Expression of porcine pro-opiomelanocortin in mouse neuroblastoma (Neuro2A) cells: targeting of the foreign neuropeptide to dense-core vesicles

Endocrine secretory granule production is caused by a lack of REST and intragranular secretory content and accelerated by PROX1

Endocrine secretory granules (ESGs) are morphological characteristics of endocrine/neuroendocrine cells and store peptide hormones/neurotransmitters. ESGs contain prohormones and ESG-related molecules, mainly chromogranin/secretogranin family proteins. However, the precise mechanism of ESG formation has not been elucidated. In this study, we experimentally induced ESGs in the non-neuroendocrine lung cancer cell line H1299. Since repressive element 1 silencing transcription factor (REST) and prospero homeobox 1 (PROX1) are closely associated with the expression of ESG-related molecules, we edited the REST gene and/or transfected PROX1 and then performed molecular biology, immunocytochemistry, and electron and immunoelectron microscopy assays to determine whether ESG-related molecules and ESGs were induced in H1299 cells. Although chromogranin/secretogranin family proteins were induced in H1299 cells by knockout of REST and the induction was accelerated by the PROX1 transgene, the ESGs could not be defined by electron microscopy. However, a small number of ESGs were detected in the H1299 cells lacking REST and expressing pro-opiomelanocortin (POMC) by electron microscopy. Furthermore, many ESGs were produced in the REST-lacking and PROX1- and POMC-expressing H1299 cells. These findings suggest that a lack of REST and the expression of genes related to ESG content are indispensable for ESG production and that PROX1 accelerates ESG production.

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Dysfunctional Rhbdf2 of proopiomelanocortin mitigates ambient particulate matter exposure-induced neurological injury and neuron loss by antagonizing oxidative stress and inflammatory reaction

Ambient particulate matter (PM_2.5)-induced metabolic syndromes is a critical contributor to the pathological processes of neurological diseases, but the underlying molecular mechanisms remain poorly understood. The rhomboid 5 homolog 2 (Rhbdf2), an essential regulator in the production of TNF-α, has recently been confirmed to exhibit a key role in regulating inflammation-associated diseases. Thus, we examined whether Rhbdf2 contributes to hypothalamic inflammation via NF-κB associated inflammation activation in long-term PM_2.5-exposed mice. Specifically, proopiomelanocortin-specific Rhbdf2 deficiency (Rhbdf2^Pomc) and corresponding littermates control mice were used for the current study. After 24 weeks of PM_2.5 inhalation, systemic-metabolism disorder was confirmed in WT mice in terms of impaired glucose tolerance, increased insulin resistance, and high blood pressure. Markedly, PM_2.5-treated Rhbdf2^Pomc mice displayed a significantly opposite trend in these parameters compared with those of the controls group. We next confirmed hypothalamic injury accompanied by abnormal POMC neurons loss, as indicated by increased inflammatory cytokines, chemokines, and oxidative-stress levels and decreased antioxidant activity. These results were further supported by blood routine examination. In summary, our findings suggest that Rhbdf2 plays an important role in exacerbating PM_2.5-stimulated POMC neurons loss associated hypothalamic injury, thus providing a possible target for blocking pathological development of air pollution-associated diseases.

Hypothalamic ER-associated degradation regulates POMC maturation, feeding, and age-associated obesity

Pro-opiomelanocortin (POMC) neurons function as key regulators of metabolism and physiology by releasing prohormone-derived neuropeptides with distinct biological activities. However, our understanding of early events in prohormone maturation in the ER remains incomplete. Highlighting the significance of this gap in knowledge, a single POMC cysteine-to-phenylalanine mutation at position 28 (POMC-C28F) is defective for ER processing and causes early onset obesity in a dominant-negative manner in humans through an unclear mechanism. Here, we report a pathologically important role of Sel1L-Hrd1, the protein complex of ER-associated degradation (ERAD), within POMC neurons. Mice with POMC neuron–specific Sel1L deficiency developed age-associated obesity due, at least in part, to the ER retention of POMC that led to hyperphagia. The Sel1L-Hrd1 complex targets a fraction of nascent POMC molecules for ubiquitination and proteasomal degradation, preventing accumulation of misfolded and aggregated POMC, thereby ensuring that another fraction of POMC can undergo normal posttranslational processing and trafficking for secretion. Moreover, we found that the disease-associated POMC-C28F mutant evades ERAD and becomes aggregated due to the presence of a highly reactive unpaired cysteine thiol at position 50. Thus, this study not only identifies ERAD as an important mechanism regulating POMC maturation within the ER, but also provides insights into the pathogenesis of monogenic obesity associated with defective prohormone folding.

