“Hyperactive” sperm are known to be the sperm that breakthrough the female egg’s exterior wall and finish the fertilizing process. A new study has shown that a particular enzyme in the tail responds to the female sex hormone progesterone, resulting in this “hyperactive” state and giving them the speed to break through the egg wall.
Progesterone is released in high levels by females just before and at the beginning of pregnancy. It was primarily thought to prepare the uterus for implantation of the embryo but now we might be starting to understand another important role, stimulating sperm. Biologist have suspected that progesterone impacts the progression of sperm but they have been unable to identify the necessary receptors on the sperm.
Upon entering the fertilizing layer of an egg cell, the sperm suddenly start a mad dash, swimming faster and their tail movements become more forceful and erratic. This “hyperactivity” gives them enough power to penetrate the outer layer of the egg so they can achieve fertilization.
Scientist recently developed a technique where they could record reactions to various chemicals by attaching tiny electrodes to the tails of sperm. This showed that CatSper, a sperm tail calcium receptor responds to calcium ions around the female egg. This is what causes the “hyperactivity” of the sperm.
But the authors of this new study suspected the process was slightly different. They thought that progesterone was binding to something that in turn was stimulating CatSper.
Using the same tiny electrodes, they showed that progesterone binds to the enzyme ABHD2 which removes the fatty sheath on the CatSper receptor. By uncovering CatSper it is now available to respond to the calcium ions and become “hyperactive.”
The researches suggest that this process is in place to prevent the sperm from using all of their energy before reaching an unfertilized egg.
“People tend to think of fertilization as like a marathon, where the fastest, most powerful sperm is going to win,” said Melissa Miller, a postdoctoral fellow at both the University of California (UC) Berkeley and UC San Francisco, and the lead author of the study, in a statement. “We think of it like the Tour de France… where the first sperm clear the way, expending their energy to break through the barrier cells,” allowing the slower ones behind to pass into the egg.
80% of male infertility cases go without finding a cause, illustrating just how little is known about the molecular workings of the sperm. Having discovered the true activation behind the “hyperactive” state, this study could pave the way for new treatments for male infertility.
Likewise, with this new understanding, a unisex contraceptive could be developed to prevent hyperactivity. This type of treatment would prevent the sperm from being able to penetrate the egg’s barrier cells and fertilizing the egg.