Metorphamide levels in chromaffin cells increase after treatment with reserpine

Five Decades of Research on Opioid Peptides: Current Knowledge and Unanswered Questions

In the mid-1970s, an intense race to identify endogenous substances that activated the same receptors as opiates resulted in the identification of the first endogenous opioid peptides. Since then, >20 peptides with opioid receptor activity have been discovered, all of which are generated from three precursors, proenkephalin, prodynorphin, and proopiomelanocortin, by sequential proteolytic processing by prohormone convertases and carboxypeptidase E. Each of these peptides binds to all three of the opioid receptor types (μ, δ, or κ), albeit with differing affinities. Peptides derived from proenkephalin and prodynorphin are broadly distributed in the brain, and mRNA encoding all three precursors are highly expressed in some peripheral tissues. Various approaches have been used to explore the functions of the opioid peptides in specific behaviors and brain circuits. These methods include directly administering the peptides ex vivo (i.e., to excised tissue) or in vivo (in animals), using antagonists of opioid receptors to infer endogenous peptide activity, and genetic knockout of opioid peptide precursors. Collectively, these studies add to our current understanding of the function of endogenous opioids, especially when similar results are found using different approaches. We briefly review the history of identification of opioid peptides, highlight the major findings, address several myths that are widely accepted but not supported by recent data, and discuss unanswered questions and future directions for research. SIGNIFICANCE STATEMENT: Activation of the opioid receptors by opiates and synthetic drugs leads to central and peripheral biological effects, including analgesia and respiratory depression, but these may not be the primary functions of the endogenous opioid peptides. Instead, the opioid peptides play complex and overlapping roles in a variety of systems, including reward pathways, and an important direction for research is the delineation of the role of individual peptides.

Regulation of neuropeptide processing enzymes by catecholamines in endocrine cells

Treatment of cultured bovine adrenal chromaffin cells with the catecholamine transport blocker reserpine was shown previously to increase enkephalin levels severalfold. To explore the biochemical mechanism of this effect, we examined the effect of reserpine treatment on the activities of three different peptide precursor processing enzymes: carboxypeptidase E (CPE) and the prohormone convertases (PCs) PC1/3 and PC2. Reserpine treatment increased both CPE and PC activity in extracts of cultured chromaffin cells; total protein levels were unaltered for any enzyme. Further analysis showed that the increase in CPE activity was due to an elevated V(max), with no change in the K(m) for substrate hydrolysis or the levels of CPE mRNA. Reserpine activation of endogenous processing enzymes was also observed in extracts prepared from PC12 cells stably expressing PC1/3 or PC2. In vitro experiments using purified enzymes showed that catecholamines inhibited CPE, PC1/3, and PC2, with dopamine quinone the most potent inhibitor (IC(50) values of ∼50-500 μM); dopamine, norepinephrine, and epinephrine exhibited inhibition in the micromolar range. The inhibition of purified CPE with catecholamines was time-dependent and, for dopamine quinone, dilution-independent, suggesting covalent modification of the protein by the catecholamine. Because the catecholamine concentrations found to be inhibitory to PC1/3, PC2, and CPE are well within the physiological range found in chromaffin granules, we conclude that catecholaminergic transmitter systems have the potential to exert considerable dynamic influence over peptidergic transmitter synthesis by altering the activity of peptide processing enzymes.

Processing of chromogranins in chromaffin cell culture: effects of reserpine and alpha-methyl-p-tyrosine

Bovine chromaffin cell cultures were treated with either reserpine or alpha-methyl-p-tyrosine for up to 10 days. Afterwards the cells were harvested and the degree of proteolytic processing of secretogranin II, chromogranin A and chromogranin B was determined by immunoblotting and HPLC followed by RIA. There was a significant increase in the proteolysis of all three chromogranins after 4-6 days in the presence of reserpine. The small peptides formed in the presence of reserpine in vitro are also produced in vivo. A similar effect was observed with alpha-methyl-p-tyrosine, an inhibitor of tyrosine hydroxylase, but the response took up to 10 days to develop. Both drugs decreased catecholamine levels but reserpine was more effective, reaching a high degree of depletion after 4 days. In addition, experiments in vitro indicate that low millimolar amounts of either adrenaline (IC50 5.2 mM) or noradrenaline (IC50 2.4 mM) can significantly impair the proteolytic activity of recombinant murine prohormone convertase 1 when assayed with synthetic fluorogenic and/or peptidyl substrates. We conclude that a lowering of catecholamine levels in chromaffin granules leads to a concomitant increase in proteolytic processing of all secretory peptides. Apparently within chromaffin granules the endoproteases are inhibited by catecholamines and thus their removal leads to increased proteolysis.

