Imaging Dynamic Redox Changes in Mammalian Cells with Green Fluorescent Protein Indicators

Abstract

Changes in the redox equilibrium of cells influence a host of cell functions. Alterations in the redox equilibrium are precipitated by changing either the glutathione/glutathione-disulfide ratio (GSH/GSSG) and/or the reduced/oxidized thioredoxin ratio. Redox-sensitive green fluorescent proteins (GFP) allow real time visualization of the oxidation state of the indicator. Ratios of fluorescence from excitation at 400 and 490 nm indicate the extent of oxidation and thus the redox potential while canceling out the amount of indicator and the absolute optical sensitivity. Because the indicator is genetically encoded, it can be targeted to specific proteins or organelles of interest and expressed in a wide variety of cells and organisms. We evaluated roGFP1 (GFP with mutations C48S, S147C, and Q204C) and roGFP2 (the same plus S65T) with physiologically or toxicologically relevant oxidants both in vitro and in living mammalian cells. Furthermore, we investigated the response of the redox probes under physiological redox changes during superoxide bursts in macrophage cells, hyperoxic and hypoxic conditions, and in responses to H 2 O 2 -stimulating agents, e.g. epidermal growth factor and lysophosphatidic acid.Cells have elaborate homeostatic mechanisms to regulate the thiol-disulfide redox status of their internal compartments. Most thiol groups within the cytoplasm are normally reduced. Very few are present as disulfides. It has been speculated that the cytoplasm is reducing because many metabolic reactions evolved before oxygen became abundant in the atmosphere (1). Modest alterations in the thiol-disulfide equilibrium could have major consequences in the cell, including defective protein folding or enzyme activity (because many enzymes have a cysteine in their active site). When excess oxidation overwhelms the reductive capabilities of the cell, death results. Despite the dangers of excessive oxidation, cells sometimes use redox adjustments as signaling events, such as in the activation of transcription factors (NF-B and AP-1), caspases, protein tyrosine phosphatases, or GTPases (Ras). Thus, changes in the redox equilibrium influence a host of cell functions, including but not limited to growth, stress responses, differentiation, metabolism, cell cycle, communication, migration, gene transcription, ion channels, and immune responses (for reviews see Refs. 2-6). Alterations in the redox equilibrium are reflected in changes of the glutathione/glutathione-disulfide ratio (GSH/ GSSG) and the reduced/oxidized thioredoxin ratio. Glutathione is found in high concentrations in cells (5-10 mM) and is considered to be the major player in maintaining intracellular redox equilibrium. Ratios of GSH to GSSG are reported to range from 100 to 300:1 (7, 8), but these measurements have been problematic because they require destruction of the tissue, during which great care must be taken not to allow further oxidation. The major source of error is the determination of GSSG concentration, because this species is at low abundance yet is ...

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