Overall lack of regulated secretion in a PC12 variant cell clone

Expression and function of the dense-core vesicle membranes are governed by the transcription repressor REST

The membrane of dense-core vesicles is present only in neural cells, where it is instrumental to the regulated discharge of important molecules such as the catecholamine neurotransmitters. The mechanism underlying the specificity of this membrane to certain cell types has so far been unclear. Studies of this problem have been carried out by employing the pheochromocytoma PC12 cell line and its clones defective of dense-core vesicles. REST, the transcription repressor expressed at high levels in non-neural and at very low levels in neural cells, was found to regulate the genes encoding almost all the proteins of both the core and the membrane of the dense-core vesicles, including the transporter for catecholamines and the SNAREs for their exocytosis. Moreover, REST appears to control the assembly of the vesicle membrane. The role of REST in the various steps of the expression and function of the dense-core vesicle membrane is critical during development and participates in the dynamic regulation of mature cell physiology.

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.

Pro-hormone secretogranin II regulates dense core secretory granule biogenesis in catecholaminergic cells

Processes underlying the formation of dense core secretory granules (DCGs) of neuroendocrine cells are poorly understood. Here, we present evidence that DCG biogenesis is dependent on the secretory protein secretogranin (Sg) II, a member of the granin family of pro-hormone cargo of DCGs in neuroendocrine cells. Depletion of SgII expression in PC12 cells leads to a decrease in both the number and size of DCGs and impairs DCG trafficking of other regulated hormones. Expression of SgII fusion proteins in a secretory-deficient PC12 variant rescues a regulated secretory pathway. SgII-containing dense core vesicles share morphological and physical properties with bona fide DCGs, are competent for regulated exocytosis, and maintain an acidic luminal pH through the V-type H(+)-translocating ATPase. The granulogenic activity of SgII requires a pH gradient along this secretory pathway. We conclude that SgII is a critical factor for the regulation of DCG biogenesis in neuroendocrine cells, mediating the formation of functional DCGs via its pH-dependent aggregation at the trans-Golgi network.

Biogenesis of Dense-Core Secretory Granules

Dense core granules (DCGs) are vesicular organelles derived from outbound traffic through the eukaryotic secretory pathway. As DCGs are formed, the secretory pathway can also give rise to other types of vesicles, such as those bound for endosomes, lysosomes, and the cell surface. DCGs differ from these other vesicular carriers in both content and function, storing highly concentrated cores’ of condensed cargo in vesicles that are stably maintained within the cell until a specific extracellular stimulus causes their fusion with the plasma membrane. These unique features are imparted by the activities of membrane and lumenal proteins that are specifically delivered to the vesicles during synthesis. This chapter will describe the DCG biogenesis pathway, beginning with the sorting of DCG proteins from proteins that are destined for other types of vesicle carriers. In the trans-Golgi network (TGN), sorting occurs as DCG proteins aggregate, causing physical separation from non-DCG proteins. Recent work addresses the nature of interactions that produce these aggregates, as well as potentially important interactions with membranes and membrane proteins. DCG proteins are released from the TGN in vesicles called immature secretory granules (ISGs). The mechanism of ISG formation is largely unclear but is not believed to rely on the assembly of vesicle coats like those observed in other secretory pathways. The required cytosolic factors are now beginning to be identified using in vitro systems with purified cellular components. ISG transformation into a mature fusion-competent, stimulus-dependent DCG occurs as endoproteolytic processing of many DCG proteins causes continued condensation of the lumenal contents. At the same time, proteins that fail to be incorporated into the condensing core are removed by a coat-mediated budding mechanism, which also serves to remove excess membrane and membrane proteins from the maturing vesicle. This chapter will summarize the work leading to our current view of granule synthesis, and will discuss questions that need to be addressed in order to gain a more complete understanding of the pathway.

