Ca(2+)-dependent activator protein for secretion is critical for the fusion of dense-core vesicles with the membrane in calf adrenal chromaffin cells

Gene-educational attainment interactions in a multi-ancestry genome-wide meta-analysis identify novel blood pressure loci

Educational attainment is widely used as a surrogate for socioeconomic status (SES). Low SES is a risk factor for hypertension and high blood pressure (BP). To identify novel BP loci, we performed multi-ancestry meta-analyses accounting for gene-educational attainment interactions using two variables, "Some College" (yes/no) and "Graduated College" (yes/no). Interactions were evaluated using both a 1 degree of freedom (DF) interaction term and a 2DF joint test of genetic and interaction effects. Analyses were performed for systolic BP, diastolic BP, mean arterial pressure, and pulse pressure. We pursued genome-wide interrogation in Stage 1 studies (N = 117 438) and follow-up on promising variants in Stage 2 studies (N = 293 787) in five ancestry groups. Through combined meta-analyses of Stages 1 and 2, we identified 84 known and 18 novel BP loci at genome-wide significance level (P < 5 × 10-8). Two novel loci were identified based on the 1DF test of interaction with educational attainment, while the remaining 16 loci were identified through the 2DF joint test of genetic and interaction effects. Ten novel loci were identified in individuals of African ancestry. Several novel loci show strong biological plausibility since they involve physiologic systems implicated in BP regulation. They include genes involved in the central nervous system-adrenal signaling axis (ZDHHC17, CADPS, PIK3C2G), vascular structure and function (GNB3, CDON), and renal function (HAS2 and HAS2-AS1, SLIT3). Collectively, these findings suggest a role of educational attainment or SES in further dissection of the genetic architecture of BP.

RNA editing-mediated regulation of calcium-dependent activator protein for secretion (CAPS1) localization and its impact on synaptic transmission

Calcium-dependent activator protein for secretion 1 (CAPS1) is a SNARE accessory protein that facilitates formation of the SNARE complex to enable neurotransmitter release. Messenger RNAs encoding CAPS1 are subject to a site-specific adenosine-to-inosine (A-to-I) editing event resulting in a glutamate-to-glycine (E-to-G) substitution in the C-terminal domain of the encoded protein product. The C-terminal domain of CAPS1 is necessary for its synaptic enrichment and Cadps RNA editing has been shown previously to enhance the release of neuromodulatory transmitters. Using mutant mouse lines engineered to solely express CAPS1 protein isoforms encoded by either the non-edited or edited Cadps transcript, primary neuronal cultures from mouse hippocampus were used to explore the effect of Cadps editing on neurotransmission and CAPS1 synaptic localization at both glutamatergic and GABAergic synapses. While the editing of Cadps does not alter baseline evoked neurotransmission, it enhances short-term synaptic plasticity, specifically short-term depression, at inhibitory synapses. Cadps editing also alters spontaneous inhibitory neurotransmission. Neurons that solely express edited Cadps have a greater proportion of synapses that contain CAPS1 than neurons that solely express non-edited Cadps for both glutamatergic and GABAergic synapses. Editing of Cadps transcripts is regulated by neuronal activity, as global network stimulation increases the extent of transcripts edited in wild-type hippocampal neurons, whereas chronic network silencing decreases the level of Cadps editing. Taken together, these results provide key insights into the importance of Cadps editing in modulating its own synaptic localization, as well as the modulation of neurotransmission at inhibitory synapses in hippocampal neurons.

Identification of a Munc13-sensitive step in chromaffin cell large dense-core vesicle exocytosis

It is currently unknown whether the molecular steps of large dense-core vesicle (LDCV) docking and priming are identical to the corresponding reactions in synaptic vesicle (SV) exocytosis. Munc13s are essential for SV docking and priming, and we systematically analyzed their role in LDCV exocytosis using chromaffin cells lacking individual isoforms. We show that particularly Munc13-2 plays a fundamental role in LDCV exocytosis, but in contrast to synapses lacking Munc13s, the corresponding chromaffin cells do not exhibit a vesicle docking defect. We further demonstrate that ubMunc13-2 and Munc13-1 confer Ca(2+)-dependent LDCV priming with similar affinities, but distinct kinetics. Using a mathematical model, we identify an early LDCV priming step that is strongly dependent upon Munc13s. Our data demonstrate that the molecular steps of SV and LDCV priming are very similar while SV and LDCV docking mechanisms are distinct.

NF-κB signaling and vesicle transport are correlated with the reactivation of the memory trace of morphine dependence

BACKGROUND

Morphine has been widely used as a clinical anesthetic and analgesic. However, abuse of morphine might result in psychological and physiological dependence. Previous studies have indicated that memory mechanisms play critical roles in morphine dependence.

METHODS

Morphine dependence was established in mice utilizing place preference conditioning (CPP). We observed changes in the methylome and transcriptome of the nucleus accumbens during the reactivation of the memory trace. We also monitored for changes in the methylome and transcriptome of mice that were acutely exposed to morphine.

RESULTS

We detected 165 and 18 differentially expressed genes (DEGs) and 6 and 24 significant methyl-sensitive cut counting (MSCC) windows in the acute morphine treatment and the CPP model, respectively. The changes in the methylome and transcriptome during the acute treatment were mainly caused by a response to the morphine stimulus; most of the DEGs were correlated with hormone or transcription factor activity regulation. The expression levels of Lcn2 and Hspb1, which participate in the activation of NF-κB, were significantly decreased in the CPP morphine treatment model. Besides, the alternative splicing of the curtailed isoform of Caps1 was significantly increased in the CPP morphine-treated group, and the methylation levels of Arf4, Vapa, and Gga3 were decreased. These genes play critical roles in the regulation of the Golgi network.

CONCLUSIONS

The current study indicates that NF-κB signaling and vesicular transport are correlated with the reactivation of the memory trace in morphine-dependent mice. The results obtained in our study agree with previous observations and identify additional candidate genes for further research.

VIRTUAL SLIDES

The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/1196707364133126.

