The mitochondrial DNA polymerase gamma degrades linear DNA fragments precluding the formation of deletions.

The mitochondrial DNA polymerase gamma degrades linear DNA fragments precluding the formation of deletions.

I posted recently how two labs have published data showing that the mitochondrial DNA  (mtDNA) polymerase gamma (POLGA) would degrade linear mtDNA fragments (Peeva et al. 2018) or mtDNA under starvation (Medeiros et al. 2018). Now a third group published similar results and they come from the lab of Carlos Moraes so you know are going to enjoy it (Nissanka et al. 2018).

In this study they used both mouse embryonic fibroblasts (MEFs) and actual mice. First, they expressed mitochondrially targeted restriction enzymes to produce linear mtDNA fragments and followed how quickly these are degraded in WT MEFs and mtDNA mutator MEFs. Clearly, MEFs expressing the exonuclease-deficient POLGA were not efficient in removing the linear mtDNA fragment which is a similar result to the Peeva et al. study.

Previously, Medeiros et al. and Peeva et al studied the degradation of mtDNA in yeast and cell culture, respectively, but it was unclear whether this would also take place in vivo. Moraes lab is rather experienced in introducing enzymes (restriction enzymes and mitoTALENs) into mitochondria in mice and as could be expected they introduced these mitochondrially targeted restriction enzymes into mice using adenovirus to study mtDNA degradation. As a result, it seems that POLGA is participating to the degradation of linear mtDNA fragments also in vivo.

Nissanka et al. also assessed whether the presence of linear mtDNA leads to mtDNA rearrangements such as circular mtDNA molecules with deletions. This seems to indeed be the case and it would be interesting to know whether these rearrangements would also take place in patients carrying pathogenic mutations in the replication machinery proteins. These results also suggest that mitochondrial zinc fingers (mtZFN) and mitoTALENs (Gammage et al. 2017) might have some unintended consequences. Both of these approaches are based on cutting the mtDNA molecules carrying a pathogenic mutation leading to the degradation of the molecule. In the ideal case, the loss of these pathogenic molecules would be replaced by the replication of the WT mtDNA molecules. Based on the results of Nissanka et al. in the non-ideal case the presence of these linear molecules could increase the amount of mtDNA rearrangements.

References:

Medeiros TC, Thomas RL, Ghillebert R, Graef M. Autophagy balances mtDNA synthesis and degradation by DNA polymerase POLG during starvation. J Cell Biol. 2018. PMID: 29519802

Nissanka N, Bacman SR, Plastini MJ, Moraes CT. The mitochondrial DNA polymerase gamma degrades linear DNA fragments precluding the formation of deletions. Nat Commun. 2018. PMID: 29950568

Gammage PA, Moraes CT, Minczuk M. Mitochondrial Genome Engineering: The Revolution May Not Be CRISPR-Ized. Trends Genet. 2017. PMID: 29179920

Peeva V, Blei D, Trombly G, Corsi S, Szukszto MJ, Rebelo-Guiomar P, Gammage PA, Kudin AP, Becker C, Altmüller J, Minczuk M, Zsurka G, Kunz WS. Linear mitochondrial DNA is rapidly degraded by components of the replication machinery. Nat Commun. 2018. PMID: 29712893

Impact of exercise on oocyte quality in the POLG mitochondrial DNA mutator mouse.

Mitochondrial DNA (mtDNA) mutator mouse carries a proofreading-deficient mitochondrial DNA polymerase leading to accumulation of mtDNA mutations. This mouse was originally engineered in two independent labs (Trifunovic et al. 2004, Kujoth et al. 2005) and has since been used to study various aspects of mitochondrial dysfunction.

Several studies have tried to find ways to improve the various phenotypes of the mtDNA mutator mouse, which include sarcopenia, hearing loss, osteoporosis, alopecia, weight loss, testicular atrophy, enlarged heart etc. For instance, our group showed recently that increasing mtDNA copy number of mtDNA mutator mice can partially rescue the testicular atrophy phenotype (Jiang et al. 2017)

It was published already seven years ago that exercise might improve the phenotype of the mitochondrial DNA (mtDNA) mutator mouse (Safdar et al. 2011). Like most papers using mtDNA mutator mouse as the model organism, Safdar et al. has the shortcoming that their WT control mice also inherited mtDNA mutations from a heterozygous mtDNA mutator mother, a.k.a. they made a “dirty” breeding (Kauppila et al. 2017). Therefore, this study had no “true” WT mouse model as a control. Also more recently, two papers from these authors have now Editorial Expression of Concerns (JBC study, PNAS study) so let’s see what happens with these findings.

Now a recent paper from a different group suggests that exercising mtDNA mutator mice might affect oocyte quality (Faraci et al. 2018). It was not reported how these mice have been bred, so one should be very cautious when interpreting the results. For instance, it would be important to know how many generations the heterozygous mtDNA mutator females have been bred and whether the WT control mice are a WT littermate from a heterozygote X heterozygote cross.

All in all, exercise has almost no effect on the oocytes of mtDNA mutator mice. For some of the differences seen it is also impossible to tell whether they are caused by changes in oocytes themselves of some systemic changes.

References:

Editorial Expression of Concern: Endurance exercise rescues progeroid aging and induces systemic mitochondrial rejuvenation in mtDNA mutator mice. Proc Natl Acad Sci U S A. 2018. PMID: 29891666

Expression of concern for: Exercise increases mitochondrial PGC-1 α content and promotes nuclear-mitochondrial cross-talk to coordinate mitochondrial biogenesis. J Biol Chem. 2018. PMID: 29602880

Faraci C, Annis S, Jin J, Li H, Khrapko K, Woods D. Impact of exercise on oocyte quality in the POLG mitochondrial DNA mutator mouse. Reproduction. 2018. PMID: 29875308

Jiang M, Kauppila TES, Motori E, Li X, Atanassov I, Folz-Donahue K, Bonekamp NA, Albarran-Gutierrez S, Stewart JB, Larsson NG. Increased Total mtDNA Copy Number Cures Male Infertility Despite Unaltered mtDNA Mutation Load. Cell Metab. 2017. PMID: 28768180

Kauppila TES, Kauppila JHK1, Larsson NG. Mammalian Mitochondria and Aging: An Update. Cell Metab. 2017. PMID: 28094012

Kujoth GC, Hiona A, Pugh TD, Someya S, Panzer K, Wohlgemuth SE, Hofer T, Seo AY, Sullivan R, Jobling WA, Morrow JD, Van Remmen H, Sedivy JM, Yamasoba T, Tanokura M, Weindruch R, Leeuwenburgh C, Prolla TA. Mitochondrial DNA mutations, oxidative stress, and apoptosis in mammalian aging. Science. 2005. PMID: 16020738

Safdar A, Bourgeois JM, Ogborn DI, Little JP, Hettinga BP, Akhtar M, Thompson JE, Melov S, Mocellin NJ, Kujoth GC, Prolla TA, Tarnopolsky MA. Endurance exercise rescues progeroid aging and induces systemic mitochondrial rejuvenation in mtDNA mutator mice. Proc Natl Acad Sci U S A. 2011. PMID: 21368114

Trifunovic A, Wredenberg A, Falkenberg M, Spelbrink JN, Rovio AT, Bruder CE, Bohlooly-Y M, Gidlöf S, Oldfors A, Wibom R, Törnell J, Jacobs HT, Larsson NG. Premature ageing in mice expressing defective mitochondrial DNA polymerase. Nature. 2004. PMID: 15164064