Molecular mechanisms of beta-endorphin biosynthesis

POMC: The Physiological Power of Hormone Processing

Pro-opiomelanocortin (POMC) is the archetypal polypeptide precursor of hormones and neuropeptides. In this review, we examine the variability in the individual peptides produced in different tissues and the impact of the simultaneous presence of their precursors or fragments. We also discuss the problems inherent in accurately measuring which of the precursors and their derived peptides are present in biological samples. We address how not being able to measure all the combinations of precursors and fragments quantitatively has affected our understanding of the pathophysiology associated with POMC processing. To understand how different ratios of peptides arise, we describe the role of the pro-hormone convertases (PCs) and their tissue specificities and consider the cellular processing pathways which enable regulated secretion of different peptides that play crucial roles in integrating a range of vital physiological functions. In the pituitary, correct processing of POMC peptides is essential to maintain the hypothalamic-pituitary-adrenal axis, and this processing can be disrupted in POMC-expressing tumors. In hypothalamic neurons expressing POMC, abnormalities in processing critically impact on the regulation of appetite, energy homeostasis, and body composition. More work is needed to understand whether expression of the POMC gene in a tissue equates to release of bioactive peptides. We suggest that this comprehensive view of POMC processing, with a focus on gaining a better understanding of the combination of peptides produced and their relative bioactivity, is a necessity for all involved in studying this fascinating physiological regulatory phenomenon.

The effect of Gly-Gln [ß-endorphin30-31] on morphine-evoked serotonin and GABA efflux in the nucleus accumbens of conscious rats

Glycyl-L-glutamine (Gly-Gln; β-endorphin30-31) is an endogenous dipeptide synthesized through the post-translational processing of β-endorphin1-31. Central Gly-Gln administration inhibits the rewarding properties of morphine and attenuates morphine tolerance, dependence and withdrawal although it does not interfere with morphine analgesia. In an earlier study, we found that Gly-Gln inhibits morphine-induced dopamine efflux in the nucleus accumbens (NAc), consistent with its ability to inhibit morphine reward. To further investigate the mechanism responsible for its central effects we tested whether i.c.v. Gly-Gln administration influences the rise in extracellular serotonin and GABA concentrations evoked by morphine in the NAc. Conscious rats were treated with Gly-Gln (100nmol/5μl) or saline i.c.v. followed, 2min later, by morphine (2.5mg/kg) or saline i.p. and extracellular serotonin and GABA concentrations were analyzed by microdialysis and HPLC. Morphine administration increased extracellular serotonin and GABA concentrations significantly within 20min, as shown previously. Unexpectedly, Gly-Gln also increased extracellular serotonin concentrations significantly in control animals. Combined treatment with Gly-Gln+morphine also elevated extracellular serotonin concentrations although the magnitude of the response did not differ significantly from the effect of Gly-Gln or morphine, given alone suggesting that Gly-Gln suppressed morphine induced serotonin efflux. Gly-Gln abolished the morphine-induced rise in extracellular GABA concentrations but had no effect on extracellular GABA when given alone to otherwise untreated animals. These data show that Gly-Gln stimulates NAc serotonin efflux and, together with earlier studies, support the hypothesis that Gly-Gln inhibits the rewarding effects of morphine by modulating morphine induced dopamine, GABA and serotonin efflux in the NAc.

Effects of N-terminal fragments of beta-endorphin on feeding in chicks

It is known that N-terminal fragments of beta-endorphin have biological activities, such as an antagonism effect of beta-endorphin (1-31) on the secretion of hormones or thermoregulation in mammals. We studied the effects of the N-terminal fragments on feeding behavior in male broiler chicks. Intracerebroventricular administration of beta-endorphin (1-27) (0.4nmol) stimulated feeding behavior compared with saline control during the 60-min experimental period. beta-Endorphin (1-17) (2.0nmol) also increased food intake at 30min postinjection. Co-injection of either beta-endorphin (1-27) or (1-17) was effective in reducing full-length beta-endorphin-induced feeding in chicks. These data suggest that the N-terminal fragments of beta-endorphin act as a partial agonist, and may regulate the activity of the central opioidergic system in chicks.

