Monitoring live mitochondrial metabolism in real-time using NMR spectroscopy

NMR spectroscopy for metabolomics in the living system: recent progress and future challenges

Metabolism is a fundamental process that underlies human health and diseases. Nuclear magnetic resonance (NMR) techniques offer a powerful approach to identify metabolic processes and track the flux of metabolites at the molecular level in living systems. An in vitro study through in-cell NMR tracks metabolites in real time and investigates protein structures and dynamics in a state close to their most natural environment. This technique characterizes metabolites and proteins involved in metabolic pathways in prokaryotic and eukaryotic cells. In vivo magnetic resonance spectroscopy (MRS) enables whole-organism metabolic monitoring by visualizing the spatial distribution of metabolites and targeted proteins. One limitation of these NMR techniques is the sensitivity, for which a possible improved approach is through isotopic enrichment or hyperpolarization methods, including dynamic nuclear polarization (DNP) and parahydrogen-induced polarization (PHIP). DNP involves the transfer of high polarization from electronic spins of radicals to surrounding nuclear spins for signal enhancements, allowing the detection of low-abundance metabolites and real-time monitoring of metabolic activities. PHIP enables the transfer of nuclear spin polarization from parahydrogen to other nuclei for signal enhancements, particularly in proton NMR, and has been applied in studies of enzymatic reactions and cell signaling. This review provides an overview of in-cell NMR, in vivo MRS, and hyperpolarization techniques, highlighting their applications in metabolic studies and discussing challenges and future perspectives.

Intracellular pyruvate-lactate-alanine cycling detected using real-time nuclear magnetic resonance spectroscopy of live cells and isolated mitochondria

Pyruvate, an end product of glycolysis, is a master fuel for cellular energy. A portion of cytosolic pyruvate is transported into mitochondria, while the remaining portion is converted reversibly into lactate and alanine. It is suggested that cytosolic lactate and alanine are transported and metabolized inside mitochondria. However, such a mechanism continues to be a topic of intense debate and investigation. As a part of gaining insight into the metabolic fate of the cytosolic lactate and alanine; in this study, the metabolism of mouse skeletal myoblast cells (C2C12) and their isolated mitochondria was probed utilizing stable isotope-labeled forms of the three glycolysis products, viz. [3-13 C1 ]pyruvate, [3-13 C1 ]lactate, and [3-13 C1 ]alanine, as substrates. The uptake and metabolism of each substrate was monitored, separately, in real-time using 1 H-13 C 2D nuclear magnetic resonance (NMR) spectroscopy. The dynamic variation of the levels of the substrates and their metabolic products were quantitated as a function of time. The results demonstrate that all three substrates were transported into mitochondria, and each substrate was metabolized to form the other two metabolites, reversibly. These results provide direct evidence for intracellular pyruvate-lactate-alanine cycling, in which lactate and alanine produced by the cytosolic pyruvate are transported into mitochondria and converted back to pyruvate. Such a mechanism suggests a role for lactate and alanine to replenish mitochondrial pyruvate, the primary source for adenosine triphosphate (ATP) synthesis through oxidative phosphorylation and the electron transport chain. The results highlight the potential of real-time NMR spectroscopy for gaining new insights into cellular and subcellular functions.