ANG-2 for quantitative Na(+) determination in living cells by time-resolved fluorescence microscopy

Spatial and temporal aspects of neuronal calcium and sodium signals measured with low-affinity fluorescent indicators

Low-affinity fluorescent indicators for Ca2+ or Na+ allow measuring the dynamics of intracellular concentration of these ions with little perturbation from physiological conditions because they are weak buffers. When using synthetic indicators, which are small molecules with fast kinetics, it is also possible to extract spatial and temporal information on the sources of ion transients, their localization, and their disposition. This review examines these important aspects from the biophysical point of view, and how they have been recently exploited in neurophysiological studies. We first analyze the environment where Ca2+ and Na+ indicators are inserted, highlighting the interpretation of the two different signals. Then, we address the information that can be obtained by analyzing the rising phase and the falling phase of the Ca2+ and Na+ transients evoked by different stimuli, focusing on the kinetics of ionic currents and on the spatial interpretation of these measurements, especially on events in axons and dendritic spines. Finally, we suggest how Ca2+ or Na+ imaging using low-affinity synthetic fluorescent indicators can be exploited in future fundamental or applied research.

Ang II acutely stimulates Na,K-pump in cells from proximal tubules by increasing its phosphorylation at S938 via a PI3K/AKT pathway

Angiotensin II (Ang II)-dependent stimulation of the AT1 receptor in proximal tubules increases sodium reabsorption and blood pressure. Reabsorption is driven by the Na,K-pump that is acutely stimulated by Ang II, which requires phosphorylation of serine-938 (S938). This site is present in humans and only known to phosphorylated by PKA. Yet, activation of AT1 decreases cAMP required to activate PKA and inhibiting PKA does not block Ang II-dependent phosphorylation of S938. We tested the hypothesis that Ang II-dependent activation is mediated via increased phosphorylation at S938 through a PI3K/AKT-dependent pathway. Experiments were conducted using opossum kidney cells, a proximal tubule cell line, stably co-expressing the AT1 receptor and either the wild-type (α-1.wild-type) or an alanine substituted (α-1.S938A) form of rat kidney Na,K-pump. A 5-min exposure to 10 pM Ang II significantly activated Na,K-pump activity (56%) measured as short-circuit current across polarized α-1.wild-type cells. Wortmannin, at a concentration that selectively inhibits PI3K, blocked that Ang II-dependent activation. Ang II did not stimulate Na,K-pump activity in α-1.S938A cells. Ang II at 10 and 100 pM increased phosphorylation at S938 in α-1.wild-type cells measured in whole cell lysates. The increase was inhibited by wortmannin plus H-89, an inhibitor of PKA, not by either alone. Ang II activated AKT inhibited by wortmannin, not H-89. These data support our hypothesis and show that Ang II-dependent phosphorylation at S938 stimulates Na,K-pump activity and transcellular sodium transport.

Chemical and Biological Tools for the Study of Voltage-Gated Sodium Channels in Electrogenesis and Nociception

The malfunction and misregulation of voltage-gated sodium channels (NaV s) underlie in large part the electrical hyperexcitability characteristic of chronic inflammatory and neuropathic pain. NaV s are responsible for the initiation and propagation of electrical impulses (action potentials) in cells. Tissue and nerve injury alter the expression and localization of multiple NaV isoforms, including NaV 1.1, 1.3, and 1.6-1.9, resulting in aberrant action potential firing patterns. To better understand the role of NaV regulation, localization, and trafficking in electrogenesis and pain pathogenesis, a number of chemical and biological reagents for interrogating NaV function have been advanced. The development and application of such tools for understanding NaV physiology are the focus of this review.

