Requirements for the identification of dense-core granules

Dense and Acidic Organelle-Targeted Visualization in Living Cells: Application of Viscosity-Responsive Fluorescence Utilizing Restricted Access to Minimum Energy Conical Intersection

Cell-imaging methods with functional fluorescent probes are an indispensable technique to evaluate physical parameters in cellular microenvironments. In particular, molecular rotors, which take advantage of the twisted intramolecular charge transfer (TICT) process, have helped evaluate microviscosity. However, the involvement of charge-separated species in the fluorescence process potentially limits the quantitative evaluation of viscosity. Herein, we developed viscosity-responsive fluorescent probes for cell imaging that are not dependent on the TICT process. We synthesized AnP₂-H and AnP₂-OEG, both of which contain 9,10-di(piperazinyl)anthracene, based on 9,10-bis(N,N-dialkylamino)anthracene that adopts a nonflat geometry at minimum energy conical intersection. AnP₂-H and AnP₂-OEG exhibited enhanced fluorescence as the viscosity increased, with sensitivities comparable to those of conventional molecular rotors. In living cell systems, AnP₂-OEG showed low cytotoxicity and, reflecting its viscosity-responsive property, allowed specific visualization of dense and acidic organelles such as lysosomes, secretory granules, and melanosomes under washout-free conditions. These results provide a new direction for developing functional fluorescent probes targeting dense organelles.

Isolation of large dense-core vesicles from bovine adrenal medulla for functional studies

Large dense-core vesicles (LDCVs) contain a variety of neurotransmitters, proteins, and hormones such as biogenic amines and peptides, together with microRNAs (miRNAs). Isolation of LDCVs is essential for functional studies including vesicle fusion, vesicle acidification, monoamine transport, and the miRNAs stored in LDCVs. Although several methods were reported for purifying LDCVs, the final fractions are significantly contaminated by other organelles, compromising biochemical characterization. Here we isolated LDCVs (chromaffin granules) with high yield and purity from bovine adrenal medulla. The fractionation protocol combines differential and continuous sucrose gradient centrifugation, allowing for reducing major contaminants such as mitochondria. Purified LDCVs show robust acidification by the endogenous V-ATPase and undergo SNARE-mediated fusion with artificial membranes. Interestingly, LDCVs contain specific miRNAs such as miR-375 and miR-375 is stabilized by protein complex against RNase A. This protocol can be useful in research on the biological functions of LDCVs.

Impaired Processing of Prohormones in Secretogranin III-Null Mice Causes Maladaptation to an Inadequate Diet and Stress

Secretogranin III (SgIII), a member of the granin family, binds both to another granin, chromogranin A (CgA), and to a cholesterol-rich membrane that is destined for secretory granules (SGs). The knockdown of SgIII in adrenocorticotropic hormone (ACTH)-producing AtT-20 cells largely impairs the regulated secretion of CgA and ACTH. To clarify the physiological roles of SgIII in vivo, we analyzed hormone secretion and SG biogenesis in newly established SgIII-knockout (KO) mice. Although the SgIII-KO mice were viable and fertile and exhibited no overt abnormalities under ordinary rearing conditions, a high-fat/high-sucrose diet caused pronounced obesity in the mice. Furthermore, in the SgIII-KO mice compared with wild-type (WT) mice, the stimulated secretion of active insulin decreased substantially, whereas the storage of proinsulin increased in the islets. The plasma ACTH was also less elevated in the SgIII-KO mice than in the WT mice after chronic restraint stress, whereas the storage level of the precursor proopiomelanocortin in the pituitary gland was somewhat increased. These findings suggest that the lack of SgIII causes maladaptation of endocrine cells to an inadequate diet and stress by impairing the proteolytic conversion of prohormones in SGs, whereas SG biogenesis and the basal secretion of peptide hormones under ordinary conditions are ensured by the compensatory upregulation of other residual granins or factors.

Secretion of Polypeptide Crystals from Tetrahymena thermophila Secretory Organelles (Mucocysts) Depends on Processing by a Cysteine Cathepsin, Cth4p

