Ca2+-dependent mobility of vesicles capturing anti-VGLUT1 antibodies

Exocytosis in Astrocytes

Until recently, astrocytes were thought to be a part of a simple "brain glue" providing only a supporting role for neurons. However, the discoveries of the last two decades have proven astrocytes to be dynamic partners participating in brain metabolism and actively influencing communication between neurons. The means of astrocyte-neuron communication are diverse, although regulated exocytosis has received the most attention but also caused the most debate. Similar to most of eukaryotic cells, astrocytes have a complex range of vesicular organelles which can undergo exocytosis as well as intricate molecular mechanisms that regulate this process. In this review, we focus on the components needed for regulated exocytosis to occur and summarise the knowledge about experimental evidence showing its presence in astrocytes.

Methods for Monitoring Endocytosis in Astrocytes

Endocytosis is a vesicle-based mechanism by which eukaryotic cells internalize extracellular material. There are several types of this universal mechanism linked to different types of endocytosed cargo, including pathogens; therefore, several approaches can be applied. Here, we describe techniques that are applicable to study the internalization of flaviviruses; dextrans; transporters, such as, glutamate transporter vGlut1; and peptidergic signaling molecules, including atrial natriuretic peptide into astrocytes, the most heterogeneous neuroglial cells, which play a key homeostatic role in the central nervous system.

Nestin affects fusion pore dynamics in mouse astrocytes

AIM

Astrocytes play a homeostatic role in the central nervous system and influence numerous aspects of neurophysiology via intracellular trafficking of vesicles. Intermediate filaments (IFs), also known as nanofilaments, regulate a number of cellular processes including organelle trafficking and adult hippocampal neurogenesis. We have recently demonstrated that the IF protein nestin, a marker of neural stem cells and immature and reactive astrocytes, is also expressed in some astrocytes in the unchallenged hippocampus and regulates neurogenesis through Notch signalling from astrocytes to neural stem cells, possibly via altered trafficking of vesicles containing the Notch ligand Jagged-1.

METHODS

We thus investigated whether nestin affects vesicle dynamics in astrocytes by examining single vesicle interactions with the plasmalemma and vesicle trafficking with high-resolution cell-attached membrane capacitance measurements and confocal microscopy. We used cell cultures of astrocytes from nestin-deficient (Nes^-/- ) and wild-type (wt) mice, and fluorescent dextran and Fluo-2 to examine vesicle mobility and intracellular Ca²⁺ concentration respectively.

RESULTS

Nes^-/- astrocytes exhibited altered sizes of vesicles undergoing full fission and transient fusion, altered vesicle fusion pore geometry and kinetics, decreased spontaneous vesicle mobility and altered ATP-evoked mobility. Purinergic stimulation evoked Ca²⁺ signalling that was slightly attenuated in Nes^-/- astrocytes, which exhibited more oscillatory Ca²⁺ responses than wt astrocytes.

CONCLUSION

These results demonstrate at the single vesicle level that nestin regulates vesicle interactions with the plasmalemma and vesicle trafficking, indicating its potential role in astrocyte vesicle-based communication.

Astroglial vesicular network: evolutionary trends, physiology and pathophysiology

Intracellular organelles, including secretory vesicles, emerged when eukaryotic cells evolved some 3 billion years ago. The primordial organelles that evolved in Archaea were similar to endolysosomes, which developed, arguably, for specific metabolic tasks, including uptake, metabolic processing, storage and disposal of molecules. In comparison with prokaryotes, cell volume of eukaryotes increased by several orders of magnitude and vesicle traffic emerged to allow for communication between distant intracellular locations. Lysosomes, first described in 1955, a prominent intermediate of endo- and exocytotic pathways, operate virtually in all eukaryotic cells including astroglia, the most heterogeneous type of homeostatic glia in the central nervous system. Astrocytes support neuronal network activity in particular through elaborated secretion, based on a complex intracellular vesicle network dynamics. Deranged homeostasis underlies disease and astroglial vesicle traffic contributes to the pathophysiology of neurodegenerative (Alzheimer's disease, Huntington's disease), neurodevelopmental diseases (intellectual deficiency, Rett's disease) and neuroinfectious (Zika virus) disorders. This review addresses astroglial cell-autonomous vesicular traffic network, as well as its into primary and secondary vesicular network defects in diseases, and considers this network as a target for developing new therapies for neurological conditions.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Astrocytic Vesicle-based Exocytosis in Cultures and Acutely Isolated Hippocampal Rodent Slices

Astrocytes are excitable neural cells that contribute to brain information processing via bidirectional communication with neurons. This involves the release of gliosignaling molecules that affect synapses patterning and activity. Mechanisms mediating the release of these molecules likely consist of non-vesicular and vesicular-based mechanisms. It is the vesicle-based regulated exocytosis that is an evolutionary more complex process. It is well established that the release of gliosignaling molecules has profound effects on information processing in different brain regions (e.g., hippocampal astrocytes contribute to long-term potentiation [LTP]), which has traditionally been considered as one of the cellular mechanisms underlying learning and memory. However, the paradigm of vesicle-based regulated release of gliosignaling molecules from astrocytes is still far from being unanimously accepted. One of the most important questions is to what extent can the conclusions obtained from cultured astrocytes be translated to in vivo conditions. Here, we overview the properties of vesicle mobility and their fusion with the plasma membrane in cultured astrocytes and compare these parameters to those recorded in astrocytes from acute brain hippocampal slices. The results from both experimental models are similar, which validates experiments on isolated astrocytes and further supports arguments in favor of in vivo vesicle-based exocytotic release of gliosignaling molecules. © 2017 Wiley Periodicals, Inc.

