immunoreactivity of the secretory granules. Black-adapted animals have 4.5 times as many DGs and MGs as white-adapted animals. In addition, the MGs in black animals show 42% more anti-alpha-MSH immunogold than the MGs in white animals. Together, these findings indicate that the three granule types represent subsequent stages in granule maturation. Adaptation to a black background stimulates the formation of young immature granules, while at the same time the processing rate during granule maturation increases.

Background adaptation by Xenopus laevis: a model for studying neuronal information processing in the pituitary pars intermedia

This review is concerned with recent literature on the neural control of the pituitary pars intermedia of the amphibian Xenopus laevis. This aquatic toad adapts skin colour to the light intensity of its environment, by releasing the proopiomelanocortin (POMC)-derived peptide alpha-MSH (alpha-melanophore-stimulating hormone) from melanotrope cells. The activity of these cells is controlled by brain centers of which the hypothalamic suprachiasmatic and magnocellular nuclei, respectively, inhibit and stimulate both biosynthesis and release of alpha-MSH. The suprachiasmatic nucleus secretes dopamine, GABA, and NPY from synaptic terminals on the melanotropes. The structure of the synapses depends on the adaptation state of the animal. The inhibitory transmitters act via cAMP. Under inhibition conditions, melanotropes actively export cAMP, which might have a first messenger action. The magnocellular nucleus produces CRH and TRH. CRH, acting via cAMP, and TRH stimulate POMC-biosynthesis and POMC-peptide release. ACh is produced by the melanotrope cell and acts in an autoexcitatory feedback on melanotrope M1 muscarinic receptors to activate secretory activity. POMC-peptide secretion is driven by oscillations of the [Ca2+]i, which are initiated by receptor-mediated stimulation of Ca2+ influx via N-type calcium channels. The hypothalamic neurotransmitters and ACh control Ca2+ oscillatory activity. The structural and functional aspects of the various neural and endocrine steps in the regulation of skin colour adaptation by Xenopus reveal a high degree of plasticity, enabling the animal to respond optimally to the external demands for physiological adaptation.

Two frog melanotrope cell subpopulations exhibiting distinct biochemical and physiological patterns in basal conditions and under thyrotropin-releasing hormone stimulation

Cell heterogeneity designates the phenomenon by which a particular cell type is composed of morphologically and physiologically distinct cell subpopulations. We have previously isolated two subsets of melanotrope cells in the intermediate lobe of the frog pituitary by means of a separation procedure based on a Percoll density gradient High density (HD) melanotrope cells were found to exhibit a more granulated cytoplasm and a lower secretory rate than low density (LD) cells. In the present study, we have investigated the biochemical and functional characteristics of each melanotrope cell subpopulation by using various approaches, including chromatographic analysis for the measurement of the proportion of acetylated alpha MSH, microfluorimetric measurement of the cytosolic free calcium concentration ([Ca2+]i) and in situ hybridization for quantification of POMC messenger RNA (mRNA). Under basal conditions, LD melanotrope cells showed higher secretory activity, acetylation rate, [Ca2+]i, and POMC mRNA content compared to HD cells. Incubation of the cells with 100 nM TRH for 2 h induced a more pronounced activation of alpha MSH secretion, [Ca2+]i mobilization, and POMC mRNA accumulation in LD than in HD melanotrope cells. Conversely, TRH increased the rate of acetylation of alpha MSH in HD cells, but did not affect acetylation in LD cells. Taken together, these results demonstrate that the frog intermediate lobe is composed of two subsets of endocrine cells with distinct biochemical and functional characteristics. The coexistence of two cell subpopulations in the frog pars intermedia is consistent with the idea of a cell secretory cycle, in which each melanotrope subset represents a specific state of cellular activity.

Immunocytochemical localization of POMC-derived peptides (adrenocorticotropic hormone, alpha-melanocyte-stimulating hormone and beta-endorphin) in the pituitary, brain and olfactory epithelium of the frog, Rana esculenta, during development