Cyclo-glycyl-glutamine inhibits ethanol intake in P and Sprague-Dawley rats

Peptide inhibitors of ethanol consumption have shown promise. The purpose of this study was to test the cyclized form of the opioid-derived dipeptide, glycyl-L-glutamine to reduce ethanol consumption after either peripheral injections or site-specific injections into the nucleus accumbens (NAC) of high drinking and low drinking rats. Following I.P. cyclo-glycyl-glutamine (c-GQ), the data show a mean decrease in ethanol intake of 34.4% in P rats, and 39.4% in Sprague-Dawley rats at doses between 5 and 25mg/kg. The data show that peripherally administered c-GQ is effective in reducing ethanol consumption in both high (P) and low (SD) drinking strains of rats and suggests a therapeutic potential.

Glycyl-glutamine reduces ethanol intake at three reward sites in P rats

beta-endorphin, implicated in modulation of ethyl alcohol reward, has neuron terminals in several reward sites. Alcohol consumption was reduced after ventricular or site-specific injections into the nucleus accumbens of an opioid-derived dipeptide, glycyl-glutamine. The current study examined the effects of this dipeptide after site-specific injections into additional reward sites. Alcohol-preferring (P) rats, stereotaxically implanted with bilateral guide cannulae into the nucleus accumbens, ventral tegmental area, and the central nucleus of the amygdala were given 30% alcohol and water in a 24h voluntary two-bottle choice paradigm. Upon achieving stable baseline intakes, glycyl-glutamine (GQ) doses were injected bilaterally, and the alcohol and water intakes and body weight recorded for the response and recovery. The data show reduced alcohol intake by 32-49.5% after 100-pmol glycyl-glutamine into reward sites (nucleus accumbens, ventral tegmental area, and central nucleus of the amygdala), but not after injections into control sites dorsal to reward sites. The order of sensitivity to the 1-fmol dose was amygdala > or = ventral tegmental area > accumbens. GQ was effective in reducing ethanol intake at reported beta-endorphin terminal regions in each of the three reward sites tested. The effective doses were similar to reported endogenous GQ levels, consistent with the notion that it may function as part of an endogenous counter regulatory mechanism and represent a "stop drinking" signal in the high drinking, P rats at these three reward sites.

Peptidomics of a single identified neuron reveals diversity of multiple neuropeptides with convergent actions on cellular excitability

In contrast to classical transmitters, the detailed structures and cellular and synaptic actions of neuropeptides are less well described. Peptide mass profiling of single identified neurons of the mollusc Lymnaea stagnalis indicated the presence of 17 abundant neuropeptides in the cardiorespiratory neuron, visceral dorsal 1 (VD1), and a subset of 14 peptides in its electrically coupled counterpart, right parietal dorsal 2. Altogether, based on this and previous work, we showed that the high number of peptides arises from the expression and processing of four distinct peptide precursor proteins, including a novel one. Second, we established a variety of posttranslational modifications of the generated peptides, including phosphorylation, disulphide linkage, glycosylation, hydroxylation, N-terminal pyroglutamylation, and C-terminal amidation. Specific synapses between VD1 and its muscle targets were formed, and their synaptic physiology was investigated. Whole-cell voltage-clamp analysis of dissociated heart muscle cells revealed, as tested for a selection of representative family members and their modifications, that the peptides of VD1 exhibit convergent activation of a high-voltage-activated Ca current. Moreover, the differentially glycosylated and hydroxylated alpha2 peptides were more potent than the unmodified alpha2 peptide in enhancing these currents. Together, this study is the first to demonstrate that single neurons exhibit such a complex pattern of peptide gene expression, precursor processing, and differential peptide modifications along with a remarkable degree of convergence of neuromodulatory actions. This study thus underscores the importance of a detailed mass spectrometric analysis of neuronal peptide content and peptide modifications related to neuromodulatory function.

