Extract from old relatives accelerates aging

Alexander Markov, "Elements"

One of the reasons for aging, many experts consider the accumulation of molecular "damages" of one or another nature with age. To test this hypothesis, American and Korean biochemists conducted experiments on three model objects – yeast, fruit flies and mice. Experimental organisms were fed extracts from young or old relatives. It turned out that in all three cases, the "old" diet based on an extract from old relatives contributes to accelerated aging. The results are consistent with the assumption that harmful substances that reduce viability accumulate in the body with age, and this is one of the causes of aging.

Although the causes and mechanisms of aging are being studied very intensively, there is still a lot of uncertainty in this matter. One of the most common causes of aging is considered to be the accumulation of various changes at the molecular level with age, whether it is changes in gene expression, their methylation, concentrations of certain metabolites, accumulation of somatic mutations or oxidative stress. However, not all researchers agree that the accumulation of molecular damage plays a significant role in aging. The main problem here is that such injuries are very diverse and depend on a huge variety of genetic and environmental factors. Therefore, their contribution to aging is difficult to prove or, even more so, to measure.

A group of American and South Korean scientists led by biochemist Vadim Gladyshev from Harvard Medical School (see: Gladyshev Lab) conducted a series of surprisingly simple and visual experiments to test the hypothesis that molecular changes occurring in the body over time contribute to aging. The authors reasoned that if this is the case, then a diet based on an extract from elderly relatives should accelerate aging. Of course, not all the "senile substances" contained in such an extract will be absorbed by the body. But there will be some, and this may be enough for the desired effect.

It is known that diet strongly affects life expectancy and the dynamics of aging. Two key signaling cascades have been identified and well studied, providing a link between what we eat and how long we live: a cascade involving insulin/insulin-Like Growth Factor Signaling and a cascade involving mTOR (the target protein of rapamycin). If we assume that some endogenous (internal) metabolites accumulating with age affect the work of these cascades (which, in turn, affect the course of aging and life expectancy), then why would these cascades not react in a similar way to the same metabolites assimilated with food? No matter how strange and unexpected such an assumption may seem at first glance, it certainly deserves experimental verification.

The experiments were carried out on three classical model objects widely used in gerontology: budding yeast Saccharomyces cerevisae, Drosophila melanogaster and mice Mus musculus (Fig. 1).

Fig. 1. The scheme of the experiment. Extracts from old or young relatives were added to the food of three model objects – yeast, fruit flies and mice. It turned out that all three species live longer on a "young" diet. Here and below are the drawings from the discussed article in Science Advances.

1. Experience by leaps and bounds. 

The idea of using a single-celled organism as a model for studying aging may seem strange, but budding yeast is well suited for this. The "lifespan" of yeast is estimated in two ways. You can put yeast cells in a medium with a limited amount of glucose and wait until all the glucose is eaten. After that, the yeast switches to nutrition with substances less pleasant to them, such as ethanol and acetate. At the same time, they stop dividing (although they remain metabolically active), their viability gradually decreases, and mortality increases. It is possible to estimate the "chronological lifespan" of yeast cells, which is defined as the time from entering the stationary phase (when cells stop multiplying) to death. The decrease in the viability of yeast under these conditions is similar in many biochemical parameters to the aging of postmitotic (stopped dividing) cells of multicellular organisms. Yeast is also used as a model of aging of dividing cells. To do this, the "replicative lifespan" is estimated, which is measured as the number of daughter cells that a given yeast cell will have time to bud off under favorable conditions until it dies (K. K. Steffen et al., 2009. Measuring replicative life span in the budding yeast).

The authors made a nutrient medium for yeast based on extracts (lysates) from young yeast cells, whose "chronological age" was 3 days, and from old ones (chronological age 8 days). Such extracts contain a sufficient amount of all nutrients for yeast growth, except glucose, which was added to the feed separately.

It turned out that yeast living on a "young" nutrient medium has a significantly higher replicative lifespan than the same yeast grown on an "old" medium (Fig. 2). It also turned out that the extract of old cells shortens the life of yeast, including in very high concentrations. Apparently, this means that the shortening of life is explained not so much by the fact that something useful is missing in the "old" extract (in this case, an increase in concentration would smooth out the effect), but by the fact that something harmful is present in it. Additional experiments have shown that the low-molecular fraction of yeast extract has a stronger effect on life expectancy than the high-molecular fraction. This is logical, because low-molecular compounds are more easily absorbed by cells.

