Peptide precursor processing enzymes within secretory vesicles

ER Lipid Defects in Neuropeptidergic Neurons Impair Sleep Patterns in Parkinson's Disease

Parkinson's disease patients report disturbed sleep patterns long before motor dysfunction. Here, in parkin and pink1 models, we identify circadian rhythm and sleep pattern defects and map these to specific neuropeptidergic neurons in fly models and in hypothalamic neurons differentiated from patient induced pluripotent stem cells (iPSCs). Parkin and Pink1 control the clearance of mitochondria by protein ubiquitination. Although we do not observe major defects in mitochondria of mutant neuropeptidergic neurons, we do find an excess of endoplasmic reticulum-mitochondrial contacts. These excessive contact sites cause abnormal lipid trafficking that depletes phosphatidylserine from the endoplasmic reticulum (ER) and disrupts the production of neuropeptide-containing vesicles. Feeding mutant animals phosphatidylserine rescues neuropeptidergic vesicle production and acutely restores normal sleep patterns in mutant animals. Hence, sleep patterns and circadian disturbances in Parkinson's disease models are explained by excessive ER-mitochondrial contacts, and blocking their formation or increasing phosphatidylserine levels rescues the defects in vivo.

The proprotein convertases, 20 years later

The proprotein convertases (PCs) are secretory mammalian serine proteinases related to bacterial subtilisin-like enzymes. The family of PCs comprises nine members, PC1/3, PC2, furin, PC4, PC5/6, PACE4, PC7, SKI-1/S1P, and PCSK9 (Fig. 3.1). While the first seven PCs cleave after single or paired basic residues, the last two cleave at non-basic residues and the last one PCSK9 only cleaves one substrate, itself, for its activation. The targets and substrates of these convertases are very varied covering many aspects of cellular biology and communication. While it took more than 22 years to begin to identify the first member in 1989-1990, in less than 14 years they were all characterized. So where are we 20 years later in 2011? We have now reached a level of maturity needed to begin to unravel the mechanisms behind the complex physiological functions of these PCs both in health and disease states. We are still far away from comprehensively understanding the various ramifications of their roles and to identify their physiological substrates unequivocally. How do these enzymes function in vivo? Are there other partners to be identified that would modulate their activity and/or cellular localization? Would non-toxic inhibitors/silencers of some PCs provide alternative therapies to control some pathologies and improve human health? Are there human SNPs or mutations in these PCs that correlate with disease, and can these help define the finesses of their functions and/or cellular sorting? The more we know about a given field, the more questions will arise, until we are convinced that we have cornered the important angles. And yet the future may well reserve for us many surprises that may allow new leaps in our understanding of the fascinating biology of these phylogenetically ancient eukaryotic proteases (Fig. 3.2) implicated in health and disease, which traffic through the cells via multiple sorting pathways (Fig. 3.3).

Mechanisms, locations, and kinetics of synaptic BDNF secretion: an update

Brain-derived neurotrophic factor (BDNF) and other members of the protein family of neurotrophins have been implicated in a multitude of processes that are important for neuronal development and synaptic plasticity in the rodent central nervous system. In comparison to the wealth of information available with respect to the biological functions of neurotrophins, our knowledge regarding the processes that govern synaptic secretion of neurotrophins is scarce. Using live cell imaging of GFP-tagged neurotrophins in primary neurons, immunocytochemical detection of endogenous BDNF in fixed cells, and by blocking the action of endogenously released BDNF by means of TrkB receptor bodies in living neurons, several studies in recent years have allowed to better understand the time course and the mechanisms of synaptic secretion of neurotrophins. This review will summarize the current knowledge regarding the intracellular processing of proneurotrophins, the targeting of neurotrophin vesicles to axons and dendrites, and the mechanisms of activity-dependent secretion of BDNF at synapses. Since these processes are known to be cell type dependent, special emphasis is given to observations gained from experiments in primary neurons.

