A dynamic pool of calcium in catecholamine storage vesicles. Exploration in living cells by a novel vesicle-targeted chromogranin A-aequorin chimeric photoprotein

Chromogranin A and its derived peptides: potential regulators of cholesterol homeostasis

Chromogranin A (CHGA), a member of the granin family of proteins, has been an attractive therapeutic target and candidate biomarker for several cardiovascular, neurological, and inflammatory disorders. The prominence of CHGA stems from the pleiotropic roles of several bioactive peptides (e.g., catestatin, pancreastatin, vasostatins) generated by its proteolytic cleavage and by their wide anatomical distribution. These peptides are emerging as novel modulators of cardiometabolic diseases that are often linked to high blood cholesterol levels. However, their impact on cholesterol homeostasis is poorly understood. The dynamic nature of cholesterol and its multitudinous roles in almost every aspect of normal body function makes it an integral component of metabolic physiology. A tightly regulated coordination of cholesterol homeostasis is imperative for proper functioning of cellular and metabolic processes. The deregulation of cholesterol levels can result in several pathophysiological states. Although studies till date suggest regulatory roles for CHGA and its derived peptides on cholesterol levels, the mechanisms by which this is achieved still remain unclear. This review aims to aggregate and consolidate the available evidence linking CHGA with cholesterol homeostasis in health and disease. In addition, we also look at common molecular regulatory factors (viz., transcription factors and microRNAs) which could govern the expression of CHGA and genes involved in cholesterol homeostasis under basal and pathological conditions. In order to gain further insights into the pathways mediating cholesterol regulation by CHGA/its derived peptides, a few prospective signaling pathways are explored, which could act as primers for future studies.

Use of aequorin-based indicators for monitoring Ca2+ in acidic organelles

Over the last years, there is accumulating evidence that acidic organelles can accumulate and release Ca²⁺ upon cell activation. Hence, reliable recording of Ca²⁺ dynamics in these compartments is essential for understanding the physiopathological aspects of acidic organelles. Genetically encoded Ca²⁺ indicators (GECIs) are valuable tools to monitor Ca²⁺ in specific locations, although their use in acidic compartments is challenging due to the pH sensitivity of most available fluorescent GECIs. By contrast, bioluminescent GECIs have a combination of features (marginal pH sensitivity, low background, no phototoxicity, no photobleaching, high dynamic range and tunable affinity) that render them advantageous to achieve an enhanced signal-to-noise ratio in acidic compartments. This article reviews the use of bioluminescent aequorin-based GECIs targeted to acidic compartments. A need for more measurements in highly acidic compartments is identified.

Systematic Comparison of Vesicular Targeting Signals Leads to the Development of Genetically Encoded Vesicular Fluorescent Zn2+ and pH Sensors

Genetically encoded fluorescent sensors have been widely used to illuminate secretory vesicle dynamics and the vesicular lumen, including Zn²⁺ and pH, in living cells. However, vesicular sensors have a tendency to mislocalize and are susceptible to the acidic intraluminal pH. In this study, we performed a systematic comparison of five different vesicular proteins to target the fluorescent protein mCherry and a Zn²⁺ Förster resonance energy transfer (FRET) sensor to secretory vesicles. We found that motifs derived from vesicular cargo proteins, including chromogranin A (CgA), target vesicular puncta with greater efficacy than transmembrane proteins. To characterize vesicular Zn²⁺ levels, we developed CgA-Zn²⁺ FRET sensor fusions with existing sensors ZapCY1 and eCALWY-4 and characterized subcellular localization and the influence of pH on sensor performance. We simultaneously monitored Zn²⁺ and pH in individual secretory vesicles by leveraging the acceptor fluorescent protein as a pH sensor and found that pH influenced FRET measurements in situ. While unable to characterize vesicular Zn²⁺ at the single-vesicle level, we were able to monitor Zn²⁺ dynamics in populations of vesicles and detected high vesicular Zn²⁺ in MIN6 cells compared to lower levels in the prostate cancer cell line LnCaP. The combination of CgA-ZapCY1 and CgA-eCALWY-4 allows for measurement of Zn²⁺ from pM to nM ranges.

Microcalcifications, calcium-sensing receptor, and cancer

Calcium stones and calculi are observed in numerous human tissues. They are the result of deposition of calcium salts and are due to high local calcium concentrations. Prostatic calculi are usually classified as endogenous or extrinsic stones. Endogenous stones are commonly caused by obstruction of the prostatic ducts around an enlarged prostate resulting from benign prostatic hyperplasia or from chronic inflammation. The latter occurs mainly around the urethra and is generally caused by reflux of urine into the prostate. Calcium concentrations higher than in the plasma at sites of infection may induce the chemotactic response that eventually leads to recruitment of inflammatory cells. The calcium sensing receptor (CaSR) may be crucial for this recruitment as its expression and activity are increased by cytokines such as IL-6 and high extracellular calcium concentrations, respectively. The links between calcium calculi, inflammation, calcium supplementation, and CaSR functions in prostate cancer patients will be discussed in this review.

Mice overexpressing chromogranin A display hypergranulogenic adrenal glands with attenuated ATP levels contributing to the hypertensive phenotype

OBJECTIVE

Elevated circulating chromogranin A (CHGA) is observed in human hypertension. CHGA is critical for granulogenesis and exocytosis of catecholamine stores from secretory large dense core vesicles (LDCV). This study aims to understand the morphological, molecular and phenotypic changes because of excess CHGA and the mechanistic link eventuating in hyper-adrenergic hypertension.

METHODS

Blood pressure and heart rate was monitored in mouse models expressing normal and elevated level of CHGA by telemetry. Catecholamine and oxidative stress radicals were measured. Adrenal ultrastructure, LDCV content and mitochondrial abundance were compared and respiration analyzed by Seahorse assay. Effect of CHGA dosage on adrenal ATP content, electron transport chain components and uncoupling protein 2 (UCP-2) were compared in vivo and in vitro.

RESULTS

Mice with excess-CHGA displayed hypertensive phenotype, higher heart rate and increased sympathetic tone. They had elevated plasma catecholamine and adrenal ROS levels. Excess-CHGA caused an increase in size and abundance of LDCV and adrenal mitochondria. Nonetheless, they had attenuated levels of ATP. Isolated adrenal mitochondria from mice with elevated CHGA showed higher maximal respiration rates in the presence of protonophore, which uncouples oxidative phosphorylation. Elevated CHGA resulted in overexpression of UCP2 and diminished ATP. In vitro in chromaffin cells overexpressing CHGA, concomitant increase in UCP2 protein and decreased ATP was detected.

