Nicotinamide adenine dinucleotide is transported into mammalian mitochondria
Comment on: “Nicotinamide adenine dinucleotide is transported into mammalian mitochondria.”
by Davila A, Liu L, Chellappa K, Redpath P, Nakamaru-Ogiso E, Paolella LM, Zhang Z, Migaud ME, Rabinowitz JD, Baur JA
Elife. 2018 Jun 12;7. pii: e33246. doi: 10.7554/eLife.33246.
Pierre Rustin1,2,3,
1INSERM, UMR 1141, Hôpital Robert Debré, Paris, France
2Université Paris Diderot, Paris, France
3CNRS, Hôpital Robert Debré, Paris, France
Correspondence to: Pierre Rustin, Inserm UMR 1141, Bât. Bingen, Hôpital Robert Debré, 48 Boulevard Sérurier, 75019, Paris, France
e-mail: pierre.rustin@inserm.fr
In a recent issue of the journal, Davila et al (Davila et al., 2018) reports a series of elegant experiments that further strengthen the observation made 22 years ago establishing that pyridine dinucleotides readily permeate mitochondrial membranes in mammal cells (Rustin et al., 1996). Considering the role now attributed to the NAD+ content of mitochondria in the control of the overall cell metabolism and fate, in disease and aging (Braidy et al., 2018; Johnson and Imai, 2018), the understanding of the mechanism by which this latter content is actually controlled obviously appears of utmost importance. In this context, Davila's new study is a welcome reminder of the ability of mitochondria to lose but also to directly capture their NAD from the cytosol to adapt to metabolic needs. In the discussion of the present paper however, they mention that it is not clear that a rapid breakdown and resynthesis could be completely excluded in our 90’s experiments. At that time, two types of experiments were carried out in support of our conclusion of a direct uptake of NAD by mitochondria. A first set of experiments were performed using whole, intact cells showing a progressive loss of mitochondrial NAD content with days of cell culture and a very rapid replenishment upon culture medium change. Although the actual speed of replenishment was hardly compatible with a biosynthetic process, the use of intact whole cells supplemented by a fresh culture medium leaves room to a possibility mechanism involving NAD biosynthesis. However, the second set was performed using digitonin-permeabilized cells (no possibility of any functional cytosolic process) in which the mitochondria were freely accessible. The effect of NAD+ was then measured after a few seconds, either by the stimulation of mitochondrial substrate oxidation using a polarographic device or by direct quantification of mitochondrial matrix NAD pool saturation after a few seconds of reduction to NADH using a spectrofluorimeter. In both cases, the effect of NAD was shown by the addition of exogenous NAD+ which obviously did not require any NAD+ synthesis. The observed replenishment of the NAD pool, conclusively demonstrated the direct uptake of NAD+ by human mitochondria. Most importantly, both studies univocally demonstrate, using different and complementary experimental approaches, that NAD+ permeate mitochondrial membranes as a function of cell metabolic demand. The movement of NAD through mitochondrial membranes controlling content of mitochondrial matrix NAD pool represents a simple and costless process for the cell to efficiently regulate mitochondrial activity. As such, mitochondrial NAD content appears a parameter that should be taken into consideration in any study focused on the importance of NAD in physiology and diseases (Braidy et al., 2018; Johnson and Imai, 2018; Lin and Apte, 2018; Rajman et al., 2018; Yaku et al., 2018).
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