Non-hormonal contraception visualised with X-rays

Many contraceptives on the market are hormonal, including birth control pills, patches, injections, implants, and hormonal IUDs, but often carry side effects. Whilst there are non-hormonal options, including copper IUDs, condoms or diaphragms, these can be inconvenient to use and sometimes less effective than their hormonal counterparts.

In reproduction, the egg coat, also known as zona pellucida (ZP), is an extracellular matrix essential for egg growth, fertilization and protection of the embryo until it implants into the uterus. It is the first interface between egg and sperm and has long been explored as a target for immunocontraception.

Now scientists are looking into monoclonal antibodies as a tool to block fertilization. The team of Luca Jovine, in the Karolinska Institut in Sweden, has studied extensively the ZP using the structural biology beamlines at the ESRF. Among the monoclonal antibodies raised against ZP proteins, they have taken interest in IE-3, which targets a domain that was suggested to directly mediate sperm binding to the ZP. As recently discovered by Jovine’s team, the same domain also regulates the cleavage of ZP2, which is crucial to permanently block polyspermy.

Effective reversible contraception

IE-3 was previously described to bind to the ZP in vitro and in vivo,with a full contraceptive effect in mice. Most importantly, it was found to cause a reversible block of animal fertility without any interference with normal follicular maturation. “All these properties established IE-3 as promising target for developing non-hormonal contraceptives. However, until now it wasn’t known how the antibody exactly binds to ZP to prevent fertilisation at the molecular level”, says Luca Jovine, researcher at Karolinska Institutet and corresponding author of the article.

Jovine joined forces with Daniele de Sanctis, scientist at the ESRF, and carried out X-ray crystallography experiments on beamline ID29, where they got precious structural information on the antibody/fertilisation protein complex. “The contact area is so flexible that you can only use X-ray crystallography to study this system”, says De Sanctis. “Whilst we’ve combined experiments with artificial intelligence (AI) in other protein complexes, in this case AI could not have helped”, he adds.

The researchers showed that an IE3-derived single-chain variable fragment is enough to block fertilization in vitro and they determine the structural basis of IE-3/ZP2 recognition. The high-affinity of this interaction depends on induced fit of the epitope (the part of ZP2 recognised by the antibody), offering insights for non-hormonal immunocontraceptive design without off-target effects.

This work contributes to the global efforts looking into the development of safe and effective non-hormonal contraceptives that offer better tolerability and distinct side effect profile compared to hormonal options.

Reference:

Dioguardi, E. et al, PNAS, April 11, 2025, 122 (15) e2426057122.

Text by Montserrat Capellas Espuny. 

Video by Olivia Moncorgé and Montserrat Capellas Espuny.

Top image: Binding of fertilization-blocking monoclonal antibody IE-3 to egg coat protein ZP2 induces the ordering of the protein’s loop that contains the epitope for IE-3 (red arrow).