CONCLUSION

Elevated CHGA expression resulted in underlying bioenergetic dysfunction in ATP production despite higher mitochondrial mass. The outcome was unregulated negative feedback of LDCV exocytosis and secretion, resulting in elevated levels of circulating catecholamine and consequently the hypertensive phenotype.

Combined targeted Omic and Functional Assays Identify Phospholipases A₂ that Regulate Docking/Priming in Calcium-Triggered Exocytosis

The fundamental molecular mechanism underlying the membrane merger steps of regulated exocytosis is highly conserved across cell types. Although involvement of Phospholipase A₂ (PLA₂) in regulated exocytosis has long been suggested, its function or that of its metabolites-a lyso-phospholipid and a free fatty acid-remain somewhat speculative. Here, using a combined bioinformatics and top-down discovery proteomics approach, coupled with lipidomic analyses, PLA₂ were found to be associated with release-ready cortical secretory vesicles (CV) that possess the minimal molecular machinery for docking, Ca²⁺ sensing and membrane fusion. Tightly coupling the molecular analyses with well-established quantitative fusion assays, we show for the first time that inhibition of a CV surface calcium independent intracellular PLA₂ and a luminal secretory PLA₂ significantly reduce docking/priming in the late steps of regulated exocytosis, indicating key regulatory roles in the critical step(s) preceding membrane merger.

Fluorescent Proteins for Investigating Biological Events in Acidic Environments

The interior lumen of acidic organelles (e.g., endosomes, secretory granules, lysosomes and plant vacuoles) is an important platform for modification, transport and degradation of biomolecules as well as signal transduction, which remains challenging to investigate using conventional fluorescent proteins (FPs). Due to the highly acidic luminal environment (pH ~ 4.5⁻6.0), most FPs and related sensors are apt to lose their fluorescence. To address the need to image in acidic environments, several research groups have developed acid-tolerant FPs in a wide color range. Furthermore, the engineering of pH insensitive sensors, and their concomitant use with pH sensitive sensors for the purpose of pH-calibration has enabled characterization of the role of luminal ions. In this short review, we summarize the recent development of acid-tolerant FPs and related functional sensors and discuss the future prospects for this field.

Using aequorin probes to measure Ca2+ in intracellular organelles

Aequorins are excellent tools for measuring intra-organellar Ca²⁺ and assessing its role in physiological and pathological functions. Here we review targeting strategies to express aequorins in various organelles. We address critical topics such as probe affinity tuning as well as normalization and calibration of the signal. We also focus on bioluminescent Ca²⁺ imaging in nucleus or mitochondria of living cells. Finally, recent advances with a new chimeric GFP-aequorin protein (GAP), which can be used either as luminescent or fluorescent Ca²⁺ probe, are presented. GAP is robustly expressed in transgenic flies and mice, where it has proven to be a suitable Ca²⁺ indicator for monitoring physiological Ca²⁺ signaling ex vivo and in vivo.

Impact of Chromogranin A deficiency on catecholamine storage, catecholamine granule morphology and chromaffin cell energy metabolism in vivo

Chromogranin A (CgA) is a prohormone and granulogenic factor in neuroendocrine tissues with a regulated secretory pathway. The impact of CgA depletion on secretory granule formation has been previously demonstrated in cell culture. However, studies linking the structural effects of CgA deficiency with secretory performance and cell metabolism in the adrenomedullary chromaffin cells in vivo have not previously been reported. Adrenomedullary content of the secreted adrenal catecholamines norepinephrine (NE) and epinephrine (EPI) was decreased 30-40 % in Chga-KO mice. Quantification of NE and EPI-storing dense core (DC) vesicles (DCV) revealed decreased DCV numbers in chromaffin cells in Chga-KO mice. For both cell types, the DCV diameter in Chga-KO mice was less (100-200 nm) than in WT mice (200-350 nm). The volume density of the vesicle and vesicle number was also lower in Chga-KO mice. Chga-KO mice showed an ~47 % increase in DCV/DC ratio, implying vesicle swelling due to increased osmotically active free catecholamines. Upon challenge with 2 U/kg insulin, there was a diminution in adrenomedullary EPI, no change in NE and a very large increase in the EPI and NE precursor dopamine (DA), consistent with increased catecholamine biosynthesis during prolonged secretion. We found dilated mitochondrial cristae, endoplasmic reticulum and Golgi complex, as well as increased synaptic mitochondria, synaptic vesicles and glycogen granules in Chga-KO mice compared to WT mice, suggesting that decreased granulogenesis and catecholamine storage in CgA-deficient mouse adrenal medulla is compensated by increased VMAT-dependent catecholamine update into storage vesicles, at the expense of enhanced energy expenditure by the chromaffin cell.

Functional ryanodine receptors in the membranes of neurohypophysial secretory granules

Highly localized Ca(2+) release events have been characterized in several neuronal preparations. In mouse neurohypophysial terminals (NHTs), such events, called Ca(2+) syntillas, appear to emanate from a ryanodine-sensitive intracellular Ca(2+) pool. Traditional sources of intracellular Ca(2+) appear to be lacking in NHTs. Thus, we have tested the hypothesis that large dense core vesicles (LDCVs), which contain a substantial amount of calcium, represent the source of these syntillas. Here, using fluorescence immunolabeling and immunogold-labeled electron micrographs of NHTs, we show that type 2 ryanodine receptors (RyRs) are localized specifically to LDCVs. Furthermore, a large conductance nonspecific cation channel, which was identified previously in the vesicle membrane and has biophysical properties similar to that of an RyR, is pharmacologically affected in a manner characteristic of an RyR: it is activated in the presence of the RyR agonist ryanodine (at low concentrations) and blocked by the RyR antagonist ruthenium red. Additionally, neuropeptide release experiments show that these same RyR agonists and antagonists modulate Ca(2+)-elicited neuropeptide release from permeabilized NHTs. Furthermore, amperometric recording of spontaneous release events from artificial transmitter-loaded terminals corroborated these ryanodine effects. Collectively, our findings suggest that RyR-dependent syntillas could represent mobilization of Ca(2+) from vesicular stores. Such localized vesicular Ca(2+) release events at the precise location of exocytosis could provide a Ca(2+) amplification mechanism capable of modulating neuropeptide release physiologically.

