An optogenetic method for the controlled release of single molecules

We developed a system for optogenetic release of single molecules in cells. We confined soluble and transmembrane proteins to the Golgi apparatus via a photocleavable protein and released them by short pulses of light. Our method allows for a light dose-dependent delivery of functional proteins to the cytosol and plasma membrane in amounts compatible with single-molecule imaging, greatly simplifying access to single-molecule microscopy of any protein in live cells. We were able to reconstitute ion conductance by delivering BK and LRRC8/volume-regulated anion channels to the plasma membrane. Finally we were able to induce NF-kB signaling in T lymphoblasts stimulated by interleukin-1 by controlled release of a signaling protein that had been knocked out. We observed light-induced formation of functional inflammatory signaling complexes that triggered phosphorylation of the inhibitor of nuclear factor kappa-B kinase only in activated cells. We thus developed an optogenetic method for the reconstitution and investigation of cellular function at the single-molecule level.

Blue-shifted genetically encoded Ca2+ indicator with enhanced two-photon absorption

Significance

Genetically encoded calcium ion (Ca 2 + ) indicators (GECIs) are powerful tools for monitoring intracellular Ca 2 + concentration changes in living cells and model organisms. In particular, GECIs have found particular utility for monitoring the transient increase of Ca 2 + concentration that is associated with the neuronal action potential. However, the palette of highly optimized GECIs for imaging of neuronal activity remains relatively limited. Expanding the selection of available GECIs to include new colors and distinct photophysical properties could create new opportunities for in vitro and in vivo fluorescence imaging of neuronal activity. In particular, blue-shifted variants of GECIs are expected to have enhanced two-photon brightness, which would facilitate multiphoton microscopy.

Aim

We describe the development and applications of T-GECO1-a high-performance blue-shifted GECI based on the Clavularia sp.-derived mTFP1.

Approach

We use protein engineering and extensive directed evolution to develop T-GECO1. We characterize the purified protein and assess its performance in vitro using one-photon excitation in cultured rat hippocampal neurons, in vivo using one-photon excitation fiber photometry in mice, and ex vivo using two-photon Ca 2 + imaging in hippocampal slices.

Results

The Ca 2 + -bound state of T-GECO1 has an excitation peak maximum of 468 nm, an emission peak maximum of 500 nm, an extinction coefficient of 49,300    M - 1   cm - 1 , a quantum yield of 0.83, and two-photon brightness approximately double that of EGFP. The Ca 2 + -dependent fluorescence increase is 15-fold, and the apparent K d for Ca 2 + is 82 nM. With two-photon excitation conditions at 850 nm, T-GECO1 consistently enabled the detection of action potentials with higher signal-to-noise (SNR) than a late generation GCaMP variant.

Conclusions

T-GECO1 is a high-performance blue-shifted GECI that, under two-photon excitation conditions, provides advantages relative to late generation GCaMP variants.

High-Performance Genetically Encoded Green Fluorescent Biosensors for Intracellular l-Lactate

l-Lactate is a monocarboxylate produced during the process of cellular glycolysis and has long generally been considered a waste product. However, studies in recent decades have provided new perspectives on the physiological roles of l-lactate as a major energy substrate and a signaling molecule. To enable further investigations of the physiological roles of l-lactate, we have developed a series of high-performance (ΔF/F = 15 to 30 in vitro), intensiometric, genetically encoded green fluorescent protein (GFP)-based intracellular l-lactate biosensors with a range of affinities. We evaluated these biosensors in cultured cells and demonstrated their application in an ex vivo preparation of Drosophila brain tissue. Using these biosensors, we were able to detect glycolytic oscillations, which we analyzed and mathematically modeled.

A Monochromatically Excitable Green-Red Dual-Fluorophore Fusion Incorporating a New Large Stokes Shift Fluorescent Protein

Genetically encoded sensors enable quantitative imaging of analytes in live cells. Sensors are commonly constructed by combining ligand-binding domains with one or more sensitized fluorescent protein (FP) domains. Sensors based on a single FP can be susceptible to artifacts caused by changes in sensor levels or distribution in vivo. To develop intensiometric sensors with the capacity for ratiometric quantification, dual-FP Matryoshka sensors were generated by using a single cassette with a large Stokes shift (LSS) reference FP nested within the reporter FP (cpEGFP). Here, we present a genetically encoded calcium sensor that employs green apple (GA) Matryoshka technology by incorporating a newly designed red LSSmApple fluorophore. LSSmApple matures faster and provides an optimized excitation spectrum overlap with cpEGFP, allowing for monochromatic coexcitation with blue light. The LSS of LSSmApple results in improved emission spectrum separation from cpEGFP, thereby minimizing fluorophore bleed-through and facilitating imaging using standard dichroic and red FP (RFP) emission filters. We developed an image analysis pipeline for yeast (Saccharomyces cerevisiae) timelapse imaging that utilizes LSSmApple to segment and track cells for high-throughput quantitative analysis. In summary, we engineered a new FP, constructed a genetically encoded calcium indicator (GA-MatryoshCaMP6s), and performed calcium imaging in yeast as a demonstration.

[Development of fluorescent imaging sensors based on proteins]

Fluorescent imaging sensors based on genetically-encoded and biocompatible proteins have become important tools in medical and biological research due to their high spatiotemporal resolution and ease of use. Protein engineering has led to the development of imaging sensors that visualize changes in the concentration of various target molecules/ions, such as calcium ions. In addition, the development of chemigenetic sensors based on complexes of proteins and synthetic molecules has been gaining momentum in recent years. In this article, the latest research trends in the development of these imaging sensors are introduced, with focus on the sensors developed by our group.

Lactate biosensors for spectrally and spatially multiplexed fluorescence imaging

L-Lactate is increasingly appreciated as a key metabolite and signaling molecule in mammals. However, investigations of the inter- and intra-cellular dynamics of L-lactate are currently hampered by the limited selection and performance of L-lactate-specific genetically encoded biosensors. Here we now report a spectrally and functionally orthogonal pair of high-performance genetically encoded biosensors: a green fluorescent extracellular L-lactate biosensor, designated eLACCO2.1, and a red fluorescent intracellular L-lactate biosensor, designated R-iLACCO1. eLACCO2.1 exhibits excellent membrane localization and robust fluorescence response. To the best of our knowledge, R-iLACCO1 and its affinity variants exhibit larger fluorescence responses than any previously reported intracellular L-lactate biosensor. We demonstrate spectrally and spatially multiplexed imaging of L-lactate dynamics by coexpression of eLACCO2.1 and R-iLACCO1 in cultured cells, and in vivo imaging of extracellular and intracellular L-lactate dynamics in mice.

