A new cell volume regulator in sperm cells that plays a key role in maintaining their shape and fertility has been uncovered by researchers at the University of Tsukuba and Cornell University. Study findings are published in The FEBS Journal.
Tsukuba - Sperm are highly specialized cells adapted to achieve a single goal: fertilize an egg. Along the road to fertilization sperm must embark on a lengthy journey to the female oviduct, avoiding several fatal stresses that occur when they pass through different physiological environments. However, the events that allow sperm cells to achieve fertilization under these conditions are only partially understood.
Most of a cell's mass consists of water, and this water can determine a cell's volume. The membrane that surrounds each cell is leaky, so cells can take in or release excess water depending on how much water surrounds them (a phenomenon known as "osmosis"). This property is analogous to how a sponge behaves--like a sponge taking on too much water, changes in water levels can drastically change the volume, and eventually the shape, of a cell. "Cells have sensor mechanisms that let them compensate for changes in their osmotic environment," says corresponding author Atsushi Asano, an assistant professor of the University of Tsukuba. "Without these compensatory sensors, cells would experience excessive volume changes leading to a ruptured membrane or other morphological problems. This type of sensor is fairly common in cells, but we were surprised to find one that plays a significant role in the function of sperm cells."
"The absorption process looks to be an important survival strategy for sperm during fertilization, since mature sperm cannot produce proteins on their own," co-author Ai Ushiyama says. "The findings convinced us that taurine was essential to the fertilization process, but we still had to figure out what exactly taurine was doing in sperm cells." The researchers keyed in on one telling clue, an unusual "kink" in the tail of the sperm. Sperm cells in CDO-deficient mice were nearly twice as likely to have a wrongly-angled tail when placed in an environment like that of a uterus. Strikingly, the defect disappeared when the sperm were supplied with extra taurine. These observations led the authors to conclude that, by controlling osmosis, taurine helped to maintain the right shape of sperm during fertilization.
"Our findings show that the production of taurine by CDO in the male reproductive tract, and its absorption by sperm, are pivotal mechanisms for male fertility," Asano concludes. "We believe that as sperm travel through different osmotic environments, the level of taurine inside the cell shifts to balance the change in cell volume. This prevents excess changes in shape caused by an influx or efflux of water, allowing the sperm to successfully complete fertilization under difficult environmental conditions."