In many organisms, sophisticated mechanisms facilitate release of peptides in response to extracellular stimuli. In the ciliate Tetrahymena thermophila, efficient peptide secretion depends on specialized vesicles called mucocysts that contain dense crystalline cores that expand rapidly during exocytosis. Core assembly depends of endoproteolytic cleavage of mucocyst proproteins by an aspartyl protease, cathepsin 3 (CTH3). Here, we show that a second enzyme identified by expression profiling, Cth4p, is also required for processing of proGrl proteins and for assembly of functional mucocysts. Cth4p is a cysteine cathepsin that localizes partially to endolysosomal structures and appears to act downstream of, and may be activated by, Cth3p. Disruption of CTH4 results in cells (Δcth4) that show aberrant trimming of Grl proproteins, as well as grossly aberrant mucocyst exocytosis. Surprisingly, Δcth4 cells succeed in assembling crystalline mucocyst cores. However, those cores do not undergo normal directional expansion during exocytosis, and they thus fail to efficiently extrude from the cells. We could phenocopy the Δcth4 defects by mutating conserved catalytic residues, indicating that the in vivo function of Cth4p is enzymatic. Our results indicate that as for canonical proteins packaged in animal secretory granules, the maturation of mucocyst proproteins involves sequential processing steps. The Δcth4 defects uncouple, in an unanticipated way, the assembly of mucocyst cores and their subsequent expansion and thereby reveal a previously unsuspected aspect of polypeptide secretion in ciliates.

Souffle/Spastizin controls secretory vesicle maturation during zebrafish oogenesis

During oogenesis, the egg prepares for fertilization and early embryogenesis. As a consequence, vesicle transport is very active during vitellogenesis, and oocytes are an outstanding system to study regulators of membrane trafficking. Here, we combine zebrafish genetics and the oocyte model to identify the molecular lesion underlying the zebrafish souffle (suf) mutation. We demonstrate that suf encodes the homolog of the Hereditary Spastic Paraplegia (HSP) gene SPASTIZIN (SPG15). We show that in zebrafish oocytes suf mutants accumulate Rab11b-positive vesicles, but trafficking of recycling endosomes is not affected. Instead, we detect Suf/Spastizin on cortical granules, which undergo regulated secretion. We demonstrate genetically that Suf is essential for granule maturation into secretion competent dense-core vesicles describing a novel role for Suf in vesicle maturation. Interestingly, in suf mutants immature, secretory precursors accumulate, because they fail to pinch-off Clathrin-coated buds. Moreover, pharmacological inhibition of the abscission regulator Dynamin leads to an accumulation of immature secretory granules and mimics the suf phenotype. Our results identify a novel regulator of secretory vesicle formation in the zebrafish oocyte. In addition, we describe an uncharacterized cellular mechanism for Suf/Spastizin activity during secretion, which raises the possibility of novel therapeutic avenues for HSP research.

Oocytes of egg laying animals frequently represent the biggest cell type of a species. The size of the egg is a consequence of active transport processes, e.g. the import of yolk proteins, which results in the massive storage of vesicles. In addition, secretory vesicles termed cortical granules are stored in the oocyte to be discharged right after fertilization during cortical reaction, which also occurs in mammals. Their secretion leads to chorion expansion, which prevents the lethal entry of additional sperm and protects the developing embryo against physical damage. Mutants with a defect in membrane transport are successful tools to discover genes regulating vesicle formation. We molecularly identify the disrupted gene in the recessive maternal-effect mutation souffle, which encodes a homolog of human SPASTIZIN. SPASTIZIN was previously implicated in endocytosis, but our cellular analysis of mutant oocytes connects this gene also with the regulation of cortical granule exocytosis. More precisely, we show that Suf/Spastizin is crucial for the maturation of cortical granules into secretion competent vesicles describing a novel role for this protein. Since SPASITIZN causes the disease Hereditary Spastic Paraplegia in humans, our results will help to decipher the pathogenesis of this neurodegenerative disorder.

Identification and staining of distinct populations of secretory organelles in astrocytes

Increasing evidence indicates that astrocytes, the most abundant glial cell type in the brain, respond to an elevation in cytoplasmic calcium concentration ([Ca(2+)]i) by releasing chemical transmitters (also called gliotransmitters) via regulated exocytosis of heterogeneous classes of organelles. By this process, astrocytes exert modulatory influences on neighboring cells and are thought to participate in the control of synaptic circuits and cerebral blood flow. Studying the properties of exocytosis in astrocytes is a challenge, because the cell biological basis of this process is incompletely defined. Astrocytic exocytosis involves multiple populations of secretory vesicles, including synaptic-like microvesicles (SLMVs), dense-core granules (DCGs), and lysosomes. Here we summarize the available information for identifying individual populations of secretory organelles in astrocytes, including DCGs, SLMVs, and lysosomes, and present experimental procedures for specifically staining such populations.

Lysosomal sorting receptors are essential for secretory granule biogenesis in Tetrahymena

The delivery of nonaggregated cargo proteins to Tetrahymena secretory granules requires receptors of the sortilin/VPS10 family, proteins classically associated with lysosome biogenesis.

