Maintained PC1 and PC2 expression in the AtT-20 variant cell line 6T3 lacking regulated secretion and POMC: restored POMC expression and regulated secretion after cAMP treatment

In PC12 cells, expression of neurosecretion and neurite outgrowth are governed by the transcription repressor REST/NRSF

A rapid drop of the transcription repressor REST/NRSF during precursor differentiation into nerve cells is known to release the repression of hundreds of specific genes and thus to orchestrate the acquisition of the specific phenotype. REST, however, is important not only for differentiation, but also for the maintenance of key properties in mature nerve cell. The PC12 line is uniquely favorable for studying REST because, in addition to the wild-type, low REST neurosecretory cells, it includes spontaneously defective clones lacking neurosecretion, where REST is as high as in non-nerve cells. In this article, we summarize our cell biologic studies of two nerve cell-specific processes dependent on REST, neurosecretion and neurite outgrowth. We demonstrate that, in wild-type PC12 transfected with REST constructs, expression of genes encoding proteins of dense-core and synaptic-like vesicles is decreased, though, to different extents, with chromogranins being the most and the SNAREs (except SNAP25) the least affected. Concomitantly, dense core-vesicles decrease markedly in size but can still be discharged by regulated exocytosis. When, in contrast, dominant-negative constructs of REST are transfected in high-REST PC12, and the main effector enzymes of REST, histone deacetylases, are blocked, dense-core vesicles reappear and are discharged upon stimulation. In high-REST PC12, also neurite outgrowth is inhibited by down regulation of the NGF receptor. Concomitantly, however, high REST induces the expression of proteins and of an exocytic organelle, the enlargeosome, which sustain a Rac1-dependent form of neurite outgrowth, unknown until now, operative in PC12, in neuroblastoma SH-SY5Y cells, and also in neurons.

Transcriptional orchestration of the regulated secretory pathway in neurons by the bHLH protein DIMM

BACKGROUND

The Drosophila basic helix-loop-helix (bHLH) gene dimmed (dimm) promotes a neurosecretory/neuroendocrine phenotype in cells but is not associated with specific neuropeptides or neurohormones. Rather, it is expressed by those peptidergic neurons that project long axons and appear to produce large amounts of secretory peptides. Here, we genetically transform nonpeptidergic neurons in Drosophila to study DIMM's action mechanisms.

RESULTS

Nonpeptidergic neurons normally fail to accumulate ectopic neuropeptides. We now show that they will do so when they are also forced to express ectopic DIMM. Furthermore, mass spectrometry shows that photoreceptors, which are normally nonpeptidergic, fail to process an ectopic neuropeptide precursor to make bioactive peptides but will do so efficiently when DIMM is co-misexpressed. Likewise, photoreceptors, which normally package the fast neurotransmitter histamine within small clear synaptic vesicles, produce numerous large dense-core vesicles (LDCVs) when they misexpress DIMM. These novel LDCVs accumulate ectopic neuropeptide when photoreceptors co-misexpress a neuropeptide transgene. DIMM-expressing photoreceptors no longer accumulate histamine and lose synaptic organelles critical to their normal physiology.

CONCLUSIONS

These findings indicate that DIMM suppresses conventional fast neurotransmission and promotes peptidergic neurosecretory properties. We conclude that DIMM normally provides a comprehensive transcriptional control to direct the differentiation of dedicated neuroendocrine neurons.

Peptidergic neurosecretory cells in insects: organization and control by the bHLH protein DIMMED

This review considers evidence that defines a role for the transcription factor DIMMED in the regulation of insect neurosecretory cells. Genetic anatomical and molecular data all suggest DIMMED is a dedicated controller of the regulated secretory pathway. DIMM is normally expressed within diverse neuropeptide-expressing cells and appears highly correlated with a neurosecretory cell fate. Loss of DIMM is associated with deficits in display of neuropeptides and neuropeptide-associated enzymes. Gain of DIMM promotes such display in peptidergic cells and can confer such neurosecretory properties onto conventional neurons. We review models proposed to explain how DIMMED regulates these essential cellular properties.

