Evidence against the swelling hypothesis for initiation of exocytosis in terminals of chromaffin cell processes

Glutamate, water and ion transport through a charged nanosize pore

The transport of transmitter, ions and water through a positively-charged nanopore was investigated through computer simulations. The physics of the problem is described by a coupled set of Poisson-Nernst-Planck and Navier-Stokes equations in a computational domain consisting a cylindrical pore, whose radius ranged from 1 to 8 nm and which was flanked by two compartments representing the vesicular interior and extra-cellular space. The concentration of co-ions is suppressed and of counter-ions enhanced, especially near the pore wall owing to electrostatic interactions. Glutamate (i.e. the transmitter considered) is negatively charged and is simulated as a counter-ion. The electro-kinetically induced pressure due to the movement of ions is negative and very pronounced near the pore wall where the concentration and flux of counter-ions is very high. The water velocity peaks in the pore center, diminishes to zero at the pore wall, but is constant along the pore axis. The mean velocity of the water/fluid is proportional to the vesicular pressure and pore cross-sectional area. Interestingly it is inversely related to the vesicular glutamate concentration. The factors determining the glutamate flux are complex. The diffusive flux generally predominates for narrow pore, and convective flux may dominate for wide pore if the vesicular pressure is high. Surprisingly at low vesicular pressure the mean total glutamate flux per unit cross-sectional pore area is higher for narrow pores. Higher flux is probably due to the rise of glutamate concentration in the nanopore, which is much more pronounced for narrow nanopores, due to the maintenance of approximate neutrality of charges in the pore and on the pore wall. In conclusion intra-vesicular pressure helps 'flushing-out' the transmitter, but the induced pressure 'drags-out' the water into the extra-cellular space.

Water secretion associated with exocytosis in endocrine cells revealed by micro forcemetry and evanescent wave microscopy

It has been a long belief that release of substances from the cell to the extracellular milieu by exocytosis is completed by diffusion of the substances from secretory vesicles through the fusion pore. Involvement of any mechanical force that may be superposed on the diffusion to enhance the releasing process has not been elucidated to date. We tackled this problem in cultured bovine chromaffin cells using direct and sensitive methods: the laser-trap forcemetry and the evanescent-wave fluorescence microscopy. With a laser beam, we trapped a micro bead in the vicinity of a cell (with 1 microm of separation) and observed movements of the bead optically. Electrical stimulation of the cell induced many of rapid and transient movements of the bead in a direction away from the cell surface. Upon the same stimulation, secretory vesicles stained with a fluorescent probe, acridine orange, and excited under the evanescent field illumination, showed a flash-like response: a transient increase in fluorescence intensity associated with a diffuse cloud of brightness, followed by a complete disappearance. These mechanical and fluorescence transients indicate a directional flow of substances. Blockers of the Cl(-) channel suppressed the rates of both responses in a characteristic way but not exocytotic fusion itself. Immunocytochemical studies revealed the presence of Cl(-) and K(+) channels on the vesicle membranes. These results suggest that the externalization of hormones or transmitters upon exocytosis of vesicles is augmented by secretion of water from the vesicle membrane through the widened fusion pore, possibly modulating the rate and reach of the hormone or transmitter release and facilitating transport of the signal molecules in intercellular spaces.

Video-rate dynamics of exocytotic events associated with phagocytosis in neutrophils

Exocytotic responses associated with phagocytosis were investigated in a single neutrophil with a special reference to their dynamic properties and their spatiotemporal relationships with ionic and chemical responses during phagocytosis. The real-time sequence of phagocytosis-exocytosis was directly visualized by video-enhanced contrast differential interference contrast (VEC-DIC) microscopy. The actual release of contents from such a granule was proven by examining a cell loaded with quinacrine with a dual imaging system that allowed us to observe DIC and fluorescence images simultaneously at a high magnification. During the process of phagosome formation in a neutrophil engulfing an opsonized zymosan, the exocytotic response was observed first in a granule located near the cell surface initially attached to the zymosan, and then in other granules sequentially along pseudopodia surrounding the zymosan. When the phagocytosis was induced in a medium containing luminol, a chemiluminescence due to active oxidants was detected exclusively in the region of phagosome, suggesting that exocytosis took place on the phagosomal membrane and not on the plasma membrane. Changes in cytosolic free calcium concentration ([Ca2+]i) were further measured using fura-2 under the dual imaging system. [Ca2+]i transients were more closely related to the extension of pseudopodia for engulfing zymosan and not directly to the exocytosis. These findings lead to a conclusion that exocytosis associated with phagocytosis is initiated by attachment of the cell membrane to the invading organism and mediated by local activation of the phagosomal membrane.

