The crucial role of chromogranins in storage and exocytosis revealed using chromaffin cells from chromogranin A null mouse

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.

N-terminal additions to the WE14 peptide of chromogranin A create strong autoantigen agonists in type 1 diabetes

Chromogranin A (ChgA) is an autoantigen for CD4(+) T cells in the nonobese diabetic (NOD) mouse model of type 1 diabetes (T1D). The natural ChgA-processed peptide, WE14, is a weak agonist for the prototypical T cell, BDC-2.5, and other ChgA-specific T-cell clones. Mimotope peptides with much higher activity share a C-terminal motif, WXRM(D/E), that is predicted to lie in the p5 to p9 position in the mouse MHC class II, IA(g7) binding groove. This motif is also present in WE14 (WSRMD), but at its N terminus. Therefore, to place the WE14 motif into the same position as seen in the mimotopes, we added the amino acids RLGL to its N terminus. Like the other mimotopes, RLGL-WE14, is much more potent than WE14 in T-cell stimulation and activates a diverse population of CD4(+) T cells, which also respond to WE14 as well as islets from WT, but not ChgA(-/-) mice. The crystal structure of the IA(g7)-RLGL-WE14 complex confirmed the predicted placement of the peptide within the IA(g7) groove. Fluorescent IA(g7)-RLGL-WE14 tetramers bind to ChgA-specific T-cell clones and easily detect ChgA-specific T cells in the pancreas and pancreatic lymph nodes of NOD mice. The prediction that many different N-terminal amino acid extensions to the WXRM(D/E) motif are sufficient to greatly improve T-cell stimulation leads us to propose that such a posttranslational modification may occur uniquely in the pancreas or pancreatic lymph nodes, perhaps via the mechanism of transpeptidation. This modification could account for the escape of these T cells from thymic negative selection.

Clinical management of paragangliomas

Paragangliomas (PGLs) are rare vascular, neuroendocrine tumors of paraganglia, which are associated with either sympathetic tissue in adrenal (pheochromocytomas (PCCs)) and extraadrenal (sympathetic paraganglioma (sPGLs)) locations or parasympathetic tissue of the head and neck paragangliomas (HNPGLs). As HNPGLs are usually benign and most tumors grow slowly, a wait-and-scan policy is often advised. However, their location in the close proximity to cranial nerves and vasculature may result in considerable morbidity due to compression or infiltration of the adjacent structures, necessitating balanced decisions between a wait-and-see policy and active treatment. The main treatment options for HNPGL are surgery and radiotherapy. In contrast to HNPGLs, the majority of sPGL/PCCs produces catecholamines, in advanced cases resulting in typical symptoms and signs such as palpitations, headache, diaphoresis, and hypertension. The state-of-the-art diagnosis and localization of sPGL/PCCs are based on measurement of plasma and/or 24-h urinary excretion of (fractionated) metanephrines and methoxytyramine (MT). sPGL/PCCs can subsequently be localized by anatomical (computed tomography and/or magnetic resonance imaging) and functional imaging studies (123I-metaiodobenzylguanidine-scintigraphy, 111In-pentetreotide scintigraphy, or positron emission tomography with radiolabeled dopamine or dihydroxyphenylalanine). Although most PGL/PCCs are benign, factors such as genetic background, tumor size, tumor location, and high MT levels are associated with higher rates of metastatic disease. Surgery is the only curative treatment. Treatment options for patients with metastatic disease are limited. PGL/PCCs have a strong genetic background, with at least one-third of all cases linked with germline mutations in 11 susceptibility genes. As genetic testing becomes more widely available, the diagnosis of PGL/PCCs will be made earlier due to routine screening of at-risk patients. Early detection of a familial PGL allows early detection of potentially malignant PGLs and early surgical treatment, reducing the complication rates of this operation.

Mice lacking chromogranins exhibit increased aggressive and depression-like behaviour

Chromogranins are acidic proteins; both chromogranins A and B constitute the main protein component in the vesicular matrix of large dense core vesicles. Chromogranins are a natural source of peptides with different physiological activities that have been associated with vascular and neurological diseases. We have used three different genetic mutant models of mice lacking chromogranin A, chromogranin B and both all on the same C57BL/6J background, to characterize the physiological roles of these proteins using metabolic, cardiovascular and behavioural tests. In mice from 3 to 18 months of age, the lack of any chromogranin promoted age-dependent hypersensitivity to insulin, while the lack of both chromogranins provoked progressive lack of response to stress, as restriction did not promote tachycardia in old mice. Moreover, the lack of chromogranin B produced a depressive-like and aggressive phenotype, while the lack either or both chromogranins increased barbering behaviour. In addition, we observed no effects on light-dark box or RotaRod tests. Mice lacking chromogranin B exhibited lower exploratory activity. Based on this extensive phenotyping with more than 2800 mice, these findings support roles of chromogranins, or the peptides derived from them, in the control of aggressive behaviour along with changes in their metabolic profile beyond their previously described activities in the secretory pathway.

Cdc42 controls the dilation of the exocytotic fusion pore by regulating membrane tension

On exocytosis, membrane fusion starts with the formation of a narrow fusion pore that must expand to allow the release of secretory compounds. The GTPase Cdc42 promotes fusion pore dilation in neuroendocrine cells by controlling membrane tension.

Membrane fusion underlies multiple processes, including exocytosis of hormones and neurotransmitters. Membrane fusion starts with the formation of a narrow fusion pore. Radial expansion of this pore completes the process and allows fast release of secretory compounds, but this step remains poorly understood. Here we show that inhibiting the expression of the small GTPase Cdc42 or preventing its activation with a dominant negative Cdc42 construct in human neuroendocrine cells impaired the release process by compromising fusion pore enlargement. Consequently the mode of vesicle exocytosis was shifted from full-collapse fusion to kiss-and-run. Remarkably, Cdc42-knockdown cells showed reduced membrane tension, and the artificial increase of membrane tension restored fusion pore enlargement. Moreover, inhibiting the motor protein myosin II by blebbistatin decreased membrane tension, as well as fusion pore dilation. We conclude that membrane tension is the driving force for fusion pore dilation and that Cdc42 is a key regulator of this force.

The interaction between chromogranin A and catecholamines governs exocytosis

Chromogranins (Cgs) are acidic proteins that have been described in the large, dense core vesicles (LDCVs) of adrenal chromaffin cells and that have been shown to promote LDCV formation, even in nonsecretory cells. Catecholamines (CAs) are adsorbed by Cgs in vitro, and the absence of Cgs modifies the storage and exocytosis of CAs in chromaffin cells. In this study, we set out to assess the role of CgA in the accumulation and exocytosis of CAs in cells when the levels of CgA and CA are manipulated. We overexpressed CgA in nonsecretory HEK293 cells and in secretory PC12 cells, to study the formation, movement, and exocytosis of newly formed granules by evanescent wave microscopy. We analyzed the association of Cgs/CA by HPLC and amperometry and their role in the accumulation and exocytosis of amines, both under resting conditions and after l-DOPA overloading. To our knowledge, this is the first demonstration that CgA expression in a nonsecretory cell line facilitates the storage and exocytosis of CA. In addition, CgA overexpression causes a doubling of the accumulation of CA, although it slows down exocytosis in PC12 cells. We propose a model to explain how the CgA/CA complex governs the accumulation and exocytosis of secreted amines.

