Molecular cloning of lungfish proopiomelanocortin cDNA

Special features of neuroendocrine interactions between stress and reproduction in teleosts

Stress and reproduction are both essential functions for vertebrate survival, ensuring on one side adaptative responses to environmental changes and potential life threats, and on the other side production of progeny. With more than 25,000 species, teleosts constitute the largest group of extant vertebrates, and exhibit a large diversity of life cycles, environmental conditions and regulatory processes. Interactions between stress and reproduction are a growing concern both for conservation of fish biodiversity in the frame of global changes and for the development of sustainability of aquaculture including fish welfare. In teleosts, as in other vertebrates, adverse effects of stress on reproduction have been largely documented and will be shortly overviewed. Unexpectedly, stress notably via cortisol, may also facilitate reproductive function in some teleost species in relation to their peculiar life cyles and this review will provide some examples. Our review will then mainly address the neuroendocrine axes involved in the control of stress and reproduction, namely the corticotropic and gonadotropic axes, as well as their interactions. After reporting some anatomo-functional specificities of the neuroendocrine systems in teleosts, we will describe the major actors of the corticotropic and gonadotropic axes at the brain-pituitary-peripheral glands (interrenals and gonads) levels, with a special focus on the impact of teleost-specific whole genome duplication (3R) on the number of paralogs and their potential differential functions. We will finally review the current knowledge on the neuroendocrine mechanisms of the various interactions between stress and reproduction at different levels of the two axes in teleosts in a comparative and evolutionary perspective.

Evolution of proopiomelanocortin

Proopiomelanocortin (POMC) belongs to the opioid/orphanin gene family whose peptide precursors include either opioid (YGGF) or the orphanin/nociceptin core sequences (FGGF). In addition to POMC the family includes the proenkephalin (PENK), prodynorphin (PDYN), and nociceptin/proorphanin (PNOC) precursors. The opioid core sequence in POMC is incorporated by the β-endorphin that occupies the C-terminal region but this propeptide also exhibits at least two "alien" melanocortin core sequences (HFRW). An ACTH/MSH fragment merged into the opioid fragment not earlier than the two tetraploidizations of the vertebrate genome. Therefore, POMC exhibit a complex "evolutionary life" since the gene has coevolved together with two different receptor systems, i.e., opioid and melanocortin following a horse trading system. In this article, we summarize the different evolutionary hypotheses proposed for POMC evolution.

60 YEARS OF POMC: POMC: an evolutionary perspective

Proopiomelanocortin (POMC) is a complex precursor that comprises several peptidic hormones, including melanocyte-stimulating hormones (MSHs), adrenocorticotropic hormone (ACTH), and β-endorphin. POMC belongs to the opioid/orphanin gene family, whose precursors include either opioid (YGGF) or the orphanin/nociceptin core sequences (FGGF). This gene family diversified during early tetraploidizations of the vertebrate genome to generate four different precursors: proenkephalin (PENK), prodynorphin (PDYN), and nociceptin/proorphanin (PNOC) as well as POMC, although both PNOC and POMC seem to have arisen due to a local duplication event. POMC underwent complex evolutionary processes, including internal tandem duplications and putative coevolutionary events. Controversial and conflicting hypotheses have emerged concerning the sequenced genomes. In this article, we summarize the different evolutionary hypotheses proposed for POMC evolution.

Evolution of the melanocortin system

The melanocortin system is one of the most complex of the hormonal systems. It involves different agonists encoded in the multiplex precursor proopiomelanocortin (POMC) or in different genes as β-defensins, endogenous antagonist, like agouti-signalling protein (ASIP) or agouti-related protein (AGRP), and five different melanocortin receptors (MCRs). Rounds of whole genome duplication events have preceded the functional and molecular diversification of the family in addition some co-evolutionary and tandem duplication processes have been proposed. The evolutionary patterns of the different partners are controversial and different hypotheses have emerged from a study of the sequenced genomes. In this review, we summarize the different evolutionary hypotheses proposed for the different melanocortin partners.

