Interferometric modulation for generating extended light sheet: improving field of view.

Significance: Light-sheet fluorescence microscopy (LSFM) has emerged as a powerful and versatile imaging technique renowned for its remarkable features, including high-speed 3D tomography, minimal photobleaching, and low phototoxicity. The interference light-sheet fluorescence microscope, with its larger field of view (FOV) and more uniform axial resolution, possesses significant potential for a wide range of applications in biology and medicine. Aim: The aim of this study is to investigate the interference behavior among multiple light sheets (LSs) in LSFM and optimize the FOV and resolution of the light-sheet fluorescence microscope. Approach: We conducted a detailed investigation of the interference effects among LSs through theoretical derivation and numerical simulations, aiming to find optimal parameters. Subsequently, we constructed a customized system of multi-LSFM that incorporates both interference light sheets (ILS) and noninterference light-sheet configurations. We performed beam imaging and microsphere imaging tests to evaluate the FOV and axial resolution of these systems. Results: Using our custom-designed light-sheet fluorescence microscope, we captured the intensity distribution profiles of both interference and noninterference light sheets (NILS). Additionally, we conducted imaging tests on microspheres to assess their imaging outcomes. The ILS not only exhibits a larger FOV compared to the NILS but also demonstrates a more uniform axial resolution. Conclusions: By effectively modulating the interference among multiple LSs, it is possible to optimize the intensity distribution of the LSs, expand the FOV, and achieve a more uniform axial resolution.

Photobleaching FRET-FLIM-ICS for quaternary structure quantification on cells. Theory and simulations.

The oligomerization of proteins is an important biological control mechanism and has several functions in activity and stability of enzymes, structural proteins, ion channels and transcription factors. The determination of the relevant oligomeric states in terms of geometry (spatial extent), oligomer size (monomer or dimer or oligomer) and affinity (amounts of monomer, dimer and oligomer) is a challenging biophysical problem. Förster resonance energy transfer and fluorescence fluctuation spectroscopy are powerful tools that are sensitive to proximity and oligomerization respectively. Here it is proposed to combine image-based lifetime-detected Forster resonance energy transfer with image correlation spectroscopy and photobleaching to determine distances, oligomer sizes and oligomer distributions. Simulations for simple oligomeric forms illustrate the potential to improve the discrimination between different quaternary states in the cellular milieu.

Lumos maxima - How robust fluorophores resist photobleaching?

Fluorescent dyes synergize with advanced microscopy for researchers to investigate the location and dynamic processes of biomacromolecules with high spatial and temporal resolution. However, the instability of fluorescent dyes, including photobleaching and photoconversion, represent fundamental limits for super-resolution and time-lapse imaging. In this review, we discuss the latest advances in improving the photostability of fluorescent dyes. We summarize the primary photobleaching processes of cyanine and rhodamine dyes and highlight a range of strategies developed in recent years to strengthen these fluorophores. Additionally, we discuss the influence of protein microenvironments and labeling methods on the photostability of fluorophores. We aim to inspire next-generation robust and bright fluorophores that ultimately enable the routine practice of time-lapse super-resolution imaging of live cells.

Diffusion of Multiple Species Resolved by Fluorescence Lifetime Recovery after Photobleaching (FLRAP).

Fluorescence recovery after photobleaching (FRAP) is now an indispensable tool to analyze the diffusion of molecules in vivo and in vitro. However, a conventional fluorescence intensity-based approach has difficulty in analyzing the diffusion of multiple species simultaneously. Here, we report fluorescence lifetime recovery after photobleaching (FLRAP) that incorporates fluorescence lifetime information into FRAP. By using FLRAP, the fluorescence intensity-recovery curves of each species can be successfully extracted from the ensemble photon data by utilizing their species-specific fluorescence decay curves, which are verified by applying FLRAP to two heterogeneous systems. Thus, FLRAP can be a powerful tool to quantitatively elucidate the molecular diffusion of multiple species in complex systems such as in living cells.

