Slow spontaneous secretion from single large dense‐core vesicles monitored in neuroendocrine cells

Calcium-dependent subquantal peptide release from single docked lawn-resident vesicles of pituitary lactotrophs

Regulated exocytosis consists of the fusion between vesicles and the plasma membranes, leading to the formation of a narrow fusion pore through which secretions exit the vesicle lumen into the extracellular space. An increase in the cytosolic concentration of free Ca²⁺ ([Ca²⁺]_i) is considered the stimulus of this process. However, whether this mechanism can be preserved in a simplified system of membrane lawns with docked secretory vesicles, devoid of cellular components, is poorly understood. Here, we studied peptide discharge from individual secretory vesicles docked at the plasma membrane, prepared from primary endocrine pituitary cells (the lactotrophs), releasing hormone prolactin. To label secretory vesicles, we transfected lactotrophs to express the fluorescent atrial natriuretic peptide (ANP.emd), previously shown to be expressed in and released from prolactin-containing vesicles. We used stimulating solutions containing different [Ca²⁺] to evoke vesicle peptide discharge, which appeared similar in membrane lawns and in intact stimulated lactotrophs. All vesicles examined discharged peptides in a subquantal manner, either exhibiting a unitary or sequential time course. In the membrane lawns, the unitary vesicle peptide discharge was predominant and slightly slower than that recorded in intact cells, but with a shorter delay with respect to the stimulation onset. This study revealed directly that Ca²⁺ triggers peptide discharge from docked single vesicles in the membrane lawns with a half-maximal response of ∼8 µM [Ca²⁺], consistent with previous whole-cell patch-clamp studies in endocrine cells where the rapid component of exocytosis, interpreted to represent docked vesicles, was fully activated at <10 µM [Ca²⁺]. Interestingly, the sequential subquantal peptide vesicle discharge indicates that fluctuations between constricted and dilated fusion pore states are preserved in membrane lawns and that fusion pore regulation appears to be an autonomously controlled process.

Probing single molecule mechanical interactions of syntaxin 1A with native synaptobrevin 2 residing on a secretory vesicle

Interactive mechanical forces between pairs of individual SNARE proteins synaptobrevin 2 (Sb2) and syntaxin 1A (Sx1A) may be sufficient to mediate vesicle docking. This notion, based on force spectroscopy single molecule measurements probing recombinant Sx1A an Sb2 in silico, questioned a predominant view of docking via the ternary SNARE complex formation, which includes an assembly of the intermediate cis binary complex between Sx1A and SNAP25 on the plasma membrane to engage Sb2 on the vesicle. However, whether a trans binary Sx1A-Sb2 complex alone could mediate vesicle docking in a cellular environment remains unclear. To address this issue, we used atomic force microscopy (AFM) in the force spectroscopy mode combined with fluorescence imaging. Using AFM tips functionalized with the full Sx1A cytosolic domain, we probed native Sb2 studding the membrane of secretory vesicles docked at the plasma membrane patches, referred to as "inside-out lawns", identified based on fluorescence stains and prepared from primary culture of lactotrophs. We recorded single molecule Sx1A-Sb2 mechanical interactions and obtained measurements of force (∼183 pN) and extension (∼21.6 nm) necessary to take apart Sx1A-Sb2 binding interactions formed at tip-vesicle contact. Measured interactive force between a single pair of Sx1A-Sb2 molecules is sufficient to hold a single secretory vesicle docked at the plasma membrane within distances up to that of the measured extension. This finding further advances a notion that native vesicle docking can be mediated by a single trans binary Sx1A-Sb2 complex in the absence of SNAP25.

Vesicle cholesterol controls exocytotic fusion pore

In some lysosomal storage diseases (LSD) cholesterol accumulates in vesicles. Whether increased vesicle cholesterol affects vesicle fusion with the plasmalemma, where the fusion pore, a channel between the vesicle lumen and the extracellular space, is formed, is unknown. Super-resolution microscopy revealed that after stimulation of exocytosis, pituitary lactotroph vesicles discharge cholesterol which transfers to the plasmalemma. Cholesterol depletion in lactotrophs and astrocytes, both exhibiting Ca²⁺-dependent exocytosis regulated by distinct Ca²⁺sources, evokes vesicle secretion. Although this treatment enhanced cytosolic levels of Ca²⁺ in lactotrophs but decreased it in astrocytes, this indicates that cholesterol may well directly define the fusion pore. In an attempt to explain this mechanism, a new model of cholesterol-dependent fusion pore regulation is proposed. High-resolution membrane capacitance measurements, used to monitor fusion pore conductance, a parameter related to fusion pore diameter, confirm that at resting conditions reducing cholesterol increases, while enrichment with cholesterol decreases the conductance of the fusion pore. In resting fibroblasts, lacking the Npc1 protein, a cellular model of LSD in which cholesterol accumulates in vesicles, the fusion pore conductance is smaller than in controls, showing that vesicle cholesterol controls fusion pore and is relevant for pathophysiology of LSD.

Exocytotic fusion pore under stress

Exocytosis is a universal process of eukaryotic cells, consisting of fusion between the vesicle and the plasma membranes, leading to the formation of a fusion pore, a channel through which vesicle cargo exits into the extracellular space. In 1986, Rand and Parsegian proposed several stages to explain the nature of membrane fusion. Following stimulation, it starts with focused stress destabilization of membranes in contact, followed by the coalescence of two membrane surfaces. In the next fraction of a millisecond, restabilization of fused membranes is considered to occur to maintain the cell's integrity. This view predicted that once a fusion pore is formed, it must widen abruptly, irreversibly and fully, whereby the vesicle membrane completely integrates with and collapses into the plasma membrane (full fusion exocytosis). However, recent experimental evidence has revealed that once the fusion pore opens, it may also reversibly close (transient or kiss-and-run exocytosis). Here, we present a historical perspective on understanding the mechanisms that initiate the membrane merger and fusion pore formation. Next, post-fusion mechanisms that regulate fusion pore stability are considered, reflecting the state in which the forces of widening and constriction of fusion pores are balanced. Although the mechanisms generating these forces are unclear, they may involve lipids and proteins, including SNAREs, which play a role not only in the pre-fusion but also post-fusion stages of exocytosis. How molecules stabilize the fusion pore in the open state is key for a better understanding of fusion pore physiology in health and disease.

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.