Developmental stages of Rana esculenta, starting with the posterior limb-bud stage (stage 26) up to a few days after metamorphosis, were examined immunohistochemically to localize cells and fibers producing some POMC-derived peptides, namely, alpha-MSH, ACTH and beta-END. Anti ACTH and anti alpha-MSH revealed a positive reaction in the pars intermedia during all stages of development included in this study, whereas no immunoreactivity in this pituitary zone was ever evidenced with anti beta-END. In the pars distalis strongly positive cells were seen with anti ACTH and anti beta-END, while anti alpha-MSH yielded weakly positive cells. Interestingly, these peptides were colocalized in the same cells. Immunoreactivity for alpha-MSH was no longer present in the pars distalis during metamorphic climax and postmetamorphosis. In the brain of premetamorphic tadpoles, belonging to stages 26 to 30, a few neurons in the posterior telencephalon showed a positive reaction only with anti alpha-MSH, but from stage 31 (prometamorphosis) onwards, ACTH and beta-endorphin-like peptide producing cells, together with alpha-MSH-immunoreactive cells, were seen in this region and in the anterior preoptic area and infundibulum. This situation persisted in the subsequent stages of development. Anti alpha-MSH also revealed weakly positive cells in the olfactory epithelium in premetamorphic tadpoles; strong immunoreactivity with anti alpha-MSH was seen in olfactory epithelium cells in animals during prometamorphosis, metamorphic climax and postmetamorphosis. The possible significance of these findings is briefly discussed.

Analysis of autofeedback mechanisms in the secretion of pro-opiomelanocortin-derived peptides by melanotrope cells of Xenopus laevis

The secretion of most pituitary hormones is under the control of feedback mechanisms. The feedback control of alpha-melanophore-stimulating hormone (alpha-MSH) from melanotrope cells is controversial. The possible existence of an autofeedback exerted by alpha-MSH or other POMC-derived peptides on melanotrope cells of the amphibian Xenopus laevis has been investigated. alpha-MSH or its potent agonist 4-norleucine,7-D-phenylalanine-alpha-MSH has no effect on the release of radiolabeled POMC-derived peptides or immunoreactive beta-endorphin from superfused neurointermediate pituitary lobes. Melanin concentrating hormone, previously reported to have an alpha-MSH-like effect on melanophores, did not affect alpha-MSH secretion. Neurointermediate lobe superfusate, which contains a mixture of POMC-derived peptides, failed to affect the secretory activity of melanotropes. It is concluded that in X. laevis the secretory activity of melanotropes is not under the control of short-term autofeedback mechanisms involving alpha-MSH or other POMC-derived peptides.

Lack of effect of TRH on alpha-MSH release from the neurointermediate lobe of the lizard Lacerta vivipara

Thyrotropin-releasing hormone (TRH) is a potent stimulator of melanotropin (alpha-MSH) release from pituitary melanotrophs in pig, frog, and fish. Concurrently, it has recently been shown that injection of TRH induces skin darkening in the lizard Anolis carolinensis (Licht and Denver, 1988). In the present study, we have thus investigated in vitro the possible effect of TRH on alpha-MSH release from the lizard (Lacerta vivipara) neurointermediate lobe, by means of the perifusion technique. Using our radioimmunoassay procedure, we found that serial dilutions of L. vivipara NIL extracts and synthetic alpha-MSH gave parallel binding curves. Administration of graded doses of TRH (10(-8)-10(-6) M) did not cause any modification of alpha-MSH release. In contrast, infusion of a depolarizing concentration of K+ induced a robust stimulation of alpha-MSH secretion. These results indicate that, in the lizard L. vivipara, the neuropeptide TRH does not stimulate pituitary melanotrophs.

Co-localization of tyrosine hydroxylase, GABA and neuropeptide Y within axon terminals innervating the intermediate lobe of the frog Rana ridibunda

Possible co-existence of gamma-aminobutyric acid (GABA), catecholamines, and neuropeptide Y (NPY) in the same nerve terminals of the frog intermediate lobe was investigated by immunocytochemistry at the electron microscopic level. Co-localization of GABA and tyrosine hydroxylase (TH) was studied by using a double immunogold labeling procedure. Co-localization of glutamate decarboxylase (GAD) and NPY was studied by combining, respectively, the peroxidase-antiperoxidase method and a radioimmunocytochemical labeling procedure. Catecholamines and GABA were systematically co-localized in nerve endings of the pars intermedia. Most of the NPY-immunoreactive fibers also contained GAD-like immunoreactivity. However, a few NPY-positive nerve terminals were not immunoreactive for GAD. These data provide evidence for co-existence of a regulatory peptide (NPY) and several neurotransmitters (i.e., GABA and catecholamines) within the same axon terminals in the intermediate lobe. Since GABA, dopamine, and NPY have all been shown to inhibit the activity of frog melanotrope cells, the present findings suggest that these neuroendocrine factors may interact either at the pre-synaptic or post-synaptic level.

