Monitoring secretion in real time: capacitance, amperometry and fluorescence compared

The readily retrievable pool of synaptic vesicles

In the CNS communication between neurons occurs at synapses by secretion of neurotransmitter via exocytosis of synaptic vesicles (SVs) at the active zone. Given the limited number of SVs in presynaptic boutons a fast and efficient recycling of exocytosed membrane and proteins by triggered compensatory endocytosis is required to maintain neurotransmission. Thus, pre-synapses feature a unique tight coupling of exo- and endocytosis in time and space resulting in the reformation of SVs with uniform morphology and well-defined molecular composition. This rapid response requires early stages of endocytosis at the peri-active zone to be well choreographed to ensure reformation of SVs with high fidelity. The pre-synapse can address this challenge by a specialized membrane microcompartment, where a pre-sorted and pre-assembled readily retrievable pool (RRetP) of endocytic membrane patches is formed, consisting of the vesicle cargo, presumably bound within a nucleated Clathrin and adaptor complex. This review considers evidence for the RRetP microcompartment to be the primary organizer of presynaptic triggered compensatory endocytosis.

5 ns electric pulses induce Ca2+-dependent exocytotic release of catecholamine from adrenal chromaffin cells

Previously we reported that adrenal chromaffin cells exposed to a 5 ns, 5 MV/m pulse release the catecholamines norepinephrine (NE) and epinephrine (EPI) in a Ca²⁺-dependent manner. Here we determined that NE and EPI release increased with pulse number (one versus five and ten pulses at 1 Hz), established that release occurs by exocytosis, and characterized the exocytotic response in real-time. Evidence of an exocytotic mechanism was the appearance of dopamine-β-hydroxylase on the plasma membrane, and the demonstration by total internal reflection fluorescence microscopy studies that a train of five or ten pulses at 1 Hz triggered the release of the fluorescent dye acridine orange from secretory granules. Release events were Ca²⁺-dependent, longer-lived relative to those evoked by nicotinic receptor stimulation, and occurred with a delay of several seconds despite an immediate rise in Ca²⁺. In complementary studies, cells labeled with the plasma membrane fluorescent dye FM 1-43 and exposed to a train of ten pulses at 1 Hz underwent Ca²⁺-dependent increases in FM 1-43 fluorescence indicative of granule fusion with the plasma membrane due to exocytosis. These results demonstrate the effectiveness of ultrashort electric pulses for stimulating catecholamine release, signifying their promise as a novel electrostimulation modality for neurosecretion.

Solvatochromic and pH-Sensitive Fluorescent Membrane Probes for Imaging of Live Cells

Membrane trafficking is essential for all cells, and visualizing it is particularly useful for studying neuronal functions. Here we report the synthesis, characterization, and application of several membrane- and pH-sensitive probes suitable for live-cell fluorescence imaging. These probes are based on a 1,8-naphthalimide fluorophore scaffold. They exhibit a solvatochromic effect, and one of them, ND6, shows a substantial fluorescence difference between pH 6 and 7. The solvatochromic effect and pH-sensitivity of those probes are explained using quantum chemical calculations, and molecular dynamics simulation confirms their integration and interaction with membrane lipids. For live-cell fluorescence imaging, we tested those probes in a cancer cell line (MCF7), cancer spheroids (MDA-MB-468), and cultured hippocampal neurons. Confocal imaging showed an excellent signal-to-noise ratio from 400:1 to about 1300:1 for cell membrane labeling. We applied ND6 during stimulation to label nerve terminals via dye uptake during evoked synaptic vesicle turnover. By ND6 imaging, we revealed cholesterol's multifaced role in replenishing synaptic vesicle pools. Our results demonstrate these fluorescent probes' great potential in studying membrane dynamic and synaptic functions in neurons and other secretory cells and tissues.

Measurements of Exocytosis by Capacitance Recordings and Calcium Uncaging in Mouse Adrenal Chromaffin Cells

Fusion of vesicles with the plasma membrane and liberation of their contents is a multistep process involving several proteins. Correctly assigning the role of specific proteins and reactions in this cascade requires a measurement method with high temporal resolution. Patch-clamp recordings of cell membrane capacitance in combination with calcium measurements, calcium uncaging, and carbon-fiber amperometry allow for the accurate determination of vesicle pool sizes, their fusion kinetics, and their secreted oxidizable content. Here, we will describe this method in a model system for neurosecretion, the adrenal chromaffin cells, which secrete adrenaline.

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.

An investigation into closed-loop treatment of neurological disorders based on sensing mitochondrial dysfunction

Dynamic feedback based closed-loop medical devices offer a number of advantages for treatment of heterogeneous neurological conditions. Closed-loop devices integrate a level of neurobiological feedback, which allows for real-time adjustments to be made with the overarching aim of improving treatment efficacy and minimizing risks for adverse events. One target which has not been extensively explored as a potential feedback component in closed-loop therapies is mitochondrial function. Several neurodegenerative and psychiatric disorders including Parkinson's disease, Major Depressive disorder and Bipolar disorder have been linked to perturbations in the mitochondrial respiratory chain. This paper investigates the potential to monitor this mitochondrial function as a method of feedback for closed-loop neuromodulation treatments. A generic model of the closed-loop treatment is developed to describe the high-level functions of any system designed to control neural function based on mitochondrial response to stimulation, simplifying comparison and future meta-analysis. This model has four key functional components including: a sensor, signal manipulator, controller and effector. Each of these components are described and several potential technologies for each are investigated. While some of these candidate technologies are quite mature, there are still technological gaps remaining. The field of closed-loop medical devices is rapidly evolving, and whilst there is a lot of interest in this area, widespread adoption has not yet been achieved due to several remaining technological hurdles. However, the significant therapeutic benefits offered by this technology mean that this will be an active area for research for years to come.

