Synaptic vesicles really do kiss and run

Transient pores in hemifusion diaphragms

Exchange of material across two membranes, as in the case of synaptic neurotransmitter release from a vesicle, involves the formation and poration of a hemifusion diaphragm (HD). The nontrivial geometry of the HD leads to environment-dependent control, regarding the stability and dynamics of the pores required for this kind of exocytosis. This work combines particle simulations, field-based calculations, and phenomenological modeling to explore the factors influencing the stability, dynamics, and possible control mechanisms of pores in HDs. We find that pores preferentially form at the HD rim, and that their stability is sensitive to a number of factors, including the three line tensions, membrane tension, HD size, and the ability of lipids to "flip-flop" across leaflets. Along with a detailed analysis of these factors, we discuss ways that vesicles or cells may use them to open and close pores and thereby quickly and efficiently transport material.

Somatostatin modulation of initial fusion pores in Ca2+-triggered exocytosis from mouse chromaffin cells

Somatostatin, a peptide hormone that activates G-protein-coupled receptors, inhibits the secretion of many hormones. This study investigated the mechanisms of this inhibition using amperometry recording of Ca2+-triggered catecholamine secretion from mouse chromaffin cells. Two distinct stimulation protocols, high-KCl depolarization and caffeine, were used to trigger exocytosis, and confocal fluorescence imaging was used to monitor the rise in intracellular free Ca2+. Analysis of single-vesicle fusion events (spikes) resolved the action of somatostatin on fusion pores at different stages. Somatostatin reduced spike frequency, and this reduction was accompanied by prolongation of pre-spike feet and slowing of spike rise times. This indicates that somatostatin stabilizes initial fusion pores and slows their expansion. This action on the initial fusion pore impacted the release mode to favour kiss-and-run over full-fusion. During a spike the permeability of a fusion pore peaks, declines and then settles into a plateau. Somatostatin had no effect on the plateau, suggesting no influence on late-stage fusion pores. These actions of somatostatin were indistinguishable between exocytosis triggered by high-KCl and caffeine, and fluorescence imaging showed that somatostatin had no effect on stimulus-induced rises in cytosolic Ca2+. Our findings thus demonstrate that the signalling cascades activated by somatostatin target the exocytotic machinery that controls the initial and expanding stages of fusion pores, while having no effect on late-stage fusion pores. As a result of its stronger inhibition of full-fusion compared to kiss-and-run, somatostatin will preferentially inhibit the secretion of large peptides over the secretion of small catecholamines. KEY POINTS: Somatostatin inhibits the secretion of various hormones by activating G-protein-coupled receptors. In this study, we used amperometry to investigate the mechanism by which somatostatin inhibits catecholamine release from mouse chromaffin cells. Somatostatin increased pre-spike foot lifetime and slowed fusion pore expansion. Somatostatin inhibited full-fusion more strongly than kiss-and-run. Our results suggest that the initial fusion pore is the target of somatostatin-mediated regulation of hormone release. The stronger inhibition of full-fusion by somatostatin will result in preferential inhibition of peptide release.

Analytical Determination of Serotonin Exocytosis in Human Platelets with BDD-on-Quartz MEA Devices

Amperometry is arguably the most widely used technique for studying the exocytosis of biological amines. However, the scarcity of human tissues, particularly in the context of neurological diseases, poses a challenge for exocytosis research. Human platelets, which accumulate 90% of blood serotonin, release it through exocytosis. Nevertheless, single-cell amperometry with encapsulated carbon fibers is impractical due to the small size of platelets and the limited number of secretory granules on each platelet. The recent technological improvements in amperometric multi-electrode array (MEA) devices allow simultaneous recordings from several high-performance electrodes. In this paper, we present a comparison of three MEA boron-doped diamond (BDD) devices for studying serotonin exocytosis in human platelets: (i) the BDD-on-glass MEA, (ii) the BDD-on-silicon MEA, and (iii) the BDD on amorphous quartz MEA (BDD-on-quartz MEA). Transparent electrodes offer several advantages for observing living cells, and in the case of platelets, they control activation/aggregation. BDD-on-quartz offers the advantage over previous materials of combining excellent electrochemical properties with transparency for microscopic observation. These devices are opening exciting perspectives for clinical applications.

Leaflet Tensions Control the Spatio-Temporal Remodeling of Lipid Bilayers and Nanovesicles

Biological and biomimetic membranes are based on lipid bilayers, which consist of two monolayers or leaflets. To avoid bilayer edges, which form when the hydrophobic core of such a bilayer is exposed to the surrounding aqueous solution, a single bilayer closes up into a unilamellar vesicle, thereby separating an interior from an exterior aqueous compartment. Synthetic nanovesicles with a size below 100 nanometers, traditionally called small unilamellar vesicles, have emerged as potent platforms for the delivery of drugs and vaccines. Cellular nanovesicles of a similar size are released from almost every type of living cell. The nanovesicle morphology has been studied by electron microscopy methods but these methods are limited to a single snapshot of each vesicle. Here, we review recent results of molecular dynamics simulations, by which one can monitor and elucidate the spatio-temporal remodeling of individual bilayers and nanovesicles. We emphasize the new concept of leaflet tensions, which control the bilayers' stability and instability, the transition rates of lipid flip-flops between the two leaflets, the shape transformations of nanovesicles, the engulfment and endocytosis of condensate droplets and rigid nanoparticles, as well as nanovesicle adhesion and fusion. To actually compute the leaflet tensions, one has to determine the bilayer's midsurface, which represents the average position of the interface between the two leaflets. Two particularly useful methods to determine this midsurface are based on the density profile of the hydrophobic lipid chains and on the molecular volumes.

