Monitoring intracellular nanomolar calcium using fluorescence lifetime imaging

Fluorescence in depth: integration of spectroscopy and imaging with Raman, IR, and CD for advanced research

Fluorescence spectroscopy serves as a vital technique for studying the interaction between light and fluorescent molecules. It encompasses a range of methods, each presenting unique advantages and applications. This technique finds utility in various chemical studies. This review discusses Fluorescence spectroscopy, its branches such as Time-Resolved Fluorescence Spectroscopy (TRFS) and Fluorescence Lifetime Imaging Microscopy (FLIM), and their integration with other spectroscopic methods, including Raman, Infrared (IR), and Circular Dichroism (CD) spectroscopies. By delving into these methods, we aim to provide a comprehensive understanding of the capabilities and significance of fluorescence spectroscopy in scientific research, highlighting its diverse applications and the enhanced understanding it brings when combined with other spectroscopic methods. This review looks at each technique's unique features and applications. It discusses the prospects of their combined use in advancing scientific understanding and applications across various domains.

Live-cell biosensors based on the fluorescence lifetime of environment-sensing dyes

In this work, we examine the use of environment-sensitive fluorescent dyes in fluorescence lifetime imaging microscopy (FLIM) biosensors. We screened merocyanine dyes to find an optimal combination of environment-induced lifetime changes, photostability, and brightness at wavelengths suitable for live-cell imaging. FLIM was used to monitor a biosensor reporting conformational changes of endogenous Cdc42 in living cells. The ability to quantify activity using phasor analysis of a single fluorophore (e.g., rather than ratio imaging) eliminated potential artifacts. We leveraged these properties to determine specific concentrations of activated Cdc42 across the cell.

Fast and slow: Recording neuromodulator dynamics across both transient and chronic time scales

Neuromodulators transform animal behaviors. Recent research has demonstrated the importance of both sustained and transient change in neuromodulators, likely due to tonic and phasic neuromodulator release. However, no method could simultaneously record both types of dynamics. Fluorescence lifetime of optical reporters could offer a solution because it allows high temporal resolution and is impervious to sensor expression differences across chronic periods. Nevertheless, no fluorescence lifetime change across the entire classes of neuromodulator sensors was previously known. Unexpectedly, we find that several intensity-based neuromodulator sensors also exhibit fluorescence lifetime responses. Furthermore, we show that lifetime measures in vivo neuromodulator dynamics both with high temporal resolution and with consistency across animals and time. Thus, we report a method that can simultaneously measure neuromodulator change over transient and chronic time scales, promising to reveal the roles of multi-time scale neuromodulator dynamics in diseases, in response to therapies, and across development and aging.

Nanocrystals for Deep-Tissue In Vivo Luminescence Imaging in the Near-Infrared Region

In vivo imaging technologies have emerged as a powerful tool for both fundamental research and clinical practice. In particular, luminescence imaging in the tissue-transparent near-infrared (NIR, 700-1700 nm) region offers tremendous potential for visualizing biological architectures and pathophysiological events in living subjects with deep tissue penetration and high imaging contrast owing to the reduced light-tissue interactions of absorption, scattering, and autofluorescence. The distinctive quantum effects of nanocrystals have been harnessed to achieve exceptional photophysical properties, establishing them as a promising category of luminescent probes. In this comprehensive review, the interactions between light and biological tissues, as well as the advantages of NIR light for in vivo luminescence imaging, are initially elaborated. Subsequently, we focus on achieving deep tissue penetration and improved imaging contrast by optimizing the performance of nanocrystal fluorophores. The ingenious design strategies of NIR nanocrystal probes are discussed, along with their respective biomedical applications in versatile in vivo luminescence imaging modalities. Finally, thought-provoking reflections on the challenges and prospects for future clinical translation of nanocrystal-based in vivo luminescence imaging in the NIR region are wisely provided.

Probing organoid metabolism using fluorescence lifetime imaging microscopy (FLIM): The next frontier of drug discovery and disease understanding

Organoid models have been used to address important questions in developmental and cancer biology, tissue repair, advanced modelling of disease and therapies, among other bioengineering applications. Such 3D microenvironmental models can investigate the regulation of cell metabolism, and provide key insights into the mechanisms at the basis of cell growth, differentiation, communication, interactions with the environment and cell death. Their accessibility and complexity, based on 3D spatial and temporal heterogeneity, make organoids suitable for the application of novel, dynamic imaging microscopy methods, such as fluorescence lifetime imaging microscopy (FLIM) and related decay time-assessing readouts. Several biomarkers and assays have been proposed to study cell metabolism by FLIM in various organoid models. Herein, we present an expert-opinion discussion on the principles of FLIM and PLIM, instrumentation and data collection and analysis protocols, and general and emerging biosensor-based approaches, to highlight the pioneering work being performed in this field.