Fluorescent Indicators For Biological Imaging of Monatomic Ions

Monatomic ions play critical biological roles including maintaining the cellular osmotic pressure, transmitting signals, and catalyzing redox reactions as cofactors in enzymes. The ability to visualize monatomic ion concentration, and dynamic changes in the concentration, is essential to understanding their many biological functions. A growing number of genetically encodable and synthetic indicators enable the visualization and detection of monatomic ions in biological systems. With this review, we aim to provide a survey of the current landscape of reported indicators. We hope this review will be a useful guide to researchers who are interested in using indicators for biological applications and to tool developers seeking opportunities to create new and improved indicators.

Ultrafast Sodium Imaging of the Axon Initial Segment of Neurons in Mouse Brain Slices

Monitoring Na⁺ influx in the axon initial segment (AIS) at high spatial and temporal resolution is fundamental to understanding the generation of an action potential (AP). Here, we present protocols to obtain this measurement, focusing on the AIS of layer 5 (L5) somatosensory cortex pyramidal neurons in mouse brain slices. We first outline how to prepare slices for this application, how to select and patch neurons, and how to optimize the image acquisition. Specifically, we describe the preparation of optimal slices, patching and loading of L5 pyramidal neurons with the Na⁺ indicator ING-2, and Na⁺ imaging at 100 µs temporal resolution with a pixel resolution of half a micron. Then, we present a data analysis strategy in order to extract information on the kinetics of activated voltage-gated Na⁺ channels by determining the change in Na⁺ by compensating for bleaching and calculating the time derivative of the resulting fit. In sum, this approach can be widely applied when investigating the function of Na⁺ channels during initiation of an AP and propagation under physiological or pathological conditions in neuronal subtypes. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Preparation of cortical slices Basic Protocol 2: Selection, patching, and Na⁺ fluorescence recording of a neuron Support Protocol: Calibrating Na⁺ fluorescence Basic Protocol 3: Data analysis.

Advances in the development of novel compounds targeting cation-chloride cotransporter physiology

For about half a century, the pharmacology of electroneutral cation-chloride cotransporters has been dominated by a few drugs that are widely used in clinical medicine. Because these diuretic drugs are so good at what they do, there has been little incentive in expanding their pharmacology. The increasing realization that cation-chloride cotransporters are involved in many other key physiological processes and the knowledge that different tissues express homologous proteins with matching transport functions have rekindled interest in drug discovery. This review summarizes the methods available to assess the function of these transporters and describe the multiple efforts that have made to identify new compounds. We describe multiple screens targeting KCC2 function and one screen designed to find compounds that discriminate between NKCC1 and NKCC2. Two of the KCC2 screens identified new inhibitors that are 3-4 orders of magnitude more potent than furosemide. Additional screens identified compounds that purportedly increase cell surface expression of the cotransporter, as well as several FDA-approved drugs that increase KCC2 transcription and expression. The technical details of each screen biased them toward specific processes in the life cycle of the transporter, making these efforts independent and complementary. In addition, each drug discovery effort contributes to our understanding of the biology of the cotransporters.

Optical measurement of physiological sodium currents in the axon initial segment

KEY POINTS

Τhe axonal Na⁺ fluorescence underlying an action potential in the axon initial segment was optically measured at unprecedented temporal resolution. The measurement allowed resolution of the kinetics of the Na⁺ current at different axonal locations. The distinct components of the Na⁺ current were correlated with the kinetics of the action potential. NEURON simulations from a modified published model qualitatively predicted the experimentally measured Na⁺ current. The present method permits the direct investigation of the kinetic behaviour of native Na⁺ channels under physiological and pathological conditions.