Glycyl-glutamine inhibits nicotine conditioned place preference and withdrawal

Glycyl-glutamine (Gly-Gln) is an inhibitory dipeptide synthesized from beta-endorphin(1-31). Previously, we showed that Gly-Gln inhibits morphine conditioned place preference, tolerance, dependence and withdrawal. In this study, we tested whether Gly-Gln's inhibitory activity extends to other rewarding drugs, specifically nicotine. Rats were conditioned with nicotine (0.6 mg/kg, s.c.) for four days and tested on day five. Glycyl-glutamine (100 nmol i.c.v.) inhibited acquisition and expression of a nicotine place preference significantly. Cyclo(Gly-Gln) (100 nmol i.c.v. or 25 mg/kg i.p.), a cyclic Gly-Gln derivative, blocked expression of nicotine place preference but Gly-d-Gln (100 nmol i.c.v.) was ineffective. To study nicotine withdrawal, rats were treated with nicotine (9 mg/kg/day) for seven days and conditioned place aversion was induced with mecamylamine (1 mg/kg, s.c.). Glycyl-glutamine blocked acquisition of place aversion to mecamylamine but not U50,488, a kappa opioid receptor agonist. Glycyl-glutamine thus inhibits the rewarding effects of nicotine and attenuates withdrawal in nicotine dependent rats.

Glycyl-glutamine in nucleus accumbens reduces ethanol intake in alcohol preferring (P) rats

Opioid peptides and glycyl-glutamine (Gly-Gln) have been implicated in the control of ethanol consumption. A recognized beta-endorphin cleavage product, Gly-Gln, inhibits voluntary alcohol consumption when microinjected into the nucleus accumbens (AcbSh) of P rats. To evaluate the site-specific efficacy of Gly-Gln on ethanol consumption following AcbSh application, ethanol preferring (P) rats were allowed to establish individual baseline ethanol/water consumption utilizing a voluntary self-administration paradigm. Subsequent to baseline ethanol consumption being established, bilateral guide cannulae were stereotaxically implanted +1 mm dorsal to the AcbSh for subsequent Gly-Gln (100 nmol/microl) or saline vehicle (1 microl) injections. Alcohol intake, body weight, and water intake were measured at 24 h post-injection intervals. Unilateral Gly-Gln injections reduced ethanol consumption 35.6% (P < 0.05) from pre-established baseline consumption (6.24 +/- 0.64 g/kg to 4.06 +/- 0.28 g/kg). Bilateral Gly-Gln injections further reduced consumption to 51.9% (6.4 +/- 1.0 g/kg to 3.08 +/- 0.65 g/kg at 24 h (P < 0.01) below established baseline values within 24 h without significant changes in body weight or water consumption. Also, the amino acid constituents of the dipeptide had no influence on ethanol consumption behavior; however, Gly-Gln efficacy was shown to be comparable to central beta-endorphin-(1-27) or intraperitoneal (i.p.) naltrexone-induced suppression of ethanol intake. These data indicate that the AcbSh exhibits a site-specific sensitivity to the suppressive actions of Gly-Gln or beta-endorphin-(1-27) injections that modulate voluntary ethanol consumption in P rats. These findings support the broader concept that select forebrain opioid-responsive neural sites may influence the development or expression of alcohol abuse syndromes in animal models or humans.

The role of melanocortins in body weight regulation: opportunities for the treatment of obesity

Five G-protein-coupled melanocortin receptors (MC(1)-MC(5)) are expressed in mammalian tissues. The melanocortin receptors support diverse physiological functions, including the regulation of hair color, adrenal function, energy homeostasis, feed efficiency, sebaceous gland lipid production and immune and sexual function. The melanocortins (adrenocorticotropic hormone (ACTH), alpha-melanocyte-stimulating hormone (alpha-MSH), beta-MSH and gamma-MSH) are agonist peptide ligands for the melanocortin receptors and these peptides are processed from the pre-prohormone proopiomelanocortin (POMC). Peptide antagonists for the melanocortin MC(1), MC(3) and MC(4) receptors include agouti-related protein (AgRP) and agouti. Diverse lines of evidence, including genetic and pharmacological data obtained in rodents and humans, support a role for the melanocortin MC(3) and MC(4) receptors in the regulation of energy homeostasis. Recent advances in the development of potent and selective peptide and non-peptide melanocortin receptor ligands are anticipated to help unravel the roles for the melanocortin receptors in humans and to accelerate the clinical use of small molecule melanocortin mimetics.

