Fluorescence intensity and lifetime redox ratios detect metabolic perturbations in T cells

Spatiotemporal relationships between neuronal, metabolic, and hemodynamic signals in the awake and anesthetized mouse brain

Neurovascular coupling (NVC) and neurometabolic coupling (NMC) provide the basis for functional magnetic resonance imaging and positron emission tomography to map brain neurophysiology. While increases in neuronal activity are often accompanied by increases in blood oxygen delivery and oxidative metabolism, these observations are not the rule. This decoupling is important when interpreting brain network organization (e.g., resting-state functional connectivity [RSFC]) because it is unclear whether changes in NMC/NVC affect RSFC measures. We leverage wide-field optical imaging in Thy1-jRGECO1a mice to map cortical calcium activity in pyramidal neurons, flavoprotein autofluorescence (representing oxidative metabolism), and hemodynamic activity during wake and ketamine/xylazine anesthesia. Spontaneous dynamics of all contrasts exhibit patterns consistent with RSFC. NMC/NVC relative to excitatory activity varies over the cortex. Ketamine/xylazine profoundly alters NVC but not NMC. Compared to awake RSFC, ketamine/xylazine affects metabolic-based connectomes moreso than hemodynamic-based measures of RSFC. Anesthesia-related differences in NMC/NVC timing do not appreciably alter RSFC structure.

A three-dimensional valve-on-chip microphysiological system implicates cell cycle progression, cholesterol metabolism and protein homeostasis in early calcific aortic valve disease progression

BACKGROUND

Calcific aortic valve disease (CAVD) is one of the most common forms of valvulopathy, with a 50 % elevated risk of a fatal cardiovascular event, and greater than 15,000 annual deaths in North America alone. The treatment standard is valve replacement as early diagnostic, mitigation, and drug strategies remain underdeveloped. The development of early diagnostic and therapeutic strategies requires the fabrication of effective in vitro valve mimetic models to elucidate early CAVD mechanisms.

METHODS

In this study, we developed a multilayered physiologically relevant 3D valve-on-chip (VOC) system that incorporated aortic valve mimetic extracellular matrix (ECM), porcine aortic valve interstitial cell (VIC) and endothelial cell (VEC) co-culture and dynamic mechanical stimuli. Collagen and glycosaminoglycan (GAG) based hydrogels were assembled in a bilayer to mimic healthy or diseased compositions of the native fibrosa and spongiosa. Multiphoton imaging and proteomic analysis of healthy and diseased VOCs were performed.

RESULTS

Collagen-based bilayered hydrogel maintained the phenotype of the VICs. Proteins related to cellular processes like cell cycle progression, cholesterol biosynthesis, and protein homeostasis were found to be significantly altered and correlated with changes in cell metabolism in diseased VOCs. This study suggested that diseased VOCs may represent an early, adaptive disease initiation stage, which was corroborated by human aortic valve proteomic assessment.

CONCLUSIONS

In this study, we developed a collagen-based bilayered hydrogel to mimic healthy or diseased compositions of the native fibrosa and spongiosa layers. When the gels were assembled in a VOC with VECs and VICs, the diseased VOCs revealed key insights about the CAVD initiation process.

STATEMENT OF SIGNIFICANCE

Calcific aortic valve disease (CAVD) elevates the risk of death due to cardiovascular pathophysiology by 50 %, however, prevention and mitigation strategies are lacking, clinically. Developing tools to assess early disease would significantly aid in the prevention of disease and in the development of therapeutics. Previously, studies have utilized collagen and glycosaminoglycan-based hydrogels for valve cell co-cultures, valve cell co-cultures in dynamic environments, and inorganic polymer-based multilayered hydrogels; however, these approaches have not been combined to make a physiologically relevant model for CAVD studies. We fabricated a bi-layered hydrogel that closely mimics the aortic valve and used it for valve cell co-culture in a dynamic platform to gain mechanistic insights into the CAVD initiation process using proteomic and multiphoton imaging assessment.

Large field-of-view metabolic profiling of murine brain tissue following morphine incubation using label-free multiphoton microscopy

BACKGROUND

Although the effects on neural activation and glucose consumption caused by opiates such as morphine are known, the metabolic machinery underlying opioid use and misuse is not fully explored. Multiphoton microscopy (MPM) techniques have been developed for optical imaging at high spatial resolution. Despite the increased use of MPM for neural imaging, the use of intrinsic optical contrast has seen minimal use in neuroscience.