Characterization of a metalloprotease from ovine chromaffin granules which cleaves a proenkephalin fragment (BAM12P) at a single arginine residue

A metalloprotease has been identified in ovine chromaffin granules which cleaves the proenkephalin fragment BAM12P to produce adrenorphin-Gly. This cleavage occurs at a single arginine residue and is an intermediate step in the production of the opiate adrenorphin in vivo. The identity of the product was confirmed by reverse-phase and ion-exchange chromatography. The adrenorphin-Gly-generating enzyme (AGE) was determined by chromatofocusing to have a pI value of 5.2 and bound strongly to a metal-chelate affinity column. After purification by gel-filtration and ion-exchange chromatography AGE was free of contaminating activities, as cleavage of radiolabelled BAM12P generated a single product as judged by reverse-phase and ion-exchange chromatography. The enzyme has a molecular mass of approx. 45 kDa and a pH optimum of 8.6 in Mops, Taps and Hepes buffers, but was inhibited by phosphate buffers. It was inhibited by micromolar concentrations of copper and zinc ions, but not by millimolar concentrations of calcium or manganese ions. The addition of BAM22P, dynorphin 1-13 or dynorphin 1-8 to the incubation mixture inhibited the cleavage of radiolabelled BAM12P. The cleavage was also inhibited by the presence of catecholamines at concentrations similar to those found within the chromaffin granule. This may explain the known effect of reserpine on chromaffin cells of reducing catecholamine levels and simultaneously increasing adrenorphin levels. It may also indicate a function for AGE and adrenorphin as reporters of intragranular conditions.

Screening of adrenal medullary neuropeptides for putative neurotrophic effects

Chromaffin granules, the secretory organelles of the neuron-like adrenal medullary chromaffin cells, have previously been shown to store and liberate neurotrophic activities that support in vitro survival of several neuron populations including those innervating the adrenal medulla. Molecules resembling fibroblast growth factor and ciliary neurotrophic factor have been identified among these activities. Since chromaffin granules store a variety of neuropeptides and many neuropeptides can have pleiotropic effects on neuronal growth and maintenance we have tested 24 different neuropeptides for their capacities to promote survival of embryonic chick ciliary, dorsal root and sympathetic ganglionic neurons. Peptides tested included several derivatives of proenkephalin (Leu- and met-enkephalin, fragments BAM 22, B, F and E), somatostatin, substance P, neuropeptide Y, neurotensin, VIP, bombesin, secretin, pancreastatin, dynorphin B, dynorphin 1-13, beta-endorphin, alpha-, beta-, and gamma-MSH. Control cultures received saturating concentrations of ciliary neurotrophic or nerve growth factor (CNTF; NGF), or no trophic supplements. At 1 x 10(-5) M leu- and met-enkephalin as well as somatostatin supported sympathetic neurons to the same extent as NGF. At the same concentrations, leu-enkephalin, the proenkephalin fragments BAM 22 and E, and somatostatin maintained about half of the dorsal root ganglionic neurons supported by NGF, but were not effective on ciliary neurons. VIP promoted the survival of approximately 50% of the ciliary and embryonic day 10 dorsal root ganglionic neurons as compared to saturating amounts of CNTF, but required the presence of non-neuronal cells in the cultures to be effective. Neurotensin (1 x 10(-5) M had a small effect on ciliary neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Reserpine-induced processing of chromogranin A in cultured bovine adrenal chromaffin cells

