Synapto-pHluorins: chimeras between pH-sensitive mutants of green fluorescent protein and synaptic vesicle membrane proteins as reporters of neurotransmitter release

Simultaneous TIRF imaging of subplasmalemmal Ca2+ dynamics and granule fusions in insulin-secreting INS-1 cells reveals coexistent synchronized and asynchronous release

The basal and glucose-induced insulin secretion from pancreatic beta cells is a tightly regulated process that is triggered in a Ca2+-dependent fashion and further positively modulated by substances that raise intracellular levels of adenosine 3',5'-cyclic monophosphate (cAMP) or by certain antidiabetic drugs. In a previous study, we have temporally resolved the subplasmalemmal [Ca2+]i dynamics in beta cells that are characterized by trains of sharply delimited spikes, reaching peak values up to 5 µM. Applying total internal reflection fluorescence (TIRF) microscopy and synaptopHluorin to visualize fusion events of individual granules, we found that several fusion events can coincide within 50 to 150 ms. To test whether subplasmalemmal [Ca2+]i microdomains around single or clustered Ca2+ channels may cause a synchronized release of insulin-containing vesicles, we applied simultaneous dual-color TIRF microscopy and monitored Ca2+ fluctuations and exocytotic events in INS-1 cells at high frame rates. The results indicate that fusions can be triggered by subplasmalemmal Ca2+ spiking. This, however, does account for a minority of fusion events. About 90 %-95 % of fusion events either happen between Ca2+ spikes or incidentally overlap with subplasmalemmal Ca2+ spikes. We conclude that only a fraction of exocytotic events in glucose-induced and tolbutamide- or forskolin-enhanced insulin release from INS-1 cells is tightly coupled to Ca2+ microdomains around voltage-gated Ca2+ channels.

Ultrastructure of light-activated axons following optogenetic stimulation to produce late-phase long-term potentiation

Analysis of neuronal compartments has revealed many state-dependent changes in geometry but establishing synapse-specific mechanisms at the nanoscale has proven elusive. We co-expressed channelrhodopsin2-GFP and mAPEX2 in a subset of hippocampal CA3 neurons and used trains of light to induce late-phase long-term potentiation (L-LTP) in area CA1. L-LTP was shown to be specific to the labeled axons by severing CA3 inputs, which prevented back-propagating recruitment of unlabeled axons. Membrane-associated mAPEX2 tolerated microwave-enhanced chemical fixation and drove tyramide signal amplification to deposit Alexa Fluor dyes in the light-activated axons. Subsequent post-embedding immunogold labeling resulted in outstanding ultrastructure and clear distinctions between labeled (activated), and unlabeled axons without obscuring subcellular organelles. The gold-labeled axons in potentiated slices were reconstructed through serial section electron microscopy; presynaptic vesicles and other constituents could be quantified unambiguously. The genetic specification, reliable physiology, and compatibility with established methods for ultrastructural preservation make this an ideal approach to link synapse ultrastructure and function in intact circuits.

Ethanol Regulates Presynaptic Activity and Sedation through Presynaptic Unc13 Proteins in Drosophila

Ethanol has robust effects on presynaptic activity in many neurons, however, it is not yet clear how this drug acts within this compartment to change neural activity, nor the significance of this change on behavior and physiology in vivo. One possible presynaptic effector for ethanol is the Munc13-1 protein. Herein, we show that ethanol binding to the rat Munc13-1 C1 domain, at concentrations consistent with binge exposure, reduces diacylglycerol (DAG) binding. The inhibition of DAG binding is predicted to reduce the activity of Munc13-1 and presynaptic release. In Drosophila, we show that sedating concentrations of ethanol significantly reduce synaptic vesicle release in olfactory sensory neurons (OSNs), while having no significant impact on membrane depolarization and Ca²⁺ influx into the presynaptic compartment. These data indicate that ethanol targets the active zone in reducing synaptic vesicle exocytosis. Drosophila, haploinsufficent for the Munc13-1 ortholog Dunc13, are more resistant to the effect of ethanol on presynaptic inhibition. Genetically reducing the activity of Dunc13 through mutation or expression of RNAi transgenes also leads to a significant resistance to the sedative effects of ethanol. The neuronal expression of Munc13-1 in heterozygotes for a Dunc13 loss-of-function mutation can largely rescue the ethanol sedation resistance phenotype, indicating a conservation of function between Munc13-1 and Dunc13 in ethanol sedation. Hence, reducing Dunc13 activity leads to naïve physiological and behavioral resistance to sedating concentrations of ethanol. We propose that reducing Dunc13 activity, genetically or pharmacologically by ethanol binding to the C1 domain of Munc13-1/Dunc13, promotes a homeostatic response that leads to ethanol tolerance.

