Mitochondria – the tiny energy generating powerhouses inside cells – are suspected of playing a key role in diseases of ageing, as well as muscle loss in the elderly, Maria Burke reports
Ageing is a critical risk factor for a variety of diseases, such as Alzheimer’s, many forms of cancer and Type 2 diabetes. Researchers working on understanding the molecular pathways underpinning the ageing process hope not only to develop novel therapeutic strategies to treat age-related disorders but also to boost general health as we age.
One area of research is mitochondria, the tiny structures or organelles in cells that produce energy from nutrients. Their primary role is to make adenosine triphosphate (ATP), the main source of intracellular energy, through oxidative phosphorylation. But they are also involved in other processes such as balancing calcium levels, and metabolising fats, proteins and carbohydrates.
As we age, mitochondria start to lose their ability to generate energy and to replicate. Dysfunctional mitochondria produce reactive oxygen species (ROS). An excess of ROS in cells may cause damage to DNA, RNA and proteins, and even cause cell death. This is one of the reasons why a build-up of dysfunctional mitochondria is suspected of playing a role in diseases of ageing, as well as muscle impairment and loss in the elderly.
Connecting the dots
An emerging theme of recent research suggests that organelles such as mitochondria are more interconnected than previously thought. In a recent study, Peter Adams of Sanford Burnham Prebys Medical Discovery Institute in La Jolla, California, US, reported that mitochondria appear to communicate with the nucleus and even control it. He suggests that a compound that interrupts the communication between the mitochondria and the nucleus would hold promise as a treatment to promote healthy ageing.
‘In school, we learned that the role of mitochondria is to generate energy and that DNA controls everything the cell does,’ says Adams.
‘Research is now showing that mitochondria are important sensors for the cell and have a lot of cross-talk with the nucleus, which makes sense given their duty to respond to the cell’s metabolic needs.’
For more than a decade, Adams has studied senescent cells; cells that have stopped dividing but remain metabolically active. Clusters of chromatin – a mix of DNA and protein normally found inside the nucleus – can leak out from senescent cells into the cell cytoplasm. In response, the cell generates a signal that produces inflammation. ‘Usually, the only time a cell encounters DNA in cytoplasm is a virus infection, which triggers anti-viral, inflammation pathways,’ Adams explains.
But the reason why chromatin clusters form remains unclear. Now Adams’ team suggest that it could be down to dysfunctional mitochondria. Using a human lung cell model of senescence, the researchers discovered that mitochondria – linked to a particular set of down-regulated genes – trigger a signalling pathway that drives cluster formation.1 ‘We have traced it back to mitochondria and we were not expecting that at all. It is the nucleus that is the master regulator so to find that the nucleus is controlled by mitochondria was surprising,’ says Adams. ‘This is the first concrete link between several known hallmarks of ageing – dysfunctional mitochondria, inflammation and senescent cells – which historically were studied as separate events.’
There’s no substitute for these exercise programmes when it comes to delaying the ageing process. These things we are seeing cannot be done by any medicine
Sreekumaran Nair, Mayo Clinic, Rochester, Minnesota, US
The team also found that two US FDA-approved cancer drugs – HDAC inhibitors trichostatin A and vorinostat - transformed senescent cells back to health and improved mitochondrial function. ‘Mitochondria in senescent cells produce more ROS and have a lower membrane potential [the difference in hydrogen ion concentration across the inner membrane of the mitochondria], indicating a breakdown in function,’ explains Adams. ‘After treatment with the drug, relatively low levels of ROS and elevated membrane potentials indicated healthy functioning mitochondria.’ The cells treated with the drug also produced less cytoplasmic chromatin and fewer inflammatory signals.
However, HDAC inhibitors have side-effects, such as fatigue and nausea, which make the drug unsuitable for healthy people. The team is screening for less toxic candidates from libraries of approved drugs. ‘Drug repurposing is particularly attractive for healthy ageing research as regulators do not allow researchers to test novel drugs on healthy older people for this purpose, as ageing is not considered to be a disease,’ Adams says.