Secretory granules, such as neuronal dense core vesicles, are specialized for storing cargo at high concentration and releasing it via regulated exocytosis in response to extracellular stimuli. Here, we used expression profiling to identify new components of the machinery for sorting proteins into mucocysts, secretory granule-like vesicles in the ciliate Tetrahymena thermophila. We show that assembly of mucocysts depends on proteins classically associated with lysosome biogenesis. In particular, the delivery of nonaggregated, but not aggregated, cargo proteins requires classical receptors of the sortilin/VPS10 family, which indicates that dual mechanisms are involved in sorting to this secretory compartment. In addition, sortilins are required for delivery of a key protease involved in T. thermophila mucocyst maturation. Our results suggest potential similarities in the formation of regulated secretory organelles between even very distantly related eukaryotes.

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

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

Evolution of intracellular compartmentalization

Cells compartmentalize their biochemical functions in a variety of ways, notably by creating physical barriers that separate a compartment via membranes or proteins. Eukaryotes have a wide diversity of membrane-based compartments, many that are lineage- or tissue-specific. In recent years, it has become increasingly evident that membrane-based compartmentalization of the cytosolic space is observed in multiple prokaryotic lineages, giving rise to several types of distinct prokaryotic organelles. Endosymbionts, previously believed to be a hallmark of eukaryotes, have been described in several bacteria. Protein-based compartments, frequent in bacteria, are also found in eukaryotes. In the present review, we focus on selected intracellular compartments from each of these three categories, membrane-based, endosymbiotic and protein-based, in both prokaryotes and eukaryotes. We review their diversity and the current theories and controversies regarding the evolutionary origins. Furthermore, we discuss the evolutionary processes acting on the genetic basis of intracellular compartments and how those differ across the domains of life. We conclude that the distinction between eukaryotes and prokaryotes no longer lies in the existence of a compartmentalized cell plan, but rather in its complexity.

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.

Controlling synaptotagmin activity by electrostatic screening

Exocytosis of neurosecretory vesicles is mediated bythe SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins syntaxin-1, synaptobrevin, and SNAP-25, with synaptotagmin functioning as the major Ca²⁺-sensor for triggering membrane fusion. Here we show that bovine chromaffin granules readily fuse with large unilamellar liposomes in a SNARE-dependent manner. Fusion is enhanced by Ca²⁺ but only if the target liposomes contain PI(4,5)P₂ and if polyphosphate anions such as nucleotides or pyrophosphate are present. Ca²⁺-dependent enhancement is mediated by endogenous synaptotagmin-1. Polyphosphates operate by an electrostatic mechanism that reverses an inactivating cis-association of synaptotagmin-1 with its own membrane whereas trans-binding is not affected. Hence, balancing trans- and cis-membrane interactions of synaptotagmin may be a crucial element in the pathway of Ca²⁺-dependent exocytosis.

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.

A novel pathway regulates thyroid hormone availability in rat and human hypothalamic neurosecretory neurons

Hypothalamic neurosecretory systems are fundamental regulatory circuits influenced by thyroid hormone. Monocarboxylate-transporter-8 (MCT8)-mediated uptake of thyroid hormone followed by type 3 deiodinase (D3)-catalyzed inactivation represent limiting regulatory factors of neuronal T3 availability. In the present study we addressed the localization and subcellular distribution of D3 and MCT8 in neurosecretory neurons and addressed D3 function in their axons. Intense D3-immunoreactivity was observed in axon varicosities in the external zone of the rat median eminence and the neurohaemal zone of the human infundibulum containing axon terminals of hypophysiotropic parvocellular neurons. Immuno-electronmicroscopy localized D3 to dense-core vesicles in hypophysiotropic axon varicosities. N-STORM-superresolution-microscopy detected the active center containing C-terminus of D3 at the outer surface of these organelles. Double-labeling immunofluorescent confocal microscopy revealed that D3 is present in the majority of GnRH, CRH and GHRH axons but only in a minority of TRH axons, while absent from somatostatin-containing neurons. Bimolecular-Fluorescence-Complementation identified D3 homodimers, a prerequisite for D3 activity, in processes of GT1-7 cells. Furthermore, T3-inducible D3 catalytic activity was detected in the rat median eminence. Triple-labeling immunofluorescence and immuno-electronmicroscopy revealed the presence of MCT8 on the surface of the vast majority of all types of hypophysiotropic terminals. The presence of MCT8 was also demonstrated on the axon terminals in the neurohaemal zone of the human infundibulum. The unexpected role of hypophysiotropic axons in fine-tuned regulation of T3 availability in these cells via MCT8-mediated transport and D3-catalyzed inactivation may represent a novel regulatory core mechanism for metabolism, growth, stress and reproduction in rodents and humans.

