Noninvasive mapping of the redox status of dimethylnitrosamine-induced hepatic fibrosis using in vivo dynamic nuclear polarization-magnetic resonance imaging

Noninvasive Diagnosis of the Mitochondrial Function of Doxorubicin-Induced Cardiomyopathy Using In Vivo Dynamic Nuclear Polarization-Magnetic Resonance Imaging

Doxorubicin (DOX) induces dose-dependent cardiotoxicity via oxidative stress and abnormal mitochondrial function in the myocardium. Therefore, a noninvasive in vivo imaging procedure for monitoring the redox status of the heart may aid in monitoring diseases and developing treatments. However, an appropriate technique has yet to be developed. In this study, we demonstrate a technique for detecting and visualizing the redox status of the heart using in vivo dynamic nuclear polarization–magnetic resonance imaging (DNP–MRI) with 3-carbamoyl-PROXYL (CmP) as a molecular imaging probe. Male C57BL/6N mice were administered DOX (20 mg/kg) or saline. DNP–MRI clearly showed a slower DNP signal reduction in the DOX group than in the control group. Importantly, the difference in the DNP signal reduction rate between the two groups occurred earlier than that detected by physiological examination or clinical symptoms. In an in vitro experiment, KCN (an inhibitor of complex IV in the mitochondrial electron transport chain) and DOX inhibited the electron paramagnetic resonance change in H9c2 cardiomyocytes, suggesting that the redox metabolism of CmP in the myocardium is mitochondrion-dependent. Therefore, this molecular imaging technique has the potential to monitor the dynamics of redox metabolic changes in DOX-induced cardiomyopathy and facilitate an early diagnosis of this condition.

In Vivo Dynamic Nuclear Polarization Magnetic Resonance Imaging for the Evaluation of Redox-Related Diseases and Theranostics

Significance:In vivo molecular and metabolic imaging is an emerging field in biomedical research that aims to perform noninvasive detection of tissue metabolism in disease states and responses to therapeutic agents. The imbalance in tissue oxidation/reduction (Redox) states is related to the onset and progression of several diseases. Tissue redox metabolism provides biomarkers for early diagnosis and drug treatments. Thus, noninvasive imaging of redox metabolism could be a useful, novel diagnostic tool for diagnosis of redox-related disease and drug discovery. Recent Advances:In vivo dynamic nuclear polarization magnetic resonance imaging (DNP-MRI) is a technique that enables the imaging of free radicals in living animals. DNP enhances the MRI signal by irradiating the target tissue or solution with the free radical molecule's electron paramagnetic resonance frequency before executing pulse sequence of the MRI. In vivo DNP-MRI with redox-sensitive nitroxyl radicals as the DNP redox contrast agent enables the imaging of the redox metabolism on various diseases. Moreover, nitroxyl radicals show antioxidant effects that suppress oxidative stress. Critical Issues: To date, considerable progress has been documented preclinically in the development of animal imaging systems. Here, we review redox imaging of in vivo DNP-MRI with a focus on the recent progress of this system and its uses in patients with redox-related diseases. Future Directions: This technique could have broad applications in the study of other redox-related diseases, such as cancer, inflammation, and neurological disorders, and facilitate the evaluation of treatment response as a theranostic tool. Antioxid. Redox Signal. 36, 172-184.

Development of 20 cm sample bore size dynamic nuclear polarization (DNP)-MRI at 16 mT and redox metabolic imaging of acute hepatitis rat model

Tissue redox metabolism is involved in various diseases, and an understanding of the spatio-temporal dynamics of tissue redox metabolism could be useful for diagnosis of progression and treatment. In in vivo dynamic nuclear polarization (DNP)-MRI, electron paramagnetic resonance (EPR) irradiation at the resonance frequency of nitroxyl radicals administered as a redox probe for induction of DNP, increases the intensity of MRI signals. For electron spin, it is necessary to apply a resonant frequency 658 times higher than that required for nuclear spin because of the higher magnetic moment of unpaired electrons. Previous studies using a disease model of small animals and in vivo DNP-MRI have revealed that an abnormal redox status is involved in many diseases, and that it could be used to visualize the dynamics of alterations in redox metabolism. To use the current methods in clinical practice, the development of a prototype DNP-MRI system for preclinical examinations of large animals is indispensable for clarifying the problems peculiar to the increase in size of the DNP-MRI device. Therefore, we developed a in vivo DNP-MRI system with a sample bore size of 20 cm and a 16-mT magnetic field using a U-shaped permanent magnet. Because the NMR frequency is very low, we adopted a digital radiofrequency transmission/reception system with excellent filter and dynamic range characteristics and equipped with a digital eddy current compensation system to suppress large eddy currents. The pulse sequence was based on the fast spin-echo sequence, which was improved for low frequency and large-eddy current equipment. The in vivo DNP-MRI system developed was used to non-invasively image the redox reaction of a carbamoyl-PROXYL probe in the livers of large rats weighing 800 g. Furthermore, DNP-MRI analysis was able to capture significant changes in redox metabolism in hepatitis-model rats.

