In vivo evaluation of different alterations of redox status by studying pharmacokinetics of nitroxides using magnetic resonance techniques

Ginsenoside Rg1 alleviates chronic inflammation-induced neuronal ferroptosis and cognitive impairments via regulation of AIM2 - Nrf2 signaling pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Ginseng is a valuable herb in traditional Chinese medicine. Modern research has shown that it has various benefits, including tonifying vital energy, nourishing and strengthening the body, calming the mind, improving cognitive function, regulating fluids, and returning blood pressure, etc. Rg1 is a primary active component of ginseng. It protects hippocampal neurons, improves synaptic plasticity, enhances cognitive function, and boosts immunity. Furthermore, it exhibits anti-aging and anti-fatigue properties and holds great potential for preventing and managing neurodegenerative diseases (NDDs).

AIM OF THE STUDY

The objective of this study was to examine the role of Rg1 in treating chronic inflammatory NDDs and its molecular mechanisms.

MATERIALS AND METHODS

In vivo, we investigated the protective effects of Rg1 against chronic neuroinflammation and cognitive deficits in mice induced by 200 μg/kg lipopolysaccharide (LPS) for 21 days using behavioral tests, pathological sections, Western blot, qPCR and immunostaining. In vitro experiments involved the stimulation of HT22 cells with 10 μg/ml of LPS, verification of the therapeutic effect of Rg1, and elucidation of its potential mechanism of action using H2DCFDA staining, BODIPY™ 581/591 C11, JC-1 staining, Western blot, and immunostaining.

RESULTS

Firstly, it was found that Rg1 significantly improved chronic LPS-induced behavioral and cognitive dysfunction in mice. Further studies showed that Rg1 significantly attenuated LPS-induced neuronal damage by reducing levels of IL-6, IL-1β and ROS, and inhibiting AIM2 inflammasome. Furthermore, chronic LPS exposure induced the onset of neuronal ferroptosis by increasing the lipid peroxidation product MDA and regulating the ferroptosis-associated proteins Gpx4, xCT, FSP1, DMT1 and TfR, which were reversed by Rg1 treatment. Additionally, Rg1 was found to activate Nrf2 and its downstream antioxidant enzymes, such as HO1 and NQO1, both in vivo and in vitro. In vitro studies also showed that the Nrf2 inhibitor ML385 could inhibit the anti-inflammatory, antioxidant, and anti-ferroptosis effects of Rg1.

CONCLUSIONS

This study demonstrated that Rg1 administration ameliorated chronic LPS-induced cognitive deficits and neuronal ferroptosis in mice by inhibiting neuroinflammation and oxidative stress. The underlying mechanisms may be related to the inhibition of AIM2 inflammasome and activation of Nrf2 signaling. These findings provide valuable insights into the treatment of chronic neuroinflammation and associated NDDs.

Rebalancing NOX2/Nrf2 to limit inflammation and oxidative stress across gut-brain axis in migraine

Migraine is one of the most common neurological illnesses, and it is characterized by complicated neurobiology. It was confirmed the influence of inflammation and oxidative stress in migraines and also in distal organs such as the intestine. Indeed, the constant bidirectional communication between the Central Nervous System (CNS) and the gastrointestinal (GI) tract, known as the gut-brain axis, has become an attractive target involved in different human disorders. Herein, we explored the role of NADPH oxidase 2 (NOX2) in nitroglycerin (NTG)-induced migraine in mice models to discover the mechanism by which, during migraine attack, oxidative stress is sustained within trigeminal neurons and GI. Considering the inverse relationship between NOX2 and Nrf2, Nrf2 upregulation seems to be a promising approach to decrease NOX2 expression and consequently limit oxidative stress and inflammation spread in neurological and non-neurological diseases. With this aim, we exploited tempol's Nrf2-inducer ability to better understand the involvement of Nrf2/NOX2 axis in migraine and associated GI comorbidities. Behavioral tests confirmed that tempol, in a dose-dependent manner, moderated clinical signs of migraine and abdominal pain. Moreover, we demonstrated that the decrease in migraine-related symptomatology was strongly linked to the modulation of Nrf2/NOX2 signaling pathway in the brain and colon. In the brain, the rebalancing of Nrf2/NOX2 prevented neuronal loss, decreased glia reactivity while inhibiting NF-κB and NLRP3 inflammasome activation. In the colon, Nrf2 upregulation and consequent NOX2 decrease reduced the histological damage, mast cells infiltration as well as tumor necrosis factor (TNF)-α and interleukin (IL)-1β release. Furthermore, the attenuation of inflammation and oxidative stress led to the restoration of the intestinal barrier through TJs replacement. Taken as a whole, data suggested that the regulation of Nrf2/NOX2 balance is a successful way to reduce neurological and related intestinal impairments during migraine and could be of relevance for migraine-like attacks in humans.

ClO2-Mediated Oxidation of the TEMPO Radical: Fundamental Considerations of the Catalytic System for the Oxidation of Cellulose Fibers

The reaction mechanism of ClO2-mediated TEMPO oxidation was investigated by EPR spectroscopy and UV-Vis spectroscopy in the context of an alternative TEMPO sequence for cellulose fiber oxidation. Without the presence of a cellulosic substrate, a reversibility between TEMPO and its oxidation product, TEMPO+, was displayed, with an effect of the pH and reagent molar ratios. The involvement of HOCl and Cl-, formed as byproducts in the oxidation mechanism, was also evidenced. Trapping HOCl partly inhibits the reaction, whereas adding methylglucoside, a cellulose model compound, inhibits the reversibility of the reaction to TEMPO.

Structural Features Governing the Metabolic Stability of Tetraethyl-Substituted Nitroxides in Rat Liver Microsomes

Nitroxides are potent tools for studying biological systems by electron paramagnetic resonance (EPR). Whatever the application, a certain stability is necessary for successful detection. Since conventional tetramethyl-substituted cyclic nitroxides have insufficient in vivo stability, efforts have recently been made to synthesize more stable, tetraethyl-substituted nitroxides. In our previous study on piperidine nitroxides, the introduction of steric hindrance around the nitroxide moiety successfully increased the resistance to reduction into hydroxylamine. However, it also rendered the carbon backbone susceptible to modifications by xenobiotic metabolism due to increased lipophilicity. Here, we focus on a new series of three nitroxide candidates with tetraethyl substitution, namely with pyrrolidine, pyrroline, and isoindoline cores, to identify which structural features afford increased stability for future probe design and application in in vivo EPR imaging. In the presence of rat liver microsomes, pyrrolidine and pyrroline tetraethyl nitroxides exhibited a higher stability than isoindoline nitroxide, which was studied in detail by HPLC-HRMS. Multiple metabolites suggest that the aerobic transformation of tetraethyl isoindoline nitroxide is initiated by hydrogen abstraction by P450-Fe^V = O from one of the ethyl groups, followed by rearrangement and further modifications by cytochrome P450, as supported by DFT calculations. Under anaerobic conditions, only reduction by rat liver microsomes was observed with involvement of P450-Fe^II.

Biocompatible Monophosphonated Trityl Spin Probe, HOPE71, for In Vivo Measurement of pO2, pH, and [Pi] by Electron Paramagnetic Resonance Spectroscopy

Hypoxia, acidosis, and elevated inorganic phosphate concentration are characteristics of the tumor microenvironment in solid tumors. There are a number of methods for measuring each parameter individually in vivo, but the only method to date for noninvasive measurement of all three variables simultaneously in vivo is electron paramagnetic spectroscopy paired with a monophosphonated trityl radical, pTAM/HOPE. While HOPE has been successfully used for in vivo studies upon intratissue injection, it cannot be delivered intravenously due to systemic toxicity and albumin binding, which causes significant signal loss. Therefore, we present HOPE71, a monophosphonated trityl radical derived from the very biocompatible trityl probe, Ox071. Here, we describe a straightforward synthesis of HOPE71 starting with Ox071 and report its EPR sensitivities to pO2, pH, and [Pi] with X-band and L-band EPR spectroscopy. We also confirm that HOPE71 lacks albumin binding, shows low cytotoxicity, and has systemic tolerance. Finally, we demonstrate its ability to profile the tumor microenvironment in vivo in a mouse model of breast cancer.

