Nucleotide sequence of cloned cDNA for pro-opiomelanocortin in the amphibian Xenopus laevis

Melanocortin Regulation of Inflammation

Adrenocorticotropic hormone (ACTH), and α-, β-, and γ-melanocyte-stimulating hormones (α-, β-, γ-MSH), collectively known as melanocortins, together with their receptors (melanocortin receptors), are components of an ancient modulatory system. The clinical use of ACTH in the treatment of rheumatoid arthritis started in 1949, originally thought that the anti-inflammatory action was through hypothalamus-pituitary-adrenal axis and glucocorticoid-dependent. Subsequent decades have witnessed extensive attempts in unraveling the physiology and pharmacology of the melanocortin system. It is now known that ACTH, together with α-, β-, and γ-MSHs, also possess glucocorticoid-independent anti-inflammatory and immunomodulatory effects by activating the melanocortin receptors expressed in the brain or peripheral immune cells. This review will briefly introduce the melanocortin system and highlight the action of melanocortins in the regulation of immune functions from in vitro, in vivo, preclinical, and clinical studies. The potential therapeutic use of melanocortins are also summarized.

Characterization of proopiomelanocortin in the snakeskin gourami (Trichopodus pectoralis) and its expression in relation to food intake

Proopiomelanocortin (POMC) is the precursor of several hormones involved in physiological systems including feed intake. The snakeskin gourami (Trichopodus pectoralis) POMC complementary DNA (TpPOMC) was cloned and characterized. Phylogenetic analysis showed that TpPOMC was clustered in a major POMC lineage in fish. Analysis of the Ka to Ks ratios for the entire POMC sequence and for each hormonal segment suggested that different POMC-derived peptide segments were subject to different evolutionary pressures. High expression level of TpPOMC was observed in all brain regions, with the highest levels in the diencephalon and pituitary gland. In situ hybridization also revealed that TpPOMC-expressing cells were distributed in discrete brain regions. The transcription level of TpPOMC was also found at moderate levels in several peripheral tissues, including gills, liver, head kidney, trunk kidney, stomach, intestine, spleen, ovary and testis, and at a low level in muscle. The expression level of TpPOMC was evaluated in each brain region (telencephalon, mesencephalon, metencephalon, and diencephalon together with the pituitary gland) at 1 h before the first and the last meals of the day and compared with expression levels at a time interval between the first and the last meals of the day. Low expression levels of TpPOMC were found at 1 h before the last meal of the day (P < 0.05). These finding suggest that decreased POMC expression level may lead to reduced melanocyte-stimulating hormones, which may in part be responsible for stimulating food intake. The effect of short-term fasting (24 h) on TpPOMC expression level in each brain region was also investigated. In telencephalon and diencephalon together with the pituitary gland, TpPOMC messenger RNA reached a nadir at 12 h of fasting, whereas TpPOMC transcript showed a nadir at 6 h of fasting in metencephalon and mesencephalon. A peak of TpPOMC level was observed at 18 h of fasting in metencephalon and diencephalon together with the pituitary gland. These findings suggest that TpPOMC expression is affected by nutritional status.

cDNA cloning, pituitary location, and extra-pituitary expression of pro-opiomelanocortin gene in rare minnow (Gobiocypris rarus)

A cDNA encoding pro-opiomelanocortin (POMC) gene was cloned from the pituitary gland of the rare minnow (Gobiocypris rarus), a small freshwater fish endemic to China. This was achieved by reverse transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE). Data showed that the predicted rare minnow POMC (rmPOMC) cDNA consisted of 846bps coding for the following sequences, flanked by proteolytic cleavage sites: signal peptide (SP, Met(1)-Ala(28)), N-terminal peptide (Gln(29)-His(105)), ACTH (Ser(108)-Met(146)), α-MSH (Ser(108)-Gal(121)), CLIP (Pro(126)-Met(146)), β-LPH (Glu(149)-His(221)), γ-LPH (Glu1(49)-Ser(186)), β-MSH (Asp(170)-Ser(186)), and β-endorphin (β-EP, Tyr(189)-Gln(221)). Sequence analysis showed no region was homologous to γ-MSH (a tetrapod POMC feature). The amino acid sequence is highly similar to POMC-I and POMC-II of the common carp (92.4%), according to homologous alignment. It was POMCα through the phylogenetic analysis. Pituitary and extra-pituitary expression were studied using RT-PCR and in situ hybridization. The rmPOMC-positive cells were mainly located in the rostral pars distalis (RPD) and pars intermedia (PI). Some rmPOMC-positive cells were detected in the proximal pars distalis (PPD) as well, according to in situ hybridization. In the extra-pituitary tissues, positive signals were observed in the brain, intestines, gonads, hepatopancreas, spleen, and gills by RT-PCR analysis.

Are lungfish living fossils? Observation on the evolution of the opioid/orphanin gene family

This minireview considers the possibility that there is a correlation between the slow rate of morphological change and speciation events that has been occurred within the lungfish lineage since the Permian period, and the apparent slow rate of divergence in the amino acid sequences of lungfish opioid precursor sequences. The status of lungfish as "living fossils" is considered.

In Silico Identification of New Secretory Peptide Genes in Drosophila melanogaster

Bioactive peptides play critical roles in regulating most biological processes in animals. The elucidation of the amino acid sequence of these regulatory peptides is crucial for our understanding of animal physiology. Most of the (neuro)peptides currently known were identified by purification and subsequent amino acid sequencing. With the entire genome sequence of some animals now available, it has become possible to predict novel putative peptides. In this way, BLAST (Basic Local Alignment Searching Tool) analysis of the Drosophila melanogaster genome has allowed annotation of 36 secretory peptide genes so far. Peptide precursor genes are, however, poorly predicted by this algorithm, thus prompting an alternative approach described here. With the described searching program we scanned the Drosophila genome for predicted proteins with the structural hallmarks of neuropeptide precursors. As a result, 76 additional putative secretory peptide genes were predicted in addition to the 43 annotated ones. These putative (neuro)peptide genes contain conserved motifs reminiscent of known neuropeptides from other animal species. Peptides that display sequence similarities to the mammalian vasopressin, atrial natriuretic peptide, and prolactin precursors and the invertebrate peptides orcokinin, prothoracicotropic hormones, trypsin modulating oostatic factor, and Drosophila immune induced peptides (DIMs) among others were discovered. Our data hence provide further evidence that many neuropeptide genes were already present in the ancestor of Protostomia and Deuterostomia prior to their divergence. This bioinformatic study opens perspectives for the genome-wide analysis of peptide genes in other eukaryotic model organisms.

