Adenine nucleotide translocator (ANT) is a mitochondrial protein involved in the exchange of ADP and ATP across the mitochondrial inner membrane. It plays a crucial role in cellular energy metabolism by facilitating the transport of ATP synthesized within the mitochondria to the cytoplasm. The isoform ANT1 predominately expresses in cardiac and skeletal muscles. Mutations or dysregulation in ANT1 have been implicated in various mitochondrial disorders and neuromuscular diseases. We aimed to examine whether ANT1 deletion may affect mitochondrial redox state in our established ANT1-deficient mice. Hearts and quadriceps resected from age-matched wild type (WT) and ANT1-deficient mice were snap-frozen in liquid nitrogen. The Chance redox scanner was utilized to perform 3D optical redox imaging. Each sample underwent scanning across 3–5 sections. Global averaging analysis showed no significant differences in the redox indices (NADH, flavin adenine dinucleotide containing-flavoproteins Fp, and the redox ratio Fp/(NADH+Fp) between WT and ANT1-deficient groups. However, quadriceps had higher Fp than hearts in both groups (p=0.0004 and 0.01, respectively). Furthermore, the quadriceps were also more oxidized (a higher redox ratio) than hearts in WT group (p=0.004). NADH levels were similar in all cases. Our data suggest that under non-stressful physical condition, the ANT1-deficient muscle cells were in the same mitochondrial state as WT ones and that the significant difference in the mitochondrial redox state between quadriceps and hearts found in WT might be diminished in ANT1-deficient ones. Redox imaging of muscles under physical stress can be conducted in future.

The heart requires continuous ATP availability that is generated in the mitochondria. Although studies using the cell culture and perfused organ models have been carried out to investigate the biochemistry in the mitochondria in response to a change in substrate supply, mitochondrial bioenergetics of heart under normal feed or fasting conditions has not been studied at the tissue level with a sub-millimeter spatial resolution either in vivo or ex vivo. Oxidation of many foodderived metabolites to generate ATP in the mitochondria is realized through the NADH/NAD＋ couple acting as a central electron carrier. We employed the Chance redox scanner — the lowtemperature fluorescence scanner to image the three-dimensional (3D) spatial distribution of the mitochondrial redox states in heart tissues of rats under normal feeding or an overnight starvation for 14.5 h. Multiple consecutive sections of each heart were imaged to map three redox indices, i.e., NADH, oxidized flavoproteins (Fp, including flavin adenine dinucleotide (FAD)) and the redox ratio NADH/Fp. The imaging results revealed the micro-heterogeneity and the spatial distribution of these redox indices. The quantitative analysis showed that in the fasted hearts the standard deviation of both NADH and Fp, i.e., SD NADH and SD Fp, significantly decreased with a p value of 0.032 and 0.045, respectively, indicating that the hearts become relatively more homogeneous after fasting. The fasted hearts contained 28.6% less NADH (p = 0.038). No signi ficant change in Fp was found (p = 0.4). The NADH/Fp ratio decreased with a marginal p value (0.076). The decreased NADH in the fasted hearts is consistent with the cardiac cells' reliance of fatty acids consumption for energy metabolism when glucose becomes scarce. The experimental observation of NADH decrease induced by dietary restriction in the heart at tissue level has not been reported to our best knowledge. The Chance redox scanner demonstrated the feasibility of 3D imaging of the mitochondrial redox state in the heart and provides a useful tool to study heart metabolism and function under normal, dietary-change and pathological conditions at tissue level.

Two-photon excited fluorescence (TPEF) spectroscopy and imaging were used to investigate the effects of gamma-irradiation on neural stem and precursor cells (NSPCs). While the observed signal from reduced nicotinamide adenine dinucleotide (NADH) was localized to the mitochondria, the signal typically associated with oxidized flavoproteins (Fp) was distributed diffusely throughout the cell. The measured TPEF emission and excitation spectra were similar to the established spectra of NAD(P)H and Fp. Fp fluorescence intensity was markedly increased by addition of the electron transport chain (ETC) modulator menadione to the medium, along with a concomitant decrease in the NAD(P)H signal. Three-dimensional (3D) neurospheres were imaged to obtain the cellular metabolic index (CMI), calculated as the ratio of Fp to NAD(P)H fluorescence intensity. Radiation effects were found to differ between low-dose (≤ 50 cGy) and high-dose (≥ 50 cGy) exposures. Low-dose irradiation caused a marked drop in CMI values accompanied by increased cellular proliferation. At higher doses, both NAD(P)H and Fp signals increased, leading to an overall elevation in CMI values. These findings underscore the complex relationship between radiation dose, metabolic state, and proliferation status in NSPCs and highlight the ability of TPEF spectroscopy and imaging to characterize metabolism in 3D spheroids.

Redox state mediates embryonic stem cell (ESC) differentiation and thus offers an important complementary approach to understanding the pluripotency of stem cells. NADH redox ratio (NADH/(Fp+ NADH)), where NADH is the reduced form of nicotinamide adenine dinucleotide and Fp is the oxidized flavoproteins, has been established as a sensitive indicator of mitochondrial redox state. In this paper, we report our redox imaging data on the mitochondrial redox state of mouse ESC (mESC) colonies and the implications thereof. The low-temperature NADH/Fp redox scanner was employed to image mESC colonies grown on a feeder layer of gamma-irradiated mouse embryonic fibroblasts (MEFs) on glass cover slips. The result showed significant heterogeneity in the mitochondrial redox state within individual mESC colonies (size: ~200-440 μm), exhibiting a core with a more reduced state than the periphery. This more reduced state positively correlates with the expression pattern of Oct4, a well-established marker of pluripotency. Our observation is the first to show the heterogeneity in the mitochondrial redox state within a mESC colony, suggesting that mitochondrial redox state should be further investigated as a potential new biomarker for the stemness of embryonic stem cells.