Complex structural and regulatory evolution of the pro-opiomelanocortin gene family

The melanocortin system is a neuroendocrine machinery that has been associated with phenotypic diversification in a number of vertebrate lineages. Central to the highly pleiotropic melanocortin system is the pro-opiomelanocortin (pomc) gene family, a family of pre-prohormones that each give rise to melanocyte stimulating hormone (MSH), adrenocorticotropic releasing hormone (ACTH), β-lipotropin hormone, and β-endorphin. Here we examine the structure, tissue expression profile, and pattern of cis transcriptional regulation of the three pomc paralogs (α1, α2, and β) in the model cichlid fish Astatotilapia burtoni and other cichlids, teleosts, and mammals. We found that the hormone-encoding regions of pomc α1, pomc α2 and pomc β are highly conserved, with a few notable exceptions. Surprisingly, the pomc β gene of cichlids and pomacentrids (damselfish) encodes a novel melanocortin peptide, ε-MSH, as a result of a tandem duplication of the segment encoding ACTH. All three genes are expressed in the brain and peripheral tissues, but pomc α1 and α2 show a more spatially restricted expression profile than pomc β. In addition, the promoters of each pomc gene have diverged in nucleotide sequence, which may have facilitated the diverse tissue-specific expression profiles of these paralogs across species. Increased understanding of the mechanisms regulating pomc gene expression will be invaluable to the study of pomc in the context of phenotypic evolution.

Posttranslational modifications of proopiomelanocortin in vertebrates and their biological significance

Proopiomelanocortin (POMC) is the precursor of several peptide hormones generated in the pituitary gland. After biosynthesis, POMC undergoes several posttranslational modifications, including proteolytic cleavage, acetylation, amidation, phosphorylation, glycosylation, and disulfide linkage formation, which generate mature POMC-derived peptides. Therefore, POMC is a useful model for the investigation of posttranslational modifications. These processes have been extensively investigated in mammals, primarily in rodents. In addition, over the last decade, much information has been obtained about the posttranslational processing of POMC in non-mammalian animals such as fish, amphibians, reptiles, and birds through sequencing and peptide identification by mass spectrometry. One POMC modification, acetylation, is known to modulate the biological activities of POMC-derived α-melanocyte-stimulating hormone (α-MSH) having an acetyl group at N-terminal through potentiation or inhibition. This bidirectional regulation depends on its intrinsic roles in the tissue or cell; for example, α-MSH, as well as desacetyl (Des-Ac)-α-MSH, stimulates pigment dispersion in the xanthophores of a flounder. In contrast, α-MSH does not stimulate pigment dispersion in the melanophores of the same species, whereas Des-Ac-α-MSH does. Regulation of pigment-dispersing activities may be associated with the subtle balance in the expression of receptor genes. In this review, we consider the posttranslational modifications of POMC in vertebrates from an evolutionary aspect, with a focus on the relationship between acetylation and the biological activities of α-MSH as an important consequence of posttranslational modification.

Evaluation of posttranslational processing of proopiomelanocortin in the banded houndshark pituitary by combined cDNA cloning and mass spectrometry

Proopiomelanocortin (POMC) is cleaved into small peptides, such as adrenocorticotropic hormone (ACTH), melanocyte-stimulating hormones (MSHs), and beta-endorphin (beta-END), by tissue-specific posttranslational processing in the corticotrophs of the pars distalis (PD) and melanotrophs of the neurointermediate lobe (NIL) of the pituitary. We examined the posttranslational processing of POMC in the pituitary of the banded houndshark Triakis scyllium by molecular cloning and subsequent mass spectrometric identification of the POMC-derived peptides in the pituitary extracts. One-fifth of the randomly selected clones from a Triakis pituitary cDNA library contained a cDNA encoding for POMC. Triakis prePOMC contained 4 MSHs and a single beta-END, as has been observed in case of other cartilaginous fish POMCs. These predicted hormonal segments were flanked by basic amino acid residues, which are the cleavage sites for the processing enzymes, i.e., protein convertases. Mass spectrometry was performed using PD (including most parts of the rostral and proximal PD) and NIL extracts to detect mass values corresponding to the POMC-derived peptides. Consequently, ACTH, beta-END, and the joining peptide (JP) were detected in the PD extract, while MSHs, processed beta-END, and some other POMC-derived peptides were identified in the NIL extract; however, neither acetylated alpha-MSH nor acetylated beta-END was detected in the latter. These tissue-specific POMC processing patterns are similar to those of the other vertebrate pituitaries; however, the absence of acetylated peptides suggests the lack of an acetylation system in the melanotrophs in the NIL of the Triakis pituitary.

cDNA cloning of proopiomelanocortin (POMC) and mass spectrometric identification of POMC-derived peptides from snake and alligator pituitaries

