Extending roGFP Emission via Förster-Type Resonance Energy Transfer Relay Enables Simultaneous Dual Compartment Ratiometric Redox Imaging in Live Cells

Upconversion nanoparticles-CuMnO2 nanoassemblies for NIR-excited imaging of reactive oxygen species in vivo

Here, we designed a ratiometric luminescent nanoprobe based on lanthanide-doped upconversion nanoparticles-CuMnO2 nanoassemblies for rapid and sensitive detection of reactive oxygen species (ROS) levels in living cells and mouse. CuMnO2 nanosheets exhibit a wide absorption range of 300-700 nm, overlapping with the visible-light emission of upconversion nanoparticles (UCNPs), resulting in a significant upconversion luminescence quenching. In an acidic environment, H2O2 can promote the redox reaction of CuMnO2, leading to its dissociation from the surface of UCNPs and the restoration of upconversion luminescence. The variation in luminescence intensity ratio (UCL475/UCL450) were monitored to detect ROS levels. The H2O2 nanoprobe exhibited a linear response in the range of 0.314-10 μM with a detection limit of 11.3 nM. The biological tests proved the excellent biocompatibility and low toxicity of obtained UCNPs-CuMnO2 nanoassemblies. This ratiometric luminescent nanoprobe was successfully applied for the detection of exogenous and endogenous ROS in live cells as well as in vivo ROS quantitation. The dual transition metal ions endow this probe efficient catalytic decomposition capabilities, and this sensing strategy broadens the application of UCNPs-based nanomaterials in the field of biological analysis and diagnosis.

Real-time simultaneous imaging of temporal alterations in cytoplasmic and mitochondrial redox in single cells during cell division and cell death

Cytosolic and organelle redox are highly interrelated, and their alterations play critical roles in both physiological and pathological cell states. This highly regulated process is crucial in life-death decisions of cells. Among organelles, the mitochondrion is the major source of intracellular-ROS and contributes to oxidation damage-induced cell death. Increase in cytosolic-redox and mitochondrial-redox is evident in cells undergoing diverse forms of cell death, such as apoptosis, necrosis, and necroptosis. The hierarchical profiling of redox signaling at the cytosol and mitochondria in a single cell is important to understand the relative contribution of each species in the initiation and shaping of cell death. Here, we demonstrate the potential application of ratiometric redox GFP (roGFP) and intensity-based redox-sensitive RFP (rxRFP) targeted to mitochondria in revealing both rapid and slow progressing changes in redox during cell division and in cells undergoing multiple modes of cell death. To generate imaging quality signal, single-cell clones stably expressing both roGFP at the cytosol and rxRFP probes targeted to mitochondria were generated. The cells provided sufficient temporal resolution with imaging-ready signal for the real-time visualization of rapidly progressing redox alterations at the cytosol and mitochondria. The long-time imaging of the cells revealed that a moderate increase in cytosolic ROS marks the division stage. Similarly, distinct forms of cell death trigger a unique and temporally regulated redox change at the cytosol and mitochondria, suggesting the potential utility of the sensor cells to dissect the nature of cell death pathways induced by specific forms of stress.

Simultaneous imaging of redox states in dystrophic neurites and microglia at Aβ plaques indicate lysosome accumulation not microglia correlate with increased oxidative stress

The inter-relationship between microglia dynamics and oxidative stress (Ox-stress) in dystrophic neurites (DNs) at Alzheimer's Disease (AD) plaques may contribute to the pathological changes in neurons. We developed new in vivo imaging strategies to combine EGFP expression in microglia with neuronal expression of genetically encoded ratiometric redox sensors (rogRFP2 or roGFP1), and immunohistochemistry to investigate how microglia influence Ox-stress at amyloid plaques in 5xFAD AD mice. By simultaneously imaging microglia morphology and neuronal Ox-stress over time in vivo and in fixed brains we found that microglia preferentially enwrapped DNs exhibiting the greatest degree of Ox-stress. After microglia were partially depleted with the CSF1 receptor antagonist PLX3397, Ox-stress in DNs increased in a manner that was inversely correlated to the extent of coverage of the adjacent Aβ plaques by the remaining microglia. These data suggest that microglia do not create Ox-stress at Aβ plaques but instead create protective barriers around Aβ plaques possibly reducing the spread of Aβ. Intracranial injection of Aβ was sufficient to induce neuronal Ox-stress suggesting it to be the initial trigger of Ox-stress generation. Although Ox-stress is increased in DNs, neuronal survival is enhanced following microglia depletion indicating complex and multifactorial roles of microglia with both neurotoxic and neuroprotective components. Increased Ox-stress of DNs was correlated with higher LAMP1 and ubiquitin immunoreactivity supporting proposed mechanistic links between lysosomal accumulation in DNs and their intrinsic generation of Ox-stress. Our results suggest protective as well as neurotoxic roles for microglia at plaques and that the generation of Ox-stress of DNs could intrinsically be generated via lysosomal disruption rather than by microglia. In Brief: Simultaneous imaging of microglia and neuronal Ox-stress revealed a double-edged role for microglia in 5xFAD mice. Plaque associated microglia were attracted to and enwrapped Aβ plaques as well as the most highly oxidized DNs. After partial depletion of microglia, DNs were larger with greater levels of Ox-stress. Despite increased Ox-stress after microglia removal neuronal survival improved. Greater Ox-stress was correlated with increased levels of LAMP1 and ubiquitin thereby linking lysosome accumulation and Ox-stress in DNs.

