Probing synaptic vesicle fusion by altering mechanical properties of the neuronal surface membrane

Mechanical Properties of 3-Hydroxybutyric Acid-Induced Vesicles

The vesicle mechanical behaviors were studied upon its exposure to 3-hydroxybutyric acid using an atomic force microscope (AFM). Dipalmitoylphosphatidylcholine (DPPC) and 3-hydroxybutyric acid were used to manufacture the vesicles at their desired ratio. The deflection of an AFM probe with respect to its displacement was measured after characterizing the vesicle adsorption. The movement was analyzed with the Hertzian model to understand the physical behavior of the vesicles. However, in the deflection just prior to the first penetration, the model was a good fit, and the vesicle mechanical moduli were calculated. The moduli became lower with the higher ratio of 3-hydroxybutyric acid to DPPC, but the moduli were saturated at 0.5 of the ratio. These results appear to be the basis for the function of the metabolism associated with 3-hydroxybutyric acid, i.e., anesthetization and glycemic control, on the physical properties of cell membranes.

Changes in Mechanical Properties of Vesicles by Mucin in Aqueous Solution

The mechanical properties of vesicles were investigated as they were prepared, according to the ratio of mucin to dipalmitoylphosphatidylcholine (DPPC), using an atomic force microscope (AFM). After the confirmation of the vesicle adsorption on a mica surface, an AFM-tip deflection, caused by the interaction between the tip and the vesicle, was measured. The deflection showed that the tip broke through into the vesicle twice. Each break meant a tip-penetration into the upper and lower portion of the vesicle. Only the first penetration allowed the Hertzian model available to estimate the vesicle mechanical moduli. Two moduli reduced as the ratio of mucin to DPPC increased to 0.5, but the moduli were little changed above the 0.5 ratio. These results seem to be a platform for the effect of the mucin on the plasma-membrane anchoring and cellular signaling.

Ectoine Effect on Mechanical Properties of Vesicles in Aqueous Solution

The mechanical properties of the vesicles incorporated with ectoine were studied using atomic force microscope (AFM). The vesicles were prepared with dipalmitoylphosphatidylcholine (DPPC) by changing only the ratio of the ectoine to DPPC. After the vesicles were adsorbed on the mica substrate and their morphology were characterized, the plot of an AFM tip displacement versus the tip deflection was acquired by monitoring the behavior of the tip into the vesicle. The breakthrough of the tip into the vesicle was observed to occur twice. Each breakthrough represented a penetration of the tip into the top and bottom portions of the vesicle, respectively. The force data between the pre-contact and the first breakthrough were comparable with the Hertzian model to estimate Young's modulus and the bending modulus of the vesicles. Both moduli decreased proportionally with the increase in the ratio of ectoine to lipid up to 0.5. However, above 0.5, the moduli were slightly changed with the increase. These results of the mechanical properties appear to be due to the osmotic and volumetric effect on the headgroup packing disruption.

Brain sterol flux mediated by cytochrome P450 46A1 affects membrane properties and membrane-dependent processes