FRET-Modulated Fluorescence Lifetime-Traceable Nanocarriers for Multidrug Release Monitoring and Synergistic Therapy

In situ monitoring multidrug release in complex cellular microenvironments is significant, and currently, it is still a great challenge. In this work, a smart nanocarrier with the capability of codelivery of small molecules and gene materials as well as with Förster resonance energy transfer (FRET)-modulated fluorescence lifetime is fabricated by integrating gold nanoparticles (the acceptor) into dual-mesoporous silica loaded with multiple drugs (the donor). Once internalized into tumor cells, in weakly acidic environments, the conformation switch of the polymer grafted on nanocarriers causes its shedding from the mesopores, triggering the release of drugs. Simultaneously, based on the strong overlap between the emission spectrum of donors and the absorption spectrum of the acceptors, any slight fluctuation of the dissociation of the drugs from nanocarriers can result in a change in the FRET-modulated lifetime signal due to the extraordinarily sensitive FRET signal to the separation distance between donors and acceptors. All these implied the potential applications of this nanoplatform in various biomedical fields that require the codelivery and real-time monitoring of multidrug-based synergistic therapy.

Volume-transmitted GABA waves pace epileptiform rhythms in the hippocampal network

Mechanisms that entrain and pace rhythmic epileptiform discharges remain debated. Traditionally, the quest to understand them has focused on interneuronal networks driven by synaptic GABAergic connections. However, synchronized interneuronal discharges could also trigger the transient elevations of extracellular GABA across the tissue volume, thus raising tonic conductance (Gtonic) of synaptic and extrasynaptic GABA receptors in multiple cells. Here, we monitor extracellular GABA in hippocampal slices using patch-clamp GABA "sniffer" and a novel optical GABA sensor, showing that periodic epileptiform discharges are preceded by transient, region-wide waves of extracellular GABA. Neural network simulations that incorporate volume-transmitted GABA signals point to a cycle of GABA-driven network inhibition and disinhibition underpinning this relationship. We test and validate this hypothesis using simultaneous patch-clamp recordings from multiple neurons and selective optogenetic stimulation of fast-spiking interneurons. Critically, reducing GABA uptake in order to decelerate extracellular GABA fluctuations-without affecting synaptic GABAergic transmission or resting GABA levels-slows down rhythmic activity. Our findings thus unveil a key role of extrasynaptic, volume-transmitted GABA in pacing regenerative rhythmic activity in brain networks.

Calcium signaling in plant immunity: a spatiotemporally controlled symphony

Calcium ions (Ca²⁺) are prominent intracellular messengers in all eukaryotic cells. Recent studies have emphasized the crucial roles of Ca²⁺ in plant immunity. Here, we review the latest progress on the spatiotemporal control of Ca²⁺ function in plant immunity. We discuss discoveries of how Ca²⁺ influx is triggered upon the activation of immune receptors, how Ca²⁺-permeable channels are activated, how Ca²⁺ signals are decoded inside plant cells, and how these signals are switched off. Despite recent advances, many open questions remain and we highlight the existing toolkit and the new technologies to address the outstanding questions of Ca²⁺ signaling in plant immunity.

Simple and Robust Deep Learning Approach for Fast Fluorescence Lifetime Imaging

Fluorescence lifetime imaging (FLIM) is a powerful tool that provides unique quantitative information for biomedical research. In this study, we propose a multi-layer-perceptron-based mixer (MLP-Mixer) deep learning (DL) algorithm named FLIM-MLP-Mixer for fast and robust FLIM analysis. The FLIM-MLP-Mixer has a simple network architecture yet a powerful learning ability from data. Compared with the traditional fitting and previously reported DL methods, the FLIM-MLP-Mixer shows superior performance in terms of accuracy and calculation speed, which has been validated using both synthetic and experimental data. All results indicate that our proposed method is well suited for accurately estimating lifetime parameters from measured fluorescence histograms, and it has great potential in various real-time FLIM applications.

