Differential regulation of chromogranin B/secretogranin I and secretogranin II by forskolin in PC12 cells

The Influence of Burst-Firing EMF on Forskolin-Induced Pheochromocytoma (PC12) Plasma Membrane Extensions

Previous research has demonstrated that pheochromocytoma (PC12) cells treated with forskolin provides a model for the in vitro examination of neuritogenesis. Exposure to electromagnetic fields (EMFs), especially those which have been designed to mimic biological function, can influence the functions of various biological systems. We aimed to assess whether exposure of PC12 cells treated with forskolin to patterned EMF would produce more plasma membrane extensions (PME) as compared to PC12 cells treated with forskolin alone (i.e., no EMF exposure). In addition, we aimed to determine whether the differences observed between the proportion of PME of PC12 cells treated with forskolin and exposed to EMF were specific to the intensity, pattern, or timing of the applied EMF. Our results showed an overall increase in PME for PC12 cells treated with forskolin and exposed to Burst-firing EMF as compared to PC12 cells receiving forskolin alone. No other patterned EMF investigated were deemed to be effective. Furthermore, intensity and timing of the Burst-firing pattern did not significantly alter the proportion of PME of PC12 cells treated with forskolin and exposed to patterned EMF.

Marfan syndrome

Marfan syndrome (MFS) is an autosomal dominant, age-related but highly penetrant condition with substantial intrafamilial and interfamilial variability. MFS is caused by pathogenetic variants in FBN1, which encodes fibrillin-1, a major structural component of the extracellular matrix that provides support to connective tissues, particularly in arteries, the pericondrium and structures in the eye. Up to 25% of individuals with MFS have de novo variants. The most prominent manifestations of MFS are asymptomatic aortic root aneurysms, aortic dissections, dislocation of the ocular lens (ectopia lentis) and skeletal abnormalities that are characterized by overgrowth of the long bones. MFS is diagnosed based on the Ghent II nosology; genetic testing confirming the presence of a FBN1 pathogenetic variant is not always required for diagnosis but can help distinguish MFS from other heritable thoracic aortic disease syndromes that can present with skeletal features similar to those in MFS. Untreated aortic root aneurysms can progress to life-threatening acute aortic dissections. Management of MFS requires medical therapy to slow the rate of growth of aneurysms and decrease the risk of dissection. Routine surveillance with imaging techniques such as transthoracic echocardiography, CT or MRI is necessary to monitor aneurysm growth and determine when to perform prophylactic repair surgery to prevent an acute aortic dissection.

Regulation and distribution of squirrel monkey chorionic gonadotropin and secretogranin II in the pituitary

Secretogranin II (SgII) is a member of the granin family of proteins found in neuroendocrine and endocrine cells. The expression and storage of SgII in the pituitary gland of Old World primates and rodents have been linked with those of luteinizing hormone (LH). However, New World primates including squirrel monkeys do not express LH in the pituitary gland, but rather CG is expressed. If CG takes on the luteotropic role of LH in New World primates, SgII may be associated with the expression and storage of CG in the pituitary gland. The goal of this study was to evaluate the regulation and distribution of CG and SgII in the squirrel monkey. A DNA fragment containing approximately 750 bp of squirrel monkey SgII promoter was isolated from genomic DNA and found to contain a cyclic-AMP response element that is also present in the human SgII promoter and important for GnRH responsiveness. The squirrel monkey and human SgII promoters were similarly activated by GnRH in luciferase reporter gene assays in LβT2 cells. Double immunofluorescence microscopy demonstrated close association of SgII and CG in gonadotrophs of squirrel monkey pituitary gland. These results suggest that CG and SgII have a similar intercellular distribution and are coregulated in squirrel monkey pituitary gland.

Aspects of the neuroendocrine cerebellum: expression of secretogranin II, chromogranin A and chromogranin B in mouse cerebellar unipolar brush cells

Morphologically distinct neuron classes can be subdivided in sublineages by differential chemical phenotypes that correlate with functional diversity. Here we show by immunocytochemistry that chromogranin A (CgA) chromogranin B (CgB) and secretogranin II (SgII), the principal granins situated in neuronal secretory granules and large dense-core vesicles, are widely but differentially expressed in cells of the mouse cerebellum and terminals of cerebellar afferents. While CgA and CgB were nearly panneuronal, SgII was more restricted in distribution. The cells most intensely immunoreactive for SgII were a class of small, excitatory interneurons enriched in the granular layer of the vestibulocerebellum, the unipolar brush cells (UBCs), although larger neurons likely to be a subset of the Golgi-Lugaro-globular cell population were also distinctly immunopositive; by contrast, Purkinje cells and granule cells were, at best, faintly stained and, stellate, basket cells were unstained. SgII was also present in subsets of mossy fibers, climbing fibers and varicose fibers. Neurons in the cerebellar nuclei and inferior olive were distinctly positive for the three granins. Double-labeling with subset-specific cell class markers indicated that, while both CgA and CgB were present in most UBCs, SgII immunoreactivity was present in the calretinin (CR)-expressing subset, but lacked in metabotropic glutamate receptor 1alpha (mGluR1alpha)-expressing UBCs. Thus, we have identified an additional cell class marker, SgII, which serves to study subtype properties in the UBC population. The abundance of SgII in only one of the two known subsets of UBCs is remarkable, as its expression in other neurons of the cortex was moderate or altogether lacking. The data suggest that the CR-positive UBCs represent a unique neuroendocrine component of the mammalian cerebellar cortex, presumably endowed with transynaptically regulated autocrine or paracrine action/s. Because of the well-known organization of the cerebellar system, several of its neuron classes may represent valuable cellular models to analyze granin functions in situ, in acute slices and in dissociated cell and organotypic slice cultures.

