A view of the N-acetylation of alpha-melanocyte-stimulating hormone and beta-endorphin from a phylogenetic perspective

Diversity of Neuropeptide Cell-Cell Signaling Molecules Generated by Proteolytic Processing Revealed by Neuropeptidomics Mass Spectrometry

Neuropeptides are short peptides in the range of 3-40 residues that are secreted for cell-cell communication in neuroendocrine systems. In the nervous system, neuropeptides comprise the largest group of neurotransmitters. In the endocrine system, neuropeptides function as peptide hormones to coordinate intercellular signaling among target physiological systems. The diversity of neuropeptide functions is defined by their distinct primary sequences, peptide lengths, proteolytic processing of pro-neuropeptide precursors, and covalent modifications. Global, untargeted neuropeptidomics mass spectrometry is advantageous for defining the structural features of the thousands to tens of thousands of neuropeptides present in biological systems. Defining neuropeptide structures is the basis for defining the proteolytic processing pathways that convert pro-neuropeptides into active peptides. Neuropeptidomics has revealed that processing of pro-neuropeptides occurs at paired basic residues sites, and at non-basic residue sites. Processing results in neuropeptides with known functions and generates novel peptides representing intervening peptide domains flanked by dibasic residue processing sites, identified by neuropeptidomics. While very short peptide products of 2-4 residues are predicted from pro-neuropeptide dibasic processing sites, such peptides have not been readily identified; therefore, it will be logical to utilize metabolomics to identify very short peptides with neuropeptidomics in future studies. Proteolytic processing is accompanied by covalent post-translational modifications (PTMs) of neuropeptides comprising C-terminal amidation, N-terminal pyroglutamate, disulfide bonds, phosphorylation, sulfation, acetylation, glycosylation, and others. Neuropeptidomics can define PTM features of neuropeptides. In summary, neuropeptidomics for untargeted, global analyses of neuropeptides is essential for elucidation of proteases that generate diverse neuropeptides for cell-cell signaling. Graphical Abstract ᅟ.

Systems approaches to genomic and epigenetic inter-regulation of peptide hormones in stress and reproduction

The evolution of the organismal stress response and fertility are two of the most important aspects that drive the fitness of a species. However, the integrated regulation of the hypothalamic pituitary adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes has been traditionally thwarted by the complexity of these systems. Pepidergic signalling systems have emerged as critical integrating systems for stress and reproduction. Current high throughput systems approaches are now providing a detailed understanding of peptide signalling in stress and reproduction. These approaches were dependent upon a long history of discovery aimed at the structural characterization of the associated molecular components. The combination of comparative genomics, microarray and epigenetic studies has led not only to a much greater understanding of the integration of stress and reproduction but also to the discovery of novel physiological systems. Recent epigenomic approaches have similarly yielded a new level of complexity in the interaction of these physiological systems. Together, such studies have provided a greater understanding of the effects of stress and reproduction.

Characterization of the sea bass melanocortin 5 receptor: a putative role in hepatic lipid metabolism

The melanocortin 5 receptor (MC5R) plays a key role in the regulation of exocrine secretion in mammalian species. This receptor has also been characterized in some fish species but its function is unknown. We report the molecular and pharmacological characterization, as well as the tissue expression pattern, of sea bass MC5R. Cloning of five active alleles showing different levels of sensitivity to endogenous melanocortin and one non-functional allele demonstrate the allelic complexity of the MC5R locus. The sea bass receptor was activated by all the melanocortins tested, with ACTH and desacetyl-MSH and beta-MSH showing the lowest efficiency. The acetylation of the MSH isoforms seems to be critical for the effectiveness of the agonist. Agouti-related protein had no effect on basal or agonist-stimulated activation of the receptor. SbMC5R was mainly expressed in the brain but lower expression levels were found in several peripheral tissues, including liver. Progressive fasting did not induce up- or downregulation of hypothalamic MC5R expression, suggesting that central MC5R is not involved in the regulation of food intake in the sea bass. MTII, a sbMC5R agonist, stimulated hepatic lipolysis in vitro, measured as free fatty acid release into the culture medium after melanocortin agonist exposure of liver fragments, suggesting that MC5R is involved in the regulation of hepatic lipid metabolism. Taken together, the data suggest that different allelic combinations may confer differential sensitivity to endogenous melanocortin in tissues where MC5R is expressed and, by extension, in hepatic lipid metabolism.