NEW METHOD

We present a label-free, multimodal microscopy technique for metabolic profiling of murine brain tissue following incubation with morphine sulfate (MSO4). We evaluate two- and three-photon excited autofluorescence, and second and third harmonic generation to determine meaningful intrinsic contrast mechanisms in brain tissue using simultaneous label-free, autofluorescence multi-harmonic (SLAM) microscopy.

RESULTS

Regional differences quantified in the cortex, caudate, and thalamus of the brain demonstrate region-specific changes to metabolic profiles measured from FAD intensity, along with brain-wide quantification. While the overall intensity of FAD signal significantly decreased after morphine incubation, this metabolic molecule accumulated near the nucleus accumbens.

COMPARISON WITH EXISTING METHODS

Histopathology requires tissue fixation and staining to determine cell type and morphology, lacking information about cellular metabolism. Tools such as fMRI or PET imaging have been widely used, but lack cellular resolution. SLAM microscopy obviates the need for tissue preparation, permitting immediate use and imaging of tissue with subcellular resolution in its native environment.

CONCLUSIONS

This study demonstrates the utility of SLAM microscopy for label-free investigations of neural metabolism, especially the intensity changes in FAD autofluorescence and structural morphology from third-harmonic generation.

METAPHOR: Metabolic evaluation through phasor-based hyperspectral imaging and organelle recognition for mouse blastocysts and oocytes

Only 30% of embryos from in vitro fertilized oocytes successfully implant and develop to term, leading to repeated transfer cycles. To reduce time-to-pregnancy and stress for patients, there is a need for a diagnostic tool to better select embryos and oocytes based on their physiology. The current standard employs brightfield imaging, which provides limited physiological information. Here, we introduce METAPHOR: Metabolic Evaluation through Phasor-based Hyperspectral Imaging and Organelle Recognition. This non-invasive, label-free imaging method combines two-photon illumination and AI to deliver the metabolic profile of embryos and oocytes based on intrinsic autofluorescence signals. We used it to classify i) mouse blastocysts cultured under standard conditions or with depletion of selected metabolites (glucose, pyruvate, lactate); and ii) oocytes from young and old mouse females, or in vitro-aged oocytes. The imaging process was safe for blastocysts and oocytes. The METAPHOR classification of control vs. metabolites-depleted embryos reached an area under the ROC curve (AUC) of 93.7%, compared to 51% achieved for human grading using brightfield imaging. The binary classification of young vs. old/in vitro-aged oocytes and their blastulation prediction using METAPHOR reached an AUC of 96.2% and 82.2%, respectively. Finally, organelle recognition and segmentation based on the flavin adenine dinucleotide signal revealed that quantification of mitochondria size and distribution can be used as a biomarker to classify oocytes and embryos. The performance and safety of the method highlight the accuracy of noninvasive metabolic imaging as a complementary approach to evaluate oocytes and embryos based on their physiology.

Dual excitation spectral autofluorescence lifetime and reflectance imaging for fast macroscopic characterization of tissues

Advancements in optical imaging techniques have revolutionized the field of biomedical research, allowing for the comprehensive characterization of tissues and their underlying biological processes. Yet, there is still a lack of tools to provide quantitative and objective characterization of tissues that can aid clinical assessment in vivo to enhance diagnostic and therapeutic interventions. Here, we present a clinically viable fiber-based imaging system combining time-resolved spectrofluorimetry and reflectance spectroscopy to achieve fast multiparametric macroscopic characterization of tissues. An essential feature of the setup is its ability to perform dual wavelength excitation in combination with recording time-resolved fluorescence data in several spectral intervals. Initial validation of this bimodal system was carried out in freshly resected human colorectal cancer specimens, where we demonstrated the ability of the system to differentiate normal from malignant tissues based on their autofluorescence and reflectance properties. To further highlight the complementarity of autofluorescence and reflectance measurements and demonstrate viability in a clinically relevant scenario, we also collected in vivo data from the skin of a volunteer. Altogether, integration of these modalities in a single platform can offer multidimensional characterization of tissues, thus facilitating a deeper understanding of biological processes and potentially advancing diagnostic and therapeutic approaches in various medical applications.