Structural analysis of the entire proopiomelanocortin gene of Xenopus laevis

In the pars intermedia of the pituitary the prohormone proopiomelanocortin (POMC) is tissue-specifically processed to, among other peptides, alpha-melanotropin (alpha MSH). In the South African clawed toad Xenopus laevis this hormone mediates the process of background adaptation: release of alpha-MSH causes darkening of the animal, while inhibition of alpha-MSH release results in a pale toad. Elevated release of alpha-MSH coincides with a higher rate of POMC gene transcription. The present study aims to find possible transcriptional regulatory elements in the Xenopus POMC gene. For that purpose the complete nucleotide sequence of the POMC gene and its 5'- and 3'- flanking regions were determined and analyzed. The Xenopus POMC gene promoter contains several regions which may be regulatory DNA elements in view of their similarity with corresponding regions of mammalian POMC gene promoters. In the rat POMC gene promoter, many of these regions represent protein-binding sequences. Besides the promoter sequence and the protein-coding sequences, no other segments with significant identity between the Xenopus and human POMC genes were found. Intron A of the Xenopus POMC gene contains a simple sequence, (TATC)76, and a JH12 repetitive element, while the 3'-flanking region contains a repetitive-EcoRI-monomer-2 element. Comparison of the JH12 sequence of the POMC gene with JH12 sequences from other Xenopus genes revealed a 335-bp consensus sequence which is flanked by a 30-bp inverted repeat. This JH12 consensus sequence is significantly larger than the previously reported JH12 core region. Alignment of intron B of the Xenopus POMC gene with database sequences revealed a consensus sequence of a novel Xenopus repetitive element of 330 bp flanked by a nearly perfect inverted repeat, indicating that this element may be a transposon-like element.

Detection of Met-enkephalin and Leu-enkephalin in the brain of the hagfish, Eptatretus stouti, and the lamprey, Petromyzon marinus

Acid extracts of the brain of the pacific hagfish, Eptatretus stouti, and the marine lamprey, Petromyzon marinus, were each fractionated by gel filtration chromatography and aliquots of column fractions were screened with radioimmunoassays (RIAs) specific for pro-dynorphin-related end products and for pro-enkephalin-related end products. Only pro-enkephalin-related immunoreactive forms were detected. The enkephalin-sized immunoreactive material, isolated for each species, was separately fractionated by reverse-phase high-performance liquid chromatography (HPLC). Aliquots of column fractions were screened with RIAs specific for Met-enkephalin, Leu-enkephalin, Met-enkephalin-Arg-Phe, and Met-enkephalin-Arg-Gly-Leu. In the hagfish brain, immunoreactive forms with the same retention times as synthetic Met-enkephalin and Leu-enkephalin were detected in a ratio of approximately 2:1. In addition, an immunoreactive form was detected with the Met-enkephalin-Arg-Phe-specific RIA. This form had the same chromatographic properties as synthetic Met-enkephalin-Arg-Phe. Analyses with the Met-enkephalin-Arg-Gly-Leu RIA were negative. HPLC analysis of the lamprey enkephalin-related material revealed the presence of authentic Met-enkaphalin and Leu-enkephalin in a molar ratio of 3:1. C-terminally extended forms of Met-enkaphalin were not detected in the lamprey extracts. Collectively these observations indicate that pro-enkephalin-related opioid peptides are present in the brain of cyclostomes.

A slow and a fast secretory compartment of POMC-derived peptides in the neurointermediate lobe of the amphibian Xenopus laevis

1. Peptide release from the neurointermediate lobe of Xenopus laevis has been studied using dual pulse-chase incubation, superfusion and HPLC techniques. 2. Lobes release pulse-labelled material in two phases, the first phase lasting about 6 hr, the second persisting up to 14 hr. 3. In both phases similar, POMC-derived peptides are released. Their release can be inhibited by dopamine. 4. When release during the first phase is inhibited, newly synthesized peptides are shunted into the second release pathway. 5. It is concluded that the neurointermediate lobe contains two release compartments. The possible locations of these compartments within melanotrope cells have been discussed.

The novel pituitary polypeptide 7B2 is a highly-conserved protein coexpressed with proopiomelanocortin

In the amphibian intermediate pituitary gland the biosynthetic activity for production of the precursor protein proopiomelanocortin (POMC) can be physiologically manipulated; POMC synthesis is high in animals adapted to a black background and low in white-adapted animals. In order to study genes associated with POMC gene expression we applied a differential hybridization technique involving screening of a pituitary cDNA library with probes derived from RNA of inactive and physiologically activated intermediate pituitary cells of the amphibian Xenopus laevis. A differentially hybridizing Xenopus pituitary cDNA clone encoded the novel polypeptide 7B2. This Mr-21,000 secretory granule-associated protein of unknown function is shown to be highly conserved between Xenopus and human (83% amino acid sequence similarity). Conserved segments within the 7B2 structure included the N-terminal portion, three pairs of basic amino acids which are potential recognition sites for proteolytic enzymes, and three regions sharing similarity with putative GTP-binding domains. Levels of 7B2 mRNA were about 3% of POMC mRNA levels in Xenopus pituitary glands. In the intermediate pituitary the amount of both POMC and 7B2 mRNA was much higher in black-adapted toads than in white-adapted animals. These physiologically-induced changes in POMC and 7B2 mRNA levels were not found in the anterior pituitary. We conclude that the POMC and 7B2 genes are coexpressed and that modulation of the activity of these genes is tissue-specific.