Proopiomelanocortin (POMC) is the precursor of melanocyte-stimulating hormone (MSH) and beta-endorphin, and is suggested to have evolved by the insertion and deletion of ancestral MSH segments. Here, the primary structure of POMC was determined with cDNA cloning of brown tree snakes of Squamata and American alligators of Crocodylia to show an overview of the molecular evolution of POMC in reptiles. Snake and alligator POMCs are composed of alpha-, beta-, and gamma-MSH segments and a single beta-END segment as in other tetrapods; however, the gamma-MSH segment in snake POMC has a mutation in the essential sequence from His-Phe-Arg-Trp to His-(d)-(d)-Arg, in which (d) means deletion. It is conceivable that the ancestry of snake gamma-MSH had weak functional constraint and lacked biological significance during evolution. Phylogenetic analyses using the neighbor-joining method show that snake prePOMC is most diverged, and alligator prePOMC is most conserved in reptilian POMCs while it shows the highest sequence identity with ostrich prePOMC. These relationships are comparable to those observed in mitochondrial DNA. On the other hand, analyses of the pituitary with mass spectrometry revealed several peptides by post-translational processing as predicted by the locations of processing sites consisting of basic amino acid residues in snake and alligator POMCs. Remarkably, the monobasic site at the N-terminal side of the snake beta-MSH is suggested to act as a processing site. Thus, the study shows the divergence of snake POMC such as the critical mutation of gamma-MSH and high conservation of hormone organization of alligator POMC.

Evolution of melanocortin systems in fish

Proopiomelanocortin (POMC) is a common precursor of melanocortin (MC), the collective term for adrenocorticotropic hormone (ACTH) and melanophore-stimulating hormone (MSH), and of beta-endorphin (beta-END). Over the past decade, considerable progress has been made in the analysis of the POMC gene from a board taxonomic group of vertebrates and invertebrates. The results suggest that three MSHs (alpha-, beta-, and gamma-MSH) and a single END were established in ancestral invertebrates. Thereafter, unequal crossing over may have resulted in class-specific numbers of MSH segments during the radiation of fish. Moreover, duplication of the entire POMC gene may have led to the differentiation of POMC as shown in lampreys; one of the two subtypes is a precursor for ACTH and beta-END, the other is a precursor for two forms of MSH and the other form of beta-END. On the other hand, at least five subtypes of MC receptor (MCR) have been observed in fish. These are G-protein-coupled receptors with seven transmembrane domains. The ancestral MCR is suggested to have appeared before vertebrates, and then MCRs may have diverged by genome duplication and local duplication of each receptor gene during the evolution of vertebrates. They are distributed in many tissues in rather a subtype-specific manner and are responsible for a variety of biological functions. Thus, MC systems may have diverged by producing structurally different MC peptides from POMC and expressing MCR subtypes differing in ligand selectivity in a variety of tissues.

The dawn and evolution of hormones in the adenohypophysis

The adenohypophysial hormones have been believed to have evolved from several ancestral genes by duplication followed by evolutionary divergence. To understand the origin and evolution of the endocrine systems in vertebrates, we have characterized adenohypophysial hormones in an agnathan, the sea lamprey Petromyzon marinus. In gnathostomes, adrenocorticotropin (ACTH) and melanotropin (MSH) together with beta-endorphins (beta-END) are encoded in a single gene, designated as proopiomelanocortin (POMC), however in sea lamprey, ACTH and MSH are encoded in two distinct genes, proopoicortin (POC) gene and proopiomelanotropin (POM) gene, respectively. The POC and POM genes are expressed specifically in the rostral pars distalis (RPD) and the pars intermedia (PI), respectively. Consequently, the final products from both tissues are the same in all vertebrates, i.e., ACTH from the PD and MSH from the PI. The POMC gene might have been established in the early stages of invertebrate evolution by internal gene duplication of the MSH domains. The ancestral gene might be then inherited in lobe-finned fish and tetrapods, while internal duplication and deletion of MSH domains as well as duplication of whole POMC gene took place in lamprey and gnathostome fish. Sea lamprey growth hormone (GH) is expressed in the cells of the dorsal half of the proximal pars distalis (PPD) and stimulates the expression of an insulin-like growth factor (IGF) gene in the liver as in other vertebrates. Its gene consists of 5 exons and 4 introns spanning 13.6 kb, which is the largest gene among known GH genes. GH appears to be the only member of the GH family in the sea lamprey, which suggests that GH is the ancestral hormone of the GH family that originated first in the molecular evolution of the GH family in vertebrates and later, probably during the early evolution of gnathostomes. The other member of the gene family, PRL and SL, appeared by gene duplication. A beta-chain cDNA belonging to the gonadotropin (GTH) and thyrotropin (TSH) family was cloned. It is expressed in cells of the ventral half of PPD. Since the expression of this gene is stimulated by lamprey gonadotropin-releasing hormone, it was assigned to be a GTHbeta. This GTHbeta is far removed from beta-subunits of LH, FSH, and TSH in an unrooted tree derived from phylogenetic analysis, and takes a position as an out group, suggesting that lampreys have a single GTH gene, which duplicated after the agnathans and prior to the evolution of gnathostomes to give rise to LH and FSH.