Observations of calcium dynamics in cortical secretory vesicles

Calcium (Ca(2+)) dynamics were evaluated in fluorescently labeled sea urchin secretory vesicles using confocal microscopy. 71% of the vesicles examined exhibited one or more transient increases in the fluorescence signal that was damped in time. The detection of transient increases in signal was dependent upon the affinity of the fluorescence indicator; the free Ca(2+) concentration in the secretory vesicles was estimated to be in the range of ∼10 to 100 μM. Non-linear stochastic analysis revealed the presence of extra variance in the Ca(2+) dependent fluorescence signal. This noise process increased linearly with the amplitude of the Ca(2+) signal. Both the magnitude and spatial properties of this noise process were dependent upon the activity of vesicle p-type (Ca(v)2.1) Ca(2+) channels. Blocking the p-type Ca(2+) channels with ω-agatoxin decreased signal variance, and altered the spatial noise pattern within the vesicle. These fluorescence signal properties are consistent with vesicle Ca(2+) dynamics and not simply due to obvious physical properties such as gross movement artifacts or pH driven changes in Ca(2+) indicator fluorescence. The results suggest that the free Ca(2+) content of cortical secretory vesicles is dynamic; this property may modulate the exocytotic fusion process.

Intracellular organelles in the saga of Ca2+ homeostasis: different molecules for different purposes?

An increase in the concentration of cytosolic free Ca(2+) is a key component regulating different cellular processes ranging from egg fertilization, active secretion and movement, to cell differentiation and death. The multitude of phenomena modulated by Ca(2+), however, do not simply rely on increases/decreases in its concentration, but also on specific timing, shape and sub-cellular localization of its signals that, combined together, provide a huge versatility in Ca(2+) signaling. Intracellular organelles and their Ca(2+) handling machineries exert key roles in this complex and precise mechanism, and this review will try to depict a map of Ca(2+) routes inside cells, highlighting the uniqueness of the different Ca(2+) toolkit components and the complexity of the interactions between them.

Vesicular Ca(2+) mediates granule motion and exocytosis

Secretory vesicles of chromaffin cells are acidic organelles that maintain an increasing pH gradient towards the cytosol (5.5 vs. 7.3) that is mediated by V-ATPase activity. This gradient is primarily responsible for the accumulation of large concentrations of amines and Ca(2+), although the mechanisms mediating Ca(2+) uptake and release from granules, and the physiological relevance of these processes, remain unclear. The presence of a vesicular matrix appears to create a bi-compartmentalised medium in which the major fractions of solutes, including catecholamines, nucleotides and Ca(2+), are strongly associated with vesicle proteins, particularly chromogranins. This association appears to be favoured at acidic pH values. It has been demonstrated that disrupting the pH gradient of secretory vesicles reduces their rate of exocytosis and promotes the leakage of vesicular amines and Ca(2+), dramatically increasing the movement of secretory vesicles and triggering exocytosis. In this short review, we will discuss the data available that highlights the importance of pH in regulating the association between chromogranins, vesicular amines and Ca(2+). We will also address the potential role of vesicular Ca(2+) in two major processes in secretory cells, vesicle movement and exocytosis.

Arachidonic acid mobilizes Ca2+ from the endoplasmic reticulum and an acidic store in rat pancreatic β cells

In rat pancreatic β cells, arachidonic acid (AA) triggered intracellular Ca(2+) release. This effect could be mimicked by eicosatetraynoic acid, indicating that AA metabolism is not required. The AA-mediated Ca(2+) signal was not affected by inhibition of ryanodine receptors or emptying of ryanodine-sensitive store but was reduced by ∼70% following the disruption of acidic stores (treatment with bafilomycin A1 or glycyl-phenylalanyl-β-naphthylamide (GPN)). The action of AA did not involve TRPM2 channels or NAADP receptors because intracellular dialysis of adenosine diphosphoribose (ADPR; an activator of TRPM2 channels) or NAADP did not affect the AA response. In contrast, stimulation of IP(3) receptors via intracellular dialysis of adenophostin A, or exogenous application of ATP largely abolished the AA-mediated Ca(2+) signal. Intracellular dialysis of heparin abolished the ATP-mediated Ca(2+) signal but not the AA response, suggesting that the action of AA did not involve the IP(3)-binding site. Treatment with the SERCA pump inhibitor, thapsigargin, reduced the amplitude of the AA-mediated Ca(2+) signal by ∼70%. Overall, our finding suggests that AA mobilizes Ca(2+) from the endoplasmic reticulum as well as an acidic store and both stores could be depleted by IP(3) receptor agonist. The possibility of secretory granules as targets of AA is discussed.

Molecular mechanisms of endolysosomal Ca2+ signalling in health and disease

Endosomes, lysosomes and lysosome-related organelles are emerging as important Ca2+ storage cellular compartments with a central role in intracellular Ca2+ signalling. Endocytosis at the plasma membrane forms endosomal vesicles which mature to late endosomes and culminate in lysosomal biogenesis. During this process, acquisition of different ion channels and transporters progressively changes the endolysosomal luminal ionic environment (e.g. pH and Ca2+) to regulate enzyme activities, membrane fusion/fission and organellar ion fluxes, and defects in these can result in disease. In the present review we focus on the physiology of the inter-related transport mechanisms of Ca2+ and H+ across endolysosomal membranes. In particular, we discuss the role of the Ca2+-mobilizing messenger NAADP (nicotinic acid adenine dinucleotide phosphate) as a major regulator of Ca2+ release from endolysosomes, and the recent discovery of an endolysosomal channel family, the TPCs (two-pore channels), as its principal intracellular targets. Recent molecular studies of endolysosomal Ca2+ physiology and its regulation by NAADP-gated TPCs are providing exciting new insights into the mechanisms of Ca2+-signal initiation that control a wide range of cellular processes and play a role in disease. These developments underscore a new central role for the endolysosomal system in cellular Ca2+ regulation and signalling.

