Semen has something in common with cerebrivers of Alzheimer’s patients: both of themcontain bundles of filaments of proteins calledfibers amyloid. But although the accumulationamyloidappears to damage brain cells , in a mechanism implicated in Alzheimer’s disease, the samefibersthey may be essential for human reproduction.
A new study suggests Amyloid fibers from semen immobilize poor quality sperm , ensuring that only the fittest sperm reach the egg.
For years, scientists have observed these fibers in semen without knowing exactly theiradvantagebiological . It all started andn 2007, when researchers discovered thatfibers amyloid adheredin the AIDS virus, HIV , to penetrate cells.But theamoliode fibers are found in the semen ofmost primates , as noted by Nadia Roan, a mucosal biologist at the University of California, San Francisco and lead author of the study. “If thefibersamyloiddid not servesome beneficial purpose, they would have been eliminatedadas throughout human evolution“.
Looking at how HIV fuses with cells , the researchers wondered if the fibers somehow facilitate the fusion of the sperm into the egg.
However, although the purpose of the fibers is to facilitate fertilization , they do not do it in the way scientists believed. Contrary to what was initially expected, the researchers found in laboratory tests that, in the presence of amyloid fibers, sperm reduce their mobility. A look under the microscope was enough to show that the fibers attach to the sperm, immobilizing them.
The scientists then concluded that the reproductive function of the amyloid fibers actually lay in helping to remove poor-quality sperm from a woman’s reproductive system . After intercourse, a woman’s immune cells flood her reproductive system to rid herself of potentially harmful microorganisms. In the laboratory, it was found that, in the presence of fibers, up to five times more sperm were eaten by lymphocytes (cells of the immune system) than in the absence of them. On the other hand, the fibers also promoted the elimination of sperm that had been previously damaged with extremely low temperatures. In this case, the immune cells ate up to 1.5 times more defective sperm in the presence of the fibers.
Therefore, the amyloid fibers in semen have at least two beneficial effects for reproduction: the first is to eliminate sperm that remain in the female reproductive tract for too long, as this could trigger an unwanted immune response that would kill all sperm. The second is to remove the defective sperm , giving healthy sperm more chances to reach the egg.
“A strategy that helps ensure that only the best and the brightest get to fertilize the egg makes a lot of sense,” in the words of Daniel Otzen, protein aggregates expert at the University of Aarhus in Denmark. “There is nothing more important. , from an evolutionary perspective, than being able to have viable offspring. “
However, more research is needed to understand how amyloid fibers trap sperm. Furthermore, it is necessary to check whether the effects observed in the laboratory can be reproduced in animals. “In any case, this new quality control function for semen fibers shows how nature is much more complex and extravagant than we think,” says Otzen.
References:
Nadia R. Roan. (2017). Semen amyloids participate in spermatozoa seletion. Science. DOI: 10.1126 / science.aan7082