Penetration Depth of Propylene Glycol, Sodium Fluorescein and Nile Red into the Skin Using Non-Invasive Two-Photon Excited FLIM

The stratum corneum (SC) forms a strong barrier against topical drug delivery. Therefore, understanding the penetration depth and pathways into the SC is important for the efficiency of drug delivery and cosmetic safety. In this study, TPT-FLIM (two-photon tomography combined with fluorescence lifetime imaging) was applied as a non-invasive optical method for the visualization of skin structure and components to study penetration depths of exemplary substances, like hydrophilic propylene glycol (PG), sodium fluorescein (NaFl) and lipophilic Nile red (NR) into porcine ear skin ex vivo. Non-fluorescent PG was detected indirectly based on the pH-dependent increase in the fluorescence lifetime of SC components. The pH similarity between PG and viable epidermis limited the detection of PG. NaFl reached the viable epidermis, which was also proved by laser scanning microscopy. Tape stripping and confocal Raman micro-spectroscopy were performed additionally to study NaFl, which revealed penetration depths of ≈5 and ≈8 μm, respectively. Lastly, NR did not permeate the SC. We concluded that the amplitude-weighted mean fluorescence lifetime is the most appropriate FLIM parameter to build up penetration profiles. This work is anticipated to provide a non-invasive TPT-FLIM method for studying the penetration of topically applied drugs and cosmetics into the skin.

A turquoise fluorescence lifetime-based biosensor for quantitative imaging of intracellular calcium

The most successful genetically encoded calcium indicators (GECIs) employ an intensity or ratiometric readout. Despite a large calcium-dependent change in fluorescence intensity, the quantification of calcium concentrations with GECIs is problematic, which is further complicated by the sensitivity of all GECIs to changes in the pH in the biological range. Here, we report on a sensing strategy in which a conformational change directly modifies the fluorescence quantum yield and fluorescence lifetime of a circular permutated turquoise fluorescent protein. The fluorescence lifetime is an absolute parameter that enables straightforward quantification, eliminating intensity-related artifacts. An engineering strategy that optimizes lifetime contrast led to a biosensor that shows a 3-fold change in the calcium-dependent quantum yield and a fluorescence lifetime change of 1.3 ns. We dub the biosensor Turquoise Calcium Fluorescence LIfeTime Sensor (Tq-Ca-FLITS). The response of the calcium sensor is insensitive to pH between 6.2-9. As a result, Tq-Ca-FLITS enables robust measurements of intracellular calcium concentrations by fluorescence lifetime imaging. We demonstrate quantitative imaging of calcium concentrations with the turquoise GECI in single endothelial cells and human-derived organoids.

Near-Infrared Inorganic Nanomaterials for Precise Diagnosis and Therapy

Traditional wavelengths (400–700 nm) have made tremendous inroads in vivo fluorescence imaging. However, the ability of visible light photon penetration hampered the bio-applications. With reduced photon scattering, minimal tissue absorption and negligible autofluorescence properties, near-infrared light (NIR 700–1700 nm) demonstrates better resolution, high signal-to-background ratios, and deep tissue penetration capability, which will be of great significance for in-vivo determination in deep tissue. In this review, we summarized the latest novel NIR inorganic nanomaterials and the emission mechanism including single-walled carbon nanotubes, rare-earth nanoparticles, quantum dots, metal nanomaterials. Subsequently, the recent progress of precise noninvasive diagnosis in biomedicine and cancer therapy utilizing near-infrared inorganic nanomaterials are discussed. In addition, this review will highlight the concerns, challenges and future directions of near-infrared light utilization.

Quantitative Confocal Microscopy for Grouping of Dose-Response Data: Deciphering Calcium Sequestration and Subsequent Cell Death in the Presence of Excess Norepinephrine

Fluorescent calcium (Ca²⁺) imaging is one of the preferred methods to record cellular activity during in vitro preclinical studies, high-content drug screening, and toxicity analysis. Visualization and analysis for dose-response data obtained using high-resolution imaging remain challenging, due to the inherent heterogeneity present in the Ca²⁺ spiking. To address this challenge, we propose measurement of cytosolic Ca²⁺ ions using spinning-disk confocal microscopy and machine learning-based analytics that is scalable. First, we implemented uniform manifold approximation and projection (UMAP) for visualizing the multivariate time-series dataset in the two-dimensional (2D) plane using Python. The dataset was obtained through live imaging experiments with norepinephrine-induced Ca²⁺ oscillation in HeLa cells for a large range of doses. Second, we demonstrate that the proposed framework can be used to depict the grouping of the spiking pattern for lower and higher drug doses. To the best of our knowledge, this is the first attempt at UMAP visualization of the time-series dose response and identification of the Ca²⁺ signature during lytic death. Such quantitative microscopy can be used as a component of a high-throughput data analysis workflow for toxicity analysis.

