Circadian Control of DRP1 Activity Regulates Mitochondrial Dynamics and Bioenergetics

http://www.cell.com/cell-metabolism/abstract/S1550-4131(18)30063-9
 
Schmitt K, Grimm A, Dallmann R, Oettinghaus B, Restelli LM, Witzig M, Ishihara N, Mihara K, Ripperger JA, Albrecht U, Frank S, Brown SA, Eckert A

• Circadian regulation of dynamin-related protein 1 (DRP1), a key mitochondrial fission protein, results in daily cycles of fission and fusion which are essential for circadian oscillations in ATP production
• Genetic and pharmacological abrogation of DRP1 activity abolished circadian network dynamics and eliminated circadian ATP production

Myosin VI-Dependent Actin Cages Encapsulate Parkin-Positive Damaged Mitochondria

https://www.sciencedirect.com/science/article/pii/S1534580718300078

Antonina J. Kruppa,Chieko Kishi-Itakura, Thomas A. Masters, Joanna E. Rorbach, Guinevere L. Grice, John Kendrick-Jones, James A. Nathan, Michal Minczuk, Folma Buss

• The authors identify a protein MYO6 which triggers the formation of F-actin cages to form around damaged mitochondria (mitochondria were damaged using a variety of pharmacological means)
• These cages form a physical barrier, preventing damaged mitochondria from refusing with the network
• MYO6 interacts with other proteins known to recruit autophagosomes to damaged mitochondria

Affinity purification of cell-specific mitochondria from whole animals resolves patterns of genetic mosaicism

https://www.nature.com/articles/s41556-017-0023-x

Arnaud Ahier, Chuan-Yang Dai, Andrea Tweedie, Ayenachew Bezawork-Geleta, Ina Kirmes & Steven Zuryn

• The authors demonstrate a technique called Cell-specific mitochondrial affinity purification (CS-MAP) to yield intact, functional, mitochondrial with >96% enrichment, >96% purity at single-cell and single-animal resolution in C. elegans.
• CS-MAP consists of tagging the outer mitochondrial membrane protein TOMM-20 with a fluorophore and a particular epitope (which is something that an antibody can bind to) called HA. They placed the fusion protein under the control of a tissue-specific promoter so that e.g. muscle-specific mitochondria could be tagged. Mitochondria can then be purified using magnetic beads coated with anti-HA antibody and performing immunoprecipitation.
• The authors crossed worms containing the CS-MAP construct with animals containing mitochondrial DNA deletions, and analysed the heteroplasmy in the individual mitochondria purified from different cell types and compared this to homogenate heteroplasmy across the whole animal. The authors found that intestine and neurons had significantly lower heteroplasmy than the homogenate, whereas the germline had significantly higher heteroplasmy than the homogenate.
• The authors also quantified mtDNA copy number per mitochondrion in a number of different tissues, finding that the germline had ~3.5 mtDNAs per mitochondrion whereas tissues such as neurons and the intestine had ~1.5 mtDNAs per mitochondrion.
• Using three-dimensional reconstruction from fluorescence images, the authors found that individual germ cells contained 71.2+/-6.5 mtDNAs per cell, whereas neurons contained 14.4+/-0.5 mtDNAs per cell.
• The authors suggest that mtDNA turnover is higher in germ cells, which may account for their observations of increased copy number and heteroplasmy in the germline
  

Thoughts:

Check out a pre-print from our group with some ideas on how increases in copy number may be related to heteroplasmy, and thoughts about comparisons between homogenate heteroplasmy and cellular heteroplasmy

An energetic view of stress: Focus on mitochondria

https://www.sciencedirect.com/science/article/pii/S0091302218300062?via%3Dihub

Martin Picard, Bruce S McEwen, Elissa S Epel & Carmen Sandi

In this review, the authors discuss the link between mitochondria and mental stress. 