Fig. 2. Survival of budding yeast on the "young" (blue line) and "old" (red line) nutrient medium. On the vertical axis – the percentage of surviving cells, on the horizontal – the number of budded daughter cells ("replicative age").

2. Experience on fruit flies. 

The simplest laboratory food for fruit flies is prepared on a yeast basis (50-60 g of fresh yeast is boiled for a long time in a liter of water) with added sugar. The authors replaced boiled yeast in this recipe with an extract of young or old fruit flies. To prepare the extract, young flies (aged 3-5 days after emerging from the pupa) were frozen alive in liquid nitrogen. Then they were ground into powder, diluted in water and centrifuged. At the same time, all insoluble precipitated, and the superadding liquid was used to prepare feed instead of yeast. For the "old" feed, flies were used that had just died of natural death at the age of 30-60 days (45 on average). They were treated the same as the young ones. The amount of fly extract added to the feed was selected so that the protein content in the nutrient medium was the same as in the standard yeast-based feed.

It turned out that on the "young" diet, female fruit flies live on average longer than on the "old" one (Fig. 3). Males were not tested due to the difficulty of mass harvesting of old flies for "old" feed. Thus, the pattern found in yeast turned out to be true for female fruit flies.

Fig. 3. Survival of female fruit flies on a "young" (blue line) and "old" (red line) diet. On the vertical axis – the percentage of surviving flies, on the horizontal – the age in days.

3. Experience on mice. 

To test a group of mice for life expectancy, a lot of food is required, because mice live for three years, and sometimes more. Therefore, it is possible to understand the authors who decided not to mess with the mass harvesting of mouse meat and used more affordable material – the meat of young (three-year-old) and old (25-year-old) red deer males. These animals are bred on farms, so it is not difficult to get a deer of the right age. The protein and fat content in the muscles of young and old deer differs (the young have more protein and less fat), which can affect the life expectancy of those who feed on them. Therefore, the researchers had to balance the "young" and "old" diets in protein, fat and calories by mixing standard mouse feed with deer meat in different proportions.

The diet did not affect the lifespan of male mice, but the results for females were the same as for yeast and fruit flies: on the "old" diet, females lived 13.3% less than on the "young" one (Fig. 4).

Fig. 4. Survival of female (left) and male mice on a "young" and "old" diet. Notation as in Fig. 3.

The authors dissected dead mice to understand the causes of death, but could not get clear results: neither the number of tumors nor the level of amyloidosis (the most typical causes of death of old mice) showed significant differences between the groups. But it was possible to show that, according to the composition of the intestinal microbiota, mice living on an "old" diet differ more from control mice (eating ordinary food) than mice living on a "young" diet.

Thus, it was shown at all three sites that the "old" diet shortens life compared to the "young" one. This result is consistent with the assumption that molecular changes occurring with age negatively affect viability, and that the "bad biochemistry" characteristic of old organisms can negatively affect the life expectancy of those who feed on them.

This study differs from most scientific articles published today in highly rated journals in its amazing simplicity and audacity. The methods used are vulnerable to criticism. For example, young fruit flies were frozen alive to prepare the extract, and old ones were frozen posthumously, so these flies differed not only in age. Symbiotic microorganisms – bacteria and yeasts living in the intestines and on the surface of the drosophila body – could survive freezing and then multiply on fly food, affecting the life expectancy of the tested flies (and it is known that the composition of the fly microbiota changes with age). In the tested yeast, the "replication" age was measured, but for the preparation of the nutrient medium, the cells were sorted by "chronological" age (because it's easier that way). Male fruit flies were not tested at all (because there were not enough old flies for them, from which the "old" nutrient medium was made). Yeast and flies were fed with extracts of representatives of the same species, and mice were fed not with mouse meat, but with venison, because it is difficult to stock so much mouse meat. It is not very clear why the differences in the mortality of mice are not consistent with the autopsy results, and so on.

The experiments carried out must be repeated many times by other researchers before we get the right to draw far-reaching conclusions based on them. But still, the coincidence of the results obtained on the three model objects is impressive. Let's hope that further research will soon show how general the discovered pattern is and which "senile substances" negatively affect life expectancy.

Source: Lee et al., Age-associated molecular changes are deleterious and may modulate life span through diet // Science Advances. 2017.

Portal "Eternal youth" http://vechnayamolodost.ru  21.02.2017