Analysis of decay kinetics of the cytosolic calcium transient induced by oxytocin in rat myometrium smooth muscle cells

The method of kinetic analysis of the relaxation phase of the mechanical response of the smooth muscle previously proposed by Burdyga and Kosterin was applied to study the dynamics of the decay of oxytocin-induced calcium transients in cytosol of the rat myometrium smooth muscle cell detected by a fluorescence signal generated by a calcium-sensitive probe fluo-4 using a laser scanning confocal microscope. The experimental data were well linearized in the coordinates ln [(F_m - F)/F] vs lnt (F and F_m are the current fluorescence intensity of the calcium probe and the fluorescence intensity at the maximum of the calcium transient, respectively, while t is the time). The empirical parameters n and τ were determined by which the maximal normalized relaxation rate V_n was calculated for five different ROIs (regions of interest) in the myocyte cytosol. It proved to be almost the same for all ROIs. The maximal normalized relaxation rate calculated from the fluorescence intensity was always lower than that calculated from the corresponding calcium concentration, i.e. the cytosolic Ca²⁺ concentration in the relaxation phase decreases faster than the corresponding fluorescence intensity. The value of the maximal normalized relaxation rate calculated both from the fluorescence intensity and from the force of oxytocin-induced contractions of isolated rat uterus longitudinal smooth muscles (according to Tsymbalyuk and Kosterin) was exactly the same. This indicates that in the relaxation phase, the decreasing curves of both the fluorescence intensity and the contraction forces coincide.

Theoretical investigations of a modified compressed ultrafast photography method suitable for single-shot fluorescence lifetime imaging

A single-shot fluorescence lifetime imaging (FLIM) method based on the compressed ultrafast photography (CUP) is proposed, named space-restricted CUP (srCUP). srCUP is suitable for imaging objects moving slowly (<∼150/Mmm/s, M is the magnification of the objective lens) in the field of view with the intensity changing within nanoseconds in a measurement window around 10 ns. We used synthetic datasets to explore the performances of srCUP compared with CUP and TCUP (a variant of CUP). srCUP not only provides superior reconstruction performances, but its reconstruction speed is also twofold and threefold faster than CUP and TCUP, respectively. The lifetime determination performances were assessed by estimating lifetime components, amplitude- and intensity-weighted average lifetimes (τ_A and τ_I), with the reconstructed scenes using the least squares method based on a bi-exponential model. srCUP has the best accuracy and precision for lifetime determinations with a relative bias less than 7% and a coefficient of variation less than 7% for τ_A, and a relative bias less than 10% and a coefficient of variation less than 11% for τ_I.

Fluorescence lifetime imaging reveals regulation of presynaptic Ca2+ by glutamate uptake and mGluRs, but not somatic voltage in cortical neurons

Brain function relies on vesicular release of neurotransmitters at chemical synapses. The release probability depends on action potential-evoked presynaptic Ca2+ entry, but also on the resting Ca2+ level. Whether these basic aspects of presynaptic calcium homeostasis show any consistent trend along the axonal path, and how they are controlled by local network activity, remains poorly understood. Here, we take advantage of the recently advanced FLIM-based method to monitor presynaptic Ca2+ with nanomolar sensitivity. We find that, in cortical pyramidal neurons, action potential-evoked calcium entry (range 10-300 nM), but not the resting Ca2+ level (range 10-100 nM), tends to increase with higher order of axonal branches. Blocking astroglial glutamate uptake reduces evoked Ca2+ entry but has little effect on resting Ca2+ whereas both appear boosted by the constitutive activation of group 1/2 metabotropic glutamate receptors. We find no consistent effect of transient somatic depolarization or hyperpolarization on presynaptic Ca2+ entry or its basal level. The results unveil some key aspects of presynaptic machinery in cortical circuits, shedding light on basic principles of synaptic connectivity in the brain.