Role of Clathrin and Dynamin in Clathrin Mediated Endocytosis/Synaptic Vesicle Recycling and Implications in Neurological Diseases

Endocytosis is a process essential to the health and well-being of cell. It is required for the internalisation and sorting of “cargo”—the macromolecules, proteins, receptors and lipids of cell signalling. Clathrin mediated endocytosis (CME) is one of the key processes required for cellular well-being and signalling pathway activation. CME is key role to the recycling of synaptic vesicles [synaptic vesicle recycling (SVR)] in the brain, it is pivotal to signalling across synapses enabling intracellular communication in the sensory and nervous systems. In this review we provide an overview of the general process of CME with a particular focus on two key proteins: clathrin and dynamin that have a central role to play in ensuing successful completion of CME. We examine these two proteins as they are the two endocytotic proteins for which small molecule inhibitors, often of known mechanism of action, have been identified. Inhibition of CME offers the potential to develop therapeutic interventions into conditions involving defects in CME. This review will discuss the roles and the current scope of inhibitors of clathrin and dynamin, providing an insight into how further developments could affect neurological disease treatments.

Remodeling of Membrane Shape and Topology by Curvature Elasticity and Membrane Tension

Cellular membranes exhibit a fascinating variety of different morphologies, which are continuously remodeled by transformations of membrane shape and topology. This remodeling is essential for important biological processes (cell division, intracellular vesicle trafficking, endocytosis) and can be elucidated in a systematic and quantitative manner using synthetic membrane systems. Here, recent insights obtained from such synthetic systems are reviewed, integrating experimental observations and molecular dynamics simulations with the theory of membrane elasticity. The study starts from the polymorphism of biomembranes as observed for giant vesicles by optical microscopy and small nanovesicles in simulations. This polymorphism reflects the unusual elasticity of fluid membranes and includes the formation of membrane necks or fluid 'worm holes'. The proliferation of membrane necks generates stable multi-spherical shapes, which can form tubules and tubular junctions. Membrane necks are also essential for the remodeling of membrane topology via membrane fission and fusion. Neck fission can be induced by fine-tuning of membrane curvature, which leads to the controlled division of giant vesicles, and by adhesion-induced membrane tension as observed for small nanovesicles. Challenges for future research include the interplay of curvature elasticity and membrane tension during membrane fusion and the localization of fission and fusion processes within intramembrane domains.

Artificial Synapse: Spatiotemporal Heterogeneities in Dopamine Electrochemistry at a Carbon Fiber Ultramicroelectrode

An artificial synapse is developed that mimics ultramicroelectrode (UME) amperometric detection of single cell exocytosis. It comprises the nanopipette of a scanning ion conductance microscope (SICM), which delivers rapid pulses of neurotransmitter (dopamine) locally and on demand at >1000 defined locations of a carbon fiber (CF) UME in each experiment. Analysis of the resulting UME current-space-time data reveals spatiotemporal heterogeneous electrode activity on the nanoscale and submillisecond time scale for dopamine electrooxidation at typical UME detection potentials. Through complementary surface charge mapping and finite element method (FEM) simulations, these previously unseen variations in electrochemical activity are related to heterogeneities in the surface chemistry of the CF UME.

Budding and Fission of Nanovesicles Induced by Membrane Adsorption of Small Solutes

Membrane budding and fission are essential cellular processes that produce new membrane compartments during cell and organelle division, for intracellular vesicle trafficking as well as during endo- and exocytosis. Such morphological transformations have also been observed for giant lipid vesicles with a size of many micrometers. Here, we report budding and fission processes of lipid nanovesicles with a size below 50 nm. We use coarse-grained molecular dynamics simulations, by which we can visualize the morphological transformations of individual vesicles. The budding and fission processes are induced by low concentrations of small solutes that absorb onto the outer leaflets of the vesicle membranes. In addition to the solute concentration, we identify the solvent conditions as a second key parameter for these processes. For good solvent conditions, the budding of a nanovesicle can be controlled by reducing the vesicle volume for constant solute concentration or by increasing the solute concentration for constant vesicle volume. After the budding process is completed, the budded vesicle consists of two membrane subcompartments which are connected by a closed membrane neck. The budding process is reversible as we demonstrate explicitly by reopening the closed neck. For poor solvent conditions, on the other hand, we observe two unexpected morphological transformations of nanovesicles. Close to the binodal line, at which the aqueous solution undergoes phase separation, the vesicle exhibits recurrent shape changes with closed and open membrane necks, reminiscent of flickering fusion pores (kiss-and-run) as observed for synaptic vesicles. As we approach the binodal line even closer, the recurrent shape changes are truncated by the fission of the membrane neck which leads to the division of the nanovesicle into two daughter vesicles. In this way, our simulations reveal a nanoscale mechanism for the budding and fission of nanovesicles, a mechanism that arises from the interplay between membrane elasticity and solute-mediated membrane adhesion.

