Fluorescence lifetime imaging microscopy: fundamentals and advances in instrumentation, analysis, and applications

Perspectives on label-free microscopy of heterogeneous and dynamic biological systems

Significance

Advancements in label-free microscopy could provide real-time, non-invasive imaging with unique sources of contrast and automated standardized analysis to characterize heterogeneous and dynamic biological processes. These tools would overcome challenges with widely used methods that are destructive (e.g., histology, flow cytometry) or lack cellular resolution (e.g., plate-based assays, whole animal bioluminescence imaging).

Aim

This perspective aims to (1) justify the need for label-free microscopy to track heterogeneous cellular functions over time and space within unperturbed systems and (2) recommend improvements regarding instrumentation, image analysis, and image interpretation to address these needs.

Approach

Three key research areas (cancer research, autoimmune disease, and tissue and cell engineering) are considered to support the need for label-free microscopy to characterize heterogeneity and dynamics within biological systems. Based on the strengths (e.g., multiple sources of molecular contrast, non-invasive monitoring) and weaknesses (e.g., imaging depth, image interpretation) of several label-free microscopy modalities, improvements for future imaging systems are recommended.

Conclusion

Improvements in instrumentation including strategies that increase resolution and imaging speed, standardization and centralization of image analysis tools, and robust data validation and interpretation will expand the applications of label-free microscopy to study heterogeneous and dynamic biological systems.

Unravelling molecular dynamics in living cells: Fluorescent protein biosensors for cell biology

Genetically encoded, fluorescent protein (FP)-based Förster resonance energy transfer (FRET) biosensors are microscopy imaging tools tailored for the precise monitoring and detection of molecular dynamics within subcellular microenvironments. They are characterised by their ability to provide an outstanding combination of spatial and temporal resolutions in live-cell microscopy. In this review, we begin by tracing back on the historical development of genetically encoded FP labelling for detection in live cells, which lead us to the development of early biosensors and finally to the engineering of single-chain FRET-based biosensors that have become the state-of-the-art today. Ultimately, this review delves into the fundamental principles of FRET and the design strategies underpinning FRET-based biosensors, discusses their diverse applications and addresses the distinct challenges associated with their implementation. We place particular emphasis on single-chain FRET biosensors for the Rho family of guanosine triphosphate hydrolases (GTPases), pointing to their historical role in driving our understanding of the molecular dynamics of this important class of signalling proteins and revealing the intricate relationships and regulatory mechanisms that comprise Rho GTPase biology in living cells.

A Lifetime Nanosensor for In Vivo pH Quantitative Imaging and Monitoring

Non-invasive, in vivo quantitative imaging for long-term biomarker monitoring is crucial for elucidating disease mechanisms, advancing precision medicine, and transforming diagnostics and therapeutic strategies. However, developing chemical sensors for sustained in vivo quantitative monitoring despite sensor concentration fluctuations, excitation variability, and tissue interference remains a major challenge. Here, a long-lifetime nanosensor based on a lanthanide-dye nanocomposite is presented that overcomes these limitations, enabling precise quantitative in vivo pH monitoring. Benefiting from a 64-fold reversible change in the dye's molar extinction coefficient, this nanosensor enables the dynamic tuning of reversible non-radiative energy transfer (RNET) efficiency (6.42%-35.23%) and luminescence lifetime (265-383 µs). This nanosensor enables 4 h of monitoring of gastrointestinal pH dynamics in mice following proton pump inhibitor (PPI) administration, offering new insights into pharmacodynamic effects across different administration routes and dosages and inter-individual variability in drug efficacy. Moreover, coordination with lanthanide nanocrystals induces a significant shift in the dye's pKa, highlighting the importance of nanomaterial interface engineering. This work establishes a versatile platform for in vivo diagnostics and therapeutic monitoring, marking a significant step forward in precision medicine.

Probing the Formation and Liquid-to-Solid Transition of FUS Condensates via the Lifetimes of Fluorescent Proteins

Liquid-liquid phase separation (LLPS) of biomolecules is a fundamental cellular process that is essential for maintaining homeostasis and facilitating biochemical activities. On the other hand, aberrant phase separation alters condensate fluidity and causes a transition from liquid-like condensates to solid-like condensates, which may lead to the formation of the pathological aggregations often observed in neurodegenerative diseases. Condensate fluidity is usually assessed by the fluorescence recovery after photobleaching. Here, we reveal that the fluorescence lifetimes of several fluorescent proteins are sensitive to LLPS and the liquid-to-solid transition. Furthermore, we identify several key residues that regulate the sensitivity of fluorescence lifetimes toward phase separation. Thus, we apply fluorescence lifetime imaging microscopy (FLIM) to visualize LLPS and the liquid-to-solid transition in living cells, demonstrating that FLIM is a nondestructive method for tracking changes in condensate fluidity in real time.

Optical Probes for Cellular Imaging of G-quadruplexes: Beyond Fluorescence Intensity Probes

The study of G-quadruplex (G4) structures that form in DNA and RNA is a rapidly growing field, which has evolved from in vitro studies of isolated G4 sequences to genome-wide detection of G4s in a cellular context. This work has revealed the tangible and significant effects that G4s may have on biological regulation. This minireview describes recent progress in the design of photoluminescent intensity-independent optical probes for G4s. We discuss the design and use of probes based on fluorescence or phosphorescence lifetime, rather than intensity-based detection; spectral ratiometric probes; and fluorescent probes for single-molecule G4-detection. We argue that each of these modalities improve unbiased G4 detection in cellular experiments, overcoming problems associated with unknown cellular uptake of probes or their organelle concentration. We discuss the improvements offered by these types of probes, as well as limitations and future research directions needed to facilitate more robust research into G4 biology.

Autofluorescence lifetime imaging classifies human B and NK cell activation state

New non-destructive tools with single-cell resolution are needed to reliably assess B cell and NK cell function for applications including adoptive cell therapy and immune profiling. Optical metabolic imaging (OMI) is a label-free method that measures the autofluorescence intensity and lifetime of the metabolic cofactors NAD(P)H and FAD to quantify metabolism at a single-cell level. Here, we demonstrate that OMI can resolve metabolic changes between primary human quiescent and IL-4/anti-CD40 activated B cells and between quiescent and IL-12/IL-15/IL-18 activated NK cells. We found that stimulated B and NK cells had an increased proportion of free compared to protein-bound NAD(P)H, a reduced redox state, and produced more lactate compared to control cells. The NAD(P)H mean fluorescence lifetime decreased in the stimulated B and NK cells compared to control cells. Random forest models classified B cells and NK cells according to activation state (CD69+) based on OMI variables with an accuracy of 93%. Our results show that autofluorescence lifetime imaging can accurately assess B and NK cell activation in a label-free, non-destructive manner.

Recent Developments in Single-Cell Metabolomics by Mass Spectrometry─A Perspective

Recent advancements in single-cell (sc) resolution analyses, particularly in sc transcriptomics and sc proteomics, have revolutionized our ability to probe and understand cellular heterogeneity. The study of metabolism through small molecules, metabolomics, provides an additional level of information otherwise unattainable by transcriptomics or proteomics by shedding light on the metabolic pathways that translate gene expression into functional outcomes. Metabolic heterogeneity, critical in health and disease, impacts developmental outcomes, disease progression, and treatment responses. However, dedicated approaches probing the sc metabolome have not reached the maturity of other sc omics technologies. Over the past decade, innovations in sc metabolomics have addressed some of the practical limitations, including cell isolation, signal sensitivity, and throughput. To fully exploit their potential in biological research, however, remaining challenges must be thoroughly addressed. Additionally, integrating sc metabolomics with orthogonal sc techniques will be required to validate relevant results and gain systems-level understanding. This perspective offers a broad-stroke overview of recent mass spectrometry (MS)-based sc metabolomics advancements, focusing on ongoing challenges from a biologist's viewpoint, aimed at addressing pertinent and innovative biological questions. Additionally, we emphasize the use of orthogonal approaches and showcase biological systems that these sophisticated methodologies are apt to explore.

