Time-resolved optical imaging provides a molecular snapshot of altered metabolic function in living human cancer cell models

Loss of the mitochondrial protein SPD-3 elevates PLK-1 levels and dysregulates mitotic events

In metazoans, Polo-like kinase (PLK1) controls several mitotic events including nuclear envelope breakdown, centrosome maturation, spindle assembly and progression through mitosis. Here we show that a mutation in the mitochondria-localized protein SPD-3 affects mitotic events by inducing elevated levels of PLK-1 in early Caenorhabditis elegans embryos. SPD-3 mutant embryos contain abnormally positioned mitotic chromosomes, show a delay in anaphase onset and asymmetrically disassemble the nuclear lamina. We found that more PLK-1 accumulated on centrosomes, nuclear envelope, nucleoplasm, and chromatin before NEBD, suggesting that PLK-1 overexpression is responsible for some of the observed mitotic phenotypes. In agreement with this, the chromosome positioning defects of the spd-3(oj35) mutant could be rescued by reducing PLK-1 levels. Our data suggests that the mitochondrial SPD-3 protein affects chromosome positioning and nuclear envelope integrity by up-regulating the endogenous levels of PLK-1 during early embryogenesis in C. elegans This finding suggests a novel link between mitochondria and nuclear envelope dynamics and chromosome positioning by increasing the amount of a key mitotic regulator, PLK-1, providing a novel link between mitochondria and mitosis.

Mitotic events depend on regulation of PLK-1 levels by the mitochondrial protein SPD-3

In metazoans, Polo Kinase (Plk1) controls several mitotic events including nuclear envelope breakdown, centrosome maturation and kinetochore assembly. Here we show that mitotic events regulated by Polo Like Kinase (PLK-1) in early C. elegans embryos depend on the mitochondrial-localized protein SPD-3. spd-3 mutant one-cell embryos contain abnormally positioned mitotic chromosomes and prematurely and asymmetrically disassemble the nuclear lamina. Nuclear envelope breakdown (NEBD) in C. elegans requires direct dephosphorylation of lamin by PLK-1. In spd-3 mutants PLK-1 levels are ~6X higher in comparison to control embryos and PLK-1::GFP was highly accumulated at centrosomes, the nuclear envelope, nucleoplasm, and chromosomes prior to NEBD. Partial depletion of plk-1 in spd-3 mutant embryos rescued mitotic chromosome and spindle positioning defects indicating that these phenotypes result from higher PLK-1 levels and thus activity. Our data suggests that the mitochondrial SPD-3 protein controls NEBD and chromosome positioning by regulating the endogenous levels of PLK-1 during early embryogenesis in C. elegans . This finding suggests a novel link between mitochondria and mitotic events by controlling the amount of a key mitotic regulator, PLK-1 and thus may have further implications in the context of cancers or age-related diseases and infertility as it provides a novel link between mitochondria and mitosis.

Transient triplet differential-based photoacoustic lifetime imaging with an automatic interleaved data acquisition method for improved scanning speed and stability

Transient triplet differential (TTD) based photoacoustic lifetime (PALT) imaging provides valuable means for background-free molecular imaging and mapping of the oxygen partial pressure (pO₂) in deep tissues. However, the broad application of this method is hindered by its long scanning time, poor accuracy, and low stability. This is mainly because most PALT systems execute the three data acquisition sequences separately without automatic control and neglect the long-time fluctuation of the laser output. In this work, we have proposed a novel automatic interleaved data acquisition method for PALT. This new method not only improved the scanning efficiency but also eliminated the long-time fluctuations of laser pulse energy. Results show that this new method can significantly improve the system's stability and help reduce the scanning time. With this new method, we obtained the 3D background-free TTD images for the first time. We also observed distinct hypoxia inside the tumor due to the high metabolic rate of cancer cells, demonstrating the high reliability of our proposed method. The proposed method in this work can significantly promote the application of PALT imaging in biomedical studies.

Direct measurement of the aerotactic response in a bacterial suspension

Aerotaxis is the ability of motile cells to navigate toward oxygen. A key question is the dependence of the aerotactic velocity with the local oxygen concentration c. Here we combine simultaneous bacteria tracking and local oxygen concentration measurements using Ruthenium encapsulated in micelles to characterize the aerotactic response of Burkholderia contaminans, a motile bacterium ubiquitous in the environment. In our experiments, an oxygen gradient is produced by the bacterial respiration in a sealed glass capillary permeable to oxygen at one end, producing a bacterial band traveling toward the oxygen source. We compute the aerotactic response χ(c) both at the population scale, from the drift velocity in the bacterial band, and at the bacterial scale, from the angular modulation of the run times. Both methods are consistent with a power-law χ∝c^{-2}, in good agreement with existing models based on the biochemistry of bacterial membrane receptors.