Cytochrome P450 46A1 encoded by CYP46A1 catalyzes cholesterol 24-hydroxylation and is a CNS-specific enzyme that controls cholesterol removal and turnover in the brain. Accumulating data suggest that increases in cytochrome P450 46A1 activity in mouse models of common neurodegenerative diseases affect various, apparently unlinked biological processes and pathways. Yet, the underlying reason for these multiple enzyme activity effects is currently unknown. Herein, we tested the hypothesis that cytochrome P450 46A1-mediated sterol flux alters physico-chemical properties of the plasma membranes and thereby membrane-dependent events. We used 9-month old 5XFAD mice (an Alzheimer's disease model) treated for 6 months with the anti-HIV drug efavirenz. These animals have previously been shown to have improved behavioral performance, increased cytochrome P450 46A1 activity in the brain, and increased sterol flux through the plasma membranes. We further examined 9-month old Cyp46a1 ^-/- mice, which have previously been observed to have cognitive deficits and decreased sterol flux through brain membranes. Synaptosomal fractions from the brain of efavirenz-treated 5XFAD mice had essentially unchanged cholesterol levels as compared to control 5XFAD mice. However with efavirenz treatment in these mice, there were changes in the membrane properties (increased cholesterol accessibility, ordering, osmotic resistance, and thickness) as well as total glutamate content and ability to release glutamate in response to mild stimulation. Similarly, the cholesterol content in synaptosomal fractions from the brain of Cyp46a1 ^-/- mice was essentially the same as in wild type mice but knockout of Cyp46a1 was associated with changes in membrane properties and glutamate content and its exocytotic release. Changes in Cyp46a1 ^-/- mice were in the opposite direction to those observed in efavirenz-treated vs control 5XFAD mice. Incubation of synaptosomal fractions with the inhibitors of glycogen synthase kinase 3, cyclin-dependent kinase 5, protein phosphatase 1/2A or calcineurin, and protein phosphatase 2B revealed that increased sterol flux in efavirenz-treated vs control 5XFAD mice affected the ability of all four enzymes to modulate glutamate release. In contrast, in Cyp46a1 ^-/- vs wild type mice, decreased sterol flux altered the ability of only cyclin-dependent kinase 5 and protein phosphatase 2B to regulate the glutamate release. Collectively, our results support cytochrome P450 46A1-mediated sterol flux as an important contributor to the fundamental properties of the membranes, protein phosphorylation, and synaptic transmission Also, our data provide an explanation of how one enzyme, cytochrome P450 46A1, can affect multiple pathways and processes and serve as a common potential target for several neurodegenerative disorders.

Second Harmonic Generation Spectroscopy of Membrane Probe Dynamics in Gram-Positive Bacteria

Bacterial membranes are complex mixtures with dispersity that is dynamic over scales of both space and time. To capture adsorption onto and transport within these mixtures, we conduct simultaneous second harmonic generation (SHG) and two-photon fluorescence measurements on two different gram-positive bacterial species as the cells uptake membrane-specific probe molecules. Our results show that SHG not only can monitor the movement of small molecules across membrane leaflets but also is sensitive to higher-level ordering of the molecules within the membrane. Further, we show that the membranes of Staphylococcus aureus remain more dynamic after longer times at room temperature in comparison to Enterococcus faecalis. Our findings provide insight into the variability of activities seen between structurally similar molecules in gram-positive bacteria while also demonstrating the power of SHG to examine these dynamics.

Interrogating Synaptic Architecture: Approaches for Labeling Organelles and Cytoskeleton Components

Synaptic transmission has been studied for decades, as a fundamental step in brain function. The structure of the synapse, and its changes during activity, turned out to be key aspects not only in the transfer of information between neurons, but also in cognitive processes such as learning and memory. The overall synaptic morphology has traditionally been studied by electron microscopy, which enables the visualization of synaptic structure in great detail. The changes in the organization of easily identified structures, such as the presynaptic active zone, or the postsynaptic density, are optimally studied via electron microscopy. However, few reliable methods are available for labeling individual organelles or protein complexes in electron microscopy. For such targets one typically relies either on combination of electron and fluorescence microscopy, or on super-resolution fluorescence microscopy. This review focuses on approaches and techniques used to specifically reveal synaptic organelles and protein complexes, such as cytoskeletal assemblies. We place the strongest emphasis on methods detecting the targets of interest by affinity binding, and we discuss the advantages and limitations of each method.

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.

