Scientists have demonstrated how certain molecules are vital for maintaining the shape of sperm cells, which may have implications for understanding male fertility.
Regulation of cell volume in sperm cells by a molecule called taurine plays a key role in their shape and ability to fertilise an egg cell, according to one new study.
The work, published in The FEBS Journal, focused on a protein called cysteine dioxygenase (CDO), which plays a key role in producing taurine. This molecule, which is an amino acid, maintains cell volume by controlling osmosis – a process where water flows in and out of the sperm cells across the cell membrane, changing their shape and structure.
Previous research had found that mice lacking CDO were ten times more likely to be infertile than mice with the protein – these CDO-deficient mice also had much lower levels of taurine in their sperm cells. However, the exact reason behind this link had not been explored.
In the new study, scientists from the University of Tsukuba in Ibaraki, Japan, and Cornell University in Ithaca, New York, identified that sperm cells in mice lacking CDO were twice as likely to have unusually kinked tail, which makes them unable to fertilise eggs. The change in tail shape happens because of changes in the amount of water absorbed through the cell membrane.
However, when extra taurine was added to the sperm of mice lacking CDO, the kinks in the sperm tails disappeared, showing that taurine is important in maintaining correct sperm shape. Since sperm cells do not produce proteins like CDO, the researchers discovered that the taurine was being absorbed as the sperm cells passed along the male reproductive tract.
'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,' said Dr Atsushi Asano, assistant professor at University of Tsukuba's faculty of life and environmental sciences. 'This prevents excess changes in shape caused by an influx or efflux of water, allowing the sperm to successfully complete fertilisation under difficult environmental conditions.'
In a separate study, scientists from Spain and Italy identified another protein called centrobin, which plays a critical role in sperm tail development. The research, published in the Journal of Cell Biology, found that the sperm of flies lacking centrobin were very fragile, with the sperm head often becoming detached from the tail.
Together the two studies suggest that sperm shape, and in particular that of the tail, is vital for male fertility.