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ANIMAL FEATURE: MITOCHONDRIA: INSIGHTS FROM ORGANELLE TO ORGANISM
MITOCHONDRIA: INSIGHTS FROM ORGANELLE TO ORGANISM
BY ALEX EVANS
The animal kingdom boasts incredible diversity in the way that organisms move, feed, fight and reproduce. All of these actions require chemical energy, and the vast majority of this is generated by one highly conserved organelle, the mitochondria. But is mitochondria a ‘one size fits all’ solution to energy production? We talked with researchers to find out just how variable mitochondrial function can be, and what that means for animals in a rapidly changing world.
EVOLUTION AND EFFICIENCY
Mitochondria have been a part of life on Earth for well over a billion years, and while they are still relatively well conserved between animal taxa, there is ample evidence that this important organelle still has the capacity to evolve to overcome environmental challenges. Neal Dawson, a senior research associate at the University of Glasgow, explores the cascade of mitochondrial function across magnitudes of scale and evolutionary time. ‘My undergraduate degree was in biochemistry, so I started at the very small level,’ he says. ‘In my first postdoc, I saw people working with these wonderful organelles, and my first naïve response was “wow, that’s a big enzyme!” but quickly learned it was much more complex than that.’ Since then, Neal’s research has focused on the impact that variation and adaptation among these tiny organelles can have on whole organisms. ‘I was especially interested in how efficiently the organelle can function rather than accepting the “with more, you can do more” hypothesis, so it became more about quality versus quantity,’ he adds.
One mystery that Neal has been particularly invested in solving is that of high-altitude adaptation in animals. Many mammals and birds have evolved
to acclimatise to high altitudes, but the ways in which they do so vary considerably. ‘We wanted to understand how evolutionary time at high altitude would influence metabolic function, especially the role of the mitochondria,’ he explains. ‘I ran mitochondrial assays up in the mountains of Peru, which was such a unique and amazing experience.’ Rather than relying on increasing their anaerobic energy production to perform well at high altitudes with lower concentrations of oxygen, Neal’s research revealed that, over time, populations of birds were actually more likely to increase their ATP production aerobically – possibly through improvements in mitochondrial efficiency.
As well as investigating populations, Neal’s research has also focused on inter-individual variation in mitochondrial function. ‘I jumped at the opportunity to work with Neil Metcalfe, and we really tried to explore if it could be possible that the variation in mitochondrial efficiency may allow some individuals to invade new niches, or in the case of the fishes I’m studying with Neil, survive in the face of climate change where others cannot,’ he explains, throwing in an analogy that Neil created to explain their interest in mitochondrial efficiency: ‘If you think about how far you could drive a car, a lot of past mitochondrial research has been focused on how big the gas tank is and not necessarily on how efficient the engine is at using the fuel.’
While consistently high mitochondrial efficiency might sound like the ideal outcome, it is not without drawbacks. Every time the mitochondria produces ATP, a small percentage of oxygen consumed becomes harmful reactive oxygen species (ROS). ‘My entire PhD was on antioxidant defence systems and I’m curious if this variation in mitochondrial quality comes down to protection against ROS accumulation,’ suggests Neal. ‘Perhaps having less-efficient mitochondria acts as a buffer against this type of damage.’
Finally, Neal has also been doing some work on ageing in birds, drawing on the curious fact that birds tend to have relatively long lifespans compared to other animals of a similar size. ‘On average, they all tend to live longer despite having more mitochondria, which should produce more ROS and bring about more rapid ageing,’ he says. ‘There will no doubt be direct human applications that can help us with disease and ageing research, simply by researching animals that are already able to delay these processes.’ By bringing a wealth of insights ranging from the protein level to the population biology and organismal evolution level, Neal is in a great place to track the role that mitochondria play in animal survival and potentially provide a better understanding of our own ageing processes. ‘I’m always trying to bridge the fields that I’ve studied,’ concludes Neal. ‘Which is why I’m really interested in looking at the relationship between efficiency and ageing, and especially in the context of climate change.’ Mitochondria may be invisible to the naked eye, but the effects they can have on the appearance of animals certainly aren’t. Rebecca Koch, a postdoctoral researcher at the University of Tulsa in the USA, is working to improve our understanding of why some external display traits, like feather coloration, have been found to vary along with the performance of internal physiological processes associated with mitochondria. ‘I focus on the mechanisms underlying variation in sexually selected mating displays and right now that includes carotenoid-based coloration in birds,’ she explains. ‘As scientists, we often try to find unifying explanations for the patterns we observe, including these traits that appear to be honest indicators of individual quality, but even in the well-studied trait of carotenoid-based coloration in birds, we haven’t been able to pin down exactly what links feather pigment deposition to this concept of quality.’
