Sturgeon proopiomelanocortin has a remnant of gamma-melanotropin

Hypothalamus-pituitary-interrenal (HPI) axis signaling in Atlantic sturgeon (Acipenser oxyrinchus) and sterlet (Acipenser ruthenus)

In vertebrates, the hypothalamic-pituitary-adrenal/interrenal (HPA/HPI) axis is a highly conserved endocrine axis that regulates glucocorticoid production via signaling by corticotropin releasing hormone (CRH) and adrenocorticotropic hormone (ACTH). Once activated by ACTH, G_s protein-coupled melanocortin 2 receptors (Mc2r) present in corticosteroidogenic cells stimulate expression of steroidogenic acute regulatory protein (Star), which initiates steroid biosynthesis. In the present study, we examined the tissue distribution of genes involved in HPI axis signaling and steroidogenesis in the Atlantic sturgeon (Acipenser oxyrinchus) and provided the first functional characterization of Mc2r in sturgeon. Mc2r of A. oxyrinchus and the sterlet sturgeon (Acipenser ruthenus) are co-dependent on interaction with the melanocortin receptor accessory protein 1 (Mrap1) and highly selective for human (h) ACTH over other melanocortin ligands. A. oxyrinchus expresses key genes involved in HPI axis signaling in a tissue-specific manner that is indicative of the presence of a complete HPI axis in sturgeon. Importantly, we co-localized mc2r, mrap1, and star mRNA expression to the head kidney, indicating that this is possibly a site of ACTH-mediated corticosteroidogenesis in sturgeon. Our results are discussed in the context of other studies on the HPI axis of basal bony vertebrates, which, when taken together, demonstrate a need to better resolve the evolution of HPI axis signaling in vertebrates.

Ancient fishes and the functional evolution of the corticosteroid stress response in vertebrates

The neuroendocrine mechanism underlying stress responses in vertebrates is hypothesized to be highly conserved and evolutionarily ancient. Indeed, elements of this mechanism, from the brain to steroidogenic tissue, are present in all vertebrate groups; yet, evidence of the function and even identity of some elements of the hypothalamus-pituitary-adrenal/interrenal (HPA/I) axis is equivocal among the most basal vertebrates. The purpose of this review is to discuss the functional evolution of the HPA/I axis in vertebrates with a focus on our understanding of this neuroendocrine mechanism in the most ancient vertebrates: the agnathan (i.e., hagfish and lamprey) and chondrichthyan fishes (i.e., sharks, rays, and chimeras). A review of the current literature presents evidence of a conserved HPA/I axis in jawed vertebrates (i.e., gnathostomes); yet, available data in jawless (i.e., agnathan) and chondrichthyan fishes are limited. Neuroendocrine regulation of corticosteroidogenesis in agnathans and chondrichthyans appears to function through similar pathways as in bony fishes and tetrapods; however, key elements have yet to be identified and the involvement of melanotropins and gonadotropin-releasing hormone in the stress axis in these ancient fishes warrants further investigation. Further, the identities of physiological glucocorticoids are uncertain in hagfishes, chondrichthyans, and even coelacanths. Resolving these and other knowledge gaps in the stress response of ancient fishes will be significant for advancing knowledge of the evolutionary origins of the vertebrate stress response.

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.

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.

Structural and functional diversity of proopiomelanocortin in fish with special reference to barfin flounder

Proopiomelanocortin (POMC) is a precursor of adrenocorticotropic hormone (ACTH), melanocyte-stimulating hormone (MSH), and endorphin (END). We have characterized POMC systems in barfin flounder. The results revealed unique aspects of POMC systems. Notable features in terms of pituitary functions are the occurrence of three functional POMC genes, the mutation of an essential sequence in the beta-END in one of the genes, occurrence of alpha-MSH in addition to ACTH in the pars distalis of the pituitary, and expression of the three genes in a single cell. While MSHs stimulate pigment dispersion, expression of the POMC gene and plasma levels of MSH do not always respond to background color changes between black and white. The functions of MSHs in skin pigmentation are very unique, because acetylation at the N-terminal of alpha-MSH inhibits its pigment dispersing activity. This is in contrast to results from other teleosts and amphibians, in which acetylation increases the activity. In the skin, the POMC gene is expressed in the non-chromatophoric dermal cells, indicating that MSH produced in the skin de novo has a paracrine function. The detection of MSH peptides in skin extracts seems to show that the control of skin pigmentation by MSHs is twofold-endocrine control by the pituitary, and paracrine control by the skin itself. Thus, fish provide an interesting model to help understand the structural and functional diversity of POMC systems. In this review, we provide an overview of our recent studies on the characterization of molecules and biological significance of POMC systems in barfin flounder.