Thiolation for Enhancing Photostability of Fluorophores at the Single-Molecule Level.

Fluorescent probes are essential for single-molecule imaging. However, their application in biological systems is often limited by the short photobleaching lifetime. To overcome this, we developed a novel thiolation strategy for squaraine dyes. By introducing thiolation of the central cyclobutene of squaraine (thio-squaraine), we observed a ≈5-fold increase in photobleaching lifetime. Our single-molecule data analysis attributes this improvement to improved photostability resulting from thiolation. Interestingly, bulk measurements show rapid oxidation of thio-squaraine to its oxo-analogue under irradiation, giving the perception of inferior photostability. This discrepancy between bulk and single-molecule environments can be ascribed to the factors in the latter, including larger intermolecular distances and restricted mobility, which reduce the interactions between a fluorophore and reactive oxygen species produced by other fluorophores, ultimately impacting photobleaching and photoconversion rate. We demonstrate the remarkable performance of thio-squaraine probes in various imaging buffers, such as glucose oxidase with catalase (GLOX) and GLOX+trolox. We successfully employed these photostable probes for single-molecule tracking of CD56 membrane protein and monitoring mitochondria movements in live neurons. CD56 tracking revealed distinct motion states and the corresponding protein fractions. This investigation is expected to propel the development of single-molecule imaging probes, particularly in scenarios where bulk measurements show suboptimal performance.

Exchangeable Self-Assembled Lanthanide Antennas for PLIM Microscopy.

Lanthanides have unique photoluminescence (PL) emission properties, including very long PL lifetimes. This makes them ideal for biological imaging applications, especially using PL lifetime imaging microscopy (PLIM). PLIM is an inherently multidimensional technique with exceptional advantages for quantitative biological imaging. Unfortunately, due to the required prolonged acquisitions times, photobleaching of lanthanide PL emission currently constitutes one of the main drawbacks of PLIM. In this study, we report a small aqueous-soluble, lanthanide antenna, 8-methoxy-2-oxo-1,2,4,5-tetrahydrocyclopenta[de]quinoline-3-phosphonic acid, PAnt, specifically designed to dynamically interact with lanthanide ions, serving as exchangeable dye aimed at mitigating photobleaching in PLIM microscopy in cellulo. Thus, self-assembled lanthanide complexes that may be photobleached during image acquisition are continuously replenished by intact lanthanide antennas from a large reservoir. Remarkably, our self-assembled lanthanide complex clearly demonstrated a significant reduction of PL photobleaching when compared to well-established lanthanide cryptates, used for bioimaging. This concept of exchangeable lanthanide antennas opens new possibilities for quantitative PLIM bioimaging.

Shining Light on Photobleaching: An Artifact That Causes Unnecessary Excitation Among Pathologists.

CONTEXT.­: Photobleaching artifact occurs when fluorescence intensity decreases following light exposure. Slides stained with fluorescent techniques may be stored in the dark until primary diagnostics. Experimental evidence suggesting the rate of photobleaching and necessity of dark storage is lacking. OBJECTIVE.­: To compare photobleaching rate on direct immunofluorescence and Thioflavin T slides stored in ambient room light conditions and exposed to excitatory wavelengths. DESIGN.­: During 2 iterations of the experiment, 45 slides were prepared, 42 with immunofluorescent antibodies plus 3 with thioflavin, from skin and kidney biopsies. The experimental group was stored in room light conditions in comparison to the control in the dark, at room temperature. Further, 1 immunofluorescence slide and 1 thioflavin slide were exposed to excitatory fluorescent light for several hours. Significant photobleaching was defined as an integer decrease in score (scale, 0-3). RESULTS.­: Exposure times ranged from 152 to 3034 hours. Nine of the 42 immunofluorescence slides (21%) photobleached after a minimum exposure of 152 hours to room light, with no significant difference between the experimental and control groups (all P values >.05). The immunofluorescence slide exposed to fluorescent light for 4 hours showed marked photobleaching in the exposed field but not elsewhere. No thioflavin slides showed clinically significant photobleaching under any conditions. CONCLUSIONS.­: Clinically significant photobleaching of slides exposed to room light may occur after a few days, but not a few hours (unless exposed to excitatory fluorescent light). Conversely, thioflavin-stained slides did not photobleach when exposed to ambient room air and photobleached only negligibly when exposed to excitatory fluorescent light.

