Correlating two-photon excited fluorescence imaging of breast cancer cellular redox state with seahorse flux analysis of normalized cellular oxygen consumption

Compact femtosecond fiber laser tunable from 800 to 850 nm with pulse energy exceeding 5 nJ

This paper introduces a compact, tunable femtosecond laser based on an Erbium-doped fiber, utilizing the Self-Soliton Frequency Shifted technique and PPLN crystal as a Second Harmonic Generation module. Achieving an unparalleled frequency conversion efficiency up to 55% for the 800 - 850 nm wavelength range, this compact laser emits sub-100 fs pulses. The laser operates simultaneously within the first and third biological windows, delivering pulse energies of 10.4 nJ and 5.1 nJ, respectively. This performance, previously unattained in similar systems, is achieved while maintaining Second Harmonic Generation power stability below 2% RMS. The presented compact laser, developed for bladder cancer detection through multiphoton microscopy, will significantly improve the system's compactness, precision, and cancer detection efficiency.

Optical imaging provides flow-cytometry-like single-cell level analysis of HIF-1α-mediated metabolic changes in radioresistant head and neck squamous carcinoma cells

Significance

Radioresistance remains a significant problem for head and neck squamous cell carcinoma (HNSCC) patients. To mitigate this, the cellular and molecular pathways used by radioresistant HNSCC that drive recurrence must be studied.

Aim

We aim to demonstrate optical imaging strategies to provide flow cytometry-like single-cell level analysis of hypoxia-inducible factor 1-alpha (HIF-1α)-mediated metabolic changes in the radioresistant and radiosensitive HNSCC cells but in a more efficient, cost-effective, and non-destructive manner. Through both optical imaging and flow cytometry studies, we will reveal the role of radiation-induced HIF-1α overexpression and the following metabolic changes in the radioresistance development for HNSCC.

Approach

We optimized the use of two metabolic probes: 2-[N-(7-nitrobenz-2-oxa-1, 3-diazol-4-yl) amino]-2-deoxy-D-glucose (2-NBDG) (to report glucose uptake) and Tetramethylrhodamine ethyl ester (TMRE) (to report mitochondrial membrane potential) with both a standard fluorescence microscope and a flow cytometry device, to report the changes in metabolism between radioresistant (rSCC-61) and radiosensitive (SCC-61) HNSCC cell lines under radiation stresses with or without HIF-1α inhibition.

Results

We found that the matched HNSCC cell lines had different baseline metabolic phenotypes, and their metabolism responded differently to radiation stress along with significantly enhanced HIF-1α expressions in the rSCC-61 cells. HIF-1α inhibition during the radiation treatment modulates the metabolic changes and radio-sensitizes the rSCC-61 cells. Through these studies, we demonstrated that a standard fluorescence microscope along with proper image processing methods can provide flow cytometry-like single-cell level analysis of HIF-1α-mediated metabolic changes in the radioresistant and radiosensitive HNSCC cells.

Conclusions

Our reported optical imaging strategies may enable one to study the role of metabolism reprogramming in cancer therapeutic resistance development at the single-cell level in a more efficient, cost-effective, and non-destructive manner. Our understanding of radiation resistance mechanisms using our imaging methods will offer opportunities to design targeted radiotherapy for improved treatment outcomes for HNSCC patients.

Passage dependence of NADH redox status and reactive oxygen species level in vitro in triple-negative breast cancer cell lines with different invasiveness

Background

The redox status of nicotinamide adenine dinucleotide (NAD; including oxidized form NAD+ and reduced form NADH) plays key roles in both health and disease and has been actively studied to develop cancer biomarkers and therapeutic strategies. With the optical redox imaging (ORI) technique, we have been investigating the relationship of NADH redox status, reactive oxygen species (ROS), and invasiveness in breast cancer cell cultures, and have associated higher invasiveness with more oxidized NADH redox state. However, the cell cultures may have phenotypic drift and metabolic change with increased passage numbers.

Methods

We investigated the passage-dependence of NADH redox status and ROS levels in two triple-negative breast cancer (TNBC) cell cultures: the more invasive/metastatic MDA-MB-231 and the less invasive/metastatic HCC1806 cell lines. We measured the NADH redox status, redox plasticity, and cytoplasmic and mitochondrial ROS levels under the basal condition and metabolic perturbations of the mitochondrial electron transport chain. We evaluated the dependence of redox and ROS profiles on the cell passage number by comparing the early (<20 passages) with the late (>60 passages) passage cells.

Results

(I) NADH redox and ROS baselines are stable and independent of cell passage number, but can vary with passage number under metabolic perturbations depending on specific perturbation and cell line; (II) NADH redox status and intracellular ROS levels can change discordantly in cancer cells; (III) under both basal and metabolically perturbed conditions, the more invasive cell line has a more oxidized NADH redox status with a higher basal cytoplasmic ROS level than the less invasive line, regardless of passage number.

Conclusions

The general correlation between redox, ROS, and invasiveness in studied TNBC cells is not very sensitive to passage number. These results indicate that NADH redox and basal ROS status in TNBC likely reflect the intrinsic progressive nature of TNBC cells.

Metabolic light absorption, scattering, and emission (MetaLASE) microscopy

Optical imaging of metabolism can provide key information about health and disease progression in cells and tissues; however, current methods have lacked gold-standard information about histological structure. Conversely, histology and virtual histology methods have lacked metabolic contrast. Here, we present metabolic light absorption, scattering, and emission (MetaLASE) microscopy, which rapidly provides a virtual histology and optical metabolic readout simultaneously. Hematoxylin-like nucleic contrast and eosin-like cytoplasmic contrast are obtained using photoacoustic remote sensing and ultraviolet reflectance microscopy, respectively. The same ultraviolet source excites endogenous Nicotinamide adenine dinucleotide (phosphate), flavin adenine dinucleotide, and collagen autofluorescence, providing a map of optical redox ratios to visualize metabolic variations including in areas of invasive carcinoma. Benign chronic inflammation and glands also are seen to exhibit hypermetabolism. MetaLASE microscopy offers promise for future applications in intraoperative margin analysis and in research applications where greater insights into metabolic activity could be correlated with cell and tissue types.

Metabolic profiling of single cells by exploiting NADH and FAD fluorescence via flow cytometry

OBJECTIVE

The metabolism of different cells within the same microenvironment can differ and dictate physiological or pathological adaptions. Current single-cell analysis methods of metabolism are not label-free.

METHODS

The study introduces a label-free, live-cell analysis method assessing endogenous fluorescence of NAD(P)H and FAD in surface-stained cells by flow cytometry.

RESULTS

OxPhos inhibition, mitochondrial uncoupling, glucose exposure, genetic inactivation of glucose uptake and mitochondrial respiration alter the optical redox ratios of FAD and NAD(P)H as measured by flow cytometry. Those alterations correlate strongly with measurements obtained by extracellular flux analysis. Consequently, metabolically distinct live B-cell populations can be resolved, showing that human memory B-cells from peripheral blood exhibit a higher glycolytic flexibility than naïve B cells. Moreover, the comparison of blood-derived B- and T-lymphocytes from healthy donors and rheumatoid arthritis patients unleashes rheumatoid arthritis-associated metabolic traits in human naïve and memory B-lymphocytes.

CONCLUSIONS

Taken together, these data show that the optical redox ratio can depict metabolic differences in distinct cell populations by flow cytometry.