HBimmCue: A Versatile Fluorescent Probe for Multi-Scale Imaging of Lipid Polarity and Membrane Order in Inner Mitochondrial Membrane

Mitochondrial membrane environmental dynamics are crucial for understanding function, yet high-resolution observation remains challenging. Here, HBimmCue is introduced as a fluorescent probe localized to inner mitochondrial membrane (IMM) that reports lipid polarity and membrane order changes, which correlate with cellular respiration levels. Using HBimmCue and fluorescence lifetime imaging microscopy (FLIM), IMM lipid heterogeneity is uncovered across scales, from nanoscale structures within individual mitochondria to mouse pre-implantation embryos. At the sub-organelle level, stimulated emission depletion (STED)-FLIM imaging highlights nanoscale polarity variations within the IMM. At the sub-cellular and cellular level, reduced IMM lipid polarity is observed in damaged mitochondria marked for lysosomal degradation and distinct IMM lipid distributions are identified in neurons and disease models. Additionally, metabolic dysfunction associated with oocytes aging and metabolic reprogramming from zygote to blastocyst is detected. Together, the work demonstrates the broad applicability of HBimmCue, offering a new paradigm for investigating lipid polarity and respiration level at multiple scales.

Fluorescence Lifetime Imaging Ophthalmoscopy, Vision, and Chorioretinal Asymmetries in Aging and Age-Related Macular Degeneration: ALSTAR2

Purpose

Eyes with age-related macular degeneration (AMD) and some healthy aged eyes exhibit risk-indicating delays in rod-mediated dark adaptation (RMDA) and prolonged long spectral channel (LSC) lifetimes by fluorescence lifetime imaging ophthalmoscopy (FLIO) in the Early Treatment Diabetic Retinopathy Study (ETDRS) outer ring, especially nasally. To learn FLIO's potential for AMD detection, we correlate FLIO to RMDA.

Methods

The ALSTAR2 follow-up cohort underwent FLIO, color fundus photography, two-wavelength autofluorescence (for macular pigment optical density [MPOD]), visual function testing, including RMDA (rod intercept time [RIT]). AMD was staged by the Age-Related Eye Disease Study (AREDS) 9-step at baseline and follow-up. In pseudophakic eyes with high-quality FLIO, mean intensity maps and meridian plots were created. Vision data were analyzed using linear regression and Spearman's r.

Results

Of 155 eyes (155 participants [75 ± 5.0 years; 60.7% female participants]), 67 eyes were healthy, 38 had early (e)AMD, and 50 had intermediate (i)AMD (P = 0.02). LSC lifetimes were longest in iAMD in all ETDRS regions (P < 0.01) and short spectral channel (SSC) lifetimes in inner and outer rings (P < 0.01). The LSC pattern manifested in 65 of 88 AMD eyes and 30 of 67 healthy eyes. Lifetimes were longest on the nasal meridian and shortest on temporal. LSC lifetimes in the inner and outer rings correlated strongly with RIT (r = 0.68). A stable subgroup had short LSC lifetimes and short RIT. SSC correlated weakly with MPOD.

Conclusions

Prolonged lifetimes in AMD exhibit spatial asymmetry, suggesting mechanisms beyond retinal cells and including choroid. Lifetimes correlate with delayed RMDA, potentially indicating risk for AMD onset and early progression. Further research into SSC signal sources is warranted.

Resolving Conformational Plasticity in Mammalian Cells with High-Resolution Fluorescence Tools

Investigating protein dynamic structural changes is fundamental for understanding protein function, drug discovery, and disease mechanisms. Traditional studies of protein dynamics often rely on investigations of purified systems, which fail to capture the complexity of the cellular environment. The intracellular milieu imposes distinct physicochemical constraints that affect macromolecular interactions and dynamics in ways not easily replicated in isolated experimental setups. We discuss the use of fluorescence resonance energy transfer, fluorescence anisotropy, and minimal photon flux imaging technologies to address these challenges and directly investigate protein conformational dynamics in mammalian cells. Key findings from the application of these techniques demonstrate their potential to reveal intricate details of protein conformational plasticity. By overcoming the limitations of traditional in vitro methods, these approaches offer a more accurate and comprehensive understanding of protein function and behavior within the complex environment of mammalian cells.

Label-free metabolic fingerprinting of motile mammalian spermatozoa with subcellular resolution

BACKGROUND

Sperm metabolic pathways that generate energy for motility are compartmentalized within the flagellum. Dysfunctions in metabolic compartments, namely mitochondrial respiration and glycolysis, can compromise motility and male fertility. Studying these compartments is thus required for fertility treatment. However, it is very challenging to capture images of metabolic compartments in motile spermatozoa because the fast beating of the flagellum introduces motion blur. Therefore, most approaches immobilize spermatozoa prior to imaging.

RESULTS

Our findings indicate that immobilizing sperm alters their metabolic profile, highlighting the necessity for measuring metabolism in spermatozoa during movement. We achieved this by encapsulating mouse epididymis in a hydrogel followed by two-photon fluorescence lifetime imaging microscopy for imaging motile sperm in situ. The autofluorescence of endogenous metabolites-FAD, NADH, and NADPH-enabled us to visualize sperm metabolic compartments without staining. We trained machine learning for automated image segmentation and generated metabolic fingerprints using object-based phasor analysis. We show that metabolic fingerprints of spermatozoa and the mitochondrial compartment (1) can distinguish individual males by genetic background, age, or fecundity status, (2) correlate with fertility, and (3) change with age likely due to increased oxidative metabolism.

CONCLUSIONS

Our approach eliminates the need for sperm immobilization and labeling and captures the native state of sperm metabolism. This technique could be adapted for metabolism-based sperm selection for assisted reproduction.

Advances in Optical Contrast Agents for Medical Imaging: Fluorescent Probes and Molecular Imaging

Optical imaging is an excellent non-invasive method for viewing visceral organs. Most importantly, it is safer as compared to ionizing radiation-based methods like X-rays. By making use of the properties of photons, this technique generates high-resolution images of cells, molecules, organs, and tissues using visible, ultraviolet, and infrared light. Moreover, optical imaging enables real-time evaluation of soft tissue properties, metabolic alterations, and early disease markers in real time by utilizing a variety of techniques, including fluorescence and bioluminescence. Innovative biocompatible fluorescent probes that may provide disease-specific optical signals are being used to improve diagnostic capabilities in a variety of clinical applications. However, despite these promising advancements, several challenges remain unresolved. The primary obstacle includes the difficulty of developing efficient fluorescent probes, and the tissue autofluorescence, which complicates signal detection. Furthermore, the depth penetration restrictions of several imaging modalities limit their use in imaging of deeper tissues. Additionally, enhancing biocompatibility, boosting fluorescent probe signal-to-noise ratios, and utilizing cutting-edge imaging technologies like machine learning for better image processing should be the main goals of future research. Overcoming these challenges and establishing optical imaging as a fundamental component of modern medical diagnoses and therapeutic treatments would require cooperation between scientists, physicians, and regulatory bodies.

Toward measurements of absolute membrane potential in Bacillus subtilis using fluorescence lifetime

Membrane potential (MP) changes can provide a simple readout of bacterial functional and metabolic state or stress levels. While several optical methods exist for measuring fast changes in MP in excitable cells, there is a dearth of such methods for absolute and precise measurements of steady-state MPs in bacterial cells. Conventional electrode-based methods for the measurement of MP are not suitable for calibrating optical methods in small bacterial cells. While optical measurement based on Nernstian indicators have been successfully used, they do not provide absolute or precise quantification of MP or its changes. We present a novel, calibrated MP recording approach to address this gap. In this study, we used a fluorescence lifetime-based approach to obtain a single-cell-resolved distribution of the membrane potential and its changes upon extracellular chemical perturbation in a population of bacterial cells for the first time. Our method is based on 1) a unique VoltageFluor (VF) optical transducer, whose fluorescence lifetime varies as a function of MP via photoinduced electron transfer and 2) a quantitative phasor-FLIM analysis for high-throughput readout. This method allows MP changes to be easily visualized, recorded and quantified. By artificially modulating potassium concentration gradients across the membrane using an ionophore, we have obtained a Bacillus subtilis-specific MP versus VF lifetime calibration and estimated the MP for unperturbed B. subtilis cells to be -65 mV (in minimal salts glycerol glutamate [MSgg]), -127 mV (in M9), and that for chemically depolarized cells as -14 mV (in MSgg). We observed a population-level MP heterogeneity of ∼6-10 mV indicating a considerable degree of diversity of physiological and metabolic states among individual cells. Our work paves the way for deeper insights into bacterial electrophysiology and bioelectricity research.