Fingolimod Suppresses the Proinflammatory Status of Interferon-γ-Activated Cultured Rat Astrocytes

Astroglia, the primary homeostatic cells of the central nervous system, play an important role in neuroinflammation. They act as facultative immunocompetent antigen-presenting cells (APCs), expressing major histocompatibility complex (MHC) class II antigens upon activation with interferon (IFN)-γ and possibly other proinflammatory cytokines that are upregulated in disease states, including multiple sclerosis (MS). We characterized the anti-inflammatory effects of fingolimod (FTY720), an established drug for MS, and its phosphorylated metabolite (FTY720-P) in IFN-γ-activated cultured rat astrocytes. The expression of MHC class II compartments, β₂ adrenergic receptor (ADR-β₂), and nuclear factor kappa-light-chain enhancer of activated B cells subunit p65 (NF-κB p65) was quantified in immunofluorescence images acquired by laser scanning confocal microscopy. In addition, MHC class II-enriched endocytotic vesicles were labeled by fluorescent dextran and their mobility analyzed in astrocytes subjected to different treatments. FTY720 and FTY720-P treatment significantly reduced the number of IFN-γ-induced MHC class II compartments and substantially increased ADR-β₂ expression, which is otherwise small or absent in astrocytes in MS. These effects could be partially attributed to the observed decrease in NF-κB p65 expression, because the NF-κB signaling cascade is activated in inflammatory processes. We also found attenuated trafficking and secretion from dextran-labeled endo-/lysosomes that may hinder efficient delivery of MHC class II molecules to the plasma membrane. Our data suggest that FTY720 and FTY720-P at submicromolar concentrations mediate anti-inflammatory effects on astrocytes by suppressing their action as APCs, which may further downregulate the inflammatory process in the brain, constituting the therapeutic effect of fingolimod in MS.

Phases of the exocytotic fusion pore

Membrane fusion and fission are fundamental processes in living organisms. Membrane fusion occurs through the formation of a fusion pore, which is the structure that connects two lipid membranes during their fusion. Fusion pores can form spontaneously, but cells endow themselves with a set of proteins that make the process of fusion faster and regulatable. The fusion pore starts with a narrow diameter and dilates relatively slowly; it may fluctuate in size or can even close completely, producing a transient vesicle fusion (kiss-and-run), or can finally expand abruptly to release all vesicle contents. A set of proteins control the formation, dilation, and eventual closure of the fusion pore and, therefore, the velocity at which the contents of secretory vesicles are released to the extracellular medium. Thus, the regulation of fusion pore expansion or closure is key to regulate the release of neurotransmitters and hormones. Here, we review the phases of the fusion pore and discuss the implications in the modes of exocytosis.

Presenilin PS1∆E9 disrupts mobility of secretory organelles in rat astrocytes

AIM

Alzheimer's disease (AD) is largely considered a neuron-derived insult, but also involves failure of astroglia. A recent study indicated that mutated presenilin 1 (PS1M146V), a putative endoplasmic reticulum (ER) Ca²⁺ channel with decreased Ca²⁺ conductance, impairs the traffic of astroglial peptidergic vesicles. Whether other pathogenically relevant PS1 mutants, such as PS1ΔE9, which code for ER channel with putative increased Ca²⁺ conductance, similarly affect vesicle traffic, is unknown.

METHODS

Here, we cotransfected rat astrocytes with plasmids encoding mutant PS1ΔE9 and atrial natriuretic peptide or vesicular glutamate transporter 1 tagged with fluorescent proteins (pANP.emd or pVGLUT1-EGFP respectively), to microscopically examine whether alterations in vesicle mobility and Ca²⁺ -regulated release of gliosignalling molecules manifest as a general vesicle-based defect; control cells were transfected to co-express exogenous or native wild-type PS1 and pANP.emd or pVGLUT1-EGFP. The vesicle mobility was analysed at rest and after ATP stimulation that increased intracellular calcium activity.

RESULTS

In PS1ΔE9 astrocytes, spontaneous mobility of both vesicle types was reduced (P < .001) when compared to controls. Post-stimulatory recovery of fast vesicle mobility was hampered in PS1ΔE9 astrocytes. The ATP-evoked peptide release was less efficient in PS1ΔE9 astrocytes than in the controls (P < .05), as was the pre-stimulatory mobility of these vesicles.

CONCLUSION

Although the PS1 mutants PS1M146V and PS1ΔE9 differently affect ER Ca²⁺ conductance, our results revealed a common, vesicle-type indiscriminate trafficking defect in PS1ΔE9 astrocytes, indicating that reduced secretory vesicle-based signalling is a general deficit in AD astrocytes.

Ångstrom-size exocytotic fusion pore: Implications for pituitary hormone secretion

In the past, vesicle content release was thought to occur immediately and completely after triggering of exocytosis. However, vesicles may merge with the plasma membrane to form an Ångstrom diameter fusion pore that prevents the exit of secretions from the vesicle lumen. The advantage of such a narrow pore is to minimize the delay between the trigger and the release. Instead of stimulating a sequence of processes, leading to vesicle merger with the plasma membrane and a formation of a fusion pore, the stimulus only widens the pre-established fusion pore. The fusion pore may be stable and may exhibit repetitive opening of the vesicle lumen to the cell exterior accompanied by a content discharge. Such release of vesicle content is partial (subquantal), and depends on fusion pore open time, diameter and the diffusibility of the cargo. Such transient mode of fusion pore opening was not confirmed until the development of the membrane capacitance patch-clamp technique, which enables high-resolution measurement of changes in membrane surface area. It allows millisecond dwell-time measurements of fusion pores with subnanometer diameters. Currently, the soluble N-ethylmaleimide-sensitive factor-attachment protein receptor (SNARE) proteins are considered to be key entities in end-stage exocytosis, and the SNARE complex assembly/disassembly may regulate the fusion pore. Moreover, lipids or other membrane constituents with anisotropic (non-axisymmetric) geometry may also favour the establishment of stable narrow fusion pores, if positioned in the neck of the fusion pore.

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.

Dominant negative SNARE peptides stabilize the fusion pore in a narrow, release-unproductive state

Key support for vesicle-based release of gliotransmitters comes from studies of transgenic mice with astrocyte-specific expression of a dominant-negative domain of synaptobrevin 2 protein (dnSNARE). To determine how this peptide affects exocytosis, we used super-resolution stimulated emission depletion microscopy and structured illumination microscopy to study the anatomy of single vesicles in astrocytes. Smaller vesicles contained amino acid and peptidergic transmitters and larger vesicles contained ATP. Discrete increases in membrane capacitance, indicating single-vesicle fusion, revealed that astrocyte stimulation increases the frequency of predominantly transient fusion events in smaller vesicles, whereas larger vesicles transitioned to full fusion. To determine whether this reflects a lower density of SNARE proteins in larger vesicles, we treated astrocytes with botulinum neurotoxins D and E, which reduced exocytotic events of both vesicle types. dnSNARE peptide stabilized the fusion-pore diameter to narrow, release-unproductive diameters in both vesicle types, regardless of vesicle diameter.