Molecular and pharmacological characterization of the melanocortin type 1 receptor in the sea bass

Melanocortin 1 receptor (MC1R) plays a key role in the physiology of the vertebrate pigment system. Point mutations producing hyperactive or inactive receptors result in darkening or paling effects, respectively. We report the molecular and pharmacological characterization, as well as the tissue expression pattern, of the sea bass Mc1r. Similar to other MC1Rs, the sea bass gene is highly polymorphic and nine DNA polymorphisms, seven of them involving an amino acid substitution, were detected. SbMc1r is mainly expressed in the testis, fat and liver with moderate levels in the ventral and dorsal skin. The sea bass receptor was activated by all the melanocortins tested, with ACTH showing the lowest efficiency. The acetylation level of the MSH isoforms seems to be critical for the effectively of the agonist. Agouti-related protein (AGRP) drastically inhibited the basal activity of the receptor in vitro, as an inverse agonist does, but only in the presence of phosphodiesterase inhibitors. This observation suggests that sbMc1r is constitutively activated and inversely regulated by AGRP, which is expressed in the skin of different fish species.

Phosphodiesterase inhibitor-dependent inverse agonism of agouti-related protein on melanocortin 4 receptor in sea bass (Dicentrarchus labrax)

The melanocortin 4 receptor (MC4R) is a G protein-coupled receptor mainly expressed in the central nervous system of vertebrates. Activation of the MC4R leads to a decrease in food intake, whereas inactivating mutations are a genetic cause of obesity. The binding of agouti-related protein (AGRP) reduces not only agonist-stimulated cAMP production (competitive antagonist) but also the basal activity of the receptor, as an inverse agonist. Transgenic zebrafish overexpressing AGRP display increased food intake and linear growth, indicative of a physiological role for the melanocortin system in the control of the energy balance in fish. We report on the cloning, pharmacological characterization, tissue distribution, and detailed brain mapping of a sea bass (Dicentrarchus labrax) MC4R ortholog. Sea bass MC4R is profusely expressed within food intake-controlling pathways of the fish brain. However, the activity of the melanocortin system during progressive fasting does not depend on the hypothalamic/pituitary proopiomelanocortin (POMC) and MC4R expression, which suggests that sea bass MC4R is constitutively activated and regulated by AGRP binding. We demonstrate that AGRP acts as competitive antagonist and reduces MTII-induced cAMP production. AGRP also decreases the basal activity of the receptor as an inverse agonist. This observation suggests that MC4R is constitutively active and supports the evolutionary conservation of the AGRP/MC4R interactions. The inverse agonism, but not the competitive antagonism, depends on the presence of a phosphodiesterase inhibitor (IBMX). This suggests that inverse agonism and competitive antagonism operate through different intracellular signaling pathways, a view that opens up new targets for the treatment of melanocortin-induced metabolic syndrome.

Analyzing the evolution of beta-endorphin post-translational processing events: studies on reptiles

In many cartilaginous fishes, most ray-finned fishes, lungfishes, and amphibians, the post-translational processing of POMC includes the monobasic cleavage of beta-endorphin to yield an opioid that is eight to ten amino acids in length. The amino acid motif within the beta-endorphin sequence required for a monobasic cleavage event is -E-R-(S/G)-Q-. Mammals and birds lack this motif and as a result beta-endorphin(1-8) is a not an end-product in either group. Since both mammals and birds were derived from ancestors with reptilian origins, an analysis of beta-endorphin sequences from extant groups of reptiles should provide insights into the manner in which beta-endorphin post-translational processing mechanisms have evolved in amniotes. To this end a POMC cDNA was cloned from the pituitary of the turtle, Chrysemys scripta. The beta-endorphin sequence in this species was compared to other reptile beta-endorphin sequences (i.e., Chinese soft shell turtle and gecko) and to known bird and mammal sequences. This analysis indicated that either the loss of the arginine residue at the cleavage site (the two turtle species, chick, and human) or a substitution at the glutamine position in the consensus sequence (gecko and ostrich) would account for the loss of the monobasic cleavage reaction in that species. Since amphibians are capable of performing the beta-endorphin monobasic reaction, it would appear that the amino acid substitutions that eliminated this post-translational process event in reptilian-related tetrapods must have occurred in the ancestral amniotes.