Metabolic changes in Toxoplasma gondii-infected host cells measured by autofluorescence imaging

Toxoplasma gondii, the causative agent of toxoplasmosis, is an obligate intracellular parasite that infects warm-blooded vertebrates across the world. In humans, seropositivity rates of T. gondii range from 10% to 90% across communities. Despite its prevalence, few studies address how T. gondii infection changes the metabolism of host cells. In this study, we investigate how T. gondii manipulates the host cell metabolic environment by monitoring the metabolic response over time using noninvasive autofluorescence lifetime imaging of single cells, metabolite analysis, extracellular flux analysis, and reactive oxygen species (ROS) production. Autofluorescence lifetime imaging indicates that infected host cells become more oxidized and have an increased proportion of bound NAD(P)H compared to uninfected controls. Over time, infected cells also show decreases in levels of intracellular glucose and lactate, increases in oxygen consumption, and variability in ROS production. We further examined changes associated with the pre-invasion "kiss and spit" process using autofluorescence lifetime imaging, which also showed a more oxidized host cell with an increased proportion of bound NAD(P)H over 48 hours compared to uninfected controls, suggesting that metabolic changes in host cells are induced by T. gondii kiss and spit even without invasion.IMPORTANCEThis study sheds light on previously unexplored changes in host cell metabolism induced by T. gondii infection using noninvasive, label-free autofluorescence imaging. In this study, we use optical metabolic imaging (OMI) to measure the optical redox ratio (ORR) in conjunction with fluorescence lifetime imaging microscopy (FLIM) to noninvasively monitor single host cell response to T. gondii infection over 48 hours. Collectively, our results affirm the value of using autofluorescence lifetime imaging to noninvasively monitor metabolic changes in host cells over the time course of a microbial infection. Understanding this metabolic relationship between the host cell and the parasite could uncover new treatment and prevention options for T. gondii infections worldwide.

Label-free metabolic optical biomarkers track stem cell fate transition in real time

Tracking stem cell fate transition is crucial for understanding their development and optimizing biomanufacturing. Destructive single-cell methods provide a pseudotemporal landscape of stem cell differentiation but cannot monitor stem cell fate in real time. We established a metabolic optical metric using label-free fluorescence lifetime imaging microscopy (FLIM), feature extraction and machine learning-assisted analysis, for real-time cell fate tracking. From a library of 205 metabolic optical biomarker (MOB) features, we identified 56 associated with hematopoietic stem cell (HSC) differentiation. These features collectively describe HSC fate transition and detect its bifurcate lineage choice. We further derived a MOB score measuring the "metabolic stemness" of single cells and distinguishing their division patterns. This score reveals a distinct role of asymmetric division in rescuing stem cells with compromised metabolic stemness and a unique mechanism of PI3K inhibition in promoting ex vivo HSC maintenance. MOB profiling is a powerful tool for tracking stem cell fate transition and improving their biomanufacturing from a single-cell perspective.

Modern optical approaches in redox biology: Genetically encoded sensors and Raman spectroscopy

The objective of the current review is to summarize the current state of optical methods in redox biology. It consists of two parts, the first is dedicated to genetically encoded fluorescent indicators and the second to Raman spectroscopy. In the first part, we provide a detailed classification of the currently available redox biosensors based on their target analytes. We thoroughly discuss the main architecture types of these proteins, the underlying engineering strategies for their development, the biochemical properties of existing tools and their advantages and disadvantages from a practical point of view. Particular attention is paid to fluorescence lifetime imaging microscopy as a possible readout technique, since it is less prone to certain artifacts than traditional intensiometric measurements. In the second part, the characteristic Raman peaks of the most important redox intermediates are listed, and examples of how this knowledge can be implemented in biological studies are given. This part covers such fields as estimation of the redox states and concentrations of Fe-S clusters, cytochromes, other heme-containing proteins, oxidative derivatives of thiols, lipids, and nucleotides. Finally, we touch on the issue of multiparameter imaging, in which biosensors are combined with other visualization methods for simultaneous assessment of several cellular parameters.

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.

Preclinical Models of Anal Cancer Combined-Modality Therapy

INTRODUCTION

There have been no significant changes in anal cancer treatment options in 4 decades. In this study, we highlight two preclinical models designed to assess anal cancer treatments.

MATERIALS AND METHODS

Transgenic K14E6/E7 mice were treated with 7, 12-dimethylbenz(a)anthracene until anal tumors developed. Mice were treated with localized radiation in addition to chemotherapy (combined-modality therapy [CMT]) and compared to no treatment control (NTC). K14E6/E7 mouse anal spheroids with and without Pik3ca mutations were isolated and treated with vehicle, LY3023414 (LY3) (a drug previously shown to be effective in cancer prevention), CMT, or CMT + LY3.

RESULTS

In the in vivo model, there was a significant increase in survival in the CMT group compared to the NTC group (P = 0.0392). In the ex vivo model, there was a significant decrease in the mean diameter of CMT and CMT + LY3-treated spheroids compared to vehicle (P ≤ 0.0001). For LY3 alone compared to vehicle, there was a statistically significant decrease in spheroid size in the K14E6/E7 group without mutation (P = 0.0004).

CONCLUSIONS

We have provided proof of concept for two preclinical anal cancer treatment models that allow for the future testing of novel therapies for anal cancer.

Metabolic changes to host cells with Toxoplasma gondii infection

Toxoplasma gondii, the causative agent of toxoplasmosis, is an obligate intracellular parasite that infects warm-blooded vertebrates across the world. In humans, seropositivity rates of T. gondii range from 10% to 90%. Despite its prevalence, few studies address how T. gondii infection changes the metabolism of host cells. Here, we investigate how T. gondii manipulates the host cell metabolic environment by monitoring metabolic response over time using non-invasive autofluorescence lifetime imaging of single cells, seahorse metabolic flux analysis, reactive oxygen species (ROS) production, and metabolomics. Autofluorescence lifetime imaging indicates that infected host cells become more oxidized and have an increased proportion of bound NAD(P)H with infection. These findings are consistent with changes in mitochondrial and glycolytic function, decrease of intracellular glucose, fluctuations in lactate and ROS production in infected cells over time. We also examined changes associated with the pre-invasion "kiss and spit" process using autofluorescence lifetime imaging, which similarly showed a more oxidized host cell with an increased proportion of bound NAD(P)H over 48 hours. Glucose metabolic flux analysis indicated that these changes are driven by NADH and NADP+ in T. gondii infection. In sum, metabolic changes in host cells with T. gondii infection were similar during full infection, and kiss and spit. Autofluorescence lifetime imaging can non-invasively monitor metabolic changes in host cells over a microbial infection time-course.

Portable optical spectroscopic assay for non-destructive measurement of key metabolic parameters on in vitro cancer cells and organotypic fresh tumor slices

To enable non-destructive metabolic characterizations on in vitro cancer cells and organotypic tumor models for therapeutic studies in an easy-to-access way, we report a highly portable optical spectroscopic assay for simultaneous measurement of glucose uptake and mitochondrial function on various cancer models with high sensitivity. Well-established breast cancer cell lines (MCF-7 and MDA-MB-231) were used to validate the optical spectroscopic assay for metabolic characterizations, while fresh tumor samples harvested from both animals and human cancer patients were used to test the feasibility of our optical metabolic assay for non-destructive measurement of key metabolic parameters on organotypic tumor slices. Our optical metabolic assay captured that MCF-7 cells had higher mitochondrial metabolism, but lower glucose uptake compared to the MDA-MB-231 cells, which is consistent with our microscopy imaging and flow cytometry data, as well as the published Seahorse Assay data. Moreover, we demonstrated that our optical assay could non-destructively measure both glucose uptake and mitochondrial metabolism on the same cancer cell samples at one time, which remains challenging by existing metabolic tools. Our pilot tests on thin fresh tumor slices showed that our optical assay captured increased metabolic activities in tumors compared to normal tissues. Our non-destructive optical metabolic assay provides a cost-effective way for future longitudinal therapeutic studies using patient-derived organotypic fresh tumor slices through the lens of tumor energetics, which will significantly advance translational cancer research.