The transcriptional repressor REST is a critical regulator of the neurosecretory phenotype

Release of distinct cellular cargoes in response to specific stimuli is a process fundamental to all higher eukaryotes and controlled by the regulated secretory pathway (RSP). However, the mechanism by which genes involved in the RSP are selectively expressed, leading to the establishment and appropriate functioning of regulated secretion remaining largely unknown. Using the rat pheochromocytoma cell line PC12, we provide evidence that, by controlling expression of many genes involved in the RSP, the transcriptional repressor REST can regulate this pathway and hence the neurosecretory phenotype. Introduction of REST transgenes into PC12 cells leads to the repression of many genes, the products of which are involved in regulated secretion. Moreover, chromatin immunoprecipitation assays show that many of the repressed genes recruit the recombinant REST protein to RE1 sites within their promoters and abrogation of REST function leads to reactivation of these transcripts. In addition to the observed transcriptional effects, PC12 cells expressing REST have fewer secretory granules and a reduction in the ability to store and release noradrenaline. Furthermore, an important trigger for synaptic release, influx of calcium through voltage-operated calcium channels, is compromised. This is the first demonstration of a transcription factor that directly controls expression of many major components of the RSP and provides further insight into the function of REST.

Secretory granule biogenesis in sympathoadrenal cells: identification of a granulogenic determinant in the secretory prohormone chromogranin A

Chromogranin A (CgA) may be critical for secretory granule biogenesis in sympathoadrenal cells. We found that silencing the expression of CgA reduced the number of secretory granules in normal sympathoadrenal cells (PC12), and we therefore questioned whether a discrete domain of CgA might promote the formation of a regulated secretory pathway in variant sympathoadrenal cells (A35C) devoid of such a phenotype. The secretory granule-forming activity of a series of human CgA domains labeled with a hemagglutinin epitope, green fluorescent protein, or embryonic alkaline phosphatase was assessed in A35C cells by deconvolution and electron microscopy and by secretagogue-stimulated release assays. Expression of CgA in A35C cells induced the formation of vesicular organelles throughout the cytoplasm, whereas two constitutive secretory pathway markers accumulated in the Golgi complex. The lysosome-associated membrane protein LGP110 did not co-localize with CgA, consistent with non-lysosomal targeting of the granin in A35C cells. Thus, CgA-expressing A35C cells showed electron-dense granules approximately 180-220 nm in diameter, and secretagogue-stimulated exocytosis of CgA from A35C cells suggested that expression of the granin may be sufficient to restore a regulated secretory pathway and thereby rescue the sorting of other secretory proteins. We show that the formation of vesicular structures destined for regulated exocytosis may be mediated by a determinant located within the CgA N-terminal region (CgA-(1-115), with a necessary contribution of CgA-(40-115)), but not the C-terminal region (CgA-(233-439)) of the protein. We propose that CgA promotes the biogenesis of secretory granules by a mechanism involving a granulogenic determinant located within CgA-(40-115) of the mature protein.

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.

Follistatin induction by nitric oxide through cyclic GMP: a tightly regulated signaling pathway that controls myoblast fusion

The mechanism of skeletal myoblast fusion is not well understood. We show that endogenous nitric oxide (NO) generation is required for myoblast fusion both in embryonic myoblasts and in satellite cells. The effect of NO is concentration and time dependent, being evident only at the onset of differentiation, and direct on the fusion process itself. The action of NO is mediated through a tightly regulated activation of guanylate cyclase and generation of cyclic guanosine monophosphate (cGMP), so much so that deregulation of cGMP signaling leads to a fusion-induced hypertrophy of satellite-derived myotubes and embryonic muscles, and to the acquisition of fusion competence by myogenic precursors in the presomitic mesoderm. NO and cGMP induce expression of follistatin, and this secreted protein mediates their action in myogenesis. These results establish a hitherto unappreciated role of NO and cGMP in regulating myoblast fusion and elucidate their mechanism of action, providing a direct link with follistatin, which is a key player in myogenesis.

Serum-free culture conditions for serial subculture of undifferentiated PC12 cells

PC12 cells, a widely used model neuronal cell line, are usually cultured in serum-supplemented medium. This report describes a serum-free medium for the culture of PC12 cells. PC12 cells grown in the two media types had similar growth rates and released dopamine in response to high potassium-induced calcium elevation. However, the levels of dopamine and of dopamine release in cells cultured in the serum-free medium were less than 10% of that in cells cultured in serum-supplemented medium. Dopamine levels recovered within 10 days if cells were returned to serum-supplemented medium, but dopamine release could not be recovered. Nerve growth factor (NGF) induced similar responses in PC12 cells cultured in both media, including phosphorylation of extracellular signal-regulated protein kinases and neurite extension. Transferrin was necessary for survival of neurite-bearing PC12 cells subcultured in serum-free medium and insulin promoted the cells proliferation. Ten days culture with NGF produced a similar increase in neurofilament expression and acetylcholinesterase activity in both media. These results suggest that PC12 in the hormonally defined serum-free media are qualitatively the same as those cultured in serum-supplemented media, and therefore this new culture protocol should enable more precise studies of PC12 cells culture in the absence of confounding unknown factors.

