Fluorescence lifetime imaging and phasor analysis of intracellular porphyrinic photosensitizers applied with different polymeric formulations

The fluorescence lifetime of a porphyrinic photosensitizer (PS) is an important parameter to assess the aggregation state of the PS even in complex biological environments. Aggregation-induced quenching of the PS can significantly reduce the yield of singlet oxygen generation and thus its efficiency as a medical drug in photodynamic therapy (PDT) of diseased tissues. Hydrophobicity and the tendency to form aggregates pose challenges on the development of efficient PSs and often require carrier systems. A systematic study was performed to probe the impact of PS structure and encapsulation into polymeric carriers on the fluorescence lifetime in solution and in the intracellular environment. Five different porphyrinic PSs including chlorin e6 (Ce6) derivatives and tetrakis(m-hydroxyphenyl)-porphyrin and -chlorin were studied in free form and combined with polyvinylpyrrolidone (PVP) or micelles composed of triblock-copolymers or Cremophor. Following incubation of HeLa cells with these systems, fluorescence lifetime imaging combined with phasor analysis and image segmentation was applied to study the lifetime distribution in the intracellular surrounding. The data suggest that for free PSs, the structure-dependent cell uptake pathways determine their state and emission lifetimes. PS localization in the plasma membrane yielded mostly monomers with long fluorescence lifetimes whereas the endocytic pathway with subsequent lysosomal deposition adds a short-lived component for hydrophilic anionic PSs. Prolonged incubation times led to increasing contributions from short-lived components that derive from aggregates mainly localized in the cytoplasm. Encapsulation of PSs into polymeric carriers led to monomerization and mostly fluorescence emission decays with long fluorescence lifetimes in solution. However, the efficiency depended on the binding strength that was most pronounced for PVP. In the cellular environment, PVP was able to maintain monomeric long-lived species over prolonged incubation times. This was most pronounced for Ce6 derivatives with a logP value around 4.5. Micellar encapsulation led to faster release of the PSs resulting in multiple components with long and short fluorescence lifetimes. The hydrophilic hardly aggregating PS exhibited a mostly stable invariant lifetime distribution over time with both carriers. The presented data are expected to contribute to optimized PDT treatment protocols and improved PS-carrier design for preventing intracellular fluorescence quenching. In conclusion, amphiphilic and concurrent hydrophobic PSs with high membrane affinity as well as strong binding to the carrier have best prospects to maintain their photophysical properties in vivo and serve thus as efficient photodynamic diagnosis and PDT drugs.

Au nanodyes as enhanced contrast agents in wide field near infrared fluorescence lifetime imaging

The near-infrared (NIR) range of the electromagnetic (EM) spectrum offers a nearly transparent window for imaging tissue. Despite the significant potential of NIR fluorescence-based imaging, its establishment in basic research and clinical applications remains limited due to the scarcity of fluorescent molecules with absorption and emission properties in the NIR region, especially those suitable for biological applications. In this study, we present a novel approach by combining the widely used IRdye 800NHS fluorophore with gold nanospheres (GNSs) and gold nanorods (GNRs) to create Au nanodyes, with improved quantum yield (QY) and distinct lifetimes. These nanodyes exhibit varying photophysical properties due to the differences in the separation distance between the dye and the gold nanoparticles (GNP). Leveraging a rapid and highly sensitive wide-field fluorescence lifetime imaging (FLI) macroscopic set up, along with phasor based analysis, we introduce multiplexing capabilities for the Au nanodyes. Our approach showcases the ability to differentiate between NIR dyes with very similar, short lifetimes within a single image, using the combination of Au nanodyes and wide-field FLI. Furthermore, we demonstrate the uptake of Au nanodyes by mineral-oil induced plasmacytomas (MOPC315.bm) cells, indicating their potential for in vitro and in vivo applications.

Fluorescence lifetime imaging microscopy for early diagnosis and severity prediction of preeclampsia with Nile Blue probe

Preeclampsia (PE) is a common pregnancy complication and the leading cause of maternal and perinatal mortality. Unfortunately, the early diagnostic methods for PE are still rare. Fluorescence lifetime imaging (FLIM) technology has proven to be applicable for diagnosis of various diseases. Here, we explore the possibility of the FLIM technique for PE early diagnosis and severity prediction with Nile Blue probe as biosensor. 23 placental slices and 162 third-trimester-collected maternal peripheral blood serum samples were stained with Nile blue and imaged by FLIM system. Fluorescence lifetimes of the probe increased significantly as the disease worsened (p < 0.0001). Characterization of the probe showed an increasing tendency in lifetimes under lower polarity conditions and revealed that the reason for the lifetime differences in serum sample was polarity changes caused by abnormal lipid metabolism in serum. For early diagnosis, we investigated 42 12th-week-collected chronic hypertension (CH) serum samples and successfully distinguished PE patients from pregnant women. With the functions of measuring fluorescence lifetime and detecting polarity changes caused by an abnormal lipid microenvironment in maternal peripheral blood, FLIM technology, together with Nile Blue probe, presents a feasible and advantageous approach for PE early noninvasive diagnosis and severity prediction.

