Pioglitazone Phases and Metabolic Effects in Nanoparticle-Treated Cells Analyzed via Rapid Visualization of FLIM Images

Fluorescence lifetime imaging microscopy (FLIM) has proven to be a useful method for analyzing various aspects of material science and biology, like the supramolecular organization of (slightly) fluorescent compounds or the metabolic activity in non-labeled cells; in particular, FLIM phasor analysis (phasor-FLIM) has the potential for an intuitive representation of complex fluorescence decays and therefore of the analyzed properties. Here we present and make available tools to fully exploit this potential, in particular by coding via hue, saturation, and intensity the phasor positions and their weights both in the phasor plot and in the microscope image. We apply these tools to analyze FLIM data acquired via two-photon microscopy to visualize: (i) different phases of the drug pioglitazone (PGZ) in solutions and/or crystals, (ii) the position in the phasor plot of non-labelled poly(lactic-co-glycolic acid) (PLGA) nanoparticles (NPs), and (iii) the effect of PGZ or PGZ-containing NPs on the metabolism of insulinoma (INS-1 E) model cells. PGZ is recognized for its efficacy in addressing insulin resistance and hyperglycemia in type 2 diabetes mellitus, and polymeric nanoparticles offer versatile platforms for drug delivery due to their biocompatibility and controlled release kinetics. This study lays the foundation for a better understanding via phasor-FLIM of the organization and effects of drugs, in particular, PGZ, within NPs, aiming at better control of encapsulation and pharmacokinetics, and potentially at novel anti-diabetics theragnostic nanotools.

Implementation of FRET Spectrometry Using Temporally Resolved Fluorescence: A Feasibility Study

Förster resonance energy transfer (FRET) spectrometry is a method for determining the quaternary structure of protein oligomers from distributions of FRET efficiencies that are drawn from pixels of fluorescence images of cells expressing the proteins of interest. FRET spectrometry protocols currently rely on obtaining spectrally resolved fluorescence data from intensity-based experiments. Another imaging method, fluorescence lifetime imaging microscopy (FLIM), is a widely used alternative to compute FRET efficiencies for each pixel in an image from the reduction of the fluorescence lifetime of the donors caused by FRET. In FLIM studies of oligomers with different proportions of donors and acceptors, the donor lifetimes may be obtained by fitting the temporally resolved fluorescence decay data with a predetermined number of exponential decay curves. However, this requires knowledge of the number and the relative arrangement of the fluorescent proteins in the sample, which is precisely the goal of FRET spectrometry, thus creating a conundrum that has prevented users of FLIM instruments from performing FRET spectrometry. Here, we describe an attempt to implement FRET spectrometry on temporally resolved fluorescence microscopes by using an integration-based method of computing the FRET efficiency from fluorescence decay curves. This method, which we dubbed time-integrated FRET (or tiFRET), was tested on oligomeric fluorescent protein constructs expressed in the cytoplasm of living cells. The present results show that tiFRET is a promising way of implementing FRET spectrometry and suggest potential instrument adjustments for increasing accuracy and resolution in this kind of study.

Impact of Surface Ligand on the Biocompatibility of InP/ZnS Quantum Dots with Platelets

InP/ZnS quantum dots (QDs) have received a large focus in recent years as a safer alternative to heavy metal-based QDs. Given their intrinsic fluorescent imaging capabilities, these QDs can be potentially relevant for in vivo platelet imaging. The InP/ZnS QDs are synthesized and their biocompatibility investigated through the use of different phase transfer agents. Analysis of platelet function indicates that platelet-QD interaction can occur at all concentrations and for all QD permutations tested. However, as the QD concentration increases, platelet aggregation is induced by QDs alone independent of natural platelet agonists. This study helps to define a range of concentrations and coatings (thioglycolic acid and penicillamine) that are biocompatible with platelet function. With this information, the platelet-QD interaction can be identified using multiple methods. Fluorescent lifetime imaging microscopy (FLIM) and confocal studies have shown QDs localize on the surface of the platelet toward the center while showing evidence of energy transfer within the QD population. It is believed that these findings are an important stepping point for the development of fluorescent probes for platelet imaging.

