Comparative measurements of membrane potentials with microelectrodes and voltage-sensitive dyes

Dependence of cell's membrane potential on extracellular voltage observed in Chara globularis

The membrane potential (Vm) of a cell results from the selective movement of ions across the cell membrane. Recent studies have revealed the presence of a gradient of voltage within a few nanometers adjacent to erythrocytes. Very notably this voltage is modified in response to changes in cell's membrane potential thus effectively extending the potential beyond the membrane and into the solution. In this study, using the microelectrode technique, we provide experimental evidence for the existence of a gradient of negative extracellular voltage (Vz) in a wide zone close to the cell wall of algal cells, extending over several micrometers. Modulating the ionic concentration of the extracellular solution with CO2 alters the extracellular voltage and causes an immediate change in Vm. Elevated extracellular CO2 levels depolarize the cell and hyperpolarize the zone of extracellular voltage (ZEV) by the same magnitude. This observation strongly suggests a coupling effect between Vz and Vm. An increase in the level of intracellular CO2 (dark respiration) leads to hyperpolarization of the cell without any immediate effect on the extracellular voltage. Therefore, the metabolic activity of a cell can proceed without inducing changes in Vz. Conversely, Vz can be modified by external stimulation without metabolic input from the cell. The evolution of the ZEV, particularly around spines and wounded cells, where ion exchange is enhanced, suggests that the formation of the ZEV may be attributed to the exchange of ions across the cell wall and cell membrane. By comparing the changes in Vm in response to external stimuli, as measured by electrodes and observed using a potential-sensitive dye, we provide experimental evidence demonstrating the significance of extracellular voltage in determining the cell's membrane potential. This may have implications for our understanding of cell membrane potential generation beyond the activities of ion channels.

The scale of zebrafish pectoral fin buds is determined by intercellular K+ levels and consequent Ca2+-mediated signaling via retinoic acid regulation of Rcan2 and Kcnk5b

K+ channels regulate morphogens to scale adult fins, but little is known about what regulates the channels and how they control morphogen expression. Using the zebrafish pectoral fin bud as a model for early vertebrate fin/limb development, we found that K+ channels also scale this anatomical structure, and we determined how one K+-leak channel, Kcnk5b, integrates into its developmental program. From FLIM measurements of a Förster Resonance Energy Transfer (FRET)-based K+ sensor, we observed coordinated decreases in intracellular K+ levels during bud growth, and overexpression of K+-leak channels in vivo coordinately increased bud proportions. Retinoic acid, which can enhance fin/limb bud growth, decreased K+ in bud tissues and up-regulated regulator of calcineurin (rcan2). rcan2 overexpression increased bud growth and decreased K+, while CRISPR-Cas9 targeting of rcan2 decreased growth and increased K+. We observed similar results in the adult caudal fins. Moreover, CRISPR targeting of Kcnk5b revealed that Rcan2-mediated growth was dependent on the Kcnk5b. We also found that Kcnk5b enhanced depolarization in fin bud cells via Na+ channels and that this enhanced depolarization was required for Kcnk5b-enhanced growth. Lastly, Kcnk5b-induced shha transcription and bud growth required IP3R-mediated Ca2+ release and CaMKK activity. Thus, we provide a mechanism for how retinoic acid via rcan2 can regulate K+-channel activity to scale a vertebrate appendage via intercellular Ca2+ signaling.

Creating cell-specific computational models of stem cell-derived cardiomyocytes using optical experiments

Human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) have gained traction as a powerful model in cardiac disease and therapeutics research, since iPSCs are self-renewing and can be derived from healthy and diseased patients without invasive surgery. However, current iPSC-CM differentiation methods produce cardiomyocytes with immature, fetal-like electrophysiological phenotypes, and the variety of maturation protocols in the literature results in phenotypic differences between labs. Heterogeneity of iPSC donor genetic backgrounds contributes to additional phenotypic variability. Several mathematical models of iPSC-CM electrophysiology have been developed to help understand the ionic underpinnings of, and to simulate, various cell responses, but these models individually do not capture the phenotypic variability observed in iPSC-CMs. Here, we tackle these limitations by developing a computational pipeline to calibrate cell preparation-specific iPSC-CM electrophysiological parameters. We used the genetic algorithm (GA), a heuristic parameter calibration method, to tune ion channel parameters in a mathematical model of iPSC-CM physiology. To systematically optimize an experimental protocol that generates sufficient data for parameter calibration, we created simulated datasets by applying various protocols to a population of in silico cells with known conductance variations, and we fitted to those datasets. We found that calibrating models to voltage and calcium transient data under 3 varied experimental conditions, including electrical pacing combined with ion channel blockade and changing buffer ion concentrations, improved model parameter estimates and model predictions of unseen channel block responses. This observation held regardless of whether the fitted data were normalized, suggesting that normalized fluorescence recordings, which are more accessible and higher throughput than patch clamp recordings, could sufficiently inform conductance parameters. Therefore, this computational pipeline can be applied to different iPSC-CM preparations to determine cell line-specific ion channel properties and understand the mechanisms behind variability in perturbation responses.

BK channels sustain neuronal Ca2+ oscillations to support hippocampal long-term potentiation and memory formation

Mutations of large conductance Ca2+- and voltage-activated K+ channels (BK) are associated with cognitive impairment. Here we report that CA1 pyramidal neuron-specific conditional BK knock-out (cKO) mice display normal locomotor and anxiety behavior. They do, however, exhibit impaired memory acquisition and retrieval in the Morris Water Maze (MWM) when compared to littermate controls (CTRL). In line with cognitive impairment in vivo, electrical and chemical long-term potentiation (LTP) in cKO brain slices were impaired in vitro. We further used a genetically encoded fluorescent K+ biosensor and a Ca2+-sensitive probe to observe cultured hippocampal neurons during chemical LTP (cLTP) induction. cLTP massively reduced intracellular K+ concentration ([K+]i) while elevating L-Type Ca2+ channel- and NMDA receptor-dependent Ca2+ oscillation frequencies. Both, [K+]i decrease and Ca2+ oscillation frequency increase were absent after pharmacological BK inhibition or in cells lacking BK. Our data suggest that L-Type- and NMDAR-dependent BK-mediated K+ outflow significantly contributes to hippocampal LTP, as well as learning and memory.

Exploitation of ATP-sensitive potassium ion (KATP) channels by HPV promotes cervical cancer cell proliferation by contributing to MAPK/AP-1 signalling

Persistent infection with high-risk human papillomaviruses (HPVs) is the causal factor in multiple human malignancies, including >99% of cervical cancers and a growing proportion of oropharyngeal cancers. Prolonged expression of the viral oncoproteins E6 and E7 is necessary for transformation to occur. Although some of the mechanisms by which these oncoproteins contribute to carcinogenesis are well-characterised, a comprehensive understanding of the signalling pathways manipulated by HPV is lacking. Here, we present the first evidence to our knowledge that the targeting of a host ion channel by HPV can contribute to cervical carcinogenesis. Through the use of pharmacological activators and inhibitors of ATP-sensitive potassium ion (KATP) channels, we demonstrate that these channels are active in HPV-positive cells and that this activity is required for HPV oncoprotein expression. Further, expression of SUR1, which forms the regulatory subunit of the multimeric channel complex, was found to be upregulated in both HPV+ cervical cancer cells and in samples from patients with cervical disease, in a manner dependent on the E7 oncoprotein. Importantly, knockdown of SUR1 expression or KATP channel inhibition significantly impeded cell proliferation via induction of a G1 cell cycle phase arrest. This was confirmed both in vitro and in in vivo tumourigenicity assays. Mechanistically, we propose that the pro-proliferative effect of KATP channels is mediated via the activation of a MAPK/AP-1 signalling axis. A complete characterisation of the role of KATP channels in HPV-associated cancer is now warranted in order to determine whether the licensed and clinically available inhibitors of these channels could constitute a potential novel therapy in the treatment of HPV-driven cervical cancer.