ABSTRACT

In most neurons of the mammalian central nervous system, the action potential (AP) is generated in the axon initial segment (AIS) by a fast Na⁺ current mediated by voltage-gated Na⁺ channels. While the axonal Na⁺ signal associated with the AP has been measured using fluorescent Na⁺ indicators, the insufficient resolution of these recordings has not allowed tracking the Na⁺ current kinetics underlying this fundamental event. In this article, we report the first optical measurement of Na⁺ currents in the AIS of pyramidal neurons of layer 5 of the somatosensory cortex from brain slices of the mouse. This measurement was obtained by achieving a temporal resolution of 100 μs in the Na⁺ imaging technique, with a pixel resolution of 0.5 μm, and by calculating the time-derivative of the Na⁺ change corrected for longitudinal diffusion. We identified a subthreshold current before the AP, a fast-inactivating current peaking during the rise of the AP and a non-inactivating current during the AP repolarization. We established a correlation between the kinetics of the non-inactivating current at different distances from the soma and the kinetics of the somatic AP. We quantitatively compared the experimentally measured Na⁺ current with the current obtained by computer simulation of published NEURON models, demonstrating how the present approach can lead to the correct estimate of the native behaviour of Na⁺ channels. Finally, we discuss how the present approach can be used to investigate the physiological or pathological function of different channel types during AP initiation and propagation.

Sensitive and non-invasive cholesterol determination in saliva via optimization of enzyme loading and platinum nano-cluster composition

To provide rapid and accurate determination of cholesterol, we have developed a simple, disposable, enzyme-based salivary cholesterol biosensor.

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.

Na+ Selective Fluorescent Tools Based on Fluorescence Intensity Enhancements, Lifetime Changes, and on a Ratiometric Response

Over the years, we developed highly selective fluorescent probes for K+ in water, which show K+ -induced fluorescence intensity enhancements, lifetime changes, or a ratiometric behavior at two emission wavelengths (cf. Scheme 1, K1-K4). In this paper, we introduce selective fluorescent probes for Na+ in water, which also show Na+ induced signal changes, which are analyzed by diverse fluorescence techniques. Initially, we synthesized the fluorescent probes 2, 4, 5, 6 and 10 for a fluorescence analysis by intensity enhancements at one wavelength by varying the Na+ responsive ionophore unit and the fluorophore moiety to adjust different Kd values for an intra- or extracellular Na+ analysis. Thus, we found that 2, 4 and 5 are Na+ selective fluorescent tools, which are able to measure physiologically important Na+ levels at wavelengths higher than 500 nm. Secondly, we developed the fluorescent probes 7 and 8 to analyze precise Na+ levels by fluorescence lifetime changes. Herein, only 8 (Kd =106 mm) is a capable fluorescent tool to measure Na+ levels in blood samples by lifetime changes. Finally, the fluorescent probe 9 was designed to show a Na+ induced ratiometric fluorescence behavior at two emission wavelengths. As desired, 9 (Kd =78 mm) showed a ratiometric fluorescence response towards Na+ ions and is a suitable tool to measure physiologically relevant Na+ levels by the intensity change of two emission wavelengths at 404 nm and 492 nm.

Genetic and pharmacological inactivation of apical Na+-K+-2Cl- cotransporter 1 in choroid plexus epithelial cells reveals the physiological function of the cotransporter

Choroid plexus epithelial cells (CPECs) secrete cerebrospinal fluid (CSF). They express Na+-K+-ATPase and Na+-K+-2Cl- cotransporter 1 (NKCC1) on their apical membrane, deviating from typical basolateral membrane location in secretory epithelia. Given this peculiarity, the direction of basal net ion fluxes mediated by NKCC1 in CPECs is controversial, and cotransporter function is unclear. Determining the direction of basal NKCC1-mediated fluxes is critical to understanding the function of apical NKCC1. If NKCC1 works in the net efflux mode, it may be directly involved in CSF secretion. Conversely, if NKCC1 works in the net influx mode, it would have an absorptive function, contributing to intracellular Cl- concentration ([Cl-]i) and cell water volume (CWV) maintenance needed for CSF secretion. We resolve this long-standing debate by electron microscopy (EM), live-cell-imaging microscopy (LCIM), and intracellular Na+ and Cl- measurements in single CPECs of NKCC1+/+ and NKCC1-/- mouse. NKCC1-mediated ion and associated water fluxes are tightly linked, thus their direction is inferred by measuring CWV changes. Genetic or pharmacological NKCC1 inactivation produces CPEC shrinkage. EM of NKCC1-/- CPECs in situ shows they are shrunken, forming large dilations of their basolateral extracellular spaces, yet remaining attached by tight junctions. Normarski LCIM shows in vitro CPECs from NKCC1-/- are ~17% smaller than NKCC1+/+. CWV measurements in calcein-loaded CPECs show that bumetanide (10 μM) produces ~16% decrease in CWV in NKCC1+/+ but not in NKCC1-/- CPECs. Our findings suggest that under basal conditions apical NKCC1 is continuously active and works in the net inward flux mode maintaining [Cl-]i and CWV needed for CSF secretion.