NAD+ modulation with nicotinamide mononucleotide coated 3D printed microneedle implants

Nicotinamide adenine dinucleotide (NAD+) deficiency has been shown to cause pathogenesis of age-related functional decline and diseases. Investigational studies have demonstrated improvements in age-associated pathophysiology and disease conditions. However, invasive methods such as immunohistochemistry, metabolic assays, and polymerase chain reaction currently used to measure cell metabolism render cells unviable and unrecoverable for longitudinal studies and are incompatible with in vivo dynamic observations. We report a non-invasive optical technique to investigate the upregulation of nicotinamide adenine dinucleotide (NAD+) in keratinocytes (both in vitro and ex vivo) upon administration of nicotinamide mononucleotide (NMN) coated microneedle (μNDs) implants. Our technique exploits intrinsic autofluorescence of cells and tissues using multiphoton microscopy. Additionally, μND coating formulations to date have been evaluated using fluorescence microscopy to determine the coated amount, often an imprecise correlation between fluorescence intensity and the coated amount on the μND surface. We also show that rheomechanical attributes of the coating formulation (containing two different viscosity enhancers: sucrose and carboxy methyl cellulose) affect the flow mechanics of the coating formulation at micron scale, and thus the amount of drug coated on the μND surface. In vitro keratinocyte cells were investigated with four concentrations of NMN (50, 250, 500 and 1000 μg), and evaluated with time-dependent NMN (500 μg) treatment at 0, 5, 10, 30, 60, 360 and 1460 min. We demonstrate that intracellular keratinocyte fluorescence of the endogenous NADH shows a decreasing trend in both the average fluorescence lifetime (τm) and the free unbound NADH (τ1), with increasing dosage of NMN administration. A similar trend in the average fluorescence lifetime (τm) of endogenous NAD(P)H was also seen in mouse ear skin ex vivo skin upon administration of NMN. We show a promising, minimally invasive, alternative delivery system for the NAD+ precursor molecule that can enhance patient compliance and therapeutic outcomes.

Computational Approach to Phosphor-Sensitized Fluorescence Based on Monomer Transition Densities

We present here an extension of the monomer transition density approach to spin multiplicity-altering excitation energy transfer (EET) processes. It builds upon complex-valued wave functions of the density functional theory-based multireference spin-orbit coupling configuration interaction method for generating the one-particle transition density matrices of the donor and acceptor molecules, which are then contracted with two-electron Coulomb and exchange integrals of the dimer. Due to the extensive use of symmetry relations between tensor components, the computation of triplet-singlet coupling remains technically feasible. As a proof-of-principle application, we have chosen an EET system, consisting of the phosphorescent platinum complex AG97 as the donor and the fluorescein derivative FITC as the acceptor. Taking experimental conditions into account, we estimate a Förster radius of about 35 Å. For intermolecular donor-acceptor separations close to the Förster radius and beyond, the error introduced by the ideal dipole approximation is rather small.

Future applications of fluorescence lifetime imaging ophthalmoscopy in neuro-ophthalmology, neurology, and neurodegenerative conditions

Fluorescence lifetime imaging ophthalmoscopy (FLIO) has emerged as an innovative advancement in retinal imaging, with the potential to provide in vivo non-invasive insights into the mitochondrial metabolism of the retina. Traditional retinal imaging, such as optical coherence tomography (OCT) and fundus autofluorescence (FAF) intensity imaging, focus solely on structural changes to the retina. In contrast, FLIO provides data that may reflect retinal fluorophore activity, some of which may indicate mitochondrial metabolism. This review builds upon the existing literature to describe the principles of FLIO and established uses in retinal diseases while introducing the potential for FLIO in neurodegenerative conditions.

Enhanced fluorescence lifetime imaging microscopy denoising via principal component analysis

Fluorescence Lifetime Imaging Microscopy (FLIM) quantifies the autofluorescence lifetime to measure cellular metabolism, therapeutic efficacy, and disease progression. These dynamic processes are intrinsically heterogeneous, increasing the complexity of the signal analysis. Often noise reduction strategies that combine thresholding and non-selective data smoothing filters are applied. These can result in error introduction and data loss. To mitigate these issues, we develop noise-corrected principal component analysis (NC-PCA). This approach isolates the signal of interest by selectively identifying and removing the noise. To validate NC-PCA, a secondary analysis of FLIM images of patient-derived colorectal cancer organoids exposed to a range of therapeutics was performed. First, we demonstrate that NC-PCA decreases the uncertainty up to 4-fold in comparison to conventional analysis with no data loss. Then, using a merged data set, we show that NC-PCA, unlike conventional methods, identifies multiple metabolic states. Thus, NC-PCA provides an enabling tool to advance FLIM analysis across fields.

Atomic Force Microscopy: A Versatile Tool in Cancer Research

The implementation of novel analytic methodologies in cancer and biomedical research has enabled the quantification of parameters that were previously disregarded only a few decades ago. A notable example of this paradigm shift is the widespread integration of atomic force microscopy (AFM) into biomedical laboratories, significantly advancing our understanding of cancer cell biology and treatment response. AFM allows for the meticulous monitoring of different parameters at the molecular and nanoscale levels, encompassing critical aspects such as cell morphology, roughness, adhesion, stiffness, and elasticity. These parameters can be systematically investigated in correlation with specific cell treatment, providing important insights into morpho-mechanical properties during normal and treated conditions. The resolution of this system holds the potential for its systematic adoption in clinics; its application could produce useful diagnostic information regarding the aggressiveness of cancer and the efficacy of treatment. This review endeavors to analyze the current literature, underscoring the pivotal role of AFM in biomedical research, especially in cancer cases, while also contemplating its prospective application in a clinical context.

Fluorescence lifetime imaging in drug delivery research

Once an exotic add-on to fluorescence microscopy for life science research, fluorescence lifetime imaging (FLIm) has become a powerful and increasingly utilised technique owing to its self-calibration nature, which affords superior quantification over conventional steady-state fluorescence imaging. This review focuses on the state-of-the-art implementation of FLIm related to the formulation, release, dosage, and mechanism of action of drugs aimed for innovative diagnostics and therapy. Quantitative measurements using FLIm have appeared instrumental for encapsulated drug delivery design, pharmacokinetics and pharmacodynamics, pathological investigations, early disease diagnosis, and evaluation of therapeutic efficacy. Attention is paid to the latest advances in lifetime-engineered nanomaterials and practical instrumentation, which begin to show preclinical and clinical translation potential beyond in vitro samples of cells and tissues. Finally, major challenges that need to be overcome in order to facilitate future perspectives are discussed.

