May 2005, Vol. 11, No. 2

Biomedicine
Sperm with Bent Tails Point to Possible Male Contraceptive

New research on mice by scientists at the Population Council and Rockefeller University sheds light on male infertility. The findings, reported in the March issue of Developmental Cell, may also lay the groundwork for a reversible male contraceptive.

Mutant sperm with tail doubled back (top); close-up (bottom). Photo credit: American Museum of Natural History

The Rockefeller University laboratory of Hermann Steller created mice that lack the gene septin 4. This gene is thought to be involved in apoptosis, the normal process that eliminates cells that are damaged, unwanted, or no longer needed. The mice were designed by postdoctoral fellow Holger Kissel to help with cancer research because apoptosis goes awry in cancer cells. Unexpectedly, Kissel discovered that the male mice were infertile even though they produced normal numbers of sperm. Kissel enlisted the help of Population Council cell and molecular biologist Gary Hunnicutt, an expert on sperm, to determine the cause of their infertility. Hunnicutt placed the sperm under the microscope and immediately noticed they are unable to swim and have tails that bend back upon themselves. The sperm have an appearance very similar to those seen in some infertile men.

During normal development, sperm cannot swim when they are extruded from the testis. It is not until they pass through the epididymis, a coiled, tube-like organ that lies between the testis and the vas deferens, that sperm mature and become motile. In the case of sperm from these mice, Hunnicutt noticed that their tails became bent as they passed through the epididymis. The tails on all the sperm bent at exactly the same position, at a site corresponding to a ring-like structure known as the annulus, whose function is unknown.

Annulus (arrow) between midpiece and principal piece of sperm (top); Sperm with missing annulus (bottom). Photo credit: American Museum of Natural History

The missing annulus
Microscopic examination revealed that these sperm are missing the annulus, suggesting that this structure may be made of septin 4 protein, the production of which is controlled by the septin 4 gene. Hunnicutt tested this hypothesis by using antibodies to the septin 4 protein to determine whether and where the protein was located on normal sperm. The analysis showed that the annulus is composed, at least in part, of septin 4 and when this protein is removed, as is the case in Kissel’s infertile mice, the annulus does not form.

It is at the annulus that the powerful whiplike beat of a sperm’s tail begins. This movement propels the sperm forward. The annulus may supply structural support to the tail at this site. Without an annulus, the sperm’s tail may “break” when it begins beating. Corroboration for this idea comes from the fact that the bending of the sperm tail within the epididymis happens at essentially the same time as the onset of motility. Hunnicutt and colleagues in his laboratory are investigating this possibility.

Septin proteins were first discovered in yeast about 30 years ago. They often form ringed structures that separate cellular components into different regions. The sperm annulus appears to compartmentalize sperm tail components. Although this has been hypothesized as a function of the annulus, it has never been testable until now.

Rearranging proteins and lipids
“Sperm, unlike other cells in the body, must respond to a host of different environments in both the male and female reproductive tracts,” says Hunnicutt. “Yet they do this without making any new proteins, which is the mechanism most cells use to function under different conditions.” Instead, sperm appear to alter their activity by rearranging the protein and lipid molecules on their surfaces. This rearrangement of molecules may be controlled by barriers that compartmentalize proteins and lipids at different locations in the sperm. The annulus has been thought to act as a gatekeeper that separates two areas of the sperm tail. When the sperm needs to alter its function, the annulus may act like a turnstile, selectively allowing certain proteins or lipids on one side to move across the annulus and thus reach a new region of the sperm tail. Once these proteins and lipids have moved into this new sperm domain, they can react with the molecules already present in this area. This can trigger new chemical reactions that cause sperm to modify their function. “Studying sperm from mice lacking septin 4, which no longer have an annulus, will allow us to finally ask if the annulus truly acts as a turnstile,” says Hunnicutt. “Our findings will advance our understanding of how sperm become cells capable of fertilization.”

Interfering with the operation of the annulus may stop sperm from functioning properly. Discovering how to do this may lay the foundation for a novel male contraceptive. Such a contraceptive would not interfere with the male endocrine system and would therefore be unlikely to have the side effects associated with potential male hormonal contraceptives.

Source
Kissel, Holger, Maria-Magdalena Georgescu, Sarit Larisch, Katia Manova, Gary R. Hunnicutt, and Hermann Steller. 2005. “The Sept4 septin locus is required for sperm terminal differentiation in mice,” Developmental Cell 8(1) 1–12.

Outside funding
National Institutes of Health

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See Also

• "Sperm maturation," project description (full text)