Label-free fluorescence lifetime imaging for the assessment of cell viability in living tumor fragments

Significance

To enable non-destructive longitudinal assessment of drug agents in intact tumor tissue without the use of disruptive probes, we have designed a label-free method to quantify the health of individual tumor cells in excised tumor tissue using multiphoton fluorescence lifetime imaging microscopy (MP-FLIM).

Aim

Using murine tumor fragments which preserve the native tumor microenvironment, we seek to demonstrate signals generated by the intrinsically fluorescent metabolic co-factors nicotinamide adenine dinucleotide phosphate [NAD(P)H] and flavin adenine dinucleotide (FAD) correlate with irreversible cascades leading to cell death.

Approach

We use MP-FLIM of NAD(P)H and FAD on tissues and confirm viability using standard apoptosis and live/dead (Caspase 3/7 and propidium iodide, respectively) assays.

Results

Through a statistical approach, reproducible shifts in FLIM data, determined through phasor analysis, are shown to correlate with loss of cell viability. With this, we demonstrate that cell death achieved through either apoptosis/necrosis or necroptosis can be discriminated. In addition, specific responses to common chemotherapeutic treatment inducing cell death were detected.

Conclusions

These data demonstrate that MP-FLIM can detect and quantify cell viability without the use of potentially toxic dyes, thus enabling longitudinal multi-day studies assessing the effects of therapeutic agents on tumor fragments.

Optical redox imaging to screen synthetic hydrogels for stem cell-derived cardiomyocyte differentiation and maturation

Significance

Heart disease is the leading cause of death in the United States, yet research is limited by the inability to culture primary cardiac cells. Cardiomyocytes (CMs) derived from human induced pluripotent stem cells (iPSCs) are a promising solution for drug screening and disease modeling.

Aim

Induced pluripotent stem cell-derived CM (iPSC-CM) differentiation and maturation studies typically use heterogeneous substrates for growth and destructive verification methods. Reproducible, tunable substrates and touch-free monitoring are needed to identify ideal conditions to produce homogenous, functional CMs.

Approach

We generated synthetic polyethylene glycol-based hydrogels for iPSC-CM differentiation and maturation. Peptide concentrations, combinations, and gel stiffness were tuned independently. Label-free optical redox imaging (ORI) was performed on a widefield microscope in a 96-well screen of gel formulations. We performed live-cell imaging throughout differentiation and early to late maturation to identify key metabolic shifts.

Results

Label-free ORI confirmed the expected metabolic shifts toward oxidative phosphorylation throughout the differentiation and maturation processes of iPSC-CMs on synthetic hydrogels. Furthermore, ORI distinguished high and low differentiation efficiency cell batches in the cardiac progenitor stage.

Conclusions

We established a workflow for medium throughput screening of synthetic hydrogel conditions with the ability to perform repeated live-cell measurements and confirm expected metabolic shifts. These methods have implications for reproducible iPSC-CM generation in biomanufacturing.

Quantitative Optical Redox Imaging of Melanoma Xenografts with Different Metastatic Potentials

To develop imaging biomarkers for tumors aggressiveness, our previous optical redox imaging (ORI) studies of the reduced nicotinamide adenine dinucleotide (NADH) and oxidized flavoproteins (Fp, containing flavin adenine dinucleotide, i.e., FAD) in tumor xenografts of human melanoma associated the high optical redox ratio (ORR = Fp/(Fp + NADH)) and its heterogeneity to the high invasive/metastatic potential, without having reported quantitative results for NADH and Fp. Here, we implemented a calibration procedure to facilitate imaging the nominal concentrations of tissue NADH and Fp in the mouse xenografts of two human melanoma lines, an indolent less metastatic A375P and a more metastatic C8161. Images of the redox indices (NADH, Fp, ORR) revealed the existence of more oxidized areas (OAs) and more reduced areas (RAs) within individual tumors. ORR was found to be higher and NADH lower in C8161 compared to that of A375P xenografts, both globally for the whole tumors and locally in OAs. The ORR in the OA can differentiate xenografts with a higher statistical significance than the global averaged ORR. H&E staining of the tumors indicated that the redox differences we identified were more likely due to intrinsically different cell metabolism, rather than variations in cell density.

