Mitochondrial NAD is a key determinant of the metabolic profile of human cells and is connected to the peroxisomal NAD pool Magali R. VanLinden, Ingvill Tolås, Lena E. Høyland, Mathias Ziegler Department of Biomedicine, University of Bergen
A common denominator to all mitochondrial processes is the presence of reactions catalyzed by enzymes using NAD as a cofactor either for metabolic redox reactions or signalling events. Fluctuations of mitochondrial NAD critically influence these processes. However, whether modulations of this pool might affect other subcellular compartments remains poorly understood. In mammals, the mitochondrial NAD pool is thought to be autonomous owing to the lack of a specific membrane carrier. To study the consequences of changes in the putatively independent mitochondrial NAD pool, we established a cell line stably expressing the catalytic domain of the NAD-consuming enzyme PARP1 within mitochondria, thereby selectively lowering the mitochondrial NAD concentration. We also established a stable cell line in which NAD is permanently increased inside the mitochondria by the expression of AtNDT2, a known plant mitochondrial NAD transporter. Characterization of these cell lines revealed that both increased and lowered mitochondrial NAD content have a strong impact on cellular processes such as proliferation and cause severe metabolic perturbation, namely, a shift from oxidative phosphorylation to glycolysis. Surprisingly, expression of the catalytic domain of PARP1 in peroxisomes led to a substantial decrease in mitochondrial NAD content along with impairment of mitochondrial redox functions. These observations suggest a hitherto unrecognized interplay between the two organelles, mediated by a communication of their NAD pools. Using the catalytic domain of PARP1 as a subcellular NAD sensor, we also detected the presence of the coenzyme in other organelles such as the endoplasmic reticulum and the Golgi complex. Current studies address the possibility of a dynamic interplay between the different subcellular NAD pools.