Analyzing the radiation of the melanocortins in amphibians: cloning of POMC cDNAs from the pituitary of the urodele amphibians, Amphiuma means and Necturus maculosus

Proopiomelanocortin (POMC) cDNAs were cloned and sequenced from brain extracts of two species of urodele amphibians: Amphiuma means and Necturus maculosus. Although the two species of urodele amphibians belong to separate families, and do not share a direct common ancestor, the level of primary sequence identity for the open reading of the POMC cDNAs was 90% at the amino acid level and 79% at the nucleotide level. It appears that the POMC gene in these urodele amphibians has been accumulating mutations at the amino acid level at a slower rate than the POMC gene in other sarcopterygian orders.

Trends in the evolution of the proopiomelanocortin gene

The POMC gene is perhaps the most extensively studied member of the opioid/orphanin gene family. In Phylum Chordata this gene has been characterized in representatives of every class within the Gnathostomata, as well as in one representative agnathan vertebrate, the marine lamprey. This review provides a systematic overview of trends in the evolution of the melanocortins (ACTH/alpha-MSH, beta-MSH, gamma-MSH, and delta-MSH) and beta-endorphin in gnathostomes, and advances the hypothesis that the appearance of gamma-MSH occurred early in the radiation of the gnathostomes. A summary of the extensive work on POMC genes in the marine lamprey is also provided, as well as a reevaluation of the conserved regions in the sequence of CLIP (corticotropin-like-intermediate lobe peptide) in the POMC sequences of the various groups of gnathostomes.

Mutational analysis of evolutionarily conserved ACTH residues

alpha-Melanocyte stimulating hormone (MSH) and adrenocorticotropin (ACTH)1-24, the minimal ACTH sequence required for full activity, differ only by the 10 C-terminal amino acids of ACTH1-24. Interestingly, these ten C-terminal residues have been highly conserved throughout vertebrate evolution. To understand the functional constraints of these 10 amino acids we analyzed the effects of mutating these residues on steroidogenic activity in vivo and in vitro. Alanine substitutions of some of the first four amino acid residues (the basic core residues KKRR, 15-18) greatly reduces ACTH activity in vitro and in vivo; replacement of mutant alanines at residues 15 and 17 with glutamine residues partially restores ACTH activity. Thus, for ACTH receptor binding and activation, the amino acid residues 15-18 are important for their side chains. Surprisingly, conversion of the five C-terminal residues (20-24) to alanines increases ACTH activity in vivo over that of native ACTH. With respect to receptor binding and activity, the last five amino acid residues are important only for the peptide length they contribute; however, with respect to serum stability, their side chains are significant.

Presence of the delta-MSH sequence in a proopiomelanocortin cDNA cloned from the pituitary of the galeoid shark, Heterodontus portusjacksoni

Since a fourth MSH sequence, delta-MSH, has been detected in the proopiomelanocortin (POMC) gene of a dogfish and a stingray, members of superorder Squalea (class Chondrichthyes), it is possible that this novel MSH sequence might be a feature common to the POMC genes of all modern sharks and rays. As an initial step towards addressing this question, a full-length POMC cDNA was cloned and sequenced from the pituitary of the Port Jackson shark, Heterodontus portusjacksoni. The Port Jackson shark represents one of the oldest lineages in superorder Galea, and this superorder together with superorder Squalea form infraclass Neoselachii (the extant sharks and rays). The Port Jackson shark POMC cDNA has an open reading frame that is 1032 nucleotides in length and encodes the deduced amino acids sequences for beta-endorphin, ACTH/alpha-MSH, beta-MSH, gamma-MSH, and delta-MSH. Port Jackson shark delta-MSH has 83% primary sequence identity with dogfish and stingray delta-MSH, and it appears that the delta-MSH sequence may have been the result of an internal domain duplication and reinsertion of the beta-MSH sequence. The presence of the delta-MSH sequence in the POMC genes of representatives of both superorders of infraclass Neoselachii would indicate that the delta-MSH sequence must have been present in the ancestral euselachian shark that gave rise to the neoselachian radiation.

Occurrence of beta-endorphin binding sites in the pituitary of the frog Rana esculenta: effect of beta-endorphin on luteinizing hormone secretion

The possible effect of proopiomelanocortin-derived peptide, beta-endorphin on frog gonadotrope cells was investigated. Binding and internalization of beta-endorphin to pituitary pars distalis cultured cells were visualized by immunofluorescence and analyzed by means of confocal laser scanning microscopy. Using biotinylated endorphin, the time-course of beta-binding showed that this opioid was internalized through receptor-mediated endocytosis, the mechanism in which actin and clathrin were involved; then, the lysosomal degradation program occurred at later stages. The beta-endorphin binding was well antagonized by Naloxone, the opiate receptor antagonist, and up-regulated since more rapid response was obtained in the previously primed cells. The double immunostaining reaction for beta-endorphin and LH beta-subunit revealed that half the beta-endorphin labeled cell population was positively immunostained for LH beta-subunit, and beta-endorphin was able to induce an increasing trend of LH secretion in cultured pars distalis cells. Therefore, it seems that beta-endorphin acts directly on pituitary pars distalis and influences gonadotropin secretion through the interaction with its own receptor.

Analyzing the evolution of the opioid/orphanin gene family

Advances in molecular biology have made it possible to rapidly obtain the amino acid sequence of neuropeptide precursors-either by cloning and sequencing the cDNA that encodes the precursor, or by reconstructing the arrangement of exons and introns in a neuropeptide-coding gene through genomic approaches. The databases generated from these molecular approaches have been used to design probes to identify the cells that express the gene, or to ascertain the rate of expression of the gene, and even to predict the post-translational modifications that can generate functional neuropeptides from a biologically inert precursor. Although the power of these approaches is substantial, it is appreciated that a gene sequence or an mRNA sequence reflects the potential products that may be assembled in a secretory cell. To understand the functional capabilities of the secretory cell, the molecular genetics approaches must be combined with procedures that actually characterize the end-products generated by the secretory cell. Recent advances in two-dimensional gel electrophoresis and mass spectrometry now make it possible to analyze neuropeptides from a relatively small amount of tissue. These procedures can reveal novel end-products, tissue-specific endoproteolytic cleavage events, and developmental shifts in post-translational processing schemes. A gene family that illustrates all of these processes and the advantages of combining genomics with proteomics is the opioid/orphanin gene family.