Electrochemical Quantification of Neurotransmitters in Single Live Cell Vesicles Shows Exocytosis is Predominantly Partial

Exocytosis plays an essential role in the communication between cells in the nervous system. Understanding the regulation of neurotransmitter release during exocytosis and the amount of neurotransmitter content that is stored in vesicles is of importance, as it provides fundamental insights to understand how the brain works and how neurons elicit a certain behavior. In this minireview, we summarize recent progress in amperometric measurements for monitoring exocytosis in single cells and electrochemical cytometry measurements of vesicular neurotransmitter content in individual vesicles. Important steps have increased our understanding of the different mechanisms of exocytosis. Increasing evidence is firmly establishing that partial release is the primary mechanism of release in multiple cell types.

SNARE-Mediated Exocytosis in Neuronal Development

The formation of the nervous system involves establishing complex networks of synaptic connections between proper partners. This developmental undertaking requires the rapid expansion of the plasma membrane surface area as neurons grow and polarize, extending axons through the extracellular environment. Critical to the expansion of the plasma membrane and addition of plasma membrane material is exocytic vesicle fusion, a regulated mechanism driven by soluble N-ethylmaleimide-sensitive factor attachment proteins receptors (SNAREs). Since their discovery, SNAREs have been implicated in several critical neuronal functions involving exocytic fusion in addition to synaptic transmission, including neurite initiation and outgrowth, axon specification, axon extension, and synaptogenesis. Decades of research have uncovered a rich variety of SNARE expression and function. The basis of SNARE-mediated fusion, the opening of a fusion pore, remains an enigmatic event, despite an incredible amount of research, as fusion is not only heterogeneous but also spatially small and temporally fast. Multiple modes of exocytosis have been proposed, with full-vesicle fusion (FFV) and kiss-and-run (KNR) being the best described. Whereas most in vitro work has reconstituted fusion using VAMP-2, SNAP-25, and syntaxin-1; there is much to learn regarding the behaviors of distinct SNARE complexes. In the past few years, robust heterogeneity in the kinetics and fate of the fusion pore that varies by cell type have been uncovered, suggesting a paradigm shift in how the modes of exocytosis are viewed is warranted. Here, we explore both classic and recent work uncovering the variety of SNAREs and their importance in the development of neurons, as well as historical and newly proposed modes of exocytosis, their regulation, and proteins involved in the regulation of fusion kinetics.

Direct Measurement of Total Vesicular Catecholamine Content with Electrochemical Microwell Arrays

We have designed and fabricated a microwell array chip (MWAC) to trap and detect the entire content of individual vesicles after disruption of the vesicular membrane by an applied electrical potential. To understand the mechanism of vesicle impact electrochemical cytometry (VIEC) in microwells, we simulated the rupture of the vesicles and subsequent diffusion of entrapped analytes. Two possibilities were tested: (i) the vesicle opens toward the electrode, and (ii) the vesicle opens away from the electrode. These two possibilities were simulated in the different microwells with varied depth and width. Experimental VIEC measurements of the number of molecules for each vesicle in the MWAC were compared to VIEC on a gold microdisk electrode as a control, and the quantified catecholamines between these two techniques was the same. We observed a prespike foot in a significant number of events (∼20%) and argue this supports the hypothesis that the vesicles rupture toward the electrode surface with a more complex mechanism including the formation of a stable pore intermediate. This study not only confirms that in standard VIEC experiments the whole content of the vesicle is oxidized and quantified at the surface of the microdisk electrode but actively verifies that the adsorbed vesicle on the surface of the electrode forms a pore in the vicinity of the electrode rather than away from it. The fabricated MWAC promotes our ability to quantify the content of vesicles accurately, which is fundamentally important in bioanalysis of the vesicles.

Optical Tracking of Nanometer-Scale Cellular Membrane Deformation Associated with Single Vesicle Release

Exocytosis involves interactions between secretory vesicles and the plasma membrane. Studying the membrane response is thus critical to understand this important cellular process and to differentiate different mediator release patterns. Here we introduce a label-free optical imaging method to detect the vesicle-membrane-interaction-induced membrane deformation associated with single exocytosis in mast cells. We show that the plasma membrane expands by a few tens of nanometers accompanying each vesicle-release event, but the dynamics of the membrane deformation varies from cell to cell, which reflect different exocytosis processes. Combining the temporal and spatial information allows us to resolve complex vesicle-release processes, such as two vesicle-release events that occur closely in time and location. Simultaneous following a vesicle release with fluorescence and membrane deformation tracking further allows us to determine the propagation speed of the vesicle-release-induced membrane deformation along the cell surface, which has an average value of 5.2 ± 1.8 μm/s.

Single Synaptic Observation of Cholinergic Neurotransmission on Living Neurons: Concentration and Dynamics

Acetylcholine, the first neurotransmitter identified more than a century ago, plays critical roles in human activities and health; however, its synaptic concentration dynamics have remained unknown. Here, we demonstrate the in situ simultaneous measurements of synaptic cholinergic transmitter concentration and release dynamics. We used nanoscale electroanalytical methods: nanoITIES electrode of 15 nm in radius and nanoresolved scanning electrochemical microscopy (SECM). Time-resolved in situ measurements unveiled information on synaptic acetylcholine concentration and release dynamics of living Aplysia neurons. The measuring technique enabled the quantitative sensing of acetylcholine with negligible interference of other ionic and redox-active species. We measured cholinergic transmitter concentrations very close to the synapse, with values as high as 2.4 mM. We observed diverse synaptic transmitter concentration dynamics consisting of singlet, doublet and multiplet events with a signal-to-noise ratio of 6 to 130. The unprecedented details about synaptic neurotransmission unveiled are instrumental for understanding brain communication and diseases in a way distinctive from extra-synaptic studies.