Viability of HepG2 and MCF-7 cells is not correlated with mitochondrial bioenergetics

Alterations in metabolism are a hallmark of cancer. It is unclear if oxidative phosphorylation (OXPHOS) is necessary for tumour cell survival. In this study, we investigated the effects of severe hypoxia, site-specific inhibition of respiratory chain (RC) components, and uncouplers on necrotic and apoptotic markers in 2D-cultured HepG2 and MCF-7 tumour cells. Comparable respiratory complex activities were observed in both cell lines. However, HepG2 cells exhibited significantly higher oxygen consumption rates (OCR) and respiratory capacity than MCF-7 cells. Significant non-mitochondrial OCR was observed in MCF-7 cells, which was insensitive to acute combined inhibition of complexes I and III. Pre-treatment of either cell line with RC inhibitors for 24-72 h resulted in the complete abolition of respective complex activities and OCRs. This was accompanied by a time-dependent decrease in citrate synthase activity, suggesting mitophagy. High-content automated microscopy recordings revealed that the viability of HepG2 cells was mostly unaffected by any pharmacological treatment or severe hypoxia. In contrast, the viability of MCF-7 cells was strongly affected by inhibition of complex IV (CIV) or complex V (CV), severe hypoxia, and uncoupling. However, it was only moderately affected by inhibition of complexes I, II, and III. Cell death in MCF-7 cells induced by inhibition of complexes II, III, and IV was partially abrogated by aspartate. These findings indicate that OXPHOS activity and viability are not correlated in these cell lines, suggesting that the connection between OXPHOS and cancer cell survival is dependent on the specific cell type and conditions.

Evolution of cisplatin resistance through coordinated metabolic reprogramming of the cellular reductive state

BACKGROUND

Cisplatin (CDDP) is a mainstay treatment for advanced head and neck squamous cell carcinomas (HNSCC) despite a high frequency of innate and acquired resistance. We hypothesised that tumours acquire CDDP resistance through an enhanced reductive state dependent on metabolic rewiring.

METHODS

To validate this model and understand how an adaptive metabolic programme might be imprinted, we performed an integrated analysis of CDDP-resistant HNSCC clones from multiple genomic backgrounds by whole-exome sequencing, RNA-seq, mass spectrometry, steady state and flux metabolomics.

RESULTS

Inactivating KEAP1 mutations or reductions in KEAP1 RNA correlated with Nrf2 activation in CDDP-resistant cells, which functionally contributed to resistance. Proteomics identified elevation of downstream Nrf2 targets and the enrichment of enzymes involved in generation of biomass and reducing equivalents, metabolism of glucose, glutathione, NAD(P), and oxoacids. This was accompanied by biochemical and metabolic evidence of an enhanced reductive state dependent on coordinated glucose and glutamine catabolism, associated with reduced energy production and proliferation, despite normal mitochondrial structure and function.

CONCLUSIONS

Our analysis identified coordinated metabolic changes associated with CDDP resistance that may provide new therapeutic avenues through targeting of these convergent pathways.

Autofluorescence imaging permits label-free cell type assignment and reveals the dynamic formation of airway secretory cell associated antigen passages (SAPs)

The specific functional properties of a tissue are distributed amongst its component cell types. The various cells act coherently, as an ensemble, in order to execute a physiologic response. Modern approaches for identifying and dissecting novel physiologic mechanisms would benefit from an ability to identify specific cell types in live tissues that could then be imaged in real time. Current techniques require the use of fluorescent genetic reporters that are not only cumbersome, but which only allow the study of three or four cell types at a time. We report a non-invasive imaging modality that capitalizes on the endogenous autofluorescence signatures of the metabolic cofactors NAD(P)H and FAD. By marrying morphological characteristics with autofluorescence signatures, all seven of the airway epithelial cell types can be distinguished simultaneously in mouse tracheal explants in real time. Furthermore, we find that this methodology for direct cell type-specific identification avoids pitfalls associated with the use of ostensibly cell type-specific markers that are, in fact, altered by clinically relevant physiologic stimuli. Finally, we utilize this methodology to interrogate real-time physiology and identify dynamic secretory cell associated antigen passages (SAPs) that form in response to cholinergic stimulus. The identical process has been well documented in the intestine where the dynamic formation of SAPs and goblet cell associated antigen passages (GAPs) enable luminal antigen sampling. Airway secretory cells with SAPs are frequently juxtaposed to antigen presenting cells, suggesting that airway SAPs, like their intestinal counterparts, not only sample antigen but convey their cargo for immune cell processing.

Fluorescence Lifetime Imaging Microscopy (FLIM) reveals spatial-metabolic changes in 3D breast cancer spheroids

Cancer cells are mechanically sensitive to physical properties of the microenvironment, which can affect downstream signaling to promote malignancy, in part through the modulation of metabolic pathways. Fluorescence Lifetime Imaging Microscopy (FLIM) can be used to measure the fluorescence lifetime of endogenous fluorophores, such as the metabolic co-factors NAD(P)H and FAD, in live samples. We used multiphoton FLIM to investigate the changes in cellular metabolism of 3D breast spheroids derived from MCF-10A and MD-MB-231 cell lines embedded in collagen with varying densities (1 vs. 4 mg/ml) over time (Day 0 vs. Day 3). MCF-10A spheroids demonstrated spatial gradients, with the cells closest to the spheroid edge exhibiting FLIM changes consistent with a shift towards oxidative phosphorylation (OXPHOS) while the spheroid core had changes consistent with a shift towards glycolysis. The MDA-MB-231 spheroids had a large shift consistent with increased OXPHOS with a more pronounced change at the higher collagen concentration. The MDA-MB-231 spheroids invaded into the collagen gel over time and cells that traveled the farthest had the largest changes consistent with a shift towards OXPHOS. Overall, these results suggest that the cells in contact with the extracellular matrix (ECM) and those that migrated the farthest had changes consistent with a metabolic shift towards OXPHOS. More generally, these results demonstrate the ability of multiphoton FLIM to characterize how spheroids metabolism and spatial metabolic gradients are modified by physical properties of the 3D ECM.

Prostate cancer cells demonstrate unique metabolism and substrate adaptability acutely after androgen deprivation therapy

BACKGROUND

Androgen deprivation therapy (ADT) has been the standard of care for advanced hormone-sensitive prostate cancer (PC), yet tumors invariably develop resistance resulting in castrate-resistant PC. The acute response of cancer cells to ADT includes apoptosis and cell death, but a large fraction remains arrested but viable. In this study, we focused on intensively characterizing the early metabolic changes that result after ADT to define potential metabolic targets for treatment.

METHODS

A combination of mass spectrometry, optical metabolic imaging which noninvasively measures drug responses in cells, oxygen consumption rate, and protein expression analysis was used to characterize and block metabolic pathways over several days in multiple PC cell lines with variable hormone response status including ADT sensitive lines LNCaP and VCaP, and resistant C4-2 and DU145.

RESULTS

Mass spectrometry analysis of LNCaP pre- and postexposure to ADT revealed an abundance of glycolytic intermediates after ADT. In LNCaP and VCaP, a reduction in the optical redox ratio [NAD(P)H/FAD], extracellular acidification rate, and a downregulation of key regulatory enzymes for fatty acid and glutamine utilization was acutely observed after ADT. Screening several metabolic inhibitors revealed that blocking fatty acid oxidation and synthesis reversed this stress response in the optical redox ratio seen with ADT alone in LNCaP and VCaP. In contrast, both cell lines demonstrated increased sensitivity to the glycolytic inhibitor 2-Deoxy- d-glucose(2-DG) and maintained sensitivity to electron transport chain inhibitor Malonate after ADT exposure. ADT followed by 2-DG results in synergistic cell death, a result not seen with simultaneous administration.

CONCLUSIONS

Hormone-sensitive PC cells displayed altered metabolic profiles early after ADT including an overall depression in energy metabolism, induction of a quiescent/senescent phenotype, and sensitivity to selected metabolic inhibitors. Glycolytic blocking agents (e.g., 2-DG) as a sequential treatment after ADT may be promising.