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.

Cloning of sole proopiomelanocortin (POMC) cDNA and the effects of stocking density on POMC mRNA and growth rate in sole, Solea solea

Proopiomelanocortin (POMC) is an important gene implicated in different functions, such as the stress response of the hypothalamus-pituitary-adrenal axis. The aim of the present study was to determine whether farming conditions, such as stocking density, can be considered a powerful stressor influencing in turn the growth rate in juvenile fish. Thus, POMC cDNA expression was investigated during adaptation to farming conditions in sole (Solea solea), as a model for studying the effects of rearing densities on stress response; different stocking densities (50, 100, and 250 animals/m(2)) were applied and, after 7 and 21 days, the fishes were examined for body weight and plasma cortisol levels as indicators of stress. In addition, proopiomelanocortin was cloned and sequenced from the brain of sole, allowing semi-quantitative RT-PCR to be performed to evaluate POMC mRNA expression in brain tissue. There was a significant increase in cortisol levels in fish reared at high stocking densities of 250/m(2) compared to fish reared at control densities of 100 and 50/m(2), in both experimental times, i.e., 7 and 21 days. The high stocking densities were also found to decrease the specific growth rate of fish. Moreover, it was demonstrated that the highest stocking density induced a significant decrease in sole POMC mRNA expression. It is concluded that POMC and cortisol are both involved in the stress response due to high rearing densities, during which cortisol may serve as a negative regulator of POMC.

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.

Gonadotropin-like and adrenocorticotropin-like cells in the pituitary gland of hagfish, Paramyxine atami; immunohistochemistry in combination with lectin histochemistry

The pituitary system of the hagfish remains an enigma. The present study has aimed to detect possible adenohypophysial hormones in the pituitary gland of the brown hagfish, Paramyxine atami, by means of immunohistochemistry in combination with lectin histochemistry. Rabbit antisera raised against ovine luteinizing hormone (LH)beta, proopiomelanocortin (POMC)-related peptides, and the growth hormone/prolactin family of tetrapod and fish species were used, and 25 kinds of lectins were tested. Three different types of adenohypophysial cells were revealed in the pituitary of brown hagfish. The first was stained with both anti-ovine LH beta and several D-mannose-binding lectins, such as Lens culinaris agglutinin and Pisum sativum agglutinin. This cell type predominated in the adenohypophysis in adults with developing gonads and thus appeared to be involved in the regulation of gonadal functions. The second was negative for anti-ovine LH beta but was stained with several N-acetylglucosamine-binding lectins, such as wheat germ agglutinin and Lycopersicon esculentum lectin. This cell type exhibited a weak positive reaction with anti-lamprey adrenocorticotropin (ACTH) and thus appeared to be related to POMC-like cells. The second cell type was found in the adenohypophysis regardless of the developmental state of the gonads. The third cell type was negative for both antisera and lectins. Since this cell type was numerous in juveniles and adults without developing gonads, most cells of this type were probably undifferentiated. These findings suggest that GTH and ACTH are major adenohypophysial hormones in the hagfish.

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.

Cloning of two melanocortin (MC) receptors in spiny dogfish: MC3 receptor in cartilaginous fish shows high affinity to ACTH-derived peptides while it has lower preference to gamma-MSH

We report the cloning and characterization of two melanocortin receptors (MCRs) from the spiny dogfish (Squalus acanthias) (Sac). Phylogenetic analysis shows that these shark receptors are orthologues of the MC3R and MC5R subtypes, sharing 65% and 70% overall amino acid identity with the human counterparts, respectively. The SacMC3R was expressed and pharmacologically characterized in HEK293 cells. The radioligand binding results show that this receptor has high affinity for adrenocorticotropic hormone (ACTH)-derived peptides while it has comparable affinity for alpha- and beta-melanocyte stimulating hormone (MSH), and slightly lower affinity for gamma-MSH when compared with the human orthologue. ACTH(1-24) has high potency in a second-messenger cAMP assay while alpha- and gamma-MSH had slightly lower potency in cells expressing the SacMC3R. We used receptor-enhanced green fluorescence protein (EGFP) fusion to show the presence of SacMC3R in plasma membrane of Chinese hamster ovary and HEK293 cells but the SacMC5R was retained in intracellular compartments of these cells hindering pharmacological characterization. The anatomical distribution of the receptors were determined using reverse transcription PCR. The results showed that the SacMC3R is expressed in the hypothalamus, brain stem and telencephalon, optic tectum and olfactory bulbs, but not in the cerebellum of the spiny dogfish while the SacMC5R was found only in the same central regions. This report describes the first molecular characterization of a MC3R in fish. The study indicates that many of the important elements of the MC system existed before radiation of gnathostomes, early in vertebrate evolution, at least 450 million years ago.

