Perhaps the point of the beginning of aging lies in the first month of embryonic development
Polina Loseva, "Elements"
The zero point of aging is the period of development in which the biological age is minimal. According to the hypothesis of Vadim Gladyshev, this condition occurs, on the one hand, not immediately after fertilization, and on the other – long before a person is born. An image from the article under discussion in Trends in Molecular Medicine.
The question of whether aging is an inherent property of life is still open. You can look for an answer to it, for example, by trying to find universal mechanisms of aging for all living things. And you can go from the other side – try to find the point of the beginning of aging and check whether it coincides with the beginning of the life of the organism. Harvard biologist Vadim Gladyshev put forward the hypothesis of the "zero point of aging", which, in his opinion, a person is somewhere in the second week of embryonic development. Many consequences can be deduced from this assumption – both about the principles of aging of all living things, and about new ways to prolong human life.
In pursuit of the ideal
The main problem with aging research is that researchers themselves cannot agree on what it is. The absence of a normal definition of the concept of "aging" gives rise to all the other problems of modern gerontology: the variety of proposed causes of aging (which, if desired, can be counted more than three hundred), disputes about methods of prolonging life, as well as the inability to find out whether aging is a universal property inherent in all living organisms, or an acquired strategy.
One could, for example, define aging as a transition from a state of youth to a state of old age. At the same time, it would make it easier to set up experiments, for example, in the field of life extension: it would be possible to try some new technology on experimental animals and calculate what proportion and in what time will pass from the category of young to the category of old.
But here the border problem arises. Experience shows that most physiological indicators change continuously. Whatever we take for the beginning of old age – be it, say, the appearance of some specific disease or just the end of reproduction - we will quickly find out that none of these signs occur overnight. Even animals, as a rule, do not lose their fertility at the same time – they simply begin to bring less and less viable offspring. And if so, is it reasonable to classify as young an individual that is still reproducing, but gives birth to one sick cub instead of ten healthy ones?
If we cannot draw a clear line between youth and old age, that is, we cannot catch the moment when age measurements reach critical values, then, going from the other side, we can try to designate the beginning and end of aging. In this case, we could define aging as a movement from point A to point B, where A is some ideal, completely healthy state, and B is a natural death. And if everything is clear with the endpoint, then we still don't know anything about the initial one. Where to count age-related changes from? Where to look for an ideal unspoiled by aging?
Apparently, if this ideal exists, it will not last very long. Whatever definition of aging we grasp, it turns out that the accompanying age-related changes occur quite quickly after a new organism is formed. If we assume, for example, that aging is the accumulation of breakdowns (in the broad sense of the word), then we have to admit that point mutations in DNA appear already in the first days after fertilization, when the embryo is just beginning to split (T. Bae et al. Different mutational rates and mechanisms in human cells at pregastrulation and neurogenesis). If we think of aging as a growing risk of dying from natural causes, then we can also see here (Fig. 2) that this risk begins to grow long before a person is born.
Signs of aging and the mortality caused by them can be detected at the very beginning of embryonic development, but it is often masked by early selection, during which many embryos with dangerous mutations die. Image from the article by E. Kinzina et al., 2019.
It turns out that the time of the appearance of many signs of aging is early embryogenesis. But even this assessment is not accurate enough and does not allow us to answer all the questions about the phenomenon of aging. I would like to know for sure exactly where his zero point is – a lot depends on this in our understanding of the process. If we could prove that the aging process begins simultaneously with the appearance (even inside the mother's womb) of a new organism, which, for example, can be considered a fertilized egg – zygote, it would mean that aging can be considered a property of life as such. If it turned out that aging begins later, at some other moment, then it would be possible to further discuss whether it should be regarded as an evolutionary acquisition and the launch of a certain program, or to reconsider our ideas about what we consider the beginning of a new life.
Between the beginning and zero
Why do we even think that the beginning of aging may not coincide with the beginning of life? If it is already clear that the first "bells" of age–related changes can be found, albeit in small quantities, at the early stages of embryogenesis, then what prevents us from deciding that they begin to accumulate at the moment when the first cell of a new organism is formed - the zygote?