Neuropeptides in hyperthermia

Brain damage as a result of hyperthermia or heat-stress has been the focus of attention in many areas of neuroscience in recent years. Heat-induced alterations in structural components of the central nervous system (CNS) will obviously also influence the relevant transmitter systems, which may be involved in a variety of different behaviors. Indeed, many studies have indicated that excitatory amino acids, and monoaminergic and peptidergic systems are affected during hyperthermia. This chapter will address past and current research on various neuropeptides that have been implicated in the consequences of hyperthermia and various other heat disorders. However, considering the large and even increasing number of identified neuroactive peptides, it is necessary to limit this chapter to a few peptides or peptide systems, which have received particular attention in relation to hyperthermia. Among these are the opioid peptides, the tachykinins, calcitonin gene-related peptide (CGRP), and peptides belonging to the angiotensin system. Most of these neuropeptides are not only affected by hyperthermia and abnormal alterations in the body temperature but also are involved in the endogenous mechanisms of regulating body temperature. This review does not endeavor to fully cover the field but it does aim to give the reader an idea of how various neuropeptides may be involved in the control of body heat and how peptidergic systems are affected during various thermal changes, including both immediate and long-term consequences.

Proteolytic processing of pro-opiomelanocortin occurs in acidifying secretory granules of AtT-20 cells

Using antibodies specific for pro-opiomelanocortin (POMC), amidated joining peptide (JP), and the prohormone convertase PC1, we showed immunocytochemically that PC1 in a corticotrophic tumor cell line, AtT-20, was co-localized either with POMC or with amidated JP in secretory granules, and also confirmed that POMC was cleaved mainly in secretory granules. Analysis using DAMP (3- [2,4-dinitroanilino]-3'-amino-N-methyldipropylamine) as the pH probe suggested a correlation between POMC processing and acidic pH in the secretory granules. Bafilomycin A1, a specific inhibitor of vacuolar-type H(+)-ATPase, completely inhibited POMC processing and caused constitutive secretion of the unprocessed precursor. By contrast, chloroquine, a weak base that is known to neutralize acidic organelles, was unable to inhibit POMC processing. Electron microscopic analysis revealed that, in AtT-20 cells treated with bafilomycin A1, the trans-Golgi cisternae were dilated and few secretory granules were present in the cytoplasm. These observations suggest that acidic pH provides a favorable environment for proteolytic processing of POMC by PC1 but is not required, and that integrity of the trans-Golgi network and sorting of POMC into secretory granules are important for POMC processing.

Processing and secretion of insulin-related peptides in an insulinoma cell line

Certain classes of prohormones and other neuroendocrine or endocrine-derived secretory proteins are post-translationally modified in the secretory storage granules. If such molecules were to be biosynthesized to acceptable quantity and yield using endocrine-derived cell lines, it would be important to understand the relationship between the secretory dynamics and the conversion and release of the immature and mature forms of the molecule. We studied aspects of such a relationship using the endocrine-derived cell line betaTC-3, which synthesizes murine proinsulin, sequesters it into secretory granules, and converts it into mature insulin. In T-flask experiments with confluent cultures of betaTC-3 cells, intracellular and secreted (pro)insulin was sampled before and after episodes of stimulated exocytosis and recharging and quantified by radioimmunoassay and reversed-phase high-performance liquid chromatography (HPLC). Under conditions of steady-state secretion in glucose-rich growth medium the cells turned over their (pro)insulin inventory (90 +/- 5% mature insulin) at 2-3% per hour through secretion of (pro)insulin which was less than 70% mature. During an episode of hyperstimulated exocytosis induced by the combined secretagogues carbachol (1 microM) and isobutylmethylxanthine (1 mM), approximately 80% of the intracellular (pro)insulin stores were depleted within 2 h and 84 +/- 4% of the secreted (pro)insulin was in the mature form. Following the discharging episode, exocytosis was suppressed to 10% of its steady-state rate with a treatment which attenuated calcium influx (20 microM verapamil with reduced levels of calcium in the medium). Under this condition the secreted protein was only approximately 50% converted to mature insulin, but 85 +/- 10% of the net (pro)insulin accumulating within the intracellular stores was converted to the mature form. The inverse relationship between rate of secretion and degree of conversion of secreted (pro)insulin is consistent with a previously observed phenomenon of preferential basal secretion from immature secretory granules. This tends to enrich the secreted peptides in immature forms relative to the total intracellular pool. Preferential early secretion can best be overcome by rapid discharging of the long-term and predominantly mature stores. Thus, a cyclic controlled secretion process wherein product is collected during intermittent discharging episodes would provide a better yield of mature product than would steady-state secretion.