Amperometry methods for monitoring vesicular quantal size and regulation of exocytosis release

Chemical signaling strength during intercellular communication can be regulated by secretory cells through controlling the amount of signaling molecules that are released from a secretory vesicle during the exocytosis process. In addition, the chemical signal can also be influenced by the amount of neurotransmitters that is accumulated and stored inside the secretory vesicle compartment. Here, we present the development of analytical methodologies and cell model systems that have been applied in neuroscience research for gaining better insights into the biophysics and the molecular mechanisms, which are involved in the regulatory aspects of the exocytosis machinery affecting the output signal of chemical transmission at neuronal and neuroendocrine cells.

Actomyosin contractility drives bile regurgitation as an early response during obstructive cholestasis

BACKGROUND & AIMS

A wide range of liver diseases manifest as biliary obstruction, or cholestasis. However, the sequence of molecular events triggered as part of the early hepatocellular homeostatic response in obstructive cholestasis is poorly elucidated. Pericanalicular actin is known to accumulate during obstructive cholestasis. Therefore, we hypothesized that the pericanalicular actin cortex undergoes significant remodeling as a regulatory response to obstructive cholestasis.

METHODS

In vivo investigations were performed in a bile duct-ligated mouse model. Actomyosin contractility was assessed using sandwich-cultured rat hepatocytes transfected with various fluorescently labeled proteins and pharmacological inhibitors of actomyosin contractility.

RESULTS

Actomyosin contractility induces transient deformations along the canalicular membrane, a process we have termed inward blebbing. We show that these membrane intrusions are initiated by local ruptures in the pericanalicular actin cortex; and they typically retract following repair by actin polymerization and actomyosin contraction. However, above a certain osmotic pressure threshold, these inward blebs pinch away from the canalicular membrane into the hepatocyte cytoplasm as large vesicles (2-8μm). Importantly, we show that these vesicles aid in the regurgitation of bile from the bile canaliculi.

CONCLUSION

Actomyosin contractility induces the formation of bile-regurgitative vesicles, thus serving as an early homeostatic mechanism against increased biliary pressure during cholestasis.

LAY SUMMARY

Bile canaliculi expand and contract in response to the amount of secreted bile, and resistance from the surrounding actin bundles. Further expansion due to bile duct blockade leads to the formation of inward blebs, which carry away excess bile to prevent bile build up in the canaliculi.

Zinc Regulates Chemical-Transmitter Storage in Nanometer Vesicles and Exocytosis Dynamics as Measured by Amperometry

We applied electrochemical techniques with nano-tip electrodes to show that micromolar concentrations of zinc not only trigger changes in the dynamics of exocytosis, but also vesicle content in a model cell line. The vesicle catecholamine content in PC12 cells is significantly decreased after 100 μm zinc treatment, but, catecholamine release during exocytosis remains nearly the same. This contrasts with the number of molecules stored in the exocytosis vesicles, which decreases, and we find that the amount of catecholamine released from zinc-treated cells reaches nearly 100 % content expelled. Further investigation shows that zinc slows down exocytotic release. Our results provide the missing link between zinc and the regulation of neurotransmitter release processes, which might be important in memory formation and storage.

Variations in Fusion Pore Formation in Cholesterol-Treated Platelets

Exocytosis is a highly regulated intercellular communication process involving various membrane proteins, lipids, and cytoskeleton restructuring. These components help control granule fusion with the cell membrane, creating a pore through which granular contents are released into the extracellular environment. Platelets are an ideal model system for studying exocytosis due to their lack of a nucleus, resulting in decreased membrane regulation in response to cellular changes. In addition, platelets contain fewer granules than most other exocytosing cells, allowing straightforward measurement of individual granule release with carbon-fiber microelectrode amperometry. This technique monitors the concentration of serotonin, an electroactive molecule found in the dense-body granules of platelets, released as a function of time, with 50 μs time resolution, revealing biophysical characteristics of the fundamental exocytotic process. Variations in fusion pore formation and closure cause deviations from the classic current versus time spike profile and may influence diffusion of serotonin molecules from the site of granule fusion. Physiologically, the delivery of smaller packets of chemical messengers or the prolonged delivery of chemical messengers may represent how cells/organisms tune biological response. The goals of this work are twofold: 1) to categorize secretion features that deviate from the traditional mode of secretion and 2) to examine how changing the cholesterol composition of the platelet membrane results in changes in the pore formation process. Results herein indicate that the expected traditional mode of release is actually in the minority of granule content release events. In addition, results indicate that as the cholesterol content of the plasma membrane is increased, pore opening is less continuous.

Cholesterol Alters the Dynamics of Release in Protein Independent Cell Models for Exocytosis

Neurons communicate via an essential process called exocytosis. Cholesterol, an abundant lipid in both secretory vesicles and cell plasma membrane can affect this process. In this study, amperometric recordings of vesicular dopamine release from two different artificial cell models created from a giant unilamellar liposome and a bleb cell plasma membrane, show that with higher membrane cholesterol the kinetics for vesicular release are decelerated in a concentration dependent manner. This reduction in exocytotic speed was consistent for two observed modes of exocytosis, full and partial release. Partial release events, which only occurred in the bleb cell model due to the higher tension in the system, exhibited amperometric spikes with three distinct shapes. In addition to the classic transient, some spikes displayed a current ramp or plateau following the maximum peak current. These post spike features represent neurotransmitter release from a dilated pore before constriction and show that enhancing membrane rigidity via cholesterol adds resistance to a dilated pore to re-close. This implies that the cholesterol dependent biophysical properties of the membrane directly affect the exocytosis kinetics and that membrane tension along with membrane rigidity can influence the fusion pore dynamics and stabilization which is central to regulation of neurochemical release.