Three distinct thyrotropin-releasing hormone-immunoreactive axonal systems project in the median eminence-pituitary complex of the frog Rana ridibunda. Immunocytochemical evidence for co-localization of thyrotropin-releasing hormone and mesotocin in fibers innervating pars intermedia cells

The localization of thyrotropin-releasing hormone-immunoreactive structures was investigated in the hypothalamo-hypophyseal complex of the frog, Rana ridibunda, by light and electron microscopy using the conventional indirect immunoperoxidase technique and the immuno-gold technique, respectively. The localization of mesotocin-, vasotocin- and neurophysin-immunoreactive elements was compared to that of thyrotropin-releasing hormone either by comparing homologous fields on serial sections or by staining the same section with two different antibodies. Thyrotropin-releasing hormone-immunoreactive perikarya occurred mainly in the anterobasal periventricular area and dorsal extension of the preoptic nucleus, and in the lateral zone of the infundibular nucleus. In the anterobasal preoptic nucleus, the distribution of thyrotropin-releasing hormone-immunoreactive perikarya partially overlapped that of vasotocin- and mesotocin-containing neurons; however, co-localization of thyrotropin-releasing hormone with either nonapeptide could not be detected there. In contrast, in the caudal extension of the preoptic nucleus, thyrotropin-releasing hormone- and mesotocin-like immunoreactivities were frequently co-localized in the same neurons. In the external zone of the median eminence, abundant networks of thyrotropin-releasing hormone- and vasotocin-immunoreactive nerve fibers were found in the vicinity of portal capillaries, while mesotocin-immunoreactive axons were only found in the internal zone. Using the immuno-gold technique at the electron microscopic level, three distinct thyrotropin-releasing hormone-immunoreactive systems were identified in the median eminence-neurointermediate lobe complex. (1) In the external zone of the median eminence, a conspicuous population of pericapillary endings contained 100-nm dense core vesicles immunoreactive solely for thyrotropin-releasing hormone. (2) In the neural lobe of the pituitary, thyrotropin-releasing hormone immunoreactivity occurred on secretory vesicles in a subpopulation of the mesotocinergic axons containing 160-nm secretory granules; co-localization with vasotocin was never seen. (3) In the intermediate lobe, thyrotropin-releasing hormone- and mesotocin (or neurophysin I)-immunoreactivities were systematically found in the same 120-nm dense core vesicles; these thyrotropin-releasing hormone-/mesotocin-immunoreactive axon terminals frequently made synaptic contacts with melanotropic cells. The possible modulatory effect of mesotocin on thyrotropin-releasing hormone-induced alpha-melanocyte-stimulating hormone secretion was investigated using perifused frog neurointermediate lobes. Administration of graded doses of mesotocin (from 10(-10) to 10(-5) M) did not affect the spontaneous release of alpha-melanocyte-stimulating hormone. In addition, mesotocin (10(-7) and 10(-6) M) did not modify thyrotropin-releasing hormone-evoked alpha-melanocyte-stimulating hormone release.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of MSH release from the neurointermediate lobe of Xenopus laevis by CRF-like peptides

Immunocytochemical studies showed the presence of a fiber system containing a CRF-like peptide in the median eminence and in the neural lobe of the pituitary gland of Xenopus laevis. During in vitro superfusion of neurointermediate lobe tissue, CRF, sauvagine and urotensin I induced a rapid and dose-dependent stimulation of secretion of MSH and endorphin. Tissue of white-background adapted animals displayed a remarkably higher sensitivity to CRF and sauvagine than tissue from animals that were adapted to a black background. During superfusion of isolated melanotrope cells in suspension, it was shown that CRF and sauvagine exerted their effect directly on the melanotrope cell. We therefore conclude that there is morphological and biochemical evidence to consider a CRF-like peptide as a physiological MSH-releasing factor.