P2X7 receptors trigger ATP exocytosis and modify secretory vesicle dynamics in neuroblastoma cells

Previously, we reported that purinergic ionotropic P2X7 receptors negatively regulate neurite formation in Neuro-2a (N2a) mouse neuroblastoma cells through a Ca²⁺/calmodulin-dependent kinase II-related mechanism. In the present study we used this cell line to investigate a parallel though faster P2X7 receptor-mediated signaling pathway, namely Ca²⁺-regulated exocytosis. Selective activation of P2X7 receptors evoked exocytosis as assayed by high resolution membrane capacitance measurements. Using dual-wavelength total internal reflection microscopy, we have observed both the increase in near-membrane Ca²⁺ concentration and the exocytosis of fluorescently labeled vesicles in response to P2X7 receptor stimulation. Moreover, activation of P2X7 receptors also affects vesicle motion in the vertical and horizontal directions, thus, involving this receptor type in the control of early steps (docking and priming) of the secretory pathway. Immunocytochemical and RT-PCR experiments evidenced that N2a cells express the three neuronal SNAREs as well as vesicular nucleotide and monoamine (VMAT-1 and VMAT-2) transporters. Biochemical measurements indicated that ionomycin induced a significant release of ATP from N2a cells. Finally, P2X7 receptor stimulation and ionomycin increased the incidence of small transient inward currents, reminiscent of postsynaptic quantal events observed at synapses. Small transient inward currents were dependent on extracellular Ca²⁺ and were abolished by Brilliant Blue G, suggesting they were mediated by P2X7 receptors. Altogether, these results suggest the existence of a positive feedback mechanism mediated by P2X7 receptor-stimulated exocytotic release of ATP that would act on P2X7 receptors on the same or neighbor cells to further stimulate its own release and negatively control N2a cell differentiation.

Sequestration of mutated alpha1-antitrypsin into inclusion bodies is a cell-protective mechanism to maintain endoplasmic reticulum function

A variant alpha1-antitrypsin with E342K mutation has a high tendency to form intracellular polymers, and it is associated with liver disease. In the hepatocytes of individuals carrying the mutation, alpha1-antitrypsin localizes both to the endoplasmic reticulum (ER) and to membrane-surrounded inclusion bodies (IBs). It is unclear whether the IBs contribute to cell toxicity or whether they are protective to the cell. We found that in hepatoma cells, mutated alpha1-antitrypsin exited the ER and accumulated in IBs that were negative for autophagosomal and lysosomal markers, and contained several ER components, but not calnexin. Mutated alpha1-antitrypsin induced IBs also in neuroendocrine cells, showing that formation of these organelles is not cell type specific. In the presence of IBs, ER function was largely maintained. Increased levels of calnexin, but not of protein disulfide isomerase, inhibited formation of IBs and lead to retention of mutated alpha1-antitrypsin in the ER. In hepatoma cells, shift of mutated alpha1-antitrypsin localization to the ER by calnexin overexpression lead to cell shrinkage, ER stress, and impairment of the secretory pathway at the ER level. We conclude that segregation of mutated alpha1-antitrypsin from the ER to the IBs is a protective cell response to maintain a functional secretory pathway.