Ketamine Inhibits ATP-Evoked Exocytotic Release of Brain-Derived Neurotrophic Factor from Vesicles in Cultured Rat Astrocytes

In the brain, astrocytes signal to neighboring cells via regulated exocytotic release of gliosignaling molecules, such as brain-derived neurotrophic factor (BDNF). Recent studies uncovered a role of ketamine, an anesthetic and antidepressant, in the regulation of BDNF expression and in the disruption of astrocytic Ca²⁺ signaling, but it is unclear whether it affects astroglial BDNF release. We investigated whether ketamine affects ATP-evoked Ca²⁺ signaling and exocytotic release of BDNF at the single-vesicle level in cultured rat astrocytes. Cells were transfected with a plasmid encoding preproBDNF tagged with the pH-sensitive fluorescent protein superecliptic pHluorin, (BDNF-pHse) to load vesicles and measure the release of BDNF-pHse when the exocytotic fusion pore opens and alkalinizes the luminal pH. In addition, cell-attached membrane capacitance changes were recorded to monitor unitary vesicle interaction with the plasma membrane. Intracellular Ca²⁺ activity was monitored with Fluo-4 and confocal microscopy, which was also used to immunocytochemically characterize BDNF-pHse-laden vesicles. As revealed by double-fluorescent micrographs, BDNF-pHse localized to vesicles positive for the soluble N-ethylmaleimide-sensitive fusion protein attachment protein receptor (SNARE) proteins, vesicle-associated membrane protein 2 (VAMP2), VAMP3, and synaptotagmin IV. Ketamine treatment decreased the number of ATP-evoked BDNF-pHse fusion/secretion events (P < 0.05), the frequency of ATP-evoked transient (P < 0.001) and full-fusion exocytotic (P < 0.05) events, along with a reduction in the ATP-evoked increase in intracellular Ca²⁺ activity in astrocytes by ~70 % (P < 0.001). The results show that ketamine treatment suppresses ATP-triggered vesicle fusion and BDNF secretion by increasing the probability of a narrow fusion pore open state and/or by reducing astrocytic Ca²⁺ excitability.

Expression of familial Alzheimer disease presenilin 1 gene attenuates vesicle traffic and reduces peptide secretion in cultured astrocytes devoid of pathologic tissue environment

In the brain, astrocytes provide metabolic and trophic support to neurones. Failure in executing astroglial homeostatic functions may contribute to the initiation and propagation of diseases, including Alzheimer disease (AD), characterized by a progressive loss of neurones over years. Here, we examined whether astrocytes from a mice model of AD isolated in the presymptomatic phase of the disease exhibit alterations in vesicle traffic, vesicular peptide release and purinergic calcium signaling. In cultured astrocytes isolated from a newborn wild-type (wt) and 3xTg-AD mouse, secretory vesicles and acidic endosomes/lysosomes were labeled by transfection with plasmid encoding atrial natriuretic peptide tagged with mutant green fluorescent protein (ANP.emd) and by LysoTracker, respectively. The intracellular Ca(2+) concentration ([Ca(2+)]i) was monitored with Fluo-2 and visualized by confocal microscopy. In comparison with controls, spontaneous mobility of ANP- and LysoTracker-labeled vesicles was diminished in 3xTg-AD astrocytes; the track length (TL), maximal displacement (MD) and directionality index (DI) were all reduced in peptidergic vesicles and in endosomes/lysosomes (P < 0.001), as was the ATP-evoked attenuation of vesicle mobility. Similar impairment of peptidergic vesicle trafficking was observed in wt rat astrocytes transfected to express mutated presenilin 1 (PS1M146V). The ATP-evoked ANP discharge from single vesicles was less efficient in 3xTg-AD and PS1M146V-expressing astrocytes than in respective wt controls (P < 0.05). Purinergic stimulation evoked biphasic and oscillatory [Ca(2+)]i responses; the latter were less frequent (P < 0.001) in 3xTg-AD astrocytes. Expression of PS1M146V in astrocytes impairs vesicle dynamics and reduces evoked secretion of the signaling molecule ANP; both may contribute to the development of AD.

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.

Vesicle size determines unitary exocytic properties and their sensitivity to sphingosine

Neuroendocrine cells contain small and large vesicles, but the functional significance of vesicle diameter is unclear. We studied unitary exocytic events of prolactin-containing vesicles in lactotrophs by monitoring discrete steps in membrane capacitance. In the presence of sphingosine, which recruits VAMP2 for SNARE complex formation, the frequency of transient and full fusion events increased. Vesicles with larger diameters proceeded to full fusion, but smaller vesicles remained entrapped in transient exocytosis. The diameter of vesicle dense cores released by full fusion exocytosis into the extracellular space was larger than the diameter of the remaining intracellular vesicles beneath the plasma membrane. Labeling with prolactin- and VAMP2-antibodies revealed a correlation between the diameters of colocalized prolactin- and VAMP2-positive structures. It is proposed that sphingosine-mediated facilitation of regulated exocytosis is not only related to the number of SNARE complexes per vesicle but also depends on the vesicle size, which may determine the transition between transient and full fusion exocytosis.

cAMP-mediated stabilization of fusion pores in cultured rat pituitary lactotrophs

Regulated exocytosis mediates the release of hormones and transmitters. The last step of this process is represented by the merger between the vesicle and the plasma membranes, and the formation of a fusion pore. Once formed, the initially stable and narrow fusion pore may reversibly widen (transient exocytosis) or fully open (full-fusion exocytosis). Exocytosis is typically triggered by an elevation in cytosolic calcium activity. However, other second messengers, such as cAMP, have been reported to modulate secretion. The way in which cAMP influences the transitions between different fusion pore states remains unclear. Here, hormone release studies show that prolactin release from isolated rat lactotrophs stimulated by forskolin, an activator of adenylyl cyclases, and by membrane-permeable cAMP analog (dbcAMP), exhibit a biphasic concentration dependency. Although at lower concentrations (2-10 μm forskolin and 2.5-5 mm dbcAMP) these agents stimulate prolactin release, an inhibition is measured at higher concentrations (50 μm forskolin and 10-15 mm dbcAMP). By using high-resolution capacitance (Cm) measurements, we recorded discrete increases in Cm, which represent elementary exocytic events. An elevation of cAMP leaves the frequency of full-fusion events unchanged while increasing the frequency of transient events. These exhibited a wider fusion pore as measured by increased fusion pore conductance and a prolonged fusion pore dwell time. The probability of observing rhythmic reopening of transient fusion pores was elevated by dbcAMP. In conclusion, cAMP-mediated stabilization of wide fusion pores prevents vesicles from proceeding to the full-fusion stage of exocytosis, which hinders vesicle content discharge at high cAMP concentrations.