Roles of acetylation and other post-translational modifications in melanocortin function and interactions with endorphins

Phylogenetic, developmental, anatomic, and stimulus-specific variations in post-translational processing of POMC are well established. For melanocortins, the role of alpha-N-acetylation and the selective activities of alpha, beta, and gamma forms are of special interest. Acetylation may shift the predominant activity of POMC products between endorphinergic and melanocortinergic actions-which are often in opposition. This review addresses: (1) variations in POMC processing; (2) the influence of acetylation on the functional activity of alpha-MSH; (3) state- and stimulus-dependent effects on the proportional distribution of forms of melanocortins and endorphins; (4) divergent effects of alpha-MSH and beta-endorphin administration; (5) potential roles of beta- and gamma-MSH.

Alpha-MSH, the melanocortin-1 receptor and background adaptation in the Mozambique tilapia, Oreochromis mossambicus

The regulation of skin darkness in vertebrates is mediated by alpha-melanophore-stimulating-hormone (alphaMSH). For this action, alphaMSH binds to the melanocortin (MC)-1 receptor, a 7-transmembrane receptor located in melanophore cell membranes. The Mozambique tilapia, Oreochromis mossambicus, can change the hue of its body in response to a change in background, a process that may involve alphaMSH and the MC1R. Scale melanophores were isolated from tilapia that were acclimatised for 25 days to a black, control grey or white background and then tested for their sensitivity to des-, mono-, and di-acetylated alphaMSH. On all backgrounds, mono-acetylated alphaMSH was the dominant isoform present in pituitary homogenates. Mono-acetylated alphaMSH also had the highest potency to disperse melanosomes. Black background adapted fish showed the highest dispersing response to alphaMSH, independent of the isoform applied. We elucidated the nucleotide and amino acid sequence of the tilapia MC1R. We show that its expression in skin does not change when tilapia are acclimatised for 25 days to a black, grey or white background, while a clear change in hue is visible. This finding, combined with the absence of differential MC1R gene expression following background acclimation indicates that the increased sensitivity to alphaMSH is most likely a result of changes in the intracellular signalling system in melanophores of black background adapted fish, rather than up-regulation of the MC1R.

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.

Generation of expression constructs that secrete bioactive alphaMSH and their use in the treatment of experimental autoimmune encephalomyelitis

alpha Melanocyte-stimulating hormone (alphaMSH) is a 13 amino acid peptide with potent anti-inflammatory effects. We created two DNA expression constructs (miniPOMC and pACTH1-17) that encode bioactive versions of the alphaMSH peptide, and tested these constructs for therapeutic effects in experimental autoimmune encephalomyelitis (EAE). Each construct contained the sequences for alphaMSH, as well as the sequences that are involved in the secretion and processing of the POMC gene with the assumption that these sequences would promote processing and release of the encoded alphaMSH peptide. The differences between the two constructs lie at the C-terminal end where amino acids necessary for amidation of alphaMSH were included in only the pACTH1-17 construct. These two constructs were tested in vitro in bioassays, and in vivo in a mouse model of EAE. The results show that although bioactive peptides are secreted from cells transfected with either construct, there appears to be a significant therapeutic effect only with the pACTH1-17 construct which contains the extra C-terminal amino acids. The data suggest that it is possible to engineer DNA expression vectors encoding small secreted peptides such as alphaMSH, and that similar type constructs may be useful as therapeutics for the treatment of inflammatory diseases.

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.