Genetically Encoded Fluorescent Redox Indicators for Unveiling Redox Signaling and Oxidative Toxicity

Redox-active molecules play essential roles in cell homeostasis, signaling, and other biological processes. Dysregulation of redox signaling can lead to toxic effects and subsequently cause diseases. Therefore, real-time tracking of specific redox-signaling molecules in live cells would be critical for deciphering their functional roles in pathophysiology. Fluorescent protein (FP)-based genetically encoded redox indicators (GERIs) have emerged as valuable tools for monitoring the redox states of various redox-active molecules from subcellular compartments to live organisms. In the first section of this review, we overview the background, focusing on the sensing mechanisms of various GERIs. Next, we review a list of selected GERIs according to their analytical targets and discuss their key biophysical and biochemical properties. In the third section, we provide several examples which applied GERIs to understanding redox signaling and oxidative toxicology in pathophysiological processes. Lastly, a summary and outlook section is included.

Therapeutic Potential of Reactive Oxygen Species: State of the Art and Recent Advances

In the last decade, several studies have proven that when at low concentration reactive oxygen species (ROS) show an adaptive beneficial effect and posited the idea that they can be utilized as inexpensive and convenient inducers of tissue regeneration. On the other hand, the recent discovery that cancer cells are more sensitive to oxidative damage paved the way for their use in the selective killing of tumor cells, and sensors to monitor ROS production during cancer treatment are under extensive investigation. Nevertheless, although ROS-activated signaling pathways are well established, less is known about the mechanisms underlying the switch from an anabolic to a cytotoxic response. Furthermore, a high variability in biological response is observed between different modalities of administration, cell types, donor ages, eventual concomitant diseases, and external microenvironment. On the other hand, available preclinical studies are scarce, whereas the quest for the most suitable systems for in vivo delivery is still elusive. Furthermore, new strategies to control the temporal pattern of ROS release need to be developed, if considering their tumorigenic potential. This review initially discusses ROS mechanisms of action and their potential application in stem cell biology, tissue engineering, and cancer therapy. It then outlines the state of art of ROS-based drugs and identifies challenges faced in translating ROS research into clinical practice.

Neuron Activity Dependent Redox Compartmentation Revealed with a Second Generation Red-Shifted Ratiometric Sensor

Oxidative stress is a hallmark of several aging and trauma related neurological disorders, but the precise details of how altered neuronal activity elicits subcellular redox changes have remained difficult to resolve. Current redox sensitive dyes and fluorescent proteins can quantify spatially distinct changes in reactive oxygen species levels, but multicolor probes are needed to accurately analyze compartment-specific redox dynamics in single cells that can be masked by population averaging. We previously engineered genetically encoded red-shifted redox-sensitive fluorescent protein sensors using a Förster resonance energy transfer relay strategy. Here, we developed a second-generation excitation ratiometric sensor called rogRFP2 with improved red emission for quantitative live-cell imaging. Using this sensor to measure activity-dependent redox changes in individual cultured neurons, we observed an anticorrelation in which mitochondrial oxidation was accompanied by a concurrent reduction in the cytosol. This behavior was dependent on the activity of Complex I of the mitochondrial electron transport chain and could be modulated by the presence of cocultured astrocytes. We also demonstrated that the red fluorescent rogRFP2 facilitates ratiometric one- and two-photon redox imaging in rat brain slices and Drosophila retinas. Overall, the proof-of-concept studies reported here demonstrate that this new rogRFP2 redox sensor can be a powerful tool for understanding redox biology both in vitro and in vivo across model organisms.

Non-Peptidic Cell-Penetrating Motifs for Mitochondrion-Specific Cargo Delivery

Mitochondrial dysfunction is linked to a variety of human illnesses, but selective delivery of therapeutics into the mitochondrion is challenging. Now a family of amphipathic cell-penetrating motifs (CPMs) is presented, consisting of four guanidinium groups and one or two aromatic hydrophobic groups (naphthalene) assembled through a central scaffold (a benzene ring). The CPMs and CPM-cargo conjugates efficiently enter the interior of cultured mammalian cells and are specifically localized into the mitochondrial matrix, as revealed by high-resolution confocal microscopy. With a membrane-impermeable peptide as cargo, the CPMs exhibited ≥170-fold higher delivery efficiency than previous mitochondrial delivery vehicles. Conjugation of a small-molecule inhibitor of heat shock protein 90 to a CPM resulted in accumulation of the inhibitor inside the mitochondrial matrix with greatly enhanced anticancer activity. The CPMs showed minimal effect on the viability or the mitochondrial membrane potential of mammalian cells.

Measuring Mitochondrial ROS in Mammalian Cells with a Genetically Encoded Protein Sensor

Reactive oxygen species (ROS) are not only known for their toxic effects on cells, but they also play an important role as second messengers. As such, they control a variety of cellular functions such as proliferation, metabolism, differentiation and apoptosis. Thus, ROS are involved in the regulation of multiple physiological and pathophysiological processes. It is now apparent that there are transient and local changes in ROS in the cell; in so-called 'microdomains' or in specific cellular compartments, which affect signaling events. These ROS hotspots need to be studied in more depth to understand their function and regulation. Therefore, it is necessary to identify and quantify redox signals in single cells with high spatial and temporal resolution. Genetically encoded fluorescence-based protein sensors provide such necessary tools to examine redox-signaling processes. A big advantage of these sensors is the possibility to target them specifically. Mitochondria are essential for energy metabolism and are one of the major sources of ROS in mammalian cells. Therefore, the evaluation of redox potential and ROS production in these organelles is of great interest. Herein, we provide a protocol for the real-time visualization of mitochondrial hydrogen peroxide (H₂O₂) using the H₂O₂-specific ratiometric sensor mitoHyPer in adherent mammalian cells.