Molecular cloning and characterization of preproopiomelanocortin (prePOMC) cDNA from the ostrich (Struthio camelus)

To date proopiomelanocortin (POMC), the precursor protein for melanotropin (MSH), adrenocorticotropin (ACTH), lipotropins (LPH), and beta-endorphin (beta-END) in the pituitary gland, has been studied extensively over a wide spectrum of vertebrate classes. A paucity of information exists, however, with regard to POMC in the avian class, where to date POMC from only one species, the domestic chicken, appears to have been fully characterized. In the present study, we report the use of three clones of cDNA to provide the complete nucleotide sequence of ostrich prePOMC cDNA, consisting of 1072 bp (excluding the poly(A) tail). The deduced amino acid sequence of 253 amino acid residues includes the N-terminal signal peptide of 17 amino acid residues. The predicted amino acid sequence in the overall arrangement of its domains, conforms to that found in other tetrapods. Sequence domains for gamma-MSH, ACTH, alpha-MSH, gamma-LPH, beta-MSH, and beta-END are located at positions 74-85, 134-172, 134-146, 175-220, 203-220, and 223-253, respectively, in ostrich prePOMC, but some of them may not be released in the ostrich pituitary gland, despite the presence of nine potential processing sites consisting of 2-4 dibasic amino acids each. Substitution of glutamic acid for a dibasic amino acid at position 202 in ostrich prePOMC could prevent release of beta-MSH. To date the release of pro-gamma-MSH, beta-LPH, ACTH, gamma-LPH, and beta-END have been confirmed by direct isolation and characterization from ostrich pituitary extracts. In the present study, we have also identified ACTH, gamma-LPH and beta-END in a single frozen ostrich pituitary slice by means of MALDI-TOF mass spectrometry. When compared to a wide range of vertebrate prePOMC molecules, ostrich prePOMC revealed a high level of amino acid sequence identity (77%) with chicken prePOMC, which is the only other avian sequence available. As with other vertebrate classes, considerable intraclass differences were also evident between chicken and ostrich prePOMCs, which belong to different avian orders. Identity of ostrich prePOMC with non-avian tetrapod counterparts is only moderate (53-56%), whereas lower identities (20-49%) are evident over a range of fish prePOMCs.

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.

Genomic structure of the proopiomelanocortin gene and expression during acute low-water stress in channel catfish

Proopiomelanocortin (POMC) is an important gene involved in the stress response of the hypothalamic-pituitary-adrenal axis. It is a precursor of several peptide hormones including adrenocorticotropic hormone, melanocyte stimulating hormones, and beta-endorphin. Our study aims to determine genomic structure and expression of POMC gene during temporal stress in channel catfish (Ictalurus punctatus). The catfish POMC gene consisting of three exons and two introns has a similar structural organization to that of other species. The catfish and mammalian POMC promoters do not exhibit regions of conservation except that of one TATA box. Genomic Southern blot analysis indicated POMC is present as a single copy gene in the catfish genome. Real-time PCR allowed us to monitor temporal expression of the POMC mRNA in catfish pituitary during low-water stress. Plasma cortisol concentrations were also measured as an indicator of stress. Within 15 min after the onset of low-water stress, POMC mRNA expression was elevated 1.87-fold above the control value. The POMC mRNA level had declined after 30 min (1.29-fold) and 1h (1.1-fold) at which time stress was removed. After 1h recovery, a significant increase in the POMC mRNA expression was detected (2.44-fold, P<0.05) followed by a decline 2h later (1.52-fold) when the experiment was terminated. Plasma cortisol levels in stressed fish were significantly above the cortisol levels in control fish during stress application (t=15 min, t=30 min, and t=1h, P<0.05), which then returned to normal during recovery. We conclude that POMC and cortisol are both involved in the low-water stress response during which cortisol may serve as a negative regulator of POMC expression in catfish.

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.