Oxidative stress in multiple sclerosis-Emerging imaging techniques

While conventional magnetic resonance imaging (MRI) is central to the evaluation of patients with multiple sclerosis, its role in detecting the pathophysiology underlying neurodegeneration is more limited. One of the common outcome measures for progressive multiple sclerosis trials, atrophy on brain MRI, is non-specific and reflects end-stage changes after considerable neurodegeneration has occurred. Identifying biomarkers that identify processes underlying neurodegeneration before it is irreversible and that reflect relevant neurodegenerative pathophysiology is an area of significant need. Accumulating evidence suggests that oxidative stress plays a major role in the pathogenesis of multiple neurodegenerative diseases, including multiple sclerosis. Imaging markers related to inflammation, myelination, and neuronal integrity have been areas of advancement in recent years but oxidative stress has remained an area of unrealized potential. In this article we will begin by reviewing the role of oxidative stress in the pathogenesis of multiple sclerosis. Chronic inflammation appears to be directly related to the increased production of reactive oxygen species and the effects of subsequent oxidative stress appear to be amplified by aging and accumulating disease. We will then discuss techniques in development used in the assessment of MS as well as other models of neurodegenerative disease in which oxidative stress is implicated. Multiple blood and CSF markers of oxidative stress have been evaluated in subjects with MS, but non-invasive imaging offers major upside in that it provides real-time assessment within the brain.

EPR and Related Magnetic Resonance Imaging Techniques in Cancer Research

Imaging tumor microenvironments such as hypoxia, oxygenation, redox status, and/or glycolytic metabolism in tissues/cells is useful for diagnostic and prognostic purposes. New imaging modalities are under development for imaging various aspects of tumor microenvironments. Electron Paramagnetic Resonance Imaging (EPRI) though similar to NMR/MRI is unique in its ability to provide quantitative images of pO₂ in vivo. The short electron spin relaxation times have been posing formidable challenge to the technology development for clinical application. With the availability of the narrow line width trityl compounds, pulsed EPR imaging techniques were developed for pO₂ imaging. EPRI visualizes the exogenously administered spin probes/contrast agents and hence lacks the complementary morphological information. Dynamic nuclear polarization (DNP), a phenomenon that transfers the high electron spin polarization to the surrounding nuclear spins (¹H and ¹³C) opened new capabilities in molecular imaging. DNP of ¹³C nuclei is utilized in metabolic imaging of 13C-labeled compounds by imaging specific enzyme kinetics. In this article, imaging strategies mapping physiologic and metabolic aspects in vivo are reviewed within the framework of their application in cancer research, highlighting the potential and challenges of each of them.

Establishment of a method to expose and measure pollution in excised porcine skin with electron paramagnetic resonance spectroscopy

Health problems associated with the amount of air pollutants are increasing worldwide. Pollution damages not only the lungs; it also has an impact on skin health and is co-responsible for the development of skin diseases. Anti-pollution products are on the rise in the cosmetic market but so far, there is no established method to directly assess the impact of pollution on the skin and to test the efficacy of anti-pollution products. To address this problem, two different chambers were developed for the reproducible exposure to realistic air pollutant concentrations. One chamber for the exclusive use of excised skin and hair samples, the second chamber for ex vivo and in vivo measurements. Measurements of nicotine next to the investigated skin area allow conclusions to be drawn on the particle concentration to which the skin is exposed. Electron paramagnetic resonance spectroscopy, which enables the detection of free radicals in different systems, was applied to assess the hazard potential of pollution in the skin. A direct proof of the formation of free radicals in the skin by the model pollutant cigarette smoke could be demonstrated. An additional application of UV irradiation even increased the formation of free radicals in the skin seven-fold (sum parameter). Depending on the question of interest, the use of different spin probes allows various assessments of the radical formation in skin: the amount of radicals but also the antioxidant status of the microenvironment can be estimated. Using two exposure chambers, the direct formation of oxidative stress by cigarette smoke on ex vivo skin, with and without additional UV exposure, could be reproducibly examined. This measurement method is promising for the assessment of anti-pollution products and could allow a direct causal connection between pollutant, effect on the skin and the protective function of skin care products.

Targeting Glioblastoma via Selective Alteration of Mitochondrial Redox State

Simple Summary

Glioblastoma is characterized by a pronounced redox imbalance due to elevated glycolytic and mitochondrial oxidative metabolism. New therapeutic strategies have been developed to modulate glioblastoma redox signaling to effectively suppress growth and prolong survival. However, drug selectivity and therapeutic relapse prove to be the major challenges. We describe a pharmacological strategy for the selective targeting and treatment of glioblastoma using the redox active combination drug menadione/ascorbate, which is characterized by tolerance to normal cells and tissues. Menadione/ascorbate treatment of glioblastoma mice suppressed tumor growth and significantly increased survival without adverse side effects. This is accompanied by increased oxidative stress, decreased reducing capacity and decreased cellular density in the tumor alone, as well as increased brain perfusion and decreased regulation of several oncoproteins and oncometabolites, which implies modulation of the immune response and reduced drug resistance. We believe that this therapeutic strategy is feasible and promising and deserves the attention of clinicians.

Abstract

Glioblastoma is one of the most aggressive brain tumors, characterized by a pronounced redox imbalance, expressed in a high oxidative capacity of cancer cells due to their elevated glycolytic and mitochondrial oxidative metabolism. The assessment and modulation of the redox state of glioblastoma are crucial factors that can provide highly specific targeting and treatment. Our study describes a pharmacological strategy for targeting glioblastoma using a redox-active combination drug. The experiments were conducted in vivo on glioblastoma mice (intracranial model) and in vitro on cell lines (cancer and normal) treated with the redox cycling pair menadione/ascorbate (M/A). The following parameters were analyzed in vivo using MRI or ex vivo on tissue and blood specimens: tumor growth, survival, cerebral perfusion, cellular density, tissue redox state, expression of tumor-associated NADH oxidase (tNOX) and transforming growth factor-beta 1 (TGF-β1). Dose-dependent effects of M/A on cell viability, mitochondrial functionality, and redox homeostasis were evaluated in vitro. M/A treatment suppressed tumor growth and significantly increased survival without adverse side effects. This was accompanied by increased oxidative stress, decreased reducing capacity, and decreased cellular density in the tumor only, as well as increased cerebral perfusion and down-regulation of tNOX and TGF-β1. M/A induced selective cytotoxicity and overproduction of mitochondrial superoxide in isolated glioblastoma cells, but not in normal microglial cells. This was accompanied by a significant decrease in the over-reduced state of cancer cells and impairment of their “pro-oncogenic” functionality, assessed by dose-dependent decreases in: NADH, NAD⁺, succinate, glutathione, cellular reducing capacity, mitochondrial potential, steady-state ATP, and tNOX expression. The safety of M/A on normal cells was compromised by treatment with cerivastatin, a non-specific prenyltransferase inhibitor. In conclusion, M/A differentiates glioblastoma cells and tissues from normal cells and tissues by redox targeting, causing severe oxidative stress only in the tumor. The mechanism is complex and most likely involves prenylation of menadione in normal cells, but not in cancer cells, modulation of the immune response, a decrease in drug resistance, and a potential role in sensitizing glioblastoma to conventional chemotherapy.