Improving fluorescence lifetime imaging microscopy phasor accuracy using convolutional neural networks

Introduction: Although a powerful biological imaging technique, fluorescence lifetime imaging microscopy (FLIM) faces challenges such as a slow acquisition rate, a low signal-to-noise ratio (SNR), and high cost and complexity. To address the fundamental problem of low SNR in FLIM images, we demonstrate how to use pre-trained convolutional neural networks (CNNs) to reduce noise in FLIM measurements. Methods: Our approach uses pre-learned models that have been previously validated on large datasets with different distributions than the training datasets, such as sample structures, noise distributions, and microscopy modalities in fluorescence microscopy, to eliminate the need to train a neural network from scratch or to acquire a large training dataset to denoise FLIM data. In addition, we are using the pre-trained networks in the inference stage, where the computation time is in milliseconds and accuracy is better than traditional denoising methods. To separate different fluorophores in lifetime images, the denoised images are then run through an unsupervised machine learning technique named "K-means clustering". Results and Discussion: The results of the experiments carried out on in vivo mouse kidney tissue, Bovine pulmonary artery endothelial (BPAE) fixed cells that have been fluorescently labeled, and mouse kidney fixed samples that have been fluorescently labeled show that our demonstrated method can effectively remove noise from FLIM images and improve segmentation accuracy. Additionally, the performance of our method on out-of-distribution highly scattering in vivo plant samples shows that it can also improve SNR in challenging imaging conditions. Our proposed method provides a fast and accurate way to segment fluorescence lifetime images captured using any FLIM system. It is especially effective for separating fluorophores in noisy FLIM images, which is common in in vivo imaging where averaging is not applicable. Our approach significantly improves the identification of vital biologically relevant structures in biomedical imaging applications.

Probing Polarity and pH Sensitivity of Carbon Dots in Escherichia coli through Time-Resolved Fluorescence Analyses

Intracellular monitoring of pH and polarity is crucial for understanding cellular processes and functions. This study employed pH- and polarity-sensitive nanomaterials such as carbon dots (CDs) for the intracellular sensing of pH, polarity, and viscosity using integrated time-resolved fluorescence anisotropy (FA) imaging (TR-FAIM) and fluorescence lifetime (FLT) imaging microscopy (FLIM), thereby enabling comprehensive characterization. The functional groups on the surface of CDs exhibit sensitivity to changes in the microenvironment, leading to variations in fluorescence intensity (FI) and FLT according to pH and polarity. The FLT of CDs in aqueous solution changed gradually from 6.38 ± 0.05 ns to 8.03 ± 0.21 ns within a pH range of 2-8. Interestingly, a complex relationship of FI and FLT was observed during measurements of CDs with decreasing polarity. However, the FA and rotational correlation time (θ) increased from 0.062 ± 0.019 to 0.112 ± 0.023 and from 0.49 ± 0.03 ns to 2.01 ± 0.27 ns, respectively. This increase in FA and θ was attributed to the higher viscosity accompanying the decrease in polarity. Furthermore, CDs were found to bind to three locations in Escherichia coli: the cell wall, inner membrane, and cytoplasm, enabling intracellular characterization using FI and FA decay imaging. FLT provided insights into cytoplasmic pH (7.67 ± 0.48), which agreed with previous works, as well as the decrease in polarity in the cell wall and inner membrane. The CD aggregation was suspected in certain areas based on FA, and the θ provided information on cytoplasmic heterogeneity due to the aggregation and/or interactions with biomolecules. The combined TR-FAIM/FLIM system allowed for simultaneous monitoring of pH and polarity changes through FLIM and viscosity variations through TR-FAIM.

Identifying lipid particle sub-types in live Caenorhabditis elegans with two-photon fluorescence lifetime imaging