Nucleotide sequence and expression of three subtypes of proopiomelanocortin mRNA in barfin flounder

Melanophore-stimulating hormone (MSH) has been shown to be associated with food intake in addition to body color change in teleosts. MSH is encoded by a proopiomelanocortin (POMC) gene together with endorphin (END). To assess the significance of MSH to biological activities, we determined the structure and evaluated the expression of POMC mRNA in barfin flounder (bf), Verasper moseri, a member of a group of teleosts, Pleuronectiformes. Three subtypes of POMC cDNAs (A, B, and C) were amplified from bf pituitary glands. These bfPOMCs contained segments for N-POMC, alpha-MSH, beta-MSH, and beta-END as do other teleost POMCs, while POMC-C showed remarkable variations in the segments corresponding to N-POMC and beta-END. A phylogenetic tree of ray-finned fish POMCs constructed by the neighbor joining method revealed that the three POMC subtypes may have appeared as a result of duplication events occurring at least twice during the course of bf evolution. The first duplication may have generated the lineage leading to an ancestor of bfPOMC-A and -B and that leading to bfPOMC-C, and then the lineage of bfPOMC-A may have diverged from that of bfPOMC-B. All peptides flanked by processing signals excluding N-POMC-C (1-14) were identified in a single pituitary extract by mass spectrometry, and the cDNAs of three POMCs were amplified from a single pituitary by reverse transcription polymerase chain reaction. These results demonstrated that the three POMC genes are expressed in a single individual. While the bfPOMC-A gene was exclusively expressed in the pituitary, the bfPOMC-B and -C genes were expressed in non-pituitary tissues such as brain, gill, heart, spleen, liver, stomach, intestine, testis, muscle, blood, and skin in addition to the pituitary. The expression levels of the POMC-A, -B, and -C genes in pituitary neurointermediate lobe were greater in the fish reared with a black background than the fish reared with a white background, indicating that MSH derived from all of the three bfPOMC genes was associated with body color change. No difference was observed in the expression levels of bfPOMC-C in the brain in response to feeding status.

Molecular cloning of proopiomelanocortin cDNA and multi-tissue mRNA expression in channel catfish

Channel catfish (Ictalurus punctatus) proopiomelanocortin (POMC) cDNA was cloned to investigate its structure, evolution, and expression in different tissues. POMC is an important gene in the hypothalamus-pituitary-adrenal axis, the main mediator of the stress response. POMC gene was isolated from a pituitary cDNA library and nucleotide sequence was determined. POMC cDNA is composed of 1164 nucleotides with a 639 nucleotide open reading frame encoding a protein of 212 amino acids. Catfish POMC protein contains a signal peptide (SP, Met(1)-Ala(28)), N-terminal peptide (Gln(29)-Glu(101)), adrenocorticotropic hormone (ACTH, Ser(104)-Met(142)), alpha-melanocyte stimulating hormone (alpha-MSH, Ser(104)-Val(116)), corticotropin-like intermediate lobe peptide (CLIP, Arg(121)-Met(142)), beta-lipotropin (beta-LPH, Glu(145)-His(212)), gamma-lipotropin (gamma-LPH, Glu(145)- Ser(177)), beta-MSH (Asp(161)-Ser(177)), and beta-endorphin (beta-EP, Tyr(180)-His(212)). Catfish POMC protein does not contain a gamma-MSH region and most of the joining peptide and part of the gamma-LPH are deleted. Protein sequence alignment showed the highest similarity with the carp (Cyprinus carpio) POMC I (66.5%) and POMC II (67%), while the sea lamprey (Petromyzon marinus) POC (17.9%) and POM (18.8%) were the most divergent. The average similarity was 46.95% among the 44 POMC proteins from 36 different species analyzed. Compared to the POMC mRNA levels in the pituitary, the concentration of the POMC mRNA was 0.0594% in the anterior kidney and 0.0012-0.0045% in all the other tissues except in the skin where the lowest expression (0.0005%) was observed. Overall architecture of channel catfish POMC is highly similar to those from other teleosts.

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.