Neuropeptide y gates a stress-induced, long-lasting plasticity in the sympathetic nervous system

Acute stress evokes the fight-or-flight reflex, which via release of the catecholamine hormones affects the function of every major organ. Although the reflex is transient, it has lasting consequences that produce an exaggerated response when stress is reexperienced. How this change is encoded is not known. We investigated whether the reflex affects the adrenal component of the sympathetic nervous system, a major branch of the stress response. Mice were briefly exposed to the cold-water forced swim test (FST) which evoked an increase in circulating catecholamines. Although this hormonal response was transient, the FST led to a long-lasting increase in the catecholamine secretory capacity measured amperometrically from chromaffin cells and in the expression of tyrosine hydroxylase. A variety of approaches indicate that these changes are regulated postsynaptically by neuropeptide Y (NPY), an adrenal cotransmitter. Using immunohistochemistry, RT-PCR, and NPY(GFP) BAC mice, we find that NPY is synthesized by all chromaffin cells. Stress failed to increase secretory capacity in NPY knock-out mice. Genetic or pharmacological interference with NPY and Y1 (but not Y2 or Y5) receptor signaling attenuated the stress-induced change in tyrosine hydroxylase expression. These results indicate that, under basal conditions, adrenal signaling is tonically inhibited by NPY, but stress overrides this autocrine negative feedback loop. Because acute stress leads to a lasting increase in secretory capacity in vivo but does not alter sympathetic tone, these postsynaptic changes appear to be an adaptive response. We conclude that the sympathetic limb of the stress response exhibits an activity-dependent form of long-lasting plasticity.

The real catecholamine content of secretory vesicles in the CNS revealed by electrochemical cytometry

Resolution of synaptic vesicle neurotransmitter content has mostly been limited to the study of stimulated release in cultured cell systems, and it has been controversial as to whether synaptic vesicle transmitter levels are saturated in vivo. We use electrochemical cytometry to count dopamine molecules in individual synaptic vesicles in populations directly sampled from brain tissue. Vesicles from the striatum yield an average of 33,000 dopamine molecules per vesicle, an amount considerably greater than typically measured during quantal release at cultured neurons. Vesicular content was markedly increased by L-DOPA or decreased by reserpine in a time-dependent manner in response to in vivo administration of drugs known to alter dopamine release. We investigated the effects of the psychostimulant amphetamine on vesicle content, finding that vesicular transmitter is rapidly depleted by 50% following in vivo administration, supporting the “weak base hypothesis” that amphetamine reduces synaptic vesicle transmitter and quantal size.

Contraction and AICAR stimulate IL-6 vesicle depletion from skeletal muscle fibers in vivo

Recent studies suggest that interleukin 6 (IL-6) is released from contracting skeletal muscles; however, the cellular origin, secretion kinetics, and signaling mechanisms regulating IL-6 secretion are unknown. To address these questions, we developed imaging methodology to study IL-6 in fixed mouse muscle fibers and in live animals in vivo. Using confocal imaging to visualize endogenous IL-6 protein in fixed muscle fibers, we found IL-6 in small vesicle structures distributed throughout the fibers under basal (resting) conditions. To determine the kinetics of IL-6 secretion, intact quadriceps muscles were transfected with enhanced green fluorescent protein (EGFP)-tagged IL-6 (IL-6-EGFP), and 5 days later anesthetized mice were imaged before and after muscle contractions in situ. Contractions decreased IL-6-EGFP-containing vesicles and protein by 62% (P < 0.05), occurring rapidly and progressively over 25 min of contraction. However, contraction-mediated IL-6-EGFP reduction was normal in muscle-specific AMP-activated protein kinase (AMPK) α2-inactive transgenic mice. In contrast, the AMPK activator AICAR decreased IL-6-EGFP vesicles, an effect that was inhibited in the transgenic mice. In conclusion, resting skeletal muscles contain IL-6-positive vesicles that are expressed throughout myofibers. Contractions stimulate the rapid reduction of IL-6 in myofibers, occurring through an AMPKα2-independent mechanism. This novel imaging methodology clearly establishes IL-6 as a contraction-stimulated myokine and can be used to characterize the secretion kinetics of other putative myokines.

Maternal-fetal unit interactions and eutherian neocortical development and evolution

The conserved brain design that primates inherited from early mammals differs from the variable adult brain size and species-specific brain dominances observed across mammals. This variability relies on the emergence of specialized cerebral cortical regions and sub-compartments, triggering an increase in brain size, areal interconnectivity and histological complexity that ultimately lies on the activation of developmental programs. Structural placental features are not well correlated with brain enlargement; however, several endocrine pathways could be tuned with the activation of neuronal progenitors in the proliferative neocortical compartments. In this article, we reviewed some mechanisms of eutherians maternal-fetal unit interactions associated with brain development and evolution. We propose a hypothesis of brain evolution where proliferative compartments in primates become activated by "non-classical" endocrine placental signals participating in different steps of corticogenesis. Changes in the inner placental structure, along with placenta endocrine stimuli over the cortical proliferative activity would allow mammalian brain enlargement with a concomitant shorter gestation span, as an evolutionary strategy to escape from parent-offspring conflict.

Single cell amperometry reveals glycocalyx hinders the release of neurotransmitters during exocytosis

The diffusional hindrance of the glycocalyx along the cell surface on exocytotic peaks, observed with single cell amperometry, was investigated. Partial digestion of the glycocalyx with neuraminidase led to the observation of faster peaks, as shown by varied peak parameters. This result indicates that diffusion of small molecules in the partially digested glycocalyx is 2.2 faster than in the intact glycocalyx. Similarly, neutralization of the negative charges present in the cell microenvironment led to faster peak kinetics. The analysis of the vesicular efflux indicates that the diffusion coefficient of dopamine at the cell surface is at most 45% of the diffusion coefficient in free solution. This study shows that the glycocalyx plays an important role in the diffusion kinetics of processes along the cell surface, including exocytotic events.