Structural Requirements for Sorting Pro-Vasopressin to the Regulated Secretory Pathway in a Neuronal Cell Line

Vasopressin is a peptide hormone normally secreted via the regulated secretory pathway in neuro-endocrine cells. In an effort to determine which region of vasopressin contains sufficient information for sorting, we created five constructs with the cDNA for vasopressin or regions of vasopressin in frame with the gene for green fluorescent protein (GFP). Fluorescence microscopy of Neuro-2a cells expressing the constructs revealed full-length vasopressin-GFP (VP-GFP), neurophysin-GFP (NP-GFP) and arginine-vasopressin/neurophysin-GFP (AN-GFP), were localized to punctate granules in the neurites and accumulated at the tips of neurites, characteristic of regulated secretory granules. These fusion proteins were secreted in a regulated manner as determined by pulse-chase labeling experiments. Two other chimeric proteins, signalpeptide-GFP and AVP-GFP were localized to a perinuclear region, characteristic of the endoplasmic reticulum. Pulse/chase [(35)S]labeling followed by immunoprecipitation using anti-GFP antibody indicated that these two fusion proteins were constitutively secreted. We conclude that the neurophysin region of pro-vasopressin contains information that is both sufficient and necessary for sorting GFP into the regulated secretory pathway.

Degradation of wild-type vasopressin precursor and pathogenic mutants by the proteasome

Mutations in the gene encoding the antidiuretic hormone arginine vasopressin cause autosomal dominant neurogenic diabetes insipidus. Autoptic data in affected individuals suggest that the neurons expressing mutant vasopressin undergo selective degeneration. Expression studies have shown that the mutants are retained in the endoplasmic reticulum, but how this trafficking defect is linked to neurotoxicity is unknown. One possibility is that unsecreted mutant precursors, or degradation products thereof, are cytotoxic. We therefore investigated the fate of endoplasmic reticulum-retained pathogenic mutants. Our data show that the mutants are retrotranslocated to the cytosol and degraded by the proteasome. In the presence of proteasomal inhibitors, three distinct un- or deglycosylated cytosolic species of vasopressin precursors were stabilized: pre-pro-vasopressin, pro-vasopressin, and an N-terminally truncated form. In addition to the retrotranslocated forms, a fraction of the newly synthesized precursor was not translocated, but was synthesized into the cytosol due to inefficient function of the vasopressin signal peptide. As a result, cytosolic pre-pro-vasopressin and its degradation product were also recovered when wild-type vasopressin was expressed. Cytosolic forms of vasopressin might trigger cytotoxicity in vivo, as has been proposed in the case of prion protein, which also contains an inefficient N-terminal signal peptide.

Plasma membrane targeting of SNAP-25 increases its local concentration and is necessary for SNARE complex formation and regulated exocytosis

SNAP-25 is an integral protein of the plasma membrane involved in neurotransmission and hormone secretion. The cysteine-rich domain of SNAP-25 is essential for membrane binding and plasma-membrane targeting. However, this domain is not required for SNARE complex formation and fusion of membranes in vitro. In this paper, we describe an `intact-cell'-based system designed to compare the effect of similar amounts of membrane-bound and soluble SNAP-25 proteins on regulated exocytosis. In transfected neuroblastoma cells,Botulinum neurotoxin E (BoNT/E), a protease that cleaves SNAP-25, blocks regulated release of hormone. However, hormone release is rescued by expressing a wild-type SNAP-25 protein resistant to the toxin. BoNT/E-resistant SNAP-25 proteins lacking the cysteine-rich domain or with all the cysteines substituted by alanines do not form SNARE complexes or rescue regulated exocytosis when expressed at the same level as membrane-bound SNAP-25, which is approximately four-fold higher than the endogenous protein. We conclude that the cysteine-rich domain of SNAP-25 is essential for Ca2+-dependent hormone release because, by targeting SNAP-25 to the plasma membrane, it increases its local concentration, leading to the formation of enough SNARE complexes to support exocytosis.