Molecular cloning of proopiomelanocortin cDNA in the ratfish, a holocephalan

Proopiomelanocortin (POMC) is a precursor for several pituitary hormones including adrenocorticotropin (ACTH), melanocyte-stimulating hormone (MSH) and endorphin (END). Fish POMCs in four taxonomic classes, Cephalaspidomorphi (lampreys), Chondrichthyes (cartilaginous fish), Sarcopterygii (lobe-finned fish), and Actinopterygii (ray-finned fish) have been identified. However, two essential species, ratfish in Chondrichthyes and hagfish in Agnatha, are still missing in the evolutionary image of this molecule. The present study reports analysis of POMC cDNA in the ratfish, Chimaera phantasma, which belongs to another subclass in the Chondrichthyes. Partial cDNA clones were amplified by PCR from single-strand cDNA prepared on total RNA from a complex of pituitary and hypothalamus, and subsequently overlapped to obtain a full-length sequence. Ratfish POMC cDNA consists of 1294bp excluding the poly(A) tail. It encodes a signal peptide of 25 amino acids and POMC of 300 amino acids. gamma-MSH, ACTH, alpha-MSH, delta-MSH, beta-MSH, and beta-END are located at prePOMC (76-87), (120-158), (120-132), (212-227), (275-290), and (293-325), respectively. delta-MSH, originally found in elasmobranch POMCs, was also present in ratfish POMC, suggesting this structure might have appeared after the divergence of chondrichthians from the ancestral lineage. Thus, we demonstrated the common occurrence of four MSHs in chondrichthian POMC and established a clear understanding of the molecular evolution of POMC in gnathostomes.

Characterizing a proopiomelanocortin cDNA cloned from the brain of the Bichir, Polypterus senegalus: evaluating phylogenetic relationships among ray-finned fish

There is general agreement that the polypteriform fishes, like Polypterus senegalus, constitute a unique lineage in the evolution of the vertebrates. However, the precise position of these fishes had been a point of controversy since the time of Darwin and Huxley. There is now consensus that the polypteriform fishes are members of superorder Actinopterygii. However, within the Actinopterygii, it is still debatable as to whether the polypteriform fishes are an early offshoot of the Actinopterygii or a more recent sister group to the sturgeon and other extant chondrostean fishes. In this study the sequence of proopiomelanocortin (POMC), the common precursor for the melanocortins and beta-endorphin, was used to evaluate the phylogenetic position of the polypteriform fishes relative to other bony fishes. 3(')RACE and 5(')RACE protocols were used to amplify overlapping regions of a POMC cDNA from the brain of P. senegalus. The full-length POMC cDNA had an open reading frame that encoded 259 amino acids. As seen in most gnathostomes, P. senegalus POMC has three melanocortin sequences (ACTH/alpha-MSH, gamma-MSH, and beta-MSH), and a beta-endorphin region. For phylogenetic analysis, the following POMC sequences were aligned at the amino acid level and analyzed using a maximum parsimony algorithm: P. senegalus, dogfish, sturgeon A, paddlefish A, sockeye salmon A, tilapia, and gar. The dogfish POMC sequence was used as the out-group. In this analysis the P. senegalus POMC sequence formed a clade with the chondrostean POMC sequences (sturgeon A and paddlefish A), and not with the neopterygian sequences (sockeye salmon A, tilapia, and gar). P. senegalus POMC is remarkably similar to sturgeon POMC A. In particular, in both precursors there is evidence for degeneration at the proteolytic cleavage site that precedes the gamma-MSH sequence. Based on the analysis of this nuclear gene it would appear that P. senegalus belongs to a branch of the chrondrostean lineage rather than representing a lineage of ray-finned fish that is ancestral to the chondrostean and neoptyergian ray-finned fishes. Alternatively, if the polypteriform fishes are in fact an early offshoot of the Actinopterygii (the traditional view), then the observations made for P. senegalus POMC relative to the chondrostean POMC sequences is the result of convergence.

The central melanocortin system regulates food intake in goldfish

Posttranscriptional processing of proopiomelanocortin (POMC) yields melanocortin peptides, which are involved in the regulation of energy balance in mammals. The sequence preservation of the main brain melanocortin, alpha-melanocyte-stimulating hormone (alpha-MSH), suggests a conserved function throughout vertebrate evolution. We studied the involvement of the central melanocortin system in the control of food intake in the goldfish. In situ hybridization studies done following molecular cloning of POMC mRNA demonstrated positive POMC mRNA cell bodies exclusively expressed within the mediobasal hypothalamus, in the anterior, posterior and inferior part of the lateral tuberal nucleus and the medial region of the lateral recess nucleus. POMC expression is localized in brain areas appropriate for involvement in food intake and neuroendocrine regulation. Progressive fasting did not affect POMC mRNA expression levels. Intracerebroventricular administration of [Nle(4), D-Phe(7)]-alpha-MSH (NDP-alpha-MSH), a universal melanocortin agonist, within nanomolar range, dose-dependently inhibited food intake 2 h after treatment. The results show for the first time a functional melanocortin system in fishes that participates in central regulation of food intake. The conserved central expression pattern of POMC mRNA and role of MSH peptides in physiological regulation of food intake suggests that melanocortin functions were gained early in vertebrate evolution.