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.

Photophysical properties of Na+ -indicator dyes suitable for quantitative two-photon fluorescence-lifetime measurements

Two-photon microscopy (2PM) offers great potential in fluorescence imaging of intracellular Na⁺ dynamics of live cells. A severe drawback, however, is that quantitative ratioing of fluorescence intensities at different wavelengths [possible in one-photon imaging with the classical Na⁺ -indicator dye sodium-binding benzofuran isophtalate (SBFI)] is not practical in 2PM. We aimed at establishing 2PM-based time-correlated fluorescence lifetime measurements as an alternative method for quantifying Na⁺ dynamics. We compared the photophysical properties of the four Na⁺ -sensitive fluorescent indicator dyes SBFI, CoroNa Green, Sodium Green and Asante NaTRIUM Green-2 (ANG-2) in cuvette calibrations. All four dyes showed Na⁺ -dependent intensity changes, with ANG-2 having the most favourable properties for 2PM. All dyes but SBFI showed significant changes in their fluorescence lifetime upon Na⁺ binding, again with ANG-2 being the most promising dye. We found that, unfortunately, the fluorescence lifetime of ANG-2 is not only affected by Na⁺ but also by protons, K⁺ and dye impurities, rendering a quantitative description of the individual lifetime components impractical. However, a simplified calibration procedure, based on a published approach for Ca²⁺ imaging, allowed relating lifetimes to Na⁺ concentration. Using ANG-2 and the simplified calibration will allow quantitative two-photon Na⁺ imaging with millimolar sensitivity.

LAY DESCRIPTION

Dynamic changes of ion concentrations, which play crucial roles in cellular physiology, can be monitored with appropriate fluorescent indicator dyes. For intracellular sodium ions (Na⁺ ), certain dyes even allow quantitative measurements with standard microscopic techniques. However, for two-photon microscopy, which allows resolving cells deep in intact tissue, imaging solutions that are fully quantitative are lacking. For the four commercially available Na⁺ dyes 'SBFI', 'CoroNa Green', 'Sodium Green', and 'Asante NaTRIUM Green-2' (ANG2) we analyzed whether their fluorescent lifetime (LT), i.e., the nanosecond decay of emission of photons after a pulsed excitation, could serve as a quantitative measure of intracellular Na⁺ . Pulsed excitation in the femtosecond range is an inherent feature of two-photon microscopy and, in combination with fast, single-photon counting microscopes, allows for easy-to-implement LT microscopy. We found that Sodium Green and ANG2 showed strong Na⁺ -dependent changes in the fluorescence LT, while SBFI showed no, and CoroNa Green only small changes. ANG2, as the brightest dye, was further characterized regarding effects of protons and potassium ions (K⁺ ), both also present in cells at significant concentrations, on the fluorescence LT. We found that the LT of ANG2 is affected in a predictable manner by Na⁺ , K⁺ , and protons. However, our data reveal that the commercial dye must also contain impurities with unexpected Na⁺ - and K⁺ -binding characteristics, rendering a quantitative description of the individual lifetime components impractical. We, therefore, adapted a simplified calibration procedure, based on a published approach for Ca²⁺ imaging, that allows relating the average lifetime to Na⁺ concentration. With this simplified calibration procedure, ANG2 is well suited for quantitative two-photon Na⁺ imaging with millimolar sensitivity.