Synchronous Photoactivation-Imaging Fluorophores Break Limitations of Photobleaching and Phototoxicity in Live-cell Microscopy.

Fluorescence microscopy is one of the most important tools in the studies of cell biology and many other fields, but two fundamental issues, photobleaching and phototoxicity, associated with the fluorophores have still limited its use for long-term and strong-illumination imaging of live cells. Here, we report a new concept of fluorophore engineering chemistry, synchronous photoactivation-imaging (SPI) fluorophores, activating and exciting fluorophores by a single light source to thus avoid the repeated switches between activation and excitation lights. The chemically reconstructed, nonemissive fluorophores can be photolyzed to allow continuous replenishing of "bright-state" probes detectable by standard fluorescent microscopes in the imaging process so as to bypass the photobleaching barrier to greatly extend the imaging period. Equally importantly, SPI fluorophores substantially reduce photocytotoxicity due to the scavenging of reactive oxygen species (ROS) by a photoactivable group and the slow release of "bright-state" probes to minimize ROS generation. Using SPI fluorophores, the time-lapsed confocal (>16 h) and super-resolution (>3 h) imaging of subcellular organelles under intensive illumination (50 MW/cm2) were achieved in live cells.

Optical traps induce fluorophore photobleaching by two-photon excitation.

Techniques combining optical tweezers with fluorescence microscopy have become increasingly popular. Unfortunately, the high-power, infrared lasers used to create optical traps can have a deleterious effect on dye stability. Previous studies have shown that dye photobleaching is enhanced by absorption of visible fluorescence excitation plus infrared trap photons, a process that can be significantly reduced by minimizing simultaneous exposure to both light sources. Here, we report another photobleaching pathway that results from direct excitation by the trapping laser alone. Our results show that this trap-induced fluorescence loss is a two-photon absorption process, as demonstrated by a quadratic dependence on the intensity of the trapping laser. We further show that, under conditions typical of many trap-based experiments, fluorescence emission of certain fluorophores near the trap focus can drop by 90% within 1 min. We investigate how photostability is affected by the choice of dye molecule, excitation and emission wavelength, and labeled molecule. Finally, we discuss the different photobleaching pathways in combined trap-fluorescence measurements, which guide the selection of optimal dyes and conditions for more robust experimental protocols.

Cryo-EM structures reveal native GABAA receptor assemblies and pharmacology.

Type A γ-aminobutyric acid receptors (GABAARs) are the principal inhibitory receptors in the brain and the target of a wide range of clinical agents, including anaesthetics, sedatives, hypnotics and antidepressants1-3. However, our understanding of GABAAR pharmacology has been hindered by the vast number of pentameric assemblies that can be derived from 19 different subunits4 and the lack of structural knowledge of clinically relevant receptors. Here, we isolate native murine GABAAR assemblies containing the widely expressed α1 subunit and elucidate their structures in complex with drugs used to treat insomnia (zolpidem (ZOL) and flurazepam) and postpartum depression (the neurosteroid allopregnanolone (APG)). Using cryo-electron microscopy (cryo-EM) analysis and single-molecule photobleaching experiments, we uncover three major structural populations in the brain: the canonical α1ß2γ2 receptor containing two α1 subunits, and two assemblies containing one α1 and either an α2 or α3 subunit, in which the single α1-containing receptors feature a more compact arrangement between the transmembrane and extracellular domains. Interestingly, APG is bound at the transmembrane α/ß subunit interface, even when not added to the sample, revealing an important role for endogenous neurosteroids in modulating native GABAARs. Together with structurally engaged lipids, neurosteroids produce global conformational changes throughout the receptor that modify the ion channel pore and the binding sites for GABA and insomnia medications. Our data reveal the major α1-containing GABAAR assemblies, bound with endogenous neurosteroid, thus defining a structural landscape from which subtype-specific drugs can be developed.