Perfusability and immunogenicity of implantable pre-vascularized tissues recapitulating features of native capillary network

Vascularization is a key pre-requisite to engineered anatomical scale three dimensional (3-D) constructs to ensure their nutrient and oxygen supply upon implantation. Presently, engineered pre-vascularized 3-D tissues are limited to only micro-scale hydrogels, which meet neither the anatomical scale needs nor the complexity of natural extracellular matrix (ECM) environments. Anatomical scale perfusable constructs are critically needed for translational applications. To overcome this challenge, we previously developed pre-vascularized ECM sheets with long and oriented dense microvascular networks. The present study further evaluated the patency, perfusability and innate immune response toward these pre-vascularized constructs. Macrophage-co-cultured pre-vascularized constructs were evaluated in vitro to confirm micro-vessel patency and perturbations in macrophage metabolism. Subcutaneously implanted pre-vascularized constructs remained viable and formed a functional anastomosis with host vasculature within 3 days of implantation. This completely biological pre-vascularized construct holds great potential as a building block to engineer perfusable anatomical scale tissues.

Two-photon lifetime-Based Photoconversion of EGFP for 3D-photostimulation in FLIM

Enhanced green fluorescence protein (EGFP) is a fluorescent tag commonly used in cellular and biomedical applications. Surprisingly, some interesting photochemical properties of EGFP have remained unexplored. Here we report on two-photon-induced photoconversion of EGFP, which can be permanently converted by intense IR irradiation to a form with a short fluorescence lifetime and spectrally conserved emission. Photoconverted EGFP thus can be distinguished from the unconverted tag by the time-resolved detection. Nonlinear dependence of the two-photon photoconversion efficiency on the light intensity allows for an accurate 3D localization of the photoconverted volume within cellular structures, which is especially useful for kinetic FLIM applications. For illustration, we used the two photon photoconversion of EGFP for measurements of redistribution kinetics of nucleophosmin and histone H2B in nuclei of live cells. Measurements revealed high mobility of fluorescently tagged histone H2B in the nucleoplasm and their redistribution between spatially separated nucleoli.

Cell density-dependent membrane distribution of ganglioside GM3 in melanoma cells

Monosialoganglioside GM3 is the simplest ganglioside involved in various cellular signaling. Cell surface distribution of GM3 is thought to be crucial for the function of GM3, but little is known about the cell surface GM3 distribution. It was shown that anti-GM3 monoclonal antibody binds to GM3 in sparse but not in confluent melanoma cells. Our model membrane study evidenced that monoclonal anti-GM3 antibodies showed stronger binding when GM3 was in less fluid membrane environment. Studies using fluorescent GM3 analogs suggested that GM3 was clustered in less fluid membrane. Moreover, fluorescent lifetime measurement showed that cell surface of high density melanoma cells is more fluid than that of low density cells. Lipidomics and fatty acid supplementation experiment suggested that monounsaturated fatty acid-containing phosphatidylcholine contributed to the cell density-dependent membrane fluidity. Our results indicate that anti-GM3 antibody senses GM3 clustering and the number and/or size of GM3 cluster differ between sparse and confluent melanoma cells.

uFLIM - Unsupervised analysis of FLIM-FRET microscopy data

Despite their widespread use in cell biology, fluorescence lifetime imaging microscopy (FLIM) data-sets are challenging to analyse, because each spatial position can contain a superposition of multiple fluorescent components. Here, we present a data analysis method employing all information in the available photon budget, as well as being fast. The method, called uFLIM, determines spatial distributions and temporal dynamics of multiple fluorescent components with no prior knowledge. It goes significantly beyond current approaches which either assume the functional dependence of the dynamics, e.g. an exponential decay, or require dynamics to be known, or calibrated. Its efficient non-negative matrix factorization algorithm allows for real-time data processing. We validate in silico that uFLIM is capable to disentangle the spatial distribution and spectral properties of five fluorescing probes, from only two excitation and detection channels and a photon budget of 100 detected photons per pixel. By adapting the method to data exhibiting Förster resonant energy transfer (FRET), we retrieve the spatial and transfer rate distribution of the bound species, without constrains on donor and acceptor dynamics.

In vivo observation of amyloid-like fibrils produced under stress

The participation of amyloids in neurodegenerative diseases and functional processes has triggered the quest for methods allowing their direct detection in vivo. Despite the plethora of data, those methods are still lacking. The autofluorescence from the extended β-sheets of amyloids is here used to follow fibrillation of S. cerevisiae Golgi Reassembly and Stacking Protein (Grh1). Grh1 has been implicated in starvation-triggered unconventional protein secretion (UPS), and here its participation also in heat shock response (HSR) is suggested. Fluorescence Lifetime Imaging (FLIM) is used to detect fibril autofluorescence in cells (E. coli and yeast) under stress (starvation and higher temperature). The formation of Grh1 large complexes under stress is further supported by size exclusion chromatography and ultracentrifugation. The data show for the first time in vivo detection of amyloids without the use of extrinsic probes as well as bring new perspectives on the participation of Grh1 in UPS and HSR.