Maturation- and degeneration-dependent articular cartilage metabolism via optical redox ratio imaging

From the two metabolic processes in healthy cartilage, glycolysis has been associated with proliferation and oxidative phosphorylation (oxphos) with matrix synthesis. Recently, metabolic dysregulation was significantly correlated with cartilage degradation and osteoarthritis progression. While these findings suggest maturation predisposes cartilage to metabolic instability with consequences for tissue maintenance, these links have not been shown. Therefore, this study sought to address three hypotheses (a) chondrocytes exhibit differential metabolic activity between immaturity (0-4 months), adolescence (5-18 months), and maturity (>18 months); (b) perturbation of metabolic activity has consequences on expression of genes pertinent to cartilage tissue maintenance; and (c) severity of cartilage damage is positively correlated with glycolysis and oxphos activity as well as optical redox ratio in postadolescent cartilage. Porcine femoral cartilage samples from pigs (3 days to 6 years) underwent optical redox ratio imaging, which measures autofluorescence of NAD(P)H and FAD. Gene expression analysis and histological scoring was conducted for comparison against imaging metrics. NAD(P)H and FAD autofluorescence both demonstrated increasing intensity with age, while optical redox ratio was lowest in adolescent samples compared to immature or mature samples. Inhibition of glycolysis suppressed expression of Col2, Col1, ADAMTS4, and ADAMTS5, while oxphos inhibition had no effect. FAD fluorescence and optical redox ratio were positively correlated with histological degeneration. This study demonstrates maturation- and degeneration-dependent metabolic activity in cartilage and explores the consequences of this differential activity on gene expression. This study aids our basic understanding of cartilage biology and highlights opportunity for potential diagnostic applications.

Subcellular analysis of nuclear and cytoplasmic redox indices differentiates breast cancer cell subtypes better than nuclear-to-cytoplasmic area ratio

SIGNIFICANCE

Stratification of malignancy is valuable for cancer treatment. Both optical redox imaging (ORI) indices and nuclear-to-cytoplasmic volume/area ratio (N:C ratio) have been investigated to differentiate between cancers with varying aggressiveness, but these two methods have not been directly compared. The redox status in the cell nucleus has not been studied by ORI, and it remains unknown whether nuclear ORI indices add new biological information.

AIM

We sought to compare the capacity of whole-cell and subcellular ORI indices and N:C ratio to differentiate between breast cancer subtypes with varying aggressiveness and between mitotic and nonmitotic cells.

APPROACH

ORI indices for whole cell, cytoplasm, and nucleus as well as the N:C area ratio were generated for two triple-negative (more aggressive) and two receptor-positive (less aggressive) breast cancer cell lines by fluorescence microscopy.

RESULTS

We found positive correlations between nuclear and cytoplasmic ORI indices within individual cells. On average, a nuclear redox status was found to be more oxidized than cytoplasm in triple-negative cells but not in receptor-positive cells. Whole-cell and subcellular ORI indices distinguished between the receptor statuses better than the N:C ratio. However, N:C ratio was a better differentiator between nonmitotic and mitotic triple-negative cells.

CONCLUSIONS

Subcellular ORI analysis differentiates breast cancer subtypes with varying aggressiveness better than N:C area ratio.

Inhibition of B-cell lymphoma 2 family proteins alters optical redox ratio, mitochondrial polarization, and cell energetics independent of cell state

SIGNIFICANCE

The optical redox ratio (ORR) [autofluorescence intensity of the reduced form of nicotinamide adenine dinucleotide (phosphate) (NAD(P)H)/flavin adenine dinucleotide (FAD)] provides a label-free method to quantify cellular metabolism. However, it is unclear whether changes in the ORR with B-cell lymphoma 2 (Bcl-2) family protein inhibition are due to metabolic stress alone or compromised cell viability.

AIM

Determine whether ABT-263 (navitoclax, Bcl-2 family inhibitor) changes the ORR due to changes in mitochondrial function that are independent of changes in cell viability.

APPROACH

SW48 colon cancer cells were used to investigate changes in ORR, mitochondrial membrane potential, oxygen consumption rates, and cell state (cell growth, viability, proliferation, apoptosis, autophagy, and senescence) with ABT-263, TAK-228 [sapanisertib, mammalian target of rapamycin complex 1/2 (mTORC 1/2) inhibitor], and their combination at 24 h.

RESULTS

Changes in the ORR with Bcl-2 inhibition are driven by increases in both NAD(P)H and FAD autofluorescence, corresponding with increased basal metabolic rate and increased mitochondrial polarization. ABT-263 treatment does not change cell viability or induce autophagy but does induce a senescent phenotype. The metabolic changes seen with ABT-263 treatment are mitigated by combination with mTORC1/2 inhibition.

CONCLUSIONS

The ORR is sensitive to increases in mitochondrial polarization, energetic state, and cell senescence, which can change independently from cell viability.

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.

Insight into redox regulation of apoptosis in cancer cells with multiparametric live-cell microscopy

Cellular redox status and the level of reactive oxygen species (ROS) are important regulators of apoptotic potential, playing a crucial role in the growth of cancer cell and their resistance to apoptosis. However, the relationships between the redox status and ROS production during apoptosis remain poorly explored. In this study, we present an investigation on the correlations between the production of ROS, the redox ratio FAD/NAD(P)H, the proportions of the reduced nicotinamide cofactors NADH and NADPH, and caspase-3 activity in cancer cells at the level of individual cells. Two-photon excitation fluorescence lifetime imaging microscopy (FLIM) was applied to monitor simultaneously apoptosis using the genetically encoded sensor of caspase-3, mKate2-DEVD-iRFP, and the autofluorescence of redox cofactors in colorectal cancer cells upon stimulation of apoptosis with staurosporine, cisplatin or hydrogen peroxide. We found that, irrespective of the apoptotic stimulus used, ROS accumulation correlated well with both the elevated pool of mitochondrial, enzyme-bound NADH and caspase-3 activation. Meanwhile, a shift in the contribution of bound NADH could develop independently of the apoptosis, and this was observed in the case of cisplatin. An increase in the proportion of bound NADPH was detected only in staurosporine-treated cells, this likely being associated with a high level of ROS production and their resulting detoxification. The results of the study favor the discovery of new therapeutic strategies based on manipulation of the cellular redox balance, which could help improve the anti-tumor activity of drugs and overcome apoptotic resistance.

Metabolic therapy and bioenergetic analysis: The missing piece of the puzzle

BACKGROUND

Aberrant metabolism is recognized as a hallmark of cancer, a pillar necessary for cellular proliferation. Regarding bioenergetics (ATP generation), most cancers display a preference not only toward aerobic glycolysis ("Warburg effect") and glutaminolysis (mitochondrial substrate level-phosphorylation) but also toward other metabolites such as lactate, pyruvate, and fat-derived sources. These secondary metabolites can assist in proliferation but cannot fully cover ATP demands.

SCOPE OF REVIEW

The concept of a static metabolic profile is challenged by instances of heterogeneity and flexibility to meet fuel/anaplerotic demands. Although metabolic therapies are a promising tool to improve therapeutic outcomes, either via pharmacological targets or press-pulse interventions, metabolic plasticity is rarely considered. Lack of bioenergetic analysis in vitro and patient-derived models is hindering translational potential. Here, we review the bioenergetics of cancer and propose a simple analysis of major metabolic pathways, encompassing both affordable and advanced techniques. A comprehensive compendium of Seahorse XF bioenergetic measurements is presented for the first time.

MAJOR CONCLUSIONS

Standardization of principal readouts might help researchers to collect a complete metabolic picture of cancer using the most appropriate methods depending on the sample of interest.

