Imaging of Mitochondria Metabolic Change after Radiation Treatment using Dynamic Nuclear Polarization (DNP)-MRI

Abstract

In vivo dynamic nuclear polarization-magnetic resonance imaging (DNP-MRI) enables the anatomical distribution of free radical species to be monitored. DNP enhances the MRI signal from nuclei such as 1H by irradiating tissue at the electron paramagnetic resonance (EPR) frequency of the free radical, before applying the MRI pulse sequence, thus increasing the image intensity where free radicals are present. Low-molecular-weight, stable nitroxyl compounds, including carbamoyl-PROXYL (CmP), are organic free radicals that possess SOD-like activity, and they have been used as probes in a variety of biophysical and biochemical experiments. The reduction rate of the radical probe depends on the tissue redox environment, which is influenced by factors such as overproduction of ROS and decreased antioxidant defences. In this study, we evaluated the feasibility of mitochondrial redox metabolic imaging, assessed by DNP-MRI with CmP as a molecular imaging probe, for detecting the metabolic change after radiation treatment to the tissue homogenate solution. Redox imaging of mitochondrial metabolism was performed with a low magnetic field using an in vivo DNP-MRI system (Keller, Japan Redox Inc.). The external magnetic field (B0) for EPR irradiation and MRI was fixed at 15 mT, and the radiofrequencies for EPR irradiation and MRI were 455 MHz and 683 kHz, respectively. DNP-MRI images of liver homogenate solution with or without radiation (10 or 20 gray) were obtained at 3, 5, 10, 15, 20, 25, 30, 35 and 40 min after starting measurement. The redox map was obtained by the slope of the enhanced DNP image intensity of each pixel from four pharmacokinetic images using a custom Excel macro program. Phantom redox experiments were conducted to examine the ability of CmP to participate in the mitochondrial redox reaction, and time-dependent changes in image contrast were assessed using liver tissue homogenate and KCN, an inhibitor of mitochondrial electron transfer chain. The DNP images clearly showed the significant enhancement in tissue homogenate solution by CmP. The enhancement of DNP-image was gradually reduced by liver homogenate solution due to reduction of CmP and the reduction rate of CmP was increased by the treatment of radiation. Interestingly, reduction of CmP in both homogenates with or without radiation were completely inhibited by KCN treatment. It is suggested that enhancement of CmP reduction is related in mitochondrial electron transfer system. We demonstrated that the mitochondrial redox metabolic change in the electron transfer chain after radiotherapy could be monitored by DNP-MRI with CmP as a redox probe.