EGFR-HER2 Transactivation Viewed in Space and Time Through the Versatile Spectacles of Imaging Cytometry-Implications for Targeted Therapy

Ligand-induced formation of signaling platforms composed of homo- and/or heterodimers of receptor tyrosine kinases is considered essential for their activation and consequential contribution to the progression of many cancers. Epidermal Growth Factor Receptor (EGFR) acts as a signal receiver upon EGF binding and produces mitogenic input for many cells also through receptor-heterodimerization with its ligandless partner, Human Epidermal growth factor Receptor 2 (HER2). Ligand-driven transactivation is a key step leading to changes in the cell surface pattern of EGFR and HER2; their interaction plays a key role in various malignancies, especially when HER2 molecules are overexpressed. Our clinically relevant model system is the SK-BR-3 breast tumor cell line, overexpressing HER2 and moderately expressing EGFR. This cell line shows significant dependency on EGF-driven HER2 signaling. We studied changes in the interaction between EGFR and HER2 in the cell membrane upon EGF binding, applying various biophysical approaches with different time scales. Changes in molecular proximity were characterized by fluorescence lifetime imaging microscopy (FLIM) techniques assessing Förster resonance energy transfer (FRET), which confirmed the ligand-enhanced interaction of EGFR and HER2, followed by an increase in HER2 homoassociation. EGF binding and transactivation were reflected in the phosphorylation of both receptor types as well. At the same time, superresolution Airyscan microscopy and fluorescence correlation and cross-correlation spectroscopy (FCS/FCCS), sensitive to changes in the size of stationary and diffusing aggregates, respectively, have revealed cyclic increases in the aggregation and stable co-diffusion of membrane-localized HER2, possibly caused by internalization and recycling, eventually leading to a new equilibrium. Such dynamic fluctuation of receptor interaction may open a window for the binding of therapeutic antibodies that are aimed at inhibiting heterodimerization, such as pertuzumab. The complementary array of state-of-the-art imaging cytometry approaches thus demonstrates a spatiotemporal pattern of spontaneous and induced receptor aggregation states that could provide mechanistic insights into the potential success of targeted therapies directed at the HER family of receptor tyrosine kinases.

Visualization of labeled micro- and nanoplastics in interaction with algae, using super-resolution stimulated emission depletion microscopy and fluorescence lifetime imaging

In contrast to microplastics, studying the interactions of nanoplastics (NPs) with primary producers such as marine microalgae remains challenging. This is attributed to the lack of adequate visualization methods that can distinguish NPs from autofluorescent biological material such as marine algae. The aim of this study was to develop a method for labeling and visualizing nonfluorescent micro- and nanoplastics (MNPs) of various polymer types, shapes, and sizes, in interaction with marine primary producers, which are autofluorescent. A labeling technique for plastics was refined, using a swell incorporation method with the commercial dye "IDye." Comprehensive quality control measures, including toxicity, leaching, and dye longevity tests, were applied to ensure the robustness of the method. Although stimulated emission depletion (STED) microscopy successfully enabled the visualization of the diverse labeled NPs smaller than 200 nm, it could not distinguish NPs from autofluorescent organic material such as marine microalgae, due to overlapping excitation and emission spectra with the photosynthetically active molecule chlorophyll-a. This study is the first to advance the field by coupling STED with fluorescence lifetime imaging microscopy (FLIM). The FLIM technique, based on the differing lifetimes of fluorescent signals, allowed us to overcome the challenge of overlapping spectra. Our work not only refines and expands existing plastic labeling protocols to accommodate a wide range of polymer types, but also introduces a more precise method for studying interactions between MNPs and autofluorescent organisms. This combined STED-FLIM approach provides a reproducible and reliable framework for examining MNP impacts in complex, ecologically relevant environments, particularly highlighting its potential for investigating MNP-microalgae interactions.

Quantitative profiling pH heterogeneity of acidic endolysosomal compartments using fluorescence lifetime imaging microscopy

The endolysosomal system plays a crucial role in maintaining cellular homeostasis and promoting organism fitness. The pH of its acidic compartments is a crucial parameter for proper function, and it is dynamically influenced by both intracellular and environmental factors. Here, we present a method based on fluorescence lifetime imaging microscopy (FLIM) for quantitatively analyzing the pH profiles of acidic endolysosomal compartments in diverse types of primary mammalian cells and in live organism Caenorhabditis elegans. This FLIM-based method exhibits high sensitivity in resolving subtle pH differences, thereby revealing heterogeneity within a cell and across cell types. This method enables rapid measurement of pH changes in the acidic endolysosomal system in response to various environmental stimuli. Furthermore, the fast FLIM measurement of pH-sensitive dyes circumvents the need for transgenic reporters and mitigates potential confounding factors associated with varying dye concentrations or excitation light intensity. This FLIM approach offers absolute pH quantification and highlights the significance of pH heterogeneity and dynamics, offering a valuable tool for investigating lysosomal functions and their regulation in various physiological and pathological contexts.

Genetically encoded protein oscillators for FM streaming of single-cell data

Radios and cellphones use frequency modulation (FM) of an oscillating carrier signal to reliably transmit multiplexed data while rejecting noise. Here, we establish a biochemical analogue of this paradigm using genetically encoded protein oscillators (GEOs) as carrier signals in circuits that enable continuous, real-time FM streaming of single-cell data. GEOs are constructed from evolutionarily diverse MinDE-family ATPase and activator modules that generate fast synthetic protein oscillations when co-expressed in human cells. These oscillations serve as a single-cell carrier signal, with frequency and amplitude controlled by GEO component levels and activity. We systematically characterize 169 ATPase/activator GEO pairs and engineer composite GEOs with multiple competing activators to develop a comprehensive platform for waveform programming. Using these principles, we design circuits that modulate GEO frequency in response to cellular activity and decode their responses using a calibrated machine-learning model to demonstrate sensitive, real-time FM streaming of transcription and proteasomal degradation dynamics in single cells. GEOs establish a dynamically controllable biochemical carrier signal, unlocking noise-resistant FM data-encoding paradigms that open new avenues for dynamic single-cell analysis.

Advances in Axial Resolution Strategies for Super-Resolution Imaging Systems

3D fluorescence super-resolution imaging technology can reconstruct the 3D structure of biological cells in space, which is crucial for observing the intricate internal structures of cells and studying the organization and function of tissues and organs. However, even with super-resolution imaging techniques that surpass the diffraction limit, the axial resolution typically only reaches one-third to one-half of the lateral resolution. Achieving true axial or 3D super-resolution imaging of samples remains a significant challenge. In light of this, this review summarizes the research progress in axial super-resolution imaging techniques, with a focus on the principles, developments, and characteristics of these techniques, and provides an outlook on their future development directions. This paper aims to provide valuable reference material for researchers in the field.

Wnt signaling modulates mechanotransduction in the epidermis to drive hair follicle regeneration

Most wounds form scars without hair follicles. However, in the wound-induced hair neogenesis (WIHN) model of skin regeneration, wounds regenerate hair follicles if tissue rigidity is optimal. Although WIHN depends on Wnt signaling, whether Wnt performs a mechanoregulatory role that contributes to regeneration remains uncharacterized. Here, we demonstrate that Wnt signaling affects mechanosensitivity at both cellular and tissue levels to drive WIHN. Atomic force microscopy revealed an attenuated substrate rigidity response in epidermal but not dermal cells of healing wounds. Super-resolution microscopy and nanoneedle probing of intracellular compartments in live human keratinocytes revealed that Wnt-induced chromatin remodeling triggers a 10-fold drop in nuclear rigidity without jeopardizing the nucleocytoskeletal mechanical coupling. Mechanistically, Wnt signaling orchestrated a massive reorganization of actin architecture and recruited adherens junctions to generate a mechanical syncytium-a cohesive contractile unit with superior capacity for force coordination and collective durotaxis. Collectively, our findings unveil Wnt signaling's mechanoregulatory role that manipulates the machinery of mechanotransduction to drive regeneration.

Surface Grafting of Graphene Flakes with Fluorescent Dyes: A Tailored Functionalization Approach

The controlled functionalization of graphene is critical for tuning and enhancing its properties, thereby expanding its potential applications. Covalent functionalization offers a deeper tuning of the geometric and electronic structure of graphene compared to non-covalent methods; however, the existing techniques involve side reactions and spatially uncontrolled functionalization, pushing research toward more selective and controlled methods. A promising approach is 1,3-dipolar cycloaddition, successfully utilized with carbon nanotubes. In the present work, this method has been extended to graphene flakes with low defect concentration. A key innovation is the use of a custom-synthesized ylide with a protected amine group (Boc), facilitating subsequent attachment of functional molecules. Indeed, after Boc cleavage, fluorescent dyes (Atto 425, 465, and 633) were covalently linked via NHS ester derivatization. This approach represents a highly selective method of minimizing structural damage. Successful functionalization was demonstrated by Raman spectroscopy, photoluminescence spectroscopy, and confocal microscopy, confirming the effectiveness of the method. This novel approach offers a versatile platform, enabling its use in biological imaging, sensing, and advanced nanodevices. The method paves the way for the development of sensors and devices capable of anchoring a wide range of molecules, including quantum dots and nanoparticles. Therefore, it represents a significant advancement in graphene-based technologies.

