Lack of the mitochondrial NAD transporter SLC25A51 redistributes NAD+ compartmentalization and cellular ADP-ribosylation

Nicotinamide adenine dinucleotide (NAD+) is a vital small molecule with important redox capacity in oxidative phosphorylation (OXPHOS) and a key co-factor in various enzymatic reactions. The recent identification of the mitochondrial NAD+ transporter SLC25A51 provides strong evidence for a direct regulation of the mitochondrial NAD+ pool. Though the effect of this transporter on glucose metabolism has been described, its contribution to other NAD+-dependent processes such as ADP-ribosylation remains elusive. Here, we report that knockdown of SLC25A51 decreased the NAD+ concentration in mitochondria but increased the NAD+ concentration in the cytoplasm and nucleus. The increase in nuclear and cytoplasmic NAD+ was not due to the upregulation of the salvage pathway, thus pointing towards an overall redistribution of NAD+ from the mitochondria towards the cyto/nuclear compartment. Furthermore, the NAD+ redistribution induced by knockdown or knockout of SLC25A51 resulted, as quantified by immunofluorescence or analyzed by mass-spectrometry, in a loss of mitochondrial ADP-ribosylation and an increase of PARP1-mediated nuclear ADP-ribosylation under basal conditions. Further, MMS/Olaparib induced PARP1 chromatin retention and the sensitivity of triple-negative MDA-MB-436 breast cancer cells to PARP inhibition were both reduced upon knockdown of SLC25A51. In addition, H2O2-induced PARP1-dependent nuclear ADP-ribosylation was prolonged while phosphorylation of H2AX was unexpectedly reduced. Together these results provide evidence that lack of SCL25A51 and subsequently altered NAD+ compartmentalization affects not only mitochondrial and nuclear ADP-ribosylation but also other chromatin associated events.