Prepulse Inhibition of Auditory Cortical Responses in the Caudolateral Superior Temporal Gyrus in Macaca mulatta

Prepulse inhibition (PPI) refers to a decreased response to a startling stimulus when another weaker stimulus precedes it. Most PPI studies have focused on the physiological startle reflex and fewer have reported the PPI of cortical responses. We recorded local field potentials (LFPs) in four monkeys and investigated whether the PPI of auditory cortical responses (alpha, beta, and gamma oscillations and evoked potentials) can be demonstrated in the caudolateral belt of the superior temporal gyrus (STGcb). We also investigated whether the presence of a conspecific, which draws attention away from the auditory stimuli, affects the PPI of auditory cortical responses. The PPI paradigm consisted of Pulse-only and Prepulse + Pulse trials that were presented randomly while the monkey was alone (ALONE) and while another monkey was present in the same room (ACCOMP). The LFPs to the Pulse were significantly suppressed by the Prepulse thus, demonstrating PPI of cortical responses in the STGcb. The PPI-related inhibition of the N1 amplitude of the evoked responses and cortical oscillations to the Pulse were not affected by the presence of a conspecific. In contrast, gamma oscillations and the amplitude of the N1 response to Pulse-only were suppressed in the ACCOMP condition compared to the ALONE condition. These findings demonstrate PPI in the monkey STGcb and suggest that the PPI of auditory cortical responses in the monkey STGcb is a pre-attentive inhibitory process that is independent of attentional modulation.

Effects of neuritin on the differentiation of bone marrow‑derived mesenchymal stem cells into neuron‑like cells

While the neurotrophic factor neuritin is known to be involved in neurodevelopment, the effects of this compound on cell differentiation remain unclear. The present study demonstrated that neuritin treatment induced the differentiation of rat bone marrow-derived mesenchymal stem cells (rBM-MSCs) into neuron-like (NL) cells. For these analyses, rBM-MSCs were incubated with 0.5 µg/ml neuritin for 24 h. Following induction, 27% of the rBM-MSCs exhibited typical NL cell morphologies. Subsequently, NL cells were characterized by examining the expression of neuronal markers and by analysis of cell functions. The findings demonstrated that the NL cells produced by neuritin treatment expressed the neuronal markers neuron-specific enolase and microtubule associate protein 2, and secreted the neurotransmitter 5-hydroxytryptamine. Furthermore, the NL cells exhibited certain partial neural-electrophysiological functions. In conclusion, neuritin treatment may be an effective method for inducing the differentiation of BM-MSCs towards NL cells. This may provide an alternative, potentially complementary tool for disease modeling and the development of cell-based therapies.

Poly(3-hexylthiophene) nanoparticles for biophotonics: study of the mutual interaction with living cells

Poly(3-hexylthiophene) nanoparticles interfacing with living cells: a new tool for biophotonics applications.

Reduced calcium current density in female versus male mouse adrenal chromaffin cells in situ

Neuroendocrine adrenal medullary chromaffin cells are a main output of the sympathetic nervous system. Acute stress activates the sympatho-adrenal stress reflex, excites adrenal chromaffin cells, and elicits catecholamine secretion into the circulation. Previous studies have demonstrated that stress-evoked serum catecholamine levels are greater in males. We investigated potential mechanistic bases for this gender dimorphism at the level of the adrenal medulla. We utilized in situ single-cell perforated patch voltage clamp to measure basic electrophysiological parameters that affect cell excitability. We found that chromaffin cells from male and female mice exhibit statistically identical depolarization-evoked calcium currents. However, the resting capacitance, an index of cell surface area, was significantly higher in cells from female mice. Thus the current density in female cells was significantly lower. We found that inhibition of protein kinase C, an enzyme shown to regulate both exocytosis and endocytosis, eliminates the cell surface area gender dimorphism. Finally, we performed kinetic simulations of the secretion process and report a predicted elevated secretory capacity in male cells. Thus, regulation of cell size may act to decrease cell excitability in female cells and may in-part represent the mechanistic basis for increased stress-evoked catecholamine secretion described in males.

Paired-pulse inhibition in the auditory cortex in Parkinson's disease and its dependence on clinical characteristics of the patients

We aimed to determine the value of the paired-pulse inhibition (PPI) in the auditory cortex in patients with Parkinson's disease (PD) and analyze its dependence on clinical characteristics of the patients. The central (Cz) auditory evoked potentials were recorded in 58 patients with PD and 22 age-matched healthy subjects. PPI of the N1/P2 component was significantly (P < .001) reduced for interstimulus intervals 500, 700, and 900 ms in patients with PD compared to control subjects. The value of PPI correlated negatively with the age of the PD patients (P < .05), age of disease onset (P < .05), body bradykinesia score (P < .01), and positively with the Mini Mental State Examination (MMSE) cognitive score (P < .01). Negative correlation between value of PPI and the age of the healthy subjects (P < .05) was also observed. Thus, results show that cortical inhibitory processes are deficient in PD patients and that the brain's ability to carry out the postexcitatory inhibition is age-dependent.

Dynasore inhibits rapid endocytosis in bovine chromaffin cells

Vesicle recycling is vital for maintaining membrane homeostasis and neurotransmitter release. Multiple pathways for retrieving vesicles fused to the plasma membrane have been reported in neuroendocrine cells. Dynasore, a dynamin GTPase inhibitor, has been shown to specifically inhibit endocytosis and vesicle recycling in nerve terminals. To characterize its effects in modulating vesicle recycling and repetitive exocytosis, changes in the whole cell membrane capacitance of bovine chromaffin cells were recorded in the perforated-patch configuration. Constitutive endocytosis was blocked by dynasore treatment, as shown by an increase in membrane capacitance. The membrane capacitance was increased during strong depolarizations and declined within 30 s to a value lower than the prestimulus level. The amplitude, but not the time constant, of the rapid exponential decay was significantly decreased by dynasore treatment. Although the maximal increase in capacitance induced by stimulation was significantly increased by dynasore treatment, the intercepts at time 0 of the curve fitted to the decay phase were all ∼110% of the membrane capacitance before stimulation, regardless of the dynasore concentration used. Membrane depolarization caused clathrin aggregation and F-actin continuity disruption at the cell boundary, whereas dynasore treatment induced clathrin aggregation without affecting F-actin continuity. The number of invagination pits on the surface of the plasma membrane determined using atomic force microscopy was increased and the pore was wider in dynasore-treated cells. Our data indicate that dynamin-mediated endocytosis is the main pathway responsible for rapid compensatory endocytosis.

