F2,6BP restores mitochondrial genome integrity in Huntingtons Disease

Abstract

Several reports have indicated that impaired mitochondrial function contributes to the development and progression of Huntingtons disease (HD). Mitochondrial genome damage, particularly DNA strand breaks, is a potential cause for its compromised functionality. Here we show that the activity of polynucleotide kinase 3-phosphatase (PNKP), a critical DNA end-processing enzyme, is significantly decreased in the mitochondrial extract of HD patients brains due to a lower level of fructose-2,6 bisphosphate (F2,6BP), a biosynthetic product of 6-phosphofructo-2-kinase fructose-2,6-bisphosphatase 3 (PFKFB3). Such decrease in PNKP activity leads to persistent DNA strand breaks that are refractory to subsequent steps for repair completion. Both PFKFB3 and F2,6BP, an allosteric modulator of glycolysis, are also present in the mitochondria and PFKFB3 is part of a mitochondrial DNA repair complex containing HTT, PNKP, DNA Pol {gamma} (POLG) and Lig III. Notably, PNKP binds F2,6BP (Kd= 525{+/-}25 nM) and utilizes it as a cofactor. The levels of both F2,6BP and PFKFB3 are significantly decreased in the mitochondrial extract of HD mouse striatal neuronal cells and patients brain. Activity of PNKP is thus severely decreased in the mitochondrial extract; however, addition of F2,6BP restored its activity. Moreover, supplementation of F2,6BP in HD cells restored PFKFB3 level, mitochondrial genome integrity and partially restored mitochondrial membrane potential, mitochondrial respiration and prevented pathogenic aggregate formation. We also observed that supplementation with F2,6BP restored mitochondrial genome integrity in an HD Drosophila model. Our findings, therefore, suggest that F2,6BP-mediated restoration of PNKP activity could have a profound impact in ameliorating neurodegenerative symptoms in HD. SignificanceWe reported earlier the loss of PNKP activity in the nuclear extracts from HD patients brain. However, a glycolytic metabolite, F2,6BP, can restore PNKP activity and rescue organismal phenotypes in HD fly models. As PNKP is present in mitochondria and several reports indicate that mitochondrial dysfunction contributes to HD, we therefore analyzed PNKP activity in the mitochondrial extract. Surprisingly, we found that PFKFB3 and its product, F2,6BP are present in mitochondria, but significantly low in patients brains. Exogenous addition of F2,6BP restored PNKP activity in patients brain mitochondrial extract. Moreover, supplementing F2,6BP in HD cells and fruit flies restored mitochondrial genome integrity suggesting maintaining adequate intracellular F2,6BP levels is critical for proper functionality of PNKP and thereby of brain health.