Biologists have discovered a protein that gives sperm the oomph to penetrate an egg _ a finding that could someday lead to new contraceptive drugs for men and treatments for male infertility.
The protein, dubbed CatSper, is found only in sperm tails. Researchers found that mice genetically engineered so that they lacked the protein produced sluggish sperm with markedly less ``whiplash'' motion in their tails. The sperm did not penetrate eggs, and conception failed.
``The reason they were infertile is that their sperm don't swim very well. They don't have enough force to penetrate an egg,'' said Dr. David Clapham of Harvard Medical School, who led the study. The findings were published in Thursday's issue of the journal Nature.
The lack of the CatSper protein did not limit the mice's ability to produce sperm or otherwise affect the animals' sexual behavior. And the sperm without the protein could indeed fertilize _ but only after an egg's tough outer membrane, the zona pellucida, was removed artificially.
Clapham said the discovery could lead to new understanding of male infertility. Scientists might zero in on possible defects in the gene that produces a similar CatSper protein in humans.
Moreover, he said, the finding may one day lead to contraceptive drugs that temporarily block the protein and render sperm far less likely to penetrate an egg. Depending on the duration of such a drug, a man or woman might take it just before or even after sex.
``If you had a good blocker of this thing, it would only have to be taken during the life of the sperm inside the female, and could be taken by either males or females,'' Clapham said.
CatSper belongs to a unique family of proteins, so a drug targeting it would be unlikely to affect other tissues in the body, he said. It thus might have fewer side effects than female birth control drugs that contain hormones.
Previous research aimed at creating male contraceptives also has focused on blocking sperm's capacity to penetrate eggs. Some of that work has examined proteins on the head of sperm that may trigger enzymes that dissolve the outer shell of the egg.
The newly discovered CatSper protein allows calcium to enter the sperm.
Other researchers said the work is an exciting first step toward developing new contraceptives.
``It's the first case where we have a calcium-permeable channel on the sperm tail, which is the right place to do this important regulatory step,'' said Dr. Harvey Florman, a cellular biologist at the University of Massachusetts. ``And if it is sperm-specific, then you could start rationally designing drugs that would block it.''