Isolation of immunoreactive beta-endorphin-related and Met-enkephalin-related peptides from the posterior pituitary of the amphibian, Xenopus laevis

Acid extracts of the posterior pituitary of the amphibian, Xenopus laevis, were analyzed with two heterologous region specific beta-endorphin RIAs. Following gel filtration chromatography and cation exchange chromatography four peaks of immunoreactivity were detected. All four peaks were detected with a N-acetyl specific beta-endorphin RIA. Peak I represented 92% of the total immunoreactivity isolated following cation exchange chromatography. This peak had a net positive charge at pH 2.5 of +1 and an apparent molecular weight of 1.4 Kd. Following reverse phase HPLC, Peak I fractionated into two peaks: Peak Ia and Peak Ib. Both peaks were detected with the N-acetyl specific beta-endorphin RIA and a Met-enkephalin RIA, however, neither peak co-migrated with either Met-enkephalin or N-acetyl-beta-endorphin(1-16). At present it is not clear whether Peak I is derived from pro-opiomelanocortin or one of the other opioid polyproteins. Peaks II, III, and IV represented 8% of the total immunoreactivity recovered following cation exchange chromatography. These peaks had net positive charges of +3, +4, and +5, respectively, and apparent molecular weights of 2.8, 3.2, and 3.5 Kd, respectively. These apparently N-acetylated beta-endorphin-sized forms are minor end products of the pro-opiomelanocortin biosynthetic pathway.

Structural organization of the proopiomelanocortin gene in Xenopus laevis. 5'-end homologies within the toad and mammalian genes

This study reports the isolation and characterization of the entire proopiomelanocortin (POMC) gene of the amphibian Xenopus laevis. The Xenopus POMC gene consists of three exons of which the main exon 3 codes for all of the bioactive domains of the precursor protein. Intron A (2.6 kb) separates the segments encoding the 5'-untranslated mRNA region and intron B (2.5 kb) interrupts the protein-coding sequence near the signal peptide coding region. In that this structural organization of the Xenopus POMC gene is similar to those of the mammalian genes, apparently the POMC gene has been remarkably stable during 350 million years of vertebrate evolution. A comparative analysis of the 5'-flanking sequences of the Xenopus and mammalian POMC genes reveals the presence of several conserved regions. One of these regions is homologous with sequences located upstream of the capping sites of other glucocorticoid-regulated genes and another region contains a segment reminiscent of a viral enhancer consensus sequence.

An NPY-like peptide may function as MSH-release inhibiting factor in Xenopus laevis

This study demonstrates the presence of a rich plexus of neuropeptide Y (NPY) immunoreactive fibers in the hypothalamus and in the intermediate lobe of the pituitary of Xenopus laevis. During superfusion of neurointermediate lobe tissue, synthetic NPY induces a rapid, powerful and dose-dependent inhibition of in vitro release of MSH, endorphin and other proopiomelanocortin (POMC) derived peptides. Therefore, NPY undoubtedly is one of the growing number of neuropeptides that are likely involved in control of the amphibian MSH cells. Although a number of stimulatory neuropeptides have been found, this is the first neuropeptide to apparently function through an inhibitory mechanism. In that a 2-hr treatment with NPY did not influence POMC biosynthesis, nor processing of this prohormone to smaller peptides, we conclude that the primary action of NPY is a direct effect on the secretory process of the MSH cell.

Assessment of TRH as a potential MSH release stimulating factor in Xenopus laevis

This study considers the possible involvement of the tripeptide TRH (thyrotropin releasing hormone) in the physiological regulation of melanophore stimulating hormone (MSH) secretion from the pars intermedia of the toad, Xenopus laevis. TRH was shown to stimulate release of MSH from superfused neurointermediate lobes obtained from white-background adapted animals, but had no effect on secretion from lobes of black-background adapted animals. Immunohistochemical analysis revealed a rich TRH-containing neuronal network terminating in the neural lobe of the Xenopus pituitary. Plasma levels of TRH, determined with a specific radioimmunoassay, proved to be extremely high and no significant difference in this level could be found between white- and black-adapted animals. Plasma TRH probably originates from the skin, and our results show that its concentration is within the effective concentration range established for this peptide in stimulating MSH release from the pars intermedia. Therefore, while both our superfusion and immunohistochemical results argue favourably for a function of TRH in the regulation of MSH secretion, we conclude that, in any regulatory role, it would likely have to function within the pars intermedia at concentrations exceeding the high plasma values. While TRH could be involved in short-term activation of the secretory process in white-background adapted animals or in animals undergoing the initial stages of black background adaptation, our results indicate that this peptide may have no function in the maintenance of secretion from the pars intermedia of animals fully adapted to black background.