The role of melanocortins in body weight regulation: opportunities for the treatment of obesity

Five G-protein-coupled melanocortin receptors (MC(1)-MC(5)) are expressed in mammalian tissues. The melanocortin receptors support diverse physiological functions, including the regulation of hair color, adrenal function, energy homeostasis, feed efficiency, sebaceous gland lipid production and immune and sexual function. The melanocortins (adrenocorticotropic hormone (ACTH), alpha-melanocyte-stimulating hormone (alpha-MSH), beta-MSH and gamma-MSH) are agonist peptide ligands for the melanocortin receptors and these peptides are processed from the pre-prohormone proopiomelanocortin (POMC). Peptide antagonists for the melanocortin MC(1), MC(3) and MC(4) receptors include agouti-related protein (AgRP) and agouti. Diverse lines of evidence, including genetic and pharmacological data obtained in rodents and humans, support a role for the melanocortin MC(3) and MC(4) receptors in the regulation of energy homeostasis. Recent advances in the development of potent and selective peptide and non-peptide melanocortin receptor ligands are anticipated to help unravel the roles for the melanocortin receptors in humans and to accelerate the clinical use of small molecule melanocortin mimetics.

Inhibition of interleukin-1beta and prostaglandin E(2) thermogenesis by glycyl-glutamine, a pro-opiomelanocortin-derived peptide

Interleukin-1beta (IL-1beta) and other cytokines produce fever by stimulating prostaglandin E(2) (PGE(2)) synthesis in thermoregulatory regions of the preoptic area and anterior hypothalamus (POA/AH). Prostaglandin E(2) is thought to raise body temperature, at least in part, by stimulating beta-endorphin release from pro-opiomelanocortin neurons that innervate the POA/AH. In this study, we investigated whether glycyl-glutamine (beta-endorphin(30-31)), an inhibitory dipeptide synthesized from beta-endorphin post-translationally, inhibits IL-1beta and PGE(2)-induced hyperthermia. Hyperthermic sites were identified by microinjecting PGE(2) (3 fmol/1 microl) into the medial preoptic area (mPOA) of conscious, unrestrained rats. Interleukin-1beta (1 U) injection into the same PGE(2) responsive thermogenic sites in the mPOA elicited a prolonged rise in colonic temperature (T(c)) (+1.02+/-0.06 degrees C) that persisted for at least 2 h. Glycyl-glutamine (3 nmol) co-injection into the mPOA inhibited IL-1beta thermogenesis completely (T(c)=-0.18+/-0.22 degrees C). Glycyl-glutamine had no effect on body temperature when given alone to normothermic rats. Co-injection of individual amino acids, glycine and glutamine (3 nmol each amino acid), failed to influence IL-1beta-induced thermogenesis, which indicates that Gly-Gln hydrolysis does not explain its inhibitory activity. Glycyl-glutamine (3 nmol) also prevented the rise in body temperature produced by PGE(2) (PGE(2)=0.89+/-0.05 degrees C; PGE(2) plus Gly-Gln=-0.16+/-0.14 degrees C), consistent with evidence that PGE(2) mediates IL-1beta-induced fever. These findings demonstrate that Gly-Gln inhibits the thermogenic response to endogenous pyrogens.

Glycyl-glutamine inhibits the respiratory depression, but not the antinociception, produced by morphine

Glycyl-glutamine (Gly-Gln; beta-endorphin(30-31)) is an endogenous dipeptide that is synthesized through the posttranslational processing of beta-endorphin in brain stem regions that control respiration and autonomic function. This study tested the hypothesis that Gly-Gln administration to conscious rats will prevent the respiratory depression caused by morphine without affecting morphine antinociception. Rats were administered Gly-Gln (1-100 nmol) or saline (10 microl) intracerebroventricularly followed, 5 min later, by morphine (40 nmol icv). Arterial blood gases and pH were measured immediately before Gly-Gln and 30 min after morphine injection. Gly-Gln pretreatment inhibited morphine-induced hypercapnia, hypoxia, and acidosis significantly. The response was dose dependent and significant at Gly-Gln doses as low as 1 nmol. In contrast, Gly-Gln (1-300 nmol) had no effect on morphine-evoked antinociception in the paw withdrawal test. When given alone to otherwise untreated animals, Gly-Gln did not affect nociceptive latencies or blood gas values. These data indicate that Gly-Gln inhibits morphine-induced respiratory depression without compromising morphine antinociception.

