Archive for the 'Institute of Healthy Ageing' Category

Damage and Fidelity: The Role of the Female Germline in mtDNA Inheritance

By Claire Asher, on 11 November 2013

Billions of years ago, one single-celled organism engulfed another, beginning a symbiotic interaction that would change live on Earth forever. The mitochondria are what remains of this symbiotic event, and are responsible for producing energy in all eukaryotic cells. Derived from a free-living organism, they carry their own genes, but these genes are at risk of damage by a natural by-product of energy production – free radicals. Mitochondrial DNA in most cells are exposed to these reactive oxygen species and may be damaged over time, resulting in some diseases of age. However, if damage occurs to the mitochondrial genes in egg and sperm cells, this damage would be passed on to the next generation. Despite this, aging is not heritable, and very few mitochondrial mutations are passed from one generation to the next. Researchers in GEE have been investigating the mechanism responsible for this apparent paradox – mitochondrial inactivation – and have discovered that this mechanism is extremely widespread in the animal kingdom.

The mitochondria are the powerhouses of the cell, generating energy through oxidative phosphorylation down an electron transport chain. The electron transport chain occurs across the mitochondrial membrane, and was a key innovation during the evolution of multicellular life. Mitochondria originated as free-living single celled organisms that were engulfed inside another cell and subsequently formed a cooperative partnership that allowed cells to produce energy more efficiently. Because of their symbiotic origin, mitochondria brought an entire genome with them, and although this has been wittled down to only a small number of genes, some genes still reside inside the mitochondria. This is a big problem, however, because in the process of producing energy, mitochondria also produce harmful reactive oxygen species (ROS), also known as free radicals. These can cause mutations in DNA, and the mitochondrial genes are therefore at great risk because of their proximity to the site of ROS production. Mutations in mitochondrial DNA are thought to be a key cause of age-related diseases.

The Electron Transport Chain
Image by Rozzychan, creative commons.

Mutations in DNA (mitochondrial or nuclear) in most cells in the body can be harmful to the health of the individual, but will have no influence on the next generation. The genes which we pass onto the next generation are separated off during early development into special ‘germ line’ cells which form sperm and eggs. Great care is taken to minimise the risk of mutation to these genes – genes in germline cells act as a blue print for the next generation. This is essentially why aging is not heritable, and it is a system that works pretty well.

Sperm Cells
Image by be_sperm

However, sperm and egg cells need mitochondria to produce energy, and so mitochondrial genes in our germ cells may still be at risk of mutation. If the free radicals generated in germ-line mitochondria harm mitochondrial DNA, these damages would be passed on to the next generation! Research in GEE has uncovered a rather elegant solution to this problem – those mitochondria that will be passed to the next generation are maintained in an inactive state. It’s a bit like buying two toasters and keeping one in a cupboard, unused, to provide a template from which to build a new toaster when the old one breaks.

Mitochondria are only inherited through the maternal line – every mitochondria in your body came from your mother, and this is true for most animal species. The mitochondria in sperm are generally discarded at some point prior to fertilisation. So, in order to preserve the fidelity of mitochondrial DNA passed on to the next generation, we only need to ‘switch-off’ mitochondria in egg cells. This is great, since sperm really need their mitochondria to provide energy for all that swimming!

Egg Cell

Previous research by John Allen and colleagues in GEE indicated that mitochondria in egg cells of the moon jellyfish (Aurelia aurita) are inactive compared to mitochondria in sperm and somatic tissues. Recently, GEE’s Prof John Allen, along with Wilson de Paula (Queen Mary University of London) and colleagues have investigated this phenomenon further and discovered that this system of mitochondrial inactivation is widespread across the animal kingdom. Using qPCR, a technique for measuring and comparing expression patterns of specific genes, they found that in both fruit flies (Drosophila melanogaster) and zebrafish (Danio rerio) expression of three key respiratory genes (nad1, cob & cox1) is much lower in mitochondria in oocytes (egg cells) than in sperm and active muscle tissue. Expression levels were 15-fold lower in eggs, whereas sperm and muscle showed similar levels of expression. They also found that membrane electrical potential, a measure of the activity of the electron transport chain, was reduced in oocytes compared to both sperm and the surrounding tissue. Further, ROS production was 50- and 100-fold lower in the eggs of fruit flies and zebrafish respectively. Finally, they confirmed that oocyte mitochodria in both species exhibit a simpler structure, indicative of reduced activity. So, it seems that in both fish and flies, the mitochondria in egg cells represent little more than a blueprint, ready to be passed on to the next generation error-free. By deactivating ovarian mitochondria, the fidelity of information is ensured across generations, and aging is not heritable.

Wilson de Paula and Prof John Allen have now identified a similar pattern of mitochondrial inactivation in species across the animal kingdom, including jellyfish, fruitflies and zebrafish. Early in multicellular evolution, animals branched into two key groups distinguished by differing patterns of embryonic development; protostomes (including arthropods, molluscs and nematodes) develop their mouth first, whereas deuterostomes (including vertebrates, tunicates and starfish) develop their anus first. This seemingly small difference represents a fundamental divide in the animal kingdom. This study therefore demonstrates that mitochondrial inactivation occurs in both of these key branches. Previous work by de Paula and Allen has shown a similar pattern in jellyfish, members of the phylum Cnidaria which pre-date the great protostome-deuterstome divide. Together, this work suggests that mitochondrial inactivation, as a mechanism to ensure fidelity of mitochondrial DNA transmission across generations, is likely to have emerged early in the evolution of multicellular life on Earth.

