Ca2+-triggered peptide secretion in single cells imaged with green fluorescent protein and evanescent-wave microscopy

Endosomal fusion upon SNARE knockdown is maintained by residual SNARE activity and enhanced docking

SNARE proteins mediate membrane fusion in the secretory pathway of eukaryotic cells. Genetic deletion and siRNA-based knockdown have been instrumental in assigning given SNAREs to defined intracellular transport steps. However, SNARE depletion occasionally results in barely detectable phenotypes. To understand how cells cope with SNARE loss, we have knocked down several SNAREs functioning in early endosome fusion. Surprisingly, knockdown of syntaxin 13, syntaxin 6 and vti1a, alone or in combinations, did not result in measurable changes of endosomal trafficking or fusion. We found that the residual SNARE levels (typically approximately 10%) were sufficient for a substantial amount of SNARE-SNARE interactions. Conversely, in wild-type cells, most SNARE molecules were concentrated in clusters, constituting a spare pool not readily available for interactions. Additionally, the knockdown organelles exhibited enhanced docking. We conclude that SNAREs are expressed at much higher levels than needed for maintenance of organelle fusion, and that loss of SNAREs is compensated for by the co-regulation of the docking machinery.

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.

Automated quantification of cellular traffic in living cells

Cellular traffic is a central aspect of cell function in health and disease. It is highly dynamic, and can be investigated at increasingly finer temporal and spatial resolution due to new imaging techniques and probes. Manual tracking of these data is labor-intensive and observer-biased and existing automation is only semi-automatic and requires near-perfect object detection and high-contrast images. Here, we describe a novel automated technique for quantifying cellular traffic. Using local intrinsic information from adjacent images in a sequence and a model for object characteristics, our approach detects and tracks multiple objects in living cells via Multiple Hypothesis Tracking and handles several confounds (merge/split, birth/death, and clutters), as reliable as expert observers. By replacing the related component (e.g. using a different appearance model) the method can be easily adapted for quantitative analysis of other biological samples.

Unique secretory dynamics of tissue plasminogen activator and its modulation by plasminogen activator inhibitor-1 in vascular endothelial cells

We analyzed the secretory dynamics of tissue plasminogen activator (tPA) in EA.hy926 cells, an established vascular endothelial cell (VEC) line producing GFP-tagged tPA, using total internal reflection-fluorescence (TIR-F) microscopy. tPA-GFP was detected in small granules in EA.hy926 cells, the distribution of which was indistinguishable from intrinsically expressed tPA. Its secretory dynamics were unique, with prolonged (> 5 minutes) retention of the tPA-GFP on the cell surface, appearing as fluorescent spots in two-thirds of the exocytosis events. The rapid disappearance (mostly by 250 ms) of a domain-deletion mutant of tPA-GFP possessing only the signal peptide and catalytic domain indicates that the amino-terminal heavy chain of tPA-GFP is essential for binding to the membrane surface. The addition of PAI-1 dose-dependently facilitated the dissociation of membrane-retained tPA and increased the amounts of tPA-PAI-1 high-molecular-weight complexes in the medium. Accordingly, suppression of PAI-1 synthesis in EA.hy926 cells by siRNA prolonged the dissociation of tPA-GFP, whereas a catalytically inactive mutant of tPA-GFP not forming complexes with PAI-1 remained on the membrane even after PAI-1 treatment. Our results provide new insights into the relationship between exocytosed, membrane-retained tPA and PAI-1, which would modulate cell surface-associated fibrinolytic potential.

Imaging a target of Ca2+ signalling: dense core granule exocytosis viewed by total internal reflection fluorescence microscopy

Ca²⁺ ions are the most ubiquitous second messenger found in all cells, and play a significant role in controlling regulated secretion from neurons, endocrine, neuroendocrine and exocrine cells. Here, we describe microscopic techniques to image regulated secretion, a target of Ca²⁺ signalling. The first of these, total internal reflection fluorescence (TIRF), is well suited for optical sectioning at cell–substrate regions with an unusually thin region of fluorescence excitation (<150 nm). It is thus particularly useful for studies of regulated hormone secretion. A brief summary of this approach is provided, as well as a description of the physical basis for the technique and the tools to implement TIRF using a standard fluorescence microscope. We also detail the different fluorescent probes which can be used to detect secretion and how to analyze the data obtained. A comparison between TIRF and other imaging modalities including confocal and multiphoton microscopy is also included.

Efficient copackaging and cotransport yields postsynaptic colocalization of neuromodulators associated with synaptic plasticity

Recent data suggest that tissue plasminogen activator (tPA) influences long-term plasticity at hippocampal synapses by converting plasminogen into plasmin, which then generates mature brain-derived neurotrophic factor (mBDNF) from its precursor, proBDNF. Motivated by this hypothesis, we used fluorescent chimeras, expressed in hippocampal neurons, to elucidate (1) mechanisms underlying plasminogen secretion from hippocampal neurons, (2) if tPA, plasminogen, and proBDNF are copackaged and cotransported in hippocampal neurons, especially within dendritic spines, and (3) mechanisms mediating the transport of these neuromodulators to sites of release. We find that plasminogen chimeras traffic through the regulated secretory pathway of hippocampal neurons in dense-core granules (DCGs) and that tPA, plasminogen, and proBDNF chimeras are extensively copackaged in DCGs throughout hippocampal neurons. We also find that 80% of spines that contain DCGs contain chimeras of these neuromodulators in the same DCG. Finally, we demonstrate, for the first time, that neuromodulators undergo cotransport along dendrites in rapidly mobile DCGs, indicating that neuromodulators can be efficiently recruited into active spines. These results support the hypothesis that tPA mediates synaptic activation of BDNF by demonstrating that tPA, plasminogen, and proBDNF colocalize in DCGs in spines, where these neuromodulators can undergo activity-dependent release and then interact and/or mediate changes that influence synaptic efficacy. The results also raise the possibility that frequency-dependent changes in extents of neuromodulator release from DCGs influence the direction of plasticity at hippocampal synapses by altering the relative proportions of two proteins, mBDNF and proBDNF, that exert opposing effects on synaptic efficacy.

