ABOVE: Live imaging of a human follicle shows an egg cell with granulosa cells on the outer edge. The activity of reactive oxygen species (ROS), in red, can be observed in the granulosa cells but not within the egg cell. Aida Rodriguez/Nature, CC-BY4.0

It was a finding too striking to be trusted. Elvan Böke, a molecular biologist at the Center for Genomic Regulation in Spain, was studying how immature egg cells in animals, called oocytes, stay healthy in the ovaries before they are fertilized, when the results of a molecular activity assay turned up something she didn’t know was possible.  

To survive, almost all cells produce energy in the form of ATP via molecular machines called complexes I, II, III, IV, and V, that rest along the inner membranes of the cells’ mitochondria. But when Böke looked for the activity of complex I in the immature egg cells of African clawed frogs (Xenopus laevis), she couldn’t detect it. “My postdoc repeated the experiment 10 times because we expected something less black and white,” says Böke. 

Her group’s findings, described July 20 in Nature, mark the first time scientists have observed that oocytes’ mitochondria skip a key metabolic reaction, carried out by complex I, that takes place in all other mitochondria in the body. This, according to Böke, also explains why immature eggs remain preserved in the ovaries without losing their reproductive potency.    

Eggs first begin to form in the ovaries during fetal development, so by the time a female baby is born, she contains one to two million immature eggs—her lifetime supply, because the ovaries do not grow more eggs after birth. But how these cells are able to keep out of harm’s way for up to 50 years and eventually grow a healthy baby, while most other cells age, die, and are replaced, has been a puzzle.  

To find out, the researchers began by imaging early stage human and Xenopus oocytes to detect whether there are any reactive oxygen species (ROS)—which can damage and kill cells over time—present in the cells. These compounds are produced in large amounts as a byproduct of reactions carried out by complex I.  

To the researchers’ surprise, the oocytes showed no detectable ROS signals. And when the scientists treated the Xenopus oocytes, which are easier to obtain than human eggs because of their abundance, with vitamin K3 to promote the formation of ROS, more than 70 percent died overnight, while untreated oocytes had a near-perfect survival rate.

Illustration of electron transport chain from complexes 1 through 4, leading to generation of ATP
Illustration of how complexes embedded in the inner mitochondrial membrane generate ATP
© ISTOCK.COM, extender01

Then the researchers analyzed the proteins that make up the mitochondrial complexes. They found that the proteins that comprise complex I in Xenopus oocytes are inactive, supporting their hypothesis that oocytes use an unknown alternative pathway to generate energy that skips complex I. Whatever it is, the process allows the cells to produce enough energy using complexes II–V to remain on “standby mode” and survive for decades, but reduces the number of ROS produced, according to Böke.

Michael Klutstein, a molecular biologist at the Hebrew University of Jerusalem who studies mouse oocytes and was not involved in the current work, says the researchers build on previous research about cell damage caused by ROS and highlight “a very important and new angle” in how oocytes solve this problem. “The reason why it hasn’t been observed before is that this field of research is very much underfunded,” he adds. 

Though the study was conducted in Xenopus and not mice, which are biologically much more similar to humans, it will have far-reaching implications, according to Klutstein, by helping to advance our overall understanding of female reproductive biology, and could perhaps even lead to the development of cancer treatments that do not damage oocytes. 

Böke similarly says she’s excited to see where these findings will take her group. The next step for the team is to look into which mechanisms allow oocytes to survive without complex I.  

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