Presynaptic inhibition of gamma lobe neurons is required for olfactory learning in Drosophila

The loss of heterotrimeric G(o) signaling through the expression of pertussis toxin (PTX) within either the α/β or γ lobe mushroom body neurons of Drosophila results in the impaired aversive olfactory associative memory formation. Herein, we focus on the cellular effects of G(o) signaling in the γ lobe mushroom body neurons during memory formation. Expression of PTX in the γ lobes specifically inhibits G(o) activation, leading to poor olfactory learning and an increase in odor-elicited synaptic vesicle release. In the γ lobe neurons, training decreases synaptic vesicle release elicited by the unpaired conditioned stimulus -, while leaving presynaptic activation by the paired conditioned stimulus + unchanged. PTX expression in γ lobe neurons inhibits the generation of this differential synaptic activation by conditioned stimuli after negative reinforcement. Hyperpolarization of the γ lobe neurons or the inhibition of presynaptic activity through the expression of dominant negative dynamin transgenes ameliorated the memory impairment caused by PTX, indicating that the disinhibition of these neurons by PTX was responsible for the poor memory formation. The role for γ lobe inhibition, carried out by G(o) activation, indicates that an inhibitory circuit involving these neurons plays a positive role in memory acquisition. This newly uncovered requirement for inhibition of odor-elicited activity within the γ lobes is consistent with these neurons serving as comparators during learning, perhaps as part of an odor salience modification mechanism.

Automated condition-invariable neurite segmentation and synapse classification using textural analysis-based machine-learning algorithms

High-resolution live-cell imaging studies of neuronal structure and function are characterized by large variability in image acquisition conditions due to background and sample variations as well as low signal-to-noise ratio. The lack of automated image analysis tools that can be generalized for varying image acquisition conditions represents one of the main challenges in the field of biomedical image analysis. Specifically, segmentation of the axonal/dendritic arborizations in brightfield or fluorescence imaging studies is extremely labor-intensive and still performed mostly manually. Here we describe a fully automated machine-learning approach based on textural analysis algorithms for segmenting neuronal arborizations in high-resolution brightfield images of live cultured neurons. We compare performance of our algorithm to manual segmentation and show that it combines 90% accuracy, with similarly high levels of specificity and sensitivity. Moreover, the algorithm maintains high performance levels under a wide range of image acquisition conditions indicating that it is largely condition-invariable. We further describe an application of this algorithm to fully automated synapse localization and classification in fluorescence imaging studies based on synaptic activity. Textural analysis-based machine-learning approach thus offers a high performance condition-invariable tool for automated neurite segmentation.

Synapto-pHluorins: genetically encoded reporters of synaptic transmission

pHluorins are pH-sensitive mutants of green fluorescent protein (GFP). Attached to proteins with defined cellular locations or itineraries, pHluorins report subcellular pH as well as protein transport between compartments of differing pH. Key applications in neurobiology include the optical detection of neurotransmitter release with synapto-pHluorins and their derivatives, as well as measurements of neurotransmitter receptor trafficking. This article describes the properties and uses of synapto-pHluorins, as well as their advantages and limitations.