Superfluous neurotransmitters?

In recent years, studies have suggested that the complexity of eukaryotic gene regulation, with its recurring and interacting motifs of cis and trans-acting regulatory elements, might result in superfluous gene expression. This conclusion is supported by a variety of experimental results that suggest that non-adaptive gene expression might be common. However, with few exceptions, the practical ramifications of unnecessary gene expression for cell biologists have not been addressed directly; this is particularly true for peptidergic neurophysiology, a field that might be plagued more than most with the consequences of this phenomenon. In this article, Chauncey W. Bowers discusses the superfluous expression of neuropeptides in the nervous system in the context of gene regulation extrapolated from studies in Drosophila.

Endocytotic pathways in the melanotroph of the rat pituitary

The internalization of the extracellular markers horseradish peroxidase (HRP) and cationized ferritin (CF) by the melanotrophs of the intermediate lobe of the rat pituitary was studied during short-time incubation of mechanically dissociated cells or in cell culture after 5 days. After a 30 min exposure, the tracers were found in electron-lucent granules or vacuoles of approximately the same size as the secretory granules, situated 200-500 nm from the cell membrane. In the cultured cells, which showed a higher rate of tracer uptake, internalization was followed for 1, 2 and 5 min after labelling and during 2 h of exposure. Initially, the label was seen only in coated pits and coated vesicles at the cell membrane. Larger vacuoles were first seen after 2-5 min of incubation. After 2 h of exposure the labelling pattern was distinctly different for the two tracers. CF was found in larger vacuoles of varying morphology, in dilatations at the base of cilia, within Golgi saccules and at the edge of the electron-dense core of forming secretory granules. HRP was found in an extensive array of tubulovesicular structures extending throughout the cytoplasm. The Golgi complex and forming granules were, however, not labelled with HRP. The study identifies part of the electron-lucent granules or vacuoles in the melanotroph as endosomes, and shows that the melanotrophs sort CF and HRP via diverting pathways after internalization, suggesting that granule membrane, and possibly its functional components, can be recycled in these cells.

Dynorphin A-(1–17) and dynorphin B are released from in vitro superfused rat hypothalami. Effects of depolarizing agents and ovariectomy

We measured the release of immunoreactive (ir) dynorphin (dyn) A-(1-17) and dyn B from the rat hypothalamus by an in vitro superfusion technique. The system was validated on the basis of the recovery and stability of radiolabeled peptides added to the superfused hypothalami. These were detected as authentic peptides by reverse-phase high-performance liquid chromatography (rp-HPLC) only in the presence of a cocktail of peptidase inhibitors added to the superfusion medium. We observed spontaneous release of ir-dyn B, evaluated by a validated radioimmunoassay in the superfusates, that was increased by potassium and veratridine depolarization. It was calcium-dependent and tetrodotoxin-sensitive. We could not evaluate ir-dyn A-(1-17) directly in the superfusates, because the peptidase inhibitors added to the medium significantly altered the tracer-antibody reaction. To obviate this problem, pooled superfusate samples were purified on C18 cartridges and assayed by rp-HPLC. Rp-HPLC analysis of superfusates revealed two molecular forms with the same retention time as authentic dyn A-(1-17) and dyn B which were four times higher in K(+)-stimulated fractions. We could not detect dyn A-(1-32), comprising dyn A-(1-17) and dyn B, even though this peptide is recognized by the antibodies used in this study and is detected in acetic acid extracts of the rat hypothalamus. The spontaneous and K(+)-evoked release of ir-dyn A-(1-17) and ir-dyn B were significantly higher in 2-week ovariectomized rats, in parallel with the increase of their content in the anterior hypothalamus preoptic area.