Gliotransmission and the tripartite synapse

In the last years, the classical view of glial cells (in particular of astrocytes) as a simple supportive cell for neurons has been replaced by a new vision in which glial cells are active elements of the brain. Such a new vision is based on the existence of a bidirectional communication between astrocytes and neurons at synaptic level. Indeed, perisynaptic processes of astrocytes express active G-protein-coupled receptors that are able (1) to sense neurotransmitters released from the synapse during synaptic activity, (2) to increase cytosolic levels of calcium, and (3) to stimulate the release of gliotransmitters that in turn can interact with the synaptic elements. The mechanism(s) by which astrocytes can release gliotransmitter has been extensively studied during the last years. Many evidences have suggested that a fraction of astrocytes in situ release neuroactive substances both with calcium-dependent and calcium-independent mechanism(s); whether these mechanisms coexist and under what physiological or pathological conditions they occur, it remains unclear. However, the calcium-dependent exocytotic vesicular release has received considerable attention due to its potential to occur under physiological conditions via a finely regulated way. By releasing gliotransmitters in millisecond time scale with a specific vesicular apparatus, astrocytes can integrate and process synaptic information and control or modulate synaptic transmission and plasticity.

Prolactin and growth hormone aggregates in secretory granules: the need to understand the structure of the aggregate

Prolactin and GH form reversible aggregates in the trans-Golgi lumen that become the dense cores of secretory granules. Aggregation is an economical means of sorting, because self-association removes the hormones from other possible pathways. Secretory granules containing different aggregates show different behavior, such as the reduction in stimulated release of granules containing R183H-GH compared with release of those containing wild-type hormone. Aggregates may facilitate localization of membrane proteins necessary for transport and exocytosis of secretory granules, and therefore understanding their properties is important. Three types of self-association have been characterized: dimers of human GH that form with Zn(2+), low-affinity self-association of human prolactin caused by acidic pH and Zn(2+) with macromolecular crowding, and amyloid fibers of prolactin. The best candidate for the form in most granules may be low-affinity self-association because it occurs rapidly at Zn(2+) concentrations that are likely to be in granules and reverses rapidly in neutral pH. Amyloid may form in older granules. Determining differences between aggregates of wild type and those of R183H-GH should help to understand why granules containing the mutant behave differently from those containing wild-type hormone. If reversible aggregation of other hormones, including those that are proteolytically processed, is the crucial act in forming granules, rather than use of a sorting signal, then prohormones should form reversible aggregates in solution in conditions that resemble those of the trans-Golgi lumen, including macromolecular crowding.

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.

Functional and structural characterization of a dense core secretory granule sorting domain from the PC1/3 protease

Several peptide hormones are initially synthesized as inactive precursors. It is only on entry of these prohormones and their processing proteases into dense core secretory granules (DCSGs) that the precursors are cleaved to generate their active forms. Prohormone convertase (PC)1/3 is a processing protease that is targeted to DCSGs. The signal for targeting PC1/3 to DCSGs resides in its carboxy-terminal tail (PC1/3(617-753)), where 3 regions (PC1/3(617-625), PC1/3(665-682), and PC1/3(711-753)) are known to aid in sorting and membrane association. In this article, we have determined a high-resolution structure of the extreme carboxy-terminal sorting domain, PC1/3(711-753) in micelles by NMR spectroscopy. PC1/3(711-753) contains 2 alpha helices located between residues 722-728 and 738-750. Functional assays demonstrate that the second helix (PC1/3(738-750)) is necessary and sufficient to target a constitutively secreted protein to granules, and that L(745) anchors a hydrophobic patch that is critical for sorting. Also, we demonstrate that calcium binding by the second helix of PC1/3(711-753) promotes aggregation of the domain via the hydrophobic patch centered on L(745). These results provide a structure-function analysis of a DCSG-sorting domain, and reveal the importance of a hydrophobic patch and calcium binding in controlling the sorting of proteins containing alpha helices to DCSGs.