C-Terminal modification of polytheonamide B uncouples its dual functions in MCF-7 cancer cells

Polytheonamide B (1) is an exceptionally large peptide that forms a transmembrane ion channel. The potent cytotoxicity of 1 against MCF-7 cancer cells originates from its two ion transport functions. Compound 1 depolarizes the plasma membrane and neutralizes acidic lysosomes. Here, we describe how we uncoupled these functions by designing and synthesizing new analogues of 1.

Lipid peroxidation increases membrane tension, Piezo1 gating, and cation permeability to execute ferroptosis

The ongoing metabolic and microbicidal pathways that support and protect cellular life generate potentially damaging reactive oxygen species (ROS). To counteract damage, cells express peroxidases, which are antioxidant enzymes that catalyze the reduction of oxidized biomolecules. Glutathione peroxidase 4 (GPX4) is the major hydroperoxidase specifically responsible for reducing lipid peroxides; this homeostatic mechanism is essential, and its inhibition causes a unique type of lytic cell death, ferroptosis. The mechanism(s) that lead to cell lysis in ferroptosis, however, are unclear. We report that the lipid peroxides formed during ferroptosis accumulate preferentially at the plasma membrane. Oxidation of surface membrane lipids increased tension on the plasma membrane and led to the activation of Piezo1 and TRP channels. Oxidized membranes thus became permeable to cations, ultimately leading to the gain of cellular Na⁺ and Ca²⁺ concomitant with loss of K⁺. These effects were reduced by deletion of Piezo1 and completely inhibited by blocking cation channel conductance with ruthenium red or 2-aminoethoxydiphenyl borate (2-APB). We also found that the oxidation of lipids depressed the activity of the Na⁺/K⁺-ATPase, exacerbating the dissipation of monovalent cation gradients. Preventing the changes in cation content attenuated ferroptosis. Altogether, our study establishes that increased membrane permeability to cations is a critical step in the execution of ferroptosis and identifies Piezo1, TRP channels, and the Na⁺/K⁺-ATPase as targets/effectors of this type of cell death.

Structural Modifications and Biological Evaluations of Rift Valley Fever Virus Inhibitors Identified from Chemical Library Screening

The Rift Valley fever virus (RVFV) is an emerging high-priority pathogen endemic in Africa with pandemic potential. There is no specific treatment or approved antiviral drugs for the RVFV. We previously developed a cell-based high-throughput assay to screen small molecules targeting the RVFV and identified a potential effective antiviral compound (1-N-(2-(biphenyl-4-yloxy)ethyl)propane-1,3-diamine) as a lead compound. Here, we investigated how structural modifications of the lead compound affected the biological properties and the antiviral effect against the RVFV. We found that the length of the 2-(3-aminopropylamino)ethyl chain of the compound was important for the compound to retain its antiviral activity. The antiviral activity was similar when the 2-(3-aminopropylamino)ethyl chain was replaced with a butyl piperazine chain. However, we could improve the cytotoxicity profile of the lead compound by changing the phenyl piperazine linker from the para-position (compound 9a) to the meta-position (compound 13a). Results from time-of-addition studies suggested that compound 13a might be active during virus post-entry and/or the replication phase of the virus life cycle and seemed to affect the K⁺ channel. The modifications improved the properties of our lead compound, and our data suggest that 13a is a promising candidate to evaluate further as a therapeutic agent for RVFV infection.

Gramicidin A accumulates in mitochondria, reduces ATP levels, induces mitophagy, and inhibits cancer cell growth

Here we revealed the spatiotemporal behavior of gramicidin A in cancer cells. Gramicidin A depolarizes both the plasma and mitochondrial membranes, inhibits ATP synthesis, and induces mitophagy, thereby causing potent inhibition of cell growth.

Plant Bioelectronics and Biohybrids: The Growing Contribution of Organic Electronic and Carbon-Based Materials

Life in our planet is highly dependent on plants as they are the primary source of food, regulators of the atmosphere, and providers of a variety of materials. In this work, we review the progress on bioelectronic devices for plants and biohybrid systems based on plants, therefore discussing advancements that view plants either from a biological or a technological perspective, respectively. We give an overview on wearable and implantable bioelectronic devices for monitoring and modulating plant physiology that can be used as tools in basic plant science or find application in agriculture. Furthermore, we discuss plant-wearable devices for monitoring a plant's microenvironment that will enable optimization of growth conditions. The review then covers plant biohybrid systems where plants are an integral part of devices or are converted to devices upon functionalization with smart materials, including self-organized electronics, plant nanobionics, and energy applications. The review focuses on advancements based on organic electronic and carbon-based materials and discusses opportunities, challenges, as well as future steps.

Simultaneous Measurement of Changes in Neutrophil Granulocyte Membrane Potential, Intracellular pH, and Cell Size by Multiparametric Flow Cytometry

Neutrophils provide rapid and efficient defense mechanisms against invading pathogens. Upon stimulation with proinflammatory mediators, including complement factors and bacterial peptides, neutrophils respond with changes in their membrane potential, intracellular pH, and cellular size. This study provides an approach to quantify these important changes simultaneously using multiparametric flow cytometry, thereby revealing a typical sequence of neutrophil activation consisting of depolarization, alkalization, and increase in cellular size. Additionally, the time resolution of the flow cytometric measurement is improved in order to allow changes that occur within seconds to be monitored, and thus to enhance the kinetic analysis of the neutrophil response. The method is appropriate for the reliable semiquantitative detection of small variations with respect to an increase, no change, and decrease in those parameters as demonstrated by the screening of various proinflammatory mediators. As a translational outlook, the findings are put into context in inflammatory conditions in vitro as well as in a clinically relevant whole blood model of endotoxemia. Taken together, the multiparametric analysis of neutrophil responsiveness regarding depolarization, alkalization, and changes in cellular size may contribute to a better understanding of neutrophils in health and disease, thus potentially yielding innovative mechanistic insights and possible novel diagnostic and/or prognostic approaches.

Exposure to 60 Hz magnetic field can affect membrane proteins and membrane potential in human cancer cells

The experimental data support the hypothesis that extremely low frequency magnetic field (ELF-MF) can affect cell membranes. Since our previous studies suggested that MF changes the permeability of cell membrane, in this study we focused on the cell membrane and investigated the effect of 60 Hz, 50 mT MF on the membrane potential and membrane proteins. The membrane potentials of three cultured human cancer cell lines, A549, MES-SA, and MES-SA/Dx5, were increased by exposure to ELF-MF. When exposed to MF and an anticancer drug, changes in the membrane potentials were detected in A549 and MES-SA cells, but not in the multi drug-resistant cells, MES-SA/Dx5. We examined whether MF has an influence on the membrane proteins extracted from cultured A549 cells, using DiBAC₄(3) dye enhanced fluorescence binding to a hydrophobic site. The increase in fluorescence observed following MF exposure for 10 min indicated that the structure of the hydrophobic site on the membrane proteins changed and became more likely to bind the probe dye. A decrease in fluorescence was detected following exposure to MF for 240 min. These results indicated that 60 Hz, 50 mT MF causes changes in the membrane potential of cultured cancer cells and the conformation of membrane proteins extracted from cultured cancer cells, and has different effects depending on the exposure time.