Modulating vesicle priming reveals that vesicle immobilization is necessary but not sufficient for fusion-competence

In neurons and neuroendocrine cells, docked vesicles need to undergo priming to become fusion competent. Priming is a multi-step process that was shown to be associated with vesicle immobilization. However, it is not known whether vesicle immobilization is sufficient to acquire complete fusion competence. To extend our understanding of the physical manifestation of vesicle priming, we took advantage of tomosyn, a SNARE-related protein that specifically inhibits vesicle priming, and measured its effect on vesicle dynamics in live chromaffin cells using total internal reflection fluorescence microscopy. We show here that while in control cells vesicles undergo immobilization before fusion, vesicle immobilization is attenuated in tomosyn overexpressing cells. This in turn increases the turnover rate of vesicles near the membrane and attenuates the fusion of newcomer vesicles. Moreover, the release probability of immobile vesicles in tomosyn cells is significantly reduced, suggesting that immobilization is an early and necessary step in priming but is insufficient, as further molecular processes are needed to acquire complete fusion competence. Using tomosyn as a molecular tool we provide a mechanistic link between functional docking and priming and suggest that functional docking is the first step in vesicle priming, followed by molecular modifications that do not translate into changes in vesicle mobility.

Local extracellular acidification caused by Ca2+-dependent exocytosis in PC12 cells

Exocytosis of acidic synaptic vesicles may produce local extracellular acidification, but this effect has not been measured directly and its magnitude may depend on the geometry and pH-buffering capacity of both the vesicles and the extracellular space. Here we have used SNARF dye immobilized by conjugation to dextran to measure the release of protons from PC12 cells. The PC12 cells were stimulated by exposure to depolarizing K(+)-rich solution and activation was verified by fluorescence measurement of intracellular Ca(2+) and the release kinetics of GFP-labeled vesicles. Confocal imaging of the pH-dependent fluorescence from the immobile extracellular SNARF dye showed transient acidification around the cell bodies and neurites of activated PC12 cells. The local acidification was abolished when extracellular solution was devoid of Ca(2+) or strong pH-buffering was imposed with 10mM of HEPES. We conclude that the release of secretory vesicles induces local rises in proton concentrations that are co-released from synaptic vesicles with the primary neurotransmitter, and propose that the co-released protons may modulate the signaling in confined micro-domains of synapses.

Sorting of the neuroendocrine secretory protein Secretogranin II into the regulated secretory pathway: role of N- and C-terminal alpha-helical domains

Secretogranin II (SgII) belongs to the granin family of prohormones widely distributed in dense-core secretory granules (DCGs) of endocrine, neuroendocrine, and neuronal cells, including sympathoadrenal chromaffin cells. The mechanisms by which secretory proteins, and granins in particular, are sorted into the regulated secretory pathway are unsettled. We designed a strategy based on novel chimeric forms of human SgII fused to fluorescent (green fluorescent protein) or chemiluminescent (embryonic alkaline phosphatase) reporters to identify trafficking determinants mediating DCG targeting of SgII in sympathoadrenal cells. Three-dimensional deconvolution fluorescence microscopy and secretagogue-stimulated release studies demonstrate that SgII chimeras are correctly targeted to DCGs and released by exocytosis in PC12 and primary chromaffin cells. Results from a Golgi-retained mutant form of SgII suggest that sorting of SgII into DCGs depends on a saturable sorting machinery at the trans-Golgi/trans-Golgi network. Truncation analyses reveal the presence of DCG-targeting signals within both the N- and C-terminal regions of SgII, with the putative alpha-helix-containing SgII-(25-41) and SgII-(334-348) acting as sufficient, independent sorting domains. This study defines sequence features of SgII mediating vesicular targeting in sympathoadrenal cells and suggests a mechanism by which discrete domains of the molecule function in sorting, perhaps by virtue of a particular arrangement in tertiary structure and/or interaction with a specific component of the DCG membrane.

Compensatory endocytosis in chromaffin cells

Exocytosis occurs via fusion of secretory granules with the cell membrane, whereupon the granule content is at least partially released and the granule membrane is temporarily added to the plasma membrane. Exocytosis is balanced by compensatory endocytosis to achieve net equilibrium of the cell surface area and to recycle and redistribute components of the exocytosis machinery. The underlying molecular mechanisms remain a matter of debate. In this review, we summarize and discuss recent progress in the understanding of compensatory endocytosis, with the focus on chromaffin cells as a useful model for studying mechanisms of regulated secretion.

A 20-nm step toward the cell membrane preceding exocytosis may correspond to docking of tethered granules

In endocrine cells, plasma membrane (PM)-bound secretory granules must undergo a number of maturation stages (i.e., priming) to become fusion-competent. Despite identification of several molecules involved in binding granules to the PM and priming them, the exact nature of events occurring at the PM still largely remains a mystery. In stimulated BON cells, we used evanescent wave microscopy to study trajectories of granules shortly before their exocytoses, which provided a physical description of vesicle-PM interactions at an unprecedented level of detail, and directly lead to an original mechanistic model. In these cells, tethered (T), nonfusogenic, vesicles are prevented from converting to fusogenic, docked (D) ones in resting conditions. Upon elevation of calcium, T-vesicles perform a 21-nm step toward the PM to become D, and fuse approximately 3 s thereafter. Our ability to directly visualize different modes of PM-attachment paves the way for clarifying the exact role of various molecules implicated in attachment and priming of granules in future studies.

Imaging Ca2+-triggered exocytosis of single secretory granules on plasma membrane lawns from neuroendocrine cells

This cell-free assay for exocytosis is particularly useful when spatial information about exocytotic sites and biochemical access to the plasma membrane within less than a minute is required. It is based on the study of plasma membrane lawns from secretory cells exhibiting secretory granules filled with neuropeptide Y-green fluorescent protein (NPY-GFP). The sample is prepared by subjecting NPY-GFP-expressing cells to a brief ultrasound pulse, leaving behind a basal, flat plasma membrane with fluorescent attached secretory organelles. These sheets can then be incubated in defined solutions with the benefit that complete solution changes can be achieved in less than 1 min. Individual secretory granules are monitored in the docked state and during exocytosis by video microscopy.