Chromogranin A promotes peptide hormone sorting to mobile granules in constitutively and regulated secreting cells: role of conserved N- and C-terminal peptides

Chromogranin A (CgA) has been proposed to play a major role in the formation of dense-core secretory granules (DCGs) in neuroendocrine cells. Here, we took advantage of unique features of the frog CgA (fCgA) to assess the role of this granin and its potential functional determinants in hormone sorting during DCG biogenesis. Expression of fCgA in the constitutively secreting COS-7 cells induced the formation of mobile vesicular structures, which contained cotransfected peptide hormones. The fCgA and the hormones coexpressed in the newly formed vesicles could be released in a regulated manner. The N- and C-terminal regions of fCgA, which exhibit remarkable sequence conservation with their mammalian counterparts were found to be essential for the formation of the mobile DCG-like structures in COS-7 cells. Expression of fCgA in the corticotrope AtT20 cells increased pro-opiomelanocortin levels in DCGs, whereas the expression of N- and C-terminal deletion mutants provoked retention of the hormone in the Golgi area. Furthermore, fCgA, but not its truncated forms, promoted pro-opiomelanocortin sorting to the regulated secretory pathway. These data demonstrate that CgA has the intrinsic capacity to induce the formation of mobile secretory granules and to promote the sorting and release of peptide hormones. The conserved terminal peptides are instrumental for these activities of CgA.

Regulated exocytosis from astrocytes physiological and pathological related aspects

Astrocytes have traditionally been considered ancillary, satellite cells of the nervous system. However, it is a very recent acquisition that glial cells generate signaling loops which are integral to the brain circuitry and participate, interactively with neuronal networks, in the processing of information. Such a conceptual breakthrough makes this field of investigation one of the hottest in neuroscience, as it calls for a revision of past theories of brain function as well as for new strategies of experimental exploration of brain function. Glial cells are electrically not excitable, and it was only the use of optical recording techniques together with calcium sensitive dyes, that allowed the chemical excitability of glial cells to become apparent. Studies using these new techniques have shown for the first time that glial cells are activated by surrounding synaptic activity and translate neuronal signals into their own calcium code. Intracellular calcium concentration([Ca2+]i) elevations in glial cells have then shown to underlie spatial transfer of information in the glial network, accompanied by release of chemical transmitters (gliotransmitters) such as glutamate and back-signaling to neurons. As a consequence, optical imaging techniques applied to cell cultures or intact tissue have become a state-of-the-art technology for studying glial cell signaling. The molecular mechanisms leading to release of "gliotransmitters," especially glutamate, from glia are under debate. Accumulating evidence clearly indicates that astrocytes secrete numerous transmitters by Ca(2+)-dependent exocytosis. This review will discuss the mechanisms underlying the release of chemical transmitters from astrocytes with a particular emphasis to the regulated exocytosis processes.

Expression, transport, and axonal sorting of neuronal CCL21 in large dense-core vesicles

Neurons are highly polarized cells, and neuron-neuron communication is based on directed transport and release of neurotransmitters, neuropeptides, and neurotrophins. Directed communication may also be attributed to neuron-microglia signaling, since neuronal damage can induce a microglia reaction at specific sites only. However, the mechanism underlying this site-specific microglia reaction is not yet understood. Neuronal CCL21 is a microglia-activating chemokine, which in brain is solely found in endangered neurons and is therefore a candidate for neuron-microglia signaling. Here we present that neuronal CCL21 is sorted into large dense-core vesicles, the secretory granules of the regulated release pathway of neurons. Live-cell imaging studies show preferential sorting of CCL21-containing vesicles into axons, indicating its directed transport. Thus, mouse neurons express and transport a microglia activating factor very similar to signaling molecules used in neuron-neuron communication. These data show for the first time the directed transport of a microglia activating factor in neurons and corroborate the function of neuronal CCL21 in directed neuron-microglia communication.

A large form of secretogranin III functions as a sorting receptor for chromogranin A aggregates in PC12 cells

Granin-family proteins, including chromogranin A and secretogranin III, are sorted to the secretory granules in neuroendocrine cells. We previously demonstrated that secretogranin III binds chromogranin A and targets it to the secretory granules in pituitary corticotrope-derived AtT-20 cells. However, secretogranin III has not been identified in adrenal chromaffin and PC12 cells, where chromogranin A is correctly sorted to the secretory granules. In this study, low levels of a large and noncleaved secretogranin III have been identified in PC12 cells and rat adrenal glands. Although the secretogranin III expression was limited in PC12 cells, when the FLAG-tagged secretogranin III lacking the secretory granule membrane-binding domain was expressed excessively, hemagglutinin-tagged chromogranin A was unable to target to the secretory granules at the tips and shifted to the constitutive secretory pathway. Secretogranin III was able to bind the aggregated form of chromogranin A, suggesting that a small quantity of secretogranin III is enough to carry a large quantity of chromogranin A. Furthermore, secretogranin III bound adrenomedullin, a major peptide hormone in chromaffin cells. Indeed, small interfering RNA-directed secretogranin III depletion impaired intracellular retention of chromogranin A and adrenomedullin, suggesting that they are constitutively released to the medium. We suggest that the sorting function of secretogranin III for chromogranin A is common in PC12 and chromaffin cells as well as in other endocrine cells, and a small amount of secretogranin III is able to sort chromogranin A aggregates together with adrenomedullin to secretory granules.