A blue-shifted genetically encoded Ca2+ indicator with enhanced two-photon absorption

Significance

Genetically encoded calcium ion (Ca2+) indicators (GECIs) are powerful tools for monitoring intracellular Ca2+ concentration changes in living cells and model organisms. In particular, GECIs have found particular utility for monitoring the transient increase of Ca2+ concentration that is associated with the neuronal action potential. However, the palette of highly optimized GECIs for imaging of neuronal activity remains relatively limited. Expanding the selection of available GECIs to include new colors and distinct photophysical properties could create new opportunities for in vitro and in vivo fluorescence imaging of neuronal activity. In particular, blue-shifted variants of GECIs are expected to have enhanced two-photon brightness, which would facilitate multiphoton microscopy.

Aim

We describe the development and applications of T-GECO1 - a high-performance blue-shifted GECI based on the Clavularia sp.-derived mTFP1.

Approach

We used protein engineering and extensive directed evolution to develop T-GECO1. We characterize the purified protein and assess its performance in vitro using one-photon excitation in cultured rat hippocampal neurons, in vivo using one-photon excitation fiber photometry in mice, and ex vivo using two-photon Ca2+ imaging in hippocampal slices.

Results

The Ca2+-bound state of T-GECO1 has an excitation peak maximum of 468 nm, an emission peak maximum of 500 nm, an extinction coefficient of 49,300 M-1cm-1, a quantum yield of 0.83, and two-photon brightness approximately double that of EGFP. The Ca2+-dependent fluorescence increase is 15-fold and the apparent Kd for Ca2+ is 82 nM. With two-photon excitation conditions at 850 nm, T-GECO1 consistently enabled detection of action potentials with higher signal-to-noise (SNR) than a late generation GCaMP variant.

Conclusion

T-GECO1 is a high performance blue-shifted GECI that, under two-photon excitation conditions, provides advantages relative to late generation GCaMP variants.

Construction of the lactate-sensing fibremats by confining sensor fluorescent protein of lactate inside nanofibers of the poly(HPMA/DAMA)/ADH-nylon 6 core-shell fibremat

The development of a new materials platform capable of sustaining the functionality of proteinous sensor molecules over an extended period without being affected by biological contaminants in living systems, such as proteases, is highly demanded. In this study, our primary focus was on fabricating new core-shell fibremats using unique polymer materials, capable of functionalizing encapsulated sensor proteins while resisting the effects of proteases. The core-fibre parts of core-shell fibremats were made using a newly developed post-crosslinkable water-soluble copolymer, poly(2-hydroxypropyl methacrylamide)-co-poly(diacetone methacrylamide), and the bifunctional crosslinking agent, adipic dihydrazide, while the shell layer of the nanofibers was made of nylon 6. Upon encapsulating the lactate-sensor protein eLACCO1.1 at the core-fibre part, the fibremat exhibited a distinct concentration-dependent fluorescence response, with a dynamic range of fluorescence alteration exceeding 1000% over the lactate concentration range of 0 to 100 mM. The estimated dissociation constant from the titration data was comparable to that estimated in a buffer solution. The response remained stable even after 5 cycles and in the presence of proteases. These results indicates that our core-shell fibremat platform could serve as effective immobilizing substrates for various sensor proteins, facilitating continuous and quantitative monitoring of various low-molecular-weight metabolites and catabolites in a variety of biological samples.

Maximizing the performance of protein-based fluorescent biosensors

Fluorescent protein (FP)-based biosensors are genetically encoded tools that enable the imaging of biological processes in the context of cells, tissues, or live animals. Though widely used in biological research, practically all existing biosensors are far from ideal in terms of their performance, properties, and applicability for multiplexed imaging. These limitations have inspired researchers to explore an increasing number of innovative and creative ways to improve and maximize biosensor performance. Such strategies include new molecular biology methods to develop promising biosensor prototypes, high throughput microfluidics-based directed evolution screening strategies, and improved ways to perform multiplexed imaging. Yet another approach is to effectively replace components of biosensors with self-labeling proteins, such as HaloTag, that enable the biocompatible incorporation of synthetic fluorophores or other ligands in cells or tissues. This mini-review will summarize and highlight recent innovations and strategies for enhancing the performance of FP-based biosensors for multiplexed imaging to advance the frontiers of research.

Rational Engineering of an Improved Genetically Encoded pH Sensor Based on Superecliptic pHluorin

Genetically encoded pH sensors based on fluorescent proteins are valuable tools for the imaging of cellular events that are associated with pH changes, such as exocytosis and endocytosis. Superecliptic pHluorin (SEP) is a pH-sensitive green fluorescent protein (GFP) variant widely used for such applications. Here, we report the rational design, development, structure, and applications of Lime, an improved SEP variant with higher fluorescence brightness and greater pH sensitivity. The X-ray crystal structure of Lime supports the mechanistic rationale that guided the introduction of beneficial mutations. Lime provides substantial improvements relative to SEP for imaging of endocytosis and exocytosis. Furthermore, Lime and its variants are advantageous for a broader range of applications including the detection of synaptic release and neuronal voltage changes.

Biosensor Optimization Using a Förster Resonance Energy Transfer Pair Based on mScarlet Red Fluorescent Protein and an mScarlet-Derived Green Fluorescent Protein

Genetically encoded biosensors based on Förster resonance energy transfer (FRET) are indispensable tools for monitoring biochemical changes in cells. Green and red fluorescent protein-based FRET pairs offer advantages over the classically employed cyan and yellow fluorescent protein pairs, such as better spectral separation, lower phototoxicity, and less autofluorescence. Here, we describe the development of an mScarlet-derived green fluorescent protein (designated as mWatermelon) and its use as a FRET donor to the red fluorescent protein mScarlet-I as a FRET acceptor. We tested the functionality of this FRET pair by engineering biosensors for the detection of protease activity, Ca²⁺, and K⁺. Furthermore, we described a strategy to enhance the FRET efficiency of these biosensors by modulating the intramolecular association between mWatermelon and mScarlet-I.