The vesicular monoamine transporter 2 contains trafficking signals in both its N-glycosylation and C-terminal domains

The vesicular acetylcholine transporter (VAChT) and the vesicular monoamine transporter (VMAT) belong to the same transporter family that packages acetylcholine into synaptic vesicles (SVs) and biogenic amines into large dense core vesicles (LDCVs) and/or SVs, respectively. These transporters share similarities in sequence and structure with their N- and C-terminal domains located in the cytoplasm. When expressed in PC12 cells, VMAT2 localizes to LDCV, whereas VAChT is found mainly on synaptic-like microvesicles. Previous studies have shown that the cytoplasmic C-terminal domain of VAChT contains signals targeting this transporter to SVs. However, the targeting signals for VMAT have not been completely elucidated. To identify signals targeting VMAT2 to LDCV, the subcellular localization of VMAT2-VAChT chimeras was analyzed in PC12 cells. Chimeras having either the N-terminal region through transmembrane domain 2 of VMAT2 or the C-terminal domain of VMAT2 do not traffic to LDCV efficiently. In contrast, chimeras having both of these regions, or the luminal glycosylated loop in conjunction with transmembrane domains 1 and 2 and the C-terminal domain of VMAT2, traffic to LDCV. Treatment of PC12 cells with 1-deoxymannojirimycin, a specific alpha-mannosidase I inhibitor, causes VMAT2 to localize to synaptic-like microvesicles. The results indicate that both mature N-linked glycosylation and the C-terminus are important for proper trafficking of VMAT2 and that the locations of trafficking signals in VMAT2 and VAChT are surprisingly different.

Protein kinase A affects trafficking of the vesicular monoamine transporters in PC12 cells

Previous studies have shown that the vesicular monoamine transporter 2 (VMAT2) is localized to both large dense core vesicles and synaptic vesicles in vivo. However, when exogenously expressed in PC12 cells, VMAT2 localizes only to large dense core vesicles. This distribution is similar to that of the endogenous vesicular monoamine transporter 1 (VMAT1) in PC12 cells. When VMAT2 was expressed in a protein kinase A (PKA)-deficient PC12 cell line it localized to synaptic-like microvesicles. Expression of recombinant VMAT1 in the same cell line showed a heterogeneous distribution to both large dense core vesicles and synaptic-like microvesicles. Coexpression of the PKA catalytic subunit partially restored trafficking of both VMAT2 and VMAT1 to large dense core vesicles; treatment of wild-type PC12 cells with the PKA inhibitor H89 increased VMAT2 on synaptic-like microvesicles. The VMAT1 and VMAT2 in large dense core vesicles exhibit a larger molecular size than those located on synaptic-like microvesicles. This difference is due to differential N-linked glycosylation. In vitro phosphorylation experiments show that PKA does not phosphorylate VMAT2. A chimera containing the VMAT2 cytoplasmic C-terminus fused to vesicular acetylcholine transporter (VAChT) shows mislocalization to synaptic-like microvesicles and VAChT-like glycosylation in the PKA-deficient cell line. However, coexpression with PKA changes the chimera's trafficking to large dense core vesicles and increases the molecular size. These results suggest that protein kinase A affects the formation and/or composition of VMAT trafficking complexes.

Neuroendocrine cell type-specific and inducible expression of chromogranin/secretogranin genes: crucial promoter motifs

The chromogranin/secretogranins (Cg/Sg) are a family of soluble, acidic proteins representing major constituents in secretory vesicle cores of virtually all neuroendocrine tissues. We and others have identified the cyclic adenosine monophosphate response element (CRE) as the crucial promoter element responsible for neuroendocrine cell type-specific expression of the Cg/Sg genes. In addition to CRE, GC-rich domains in chromogranin B (CgB) and serum response element (SRE) in secretogranin II (SgII) promoters appear to play important roles in neuroendocrine cell type-specific expression of CgB and SgII genes. Nicotinic-cholinergic and peptidergic chromaffin cell stimuli evoke catecholamine secretion and augment biosynthesis of Cg/Sg genes. These stimuli signal to CgA gene transcription through the CRE in cis and through protein kinase A, protein kinase C, and mitogen-activated protein kinase and CRE-binding protein in trans. In addition to CRE, a GC-rich domain in CgB and SRE in SgII promoters also play important roles in mediating inducible expression of the CgB and SgII genes. We conclude that CRE, GC-rich domains, and SRE are crucial determinants of both cell type-specific and secretagogue-inducible expression of the Cg/Sg genes.