Two-photon probes for in vivo multicolor microscopy of the structure and signals of brain cells

Imaging the brain of living laboratory animals at a microscopic scale can be achieved by two-photon microscopy thanks to the high penetrability and low phototoxicity of the excitation wavelengths used. However, knowledge of the two-photon spectral properties of the myriad fluorescent probes is generally scarce and, for many, non-existent. In addition, the use of different measurement units in published reports further hinders the design of a comprehensive imaging experiment. In this review, we compile and homogenize the two-photon spectral properties of 280 fluorescent probes. We provide practical data, including the wavelengths for optimal two-photon excitation, the peak values of two-photon action cross section or molecular brightness, and the emission ranges. Beyond the spectroscopic description of these fluorophores, we discuss their binding to biological targets. This specificity allows in vivo imaging of cells, their processes, and even organelles and other subcellular structures in the brain. In addition to probes that monitor endogenous cell metabolism, studies of healthy and diseased brain benefit from the specific binding of certain probes to pathology-specific features, ranging from amyloid-β plaques to the autofluorescence of certain antibiotics. A special focus is placed on functional in vivo imaging using two-photon probes that sense specific ions or membrane potential, and that may be combined with optogenetic actuators. Being closely linked to their use, we examine the different routes of intravital delivery of these fluorescent probes according to the target. Finally, we discuss different approaches, strategies, and prerequisites for two-photon multicolor experiments in the brains of living laboratory animals.

Local tissue manipulation via a force- and pressure-controlled AFM micropipette for analysis of cellular processes

Local manipulation of complex tissues at the single-cell level is challenging and requires excellent sealing between the specimen and the micromanipulation device. Here, biological applications for a recently developed loading technique for a force- and pressure-controlled fluidic force microscope micropipette are described. This technique allows for the exact positioning and precise spatiotemporal control of liquid delivery. The feasibility of a local loading technique for tissue applications was investigated using two fluorescent dyes, with which local loading behaviour could be optically visualised. Thus, homogeneous intracellular distribution of CellTracker Red and accumulation of SYTO 9 Green within nuclei was realised in single cells of a tissue preparation. Subsequently, physiological micromanipulation experiments were performed. Salivary gland tissue was pre-incubated with the Ca2+-sensitive dye OGB-1. An intracellular Ca2+ rise was then initiated at the single-cell level by applying dopamine via micropipette. When pre-incubating tissue with the nitric oxide (NO)-sensitive dye DAF-FM, NO release and intercellular NO diffusion was observed after local application of the NO donor SNP. Finally, local micromanipulation of a well-defined area along irregularly shaped cell surfaces of complex biosystems was shown for the first time for the fluidic force microscope micropipette. Thus, this technique is a promising tool for the investigation of the spatiotemporal effects of locally applied substances in complex tissues.

Highly K+ -Selective Fluorescent Probes for Lifetime Sensing of K+ in Living Cells

The new K⁺ -selective fluorescent probes 1 and 2 were obtained by Cu^I -catalyzed 1,3-dipolar azide alkyne cycloaddition (CuAAC) reactions of an alkyne-substituted [1,3]dioxolo[4,5-f][1,3]benzodioxole (DBD) ester fluorophore with azido-functionalized N-phenylaza-18-crown-6 ether and N-(o-isopropoxy) phenylaza-18-crown-6 ether, respectively. Probes 1 and 2 allow the detection of K⁺ in the presence of Na⁺ in water by fluorescence enhancement (2.2 for 1 at 2000 mm K⁺ and 2.5 for 2 at 160 mm K⁺ ). Fluorescence lifetime measurements in the absence and presence of K⁺ revealed bi-exponential decay kinetics with similar lifetimes, however with different proportions changing the averaged fluorescence decay times (τ_f(av) ). For 1 a decrease of τ_f(av) from 12.4 to 9.3 ns and for 2 an increase from 17.8 to 21.8 ns was observed. Variation of the substituent in ortho position of the aniline unit of the N-phenylaza-18-crown-6 host permits the modulation of the K_d value for a certain K⁺ concentration. For example, substitution of H in 1 by the isopropoxy group (2) decreased the K_d value from >300 mm to 10 mm. 2 was chosen for studying the efflux of K⁺ from human red blood cells (RBC). Upon addition of the Ca²⁺ ionophor ionomycin to a RBC suspension in a buffer containing Ca²⁺ , the fluorescence of 2 slightly rose within 10 min, however, after 120 min a significant increase was observed.