Cladistic analysis of anuran POMC sequences

Procedures for performing cladistic analyses can provide powerful tools for understanding the evolution of neuropeptide and polypeptide hormone coding genes. These analyses can be done on either amino acid data sets or nucleotide data sets and can utilize several different algorithms that are dependent on distinct sets of operating assumptions and constraints. In some cases, the results of these analyses can be used to gauge phylogenetic relationships between taxa. Selecting the proper cladistic analysis strategy is dependent on the taxonomic level of analysis and the rate of evolution within the orthologous genes being evaluated. For example, previous studies have shown that the amino acid sequence of proopiomelanocortin (POMC), the common precursor for the melanocortins and beta-endorphin, can be used to resolve phylogenetic relationships at the class and order level. This study tested the hypothesis that POMC sequences could be used to resolve phylogenetic relationships at the family taxonomic level. Cladistic analyses were performed on amphibian POMC sequences characterized from the marine toad, Bufo marinus (family Bufonidae; this study), the spadefoot toad, Spea multiplicatus (family Pelobatidae), the African clawed frog, Xenopus laevis (family Pipidae) and the laughing frog, Rana ridibunda (family Ranidae). In these analyses the sequence of Australian lungfish POMC was used as the outgroup. The analyses were done at the amino acid level using the maximum parsimony algorithm and at the nucleotide level using the maximum likelihood algorithm. For the anuran POMC genes, analysis at the nucleotide level using the maximum likelihood algorithm generated a cladogram with higher bootstrap values than the maximum parsimony analysis of the POMC amino acid data set. For anuran POMC sequences, analysis of nucleotide sequences using the maximum likelihood algorithm would appear to be the preferred strategy for resolving phylogenetic relationships at the family taxonomic level.

Dynamics and plasticity of peptidergic control centres in the retino-brain-pituitary system of Xenopus laevis

This review deals particularly with the recent literature on the structural and functional aspects of the retino-brain-pituitary system that controls the physiological process of background adaptation in the aquatic toad Xenopus laevis. Taking together the large amount of multidisciplinary data, a consistent picture emerges of a highly plastic system that efficiently responds to changes in the environmental light condition by releasing POMC-derived peptides, such as the peptide alpha-melanophore-stimulating hormone (alpha-MSH), into the circulation. This plasticity is exhibited by both the central nervous system and the pituitary pars intermedia, at the level of molecules, subcellular structures, synapses, and cells. Signal transduction in the pars intermedia of the pituitary gland of Xenopus laevis appears to be a complex event, involving various environmental factors (e.g., light and temperature) that act via distinct brain centres and neuronal messengers converging on the melanotrope cells. In the melanotropes, these messages are translated by specific receptors and second messenger systems, in particular via Ca(2+) oscillations, controlling main secretory events such as gene transcription, POMC-precursor translation and processing, posttranslational peptide modifications, and release of a bouquet of POMC-derived peptides. In conclusion, the Xenopus hypothalamo-hypophyseal system involved in background adaptation reveals how neuronal plasticity at the molecular, cellular and organismal levels, enable an organism to respond adequately to the continuously changing environmental factors demanding physiological adaptation.

Molecular cloning of proopiomelanocortin cDNA from an elasmobranch, the stingray, Dasyatis akajei

Recently, we have characterized a new MSH (named delta-MSH) which joins the group of MSHs (alpha, beta, gamma) in dogfish proopiomelanocortin (POMC). The present study has confirmed the presence of delta-MSH in POMC of another member of the elasmobranchian order, the stingray, Dasyatis akajei, by cDNA cloning from pituitary mRNAs. Overlapping partial cDNA clones corresponding to stingray POMC were amplified by PCR from single-strand cDNA prepared from pituitary poly (A)(+) RNA. Excluding the poly A tail, stingray POMC cDNA consists of 1077 base pairs (bp). It contains a 912-bp open reading frame encoding a signal peptide of 24 amino acids (aa) and a POMC of 280 aa. gamma-MSH, alpha-MSH, ACTH, delta-MSH, beta-MSH, and beta-endorphin are located at POMC (50-61), (115-127), (115-153), (182-193), (226-242), and (245-280), respectively. The stingray POMC is smaller than that of the dogfish POMC (294 aa) mainly due to the absence of a sequence of 11 consecutive aa between delta-MSH and beta-MSH. delta-MSH has been found only in the elasmobranchs and, therefore, delta-MSH might have evolved after the divergence of chondrichthians from the ancestral vertebrate lineage and before divergence of sharks and rays.

Cloning of a neoteleost (Oreochromis mossambicus) pro-opiomelanocortin (POMC) cDNA reveals a deletion of the gamma-melanotropin region and most of the joining peptide region: implications for POMC processing

A signature feature of tetrapod pro-opiomelanocortin (POMC) is the presence of three melantropin (MSH) coding regions (alpha-MSH, beta-MSH, gamma-MSH). The MSH duplication events occurred early during the radiation of the jawed vertebrates well over 400 million years ago. However, in at least one order of modern bony fish (subdivision Teleostei; order Salmoniformes; i.e. salmon and trout) the gamma-MSH sequence has been deleted from POMC. To determine whether the gamma-MSH deletion has occurred in other teleost orders, a POMC cDNA was cloned from the pituitary of the neoteleost Oreochromis mossambicus (order Perciformes). In O. mossambicus POMC, the deletion is more extensive and includes the gamma-MSH sequence and most of the joining peptide region. Because the salmoniform and perciform teleosts do not share a direct common ancestor, the gamma-MSH deletion event must have occurred early in the evolution of the neoteleost fishes. The post-translational processing of O. mossambicus POMC occurs despite the fact that the proteolytic recognition sequence, (R/K)-Xn-(R/K) where n can be 0, 2, 4, or 6, a common feature in mammalian neuropeptide and polypeptide hormone precursors, is not present at several cleavage sites in O. mossambicus POMC. These observations would indicate that either the prohormone convertases in teleost fish use distinct recognition sequences or vertebrate prohormone convertases are capable of recognizing a greater number of primary sequence motifs around proteolytic cleavage sites.