Genes preferentially induced by depolarization after concussive brain injury: effects of age and injury severity

Fluid percussion (FP) brain injury leads to immediate indiscriminate depolarization and massive potassium efflux from neurons. Using Northern blotting, we examined the post-FP expression of primary response/immediate early genes previously described as induced by depolarization in brain. RNA from ipsilateral and contralateral hippocampus was harvested from immature and adult rats 1 h following mild, moderate, or severe lateral fluid percussion injury and compared against age-matched sham animals. C-fos gene expression was used as a positive control and showed marked induction in both pups (6-25-fold with increasing severity) and adults (9.7-17.1-fold). Kinase-induced-by-depolarization-1 (KID-1) and salt-inducible kinase (SIK) gene expression was increased in adult (KID-1 1.5-1.6-fold; SIK 1.3-3.9-fold) but not developing rats. NGFI-b RNA was elevated after injury in both ages (pups 1.8-6.1-fold; adults 3.5-5-fold), in a pattern similar to that seen for c-fos. Secretogranin I (sec I) demonstrated no significant changes. Synaptotagmin IV (syt IV) was induced only following severe injury in the immature rats (1.4-fold). Our results reveal specific severity- and age-dependent patterns of hippocampal immediate early gene expression for these depolarization-induced genes following traumatic brain injury. Differential expression of these genes may be an important determinant of the distinct molecular responses of the brain to varying severities of trauma experienced at different ages.

Proteomic analysis of proteins in PC12 cells before and after treatment with nerve growth factor: increased levels of a 43-kDa chromogranin B-derived fragment during neuronal differentiation

Proteomic analysis is an important approach to characterizing the proteome and studying protein function in the post-genomic era. It is also a powerful screening method for detecting unexpected alterations in protein expression that may be missed by conventional biochemical techniques. The aim of this study was to perform a preliminary proteomic analysis of PC12 cells in order to investigate the effect of nerve growth factor (NGF) on protein expression in PC12 cells during neurite outgrowth. PC12 cell proteins were separated by two-dimensional electrophoresis (2DE) and visualized by silver staining, then certain proteins were identified by N-terminal amino acid microsequencing and a homology search of a protein sequence database. Over 400 proteins were detected, 10% of which showed a significant (greater than 30%) increase or decrease in expression during NGF-induced neuronal differentiation. Seven proteins in the 2DE map were identified; the levels of five of these were unaffected by NGF treatment, whereas the levels of the other two, beta-tubulin and a novel 43-kDa chromogranin B-derived fragment, were significantly increased by more than 30 and 200%, respectively. Our results suggest that chromogranin B processing is enhanced in PC12 cells during NGF-induced neuronal differentiation. In addition, since this increase in the levels of the chromogranin B-derived fragment was specifically blocked by PD98059, we suggest that the increased processing can be ascribed to activation of the MAP kinase pathway, and that the 43-kDa chromogranin B-derived fragment can serve as a new marker of neuronal differentiation for proteomic studies.

Neuroendocrine cell type-specific and inducible expression of the chromogranin B gene: crucial role of the proximal promoter

Chromogranin B, a soluble acidic secretory protein, is widely distributed in neuroendocrine and neuronal cells, although not in other cell types. To identify the elements governing such widespread, yet selective, expression of the gene, we characterized the isolated mouse chromogranin B promoter. 5'-Promoter deletions localized neuroendocrine cell type-specific expression to the proximal chromogranin B promoter (from -216 to -91 bp); this region contains an E box (at [-206 bp]CACCTG[-201 bp]), four G/C-rich regions (at [-196 bp]CCCCGC[-191 bp], [-134 bp]CCGCCCGC[-127 bp], [-125 bp]GGCGCCGCC[-117 bp], and [-115 bp]CGGGGC[-110 bp]), and a cAMP response element (CRE; at [-102 bp]TGACGTCA[-95 bp]). A 60-bp core promoter region, defined by an internal deletion from - 134 to -74 bp upstream of the cap site and spanning the CRE and three G/C-rich regions, directed tissue-specific expression of the gene. The CRE motif directed cell type-specific expression of the chromogranin B gene in neurons, whereas three of the G/C-rich regions played a crucial role in neuroendocrine cells. Both the endogenous chromogranin B gene and the transfected chromogranin B promoter were induced by preganglionic secretory stimuli (pituitary adenylyl cyclase-activating polypeptide, vasoactive intestinal peptide, or a nicotinic cholinergic agonist), establishing stimulus-transcription coupling for this promoter. The adenylyl cyclase activator forskolin, nerve growth factor, and retinoic acid also activated the chromogranin B gene. Secretagogue-inducible expression of chromogranin B also mapped onto the proximal promoter; inducible expression was entirely lost upon internal deletion of the 60-bp core (from 134 to -74 bp). We conclude that CRE and G/C-rich domains are crucial determinants of both cell type-specific and secretagogue-inducible expression of the chromogranin B gene.