Substratum stiffness tunes membrane voltage in mammary epithelial cells

Membrane voltage (Vm) plays a critical role in the regulation of several cellular behaviors, including proliferation, apoptosis and phenotypic plasticity. Many of these behaviors are affected by the stiffness of the underlying extracellular matrix, but the connections between Vm and the mechanical properties of the microenvironment are unclear. Here, we investigated the relationship between matrix stiffness and Vm by culturing mammary epithelial cells on synthetic substrata, the stiffnesses of which mimicked those of the normal mammary gland and breast tumors. Although proliferation is associated with depolarization, we surprisingly observed that cells are hyperpolarized when cultured on stiff substrata, a microenvironmental condition that enhances proliferation. Accordingly, we found that Vm becomes depolarized as stiffness decreases, in a manner dependent on intracellular Ca2+. Furthermore, inhibiting Ca2+-gated Cl- currents attenuates the effects of substratum stiffness on Vm. Specifically, we uncovered a role for cystic fibrosis transmembrane conductance regulator (CFTR) in the regulation of Vm by substratum stiffness. Taken together, these results suggest a novel role for CFTR and membrane voltage in the response of mammary epithelial cells to their mechanical microenvironment.

Phosphatidylserine Synthase from Salicornia europaea Is Involved in Plant Salt Tolerance by Regulating Plasma Membrane Stability

Salinity-induced lipid alterations have been reported in many plant species; however, how lipid biosynthesis and metabolism are regulated and how lipids work in plant salt tolerance are much less studied. Here, a constitutively much higher phosphatidylserine (PS) content in the plasma membrane (PM) was found in the euhalophyte Salicornia europaea than in Arabidopsis. A gene encoding PS synthase (PSS) was subsequently isolated from S. europaea, named SePSS, which was induced by salinity. Multiple alignments and phylogenetic analysis suggested that SePSS belongs to a base exchange-type PSS, which localises to the endoplasmic reticulum. Knockdown of SePSS in S. europaea suspension cells resulted in reduced PS content, decreased cell survival rate, and increased PM depolarization and K+ efflux under 400 or 800 mM NaCl. By contrast, the upregulation of SePSS leads to increased PS and phosphatidylethanolamine levels and enhanced salt tolerance in Arabidopsis, along with a lower accumulation of reactive oxygen species, less membrane injury, less PM depolarization and higher K+/Na+ in the transgenic lines than in wild-type (WT). These results suggest a positive correlation between PS levels and plant salt tolerance, and that SePSS participates in plant salt tolerance by regulating PS levels, hence PM potential and permeability, which help maintain ion homeostasis. Our work provides a potential strategy for improving plant growth under multiple stresses.

Allelochemicals of Alexandrium minutum: Kinetics of membrane disruption and photosynthesis inhibition in a co-occurring diatom

Allelopathy is an efficient strategy by which some microalgae can outcompete other species. Allelochemicals from the toxic dinoflagellate Alexandrium minutum have deleterious effects on diatoms, inhibiting metabolism and photosynthesis and therefore give a competitive advantage to the dinoflagellate. The precise mechanisms of allelochemical interactions and the molecular target of allelochemicals remain however unknown. To understand the mechanisms, the short-term effects of A. minutum allelochemicals on the physiology of the diatom Chaetoceros muelleri were investigated. The effects of a culture filtrate were measured on the diatom cytoplasmic membrane integrity (polarity and permeability) using flow-cytometry and on the photosynthetic performance using fluorescence and absorption spectroscopy. Within 10 min, the unknown allelochemicals induced a depolarization of the cytoplasmic membranes and an impairment of photosynthesis through the inhibition of the plastoquinone-mediated electron transfer between photosystem II and cytochrome b₆f. At longer time of exposure, the cytoplasmic membranes were permeable and the integrity of photosystems I, II and cytochrome b₆f was compromised. Our demonstration of the essential role of membranes in this allelochemical interaction provides new insights for the elucidation of the nature of the allelochemicals. The relationship between cytoplasmic membranes and the inhibition of the photosynthetic electron transfer remains however unclear and warrants further investigation.

GPR55-mediated effects on brain microvascular endothelial cells and the blood-brain barrier

GPR55, an atypical cannabinoid receptor activated by lysophosphatidylinositol (LPI) has been involved in various physiological and pathological processes. We examined the effect of GPR55 activation on rat brain microvascular endothelial cells (RBMVEC), an essential component of the blood-brain barrier (BBB). GPR55 was detected in RBMVEC by western blot and immunocytochemistry. Treatment of RBMVEC with LPI increased cytosolic Ca²⁺ concentration, [Ca²⁺]_i, in a concentration-dependent manner; the effect was abolished by the GPR55 antagonist, ML-193. Repetitive application of LPI induced tachyphylaxis. LPI-induced increase in [Ca²⁺]_i was not sensitive to U-73122, a phospholipase C inhibitor, but was abolished by the blockade of voltage-gated Ca²⁺ channels or in Ca²⁺-free saline, indicating that Ca²⁺ influx was involved in this response. LPI induced a biphasic change in RBMVEC membrane potential: a fast depolarization followed by a long-lasting hyperpolarization. The hyperpolarization phase was prevented by apamin and charibdotoxin, inhibitors of small- and intermediate-conductance Ca²⁺-activated K⁺ channels (K_Ca). Immunofluorescence studies indicate that LPI produced transient changes in tight and adherens junctions proteins and F-actin stress fibers. LPI decreased the electrical resistance of RBMVEC monolayer assessed with Electric Cell-Substrate Impedance Sensing (ECIS) in a dose-dependent manner. In vivo studies indicate that systemic administration of LPI increased the permeability of the BBB, assessed with Evans Blue method. Taken together, our results indicate that GPR55 activation modulates the function of endothelial cells of brain microvessels, produces a transient reduction in endothelial barrier function and increases BBB permeability.

Mitochondrial membrane potential

The mitochondrial membrane potential (ΔΨm) generated by proton pumps (Complexes I, III and IV) is an essential component in the process of energy storage during oxidative phosphorylation. Together with the proton gradient (ΔpH), ΔΨm forms the transmembrane potential of hydrogen ions which is harnessed to make ATP. The levels of ΔΨm and ATP in the cell are kept relatively stable although there are limited fluctuations of both these factors that can occur reflecting normal physiological activity. However, sustained changes in both factors may be deleterious. A long-lasting drop or rise of ΔΨm vs normal levels may induce unwanted loss of cell viability and be a cause of various pathologies. Among other factors, ΔΨm plays a key role in mitochondrial homeostasis through selective elimination of dysfunctional mitochondria. It is also a driving force for transport of ions (other than H+) and proteins which are necessary for healthy mitochondrial functioning. We propose additional potential mechanisms for which ΔΨm is essential for maintenance of cellular health and viability and provide recommendations how to accurately measure ΔΨm in a cell and discuss potential sources of artifacts.

Pregnancy-adapted uterine artery endothelial cell Ca2+ signaling and its relationship with membrane potential

Pregnancy‐derived uterine artery endothelial cells (P‐UAEC) express P2Y2 receptors and at high cell density show sustained and synchronous [Ca2+]i burst responses in response to ATP. Bursts in turn require coupling of transient receptor potential canonical type3 channel (TRPC3) and inositol 1,4,5‐triphosphate receptor type 2 (IP3R2), which is upregulated in P‐UAEC in a manner dependent on connexin 43 (Cx43) gap junctions. While there is no known direct interaction of TRPC3 with Cx43, early descriptions of TRPC3 function showed it may also be influenced by altered membrane potential (V_m). Herein, we ask if enhanced TRPC3 Ca2+ bursting due to enhanced Cx43 coupling may be coupled via dynamic alterations in V_m in P‐UAEC, as reported in some (HUVEC) but not all endothelial cells. Following basic electrical characterization of UAEC, we employed a high sensitivity cell imaging system to simultaneously monitor cell V_m and [Ca2+]i in real time in continuous monolayers of UAEC. Our findings show that while acute and sustained phase [Ca2+]i bursting occur dose‐dependently in response to ATP, V_m is not coregulated with any periodicity related to [Ca2+]i bursting. Only a small but significant progressive change in V_m is seen, and this is more closely related to overall mobilization of Ca2+. Surprisingly, this is also most apparent in NP‐UAEC > P‐UAEC. In contrast [Ca2+]i bursting is more synchronous in P‐UAEC and even achieves [Ca2+]i waves passing through the P‐UAEC monolayer. The relevance of these findings to mechanisms of pregnancy adaptation and its failure in hypertensive pregnancy are discussed.