Ontogenesis of proopiomelanocortin and its processing to beta-endorphin by the fetal and neonatal rat brain

A number of reports suggest that beta-endorphin (beta-END) may play an important role in the regulation of cell proliferation and neuronal differentiation. Proopiomelanocortin (POMC), the common precursor ofadrenocorticotropic hormone and beta-END, is detected very early in embryonic life in hypothalamic neurons of the developing rat. However, very little is known about the degree to which POMC is processed to beta-END during fetal and early postnatal life. Thus, it was the objective of the present study to estimate the hypothalamic content of POMC mRNA, as well as the biosynthesis and posttranslational processing of POMC by hypothalamic neurons on fetal day 20 and on days 1, 8 and 22 of postnatal life. Hypothalamic POMC mRNA, as determined by Northern blot analysis, was higher on fetal day 20 than on postnatal days 1, 8 and 22. A higher rate of incorporation of [(3)H]phenylalanine into beta-END immunoreactive peptides was observed on fetal day 20 than on postnatal day 1. However, the rate of incorporation was significantly increased by day 8 of postnatal life and was similar to that on day 22. POMC was processed to beta-lipotropin (beta-LPH) and beta-END at all ages examined, but the relative proportions of POMC:beta-LPH:beta-END changed during development. Thus, beta-END accounted only for 34.89 +/- 6.14% of the total [(3)H]phenylalanine-labeled beta-END immunoreactive peptides on fetal day 20, while it accounted for 57. 37 +/- 5.20, 62.81 +/- 1.38 and 79.25 +/- 6.57% on days 1, 8 and 22 of postnatal life, respectively. Thus, POMC is processed to a considerable extent into beta-END-sized peptides by the fetal hypothalamus and may influence brain development. Furthermore, the rate of processing of hypothalamic POMC into beta-END increases with development, probably due to the increased activity of the enzymes specific for POMC processing.

Glycyl-L-glutamine [beta-endorphin-(30-31)] attenuates hemorrhagic hypotension in conscious rats

The profound hypotension caused by acute hemorrhage is thought to involve opioid peptide neurons. In this study, we tested whether glycyl-L-glutamine [Gly-Gln; beta-endorphin-(30-31)], a nonopioid peptide derived from beta-endorphin processing, prevents the cardiovascular depression induced by hemorrhage in conscious and anesthetized rats. Previously, we found that Gly-Gln inhibits the hypotension and respiratory depression produced by beta-endorphin and morphine but does not affect opioid antinociception. Hemorrhage (2.5 ml/100 g body wt over 20 min) lowered arterial pressure in conscious rats (from 120.1 +/- 2.9 to 56.2 +/- 4.7 mmHg) but did not change heart rate significantly. Intracerebroventricular Gly-Gln (3, 10, or 30 nmol) pretreatment inhibited the fall in arterial pressure and increased heart rate significantly. The response was dose related and was sustained during the 35-min posthemorrhage interval. Pentobarbital sodium anesthesia potentiated the hemodynamic response to hemorrhage and attenuated the effect of Gly-Gln. Gly-Gln (10 or 100 nmol icv) did not influence arterial pressure or heart rate in normotensive rats. These data indicate that Gly-Gln is an effective antagonist of hemorrhagic hypotension.