Stimulation-dependent regulation of the pH, volume and quantal size of bovine and rodent secretory vesicles

Trapping of weak bases was utilized to evaluate stimulus-induced changes in the internal pH of the secretory vesicles of chromaffin cells and enteric neurons. The internal acidity of chromaffin vesicles was increased by the nicotinic agonist 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP; in vivo and in vitro) and by high K+ (in vitro); and in enteric nerve terminals by exposure to veratridine or a plasmalemmal [Ca2+]o receptor agonist (Gd3+). Stimulation-induced acidification of chromaffin vesicles was [Ca2+]o-dependent and blocked by agents that inhibit the vacuolar proton pump (vH+-ATPase) or flux through Cl- channels. Stimulation also increased the average volume of chromaffin vesicles and the proportion that displayed a clear halo around their dense cores (called active vesicles). Stimulation-induced increases in internal acidity and size were greatest in active vesicles. Stimulation of chromaffin cells in the presence of a plasma membrane marker revealed that membrane was internalized in endosomes but not in chromaffin vesicles. The stable expression of botulinum toxin E to prevent exocytosis did not affect the stimulation-induced acidification of the secretory vesicles of mouse neuroblastoma Neuro2A cells. Stimulation-induced acidification thus occurs independently of exocytosis. The quantal size of secreted catecholamines, measured by amperometry in cultured chromaffin cells, was found to be increased either by prior exposure to L-DOPA or stimulation by high K+, and decreased by inhibition of vH+-ATPase or flux through Cl- channels. These observations are consistent with the hypothesis that the content of releasable small molecules in secretory vesicles is increased when the driving force for their uptake is enhanced, either by increasing the transmembrane concentration or pH gradients.

SNAP-25 and synaptotagmin 1 function in Ca2+-dependent reversible docking of granules to the plasma membrane

In neuroendocrine cells, Ca2+ triggers fusion of granules with the plasma membrane and functions at earlier steps by increasing the size of the readily releasable pool of vesicles. The effect of Ca2+ at early steps of secretion may be due to the recruitment at the plasma membrane of granules localized in the cytoplasm. To study the mechanism of granule docking, a new in vitro assay is designed using membrane fractions from mouse pituitary AtT-20 cells. By using this assay, it is found that granule docking to the plasma membrane is controlled by Ca2+ concentrations in the micromolar range, is reversible and requires intact SNAP-25, but not VAMP-2. In the docking assay, addition of Ca2+ induces the formation of a SNAP-25-Synaptotagmin 1 complex. The cytosolic domain C2AB of Synaptotagmin 1 and anti-Synaptotagmin 1 antibodies block granule docking. These results show that Ca2+ modulates dynamic docking of granules to the plasma membrane and that this process is due to a Ca2+-dependent interaction between SNAP-25 and Synaptotagmin 1.

Visualization of time-dependent redistribution of delta-opioid receptors in neuronal cells during prolonged agonist exposure

To date, the visualization of delta-opioid receptor (DOR) internalization has been largely focused on the events of short-term agonist treatment in transfected non-neuronal cells. In this study, we followed DOR trafficking upon prolonged agonist exposure in the neuronally derived neuro2a cells, stably transfected with the fusion DOR (HA-DOR) cDNA. Internalization of surface DOR was clearly visualized in 5 min of exposure to agonist (100 nM DADLE), and the cell surface DOR remained low throughout the entire 24 h agonist exposure. Significant intracellular accumulation was visible at 20 min exposure, and increased to a maximum at 4 h, after which intracellular DOR staining gradually diminished. DOR intracellular staining was enhanced in the presence of agonist and chloroquine, a lysosomotropic agent, suggesting that internalized receptors were targeted to lysosomes and degraded upon prolonged treatment. Time-dependent colocalization of DOR with transferrin and LAMP-2 following short-term and prolonged agonist exposure further confirmed that receptor was distributed to early endosomes (sequestration) and subjected to lysosomes for degradation (down-regulation), respectively.

Dense-cored vesicles in human dental macrophage-like pulpal cells

In the human dental pulp of five healthy premolars, we detected the presence of vesicles in the cytoplasm of macrophage-like cells. There were, in general, two types of vesicles, with regard to their size and content. The first was of smaller size (35 to 55 nm), and they were electron optically empty. The second type included larger vesicles (100 to 150 nm) presenting a heavily and homogeneously osmiophilic dense core. Their overall morphology was strikingly similar to the well-known, dense-cored vesicles of the sympathetic and neuroendocrine systems. This morphological evidence may be considered as an adjunctive support for the neural crest derivation of the dental pulp.