Real-time imaging of the dynamics of secretory granules in growth cones

Secretory granules containing a hybrid protein consisting of the regulated secretory protein tissue plasminogen activator and an enhanced form of green fluorescent protein were tracked at high spatial resolution in growth cones of differentiated PC12 cells. Tracking shows that granules, unlike synaptic vesicles, generally are mobile in growth cones. Quantitative analysis of trajectories generated by granules revealed two dominant modes of motion: diffusive and directed. Diffusive motion was observed primarily in central and peripheral parts of growth cones, where most granules diffused two to four orders of magnitude more slowly than comparably sized spheres in dilute solution. Directed motion was observed primarily in proximal parts of growth cones, where a subset of granules underwent rapid, directed motion at average speeds comparable to those observed for granules in neurites. This high-resolution view of the dynamics of secretory granules in growth cones provides insight into granule organization and release at nerve terminals. In particular, the mobility of granules suggests that granules, unlike synaptic vesicles, are not tethered stably to cytoskeletal structures in nerve terminals. Moreover, the slow diffusive nature of this mobility suggests that secretory responses involving centrally distributed granules in growth cones will occur slowly, on a time scale of minutes or longer.

Spatio-temporal resolution of exocytosis from individual cells

Biophysical events involved in late stages of exocytosis occur at highly localized areas of cells on millisecond and submillisecond time scales. Thus, methodologies with high spatio-temporal resolution are required to achieve measurements at individual secretory cells. Much has been learned about the mechanisms and kinetics of vesicular release through analysis with the carbon fiber microelectrode techniques amperometry and cyclic voltammetry. Coupling of these techniques with other methods such as patch-clamp continues to reveal details of the secretion process. It is now clear that extrusion of the vesicular contents is a more complex process than previously believed. Vesicle-cell fusion, revealed by cell capacitance measurements, is temporally dissociated from secretion measured amperometrically. The stability imparted by interaction and association of vesicle contents at rest results in a rate-limiting extrusion process after full fusion. Furthermore, the presence of partial fusion events and the occurrence of nonquantized release have been revealed with electrochemical tools.

Pepsinogen secretion: coupling of exocytosis visualized by video microscopy and [Ca2+]i in single cells

Conventional in vitro studies of pepsinogen secretion have measured secretion into the bulk medium and have demonstrated the critical role of Ca2+ in the process. The present study was undertaken to obtain further details of the process of secretion and its relation to Ca2+ changes over very short time periods. The relation between Ca2+ mobilization and exocytosis in an isolated individual peptic cell of the bullfrog was investigated by a method to measure both intracellular Ca2+ ([Ca2+]i), using a fluorescent Ca2+ indicator, fura 2, and exocytosis from single cells using a video microscope analyzing system. Bombesin (3.2 x 10(-7) M) and bethanechol (3.2 x 10(-4) M) caused a rapid increase in [Ca2+]i (initial peak) and a corresponding high frequency of initial exocytosis. After the initial peak, [Ca2+]i was maintained at a somewhat elevated level over the baseline (sustained phase), with a corresponding low frequency of exocytosis. Both the sustained phase of elevated [Ca2+]i and the related exocytosis were eliminated by the depletion of extracellular Ca2+. Low concentrations of bombesin (3.2 x 10(-10) M) and bethanechol (3.2 x 10(-7) M) caused sustained low-amplitude Ca2+ oscillations with correspondingly low frequencies but also caused sustained exocytosis. These data show that 1) cellular response differs between high and low concentrations of stimulus, 2) there is a close relation between [Ca2+]i and exocytosis, 3) exocytosis follows elevation of [Ca2+]i by 14-45 s (n = 6), and 4) there is a significant positive correlation between the peak [Ca2+]i and the number of exocytoses.