Regulation of the biosynthesis and processing of chromogranins in organotypic slices: influence of depolarization, forskolin and differentiating factors

Slices from rat hippocampus in organotypic culture were used to study the biosynthesis regulation of chromogranins A and B and secretogranin II. Additionally, we investigated the proteolytic conversion of secretogranin II and the levels of prohormone convertases putatively involved. Forskolin treatment and depolarization with potassium plus BayK 8644 led to significant increases in secretogranin II mRNA in the principal cells of the hippocampus. Enhanced expression of secretogranin II was also reflected by a rise in peptide levels. Despite this induction of biosynthesis the extensive processing to secretoneurin normally observed in brain was maintained. Both forskolin and depolarization upregulated the prohormone convertase (PC)1, but not PC2, indicating that PC1 levels are critical for secretoneurin production under stimulating conditions. Results obtained for chromogranins A and B were less consistent. For chromogranin A mRNA, changes were restricted to granule cells; for chromogranin B, a response in granule cells was observed to depolarization but not to forskolin, and effects in pyramidal neurons were weak. Accordingly, we were unable to detect alterations in chromogranin A and B protein levels. Furthermore, we tested several neurotrophic growth factors and found that only basic fibroblast growth factor raised secretogranin II expression without affecting chromogranins A and B. The hippocampal slice preparation allowed well controlled treatment with identification of neuronal subpopulations and yielded data largely matching experiments in vivo and in cell culture. The pronounced regulation of secretogranin II and its effective processing underlines the importance of the resulting peptide secretoneurin as an active neuropeptide in the nervous system.

rTLE3, a newly identified transducin-like enhancer of split, is induced by depolarization in brain

The transducin-like enhancers of split are a family of mammalian proteins that share sequence homology with the Drosophila protein Groucho. Using representational difference analysis, we isolated the cDNA for a previously unidentified gene, rTLE3 (rat transducin-like enhancer of split 3), as a sequence induced by depolarization and forskolin, but not by neurotrophins or growth factors, in PC12 pheochromocytoma cells. rTLE3 encodes the protein rTLE3, a 764-amino acid orthologue of mouse and human TLE3. R-esp2, the gene encoding the closest related rat protein, is not induced by any of the four treatments in PC12 cells. rTLE3 and R-esp2 have different patterns of expression in the adult rat CNS and other tissues. After systemic administration of kainic acid, rTLE3 is induced specifically in the dentate gyrus of the hippocampus. We propose that members of the transducin-like enhancer of split family of proteins may have distinct functions in the mature CNS, in addition to their functions during development.

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.

Isolation and characterization of the human secretogranin II gene promoter

The goal of this study was to isolate and functionally characterize the human secretogranin II (SgII) gene promoter. SgII is a member of the granin family of proteins which are selectively expressed in neurosecretory cells. The human SgII promoter contains a consensus TATA box and cyclic AMP response element (CRE) 35 and 74 bp upstream of the transcription start site, respectively, elements also found in the mouse and rat SgII gene promoters. Transfection studies showed that 869 bp of the human SgII promoter were sufficient to confer cell type-specific expression of an SgII promoter-luciferase reporter gene in neurosecretory PC-12, GH and BE(2)-M17 cells. The activity of the human SgII promoter was also compared in three N-type, human neuroblastoma cell lines [BE(2)-M17, SMS-KAN and SH-SY5Y], which differ markedly in the level of SgII expression. SgII promoter activities in the neuroblastoma cell lines correlated not only with the levels of SgII but also the levels of the cyclic AMP response element-binding protein CREB which were highest in BE(2)-M17 cells and lowest in SH-SY5Y cells. To establish that the activity of the human SgII promoter in these neuroblastoma cell lines is dependent on the level of CREB, rat CREB was overexpressed in SH-SY5Y cells. SgII promoter activity was up to 8-fold higher in SH-SY5Y cells overexpressing CREB. These results suggest that SgII expression is a marker for neuronal differentiation in human neuroblastoma cell lines and is dependent on the level of CREB expression.

KID-1, a protein kinase induced by depolarization in brain

Membrane depolarization leads to changes in gene expression that modulate neuronal plasticity. Using representational difference analysis, we have identified a previously undiscovered cDNA, KID-1 (kinase induced by depolarization), that is induced by membrane depolarization or forskolin, but not by neurotrophins or growth factors, in PC12 pheochromocytoma cells. KID-1 is an immediate early gene that shares a high degree of sequence similarity with the family of PIM-1 serine/threonine protein kinases. Recombinant KID-1 fusion protein is able to catalyze both histone phosphorylation and autophosphorylation. KID-1 mRNA is present in a number of unstimulated tissues, including brain. In response to kainic acid and electroconvulsive shock-induced seizures, KID-1 is induced in specific regions of the hippocampus and cortex.

