Q: Press coverage of a recent study on the ability of the eggs (oocytes) of the roundworm C. elegans suggested that the researchers had discovered a new way that these cells clear out damaged proteins, and thereby “turn back time” and become “young again.” Is there some way that we could take advantage of this mechanism to remove the junk that accumulates inside cells as part of the degenerative aging process in humans?
A: The headlines were certainly provocative, no doubt emboldened by the paper’s opening line (“Although individuals age and die with time, an animal species can continue indefinitely, because of its immortal germ-cell lineage”[1]) and the fact that the senior author has made landmark contributions to understanding the genes that regulate the rate of aging. Some readers got the impression that this study had uncovered a special molecular mechanism that allows these roundworms’ oocytes uniquely to stay “young,” even as the body as a whole grew old. This impression may have been reinforced by a quote from one researcher, contrasting the aging of the human body with the (seeming) “immortality” of the germline (the “line” of sperm and egg genes that actually passes from generation to generation): “You take humans — they age two, three or four decades, and then they have a baby that’s brand new.”
Taken together, some readers came away with the suggestion that the fact that babies are born young implies the ability of oocytes to “sweep themselves clean” of their adult parents’ lifetime burden of deformed proteins, and excitedly hoped that the tricks that oocytes use to execute this feat could somehow be engineered into aging cells elsewhere in the body to keep our muscle and brain cells young.
Unfortunately, no such tricks emerged from this study, nor are they likely to. This study1 adds substantial insight to a body of work on roundworm (and later frog) oocyte biology sparked by a discovery made by French scientists in 2010[2] and prior work in yeast and in mouse embryos. However, there is nothing here that can be exploited for developing anti-aging therapies.
Off Again, On Again
The real finding of the paper is better captured by its own title than the newspaper headlines: “A lysosomal switch triggers proteostasis renewal in the immortal C. elegans germ lineage.” The key word in there is not “immortal,” but “renewal” — renewal of “proteostasis,” the somewhat equivocal concept of the young cell’s dynamic maintenance of stably low levels of damaged proteins. As it turns out, the “renewal” in question is a reactivation of the normal “proteostatic” activity of the lysosome — the cell’s recycling center, where old and damaged proteins are broken down into raw materials that can then be reused to build new proteins.
While oocytes are held in storage, they adopt a metabolically dormant state to conserve energy and reduce the production of metabolic wastes. This much is just as true in mammals as it is in the roundworms and frogs studied in this new report. What the new study uncovered is a particular energy-conservation strategy these animals’ oocytes use.
When there is no opportunity for fertilization, the oocytes of roundworms and frogs temporarily deactivate the energy-intensive molecular pumps that keep their lysosomes’ interiors acidic. This acidity is needed for lysosomes break down damaged proteins, so when some oocyte proteins become damaged (notably, via an alteration known as carbonylation), there’s no way for them to be degraded. Instead, these damaged proteins aggregate into clumps and slowly accumulate inside the cell.1
But things change suddenly as fertilization approaches. As a sperm draws near to an oocyte, the oocyte senses the sperm cell’s “major sperm proteins” (yes, that’s the term!), and jumps into action in preparation for fertilization. One of its first actions is to rapidly kick its lysosomes’ ion pumps into gear, awakening their ability break down the waste that’s accumulated while the oocyte was in waiting mode. Once acidified and ready to take in wastes again, the lysosomes actively reach out to nearby aggregated proteins, quickly cleaning house so that the oocyte is ready just in time for the big event.1
Being able to put off normal housekeeping in oocytes until they are just about to be fertilized means that no energy is wasted in maintaining oocytes that may not be fertilized for some time, and may never be fertilized at all. The authors also speculate that the rapid breakdown of the accumulated carbonylated proteins may even provide a source of energy for the oocyte, just in time for the energy-intensive processes that follow fertilization.1 But no special rejuvenative power is involved in this process: other cells clean up these same wastes routinely, as a matter of day-to-day housekeeping, instead of letting them build up until it’s absolutely necessary to get rid of them. You doubtless are familiar with similar patterns of housekeeping in your own circles. Both systems can work, especially in a household that doesn’t create much mess in the first place.