Nitroxyl Radical as a Theranostic Contrast Agent in Magnetic Resonance Redox Imaging

Significance:In vivo assessment of paramagnetic and diamagnetic conversions of nitroxyl radicals based on cyclic redox mechanism can be an index of tissue redox status. The redox mechanism of nitroxyl radicals, which enables their use as a normal tissue-selective radioprotector, is seen as being attractive on planning radiation therapy. Recent Advances:In vivo redox imaging using nitroxyl radicals as redox-sensitive contrast agents has been developed to assess tissue redox status. Chemical and biological behaviors depending on chemical structures of nitroxyl radical compounds have been understood in detail. Polymer types of nitroxyl radical contrast agents and/or nitroxyl radical-labeled drugs were designed for approaching theranostics. Critical Issues: Nitroxyl radicals as magnetic resonance imaging (MRI) contrast agents have several advantages compared with those used in electron paramagnetic resonance (EPR) imaging, while support by EPR spectroscopy is important to understand information from MRI. Redox-sensitive paramagnetic contrast agents having a medicinal benefit, that is, nitroxyl-labeled drug, have been developed and proposed. Future Directions: A development of suitable nitroxyl contrast agent for translational theranostic applications with high reaction specificity and low normal tissue toxicity is under progress. Nitroxyl radicals as redox-sensitive magnetic resonance contrast agents can be a useful tool to detect an abnormal tissue redox status such as disordered oxidative stress. Antioxid. Redox Signal. 36, 95-121.

Wavelength, dose, skin type and skin model related radical formation in skin

The exposure to sun radiation is indispensable to our health; however, a long-term and high exposure could lead to cell damage, erythema, premature skin aging, and promotion of skin tumors. An underlying pathomechanism is the formation of free radicals which may induce oxidative stress at elevated concentrations. Different skin models, such as porcine-, murine-, human- ex vivo skin, reconstructed human skin (RHS) and human skin in vivo, were investigated during and after irradiation using X- and L-band EPR spectroscopy within different spectral regions (UVC to NIR). The amount of radical formation was quantified with the spin probe PCA and the radical types were measured ex vivo with the spin trap DMPO. The radiation dose influences the types of radicals formed in the skin. While reactive oxygen species (ROS) are always pronounced at low doses, there is an increase in lipid oxygen species (LOS) at high doses. Furthermore, the radical types arise independent from the irradiation wavelength, whereas the general amount of radical formation differs with the irradiation wavelength. Heat pre-stressed porcine skin already starts with higher LOS values. Thus, the radical type ratio might be an indicator of stress and the reversal of ROS/LOS constitutes the point where positive stress turns into negative stress.Compared to light skin types, darker types produce less radicals in the ultraviolet, similar amounts in the visible and higher ones in the infrared spectral region, rendering skin type-specific sun protection a necessity.

Three-dimensional electron paramagnetic resonance imaging of mice using ascorbic acid sensitive nitroxide imaging probes

Nitroxide compounds have been used as redox-sensitive imaging probes by electron paramagnetic resonance (EPR) for assessing oxidative stress in vivo. Fast redox reactions of nitroxide radicals are favorable for assessment of higher redox sensitivity; however, a variety of nitroxides have not been trialed for use as imaging probes due to their very rapid in vivo reduction, which cannot be captured at the slow operation speed of existing EPR imagers. To overcome this limitation, we improved our EPR system to provide a stable and highly sensitive imaging operation. We challenged the improved EPR imager to perform three-dimensional (3D) EPR imaging of mouse brain using two useful nitroxide imaging probes, 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (Tempol) and 2,6-dispiro-4',4"-dipyrane-piperidine-4-one-N-oxyl (DiPy). The second-order rate constant of DiPy with ascorbic acid is 10 times larger than that of Tempol. The improved EPR imager obtained clear 3D EPR images of mouse brain and demonstrated that Tempol could exist with an unpaired electron. The imager also successfully obtained 3D EPR images of mouse head after administration of DiPy. As 126 projections can be acquired in a period of 6 s, 3D EPR imaging can visualize the sequential process of DiPy entering the brain, being distributed within the brain, and being reduced within the brain. These improvements to the EPR imager will enable useful nitroxide imaging probes that were previously unsuitable as imaging probes due to their rapid reduction to be considered for use for sensitive redox assessment in an in vivo system.

The effects of nitroxide structure upon 1H Overhauser dynamic nuclear polarization efficacy at ultralow-field

The efficacy in ¹H Overhauser dynamic nuclear polarization in liquids at ultralow magnetic field (ULF, B₀ = 92 ± 0.8 µT) and polarization field (B_p = 1-10 mT) was studied for a broad variety of 26 different spin probes. Among others, piperidine, pyrrolidine, and pyrroline radicals specifically synthesized for this study, along with some well-established commercially available nitroxides, were investigated. Isotope-substituted variants, some sterically shielded reduction-resistant nitroxides, and some biradicals were included in the measurements. The maximal achievable enhancement, E_max, and the radio frequency power, P_1/2, needed for reaching E_max/2 were measured. Physico-chemical features such as molecular weight, spectral linewidth, heterocyclic structure, different types of substituents, deuteration, and ¹⁵N-labeling as well as the difference between monoradicals and biradicals were investigated. For the unmodified nitroxide radicals, the E_max values correlate with the molecular weight. The P_1/2 values correlate with the spectral linewidth and are additionally influenced by the type of substituents neighboring the nitroxide group. The nitroxide biradicals with high intramolecular spin-spin coupling show low performance. Nitroxides enriched with ¹⁵N and/or ²H afford significantly higher |E_max| and require lower power to do so, compared to their unmodified counterparts containing at natural abundance predominantly ¹⁴N and ¹H. The results allow for a correlation of chemical features with physical hyperpolarization-related properties and indicate that small nitroxides with narrow spectral lines have clear advantages for the use in Overhauser dynamic nuclear polarization experiments. Perdeuteration and ¹⁵N-labeling can be used to additionally boost the spin probe performance.

A Review of Low-Frequency EPR Technology for the Measurement of Brain pO2 and Oxidative Stress

EPR can uniquely measure paramagnetic species. Although commercial EPR was introduced in 1950s, the early studies were mostly restricted to chemicals in solution or cellular experiments using X-band EPR equipment. Due to its limited penetration (<1 mm), experiments with living animals were almost impossible. To overcome these difficulties, Swartz group, along with several other leaders in field, pioneered the technology of low frequency EPR (e.g., L-band, 1-2 GHz). The development of low frequency EPR and the associated probes have dramatically expanded the application of EPR technology into the biomedical research field, providing answers to important scientific questions by measuring specific parameters that are impossible or very difficult to obtain by other approaches. In this review, which is aimed at highlighting the seminal contribution from Swartz group over the last several decades, we will focus on the development of EPR technology that was designed to deal with the potential challenges arising from conducting EPR spectroscopy in living animals. The second half of the review will be concentrated on the application of low frequency EPR in measuring cerebral tissue pO₂ changes and oxidative stress in various physiological and pathophysiological conditions in the brain of animal disease models.

A Simple Method of Synthesis of 3-Carboxy-2,2,5,5-Tetraethylpyrrolidine-1-oxyl and Preparation of Reduction-Resistant Spin Labels and Probes of Pyrrolidine Series

Stable free radicals are widely used as molecular probes and labels in various biophysical and biomedical research applications of magnetic resonance spectroscopy and imaging. Among these radicals, sterically shielded nitroxides of pyrrolidine series demonstrate the highest stability in biological systems. Here, we suggest new convenient procedure for preparation of 3-carboxy-2,2,5,5-tetraethylpyrrolidine-1-oxyl, a reduction-resistant analog of widely used carboxy-Proxyl, from cheap commercially available reagents with the yield exceeding the most optimistic literature data. Several new spin labels and probes of 2,2,5,5-tetraethylpyrrolidine-1-oxyl series were prepared and reduction of these radicals in ascorbate solutions, mice blood and tissue homogenates was studied.

