FM1-43 dye ultrastructural localization in and release from frog motor nerve terminals

Elevated amyloid beta disrupts the nanoscale organization and function of synaptic vesicle pools in hippocampal neurons

Alzheimer's disease is linked to increased levels of amyloid beta (Aβ) in the brain, but the mechanisms underlying neuronal dysfunction and neurodegeneration remain enigmatic. Here, we investigate whether organizational characteristics of functional presynaptic vesicle pools, key determinants of information transmission in the central nervous system, are targets for elevated Aβ. Using an optical readout method in cultured hippocampal neurons, we show that acute Aβ42 treatment significantly enlarges the fraction of functional vesicles at individual terminals. We observe the same effect in a chronically elevated Aβ transgenic model (APPSw,Ind) using an ultrastructure-function approach that provides detailed information on nanoscale vesicle pool positioning. Strikingly, elevated Aβ is correlated with excessive accumulation of recycled vesicles near putative endocytic sites, which is consistent with deficits in vesicle retrieval pathways. Using the glutamate reporter, iGluSnFR, we show that there are parallel functional consequences, where ongoing information signaling capacity is constrained. Treatment with levetiracetam, an antiepileptic that dampens synaptic hyperactivity, partially rescues these transmission defects. Our findings implicate organizational and dynamic features of functional vesicle pools as targets in Aβ-driven synaptic impairment, suggesting that interventions to relieve the overloading of vesicle retrieval pathways might have promising therapeutic value.

Membrane Traffic and Ca2+ -Signals in Ciliates

A Paramecium cell has as many types of membrane interactions as mammalian cells, as established with monoclonal antibodies by R. Allen and A. Fok. Since then, we have identified key-players, such as SNARE-proteins, Ca²⁺ -regulating proteins, including Ca²⁺ -channels, Ca²⁺ -pumps, Ca²⁺ -binding proteins of different affinity etc. at the molecular level, probed their function and localized them at the light and electron microscopy level. SNARE-proteins, in conjunction with a synaptotagmin-like Ca²⁺ -sensor protein, mediate membrane fusion. This interaction is additionally regulated by monomeric GTPases whose spectrum in Tetrahymena and Paramecium has been established by A. Turkewitz. As known from mammalian cells, GTPases are activated on membranes in conjunction with lumenal acidification by an H⁺ -ATPase. For these complex molecules we found in Paramecium an unsurpassed number of 17 a-subunit paralogs which connect the polymeric head and basis part, V1 and V0. (This multitude may reflect different local functional requirements.) Together with plasmalemmal Ca²⁺ -influx-channels, locally enriched intracellular InsP₃ -type (InsP₃ R, mainly in osmoregulatory system) and ryanodine receptor-like Ca²⁺ -release channels (ryanodine receptor-like proteins, RyR-LP), this complexity mediates Ca²⁺ signals for most flexible local membrane-to-membrane interactions. As we found, the latter channel types miss a substantial portion of the N-terminal part. Caffeine and 4-chloro-meta-cresol (the agent used to probe mutations of RyRs in man during surgery in malignant insomnia patients) initiate trichocyst exocytosis by activating Ca²⁺ -release channels type CRC-IV in the peripheral part of alveolar sacs. This is superimposed by Ca²⁺ -influx, i.e. a mechanism called "store-operated Ca²⁺ -entry" (SOCE). For the majority of key players, we have mapped paralogs throughout the Paramecium cell, with features in common or at variance in the different organelles participating in vesicle trafficking. Local values of free Ca²⁺ -concentration, [Ca²⁺ ]_i , and their change, e.g. upon exocytosis stimulation, have been registered by flurochromes and chelator effects. In parallel we have registered release of Ca²⁺ from alveolar sacs by quenched-flow analysis combined with cryofixation and x-ray microanalysis.

Cnr2 Is Important for Ribbon Synapse Maturation and Function in Hair Cells and Photoreceptors

The role of the cannabinoid receptor 2 (CNR2) is still poorly described in sensory epithelia. We found strong cnr2 expression in hair cells (HCs) of the inner ear and the lateral line (LL), a superficial sensory structure in fish. Next, we demonstrated that sensory synapses in HCs were severely perturbed in larvae lacking cnr2. Appearance and distribution of presynaptic ribbons and calcium channels (Ca_v1.3) were profoundly altered in mutant animals. Clustering of membrane-associated guanylate kinase (MAGUK) in post-synaptic densities (PSDs) was also heavily affected, suggesting a role for cnr2 for maintaining the sensory synapse. Furthermore, vesicular trafficking in HCs was strongly perturbed suggesting a retrograde action of the endocannabinoid system (ECs) via cnr2 that was modulating HC mechanotransduction. We found similar perturbations in retinal ribbon synapses. Finally, we showed that larval swimming behaviors after sound and light stimulations were significantly different in mutant animals. Thus, we propose that cnr2 is critical for the processing of sensory information in the developing larva.

Current molecular approaches to investigate pre-synaptic dysfunction

Over the course of the last few decades it has become clear that many neurodevelopmental and neurodegenerative disorders have a synaptic defect, which contributes to pathogenicity. A rise in new techniques, and in particular '-omics'-based methods providing large datasets, has led to an increase in potential proteins and pathways implicated in synaptic function and related disorders. Additionally, advancements in imaging techniques have led to the recent discovery of alternative modes of synaptic vesicle recycling. This has resulted in a lack of clarity over the precise role of different pathways in maintaining synaptic function and whether these new pathways are dysfunctional in neurodevelopmental and neurodegenerative disorders. A greater understanding of the molecular detail of pre-synaptic function in health and disease is key to targeting new proteins and pathways for novel treatments and the variety of new techniques currently available provides an ideal opportunity to investigate these functions. This review focuses on techniques to interrogate pre-synaptic function, concentrating mainly on synaptic vesicle recycling. It further examines techniques to determine the underlying molecular mechanism of pre-synaptic dysfunction and discusses methods to identify molecular targets, along with protein-protein interactions and cellular localization. In combination, these techniques will provide an expanding and more complete picture of pre-synaptic function. With the application of recent technological advances, we are able to resolve events with higher spatial and temporal resolution, leading research towards a greater understanding of dysfunction at the presynapse and the role it plays in pathogenicity.

