Interpreting Heterogeneity in Response of Cells Expressing a Fluorescent Hydrogen Peroxide Biosensor

HyPer as a tool to determine the reductive activity in cellular compartments

A multitude of cellular metabolic and regulatory processes rely on controlled thiol reduction and oxidation mechanisms. Due to our aerobic environment, research preferentially focuses on oxidation processes, leading to limited tools tailored for investigating cellular reduction. Here, we advocate for repurposing HyPer1, initially designed as a fluorescent probe for H2O2 levels, as a tool to measure the reductive power in various cellular compartments. The response of HyPer1 depends on kinetics between thiol oxidation and reduction in its OxyR sensing domain. Here, we focused on the reduction half-reaction of HyPer1. We showed that HyPer1 primarily relies on Trx/TrxR-mediated reduction in the cytosol and nucleus, characterized by a second order rate constant of 5.8 × 102 M-1s-1. On the other hand, within the mitochondria, HyPer1 is predominantly reduced by glutathione (GSH). The GSH-mediated reduction rate constant is 1.8 M-1s-1. Using human leukemia K-562 cells after a brief oxidative exposure, we quantified the compartmentalized Trx/TrxR and GSH-dependent reductive activity using HyPer1. Notably, the recovery period for mitochondrial HyPer1 was twice as long compared to cytosolic and nuclear HyPer1. After exploring various human cells, we revealed a potent cytosolic Trx/TrxR pathway, particularly pronounced in cancer cell lines such as K-562 and HeLa. In conclusion, our study demonstrates that HyPer1 can be harnessed as a robust tool for assessing compartmentalized reduction activity in cells following oxidative stress.

Delivery of Lipid Nanoparticles with ROS Probes for Improved Visualization of Hepatocellular Carcinoma

Reactive oxygen species (ROS) are highly reactive products of the cell metabolism derived from oxygen molecules, and their abundant level is observed in many diseases, particularly tumors, such as hepatocellular carcinoma (HCC). In vivo imaging of ROS is a necessary tool in preclinical research to evaluate the efficacy of drugs with antioxidant activity and for diagnosis and monitoring of diseases. However, most known sensors cannot be used for in vivo experiments due to low stability in the blood and rapid elimination from the body. In this work, we focused on the development of an effective delivery system of fluorescent probes for intravital ROS visualization using the HCC model. We have synthesized various lipid nanoparticles (LNPs) loaded with ROS-inducible hydrocyanine pro-fluorescent dye or plasmid DNA (pDNA) with genetically encoded protein sensors of hydrogen peroxide (HyPer7). LNP with an average diameter of 110 ± 12 nm, characterized by increased stability and pDNA loading efficiency (64 ± 7%), demonstrated preferable accumulation in the liver compared to 170 nm LNPs. We evaluated cytotoxicity and demonstrated the efficacy of hydrocyanine-5 and HyPer7 formulated in LNP for ROS visualization in mouse hepatocytes (AML12 cells) and in the mouse xenograft model of HCC. Our results demonstrate that obtained LNP could be a valuable tool in preclinical research for visualization ROS in liver diseases.

