US: Hormone boosts sperm activity

Two teams of researchers have separately discovered why human male sperm approaching the female egg switch from a smooth swimming motion to a frantic flicking to push through the thick jelly-like coating around the egg. For the past decade, scientists suspected the female hormone progesterone released by the egg prompted the tail to change its movement but exactly how this occurred had remained a mystery.

Now researchers have shown the hormone acts directly on a protein on the sperm's surface. The finding could lead to a solution to some male infertility and even to a new non-hormonal contraceptive.

Because sperm responds to progesterone within seconds, scientists reasoned the hormone must bind to a surface protein and not one within the cells because this would take longer for the progesterone to reach. Earlier research had found that infertile men sometimes had mutations that disrupted a protein called CatSper, which ferried calcium ions in and out of sperm.

This "calcium channel" is only located within a sperm's tail but determining if it responded to progesterone was difficult given that sperm are not easy cells to work with, not least because they do not stay still.

The two research teams were each successful in linking progesterone to CatSper by inserting a tiny electrode into individual sperm. Two of the laboratories in the world able to do this are based at the Center of Advanced European Studies and Research in Bonn, Germany, and at the University of California, San Francisco.

In separate reports online in Nature the groups show the change in current inside a sperm as progesterone causes positively charged calcium ions to pass into the cell. A working ion channel produces a characteristic electrical 'fingerprint' so the researchers used their electrodes to reveal that CatSper was the agent for letting in the calcium.

Researchers say this might explain why some men whose sperm do not respond to progesterone have low fertility, at least in the 40% of cases where no underlying cause is known. Benjamin Kaupp, a biophysicist at the Bonn centre who led one of the teams, said doctors could investigate whether a man's sperm was insensitive to progesterone because of problems linked to CatSper.

Another possibility was that CatSper could be used to develop a non-hormone contraceptive. If researchers locate where progesterone binds to the CatSper channel, they could find molecules to block this interaction and render sperm inactive.