Spontaneous and electrically-evoked catecholamine secretion from long-term cultures of bovine adrenal chromaffin cells

Catecholamine release was measured from bovine adrenal medullary chromaffin cell (CC) cultures maintained over a period of three months. Cells were plated over simple biocompatible cell platforms with electrical stimulation capability and at specified times transferred to an acrylic superfusion chamber designed to allow controlled flow of superfusate over the culture. Catecholamine release was measured from the superfusates using fast cyclic voltammetry before, during and after electrical stimulation of the cells. Immunocytochemical staining of CC cultures revealed that they were composed of epinephrine (EP) and/or norepinephrine (NE) type cells. Both spontaneous and evoked-release of catecholamines from CCs were observed throughout the testing period. EP predominated during spontaneous release, whereas NE was more prevalent during electrically-evoked release. Electrical stimulation for 20 s, increased total catecholamine release by 60-130% (measured over a period of 500 s) compared to that observed for an equivalent 20 s period of spontaneous release. Stimulus intensity was correlated with the amount of evoked release, up to a plateau which was observed near the highest intensities. Shorter intervals between stimulation trials did not significantly affect the initial amount of release, and the amount of evoked release was relatively stable over time and did not decrease significantly with age of the culture. The present study demonstrates long-term survival of CC cultures in vitro and describes a technique useful for rapid assessment of cell functionality and release properties of cultured monoaminergic cell types that later can be transplanted for neurotransmitter replacement following injury or disease.

Munc13 homology domain-1 in CAPS/UNC31 mediates SNARE binding required for priming vesicle exocytosis

Neuropeptide and peptide hormone secretion from neural and endocrine cells occurs by Ca(2+)-triggered dense-core vesicle exocytosis. The membrane fusion machinery consisting of vesicle and plasma membrane SNARE proteins needs to be assembled for Ca(2+)-triggered vesicle exocytosis. The related Munc13 and CAPS/UNC31 proteins that prime vesicle exocytosis are proposed to promote SNARE complex assembly. CAPS binds SNARE proteins and stimulates SNARE complex formation on liposomes, but the relevance of SNARE binding to CAPS function in cells had not been determined. Here we identify a core SNARE-binding domain in CAPS as corresponding to Munc13 homology domain-1 (MHD1). CAPS lacking a single helix in MHD1 was unable to bind SNARE proteins or to support the Ca(2+)-triggered exocytosis of either docked or newly arrived dense-core vesicles. The results show that MHD1 is a SNARE-binding domain and that SNARE protein binding is essential for CAPS function in dense-core vesicle exocytosis.

RNAi screen identifies a role for adaptor protein AP-3 in sorting to the regulated secretory pathway

AP-3 concentrates proteins within large dense-core vesicles to promote regulated exocytosis.

The regulated release of proteins depends on their inclusion within large dense-core vesicles (LDCVs) capable of regulated exocytosis. LDCVs form at the trans-Golgi network (TGN), but the mechanism for protein sorting to this regulated secretory pathway (RSP) and the cytosolic machinery involved in this process have remained poorly understood. Using an RNA interference screen in Drosophila melanogaster S2 cells, we now identify a small number of genes, including several subunits of the heterotetrameric adaptor protein AP-3, which are required for sorting to the RSP. In mammalian neuroendocrine cells, loss of AP-3 dysregulates exocytosis due to a primary defect in LDCV formation. Previous work implicated AP-3 in the endocytic pathway, but we find that AP-3 promotes sorting to the RSP within the biosynthetic pathway at the level of the TGN. Although vesicles with a dense core still form in the absence of AP-3, they contain substantially less synaptotagmin 1, indicating that AP-3 concentrates the proteins required for regulated exocytosis.

Two distinct secretory vesicle-priming steps in adrenal chromaffin cells

The calcium-dependent activator proteins for secretion, CAPS1 and CAPS2, facilitate syntaxin opening during synaptic vesicle priming.

Priming of large dense-core vesicles (LDCVs) is a Ca²⁺-dependent step by which LDCVs enter a release-ready pool, involving the formation of the soluble N-ethyl-maleimide sensitive fusion protein attachment protein (SNAP) receptor complex consisting of syntaxin, SNAP-25, and synaptobrevin. Using mice lacking both isoforms of the calcium-dependent activator protein for secretion (CAPS), we show that LDCV priming in adrenal chromaffin cells entails two distinct steps. CAPS is required for priming of the readily releasable LDCV pool and sustained secretion in the continued presence of high Ca²⁺ concentrations. Either CAPS1 or CAPS2 can rescue secretion in cells lacking both CAPS isoforms. Furthermore, the deficit in the readily releasable LDCV pool resulting from CAPS deletion is reversed by a constitutively open form of syntaxin but not by Munc13-1, a priming protein that facilitates the conversion of syntaxin to the open conformation. Our data indicate that CAPS functions downstream of Munc13s but also interacts functionally with Munc13s in the LDCV-priming process.

Effects of galanin on monoaminergic systems and HPA axis: Potential mechanisms underlying the effects of galanin on addiction- and stress-related behaviors

Like a number of neuropeptides, galanin can alter neural activity in brain areas that are important for both stress-related behaviors and responses to drugs of abuse. Accordingly, drugs that target galanin receptors can alter behavioral responses to drugs of abuse and can modulate stress-related behaviors. Stress and drug-related behaviors are interrelated: stress can promote drug-seeking, and drug exposure and withdrawal can increase activity in brain circuits involved in the stress response. We review here what is known about the ability of galanin and galanin receptors to alter neuronal activity, and we discuss potential mechanisms that may underlie the effects of galanin on behaviors involved in responses to stress and addictive drugs. Understanding the mechanisms underlying galanin's effects on neuronal function in brain regions related to stress and addiction may be useful in developing novel therapeutics for the treatment of stress- and addiction-related disorders.

Developmentally regulated Ca2+-dependent activator protein for secretion 2 (CAPS2) is involved in BDNF secretion and is associated with autism susceptibility

The postnatal development of the cerebellum is accomplished via a series of cytogenetic and morphogenetic events encoded in the genome. To decipher the underlying genetic basis of these events we have systematized the spatio-temporal gene expression profiles during mouse cerebellar development in the Cerebellar Development Transcriptome Database (CDT-DB). Using the CDT-DB, Ca(2+)-dependent activator protein for secretion 2 (CAPS2 or CADPS2) was identified as a developmentally regulated gene that is predominantly expressed in cerebellar granule cells (GCs) with an expression peak around the first or second postnatal week. CAPS2 protein is concentrated in parallel fiber (PF) terminals and is associated with secretory vesicles containing brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3). CAPS2 enhances release of BDNF and NT-3, both of which are essential for normal cerebellar development. CAPS2-deficient (CAPS2(-/-)) mice show reduced secretion of BDNF and NT-3; consequently, the cerebella of these mice exhibit developmental deficits, such as delayed development and increased cell death in GCs, fewer branched dendrites on Purkinje cells (PCs), and loss of the intercrural fissure. The PF-PC synapses have aberrant cytoarchitectures and electrophysiological properties. These abnormal cellular and morphological phenotypes are more severe around the cerebellar vermis, in which hypoplasia has been reported in autism patients. Moreover, CAPS2(-/-) mice had fewer cortical and hippocampal parvalbumin-positive interneurons and some autistic-like behavioral phenotypes. In the CAPS2 genes of some autistic patients an aberrant splicing variant and non-synonymous SNPs have been identified. These recent studies implicate CAPS2 in autism susceptibility. Therefore, CAPS2(-/-) mice will be a useful model animal in which to study aspects of the neuropathology and behaviors characteristic of developmental disorders.