Exocytosis in chromaffin cells of the adrenal medulla

The chromaffin cell has been used as a model to characterize releasable components present in secretory granules and to understand the cellular mechanisms involved in catecholamine release. Recent physiological and biochemical developments have revealed that molecular mechanisms implicated in granule trafficking are conserved in all eukaryotic species: a rise in intracellular calcium triggers regulated exocytosis, and highly conserved proteins are essential elements which interact with each other to form a molecular scaffolding, ensuring the docking of granules at the plasma membrane, and perhaps membrane fusion. However, the mechanisms regulating secretion are multiple and cell specific. They operate at different steps along the life of a granule, from the time of granule biosynthesis up to the last step of exocytosis. With regard to cell specificity, noradrenaline and adrenaline chromaffin cells display different receptor and signaling characteristics that may be important to exocytosis. Characterization of regulated exocytosis in chromaffin cells provides not only fundamental knowledge of neurosecretion but is of additional importance as these cells are used for therapeutic purposes.

Effects of the N-terminal cysteine mutation on prolactin expression and secretion in transfected cells

We developed an experimental cell model to look for motif(s) of rat PRL sequence encoding a sorting signal to secretory granules. An efficient expression vector (pCMV-rPRL) was used to transfect several eukaryotic cell lines in culture, i.e., one neuronal cell line (C6) and three glandular pituitary derived cell lines (AtT20, GC, GH3CDL). Despite the ubiquitous transcription of pCMV-rPRL, the synthesis and secretion of rPRL were detected primarily in GH3CDL cells that derived from lactotrophs, suggesting a cell-specific post-transcriptional control of rPRL expression. During transient expression, exogenous native PRL was transported through intracellular compartments of the secretory pathway and underwent regulated release. Abolition by mutagenesis (C4S) of the N-terminal disulfide bond increased by 50% the PRL secretion rate (medium to cell ratio) and multiplied by 5 the specific activity of medium PRL from pulse-labeled cells. These results support the hypothesis that N-terminal disulfide bond plays a role in the control of PRL intracellular transit and storage.

Identification of a functional cAMP response element in the secretogranin II gene

Secretogranin II is an acidic secretory protein with a widespread distribution in secretory granules of neuronal and endocrine cells. The secretogranin II gene contains, like other members of the granin family, a cAMP response element (CRE) in its upstream region. To investigate the functional significance of this motif, intracellular cAMP levels were increased in a neuronal cell line derived from the septal region of the brain and the level of secretogranin II gene expression was analysed. It was found that increased cAMP levels did, in fact, induce secretogranin II gene expression. To analyse the cis-acting sequence responsible for this induction, a hybrid gene containing the upstream region of the mouse secretogranin II gene fused to beta-globin as a reporter was constructed. Transfection analysis revealed that cAMP-induced transcription of the secretogranin II promoter/beta-globin gene in septal and insulinoma cells. DNA-protein binding assays showed that recombinant CRE-binding protein (CREB), produced in bacteria or human cells, bound in a sequence-specific manner to the secretogranin II promoter CRE. Moreover, deletion mutagenesis revealed that the CRE motif is a bifunctional genetic regulatory element in that it mediates basal as well as cAMP-stimulated transcription. Interestingly, cAMP had no effect upon secretogranin II gene transcription in PC12 and neuroblastoma cells. An increase in the intracellular cAMP concentration activated a GAL4-CREB fusion protein upon transcription in neuroblastoma cells indicating the integrity of the cAMP signaling pathway to the nucleus. Basal as well as cAMP-stimulated transcription, directed from the secretogranin II promoter was, however, impaired in insulinoma cells by overexpression of CREB-2, a negative-acting CRE-binding protein. These results indicate that competitive effects are likely to occur between CRE-bound transcriptional activators and repressors. We conclude that cAMP-stimulated induction of secretogranin II gene transcription is mediated by the CRE motif in a cell-type-specific manner, and is likely to depend on the balance between positive and negative CRE-binding proteins in a particular cell type.

Regulation of secretogranin II mRNA in rat neuronal cultures

The regulation of SgII mRNA expression was investigated in primary cultures of neurons prepared from the hypothalamus and brainstem of 1-day-old rats. The administration of forskolin (FSK) resulted in a time- and dose-dependent increase in SgII mRNA expression, a 9-fold effect within 6 h being achieved with 10 microM FSK, which maximally increased cellular cAMP levels. SgII mRNA levels remained elevated for 24 h. Activation of protein kinase C with 100 nM phorbol 12-myristate 13-acetate (PMA) also increased SgII mRNA expression, although induction with PMA was slower and more moderate (3.8-fold above control after 24 h). Neither 10 microM 1,9-dideoxyforskolin nor 100 nM 4 alpha-phorbol 12,13-didecanoate, inactive analogues of FSK and PMA respectively, had an effect on SgII mRNA. Depolarization of neuronal cultures with 50 mM KCl had a small and variable effect on SgII mRNA levels (1.8-fold above control) in neuronal cultures and did not influence induction with FSK. To investigate whether neuron-like regulation of SgII mRNA expression could be reproduced in PC12 cells, PC12 cells were treated with 100 nM nerve growth factor (NGF) for 7 days prior to challenge with FSK or PMA. Whereas NGF alone modestly increased SgII mRNA expression in PC12 cells (1.8-fold above control), it did not uncover a stimulatory effect of FSK or PMA. These studies indicate that SgII mRNA expression is enhanced by an increase in cellular cAMP and activation of protein kinase C in primary cultures of neurons and emphasize that SgII mRNA is regulated in a cell-specific manner.