Neprilysin, a novel target for ultraviolet B regulation of melanogenesis via melanocortins

Compelling evidence suggest a role for melanocortins in the regulation of melanogenesis by ultraviolet radiation. Within the epidermis, melanocytes and keratinocytes produce alpha-melanocyte-stimulating hormone and adrenocorticotropic hormone. The persistence and the strength of the biologic signal delivered by these peptides depend on their local concentration, which is controlled by the rate of peptide production and by the rate of its degradation. In this study, we investigated the mechanism of melanocortin degradation by melanocytes and the effect of ultraviolet on this process. We have focused our attention on a neutral endopeptidase, neprilysin, which has been implicated in the ending of numerous peptidergic signals. We have shown that this enzyme is expressed at the surface of human melanocytes. Interestingly, its activity and its expression are dramatically downregulated by ultraviolet B treatment. Moreover, in the presence of phosphoramidon, a stable inhibitor of neprilysin, we observed an increased efficiency of alpha-melanocyte-stimulating hormone and adrenocorticotropic hormone to stimulate both tyrosinase activity and microphthalmia expression. Taken together, these data indicate that neprilysin expressed by melanocytes has a physiologic role in the regulation of melanogenesis by proopiomelanocortin peptide. Further, its downregulation by ultraviolet B irradiation shed light on a new and appealing mechanism of ultraviolet B induced melanogenesis via the control of melanocortins degradation.

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.

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.

Does alpha-MSH have a role in regulating skin pigmentation in humans?

Over the years there has been much debate as to whether alpha-MSH has a role as a pigmentary hormone in humans. There are two main reasons for this. First, despite the observations in the 1960s that alpha-MSH increased skin darkening in humans, there are reports that the peptide has no effect on melanogenesis in cultured human melanocytes. Second, the human pituitary, unlike that of most mammals, secretes very little alpha-MSH and circulatory levels of the peptide in humans are extremely low. However, there is now evidence from several groups that alpha-MSH is capable of stimulating melanogenesis in cultured human melanocytes. Rather than producing an overall increase in melanin production, it appears that the peptide acts specifically to increase the synthesis of eumelanin. Such an action could well explain the previously observed skin darkening effects of alpha-MSH. It is also now known that alpha-MSH is not produced exclusively in the pituitary but has been found at numerous sites, including the skin where it is produced by several cell types. Related Proopiomelanocortin (POMC) peptides such as ACTH are also produced in human skin. The ACTH peptides act at the same receptor (MC-1) as alpha-MSH and certain of these would appear to be more potent than alpha-MSH in stimulating melanogenesis. The ACTH peptides are also present in greater amounts than alpha-MSH in human epidermis and it is likely that they play an important role in regulating pigmentary responses. These POMC peptides are released from keratinocytes in response to ultraviolet radiation (UVR) and it has been proposed that they serve as paracrine factors in mediating UV induced pigmentation. Their production by keratinocytes could therefore be critical in determining pigmentary responses and any changes in the availability of these POMC peptides might explain the variations in tanning ability seen in different individuals. However, the possibility that tanning ability is also dependent upon differences at the level of the MC-1 receptor cannot be ruled out and it has been suggested that an inability to tan may depend upon the presence of non-functional changes at the MC-1 receptor. alpha-MSH does, of course, affect human melanocytes in several ways and its stimulation of melanogenesis could be the consequence of some other fundamental action in the melanocyte. The peptide also has many other target sites in the skin and while it may have a role in regulating skin pigmentation in humans, it should not be viewed solely as a pigmentary peptide. alpha-MSH clearly has many different actions and its primary role in the skin may be to maintain homeostasis.

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.

Characterisation of ACTH peptides in human skin and their activation of the melanocortin-1 receptor

Alpha-Melanocyte-stimulating hormone (alpha-MSH) is a proopiomelanocortin (POMC)-derived peptide, which is produced in the pituitary and at other sites including the skin. It has numerous effects and in the skin has a pigmentary action through the activation of the melanocortin-1 (MC-1) receptor, which is expressed by melanocytes. Recent evidence suggests that the related POMC peptides such as adrenocorticotrophin (ACTH), which is the precursor of alpha-MSH, is also an agonist at the MC-1 receptor. By using immunocytochemistry, we confirmed the presence of alpha-MSH in human skin where staining was evident in keratinocytes and especially strong in melanocytes and possibly Langerhans cells. ACTH was also present and tended to show the strongest reaction in differentiated keratinocytes. Immunostaining was also observed for the prohormone convertases, PC1 and PC2, which are involved in the formation of ACTH and its cleavage to alpha-MSH, respectively. The amounts of immunoreactive ACTH exceeded those of alpha-MSH. Using HPLC we identified for the first time the presence of ACTH1-39, ACTH1-17, ACTH1-10, acetylated ACTH1-10, alpha-MSH, and desacetyl alpha-MSH in epidermis and in cultured keratinocytes. The ability of these peptides to activate the human MC-1 receptor was examined in HEK 293 cells that had been transfected with the receptor. All peptides increased adenylate cyclase in these cells with the following order of potency: ACTH1-17 > alpha-MSH > ACTH1-39 > desacetyl alpha-MSH > acetylated ACTH1-10 > ACTH1-10. ACTH1-17 also increased the dendricity and melanin content of cultured human melanocytes indicating that the peptide was able to activate MC-1 receptors when present in their normal location. However, as found with alpha-MSH, not all cultures were responsive and, as we have previously suggested, we suspect that this was the result of changes at the MC-1 receptor. Nevertheless, it would appear that ACTH peptides can serve as natural ligands of the MC-1 receptor on human melanocytes and their presence in the skin suggests that, together with alpha-MSH, they may have a role in the regulation of human melanocytes.

