A molecular mechanism that makes your heart beat or your biceps flex may also be essential to the fusion of sperm and egg cells. Researchers studying signals in the heart and nervous system have discovered a new type of ion channel that gives sperm the boost to penetrate the egg¹s thick outer coat.
Calcium ions are crucial for sending signals within neurons and other cells. Muscle cells, for example, have special proteins embedded in the cell membrane that act as channels for the ions. When these channels open, calcium ions pour in and set off a biochemical cascade that causes the muscle fibers to contract. Sperm cells also have calcium channels, and researchers suspected they were needed for movement. But no one had identified a specific channel, and the channels were thought to be located in the sperm head.
While searching the mouse genome for potential calcium channel proteins, ion-channel researcher David Clapham of Harvard Medical School and his colleagues found a gene whose protein resembled other calcium channels, which turned out to be expressed only in the sperm tail. Intrigued, they investigated the new protein, which they call CatSper. In the 11 October issue of Nature, the team reports that mice lacking the channel are infertile; they make the usual number of sperm, and the sperm cells look normal, but they are sluggish. They were also unable to propel themselves through the protective protein coat surrounding an egg cell; when that coat was removed, they easily fertilized the egg. Clapham¹s team also found a human version of the gene.
Because the channel is apparently unique to sperm, Clapham suggests that it may lead to a drug for treating men with poor sperm motility, or for a contraceptive. He adds that a drug that could temporarily disable the protein would probably have few side effects and could be used by men or women. Others are cautiously encouraged by the prospect. "This, on the face of it, would appear to have some potential," says reproductive biologist J. Michael Bedford of Cornell University.