Neuroarchitecture of aminergic systems in the larval ventral ganglion of Drosophila melanogaster

Biogenic amines are important signaling molecules in the central nervous system of both vertebrates and invertebrates. In the fruit fly Drosophila melanogaster, biogenic amines take part in the regulation of various vital physiological processes such as feeding, learning/memory, locomotion, sexual behavior, and sleep/arousal. Consequently, several morphological studies have analyzed the distribution of aminergic neurons in the CNS. Previous descriptions, however, did not determine the exact spatial location of aminergic neurite arborizations within the neuropil. The release sites and pre-/postsynaptic compartments of aminergic neurons also remained largely unidentified. We here used gal4-driven marker gene expression and immunocytochemistry to map presumed serotonergic (5-HT), dopaminergic, and tyraminergic/octopaminergic neurons in the thoracic and abdominal neuromeres of the Drosophila larval ventral ganglion relying on Fasciclin2-immunoreactive tracts as three-dimensional landmarks. With tyrosine hydroxylase- (TH) or tyrosine decarboxylase 2 (TDC2)-specific gal4-drivers, we also analyzed the distribution of ectopically expressed neuronal compartment markers in presumptive dopaminergic TH and tyraminergic/octopaminergic TDC2 neurons, respectively. Our results suggest that thoracic and abdominal 5-HT and TH neurons are exclusively interneurons whereas most TDC2 neurons are efferent. 5-HT and TH neurons are ideally positioned to integrate sensory information and to modulate neuronal transmission within the ventral ganglion, while most TDC2 neurons appear to act peripherally. In contrast to 5-HT neurons, TH and TDC2 neurons each comprise morphologically different neuron subsets with separated in- and output compartments in specific neuropil regions. The three-dimensional mapping of aminergic neurons now facilitates the identification of neuronal network contacts and co-localized signaling molecules, as exemplified for DOPA decarboxylase-synthesizing neurons that co-express crustacean cardioactive peptide and myoinhibiting peptides.

Discovery and progress in our understanding of the regulated secretory pathway in neuroendocrine cells

In this review we start with a historical perspective beginning with the early morphological work done almost 50 years ago. The importance of these pioneering studies is underscored by our brief summary of the key questions addressed by subsequent research into the mechanism of secretion. We then highlight important advances in our understanding of the formation and maturation of neuroendocrine secretory granules, first using in vitro reconstitution systems, then most recently biochemical approaches, and finally genetic manipulations in vitro and in vivo.

Origins of the regulated secretory pathway

Modes of transport of soluble (or luminal) secretory proteins synthesized in the endoplasmic reticulum (ER) could be divided into two groups. The socalled constitutive secretory pathway (CSP) is common to all eukaryotic cells, constantly delivering constitutive soluble secretory proteins (CSSPs) linked to the rate of protein synthesis but largely independent of external stimuli. In regulated secretion, protein is sorted from the Golgi into storage/secretory granules (SGs) whose contents are released when stimuli trigger their final fusion with the plasma membrane (Hannah et al. 1999).

The nature and efficiency of neurotransmitter exocytosis also depend on physicochemical parameters

Exocytosis is an important biological process used by cells to deliver messengers or effectors to target cells with high spatial, quantitative, and kinetic precision. This process occurs by interaction and fusion of vesicles containing the (bio)chemical information with the cell membrane to release their contents into the surrounding medium. Because of its importance for life, this mechanism underlies many biological controlling factors, including different families of proteins and enzymes. Tremendous efforts have been made over the last decade toward their determination. However, in parallel, many studies have also shown that the physical and chemical characteristics of the exocytosis actors (vesicle, membrane, and extracellular medium) could directly affect the quantitative or kinetic features of secretion. The major pieces evidence for this influence, which have been reported in the literature, are reviewed herein. It demonstrates undoubtedly that pure biological aspects cannot be segregated from the physicochemical context of living mechanisms.

Sending proteins to dense core secretory granules: still a lot to sort out

The intracellular sorting of peptide hormone precursors to the dense core secretory granules (DCSGs) is essential for their bioactivation. Despite the fundamental importance of this cellular process, the nature of the sorting signals for entry of proteins into DCSGs remains a source of vigorous debate. This review highlights recent discoveries that are consistent with a model in which several protein domains, acting in a cell-specific fashion and at different steps in the sorting process, act in concert to regulate the entry of proteins into DCSGs.

