Shifts in the fluorescence lifetime of EGFP during bacterial phagocytosis measured by phase-sensitive flow cytometry

No Evidence of Neutrophil Response Modulation in Goats after Immunization against Paratuberculosis with a Heat-Inactivated Vaccine

Neutrophils are believed to play a role in the initial stages of paratuberculosis, and it has recently been demonstrated that vaccination can modulate their function via priming or through epigenetic and metabolic reprogramming (training). Modulation of the neutrophil response against Mycobacterium avium subspecies paratuberculosis (Map) through vaccination has been demonstrated in a rabbit model but not in ruminants. Therefore, in the present work, the effect of vaccination on the response of caprine neutrophils against Map was studied. Neutrophils were isolated from non-vaccinated (n = 7) and Gudair®-vaccinated goat kids (n = 7), before vaccination and 30 days post-vaccination. Then, several neutrophil functions were quantified ex vivo: cell-free and anchored neutrophil extracellular trap (NET) release, phagocytosis, and the differential expression of several cytokines and TLR2. The induction of cell-free NETosis and TLR2 expression by Map is reported for the first time. However, vaccination showed no significant effect on any of the functions studied. This suggests that the protection conferred by Gudair® vaccination is based on mechanisms that are independent of the neutrophil function modulation. Further research into the impact of alternative vaccination strategies or the paratuberculosis infection stage on ruminant neutrophil function could provide valuable insights into its role in paratuberculosis.

Near Infrared Autofluorescence Lifetime Imaging of Human Retinal Pigment Epithelium Using Adaptive Optics Scanning Light Ophthalmoscopy

Purpose

To demonstrate the first near-infrared adaptive optics fluorescence lifetime imaging ophthalmoscopy (NIR-AOFLIO) measurements in vivo of the human retinal pigment epithelial (RPE) cellular mosaic and to visualize lifetime changes at different retinal eccentricities.

Methods

NIR reflectance and autofluorescence were captured using a custom adaptive optics scanning light ophthalmoscope in 10 healthy subjects (23-64 years old) at seven eccentricities and in two eyes with retinal abnormalities. Repeatability was assessed across two visits up to 8 weeks apart. Endogenous retinal fluorophores and hydrophobic whole retinal extracts of Abca4-/- pigmented and albino mice were imaged to probe the fluorescence origin of NIR-AOFLIO.

Results

The RPE mosaic was resolved at all locations in five of seven younger subjects (<35 years old). The mean lifetime across near-peripheral regions (8° and 12°) was longer compared to near-foveal regions (0° and 2°). Repeatability across two visits showed moderate to excellent correlation (intraclass correlation: 0.88 [τm], 0.75 [τ1], 0.65 [τ2], 0.98 [a1]). The mean lifetime across drusen-containing eyes was longer than in age-matched healthy eyes. Fluorescence was observed in only the extracts from pigmented Abca4-/- mouse.

Conclusions

NIR-AOFLIO was repeatable and allowed visualization of the RPE cellular mosaic. An observed signal in only the pigmented mouse extract infers the fluorescence signal originates predominantly from melanin. Variations observed across the retina with intermediate age-related macular degeneration suggest NIR-AOFLIO may act as a functional measure of a biomarker for in vivo monitoring of early alterations in retinal health.

Fluorescence Lifetime Measurements and Analyses: Protocols Using Flow Cytometry and High-Throughput Microscopy

This chapter focuses on applications and protocols that involve the measurement of the fluorescence lifetime as an informative cytometric parameter. The timing of fluorescence decay has been well-studied for cell counting, sorting, and imaging. Therefore, provided herein is an overview of the techniques used, how they enhance cytometry protocols, and the modern techniques used for lifetime analysis. The background and theory behind fluorescence decay kinetic measurements in cells is first discussed followed by the history of the development of time-resolved flow cytometry. These sections are followed by a review of applications that benefit from the quantitative nature of fluorescence lifetimes as a photophysical trait. Lastly, perspectives on the modern ways in which the fluorescence lifetime is scanned at high throughputs which include high-speed microscopy and machine learning are provided.

Genetically encoded fluorescence lifetime biosensors: overview, advances, and opportunities

Genetically encoded biosensors based on fluorescent proteins (FPs) are powerful tools for tracking analytes and cellular events with high spatial and temporal resolution in living cells and organisms. Compared with intensiometric readout and ratiometric readout, fluorescence lifetime readout provides absolute measurements, independent of the biosensor expression level and instruments. Thus, genetically encoded fluorescence lifetime biosensors play a vital role in facilitating accurate quantitative assessments within intricate biological systems. In this review, we first provide a concise description of the categorization and working mechanism of genetically encoded fluorescence lifetime biosensors. Subsequently, we elaborate on the combination of the fluorescence lifetime imaging technique and lifetime analysis methods with fluorescence lifetime biosensors, followed by their application in monitoring the dynamics of environment parameters, analytes and cellular events. Finally, we discuss worthwhile considerations for the design, optimization and development of fluorescence lifetime-based biosensors from three representative cases.