Combined FLIM, Confocal Microscopy, and STED Nanoscopy for Live-Cell Imaging

Time-lapse fluorescence microscopy is a relevant technique to visualize biological events in living samples. Maintaining cell survival by limiting light-induced cellular stress is challenging and requires protocol development and image acquisition optimization. Here, we provide a guide by considering the quartet sample, probe, instrument, and image processing to obtain appropriate resolutions and information for live cell fluorescence imaging. The pleural mesothelial cell line H28, an adherent cell line that is easy to seed, was used to develop innovative advanced light microscopy strategies. The chosen red and near-infrared probes, capable of passively penetrating through the cell plasma membrane, are particularly suitable because their stimulation from 600 to 800 nm induces less cytotoxicity. The labeling protocol describes the concentration, time, and incubation conditions of the probes and associated adjustments for multi-labeling. To limit phototoxicity, acquisition parameters in advanced confocal laser scanning microscopy with a white laser are determined. Light power must be adjusted and minimized at red wavelengths for reduced irradiance (including a 775 nm depletion laser for STED nanoscopy), in simultaneous mode with hybrid detectors and combined with the fast FLIM module. These excellent conditions allow us to follow cellular and intracellular dynamics for a few minutes to several hours while maintaining good spatial and temporal resolutions. Lifetime analysis in lifetime imaging (modification of the lifetime depending on environmental conditions), lifetime dye unmixing (separation with respect to the lifetime value for the spectrally closed dye), and lifetime denoising (improvement of image quality) provide flexibility for multiplexing experiments. Key features • Cell preservation after labeling with less cytotoxic red, near-infrared dye viable probes. • Determination of lower but efficient probe concentration; adjust good balance between probes concentration and incubation time to achieve multi-labeling. • Long time-lapse acquisition in advanced confocal microscopy with sensitive new-generation detectors. • Confocal image combined with fast FLIM for multi-labeling with spectrally closed dyes, unmixed from lifetime values. • Confocal-STED image acquisition combined with fast FLIM to improve signal-to-noise ratio.

Measuring plasma membrane fluidity using confocal microscopy

Membrane fluidity is a crucial parameter for cellular physiology. Recent evidence suggests that fluidity varies between cell types and states and in diseases. As membrane fluidity has gradually become an important consideration in cell biology and biomedicine, it is essential to have reliable and quantitative ways to measure it in cells. In the past decade, there has been substantial progress both in chemical probes and in imaging tools to make membrane fluidity measurements easier and more reliable. We have recently established a robust pipeline, using confocal imaging and new environment-sensitive probes, that has been successfully used for several studies. Here we present our detailed protocol for membrane fluidity measurement, from labeling to imaging and image analysis. The protocol takes ~4 h and requires basic expertise in cell culture, wet lab and microscopy.

UNET-FLIM: A Deep Learning-Based Lifetime Determination Method Facilitating Real-Time Monitoring of Rapid Lysosomal pH Variations in Living Cells

Lifetime determination plays a crucial role in fluorescence lifetime imaging microscopy (FLIM). We introduce UNET-FLIM, a deep learning architecture based on a one-dimensional U-net, specifically designed for lifetime determination. UNET-FLIM focuses on handling low photon count data with high background noise levels, which are commonly encountered in fast FLIM applications. The proposed network can be effectively trained using simulated decay curves, making it adaptable to various time-domain FLIM systems. Our evaluations of simulated data demonstrate that UNET-FLIM robustly estimates lifetimes and proportions, even when the signal photon count is extremely low and background noise levels are high. Remarkably, UNET-FLIM's insensitivity to noise and minimal photon count requirements facilitate fast FLIM imaging and real-time lifetime analysis. We demonstrate its potential by applying it to monitor rapid lysosomal pH variations in living cells during in situ acetic acid treatment, all without necessitating any modifications to existing FLIM systems.

Understanding the Influence of Electron Traps and Urbach States on the Kinetics of Ti3+ Persistent Luminescence in LaAlO3:Ti3

The luminescence kinetics of Ti3+ ions, resulting from the spin-allowed 2E → 2T2 electron transition, are generally expected to be fast, within the microsecond range. However, in this study, we observed average lifetimes of up to 30 ms in nanocrystalline LaAlO3:Ti3+ powders. Our detailed analysis of the spectroscopic and thermoluminescence properties of LaAlO3:Ti3+ suggests that this prolonged Ti3+ kinetics is associated with the presence of electron traps and the proximity of the Ti3+ excited state to the conduction band, which facilitates energy transfer between them. Furthermore, the observed shift in Urbach states with an increasing Ti3+ concentration correlates with the efficiency of energy transfer between deeper traps and Ti3+ ions. This study provides a comprehensive strategy for controlling the luminescence kinetics of Ti3+ ions through electron trap engineering, induced by dopant ion concentration, which can be applied in various fields including luminescence thermometry.

Development of a time-resolved laser-induced fluorescence fingerprinting method for detecting low-level adulteration in extra virgin olive oil

Adulteration identification in extra virgin olive oil (EVOO) is a significant concern in the olive oil industry. This study aimed to detect low-level adulteration of EVOO with other edible oils by using a novel time-resolved laser-induced fluorescence (TRLIF) fingerprinting method. Five EVOO brands were first analyzed to assess its potential for classification based on their differing fluorophore content. The developed method effectively reduces the effects associated with the optical path length and excitation light intensity. Subsequently, three sets of binary mixtures were tested: EVOO adulterated with refined olive oil, soybean oil, and sunflower oil. Quantitative analysis was performed using parallel factor analysis, which achieved a cross-validation coefficient of determination (R2) exceeding 0.90, with a prediction error < 1.40 %. These findings demonstrate that this method has the potential to determine the purity and quality of EVOO.

GSLab: Open-Source Platform for Advanced Phasor Analysis in Fluorescence Microscopy

GSLab addresses the need for effective image analysis tools in fluorescence microscopy by providing an open-source platform that enhances traditional phasor analysis with advanced features. Key capabilities include machine learning-based clustering, real-time monitoring, and quantitative unmixing of fluorescent species. Designed for both commercial and custom systems, GSLab provides researchers with comprehensive lifetime and spectral phasor image analysis tools to tackle complex biological problems.

Multispectral laser-scanning pulse-sampling fluorescence lifetime system for large-scale tissue imaging

We report a multispectral laser-scanning pulse-sampling fluorescence lifetime imaging (LSPS-FLIm) system designed for rapid, high-resolution imaging of large tissue specimens. This system provides a substantial imaging field of view (FoV) of 6 × 15 cm2 with a high spatial resolution of ∼17.5 µm. The LSPS-FLIm system has been tested on a range of fluorescent dyes, endogenous tissue fluorophores, and tissue specimens with varied sizes and properties. These tests demonstrate the system's versatility in resolving morphological and molecular features, enabling centimeter-scale FoV imaging, and distinguishing complex microstructures in tissue specimens. With the capability to maintain high imaging quality and acquisition speed while minimizing tissue damage, LSPS-FLIm represents a promising advancement in the field of fluorescence lifetime imaging for biological and clinical applications.

A Long Fluorescence Lifetime Probe for Labeling of Gram-Negative Bacteria

Bacterial resistance, primarily stemming from misdiagnosis, misuse, and overuse of antibacterial medications in humans and animals, is a pressing issue. To address this, we focused on developing a fluorescent probe for the detection of bacteria, with a unique feature-an exceptionally long fluorescence lifetime, to overcome autofluorescence limitations in biological samples. The polymyxin-based probe (ADOTA-PMX) selectively targets Gram-negative bacteria and used the red-emitting fluorophore azadioxatriangulenium (with a reported fluorescence lifetime of 19.5 ns). Evaluation of ADOTA-PMX's bacterial labeling efficacy revealed strong specificity for Gram-negative bacteria, and full spectral fluorescence lifetime imaging microscopy demonstrated the potential of ADOTA-PMX for bacterial imaging applications. The probe exhibited a lifetime of 4.5 ns when bound to bacteria, possibly indicating interactions with the bacterial outer membrane. Furthermore, the fluorescence lifetime measurements of ADOTA-PMX labeled bacteria could be performed using a benchtop fluorimeter without the need of sophisticated microscopes. This study represents the first targeted probe for fluorescence lifetime imaging, offering sensitivity for detecting Gram-negative bacteria and enabling multiplexing via fluorescence lifetime imaging.