Zebrafish as an appealing model for optogenetic studies

Optogenetics, the use of light-based protein tools, has begun to revolutionize biological research. The approach has proven especially useful in the nervous system, where light has been used both to detect and to manipulate activity in targeted neurons. Optogenetic tools have been deployed in systems ranging from cultured cells to primates, with each offering a particular combination of advantages and drawbacks. In this chapter, we provide an overview of optogenetics in zebrafish. Two of the greatest attributes of the zebrafish model system are external fertilization and transparency in early life stages. Combined, these allow researchers to observe the internal structures of developing zebrafish embryos and larvae without dissections or other interference. This transparency, combined with the animals' small size, simple husbandry, and similarity to mammals in many structures and processes, has made zebrafish a particularly popular model system in developmental biology. The easy optical access also dovetails with optogenetic tools, allowing their use in intact, developing, and behaving animals. This means that optogenetic studies in embryonic and larval zebrafish can be carried out in a high-throughput fashion with relatively simple equipment. As a consequence, zebrafish have been an important proving ground for optogenetic tools and approaches and have already yielded important new knowledge about the neural circuits underlying behavior. Here, we provide a general introduction to zebrafish as a model system for optogenetics. Through descriptions and analyses of important optogenetic studies that have been done in zebrafish, we highlight the advantages and liabilities that the system brings to optogenetic experiments.

Investigating the genetics of visual processing, function and behaviour in zebrafish

Over the past three decades, the zebrafish has been proven to be an excellent model to investigate the genetic control of vertebrate embryonic development, and it is now also increasingly used to study behaviour and adult physiology. Moreover, mutagenesis approaches have resulted in large collections of mutants with phenotypes that resemble human pathologies, suggesting that these lines can be used to model diseases and screen drug candidates. With the recent development of new methods for gene targeting and manipulating or monitoring gene expression, the range of genetic modifications now possible in zebrafish is increasing rapidly. Combined with the classical strengths of the zebrafish as a model organism, these advances are set to substantially expand the type of biological questions that can be addressed in this species. In this review, we outline how the potential of the zebrafish can be harvested in the context of eye development and visual function. We review recent technological advances used to study the formation of the eyes and visual areas of the brain, visual processing on the cellular, subcellular and molecular level, and the genetics of visual behaviour in vertebrates.

Focusing on optic tectum circuitry through the lens of genetics

The visual pathway is tasked with processing incoming signals from the retina and converting this information into adaptive behavior. Recent studies of the larval zebrafish tectum have begun to clarify how the 'micro-circuitry' of this highly organized midbrain structure filters visual input, which arrives in the superficial layers and directs motor output through efferent projections from its deep layers. The new emphasis has been on the specific function of neuronal cell types, which can now be reproducibly labeled, imaged and manipulated using genetic and optical techniques. Here, we discuss recent advances and emerging experimental approaches for studying tectal circuits as models for visual processing and sensorimotor transformation by the vertebrate brain.

The Gal4/UAS toolbox in zebrafish: new approaches for defining behavioral circuits

Over recent years, several groundbreaking techniques have been developed that allow for the anatomical description of neurons, and the observation and manipulation of their activity. Combined, these approaches should provide a great leap forward in our understanding of the structure and connectivity of the nervous system and how, as a network of individual neurons, it generates behavior. Zebrafish, given their external development and optical transparency, are an appealing system in which to employ these methods. These traits allow for direct observation of fluorescence in describing anatomy and observing neural activity, and for the manipulation of neurons using a host of light-triggered proteins. Gal4/Upstream Activating Sequence techniques, as they are based on a binary system, allow for the flexible deployment of a range of transgenes in expression patterns of interest. As such, they provide a promising approach for viewing neurons in a variety of ways, each of which can reveal something different about their structure, connectivity, or function. In this study, the author will review recent progress in the development of the Gal4/Upstream Activating Sequence system in zebrafish, feature examples of promising studies to date, and examine how various new technologies can be used in the future to untangle the complex mechanisms by which behavior is generated.

