By William Carter
It is doubtful that any single mechanism exists in our biology to enact senescence. Rather, it is thought the revolutions and cycles of an intricate multiplicity of interlocking cellular cogs and gears is what dictates the slow, assured countdown of our personal doomsday clocks. One particular complication orchestrating our mortality is the class of Reactive Oxygen Species.
In order for our bodies to function, a means by which energy can be transferred from our food to our cells is required. This essential role is played by countless tiny organelles called mitochondria. Mitochondria are specifically designed to perform cellular respiration, the process of converting energy from broken-down food molecules, such as glucose, into an incredibly important compound called adenosine triphosphate (ATP). ATP is used by our cells as a source of energy and is essential to our very existence.
There is a slight issue with cellular respiration, however. The very core process that fuels our cells also produces dangerous by-products, whose accumulation in our body is now thought to contribute to ageing. These are Reactive Oxygen Species (ROS) or Free Radicals – highly reactive molecules or ions that are mainly formed when electrons ‘leak’ out of the mitochondria during cellular respiration (Cooke et al, 2003) and combine with oxygen (Krokan et al, 1997)
Though typically generated for purposeful use in cellular processes, ROS, once free in the cytoplasm react with the proteins and lipids forming the structures of the cell, inflicting oxidative damage over time to the cellular apparatus and impairing the cell’s proper functioning (Alexeyev, 2009). In addition to this, ROS will readily attack DNA, generating lesions in the cell’s genetic code, making mutations much more likely.
Mammals such as ourselves have evolved to combat this assault with an array of sophisticated defensive mechanisms. Not only can the vast majority of the DNA lesions formed by ROS be repaired by specialised suites of enzymes, but we have even developed a method of neutralising the ROS present in our cells using ROS-scavenging enzymes, such as Superoxide dismutase (Finkel et al, 2000).
However, our defences are imperfect and will inevitably be overwhelmed. The imbalance between the production of ROS by the mitochondria and our cells’ ability to counteract their harmful effects is called oxidative stress – when a cell cannot repair oxidative damage as quickly as it is being generated and cannot neutralise ROS as quickly as they are being produced. Under oxidative stress, oxidative damage accumulates.
It was on this basis that Denham Harman first proposed the Free Radical Theory of Ageing more than 50 years ago (Harman, 1956). The Free Radical theory postulates that ageing results from the accumulation of the deleterious effects caused by ROS, and that the ability of an individual organism to effectively repair oxidative damage and neutralise ROS (to resist oxidative stress) plays an important role in determining their lifespan (Ishii, 2000).
There is good evidence supporting the assertions of Harman’s theory, with numerous genetic studies showing a correlation of enhancement in resistance to oxidative stress and decreased production of ROS, to longer lifespans (Elchuri et al, 2005).
Further adding to the growing body of evidence in this case, replicative senescence, when a cell stops growth and replication after a certain number of divisions, is now also thought to be influenced by ROS. Telomere shortening is the main cause of replicative senescence, which is thought to greatly
influence the manifestation of many of ageing’s symptoms. Research has shown that ROS play a major role in the attrition of telomeres, with research suggesting that the rate of telomere shortening is directly linked to the level of oxidative stress in the cell – the more oxidative stress, the quicker the loss of telomeric sequence (Haendler et al, 2004).
Significant increases in telomere shortening are observed even under only mild oxidative stress, indicating the influence ROS have over the onset of replicative senescence. Further supporting this is the fact that decreased levels of ROS in human fibroblasts, achieved by increasing production of the ROS-scavenging enzyme SOD, was found to greatly decrease the rate of telomere shortening (Chen et al, 1995), and thus delay replicative senescence.
Additionally, ROS appear to affect the maintenance of the telomeres as well, as oxidative damage to the telomeric sequence makes it much harder for telomerase to bind to and extend the telomeres. ROS also affects the ability of the telomerase enzyme to form (Itahana et al, 2003).
Senescent cells are found to have much higher levels of ROS than normal body cells and exhibit more accumulated oxidative damage to their DNA and proteins (Weindruch, 1997). In contrast to this, immortal cells such as cancer cells (which do not experience replicative senescence) suffer less oxidative damage than their non-immortal counterparts and also have greater resistance to the deleterious effects of ROS. This clearly demonstrates how ROS play a critical role in cellular senescence by directly impacting the rates of telomere shortening.
The site most severely affected by oxidative damage, however, is the mitochondria themselves. Mitochondria each possess their own genetic material, but this mitochondrial DNA (mtDNA) is not as well protected as the DNA in the cell’s nucleus, making it far more vulnerable to attack from ROS (Maynard et al, 2009). In addition to this vulnerability, the proximity of the mitochondria to ROS (being the site of their generation) makes them a primary target for oxidative damage.
The degradation of the mitochondria and their mtDNA by ROS affects their ability to effectively perform cellular respiration and in turn makes them more liable to produce ROS as by-products. This results in a vicious cycle of oxidative stress that, over time, can lead to the deterioration of cells, and later, entire organs (Hang et al, 2012). This is reflected in aged tissues, which exhibit mitochondria that not only produce more ROS, but also show greater frequency of mutation due to oxidative stress in their mtDNA, suggesting the progressive accumulation of oxidative damage to the mitochondria and the mtDNA contributes to the ageing process (Zhang et al, 2002).
In addition to cellular respiration, mitochondria also play a critical role in the activation of specific mechanisms that help regulate apoptosis. Apoptosis is a carefully controlled process used by multi-cellular organisms to kill off unwanted cell. It is used, for example, during the development of our fingers in the womb, which initially form as tiny flippers, each finger joined to the next by tissue, before apoptosis destroys the cells between each digit.
Certain chemical changes in the cell generate apoptotic inducers which trigger the release of key enzymes from the mitochondria that cause apoptosis. Mitochondria heavily affected by oxidative damage are far more sensitive to potential apoptotic inducers and so may initiate apoptosis even when it was not required. The oxidative damage to mitochondria by ROS is therefore also implicated in the ageing process through its potential effects on the regulatory machinery of apoptosis.
ROS, an unavoidable by-product of the very process that powers our every cell, are highly culpable for our mortality. Over the course of our lifetime, the damage caused by ROS eventually overwhelms our defences, creating oxidative stress. Oxidative Stress is implicated in an incredibly wide range of processes throughout not only individual cells, but the whole body, that play a part in our steady descent from health to inevitable death.
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