Cyfip1 Regulates Presynaptic Activity during Development

Copy number variations encompassing the gene encoding Cyfip1 have been associated with a variety of human diseases, including autism and schizophrenia. Here we show that juvenile mice hemizygous for Cyfip1 have altered presynaptic function, enhanced protein translation, and increased levels of F-actin. In developing hippocampus, reduced Cyfip1 levels serve to decrease paired pulse facilitation and increase miniature EPSC frequency without a change in amplitude. Higher-resolution examination shows these changes to be caused primarily by an increase in presynaptic terminal size and enhanced vesicle release probability. Short hairpin-mediated knockdown of Cyfip1 coupled with expression of mutant Cyfip1 proteins indicates that the presynaptic alterations are caused by dysregulation of the WAVE regulatory complex. Such dysregulation occurs downstream of Rac1 as acute exposure to Rac1 inhibitors rescues presynaptic responses in culture and in hippocampal slices. The data serve to highlight an early and essential role for Cyfip1 in the generation of normally functioning synapses and suggest a means by which changes in Cyfip1 levels could impact the generation of neural networks and contribute to abnormal and maladaptive behaviors.

SIGNIFICANCE STATEMENT

Several developmental brain disorders have been associated with gene duplications and deletions that serve to increase or decrease levels of encoded proteins. Cyfip1 is one such protein, but the role it plays in brain development is poorly understood. We asked whether decreased Cyfip1 levels altered the function of developing synapses. The data show that synapses with reduced Cyfip1 are larger and release neurotransmitter more rapidly. These effects are due to Cyfip1's role in actin polymerization and are reversed by expression of a Cyfip1 mutant protein retaining actin regulatory function or by inhibiting Rac1. Thus, Cyfip1 has a more prominent early role regulating presynaptic activity during a stage of development when activity helps to define neural pathways.

Fast, Ca2+-dependent exocytosis at nerve terminals: shortcomings of SNARE-based models

Investigations over the last two decades have made major inroads in clarifying the cellular and molecular events that underlie the fast, synchronous release of neurotransmitter at nerve endings. Thus, appreciable progress has been made in establishing the structural features and biophysical properties of the calcium (Ca2+) channels that mediate the entry into nerve endings of the Ca2+ ions that trigger neurotransmitter release. It is now clear that presynaptic Ca2+ channels are regulated at many levels and the interplay of these regulatory mechanisms is just beginning to be understood. At the same time, many lines of research have converged on the conclusion that members of the synaptotagmin family serve as the primary Ca2+ sensors for the action potential-dependent release of neurotransmitter. This identification of synaptotagmins as the proteins which bind Ca2+ and initiate the exocytotic fusion of synaptic vesicles with the plasma membrane has spurred widespread efforts to reveal molecular details of synaptotagmin's action. Currently, most models propose that synaptotagmin interfaces directly or indirectly with SNARE (soluble, N-ethylmaleimide sensitive factor attachment receptors) proteins to trigger membrane fusion. However, in spite of intensive efforts, the field has not achieved consensus on the mechanism by which synaptotagmins act. Concurrently, the precise sequence of steps underlying SNARE-dependent membrane fusion remains controversial. This review considers the pros and cons of the different models of SNARE-mediated membrane fusion and concludes by discussing a novel proposal in which synaptotagmins might directly elicit membrane fusion without the intervention of SNARE proteins in this final fusion step.

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.

Active transport of vesicles in neurons is modulated by mechanical tension

Effective intracellular transport of proteins and organelles is critical in cells, and is especially important for ensuring proper neuron functionality. In neurons, most proteins are synthesized in the cell body and must be transported through thin structures over long distances where normal diffusion is insufficient. Neurons transport subcellular cargo along axons and neurites through a stochastic interplay of active and passive transport. Mechanical tension is critical in maintaining proper function in neurons, but its role in transport is not well understood. To this end, we investigate the active and passive transport of vesicles in Aplysia neurons while changing neurite tension via applied strain, and quantify the resulting dynamics. We found that tension in neurons modulates active transport of vesicles by increasing the probability of active motion, effective diffusivity, and induces a retrograde bias. We show that mechanical tension modulates active transport processes in neurons and that external forces can couple to internal (subcellular) forces and change the overall transport dynamics.