Astrocytic Pathological Calcium Homeostasis and Impaired Vesicle Trafficking in Neurodegeneration

Although the central nervous system (CNS) consists of highly heterogeneous populations of neurones and glial cells, clustered into diverse anatomical regions with specific functions, there are some conditions, including alertness, awareness and attention that require simultaneous, coordinated and spatially homogeneous activity within a large area of the brain. During such events, the brain, representing only about two percent of body mass, but consuming one fifth of body glucose at rest, needs additional energy to be produced. How simultaneous energy procurement in a relatively extended area of the brain takes place is poorly understood. This mechanism is likely to be impaired in neurodegeneration, for example in Alzheimer’s disease, the hallmark of which is brain hypometabolism. Astrocytes, the main neural cell type producing and storing glycogen, a form of energy in the brain, also hold the key to metabolic and homeostatic support in the central nervous system and are impaired in neurodegeneration, contributing to the slow decline of excitation-energy coupling in the brain. Many mechanisms are affected, including cell-to-cell signalling. An important question is how changes in cellular signalling, a process taking place in a rather short time domain, contribute to the neurodegeneration that develops over decades. In this review we focus initially on the slow dynamics of Alzheimer’s disease, and on the activity of locus coeruleus, a brainstem nucleus involved in arousal. Subsequently, we overview much faster processes of vesicle traffic and cytosolic calcium dynamics, both of which shape the signalling landscape of astrocyte-neurone communication in health and neurodegeneration.

The structural and functional evidence for vesicular release from astrocytes in situ

The concept of the tripartite synapse states that bi-directional signalling between perisynaptic astrocyte processes, presynaptic axonal boutons and postsynaptic neuronal structures defines the properties of synaptic information processing. Ca²⁺-dependent vesicular release from astrocytes, as one of the mechanisms of astrocyte-neuron communication, has attracted particular attention but has also been the subject of intense debate. In neurons, regulated vesicular release is a strongly coordinated process. It requires a complex release machinery comprised of many individual components ranging from vesicular neurotransmitter transporters and soluble NSF attachment protein receptors (SNARE) proteins to Ca²⁺-sensors and the proteins that spatially and temporally control exocytosis of synaptic vesicles. If astrocytes employ similar mechanisms to release neurotransmitters is less well understood. The aim of this review is therefore to discuss recent experimental evidence that sheds light on the central structural components responsible for vesicular release from astrocytes in situ.

Time-dependent uptake and trafficking of vesicles capturing extracellular S100B in cultured rat astrocytes

Astrocytes, the most heterogeneous glial cells in the central nervous system, contribute to brain homeostasis, by regulating a myriad of functions, including the clearance of extracellular debris. When cells are damaged, cytoplasmic proteins may exit into the extracellular space. One such protein is S100B, which may exert toxic effects on neighboring cells unless it is removed from the extracellular space, but the mechanisms of this clearance are poorly understood. By using time-lapse confocal microscopy and fluorescently labeled S100B (S100B-Alexa⁴⁸⁸ ) and fluorescent dextran (Dextran⁵⁴⁶ ), a fluid phase uptake marker, we examined the uptake of fluorescently labeled S100B-Alexa⁴⁸⁸ from extracellular space and monitored trafficking of vesicles that internalized S100B-Alexa⁴⁸⁸ . Initially, S100B-Alexa⁴⁸⁸ and Dextran⁵⁴⁶ internalized with distinct rates into different endocytotic vesicles; S100B-Alexa⁴⁸⁸ internalized into smaller vesicles than Dextran⁵⁴⁶ . At a later stage, S100B-Alexa⁴⁸⁸ -positive vesicles substantially co-localized with Dextran⁵⁴⁶ -positive endolysosomes and with acidic LysoTracker-positive vesicles. Cell treatment with anti-receptor for advanced glycation end products (RAGE) antibody, which binds to RAGE, a 'scavenger receptor', partially inhibited uptake of S100B-Alexa⁴⁸⁸ , but not of Dextran⁵⁴⁶ . The dynamin inhibitor dynole 34-2 inhibited internalization of both fluorescent probes. Directional mobility of S100B-Alexa⁴⁸⁸ -positive vesicles increased over time and was inhibited by ATP stimulation, an agent that increases cytosolic free calcium concentration ([Ca²⁺ ]_i ). We conclude that astrocytes exhibit RAGE- and dynamin-dependent vesicular mechanism to efficiently remove S100B from the extracellular space. If a similar process occurs in vivo, astroglia may mitigate the toxic effects of extracellular S100B by this process under pathophysiologic conditions. This study reveals the vesicular clearance mechanism of extracellular S100B in astrocytes. Initially, fluorescent S100B internalizes into smaller endocytotic vesicles than dextran molecules. At a later stage, both probes co-localize within endolysosomes. S100B internalization is both dynamin- and RAGE-dependent, whereas dextran internalization is dependent on dynamin. Vesicle internalization likely mitigates the toxic effects of extracellular S100B and other waste products.

Impaired αGDI Function in the X-Linked Intellectual Disability: The Impact on Astroglia Vesicle Dynamics

X-linked non-syndromic intellectual disability (XLID) is a common mental disorder recognized by cognitive and behavioral deficits. Mutations in the brain-specific αGDI, shown to alter a subset of RAB GTPases redistribution in cells, are linked to XLID, likely via changes in vesicle traffic in neurons. Here, we show directly that isolated XLID mice astrocytes, devoid of pathologic tissue environment, exhibit vesicle mobility deficits. Contrary to previous studies, we show that astrocytes express two GDI proteins. The siRNA-mediated suppression of expression of αGDI especially affected vesicle dynamics. A similar defect was recorded in astrocytes from the Gdi1 ^-/Y mouse model of XLID and in astrocytes with recombinant mutated human XLID αGDI. Endolysosomal vesicles studied here are involved in the release of gliosignaling molecules as well as in regulating membrane receptor density; thus, the observed changes in astrocytic vesicle mobility may, over the long time-course, profoundly affect signaling capacity of these cells, which optimize neural activity.

Astrocytes as secretory cells of the central nervous system: idiosyncrasies of vesicular secretion

Astrocytes are housekeepers of the central nervous system (CNS) and are important for CNS development, homeostasis and defence. They communicate with neurones and other glial cells through the release of signalling molecules. Astrocytes secrete a wide array of classic neurotransmitters, neuromodulators and hormones, as well as metabolic, trophic and plastic factors, all of which contribute to the gliocrine system. The release of neuroactive substances from astrocytes occurs through several distinct pathways that include diffusion through plasmalemmal channels, translocation by multiple transporters and regulated exocytosis. As in other eukaryotic cells, exocytotic secretion from astrocytes involves divergent secretory organelles (synaptic-like microvesicles, dense-core vesicles, lysosomes, exosomes and ectosomes), which differ in size, origin, cargo, membrane composition, dynamics and functions. In this review, we summarize the features and functions of secretory organelles in astrocytes. We focus on the biogenesis and trafficking of secretory organelles and on the regulation of the exocytotic secretory system in the context of healthy and diseased astrocytes.

