Imaging of oxygen gradients in monolayer cultured cells using green fluorescent protein

Turn-on Fluorescent Biosensors for Imaging Hypoxia-like Conditions in Living Cells

We present the synthesis, photophysical properties, and biological application of nontoxic 3-azo-conjugated BODIPY dyes as masked fluorescent biosensors of hypoxia-like conditions. The synthetic methodology is based on an operationally simple N=N bond-forming protocol, followed by a Suzuki coupling, that allows for a direct access to simple and underexplored 3-azo-substituted BODIPY. These dyes can turn on their emission properties under both chemical and biological reductive conditions, including bacterial and human azoreductases, which trigger the azo bond cleavage, leading to fluorescent 3-amino-BODIPY. We have also developed a practical enzymatic protocol, using an immobilized bacterial azoreductase that allows for the evaluation of these azo-based probes and can be used as a model for the less accessible and expensive human reductase NQO1. Quantum mechanical calculations uncover the restructuration of the topography of the S₁ potential energy surface following the reduction of the azo moiety and rationalize the fluorescent quenching event through the mapping of an unprecedented pathway. Fluorescent microscopy experiments show that these azos can be used to visualize hypoxia-like conditions within living cells.

Warburg Effect, Glutamine, Succinate, Alanine, When Oxygen Matters

Cellular bioenergetics requires an intense ATP turnover that is increased further by hypermetabolic states caused by cancer growth or inflammation. Both are associated with metabolic alterations and, notably, enhancement of the Warburg effect (also known as aerobic glycolysis) of poor efficiency with regard to glucose consumption when compared to mitochondrial respiration. Therefore, beside this efficiency issue, other properties of these two pathways should be considered to explain this paradox: (1) biosynthesis, for this only indirect effect should be considered, since lactate release competes with biosynthetic pathways in the use of glucose; (2) ATP production, although inefficient, glycolysis shows other advantages when compared to mitochondrial respiration and lactate release may therefore reflect that the glycolytic flux is higher than required to feed mitochondria with pyruvate and glycolytic NADH; (3) Oxygen supply becomes critical under hypermetabolic conditions, and the ATP/O₂ ratio quantifies the efficiency of oxygen use to regenerate ATP, although aerobic metabolism remains intense the participation of anaerobic metabolisms (lactic fermentation or succinate generation) could greatly increase ATP/O₂ ratio; (4) time and space constraints would explain that anaerobic metabolism is required while the general metabolism appears oxidative; and (5) active repression of respiration by glycolytic intermediates, which could ensure optimization of glucose and oxygen use.

Phenotyping placental oxygenation in Lgals1 deficient mice using 19F MRI

Placental hypoperfusion and hypoxia are key drivers in complications during fetal development such as fetal growth restriction and preeclampsia. In order to study the mechanisms of disease in mouse models, the development of quantitative biomarkers of placental hypoxia is a prerequisite. The goal of this exploratory study was to establish a technique to noninvasively characterize placental partial pressure of oxygen (PO2) in vivo in the Lgals1 (lectin, galactoside-binding, soluble, 1) deficient mouse model of preeclampsia using fluorine magnetic resonance imaging. We hypothesized a decrease in placental oxygenation in knockout mice. Wildtype and knockout animals received fluorescently labeled perfluoro-5-crown-15-ether nanoemulsion i.v. on day E14-15 during pregnancy. Placental PO2 was assessed via calibrated 19F MRI saturation recovery T1 mapping. A gas challenge with varying levels of oxygen in breathing air (30%, 60% and 100% O2) was used to validate that changes in oxygenation can be detected in freely breathing, anesthetized animals. At the end of the experiment, fluorophore-coupled lectin was injected i.v. to label the vasculature for histology. Differences in PO2 between breathing conditions and genotype were statistically analyzed with linear mixed-effects modeling. As expected, a significant increase in PO2 with increasing oxygen in breathing air was found. PO2 in Lgals1 knockout animals was decreased but this effect was only present at 30% oxygen in breathing air, not at 60% and 100%. Histological examinations showed crossing of the perfluorocarbon nanoemulsion to the fetal blood pool but the dominating contribution of 19F MR signal is estimated at > 70% from maternal plasma based on volume fraction measurements of previous studies. These results show for the first time that 19F MRI can characterize oxygenation in mouse models of placental malfunction.