Aquaporin 1 is important for maintaining secretory granule biogenesis in endocrine cells

Aquaporins (AQPs), a family of water channels expressed in epithelial cells, function to transport water in a bidirectional manner to facilitate transepithelial fluid absorption and secretion. Additionally, AQP1 and AQP5 are found in pancreatic zymogen granules and synaptic vesicles and are involved in vesicle swelling and exocytosis in exocrine cells and neurons. Here, we show AQP1 is in dense-core secretory granule (DCSG) membranes of endocrine tissue: pituitary and adrenal medulla. The need for AQP1 in endocrine cell function was examined by stable transfection of AQP1 antisense RNA into AtT20 cells, a pituitary cell line, to down-regulate AQP1 expression. These AQP1-deficient cells showed more than 60% depletion of DCSGs and significantly decreased DCSG protein levels, including proopiomelanocotin/pro-ATCH and prohormone convertase 1/3, but not non-DCSG proteins. Pulse-chase studies revealed that whereas DCSG protein synthesis was unaffected, approximately 50% of the newly synthesized proopiomelanocortin was degraded within 1 h. Low levels of ACTH were released upon stimulation, indicating that the small number of DCSGs that were made in the presence of the residual AQP1 were functionally competent for exocytosis. Analysis of anterior pituitaries from AQP1 knockout mice showed reduced prohormone convertase 1/3, carboxypeptidase E, and ACTH levels compared to wild-type mice demonstrating that our results observed in AtT20 cells can be extended to the animal model. Thus, AQP1 is important for maintaining DCSG biogenesis and normal levels of hormone secretion in pituitary endocrine cells.

Dense-core secretory granule biogenesis

The dense-core secretory granule is a key organelle for secretion of hormones and neuropeptides in endocrine cells and neurons, in response to stimulation. Cholesterol and granins are critical for the assembly of these organelles at the trans-Golgi network, and their biogenesis is regulated quantitatively by posttranscriptional and posttranslational mechanisms.

Somatostatin and its receptors in the development of the endocrine pancreas

The development of the endocrine pancreas is regulated by numerous transcription and growth factors. Somatostatin (SST) is present in many tissues and acts as a neurotransmitter and autocrine/paracrine/endocrine regulator in response to ions, nutrients, peptides, and hormones as well as neurotransmitters. In the pancreas, there is evidence that SST acts an inhibitory paracrine regulator of hormone secretion. Somatostatin receptors (SSTRs) are a family of 5 transmembrane G protein-coupled receptors, which are widely expressed in mammals including humans. SSTRs regulate multiple downstream signal transduction pathways that mediate inhibitory effects. These receptors also exhibit age- and tissue-specific expression patterns. Interactions of SST and SSTRs are not only important during normal pancreas development, but have also been implicated in many pancreatic diseases such as diabetes mellitus and pancreatic cancer. In this review article, we use evidence from recently published animal studies to present the critical roles of SST and SSTRs proteins in the development of the endocrine pancreas.

Protease nexin-1 promotes secretory granule biogenesis by preventing granule protein degradation

Dense-core secretory granule (DCG) biogenesis is a prerequisite step for the sorting, processing, and secretion of neuropeptides and hormones in (neuro)endocrine cells. Previously, chromogranin A (CgA) has been shown to play a key role in the regulation of DCG biogenesis in vitro and in vivo. However, the underlying mechanism of CgA-mediated DCG biogenesis has not been explored. In this study, we have uncovered a novel mechanism for the regulation of CgA-mediated DCG biogenesis. Transfection of CgA into endocrine 6T3 cells lacking CgA and DCGs not only recovered DCG formation and regulated secretion but also prevented granule protein degradation. Genetic profiling of CgA-expressing 6T3 versus CgA- and DCG-deficient 6T3 cells, followed by real-time reverse transcription-polymerase chain reaction and Western blotting analyses, revealed that a serine protease inhibitor, protease nexin-1 (PN-1), was significantly up-regulated in CgA-expressing 6T3 cells. Overexpression of PN-1 in CgA-deficient 6T3 cells prevented degradation of DCG proteins at the Golgi apparatus, enhanced DCG biogenesis, and recovered regulated secretion. Moreover, depletion of PN-1 by antisense RNAs in CgA-expressing 6T3 cells resulted in the specific degradation of DCG proteins. We conclude that CgA increases DCG biogenesis in endocrine cells by up-regulating PN-1 expression to stabilize granule proteins against degradation.