Memory Formation Shaped by Astroglia

Astrocytes, the most heterogeneous glial cells in the central nervous system (CNS), execute a multitude of homeostatic functions and contribute to memory formation. Consolidation of synaptic and systemic memory is a prolonged process and hours are required to form long-term memory. In the past, neurons or their parts have been considered to be the exclusive cellular sites of these processes, however, it has now become evident that astrocytes provide an important and essential contribution to memory formation. Astrocytes participate in the morphological remodeling associated with synaptic plasticity, an energy-demanding process that requires mobilization of glycogen, which, in the CNS, is almost exclusively stored in astrocytes. Synaptic remodeling also involves bidirectional astroglial-neuronal communication supported by astroglial receptors and release of gliosignaling molecules. Astroglia exhibit cytoplasmic excitability that engages second messengers, such as Ca²⁺, for phasic, and cyclic adenosine monophosphate (cAMP), for tonic signal coordination with neuronal processes. The detection of signals by astrocytes and the release of gliosignaling molecules, in particular by vesicle-based mechanisms, occurs with a significant delay after stimulation, orders of magnitude longer than that present in stimulus–secretion coupling in neurons. These particular arrangements position astrocytes as integrators ideally tuned to support time-dependent memory formation.

Neuroglial Transmission

Neuroglia, the "glue" that fills the space between neurons in the central nervous system, takes active part in nerve cell signaling. Neuroglial cells, astroglia, oligodendroglia, and microglia, are together about as numerous as neurons in the brain as a whole, and in the cerebral cortex grey matter, but the proportion varies widely among brain regions. Glial volume, however, is less than one-fifth of the tissue volume in grey matter. When stimulated by neurons or other cells, neuroglial cells release gliotransmitters by exocytosis, similar to neurotransmitter release from nerve endings, or by carrier-mediated transport or channel flux through the plasma membrane. Gliotransmitters include the common neurotransmitters glutamate and GABA, the nonstandard amino acid d-serine, the high-energy phosphate ATP, and l-lactate. The latter molecule is a "buffer" between glycolytic and oxidative metabolism as well as a signaling substance recently shown to act on specific lactate receptors in the brain. Complementing neurotransmission at a synapse, neuroglial transmission often implies diffusion of the transmitter over a longer distance and concurs with the concept of volume transmission. Transmission from glia modulates synaptic neurotransmission based on energetic and other local conditions in a volume of tissue surrounding the individual synapse. Neuroglial transmission appears to contribute significantly to brain functions such as memory, as well as to prevalent neuropathologies.

Glutamatergic Transmission: A Matter of Three

Glutamatergic transmission in the vertebrate brain requires the involvement of glia cells, in a continuous molecular dialogue. Glial glutamate receptors and transporters are key molecules that sense synaptic activity and by these means modify their physiology in the short and long term. Posttranslational modifications that regulate protein-protein interactions and modulate transmitter removal are triggered in glial cells by neuronal released glutamate. Moreover, glutamate signaling cascades in these cells are linked to transcriptional and translational control and are critically involved in the control of the so-called glutamate/glutamine shuttle and by these means in glutamatergic neurotransmission. In this contribution, we summarize our current understanding of the biochemical consequences of glutamate synaptic activity in their surrounding partners and dissect the molecular mechanisms that allow neurons to take control of glia physiology to ensure proper glutamate-mediated neuronal communication.

Pathologic Potential of Astrocytic Vesicle Traffic: New Targets to Treat Neurologic Diseases?

Vesicles are small intracellular organelles that are fundamental for constitutive housekeeping of the plasmalemma, intercellular transport, and cell-to-cell communications. In astroglial cells, traffic of vesicles is associated with cell morphology, which determines the signaling potential and metabolic support for neighboring cells, including when these cells are considered to be used for cell transplantations or for regulating neurogenesis. Moreover, vesicles are used in astrocytes for the release of vesicle-laden chemical messengers. Here we review the properties of membrane-bound vesicles that store gliotransmitters, endolysosomes that are involved in the traffic of plasma membrane receptors, and membrane transporters. These vesicles are all linked to pathological states, including amyotrophic lateral sclerosis, multiple sclerosis, neuroinflammation, trauma, edema, and states in which astrocytes contribute to developmental disorders. In multiple sclerosis, for example, fingolimod, a recently introduced drug, apparently affects vesicle traffic and gliotransmitter release from astrocytes, indicating that this process may well be used as a new pathophysiologic target for the development of new therapies.

Astrocytic vesicles and gliotransmitters: Slowness of vesicular release and synaptobrevin2-laden vesicle nanoarchitecture

Neurotransmitters released at synapses activate neighboring astrocytes, which in turn, modulate neuronal activity by the release of diverse neuroactive substances that include classical neurotransmitters such as glutamate, GABA or ATP. Neuroactive substances are released from astrocytes through several distinct molecular mechanisms, for example, by diffusion through membrane channels, by translocation via plasmalemmal transporters or by vesicular exocytosis. Vesicular release regulated by a stimulus-mediated increase in cytosolic calcium involves soluble N-ethyl maleimide-sensitive fusion protein attachment protein receptor (SNARE)-dependent merger of the vesicle membrane with the plasmalemma. Up to 25 molecules of synaptobrevin 2 (Sb2), a SNARE complex protein, reside at a single astroglial vesicle; an individual neuronal, i.e. synaptic, vesicle contains ∼70 Sb2 molecules. It is proposed that this paucity of Sb2 molecules in astrocytic vesicles may determine the slow secretion. In the present essay we shall overview multiple aspects of vesicular architecture and types of vesicles based on their cargo and dynamics in astroglial cells.

Single-vesicle architecture of synaptobrevin2 in astrocytes

Exocytic transmitter release is regulated by the SNARE complex, which contains a vesicular protein, synaptobrevin2 (Sb2). However, Sb2 vesicular arrangement is unclear. Here we use super-resolution fluorescence microscopy to study the prevalence and distribution of endogenous and exogenous Sb2 in single vesicles of astrocytes, the most abundant glial cells in the brain. We tag Sb2 protein at C- and N termini with a pair of fluorophores, which allows us to determine the Sb2 length and geometry. To estimate total number of Sb2 proteins per vesicle and the quantity necessary for the formation of fusion pores, we treat cells with ATP to stimulate Ca2+-dependent exocytosis, increase intracellular alkalinity to enhance the fluorescence presentation of yellow-shifted pHluorin (YpH), appended to the vesicle lumen domain of Sb2, and perform photobleaching of YpH fluorophores. Fluorescence intensity analysis reveals that the total number of endogenous Sb2 units or molecules per vesicle is ≤25.