Cerebrospinal fluid and plasma beta-endorphin in combat veterans with post-traumatic stress disorder

Opioid-mediated analgesia develops in experimental animals following traumatic stress and increased opioid-mediated analgesia has been observed in combat veterans with post-traumatic stress disorder (PTSD). These observations have led to the hypothesis that increased central nervous system (CNS) opioidergic activity exists in patients with PTSD. However, direct CNS data on opioid peptide concentrations and dynamics in patients with PTSD are lacking. We withdrew cerebrospinal fluid (CSF) via a flexible, indwelling subarachnoid catheter over a 6-h period and determined hourly CSF concentrations of immunoreactive beta-endorphin (ir beta END) in 10 well-characterized combat veterans with PTSD and nine matched normal volunteers. Blood was simultaneously withdrawn to obtain plasma for ir beta END. PTSD symptom clusters, as measured by the CAPS, were correlated with neuroendocrine data. Mean CSF ir beta END was significantly greater in patients with PTSD compared with normals and there was a negative correlation between the ir beta END and PTSD intrusive and avoidant symptoms of PTSD. No intergroup difference between plasma ir beta END was found, nor was there a significant correlation between CSF and plasma ir beta END. Immunoreactive beta-lipotropin (ir beta LPH) and pro-opiomelanocortin (irPOMC), both precursors of beta END, were much more plentiful in human CSF than was beta-endorphin itself, as has been previously reported. It remains to be determined whether the increased CNS opioid concentrations predate traumatic stress, thereby conferring a vulnerability to dissociative states and PTSD itself, or result from the trauma. The negative correlation between CSF ir beta END and avoidant and intrusive symptoms suggests that CNS hypersecretion of opioids might constitute an adaptive response to traumatic experience. Poor correlation between CSF and plasma ir beta END limits use of plasma measures to assess CNS opioid activity.

Pattern changes of pituitary peptides in rat after salt-loading as detected by means of direct, semiquantitative mass spectrometric profiling

We have established a differential peptide display method, based on a mass spectrometric technique, to detect peptides that show semiquantitative changes in the neurointermediate lobe (NIL) of individual rats subjected to salt-loading. We employed matrix-assisted laser desorption/ionization mass spectrometry, using a single-reference peptide in combination with careful scanning of the whole crystal rim of the matrix-analyte preparation, to detect in a semiquantitative manner the molecular ions present in the unfractionated NIL homogenate. Comparison of the mass spectra generated from NIL homogenates of salt-loaded and control rats revealed a selective and significant decrease in the intensities of several molecular ion species of the NIL homogenates from salt-loaded rats. These ion species, which have masses that correspond to the masses of oxytocin, vasopressin, neurophysins, and an unidentified putative peptide, were subsequently chemically characterized. We confirmed that the decreased molecular ion species are peptides derived exclusively from propressophysin and prooxyphysin (i.e., oxytocin, vasopressin, and various neurophysins). The putative peptide is carboxyl-terminal glycopeptide. The carbohydrate moiety of the latter peptide was determined by electrospray tandem MS as bisected biantennary Hex3HexNAc5Fuc. This posttranslational modification accounts for the mass difference between the predicted mass of the peptide based on cDNA studies and the measured mass of the mature peptide.

Cyclo(Gly-Gln) inhibits the cardiorespiratory depression produced by beta-endorphin and morphine

Glycyl-L-glutamine (Gly-Gln; beta-endorphin 30-31) is an endogenous dipeptide that is synthesized through the post-translational processing of beta-endorphin. Previously, we showed that Gly-Gln inhibits the hypotension and respiratory depression produced by central beta-endorphin administration. In this study, we tested whether cyclo(Gly-Gln), a non-polar, cyclic Gly-Gln derivative, was similarly effective following intracerebro-ventricular (i.c.v.) or intra-arterial (i.a.) administration to pentobarbital-anesthetized rats pretreated with beta-endorphin (0.5 nmol i.c.v.). Intracerebroventricular cyclo(Gly-Gln) (0.3, 0.6 or 1.0 nmol) injection produced a dose-dependent inhibition of beta-endorphin-induced hypotension, but not bradycardia, with a potency similar to that of Gly-Gln. Cyclo(Gly-Gln) (5 mg/kg) was also effective following i.a. injection and significantly attenuated the fall in arterial pressure elicited by i.c.v. beta-endorphin, consistent with evidence that cyclic dipeptides permeate the blood-brain barrier; i.a. Gly-Gln was ineffective. Intra-arterial cyclo(Gly-Gln) (5 mg/kg) and i.c.v. Gly-Gln (10 nmol) also attenuated the hypotension and respiratory depression induced by morphine (50 or 100 nmol i.c.v.). Cyclo(Gly-Gln) (0.5, 5.0 or 50.0 mg/kg i.a.) had no effect on arterial pressure or heart rate when given alone. These findings indicate that cyclo(Gly-Gln) is a biologically active peptide capable of reversing the cardiorespiratory depression produced by beta-endorphin or morphine.