Parotid secretory granules: crossroads of secretory pathways and protein storage

Saliva plays an important role in digestion, host defense, and lubrication. The parotid gland contributes a variety of secretory proteins-including amylase, proline-rich proteins, and parotid secretory protein (PSP)-to these functions. The regulated secretion of salivary proteins ensures the availability of the correct mix of salivary proteins when needed. In addition, the major salivary glands are targets for gene therapy protocols aimed at targeting therapeutic proteins either to the oral cavity or to circulation. To be successful, such protocols must be based on a solid understanding of protein trafficking in salivary gland cells. In this paper, model systems available to study the secretion of salivary proteins are reviewed. Parotid secretory proteins are stored in large dense-core secretory granules that undergo stimulated secretion in response to extracellular stimulation. Secretory proteins that are not stored in large secretory granules are secreted by either the minor regulated secretory pathway, constitutive secretory pathways (apical or basolateral), or the constitutive-like secretory pathway. It is proposed that the maturing secretory granules act as a distribution center for secretory proteins in salivary acinar cells. Protein distribution or sorting is thought to involve their selective retention during secretory granule maturation. Unlike regulated secretory proteins in other cell types, salivary proteins do not exhibit calcium-induced aggregation. Instead, sulfated proteoglycans play a role in the storage of secretory proteins in parotid acinar cells. This work suggests that unique sorting and retention mechanisms are responsible for the distribution of secretory proteins to different secretory pathways from the maturing secretory granules in parotid acinar cells.

Antral G-cell in gastrin and gastrin-cholecystokinin knockout animals

The antral hormone gastrin is the key regulator of gastric acid secretion, mucosal growth and differentiation. Gastrin is synthesized in the endocrine G-cells in the antroduodenal mucosa. We have now examined the way in which the loss of gastrin alone or gastrin plus cholecystokinin (CCK) affects the antral G-cell. Immunohistochemistry, radioimmunoassay and quantitative real-time polymerase chain reaction techniques were employed to examine the expression of genes belonging to the G-cell secretory pathway in gastrin and gastrin-CCK knockout mice. Transmission electron microscopy was used to examine the ultrastructure of the G-cells. The number of G-cells increased but the secretory granules were few and abnormally small in the G-cells of both mouse models compared with wildtypes. Thus, gastrin is not necessary for the formation of G-cells as such but the lack of gastrin reduces the number and size of their secretory granules suggesting that gastrin is vital for the formation and/or maintenance of secretory granules in G-cells.

Chromogranin B-induced secretory granule biogenesis: comparison with the similar role of chromogranin A

The two major proteins of secretory granules of secretory cells, chromogranins A (CGA) and B (CGB), have previously been proposed to play key roles in secretory granule biogenesis. Recently, CGA was reported to play an on/off switch role for secretory granule biogenesis. In the present study we found CGB being more effective than CGA in inducing secretory granule formation in non-neuroendocrine NIH3T3 and COS-7 cells. The mean number of dense core granules formed/cell of CGA-transfected NIH3T3 cells was 2.51, whereas that of CGB-transfected cells was 4.02, indicating the formation of 60% more granules in the CGB-transfected cells. Similarly, there were 55% more dense core granules formed in the CGB-transfected COS-7 cells than in the CGA-transfected cells. Moreover, transfection of CGA- and CGB-short interfering RNA (siRNA) into neuroendocrine PC12 cells not only decreased the amount of CGA and CGB expressed but also reduced the number of secretory granules by 41 and 78%, respectively, further suggesting the importance of CGB expression in secretory granule formation.

Secretory granule biogenesis and chromogranin A: master gene, on/off switch or assembly factor?

Secretory granules are found in specialized cell types, including endocrine cells, suggesting that a coordinated programme of gene expression is involved in their biogenesis. Indeed, it has been proposed that chromogranin A (CgA) acts as an on/off switch for secretory granule biogenesis. However, this proposed function is difficult to reconcile with the large body of evidence suggesting that secretory granules exist in the absence of CgA and that cells can synthesize CgA in the absence of secretory granules. Indeed, recent evidence suggests that, rather than a master gene or universal on/off switch, a series of on/off switches combines to induce expression of subsets of secretory granule-associated genes. The assembly of newly synthesized proteins and the inclusion of existing granule proteins would produce functional secretory granules. CgA and related proteins might act as assembly factors in this process.