Nanoscale Remodeling of Functional Synaptic Vesicle Pools in Hebbian Plasticity

Vesicle pool properties are known determinants of synaptic efficacy, but their potential role as modifiable substrates in forms of Hebbian plasticity is still unclear. Here, we investigate this using a nanoscale readout of functionally recycled vesicles in natively wired hippocampal CA3→CA1 circuits undergoing long-term potentiation (LTP). We show that the total recycled vesicle pool is larger after plasticity induction, with the smallest terminals exhibiting the greatest relative expansion. Changes in the spatial organization of vesicles accompany potentiation including a specific increase in the number of recycled vesicles at the active zone, consistent with an ultrastructural remodeling component of synaptic strengthening. The cAMP-PKA pathway activator, forskolin, selectively mimics some features of LTP-driven changes, suggesting that distinct and independent modules of regulation accompany plasticity expression. Our findings provide evidence for a presynaptic locus of LTP encoded in the number and arrangement of functionally recycled vesicles, with relevance for models of long-term plasticity storage.

Experimental toolbox for quantitative evaluation of clathrin-mediated endocytosis in the plant model Arabidopsis

Clathrin-mediated endocytosis (CME) is a crucial cellular process implicated in many aspects of plant growth, development, intra- and intercellular signaling, nutrient uptake and pathogen defense. Despite these significant roles, little is known about the precise molecular details of how CME functions in planta To facilitate the direct quantitative study of plant CME, we review current routinely used methods and present refined, standardized quantitative imaging protocols that allow the detailed characterization of CME at multiple scales in plant tissues. These protocols include: (1) an efficient electron microscopy protocol for the imaging of Arabidopsis CME vesicles in situ, thus providing a method for the detailed characterization of the ultrastructure of clathrin-coated vesicles; (2) a detailed protocol and analysis for quantitative live-cell fluorescence microscopy to precisely examine the temporal interplay of endocytosis components during single CME events; (3) a semi-automated analysis to allow the quantitative characterization of global internalization of cargos in whole plant tissues; and (4) an overview and validation of useful genetic and pharmacological tools to interrogate the molecular mechanisms and function of CME in intact plant samples.This article has an associated First Person interview with the first author of the paper.

Time and exposure to serotonin affect releasability of recycled vesicles at crayfish claw opener muscle synapses

The crayfish claw opener neuromuscular junction is a biological model for studying presynaptic neuromodulation by serotonin (5HT) and synaptic vesicle recycling. It has been hypothesized that 5HT enhances release by recruiting a population of either previously nonrecycling or "reluctant" vesicles to increase the readily releasable pool. To determine if 5HT activates a distinct population of synaptic vesicles, recycling membranes were labeled with the membrane dye, FM1-43. Unloading (destaining) protocols could not resolve a population of vesicles that were only releasable in the presence of 5HT. Instead, we conclude synaptic vesicles change behavior in axon terminals independent of 5HT, becoming less likely to exocytose and unload dye over periods of >1 hr after recycling. We hypothesized this to be due to the slow conversion of a portion of recycled vesicles to a difficult to release state. The possibility that vesicles in these pools were spatially separated within the terminal was tested using photoconversion of FM1-43 and transmission electron microscopy. The location of FM1-43-labeled vesicles fixed 2 min following 3 min of 20-Hz stimulation did not reveal preferential localization of recycling vesicles specifically near release sites and the distribution of labeled vesicles was not significantly different between early (2 min) and late (180 min) time points. Terminals fixed 30 s following stimulation contained a significant proportion of vesicular structures equivalent in diameter to 2-5 regular vesicles, with multivesicular bodies and calveoli rarely seen, suggesting that endocytosis during sustained release at crayfish terminals occurs via multiple routes, most commonly through large "vesicle" intermediates.

Photoconversion of FM1-43 Reveals Differences in Synaptic Vesicle Recycling and Sensitivity to Pharmacological Disruption of Actin Dynamics in Individual Synapses

The cycling of synaptic vesicles ensures that neurons can communicate adequately through their synapses on repeated occasions when activity is sustained, and several steps in this cycle are modulated by actin. The effects of pharmacological stabilization of actin with jasplakinolide or its depolymerization with latrunculin A was assessed on the synaptic vesicle cycle at individual boutons of cerebellar granule cells, using FM1-43 imaging to track vesicle recycling and its photoconversion to specifically label recycled organelles. Remarkable differences in the recycling capacity of individual boutons are evident, and their dependence on the actin cytoskeleton for recycling is clear. Disrupting actin dynamics causes a loss of functional boutons, and while this indicates that exo/endocytotic cycling in boutons is fully dependent on such events, this dependence is only partial in other boutons. Indeed, exocytosis and vesicle trafficking are impaired significantly by stabilizing or depolymerizing actin, whereas repositioning recycled vesicles at the active zone seems to be dependent on actin polymerization alone. These findings support the hypothesis that different steps of synaptic vesicle cycling depend on actin dynamics and that such dependence varies among individual boutons.

Altered Synaptic Vesicle Release and Ca2+ Influx at Single Presynaptic Terminals of Cortical Neurons in a Knock-in Mouse Model of Huntington's Disease

Huntington's disease (HD) is an inherited neurodegenerative disorder caused by the abnormal expansion of CAG repeats in the huntingtin (HTT) gene, which leads to progressive loss of neurons starting in the striatum and cortex. One possible mechanism for this selective loss of neurons in the early stage of HD is altered neurotransmission at synapses. Despite the recent finding that presynaptic terminals play an important role in HD, neurotransmitter release at synapses in HD remains poorly understood. Here, we measured synaptic vesicle release in real time at single presynaptic terminals during electrical field stimulation. We found the increase in synaptic vesicle release at presynaptic terminals in primary cortical neurons in a knock-in mouse model of HD (zQ175). We also found the increase in Ca²⁺ influx at presynaptic terminals in HD neurons during the electrical stimulation. Consistent with increased Ca²⁺-dependent neurotransmission in HD neurons, the increase in vesicle release and Ca²⁺ influx was rescued with Ca²⁺ chelators or by blocking N-type voltage-gated Ca²⁺ channels, suggesting N-type voltage-gated Ca²⁺ channels play an important role in HD. Taken together, our results suggest that the increased synaptic vesicles release due to increased Ca²⁺ influx at presynaptic terminals in cortical neurons contributes to the selective neurodegeneration of these neurons in early HD and provide a possible therapeutic target.