Effects of Platelet-Activating Factor on Brain Microvascular Endothelial Cells

Platelet-activating factor (PAF) is a potent phospholipid mediator that exerts various pathophysiological effects by interacting with a G protein-coupled receptor. PAF has been reported to increase the permeability of the blood-brain barrier (BBB) via incompletely characterized mechanisms. We investigated the effect of PAF on rat brain microvascular endothelial cells (RBMVEC), a critical component of the BBB. PAF produced a dose-dependent increase in cytosolic Ca²⁺ concentration; the effect was prevented by the PAF receptor antagonist, WEB2086. The effect of PAF on cytosolic Ca²⁺ was abolished in Ca²⁺-free saline or in the presence of L-type voltage-gated Ca²⁺ channel inhibitor, nifedipine, indicating that Ca²⁺ influx is critical for PAF-induced increase in cytosolic Ca²⁺. PAF produced RBMVEC depolarization; the effect was inhibited by WEB2086. In cells loaded with [(4-amino-5-methylamino-2',7'-difluoro-fluorescein)diacetate] (DAF-FM), a nitric oxide (NO)-sensitive fluorescent dye, PAF increased the NO level; the effect was prevented by WEB2086, nifedipine or by l-NAME, an inhibitor of NO synthase. Immunocytochemistry studies indicate that PAF reduced the immunostaining of ZO-1, a tight junction-associated protein, increased F-actin fibers, and produced intercellular gaps. PAF produced a decrease in RBMVEC monolayer electrical resistance assessed with Electric Cell-Substrate Impedance Sensing (ECIS), indicative of a disruption of endothelial barrier function. In vivo studies indicate that PAF increased the BBB permeability, assessed with sodium fluorescein and Evans Blue methods, via PAF receptor-dependent mechanisms, consequent to Ca²⁺ influx and increased NO levels. Our studies reveal that PAF alters the BBB permeability by multiple mechanisms, which may be relevant for central nervous system (CNS) inflammatory disorders.

TRPV4 Contributes to Resting Membrane Potential in Retinal Müller Cells: Implications in Cell Volume Regulation

Neural activity alters osmotic gradients favoring cell swelling in retinal Müller cells. This swelling is followed by a regulatory volume decrease (RVD), partially mediated by an efflux of KCl and water. The transient receptor potential channel 4 (TRPV4), a nonselective calcium channel, has been proposed as a candidate for mediating intracellular Ca²⁺ elevation induced by swelling. We previously demonstrated in a human Müller cell line (MIO-M1) that RVD strongly depends on ion channel activation and, consequently, on membrane potential (V_m ). The aim of this study was to investigate if Ca²⁺ influx via TRPV4 contributes to RVD by modifying intracellular Ca²⁺ concentration and/or modulating V_m in MIO-M1 cells. Cell volume, intracellular Ca²⁺ levels, and V_m changes were evaluated using fluorescent probes. Results showed that MIO-M1 cells express functional TRPV4 which determines the resting V_m associated with K⁺ channels. Swelling-induced increases in Ca²⁺ levels was due to both Ca²⁺ release from intracellular stores and Ca²⁺ influx by a pathway alternative to TRPV4. TRPV4 blockage affected swelling-induced biphasic response (depolarization-repolarization), suggesting its participation in modulating V_m changes during RVD. Agonist stimulation of Ca²⁺ influx via TRPV4 activated K⁺ channels hyperpolarizing V_m and accelerating RVD. We propose that TRPV4 forms a signaling complex with Ca²⁺ and/or voltage-dependent K⁺ channels to define resting V_m and V_m changes during RVD. TRPV4 involvement in RVD depends on the type of stimuli and/or degree of channel activation, leading to a maximum RVD response when Ca²⁺ influx overcomes a threshold and activates further signaling pathways in cell volume regulation. J. Cell. Biochem. 118: 2302-2313, 2017. © 2017 Wiley Periodicals, Inc.

Phenotypic assays for analyses of pluripotent stem cell-derived cardiomyocytes

Stem cell-derived cardiomyocytes (CMs) hold great hopes for myocardium regeneration because of their ability to produce functional cardiac cells in large quantities. They also hold promise in dissecting the molecular principles involved in heart diseases and also in drug development, owing to their ability to model the diseases using patient-specific human pluripotent stem cell (hPSC)-derived CMs. The CM properties essential for the desired applications are frequently evaluated through morphologic and genotypic screenings. Even though these characterizations are necessary, they cannot in principle guarantee the CM functionality and their drug response. The CM functional characteristics can be quantified by phenotype assays, including electrophysiological, optical, and/or mechanical approaches implemented in the past decades, especially when used to investigate responses of the CMs to known stimuli (eg, adrenergic stimulation). Such methods can be used to indirectly determine the electrochemomechanics of the cardiac excitation-contraction coupling, which determines important functional properties of the hPSC-derived CMs, such as their differentiation efficacy, their maturation level, and their functionality. In this work, we aim to systematically review the techniques and methodologies implemented in the phenotype characterization of hPSC-derived CMs. Further, we introduce a novel approach combining atomic force microscopy, fluorescent microscopy, and external electrophysiology through microelectrode arrays. We demonstrate that this novel method can be used to gain unique information on the complex excitation-contraction coupling dynamics of the hPSC-derived CMs.

Noninvasive Measurement of Electrical Events Associated with a Single Chlorovirus Infection of a Microalgal Cell

Chlorovirus Paramecium bursaria chlorella virus 1 (PBCV-1) contains a viral-encoded K(+) channel imbedded in its internal membrane, which triggers host plasma membrane depolarization during virus infection. This early stage of infection was monitored at high resolution by recording the cell membrane depolarization of a single Chlorella cell during infection by a single PBCV-1 particle. The measurement was achieved by depositing the cells onto a network of one-dimensional necklaces of Au nanoparticles, which spanned two electrodes 70 μm apart. The nanoparticle necklace array has been shown to behave as a single-electron device at room temperature. The resulting electrochemical field-effect transistor (eFET) was gated by the cell membrane potential, which allowed a quantitative measurement of the electrophysiological changes across the rigid cell wall of the microalgae due to a single viral attack at high sensitivity. The single viral infection signature was quantitatively confirmed by coupling the eFET measurement with a method in which a single viral particle was delivered for infection by a scanning probe microscope cantilever.

Analysis of Antimicrobial-Triggered Membrane Depolarization Using Voltage Sensitive Dyes

The bacterial cytoplasmic membrane is a major inhibitory target for antimicrobial compounds. Commonly, although not exclusively, these compounds unfold their antimicrobial activity by disrupting the essential barrier function of the cell membrane. As a consequence, membrane permeability assays are central for mode of action studies analysing membrane-targeting antimicrobial compounds. The most frequently used in vivo methods detect changes in membrane permeability by following internalization of normally membrane impermeable and relatively large fluorescent dyes. Unfortunately, these assays are not sensitive to changes in membrane ion permeability which are sufficient to inhibit and kill bacteria by membrane depolarization. In this manuscript, we provide experimental advice how membrane potential, and its changes triggered by membrane-targeting antimicrobials can be accurately assessed in vivo. Optimized protocols are provided for both qualitative and quantitative kinetic measurements of membrane potential. At last, single cell analyses using voltage-sensitive dyes in combination with fluorescence microscopy are introduced and discussed.