Axonal cap-dependent translation regulates presynaptic p35

Axonal growth cones synthesize proteins during development and in response to injury in adult animals. Proteins locally translated in axons are used to generate appropriate responses to guidance cues, contribute to axon growth, and can serve as retrograde messengers. In addition to growth cones, mRNAs and translational machinery are also found along the lengths of axons where synapses form en passant, but contributions of intra-axonal translation to developing synapses are poorly understood. Here, we engineered a subcellular-targeting translational repressor to inhibit mRNA translation in axons, and we used this strategy to investigate presynaptic contributions of cap-dependent protein translation to developing CNS synapses. Our data show that intra-axonal mRNA translation restrains synaptic vesicle recycling pool size and that one target of this regulation is p35, a Cdk5 activating protein. Cdk5/p35 signaling regulates the size of vesicle recycling pools, p35 levels diminish when cap-dependent translation is repressed, and restoring p35 levels rescues vesicle recycling pools from the effects of spatially targeted translation repression. Together our findings show that intra-axonal synthesis of p35 is required for normal vesicle recycling in developing neurons, and that targeted translational repression provides a novel strategy to investigate extrasomal protein synthesis in neurons.

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.

Pool-independent labelling of synaptic vesicle exocytosis with single vesicle resolution in rat hippocampal neurons

FM dyes are an established tool to analyze synaptic vesicle pools. However, quantitative measurements using FM dyes are typically based on the re-release properties of previously labelled vesicles, which might vary depending on the experimental setup. An FM dye protocol independent of the previous labelling of vesicle membrane has not been applied for quantitative measurements of individual synaptic vesicles before. We therefore analyzed the direct staining of newly exocytosed vesicle membrane with FM dyes in cultured rat hippocampal neurons. In the presence of FM 1-43, stimulation-induced synaptic activity led to a stable fluorescence increase. The quantal release of synaptic vesicles was preserved and its amplitude correlated highly with the exocytic dye loss induced by a subsequent stimulation. Thus, the method presented here provides a tool for the pool-independent measurement of synaptic vesicle exocytosis.

Live imaging of bulk endocytosis in frog motor nerve terminals using FM dyes

We observed endocytosis in real time in stimulated frog motor nerve terminals by imaging the growth of large membrane infoldings labeled with a low concentration of FM dye. The spatial and temporal information made available by these experiments allowed us to image several new aspects of this synaptic vesicle recycling pathway. Membrane infoldings appeared near synaptic vesicle clusters and grew rapidly during long-duration, high-frequency stimulation. In some cases, we observed large, elongated infoldings growing laterally into the terminal. We used these observations to calculate infolding growth rates. A decrease in stimulation frequency caused a decrease in growth rates, but the overall length of these structures was unaffected by frequency changes. Attempts to wash the dye from these infoldings after stimulation were unsuccessful, demonstrating that the fluorescent structures had been endocytosed. We also used this technique to trigger and image infoldings during repeated, short trains. We found that membrane uptake occurred repeatedly at individual endocytosis sites, but only during a portion of the total number of trains delivered to the terminal. Finally, we showed that phosphatidylinositol 3-kinase, but not actin, was involved in this endocytosis pathway. The ability to monitor many individual bulk endocytosis sites in real time should allow for new types of endocytosis measurements and could reveal novel and unexpected mechanisms for coordinating membrane recovery during synaptic activity.

The same synaptic vesicles drive active and spontaneous release

Synaptic vesicles release neurotransmitter both actively (on stimulation) and spontaneously (at rest). It has been assumed that identical vesicles use both modes of release; however, recent evidence has challenged this view. Using several assays (FM dye imaging, pHluorin imaging and antibody-labeling of synaptotagmin) in neuromuscular preparations from Drosophila, frog and mouse, as well as rat cultured neurons, we found that the same vesicles participate in active and spontaneous release.