Large dense-core secretory granule biogenesis is under the control of chromogranin A in neuroendocrine cells

The large dense-core secretory granule is an organelle in neuroendocrine/endocrine cells, where prohormones and proneuropeptides are stored, processed, and secreted in a regulated manner. Here we present evidence that chromogranin A (CgA), one of the most abundant acidic glycoproteins ubiquitously present in neuroendocrine/endocrine cells, regulates dense-core secretory granule biogenesis. Specific depletion of CgA expression by antisense RNAs in PC12 cells led to a profound loss of secretory granule formation. An exogenously expressed prohormone, pro-opiomelanocortin, was neither stored nor secreted in a regulated manner in CgA-deficient PC12 cells. Overexpression of bovine CgA into CgA-deficient PC12 cells rescued regulated secretion. Other secretory granule proteins, such as chromogranin B (CgB), carboxypeptidase E, and synaptotagmin, were rapidly degraded, whereas nongranule proteins were not affected in CgA-deficient PC12 cells. Unlike CgA, another granin protein CgB could not substitute for the role of CgA in secretory granule biogenesis. Thus, we conclude that CgA is a master "on/off" switch regulating the formation of the dense-core secretory granule in neuroendocrine cells.

Neurosecretion competence. A comprehensive gene expression program identified in PC12 cells

The phenotype of neurosecretory cells is characterized by clear vesicles and dense granules, both discharged by regulated exocytosis. However, these organelles are lacking completely in a few neurosecretion-incompetent clones of the pheochromocytoma PC12 line, in which other specific features are maintained (incompetent clones). In view of the heterogeneity of PC12 cells, a differential characterization of the incompetent phenotype based on the comparison of a single incompetent and a single wild-type clone would have been inconclusive. Therefore, we have compared two pairs of PC12 clones, studying in parallel the transcript levels of 4,200 genes and 19,000 express sequence tags (ESTs) by high density oligonucleotide arrays. After accurate data processing for quality control and filtration, a total of 755 transcripts, corresponding to 448 genes and 307 ESTs, was found consistently changed, with 46% up-regulated and 54% down-regulated in incompetent versus wild-type clones. Many but not all neurosecretion genes were profoundly down-regulated in incompetent cells. Expression of endocytosis genes was normal, whereas that of many nuclear and transcription factors, including some previously shown to play key roles in neurogenesis, was profoundly changed. Additional differences appeared in genes involved in signaling and metabolism. Taken together these results demonstrate for the first time that expression of neurosecretory vesicles and granules is part of a complex gene expression program that includes many other features that so far have not been recognized.

Genes expressed in the mouse pituitary corticotrope AtT-20/D-16v tumor cell line

The pituitary corticotrope AtT-20 stable cell line has been used as a model system to study peptide secretion, glucocorticoid regulation, and several other processes. In order to better understand this model cell line, a phage cDNA library was generated from AtT-20/D-16v cell mRNA and cDNA sequences were obtained for 317 clones representing 203 known genes and 48 novel cDNAs. The sequencing results revealed the prevalence of the mouse leukemia virus in this cell line and also identified a number of putatively secreted molecules that were not previously recognized as being secreted from AtT-20/D-16v cells or pituitary corticotropes. Nine completely novel cDNAs and 39 cDNAs homologous to known ESTs were also identified. A listing of other genes known to be expressed in AtT-20/D-16v cells is also provided.

Cell type-specific metabolism of peptidylglycine alpha-amidating monooxygenase in anterior pituitary

Peptidylglycine alpha-amidating monooxygenase (PAM) is a bifunctional enzyme expressed in each major anterior pituitary cell type. We used primary cultures of adult male rat anterior pituitary to examine PAM expression, processing, and secretion in the different pituitary cell types and to compare these patterns to those observed in transfected AtT-20 corticotrope tumor cells. Immunostaining and subcellular fractionation identified PAM in pituitary secretory granules and additional vesicular compartments; in contrast, in AtT-20 cells, transfected PAM was primarily localized to the trans-Golgi network. PAM expression was highest in gonadotropes, with moderate levels in somatotropes and thyrotropes and lower levels in corticotropes and lactotropes. Under basal conditions, less than 1% of the cell content of monooxygenase activity was secreted per h, a rate comparable to the basal rate of release of individual pituitary hormones. General secretagogues stimulated PAM secretion 3- to 5-fold. Stimulation with specific hypothalamic releasing hormones demonstrated that different pituitary cell types secrete characteristic sets of PAM proteins. Gonadotropes and thyrotropes release primarily monofunctional monooxygenase. Somatotropes secrete primarily bifunctional PAM, whereas corticotropes secrete a mixture of mono- and bifunctional proteins. As observed in transfected AtT-20 cells, pituitary cells rapidly internalize the PAM/PAM-antibody complex from the cell surface. The distinctly different steady-state localizations of endogenous PAM in primary pituitary cells and transfected PAM in AtT-20 cell lines may simply reflect the increased storage capacity of primary pituitary cells.