Biophysical studies of phase separation integrating experimental and computational methods

Biomolecular phase separation that contributes to the formation of membraneless organelles and biomolecular condensates has recently gained tremendous attention because of the importance of these assemblies in physiology, disease, and engineering applications. Understanding and directing biomolecular phase separation requires a multiscale view of the biophysical properties of these phases. Yet, many classic tools to characterize biomolecular properties do not apply in these condensed phases. Here, we discuss insights obtained from spectroscopic methods, in particular nuclear magnetic resonance and optical spectroscopy, in understanding the molecular and atomic interactions that underlie the formation of protein-rich condensates. We also review approaches closely coupling nuclear magnetic resonance data with computational methods especially coarse-grained and all-atom molecular simulations, which provide insight into molecular features of phase separation. Finally, we point to future methodolical developments, particularly visualizing biophysical properties of condensates in cells.

PSD-95 protects synapses from β-amyloid

Beta-amyloid (Aβ) depresses excitatory synapses by a poorly understood mechanism requiring NMDA receptor (NMDAR) function. Here, we show that increased PSD-95, a major synaptic scaffolding molecule, blocks the effects of Aβ on synapses. The protective effect persists in tissue lacking the AMPA receptor subunit GluA1, which prevents the confounding synaptic potentiation by increased PSD-95. Aβ modifies the conformation of the NMDAR C-terminal domain (CTD) and its interaction with protein phosphatase 1 (PP1), producing synaptic weakening. Higher endogenous levels or overexpression of PSD-95 block Aβ-induced effects on the NMDAR CTD conformation, its interaction with PP1, and synaptic weakening. Our results indicate that increased PSD-95 protects synapses from Aβ toxicity, suggesting that low levels of synaptic PSD-95 may be a molecular sign indicating synapse vulnerability to Aβ. Importantly, pharmacological inhibition of its depalmitoylation increases PSD-95 at synapses and rescues deficits caused by Aβ, possibly opening a therapeutic avenue against Alzheimer's disease.

A Tool to Locate Parathyroid Glands Using Dynamic Optical Contrast Imaging

OBJECTIVES/HYPOTHESIS

Identification of parathyroid glands and adjacent tissues intraoperatively can be quite challenging because of their small size, variable locations, and indistinct external features. The objective of this study is to test the efficacy of the dynamic optical contrast imaging (DOCI) technique as a tool in specifically differentiating parathyroid tissue and adjacent structures, facilitating efficient and reliable tissue differentiation.

STUDY DESIGN

Prospective study.

METHODS

Both animal and human tissues were included in this study. Fresh specimens were imaged with DOCI and subsequently processed for hematoxylin and eosin (H&E) stain. The DOCI images were analyzed and compared to the H&E results as ground truth.

RESULTS

In both animal and human experiments, significant DOCI contrast was observed between parathyroid glands and adjacent tissue of all types. Region of interest analysis revealed most distinct DOCI values for each tissue when using 494 and 572 nm-specific band pass filter for signal detection (P < .005 for porcine tissues, and P = .02 for human specimens). Linear discriminant classifier for tissue type prediction based on DOCI also matched the underlying histology.

CONCLUSIONS

We demonstrate that the DOCI technique reliably facilitates specific parathyroid gland localization. The DOCI technique constitutes important groundwork for in vivo precision endocrine surgery.

LEVEL OF EVIDENCE

4 Laryngoscope, 2021.

Intraoperative Mapping of Parathyroid Glands Using Fluorescence Lifetime Imaging

BACKGROUND

Hypoparathyroidism is a common complication following thyroidectomy. There is a need for technology to aid surgeons in identifying the parathyroid glands. In contrast to near infrared technologies, fluorescence lifetime imaging (FLIm) is not affected by ambient light and may be valuable in identifying parathyroid tissue, but has never been evaluated in this capacity.

METHODS

We used FLIm to measure the UV induced (355 nm) time-resolved autofluorescence signatures (average lifetimes in 3 spectral emission channels) of thyroid, parathyroid, lymphoid and adipose tissue in 21 patients undergoing thyroid and parathyroid surgery. The Mann-Whitney U test was used to assess the ability of FLIm to discriminate normocellular parathyroid from each of the other tissues. Various machine learning classifiers (random forests, neural network, support vector machine) were then evaluated to recognize parathyroid through a leave-one-out cross-validation.

RESULTS

Statistically significant differences in average lifetime were observed between parathyroid and each of the other tissue types in spectral channels 2 and 3 respectively. The largest change was observed between adipose tissue and parathyroid (P < 0.001), while less pronounced but still significant changes were observed when comparing parathyroid with lymphoid tissue (P < 0.05) and thyroid (P < 0.01). A random forest classifier trained on average lifetimes was found to detect parathyroid tissue with 100% sensitivity and 93% specificity at the acquisition run level.

