Characterization of pro-opiomelanocortin cDNA from the Old World monkey, Macaca nemestrina

Beta-turn secondary structure and melanocortin ligands

The melanocortin pathway has emerged during this past decade as an important target area for the discovery and development of therapeutic agents related to obesity and type 2 diabetes. This peptide-G-protein coupled receptor (GPCR) pathway has evolved from peptide-based ligands to small molecules possessing a variety of different molecular scaffolds. Herein, we summarize the originating hypothesis of the importance of the reverse beta-turn secondary structure for agonist ligand potency at the melanocortin receptors and how that information was utilized for the discovery of small molecules based upon this type of turn structure.

Beta-endorphin levels in longtailed and pigtailed macaques vary by abnormal behavior rating and sex

Frequent or severe abnormal behavior may be associated with the release of endorphins that positively reinforce the behavior with an opiate euphoria or analgesia. One line of research exploring this association involves the superhormone, proopiomelanocortin (POMC). The products of POMC appear to be dysregulated in some human subjects who exhibit self-injurious behavior (SIB). Macaque monkeys have POMC very similar to humans, and some laboratory macaques display SIB or frequent stereotypies. We investigated associations between plasma levels of three immunoreactive POMC fragments with possible opioid action and abnormal behavior ratings in macaques. In 58 adult male and female macaques (24 Macaca fascicularis and 34 Macaca nemestrina), plasma levels of intact beta-endorphin (betaE) and the N-terminal fragment (BEN) were significantly higher in animals with higher levels of abnormal behavior. The C-terminal fragment (BEC) was significantly higher in males but unrelated to ratings of abnormal behavior. Levels of ACTH, cortisol, and (betaE-ACTH)/betaE dysregulation index were unrelated to abnormal behavior. None of the POMC products differed significantly by subjects' species, age, or weight. The finding that intact beta-endorphin is positively related to abnormal behavior in two species of macaque is consistent with some previous research on human subjects and nonprimates. The positive relation of the N-terminal fragment of betaE to abnormal behavior is a new finding.

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.

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 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.

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

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

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.

Molecular evolution of the opioid/orphanin gene family

Gene duplication is a recurring theme in the evolution of vertebrate polypeptide hormones and neuropeptides. These duplication events can lead to the formation of gene families in which divergence of function is the usual outcome. In the case of the opioid/orphanin family of genes, duplication events have proceeded along two paths: (a) an apparent duplication of function as seen in the analgesic activity of Proenkephalin and Prodynorphin end-products; and (b) divergence of function as seen in the nociceptic activity of Proorphanin end-products or the melanocortin (color change and chronic stress regulation) activity of Proopiomelanocortin end-products. Although genes coding for Proopiomelanocortin, Proenkephalin, Prodynorphin, and Proorphanin have been extensively studied in mammals, the distribution and radiation of these genes in nonmammalian vertebrates is less well understood. This review will present the hypothesis that the radiation of the opioid/orphanin gene family is the result of the duplication and divergence of the Proenkephalin gene during the radiation of the chordates. To evaluate the Proenkephalin gene duplication hypothesis, a 3'RACE procedure was used to screen for the presence of Prodynorphin-related, Proenkephalin-related, and Proorphanin-related cDNAs expressed in the brains of nonmammalian vertebrates.

Sturgeon proopiomelanocortin has a remnant of gamma-melanotropin

For the investigation of the evolution of the proopiomelanocortin (POMC) gene in early ray-finned fishes, nucleotide sequence of POMC cDNA from a chondrostean fish, the sturgeon has been determined. POMC cDNA was amplified by PCR from double-strand cDNA prepared from sturgeon pituitary and ligated with lambdaZAP II. The POMC cDNA consists of 1079 bp without a poly-A. An open reading frame of the POMC cDNA encodes 263 amino acid residues. Sturgeon POMC contains ACTH, alpha-melanotropin (MSH), beta-MSH and beta-END at positions (115-153), (115-127), (186-202) and (205-238), respectively. Location of POMC(51-72) is homologous to gamma-MSH, whereas the third residue of MSH-core sequence, His-Phe-Arg-Trp, is changed to His. Moreover, there are no basic amino acids to serve as a processing signal on the N-terminal side of POMC(51-72). These structural characteristics suggest that an ancestor of the ray-finned fishes had gamma-MSH, whereas significant mutations occurred during the evolution of chondrostean fish.

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

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

The production of the ostrich NH2-terminal POMC fragment requires cleavage at a unique signal peptidase site

The NH2-terminal fragment of ostrich proopiomelanocortin was isolated and purified following acid/acetone extraction. The amino acid sequence was deduced by automatic Edman degradation of the native as well as CNBr-, tryptic-, and S. aureus protease-derived peptides. Primary structure analysis reveals its close resemblance to other known sequences, especially to amphibian POMC. The usual Trp/Gln-Cys NH2-terminal sequence found in all other homologous sequences, is replaced here by an His-Gly-Pro-Cys sequence. In addition, the gamma-MSH sequence, contrary to salmon POMC, is present and contains three substitutions, namely a Ser, an Asn, and a Lys residue substituting the normally occurring mammalian Gly, Asp, and Arg residue, respectively. Finally, the molecular weight of this fragment as deduced from ion-spray mass spectrometry and sedimentation equilibrium centrifugation is in close agreement with the proposed structure.