Quantification of NADH:ubiquinone oxidoreductase (complex I) content in biological samples

Impairments in mitochondrial energy metabolism have been implicated in human genetic diseases associated with mitochondrial and nuclear DNA mutations, neurodegenerative and cardiovascular disorders, diabetes, and aging. Alteration in mitochondrial complex I structure and activity has been shown to play a key role in Parkinson's disease and ischemia/reperfusion tissue injury, but significant difficulty remains in assessing the content of this enzyme complex in a given sample. The present study introduces a new method utilizing native polyacrylamide gel electrophoresis in combination with flavin fluorescence scanning to measure the absolute content of complex I, as well as α-ketoglutarate dehydrogenase complex, in any preparation. We show that complex I content is 19 ± 1 pmol/mg of protein in the brain mitochondria, whereas varies up to 10-fold in different mouse tissues. Together with the measurements of NADH-dependent specific activity, our method also allows accurate determination of complex I catalytic turnover, which was calculated as 104 min-1 for NADH:ubiquinone reductase in mouse brain mitochondrial preparations. α-ketoglutarate dehydrogenase complex content was determined to be 65 ± 5 and 123 ± 9 pmol/mg protein for mouse brain and bovine heart mitochondria, respectively. Our approach can also be extended to cultured cells, and we demonstrated that about 90 × 103 complex I molecules are present in a single human embryonic kidney 293 cell. The ability to determine complex I content should provide a valuable tool to investigate the enzyme status in samples after in vivo treatment in mutant organisms, cells in culture, or human biopsies.

Label-free two-photon imaging of mitochondrial activity in murine macrophages stimulated with bacterial and viral ligands

Mitochondria are the metabolic hub of the cell, playing a central role in regulating immune responses. Dysfunction of mitochondrial reprogramming can occur during bacterial and viral infections compromising hosts’ immune signaling. Comparative evaluation of these alterations in response to bacterial and viral ligands can provide insights into a cell’s ability to mount pathogen-specific responses. In this study, we used two-photon excitation fluorescence (TPEF) imaging to quantify reduced nicotinamide adenine dinucleotide phosphate (NAD(P)H) and flavin adenine dinucleotide (FAD) levels in the cell and to calculate the optical redox ratio (ORR), an indicator of mitochondrial dysfunction. Analyses were performed on RAW264.7 cells and murine bone marrow derived macrophages (BMM) stimulated with bacterial (LPS) and viral (Poly(I:C)) ligands. Responses were cell type dependent, with primary cells having significantly higher levels of FAD and higher oxygen consumption rates suggesting BMM may be more dependent on mitochondrial metabolism. Our findings also suggest that FAD-TPEF intensity may be a better predictor of mitochondrial activity and localization since it demonstrates unique mitochondrial clustering patterns in LPS vs. Poly(I:C) stimulated macrophages. Collectively, we demonstrate that TPEF imaging is a powerful label-free approach for quantifying changes in mitochondrial function and organization in macrophages following bacterial and viral stimuli.

Medulloblastoma uses GABA transaminase to survive in the cerebrospinal fluid microenvironment and promote leptomeningeal dissemination

Medulloblastoma (MB) is a malignant pediatric brain tumor arising in the cerebellum. Although abnormal GABAergic receptor activation has been described in MB, studies have not yet elucidated the contribution of receptor-independent GABA metabolism to MB pathogenesis. We find primary MB tumors globally display decreased expression of GABA transaminase (ABAT), the protein responsible for GABA metabolism, compared with normal cerebellum. However, less aggressive WNT and SHH subtypes express higher ABAT levels compared with metastatic G3 and G4 tumors. We show that elevated ABAT expression results in increased GABA catabolism, decreased tumor cell proliferation, and induction of metabolic and histone characteristics mirroring GABAergic neurons. Our studies suggest ABAT expression fluctuates depending on metabolite changes in the tumor microenvironment, with nutrient-poor conditions upregulating ABAT expression. We find metastatic MB cells require ABAT to maintain viability in the metabolite-scarce cerebrospinal fluid by using GABA as an energy source substitute, thereby facilitating leptomeningeal metastasis formation.

Autofluorescence Imaging of Treatment Response in Neuroendocrine Tumor Organoids

Gastroenteropancreatic neuroendocrine tumors (GEP-NET) account for roughly 60% of all neuroendocrine tumors. Low/intermediate grade human GEP-NETs have relatively low proliferation rates that animal models and cell lines fail to recapitulate. Short-term patient-derived cancer organoids (PDCOs) are a 3D model system that holds great promise for recapitulating well-differentiated human GEP-NETs. However, traditional measurements of drug response (i.e., growth, proliferation) are not effective in GEP-NET PDCOs due to the small volume of tissue and low proliferation rates that are characteristic of the disease. Here, we test a label-free, non-destructive optical metabolic imaging (OMI) method to measure drug response in live GEP-NET PDCOs. OMI captures the fluorescence lifetime and intensity of endogenous metabolic cofactors NAD(P)H and FAD. OMI has previously provided accurate predictions of drug response on a single cell level in other cancer types, but this is the first study to apply OMI to GEP-NETs. OMI tested the response to novel drug combination on GEP-NET PDCOs, specifically ABT263 (navitoclax), a Bcl-2 family inhibitor, and everolimus, a standard GEP-NET treatment that inhibits mTOR. Treatment response to ABT263, everolimus, and the combination were tested in GEP-NET PDCO lines derived from seven patients, using two-photon OMI. OMI measured a response to the combination treatment in 5 PDCO lines, at 72 h post-treatment. In one of the non-responsive PDCO lines, heterogeneous response was identified with two distinct subpopulations of cell metabolism. Overall, this work shows that OMI provides single-cell metabolic measurements of drug response in PDCOs to guide drug development for GEP-NET patients.

Bioengineering tools for probing intracellular events in T lymphocytes

T lymphocytes are the central coordinator and executor of many immune functions. The activation and function of T lymphocytes are mediated through the engagement of cell surface receptors and regulated by a myriad of intracellular signaling network. Bioengineering tools, including imaging modalities and fluorescent probes, have been developed and employed to elucidate the cellular events throughout the functional lifespan of T cells. A better understanding of these events can broaden our knowledge in the immune systems biology, as well as accelerate the development of effective diagnostics and immunotherapies. Here we review the commonly used and recently developed techniques and probes for monitoring T lymphocyte intracellular events, following the order of intracellular events in T cells from activation, signaling, metabolism to apoptosis. The techniques introduced here can be broadly applied to other immune cells and cell systems. This article is categorized under: Immune System Diseases > Molecular and Cellular Physiology Immune System Diseases > Biomedical Engineering Infectious Diseases > Biomedical Engineering.

Sex and SP-A2 Dependent NAD(H) Redox Alterations in Mouse Alveolar Macrophages in Response to Ozone Exposure: Potential Implications for COVID-19

Co-enzyme nicotinamide adenine dinucleotide (NAD(H)) redox plays a key role in macrophage function. Surfactant protein (SP-) A modulates the functions of alveolar macrophages (AM) and ozone (O₃) exposure in the presence or absence of SP-A and reduces mouse survival in a sex-dependent manner. It is unclear whether and how NAD(H) redox status plays a role in the innate immune response in a sex-dependent manner. We investigated the NAD(H) redox status of AM from SP-A2 and SP-A knockout (KO) mice in response to O₃ or filtered air (control) exposure using optical redox imaging technique. We found: (i) In SP-A2 mice, the redox alteration of AM in response to O₃ showed sex-dependence with AM from males being significantly more oxidized and having a higher level of mitochondrial reactive oxygen species than females; (ii) AM from KO mice were more oxidized after O₃ exposure and showed no sex differences; (iii) AM from female KO mice were more oxidized than female SP-A2 mice; and (iv) Two distinct subpopulations characterized by size and redox status were observed in a mouse AM sample. In conclusions, the NAD(H) redox balance in AM responds to O₃ in a sex-dependent manner and the innate immune molecule, SP-A2, contributes to this observed sex-specific redox response.

Classification of T-cell activation via autofluorescence lifetime imaging

The function of a T cell depends on its subtype and activation state. Here, we show that the imaging of autofluorescence-lifetime signals from quiescent and activated T cells can be used to classify the cells. T cells isolated from human peripheral blood and activated in culture via a tetrameric antibody against the surface ligands CD2, CD3 and CD28 showed specific activation-state-dependent patterns of autofluorescence lifetime. Logistic-regression models and random-forest models classified T cells according to activation state with 97–99% accuracy, and according to activation state (quiescent or activated) and subtype (CD3⁺ CD8⁺ or CD3⁺ CD4⁺) with 97% accuracy. Autofluorescence-lifetime imaging could be used to non-destructively determine T-cell function.