Granins and catecholamines: functional interaction in chromaffin cells and adipose tissue

Catecholamines (CAs) and granin peptides are costored in dense-core vesicles within the chromaffin cells of the adrenal medulla and in other endocrine organs and neurons. Granins play a major functional and structural role in chromaffin cells but are ubiquitous proteins, which are present also in secretory cells of the nervous, endocrine, and immune systems, where they regulate a number of cellular functions. Furthermore, recent studies also demonstrate that granin-derived peptides can functionally interact with CA to modulate key physiological functions such as lipolysis and blood pressure. In this chapter, we will provide a brief update on the interaction between CA and granins at the cellular and organ levels. We will first discuss recent data on the regulation of exocytosis of CA and peptides from the chromaffin cells by the sympathetic nervous system with a specific reference to the prominent role played by splanchnic nerve-derived pituitary adenylate cyclase-activating peptide (PACAP). Secondly, we will discuss the role of granins in the storage and regulation of exocytosis in large dense-core vesicles. Finally, we will provide an up-to-date review of the roles played by two granin-derived peptides, the chromogranin A-derived peptide catestatin and the VGF-derived peptide TLQP-21, on lipolysis and obesity. In conclusion, the knowledge gathered from recent findings on the role played by proteins/peptides in the sympathetic/target cell synapses, discussed in this chapter, would contribute to and provide novel mechanistic support for an increased appreciation of the physiological role of CA in human pathophysiology.

Multiple sorting systems for secretory granules ensure the regulated secretion of peptide hormones

Prior to secretion, regulated peptide hormones are selectively sorted to secretory granules (SGs) at the trans-Golgi network (TGN) in endocrine cells. Secretogranin III (SgIII) appears to facilitate SG sorting process by tethering of protein aggregates containing chromogranin A (CgA) and peptide hormones to the cholesterol-rich SG membrane (SGM). Here, we evaluated the role of SgIII in SG sorting in AtT-20 cells transfected with small interfering RNA targeting SgIII. In the SgIII-knockdown cells, the intracellular retention of CgA was greatly impaired, and only a trace amount of CgA was localized within the vacuoles formed in the TGN, confirming the significance of SgIII in both the tethering of CgA-containing aggregates and the establishment of the proper SG morphology. Although the intracellular retention of proopiomelanocortin (POMC) was considerably impaired in SgIII-knockdown cells, residual adrenocorticotropic hormone (ACTH)/POMC was still localized to some few remaining SGs together with another granin protein, secretogranin II (SgII), and was secreted in a regulated manner. Biochemical analyses indicated that SgII bound directly to the SGM in a cholesterol-dependent manner and was able to retain the aggregated form of POMC, revealing a latent redundancy in the SG sorting and retention mechanisms, that ensures the regulated secretion of bioactive peptides.

Amperometric post spike feet reveal most exocytosis is via extended kiss-and-run fusion

The basis for communication between nerve cells lies in the process of exocytosis, the fusion of neurotransmitter filled vesicles with the cell membrane resulting in release of the signaling molecules. Even though much is known about this process, the extent that the vesicles are emptied upon fusion is a topic that is being debated. We have analyzed amperometric peaks corresponding to release at PC12 cells and find stable plateau currents during the decay of peaks, indicating closing of the vesicle after incomplete release of its content. Using lipid incubations to alter the amount of transmitter released we were able to estimate the initial vesicular content, and from that, the fraction of release. We propose a process for most exocytosis events where the vesicle partially opens to release transmitter and then closes directly again, leaving the possibility for regulation of transmission within events.

Juxtamembrane tryptophans of synaptobrevin 2 control the process of membrane fusion

Synaptobrevin 2 (Syb2), syntaxin (Sx1A), and SNAP-25, generate a force to induce fusion pore formation. The v-SNARE, Syb2, is anchored to the vesicle membrane by a single transmembrane domain. Here we show that 2 tryptophans (W89/W90) located in the juxtamembrane domain of Syb2, which stabilize the transmembrane (TM) domain position, control the ratio of spontaneous vs. stimulated membrane fusion events in chromaffin cells. Changing the 2 hydrophobic tryptophans to neutral alanines promotes spontaneous membrane fusion, faster transmitter release kinetics and complete release from individual vesicles. The results indicate that the two tryptophans act as a fusion clamp making fusion stimulus-dependent.

A functioning artificial secretory cell

We present an amperometric study of content release from individual vesicles in an artificial secretory cell designed with the minimal components required to carry out exocytosis. Here, the membranes of the cell and vesicles are substituted for protein-free giant and large unilamellar vesicles respectively. In replacement of the SNARE-complex, the cell model was equipped with an analog composed of complimentary DNA constructs. The DNA constructs hybridize in a zipper-like fashion to bring about docking of the artificial secretory vesicles and following the addition of Ca²⁺ artificial exocytosis was completed. Exocytotic events recorded from the artificial cell closely approximate exocytosis in live cells. The results together with simulations of vesicular release demonstrate that the molecular flux in this model is attenuated and we suggest that this is the result of restricted diffusion through a semi-stable fusion pore or a partitioning of the signalling molecule out of the fused vesicle membrane.

Chromogranin A induces the biogenesis of granules with calcium- and actin-dependent dynamics and exocytosis in constitutively secreting cells

Chromogranins are a family of acidic glycoproteins that play an active role in hormone and neuropeptide secretion through their crucial role in secretory granule biogenesis in neuroendocrine cells. However, the molecular mechanisms underlying their granulogenic activity are still not fully understood. Because we previously demonstrated that the expression of the major component of secretory granules, chromogranin A (CgA), is able to induce the formation of secretory granules in nonendocrine COS-7 cells, we decided to use this model to dissect the mechanisms triggered by CgA leading to the biogenesis and trafficking of such granules. Using quantitative live cell imaging, we first show that CgA-induced organelles exhibit a Ca(2+)-dependent trafficking, in contrast to native vesicle stomatitis virus G protein-containing constitutive vesicles. To identify the proteins that confer such properties to the newly formed granules, we developed CgA-stably-expressing COS-7 cells, purified their CgA-containing granules by subcellular fractionation, and analyzed the granule proteome by liquid chromatography-tandem mass spectrometry. This analysis revealed the association of several cytosolic proteins to the granule membrane, including GTPases, cytoskeleton-based molecular motors, and other proteins with actin- and/or Ca(2+)-binding properties. Furthermore, disruption of cytoskeleton affects not only the distribution and the transport but also the Ca(2+)-evoked exocytosis of the CgA-containing granules, indicating that these granules interact with microtubules and cortical actin for the regulated release of their content. These data demonstrate for the first time that the neuroendocrine factor CgA induces the recruitment of cytoskeleton-, GTP-, and Ca(2+)-binding proteins in constitutively secreting COS-7 cells to generate vesicles endowed with typical dynamics and exocytotic properties of neuroendocrine secretory granules.

Hippocampal CA1 transcriptional profile of sleep deprivation: relation to aging and stress

Background

Many aging changes seem similar to those elicited by sleep-deprivation and psychosocial stress. Further, sleep architecture changes with age suggest an age-related loss of sleep. Here, we hypothesized that sleep deprivation in young subjects would elicit both stress and aging-like transcriptional responses.