A distinctive distribution of hypoxia-inducible factor-1α in cultured renal tubular cells with hypoperfusion simulated by coverslip placement

Chronic hypoxia in the renal tubulointerstitium plays a key role in the progression of chronic kidney disease (CKD). It is therefore important to investigate tubular hypoxia and the activity of hypoxia-inducible factor (HIF)-1α in response to hypoxia. Rarefaction of the peritubular capillary causes hypoperfusion in CKD; however, the effect of hypoperfusion on HIFs has rarely been investigated. We induced hypoperfusion caused by coverslip placement in human kidney-2 cells, and observed an oxygen gradient under the coverslip. Immunocytochemistry of HIF-1α showed a doughnut-shaped formation on the edge of a pimonidazole-positive area, which we named the "HIF-ring". The oxygen tension of the HIF-ring was estimated to be between approximately 4 mmHg and 20 mmHg. This result was not compatible with those of past research showing HIF-1α accumulation in the anoxic range with homogeneous oxygen tension. We further observed the presence of a pH gradient under a coverslip, as well as a shift of the HIF ring due to changes in the pH of the culture medium, suggesting that the HIF ring was formed by suppression of HIF-1α related to low pH. This research demonstrated that HIF-1α activation mimics the physiological state in cultured cells with hypoperfusion.

Genetically Encoded Tools for Research of Cell Signaling and Metabolism under Brain Hypoxia

Hypoxia is characterized by low oxygen content in the tissues. The central nervous system (CNS) is highly vulnerable to a lack of oxygen. Prolonged hypoxia leads to the death of brain cells, which underlies the development of many pathological conditions. Despite the relevance of the topic, different approaches used to study the molecular mechanisms of hypoxia have many limitations. One promising lead is the use of various genetically encoded tools that allow for the observation of intracellular parameters in living systems. In the first part of this review, we provide the classification of oxygen/hypoxia reporters as well as describe other genetically encoded reporters for various metabolic and redox parameters that could be implemented in hypoxia studies. In the second part, we discuss the advantages and disadvantages of the primary hypoxia model systems and highlight inspiring examples of research in which these experimental settings were combined with genetically encoded reporters.

Nitroimidazole derivative incorporated liposomes for hypoxia-triggered drug delivery and enhanced therapeutic efficacy in patient-derived tumor xenografts

Hypoxia is not merely a tumor microenvironment byproduct, but rather an active participant in tumor development, invasion, and metastasis. Hypoxia contributes to poor outcomes in tumor treatment and has currently emerged as an important therapeutic target. In this work, a facile hypoxia-responsive liposomal drug delivery system was developed by incorporating derivatized nitroimidazole into liposome membranes. Under hypoxic conditions, hypoxia-induced reductive metabolism of the nitroimidazole derivative facilitated disassembly of the liposomes for triggered drug release. The liposomes showed high sensitivity to hypoxia, even at the cellular level, and could release payload in an oxygen-dependent manner, leading to high cytotoxicity in hypoxic conditions. In vivo fluorescence imaging revealed that there was a selective release of the liposomes at the hypoxic tumor site. As a result, the liposomes exhibited enhanced therapeutic efficacy in treating a hypoxic tumor in both cell line-derived and clinically relevant patient-derived xenograft models. Thus, hypoxia-responsive liposomes are a promising drug delivery system for hypoxia targeted tumor therapy. STATEMENT OF SIGNIFICANCE: 1. A facile but smart hypoxia-responsive liposomal drug delivery system is developed by incorporating nitroimidazole derivative, one of representative hypoxia-responsive moieties, into phospholipid bilayer of the liposomes. 2. The liposomes show extremely high sensitivity to hypoxia and can selectively release payload in hypoxic cells and hypoxic tumor. 3. The liposomes show enhanced therapeutic efficacy not only in cell line-derived xenograft model but also in clinically relevant patient-derived xenograft model, indicating their promising prospect in clinical application.

