A positively tuned voltage indicator for extended electrical recordings in the brain.

Genetically encoded voltage indicators (GEVIs) enable optical recording of electrical signals in the brain, providing subthreshold sensitivity and temporal resolution not possible with calcium indicators. However, one- and two-photon voltage imaging over prolonged periods with the same GEVI has not yet been demonstrated. Here, we report engineering of ASAP family GEVIs to enhance photostability by inversion of the fluorescence-voltage relationship. Two of the resulting GEVIs, ASAP4b and ASAP4e, respond to 100-mV depolarizations with ≥180% fluorescence increases, compared with the 50% fluorescence decrease of the parental ASAP3. With standard microscopy equipment, ASAP4e enables single-trial detection of spikes in mice over the course of minutes. Unlike GEVIs previously used for one-photon voltage recordings, ASAP4b and ASAP4e also perform well under two-photon illumination. By imaging voltage and calcium simultaneously, we show that ASAP4b and ASAP4e can identify place cells and detect voltage spikes with better temporal resolution than commonly used calcium indicators. Thus, ASAP4b and ASAP4e extend the capabilities of voltage imaging to standard one- and two-photon microscopes while improving the duration of voltage recordings.

Time-dependent effect of inhaled cigarette smoke exposure in the bleomycin-induced lung injury rat model.

Cigarette smoke (CS) substances are known to induce diverse ailments such as cancer, decreased immunity, and lung diseases. Although some studies have been actively conducted to evaluate cigarette toxicity, the current animal exposure methods, that is, exposure of 28- or 90-days, require considerable research cost and lead to obscure results of the CS effects. In a previous study, we compared the effects of CS in a rat model of bleomycin (BLM) and lipopolysaccharide (LPS) induced lung disease. We determined that compared to the LPS-induced rat model, the BLM-induced rat model was more sensitive to alterations in secreting cytokines and total cell number. In the current study, we further confirmed the time-point of effective inhalation exposure by CS in the BLM-induced lung injury rat model. Using an automatic video instillator, rats were administered a single dose of 2.5 mg/kg BLM (day 1), and subsequently exposed to CS via inhalation (nose-only) 4 h/day, for 1, 2, 3, and 4 weeks. The bronchoalveolar lavage fluid (BALF) was obtained from the right lung lobes, total cell numbers were counted, and chemokine and cytokine expressions were evaluated using Enzyme-Linked Immunosorbent Assay. For the 1-week exposure, we observed a greater increase of neutrophils in the BLM + CS 300 µg/L group than in the BLM or CS 300 µg/L groups. Exposure of CS in the BLM-induced lung injury rat model enhanced the secretions of chemokines and cytokines, such as CCL2/MCP-1, CXCL2/MIP-2 and TNF-α, at 1 week. Immunohistochemistry and Hematoxylin and Eosin staining of lungs at 1-2 weeks after exposure clearly confirmed this tendency in the increased levels of CCL2/MCP-1 and TNF-α. Taken together, these results indicate that the rat model of BLM-induced lung injury is more sensitive to CS exposure than other rat models, and may be an appropriate model to evaluate the effect of CS exposure at 1-2 weeks.

Biophysical Parameters of GEVIs: Considerations for Imaging Voltage.

Genetically encoded voltage indicators (GEVIs) continue to evolve, resulting in many different probes with varying strengths and weaknesses. Developers of new GEVIs tend to highlight their positive features. A recent article from an independent laboratory has compared the signal/noise ratios of a number of GEVIs. Such a comparison can be helpful to investigators eager to try to image the voltage of excitable cells. In this perspective, we will present examples of how the biophysical features of GEVIs affect the imaging of excitable cells in an effort to assist researchers when considering probes for their specific needs.

A fast and responsive voltage indicator with enhanced sensitivity for unitary synaptic events.

A remaining challenge for genetically encoded voltage indicators (GEVIs) is the reliable detection of excitatory postsynaptic potentials (EPSPs). Here, we developed ASAP5 as a GEVI with enhanced activation kinetics and responsivity near resting membrane potentials for improved detection of both spiking and subthreshold activity. ASAP5 reported action potentials (APs) in vivo with higher signal-to-noise ratios than previous GEVIs and successfully detected graded and subthreshold responses to sensory stimuli in single two-photon trials. In cultured rat or human neurons, somatic ASAP5 reported synaptic events propagating centripetally and could detect ∼1-mV EPSPs. By imaging spontaneous EPSPs throughout dendrites, we found that EPSP amplitudes decay exponentially during propagation and that amplitude at the initiation site generally increases with distance from the soma. These results extend the applications of voltage imaging to the quantal response domain, including in human neurons, opening up the possibility of high-throughput, high-content characterization of neuronal dysfunction in disease.

A trafficking motif alters GEVI activity implicating persistent protein interactions at the membrane.