The Ca(2+)-dependent activator protein for secretion CAPS: do I dock or do I prime?

The "Ca(2+)-dependent activator protein for secretion" (CAPS) is a protein which reconstitutes regulated secretion in permeabilized neuroendocrine cells. It is generally accepted that CAPS plays an important role in the release of the contents of dense core vesicles in the nervous system as well as in a variety of other secretory tissues. At which step in the exocytotic process CAPS functions as well as its role in the fusion of synaptic vesicles is still under dispute. A recent growth spurt in the CAPS field has been fueled by genetic approaches in Caenorhabditis elegans and Drosophila as well as the application of knockout and knockdown approaches in mouse cells and in cell lines, respectively. We have attempted to review the body of work that established CAPS as an important regulator of secretion and to describe new information that has furthered our understanding of how CAPS may function. We discuss the conclusions, point out areas where controversy remains, and suggest directions for future experiments.

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.

A network of G-protein signaling pathways control neuronal activity in C. elegans

The Caenorhabditis elegans neuromuscular junction (NMJ) is one of the best studied synapses in any organism. A variety of genetic screens have identified genes required both for the essential steps of neurotransmitter release from motorneurons as well as the signaling pathways that regulate rates of neurotransmitter release. A number of these regulatory genes encode proteins that converge to regulate neurotransmitter release. In other cases genes are known to regulate signaling at the NMJ but how they act remains unknown. Many of the proteins that regulate activity at the NMJ participate in a network of heterotrimeric G-protein signaling pathways controlling the release of synaptic vesicles and/or dense-core vesicles (DCVs). At least four heterotrimeric G-proteins (Galphaq, Galpha12, Galphao, and Galphas) act within the motorneurons to control the activity of the NMJ. The Galphaq, Galpha12, and Galphao pathways converge to control production and destruction of the lipid-bound second messenger diacylglycerol (DAG) at sites of neurotransmitter release. DAG acts via at least two effectors, MUNC13 and PKC, to control the release of both neurotransmitters and neuropeptides from motorneurons. The Galphas pathway converges with the other three heterotrimeric G-protein pathways downstream of DAG to regulate neuropeptide release. Released neurotransmitters and neuropeptides then act to control contraction of the body-wall muscles to control locomotion. The lipids and proteins involved in these networks are conserved between C. elegans and mammals. Thus, the C. elegans NMJ acts as a model synapse to understand how neuronal activity in the human brain is regulated.

Measuring secretion in chromaffin cells using electrophysiological and electrochemical methods

Our present understanding of exocytosis of catecholamines has benefited tremendously from the arrival of single-cell electrochemical methods (amperometry and voltammetry), electrophysiological techniques (whole-cell and patch capacitance) and from the combination of both techniques (patch amperometry). In this brief review, we will outline the strengths and limitations of amperometric and electrophysiological methods and highlight the major contribution obtained with the use of these techniques in chromaffin cells.

CAPS1 and CAPS2 regulate stability and recruitment of insulin granules in mouse pancreatic beta cells

CAPS1 and CAPS2 regulate dense-core vesicle release of transmitters and hormones in neuroendocrine cells, but their precise roles in the secretory process remain enigmatic. Here we show that CAPS2(-/-) and CAPS1(+/-);CAPS2(-/-) mice, despite having increased insulin sensitivity, are glucose intolerant and that this effect is attributable to a marked reduction of glucose-induced insulin secretion. This correlates with diminished Ca(2+)-dependent exocytosis, a reduction in the size of the morphologically docked pool, a decrease in the readily releasable pool of secretory vesicles, slowed granule priming, and suppression of second-phase (but not first-phase) insulin secretion. In beta cells of CAPS1(+/-);CAPS2(-/-) mice, the lowered insulin content and granule numbers were associated with an increase in lysosome numbers and lysosomal enzyme activity. We conclude that although CAPS proteins are not required for Ca(2+)-dependent exocytosis to proceed, they exert a modulatory effect on insulin granule priming, exocytosis, and stability.

Ca2+-dependent activator protein for secretion 1 is critical for constitutive and regulated exocytosis but not for loading of transmitters into dense core vesicles

Although CAPS1 was originally identified as a soluble factor that reconstitutes Ca(2+)-dependent secretion from permeabilized neuroendocrine cells, its exact function in intact mammalian cells remains controversial. Here we investigate the role for CAPS1 by generating stable cell lines in which CAPS1 is strongly down-regulated. In these cells, Ca(2+)-dependent secretion was strongly reduced not only of catecholamine but also of a transfected neuropeptide. These secretion defects were rescued by infusion of CAPS1-containing brain cytosol or by transfection-mediated expression of CAPS1. Whole cell patch clamp recording revealed significant reductions in slow burst and sustained release components of exocytosis in the knockdown cells. Unexpectedly, they also accumulated higher amounts of endogenous and exogenous transmitters, which were attributable to reductions in constitutive secretion. Electron microscopy did not reveal abnormalities in the number or docking of dense core vesicles. Our results indicate that CAPS1 plays critical roles not only in Ca(2+)-dependent, regulated exocytosis but also in constitutive exocytosis downstream of vesicle docking. However, they do not support the role for CAPS1 in loading transmitters into dense core vesicles.

Defective processing of neuropeptide precursors in Caenorhabditis elegans lacking proprotein convertase 2 (KPC-2/EGL-3): mutant analysis by mass spectrometry

Biologically active peptides are synthesized as larger inactive proprotein peptide precursors which are processed by the concerted action of a cascade of enzymes. Among the proprotein convertases, PC2 is widely expressed in neuro-endocrine tissues and has been proposed to be the major convertase involved in the biosynthesis of neuropeptides. In this study, we have examined the role of the Caenorhabditis elegans orthologue PC2/EGL-3 in the processing of proprotein peptide precursors. We recently isolated and identified 60 endogenous peptides in the nematode C. elegans by two-dimensional nanoscale liquid chromatography - quadrupole time-of-flight tandem mass spectrometry. In the present study, we compare the peptide profile of different C. elegans strains, including PC2/EGL-3 mutants. For this purpose, we used an offline approach in which HPLC fractions are analysed by a matrix-assisted laser desorption ionisation - time of flight mass spectrometer. This differential peptidomic approach unambiguously provides evidence for the role of PC2/EGL-3 in the processing of FMRFamide-like peptide (FLP) precursors and neuropeptide-like protein (NLP) precursors in nematodes.