Using in vivo multiphoton fluorescence lifetime imaging to unravel disease-specific changes in the liver redox state

Multiphoton fluorescence lifetime microscopy has revolutionized studies of pathophysiological and xenobiotic dynamics, enabling the spatial and temporal quantification of these processes in intact organs in vivo. We have previously used multiphoton fluorescence lifetime microscopy to characterise the morphology and amplitude weighted mean fluorescence lifetime of the endogenous fluorescent metabolic cofactor nicotinamide adenine dinucleotide (phosphate) (NAD(P)H) of mouse livers in vivo following induction of various disease states. Here, we extend the characterisation of liver disease models by using nonlinear regression to estimate the unbound, bound fluorescence lifetimes for NAD(P)H, flavin adenine dinucleotide (FAD), along with metabolic ratios and examine the impact of using multiple segmentation methods. We found that NAD(P)H amplitude ratio, and fluorescence lifetime redox ratio can be used as discriminators of diseased liver from normal liver. The redox ratio provided a sensitive measure of the changes in hepatic fibrosis and biliary fibrosis. Hepatocellular carcinoma was associated with an increase in spatial heterogeneity and redox ratio coupled with a decrease in mean fluorescence lifetime. We conclude that multiphoton fluorescence lifetime microscopy parameters and metabolic ratios provided insights into the in vivo redox state of diseased compared to normal liver that were not apparent from a global, mean fluorescence lifetime measurement alone.

Single cell-based fluorescence lifetime imaging of intracellular oxygenation and metabolism

Oxidation-reduction chemistry is fundamental to the metabolism of all living organisms, and hence quantifying the principal redox players is important for a comprehensive understanding of cell metabolism in normal and pathological states. In mammalian cells, this is accomplished by measuring oxygen partial pressure (pO2) in parallel with free and enzyme-bound reduced nicotinamide adenine dinucleotide (phosphate) [H] (NAD(P)H) and flavin adenine dinucleotide (FAD, a proxy for NAD+). Previous optical methods for these measurements had accompanying problems of cytotoxicity, slow speed, population averaging, and inability to measure all redox parameters simultaneously. Herein we present a Förster resonance energy transfer (FRET)-based oxygen sensor, Myoglobin-mCherry, compatible with fluorescence lifetime imaging (FLIM)-based measurement of nicotinamide coenzyme state. This offers a contemporaneous reading of metabolic activity through real-time, non-invasive, cell-by-cell intracellular pO2 and coenzyme status monitoring in living cells. Additionally, this method reveals intracellular spatial heterogeneity and cell-to-cell variation in oxygenation and coenzyme states.

Selecting optimal spectral bands for improved detection of autofluorescent biomarkers in multiphoton microscopy

SIGNIFICANCE

In multiphoton microscopy, two-photon excited fluorescence (TPEF) spectra carry valuable information on morphological and functional biological features. For measuring these biomarkers, separation of different parts of the fluorescence spectrum into channels is typically achieved by the use of optical band pass filters. However, spectra from different biomarkers can be unknown or overlapping, creating a crosstalk in between the channels. Previously, establishing these channels relied on prior knowledge or heuristic testing.

AIM

The presented method aims to provide spectral bands with optimal separation between groups of specimens expressing different biomarkers.

APPROACH

We have developed a system capable of resolving TPEF with high spectral resolution for the characterization of biomarkers. In addition, an algorithm is created to simulate and optimize optical band pass filters for fluorescence detection channels. To demonstrate the potential improvements in cell and tissue classification using these optimized channels, we recorded spectrally resolved images of cancerous (HT29) and normal epithelial colon cells (FHC), cultivated in 2D layers and in 3D to form spheroids. To provide an example of an application, we relate the results with the widely used redox ratio.

RESULTS

We show that in the case of two detection channels, our system and algorithm enable the selection of optimized band pass filters without the need of knowing involved fluorophores. An improvement of 31,5% in separating different 2D cell cultures is achieved, compared to using established spectral bands that assume NAD(P)H and FAD as main contributors of autofluorescence. The compromise is a reduced SNR in the images.

CONCLUSIONS

We show that the presented method has the ability to improve imaging contrast and can be used to tailor a given label-free optical imaging system using optical band pass filters targeting a specific biomarker or application.

Kinetic Analysis of Lipid Metabolism in Breast Cancer Cells via Nonlinear Optical Microscopy

Investigating the behavior of breast cancer cells via reaction kinetics may help unravel the mechanisms that underlie metabolic changes in tumors. However, obtaining human in vivo kinetic data is challenging because of difficulties associated with measuring these parameters. Nondestructive methods of measuring lipid content in live cells provide a novel approach to quantitatively model lipid synthesis and consumption. In this study, coherent Raman scattering microscopy was used to probe de novo intracellular lipid content. Combining nonlinear optical microscopy and Michaelis-Menten kinetics-based simulations, we isolated fatty acid synthesis/consumption rates and elucidated effects of altered lipid metabolism in T47D breast cancer cells. When treated with 17β-estradiol, the lipid utilization in cancer cells jumped by twofold. Meanwhile, the rate of de novo lipid synthesis in cancer cells treated with 17β-estradiol was increased by 42%. To test the model in extreme metabolic conditions, we treated T47D cells with etomoxir. Our kinetic analysis demonstrated that the rate of key enzymatic reactions dropped by 75%. These results underline the capability to probe lipid alterations in live cells with minimum interruption and to characterize lipid metabolism in breast cancer cells via quantitative kinetic models and parameters.

Metabolic Heterogeneity in Patient Tumor-Derived Organoids by Primary Site and Drug Treatment

New tools are needed to match cancer patients with effective treatments. Patient-derived organoids offer a high-throughput platform to personalize treatments and discover novel therapies. Currently, methods to evaluate drug response in organoids are limited because they overlook cellular heterogeneity. In this study, non-invasive optical metabolic imaging (OMI) of cellular heterogeneity was characterized in breast cancer (BC) and pancreatic cancer (PC) patient-derived organoids. Baseline heterogeneity was analyzed for each patient, demonstrating that single-cell techniques, such as OMI, are required to capture the complete picture of heterogeneity present in a sample. Treatment-induced changes in heterogeneity were also analyzed, further demonstrating that these measurements greatly complement current techniques that only gauge average cellular response. Finally, OMI of cellular heterogeneity in organoids was evaluated as a predictor of clinical treatment response for the first time. Organoids were treated with the same drugs as the patient's prescribed regimen, and OMI measurements of heterogeneity were compared to patient outcome. OMI distinguished subpopulations of cells with divergent and dynamic responses to treatment in living organoids without the use of labels or dyes. OMI of organoids agreed with long-term therapeutic response in patients. With these capabilities, OMI could serve as a sensitive high-throughput tool to identify optimal therapies for individual patients, and to develop new effective therapies that address cellular heterogeneity in cancer.

Detecting metabolic differences in fetal and adult sheep adipose and skeletal muscle tissues

The primary metabolic pathway required to produce ATP differs as a result of tissue type, developmental stage and substrate availability. We utilized molecular and histological techniques to define the metabolic status in foetal and adult, adipose and skeletal muscle tissues. Redox ratios of these tissues were also determined optically by two-photon microscopy. Adult perirenal adipose tissue had a higher optical redox ratio than fetal perirenal adipose tissue, which aligned with glycolysis being used for ATP production; whereas adult skeletal muscle had a lower optical redox ratio than fetal skeletal muscle, which aligned with oxygen demanding oxidative phosphorylation activity being utilized for ATP production. We have compared traditional molecular and microscopy techniques of metabolic tissue characterization with optical redox ratios to provide a more comprehensive report on the dynamics of tissue metabolism.