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.

Evaluating the radiation of the POMC gene in teleosts: characterization of American eel POMC

A distinctive feature of the pituitary hormone precursor, proopiomelanocortin (POMC), is the presence of multiple melanocortin core sequences (HFRW), and one copy of the opioid, beta-endorphin. In the older lineages of ray-finned fish (i.e., orders Acipenseriformes and Semionotiformes), certain extant lobe-finned fish (Australian lungfish and African lungfish), and the tetrapods there are three melanocortin regions in POMC: ACTH/alphaMSH, beta-MSH, and gamma-MSH. However, among the teleosts, the most recent radiation of the ray-finned fishes, the gamma-MSH sequence is absent from the POMC genes of euteleosts like the carp, tilapia, chum salmon, sockeye salmon, and rainbow trout. The objective of this study was to determine whether the gamma-MSH sequence still may be present in the POMC gene of a more basal lineage of the teleosts such as a representative from subdivision Elopomorpha. To this end, a POMC cDNA was cloned and sequenced from the pituitary of the American eel, Anguilla rostrata (order Anguilliformes, family Anguillidae). The open reading frame of the eel POMC cDNA was 648 nucleotides in length and encoded 216 amino acids. As predicted, eel POMC contained the deduced amino acid sequences for beta-endorphin, ACTH/alpha-MSH, and beta-MSH. These end-products displayed primary sequence features that are common to ray-finned fish. Eel POMC lacks a gamma-MSH sequence and a large portion of the joining peptide region. In this regard, the eel POMC gene thus displays features very similar to the POMC genes that have been sequenced from euteleosts. Although it is conceivable that the gamma-MSH sequence may be present in representatives from the other basal extant lineages of teleosts (i.e., subdivisions Osteoglossomorpha or Clupeomorpha), it is also possible that the deletion that resulted in the loss of the gamma-MSH sequence occurred in the ancestral neopterygian that gave rise to the teleosts. In this case, the gamma-MSH sequence should be absent in all extant teleosts.

Ontogeny of a novel decapeptide derived from POMC-A in the brain and pituitary of the rainbow trout

Trout POMC-A exhibits a unique C-terminal extension of 25-amino acids which is processed in the pituitary and hypothalamus to generate two novel decapeptides, EQWGREEGEE and ALGERKYHFQ-NH(2). The fibers containing these two decapeptides are widely distributed in the brain, suggesting that they may exert neurotransmitter or neuromodulator activities. In the present study, we have investigated the ontogeny of the decapeptide EQWGREEGEE in the trout pituitary and brain. In the pituitary of 29-day embryos and 33-day alevins, EQWGREEGEE-immunoreactive material was observed in a cluster of cells located in the central and rostral region of the gland, respectively. In 47-day alevins, a second group of cells exhibiting EQWGREEGEE-like immunoreactivity was detected in the caudal region of the pituitary and the intensity of labeling in these cells increased in 61-day fry. In the brain, EQWGREEGEE immunoreactivity was detected in 47-day alevins. In 47- and 61-day larvae, immunoreactive elements were mainly detected in the diencephalon. Characterization of the immunoreactive material by reversed-phase high-performance liquid chromatographic analysis combined with radioimmunoassay detection revealed the existence of two major forms which exhibited different retention times than synthetic EQWGREEGEE. The present study indicates that EQWGREEGEE-related peptides are present in the trout pituitary early during ontogeny and appear in the brain only later, and that processing of the C-terminal extension of POMC-A generates distinct molecular species at different developmental stages. These data suggest that alternative processing of the C-terminal domain of POMC-A gives rise to various peptide products that may exert specific activities during trout development.

Molecular cloning of proopiomelanocortin (POMC) cDNA from mud turtle, Pelodiscus sinensis