Light- and electron-microscopic immunocytochemistry of a molluscan insulin-related peptide in the central nervous system of Planorbarius corneus

Two groups of cerebral dorsal cells of the pulmonate snail Planorbarius corneus stain positively with antisera raised against synthetic fragments of the B- and C-chain of the molluscan pro-insulin-related prohormone, proMIP-I, of another pulmonate snail, Lymnaea stagnalis. At the light-microscopic level the somata of the dorsal cells and their axons and neurohemal axon terminals in the periphery of the paired median lip nerves are immunoreactive with both antisera. Furthermore, the canopy cells in the lateral lobes of the cerebral ganglia are positive. In addition, MIPB-immunoreactive neurons are found in most other ganglia of the central nervous system. At the ultrastructural level, pale and dark secretory granules are found in somata and axon terminals of the dorsal cells. Dark granules are about 4 times as immunoreactive to both antisera as pale granules. Release of anti-MIPB- and anti-MIPC-immunopositive contents of the secretory granules by exocytosis is apparent in material treated according to the tannic acid method. It is concluded that the dorsal and canopy cells synthesize a molluscan insulin-related peptide that is packed in the cell body into secretory granules and that is subsequently transported to the neurohemal axon terminals and released into the hemolymph by exocytosis. Thus, MIP seems to act as a neurhormone on peripheral targets. On the basis of the analogy between the dorsal cells and the MIP-producing cells in L. stagnalis, it is proposed that the dorsal cells of P. corneus are involved in the control of body growth and associated processes.

Segment-specific modifications of a neuropeptide phenotype in embryonic neurons of the moth, Manduca sexta

We have studied differences in the development of segmentally homologous neurons to identify factors that may regulate a neuropeptide phenotype. Bilaterally paired homologs of the peripheral neuron L1 were identified in the thoracic and abdominal segments in embryos of the moth Manduca: each bipolar neuron arises at a stereotyped location and, at 40% of embryogenesis, projects its major process within the transverse nerve of its own segment. Shortly after the initiation of axonogenesis (approximately 41%), L1 homologs in all but the prothoracic segment (T1) were labelled specifically by an antiserum to the molluscan neuropeptide Phe-Met-Arg-Phe-NH2 (authentic FMRFamide). Levels of peptide-immunoreactivity (IR) were comparable in all such segmental homologs up to the approximately 60% stage of embryogenesis, whereupon two distinct levels of peptide IR were displayed: homologs in the three most rostral segments (T2, T3, and A1; [abdominal segment 1]) showed high levels and were called Type I L1 neurons; homologs in the more caudal segments (A2-A8) typically showed low levels of IR and were called Type II L1 neurons. This segment-specific difference represented mature differentiated states and was retained in postembryonic stages. Intracellular dye fills of embryonic L1 neurons revealed that the morphogenesis of the Type I and II L1 neuron homologs was similar until approximately 48% of embryogenesis; thereafter it differed in two salient ways: (1) the cell bodies of Type II L1 neurons migrated approximately 150 microns laterally from their point of origin, and (2) the distal processes of the Type II L1 neurons contacted the heart, whereas those of Type I L1 neurons did not. Ultrastructural studies of both mature and developing L1 homologs showed that the FMRFamide-like antigen(s) localized specifically to secretory granules. Further, whereas the secretory granules in segmental homologs appeared similar initially (i.e., at approximately 50% of development), following the establishment of segment-specific differences, secretory granules found in mature Type I and II L1 neurons were cell type-specific.