Improving the performance of rapid lifetime determination for wide-field time-gated imaging in live cells

In biological research, rapid wide-field fluorescence lifetime imaging has become an important imaging tool. However, the biological samples with weak fluorescence signals and lower sensitivity often suffer from very low precision in lifetime determinations which restricts its widespread utilization in many bioimaging applications. To address this issue, a method is presented in this paper to substantially enhance the precision of rapid lifetime determination (RLD). It expedites the discrimination of fluorescence lifetimes, even for the weak signals coming from the cells, stained with long-lived biocompatible AIS/ZnS QDs. The proposed method works in two phases. The first phase deals with the systematic noise analysis based on the signal and contrast of the images in a time-gated imaging system, wherein acquiring the high-quality imaging data through optimization of hardware parameters improves the overall system performance. In the second phase, the chosen images are treated using total variation denoising method combined with the Max/Min filtering method for extracting the region of interest to reconstruct the intensity images for RLD. We performed several experiments on live cells to demonstrate the improvements in imaging performance by the systematic optimizations and data treatment. Obtained results demonstrated a great enhancement in signal-to-noise and contrast-to-noise ratios beside witnessing an obvious improvement in RLD for weak signals. This approach can be used not only to improve the quality of time-gated imaging data but also for efficient fluorescence lifetime imaging of live biological samples without compromising imaging speed and light exposure.

Characterization of endogenous fluorescence in nonsmall lung cancerous cells: A comparison with nonmalignant lung normal cells

Monitoring fluorescence properties of endogenous fluorophores such as nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD) in normal and cancerous cells provide substantial information noninvasively on biochemical and biophysical aspects of metabolic dysfunction of cancerous cells. Time-resolved spectral profiles and fluorescence lifetime images of NADH and FAD were obtained in human lung nonsmall carcinomas (H661 and A549) and normal lung cells (MRC-5). Both fluorophores show the fast and slowly decaying emission components upon pulsed excitation, and fluorescence spectra of NADH and FAD show blue- and red-shifts, respectively, during their decay. All identified lifetime components of NADH and FAD were found to be shorter in cancerous cells than in normal cells, no matter how they were measured under different extra-cellular conditions (cells suspended in cuvette and cells attached on glass substrate), indicating that the changes in metabolism likely altered the subcellular milieu and potentially also affected the interaction of NADH and FAD with enzymes to which these cofactors were bound. The intensity ratio of NADH and FAD of cancerous cells was also shown to be larger than that of normal cells.

Laser induced autofluorescence lifetime to identify larynx squamous cell carcinoma: Short series ex vivo study

Laser induced autofluorescence (LIAF) lifetime is useful to distinguish between normal laryngeal tissues and squamous cell carcinoma (SCC) based on variations of their biochemical composition and structure alterations. LIAF was collected from samples constituted by pairs of normal and malignant tissue, which were excised from three patients. Exclusion criteria for samples harvest were: (i) macroscopic changes of normal vocal cord observed during surgery; (ii) previous surgical intervention on vocal cord, (iii) patients treated only with chemotherapy or radiotherapy for carcinoma. Inclusion conditions: men, aged 57-68, non-smokers. A pulsed laser diode excited LIAF at 375 nm and 31 MHz repetition rate; beam full-time width at half-maximum was 87 ps at an average power of 0.49 mW. Mean LIAF lifetime for normal tissues was (3.75 ± 0.49) ns and for malignant (4.37 ± 0.85) ns: it is longer in malignant than in normal tissue. Variance analysis made with Fisher's test has shown no significant difference between patients for normal tissues; the same was true for malignant. Though, when malignant tissue was compared to normal for the same patients as well as between patients, a significant difference (significance level of 5%) was evidenced. Time-resolved LIAF may allow better differentiation between normal and malignant tissues in patients diagnosed with larynx SCC.

Noninvasive Optical Assessment of Implanted Engineered Tissues Correlates with Cytokine Secretion

Fluorescence lifetime sensing has been shown to noninvasively characterize the preimplantation health and viability of engineered tissue constructs. However, current practices to monitor postimplantation construct integration are either qualitative (visual assessment) or destructive (tissue histology). We employed label-free fluorescence lifetime spectroscopy for quantitative, noninvasive optical assessment of engineered tissue constructs that were implanted into a murine model. The portable system was designed to be suitable for intravital measurements and included a handheld probe to precisely and rapidly acquire data at multiple sites per construct. Our model tissue constructs were manufactured from primary human cells to simulate patient variability based on a standard protocol, and half of the manufactured constructs were stressed to create a range of health states. Secreted amounts of three cytokines that relate to cellular viability were measured in vitro to assess preimplantation construct health: interleukin-8 (IL-8), human β-defensin 1 (hBD-1), and vascular endothelial growth factor (VEGF). Preimplantation cytokine secretion ranged from 1.5 to 33.5 pg/mL for IL-8, from 3.4 to 195.0 pg/mL for hBD-1, and from 0.1 to 154.3 pg/mL for VEGF. In vivo optical sensing assessed constructs at 1 and 3 weeks postimplantation. We found that at 1 week postimplantation, in vivo optical parameters correlated with in vitro preimplantation secretion levels of all three cytokines (p < 0.05). This correlation was not observed in optical measurements at 3 weeks postimplantation when histology showed that the constructs had re-epithelialized, independent of preimplantation health state, supporting the lack of a correlation. These results suggest that clinical optical diagnostic tools based on label-free fluorescence lifetime sensing of endogenous tissue fluorophores could noninvasively monitor postimplantation integration of engineered tissues.