The evidence for open and closed exocytosis as the primary release mechanism

Exocytosis is the fundamental process by which cells communicate with each other. The events that lead up to the fusion of a vesicle loaded with chemical messenger with the cell membrane were the subject of a Nobel Prize in 2013. However, the processes occurring after the initial formation of a fusion pore are very much still in debate. The release of chemical messenger has traditionally been thought to occur through full distention of the vesicle membrane, hence assuming exocytosis to be all or none. In contrast to the all or none hypothesis, here we discuss the evidence that during exocytosis the vesicle-membrane pore opens to release only a portion of the transmitter content during exocytosis and then close again. This open and closed exocytosis is distinct from kiss-and-run exocytosis, in that it appears to be the main content released during regular exocytosis. The evidence for this partial release via open and closed exocytosis is presented considering primarily the quantitative evidence obtained with amperometry.

Membrane Transport

Life depends on a membrane's ability to precisely control the level of solutes in the aqueous compartments, inside and outside, bathing the membrane. The membrane determines what solutes enter and leave a cell. Transmembrane transport is controlled by complex interactions between membrane lipids, proteins, and carbohydrates. How the membrane accomplishes these tasks is the topic of this chapter.

Electrochemical nanoprobes for the chemical detection of neurotransmitters

Neurotransmitters, acting as chemical messengers, play an important role in neurotransmission, which governs many functional aspects of nervous system activity. Electrochemical probes have proven a very useful technique to study neurotransmission, especially to quantify and qualify neurotransmitters. With the emerging interests in probing neurotransmission at the level of single cells, single vesicles, as well as single synapses, probes that enable detection of neurotransmitters at the nanometer scale become vitally important. Electrochemical nanoprobes have been successfully employed in nanometer spatial resolution imaging of single nanopores of Si membrane and single Au nanoparticles, providing both topographical and chemical information, thus holding great promise for nanometer spatial study of neurotransmission. Here we present the current state of electrochemical nanoprobes for chemical detection of neurotransmitters, focusing on two types of nanoelectrodes, i.e. carbon nanoelectrode and nano-ITIES pipet electrode.

Real-time Monitoring of Discrete Synaptic Release Events and Excitatory Potentials within Self-reconstructed Neuromuscular Junctions

Chemical synaptic transmission is central to the brain functions. In this regard, real-time monitoring of chemical synaptic transmission during neuronal communication remains a great challenge. In this work, in vivo-like oriented neural networks between superior cervical ganglion (SCG) neurons and their effector smooth muscle cells (SMC) were assembled in a microfluidic device. This allowed amperometric detection of individual neurotransmitter release events inside functional SCG-SMC synapse with carbon fiber nanoelectrodes as well as recording of postsynaptic potential using glass nanopipette electrodes. The high vesicular release activities essentially involved complex events arising from flickering fusion pores as quantitatively established based on simulations. This work allowed for the first time monitoring in situ chemical synaptic transmission under conditions close to those found in vivo, which may yield important and new insights into the nature of neuronal communications.

Nanopipet-Based Liquid-Liquid Interface Probes for the Electrochemical Detection of Acetylcholine, Tryptamine, and Serotonin via Ionic Transfer

A nanoscale interface between two immiscible electrolyte solutions (ITIES) provides a unique analytical platform for the detection of ionic species of biological interest such as neurotransmitters and neuromodulators, especially those that are otherwise difficult to detect directly on a carbon electrode without electrode modification. We report the detection of acetylcholine, serotonin, and tryptamine on nanopipet electrode probes with sizes ranging from a radius of ≈7 to 35 nm. The transfer of these analytes across a 1,2-dichloroethane/water interface was studied by cyclic voltammetry and amperometry. Well-defined sigmoidal voltammograms were observed on the nanopipet electrodes within the potential window of artificial seawater for acetylcholine and tryptamine. The half wave transfer potential, E1/2, of acetylcholine, tryptamine, and serotonin were found to be -0.11, -0.25, and -0.47 V vs E(1/2,TEA) (term is defined later in experimental), respectively. The detection was linear in the range of 0.25-6 mM for acetylcholine and of 0.5-10 mM for tryptamine in artificial seawater. Transfer of serotonin was linear in the range of 0.15-8 mM in LiCl solution. The limit of detection for serotonin in LiCl on a radius ≈21 nm nanopipet electrode was 77 μM, for acetylcholine on a radius ≈7 nm nanopipet electrode was 205 μM, and for tryptamine on a radius ≈19 nm nanopipet electrode was 86 μM. Nanopipet-supported ITIES probes have great potential to be used in nanometer spatial resolution measurements for the detection of neurotransmitters.

Nanoelectrode for amperometric monitoring of individual vesicular exocytosis inside single synapses

Chemical neurotransmission occurs at chemical synapses and endocrine glands, but up to now there was no means for direct monitoring of neurotransmitter exocytosis fluxes and their precise kinetics from inside an individual synapse. The fabrication of a novel finite conical nanoelectrode is reported perfectly suited in size and electrochemical properties for probing amperometrically inside what appears to be single synapses and monitoring individual vesicular exocytotic events in real time. This allowed obtaining direct and important physiological evidences which may yield important and new insights into the nature of synaptic communications.