Molecular basis of autosomal dominant neurohypophyseal diabetes insipidus. Cellular toxicity caused by the accumulation of mutant vasopressin precursors within the endoplasmic reticulum

Mutations in the arginine vasopressin (AVP) gene cause autosomal dominant familial neurohypophyseal diabetes insipidus (FNDI). The dominant inheritance pattern has been postulated to reflect neuronal toxicity of the mutant proteins, but the mechanism for such cytotoxicity is unknown. In this study, wild-type or several different mutant AVP genes were stably expressed in neuro2A neuroblastoma cells. When cells were treated with valproic acid to induce neuronal differentiation, each of the mutants caused reduced viability. Metabolic labeling revealed diminished intracellular trafficking of mutant AVP precursors and confirmed inefficient secretion of immunoreactive AVP. Immunofluorescence studies demonstrated marked accumulation of mutant AVP precursors within the endoplasmic reticulum. These studies suggest that the cellular toxicity in FNDI may be caused by the intracellular accumulation of mutant precursor proteins.

Identification of the sorting signal motif within pro-opiomelanocortin for the regulated secretory pathway

The NH2-terminal region of pro-opiomelanocortin (POMC) is highly conserved across species, having two disulfide bridges that cause the formation of an amphipathic hairpin loop structure between the 2nd and 3rd cysteine residues (Cys8 to Cys20). The role that the NH2-terminal region of pro-opiomelanocortin plays in acting as a molecular sorting signal for the regulated secretory pathway was investigated by using site-directed mutagenesis either to disrupt one or more of the disulfide bridges or to delete the amphipathic loop entirely. When POMC was expressed in Neuro-2a cells, ACTH immunoreactive material was localized in punctate secretory granules in the cell body and along the neurites, with heavy labeling at the tips. ACTH was secreted from these POMC-transfected cells in a regulated manner. Disruption of both disulfide bridges or the second disulfide bridge or removal of the amphipathic hairpin loop resulted in constitutive secretion of the mutant POMC from the cells and a lack of punctate secretory granule immunostaining within the cells. We have modeled the NH2-terminal POMC Cys8 to Cys20 domain and have identified it as an amphipathic loop containing four highly conserved hydrophobic and acidic amino acid residues (Asp10-Leu11-Glu14-Leu1). Thus the sorting signal for POMC to the regulated secretory pathway appears to be encoded by a specific conformational motif comprised of a 13-amino acid amphipathic loop structure stabilized by a disulfide bridge, located at the NH2 terminus of the molecule.

Differential trafficking of soluble and integral membrane secretory granule-associated proteins

The posttranslational processing enzyme peptidylglycine alpha-amidating monooxygenase (PAM) occurs naturally in integral membrane and soluble forms. With the goal of understanding the targeting of these proteins to secretory granules, we have compared the maturation, processing, secretion, and storage of PAM proteins in stably transfected AtT-20 cells. Integral membrane and soluble PAM proteins exit the ER and reach the Golgi apparatus with similar kinetics. Biosynthetic labeling experiments demonstrated that soluble PAM proteins were endoproteolytically processed to a greater extent than integral membrane PAM; this processing occurred in the regulated secretory pathway and was blocked by incubation of cells at 20 degrees C. 16 h after a biosynthetic pulse, a larger proportion of soluble PAM proteins remained cell-associated compared with integral membrane PAM, suggesting that soluble PAM proteins were more efficiently targeted to storage granules. The nonstimulated secretion of soluble PAM proteins peaked 1-2 h after a biosynthetic pulse, suggesting that release was from vesicles which bud from immature granules during the maturation process. In contrast, soluble PAM proteins derived through endoproteolytic cleavage of integral membrane PAM were secreted in highest amount during later times of chase. Furthermore, immunoprecipitation of cell surface-associated integral membrane PAM demonstrated that very little integral membrane PAM reached the cell surface during early times of chase. However, when a truncated PAM protein lacking the cytoplasmic tail was expressed in AtT-20 cells, > 50% of the truncated PAM-1 protein reached the cell surface within 3 h. We conclude that the trafficking of integral membrane and soluble secretory granule-associated enzymes differs, and that integral membrane PAM proteins are less efficiently retained in maturing secretory granules.