There are many methods to approach this question of quality but, according to Rebecca, there has been a lack of consistent and satisfying answers. However, subcellular research may offer new insights. ‘Mitochondrial biology entered the scene while I was in graduate school through a few somewhat unrelated events and a book club led by my advisor Geoff Hill,’ she explains. ‘It has been exciting to explore the possibility that examining a subcellular process could offer a more fundamental explanation for the larger-scale variation we observe.’
Many of the aspects of this area of research are already intrinsically linked to mitochondria, through metabolic rates and hormone functions, so Rebecca and her team believe that this may help them to interpret some of the patterns they’ve already observed from other investigations. ‘Right now, I’m focused somewhat obliquely on discovering and testing the genes involved in becoming a brightly coloured male house finch,’ she says. ‘These will be genes involved in carotenoid pigment absorption, metabolism and transport, some of which may actually involve mitochondria directly.’
Ultimately, the goal of animals is to survive and reproduce – and both of these factors rely on how well the animal can cope with environmental or immune stress and show off to others while doing it. ‘By performing targeted manipulations and measurements of mitochondrial traits and environmental stressors, we will gain a better understanding of how mitochondrial respiratory performance varies in wild animals,’ she explains, ‘and how this may or may not affect sexually selected display quality.’ The methods that Rebecca uses to carry out these experiments rely on measuring the consumption of oxygen by the mitochondria, but they also allow her to craft more creative investigations. ‘To quantify mitochondrial aerobic
respiration, we use a high-resolution respirometer to measure gas exchange in tiny samples,’ she explains. ‘These machines are fascinating because they allow us to add specific substrates or inhibitors that are involved in the mitochondrial respiration process, and measure how this affects gas exchange.’
This broad appreciation for mitochondria, even when dealing with something that is seemingly unrelated at first glance, just further demonstrates the interconnectedness of animal physiology and the importance of cross-theme research. ‘I think the fact that a bunch of behavioural ecologists and evolutionary biologists are interested in subcellular biology in the first place is quite surprising,’ she says. ‘The core functions of mitochondria are so key to eukaryotic life that there is some surprise that any variation exists at all but I’ve been fascinated by how plastic and flexible mitochondria are.’
Rebecca is always keeping her eye on technological developments that advance the ability of researchers to make these measurements both in the laboratory and in the field. ‘I’m very excited for any new breakthroughs in technology and logistics that might allow us to measure mitochondrial performance at a higher throughput and with greater mobility than we currently can with lab-based high-resolution respirometers,’ adds Rebecca. ‘A team I collaborate with for my current project has built a MitoMobile1 that can drive those laboratory spaces to the field, and I think we are still scratching the surface for bringing mitochondrial biology to ecology.’
THE HEART OF THE ISSUE
Mitochondrial function can be highly variable, but because it plays such a vital role in animal physiology, it is also incredibly well conserved throughout the animal kingdom, meaning that we can learn a lot about our own physiology by looking at how other animals cope with challenges. Lucie Gerber, a postdoctoral researcher at the Department of Biosciences, University of Oslo, Norway, has been conducting investigative work into the cardiac mitochondria of freshwater fish that could potentially impact beyond just our understanding of mitochondrial function.
‘I am a broadly trained integrative and comparative animal physiologist,’ she says. ‘My research questions naturally led me to study mitochondrial function and there is currently a high demand for a better understanding of mitochondrial function in ecophysiology.’ Lucie especially enjoys working on a topic that is currently in the experimental biology spotlight, because it means that she is connected to a broader community of researchers and the impact of her research has considerable reach. ‘It was also a natural thing to pursue; I have always focused and been interested in basic physiological processes and how organisms cope with challenges in their environment,’ she explains. ‘Mitochondria are central to most physiological processes and understanding their acclimation and adaptation capacity to stressors is emerging as a tool in ecophysiology to help predict animal responses in a changing world.’
In a rapidly changing world, the ability of organisms to adapt to new environmental conditions is crucial for their survival – whether this is through their behaviour, physiology or, in this case, cellular function. ‘The role of my postdoctoral research in Canada was to understand mitochondrial acclimation capacity and plasticity to two major environmental stressors: environmental warming and hypoxia,’ she says. ‘Our study on acclimation capacity in salmon published in the Journal of Experimental Biology, with a focus on heart mitochondria as a metabolic predictor of performance and acclimation capacity, was highlighted as the most cited research papers in 2021 by JEB!’
