Development of a family of redox-sensitive green fluorescent protein indicators for use in relatively oxidizing subcellular environments

Real-time monitoring of redox changes in the mammalian endoplasmic reticulum

Redox-sensitive GFPs with engineered disulphide bonds have been used previously to monitor redox status in the cytosol and mitochondria of living cells. The usefulness of these redox probes depends on the reduction potential of the disulphide, with low values suiting the cytosol and mitochondrion, and higher values suiting the more oxidising environment of the endoplasmic reticulum (ER). Here, we targeted a modified redox-sensitive GFP (roGFP1-iL), with a relatively high reduction potential, to the ER of mammalian cells. We showed that the disulphide is partially oxidised, allowing roGFP1-iL to monitor changes in ER redox status. When cells were treated with puromycin, the redox balance became more reducing, suggesting that the release of nascent chains from ribosomes alters the ER redox balance. In addition, downregulating Ero1α prevented normal rapid recovery from dithiothreitol (DTT), whereas downregulating peroxiredoxin IV had no such effect. This result illustrates the contribution of the Ero1α oxidative pathway to ER redox balance. This first report of the use of roGFP to study the ER of mammalian cells demonstrates that roGFP1-iL can be used to monitor real-time changes to the redox status in individual living cells.

Glutathione- and non-glutathione-based oxidant control in the endoplasmic reticulum

The redox-active tripeptide glutathione is an endogenous reducing agent that is found in abundance and throughout the cell. In the endoplasmic reticulum (ER), the ratio of glutathione to glutathione disulfide is lower compared with non-secretory organelles. This relatively oxidizing thiol-disulfide milieu is essential for the oxidative folding of nascent proteins in the ER and, at least in part, maintained by the activity of ER-resident endoplasmic oxidoreductin 1 (Ero1) enzymes that oxidize cysteine side chains at the expense of molecular oxygen. Glutathione disulfide and hydrogen peroxide formed as a consequence of Ero1 activity are widely considered as being inoperative and potentially dangerous by-products of oxidative protein folding in the ER. In contrast to this common view, this Commentary highlights the importance of glutathione- and non glutathione-based homeostatic redox control mechanisms in the ER. Stability in the thiol-disulfide system that prominently includes the protein disulfide isomerases is ensured by the contribution of tightly regulated Ero1 activity, ER-resident peroxidases and the glutathione-glutathione-disulfide redox pair that acts as a potent housekeeper of redox balance. Accordingly, the widely held concept that Ero1-mediated over-oxidation in the ER constitutes a common cause of cellular demise is critically re-evaluated.

ROS signaling: the new wave?

Reactive oxygen species (ROS) play a multitude of signaling roles in different organisms from bacteria to mammalian cells. They were initially thought to be toxic byproducts of aerobic metabolism, but have now been acknowledged as central players in the complex signaling network of cells. In this review, we will attempt to address several key questions related to the use of ROS as signaling molecules in cells, including the dynamics and specificity of ROS signaling, networking of ROS with other signaling pathways, ROS signaling within and across different cells, ROS waves and the evolution of the ROS gene network.

Development of a high-dynamic range, GFP-based FRET probe sensitive to oxidative microenvironments

We report the optimization of a novel redox-sensitive probe with enhanced dynamic range and an exceptionally well-positioned oxidative midpoint redox potential. The present work characterizes factors that contribute to the improved Förster resonance energy transfer (FRET) performance of this green fluorescent protein (GFP)-based redox sensor. The α-helical linker, which separates the FRET donor and acceptor, has been extended in the new probe and leads to a decreased FRET efficiency in the linker's reduced, 'FRET-off' state. Unexpectedly, the FRET efficiency is increased in the new linker's oxidized, 'FRET-on' state compared with the parent probe, in spite of the longer linker sequence. The combination of a lowered baseline 'FRET-off' and an increased 'FRET-on' signal significantly improves the dynamic range of the probe for a more robust discrimination of its reduced and oxidized linker states. Mutagenesis of the cysteine residues within the α-helix linker reveals the importance of the fourth, C-terminal cysteine and the relative insignificance of the second cysteine in forming the disulfide bridge to clamp the linker into the high-FRET, oxidized state. To further optimize the performance of the redox probe, various cyan fluorescent protein (CFP)/yellow fluorescent protein (YFP) FRET pairs, placed at opposite ends of the improved redox linker (RL7), were quantitatively compared and exchanged. We found that the CyPet/YPet and ECFP/YPet FRET pairs when attached to RL7 do not function well as sensitive redox probes due to a strong tendency to form heterodimers, which disrupt the α-helix. However, monomeric versions of CyPet and YPet (mCyPet and mYPet) eliminate dimerization and restore redox sensitivity of the probe. The best performing probe, ECFP-RL7-EYFP, exhibits an approximately six-fold increase in FRET efficiency in vitro when passing from the oxidized to the reduced state. We determined the midpoint redox potential of the probe to be -143 ± 6 mV, which is ideal for measuring glutathione (GSH/GSSG) redox potentials in oxidative compartments of mammalian cells (e.g. the endoplasmic reticulum).