Prosomatostatin processing in Neuro2A cells. Role of beta-turn structure in the vicinity of the Arg-Lys cleavage site

Proline residues located near the processing sites of human prosomatostatin were previously shown to be important for cleavage of the precursor into somatostatin 28 and somatostatin 14 [Gomez, S., Boileau, G., Zollinger, L., Nault, C., Rholam, M. & Cohen, P. (1989) EMBO J. 8, 2911-2916]. In this study, site-directed and regional mutagenesis of the human prosomatostatin cDNA coupled with analysis by circular-dichroism and Fourier-transform-infrared spectroscopies of the native and mutated peptide sequences were used to elucidate the role of proline in proteolytic processing. Glycine was substituted for proline a position -5 and the beta-turn-promoting sequence Pro-Arg-Glu-Arg, located near the somatostatin-14 cleavage site and predicted to form a beta-turn structure, was replaced by Ser-Ser-Asn-Arg or Tyr-Lys-Gly-Arg, which have been shown by X-ray diffraction to form beta turns in other proteins. Analysis of the prosomatostatin-derived peptides produced by expression of the mutated cDNA species in Neuro2A cells indicated that while Pro-5-->Ala abolished cleavage at the dibasic site, the formation of mutants [Gly-5] prosomatostatin, [Ser-5, Ser-4, Arg-3] prosomatostatin and [Tyr-5, Lys-4, Gly-3] prosomatostatin did not affect cleavage at the dibasic site but produced modifications in both the relative proportions of the generated hormones and in precursor processing efficiency. Moreover, spectroscopical analysis showed that whereas these substitutions did not modify the presence of a beta turn structure in the corresponding peptide sequences, replacement of Pro-5-->Ala resulted in a dramatic increase in alpha-helix accompanied by the significant decrease of other structures including beta turn. The data support the hypothesis that the proline residue near the processing site for somatostatin-14 production is an important structural feature for conferring on the cleavage domain the adequate conformation for accessibility to processing enzymes and permitting production of equivalent amounts of both hormones.

Targeting of pro-opiomelanocortin to the regulated secretory pathway may involve cooperation between different protein domains

The structure of pro-opiomelanocortin (POMC) can be divided into three main domains: an NH2-terminal domain formed by the NH2-terminal glycopeptide and the joining peptide, a central domain corresponding to the adrenocorticotropin sequences and a COOH-terminal domain containing the beta-lipotropin sequences. Expression of POMC in neuroendocrine cell lines such as the mouse neuroblastoma Neuro2A cells results in its targeting to the regulated secretory pathway of these cells. Intracellular targeting of proteins along non default pathways are widely believed to involve the recognition of specific structural features by a sorting machinery. To understand the nature of the signal involved in targeting prohormone to the regulated secretory pathway, we have constructed mutants of POMC in which sequences from the NH2-terminal, the central and the COOH-terminal domains were deleted and examined the sorting of these mutant POMC molecules in Neuro2A cells by immunofluorescence and immunoelectron microscopy. Our results indicate that POMC NH2-terminal glycopeptide or beta-LPH domain do not contain sufficient information for targeting to the regulated pathway since these peptides are not sorted to secretory vesicles when expressed in Neuro2A cells: Similarly, the ACTH domain does not contain essential targeting information since POMC mutants lacking these sequences were sorted to secretory vesicles. Mutant POMCs containing the sequences of more than one of the main protein domains were, however, correctly targeted to the regulated secretory pathway. Our results indicate that POMC is not targeted to the regulated secretory pathway through recognition of a unique continuous 'molecular address'.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of individual forms of peptidylglycine alpha-amidating monooxygenase in AtT-20 cells: endoproteolytic processing and routing to secretory granules