Methodology/Principal Findings

F344 rats were divided into control and sleep deprivation groups. Body weight, adrenal weight, corticosterone level and hippocampal CA1 transcriptional profiles were measured. A second group of animals was exposed to novel environment stress (NES), and their hippocampal transcriptional profiles measured. A third cohort exposed to control or SD was used to validate transcriptional results with Western blots. Microarray results were statistically contrasted with prior transcriptional studies. Microarray results pointed to sleep pressure signaling and macromolecular synthesis disruptions in the hippocampal CA1 region. Animals exposed to NES recapitulated nearly one third of the SD transcriptional profile. However, the SD -aging relationship was more complex. Compared to aging, SD profiles influenced a significant subset of genes. mRNA associated with neurogenesis and energy pathways showed agreement between aging and SD, while immune, glial, and macromolecular synthesis pathways showed SD profiles that opposed those seen in aging.

Conclusions/Significance

We conclude that although NES and SD exert similar transcriptional changes, selective presynaptic release machinery and Homer1 expression changes are seen in SD. Among other changes, the marked decrease in Homer1 expression with age may represent an important divergence between young and aged brain response to SD. Based on this, it seems reasonable to conclude that therapeutic strategies designed to promote sleep in young subjects may have off-target effects in the aged. Finally, this work identifies presynaptic vesicular release and intercellular adhesion molecular signatures as novel therapeutic targets to counter effects of SD in young subjects.

Granule matrix property and rapid “kiss-and-run” exocytosis contribute to the different kinetics of catecholamine release from carotid glomus and adrenal chromaffin cells at matched quantal size

Catecholamine-containing small dense core granules (SDCGs, vesicular diameter of ∼100 nm) are prominent in carotid glomus (chemosensory) cells and some neurons, but the release kinetics from individual SDCGs has not been studied in detail. In this study, we compared the amperometric signals from glomus cells with those from adrenal chromaffin cells, which also secrete catecholamine but via large dense core granules (LDCGs, vesicular diameter of ∼200–250 nm). When exocytosis was triggered by whole-cell dialysis (which raised the concentration of intracellular Ca2+ ([Ca2+]i) to ∼0.5 µmol/L), the proportion of the type of signal that represents a flickering fusion pore was 9-fold higher for glomus cells. Yet, at the same range of quantal size (Q, the total amount of catecholamine that can be released from a granule), the kinetics of every phase of the amperometric spike signals from glomus cells was faster. Our data indicate that the last phenomenon involved at least 2 mechanisms: (i) the granule matrix of glomus cells can supply a higher concentration of free catecholamine during exocytosis; (ii) a modest elevation of [Ca2+]i triggers a form of rapid “kiss-and-run” exocytosis, which is very prevalent among glomus SDCGs and leads to incomplete release of their catecholamine content (and underestimation of their Q value).

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.

High resolution electrophysiological techniques for the study of calcium-activated exocytosis

BACKGROUND

Neurotransmitters, neuropeptides and hormones are released from secretory vesicles of nerve terminals and neuroendocrine cells by calcium-activated exocytosis. A key step in this process is the formation of a fusion pore between the vesicle membrane and the plasma membrane. Exocytotic fusion leads to an increase in plasma membrane area that can be measured as a proportional increase in plasma membrane capacitance.

SCOPE OF REVIEW

High resolution capacitance measurements in single cells, nerve terminals and small membrane patches have become possible with the development of the patch clamp technique. This review discusses the methods of whole cell patch clamp capacitance measurements and their use in conjunction with voltage clamp pulse stimulation and with stimulation by photorelease of caged calcium. It also discusses patch capacitance measurements for the study of single exocytotic events and fusion pore properties in neuroendocrine cells and nerve terminals.

MAJOR CONCLUSIONS

Capacitance measurements provide high resolution information on the extent and time course of fusion for the characterization of vesicle pools and the kinetics of exocytosis. They allow the characterization of the mode of fusion including distinction of single vesicle full fusion, transient kiss-and-run fusion or multivesicular compound exocytosis. Furthermore, measurement of fusion pore conductances and their dynamic behavior has enabled the characterization of fusion pore properties in a way that resembles the characterization of ion channel function through single channel recordings.

GENERAL SIGNIFICANCE

The combination of patch clamp capacitance measurements with pharmacological and molecular manipulations of exocytosis is emerging as a powerful approach to investigate the molecular mechanisms of calcium-activated exocytotic fusion pore formation. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signalling.

The extended granin family: structure, function, and biomedical implications

The chromogranins (chromogranin A and chromogranin B), secretogranins (secretogranin II and secretogranin III), and additional related proteins (7B2, NESP55, proSAAS, and VGF) that together comprise the granin family subserve essential roles in the regulated secretory pathway that is responsible for controlled delivery of peptides, hormones, neurotransmitters, and growth factors. Here we review the structure and function of granins and granin-derived peptides and expansive new genetic evidence, including recent single-nucleotide polymorphism mapping, genomic sequence comparisons, and analysis of transgenic and knockout mice, which together support an important and evolutionarily conserved role for these proteins in large dense-core vesicle biogenesis and regulated secretion. Recent data further indicate that their processed peptides function prominently in metabolic and glucose homeostasis, emotional behavior, pain pathways, and blood pressure modulation, suggesting future utility of granins and granin-derived peptides as novel disease biomarkers.

Quantal regulation and exocytosis of platelet dense-body granules

This study reports how quantal size, or the quantity of chemical messengers within a storage granule, is regulated in platelet dense-body granules via dynamic adaption of granule size according to changing levels of granule contents. Mechanistic studies using carbon-fiber microelectrode fast-scan cyclic voltammetry and amperometry methods correlated with transmission electron microscopy analysis reveal the impact of granule structural changes on granular content secretion kinetics and highlight the dynamic interplay between soluble granule contents and membrane components in exocytosis. Despite the distinct chemical profile of platelet dense-body granules, these secretory granules act according to general biochemical/biophysical phenomena using charge-charge interactions to sequester chemical messengers and employ known conserved exocytotic machinery to deliver them; therefore, the mechanistic information obtained herein further advances the general understanding of exocytosis while revealing fundamental details about blood platelets.

Chromogranins A and B are key proteins in amine accumulation, but the catecholamine secretory pathway is conserved without them

Chromogranins are the main soluble proteins in the large dense core secretory vesicles (LDCVs) found in aminergic neurons and chromaffin cells. We recently demonstrated that chromogranins A and B each regulate the concentration of adrenaline in chromaffin granules and its exocytosis. Here we have further studied the role played by these proteins by generating mice lacking both chromogranins. Surprisingly, these animals are both viable and fertile. Although chromogranins are thought to be essential for their biogenesis, LDCVs were evident in these mice. These vesicles do have a somewhat atypical appearance and larger size. Despite their increased size, single-cell amperometry recordings from chromaffin cells showed that the amine content in these vesicles is reduced by half. These data demonstrate that although chromogranins regulate the amine concentration in LDCVs, they are not completely essential, and other proteins unrelated to neurosecretion, such as fibrinogen, might compensate for their loss to ensure that vesicles are generated and the secretory pathway conserved.