Current Advances in the Biomedical Applications of Quantum Dots: Promises and Challenges.

Quantum dots (QDs) are a type of nanoparticle with exceptional photobleaching-resistant fluorescence. They are highly sought after for their potential use in various optical-based biomedical applications. However, there are still concerns regarding the use of quantum dots. As such, much effort has been invested into understanding the mechanisms behind the behaviors of QDs, so as to develop safer and more biocompatible quantum dots. In this mini-review, we provide an update on the recent advancements regarding the use of QDs in various biomedical applications. In addition, we also discuss# the current challenges and limitations in the use of QDs and propose a few areas of interest for future research.

Single-molecule tracking to determine the abundances and stoichiometries of freely-diffusing protein complexes in living cells: Past applications and future prospects.

Most biological processes in living cells rely on interactions between proteins. Live-cell compatible approaches that can quantify to what extent a given protein participates in homo- and hetero-oligomeric complexes of different size and subunit composition are therefore critical to advance our understanding of how cellular physiology is governed by these molecular interactions. Biomolecular complex formation changes the diffusion coefficient of constituent proteins, and these changes can be measured using fluorescence microscopy-based approaches, such as single-molecule tracking, fluorescence correlation spectroscopy, and fluorescence recovery after photobleaching. In this review, we focus on the use of single-molecule tracking to identify, resolve, and quantify the presence of freely-diffusing proteins and protein complexes in living cells. We compare and contrast different data analysis methods that are currently employed in the field and discuss experimental designs that can aid the interpretation of the obtained results. Comparisons of diffusion rates for different proteins and protein complexes in intracellular aqueous environments reported in the recent literature reveal a clear and systematic deviation from the Stokes-Einstein diffusion theory. While a complete and quantitative theoretical explanation of why such deviations manifest is missing, the available data suggest the possibility of weighing freely-diffusing proteins and protein complexes in living cells by measuring their diffusion coefficients. Mapping individual diffusive states to protein complexes of defined molecular weight, subunit stoichiometry, and structure promises to provide key new insights into how protein-protein interactions regulate protein conformational, translational, and rotational dynamics, and ultimately protein function.

Photobleaching-induced changes in the optical and photochemical properties of algal organic matter.

Algal organic matter (AOM), a significant source of endogenous dissolved organic matter (DOM) is released in high concentrations during cyanobacterial blooms, along with cyanotoxins. Subsequent photobleaching of AOM is an important phenomenon to investigate. In this study, intracellular organic matter (IOM) and extracellular organic matter (EOM) were extracted from cultured cyanobacteria taken from Taihu Lake in China. The formation of photochemically produced reactive intermediates in different stages of IOM and EOM photobleaching was compared to Suwannee River DOM (SRDOM, reference standard DOM). Results revealed notable differences influenced by the pigment component among IOM, EOM, and SRDOM. The pigment in IOM contributed to a triplet state pool with strong energy-transfer but limited electron-transfer capabilities. Notably, IOM exhibited the highest triplets state quantum yield value in the visible region, suggesting its potential significance in pollutant degradation in deeper water layers. For EOM, one of the pools exhibits photolability and remarkable electron-transfer capability, indicating it as a high-energy triplet state component. Moreover, three cyanotoxins (MC-LR, ACA, and ATX-a) were detected in the extracted AOM, and their photodegradation was monitored during the AOM photobleaching process. This highlights the potential role of AOM as a photosensitizer in the natural self-cleaning mechanisms of water bodies, facilitating the degradation of organic pollutants through photochemical reactions. The findings of this study contribute to understanding the dynamic nature of AOM and its implications in environmental processes.

Self-Renewable Tag for Photostable Fluorescence Imaging of Proteins.