Out with a bang! Tetrahymena as a model system to study secretory granule biogenesis

The release of polypeptides in response to extracellular cues is a notable feature of endocrine, exocrine and neuronal cells, and is based on regulated exocytosis via dense-core secretory granules. There is interest in this mode of secretion because of its importance in human physiology and also because regulated exocytosis reflects a complex pathway of membrane traffic that includes compartment-specific reversible macromolecular assembly, coat-independent vesicle budding, maturation/remodeling of both lumenal and membrane constituents, and stimulus-dependent membrane fusion. Secretory granules are absent in most unicellular model organisms but are highly developed in the Ciliates, which therefore offer attractive systems to study these phenomena. In Tetrahymena thermophila, biochemical and genetic approaches have begun yielding insights into issues ranging from control of granule core assembly, based on reverse genetic analysis of granule cargo, to questions about factors involved in granule biogenesis, based on random mutational approaches.

The secretory granule-associated protein CAPS2 regulates neurotrophin release and cell survival

Neurotrophins are key modulators of various neuronal functions, including differentiation, survival, and synaptic plasticity, but the molecules that regulate their secretion are poorly understood. We isolated a clone that is predominantly expressed in granule cells of postnatally developing mouse cerebellum, which turned out to be a paralog of CAPS (Ca2+-dependent activator protein for secretion), and named CAPS2. CAPS2 is enriched on vesicular structures of presynaptic parallel fiber terminals of granule cells connecting postsynaptic spines of Purkinje cell dendrites. Vesicle factions affinity-purified by the CAPS2 antibody from mouse cerebella contained significant amounts of neurotrophin-3 (NT-3), brain-derived neurotrophic factor (BDNF), and chromogranin B but not marker proteins for synaptic vesicle synaptophysin and synaptotagmin. In cerebellar primary cultures, punctate CAPS2 immunoreactivities are primarily colocalized with those of NT-3 and BDNF and near those of a postsynaptic marker, postsynaptic density-95, around dendritic arborization of Purkinje cells. Exogenously expressed CAPS2 enhanced release of exogenous NT-3 and BDNF from PC12 cells and endogenous NT-3 from cultured granule cells in a depolarization-dependent manner. Moreover, the overexpression of CAPS2 in granule cells promotes the survival of Purkinje cells in cerebellar cultures. Thus, we suggest that CAPS2 mediates the depolarization-dependent release of NT-3 and BDNF from granule cells, leading to regulation in cell differentiation and survival during cerebellar development.

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.

Immunosuppressant FK506 induces neurite outgrowth in PC12 mutant cells with impaired NGF-promoted neuritogenesis via a novel MAP kinase signaling pathway

We obtained a drug-hypersensitive PC12 mutant cell (PC12m3), in which neurite outgrowth was strongly stimulated by various drugs such as FK506, calcimycin and cAMP, under the condition of NGF treatment. The frequency of neurite outgrowth stimulated by FK506 was approximately 40 times greater than by NGF alone. The effects of FK506 on neurite outgrowth in PC12m3 cells were inhibited by rapamycin, an FK506 antagonist, and by calcimycin, a calcium ionophore. PC12m3 cells had a strong NGF-induced MAP kinase activity, the same as PC12 parental cells. However, FK506-induced MAP kinase activity was detected only in PC12m3 cells. The activation of MAP kinase by FK506 in PC12m3 cells was markedly inhibited by rapamicin and calcimycin. FK506-induced MAP kinase activity was also inhibited by MAP kinase inhibitor U0126. These results demonstrate that drug-hypersensitive PC12m3 cells have a novel FK506-induced MAP kinase pathway for neuritogenesis.

Immunosuppressant FK506 induces sustained activation of MAP kinase and promotes neurite outgrowth in PC12 mutant cells incapable of differentiating

During the continuous culturing of neural PC12 cells, a drug hypersensitive PC12 mutant cell line (PC12m3) was obtained, which demonstrated high neurite outgrowth when stimulated by various drugs. When the immunosuppressant drug FK506 and nerve growth factor (NGF) were introduced to the PC12m3 cells, the frequency of neurite outgrowth increased approximately 40-fold for NGF alone. However, the effect of FK506 on neuritogenesis in PC12 parental and drug insensitive PC12m1 mutant cells was much lower than in PC12m3 cells. The sustained activation of mitogen-activated protein (MAP) kinase plays an important role in neurite outgrowth of PC12 cells. Interestingly, the drug hypersensitive PC12m3 cells exhibited the sustained activation of MAP kinase with FK506 in comparison to low or no activities in PC12 parental or drug insensitive PC12m1 cells. These results indicate that PC12m3 cells have a novel FK506-induced MAP kinase pathway for neuritogenesis.

