In vivo visualization of redox status by high-resolution whole body magnetic resonance imaging using nitroxide radicals

Non-invasive differentiation of hepatic steatosis and steatohepatitis in a mouse model using nitroxyl radical as an MRI-contrast agent

Drug-induced steatohepatitis is considered more serious than drug-induced hepatic steatosis, so that differentiating between the two is crucial in drug development. In addition, early detection of drug-induced steatohepatitis is considered important since recovery is possible with drug withdrawal. However, no method has been established to differentiate between the two. In the development of drug-induced steatohepatitis, reactive oxygen species (ROS) is excessively generated in the liver. It has been reported that ROS can be monitored with electron spin resonance (ESR) and dynamic nuclear polarization-magnetic resonance imaging (DNP-MRI) by using nitroxyl radicals, which are known to participate in various in vivo redox reactions. The decay/reduction rate, which is an index for monitoring nitroxyl radicals, has been reported to be increased in tissues with excessive ROS levels other than liver, but decreased in methionine choline deficient (MCD) diet-induced steatohepatitis with excess ROS. Therefore, looking to differentiate between drug-induced hepatic steatosis and steatohepatitis, we examined whether the reduction rate decreases in steatohepatitis other than the MCD-diet induced disease and whether the decrease could be detected by MRI. We used STAM™ mice in which hepatic steatosis and steatohepatitis developed sequentially under diabetic conditions. 3-carbamoyl-PROXYL (CmP), one of the nitroxyl radicals, was injected intravenously during the MRI procedure and the reduction rate was calculated. The reduction rate was significantly higher in early steatohepatitis than in hepatic steatosis and the control. Excess ROS in early steatohepatitis was detected by an immunohistochemical marker for ROS. Therefore, it was indicated that the increase or decrease in the reduction rate in steatohepatitis differs depending on the model, and early steatohepatitis could be noninvasively differentiated from hepatic steatosis using CmP in MRI. Since the change in direction of the reduction rate in steatohepatitis in clinical studies could be predicted by confirming the reduction rate in preclinical studies, the present method, which can be used consistently in clinical and preclinical studies, warrants consideration as a candidate monitoring method for differentiating between early drug-induced steatohepatitis and hepatic steatosis in drug development.

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.

Oxidative stress evaluation of skeletal muscle in ischemia-reperfusion injury using enhanced magnetic resonance imaging

Acute extremity arterial occlusion requires prompt revascularization. Delayed revascularization induces ischemia–reperfusion injury in the skeletal muscle. Organ injury-induced oxidative stress is widely reported, and oxidative stress is heavily involved in ischemia–reperfusion injury. This study aimed to evaluate oxidative stress in ischemia–reperfusion rat models using 3-carbamoyl PROXYL enhanced magnetic resonance imaging (3-CP enhanced MRI). Ischemia–reperfusion injury was induced through clamping the right femoral artery in rats, with a 4-h ischemia time in all experiments. 3-CP enhanced MRI was performed to evaluate oxidative stress, and the rats were divided into 3 reperfusion time groups: 0.5, 2, and 24 h. Signal intensity was evaluated using 3-CP enhanced MRI and compared in the ischemia–reperfusion and intact limbs in the same rat. Furthermore, the effect of edaravone (radical scavenger) was evaluated in the 4-h ischemia—24-h reperfusion injury rat model. The signal intensity of the ischemia–reperfusion limb was significantly stronger than that of the intact limb, suggesting that oxidative stress was induced in the ischemia–reperfusion muscle. Edaravone administration reduced the oxidative stress in the ischemia–reperfusion limb. The signal intensity of the ischemia–reperfusion limb was stronger than that of the intact limb, presumably reflecting the oxidative stress in the former. 3-CP MRI examination shows promise for effective assessment of oxidative stress and may facilitate early diagnosis of ischemia–reperfusion injury.