Depletion of A-type lamins and Lap2α reduces 53BP1 accumulation at UV-induced DNA lesions and Lap2α protein is responsible for compactness of irradiated chromatin

We studied how deficiency in lamins A/C and lamina-associated polypeptide 2α (Lap2α) affects DNA repair after irradiation. A-type lamins and Lap2α were not recruited to local DNA lesions and did not accumulate to γ-irradiation-induced foci (IRIF), as it is generally observed for well-known marker of DNA lesions, 53BP1 protein. At micro-irradiated chromatin of lmna double knockout (dn) and Lap2α dn cells, 53BP1 protein levels were reduced, compared to locally irradiated wild-type counterpart. Decreased levels of 53BP1 we also observed in whole populations of lmna dn and Lap2α dn cells, irradiated by UV light. We also studied distribution pattern of 53BP1 protein in a genome outside micro-irradiated region. In Lap2α deficient cells, identical fluorescence of mCherry-tagged 53BP1 protein was found at both microirradiated region and surrounding chromatin. However, a well-known marker of double strand breaks, γH2AX, was highly abundant in the lesion-surrounding genome of Lap2α deficient cells. Described changes, induced by irradiation in Lap2α dn cells, were not accompanied by cell cycle changes. In Lap2α dn cells, we additionally performed analysis by FLIM (Fluorescence Lifetime Imaging Microscopy) that showed different dynamic behavior of mCherry-tagged 53BP1 protein pools when it was compared with wild-type (wt) fibroblasts. This analysis revealed three different fractions of mCherry-53BP1 protein. Two of them showed identical exponential decay times (τ1 and τ3), but the decay rate of τ2 and amplitudes of fluorescence decays (A1-A3) were statistically different in wt and Lap2α dn fibroblasts. Moreover, γ-irradiation weakened an interaction between A-type lamins and Lap2α. Together, our results demonstrate how depletion of Lap2α affects DNA damage response (DDR) and how chromatin compactness is changed in Lap2α deficient cells exposed to radiation.

Types of advanced optical microscopy techniques for breast cancer research: a review

A cancerous cell is characterized by morphological and metabolic changes which are the key features of carcinogenesis. Adenosine triphosphate (ATP) in cancer cells is primarily produced by aerobic glycolysis rather than oxidative phosphorylation. In normal cellular metabolism, nicotinamide adenine dinucleotide (NADH) is considered as a principle electron donor and flavin adenine dinucleotide (FAD) as an electron acceptor. During metabolism in a cancerous cell, a net increase in NADH is found as the pathway switched from oxidative phosphorylation to aerobic glycolysis. Often during initiation and progression of cancer, the developmental regulation of extracellular matrix (ECM) is restricted and becomes disorganized. Tumor cell behavior is regulated by the ECM in the tumor micro environment. Collagen, which forms the scaffold of tumor micro-environment also influences its behavior. Advanced optical microscopy techniques are useful for determining the metabolic characteristics of cancerous, normal cells and tissues. They can be used to identify the collagen microstructure and the function of NADH, FAD, and lipids in living system. In this review article, various optical microscopy techniques applied for breast cancer research are discussed including fluorescence, confocal, second harmonic generation (SHG), coherent anti-Stokes Raman scattering (CARS), and fluorescence lifetime imaging (FLIM).

Correlation of intracellular oxygen and cell metabolism by simultaneous PLIM of phosphorescent TLD1433 and FLIM of NAD(P)H

During photodynamic therapy (PDT), disruption of cell respiration and metabolic changes could be one of the first events. Photophysical characteristics of the photosensitizer (PS) and its specific redox potential define consumption of molecular oxygen followed by generation of reactive oxygen species. The potential PS TLD1433 is based on transition metal Ru(II) and possess an oxygen-dependent luminescence. This enables the study of oxygen consumption by PS-phosphorescence lifetime imaging (PLIM) and simultaneously changes the cellular metabolic state by nicotinamide adenine dinucleotide (NAD(P)H)-fluorescence lifetime imaging (FLIM). Within this study, localization and cellular function of TLD1433 is investigated in bladder carcinoma cells using time-resolved and confocal laser scanning microscopy. Simultaneous FLIM/PLIM of NAD(P)H and TLD1433 during PDT correlated oxygen consumption, redox state and cellular energy metabolism. Our investigations aimed to provide a personalized protocol in theranostic PDT procedures and demonstrate the potential use of TLD1433 PDT also under hypoxic conditions, which are otherwise difficult to treat.

The inducible chemical-genetic fluorescent marker FAST outperforms classical fluorescent proteins in the quantitative reporting of bacterial biofilm dynamics

To increase our understanding of bacterial biofilm complexity, real- time quantitative analyses of the living community functions are required. To reach this goal, accurate fluorescent reporters are needed. In this paper, we used the classical fluorescent genetic reporters of the GFP family and demonstrated their limits in the context of a living biofilm. We showed that fluorescence signal saturated after only a few hours of growth and related this saturation to the reduction of oxygen concentration induced by bacterial consumption. This behaviour prevents the use of GFP-like fluorescent proteins for quantitative measurement in living biofilms. To overcome this limitation, we propose the use of a recently introduced small protein tag, FAST, which is fluorescent in the presence of an exogenously applied fluorogenic dye, enabling to avoid the oxygen sensitivity issue. We compared the ability of FAST to report on biofilm growth with that of GFP and mCherry, and demonstrated the superiority of the FAST:fluorogen probes for investigating dynamics in the complex environment of a living biofilm.