CONCLUSION

We found that FLIm derived parameters can distinguish the parathyroid glands and other adjacent tissue types and has promise in scanning the surgical field to identify parathyroid tissue in real-time.

Aggregation-related quenching of LHCII fluorescence in liposomes revealed by single-molecule spectroscopy

Incorporation of membrane proteins into reconstituted lipid membranes is a common approach for studying their structure and function relationship in a native-like environment. In this work, we investigated fluorescence properties of liposome-reconstituted major light-harvesting complexes of plants (LHCII). By utilizing liposome labelling with the fluorescent dye molecules and single-molecule microscopy techniques, we were able to study truly liposome-reconstituted LHCII and compare them with bulk measurements and liposome-free LHCII aggregates bound to the surface. Our results showed that fluorescence lifetime obtained in bulk and in single liposome measurements were correlated. The fluorescence lifetimes of LHCII were shorter for liposome-free LHCII than for reconstituted LHCII. In the case of liposome-reconstituted LHCII, fluorescence lifetime showed dependence on the protein density reminiscent to concentration quenching. The dependence of fluorescence lifetime of LHCII on the liposome size was not significant. Our results demonstrated that fluorescence quenching can be induced by LHCII - LHCII interactions in reconstituted membranes, most likely occurring via the same mechanism as photoprotective non-photochemical quenching in vivo.

Characterization of radical types, penetration profile and distribution pattern of the topically applied photosensitizer THPTS in porcine skin ex vivo

The topical photodynamic therapy (PDT) is mainly used in the treatment of dermato-oncological diseases. The distribution and functionality of the photosensitizer Tetrahydroporphyrin-Tetratosylat (THPTS) was investigated using microscopic and spectroscopic methods after topical application to excised porcine skin followed by irradiation. The distribution of THPTS was determined by two-photon tomography combined with fluorescence lifetime imaging (TPT/FLIM) and confocal Raman microspectroscopy (CRM). The radicals were quantified and characterized by electron paramagnetic resonance (EPR) spectroscopy. Results show a penetration depth of THPTS into the skin down to around 12 ± 5 µm. A penetration of THPTS through the stratum corneum was not clearly observable after 1 h penetration time, but cannot be excluded. The irradiation within the phototherapeutic window (spectral range of visible and near infrared light in the range ≈ 650-850 nm) is needed to activate THPTS. An incubation time of 10 min showed the highest radical production. A longer incubation time affected the functionality of THPTS, whereby significant less radicals were detectable. During PDT mainly reactive oxygen species (ROS) and lipid oxygen species (LOS) are produced. Overall, the irradiation dose per se influences the radical types formed in skin. While ROS are always prominent at low doses, LOS increase at high doses, independent of previous skin treatment and the irradiation wavelength used.

Tracking of fluorescent antibiotic conjugate in planta utilizing fluorescence lifetime imaging

Excited state lifetime-based separation of fluorophore-tagged antibiotic conjugate emission from the spectrally broad plant autofluorescence enables in planta tracking of the translocation of systemic cargo such as antibiotics via fluorescence lifetime imaging. The efficacy of antibiotic treatments in citrus crops is uncertain due to mixed results from in-field experiments and a lack of study on their systemic movement. As of yet there has been an inability to track treatments using traditional fluorescence microscopy due to treatments having little fluorescence characteristics, and signal convolution due to plant autofluorescence. In this study, we used streptomycin sulfate, a commercially available antibiotic, and conjugated it to a modified tris(bipyridine) ruthenium (II) chloride, a dye with an excited state lifetime magnitudes higher than other commonly used organic fluorescent probes. The resultant is a fluorescence lifetime imaging (FLIM) trackable antibiotic conjugate, covalently attached via an amide linkage that is uniquely distinguishable from plant autofluorescence. Characterization of the fluorescent antibiotic conjugate showed no mitigation of excited state lifetime, and a distinct IR peak not found in any synthetic components. Subsequent tracking using FLIM in citrus tissue was achieved, with identification of movement through citrus plant vasculature via tissue localization in xylem and phloem. Results indicated upwards systemic movement of the conjugate in both xylem and phloem after 48 h of incubation. However, the conjugate failed to move down towards the root system of the plant by 168 h. Mechanistically, it is likely that xylem contributes heavily in the translocation of the conjugate upwards; however, phloem led flow due to growth changes could act as a contributor. This proof-of-concept sets groundwork for subsequent studies regarding antibiotic localization and movement in citrus.

Long fluorescence lifetime triangulenium dyes in imaging and fluorescence polarization assay

The development of new fluorescent dyes-new fluorochromes-has a large potential to improve the established methods in enzymology, by empowering both detection capability and the scope of the individual method. Unfortunately, there are huge barriers when adopting new improved fluorescent dyes in established methods. The dyes have to be generally available, protocols for labeling and analysis must be in place, and the field has to be aware how the new improved dye can enhance their method of choice. In this chapter, we will address these issues for the triangulenium dyes. A class of dyes that has a long fluorescence lifetime and emission in the red. A unique combination that opens up new possibilities for the study of protein rotational motion, when developing fluorescence polarization (FP) assays, and for all time-resolved imaging or analysis platforms. To make these dyes generally available, the features of the long fluorescence lifetime triangulenium dyes are described and an optimized labelling protocol are reported.

