PARP Inhibition Delays Progression of Mitochondrial Encephalopathy in Mice

Abstract

Mitochondrial disorders are deadly childhood diseases for which therapeutic remedies are an unmet need. Given that genetic suppression of the nuclear enzyme poly (adenine diphosphate-ribose) polymerase(PARP)-1 improves mitochondrial functioning, we investigated whether pharmacological inhibition of the enzyme affords protection in a mouse model of a mitochondrial disorder. We used mice lacking the Ndufs4 subunit of the respiratory complex I (Ndufs4 knockout [ KO] mice); these mice undergo progressive encephalopathy and die around postnatal day 50. Mice were treated daily with the potent PARP inhibitor N-(6-oxo-5,6-dihydrophenanthridin-2-yl)-(N,Ndimethylamino)acetamide hydrochloride (PJ34); neurological parameters, PARP activity, and mitochondrial homeostasis were evaluated. We found that mice receiving N-(6-oxo-5,6-dihydrophenanthridin-2-yl)-(N,N-dimethylamino)acetamide hydrochloride from postnatal day 30 to postnatal day 50 show reduced neurological impairment, and increased exploratory activity and motor skills compared with vehicle-treated animals. However, drug treatment did not delay or reduce death. We found no evidence of increased PARP activity within the brain of KO mice compared with heterozygous, healthy controls. Conversely, a 10-day treatment with the PARP inhibitor significantly reduced basal poly(ADP-ribosyl)ation in different organs of the KO mice, including brain, skeletal muscle, liver, pancreas, and spleen. In keeping with the epigenetic role of PARP-1, its inhibition correlated with increased expression of mitochondrial respiratory complex subunits and organelle number. Remarkably, pharmacological targeting of PARP reduced astrogliosis in olfactory bulb and motor cortex, but did not affect neuronal loss of KO mice. In light of the advanced clinical development of PARP inhibitors, these data emphasize their relevance to treatment of mitochondrial respiratory defects.

Pharmacological NAD-Boosting Strategies Improve Mitochondrial Homeostasis in Human Complex I–Mutant Fibroblasts

“…2E), which was consistent with the notion that suppression of PARP-1 activity promotes mitochondrial biogenesis in mice (Bai et al, 2011b). In light of our recent finding that both Phe and PJ34 increase expression of respiratory complex subunits in different tissues of Ndufs4 knockout mice (Felici et al, 2014), we asked whether this also occurs in human cells harboring a mitochondrial defect. NDUFS1-deficient fibroblasts were therefore exposed to PARP-1 inhibitors for 7 days, and expression levels of respiratory complex subunits encoded by nDNA or mtDNA were analyzed.…”

“…Still, both NR and PARP-1 inhibitors increased mitochondrial membrane potential, organelle content, as well as MTT reduction capacity. The inability of PARP-1 inhibitors to increase cellular NAD content is in contrast with prior work (Bai et al, 2011b), but consistent with a recent contribution showing that PJ34 does not augment NAD content in different organs of Ndufs4 knockout mice (Felici et al, 2014). At present, we do not know the reason(s) of this apparent inconsistency.…”

“…The latter result is in keeping with impairment of complex I-dependent respiration in NDUFS1 mutant fibroblasts (Iuso et al, 2006). We also quantified mitochondrial DNA (mtDNA) as an indicator of mitochondrial content (Lapucci et al, 2011;Felici et al, 2014), and found that it did not differ between mutant and control fibroblasts (Fig. 1D), suggesting therefore that mitochondrial dysfunction prompted by NDUFS1 mutation does not affect mitochondrial biogenesis.…”

“…At present, we do not know the reason(s) of this apparent inconsistency. Evidence that inhibitors of PARP-1 do not increase NAD content in mouse tissues (Felici et al, 2014) and human cells rules out the possibility that the inconsistency might be merely due to species differences. We speculate that different treatment paradigms and/or disease models may explain why treatment with chemical PARP-1 inhibitors is not invariantly related to NAD increases in cells or tissues.…”

“…Mitochondrial membrane potential was evaluated by means of flow cytometry (Felici et al, 2014). NADH dehydrogenase (ubiquinone) Fe-S protein 1 (NDUFS1) mutant fibroblasts were treated with vehicle, NR, or the PARP-1 inhibitors PJ34 (20 mM) or Phe (20 mM).…”