Fat metabolism is an important modifier of aging and longevity in Caenorhabditis elegans. Given the anatomy and hermaphroditic nature of C. elegans, a major challenge is to distinguish fats that serve the energetic needs of the parent from those that are allocated to the progeny. Broadband coherent anti-Stokes Raman scattering (BCARS) microscopy has revealed that the composition and dynamics of lipid particles are heterogeneous both within and between different tissues of this organism. Using BCARS, we have previously succeeded in distinguishing lipid-rich particles that serve as energetic reservoirs of the parent from those that are destined for the progeny. While BCARS microscopy produces high-resolution images with very high information content, it is not yet a widely available platform. Here we report a new approach combining the lipophilic vital dye Nile Red and two-photon fluorescence lifetime imaging microscopy (2p-FLIM) for the in vivo discrimination of lipid particle sub-types. While it is widely accepted that Nile Red staining yields unreliable results for detecting lipid structures in live C. elegans due to strong interference of autofluorescence and non-specific staining signals, our results show that simple FLIM phasor analysis can effectively separate those signals and is capable of differentiating the non-polar lipid-dominant (lipid-storage), polar lipid-dominant (yolk lipoprotein) particles, and the intermediates that have been observed using BCARS microscopy. An advantage of this approach is that images can be acquired using common, commercially available 2p-FLIM systems within about 10% of the time required to generate a BCARS image. Our work provides a novel, broadly accessible approach for analyzing lipid-containing structures in a complex, live whole organism context.

Visualization of Polymer Dynamics in Cellulose Nanocrystal Matrices Using Fluorescence Lifetime Measurements

Polymer nanocomposites containing self-assembled cellulose nanocrystals (CNCs) are ideal for advanced applications requiring both strength and toughness as the helicoidal structure of the CNCs deflects crack propagation and the polymer matrix dissipates impact energy. However, any adsorbed water layer surrounding the CNCs may compromise the interfacial adhesion between the polymer matrix and the CNCs, thus impacting stress transfer at that interface. Therefore, it is critical to study the role of water at the interface in connecting the polymer dynamics and the resulting mechanical performance of the nanocomposite. Here, we explore the effect of polymer confinement and water content on polymer dynamics in CNC nanocomposites by covalently attaching a fluorogenic water-sensitive dye to poly(diethylene glycol methyl ether methacrylate) (PMEO₂MA), to provide insights into the observed mechanical performance. Utilizing fluorescence lifetime imaging microscopy (FLIM), the lifetime of dye fluorescence decay was measured to probe the polymer chain dynamics of PMEO₂MA in CNC nanocomposite films. The PMEO₂MA chains experienced distinct regions of differing dynamics within Bouligand structures. A correlation was observed between the average fluorescence lifetime and the mechanical performance of CNC films, indicating that polymer chains with high mobility improved the strain and toughness. These studies demonstrated FLIM as a method to investigate polymer dynamics at the nanosecond timescale that can readily be applied to other composite systems.

Phasor Histone FLIM-FRET Microscopy Maps Nuclear-Wide Nanoscale Chromatin Architecture With Respect to Genetically Induced DNA Double-Strand Breaks

A DNA double-strand break (DSB) takes place in the context of chromatin, and there is increasing evidence for chromatin structure to play a functional role in DSB signaling and repair. Thus, there is an emerging need for quantitative microscopy methods that can directly measure chromatin network architecture and detect changes in this structural framework upon DSB induction within an intact nucleus. To address this demand, here we present the phasor approach to fluorescence lifetime imaging microscopy (FLIM) of Förster resonance energy transfer (FRET) between fluorescently labeled histones in the DSB inducible via AsiSI cell system (DIvA), which has sufficient spatial resolution to map nuclear-wide chromatin compaction at the level of nucleosome proximity with respect to multiple site-specific DSBs. We also demonstrate that when phasor histone FLIM-FRET is coupled with immunofluorescence, this technology has the unique advantage of enabling exploration of any heterogeneity that exists in chromatin structure at the spatially distinct and genetically induced DSBs.

Revealing Inflammatory Indications Induced by Titanium Alloy Wear Debris in Periprosthetic Tissue by Label-Free Correlative High-Resolution Ion, Electron and Optical Microspectroscopy