Digital scanned laser light-sheet fluorescence lifetime microscopy with wide-field time-gated imaging

We develop a multidimensional fluorescence imaging technique by implementing a wide-field time-gated fluorescence lifetime imaging into digital scanned laser light-sheet microscopy (FLIM-DSLM) to measure 3D fluorescence lifetime distribution in mesoscopic specimens with high resolution. This is achieved by acquiring a series of time-gated images at different relative time delays with respect of excitation pulses at different depths. The lifetime is determined for each voxel by iteratively fitting to single exponential decay. The performance of the developed system is evaluated with the measurements of a lifetime reference Rhodamine 6G solution and a subresolution fluorescent bead phantom. We also demonstrate the application performances of this system to ex vivo and in vivo imaging of Tg(kdrl:EGFP) transgenic zebrafish embryos, illustrating the lifetime differences between the GFP signal and the autofluorescence signal. The results show that FLIM-DSLM can be used for sample size up to a few millimetres and can be utilised as a powerful and robust method for biomedical research, for example as a readout of protein-protein interactions via Förster resonance energy transfer.

Multispectral autofluorescence dermoscope for skin lesion assessment

Basal cell carcinoma (BCC) is the most common type of skin cancer. Diagnosis and edge assessment of BCC lesions are based on clinical and dermoscopy evaluation, which are strongly dependent on the expertise and training of the physician. There is a high rate of underdiagnosis because BCC is frequently confused with certain common benign lesions and is often indistinguishable from the surrounding healthy tissue. In the present study, a multispectral fluorescence lifetime imaging (FLIm) dermoscopy system, designed for imaging and analyzing the autofluorescence emission of skin tissue, was used to image thirty-eight patients with diagnosed nodular BCC (nBCC) lesions, using clinically acceptable levels of excitation light exposure. With this system, skin autofluorescence was imaged simultaneously using three emission bands: 390 ± 20 nm, 452 ± 22 nm, and >496 nm, preferentially targeting collagen, NADH, and FAD autofluorescence, respectively. Statistical classifiers based on FLIm features developed to discriminate BCC from healthy tissue showed promising performance (ROC area-under-the-curve of 0.82). This study demonstrates the feasibility of clinically performing multispectral endogenous FLIm dermoscopy providing baseline results indicating the potential of this technology as an image-guided tool to improve the delineation of nBCC during surgical lesion resection.

Automated detection of superficial macrophages in atherosclerotic plaques using autofluorescence lifetime imaging

BACKGROUND AND AIMS

Macrophages play an important role in the development and destabilization of advanced atherosclerotic plaques. Hence, the clinical imaging of macrophage content in advanced plaques could potentially aid in identifying patients most at risk of future clinical events. The lifetime of the autofluorescence emission from atherosclerotic plaques has been correlated with lipids and macrophage accumulation in ex vivo human coronary arteries, suggesting the potential of intravascular endogenous fluorescence or autofluorescence lifetime imaging (FLIM) for macrophage imaging. The aim of this study was to quantify the accuracy of the coronary intima autofluorescence lifetime to detect superficial macrophage accumulation in atherosclerotic plaques.

METHODS

Endogenous FLIM imaging was performed on 80 fresh postmortem coronary segments from 23 subjects. The plaque autofluorescence lifetime at an emission spectral band of 494 ± 20.5 nm was used as a discriminatory feature to detect superficial macrophage accumulation in atherosclerotic plaques. Detection of superficial macrophage accumulation in the imaged coronary segments based on immunohistochemistry (CD68 staining) evaluation was taken as the gold standard. Receiver Operating Characteristic (ROC) curve analysis was applied to select an autofluorescence lifetime threshold value to detect superficial macrophages accumulation.

RESULTS

A threshold of 6 ns in the plaque autofluorescence lifetime at the emission spectral band of 494 ± 20.5 nm was applied to detect plaque superficial macrophages accumulation, resulting in ∼91.5% accuracy.

CONCLUSIONS

This study demonstrates the capability of endogenous FLIM imaging to accurately identify superficial macrophages accumulation in human atherosclerotic plaques, a key biomarker of atherosclerotic plaque vulnerability.

Metabolic Profiling of Live Cancer Tissues Using NAD(P)H Fluorescence Lifetime Imaging

Altered metabolism is a hallmark of cancer, both resulting from and driving oncogenesis. The NAD and NADP redox couples play a key role in a large number of the metabolic pathways involved. In their reduced forms, NADH and NADPH, these molecules are intrinsically fluorescent. As the average time for fluorescence to be emitted following excitation by a laser pulse, the fluorescence lifetime, is exquisitely sensitive to changes in the local environment of the fluorophore, imaging the fluorescence lifetime of NADH and NADPH offers the potential for label-free monitoring of metabolic changes inside living tumors. Here, we describe the biological, photophysical, and methodological considerations required to establish fluorescence lifetime imaging (FLIM) of NAD(P)H as a routine method for profiling the metabolism of living cancer cells and tissues.