Imaging of oxygen and hypoxia in cell and tissue samples

Molecular oxygen (O2) is a key player in cell mitochondrial function, redox balance and oxidative stress, normal tissue function and many common disease states. Various chemical, physical and biological methods have been proposed for measurement, real-time monitoring and imaging of O2 concentration, state of decreased O2 (hypoxia) and related parameters in cells and tissue. Here, we review the established and emerging optical microscopy techniques allowing to visualize O2 levels in cells and tissue samples, mostly under in vitro and ex vivo, but also under in vivo settings. Particular examples include fluorescent hypoxia stains, fluorescent protein reporter systems, phosphorescent probes and nanosensors of different types. These techniques allow high-resolution mapping of O2 gradients in live or post-mortem tissue, in 2D or 3D, qualitatively or quantitatively. They enable control and monitoring of oxygenation conditions and their correlation with other biomarkers of cell and tissue function. Comparison of these techniques and corresponding imaging setups, their analytical capabilities and typical applications are given.

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

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

Synthesis of unsymmetrical Si-rhodamine fluorophores and application to a far-red to near-infrared fluorescence probe for hypoxia

A versatile synthesis of unsymmetrical Si-rhodamines was established and applied for development of a hypoxia-sensing far-red to NIR fluorescence probe.

Impact of Oxygen Levels on Human Hematopoietic Stem and Progenitor Cell Expansion

Oxygen levels are an important variable during the in vitro culture of stem cells. There has been increasing interest in the use of low oxygen to maximize proliferation and, in some cases, effect differentiation of stem cell populations. It is generally assumed that the defined pO₂ in the incubator reflects the pO₂ to which the stem cells are being exposed. However, we demonstrate that the pO₂ experienced by cells in static culture can change dramatically during the course of culture as cell numbers increase and as the oxygen utilization by cells exceeds the diffusion of oxygen through the media. Dynamic culture (whereby the cell culture plate is in constant motion) largely eliminates this effect, and a combination of low ambient oxygen and dynamic culture results in a fourfold increase in reconstituting capacity of human hematopoietic stem cells compared with those cultured in static culture at ambient oxygen tension. Cells cultured dynamically at 5% oxygen exhibited the best expansion: 30-fold increase by flow cytometry, 120-fold increase by colony assay, and 11% of human CD45 engraftment in the bone marrow of NOD/SCID mice. To our knowledge, this is the first study to compare individual and combined effects of oxygen and static or dynamic culture on hematopoietic ex vivo expansion. Understanding and controlling the effective oxygen tension experienced by cells may be important in clinical stem cell expansion systems, and these results may have relevance to the interpretation of low oxygen culture studies.

Direct visualization of the phenotype of hypoxic tumor cells at single cell resolution in vivo using a new hypoxia probe

Tumor hypoxia is linked to tumor progression, metastasis, and therapy resistance. However, the underlying mechanisms behind this linkage are not fully understood. Here we present a novel fluorescent mCherry hypoxia-responsive marker that can be used in real time imaging to specifically and sensitively identify hypoxic cells in vivo at single cell resolution. Tumors derived from triple negative tumor cells expressing the hypoxia marker reveal that the hypoxic tumor cells congregate near flowing blood vessels. Using multiphoton microscopy, hypoxic MDA-MB-231 cells were directly visualized and showed a more persistent slow migration phenotype as compared to normoxic cells in the same field in vivo. Hypoxic tumor cells are enriched in the cell population that migrates toward human epithelial growth factor gradients in vivo, and has increased collagen degradation and intravasation activity, characteristics of dissemination and metastasis competent tumor cells. The hypoxia probe introduced in this study provides a specific reporter of hypoxic cell phenotypes in vivo which reveals new insights into the mechanisms by which hypoxia is linked to metastasis.