Relationship between Optical Redox Status and Reactive Oxygen Species in Cancer Cells

Shifted NAD(H) redox status and enhanced reactive oxygen species (ROS) scavenging systems have been observed in cancers. However, how such redox shift is related to the ROS level in cancer cells is less clear. Based on collecting the intrinsic fluorescence of oxidized flavoproteins (Fp containing flavin adenine dinucleotide) and reduced nicotinamide adenine dinucleotide (NADH), optical redox imaging (ORI) provides a quantitative measure of the mitochondrial redox state by the optical redox ratio, Fp/(NADH+Fp), a surrogate marker of the NAD⁺-coupled redox state NAD⁺/NADH. Our study aims to explore the relationship between NAD(H) redox status and ROS by imaging NADH, Fp, and ROS levels using cultured breast cancer cell models. By manipulating either ROS levels via application of exogenous H₂O₂ or redox status via metabolic perturbation compounds, we found that: (1) oxidation of NAD(H) redox status correlates with ROS levels at lower H₂O₂ concentrations (up to ~700 μM), but not necessarily at higher concentrations; (2) an elevated ROS level diminishes NADH and reduces redox ratio plasticity; (3) either more oxidized or more reduced status can correlate to an increased ROS level; and (4) sometimes, a more oxidized status can correlate to a decreased ROS level depending on cell lines. These observations indicated that cellular NAD(H) redox state and ROS are intricately related but can also change separately. This study can benefit cancer research as both NAD(H) redox status and ROS have been implicated in cancer transformation and progression.

Non-invasive Optical Biomarkers Distinguish and Track the Metabolic Status of Single Hematopoietic Stem Cells

Metabolism is a key regulator of hematopoietic stem cell (HSC) functions. There is a lack of real-time, non-invasive approaches to evaluate metabolism in single HSCs. Using fluorescence lifetime imaging microscopy, we developed a set of metabolic optical biomarkers (MOBs) from the auto-fluorescent properties of metabolic coenzymes NAD(P)H and FAD. The MOBs revealed the enhanced glycolysis, low oxidative metabolism, and distinct mitochondrial localization of HSCs. Importantly, the fluorescence lifetime of enzyme-bound NAD(P)H (τ_bound) can non-invasively monitor the glycolytic/lactate dehydrogenase activity in single HSCs. As a proof of concept for metabolism-based cell sorting, we further identified HSCs within the Lineage-cKit+Sca1+ (KLS) hematopoietic stem/progenitor population using MOBs and a machine-learning algorithm. Moreover, we revealed the dynamic changes of MOBs, and the association of longer τ_bound with enhanced glycolysis under HSC stemness-maintaining conditions during HSC culture. Our work thus provides a new paradigm to identify and track the metabolism of single HSCs non-invasively and in real time.

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Metabolic optical biomarkers non-invasively distinguish HSCs from early progenitors

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NAD(P)H τ_bound reflects lactate dehydrogenase activity in single fresh/cultured HSCs

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pHi correlates with τ_bound in hematopoietic populations, with HSCs being the highest

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Optical biomarkers track metabolic changes and response to drugs in cultured HSCs

Two-Photon Autofluorescence Imaging of Fixed Tissues: Feasibility and Potential Values for Biomedical Applications

The value of optical redox imaging (ORI) of cells/tissues based on the intrinsic fluorescences of NADH (nicotinamide adenine dinucleotide) and oxidized flavoproteins (containing flavin adenine dinucleotide, i.e., FAD) has been demonstrated for potential biomedical applications including diagnosis, prognosis, and determining treatment response. However, the Chance redox scanner (a 3D cryogenic tissue imager) is limited by spatial resolution (~50 μm), and tissue ORI using fluorescence microscopy (single or multi-photon) is limited by the light penetration depth. Furthermore, viable or snap-frozen tissues are usually required. In this project, we aimed to study whether ORI may be achieved for unstained fixed tissue using a state-of-the-art modern Serial Two-Photon (STP) Tomography scanner that can rapidly acquire multi-plane images at micron resolution. Tissue specimens of mouse muscle, liver, and tumor xenografts were harvested and fixed in 4% paraformaldehyde (PFA) for 24 h. Tissue blocks were scanned by STP Tomography under room temperature to acquire the autofluorescence signals (NADH channel: excitation 750 nm, blue emission filter; FAD channel: excitation 860 nm, green emission filter). We observed remarkable signals with significant intra-tissue heterogeneity in images of NADH, FAD and redox ratio (FAD/(NADH+FAD)), which are worthy of further investigation for extracting biological information.

Real-time optical redox imaging of cartilage metabolic response to mechanical loading

OBJECTIVE

Metabolic dysregulation has recently been identified as a key feature of osteoarthritis. Mechanical overloading has been postulated as a primary cause of this metabolic response. Current methods of real-time metabolic activity analysis in cartilage are limited and challenging. However, optical redox imaging leverages the autofluorescence of co-enzymes NAD(P)H and FAD to provide dye-free real-time analysis of metabolic activity. This technique has not yet been applied to cartilage. This study aimed to assess the effects of a compressive load on cartilage using optical redox imaging.

METHOD

Cartilage samples were excised from porcine femoral condyles. To validate this imaging modality in cartilage, glycolysis was inhibited via 2-deoxy-D-glucose (2DG) and oxidative phosphorylation was inhibited by rotenone. Optical redox images were collected pre- and post-inhibition. To assess the effects of mechanical loading, samples were subjected to a compressive load and imaged for approximately 30 min. Load and strain parameters were determined using high-speed camera images in Matlab. A range of loading magnitudes and rates were applied across samples.

RESULTS

2DG and rotenone demonstrated the expected inhibitory effects on fluorescence intensity in the channels corresponding to NAD(P)H and FAD, respectively. Mechanical loading induced an increase in NAD(P)H channel fluorescence which subsided by 30 min post-loading. Magnitude of loading parameters had mixed effects on metabolites.

CONCLUSIONS

Optical redox imaging provides an opportunity to assess real-time metabolic activity in cartilage. This approach revealed a metabolic response to a single load and can be used to provide insight into the role of metabolism in mechanically-mediated cartilage degradation.

Optical Imaging Approaches to Investigating Radiation Resistance

Radiation therapy is frequently the first line of treatment for over 50% of cancer patients. While great advances have been made in improving treatment response rates and reducing damage to normal tissue, radiation resistance remains a persistent clinical problem. While hypoxia or a lack of tumor oxygenation has long been considered a key factor in causing treatment failure, recent evidence points to metabolic reprogramming under well-oxygenated conditions as a potential route to promoting radiation resistance. In this review, we present recent studies from our lab and others that use high-resolution optical imaging as well as clinical translational optical spectroscopy to shine light on the biological basis of radiation resistance. Two-photon microscopy of endogenous cellular metabolism has identified key changes in both mitochondrial structure and function that are specific to radiation-resistant cells and help promote cell survival in response to radiation. Optical spectroscopic approaches, such as diffuse reflectance and Raman spectroscopy have demonstrated functional and molecular differences between radiation-resistant and sensitive tumors in response to radiation. These studies have uncovered key changes in metabolic pathways and present a viable route to clinical translation of optical technologies to determine radiation resistance at a very early stage in the clinic.

Quantitative Spatial Analysis of Metabolic Heterogeneity Across in vivo and in vitro Tumor Models

Metabolic preferences of tumor cells vary within a single tumor, contributing to tumor heterogeneity, drug resistance, and patient relapse. However, the relationship between tumor treatment response and metabolically distinct tumor cell populations is not well-understood. Here, a quantitative approach was developed to characterize spatial patterns of metabolic heterogeneity in tumor cell populations within in vivo xenografts and 3D in vitro cultures (i.e., organoids) of head and neck cancer. Label-free images of cell metabolism were acquired using two-photon fluorescence lifetime microscopy of the metabolic co-enzymes NAD(P)H and FAD. Previous studies have shown that NAD(P)H mean fluorescence lifetimes can identify metabolically distinct cells with varying drug response. Thus, density-based clustering of the NAD(P)H mean fluorescence lifetime was used to identify metabolic sub-populations of cells, then assessed in control, cetuximab-, cisplatin-, and combination-treated xenografts 13 days post-treatment and organoids 24 h post-treatment. Proximity analysis of these metabolically distinct cells was designed to quantify differences in spatial patterns between treatment groups and between xenografts and organoids. Multivariate spatial autocorrelation and principal components analyses of all autofluorescence intensity and lifetime variables were developed to further improve separation between cell sub-populations. Spatial principal components analysis and Z-score calculations of autofluorescence and spatial distribution variables also visualized differences between models. This analysis captures spatial distributions of tumor cell sub-populations influenced by treatment conditions and model-specific environments. Overall, this novel spatial analysis could provide new insights into tumor growth, treatment resistance, and more effective drug treatments across a range of microscopic imaging modalities (e.g., immunofluorescence, imaging mass spectrometry).