Ensuring the faithful transmission of genes to the next generation is a key problem for all life on Earth. Although the mitochondrial symbiosis event which marked the emergence of eukaryotic life was a major breakthrough in efficient cellular energy production, it brought problems of its own. Mitochondria must carry a few genes in order to maximise responsivness to cellular demands, but these genes are at risk of damage from a natural by-product of energy production – free radicals. A system of mitochondrial inactivation in female germ cells (eggs) may serve to resolve this conundrum, and seems to be shared across all animal life.

Original Article:

() Genome Biology and Evolution

This research was made possible by funding from the Natural Environment Research Council (NERC), and the The Leverhulme Trust

Go with your Gut:
How Gut Bacteria Influence Health and Ageing

By Claire Asher, on 9 July 2013

Inside the human gut is a thriving community of bacteria, many of which provide nutrients in exchange for a safe place to live. But they are not merely passive inhabitants – these microbes can also impact upon our metabolism. GEE researchers in the Institute of Healthy Ageing have been investigating how gut bacteria interact with medicinal drugs to influence lifespan and health.

The Nematode Worm,
Caenorhabditis elegans

There are 100 trillion microorganisms inhabiting the human gut – outnumbering human cells ten to one. These microorganisms are generally harmless, and in fact many are beneficial, forming a symbiotic relationship with their host. Occasionally, this relationship is less friendly, however, and gut bacteria can sometimes cause illness. Whether beneficial or harmful, gut bacteria can alter metabolic processes and chemical signalling pathways in the host, and recent research by Filipe Cabreiro, Catherine Au, David Gems and colleagues suggests gut microorganisms may interact with medicines to effect health and ageing.

Metformin: Beneficial Side Effects in Diabetes Treatment
A drug called Metformin is the most commonly prescribed treatment for type 2 diabetes, but it has long been noted that Metformin has some positive effects beyond those it is prescribed for, including reducing the risk of cancer. Metformin has also been found to slow the ageing process in rats and the nematode worm, Caenorhabditis elegans. This is partly mediated through Metformin’s effect on an enzyme, AMP-activated protein kinase (AMPK), which is important in influencing several key signalling pathways which have a knock-on effect on the ageing process. However, this is not the whole story – in nematodes Metformin also influences health and ageing through an interaction with microbes in the gut.

The Bacterium, Esterichia coli

Using a combination of techniques, including liquid-chromatograph mass spectrometry (LCMC) to visualise folate composition, and mutant E.coli strains and nematode worms lacking key metabolic cycles, Cabriero et al showed that E.coli are a critical aspect of the anti-aging properties of Metformin. The nematode, C.elegans, usually feeds on E.coli, but some of their microbial diet manages to avoid digestion and stick around in the nematode’s gut. Although the microbes aren’t really welcome there, and ultimately contribute to the worm’s death, they inadvertently have some beneficial effects during their stay. Worms reared on an E.coli-free diet do not show the usual positive effects of Metformin – in fact, without E.coli, metformin is toxic to the nematode. The researchers were able to track this bacteria-drug interaction to the B-vitamin, folate. Metformin substantially altered the folate content of E.coli, and when the researchers created a mutant strain of E.coli that lacked the ability to metabolise folate, nematode hosts experienced the same toxic effects were seen as when nematodes lacked the gut-bacteria entirely.

Interactions Between Host and Guest
So, metformin alters the folate cycle in the gut bacteria of nematode worms, and this in turn influences ageing in the worm. But how? There were no changes in the folate levels of the nematode worms treated with Metformin, indicating there must be another step in the process. This step appears to be mediated by methionine, an amino acid which is an important part of the nematode’s diet, obtained almost entirely from its gut bacteria. Metformin, by altering the folate cycle, reduces methionine production in E.coli, and this seems to be responsible for the anti-ageing effects on the worm host.

Metformin, Dietary Restriction and Ageing
Metformin, in interaction with the gut-bacteria, seems to mimic the effects of dietary restriction – the well-documented but still relatively poorly understood phenomenon by which reducing food intake increases lifespan. Anti-aging effects of dietary restriction have now been documented in rats, worms, and monkeys, and some people have even made the decision to reduce their food intake in the hopes that it will extend their lives. Research into the mechanisms by which dietary restriction is able to increase lifespan is on-going, and is a major focus of research in UCL’s Institute of Healthy Ageing. It is currently thought that dietary protein (and amino acids) may be important in controlling the effects of dietary restriction on ageing, and several chemical signaling pathways, including those regulated by AMPK, have been implicated in this process.

Thus, the anti-ageing properties of Metformin appear to be two-fold. By activating AMPK, Metformin may mediate life extension through its influences on key signalling pathways, essentially mimicking dietary restriction. Secondly, Metformin alters folate metabolism in microbial symbionts, reducing methionine production and again mimicking a restricted diet. Importantly, in the absence of the gut microbes, the drug showed strongly toxic effects, indicating that in some way, E.coli is able to ameliorate the drug’s toxicity. The effect of the drug Metformin is the result of a complex interplay between both toxic and beneficial effects on the host and the gut microbiota. Finally, all of this appears to interact with the diet of the nematode – feeding nematodes on a high-sugar diet eliminated the positive effects of Metformin.

Our gut bacteria are not passive hitch-hikers, but instead interact with our metabolic processes and dietary intake in complex ways. The effects of medicine can be strongly dependent upon our gut microbiome, and understanding this may help us to understand why the same drugs have different effects on different people. Drugs chosen for one beneficial effect may also have other, unintended effects (both positive and negative), but the extent and type of these may be heavily dependent upon our diet and gut bacteria. By viewing our bodies as a complex ecosystem of human cells interacting with a large and varied microbial population, we can gain greater insights into the workings of our bodies, and how to improve our health and lifespan.

Original Article:

() Cell 153: 228 – 239

This research was made possible by funding from the Wellcome Trust, The European Union and the Medical Research Council.