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Stuck on you: sperm-to-egg-binding protein discovered

28 April 2014
Appeared in BioNews 751

A protein that allows eggs and sperm to fuse together has been identified by scientists at the Wellcome Trust Sanger Institute, Cambridge.

Successful fertilisation occurs only if the egg and sperm recognise each other, then fuse together to form the embryo.

In 2005, Japanese researchers identified a sperm protein important for this process, named Izumo after a Japanese marriage shrine. Until recently, the binding partner present on the egg had proved more difficult to identify.

However, by creating an artificial version of the Izumo protein, researchers were able to find the protein that binds to it to initiate fertilisation.

They have named this new protein Juno, after the Roman goddess of fertility and marriage.

'We have solved a long-standing mystery in biology by identifying the molecules displayed on the sperm and egg which must bind each other at the moment we were conceived', says Dr Gavin Wright, senior author from the Wellcome Trust Sanger Institute. 'Without this essential interaction, fertilisation just cannot happen'.

Dr Enrica Bianchi, the first author of the paper added: 'The binding of the two proteins is very weak, which probably explains why this has remained a mystery until now'.

The study reports that eggs from mice lacking Juno cannot fuse with sperm, making them infertile. Male mice lacking Izumo are also infertile, highlighting the importance of these proteins for fertility.

Lastly, the researchers also discovered that the Juno protein is quickly lost from the egg's surface around 40 minutes after fertilisation.

This may explain how the egg prevents additional sperm fusing with it once it has already been fertilised. This is important to ensure the embryo has the correct number of chromosomes to survive.

As a result, Dr Wright has speculated that 'we may be able to use this discovery to improve fertility treatments and develop new contraceptives'.

The team is now screening infertile women to determine whether problems with Juno are responsible for cases of currently unexplained infertility.

Dr Allan Pacey, a leading fertility expert not involved in this study, has said: 'We know that fertilisation failure in IVF is quite rare, and so I suspect the lack or dysfunction of this protein is probably not a major cause of infertility in couples. However, it would be useful to know how many women have eggs that lack this protein so we can properly assess this'.

In such cases, regular IVF treatment would fail. Therefore, doctors could instead suggest using intracytoplasmic sperm injection (ICSI), where the sperm is directly injected into the egg. This would bypass the need for Izumo and Juno to bind, and could reduce the expense and stress often involved in assisted fertility treatments.

The research was published in the journal Nature.

SOURCES & REFERENCES
Fertility breakthrough: scientists discover how sperm and egg bind
Telegraph |  16 April 2014
Fertility mystery solved: protein discovered that joins sperm with eggs
The Guardian |  16 April 2014
Juno is the egg Izumo receptor and is essential for mammalian fertilization
Nature |  24 April 2014
Oh baby: Scientists find protein that lets egg and sperm hook up
Reuters |  16 April 2014
Sperm meets egg: protein essential for fertilisation discovered
Wellcome Trust Sanger Institute (press release) |  16 April 2014
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