The process of getting eggs ready for fertilization in mammals is extremely complicated. With so many moving parts, it’s amazing that it actually goes right so much of the time. But as biologists, we are of course more interested in when things go wrong. Researchers studying reproduction at the Chinese Academy of Sciences had an inkling that a small protein called Survivin was important for female fertility, based on its known functions. So, like anyone who’s ever wondered “what happens if I just take out this piece”, they took Survivin out of the mouse eggs. And they were (I assume) delighted to see that things went terribly wrong.
Qing-Yuan Sun and colleagues reported that Survivin is essential for female fertility in a recent issue of Cell Death & Disease. In order to find out just what Survivin was doing in the developing egg, the researchers made two different genetically engineered mouse strains. The first strain had Survivin deleted from the oocyte, the part of the egg that eventually becomes the baby mouse. The second strain deleted Survivin from the granulosa cells (GCs), a type of support cell for the oocyte.
Deletion of Survivin from either cell type caused problems for female fertility, but in different ways. When Survivin was removed from the oocytes the eggs, well, stopped survivin’. Females with no Survivin in the ovaries were not able to have any pups. Survivin was also important in GCs, as females lacking Survivin in those cells had about 70% fewer pups than normal females.
The researchers then looked inside the eggs to see what was actually broken without the Survivin protein. In the eggs with Survivin deleted from the oocytes, the situation was a serious mess. Unlike in normal oocytes, the Survivin-less oocytes had their chromosomes all over the place, instead of nicely lined up. The spindle, part of the mitosis machinery, wasn’t assembled correctly. When they looked at spindle assembly proteins, proteins whose job is to make sure that the chromosomes are set up right for mitosis, they found that they weren’t at their posts. In the figure on the right, one of these checkpoint proteins, Bub3, can be seen in the top (normal) panel as green dots at the ends of the red chromosomes. In the bottom (Survivin mutant) panel, Bub3 is spread out all over the oocyte, with no clear dots at the chromosome ends. The total chaos of the egg is clearly illustrated in the cartoon below:
Deletion of Survivin from the GCs messed up a whole different, earlier process. In the oocyte-deletion strain, female mice could still ovulate, even though the eggs they ovulated were totally useless. In the second strain, with Survivin missing from the GCs, females could barely ovulate any eggs, even when stimulated with fertility drugs. The ovaries of these mice were unable to make normal follicles, and when they did, the number of GCs in the outer layers of the egg were very reduced. Since the major function of GCs is making sex steroids that promote development of the egg, the extremely low numbers of GCs meant that the eggs weren’t getting the growth signals they needed.
The reason for the lack of GCs? Survivin is required for GCs to survive. Nice how that worked out, huh? The GCs were able to form at the normal rate, but they couldn’t stay alive for long without Survivin. This fits well with one of the known functions of Survivin, preventing apoptosis (programmed cell death). As you can see in the image below, the normal ovaries on the left have barely any cleaved caspase 3 (CC3), a marker of apoptosis, in their developing GCs. However, the ovaries on the left, which are missing Survivin from the GCs, are chock full of death proteins.
So it turns out that Survivin really lives up to its name. The next step, of course, is to determine if Survivin plays a similar role in human female fertility and whether some cases of infertility could be due to a Survivin defect. If so, it’s possible that those types of infertility could be treated in the future by increasing the amount of Survivin protein in the ovaries.
In the meantime, enjoy this video of mommy and daddy mouse fighting over parenting styles. Even when everything goes right in the ovaries, there’s still the whole parenting thing to worry about.
Jiang ZZ, Hu MW, Wang ZB, Huang L, Lin F, Qi ST, Ouyang YC, Fan HY, Schatten H, Mak TW, & Sun QY (2014). Survivin is essential for fertile egg production and female fertility in mice. Cell death & disease, 5 PMID: 24675472