Functional Studies of Sodium Channels: From Target to Compound Identification

Over the last six decades, voltage-gated sodium (Na_v ) channels have attracted a great deal of scientific and pharmaceutical interest, driving fundamental advances in both biology and technology. The structure and physiological function of these channels have been extensively studied; clinical and genetic data have uncovered their implication in diseases such as epilepsy, arrhythmias, and pain, bringing them into focus as current and future drug targets. While different techniques have been established to record the activity of Na_v channels, proper determination of their properties still presents serious challenges, depending upon the experimental conditions and the desired subtype of channel to be characterized. The aim of this unit is to review the characteristics of Na_v channels, their properties, the cells in which they can be studied, and the currently available techniques. Topics covered include the determination of Na_v -channel biophysical properties as well as the use of toxins to discriminate between subtypes using electrophysiological or optical methods. Perspectives on the development of high-throughput screening assays with their advantages and limitations are also discussed to allow a better understanding of the challenges encountered in voltage-gated sodium channel preclinical drug discovery. © 2016 by John Wiley & Sons, Inc.

Comparison of fluorescence probes for intracellular sodium imaging in prostate cancer cell lines

Sodium (Na⁺) ions are known to regulate many signaling pathways involved in both physiological and pathological conditions. In particular, alterations in intracellular concentrations of Na⁺ and corresponding changes in membrane potential are known to be major actors of cancer progression to metastatic phenotype. Though the functionality of Na⁺ channels and the corresponding Na⁺ currents can be investigated using the patch-clamp technique, the latter is rather invasive and a technically difficult method to study intracellular Na⁺ transients compared to Na⁺ fluorescence imaging. Despite the fact that Na⁺ signaling is considered an important controller of cancer progression, only few data using Na⁺ imaging approaches are available so far, suggesting the persisting challenge within the scientific community. In this study, we describe in detail the approach for application of Na⁺ imaging technique to measure intracellular Na⁺ variations in human prostate cancer cells. Accordingly, we used three Na⁺-specific fluorescent dyes-Na⁺-binding benzofuran isophthalate (SBFI), CoroNa™ Green (Corona) and Asante NaTRIUM Green-2 (ANG-2). These dyes have been assessed for optimal loading conditions, dissociation constant and working range after different calibration methods, and intracellular Na⁺ sensitivity, in order to determine which probe can be considered as the most reliable to visualize Na⁺ fluctuations in vitro.

Pyroptosis is driven by non-selective gasdermin-D pore and its morphology is different from MLKL channel-mediated necroptosis

Necroptosis and pyroptosis are two forms of programmed cell death with a common feature of plasma membrane rupture. Here we studied the morphology and mechanism of pyroptosis in comparison with necroptosis. Different from necroptosis, pyroptosis undergoes membrane blebbing and produces apoptotic body-like cell protrusions (termed pyroptotic bodies) prior to plasma membrane rupture. The rupture in necroptosis is explosion-like, whereas in pyroptosis it leads to flattening of cells. It is known that the execution of necroptosis is mediated by mixed lineage kinase domain-like (MLKL) oligomers in the plasma membrane, whereas gasdermin-D (GSDMD) mediates pyroptosis after its cleavage by caspase-1 or caspase-11. We show that N-terminal fragment of GSDMD (GSDMD-N) generated by caspase cleavage also forms oligomer and migrates to the plasma membrane to kill cells. Both MLKL and GSDMD-N are lipophilic and the N-terminal sequences of both proteins are important for their oligomerization and plasma membrane translocation. Unlike MLKL which forms channels on the plasma membrane that induces influx of selected ions which osmotically swell the cells to burst, GSDMD-N forms non-selective pores and does not rely on increased osmolarity to disrupt cells. Our study reveals the pore-forming activity of GSDMD and channel-forming activity of MLKL determine different ways of plasma membrane rupture in pyroptosis and necroptosis.