We report the development of a self-renewable tag (srTAG) for protein fluorescence imaging. srTAG leverages the "on-protein" fluorophore equilibrium between the fluorescent zwitterion and non-fluorescent spirocyclic form and the reversible fluorescence labeling to enable self-recovery of fluorescence after photobleaching. This small-sized srTAG allows 2-6 times longer imaging duration compared to other commonly used self-labeling tags and is compatible with fluorophores with different spectral properties. This study provides a new strategy for fine tuning of self-labeling tags.

Determination of Interstitial Collagen Deposition and Related Topography Using the Second Harmonic Generation-Based HistoIndex Platform.

Interstitial fibrosis is characterized by the increased deposition of extracellular matrix (ECM) components within the interstitial space of various organs, such as the kidneys, heart, lungs, liver, and skin. The primary component of interstitial fibrosis-related scarring is interstitial collagen. Therefore, the therapeutic application of anti-fibrotic medication hinges on the accurate measurement of interstitial collagen levels within tissue samples. Current histological measurement techniques for interstitial collagen are generally semi-quantitative in nature and only provide a ratio of collagen levels within tissues. However, the Genesis™ 200 imaging system and supplemental image analysis software, FibroIndex™, from HistoIndex™, is a novel, automated platform for imaging and characterizing interstitial collagen deposition and related topographical properties of the collagen structures within an organ, in the absence of any staining. This is achieved by using a property of light known as second harmonic generation (SHG). Using a rigorous optimization protocol, collagen structures in tissue sections can be imaged with a high degree of reproducibility and ensures homogeneity across all samples while minimizing the introduction of any imaging artefacts or photobleaching (decreased tissue fluorescence due to prolonged exposure to the laser). This chapter outlines the protocol that should be undertaken to optimize HistoIndex scanning of tissue sections, and the outputs that can be measured and analyzed using the FibroIndex™ software.

Expanding the applicability of multiphoton fluorescence recovery after photobleaching by incorporating shear stress in laminar flow.

Significance: Multi-photon fluorescence recovery after photobleaching (MPFRAP) is a nonlinear microscopy technique used to measure the diffusion coefficient of fluorescently tagged molecules in solution. Previous MPFRAP fitting models calculate the diffusion coefficient in systems with diffusion or diffusion in laminar flow. Aim: We propose an MPFRAP fitting model that accounts for shear stress in laminar flow, making it a more applicable technique for in vitro and in vivo studies involving diffusion. Approach: Fluorescence recovery curves are generated using high-throughput molecular dynamics simulations and then fit to all three models (diffusion, diffusion and flow, and diffusion and shear flow) to define the limits within which accurate diffusion coefficients are produced. Diffusion is simulated as a random walk with a variable horizontal bias to account for shear flow. Results: Contour maps of the accuracy of the fitted diffusion coefficient as a function of scaled velocity and scaled shear rate show the parameter space within which each model produces accurate diffusion coefficients; the shear-flow model covers a larger area than the previous models. Conclusion: The shear-flow model allows MPFRAP to be a viable optical tool for studying more biophysical systems than previous models.

Photobleaching Kinetics and Effect of Solvent in the Photophysical Properties of Indocyanine Green for Photodynamic Therapy.

Indocyanine green is an attractive molecule for photodynamic therapy due to its near infrared absorption, resulting in a higher tissue penetration. However, its quantum yields of the triplet and singlet state have been reported to be low and then, reactive oxygen species are unlikely to be formed. Aiming to understand the ICG role in photodynamic response, its photobleaching behavior in solution has been studied under distinct conditions of CW laser irradiation at 780 and 808 nm, oxygen saturations and solvents. Sensitizer bleaching and photoproduct formation were measured by absorption spectroscopy and analyzed using the PDT bleaching macroscopic model to extract physical parameters. ICG photobleaching occurs even at lower oxygen concentrations, indicating that the molecule presents more than one way of degradation. Photoproducts were produced even in solution of less than 4 % oxygen saturation for both solvents and excitation wavelengths. Also, the amplitude of absorption related to J-dimers was increased during irradiation, but only in 50 % PBS solution. The formation of photoproducts was enhanced in the presence of J-type dimers under low oxygen concentration, and the quantum yields of triplet and singlet states were one order of magnitude and two times higher, respectively, when compared to ICG in distilled H2 O.