Label-Free Assessment of Collagenase Digestion on Bovine Pericardium Properties by Fluorescence Lifetime Imaging

The extracellular matrix architecture of bovine pericardium (BP) has distinct biochemical and biomechanical properties that make it a useful biomaterial in the field of regenerative medicine. Collagen represents the dominant structural protein of BP and is therefore intimately associated with the properties of this biomaterial. Enzymatic degradation of collagen molecules is critical for extracellular matrix turnover, remodeling and ultimately tissue regeneration. We present a quantitative, label-free and non-destructive method for monitoring changes in biochemical and biomechanical properties of BP during tissue degradation, based on multi-spectral fluorescence lifetime imaging (ms-FLIm). Strong correlations of fluorescence intensity ratio and average fluorescence lifetime were identified with collagen content, Young's Modulus and Ultimate tensile strength during collagenase degradation, indicating the potential of optically monitoring collagen degradation using ms-FLIm. The obtained results demonstrate the value of ms-FLIm to assess the quality of biomaterials in situ for applications in regenerative medicine.

Using in vivo fluorescence lifetime imaging to detect HER2-positive tumors

Background

Assessment of the status of tumor biomarkers in individual patients would facilitate personalizing treatment strategy, and continuous monitoring of those biomarkers and their binding process to the therapeutic drugs would provide a means for early evaluation of the efficacy of therapeutic intervention. Fluorescent probes can accumulate inside the tumor region due to the leakiness of its vascularization and this can make it difficult to distinguish if the measured fluorescence intensity is from probes bound to target receptors or just accumulated unbound probes inside the tumor. In this paper, we have studied the fluorescence lifetime as a means to distinguish bound HER2 specific affibody probes to HER2 receptors.

Our imaging system is a time-resolved fluorescence system using a Ti-Sapphire femtosecond pulse laser as source and Time correlated Single photon Counting (TCSPC) system as detector for calculating the lifetime of the contrast agent. HER2-specific Affibody (His6-ZHER2:GS-Cys) (Affibody, Stockholm, Sweden) conjugated with a Dylight750 fluorescent probe (Thermo-Fisher-Scientific, Waltham, Massachusetts) was used as contrast agent and six human cancer cell lines, BT-474, SKOV-3, NCI-N87, MDA-MB-361, MCF-7, and MDA-MB-468, known to express different levels of HER2/neu, are used in athymic mice xenografts.

Results

By comparing the lifetime of unbound contrast agent (at the contralateral site) to the fluorescence lifetime at the tumor site, our results show that the fluorescence lifetime decreases as HER2 specific Affibody probes bind to the tumor receptors. A decrease of ~15% (100ps) in tumor fluorescence lifetime was observed in tumors with mid to high HER2 expression. Smaller decreases were observed in tumors with low-level of HER2 receptors and no change was observed in the non-HER2-expressing tumors.

Conclusions

Using HER2-specific Affibody conjugated with the Dylight750 fluorescent probe as contrast agent, we demonstrated in live animals that change in fluorescence lifetime of the bound contrast agent can be used to assess the high to mid-level expression of HER2 expressing tumors in-vivo in only one measurement. The rationale is that the fluorescence lifetime of our specific probe is sensitive to affinity to, and specific interaction with, other molecules.

Application of Fluorescence Lifetime Imaging (FLIM) to Measure Intracellular Environments in a Single Cell

Fluorescence lifetime imaging (FLIM) has now been used in many bioscience fields, which comes from the quantification of fluorescence lifetime. The procedure for obtaining lifetime images is very similar to that used in fluorescence microscopy. However, obtaining reliable lifetime images requires an understanding of the theory of fluorescence lifetime, principle of FLIM systems, and evaluation procedure of intracellular environments. In this chapter, the materials, methods, and notes on FLIM measurements have been described, in conjunction with a brief explanation of the background of FLIM.

Monitoring Ca2+ elevations in individual astrocytes upon local release of amyloid beta in acute brain slices

The pathogenesis of Alzheimer's disease (AD) is thought to involve acute neurotoxic effects exerted by oligomeric forms of amyloid-β 1-42 (Aβ). Application of Aβ oligomers in physiological concentrations have been shown to transiently elevate internal Ca2+ in cultured astroglia. While the cellular machinery involved has been extensively explored, to what degree this important signalling cascade occurs in organised brain tissue has remained unclear. Here we adapted two-photon excitation microscopy and calibrated time-resolved imaging (FLIM), coupled with patch-clamp electrophysiology, to monitor Ca2+ concentration ([Ca2+]) inside individual astrocytes and principal neurons in acute brain slices. Inside the slice tissue local micro-ejection of Aβ in sub-micromolar concentrations triggered prominent [Ca2+] elevations in an adjacent astrocyte translated as an approximately two-fold increase (averaged over ∼5min) in basal [Ca2+]. This elevation did not spread to neighbouring cells and appeared comparable in amplitude with commonly documented spontaneous [Ca2+] rises in astroglia. Principal nerve cells (pyramidal neurons) also showed Ca2+ sensitivity, albeit to a lesser degree. These observations shed light on the extent and dynamics of the acute physiological effects of Aβ on brain cells in situ, in the context of AD.