Comparative EPR Study on the Scavenging Effect of Methotrexate with the Isomers of Its Photoswitchable Derivative

The scavenging effect of the antimetabolite dihydrofolate reductase inhibitor methotrexate (MTX) and the isomers of its photoswitchable derivate, cis- and trans-phototrexate (PHX), have been compared by ESR spectroscopy, with the application of a cyclic hydroxylamine spin probe. The results showed the most pronounced scavenging effect in the presence of trans-phototrexate (trans-PHX). At a low concentration (100 µM) cis-PHX also showed a greater scavenging effect than the parent molecule MTX. Direct antioxidant properties of the investigated molecules were measured by ABTS scavenging assay, which showed no significant difference between trans-PHX and cis-PHX, but both of the isomers of PHX showed a higher antioxidant capacity than MTX. These findings imply that trans-PHX may have more pronounced anti-inflammatory and tissue-protective effects than MTX, despite the lack of its cytotoxic, antineoplastic effect.

Exercise-Stimulated ROS Sensitive Signaling Pathways in Skeletal Muscle

Physical exercise represents a major challenge to whole-body homeostasis, provoking acute and adaptative responses at the cellular and systemic levels. Different sources of reactive oxygen species (ROS) have been described in skeletal muscle (e.g., NADPH oxidases, xanthine oxidase, and mitochondria) and are closely related to the physiological changes induced by physical exercise through the modulation of several signaling pathways. Many signaling pathways that are regulated by exercise-induced ROS generation, such as adenosine monophosphate-activated protein kinase (AMPK), mitogen activated protein kinase (MAPK), nuclear respiratory factor2 (NRF2), and PGC-1α are involved in skeletal muscle responses to physical exercise, such as increased glucose uptake, mitochondriogenesis, and hypertrophy, among others. Most of these adaptations are blunted by antioxidants, revealing the crucial role played by ROS during and after physical exercise. When ROS generation is either insufficient or exacerbated, ROS-mediated signaling is disrupted, as well as physical exercise adaptations. Thus, an understanding the limit between "ROS that can promote beneficial effects" and "ROS that can promote harmful effects" is a challenging question in exercise biology. The identification of new mediators that cause reductive stress and thereby disrupt exercise-stimulated ROS signaling is a trending on this topic and are covered in this current review.

Multimodal Functional Imaging for Cancer/Tumor Microenvironments Based on MRI, EPRI, and PET

Radiation therapy is one of the main modalities to treat cancer/tumor. The response to radiation therapy, however, can be influenced by physiological and/or pathological conditions in the target tissues, especially by the low partial oxygen pressure and altered redox status in cancer/tumor tissues. Visualizing such cancer/tumor patho-physiological microenvironment would be a useful not only for planning radiotherapy but also to detect cancer/tumor in an earlier stage. Tumor hypoxia could be sensed by positron emission tomography (PET), electron paramagnetic resonance (EPR) oxygen mapping, and in vivo dynamic nuclear polarization (DNP) MRI. Tissue oxygenation could be visualized on a real-time basis by blood oxygen level dependent (BOLD) and/or tissue oxygen level dependent (TOLD) MRI signal. EPR imaging (EPRI) and/or T1-weighted MRI techniques can visualize tissue redox status non-invasively based on paramagnetic and diamagnetic conversions of nitroxyl radical contrast agent. 13C-DNP MRI can visualize glycometabolism of tumor/cancer tissues. Accurate co-registration of those multimodal images could make mechanisms of drug and/or relation of resulted biological effects clear. A multimodal instrument, such as PET-MRI, may have another possibility to link multiple functions. Functional imaging techniques individually developed to date have been converged on the concept of theranostics.

Detection of Vascular Reactive Oxygen Species in Experimental Atherosclerosis by High-Resolution Near-Infrared Fluorescence Imaging Using VCAM-1-Targeted Liposomes Entrapping a Fluorogenic Redox-Sensitive Probe

Excessive production of reactive oxygen species (ROS) and the ensuing oxidative stress are instrumental in all phases of atherosclerosis. Despite the major achievements in understanding the regulatory pathways and molecular sources of ROS in the vasculature, the specific detection and quantification of ROS in experimental models of disease remain a challenge. We aimed to develop a reliable and straightforward imaging procedure to interrogate the ROS overproduction in the vasculature and in various organs/tissues in atherosclerosis. To this purpose, the cell-impermeant ROS Brite™ 700 (RB700) probe that produces bright near-infrared fluorescence upon ROS oxidation was encapsulated into VCAM-1-targeted, sterically stabilized liposomes (VLp). Cultured human endothelial cells (EC) and macrophages (Mac) were used for in vitro experiments. C57BL6/J and ApoE-/- mice were randomized to receive normal or high-fat, cholesterol-rich diet for 10 or 32 weeks. The mice received a retroorbital injection with fluorescent tagged VLp incorporating RB700 (VLp-RB700). After two hours, the specific signals of the oxidized RB700 and 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-(7-nitro-2-1,3-benzoxadiazol-4-yl) (NBD-DSPE), inserted into liposome bilayers, were measured ex vivo in the mouse aorta and various organs by high-resolution fluorescent imaging. VLp-RB700 was efficiently taken up by cultured human EC and Mac, as confirmed by fluorescence microscopy and spectrofluorimetry. After systemic administration in atherosclerotic ApoE-/- mice, VLp-RB700 were efficiently concentrated at the sites of aortic lesions, as indicated by the augmented NBD fluorescence. Significant increases in oxidized RB700 signal were detected in the aorta and in the liver and kidney of atherosclerotic ApoE-/- mice. RB700 encapsulation into sterically stabilized VCAM-1-sensitive Lp could be a novel strategy for the qualitative and quantitative detection of ROS in the vasculature and various organs and tissues in animal models of disease. The accurate and precise detection of ROS in experimental models of disease could ease the translation of the results to human pathologies.

In Vivo/Ex Vivo EPR Investigation of the Brain Redox Status and Blood-Brain Barrier Integrity in the 5xFAD Mouse Model of Alzheimer's Disease

BACKGROUND

Alzheimer's disease (AD) is the most common neurodegenerative disorder characterized by cognitive decline and total brain atrophy. Despite the substantial scientific effort, the pathological mechanisms underlying neurodegeneration in AD are currently unknown. In most studies, amyloid β peptide has been considered the key pathological change in AD. However, numerous Aβ-targeting treatments have failed in clinical trials. This implies the need to shift the research focus from Aβ to other pathological features of the disease.

OBJECTIVE

The aim of this study was to examine the interplay between mitochondrial dysfunction, oxidative stress and blood-brain barrier (BBB) disruption in AD pathology, using a novel approach that involves the application of electron paramagnetic resonance (EPR) spectroscopy.

METHODS

In vivo and ex vivo EPR spectroscopy using two spin probes (aminoxyl radicals) exhibiting different cell-membrane and BBB permeability were employed to assess BBB integrity and brain tissue redox status in the 5xFAD mouse model of AD. In vivo spin probe reduction decay was analyzed using a two-compartment pharmacokinetic model. Furthermore, 15 K EPR spectroscopy was employed to investigate the brain metal content.

RESULTS

This study has revealed an altered brain redox state, BBB breakdown, as well as ROS-mediated damage to mitochondrial iron-sulfur clusters, and up-regulation of MnSOD in the 5xFAD model.