{alpha}-synuclein and its A30P mutant affect actin cytoskeletal structure and dynamics

The function of alpha-synuclein, a soluble protein abundant in the brain and concentrated at presynaptic terminals, is still undefined. Yet, alpha-synuclein overexpression and the expression of its A30P mutant are associated with familial Parkinson's disease. Working in cell-free conditions, in two cell lines as well as in primary neurons we demonstrate that alpha-synuclein and its A30P mutant have different effects on actin polymerization. Wild-type alpha-synuclein binds actin, slows down its polymerization and accelerates its depolymerization, probably by monomer sequestration; A30P mutant alpha-synuclein increases the rate of actin polymerization and disrupts the cytoskeleton during reassembly of actin filaments. Consequently, in cells expressing mutant alpha-synuclein, cytoskeleton-dependent processes, such as cell migration, are inhibited, while exo- and endocytic traffic is altered. In hippocampal neurons from mice carrying a deletion of the alpha-synuclein gene, electroporation of wild-type alpha-synuclein increases actin instability during remodeling, with growth of lamellipodia-like structures and apparent cell enlargement, whereas A30P alpha-synuclein induces discrete actin-rich foci during cytoskeleton reassembly. In conclusion, alpha-synuclein appears to play a major role in actin cytoskeletal dynamics and various aspects of microfilament function. Actin cytoskeletal disruption induced by the A30P mutant might alter various cellular processes and thereby play a role in the pathogenesis of neurodegeneration.

The unitary event underlying multiquantal EPSCs at a hair cell's ribbon synapse

EPSCs at the synapses of sensory receptors and of some CNS neurons include large events thought to represent the synchronous release of the neurotransmitter contained in several synaptic vesicles by a process known as multiquantal release. However, determination of the unitary, quantal size underlying such putatively multiquantal events has proven difficult at hair cell synapses, hindering confirmation that large EPSCs are in fact multiquantal. Here, we address this issue by performing presynaptic membrane capacitance measurements together with paired recordings at the ribbon synapses of adult hair cells. These simultaneous presynaptic and postsynaptic assays of exocytosis, together with electron microscopic estimates of single vesicle capacitance, allow us to estimate a single vesicle EPSC charge of approximately -45 fC, a value in close agreement with the mean postsynaptic charge transfer of uniformly small EPSCs recorded during periods of presynaptic hyperpolarization. By thus establishing the magnitude of the fundamental quantal event at this peripheral sensory synapse, we provide evidence that the majority of spontaneous and evoked EPSCs are multiquantal. Furthermore, we show that the prevalence of uniquantal versus multiquantal events is Ca2+ dependent. Paired recordings also reveal a tight correlation between membrane capacitance increase and evoked EPSC charge, indicating that glutamate release during prolonged hair cell depolarization does not significantly saturate or desensitize postsynaptic AMPA receptors. We propose that the large EPSCs reflect the highly synchronized release of multiple vesicles at single presynaptic ribbon-type active zones through a compound or coordinated vesicle fusion mechanism.

Fluorescent cargo proteins in peptidergic endocrine cells: cell type determines secretion kinetics at exocytosis

Fluorescent fusion proteins are an important tool for the study of vesicle trafficking and exocytosis, especially when combined with newer types of microscopy. We previously reported that the design of a vesicle-targeted fluorescent fusion construct strongly influences the kinetics of fluorescence change at exocytosis. In the present study we demonstrate that the cell in which a construct is expressed also affects the kinetics of fluorescence change at exocytosis. We fused enhanced green fluorescent protein to the carboxy terminus of the vesicular cargo protein rodent islet amyloid polypeptide. The two proteins were separated by a "linker" sequence of 18 amino acids. We then compared kinetics of fluorescence change at exocytosis for this fluorescent cargo protein expressed in three different types of peptidergic endocrine cell: pancreatic alpha cell, pancreatic beta cell, and adrenal chromaffin cell. In resting cells of all three types, fluorescent spots of similar size and membrane-proximal density appeared near the plasma membrane as expected if the probe is stored in large dense-core secretory vesicles. Upon stimulation, the fluorescent spots displayed sudden changes in fluorescence intensity that were consistent with exocytosis. In beta and alpha cells the fluorescent spots consistently brightened and persisted, whereas in chromaffin cells the fluorescent spots always dispersed rapidly. Thus, for fluorescent cargo proteins in peptidergic endocrine cells, cell type influences the kinetics of fluorescence change at exocytosis. Together with our previous findings, this observation strongly highlights the fact that the behavior of vesicle-targeted fluorescent cargo may be unrelated to that of native cargo, and it emphasizes the need for caution in interpreting fluorescence kinetics in terms of an exocytosis mechanism.

Use of FM1-43 and other derivatives to investigate neuronal function

The fluorescent dye FM1-43 and its derivatives can be used to monitor the physiology of synaptic vesicle turnover in central nerve terminals. They do so by their ability to reversibly partition into membranes, a process that results in a huge increase in fluorescence in comparison to their quantum yield in solution. This unit provides protocols for quantifying total synaptic vesicle turnover, the kinetics and extent of synaptic vesicle exocytosis, and the kinetics and mode of synaptic vesicle endocytosis. Descriptions of other ways these protocols have been used to derive information about the life cycle of the synaptic vesicle are also provided.

Fourier microfluidics

We present a new experimental technique for the separation of dynamic chemical signals based on their frequency domain characteristics. Such a technique can be used to create filters that separate slow signals from fast signals from a common input flow stream. The propagation of time-varying chemical waves through networks of microfluidic channels is first examined. Mathematical models and a set of simple experiments are developed that demonstrate that short microfluidic channels behave as linear delay lines. The observed dispersive broadening and delay behavior can be explained in Fourier space in terms of corresponding phase delay, amplitude decay and characteristic transfer functions. Such delay components can be utilized to implement frequency dependent interference filters. An 8th order PDMS bandpass filter chip demonstrating these ideas was constructed. The filter chip has a central frequency of 0.17 Hz and a bandwith of 0.04 Hz at a flow rate of 4 microL h(-1).