Label-free spatially maintained measurements of metabolic phenotypes in cells

Metabolic reprogramming at a cellular level contributes to many diseases including cancer, yet few assays are capable of measuring metabolic pathway usage by individual cells within living samples. Here, autofluorescence lifetime imaging is combined with single-cell segmentation and machine-learning models to predict the metabolic pathway usage of cancer cells. The metabolic activities of MCF7 breast cancer cells and HepG2 liver cancer cells were controlled by growing the cells in culture media with specific substrates and metabolic inhibitors. Fluorescence lifetime images of two endogenous metabolic coenzymes, reduced nicotinamide adenine dinucleotide (NADH) and oxidized flavin adenine dinucleotide (FAD), were acquired by a multi-photon fluorescence lifetime microscope and analyzed at the cellular level. Quantitative changes of NADH and FAD lifetime components were observed for cells using glycolysis, oxidative phosphorylation, and glutaminolysis. Conventional machine learning models trained with the autofluorescence features classified cells as dependent on glycolytic or oxidative metabolism with 90%-92% accuracy. Furthermore, adapting convolutional neural networks to predict cancer cell metabolic perturbations from the autofluorescence lifetime images provided improved performance, 95% accuracy, over traditional models trained via extracted features. Additionally, the model trained with the lifetime features of cancer cells could be transferred to autofluorescence lifetime images of T cells, with a prediction that 80% of activated T cells were glycolytic, and 97% of quiescent T cells were oxidative. In summary, autofluorescence lifetime imaging combined with machine learning models can detect metabolic perturbations between glycolysis and oxidative metabolism of living samples at a cellular level, providing a label-free technology to study cellular metabolism and metabolic heterogeneity.

Deconvolution of the fluorescence spectra measured through a needle probe to assess the functional state of the liver

OBJECTIVES

Currently, one of the most pressing issues for surgeons in the treatment of obstructive jaundice is the ability to assess the functional state of the liver and to detect and determine the degree of liver failure in a timely manner with simple and objective techniques. In this regard, the use of fluorescence spectroscopy method can be considered as one of the ways to improve the informativity of existing diagnostic algorithms in clinical practice and to introduce new diagnostic tools. Thus, the aim of the work was to study in vivo the functional state of liver parenchyma by the method of fluorescence spectroscopy implemented through a needle probe and assess the contribution of the main tissue fluorophores to reveal new diagnostic criteria.

MATERIALS AND METHODS

We compared data from 20 patients diagnosed with obstructive jaundice and 11 patients without this syndrome. Measurements were performed using a fluorescence spectroscopy method at excitation wavelengths of 365 and 450 nm. Data were collected using a 1 mm fiber optic needle probe. The analysis was based on the comparison of the results of deconvolution with the combinations of Gaussian curves reflecting the contribution of the pure fluorophores in the liver tissues.

RESULTS

The results showed a statistically significant increase in the contribution of curves reflecting NAD(P)H fluorescence, bilirubin, and flavins in the group of patients with obstructive jaundice. This and the calculated redox ratio values indicated that the energy metabolism of the hepatocytes may have shifted to glycolysis due to hypoxia. An increase in vitamin A fluorescence was also observed. It may also serve as a marker of liver damage, indicating impaired vitamin A mobilization from the liver due to cholestasis.

CONCLUSIONS

The results obtained reflect changes associated with shifts in the content of the main fluorophores characterizing hepatocyte dysfunction caused by accumulation of bilirubin and bile acids and after disturbance of oxygen utilization. The contributions of NAD(P)H, flavins, and bilirubin as well as vitamin A can be used for further studies as promising diagnostic and prognostic markers for the course of liver failure. Further work will include collecting fluorescence spectroscopy data in patients with different clinical effects of obstructive jaundice on postoperative clinical outcome after biliary decompression.