CONCLUSION

The EPR spin probes were shown to be excellent in vivo reporters of the 5xFAD neuronal tissue redox state, as well as the BBB integrity, indicating the importance of in vivo EPR spectroscopy application in preclinical studies of neurodegenerative diseases.

In vivo analysis of redox status in organs - from bench to bedside

Reactive oxygen species (ROS) such as superoxide, hydroxyl radical, and hydrogen peroxide play an important role in the maintenance of life. However, production of excessive ROS and/or deficiency of the antioxidant system lead to oxidative stress and cause a variety of diseases. In the present study, we used electron spin resonance (ESR) to detect ROS in vivo to clarify its roles in redox dynamics and organ damage. However, the limited permeability of microwaves and low anatomic resolution of ESR equipment made it difficult to apply clinically. Nitroxide is widely used as a sensitive redox sensor for in vivo ESR analysis. The unpaired electrons of nitroxide are known to cause the T1 relaxation time-shortening effect of water protons, creating magnetic resonance imaging (MRI) effects. The remarkable development of MRI has facilitated the spatiotemporal analysis of nitroxide, which was previously impossible. In a rat model, we have been able to image and analyze the process of nitroxide reduction using MRI. MRI using nitroxide as a contrast medium is considered to be clinically applicable for evaluation of organ redox, imaging of ROS (which cause organ damage), and evaluation of therapeutic effects. In this review, we describe current advances in the analysis of in vivo redox capacity in animals using ESR and MRI equipment. We consider that redox evaluation using MRI can contribute to advances in clinical medicine.

Nitroxide-enhanced MRI of cardiovascular oxidative stress

BACKGROUND

In vivo imaging of oxidative stress can facilitate the understanding and treatment of cardiovascular diseases. We evaluated nitroxide-enhanced MRI with 3-carbamoyl-proxyl (3CP) for the detection of myocardial oxidative stress.

METHODS

Three mouse models of cardiac oxidative stress were imaged, namely angiotensin II (Ang II) infusion, myocardial infarction (MI), and high-fat high-sucrose (HFHS) diet-induced obesity (DIO). For the Ang II model, mice underwent MRI at baseline and after 7 days of Ang II (n = 8) or saline infusion (n = 8). For the MI model, mice underwent MRI at baseline (n = 10) and at 1 (n = 8), 4 (n = 9), and 21 (n = 8) days after MI. For the HFHS-DIO model, mice underwent MRI at baseline (n = 20) and 18 weeks (n = 13) after diet initiation. The 3CP reduction rate, K_red , computed using a tracer kinetic model, was used as a metric of oxidative stress. Dihydroethidium (DHE) staining of tissue sections was performed on Day 1 after MI.

RESULTS

For the Ang II model, K_red was higher after 7 days of Ang II versus other groups (p < 0.05). For the MI model, K_red , in the infarct region was significantly elevated on Days 1 and 4 after MI (p < 0.05), whereas K_red in the noninfarcted region did not change after MI. DHE confirmed elevated oxidative stress in the infarct zone on Day 1 after MI. After 18 weeks of HFHS diet, K_red was higher in mice after diet versus baseline (p < 0.05).

CONCLUSIONS

Nitroxide-enhanced MRI noninvasively quantifies tissue oxidative stress as one component of a multiparametric preclinical MRI examination. These methods may facilitate investigations of oxidative stress in cardiovascular disease and related therapies.

Enhancing the Efficacy of Metal-Free MRI Contrast Agents via Conjugating Nitroxides onto PEGylated Cross-Linked Poly(Carboxylate Ester)

Herein, two water-soluble PROXYL-based magnetic resonance imaging (MRI) macromolecular organic contrast agents (mORCAs) are designed and synthesized: linear and cross-linked PCE-mPEG-Ppa-PROXYL. They are prepared by conjugating linear and cross-linked poly(carboxylate ester) (PCE) with poly(ethylene glycol) (mPEG2000)-modified nitroxides (PROXYL), respectively. Both mORCAs form self-assembled aggregates in an aqueous phase and PROXYL is protected inside a hydrophobic core to achieve great resistance to reduction in the physiological environment, and they have low toxicity. Since cross-linked PCE-mPEG-Ppa-PROXYL possess a branched architecture, its self-assembled aggregate is more stable and compact with a greater particle size. Cross-linked PCE-mPEG-Ppa-PROXYL outperform the linear one in the following aspects: 1) its longitudinal relaxivity (r 1 = 0.79 mm -1 s-1) is higher than that of the linear one (r 1 = 0.64 mm -1 s-1) and both excel the best mORCA reported so far (r 1 = 0.42 mm -1 s-1); 2) its blood retention time (≈48 h) is longer than that of its linear counterpart (≈10 h); 3) cross-linked PCE-mPEG-Ppa-PROXYL provided better MR imaging contrast resolution in normal organs (liver and kidney) and tumor of mice than the linear one. Overall, cross-linked PCE-mPEG-Ppa-PROXYL may have great potential to be a novel metal-free macromolecular contrast agent for MR imaging.

Analysis of the Status of the Cutaneous Endogenous and Exogenous Antioxidative System of Smokers and the Short-Term Effect of Defined Smoking Thereon

The daily consumption of tobacco products leads to a boost in free radical production in tissues, promoting the risk for malignancies, metabolic alterations and chronic-inflammatory diseases. This study aimed to broaden the knowledge of the status of the antioxidative (AO) system in the skin, compared to the blood, of healthy appearing smokers. Both, the basic status compared to non-smokers and the short-term impact of controlled cigarette consumption in smokers were analyzed. Our study showed that the basic level of the AO system of smokers significantly differed from that of non-smokers. As determined by resonant Raman spectroscopy (RRS), the levels of exogenous AOs were decreased in both, the skin, in vivo (β-carotene and lycopene), and blood plasma (β-carotene only). In contrast, the levels of glutathione (GSH), the prototypical endogenous AO, which were analyzed by fluorimetric assays in cutaneous tape strips and blood plasma, were increased in the skin, although unchanged in the blood of smokers. Elevated cutaneous GSH levels were reflected by an elevated overall radical scavenging activity in the skin, as quantified by non-invasive electron paramagnetic resonance (EPR) spectroscopy. Analysis of the expression of selected stress-associated genes in blood immune cells by quantitative RT-PCR in subgroups of non-smokers and smokers additionally demonstrated the downregulation of AKR1C2 in smokers, and its negative correlation with blood plasma levels of the protective immune mediator interleukin-22, assessed by the ELISA technique. Controlled cigarette consumption did not alter exogenous or endogenous AOs in the skin of smokers, but decreased lycopene levels in blood plasma. Moreover, there was a decline in blood IL-22 levels, while no relevant response of blood cell gene expressions was found after the considered short time. Our data therefore demonstrate a strengthened endogenous AO status in the skin of smokers, which may indicate a long-term adaptation to chronic oxidative stress in this specific organ. While this effect was not clearly visible in the blood, this compartment seems to be useful as an immediate indicator of the body's AO consumption. Moreover, decreased levels of AKR1C2, which we show for the first time to be expressed in immune cells, may be a candidate marker for long-term smoking. In addition, this study demonstrates that the rate constant of a spin probe decline determined by EPR spectroscopy mainly represents the endogenous AO status of a tissue.