Cloning of a proopiomelanocortin cDNA from the pituitary of the Australian lungfish, Neoceratodus forsteri: analyzing trends in the organization of this prohormone precursor

The polypeptide hormone precursor, proopiomelanocortin (POMC), was cloned and sequenced from the pituitary of the Australian lungfish, Neoceratodus forsteri, the only surviving species of the oldest extant lineage of lungfish. The Australian lungfish POMC cDNA had an open reading frame that coded for a 255-amino acid precursor. A comparison of POMC sequences from the Australian lungfish and the African lungfish indicated that the deduced amino acid sequences for ACTH, beta-MSH, and beta-endorphin were over 90% identical. Furthermore, within the open reading frames of the two lungfish POMCs, there was 84% identity at the nucleotide level. Although a gamma-MSH-like region was detected in the Australian lungfish POMC cDNA, this sequence contained mutations that have been detected in the gamma-MSH sequences of some ray-finned fish and are not found in the gamma-MSH sequence of the African lungfish or those of tetrapods. In addition, the sequence of beta-endorphin in the two species of lungfish has amino acid motifs that are found in the beta-endorphin sequences of cartilaginous fish and ray-finned fish but not in tetrapods. However, maximum parsimony analysis of the entire POMC open reading indicated that the lungfish POMC sequences form a clade with two amphibian POMC sequences rather than with POMC sequences from ray-finned fish. This result is consistent with the accepted view that the sarcopterygians (lungfishes and tetrapods) are a monophyletic assemblage. Analysis of rates of divergence for various POMC sequences indicate that point mutations are accumulating in the lungfish POMC sequences at a slower rate than in either amphibian or mammalian POMC sequences. The phylogenetic implications of these observations are discussed.

Duplication of the POMC gene in the paddlefish (Polyodon spathula): analysis of gamma-MSH, ACTH, and beta-endorphin regions of ray-finned fish POMC

The proopiomelanocortin (POMC) gene, which encodes the common precursor for MSH-related and beta-endorphin-related end products, appeared early in chordate evolution and features a variety of lineage-specific modifications. Key among these has been the apparent degeneration and subsequent deletion of the gamma-MSH region during the evolution of POMC in the ray-finned fish. A second area of increasing focus has been the role of gene duplication in the evolution of POMC in particular and the opioid/orphanin gene family in general. The cloning and phylogenetic analysis of two POMC cDNAs from the paddlefish (Polyodon spathula) is reported here and biochemical data on their processed end products are presented. Based on conceptual amino acid translations, the paddlefish cDNAs encode all functional domains and, in most cases, the flanking paired-basic amino acid cleavage sites characteristic of gnathostome POMCs (i.e., signal sequence, gamma-MSH-like region, ACTH (alpha-MSH and CLIP), gamma-LPH, beta-MSH, and beta-endorphin). Phylogenetic analysis of the paddlefish POMC sequences in the context of the duplicated POMCs of sturgeon and salmonids suggests that degeneration of the gamma-MSH core sequence and its amino-terminal proteolytic cleavage site was initiated prior to divergence of the sturgeon and paddlefish lineages over 150 mya. Finally, a comparison of the relative rates of evolutionary divergence between paralogously related POMC genes within chondrostean and salmonid lineages provides potential support for the hypothesis that some taxa (e.g., the Chondrosteii) represent relic species as a result of an exceptionally slow rate of evolutionary change.

Enhancement by proopiomelanocortin-derived peptides of growth hormone and prolactin secretion by bullfrog pituitary cells

Corticotrophs in the bullfrog (Rana catesbeiana) are situated mainly in the rostral region of the anterior lobe of the pituitary gland, which receives its blood supply primarily from the portal vessel. On the assumption that the proopiomelanocortin (POMC)-derived peptides released into the pituitary circulation may influence the function of other pituitary cells situated downstream, the effects of three POMC-derived peptides, namely, N-terminal peptide of POMC (NPP), adrenocorticotropic hormone (ACTH), and joining peptide (JP), on the secretion of growth hormone (GH) and prolactin (PRL) by bullfrog dispersed anterior pituitary cells were examined. NPP and ACTH, but not JP, stimulated the release of GH and PRL in a concentration-dependent manner. It was also found that ACTH1-17, but not alpha-melanocyte-stimulating hormone, was effective in enhancing GH and PRL release. A marked difference between the response to NPP and ACTH and the response to thyrotropin-releasing hormone employed as a reference secretagogue in terms of the time required for stimulating the release of GH and PRL was noted. Northern blot analysis of GH and PRL mRNA levels and radioimmunoassay for GH and PRL in the cultured cells revealed that ACTH increases the syntheses of both pituitary hormones as well. The possibility that NPP and ACTH act on neighboring cells to maintain their overall secretory function is discussed.

Analysis by mass spectrometry of POMC-derived peptides in amphibian melanotrope subpopulations

We have previously shown that the melanotrope population of the pituitary intermediate lobe of Rana ridibunda is composed of two subpopulations, of low (LD) and high density (HD), that show distinct ultrastructural features and display different synthetic and secretory rates. To investigate whether LD and HD melanotrope cells also differ in proopiomelanocortin (POMC) processing, we have analyzed the POMC-end products in single cells from both subpopulations by means of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). The mass spectra revealed the presence of 8 POMC-derived peptides in HD and LD melanotrope cells, indicating a similar processing of the precursor in both subpopulations. However, the relative abundance of three POMC-end products (i.e. lys-gamma1-MSH, acetyl-alpha-MSH, and CLIP fragment) was higher in the HD subset. Moreover, two peptides with molecular weights of 1030 and 1818 Da, respectively, were detected that could not be assigned to any product deduced from the frog POMC sequence. The relative amount of the 1030 Da peptide was higher in LD melanotrope cells. Taken together, our results suggest that POMC processing is differentially regulated in the two melanotrope cell subsets.

Cloning and expression of two proopiomelanocortin mRNAs in the common carp (Cyprinus carpio L.)

Proopiomelanocortin (POMC) is the precursor for a number of biologically active peptides such as adrenocorticotropic hormone (ACTH), alpha-melanocyte-stimulating hormone (alpha-MSH) and beta-endorphin. It is well known that these peptides are involved in the stress response in fish as well as in mammals. We have cloned two different carp POMC cDNAs called, POMC-I and POMC-II. The nucleotide sequences of 955 bp for POMC-I and 959 bp for POMC-II share 93.5% identity in their cDNAs, and the deduced amino acid sequences (both 222 amino acids) are 91.4% identical. In the ACTH and beta-MSH domain, two amino acid substitutions are found, whereas alpha-MSH and beta-endorphin are identical. For beta-MSH, the serine replacement (in POMC-I) by a glycine (in POMC-II) results in a putative amidation site Pro-X-Gly for POMC-II. We used RT-PCR to show that both POMC mRNAs are expressed in the hypophysis, hypothalamus and other parts of the brain of a single fish. Furthermore, in a phylogenetic tree based on POMC sequences the divergence of carp POMC-I and -II from tetraploid animals (salmon, trout and xenopus) is demonstrated.