A functional cyclic AMP response element plays a crucial role in neuroendocrine cell type-specific expression of the secretory granule protein chromogranin A

Chromogranin A, a soluble acidic protein, is a ubiquitous component of secretory vesicles throughout the neuroendocrine system. We reported previously the cloning and initial characterization of the mouse chromogranin A gene promoter, which showed that the promoter contains both positive and negative domains and that a proximal promoter spanning nucleotides -147 to +42 bp relative to the transcriptional start site is sufficient for neuroendocrine cell type-specific expression. The current study was undertaken to identify the particular elements within this proximal promoter that control tissue-specific expression. We found that deletion or point mutations in the potential cAMP response element (CRE) site at -68 bp virtually abolished promoter activity specifically in neuroendocrine (PC12 chromaffin or AtT20 corticotrope) cells, with little effect on activity in control (NIH3T3 fibroblast) cells; thus, the CRE box is necessary for neuroendocrine cell type-specific activity of the chromogranin A promoter. Furthermore, the effect of the CRE site is enhanced in the context of intact (wild-type) promoter sequences between -147 and -100 bp. DNase I footprint analysis showed that these regions (including the CRE box) bind nuclear proteins present in both neuroendocrine (AtT20) and control (NIH3T3) cells. In AtT20 cells, electrophoretic mobility shift assays and factor-specific antibody supershifts showed that an oligonucleotide containing the chromogranin A CRE site formed a single, homogeneous protein-DNA complex containing the CRE-binding protein CREB. However, in control NIH3T3 cells we found evidence for an additional immunologically unrelated protein in this complex. A single copy of this oligonucleotide was able to confer neuroendocrine-specific expression to a heterologous (thymidine kinase) promoter, albeit with less fold selectivity than the full proximal chromogranin A promoter. Hence, the CRE site was partially sufficient to explain the neuroendocrine cell type specificity of the promoter. The functional activity of the CRE site was confirmed through studies of the endogenous chromogranin A gene. Northern mRNA analysis showed that expression of the endogenous chromogranin A gene was stimulated seven- to eightfold by cAMP in PC12 cells, whereas no induction occurred in the NIH3T3 cells. Similar cAMP induction was obtained with the transfected chromogranin A promoter in PC12 cells, and abolition of the CRE site (by deletion or point mutation) eliminated the induction. Thus, the CRE site in the chromogranin A proximal promoter is functional and plays a crucial, indeed indispensable, role in neuroendocrine-specific expression of the gene. These results also provide insight into transcriptional mechanisms governing acquisition of the neuroendocrine secretory phenotype.

Biosynthesis of large dense-core vesicles in PC12 cells: effects of depolarization and second messengers on the mRNA levels of their constituents

mRNA levels of various constituents of large dense-core vesicles were determined in PC12 cells during depolarization and/or in the presence of BayK 8644, forskolin or phorbolester. For the soluble (secretory) proteins of the vesicles the mRNAs of chromogranin A and B, secretogranin II, neuropeptide Y and VGF were analyzed. Depolarization in the presence of BayK induced a strong up-regulation of the messages for chromogranin B, neuropeptide Y and VGF. Addition of forskolin enhanced this response for neuropeptide Y and VGF, phorbolester did the same only for VGF. Partly membrane-bound and membrane-spanning components analyzed were carboxypeptidase H, dopamine beta-hydroxylase and glycoprotein III (clusterin), peptidylglycine alpha-amidating mono-oxygenase and cytochrome b-561, respectively. Changes of mRNAs for these components were in general smaller and delayed. Six days of depolarization caused an up-regulation of glycoprotein III, peptidylglycine alpha-amidating mono-oxygenase and carboxypeptidase H mRNA levels which were not further increased by cyclic AMP and phorbolester. The dopamine beta-hydroxylase message increased after 6 days of depolarization, however, addition of phorbolester reduced this effect. For cytochrome b-561 there was no change after any of the conditions employed. These in vitro results are compared with those obtained for the biosynthesis regulation of large dense-core vesicles under in vivo conditions. It is suggested that in vivo acetylcholine and vasoactive intestinal polypeptide released from splanchnic nerve induce a differential change in the biosynthesis of large dense-core vesicles by acting via calcium and protein kinase A and C.

Secretogranin II: molecular properties, regulation of biosynthesis and processing to the neuropeptide secretoneurin

Secretogranin II is an acidic secretory protein in large dense core vesicles of endocrine, neuroendocrine and neuronal tissues. It comprises, together with chromogranins A and B, the class of proteins collectively called chromogranins. In this review the physico-chemical properties, genomic organization, tissue distribution, synthesis regulation, ontogeny and physiological function of this protein are discussed. Secretogranin II gained interest recently for mainly three reasons: (1) secretogranin II is an excellent marker for the regulated secretory pathway due to its simple and specific metabolic labeling by inorganic sulfate; (2) secretogranin II occurs in a variety of neoplasms arising from endocrine and neuroendocrine cells and was shown to be a useful histological tumor marker for these cells; (3) secretogranin II is the precursor of the recently discovered neuropeptide secretoneurin which induces dopamine release in the striatum of the rat brain.