Age-related murine hippocampal CA1 laminae oxidative stress measured in vivo by QUEnch-assiSTed (QUEST) MRI: impact of isoflurane anesthesia

Age-related impairments in spatial learning and memory often precede non-familial neurodegenerative disease. Ex vivo studies suggest that physiologic age-related oxidative stress in hippocampus area CA1 may contribute to prodromal spatial disorientation and to morbidity. Yet, conventional blood or cerebrospinal fluid assays appear insufficient for early detection or management of oxidative stress within CA1 sub-regions in vivo. Here, we address this biomarker problem using a non-invasive MRI index of CA1 laminae oxidative stress based on reduction in R1 (= 1/T1) after anti-oxidant administration. An R1 reduction reflects quenching of continuous and excessive production of endogenous paramagnetic free radicals. Careful motion-correction image acquisition, and avoiding repeated exposure to isoflurane, facilitates detection of hippocampus CA1 laminae oxidative stress with QUEnch-assiSTed (QUEST) MRI. Intriguingly, age- and isoflurane-related oxidative stress is localized to the stratum lacunosum of the CA1 region. Our data raise the possibility of using QUEST MRI and FDA-approved anti-oxidants to remediate spatial disorientation and later neurodegeneration with age in animals and humans.

Investigation of TEMPO partitioning in different skin models as measured by EPR spectroscopy - Insight into the stratum corneum

Electron paramagnetic resonance (EPR) spectroscopy represents an established tool to study properties of microenvironments, e.g. to investigate the structure and dynamics of biological and artificial membranes. In this study, the partitioning of the spin probe 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in ex vivo human abdominal and breast skin, ex vivo porcine abdominal and ear skin as well as normal and inflammatory in vitro skin equivalents was investigated by EPR spectroscopy. Furthermore, the stratum corneum (SC) lipid composition (as determined by high-performance thin-layer chromatography), SC lipid chain order (probed by infrared spectroscopy) and the SC thickness (investigated by histology) were determined in the skin models. X-band EPR measurements have shown that TEMPO partitions in the lipophilic and hydrophilic microenvironment in varying ratios in different ex vivo and in vitro skin models. Ex vivo human abdominal skin exhibited the highest amount of TEMPO in the lipophilic microenvironment. In contrast, the lowest amount of TEMPO in the lipophilic microenvironment was determined in ex vivo human breast skin and the inflammatory in vitro skin equivalents. Individual EPR spectra of epidermis including SC and dermis indicated that the lipophilic microenvironment of TEMPO mainly corresponds to the most lipophilic part of the epidermis, the SC. The amount of TEMPO in the lipophilic microenvironment was independent of the SC lipid composition and the SC lipid chain order but correlated with the SC thickness. In conclusion, EPR spectroscopy could be a novel technique to determine differences in the SC thickness, thus suitably complementing existing methods.

Early administration of galantamine from preplaque phase suppresses oxidative stress and improves cognitive behavior in APPswe/PS1dE9 mouse model of Alzheimer's disease

Alzheimer's disease (AD) is a common neurodegenerative disease that progressively impairs memory and cognition. Deposition of amyloid-β (Aβ) peptides is the most important pathophysiological hallmark of AD. Oxidative stress induced by generation of reactive oxygen species (ROS) is a prominent phenomenon in AD and known to occur early in the course of AD. Several reports suggest a relationship between change in redox status and AD pathology including progressive Aβ deposition, glial cell activation, and inflammation. Galantamine is an acetylcholinesterase inhibitor and has been reported to have an oxidative stress inhibitory function. In the present study, galantamine was administered orally to AD model mice from before the appearance of Aβ plaques (preplaque phase), and in vivo change in redox status of the brain was measured using electron paramagnetic resonance (EPR) imaging. Administration of galantamine from the preplaque phase ameliorated memory decline in Morris water maze test and novel object recognition test. Monitoring of the redox status of the brain using EPR imaging showed that galantamine treatment improved the unbalanced redox state. Additionally, galantamine administration enhanced microglial function to promote Aβ clearance, reducing the Aβ-positive area in the cortex and amount of insoluble Aβ in the brain. In contrast, galantamine treatment from the preplaque phase suppressed the production of proinflammatory cytokines through neurotoxic microglial activity. Therefore, galantamine administration from the preplaque phase may have the potential of clinical application for the prevention of AD. In addition, our results demonstrate the usefulness of EPR imaging for speedy and quantitative evaluation of the efficacy of disease-modifying drugs for AD.

Radiation-induced redox alteration in the mouse brain

Time courses of the redox status in the brains of mice after X-ray or carbon-ion beam irradiation were observed by magnetic resonance redox imaging (MRRI). The relationship between radiation-induced oxidative stress on the cerebral nervous system and the redox status in the brain was discussed. The mice were irradiated by 8-Gy X-ray or carbon-ion beam (C-beam) on their head under anesthesia. C-beam irradiation was performed at HIMAC (Heavy-Ion Medical Accelerator in Chiba, NIRS/QST, Chiba, Japan). MRRI measurements using a blood-brain-barrier-permeable nitroxyl contrast agent, MCP or TEMPOL, were performed using 7-T scanner at several different times, i.e., 5-10 h, 1, 2, 4, and 8 day(s) after irradiation. Decay rates of the nitroxyl-enhanced T₁-weighted MR signals in the brains were estimated from MRRI data sets, and variation in the decay rates after irradiation was assessed. The variation in decay rates of MCP and TEMPOL after X-ray or C-beam irradiation was similar, but different variation patterns were observed between X-ray and C-beam. The apparent decay rate of both MCP and TEMPOL decreased due to the temporal reduction of blood flow in the brain several hours after X-ray and/or C-beam irradiation. After decreasing, the apparent decay rates of nitroxyl radicals in the brain gradually increased during the following days after X-ray irradiation or rapidly increased 1 day after C-beam irradiation. The sequential increase in nitroxyl decay rates may have been due to the oxidative atmosphere in the tissue due to ROS generation. X-ray and C-beam irradiation resulted in different redox responses, which may have been due to time-varying oxidative stress/injury, in the mouse brain. The C-beam irradiation effects were more acute and larger than those of X-ray irradiation.

Direct and indirect assessment of cancer metabolism explored by MRI

Magnetic resonance-based approaches to obtain metabolic information on cancer have been explored for decades. Electron paramagnetic resonance (EPR) has been developed to pursue metabolic profiling and successfully used to monitor several physiologic parameters such as pO₂ , pH, and redox status. All these parameters are associated with pathophysiology of various diseases. Especially in oncology, cancer hypoxia has been intensively studied because of its relationship with metabolic alterations, acquiring treatment resistance, or a malignant phenotype. Thus, pO₂ imaging leads to an indirect metabolic assessment in this regard. Proton electron double-resonance imaging (PEDRI) is an imaging technique to visualize EPR by using the Overhauser effect. Most biological parameters assessed in EPR can be visualized using PEDRI. However, EPR and PEDRI have not been evaluated sufficiently for clinical application due to limitations such as toxicity of the probes or high specific absorption rate. Hyperpolarized (HP) ¹³ C MRI is a novel imaging technique that can directly visualize the metabolic profile. Production of metabolites of the HP ¹³ C probe delivered to target tissue are evaluated in this modality. Unlike EPR or PEDRI, which require the injection of radical probes, ¹³ C MRI requires a probe that can be physiologically metabolized and efficiently hyperpolarized. Among several methods for hyperpolarizing probes, dissolution dynamic nuclear hyperpolarization is a widely used technique for in vivo imaging. Pyruvate is the most suitable probe for HP ¹³ C MRI because it is part of the glycolytic pathway and the high efficiency of pyruvate-to-lactate conversion is a distinguishing feature of cancer. Its clinical applicability also makes it a promising metabolic imaging modality. Here, we summarize the applications of these indirect and direct MR-based metabolic assessments focusing on pO₂ and pyruvate-to-lactate conversion. The two parameters are strongly associated with each other, hence the acquired information is potentially interchangeable when evaluating treatment response to oxygen-dependent cancer therapies.