GABA and dopamine act directly on melanotropes of Xenopus to inhibit MSH secretion

The release of melanophore stimulating hormone (MSH) from the pars intermedia of the amphibian Xenopus laevis is regulated by multiple factors of hypothalamic origin. The aim of this study was to determine if potential secretagogues function through a direct action on the melanotrope cell. For this purpose an in vitro superfusion system containing isolated melanotropes (cell suspension) was utilized. The viability of the cells in suspension was tested by examining their ability to synthesize, process and release pro-opiomelanocortin (POMC) related peptides. All biosynthetic functions appeared normal, with the exception that the isolated melanotropes are unable to N-terminally acetylate MSH. Release of immunoreactive-MSH from these cells was shown to be Ca2+-dependent, and high K+ stimulated release. Both the neurotransmitters dopamine and gamma-aminobutyric acid (GABA), which are thought to be physiologically important MSH-release inhibiting factors, were shown to inhibit MSH release from isolated melanotropes. Dopamine appeared to function through a dopamine D2 type receptor mechanism while for GABA, both a GABAa and GABAb receptor mechanism are involved.

Regulation of melanotropin release from the pars intermedia of the amphibian Xenopus laevis: evaluation of the involvement of serotonergic, cholinergic, or adrenergic receptor mechanisms

Melanophore-stimulating hormone (MSH) release from the pars intermedia of the pituitary gland is probably regulated by multiple factors of hypothalamic origin. We have examined a number of potential regulatory factors for their effects on MSH release from the amphibian Xenopus laevis. Serotonin and acetylcholine have no effect on MSH release. Both adrenaline and noradrenaline inhibit release of MSH in a dose-dependent manner. Studies with specific receptor agonists and antagonists reveal that these neurotransmitters exert their in vitro effects primarily through a dopamine D-2 receptor, although an alpha-adrenergic receptor could not be excluded. We further conclude that the pars intermedia of X. laevis lacks a beta-adrenergic receptor for the regulation of MSH secretion from the pars intermedia. In mammals, this receptor activates the adenylate cyclase system. Our studies reveal that despite the lack of beta-adrenergic receptors, cyclic-AMP is likely an intracellular factor involved in the stimulation of MSH release.

An investigation on pro-opiomelanocortin and processed peptides from the teleost fish Prochilodus platensis

Acid extracts of carefully dissected proadenohypophysis (PA) and metaadenohypophysis (MA) of the teleost Prochilodus platensis were subjected to chromatography in Sephadex G-50 after which several pro-opiomelanocortin (POMC) peptides were detected by means of three heterologous RIA systems: alpha-MSH, ACTH and beta-endorphin. Parallelism among extracts displacement curves ranged from 26% to 95% of those of the standard curves for the different systems employed. In PA chromatograms, peaks of ACTH immunoreactivity (IR) were detected at the positions of 30 kilodalton (K), 20K, 9K, a large 4.5K peak and 2K. Only one peak of beta-endorphin IR was detected at 30K. In MA chromatograms, ACTH IR detected similar peaks as in PA runs, but 4.5K peak was much smaller, whereas a large 2K peak roughly coincided with all alpha-MSH detected in the chromatograms. beta-Endorphin IR was detected mainly as a large peak coinciding with synthetic beta-endorphin in MA runs. Bioactivity was detected in both PA and MA 4.5K ACTH peaks, whereas little activity could be demonstrated associated with the 30K, 20K and 9K ACTH IR peaks. Prochilodus PAs and MAs were incubated with tritiated aminoacids and the extracts immunoprecipitated with ACTH, beta-endorphin and N-terminal POMC (N-POMC) antisera. The dissociated complexes were run in SDS polyacrylamide slab gel electrophoresis. The tritiated bands detected confirmed the results obtained with Sephadex chromatography. N-POMC immunoprecipitated peptides were located at 28K, 18K and 9K positions. The first two probably accounted for POMC and the N-POMC/ACTH intermediate respectively; the third corresponded to the mammalian 1-76N-POMC.(ABSTRACT TRUNCATED AT 250 WORDS)

Separation and partial characterization by high-performance liquid chromatography and radioimmunoassay of different forms of melanocyte-stimulating hormone from fish (Cyprinidae) neurointermediate lobes

From neurointermediate lobe (NIL) extracts of two species of Cyprinidae, Carassius auratus and Cyprinus carpio, several peptides were separated by high-performance liquid chromatography (HPLC) on a C18 muBondapak column eluted with a methanol/acetic acid/triethylamine mixture. Monitoring all fractions by radioimmunoassay (RIA) with an antibody against melanocyte-stimulating hormone (MSH) C terminal gave positive reactions for fractions 7, 11-12, 15-16, 23-24, and 25-27. For further characterization, the elution positions of these peaks were compared to those of known synthetic reference substances. Peak 7 elutes in the same position as oxidized alpha MSH, whereas peak 15-16 matches the elution position of des-acetyl alpha MSH and 23-24 that of alpha MSH. The product from peak 26-27 has several characteristics of the diacetylated form of alpha MSH: its immunoreactivity in RIA, its sensitivity to weak bases and to HCl and its mass spectrum which is identical with that of mammalian diacetyl alpha MSH. In both species, the diacetylated form is predominant in the intracellular pool. This study establishes the coexistence of three different forms of alpha MSH, a des-acetylated, monoacetylated, and diacetylated in the cyprinid NIL extracts.