Styryl dye-based synaptic vesicle recycling assay in cultured cerebellar granule neurons

Neurons transmit information by exocytosis of synaptic vesicles (SV), which contain neurotransmitter. Exocytosis is followed by efficient retrieval of the plasma membrane by endocytosis to generate a new SV. SV retrieval supports multiple cycles of synaptic transmission. Over the years, styryl dyes have been widely used to probe the mechanism of SV recycling in the processes of cultured neurons. The styryl dye method is complementary to electrophysiological measurements or genetic reporter approaches. Owing to their ease to culture, cerebellar granule neurons provide a robust neuronal model system for the assay. These cells are readily transfected with various DNA constructs to study the function of exocytic or endocytic proteins in SV recycling.

Quantifying exocytosis by combination of membrane capacitance measurements and total internal reflection fluorescence microscopy in chromaffin cells

Total internal reflection fluorescence microscopy (TIRF-Microscopy) allows the observation of individual secretory vesicles in real-time during exocytosis. In contrast to electrophysiological methods, such as membrane capacitance recording or carbon fiber amperometry, TIRF-Microscopy also enables the observation of vesicles as they reside close to the plasma membrane prior to fusion. However, TIRF-Microscopy is limited to the visualization of vesicles that are located near the membrane attached to the glass coverslip on which the cell grows. This has raised concerns as to whether exocytosis measured with TIRF-Microscopy is comparable to global secretion of the cell measured with membrane capacitance recording. Here we address this concern by combining TIRF-Microscopy and membrane capacitance recording to quantify exocytosis from adrenal chromaffin cells. We found that secretion measured with TIRF-Microscopy is representative of the overall secretion of the cells, thereby validating for the first time the TIRF method as a measure of secretion. Furthermore, the combination of these two techniques provides a new tool for investigating the molecular mechanism of synaptic transmission with combined electrophysiological and imaging techniques.

Imaging single events at the cell membrane

The ability to sense and respond to the environment is a hallmark of living systems. These processes occur at the levels of the organism, cells and individual molecules. Sensing of extracellular changes could result in a structural or chemical alteration in a molecule, which could in turn trigger a cascade of intracellular signals or regulated trafficking of molecules at the cell surface. These and other such processes allow cells to sense and respond to environmental changes. Often, these changes and the responses to them are spatially and/or temporally localized, and visualization of such events necessitates the use of high-resolution imaging approaches. Here we discuss optical imaging approaches and tools for imaging individual events at the cell surface with improved speed and resolution.

NMDA receptors trigger neurosecretion of 5-HT within dorsal raphe nucleus of the rat in the absence of action potential firing

Activity and calcium-dependent release of neurotransmitters from the somatodendritic compartment is an important signalling mechanism between neurones throughout the brain. NMDA receptors and vesicles filled with neurotransmitters occur in close proximity in many brain areas. It is unknown whether calcium influx through these receptors can trigger the release of somatodendritic vesicles directly, or whether postsynaptic action potential firing is necessary for release of these vesicles. Here we addressed this question by studying local release of serotonin (5-HT) from dorsal raphé nucleus (DRN) neurones. We performed capacitance measurements to monitor the secretion of vesicles in giant soma patches, in response to short depolarizations and action potential waveforms. Amperometric measurements confirmed that secreted vesicles contained 5-HT. Surprisingly, two-photon imaging of DRN neurones in slices revealed that dendritic calcium concentration changes in response to somatic firing were restricted to proximal dendritic areas. This implied that alternative calcium entry pathways may dominate the induction of vesicle secretion from distal dendrites. In line with this, transient NMDA receptor activation, in the absence of action potential firing, was sufficient to induce capacitance changes. By monitoring GABAergic transmission onto DRN 5-HT neurones in slices, we show that endogenous NMDA receptor activation, in the absence of postsynaptic firing, induced release of 5-HT, which in turn increased the frequency of GABAergic inputs through activation of 5-HT(2) receptors. We propose here that calcium influx through NMDA receptors can directly induce postsynaptic 5-HT release from DRN neurones, which in turn may facilitate GABAergic input onto these cells.

Methods for detecting internalized, FM 1-43 stained particles in epithelial cells and monolayers

The membrane dye FM 1-43 has frequently been used to quantify exocytosis in neurons. In epithelia, intense lateral intracellular space staining and fluctuations in baseline labeling produced inconsistent results. Membrane retrieved in the presence of FM 1-43 retains the dye, however, and cells that undergo compensatory endocytosis during and following evoked exocytosis contain punctate, fluorescent particles after washout of external stain. As an alternative measure of trafficking, we quantified the fluorescent puncta retained after dye washout and tested our method on both coverslip-grown cell clusters and filter-grown intact monolayers. Images for analysis were acquired using serial sectioning with either epifluorescence or confocal microscopy. Tests with an intestinal goblet cell line that exhibits basal and ATP-stimulated granule trafficking confirmed that 1), the algorithm identified the same number of internalized particles with either epifluorescence or confocal microscopy acquired images; 2), low density clusters exhibited significantly more internalized particles per cell than either filter-grown monolayers or high density clusters; 3), ATP stimulation significantly increased the number of internalized particles in all preparations; and 4), the number of particles internalized was comparable to capacitance measurements of exocytosis. This method provides a single technique for quantifying membrane trafficking in both monolayers and unpolarized cells.