Two modes of exocytosis in an artificial cell

The details of exocytosis, the vital cell process of neuronal communication, are still under debate with two generally accepted scenarios. The first mode of release involves secretory vesicles distending into the cell membrane to release the complete vesicle contents. The second involves partial release of the vesicle content through an intermittent fusion pore, or an opened or partially distended fusion pore. Here we show that both full and partial release can be mimicked with a single large-scale cell model for exocytosis composed of material from blebbing cell plasma membrane. The apparent switching mechanism for determining the mode of release is demonstrated to be related to membrane tension that can be differentially induced during artificial exocytosis. These results suggest that the partial distension mode might correspond to an extended kiss-and-run mechanism of release from secretory cells, which has been proposed as a major pathway of exocytosis in neurons and neuroendocrine cells.

Role of monosaccharide transport proteins in carbohydrate assimilation, distribution, metabolism, and homeostasis

The facilitated diffusion of glucose, galactose, fructose, urate, myoinositol, and dehydroascorbicacid in mammals is catalyzed by a family of 14 monosaccharide transport proteins called GLUTs. These transporters may be divided into three classes according to sequence similarity and function/substrate specificity. GLUT1 appears to be highly expressed in glycolytically active cells and has been coopted in vitamin C auxotrophs to maintain the redox state of the blood through transport of dehydroascorbate. Several GLUTs are definitive glucose/galactose transporters, GLUT2 and GLUT5 are physiologically important fructose transporters, GLUT9 appears to be a urate transporter while GLUT13 is a proton/myoinositol cotransporter. The physiologic substrates of some GLUTs remain to be established. The GLUTs are expressed in a tissue specific manner where affinity, specificity, and capacity for substrate transport are paramount for tissue function. Although great strides have been made in characterizing GLUT-catalyzed monosaccharide transport and mapping GLUT membrane topography and determinants of substrate specificity, a unifying model for GLUT structure and function remains elusive. The GLUTs play a major role in carbohydrate homeostasis and the redistribution of sugar-derived carbons among the various organ systems. This is accomplished through a multiplicity of GLUT-dependent glucose sensing and effector mechanisms that regulate monosaccharide ingestion, absorption,distribution, cellular transport and metabolism, and recovery/retention. Glucose transport and metabolism have coevolved in mammals to support cerebral glucose utilization.

Platelet granule exocytosis: a comparison with chromaffin cells

The rapid secretion of bioactive amines from chromaffin cells constitutes an important component of the fight or flight response of mammals to stress. Platelets respond to stresses within the vasculature by rapidly secreting cargo at sites of injury, inflammation, or infection. Although chromaffin cells derive from the neural crest and platelets from bone marrow megakaryocytes, both have evolved a heterogeneous assemblage of granule types and a mechanism for efficient release. This article will provide an overview of granule formation and exocytosis in platelets with an emphasis on areas in which the study of chromaffin cells has influenced that of platelets and on similarities between the two secretory systems. Commonalities include the use of transporters to concentrate bioactive amines and other cargos into granules, the role of cytoskeletal remodeling in granule exocytosis, and the use of granules to provide membrane for cytoplasmic projections. The SNAREs and SNARE accessory proteins used by each cell type will also be considered. Finally, we will discuss the newly appreciated role of dynamin family proteins in regulated fusion pore formation. This evaluation of the comparative cell biology of regulated exocytosis in platelets and chromaffin cells demonstrates a convergence of mechanisms between two disparate cell types both tasked with responding rapidly to physiological stimuli.

Membrane Transport

Carefully controlled solute movement into and out of cells is an essential feature of life. There are many ways solutes are transported across the thin (~40 Å) hydrophobic barrier. Transport is divided into passive diffusion and active transport. A biological membrane is semi-permeable, being permeable to some molecules, most notably water, while being very impermeable to most solutes that require some form of transporter.

Passive diffusion (simple and facilitated) only requires the energy inherent in the solute’s electrochemical gradient and results in equilibrium across the membrane. In contrast, active transport requires additional energy (i.e. ATP), and results in a non-equilibrium, net accumulation of the solute. Passive transport can involve simple diffusion or facilitated carriers including ionophores and channels. Active transport comes in many, often complex forms. Examples of active transport include primary active transport (uniport), secondary active transport (cotransport, antiport) and group translocation.

Besides the multitude of transport systems, transport can be accomplished by Gap Junctions, receptor-mediated endocytosis, phagocytosis, pinocytosis, exocytosis and apoptotic membrane blebbing.

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

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

Hydrodynamic flow in a synaptic cleft during exocytosis

It is shown that exocytosis in a chemical synapse may be accompanied by "microjet" formation due to the overpressure that exists in the vesicles. This mechanism may take place either at complete fusion of a vesicle with the presynaptic membrane or in the so-called kiss-and-run mode of neurotransmitter release. A simple hydrodynamic model of the viscous incompressible flow arising in the synaptic cleft is suggested. The occurrence of hydrodynamic flow (microjet) leads to more efficient transport of neurotransmitter than in the case of classical diffusive transport.