The endoplasmic reticulum kinase PERK interacts with the oxidoreductase ERO1 to metabolically adapt mitochondria

Endoplasmic reticulum (ER) homeostasis requires molecular regulators that tailor mitochondrial bioenergetics to the needs of protein folding. For instance, calnexin maintains mitochondria metabolism and mitochondria-ER contacts (MERCs) through reactive oxygen species (ROS) from NADPH oxidase 4 (NOX4). However, induction of ER stress requires a quick molecular rewiring of mitochondria to adapt to new energy needs. This machinery is not characterized. We now show that the oxidoreductase ERO1⍺ covalently interacts with protein kinase RNA-like ER kinase (PERK) upon treatment with tunicamycin. The PERK-ERO1⍺ interaction requires the C-terminal active site of ERO1⍺ and cysteine 216 of PERK. Moreover, we show that the PERK-ERO1⍺ complex promotes oxidization of MERC proteins and controls mitochondrial dynamics. Using proteinaceous probes, we determined that these functions improve ER-mitochondria Ca²⁺ flux to maintain bioenergetics in both organelles, while limiting oxidative stress. Therefore, the PERK-ERO1⍺ complex is a key molecular machinery that allows quick metabolic adaptation to ER stress.

Quantification of Intracellular Citrate Concentrations with Genetically Encoded Biosensors

Citrate is a central intracellular metabolite with roles in a variety of normal and aberrant biological processes. The methods for quantifying citrate concentration in cells can enable the study of the molecular mechanisms of citrate-related biological processes and diseases. Compared to existing analytical methods such as enzymatic assays and mass spectrometry, genetically encoded biosensors based on fluorescent proteins (FPs) offer the advantage of noninvasively tracking intracellular ion and small molecule dynamics with high spatial-temporal resolution. These biosensors are less toxic than chemosensors and can be targeted to specific organelles for subcellular imaging. Here we present a protocol for quantification of cytosolic and mitochondrial citrate in mammalian cells with recently reported genetically encoded biosensors for citrate.

Chemigenetic indicators based on synthetic chelators and green fluorescent protein

Molecular fluorescent indicators are versatile tools for dynamic imaging of biological systems. We now report a class of indicators that are based on the chemigenetic combination of a synthetic ion-recognition motif and a protein-based fluorophore. Specifically, we have developed a calcium ion (Ca²⁺) indicator that is based on genetic insertion of circularly permuted green fluorescent protein into HaloTag protein self-labeled with a ligand containing the Ca²⁺ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid. We have demonstrated the versatility of this design by also developing a sodium ion (Na⁺) indicator using a crown-ether-containing ligand. This approach affords bright and sensitive ion indicators that can be applicable to cell imaging. This design can enable the development of chemigenetic indicators with ion or molecular specificities that have not been realized with fully protein-based indicators.

A genetically encoded far-red fluorescent calcium ion biosensor derived from a biliverdin-binding protein

Far-red and near-infrared (NIR) genetically encoded calcium ion (Ca2+ ) indicators (GECIs) are powerful tools for in vivo and multiplexed imaging of neural activity and cell signaling. Inspired by a previous report to engineer a far-red fluorescent protein (FP) from a biliverdin (BV)-binding NIR FP, we have developed a far-red fluorescent GECI, designated iBB-GECO1, from a previously reported NIR GECI. iBB-GECO1 exhibits a relatively high molecular brightness, an inverse response to Ca2+ with ΔF/Fmin  = -13, and a near-optimal dissociation constant (Kd ) for Ca2+ of 105 nM. We demonstrate the utility of iBB-GECO1 for four-color multiplexed imaging in MIN6 cells and five-color imaging in HEK293T cells. Like other BV-binding GECIs, iBB-GECO1 did not give robust signals during in vivo imaging of neural activity in mice, but did provide promising results that will guide future engineering efforts. SIGNIFICANCE: Genetically encoded calcium ion (Ca2+ ) indicators (GECIs) compatible with common far-red laser lines (~630-640 nm) on commercial microscopes are of critical importance for their widespread application to deep-tissue multiplexed imaging of neural activity. In this study, we engineered a far-red excitable fluorescent GECI, designated iBB-GECO1, that exhibits a range of preferable specifications such as high brightness, large fluorescence response to Ca2+ , and compatibility with multiplexed imaging in mammalian cells.

A sensitive and specific genetically-encoded potassium ion biosensor for in vivo applications across the tree of life

Potassium ion (K⁺) plays a critical role as an essential electrolyte in all biological systems. Genetically-encoded fluorescent K⁺ biosensors are promising tools to further improve our understanding of K⁺-dependent processes under normal and pathological conditions. Here, we report the crystal structure of a previously reported genetically-encoded fluorescent K⁺ biosensor, GINKO1, in the K⁺-bound state. Using structure-guided optimization and directed evolution, we have engineered an improved K⁺ biosensor, designated GINKO2, with higher sensitivity and specificity. We have demonstrated the utility of GINKO2 for in vivo detection and imaging of K⁺ dynamics in multiple model organisms, including bacteria, plants, and mice.

Potassium ions play a critical role as an essential electrolyte in all biological systems. This study describes high performance genetically encoded potassium ion sensors to enable in vivo measurement of potassium ion concentrations across multiple model organisms.

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.

Live cell tracking of macrophage efferocytosis during Drosophila embryo development in vivo

Apoptosis of cells and their subsequent removal via efferocytosis occurs in nearly all tissues during development, homeostasis, and disease. However, it has been difficult to track cell death and subsequent corpse removal in vivo. Here, we developed a genetically encoded fluorescent reporter, CharON, that could track emerging apoptotic cells and their efferocytic clearance by phagocytes. Using Drosophila expressing CharON, we uncovered multiple qualitative and quantitative features of coordinated clearance of apoptotic corpses during embryonic development. To confront high rate of emerging apoptotic corpses, the macrophages displayed heterogeneity in engulfment, with some efferocytic macrophages carrying high corpse burden. However, overburdened macrophages were compromised in clearing wound debris, revealing an inherent vulnerability. These findings reveal known and unexpected features of apoptosis and macrophage efferocytosis in vivo.

