Nat Metab. 2024 Dec;6(12): 2319-2337
Lena E Høyland,
Magali R VanLinden,
Marc Niere,
Øyvind Strømland,
Suraj Sharma,
Jörn Dietze,
Ingvill Tolås,
Eva Lucena,
Ersilia Bifulco,
Lars J Sverkeli,
Camila Cimadamore-Werthein,
Hanan Ashrafi,
Kjellfrid F Haukanes,
Barbara van der Hoeven,
Christian Dölle,
Cédric Davidsen,
Ina K N Pettersen,
Karl J Tronstad,
Svein A Mjøs,
Faisal Hayat,
Mikhail V Makarov,
Marie E Migaud,
Ines Heiland,
Mathias Ziegler.
The coenzyme NAD+ is consumed by signalling enzymes, including poly-ADP-ribosyltransferases (PARPs) and sirtuins. Ageing is associated with a decrease in cellular NAD+ levels, but how cells cope with persistently decreased NAD+ concentrations is unclear. Here, we show that subcellular NAD+ pools are interconnected, with mitochondria acting as a rheostat to maintain NAD+ levels upon excessive consumption. To evoke chronic, compartment-specific overconsumption of NAD+, we engineered cell lines stably expressing PARP activity in mitochondria, the cytosol, endoplasmic reticulum or peroxisomes, resulting in a decline of cellular NAD+ concentrations by up to 50%. Isotope-tracer flux measurements and mathematical modelling show that the lowered NAD+ concentration kinetically restricts NAD+ consumption to maintain a balance with the NAD+ biosynthesis rate, which remains unchanged. Chronic NAD+ deficiency is well tolerated unless mitochondria are directly targeted. Mitochondria maintain NAD+ by import through SLC25A51 and reversibly cleave NAD+ to nicotinamide mononucleotide and ATP when NMNAT3 is present. Thus, these organelles can maintain an additional, virtual NAD+ pool. Our results are consistent with a well-tolerated ageing-related NAD+ decline as long as the vulnerable mitochondrial pool is not directly affected.