Langendorff heart: a model system to study cardiovascular effects of engineered nanoparticles

Engineered nanoparticles (ENPs) are produced and used in increasing quantities for industrial products, food, and drugs. The fate of ENPs after usage and impact on health is less known. Especially as air pollution, suspended nanoparticles have raised some attention, causing diseases of the lung and cardiovascular system. Human health risks may arise from inhalation of ENPs with associated inflammation, dispersion in the body, and exposure of vulnerable organs (e.g., heart, brain) and tissues with associated toxicity. However, underlying mechanisms are largely unknown. Furthermore future use of ENPs in therapeutic applications is being researched. Therefore knowledge about potential cardiovascular risks due to exposure to ENPs is highly demanded, but there are no established biological testing models yet. Therefore, we established the isolated beating heart (Langendorff heart) as a model system to study cardiovascular effects of ENPs. This model enables observation and analysis of electrophysiological parameters over a minimal time period of 4 h without influence by systemic effects and allows the determination of stimulated release of substances under influence of ENPs. We found a significant dose and material dependent increase in heart rate accompanied by arrhythmia evoked by ENPs made of flame soot (Printex 90), spark discharge generated soot, anatas (TiO(2)), and silicon dioxide (SiO(2)). However, flame derived SiO(2) (Aerosil) and monodisperse polystyrene lattices exhibited no effects. The increase in heart rate is assigned to catecholamine release from adrenergic nerve endings within the heart. We propose the isolated Langendorff heart and its electrophysiological characterization as a suitable test model for studying cardiovascular ENP toxicity.

On the role of intravesicular calcium in the motion and exocytosis of secretory organelles

Secretory vesicles of sympathetic neurons and chromaffin granules maintain a pH gradient towards the cytosol (5.5 vs. 7.2) promoted by the V-ATPase activity. This gradient of pH is mainly responsible for the accumulation of amines. The secretory vesicles contain large amounts of total Ca(2+), but the free intragranular [Ca(2+)], the mechanisms for Ca(2+) uptake and release from the granules and their physiological relevance regarding exocytosis are still matters of debate.We have recently shown that disruption of the pH gradient of secretory vesicles slowed down exocytosis. Fluorimetric measurements, using the dye Oregon green BAPTA-2, showed that the V-ATPase inhibitor bafilomycin A1 directly released Ca(2+) from freshly isolated vesicles. Accordingly, vesicle alkalinization released Ca(2+) from the granules to the cytosol, measured with fura-2 in intact chromaffin cells. Using TIRFM in cells overexpressing the EGFP-labeled synaptobrevin (VAMP2-EGFP) protein, we have then shown that the Ca(2+) released from the vesicles to the cytosol in the presence of bafilomycin, dramatically increased the granule motion of chromaffin- or PC12-derived granules, and triggered exocytosis (measured by amperometry).We conclude that the gradient of pH of secretory vesicles might be involved in the homeostatic regulation of the local cytosolic Ca(2+) around the vesicles and in two of the major functions of secretory cells, vesicle motion and exocytosis.1.

Dramatic changes in catestatin are associated with hemodynamics in acute myocardial infarction

Acute myocardial infarction (AMI) is characterized by complex neuroendocrine activation. To investigate catestatin profiles, serial catestatin levels were determined by enzyme-linked immunosorbent assay in the first week after AMI in 50 patients. Catestatin levels reduced at admission and negatively correlated with heart rates; it increased significantly on the third day but remained decreased at 1 week and positively with blood pressure. In a subgroup of 20 patients admitted within 4 h after onset, circulating catestatin correlated inversely with norepinephrine. Catestatin might be involved in the course of AMI and act as a tool in monitoring the progression of AMI.

Role of annexin A6 isoforms in catecholamine secretion by PC12 cells: distinct influence on calcium response

Noradrenaline and adrenaline are secreted by adrenal medulla chromaffin cells via exocytosis. Exocytosis of catecholamines occurs after cell stimulation with various endogenous activators such as nicotine or after depolarization of the plasma membrane and is regulated by calcium ions. Cytosolic [Ca(2+)] increases in response to cell excitation and triggers a signal-initiated secretion. Annexins are known to participate in the regulation of membrane dynamics and are also considered to be involved in vesicular trafficking. Some experimental evidence suggests that annexins may participate in Ca(2+)-regulated catecholamine secretion. In this report the effect of annexin A6 (AnxA6) isoforms 1 and 2 on catecholamine secretion has been described. Overexpression of AnxA6 isoforms and AnxA6 knock-down in PC12 cells were accompanied by almost complete inhibition or a 20% enhancement of dopamine secretion, respectively. AnxA6-1 and AnxA6-2 overexpression reduced Delta[Ca(2+)](c) upon depolarization by 32% and 58%, respectively, while AnxA6 knock-down increased Delta[Ca(2+)](c) by 44%. The mechanism of AnxA6 action on Ca(2+) signalling is not well understood. Experimental evidence suggests that two AnxA6 isoforms interact with different targets engaged in regulation of calcium homeostasis in PC12 cells.

Role of secretory granules in inositol 1,4,5-trisphosphate-dependent Ca(2+) signaling: from phytoplankton to mammals

The majority of secretory cell calcium is stored in secretory granules that serve as the major IP(3)-dependent intracellular Ca(2+) store. Even in unicellular phytoplankton secretory granules are responsible for the IP(3)-induced Ca(2+) release that triggers exocytosis. The number of secretory granules in the cell is directly related not only to the magnitude of IP(3)-induced Ca(2+) release, which accounts for the majority of the IP(3)-induced cytoplasmic Ca(2+) release in neuroendocrine cells, but also to the IP(3) sensitivity of the cytoplasmic IP(3) receptor (IP(3)R)/Ca(2+) channels. Moreover, secretory granules contain the highest IP(3)R concentrations and the largest amounts of IP(3)Rs in any subcellular organelles in neuroendocrine cells. Secretory granules from phytoplankton to mammals contain large amounts of polyanionic molecules, chromogranins being the major molecules in mammals, in addition to acidic intragranular pH and high Ca(2+) concentrations. The polyanionic molecules undergo pH- and Ca(2+)-dependent conformational changes that serve as a molecular basis for condensation-decondensation phase transitions of the intragranular matrix. Likewise, chromogranins undergo pH- and Ca(2+)-dependent conformational changes with increased exposure of the structure and increased interactions with Ca(2+) and other granule components at acidic pH. The unique physico-chemical properties of polyanionic molecules appear to be at the center of biogenesis, and physiological functions of secretory granules in living organisms from primitive to advanced species.