Conversion and storage of somatostatin are established before response to secretagogue stimuli in P19 neurons

In mature neurons, neuropeptides are synthesized via limited proteolysis of propolypeptides by convertases. The bioactive peptides are then stored in secretory granules until they are released extracellularly upon the induction of a fusion between granules and the plasma membrane, in response to secretagogues. We used the mouse P19 embryonic carcinoma cells as a model to determine if the capacities to convert and store neuropeptides and to secrete them in a regulated fashion are established coordinately during neuronal differentiation. We have previously shown that both undifferentiated P19 cells and their neuronal derivatives express the largely distributed furin, PACE4 and PC5 convertases, whereas only neuronal derivatives express the neuroendocrine convertase PC2. In addition, undifferentiated cells displayed furin- rather than PC2-like converting capacities. The present work demonstrates that day 8 P19 neurons mainly convert prosomatostatin (proSS) to somatostatin-14 (SS-14) using HPLC and radioimmunoassay (RIA) analyses, indicating that P19 cells acquire PC2-like converting capacities as a consequence of neuronal differentiation. SS-14 was predominantly intracellular in neuronal cells which were shown to express several granins, markers of granules, by Western blotting. However, cell membrane depolarization with 50 mM K+, a general secretagogue stimulus, evoked the release of SS-14 by day 12, but not by day 8, P19 neurons. The results thus demonstrate that capacities to convert and store neuropeptides can be established before coupling of stimulus-secretion during neuronal differentiation.

A PC12 variant lacking regulated secretory organelles: aberrant protein targeting and evidence for a factor inhibiting neuroendocrine gene expression

A variant of the PC12 pheochromocytoma cell line (termed A35C) has been isolated that lacks regulated secretory organelles and several constituent proteins. Northern and Southern blot analyses suggested a block at the transcriptional level. The proprotein-converting enzyme carboxypeptidase H was synthesised in the A35C cell line but was secreted by the constitutive pathway. Transient transfection of A35C cells with cDNAs encoding the regulated secretory proteins dopamine beta-hydroxylase and synaptotagmin I resulted in distinct patterns of mistargeting of these proteins. It is surprising that hybrid cells created by fusing normal PC12 cells with A35C cells exhibited the variant phenotype, suggesting that A35C cells express an inhibitory factor that represses neuroendocrine-specific gene expression.

Cellular localization of the prohormone convertases in the hypothalamic paraventricular and supraoptic nuclei: selective regulation of PC1 in corticotrophin-releasing hormone parvocellular neurons mediated by glucocorticoids

The prohormone convertases (PCs) are processing enzymes that activate proproteins via cleavage at specific single or pairs of basic residues. The hypothalamic paraventricular nucleus (PVN) and supraoptic nucleus (SON) are primary sites of biosynthesis of several neuroendocrine hormone precursors, including provasopressin (pro-AVP), pro-oxytocin (pro-OT), and procorticotrophin-releasing hormone (pro-CRH), which require post-translational processing to yield active products. Using in situ hybridization, we observed PC1 and PC5 mRNAs in PVN and SON magnocellular neurons, while PC2 mRNA was observed in both magnocellular and parvocellular PVN neurons as well as magnocellular SON neurons. Similar to furin, PC7 mRNA was expressed throughout the PVN and SON, whereas PACE4 mRNA levels were undetectable. Both immunohistochemical and Western blot studies were performed to demonstrate the presence of PC proteins and forms in the PVN and SON. Using double-labeling in situ hybridization, we examined the cellular colocalization of each PC mRNA with pro-AVP, pro-OT, and pro-CRH mRNAs in PVN and SON. PC1 mRNA was colocalized with both AVP and OT mRNA in PVN and SON magnocellular neurons. All AVP, OT, and CRH neurons expressed PC2. In contrast, PC5 mRNA was colocalized only with OT mRNA. We examined the effects of adrenalectomy (ADX) on PVN PC mRNA levels. PC1 mRNA levels were increased selectively within CRH/AVP parvocellular neurons but were unchanged in PVN magnocellular AVP or OT neurons. These results established the anatomical organization of each convertase and proneuropeptide substrates in the PVN and SON and suggested potential roles for each enzyme under resting and stimulated conditions.