High-resolution membrane capacitance measurements for the study of exocytosis and endocytosis

In order to understand exocytosis and endocytosis, it is necessary to study these processes directly. An elegant way to do this is by measuring plasma membrane capacitance (C(m)), a parameter proportional to cell surface area, the fluctuations of which are due to fusion and fission of secretory and other vesicles. Here we describe protocols that enable high-resolution C(m) measurements in macroscopic and microscopic modes. Macroscopic mode, performed in whole-cell configuration, is used for measuring bulk C(m) changes in the entire membrane area, and it enables the introduction of exocytosis stimulators or inhibitors into the cytosol through the patch pipette. Microscopic mode, performed in cell-attached configuration, enables measurements of C(m) with attofarad resolution and allows characterization of fusion pore properties. Although we usually apply these protocols to primary pituitary cells and astrocytes, they can be adapted and used for other cell types. After initial hardware setup and culture preparation, several C(m) measurements can be performed daily.

Comparison of unitary exocytic events in pituitary lactotrophs and in astrocytes: modeling the discrete open fusion-pore states

In regulated exocytosis the merger between the vesicle and the plasma membranes leads to the formation of an aqueous channel (a fusion-pore), through which vesicular secretions exit into the extracellular space. A fusion pore was thought to be a short-lived intermediate preceding full-fusion of the vesicle and the plasma membranes (full-fusion exocytosis). However, transient exocytic events were also observed, where the fusion-pore opens and closes, repetitively. Here we asked whether there are different discrete states of the open fusion-pore. Unitary exocytic events were recorded by the high-resolution cell-attached patch-clamp method in pituitary lactotrophs and brain astrocytes. We monitored reversible unitary exocytic events, characterized by an on-step, which is followed by an off-step in membrane capacitance (C_m), a parameter linearly related to the membrane area. The results revealed three categories of reversible exocytic events (transient fusion-pore openings), which do not end with the complete integration of the vesicle membrane into the plasma membrane. These were categorized according to the observed differences in the amplitude and sign of the change in the real (Re) parts of the admittance signals: in case I events (Re ≈ 0) fusion pores are relatively wide; in case II (Re > 0) and case III (Re < 0) events fusion pores are relatively narrow. We show that case III events are more likely to occur for small vesicles, whereas, case II events are more likely to occur for larger vesicles. Case III events were considerably more frequent in astrocytes than in lactotrophs.

Peptide hormone release monitored from single vesicles in "membrane lawns" of differentiated male pituitary cells: SNAREs and fusion pore widening

In this study we used live-cell immunocytochemistry and confocal microscopy to study the release from a single vesicle in a simplified system called membrane lawns. The lawns were prepared by exposing differentiated pituitary prolactin (PRL)-secreting cells to a hypoosmotic shear stress. The density of the immunolabeled ternary soluble N-ethylmaleimide-sensitive factor-attachment protein receptor (SNARE) complexes that bind complexin was approximately 10 times lower than the PRL-positive, lawn-resident vesicles; this indicates that some but not all vesicles are associated with ternary SNARE complexes. However, lawn-resident PRL vesicles colocalized relatively well with particular SNARE proteins: synaptobrevin 2 (35%), syntaxin 1 (22%), and 25-kDa synaptosome associated protein (6%). To study vesicle discharge, we prepared lawn-resident vesicles, derived from atrial natriuretic peptide tagged with emerald fluorescent protein (ANP.emd)-transfected cells, which label vesicles. These maintained the structural passage to the exterior because approximately 40% of ANP.emd-loaded vesicles were labeled by extracellular PRL antibodies. Cargo release from the lawn-resident vesicles, monitored by the decline in the ANP.emd fluorescence intensity, was similar to that in intact cells. It is likely that SNARE proteins are required for calcium-dependent release from these vesicles. This is because the expression of the dominant-negative SNARE peptide, which interferes with SNARE complex formation, reduced the number of PRL-positive spots per cell (PRL antibodies placed extracellularly) significantly, from 58 ± 9 to 4 ± 2. In dominant-negative SNARE-treated cells, the PRL-positive area was reduced from 0.259 ± 0.013 to 0.123 ± 0.014 μm(2), which is consistent with a hindered vesicle luminal access for extracellular PRL antibodies. These results indicate that vesicle discharge is regulated by SNARE-mediated fusion pore widening.

Munc18-1, exocytotic fusion pore regulation and local membrane anisotropy

The release of hormones and neurotransmitters from vesicles can be modified by the regulation of the fusion pore, an aqueous channel that forms upon the fusion of the vesicle membrane with the plasma membrane. However, the mechanisms are unclear. Munc18-1 protein interacts with Syntaxin1 (Synt 1), a member of the SNARE proteins, which plays an important role in exocytosis. It has been shown that Munc18-1 has multiple roles, both in pre- and post-fusion stages of exocytosis. It regulates the traffic of Synt1 to the plasma membrane. By inhibiting the tethering of the vesicle SNARE protein Synaptobrevin 2 (Syb2) solely to Synt1 at the plasma membrane, but favoring the vesicular tethering to the preformed binary cis SNARE complex of Synt1A-SNAP25B, Munc18-1 is tuning vesicle docking and the membrane merger process. Additionally, Munc18-1 affects exocytosis at the post-fusion stage by regulating the fusion pore properties (i.e., dwell-time and fusion pore diameter). Among many possible mechanisms that may regulate the fusion pore, but have never been considered previously, is the influence of Munc18-1 on the membrane anisotropy, which determines the local spontaneous membrane curvature and the architecture of the fusion pore. We here propose that Munc18-1 affects the fusion pore by modulating the dynamic local (re)arrangement of anisotropic membrane components within the highly curved fusion pore nanostructure, to which proteins, lipids or their complexes can participate.

Fusion pores, SNAREs, and exocytosis

Exocytosis is a multistage process involving a merger between the vesicle and the plasma membranes, leading to the formation of a fusion pore, a channel, through which secretions are released from the vesicle to the cell exterior. A stimulus may influence the pore by either dilating it completely (full-fusion exocytosis) or mediating a reversible closure (transient exocytosis). In neurons, these transitions are short-lived and not accessible for experimentation. However, in some neuroendocrine cells, initial fusion pores may reopen several hundred times, indicating their stability. Moreover, these pores are too narrow to pass luminal molecules to the extracellular space, termed release-unproductive. However, on stimulation, their diameter dilates, initiating the release of cargo without de novo fusion pore formation. To explain the stability of the initial narrow fusion pores, anisotropic membrane constituents with non-axisymmetrical shape were proposed to accumulate in the fusion pore membrane. Although the nature of these is unclear, they may consist of lipids and proteins, including SNAREs, which may facilitate and regulate the pre- and post-fusional stages of exocytosis. In the future, a more detailed insight into the molecular control of fusion pore stabilization and regulation will generate a better understanding of fusion pore physiology in health and disease.