Processing of prothrombin in the secretory pathway

Antibodies raised against plasma prothrombin and the prothrombin propeptide were used to identify prothrombin precursors in rough and smooth microsomes and in the Golgi apparatus. The data demonstrate that the propeptide is part of the prothrombin molecule when undergoing a variety of modifications in the Golgi apparatus. It is shown that these modifications result in an increase in the apparent molecular mass of the prothrombin precursor from 78 kDa in early processing to 83 kDa in late processing. The 83 kDa prothrombin precursor was not recognized by the anti-propeptide antiserum and most likely represents the final product of the precursor in the secretory pathway. Evidence is presented that the propeptide is released from the parent molecule in the Golgi apparatus by a membrane-bound Ca(2+)-dependent serine proteinase(s) with characteristics similar to those of the proalbumin-to-albumin-converting enzyme. Vitamin K-dependent carboxylase activity was measured in membrane fragments obtained from the Golgi apparatus preparation. Sucrose-density-gradient centrifugation and the use of marker enzymes showed that carboxylase activity was highest in fractions enriched in cis-Golgi cisternae. Two different synthetic peptides were used as substrates for the carboxylase. These peptides were from the N-terminal and the C-terminal part of the gamma-carboxyglutamic acid (Gla) region of prothrombin. It is shown that the N-terminal and the C-terminal peptides were preferred as substrates for the carboxylase in the microsomal and the Golgi apparatus preparations respectively. It is also shown that the prothrombin precursor acquires negative charges in the Golgi apparatus that do not result from addition of sugars in late processing. These negative charges could be eliminated by thermal decarboxylation, suggesting that Gla residues may also be synthesized in late processing.

The isolation and partial characterization of bovine parathyroid secretory granules

We have developed a procedure which allows the isolation of secretion granules from fresh parathyroid glands. Following collagenase digestion of the tissue, the cells were broken with osmotic shock and a crude granule/mitochondrial pellet was obtained by differential centrifugation. Before loading this fraction onto a metrizamide density gradient it was subjected to brief sonication to disrupt the mitochondria. This procedure was necessary in order to achieve separation of the granules from the mitochondria during ultracentrifugation of the gradient. When the fractionated gradient was analysed for PTH by radioimmunoassay, three bands containing parathyroid hormone were found, at densities of 1.0, 1.05 and 1.18. Upon electron microscopic examination of the gradient fractions, granules were found only in those fractions containing hormone. A typical granule appearance was observed for two of the populations, but the third population (density 1.18), consisted of granules without membranes and which appeared less electron dense than those of populations 1 (density of 1.0) and 2 (density of 1.05). Moreover, the lack of a limiting membrane imparted a fuzzy appearance to the population 3 granules. When fresh tissue sections were examined as control samples, granules with and without membranes were also observed. Standard marker enzyme assays further confirmed that populations 2 and 3 were relatively free of other cellular contaminants, but population 1 contained endoplasmic reticulum and lysosomal material. Because the number of granules contained in this population is very small, we have not been successful in achieving further purification of population 1. Based on radioimmunoassay of extracts of each granule population, PTH was concentrated in population 3, while the other two contained lesser amounts. Interestingly, results obtained with a radioimmunoassay for SP-1 revealed a striking difference in the distribution of SP-1 in the three granule populations. This protein, which is also secreted by the parathyroid gland, was concentrated in population 1 and 2. Only very low levels were found in population 3. Thus, the two major secretory products are localized in different granule populations. The isolated granules were stable to pH changes, cycles of freeze/thaw and sonication. The yields of PTH extracted from each of the granule populations by freezing and thawing in buffer or by Triton containing solutions were low. PTH was completely extracted from each population only by using 8 M urea in HCl. Lower concentrations of urea were less effective. These results indicate that the molecular architecture of the granules is highly resistant to disruption.