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.

POMC-related products in the intermediate pituitary of the amphibian, Bufo marinus: differential subcellular processing in the Golgi and secretory granules

In the intermediate pituitary of the anuran amphibian, Bufo marinus, the N-acetylation of ACTH(1-13)-NH2 to yield alpha-MSH occurs as a cosecretory processing event, whereas the N-acetylation of beta-endorphin occurs as a posttranslational processing event. To understand how these two N-acetylation reactions are segregated, B. marinus intermediate pituitary cells were analyzed by immunogold labeling electron microscopy, and by using an ultracentrifugation procedure. The immunogold labeling studies indicated that ACTH(1-13)-NH2-related immunoreactivity was colocalized with N-acetylated beta-endorphin-related immunoreactivity in secretory granules. Furthermore, ACTH(1-13)-NH2-related immunoreactivity was not detected in either the ER or the Golgi. N-Acetylated beta-endorphin-related immunoreactivity, however, was detected in the Golgi. Ultracentrifugation analysis revealed that in an ER/microsomal fraction, beta-LPH-sized and nonacetylated beta-endorphin-sized immunoreactive material were present in a molar ratio of 1:2. No N-acetylated forms of beta-endorphin were detected in the ER/microsomal fraction. In a Golgi/secretory granule fraction, the molar ratio of beta-LPH to beta-endorphin was 1:9 with 58% of the beta-endorphin being N-acetylated. Collectively, these data support the following hypotheses. The proteolytic cleavage of ACTH (1-39) to yield ACTH (1-13)-NH2 is a late processing event occurring in secretory granules. The cleavage of beta-LPH to yield nonacetylated beta-endorphin is an early processing event that may occur in the ER or the Golgi. Because N-acetylated beta-endorphin and nonacetylated ACTH(1-13)-NH2 are colocalized in secretory granules, it appears, therefore, that the N-acetylation of beta-endorphin is completed prior to loading into secretory granules. Thus, there is a spatial and temporal separation of the posttranslational processing events associated with the beta-LPH portion and ACTH portion of the POMC biosynthetic pathway in amphibian intermediate pituitary cells.

Endorphin and MSH in concert form the corticotropic principle released by tilapia (Oreochromis mossambicus; Teleostei) melanotropes

HPLC characterization of tilapia pituitary endorphins using an antibody specific for N-terminally acetylated endorphins yielded three major peaks in the neurointermediate lobe, but none in the pars distalis. The melanotropes secreted two of the immunoreactive products in vitro, one of which coeluted with Xenopus laevis N-ac-beta-END(1-8). This immunoreactive fraction also coeluted with diacetyl-alpha-MSH. Evidence is presented that the noteworthy corticotropic potency of this HPLC fraction, previously attributed to diacetyl-alpha-MSH, results from END and MSH acting in a coordinated fashion. Confinement stress had no effect on plasma N-ac-beta-END immunoreactivity, but led to a decrease in plasma alpha-MSH levels. Therefore, it seems unlikely that the corticotropic action of the peptides regulates the elevation of cortisol production that takes place during confinement, but it may play a role during other forms of stress that are known to activate the melanotropes.