Indeed, part of what makes this system workable is that oocytes don’t generate much waste in their metabolically dormant state, so there’s little for them to clean up when the opportunity for fertilization comes around. Such metabolic dormancy also helps maintain the integrity of oocytes in mammals, although there’s no evidence yet that mammalian oocytes put lysosomal acidification on hold until they’re ready to be fertilized. By contrast, deactivation of the lysosome in cells with very high metabolic activity (like neurons and muscle cells) would be disastrous, because such cells produce much higher levels of waste, and are accordingly already vulnerable to the accumulation of much tougher aggregates even with their lysomes working full-time.
Similarly, the very low metabolic activity of oocytes compared to cells in the rest of the body (especially as compared with highly energy-demanding tissues like muscle and brain) reduces the rate at which mitochondria generate free radicals during energy generation, and thus the rate of formation of mitochondrial mutations and other aging damage. Obviously, our lives would not be possible if we spent our days with energy-intensive tissues like brain and muscle in a similar state of dormancy.
Dying that Others May Live
Coming back to the question of the so-called “immortality” of the germ line: an important feature of many of the mechanisms that keep the germ line clear of major mitochondrial mutations and other aging damage is that they come at the expense of the death of many individual oocytes, which are ruthlessly culled in order to avoid passing such damage on to the next generation. When mitochondrial mutations do occur in aging oocytes despite the protective effect of low mitochondrial respiration, their appearance drives both the programmed cell death of individual oocytes and the culling (atresia) of entire follicles that release new oocytes from the ovaries.[3],[4]
This culling process prevents oocytes dominated by mitochondrial mutations from being fertilized and developing into babies with terrible inherited mitochondrial disease (though tragically, even this mechanism sometimes fails). But it also drives the eventual exhaustion of a woman’s reserve of follicles (and thus, oocytes) with age.3 A woman is born with 1-2 million ovarian follicles; by the time she reaches puberty, she is already down to just 300,000 — and despite the fact that she only loses one (or a few) eggs per month to ovulation, only a few hundred of the healthiest of them remain when a woman is in her fifties or sixties. Once the ovaries are unable to respond to the hormonal signal to ovulate, this lack of response triggers the hormonal chaos of menopause, and all the age-related problems associated with it.
Programmed cell death also occurs throughout the body, just as it does in oocytes, as individual cells that have suffered mutations or other damage that puts them at risk of turning cancerous sacrifice themselves to protect the body from their own deadly predilection. But we can’t afford to “solve” the rising burden of mitochondrial mutations and other aging damage in our brain neurons and muscle cells with age by killing off even more such cells as soon as they suffer such damage.
The Reality of Reproductive Aging
Despite the lengths to which the body goes to maintain only viable, “young” eggs, oocytes do still manage to degenerate with age, which is part of the reason why older parents are less fertile (rising numbers of the sperm and eggs become duds), have more miscarriages (in addition to problems in the ability of older mothers’ bodies to support the developing embryo, the embryo itself is more likely to inherit fatal flaws acquired in the aging sperm and egg), and are more at risk for birth defects. The example you probably know about already is the increasing risk of congenital abnormalities as women reach middle age, most especially Down syndrome. These increased risks are driven by the accumulation of abnormalities in the oocytes, such as aneuploidy — an abnormal number of chromosomes in the cell.
Mitochondrial free radicals also contribute to abnormalities in organelles and other constituents of aging oocytes.2 Calorie restriction (CR) reduces the rate of accumulation of mitochondrial and chromosomal damage in oocytes in laboratory rodents,[5] consistent with its known ability to lower the rate of production of mitochondrial free radicals and to retard the aging process in many species (although with uncertain effects in primates). In fact, CR is so effective at preserving oocytes and other parts of the female rodent reproductive system against aging damage “that ovulated oocytes of aged female mice previously maintained on CR … are comparable to those of young females during prime reproductive life.”5 When mice are started on CR around the time they first become fertile and then returned to a conventional diet just as the last of littermates kept on a lifelong conventional diet are losing fertility, animals previously kept on CR are able to keep delivering viable pups for much of the remaining lifespan, and do so in higher numbers and with greater offspring survival rates than conventionally-fed animals half their age.[6]
And it isn’t just women whose reproductive systems are ravaged by the degenerative aging process. While men don’t suffer the sudden and dramatic loss of fertility that women do with age, and while congenital diseases are less often directly attributable to the aging of sperm than to the aging of eggs, the fact remains that sperm cells from middle-aged and older exhibit a wide range of abnormalities, and that embryos of older fathers are more likely to spontaneously abort or to suffer genetic disorders — many, again, caused by aneuploidy of the sperm, and also by mutations and by abnormalities in the structure of individual chromosomes.[7]
The silver lining in all of this bad news: because the nature of the degenerative aging process is not different from the aging of the rest of the body at the cellular and molecular level, the “damage-repair” heuristic of rejuvenation biotechnology can be applied to rejuvenate the aging reproductive system just as it can to the rejuvenation of the rest of our bodies.