Ultrastructural evidence for synthesis, storage and release of insulin-related peptides in the central nervous system of Lymnaea stagnalis

The cerebral neuroendocrine Light Green Cells of the pulmonate snail Lymnaea stagnalis, which control body growth and associated processes, stain positively with an affinity-purified antiserum raised to a large part of the C-chain of pro-molluscan insulin-related peptides. At the ultrastructural level, the rough endoplasmic reticulum is immunonegative, the Golgi apparatus is slightly positive and secretory granules in the process of budding from the Golgi apparatus are strongly positive. These observations indicate that the Light Green Cells synthesize molluscan insulin-related peptides, which are processed before packing by the Golgi apparatus into secretory granules. The two morphologically distinct secretory granule types, i.e. with pale and dark contents, respectively, are equally immunoreactive with antiserum raised to the C-chain of molluscan insulin-related peptides. Secretory granules within lysosomal structures reveal various degrees of immunoreactivity, indicating their graded breakdown. The Light Green Cells release secretory material by the process of exocytosis into the haemolymph from neurohaemal axon terminals located in the periphery of the median lip nerve. The electron-dense (tannic acid method) released contents are clearly immunopositive. The same holds for secretory granule contents released from Light Green Cells axon profiles in the centre of the lip nerve. Some immunoreactivity is also present in the intercellular space between these axon profiles. It is concluded that molluscan insulin-related peptides may act in two ways, namely (1) as neurohormones via the haemolymph at peripheral targets and (2) in a non-synaptic (paracrine) fashion at targets within the central nervous system.

Analysis of endothelin related peptides in culture supernatant of porcine aortic endothelial cells: evidence for biosynthetic pathway of endothelin-1

We investigated the molecular forms of endothelin (ET) related peptides in culture supernatant of porcine aortic endothelial cells by high performance liquid chromatography coupled with radioimmunoassays for ET related peptides. We isolated and sequenced a C-terminal peptide (big ET-1(22-39] of big ET-1(1-39) and its N-terminal truncated form (big ET-1(23-39] in addition to ET-1(1-21) and its oxidized form, [Met7 (0)]ET-1(1-21). The total contents of the two C-terminal peptides of big ET-1(1-39) are approximately equal to those of ET-1(1-21) and its oxidized form on a molar basis in the culture supernatant. Furthermore, we isolated big ET-1(1-39) although its content is approximately 2% of that of ET-1(1-21). These results strongly suggest that ET-1(1-21) and big ET-1(22-39) are generated from big ET-1(1-39) by specific processing between Trp21-Val22.

Regulated secretion of pro-opiomelanocortin converting enzyme and an aminopeptidase B-like enzyme from dispersed bovine intermediate lobe pituitary cells

Coordinate secretion of two prohormone/proneuropeptide processing enzymes [pro-opiomelanocortin converting enzyme (PCE) and an aminopeptidase B-like enzyme (APBE)] and alpha-melanotropin (alpha-MSH) from bovine intermediate lobe pituitary cells was studied. Stimulation of secretion with 8-bromo-cyclic AMP produced significant increases in levels of immunoreactive alpha-MSH, PCE, and APBE. Treatment of cells with the dopaminergic agonist 2-bromo-alpha-ergocryptine resulted in significant decreases in secretion of alpha-MSH, PCE, and APBE. In neither case were there significant changes in levels of cytosolic lactic dehydrogenase or lysosomal beta-glucuronidase in the medium. The secreted PCE activity was shown to process frog and mouse pro-opiomelanocortin primarily to 23,000-Mr corticotropin (ACTH), 13,000-Mr ACTH, beta-lipotropin, a beta-endorphin-like peptide, and beta-endorphin, products comparable to those synthesized by the mouse and frog intermediate lobe in situ. The secreted enzymatic activity had a pH optimum between 4.0 and 5.0, was strongly inhibited by pepstatin A, and had an inhibitor profile similar to the purified bovine intermediate lobe PCE. The secreted APBE activity cleaved Argo-[Met]-enkephalin to [Met]-enkephalin and had a pH optimum and inhibitor profile similar to that previously reported for an activity from purified secretory vesicle fractions of bovine intermediate and neural lobes. The coordinate regulated secretion of alpha-MSH and enzyme activities (PCE and APBE) strongly indicates their colocalization in the same secretory vesicle compartment within the cell. The characteristics of the two enzymes secreted in the medium paralleled those seen in the tissue and further support their role in pro-opiomelanocortin processing in vivo.