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.

VMAT2: a dynamic regulator of brain monoaminergic neuronal function interacting with drugs of abuse

The monoaminergic neuron, in particular the dopaminergic neuron, is central to mediating the hedonic and addictive properties of drugs of abuse. The effects of amphetamine (AMPH) and cocaine (COC), for example, depend on the ability to increase dopamine in the synapse, by effects on either the plasma membrane transporter DAT or the vesicular transporter for monoamine storage, VMAT2. The potential role of DAT as a target for AMPH and COC has been reviewed extensively. Here, we present VMAT2 as a target that enables the rewarding and addictive actions of these drugs, based on imaging, neurochemical, biochemical, cell biological, genetic, and immunohistochemical evidence. The presence of VMAT2 in noradrenergic, serotoninergic, histaminergic, and potentially trace aminergic neurons invites consideration of a wider role for aminergic neurotransmission in AMPH and COC abuse and addiction.

Chromogranin peptides in brain diseases

Synaptic disturbances may play a key role in the pathophysiology of neuropsychiatric diseases. In this article, we review immunohistological findings of chromogranin peptides in neurodegenerative and neurodevelopmental disorders, with particular emphasis on Alzheimer's disease, the disorder chromogranins have been studied most extensively. Data was collected from existing and new experimental data and medline research. This review focuses on synaptic changes elicited by chromogranin peptides immunoreactivity in Alzheimer's disease, as well in schizophrenia and amyotrophic lateral sclerosis (ALS). An imbalanced availability of chromogranin peptides may be responsible for impaired neurotransmission and a reduced functioning of dense core vesicles. Since chromogranin A was postulated as a potent proinflammatory agent, we focused on chromogranin A in neuroinflammation in Alzheimer's disease and ALS. Further understanding of role and function of chromogranin peptides in neuropathological conditions is still required.

Plasma chromogranin A levels are increased in a small portion of patients with hereditary head and neck paragangliomas

CONTEXT

The majority of patients with head and neck paragangliomas (HNPGL) have biochemically silent tumours. Chromogranin A (CgA) is a tumour marker for neuroendocrine tumours.

OBJECTIVE

To assess the role of CgA as a tumour marker in patients with hereditary HNPGL.

PATIENTS AND METHODS

We included 95 consecutive patients with hereditary HNPGL for screening of plasma CgA levels and catecholamine excess by measurement of 24-h urinary excretion of (nor)metanephrine, (nor)adrenaline, VMA, dopamine and 3-methoxytyramine. In all patients with catecholamine excess, abdominal/intrathoracic paragangliomas were excluded by (123) I-MIBG scintigraphy, MRI and/or CT.

RESULTS

Plasma CgA levels were increased in only 15 of 95 patients (16%). Thirty-three of the 95 patients (35%) had increased urinary excretion rates of catecholamines. Six of these 33 patients (18%) had increased plasma CgA levels. Nine of the 62 patients (15%) with a biochemically silent tumour, i.e. no increased urinary excretion of catecholamines or their metabolites, had increased CgA levels. Increased plasma CgA levels were positively correlated with urinary excretion rates of noradrenaline (r = 0·68, P = 0·005) and normetanephrine (r = 0·68, P = 0·005). There was a positive correlation between maximal HNPGL diameter and plasma CgA levels in the 57 patients with a single HNPGL (r = 0·57, P = 0·001).

CONCLUSIONS

Plasma CgA levels are increased in only a small portion of patients with hereditary HNPGL and have limited additional value to the combination of radiological and routine biochemical assessment of patients with HNPGL. Increased plasma CgA levels are associated with increased noradrenergic activity and tumour size in patients with a single HNPGL.

Effects of chromogranin A deficiency and excess in vivo: biphasic blood pressure and catecholamine responses

OBJECTIVE

The phenotype of the chromogranin A (Chga) null (knockout) mouse is hypertensive. However, hypertensive humans and spontaneously hypertensive rats display elevated CHGA expression. This study addresses the paradox that both ablation and elevation of CHGA result in hypertension.

METHODS

Mice with varying copy number of the CHGA gene were generated. In these mice CHGA, catecholamine and blood pressure (BP) were measured. Also a cohort of healthy human individuals was stratified into tertiles based on plasma CHGA expression and phenotyped for characteristics including their BP response to environmental (cold) stress.

RESULTS

The mice displayed a direct CHGA gene dose-dependent (0-4 copies/genome) activation of CHGA expression in both plasma and adrenal gland, yet the BP dependence of CHGA gene dose was U-shaped, maximal at 0 and four copies of the gene, whereas minimal at two copies (i.e., the wild-type gene dosage). Plasma catecholamine showed a parallel U-shaped dose/response in mice, whereas adrenal epinephrine exhibited a reciprocal (inverted) U-shaped response, suggesting dysregulated neurotransmission at both extremes of CHGA expression. The human individuals also showed a nonlinear relationship between CHGA expression and pressor responses to environmental (cold) stress, that were maximal in the highest and lowest tertiles, though basal BPs did not differ among the groups. The human CHGA tertiles also differed in epinephrine secretion as well as degree of CHGA processing to catestatin (catecholamine release-inhibitory peptide derived from CHGA processing).

CONCLUSION

Thus, across mammalian species, an optimal amount of CHGA may be required to establish appropriate catecholamine storage and release, and hence BP homeostasis.

Chromogranin A as a crucial factor in the sorting of peptide hormones to secretory granules

Chromogranin A (CgA) is a soluble glycoprotein stored along with hormones and neuropeptides in secretory granules of endocrine cells. In the last four decades, intense efforts have been concentrated to characterize the structure and the biological function of CgA. Besides, CgA has been widely used as a diagnostic marker for tumors of endocrine origin, essential hypertension, various inflammatory diseases, and neurodegenerative disorders such as amyotrophic lateral sclerosis and Alzheimer's disease. CgA displays peculiar structural features, including numerous multibasic cleavage sites for prohormone convertases as well as a high proportion of acidic residues. Thus, it has been proposed that CgA represents a precursor of biologically active peptides, and a "granulogenic protein" that plays an important role as a chaperone for catecholamine storage in adrenal chromaffin cells. The widespread distribution of CgA throughout the neuroendocrine system prompted several groups to investigate the role of CgA in peptide hormone sorting to the regulated secretory pathway. This review summarizes the findings and theoretical concepts around the molecular machinery used by CgA to exert this putative intracellular function. Since CgA terminal regions exhibited strong sequence conservation through evolution, our work focused on the implication of these domains as potential functional determinants of CgA. Characterization of the molecular signals implicating CgA in the intracellular traffic of hormones represents a major biological issue that may contribute to unraveling the mechanisms defining the secretory competence of neuroendocrine cells.