Recruitment of synapses in the neurosecretory process during long-term facilitation at the lobster neuromuscular junction

We investigated long-term facilitation at the lobster neuromuscular synapse employing a combination of FM1-43 staining of synaptic vesicles, electron microscopy analysis, and electrical recordings of synaptic activity. Synaptic terminals were loaded with the fluorescent dye FM1-43 producing clusters of activity-dependent fluorescent spots. Electron microscopy analysis of synaptic ultrastructure suggested that fluorescent spots represent compartments of synaptic terminals filled with vesicles. Excitatory postsynaptic currents were recorded from the stained synaptic terminals using focal macropatch electrodes. Terminals were stained during the nerve stimulation at a low stimulation frequency (2, 5 or 10 Hz) before and after long-term facilitation was elicited by high-frequency stimulation (20 or 30 Hz for 5 min). We found that staining after long-term facilitation results in the appearance of new fluorescent spots, as well as in the increase in fluorescence of the spots that appeared before long-term facilitation. This increase in fluorescence accounted for the increase in quantal release. Activation of individual fluorescent spots was found to be non-uniform. In spite of overall increase in fluorescence, some synaptic compartments decreased their staining after long-term facilitation. Thus, our study demonstrates that long-term facilitation produces non-uniform activation of FM1-43 uptake in synaptic compartments that correlates with the increase in quantal neurosecretion.

The mechanisms of cell membrane resealing in rabbit corneal epithelial cells

PURPOSE

To examine membrane repair mechanisms in rabbit corneal epithelial (RCE) cells.

METHODS

Microneedle puncture and fluorescent dye loss were used to wound membranes and assay resealing, respectively. Different repair mechanisms were detected pharmacologically and with antisense oligonucleotides.

RESULTS

The RCE cells rapidly reseal plasma membranes by calcium-dependent exocytotic mechanisms that exhibit both facilitated and potentiated responses to multiple wounding. The facilitated response was inhibited by specific inhibitors of protein kinase C (PKC) and brefeldin A, and the potentiated response was blocked by inhibitors of cAMP-dependent protein kinase (PKA). Reduction of myosin IIA inhibited the facilitated response, and reduction of IIB inhibited the initial resealing.

CONCLUSIONS

RCE cells rapidly repair plasma membrane disruptions. At a second wound at the same site, PKC stimulated vesicle formation from the Golgi apparatus, resulting in more rapid membrane resealing for a facilitated response. The RCE cell also contains a PKA-dependent global potentiation of membrane resealing.

Real-time measurements of noradrenaline release in periphery and central nervous system

Noradrenaline (NA) plays important hormonal and neurotransmitter roles in the periphery and central nervous system, respectively. The cells that produce and release NA, namely, adrenal chromaffin cells (ACCs), sympathetic postganglionic neurones and central neurones, show both commonalities as well as profound differences in morphology, physiological function and characteristics of NA secretion. In order to address disorders which have been associated with the dysregulation of NA release, such as essential hypertension, a better understanding of the molecular mechanisms governing and modulating NA release in neurones is urgently required. Due to profound technical challenges, the molecular basis of NA release has been investigated much more thoroughly in ACCs than in neurones. This review discusses suitable approaches for detecting NA secretion in periphery as well as brain tissues. Membrane capacitance and high-resolution electrochemical measurements have proven particularly useful when combined with fluorescence microscopy. ACCs and peripheral and central NAergic neurones are compared regarding their vesicle morphologies, as well as possible locations of release sites, and the trajectory of secreted NA. Further, current views on the properties of single vesicle release events, including proposed release probabilities in these cell types, are presented.

Synaptic vesicle cycling at type-identified diaphragm neuromuscular junctions

Differences in neuromuscular transmission and neuromuscular junction morphology exist across muscle fiber types. We hypothesized that these fiber-type differences are reflected in the size of the cycling synaptic vesicle pool. Synaptic vesicle cycling at type-identified rat diaphragm neuromuscular junctions was examined by fluorescently labeling presynaptic vesicles with FM4-64. We found that FM4-64 fluorescence uptake was higher at presynaptic terminals of type I/IIa fibers than type IIx/IIb fibers. However, no fiber-type differences in the rate of FM4-64 destaining were found with repetitive nerve stimulation. Synaptic vesicle density at active zones was examined by transmission electron microscopy. In accordance with FM4-64 uptake, synaptic vesicle density was greater at type I/IIa than IIx/IIb fibers. These results demonstrate differences in synaptic vesicle cycling across diaphragm muscle fiber types, which may underlie previously observed differences in neuromuscular transmission across diaphragm muscle fiber types. In the diaphragm, motor units comprising type I and type IIa fibers are most frequently recruited with a duty cycle of approximately 40%. Motor units comprising IIx/IIb fibers are infrequently recruited and only for short durations. The capacity for synaptic vesicle release and cycling at different muscle fiber types matches the functional requirements of these motor units. If the demand for recruitment of motor units comprising IIx/IIb fibers increases, for example, with mechanical loading, there is an increased risk for neuromuscular transmission failure that my relate to the capacity for synaptic vesicle release and cycling. Muscle fiber type-specific adaptations should be considered when examining neuromuscular disorders.

Constitutive apical membrane recycling in Aplysia enterocytes

In Aplysia californica enterocytes, alanine-stimulated Na+ absorption increases both apical membrane exocytosis and fractional capacitance (fCa; a measure of relative apical membrane surface area). These increases are thought to reduce membrane tension during periods of nutrient absorption that cause the enterocytes to swell osmotically. In the absence of alanine, exocytosis and fCa are constant. These findings imply equal rates of constitutive endocytosis and exocytosis and constitutive recycling of the apical plasma membrane. Thus, the purpose of this study was to confirm and determine the relative extent of constitutive apical membrane recycling in Aplysia enterocytes. Biotinylated lectins are commonly used to label plasma membranes and to investigate plasma membrane recycling. Of fourteen biotinylated lectins tested, biotinylated wheat germ agglutinin (bWGA) bound preferentially to the enterocytes apical surface. Therefore, we used bWGA, avidin D (which binds tightly to biotin), and the UV fluorophore 7-amino-4-methylcoumarin-3-acetic acid (AMCA)-conjugated avidin D to assess the extent of constitutive apical membrane recycling. A temperature-dependent (20 vs. 4 degrees C) experimental protocol employed the use of two tissues from each of five snails and resulted in a approximately 60% difference in apical surface fluorescence intensity. Because the extent of membrane recycling is proportional to the difference in surface fluorescence intensity, this difference reveals a relatively high rate of constitutive apical membrane recycling in Aplysia enterocytes.