Difficulties begin at the moment when we remember that a new life does not arise from scratch, but is formed from the fusion of two cells belonging to organisms that have lived in this world for a long time. A logical question arises: where does the "accumulated age" of the parent cells go at this moment? There is no doubt that it must disappear, "reset", otherwise each next generation would be born a little closer to the natural end than the previous one, and all populations of organisms would move towards inevitable extinction.
There are several possible options here.
1. "Zeroing" occurs during gametogenesis.
In other words, while the whole body is aging, wearing out and accumulating breakdowns, somewhere in its bowels there is a spark of eternal youth, concentrated in a group of germ cells, gametes. This division of functions between cells is the basis of the theory of disposable soma, according to which only gametes (cells of the so–called germ line) are truly ageless, and the rest of the body is just a superstructure (soma) that allows them to form a new organism and after that can be discarded as unnecessary.
We do not yet know what mechanisms are involved in maintaining this light (see B. Zhang, V. Gladyshev, 2020). It could be assumed that resources are simply redistributed in germ cells: energy is spent not on the growth and construction of new molecules (which, as a rule, is fraught with the accumulation of production waste), but on maintaining intracellular order (autophagy and DNA repair system, for example, could be responsible for it).
Nevertheless, we know that age still affects the "health" of specific germ cells. Elderly spermatozoa accumulate point mutations in themselves – a consequence of frequent divisions, and it becomes increasingly difficult for eggs to keep their chromosomes intact with age. But, despite this, the children of elderly parents do not inherit their age directly. Those who managed to inherit chromosomes without abnormalities do not age prematurely and do not produce prematurely aging children.
This means that we cannot consider germ cells not aging in principle and must assume that at some point in their life they also need a "reset" of age.
2. "Zeroing" occurs during fertilization.
Not much time passes between the fusion of the membranes of two germ cells and the formation of a single genome of a new embryo (for example, in humans, the embryo's own genes begin to work a few days after fertilization), but in this short interval many intracellular rearrangements take place. For example, in human zygotes, the properties of the membrane change at this time: in order to avoid repeated fertilization, the division of the egg's genetic material ends and the cytoskeleton is rebuilt. Why not at this moment some other processes that would help to cope with the accumulated molecular debris in the germ cells?
Something like this seems to happen during fertilization in nematodes (see K. A. Bohnert, C. Kenyon, 2017). The composition of their sperm, in addition to the germ cells themselves, includes signaling proteins, under the influence of which lysosomes "wake up" in oocytes, which begin to pump hydrogen ions through the membrane, creating an acidic environment inside the oocyte. After that, the oocyte gets rid of protein aggregates (that is, molecular debris) that have accumulated in it during maturation, and approaches fertilization already pristine.
Similar processes were found in the smooth spur frog (Xenopus laevis, Fig. 3), which means that this could serve as a universal mechanism for "zeroing" the age of oocytes. However, according to other works, protein degradation in amphibian oocytes mainly affects regulatory proteins of the cell cycle – that is, its meaning is not so much in updating as in lifting the ban on the division of the future zygote (see M. Presler et al., 2017).
Activation of lysosomes in the Xenopus laevis frog oocyte preparing for fertilization. Protein aggregates (green) and fragmented mitochondria (yellow) are accumulated in the resting oocyte (left). Under the influence of sperm or hormonal signals, lysosomes (red) begin to acidify in the oocyte (right), protein aggregates and damaged mitochondria disappear, and the remaining mitochondria grow (brown). Image from an article by K. A. Bohnert, C. Kenyon, 2017.
Therefore, we do not yet have unambiguous arguments in favor of putting the zero point of aging immediately before or after fertilization. In addition, some other processes that could be attributed to rejuvenation are just beginning after fertilization.
3. "Zeroing" occurs in the first days of embryo development.
For example, we know that after fertilization, telomerase is triggered in the embryo cells and telomeres begin to grow (see L. Liu et al., 2007). This is absolutely necessary for cells that have to repeatedly divide, increasing the mass of the embryo, since telomeres inevitably shorten during division. But at the same time, this can be considered a decrease in biological age, which is often determined just by the length of telomeres.
In addition, at least in all vertebrates and some invertebrates (for example, echinoderms and tunicates), fertilization is followed by a wave of DNA demethylation (see X. Xu et al., 2019, and Z. Smith et al., 2014).