The complete complementary DNA (cDNA) of proopiomelanocortin (POMC), a common precursor of opioid hormone beta-endorphin, melanotropin (MSH), and corticortropin (ACTH), was cloned and sequenced from pituitary and hypothalamus of mud turtle (Pelodiscus sinensis) by RT-PCR and rapid amplification of cDNA end (RACE) methods. Two transcripts of POMC mRNAs with different polyadenylation sites were observed. Both transcripts had an open reading frame encoding a 261-amino acid peptide containing nine dibasic amino acids (pair of Arg and Lys), putative proteolytic cleavage sites for processing to functional peptides. All the functional peptide fragments of mud turtle POMC, gamma-MSH, alpha-MSH, ACTH, beta-MSH, and beta-endorphin, are flanked by dibasic residues as found in other tetrapods, implying that it could be processed to give rise to all members of POMC-derived peptides. The deduced amino acid sequences of mud turtle POMC displays 63-67% identity with amphibian, 59% with chicken, 48-53% with mammals, and 37-59% identity with fish. However, functional peptide fragments are much more conserved than overall sequence and intervening fragments. In addition to pituitary and brain, mud turtle POMC mRNAs are also expressed in many peripheral tissues, such as skin, thyroid, and testis. This is the first report on the complete sequence of cDNA nucleotides and deduced amino acids of POMC in reptile.

Identification of proopiomelanocortin-related peptides in the rostral pars distalis of the pituitary in coelacanth: evolutional implications

The coelacanth fish, genus Latimeria, flourished during the Devonian Period and is considered among the closest living relatives of tetrapods. It may therefore provide important information on the evolution of fishes into tetrapods. However, little is known about the components of the endocrine system in this fish. Here we describe the structural characterization of pituitary hormones derived from proopiomelanocortin (POMC) in Latimeria chalumnae. We identified alpha-melanocyte-stimulating hormone (MSH), N-Des-acetyl-alpha-MSH, beta-MSH, N-terminal peptide containing gamma-MSH, corticotropin-like intermediate lobe peptide (CLIP), and N-acetyl-beta-endorpin (END) in an extract from the rostral pars distalis of the pituitary by reversed-phase high-performance liquid chromatography, amino acid sequence analysis, and mass spectrometry. The occurrence of three different MSHs and one beta-END indicates that the structural organization of coelacanth POMC is the same as that of lungfish, tetrapods, and primitive ray-finned fish. The coelacanth alpha-MSH is identical to its mammalian counterpart. The coelacanth beta-MSH shows the highest sequence identity with the amphibian counterpart, and gamma-MSH and CLIP show the highest sequence identity with their amphibian and bird counterparts, whereas coelacanth beta-END is most similar to the sturgeon peptide. The coexistence of tetrapod-type and fish-type characteristics in the putative coelacanth POMC molecule reflects the phylogenetic position of this fish. When each hormonal segment was compared between coelacanth, lungfish, and tetrapod, MSH and CLIP of coelacanth were closer to their tetrapod counterparts than those of lungfish, whereas beta-MSH and beta-END of coelacanth are less closely related to their tetrapod counterparts than those of lungfish. gamma-MSH and CLIP may have evolved at a different rate from beta-MSH and beta-END in both the coelacanth and lungfish.

Isolation and structure-bioactivity characterization of glycosylated N-pro-opiomelanocortin isoforms

The N-terminal fragment of mouse pro-opiomelanocortin (N-POMC) was isolated from AtT-20 cell-conditioned medium on the basis of immunoreactivity to an anti-POMC1-50 monoclonal antibody by a concentration step, a cation exchange step, reversed phase high-performance liquid chromatography (HPLC) and size exclusion HPLC. Two groups of N-POMC isoforms with a molecular weight (MW) of approximately 11 kDa and 13 kDa, respectively, were identified by mass spectrometry and N-terminal amino acid sequencing. C-terminal sequencing indicated that 11 kDa isoforms correspond to POMC1-74 and 13 kDa isoforms to POMC1-95. Isoforms from both groups enhanced the prolactin mRNA content (measured by means of TaqMan real-time reverse transcription-polymerase chain reaction) in cultured rat pituitary cell aggregates in a dose-dependent manner, but not all of them showed this activity. POMC1-74 compounds were significantly more potent than POMC1-95 isoforms. The observed effects were abolished by coincubation with the monoclonal anti-POMC1-50 antibody, showing the specificity of this biological action. Incorporation of bromodeoxyuridine into DNA of immunostained lactotrophs was enhanced by only a minor part of the isoforms. Some of these had no effect on prolactin mRNA expression. The N-POMC isoforms appeared to be N- and at least in part O-glycosylated. After enzymatic N-deglycosylation of selected N-POMC isoforms, the stimulatory effect on the prolactin mRNA level was depressed (in case of the POMC1-95 isoforms) or totally abolished (in case of the POMC1-74 isoforms). The present findings show that N-POMC is a mixture of differentially glycosylated isoforms, that the isoforms of POMC1-74 are the biologically more effective forms and that different isoforms induce different biological responses in the same cell population. The data also show the essential role of N-glycosylation in the biological response.

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.

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 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.