Analytical approaches to investigate transmitter content and release from single secretory vesicles

The vesicle serves as the primary intracellular unit for the highly efficient storage and release of chemical messengers triggered during signaling processes in the nervous system. This review highlights conventional and emerging analytical methods that have used microscopy, electrochemistry, and spectroscopy to resolve the location, time course, and quantal content characteristics of neurotransmitter release. Particular focus is on the investigation of the synaptic vesicle and its involvement in the fundamental molecular mechanisms of cell communication.

Monitoring of vesicular exocytosis from single cells using micrometer and nanometer-sized electrochemical sensors

Communication between cells by release of specific chemical messengers via exocytosis plays crucial roles in biological process. Electrochemical detection based on ultramicroelectrodes (UMEs) has become one of the most powerful techniques in real-time monitoring of an extremely small number of released molecules during very short time scales, owing to its intrinsic advantages such as fast response, excellent sensitivity, and high spatiotemporal resolution. Great successes have been achieved in the use of UME methods to obtain quantitative and kinetic information about released chemical messengers and to reveal the molecular mechanism in vesicular exocytosis. In this paper, we review recent developments in monitoring exocytosis by use of UMEs-electrochemical-based techniques including electrochemical detection using micrometer and nanometer-sized sensors, scanning electrochemical microscopy (SECM), and UMEs implemented in lab-on-a-chip (LOC) microsystems. These advances are of great significance in obtaining a better understanding of vesicular exocytosis and chemical communications between cells, and will facilitate developments in many fields, including analytical chemistry, biological science, and medicine. Furthermore, future developments in electrochemical probing of exocytosis are also proposed.

Fully automated microchip system for the detection of quantal exocytosis from single and small ensembles of cells

A lab-on-a-chip device that enables positioning of single or small ensembles of cells on an aperture in close proximity to a mercaptopropionic acid (MPA) modified sensing electrode has been developed and characterized. The microchip was used for the detection of Ca(2+)-dependent quantal catecholamine exocytosis from single as well as small assemblies of rat pheochromocytoma (PC12) cells. The frequency of events increased considerably upon depolarization of the PC12 cell membrane using a high extracelluar concentration of potassium. The number of recorded events could be correlated with the number of cells immobilized on the electrode. Quantal characteristics, such as the number of released molecules per recorded event, are equivalent to data obtained using conventional carbon fiber microelectrodes. The detection sensitivity of the device allows for the detection of less than 10 000 dopamine molecules in a quantal release. The distribution of peak rise-time and full width at half maximum was constant during measurement periods of several minutes demonstrating the stability of the MPA modified surface.

Resident neuroelectrochemical interfacing using carbon nanofiber arrays

Carbon nanofiber electrode architectures are used to provide for long-term, neuroelectroanalytical measurements of the dynamic processes of intercellular communication between excitable cells. Individually addressed, vertically aligned carbon nanofibers are incorporated into multielement electrode arrays upon which excitable cell matrixes of both neuronal-like derived cell lines (rat pheochromocytoma, PC-12) and primary cells (dissociated cells from embryonic rat hippocampus) are cultured over extended periods (days to weeks). Electrode arrays are characterized with respect to their response to easily oxidized neurotransmitters, including dopamine, norepinephrine, and 5-hydroxytyramide. Electroanalysis at discrete electrodes following long-term cell culture demonstrates that this platform remains responsive for the detection of easily oxidized species generated by the cultured cells. Preliminary data also suggests that quantal release of easily oxidized transmitters can be observed at nanofiber electrodes following direct culture and differentiation on the arrays for periods of at least 16 days.

Vesicular exocytosis under hypotonic conditions shows two distinct populations of dense core vesicles in bovine chromaffin cells

Several previous reports have discussed the effects of external osmolarity on vesicular exocytotic processes. However, few of these studies considered hypotonic conditions on chromaffin cells. Herein, the exocytosis of catecholamines by chromaffin cells was investigated in a medium of low osmolarity (200 mOsm) by amperometry at carbon fiber microelectrodes. It is observed that the frequency of the exocytotic events is significantly higher under hypotonic conditions than under physiological conditions (315 mOsm). This further confirms that the swelling of the polyelectrolytic matrix (which follows ionic exchanges) contained in dense core vesicles is the energetic driving force of the exocytotic phenomenon, being favored by a lower osmolarity. The mean amount of catecholamines released during secretory events also increases importantly under the hypotonic condition. This may be rationalized by the coexistence of two distinct populations of dense core vesicles with a relative content ratio of 4.7. The larger content population is favored under hypotonic conditions but plays only a side role under isotonic conditions.

PICK1 interacts with alpha7 neuronal nicotinic acetylcholine receptors and controls their clustering

Central to synaptic function are protein scaffolds associated with neurotransmitter receptors. Alpha7 neuronal nicotinic acetylcholine receptors (nAChRs) modulate network activity, neuronal survival and cognitive processes in the CNS, but protein scaffolds that interact with these receptors are unknown. Here we show that the PDZ-domain containing protein PICK1 binds to alpha7 nAChRs and plays a role in their clustering. PICK1 interacted with the alpha7 cytoplasmic loop in yeast in a PDZ-dependent way, and the interaction was confirmed in recombinant pull-down experiments and by co-precipitation of native proteins. Some alpha7 and PICK1 clusters were adjacent at the surface of SH-SY5Y cells and GABAergic interneurons in hippocampal cultures. Expression of PICK1 caused decreased alpha7 clustering on the surface of the interneurons in a PDZ-dependent way. These data show that PICK1 negatively regulates surface clustering of alpha7 nAChRs on hippocampal interneurons, which may be important in inhibitory functions of alpha7 in the hippocampus.