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.

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.

The melanocortin system in Fugu: determination of POMC/AGRP/MCR gene repertoire and synteny, as well as pharmacology and anatomical distribution of the MCRs

The G-protein-coupled melanocortin receptors (MCRs) play an important role in a variety of essential functions such as the regulation of pigmentation, energy homeostasis, and steroid production. We performed a comprehensive characterization of the MC system in Fugu (Takifugu rubripes). We show that Fugu has an AGRP gene with high degree of conservation in the C-terminal region in addition to a POMC gene lacking gamma-MSH. The Fugu genome contains single copies of four MCRs, whereas the MC3R is missing. The MC2R and MC5R are found in tandem and remarkably contain one and two introns, respectively. We suggest that these introns were inserted through a reverse splicing mechanism into the DRY motif that is widely conserved through GPCRs. We were able to assemble large blocks around the MCRs in Fugu, showing remarkable synteny with human chromosomes 16 and 18. Detailed pharmacological characterization showed that ACTH had surprisingly high affinity for the Fugu MC1R and MC4R, whereas alpha-MSH had lower affinity. We also showed that the MC2R gene in Fugu codes for an ACTH receptor, which did not respond to alpha-MSH. All the Fugu receptors were able to couple functionally to cAMP production in line with the mammalian orthologs. The anatomical characterization shows that the MC2R is expressed in the brain in addition to the head-kidney, whereas the MC4R and MC5R are found in both brain regions and peripheral tissues. This is the first comprehensive genomic and functional characterization of a GPCR family within the Fugu genome. The study shows that some parts of the MC system are highly conserved through vertebrate evolution, such as regions in POMC coding for ACTH, alpha-MSH, and beta-MSH, the C-terminal region of AGRP, key binding units within the MC1R, MC2R, MC4R, and MC5R, synteny blocks around the MCRs, pharmacological properties of the MC2R, whereas other parts in the system are either missing, such as the MC3R and gamma-MSH, or different as compared to mammals, such as the affinity of ACTH and MSH peptides to MC1R and MC4R and the anatomical expression pattern of the MCRs.

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.

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.

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.

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.

Modulation of fish phagocytic cells by N-terminal peptides of proopiomelanocortin (NPP)

N-terminal peptide of proopiomelanocortin (NPP, or pro-gamma-MSH) has shown to exhibit biological activity such as stimulation of adrenal mitogenesis and prolactin release-inhibiting factor activity. Structurally, studies reveal a significant difference between fish NPP from that of tetrapods, as NPPs from carp and salmonid lack gamma-MSH. Thus, fish NPP may exhibit functions different from that of mammals. The activation of phagocytic cells by NPP was analysed using rainbow trout Oncorhynchus mykiss and carp Cyprinus carpio. Rainbow trout and carp macrophages incubated with chum salmon NPP significantly enhanced the production of superoxide anion in comparison with control macrophages (without hormones). Both rainbow trout and carp macrophages had shown increased phagocytosis when stimulated administered with NPP. The above results were complemented by in vivo studies where NPP was administered to rainbow trout and carp. NPP significantly increased superoxide anion production as well as phagocytosis in macrophages. These results show that NPP in lower vertebrates activates the function of the phagocytic cells.

Update: brain and pituitary hormones of lampreys

Lampreys and hagfish of the class Agnatha are of particular importance in understanding endocrinological relationships since they represent the oldest lineages of extant vertebrates which evolved over 550 million years ago. This review briefly summarizes the latest findings on the reproductive endocrinology of the sea lampreys. Since the First International Symposium of Fish Endocrinology in 1988, when virtually little was known of the hypothalamic-pituitary-gonadal axis, substantial new biochemical, molecular, physiological and immunological evidence has now clearly shown that lamprey reproduction is controlled by the neuroendocrine axis. In addition, five brain and six pituitary hormones of lampreys have been identified mainly by Sower and Kawauchi and colleagues between 1986 and 2000. We now hypothesize that lamprey reproduction is a highly synchronized process that is initiated or mediated by a coordination of complex integration of environmental cues and hormonal mechanisms which is broadly similar to that exhibited by gnathostome vertebrates.