The role of synaptic ion channels in synaptic plasticity

The nervous system receives a large amount of information about the environment through elaborate sensory routes. Processing and integration of these wide-ranging inputs often results in long-term behavioural alterations as a result of past experiences. These relatively permanent changes in behaviour are manifestations of the capacity of the nervous system for learning and memory. At the cellular level, synaptic plasticity is one of the mechanisms underlying this process. Repeated neural activity generates physiological changes in the nervous system that ultimately modulate neuronal communication through synaptic transmission. Recent studies implicate both presynaptic and postsynaptic ion channels in the process of synapse strength modulation. Here, we review the role of synaptic ion channels in learning and memory, and discuss the implications and significance of these findings towards deciphering the molecular biology of learning and memory.

Rapid activation of inwardly rectifying potassium channels by immobile G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) mediate slow synaptic transmission and many other effects of small molecule and peptide neurotransmitters. In the standard model of GPCR signaling, receptors and G-proteins diffuse laterally within the plane of the plasma membrane and encounter each other by random collision. This model predicts that signaling will be most efficient if both GPCRs and G-proteins are free to diffuse, thus maximizing collision frequency. However, neuronal GPCRs are often recruited to and enriched at specific synaptic locations, suggesting receptor mobility is restricted in these cells. Here, we test the hypothesis that restricting GPCR mobility impairs signaling in neurons by limiting the frequency of collisions between receptors and G-proteins. Mu-opioid receptors (MORs) were immobilized on the surface of cerebellar granule neurons by avidin-mediated cross-linking, and inwardly rectifying potassium (GIRK) channels were used as rapid indicators of G-protein activation. Mobile and immobile MORs activated GIRK channels with the same onset kinetics and agonist sensitivity in these neurons. In a heterologous expression system, GFP (green fluorescent protein)-tagged G alpha(oA) subunits remained mobile after cross-linking, but their mobility was reduced in the presence of immobile MORs, suggesting that these receptors and subunits were transiently precoupled. In addition, channel activation could be reconstituted with immobile GPCRs, G-protein heterotrimers, and GIRK channels. These results show that collision frequency is not rate-limiting for G-protein activation in CNS neurons, and are consistent with the idea that signaling components are compartmentalized or preassembled.

Tetanus Toxin Is Transported in a Novel Neuronal Compartment Characterized by a Specialized pH Regulation

Tetanus toxin binds specifically to motor neurons at the neuromuscular junction. There, it is internalized into vesicular carriers undergoing fast retrograde transport to the spinal cord. Despite the importance of this axonal transport pathway in health and disease, its molecular and biophysical characterization is presently lacking. We sought to fill this gap by determining the pH regulation of this compartment in living motor neurons using a chimera of the tetanus toxin binding fragment (TeNT HC) and a pH-sensitive variant of the green fluorescent protein (ratiometric pHluorin). We have demonstrated that moving retrograde carriers display a narrow range of neutral pH values, which is kept constant during transport. Stationary TeNT HC-positive organelles instead exhibit a wide spectrum of pH values, ranging from acidic to neutral. This distinct pH regulation is due to a differential targeting of the vacuolar (H+) ATPase, which is not present on moving TeNT HC compartments. Accordingly, inhibition of the vacuolar (H+) ATPase under conditions that completely abolish the intracellular accumulation of acidotrophic dyes does not affect axonal retrograde transport of TeNT HC. However, a functional vacuolar (H+) ATPase is required for early steps of TeNT HC trafficking following endocytosis, and it is localized to axonal vesicles containing TeNT HC. Altogether, these findings indicate that the vacuolar (H+ ATPase plays a specific role in early sorting events directing TeNT HC to axonal carriers but not in their subsequent progression along the retrograde transport route, which escapes acidification and targeting to degradative organelles.

