Ageing - The long and the short of it

Ageing - The long and the short of it

Male silverback gorilla, Kwan. Photo: Lincoln Park Zoo, Chicago

By Caroline Wood

What are the factors that limit how long we can live for? It’s clear that our bodies wear out over time – but can we reduce this accumulated damage? Certain plants and animals suggest we can, so perhaps we should learn from their strategies. 

Size matters

The “Rate of Living” concept is one of the longest-lived theories of ageing, first proposed in 1908 by Max Rubner. According to his thinking, the faster an organism’s metabolism, the shorter is its lifespan. Hence, larger species generally outlive smaller ones as they have lower basal metabolic rates. However, humans can live for considerably longer then mammals of an equivalent size – so does this disprove the metabolic theory of aging? Possibly not: new research suggests that humans and other primates have exceptionally low metabolic rates for their size, which may be their secret to reaching those “golden years”.

Using a variety of ape, monkey and lemur species, Herman Pontzer (Hunter College, USA) and his colleagues found that primates expend approximately 50 %less energy each day than placental mammals of a similar mass, even when activity levels did not differ significantly. To put that in perspective, says Herman, “human beings habitually expend 2500 kilocalories a day, whereas a mammal of a similar size, such as an antelope, expends 5,000 kilocalories. This level would only be reached by a human during an extreme endurance event such as the Tour de France”.

Furthermore, when comparing across different primate species, it was found that total energy expenditure was a strong indicator of life history traits, such as growth rates and lifespan, suggesting that this reduced metabolic rate contributes to longevity. The molecular basis for this remains unknown, but one possible explanation is that faster metabolic rates cause more of the oxidative stress and cellular damage that lead to aging. 

It is currently unclear how primates achieve this ‘go-slow’ state, and how this strategy evolved against ‘live fast, die young’ scenarios. “Out of the many genes which regulate metabolism, we don’t yet know which ones have changed in primates”, says Herman. It may be that a reduced metabolic rate – and hence a lower calorie expenditure – evolved as a mechanism to cope with periods of famine. With an increased lifespan, however, would come other benefits, such as time to invest in teaching juveniles foraging and survival skills. Herman concludes: “Our results are really just the tip of the iceberg – we’ve stumbled onto something really surprising about the way humans and primates use energy compared to other mammals”. He now hopes to investigate how these differences evolved and whether differences in metabolic rate between and within primate species reflect those in ecology, diet, life history or other factors.

A whale of a time

Cetaceans may provide a genetic clue regarding the mechanisms underlying a reduced metabolism. The Bowhead Whale (Balaena mysticetus) has been recorded as the longest-lived mammal, based on one male estimated to be 211 years-old +/- 35 years. In addition, this species has a noticeably reduced metabolism, with genetic analysis suggesting that this may be due to mutations in the sequence coding for UCP1 (UnCoupling Protein 1, also known as thermogenin). This anion transporter, found on the inner mitochondrial membrane, allows excess energy to be converted solely into heat during times of calorie excess or extreme cold.

Curiously, human genetic studies have found that certain UCP1 nucleotide variants are associated with increased longevity [1], even though it was previously thought that this gene is only expressed in the brown adipose tissue of newborns. However, comparative biologist João Pedro de Magalhães (University of Liverpool, UK) remains unconvinced by the metabolic theory of aging. “Many bats and birds are exceptions to the idea as certain species have high metabolic rates and relatively long lifespans”, he says. One study on a population of outbred rodents even concluded that mice in the upper quartile of metabolic rate had greater daily energy expenditures, yet lived 36% longer. “For the whales, my intuition is that metabolic adaptations from changes in UCP1 are more likely related to their large size than their longevity”, says João.

Life in the slow lane 

“The Greenland shark may be one of the longest-living vertebrates on the planet, with our limited data suggesting that they can live in excess of 100 years”, says biochemist Cinzia Verde (Institute of Biosciences and BioResources, CNR, Naples, Italy). The shark, which lives further north than any other of its species in the North Atlantic waters around Iceland and Greenland, reaches immense sizes in excess of twenty feet, and can also live to incredible ages. Curiously, although they are among the slowest-swimming shark species, the remains of polar bears, reindeer and moose have been found in their stomachs. 

