A team of Japanese scientists at Osaka University led by Dr Masaru Okabe have discovered a protein essential for human sperm to fuse to an egg. The findings could well lead to new methods of treating infertility and the development of new contraceptive strategies.
The protein, Izumo, is named after a Japanese shrine dedicated to marriage. Human sperm has a number of obstacles to overcome in it's Herculian race to fertilise the egg. It must pass through a barrier of follicle cells surrounding the freshly ovulated egg. It then has to penetrate the egg's outer wall, the zona pellucida, a thick layer that protects the egg, so that it can bind to the plasma membrane beyond it.
The team bred male mice genetically engineered to lack the Izumo protein which were found to be infertile. In laboratory tests mouse sperm which did not have the protein were unable to fuse with the egg. The sperm from the mice was able to penetrate both of the barriers around the egg, but not one Izumo-deficient sperm was seen to fuse with the egg membrane. They have discovered that a form of the Izumo protein also exists on human sperm. Tests showed that an antibody designed to react against the protein blocked the fusion of human sperm with hamster eggs, which is a recognised test for male fertility.
Writing in Nature, Dr Masaru Okabe, said "Our finding not only provides insight into the enigmatic fusion mechanism (of the egg and the sperm), but also promises benefits in the clinical treatment of infertility and the potential development of new contraceptive strategies."