Efficient plasma-membrane expression is critical for genetically encoded voltage indicators (GEVIs). To improve the plasma-membrane expression, we introduced multiple combinations of plasma-membrane trafficking motifs at different positions to members of the Bongwoori family of GEVIs. An improvement from 20% to 27% in the ΔF/F/100 mV depolarization of the plasma membrane was observed when a Golgi transport motif was inserted near the N-terminus in conjunction with an endoplasmic reticulum release motif near the C-terminus of the protein. Unfortunately, this variant was also slower. The weighted tau on of the variant (25 ms) was more than double the original construct (11 ms). The weighted tau off was >20 ms compared with 10 ms for the original GEVI. The voltage range of the GEVI was also shifted to more negative potentials. Insertion of spacer amino acids between the fluorescent-protein domain and the endoplasmic reticulum release motif at the C-terminus rescued the speed of both the tau on and tau off while restoring the voltage range and maintaining the improved voltage-dependent optical signal. These results suggest that while trafficking motifs do improve plasma-membrane expression, they may also mediate persistent associations that affect the functioning of the protein.

Effect of the phenylpyrrole fungicide fludioxonil on cell proliferation and cardiac differentiation in mouse embryonic stem cells.

Fludioxnil is extensively used as a fungicide in agricultural application, but its possible impact on embryonic development is not yet well understood. In this study, the potential effect of fludioxonil on cardiac differentiation was evaluated in mouse embryonic stem cells (mESCs). The water-soluble tetrazolium (WST) and colony formation assays were conducted to confirm the effect of fludioxonil on proliferation of mESCs. The effect of fludioxonil on the ability of mESCs to form mouse embryoid bodies (mEBs) was determined by the hanging drop assay, whereas the ability of cardiomyocyte differentiation in the early stage was evaluated by determining the beating ratio (ratio of the number of contracting cells to the number of attached EBs) of cardiomyocytes. The viability of mESCs was significantly decreased (less than 50 %) at 10-5 M fludioxonil. Results of the colony formation assay revealed suppressed colony formation at 10-5 M fludioxonil (about 50 % at 5 days). Furthermore, the expressions of cell-cycle related proteins, i.e., cyclin D1, cyclin E, p21 and p27, were altered and trending towards inhibiting cell growth. Exposure to fludioxonil also resulted in reduced size of the mEB and induced increasing expression levels of the pluripotency markers Oct4, Sox2 and Nanog. Development of the beating ratio in the process of differentiation to cardiomyocytes derived from mESCs was completely inhibited after exposure to 10-5 M fludioxonil during the early stage of differentiation (day 5), whereas the beating ratio gradually increased after 5-day treatment. Simultaneously, expressions of the cardiomyocyte-related proteins, Gata4, Hand1 and cTnI, were inhibited after exposure to 10-5 M fludioxonil. Taken together, these results imply that fludioxonil may impact on the developmental process of mESCs, particularly the cardiac lineage.

Fenhexamid induces cancer growth and survival via estrogen receptor-dependent and PI3K-dependent pathways in breast cancer models.

Fenhexamid (Fen), a fungicide used to treat gray mold of fruits and vegetables, is reported to function as an endocrine disrupting chemical via the estrogen receptors (ER), despite low-toxicity of the pesticide. In this study, we elucidated that the disrupting effects of Fen are exerted via the ER and phosphatidylinositol 3-kinase (PI3K) pathways in breast cancer models. The WST assay, live cell monitoring, cell cycle analysis, colony formation assay, apoptotic analysis by JC-1 dyeing, and Western blot analysis were applied in ER positive MCF-7 and ER negative MDA-MB-231 breast cancer cells, after exposure to 17ß-estradiol (E2), Fen, ICI 182,780 (ICI; an ER antagonist) and/or Pictilisib (Pic; a PI3K inhibitor). Exposure to E2 and Fen induced the cell growth and survival ability of MCF-7 cells by increasing the S-phase cells and regulating the cell cycle-related proteins (Cyclin D1 and E1, p21 and p27). In addition, E2 and Fen treatment resulted in elevated levels of the survival-related proteins (Survivin and PCNA), and inhibited apoptosis by increasing the mitochondrial membrane potential and regulating the apoptosis-related proteins (BAX, BCL-2, and Caspase-9). These changes were reversed to the same level as the control group when exposed to their respective inhibitors, thereby indicating that the changes are exerted via the ER and PI3K pathways. In particular, co-treatment with these inhibitors induced greater inhibition than single treatment. Conversely, no alterations were observed in the ER-negative MDA-MB-231 breast cancer cells. Taken together, these results indicate that Fen promotes the growth of breast cancer cells via the ER and/or PI3K pathways, similar to the E2 mechanism. Although a relatively safe pesticide, Fen possibly exerts its influence as an endocrine disrupting chemical in ER-positive breast cancer cells via the ER and PI3K pathways.