Peptidylglycine alpha-amidating monooxygenase (PAM: EC 1.14.17.3) is a bifunctional protein which catalyzes the COOH-terminal amidation of bioactive peptides; the NH2-terminal monooxygenase and mid-region lyase act in sequence to perform the peptide alpha-amidation reaction. Alternative splicing of the single PAM gene gives rise to mRNAs generating PAM proteins with and without a putative transmembrane domain, with and without a linker region between the two enzymes, and forms containing only the monooxygenase domain. The expression, endoproteolytic processing, storage, and secretion of this secretory granule-associated protein were examined after stable transfection of AtT-20 mouse pituitary cells with naturally occurring and truncated PAM proteins. The transfected proteins were examined using enzyme assays, subcellular fractionation, Western blotting, and immunocytochemistry. Western blots of crude membrane and soluble fractions of transfected cells demonstrated that all PAM proteins were endoproteolytically processed. When the linker region was present between the monooxygenase and lyase domains, monofunctional soluble enzymes were generated from bifunctional PAM proteins; without the linker region, bifunctional enzymes were generated. Soluble forms of PAM expressed in AtT-20 cells and soluble proteins generated through selective endoproteolysis of membrane-associated PAM were secreted in an active form into the medium; secretion of the transfected proteins and endogenous hormone were stimulated in parallel by secretagogues. PAM proteins were localized by immunocytochemistry in the perinuclear region near the Golgi apparatus and in secretory granules, with the greatest intensity of staining in the perinuclear region in cell lines expressing integral membrane forms of PAM. Monofunctional and bifunctional PAM proteins that were soluble or membrane-associated were all packaged into regulated secretory granules in AtT-20 cells.

Investigation of a possible role of the amino-terminal pro-region of proopiomelanocortin in its processing and targeting to secretory granules

Proopiomelanocortin (POMC) is a polyprotein which is targeted to the regulated secretory pathway of neuroendocrine cells where it undergoes tissue-specific proteolysis to yield peptides such as adrenocorticotropic hormone, beta-lipotropin and beta-endorphin. The pro-region of POMC is 49 amino acid long with two disulfide bonds between cysteine residues 2 and 24 and 8 and 20. These cysteine residues are conserved across the species. The pro-region contains no known hormonal sequence. Sorting to the regulated secretory pathway is thought to involve targeting signals encoded in the structure of secretory proteins. In the present study, we have examined the possibility that the disulfide bridges located in the NH2-terminal portion of the pro-region of POMC are essential for maintaining a determinant involved in the sorting of POMC to the regulated secretory pathway. Using site-directed and deletion mutagenesis of the porcine POMC cDNA, we created mutants in which one or both disulfide bridges were disrupted or in which the first 26 amino acid residues of the pro-region were deleted. Recombinant retroviruses carrying the mutated POMC cDNAs were used to infect Neuro2A cells. Immunofluorescence and immunoelectron microscopy studies performed on infected cells revealed that the unmutated and mutated POMC-immunoreactive peptides were localized in dense-core vesicles at the tips of cellular extensions. Analysis of the POMC-immunoreactive peptides extracted from the infected Neuro2A cells indicated that the mutated precursors in which one disulfide bridge was disrupted (POMC-S2 or POMC-S8) were stored and processed as efficiently as the unmutated POMC. By contrast, the mutated precursor in which both disulfide bridges were disrupted (POMC-S2,8) did not accumulate in intracellular compartments to the same extent as unmutated POMC. Moreover, this mutant was very inefficiently processed and no release could be observed upon stimulation of the cells with K+/Ca2+. These results suggest that POMC-S2,8 entered the regulated secretory pathway less efficiently than the unmutated precursor. However, when both disulfide bridges were removed from the precursor from the precursor by deletion of the first 26 amino acid residues of POMC, the truncated precursor (POMC delta 1-26) behaved as the unmutated POMC. Taken together our results indicate that the NH2-terminal portion of the pro-region including both disulfide bridges can be deleted without affecting the targeting of the molecule to secretory granules. However, when the entire POMC sequence is expressed in Neuro2A cells, the proper folding of the NH2-terminal region might be important for efficient processing and targeting.