Analysis of Drug Molecules in Living Cells

Cells are the fundamental units of life, comprising a highly concentrated and complex assembly of biomolecules that interact dynamic ally across spatial and temporal scales. Living cells are constantly undergoing dynamic processes, therefore, to understand the interactions between drug molecules and living cells is of paramount importance in the biomedical sciences and pharmaceutical development. Compared with traditional end-point assays and fixed cell analysis, analysis of drug molecules in living cells can provide more insight into the effects of drugs on cells in real-time and allowing for a better understanding of drug mechanisms and effects, which will contribute to the development of drug developing and testing and personalize medicine. However, the high demands of living cell analysis, including high costs, technical complexity, and throughput limitations, hinder the widespread application of this technology. In recent years, the rapid development of analytical methods such as Raman spectroscopy and fluorescence has made the in situ and real-time detection possible, allowing the analysis of single cell or cell populations at various conditions. In this review, we summarize the advanced analytical methods and technologies from last few years for drug detection in living cells.

Resistive-Pulse Sensing Coupled with Fluorescence Lifetime Imaging Microscopy for Differentiation of Individual Liposomes

Characterization of individual biological nanoparticles can be significantly improved by coupling complementary analytical methods. Here, we combine resistive-pulse sensing (RPS) with fluorescence lifetime imaging microscopy (FLIM) to differentiate liposomes at the single-particle level. RPS measures the particle volume, shape, and surface-charge density, and FLIM determines the fluorescence lifetime of the fluorophore associated with the lipid membrane. The RPS devices are fabricated in-plane on a glass substrate to facilitate coupling of RPS with FLIM measurements. For proof-of-concept, we studied liposomes containing various cholesterol concentrations with membrane-intercalated Di-8-ANEPPS, whose fluorescence lifetime is known to be sensitive to cholesterol concentrations in the membrane. RPS-FLIM revealed that increasing cholesterol concentrations in the liposome from 0% to 50% increased the fluorescence lifetimes from 2.1 ± 0.2 to 3.4 ± 0.5 ns, respectively. Moreover, RPS-FLIM discerned liposome populations with the same cholesterol concentration but labeled with dyes that have different fluorescence lifetimes (Di-8-ANEPPS and COE-S6), parsing two particle populations with statistically identical volumes, cholesterol concentration, and lipid composition. Interrogation with RPS-FLIM occurred with individual particles making a single pass through the detection region and overcomes issues with fluorescence spectral overlap that limits traditional methods. We envision RPS-FLIM as a versatile and scalable technique with the potential to differentiate biological particles at the single-particle level to simultaneously inform on particle size, surface-charge density, membrane composition, and identity.

FLIM nanoscopy resolves the structure and preferential adsorption in the co-nonsolvency of PNIPAM microgels in methanol-water

Polymer microgels are swollen macromolecular networks with a typical size of hundred of nanometers to several microns that show an extraordinary open and responsive architecture to different external stimuli, being therefore important candidates for nanobiotechnology and nanomedical applications such as biocatalysis, sensing and drug delivery. It is therefore crucial to understand the delicate balance of physical-chemical interactions between the polymer backbone and solvent molecules that to a high extent determine their responsivity. In particular, the co-nonsolvency effect of poly(N-isopropylacrylamide) in aqueous alcohols is highly discussed, and there is a disagreement between molecular dynamics (MD) simulations (from literature) of the preferential adsorption of alcohol on the polymer chains and the values obtained by several empirical methods that mostly probe the bulk solvent properties. It is our contention that the most efficacious method for addressing this problem requires a nanoscopic method that can be combined with spectroscopy and record fluorescence spectra and super-resolved fluorescence lifetime images of microgels labeled covalently with the solvatochromic dye Nile Red. By employing this approach, we could simultaneously resolve the structure of sub-micron size objects in the swollen and in the collapsed state and estimate the solvent composition inside of them in - mixtures for two very different polymer architectures. We found an outstanding agreement between the MD simulations and our results that estimate a co-solvent molar fraction excess of approximately 3 with a very flat profile in the lateral direction of the microgel.

The optimization and application of photodynamic diagnosis and autofluorescence imaging in tumor diagnosis and guided surgery: current status and future prospects

Photodynamic diagnosis (PDD) and autofluorescence imaging (AFI) are emerging cancer diagnostic technologies that offer significant advantages over traditional white-light endoscopy in detecting precancerous lesions and early-stage cancers; moreover, they hold promising potential in fluorescence-guided surgery (FGS) for tumors. However, their shortcomings have somewhat hindered the clinical application of PDD and AFI. Therefore, it is imperative to enhance the efficacy of PDD and AFI, thereby maximizing their potential for practical clinical use. This article reviews the principles, characteristics, current research status, and advancements of PDD and AFI, focusing on analyzing and discussing the optimization strategies of PDD and AFI in tumor diagnosis and FGS scenarios. Considering the practical and technical feasibility, optimizing PDD and AFI may result in an effective real-time diagnostic tool to guide clinicians in tumor diagnosis and surgical guidance to achieve the best results.

Defining the Zone of Acute Peripheral Nerve Injury Using Fluorescence Lifetime Imaging in a Crush Injury Sheep Model

PURPOSE

Current technologies to define the zone of acute peripheral nerve injury intraoperatively are limited by surgical experience, time, cumbersome electrodiagnostic equipment, and interpreter reliability. In this pilot study, we evaluated a real-time, label-free optical technique for intraoperative nerve injury imaging. We hypothesize that fluorescence lifetime imaging (FLIm) will detect a difference between the time-resolved fluorescence signatures for acute crush injuries versus uninjured segments of peripheral nerves in sheep.

METHODS

Label-free FLIm uses ultraviolet laser pulses to excite endogenous tissue fluorophores and detect their fluorescent decay over time, generating real-time tissue-specific signatures. A crush injury was produced in eight peripheral nerves of two sheep. A hand-held FLIm instrument captured the time-resolved fluorescence signatures of injured and uninjured nerve segments across three spectral emission channels (390/40 nm, 470/28 nm, and 540/50 nm). The average FLIm parameters (ie, lifetime and intensity ratios) for injured and uninjured nerve segments were compared. We used linear discriminant analysis to differentiate between crushed and uninjured nerve segments.

RESULTS

A total of 23,692 point measurements were collected from eight crushed peripheral nerves of two sheep. Histology confirmed the zone of injury. Average lifetime at 470 nm and 540 nm were significantly different between crushed and uninjured sheep nerve segments. The linear discriminant analysis differentiated between crushed and uninjured areas of eight nerve segments with 92% sensitivity, 85% specificity, and 88% accuracy.

CONCLUSIONS

In this pilot study, FLIm detected differing average lifetime values for crushed versus uninjured sheep peripheral nerves with high sensitivity, specificity, and accuracy.

CLINICAL RELEVANCE

With further investigation, FLIm may guide the peripheral nerve surgeon to the precise zone of injury for reconstruction.

Deep learning-enabled filter-free fluorescence microscope

Optical filtering is an indispensable part of fluorescence microscopy for selectively highlighting molecules labeled with a specific fluorophore and suppressing background noise. However, the utilization of optical filtering sets increases the complexity, size, and cost of microscopic systems, making them less suitable for multifluorescence channel, high-speed imaging. Here, we present filter-free fluorescence microscopic imaging enabled with deep learning-based digital spectral filtering. This approach allows for automatic fluorescence channel selection after image acquisition and accurate prediction of fluorescence by computing color changes due to spectral shifts with the presence of excitation scattering. Fluorescence prediction for cells and tissues labeled with various fluorophores was demonstrated under different magnification powers. The technique offers accurate identification of labeling with robust sensitivity and specificity, achieving consistent results with the reference standard. Beyond fluorescence microscopy, the deep learning-enabled spectral filtering strategy has the potential to drive the development of other biomedical applications, including cytometry and endoscopy.