The metallic-associated adverse local tissue reactions (ALTR) and events accompanying worn-broken implant materials are still poorly understood on the subcellular and molecular level. Current immunohistochemical techniques lack spatial resolution and chemical sensitivity to investigate causal relations between material and biological response on submicron and even nanoscale. In our study, new insights of titanium alloy debris-tissue interaction were revealed by the implementation of label-free high-resolution correlative microscopy approaches. We have successfully characterized its chemical and biological impact on the periprosthetic tissue obtained at revision surgery of a fractured titanium-alloy modular neck of a patient with hip osteoarthritis. We applied a combination of photon, electron and ion beam micro-spectroscopy techniques, including hybrid optical fluorescence and reflectance micro-spectroscopy, scanning electron microscopy (SEM), Energy-dispersive X-ray Spectroscopy (EDS), helium ion microscopy (HIM) and micro-particle-induced X-ray emission (micro-PIXE). Micron-sized wear debris were found as the main cause of the tissue oxidative stress exhibited through lipopigments accumulation in the nearby lysosome. This may explain the indications of chronic inflammation from prior histologic examination. Furthermore, insights on extensive fretting and corrosion of the debris on nm scale and a quantitative measure of significant Al and V release into the tissue together with hydroxyapatite-like layer formation particularly bound to the regions with the highest Al content were revealed. The functional and structural information obtained at molecular and subcellular level contributes to a better understanding of the macroscopic inflammatory processes observed in the tissue level. The established label-free correlative microscopy approach can efficiently be adopted to study any other clinical cases related to ALTR.

Model Lipid Membranes Assembled from Natural Plant Thylakoids into 2D Microarray Patterns as a Platform to Assess the Organization and Photophysics of Light-Harvesting Proteins

Natural photosynthetic "thylakoid" membranes found in green plants contain a large network of light-harvesting (LH) protein complexes. Rearrangement of this photosynthetic machinery, laterally within stacked membranes called "grana", alters protein-protein interactions leading to changes in the energy balance within the system. Preparation of an experimentally accessible model system that allows the detailed investigation of these complex interactions can be achieved by interfacing thylakoid membranes and synthetic lipids into a template comprised of polymerized lipids in a 2D microarray pattern on glass surfaces. This paper uses this system to interrogate the behavior of LH proteins at the micro- and nanoscale and assesses the efficacy of this model. A combination of fluorescence lifetime imaging and atomic force microscopy reveals the differences in photophysical state and lateral organization between native thylakoid and hybrid membranes, the mechanism of LH protein incorporation into the developing hybrid membranes, and the nanoscale structure of the system. The resulting model system within each corral is a high-quality supported lipid bilayer that incorporates laterally mobile LH proteins. Photosynthetic activity is assessed in the hybrid membranes versus proteoliposomes, revealing that commonly used photochemical assays to test the electron transfer activity of photosystem II may actually produce false-positive results.

Building Accurate Intracellular Polarity Maps through Multiparametric Microscopy

The precise knowledge of intracellular polarity, a physiological parameter that involves complex and intertwined intracellular mechanisms, may be relevant in the study of important diseases like cancer or Alzheimer’s. In this technical note, we illustrate our recently developed, accurate method for obtaining intracellular polarity maps employing potent fluorescence microscopy techniques. Our method is based on the selection of appropriate luminescent probes, in which several emission properties vary with microenvironment polarity, specifically spectral shifts and luminescence lifetime. A multilinear calibration is performed, correlating polarity vs. spectral shift vs. luminescence lifetime, to generate a powerful and error-free 3D space for reliable interpolation of microscopy data. Multidimensional luminescence microscopy is then used to obtain simultaneously spectral shift and luminescence lifetime images, which are then interpolated in the 3D calibration space, resulting in accurate, quantitative polarity maps.

Yeast Sphingolipid-Enriched Domains and Membrane Compartments in the Absence of Mannosyldiinositolphosphorylceramide

The relevance of mannosyldiinositolphosphorylceramide [M(IP)₂C] synthesis, the terminal complex sphingolipid class in the yeast Saccharomyces cerevisiae, for the lateral organization of the plasma membrane, and in particular for sphingolipid-enriched gel domains, was investigated by fluorescence spectroscopy and microscopy. We also addressed how changing the complex sphingolipid profile in the plasma membrane could influence the membrane compartments (MC) containing either the arginine/ H⁺ symporter Can1p (MCC) or the proton ATPase Pma1p (MCP). To achieve these goals, wild-type (wt) and ipt1Δ cells, which are unable to synthesize M(IP)₂C accumulating mannosylinositolphosphorylceramide (MIPC), were compared. Living cells, isolated plasma membrane and giant unilamellar vesicles reconstituted from plasma membrane lipids were labelled with various fluorescent membrane probes that report the presence and organization of distinct lipid domains, global order, and dielectric properties. Can1p and Pma1p were tagged with GFP and mRFP, respectively, in both yeast strains, to evaluate their lateral organization using confocal fluorescence intensity and fluorescence lifetime imaging. The results show that IPT1 deletion strongly affects the rigidity of gel domains but not their relative abundance, whereas no significant alterations could be perceived in ergosterol-enriched domains. Moreover, in these cells lacking M(IP)₂C, a clear alteration in Pma1p membrane distribution, but no significant changes in Can1p distribution, were observed. Thus, this work reinforces the notion that sphingolipid-enriched domains distinct from ergosterol-enriched regions are present in the S. cerevisiae plasma membrane and suggests that M(IP)₂C is important for a proper hydrophobic chain packing of sphingolipids in the gel domains of wt cells. Furthermore, our results strongly support the involvement of sphingolipid domains in the formation and stability of the MCP, possibly being enriched in this compartment.