The effect of reserpine on the processing of the secretory granule protein chromogranin A (CgA) in isolated bovine adrenal chromaffin cells was investigated using two radioimmunoassays employing site-specific antisera. The two antisera were directed against closely associated regions of the CgA molecule which would be exposed by specific processing: antiserum L331 was raised against the C-terminus of the regulatory peptide pancreastatin, and the second antiserum, L300, was raised against the synthetic peptide [Tyr0]CgA306-313 (YLSKEWEDA), a sequence that lies immediately C-terminal to pancreastatin and adjacent to a dibasic amino acid cleavage site. Chronic reserpine treatment of chromaffin cells produced a time- and dose-dependent increase in processing, as demonstrated by an increase in pancreastatin- and YLSKEWEDA-immunoreactivity (ir). The reserpine-induced rise in pancreastatin-ir was due predominantly to an increase in pancreastatin 1-47, whereas the rise in YLSKEWEDA-ir was due to increases in three polypeptides: a 51-kDa YLSKEWEDA-ir polypeptide, CgA297-313, and CgA248-313. The latter predominated. The action of reserpine on both pancreastatin- and YLSKEWEDA-ir was found to be largely inhibited by the protein synthesis inhibitor cycloheximide. The results show that treatment of isolated chromaffin cells with reserpine induces both the selective proteolytic processing and peptidyl-glycine amidation of CgA and its derived fragments. As reserpine has a similar effect on proenkephalin in chromaffin cells, the results suggest that reserpine induces a general increase in the activity of the processing enzymes, partially by an increase in protein synthesis.

Processing of proenkephalin in adrenal chromaffin cells

The processing of proenkephalin was studied using [35S]methionine pulse-chase techniques in primary cultures of bovine adrenal medullary chromaffin cells. Following radiolabeling, proenkephalin-derived peptides were extracted from the cells and separated by reverse-phase HPLC. Fractions containing proenkephalin fragments were digested with trypsin and carboxypeptidase B to liberate Met-enkephalin sequences and subjected to a second HPLC step to demonstrate association of radiolabel with Met-enkephalin. Processing of proenkephalin is complete within 2 h of synthesis, suggesting completion at or soon after incorporation into storage vesicles. Pretreatment of the cells with nicotine, histamine, or vasoactive intestinal peptide to enhance the rate of proenkephalin synthesis failed to alter the time course of processing and had minimal effects on the distribution of products formed. Addition of tetrabenazine, an inhibitor of catecholamine uptake into chromaffin vesicles, during radiolabeling and a 6-h chase period caused enhanced proenkephalin processing. These results suggest that the full range of proenkephalin fragments normally found in the adrenal medulla (up to 23.3 kDa) represents final processing products of the tissue and that termination of processing may depend on the co-storage of catecholamines.

The enkephalin-containing cell: strategies for polypeptide synthesis and secretion throughout the neuroendocrine system

1. Enkephalinergic cells are found throughout the diffuse neuroendocrine system, in the adrenal medulla, brain, spinal cord, peripheral and enteric nervous systems, and endocrine pancreas. 2. In each of these diverse cell types, the enkephalin phenotype is (i) established during development, (ii) modified by the particular environment in which the cell is located, and (iii) maintained by ongoing biosynthesis at a rate consistent with loss of enkephalins from the cell during periods of secretion. 3. Enkephalin expression and biosynthesis have been studied in several neuroendocrine cell types and tumor cell lines. Transcriptional, translational, and posttranslational factors can play a role at all three stages (establishment, modification, and maintenance) in the regulation of enkephalin expression during the lifetime of the cell. 4. Cyclic nucleotides, glucocorticoids, and calcium may all act to control the overall level of enkephalin biosynthesis pretranslationally, while regulation of posttranslational processing of proenkephalin seems to be important in determining the pattern of proenkephalin-derived opiate peptides produced in a given tissue. 5. The themes (and variations) of cell regulation that apply to enkephalin expression may be similar for other bioactive peptides produced in neural and endocrine tissues.

Reserpine increases chromaffin cell enkephalin stores without a concomitant decrease in other proenkephalin-derived peptides

Reserpine increases the levels of enkephalins in adrenal medullary chromaffin cells; however, the origin of the newly apparent pentapeptides has been the subject of debate, because no increase in the levels of proenkephalin mRNA has been observed. The present study was performed for determining if the reserpine-induced increase in context of enkephalins was derived from processing of preexisting fragments of proenkephalin. Bovine chromaffin cell enkephalins and larger enkephalin-containing peptides were separated by reversed-phase HPLC and identified by approximate molecular weight, elution with peptide standards, and enkephalin sequences contained. Treatment of the cells with reserpine increased the levels of enkephalins and of enkephalin-containing peptides of up to approximately 3 kilo-daltons without reducing the levels of larger enkephalin-containing peptides. Similar results were obtained with another catecholamine-depleting drug, tetrabenazine. In contrast, treatment of chromaffin cells with theophylline or forskolin increased the levels of both enkephalins and enkephalin-containing peptides of all sizes. The results suggest that new synthesis of proenkephalin is required for the effects of reserpine, although proenkephalin processing is also altered by this drug.