But why is it that these sharks, and polar species in general, can live so long when the more typical scenario for fish is to grow and reproduce as fast as possible? “The main reason we see enhanced longevity in polar species is seasonal food scarcity”, says Cinzia. Given that small fish are at a high risk of predation, evolution has generally selected for strategies that achieve maximum body size as fast as possible. However, increased growth rates also require a higher food uptake, which is often not sustainable in polar areas. Furthermore, in ectotherms metabolic rate is dependent upon the environmental temperature, increasing 2-3 times for every 10 °C increase. Hence, this combination of a suppressed metabolic rate and resource scarcity may explain why polar species can have such slow growth rates and remarkably long life spans.

Live specimen of Greenland shark (Somniosus microcephalus) Photo: Erik Bonsdorff

Don’t love thy neighbour

Where resources are not as limited as in the arctic, there are many sound arguments for adopting a short-lifestyle strategy that invests in producing as much offspring as possible at the earliest opportunity. As ecologist Patrick Doncaster (University of Southampton) puts it: “Since reproduction carries physiological costs, natural selection favours reaping early benefits, and delaying the cost in physiological decline until later in life when there is a greater chance of being dead anyway from environmental hazards”. Despite this, age-defying species still emerge against this evolutionary pressure. Working with mathematician Robert Seymour (University College London), Patrick has been investigating how this is possible, using long-lived trees such as the Bristlecone Pine. Although there is some dispute, Bristlecones are often claimed to include the world’s oldest individual tree: a species of Pinus longaeva was recorded to be 5,065 years old in California. Even more impressively, these ancient veterans are still flush with fertility, able to produce viable cones at over 4,000 years of age. 

By using computing simulations, Patrick and Robert concluded that such a lifestyle could evolve over many generations by older organisms ‘crowding-out’ [2] younger individuals that try to grow alongside them. “Mathematical analysis shows that the crowding out of young individuals favours selection on ever-reducing senescence. Our computer simulations indicate that this runaway process could even lead to immortality”, says Patrick. This is particularly relevant for species occupying inhospitable areas – such as the typically arid, rocky regions where Bristlecone Pines are found – making it difficult for juveniles to establish themselves. Patrick ends: “If you care about your reproductive legacy, the secret of immortality is... stay put and outlive your neighbours, then plant your seed in their place."

Damage limitation

One of the reasons why metabolic rate may be linked to ageing is that many of the vital chemical reactions that support life also produce toxic by-products. According to the ‘oxidative stress theory of ageing’ the absolute barrier to longevity is set by the accumulation of cellular damage caused by these reactive molecules that lead to progressive loss of function. “Oxidative damage occurs when production of oxidants exceeds that of antioxidant defences”, says plant biochemist and redox biologist Christine Foyer (University of Leeds, UK). “This produces reactive oxygen species (ROS) that are inherently more ‘reactive’ than ground state oxygen”. Mitochondria are a major source of ROS, particularly highly reactive superoxide anions, hydroxyl radicals and hydrogen peroxide. Although these can directly damage DNA, proteins and lipids, their production is conserved across species suggesting a physiological function. “ROS also act as signalling molecules that function by their oxidative action and irreversible oxidation can mark proteins for turnover”, says Christine.

Nevertheless, as oxidative damage accumulates over time, cells gradually lose functionality and wear out. As we become older however, it becomes more difficult to replace these exhausted cells. “As animals age, their cells often become more resistant to programmed cell death and cannot be renewed”, says Christine. “Hence, the accumulation of senescent cells in animal organs may contribute to the ageing process by depleting the renewal capacity of tissues”.

Clamming up

Protecting against oxidative damage could be an effective strategy for longevity and, indeed, appears to be the case for certain bivalves. Mussels show an astonishing range of lifespans, which has made them the new model species for longevity over the past decade. “Not only are clams readily available, but they include species with very diverse longevities whilst having remarkably conserved evolutionary lineages and body sizes, allowing for direct comparative studies”, says mussel researcher Daniel Munro (University of Manitoba, Canada). This is thought to be largely due to environmental conditions: shorter-lived species, such as Spisula solidissima, which rarely lives beyond 37 years, tend to be found on the shoreline, and experience pronounced daily fluctuations in temperature and humidity. On the other hand, long-lived species, such as Arctica islandica, can be found at depths of up to 300m, where energy throughput is much less. In fact, A. islandica holds the record for the longest-living animal; this goes to ‘Ming the Mollusc’, who was recorded to be 507 years old and named after the Chinese dynasty which ruled at the time of his birth. 