Serine Phosphorylation of L-Selectin Regulates ERM Binding, Clustering, and Monocyte Protrusion in Transendothelial Migration

The migration of circulating leukocytes toward damaged tissue is absolutely fundamental to the inflammatory response, and transendothelial migration (TEM) describes the first cellular barrier that is breached in this process. Human CD14⁺ inflammatory monocytes express L-selectin, bestowing a non-canonical role in invasion during TEM. In vivo evidence supports a role for L-selectin in regulating TEM and chemotaxis, but the intracellular mechanism is poorly understood. The ezrin-radixin-moesin (ERM) proteins anchor transmembrane proteins to the cortical actin-based cytoskeleton and additionally act as signaling adaptors. During TEM, the L-selectin tail within transmigrating pseudopods interacts first with ezrin to transduce signals for protrusion, followed by moesin to drive ectodomain shedding of L-selectin to limit protrusion. Collectively, interaction of L-selectin with ezrin and moesin fine-tunes monocyte protrusive behavior in TEM. Using FLIM/FRET approaches, we show that ERM binding is absolutely required for outside-in L-selectin clustering. The cytoplasmic tail of human L-selectin contains two serine (S) residues at positions 364 and 367, and here we show that they play divergent roles in regulating ERM binding. Phospho-S364 blocks direct interaction with ERM, whereas molecular modeling suggests phospho-S367 likely drives desorption of the L-selectin tail from the inner leaflet of the plasma membrane to potentiate ERM binding. Serine-to-alanine mutagenesis of S367, but not S364, significantly reduced monocyte protrusive behavior in TEM under flow conditions. Our data propose a model whereby L-selectin tail desorption from the inner leaflet of the plasma membrane and ERM binding are two separable steps that collectively regulate protrusive behavior in TEM.

Leaves of isoprene-emitting tobacco plants maintain PSII stability at high temperatures

At high temperatures, isoprene-emitting plants display a higher photosynthetic rate and a lower nonphotochemical quenching (NPQ) compared with nonemitting plants. The mechanism of this phenomenon, which may be very important under current climate warming, is still elusive. NPQ was dissected into its components, and chlorophyll fluorescence lifetime imaging microscopy (FLIM) was used to analyse the dynamics of excited chlorophyll relaxation in isoprene-emitting and nonemitting plants. Thylakoid membrane stiffness was also measured using atomic force microscope (AFM) to identify a possible mode of action of isoprene in improving photochemical efficiency and photosynthetic stability. We show that, when compared with nonemitters, isoprene-emitting tobacco plants exposed at high temperatures display a reduced increase of the NPQ energy-dependent component (qE) and stable (1) chlorophyll fluorescence lifetime; (2) amplitude of the fluorescence decay components; and (3) thylakoid membrane stiffness. Our study shows for the first time that isoprene maintains PSII stability at high temperatures by preventing the modifications of the surrounding environment, namely providing a more steady and homogeneous distribution of the light-absorbing centres and a stable thylakoid membrane stiffness. Isoprene photoprotects leaves with a mechanism alternative to NPQ, enabling plants to maintain a high photosynthetic rate at rising temperatures.