Phase intensity nanoscope (PINE) opens long-time investigation windows of living matter.

Fundamental to all living organisms and living soft matter are emergent processes in which the reorganization of individual constituents at the nanoscale drives group-level movements and shape changes at the macroscale over time. However, light-induced degradation of fluorophores, photobleaching, is a significant problem in extended bioimaging in life science. Here, we report opening a long-time investigation window by nonbleaching phase intensity nanoscope: PINE. We accomplish phase-intensity separation such that nanoprobe distributions are distinguished by an integrated phase-intensity multilayer thin film (polyvinyl alcohol/liquid crystal). We overcame a physical limit to resolve sub-10 nm cellular architectures, and achieve the first dynamic imaging of nanoscopic reorganization over 250 h using PINE. We discover nanoscopic rearrangements synchronized with the emergence of group-level movements and shape changes at the macroscale according to a set of interaction rules with importance in cellular and soft matter reorganization, self-organization, and pattern formation.

Monomer and Oligomer Transition of Zinc Phthalocyanine Is Key for Photobleaching in Photodynamic Therapy.

Photodynamic therapy (PDT) is recognized as a powerful method to inactivate cells. However, the photosensitizer (PS), a key component of PDT, has suffered from undesired photobleaching. Photobleaching reduces reactive oxygen species (ROS) yields, leading to the compromise of and even the loss of the photodynamic effect of the PS. Therefore, much effort has been devoted to minimizing photobleaching in order to ensure that there is no loss of photodynamic efficacy. Here, we report that a type of PS aggregate showed neither photobleaching nor photodynamic action. Upon direct contact with bacteria, the PS aggregate was found to fall apart into PS monomers and thus possessed photodynamic inactivation against bacteria. Interestingly, the disassembly of the bound PS aggregate in the presence of bacteria was intensified by illumination, generating more PS monomers and leading to an enhanced antibacterial photodynamic effect. This demonstrated that on a bacterial surface, the PS aggregate photo-inactivated bacteria via PS monomer during irradiation, where the photodynamic efficiency was retained without photobleaching. Further mechanistic studies showed that PS monomers disrupted bacterial membranes and affected the expression of genes related to cell wall synthesis, bacterial membrane integrity, and oxidative stress. The results obtained here are applicable to other types of PSs in PDT.

Characterization of mApple as a Red Fluorescent Protein for Cryogenic Single-Molecule Imaging with Turn-Off and Turn-On Active Control Mechanisms.

Single-molecule superresolution microscopy is a powerful tool for the study of biological structures on size scales smaller than the optical diffraction limit. Imaging samples at cryogenic temperatures (77 K) reduces the quantum yield of photobleaching for many fluorescent labels, yielding localization precisions below 10 nm. Cryogenic imaging further enables correlation with cryogenic electron tomography. A key limitation in applying methods such as PALM and STORM to samples maintained at 77 K is the limited number of fluorophores known to undergo efficient turn-on and turn-off mechanisms necessary to control the sparsity of active emitters. We find that mApple, a red-emitting fluorescent protein, undergoes a novel turn-off mechanism in response to simultaneous illumination with two colors of light. This turn-off mechanism enables localization of many individual molecules in initially bright samples, but the final density of localizable emitters is limited by relatively inefficient turn-on (photoactivation). Bulk excitation and emission spectroscopy shows that mApple has access to two distinct emissive states as well as dark states accessible optically or through changes in pH. The bright and stable emission of mApple enables widefield collection of single-molecule emission spectra, which highlight the complex nature and environmental sensitivity of states observed in red fluorescent proteins.