Cyan fluorescent proteins derived from mNeonGreen

mNeonGreen, an engineered green fluorescent protein (GFP) derived from lancelet, is one of the most brightly fluorescent homologs of Aequorea victoria jellyfish GFP (avGFP) yet reported. In this work, we investigated whether this bright fluorescence might be retained in homologs of mNeonGreen with modified chromophore structures and altered fluorescent hues. We found mNeonGreen to be generally less tolerant than avGFP to chromophore modification by substitution of the key chromophore-forming tyrosine residue with other aromatic amino acids. However, we were ultimately successful in creating a variant, designated as NeonCyan1, with a tryptophan-derived cyan fluorescent protein (CFP)-type chromophore, and two additional mutants with distinct spectral hues. Structural, computational, and photophysical characterization of NeonCyan1 and its variants provided insight into the factors that control the fluorescence emission color. Though not recommended as replacements for contemporary CFP variants, we demonstrate that NeonCyan1 variants are potentially suitable for live cell imaging applications.

Neurophotonic tools for microscopic measurements and manipulation: status report

Neurophotonics was launched in 2014 coinciding with the launch of the BRAIN Initiative focused on development of technologies for advancement of neuroscience. For the last seven years, Neurophotonics' agenda has been well aligned with this focus on neurotechnologies featuring new optical methods and tools applicable to brain studies. While the BRAIN Initiative 2.0 is pivoting towards applications of these novel tools in the quest to understand the brain, this status report reviews an extensive and diverse toolkit of novel methods to explore brain function that have emerged from the BRAIN Initiative and related large-scale efforts for measurement and manipulation of brain structure and function. Here, we focus on neurophotonic tools mostly applicable to animal studies. A companion report, scheduled to appear later this year, will cover diffuse optical imaging methods applicable to noninvasive human studies. For each domain, we outline the current state-of-the-art of the respective technologies, identify the areas where innovation is needed, and provide an outlook for the future directions.

A genetically encoded fluorescent biosensor for extracellular L-lactate

L-Lactate, traditionally considered a metabolic waste product, is increasingly recognized as an important intercellular energy currency in mammals. To enable investigations of the emerging roles of intercellular shuttling of L-lactate, we now report an intensiometric green fluorescent genetically encoded biosensor for extracellular L-lactate. This biosensor, designated eLACCO1.1, enables cellular resolution imaging of extracellular L-lactate in cultured mammalian cells and brain tissue.

Vascular rinsing and chilling effects on meat quality attributes from cull dairy cows associated with the two lowest-valued marketing classes

Commercially harvested cull dairy cow carcasses (n = 64) from the two lowest-valued marketing classes (MC: Lean, LE; Light, LI) were conventionally chilled (CN) or vascularly rinsed with a chilled isotonic substrate solution (Rinse & Chill®; RC). Longissimus lumborum (LL) and Triceps brachii (TB) muscles were processed (steaks, ground). Early postmortem (first 24 h), RC resulted in a lower pH at each time measured. RC steaks had longer sarcomeres and lower shear force than CN. RC produced greater redness associated with blooming and display times. RC LE beef resulted in greater oxymyoglobin during display times. RC ground TB had greater moisture fat-free than CN. RC Lean LL had less purge loss compared to CN LE. RC had greater total pigments than CN. RC ground TB had greater oxygen consumption and lower thiobarbituric acid reactive substances compared to CN. RC has the potential to improve tenderness and color as well as limit lipid oxidation with similar benefits across the two marketing classes.

Photocleavable proteins that undergo fast and efficient dissociation

Photocleavable molecules can enable the light-dependent modulation of biomolecular activities with high spatiotemporal precision. We have previously reported a photocleavable protein (PhoCl1) that, uniquely, is a fully genetically encoded photocleavable molecule that can be introduced into cells in the form of its corresponding gene to enable optogenetic control of biomolecular activities. However, the first generation PhoCl1 exhibited a relatively slow rate of dissociation, potentially limiting its utility. Here, we report the X-ray crystal structures of the PhoCl1 green state, red state, and cleaved empty barrel. Molecular dynamics (MD) simulations were performed to provide insight into the precise dissociation mechanism. Using structure-guided engineering and directed evolution, we have developed PhoCl2c with higher contrast ratio and PhoCl2f with faster dissociation. We characterized the performance of these new variants as purified proteins and in cultured cells. Our results demonstrate that PhoCl2 variants exhibit faster and more efficient dissociation, which should enable improved optogenetic manipulations of protein localization and protein-protein interactions in living cells.

Early initiation of anti-androgen treatment is associated with increased probability of spontaneous conception leading to childbirth in women with polycystic ovary syndrome: a population-based multiregistry cohort study in Sweden

STUDY QUESTION

Is anti-androgen treatment during adolescence associated with an improved probability of spontaneous conception leading to childbirth in women with polycystic ovary syndrome (PCOS)?

SUMMARY ANSWER

Early initiation of anti-androgen treatment is associated with an increased probability of childbirth after spontaneous conception among women with PCOS.

WHAT IS KNOWN ALREADY

PCOS is the most common endocrinopathy affecting women of reproductive age. Hyperandrogenism and menstrual irregularities associated with PCOS typically emerge in early adolescence. Previous work indicates that diagnosis at an earlier age (<25 years) is associated with higher fecundity compared to a later diagnosis.

STUDY DESIGN, SIZE, DURATION

This population-based study utilized five linked Swedish national registries. A total of 15 106 women with PCOS and 73 786 control women were included. Women were followed from when they turned 18 years of age until the end of 2015, leading to a maximum follow-up of 10 years. First childbirth after spontaneous conception was the main outcome, as identified from the Medical Birth Registry.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Participants included all women born between 1987 and 1996 with a diagnosis of PCOS in the Swedish Patient Registry and randomly selected non-PCOS controls (ratio 1:5). Information on anti-androgenic treatment was retrieved from the Swedish Prescribed Drug Registry with the use of Anatomic Therapeutic Chemical (ATC) codes. Women with PCOS who were not treated with any anti-androgenic medication were regarded as normo-androgenic, while those treated were regarded as hyperandrogenic. Women were further classified as being mildly hyperandrogenic if they received anti-androgenic combined oral contraceptive (aaCOC) monotherapy, or severely hyperandrogenic if they received other anti-androgens with or without aaCOCs. Early and late users comprised women with PCOS who started anti-androgenic treatment initiated either during adolescence (≤ 18 years of age) or after adolescence (>18 years), respectively. The probability of first childbirth after spontaneous conception was analyzed with the use of Kaplan–Meier hazard curve. The fecundity rate (FR) and 95% confidence interval for the time to first childbirth that were conceived spontaneously were calculated using Cox proportional hazards regression models, with adjustment for obesity, birth year, country of birth and education level.