Regulated exocytosis per partes

This Special Issue (SI) of Cell Calcium focuses on regulated exocytosis, a recent evolutionary invention of eukaryotic cells. This essential cellular process consists of several stages: (i) the delivery of membrane bound vesicles to specific plasma membrane sites, (ii) where the merger between the vesicle and the plasma membranes occurs, (iii) leading to the formation of an aqueous channel through which vesicle content starts to be discharged to the cell exterior, (iv) after the full incorporation of the vesicle membrane into the plasma membrane, the added vesicle membrane is retrieved back into the cytoplasm by endocytosis. (v) When a fusion pore opens it may close again, a process known as transient fusion pore opening (also kiss-and-run exocytosis). In some cell types these stages are extremely shortlived, as in some neurons, and thus relatively inaccessible to experimentation. In other cell types the transition between these stages is orders of magnitude slower and can be studied in more detail. However, despite the intense investigations of this critical biological process over the last decades, the molecular mechanisms underlying regulated exocytosis have yet to be fully resolved. We thus still lack a comprehensive physiological insight into the nature of the progressive and coupled stages of exocytosis. Such a molecular-level understanding would help to fully reconstruct this process in vitro, as well as identify potential therapeutic targets for a range of diseases and dysfunctions. There are 18 papers in this SI which have been organized into three sections: Rapid regulated exocytosis and calcium homeostasis with an introduction by Erwin Neher, Molecular mechanisms of regulated exocytosis, and Cell models for regulated exocytosis. Here we briefly outline and integrate the messages of these sections.

Fingolimod--a sphingosine-like molecule inhibits vesicle mobility and secretion in astrocytes

In the brain, astrocytes signal to the neighboring cells by the release of chemical messengers (gliotransmitters) via regulated exocytosis. Recent studies uncovered a potential role of signaling lipids in modulation of exocytosis. Hence, we investigated whether sphingosine and the structural analog fingolimod/FTY720, a recently introduced therapeutic for multiple sclerosis, affect (i) intracellular vesicle mobility and (ii) vesicle cargo discharge from cultured rat astrocytes. Distinct types of vesicles, peptidergic, glutamatergic, and endosomes/lysosomes, were fluorescently prelabeled by cell transfection with plasmids encoding atrial natriuretic peptide tagged with mutant green fluorescent protein and vesicular glutamate transporter tagged with enhanced green fluorescent protein or by LysoTracker staining, respectively. The confocal and total internal reflection fluorescence microscopies were used to monitor vesicle mobility in the cytoplasm and near the basal plasma membrane, respectively. Sphingosine and FTY720, but not the membrane impermeable lipid analogs, dose-dependently attenuated vesicle mobility in the subcellular regions studied, and significantly inhibited stimulated exocytotic peptide and glutamate release. We conclude that in astrocytes, cell permeable sphingosine-like lipids affect regulated exocytosis by attenuating vesicle mobility, thereby preventing effective vesicle access/interaction with the plasma membrane docking/release sites.

Fusion pore regulation in peptidergic vesicles

Regulated exocytosis, which involves fusion of secretory vesicles with the plasma membrane, is an important mode of communication between cells. In this process, signalling molecules that are stored in secretory vesicles are released into the extracellular space. During the initial stage of fusion, the interior of the vesicle is connected to the exterior of the cell with a narrow, channel-like structure: the fusion pore. It was long believed that the fusion pore is a short-lived intermediate state leading irreversibly to fusion pore dilation. However, recent results show that the diameter of the fusion pore can fluctuate, suggesting that the fusion pore is a subject of stabilization. A possible mechanism is addressed in this article, involving the local anisotropicity of membrane constituents that can stabilize the fusion pore. The molecular nature of such a stable fusion pore to predict how interacting molecules (proteins and/or lipids) mediate changes that affect the stability of the fusion pore and exocytosis is also considered. The fusion pore likely attains stability via multiple mechanisms, which include the shape of the lipid and protein membrane constituents and the interactions between them.

Exocytotic fusion pore stability and topological defects in the membrane with orientational degree of ordering

Regulated exocytosis is a process that strongly depends on the formation and stability of the fusion pore. It was indicated experimentally and theoretically that narrow and highly curved fusion pore may be stabilized by accumulation of anisotropic membrane components possessing orientational ordering. On the other hand, narrow fusion pore may also undergo repetitive opening and closing, disruption in the so called kiss and run process or become completely opened in the process of full fusion of the vesicle with the membrane. In this paper we attempt to elucidate the subtle interplay between the stabilizing and destabilizing processes in the fusion neck. A possible physical mechanism which may lead to disruption of the stable fusion pore or complete fusion of the vesicle with the membrane is discussed. It is indicated that topologically driven defects of the in-plane orientational membrane ordering in the region of the fusion pore may disrupt the fusion. Alternatively, it may facilitate repetitive opening and closing of the fusion pore or induce full fusion of the vesicle with the target membrane.

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.

Fusion pore diameter regulation by cations modulating local membrane anisotropy

The fusion pore is an aqueous channel that is formed upon the fusion of the vesicle membrane with the plasma membrane. Once the pore is open, it may close again (transient fusion) or widen completely (full fusion) to permit vesicle cargo discharge. While repetitive transient fusion pore openings of the vesicle with the plasma membrane have been observed in the absence of stimulation, their frequency can be further increased using a cAMP-increasing agent that drives the opening of nonspecific cation channels. Our model hypothesis is that the openings and closings of the fusion pore are driven by changes in the local concentration of cations in the connected vesicle. The proposed mechanism of fusion pore dynamics is considered as follows: when the fusion pore is closed or is extremely narrow, the accumulation of cations in the vesicle (increased cation concentration) likely leads to lipid demixing at the fusion pore. This process may affect local membrane anisotropy, which reduces the spontaneous curvature and thus leads to the opening of the fusion pore. Based on the theory of membrane elasticity, we used a continuum model to explain the rhythmic opening and closing of the fusion pore.

Pool-independent labelling of synaptic vesicle exocytosis with single vesicle resolution in rat hippocampal neurons

FM dyes are an established tool to analyze synaptic vesicle pools. However, quantitative measurements using FM dyes are typically based on the re-release properties of previously labelled vesicles, which might vary depending on the experimental setup. An FM dye protocol independent of the previous labelling of vesicle membrane has not been applied for quantitative measurements of individual synaptic vesicles before. We therefore analyzed the direct staining of newly exocytosed vesicle membrane with FM dyes in cultured rat hippocampal neurons. In the presence of FM 1-43, stimulation-induced synaptic activity led to a stable fluorescence increase. The quantal release of synaptic vesicles was preserved and its amplitude correlated highly with the exocytic dye loss induced by a subsequent stimulation. Thus, the method presented here provides a tool for the pool-independent measurement of synaptic vesicle exocytosis.

Amyotrophic lateral sclerosis immunoglobulins G enhance the mobility of Lysotracker-labelled vesicles in cultured rat astrocytes

AIM

We examined the effect of purified immunoglobulins G (IgG) from patients with amyotrophic lateral sclerosis (ALS) on the mobility and exocytotic release from Lysotracker-stained vesicles in cultured rat astrocytes.

METHODS

Time-lapse confocal images were acquired, and vesicle mobility was analysed before and after the application of ALS IgG. The vesicle counts were obtained to assess cargo exocytosis from stained organelles.