Spatial and temporal distribution of proopiomelanotropin and proopiocortin mRNA during the life cycle of the sea lamprey: a qualitative and quantitative in situ hybridization study

Two POMC-like pituitary prohormones proopiocortin (POC) and proopiomelanotropin (POM) have been characterized from adult sea lampreys (Petromyzon marinus). POC encodes a nasohypophysial factor (NHF), ACTH, an MSH, and beta-END; and POM encodes MSH-A, MSH-B, and beta-END. Two radiolabeled riboprobes, one encoding a unique portion of POC mRNA and the other encoding the MSH-B domain unique to POM mRNA, were generated in order to examine the expression of POC and POM during the life cycle of the sea lamprey by in situ hybridization. POC expression appears evenly distributed throughout most cells of the rostral pars distalis (RPD) during the entire life cycle. POC expression also occurs in scattered cells of the caudal (proximal) pars distalis (CPD) at stage 5 of metamorphosis. By the prespawner period, POC expression is mainly distributed in the dorsal aspect of this region. POM expression was completely confined to most cells of the pars intermedia (PI) at all periods examined. Quantitative, computer-assisted, image analysis of POM expression revealed high signal densities in all larvae which decreased by early metamorphosis, steadily increased and reached high levels by late metamorphosis (stages 6 and 7), and attained even higher levels in prespawners. Volumetric analysis revealed that the net volume of POM expressing cells is at its lowest in larvae and increases during subsequent development. Analysis of signal density and volumetric measurements of POC expression revealed that POC expression in the RPD is low in larvae and steadily increases during subsequent intervals of the life cycle reaching very high levels by the prespawning period. POC expression in the CPD, first visible at stage 5, increases steadily throughout the remainder of metamorphosis and reaches the highest levels of expression in prespawning animals. These results would implicate the role of POM and POC in some developmental processes but not in the initiation of metamorphosis. The very high levels of POM and POC expression in prespawner animals suggest that the two genes may have important roles at this time in the life cycle of lampreys.

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.

Isolation and structural characterization of two novel peptides derived from proopiomelanocortin in the pituitary of the rainbow trout

The trout possesses two POMC genes as a result of duplication of its genome some 25-100 million years ago. One of the POMC molecules exhibits a unique C-terminal extension of 25 amino acid residues which is not found in any other POMC characterized so far. In order to isolate possible novel peptides derived from trout POMC-A, we have raised antibodies against two synthetic epitopes derived from the C-terminal region of the precursor. Two native decapeptides were isolated in pure form from an extract of trout pituitary. The primary structures of these peptides were established as Glu-Gln-Trp-Gly-Arg-Glu-Glu-Gly-Glu-Glu and Ala-Leu-Gly-Glu-Arg-Lys-Tyr-His-Phe-Gln-NH2. The structure of the trout POMC-A cDNA reveals that both peptides are flanked by pairs of basic amino acids or a glycine residue, indicating that they can actually be generated during post-translational processing of POMC-A.

Deciphering posttranslational processing events in the pituitary of a neopterygian fish: cloning of a gar proopiomelanocortin cDNA

A cDNA that codes for the polypeptide hormone precursor proopiomelanocortin (POMC) was cloned and sequenced from a gar (Lepisosteus osseus) pituitary cDNA library. The gar POMC cDNA is 1237 bp and contains a 780-bp open reading frame. The deduced amino acid sequence for gar POMC is 259 amino acids in length. The general organization of gar POMC is very similar to that of other gnathostome POMC sequences. The beta-endorphin sequence had 91% sequence identity with sockeye A beta-endorphin and 71% sequence identity with Xenopus laevis beta-endorphin. Three melanocyte-stimulating hormone (MSH) core sequences [HFR(W)] were detected. The gar alpha-MSH sequence was identical to the alpha-MSH sequence in rat POMC. The gar beta-MSH sequence had 77% sequence identity with salmonid forms of beta-MSH and 53% sequence identity with tetrapod forms of beta-MSH. The gamma-MSH region of gar POMC only had 26% primary sequence identity with tetrapod gamma-MSH sequences. Gar gamma-MSH had an incomplete MSH core sequence (HRF), an apparent internal deletion of five amino acids, and lacked flanking paired basic amino acids essential for proteolytic cleavage. The apparent degenerate nature of gar gamma-MSH is discussed in light of the absence of this sequence in salmonid fish.

Pro-opiomelanocortin (POMC) expression and immunolocalization of POMC-related peptides in the ovary of Protopterus annectens, an African lungfish

Antisera against adrenocorticotropic hormone (ACTH), alpha-melanocyte-stimulating hormone (alpha MSH) and beta-endorphin were used to localize pro-opiomelanocortin (POMC)-derived peptides in the ovary of the African lungfish Protopterus annectens by immunohistochemistry. Immunoreactivity was observed in the granulosa and the internal theca of the vitellogenic follicles. No immunoreactivity was observed in immature follicles. Using human POMC cDNA as the hybridization probe POMC-like mRNA was identified in situ in cells of the granulosa and internal theca of the vitellogenic follicles. No labeling was observed in primordial follicles. The demonstration in the same cells of POMC mRNA and POMC-related peptides immunoreactivity indicates a local production of the opiate hormones.

Pro-opiomelanocortin-derived peptides, cytokines, and nitric oxide in immune responses and stress: an evolutionary approach

In vertebrates, including man, the study of stress has contributed substantially to unravelling the complex relationship between immune-neuroendocrine interactions and the systems involved. On the basis of data on the presence and distribution of the main actors (POMC products, cytokines, biogenic amines, and steroid hormones) in different species and taxa from invertebrates to vertebrates, we argue that these responses have been deeply connected and interrelated since the beginning of life. Moreover, the study of nitric oxide suggests that the inflammatory reaction is located precisely between the immune and stress responses, sharing the same fundamental evolutionary roots. The major argument in favor of this hypothesis is that the immune, stress, and inflammation responses appear to be mediated by a common pool of molecules that have been conserved throughout evolution and that from a network of adaptive mechanisms. One cell type, the macrophage, appears to emerge as that most capable of supporting this network critical for survival; it was probably a major target of selective pressure. All these data fit the unitarian hypothesis we propose, by which evolution favors what has been conserved, rather than what has changed, as far as both molecules and functions are concerned.

