Friday 25 May 2018

Mitochondrial Translation Efficiency Controls Cytoplasmic Protein Homeostasis

https://www.sciencedirect.com/science/article/pii/S1550413118302547?via%3Dihub

Suhm T, Kaimal JM, Dawitz H, Peselj C, Masser AE, Hanzén S, Ambrožič M, Smialowska A, Björck ML, Brzezinski P, Nyström T, Büttner S, Andréasson C, Ott M

  • As mitochondria possess their own DNA, they also possess their own transcription and translation machinery. 
  • This study focuses on the mitochondrial translation machinery. There exists a tradeoff between the accuracy with which mRNAs are translated into protein, and the speed at which proteins can be formed.
  • The mitoribosome is structurally similar to the bacterial ribosome. Several mutations are known for the bacterial ribosome which can increase or decrease the fidelity of translation. The authors sought to determine the effects of such mutations in yeast. 
  • The authors investigated two mutations in particular: P50R which caused reduced fidelity of translation, and K71T which increased fidelity (likely at the cost of slower translation). Both mutants caused growth defects on galactose (a respiratory substrate which forces oxidative phosphorylation). 
  • The authors observed that the K71T hyperaccurate mutant displayed extended lifespan. These mutants also had lower levels of reactive oxygen species and fewer protein aggregates. The authors suggest that hyperaccurate translation inside mitochondria mitigates ROS-induced aggregation of proteins.

Wednesday 23 May 2018

Restoring mitochondrial DNA copy number preserves mitochondrial function and delays vascular aging in mice

https://onlinelibrary.wiley.com/doi/abs/10.1111/acel.12773

Kirsty Foote, Johannes Reinhold, Emma P. K. Yu, Nichola L. Figg, Alison Finigan, Michael P. Murphy, Martin R. Bennett.

INTRODUCTION

Ageing of the large conduit arteries is a major cause of morbidity and mortality, contributing to hypertension. Arterial ageing is associated with multiple structural and functional changes, including vessel dilatation and wall thickening, loss of elastin and deposition of collagen.

Several invasive and noninvasive parameters of vascular stiffness can reliably predict cardiovascular events. However, furthering research in mouse models is not easy. Improved animal welfare means that laboratory mice can now live more than 2 years. This complicates ageing research, since such aged animals might be too frail for functional analyses, and makes ageing studies very long and expensive to perform. Furthermore, it is unclear what the earliest time points that constitute vascular ageing in laboratory mice are, which physiological measures of large artery stiffness correspond most closely to humans.

It is unclear whether decreased mitochondrial function promotes vascular ageing directly or is just a consequence of ageing.

The authors examine multiple parameters of vascular function, histological markers, and markers of mitochondrial damage and function during normal vascular ageing, and the effects of reducing or augmenting mitochondrial function on the onset and progression of vascular ageing.


RESULTS
The authors show that:

  • Vascular ageing in mice can be demonstrated by changes in a variety of physiological parameters, with multiple robust reproducible markers appearing as early as 44 wk (earlier than previously thought, allowing for shorter vascular ageing protocols).
  • Mouse vascular ageing is associated with characteristic structural changes over the same time, confirming that these changes in physiological parameters represent structural changes associated with ageing.
  • Mitochondrial copy number (mtCN), the proteins that regulate it, and mitochondrial respiration are all reduced at the same age that changes in functional and structural parameters were observed.
  • Finally, using gain- and loss-of-mitochondrial-function mouse models, we identify that mtCN and mtDNA integrity directly regulate the onset and progression of vascular ageing in mice. In other words, manipulations that result in increased or decreased respiration delay or accelerate changes associated with ageing, respectively.
  • The mouse models used were mice overexpressing the helicase Twinkle, to increase mtCN and mitochondrial respiration, and the PolG mutator mice, to compromise mtDNA integrity. These mice do not show change of ROS (at least in early life for the PolG mice, which is when these mice were examined). This suggests that the role of mitochondria in vascular ageing goes beyond ROS.

Thursday 3 May 2018

Linear mitochondrial DNA is rapidly degraded by components of the replication machinery

https://www.nature.com/articles/s41467-018-04131-w

Peeva V, Blei D, Trombly G, Corsi S, Szukszto MJ, Rebelo-Guiomar P, Gammage PA, Kudin AP, Becker C, Altmüller J, Minczuk M, Zsurka , Kunz WS

  • Tools such as mitoTALENs and mitochondrial ZFNs are able to cut mtDNA in a sequence-specific manner. Whilst these methods are known to be able to reduce the proportion of mutant mtDNAs, the mechanism by which linear mtDNAs are degraded is unknown.
  • The authors show that linear mtDNA is eliminated within hours
  • Inactivation of the mitochondrial exonuclease MGME1; inhibition of the exonuclease activity of POLG (see also this complementary study); or knockdown of the mitochondrial DNA helicase TWNK leads to severe imediment of mtDNA degradation. 
  • Failure to remove damaged mtDNA leads to the accumulation of abnormal linear and rearranged molecules.
  • Degradation of linearized mtDNA is performed by the same machinery that is responsible for mtDNA replication.

Mitochondrial nicotinamide adenine dinucleotide reduced (NADH) oxidation links the tricarboxylic acid (TCA) cycle with methionine metabolism and nuclear DNA methylation

http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.2005707

Lozoya OA, Martinez-Reyes I, Wang T, Grenet D, Bushel P, Li J, Chandel N, Woychik RP, Santos JH

  • The authors use an inducible dominant-negative mutant of the mtDNA polymerase POLG in human embryonic kidney cells (HEK293), see here (DN-POLG cells).
  • The authors perform RNA-seq on cells with induced DN-POLG at 0, 3, 6 and 9 days after treatment, with approximately 100%, 30%, 10% and 0% mtDNA copy number respectively.
  • Loss of mtDNA resulted in DNA hypermethylation. 57% of differentially expressed genes showed significant alteration in their promoter methylation compared with day 0.
  • The authors expressed two non-mammalian proteins (NDI1/AOX, see here) which by-pass the mitochondria allowing a normal NAD+/NADH balance to be maintained. In doing this, the authors found far fewer differentially expressed genes when comparing day 0 to day 9 cells. Furthermore, no significant changes in DNA methylation were observed in cells expresing NDI1/AOX. 
  • The authors suggest that mtDNA depletion induces imbalance in the NAD+/NADH pool, causing DNA methylation and pathological gene transcription