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.

Analysis of a mutant exhibiting conditional sorting to dense core secretory granules in Tetrahymena thermophila

The formation of dense core granules (DCGs) requires both the sorting of granule contents from other secretory proteins and a postsorting maturation process. The Tetrahymena thermophila strain SB281 fails to synthesize DCGs, and previous analysis suggested that the defect lay at or near the sorting step. Because this strain represents one of the very few mutants in this pathway, we have undertaken a more complete study of the phenotype. Genetic epistasis analysis places the defect upstream of those in two other characterized Tetrahymena mutants. Using immunofluorescent detection of granule content proteins, as well as GFP tagging, we describe a novel cytoplasmic compartment to which granule contents can be sorted in growing SB281 cells. Cell fusion experiments indicate that this compartment is not a biosynthetic intermediate in DCG synthesis. Sorting in SB281 is strongly conditional with respect to growth. When cells are starved, the storage compartment is degraded and de novo synthesized granule proteins are rapidly secreted. The mutation in SB281 therefore appears to affect DCG synthesis at the level of both sorting and maturation.

Syntaxin 1A is delivered to the apical and basolateral domains of epithelial cells: the role of munc-18 proteins

SNARE (Soluble N-ethyl-maleimide sensitive factor Attachment protein Receptor) proteins assemble in tight core complexes, which promote fusion of carrier vesicles with target compartments. Members of this class of proteins are expressed in all eukaryotic cells and are distributed in distinct subcellular compartments. The molecular mechanisms underlying sorting of SNAREs to their physiological sites of action are still poorly understood. Here have we analyzed the transport of syntaxin1A in epithelial cells. In line with previous data we found that syntaxin1A is not transported to the plasma membrane, but rather is retained intracellularly when overexpressed in MDCK and Caco-2 cells. Its delivery to the cell surface is recovered after munc-18-1 cotransfection. Furthermore, overexpression of the ubiquitous isoform of munc-18, munc-18-2, is also capable of rescuing the transport of the t-SNARE. The interaction between syntaxin 1A and munc-18 occurs in the biosynthetic pathway and is required to promote the exit of the t-SNARE from the Golgi complex. This enabled us to investigate the targeting of syntaxin1A in polarized cells. Confocal analysis of polarized monolayers demonstrates that syntaxin1A is delivered to both the apical and basolateral domains independently of the munc-18 proteins used in the cotranfection experiments. In search of the mechanisms underlying syntaxin 1A sorting to the cell surface, we found that a portion of the protein is included in non-ionic detergent insoluble complexes. Our results indicate that the munc-18 proteins represent limiting but essential factors in the transport of syntaxin1A from the Golgi complex to the epithelial cell surface. They also suggest the presence of codominant apical and basolateral sorting signals in the syntaxin1A sequence.

Chromogranin A, an "on/off" switch controlling dense-core secretory granule biogenesis

We present evidence that regulation of dense-core secretory granule biogenesis and hormone secretion in endocrine cells is dependent on chromogranin A (CGA). Downregulation of CGA expression in a neuroendocrine cell line, PC12, by antisense RNAs led to profound loss of dense-core secretory granules, impairment of regulated secretion of a transfected prohormone, and reduction of secretory granule proteins. Transfection of bovine CGA into a CGA-deficient PC12 clone rescued the regulated secretory phenotype. Stable transfection of CGA into a CGA-deficient pituitary cell line, 6T3, lacking a regulated secretory pathway, restored regulated secretion. Overexpression of CGA induced dense-core granules, immunoreactive for CGA, in nonendocrine fibroblast CV-1 cells. We conclude that CGA is an "on/off" switch that alone is sufficient to drive dense-core secretory granule biogenesis and hormone sequestration in endocrine cells.