Secretogranin III is a sulfated protein undergoing proteolytic processing in the regulated secretory pathway

Secretogranin III (SgIII) is an acidic protein of unknown function that is present in the storage vesicles of many neuroendocrine cells. It is coexpressed with the prohormone proopiomelanocortin in the intermediate pituitary of Xenopus laevis. We developed an antiserum to investigate the biosynthesis of SgIII in pulse-chase incubated Xenopus neurointermediate lobes. SgIII was synthesized as a 61- or 63-kDa (N-glycosylated) protein and processed to a 48-kDa form which, in turn, was partially cleaved to fragments of 28 and 20 kDa. The 48-, 28-, and 20-kDa cleavage products, but not their precursors, were secreted. This secretion is regulated and can be blocked in parallel with that of proopiomelanocortin-derived peptides by the hypothalamic factors dopamine, gamma-aminobutyric acid, and neuropeptide Y. Coexpression of Xenopus SgIII with prohormone convertase (PC)1 or PC2 in transfected fibroblasts was sufficient to reconstitute the processing events observed in the neurointermediate lobes. Site-directed mutagenesis revealed that Xenopus SgIII is cleaved at two dibasic sites, namely Lys68-Arg69 and Arg237-Arg238. Pulse-chase incubations of lobes with Na2[35S]SO4 showed that SgIII is sulfated in the trans-Golgi network before it is processed. Finally, SgIII processing was found in several neuroendocrine cell types from various species. We conclude that SgIII is a precursor protein and that the intact molecule can only have an intracellular function, whereas an extracellular role can only be attributed to its cleavage products.

Two types of cDNAs encoding proopiomelanocortin of sockeye salmon, Oncorhynchus nerka

To investigate regulatory mechanisms of proopiomelanocortin (POMC) gene expression in sockeye salmon, we have isolated and characterized cDNAs encoding two types of sockeye salmon POMC, which are referred to as ssPOMC-A and -B. Two types of PCR products were amplified from total RNA of sockeye salmon pituitaries by use of rainbow trout sequences. Full length cDNA clones encoding ssPOMC-A and ssPOMC-B were obtained from a pituitary cDNA library of sockeye salmon using the PCR products as probes. The ssPOMC-A and -B cDNAs have a length of 1072 and 1709 bps, respectively. Northern blot analysis showed that both ssPOMC-A and -B mRNAs were expressed only in the pituitary, and their sizes were about 1.2 kb and 1.8 kb, respectively. The presence of two ssPOMC genes was confirmed by Southern blot analysis of genomic DNA obtained from a single sockeye salmon. The deduced amino acid sequences of the ssPOMC-A and -B contained 230 and 226 residues, respectively. The amino terminal of beta-endorphin in ssPOMC-B which corresponds to Met-enkephalin domain is YSGFM, which is different from YGGFM of Met-enkephalin found in many other vertebrate species. The homology of nucleotide sequences between ssPOMC-A and -B is 59% in the entire coding region, whereas alpha-MSH coding regions are highly homologous (91%). Although the deduced amino acid sequences of ssPOMs show 43% overall similarity, their hydropathy profiles are coincident with those of several other vertebrate species, particularly the amino terminal of N-terminal peptide (NPP) shows almost the same pattern with other vertebrate NPPs.

Identification of POMC processing products in single melanotrope cells by matrix-assisted laser desorption/ionization mass spectrometry

The use of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) in identifying proopiomelanocortin (POMC) processing products in melanotrope cells of the pituitary intermediate lobe of Xenopus laevis was explored. Mass spectra were obtained with such a high sensitivity of detection that the peptides could be identified in a single melanotrope cell. In addition to known POMC processing products of the Xenopus melanotrope cell, the presence of previously unidentified POMC-derived peptides was demonstrated. Together these POMC processing products accounted for the entire length of the POMC precursor. Furthermore, Xenopus possesses two genes for POMC and the sensitivity and accuracy of the MALDI-MS technique allowed identification of processing products of both the POMCA and POMCB gene. In addition, differences were obtained between the mass spectra of melanotrope cells from Xenopus laevis adapted to different conditions of background illumination. These results show that MALDI-MS is a valuable tool in the study of the expression of peptides in single (neuroendocrine) cells.

Immunocytochemical evidence for the presence of Met-enkephalin and Leu-enkephalin in distinct neurons in the brain of the elasmobranch fish Scyliorhinus canicula

Immunohistochemical methods have been used to investigate the distribution of various opioid peptides derived from mammalian proenkephalin in the central nervous system of Scyliorhinus canicula. The results indicate that both Leu- and Met-enkephalin-immunoreactive peptides are present in the dogfish brain. In contrast, enkephalin forms similar to Met-enkephalin-Arg-Phe or Met-enkephalin-Arg-Gly-Leu, and mammalian alpha-neo-endorphin, dynorphin A (1-8), dynorphin A (1-13), and dynorphin A (1-17) were not detected. Met- and Leu-enkephalin immunoreactivities were found in distinct neurons of the telencephalon and hypothalamus. In particular, cell bodies reacting only with the Met-enkephalin antiserum were localized in the preoptic nucleus and in the suprachiasmatic region of the hypothalamus. Conversely, cell bodies reacting only with the Leu-enkephalin antiserum were localized in the pallium and the nucleus lobi lateralis hypothalami. Several areas of the telencephalon and diencephalon exhibited both Met- and Leu-enkephalin-like immunoreactivity, but the two immunoreactive peptides were clearly contained in distinct perikarya. The overall distribution of Met-enkephalin-immunoreactive elements in the dogfish exhibited similarities to the distribution of proenkephalin-derived peptides previously reported for the brain of tetrapods. The fact that Met- and Leu-enkephalin-like peptides were detected in distinct neurons, together with the absence of dynorphin-related peptides, suggests the existence of a novel Leu-enkephalin-containing precursor in the dogfish brain.