So what is their secret? According to Daniel, protecting the mitochondria from damage caused by ROS is key. In cells and organelles, primary ROS species can set off a chain reaction due to the inherent instability of double bonds in the polyunsaturated fatty acids (PUFAs) that make up their membranes. Weakly bonded hydrogen atoms are snatched by ROS, causing the lipid chain to reorganise itself into a lipid peroxyl. This may then fragment, releasing reactive aldehydes which act as secondary ROS that go on to damage the molecular machinery of the cell. Mitochondrial DNA is particularly vulnerable to mutations and damage due to its close proximity to a primary source of ROS. As these mutations accumulate, the coding sequence produces less effective enzymes for energy generation from ATP. “As all physiological functions rely on enzymatic processes that require ATP, this explains the simultaneous decline of cognition, strength, the immune system, etc. seen in ageing”, says Daniel. 

Protect and survive

To investigate how such accumulated mitochondrial damage contributes to ageing, Daniel and his colleagues compared five species of bivalves belonging to the order Veneroida which were similar in size, shape and morphology but with different lifespans. “We found a strong inverse relationship between the abundance of PUFAs in the mitochondrial membrane and longevity in these five species, particularly DHA, the fatty acid most susceptible to oxidation”, says Daniel.

Whilst previous relationships have found similar relationships between PUFA abundance and longevity in birds and mammals, crucially these did not distinguish the type of membrane, making it difficult to determine the exact role of mitochondria in aging. “In this study, we separated the mitochondrial membrane of the gills from the rest of the cell membranes and found more pronounced results for the mitochondrial membranes, supporting for the first time the primary involvement of mitochondria in oxidative processes related to longevity”, says Daniel.  

Bivalves may also be protected from oxidative damage by an unusually high content of lipids known as plasmalogens, which can act as protective antioxidants. In mammals, plasmalogens are mainly restricted to the retina, the nervous system and the heart but in bivalves they are found abundantly in all tissue types. 

There are other lines of evidence which suggest that reducing oxidation damage to membranes can promote longevity. Prolonged calorie restriction has been demonstrated to extend the lifespan of rodents, and also seems to decrease the oxidation potential of membranes. Ayala et al. from the University of Lleida and the Complutense University of Madrid, Spain, found that decreasing protein intake of rats by 40% over 7 weeks caused a decrease in the abundance of highly unsaturated (and hence highly reactive) PUFAs, such as docosahexaenoic acid (DHA) and arachidonic acid. Instead, these were substituted with monounsaturated fatty acids or those with only two double bonds. This is thought to be caused by simultaneous upregulation of the delta 9 desaturase enzyme which produces monounsaturated fatty acids with downregulation of the desaturases which make PUFAs [3]. Meanwhile, studies on nematode Caenorhabditis elegans mutants with 10-fold differences in longevity by Reis et al. (the University of Arkansas) found a clear correlation between the proportion of monounsaturated fatty acids over PUFAs and maximum lifespan [4].

But what can we, as humans, learn from this? Since it appears that reducing oxidative damage can defend against aging, perhaps there is something to be said for those ‘superfood diets’ based on foods “packed with antioxidants” (such as blueberries, grapes, green tea and nuts). But whether or not we can artificially extend our lifespans, Daniel Munro offers some personal philosophy: “we are already a very long-lived species and should not seek what we don’t have.” 

Suspended in time

In the most basic sense, longevity can also refer to the ability of genetic material to remain viable for extended periods of time, such as the dormancy mechanisms that enable plants to disperse their offspring through time as well as space. There are countless incredible accounts of how long seeds can remain quiescent, in suspended animation as they wait for the opportune moment to germinate. These incredible stories include that of a Japanese Magnolia seed (Magnolia kobus) estimated to be 2,000 years old, found by archaeologists in a pit in the ancient bronze-age village of Asada. Curiously, not only did the seed germinate, but when it eventually produced flowers, these were distinctly different to those of modern trees, having seven or eight petals, rather than six. It is unknown whether this was due to genetic mutations accumulated during dormancy or, more excitingly, that this seed was so old that it pre-dated an evolutionary step in flower development for this species. 