Tick-borne encephalitis virus infects rat astrocytes but does not affect their viability

Tick-borne encephalitis virus (TBEV) causes one of the most dangerous human neuroinfections in Europe and Asia. To infect neurons it must cross the blood-brain-barrier (BBB), and presumably also cells adjacent to the BBB, such as astrocytes, the most abundant glial cell type. However, the knowledge about the viral infection of glial cells is fragmental. Here we studied whether TBEV infects rat astrocytes. Rats belong to an animal group serving as a TBEV amplifying host. We employed high resolution quantitative fluorescence microscopy to investigate cell entry and cytoplasmic mobility of TBEV particles along with the effect on the cell cytoskeleton and cell survival. We report that infection of astrocytes with TBEV increases with time of exposure to TBEV and that with post-infection time TBEV particles gained higher mobility. After several days of infection actin cytoskeleton was affected, but cell survival was unchanged, indicating that rat astrocytes resist TBEV-mediated cell death, as reported for other mammalian cells. Therefore, astrocytes may present an important pool of dormant TBEV infections and a new target for therapeutic intervention.

Gliotransmission: focus on exocytotic release of L-glutamate and D-serine from astrocytes

The release of neuromodulators, called gliotransmitters, by astrocytes is proposed to modulate neurotransmission and synaptic plasticity, and thereby cognitive functions; but they are also proposed to have a role in diverse neurological disorders. Two main routes have been proposed to ensure gliotransmitter release: non-exocytotic release from cytosolic pools through plasma membrane proteins, and Ca2+-regulated exocytosis through the fusion of gliotransmitter-storing secretory organelles. Regulated Ca2+-dependent glial exocytosis has received much attention and is appealing since its existence endows astrocytes with some of the basic properties thought to be exclusive to neurons and neuroendocrine cells. The present review summarizes recent findings regarding the exocytotic mechanisms underlying the release of two excitatory amino acids, L-glutamate and D-serine.

Corticosterone treatment results in enhanced release of peptidergic vesicles in astrocytes via cytoskeletal rearrangements

While the effect of stress on neuronal physiology is widely studied, its effect on the functionality of astrocytes is not well understood. We studied the effect of high doses of stress hormone corticosterone, on two physiological properties of astrocytes, i.e., gliotransmission and interastrocytic calcium waves. To study the release of peptidergic vesicles from astrocytes, hippocampal astrocyte cultures were transfected with a plasmid to express pro-atrial natriuretic peptide (ANP) fused with the emerald green fluorescent protein (ANP.emd). The rate of decrease in fluorescence of ANP.emd on application of ionomycin, a calcium ionophore was monitored. Significant increase in the rate of calcium-dependent exocytosis of ANP.emd was observed with the 100 nM and 1 μM corticosterone treatments for 3 h, which depended on the activation of the glucocorticoid receptor. ANP.emd tagged vesicles exhibited increased mobility in astrocyte culture upon corticosterone treatment. Increasing corticosterone concentrations also resulted in concomitant increase in the calcium wave propagation velocity, initiated by focal ATP application. Corticosterone treatment also resulted in increased GFAP expression and F-actin rearrangements. FITC-Phalloidin immunostaining revealed increased formation of cross linked F-actin networks with the 100 nM and 1 μM corticosterone treatment. Alternatively, blockade of actin polymerization and disruption of microtubules prevented the corticosterone-mediated increase in ANP.emd release kinetics. This study reports for the first time the effect of corticosterone on gliotransmission via modulation of cytoskeletal elements. As ANP acts on both neurons and blood vessels, modulation of its release could have functional implications in neurovascular coupling under pathophysiological conditions of stress.

Bacterial cytolysin during meningitis disrupts the regulation of glutamate in the brain, leading to synaptic damage

Streptococcus pneumoniae (pneumococcal) meningitis is a common bacterial infection of the brain. The cholesterol-dependent cytolysin pneumolysin represents a key factor, determining the neuropathogenic potential of the pneumococci. Here, we demonstrate selective synaptic loss within the superficial layers of the frontal neocortex of post-mortem brain samples from individuals with pneumococcal meningitis. A similar effect was observed in mice with pneumococcal meningitis only when the bacteria expressed the pore-forming cholesterol-dependent cytolysin pneumolysin. Exposure of acute mouse brain slices to only pore-competent pneumolysin at disease-relevant, non-lytic concentrations caused permanent dendritic swelling, dendritic spine elimination and synaptic loss. The NMDA glutamate receptor antagonists MK801 and D-AP5 reduced this pathology. Pneumolysin increased glutamate levels within the mouse brain slices. In mouse astrocytes, pneumolysin initiated the release of glutamate in a calcium-dependent manner. We propose that pneumolysin plays a significant synapto- and dendritotoxic role in pneumococcal meningitis by initiating glutamate release from astrocytes, leading to subsequent glutamate-dependent synaptic damage. We outline for the first time the occurrence of synaptic pathology in pneumococcal meningitis and demonstrate that a bacterial cytolysin can dysregulate the control of glutamate in the brain, inducing excitotoxic damage.

Astrocytic vesicle mobility in health and disease

Astrocytes are no longer considered subservient to neurons, and are, instead, now understood to play an active role in brain signaling. The intercellular communication of astrocytes with neurons and other non-neuronal cells involves the exchange of molecules by exocytotic and endocytotic processes through the trafficking of intracellular vesicles. Recent studies of single vesicle mobility in astrocytes have prompted new views of how astrocytes contribute to information processing in nervous tissue. Here, we review the trafficking of several types of membrane-bound vesicles that are specifically involved in the processes of (i) intercellular communication by gliotransmitters (glutamate, adenosine 5′-triphosphate, atrial natriuretic peptide), (ii) plasma membrane exchange of transporters and receptors (EAAT2, MHC-II), and (iii) the involvement of vesicle mobility carrying aquaporins (AQP4) in water homeostasis. The properties of vesicle traffic in astrocytes are discussed in respect to networking with neighboring cells in physiologic and pathologic conditions, such as amyotrophic lateral sclerosis, multiple sclerosis, and states in which astrocytes contribute to neuroinflammatory conditions.