Isolation and structural characterization of a novel peptide related to gamma-melanocyte stimulating hormone from the brain of the leech Theromyzon tessulatum

This paper reports the purification of a novel pro-opiomelanocortin derivative peptide (a gamma-melanocyte stimulating hormone-like (gamma-MSH-like) molecule) from the brain of the leech Theromyzon tessulatum. After reverse-phase HPLC purification, the sequence of the gamma-MSH-like peptide (YVMGHFRWDKFamide) was established by a combination of automated Edman degradation, electrospray mass spectrometry measurement, enzymatic treatment and co-elution experiments in reverse-phase HPLC with synthetic peptides.

The posttranslational modification of beta-endorphin in the intermediate pituitary of the toad, Bufo marinus, includes processing at a monobasic cleavage site

Fractionation of an acid extract of 15 B. marinus intermediate pituitaries by a combination of gel filtration chromatography and cation exchange chromatography revealed one major and five minor forms of beta-endorphin in this tissue. Based on reversed-phase HPLC and immunological properties, as well as amino acid composition and primary sequence analysis, it was deduced that the sequence of the major form of B. marinus beta-endorphin is N-acetyl-YGGFMTPE. Overall, the steady-state analyses of the minor forms of beta-endorphin indicated that the posttranslational processing of beta-endorphin in the toad intermediate pituitary includes endoproteolytic cleavage at both paired basic and monobasic cleavage sites.

Alpha-melanocyte-stimulating hormone (alpha-MSH) in the brain of the African lungfish, Protopterus annectens: immunohistochemical localization and biochemical characterization

The distribution of alpha-melanocyte-stimulating hormone (alpha-MSH) containing neurons and the molecular forms of alpha-MSH-related peptides exhibit substantial differences in the brains of fish and amphibians. Lungfishes, which share similarities with both fishes and tetrapods, represent a valuable group in which to investigate the neuroanatomical and neurochemical facets of evolution. In the present study, we have localized and characterized alpha-MSH-immunoreactive peptides in the central nervous system of the African lungfish Protopterus annectens. Perikarya exhibiting alpha-MSH-like immunoreactivity were observed in two distinct regions of the hypothalamus: the rostral part of the preoptic nucleus and the caudal part of the hypothalamus. In the caudal hypothalamus most alpha-MSH-immunopositive perikarya were located in both the subependymal and deepest layers of the ventral periventricular region. Scattered alpha-MSH-immunopositive cells were occasionally detected in the dorsal side of the caudal hypothalamus. The alpha-MSH-immunoreactive material localized in the brain was characterized by combining high-performance liquid chromatography (HPLC) analysis and radioimmunological detection. The displacement curves obtained with synthetic alpha-MSH and serial dilutions of brain and pituitary extracts were parallel. HPLC analysis of lungfish hypothalamic extracts showed that the major immunoreactive peak coeluted with synthetic desacetyl alpha-MSH and its sulfoxide derivative. An additional peak coeluted with synthetic sulfoxide alpha-MSH. In contrast, in the pituitary, the predominant form of alpha-MSH-like material coeluted with the N,O-diacetyl alpha-MSH standard. These results provide the first evidence for the presence of alpha-MSH-related peptides in the brain of a lungfish. The distribution of alpha-MSH neuronal systems in the lungfish is very similar to that reported in amphibians, supporting the existence of phylogenetic convergences between these two vertebrate groups.

Presence of Vi-transposon-like elements in the proopiomelanocortin gene A of Xenopus laevis does not affect gene activity

Restriction mapping of the two proopiomelanocortin (POMC) genes of the South African clawed toad Xenopus laevis revealed that POMC gene A is much larger than POMC gene B. Here we report that this size difference is mainly due to the presence of four vitellogenin (Vi)-transposon-like elements in POMC gene A, while Vi elements are absent from POMC gene B. Alignment of these elements with other Vi elements revealed a consensus sequence of 463 bp, which is bounded by a 16 bp inverted repeat and flanked by a 3 bp direct repeat. Since the amounts of mRNA produced by both POMC genes in the pars intermedia of the Xenopus pituitary are similar, the presence of the Vi-transposon-like elements in POMC gene A apparently has no effect on POMC gene expression at transcriptional or post-transcriptional levels.

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.

Characterization of Xenopus laevis proenkephalin gene

Enkephalins are opiate peptides found in a variety of tissues including brain and pituitary. In brain, they function as neurotransmitters, neuromodulators and neurohormones. Recent studies show that proenkephalin mRNA is expressed early in development both in mammals and the amphibian, suggesting that enkephalins may play a unique role in embryogenesis. In order to characterize factors which regulate the onset and patterning of expression of this gene in adult and developing frog embryos, the proenkephalin A gene was cloned from Xenopus laevis. The clones have been characterized by DNA sequencing and restriction endonuclease mapping. The gene is made up of three exons which span approximately 12 kb. Exon I encodes the 5' untranslated region of the mRNA. Exon II contains the signal peptide and the N terminus of the mature protein. Biologically active opioid peptides are generated from exon III. Comparison to mammalian proenkephalin genomic sequence indicated that nucleotide sequences of the 5' flanking region, noncoding exon I and exon II were not well conserved but exon III was highly conserved. Primer extension and RNase protection assay analyses of the RNA transcripts revealed two major 5' ends. The putative TATA box, CAAT box, CRE and Pit 1 elements have been identified on this gene by sequence homology to published consensus sequences. To assay for sequences that could potentially regulate Xenopus proenkephalin expression, we transfected constructs that contained upstream genomic sequences linked to the CAT reporter gene into various eukaryotic cell lines. The expression of the fusion gene constructs were detected and could be induced 10- to 30-fold upon treatment with forskolin.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization and evolutionary aspects of a transcript encoding a neuropeptide precursor of Lymnaea neurons, VD1 and RPD2

We isolated and characterized a cDNA clone encoding the major prohormone of VD1 and RPD2, two electrotonically coupled identified neurons in the central nervous system of the freshwater snail, Lymnaea stagnalis. The VD1/RPD2 prohormone may be cleaved to generate a set of 4 different neuropeptides, called epsilon, delta, alpha 1 and beta peptides, as well as a single aspartate. Since VD1 and RPD2 probably are involved in O2 perception and modulation of cardio-respiratory functions, it is thought that the neuropeptides synthesized and released by these neurons coordinate the adaptive physiological and behavioural processes that occur in response to changes in O2 availability. Comparison of the Lymnaea VD1/RPD2 precursor with two related precursors, prohormones R15-1 and R15-2, identified from neuron R15 in the marine mollusc Aplysia californica revealed a similar pattern of organization of the preprohormones. The overall homology is rather low, however, detailed comparisons show a highly differential pattern of conservation of peptide regions on the precursors.