RESULTS

At rest, when mobility was monitored for 2 min in bath with Ca(2+), two vesicle populations were discovered: (1) non-mobile vesicles (6.1%) with total track length (TL) < 1 μm, averaging at 0.33 ± 0.01 μm (n = 1305) and (2) mobile vesicles (93.9%) with TL > 1 μm, averaging at 3.03 ± 0.01 μm (n = 20,200). ALS IgG (0.1 mg mL(-1)) from 12 of 13 patients increased the TL of mobile vesicles by approx. 24% and maximal displacement (MD) by approx. 26% within 4 min, while the IgG from control group did not alter the vesicle mobility. The mobility enhancement by ALS IgG was reduced in extracellular solution devoid of Ca(2+), indicating that ALS IgG vesicle mobility enhancement involves changes in Ca(2+) homeostasis. To examine whether enhanced mobility relates to elevated Ca(2+) activity, cells were stimulated by 1 mm ATP, a cytosolic Ca(2+) increasing agent, in the presence (2 mm) and in the absence of extracellular Ca(2+). ATP stimulation triggered an increase in TL by approx. 7% and 12% and a decrease in MD by approx. 11% and 1%, within 4 min respectively. Interestingly, none of the stimuli triggered the release of vesicle cargo.

CONCLUSION

Amyotrophic lateral sclerosis-IgG-enhanced vesicle mobility in astrocytes engages changes in calcium homeostasis.

Aluminium-induced changes of fusion pore properties attenuate prolactin secretion in rat pituitary lactotrophs

Hormone secretion is mediated by Ca(2+)-regulated exocytosis. The key step of this process consists of the merger of the vesicle and the plasma membranes, leading to the formation of a fusion pore. This is an aqueous channel through which molecules stored in the vesicle lumen exit into the extracellular space on stimulation. Here we studied the effect of sub-lethal dose of aluminium on prolactin secretion in isolated rat pituitary lactotrophs with an enzyme immunoassay and by monitoring electrophysiologically the interaction of a single vesicle with the plasma membrane in real time, by monitoring membrane capacitance. After 24-h exposure to sub-lethal AlCl(3) (30 μM), the secretion of prolactin was reduced by 14±8% and 46±11% under spontaneous and K(+)-stimulated conditions, respectively. The frequency of unitary exocytotic events, recorded by the high-resolution patch-clamp monitoring of membrane capacitance, a parameter linearly related to the membrane area, under spontaneous and stimulated conditions, was decreased in aluminium-treated cells. Moreover, while the fusion pore dwell-time was increased in the presence of aluminium, the fusion pore conductance, a measure of fusion pore diameter, was reduced, both under spontaneous and stimulated conditions. These results suggest that sub-lethal aluminium concentrations reduce prolactin secretion downstream of the stimulus secretion coupling by decreasing the frequency of unitary exocytotic events and by stabilizing the fusion pore diameter to a value smaller than prolactin molecule, thus preventing its discharge into the extracellular space.

Synaptic neuropeptide release induced by octopamine without Ca2+ entry into the nerve terminal

Synaptic release of neurotransmitters is evoked by activity-dependent Ca(2+) entry into the nerve terminal. However, here it is shown that robust synaptic neuropeptide release from Drosophila motoneurons is evoked in the absence of extracellular Ca(2+) by octopamine, the arthropod homolog to norepinephrine. Genetic and pharmacology experiments demonstrate that this surprising peptidergic transmission requires cAMP-dependent protein kinase, with only a minor contribution of exchange protein activated by cAMP (epac). Octopamine-evoked neuropeptide release also requires endoplasmic reticulum Ca(2+) mobilization by the ryanodine receptor and the inositol trisphosphate receptor. Hence, rather than relying exclusively on activity-dependent Ca(2+) entry into the nerve terminal, a behaviorally important neuromodulator uses synergistic cAMP-dependent protein kinase and endoplasmic reticulum Ca(2+) signaling to induce synaptic neuropeptide release.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

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.

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.

Imaging localized astrocyte ATP release with firefly luciferase beads attached to the cell surface

Extracellular adenosine triphosphate (ATP) functions as a signaling molecule in many cell regulation processes. The traditional firefly luciferase assays measure the ATP release as a signal increase with time using a luminometer. Recently, advanced cell imaging techniques using charge-coupled device (CCD) cameras have enabled two-dimensional (2D) high-resolution detection providing both spatial and temporal information. Real-time imaging of ATP release from astrocyte cells has been reported. However, the observed chemiluminescence propagation wave reflects both ATP release and diffusion in the extracellular bulk solution. The dynamic ATP efflux at the cell surface could not be accurately measured. Hence, we constructed biotinylated fused firefly luciferase proteins, immobilized the proteins on 1 microm beads, and attached the beads to the cell surface to detect ATP release from mechanically stimulated astrocyte cells. This novel detection method enables us to monitor the actual ATP concentration at the surface of single live cells. The localized ATP release was found to be prominent but lasted only <20 s, which is very different from the results obtained by free firefly luciferase detection.

Multiple roles of Gi/o protein-coupled receptors in control of action potential secretion coupling in pituitary lactotrophs

G(i/o) protein-coupled receptors, signaling through G protein-dependent and protein-independent pathways, have prominent effects on secretion by modulating calcium signaling and regulating the size of the releasable secretory pool, the rates of exocytosis and endocytosis, and de novo synthesis. Pituitary cells fire action potentials spontaneously, and the associated calcium influx is sufficient to maintain prolactin (PRL) release but not gonadotropin release at high and steady levels for many hours. Such secretion, termed intrinsic, spontaneous, or basal, reflects fusion of secretory vesicles triggered by the cell type-specific pattern of action potentials. In lactotrophs, activation of endothelin ET(A) and dopamine D(2) receptors causes inhibition of spontaneous electrical activity and basal adenylyl cyclase activity accompanied with inhibition of basal PRL release. Agonist-induced inhibition of cAMP production and firing of action potentials is abolished in cells with blocked pertussis toxin (PTX)-sensitive G(i/o) signaling pathway. However, agonist-induced inhibition of PRL release is only partially relieved in such treated cells, indicating that both receptors also inhibit exocytosis downstream of cAMP/calcium signaling. The PTX-insensitive step in agonist-induced inhibition of PRL release is not affected by inhibition of phosphoinositide 3-kinase and glycogen synthase kinase-3 but is partially rescued by downregulation of the G(z)alpha expression. Thus, ET(A) and D(2) receptors inhibit basal PRL release not only by blocking electrical activity but also by desensitizing calcium-secretion coupling.