Even more impressive is the account of how Yashina et al. (Russian Academy of Sciences) used tissue culture methods to regenerate whole, fertile narrow-leafed campion plants (Silene stenophylla) from placental tissue extracted from fruits buried in the Siberian permafrost. The seeds were found in fossilised squirrel burrows and were radio-carbon dated as approximately 31,800 years old [5]. 

So how do they manage it? According to seed biologist Steve Penfield (John Innes Centre, UK), the seed’s protective coating is key. “The seed coat is really important for dormancy and longevity and its properties enable seeds to weather difficult conditions over a long time”, he says. One of its prime roles is to exclude oxygen, which promotes germination and can also damage molecules such as DNA and proteins. “This damage is the main enemy of the long-lived seed”, Steve says. “Seed coats seem to be very good at excluding some things while letting others in, and nobody really knows how they do it”.

Besides this, seed coats also produce ABA, the key plant hormone which prepares the seed for quiescence by arresting cell division and suspending growth. Throughout dormancy, ABA is continuously produced by the endosperm (a layer of tissue surrounding the embryo which acts as a food store) but the mechanism by which it is transported to the embryo to keep it quiescent has only recently been elucidated. Researchers at Pohang University of Science and Technology (POSTECH), the University of Zurich and the University of Geneva (UNIGE) hypothesised that, since ABA delivery to the embryo was so critical to avoid premature germination, it was likely to depend on a coordinated transport system, rather than simple diffusion.

In fact, no fewer than four different types of transporters (belonging to the AtABCG family of transporters) are involved in this process. These act in concert, with two exporting ABA from the endosperm to the embryo, and the other two import the hormone into the embryo [6].

It is hoped that this knowledge could be used to safeguard crop harvests in the future. “Controlling dormancy before harvest is very important, particularly for cereal crops”, says Steve. “Here in the UK for instance, wet summers cause pre-harvest sprouting of wheat. The problem with this is that germination causes wheat to produce enzymes which breakdown the protein and starch needed to make bread, so these batches have to be sent for animal feed”. By integrating knowledge of seed dormancy mechanisms into plant breeding programmes, it may be possible to select plants which avoid premature germination, and thus economic losses. This is particularly pertinent in a world experiencing more variable weather due to climate change. “Temperature changes of a little as 1 °C can have massive effects on seed germination so one of the aims of our lab is to help future-proof seed production against climate change”, Steve concludes.  

Time after time 

Trees can also put us to shame when it comes to longevity, with the Yew Tree being one of the top stars in Europe. “It is generally accepted that Yews can live for at least 3,000 years”, says author and ancient Yew historian, Janis Fry. “However, this can only be an estimate because the heart wood rots away from around the age of a thousand years, so normal dendrochronology methods can’t be used to take an accurate measure”. Instead, other techniques have to be used, such as measuring the girth and using the average number of rings per inch in the tree, as well as taking account of other factors relating to the tree’s condition, the surrounding environment and its history. 

Yew tree that has stood for at 1,000 years in St Nicholas churchyard near Brockenhurst, UK Photo: Patrick Doncaster

But how is it that Yews can reach such staggering ages? According to Janis, “Yew trees are capable of continual regeneration – we see them as immortal giants”. Central to this is the Yew’s capacity to produce aerial roots almost anywhere, including on branches that can be several metres away from the main trunk. These roots emerge from the cambium layer beneath the bark and grow downwards until they touch the soil, at which point they may embed themselves and re-root. Alternatively, they may grow along the surface of the ground and produce new shoots along their length. In this way, a single Yew branch can give rise to a whole grove of trees, such as at Newlands Corner, in Guildford [7]. 

Besides this, Yews can also produce ‘interior roots’ inside the main trunk that grow downwards to reach the ground. According to Janis, “these aerial roots inside the old tree grow into a new tree, either by breaking up the old tree or amalgamating with it”. Occasionally, the original tree may fragment into two or more trees with the same DNA, which may still share a common base or become entirely separate.