Two overlapping reading frames in a single exon encode interacting proteins--a novel way of gene usage

The >1 kb XL-exon of the rat XLalphas/Galphas gene encodes the 37 kDa XL-domain, the N-terminal half of the 78 kDa neuroendocrine-specific G-protein alpha-subunit XLalphas. Here, we describe a novel feature of the XL-exon, the presence of an alternative >1 kb open reading frame (ORF) that completely overlaps with the ORF encoding the XL-domain. The alternative ORF starts 32 nucleotides downstream of the start codon for the XL-domain and is terminated by a stop codon exactly at the end of the XL-exon. The alternative ORF encodes ALEX, a very basic (pI 11.8), proline-rich protein of 356 amino acids. Both XLalphas and ALEX are translated from the same mRNA. Like XLalphas, ALEX is expressed in neuroendocrine cells and tightly associated with the cytoplasmic leaflet of the plasma membrane. Remarkably, ALEX binds to the XL-domain of XLalphas. Our results reveal a mechanism of gene usage that is without precedent in mammalian genomes.

Signaling mediated by the cytosolic domain of peptidylglycine alpha-amidating monooxygenase

The luminal domains of membrane peptidylglycine alpha-amidating monooxygenase (PAM) are essential for peptide alpha-amidation, and the cytosolic domain (CD) is essential for trafficking. Overexpression of membrane PAM in corticotrope tumor cells reorganizes the actin cytoskeleton, shifts endogenous adrenocorticotropic hormone (ACTH) from mature granules localized at the tips of processes to the TGN region, and blocks regulated secretion. PAM-CD interactor proteins include a protein kinase that phosphorylates PAM (P-CIP2) and Kalirin, a Rho family GDP/GTP exchange factor. We engineered a PAM protein unable to interact with either P-CIP2 or Kalirin (PAM-1/K919R), along with PAM proteins able to interact with Kalirin but not with P-CIP2. AtT-20 cells expressing PAM-1/K919R produce fully active membrane enzyme but still exhibit regulated secretion, with ACTH-containing granules localized to process tips. Immunoelectron microscopy demonstrates accumulation of PAM and ACTH in tubular structures at the trans side of the Golgi in AtT-20 cells expressing PAM-1 but not in AtT-20 cells expressing PAM-1/K919R. The ability of PAM to interact with P-CIP2 is critical to its ability to block exit from the Golgi and affect regulated secretion. Consistent with this, mutation of its P-CIP2 phosphorylation site alters the ability of PAM to affect regulated secretion.

Establishment of variant PC12 subclones deficient in stimulation-secretion coupling

Clonal rat pheochromocytoma (PC12) cells have been widely used to study the molecular mechanism of exocytosis. We have isolated variant PC12 subclones with deficiencies in stimulation-secretion coupling, by a single cell recloning, and investigated the defects. PC12-1G2 hardly released dopamine following high-K(+)-induced depolarization, but normal release was evoked by the Ca(2+)-ionophore, ionomycin. Fura-2 fluorometry indicated that a nicardipine-sensitive component of Ca(2+) influx was missing, suggesting that PC12-1G2 has defects in L-type Ca(2+) channel function. PC12-2B3 was not responsive to high-K(+)-induced depolarization and ionomycin, and voltage-dependent Ca(2+) entry was identical to that of the normal clone. Electron microscopy revealed that the number of vesicles adjacent or directly attached to the plasma membrane was decreased in PC12-2B3. The expression of presynaptic proteins was analyzed by immunoblotting using a panel of antibodies. Syntaxin 1, VAMP-2, SNAP-25, Munc18, Rab3C and Sec-6 were decreased compared to the control clone and that of synaptophysin was extremely low. PC12-D60 synthesized and released dopamine normally, but had almost lost its catecholamine-uptake activity. These results show that multiple PC12 cells variants are spontaneously generated, and that recloning can select PC12 subclones useful for the study of the molecular mechanisms of neurotransmitter release.

Trafficking/sorting and granule biogenesis in the beta-cell

Proinsulin is packaged into nascent (immature, clathrin-coated) secretory granules in the trans-Golgi network (TGN) of the beta -cell along with other granular constituents including the proinsulin conversion enzymes. It is assumed that such packaging is dependent on an active sorting process, separating granular proteins from other secretory or membrane proteins, but the mechanism remains elusive. As granules mature, the clathrin coat is lost, the intragranular milieu is progressively acidified, and proinsulin is converted to insulin and C-peptide. Loss of clathrin is believed to arise by budding of clathrin-coated vesicles from maturing granules, carrying with them any inappropriate or unnecessary products and providing an additional means for refinement of granular content.

