Mitochondrial PARP1 regulates NAD+-dependent poly ADP-ribosylation of mitochondrial nucleoids

PARPs play fundamental roles in multiple DNA damage recognition and repair pathways. Persistent nuclear PARP activation causes cellular NAD + depletion and exacerbates cellular aging. However, very little is known about mitochondrial PARP (mtPARP) and poly ADP-ribosylation (PARylation). The existence of mtPARP is controversial, and the biological roles of mtPARP-induced mitochondrial PARylation are unclear. Here, we demonstrate the presence of PARP1 and PARylation in purified mitochondria. The addition of the PARP1 substrate NAD + to isolated mitochondria induced PARylation, which was suppressed by treatment with the inhibitor olaparib. Mitochondrial PARylation was also evaluated by enzymatic labeling of terminal ADP-ribose (ELTA). To further confirm the presence of mtPARP1, we evaluated mitochondrial nucleoid PARylation by ADP ribose-chromatin affinity purification (ADPr-ChAP) and PARP1 chromatin immunoprecipitation (ChIP). We observed that NAD + stimulated PARylation and TFAM occupancy on the mtDNA regulatory region D-loop, inducing mtDNA transcription. These findings suggest that PARP1 is integrally involved in mitochondrial PARylation and that NAD + -dependent mtPARP1 activity contributes to mtDNA transcriptional regulation. Aging: Key DNA repair protein found in mitochondria An enzyme critical to healthy DNA repair may help scientists clarify the mechanisms behind mitochondrial dysfunction in aging. As humans age, DNA damage increases, triggering increased activity of the poly [ADP-ribose] polymerase 1 enzyme (PARP1). However, persistent PARP1 activation may be detrimental and could even accelerate aging. PARP1 activity in cell nuclei is well documented, but whether the enzyme is present in mitochondria is disputed. Vilhelm Bohr at the National Institute of Aging, Baltimore, USA, and co-workers conducted experiments on purified mitochondria from human cells and found that mitochondrial PARP1 (mtPARP1) is integral to the mitochondrial matrix and binds to mtDNA. Just as in the cell nucleus, mtPARP1 requires a specific metabolic cofactor for its activity. The mitochondrial enzyme may act as a sensor for this cofactor, regulating signaling for mtDNA transcription.