The work, from the laboratory of David Sinclair at Harvard Medical School, with collaborators from Weill Medical College of Cornell University, New York, and the NIH, suggests that boosting mitochondrial NAD+ in the face of stress, or otherwise activating mitochondrial sirtuins, can rescue cells at risk. The strategy might be applicable in Alzheimer disease and other neurodegenerative disorders, where aging and mitochondrial decline both contribute to disease, Sinclair said. The paper appears today in Cell.

Nicotinamide adenine dinucleotide (NAD+) has been implicated in life-or-death decisions before: activation of the proapoptotic enzyme poly(ADP-ribose) polymerase-1 (PARP-1) depletes cellular NAD+, causing the translocation of the apoptosis initiation factor (AIF) from the mitochondria to the nucleus and setting in motion the apoptotic cascade. NAD+ is also a necessary cofactor for sirtuins, whose activity extends lifespan and wards off aging-related diseases, including AD, in animals (Kim et al., 2007). Elevated sirtuin-1 (SIRT1) activity in animals mediates the lifespan-prolonging effects of calorie restriction (see ARF related news story), and the family of seven sirtuins has become a prime target for anti-aging therapies. Today’s paper is a surprise because it implicates two new members of the sirtuin family, SIRT3 and SIRT4, in the control of aging and cell survival, where the prior focus of drug development had been squarely on SIRT1.

In the new study, first author Hongying Yang and colleagues set out to understand how NAD+ biosynthetic pathways affect sirtuin activity, and ended up uncovering a surprising new role of mitochondria in the process. They found that the stress of DNA damage in cells, or calorie restriction in intact animals, increased levels of a rate-limiting NAD+ biosynthetic enzyme called nicotinamide phosphoribosyltransferase (Nampt). Forced expression of Nampt protected cells against apoptosis induced by the genotoxic agents methylmethane sulfonate (MMS), etoposide, or camptothecin. The protection required the activity of mitochondrial sirtuins (SIRT3 and SIRT4), but not other sirtuins, and they showed that overexpression of Nampt increased activity of SIRT3. The results fit a scenario where elevated Nampt would help cells survive by preventing NAD+ depletion, and preserving sirtuin function.

However, when the researchers actually measured cellular NAD+, the results were unexpected. While overexpression of Nampt did increase total NAD+ content in cells, as expected, it did not prevent the depletion of NAD+ triggered by MMS treatment. That prompted Yang and colleagues to develop a new technique to measure NAD+ levels specifically in mitochondria, and when they did, the results were quite different. Even when the rest of the cell was severely depleted of NAD+, mitochondria from cells that overexpress Nampt maintained normal levels of this important cofactor. By doing so, they protected cells from the initiation of apoptosis stimulated by MMS.

“One of the most surprising findings of the study was the observation that mitochondria can maintain physiological levels of NAD+ during genotoxic stress and promote cell survival, even if NAD+ in the cytoplasm and nucleus has fallen well below normal physiological levels,” the authors write. The mitochondria accomplish this feat by locally synthesizing NAD+ using their own supply of Nampt, the researchers show.

Fasted rats showed the same increased Nampt and mitochondrial NAD. This indicates that mitochondrial sirtuins could play a role in some of the health benefits of caloric restriction, which has been shown to lead to a longer life and delayed appearance of aging-related degenerative diseases. A paper in this week’s Journal of Neuroscience, from Angel Carrion and colleagues at the Universidad Pablo de Olavide de Sevilla in Spain, suggests that calorie restriction can even ameliorate aging-related memory and cognition defects by affecting synaptic plasticity in mice.

What of the Nampt/sirtuin pathway in neurons, and in neurodegenerative disease? “I don’t want to scoop myself, but I can say that the research raises the possibility that this major mode of cell protection is relevant to neurons,” Sinclair told ARF. “We are currently testing that and the initial results look promising,” he said. It is unclear at present how these new data fit with current drug development research on the SIRT1 inducer resveratrol and newer, more potent, compounds.—Pat McCaffrey.

References:
Yang H, Yang T, Baur JA, Perez E, Matsui T, Carmona JJ, Lamming DW, Souza-Pinto NC, Bohr VA, Rosenzweig A, de Cabo R, Sauve AA, Sinclair DA. Nutrient-Sensitive Mitochondrial NAD+ Levels Dictate Cell Survival. Cell. 2007 Sep 21;130:1095-1107. Abstract

Fontan-Lozano A, Saez-Cassanelli JL, Inda MC, de los Santos-Arteaga M, Sierra-Dominguez SA, Lopez-Lluch G, Delgado-Garcia JM, Carrion AM. Caloric restriction increases learning consolidation and facilitates synaptic plasticity through mechanisms dependent on NR2B subunits of the NMDA receptor. J Neurosci. 2007 Sep 19;27(38):10185-95. Abstract