Direct Modulators of K-Ras-Membrane Interactions

Protein-membrane interactions (PMIs) are ubiquitous in cellular signaling. Initial steps of signal transduction cascades often rely on transient and dynamic interactions with the inner plasma membrane leaflet to populate and regulate signaling hotspots. Methods to target and modulate these interactions could yield attractive tool compounds and drug candidates. Here, we demonstrate that the conjugation of a medium-chain lipid tail to the covalent K-Ras(G12C) binder MRTX849 at a solvent-exposed site enables such direct modulation of PMIs. The conjugated lipid tail interacts with the tethered membrane and changes the relative membrane orientation and conformation of K-Ras(G12C), as shown by molecular dynamics (MD) simulation-supported NMR studies. In cells, this PMI modulation restricts the lateral mobility of K-Ras(G12C) and disrupts nanoclusters. The described strategy could be broadly applicable to selectively modulate transient PMIs.

Comparison of fluorescence lifetime and multispectral imaging for quantitative multiplexing in biological tissue

Fluorescence lifetime (FLT) multiplexing and multispectral imaging (MSI) are both frequently employed for in vitro and ex vivo biological studies. In vivo applications of MSI for deep seated fluorophores require consideration of diffusive light propagation in biological tissue. We have previously shown that a well-known redshift of fluorescence spectra in diffusive medium induces a fluorophore cross-talk, which cannot be accounted for even with known optical properties of the medium. In contrast, FLT measurements remain largely unaffected by light propagation in tissue, enabling zero cross-talk and accurate relative quantification. While a fully quantitative estimation of fluorophore concentrations requires depth resolved tomographic imaging, this is often not possible due to the difficulty of estimating tissue optical properties and modelling light propagation in complex tissue geometries. Here, we experimentally investigate the performance of planar (non-tomographic) MSI and FLT multiplexing for the quantitative recovery of multiple near-infrared fluorophores embedded in 4-8 mm thick tissue. We show that FLT multiplexing provides a superior quantification accuracy (error < 10%) compared to MSI (error = 20-107%) in tissue. The error rates for MSI increased with tissue thickness and can be directly attributed to the spectral redshift induced cross-talk between emission spectra. Our data indicate that planar FLT multiplexing can provide high quantification accuracy in thick biological tissue without a need for optical property estimation, thereby offering an important validation tool for rapid quantification of fluorophore concentrations in bulk tissue.

Toward absolute viability measurements for bacteria

We aim to develop a quantitative viability method that distinguishes individual quiescent from dead cells and is measured in time (ns) as a referenceable, comparable quantity. We demonstrate that fluorescence lifetime imaging of an anionic, fluorescent membrane voltage probe fulfills these requirements for Streptococcus mutans. A random forest machine-learning model assesses whether individual S. mutans can be correctly classified into their original populations: stationary phase (quiescent), heat killed and inactivated via chemical fixation. We compare the results to intensity using three models: lifetime variables (τ₁ , τ₂ and p₁ ), phasor variables (G, S) or all five variables, with the five variable models having the most accurate classification. This initial work affirms the potential for using fluorescence lifetime of a membrane voltage probe as a viability marker for quiescent bacteria, and future efforts on other bacterial species and fluorophores will help refine this approach.

A Review of New High-Throughput Methods Designed for Fluorescence Lifetime Sensing From Cells and Tissues

Though much of the interest in fluorescence in the past has been on measuring spectral qualities such as wavelength and intensity, there are two other highly useful intrinsic properties of fluorescence: lifetime (or decay) and anisotropy (or polarization). Each has its own set of unique advantages, limitations, and challenges in detection when it comes to use in biological studies. This review will focus on the property of fluorescence lifetime, providing a brief background on instrumentation and theory, and examine the recent advancements and applications of measuring lifetime in the fields of high-throughput fluorescence lifetime imaging microscopy (HT-FLIM) and time-resolved flow cytometry (TRFC). In addition, the crossover of these two methods and their outlooks will be discussed.