Lack of evidence for vesicular glutamate transporter expression in mouse astrocytes

The concept of a tripartite synapse including a presynaptic terminal, a postsynaptic spine, and an astrocytic process that responds to neuronal activity by fast gliotransmitter release, confers to the electrically silent astrocytes an active role in information processing. However, the mechanisms of gliotransmitter release are still highly controversial. The reported expression of all three vesicular glutamate transporters (VGLUT1-3) by astrocytes suggests that astrocytes, like neurons, may release glutamate by exocytosis. However, the demonstration of astrocytic VGLUT expression is largely based on immunostaining, and the possibility of nonspecific labeling needs to be systematically addressed. We therefore examined the expression of VGLUT1-3 in astrocytes, both in culture and in situ. We used Western blots and single-vesicle imaging by total internal reflection fluorescence microscopy in live cultured astrocytes, and confocal microscopy, at the cellular level in cortical, hippocampal, and cerebellar brain slices, combined with quantitative image analysis. Control experiments were systematically performed in cultured astrocytes using wild-type, VGLUT1-3 knock-out, VGLUT1(Venus) knock-in, and VGLUT2-EGFP transgenic mice. In fixed brain slices, we quantified the degree of overlap between VGLUT1-3 and neuronal or astrocytic markers, both in an object-based manner using fluorescence line profiles, and in a pixel-based manner using dual-color scatter plots followed by the calculation of Pearson's correlation coefficient over all pixels with intensities significantly different from background. Our data provide no evidence in favor of the expression of any of the three VGLUTs by gray matter protoplasmic astrocytes of the primary somatosensory cortex, the thalamic ventrobasal nucleus, the hippocampus, and the cerebellum.

Astrocytes negatively regulate neurogenesis through the Jagged1-mediated Notch pathway

Adult neurogenesis is regulated by a number of cellular players within the neurogenic niche. Astrocytes participate actively in brain development, regulation of the mature central nervous system (CNS), and brain plasticity. They are important regulators of the local environment in adult neurogenic niches through the secretion of diffusible morphogenic factors, such as Wnts. Astrocytes control the neurogenic niche also through membrane-associated factors, however, the identity of these factors and the mechanisms involved are largely unknown. In this study, we sought to determine the mechanisms underlying our earlier finding of increased neuronal differentiation of neural progenitor cells when cocultured with astrocytes lacking glial fibrillary acidic protein (GFAP) and vimentin (GFAP(-/-) Vim(-/-) ). We used primary astrocyte and neurosphere cocultures to demonstrate that astrocytes inhibit neuronal differentiation through a cell-cell contact. GFAP(-/-) Vim(-/-) astrocytes showed reduced endocytosis of Notch ligand Jagged1, reduced Notch signaling, and increased neuronal differentiation of neurosphere cultures. This effect of GFAP(-/-) Vim(-/-) astrocytes was abrogated in the presence of immobilized Jagged1 in a manner dependent on the activity of γ-secretase. Finally, we used GFAP(-/-) Vim(-/-) mice to show that in the absence of GFAP and vimentin, hippocampal neurogenesis under basal conditions as well as after injury is increased. We conclude that astrocytes negatively regulate neurogenesis through the Notch pathway, and endocytosis of Notch ligand Jagged1 in astrocytes and Notch signaling from astrocytes to neural stem/progenitor cells depends on the intermediate filament proteins GFAP and vimentin.

Computational quest for understanding the role of astrocyte signaling in synaptic transmission and plasticity

The complexity of the signaling network that underlies astrocyte-synapse interactions may seem discouraging when tackled from a theoretical perspective. Computational modeling is challenged by the fact that many details remain hitherto unknown and conventional approaches to describe synaptic function are unsuitable to explain experimental observations when astrocytic signaling is taken into account. Supported by experimental evidence is the possibility that astrocytes perform genuine information processing by means of their calcium signaling and are players in the physiological setting of the basal tone of synaptic transmission. Here we consider the plausibility of this scenario from a theoretical perspective, focusing on the modulation of synaptic release probability by the astrocyte and its implications on synaptic plasticity. The analysis of the signaling pathways underlying such modulation refines our notion of tripartite synapse and has profound implications on our understanding of brain function.

IFN-γ-induced increase in the mobility of MHC class II compartments in astrocytes depends on intermediate filaments

Background

In immune-mediated diseases of the central nervous system, astrocytes exposed to interferon-γ (IFN-γ) can express major histocompatibility complex (MHC) class II molecules and antigens on their surface. MHC class II molecules are thought to be delivered to the cell surface by membrane-bound vesicles. However, the characteristics and dynamics of this vesicular traffic are unclear, particularly in reactive astrocytes, which overexpress intermediate filament (IF) proteins that may affect trafficking. The aim of this study was to determine the mobility of MHC class II vesicles in wild-type (WT) astrocytes and in astrocytes devoid of IFs.

Methods

The identity of MHC class II compartments in WT and IF-deficient astrocytes 48 h after IFN-γ activation was determined immunocytochemically by using confocal microscopy. Time-lapse confocal imaging and Alexa Fluor⁵⁴⁶-dextran labeling of late endosomes/lysosomes in IFN-γ treated cells was used to characterize the motion of MHC class II vesicles. The mobility of vesicles was analyzed using ParticleTR software.

Results

Confocal imaging of primary cultures of WT and IF-deficient astrocytes revealed IFN-γ induced MHC class II expression in late endosomes/lysosomes, which were specifically labeled with Alexa Fluor⁵⁴⁶-conjugated dextran. Live imaging revealed faster movement of dextran-positive vesicles in IFN-γ-treated than in untreated astrocytes. Vesicle mobility was lower in IFN-γ-treated IF-deficient astrocytes than in WT astrocytes. Thus, the IFN-γ-induced increase in the mobility of MHC class II compartments is IF-dependent.

Conclusions

Since reactivity of astrocytes is a hallmark of many CNS pathologies, it is likely that the up-regulation of IFs under such conditions allows a faster and therefore a more efficient delivery of MHC class II molecules to the cell surface. In vivo, such regulatory mechanisms may enable antigen-presenting reactive astrocytes to respond rapidly and in a controlled manner to CNS inflammation.