Tracking Functional Tumor Cell Subpopulations of Malignant Glioma by Phasor Fluorescence Lifetime Imaging Microscopy of NADH

Intra-tumoral heterogeneity is associated with therapeutic resistance of cancer and there exists a need to non-invasively identify functional tumor subpopulations responsible for tumor recurrence. Reduced nicotinamide adenine dinucleotide (NADH) is a metabolic coenzyme essential in cellular respiration. Fluorescence lifetime imaging microscopy (FLIM) of NADH has been demonstrated to be a powerful label-free indicator for inferring metabolic states of living cells. Using FLIM, we identified a significant shift towards longer NADH fluorescence lifetimes, suggesting an increase in the fraction of protein-bound NADH, in the invasive stem-like tumor-initiating cell (STIC) subpopulation relative to the tumor mass-forming cell (TMC) subpopulation of malignant gliomas. By applying our previously studied model to transition glioma from a majority of STIC to a majority of TMC in serum-adherent culture conditions following serial passages, we compared changes in NADH states, cellular respirations (oxidative phosphorylation and glycolysis), EGFR expression, and cell-growth speed over passages. We identified a significant positive correlation between free-NADH fraction and cell growth, which was related to an increase of TMC fraction. In comparison, the increase of EGFR and cellular respirations preceded all these changes. In conclusion, FLIM of NADH provides a non-invasive method to monitor the dynamics of tumor heterogeneity before and after treatment.

Enhancing Quantum Dots for Bioimaging using Advanced Surface Chemistry and Advanced Optical Microscopy: Application to Silicon Quantum Dots (SiQDs)

Fluorescence lifetime imaging microscopy is successfully demonstrated in both one- and two-photon cases with surface modified, nanocrystalline silicon quantum dots in the context of bioimaging. The technique is further demonstrated in combination with Förster resonance energy transfer studies where the color of the nanoparticles is tuned by using organic dye acceptors directly conjugated onto the nanoparticle surface.

Fluorescence lifetime imaging and FRET-induced intracellular redistribution of Tat-conjugated quantum dot nanoparticles through interaction with a phthalocyanine photosensitiser

The interaction of Tat-conjugated PEGylated CdSe/ZnS quantum dots (QD) with the amphiphilic disulfonated aluminium phthalocyanine photosensitiser is investigated in aqueous solution and in a human breast cancer cell line. In aqueous solution, the QDs and phthalocyanine form stable nanocomposites. Using steady-state and time-resolved fluorescence measurements combined with singlet oxygen detection, efficient Förster resonance energy transfer (FRET) is observed with the QDs acting as donors, and the phthalocyanine photosensitiser, which mediates production of singlet oxygen, as acceptors. In cells, the Tat-conjugated QDs localise in lysosomes and the QD fluorescence lifetimes are close to values observed in aqueous solution. Strong FRET-induced quenching of the QD lifetime is observed in cells incubated with the nanocomposites using fluorescence lifetime imaging microscopy (FLIM). Using excitation of the QDs at wavelengths where phthalocyanine absorption is negligible, FRET-induced release of QDs from endo/lysosomes is confirmed using confocal imaging and FLIM, which is attributed to photooxidative damage to the endo/lysosomal membranes mediated by the phthalocyanine acceptor.

Multimode Optical Imaging for Translational Chemotherapy: In Vivo Tumor Detection and Delineation by Targeted Gallium Corroles

We report the feasibility of tumor detection and delineation in vivo using multimode optical imaging of targeted gallium corrole (HerGa). HerGa is highly effective for targeted HER2+ tumor elimination in vivo, and it emits intense fluorescence. These unique characteristics of HerGa prompted us to investigate the potential of HerGa for tumor detection and delineation, by performing multimode optical imaging ex vivo and in vivo; the imaging modes included fluorescence intensity, spectral (including ratiometric), lifetime, and two-photon excited fluorescence, using our custom-built imaging system. While fluorescence intensity imaging provided information about tumor targeting capacity and tumor retention of HerGa, ratiometric spectral imaging offered more quantitative and specific information about HerGa location and accumulation. Most importantly, the fluorescence lifetime imaging of HerGa allowed us to discriminate between tumor and non-tumor regions by fluorescence lifetime differences. Finally, two-photon excited fluorescence images provided highly resolved and thus topologically detailed information around the tumor regions where HerGa accumulates. Taken together, the results shown in this report suggest the feasibility of tumor detection and delineation by multimode optical imaging of HerGa, and fluorescent chemotherapy agents in general. Specifically, the multimode optical imaging can offer complementary and even synergetic information simultaneously in the tumor detection and delineation by HerGa, thus enhancing contrast.