Oocytes are immature egg cells that develop in almost all female mammals before birth. The growth of future generations depends on this limited pool of cells that survive for many years without damage. In mice, this can be as long as eighteen months, while in humans it can be about half a century, the average time between birth and menopause. How cells accomplish this remarkable feat of longevity has been a long-standing question.

Researchers at the Center for Genomic Regulation (CRG) in Barcelona have discovered a new mechanism that explains how oocytes can survive in pristine conditions for decades without undergoing the breakdown that causes other cell types. They fail. The results, reported today in the journal SaleRepresent a new frontier in exploring unexplained causes of infertility.

The researchers looked at protein aggregates, which are clusters of misfolded or damaged proteins. If left unchecked, these harmful substances accumulate in the cytoplasm and have highly toxic effects. Protein aggregates accumulate in neurons and their effects are linked to several neurodegenerative diseases. Cells normally manage aggregates by breaking them down with special enzymes. They can also divide into two new cells, concentrating the aggregate in one cell and leaving the other.

But oocytes are not like other cells. Their long life means they cannot eliminate toxins through cell division. Constantly breaking down protein aggregates is an indispensable strategy, as it requires the use of large amounts of energy that may not be readily available. Oocytes also have the task of donating their entire cytoplasm to an embryo after fusing with sperm, and therefore prefer to reduce their metabolic activity, a strategy that avoids producing such byproducts. Avoids foods that can damage the mother’s DNA and compromise future reproductive success. This makes oocytes particularly susceptible to the effects of misfolded or damaged proteins.

However, “in contrast to the tens of thousands of papers on protein aggregation in neurons, how mammalian oocytes cope with protein aggregation remains largely unstudied, despite long-term survival and distribution. There’s only one problem with not being there,” explains Dr. Alvan Bock. , group leader of the Oocyte Biology and Cellular Dormancy Program at the Center for Genomic Regulation and study author. “We wanted to explore how oocytes deal with these misfolded or damaged proteins,” says Dr. Bock.

patrol ‘cleaning crew’

Dr. Bock’s team, led by Dr. Gabriel Zaffignini, began by collecting thousands of immature oocytes, mature eggs, and early embryos from adult mice. Using special dyes, they observed how protein aggregates behaved in real time using a technique called live-cell imaging. They also used electron microscopy to get a closer look and see the nanoscopic details inside the cells, a task that took five and a half years to complete.

The researchers discovered special structures in oocytes that they named EndoLysosomal Vesicular Assemblies — or ELVAs for short. These structures — there are about 50 in each oocyte — move around in the cytoplasm, where they capture and hold protein aggregates, rendering them harmless. Cells contain many subcellular structures called organelles, which perform the same functions as an organ in the body. Researchers conceptualize ELVAs as a “superorganelle” because it is a network of many different types of cellular components that work together as a unit.

The study revealed a critical moment during the oocyte maturation phase, which is when an oocyte transforms into a mature egg, preparing for ovulation and possible fertilization. During this phase, the researchers observed that ELVAs move to the cell surface and break up protein aggregates, essentially cleaning the cytoplasm deeply. This is the first observation of a unique strategy that oocytes use to get rid of protein aggregates.

“An oocyte must donate all of its cytoplasm to the embryo at fertilization, so it cannot afford to accumulate garbage, which could pose an existential threat to its function. To ensure that patrolling the cytoplasm so that no aggregates are free-floating. ELVAs keep these aggregates in a confined environment until the oocyte is ready to dispose of them in one fell swoop. This is an efficient and is an energy-intensive strategy,” says Dr. Zafagnini, a postdoctoral researcher at the Center for Genomic Regulation.

Protein aggregates can contribute to infertility.

Fertility declines with age, and the leading cause of female infertility is poor oocyte quality. Global infertility rates are also increasing, with delayed motherhood being a major factor. Understanding how oocytes remain healthy, and what causes these strategies to fail with age, is critical to understanding the unexplained causes of infertility and opening new avenues for treatment.

The results of the study suggest that the presence of protein aggregates can interfere with both egg and embryo quality. When the researchers experimentally blocked the ability of ELVAs to degrade protein aggregates during the oocyte maturation process, this led to defective eggs. When the researchers intervened and “forced” the embryos to inherit the accumulated protein, 3 out of 5 (60%) failed to complete the early stages of development.

“Many studies have historically focused on one small aspect of why oocyte quality declines, namely meiosis and euploidy. However, a recent review of eleven thousand embryo transfers revealed that Decline in female fertility with age is strongly influenced by as yet unknown factors. Our research opens an exciting future direction to explore whether protein degradation, and how it is regulated in oocytes, affects embryonic development. may explain age-related declines in health,” concluded Dr. Boke.

Another type of long-lived cell that does not yet divide and has to deal with protein aggregates are neurons. Accumulation of harmful substances in these cells is linked to the development of several types of neurodegenerative diseases, including Alzheimer’s. Could ELVA-like compartments also exist in neurons and other cell types? This study opens avenues for future research beyond the field of reproduction.