Label-free imaging of neurotransmitters in live brain tissue by multi-photon ultraviolet microscopy

Visualizing small biomolecules in living cells remains a difficult challenge. Neurotransmitters provide one of the most frustrating examples of this difficulty, as our understanding of signaling in the brain critically depends on our ability to follow the neurotransmitter traffic. Last two decades have seen considerable progress in probing some of the neurotransmitters, e.g. by using false neurotransmitter mimics, chemical labeling techniques, or direct fluorescence imaging. Direct imaging harnesses the weak UV fluorescence of monoamines, which are some of the most important neurotransmitters controlling mood, memory, appetite, and learning. Here we describe the progress in imaging of these molecules using the least toxic direct excitation route found so far, namely multi-photon (MP) imaging. MP imaging of serotonin, and more recently that of dopamine, has allowed researchers to determine the location of the vesicles, follow their intracellular dynamics, probe their content, and monitor their release. Recent developments have even allowed ratiometric quantitation of the vesicular content. This review shows that MP ultraviolet (MP-UV) microscopy is an effective but underutilized method for imaging monoamine neurotransmitters in neurones and brain tissue.

The mechanism of botulinum A on Raynaud syndrome

Background

Botulinum neurotoxin type A (BoNT/A) is emerging as a treatment modality for Raynaud's phenomenon (RP). However, the mechanism of the role of BoNT/A in antagonizing the constriction of arteriola in RP remains unclear.

Materials and methods

We tested the constriction of arteriole diameter and the distribution of adrenergic receptors on the rat cremaster modle. Moreover, we measured the secretion of norepinephrine (NE), protein level changes and related receptors on cultured rat superior cervical ganglia neurons(SCGNs), a model of sympathetic neuron.

Results

Based on our results, the inhibition of arteriole vasoconstriction was increased with increasing doses of BoNT/A. BoNT/A, prazosin, and BQ123 treatment can result in significant inhibition of arteriole vasoconstriction with the same electrical stimulation. The inhibition effect of prazosin was equivalent to BoNT/A, while BQ123 has a synergistic effect with BoNT/A. After treating SCGNs using BoNT/A for 30 min, the decrease in fluorescence intensity of FM1-43 slowed down which was correlated with the doses of BoNT/A. Furthermore, release of NE in the supernatant was significantly decreased as measured by enzyme-linked immunosorbent assay, 24 h after a high dose of BoNT/A (25 µ/mL). Cleaved-SNAP-25 was detected by Western blotting 24 h following BoNT/A (50 µ/mL) treatment. Moreover, receptor SV2C, GM1, and FGFR3 were detected on sympathetic neurons, similarly to cholinergic neurons.

Conclusion

Our study showed that BoNT/A could significantly inhibit electrical stimulation-induced arteriole vasoconstriction through the sympathetic pathway. The mechanism was similar to the cholinergic one, in which the vesicle release of sympathetic neurons could be inhibited by cleavage of SNAP-25. The end result was blocked vesicle fusion with the presynaptic membrane after BoNT/A treatment, inhibiting the release of the NE.

Exosomes taken up by neurons hijack the endosomal pathway to spread to interconnected neurons

In Alzheimer disease and related disorders, the microtubule-associated protein tau aggregates and forms cytoplasmic lesions that impair neuronal physiology at many levels. In addition to affecting the host neuron, tau aggregates also spread to neighboring, recipient cells where the misfolded tau aggregates, in a manner similar to prions, actively corrupt the proper folding of soluble tau, and thereby impair cellular functions. One vehicle for the transmission of tau aggregates are secretory nanovesicles known as exosomes. Here, we established a simple model of a neuronal circuit using a microfluidics culture system in which hippocampal neurons A and B were seeded into chambers 1 and 2, respectively, extending axons via microgrooves in both directions and thereby interconnecting. This system served to establish two models to track exosome spreading. In the first model, we labeled the exosomal membrane by coupling tetraspanin CD9 with either a green or red fluorescent tag. This allowed us to reveal that interconnected neurons exchange exosomes only when their axons extend in close proximity. In the second model, we added exosomes isolated from the brains of tau transgenic rTg4510 mice (i.e. exogenous, neuron A-derived) to neurons in chamber 1 (neuron B) interconnected with neuron C in chamber 2. This allowed us to demonstrate that a substantial fraction of the exogenous exosomes were internalized by neuron B and passed then on to neuron C. This transportation from neuron B to C was achieved by a mechanism that is consistent with the hijacking of secretory endosomes by the exogenous exosomes, as revealed by confocal, super-resolution and electron microscopy. Together, these findings suggest that fusion events involving the endogenous endosomal secretory machinery increase the pathogenic potential and the radius of action of pathogenic cargoes carried by exogenous exosomes.

Novel pH-Sensitive Lipid Based Exo-Endocytosis Tracers Reveal Fast Intermixing of Synaptic Vesicle Pools

Styryl dyes and genetically encoded pH-sensitive fluorescent proteins like pHluorin are well-established tools for the optical analysis of synaptic vesicle (SV) recycling at presynaptic boutons. Here, we describe the development of a new class of fluorescent probes based on pH-sensitive organic dyes covalently bound to lipids, providing a promising complementary assay to genetically encoded fluorescent probes. These new optical tracers allow a pure read out of membrane turnover during synaptic activity and visualization of multiple rounds of stimulation-dependent SV recycling without genetic perturbation. Measuring the incorporation efficacy of different dye-labeled lipids into budding SVs, we did not observe an enrichment of lipids with affinity for liquid ordered membrane domains. But most importantly, we found no evidence for a static segregation of SVs into recycling and resting pools. A small but significant fraction of SVs that is reluctant to release during a first round of evoked activity can be exocytosed during a second bout of stimulation, showing fast intermixing of SV pools within seconds. Furthermore, we found that SVs recycling spontaneously have a higher chance to re-occupy release sites than SVs recycling during high-frequency evoked activity. In summary, our data provide strong evidence for a highly dynamic and use-dependent control of the fractions of releasable or resting SVs.

Using FM Dyes to Monitor Clathrin-Mediated Endocytosis in Primary Neuronal Culture

This protocol utilizes lipophilic FM dyes to monitor membrane recycling in real time. FM dyes are virtually nonfluorescent in solution but when membrane bound are intensely fluorescent, combined with the flexibility of different emission wavelengths make these dyes an excellent choice for investigating clathrin-mediated endocytosis, among other membrane trafficking and recycling pathways.