Calcium Signaling and Exocytosis in Adrenal Chromaffin Cells

At a given cytosolic domain of a chromaffin cell, the rate and amplitude of the Ca2+concentration ([Ca2+]c) depends on at least four efficient regulatory systems: 1) plasmalemmal calcium channels, 2) endoplasmic reticulum, 3) mitochondria, and 4) chromaffin vesicles. Different mammalian species express different levels of the L, N, P/Q, and R subtypes of high-voltage-activated calcium channels; in bovine and humans, P/Q channels predominate, whereas in felines and murine species, L-type channels predominate. The calcium channels in chromaffin cells are regulated by G proteins coupled to purinergic and opiate receptors, as well as by voltage and the local changes of [Ca2+]c. Chromaffin cells have been particularly useful in studying calcium channel current autoregulation by materials coreleased with catecholamines, such as ATP and opiates. Depending on the preparation (cultured cells, adrenal slices) and the stimulation pattern (action potentials, depolarizing pulses, high K+, acetylcholine), the role of each calcium channel in controlling catecholamine release can change drastically. Targeted aequorin and confocal microscopy shows that Ca2+entry through calcium channels can refill the endoplasmic reticulum (ER) to nearly millimolar concentrations, and causes the release of Ca2+(CICR). Depending on its degree of filling, the ER may act as a sink or source of Ca2+that modulates catecholamine release. Targeted aequorins with different Ca2+affinities show that mitochondria undergo surprisingly rapid millimolar Ca2+transients, upon stimulation of chromaffin cells with ACh, high K+, or caffeine. Physiological stimuli generate [Ca2+]cmicrodomains in which the local subplasmalemmal [Ca2+]crises abruptly from 0.1 to ∼50 μM, triggering CICR, mitochondrial Ca2+uptake, and exocytosis at nearby secretory active sites. The fact that protonophores abolish mitochondrial Ca2+uptake, and increase catecholamine release three- to fivefold, support the earlier observation. This increase is probably due to acceleration of vesicle transport from a reserve pool to a ready-release vesicle pool; this transport might be controlled by Ca2+redistribution to the cytoskeleton, through CICR, and/or mitochondrial Ca2+release. We propose that chromaffin cells have developed functional triads that are formed by calcium channels, the ER, and the mitochondria and locally control the [Ca2+]cthat regulate the early and late steps of exocytosis.

Differential Expression of Neuronal Genes Defines Subtypes of Disseminated Neuroblastoma with Favorable and Unfavorable Outcome

PURPOSE

Identification of molecular characteristics of spontaneously regressing stage IVS and progressing stage IV neuroblastoma to improve discrimination of patients with metastatic disease following favorable and unfavorable clinical courses.

EXPERIMENTAL DESIGN

Serial analysis of gene expression profiles were generated from five stage IVS and three stage IV neuroblastoma. Differential expression of candidate genes was evaluated by real-time quantitative reverse transcription-PCR in 76 pretreatment tumor samples (stage IVS n=27 and stage IV n=49). Gene expression-based outcome prediction was determined by Prediction Analysis for Microarrays using 38 tumors as a training set and 38 tumors as a test set.

RESULTS

Comparison of serial analysis of gene expression profiles from stage IV and IVS neuroblastoma revealed approximately 500 differentially expressed transcripts. Genes related to neuronal differentiation were observed more frequently in stage IVS tumors as determined by associating transcripts to Gene Ontology annotations. Forty-one candidate genes were evaluated by quantitative reverse transcription-PCR and 18 were confirmed to be differentially expressed (P<or=0.001). Classification of patients according to expression patterns of these 18 genes using Prediction Analysis for Microarrays discriminated two subgroups with significantly differing event-free survival (96+/-6% versus 40+/-8% at 3 years; P<0.0001) and overall survival (100% versus 72+/-7% at 3 years; P=0.0003). This classifier was the only independent covariate marker in a multivariate analysis considering the variables stage, age, MYCN amplification, and gene signature.

CONCLUSIONS

Spontaneously regressing and progressing metastatic neuroblastoma differ by specific gene expression patterns, indicating distinct levels of neuronal differentiation and allowing for an improved risk estimation of children with disseminated disease.

Role of Phosphoinositide Signaling in the Control of Insulin Exocytosis

Phosphoinositides (PI) are important signaling molecules involved in the regulation of vesicular trafficking. We found that phosphatidylinositol 4-phosphate (PI4P) and phosphatidylinositol 4,5-biphosphate [PI(4,5)P(2)] increase the secretory response triggered by 10 mum Ca(2+) in streptolysin-O-permeabilized insulin-secreting INS-1E cells. In addition, nutrient-induced exocytosis was diminished in intact cells expressing constructs that sequester PI(4,5)P(2) and in cells transfected with constructs that reduce by RNA interference the level of two enzymes involved in PI(4,5)P(2) production, type III PI4-kinase beta and type I phosphatidylinositol 4-bisphosphate 5-kinase-gamma. To clarify the mechanism of action of PI, we investigated the involvement in the regulation of insulin exocytosis of three potential PI targets, phospholipase D1, the Ca(2+)-dependent activator protein for secretion 1, and Munc18-interacting protein 1. Transfection of insulin-secreting cells with plasmids that direct the synthesis of small interfering RNAs capable of reducing the endogenous levels of these proteins inhibited hormone release elicited by glucose- and cAMP-elevating agents without affecting basal release. Our data indicate that the production of PI(4,5)P(2) is necessary for proper control of beta-cell secretion and suggest that at least part of the effect of PI on insulin exocytosis could be exerted through the activation of phospholipase D1, Ca(2+)-dependent activator protein for secretion 1, and Munc18-interacting protein 1.