Kinetic regulation of vesicle endocytosis at synapses

Studies from a variety of synapses indicate that the time course of endocytosis ranges from less than a second to hundreds of seconds. This raises questions about how the time course of endocytosis is regulated and why different rates of endocytosis are needed. Recent progress sheds light on these issues. Neuronal firing frequency and duration determine the time course of endocytosis. The dynamic nature of this time course could be a result of multiple endocytic pathways and/or of regulation by a variety of modulators. Because endocytosis is crucial for maintaining transmitter release during repetitive stimulation, regulation of endocytosis could thus provide a mechanism by which synaptic plasticity is achieved.

Capacitance measurements at the calyx of Held in the medial nucleus of the trapezoid body

We have recently applied Lindau-Neher's capacitance measurement technique to study vesicle trafficking at the calyx-type synapse in the rat medial nucleus of the trapezoid body (MNTB) in slice conditions. This application made the MNTB synapse an excellent model for the study of exocytosis and endocytosis at conventional active zones. However, the application was only made at calyces that are presumably equivalent to a single-compartment circuit because their passive current transients decayed mono-exponentially. Here, we determined whether the application could be extended to majority of calyces whose passive current transients decayed bi-exponentially. By comparison of calyces with mono- or bi-exponential decay in their passive current transients, we found similar properties in respect to: (1) the capacitance jump induced by trains of action-potential equivalent stimuli, which reflects exocytosis; (2) the size of a releasable vesicle pool; (3) the time course of the decay after the capacitance jump, which reflects endocytosis; and (4) the transient capacitance artifact observed in the presence of Cd(2+) that blocks exocytosis. These similar properties were also obtained from modeling calyces as a single- or two-compartment circuit. Thus, capacitance measurements may be extended to the majority of calyces, which may facilitate the study of rapid vesicle trafficking at conventional active zones.

Real-Time Measurement of Exocytosis and Endocytosis Using Interference of Light

We describe a new approach for making real-time measurements of exocytosis and endocytosis in neurons and neuroendocrine cells. The method utilizes interference reflection microscopy (IRM) to image surface membrane in close contact with a glass coverslip (the "footprint"). At the synaptic terminal of retinal bipolar cells, the footprint expands during exocytosis and retracts during endocytosis, paralleling changes in total surface area measured by capacitance. In chromaffin cells, IRM detects the fusion of individual granules as the appearance of bright spots within the footprint with spatial and temporal resolution similar to total internal reflection fluorescence microscopy. Advantages of IRM over capacitance are that it can monitor changes in surface area while cells are electrically active and it can be applied to mammalian neurons with relatively small synaptic terminals. IRM reveals that vesicles at the synapse of bipolar cells rapidly collapse into the surface membrane while secretory granules in chromaffin cells do not.

Plasma Membrane Disruption: Repair, Prevention, Adaptation

Many metazoan cells inhabit mechanically stressful environments and, consequently, their plasma membranes are frequently disrupted. Survival requires that the cell rapidly repair or reseal the disruption. Rapid resealing is an active and complex structural modification that employs endomembrane as its primary building block, and cytoskeletal and membrane fusion proteins as its catalysts. Endomembrane is delivered to the damaged plasma membrane through exocytosis, a ubiquitous Ca2+-triggered response to disruption. Tissue and cell level architecture prevent disruptions from occurring, either by shielding cells from damaging levels of force, or, when this is not possible, by promoting safe force transmission through the plasma membrane via protein-based cables and linkages. Prevention of disruption also can be a dynamic cell or tissue level adaptation triggered when a damaging level of mechanical stress is imposed. Disease results from failure of either the preventive or resealing mechanisms.

Secretory granule exocytosis

Regulated exocytosis of secretory granules or dense-core granules has been examined in many well-characterized cell types including neurons, neuroendocrine, endocrine, exocrine, and hemopoietic cells and also in other less well-studied cell types. Secretory granule exocytosis occurs through mechanisms with many aspects in common with synaptic vesicle exocytosis and most likely uses the same basic protein components. Despite the widespread expression and conservation of a core exocytotic machinery, many variations occur in the control of secretory granule exocytosis that are related to the specialized physiological role of particular cell types. In this review we describe the wide range of cell types in which regulated secretory granule exocytosis occurs and assess the evidence for the expression of the conserved fusion machinery in these cells. The signals that trigger and regulate exocytosis are reviewed. Aspects of the control of exocytosis that are specific for secretory granules compared with synaptic vesicles or for particular cell types are described and compared to define the range of accessory control mechanisms that exert their effects on the core exocytotic machinery.

Temporal and spatial coordination of exocytosis and endocytosis

In secretory cells, exocytosis and compensatory endocytosis are tightly coupled membrane trafficking processes that control the surface area and composition of the plasma membrane. While exocytic and endocytic processes have been studied independently in great detail, at present there is much interest in understanding the mode of their coupling. This review discusses emerging insights into the coupling of these processes, both in the chemical synapses of neurons and in non-neuronal cells.

An improved method for real-time monitoring of membrane capacitance in Xenopus laevis oocytes