Probes for monitoring regulated exocytosis

Regulated secretion is a fundamental cellular process that serves diverse functions in neurobiology, endocrinology, immunology, and numerous other aspects of animal physiology. In response to environmental or biological cues, cells release contents of secretory granules into an extracellular medium to communicate with or impact neighboring or distant cells through paracrine or endocrine signaling. To investigate mechanisms governing stimulus-secretion coupling, to better understand how cells maintain or regulate their secretory activity, and to characterize secretion defects in human diseases, probes for tracking various exocytotic events at the cellular or sub-cellular level have been developed over the years. This review summarizes different strategies and recent progress in developing optical probes for monitoring regulated secretion in mammalian cells.

Implant-derived magnesium induces local neuronal production of CGRP to improve bone-fracture healing in rats

Orthopedic implants containing biodegradable magnesium have been used for fracture repair with considerable efficacy; however, the underlying mechanisms by which these implants improve fracture healing remain elusive. Here we show the formation of abundant new bone at peripheral cortical sites after intramedullary implantation of a pin containing ultrapure magnesium into the intact distal femur in rats. This response was accompanied by substantial increases of neuronal calcitonin gene-related polypeptide-α (CGRP) in both the peripheral cortex of the femur and the ipsilateral dorsal root ganglia (DRG). Surgical removal of the periosteum, capsaicin denervation of sensory nerves or knockdown in vivo of the CGRP-receptor-encoding genes Calcrl or Ramp1 substantially reversed the magnesium-induced osteogenesis that we observed in this model. Overexpression of these genes, however, enhanced magnesium-induced osteogenesis. We further found that an elevation of extracellular magnesium induces magnesium transporter 1 (MAGT1)-dependent and transient receptor potential cation channel, subfamily M, member 7 (TRPM7)-dependent magnesium entry, as well as an increase in intracellular adenosine triphosphate (ATP) and the accumulation of terminal synaptic vesicles in isolated rat DRG neurons. In isolated rat periosteum-derived stem cells, CGRP induces CALCRL- and RAMP1-dependent activation of cAMP-responsive element binding protein 1 (CREB1) and SP7 (also known as osterix), and thus enhances osteogenic differentiation of these stem cells. Furthermore, we have developed an innovative, magnesium-containing intramedullary nail that facilitates femur fracture repair in rats with ovariectomy-induced osteoporosis. Taken together, these findings reveal a previously undefined role of magnesium in promoting CGRP-mediated osteogenic differentiation, which suggests the therapeutic potential of this ion in orthopedics.

Functional Nanoscale Imaging of Synaptic Vesicle Cycling with Superfast Fixation

Functional imaging is the measurement of structural changes during an ongoing physiological process over time. In many cases, functional imaging has been implemented by tracking a fluorescent signal in live imaging sessions. Electron microscopy, however, excludes live imaging which has hampered functional imaging approaches on the ultrastructural level. This barrier was broken with the introduction of superfast fixation. Superfast fixation is capable of stopping and fixing membrane traffic at sufficient speed to capture a physiological process at a distinct functional state. Applying superfast fixation at sequential time points allows tracking of membrane traffic in a step-by-step fashion.This technique has been applied to track labeled endocytic vesicles at central synapses as they pass through the synaptic vesicle cycle. At synapses, neurotransmitter is released from synaptic vesicles (SVs) via fast activity-dependent exocytosis. Exocytosis is coupled to fast endocytosis that retrieves SVs components from the plasma membrane shortly after release. Fluorescent FM dyes that bind to the outer leaflet of the plasma membrane enter the endocytic vesicle during membrane retrieval and remain trapped in endocytic vesicles have been widely used to study SV exo-endocytic cycling in live imaging sessions. FM dyes can also be photoconverted into an electron-dense diaminobenzidine polymer which allows the investigation of SV cycling in the electron microscope. The combination of FM labeling with superfast fixation made it possible to track the fine structure of endocytic vesicles at 1 s intervals. Because this combination is not specialized to SV cycling, many other cellular processes can be studied. Furthermore, the technique is easy to set up and cost effective.This chapter describes activity-dependent FM dye labeling of SVs in cultured hippocampal neurons, superfast microwave-assisted fixation, photoconversion of the fluorescent endocytic vesicles, and the analysis of individual synapses after serial section 3D reconstruction of individual synapses from electron micrographs.

The Membrane Marker mCLING Reveals the Molecular Composition of Trafficking Organelles

mCLING is a fixable endocytosis marker that can be combined with immunolabeling techniques to study the molecular composition of trafficking organelles. mCLING can be used both in cultured cells and in tissue if critical sample preparation steps, such as fixation, are correctly performed. This unit describes protocols for the application of mCLING and for the subsequent sample processing. We include immunostaining protocols and embedding procedures for confocal and high-resolution microscopy.

Ultrastructural and functional fate of recycled vesicles in hippocampal synapses

Efficient recycling of synaptic vesicles is thought to be critical for sustained information transfer at central terminals. However, the specific contribution that retrieved vesicles make to future transmission events remains unclear. Here we exploit fluorescence and time-stamped electron microscopy to track the functional and positional fate of vesicles endocytosed after readily releasable pool (RRP) stimulation in rat hippocampal synapses. We show that most vesicles are recovered near the active zone but subsequently take up random positions in the cluster, without preferential bias for future use. These vesicles non-selectively queue, advancing towards the release site with further stimulation in an actin-dependent manner. Nonetheless, the small subset of vesicles retrieved recently in the stimulus train persist nearer the active zone and exhibit more privileged use in the next RRP. Our findings reveal heterogeneity in vesicle fate based on nanoscale position and timing rules, providing new insights into the origins of future pool constitution.

Acquiring snapshots of the orientation of trans-membrane protein domains using a hybrid FRET pair

One challenge in studying the function of membrane-embedded proteins is determining the orientation of key domains in the context of the changing and dynamic membrane environment. We describe a confocal microscopy setup that utilizes external electric field pulses to direct dipicrylamine (DPA) to a membrane leaflet. The detection of FRET between DPA and a fluorescent probe attributes it to the inner or outer leaflet of a membrane. By utilizing short acquisition times and confocal imaging, this attribution could be made even in changing membrane environments. Our setup adds versatility to the study of the biological activity of membrane-embedded proteins.

Examination of synaptic vesicle recycling using FM dyes during evoked, spontaneous, and miniature synaptic activities

Synaptic vesicles in functional nerve terminals undergo exocytosis and endocytosis. This synaptic vesicle recycling can be effectively analyzed using styryl FM dyes, which reveal membrane turnover. Conventional protocols for the use of FM dyes were designed for analyzing neurons following stimulated (evoked) synaptic activity. Recently, protocols have become available for analyzing the FM signals that accompany weaker synaptic activities, such as spontaneous or miniature synaptic events. Analysis of these small changes in FM signals requires that the imaging system is sufficiently sensitive to detect small changes in intensity, yet that artifactual changes of large amplitude are suppressed. Here we describe a protocol that can be applied to evoked, spontaneous, and miniature synaptic activities, and use cultured hippocampal neurons as an example. This protocol also incorporates a means of assessing the rate of photobleaching of FM dyes, as this is a significant source of artifacts when imaging small changes in intensity.