High-throughput analysis of T cell-monocyte interaction in human tuberculosis

The lack of efficient tools for identifying immunological correlates of tuberculosis (TB) protection or risk of disease progression impedes the development of improved control strategies. To more clearly understand the host response in TB, we recently established an imaging flow cytometer‐based in‐vitro assay, which assesses multiple aspects of T cell–monocyte interaction. Here, we extended our previous work and characterized communication between T cells and monocytes using clinical samples from individuals with different TB infection status and healthy controls from a TB endemic setting. To identify T cell–monocyte conjugates, peripheral blood mononuclear cells (PBMC) were stimulated with ds‐Red‐expressing Mycobacterium bovis bacille Calmette–Guérin or 6‐kDa early secreted antigenic target (ESAT 6) peptides for 6 h, and analyzed by imaging flow cytometer (IFC). We then enumerated T cell–monocyte conjugates using polarization of T cell receptor (TCR) and F‐actin as markers for synapse formation, and nuclear factor kappa B (NF‐κB) nuclear translocation in the T cells. We observed a reduced frequency of T cell–monocyte conjugates in cells from patients with active pulmonary tuberculosis (pTB) compared to latent TB‐infected (LTBI) and healthy controls. When we monitored NF‐κB nuclear translocation in T cells interacting with monocytes, the proportion of responding cells was significantly higher in active pTB compared with LTBI and controls. Overall, these data underscore the need to consider multiple immunological parameters against TB, where IFC could be a valuable tool.

Enrichment of rare events using a multi-parameter high throughput microfluidic droplet sorter

High information content analysis, enrichment, and selection of rare events from a large population are of great importance in biological and biomedical research. The fluorescence lifetime of a fluorophore, a photophysical property which is independent of and complementary to fluorescence intensity, has been incorporated into various imaging and sensing techniques through microscopy, flow cytometry and droplet microfluidics. However, the throughput of fluorescence lifetime activated droplet sorting is orders of magnitude lower than that of fluorescence activated cell sorting, making it unattractive for applications such as directed evolution of enzymes, despite its highly effective compartmentalization of library members. We developed a microfluidic sorter capable of selecting fluorophores based on fluorescence lifetime and brightness at two excitation and emission colors at a maximum droplet rate of 2.5 kHz. We also present a novel selection strategy for efficiently analyzing and/or enriching rare fluorescent members from a large population which capitalizes on the Poisson distribution of analyte encapsulation into droplets. The effectiveness of the droplet sorter and the new selection strategy are demonstrated by enriching rare populations from a ∼108-member site-directed mutagenesis library of fluorescent proteins expressed in bacteria. This selection strategy can in principle be employed on many droplet sorting platforms, and thus can potentially impact broad areas of science where analysis and enrichment of rare events is needed.

Fluorescence lifetime imaging reveals heterogeneous functional distribution of eGFP expressed in Xenopus oocytes

The oocytes from Xenopus laevis are well known for their polarity, presenting a distinct animal and vegetal pole. Other heterogeneities are less known. To study the heterogeneity of the Xenopus oocyte, we expressed eGFP and analyzed the protein distribution with fluorescence lifetime microscopy. The vegetal pole exhibited higher levels of fluorescence, than the animal pole. However, the fluorescence lifetimes between the two areas were indistinguishable, suggesting similar environments. In contrast, we observed a substantial and gradual decrease in the fluorescence lifetime from 2.9 ns to 2.6 ns as slices approached the periphery. This has an important implication for future oocyte studies as it demonstrates the environment inside the oocyte is not uniform and might affect the fluorescence intensity. As a result, it cannot be assumed that the observed fluorescence intensity reflects the expression of the proteins but might reflect the environment within the oocyte.

Green fluorescent protein emission obscures metabolic fluorescent lifetime imaging of NAD(P)H

Autofluorescence of endogenous molecules can provide valuable insights in both basic research and clinical applications. One such technique is fluorescence lifetime imaging (FLIM) of NAD(P)H, which serves as a correlate of glycolysis and electron transport chain rates in metabolically active tissue. A powerful advantage of NAD(P)H-FLIM is the ability to measure cell-specific metabolism within heterogeneous tissues. Cell-type specific identification is most commonly achieved with directed green fluorescent protein (GFP) expression. However, we demonstrate that NAD(P)H-FLIM should not be analyzed in GFP-expressing cells, as GFP molecules themselves emit photons in the blue spectrum with short fluorescence lifetimes when imaged using two-photon excitation at 750 nm. This is substantially different from the reported GFP emission wavelength and lifetime after two-photon excitation at 910 nm. These blue GFP photons are indistinguishable from free NAD(P)H by both emission spectra and fluorescence lifetime. Therefore, NAD(P)H-FLIM in GFP-expressing cells will lead to incorrect interpretations of metabolic rates, and thus, these techniques should not be combined.

