Friday, 30 January 2015

Replication-transcription switch in human mitochondria

Karen Agaronyan, Yaroslav I. Morozov, Michael Anikin and Dmitry Temiakov

Mitochondrial DNA (mtDNA) replication coincides with transcription in both time and space. As such, there is potential for collisions between the separate machineries of these processes to cause detrimental effects. This study elucidates the mechanisms of decision making, between transcription and replication. The authors investigate the effects of human transcription elongation factor (TEFM) binding to mitochondrial RNA polymerase (mtRNAP). The authors suggest that mtRNAP initiates replication of mtDNA when unbound to TEFM,  by transcribing short 120nt replication primers and then falling off of the mtDNA. However, when bound to TEFM, mtRNAP transcribes the entire heavy strand, ready for translation into protein. Thus, TEFM behaves as a molecular switch, toggling between replication and transcription of mtDNA, suggesting they are mutually exclusive processes.

Thursday, 29 January 2015

Antibiotics that target mitochondria effectively eradicate cancer stem cells, across multiple tumor types: Treating cancer like an infectious disease

R. Lamb , B. Ozsvari, C.L. Lisanti et al.

It is becoming increasingly clear that cancer stem cells play an important role in tumorigenesis. A key observation is that these cells are highly dependent on mitochondrial oxidative phosphorylation. In this paper, the authors raise the fact that a number of FDA-approved drugs inhibit mitochondrial biogenesis as one of their side-effects. This is because many antibiotics target bacterial ribosomes, which share many similarities with the mitochondrial ribosome. They show that a number of existing antibiotics can reduce tumour-sphere number in a dose-dependent manner: for instance Doxycycline can eradicate two breast cancer cell lines at doses as low as 50um. This approach is perhaps a step towards mutation-independent cancer therapy.

Wednesday, 28 January 2015

Mitochondrial dynamics and viral infections: A close nexus

Mohsin Khan, Gulam Hussain Syed, Seong-Jun Kim and Aleem Siddiqui

This paper discusses how viruses manipulate cellular machinery, in particular mitochondria, for their own good. It makes sense for viruses to target mitochondria because it gives them control over the energy production of the cell: the more energy, the more viruses can be made. Perhaps more importantly, by targeting mitochondria the virus may have some control over the survival of the cell as mitochondria are involved in apoptosis. The virus wants to keep the cell alive for as long as possible in order to produce many copies of itself before bursting out the cell and killing it. The paper focusses on how viruses influence mitochondrial dynamics, and how this may influence cell survival. Several viruses and their effects on mitochondrial dynamics are discussed.

Hepatitus C viruses (HCV) cause ER stress, release of calcium from the ER and subsequent uptake of calcium by mitochondria which then depolarize and become dysfunctional. Proteins of HCV can associate directly with the mitochondria and localize to the outer mitochondria membrane, or the mitochondria-associated-ER membrane (MAM). Once associated with MAM, HCV proteases are able to cleave mitochondria associated antiviral signalling proteins (MAVS). MAVS play an important role in immune signalling, and by cleaving MAVS, the virus may be able to evade an immune response. Other HCV proteins are able to perturb the activity of complex I of the respiratory chain, promote mitochondrial calcium uptake and promote ROS production. The virus also messes with mitochondrial quality control, by altering Drp-1 phosphorylation which causes mitochondria to fragment, and increasing the expression of Parkin and PINK1 which promotes mitophagy. When Parkin and Drp1 were depleted from HCV-infected cells, the virus could not induce fragmentation and mitophagy any more, but suddenly apoptotic signalling and cytochrome c release increased, with apoptosis as a consequence. It may thus be that the HCV induced fission followed by mitophagy boosts mitochondrial quality control and helps to prevent cytochrome c release, so that the cell survives longer.