Communication between the nucleus and mitochondria also appears to have a role in hair loss and wrinkles. Researchers at the University of Alabama at Birmingham (UAB), US, have developed a mouse model that they can genetically manipulate to develop wrinkles and hair loss within weeks. But the effects can be reversed by turning off the gene mutation.
The team induced the mutation in a nuclear gene that affects mitochondrial function by giving the mice an antibiotic – doxycycline. This inactivates an enzyme that replicates DNA and the result is depleted amounts of mitochondrial DNA. After four weeks, the researchers observed grey hair, thinning hair and hair loss in the mice, as well as lethargy.2 Wrinkled skin appeared between four to eight weeks later, with females being more severely affected than males.
However, a month after the team turned off the gene mutation, the mice regained their smooth skin and thick fur as their mitochondrial DNA was restored. ‘It suggests that epigenetic mechanisms underlying mitochondria-to-nucleus cross-talk must play an important role in the restoration of normal skin and hair phenotype,’ comments Keshav Singh, a professor of genetics in the UAB School of Medicine. ‘This mouse model should provide an unprecedented opportunity for the development of preventive and therapeutic drug strategies to augment the mitochondrial functions for the treatment of ageing-associated skin and hair pathology, and other human diseases in which mitochondrial dysfunction plays a significant role.’
Recycling and control
Normally cells recycle old and dysfunctional mitochondria before they can damage cells, in a process called mitophagy. This quality control process is key to keeping cells healthy but it declines as we age.
This is why researchers are looking at ways to stimulate mitophagy. A recent report from a team at the University of Birmingham’s School of Sport, Exercise and Rehabilitation Sciences has provided further evidence of a key mechanism that drives mitophagy in muscle cells. They used fluorescent tags to flag healthy and degrading mitochondria. The team found that activating an energy-sensing molecule called AMP-activated protein kinase (AMPK) helped to stimulate mitochondrial breakdown.3 This suggests that it could be used to encourage the clearance of damaged mitochondria, so keeping mitochondria healthy in muscles and prolonging older people’s physical capabilities.
It is known that the diabetes drug metformin, some natural products, calorie restriction and exercise all increase AMPK activity. But, so far, developing direct activators of AMPK to elicit beneficial effects has been challenging.
Now, however, the Swiss company Amazentis has developed a product that appears to stimulate mitophagy, showing potential to boost ageing muscles. The company was set up by researchers from Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland, after they identified a molecule – urolithin A (UA) – that appears to re-establish the cell’s ability to recycle the components of defective mitochondria.4
In their original studies, the team found that the lifespan of nematode worms exposed to UA increased by more than 45%, compared with a control group. They also reported similar results with mice. Older mice, around two years of age and exposed to UA, showed 42% better endurance while running than the same age control mice. The researchers say UA boosts mitochondrial health by upregulating specific genes involved in mitochondrial protein synthesis.
‘It’s the only known molecule that can relaunch the mitochondrial clean-up process, mitophagy,’ says Patrick Aebischer of EPFL. ‘It’s a completely natural substance, and its effect is powerful and measurable.’
Precursors to UA called ellagitannins are found in pomegranates, and, in smaller amounts, in many nuts and berries. Microbes in the gut convert the precursor into UA, but the amount produced can vary widely, depending on the species of animal and the flora present in the gut microbiome.
The team at Amazentis believes that by helping the body to renew itself, UA could succeed where so many pharmaceutical products, most of which have tried to increase muscle mass, have failed. Chris Rinsch, CEO of Amazentis, says: ‘The nutritional approach opens up territory that traditional pharma has never explored.’
Trials in 2019 showed that regular doses with an oral formulation were safe and effective at improving mitochondrial health.5 The company is now evaluating UA in two Phase 2 clinical trials, and has recently partnered with Nestlé Health Science to develop products containing UA.
Diet and exercise
While certain foods or supplements might help boost mitochondrial health, what about cutting out food altogether? Periods of fasting have been shown to help healthy ageing, but little is known about the underlying biology. Now researchers at Harvard T.H. Chan School of Public Health, US, suggest the involvement of mitochondria.
Mitochondria exist in dynamic networks that change shape according to energy demand, but their ability to flex reduces with age. Mitochondrial networks switch between two states: fused (associated with youthfulness) and fragmented (associated with ageing), depending on age, nutrients and damage. Growing evidence links the dynamic state of mitochondrial networks to metabolic efficiency and lifespan.