CAPS1 regulates catecholamine loading of large dense-core vesicles

CAPS1 is thought to play an essential role in mediating exocytosis from large dense-core vesicles (LDCVs). We generated CAPS1-deficient (KO) mice and studied exocytosis in a model system for Ca2+-dependent LDCV secretion, the adrenal chromaffin cell. Adult heterozygous CAPS1 KO cells display a gene dosage-dependent decrease of CAPS1 expression and a concomitant reduction in the number of docked vesicles and secretion. Embryonic homozygous CAPS1 KO cells show a strong reduction in the frequency of amperometrically detectable release events of transmitter-filled vesicles, while the total number of fusing vesicles, as judged by capacitance recordings or total internal reflection microscopy, remains unchanged. We conclude that CAPS1 is required for an essential step in the uptake or storage of catecholamines in LDCVs.

CAPS in search of a lost function

Ca2+-dependent activator protein for secretion (CAPS) is an evolutionarily conserved secretory protein that was previously thought to mediate Ca2+-triggered fusion of dense-core vesicles. In an elegant study of CAPS1-deficient mice, Speidel et al. (this issue of Neuron) now show that CAPS function may have been misunderstood. CAPS appears to act upstream of fusion in the biogenesis or maintenance of mature secretory vesicles, raising the possibility of a completely new type of function for an essential component of the secretory machinery.

Regulation of exocytosis in neurons and neuroendocrine cells

Neurons communicate with one another through the release of molecules from synaptic vesicles and large dense core granules through the process of exocytosis. During exocytosis, molecules are released to the extracellular space through a fusion pore, which can either dilate, resulting in full fusion, or close, resulting in incomplete exocytosis, often referred to as 'kiss and run' exocytosis. Recently, there has been much interest in the regulation of this process in both neurons and neuroendocrine cells. There has been much recent work that addresses the existence of incomplete exocytosis in neurons and neuroendocrine cells, as well as recent work probing the molecular components and modulation of the fusion pore.

CAPS Acts at a Prefusion Step in Dense-Core Vesicle Exocytosis as a PIP2 Binding Protein

CAPS-1 is required for Ca2+-triggered fusion of dense-core vesicles with the plasma membrane, but its site of action and mechanism are unknown. We analyzed the kinetics of Ca2+-triggered exocytosis reconstituted in permeable PC12 cells. CAPS-1 increased the initial rate of Ca2+-triggered vesicle exocytosis by acting at a rate-limiting, Ca2+-dependent prefusion step. CAPS-1 activity depended upon prior ATP-dependent priming during which PIP2 synthesis occurs. CAPS-1 activity and binding to the plasma membrane depended upon PIP2. Ca2+ was ineffective in triggering vesicle fusion in the absence of CAPS-1 but instead promoted desensitization to CAPS-1 resulting from decreased plasma membrane PIP2. We conclude that CAPS-1 functions following ATP-dependent priming as a PIP2 binding protein to enhance Ca2+-dependent DCV exocytosis. Essential prefusion steps in dense-core vesicle exocytosis involve sequential ATP-dependent synthesis of PIP2 and the subsequent PIP2-dependent action of CAPS-1. Regulation of PIP2 levels and CAPS-1 activity would control the secretion of neuropeptides and monoaminergic transmitters.

A Family of Ca2+-Dependent Activator Proteins for Secretion

Ca2+-dependent activator protein for secretion (CAPS) 1 is an essential cytosolic component of the protein machinery involved in large dense-core vesicle (LDCV) exocytosis and in the secretion of a subset of neurotransmitters. In the present study, we report the identification, cloning, and comparative characterization of a second mammalian CAPS isoform, CAPS2. The structure of CAPS2 and its function in LDCV exocytosis from PC12 cells are very similar to those of CAPS1. Both isoforms are strongly expressed in neuroendocrine cells and in the brain. In subcellular fractions of the brain, both CAPS isoforms are enriched in synaptic cytosol fractions and also present on vesicular fractions. In contrast to CAPS1, which is expressed almost exclusively in brain and neuroendocrine tissues, CAPS2 is also expressed in lung, liver, and testis. Within the brain, CAPS2 expression seems to be restricted to certain brain regions and cell populations, whereas CAPS1 expression is strong in all neurons. During development, CAPS2 expression is constant between embryonic day 10 and postnatal day 60, whereas CAPS1 expression is very low before birth and increases after postnatal day 0 to reach a plateau at postnatal day 21. Light microscopic data indicate that both CAPS isoforms are specifically enriched in synaptic terminals. Ultrastructural analyses show that CAPS1 is specifically localized to glutamatergic nerve terminals. We conclude that at the functional level, CAPS2 is largely redundant with CAPS1. Differences in the spatial and temporal expression patterns of the two CAPS isoforms most likely reflect as yet unidentified subtle functional differences required in particular cell types or during a particular developmental period. The abundance of CAPS proteins in synaptic terminals indicates that they may also be important for neuronal functions that are not exclusively related to LDCV exocytosis.

Tuning exocytosis for speed: fast and slow modes

Exocytic fusion reactions triggered by Ca(2+) are widespread in neural, endocrine, exocrine, hemapoetic and perhaps all cell types. These processes exhibit tremendous variation in latencies to fusion following a Ca(2+) rise and in rates of fusion. We review reported differences for synaptic vesicle (SV) and dense-core vesicle (DCV) exocytosis and attempt to identify key features in the molecular mechanisms of docking, priming and fusion of SVs and DCVs that may account for differences in speed.

Differential sensitivity to calcium and osmotic pressure of fast and slow ATP currents at sympathetic varicosities in mouse vas deferens

Secretion of noradrenaline from large dense-core vesicles in chromaffin cells involves both rapid and slow components of exocytosis which are differentially sensitive to changes in external calcium, osmotic pressure and interruption of the interacting SNARE proteins. Electrical signs of secretion of ATP from sympathetic nerve terminals of mouse vas deferens, the excitatory junctional currents (EJCs), also indicate both rapid and slow mechanisms of exocytosis, which might also show such differential sensitivity. We report here that the large and fast EJCs are highly sensitive to changes in extracellular calcium ions whereas the small and slow EJCs are not. Furthermore, the frequency of fast EJCs is accelerated by hypotonic solutions whereas the slow EJCs are accelerated by hypertonic solution. Fast EJCs, but not slow EJCs, are blocked by peptide fragments of alpha-SNAP and syntaxin whereas slow EJCs are not. These observations point to two classes of exocytosis from sympathetic nerve terminals that parallel those of exocytosis from chromaffin cells.