AQP8 is a crucial H2O2 transporter in insulin-producing RINm5F cells

Peroxiporins are distinct aquaporins (AQP) which, beside water, also facilitate the bidirectional transport of hydrogen peroxide (H₂O₂) across cellular membranes. H₂O₂ serves as the major reactive oxygen species that mediates essential cell signaling events. In pancreatic β-cells, H₂O₂ has been associated with the regulation of cell growth but in excess it leads to failure of insulin secretion, making it important for diabetes mellitus (DM) pathogenesis. In the present study, the role of aquaporin-8 (AQP8) as a peroxiporin was investigated in RINm5F cells. The role of AQP8 was studied in an insulin-producing cell model, on the basis of stable AQP8 overexpression (AQP8↑) and CRISPR/Cas9-mediated AQP8 knockdown (KD). A complete AQP8 knock-out was found to result in cell death, however we demonstrate that mild lentiviral re-expression through a Tet-On-regulated genetically modified AQP8 leads to cell survival, enabling functional characterization. Proliferation and insulin content were found to be increased in AQP8↑ cells underlining the importance of AQP8 in the regulation of H₂O₂ homeostasis in pancreatic β-cells. Colocalization analyses of V5-tagged AQP8 proteins based on confocal microscopic imaging revealed its membrane targeting to both the mitochondria and the plasma membrane, but not to the ER, the Golgi apparatus, insulin vesicles, or peroxisomes. By using the fluorescence H₂O₂ specific biosensor HyPer together with endogenous generation of H₂O₂ using d-amino acid oxidase, live cell imaging revealed enhanced H₂O₂ flux to the same subcellular regions in AQP8 overexpressing cells pointing to its importance in the development of type-1 DM. Moreover, the novel ultrasensitive H₂O₂ sensor HyPer7.2 clearly unveiled AQP8 as a H₂O₂ transporter in RINm5F cells. In summary, these studies establish that AQP8 is an important H₂O₂ pore in insulin-producing RINm5F cells involved in the transport of H₂O₂ through the mitochondria and cell membrane and may help to explain the H₂O₂ transport and toxicity in pancreatic β-cells.

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AQP8 KO is lethal for insulin-producing RINm5F cells.

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The peroxiporin AQP8 is localized in the plasma and mitochondrial membrane channeling H₂O₂ in RINm5F cells.

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Tet-On regulated low AQP8 re-expression and APQ8 overexpression are feasible models to study H₂O₂ transport in β-cells.

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Overexpression of AQP8 increases cell proliferation and cellular insulin content.

Reactive oxygen species (ROS) as pleiotropic physiological signalling agents

'Reactive oxygen species' (ROS) is an umbrella term for an array of derivatives of molecular oxygen that occur as a normal attribute of aerobic life. Elevated formation of the different ROS leads to molecular damage, denoted as 'oxidative distress'. Here we focus on ROS at physiological levels and their central role in redox signalling via different post-translational modifications, denoted as 'oxidative eustress'. Two species, hydrogen peroxide (H₂O₂) and the superoxide anion radical (O₂^·-), are key redox signalling agents generated under the control of growth factors and cytokines by more than 40 enzymes, prominently including NADPH oxidases and the mitochondrial electron transport chain. At the low physiological levels in the nanomolar range, H₂O₂ is the major agent signalling through specific protein targets, which engage in metabolic regulation and stress responses to support cellular adaptation to a changing environment and stress. In addition, several other reactive species are involved in redox signalling, for instance nitric oxide, hydrogen sulfide and oxidized lipids. Recent methodological advances permit the assessment of molecular interactions of specific ROS molecules with specific targets in redox signalling pathways. Accordingly, major advances have occurred in understanding the role of these oxidants in physiology and disease, including the nervous, cardiovascular and immune systems, skeletal muscle and metabolic regulation as well as ageing and cancer. In the past, unspecific elimination of ROS by use of low molecular mass antioxidant compounds was not successful in counteracting disease initiation and progression in clinical trials. However, controlling specific ROS-mediated signalling pathways by selective targeting offers a perspective for a future of more refined redox medicine. This includes enzymatic defence systems such as those controlled by the stress-response transcription factors NRF2 and nuclear factor-κB, the role of trace elements such as selenium, the use of redox drugs and the modulation of environmental factors collectively known as the exposome (for example, nutrition, lifestyle and irradiation).

Measuring intracellular concentration of hydrogen peroxide with the use of genetically encoded H2O2 biosensor HyPer

In this study, we propose a method for quantification of average hydrogen peroxide concentration within a living cell that is based on the use of genetically encoded H₂O₂ biosensor HyPer. The method utilizes flow cytometric measurements of HyPer fluorescence in H₂O₂-exposed cells to analyze the biosensor oxidation kinetics. Fitting the experimental curves with kinetic equations allows determining the rate constants of HyPer oxidation/reduction which are used further for the calculation of peroxide concentrations in the cells of interest both in the presence and absence of external H₂O₂. Applying this method to K562 cells, we have estimated the gradient as about 390-fold between the extracellular and intracellular level of exogenous H₂O₂ in cells exposed to the micromole doses of peroxide, as well as the average basal level of H₂O₂ in the cytosol of undisturbed cells ([H2O2]basal=2.2±0.4nM). The method can be extended to other H₂O₂-sensitive redox probes or to procedures in which, rather than adding external peroxide, intracellular production of peroxide is triggered, providing a tool to quantitate not only basal average H₂O₂ concentrations but also the concentration of peroxide build up in the vicinity of redox probes.