High throughput screening technologies for ion channels

Ion channels are involved in a variety of fundamental physiological processes, and their malfunction causes numerous human diseases. Therefore, ion channels represent a class of attractive drug targets and a class of important off-targets for in vitro pharmacological profiling. In the past decades, the rapid progress in developing functional assays and instrumentation has enabled high throughput screening (HTS) campaigns on an expanding list of channel types. Chronologically, HTS methods for ion channels include the ligand binding assay, flux-based assay, fluorescence-based assay, and automated electrophysiological assay. In this review we summarize the current HTS technologies for different ion channel classes and their applications.

Simultaneous Sodium and Calcium Imaging from Dendrites and Axons

Dynamic calcium imaging is a major technique of neuroscientists. It can reveal information about the location of various calcium channels and calcium permeable receptors, the time course, magnitude, and location of intracellular calcium concentration ([Ca²⁺]_i) changes, and indirectly, the occurrence of action potentials. Dynamic sodium imaging, a less exploited technique, can reveal analogous information related to sodium signaling. In some cases, like the examination of AMPA and NMDA receptor signaling, measurements of both [Ca²⁺]_i and [Na⁺]_i changes in the same preparation may provide more information than separate measurements. To this end, we developed a technique to simultaneously measure both signals at high speed and sufficient sensitivity to detect localized physiologic events. This approach has advantages over sequential imaging because the preparation may not respond identically in different trials. We designed custom dichroic and emission filters to allow the separate detection of the fluorescence of sodium and calcium indicators loaded together into a single neuron in a brain slice from the hippocampus of Sprague-Dawley rats. We then used high-intensity light emitting diodes (LEDs) to alternately excite the two indicators at the appropriate wavelengths. These pulses were synchronized with the frames of a CCD camera running at 500 Hz. Software then separated the data streams to provide independent sodium and calcium signals. With this system we could detect [Ca²⁺]_i and [Na⁺]_i changes from single action potentials in axons and synaptically evoked signals in dendrites, both with submicron resolution and a good signal-to-noise ratio (S/N).

Simultaneous Fluorescence and Phosphorescence Lifetime Imaging Microscopy in Living Cells

In living cells, there are always a plethora of processes taking place at the same time. Their precise regulation is the basis of cellular functions, since small failures can lead to severe dysfunctions. For a comprehensive understanding of intracellular homeostasis, simultaneous multiparameter detection is a versatile tool for revealing the spatial and temporal interactions of intracellular parameters. Here, a recently developed time-correlated single-photon counting (TCSPC) board was evaluated for simultaneous fluorescence and phosphorescence lifetime imaging microscopy (FLIM/PLIM). Therefore, the metabolic activity in insect salivary glands was investigated by recording ns-decaying intrinsic cellular fluorescence, mainly related to oxidized flavin adenine dinucleotide (FAD) and the μs-decaying phosphorescence of the oxygen-sensitive ruthenium-complex Kr341. Due to dopamine stimulation, the metabolic activity of salivary glands increased, causing a higher pericellular oxygen consumption and a resulting increase in Kr341 phosphorescence decay time. Furthermore, FAD fluorescence decay time decreased, presumably due to protein binding, thus inducing a quenching of FAD fluorescence decay time. Through application of the metabolic drugs antimycin and FCCP, the recorded signals could be assigned to a mitochondrial origin. The dopamine-induced changes could be observed in sequential FLIM and PLIM recordings, as well as in simultaneous FLIM/PLIM recordings using an intermediate TCSPC timing resolution.