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.

Acidic calcium stores open for business: expanding the potential for intracellular Ca2+ signaling

Changes in cytosolic calcium concentration are crucial for a variety of cellular processes in all cells. It has long been appreciated that calcium is stored and released from intracellular calcium stores such as the endoplasmic reticulum. However, emerging evidence indicates that calcium is also dynamically regulated by a seemingly disparate collection of acidic organelles. In this paper, we review the defining features of these 'acidic calcium stores' and highlight recent progress in understanding the mechanisms of uptake and release of calcium from these stores. We also examine the nature of calcium buffering within the stores, and summarize the physiological and pathophysiological significance of these ubiquitous organelles in calcium signaling.

Chromogranin A: a novel susceptibility gene for essential hypertension

Chromogranin A (CHGA) is ubiquitously expressed in secretory cells of the endocrine, neuroendocrine, and neuronal tissues. Although this protein has long been known as a marker for neuroendocrine tumors, its role in cardiovascular disease states including essential hypertension (EH) has only recently been recognized. It acts as a prohormone giving rise to bioactive peptides such as vasostatin-I (human CHGA(1-76)) and catestatin (human CHGA(352-372)) that exhibit several cardiovascular regulatory functions. CHGA is over-expressed but catestatin is diminished in EH. Moreover, genetic variants in the promoter, catestatin, and 3'-untranslated regions of the human CHGA gene alter autonomic activity and blood pressure. Consistent with these findings, targeted ablation of this gene causes severe arterial hypertension and ventricular hypertrophy in mice. Transgenic expression of the human CHGA gene or exogenous administration of catestatin restores blood pressure in these mice. Thus, the accumulated evidence establishes CHGA as a novel susceptibility gene for EH.

Secretory granules in inositol 1,4,5-trisphosphate-dependent Ca2+ signaling in the cytoplasm of neuroendocrine cells

Of all the intracellular organelles, secretory granules contain by far the highest calcium concentration; secretory granules of typical neuroendocrine chromaffin cells contain approximately 40 mM Ca(2+) and occupy approximately 20% cell volume, accounting for >60% of total cellular calcium. They also contain the majority of cellular inositol 1,4,5-trisphosphate receptors (IP(3)Rs) in addition to the presence of >2 mM of chromogranins A and B that function as high-capacity, low-affinity Ca(2+) storage proteins. Chromogranins A and B also interact with the IP(3)Rs and activate the IP(3)R/Ca(2+) channels. In experiments with both neuroendocrine PC12 and nonneuroendocrine NIH3T3 cells, in which the number of secretory granules present was changed by either suppression or induction of secretory granule formation, secretory granules were demonstrated to account for >70% of the IP(3)-induced Ca(2+) releases in the cytoplasm. Moreover, the IP(3) sensitivity of secretory granule IP(3)R/Ca(2+) channels is at least approximately 6- to 7-fold more sensitive than those of the endoplasmic reticulum, thus enabling secretory granules to release Ca(2+) ahead of the endoplasmic reticulum. Further, there is a direct correlation between the number of secretory granules and the IP(3) sensitivity of cytoplasmic IP(3)R/Ca(2+) channels and the increased ratio of IP(3)-induced cytoplasmic Ca(2+) release, highlighting the importance of secretory granules in the IP(3)-dependent Ca(2+) signaling. Given that secretory granules are present in all secretory cells, these results presage critical roles of secretory granules in the control of cytoplasmic Ca(2+) concentrations in other secretory cells.-Yoo, S. H. Secretory granules in inositol 1,4,5-trisphosphate-dependent Ca(2+) signaling in the cytoplasm of neuroendocrine cells.

Proteolytic fragments of chromogranins A and B represent major soluble components of chromaffin granules, illustrated by two-dimensional proteomics with NH(2)-terminal Edman peptide sequencing and MALDI-TOF MS

Neuroendocrine chromaffin granules of adrenal medulla represent regulated secretory vesicles that secrete neuropeptides and catecholamines which mediate cell-cell communication for physiological functions. This study addressed the identification of the major proteins in these secretory vesicles that provide dynamic storage and secretion of bioactive molecules. Proteins of the soluble compartment of the vesicles were separated by two-dimensional gels and subjected to NH(2)-terminal Edman sequencing for identification and determination of NH(2)-termini. Results showed that proteolytic fragments of chromogranin A (CgA) and chromogranin B (CgB) represent the major proteins of these secretory vesicles. These fragments resulted from cleavage of their respective precursor proteins at dibasic and monobasic sites, which is consistent with the known cleavage specificities of prohormone processing enzymes. MALDI-TOF MS analyses of protein spots similar in molecular weight that possessed a range of pI values were represented by molecular forms of CgA and CgB proteins. These findings indicate the high prevalence of endogenous CgA and CgB proteolytic fragments that function in chromaffin secretory vesicles for release of bioactive molecules for cell-cell communication.

Calcium dynamics in bovine adrenal medulla chromaffin cell secretory granules

The secretory granules constitute one of the less well-known compartments in terms of Ca2+ dynamics. They contain large amounts of total Ca2+, but the free intragranular [Ca2+] ([Ca2+]SG), the mechanisms for Ca2+ uptake and release from the granules and their physiological significance regarding exocytosis are still matters of debate. We used in the present work an aequorin chimera targeted to the granules to investigate [Ca2+]SG homeostasis in bovine adrenal chromaffin cells. We found that most of the intracellular aequorin chimera is present in a compartment with 50-100 microM Ca2+. Ca2+ accumulation into this compartment takes place mainly through an ATP-dependent mechanism, namely, a thapsigargin-sensitive Ca2+-ATPase. In addition, fast Ca2+ release was observed in permeabilized cells after addition of inositol 1,4,5-trisphosphate (InsP3) or caffeine, suggesting the presence of InsP3 and ryanodine receptors in the vesicular membrane. Stimulation of intact cells with the InsP3-producing agonist histamine or with caffeine also induced Ca2+ release from the vesicles, whereas acetylcholine or high-[K+] depolarization induced biphasic changes in vesicular[Ca2+], suggesting heterogeneous responses of different vesicle populations, some of them releasing and some taking up Ca2+during stimulation. In conclusion, our data show that chromaffin cell secretory granules have the machinery required for rapid uptake and release of Ca2+, and this strongly supports the hypothesis that granular Ca2+ may contribute to its own secretion.