Imaging of Hydroxyl-Radical Generation Using Dynamic Nuclear Polarization-Magnetic Resonance Imaging and a Spin-Trapping Agent

Reactive oxygen species (ROS) play an important role in cell metabolism, but they can cause oxidative damage to biomolecules. Among ROS, the hydroxyl radical (·OH) is one of the most reactive molecules in biological systems because of its high reaction rate constant. Therefore, imaging of ·OH could be useful for evaluation of the redox mechanism and diagnosis of oxidative diseases. In vivo dynamic nuclear polarization-magnetic resonance imaging (DNP-MRI) is a noninvasive imaging method to obtain spatiotemporal information about free radicals with MRI anatomical resolution. In this study, we investigated the visualization of hydroxyl radicals generated from the Fenton reaction by combining DNP-MRI with a spin-trapping agent (DMPO: 5,5-dimethyl-1-pyrroline N-oxide) for ·OH. Additionally, we demonstrated the radical-scavenging effect using four thiol-related reagents by DNP-MRI. We demonstrated that DNP enhancement could be induced by the DMPO-OH radical using the DNP-MRI/spin-trapping method and visualized ·OH generation for the first time. Maximum DNP enhancement was observed at an electron paramagnetic resonance irradiation frequency of 474.5 MHz. Furthermore, the radical-scavenging effect was simultaneously evaluated by the decrease in the DNP image value of DMPO-OH. An advantage of our methods is that they simultaneously investigate compound activity and the radical-scavenging effect.

Metabolism of N-nitrosodimethylamine, methylation of macromolecules, and development of hepatic fibrosis in rodent models

Hepatic fibrosis and cirrhosis are chronic diseases affecting liver and a major health problem throughout the world. The hallmark of fibrosis and cirrhosis is inordinate synthesis and deposition of fibril forming collagens in the extracellular matrix of the liver leading to nodule formation and loss of normal architecture. Hepatic stellate cells play a crucial role in the pathogenesis and progression of liver fibrosis through secretion of several potent fibrogenic factors that trigger hepatocytes, portal fibrocytes, and bone marrow-derived fibroblasts to synthesize and deposit several connective tissue proteins, especially collagens between hepatocytes and space of Disse. Regulation of various events involved in the activation and transformation of hepatic stellate cells seems to be an appropriate strategy for the arrest of hepatic fibrosis and liver cirrhosis. In order to unravel the molecular mechanisms involved in the pathogenesis and progression of hepatic fibrosis, to determine proper and potent targets to arrest fibrosis, and to discover powerful therapeutic agents, a quick and reproducible animal model of hepatic fibrosis and liver cirrhosis that display all decompensating features of human condition is required. This review thoroughly evaluates the biochemical, histological, and pathological features of N-nitrosodimethylamine-induced model of liver injury, hepatic fibrosis, and early cirrhosis in rodents.

Development of multifunctional Overhauser-enhanced magnetic resonance imaging for concurrent in vivo mapping of tumor interstitial oxygenation, acidosis and inorganic phosphate concentration

Tumor oxygenation (pO₂), acidosis (pH) and interstitial inorganic phosphate concentration (Pi) are important parameters of the malignant behavior of cancer. A noninvasive procedure that enables visualization of these parameters may provide unique information about mechanisms of tumor pathophysiology and provide clues to new treatment targets. In this research, we present a multiparametric imaging method allowing for concurrent mapping of pH, spin probe concentration, pO₂, and Pi using a single contrast agent and Overhauser-enhanced magnetic resonance imaging technique. The developed approach was applied to concurrent multifunctional imaging in phantom samples and in vivo in a mouse model of breast cancer. Tumor tissues showed higher heterogeneity of the distributions of the parameters compared with normal mammary gland and demonstrated the areas of significant acidosis, hypoxia, and elevated Pi content.