Chapter 7: Total internal reflection fluorescence microscopy

Total internal reflection fluorescence microscopy (TIRFM), also known as evanescent wave microscopy, is used in a wide range of applications, particularly to view single molecules attached to planar surfaces and to study the position and dynamics of molecules and organelles in living culture cells near the contact regions with the glass coverslip. TIRFM selectively illuminates fluorophores only in a very thin (less than 100 nm deep) layer near the substrate, thereby avoiding excitation of fluorophores outside this subresolution optical section. This chapter reviews the history, current applications in cell biology and biochemistry, basic optical theory, combinations with numerous other optical and spectroscopic approaches, and a range of setup methods, both commercial and custom.

A novel site of action for alpha-SNAP in the SNARE conformational cycle controlling membrane fusion

Regulated exocytosis in neurons and neuroendocrine cells requires the formation of a stable soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex consisting of synaptobrevin-2/vesicle-associated membrane protein 2, synaptosome-associated protein of 25 kDa (SNAP-25), and syntaxin 1. This complex is subsequently disassembled by the concerted action of alpha-SNAP and the ATPases associated with different cellular activities-ATPase N-ethylmaleimide-sensitive factor (NSF). We report that NSF inhibition causes accumulation of alpha-SNAP in clusters on plasma membranes. Clustering is mediated by the binding of alpha-SNAP to uncomplexed syntaxin, because cleavage of syntaxin with botulinum neurotoxin C1 or competition by using antibodies against syntaxin SNARE motif abolishes clustering. Binding of alpha-SNAP potently inhibits Ca(2+)-dependent exocytosis of secretory granules and SNARE-mediated liposome fusion. Membrane clustering and inhibition of both exocytosis and liposome fusion are counteracted by NSF but not when an alpha-SNAP mutant defective in NSF activation is used. We conclude that alpha-SNAP inhibits exocytosis by binding to the syntaxin SNARE motif and in turn prevents SNARE assembly, revealing an unexpected site of action for alpha-SNAP in the SNARE cycle that drives exocytotic membrane fusion.

Direct quantification of fusion rate reveals a distal role for AS160 in insulin-stimulated fusion of GLUT4 storage vesicles

Insulin-stimulated GLUT4 translocation to the plasma membrane constitutes a key process for blood glucose control. However, convenient and robust assays to monitor this dynamic process in real time are lacking, which hinders current progress toward elucidation of the underlying molecular events as well as screens for drugs targeting this particular pathway. Here, we have developed a novel dual colored probe to monitor the translocation process of GLUT4 based on dual color fluorescence measurement. We demonstrate that this probe is more than an order of magnitude more sensitive than the current technology for detecting fusion events from single GLUT4 storage vesicles (GSVs). A small fraction of fusion events were found to be of the "kiss-and-run" type. For the first time, we show that insulin stimulation evokes a approximately 40-fold increase in the fusion of GSVs in 3T3-L1 adipocytes, compared with basal conditions. The probe can also be used to monitor the prefusion behavior of GSVs. By quantifying both the docking and fusion rates simultaneously, we demonstrate a proportional inhibition in both docking and fusion of GSVs by a dominant negative mutant of AS160, indicating a role for AS160 in the docking of GSVs but not in the regulation of GSV fusion after docking.

Activity-dependent release of tissue plasminogen activator from the dendritic spines of hippocampal neurons revealed by live-cell imaging

Tissue plasminogen activator (tPA) has been implicated in a variety of important cellular functions, including learning-related synaptic plasticity and potentiating N-methyl-D-aspartate (NMDA) receptor-dependent signaling. These findings suggest that tPA may localize to, and undergo activity-dependent secretion from, synapses; however, conclusive data supporting these hypotheses have remained elusive. To elucidate these issues, we studied the distribution, dynamics, and depolarization-induced secretion of tPA in hippocampal neurons, using fluorescent chimeras of tPA. We found that tPA resides in dense-core granules (DCGs) that traffic to postsynaptic dendritic spines and that can remain in spines for extended periods. We also found that depolarization induced by high potassium levels elicits a slow, partial exocytotic release of tPA from DCGs in spines that is dependent on extracellular Ca(+2) concentrations. This slow, partial release demonstrates that exocytosis occurs via a mechanism, such as fuse-pinch-linger, that allows partial release and reuse of DCG cargo and suggests a mechanism that hippocampal neurons may rely upon to avoid depleting tPA at active synapses. Our results also demonstrate release of tPA at a site that facilitates interaction with NMDA-type glutamate receptors, and they provide direct confirmation of fundamental hypotheses about tPA localization and release that bear on its neuromodulatory functions, for example, in learning and memory.

Imaging Exocytosis with Total Internal Reflection Microscopy (TIRFM)

INTRODUCTIONAlthough electrophysiological techniques such as membrane capacitance measurements, electrochemical detection, and post-synaptic recordings are powerful ways of studying exocytosis, information concerning any steps prior to vesicle fusion must be inferred indirectly. Total internal reflection fluorescence microscopy (TIRFM) is a powerful technique for studying events that, like exocytosis, occur near a cell surface. The technique allows selective imaging of fluorescent molecules that are closest to a high refractive index substance such as glass. In this protocol, TIRFM is used to investigate the steps leading up to vesicle fusion in both retinal bipolar neurons and chromaffin cells by directly imaging synaptic vesicles and dense core granules prior to and including exocytosis.

Studying individual events in biology

Studying the properties of individual events and molecules offers a host of advantages over taking only macroscopic measurements of populations. Here we review such advantages, as well as some pitfalls, focusing on examples from biological imaging. Examples include single proteins, their interactions in cells, organelles, and their interactions both with each other and with parts of the cell. Additionally, we discuss constraints that limit the study of single events, along with the criteria that must be fulfilled to determine whether single molecules or events are being detected.

Resolving vesicle fusion from lysis to monitor calcium-triggered lysosomal exocytosis in astrocytes

Optical imaging of individual vesicle exocytosis is providing new insights into the mechanism and regulation of secretion by cells. To study calcium-triggered secretion from astrocytes, we used acridine orange (AO) to label vesicles. Although AO is often used for imaging exocytosis, we found that imaging vesicles labeled with AO can result in their photolysis. Here, we define experimental and analytical approaches that permit us to distinguish unambiguously between fusion, leakage, and lysis of individual vesicles. We have used this approach to demonstrate that lysosomes undergo calcium-triggered exocytosis in astrocytes.