Intravesicular factors controlling exocytosis in chromaffin cells

Chromaffin granules are similar organelles to the large dense core vesicles (LDCV) present in many secretory cell types including neurons. LDCV accumulate solutes at high concentrations (catecholamines, 0.5-1 M; ATP, 120-300 mM; or Ca(2+), 40 mM (Bulenda and Gratzl Biochemistry 24:7760-7765, 1985). Solutes seem to aggregate to a condensed matrix to elude osmotic lysis. The affinity of solutes for LDCV matrix is responsible for the delayed release of catecholamines during exocytosis. The aggregation of solutes occurs due to a specific H(+) pump denominated V-ATPase that maintains an inner acidic media (pH ≈5.5). This pH gradient against cytosol is also responsible for the vesicular accumulation of amines and Ca(2+). When this gradient is reduced by modulation of the V-ATPase activity, catecholamines and Ca(2+) are moved toward the cytosol. In addition, some drugs largely accumulate inside LDCV and not only impair the accumulation of natural solutes, but also act as false neurotransmitters when they are co-released with catecholamines. There is much experimental evidence to conclude that the physiological modulation of vesicle pH and the manipulation of intravesicular media with drugs affect the LDCV cargo and change the kinetics of exocytosis. Here, we will present some experimental data demonstrating the participation of drugs in the kinetics of exocytosis through changes in the composition of vesicular media. We also offer a model to explain the regulation of exocytosis by the intravesicular media that conciliate the experimentally obtained data.

Chromogranins A and B as regulators of vesicle cargo and exocytosis

Chromogranins (Cgs) are acidic proteins that have been implicated in several physiological processes such as vesicle sorting, the production of bioactive peptides and the accumulation of soluble species inside large dense core vesicles (LDCV). They constitute the main protein component in the vesicular matrix of LDCV. This latter characteristic of Cgs accounts for the ability of vesicles to concentrate catecholamines and Ca(2+). It is likely that Cgs are behind the delay in the neurotransmitter exit towards the extracellular milieu after vesicle fusion, due to their low affinity and high capacity to bind solutes present inside LDCV. The recent availability of mouse strains lacking Cgs, combined with the arrival of several techniques for the direct monitoring of exocytosis, have helped to expand our knowledge about the mechanisms used by granins to concentrate catecholamines and Ca(2+) in LDCV, and how they affect the kinetics of exocytosis. We will discuss the roles of Cgs A and B in maintaining the intravesicular environment of secretory vesicles and in exocytosis, bringing together the most recent findings from adrenal chromaffin cells.

Depolarization-induced Ca2+ entry preferentially evokes release of large quanta in the developing Xenopus neuromuscular junction

The amplitude histogram of spontaneously occurring miniature synaptic currents (mSCs) is skewed positively at developing Xenopus neuromuscular synapses formed in culture. To test whether the quantal size of nerve-evoked quanta (eSCs) distributes similarly, we compared the amplitude histogram of single quantum eSCs in low external Ca(2+) with that of mSCs and found that nerve stimulation preferentially released large quanta. Depolarization of presynaptic terminals by elevating [K(+)] in the external solution or by direct injection of current through a patch pipette increased the mSC frequency and preferentially, but not exclusively, evoked the release of large quanta, resulting in a second broad peak in the amplitude histogram. Formation of the second peak under these conditions was blocked by the N-type Ca(2+) channel blocker, ω-conotoxin GVIA. In contrast, when the mSC frequency was elevated by thapsigargin- or caffeine-induced mobilization of internal Ca(2+), formation of the second peak did not occur. We conclude that the second peak in the amplitude histogram is generated by Ca(2+) influx through N-type Ca(2+) channels, causing a local elevation of internal Ca(2+). The mSC amplitude in the positively skewed portion of the histogram varied over a wide range. A competitive blocker of acetylcholine (ACh) receptors, d-tubocurarine, reduced the amplitude of smaller mSCs in this range relatively more than that of larger mSCs, suggesting that this variation in the mSC amplitude is due to variable amounts of ACh released from synaptic vesicles. We suggest that Ca(2+) influx through N-type Ca(2+) channels preferentially induces release of vesicles with large ACh content.

Reprint of: Chromogranin A: a new proposal for trafficking, processing and induction of granule biogenesis

Chromogranin A (CgA), a member of the granin family serves several important cell biological roles in (neuro)endocrine cells which are summarized in this review. CgA is a "prohormone" that is synthesized at the rough endoplasmic reticulum and transported into the cisternae of this organelle via its signal peptide. It is then trafficked to the Golgi complex and then to the trans-Golgi network (TGN) where CgA aggregates at low pH in the presence of calcium. The CgA aggregates provide the physical driving force to induce budding of the TGN membrane resulting in dense core granule (DCG) formation. Within the granule, a small amount of the CgA is processed to bioactive peptides, including a predicted C-terminal peptide, serpinin. Upon stimulation, DCGs undergo exocytosis and CgA and its derived peptides are released. Serpinin, acting extracellularly is able to signal the increase in transcription of a serine protease inhibitor, protease nexin-1 (PN-1) that protects DCG proteins against degradation in the Golgi complex, which then enhances DCG biogenesis to replenish those that were released. Thus CgA and its derived peptide, serpinin, plays a significant role in granule formation and regulation of granule biogenesis, respectively, in (neuro) endocrine cells.

Urocortin 2 lowers blood pressure and reduces plasma catecholamine levels in mice with hyperadrenergic activity

Exaggerated adrenergic activity is associated with human hypertension. The peptide urocortin 2 (Ucn 2) inhibits catecholamine synthesis and secretion from adrenal chromaffin cells in vitro and administration to mammals lowers blood pressure (BP). The chromogranin A-null mouse (Chga-/-) manifests systemic hypertension because of excessive catecholamine secretion from the adrenal and decreased catecholamine storage. In the present study, we investigated whether systemic administration of Ucn 2 could reduce BP and adrenal and plasma levels of catecholamines in vivo. Ucn 2 peptide was administered to freely moving, conscious Chga-/- and wild-type control mice. Telemetry and HPLC measured changes in BP and catecholamine levels, respectively. In both groups of mice, Ucn 2 dose-dependently decreased BP, and this effect was mediated by corticotropin factor-receptor type 2. However, in Chga-/- mice, the maximal percentage decrease of systolic BP from basal systolic BP was 37% compared with only a 23% reduction in wild-type mice (P=0.04). In Chga-/- mice only, Ucn 2 decreased adrenal and plasma levels of catecholamines as well as adrenal levels of tyrosine hydroxylase protein and phosphorylation. In vitro mechanistic studies demonstrated that Ucn 2 reduces both catecholamine secretion and tyrosine hydroxylase promoter activity, suggesting that the exaggerated action of Ucn 2 to reduce BP in the Chga-/- mouse is mediated through inhibition of both catecholamine synthesis and secretion. The data suggest that Ucn 2 may be therapeutically useful in regulating the exaggerated sympathoadrenal function of hyperadrenergic hypertension.