[The intermediate lobe of the pituitary, model of neuroendocrine communication]

The intermediate lobe of the pituitary is composed of a homogeneous population of endocrine cells, the melanotrophs, which secrete several bioactive peptides including alpha-melanocyte-stimulating hormone (alpha-MSH) and beta-endorphin. In contrast to most endocrine glands which are richly vascularized, the intermediate lobe of the pituitary contains very few blood vessels; in some species, the pars intermedia is virtually totally avascular. In contrast, pituitary melanotrophs are richly supplied by nerve fibers originating from the hypothalamus. The pars intermedia thus appears as a pure model of neuroendocrine communication, i.e. it is an archetype of the mode of transducing interface between the central nervous system and endocrine effectors. In mammalian species, different types of nerve terminals containing dopamine, norepinephrine, gamma-aminobutyric acid (GABA) and serotonin have been identified. In lower vertebrates, particularly in fish and amphibians, the pars intermedia is also innervated by peptidergic fibers which are though to take part in regulation of the secretory activity of the melanotroph. In these animals, the pars intermedia is regarded as a major center of neuroendocrine integration and an exceptional model to investigate the process of communication between the brain and the endocrine glands. The purpose of the present review is to summarize our current knowledge on the synthesis, processing and release of peptide hormones from pars intermedia cells and to survey the multiple regulatory mechanisms which are involved in the control of the activity of pituitary melanotrophs. Proopiomelanocortin, a multifunctional precursor. Pituitary melanotrophs synthetise a major precursor protein called proopiomelanocortin (POMC) which generates through proteolytic cleavage several biologically active peptides including adrenocorticotropic hormone (ACTH), endorphins and MSHs. In lower vertebrates, alpha-MSH is generally considered as the major hormone secreted by melanotrophs, in that it is involved in the process of skin colour adaptation. The post-translational processing of POMC, which yields to the mature hormones released by melanotrophs, includes a number of steps: glycosylation, phosphorylation, tissue-specific proteolytic cleavage, amidation and acetylation. Some of these posttranslational modifications can be regulated by neuroendocrine factors. For instance, in frogs, it has been shown that dopamine inhibits acetylation of alpha-MSH and thus reduces the secretion of the biologically active form of the peptide. The intermediate lobe of the pituitary: a model of neuroendocrine integration. In most vertebrate species, the intermediate lobe of the pituitary is innervated by catecholamine-containing fibers. In particular, the presence of dopaminergic nerve fibers has been observed in the pars intermedia of mammals and poikilotherms.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of chicken ACTH and alpha-MSH: the primary sequence of chicken ACTH is more similar to Xenopus ACTH than to other avian ACTH

Adrenocorticotropic (ACTH) and melanocyte stimulating (MSH) hormones have been demonstrated in the same cells in the cephalic half of the pars distalis of the chicken pituitary glands in three ways: (I) immunohistochemistry, (II) radioimmunoassay (RIA) using both anti-human or porcine ACTH and synthetic alpha-MSH antibodies, and (III) isolation and purification, followed by the determination of amino acid compositions of both hormones. The contents of ACTH and alpha-MSH are estimated by RIA to be 1600 and 10 ng/gland, respectively. ACTH missed 1 (des-Phe39-ACTH) or 2 residues (des-Glu38, Phe39-ACTH) from the C-terminal portion was also isolated. The recoveries of these ACTHs are differed from preparation to preparation. The complete amino acid sequence of chicken ACTH (39 residues) has been determined as NH2-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-Gly-Arg-Lys-Arg- Arg- Pro-Ile-Lys-Val-Tyr-Pro-Asn-Gly-Val-Asp-Glu-Glu-Ser-Ala-Glu-Ser-Tyr-Pro- Met-Glu-Phe-OH Strikingly the amino acid sequence of chicken ACTH shows a closer resemblance to that from an amphibian, Xenopus (3 residue substitution) than that from another bird, the ostrich (7 residue substitution) or the turkey (at least 9 residue substitution).

Study of frog (Rana esculenta) proopiomelanocortin processing in the intermediate pituitary. Identification of alpha-melanotropin, beta-melanotropin, Lys-gamma-melanotropin, and corticotropin-like intermediate lobe peptide

The proteolytic processing of frog (Rana esculenta) proopiomelanocortin in melanotropic cells of the intermediate pituitary gland has been examined through purification of the mature fragments by reverse-phase high-pressure liquid chromatography and microsequencing of isolated peptides. alpha-Melanotropin, beta-melanotropin, Lys-gamma-melanotropin, corticotropin-like intermediate lobe peptide, and hinge peptide have been isolated and chemically characterized. The results show a high preservation in the processing sites of frog proopiomelanotropin when compared to bovine counterparts. They reveal also a great conservation of the processing enzyme equipment of melanotropic cells in tetrapods species. Identification of Lys-gamma-melanotropin suggests the occurrence of an endopeptidase able to cleave between two basic residues. On the other hand alpha-melanotropin does not appear to be N-acetylated, as previously found in the clawed-toad Xenopus laevis, and this feature might distinguish amphibian from mammalian proopiomelanocortin processing.

Characterization of the cDNA encoding proopiomelanocortin in the frog Rana ridibunda

In the amphibian pars intermedia, secretion of proopiomelanocortin (POMC)-derived peptides is controlled by multiple factors including classical neurotransmitters and neuropeptides. To pursue questions concerning the regulation of POMC gene expression in Rana ridibunda, we have isolated and characterized a full-length cDNA for frog POMC. A cDNA clone isolated from a frog pituitary library contains an open-reading frame of 780-bp that predicts a 260 amino acid POMC protein. The structure of frog POMC demonstrates considerable amino acid sequence similarity with POMC from other species. In particular, the sequence of alpha-melanotropin (alpha-MSH) is identical in frog and all mammalian species studied so far, while adrenocorticotropin (ACTH) and beta-endorphin exhibit 79% and 84% homology with their human counterpart. Frog POMC contains only one potential asparagine-linked N-glycosylation signal (Asn-Ser-Thr) within the gamma-MSH domain. The alpha-MSH sequence is C-terminally flanked by the Gly-Lys-Lys amidation signal while the joining peptide is not amidate.