MAIN RESULTS AND THE ROLE OF CHANCE

The probability of childbirth after spontaneous conception in the PCOS group compared to non-PCOS controls was 11% lower among normo-androgenic (adjusted FR 0.68 (95% CI 0.64–0.72)), and 40% lower among hyperandrogenic women with PCOS (adjusted FR 0.53 (95% CI 0.50–0.57)). FR was lowest among severely hyperandrogenic women with PCOS compared to normo-androgenic women with PCOS (adjusted FR 0.60 (95% CI 0.52–0.69)), followed by mildly hyperandrogenic women with PCOS (adjusted FR 0.84 (95% CI 0.77–0.93)). Compared to early anti-androgenic treatment users, late users exhibited a lower probability of childbirth after spontaneous conception (adjusted FR 0.79 (95% CI 0.68–0.92)).

LIMITATIONS, REASONS FOR CAUTION

We lacked direct information on the intention to conceive and the androgenic biochemical status of the PCOS participants, applying instead the use of anti-androgenic medications as a proxy of hyperandrogenism. The duration of anti-androgenic treatment utilized is not known, only the age at prescription. Results are not adjusted for BMI, but for obesity diagnosis. The period of follow-up (10 years) was restricted by the need to include only those women for whom data were available on the dispensing of medications during adolescence (born between 1987 and 1996). Women with PCOS who did not seek medical assistance might have been incorrectly classified as not having the disease. Such misclassification would lead to an underestimation of the true association between PCOS and outcomes.

WIDER IMPLICATIONS OF THE FINDINGS

Early initiation of anti-androgen treatment is associated with better spontaneous fertility rate. These findings support the need for future interventional randomized prospective studies investigating critical windows of anti-androgen treatment.

STUDY FUNDING/COMPETING INTEREST(S)

This study was funded by the Health Research Council of New Zealand (18-671), the Swedish Society of Medicine and the Uppsala University Hospital. Evangelia Elenis has, over the past year, received lecture fee from Gedeon Richter outside the submitted work. Inger Sundström Poromaa has, over the past 3 years, received compensation as a consultant and lecturer for Bayer Schering Pharma, MSD, Gedeon Richter, Peptonics and Lundbeck A/S. The other authors declare no competing interests.

TRIAL REGISTRATION NUMBER

N/A

Controlled Osteogenic Differentiation of Human Mesenchymal Stem Cells Using Dexamethasone-Loaded Light-Responsive Microgels

Human mesenchymal stem cells (hMSCs), which have the ability to differentiate into osteoblasts, show promise for bone tissue engineering and bone defect treatment. While there are a number of approaches currently available to accomplish this, e.g., utilizing biodegradable materials loaded with the synthetic glucocorticoid osteogenic inducer dexamethasone (DEX), there are still many disadvantages with the current technologies. Here, we generated light-responsive microgels that we showed are capable of loading and releasing DEX in a light-triggered fashion, with the released DEX being able to induce hMSC differentiation into osteoblasts. Specifically, light-responsive poly(N-isopropylacrylamide-co-nitrobenzyl methacrylate) (pNIPAm-co-NBMA) microgels were synthesized via free radical precipitation polymerization and their size, morphology, and chemical composition were characterized. We then went on to show that the microgels could be loaded with DEX (via what we think are hydrophobic interactions) and released upon exposure to UV light. We went on to show that the DEX released from the microgels was still capable of inducing osteogenic differentiation of hMSCs using an alamarBlue assay and normalized alkaline phosphatase (ALP) activity assay. We also investigated how hMSC differentiation was impacted by intermittent DEX released from UV-exposed microgels. Finally, we confirmed that the microgels themselves were not cytotoxic to hMSCs. Taken together, the DEX-loaded light-responsive microgels reported here may find a use for niche clinical applications, e.g., bone tissue repair.

Switching between Ultrafast Pathways Enables a Green-Red Emission Ratiometric Fluorescent-Protein-Based Ca2+ Biosensor

Ratiometric indicators with long emission wavelengths are highly preferred in modern bioimaging and life sciences. Herein, we elucidated the working mechanism of a standalone red fluorescent protein (FP)-based Ca2+ biosensor, REX-GECO1, using a series of spectroscopic and computational methods. Upon 480 nm photoexcitation, the Ca2+-free biosensor chromophore becomes trapped in an excited dark state. Binding with Ca2+ switches the route to ultrafast excited-state proton transfer through a short hydrogen bond to an adjacent Glu80 residue, which is key for the biosensor's functionality. Inspired by the 2D-fluorescence map, REX-GECO1 for Ca2+ imaging in the ionomycin-treated human HeLa cells was achieved for the first time with a red/green emission ratio change (ΔR/R0) of ~300%, outperforming many FRET- and single FP-based indicators. These spectroscopy-driven discoveries enable targeted design for the next-generation biosensors with larger dynamic range and longer emission wavelengths.

Vascular infusion with concurrent vascular rinsing on color, tenderness, and lipid oxidation of hog meat

Market hogs were conventionally chilled (CN, n = 12) or Rinse & Chill® processed (RC, n = 13, MPSC Inc.). Muscles (Longissimus lumborum, LL; picnic shoulder, PS) were processed (chops, ground), packaged, and displayed or stored in the dark. Color, pH, moisture fat free (MFF), expressible moisture (EM), oxygen consumption, Warner-Bratzler shear (WBS), total pigment, TBARS, and hexanal content were determined. RC generally resulted in a lower pH during the first 4 h compared to CN. RC compared to CN had lower fat, but were not different in moisture fat free, expressible moisture, and total pigments. RC did not affect cooler shrink, cook loss and WBS force. RC PS was redder than RC LM. RC had greater deoxymyoglobin than CN on 7 d display. RC chops (LL) were lighter and had less deoxymyoglobin compared to CN. RC ground pork had greater oxygen consumption, lower TBARS and hexanal values compared to CN. RC has the potential to improve color and reduce lipid oxidation.