Exocytosis in astrocytes: transmitter release and membrane signal regulation

Astrocytes, a type of glial cells in the brain, are eukaryotic cells, and a hallmark of these are subcellular organelles, such as secretory vesicles. In neurons vesicles play a key role in signaling. Upon a stimulus-an increase in cytosolic concentration of free Ca(2+) ([Ca(2+)](i))-the membrane of vesicle fuses with the presynaptic plasma membrane, allowing the exit of neurotransmitters into the extracellular space and their diffusion to the postsynaptic receptors. For decades it was thought that such vesicle-based mechanisms of gliotransmitter release were not present in astrocytes. However, in the last 30 years experimental evidence showed that astrocytes are endowed with mechanisms for vesicle- and non-vesicle-based gliotransmitter release mechanisms. The aim of this review is to focus on exocytosis, which may play a role in gliotransmission and also in other forms of cell-to-cell communication, such as the delivery of transporters, ion channels and antigen presenting molecules to the cell surface.

Rab4 and Rab5 GTPase are required for directional mobility of endocytic vesicles in astrocytes

Rab4 and Rab5 GTPases are key players in the regulation of endocytosis. Although their role has been studied intensively in the past, it is still unclear how they regulate vesicle mobility. In particular, in astrocytes, the most abundant glial cells in the brain, vesicles have been shown to exhibit nondirectional and directional mobility, which can be intermittent, but the underlying switching mechanisms are not known. By using quantitative imaging, we studied the dynamics of single vesicle movements in astrocytes in real time, by transfecting them with different GDP- and GTP-locked mutants of Rab4 and Rab5. Along with the localization of Rab4 and Rab5 on early and late endocytic compartments, we measured the apparent vesicle size by monitoring the area of fluorescent puncta and determined the patterns of vesicle mobility in the presence of wild-type and Rab mutants. Dominant-negative and dominant-positive mutants, Rab4 S22N, Rab5 S34N and Rab4 Q67L, Rab5 Q79L, induced an increase in the apparent vesicle size, especially Rab5 mutants. These mutants also significantly reduced vesicle mobility in terms of vesicle track length, maximal displacement, and speed. In addition, significant reductions in the fraction of vesicles exhibiting directional mobility were observed in cells expressing Rab4 S22N, Rab4 Q67L, Rab5 S34N, and Rab5 Q79L. Our data indicate that changes in the GDP-GTP switch apparently not only affect fusion events in endocytosis and recycling, as already proposed, but also affect the molecular interactions determining directional vesicle mobility, likely involving motor proteins and the cytoskeleton.

Astroglial excitability and gliotransmission: an appraisal of Ca2+ as a signalling route

Astroglial cells, due to their passive electrical properties, were long considered subservient to neurons and to merely provide the framework and metabolic support of the brain. Although astrocytes do play such structural and housekeeping roles in the brain, these glial cells also contribute to the brain's computational power and behavioural output. These more active functions are endowed by the Ca²⁺-based excitability displayed by astrocytes. An increase in cytosolic Ca²⁺ levels in astrocytes can lead to the release of signalling molecules, a process termed gliotransmission, via the process of regulated exocytosis. Dynamic components of astrocytic exocytosis include the vesicular-plasma membrane secretory machinery, as well as the vesicular traffic, which is governed not only by general cytoskeletal elements but also by astrocyte-specific IFs (intermediate filaments). Gliotransmitters released into the ECS (extracellular space) can exert their actions on neighbouring neurons, to modulate synaptic transmission and plasticity, and to affect behaviour by modulating the sleep homoeostat. Besides these novel physiological roles, astrocytic Ca²⁺ dynamics, Ca²⁺-dependent gliotransmission and astrocyte–neuron signalling have been also implicated in brain disorders, such as epilepsy. The aim of this review is to highlight the newer findings concerning Ca²⁺ signalling in astrocytes and exocytotic gliotransmission. For this we report on Ca²⁺ sources and sinks that are necessary and sufficient for regulating the exocytotic release of gliotransmitters and discuss secretory machinery, secretory vesicles and vesicle mobility regulation. Finally, we consider the exocytotic gliotransmission in the modulation of synaptic transmission and plasticity, as well as the astrocytic contribution to sleep behaviour and epilepsy.

Temporal characteristics of vesicular fusion in astrocytes: examination of synaptobrevin 2-laden vesicles at single vesicle resolution

Astrocytes can release various gliotransmitters in response to stimuli that cause increases in intracellular Ca(2+) levels; this secretion occurs via a regulated exocytosis pathway. Indeed, astrocytes express protein components of the vesicular secretory apparatus. However, the detailed temporal characteristics of vesicular fusions in astrocytes are not well understood. In order to start addressing this issue, we used total internal reflection fluorescence microscopy (TIRFM) to visualize vesicular fusion events in astrocytes expressing the fluorescent synaptobrevin 2 derivative, synapto-pHluorin. Although our cultured astrocytes from visual cortex express synaptosome-associated protein of 23 kDa (SNAP23), but not of 25 kDa (SNAP25), these glial cells exhibited a slow burst of exocytosis under mechanical stimulation; the expression of SNAP25B did not affect bursting behaviour. The relative amount of two distinct types of events observed, transient and full fusions, depended on the applied stimulus. Expression of exogenous synaptotagmin 1 (Syt1) in astrocytes endogenously expressing Syt4, led to a greater proportion of transient fusions when astrocytes were stimulated with bradykinin, a stimulus otherwise resulting in more full fusions. Additionally, we studied the stability of the transient fusion pore by measuring its dwell time, relation to vesicular size, flickering and decay slope; all of these characteristics were secretagogue dependent. The expression of SNAP25B or Syt1 had complex effects on transient fusion pore stability in a stimulus-specific manner. SNAP25B obliterated the appearance of flickers and reduced the dwell time when astrocytes were mechanically stimulated, while astrocytes expressing SNAP25B and stimulated with bradykinin had a reduction in decay slope. Syt1 reduced the dwell time when astrocytes were stimulated either mechanically or with bradykinin. Our detailed study of temporal characteristics of astrocytic exocytosis will not only aid the general understanding of this process, but also the interpretation of the events at the tripartite synapse, both in health and disease.