Phosphatidylinositol 4-kinase serves as a metabolic sensor and regulates priming of secretory granules in pancreatic beta cells

Insulin secretion is controlled by the beta cell's metabolic state, and the ability of the secretory granules to undergo exocytosis increases during glucose stimulation in a membrane potential-independent fashion. Here, we demonstrate that exocytosis of insulin-containing secretory granules depends on phosphatidylinositol 4-kinase (PI 4-kinase) activity and that inhibition of this enzyme suppresses glucose-stimulated insulin secretion. Intracellular application of phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate [PI(4,5)P(2)] stimulated exocytosis by promoting the priming of secretory granules for release and increasing the number of granules residing in a readily releasable pool. Reducing the cytoplasmic ADP concentration in a way mimicking the effects of glucose stimulation activated PI 4-kinase and increased exocytosis whereas changes of the ATP concentration in the physiological range had little effect. The PI(4,5)P(2)-binding protein Ca(2+)-dependent activator protein for secretion (CAPS) is present in beta cells, and neutralization of the protein abolished both Ca(2+)- and PI(4,5)P(2)-induced exocytosis. We conclude that ADP-induced changes in PI 4-kinase activity, via generation of PI(4,5)P(2), represents a metabolic sensor in the beta cell by virtue of its capacity to regulate the release competence of the secretory granules.

Secretory granule exocytosis

Regulated exocytosis of secretory granules or dense-core granules has been examined in many well-characterized cell types including neurons, neuroendocrine, endocrine, exocrine, and hemopoietic cells and also in other less well-studied cell types. Secretory granule exocytosis occurs through mechanisms with many aspects in common with synaptic vesicle exocytosis and most likely uses the same basic protein components. Despite the widespread expression and conservation of a core exocytotic machinery, many variations occur in the control of secretory granule exocytosis that are related to the specialized physiological role of particular cell types. In this review we describe the wide range of cell types in which regulated secretory granule exocytosis occurs and assess the evidence for the expression of the conserved fusion machinery in these cells. The signals that trigger and regulate exocytosis are reviewed. Aspects of the control of exocytosis that are specific for secretory granules compared with synaptic vesicles or for particular cell types are described and compared to define the range of accessory control mechanisms that exert their effects on the core exocytotic machinery.

Neurotrophin secretion: current facts and future prospects

The proteins of the mammalian neurotrophin family (nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5)) were originally identified as neuronal survival factors. During the last decade, evidence has accumulated implicating them (especially BDNF) in addition in the regulation of synaptic transmission and synaptogenesis in the CNS. However, a detailed understanding of the secretion of neurotrophins from neurons is required to delineate their role in regulating synaptic function. Some crucial questions that need to be addressed include the sites of neurotrophin secretion (i.e. axonal versus dendritic; synaptic versus extrasynaptic) and the neuronal and synaptic activity patterns that trigger the release of neurotrophins. In this article, we review the current knowledge in the field of neurotrophin secretion, focussing on activity-dependent synaptic release of BDNF. The modality and the site of neurotrophin secretion are dependent on the processing and subsequent targeting of the neurotrophin precursor molecules. Therefore, the available data regarding formation and trafficking of neurotrophins in the secreting neurons are critically reviewed. In addition, we discuss existing evidence that the characteristics of neurotrophin secretion are similar (but not identical) to those of other neuropeptides. Finally, since BDNF has been proposed to play a critical role as an intercellular synaptic messenger in long-term potentiation (LTP) in the hippocampus, we try to reconcile this possible role of BDNF in LTP with the recently described features of synaptic BDNF secretion.

Cloning and characterization of human CADPS and CADPS2, new members of the Ca2+-dependent activator for secretion protein family

The recent identification of some of the components involved in regulated and constitutive exocytotic pathways has yielded important insights into the mechanisms of membrane trafficking and vesicle secretion. To understand precisely the molecular events taking place during vesicle exocytosis, we must identify all of the proteins implicated in these pathways. In this paper we describe the full-length cloning and characterization of human CADPS and CADPS2, two new homologs of the mouse Cadps protein involved in large dense-core vesicle (LDCV)-regulated exocytosis. We show that these two genes have disparate RNA expression patterns, with CADPS restricted to neural and endocrine tissues and CADPS2 expressed ubiquitously. We also identify a C2 domain, a known protein motif involved in calcium and phospholipid interactions, in both CADPS and CADPS2. We propose that CADPS functions as a calcium sensor in regulated exocytosis, whereas CADPS2 acts as a calcium sensor in constitutive vesicle trafficking and secretion. CADPS and CADPS2 were determined to span 475 kb and 561 kb on human chromosomes 3p21.1 and 7q31.3, respectively. The q31-q34 of human chromosome 7 has recently been identified to contain a putative susceptibility locus for autism (AUTS1). The function, expression profile, and location of CADPS2 make it a candidate gene for autism, and thus we conducted mutation screening for all 28 exons in 90 unrelated autistic individuals. We identified several nucleotide substitutions, including only one that would affect the amino acid sequence. No disease-specific variants were identified.

Synaptotagmin I increases the probability of vesicle fusion at low [Ca2+] in pituitary cells

Synaptotagmin I (Syt I), a low-affinity Ca(2+)-binding protein, is thought to serve as the Ca(2+) sensor in the release of neurotransmitter. However, functional studies on the calyx of Held synapse revealed that the rapid release of neurotransmitter requires only approximately micromolar [Ca(2+)], suggesting that Syt I may play a more complex role in determining the high-affinity Ca(2+) dependence of exocytosis. Here we tested this hypothesis by studying pituitary cells, which possess high- and low-affinity Ca(2+)-dependent exocytic pathways and express Syt I. Using patch-clamp capacitance measurements to monitor secretion and the acute antisense deletion of Syt I from differentiated cells, we have shown that the rapid and the most Ca(2+)-sensitive pathway of exocytosis in rat melanotrophs requires Syt I. Furthermore, stimulation of the Ca(2+)-dependent exocytosis by cytosol dialysis with solutions containing 1 microM [Ca(2+)] was completely abolished in the absence of Syt I. Similar results were obtained by the preinjection of antibodies against the CAPS (Ca(2+)-dependent activator protein for secretion) protein. These results indicate that synaptotagmin I and CAPS proteins increase the probability of vesicle fusion at low cytosolic [Ca(2+)].

Emerging roles of presynaptic proteins in Ca++-triggered exocytosis

The twinning of techniques from biophysics and molecular biology has led to remarkable progress in understanding the molecular mechanisms of synaptic transmission. Here we review the current picture of Ca++-triggered exocytosis, which has emerged from studies of a simple cellular model, the adrenal chromaffin cell. We discuss the molecular players that have been assigned a specific role in a particular step of this process and give a brief outlook on what these insights might tell us about mechanisms of short-term plasticity at classical synapses.