Ectoenzymes as sites of peptide regulation

The ectoenzyme-mediated metabolism of neuropeptides may be an important regulatory site of peptide-mediated activity. These membrane-bound, extracellularly oriented peptidases are not only responsible for inactivating peptide substrates, but also lead to the formation of metabolic fragments. Peptide fragments formed after enzymatic proteolysis have been shown to display novel bioactivity as a consequence of a shift in receptor selectivity. This example of nervous system plasticity through peptide biotransformation can have multiple consequences. Centrally acting drugs have been shown to have profound effects on peptide-mediated systems throughout the brain and spinal cord including a differential alteration in ectoenzyme activity and ectoenzyme-mediated metabolism of neuropeptides. In this review, Tom Davis and Chris Konkoy suggest that the modulation of ectoenzyme-mediated peptide metabolism represents an additional level at which the concentration of extracellular neuropeptides, and thus peptide-mediated transmission, can be regulated.

Peptide quantification by tandem mass spectrometry

This manuscript reviews the literature on the mass spectrometry (MS) and tandem mass spectrometry (MS/MS) quantification of biologically important peptides that have been extracted from tissues. The most important aspect of this quantification process is the use of MS/MS to link the protonated molecule ion, (M + H)(+) , of the peptide with one or more of its amino acid sequence-determining fragment ions. The actual name of a peptide cannot be used in any study until the amino acid sequence of that peptide has been firmly established. This article reviews the analytical data obtained from the measurement of opioid peptides in human pituitary tissues. For example, the proopiomelanocortin (POMC)-derived beta-endorphin (BE) and the proenkephalin-derived methionine enkephalin (ME) opioid peptides have been quantified. The biogenesis of opioid neuropeptides is briefly reviewed; critical aspects of pituitary neuropeptides are discussed, including their localization and regulation, and their role in tumor formation; other analytical methods used to detect and measure neuropeptides are mentioned, including radioimmunoassay (RIA), radioreceptorassay (RRA), in situ hybridization, mRNA, and cDNA methods; and the MS and MS/MS methods are described. The use of stable isotope-incorporated synthetic peptide internal standards is described. Data are presented on the measurement of BE and ME in control pituitaries and in pituitary tumors (PRL-secreting and nonsecreting tumors). A significant alteration in the POMC peptide BE was found between the control and tumor tissues. That difference suggests that the POMC neuropeptidergic system had been down-regulated in those tumors. © 1997 John Wiley & Sons, Inc.

Liquid chromatographic-mass spectrometric studies on the enzymatic degradation of beta-endorphin by endothelial cells

An on-line HPLC-mass spectrometric procedure with an electrospray atmospheric pressure ionization (ESI-API) ion source was developed to identify the enzymatic degradation products (peptides) generated by incubation of human beta-endorphin (h beta E) with cultured aortic endothelial cells. The samples from the complex incubation mixture were prepurified and enriched using a small reversed-phase (RP) perfusion precolumn. Flow switching was applied to transfer the peptides from this precolumn to the analytical RP column of 2 or 0.32 mm I.D. and to separate them by gradient elution. The peptides were detected by means of an on-line coupled triple quadrupole mass spectrometer (TSQ 700) with an ESI-API ion source operated in the positive ion mode. This MS system behaves as a concentration sensitive detector at flow-rates from 5 to 150 microliters/min. MS-MS experiments supported the unambiguous assignment of the peptide structures. Thus most of the peptide fractions were identified and the region 16-17-18 (-L-F-K-) of h beta E was found to be primarily attacked by the enzymes of the endothelial cells.