Synaptic vesicle movements monitored by fluorescence recovery after photobleaching in nerve terminals stained with FM1-43

We used the fluorescence recovery after photobleaching technique to monitor movements of synaptic vesicles in top views of living frog motor nerve terminals that had been prestained with the fluorescent dye FM1-43. In each experiment, a small portion of a single stained vesicle cluster was bleached with a laser and monitored subsequently for signs of recovery as neighboring, unbleached vesicles moved into the bleached region. In resting terminals, little or no recovery from photobleaching occurred. Repetitive nerve stimulation, which caused all fluorescent spots to grow dim as dye was released from exocytosing vesicles, did not promote recovery from photobleaching. Pretreatment with botulinum toxin (type A, C, or D) blocked exocytosis and destaining, but intense nerve stimulation still did not cause significant recovery in bleached regions. These results suggest that lateral movements of synaptic vesicles are restricted severely in both resting and stimulated nerve terminals. We tested for laser-induced photodamage in several ways. Bleached regions could be restained fully with FM1-43, and these restained regions could be destained fully by nerve stimulation. Partially bleached regions could be destained, although the rate of destaining was lower than normal. Brisk recovery from photobleaching occurred after treatment with okadaic acid, which disrupts synaptic vesicle clusters and causes vesicles to spread throughout the nerve terminal. These results suggest that vesicle translocation and recycling machinery was intact in photobleached regions.

Imaging exocytosis and endocytosis

From the secretion of neurotransmitters via synaptic vesicles to the expulsion of cellular waste via contractile vacuoles, exocytosis and its sequel, endocytosis, are being explored with a variety of new optical tools. Fluorescent markers, especially styryl dyes such as FM1-43 (which reversibly labels endosomal membranes), have been used to follow exo- and endocytic events in many cell types. Even though the development of new dyes is still largely empirical, some theoretical principles have emerged to guide future dye chemistry. Moreover, advances in optical imaging technology that augment conventional fluorescence microscopy are appearing. For example, interference reflection microscopy (which requires no flurophore) and total internal reflection microscopy have recently been used to observe single exocytic events at the contact point between a glass coverslip and the plasma membrane.

Temporally resolved, independent stages of individual exocytotic secretion events

The stages of the complex events involved in exocytotic secretion after vesicle-cell membrane fusion have been examined at the level of individual vesicles. Catecholamine flux from single bovine adrenal medullary cells was measured with carbon-fiber microelectrodes firmly touching the cell surface. The data reveal that secretion during exocytotic events has three distinct stages: a small increase in catecholamine flux, a rapid, but not instantaneous, rise to a maximum, followed by an exponential decrease in the flux. These stages are interpreted in the following ways. The initial stage corresponds to catecholamine secretion through a fusion pore. The rate of pore expansion appears to control the rise time of the flux to its maximum value. The final exponential stage is consistent with chemical dissociation of the intravesicular matrix or gel.

Calcium loading of secretory granules in stimulated neurohypophysial nerve endings

The total calcium content of secretory granules, Cag, was evaluated in isolated neurohypophysial nerve endings. The Cag in the resting state, as measured by X-ray microanalysis, is relatively high with an average of 7.4 +/- 0.6 mmol/kg wet weight. Following a depolarizing potassium challenge, a subpopulation of granules with even higher Cag could be detected, dispersed over a wider range of concentrations (up to 70 mmol/kg wet weight). After subsequent rinsing in physiological saline, Cag decreased to control values. This could have resulted from Ca2+ extrusion, or from preferential secretion of calcium-enriched granules. Our data can be interpreted in favor of the second explanation since no decrease in Cag was observed when secretion was blocked by a hyperosmotic saline. The effect of hyperosmotic conditions on isolated nerve endings was further studied by monitoring free cytoplasmic Ca2+ with the calcium-sensitive dye Fura-2 and by conventional electron microscopy. It was demonstrated that hyperosmotic treatment alone did not increase basal cytosolic Ca2+ concentrations but did significantly reduce the potassium-induced cytosolic rise in Ca2+. Electron microscopy of nerve endings in hyperosmotic conditions showed numerous exocytotic figures at various stages. The observed changes in Cag are in accord with a published hypothesis which proposes that intragranular calcium is a significant variable in regulated secretion.