Despite their high toxicity which deters predators and parasites, Yews are also a source of chemical compounds that are helping us to live longer: paclitaxel (Taxol), one of the most well-known cancer treatments, originates from the bark of Pacific Yew trees. Little wonder then, that the Ancient druids revered Yews as symbols of everlasting life [8]. 

Stemming the tide

Cases of constant regeneration are also known in the animal world, particularly in so-called “immortal” Planarian worms, which seem capable of renewing themselves indefinitely. Throughout our adult life, we are constantly shedding cells from the gut, skin, hair, etc. which are replaced by small populations of stem cells. There is a limit however, to how long they can do this and over time it becomes more difficult to renew exhausted tissue, with one example being the aging skin. Yet in Planarian Worms, the stem cells seem capable of dividing indefinitely, allowing these worms to defy the ageing process.

According to Planarian researcher Aziz Aboobaker (University of Oxford, UK), the secret lies in the telomeres – ‘protective caps’ of DNA found at the ends of chromosomes which are vital for cell division. Normally, telomeres shorten slightly each time the cell divides, setting an ultimate limit for regeneration. However, in Planarian Worms, telomere length is maintained indefinitely, allowing the cells to divide again and again. This seems to be caused by the enzyme telomerase, which is capable of restoring the ends of telomeres. For most sexually reproducing organisms, this enzyme is mainly active during early development and is then turned off. But in Planarian Worms “it seems that telomerase activity is left switched on in stem cells and upregulated during regeneration”, says Aziz. “This is done by increasing the amount of telomerase transcript and also through the production of more active splice forms of telomerase mRNA. There may be other mechanisms that we don’t know about yet”. 

Maintaining telomere length may also be a strategy used by other animals. For instance, a study by Plot et al. (University of Strasbourg) compared Leatherback Turtle (Dermochelys coriacea) hatchlings and adults found no significant difference in telomere length, suggesting that these did not shorten over time. One explanation may be that a prodigiously early growth phase in this species promotes high telomerase activity levels to facilitate rapid cell division [9]. Other species go even further and can actually make their telomere ends even longer. As discovered by Ujvari et al., Water Python Liasis fuscus hatchlings have significantly shorter telomeres than adults, which then increase rapidly during their first year [10]. If telomeres are not necessarily subject to inevitable shortening, then perhaps telomerase reactivation is a viable anti-ageing strategy for humans? 

Meanwhile, the stem cells of Planarian Worms are special for another reason, as Aziz explains. “Planarian stem cells are pluripotent, which is unusual if not unique for a stem cell contributing to the adult body”, he says. “This means that they can make any other cell type in the body”. Because of this property, Planarians have the capacity to regenerate from almost any starting fragment. In fact, some species have dispensed with sex entirely and merely split in half to reproduce. Curiously, when the molecular profile of these cells was compared with stem cell types from other animals, they showed greater similarities to germline cells than the somatic stem cells that contribute to adult tissues. In particular, they have enlarged nuclei with peri-nuclear RNA rich granules (chromatoid bodies) which contain many components associated with germline cells in other animals. 

It seems that the mysteries of Planarians will be keeping researchers busy for some years yet. “It is unclear why not ageing might be an advantage for these animals but it seems likely that it is an emergent property of being highly regenerative” says Aziz.  In future work, he hopes to use other species to further investigate the interplay between age and regeneration. 

The medusa touch

As a grand finale to this story of the wonderful world of age-defying organisms, take a look at an animal which goes one step further in the “Regeneration Game”. Rather than simply replacing what was once there, the tiny jellyfish Turritopsis dorhnii actually reverses its life cycle and reverts back to childhood. “It is more than simple regeneration, it is an overall reverse transformation or reverse metamorphosis of the body”, says jellyfish researcher, Stefano Piraino (University of Salento, Italy). Otherwise known as the “Benjamin Button” jellyfish (after the film of that name), Turritopsis has a complex life cycle with distinct sexual and asexual stages.

The immortal jellyfish (Turritopsis dohrnii) Photo: Stefanpo Piraino

References:

1.https://goo.gl/lcz2ax
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