Fluorescent proteins as a toolkit for in vivo imaging

Green fluorescent protein (GFP) from the jellyfish Aequorea victoria, and its mutant variants, are the only fully genetically encoded fluorescent probes available and they have proved to be excellent tools for labeling living specimens. Since 1999, numerous GFP homologues have been discovered in Anthozoa, Hydrozoa and Copepoda species, demonstrating the broad evolutionary and spectral diversity of this protein family. Mutagenic studies gave rise to diversified and optimized variants of fluorescent proteins, which have never been encountered in nature. This article gives an overview of the GFP-like proteins developed to date and their most common applications to study living specimens using fluorescence microscopy.

Acid-sensing ion channel 2 contributes a major component to acid-evoked excitatory responses in spiral ganglion neurons and plays a role in noise susceptibility of mice

Ion channels in the degenerin-epithelial sodium channel (DEG-ENaC) family perform diverse functions, including mechanosensation. Here we explored the role of the vertebrate DEG-ENaC protein, acid-sensing ion channel 2 (ASIC2), in auditory transduction. Contributions of ASIC2 to hearing were examined by comparing hearing threshold and noise sensitivity of wild-type and ASIC2 null mice. ASIC2 null mice showed no significant hearing loss, indicating that the ASIC2 was not directly involved in the mechanotransduction of the mammalian cochlea. However, we found that (1) ASIC2 was present in the spiral ganglion (SG) neurons in the adult cochlea and that externally applied protons induced amiloride-sensitive sodium currents and action potentials in SG neurons in vitro, (2) proton-induced responses were greatly reduced in SG neurons obtained from ASIC2 null mice, indicating that activations of ASIC2 contributed a major portion of the proton-induced excitatory response in SG neurons, and (3) ASIC2 null mice were considerably more resistant to noise-induced temporary, but not permanent, threshold shifts. Together, these data suggest that ASIC2 contributes to suprathreshold functions of the cochlea. The presence of ASIC2 in SG neurons could provide sensors to directly convert local acidosis to excitatory responses, therefore offering a cellular mechanism linking hearing losses caused by many enigmatic causes (e.g., ischemia or inflammation of the inner ear) to excitotoxicity.

Transgenic mouse lines expressing synaptopHluorin in hippocampus and cerebellar cortex

We generated six transgenic mouse lines in which synaptopHluorin (SpH), one of green fluorescent protein-based sensors of vesicular exocytosis, was expressed under the control of neuron-specific Thy-1.2 promoter. In situ hybridization study revealed that SpH mRNA was expressed in a broad spectrum of brain regions in four of them, whereas in others it was expressed in the specific regions of the hippocampus. In one particular line, SpH immunoreactivity was specifically observed in the mossy fiber presynaptic terminals of both hippocampus and cerebellar cortex. The fluorescence intensity of these presynaptic terminals was somewhat decreased by acidic buffer superfusion and greatly increased by vesicular neutralization of pH, indicating that the SpH molecules are mainly distributed in the synaptic vesicles. The exocytosis-dependent fluorescence increment was measured upon activation of a single presynaptic terminal. These transgenic lines are expected to facilitate morphological and physiological studies of presynaptic terminals in a variety of regions of the brain.

Video-rate nonlinear microscopy of neuronal membrane dynamics with genetically encoded probes

Biological membranes decorated with suitable contrast agents give rise to nonlinear optical signals such as two-photon fluorescence and harmonic up-conversion when illuminated with ultra-short, high-intensity pulses of infrared laser light. Microscopic images based on these nonlinear contrasts were acquired at video or higher frame rates by scanning a focused illuminating spot rapidly across neural tissues. The scan engine relied on an acousto-optic deflector (AOD) to produce a fast horizontal raster and on corrective prisms to offset the AOD-induced dispersion of the ultra-short excitation light pulses in space and time. Two membrane-bound derivatives of the green fluorescent protein (GFP) were tested as nonlinear contrast agents. Synapto-pHluorin, a pH-sensitive GFP variant fused to a synaptic vesicle membrane protein, provided a time-resolved fluorescent read-out of neurotransmitter release at genetically specified synaptic terminals in the intact brain. Arrays of dually lipidated GFP molecules at the plasma membrane generated intense two-photon fluorescence but no detectable second-harmonic power. Comparison with second-harmonic generation by membranes stained with a synthetic styryl dye suggested that the genetically encoded chromophore arrangement lacked the orientational anisotropy and/or dipole density required for efficient coherent scattering of the incident optical field.