Cholesterol binds to synaptophysin and is required for biogenesis of synaptic vesicles

Here, to study lipid-protein interactions that contribute to the biogenesis of regulated secretory vesicles, we have developed new approaches by which to label proteins in vivo, using photoactivatable cholesterol and glycerophospholipids. We identify synaptophysin as a major specifically cholesterol-binding protein in PC12 cells and brain synaptic vesicles. Limited cholesterol depletion, which has little effect on total endocytic activity, blocks the biogenesis of synaptic-like microvesicles (SLMVs) from the plasma membrane. We propose that specific interactions between cholesterol and SLMV membrane proteins, such as synaptophysin, contribute to both the segregation of SLMV membrane constituents from plasma-membrane constituents, and the induction of synaptic-vesicle curvature.

Synaptic vesicle biogenesis

Synaptic vesicles, which have been a paradigm for the fusion of a vesicle with its target membrane, also serve as a model for understanding the formation of a vesicle from its donor membrane. Synaptic vesicles, which are formed and recycled at the periphery of the neuron, contain a highly restricted set of neuronal proteins. Insight into the trafficking of synaptic vesicle proteins has come from studying not only neurons but also neuroendocrine cells, which form synaptic-like microvesicles (SLMVs). Formation and recycling of synaptic vesicles/SLMVs takes place from the early endosome and the plasma membrane. The cytoplasmic machinery of synaptic vesicle/SLMV formation and recycling has been studied by a variety of experimental approaches, in particular using cell-free systems. This has revealed distinct machineries for membrane budding and fission. Budding is mediated by clathrin and clathrin adaptors, whereas fission is mediated by dynamin and its interacting protein SH3p4, a lysophosphatidic acid acyl transferase.

Neurosecretory cells without neurosecretion: evidence of an independently regulated trait of the cell phenotype

Neurosecretion competence is a fundamental property that enables differentiated neurones and professional neurosecretory cells to store neurotransmitters and hormones in specialized organelles, the synaptic-like vesicles and dense granules, and to release them by regulated exocytosis. In our laboratory, the study of rat phaeochromocytoma (PC12) clones that fail to express the above organelles or any other components involved in neurosecretion, whilst maintaining most of the general markers of the parental population, has served to demonstrate that this trait is controlled independently from the rest of the phenotype. The present review focuses on recent advances in elucidating the molecular mechanisms governing neurosecretion competence. Moreover, the opportunities that such neurosecretion-defective PC12 clones offer for the investigation of new aspects of regulated exocytosis and the localization of its components are summarized.

Calmodulin and protein kinase C increase Ca(2+)-stimulated secretion by modulating membrane-attached exocytic machinery

The molecular mechanisms underlying the Ca(2+) regulation of hormone and neurotransmitter release are largely unknown. Using a reconstituted [(3)H]norepinephrine release assay in permeabilized PC12 cells, we found that essential proteins that support the triggering stage of Ca(2+)-stimulated exocytosis are enriched in an EGTA extract of brain membranes. Fractionation of this extract allowed purification of two factors that stimulate secretion in the absence of any other cytosolic proteins. These are calmodulin and protein kinase Calpha (PKCalpha). Their effects on secretion were confirmed using commercial and recombinant proteins. Calmodulin enhances secretion in the absence of ATP, whereas PKC requires ATP to increase secretion, suggesting that phosphorylation is involved in PKC- but not calmodulin-mediated stimulation. Both proteins modulate release events that occur in the triggering stage of exocytosis. The half-maximal increase was elicited by 3 nM PKC and 75 nM calmodulin. These results suggest that calmodulin and PKC increase Ca(2+)-activated exocytosis by directly modulating the membrane- or cytoskeleton-attached exocytic machinery downstream of Ca(2+) elevation.