Ir(iii) complex-based oxygen imaging of living cells and ocular fundus with a gated ICCD camera

Phosphorescence lifetime imaging methods using oxygen-sensitive probes are very useful for visualizing the oxygen status of living cells and tissues with high spatial resolution. We aim to develop a useful oxygen detection technique combining a phosphorescent oxygen probe and an optimal detection method. Herein we present a biological oxygen imaging method using a microscope equipped with a gated intensified charge-coupled device (ICCD) camera as a detector and an Ir(iii) complex as a phosphorescent oxygen probe. Microscopic luminescence images of monolayer HT-29 cells (human colorectal adenocarcinoma cells) obtained using the cell-penetrating Ir(iii) complex BTPDM1 and an inverted microscope demonstrated that this method allowed visualization of the oxygen gradient produced in a monolayer of cultured cells when the monolayer is covered with a thin coverslip. Furthermore, combining the IR-emitting Ir(iii) complex DTTPH-PEG24 with a macrozoom microscope equipped with a gated ICCD camera enabled both the visualization of retinal vessels near the optic disc and the monitoring of oxygen level changes in a rabbit retina upon changing the inhaled oxygen content.

Bacterial biofilm under flow: First a physical struggle to stay, then a matter of breathing

Bacterial communities attached to surfaces under fluid flow represent a widespread lifestyle of the microbial world. Through shear stress generation and molecular transport regulation, hydrodynamics conveys effects that are very different by nature but strongly coupled. To decipher the influence of these levers on bacterial biofilms immersed in moving fluids, we quantitatively and simultaneously investigated physicochemical and biological properties of the biofilm. We designed a millifluidic setup allowing to control hydrodynamic conditions and to monitor biofilm development in real time using microscope imaging. We also conducted a transcriptomic analysis to detect a potential physiological response to hydrodynamics. We discovered that a threshold value of shear stress determined biofilm settlement, with sub-piconewton forces sufficient to prevent biofilm initiation. As a consequence, distinct hydrodynamic conditions, which set spatial distribution of shear stress, promoted distinct colonization patterns with consequences on the growth mode. However, no direct impact of mechanical forces on biofilm growth rate was observed. Consistently, no mechanosensing gene emerged from our differential transcriptomic analysis comparing distinct hydrodynamic conditions. Instead, we found that hydrodynamic molecular transport crucially impacts biofilm growth by controlling oxygen availability. Our results shed light on biofilm response to hydrodynamics and open new avenues to achieve informed design of fluidic setups for investigating, engineering or fighting adherent communities.

Investigating mitochondrial redox state using NADH and NADPH autofluorescence

The redox states of the NAD and NADP pyridine nucleotide pools play critical roles in defining the activity of energy producing pathways, in driving oxidative stress and in maintaining antioxidant defences. Broadly speaking, NAD is primarily engaged in regulating energy-producing catabolic processes, whilst NADP may be involved in both antioxidant defence and free radical generation. Defects in the balance of these pathways are associated with numerous diseases, from diabetes and neurodegenerative disease to heart disease and cancer. As such, a method to assess the abundance and redox state of these separate pools in living tissues would provide invaluable insight into the underlying pathophysiology. Experimentally, the intrinsic fluorescence of the reduced forms of both redox cofactors, NADH and NADPH, has been used for this purpose since the mid-twentieth century. In this review, we outline the modern implementation of these techniques for studying mitochondrial redox state in complex tissue preparations. As the fluorescence spectra of NADH and NADPH are indistinguishable, interpreting the signals resulting from their combined fluorescence, often labelled NAD(P)H, can be complex. We therefore discuss recent studies using fluorescence lifetime imaging microscopy (FLIM) which offer the potential to discriminate between the two separate pools. This technique provides increased metabolic information from cellular autofluorescence in biomedical investigations, offering biochemical insights into the changes in time-resolved NAD(P)H fluorescence signals observed in diseased tissues.

Time-resolved spectrometry of mitochondrial NAD(P)H fluorescence and its applications for evaluating the oxidative state in living cells

Time-resolved fluorescence spectrometry is a highly valuable technological tool to detect and characterize mitochondrial metabolic oxidative changes by means of endogenous fluorescence (Chorvat and Chorvatova, Laser Phys Lett 6: 175-193, 2009). Here, we describe the detection and measurement of endogenous mitochondrial NAD(P)H (nicotinamide adenine dinucleotide (phosphate)) fluorescence directly in living cultured cells using fluorescence lifetime spectrometry imaging after excitation with 405 nm picosecond (ps) laser. Time-correlated single photon counting (TCSPC) method is employed.

FRAP, FLIM, and FRET: Detection and analysis of cellular dynamics on a molecular scale using fluorescence microscopy

The combination of fluorescent-probe technology plus modern optical microscopes allows investigators to monitor dynamic events in living cells with exquisite temporal and spatial resolution. Fluorescence recovery after photobleaching (FRAP), for example, has long been used to monitor molecular dynamics both within cells and on cellular surfaces. Although bound by the diffraction limit imposed on all optical microscopes, the combination of digital cameras and the application of fluorescence intensity information on large-pixel arrays have allowed such dynamic information to be monitored and quantified. Fluorescence lifetime imaging microscopy (FLIM), on the other hand, utilizes the information from an ensemble of fluorophores to probe changes in the local environment. Using either fluorescence-intensity or lifetime approaches, fluorescence resonance energy transfer (FRET) microscopy provides information about molecular interactions, with Ångstrom resolution. In this review, we summarize the theoretical framework underlying these methods and illustrate their utility in addressing important problems in reproductive and developmental systems.

