When Sperm Meets Egg

In recent years, researchers have learned that nitric oxide (NO) is much more than an air pollutant. Indeed, a 1998 Nobel prize went to the three scientists who found it was a potent signaling molecule in the human body. This versatile gas regulates blood pressure by keeping the arteries open, serves as an important neurotransmitter in the brain and, when produced by white blood cells, battles invading bacteria and parasites. Now, scientists find NO has yet another purpose: it seems to play a crucial role in sex.

Since the turn of the century, one of the standing puzzles about how fertilization takes place involved the moment when a sperm enters an egg. For many years, reseachers knew that shortly thereafter, internal calcium ion stores in the egg opened, releasing calcium into the cytoplasm. Thus activated, the egg would start dividing and growing into an embryo. But how the sperm triggered this calcium release had remained a mystery.

David Epel and his colleagues from Stanford University may have found the answer, studying the gametes--eggs and sperm--of sea urchins. Epel and the lead author of the study, graduate student Richard C. Kuo, report the findings in the August 10th issue of Nature.

Sea urchins--purple, spiny invertebrates--are found in the Pacific Ocean, yards away from Epel's laboratory at Stanford's Hopkins Marine Station in Monterey Bay. They have long been popular models in early development studies because, unlike humans, their fertilization takes place in the open sea where it is easy to observe. To date, sea urchins have helped solve several important questions about human fertilization, including how an egg becomes activated.

This time, the scientists found that both sperm and eggs contain an enzyme that can produce NO from precursor molecules: nitric oxide synthase NOS). Sperm start producing NO, largely inside their bulky heads, within seconds of coming into contact with the egg coat, a jellylike layer on the outside of the egg that mainly consists of sugar-coated proteins. Touching this layer also makes the sperm release its protein-destroying enzymes, which it stores in a vesicle called the acrosome near the front of its head. These enzymes pave the way for the sperm to penetrate the egg membrane and release its genetic material inside.

Of interest, when Epel's group inhibited this acrosome reaction using chemicals, the sperm no longer made a lot of NO. So this reaction seems crucial for NO production in the sperm. Eggs, however, only showed large increases in NO about 40 seconds after a sperm had fertilized them or after they had been artificially activated. So the idea is that NO from the sperm triggers the release of calcium in the egg, which in turn activates the NOS in the egg, producing more NO and further calcium release.

One question was whether NO or the extra NOS from the sperm were enough to activate the egg. To answer it, the researchers injected one of two molecules into the eggs: a molecule that releases NO gas spontaneously or the NOS-enzyme. They found that in both cases, a large fraction of the eggs started to lift up their fertilization membrane, a sure sign of activation.

Additional experiments suggested that, NO aside, nothing else in the sperm seems able to activate the eggs. To prove this, the scientists turned to oxyhemoglobin, the familiar oxygen transporter in blood. This protein mops up and "neutralizes" all NO in its vicinity before the gas can perform any other function. And Epel's team found that when they injected oxyhemoglobin into the eggs, the sperm could no longer activate them and the calcium levels in their cytoplasms remained low; nothing replaced the function of NO.

It will take more study to determine if NO really is the crucial factor in switching an egg on, but if it is, it might explain some cases of male infertility: if a man's sperm doesn't produce enough NO, it may be unable to successfully fertilize an egg. The good news is that it is probably not a problem that can't be fixed. A little extra NO, introduced to the egg during in vitro fertilization (IVF,) might just do the trick, giving some couples the chance to have children of their own.