FM dye photo-oxidation as a tool for monitoring membrane recycling in inner hair cells

Styryl (FM) dyes have been used for more than two decades to investigate exo- and endocytosis in conventional synapses. However, they are difficult to use in the inner hair cells of the auditory pathway (IHCs), as FM dyes appear to penetrate through mechanotransducer channels into the cytosol of IHCs, masking endocytotic uptake. To solve this problem we applied to IHCs the FM dye photo-oxidation technique, which renders the dyes into electron microscopy markers. Photo-oxidation allowed the unambiguous identification of labeled organelles, despite the presence of FM dye in the cytosol. This enabled us to describe the morphologies of several organelles that take up membrane in IHCs, both at rest and during stimulation. At rest, endosome-like organelles were detected in the region of the cuticular plate. Larger tubulo-cisternal organelles dominated the top and nuclear regions. Finally, the basal region, where the IHC active zones are located, contained few labeled organelles. Stimulation increased significantly membrane trafficking in the basal region, inducing the appearance of labeled vesicles and cistern-like organelles. The latter were replaced by small, synaptic-like vesicles during recovery after stimulation. In contrast, no changes in membrane trafficking were induced by stimulation in the cuticular plate region or in the top and nuclear regions. We conclude that synaptic vesicle recycling takes place mostly in the basal region of the IHCs. Other organelles participate in abundant constitutive membrane trafficking throughout the rest of the IHC volume.

Visualizing presynaptic function

Synaptic communication in the nervous system is initiated by the fusion of synaptic vesicles with the presynaptic plasma membrane and subsequent neurotransmitter release. In the 1980s, this process was characterized by electron microscopy, albeit without the ability to follow processes in living cells. In the last two decades, fluorescence imaging methods have been developed that report synaptic vesicle fusion, endocytosis and recycling. These probes have provided unprecedented insight into synaptic vesicle trafficking in individual synaptic terminals and revealed heterogeneity in recycling pathways as well as synaptic vesicle populations. These methods either take advantage of uptake of fluorescent probes into recycling vesicles or exogenous expression of synaptic vesicle proteins tagged with a pH-sensitive fluorescent marker at regions facing the vesicle lumen. We provide an overview of these methods, with particular emphasis on the challenges associated with their use and the opportunities for future investigations.

Membrane potential measurements of isolated neurons using a voltage-sensitive dye

The ability to monitor changes in membrane potential is a useful tool for studying neuronal function, but there are only limited options available at present. Here, we have investigated the potential of a commercially available FLIPR membrane potential (FMP) dye, developed originally for high throughput screening using a plate reader, for imaging the membrane potential of cultured cells using an epifluorescence-based single cell imaging system. We found that the properties of the FMP dye make it highly suitable for such imaging since 1) its fluorescence displayed a high signal-to-noise ratio, 2) robust signals meant only minimal exposure times of around 5 ms were necessary, and 3) bidirectional changes in fluorescence were detectable resulting from hyper- or depolarising conditions, reaching equilibrium with a time constant of 4–8 s. Measurements were possible independently of whether membrane potential changes were induced by voltage clamping, or manipulating the ionic distribution of either Na⁺ or K⁺. Since FMP behaves as a charged molecule which accumulates in the cytosol, equations based on the Boltzmann distribution were developed determining that the apparent charge of FMP which represents a measure of the voltage sensitivity of the dye, is between −0.62 and −0.72. Finally, we demonstrated that FMP is suitable for use in a variety of neuronal cell types and detects membrane potential changes arising from spontaneous firing of action potentials and through stimulation with a variety of excitatory and inhibitory neurotransmitters.

The pH probe CypHer™5E is effectively quenched by FM dyes

Concurrent imaging of spectrally distinct fluorescence probes has become an important method for live-cell microscopy experiments in many biological disciplines. The technique enables the identification of a multitude of causal relationships. However, interactions between fluorescent dyes beyond an obvious overlap of their fluorescent spectra are often neglected. Here we present the effects of the well-established fluorescent dyes FM®2-10 or FM®1-43 on the recently introduced pH-dependent probe CypHer™5E. Spectrophotometry as well as live-cell fluorescence microscopy revealed that both FM dyes are effective quenchers of CypHer™5E. Control experiments indicated that this effect is reversible and not due to bleaching. We conclude that, in general, parallel measurements of both dyes are possible, with low FM dye concentrations. Nevertheless, our results implicate that special care has to be taken in such dual colour experiments especially when analysing dynamic CypHer™5E signals in live-cell microscopy.

A preferentially segregated recycling vesicle pool of limited size supports neurotransmission in native central synapses

At small central synapses, efficient turnover of vesicles is crucial for stimulus-driven transmission, but how the structure of this recycling pool relates to its functional role remains unclear. Here we characterize the organizational principles of functional vesicles at native hippocampal synapses with nanoscale resolution using fluorescent dye labeling and electron microscopy. We show that the recycling pool broadly scales with the magnitude of the total vesicle pool, but its average size is small (∼45 vesicles), highly variable, and regulated by CDK5/calcineurin activity. Spatial analysis demonstrates that recycling vesicles are preferentially arranged near the active zone and this segregation is abolished by actin stabilization, slowing the rate of activity-driven exocytosis. Our approach reveals a similarly biased recycling pool distribution at synapses in visual cortex activated by sensory stimulation in vivo. We suggest that in small native central synapses, efficient release of a limited pool of vesicles relies on their favored spatial positioning within the terminal.