The synaptic vesicle cycle: exocytosis and endocytosis in Drosophila and C. elegans

Advances in the study of Drosophila melanogaster and Caenorhabditis elegans have provided key insights into the processes of neurotransmission and neuromodulation. Work in the past year has revealed that Unc-13 and Rab3a-interacting molecule regulate the conformational state of syntaxin to prime synaptic vesicle fusion. Analyses of synaptotagmin support its role as a putative calcium sensor triggering vesicular fusion and highlight the possible role of SNARE complex oligomerization in the fusion mechanism. Characterization of endophilin mutants demonstrates that kiss-and-run endocytosis is a major component of synaptic vesicle recycling. In neuromodulation, dcaps mutants provide the first genetic insight into possible roles of the CAPS protein in mediating dense core vesicle fusion and modulating synaptic vesicle fusion.

Role of CAPS in dense-core vesicle exocytosis

Calcium-dependent activator protein for secretion (CAPS) was initially identified in brain cytosol based on its ability to reconstitute calcium-triggered dense-core vesicle (DCV) exocytosis in permeable cell lines (PC12) of adrenal chromaffin origin. Current evidence indicates that CAPS functions selectively in DCV exocytosis by interacting with DCVs, the plasma membrane, and protein components of the fusion machinery. To further delineate the role of CAPS in endocrine and neural secretion, the tissue distribution of CAPS was determined. Immunoreactive CAPS I localized exclusively to neural and endocrine tissues including adrenal medulla, pancreatic islets, anterior pituitary, thyroid parafollicular C cells, gastrointestinal G cells, renal juxtaglomerular cells, and gray matter throughout the central nervous system. The results are consistent with a widespread functional role of CAPS in the regulated exocytosis of DCVs in the nervous and endocrine systems.

Membrane association domains in Ca2+-dependent activator protein for secretion mediate plasma membrane and dense-core vesicle binding required for Ca2+-dependent exocytosis

Ca2+-dependent activator protein for secretion (CAPS) is a cytosolic protein essential for the Ca2+-dependent fusion of dense-core vesicles (DCVs) with the plasma membrane and the regulated secretion of a subset of neurotransmitters. The mechanism by which CAPS functions in exocytosis and the means by which it associates with target membranes are unknown. We identified two domains in CAPS with distinct membrane-binding properties that were each essential for CAPS activity in regulated exocytosis. The first of these, a centrally located pleckstrin homology domain, exhibited three properties: charge-based binding to acidic phospholipids, binding to plasma membrane but not DCV membrane, and stereoselective binding to phosphatidylinositol 4,5-bisphosphate. Mutagenesis studies revealed that the former two properties but not the latter were essential for CAPS function. The central pleckstrin homology domain may mediate transient CAPS interactions with the plasma membrane during Ca2+-triggered exocytosis. The second membrane association domain comprising distal C-terminal sequences mediated CAPS targeting to and association with neuroendocrine DCVs. The CAPS C-terminal domain was also essential for optimal activity in regulated exocytosis. The presence of two membrane association domains with distinct binding specificities may enable CAPS to bind both target membranes to facilitate DCV-plasma membrane fusion.

Complexin regulates the closure of the fusion pore during regulated vesicle exocytosis

Membrane fusion during exocytosis and throughout the cell is believed to involve members of the SNARE (soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptors) family of proteins. The assembly of these proteins into a four-helix bundle may be part of the driving force for bilayer fusion. Regulated exocytosis in neurons and related cell types is specialized to be fast and Ca(2+)-dependent suggesting the involvement of other regulatory proteins specific for regulated exocytosis. Among these are the complexins, two closely related proteins that bind only to the assembled SNARE complex. We have investigated the function of complexin by analysis of single vesicle release events in adrenal chromaffin cells using carbon fiber amperometry. These cells express complexin II, and overexpression of this protein modified the kinetics of vesicle release events so that their time course was shortened. This effect depended on complexin interaction with the SNARE complex as introduction of a mutation of Arg-59, a residue that interacts with synaptobrevin in the SNARE complex, abolished its effects. The data are consistent with a function for complexin in stabilizing an intermediate of the SNARE complex to allow kiss-and-run recycling of the exocytosed vesicle.

Splitting the quantum: regulation of quantal release during vesicle fusion

One of the most enduring central hypotheses in neurobiology has been that neurotransmission is determined by the quantal release of neurotransmitter. An important assumption has been that quantal size (amount of transmitter released per vesicle) is constant. Recent work has established that, for dense-core granules, quantal size can be varied by stimulation frequency, changes in second messenger levels and modification of the proteins of the exocytotic machinery. These data argue against the long-held belief in the universality of the quantal hypothesis, and raise important but controversial questions as to whether the same mechanisms could also regulate release from synaptic vesicles and contribute to synaptic plasticity.

Ageing changes the cellular basis of the "fight-or-flight" response in human adrenal chromaffin cells

Stress-induced increases in plasma epinephrine in man have been reported to decrease with age. To investigate the possible cellular basis for this decline we determined the characteristics of calcium currents and their relationship to catecholamine secretion in isolated human adrenal chromaffin (AC) cells. Cells derived from young individuals displayed prominent prepulse facilitation of L-type Ca channels but this property was absent in cells from older subjects. Robust quantal secretion in young cells as determined by amperometry was strongly coupled to the activation of these channels with an average delay of only approximately 3 msec. N- and P-type Ca channels also contributed to secretion but were more weakly coupled to catecholamine release sites. Cells from older subjects secreted much less efficiently and showed only weak coupling between Ca channels and secretion. These studies suggest that the magnitude and timing of adrenal secretion changes with age and that the facilitation Ca channel is key to rapid activation of the fight-or-flight response in young individuals.