Advancements in phasor-based FLIM: multi-component analysis and lifetime probes in biological imaging

Fluorescence lifetime imaging microscopy (FLIM) is a reliable method that achieves imaging by detecting fluorescence lifetimes within samples. Owing to its unique temporal characteristic, it can complement fluorescence intensity measurement. Technological and methodological advancements in FLIM have broadened its applications across various domains. The processing of fluorescence lifetime data is crucial for enhancing the speed and accuracy of imaging. Thus, various lifetime fitting algorithms have been developed to improve the imaging speed. The phasor analysis (PA) method is an approach for processing fluorescence lifetime data, capable of directly converting lifetime signals into visual graphics without fitting, which outperforms traditional approaches in speed. Furthermore, lifetime probes with distinct lifetimes are readily implemented for visualization and cluster analysis combined with PA, facilitating the prediction of specific biological states or functions. This review examines various lifetime probes employed in phasor-based FLIM and discusses their roles in the PA method. The methods for multi-component PA within complex biological environments were also described. Additionally, we focused on the advantages of the phasor vector rule and the unmixing of multi-component analysis based on PA. The integration of lifetime probes with phasor-based FLIM facilitates rapid and intuitive detection methods for analyzing complex biological environments.

Cognitive disorders: Potential astrocyte-based mechanism

Cognitive disorders are a common clinical manifestation, including a deterioration in the patient's memory ability, attention, executive power, language, and other functions. The contributing factors of cognitive disorders are numerous and diverse in nature, including organic diseases and other mental disorders. Neurodegenerative diseases are a common type of organic disease related to the pathology of neuronal death and disruption of glial cell balance, ultimately accompanied with cognitive impairment. Thus, cognitive disorder frequently serves as an extremely critical indicator of neurodegenerative disorders. Cognitive impairments negatively affect patients' daily lives. However, our understanding of the precise pathogenic pathways of cognitive defects remains incomplete. The most prevalent kind of glial cells in the central nervous system are called astrocytes. They have a unique significance in cerebral function because of their wide range of functions in maintaining homeostasis in the central nervous system, regulating synaptic plasticity, and so on. Dysfunction of astrocytes is intimately linked to cognitive disorders, and we are attempting to understand this phenomenon predominantly from those perspectives: neuroinflammation, astrocytic senescence, connexin, Ca2 + signaling, mitochondrial dysfunction, and the glymphatic system.

Correlating NAD(P)H lifetime shifts to tamoxifen resistance in breast cancer cells: A metabolic screening study with time-resolved flow cytometry

Time-resolved flow cytometry (TRFC) was used to measure metabolic differences in estrogen receptor-positive breast cancer cells. This specialty cytometry technique measures fluorescence lifetimes as a single-cell parameter thereby providing a unique approach for high-throughput cell counting and screening. Differences in fluorescence lifetime were detected and this was associated with sensitivity to the commonly prescribed therapeutic tamoxifen. Differences in fluorescence lifetime are attributed to the binding states of the autofluorescent metabolite NAD(P)H. The function of NAD(P)H is well described and in general involves cycling from a reduced to oxidized state to facilitate electron transport for the conversion of pyruvate to lactate. NAD(P)H fluorescence lifetimes depend on the bound or unbound state of the metabolite, which also relates to metabolic transitions between oxidative phosphorylation and glycolysis. To determine if fundamental metabolic profiles differ for cells that are sensitive to tamoxifen compared to those that are resistant, large populations of MCF-7 breast cancer cells were screened and fluorescence lifetimes were quantified. Additionally, metabolic differences associated with tamoxifen sensitivity were measured with a Seahorse HS mini metabolic analyzer (Agilent Technologies Inc. Santa Clara, CA) and confocal imaging. Results show that tamoxifen-resistant breast cancer cells have increased utilization of glycolysis for energy production compared to tamoxifen-sensitive breast cancer cells. This work is impacting because it establishes an early step toward developing a reliable screening technology in which large cell censuses can be differentiated for drug sensitivity in a label-free fashion.

Novel application of metabolic imaging of early embryos using a light-sheet on-a-chip device: a proof-of-concept study

STUDY QUESTION

Is it feasible to safely determine metabolic imaging signatures of nicotinamide adenine dinucleotide [NAD(P)H] associated auto-fluorescence in early embryos using a light-sheet on-a-chip approach?

SUMMARY ANSWER

We developed an optofluidic device capable of obtaining high-resolution 3D images of the NAD(P)H autofluorescence of live mouse embryos using a light-sheet on-a-chip device as a proof-of-concept.

WHAT IS KNOWN ALREADY

Selecting the most suitable embryos for implantation and subsequent healthy live birth is crucial to the success rate of assisted reproduction and offspring health. Besides morphological evaluation using optical microscopy, a promising alternative is the non-invasive imaging of live embryos to establish metabolic activity performance. Indeed, in recent years, metabolic imaging has been investigated using highly advanced microscopy technologies such as fluorescence-lifetime imaging and hyperspectral microscopy.

STUDY DESIGN, SIZE, DURATION

The potential safety of the system was investigated by assessing the development and viability of live embryos after embryo culture for 67 h post metabolic imaging at the two-cell embryo stage (n = 115), including a control for culture conditions and sham controls (system non-illuminated). Embryo quality of developed blastocysts was assessed by immunocytochemistry to quantify trophectoderm and inner mass cells (n = 75). Furthermore, inhibition of metabolic activity (FK866 inhibitor) during embryo culture was also assessed (n = 18).

PARTICIPANTS/MATERIALS, SETTING, METHODS

The microstructures were fabricated following a standard UV-photolithography process integrating light-sheet fluorescence microscopy into a microfluidic system, including on-chip micro-lenses to generate a light-sheet at the centre of a microchannel. Super-ovulated F1 (CBA/C57Bl6) mice were used to produce two-cell embryos and embryo culture experiments. Blastocyst formation rates and embryo quality (immunocytochemistry) were compared between the study groups. A convolutional neural network (ResNet 34) model using metabolic images was also trained.

MAIN RESULTS AND THE ROLE OF CHANCE

The optofluidic device was capable of obtaining high-resolution 3D images of live mouse embryos that can be linked to their metabolic activity. The system's design allowed continuous tracking of the embryo location, including high control displacement through the light-sheet and fast imaging of the embryos (<2 s), while keeping a low dose of light exposure (16 J · cm-2 and 8 J · cm-2). Optimum settings for keeping sample viability showed that a modest light dosage was capable of obtaining 30 times higher signal-noise-ratio images than images obtained with a confocal system (P < 0.00001; t-test). The results showed no significant differences between the control, illuminated and non-illuminated embryos (sham control) for embryo development as well as embryo quality at the blastocyst stage (P > 0.05; Yate's chi-squared test). Additionally, embryos with inhibited metabolic activity showed a decreased blastocyst formation rate of 22.2% compared to controls, as well as a 47% reduction in metabolic activity measured by metabolic imaging (P < 0.0001; t-test). This indicates that the optofluidic device was capable of producing metabolic images of live embryos by measuring NAD(P)H autofluorescence, allowing a novel and affordable approach. The obtained metabolic images of two-cell embryos predicted blastocyst formation with an AUC of 0.974.

LARGE SCALE DATA

N/A.

LIMITATIONS, REASONS FOR CAUTION

The study was conducted using a mouse model focused on early embryo development assessing illumination at the two-cell stage. Further safety studies are required to assess the safety and use of 405 nm light at the blastocyst stage by investigating any potential negative impact on live birth rates, offspring health, aneuploidy rates, mutational load, changes in gene expression, and/or effects on epigenome stability in newborns.