Separating NADH and NADPH fluorescence in live cells and tissues using FLIM

NAD is a key determinant of cellular energy metabolism. In contrast, its phosphorylated form, NADP, plays a central role in biosynthetic pathways and antioxidant defence. The reduced forms of both pyridine nucleotides are fluorescent in living cells but they cannot be distinguished, as they are spectrally identical. Here, using genetic and pharmacological approaches to perturb NAD(P)H metabolism, we find that fluorescence lifetime imaging (FLIM) differentiates quantitatively between the two cofactors. Systematic manipulations to change the balance between oxidative and glycolytic metabolism suggest that these states do not directly impact NAD(P)H fluorescence decay rates. The lifetime changes observed in cancers thus likely reflect shifts in the NADPH/NADH balance. Using a mathematical model, we use these experimental data to quantify the relative levels of NADH and NADPH in different cell types of a complex tissue, the mammalian cochlea. This reveals NADPH-enriched populations of cells, raising questions about their distinct metabolic roles.

NAD and NADP play fundamentally different roles in cellular metabolism, and yet these pyridine nucleotides cannot be distinguished spectroscopically in living cells. Blacker et al. demonstrate that fluorescence lifetime imaging can be used to quantify NADPH/NADH balance in cultured cells and in the mammalian cochlea.

Fluorescence lifetime imaging microscopy for quantitative biological imaging

Fluorescence lifetime imaging microscopy (FLIM) is a method for measuring fluorophore lifetimes with microscopic spatial resolution, providing a useful tool for cell biologists to detect, visualize, and investigate structure and function of biological systems. In this chapter, we begin by introducing the basic theory of fluorescence lifetime, including the characteristics of fluorophore decay, followed by a discussion of factors affecting fluorescence lifetimes and the potential advantages of fluorescence lifetime as a source of image contrast. Experimental methods for creating lifetime maps, including both time- and frequency-domain experimental approaches, are then introduced. Then, FLIM data analysis methods are discussed, including rapid lifetime determination, multiexponential fitting, Laguerre polynomial fitting, and phasor plot analysis. After, data analysis methods are introduced that improve lifetime precision of FLIM maps based upon optimal virtual gating and total variation denoising. The chapter concludes by highlighting several recent FLIM applications for quantitative biological imaging, including Förster resonance energy transfer-FLIM, fluorescence correlation spectroscopy-FLIM, multispectral-FLIM, and multiphoton-FLIM.

3D-resolved fluorescence and phosphorescence lifetime imaging using temporal focusing wide-field two-photon excitation

Fluorescence and phosphorescence lifetime imaging are powerful techniques for studying intracellular protein interactions and for diagnosing tissue pathophysiology. While lifetime-resolved microscopy has long been in the repertoire of the biophotonics community, current implementations fall short in terms of simultaneously providing 3D resolution, high throughput, and good tissue penetration. This report describes a new highly efficient lifetime-resolved imaging method that combines temporal focusing wide-field multiphoton excitation and simultaneous acquisition of lifetime information in frequency domain using a nanosecond gated imager from a 3D-resolved plane. This approach is scalable allowing fast volumetric imaging limited only by the available laser peak power. The accuracy and performance of the proposed method is demonstrated in several imaging studies important for understanding peripheral nerve regeneration processes. Most importantly, the parallelism of this approach may enhance the imaging speed of long lifetime processes such as phosphorescence by several orders of magnitude.

Imaging a photodynamic therapy photosensitizer in vivo with a time-gated fluorescence tomography system

We report the tomographic imaging of a photodynamic therapy (PDT) photosensitizer, 2-(1-hexyloxyethyl)-2-devinyl pyropheophorbide-a (HPPH) in vivo with time-domain fluorescence diffuse optical tomography (TD-FDOT). Simultaneous reconstruction of fluorescence yield and lifetime of HPPH was performed before and after PDT. The methodology was validated in phantom experiments, and depth-resolved in vivo imaging was achieved through simultaneous three-dimensional (3-D) mappings of fluorescence yield and lifetime contrasts. The tomographic images of a human head-and-neck xenograft in a mouse confirmed the preferential uptake and retention of HPPH by the tumor 24-h post-injection. HPPH-mediated PDT induced significant changes in fluorescence yield and lifetime. This pilot study demonstrates that TD-FDOT may be a good imaging modality for assessing photosensitizer distributions in deep tissue during PDT monitoring.

Total variation versus wavelet-based methods for image denoising in fluorescence lifetime imaging microscopy

We report the first application of wavelet-based denoising (noise removal) methods to time-domain box-car fluorescence lifetime imaging microscopy (FLIM) images and compare the results to novel total variation (TV) denoising methods. Methods were tested first on artificial images and then applied to low-light live-cell images. Relative to undenoised images, TV methods could improve lifetime precision up to 10-fold in artificial images, while preserving the overall accuracy of lifetime and amplitude values of a single-exponential decay model and improving local lifetime fitting in live-cell images. Wavelet-based methods were at least 4-fold faster than TV methods, but could introduce significant inaccuracies in recovered lifetime values. The denoising methods discussed can potentially enhance a variety of FLIM applications, including live-cell, in vivo animal, or endoscopic imaging studies, especially under challenging imaging conditions such as low-light or fast video-rate imaging.