Somatostatin-14, somatostatin-28, and prosomatostatin[1-10] are independently and efficiently processed from prosomatostatin in the constitutive secretory pathway in islet somatostatin tumor cells (1027B2)

We have characterized the biosynthetic origin of somatostatin-14 (SS-14), SS-28, and pro-SS[1-10] from pro-SS (PSS) in 1027B2 rat islet tumor cells. Because these cells lack regulated secretion and show unresponsiveness of the SS gene to cAMP, we have additionally carried out morphological and functional studies to elucidate the molecular defect in cAMP signalling and to localize the sites of PSS maturation along the secretory pathway. Cell extracts and secretion media were analysed by high performance liquid chromatography and specific C- and N-terminal radioimmunoassays. Electron microscopic sampling of 1027B2 cell cultures showed that most cells had very few dense core secretory granules for heterogeneous sizes. The cells expressed the endoproteases furin, PC1, and PC2 and contained large quantities of fully processed SS-14 and SS-28 with very little unprocessed PSS (ratio SS-14:SS-28:PSS = 39:51:10%). They secreted high concentrations of SS-14, SS-28, and PSS[1-10] constitutively along with PC1 and PC2. Pulse-chase studies demonstrated that PSS is rapidly (within 15 min), and efficiently processed to SS-14, SS-28, and PSS[1-10] via separate biosynthetic pathways: PSS --> SS-14 + 8 kDa; PSS --> SS-28 + 7 kDa; PSS --> PSS[1-10]. Monensin reduced intracellular SS-like immunoreactivity without altering processing efficiency. Transfection with the catalytic subunit of protein kinase A (PKA-C) activated SS promoter-CAT activating indicating that the defect in cAMP-dependent signaling in 1027B2 cells lies at the level of PKA-C. PKA-C overexpression failed to alter the ratio of processed SS-14 and SS-28. These results demonstrate that SS-14, SS-28, and PSS[1-10] are independently synthesized from PSS and that efficient precursor processing can occur within the constitutive secretory pathway in the relative absence of dense core secretory vesicles.

Heterologous expression of human vasopressin-neurophysin precursors in a pituitary cell line: defective transport of a mutant protein from patients with familial diabetes insipidus

Familial hypothalamic diabetes insipidus is an autosomal dominant disorder characterized by deficient vasopressin synthesis. Different point mutations in the vasopressin-neurophysin (VP-NP) precursor gene have been found in affected families. In a Dutch kindred, a single G to T transversion in the NP-encoding exon B of one allele converts the highly conserved glycine 17 to a valine residue. In order to examine whether this point mutation affects the processing and transport of the VP-NP precursor, the normal (HV2) and mutant (MT6) vasopressin cDNAs were stably expressed in the mouse pituitary cell line AtT20. The normal precursor was correctly glycosylated and processed, and NP was detected in the culture medium. Secretion of NP was stimulated by 8-bromo-cAMP, indicating that the normal precursor was targeted to the regulated secretory pathway. In contrast, the mutant precursor was synthesized, but processing and secretion were dramatically reduced. The mutant precursor was core-glycosylated but remained endoglycosidase H-sensitive, suggesting that the protein did not reach the trans-Golgi network. These results were supported by immunocytochemical studies. In HV2 cells, NP derived from the precursor was concentrated in the tips of the cell processes where secretory granules accumulate. In MT6 cells, NP staining was restricted to the endoplasmic reticulum (ER) as determined by colocalization with an ER-resident protein, BiP. These results suggest that the mutation within the conserved part of NP alters the conformation of the precursor and thus triggers its retention in the ER.