Differential regulation of chromogranin B and synapsin I gene promoter activity by cAMP and cAMP-dependent protein kinase

cAMP has neutrotrophic effects in the nervous system. We have investigated whether there is a correlation between cAMP-induced neurite outgrowth and induction of chromogranin B and synapsin I gene expression. These genes encode marker proteins of distinct populations of vesicles in neurons, neuroendocrine and endocrine cells, and in addition, they contain a cAMP response element (CRE) in their upstream regions, making it likely that cAMP-induced neuronal differentiation might be accompanied by increased transcription of these genes. We increased intracellular cAMP levels in neuronal and neuroendocrine cells and analyzed the levels of chromogranin B and synapsin I mRNA. Our data revealed that, while chromogranin B mRNA was in fact induced following cAMP stimulation, synapsin I mRNA was not affected. To analyze the cis-acting sequences, we constructed hybrid genes containing the upstream region of the mouse chromogranin B gene fused to a reporter gene. Similar plasmids containing the synapsin I or the glucagon promoter were constructed. Transfections of neuronal and endocrine cells, together with deletion mutagenesis, revealed that the CRE of the chromogranin B gene mediated the effect of cAMP upon transcription. This effect was mimicked by overexpression of the catalytic subunit of the cAMP-dependent protein kinase. In addition, overexpression of the negative-acting CRE-binding protein CREB-2 revealed that the chromogranin B CRE functions as a bifunctional genetic regulatory element in that it mediates basal as well as cAMP-stimulated transcription. Synapsin I gene expression, however, was not induced by either elevated intracellular cAMP concentration or by overexpression of protein kinase A, although a similar pattern of proteins, including CREB, bound to the synapsin I and chromogranin B CRE in vitro. Thus while the CRE element in the chromogranin B gene promoter is responsive to cAMP, the same element, when present in the synapsin I promoter, does not confer cAMP inducibility.

Induction of secretogranin II mRNA by protein synthesis inhibitors in GH4C1 cells

The effect of cycloheximide (CHX) on the expression of secretogranin II (SgII), a member of the granin family of secretory proteins, was investigated in rat pituitary GH4C1 (GH) cells. The administration of CHX resulted in a dose- and time-dependent increase in SgII mRNA expression, the greatest effect (3.8-fold above control) being achieved with 1 microgram/ml CHX, which resulted in > 90% inhibition of protein synthesis. Emetine (1 microgram/ml), pactamycin (0.6 microgram/ml), anisomycin (2.5 micrograms/ml), and puromycin (100 micrograms/ml), protein synthesis inhibitors structurally and mechanistically unrelated to CHX, also increased the level of SgII mRNA expression. Treatment with forskolin (10 microM) alone had no effect on SgII mRNA levels but potentiated the effect of CHX. Neither 100 nM phorbol 12-myristate 13-acetate nor 45 mM KCl affected SgII mRNA levels in the absence or presence of 1 microgram/ml CHX. The effect of CHX was blocked by the transcription inhibitors actinomycin D (5 micrograms/ml) and 5,6-dichlorobenzimidazole riboside (20 micrograms/ml) but not by the coadministration of the polysome destabilizer pactamycin (0.6 microgram/ml), suggesting that the effect of CHX was transcriptional. These studies show that the expression of SgII mRNA is induced by protein synthesis inhibitors in GH cells, suggesting the presence of a labile repressor, which not only controls the basal expression of the SgII gene but also completely inhibits the stimulatory effect of forskolin.

Cell type-specific gene expression in the neuroendocrine system. A neuroendocrine-specific regulatory element in the promoter of chromogranin A, a ubiquitous secretory granule core protein

The acidic secretory protein chromogranin A universally occurs in amine and peptide hormone and neurotransmitter storage granules throughout the neuroendocrine system. What factors govern the activity of the chromogranin A gene, to yield such a widespread yet neuroendocrine-selective pattern of expression? To address this question, we isolated the mouse chromogranin A gene promoter. The promoter conferred cell type-specific expression in several neuroendocrine cell types (adrenal medullary chromaffin cells, anterior pituitary corticotropes, and anterior pituitary somatolactotropes) but not in control (fibroblast or kidney) cells. In neuroendocrine cells, analysis of promoter deletions established both positive and negative transcriptional regulatory domains. A distal positive domain (-4.8/-2.2 kbp) was discovered, as well as negative (-258/-181 bp) and positive (-147/-61 bp) domains in the proximate promoter. The proximate promoter contained a minimal neuroendocrine-specific element between -77 and -61 bp. Sequence alignment of the mouse promoter with corresponding regions in rat and bovine clones indicated that the mouse sequence shares over 85% homology with rat and 52% with bovine promoters. DNaseI footprinting and electrophoretic gel mobility shift assays demonstrated the presence of nuclear factors in neuroendocrine cells that recognized the proximate promoter. We conclude that the chromogranin A promoter contains both positive and negative domains governing its cell type-specific pattern of transcription, and that a small proximate region of the promoter, containing novel as well as previously described elements, interacts specifically with neuroendocrine nuclear proteins, and is thereby sufficient to ensure widespread neuroendocrine expression of the gene.