GLUT4 translocation: the last 200 nanometers

Insulin regulates circulating glucose levels by suppressing hepatic glucose production and increasing glucose transport into muscle and adipose tissues. Defects in these processes are associated with elevated vascular glucose levels and can lead to increased risk for the development of Type 2 diabetes mellitus and its associated disease complications. At the cellular level, insulin stimulates glucose uptake by inducing the translocation of the glucose transporter 4 (GLUT4) from intracellular storage sites to the plasma membrane, where the transporter facilitates the diffusion of glucose into striated muscle and adipocytes. Although the immediate downstream molecules that function proximal to the activated insulin receptor have been relatively well-characterized, it remains unknown how the distal insulin-signaling cascade interfaces with and recruits GLUT4 to the cell surface. New biochemical assays and imaging techniques, however, have focused attention on the plasma membrane as a potential target of insulin action leading to GLUT4 translocation. Indeed, it now appears that insulin specifically regulates the docking and/or fusion of GLUT4-vesicles with the plasma membrane. Future work will focus on identifying the key insulin targets that regulate the GLUT4 docking/fusion processes.

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.

Visualization of Rab3A dissociation during exocytosis: a study by total internal reflection microscopy

Rab3A is a small G protein in the Rab3 subfamily, and is thought to act at late stage of exocytosis. However, the detailed mechanism of its action is not completely understood. To study the role of Rab3A in exocytosis, we used a total internal reflection fluorescence microscope to examine the fluorescence changes of EGFP-Rab3A-labeled and NPY-EGFP-labeled vesicles in PC12 cells upon stimulation. The fluorescence of EGFP-Rab3A-labeled and NPY-EGFP-labeled vesicles decreased while showing different patterns. The NPY-EGFP-labeled vesicles that exocytosed showed a transient fluorescence increase before NPY-EGFP fluorescence disappearance, which represents fusion and NPY release. This transient increase was diminished in cells that co-expressed the GDP-bound Rab3A mutant. The fluorescence of EGFP-Rab3A-labeled vesicles dispersed before disappearance, which represents the dissociation of Rab3A from the vesicles. The dispersion was not found in GTP-bound Rab3A mutant-labeled vesicles. Interestingly, EGFP-Rab3A F59S, a mutant unable to bind rabphilin, dissociates slower from the vesicles than wild type Rab3A and caused a slower release of NPY-EGFP. The results provide direct evidence to support the hypothesis that GTP hydrolysis and rabphilin are involved in Rab3A dissociation from the vesicles and the occurrence of exocytosis.

Rab18 inhibits secretory activity in neuroendocrine cells by interacting with secretory granules

Rab proteins comprise a complex family of small GTPases involved in the regulation of intracellular membrane trafficking and reorganization. In this study, we identified Rab18 as a new inhibitory player of the secretory pathway in neuroendocrine cells. In adrenal chromaffin PC12 cells and pituitary AtT20 cells, Rab18 is located at the cytosol but associates with a subpopulation of secretory granules after stimulation of the regulated secretory pathway, strongly suggesting that induction of secretion provokes Rab18 activation and recruitment to these organelles. In support of this, a dominant-inactive Rab18 mutant was found to distribute diffusely in the cytosol, whereas a dominant-active Rab18 mutant was predominantly associated to secretory granules. Furthermore, interaction of Rab18 with secretory granules was associated to an inhibition in the secretory activity of PC12 and AtT20 cells in response to stimulatory challenges. Association of Rab18 with secretory granules was also observed by immunoelectron microscopy in normal, non-tumoral endocrine cells (pituitary melanotropes), wherein Rab18 protein content is inversely correlated to the level of secretory activity of cells. Taken together, these findings suggest that, in neuroendocrine cells, Rab18 acts as a negative regulator of secretory activity, likely by impairing secretory granule transport.

Vesicle movements are governed by the size and dynamics of F-actin cytoskeletal structures in bovine chromaffin cells

Dense vesicles can be observed in live bovine chromaffin cells using fluorescent reflection confocal microscopy. These vesicles display a similar distribution, cytoplasmic density and average size as the chromaffin granules visualized by electron microscopy. In addition, the acidic vesicles labeled with Lysotracker Red comprised a subpopulation of the vesicles that are visualized by reflection fluorescence. A combination of fluorescence reflection and transmitted light images permitted the movements of vesicles in relation to the cortical cytoskeleton to be studied. The movement of vesicles located on the outside of this structure was restricted, with an apparent diffusion coefficient of 1.0+/-0.4 x 10(-4) microm(2)/s. In contrast, vesicles located in the interior moved much more freely and escaped from the visual confocal plane. Lysotracker labeling was more appropriate to study the movement of the faster moving vesicles, whose diffusion coefficient was five times higher. Using this type of labeling we confirmed the restriction on cortical movement and showed a clear relationship between vesicle mobility and the kinetics of cytoskeletal movement on both sides of the cortical cytoskeleton. This relationship was further emphasized by studying cytoskeletal organization and kinetics. Indeed, an estimate of the size of the cytoskeletal polygonal cages present in the cortical region and in the cell interior agreed well with the calculation of the theoretical radius of the cages imprisoning vesicle movement. Therefore, these data suggest that the structure and kinetics of the cytoskeleton governs vesicle movements in different regions of chromaffin cells.

Characterization of sequential exocytosis in a human neuroendocrine cell line using evanescent wave microscopy and “virtual trajectory” analysis

Secretion of hormones and other bioactive substances is a fundamental process for virtually all multicellular organisms. Using total internal reflection fluorescence microscopy (TIRFM), we have studied the calcium-triggered exocytosis of single, fluorescently labeled large, dense core vesicles in the human neuroendocrine BON cell line. Three types of exocytotic events were observed: (1) simple fusions (disappearance of a fluorescent spot by rapid diffusion of the dye released to the extracellular space), (2) "orphan" fusions for which only rapid dye diffusion, but not the parent vesicle, could be detected, and (3) events with incomplete or multi-step disappearance of a fluorescent spot. Although all three types were reported previously, only the first case is clearly understood. Here, thanks to a combination of two-color imaging, variable angle TIRFM, and novel statistical analyses, we show that the latter two types of events are generated by the same basic mechanism, namely shape retention of fused vesicle ghosts which become targets for sequential fusions with deeper lying vesicles. Overall, approximately 25% of all exocytotic events occur via sequential fusion. Secondary vesicles, located 200-300 nm away from the cell membrane are as fusion ready as primary vesicles located very near the cell membrane. These findings call for a fundamental shift in current models of regulated secretion in endocrine cells. Previously, sequential fusion had been studied mainly using two-photon imaging. To the best of our knowledge, this work constitutes the first quantitative report on sequential fusion using TIRFM, despite its long running and widespread use in studies of secretory mechanisms.