Intracellular pH sensing using autofluorescence lifetime microscopy

Fluorescence lifetime images of reduced nicotinamide adenine dinucleotide (NADH) that is a key cofactor in cellular metabolism were obtained in a cell at various values of intracellular pH. The average fluorescence lifetime of NADH is found to become shorter monotonically with increasing pH, indicating that pH in a single cell can be determined by fluorescence lifetime imaging of NADH without adding exogenous fluorescent probes. The magnitude of the pH-induced lifetime change is higher in cells than that in buffer solution. The fluorescence lifetime of NADH is not uniform inside a cell; the fluorescence lifetime of nuclear NADH is shorter than that of mitochondrial NADH at each pH, and the magnitude of the pH-induced change is larger in nuclei than in other areas. The local electric field effect on the fluorescence lifetime is discussed since this effect may be one of the strong possibilities for the nonuniformity of the autofluorescence lifetime of NADH in cells.

Ruthenium(II) Complex Enantiomers as Cellular Probes for Diastereomeric Interactions in Confocal and Fluorescence Lifetime Imaging Microscopy

Ruthenium dipyridophenazine (dppz) complexes are sensitive luminescent probes for hydrophobic environments. Here, we apply multiple-frequency fluorescence lifetime imaging microscopy (FLIM) to Δ and Λ enantiomers of lipophilic ruthenium dppz complexes in live and fixed cells, and their different lifetime staining patterns are related to conventional intensity-based microscopy. Excited state lifetimes of the enantiomers determined from FLIM measurements correspond well with spectroscopically measured emission decay curves in pure microenvironments of DNA, phospholipid membrane or a model protein. We show that FLIM can be applied to monitor the long-lived excited states of ruthenium complex enantiomers and, combined with confocal microscopy, give new insight into their biomolecular binding and reveal differences in the microenvironment probed by the complexes.

Fluorescence lifetime spectroscopy for guided therapy of brain tumors

This study evaluates the potential of time-resolved laser induced fluorescence spectroscopy (TR-LIFS) as intra-operative tool for the delineation of brain tumor from normal brain. Forty two patients undergoing glioma (WHO grade I-IV) surgery were enrolled in this study. A TR-LIFS prototype apparatus (gated detection, fast digitizer) was used to induce in-vivo fluorescence using a pulsed N2 laser (337 nm excitation, 0.7 ns pulse width) and to record the time-resolved spectrum (360-550 nm range, 10 nm interval). The sites of TR-LIFS measurement were validated by conventional histopathology (H&E staining). Parameters derived from the TR-LIFS data including intensity values and time-resolved intensity decay features (average fluorescence lifetime and Laguerre coefficients values) were used for tissue characterization and classification. 71 areas of tumor and normal brain were analyzed. Several parameters allowed for the differentiation of distinct tissue types. For example, normal cortex (N=35) and normal white matter (N=12) exhibit a longer-lasting fluorescence emission at 390 nm (τ390=2.12±0.10 ns) when compared with 460 nm (τ460=1.16±0.08 ns). High grade glioma (grades III and IV) samples (N=17) demonstrate emission peaks at 460 nm, with large variation at 390 nm while low grade glioma (I and II) samples (N=7) demonstrated a peak fluorescence emission at 460 nm. A linear discriminant algorithm allowed for the classification of low-grade gliomas with 100% sensitivity and 98% specificity. High-grade glioma demonstrated a high degree of heterogeneity thus reducing the discrimination accuracy of these tumors to 47% sensitivity and 94% specificity. Current findings demonstrate that TR-LIFS holds the potential to diagnose brain tumors intra-operatively and to provide a valuable tool for aiding the neurosurgeon-neuropathologist team in to rapidly distinguish between tumor and normal brain during surgery.

Optical oxygen sensors for applications in microfluidic cell culture

The presence and concentration of oxygen in biological systems has a large impact on the behavior and viability of many types of cells, including the differentiation of stem cells or the growth of tumor cells. As a result, the integration of oxygen sensors within cell culture environments presents a powerful tool for quantifying the effects of oxygen concentrations on cell behavior, cell viability, and drug effectiveness. Because microfluidic cell culture environments are a promising alternative to traditional cell culture platforms, there is recent interest in integrating oxygen-sensing mechanisms with microfluidics for cell culture applications. Optical, luminescence-based oxygen sensors, in particular, show great promise in their ability to be integrated with microfluidics and cell culture systems. These sensors can be highly sensitive and do not consume oxygen or generate toxic byproducts in their sensing process. This paper presents a review of previously proposed optical oxygen sensor types, materials and formats most applicable to microfluidic cell culture, and analyzes their suitability for this and other in vitro applications.

Enhancing precision in time-domain fluorescence lifetime imaging

In biological applications of fluorescence lifetime imaging, low signals from samples can be a challenge, causing poor lifetime precision. We demonstrate how optimal signal gating (a method applied to the temporal dimension of a lifetime image) and novel total variation denoising models (a method applied to the spatial dimension of a lifetime image) can be used in time-domain fluorescence lifetime imaging microscopy (FLIM) to improve lifetime precision. In time-gated FLIM, notable fourfold precision improvements were observed in a low-light example. This approach can be employed to improve FLIM data while minimizing sample light exposure and increasing imaging speed.

