Research gives sperm a boost
Dr. Kirsty Horey
Progress Educational Trust13 February 2006
Scientists in the US have discovered what gives sperm cells the burst of energy they need to be able to reach and penetrate an egg. Researchers at the Boston Children's Hospital and the Howard Hughes Medical Institute measured the electrical activity taking place in a single sperm cell. Reported in the journal Nature, the study, undertaken by Drs Yuriy Kirichok and Betsy Navarro, was the first to measure the currents that flow across the sperm's outer membrane.
The ability to measure these electrical currents has also enabled the researchers to study the role of CatSper, a protein that is vital for male fertility. CatSper is only found in the tails of mature sperm, but it has never been proved what the protein was needed for or how it works. Previous studies have shown that mice lacking the CatSper protein were infertile because their sperm were poor swimmers and were unable to penetrate the protective outer membranes of the egg.
The new studies, which directly measured electrical activity conducted by sperm, showed that CatSper is a key channel by which calcium ions enter the sperm's tail. Electrical activity has long been known to be important in sperm cells, and scientists have tried unsuccessfully to measure electrical currents for twenty years, said Dr Kirichok. The influx of calcium influx, measurable as an electrical current, was found to 'hyperactivate' the sperm, giving them a burst of power and energy needed in order for them to reach and penetrate the egg. The electrical current was detectable in sperm from normal mice, but not in sperm from mice that lacked CatSper. The chemical change was found to convert the steady undulating motion of the sperm's tail into a 'whip-cracking snap'.
The findings suggest that future research could focus on using CatSper as a target for developing a male contraceptive. 'If they find something that blocks the current, it could be used as a male contraceptive', said Dr Navarro, adding 'before, people didn't know what the exact target was, and didn't have a system to test drugs'. However, the challenge, she said, would be to develop a drug that blocks only CatSper and not other calcium channels, which occur throughout the body. Further testing showed that CatSper is triggered by alkaline conditions inside sperm cells, which make it open its 'gates' to calcium. The next step for the researchers is to find out what causes these alkaline conditions, said Dr Kirichok, adding that 'quite possibly the sperm senses something released by the egg'.
Previous attempts to measure electrical currents have failed, because it has proved to be too difficult to get a tight enough seal between the sperm's outer membrane and the pipette used. But the scientists found that they could use a bubble found just below a sperm's head - which enabled tight contact to be made with the pipette via a process known as 'patch clamping'. The ability to 'patch clamp' sperm and study its electrical activity will allow the scientists to measure other aspects of the sperm's behaviour, the functions of which are currently unknown. For example, other calcium channels may be involved in the process in which sperm releases enzymes that dissolve the barrier around the egg, said Dr Kirichok.
The research may also help in understanding and treating some forms of male infertility. Dr Allan Pacey, senior lecturer at the University of Sheffield, said that 'we know that hyperactivation is crucial to successful fertilisation, and we suspect that in some men this might be why they have difficulty in conceiving with their partners'. He added: 'if we could better understand the molecules involved in that process we might be able to diagnose the problem earlier and therefore save the couple both time and heartache'.
Reproduced with permission from BioNews, an email and online sources of news, information and comment on assisted reproduction and genetics.