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

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

Detection of N-acetylated forms of alpha-MSH and beta-endorphin in the intermediate pituitary of the holostean fishes, Lepisosteus spatula, Lepisosteus osseus, and Amia calva

Acid extracts of the intermediate pituitaries of the gars, L. spatula and L. osseus, were fractionated by Sephadex G-50 column chromatography and analyzed by radioimmunoassay. This procedure revealed that immunoreactive forms of N-acetylated beta-endorphin- and alpha-MSH-sized material were present in equimolar amounts and represented the major end products of the POMC biosynthetic pathway in these species. Cation-exchange chromatography indicated that multiple N-acetylated forms of beta-endorphin were present in the intermediate pituitaries of the two species of gar, and that these forms differed in their net positive charge and in their apparent molecular weight. Reversed-phase HPLC analysis of the alpha-MSH-related material indicated that up to 90% of the total MSH in the pituitary of the gar was N-acetylated. Furthermore, the predominant form of alpha-MSH in both species of gar was N,O-diacetyl-ACTH(1-13)-NH2. Nearly identical results were obtained following the analysis of alpha-MSH-related peptides in the intermediate pituitary of the bowfin, A. calva. The pattern of posttranslational processing of POMC observed in the intermediate pituitaries of holostean fishes is very similar to the processing events observed in lungfishes, turtles, and mammals; hence, the processing of POMC has been remarkably conserved during vertebrate evolution.

Proteolytic cleavage of ACTH in corticotropes of sexually mature axolotls (Ambystoma mexicanum)

Immunohistochemical analysis of the pituitary of sexually mature axolotls revealed both ACTH(1-39)-related and alpha-MSH-related immunoreactivity present in corticotropic cells located in the rostral anterior pituitary. Gel filtration analysis indicated that the ACTH(1-39)-sized immunoreactivity and the alpha-MSH-sized immunoreactivity detected in acid extracts of the axolotl anterior pituitary were present in a ratio in a range between 1:1 and 1:0.6. Reversed-phase HPLC analyses indicated that the alpha-MSH-sized immunoreactivity had the same retention time as synthetic ACTH(1-13)-NH2. The corticotropic activity of the ACTH(1-39)-sized immunoreactivity and the purified ACTH(1-13)-NH2 was tested in a heterologous, larval bullfrog adrenal bioassay system. As expected, the ACTH(1-39)-sized immunoreactivity stimulated corticosterone release; however, the purified ACTH(1-13)-NH2 lacked glucocorticoid activity. The proteolytic cleavage of ACTH in corticotropes of sexually mature axolotls was identical to the cleavage events observed in neotenic Ambystoma tigrinum that had not reached sexual maturity. These studies indicate that the transient expression of ACTH cleavage activity is not affected by the reproductive state of the animal. Since axolotls do not undergo metamorphosis, it is possible that events associated with metamorphosis may induce the decline in ACTH cleavage activity observed in amphibian corticotropes.

Differential N-acetylation of alpha-MSH and beta-endorphin in the intermediate pituitary of the turtle, Pseudemys scripta

Steady-state analyses of the intermediate pituitary of the turtle, Pseudemys scripta, indicated that alpha-MSH-sized immunoreactive forms and beta-endorphin-sized immunoreactive forms are major end products of melanotropic cells. Three forms of alpha-MSH-related immunoreactivity were detected. The two major forms had the same reversed-phase HPLC properties as synthetic N,O-diacetyl-ACTH(1-13)-NH2 and N-acetyl-ACTH(1-13)-NH2. These forms accounted for 97% of the total alpha-MSH-related immunoreactivity detected. A minor peak of ACTH(1-13)-NH2 was also detected. Multiple forms of beta-endorphin-related immunoreactivity were detected, which varied in net positive charge (+1 to +5), apparent molecular weight (2.4 to 3.5 kDa), and degree of N-terminal acetylation. Although N-acetylated forms of beta-endorphin were detected in the turtle intermediate pituitary, the major forms of turtle beta-endorphin were nonacetylated. These features of the turtle intermediate pituitary POMC-specific N-acetylation mechanism are similar to, yet distinct from, the POMC N-acetylation mechanisms observed for mammals. These data suggest that POMC-specific N-acetylation mechanisms were present in reptiles prior to the divergence of the anapsid and synapsid lines.