Stimulation of intracellular free calcium in GH3 cells by gamma3-melanocyte-stimulating hormone. Involvement of a novel melanocortin receptor?

The melanocortin (MC) gamma3MSH is a peptide that can be generated from the N-terminal domain of POMC and is believed to signal through the MC3 receptor. We recently showed that it induces a sustained rise in intracellular free calcium levels ([Ca(2+)](i)) in a subpopulation of pituitary cells, particularly in the lactosomatotroph lineage. In the present study we report that gamma3MSH and some analogs increase [Ca(2+)](i) in the GH- and PRL-secreting GH3 cell line and evaluate on the basis of pharmacological experiments and gene expression studies which MC receptor may be involved. A dose as low as 1 pM gamma3MSH induced an oscillating [Ca(2+)](i) increase in a significant percentage of GH3 cells. Increasing the dose recruited an increasing number of responding cells; a maximum was reached at 0.1 nM. gamma2MSH, alphaMSH, and NDP-alphaMSH displayed a similar effect. SHU9119, an MC3 and MC4 receptor antagonist, and an MC5 receptor agonist, did not affect the number of cells showing a [Ca(2+)](i) rise in response to gamma3MSH. SHU9119 had also no effect when added alone. MTII, a potent synthetic agonist of the MC3, MC4, and MC5 receptor as well as an N-terminally extended recombinant analog of gamma3MSH showed low potency in increasing [Ca(2+)](i) in GH3 cells, but high potency in stimulating cAMP accumulation in HEK 293 cells stably transfected with the MC3 receptor. In contrast, a peptide corresponding to the gamma2MSH sequence of POMC-A of Acipenser transmontanus increased [Ca(2+)](i) in GH3 cells, but was about 50 times less potent than gamma2- or gamma3MSH in stimulating cAMP accumulation in the MC3 receptor expressing HEK 293 cells. By means of RT-PCR performed on a RNA extract from GH3 cells, the messenger RNA of the MC2, MC3, and MC4 receptor was undetectable, but messenger RNA of the MC5 receptor was clearly present. These data suggest that the GH3 cell line does not mediate the effect of gamma3MSH through the MC3 receptor. The involvement of the MC5 receptor is unlikely, but cannot definitely be excluded. The findings animate the hypothesis that there exists a second, hitherto unidentified, MC receptor that displays high affinity for gamma3MSH.

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.

Molecular cloning of lungfish proopiomelanocortin cDNA

To investigate the evolution of proopiomelanocortin (POMC) from fish to tetrapods, nucleotide sequence of POMC cDNA from a lobe-finned fish, the African lungfish, was determined. POMC cDNA was prepared from lungfish pituitary glands. The POMC cDNA is composed of 1114 bp, excluding a poly-A tail, and encodes 255 amino acids (aa) including a signal peptide of 25 aa. The lungfish POMC contains the segment corresponding to gamma-melanotropin (MSH), corticotropin, alpha-MSH, beta-MSH, and beta-endorphin at positions (50-61), (108-146), (108-120), (178-194), and (197-230), respectively. The lungfish POMC shows greater sequence identity on average with amphibian (62%), ancient ray-finned fishes including acipenseriformes and semionotiformes (62%), and mammalian POMC (52%) than with teleostean (49%), elasmobranch (46%), and agnathan POMC (31%). Thus, the overall structural feature of lungfish POMC is close to the tetrapod POMCs which contain gamma-MSH and the ancient ray-finned fishes POMCs containing gamma-MSH-like sequence. However, amino acid sequence of lungfish beta-endorphin exhibits properties which are specifically observed in the ray-finned fishes and the elasmobranchs.

