A new generation of contraceptives could be on the way now that researchers have identified a receptor on the surface of eggs that binds to a sperm surface protein. By targeting such receptors, it might be possible to trick an egg into believing it has been fertilised, making it change its outer coat to keep sperm out.
Existing birth control methods, such as barrier contraceptives and hormones, all have their drawbacks. So researchers have turned their attention to biochemical interactions between sperm and eggs. Blocking these interactions could prevent fertilisation. So far, scientists have identified at least three binding proteins on the sperm.
But identifying the corresponding receptors on the egg has been difficult, partly because eggs are harder to come by. Now, for the first time, Nicole Sampson and Hui Chen at the State University of New York in Stony Brook have found a receptor for a critical sperm surface protein called fertilin-beta. In mice with a faulty fertilin-beta gene, sperm rarely fuse with eggs. The team synthesised the part of the sperm protein thought to bind to the egg, attached a radioactive tag, and then mixed the tagged peptide with mouse eggs.
The tagged fragment bound only to a re-ceptor called alpha-6/beta-1 integrin (Chemistry & Biology, vol 6, p 1). "The integrins are really an intriguing target," says Sampson. Some evidence from frog eggs hints that an integrin receptor can activate the egg and prompt it to change its outer coat to prevent more than one sperm from getting in.
If this happens in humans, the integrin receptor could be a target for a new contraceptive that deceives the egg. "It would be great if you could prematurely activate an egg," says Janice Evans of the Johns Hopkins School of Public Health in Baltimore, Maryland. But so far, researchers have failed to activate mammalian eggs by targeting integrins.
Evans adds that sperm probably have several ways of getting into an egg, so researchers might have to come up with a cocktail of drugs to block all the different sperm-egg interactions. "If there was only one way to get a sperm to an egg, none of us would be here," she says.
Richard Schultz of the University of Pennsylvania in Philadelphia agrees that the work opens up new avenues to explore. But he believes targeting sperm proteins would be a better approach, because the alpha-6/beta-1 integrin receptor is also found on cells other than the egg. Drugs that block this receptor might disrupt other cell interactions and cause side effects.
Author: Nell Boyce, Washington DC
New Scientist magazine, issue 23rd Jan 99
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