Neurite extension occurs in the absence of regulated exocytosis in PC12 subclones

We have investigated the process leading to differentiation of PC12 cells. This process is known to include extension of neurites and changes in the expression of subsets of proteins involved in cytoskeletal rearrangements or in neurosecretion. To this aim, we have studied a PC12 clone (trk-PC12) stably transfected with the nerve growth factor receptor TrkA. These cells are able to undergo both spontaneous and neurotrophin-induced morphological differentiation. However, both undifferentiated and nerve growth factor-differentiated trk-PC12 cells appear to be completely defective in the expression of proteins of the secretory apparatus, including proteins of synaptic vesicles and large dense-core granules, neurotransmitter transporters, and neurotransmitter-synthesizing enzymes. These results indicate that neurite extension can occur independently of the presence of the neurosecretory machinery, including the proteins that constitute the fusion machine, suggesting the existence of differential activation pathways for the two processes during neuronal differentiation. These findings have been confirmed in independent clones obtained from PC12-27, a previously characterized PC12 variant clone globally incompetent for regulated secretion. In contrast, the integrity of the Rab cycle appears to be necessary for neurite extension, because antisense oligonucleotides against the neurospecific isoform of Rab-guanosine diphosphate-dissociation inhibitor significantly interfere with process formation.

A regulated secretory pathway in cultured hippocampal astrocytes

Glial cells have been reported to express molecules originally discovered in neuronal and neuroendocrine cells, such as neuropeptides, neuropeptide processing enzymes, and ionic channels. To verify whether astrocytes may have regulated secretory vesicles, the primary cultures prepared from hippocampi of embryonic and neonatal rats were used to investigate the subcellular localization and secretory pathway followed by secretogranin II, a well known marker for dense-core granules. By indirect immunofluorescence, SgII was detected in a large number of cultured hippocampal astrocytes. Immunoreactivity for the granin was detected in the Golgi complex and in a population of dense-core vesicles stored in the cells. Subcellular fractionation experiments revealed that SgII was stored in a vesicle population with a density identical to that of the dense-core secretory granules present in rat pheochromocytoma cells. In line with these data, biochemical results indicated that 40-50% of secretogranin II synthesized during 18-h labeling was retained intracellularly over a 4-h chase period and released after treatment with different secretagogues. The most effective stimulus appeared to be phorbol ester in combination with ionomycin in the presence of extracellular Ca(2+), a treatment that was found to produce a large and sustained increase in intracellular calcium [Ca(2+)](i) transients. Our findings indicate that a regulated secretory pathway characterized by (i) the expression and stimulated exocytosis of a typical marker for regulated secretory granules, (ii) the presence of dense-core vesicles, and (iii) the ability to undergo [Ca(2+)](i) increase upon specific stimuli is present in cultured hippocampal astrocytes.

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.

Blockade of membrane transport and disassembly of the Golgi complex by expression of syntaxin 1A in neurosecretion-incompetent cells: prevention by rbSEC1

The t-SNAREs syntaxin1A and SNAP-25, i.e. the members of the complex involved in regulated exocytosis at synapses and neurosecretory cells, are delivered to their physiological site, the plasma membrane, when transfected into neurosecretion-competent cells, such as PC12 and AtT20. In contrast, when transfection is made into cells incompetent for neurosecretion, such as those of a defective PC12 clone and the NRK fibroblasts, which have no endogenous expression of these t-SNAREs, syntaxin1A (but neither two other syntaxin family members nor SNAP-25) remains stuck in the Golgi-TGN area with profound consequences to the cell: blockade of both membrane (SNAP-25, GAT-1) and secretory (chromogranin B) protein transport to the cell surface; progressive disassembly of the Golgi complex and TGN; ultimate disappearance of the latter structures, with intermixing of their markers (mannosidase II; TGN-38) with those of the endoplasmic reticulum (calreticulin) and with syntaxin1A itself. When, however, syntaxin 1A is transfected together with rbSec1, a protein known to participate in neurosecretory exocytosis via its dynamic interaction with the t-SNARE, neither the blockade nor the alterations of the Golgi complex take place. Our results demonstrate that syntaxin1A, in addition to its role in exocytosis at the cell surface, possesses a specific potential to interfere with intracellular membrane transport and that its interaction with rbSec1 is instrumental to its physiological function not only at the plasma membrane but also within the cell. At the latter site, the rbSec1-induced conversion of syntaxin1A into a form that can be transported and protects the cell from the development of severe structural and membrane traffic alterations.

Molecular mechanisms of neurite extension

The extension of neurites is a major task of developing neurons, requiring a significant metabolic effort to sustain the increase in molecular synthesis necessary for plasma membrane expansion. In addition, neurite extension involves changes in the subsets of expressed proteins and reorganization of the cytomatrix. These phenomena are driven by environmental cues which activate signal transduction processes as well as by the intrinsic genetic program of the cell. The present review summarizes some of the most recent progress made in the elucidation of the molecular mechanisms underlying these processes.

