Absence of mitochondrial SLC25A51 enhances PARP1-dependent DNA repair by increasing nuclear NAD+ levels

Though the effect of the recently identified mitochondrial NAD+ transporter SLC25A51 on glucose metabolism has been described, its contribution to other NAD+-dependent processes throughout the cell such as ADP-ribosylation remains elusive. Here, we report that absence of SLC25A51 leads to increased NAD+ concentration not only in the cytoplasm and but also in the nucleus. The increase is not associated with upregulation of the salvage pathway, implying an accumulation of constitutively synthesized NAD+ in the cytoplasm and nucleus. This results in an increase of PARP1-mediated nuclear ADP-ribosylation, as well as faster repair of DNA lesions induced by different single-strand DNA damaging agents. Lastly, absence of SLC25A51 reduces both MMS/Olaparib induced PARP1 chromatin retention and the sensitivity of different breast cancer cells to PARP1 inhibition. Together these results provide evidence that SLC25A51 might be a novel target to improve PARP1 inhibitor based therapies by changing subcellular NAD+ redistribution.

Dissection of DNA damage and repair pathways in live cells by femtosecond laser microirradiation and free-electron modeling

Understanding and predicting the outcome of the interaction of light with DNA has a significant impact on the study of DNA repair and radiotherapy. We report on a combination of femtosecond pulsed laser microirradiation at different wavelengths, quantitative imaging, and numerical modeling that yields a comprehensive picture of photon-mediated and free-electron-mediated DNA damage pathways in live cells. Laser irradiation was performed under highly standardized conditions at four wavelengths between 515 nm and 1,030 nm, enabling to study two-photon photochemical and free-electron-mediated DNA damage in situ. We quantitatively assessed cyclobutane pyrimidine dimer (CPD) and γH2AX-specific immunofluorescence signals to calibrate the damage threshold dose at these wavelengths and performed a comparative analysis of the recruitment of DNA repair factors xeroderma pigmentosum complementation group C (XPC) and Nijmegen breakage syndrome 1 (Nbs1). Our results show that two-photon-induced photochemical CPD generation dominates at 515 nm, while electron-mediated damage dominates at wavelengths ≥620 nm. The recruitment analysis revealed a cross talk between nucleotide excision and homologous recombination DNA repair pathways at 515 nm. Numerical simulations predicted electron densities and electron energy spectra, which govern the yield functions of a variety of direct electron-mediated DNA damage pathways and of indirect damage by ^•OH radicals resulting from laser and electron interactions with water. Combining these data with information on free electron-DNA interactions gained in artificial systems, we provide a conceptual framework for the interpretation of the wavelength dependence of laser-induced DNA damage that may guide the selection of irradiation parameters in studies and applications that require the selective induction of DNA lesions.

Subcellular Quantitation of ADP-Ribosylation by High-Content Microscopy

ADP-ribosylation is a posttranslational modification with many functions ranging from the DNA damage response to transcriptional regulation. While nuclear ADP-ribosylation has been extensively studied in the context of genotoxic stress mediated by PARP1, signaling by other members of the family and in other cellular compartments is still not as well understood. In recent years, however, progress has been made with the development of new tools for detection of ADP-ribosylation by immunofluorescence, which allows for a spatial differentiation of signal intensity for different cellular compartments. Here, we present our method for the detection and quantification of compartment-specific ADP-ribosylation by immunofluorescence and show why the engineered macrodomain eAf5121 might be the best tool to date.

Mitochondrial NAD+ Controls Nuclear ARTD1-Induced ADP-Ribosylation

In addition to its role as an electron transporter, mitochondrial nicotinamide adenine dinucleotide (NAD⁺) is an important co-factor for enzymatic reactions, including ADP-ribosylation. Although mitochondria harbor the most intra-cellular NAD⁺, mitochondrial ADP-ribosylation remains poorly understood. Here we provide evidence for mitochondrial ADP-ribosylation, which was identified using various methodologies including immunofluorescence, western blot, and mass spectrometry. We show that mitochondrial ADP-ribosylation reversibly increases in response to respiratory chain inhibition. Conversely, H₂O₂-induced oxidative stress reciprocally induces nuclear and reduces mitochondrial ADP-ribosylation. Elevated mitochondrial ADP-ribosylation, in turn, dampens H₂O₂-triggered nuclear ADP-ribosylation and increases MMS-induced ARTD1 chromatin retention. Interestingly, co-treatment of cells with the mitochondrial uncoupler FCCP decreases PARP inhibitor efficacy. Together, our results suggest that mitochondrial ADP-ribosylation is a dynamic cellular process that impacts nuclear ADP-ribosylation and provide evidence for a NAD⁺-mediated mitochondrial-nuclear crosstalk.