Tuesday, 29 April 2014

The physiological role of mitochondrial calcium revealed by mice lacking the mitochondrial calcium uniporter


In this paper, mice lacking the calcium uniporter are investigated to see what consequences this has on mitochondrial calcium uptake, bioenergetics and cell death.

Mice lacking the calcium uniporter could not take up calcium rapidly. The basal mitochondrial matrix calcium concentration (from skeletal muscle cells) was reduced by 75% compared to that of wild type cells. Despite this, the uniporter-lacking cells showed no change in basal oxygen consumption and also the maximal maximal oxidative capacity was the same in wilde-type and uniporter-lacking mice.

Because of the lower calcium levels in uniporter-lacking mice, phosphorylation levels of the pyruvate dehydrogenase were increased which reduces its activity. This difference in phosphorylation levels was only seen in starved conditions, but dissapeared once the mice were fed. Because there were hardly any defects in basal metabolism of uniporter-lacking cells, the in vivo effects of altering matrix calcium may be most important under certain stress conditions.

When mice were placed on a treadmill, the uniporter-lacking mice were less able to exercise than the control group.

The mitochondrial permeability transition pore (PTP) opens if extramitochondrial calcium concentrations rise above a certain value. This behaviour was not seen in cells lacking the uniporter.

Effects on apoptosis
Cells were exposed to agents that induce cell death (such as tunicamycin,   doxorubicin and  thapsigargin). There was no change in the kinetics or magnitude of cell death in MEFs with or without the uniporter. Levels of cytosolic calcium and the release of cytochrome c were also the same in control and uniporter-lacking cells during cell death.

Thursday, 24 April 2014

calorie restriction changes mitochondrial ultrastructure


CR (Calorie restriction) leads to a longer life span, less cancer and fewer mitochondrial diseases. What is the effect of CR on mitochondria?

In this study they show that CR induced
  • increases in mitochondrial mass
  • increases in mitochondrial size
  • increases in the number of mitochondria per cell
  • increases in the number of cristae per mitochondrion
  • increases in the mean cristae length
  • increases in mitochondrial biogenesis.

They found that CR caused slight decreases in levels of Drp1 and Fis1. They measured only total levels of Drp1, not its phosphorylation levels. They did not find significant changes in the levels of the fusion proteins OPA1, MFN1 and MFN2.

Inverse hormesis of cancer growth mediated by narrow ranges of tumor-directed antibodies

Inverse hormesis of cancer growth mediated by narrow ranges of tumor-directed antibodies

Low levels of antibodies inside of a tumour can in fact stimulate tumour growth, by causing inflammation. This switches on pathways such as NF-κB and STAT3. However, it has long been thought that the immune system, if properly engaged, could be used to eliminate cancer cells. Here, the authors show that antibodies targeting Neu5Gc (which accumulates in human tumours) have a dose-dependent affect on tumour mass. In particular, low levels promote tumour growth, but over a surprisingly narrow dose range, the antibody suddenly begins to inhibit tumour growth.

Leaky mitochondria make you lose weight - Mitochondrial uncoupling as a treatment for obesity


In the 1930s, DNP (2,4-dinitrophenol) was introduced as a drug to treat obesity. What does DNP do? It causes mitochondrial uncoupling, i.e. it increases proton leak and as a result energy production becomes less efficient. Patients treated with DNP therefore 'waste' more energy and metabolic rate increases. An increase in energy output, while keeping the energy intake constant, means that people will start losing weight.

How does DNP work exactly? DNP is a lipid-soluble weak acid. It can pick up protons which it then carries to the other side of the membrane.  It then drops the proton, crosses the membrane again, picks up another proton, etc..

Patients treated with DNP reported feelings of warmth and increased sweating. Doses of 5 mg/kg were tolerated well and there was no increase in heart rate. Patients receiving daily  doses of 3-5 mg/kg experienced a 40% increase in metabolic rate that was maintained for at least 10 weeks. After those 10 weeks, a mean weight loss of 7.8 kg was observed. There was no need for additional dieting.

The increase in metabolic rate was clearly dependent on increase in DNP dose with an average increase of 11% for each dosage increment of 0.1g of DNP.

Because of this success, the drug became popular and it was used more and more. By 1934, about 100,000 people had been treated with the drug. However, the drug was used by inexperienced physicians who had no access to metabolic rate measurements to determine optimal doses. This lead to several people being `literally cooked to death' and the drug was taken off the market (there are reports, however, that the drug is still prescribed by US clinics)

The changing colour of fat

The changing colour of fat

It appears that fat cells come in three distinct forms: white fat cells, which specifically store energy in the form of lipids; brown fat cells, found around the head and neck which burn energy to release heat; and beige fat cells which are an immature form of brown fat cells, existing in white fat tissue. 

None of these cells are inherently "good" or "bad" under normal conditions; however, during obesity, white cells expand and proliferate at a rate faster than the development of vasculature causing a hypoxic environment. HIF-1α, a factor associated with cancer, is recruited to the tissue causing excess production of connective tissue (fibrosis). These fat cells also attract immune cells, become inflamed, develop metabolic disease and insulin resistance, which can result in diabetes.

Caenorhabditis elegans pathways that surveil and defend mitochondria

Caenorhabditis eleganspathways that surveil and defend mitochondria

Iron is a valuable resource inside the cell. The mitochondrial electron transport chain is rich in haem and iron sulphur proteins, making it an attractive target upon bacterial infection. The authors show that disruption of mitochondrial function, through drugs such as antimycin or interference RNAs (RNAis), induces xenobiotic-detoxification, mitochondrial repair and pathogen response pathways in C. Elegans. They performed an RNAi screen to identify 45 RNAis that are involved in such pathways.