Despite her focus on mitochondria from freshwater fish, the bigger picture of Lucie’s research is very much painted with human applications in mind. ‘Mitochondria are also emerging as therapeutic targets and my current postdoctoral research project on crucian carp, the champion of anoxia tolerance, has the potential to shed light on mitochondrial adaptation to anoxia and the treatment of oxygenrelated diseases humans,’ she explains. ‘This very exciting and ambitious project is part of an interdisciplinary project called 3DR that aims to develop strategies to improve organ preservation protocols in response to a growing demand for viable and functioning organs in transplantation.’
One of the benefits of studying a topic with wide appeal is that Lucie has a diverse arsenal of reliable technology and techniques at her disposal. ‘I often combined methods in molecular and cellular biology, such as qPCR, Western blotting and enzymatic assays, to study gene and protein expression and activity, with techniques from physiology or respirometry, depending on the research questions, to integrate observations at different levels of biological organisation,’ Lucie explains. ‘Having such a repertoire of techniques is definitely an advantage because it allows me to be flexible and integrative in my research. I am now delving into omics (RNA-seq and Ribo-seq), which are great tools to find new research questions and add a discovery-driven approach to my repertoire.’
For Lucie and her team, as well as uncovering some fascinating discoveries that have the potential to benefit human health and transplantation, their
research has raised even more mitochondrial mysteries to be solved. ‘The lower mitochondrial ROS release rate in isolated cardiac mitochondria from warm-acclimated salmon has been puzzling us,’ she says. ‘Digging into the mechanisms underlying this acclimatory response by cardiac mitochondria in salmon is definitely on our list of next steps!’
EFFICIENCY OF A FISH IN SEA
Energy is sometimes referred to as the currency of the universe, and, just like other currencies, biological energy production is constantly subject to fluctuations caused by a range of factors. Some animals are able to generate energy within their mitochondria with great efficiency while others cannot – so what affects this individual variation in metabolic exchange rate? Karine Salin, a researcher at IFREMER, the Institute for Ocean Science in Plouzané, France, is on the case. ‘I’m really interested in understanding individual variation in animal performance,’ she says. ‘Taking individuals from the same population or experimental group, some are better than others, and I want to understand the physiological mechanisms at work.’
Karine’s main focus right now is understanding the flexibility of energy metabolism in marine animals by investigating how mitochondrial function changes under environmental stress. ‘I want to figure out if marine animals can produce more ATP if they need more in order to cope with their environment, and if mitochondrial function can explain individual variation in whole animal performance,’ she says. ‘I’m looking at mitochondrial function especially because their phenotype links to specific genotypes that could be inherited into the next generation, so this variation between individuals may play a role in natural selection.’
To better understand the mitochondrial function of her animals, Karine frequently measures the oxygen consumption of the mitochondria performing oxidative phosphorylation, which is a technique that has been used for decades for similar means. ‘However, we also measure the mitochondria’s ATP production,’ she says. ‘Two individuals may be able to produce different amounts of ATP using the same amount of oxygen, so this helps us to work out the “mitochondrial efficiency” of an individual.’ While it might be expected for similar animals to have a similar mitochondrial efficiency, Karine has found that this is not always the case. ‘We have been very surprised to find that when we keep individuals in the same environmental conditions for months, they will still be highly variable in their ability to produce ATP – some generating up to five times as much ATP than others,’ she explains.
Karine’s mitochondrial research is not only important for understanding how variation between individuals may affect their chances of survival or reproduction in the wild, but also for understanding how to find the optimal fish phenotypes for farming. ‘I mostly work with European sea bass, which is a very important food resource both for wild fishing and aquaculture,’ she explains. ‘Fish that require less food and oxygen to produce energy would be more economically beneficial and is this a heritable trait that could be bred to make more productive stocks.’
However, Karine explains that while there are certainly benefits to being efficient at making ATP, there are also drawbacks and being energyefficient can actually be a double-edged sword. ‘One of the downsides of making a lot of ATP is producing a lot of ROS which cause oxidative stress and harm the cell,’ she says. ‘Our hypothesis is that individuals that make a lot of ATP are able to grow fast, but also have a faster rate of senescence – so these individuals may function well in some environments that best suit shorter generations but not in others where living longer lives is a better strategy.’
Reference:
1. https://wp.auburn.edu/mitomobile Above Lucie Gerber and PhD student Magdalena Wiklhofer catching v in Oslo, Norway Photo credit: Laura Valencia
Left Lucie Gerber holding crucian carp at the InVivo aquarium at the Department of Biosciences, University of Oslo (https://titan.uio.no/ naturvitenskap/2021/ karussens-strategioverleve-naroksygenet-blirborte-kan-loseutfordringer-vedorgandonasjon) Photo credit: Gina Aakre