TriPer, an optical probe tuned to the endoplasmic reticulum tracks changes in luminal H2O2

Background

The fate of hydrogen peroxide (H₂O₂) in the endoplasmic reticulum (ER) has been inferred indirectly from the activity of ER-localized thiol oxidases and peroxiredoxins, in vitro, and the consequences of their genetic manipulation, in vivo. Over the years hints have suggested that glutathione, puzzlingly abundant in the ER lumen, might have a role in reducing the heavy burden of H₂O₂ produced by the luminal enzymatic machinery for disulfide bond formation. However, limitations in existing organelle-targeted H₂O₂ probes have rendered them inert in the thiol-oxidizing ER, precluding experimental follow-up of glutathione’s role in ER H₂O₂ metabolism.

Results

Here we report on the development of TriPer, a vital optical probe sensitive to changes in the concentration of H₂O₂ in the thiol-oxidizing environment of the ER. Consistent with the hypothesized contribution of oxidative protein folding to H₂O₂ production, ER-localized TriPer detected an increase in the luminal H₂O₂ signal upon induction of pro-insulin (a disulfide-bonded protein of pancreatic β-cells), which was attenuated by the ectopic expression of catalase in the ER lumen. Interfering with glutathione production in the cytosol by buthionine sulfoximine (BSO) or enhancing its localized destruction by expression of the glutathione-degrading enzyme ChaC1 in the lumen of the ER further enhanced the luminal H₂O₂ signal and eroded β-cell viability.

Conclusions

A tri-cysteine system with a single peroxidatic thiol enables H₂O₂ detection in oxidizing milieux such as that of the ER. Tracking ER H₂O₂ in live pancreatic β-cells points to a role for glutathione in H₂O₂ turnover.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-017-0367-5) contains supplementary material, which is available to authorized users.

In vivo multiphoton tomography and fluorescence lifetime imaging of human brain tumor tissue

High resolution multiphoton tomography and fluorescence lifetime imaging differentiates glioma from adjacent brain in native tissue samples ex vivo. Presently, multiphoton tomography is applied in clinical dermatology and experimentally. We here present the first application of multiphoton and fluorescence lifetime imaging for in vivo imaging on humans during a neurosurgical procedure. We used a MPTflex™ Multiphoton Laser Tomograph (JenLab, Germany). We examined cultured glioma cells in an orthotopic mouse tumor model and native human tissue samples. Finally the multiphoton tomograph was applied to provide optical biopsies during resection of a clinical case of glioblastoma. All tissues imaged by multiphoton tomography were sampled and processed for conventional histopathology. The multiphoton tomograph allowed fluorescence intensity- and fluorescence lifetime imaging with submicron spatial resolution and 200 picosecond temporal resolution. Morphological fluorescence intensity imaging and fluorescence lifetime imaging of tumor-bearing mouse brains and native human tissue samples clearly differentiated tumor and adjacent brain tissue. Intraoperative imaging was found to be technically feasible. Intraoperative image quality was comparable to ex vivo examinations. To our knowledge we here present the first intraoperative application of high resolution multiphoton tomography and fluorescence lifetime imaging of human brain tumors in situ. It allowed in vivo identification and determination of cell density of tumor tissue on a cellular and subcellular level within seconds. The technology shows the potential of rapid intraoperative identification of native glioma tissue without need for tissue processing or staining.

Simultaneous morphological and biochemical endogenous optical imaging of atherosclerosis

AIMS

The aim of this study was to validate novel imaging technology for simultaneous morphological and biochemical endogenous optical imaging of coronary atherosclerotic plaque.

METHODS AND RESULTS

Optical coherence tomography (OCT) generates high-resolution 3D images of plaque morphology and endogenous fluorescence lifetime imaging microscopy (FLIM) characterizes biochemical composition. Both imaging modalities rely on plaque's intrinsic optical characteristics, making contrast agents unnecessary. A multimodal OCT/FLIM system was utilized to generate luminal biochemical maps superimposed on high-resolution (7 µm axial and 13 µm lateral) structural volumetric images. Forty-seven fresh postmortem human coronary segments were imaged: pathological intimal thickening (PIT, n = 26), fibroatheroma (FA, n = 12), thin-cap FA (TCFA, n = 2), and fibrocalcific plaque (CA, n = 7), determined by histopathology. Multimodal images were evaluated, and each plaque identified as PIT, FA, TCFA, or CA based on expert OCT readers, and as having high-lipid (HL), high-collagen (HC), or low-collagen/low-lipid (LCL) luminal composition based on linear discriminant analysis of FLIM. Of 47 plaques, 89.4% (42/47) of the plaques were correctly identified based on OCT/FLIM evaluation using tissue histopathology and immunohistochemistry as the gold standard. Four of the misclassifications corresponded to confusing PIT with HL luminal composition for FA with HL cap. The other corresponded to confusing FA with a HC cap for FA with an LCL cap.