Overall lack of regulated secretion in a PC12 variant cell clone

A stable clone of PC12 neuroendocrine cells, named 27, known from previous studies to exhibit a defect of regulated secretion (lack of regulated secretory proteins, of synaptophysin, of dense granules and of catecholamine uptake and release; Clementi, E., Racchetti, G., Zacchetti, D., Panzeri, M. C., and Meldolesi, J. (1992) Eur. J. Neurosci. 4, 944-953) was characterized in detail to clarify the nature of its phenotype and the mechanisms of its establishment. The neuroendocrine nature of the PC12-27 phenotype was documented by specific markers: synapsins, neurofilament subunit H, neuronal kinesin, and alpha-latrotoxin receptor. Moreover, various intracellular membrane systems of PC12-27, including the endoplasmic reticulum and the Golgi complex, appeared similar to control PC12 in both morphology and marker expression. In contrast, all the investigated markers located either in dense granules (dopamine-beta-hydroxylase), in synaptic-like microvesicles (the acetylcholine transporter) or in both these regulated secretory organelles (VAMP2/synaptobrevin-2, synaptotagmin) were missing in PC12-27 cells, and the same was true also for the cytosolic and plasmalemma proteins involved in regulated exocytosis (Rab3, SNAP25, syntaxin). Pulse labeling and in vitro translation experiments revealed the defect to consist in a protein synthesis blockade that mRNA studies (reverse transcription-polymerase chain reaction, Northern blotting, and actinomycin D experiments) revealed to take place primarily at the transcriptional level. The secretion defect of PC12-27 cells was modified neither by various types of long term stimulation nor by nerve growth factor treatment. Moreover, when one of the missing regulated secretory proteins, chromogranin B, was expressed by cDNA transfection, it was secreted, however via the constitutive pathway. Our results demonstrate that PC12-27 cells are fully incompetent for both branches of regulated secretion, those of dense granules and synaptic-like microvesicles, possibly because of the impairment of a general expression control system that appears to operate independently of neuroendocrine cell differentiation.

Chromogranin A processing and secretion: specific role of endogenous and exogenous prohormone convertases in the regulated secretory pathway

Chromogranins A and B and secretogranin II are a family of acidic proteins found in neuroendocrine secretory vesicles; these proteins contain multiple potential cleavage sites for proteolytic processing by the mammalian subtilisin-like serine endoproteases PC1 and PC2 (prohormone convertases 1 and 2), and furin. We explored the role of these endoproteases in chromogranin processing in AtT-20 mouse pituitary corticotropes. Expression of inducible antisense PC1 mRNA virtually abolished PC1 immunoreactivity on immunoblots. Chromogranin A immunoblots revealed chromogranin A processing, from both the NH2 and COOH termini, in both wild-type AtT-20 and AtT-20 antisense PC1 cells. After antisense PC1 induction, an approximately 66-kD chromogranin A NH2-terminal fragment as well as the parent chromogranin A molecule accumulated, while an approximately 50 kD NH2-terminal and an approximately 30 kD COOH-terminal fragment declined in abundance. Chromogranin B and secretogranin II immunoblots showed no change after PC1 reduction. [35S]Methionine/cysteine pulse-chase metabolic labeling in AtT-20 antisense PC1 and antisense furin cells revealed reciprocal changes in secreted chromogranin A COOH-terminal fragments (increased approximately 82 kD and decreased approximately 74 kD forms, as compared with wild-type AtT-20 cells) indicating decreased cleavage, while AtT-20 cells overexpressing PC2 showed increased processing to and secretion of approximately 71 and approximately 27 kD NH2-terminal chromogranin A fragments. Antisense PC1 specifically abolished regulated secretion of both chromogranin A and beta-endorphin in response to the usual secretagogue, corticotropin-releasing hormone. Moreover, immunocytochemistry demonstrated a relative decrease of chromogranin A in processes (where regulated secretory vesicles accumulate) of AtT-20 cells overexpressing either PC1 or PC2. These results demonstrate that chromogranin A is a substrate for the endogenous endoproteases PC1 and furin in vivo, and that such processing influences its trafficking into the regulated secretory pathway; furthermore, lack of change in chromogranin B and secretogranin II cleavage after diminution of PCl suggests that the action of PC1 on chromogranin A may be specific within the chromogranin/secretogranin protein family.