Multifunctional Phosphorescent Conjugated Polymer Dots for Hypoxia Imaging and Photodynamic Therapy of Cancer Cells

Molecular oxygen (O2) plays a key role in many physiological processes, and becomes a toxicant to kill cells when excited to 1O2. Intracellular O2 levels, or the degree of hypoxia, are always viewed as an indicator of cancers. Due to the highly efficient cancer therapy ability and low side effect, photodynamic therapy (PDT) becomes one of the most promising treatments for cancers. Herein, an early-stage diagnosis and therapy system is reported based on the phosphorescent conjugated polymer dots (Pdots) containing Pt(II) porphyrin as an oxygen-responsive phosphorescent group and 1O2 photosensitizer. Intracellular hypoxia detection has been investigated. Results show that cells treated with Pdots display longer lifetimes under hypoxic conditions, and time-resolved luminescence images exhibit a higher signal-to-noise ratio after gating off the short-lived background fluorescence. Quantification of O2 is realized by the ratiometric emission intensity of phosphorescence/fluorescence and the lifetime of phosphorescence. Additionally, the PDT efficiency of Pdots is estimated by flow cytometry, MTT cell viability assay, and in situ imaging of PDT induced cell death. Interestingly, Pdots exhibit a high PDT efficiency and would be promising in clinical applications.

Directional Migration of MDA-MB-231 Cells Under Oxygen Concentration Gradients

To elucidate the initial mechanism of hematogenous metastasis of cancer cells, we hypothesized that cancer cells migrate toward regions with higher oxygen concentration such as intratumor micro vessels along the oxygen concentration gradient. To produce gradients of oxygen concentration in vitro, we devised the gap cover glass (GCG). After placing a GCG onto cultured MDA-MB-231 cells (a metastatic breast cancer cell line), the migration of individual cells under the GCG was tracked up to 12 h at 3 % oxygen in the micro incubator. We quantified the migration of individual cells using forward migration index (FMI). The cell migration perpendicular to the oxygen gradients was random in the direction whereas FMIs of the cell located at 300, 500, 700, and 1500 μm from the oxygen inlet were positive (p < 0.05) indicating a unidirectional migration toward the oxygen inlet. Present results are consistent with our hypothesis that MDA-MB-231 cells migrate toward regions with higher oxygen concentration.

A Phosphorescent Iridium(III) Complex-Modified Nanoprobe for Hypoxia Bioimaging Via Time-Resolved Luminescence Microscopy

Oxygen plays a crucial role in many biological processes. Accurate monitoring of oxygen level is important for diagnosis and treatment of diseases. Autofluorescence is an unavoidable interference in luminescent bioimaging, so that an amount of research work has been devoted to reducing background autofluorescence. Herein, a phosphorescent iridium(III) complex-modified nanoprobe is developed, which can monitor oxygen concentration and also reduce autofluorescence under both downconversion and upconversion channels. The nanoprobe is designed based on the mesoporous silica coated lanthanide-doped upconversion nanoparticles, which contains oxygen-sensitive iridium(III) complex in the outer silica shell. To image intracellular hypoxia without the interferences of autofluorescence, time-resolved luminescent imaging technology and near-infrared light excitation, both of which can reduce autofluorescence effectively, are adopted in this work. Moreover, gradient O2 concentration can be detected clearly through confocal microscopy luminescence intensity imaging, phosphorescence lifetime imaging microscopy, and time-gated imaging, which is meaningful to oxygen sensing in tissues with nonuniform oxygen distribution.