Besides hepatitus C, they also discuss the hepatitus B virus, the Epstein–Barr virus, Human cytomegalovirus, Pseudorabies virus, Influenza virus, Measles virus, Newcastle disease virus, and the SARS coronavirus. Most of these virusses trigger mitochondrial fission, except for the SARS coronavirus, which causes mitochondria to fuse. Why does this virus work differently? Read the paper, and perhaps the answer will be there..

Inhibition of oxidative metabolism leads to p53 genetic inactivation and transformation in neural stem cells

Stefano Bartesaghi, Vincenzo Graziano, Sara Galavotti et al.

Evasion of cell death is known to be one of the hallmarks of cancer. As we know, mitochondria are central organelles to the execution of cell death, and are observed to be perturbed in cancer. In this study, the authors perturb the mitochondria of neural progenitor stem cells in two ways. Firstly, through knockdown of NDUFA10 (a subunit of complex I), they find a shift to glycolytic metabolism and an increase in cell confluence (i.e. cell growth). They then interfere with the gene TK2, which is involved in mtDNA biosynthesis. Perhaps surprisingly, they find TK2 KO cells have a higher ATP yield that the wild-type, despite their glycolytic shift. These cells also had increased cell diameter, an accumulation of cells in S-phase and resistance to differentiation. They found that these KO cells did not tend to express the pro-apoptotic transcription factor p53, rather a shorter isoform Δp53, and were more susceptible to neoplastic transformation.

Wednesday, 14 January 2015

Transfer of mitochondria via tunneling nanotubes rescues apoptotic PC12 cells

X Wang and H-H Gerdes

It has previously been shown that cells have the ability to transfer mitochondria through the formation of tunnelling nanotubes (TNTs), which can extend from one cell to another. The authors use UV light to promote apoptosis in a culture of pheochromocytoma 12 cells, and show that cell death can be rescued through coculture with healthy cells. They show that after cytochrome-c release, but before caspase-3 activation, stressed cells extend microtubules out to healthy cells. The healthy cells then donate healthy mitochondria to the stressed cells, causing a reduction in cell death. Inhibition of TNT formation almost eliminates the ability of healthy cells in coculture to rescue cell death.

Friday, 9 January 2015

Variation in cancer risk among tissues can be explained by the number of stem cell divisions

Cristian Tomasetti and Bert Vogelstein

Most cells in tissues are partially or fully differentiated, typically short-lived and unlikely to be able to initiate a tumour. However, stem cells have the capacity to self-renew and can be involved in tumourigenesis. It is well known that environmental factors, such as carcinogens, can increase cancer risk; but also that different tissues are intrinsically more predisposed to neoplastic transformation than others. The authors investigate which of these factors explains lifetime cancer risk.

They find that the total number of stem cell differentiations in a particular tissue, correlates strongly (0.804) with the lifetime risk of cancer in that tissue. They calculate that 65% (39% to 81%; 95% CI) of the variance in cancer risk can be attributed to the total number of stem cell divisions in the tissue. The authors propose classifying cancers into two categories: deterministic tumours (D-tumours) which are heavily influenced by the environment, and replicative tumours (R-tumours) which are driven by mainly stochastic factors. They use unsupervised clustering to find that most are R-tumours.

Wednesday, 7 January 2015

Evidence for frequent and tissue-specific sequence heteroplasmy in human mitochondrial DNA

Jana Naue, Steffen Hörer, Timo Sänger et al.

The authors measured heteroplasmy levels across 100 individuals taken during autopsy. They examined a number of tissues: blood (control), buccal cells, liver, brain, muscle, heart, lung, bone and hair, with 883 samples in total across a range of ages. They find that muscle and liver cells are the most susceptible tissues to developing mtDNA mutations (79% and 69% of individuals respectively), with only 12 individuals displaying no mutations whatsoever in the measured tissues. Bone (19.8%), blood (18%), lung (17%) and buccal cells (16.2%) showed the fewest number of individuals with mtDNA mutations. They find a strong correlation between the mean number of heteroplasmies in muscle and age (r=0.746).