Two-photon fluorescence lifetime imaging microscopy of NADH metabolism in HIV-1 infected cells and tissues

Rapid detection of microbial-induced cellular changes during the course of an infection is critical to understanding pathogenesis and immunological homeostasis. In the last two decades, fluorescence imaging has received significant attention for its ability to help characterize microbial induced cellular and tissue changes in in vitro and in vivo settings. However, most of these methods rely on the covalent conjugation of large exogenous probes and detection methods based on intensity-based imaging. Here, we report a quantitative, intrinsic, label-free, and minimally invasive method based on two-photon fluorescence lifetime (FLT) imaging microscopy (2p-FLIM) for imaging 1,4-dihydro-nicotinamide adenine dinucleotide (NADH) metabolism of virally infected cells and tissue sections. To better understand virally induced cellular and tissue changes in metabolism we have used 2p-FLIM to study differences in NADH intensity and fluorescence lifetimes in HIV-1 infected cells and tissues. Differences in NADH fluorescence lifetimes are associated with cellular changes in metabolism and changes in cellular metabolism are associated with HIV-1 infection. NADH is a critical co-enzyme and redox regulator and an essential biomarker in the metabolic processes. Label-free 2p-FLIM application and detection of NADH fluorescence using viral infection systems are in their infancy. In this study, the application of the 2p-FLIM assay and quantitative analyses of HIV-1 infected cells and tissue sections reveal increased fluorescence lifetime and higher enzyme-bound NADH fraction suggesting oxidative phosphorylation (OxPhos) compared to uninfected cells and tissues. 2p-FLIM measurements improve signal to background, fluorescence specificity, provide spatial and temporal resolution of intracellular structures, and thus, are suitable for quantitative studies of cellular functions and tissue morphology. Furthermore, 2p-FLIM allows distinguishing free and bound populations of NADH by their different fluorescence lifetimes within single infected cells. Accordingly, NADH fluorescence measurements of individual single cells should provide necessary insight into the heterogeneity of metabolic activity of infected cells. Implementing 2p-FLIM to viral infection systems measuring NADH fluorescence at the single or subcellular level within a tissue can provide visual evidence, localization, and information in a real-time diagnostic or therapeutic metabolic workflow.

Fluorescence Lifetime Imaging of NAD(P)H T Cells Autofluorescence in the Lymphatic Nodes to Assess the Effectiveness of Anti-CTLA-4 Immunotherapy

The main problem in the field of tumor immunotherapy is the lack of reliable biomarkers that allow pre-determining the susceptibility of individual patients to treatment, as well as insufficient knowledge about the resistance mechanisms. Biomarkers based on the autofluorescence of metabolic coenzymes in immune cells can become a powerful new predictor of early tumor response to treatment, whereas the optical FLIM method can be a tool to predict the effectiveness of immunotherapy, which allows preserving the spatial structure of the sample and obtaining results on the metabolic status of immune cells in real time. The aim of the study is to conduct a metabolic autofluorescence imaging study of the NAD(P)H metabolic coenzyme in immune cells of freshly isolated lymph nodes as a potential marker for assessing the effectiveness of an early response to immunotherapy.

Materials and Methods

The study was carried out on C57Bl/6 FoxP3-EGFP mice with B16F0 melanoma implanted near the inguinal lymph node. The mice were injected with antibodies to CTLA-4 (Bio X Cell, USA) (250 μg per mouse, intraperitoneally on days 7, 8, 11, and 12 of the tumor growth). FLIM images in the nicotinamide adenine dinucleotide (phosphate) coenzyme (NAD(P)H) channel (excitation - 375 nm, reception - 435-485 nm) were received using an LSM 880 fluorescent confocal laser scanning microscope (Carl Zeiss, Germany) equipped with a FLIM Simple-Tau module 152 TCSPC (Becker & Hickl GmbH, Germany). Flow cytometry was conducted using a BD FACSAria III cell sorter (BD Biosciences, USA).

Results

Immunotherapy with checkpoint inhibitors resulted in marked metabolic rearrangements in T cells of freshly isolated lymph nodes in responder mice, with inhibition of the tumor growth. Fluorescence lifetime imaging data on NAD(P)H indicated an increase in the free fraction of NADH α1, a form associated with glycolysis to meet high demands of the activated T cells and pro-inflammatory cytokine synthesis. In contrast, non-responder mice with advanced tumors showed low values of the ratio of free fraction to bound α1/α2, which may be related to mechanisms of resistance to therapy.The response to immunotherapy was verified by data on the expression of activation and proliferation markers by means of flow cytometry. The authors observed an increase in the production of the pro-inflammatory cytokine IFN-γ in effector T cells in responder mice compared to untreated controls and non-responders. In addition, an increase in the expression of the surface activation markers CD25 and CD69 was registered compared to untreated controls.