A Guide to Fluorescence Lifetime Microscopy and Förster's Resonance Energy Transfer in Neuroscience

Fluorescence lifetime microscopy (FLIM) and Förster's resonance energy transfer (FRET) are advanced optical tools that neuroscientists can employ to interrogate the structure and function of complex biological systems in vitro and in vivo using light. In neurobiology they are primarily used to study protein-protein interactions, to study conformational changes in protein complexes, and to monitor genetically encoded FRET-based biosensors. These methods are ideally suited to optically monitor changes in neurons that are triggered optogenetically. Utilization of this technique by neuroscientists has been limited, since a broad understanding of FLIM and FRET requires familiarity with the interactions of light and matter on a quantum mechanical level, and because the ultra-fast instrumentation used to measure fluorescent lifetimes and resonance energy transfer are more at home in a physics lab than in a biology lab. In this overview, we aim to help neuroscientists overcome these obstacles and thus feel more comfortable with the FLIM-FRET method. Our goal is to aid researchers in the neuroscience community to achieve a better understanding of the fundamentals of FLIM-FRET and encourage them to fully leverage its powerful ability as a research tool. Published 2020. U.S. Government.

High-Accuracy Detection of Neuronal Ensemble Activity in Two-Photon Functional Microscopy Using Smart Line Scanning

Two-photon functional imaging using genetically encoded calcium indicators (GECIs) is one prominent tool to map neural activity. Under optimized experimental conditions, GECIs detect single action potentials in individual cells with high accuracy. However, using current approaches, these optimized conditions are never met when imaging large ensembles of neurons. Here, we developed a method that substantially increases the signal-to-noise ratio (SNR) of population imaging of GECIs by using galvanometric mirrors and fast smart line scan (SLS) trajectories. We validated our approach in anesthetized and awake mice on deep and dense GCaMP6 staining in the mouse barrel cortex during spontaneous and sensory-evoked activity. Compared to raster population imaging, SLS led to increased SNR, higher probability of detecting calcium events, and more precise identification of functional neuronal ensembles. SLS provides a cheap and easily implementable tool for high-accuracy population imaging of neural GCaMP6 signals by using galvanometric-based two-photon microscopes.

New luminescence lifetime macro-imager based on a Tpx3Cam optical camera

The properties of a novel ultra-fast optical imager, Tpx3Cam, were investigated for macroscopic wide-field phosphorescent lifetime imaging (PLIM) applications. The camera is based on a novel optical sensor and Timepix3 readout chip with a time resolution of 1.6 ns, recording of photon arrival time and time over threshold for each pixel, and readout rate of 80 megapixels per second. In this study, we coupled the camera to an image intensifier, a 760 nm emission filter and a 50 mm lens, and with a super-bright 627nm LED providing pulsed excitation of a 18 × 18 mm sample area. The resulting macro-imager with compact and rigid optical alignment of its main components was characterised using planar phosphorescent O2 sensors and a resolution plate mask. Several acquisition and image processing algorithms were evaluated to optimise the system resolution and performance for the wide-field PLIM, followed by imaging a variety of phosphorescent samples. The new PLIM system looks promising, particularly for phosphorescence lifetime-based imaging of O2 in various chemical and biological samples.

Polymer microchamber arrays for geometry-controlled drug release: a functional study in human cells of neuronal phenotype

Polyelectrolyte multilayer (PEM) microchambers can provide a versatile cargo delivery system enabling rapid, site-specific drug release on demand.