Modulation of Potassium Channels Inhibits Bunyavirus Infection

Bunyaviruses are considered to be emerging pathogens facilitated by the segmented nature of their genome that allows reassortment between different species to generate novel viruses with altered pathogenicity. Bunyaviruses are transmitted via a diverse range of arthropod vectors, as well as rodents, and have established a global disease range with massive importance in healthcare, animal welfare, and economics. There are no vaccines or anti-viral therapies available to treat human bunyavirus infections and so development of new anti-viral strategies is urgently required. Bunyamwera virus (BUNV; genus Orthobunyavirus) is the model bunyavirus, sharing aspects of its molecular and cellular biology with all Bunyaviridae family members. Here, we show for the first time that BUNV activates and requires cellular potassium (K(+)) channels to infect cells. Time of addition assays using K(+) channel modulating agents demonstrated that K(+) channel function is critical to events shortly after virus entry but prior to viral RNA synthesis/replication. A similar K(+) channel dependence was identified for other bunyaviruses namely Schmallenberg virus (Orthobunyavirus) as well as the more distantly related Hazara virus (Nairovirus). Using a rational pharmacological screening regimen, two-pore domain K(+) channels (K2P) were identified as the K(+) channel family mediating BUNV K(+) channel dependence. As several K2P channel modulators are currently in clinical use, our work suggests they may represent a new and safe drug class for the treatment of potentially lethal bunyavirus disease.

G protein-coupled estrogen receptor-mediated effects on cytosolic calcium and nanomechanics in brain microvascular endothelial cells

G protein-coupled estrogen receptor (GPER) is a relatively recently identified non-nuclear estrogen receptor, expressed in several tissues, including brain and blood vessels. The mechanisms elicited by GPER activation in brain microvascular endothelial cells are incompletely understood. The purpose of this work was to assess the effects of GPER activation on cytosolic Ca(2+) concentration, [Ca(2+)](i), nitric oxide production, membrane potential and cell nanomechanics in rat brain microvascular endothelial cells (RBMVEC). Extracellular but not intracellular administration of G-1, a selective GPER agonist, or extracellular administration of 17-β-estradiol and tamoxifen, increased [Ca(2+)](i) in RBMVEC. The effect of G-1 on [Ca(2+)](i) was abolished in Ca(2+) -free saline or in the presence of a L-type Ca(2+) channel blocker. G-1 increased nitric oxide production in RBMVEC; the effect was prevented by NG-nitro-l-arginine methyl ester. G-1 elicited membrane hyperpolarization that was abolished by the antagonists of small and intermediate-conductance Ca(2+) -activated K(+) channels, apamin, and charibdotoxin. GPER-mediated responses were sensitive to G-36, a GPER antagonist. In addition, atomic force microscopy studies revealed that G-1 increased the modulus of elasticity, indicative of cytoskeletal changes and increase in RBMVEC stiffness. Our results unravel the mechanisms underlying GPER-mediated effects in RBMVEC with implications for the effect of estrogen on cerebral microvasculature. Activation of the G protein-coupled estrogen receptor (GPER) in rat brain microvascular endothelial cells (RBMVEC) increases [Ca(2+)](i) by promoting Ca(2+) influx. The increase in [Ca(2+)](i) leads to membrane hyperpolarization, nitric oxide (NO) production, and to cytoskeletal changes and increased cell stiffness. Our results unravel the mechanisms underlying GPER-mediated effects in RBMVEC with implications for the effect of estrogen on cerebral microvasculature.

Oral dosing of chemical indicators for in vivo monitoring of Ca2+ dynamics in insect muscle

This paper proposes a remarkably facile staining protocol to visually investigate dynamic physiological events in insect tissues. We attempted to monitor Ca²⁺ dynamics during contraction of electrically stimulated living muscle. Advances in circuit miniaturization and insect neuromuscular physiology have enabled the hybridization of living insects and man-made electronic components, such as microcomputers, the result of which has been often referred as a Living Machine, Biohybrid, or Cyborg Insect. In order for Cyborg Insects to be of practical use, electrical stimulation parameters need to be optimized to induce desired muscle response (motor action) and minimize the damage in the muscle due to the electrical stimuli. Staining tissues and organs as well as measuring the dynamics of chemicals of interest in muscle should be conducted to quantitatively and systematically evaluate the effect of various stimulation parameters on the muscle response. However, existing staining processes require invasive surgery and/or arduous procedures using genetically encoded sensors. In this study, we developed a non-invasive and remarkably facile method for staining, in which chemical indicators can be orally administered (oral dosing). A chemical Ca²⁺ indicator was orally introduced into an insect of interest via food containing the chemical indicator and the indicator diffused from the insect digestion system to the target muscle tissue. We found that there was a positive relationship between the fluorescence intensity of the indicator and the frequency of electrical stimulation which indicates the orally dosed indicator successfully monitored Ca²⁺ dynamics in the muscle tissue. This oral dosing method has a potential to globally stain tissues including neurons, and investigating various physiological events in insects.

Relaxant effect of flavonoid naringenin on contractile activity of rat colonic smooth muscle

ETHNOPHARMACOLOGICAL RELEVANCE

Disturbed gastrointestinal (GI) motility can be associated with smooth muscle abnormalities and dysfunction. Exploring innovative approaches that can modulate the disturbed colonic motility are of great importance for clinical therapeutics. Naringenin, a flavonoid presented in many traditional Chinese herbal medicines, has been shown to have a relaxant effect on different smooth muscles. The aim of the present study was to investigate the effect of naringenin on regulation of GI motility.

MATERIAL AND METHODS

Mechanical recording was used to investigate the effect of naringenin on isolated rat colonic smooth muscle spontaneous contractions. Whole cell patch clamp, intracellular [Ca(2+)] concentration ([Ca(2+)]i) and membrane potential measurements were examined on primary cultures of colonic smooth muscle cells (SMCs). A neostigmine-stimulated rat model was utilized to investigate the effect of naringenin in vivo.

RESULTS

Naringenin induced a concentration-dependent inhibition (1-1000 μM) on rat colonic spontaneous contraction, which was reversible after wash out. The external Ca(2+) influx induced contraction and [Ca(2+)]i increase were inhibited by naringenin (100 μM). In rat colonic SMCs, naringenin-induced membrane potential hyperpolarization was sensitive to TEA and selective large-conductance calcium-activated K(+) (BKCa) channel inhibitor iberiotoxin. Under whole cell patch-clamp condition, naringenin stimulated an iberiotoxin-sensitive BKCa current, which was insensitive to changes in the [Ca(2+)]i concentration. Furthermore, naringenin significantly suppressed neostigmine-enhanced rat colon transit in vivo.

CONCLUSION

Our results for the first time demonstrated the relaxant effect of flavonoid naringenin on colon smooth muscle both in vitro and in vivo. The relaxant effect of naringenin was attributed to direct activation of BKCa channels, which subsequently hyperpolarized the colonic SMCs and decreased Ca(2+) influx through VDCC. Naringenin might be of therapeutic value in the treatment of GI motility disorders.