Conclusion

Use of the FLIM method allowed to demonstrate that autofluorescence of the NAD(P)H coenzyme is sensitive to the response to checkpoint immunotherapy and can be used as a reliable marker of the effectiveness of response to treatment.

Osr1 Regulates Macrophage-mediated Liver Inflammation in Nonalcoholic Fatty Liver Disease Progression

BACKGROUND & AIMS

Liver macrophage-mediated inflammation contributes to the pathogenesis of the nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH). Odd skipped-related 1 (Osr1) is a putative transcription factor previously reported to be involved in NASH progression; however, the underlying mechanisms remain unknown. The current study focused on the role of Osr1 in macrophage polarization and metabolism and its associated functions in the inflammation-induced pathogenesis of NASH.

METHODS

OSR1/Osr1 expression patterns were compared in normal and NASH patients and mouse livers. NASH was established and compared between hepatocyte-specific Osr1 knockout (Osr1^ΔHep), macrophage-specific Osr1 knockout (Osr1^ΔMφ), and wild-type (Osr1^F) mice fed with 3 different chronic obesogenic diets and methionine choline-deficient diet. Using genetic and therapeutic strategies in vitro and in vivo, the downstream targets of Osr1 and the associated mechanisms in inflammation-induced NASH were established.

RESULTS

Osr1 was expressed in both hepatocytes and macrophages and exhibited different expression patterns in NASH. In NAFLD and NASH murine models, deleting Osr1 in myeloid cells (Osr1^ΔMφ), but not hepatocytes, aggravated steatohepatitis with pronounced liver inflammation. Myeloid Osr1 deletion resulted in a polarization switch toward a pro-inflammatory phenotype associated with reduced oxidative phosphorylation activity. These inflamed Osr1^ΔMφ macrophages promoted steatosis and inflammation in hepatocytes via cytokine secretion. We identified 2 downstream transcriptional targets of Osr1, c-Myc, and PPARγ and established the Osr1-PPARγ cascade in macrophage polarization and liver inflammation by genetic study and rosiglitazone treatment in vivo. We tested a promising intervention strategy targeting Osr1-PPARγ by AAV8L-delivered Osr1 expression or rosiglitazone that significantly repressed NAFLD/NASH progression in Osr1^F and Osr1^ΔMφ mice.

CONCLUSIONS

Myeloid Osr1 mediates liver immune homeostasis and disrupting Osr1 aggravates the progression of NAFLD/NASH.

FLIM of NAD(P)H in Lymphatic Nodes Resolves T-Cell Immune Response to the Tumor

Assessment of T-cell response to the tumor is important for diagnosis of the disease and monitoring of therapeutic efficacy. For this, new non-destructive label-free methods are required. Fluorescence lifetime imaging (FLIM) of metabolic coenzymes is a promising innovative technology for the assessment of the functional status of cells. The purpose of this work was to test whether FLIM can resolve metabolic alterations that accompany T-cell reactivation to the tumors. The study was carried out on C57Bl/6 FoxP3-EGFP mice bearing B16F0 melanoma. Autofluorescence of the immune cells in fresh lymphatic nodes (LNs) was investigated. It was found that fluorescence lifetime parameters of nicotinamide adenine dinucleotide (phosphate) NAD(P)H are sensitive to tumor development. Effector T-cells in the LNs displayed higher contribution of free NADH, the form associated with glycolysis, in all tumors and the presence of protein-bound NADPH, associated with biosynthetic processes, in the tumors of large size. Flow cytometry showed that the changes in the NADH fraction of the effector T-cells correlated with their activation, while changes in NADPH correlated with cell proliferation. In conclusion, FLIM of NAD(P)H in fresh lymphoid tissue is a powerful tool for assessing the immune response to tumor development.