Identification of a novel targeting sequence for regulated secretion in the serine protease inhibitor neuroserpin

Ns (neuroserpin) is a member of the serpin (serine protease inhibitor) gene family that is primarily expressed within the central nervous system. Its principal target protease is tPA (tissue plasminogen activator), which is thought to contribute to synaptic plasticity and to be secreted in a stimulus-dependent manner. In the present study, we demonstrate in primary neuronal cultures that Ns co-localizes in LDCVs (large dense core vesicles) with the regulated secretory protein chromogranin B. We also show that Ns secretion is regulated and can be specifically induced 4-fold by secretagogue treatment. A novel 13-amino-acid sorting signal located at the C-terminus of Ns is identified that is both necessary and sufficient to target Ns to the regulated secretion pathway. Its deletion renders Ns no longer responsive to secretagogue stimulation, whereas PAI-Ns [Ns (neuroserpin)-PAI-1 (plasminogen activator inhibitor-1) chimaera appending the last 13 residues of Ns sequence to the C-terminus of PAI-1] shifts PAI-1 secretion into a regulated secretory pathway.

Three-Dimensional Tracking of Single Granules in Living PC-12 Cells Employing TIRFM and WFFM

A comparative study was carried out on evaluating the performance of total internal reflection fluorescence microscopy (TIRFM) and deconvolution wide-field fluorescence microscopy (WFFM) in tracking single secretory granules. Both techniques have been applied to follow the three-dimensional mobility of single secretory granules in living neuroendocrine PC-12 cells. Both techniques return the similar result that most acridine orange-labeled granules were found to travel in random and caged diffusion, and only a small fraction of granules traveled in directed diffusion. Furthermore, the size and 3-D diffusion coefficient of secretory granules, obtained by these two imaging techniques, yield the same value. Together, our results demonstrate the potential of the combination TIRFM and WFFM in tracking long-termed motion of granules throughout live whole cells.

Granulogenesis in Non-neuroendocrine COS-7 Cells Induced by EGFP-tagged Chromogranin A Gene Transfection: Identical and Distinct Distribution of CgA and EGFP

We examined whether an enhanced green fluorescent protein (EGFP)-tagged chromogranin A (CgA) gene construct could serve as a marker protein to follow the synthesis of CgA and the process of granulogenesis in non-neuroendocrine (NE) cells. We transfected a CgA-EGFP expression vector into non-NE COS-7 cells and investigated the localization of a chimeric CgA-EGFP protein using confocal laser scanning microscopy (CLSM). The fluorescent signal of CgA-EGFP was distributed granularly in the cytoplasm. An immunocytochemical study using anti-CgA antibody with a quantum dot (Qd)525 shows colocalization of fluorescent signal of chimeric CgA-EGFP and CgA-Qd525 signals in granular structures, particularly at the periphery of the cytoplasm. We interpreted granules that were immunoreactive to CgA in electron micrographs as secretory. Spectral analysis of EGFP fluorescence revealed distinct EGFP signals without CgA colocalization. This is the first report to show that a granular structure can be induced by transfecting the EGFP-tagged human CgA gene into non-NE cells. The EGFP-tagged CgA gene could be a useful tool to investigate processes of the regulatory pathway. A more precise analysis of the fluorescence signal of EGFP by combination with the Qd system or by spectral analysis with CLSM can provide insight into biological phenomena.

Munc18-bound syntaxin readily forms SNARE complexes with synaptobrevin in native plasma membranes

Munc18-1, a protein essential for regulated exocytosis in neurons and neuroendocrine cells, belongs to the family of Sec1/Munc18-like (SM) proteins. In vitro, Munc18-1 forms a tight complex with the SNARE syntaxin 1, in which syntaxin is stabilized in a closed conformation. Since closed syntaxin is unable to interact with its partner SNAREs SNAP-25 and synaptobrevin as required for membrane fusion, it has hitherto not been possible to reconcile binding of Munc18-1 to syntaxin 1 with its biological function. We now show that in intact and exocytosis-competent lawns of plasma membrane, Munc18-1 forms a complex with syntaxin that allows formation of SNARE complexes. Munc18-1 associated with membrane-bound syntaxin 1 can be effectively displaced by adding recombinant synaptobrevin but not syntaxin 1 or SNAP-25. Displacement requires the presence of endogenous SNAP-25 since no displacement is observed when chromaffin cell membranes from SNAP-25-deficient mice are used. We conclude that Munc18-1 allows for the formation of a complex between syntaxin and SNAP-25 that serves as an acceptor for vesicle-bound synaptobrevin and that thus represents an intermediate in the pathway towards exocytosis.

Dissecting multiple steps of GLUT4 trafficking and identifying the sites of insulin action

Insulin-stimulated GLUT4 translocation is central to glucose homeostasis. Functional assays to distinguish individual steps in the GLUT4 translocation process are lacking, thus limiting progress toward elucidation of the underlying molecular mechanism. Here we have developed a robust method, which relies on dynamic tracking of single GLUT4 storage vesicles (GSVs) in real time, for dissecting and systematically analyzing the docking, priming, and fusion steps of GSVs with the cell surface in vivo. Using this method, we have shown that the preparation of GSVs for fusion competence after docking at the surface is a key step regulated by insulin, whereas the docking step is regulated by PI3K and its downstream effector, the Rab GAP AS160. These data show that Akt-dependent phosphorylation of AS160 is not the major regulated step in GLUT4 trafficking, implicating alternative Akt substrates or alternative signaling pathways downstream of GSV docking at the cell surface as the major regulatory node.