PF-4/CXCL4 and CXCL4L1 exhibit distinct subcellular localization and a differentially regulated mechanism of secretion

PF-4/CXCL4 is a member of the CXC chemokine family, which is mainly produced by platelets and known for its pleiotropic biological functions. Recently, the proteic product of a nonallelic variant gene of CXCL4 was isolated from human platelets and named as CXCL4L1. CXCL4L1 shows only 4.3% amino acid divergence in the mature protein, but exhibits a 38% amino acid divergence in the signal peptide region. We hypothesized that this may imply a difference in the cell type in which CXCL4L1 is expressed or a difference in its mode of secretion. In different types of transfected cells, CXCL4 and CXCL4L1 exhibited a distinct subcellular localization and a differential regulation of secretion, CXCL4 being stored in secretory granules and released in response to protein kinase C activation, whereas CXCL4L1 was continuously synthesized and secreted through a constitutive pathway. A protein kinase C-regulated CXCL4 secretion was observed also in lymphocytes, a cell type expressing mainly CXCL4 mRNA, whereas smooth muscle cells, which preferentially expressed CXCL4L1, exhibited a constitutive pathway of secretion. These results demonstrate that CXCL4 and CXCL4L1 exhibit a distinct subcellular localization and are secreted in a differentially regulated manner, suggesting distinct roles in inflammatory or homeostatic processes.

Regulation of large dense-core vesicle volume and neurotransmitter content mediated by adaptor protein 3

Adaptor protein 3 (AP-3) is a vesicle-coat protein that forms a heterotetrameric complex. Two types of AP-3 subunits are found in mammalian cells. Ubiquitous AP-3 subunits are expressed in all tissues of the body, including the brain. In addition, there are neuronal AP-3 subunits that are thought to serve neuron-specific functions such as neurotransmitter release. In this study, we show that overexpression of neuronal AP-3 in mouse chromaffin cells results in a striking decrease in the neurotransmitter content of individual vesicles (quantal size), whereas deletion of all AP-3 produces a dramatic increase in quantal size; these changes were correlated with alterations in dense-core vesicle size. AP-3 appears to localize in the trans-Golgi network and possibly immature secretory vesicles, where it may be involved in the formation of neurosecretory vesicles.

Biogenesis of secretory granules

Secretory granules of neuroendocrine cells store and release peptide hormones and neuropeptides in response to various stimuli. Generation of granules from the Golgi complex involves the aggregation of cargo proteins and their sorting from non-regulated secretory molecules. Recent findings on knockout mice lacking individual granule constituents have challenged the hypothesis that an 'essential' protein for the assembly of these organelles exists, while studies on polypyrimidine tract-binding protein and ICA512/IA-2 have provided insight into the mechanisms for adjusting granule production in relation to stimulation and secretory activity.

Interaction between secretogranin III and carboxypeptidase E facilitates prohormone sorting within secretory granules

Secretogranin III (SgIII) and carboxypeptidase E (CPE) bind specifically to cholesterol-rich secretory granule (SG) membranes. We previously showed that SgIII binds chromogranin A (CgA) and targets CgA to the SGs in endocrine cells. We investigated the binding of SgIII and CPE because they frequently localize close to the periphery of SGs, and they bind each other in mouse corticotrope-derived AtT-20 cells. In Cpe fat mouse corticotropes, which have defective CPE, proopiomelanocortin (POMC)-derived adrenocorticotrophin hormone (ACTH)-containing peptides were distributed over the entire surface of the SGs, and displayed a regulated secretion by secretagogues. The Cpe fat pituitary exhibited elevated levels of SgIII and CgA, which suggests that they compensate for a sorting function of CPE for POMC and its intermediates to ACTH. Indeed, both SgIII and CgA were able to bind POMC-derived intermediates. In a competitive pull-down assay, excessive SgIII led to a decrease in CPE-bound POMC-derived intermediate molecules, and SgIII pulled-down by anti-ACTH antibody increased proportionately. We suggest that SgIII and CPE form the separate functional sorting complex by anchoring to cholesterol-rich SG membranes, and POMC-derived peptides are transferred from CPE to SgIII, and subsequently to CgA.