Oxidative protein folding by an endoplasmic reticulum-localized peroxiredoxin

Endoplasmic reticulum (ER) oxidation 1 (ERO1) transfers disulfides to protein disulfide isomerase (PDI) and is essential for oxidative protein folding in simple eukaryotes such as yeast and worms. Surprisingly, ERO1-deficient mammalian cells exhibit only a modest delay in disulfide bond formation. To identify ERO1-independent pathways to disulfide bond formation, we purified PDI oxidants with a trapping mutant of PDI. Peroxiredoxin IV (PRDX4) stood out in this list, as the related cytosolic peroxiredoxins are known to form disulfides in the presence of hydroperoxides. Mouse embryo fibroblasts lacking ERO1 were intolerant of PRDX4 knockdown. Introduction of wild-type mammalian PRDX4 into the ER rescued the temperature-sensitive phenotype of an ero1 yeast mutation. In the presence of an H(2)O(2)-generating system, purified PRDX4 oxidized PDI and reconstituted oxidative folding of RNase A. These observations implicate ER-localized PRDX4 in a previously unanticipated, parallel, ERO1-independent pathway that couples hydroperoxide production to oxidative protein folding in mammalian cells.

Mitochondrial redox potential during contraction in single intact muscle fibers

Although the production of reactive oxygen species (ROS) during muscle contractile activity has been linked to both positive and negative adaptive responses, the sites for ROS generation within working muscle are not clearly defined. We assessed cytosolic ROS production and mitochondrial redox potential with a targeted redox-sensitive green fluorescent protein during repetitive field stimulation of single mature myofibers. Cytosolic ROS production increased by 94%, an effect that was abolished by pretreatment with the reducing agent dithiothreitol. Mitochondrial redox potential was not altered during muscle contraction. In contrast, activity-dependent ROS production was ablated by an inhibitor of NADPH oxidase. We provide the first report on dynamic ROS production from mitochondria in single living myofibers and suggest that the mitochondria are not the major source of ROS during skeletal muscle contraction. Alternatively, our data support a role for NADPH oxidase-derived ROS during contractile activity.

Fluorescent protein-based redox probes

Redox biochemistry is increasingly recognized as an integral component of cellular signal processing and cell fate decision making. Unfortunately, our capabilities to observe and measure clearly defined redox processes in the natural context of living cells, tissues, or organisms are woefully limited. The most advanced and promising tools for specific, quantitative, dynamic and compartment-specific observations are genetically encoded redox probes derived from green fluorescent protein (GFP). Within only few years from their initial introduction, redox-sensitive yellow FP (rxYFP), redox-sensitive GFPs (roGFPs), and HyPer have generated enormous interest in applying these novel tools to monitor dynamic redox changes in vivo. As genetically encoded probes, these biosensors can be specifically targeted to different subcellular locations. A critical advantage of roGFPs and HyPer is their ratiometric fluorogenic behavior. Moreover, the probe scaffold of redox-sensitive fluorescent proteins (rxYFP and roGFPs) is amenable to molecular engineering, offering fascinating prospects for further developments. In particular, the engineering of redox relays between roGFPs and redox enzymes allows control of probe specificity and enhancement of sensitivity. Genetically encoded redox probes enable the functional analysis of individual proteins in cellular redox homeostasis. In addition, redox biosensor transgenic model organisms offer extended opportunities for dynamic in vivo imaging of redox processes.

Hypoxia-induced changes in pulmonary and systemic vascular resistance: where is the O2 sensor?

Pulmonary arteries (PA) constrict in response to alveolar hypoxia, whereas systemic arteries (SA) undergo dilation. These physiological responses reflect the need to improve gas exchange in the lung, and to enhance the delivery of blood to hypoxic systemic tissues. An important unresolved question relates to the underlying mechanism by which the vascular cells detect a decrease in oxygen tension and translate that into a signal that triggers the functional response. A growing body of work implicates the mitochondria, which appear to function as O2 sensors by initiating a redox-signaling pathway that leads to the activation of downstream effectors that regulate vascular tone. However, the direction of this redox signal has been the subject of controversy. Part of the problem has been the lack of appropriate tools to assess redox signaling in live cells. Recent advancements in the development of redox sensors have led to studies that help to clarify the nature of the hypoxia-induced redox signaling by reactive oxygen species (ROS). Moreover, these studies provide valuable insight regarding the basis for discrepancies in earlier studies of the hypoxia-induced mechanism of redox signaling. Based on recent work, it appears that the O2 sensing mechanism in both the PA and SA are identical, that mitochondria function as the site of O2 sensing, and that increased ROS release from these organelles leads to the activation of cell-specific, downstream vascular responses.