Multiple and diverse forms of regulated exocytosis in wild-type and defective PC12 cells

Regulated exocytosis triggered by the photolysis of a caged Ca2+ compound, DM-nitrophen, was investigated by patch-clamp capacitance measurements in two clones of PC12, the first wild-type and the second (PC12-27) defective of both types of classical secretory vesicles together with the neuronal-type receptors for the attachment proteins of the N-ethylmaleimide-sensitive fusion protein, the so called SNAREs. Moreover, the electrophysiological data were correlated with the ultrastructure of resting quick-frozen-freeze-dried cells of the two clones. Wild-type PC12 exhibited two-component capacitance responses, time constants of 30-100 ms and >10 s, that previous studies had suggested to reflect primarily the fusion of the small and large secretory vesicles, each contributing cell surface increases of approximately 10%. Both of these components were largely and specifically inhibited whether cells previously were microinjected with tetanus toxin light chain. In the defective clone, large responses also were recorded ( approximately 19% surface expansion; time constant, approximately 1 s) that, in contrast to those of the wild-type, were entirely resistant to the toxin. Although secretory organelles, i.e., large vesicles and also profiles of small vesicles, were abundant at the cell periphery and often docked to the plasmalemma of resting wild-type PC12, in the defective clone, no superficial accumulation of vesicles was observed. Our coordinate structural and functional results have revealed diversities between the two classical forms of regulated secretion in wild-type PC12 and have provided evidence of a toxin-insensitive form of Ca2+-induced exocytosis, prominent in the defective clone, that may play an important role(s) in cellular physiology.

Neurosecretion competence, an independently regulated trait of the neurosecretory cell phenotype

Neurosecretion competence is intended as the ability of neurosecretory cells to express dense and clear vesicles discharged by regulated exocytosis (neurotransmitter release). Such a property, which so far has never been studied independently, is investigated here by a heterotypic cell fusion approach, using a clone of rat pheochromocytoma PC12 cells totally incompetent for neurosecretion that still largely maintains its typical molecular and cellular phenotype. When fused with wild-type partners of various species (rat, human) and specialization (PC12, neuroblastoma SH-SY5Y, HeLa), the defective cells reacquire their competence as revealed by the expression of their secretion-specific proteins. Fused wild-type cells therefore appear able to complement defective cells by providing them with factor(s) inducing the reactivation of their secretory program. The mechanism of action of these factors may consist not in a coordinate unblocking of transcription but in the prevention of a rapid post-transcriptional degradation of the mRNAs for secretion-specific genes.

Increased MAP1B expression without increased phosphorylation in manganese-treated PC12Mn cells

Regulation of MAP1B expression and phosphorylation is thought to play an important role in neuronal development, particularly with regard to axon growth. The present work utilizes a novel PC12 cell variant [26] which exhibits many of the early morphological features of neurite outgrowth when stimulated with manganese chloride. Expression of MAP1B was determined by immunoblots and phosphorylation was assessed by metabolic radiolabeling with [32P]orthophosphate or with a phospho-specific antibody. The results indicate that MAP1B protein levels rise within 12 to 24 h, but there is no significant change in the phosphorylation of MAP1B. The latter conclusion is based on (i) experiments utilizing SMI 31, a monoclonal antibody that specifically reacts with phospho-MAP1B and (ii) assessments of both MAP1B phosphorylation and MAP1B protein within that same isloated protein band on Western blots. Thus, manganese increases MAP1B expression without affecting its relative phosphorylation. Although manganese does not cause neurite formation in the parental PC12 cell line, manganese is capable of inducing transient neurite regeneration from NGF-primed cells. These studies provide further evidence that the onset of neurite outgrowth may proceed without increased phosphorylation of MAP1B. During sustained neurite regeneration, however, NGF increases phosphate incorporation into MAP1B. Based on all of these findings, we conclude that early phases of neurite outgrowth (cell spreading and formation of short tapered extensions) do not necessarily require elevated phosphorylation of MAP1B.

Regulated exocytosis in neurons and neurosecretory cells: structural events and expression competence

This paper summarizes the contribution of the laboratory first in the development of the 'kiss-and-run model' of exocytosis, with its fascinating aspects of specificity and rapidity, then in proposing the existence of 'competence factors' that appear to govern the appearance of the secretory vesicles and exocytic process in neurosecretory cells.