Modulation of calcium signalling by intracellular organelles seen with targeted aequorins

The cytosolic Ca(2+) signals that trigger cell responses occur either as localized domains of high Ca(2+) concentration or as propagating Ca(2+) waves. Cytoplasmic organelles, taking up or releasing Ca(2+) to the cytosol, shape the cytosolic signals. On the other hand, Ca(2+) concentration inside organelles is also important in physiology and pathophysiology. Comprehensive study of these matters requires to measure [Ca(2+)] inside organelles and at the relevant cytosolic domains. Aequorins, the best-known chemiluminescent Ca(2+) probes, are excellent for this end as they do not require stressing illumination, have a large dynamic range and a sharp Ca(2+)-dependence, can be targeted to the appropriate location and engineered to have the proper Ca(2+) affinity. Using this methodology, we have evidenced the existence in chromaffin cells of functional units composed by three closely interrelated elements: (1) plasma membrane Ca(2+) channels, (2) subplasmalemmal endoplasmic reticulum and (3) mitochondria. These Ca(2+)-signalling triads optimize Ca(2+) microdomains for secretion and prevent propagation of the Ca(2+) wave towards the cell core. Oscillatory cytosolic Ca(2+) signals originate also oscillations of mitochondrial Ca(2+) in several cell types. The nuclear envelope slows down the propagation of the Ca(2+) wave to the nucleus and filters high frequencies. On the other hand, inositol-trisphosphate may produce direct release of Ca(2+) to the nucleoplasm in GH(3) pituitary cells, thus providing mechanisms for selective nuclear signalling. Aequorins emitting at different wavelengths, prepared by fusion either with green or red fluorescent protein, permit simultaneous and independent monitorization of the Ca(2+) signals in different subcellular domains within the same cell.

The co-presence of Na+/Ca2+-K+ exchanger and Na+/Ca2+ exchanger in bovine adrenal chromaffin cells

We have previously shown that there is high Na(+)/Ca(2+) exchange (NCX) activity in bovine adrenal chromaffin cells. In this study, by monitoring the [Ca(2+)](i) change in single cells and in a population of chromaffin cells, when the reverse mode of exchanger activity has been initiated, we have shown that the NCX activity is enhanced by K(+). The K(+)-enhanced activity accounted for a significant proportion of the Na(+)-dependent Ca(2+) uptake activity in the chromaffin cells. The results support the hypothesis that both NCX and Na(+)/Ca(2+)-K(+) exchanger (NCKX) are co-present in chromaffin cells. The expression of NCKX in chromaffin cells was further confirmed using PCR and northern blotting. In addition to the plasma membrane, the exchanger activity, measured by Na(+)-dependent (45)Ca(2+) uptake, was also present in membrane isolated from the chromaffin granules enriched fraction and the mitochondria enriched fraction. The results support that both NCX and NCKX are present in bovine chromaffin cells and that the regulation of [Ca(2+)](i) is probably more efficient with the participation of NCKX.

Intravesicular calcium release mediates the motion and exocytosis of secretory organelles: a study with adrenal chromaffin cells

Secretory vesicles of sympathetic neurons and chromaffin granules maintain a pH gradient toward the cytosol (pH 5.5 versus 7.2) promoted by the V-ATPase activity. This gradient of pH is also responsible for the accumulation of amines and Ca2+ because their transporters use H+ as the counter ion. We have recently shown that alkalinization of secretory vesicles slowed down exocytosis, whereas acidification caused the opposite effect. In this paper, we measure the alkalinization of vesicular pH, caused by the V-ATPase inhibitor bafilomycin A1, by total internal reflection fluorescence microscopy in cells overexpressing the enhanced green fluorescent protein-labeled synaptobrevin (VAMP2-EGFP) protein. The disruption of the vesicular gradient of pH caused the leak of Ca2+, measured with fura-2. Fluorimetric measurements, using the dye Oregon green BAPTA-2, showed that bafilomycin directly released Ca2+ from freshly isolated vesicles. The Ca2+ released from vesicles to the cytosol dramatically increased the granule motion of chromaffin- or PC12-derived granules and triggered exocytosis (measured by amperometry). We conclude that the gradient of pH of secretory vesicles might be involved in the homeostatic regulation of cytosolic Ca2+ and in two of the major functions of secretory cells, vesicle motion and exocytosis.

The trans-Golgi proteins SCLIP and SCG10 interact with chromogranin A to regulate neuroendocrine secretion

Secretion of proteins and peptides from eukaryotic cells takes place by both constitutive and regulated pathways. Regulated secretion may involve interplay of proteins that are currently unknown. Recent studies suggest an important role of chromogranin A (CHGA) in the regulated secretory pathway in neuroendocrine cells, but the mechanism by which CHGA enters the regulated pathway, or even triggers the formation of the pathway, remains unclear. In this study, we used a transcriptome/proteome-wide approach, to discover binding partners for CHGA, by employing a phage display cDNA library method. Several proteins within or adjacent to the secretory pathway were initially detected as binding partners of recombinant human CHGA. We then focused on the trans-Golgi protein SCLIP (STMN3) and its stathmin paralog SCG10 (STMN2) for functional study. Co-immunoprecipitation experiments confirmed the interaction of each of these two proteins with CHGA in vitro. SCLIP and SCG10 were colocalized to the Golgi apparatus of chromaffin cells in vivo and shared localization with CHGA as it transited the Golgi. Downregulation of either SCLIP or SCG10 by synthetic siRNAs virtually abolished chromaffin cell secretion of a transfected CHGA-EAP chimera (expressing CHGA fused to an enzymatic reporter, and trafficked to the regulated pathway). SCLIP siRNA also decreased the level of secretion of endogenous CHGA and SCG2, as well as transfected human growth hormone, while SCG10 siRNA decreased the level of regulated secretion of endogenous CHGB. Moreover, a dominant negative mutant of SCG10 (Cys 22,Cys 24-->Ala 22,Ala 24) significantly blocked secretion of the transfected CHGA-EAP chimera. A decrease in the buoyant density of chromaffin granules was observed after downregulation of SCG10 by siRNA, suggesting participation of these stathmins in granule formation or maturation. We conclude that SCLIP and SCG10 interact with CHGA, share partial colocalization in the Golgi apparatus, and may be necessary for typical transmitter storage and release from chromaffin cells.