Actin- and dynamin-dependent maturation of bulk endocytosis restores neurotransmission following synaptic depletion

Bulk endocytosis contributes to the maintenance of neurotransmission at the amphibian neuromuscular junction by regenerating synaptic vesicles. How nerve terminals internalize adequate portions of the presynaptic membrane when bulk endocytosis is initiated before the end of a sustained stimulation is unknown. A maturation process, occurring at the end of the stimulation, is hypothesised to precisely restore the pools of synaptic vesicles. Using confocal time-lapse microscopy of FM1-43-labeled nerve terminals at the amphibian neuromuscular junction, we confirm that bulk endocytosis is initiated during a sustained tetanic stimulation and reveal that shortly after the end of the stimulation, nerve terminals undergo a maturation process. This includes a transient bulging of the plasma membrane, followed by the development of large intraterminal FM1-43-positive donut-like structures comprising large bulk membrane cisternae surrounded by recycling vesicles. The degree of bulging increased with stimulation frequency and the plasmalemma surface retrieved following the transient bulging correlated with the surface membrane internalized in bulk cisternae and recycling vesicles. Dyngo-4a, a potent dynamin inhibitor, did not block the initiation, but prevented the maturation of bulk endocytosis. In contrast, cytochalasin D, an inhibitor of actin polymerization, hindered both the initiation and maturation processes. Both inhibitors hampered the functional recovery of neurotransmission after synaptic depletion. Our data confirm that initiation of bulk endocytosis occurs during stimulation and demonstrates that a delayed maturation process controlled by actin and dynamin underpins the coupling between exocytosis and bulk endocytosis.

Induction of adipose-derived stem cell into motoneuron-like cells using selegiline as preinducer

Cell therapy is one of the approaches taken to treatment of spinal cord disorders. In this study, adipose-derived stem cells (ADSCs) were induced to form motoneuron-like cells (MNLCs) using selegiline as preinducer, as well as Shh and all trans-retinoic acid (RA) as inducers. Selegiline was reported to induce the embryonic stem cells and bone marrow stromal cells into neuronal phenotype. ADSCs were evaluated using CD90, CD44, CD 49d, CD106, CD31, CD45, lipogenesis and osteogenesis. Dose response and time course studies were used in selecting the optimal concentration for selegiline using the percentage of viable cells (PVC) and percentages of immunoreactive cells (PIC) to nestin and neurofilament 68. Accordingly, such studies were used in selecting the optimal dose for RA using PVC and PIC to islet-1 and oligo-2. The expression of islet-1, oligo-2 and HLXB9 was evaluated using RT-PCR and immunocytochemistry. Real-time PCR was utilized in order to quantify the expression of islet-1, oligo-2 and HLXB9. ADSCs were immunoreactive to CD90, CD44 and CD 49d with consistent differentiation osteogenic and lipogenic cells. The optimal concentrations of selegiline and RA were 10⁻⁹ mM and 2 × 10⁻⁸ M, respectively. After two days, MNLCs showed high oligo-2 expression. MNLCs innervated myotubes; also, the release rate of synaptic vesicles using FM1-43 followed exponential decay model, and this rate in the induced MNLCs was approximately three times of that in the preinduced cells.

Imaging synaptic vesicle recycling by staining and destaining vesicles with FM dyes

The synaptic vesicle is the essential organelle of the synapse. Many approaches for studying synaptic vesicle recycling have been devised, one of which, the styryl (FM) dye, is well suited for this purpose. FM dyes reversibly stain, but do not permeate, membranes; hence they can specifically label membrane-bound organelles. Their quantum yield is drastically higher when bound to membranes than when in aqueous solution. This protocol describes the imaging of synaptic vesicle recycling by staining and destaining vesicles with FM dyes. Nerve terminals are stimulated (electrically or by depolarization with high K(+)) in the presence of dye, their vesicles are then allowed to recycle, and finally dye is washed from the chamber. In neuromuscular junction (NMJ) preparations, movements of the muscle must be inhibited if imaging during stimulation is desired (e.g., by application of curare, a potent acetylcholine receptor inhibitor). The main characteristics of FM dyes are also reviewed here, as are recent FM dye monitoring techniques that have been used to investigate the kinetics of synaptic vesicle fusion.

Colocalization of FM1-43, Bassoon, and GnRH-1: GnRH-1 release from cell bodies and their neuroprocesses

Pulsatile release of GnRH-1 is critical for reproductive function. However, the cellular mechanism of GnRH-1 neurosecretion is still elusive. In this study, we examined the neurosecretory process of GnRH-1 neurons using time-lapse image acquisition followed by immunocytochemistry with confocal microscopy. To monitor exocytotic processes, cultured GnRH-1 neurons derived from monkey embryos were labeled with the lipophilic dye, FM1-43, or its fixable form FM1-43Fx, in the presence or absence of depolarization signals, and changes in vesicles labeled with FM1-43 were analyzed. The results show FM1-43 was taken up into the cell and labeled puncta in the soma and neuroprocesses in the absence of depolarization signals, indicating that GnRH-1 neurons were spontaneously active. Depolarization of GnRH-1 neurons with high K+ or veratridine challenge increased the intensity and size of puncta in both soma and neuroprocesses, and the veratridine-induced changes in puncta were blocked by tetrodotoxin, indicating that changes in the puncta intensity and size reflect neurosecretory activity. Subsequent double immunocytochemistry for GnRH-1 and the synaptic vesicle marker, vesicle-associated membrane protein, demonstrated that the FM1-43Fx-labeled puncta were synaptic vesicles with the GnRH-1 peptide. Additional double immunocytochemistry for GnRH-1 and the marker of the neurosecretory active zone, Bassoon, indicated that the FM1-43Fx-labeled puncta were located at the sites of neurosecretory active zones in GnRH-1 neurons. These results suggest that GnRH-1 neurons have the capacity to release the peptide from the soma and dendrites. Collectively, we hypothesize that soma-dendritic release of the peptide may be a mechanism of synchronized activity among GnRH-1 neurons.

A small pool of vesicles maintains synaptic activity in vivo

Chemical synapses contain substantial numbers of neurotransmitter-filled synaptic vesicles, ranging from approximately 100 to many thousands. The vesicles fuse with the plasma membrane to release neurotransmitter and are subsequently reformed and recycled. Stimulation of synapses in vitro generally causes the majority of the synaptic vesicles to release neurotransmitter, leading to the assumption that synapses contain numerous vesicles to sustain transmission during high activity. We tested this assumption by an approach we termed cellular ethology, monitoring vesicle function in behaving animals (10 animal models, nematodes to mammals). Using FM dye photooxidation, pHluorin imaging, and HRP uptake we found that only approximately 1-5% of the vesicles recycled over several hours, in both CNS synapses and neuromuscular junctions. These vesicles recycle repeatedly, intermixing slowly (over hours) with the reserve vesicles. The latter can eventually release when recycling is inhibited in vivo but do not seem to participate under normal activity. Vesicle recycling increased only to ≈ 5% in animals subjected to an extreme stress situation (frog predation on locusts). Synapsin, a molecule binding both vesicles and the cytoskeleton, may be a marker for the reserve vesicles: the proportion of vesicles recycling in vivo increased to 30% in synapsin-null Drosophila. We conclude that synapses do not require numerous reserve vesicles to sustain neurotransmitter release and thus may use them for other purposes, examined in the accompanying paper.