Eventual analysis of global cerebral ischemia-reperfusion injury in rat brain: a paradigm of a shift in stress and its influence on cognitive functions

Cognitive issues in stroke arise as a result of reperfusion of a clogged artery, which is reported to exacerbate the injury in the brain leading to oxidative stress. Through the present work, we try to understand the regional variations across brain regions mainly cortex and striatum associated with the progression of ischemia-reperfusion injury (IRI). In a rat model of IRI, the influence of varying ischemia and reperfusion times on the biochemical phases across the brain regions were monitored. IRI resulted in the blood-brain barrier disruption and developed mild areas of risk. The brain's tolerance towards IRI indicated a progressive trend in the injury and apoptosis from ischemia to reperfusion that was supported by the activities of plasma lactate dehydrogenase and tissue caspase-3. Cognitive impairment in these rats was an implication of cellular oxidative stress (higher lipid peroxidation and lower antioxidant enzyme activity) that persisted by 24-h reperfusion. The oxidative stress was prominent in the cortex than the striatum and was supported by the lower ATP level. Upregulated Mn-SOD expression leading to a preserved mitochondria in the striatum could be attributed to the regional protection. Overall, a progression of IRI was observed from striatum to cortex leading to 5-day cognitive decline.

"Redox Imaging" to Distinguish Cells with Different Proliferative Indexes: Superoxide, Hydroperoxides, and Their Ratio as Potential Biomarkers

The present study was directed to the development of EPR methodology for distinguishing cells with different proliferative activities, using “redox imaging.” Three nitroxide radicals were used as redox sensors: (a) mito-TEMPO—cell-penetrating and localized mainly in the mitochondria; (b) methoxy-TEMPO—cell-penetrating and randomly distributed between the cytoplasm and the intracellular organelles; and (c) carboxy-PROXYL—nonpenetrating in living cells and evenly distributed in the extracellular environment. The experiments were conducted on eleven cell lines with different proliferative activities and oxidative capacities, confirmed by conventional analytical tests. The data suggest that cancer cells and noncancer cells are characterized by a completely different redox status. This can be analyzed by EPR spectroscopy using mito-TEMPO and methoxy-TEMPO, but not carboxy-PROXYL. The correlation analysis shows that the EPR signal intensity of mito-TEMPO in cell suspensions is closely related to the superoxide level. The described methodology allows the detection of overproduction of superoxide in living cells and their identification based on the intracellular redox status. The experimental data provide evidences about the role of superoxide and hydroperoxides in cell proliferation and malignancy.

Molecular Probes for Evaluation of Oxidative Stress by In Vivo EPR Spectroscopy and Imaging: State-of-the-Art and Limitations

Oxidative stress, defined as a misbalance between the production of reactive oxygen species and the antioxidant defenses of the cell, appears as a critical factor either in the onset or in the etiology of many pathological conditions. Several methods of detection exist. However, they usually rely on ex vivo evaluation or reports on the status of living tissues only up to a few millimeters in depth, while a whole-body, real-time, non-invasive monitoring technique is required for early diagnosis or as an aid to therapy (to monitor the action of a drug). Methods based on electron paramagnetic resonance (EPR), in association with molecular probes based on aminoxyl radicals (nitroxides) or hydroxylamines especially, have emerged as very promising to meet these standards. The principles involve monitoring the rate of decrease or increase of the EPR signal in vivo after injection of the nitroxide or the hydroxylamine probe, respectively, in a pathological versus a control situation. There have been many successful applications in various rodent models. However, current limitations lie in both the field of the technical development of the spectrometers and the molecular probes. The scope of this review will mainly focus on the latter.

Brain Redox Imaging Using In Vivo Electron Paramagnetic Resonance Imaging and Nitroxide Imaging Probes

Reactive oxygen species (ROS) are produced by living organisms as a result of normal cellular metabolism. Under normal physiological conditions, oxidative damage is prevented by the regulation of ROS by the antioxidant network. However, increased ROS and decreased antioxidant defense may contribute to many brain disorders, such as stroke, Parkinson’s disease, and Alzheimer’s disease. Noninvasive assessment of brain redox status is necessary for monitoring the disease state and the oxidative damage. Continuous-wave electron paramagnetic resonance (CW-EPR) imaging using redox-sensitive imaging probes, such as nitroxides, is a powerful method for visualizing the redox status modulated by oxidative stress in vivo. For conventional CW-EPR imaging, however, poor signal-to-noise ratio, low acquisition efficiency, and lack of anatomic visualization limit its ability to achieve three-dimensional redox mapping of small rodent brains. In this review, we discuss the instrumentation and coregistration of EPR images to anatomical images and appropriate nitroxide imaging probes, all of which are needed for a sophisticated in vivo EPR imager for all rodents. Using new EPR imaging systems, site-specific distribution and kinetics of nitroxide imaging probes in rodent brains can be obtained more accurately, compared to previous EPR imaging systems. We also describe the redox imaging studies of animal models of brain disease using newly developed EPR imaging.

Neurotoxicity of anesthetics: Mechanisms and meaning from mouse intervention studies

Volatile anesthetics are widely used in human medicine and generally considered to be safe in healthy individuals. In recent years, the safety of volatile anesthesia in pediatric patients has been questioned following reports of anesthetic induced neurotoxicity in pre-clinical studies. These studies in mice, rats, and primates have demonstrated that exposure to anesthetic agents during early post-natal periods can cause acute neurotoxicity, as well as later-life cognitive defects including deficits in learning and memory. In recent years, the focus of many pre-clinical studies has been on identifying candidate pathways or potential therapeutic targets through intervention trials. These reports have shed light on the mechanisms underlying anesthesia induced neurotoxicity as well as highlighting the challenges of pre-clinical modeling of anesthesia induced neurotoxicity in mice. Here, we summarize the data derived from intervention studies in neonatal mouse models of anesthetic exposure and provide an overview of mechanisms proposed to mediate anesthesia induced neurotoxicity in mice based on these reports. The majority of these studies implicate one of three mechanisms: reactive oxygen species (ROS) mediated stress and signaling, growth/nutrient signaling, or direct neuronal modulation.

Electron Paramagnetic Resonance Measurements of Reactive Oxygen Species by Cyclic Hydroxylamine Spin Probes

SIGNIFICANCE

Oxidative stress contributes to numerous pathophysiological conditions such as development of cancer, neurodegenerative, and cardiovascular diseases. A variety of measurements of oxidative stress markers in biological systems have been developed; however, many of these methods are not specific, can produce artifacts, and do not directly detect the free radicals and reactive oxygen species (ROS) that cause oxidative stress. Electron paramagnetic resonance (EPR) is a unique tool that allows direct measurements of free radical species. Cyclic hydroxylamines are useful and convenient molecular probes that readily react with ROS to produce stable nitroxide radicals, which can be quantitatively measured by EPR. In this work, we critically review recent applications of various cyclic hydroxylamine spin probes in biology to study oxidative stress, their advantages, and the shortcomings. Recent Advances: In the past decade, a number of new cyclic hydroxylamine spin probes have been developed and their successful application for ROS measurement using EPR has been published. These new state-of-the-art methods provide improved selectivity and sensitivity for in vitro and in vivo studies.

CRITICAL ISSUES

Although cyclic hydroxylamine spin probes EPR application has been previously described, there has been lack of translation of these new methods into biomedical research, limiting their widespread use. This work summarizes "best practice" in applications of cyclic hydroxylamine spin probes to assist with EPR studies of oxidative stress.

FUTURE DIRECTIONS

Additional studies to advance hydroxylamine spin probes from the "basic science" to biomedical applications are needed and could lead to better understanding of pathological conditions associated with oxidative stress. Antioxid. Redox Signal. 28, 1433-1443.