Measurements of membrane capacitance (C(m)) in Xenopus laevis oocytes offer unique experimental possibilities but are difficult to perform with current methods. To improve C(m) measurements in the two-electrode voltage clamp (TEVC) mode, we developed a paired-ramp protocol and tested its performance in a model circuit (with tunable C(m), membrane resistance R(m), and series resistance R(s)) and in Xenopus oocytes. In the cell model and with R(s) = 0 Omega, inaccuracy of C(m) estimates was <1% under widely varying conditions (R(m) ranging from 100 to 2000 kOmega, and C(m) from 50 to 1000 nF). With R(s) > 0 Omega, C(m) was underestimated by a relative error epsilon closely approximated as epsilon approximate 2 x R(s)/(R(s) + R(m)), in keeping with the theoretical prediction. Thus, epsilon may be neglected under standard conditions or, under extreme conditions, corrected for if R(s) is known. Relative imprecision of C(m) estimates was small, independent of R(s), and inversely related to C(m) (<1.5% at 50 nF, <0.4% at 200 nF). Averaging allowed reliable detection of C(m) deviations from 200 nF of 0.1 nF, i.e., 0.05%. In Xenopus oocytes, we could resolve C(m) changes that were small (e.g., DeltaC(m) approximate 2 nF upon 100 muM 8-Br-cAMP), fast (e.g., DeltaC(m)/Deltat approximate 20nF/30s upon 1 muM phorbol myristate acetate (PMA)) or extended and complex (e.g., fast increase, followed by prolonged C(m) decrease upon 1 muM PMA). Rapidly alternating between paired ramps and a second, step protocol allowed quasi-simultaneous monitoring of additional electrical parameters such as R(m), slope conductance g(m), and reversal potential E(rev). Taken together, our method is suited to monitor C(m) in Xenopus oocytes conveniently, with high temporal resolution, accuracy and precision, and in parallel with other electrical parameters. Thus, it may be useful for the study of endo- and exocytosis and of membrane protein regulation and for electrophysiological high-throughput screening.

Toward understanding vesicle traffic and the guard cell model

Vesicular trafficking and membrane fusion are integral to cell growth and development with SNARE proteins, RabGTPases and their associates implicated in membrane fusion and secretion throughout the plant endomembrane system. Although the overall pattern of function is similar to that of animals and yeast, many aspects of endomembrane organization and vesicle trafficking appear unique to plants, for example, the control of cell and vacuolar expansion, asymmetric growth and cell division. However, the dominant membrane trafficking pathways have yet to be defined. Comparative genomics provide important information about vesicle trafficking elements but assigning biological roles based on sequence similarities is extremely difficult. Cellular and genetic approaches are reviewed here that have allowed visualization of vesicle trafficking in plants, including capacitance and dye methods, imaging and marker techniques, protein interactions and reverse genetics. Stomatal guard cells are discussed as cell models for identifying vesicle trafficking pathways and evidence points to a role for vesicle trafficking in stomatal function. For plants generally, kinetic analyses and biochemical studies suggest that several different pools of vesicles, and possibly different mechanisms for delivery, are available for vesicle traffic between endomembrane compartments and the plasma membrane. Characterizing these pathways, their functions and controls provides a major challenge for the future.

Enhancement of inward Ca(2+) currents in bovine chromaffin cells by green tea polyphenol extracts

Green tea contains four major polyphenol compounds: they are (-)-epigallocatechin-3-gallate (EGCG), (-)-epigallocatechin (EGC), (-)-epicatechin-3-gallate (ECG), and (-)-epicatechin (EC). Although all four polyphenol compounds are known to affect tumor suppression, little is known about whether they alter membrane properties. In this study, we examined the effects of ECG and EGCG on ionic currents and secretion. Membrane capacitance changes were used to monitor secretion in bovine chromaffin cells. ECG had the ability to reversibly enhance the inward Ca(2+) current by 21%, and inhibited the peak sodium current by 34%. EGCG had no effect on Ca(2+) current even though it differs from ECG by just a hydroxyl group. The EC(50) of ECG in enhancing Ca(2+) current was 7.6 microM. The maximum enhancement of Ca(2+) current was observed at 0 mV and the maximum current was shifted approximately 10 mV in the hyperpolarizing direction. When cells were stimulated by trains of depolarizations, the exocytosis elicited was enhanced by ECG treatment and the largest enhancement of secretion was observed in later stimulations. EGCG, although it had no significant effect on Ca(2+) current, enhanced exocytosis and slowed endocytosis. These results suggest that green tea polyphenol compounds modulate stimulus-secretion coupling in bovine chromaffin cells.

A variant of yellow fluorescent protein with fast and efficient maturation for cell-biological applications

The green fluorescent protein (GFP) from the jellyfish Aequorea victoria has provided a myriad of applications for biological systems. Over the last several years, mutagenesis studies have improved folding properties of GFP (refs 1,2). However, slow maturation is still a big obstacle to the use of GFP variants for visualization. These problems are exacerbated when GFP variants are expressed at 37 degrees C and/or targeted to certain organelles. Thus, obtaining GFP variants that mature more efficiently is crucial for the development of expanded research applications. Among Aequorea GFP variants, yellow fluorescent proteins (YFPs) are relatively acid-sensitive, and uniquely quenched by chloride ion (Cl-). For YFP to be fully and stably fluorescent, mutations that decrease the sensitivity to both pH and Cl- are desired. Here we describe the development of an improved version of YFP named "Venus". Venus contains a novel mutation, F46L, which at 37 degrees C greatly accelerates oxidation of the chromophore, the rate-limiting step of maturation. As a result of other mutations, F64L/M153T/V163A/S175G, Venus folds well and is relatively tolerant of exposure to acidosis and Cl-. We succeeded in efficiently targeting a neuropeptide Y-Venus fusion protein to the dense-core granules of PC12 cells. Its secretion was readily monitored by measuring release of fluorescence into the medium. The use of Venus as an acceptor allowed early detection of reliable signals of fluorescence resonance energy transfer (FRET) for Ca2+ measurements in brain slices. With the improved speed and efficiency of maturation and the increased resistance to environment, Venus will enable fluorescent labelings that were not possible before.

Calcium influx selects the fast mode of endocytosis in the synaptic terminal of retinal bipolar cells

To investigate the regulation of endocytosis by Ca(2+), we have made capacitance measurements in the synaptic terminal of depolarizing bipolar cells from the retina of goldfish. After a brief depolarization, all of the excess membrane was retrieved rapidly (tau approximately 1 s). But when the rise in free [Ca(2+)] was reduced by the introduction of Ca(2+) buffers [1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate (BAPTA) or EGTA], a large fraction of the membrane was retrieved by a second, slower mechanism (tau > or = 10 s). The block of fast endocytosis by EGTA could be overcome by increasing the amplitude of the Ca(2+) current, demonstrating that Ca(2+) influx was the trigger for fast endocytosis. These manipulations of the Ca(2+) signal altered the relative proportions of fast and slow endocytosis but did not modulate the rate constants of these processes. A brief stimulus that triggered fast endocytosis did not generate a significant rise in the spatially averaged [Ca(2+)], indicating that Ca(2+) regulated endocytosis through an action close to the active zone. The slow mode of retrieval occurred at the resting [Ca(2+)]. These results demonstrate that Ca(2+) influx couples fast endocytosis and exocytosis at this synapse.

