Researchers at the University of the State of São Paulo (Brazil) have studied a sperm protein and discovered two new targets that can be used in combination to develop male contraceptives.
The study, published in the scientific journal 'Molecular Human Reproduction', also demonstrates the feasibility of using mice as a model for in vivo testing during the drug development process. Scientists interested in the protein have so far used primates in their experiments, making trials more complex, lengthy, and expensive.
The project focuses on the epididymal protease inhibitor (EPPIN), a protein whose main function is to modulate sperm motility, that is, its ability to move through the female reproductive tract and reach the egg . Scientists and the pharmaceutical industry intend to develop male contraceptives that inhibit sperm motility, since it is very difficult to develop a drug that prevents their production.
Sperm production is much more complex than egg production. Spermatogenesis lasts about two months and is continuous. AnA male contraceptive designed to block sperm production would not take effect for three to four months, explains Erick José Ramo da Silva, last author of the article.
A diagram of the human fertilization process helps explain how EPPIN works. When a man ejaculates, semen is forced out of the epididymis, where it is stored, and through the urethra, receiving fluids from the seminal vesicles, prostate gland, and other glands along the way.
In the case of mammals, especially primates, freshly ejaculated semen is gelatinous and viscous, and contains various proteins that form the so-called seminal clot. One of them is semenogelin, with which EPPIN interacts to block sperm motility. Until ejaculation, sperm do not swim, although they have the machinery to do so, explains Ramo da Silva.
For the sperm to continue their journey to the egg, they must break free from the clot, which is gel-like due to the presence of proteins secreted by the seminal vesicles. Here, the prostate-specific antigen (PSA), well-known as a diagnostic marker for prostate cancer, plays a fundamental role. The protease cleaves clot-forming proteins, including semenogelin, to cause semen liquefaction.
This PSA cleavage releases the sperm in a process we call progressive motility. They are then able to penetrate the outer layers of the ovum in amovement known as hyperactivated motility, points out the Brazilian scientist.
PSA-mediated cleavage occurs in the female reproductive tract five to ten minutes after ejaculation. Until the time of PSA-mediated cleavage, the only mechanism that moves sperm into the female reproductive tract is ejaculation. Sperm motility is not necessary before that. They save energy for the rest of the journey to the uterus and the egg, he details."
In previous research, monkeys were vaccinated with human recombinant EPPIN (produced in the laboratory by genetically modified microorganisms) and developed antibodies that bound to the protein, causing infertility by blocking the cleavage of semenogelin and delaying the semen liquefaction. This showed that semenogelin plays a role in the control of sperm motility.
Given the differences between rodents and primates, the scientists set out to work with the latter first, but the study by Ramo da Silva's group opted for a mouse model because mice have a protein called SVS2 that plays the same role than semenogelin in humans, binding to EPPIN and blocking sperm motility.
In the study conducted at UNESP, mice received three types of antibodies to see if they would bind to EPPIN, in which case they would show which domains of the protease inhibitor could be targeted to reduce or block motility of thesperm.
The antibodies act against a specific part of the EPPIN, and do not bind to other parts, comments Ramo da Silva. Antibodies that inhibited sperm motility bound to an initial region called the C-terminal extension of the Kunitz domain, as expected, but other bodies, which bound to the N-terminus of the WFDC domain, were also capable of inhibiting sperm motility. sperm motility, which was a first for scientists.
Furthermore, both antibodies against the C-terminal and N-terminal regions inhibited the rate of in vitro fertilization, confirming that their connection with EPPIN affects the potential fertility of sperm.
Sperm motility was measured and found to be decreased, showing that protease inhibitors regulate sperm motility in both regions and could be targets for new drugs. In other words, the study showed that it is possible to design molecules that bind not only to the C-terminus but to both domains, blocking sperm motility.
The study showed which sequences of the EPPIN chain, of 133 amino acids, should be the target of a male contraceptive based on the inhibition of sperm motility. Finally, it showed that mice can be used as models for in vivo testing, which promises to make preclinical studies easier, faster, and cheaper.