Ultrastructural evidence of piecemeal degranulation in large dense-core vesicles of brain neurons

Large dense-core vesicles (LDCV) are a group of neuronal secretory organelles with different size and characteristically condensed morphology. LDCV release their specific cargo by regulated exocytosis, either in the form of "full fusion" or "kiss-and-run" exocytosis. In this paper, we provide ultrastructural evidence indicative of a slow and particulate mode of secretion from LDCV, called piecemeal degranulation (PMD). A number of LDCV in the nerve boutons of mouse brain presented marked increase in their size accompanied by reduction and also disappearance of content material. Residual secretory constituents in altered LDCV displayed eroded marginated patterns, leading to eccentric "haloed" morphologies. Remarkably, altered LDCV never appeared to be fused with each other or with the nerve plasma membrane. Very small vesicles, empty or apparently loaded with the same material making-up the LDCV content, could be seen near or attached to LDCV and the plasma membrane. First described in basophils, mast cells and eosinophils, PMD has recently been recognized in various neuro-endocrine cells, like adrenal chromaffin cells and endocrine cells of the gastro-intestinal epithelia. Here we suggest that PMD may be a hitherto unrecognized pathway of neuron secretion. It would represent the morphological correlate of a long-lasting and low-level process of neuro-transmitter release. It extends the patterns of neuron secretion and possibly opens new perspectives in understanding neuron plasticity.

An electron microscopy study of changes in dense core vesicles of PC12 cells following pulse depolarization

Depolarization of PC12 cells by brief pulses of high K+ or ATP produces electrophysiological responses consistent with the quantal release of ATP. Quantitative electron microscopy was used to validate whether the same protocol changed dense core vesicles containing endogenous ATP. The total vesicle number fell by 54% after high K+ and by 21% after ATP. Perimembrane vesicles were significantly depleted by high K+, yet unchanged by ATP, suggesting differential contribution by vesicle pools to distinct stimuli during the release process. Vesicle changes were consistent with vesicular release mechanisms for the liberation of discrete packets of endogenous ATP. These data thus support the use of clustered PC12 cells as a model to study the process of ATP release.

Secretion and molecular forms of NESP55, a novel genomically imprinted neuroendocrine-specific protein from AtT-20 cells

NESP55 (neuroendocrine secretory protein of M(r) 55,000) is a paternally imprinted proteoglycan, expressed specifically in endocrine cells and the nervous system. We investigated the subcellular localization and secretion of NESP55 in AtT-20 cells. NESP55 accumulated in the medium linearly over 24 h exceeding its intracellular content 3.7-fold by that time. Incubation of cells at 16 degrees C, to block protein export, inhibited basal secretion by 79%. Stimulation of AtT-20 cells with 8-Br-cAMP increased secretion of NESP55 by only 45%. The NESP55 secretory vesicles sedimented at a density of 1.2-1.4 M, which is slightly lighter than that of the large dense core vesicles. Immunofluorescence studies revealed immunoreactivity in the Golgi apparatus and a punctuate staining of processes or neurites. Our data demonstrate that NESP55 is mainly sorted to and released from a population of constitutive secretory vesicles, which are transported out of the perikarya into processes or axons. In addition, some NESP55 is also routed to the regulated pathway. The signal peptide of NESP55, as determined with peptide antisera, is 46 amino acids long and represents the best conserved region of this molecule suggesting that the signal peptide may have a function of its own. The subcellular localization and export of NESP55 from cells are reminiscent of neuronal proteoglycans forming the extracellular matrix, which are implicated in the development and maintenance of neuronal circuits and mechanisms of axonal guidance.

Thapsigargin-induced mobilization of dendritic dense-cored vesicles in rat supraoptic neurons

Dense-cored vesicles (DCVs) containing oxytocin or vasopressin are secreted from both the nerve terminals in the posterior pituitary and dendrites in the hypothalamus of magnocellular supraoptic neurons. Dendritic secretion can be enhanced (primed) by pretreatment with either thapsigargin or oxytocin for subsequent activity-dependent release. Here, we determined whether priming involves a translocation of DCV closer to the dendritic membrane. To reduce total vesicle content, rats were salt-loaded for 24 h before application of thapsigargin or vehicle onto the ventrally exposed surface of the supraoptic nucleus in vivo. Tissues were then prepared for quantitative electron microscopic analysis of the total incidence of DCVs within supraoptic dendritic cross-sections, and of the incidence and distance (within a 500-nm margin) of each DCV to the dendritic plasma membrane. Salt loading per se did not alter the frequency distribution or average proportion of DCVs found in the 500-nm margin but significantly decreased the average incidence of DCVs per dendrite by 30% (P < 0.05). However, thapsigargin treatment resulted in a significant increase in the total incidence of DCVs within the 500-nm margins and a higher incidence of DCVs within the first 200 nm of the plasma membrane (P < 0.05), indicating that the thapsigargin-induced priming involves a relocation of DCVs closer to sites of secretion.