In vivo melanoma imaging based on dynamic nuclear polarization enhancement in melanin pigment of living mice using in vivo dynamic nuclear polarization magnetic resonance imaging

Melanin is a pigment that includes free radicals and is widely distributed in living animals. Malignant melanoma is one of the most progressive tumors in humans with increasing incidence worldwide, and has shown resistance to chemotherapy, resulting in high mortality at the metastatic stage. In general, melanoma involves the abnormal accumulation of melanin pigment produced by malignant melanocytes. Electron paramagnetic resonance (EPR) spectroscopy and imaging is a powerful technique to directly visualize melanomas using endogenous free radicals in the melanin pigment. Because melanin radicals have a large linewidth, the low spatial resolution of EPR imaging results in blurred images and a lack of anatomical information. Dynamic nuclear polarization (DNP)-MRI is a noninvasive imaging method to obtain the spatio-temporal information of free radicals with MRI anatomical resolution. Proton signals in tissues, including free radicals, can be dramatically enhanced by EPR irradiation at the resonance frequency of the free radical prior to applying the MRI pulse sequence. However, the DNP effects of free radicals in the pigment of living organisms is unclear. Therefore, if endogenous free radicals in melanin pigment could be utilized as a bio-probe for DNP-MRI, this will be an advantage for the specific enhancement of melanoma tissues and might allow the separate noninvasive visualization of melanoma tissues without the need for probe administration. Here, we report that biological melanin pigment induced a in vivo DNP effect by interacting with water molecules. In addition, we demonstrated in vivo melanoma imaging based on the DNP effects of endogenous free radicals in the melanin pigment of living mice.

Molecular mechanisms in the pathogenesis of N-nitrosodimethylamine induced hepatic fibrosis

Hepatic fibrosis is marked by excessive synthesis and deposition of connective tissue proteins, especially interstitial collagens in the extracellular matrix of the liver. It is a result of an abnormal wound healing in response to chronic liver injury from various causes such as ethanol, viruses, toxins, drugs, or cholestasis. The chronic stimuli involved in the initiation of fibrosis leads to oxidative stress and generation of reactive oxygen species that serve as mediators of molecular events involved in the pathogenesis of hepatic fibrosis. These processes lead to cellular injury and initiate inflammatory responses releasing a variety of cytokines and growth factors that trigger activation and transformation of resting hepatic stellate cells into myofibroblast like cells, which in turn start excessive synthesis of connective tissue proteins, especially collagens. Uncontrolled and extensive fibrosis results in distortion of lobular architecture of the liver leading to nodular formation and cirrhosis. The perpetual injury and regeneration process could also results in genomic aberrations and mutations that lead to the development of hepatocellular carcinoma. This review covers most aspects of the molecular mechanisms involved in the pathogenesis of hepatic fibrosis with special emphasize on N-Nitrosodimethylamine (NDMA; Dimethylnitorsmaine, DMN) as the inducing agent.

Effects of oxygen challenging to tissue redox and pO2 status

Nitroxide free radicals can serve as redox-sensitive MRI contrast agents useful to image the redox status of tissue of interest. In this study, the effect of oxygen content in the inspired gas on the kinetics of metabolism of three nitroxides has been evaluated in the muscle and tumor in mice. SCC tumors (approximate size of 1.0 cm³) on the right hind leg of female C3H/Hen MTV^- mice were prepared. Three nitroxides, 3-carboxy-2,2,5,5-tetramethylpyrrolidine-N-oxyl (CxP), 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-N-oxyl (CmP), and 4-hydroxy-tetramethylpiperidine-N-oxyl (TEMPOL), having different lipophilicities were compared using MR redox imaging. T₁-mapping of the tissues was obtained using a multi-slice multi-echo (MSME) sequence with several TRs. The three nitroxides showed differences in accumulation and metabolism/clearance in muscle and tumor. The cell impermeable nitroxide CxP displayed kinetic patterns of slow enhancement followed by a slow decline typical of clearance rather than metabolism. The cell permeable CmP on the other hand showed a relatively faster uptake and metabolism with a modestly higher rate of metabolism in the tumor than muscle. The TEMPOL on the other hand displayed a rapid uptake and reduction with a trend of significantly rapid decay rate in tumor tissue, while slightly higher maximum signal intensity and slower decay rate was observed in normal muscle. The reduction rate of TEMPOL in the tumor was significantly enhanced when the breathing gas had 100%-oxygen while it was not significantly different in the muscle. EPR oximetry studies monitoring the oxygen dependent linewidth of TEMPOL showed that the pO₂ in the healthy tissue during carbogen breathing significantly increased normal tissue pO₂ compared to air breathing whereas breathing 100%-oxygen made normal tissue slight hypoxic. Since TEMPOL is a radioprotector, our studies show that a combination of 100%-oxygen breathing and TEMPOL has a potential to enhance radioprotective effects to normal tissue.