Polyethyleneimine and polyethyleneimine-based nanoparticles: novel bacterial and yeast biofilm inhibitors

Biofilms are commonly involved in medical device-related infections. The purpose of this study was to determine the antimicrobial and anti-biofilm activity of polyethyleneimine (PEI) and PEI-based nanoparticles (nanoPEI) against Staphylococcus aureus, Staphylococcus epidermidis, Acinetobacter baumannii and Candida albicans (clinical and ATCC strains), and to evaluate their effect upon biofilm formation on polyurethane (PUR)-like catheters. MICs and minimal lethal concentrations of PEI and nanoPEI were determined according to CLSI microdilution reference protocols. For PEI, the MIC value was 195.31 mg l−1 for all the bacteria and 48.83 mg l−1 for the yeast strains. For nanoPEI, the MIC value was 1250 mg l−1 for all the strains except A. baumannii, for which it was 2500 mg l−1. Biofilm formation was assessed with PUR-like catheter segments and biofilm metabolic activity was quantified by colorimetry with a tetrazolium reduction assay. Plasma membrane integrity and membrane potential were assessed by flow cytometry after staining microbial cells with a membrane-impermeable dye, propidium iodide, and a membrane-potential marker, DiBAC4(3). PEI inhibited growth of all microbial species; higher concentrations of nanoPEI were needed to inhibit growth of all species. Biofilm formation in the presence of anti-bacterial PEI activity was dose-dependent (except for S. epidermidis) and species-related. NanoPEI at 0.5×MIC and MIC significantly reduced the metabolic activity of biofilms of S. aureus, S. epidermidis and A. baumannii, whereas 2×MIC was required in order to inhibit biofilm metabolic activity.

Cellular mechanism underlying hydrogen sulfide induced mouse tracheal smooth muscle relaxation: role of BKCa

Recent studies have suggested that hydrogen sulfide (H2S), an important endogenous signaling gaseous molecule, participates in relaxation of smooth muscle. Nevertheless, the mechanism of this relaxation effect on respiratory system is still unclear. The present study aims to investigate the physiological function as well as cellular mechanism of H2S in tracheal smooth muscle. Application of the H2S donor, sodium hydrosulphide (NaHS) and the precursor of H2S, l-cysteine (l-Cys) induced mouse tracheal smooth muscle (TSM) relaxation in an epithelium-independent manner. The relaxation of TSM induced by NaHS was abrogated by iberiotoxin (IbTX), the large conductance calcium activated potassium channel (BKCa) blocker. In primary cultured mouse TSM cells, NaHS remarkably increased potassium outward currents in whole-cell patch clamp, hyperpolarized TSM cells and inhibited the calcium influx. All of these effects were significantly blocked by IbTX. Consistent with the results in vitro, administration of NaHS in vivo also reduced airway hyperresponsiveness in Ovalbumin (OVA)-challenged asthmatic mice. Our present study indicates that NaHS can induce mouse TSM relaxation by activating BKCa. These observations reveal the physiological function of H2S in airway, which provides a promising pharmacological target for the treatment of asthma and other respiratory diseases associated with over-contraction of TSM.

Modulation of medium pH by Caulobacter crescentus facilitates recovery from uranium-induced growth arrest

The oxidized form of uranium [U(VI)] predominates in oxic environments and poses a major threat to ecosystems. Due to its ability to mineralize U(VI), the oligotroph Caulobacter crescentus is an attractive candidate for U(VI) bioremediation. However, the physiological basis for U(VI) tolerance is unclear. Here we demonstrated that U(VI) caused a temporary growth arrest in C. crescentus and three other bacterial species, although the duration of growth arrest was significantly shorter for C. crescentus. During the majority of the growth arrest period, cell morphology was unaltered and DNA replication initiation was inhibited. However, during the transition from growth arrest to exponential phase, cells with shorter stalks were observed, suggesting a decoupling between stalk development and the cell cycle. Upon recovery from growth arrest, C. crescentus proliferated with a growth rate comparable to that of a control without U(VI), although a fraction of these cells appeared filamentous with multiple replication start sites. Normal cell morphology was restored by the end of exponential phase. Cells did not accumulate U(VI) resistance mutations during the prolonged growth arrest, but rather, a reduction in U(VI) toxicity occurred concomitantly with an increase in medium pH. Together, these data suggest that C. crescentus recovers from U(VI)-induced growth arrest by reducing U(VI) toxicity through pH modulation. Our finding represents a unique U(VI) detoxification strategy and provides insight into how microbes cope with U(VI) under nongrowing conditions, a metabolic state that is prevalent in natural environments.

Evidence for role of acid-sensing ion channels in nucleus ambiguus neurons: essential differences in anesthetized versus awake rats

Acid-sensing ion channels (ASIC) are widely expressed in several brain regions including medulla; their role in physiology and pathophysiology is incompletely understood. We examined the effect of acidic pH of 6.2 on the medullary neurons involved in parasympathetic cardiac control. Our results indicate that retrogradely labeled cardiac vagal neurons of nucleus ambiguus are depolarized by acidic pH. In addition, acidic saline of pH 6.2 increases cytosolic Ca(2+) concentration by promoting Ca(2+) influx in nucleus ambiguus neurons. In vivo studies indicate that microinjection of acidic artificial cerebrospinal fluid (pH 6.2) into the nucleus ambiguus decreases the heart rate in conscious rats, whereas it has no effect in anesthetized animals. Pretreatment with either amiloride or benzamil, two widely used ASIC blockers, abolishes both the in vitro and in vivo effects elicited by pH 6.2. Our findings support a critical role for ASIC in modulation of cardiac vagal tone and provide a potential mechanism for acidosis-induced bradycardia, while identifying important differences in the response to acidic pH between anesthetized and conscious rats.

HIV-1-Tat excites cardiac parasympathetic neurons of nucleus ambiguus and triggers prolonged bradycardia in conscious rats

The mechanisms of autonomic imbalance and subsequent cardiovascular manifestations in HIV-1-infected patients are poorly understood. We report here that HIV-1 transactivator of transcription (Tat, fragment 1-86) produced a concentration-dependent increase in cytosolic Ca(2+) in cardiac-projecting parasympathetic neurons of nucleus ambiguus retrogradely labeled with rhodamine. Using store-specific pharmacological agents, we identified several mechanisms of the Tat-induced Ca(2+) elevation: 1) lysosomal Ca(2+) mobilization, 2) Ca(2+) release via inositol 1,4,5-trisphosphate-sensitive endoplasmic reticulum pools, and 3) Ca(2+) influx via transient receptor potential vanilloid type 2 (TRPV2) channels. Activation of TRPV2, nonselective cation channels, induced a robust and prolonged neuronal membrane depolarization, thus triggering an additional P/Q-mediated Ca(2+) entry. In vivo microinjection studies indicate a dose-dependent, prolonged bradycardic effect of Tat administration into the nucleus ambiguus of conscious rats, in which neuronal TRPV2 played a major role. Our results support previous studies, indicating that Tat promotes bradycardia and, consequently, may be involved in the QT interval prolongation reported in HIV-infected patients. In the context of an overall HIV-dependent autonomic dysfunction, these Tat-mediated mechanisms may account for the higher prevalence of sudden cardiac death in HIV-1-infected patients compared with general population with similar risk factors. Our results may be particularly relevant in view of the recent findings that significant Tat levels can still be identified in the cerebrospinal fluid of HIV-infected patients with viral load suppression due to efficient antiretroviral therapy.