Quantifying Acute Fuel and Respiration Dependent pH Homeostasis in Live Cells Using the mCherryTYG Mutant as a Fluorescence Lifetime Sensor

Intracellular pH plays a key role in physiology, and its measurement in living specimens remains a crucial task in biology. Fluorescent protein-based pH sensors have gained widespread use, but there is limited spectral diversity for multicolor detection, and it remains a challenge to measure absolute pH values. Here we demonstrate that mCherryTYG is an excellent fluorescence lifetime pH sensor that significantly expands the modalities available for pH quantification in live cells. We first report the 1.09 Å X-ray crystal structure of mCherryTYG, exhibiting a fully matured chromophore. We next determine that it has an extraordinarily large dynamic range with a 2 ns lifetime change from pH 5.5 to 9.0. Critically, we find that the sensor maintains a p Ka of 6.8 independent of environment, whether as the purified protein in solution or expressed in live cells. Furthermore, the lifetime measurements are robustly independent of total fluorescence intensity and scatter. We demonstrate that mCherryTYG is a highly effective sensor using time-resolved fluorescence spectroscopy on live-cell suspensions, which has been previously overlooked as an easily accessible approach for quantifying intracellular pH. As a red fluorescent sensor, we also demonstrate that mCherryTYG is spectrally compatible with the ATeam sensor and EGFP for simultaneous dual-color measurements of intracellular pH, ATP, and extracellular pH. In a proof-of-concept, we quantify acute respiration-dependent pH homeostasis that exhibits a stoichiometric relationship with the ATP-generating capacity of the carbon fuel choice in E. coli. Broadly speaking, our work presents a previously unemployed methodology that will greatly facilitate continuous pH quantification.

Multifunctional CdSNPs@ZIF-8: Potential Antibacterial Agent against GFP-Expressing Escherichia coli and Staphylococcus aureus and Efficient Photocatalyst for Degradation of Methylene Blue

Multifunctional novel core-shell composites, CdSNPs@ZIF-8, have been synthesized by in situ encapsulation of different amounts of CdSNPs (150, 300, and 500 μL suspension of CdSNPs in methanol) in ZIF-8 at room temperature. These composites have been characterized by powder X-ray diffraction, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM), and diffuse reflectance spectroscopy techniques and Brunauer-Emmett-Teller surface analysis. XPS and HRTEM indicate the encapsulation of CdSNPs within ZIF-8 crystal without disturbing the crystal order of ZIF-8. The average size of embedded CdSNPs (determined by the particle size distribution from HRTEM) is found to be 16.34 nm. CdSNPs@ZIF-8 showed potential to be used as an antibacterial agent against the broad spectrum of bacterial strains such as Gram-positive Staphylococcus aureus and Gram-negative green fluorescent protein-expressing Escherichia coli in aqueous medium, as evident by various biophysical experiments, viz., 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, optical density and fluorescence spectroscopic studies, fluorescence and optical microscopic image analysis, disk diffusion assay, field emission scanning electron microscopy, and flow cytometry for reactive oxygen species induction assay. Further, the composite has been used as an efficient photocatalyst for the degradation of organic pollutants, such as methylene blue dye, in aqueous medium and found that the core-shell composite, CdSNPs@ZIF-8 (150 μL) (abbreviated as NC-1) (5 mg), exhibited higher photocatalytic activity (≈1.8 times) than CdSNPs for degradation of 90% of methylene blue (10 mL of 10 ppm) at pH ≥ 7 due to the synergetic effect. Therefore, in situ encapsulation of CdSNPs in ZIF-8 provides an easy executable measure for purification of wastewater effluents for the effective photocatalytic degradation of organic pollutants as well as to remove the bacterial contamination under sunlight.

Characterization of the LECT2 gene and its protective effects against microbial infection via large lymphocytes in Lampetra japonica

Leukocyte cell-derived chemotaxin 2 (LECT2) is a multifunctional protein of the innate immune system that defends against bacterial infections and chemotactic activity. However, its precise function in lamprey remains unclear. In this study, a novel LECT2 gene was first cloned from Lampetra japonica. The full-length cDNA sequence of L-LECT2 consists of a 606-bp ORF encoding a protein of 201 amino acid residues. L-LECT2 has greater than 50% sequence identity with its homologs in jawed vertebrates. FACS and immunohistochemistry assays were used to determine that the L-LECT2 protein was primarily distributed in the intestines and supraneural body tissues of lamprey, also marginally detectable in leukocytes. However, the expression of L-LECT2 was differentially upregulated in the intestines and heart after treatment with LPS. The recombinant L-LECT2 resulted in significant promoting migration of the leukocytes in vitro. Our data demonstrate that L-LECT2 treatment could enhance phagocytosis in lamprey large lymphocytes. Thus, our results suggest that LECT2 can modulate the host defense in lamprey and mediate antibacterial protection against E.coli through large lymphocytes.