How to measure Ca2+ in cellular organelles?

The review will aim to briefly summarise information on calcium measurements in cellular organelles with emphases on studies conducted in live cells using optical probes. When appropriate we will try to compare the effectiveness of different indicators for intraorganellar calcium measurements. We will consider calcium measurements in endoplasmic reticulum, Golgi apparatus, endosomes/lysosomes, nucleoplasm, nuclear envelope, mitochondria and secretory granules.

Calcium transport mechanisms of PC12 cells

Many studies of Ca²⁺ signaling use PC12 cells, yet the balance of Ca²⁺ clearance mechanisms in these cells is unknown. We used pharmacological inhibition of Ca²⁺ transporters to characterize Ca²⁺ clearance after depolarizations in both undifferentiated and nerve growth factor-differentiated PC12 cells. Sarco-endoplasmic reticulum Ca²⁺ ATPase (SERCA), plasma membrane Ca²⁺ ATPase (PMCA), and Na⁺/Ca²⁺ exchanger (NCX) account for almost all Ca²⁺ clearance in both cell states, with NCX and PMCA making the greatest contributions. Any contribution of mitochondrial uniporters is small. The ATP pool in differentiated cells was much more labile than that of undifferentiated cells in the presence of agents that dissipated mitochondrial proton gradients. Differentiated PC12 cells have a small component of Ca²⁺ clearance possessing pharmacological characteristics consistent with secretory pathway Ca²⁺ ATPase (SPCA), potentially residing on Golgi and/or secretory granules. Undifferentiated and differentiated cells are similar in overall Ca²⁺ transport and in the small transport due to SERCA, but they differ in the fraction of transport by PMCA and NCX. Transport in neurites of differentiated PC12 cells was qualitatively similar to that in the somata, except that the ER stores in neurites sometimes released Ca²⁺ instead of clearing it after depolarization. We formulated a mathematical model to simulate the observed Ca²⁺ clearance and to describe the differences between these undifferentiated and NGF-differentiated states quantitatively. The model required a value for the endogenous Ca²⁺ binding ratio of PC12 cell cytoplasm, which we measured to be 268 ± 85. Our results indicate that Ca²⁺ transport in undifferentiated PC12 cells is quite unlike transport in adrenal chromaffin cells, for which they often are considered models. Transport in both cell states more closely resembles that of sympathetic neurons, for which differentiated PC12 cells often are considered models. Comparison with other cell types shows that different cells emphasize different Ca²⁺ transport mechanisms.

Cytoplasmic organelles determine complexity and specificity of calcium signalling in adrenal chromaffin cells

Complex and coordinated fluctuations of intracellular free Ca2+ concentration ([Ca2+]c) regulate secretion of adrenaline from chromaffin cells. The physiologically relevant intracellular Ca2+ signals occur either as localized microdomains of high Ca2+ concentrations or as propagating Ca2+ waves, which give rise to global Ca2+ elevations. Intracellular organelles, the endoplasmic reticulum (ER), mitochondria and nuclear envelope, are endowed with powerful Ca2+ transport systems. Calcium uptake and Ca2+ release from these organelles determine the spatial and temporal parameters of Ca2+ signalling events. Furthermore, the ER and mitochondria form close relations with the sites of plasmalemmal Ca2+ entry, creating 'Ca2+ signalling triads' which act as elementary operational units, which regulate exocytosis. Ca2+ ions accumulating in the ER and mitochondria integrate exocytotic activity with energy production and protein synthesis.

Effect of group II metal cations on catecholate oxidation

The unexpected effects of Ca(2+) on the free-radical chain reactions of dopamine, norepinephrine, isoproterenol, and pyrocatechol oxidation are studied using oxygen consumption measurements, EPR-spectroscopy, UV/VIS spectrophotometry, and by potentiometric titration. It is found that the formation of Ca(2+)-catecholate complexes is accompanied by an increase in the dissociation constants (K(ai) ) of their phenolic hydroxyls. At pH>pK(ai) and in the presence of alkaline-earth metal cations, the rate of catecholate oxidation increases (Ca(2+), Mg(2+)> Sr(2+), Ba(2+)), whereas on addition of Zn ions the rate decreases. The effects of Group II metal cations on catecholate autoxidation are concomitant with a transient increase of the EPR signal for metal-semiquinonate complexes. Therefore, the effects of Ca(2+) and other alkaline-earth metal cations on catecholate autoxidation can be defined as 1) additional deprotonation of catechol OH-groups involved in the formation of M(2+)-catecholate complexes, the latter exceeding catechols in the susceptibility to dioxygen-induced oxidation and 2) formation of relatively stable free-radical intermediates responsible for chain propagation.

Secretory mechanisms and Ca2+ signaling in gametes: similarities to regulated neuroendocrine secretion in somatic cells and involvement in emerging pathologies

Recent studies demonstrate that regulated secretion in probably all mammalian cells, from gonadotropes to gametes, utilizes similar signaling systems, intracellular Ca(2+) regulation, Ca(2+)-dependent proteins, cytoskeletal participation, and SNARE-mediated fusion. Thus, highly specialized cells, like sperm and eggs, should no longer be considered to have evolved a cell-type specific secretory mechanism. In gametes, Ca(2+)-dependent proteins and enzymes transduce elevations of intracellular Ca(2+) into secretory events, i.e., exocytosis of the acrosome in sperm and cortical granules in the egg. Just as secretory deficiencies have clinical consequences in endocrine and exocrine cells, failure of secretion of cortical granules or the acrosome can result in failure of normal fertilization or fertilization followed by abnormal development. With the advent of human in vitro fertilization, such gamete pathologies have been recently identified and have led to new clinical procedures to achieve normal fertilization and pregnancies. A better understanding of the common Ca(2+)-dependent secretory pathways in both gametes and somatic cells should be beneficial to investigating mis-regulation in either cell type.