Mechanisms of peptide hormone secretion

According to the classical view, peptide hormones are stored in large dense-core vesicles that release all of their cargo rapidly and completely when they fuse with and flatten into the plasma membrane. However, recent imaging studies suggest that this view is too simple. Even after vesicles fuse with the plasma membrane, cells might control the rate of dispersal of vesicle cargo - either by modulating the properties of the fusion pore that connects the vesicle lumen to the extracellular solution or by storing cargo in states that disperse slowly in the extracellular space. Understanding these mechanisms is important, owing to the increasing prevalence of diseases, such as type 2 diabetes mellitus, which arise from insufficient secretion of peptide hormones.

PKC-1 regulates secretion of neuropeptides

The secretion of neurotransmitters and neuropeptides is mediated by distinct organelles-synaptic vesicles (SVs) and dense-core vesicles (DCVs), respectively. Relatively little is known about the factors that differentially regulate SV and DCV secretion. Here we show that protein kinase C-1 (PKC-1), which is most similar to the vertebrate PKC eta and epsilon isoforms, regulates exocytosis of DCVs in Caenorhabditis elegans motor neurons. Mutants lacking PCK-1 activity had delayed paralysis induced by the acetylcholinesterase inhibitor aldicarb, whereas mutants with increased PKC-1 activity had more rapid aldicarb-induced paralysis. Imaging and electrophysiological assays indicated that SV release occurred normally in pkc-1 mutants. By contrast, genetic analysis of aldicarb responses and imaging of fluorescently tagged neuropeptides indicated that mutants lacking PKC-1 had reduced neuropeptide secretion. Similar neuropeptide secretion defects were found in mutants lacking unc-31 (encoding the protein CAPS) or unc-13 (encoding Munc13). These results suggest that PKC-1 selectively regulates DCV release from neurons.

Insulin exocytotic mechanism by imaging technique

Insulin is stored in pancreatic beta cell granules, and released biphasically by the exocytotic mechanism induced by nutrient glucose. Insulin exocytosis must be critically regulated to finely control body glucose homeostasis because insulin is the only hormone that can promptly reduce the blood glucose level. Recent advanced techniques in molecular biology and electrophysiology revealed the molecular mechanism of insulin release in the process from glucose entry to increased [Ca(2+)](i). However, the insulin exocytotic process such as translocation, docking and fusion of insulin granules was largely unknown. In order to reveal the molecular mechanism of this process, we utilized a newly innovated imaging technique, TIRF imaging system. Here we review recent results of our studies into docking and fusion of insulin granules analyzed by TIRF system.

Secretory granule biogenesis in sympathoadrenal cells: identification of a granulogenic determinant in the secretory prohormone chromogranin A

Chromogranin A (CgA) may be critical for secretory granule biogenesis in sympathoadrenal cells. We found that silencing the expression of CgA reduced the number of secretory granules in normal sympathoadrenal cells (PC12), and we therefore questioned whether a discrete domain of CgA might promote the formation of a regulated secretory pathway in variant sympathoadrenal cells (A35C) devoid of such a phenotype. The secretory granule-forming activity of a series of human CgA domains labeled with a hemagglutinin epitope, green fluorescent protein, or embryonic alkaline phosphatase was assessed in A35C cells by deconvolution and electron microscopy and by secretagogue-stimulated release assays. Expression of CgA in A35C cells induced the formation of vesicular organelles throughout the cytoplasm, whereas two constitutive secretory pathway markers accumulated in the Golgi complex. The lysosome-associated membrane protein LGP110 did not co-localize with CgA, consistent with non-lysosomal targeting of the granin in A35C cells. Thus, CgA-expressing A35C cells showed electron-dense granules approximately 180-220 nm in diameter, and secretagogue-stimulated exocytosis of CgA from A35C cells suggested that expression of the granin may be sufficient to restore a regulated secretory pathway and thereby rescue the sorting of other secretory proteins. We show that the formation of vesicular structures destined for regulated exocytosis may be mediated by a determinant located within the CgA N-terminal region (CgA-(1-115), with a necessary contribution of CgA-(40-115)), but not the C-terminal region (CgA-(233-439)) of the protein. We propose that CgA promotes the biogenesis of secretory granules by a mechanism involving a granulogenic determinant located within CgA-(40-115) of the mature protein.

Dissecting docking and tethering of secretory vesicles at the target membrane

Secretory vesicles dock at their target in preparation for fusion. Using single-vesicle total internal reflection fluorescence microscopy in chromaffin cells, we show that most approaching vesicles dock only transiently, but that some are captured by at least two different tethering modes, weak and strong. Both vesicle delivery and tethering depend on Munc18-1, a known docking factor. By decreasing the amount of cortical actin by Latrunculin A application, morphological docking can be restored artificially in docking-deficient munc18-1 null cells, but neither strong tethering nor fusion, demonstrating that morphological docking is not sufficient for secretion. Deletion of the t-SNARE and Munc18-1 binding partner syntaxin, but not the v-SNARE synaptobrevin/VAMP, also reduces strong tethering and fusion. We conclude that docking vesicles either undock immediately or are captured by minimal tethering machinery and converted in a munc18-1/syntaxin-dependent, strongly tethered, fusion-competent state.

Analysis of transient behavior in complex trajectories: application to secretory vesicle dynamics

Analysis of trajectories of dynamical biological objects, such as breeding ants or cell organelles, is essential to reveal the interactions they develop with their environments. Many previous works used a global characterization based on parameters calculated for entire trajectories. In cases where transient behavior was detected, this usually concerned only a particular type, such as confinement or directed motion. However, these approaches are not appropriate in situations in which the tracked objects may display many different types of transient motion. We have developed a method to exhaustively analyze different kinds of transient behavior that the tracked objects may exhibit. The method discriminates stalled periods, constrained and directed motions from random dynamics by evaluating the diffusion coefficient, the mean-square displacement curvature, and the trajectory asymmetry along individual trajectories. To detect transient motions of various durations, these parameters are calculated along trajectories using a rolling analysis window whose width is variable. The method was applied to the study of secretory vesicle dynamics in the subplasmalemmal region of human carcinoid BON cells. Analysis of transitions between transient motion periods, combined with plausible assumptions about the origin of each motion type, leads to a model of dynamical subplasmalemmal organization.