Urotensin II promotes vagal-mediated bradycardia by activating cardiac-projecting parasympathetic neurons of nucleus ambiguus

Urotensin II (U-II) is a cyclic undecapeptide that regulates cardiovascular function at central and peripheral sites. The functional role of U-II nucleus ambiguus, a key site controlling cardiac tone, has not been established, despite the identification of U-II and its receptor at this level. We report here that U-II produces an increase in cytosolic Ca(2+) concentration in retrogradely labeled cardiac vagal neurons of nucleus ambiguus via two pathways: (i) Ca(2+) release from the endoplasmic reticulum via inositol 1,4,5-trisphosphate receptor; and (ii) Ca(2+) influx through P/Q-type Ca(2+) channels. In addition, U-II depolarizes cultured cardiac parasympathetic neurons. Microinjection of increasing concentrations of U-II into nucleus ambiguus elicits dose-dependent bradycardia in conscious rats, indicating the in vivo activation of the cholinergic pathway controlling the heart rate. Both the in vitro and in vivo effects were abolished by the urotensin receptor antagonist, urantide. Our findings suggest that, in addition, to the previously reported increase in sympathetic outflow, U-II activates cardiac vagal neurons of nucleus ambiguus, which may contribute to cardioprotection.

Maintaining energy homeostasis is an essential component of Wld(S)-mediated axon protection

Wld(S) mutation protects axons from degeneration in diverse experimental models of neurological disorders, suggesting that the mutation might act on a key step shared by different axon degeneration pathways. Here we test the hypothesis that Wld(S) protects axons by preventing energy deficiency commonly encountered in many diseases. We subjected compartmentally cultured, mouse cortical axons to energy deprivation with 6mM azide and zero glucose. In wild-type (WT) culture, the treatment, which reduced axon ATP level ([ATP]axon) by 65%, caused immediate axon depolarization followed by gradual free calcium accumulation and subsequent irreversible axon damage. The calcium accumulation resulted from calcium influx partially via L-type voltage-gated calcium channel (L-VGCC). Blocking L-VGCC with nimodipine reduced calcium accumulation and protected axons. Without altering baseline [ATP]axon, the presence of Wld(S) mutation significantly reduced the axon ATP loss and depolarization, restrained the subsequent calcium accumulation, and protected axons against energy deprivation. Wld(S) neurons possessed higher than normal nicotinamide mononucleotide adenylyltransferase (NMNAT) activity. The intrinsic Wld(S) NMNAT activity was required for the Wld(S)-mediated energy preservation and axon protection during but not prior to energy deprivation. NMNAT catalyzes the reversible reaction that produces nicotinamide adenine dinucleotide (NAD) from nicotinamide mononucleotide (NMN). Interestingly, preventing the production of NAD from NMN with FK866 increased [ATP]axon and protected axons from energy deprivation. These results indicate that the Wld(S) mutation depends on its intrinsic Wld(S) NMNAT activity and the subsequent increase in axon ATP but not NAD to protect axons, implicating a novel role of Wld(S) NMNAT in axon bioenergetics and protection.

Aldosterone increases cardiac vagal tone via G protein-coupled oestrogen receptor activation

In addition to acting on mineralocorticoid receptors, aldosterone has been recently shown to activate the G protein-coupled oestrogen receptor (GPER) in vascular cells. In light of the newly identified role for GPER in vagal cardiac control, we examined whether or not aldosterone activates GPER in rat nucleus ambiguus. Aldosterone produced a dose-dependent increase in cytosolic Ca(2+) concentration in retrogradely labelled cardiac vagal neurons of nucleus ambiguus; the response was abolished by pretreatment with the GPER antagonist G-36, but was not affected by the mineralocorticoid receptor antagonists, spironolactone and eplerenone. In Ca(2+)-free saline, the response to aldosterone was insensitive to blockade of the Ca(2+) release from lysosomes, while it was reduced by blocking the Ca(2+) release via ryanodine receptors and abolished by blocking the IP3 receptors. Aldosterone induced Ca(2+) influx via P/Q-type Ca(2+) channels, but not via L-type and N-type Ca(2+) channels. Aldosterone induced depolarization of cardiac vagal neurons of nucleus ambiguus that was sensitive to antagonism of GPER but not of mineralocorticoid receptor. in vivo studies, using telemetric measurement of heart rate, indicate that microinjection of aldosterone into the nucleus ambiguus produced a dose-dependent bradycardia in conscious, freely moving rats. Aldosterone-induced bradycardia was blocked by the GPER antagonist, but not by the mineralocorticoid receptor antagonists. In summary, we report for the first time that aldosterone decreases heart rate by activating GPER in cardiac vagal neurons of nucleus ambiguus.

Nesfatin-1 activates cardiac vagal neurons of nucleus ambiguus and elicits bradycardia in conscious rats

Nesfatin-1, a peptide whose receptor is yet to be identified, has been involved in the modulation of feeding, stress, and metabolic responses. More recently, increasing evidence supports a modulatory role for nesfatin-1 in autonomic and cardiovascular activity. This study was undertaken to test if the expression of nesfatin-1 in the nucleus ambiguus, a key site for parasympathetic cardiac control, may be correlated with a functional role. As we have previously demonstrated that nesfatin-1 elicits Ca²⁺ signaling in hypothalamic neurons, we first assessed the effect of this peptide on cytosolic Ca²⁺ in cardiac pre-ganglionic neurons of nucleus ambiguus. We provide evidence that nesfatin-1 increases cytosolic Ca²⁺ concentration via a Gi/o-coupled mechanism. The nesfatin-1-induced Ca²⁺ rise is critically dependent on Ca²⁺ influx via P/Q-type voltage-activated Ca²⁺ channels. Repeated administration of nesfatin-1 leads to tachyphylaxis. Furthermore, nesfatin-1 produces a dose-dependent depolarization of cardiac vagal neurons via a Gi/o-coupled mechanism. In vivo studies, using telemetric and tail-cuff monitoring of heart rate and blood pressure, indicate that microinjection of nesfatin-1 into the nucleus ambiguus produces bradycardia not accompanied by a change in blood pressure in conscious rats. Taken together, our results identify for the first time that nesfatin-1 decreases heart rate by activating cardiac vagal neurons of nucleus ambiguus. Our results indicate that nesfatin-1, one of the most potent feeding peptides, increases cytosolic Ca²⁺ by promoting Ca²⁺ influx via P/Q channels and depolarizes nucleus ambiguus neurons; both effects are Gi/o-mediated. In vivo studies indicate that microinjection of nesfatin-1 into nucleus ambiguus produces bradycardia in conscious rats. This is the first report that nesfatin-1 increases the parasympathetic cardiac tone.

A transparent organic transistor structure for bidirectional stimulation and recording of primary neurons

Real-time stimulation and recording of neural cell bioelectrical activity could provide an unprecedented insight in understanding the functions of the nervous system, and it is crucial for developing advanced in vitro drug screening approaches. Among organic materials, suitable candidates for cell interfacing can be found that combine long-term biocompatibility and mechanical flexibility. Here, we report on transparent organic cell stimulating and sensing transistors (O-CSTs), which provide bidirectional stimulation and recording of primary neurons. We demonstrate that the device enables depolarization and hyperpolarization of the primary neuron membrane potential. The transparency of the device also allows the optical imaging of the modulation of the neuron bioelectrical activity. The maximal amplitude-to-noise ratio of the extracellular recording achieved by the O-CST device exceeds that of a microelectrode array system on the same neuronal preparation by a factor of 16. Our organic cell stimulating and sensing device paves the way to a new generation of devices for stimulation, manipulation and recording of cell bioelectrical activity in vitro and in vivo.