SNARE protein expression in synaptic terminals and astrocytes in the adult hippocampus: a comparative analysis

Several evidences suggest that astrocytes release small transmitter molecules, peptides, and protein factors via regulated exocytosis, implying that they function as specialized neurosecretory cells. However, very little is known about the molecular and functional properties of regulated secretion in astrocytes in the adult brain. Establishing these properties is central to the understanding of the communication mode(s) of these cells and their role(s) in the control of synaptic functions and of cerebral blood flow. In this study, we have set-up a high-resolution confocal microscopy approach to distinguish protein expression in astrocytic structures and neighboring synaptic terminals in adult brain tissue. This approach was applied to investigate the expression pattern of core SNARE proteins for vesicle fusion in the dentate gyrus and CA1 regions of the mouse hippocampus. Our comparative analysis shows that astrocytes abundantly express, in their cell body and main processes, all three protein partners necessary to form an operational SNARE complex but not in the same isoforms expressed in neighbouring synaptic terminals. Thus, SNAP25 and VAMP2 are absent from astrocytic processes and typically concentrated in terminals, while SNAP23 and VAMP3 have the opposite expression pattern. Syntaxin 1 is present in both synaptic terminals and astrocytes. These data support the view that astrocytes in the adult hippocampus can communicate via regulated exocytosis and also indicates that astrocytic exocytosis may differ in its properties from action potential-dependent exocytosis at neuronal synapses, as it relies on a distinctive set of SNARE proteins.

Astrocytes and energy metabolism

Astrocytes are glial cells, which play a significant role in a number of processes, including the brain energy metabolism. Their anatomical position between blood vessels and neurons make them an interface for effective glucose uptake from blood. After entering astrocytes, glucose can be involved in different metabolic pathways, e.g. in glycogen production. Glycogen in the brain is localized mainly in astrocytes and is an important energy source in hypoxic conditions and normal brain functioning. The portion of glucose metabolized into glycogen molecules in astrocytes is as high as 40%. It is thought that the release of gliotransmitters (such as glutamate, neuroactive peptides and ATP) into the extracellular space by regulated exocytosis supports a significant part of communication between astrocytes and neurons. On the other hand, neurotransmitter action on astrocytes has a significant role in brain energy metabolism. Therefore, understanding the astrocytes energy metabolism may help understanding neuron-astrocyte interactions.

Expression of VGLUTs contributes to degeneration and acquisition of learning and memory

Vesicular glutamate transporters (VGLUTs), which include VGLUT1, VGLUT2 and VGLUT3, are responsible for the uploading of L-glutamate into synaptic vesicles. The expression pattern of VGLUTs determines the level of synaptic vesicle filling (i.e., glutamate quantal size) and directly influences glutamate receptors and glutamatergic synaptic transmission; thus, VGLUTs may play a key role in learning and memory in the central nervous system. To determine whether VGLUTs contribute to the degeneration or acquisition of learning and memory, we used an animal model for the age-related impairment of learning and memory, senescence-accelerated mouse/prone 8 (SAMP8). KM mice were divided into groups based on their learning and memory performance in a shuttle-box test. The expression of VGLUTs and synaptophysin (Syp) mRNA and protein in the cerebral cortex and hippocampus were investigated with real-time fluorescence quantitative PCR and western blot, respectively. Our results demonstrate that, in the cerebral cortex, protein expression of VGLUT1, VGLUT2, VGLUT3 and Syp was decreased in SAMP8 with age and increased in KM mice, which displayed an enhanced capacity for learning and memory. The protein expression of VGLUT2 and Syp was decreased in the hippocampus of SAMP8 with aging. The expression level of VGLUT1 and VGLUT2 proteins were highest in KM mouse group with a 76-100% avoidance score in the shuttle-box test. These data demonstrate that protein expression of VGLUT1, VGLUT2 and Syp decreases age-dependently in SAMP8 and increases in a learning- and memory-dependent manner in KM mice. Correlation analysis indicated the protein expression of VGLUT1, VGLUT2 and Syp has a positive correlation with the capacity of learning and memory.

Analysis of confocal images using variable-width line profiles

A line profile of fluorescent intensities in confocal images is frequently examined. We have developed the computer software tool to analyse the profiles of intensities of fluorescent probes in confocal images. The software averages neighbouring pixels, adjacent to the central line, without reducing the spatial resolution of the image. As an experimental model, we have used the skeletal muscle fibre isolated from the rat skeletal muscle extensor digitorum brevis. As a marker of myofibrils' structure, we have used phalloidin-rhodamine staining and the anti-TIM antibody to label mitochondria. We also tested the distribution of the protein kinase B/Akt. Since signalling is ordered in modules and large protein complexes appear to direct signalling to organelles and regulate specific physiological functions, a software tool to analyse such complexes in fluorescent confocal images is required. The software displays the image, and the user defines the line for analysis. The image is rotated by the angle of the line. The line profile is calculated by averaging one dimension of the cropped rotated image matrix. The spatial resolution in averaged line profile is not decreased compared with single-pixel line profile, which was confirmed by the discrete Fourier transform computed with a fast Fourier transform algorithm. We conclude that the custom software tool presented here is a useful tool to analyse line profiles of fluorescence intensities in confocal images.

Intermediate filaments attenuate stimulation-dependent mobility of endosomes/lysosomes in astrocytes

Intermediate filament (IF) proteins upregulation is a hallmark of astrocyte activation and reactive gliosis, but its pathophysiological implications remain incompletely understood. A recently reported association between IFs and directional mobility of peptidergic vesicles allows us to hypothesize that IFs affect vesicle dynamics and exocytosis-mediated astrocyte communication with neighboring cells. Here, we ask whether the trafficking of recycling vesicles (i.e., those fused to and then retrieved from the plasma membrane) and endosomes/lysosomes depends on IFs. Recycling vesicles were labeled by antibodies against vesicle glutamate transporter 1 (VGLUT1) and atrial natriuretic peptide (ANP), respectively, and by lysotracker, which labels endosomes/lysosomes. Quantitative fluorescence microscopy was used to monitor the mobility of labeled vesicles in astrocytes, derived from either wild-type (WT) mice or mice deficient in glial fibrillary acidic protein and vimentin (GFAP(-/-)Vim(-/-)), the latter lacking astrocyte IFs. Stimulation with ionomycin or ATP enhanced the mobility of VGLUT1-positive vesicles and reduced the mobility of ANP-positive vesicles in WT astrocytes. In GFAP(-/-)Vim(-/-) astrocytes, both vesicle types responded to stimulation, but the relative increase in mobility of VGLUT1-positive vesicles was more prominent compared with nonstimulated cells, whereas the stimulation-dependent attenuation of ANP-positive vesicles mobility was reduced compared with nonstimulated cells. The mobility of endosomes/lysosomes decreased following stimulation in WT astrocytes. However, in GFAP(-/-)Vim(-/-) astrocytes, a small increase in the mobility of endosomes/lysosomes was observed. These findings show that astrocyte IFs differentially affect the stimulation-dependent mobility of vesicles. We propose that upregulation of IFs in pathologic states may alter the function of astrocytes by deregulating vesicle trafficking.