Characterization of pro-opiomelanocortin processing in heterologous neuronal cells that express PC2 mRNA

We have investigated processing of monkey pro-opiomelanocortin (POMC) following transfection into heterologous neuronal Neuro 2A (N2A) cells. In several separately transfected stable cell lines (termed N2A/POMC2-like; n = 4), POMC was processed to beta E only, by direct cleavage from the precursor. Thus, these cell lines did not produce beta E in the orderly manner observed in the pituitary, that is, via the intermediate peptide beta LPH. Analysis of one representative N2A/POMC2 cell line revealed that the extent of processing to beta E appeared to be negatively correlated with precursor expression level, suggesting that the processing enzyme(s) in these cells was present in limiting amounts. Northern analysis of PC1 and PC2, two recently cloned processing enzymes, showed that N2A/POMC2 cells expressed low levels of PC2 mRNA, but no detectable PC1 mRNA. These data suggest that (1) the order of processing observed in the pituitary is not exclusively determined by tertiary folding of the precursor, but rather by the complement of processing enzymes in a particular cell, and (2) if PC2 is responsible for POMC processing in N2A/POMC2 cells, this enzyme, expressed in limiting amounts, appeared to show selectivity for the beta E amino terminal processing site.

Purification and characterization of joining peptide and N-terminal peptide of proopiomelanocortin from the pars distalis of the bullfrog pituitary

The joining peptide (JP) and the N-terminal peptide of proopiomelanocortin (NPP) were isolated from an acid-acetone extract of the distal lobe of the pituitary of the bullfrog, Rana catesbeiana, and purified by gel filtration and reverse-phase high performance liquid chromatography. The amino acid sequence of the bullfrog JP resembled the sequences of the JPs of Rana ridibunda (86% similarity) and Xenopus laevis (54% similarity), as deduced from the nucleotide sequences of their cDNAs. The amino acid sequence of bullfrog NPP showed 100%, 85%, and 50% similarity with those of Rana ridibunda, Xenopus laevis, and human NPPs, respectively. Administration of bullfrog NPP (0.05-5 micrograms/ml) to perifused Rana ridibunda interrenal slices induced a dose-dependent stimulation of corticosterone and aldosterone release. The present results indicate that the primary structure of NPP has been highly conserved during evolution. These data also reveal that NPP, which has no sequence homology with ACTH, exhibits a substantial corticotropic activity.

Molecular cloning and sequencing of a guinea-pig pro-opiomelanocortin cDNA

The guinea-pig has high levels of circulating cortisol. Though adrenocorticotropin (ACTH) levels are similar to those in other mammals, guinea-pig ACTH has been reported to have a single amino-acid substitution which results in increased bioactivity of the peptide. Pro-opiomelanocortin (POMC) is the precursor for ACTH, gamma-melanocyte-stimulating hormone (gamma-MSH) and the endogenous opioid peptide beta-endorphin. Both to confirm this substitution in guinea-pig ACTH and to establish whether other non-conservative substitutions occur elsewhere in the precursor we cloned guinea-pig POMC. The guinea-pig alanine for proline substitution at position 24 of ACTH was confirmed. Potentially significant mutations were also identified in gamma-MSH and beta-endorphin. A similar pattern of POMC mRNA expression was obtained for guinea-pig and rat as determined by Northern analysis and in situ hybridization. Southern blot analysis indicated that guinea-pig POMC is a single-copy gene. Cloning and sequencing of guinea-pig POMC thus further demonstrate the divergence of the New World hystricomorph peptides from those in New World primates, and underscore the differences observed in other endocrine axes in the guinea-pig.

Characterization of the cDNA encoding proopiomelanocortin in the frog Rana ridibunda

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

Isolation and full structural characterisation of six adrenocorticotropin-like peptides from porcine pituitary gland. Identification of three novel fragments of adrenocorticotropin and of two forms of a novel adrenocorticotropin-like peptide

A partially purified fraction of extracted porcine pituitary glands which possesses lipolytic and adrenocorticotropic activity has been characterised. It consists of six adrenocorticotropin(ACTH)-like peptides (five of which have not been previously described) which were each purified by sequential reverse-phase (rp) HPLC. Their complete primary structures were determined following amino acid compositional analysis, extensive peptide mapping and partial sequencing. Four of the fragments represent the following ACTH fragments; ACTH(1-31), ACTH(7-34), ACTH(7-36) and ACTH(7-38). By combined analytical rpHPLC and an ACTH radioimmunoassay (with an antiserum exhibiting full cross-reaction with all six ACTH variants isolated here), evidence was obtained from analysis of extracts of whole pituitary that these fragments of ACTH exist in significant amounts relative to intact ACTH(1-39). This suggests that ACTH can undergo more extensive differential proteolytic processing than previously thought. These peptides were found to possess reduced or a complete absence of ACTH-like biological activity. Therefore the biological significance of this processing needs to be resolved. The other two fragments also resembled fragments of ACTH but each possessed the same, single amino acid substitution: a threonine replacing the arginine at the position corresponding to position 8 in the ACTH sequence and had the structures [Thr8]ACTH(1-31) and [Thr8]ACTH(7-31). They possess little ACTH-like biological activity. If these variants are derived from a variant ACTH, this would be a significant finding in view of the site of the amino acid substitution and the highly conserved nature of the ACTH primary structure. The possible physiological and genetic implications are briefly discussed. In this study attempts were also made to identify the DNA coding for the mutant ACTH sequence.