Live Optical Imaging of Nervous System Development

Although development of the nervous system is inherently a process of dynamic change, until recently it has generally been investigated by inference from static images. However, advances in live optical imaging are now allowing direct observation of growth, synapse formation, and even incipient function in the developing nervous system, at length scales from molecules to cortical regions, and over timescales from milliseconds to months. In this review, we provide technical background and present examples of how these new methods, including confocal and two-photon microscopy, GFP-based markers, and functional indicators, are being applied to provide fresh insight into long-standing questions of neural development.

Visualization of Molecular Activities Inside Living Cells with Fluorescent Labels

The major task of modern cell biology is to identify the function and relation of the many different gene products, discovered by genomics and proteomics approaches, in the context of the living cell. To achieve this goal, an increasing toolbox of custom-designed biosensors based on fluorescent labels is available to study the molecular activities of the cellular machinery. An overview of the current status of the young field of molecular-cellular physiology is presented that includes the application of fluorescent labels in the design of biosensors and the major detection schemes used to extract their sensing information. In particular, the use of the photophysical phenomenon of Förster resonance energy transfer (FRET) as a powerful indicator of cellular biochemical events is discussed. In addition, we will point out the challenges and directions of the field and project the short-term future for the application of fluorescence-based biosensors in biology.

Transmission of olfactory information between three populations of neurons in the antennal lobe of the fly

Three classes of neurons form synapses in the antennal lobe of Drosophila, the insect counterpart of the vertebrate olfactory bulb: olfactory receptor neurons, projection neurons, and inhibitory local interneurons. We have targeted a genetically encoded optical reporter of synaptic transmission to each of these classes of neurons and visualized population responses to natural odors. The activation of an odor-specific ensemble of olfactory receptor neurons leads to the activation of a symmetric ensemble of projection neurons across the glomerular synaptic relay. Virtually all excited glomeruli receive inhibitory input from local interneurons. The extent, odor specificity, and partly interglomerular origin of this input suggest that inhibitory circuits assemble combinatorially during odor presentations. These circuits may serve as dynamic templates that extract higher order features from afferent activity patterns.

Selective photostimulation of genetically chARGed neurons

To permit direct functional analyses of neural circuits, we have developed a method for stimulating groups of genetically designated neurons optically. Coexpression of the Drosophila photoreceptor genes encoding arrestin-2, rhodopsin (formed by liganding opsin with retinal), and the alpha subunit of the cognate heterotrimeric G protein--an explosive combination we term "chARGe"--sensitizes generalist vertebrate neurons to light. Illumination of a mixed population of neurons elicits action potentials selectively and cell-autonomously in its genetically chARGed members. In contrast to bath-applied photostimulants or caged neurotransmitters, which act indiscriminately throughout the illuminated volume, chARGe localizes the responsiveness to light. Distributed activity may thus be fed directly into a circumscribed population of neurons in intact tissue, irrespective of the spatial arrangement of its elements.

Imaging into the future: visualizing gene expression and protein interactions with fluorescent proteins

Since its introduction into heterologous organisms as a marker of gene expression, the green fluorescent protein (GFP) has led a dramatic revolution in cell, developmental and neurobiology. By allowing breathtaking visualization of fluorescent fusion proteins as they move within and between cells, GFP has fundamentally transformed the spatial analysis of protein function. Now, new GFP technologies allow far more than simple observations of fusion protein localization. The growing family of fluorescent protein variants is enabling more sophisticated studies of protein function and illuminating wide-ranging processes from gene expression to second-messenger cascades and intercellular signalling. Together with advances in microscopy, new GFP-based experimental approaches are forging a second GFP revolution.