Differential properties of GTP- and Ca(2+)-stimulated exocytosis from large dense core vesicles

Many cells utilize a GTP-dependent pathway to trigger exocytosis in addition to Ca(2+)-triggered exocytosis. However, little is known about the mechanism by which GTP triggers exocytosis independent of Ca(2+). We used dual-color evanescent field microscopy to compare the motion and fusion of large dense core vesicles stimulated by either mastoparan (Mas) in Ca(2+)-free conditions or high K(+) in the presence of Ca(2+). We demonstrate that Mas is hardly effective in triggering the fusion of the predocked vesicles but predominantly mobilizes cytosolic vesicles. In contrast, Ca(2+)-dependent exocytosis is largely due to predocked vesicles. Fusion kinetics analysis and carbon-fiber amperometry reveal that Mas induces a brief 'kiss-and-run' fusion and releases only a small amount of the cargo, whereas Ca(2+) stimulates a more persistent opening of the fusion pore and larger release of the contents. Furthermore, we show that Mas-released vesicles require a much shorter time to reach fusion competence once they approach the plasma membrane. Our data suggest the involvement of different mechanisms not only in triggering and fusion but also in the docking and priming process for Ca(2+)- and GTP-dependent exocytosis.

PDF cycling in the dorsal protocerebrum of the Drosophila brain is not necessary for circadian clock function

In Drosophila, the neuropeptide pigment-dispersing factor (PDF) is a likely circadian molecule, secreted by central pacemaker neurons (LNvs). PDF is expressed in both small and large LNvs (sLNvs and lLNvs), and there are striking circadian oscillations of PDF staining intensity in the small cell termini, which require a functional molecular clock. This cycling may be relevant to the proposed role of PDF as a synchronizer of the clock system or as an output signal connecting pacemaker cells to locomotor activity centers. In this study, the authors use a generic neuropeptide fusion protein (atrial natriuretic factor-green fluorescent protein [ANF-GFP]) and show that it can be expressed in the same neurons as PDF itself. Yet, ANF-GFP as well as PDF itself does not manifest any cyclical accumulation in sLNv termini in adult transgenic flies. Surprisingly, the absence of detectable PDF cycling is not accompanied by any detectable behavioral pheno-type, since these transgenic flies have normal morning and evening anticipation in a light-dark cycle (LD) and are fully rhythmic in constant darkness (DD). The molecular clock is also not compromised. The results suggest that robust PDF cycling in sLNv termini plays no more than a minor role in the Drosophila circadian system and is apparently not even necessary for clock output function.

The Slp4-a linker domain controls exocytosis through interaction with Munc18-1.syntaxin-1a complex

Synaptotagmin-like protein 4-a (Slp4-a)/granuphilin-a is specifically localized on dense-core vesicles in certain neuroendocrine cells and negatively controls dense-core vesicle exocytosis through specific interaction with Rab27A. However, the precise molecular mechanism of its inhibitory effect on exocytosis has never been elucidated and is still a matter of controversy. Here we show by deletion and chimeric analyses that the linker domain of Slp4-a interacts with the Munc18-1.syntaxin-1a complex by directly binding to Munc18-1 and that this interaction promotes docking of dense-core vesicles to the plasma membrane in PC12 cells. Despite increasing the number of plasma membrane docked vesicles, expression of Slp4-a strongly inhibited high-KCl-induced dense-core vesicle exocytosis. The inhibitory effect by Slp4-a is absolutely dependent on the linker domain of Slp4-a, because substitution of the linker domain of Slp4-a by that of Slp5 (the closest isoform of Slp4-a that cannot bind the Munc18-1.syntaxin-1a complex) completely abrogated the inhibitory effect. Our findings reveal a novel docking machinery for dense-core vesicle exocytosis: Slp4-a simultaneously interacts with Rab27A and Munc18-1 on the dense-core vesicle and with syntaxin-1a in the plasma membrane.

Ras and its signals diffuse through the cell on randomly moving nanoparticles

Spatiotemporal modulation of Ras signaling from different intracellular compartments requires mechanisms allowing Ras and its signals to navigate across cells. Here, we describe one mechanism by which clusters of palmitoylated H-Ras and N-Ras isoforms but not nonpalmitoylated K-Ras diffuse through the cytoplasm, independently of ATP, on fast, randomly moving, small cytosolic nanoparticles ("rasosomes"). Rasosomes forced to diffuse out of live cells and trapped by Ras antibody beads appear as round structures of 80- to 100-nm diameter. Association of H-Ras with rasosomes requires Ras palmitoylation and the hypervariable sequence (hvr) upstream of the palmitoylated cysteines. H-Ras hvr mutants that fail to interact with rasosomes are biologically inactive. Epidermal growth factor stimulation rapidly increases active H-Ras-GTP and phosphorylated extracellular signal-regulated kinase (ERK) on rasosomes. Similarly, rasosomes carrying H-Ras(G12V) but not H-Ras are loaded with active ERK. Thus, the rasosome represents a hitherto unknown particle that enables Ras signal information to spread rapidly across cells.

Mechanisms of transport and exocytosis of dense-core granules containing tissue plasminogen activator in developing hippocampal neurons

Dense-core granules (DCGs) are organelles found in specialized secretory cells, including neuroendocrine cells and neurons. Neuronal DCGs facilitate many critical processes, including the transport and secretion of proteins involved in learning, and yet their transport and exocytosis are poorly understood. We have used wide-field and total internal reflection fluorescence microscopy, in conjunction with transport theory, to visualize the transport and exocytosis of DCGs containing a tissue plasminogen activator-green fluorescent protein hybrid in cell bodies, neurites, and growth cones of developing hippocampal neurons and to quantify the roles that diffusion, directed motion, and immobility play in these processes. Our results demonstrate that shorter-ranged transport of DCGs near sites of exocytosis in hippocampal neurons and neuroendocrine cells differs markedly. Specifically, the immobile fraction of DCGs within growth cones and near the plasma membrane of hippocampal neurons is small and relatively unaltered by actin disruption, unlike in neuroendocrine cells. Moreover, transport of DCGs in these domains of hippocampal neurons is unusually heterogeneous, being significantly rapid and directed as well as slow and diffusive. Our results also demonstrate that exocytosis is preceded by substantial movement and heterogeneous transport; this movement may facilitate delivery of DCG cargo in hippocampal neurons, given the relatively low abundance of neuronal DCGs. In addition, the extensive mobility of DCGs in hippocampal neurons argues strongly against the hypothesis that cortical actin is a major barrier to membrane-proximal DCGs in these cells. Instead, our results suggest that extended release of DCG cargo from hippocampal neurons arises from heterogeneity in DCG mobility.