Molecular cloning and characterization of the chicken pro-opiomelanocortin (POMC) gene

The gene for pro-opiomelanocortin (POMC), a common precursor of melanocortins, lipotropins and beta-endorphin, was isolated in the chicken first among avian species. The chicken POMC gene was found to be a single copy gene and appeared to show the same structural organization as that of other species of different classes. The predicted POMC displayed the highest identity to Xenopus POMC(A) (60. 1%), and consisted of 251 amino acid residues with nine proteolytic cleavage sites, suggesting that it could be processed to give rise to all members of the melanocortin family, including adrenocorticotropic hormone and alpha-, beta- and gamma-melanocyte-stimulating hormones, as well as the other POMC-derived peptides. RT-PCR analysis detected the POMC mRNA in the brain, adrenal gland, gonads, kidney, uropygial gland and adipose tissues, each of which has been demonstrated to express melanocortin receptors. These results suggest that melanocortins act in a paracrine and/or autocrine manner to control a variety of functions both in the brain and in the peripheral tissues in the chicken.

A newly characterized melanotropin in proopiomelanocortin in pituitaries of an elasmobranch, Squalus acanthias

Proopiomelanocortin (POMC) is a precursor for corticotropin (ACTH), three or fewer molecular types of melanotropin (MSH), and beta-endorphin. This protein is thought to have evolved by duplication of MSH genomic segments. Here we report that the POMC in the dogfish, an elasmobranch, contains a fourth type of MSH in addition to classical alpha-, beta-, and gamma-MSH. POMC cDNA was amplified by PCR from double-strand cDNA prepared from dogfish pituitary and ligated into lambdaZAP II. The POMC cDNA is composed of 1315 bp without a poly(A) tail. Northern blot analysis detected a 1.4-kb signal of dogfish POMC mRNA. An open reading frame of the POMC cDNA encodes 320 amino acids, including a signal peptide of 26 amino acids. The dogfish POMC includes gamma-MSH, ACTH, alpha-MSH, beta-MSH, and beta-endorphin at positions 50-61, 115-153, 115-127, 239-256, and 259-294, respectively. In addition to these classical peptides, a newly discovered MSH, which we have termed delta-MSH, is present in dogfish POMC at position (184-195). The four dogfish MSHs can be separated into two groups based on their sequence identities: one pair consists of alpha-MSH and gamma-MSH, and the other consists of beta-MSH and delta-MSH, suggesting that gamma-MSH and delta-MSH may have been duplicated evolutionarily from alpha-MSH and beta-MSH, respectively. gamma-MSH might first have appeared in early gnathostomes because it is absent in the most primitive vertebrate group, the agnathans. delta-MSH, which at this time is found only in chondrichthians, might have appeared after the divergence of chondrichthians from a lineage leading to osteichthyans and tetrapods.

Cloning of a second proopiomelanocortin cDNA from the pituitary of the sturgeon, Acipenser transmontanus

A recent study on the pituitary of the sturgeon, Acipenser transmontanus, resulted in the cloning of a cDNA that codes for the prohormone, proopiomelanocortin (POMC). This cDNA is designated sturgeon POMC A. Subsequent analysis of the sturgeon pituitary uncovered a second distinct POMC cDNA (sturgeon POMC B). In both sturgeon POMC cDNAs the open reading frame is 795 nucleotides in length. However, the two sturgeon POMC cDNAs differ at 26 amino acid positions in the opening frame. In addition, the 2 forms of POMC differ at 45 nucleotide positions within the open reading frame. The number and types of point mutations are compared in the 2 sturgeons POMC cDNAs, and the origin of the two POMC genes is discussed.

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.

A new family of growth and differentiation factors derived from the N-terminal domain of proopiomelanocortin (N-POMC)

There is a large body of in vitro evidence that pituitary function is not only dependent on hormonal signals from the brain but also on paracrine signals produced in the tissue itself. These signals appear to be involved in the control of pituitary hormone secretion as well as in pituitary cell differentiation and development (for review see Denef C. Paracrine mechanisms in the pituitary. In: Imura H, editor. The pituitary gland, 2nd ed. Raven Press, 1994: 351-378; Denef C. Autocrine/paracrine intermediates in hormone action and modulation of cellular responses to hormones. In: Conn M, editor. Handbook of Physiology. New York: Oxford University Press, 1998 (in press)). The paracrine factors which have been identified in the pituitary belong to diverse biological molecules such as neuropeptides, acetylcholine, growth factors, cytokines and posttranslationally modified derivatives of pituitary hormone such as cleaved prolactin (PRL) and the gonadotropin alpha-subunit. Recently, we have identified several N-terminal fragments of the polypeptide proopiomelanocortin (POMC) as a novel family of growth and differentiation factors in the rat anterior pituitary.

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.