Compound exocytosis in pituitary cells

Neurotransmitter and hormone release from vesicles involves fusion between the vesicle and the plasma membranes, a process termed exocytosis. Recently we reported that most of the spontaneous and stimulated exocytotic events in pituitary lactotrophs are transient and repetitive, appearing in bursts lasting more than 100 s. However, whether this is also the case in compound vesicle-to-vesicle exocytosis is unknown. Here we investigated compound exocytotic events in resting and stimulated lactotrophs by using optical and cell-attached patch-clamp capacitance measurements. Elementary compound exocytotic events were characterized by multiple-amplitude on-steps in synaptopHluorin fluorescence and in membrane capacitance signals. Multiple-amplitude on-steps appeared either as a relatively large upward step, indicating that vesicles were fused with each other prior to fusion of the vesicle membrane with the plasma membrane (multivesicular exocytosis), or as a time-dependent stepwise signal increase, indicating sequential fusion of two or more vesicles with the plasma membrane (sequential exocytosis). In the majority of membrane capacitance recordings (>90%), multiple-amplitude on-steps terminated as multiple-amplitude off-steps. These complex amplitude events were repetitive, indicating that transient fusion pore openings reflect repetitive interactions of a single vesicle or vesicles in a cluster with the plasma membrane. Out of many mechanisms, these interactions may enable the diffusion of fusion proteins from the plasma membrane to the membrane of the primary fused vesicles, consequently enabling vesicle-to-vesicle fusion. The incidence of compound exocytotic events increased by 33% after stimulation, which is consistent with the enhanced efficiency of hormone secretion after the stimulus.

The fusion pore and vesicle cargo discharge modulation

Exocytosis, the merger of the vesicle membrane with the plasma membrane, is thought to mediate the release of hormones and neurotransmitters from secretory vesicles. The work of Bernard Katz and colleagues decades ago considered that vesicle cargo discharge initially requires the delivery of secretory vesicles to the plasma membrane where vesicles dock and are primed for fusion with the plasma membrane. Then, upon stimulation, the vesicle and the plasma membranes fuse to form a transient fusion pore through which cargo molecules diffuse out of the vesicle lumen into the extracellular space. Katz and colleagues considered this process to occur in an all-or-none fashion. However, recent studies show that this may not be so simple. The aim of this overview is to highlight the novel findings that indicate that fusion pores are subject to regulations, which affect the release competence of a single vesicle. Here we discuss the elementary properties of spontaneous and stimulated peptidergic vesicle discharge, which appears to be modulated, at least in pituitary lactotrophs, by fusion pore conductance (pore diameter) and fusion pore gating (kinetics).

Prolactin secretion sites contain syntaxin-1 and differ from ganglioside monosialic acid rafts in rat lactotrophs

In neuroendocrine cells, discharge of hormones follows the fusion of exocytotic vesicles with the plasma membrane at confined sites; however, the molecular nature of these distinct sites remains poorly understood. We studied intact pituitary lactotrophs and plasma membrane lawns by confocal microscopy in conjunction with antibodies against rat prolactin (rPRL), soluble N-ethylmaleimide-sensitive factor-attachment protein receptor (SNARE) proteins (syntaxin-1 and synaptobrevin-2,) and fluorescent cholera toxin subunit B (CT-B), a marker of ganglioside monosialic acid (GM1) lipid rafts, to examine 1) whether rPRL vesicles discharge cargo at GM1 rafts, 2) whether discharging rPRL vesicles interact with SNAREs, and 3) to examine the overlap of GM1 rafts, rPRL, and syntaxin-1 sites in plasma membrane lawns. In intact cells, immunofluorescently labeled rPRL poorly colocalized (<6%) with CT-B. In conditions favoring endocytotic trafficking, vesicle SNARE synaptobrevin-2 modestly colocalized (35%) with CT-B, whereas it highly colocalized (58%) with retrieved rPRL. Although partial mixing between rPRL and CT-B intracellular trafficking pathways is likely, our results indicated that rPRL discharge involves interactions with plasma membrane SNAREs, but not with GM1 rafts. In support of this, the plasma membrane SNARE syntaxin-1 poorly colocalized with CT-B (<5%), whereas it highly colocalized (75%) with rPRL in inside-out plasma membrane lawns. Spontaneous and stimulated rPRL discharge in live lactotrophs is thus associated with plasma membrane sites enriched with SNARE proteins, however, spatially confined to plasma membrane areas other than GM1 rafts.

A fast mode of membrane fusion dependent on tight SNARE zippering

SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins have a key role in membrane fusion. It is commonly assumed that pairing of SNARE proteins anchored in opposing membranes overcomes the repulsion energy between membranes, thereby catalyzing fusion. In this study, we have increased the distance between the coiled-coil SNARE motif and the transmembrane domain of the vesicular SNARE synaptobrevin-2 by insertion of a flexible linker to analyze how an increased intermembrane distance affects exocytosis. Synaptobrevin-2 lengthening did not change the frequency of exocytotic events measured at 1 mum free calcium but prevented the increase in the secretory activity triggered by higher calcium concentration. Exocytotic events monitored in adrenal chromaffin cells by means of carbon fiber amperometry were classified in two groups according to the rate and extent of fusion pore expansion. Lengthening the juxtamembrane region of synaptobrevin-2 severely reduced the occurrence of rapid single events, leaving slow ones unchanged. It also impaired the increase in the fast-fusion mode that normally follows elevation of intracellular Ca2+ levels. We conclude that mild stimuli trigger slow fusion events that do not rely on a short intermembrane distance. In contrast, a short intermembrane distance mediated by tight zippering of SNAREs is essential to a component of the secretory response elicited by robust stimuli and characterized by rapid dilation of the fusion pore.

Hypotonicity and peptide discharge from a single vesicle

Neuroendocrine secretory vesicles discharge their cargo in response to a stimulus, but the nature of this event is poorly understood. We studied the release of the pituitary hormone prolactin by hypotonicity, because this hormone also contributes to osmoregulation. In perfused rat lactotrophs, hypotonicity resulted in a transient increase followed by a sustained depression of prolactin release, as monitored by radioimmunoassay. In single cells imaged by confocal microscopy, hypotonicity elicited discharge of the fluorescently labeled atrial natriuretic peptide cargo from approximately 2% of vesicles/cell. In contrast, KCl-induced depolarization resulted in a response of approximately 10% of vesicles/cell, with different unloading/loading time course of the two fluorescent probes. In cell-attached studies, discrete changes in membrane capacitance were recorded in both unstimulated and stimulated conditions, reflecting single vesicle fusion/fissions with the plasma membrane. In stimulated cells, the probability of occurrence of full fusion events was low and unchanged, whereas over 95% of fusion events were transient, with the open fusion pore probability, the average pore dwell-time, the frequency of occurrence, and the fusion pore conductance increased. Hypotonicity only rarely elicited new fusion events in silent membrane patches. The results indicate that, in hypotonicity-stimulated lactotrophs, transient vesicle fusion mediates hormone release.