Improved genetically encoded near-infrared fluorescent calcium ion indicators for in vivo imaging

Near-infrared (NIR) genetically encoded calcium ion (Ca²⁺) indicators (GECIs) can provide advantages over visible wavelength fluorescent GECIs in terms of reduced phototoxicity, minimal spectral cross talk with visible light excitable optogenetic tools and fluorescent probes, and decreased scattering and absorption in mammalian tissues. Our previously reported NIR GECI, NIR-GECO1, has these advantages but also has several disadvantages including lower brightness and limited fluorescence response compared to state-of-the-art visible wavelength GECIs, when used for imaging of neuronal activity. Here, we report 2 improved NIR GECI variants, designated NIR-GECO2 and NIR-GECO2G, derived from NIR-GECO1. We characterized the performance of the new NIR GECIs in cultured cells, acute mouse brain slices, and Caenorhabditis elegans and Xenopus laevis in vivo. Our results demonstrate that NIR-GECO2 and NIR-GECO2G provide substantial improvements over NIR-GECO1 for imaging of neuronal Ca²⁺ dynamics.

This study describes improved genetically encoded near-infrared fluorescent calcium ion indicators, demonstrating that they enable robust detection of neuronal activity in cultured cells, rodent brain slices, Caenorhabditis elegans, and Xenopus laevis.

Engineering Photosensory Modules of Non-Opsin-Based Optogenetic Actuators

Optogenetic (photo-responsive) actuators engineered from photoreceptors are widely used in various applications to study cell biology and tissue physiology. In the toolkit of optogenetic actuators, the key building blocks are genetically encodable light-sensitive proteins. Currently, most optogenetic photosensory modules are engineered from naturally-occurring photoreceptor proteins from bacteria, fungi, and plants. There is a growing demand for novel photosensory domains with improved optical properties and light-induced responses to satisfy the needs of a wider variety of studies in biological sciences. In this review, we focus on progress towards engineering of non-opsin-based photosensory domains, and their representative applications in cell biology and physiology. We summarize current knowledge of engineering of light-sensitive proteins including light-oxygen-voltage-sensing domain (LOV), cryptochrome (CRY2), phytochrome (PhyB and BphP), and fluorescent protein (FP)-based photosensitive domains (Dronpa and PhoCl).

The Role of Amino Acids in Neurotransmission and Fluorescent Tools for Their Detection

Neurotransmission between neurons, which can occur over the span of a few milliseconds, relies on the controlled release of small molecule neurotransmitters, many of which are amino acids. Fluorescence imaging provides the necessary speed to follow these events and has emerged as a powerful technique for investigating neurotransmission. In this review, we highlight some of the roles of the 20 canonical amino acids, GABA and β-alanine in neurotransmission. We also discuss available fluorescence-based probes for amino acids that have been shown to be compatible for live cell imaging, namely those based on synthetic dyes, nanostructures (quantum dots and nanotubes), and genetically encoded components. We aim to provide tool developers with information that may guide future engineering efforts and tool users with information regarding existing indicators to facilitate studies of amino acid dynamics.

High-Performance Intensiometric Direct- and Inverse-Response Genetically Encoded Biosensors for Citrate

Motivated by the growing recognition of citrate as a central metabolite in a variety of biological processes associated with healthy and diseased cellular states, we have developed a series of high-performance genetically encoded citrate biosensors suitable for imaging of citrate concentrations in mammalian cells. The design of these biosensors was guided by structural studies of the citrate-responsive sensor histidine kinase and took advantage of the same conformational changes proposed to propagate from the binding domain to the catalytic domain. Following extensive engineering based on a combination of structure guided mutagenesis and directed evolution, we produced an inverse-response biosensor (ΔF/F _min ≈ 18) designated Citroff1 and a direct-response biosensor (ΔF/F _min ≈ 9) designated Citron1. We report the X-ray crystal structure of Citron1 and demonstrate the utility of both biosensors for qualitative and quantitative imaging of steady-state and pharmacologically perturbed citrate concentrations in live cells.

Bright and High-Performance Genetically Encoded Ca2+ Indicator Based on mNeonGreen Fluorescent Protein

Genetically encodable calcium ion (Ca²⁺) indicators (GECIs) based on green fluorescent proteins (GFP) are powerful tools for imaging of cell signaling and neural activity in model organisms. Following almost 2 decades of steady improvements in the Aequorea victoria GFP-based GCaMP series of GECIs, the performance of the most recent generation (i.e., jGCaMP7) may have reached its practical limit due to the inherent properties of GFP. In an effort to sustain the steady progression toward ever-improved GECIs, we undertook the development of a new GECI based on the bright monomeric GFP, mNeonGreen (mNG). The resulting indicator, mNG-GECO1, is 60% brighter than GCaMP6s in vitro and provides comparable performance as demonstrated by imaging Ca²⁺ dynamics in cultured cells, primary neurons, and in vivo in larval zebrafish. These results suggest that mNG-GECO1 is a promising next-generation GECI that could inherit the mantle of GCaMP and allow the steady improvement of GECIs to continue for generations to come.

Challenges for Therapeutic Applications of Opsin-Based Optogenetic Tools in Humans

As the technological hurdles are overcome and optogenetic techniques advance to have more control over neurons, therapies based on these approaches will begin to emerge in the clinic. Here, we consider the technical challenges surrounding the transition of this breakthrough technology from an investigative tool to a true therapeutic avenue. The emerging strategies and remaining tasks surrounding genetically encoded molecules which respond to light as well as the vehicles required to deliver them are discussed.The use of optogenetics in humans would represent a completely new paradigm in medicine and would be associated with unprecedented technical considerations. To be applied for stimulation of neurons in humans, an ideal optogenetic tool would need to be non-immunogenic, highly sensitive, and activatable with red light or near-infrared light (to maximize light penetration while minimizing photodamage). To enable sophisticated levels of neuronal control, the combined use of optogenetic actuators and indicators could enable closed-loop all-optical neuromodulation. Such systems would introduce additional challenges related to spectral orthogonality between actuator and indicator, the need for decision making computational algorithms and requirements for large gene cassettes. As in any gene therapy, the therapeutic efficiency of optogenetics will rely on vector delivery and expression in the appropriate cell type. Although viral vectors such as those based on AAVs are showing great potential in human trials, barriers to their general use remain, including immune responses, delivery/transport, and liver clearance. Limitations associated with the gene cassette size which can be packaged in currently approved vectors also need to be addressed.