An integrated imaging approach to the study of oxidative stress generation by mitochondrial dysfunction in living cells

BACKGROUND

The mechanisms of action of many environmental agents commonly involve oxidative stress resulting from mitochondrial dysfunction. Zinc is a common environmental metallic contaminant that has been implicated in a variety of oxidant-dependent toxicological responses. Unlike ions of other transition metals such as iron, copper, and vanadium, Zn(2+) does not generate reactive oxygen species (ROS) through redox cycling.

OBJECTIVE

To characterize the role of oxidative stress in zinc-induced toxicity.

METHODS

We used an integrated imaging approach that employs the hydrogen peroxide (H2O2)-specific fluorophore Peroxy Green 1 (PG1), the mitochondrial potential sensor 5,5 ,6,6 -tetrachloro-1,1 ,3,3 -tetraethylbenzimidazolylcarbocyanine iodide (JC-1), and the mitochondria-targeted form of the redox-sensitive genetically encoded fluorophore MTroGFP1 in living cells.

RESULTS

Zinc treatment in the presence of the Zn(2+) ionophore pyrithione of A431 skin carcinoma cells preloaded with the H(2)O(2)-specific indicator PG1 resulted in a significant increase in H(2)O(2) production that could be significantly inhibited with the mitochondrial inhibitor carbonyl cyanide 3-chlorophenylhydrazone. Mitochondria were further implicated as the source of zinc-induced H(2)O(2) formation by the observation that exposure to zinc caused a loss of mitochondrial membrane potential. Using MTroGFP1, we showed that zinc exposure of A431 cells induces a rapid loss of reducing redox potential in mitochondria. We also demonstrated that zinc exposure results in rapid swelling of mitochondria isolated from mouse hearts.

CONCLUSION

Taken together, these findings show a disruption of mitochondrial integrity, H(2)O(2) formation, and a shift toward positive redox potential in cells exposed to zinc. These data demonstrate the utility of real-time, live-cell imaging to study the role of oxidative stress in toxicological responses.

A novel fluorescent sensor protein for visualization of redox states in the cytoplasm and in peroxisomes

Reactive oxygen species are generated within peroxisomes during peroxisomal metabolism. However, due to technological difficulties, the intraperoxisomal redox state remain elusive, and the effect of peroxisome deficiency on the intracellular redox state is controversial. A newly developed, genetically encoded fluorescence resonance energy transfer (FRET) probe, Redoxfluor, senses the physiological redox state via its internal disulfide bonds, resulting in a change in the conformation of the protein leading to a FRET response. We made use of Redoxfluor to measure the redox states at the subcellular level in yeast and Chinese hamster ovary (CHO) cells. In wild-type peroxisomes harboring an intact fatty acid beta-oxidation system, the redox state within the peroxisomes was more reductive than that in the cytosol, despite the fact that reactive oxygen species were generated within the peroxisomes. Interestingly, we observed that the redox state of the cytosol of cell mutants for peroxisome assembly, regarded as models for a neurological metabolic disorder, was more reductive than that of the wild-type cells in yeast and CHO cells. Furthermore, Redoxfluor was utilized to develop an efficient system for the screening of drugs that moderate the abnormal cytosolic redox state in the mutant CHO cell lines for peroxisome assembly without affecting the redox state of normal cells.

Hypoxia triggers subcellular compartmental redox signaling in vascular smooth muscle cells

RATIONALE

Recent studies have implicated mitochondrial reactive oxygen species (ROS) in regulating hypoxic pulmonary vasoconstriction (HPV), but controversy exists regarding whether hypoxia increases or decreases ROS generation.

OBJECTIVE

This study tested the hypothesis that hypoxia induces redox changes that differ among subcellular compartments in pulmonary (PASMCs) and systemic (SASMCs) smooth muscle cells.

METHODS AND RESULTS

We used a novel, redox-sensitive, ratiometric fluorescent protein sensor (RoGFP) to assess the effects of hypoxia on redox signaling in cultured PASMCs and SASMCs. Using genetic targeting sequences, RoGFP was expressed in the cytosol (Cyto-RoGFP), the mitochondrial matrix (Mito-RoGFP), or the mitochondrial intermembrane space (IMS-RoGFP), allowing assessment of oxidant signaling in distinct intracellular compartments. Superfusion of PASMCs or SASMCs with hypoxic media increased oxidation of both Cyto-RoGFP and IMS-RoGFP. However, hypoxia decreased oxidation of Mito-RoGFP in both cell types. The hypoxia-induced oxidation of Cyto-RoGFP was attenuated through the overexpression of cytosolic catalase in PASMCs.

CONCLUSIONS

These results indicate that hypoxia causes a decrease in nonspecific ROS generation in the matrix compartment, whereas it increases regulated ROS production in the IMS, which diffuses to the cytosol of both PASMCs and SASMCs.