Imaging of oxygen gradients in giant umbrella cells: an ex vivo PLIM study

O2 plays a pivotal role in aerobic metabolism and regulation of cell and tissue function. Local differences and fluctuations in tissue O2 levels are well documented; however, the physiological significance of O2 microgradients, particularly at the subcellular level, remains poorly understood. Using the cell-penetrating phosphorescent O2 probe Pt-Glc and confocal fluorescence microscopy, we visualized O2 distribution in individual giant (>100-μm) umbrella cells located superficially in the urinary bladder epithelium. We optimized conditions for in vivo phosphorescent staining of the inner surface of the mouse bladder and subsequent ex vivo analysis of excised live tissue. Imaging experiments revealed significant (≤85 μM) and heterogeneous deoxygenation within respiring umbrella cells, with radial O2 gradients of up to 40 μM across the cell, or ∼0.6 μM/μm. Deeply deoxygenated (5–15 μM O2) regions were seen to correspond to the areas enriched with polarized mitochondria. Pharmacological activation of mitochondrial respiration decreased oxygenation and O2 gradients in umbrella cells, while inhibition with antimycin A dissipated the gradients and caused gradual reoxygenation of the tissue to ambient levels. Detailed three-dimensional maps of O2 distribution potentially can be used for the modeling of intracellular O2-dependent enzymatic reactions and downstream processes, such as hypoxia-inducible factor signaling. Further ex vivo and in vivo studies on intracellular and tissue O2 gradients using confocal imaging can shed light on the molecular mechanisms regulating O2-dependent (patho)physiological processes in the bladder and other tissues.

Oxygen sensing by protozoans: how they catch their breath

Cells must know the local levels of available oxygen and either adapt accordingly or relocate to more favorable environments. Prolyl 4-hydroxylases (P4Hs) are emerging as universal cellular oxygen sensors. In animals, these oxygen sensors respond to decreased oxygen availability by up-regulating hypoxia-inducible transcription factors. In protozoa, the P4Hs appear to activate E3-SCF ubiquitin ligase complexes via a glycosylation-dependent mechanism, potentially to turn over their proteomes. Intracellular parasites are impacted by both types of oxygen-sensing pathways. Since parasites are exposed to diverse oxygen tensions during their life cycles, this review identifies emerging oxygen-sensing mechanisms and discusses how these mechanisms probably contribute to the regulation of unicellular eukaryotes.

Anaerobic respiration sustains mitochondrial membrane potential in a prolyl hydroxylase pathway-activated cancer cell line in a hypoxic microenvironment

To elucidate how tumor cells produce energy in oxygen-depleted microenvironments, we studied the possibility of mitochondrial electron transport without oxygen. We produced well-controlled oxygen gradients (ΔO2) in monolayer-cultured cells. We then visualized oxygen levels and mitochondrial membrane potential (ΔΦm) in individual cells by using the red shift of green fluorescent protein (GFP) fluorescence and a cationic fluorescent dye, respectively. In this two-dimensional tissue model, ΔΦm was abolished in cells >500 μm from the oxygen source [the anoxic front (AF)], indicating limitations in diffusional oxygen delivery. This result perfectly matched GFP-determined ΔO2. In cells pretreated with dimethyloxaloylglycine (DMOG), a prolyl hydroxylase domain-containing protein (PHD) inhibitor, the AF was expanded to 1,500-2,000 μm from the source. In these cells, tissue ΔO2 was substantially decreased, indicating that PHD pathway activation suppressed mitochondrial respiration. The expansion of the AF and the reduction of ΔO2 were much more prominent in a cancer cell line (Hep3B) than in the equivalent fibroblast-like cell line (COS-7). Hence, the results indicate that PHD pathway-activated cells can sustain ΔΦm, despite significantly decreased electron flux to complex IV. Complex II inhibition abolished the effect of DMOG in expanding the AF, although tissue ΔO2 remained shallow. Separate experiments demonstrated that complex II plays a substantial role in sustaining ΔΦm in DMOG-pretreated Hep3B cells with complex III inhibition. From these results, we conclude that PHD pathway activation can sustain ΔΦm in an otherwise anoxic microenvironment by decreasing tissue ΔO2 while activating oxygen-independent electron transport in mitochondria.