Intelligent image-activated cell sorting 2.0

The advent of intelligent image-activated cell sorting (iIACS) has enabled high-throughput intelligent image-based sorting of single live cells from heterogeneous populations. iIACS is an on-chip microfluidic technology that builds on a seamless integration of a high-throughput fluorescence microscope, cell focuser, cell sorter, and deep neural network on a hybrid software-hardware data management architecture, thereby providing the combined merits of optical microscopy, fluorescence-activated cell sorting (FACS), and deep learning. Here we report an iIACS machine that far surpasses the state-of-the-art iIACS machine in system performance in order to expand the range of applications and discoveries enabled by the technology. Specifically, it provides a high throughput of ∼2000 events per second and a high sensitivity of ∼50 molecules of equivalent soluble fluorophores (MESFs), both of which are 20 times superior to those achieved in previous reports. This is made possible by employing (i) an image-sensor-based optomechanical flow imaging method known as virtual-freezing fluorescence imaging and (ii) a real-time intelligent image processor on an 8-PC server equipped with 8 multi-core CPUs and GPUs for intelligent decision-making, in order to significantly boost the imaging performance and computational power of the iIACS machine. We characterize the iIACS machine with fluorescent particles and various cell types and show that the performance of the iIACS machine is close to its achievable design specification. Equipped with the improved capabilities, this new generation of the iIACS technology holds promise for diverse applications in immunology, microbiology, stem cell biology, cancer biology, pathology, and synthetic biology.

Engineering genetically encoded fluorescent indicators for imaging of neuronal activity: Progress and prospects

Genetically encoded fluorescent indicators have transformed the way neuroscientists record neuronal activities and interrogate the nervous system in vivo. In this review, we discuss recent advances and new additions to the toolkit of indicators for calcium ion entry, membrane voltage change, neurotransmitter release, and other neuronal molecular processes. We highlight new engineering approaches for indicator design and development, and identify key areas for future improvement. From molecular tool developers' perspective, we aim to provide practical information for neuroscientists to evaluate and choose the most appropriate indicators for enabling new insights into brain function.

A single-phase flow microfluidic cell sorter for multiparameter screening to assist the directed evolution of Ca2+ sensors

We introduce a single-phase flow microfluidic cell sorter with a two-point detection system capable of two-parameter screening to assist with directed evolution of a fluorescent protein based Ca²⁺ sensor expressed in bacterial cells. The new cell sorting system utilizes two fluorescence microscopes to obtain signals at two different points along a flow path in which a change in concentration of the analyte, Ca²⁺, is induced. The two detectors thus determine the magnitude of fluorescence change of the sensor following the reaction, along with the overall brightness of the sensor. A design for a 3D focusing flow was configured to enhance the spatial control of cells and signal pair-matching. The cell sorter screens the sensors at a moderate throughput, 10 cells per s and 10⁵ cells per round, enriching top variants for the subsequent manual screening with higher accuracy. Our new μFACS greatly accelerates the directed evolution of genetically encoded Ca²⁺ sensors compared to the previous version with single point detection for brightness-based screening. Two rounds of directed evolution led to a variant, named Y-GECO2f, which exhibits a 26% increase in brightness and a greater than 300% larger Ca²⁺-dependent fluorescence change in vitro relative to the variant before evolution.

Ratiometric Detection of Nerve Agents by Coupling Complementary Properties of Silicon-Based Quantum Dots and Green Fluorescent Protein

Ratiometric photoluminescent detection of the toxicologically potent organophosphate ester nerve agents paraoxon (PX) and parathion (PT) using the complementary optical and chemical properties of the long Stokes shift green fluorescent protein variant, mAmetrine1.2 (mAm), and red-emitting silicon-based quantum dots (SiQDs) is reported. PX and PT selectively quench SiQD photoluminescence (PL) through a dynamic quenching mechanism, thereby, facilitating the development of a ratiometric sensor platform that shows micromolar limits of detection for PX and PT and that is unaffected by the presence of common inorganic and organic interferents. As a part of the present study, we also demonstrate that the paper-based sensors derived from mAm and SiQDs detect PX and PT at concentrations as low as 5 μM using a readily available commercial color analysis smartphone "app". The ratiometric sensor reported herein can potentially be used for the convenient and rapid on-site detection and quantification of PX and PT in real-world samples.

Spying on Cells: Toward a Perfect Sleeper Agent

Creative engineering of fluorescent proteins has yielded a variety of tools for visualization of biochemical events in vivo. In this issue of Cell Chemical Biology, To et al. (2016) describe a fluorogenic green fluorescent protein that is activated by caspase-3 activity and enables imaging of apoptosis in developing zebrafish embryos (To et al., 2016).

Understanding the Fluorescence Change in Red Genetically Encoded Calcium Ion Indicators

For over 20 years, genetically encoded Ca²⁺ indicators have illuminated dynamic Ca²⁺ signaling activity in living cells and, more recently, whole organisms. We are just now beginning to understand how they work. Various fluorescence colors of these indicators have been developed, including red. Red ones are promising because longer wavelengths of light scatter less in tissue, making it possible to image deeper. They are engineered from a red fluorescent protein that is circularly permuted and fused to a Ca²⁺-sensing domain. When Ca²⁺ binds, a conformational change in the sensing domain causes a change in fluorescence. Three factors can contribute to this fluorescence change: 1) a shift in the protonation equilibrium of the chromophore, 2) a change in fluorescence quantum yield, and 3) a change in the extinction coefficient or the two-photon cross section, depending on if it is excited with one or two photons. Here, we conduct a systematic study of the photophysical properties of a range of red Ca²⁺ indicators to determine which factors are the most important. In total, we analyzed nine indicators, including jRGECO1a, K-GECO1, jRCaMP1a, R-GECO1, R-GECO1.2, CAR-GECO1, O-GECO1, REX-GECO1, and a new variant termed jREX-GECO1. We find that these could be separated into three classes that each rely on a particular set of factors. Furthermore, in some cases, the magnitude of the change in fluorescence was larger with two-photon excitation compared to one-photon because of a change in the two-photon cross section, by up to a factor of two.

A genetically encoded near-infrared fluorescent calcium ion indicator

We report an intensiometric, near-infrared (NIR) fluorescent, genetically encoded calcium ion (Ca²⁺) indicator (GECI) with excitation and emission maxima at 678 nm and 704 nm, respectively. This GECI, designated NIR-GECO1, enables imaging of Ca²⁺ transients in cultured mammalian cells and brain tissue with sensitivity comparable to currently available visible-wavelength GECIs. We demonstrate that NIR-GECO1 opens up new vistas for multicolor Ca²⁺ imaging in combination with other optogenetic indicators and actuators.