Fast kinetics of exocytosis revealed by simultaneous measurements of presynaptic capacitance and postsynaptic currents at a central synapse

The rate of release from nerve terminals depends on both the number of release sites and the rate of release at each site. The latter remains largely unknown at central synapses. We addressed this issue by simultaneously measuring the nerve terminal membrane capacitance and the postsynaptic current at single calyceal synapses in rat brainstem. We found that a 10 ms presynaptic step depolarization depleted a releasable pool containing 3300-5200 vesicles. Released vesicles were endocytosed with a time constant of a few seconds to tens of seconds. Release of only one third of this pool saturated both postsynaptic AMPA and NMDA receptors. A release site can release more than three vesicles in 10 ms (>300 vesicles per second). We conclude that both a large number of release sites and a fast release rate at each site enable synapses to release at a high rate.

Differential coupling of voltage-gated Ca(2+) channels to catecholamine secretion from separate PC12 cell batches

Amperometric recordings were employed to investigate the coupling of Ca(2+) channels to catecholamine secretion in two batches of pheochromocytoma (PC12) cells. In 'new' (freshly obtained) cells (PC12n cells), secretion was dependent on Ca(2+) influx through L-type and N-type Ca(2+) channels. By contrast, in 'aged' cells (maintained in liquid nitrogen for 6-8 years; PC12a cells), secretion was mostly dependent on Ca(2+) influx through N-type channels. Patch clamp recordings revealed that L-type channels accounted for only ca. 26% of total whole-cell current in PC12a cells (determined by blockade caused by 2 microM nifedipine). In contrast, nifedipine suppressed currents by ca. 59% in PC12n cells. Thus important differences in fundamental physiological properties can be observed in PC12 cell batches even when obtained from the same source and maintained under identical conditions.

Cellular signaling and volume control in stomatal movements in plants

Stomatal guard cells are unique as a plant cell model and, because of the depth of present knowledge on ion transport and its regulation, offer a first look at signal integration in higher plants. A large body of data indicates that Ca(2+) and H(+) act independently, integrating with protein kinases and phosphatases, to control the gating of the K(+) and Cl(-) channels that mediate solute flux for stomatal movements. Oscillations in the cytosolic-free concentration of Ca(2+) contribute to a signaling cassette, integrated within these events through an unusual coupling with membrane voltage for solute homeostasis. Similar cassettes are anticipated to include control pathways linked to cytosolic pH. Additional developments during the last two years point to events in membrane traffic that play equally important roles in stomatal control. Research in these areas is now adding entirely new dimensions to our understanding of guard cell signaling.

Clathrin-mediated endocytosis near active zones in snake motor boutons

We have used the activity-dependent probe FM1-43 with electron microscopy (EM) to examine endocytosis at the vertebrate nerve-muscle synapse. Preparations were fixed after very brief neural stimulation at reduced temperature, and internalized FM1-43 was photoconverted into an electron-dense reaction product. To locate the reaction product, we reconstructed computer renderings of individual terminal boutons from serial EM sections. Most of the reaction product was seen in 40-60 nm vesicles. All of the labeled vesicles were clathrin-coated, and 92% of them were located within 300 nm of the plasma membrane, suggesting that they had undergone little processing after retrieval from their endocytic sites. The vesicles (and by inference the sites) were not dispersed randomly near the plane of the membrane but instead were clustered significantly near active zones. Additional reaction product was found within putative macropinosomes; these appeared to form from deep membrane invaginations near active zones. Thus two mechanisms of endocytosis were evident after brief stimulation. Endocytosis near active zones is consistent with the existence of local exo/endocytic cycling pools. This mechanism also might serve to maintain alignment of active zones with postsynaptic folds during periods of activity when vesicular and plasma membranes are interchanged.

A decrease in membrane tension precedes successful cell-membrane repair

We hypothesized that the requirement for Ca(2+)-dependent exocytosis in cell-membrane repair is to provide an adequate lowering of membrane tension to permit membrane resealing. We used laser tweezers to form membrane tethers and measured the force of those tethers to estimate the membrane tension of Swiss 3T3 fibroblasts after membrane disruption and during resealing. These measurements show that, for fibroblasts wounded in normal Ca(2+) Ringer's solution, the membrane tension decreased dramatically after the wounding and resealing coincided with a decrease of approximately 60% of control tether force values. However, the tension did not decrease if cells were wounded in a low Ca(2+) Ringer's solution that inhibited both membrane resealing and exocytosis. When cells were wounded twice in normal Ca(2+) Ringer's solution, decreases in tension at the second wound were 2.3 times faster than at the first wound, correlating well with twofold faster resealing rates for repeated wounds. The facilitated resealing to a second wound requires a new vesicle pool, which is generated via a protein kinase C (PKC)-dependent and brefeldin A (BFA)-sensitive process. Tension decrease at the second wound was slowed or inhibited by PKC inhibitor or BFA. Lowering membrane tension by cytochalasin D treatment could substitute for exocytosis and could restore membrane resealing in low Ca(2+) Ringer's solution.

Invited review: plasma membrane stress failure in alveolar epithelial cells

In this review, we examine the hypothesis that plasma membrane stress failure is a central event in the pathophysiology of injury from alveolar overdistension. This hypothesis leads us to consider alveolar micromechanics and specifically the mechanical interactions between lung matrix and alveolar epithelial cell cytoskeleton and plasma membrane. We then explore events that are central to the regulation of plasma membrane tension and detail the lipid-trafficking responses of in vitro deformed and/or injured cells. We conclude with a reference to upregulation of stress-responsive genes after membrane injury and resealing.