Diseases involving the Golgi calcium pump

Secretory-pathway Ca2(+)-transport ATPases (SPCA) provide the Golgi apparatus with Ca2+ and Mn2+ needed for the normal functioning of this organelle. Loss of one functional copy of the human SPCA1 gene (ATP2C1) causes Hailey-Hailey disease, a rare skin disorder characterized by recurrent blisters and erosions in the flexural areas. Here, we will review the properties and functional role of the SPCAs. The relationship between Hailey-Hailey disease and its defective gene (ATP2C1) will be adressed as well.

Calcium microdomains in regulated exocytosis

Katz and co-workers showed that Ca(2+) triggers exocytosis. The existence of sub-micrometer domains of greater than 100 microM [Ca(2+)](i) was postulated on theoretical grounds. Using a modified, low-affinity aequorin, Llinas et al. were the first to demonstrate the existence of Ca(2+) 'microdomains' in squid presynaptic terminals. Over the past several years, it has become clear that individual Ca(2+) nano- and microdomains forming around the mouth of voltage-gated Ca(2+) channels ascertain the tight coupling of fast synaptic vesicle release to membrane depolarization by action potentials. Recent work has established different geometric arrangements of vesicles and Ca(2+) channels at different central synapses and pointed out the role of Ca(2+) syntillas - localized, store operated Ca(2+) signals - in facilitation and spontaneous release. The coupling between Ca(2+) increase and evoked exocytosis is more sluggish in peripheral terminals and neuroendocrine cells, where channels are less clustered and Ca(2+) comes from different sources, including Ca(2+) influx via the plasma membrane and the mobilization of Ca(2+) from intracellular stores. Finally, also non- (electrically) excitable cells display highly localized Ca(2+) signaling domains. We discuss in particular the organization of structural microdomains of Bergmann glia, specialized astrocytes of the cerebellum that have only recently been considered as secretory cells. Glial microdomains are the spatial substrate for functionally segregated Ca(2+) signals upon metabotropic activation. Our review emphasizes the large diversity of different geometric arrangements of vesicles and Ca(2+) sources, leading to a wide spectrum of Ca(2+) signals triggering release.

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.

Vesicular Ca(2+) -induced secretion promoted by intracellular pH-gradient disruption

The actions of the protonophore CCCP on intracellular Ca2+ regulation and exocytosis in chromaffin cells have been examined. Simultaneous fura-2 imaging and amperometry reveal that exposure to CCCP not only perturbs mitochondrial function but that it also alters vesicular storage of Ca2+ and catecholamines. By disrupting the pH gradient of the secretory vesicle membrane, the protonophore allows both Ca(2+) and catecholamine to leak into the cytosol. Unlike the high cytosolic Ca2+ concentrations resulting from mitochondrial membrane disruption, Ca2+ leakage from secretory vesicles may initiate exocytotic release. In conjunction with previous studies, this work reveals that catalytic and self-sustained vesicular Ca(2+) -induced exocytosis occurs with extended exposure to weak acid or base protonophores.

Astrocytic glutamate release-induced transient depolarization and epileptiform discharges in hippocampal CA1 pyramidal neurons

A paroxysmal depolarization shift (PDS) has been suggested to be a hallmark for epileptic activity in partial-onset seizures. By monitoring membrane potentials and currents in pairs of pyramidal neurons and astrocytes with dual patch-clamp recording and exocytosis of vesicles from astrocytes with two-photon laser scanning microscopy in hippocampal slices, we found that infusion of inositol 1,4,5-trisphosphate (IP(3)) into astrocytes by patch pipettes induced astrocytic glutamate release that triggered a transient depolarization (TD) and epileptiform discharges in CA1 pyramidal neurons. The TD is due to a tetrodotoxin (TTX)-insensitive slowly decaying transient inward current (STC). Astrocytic glutamate release simultaneously triggers both the STC in pyramidal neurons and a transport current (TC) in astrocytes. The neuronal STC is mediated by ionotropic glutamate receptors leading to the TD and epileptiform discharges; while the astrocytic TC is a glutamate reuptake current resulting from transporting released glutamate into the patched astrocyte. Fusion of a large vesicle in astrocytes was immediately followed by an astrocytic TC, suggesting that the fused vesicle contains glutamate. Both fusion of large vesicles and astrocytic TCs were blocked by tetanus toxin (TeNT), suggesting that astrocytic glutamate release is via SNARE-dependent exocytosis of glutamate-containing vesicles. In the presence of TTX, the epileptogenic reagent, 4-AP, also induced similar neuronal STCs and astrocytic TCs, suggesting that astrocytic glutamate release may play an epileptogenic role in initiation of epileptic seizures under pathological conditions. Our study provides a novel mechanism, astrocytic release of glutamate, for seizure initiation.

Calcium dynamics in catecholamine-containing secretory vesicles

We have used an aequorin chimera targeted to the membrane of the secretory granules to monitor the free [Ca(2+)] inside them in neurosecretory PC12 cells. More than 95% of the probe was located in a compartment with an homogeneous [Ca(2+)] around 40 microM. Cell stimulation with either ATP, caffeine or high-K(+) depolarization increased cytosolic [Ca(2+)] and decreased secretory granule [Ca(2+)] ([Ca(2+)](SG)). Inositol-(1,4,5)-trisphosphate, cyclic ADP ribose and nicotinic acid adenine dinucleotide phosphate were all ineffective to release Ca(2+) from the granules. Changes in cytosolic [Na(+)] (0-140 mM) or [Ca(2+)] (0-10 microM) did not modify either ([Ca(2+)](SG)). Instead, [Ca(2+)](SG) was highly sensitive to changes in the pH gradient between the cytosol and the granules. Both carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP) and nigericin, as well as cytosolic acidification, reversibly decreased [Ca(2+)](SG), while cytosolic alcalinization reversibly increased [Ca(2+)](SG). These results are consistent with the operation of a H(+)/Ca(2+) antiporter in the vesicular membrane. This antiporter could also mediate the effects of ATP, caffeine and high-K(+) on [Ca(2+)](SG), because all of them induced a transient cytosolic acidification. The FCCP-induced decrease in [Ca(2+)](SG) was reversible in 10-15 min even in the absence of cytosolic Ca(2+) or ATP, suggesting that most of the calcium content of the vesicles is bound to a slowly exchanging Ca(2+) buffer. This large store buffers [Ca(2+)](SG) changes in the long-term but allows highly dynamic free [Ca(2+)](SG) changes to occur in seconds or minutes.