Precise fluorophore lifetime mapping in live-cell, multi-photon excitation microscopy

Fluorophore excited state lifetime is a useful indicator of micro-environment in cellular optical molecular imaging. For quantitative sensing, precise lifetime determination is important, yet is often difficult to accomplish when using the experimental conditions favored by live cells. Here we report the first application of temporal optimization and spatial denoising methods to two-photon time-correlated single photon counting (TCSPC) fluorescence lifetime imaging microscopy (FLIM) to improve lifetime precision in live-cell images. The results demonstrated a greater than five-fold improvement in lifetime precision. This approach minimizes the adverse effects of excitation light on live cells and should benefit FLIM applications to high content analysis and bioimage informatics.

Fluorescence lifetime discrimination using expectation-maximization algorithm with joint deconvolution

The fluorescence lifetime technique offers an effective way to resolve fluorescent components with overlapping emission spectra. The presence of multiple fluorescent components in biological compounds can hamper their discrimination. The conventional method based on the nonlinear least-squares technique is unable to consistently determine the correct number of fluorescent components in a fluorescence decay profile. This can limit the applications of the fluorescence lifetime technique in biological assays and diagnoses where more than one fluorescent component is typically encountered. We describe the use of an expectation-maximization (EM) method with joint deconvolution to estimate the fluorescence decay parameters, and the Bayesian information criterion (BIC) to accurately determine the number of fluorescent components. A comprehensive simulation and experimental study is carried out to compare the performance and accuracy of the proposed method. The results show that the EM-BIC method is able to accurately identify the correct number of fluorescent components in samples with weakly fluorescing components.

Hard top soft bottom microfluidic devices for cell culture and chemical analysis

We report fabrication and characterization of microfluidic devices made of thermoplastic and elastomeric polymers. These hard-soft hybrid material devices are motivated by the combined need for large scale manufacturability, enhanced barrier properties to gas permeation and evaporation of aqueous solutions compared to poly(dimethyl siloxane) (PDMS) devices, and compatibility with deformation-based actuation. Channel features are created on rigid polymers such as polyethylene terephthalate glycol (PETG), cyclic olefin copolymer (COC), and polystyrene (PS) by hot embossing. These "hard tops" are bonded to elastomeric "soft bottoms" (polyurethane (PU) or PDMS-parylene C-PDMS) to create devices that can be used for microfluidic cell culture where deformation-based fluid actuation schemes are used to perfuse and recirculate media. The higher barrier properties of this device compared to PDMS devices enable cell culture with less evaporation and creation of hypoxic conditions.

Fine temporal control of the medium gas content and acidity and on-chip generation of series of oxygen concentrations for cell cultures

We describe the design, operation, and applications of two microfluidic devices that generate series of concentrations of oxygen, [O(2)], by on-chip gas mixing. Both devices are made of polydimethylsiloxane (PDMS) and have two layers of channels, the flow layer and the gas layer. By using in-situ measurements of [O(2)] with an oxygen-sensitive fluorescent dye, we show that gas diffusion through PDMS leads to equilibration of [O(2)] in an aqueous solution in the flow layer with [O(2)] in a gas injected into the gas layer on a time scale of approximately 1 sec. Injection of carbon dioxide into the gas layer causes the pH in the flow layer to drop within approximately 0.5 sec. Gas-mixing channel networks of both devices generate series of 9 gas mixtures with different [O(2)] from two gases fed to the inlets, thus creating regions with 9 different [O(2)] in the flow layer. The first device generates nitrogen-oxygen mixtures with [O(2)] varying linearly between 0 and 100%. The second device generates nitrogen-air mixtures with [O(2)] varying exponentially between 0 and 20.9%. The flow layers of the devices are designed for culturing bacteria in semi-permeable microchambers, and the second device is used to measure growth curves of E. coli colonies at 9 different [O(2)] in a single experiment. The cell division rates at [O(2)] of 0, 0.2, and 0.5% are found to be significantly different, further validating the capacity of the device to set [O(2)] in the flow layer with high precision and resolution. The degree of control of [O(2)] achieved in the devices and the robustness with respect to oxygen consumption due to respiration would be difficult to match in a traditional large-scale culture. The proposed devices and technology can be used in research on bacteria and yeast under microaerobic conditions and on mammalian cells under hypoxia.

Calibration and validation of an optical sensor for intracellular oxygen measurements

Calibration of fluorescent optical sensors for accurate, quantitative intracellular measurements in vivo suffers from lack of a representative medium that appropriately simulates the molecular complexity of the cytosol. We present a novel protocol for accurate intracellular oxygen sensing via fluorescence lifetime imaging microscopy (FLIM) using cell lysate-FLIM measurements to correct the in vitro calibration of a fluorescent oxygen sensor, and we describe electron paramagnetic resonance (EPR) validation studies. Lysate-FLIM studies provided biochemical information, while EPR provided a "gold standard" for intracellular oxygen estimation. Oxygen levels were evaluated in living human normal squamous and adenocarcinoma esophageal epithelial cells, and good agreement was observed between oxygen levels derived from the optical protocol and EPR. The proposed protocol introduces the concept of a living cell line as a reference for estimating unknown oxygen levels in other cell lines and accounts for high degrees of variability between different cell lines.

Synthesis and characterization of a redox- and light-sensitive MRI contrast agent

A redox- and light-sensitive, T(1)-weighted magnetic resonance imaging (MRI) contrast agent which tethers a spiropyran(SP)/merocyanine(MC) motif to a Gd-DO3A moiety was synthesized and characterized. When in the dark, the probe is in its MC form which has an r(1) relaxivity of 2.51 mM(-1)s(-1) (60MHz, 37°C). After irradiation with visible light or mixing with NADH, the probe experiences an isomerization and the r(1) relaxivity decreased 18% and 26%, respectively. Additionally, the signal intensity in MRI showed an observable decrease after the compound was mixed with NADH.