Local redox conditions in cells imaged via non-fluorescent transient states of NAD(P)H

The autofluorescent coenzyme nicotinamide adenine dinucleotide (NADH) and its phosphorylated form (NADPH) are major determinants of cellular redox balance. Both their fluorescence intensities and lifetimes are extensively used as label-free readouts in cellular metabolic imaging studies. Here, we introduce fluorescence blinking of NAD(P)H, as an additional, orthogonal readout in such studies. Blinking of fluorophores and their underlying dark state transitions are specifically sensitive to redox conditions and oxygenation, parameters of particular relevance in cellular metabolic studies. We show that such dark state transitions in NAD(P)H can be quantified via the average fluorescence intensity recorded upon modulated one-photon excitation, so-called transient state (TRAST) monitoring. Thereby, transitions in NAD(P)H, previously only accessible from elaborate spectroscopic cuvette measurements, can be imaged at subcellular resolution in live cells. We then demonstrate that these transitions can be imaged with a standard laser-scanning confocal microscope and two-photon excitation, in parallel with regular fluorescence lifetime imaging (FLIM). TRAST imaging of NAD(P)H was found to provide additional, orthogonal information to FLIM and allows altered oxidative environments in cells treated with a mitochondrial un-coupler or cyanide to be clearly distinguished. We propose TRAST imaging as a straightforward and widely applicable modality, extending the range of information obtainable from cellular metabolic imaging of NAD(P)H fluorescence.

High-throughput, label-free, single-cell photoacoustic microscopy of intratumoral metabolic heterogeneity

Intratumoral heterogeneity, which is manifested in almost all of the hallmarks of cancer, including the significantly altered metabolic profiles of cancer cells, represents a challenge to effective cancer therapy. High-throughput measurements of the metabolism of individual cancer cells would allow direct visualization and quantification of intratumoral metabolic heterogeneity, yet the throughputs of current measurement techniques are limited to about 120 cells per hour. Here, we show that single-cell photoacoustic microscopy can reach throughputs of approximately 12,000 cells per hour by trapping single cells with blood in an oxygen-diffusion-limited high-density microwell array and by using photoacoustic imaging to measure the haemoglobin oxygen change (that is, the oxygen consumption rate) in the microwells. We demonstrate the capability of this label-free technique by performing high-throughput single-cell oxygen-consumption-rate measurements of cultured cells and by imaging intratumoral metabolic heterogeneity in specimens from patients with breast cancer. High-throughput single-cell photoacoustic microscopy of oxygen consumption rates should enable the faster characterization of intratumoral metabolic heterogeneity.

Simultaneous in vivo optical quantification of key metabolic and vascular endpoints reveals tumor metabolic diversity in murine breast tumor models

Therapeutically exploiting vascular and metabolic endpoints becomes critical to translational cancer studies because altered vascularity and deregulated metabolism are two important cancer hallmarks. The metabolic and vascular phenotypes of three sibling breast tumor lines with different metastatic potential are investigated in vivo with a newly developed quantitative spectroscopy system. All tumor lines have different metabolic and vascular characteristics compared to normal tissues, and there are strong positive correlations between metabolic (glucose uptake and mitochondrial membrane potential) and vascular (oxygen saturations and hemoglobin concentrations) parameters for metastatic (4T1) tumors but not for micrometastatic (4T07) and nonmetastatic (67NR) tumors. A longitudinal study shows that both vascular and metabolic endpoints of 4T1 tumors increased up to a specific tumor size threshold beyond which these parameters decreased. The synchronous changes between metabolic and vascular parameters, along with the strong positive correlations between these endpoints suggest that 4T1 tumors rely on strong oxidative phosphorylation in addition to glycolysis. This study illustrates the great potential of our optical technique to provide valuable dynamic information about the interplay between the metabolic and vascular status of tumors, with important implications for translational cancer investigations.

Optical Imaging of Glucose Uptake and Mitochondrial Membrane Potential to Characterize Her2 Breast Tumor Metabolic Phenotypes

With the large number of women diagnosed and treated for breast cancer each year, the importance of studying recurrence has become evident due to most deaths from breast cancer resulting from tumor recurrence following therapy. To mitigate this, cellular and molecular pathways used by residual disease prior to recurrence must be studied. An altered metabolism has long been considered a hallmark of cancer, and several recent studies have gone further to report metabolic dysfunction and alterations as key to understanding the underlying behavior of dormant and recurrent cancer cells. Our group has used two probes, 2-[N-(7-nitrobenz-2-oxa-1, 3-diaxol-4-yl) amino]-2-deoxyglucose (2-NBDG) and tetramethyl rhodamine ethyl ester (TMRE), to image glucose uptake and mitochondrial membrane potential, respectively, to report changes in metabolism between primary tumors, regression, residual disease, and after regrowth in genetically engineered mouse (GEM)-derived mammospheres. Imaging revealed unique metabolic phenotypes across the stages of tumor development. Although primary mammospheres overexpressing Her2 maintained increased glucose uptake ("Warburg effect"), after Her2 downregulation, during regression and residual disease, mammospheres appeared to switch to oxidative phosphorylation. Interestingly, in mammospheres where Her2 overexpression was turned back on to model recurrence, glucose uptake was lowest, indicating a potential change in substrate preference following the reactivation of Her2, reeliciting growth. Our findings highlight the importance of imaging metabolic adaptions to gain insight into the fundamental behaviors of residual and recurrent disease. IMPLICATIONS: This study demonstrates these functional fluorescent probes' ability to report metabolic adaptations during primary tumor growth, regression, residual disease, and regrowth in Her2 breast tumors.

Autofluorescence lifetime imaging of cellular metabolism: Sensitivity toward cell density, pH, intracellular, and intercellular heterogeneity

Autofluorescence imaging (AFI) has greatly accelerated in the last decade, way past its origins in detecting endogenous signals in biological tissues to identify differences between samples. There are many endogenous fluorescence sources of contrast but the most robust and widely utilized have been those associated with metabolism. The intrinsically fluorescent metabolic cofactors nicotinamide adenine dinucleotide (NAD+ /NADH) and flavin adenine dinucleotide (FAD/FADH2 ) have been utilized in a number of AFI applications including basic research, clinical, and pharmaceutical studies. Fluorescence lifetime imaging microscopy (FLIM) has emerged as one of the more powerful AFI tools for NADH and FAD characterization due to its unique ability to noninvasively detect metabolite bound and free states and quantitate cellular redox ratio. However, despite this widespread biological use, many standardization methods are still needed to extend FLIM-based AFI into a fully robust research and clinical diagnostic tools. FLIM is sensitive to a wide range of factors in the fluorophore microenvironment, and there are a number of analysis variables as well. To this end, there has been an emphasis on developing imaging standards and ways to make the image acquisition and analysis more consistent. However, biological conditions during FLIM-based AFI imaging are rarely considered as key sources of FLIM variability. Here, we present several experimental factors with supporting data of the cellular microenvironment such as confluency, pH, inter-/intracellular heterogeneity, and choice of cell line that need to be considered for accurate quantitative FLIM-based AFI measurement of cellular metabolism. © 2018 International Society for Advancement of Cytometry.

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).