Posttranslational processing of endogenous and of baculovirus-expressed human gastrin-releasing peptide precursor

The 27-amino-acid gastrin-releasing peptide (GRP1-27) is a neuropeptide and growth factor that is synthesized by various neural and neuroendocrine cells. The major pro-GRP hormone (isoform I) contains both GRP1-27 and a novel C-terminal extension peptide termed pro-GRP31-125. In order to define potentially active neuropeptides that could be generated from this novel protein domain, we analyzed the posttranslational processing of endogenous human pro-GRP1-125 in a small-cell lung cancer cell line. Because such studies are much easier in an overexpression system, we investigated at the same time the posttranslational processing of baculovirus-expressed human pro-GRP1-125 in an insect ovary cell line. In the small-cell lung cancer cell line, GRP1-27 was cleaved as expected from the endogenous prohormone at a pair of basic amino acids (29 and 30) and alpha-amidated at its C-terminal methionine; however, a number of novel peptides were generated by additional cleavages in the pro-GRP31-125 domain. In the insect ovary cell line, GRP1-27 was cleaved from the expressed prohormone by a different mechanism, as were a number of other peptides that appeared to be similar in size to those produced by the human neuroendocrine tumor cell line. These data show for the first time that an insect ovary cell line that is widely used to overexpress proteins can process a human neuropeptide precursor. They also reveal the existence of novel pro-GRP-derived peptides that are candidates for biologically active ligands.

Posttranslational processing of endogenous and of baculovirus-expressed human gastrin-releasing peptide precursor

The 27-amino-acid gastrin-releasing peptide (GRP1-27) is a neuropeptide and growth factor that is synthesized by various neural and neuroendocrine cells. The major pro-GRP hormone (isoform I) contains both GRP1-27 and a novel C-terminal extension peptide termed pro-GRP31-125. In order to define potentially active neuropeptides that could be generated from this novel protein domain, we analyzed the posttranslational processing of endogenous human pro-GRP1-125 in a small-cell lung cancer cell line. Because such studies are much easier in an overexpression system, we investigated at the same time the posttranslational processing of baculovirus-expressed human pro-GRP1-125 in an insect ovary cell line. In the small-cell lung cancer cell line, GRP1-27 was cleaved as expected from the endogenous prohormone at a pair of basic amino acids (29 and 30) and alpha-amidated at its C-terminal methionine; however, a number of novel peptides were generated by additional cleavages in the pro-GRP31-125 domain. In the insect ovary cell line, GRP1-27 was cleaved from the expressed prohormone by a different mechanism, as were a number of other peptides that appeared to be similar in size to those produced by the human neuroendocrine tumor cell line. These data show for the first time that an insect ovary cell line that is widely used to overexpress proteins can process a human neuropeptide precursor. They also reveal the existence of novel pro-GRP-derived peptides that are candidates for biologically active ligands.

Pro-opiomelanocortin and pro-vasopressin converting enzyme in pituitary secretory vesicles

Peptide hormones are synthesized from larger precursors by cleavages at paired basic residues. We have isolated a pro-hormone converting enzyme from bovine neural and intermediate lobe secretory vesicles that cleaves pro-vasopressin and pro-opiomelanocortin at Lys-Arg residues to yield vasopressin, and adrenocorticotropin/endorphin-related peptides, respectively. The enzyme from both lobes is an aspartyl protease of approximately 70,000 Da, is a glycoprotein and has an optimum pH range of 4.0-5.0. Present within the same secretory vesicles is an aminopeptidase B-like enzyme which is a metalloprotease that is inhibited by Co2+ and Zn2+. This enzyme may play a role in trimming off the N-terminal extended basic residues from peptides liberated by the pro-hormone converting enzyme.