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REPRODUCTION How the Sperm Gets Its WiggleAfter the flowers, the candlelight dinner, the meandering walk home, it could be time for millions of sperm to join the action. But their role won't be so leisurely. Once dispatched, they'll quickly rush ahead on an all-or-nothing-mission: to find and fertilize the egg.
Found only in the tails of sperm, a newly discovered family of ion channels (indicated in orange, above), gives sperm the wiggle and wave necessary for fertilization. The authors, who include (below, l to r) David Clapham, Betsy Navarro, and Dejian Ren, believe the channel would make a good target for male and female contraceptives. Photo by Justin Knight
For the fast-moving sperm, the long journey from the testes to the fallopian tubes, where the egg beckons, can be as quick as a minute. Although wriggling sperm receive power boosts along the way from helpful chemical changes and contractions in the uterus, ultimately the sperm is on its own for the final plunge into the egg. Now it appears that a single-purpose ion channel gives the sperm tail its crucial wiggle and wave. Yet as is common in science and love, many mysteries remain. A Safe Drug TargetThe newfound ion channel may help scientists figure out more details in the poorly understood process of fertilization. More practically, its singular location in sperm tails makes it a bull's-eye for birth control drugs that may avoid the dangerous side effects of hormonal contraceptives, such as blood clots and cancer."To me what's cool is that it's so specific," said senior author David Clapham, a Howard Hughes researcher at Children's and the Aldo R. Castaneda professor of cardiovascular research. "It's only in the sperm, and it seems to do one thing: it makes the sperm motile enough to get through the egg membrane." The researchers dubbed their channel "CatSper," short for cation channel of sperm. The protein has been patented by Children's Hospital. The article appears in the Oct. 11 Nature. "The proteins in this new family are something of an oddity," writes David Garbers, HHMI researcher at the University of Texas Southwestern Medical Center in an accompanying perspective. "The authors have discovered a channel, which probably allows calcium ions to pass through, that is expressed only in sperm and is needed for them to move normally." In a paper in press in the Proceedings of the National Academy of Sciences, Garbers' lab has identified a second member of the new ion channel family (CatSper II) that also lives in the tail end of sperm. Clapham and Dejian Ren, a postdoctoral fellow in Clapham's lab who is the first author on the Nature paper, are co-authors on the study in PNAS. Readers might well ask how researchers who specialize in the heart and brain found themselves writing rather authoritatively about sperm motility and fertilization. "We landed there accidentally," Ren said. "We do have some cardiology research going on." Mystery ChannelIn a quintessential postgenomic science story, Ren had been seeking the source of a small electrical current he had measured that appeared to be an immune-related calcium channel. Ion channels are proteins that live on cell membranes. With split-second reaction times, they generate and orchestrate the electrical signals passing through the thinking brain, the beating heart, and the contracting muscle. Defective channels are responsible for diseases ranging from cystic fibrosis to hypertension. Drugs for many other disorders, such as cardiac arrhythmias, type 2 diabetes, and pain work by tweaking ion channels.So Ren turned to the mouse and human genome sequences to scout for a candidate sequence that might define the unknown channel. From one point of view, ion channels can be very picky about their clientele. Any given cell surface may sport 30 different types of ion channel, selective in different conditions for calcium, potassium, sodium, chloride, or other ions. The number of potential ion tunnels on a cell can reach 10,000. From another point of view, the different ion channels have almost a generic molecular blueprint. The presumed structure of one of the smallest channels for potassium is a circled wagon of four repeats of six transmembrane segment domains of the protein. One special loop between two of the segments shapes the pore in the cell membrane. The larger voltage-dependent calcium channel of skeletal muscle and neuronal fame is transcribed from a gene of four connected six-transmembrane segments. The new family of ion channels is odd in part because it seems to be as small as a single-domain potassium channel but to function like a calcium channel. No detailed x-ray crystallography structure of a mammalian ion channel has yet been solved. Homing InAfter scanning the mouse and human genome banks for the signature ion channel amino acid sequences, Ren found a candidate gene. He evaluated 50 human tissue samples, including heart, liver, kidney, bone marrow, and bladder. The gene turned on only in the testes.Fluorescent antibody studies with mouse sperm further pinpointed the ion channel protein in the tail. The tiny wiggling sperm proved resistant to patch clamping, the 20-year-old technique that heralded the new era of ion channel research. (Patch clamping was developed by German researchers Erwin Neher and Bert Sakmann, Clapham's mentors and winners of the 1991 Nobel Prize.) Other experiments in Clapham's lab by postdoctoral fellow Betsy Navarro and neurobiology student Alex Jackson showed that the newly discovered ion channel let calcium into the cell. Knockout mice missing the protein seemed to be normal in every respect--size, sex drive, and "mounting behavior," according to the research paper. But their sperm were lethargic and poorly motile, according to studies by Massachusetts General Hospital researchers Gloria Perez and Jonathan Tilly. They also were infertile. Test tube studies soon suggested what the hang-up was. The sperm could not get to second base with the egg. Once it completes the Herculean task of finding the egg, the preeminent sperm gets down to the daunting challenge of microscopic intimacy. The fast-wiggling gamete turns hyperactive and begins thrashing wildly. In that aroused state, the sperm binds to the egg's luxuriant gelatinous coat, the zona pellucida, in the first step of fertilization. Then, in an old story, the sperm head's calcium channels open in what is known as the acrosome reaction, which may allow the sperm to penetrate the extracellular coat. Other unknown mechanisms--mechanical or enzymatic--may also be at work here. "It seems that the reduction in progressive motility [in mouse sperm missing CatSper] is sufficient to block penetration of the zona pellucida," Garbers said. But for the normal sperm, another layer awaits. It must bind to and then fuse with the egg's plasma membrane. Only then is fertilization complete, with other suitors locked out through a combination of brute force and biochemistry. "There's no romance at the molecular level," Ren said. In the ultimate blind date, when researchers mixed sperm from normal mice with a mouse egg in a test tube, they found a two-celled zygote the next morning. No such luck with the knockout mice. This makes Clapham believe that a drug blocking CatSper would be an ideal contraceptive for men and women. "Because the channel is only in sperm, a specific blocker should have no side effects," he said. "In women, it might even be used after sex, although fertilization can occur in a short time under the right conditions." When the researchers removed the thick zona pellucida coat from a mouse egg, giving the CatSper-less sperm a last chance at love, both the normal and mutant sperm succeeded in fertilizing their respective eggs in a single night. --Carol Cruzan Morton |