Biosynthesis, processing and release of pro-opiomelanocortin related peptides in the intermediate lobe of the pituitary gland of the frog (Rana ridibunda)

The biosynthesis of pro-opiomelanocortin (POMC) and related peptides by the intermediate lobe of the pituitary gland was studied in the frog Rana ridibunda using the pulse-chase technique. Analysis of radioactive proteins by dodecyl sulfate polyacrylamide gel electrophoresis showed that during pulse incubations a 36,000 dalton (36K) glycosylated prohormone was synthesized. It disappeared slowly during chase incubations, giving rise to another glycosylated protein (Mr 18K), identified as the N-terminal fragment of POMC. This latter protein was secreted to the incubation medium. High performance liquid chromatography analysis of peptides synthesized during chase incubations revealed the biosynthesis of two peptides related to gamma-MSH, three peptides related to alpha-MSH, one endorphin-related and one CLIP-related peptides. These newly synthesized peptides were slowly secreted to the incubation medium. Among the alpha-MSH related peptides, only the des-N alpha-acetyl alpha-MSH form of the peptide was found to be present within the cells, in contrast to the incubation medium where the presence of des-N alpha-acetyl alpha-MSH and a modified alpha-MSH was demonstrated.

The development of the pars intermedia and its role in the regulation of dermal melanophores in the larvae of the amphibian Xenopus laevis

The ontogenesis of biosynthesis of pro-opiomelanocortin (POMC)-related peptides in the pars intermedia of Xenopus laevis tadpoles was studied. The results were related to the capacity of the animal to adapt to background color through regulation of pigment dispersion in dermal melanophores. Using immunocytochemical techniques with antisera to alpha-melanophore-stimulating hormone (alpha-MSH), it was revealed that this peptide first appeared at developmental stage 37/38, just prior to the animal's ability to adapt to background. It was shown that pigment dispersion in melanophores between stages 33 and 39 was not dependent on melanotropins of pituitary origin. Using in vitro biosynthetic studies it was possible to follow POMC biosynthetic activity, its processing and the release of peptides from stage 48 onward. Among the newly synthesized peptides observed were a gamma 3-MSH-like peptide, des-N-alpha-acetyl-alpha-MSH, alpha-MSH, and two endorphin-like peptides. By stage 57 a biosynthetic pattern almost identical to that of the adult pars intermedia had evolved. It was concluded that stage 39/40 is a critical stage in the simultaneous development of a number of the components involved in the neuroendocrine control of background adaptation.

Evidence for a proenkephalin-like precursor in amphibian brain

The mammalian proenkephalin-derived peptides [Leu]enkephalin, [Met]enkephalin, and [Met]enkephalin-Arg6-Phe7 were identified in acid extracts of the brain of Bufo marinus by using reversed-phase HPLC and specific radioimmunoassays. [Met]Enkephalin was the predominant opioid peptide present (270 pmol/g). In contrast, the octapeptide [Met]enkephalin-Arg6-Gly7-Leu8, which is also derived from mammalian proenkephalin, was not detected. The ratio of free [Met]enkephalin to [Met]enkephalin-Arg6-Phe7 was found to be 3.5 to 1, which is similar to that observed in mammalian proenkephalin-containing tissues. Together these data (i) indicate that amphibian brain contains a proenkephalin related to the mammalian precursor and (ii) establish the existence of enkephalins and proenkephalin-derived enkephalin-containing peptides in a submammalian species.