• Allostasis is defined as the active (i.e. energy-requiring) process of achieving stability, or homeostasis, through physiological or behavioural change. This includes neuroendocrine, autonomic, epigenetic, metabolic and immune changes, and is generally a short-term adaptation when regulated in a healthy setting.
• When allostatic mediators are not turned off, these same mediators can cause unhealthy changes in the brain and body: these are the pathophysiological consequences of stress. The authors refer to "allostatic load" as the pathophysiological consequences of chronic dysregulation of allostatic mediators.
• Metabolic intermediates that are the substrates or co-factors for epigenetic modifications are all derived from the Krebs cycle and other metabolic pathways within mitochondria. Some examples are discussed within the review. Hence, both the addition and removal of epigenetic marks are metabolically/mitochondrially regulated.
• Mitochondria are the site of synthesis for all steroid hormones, including glucocorticoids such as cortisol (the archetypal stress hormone), androgens such as testosterone and estrogens such as estriol. Norepinephrine and epinephrine are also hormones (called catecholamines) which are released in response to certain stressors. Enzymes involved in the degradation of catecholamines (MAO-A and MAO-B) are anchored to the outer mitochondrial membrane.
• Glucocorticoids (GCs) increase blood glucose levels by acting on the liver, skeletal muscles and adipose tissue by targetting the glucocorticoid receptor (GR). In the liver, GR activation has been shown to induce chromatin remodelling (an epigenetic effect). In skeletal muscle, GCs antagonize several elements of insulin signalling, and inhibits the uptake of pyruvate by mitochondria.
• Humans with higher circulating levels of cortisol under resting conditions also have higher levels of glucose, triglycerides and higher insulin resistance (essentially a pre-diabetic state). In mice, chronic GC administration results in glucose intolerance, elevated triglycerides, weight gain and depressive behaviour.
• Under healthy conditions, GCs are associated with the proper maintainance of a diurnal cycle.
• Some, but not all, synapses in many parts of the cerebral cortex turn over during the diurnal cycle. Interfering with the daily cycle of GCs can impair motor learning in humans.
• An animal model of shift work caused dendrites to shrink in the prefrontal cortex and the animal to become cognitively rigid, as well as gaining weight and becoming insulin resistant.

Mitochondrial levels determine variability in cell death by modulating apoptotic gene expression

https://www.nature.com/articles/s41467-017-02787-4

Silvia Márquez-Jurado, Juan Díaz-Colunga, Ricardo Pires das Neves, Antonio Martinez-Lorente, Fernando Almazán, Raúl Guantes & Francisco J. Iborra

Chemotherapies often leave a proportion of cancer cells behind. Even genetically identical cells grown in vitro show this effect, suggesting that there exists some level of non-genetic heterogeneity in cancer cells. 

Two well-known pathways are able to induce cell death: the intrinsic and extrinsic pathways. The intrinsic pathway (which does not involve signalling from outside of the cell) directly involves mitochondria. In contrast, the extrinsic pathway may be activated via the binding of specific ligands to cell death receptors on the plasma membrane of the cell and does not directly involve mitochondria. However, several proteins may participate in both the intrinsic and extrinsic pathways, meaning that these pathways have cross-talk. 

TNF-related apoptosis-inducing ligand (TRAIL) is a protein which may induce the extrinsic apoptosis pathway. When cells are treated with TRAIL, the authors observe that the fraction of cells which are killed saturates at 35% with the concentration of TRAIL, and that there is heterogeneity in the time to death at every concentration of TRAIL. The authors also observed (as noted previously by other authors) that sister cells tended to have the same fate and very similar times to death (Pearson correlation >0.8).

The authors investigated how mitochondrial content affected cell death propensity. They found that:

• Cells with higher mitochondrial mass (as determined by Mitotracker Green with 24 hr live-cell imaging) were more likely to die under TRAIL (as well as other cell death inducing drugs such as CHX and DRB).
• Cells show a weak correlation between time to death and mitochondrial mass (rho = -0.47) for intermediate TRAIL concentrations.
• After sorting cells into mito-high and mito-low fractions, (a fold-change of ~x5 between fractions), mito-high fractions had ~x3 more RNA than mito-low.
• Mitochondrial mass contributed to around 50% of the total variability observed in proteins which participate in apoptosis (both pro-apoptotic and anti-apoptotic).
• Including the observed correlation between mitochondrial mass and protein levels of apoptosis genes in a pre-existing mathematical model of the extrinsic cell death pathway in HeLa cells was able to recapitulate many of the authors' experimental observations.

The authors also investigated whether these results hold in real tumours, as opposed to cell culture conditions where environmental noise is minimised. The authors stained sections from colon cancer biopsies with antibodies against Aconitase 2 (for mitochondrial mass) and various cell death proteins. Whilst the mitochondrial contribution to variability for some apoptotic proteins was lost, others were retained (where pro-apoptotic proteins tended to have a higher correlation with mitochondrial mass than anti-apoptotic proteins).
 
Thoughts:
Does cell volume confound any of these observations?