Triggering and augmentation mechanisms, granule pools, and biphasic insulin secretion

The insulin secretory response by pancreatic beta-cells to an acute "square wave" stimulation by glucose is characterized by a first phase that occurs promptly after exposure to glucose, followed by a decrease to a nadir, and a prolonged second phase. The first phase of release is due to the ATP-sensitive K(+) (K(ATP)) channel-dependent (triggering) pathway that increases [Ca(2+)](i) and has been thought to discharge the granules from a "readily releasable pool." It follows that the second phase entails the preparation of granules for release, perhaps including translocation and priming for fusion competency before exocytosis. The pathways responsible for the second phase include the K(ATP) channel-dependent pathway because of the need for elevated [Ca(2+)](i) and additional signals from K(ATP) channel-independent pathways. The mechanisms underlying these additional signals are unknown. Current hypotheses include increased cytosolic long-chain acyl-CoA, the pyruvate-malate shuttle, glutamate export from mitochondria, and an increased ATP/ADP ratio. In mouse islets, the beta-cell contains some 13,000 granules, of which approximately 100 are in a "readily releasable" pool. Rates of granule release are slow, e.g., one every 3 s, even at the peak of the first phase of glucose-stimulated release. As both phases of glucose-stimulated insulin secretion can be enhanced by agents such as glucagon-like peptide 1, which increases cyclic AMP levels and protein kinase A activity, or acetylcholine, which increases diacylglycerol levels and protein kinase C activity, a single "readily releasable pool" hypothesis is an inadequate explanation for insulin secretion. Multiple pools available for rapid release or rapid conversion of granules to a rapidly releasable state are required.

Quantal size is dependent on stimulation frequency and calcium entry in calf chromaffin cells

To what extent the quantal hypothesis of transmitter release applies to dense-core vesicle (DCV) secretion is unknown. We determined the characteristics of individual secretory events in calf chromaffin cells using catecholamine amperometry combined with different patterns of stimulation. Raising the frequency of action potential trains from 0.25-10 Hz in 2 mM [Ca(2+)]o or [Ca(2+)]o from 0.25-7 mM at 7 Hz elevated the amount released per event (quantal size). With increased stimulation, quantal size rose continuously, not abruptly, suggesting that release efficiency from a single population of DCVs rather than recruitment of different-sized vesicles contributed to the effect. These results suggest that catecholamine secretion does not conform to the quantal model. Inhibition of rapid endocytosis damped secretion in successive episodes, implying an essential role for this process in the recycling of vesicles needed for continuous secretion.

Drosophila CAPS is an essential gene that regulates dense-core vesicle release and synaptic vesicle fusion

Calcium-activated protein for secretion (CAPS) is proposed to play an essential role in Ca2+-regulated dense-core vesicle exocytosis in vertebrate neuroendocrine cells. Here we report the cloning, mutation, and characterization of the Drosophila ortholog (dCAPS). Null dCAPS mutants display locomotory deficits and complete embryonic lethality. The mutant NMJ reveals a 50% loss in evoked glutamatergic transmission, and an accumulation of synaptic vesicles at active zones. Importantly, dCAPS mutants display a highly specific 3-fold accumulation of dense-core vesicles in synaptic terminals, which was not observed in mutants that completely arrest synaptic vesicle exocytosis. Targeted transgenic CAPS expression in identified motoneurons fails to rescue dCAPS neurotransmission defects, demonstrating a cell nonautonomous role in synaptic vesicle fusion. We conclude that dCAPS is required for dense-core vesicle release and that a dCAPS-dependent mechanism modulates synaptic vesicle release at glutamatergic synapses.

A double-labeled preparation for simultaneous measurement of [3H]-noradrenaline and [14C]-glutamic acid exocytosis from streptolysin-O (SLO)-perforated synaptosomes

We have developed a novel method to examine [3H]-noradrenaline and [14C]-glutamate release from the same sample of streptolysin-O (SLO) perforated rat cortical synaptosomes. Ca2+ -dependent [3H]-noradrenaline and [14C]-glutamate release was examined at different temperatures and was found to be greater at 30 degrees C than at 25 degrees C. Ca2+ -dependent release of [3H]-noradrenaline is more ATP dependent than Ca2+ -dependent release of [14C]-glutamate. No significant reuptake of either neurotransmitter by the perforated synaptosomes was detected, indicating all the synaptosomes were indeed perforated. Incubations with 1 mM ouabain, a specific Na+,K+ -ATPase inhibitor, slightly increased Ca2+ -dependent release of both neurotransmitters. [3H]-noradrenaline is released from large dense-core vesicles and [14C]-glutamate is released from small clear synaptic vesicles, so one can directly compare and contrast neurotransmitter release mechanisms between large dense-core vesicles and small clear synaptic vesicles using this preparation.

Calcium in ciliated protozoa: sources, regulation, and calcium-regulated cell functions

In ciliates, a variety of processes are regulated by Ca2+, e.g., exocytosis, endocytosis, ciliary beat, cell contraction, and nuclear migration. Differential microdomain regulation may occur by activation of specific channels in different cell regions (e.g., voltage-dependent Ca2+ channels in cilia), by local, nonpropagated activation of subplasmalemmal Ca stores (alveolar sacs), by different sensitivity thresholds, and eventually by interplay with additional second messengers (cilia). During stimulus-secretion coupling, Ca2+ as the only known second messenger operates at approximately 5 microM, whereby mobilization from alveolar sacs is superimposed by "store-operated Ca2+ influx" (SOC), to drive exocytotic and endocytotic membrane fusion. (Content discharge requires binding of extracellular Ca2+ to some secretory proteins.) Ca2+ homeostasis is reestablished by binding to cytosolic Ca2+-binding proteins (e.g., calmodulin), by sequestration into mitochondria (perhaps by Ca2+ uniporter) and into endoplasmic reticulum and alveolar sacs (with a SERCA-type pump), and by extrusion via a plasmalemmal Ca2+ pump and a Na+/Ca2+ exchanger. Comparison of free vs total concentration, [Ca2+] vs [Ca], during activation, using time-resolved fluorochrome analysis and X-ray microanalysis, respectively, reveals that altogether activation requires a calcium flux that is orders of magnitude larger than that expected from the [Ca2+] actually required for local activation.

Electron microscopic evidence for multiple types of secretory vesicles in bovine chromaffin cells

It has been previously shown that the neuron-like chromaffin cells from the bovine adrenal medulla are heterogeneous. Among other differences, the cells also differed in secretory vesicles represented in their cytoplasm. The present study investigates the types of secretory vesicles in bovine chromaffin cells by electron microscopy. Morphometric analysis revealed five types of electron-dense secretory vesicles in chromaffin cells. These were as follows: elementary large catecholamine-storing chromaffin granules of rounded shape, large dense core vesicles of ovoid and rod-like shapes, small dense core vesicles as well as ribosome-coated vesicles of intermediate density. Among the electron-lucent vesicles there were small synaptic-like microvesicles, endocytotic clathrin-coated vesicles, growth cone vesicles, and emptied large light core vesicles. The structural and functional backgrounds of different types of secretory vesicles are described, focusing on their formation and potential role.