Do astrocytes really exocytose neurotransmitters?

In the past 20 years, an extra layer of information processing, in addition to that provided by neurons, has been proposed for the CNS. Neuronally evoked increases of the intracellular calcium concentration in astrocytes have been suggested to trigger exocytotic release of the 'gliotransmitters' glutamate, ATP and D-serine. These are proposed to modulate neuronal excitability and transmitter release, and to have a role in diseases as diverse as stroke, epilepsy, schizophrenia, Alzheimer's disease and HIV infection. However, there is intense controversy about whether astrocytes can exocytose transmitters in vivo. Resolving this issue would considerably advance our understanding of brain function.

Regulated exocytosis in astrocytic signal integration

Astrocytes can be considered as signal integrators in central nervous system activity. These glial cells can respond to signals from the heterocellular milieu of the brain and subsequently release various molecules to signal to themselves and/or other neighboring neural cells. An important functional module that enables signal integration in astrocytes is exocytosis, a Ca(2+)-dependent process consisting of vesicular fusion to the plasma membrane. Astrocytes utilize regulated exocytosis to release various signaling molecules stored in the vesicular lumen. Here we review the properties of exocytotic release of three classes of gliotransmitters: (i) amino acids, (ii) nucleotides and (iii) peptides. Vesicles may carry not only lumenal cargo, but also membrane-associated molecules. Therefore, we also discuss exocytosis as a delivery mechanism for transporters and receptors to the plasma membrane, where these proteins are involved in astrocytic intercellular signaling.

Gliotransmission: Exocytotic release from astrocytes

Gliotransmitters are chemicals released from glial cells fulfilling a following set of criteria: (i) they are synthesized by and/or stored in glia; (ii) their regulated release is triggered by physiological and/or pathological stimuli; (iii) they activate rapid (milliseconds to seconds) responses in neighboring cells; and (iv) they play a role in (patho)physiological processes. Astrocytes can release a variety of gliotransmitters into the extracellular space using several different mechanisms. In this review, we focus on exocytotic mechanism(s) underlying the release of three classes of gliotransmitters: (i) amino acids, such as, glutamate and d-serine; (ii) nucleotides, like adenosine 5'-triphosphate; and (iii) peptides, such as, atrial natriuretic peptide and brain-derived neurotrophic factor. It is becoming clear that astrocytes are endowed with elements that qualify them as cells communicating with neurons and other cells within the central nervous system by employing regulated exocytosis.

Trafficking of astrocytic vesicles in hippocampal slices

The increasingly appreciated role of astrocytes in neurophysiology dictates a thorough understanding of the mechanisms underlying the communication between astrocytes and neurons. In particular, the uptake and release of signaling substances into/from astrocytes is considered as crucial. The release of different gliotransmitters involves regulated exocytosis, consisting of the fusion between the vesicle and the plasma membranes. After fusion with the plasma membrane vesicles may be retrieved into the cytoplasm and may continue to recycle. To study the mobility implicated in the retrieval of secretory vesicles, these structures have been previously efficiently and specifically labeled in cultured astrocytes, by exposing live cells to primary and secondary antibodies. Since the vesicle labeling and the vesicle mobility properties may be an artifact of cell culture conditions, we here asked whether the retrieving exocytotic vesicles can be labeled in brain tissue slices and whether their mobility differs to that observed in cell cultures. We labeled astrocytic vesicles and recorded their mobility with two-photon microscopy in hippocampal slices from transgenic mice with fluorescently tagged astrocytes (GFP mice) and in wild-type mice with astrocytes labeled by Fluo4 fluorescence indicator. Glutamatergic vesicles and peptidergic granules were labeled by the anti-vesicular glutamate transporter 1 (vGlut1) and anti-atrial natriuretic peptide (ANP) antibodies, respectively. We report that the vesicle mobility parameters (velocity, maximal displacement and track length) recorded in astrocytes from tissue slices are similar to those reported previously in cultured astrocytes.

Regulated exocytosis and vesicle trafficking in astrocytes

Astrocytes are increasingly viewed as crucial cells supporting and integrating brain functions. It is thought that the release of gliotransmitters into the extracellular space by regulated exocytosis supports a significant part of communication between astrocytes and neurons. Prior to exocytosis, the membrane-bound vesicles are transported through the astrocyte cytoplasm. Our recent studies have revealed new insights into vesicle trafficking in the cytoplasm of astrocytes and are reviewed in this article. The prefusion mobility of fluorescently labeled peptidergic vesicles was studied in cultured rat and mouse astrocytes. Vesicle delivery to the plasma membrane involved an interaction with the cytoskeleton, in particular with microtubules and actin filaments. Interestingly, vesicle mobility in mouse astrocytes deficient in intermediate filaments show impaired directionality of peptidergic vesicle mobility. To explore whether stimuli that increase the concentration of free calcium ions in the cytoplasm triggered vesicular ATP release from astrocytes, human embryonic kidney-293T cells transfected with a P2X(3) receptor were used as sniffers to detect ATP release. Glutamate stimulation of astrocytes was followed by an increase in the incidence of small, transient, inward currents in sniffer cells, reminiscent of postsynaptic quantal events observed at synapses. Some of the membrane-bound vesicles are retrieved from the plasma membrane to be recycled back into the cytosol. Trafficking velocity of postfusion (recycling) atrial natriuretic peptide vesicles was one order of magnitude slower in comparison to the mobility of prefusion vesicles. However, transport of all vesicle types studied required an intact cytoskeleton.