Biosynthesis of proteins of large dense-core vesicles in rat PC12 cells: regulation by forskolin and phorbol ester

We have investigated the influence of various second messengers on the biosynthesis of large dense-core vesicle constituents in rat PC12 cells. After treatment with forskolin, phorbol ester or a combination of both substances for up to six days, the messenger RNA levels of several vesicle components were determined by northern blotting. Forskolin increased the expression of messenger RNA encoding the soluble proteins chromogranin B, neuropeptide Y and VGF. Addition of phorbol ester markedly enhanced the effects of forskolin. On the other hand, the expression of two further soluble proteins, chromogranin A and secretogranin II, remained fairly unchanged with all treatments tested. Amongst partly membrane-bound vesicle components, the biosynthesis of glycoprotein III and peptidylglycine alpha-amidating mono-oxygenase was significantly up-regulated by combined treatment with forskolin plus phorbol ester. The carboxypeptidase H messenger RNA increased due to phorbol ester and after long-term application of both drugs. In contrast, phorbol ester alone or plus forskolin down-regulated the expression of dopamine beta-hydroxylase. Essentially the same applies to the intrinsic membrane protein cytochrome b-561, whose messenger RNA level declined in all treatment groups. In conclusion, our results show that forskolin and phorbol ester can regulate the composition of large dense-core vesicles in quite distinct patterns.

Secretogranin II: regulation of synthesis and post-translational proteolysis in bovine adrenal chromaffin cells

Secretogranin II (SgII), also called chromogranin C, is an acidic tyrosine-sulfated secretory protein found in secretory granules in a wide variety of endocrine cells and neurones. Although less abundant than chromogranin A (CGA) and chromogranin B (CGB), SgII is found in adrenal medullary chromaffin granules. In the present study we investigated the regulation of SgII biosynthesis in bovine chromaffin cells maintained in primary culture. Cellular proteins were labelled with [35S]methionine and the heat stable chromogranin enriched fraction was isolated. Following electrophoretic separation, the 86 kDa SgII band was identified by sequence analysis using the Edman degradation procedure. The radioactivity incorporated in the 86 kDa SgII band was used as an index of the SgII synthesis rate. We found that stimulation of chromaffin cells with nicotine and histamine and to a smaller extent with angiotensin II and bradykinin significantly enhanced the rate of SgII synthesis. In contrast direct depolarization with K+ may not be sufficient to induce modifications in SgII synthesis suggesting that the raise of cytosolic calcium evoked by high K+ may not be sufficient to induce modifications in SgII synthesis . The possible second messenger pathways involved in the control of SgII biosynthesis were investigated by using protein kinase C and adenylate cyclase activators. We observed that 12-O-tetradecanoylphorbol 13-acetate (TPA) and forskolin increased the basal rate of SgII synthesis. Incubation with both TPA and forskolin was required to obtain an effect comparable to that produced by nicotine or histamine suggesting that these secretagogues recruit both protein kinase C- and cyclic AMP-dependent mechanisms to stimulate SgII synthesis.

In situ hybridization: mRNA levels of secretogranin II, VGF and peptidylglycine alpha-amidating monooxygenase in brain of salt-loaded rats

The mRNA levels of secretogranin II (SgII), VGF and peptidylglycine alpha-amidating monooxygenase (PAM) were studied in brains of salt loaded rats by in situ hybridization. In these rats the levels of the message for secretogranin II and VGF were increased in the paraventricular, supraoptic and retrochiasmatic nuclei and in the subfornical organ. The increases ranged from 416 to 721% for SgII and from 778 to 890% for VGF. The PAM message was also elevated in these brain regions; however, the maximal increase was only 221%. We conclude that the message for all secretory peptides investigated so far, i.e. vasopressin, galanin, secretogranin II and VGF are upregulated to a similar degree in the hypothalamus of salt-located rats. The relative increase in mRNA for the enzyme peptidylglycine alpha-amidating monooxygenase occurred to a much lower extent, and was comparable to the limited changes previously seen for carboxypeptidase H.

Granins markers of the regulated secretory pathway

The granins are a family of acidic secretory proteins made up of chromogranin A, chromogranin B, and secretogranin II, which exhibit widespread distribution in endocrine and neuronal cells. The numerous potential sites for proteolytic processing have suggested a role for these peptides as prohormones: several potential degradation products of chromogranin A, pancreastatin, and chromostatin have autocrine activity. On the other hand, an intracellular role for the granins is supported by their propensity to aggregate in a low-pH, high-calcium environment such as found in the trans-Golgi network followed by their efficient sorting to the regulated pathway. As a result, the granins are considered markers for the regulated pathway and may play a role in secretory granule formation.