Image restoration for fluorescence lifetime imaging microscopy (FLIM)

Computational image restoration finds wide applicability for fluorescence intensity imaging. Relatively little work in this regard has been performed on FLIM images, which also suffer from diminished spatial resolution. In this work, we report two separate approaches to enhance FLIM image quality while maintaining lifetime accuracy. A 2D-image restoration algorithm was employed to improve resolution in gated intensity images of various samples including fluorescent beads, living cells and fixed tissue samples. The restoration approach improved lifetime image quality without significant variation in lifetime. Further, overlaying a restored-intensity image over the native lifetime image provided even better results, where the resulting lifetime map had spatial features similar to the intensity map. 2D and 3D image restoration also benefit from advances in computational power and hence holds potential for enhancing FLIM resolution, particularly in ICCD-based wide-field FLIM systems, without sacrificing vital quantitative information.

Photoacoustic probing of fluorophore excited state lifetime with application to oxygen sensing

A new method is developed to perform local measurements of fluorophore excited state lifetimes in turbid media without collecting the fluorescence emission. The method is based on a pump-probe approach where a first laser pulse excites the dye and then a second laser pulse is used for photoacoustic probing of the transient absorption. The photoacoustic response generated by the probe pulse is recorded by an ultrasound receiver. Repeating the measurement for increasing pump-probe time delays yields a series of photoacoustic signals that are used to extract the lifetime of the excited state. The method is validated by measuring the lifetime of an oxygen sensitive dye solution at different concentrations of dissolved oxygen. The dye is pumped with a 532-nm pulsed laser and the transient absorption at 740 nm is probed using a second pulsed laser system. The photoacoustic-based results are in close agreement with those obtained from time-dependent fluorescent measurements. The method can be extended to photoacoustic lifetime imaging by using a receiver array instead of a single receiver. Potential applications of this method include tissue oxygen imaging for cancer diagnostics and mapping molecular events such as resonant energy transfer and ion collisions in a biological environment.

Quantitative measurement and control of oxygen levels in microfluidic poly(dimethylsiloxane) bioreactors during cell culture

Microfluidic bioreactors fabricated from highly gas-permeable poly(dimethylsiloxane) (PDMS) materials have been observed, somewhat unexpectedly, to give rise to heterogeneous long term responses along the length of a perfused mammalian cell culture channel, reminiscent of physiologic tissue zonation that arises at least in part due to oxygen gradients. To develop a more quantitative understanding and enable better control of the physical-chemical mechanisms underlying cell biological events in such PDMS reactors, dissolved oxygen concentrations in the channel system were quantified in real time using fluorescence intensity and lifetime imaging of an oxygen sensitive dye, ruthenium tris(2,2'-dipyridyl) dichloride hexahydrate (RTDP). The data indicate that despite oxygen diffusion through PDMS, uptake of oxygen by cells inside the perfused PDMS microchannels induces an axial oxygen concentration gradient, with lower levels recorded in downstream regions. The oxygen concentration gradient generated by a balance of cellular uptake, convective transport by media flow, and permeation through PDMS in our devices ranged from 0.0003 (mg/l)/mm to 0.7 (mg/l)/mm. The existence of such steep gradients induced by cellular uptake can have important biological consequences. Results are consistent with our mathematical model and give insight into the conditions under which flux of oxygen through PDMS into the microchannels will or will not contribute significantly to oxygen delivery to cells and also provide a design tool to manipulate and control oxygen for cell culture and device engineering. The combination of computerized microfluidics, in situ oxygen sensing, and mathematical models opens new windows for microphysiologic studies utilizing oxygen gradients and low oxygen tensions.

Optical imaging in microfluidic bioreactors enables oxygen monitoring for continuous cell culture

For the first time, a fluorescence lifetime calibration method for an oxygen-sensitive dye ruthenium tris(2,2'-dipyridyl) dichloride hexahydrate (RTDP) is applied to image oxygen levels in poly(dimethyl siloxane) (PDMS) bioreactors containing living C2C12 mouse myoblasts. PDMS microsystems are broadly used in bioengineering applications due to their biocompatibility and ease of handling. For these systems, oxygen concentrations are of significance and are likely to play an important role in cell behavior and gene expression. Fluorescence lifetime imaging microscopy (FLIM) bases image contrast on fluorophore excited state lifetimes, which reflect local biochemistry. Unique attributes of the widefield, time-domain FLIM system include tunable excitation (337.1 to 960 nm), large temporal dynamic range (> or =600 ps), high spatial resolution (1.4 microm), calibrated detection (0 to 300+/-8 microM of oxygen), and rapid data acquisition and processing times (10 s). Oxygen levels decrease with increasing cell densities and are consistent with model outcomes obtained by simulating bioreactor oxygen diffusion and cell proliferation. In single bioreactor loops, FLIM detects spatial heterogeneity in oxygen levels with variations as high as 20%. The fluorescence lifetime-based imaging approach we describe avoids intensity-based artifacts (including photobleaching and concentration variations) and provides a technique with high spatial discrimination for oxygen monitoring in continuous cell culture systems.