Chromogranins as regulators of exocytosis

Chromogranins (Cgs) constitute the main protein component in the vesicular matrix of large dense core vesicles (LDCV). These acidic proteins have been implicated in several physiological processes such as vesicle sorting, the generation of bioactive peptides and the accumulation of soluble species inside LDCV. This latter feature of Cgs accounts for the ability of vesicles to concentrate catecholamines and Ca(2+). Indeed, the low affinity and high capacity of Cgs to bind solutes at the low pH of the LDCV lumen seems to be behind the delay in the neurotransmitter exit towards the extracellular milieu after vesicle fusion. The availability of new mouse strains lacking Cgs in combination with the arrival of several techniques for the direct monitoring of exocytosis (like amperometry, patch-amperometry and intracellular electrochemistry), have helped advance our understanding of how these granins concentrate catecholamines and Ca(2+) in LDCV, and how they influence the kinetics of exocytosis. In this review, we will discuss the roles of Cgs A and B in maintaining the intravesicular environment of secretory vesicles and in exocytosis, bringing together the most recent findings from adrenal chromaffin cells.

Real-time monitoring of chemical transmission in slices of the murine adrenal gland

The real-time electrochemical detection of catecholamine secretion from murine adrenal slices using fast-scan cyclic voltammetry (FSCV) and amperometry at carbon fiber microelectrodes is described. Bright-field and immunofluorescent microscopy supported that chromaffin cells in the adrenal medulla are organized into clusters and positively stain for tyrosine hydroxylase confirming that they are catecholaminergic. Spontaneous exocytotic catecholamine events were observed inside chromaffin cell clusters with both FSCV and amperometry and were modulated by the nicotinic acetylcholine receptor antagonist hexamethonium and low extracellular calcium. Reintroduction of extracellular calcium and pressure ejection of acetylcholine caused the frequency of spikes to increase back to predrug levels. Electrical stimulation caused the synchronous secretion from multiple cells within the gland, which were modulated by nicotinic acetylcholine receptors but not muscarinic receptors or gap junctions. Furthermore, electrically stimulated release was abolished with perfusion of low extracellular calcium or tetrodotoxin, indicating that the release requires electrical excitability. An extended waveform was used to study the spontaneous and stimulated release events to determine their chemical content by FSCV. Consistent with total content analysis and immunohistochemical studies, about two thirds of the cells studied spontaneously secreted epinephrine, whereas one third secreted norepinephrine. Whereas adrenergic sites contained mostly epinephrine during electrical stimulation, noradrenergic sites contained a mixture of the catecholamines showing the heterogeneity of the adrenal medulla.

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.

Only a Fraction of Quantal Content is Released During Exocytosis as Revealed by Electrochemical Cytometry of Secretory Vesicles

The primary method for neuronal communication involves the release of chemical messengers that are packaged intracellularly in vesicles. Although experiments measuring release at single cells have classically been thought to assess the entire content of vesicles, there is evidence in the literature that suggests that the total transmitter stored in vesicles is not expelled during exocytosis. In this work, we introduce a novel technology using a microfluidic-based platform to electrochemically probe individual PC12 cell vesicles isolated from the cell environment. We measure the total vesicular content using methodology that circumvents the biophysical processes of the cell associated with exocytosis. Direct comparisons of amperometric data from release experiments at single PC12 cells versus our cell-free model reveal that on average vesicles release only 40% of their total transmitter load. The data support the intriguing hypothesis that the average vesicle does not open all the way during the normal exocytosis process, resulting in incomplete distention of the vesicular contents. In addition, we have shown that vesicular catecholamine levels can be altered with pharmacological manipulation and variances observed from these treatments can be resolved at the single vesicle level in a high-throughput manner, a process that we have termed electrochemical cytometry. Upon establishing that release in exocytotic processes proceeds in an incomplete manner, electrochemical data quantified from both single cell release experiments and electrochemical cytometry of vesicles were related to vesicular volume from electron microscopy measurements to investigate the location of intravesicular catecholamine stores retained post-fusion.

Chromogranin B gene ablation reduces the catecholamine cargo and decelerates exocytosis in chromaffin secretory vesicles

Chromogranins/secretogranins (Cgs) are the major soluble proteins of large dense-core secretory vesicles (LDCVs). We have recently reported that the absence of chromogranin A (CgA) caused important changes in the accumulation and in the exocytosis of catecholamines (CAs) using a CgA-knock-out (CgA-KO) mouse. Here, we have analyzed a CgB-KO mouse strain that can be maintained in homozygosis. These mice have 36% less adrenomedullary epinephrine when compared to Chgb(+/+) [wild type (WT)], whereas the norepinephrine content was similar. The total evoked release of CA was 33% lower than WT mice. This decrease was not due to a lower frequency of exocytotic events but to less secretion per quantum (approximately 30%) measured by amperometry; amperometric spikes exhibited a slower ascending but a normal decaying phase. Cell incubation with L-DOPA increased the vesicle CA content of WT but not of the CgB-KO cells. Intracellular electrochemistry, using patch amperometry, showed that L-DOPA overload produced a significantly larger increase in cytosolic CAs in cells from the KO animals than chromaffin cells from the WT. These data indicate that the mechanisms for vesicular accumulation of CAs in the CgB-KO cells were saturated, while there was ample capacity for further accumulation in WT cells. Protein analysis of LDCVs showed the overexpression of CgA as well as other proteins apparently unrelated to the secretory process. We conclude that CgB, like CgA, is a highly efficient system directly involved in monoamine accumulation and in the kinetics of exocytosis from LDCVs.

Defective secretion of islet hormones in chromogranin-B deficient mice

Granins are major constituents of dense-core secretory granules in neuroendocrine cells, but their function is still a matter of debate. Work in cell lines has suggested that the most abundant and ubiquitously expressed granins, chromogranin A and B (CgA and CgB), are involved in granulogenesis and protein sorting. Here we report the generation and characterization of mice lacking chromogranin B (CgB-ko), which were viable and fertile. Unlike neuroendocrine tissues, pancreatic islets of these animals lacked compensatory changes in other granins and were therefore analyzed in detail. Stimulated secretion of insulin, glucagon and somatostatin was reduced in CgB-ko islets, in parallel with somewhat impaired glucose clearance and reduced insulin release, but normal insulin sensitivity in vivo. CgB-ko islets lacked specifically the rapid initial phase of stimulated secretion, had elevated basal insulin release, and stored and released twice as much proinsulin as wildtype (wt) islets. Stimulated release of glucagon and somatostatin was reduced as well. Surprisingly, biogenesis, morphology and function of insulin granules were normal, and no differences were found with regard to beta-cell stimulus-secretion coupling. We conclude that CgB is not required for normal insulin granule biogenesis or maintenance in vivo, but is essential for adequate secretion of islet hormones. Consequentially CgB-ko animals display some, but not all, hallmarks of human type-2 diabetes. However, the molecular mechanisms underlying this defect remain to be determined.