In a study on nematode worms, the Harvard team found they could maintain the networks in a fused youthful state by either restricting the worms’ diet, or mimicking fasting by genetic manipulation of AMPK.6 The worms with more fused networks lived longer.
‘The finding surprised us initially as it does seem counter-intuitive that locking networks would lead to long lifespan,’ comments Pallas Yao, a researcher in team leader William Mair’s lab. ‘So we looked at whether there was any metabolic shift in the long-lived mutants, compared with normal wildtype worms. [We found] the mutants showed consistently elevated levels of very long chain fatty acids [which] tend not to be processed in the mitochondria, but in other organelles called peroxisomes.’
The team suggests that changes in mitochondrial network shape and dynamics were being communicated to peroxisomes, causing them to alter their metabolism. Peroxisome shape also appeared ‘strikingly different’ in the mutants, Yao says. Significantly, when the researchers ‘knocked down’ peroxisome function in the mutants, they no longer had long lifespans. But they still don’t know how exactly mitochondria communicate with peroxisomes.
Intermittent fasting is known to improve health and increase longevity in some animals, including nematode worms. In this study, the team showed that cycles of feeding and fasting caused mitochondrial networks to switch morphology between fused and fragmented in normal worms, but not in the mutants with locked networks. ‘Our work shows how crucial the plasticity of mitochondria networks is for the benefits of fasting,’ says Mair. ‘If we lock mitochondria in one state, we completely block the effects of fasting or dietary restriction on longevity.’
Yao concludes that it’s too simplistic to assign a ‘one size fits all’ to mitochondrial networks during ageing. ‘We instead think that the requirements of mitochondrial dynamics – fusion, fission, plasticity – depend on context.’
As well as diet, the role of exercise in healthy ageing is another key area of research. It is known to stimulate AMPK, for example, as well as having lots of other positive effects. Researchers at the Mayo Clinic in Rochester, Minnesota, US, are looking at the cellular mechanisms involved in different types of exercise. They found that exercise – and in particular high-intensity interval training – caused cells to make more proteins for their mitochondria and their protein-building ribosomes, effectively stopping ageing at the cellular level.
The 12-week study involved 36 men and 36 women from two age groups – 18-30 years old and 65-80 years old. They were divided into three exercise programmes: high-intensity interval biking, strength training with weights, and combined strength training and interval training. The team compared thigh muscle cells with those from a sedentary group. They found that high-intensity interval training yielded the biggest benefits at the cellular level.7 The younger volunteers in the interval training group saw a 49% increase in mitochondrial capacity, and the older volunteers a 69% increase. Interval training also improved volunteers’ insulin sensitivity, which indicates a lower likelihood of developing diabetes. But it was less effective at improving muscle strength, which typically declines with ageing.
The team also compared proteomic and RNA-sequencing data from people on the different exercise programmes and found evidence that exercise encourages the cell to make more RNA copies of genes coding for mitochondrial proteins, as well as proteins responsible for muscle growth. The clearest finding was the increase in muscle protein content. In some cases, high-intensity biking seemed to reverse the age-related decline in mitochondrial function and proteins needed for muscle building.
‘If people have to pick one exercise, I would recommend high-intensity interval training, but I think it would be more beneficial if they could do three to four days of interval training and then a couple days of strength training,’ says senior author Sreekumaran Nair. Any exercise was better than no exercise. ‘There’s no substitute for these exercise programmes when it comes to delaying the ageing process. These things we are seeing cannot be done by any medicine.’
1 Genes & Development, doi: 10.1101/gad.331272.119
2 Cell Death & Disease, doi: 10.1038/s41419-018-0765-9
3 The FASEB Journal 2020, https://doi.org/10.1096/fj.201903051R
4 Nature Medicine, doi: 10.1038/nm.4132
5 Nature Metabolism, doi: 10.1038/s42255-019-0073-4
6 Cell Metabolism, doi: 10.1016/j.cmet.2017.09.024
7 Cell Metabolism, doi: 10.1016/j.cmet.2017.02.009)