Inhalation exposure by cigarette smoke: Effects on the progression of bleomycin- and lipopolysaccharide-induced lung injuries in rat models.

The toxic substances of cigarette smoke (CS) induce inflammatory responses in the lung by recruiting inflammatory cells. In this study, we investigated the effects of CS on the progression of lung disease in bleomycin (BLM) and lipopolysaccharide (LPS)-induced lung injury rat models. Briefly, rats were exposed to CS via inhalation (nose-only) for 28 consecutive days, for 4 h per day. Using an automatic video instillator, rats were administered a single dose of 2.5 mg/kg BLM (day 1) or 0.5 mg/kg LPS (day 26), prepared in 50 µL phosphate-buffered saline (PBS) solution. Examination of the bronchoalveolar lavage fluid (BALF) revealed that the number of neutrophils increased in a concentration-dependent manner of CS. Exposure to CS also enhanced the expression of cytokines, i.e., CCL2 (MCP-1), CCL3 (MIP-1α), CXCL2 (CINC3), CXCL10 (IP-10), TNF-α, IFN-γ, IL-2, IL-4 in the BALF of the vehicle (VC) and BLM groups in a concentration-dependent manner. In particular, the expressions of CCL2, CXCL10 and TNF-α were remarkably upregulated in the BLM + CS 300 treatment as compared to VC, while there were no differences in these cytokine levels in the serum following CS exposure. Exposure to CS resulted in compacted alveolar spaces and macrophage aggregation in the lung tissues following BLM and LPS treatments. Compared to VC, pulmonary fibrosis and chronic inflammation of bronchioloalveoli were observed in the BLM + CS treatment and inflammatory cell infiltration of bronchioloalveoli was observed in the LPS + CS treatment in a concentration-dependent manner by CS. The expression levels of CCL2 and IFN-γ in the lung tissues were increased similar to the levels obtained in BALF, in a concentration-dependent manner by CS. Taken together, these results indicate that repeated exposure to CS may exacerbate the lung injury initially caused by BLM and LPS.

Inhibitory effects of cigarette smoke extracts on neural differentiation of mouse embryonic stem cells.

Maternal smoking during the perinatal period is linked to adverse neonatal outcomes such as low birth weight and birth defects. Numerous studies have shown that cigarette smoke or nicotine exposure has a widespread effect on fetal nerve development. However, there exists a lack of understanding of what specific changes occur at the cellular level on persistent exposure to cigarette smoke during the differentiation of embryonic stem cells (ESCs) into neural cells. We previously investigated the effects of cigarette smoke extract (CSE) and its major component, nicotine, on the neural differentiation of mouse embryonic stem cells (mESCs). Differentiation of mESCs into neural progenitor cells (NPCs) or neural crest cells (NCCs) was induced with chemically defined media, and the cells were continuously exposed to CSE or nicotine during neural differentiation and development. Disturbed balance of the pluripotency state was observed in the NPCs, with consequent inhibition of neurite outgrowth and glial fibrillary acidic protein (Gfap) expression. These inhibitions correlated with the altered expression of proteins involved in the Notch-1 signaling pathways. The migration ability of NCCs was significantly decreased by CSE or nicotine exposure, which was associated with reduced protein expression of migration-related proteins. Taken together, we concluded that CSE and nicotine inhibit differentiation of mESCs into NPCs or NCCs, and may disrupt functional development of neural cells. These results imply that cigarette smoking during the perinatal period potentially inhibits neural differentiation and development of ESCs cells, leading to neonatal abnormal brain development and behavioral abnormalities.

Optical consequences of a genetically-encoded voltage indicator with a pH sensitive fluorescent protein.

Genetically-Encoded Voltage Indicators (GEVIs) are capable of converting changes in membrane potential into an optical signal. Here, we focus on recent insights into the mechanism of ArcLight-type probes and the consequences of utilizing a pH-dependent Fluorescent Protein (FP). A negative charge on the exterior of the ß-can of the FP combined with a pH-sensitive FP enables voltage-dependent conformational changes to affect the fluorescence of the probe. This hypothesis implies that interaction/dimerization of the FP creates a microenvironment for the probe that is altered via conformational changes. This mechanism explains why a pH sensitive FP with a negative charge on the outside of the ß-can is needed, but also suggests that pH could affect the optical signal as well. To better understand the effects of pH on the voltage-dependent signal of ArcLight, the intracellular pH (pHi) was tested at pH 6.8, 7.2, or 7.8. The resting fluorescence of ArcLight gets brighter as the pHi increases, yet only pH 7.8 significantly affected the ΔF/F. ArcLight could also simultaneously report voltage and pH changes during the acidification of a neuron firing multiple action potentials revealing different buffering capacities of the soma versus the processes of the cell.