Opioid and anti-opioid peptides

The numerous endogenous opioid peptides (beta-endorphin, enkephalins, dynorphins ... ) and the exogenous opioids (such as morphine) exert their effects through the activation of receptors belonging to four main types, mu, delta, kappa and epsilon. Opioidergic neurones and opioid receptors are largely distributed centrally and peripherally. It is thus not surprising that opioids have numerous pharmacological effects and that endogenous opioids are thought to be involved in the physiological control of various functions, among which nociception is particularly emphasized. Some opioid targets may be components of homeostatic systems tending to reduce the effects of opioids. "Anti-opioid" properties have been attributed to various peptides, especially cholecystokinin (CCK), neuropeptide FF (NPFF) and melanocyte inhibiting factor (MIF)-related peptides. In addition, a particular place should be attributed, paradoxically, to opioid peptides themselves among the anti-opioid peptides. These peptides can oppose some of the acute effects of opioids, and a hyperactivation of anti-opioid peptidergic neurones due to the chronic administration of opioids may be involved in the development of opioid tolerance and/or dependence. In fact, CCK, NPFF and the MIF family of peptides have complex properties and can act as opioid-like as well as anti-opioid peptides. Thus, "opioid modulating peptides" would be a better term to designate these peptides, which probably participate, together with the opioid systems, in multiple feed-back loops for the maintenance of homeostasis. "Opioid modulating peptides" have generally been shown to act through the activation of their own receptors. For example, CCK appears to exert its anti-opioid actions mainly through the activation of CCK-B receptors, whereas its opioid-like effects seem to result from the stimulation of CCK-A receptors. However, the partial agonistic properties at opioid receptors of some MIF-related peptides very likely contribute to their ability to modulate the effects of opioids. CCK- and NPFF-related drugs have potential therapeutic interest as adjuncts to opioids for alleviating pain and/or for the treatment of opioid abuse.

Inhibition of proopiomelanocortin expression by an oligodeoxynucleotide complementary to beta-endorphin mRNA

Gene expression in mammalian cells can be suppressed by oligonucleotides complementary to the target mRNA. This strategy was explored as a means of arresting translation of the prohormone precursor proopiomelanocortin (POMC), used as a model system of peptide messengers that are synthesized and released from endocrine and neuronal cells. The synthesis of the POMC-derived peptides adrenocorticotropin (ACTH) and beta-endorphin (beta-END) was markedly reduced by an oligodeoxynucleotide (ODN) complementary to a region of beta-END mRNA in AtT-20 cells, which retain many of the differentiated phenotypes of corticotrophs; this treatment did not affect the steady-state levels of POMC mRNA. Antisense ODN was stable in cell culture medium for 24 h, and cellular uptake was low (approximately 2.5% of the added ODN); however, the intracellular levels of the ODN were sufficient to form a ribonuclease-resistant duplex with complementary cellular mRNA. Addition of ODN to the cell culture did not affect the cellular levels of chromogranin A-(264-314)/pancreastatin or cell viability and proliferation, as evidenced by bromodeoxyuridine incorporation and ornithine decarboxylase activity. Microinfusion of the antisense ODN in the rat hypothalamic arcuate nucleus, where the majority of POMC-positive brain perikarya are located, significantly reduced ACTH- and beta-END-immunopositive neurons, and antisense ODN-treated rats showed substantially less of the grooming behavior usually observed in a novel environment.

Identification of a sorting signal for the regulated secretory pathway at the N-terminus of pro-opiomelanocortin

The N-terminal 26 amino acids of the prohormone pro-opiomelanocortin (POMC) were investigated to determine whether this region has the capacity to act as a sorting signal for the regulated secretory pathway. Constructs were made using the N-terminal 101, 50, 26 or 10 amino acids of POMC fused to the chloramphenicol acetyltransferase (CAT) reporter protein and expressed in AtT20 cells to show that at least the first 26 amino acids were required to sort CAT to the regulated secretory pathway. Full length POMC was mutated by deleting amino acids 2-26 from the N-terminal region. Analysis of Neuro-2a cells expressing this mutation compared to wild type POMC indicated that these 26 amino acids contain information essential for sorting POMC to the regulated secretory pathway. The results presented here suggest the presence of a conformation-dependent signal in the N-terminal 26 amino acids of POMC responsible for sorting POMC to the regulated secretory pathway.

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.

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.