Imaging approaches for monitoring three-dimensional cell and tissue culture systems

The past decade has seen an increasing demand for more complex, reproducible and physiologically relevant tissue cultures that can mimic the structural and biological features of living tissues. Monitoring the viability, development and responses of such tissues in real-time are challenging due to the complexities of cell culture physical characteristics and the environments in which these cultures need to be maintained in. Significant developments in optics, such as optical manipulation, improved detection and data analysis, have made optical imaging a preferred choice for many three-dimensional (3D) cell culture monitoring applications. The aim of this review is to discuss the challenges associated with imaging and monitoring 3D tissues and cell culture, and highlight topical label-free imaging tools that enable bioengineers and biophysicists to non-invasively characterise engineered living tissues.

Development of an optical fiber-based redox monitoring system for tissue metabolism

An optical redox ratio can potentially be used to report on the dynamics of cell and tissue metabolism and define altered metabolic conditions for different pathologies. While there are methods to measure the optical redox ratio, they are not particularly suited to real-time in situ or in vivo analysis. Here, we have developed a fiber-optic system to measure redox ratios in cells and tissues and two mathematical models to enable real-time, in vivo redox measurements. The optical redox ratios in tissue explants are correlated directly with endogenous NADH/FAD fluorescence emissions. We apply the mathematical models to the two-photon microscopy data and show consistent results. We also used our fiber-optic system to measure redox in different tissues and show consistent results between the two models, hence demonstrating proof-of-principle. This innovative redox monitoring system will have practical applications for defining different metabolic disease states.

Identification of Rare Cell Populations in Autofluorescence Lifetime Image Data

Drug‐resistant cells and anti‐inflammatory immune cells within tumor masses contribute to tumor aggression, invasion, and worse patient outcomes. These cells can be a small proportion (<10%) of the total cell population of the tumor. Due to their small quantity, the identification of rare cells is challenging with traditional assays. Single cell analysis of autofluorescence images provides a live‐cell assay to quantify cellular heterogeneity. Fluorescence intensities and lifetimes of the metabolic coenzymes reduced nicotinamide adenine dinucleotide and oxidized flavin adenine dinucleotide allow quantification of cellular metabolism and provide features for classification of cells with different metabolic phenotypes. In this study, Gaussian distribution modeling and machine learning classification algorithms are used for the identification of rare cells within simulated autofluorescence lifetime image data of a large tumor comprised of tumor cells and T cells. A Random Forest machine learning algorithm achieved an overall accuracy of 95% for the identification of cell type from the simulated optical metabolic imaging data of a heterogeneous tumor of 20,000 cells consisting of 70% drug responsive breast cancer cells, 5% drug resistant breast cancer cells, 20% quiescent T cells and 5% activated T cells. High resolution imaging methods combined with single‐cell quantitative analyses allows identification and quantification of rare populations of cells within heterogeneous cultures

Effects of photobiomodulation on the redox state of healthy and cancer cells

Photobiomodulation therapy (PBMT) uses light to stimulate cells. The molecular basis of the effects of PBMT is being unveiled, but it is stated that the cytochrome-c oxidase enzyme in mitochondria, a photon acceptor of PBMT, contributes to an increase in ATP production and modulates the reduction and oxidation of electron carriers NADH and FAD. Since its effects are not fully understood, PBMT is not used on tumors. Thus, it is interesting to investigate if its effects correlate to mitochondrial metabolism and if so, how it could be linked to the optical redox ratio (ORR), defined as the ratio of FAD/(NADH + FAD) fluorescences. To that end, fibroblasts (HDFn cell line) and oral squamous cell carcinoma (SCC-25 cell line) were irradiated with a light source of 780 nm and a total dose of 5 J/cm2, and imaged by optical microscopy. PBMT down-regulated the SCC-25 ORR by 10%. Furthermore, PBMT led to an increase in ROS and ATP production in carcinoma cells after 4 h, while fibroblasts only had a modest ATP increase 6 h after irradiation. Cell lines did not show distinct cell cycle profiles, as both had an increase in G2/M cells. This study indicates that PBMT decreases the redox state of oral cancer by possibly increasing glycolysis and affects normal and tumor cells through distinct pathways. To our knowledge, this is the first study that investigated the effects of PBMT on mitochondrial metabolism from the initiation of the cascade to DNA replication. This is an essential step in the investigation of the mechanism of action of PBMT in an effort to avoid misinterpretations of a variety of combined protocols.