Correlative microscopy: a powerful tool for exploring neurological cells and tissues

Imaging tools for exploring the neurological samples have seen a rapid transformation over the last decade. Approaches that allow clear and specific delineation of targeted tissues, individual neurons, and their cell-cell connections as well as subcellular constituents have been especially valuable. Considering the significant complexity and extent to which the nervous system interacts with every organ system in the body, one non-trivial challenge has been how to identify and target specific structures and pathologies by microscopy. To this end, correlative methods enable one to view the same exact structure of interest utilizing the capabilities of typically separate, but powerful, microscopy platforms. As such, correlative microscopy is well-positioned to address the three critical problems of identification, scale, and resolution inherent to neurological systems. Furthermore, the application of multiple imaging platforms to the study of singular biological events enables more detailed investigations of structure-function relationships to be conducted, greatly facilitating our understanding of relevant phenomenon. This comprehensive review provides an overview of methods for correlative microscopy, including histochemistry, transgenic markers, immunocytochemistry, photo-oxidation as well as various probes and tracers. An emphasis is placed on correlative light and electron microscopic strategies used to facilitate relocation of neurological structures. Correlative microscopy is an invaluable tool for neurological research, and we fully anticipate developments in automation of the process, and the increasing availability of genomic and transgenic tools will facilitate the adoption of correlative microscopy as the method of choice for many imaging experiments.

Systematic heterogeneity of fractional vesicle pool sizes and release rates of hippocampal synapses

Hippocampal neurons in tissue culture develop functional synapses that exhibit considerable variation in synaptic vesicle content (20-350 vesicles). We examined absolute and fractional parameters of synaptic vesicle exocytosis of individual synapses. Their correlation to vesicle content was determined by activity-dependent discharge of FM-styryl dyes. At high frequency stimulation (30 Hz), synapses with large recycling pools released higher amounts of dye, but showed a lower fractional release compared to synapses that contained fewer vesicles. This effect gradually vanished at lower frequencies when stimulation was triggered at 20 Hz and 10 Hz, respectively. Live-cell antibody staining with anti-synaptotagmin-1-cypHer 5, and overexpression of synaptopHluorin as well as photoconversion of FM 1-43 followed by electron microscopy, consolidated the findings obtained with FM-styryl dyes. We found that the readily releasable pool grew with a power function with a coefficient of 2/3, possibly indicating a synaptic volume/surface dependency. This observation could be explained by assigning the rate-limiting factor for vesicle exocytosis at high frequency stimulation to the available active zone surface that is proportionally smaller in synapses with larger volumes.

Using the fluorescent styryl dye FM1-43 to visualize synaptic vesicles exocytosis and endocytosis in motor nerve terminals

The styryl dye FM1-43 is a powerful tool to track exocytosis, endocytosis and recycling of secretory granules or vesicles. Due to its unique structure, dye molecules reversibly partition into the outer leaflet of surface membrane without permeating due to two cationic charges located in their headgroup. When a secretory cell is stimulated to evoke exocytosis, FM1-43 molecules that were inserted in the membrane are internalized during compensatory endocytosis and newly formed secretory granules or vesicles become stained with dye (staining/endocytosis). If stained secretory granules or vesicles undergo exocytosis in dye-free medium, due to concentration gradient, FM1-43 molecules dissociate from the membrane and loose fluorescence (destaining/exocytosis). Using a fluorescence microscope attached to a CCD camera or a confocal, it is possible to follow secretion in live cell or tissue preparations and in this chapter, we will make a description of FM1-43 staining and destaining protocol using the neuromuscular junction as experimental model. This technique has allowed answering important questions concerning synaptic vesicle recycling, which is a key step for neuronal communication. In addition, FM1-43 has proven to be an excellent tool for investigating membrane internalization and endosome recycling in a variety of cell types.

Examining size-strength relationships at hippocampal synapses using an ultrastructural measurement of synaptic release probability

Release probability (p(r)) is a fundamental presynaptic parameter which is critical in defining synaptic strength. Knowledge of how synapses set and regulate their p(r) is a fundamental step in understanding synaptic transmission and communication between neurons. Despite its importance, p(r) is difficult to measure directly at single synapses. One important strategy to achieve this has relied on the application of fluorescence-based imaging methods, but this is always limited by the lack of detailed information on the morphological and structural properties of the individual synapses under study, and thus precludes an investigation of the relationship between p(r) and synaptic anatomy. Here we outline a powerful methodology based on using FM-styryl dyes, photoconversion and correlative ultrastructural analysis in dissociated hippocampal cultured neurons, which provides both a direct readout of p(r) as well as nanoscale detail on synaptic organization and structure. We illustrate the value of this approach by investigating, at the level of individual reconstructed terminals, the relationship between release probability and defined vesicle pools. We show that in our population of synapses, p(r) is highly variable, and while it is positively correlated with the number of vesicles docked at the active zone it shows no relationship with the total number of synaptic vesicles. The lack of a direct correlation between total synaptic size and performance in these terminals suggests that factors other than the absolute magnitude of the synapse are the most important determinants of synaptic efficacy.

Identification of neuromuscular junctions by correlative confocal and transmission electron microscopy

The physiological processes regulating neuromuscular transmission are highly dependent on the structural features of the motor neuron and motor endplate, and detailing the structure of neuromuscular junctions (NMJs) in muscle biopsies is a powerful method for research and diagnostics. The observation of NMJ ultrastructure, however, is complicated by the difficulty in locating NMJs for analysis by electron microscopy. Consequently, a correlative confocal-transmission electron microscopy method was developed. Fixed muscle samples were cryo-protected in sucrose, sectioned on a cryostat, and stained with fluorescent alpha-bungarotoxin for confocal microscopy. Sections containing junctions were mapped and then processed for transmission electron microscopy (TEM). Cryostat sections allowed large expanses of muscle tissue to be rapidly screened and enabled specific junctions to be targeted for TEM. The morphology of the junctions was well preserved with all essential features of the pre- and postsynaptic elements readily identifiable without freeze damage. Unlike NMJ correlative methods using histochemical stains and DAB photo-oxidation, no electron dense precipitate was deposited over the NMJ, enabling an unobstructed view of the pre- and postsynaptic structures.