An elastase activity reporter for Electronic Paramagnetic Resonance (EPR) and Overhauser-enhanced Magnetic Resonance Imaging (OMRI) as a line-shifting nitroxide

Pulmonary inflammatory diseases are a major burden worldwide. They have in common an influx of neutrophils. Neutrophils secrete unchecked proteases at inflammation sites consequently leading to a protease/inhibitor imbalance. Among these proteases, neutrophil elastase is responsible for the degradation of the lung structure via elastin fragmentation. Therefore, monitoring the protease/inhibitor status in lungs non-invasively would be an important diagnostic tool. Herein we present the synthesis of a MeO-Suc-(Ala)₂-Pro-Val-nitroxide, a line-shifting elastase activity probe suitable for Electron Paramagnetic Resonance spectroscopy (EPR) and Overhauser-enhanced Magnetic Resonance Imaging (OMRI). It is a fast and sensitive neutrophil elastase substrate with K_m = 15 ± 2.9 µM, k_cat/K_m = 930,000 s^-1 M^-1 and K_m = 25 ± 5.4 µM, k_cat/K_m = 640,000 s^-1 M^-1 for the R and S isomers, respectively. These properties are suitable to detect accurately concentrations of neutrophil elastase as low as 1 nM. The substrate was assessed with broncho-alveolar lavages samples derived from a mouse model of Pseudomonas pneumonia. Using EPR spectroscopy we observed a clear-cut difference between wild type animals and animals deficient in neutrophil elastase or deprived of neutrophil Elastase, Cathepsin G and Proteinase 3 or non-infected animals. These results provide new preclinical ex vivo and in vivo diagnostic methods. They can lead to clinical methods to promote in time lung protection.

Reactivity of redox sensitive paramagnetic nitroxyl contrast agents with reactive oxygen species

The reactivity of nitroxyl free radicals, 4-hydroxyl-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPOL) and 3-carbamoyl-2,2,5,5-tetramethylpyrrolidine-N-oxyl (CmP), with reactive oxygen species (ROS) were compared as typical 6-membered and 5-membered ring nitroxyl compounds, respectively. The reactivity of the hydroxylamine forms of both these nitroxyl radicals (TEMPOL-H and CmP-H) was also assessed. Two free radical species of ROS, hydroxyl radical (^•OH) and superoxide (O₂^•−), were subjected to a competing reaction. ^•OH was generated by UV irradiation from an aqueous H₂O₂ solution (H₂O₂-UV system), and O₂^•− was generated by a reaction between hypoxanthine and xanthine oxidase (HX-XO system). ^•OH and O₂^•− generated by the H₂O₂-UV and HX-XO systems, respectively, were measured by electron paramagnetic resonance (EPR) spin-trapping, and the amount of spin adducts generated by each system was adjusted to be equal. The time courses of the one-electron oxidation of TEMPOL, CmP, TEMPOL-H, and CmP-H in each ROS generation system were compared. A greater amount of TEMPOL was oxidized in the HX-XO system compared with the H₂O₂-UV system, whereas the reverse was observed for CmP. Although the hydroxylamine forms of the tested nitroxyl radicals were oxidized evenly in the H₂O₂-UV and HX-XO systems, the amount of oxidized CmP-H was approximately 3 times greater compared with TEMPOL-H.

Selective On/Off-Nitroxides as Radical Probes to Investigate Non-radical Enzymatic Activity by Electron Paramagnetic Resonance

A nitroxide carrying a peptide specific to the binding pocket of the serine proteases chymotrypsin and cathepsin G is prepared. This peptide is attached as an enol ester to the nitroxide. Upon enzymatic hydrolysis of the peptide, the enol ester moiety is transformed into a ketone moiety. This transformation affords a difference of 5 G in phosphorus hyperfine coupling constant between the electronic paramagnetic resonance (EPR) signals of each nitroxide. This property is used to monitor the enzymatic activity of chymotrypsin and cathepsin G by EPR. Michaelis constants were determined and match those reported for conventional optical probes.

In vivo redox metabolic imaging of mitochondria assesses disease progression in non-alcoholic steatohepatitis

Given the rising incidence of non-alcoholic fatty liver disease (NAFLD) in both adults and children, the development of a non-invasive diagnostic method for assessing disease progression to non-alcoholic steatohepatitis (NASH) has become an important research goal. Currently available non-invasive imaging technologies are only able to assess fat accumulation in the liver. Therefore, these methods are not suitable for a precise diagnosis of NASH. The standard diagnostic technique for NASH, liver biopsy, has several drawbacks, including the higher risk of complications that accompanies invasive procedures. Here, we demonstrated that in vivo mitochondrial redox metabolism was dramatically altered at an early stage, before histopathological changes, and NASH could be accurately diagnosed by in vivo dynamic nuclear polarization-magnetic resonance imaging, with carbamoyl-PROXYL as a molecular imaging probe. In addition, this technique was feasible for the diagnosis of NASH compared with histopathological findings from biopsies. Our data reveal a novel method for monitoring the dynamics of redox metabolic changes in NAFLD/NASH.