Melina Schuh has been fascinated with ovarian germ cells, or oocytes, since she began her graduate studies in 2004. “[I] realized that only little was known about these important cells that are crucial for reproduction and the creation of a new life,” she said. Now a biochemist at the Max Planck Institute for Multidisciplinary Science, Schuh is curious about how oocytes—which are all formed before birth—are able to remain functional for decades, and interested in the factors that might lead to the eventual decline of these understudied cells.
In a recent study, Schuh and her team showed that cells in the mammalian ovary contain proteins with extremely long lives.1 The findings, published in Nature Cell Biology, shed light on the adaptations that help maintain oocytes with minimal damage throughout a female animal’s reproductive life and offer clues about fertility decline in aging ovaries.
“Although the biology of extremely long-lived proteins in aging has been known for a while, this is the first paper to carefully characterize the nature and identity of those proteins in the ovary,” said Lei Lei, a reproductive biologist at the University of Missouri School of Medicine, who was not associated with the study. Making new proteins comes with the risk of making mistakes, which oocytes cannot afford to do, she added. “Because after all, you’re going to support a new life.”
Reports of a long-lived protein complex in oocytes left Schuh wondering about the prevalence of this phenomenon and whether protein longevity plays a functional role in maintaining these cells.2 To determine how long proteins could persist in oocytes, Schuh and her team fed pregnant female mice a diet with an amino acid containing a heavy isotope of carbon. These mice, along with their in utero pups, integrated this heavy element into their proteins. When the animals gave birth, scientists switched the newborns to food containing a lighter isotope of carbon. This strategy meant that proteins made by the pups before the diet change would contain heavy carbon, while proteins synthesized later would carry the lighter version.
The researchers then collected oocytes from these pups when they reached puberty at eight weeks old. Mass spectrometry analysis of these cells revealed that almost 10 percent of the proteins were made before the mice were born. The long-lived proteins belonged to different cell components like mitochondria, ribosomes, and chromatin, and were involved in functions like metabolism and DNA repair.
The ovary consists of cells other than oocytes, like stromal and thecal cells, that play essential roles in fertility. The team wondered whether these cells also housed long-lived proteins. They analyzed proteins from the ovaries of mice up to 15 months old, an advanced age for mice. Mathematical modeling showed that more than 10 percent of the proteins had a half-life of more than 100 days, with many persisting in the ovaries for most of the animals’ lives. In comparison, less than one percent of proteins in the cartilage, brain, and muscle had such long lives. These long-lived ovarian proteins have essential functions in structures like the mitochondria and cytoskeleton, and processes like protein homeostasis and chromatin maintenance. RNA sequencing revealed that aside from oocytes, a subset of somatic cells in the ovary also carried such long-lived proteins.
The researchers next wondered how these proteins were able to persist for such a long time. To determine if altered protein homeostasis played a role, they tested whether aged oocytes contained aggregates of misfolded proteins. Microscopy revealed no such aggregates in aged oocytes. The researchers further confirmed that age did not reduce the activity of proteasomes—complexes that degrade misfolded proteins to maintain protein homeostasis in cells.
Analyzing protein abundance in the ovaries showed enrichment of antioxidants and chaperones that help in protein folding, suggesting that proteins are maintained over long periods by preventing protein misfolding and protecting against oxidative damage.
To understand the effect of ovarian aging on these long-lived proteins, the researchers tested their abundance in ovaries from mice at several points across the lifespan, from one day to 11.5 months old. Mass spectrometry revealed that ovarian aging is associated with a reduction of many long-lived proteins. This causes an extensive remodeling of the ovarian protein landscape, which eventually leads to gradual fertility decline after the age of three months in mice.
Finding long-lived proteins in the ovary was not entirely unexpected, said Schuh. “But that so many proteins persist for a very long time period, that was surprising,” she said. Her team has started looking into some of these long-lived proteins to understand why they do not degrade more often, and what the functional implications of their longevity are.
“These results are important for understanding human ovarian biology and ovarian aging,” said Lei, but cautioned against designing medical therapies to delay ovarian aging or improve egg quality based on these results. This is mainly because human ovarian biology is more complex than that of mice, she explained.
“How [these results] relate directly to humans, we do not know yet,” agreed Schuh. However, she expects that human ovarian proteins would also be long-lived. Although it’s difficult to study this in people at the moment, she noted that, “expanding this to humans one day would be absolutely exciting.”
- Harasimov K, et al. The maintenance of oocytes in the mammalian ovary involves extreme protein longevity. Nat Cell Biol. 2024;26(7):1124-1138.
- Tachibana-Konwalski K, et al. Rec8-containing cohesin maintains bivalents without turnover during the growing phase of mouse oocytes. Genes Dev. 2010;24(22):2505-2516.