Flow cytometric HyPer-based assay for hydrogen peroxide

HyPer is a genetically encoded fluorogenic sensor for hydrogen peroxide which is generally used for the ratiometric imaging of H₂O₂ fluxes in living cells. Here, we demonstrate the advantages of HyPer-based ratiometric flow cytometry assay for H₂O₂, by using K562 and human mesenchymal stem cell lines expressing HyPer. We show that flow cytometry analysis is suitable to detect HyPer response to submicromolar concentrations of extracellularly added H₂O₂ that is much lower than concentrations addressed previously in the other HyPer-based assays (such as cell imaging or fluorimetry). Suggested technique is also much more sensitive to hydrogen peroxide than the widespread flow cytometry assay exploiting H₂O₂-reactive dye H₂DCFDA and, contrary to the H₂DCFDA-based assay, can be employed for the kinetic studies of H₂O₂ utilization by cells, including measurements of the rate constants of H₂O₂ removal. In addition, flow cytometry multi-parameter ratiometric measurements enable rapid and high-throughput detection of endogenously generated H₂O₂ in different subpopulations of HyPer-expressing cells. To sum up, HyPer can be used in multi-parameter flow cytometry studies as a highly sensitive indicator of intracellular H₂O₂.

Monitoring the action of redox-directed cancer therapeutics using a human peroxiredoxin-2-based probe

Redox cancer therapeutics target the increased reliance on intracellular antioxidant systems and enhanced susceptibility to oxidant-induced stress of some cancer cells compared to normal cells. Many of these therapeutics are thought to perturb intracellular levels of the oxidant hydrogen peroxide (H2O2), a signaling molecule that modulates a number of different processes in human cells. However, fluorescent probes for this species remain limited in their ability to detect the small perturbations induced during successful treatments. We report a fluorescent sensor based upon human peroxiredoxin-2, which acts as the natural indicator of small H2O2 fluctuations in human cells. The new probe reveals peroxide-induced oxidation in human cells below the detection limit of current probes, as well as peroxiredoxin-2 oxidation caused by two different redox cancer therapeutics in living cells. This capability will be useful in elucidating the mechanism of current redox-based therapeutics and in developing new ones.

Recent Advances in Development of Genetically Encoded Fluorescent Sensors

Genetically encoded fluorescent sensors are essential tools in modern biological research, and recent advances in fluorescent proteins (FPs) have expanded the scope of sensor design and implementation. In this review we compare different sensor platforms, including Förster resonance energy transfer (FRET) sensors, fluorescence-modulated single FP-based sensors, translocation sensors, complementation sensors, and dimerization-based sensors. We discuss elements of sensor design and engineering for each platform, including the incorporation of new types of FPs and sensor screening techniques. Finally, we summarize the wide range of sensors in the literature, exploring creative new sensor architectures suitable for different applications.

Modulating and Measuring Intracellular H2O2 Using Genetically Encoded Tools to Study Its Toxicity to Human Cells

Reactive oxygen species (ROS) such as H₂O₂ play paradoxical roles in mammalian physiology. It is hypothesized that low, baseline levels of H₂O₂ are necessary for growth and differentiation, while increased intracellular H₂O₂ concentrations are associated with pathological phenotypes and genetic instability, eventually reaching a toxic threshold that causes cell death. However, the quantities of intracellular H₂O₂ that lead to these different responses remain an unanswered question in the field. To address this question, we used genetically encoded constructs that both generate and quantify H₂O₂ in a dose-response study of H₂O₂-mediated toxicity. We found that, rather than a simple concentration-response relationship, a combination of intracellular concentration and the cumulative metric of H₂O₂ concentration multiplied by time (i.e., the area under the curve) determined the occurrence and level of cell death. Establishing the quantitative relationship between H₂O₂ and cell toxicity promotes a deeper understanding of the intracellular effects of H₂O₂ specifically as an individual reactive oxygen species, and it contributes to an understanding of its role in various redox-related diseases.