Polyelectrolyte multilayer (PEM) microchambers can provide a versatile cargo delivery system enabling rapid, site-specific drug release on demand. However, experimental evidence for their potential benefits in live human cells is scarce. Equally, practical applications often require substance delivery that is geometrically constrained and highly localized. Here, we establish human-cell biocompatibility and on-demand cargo release properties of the PEM or polylactic acid (PLA)-based microchamber arrays fabricated on a patterned film base. We grow human N2A cells (a neuroblastoma cell line widely used for studies of neurotoxicity) on the surface of the patterned microchamber arrays loaded with either a fluorescent indicator or the ubiquitous excitatory neurotransmitter glutamate. The differentiating human N2A cells show no detrimental effects on viability when growing on either PEM@PLA or PLA-based arrays for up to ten days in vitro. Firstly, we use two-photon (2P) excitation with femtosecond laser pulses to open individual microchambers in a controlled way while monitoring release and diffusion of the fluorescent cargo (rhodamine or FITC fluorescent dye). Secondly, we document the increases in intracellular Ca²⁺ in local N2A cells in response to the laser-triggered glutamate release from individual microchambers. The functional cell response is site-specific and reproducible on demand and could be replicated by applying glutamate to the cells using a pressurised micropipette. Time-resolved fluorescence imaging confirms the physiological range of the glutamate-evoked intracellular Ca²⁺ dynamics in the differentiating N2A cells. Our data indicate that the nano-engineering design of the fabricated PEM or PLA-based patterned microchamber arrays could provide a biologically safe and efficient tool for targeted, geometrically constrained drug delivery.

A smart nanoprobe based on a gadolinium complex encapsulated by ZIF-8 with enhanced room temperature phosphorescence for synchronous oxygen sensing and photodynamic therapy

The phosphorescence lifetime approach based on the room temperature phosphorescence (RTP) property has received considerable attention in recent years due to its excellent performance in the precise measurement of oxygen. Herein, a smart nanoprobe, Gd[PC]@ZIF-8, was designed and assembled by homogenously encapsulating a rare-earth complex phosphor Gd[(Pyr)₄cyclen] (Pyr = pyrenol) into a zeolitic imidazolate framework (ZIF-8). Because of the restriction of the metal-organic framework (MOF) matrix and host-guest interactions, the nanoprobe Gd[PC]@ZIF-8 exhibited highly enhanced RTP properties, including intensity, quantum yield, and elongated decay lifetime. It displayed an outstanding linear relationship between the phosphorescence decay lifetime, intensity and oxygen concentration, which can be applied in the field of oxygen sensing. Moreover, the complex Gd[(Pyr)₄cyclen] in the nanoprobe Gd[PC]@ZIF-8 served as a favorable photosensitizer that resulted in the simultaneous conversion of sufficient oxygen molecules into single state oxygen (¹O₂) under irradiation during the phosphorescence quenching process, which is conducive to photodynamic therapy (PDT). Thus, the design of the smart nanoprobe Gd[PC]@ZIF-8 in this study provides an ingenious strategy of utilizing a MOF as a matrix to enhance the RTP properties of phosphors for synchronous oxygen sensing and PDT.

Quantitative determination of cellular [Na+] by fluorescence lifetime imaging with CoroNaGreen

Meyer et al. establish the suitability of the sodium-sensitive indicator dye CoroNaGreen for fluorescence lifetime imaging inside cells. This approach represents a valuable tool for quantitative and dynamic determination of intracellular sodium concentrations independent of dye concentration.

Fluorescence lifetime imaging microscopy (FLIM) with fluorescent ion sensors enables the measurement of ion concentrations based on the detection of photon emission events after brief excitation with a pulsed laser source. In contrast to intensity-based imaging, it is independent of dye concentration, photobleaching, or focus drift and has thus been successfully employed for quantitative analysis of, e.g., calcium levels in different cell types and cellular microdomains. Here, we tested the suitability of CoroNaGreen for FLIM-based determination of sodium concentration ([Na⁺]) inside cells. In vitro measurements confirmed that fluorescence lifetimes of CoroNaGreen (CoroNaFL) increased with increasing [Na⁺]. Moreover, CoroNaFL was largely independent of changes in potassium concentration or viscosity. Changes in pH slightly affected FL in the acidic range (pH ≤ 5.5). For intracellular determination of [Na⁺], HEK293T cells were loaded with the membrane-permeable form of CoroNaGreen. Fluorescence decay curves of CoroNaGreen, derived from time-correlated single-photon counting, were approximated by a bi-exponential decay. In situ calibrations revealed a sigmoidal dependence of CoroNaFL on [Na⁺] between 0 and 150 mM, exhibiting an apparent K_d of ∼80 mM. Based on these calibrations, a [Na⁺] of 17.6 mM was determined in the cytosol. Cellular nuclei showed a significantly lower [Na⁺] of 13.0 mM, whereas [Na⁺] in perinuclear regions was significantly higher (26.5 mM). Metabolic inhibition or blocking the Na⁺/K⁺-ATPase by removal of extracellular K⁺ caused significant [Na⁺] increases in all cellular subcompartments. Using an alternative approach for data analysis (“Ratio FLIM”) increased the temporal resolution and revealed a sequential response to K⁺ removal, with cytosolic [Na⁺] increasing first, followed by the nucleus and finally the perinuclear regions. Taken together, our results show that CoroNaGreen is suitable for dynamic, FLIM-based determination of intracellular [Na⁺]. This approach thus represents a valuable tool for quantitative determination of [Na⁺] and changes thereof in different subcellular compartments.