Engineering Photoactivatability in Genetically Encoded Voltage and pH Indicators.

Genetically-encoded indicators of neuronal activity enable the labeling of a genetically defined population of neurons to optically monitor their activities. However, researchers often find difficulties in identifying relevant signals from excessive background fluorescence. A photoactivatable version of a genetically encoded calcium indicator, sPA-GCaMP6f is a good example of circumventing such an obstacle by limiting the fluorescence to a region of interest defined by the user. Here, we apply this strategy to genetically encoded voltage (GEVI) and pH (GEPI) indicators. Three photoactivatable GEVI candidates were considered. The first one used a circularly-permuted fluorescent protein, the second design involved a Förster resonance energy transfer (FRET) pair, and the third approach employed a pH-sensitive variant of GFP, ecliptic pHluorin. The candidate with a variant of ecliptic pHluorin exhibited photoactivation and a voltage-dependent fluorescence change. This effort also yielded a pH-sensitive photoactivatable GFP that varies its brightness in response to intracellular pH changes.

A dimeric fluorescent protein yields a bright, red-shifted GEVI capable of population signals in brain slice.

A bright, red-shifted Genetically Encoded Voltage Indicator (GEVI) was developed using a modified version of the fluorescent protein, tdTomato. Dimerization of the fluorescent domain for ArcLight-type GEVIs has been shown to affect the signal size of the voltage-dependent optical signal. For red-shifted GEVI development, tdTomato was split fusing a single dTomato chromophore to the voltage sensing domain. Optimization of the amino acid length and charge composition of the linker region between the voltage sensing domain and the fluorescent protein resulted in a probe that is an order of magnitude brighter than FlicR1 at a resting potential of -70 mV and exhibits a ten-fold larger change in fluorescence (ΔF) upon 100 mV depolarization of the plasma membrane in HEK 293 cells. Unlike ArcLight, the introduction of charged residues to the exterior of dTomato did not substantially improve the dynamic range of the optical signal. As a result, this new GEVI, Ilmol, yields a 3-fold improvement in the signal-to-noise ratio compared to FlicR1 despite a smaller fractional change in fluorescence of 4% per 100 mV depolarization of the plasma membrane. Ilmol expresses well in neurons resolving action potentials in neuronal cultures and reporting population signals in mouse hippocampal acute brain slice recordings. Ilmol is the brightest red-shifted GEVI to date enabling imaging with 160-fold less light than Archon1 for primary neuron recordings (50 mW/cm2 versus 8 W/cm2) and 600-fold less light than QuasAr2 for mouse brain slice recordings (500 mW/cm2 versus 300 W/cm2). This new GEVI uses a distinct mechanism from other approaches, opening an alternate engineering path to improve sensitivity and speed.

Improving a genetically encoded voltage indicator by modifying the cytoplasmic charge composition.

An improved genetically encoded voltage indicator (GEVI) was achieved by altering the charge composition of the region linking the voltage-sensing domain of the GEVI to a pH-sensitive fluorescent protein. Negatively charged linker segments reduced the voltage-dependent optical signal while positively charged linkers increased the signal size. Arginine scanning mutagenesis of the linker region improved the signal size of the GEVI, Bongwoori, yielding fluorescent signals as high as 20% ΔF/F during the firing of action potentials. The speed of this new sensor was also capable of optically resolving action potentials firing at 65 Hz. This large signal size enabled individual pixels to become surrogate electrodes. Plotting the highest correlated pixels based only on fluorescence changes reproduced the image of the neuron exhibiting activity. Furthermore, the use of a pH-sensitive fluorescent protein facilitated the detection of the acidification of the neuron during the firing of action potentials.

Imaging Membrane Potential with Two Types of Genetically Encoded Fluorescent Voltage Sensors.

Genetically encoded voltage indicators (GEVIs) have improved to the point where they are beginning to be useful for in vivo recordings. While the ultimate goal is to image neuronal activity in vivo, one must be able to image activity of a single cell to ensure successful in vivo preparations. This procedure will describe how to image membrane potential in a single cell to provide a foundation to eventually image in vivo. Here we describe methods for imaging GEVIs consisting of a voltage-sensing domain fused to either a single fluorescent protein (FP) or two fluorescent proteins capable of Förster resonance energy transfer (FRET) in vitro. Using an image splitter enables the projection of images created by two different wavelengths onto the same charge-coupled device (CCD) camera simultaneously. The image splitter positions a second filter cube in the light path. This second filter cube consists of a dichroic and two emission filters to separate the donor and acceptor fluorescent wavelengths depending on the FPs of the GEVI. This setup enables the simultaneous recording of both the acceptor and donor fluorescent partners while the membrane potential is manipulated via whole cell patch clamp configuration. When using a GEVI consisting of a single FP, the second filter cube can be removed allowing the mirrors in the image splitter to project a single image onto the CCD camera.