SNARE-dependent glutamate release in megakaryocytes

Objective

The identification of signaling pathways involved in megakaryocytopoiesis is essential for development of novel therapeutics to treat hematological disorders. Following our previous findings that megakaryocytes express functional channel-forming N-methyl-D-aspartate-type glutamate receptors, here we aimed to determine the glutamate release capacity in undifferentiated and differentiated megakaryocytes and the role of soluble N-ethyl maleimide-sensitive factor attachment protein receptor (SNARE) proteins that are known to be associated with vesicular exocytosis.

Materials and Methods

Using the megakaryocytic cell line MEG-01, primary megakaryocytes, and tissue sections of bone marrow, reverse transcription polymerase chain reaction, Western blot analysis, and immunolocalization were employed to detect factors required for vesicular glutamate release. Vesicle recycling was monitored by acridine orange and FM1-43 staining and glutamate release activity was assessed by an enzyme-linked fluorimetric assay. Genetically modified MEG-01 cells, with deletion or overexpression of SNARE and vesicular proteins, were also examined for glutamate release activity.

Results

We demonstrated that megakaryocytes express numerous proteins required for vesicular glutamate release, including core SNARE proteins, vesicle-associated membrane protein, soluble N-ethyl maleimide-sensitive factor attachment protein−23, and syntaxin, as well as specific glutamate-loading vesicle proteins, VGLUT1 and VGLUT2. Moreover, active vesicle recycling and differentiation-dependent glutamate release were observed in megakaryocytes. Vesicle-associated membrane protein−deficient MEG-01 cells, which are impaired in vesicle recycling, showed a 30% decrease in released glutamate, whereas overexpression of VGLUT1 exhibited up to a 2.2-fold increase in glutamate release.

Conclusion

These data show that glutamate release from megakaryocytes occurs in a SNARE-dependent, exocytotic manner and is increased during differentiation, suggesting that manipulation of glutamate signaling could influence megakaryocytopoiesis and, therefore, offer a suitable target for the treatment of thrombosis and other hematological disorders.

Sustained synaptic-vesicle recycling by bulk endocytosis contributes to the maintenance of high-rate neurotransmitter release stimulated by glycerotoxin

Glycerotoxin (GLTx), a large neurotoxin isolated from the venom of the sea worm Glycera convoluta, promotes a long-lasting increase in spontaneous neurotransmitter release at the peripheral and central synapses by selective activation of Ca(v)2.2 channels. We found that GLTx stimulates the very high frequency, long-lasting (more than 10 hours) spontaneous release of acetylcholine by promoting nerve terminal Ca(2+) oscillations sensitive to the inhibitor omega-conotoxin GVIA at the amphibian neuromuscular junction. Although an estimate of the number of synaptic vesicles undergoing exocytosis largely exceeds the number of vesicles present in the motor nerve terminal, ultrastructural examination of GLTx-treated synapses revealed no significant change in the number of synaptic vesicles. However, we did detect the appearance of large pre-synaptic cisternae suggestive of bulk endocytosis. Using a combination of styryl dyes, photoconversion and horseradish peroxidase (HRP)-labeling electron microscopy, we demonstrate that GLTx upregulates presynaptic-vesicle recycling, which is likely to emanate from the limiting membrane of these large cisternae. Similar synaptic-vesicle recycling through bulk endocytosis also occurs from nerve terminals stimulated by high potassium. Our results suggest that this process might therefore contribute significantly to synaptic recycling under sustained levels of synaptic stimulation.

Studying synaptic vesicle pools using photoconversion of styryl dyes

The fusion of synaptic vesicles with the plasma membrane (exocytosis) is a required step in neurotransmitter release and neuronal communication. The vesicles are then retrieved from the plasma membrane (endocytosis) and grouped together with the general pool of vesicles within the nerve terminal, until they undergo a new exo- and endocytosis cycle (vesicle recycling). These processes have been studied using a variety of techniques such as electron microscopy, electrophysiology recordings, amperometry and capacitance measurements. Importantly, during the last two decades a number of fluorescently labeled markers emerged, allowing optical techniques to track vesicles in their recycling dynamics. One of the most commonly used markers is the styryl or FM dye ¹; structurally, all FM dyes contain a hydrophilic head and a lipophilic tail connected through an aromatic ring and one or more double bonds (Fig. 1B). A classical FM dye experiment to label a pool of vesicles consists in bathing the preparation (Fig. 1Ai) with the dye during the stimulation of the nerve (electrically or with high K⁺). This induces vesicle recycling and the subsequent loading of the dye into recently endocytosed vesicles (Fig. 1A_i-iii). After loading the vesicles with dye, a second round of stimulation in a dye-free bath would trigger the FM release through exocytosis (Fig. 1A_iv-v), process that can be followed by monitoring the fluorescence intensity decrease (destaining).

Although FM dyes have contributed greatly to the field of vesicle recycling, it is not possible to determine the exact localization or morphology of individual vesicles by using conventional fluorescence microscopy. For that reason, we explain here how FM dyes can also be used as endocytic markers using electron microscopy, through photoconversion. The photoconversion technique exploits the property of fluorescent dyes to generate reactive oxygen species under intense illumination. Fluorescently labeled preparations are submerged in a solution containing diaminobenzidine (DAB) and illuminated. Reactive species generated by the dye molecules oxidize the DAB, which forms a stable, insoluble precipitate that has a dark appearance and can be easily distinguished in electron microscopy ^2,3. As DAB is only oxidized in the immediate vicinity of fluorescent molecules (as the reactive oxygen species are short-lived), the technique ensures that only fluorescently labeled structures are going to contain the electron-dense precipitate. The technique thus allows the study of the exact location and morphology of actively recycling organelles.

Monitoring rapid endocytosis in the electron microscope via photoconversion of vesicles fluorescently labeled with FM1-43

Cells communicate via endo- and exocytosis with their environment and neighboring cells. At synapses of the nervous system, fast exocytosis is coupled to fast endocytosis, which forms the basis for neurotransmitter release. The introduction of the unique fluorescent FM dyes allowed the monitoring of fast synaptic vesicle exo-endocytic cycling during live imaging sessions and after photoconversion of FM dyes into an electron-dense diaminobenzidine polymer synaptic vesicle cycling can be studied in the electron microscope. This protocol describes FM dye labeling of synaptic vesicles of cultured hippocampal neurons and photoconversion of the fluorescent synaptic vesicles for analysis in the electron microscope (EM).