Hydroxylamine cleavage of proteins in polyacrylamide gels

A modification of the hydroxylamine cleavage of proteins is presented in which proteins were cleaved while immobilized in the matrix of a polyacrylamide gel. The reaction under these conditions retains its high specificity for Asn-Gly bonds and has the advantage that the gel matrix, acting as a carrier, facilitates simultaneous treatment of many samples, and contributes to a high recovery efficiency (60-90%) of the cleavage products. The cleavage is performed with individual protein bands excised from dried slab gels after detection by staining, autoradiography, or fluorography. The procedure can be easily combined with other techniques to further characterize the cleavage fragments. Also a two-dimensional version of the cleavage method was developed, which allows rapid recognition of interrelationships between proteins in a complicated mixture. The versatility of the procedure is demonstrated in a number of applications. Highly related strains of murine leukemia viruses were easily distinguished from one another by the unique cleavage patterns of their gag- and env-precursor polypeptides. Comparing the env-precursor gPr82env synthesized in the presence or absence of tunicamycin with its cell-free synthesized counterpart, revealed the presence of an amino-terminal signal sequence. Cleavage patterns of pro-opiomelanocortin (POMC) from three different species revealed a high degree of homology between rat and mouse POMC, whereas Xenopus POMC was very different. Regions to which carbohydrates are attached could be identified by comparing glycosylated and unglycosylated forms of POMC. Combining the hydroxylamine cleavage procedure with immunological characterization of the fragments showed a small but significant difference between the amino-terminal sequences of the recombinant transforming protein P120 of Abelson murine leukemia virus and of its parent molecule Pr65gag of Moloney murine leukemia virus.

In vivo biosynthesis of melanotropins and related peptides in the pars intermedia of Xenopus laevis

To study in vivo biosynthesis of pars intermedia peptides in Xenopus laevis, [3H]lysine was administered by an osmotic minipump via a cannula inserted near the pituitary gland. Following extraction of the neurointermediate lobe, high-performance liquid chromatography was used to separate the newly synthesized peptides. In black-background adapted animals, [3H]lysine was incorporated into a number of peptides. The elution characteristics of these peptides corresponded exactly with those of peptides synthesized during in vitro incubation of neurointermediate lobes, and which were identified as des-N alpha-acetyl-alpha-MSH, a gamma-MSH-like peptide, two corticotropin-like intermediate lobe peptides, and two forms of endorphin. In white-background adapted Xenopus, practically no synthesis of pars intermedia peptides occurred. Transfer of black-adapted toads to a white background at the beginning of infusion led to storage of newly synthesized peptides. When such animals were maintained on a white background for 10 days, des-N alpha-acetyl-alpha-MSH, but not alpha-MSH, was present in the pars intermedia; this supports the notion that des-N alpha-acetyl-alpha-MSH constitutes the "storage form" of alpha-MSH.

Biosynthesis of a gamma 3-melanotropin-like peptide in the pars intermedia of the amphibian pituitary gland

This study reveals the biosynthesis of a gamma 3-melanotropin-like peptide in the pars intermedia of the pituitary gland of the aquatic toad Xenopus laevis. Pulse-chase experiments in vitro showed that this product is synthesized through processing of a prohormone, pro-opiomelanocortin, and that it is released into the incubation medium. The peptide immunoprecipitated with antiserum to gamma 3-melanotropin, appeared to be a glycopeptide and displayed melanotropic activity. This last observation together with the results of tryptic and chymotryptic peptide mapping of the newly synthesized product indicates that Xenopus gamma 3-melanotropin is structurally different from the proposed mammalian gamma-melanotropins.

Biosynthesis of two structurally different pro-opiomelanocortins in the pars intermedia of the amphibian pituitary gland

This study reports the biosynthesis of two forms of pro-opiomelanocortin in the pars intermedia of the pituitary gland of the African clawed toad Xenopus laevis. The two forms could be resolved by dodecyl sulphate gel electrophoresis on a 9-16% acrylamide gradient and their molecular weights were 38200 and 37300. Incubation of neurointermediate lobes with [3H]glucosamine followed by tryptic digestion of the newly synthesized glycoproteins, revealed that both prohormones have only one glycosylated site, namely within the region corresponding to gamma 3-melanotropin. Biosynthesis of proteins in lobes treated with tunicamycin to prevent glycosylation again resulted in the production of two pro-opiomelanocortins (Mr 35000 and 34200), indicating that the two forms differ in their primary structure. This notion was corroborated by the results of tryptic mapping of the newly synthesized prohormones. The maps showed that the primary structures of the two forms of pro-opiomelanocortin differ in at least two parts of the molecules, one part concerning the endorphin region.

Pro-opiocortin processing in the pituitary: a model for neuropeptide biosynthesis

The biosynthesis of alpha-MSH, beta-endorphin and ACTH in the pituitary is reviewed. These neuropeptides are synthesized from a common pro-protein, pro-opiomelanocortin. The pro-protein is cleaved intragranularly, at pairs of basic residues in the molecule to yield the respective peptide products. An unique, thiol protease (pro-opiocortin converting enzyme) and a carboxypeptidase B-like enzyme, both localized within pituitary secretory granules and having a pH optimum of 5-6, appear to be involved in the proteolytic processing of pro-opiomelanocortin.