Reversible off-on fluorescence probe for hypoxia and imaging of hypoxia-normoxia cycles in live cells

We report a fully reversible off-on fluorescence probe for hypoxia. The design employs QSY-21 as a Förster resonance energy transfer (FRET) acceptor and cyanine dye Cy5 as a FRET donor, based on our finding that QSY-21 undergoes one-electron bioreduction to the radical under hypoxia, with an absorbance decrease at 660 nm. At that point, FRET can no longer occur, and the dye becomes strongly fluorescent. Upon recovery of normoxia, the radical is immediately reoxidized to QSY-21, with loss of fluorescence due to restoration of FRET. We show that this probe, RHyCy5, can monitor repeated hypoxia-normoxia cycles in live cells.

Optical oxygen micro- and nanosensors for plant applications

Pioneered by Clark's microelectrode more than half a century ago, there has been substantial interest in developing new, miniaturized optical methods to detect molecular oxygen inside cells. While extensively used for animal tissue measurements, applications of intracellular optical oxygen biosensors are still scarce in plant science. A critical aspect is the strong autofluorescence of the green plant tissue that interferes with optical signals of commonly used oxygen probes. A recently developed dual-frequency phase modulation technique can overcome this limitation, offering new perspectives for plant research. This review gives an overview on the latest optical sensing techniques and methods based on phosphorescence quenching in diverse tissues and discusses the potential pitfalls for applications in plants. The most promising oxygen sensitive probes are reviewed plus different oxygen sensing structures ranging from micro-optodes to soluble nanoparticles. Moreover, the applicability of using heterologously expressed oxygen binding proteins and fluorescent proteins to determine changes in the cellular oxygen concentration are discussed as potential non-invasive cellular oxygen reporters.

Optical probes and techniques for O2 measurement in live cells and tissue

In recent years, significant progress has been achieved in the sensing and imaging of molecular oxygen (O₂) in biological samples containing live cells and tissue. We review recent developments in the measurement of O₂ in such samples by optical means, particularly using the phosphorescence quenching technique. The main types of soluble O₂ sensors are assessed, including small molecule, supramolecular and particle-based structures used as extracellular or intracellular probes in conjunction with different detection modalities and measurement formats. For the different O₂ sensing systems, particular attention is paid to their merits and limitations, analytical performance, general convenience and applicability in specific biological applications. The latter include measurement of O₂ consumption rate, sample oxygenation, sensing of intracellular O₂, metabolic assessment of cells, and O₂ imaging of tissue, vasculature and individual cells. Altogether, this gives the potential user a comprehensive guide for the proper selection of the appropriate optical probe(s) and detection platform to suit their particular biological applications and measurement requirements.

From cancer metabolism to new biomarkers and drug targets

Great interest is presently given to the analysis of metabolic changes that take place specifically in cancer cells. In this review we summarize the alterations in glycolysis, glutamine utilization, fatty acid synthesis and mitochondrial function that have been reported to occur in cancer cells and in human tumors. We then propose considering cancer as a system-level disease and argue how two hallmarks of cancer, enhanced cell proliferation and evasion from apoptosis, may be evaluated as system-level properties, and how this perspective is going to modify drug discovery. Given the relevance of the analysis of metabolism both for studies on the molecular basis of cancer cell phenotype and for clinical applications, the more relevant technologies for this purpose, from metabolome and metabolic flux analysis in cells by Nuclear Magnetic Resonance and Mass Spectrometry technologies to positron emission tomography on patients, are analyzed. The perspectives offered by specific changes in metabolism for a new drug discovery strategy for cancer are discussed and a survey of the industrial activity already going on in the field is reported.