Comparative studies with EPR and MRI on the in vivo tissue redox status estimation using redox-sensitive nitroxyl probes: influence of the choice of the region of interest

In vivo decay rates of a nitroxyl contrast agent were estimated by a MR redox imaging (MRRI) technique and compared with the decay rates obtained by the electron paramagnetic resonance spectroscopy (EPRS) and imaging (EPRI). MRRI is a dynamic imaging technique employing T₁-weighted pulse sequence, which can visualise a nitroxyl-induced enhancement of signal intensity by T₁-weighted contrast. EPR techniques can directly measure the paramagnetic nitroxyl radical. Both the squamous cell carcinoma (SCC) tumour-bearing and normal legs of a female C3H mouse were scanned by T₁-weighted SPGR sequence at 4.7 T with the nitroxyl radical, carbamoyl-proxyl (CmP), as the contrast agent. Similarly, the time course of CmP in normal muscle and tumour tissues was obtained using a 700-MHz EPR spectrometer with a surface coil. The time course imaging of CmP was also performed by 300 MHz CW EPR imager. EPRS and EPRI gave slower decay rates of CmP compared to the MRRI. Relatively slow decay rate at peripheral region of the tumour tissues, which was found in the image obtained by MRRI, may contribute to the slower decay rates observed by EPRS and/or the EPRI measurements. To reliably determine the tissue redox status from the reduction rates of nitroxyls such as CmP, heterogenic structure in the tumour tissue must be considered. The high spatial and temporal resolution of T₁-weighted MRI and the T₁-enhancing capabilities of nitroxyls support the use of this method to map tissue redox status which can be a useful biomarker to guide appropriate treatments based on the tumour microenvironment.

Cellular accumulation and antioxidant activity of acetoxymethoxycarbonyl pyrrolidine nitroxides

Nitroxides are widely used in biology as antioxidants, spin labels, functional spin probes for pH, oxygen and thiol levels, and tissue redox status imaging using electron paramagnetic resonance (EPR); however, biological applications of nitroxides is hindered by fast bioreduction to EPR-silent hydroxylamines and rapid clearance. In this work, we have studied pyrrolidine nitroxides with acetoxymethoxycarbonyl groups which can undergo hydrolysis by cellular esterases to hydrophilic carboxylate derivatives resistant to bioreduction. Nitroxides containing acetoxymethoxycarbonyl groups were rapidly absorbed by cells from the media, 3,4-bis-(acetoxymethoxycarbonyl)-proxyl (DCP-AM2) and 3-(2-(bis(2-(acetoxymethoxy)-2-oxoethyl)amino)acetamido)-proxyl (DCAP-AM2) showing the strongest EPR signal of the cellular fraction. Remarkably, the EPR parameters of 3,4-dicarboxy-proxyl (DCP) and its mono- and di-acetoxymethyl esters are different, and consequent intracellular hydrolysis of acetoxymethoxycarbonyl groups in DCP-AM2 can be followed by EPR. To elucidate intracellular location of the resultant DCP, the mitochondrial fraction has been isolated. EPR measurements showed that mitochondria were the main place where DCP was finally accumulated. TEMPO derivatives showed expectedly much faster decay of EPR signal in the cellular fraction, compared to pyrrolidine nitroxides. It was found that supplementation of endothelial cells with 50 nM of DCP-AM2 completely normalised the mitochondrial superoxide level. Moreover, administration of DCP-AM2 to mice (1.4 mg/kg/day) resulted in substantial nitroxide accumulation in the tissues and significantly reduced hypertension. We found that hydroxylamine derivatives of dicarboxyproxyl nitroxide DCP-AM-H can be used for the detection of superoxide in vivo in angiotensin II model of hypertension. Infusion of DCP-AM-H in mice leads to accumulation of persistent EPR signal of nitroxide in the blood and vascular tissue in angiotensin II-infused wild-type but not in SOD2 overexpressing mice. Our data demonstrate that acetoxymethoxycarbonyl group containing nitroxides accumulate in mitochondria and demonstrate site-specific antioxidant activity.

Application of a Compact Magnetic Resonance Imaging System with 1.5 T Permanent Magnets to Visualize Release from and the Disintegration of Capsule Formulations in Vitro and in Vivo

Although magnetic resonance imaging (MRI) has potential in assessments of formulations, few studies have been conducted because of the size and expense of the instrument. In the present study, the processes of in vitro and in vivo release in a gelatin capsule formulation model were visualized using a compact MRI system with 1.5 T permanent magnets, which is more convenient than the superconducting MRI systems typically used for clinical and experimental purposes. A Gd-chelate of diethylenetriamine-N,N,N',N″,N″-pentaacetic acid, a contrast agent that markedly enhances proton signals via close contact with water, was incorporated into capsule formulations as a marker compound. In vitro experiments could clearly demonstrate the preparation-dependent differences in the release/disintegration of the formulations. In some preparations, the penetration of water into the formulation and generation of bubbles in the capsule were also observed prior to the disintegration of the formulation. When capsule formulations were orally administered to rats, the release of the marker into the stomach and its transit to the duodenum were visualized. These results strongly indicate that the compact MRI system is a powerful tool for pharmaceutical studies.

In vivo EPR pharmacokinetic evaluation of the redox status and the blood brain barrier permeability in the SOD1G93A ALS rat model

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder affecting the motor pathways of the central nervous system. Although a number of pathophysiological mechanisms have been described in the disease, post mortem and animal model studies indicate blood-brain barrier (BBB) disruption and elevated production of reactive oxygen species as major contributors to disease pathology. In this study, the BBB permeability and the brain tissue redox status of the SOD1^G93A ALS rat model in the presymptomatic (preALS) and symptomatic (ALS) stages of the disease were investigated by in vivo EPR spectroscopy using three aminoxyl radicals with different cell membrane and BBB permeabilities, Tempol, 3-carbamoyl proxyl (3CP), and 3-carboxy proxyl (3CxP). Additionally, the redox status of the two brain regions previously implicated in disease pathology, brainstem and hippocampus, was investigated by spectrophotometric biochemical assays. The EPR results indicated that among the three spin probes, 3CP is the most suitable for reporting the intracellular redox status changes, as Tempol was reduced in vivo within minutes (t_1/2 =2.0±0.5min), thus preventing reliable kinetic modeling, whereas 3CxP reduction kinetics gave divergent conclusions, most probably due to its membrane impermeability. It was observed that the reduction kinetics of 3CP in vivo, in the head of preALS and ALS SOD1^G93A rats was altered compared to the controls. Pharmacokinetic modeling of 3CP reduction in vivo, revealed elevated tissue distribution and tissue reduction rate constants indicating an altered brain tissue redox status, and possibly BBB disruption in these animals. The preALS and ALS brain tissue homogenates also showed increased nitrilation, superoxide production, lipid peroxidation and manganese superoxide dismutase activity, and a decreased copper-zinc superoxide dismutase activity. The present study highlights in vivo EPR spectroscopy as a reliable tool for the investigation of changes in BBB permeability and for the unprecedented in vivo monitoring of the brain tissue redox status, as early markers of ALS.

Exchange Phenomena in the Electron Paramagnetic Resonance Spectra of the Nitroxyl and Trityl Radicals: Multifunctional Spectroscopy and Imaging of Local Chemical Microenvironment

This Feature overviews the basic principles of using stable organic radicals involved in reversible exchange processes as functional paramagnetic probes. We demonstrate that these probes in combination with electron paramagnetic resonance (EPR)-based spectroscopy and imaging techniques provide analytical tools for quantitative mapping of critical parameters of local chemical microenvironment. The Feature is written to be understandable to people who are laymen to the EPR field in anticipation of future progress and broad application of these tools in biological systems, especially in vivo, over the next years.