Leptin facilitates learning and memory performance and enhances hippocampal CA1 long-term potentiation and CaMK II phosphorylation in rats

Leptin, an adipocytokine encoded by an obesity gene and expressed in adipose tissue, affects feeding behavior, thermogenesis, and neuroendocrine status via leptin receptors distributed in the brain, especially in the hypothalamus. Leptin may also modulate the synaptic plasticity and behavioral performance related to learning and memory since: leptin receptors are found in the hippocampus, and both leptin and its receptor share structural and functional similarities with the interleukin-6 family of cytokines that modulate long-term potentiation (LTP) in the hippocampus. We therefore examined the effect of leptin on (1) behavioral performance in emotional and spatial learning tasks, (2) LTP at Schaffer collateral-CA1 synapses, (3) presynaptic and postsynaptic activities in hippocampal CA1 neurons, (4) the intracellular Ca(2+) concentration ([Ca(2+)](i)) in CA1 neurons, and (5) the activity of Ca(2+)/calmodulin protein kinase II (CaMK II) in the hippocampal CA1 tissue that exhibits LTP. Intravenous injection of 5 and/or 50mug/kg, but not of 500mug/kg leptin, facilitated behavioral performance in passive avoidance and Morris water-maze tasks. Bath application of 10(-12)M leptin in slice experiments enhanced LTP and increased the presynaptic transmitter release, whereas 10(-10)M leptin suppressed LTP and reduced the postsynaptic receptor sensitivity to N-methyl-d-aspartic acid. The increase in the [Ca(2+)](i) induced by 10(-10)M leptin was two times greater than that induced by 10(-12)M leptin. In addition, the facilitation (10(-12)M) and suppression (10(-10)M) of LTP by leptin was closely associated with an increase and decrease in Ca(2+)-independent activity of CaMK II. Our results show that leptin not only affects hypothalamic functions (such as feeding, thermogenesis, and neuroendocrine status), but also modulates higher nervous functions, such as the behavioral performance related to learning and memory and hippocampal synaptic plasticity.

Imaging of dynamic secretory vesicles in living pollen tubes of Picea meyeri using evanescent wave microscopy

Evanescent wave excitation was used to visualize individual, FM4-64-labeled secretory vesicles in an optical slice proximal to the plasma membrane of Picea meyeri pollen tubes. A standard upright microscope was modified to accommodate the optics used to direct a laser beam at a variable angle. Under evanescent wave microscopy or total internal reflection fluorescence microscopy, fluorophores localized near the surface were excited with evanescent waves, which decay exponentially with distance from the interface. Evanescent waves with penetration depths of 60 to 400 nm were generated by varying the angle of incidence of the laser beam. Kinetic analysis of vesicle trafficking was made through an approximately 300-nm optical section beneath the plasma membrane using time-lapse evanescent wave imaging of individual fluorescently labeled vesicles. Two-dimensional trajectories of individual vesicles were obtained from the resulting time-resolved image stacks and were used to characterize the vesicles in terms of their average fluorescence and mobility, expressed here as the two-dimensional diffusion coefficient D2. The velocity and direction of vesicle motions, frame-to-frame displacement, and vesicle trajectories were also calculated. Analysis of individual vesicles revealed for the first time, to our knowledge, that two types of motion are present, and that vesicles in living pollen tubes exhibit complicated behaviors and oscillations that differ from the simple Brownian motion reported in previous investigations. Furthermore, disruption of the actin cytoskeleton had a much more pronounced effect on vesicle mobility than did disruption of the microtubules, suggesting that actin cytoskeleton plays a primary role in vesicle mobility.

Src family tyrosine kinases differentially modulate exocytosis from rat brain nerve terminals

We have studied the role of src family tyrosine kinases in regulating synaptic transmitter release from rat brain synaptosomes by using two assays that measure different aspects of synaptic vesicle exocytosis: glutamate release (that directly measures exocytosis of vesicle contents) and release of FM 2-10 styryl dye (that is proportional to the time the synaptic vesicle is fused to the plasma membrane). Depolarisation was induced by KCl (30 mM) or 4-aminopyridine (4AP: 0.3mM) to induce release by full fusion (FF) exocytosis, or by 1 mM 4AP to induce release by both FF and kiss-and-run (KR)-like exocytosis. The src family selective inhibitor, PP1 (10 microM), increased KCl and 0.3 mM 4AP-evoked Ca2+ -dependent release of glutamate, but had little effect upon exocytosis evoked by 1mM 4AP. PP1 did not affect the release of FM 2-10 under any of the depolarisation conditions used. PP1 also had no effect on overall intracellular calcium levels, as measured by FURA2, suggesting that the effects of the inhibitor are downstream of calcium entry. At the same concentration the inactive analogue of this compound, PP3, had no effect on any measure. Immunoblotting with an antibody to phosphotyrosine revealed that phosphorylation of many synaptosomal proteins was reduced by PP1. The immunoreactivity of three protein bands increased upon depolarisation and this increase was blocked by PP1. Phosphorylation of src at tyrosine-416 was reduced by PP1 but changes in its phosphorylation did not correlate with the effects of PP1 on release. These results suggest one or more members of the src family of tyrosine kinases is a negative regulator of the KR mode of exocytosis in synaptosomes, perhaps by tonically inhibiting KR under normal stimulation conditions.