Genetically encoded fluorescent indicators for imaging intracellular potassium ion concentration

Potassium ion (K+) homeostasis and dynamics play critical roles in biological activities. Here we describe three genetically encoded K+ indicators. KIRIN1 (potassium (K) ion ratiometric indicator) and KIRIN1-GR are Förster resonance energy transfer (FRET)-based indicators with a bacterial K+ binding protein (Kbp) inserting between the fluorescent protein FRET pairs mCerulean3/cp173Venus and Clover/mRuby2, respectively. GINKO1 (green indicator of K+ for optical imaging) is a single fluorescent protein-based K+ indicator constructed by insertion of Kbp into enhanced green fluorescent protein (EGFP). These indicators are suitable for detecting K+ at physiologically relevant concentrations in vitro and in cells. KIRIN1 enabled imaging of cytosolic K+ depletion in live cells and K+ efflux and reuptake in cultured neurons. GINKO1, in conjunction with red fluorescent Ca2+ indicator, enable dual-color imaging of K+ and Ca2+ dynamics in neurons and glial cells. These results demonstrate that KIRIN1 and GINKO1 are useful tools for imaging intracellular K+ dynamics.

Monomerization of far-red fluorescent proteins

Anthozoa-class red fluorescent proteins (RFPs) are frequently used as biological markers, with far-red (λem ∼ 600-700 nm) emitting variants sought for whole-animal imaging because biological tissues are more permeable to light in this range. A barrier to the use of naturally occurring RFP variants as molecular markers is that all are tetrameric, which is not ideal for cell biological applications. Efforts to engineer monomeric RFPs have typically produced dimmer and blue-shifted variants because the chromophore is sensitive to small structural perturbations. In fact, despite much effort, only four native RFPs have been successfully monomerized, leaving the majority of RFP biodiversity untapped in biomarker development. Here we report the generation of monomeric variants of HcRed and mCardinal, both far-red dimers, and describe a comprehensive methodology for the monomerization of red-shifted oligomeric RFPs. Among the resultant variants is mKelly1 (emission maximum, λem = 656 nm), which, along with the recently reported mGarnet2 [Matela G, et al. (2017) Chem Commun (Camb) 53:979-982], forms a class of bright, monomeric, far-red FPs.

In vivo photoacoustic difference-spectra imaging of bacteria using photoswitchable chromoproteins

Photoacoustic (PA) imaging offers great promise for deep molecular imaging of optical reporters but has difficulties in imaging multiple molecular probes simultaneously in a strong blood background. Photoswitchable chromoproteins like BphP1 have recently allowed for sensitive PA detection by reducing high-blood background signals but lack multiplexing capabilities. We propose a method known as difference-spectra demixing for multiplexing multiple photoswitchable chromoproteins and introduce a second photoswitchable chromoprotein, sGPC2. sGPC2 has a far-red and orange state with peaks at 700 and 630 nm, respectively. It is roughly one-tenth the size of BphP1 and photoswitches four times as fast (2.4% per mJ  /  cm2). We simultaneously image Escherichia coli expressing sGPC2 and BphP1 injected in mice in vivo. Difference-spectra demixing obtained successful multiplexed images of photoswitchable molecular probes, resulting in a 21.6-fold increase in contrast-to-noise ratio in vivo over traditional PA imaging and an 8% to 40% reduction in erroneously demixed signals in comparison with traditional spectral demixing. PA imaging and characterization were conducted using a custom-built photoswitching PA imaging system.

Inverse-response Ca2+ indicators for optogenetic visualization of neuronal inhibition

We have developed a series of yellow genetically encoded Ca2+ indicators for optical imaging (Y-GECOs) with inverted responses to Ca2+ and apparent dissociation constants (Kd') ranging from 25 to 2400 nM. To demonstrate the utility of this affinity series of Ca2+ indicators, we expressed the four highest affinity variants (Kd's = 25, 63, 121, and 190 nM) in the Drosophila medulla intrinsic neuron Mi1. Hyperpolarization of Mi1 by optogenetic stimulation of the laminar monopolar neuron L1 produced a decrease in intracellular Ca2+ in layers 8-10, and a corresponding increase in Y-GECO fluorescence. These experiments revealed that lower Kd' was associated with greater increases in fluorescence, but longer delays to reach the maximum signal change due to slower off-rate kinetics.

A Bioluminescent Ca2+ Indicator Based on a Topological Variant of GCaMP6s

Fluorescent genetically encoded calcium ion indicators (GECIs) enable Ca²⁺ dynamics to be monitored in a diverse array of cell types and tissues. One drawback of green fluorescent GECIs, such as the widely used GCaMP6, is that the blue wavelengths of light used to excite the GECI also activate optogenetic actuators such as channelrhodopsins. Accordingly, it is particularly challenging simultaneously to use both optogenetic actuators and GECIs to both control and image cell signaling. Bioluminescence is an alternative imaging modality that circumvents this problem by avoiding the need for illumination for fluorescence excitation. Here, we report the development of a bioluminescent GECI, designated LUCI-GECO1, based on efficient bioluminescent resonance energy transfer (BRET) between the NanoLuc luciferase and a topological variant of GCaMP6s. LUCI-GECO1 is a sensitive ratiometric GECI that retains the highly optimized properties of GCaMP6s, as we demonstrate by imaging of chemically and optogenetically induced Ca²⁺ concentration changes in cultured cells and neurons.

Genetically Encoded Glutamate Indicators with Altered Color and Topology

Glutamate is one of the 20 common amino acids and of utmost importance for chemically mediated synaptic transmission in nervous systems. To expand the color palette of genetically encoded indicators for glutamate, we used protein engineering to develop a red intensity-based glutamate-sensing fluorescent reporter (R-iGluSnFR1). Manipulating the topology of R-iGluSnFR1, and a previously reported green fluorescent indicator, led to the development of noncircularly permutated (ncp) variants. R- and R^ncp-iGluSnFR1 display glutamate affinities of 11 μM and 0.9 μM, respectively. We demonstrate that these glutamate indicators are functional when targeted to the surface of HEK-293 cells. Furthermore, we show that G^ncp-iGluSnFR enabled reliable visualization of extrasynaptic glutamate in organotypic hippocampal slice cultures, while R-iGluSnFR can reliably resolve action potential-evoked glutamate transients by electrical field stimuli in cultures of dissociated hippocampal neurons.