Real Damage Repair
As you can see, we’re not going to solve the degenerative aging process by borrowing any special tricks from the oocyte.[8] The oocyte doesn’t really have any tricks for us to profitably exploit — and more importantly, no cell in the body is naturally able to remove or repair many of the kinds of damage that accumulate in aging bodies and ultimately lead to age-related disease, debility, and death. The oocyte has no way to clear beta-amyloid from aging brains, or TTR amyloid from aging hearts — nor to cleave AGE crosslinks from aging arteries, to none of which damage they are subject. It has no internal means to replace cells that are lost to aging damage,8 and is no more able to degrade the truly stubborn intracellular aggregates that accumulate in aging cells than any other cell type.
For that, we need a new class of medicines that can do what we can’t do on our own: remove, repair, replace, or render harmless the cellular and molecular damage of aging in our tissues. It is when we develop rejuvenation biotechnologies and deploy them comprehensively that we will finally be able to effectively “turn back time” for aging bodies as a whole.
References
- Bohnert KA, Kenyon C. A lysosomal switch triggers proteostasis renewal in the immortal C. elegans germ lineage. Nature. 2017 Nov 30;551(7682):629-633. doi: 10.1038/nature24620. Epub 2017 Nov 22. PubMed PMID: 29168500.
- Goudeau J, Aguilaniu H. Carbonylated proteins are eliminated during reproduction in C. elegans. Aging Cell. 2010 Dec;9(6):991-1003. doi: 10.1111/j.1474-9726.2010.00625.x. Epub 2010 Oct 29. PubMed PMID: 21040398.
- May-Panloup P, Boucret L, Chao de la Barca JM, Desquiret-Dumas V, Ferré-L’Hotellier V, Morinière C, Descamps P, Procaccio V, Reynier P. Ovarian ageing: the role of mitochondria in oocytes and follicles. Hum Reprod Update. 2016 Nov;22(6):725-743. Epub 2016 Aug 25. Review. PubMed PMID: 27562289.
- Ramalho-Santos J, Varum S, Amaral S, Mota PC, Sousa AP, Amaral A. Mitochondrial functionality in reproduction: from gonads and gametes to embryos and embryonic stem cells. Hum Reprod Update. 2009 Sep-Oct;15(5):553-72. doi: 10.1093/humupd/dmp016. Epub 2009 May 4. Review. PubMed PMID: 19414527.
- Selesniemi K, Lee HJ, Muhlhauser A, Tilly JL. Prevention of maternal aging-associated oocyte aneuploidy and meiotic spindle defects in mice by dietary and genetic strategies. Proc Natl Acad Sci U S A. 2011 Jul 26;108(30):12319-24. doi: 10.1073/pnas.1018793108. Epub 2011 Jul 5. PubMed PMID: 21730149; PubMed Central PMCID: PMC3145697.
- Selesniemi K, Lee HJ, Tilly JL. Moderate caloric restriction initiated in rodents during adulthood sustains function of the female reproductive axis into advanced chronological age. Aging Cell. 2008 Oct;7(5):622-9. doi: 10.1111/j.1474-9726.2008.00409.x. Epub 2008 Jul 24. PubMed PMID: 18549458; PubMed Central PMCID: PMC2990913.
- Amaral S, Amaral A, Ramalho-Santos J. Aging and male reproductive function: a mitochondrial perspective. Front Biosci (Schol Ed). 2013 Jan 1;5:181-97. Review. PubMed PMID: 23277044.
- The one pseudo-exception is the use of oocytes to engineer pluripotent stem cells that are a perfect match for the patient, in the process known as somatic cell nuclear transfer or “therapeutic cloning.”