Synaptic vesicle pools: an update

During the last few decades synaptic vesicles have been assigned to a variety of functional and morphological classes or “pools”. We have argued in the past (Rizzoli and Betz, 2005) that synaptic activity in several preparations is accounted for by the function of three vesicle pools: the readily releasable pool (docked at active zones and ready to go upon stimulation), the recycling pool (scattered throughout the nerve terminals and recycling upon moderate stimulation), and finally the reserve pool (occupying most of the vesicle clusters and only recycling upon strong stimulation). We discuss here the advancements in the vesicle pool field which took place in the ensuing years, focusing on the behavior of different pools under both strong stimulation and physiological activity. Several new findings have enhanced the three-pool model, with, for example, the disparity between recycling and reserve vesicles being underlined by the observation that the former are mobile, while the latter are “fixed”. Finally, a number of altogether new concepts have also evolved such as the current controversy on the identity of the spontaneously recycling vesicle pool.

Multivalent ions control the transport through lysenin channels

We report the effect of different ions on the conducting properties of lysenin channels inserted into planar lipid bilayer membranes. Our observations indicated that multivalent ions inhibited the lysenin channels conductance in a concentration dependent manner. The analysis performed on single channels revealed that multivalent ions induced reversible sub-conducting or closed states depending on the ionic charge and size. Good agreement is reported between experimental results and a theoretical model that is proposed to describe the interaction between divalent ions and lysenin channels as a simple isothermal absorption process.

Probing plant membranes with FM dyes: tracking, dragging or blocking?

Remarkable progress in various techniques of in vivo fluorescence microscopy has brought an urgent need for reliable markers for tracking cellular structures and processes. The goal of this manuscript is to describe unexplored effects of the FM (Fei Mao) styryl dyes, which are widely used probes that label processes of endocytosis and vesicle trafficking in eukaryotic cells. Although there are few reports on the effect of styryl dyes on membrane fluidity and the activity of mammalian receptors, FM dyes have been considered as reliable tools for tracking of plant endocytosis. Using plasma membrane-localized transporters for the plant hormone auxin in tobacco BY-2 and Arabidopsis thaliana cell suspensions, we show that routinely used concentrations of FM 4-64 and FM 5-95 trigger transient re-localization of these proteins, and FM 1-43 affects their activity. The active process of re-localization is blocked neither by inhibitors of endocytosis nor by cytoskeletal drugs. It does not occur in A. thaliana roots and depends on the degree of hydrophobicity (lipophilicity) of a particular FM dye. Our results emphasize the need for circumspection during in vivo studies of membrane proteins performed using simultaneous labelling with FM dyes.

FM dyes enter via a store-operated calcium channel and modify calcium signaling of cultured astrocytes

The amphiphilic fluorescent styryl pyridinium dyes FM1-43 and FM4-64 are used to probe activity-dependent synaptic vesicle cycling in neurons. Cultured astrocytes can internalize FM1-43 and FM4-64 inside vesicles but their uptake is insensitive to the elevation of cytosolic calcium (Ca(2+)) concentration and the underlying mechanism remains unclear. Here we used total internal reflection fluorescence microscopy and pharmacological tools to study the mechanisms of FM4-64 uptake into cultured astrocytes from mouse neocortex. Our data show that: (i) endocytosis is not a major route for FM4-64 uptake into astrocytes; (ii) FM4-64 enters astrocytes through an aqueous pore and strongly affects Ca(2+) homeostasis; (iii) partitioning of FM4-64 into the outer leaflet of the plasma membrane results in a facilitation of store-operated Ca(2+) entry (SOCE) channel gating; (iv) FM4-64 permeates and competes with Ca(2+) for entry through a SOCE channel; (v) intracellular FM4-64 mobilizes Ca(2+) from the endoplasmic reticulum stores, conveying a positive feedback to activate SOCE and to sustain dye uptake into astrocytes. Our study demonstrates that FM dyes are not markers of cycling vesicles in astrocytes and calls for a careful interpretation of FM fluorescence.