Plasmalemmal phosphatidylinositol-4,5-bisphosphate level regulates the releasable vesicle pool size in chromaffin cells

During exocytosis, certain phospholipids may act as regulators of secretion. Here, we used several independent approaches to perturb the phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] level in bovine chromaffin cells to investigate how changes of plasmalemmal PI(4,5)P2 affect secretion. Membrane levels of PI(4,5)P2 were estimated by analyzing images of lawns of plasma membranes labeled with fluorescent probes specific for PI(4,5)P2. The specific PI(4,5)P2 signal was enriched in submicrometer-sized clusters. In parallel patch-clamp experiments on intact cells, we measured the secretion of catecholamines. Overexpression of phosphatidylinositol-4-phosphate-5-kinase I, or infusion of PI(4,5)P2 through the patch pipette, increased the PI(4,5)P2 level in the plasma membrane and potentiated secretion. Expression of a membrane-targeted inositol 5-phosphatase domain of synaptojanin 1 eliminated PI(4,5)P2 from the membrane and abolished secretion. An inhibitor of phosphatidylinositol-3 kinase, 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one, led to a transient increase in the PI(4,5)P2 level that was associated with a potentiation of secretion. After prolonged incubation, the level of PI(4,5)P2 decreased and secretion was inhibited. Kinetic analysis showed that changes in PI(4,5)P2 levels led to correlated changes in the size of two releasable vesicle pools, whereas their fusion kinetics remained unaffected. We conclude that during both short- and long-term manipulations of PI(4,5)P2 level secretion scales with plasma membrane PI(4,5)P2 content and that PI(4,5)P2 has an early effect on secretion by regulating the number of vesicles ready for release.

Differential vesicular targeting and time course of synaptic secretion of the mammalian neurotrophins

Neurotrophins are a family of secreted neuronal survival and plasticity factors comprising NGF, BDNF, neurotrophin-3 (NT-3), and NT-4. Whereas synaptic secretion of BDNF has been described, the routes of intracellular targeting and secretion of NGF, NT-3, and NT-4 in neurons are poorly understood. To allow for a direct comparison of intracellular targeting and release properties, all four mammalian neurotrophins were expressed as green fluorescent protein fusion proteins in cultured rat hippocampal neurons. We show that BDNF and NT-3 are targeted more efficiently to dendritic secretory granules of the regulated pathway of secretion (BDNF, in 98% of cells; NT-3, 85%) than NGF (46%) and NT-4 (23%). For all NTs, the remaining cells showed targeting to the constitutive secretory pathway. Fusing the BDNF pre-pro sequence to NT-4 directed NT-4 more efficiently to the regulated pathway of secretion. All neurotrophins, once directed to the regulated secretion pathway, were detected near synapsin I-positive presynaptic terminals and colocalized with PSD-95-DsRed (postsynaptic density-95-Discosoma red), suggesting postsynaptic targeting of the neurotrophins to glutamatergic synapses. Depolarization-induced release of all neurotrophins from synaptic secretory granules was slow (delay in onset, 10-30 s; tau = 120-307 s) compared with transmitter release kinetics monitored with FM4-64 [N-(3-triethylammoniumpropyl)-4-(6-(4-diethylamino)phenyl)hexatrienyl)pyridinium dibromide] destaining (onset, <5 s; tau = 13 +/- 2 s). Among the neurotrophins, NT-4 secretion was most rapid but still proceeded 10 times more slowly than transmitter secretion. Preincubation of neurons with monensin (neutralizing intragranular pH, thus solubilizing the peptide core) increased the speed of secretion of BDNF, NGF, and NT-3 to the value of NT-4. These data suggest that peptide core dissolution in secretory granules is the critical determinant of the speed of synaptic secretion of all mammalian NTs and that the speed of release is not compatible with fast transmitter-like actions of neurotrophins.

Imaging the secretory pathway: The past and future impact of live cell optical techniques

Classically, the secretory pathway has been studied using a combination of electron microscopic, biochemical and genetic approaches. In the last 20 years with the arrival of molecular biology and epitope tagging, fluorescence microscopy has become more important than previously. Moreover, with the common availability of Green Fluorescent Protein (GFP) and confocal microscopes in the last 10 years, live cell imaging has become a major experimental approach. This review highlights the impact of the recent introduction of single-cell quantitative time-lapse imaging and photobleach techniques on the study of the secretory pathway, and the potential impact of those optical techniques which may play a significant future role in the study of the Golgi apparatus and the secretory pathway. Particular attention is paid to techniques (Fluorescence Resonance Energy Transfer, Fluorescence Correlation Spectroscopy) which can monitor protein-protein interactions in living cells.

EGFP-tagged vasopressin precursor protein sorting into large dense core vesicles and secretion from PC12 cells

1. Hypothalamic magnocellular neurons synthesize, store, and secrete large quantities of the neuropeptides, vasopressin (VP) and oxytocin (OT), which are synthesized as protein precursors also containing proteins called neurophysins. These protein precursors are sorted through the regulated secretory pathway (RSP), packaged into large dense core vesicles LDCVs, and their peptide products are secreted from nerve terminals in the posterior pituitary. 2. It has been hypothesized that this efficient packaging is dependent on the interaction of the peptide with neurophysin in a complex that forms the granule core. To test this, PC12 cells were transfected with vasopressin precursor DNA constructs that either contained or deleted the neurophysin moiety and tagged with enhanced green fluorescent protein (EGFP) as reporters. The intracellular routing and secretion of the EGFP-tagged VP precursor proteins were studied by in differentiated PC12 cells by fluorescence microscopy, electron microscopic immunocytochemistry, and fluorescent imaging techniques. 3. The data showed that only when the neurophysin was present in the VP precursor construct did the fluorescent fusion protein become routed to the RSP and get efficiently packaged into LDCVs and secreted. These data are consistent with the view that routing of the precursor to LDCVs requires the amino acids that encode the intravesicular chaperone, neurophysin.