Infamous High School Biology
Over a billion years ago, somewhere in a random hot puddle, a bacterium was engulfed by a cell that is an early ancestor to all of us. Exactly how it happened, we don’t know. Maybe the cell meant for the bacterium to be a simple meal. Or maybe the aggressor was the bacterium; a parasite looking for a new home. Whatever happened, the bacterium ended up inside the cell and it has stuck around for a long time. In fact, its descendants are still a part of you and me today.
You see, while the bacterium and ancestor cell were two different species, today they are one. Through millions of years of evolution, the two have merged with each other and can no longer be separated.
We call the descendants of this bacterium ‘mitochondria’, and they are a vital part of our cells. If we were to look inside your cells right now, we would find anything from a few to several thousand mitochondria. These mitochondria still bear resemblance to their bacterial past: they are shaped and structured like bacteria, and even still act a little bit like bacteria. For instance, your mitochondria make more mitochondria in the same way as bacteria make more bacteria: by dividing. That said, your mitochondria are not something separate from you. They are closely integrated with the rest of the cell as an organelle (a cellular organ). And your mitochondria are not viable on their own anymore, either. They can only exist as part of your cells. Through millions of years of evolution, the vast majority of mitochondrial DNA has been moved to the nucleus of the cell with the rest of your genetic blueprint. Only a tiny bit remains in the mitochondria themselves as a reminder of their independent heritage.
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You might already know the role of mitochondria thanks to that most infamous quote of high school biology: Mitochondria are the powerhouse of the cell. While many people are annoyed they have to learn this, it actually makes mitochondria one of the most important organelles in your cells. They are responsible for the last part of the cellular task that makes everything else possible: harvesting energy from the food you eat. As a result, the amount of mitochondria in cells varies by function. Your muscle cells, especially heart muscle cells, have a lot of mitochondria because they use a lot of energy. Other cell types, such as skin cells, are mostly tasked with sitting around, and thus they have few mitochondria.
Power plants really are the best analogy for mitochondria and you want all the properties from your mitochondria that you’d want from your local power plant: reliability, safety and minimal impact on the environment. Evolution has made sure your mitochondria are highly optimised to fulfil this job. However, as with most other things in our cells, ageing tends to ruin the system. As we age, we lose mitochondria, and those that remain tend to become dysfunctional. You can think of it as going from lots of shiny new power plants to a few old and worn-out ones.
This decline in mitochondrial function spells trouble, as energy is necessary for everything the cell does. Studies show that dysfunctional mitochondria promote ageing in laboratory organisms, and we see the effect of their dysfunction in humans too. For instance, loss of mitochondria is one of the reasons our muscles tend to get weaker with age. So, what can we do to keep the cellular power plants humming?
The answer to that question is a list of our old acquaintances. Like many other biological systems, mitochondria experience hormesis. The main way to beneficially challenge these organelles is by increasing your energy needs, especially acutely. Two things come to mind. First, exercise, especially exercise with a high intensity. And second, cold exposure, for instance in the form of winter swimming.
One of the ways mitochondria respond to being challenged is through something called ‘mitochondrial biogenesis’. This just means mitochondria dividing to make more of themselves. That’s good, because it increases the capacity of the cells, and it also counteracts the loss of mitochondria that normally happens during ageing. In fact, it seems you can pretty much entirely cancel out the age-related loss of mitochondria if you exercise sufficiently.
Another response to mitochondrial hormesis is autophagy, or ‘mitophagy’, as some call it when related to mitochondria. This process ensures that old and dysfunctional cellular power plants are regularly removed. In fact, the removal of damaged mitochondria is one of the most important roles played by autophagy. As a result, autophagy boosters like spermidine particularly impact mitochondria. When researchers give mice spermidine to extend their lives, it turns out the most important effect is mediated through mitophagy, in particular the removal of dysfunctional cellular power plants in heart muscle cells. Treatment with spermidine improves heart health in the mice and ensures a clean energy supply. This is pretty important, as we would prefer for the heart to keep beating. (In fact, it is not just the spermidine mice that have healthier hearts. We also know that, among humans, a higher spermidine intake in the diet is associated with a lower risk of cardiovascular disease.)
Scientists have also identified another compound, called urolithin A, which can increase mitophagy. When researchers give urolithin A to elderly, non-active people, there’s an increase in mitophagy in their muscles. The same thing happens in mice, improving their endurance as a result. And it turns out urolithin A doesn’t just improve mitophagy: it also stimulates mitochondria to divide, which is the same thing that happens after exercise.
Unfortunately, urolithin A doesn’t occur naturally in food. Or at least, no one has found it yet. However, precursors to the urolithin A molecule are found in pomegranates, walnuts and raspberries in the form of polyphenols called ellagitannins. And it turns out some gut bacteria can convert ellagitannins into urolithin A. Not everyone has these species in their gut, though, but generally speaking, eating more pomegranates, walnuts and raspberries can never be a bad thing.
While the main role of your mitochondria is as the powerhouse of the cell, nature is fond of recycling. For whatever reason, mitochondria have other functions that don’t relate much to their power plant job. One example is that the trigger for cellular suicide – apoptosis – sits on the mitochondria. And in addition, mitochondria are also involved in our immune system – both in killing enemies and in signalling pathways that control the whole thing.