WIDER IMPLICATIONS OF THE FINDINGS

This light-sheet on-a-chip approach is novel and after rigorous safety studies and a roadmap for technology development, potential future applications could be developed for ART. The overall cost-efficient fabrication of the device will facilitate scalability and integration into future devices if full-safety application is demonstrated.

STUDY FUNDING/COMPETING INTEREST(S)

This work was partially supported by an Ideas Grant (no 2004126) from the National Health and Medical Research Council (NHMRC), by the Education Program in Reproduction and Development (EPRD), Department Obstetrics and Gynaecology, Monash University, and by the Department of Mechanical and Aerospace Engineering, Faculty of Engineering, Monash University. The authors E.V-O, R.N., V.J.C., A.N., and F.H. have applied for a patent on the topic of this technology (PCT/AU2023/051132). The remaining authors have nothing to disclose.

FRET-FLIM for the Study of Protein-Protein Interactions Underpinning Mitosis Checkpoints

Cell division is a key cellular process that ensures the continuation of life on Earth. In order to protect the genetic integrity of organisms, cell division must happen accurately, ensuring each daughter cell receives a complete copy of the original genome. The accuracy of this process is, in part, preserved by various cell cycle checkpoints. These checkpoints rely on the physical interactions of their components to ensure proper function. The spindle assembly checkpoint (SAC), for example, produces an inhibitory complex of BUBR1-BUB3 and MAD2 bound to CDC20. Many of these cell cycle checkpoint components have been identified in plants, but it has not yet been established whether plants have a mitotic checkpoint architecture that is similar to mammalian cells. To understand the function of plant cell cycle homologues, it is imperative to characterize their interactions in vivo. FRET-FLIM (Förster resonance energy transfer-fluorescence lifetime imaging microscopy), is a rapidly expanding technique that can be used to rapidly and simply characterize protein-protein interactions.

Exploring the Frontiers of Cell Temperature Measurement and Thermogenesis

The precise measurement of cell temperature and an in-depth understanding of thermogenic processes are critical in unraveling the complexities of cellular metabolism and its implications for health and disease. This review focuses on the mechanisms of local temperature generation within cells and the array of methods developed for accurate temperature assessment. The contact and noncontact techniques are introduced, including infrared thermography, fluorescence thermometry, and other innovative approaches to localized temperature measurement. The role of thermogenesis in cellular metabolism, highlighting the integral function of temperature regulation in cellular processes, environmental adaptation, and the implications of thermogenic dysregulation in diseases such as metabolic disorders and cancer are further discussed. The challenges and limitations in this field are critically analyzed while technological advancements and future directions are proposed to overcome these barriers. This review aims to provide a consolidated resource for current methodologies, stimulate discussion on the limitations and challenges, and inspire future innovations in the study of cellular thermodynamics.

Gold Nanoparticle Detection with Two-Photon Excitation Fluorescence Lifetime Imaging of NAD(P)H in Cancer Cells: An Analytical Approach to Separate Nanoparticle and NAD(P)H Signals

Gold nanoparticles (AuNPs) have shown promise for applications in the diagnosis and treatment of different diseases, including cancer. Understanding the effect of AuNPs on metabolic reprogramming in cancer cells at the single cell level is of high importance for improving the efficacy and safety. Fluorescence lifetime imaging microscopy (FLIM) of nicotinamide adenine dinucleotide (phosphate) hydrogen (NAD(P)H) as a main metabolic cofactor and an indicator of metabolic reprogramming in cancer cells enables real-time monitoring of cancer cell metabolism in response to different treatments, including AuNPs. However, NPs such as AuNPs can be a potential source of signals themselves, which provides opportunities to measure the NP internalization, but it is also important to minimize confounding effects on metabolic measurements. In this study, we detected inherent photoluminescence (PL) from the AuNPs in treated prostate cancer cells (PC-3 cell line) as well as in solution at the NAD(P)H emission wavelength. We developed an analysis approach to minimize the confounding effect of the AuNPs' PL on metabolic measurements. On the other hand, we assessed the reliability of the intracellular AuNPs' PL as an estimator of AuNP uptake. To assess if intracellular AuNPs' PL may be dependent on the exposed cell type, we performed NAD(P)H FLIM imaging of AuNP-exposed SKBR-3 breast cancer cells, where we observed a similar AuNP PL but at a much lower level compared to PC-3 cells. We proposed that this difference can be attributed to the different levels of AuNP uptake or varying intracellular microenvironments.

Optimizing Photobiomodulation Radiometric and Spectral Parameters In Vitro to Enhance Angiogenesis and Mitochondrial Function

Photobiomodulation (PBM) therapy, a therapeutic approach utilizing low-level light, has garnered significant attention for its potential to modulate various biological processes. This study aimed at optimizing and investigating the effects of PBM on angiogenesis and mitochondrial metabolic activity. In vitro experiments using human umbilical vein endothelial cells (HUVECs) and vascular smooth muscle cells (VSMCs) were performed to assess PBM's impacts on cell migration, proliferation, endogenous protoporphyrin IX production, mitochondrial membrane potential, Rhodamine 123 fluorescence lifetime, mitochondrial morphology, and oxygen consumption. Our findings demonstrated that the PBM approach significantly stimulates HUVECs and VSMCs, highlighting the importance of precise light dosimetry for optimal outcomes. Interestingly, our results indicate that in our conditions, the optimal radiometric and spectral parameters are similar for HUVECs and VSMCs for the different endpoints mentioned above. In conclusion, our study strongly suggests that PBM holds promise as a therapeutic intervention for conditions characterized by impaired angiogenesis, such as wound healing, ischemia, and cardiovascular disease. Further research is necessary to fully elucidate the underlying mechanisms and optimize the radiometric and spectral parameters for clinical applications.

Towards measurements of absolute membrane potential in Bacillus subtilis using fluorescence lifetime

Membrane potential (MP) changes can provide a simple readout of bacterial functional and metabolic state or stress levels. While several optical methods exist for measuring fast changes in MP in excitable cells, there is a dearth of such methods for absolute and precise measurements of steady-state membrane potentials (MPs) in bacterial cells. Conventional electrode-based methods for the measurement of MP are not suitable for calibrating optical methods in small bacterial cells. While optical measurement based on Nernstian indicators have been successfully used, they do not provide absolute or precise quantification of MP or its changes. We present a novel, calibrated MP recording approach to address this gap. In this study, we used a fluorescence lifetime-based approach to obtain a single-cell resolved distribution of the membrane potential and its changes upon extracellular chemical perturbation in a population of bacterial cells for the first time. Our method is based on (i) a unique VoltageFluor (VF) optical transducer, whose fluorescence lifetime varies as a function of MP via photoinduced electron transfer (PeT) and (ii) a quantitative phasor-FLIM analysis for high-throughput readout. This method allows MP changes to be easily visualized, recorded and quantified. By artificially modulating potassium concentration gradients across the membrane using an ionophore, we have obtained a Bacillus subtilis-specific MP versus VF lifetime calibration and estimated the MP for unperturbed B. subtilis cells to be -65 mV (in MSgg), 127 mV (in M9) and that for chemically depolarized cells as -14 mV (in MSgg). We observed a population level MP heterogeneity of ~6-10 mV indicating a considerable degree of diversity of physiological and metabolic states among individual cells. Our work paves the way for deeper insights into bacterial electrophysiology and bioelectricity research.

Recent Technologies on 2D and 3D Imaging Flow Cytometry

Imaging flow cytometry is a technology that performs microscopy image analysis of cells within flow cytometry and allows high-throughput, high-content cell analysis based on their intracellular molecular distribution and/or cellular morphology. While the technology has been available for a couple of decades, it has recently gained significant attention as technical limitations for higher throughput, sorting capability, and additional imaging dimensions have been overcome with various approaches. These evolutions have enabled imaging flow cytometry to offer a variety of solutions for life science and medicine that are not possible with conventional flow cytometry or microscopy-based screening. It is anticipated that the extent of applications will expand in the upcoming years as the technology becomes more accessible through dissemination. In this review, we will cover the technical advances that have led to this new generation of imaging flow cytometry, focusing on the advantages and limitations of each technique.