CONCLUSION

We have demonstrated the feasibility of accurate simultaneous OCT/FLIM morphological and biochemical characterization of coronary plaques at spatial resolutions and acquisition speeds compatible with catheter-based intravascular imaging. The success of this pilot study sets up future development of a multimodal intravascular imaging system that will enable studies that could help improve our understanding of plaque pathogenesis.

Response properties of the genetically encoded optical H2O2 sensor HyPer

Reactive oxygen species mediate cellular signaling and neuropathologies. Hence, there is tremendous interest in monitoring (sub)cellular redox conditions. We evaluated the genetically engineered redox sensor HyPer in mouse hippocampal cell cultures. Two days after lipofection, neurons and glia showed sufficient expression levels, and H2O2 reversibly and dose-dependently increased the fluorescence ratio of cytosolic HyPer. Yet, repeated H2O2 treatment caused progressively declining responses, and with millimolar doses an apparent recovery started while H2O2 was still present. Although HyPer should be H2O2 specific, it seemingly responded also to other oxidants and altered cell-endogenous superoxide production. Control experiments with the SypHer pH sensor confirmed that the HyPer ratio responds to pH changes, decreasing with acidosis and increasing during alkalosis. Anoxia/reoxygenation evoked biphasic HyPer responses reporting apparent reduction/oxidation; replacing Cl(-) exerted only negligible effects. Mitochondria-targeted HyPer readily responded to H2O2-albeit less intensely than cytosolic HyPer. With ratiometric two-photon excitation, H2O2 increased the cytosolic HyPer ratio. Time-correlated fluorescence-lifetime imaging microscopy (FLIM) revealed a monoexponential decay of HyPer fluorescence, and H2O2 decreased fluorescence lifetimes. Dithiothreitol failed to further reduce HyPer or to induce reasonable FLIM and two-photon responses. By enabling dynamic recordings, HyPer is superior to synthetic redox-sensitive dyes. Its feasibility for two-photon excitation also enables studies in more complex preparations. Based on FLIM, quantitative analyses might be possible independent of switching excitation wavelengths. Yet, because of its pronounced pH sensitivity, adaptation to repeated oxidation, and insensitivity to reducing stimuli, HyPer responses have to be interpreted carefully. For reliable data, side-by-side pH monitoring with SypHer is essential.

Fluorescence kinetics of PSII crystals containing Ca(2+) or Sr(2+) in the oxygen evolving complex

Photosystem II (PSII) is the pigment-protein complex which converts sunlight energy into chemical energy by catalysing the process of light-driven oxidation of water into reducing equivalents in the form of protons and electrons. Three-dimensional structures from x-ray crystallography have been used extensively to model these processes. However, the crystal structures are not necessarily identical to those of the solubilised complexes. Here we compared picosecond fluorescence of solubilised and crystallised PSII core particles isolated from the thermophilic cyanobacterium Thermosynechococcus elongatus. The fluorescence of the crystals is sensitive to the presence of artificial electron acceptors (K3Fe(CN)3) and electron transport inhibitors (DCMU). In PSII with reaction centres in the open state, the picosecond fluorescence of PSII crystals and solubilised PSII is indistinguishable. Additionally we compared picosecond fluorescence of native PSII with PSII in which Ca(2) in the oxygen evolving complex (OEC) is biosynthetically replaced by Sr(2+). With the Sr(2+) replaced OEC the average fluorescence decay slows down slightly (81ps to 85ps), and reaction centres are less readily closed, indicating that both energy transfer/trapping and electron transfer are affected by the replacement.

Structural and dynamical aspects of skin studied by multiphoton excitation fluorescence microscopy-based methods

This mini-review reports on applications of particular multiphoton excitation microscopy-based methodologies employed in our laboratory to study skin. These approaches allow in-depth optical sectioning of the tissue, providing spatially resolved information on specific fluorescence probes' parameters. Specifically, by applying these methods, spatially resolved maps of water dipolar relaxation (generalized polarization function using the 6-lauroyl-2-(N,N-dimethylamino)naphthale probe), activity of protons (fluorescence lifetime imaging using a proton sensitive fluorescence probe--2,7-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein) and diffusion coefficients of distinct fluorescence probes (raster imaging correlation spectroscopy) can be obtained from different regions of the tissue. Comparative studies of different tissue strata, but also between equivalent regions of normal and abnormal excised skin, including applications of fluctuation correlation spectroscopy on transdermal penetration of liposomes are presented and discussed. The data from the different studies reported reveal the intrinsic heterogeneity of skin and also prove these strategies to be powerful noninvasive tools to explore structural and dynamical aspects of the tissue.