Differential activation of cultured neonatal cardiomyocytes by plasmalemmal versus intracellular G protein-coupled receptor 55

The L-α-lysophosphatidylinositol (LPI)-sensitive receptor GPR55 is coupled to Ca(2+) signaling. Low levels of GPR55 expression in the heart have been reported. Similar to other G protein-coupled receptors involved in cardiac function, GPR55 may be expressed both at the sarcolemma and intracellularly. Thus, to explore the role of GPR55 in cardiomyocytes, we used calcium and voltage imaging and extracellular administration or intracellular microinjection of GPR55 ligands. We provide the first evidence that, in cultured neonatal ventricular myocytes, LPI triggers distinct signaling pathways via GPR55, depending on receptor localization. GPR55 activation at the sarcolemma elicits, on one hand, Ca(2+) entry via L-type Ca(2+) channels and, on the other, inositol 1,4,5-trisphosphate-dependent Ca(2+) release. The latter signal is further amplified by Ca(2+)-induced Ca(2+) release via ryanodine receptors. Conversely, activation of GPR55 at the membrane of intracellular organelles promotes Ca(2+) release from acidic-like Ca(2+) stores via the endolysosomal NAADP-sensitive two-pore channels. This response is similarly enhanced by Ca(2+)-induced Ca(2+) release via ryanodine receptors. Extracellularly applied LPI produces Ca(2+)-independent membrane depolarization, whereas the Ca(2+) signal induced by intracellular microinjection of LPI converges to hyperpolarization of the sarcolemma. Collectively, our findings point to GPR55 as a novel G protein-coupled receptor regulating cardiac function at two cellular sites. This work may serve as a platform for future studies exploring the potential of GPR55 as a therapeutic target in cardiac disorders.

Cell volume regulation in cultured human retinal Müller cells is associated with changes in transmembrane potential

Müller cells are mainly involved in controlling extracellular homeostasis in the retina, where intense neural activity alters ion concentrations and osmotic gradients, thus favoring cell swelling. This increase in cell volume is followed by a regulatory volume decrease response (RVD), which is known to be partially mediated by the activation of K⁺ and anion channels. However, the precise mechanisms underlying osmotic swelling and subsequent cell volume regulation in Müller cells have been evaluated by only a few studies. Although the activation of ion channels during the RVD response may alter transmembrane potential (V_m), no studies have actually addressed this issue in Müller cells. The aim of the present work is to evaluate RVD using a retinal Müller cell line (MIO-M1) under different extracellular ionic conditions, and to study a possible association between RVD and changes in V_m. Cell volume and V_m changes were evaluated using fluorescent probe techniques and a mathematical model. Results show that cell swelling and subsequent RVD were accompanied by V_m depolarization followed by repolarization. This response depended on the composition of extracellular media. Cells exposed to a hypoosmotic solution with reduced ionic strength underwent maximum RVD and had a larger repolarization. Both of these responses were reduced by K⁺ or Cl⁻ channel blockers. In contrast, cells facing a hypoosmotic solution with the same ionic strength as the isoosmotic solution showed a lower RVD and a smaller repolarization and were not affected by blockers. Together, experimental and simulated data led us to propose that the efficiency of the RVD process in Müller glia depends not only on the activation of ion channels, but is also strongly modulated by concurrent changes in the membrane potential. The relationship between ionic fluxes, changes in ion permeabilities and ion concentrations –all leading to changes in V_m– define the success of RVD.

Bradycardic effects mediated by activation of G protein-coupled estrogen receptor in rat nucleus ambiguus

The G protein-coupled estrogen receptor (GPER) has been identified in several brain regions, including cholinergic neurons of the nucleus ambiguus, which are critical for parasympathetic cardiac regulation. Using calcium imaging and electrophysiological techniques, microinjection into the nucleus ambiguus and blood pressure measurement, we examined the in vitro and in vivo effects of GPER activation in nucleus ambiguus neurons. A GPER selective agonist, G-1, produced a sustained increase in cytosolic Ca(2+) concentration in a concentration-dependent manner in retrogradely labelled cardiac vagal neurons of nucleus ambiguus. The increase in cytosolic Ca(2+) produced by G-1 was abolished by pretreatment with G36, a GPER antagonist. G-1 depolarized cultured cardiac vagal neurons of the nucleus ambiguus. The excitatory effect of G-1 was also identified by whole-cell visual patch-clamp recordings in nucleus ambiguus neurons, in medullary slices. To validate the physiological relevance of our in vitro studies, we carried out in vivo experiments. Microinjection of G-1 into the nucleus ambiguus elicited a decrease in heart rate; the effect was blocked by prior microinjection of G36. Systemic injection of G-1, in addition to a previously reported decrease in blood pressure, also reduced the heart rate. The G-1-induced bradycardia was prevented by systemic injection of atropine, a muscarinic antagonist, or by bilateral microinjection of G36 into the nucleus ambiguus. Our results indicate that GPER-mediated bradycardia occurs via activation of cardiac parasympathetic neurons of the nucleus ambiguus and support the involvement of the GPER in the modulation of cardiac vagal tone.

Creatine pretreatment protects cortical axons from energy depletion in vitro

Creatine is a natural nitrogenous guanidino compound involved in bioenergy metabolism. Although creatine has been shown to protect neurons of the central nervous system (CNS) from experimental hypoxia/ischemia, it remains unclear if creatine may also protect CNS axons, and if the potential axonal protection depends on glial cells. To evaluate the direct impact of creatine on CNS axons, cortical axons were cultured in a separate compartment from their somas and proximal neurites using a modified two-compartment culture device. Axons in the axon compartment were subjected to acute energy depletion, an in vitro model of white matter ischemia, by exposure to 6mM sodium azide for 30 min in the absence of glucose and pyruvate. Energy depletion reduced axonal ATP by 65%, depolarized axonal resting potential, and damaged 75% of axons. Application of creatine (10 mM) to both compartments of the culture at 24h prior to energy depletion significantly reduced axonal damage by 50%. In line with the role of creatine in the bioenergy metabolism, this application also alleviated the axonal ATP loss and depolarization. Inhibition of axonal depolarization by blocking sodium influx with tetrodotoxin also effectively reduced the axonal damage caused by energy depletion. Further study revealed that the creatine effect was independent of glial cells, as axonal protection was sustained even when creatine was applied only to the axon compartment (free from somas and glial cells) for as little as 2h. In contrast, application of creatine after energy depletion did not protect axons. The data provide the first evidence that creatine pretreatment may directly protect CNS axons from energy deficiency.

Quantitative measurement of mitochondrial membrane potential in cultured cells: calcium-induced de- and hyperpolarization of neuronal mitochondria

Mitochondrial membrane potential (ΔΨM) is a central intermediate in oxidative energy metabolism. Although ΔΨM is routinely measured qualitatively or semi-quantitatively using fluorescent probes, its quantitative assay in intact cells has been limited mostly to slow, bulk-scale radioisotope distribution methods. Here we derive and verify a biophysical model of fluorescent potentiometric probe compartmentation and dynamics using a bis-oxonol-type indicator of plasma membrane potential (ΔΨP) and the ΔΨM probe tetramethylrhodamine methyl ester (TMRM) using fluorescence imaging and voltage clamp. Using this model we introduce a purely fluorescence-based quantitative assay to measure absolute values of ΔΨM in millivolts as they vary in time in individual cells in monolayer culture. The ΔΨP-dependent distribution of the probes is modelled by Eyring rate theory. Solutions of the model are used to deconvolute ΔΨP and ΔΨM in time from the probe fluorescence intensities, taking into account their slow, ΔΨP-dependent redistribution and Nernstian behaviour. The calibration accounts for matrix:cell volume ratio, high- and low-affinity binding, activity coefficients, background fluorescence and optical dilution, allowing comparisons of potentials in cells or cell types differing in these properties. In cultured rat cortical neurons, ΔΨM is −139 mV at rest, and is regulated between −108 mV and −158 mV by concerted increases in ATP demand and Ca2+-dependent metabolic activation. Sensitivity analysis showed that the standard error of the mean in the absolute calibrated values of resting ΔΨM including all biological and systematic measurement errors introduced by the calibration parameters is less than 11 mV. Between samples treated in different ways, the typical equivalent error is ∼5 mV.