Dopamine inhibits basal prolactin release in pituitary lactotrophs through pertussis toxin-sensitive and -insensitive signaling pathways

Dopamine D2 receptors signal through the pertussis toxin (PTX)-sensitive G(i/o) and PTX-insensitive G(z) proteins, as well as through a G protein-independent, beta-arrestin/glycogen synthase kinase-3-dependent pathway. Activation of these receptors in pituitary lactotrophs leads to inhibition of prolactin (PRL) release. It has been suggested that this inhibition occurs through the G(i/o)-alpha protein-mediated inhibition of cAMP production and/or G(i/o)-betagamma dimer-mediated activation of inward rectifier K(+) channels and inhibition of voltage-gated Ca(2+) channels. Here we show that the dopamine agonist-induced inhibition of spontaneous Ca(2+) influx and release of prestored PRL was preserved when cAMP levels were elevated by forskolin treatment. We further observed that dopamine agonists inhibited both spontaneous and depolarization-induced Ca(2+) influx in untreated but not in PTX-treated cells. This inhibition was also observed in cells with blocked inward rectifier K(+) channels, suggesting that the dopamine effect on voltage-gated Ca(2+) channel gating is sufficient to inhibit spontaneous Ca(2+) influx. However, agonist-induced inhibition of PRL release was only partially relieved in PTX-treated cells, indicating that dopamine receptors also inhibit exocytosis downstream of voltage-gated Ca(2+) influx. The PTX-insensitive step in agonist-induced inhibition of PRL release was not affected by the addition of wortmannin, an inhibitor of phosphatidylinositol 3-kinase, and lithium, an inhibitor of glycogen synthase kinase-3, but was attenuated in the presence of phorbol 12-myristate 13-acetate, which inhibits G(z) signaling pathway in a protein kinase C-dependent manner. Thus, dopamine inhibits basal PRL release by blocking voltage-gated Ca(2+) influx through the PTX-sensitive signaling pathway and by desensitizing Ca(2+) secretion coupling through the PTX-insensitive and protein kinase C-sensitive signaling pathway.

Stimulation inhibits the mobility of recycling peptidergic vesicles in astrocytes

Astrocytes are increasingly viewed as playing many roles in the integration of brain function. These cells store among other gliotransmitters also neuroactive peptides in membrane bound vesicles, the trafficking and release of which, may be changed in altered conditions, therefore affecting the physiological status of neurons. In general, peptidergic membrane-bound secretory vesicles fuse with the plasma membrane in the process of exocytosis. Some of them are retrieved from the plasma membrane to be recycled back into the cytosol. The mobility of retrieving vesicles in astrocytes was not studied yet, however, understanding the mechanisms of such trafficking would highlight the communication paths between astrocytes and neurons. We labeled vesicles with antibodies against the vesicle atrial natriuretic peptide (ANP), which is stored inside secretory vesicles. ANP-vesicles in astrocytes have been proposed to enter Ca2+-dependent secretion and here we show that they are associated with synaptotagmin IV (SytIV), a regulator of exocytosis in astrocytes. Moreover, the results show that recycling ANP-vesicles are to a significant extent acidic. Their velocity (0.06+/-0.001 microm/s) is one order of magnitude lower than the velocity of vesicles trafficking to the plasma membrane (Potokar et al. (2005) Biochem Biophys Res Commun 329:678-683; Potokar et al. (2007) Traffic 8:12-20). Interestingly, ionomycin or ATP application further attenuated ANP-vesicle mobility to 0.02+/-0.002 and to 0.03+/-0.001 microm/s, respectively. In summary, the mobility of recycling peptidergic vesicles appears to be slower than the vesicle traffic to the plasma membrane and it requires an intact cytoskeleton. Physiological implications of attenuated traffic of ANP-vesicles are considered in the discussion.

Distinct labelling of fusion events in rat lactotrophs by FM 1-43 and FM 4-64 is associated with conformational differences

AIM

Conformational analysis of fluorescent styryl dyes FM 1-43 and FM 4-64 was undertaken to clarify if distinct activity-dependent labelling of single lactotrophs vesicles and plasma membrane by two dyes is associated with their structural differences.

METHODS

The activity-dependent labelling of single vesicles and plasma membrane by FM 1-43 and FM 4-64 was studied using confocal microscopy. The fluorescence intensity of vesicles fused with the plasma membrane, and the plasma membrane alone was measured; the ratio of their respective peak amplitudes was calculated. The conformational analysis of FM 1-43 and FM 4-64 was further undertaken by employing the Monte Carlo approach to search the conformational space of these molecules.

RESULTS

In FM 1-43 staining of vesicles and plasma membrane, the ratio of the fluorescence peak amplitudes (vesicle vs. plasma membrane) was 2.6 times higher in comparison with FM 4-64 staining. In FM 4-64 molecule the low-energy conformations are distributed in three conformational states (consisting of 3, 4 and 2 conformers respectively) in which the proportion of the molecules residing in a given state is 62%, 28% and 9% respectively. In FM 1-43 the conformation distribution is limited to just one conformational state with three approximately equally populated conformers what can be explained by greater intrinsic rigidity of the molecule.

CONCLUSIONS

The observed structural characteristics of FM 1-43 molecules may account for a higher increase in quantum yield and/or binding affinity upon incorporation of the dye into the vesicle matrix and therefore stronger fluorescence emission in comparison with FM 4-64.

Exocytotic release of ATP from cultured astrocytes

Astrocytes appear to communicate with each other as well as with neurons via ATP. However, the mechanisms of ATP release are controversial. To explore whether stimuli that increase [Ca(2+)](i) also trigger vesicular ATP release from astrocytes, we labeled ATP-containing vesicles with the fluorescent dye quinacrine, which exhibited a significant co-localization with atrial natriuretic peptide. The confocal microscopy study revealed that quinacrine-loaded vesicles displayed mainly non-directional spontaneous mobility with relatively short track lengths and small maximal displacements, whereas 4% of vesicles exhibited directional mobility. After ionomycin stimulation only non-directional vesicle mobility could be observed, indicating that an increase in [Ca(2+)](i) attenuated vesicle mobility. Total internal reflection fluorescence (TIRF) imaging in combination with epifluorescence showed that a high percentage of fluorescently labeled vesicles underwent fusion with the plasma membrane after stimulation with glutamate or ionomycin and that this event was Ca(2+)-dependent. This was confirmed by patch-clamp studies on HEK-293T cells transfected with P2X(3) receptor, used as sniffers for ATP release from astrocytes. Glutamate stimulation of astrocytes was followed by an increase in the incidence of small transient inward currents in sniffers, reminiscent of postsynaptic quantal events observed at synapses. Their incidence was highly dependent on extracellular Ca(2+). Collectively, these findings indicate that glutamate-stimulated ATP release from astrocytes was most likely exocytotic and that after stimulation the fraction of quinacrine-loaded vesicles, spontaneously exhibiting directional mobility, disappeared.