Multiplex imaging relates quantal glutamate release to presynaptic Ca2+ homeostasis at multiple synapses in situ

Information processing by brain circuits depends on Ca2+-dependent, stochastic release of the excitatory neurotransmitter glutamate. Whilst optical glutamate sensors have enabled detection of synaptic discharges, understanding presynaptic machinery requires simultaneous readout of glutamate release and nanomolar presynaptic Ca2+ in situ. Here, we find that the fluorescence lifetime of the red-shifted Ca2+ indicator Cal-590 is Ca2+-sensitive in the nanomolar range, and employ it in combination with green glutamate sensors to relate quantal neurotransmission to presynaptic Ca2+ kinetics. Multiplexed imaging of individual and multiple synapses in identified axonal circuits reveals that glutamate release efficacy, but not its short-term plasticity, varies with time-dependent fluctuations in presynaptic resting Ca2+ or spike-evoked Ca2+ entry. Within individual presynaptic boutons, we find no nanoscopic co-localisation of evoked presynaptic Ca2+ entry with the prevalent glutamate release site, suggesting loose coupling between the two. The approach enables a better understanding of release machinery at central synapses.

Bright Dots and Smart Optical Microscopy to Probe Intracellular Events in Single Cells

Probing intracellular events is a key step in developing new biomedical methodologies. Optical microscopy has been one of the best options to observe biological samples at single cell and sub-cellular resolutions. Morphological changes are readily detectable in brightfield images. When stained with fluorescent molecules, distributions of intracellular organelles, and biological molecules are made visible using fluorescence microscopes. In addition to these morphological views of cells, optical microscopy can reveal the chemical and physical status of defined intracellular spaces. This review begins with a brief overview of genetically encoded fluorescent probes and small fluorescent chemical dyes. Although these are the most common approaches, probing is also made possible by using tiny materials that are incorporated into cells. When these tiny materials emit enough photons, it is possible to draw conclusions about the environment in which the tiny material resides. Recent advances in these tiny but sufficiently bright fluorescent materials are nextly reviewed to show their applications in tracking target molecules and in temperature imaging of intracellular spots. The last section of this review addresses purely optical methods for reading intracellular status without staining with probes. These non-labeling methods are especially essential when biospecimens are thereafter required for in vivo uses, such as in regenerative medicine.

The Use of ex Vivo Rodent Platforms in Neuroscience Translational Research With Attention to the 3Rs Philosophy

The principles of the 3Rs—Replacement, Reduction, and Refinement—are at the basis of most advanced national and supranational (EU) regulations on animal experimentation and welfare. In the perspective to reduce and refine the use of these animals in translational research, we here discuss the use of rodent acute and organotypically cultured central nervous system slices. We describe novel applications of these ex vivo platforms in medium-throughput screening of neuroactive molecules of potential pharmacological interest, with particular attention to more recent developments that permit to fully exploit the potential of direct genetic engineering of organotypic cultures using transfection techniques. We then describe the perspectives for expanding the use ex vivo platforms in neuroscience studies under the 3Rs philosophy using the following approaches: (1) Use of co-cultures of two brain regions physiologically connected to each other (source-target) to analyze axon regeneration and reconstruction of circuitries; (2) Microinjection or co-cultures of primary cells and/or cell lines releasing one or more neuroactive molecules to screen their physiological and/or pharmacological effects onto neuronal survival and slice circuitry. Microinjected or co-cultured cells are ideally made fluorescent after transfection with a plasmid construct encoding green or red fluorescent protein under the control of a general promoter such as hCMV; (3) Use of “sniffer” cells sensing the release of biologically active molecules from organotypic cultures by means of fluorescent probes. These cells can be prepared with activatable green fluorescent protein, a unique chromophore that remains in a “dark” state because its maturation is inhibited, and can be made fluorescent (de-quenched) if specific cellular enzymes, such as proteases or kinases, are activated.