This process has a simple functional meaning: since gametes are differentiated cells, some of their genes are tightly packed, and the information recorded in them is not available. To give the embryo access to those genes that an adult organism does not need, it is necessary to remove epigenetic markers from DNA and unpack it back. And at the same time, demethylation "zeroes" the biological age of cells – if measured by another popular marker, the epigenetic clock (that is, a set of tags on DNA and histones).
Harvard biologist Vadim Gladyshev suggested that at some point during the early development of the embryo, these processes should "meet", and that's where the zero point of aging will be (Fig. 4). This state should be the culmination of all rejuvenation processes that "reset" the age of cells on all scales of biological age. Telomeres, epigenetic labels, intracellular molecular debris, even the level of entropy – all these signs should converge in an "ideal", zero state. The only exception that Gladyshev cites are mutations inherited from parents or acquired de novo during the first divisions of crushing. It will not be so easy to "reset" them and delete them from DNA, so here we have to rely on natural selection (carriers of the most unsuccessful mutations will not survive), which reduces the age not of each embryo individually, but of the embryo population as a whole.
The zero point of aging according to Gladyshev. In this state, in particular, the telomere length is maximal, and the biological age of the embryo is minimal. Entropy on the upper graph (orange line) – a conditional measure of the complexity of a developing organism. An image from the article under discussion in Trends in Molecular Medicine.
However, Gladyshev admits that not all rejuvenation processes can occur simultaneously. In his concept, there is a place for all the options that we discussed above: something happens in the germ cells even before fertilization, something "winds up" in the zygote after it, something begins already in the fragmenting embryo. Therefore, the zero point may in practice turn out to be a whole zero period of aging, a special state of the embryo.
At what exact moment the embryo reaches this state is still an open question. It can probably be determined if we extrapolate the graphs of the epigenetic clocks we have (Fig. 5). It is known, for example, that the epigenetic clock can "detect" the first signs of aging already on the 45th day after conception (A. Hoshino et al., 2019), and it would be possible to extend this graph into the negative region and try to find the point of its intersection with the graphs of other markers of biological age. However, there is no guarantee that this point will be found – or that it will turn out to be a single moment, and not a whole period of development.
Extrapolation of the graph for biological age in the embryonic period allows us to predict where its minimum is. An image from the article under discussion in Trends in Molecular Medicine.
Gladyshev suggests that the zero point (or zero period) may lie somewhere between the blastocyst stage (if we are talking about mammals) and pharyngula (pharyngula), in humans it is a week and about a month of development, respectively. The blastocyst is the stage at which the embryo is represented by pluripotent cells, that is, those from which any cell of an adult organism can be obtained (Fig. 6). This is the stage to which adult cells "roll back" after reprogramming, so it is logical to consider it a certain "minimum of development".
Simplified scheme of human embryo development in the first five months of pregnancy. The extreme right stage in the upper row is the blastocyst, the extreme left in the lower row roughly corresponds to the pharyngula. Somewhere between them, according to Gladyshev, may lie the beginning of human aging. Drawing from the website byjus.com .
The pharyngula is the stage that is considered the most conservative in the development of vertebrates, in humans it corresponds to the period of development after neurulation. At this point, the embryo has already formed a body plan and the nervous and immune systems gradually begin to work. In Gladyshev's concept, this is a transitional moment when the "beginning of cellular life" turns into the "beginning of the life of the organism".
Time to move zero
The idea that the vertebrate organism goes through the zero of its life somewhere between the blastula and the pharyngula suggests that aging is a universal feature of multicellular. Gladyshev himself, however, believes that ageless multicellular ones are also possible – among, for example, coelenterates or sponges – but considers them rather primitive and almost transitional forms between multicellular and unicellular (see F. Galkin et al., 2019). Nevertheless, he considers only unicellular and hypothetical multicellular ancestor to be truly ageless organisms – that, in Gladyshev's view, spent his whole life in approximately such a "zero" state as cells in the embryo of vertebrates.