Chromogranins A and B and secretogranin II as prohormones for regulatory peptides from the diffuse neuroendocrine system

Chromogranin A (CgA), chromogranin B (CgB), and secretogranin II (SgII) belong to a family of uniquely acidic secretory proteins in elements of the diffuse neuroendocrine system. These "granins" are characterized by numerous pairs of basic amino acids as potential sites for intra- and extragranular processing. In response to adequate stimuli, the granins are coreleased with neurotransmitters and hormones and appear in the circulation as potential modulators of homeostatic processes. This review is directed towards functional aspects of the secreted CgA, CgB, and SgII and their biologically active sequences. Widely different effects and targets have been reported for granin-derived peptides. So far, the CgA peptides vasostatin-I, pancreastatin, and catestatin, the CgB peptides CgB(1-41) and secretolytin, and the SgII peptide secretoneurin are the most likely candidates for granin-derived regulatory peptides. Most of their effects fit into patterns of direct or indirect modulations of major functions, in particular associated with inflammatory conditions.

Secretogranin III: a bridge between core hormone aggregates and the secretory granule membrane

Secretory granules in endocrine cells selectively store bioactive peptide hormones and amines, which are secreted in a regulated manner upon appropriate stimulation. In addition to bioactive substances, various proteins and lipids characteristic of secretory granules are likely recruited to a restricted space at the trans-Golgi Network (TGN), and the space then matures to the secretory granule. Although experimental findings so far have strongly suggested that aggregation- and receptor-mediated processes are essential for the formation of secretory granules, the putative link between these two processes remains to be clarified. Recently, secretogranin III (SgIII) has been identified as a specific binding protein for chromogranin A (CgA), a representative constituent of the core aggregate within secretory granules, and it was later revealed that SgIII can also bind to the cholesterol-rich membrane domain at the TGN. Based on its multifaceted binding properties, SgIII may act as a central player in the formation of cholesterol-rich membrane platforms. Upon these platforms, essential processes for secretory granule biogenesis coordinately occur; that is, selective recruitment of prohormones, processing and modifying of prohormones, and condensation of mature hormones as an aggregate. This review summarizes the findings and theoretical concepts on the issue to date and then focuses on the putative role of SgIII in secretory granule biogenesis in endocrine cells.

Cellular distribution of chromogranin A in excitatory, inhibitory, aminergic and peptidergic neurons of the rodent central nervous system

Immunoreactivity for both processed and unprocessed forms of chromogranin A (CGA) was examined, using an antibody recognizing the WE14 epitope, among terminal fields and cell bodies of anatomically defined GABAergic, glutamatergic, cholinergic, catecholaminergic, and peptidergic cell groups in the rodent central nervous system. CGA is ubiquitous within neuronal cell bodies, with no obvious anatomical or chemically-coded subdivision of the nervous system in which CGA is not expressed in most neurons. CGA expression is essentially absent from catecholaminergic terminal fields in the CNS, suggesting a relative paucity of large dense-core vesicles in CNS compared to peripheral catecholaminergic neurons. Extensive synaptic co-localization with classical transmitter markers is not observed even in areas such as amygdala, where CGA fibers are numerous, suggesting preferential segregation of CGA to peptidergic terminals in CNS. Localization of CGA in dendrites in some areas of CNS may indicate its involvement in regulation of dendritic release mechanisms. Finally, the ubiquitous presence of CGA in neuronal cell somata, especially pronounced in GABAergic neurons, suggests a second non-secretory vesicle-associated function for CGA in CNS. We propose that CGA may function in the CNS as a prohormone and granulogenic factor in some terminal fields, but also possesses as-yet unknown unique cellular functions within neuronal somata and dendrites.

Chromogranin A: a new proposal for trafficking, processing and induction of granule biogenesis

Chromogranin A (CgA), a member of the granin family serves several important cell biological roles in (neuro)endocrine cells which are summarized in this review. CgA is a "prohormone" that is synthesized at the rough endoplasmic reticulum and transported into the cisternae of this organelle via its signal peptide. It is then trafficked to the Golgi complex and then to the trans-Golgi network (TGN) where CgA aggregates at low pH in the presence of calcium. The CgA aggregates provide the physical driving force to induce budding of the TGN membrane resulting in dense core granule (DCG) formation. Within the granule, a small amount of the CgA is processed to bioactive peptides, including a predicted C-terminal peptide, serpinin. Upon stimulation, DCGs undergo exocytosis and CgA and its derived peptides are released. Serpinin, acting extracellularly is able to signal the increase in transcription of a serine protease inhibitor, protease nexin-1 (PN-1) that protects DCG proteins against degradation in the Golgi complex, which then enhances DCG biogenesis to replenish those that were released. Thus CgA and its derived peptide, serpinin, plays a significant role in granule formation and regulation of granule biogenesis, respectively, in (neuro) endocrine cells.

Global disturbances in autonomic function yield cardiovascular instability and hypertension in the chromogranin a null mouse

We reported previously that chromogranin A (Chga) knockout (KO) mice are hypertensive and hyperadrenergic. Here we sought to determine the basis of such alterations by probing physiological, biochemical, and pharmacological responses to perturbations of the autonomic nervous system. In the conscious state, KO mice had substantially elevated basal high blood pressure (BP) and heart rate (HR); immobilization stress caused increments in systolic BP and HR in both wild-type (WT) and KO mice, with higher maxima but blunted increments in the KO state. Catestatin (CST; CHGA(352-372)) selectively diminished stress-induced increments in BP and HR in KO mice, implicating CST as an antihypertensive peptide, even in stressful conditions. Heightened plasma catecholamines in KO mice returned to WT level after CST. Stress caused further increments in catecholamines in WT mice but no change in KO mice. KO mice displayed diminished baroreflex sensitivity in response to either phenylephrine or sodium nitroprusside, accounting for exaggerated pressor and depressor responses to these compounds; baroreceptor function was normalized by CST. To probe the relative roles of endogenous/basal sympathetic vs. parasympathetic tone in control of BP and HR, we used the muscarinic-cholinergic antagonist atropine or the beta-adrenergic antagonist propranolol; HR and BP responses to each antagonist were exaggerated in KO animals. We conclude that ablation of Chga expression results in global disturbances in autonomic function, both sympathetic and parasympathetic, that can be abrogated (or rescued), at least in part, by replacement of CST. The results point to mechanisms whereby CHGA and its CST fragment act to control cardiovascular homeostasis.