Many questions arise to this statement. On the one hand, the definition of a "zero point" or "zero period" does not allow us to conclude that it is possible to exist in such a state for a long time. At least as far as the demethylation of the genome is concerned, we know that this is a fairly short period of time (a few days) – it is immediately followed by a wave of new methylation, which "closes" back the genes that are unnecessary for the development of the embryo. On the other hand, not all scientists recognize unicellular organisms as obviously ageless organisms. Does this mean that there are several different types of aging – "unicellular" (this is probably how germ line cells can age) and "multicellular" (this is how soma ages, the superstructure over the germ cells), as well as different rejuvenation processes corresponding to them? Or do multicellular organisms have to combine several types of aging at once?
In a sense, Gladyshev answers this question as well. According to him, the model of the "zero point of aging" does not refute, but expands the theory of disposable soma. The zero state is the moment when there is no separation between the soma and the germ line. This is the period when both the soma and the germ line are rejuvenated at the same time – before splitting into aging and (practically) ageless cells. And this is the very time when the aging of the soma and the germ line can be affected simultaneously.
It is possible, Gladyshev believes, that the zero point from which the trajectories of aging of soma and germ cells diverge does not correspond to the "absolute" biological zero. Natural selection does not necessarily have to favor a decrease in this zero, since the consequences of being born with a non–zero biological age do not affect the health of the body immediately, most often after the reproductive period (it is not for nothing that gerontologist Joao Pedro De Magalhaes suggests calling evolution a "short-sighted watchmaker"). And if so, then you can try to move the zero point even lower (Fig. 7). This is unlikely to lead to the fact that the embryos will develop better, and the offspring will be born healthier, but it may delay the onset of the moment when accumulated age–related changes begin to reduce viability, in other words, it may postpone aging in the everyday sense of the word.
If rejuvenation occurs at the beginning of the development of the body, then why not lead it and move the point of aging even lower? An image from the article under discussion in Trends in Molecular Medicine.
Such proposals do sound from time to time. For example, you can come across the idea of additionally increasing the telomeres of the embryo – since their length may not have a limit value that would correspond to a biological "zero". And some time ago, it was really possible to breed mice with extra-long telomeres (M. Muñoz-Lorente et al., 2019). To do this, the researchers took cells from mouse embryos at the blastocyst stage, when telomerase is particularly active in them, and cultured them, not allowing them to differentiate until the telomeres were about twice as long as the average. Then these cells were injected back into the blastocysts – and they turned out to be healthy, viable mice that lived a little longer than their peers (Fig. 8). And after that, a suggestion immediately arose: why not artificially slow down the differentiation of cells in the embryo by changing the concentration of substances inside the uterus in order to grow potential centenarians with long telomeres (M. Bonafè et al., 2020)?
Mice with extra-long telomeres (left in the left photo and right in the right) are less susceptible to obesity than their control peers. Image from the article by M. Muñoz-Lorente et al., 2019.
Other options for manipulating embryos are also possible. For example, not so long ago it turned out that the chicks of the white-seamstress flycatcher (Ficedula albicollis), under the influence of maternal thyroid hormones, telomeres also grow longer than they should (A. Stier et al., 2020.). One can only guess how many more unexpected ways there are to lower the "biological zero".
Of course, we understand that in relation to a person, all these arguments can only be of a theoretical nature so far. It is unlikely that anyone will seriously start advising pregnant women to stimulate their thyroid gland or slow down the development of the embryo in the near future. However, in the future, this kind of approach could be a complement to other life extension strategies. All other methods that are being discussed today are aimed rather at slowing down changes in the adult body. And reducing the "biological zero" could extend the "basic" life expectancy, the original "shelf life" that is measured to the human body.
Another question is whether there is a limit to this decrease, "absolute zero aging", similar to absolute zero temperature? For biological reasons, it follows that it must exist – if only because it is possible to imagine a cell without a single "garbage" molecule and without a single epigenetic label on DNA, which there is nowhere else to rejuvenate. How far does today's "zero" of our embryos lie from this speculative "ideal" state? How much closer could we theoretically get to it? How much could this reduction prolong our days – and would this bonus be able to compensate for the risks that such an intervention would inevitably bring with it?
There are no definite answers to all these questions yet, and there is no way to theoretically calculate them now. However, it is already clear that the relationship between life, birth, aging and death is much more complicated than it may seem at first glance.
Source: V. Gladyshev. The ground zero of organismal life and aging // Trends in Molecular Medicine. 2020.
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