A protein which allows a sperm and egg to bond together
has been discovered
by scientists in a breakthrough which could improve fertility
treatments
A protein which allows sperm to ‘dock’ with an egg has
been found in a discovery
which could help infertile couples and lead to new contraceptives.
The molecule, named Juno, after the Roman goddess of
fertility, is present on the surface of the egg and binds with a similar
protein on the sperm cell.
Their meeting is the very first moment of conception.
In 2005, Japanese scientists discovered the male part
of the processes but until now its counterpart has proved elusive.
"We have solved a long-standing mystery in biology
by identifying the molecules displayed on all sperm and egg that must bind each
other at the moment we were conceived," said lead researcher Dr Gavin
Wright, from the Wellcome Trust Sanger Institute in Hinxton, Cambridgeshire.
"Without this essential interaction, fertilisation
just cannot happen. We may be able to use this discovery to improve fertility
treatments and develop new contraceptives."
Researchers believe that a protein on the male sperm –
known as ‘Izumo’ – acts like a metal detector seeking out ‘Juno’ on a female
egg.
They found that mice which did not have either ‘Juno’
or ‘Izumo’ molecules were unable to fuse together.
The research, reported in the journal Nature, also
suggests that Juno plays a role in preventing additional sperm fusing with an
already fertilised egg.
"The Izumo-Juno pairing is the first known
essential interaction for sperm-egg recognition in any organism," said
co-author Dr Enrica Bianchi, also from the Sanger Institute.
“The binding of the two proteins is very weak, which
probably explains why this has remained a mystery until now."
Scientists discovered that 40 minutes after the initial
binding of the sperm and egg, ‘Juno’ vanishes, stopping any other sperm from
latching on.
Researchers claim this may help explain why as soon as
an egg is fertilised by one sperm cell it puts up a barrier against others.
Fertilisation involving more than one sperm would lead
to the formation of abnormal doomed embryos with too many chromosomes.
The scientists are now screening infertile women to see
whether ‘Juno’ defects underlie their condition.
If they do, a simple genetic screening test could help
doctors provide them with the most appropriate treatment while avoiding
wasteful expense and stress.
Regular In-Vitro Fertilisation (IVF) treatment, with
sperm randomly fertilising eggs in a laboratory dish, could not work without
‘Juno’.
However, it may be possible to bypass the natural
mating of Izumo and Juno using intra-cytoplasmic sperm injection (Icsi). This
is an increasingly popular method of IVF which involves injecting a sperm
directly into an egg.
While ‘Juno’ is vital to initial egg and sperm binding,
the scientists believe other proteins might be necessary to trigger the full
fusion than leads to fertilisation.
But the Izumo-Juno interaction is an essential first
step without which fertilisation cannot occur.
Both molecules are thought to exist throughout the
mammalian animal kingdom. The scientists identified versions of ‘Izumo’ and
‘Juno’ in pigs, opposums and humans as well as mice.
Leading fertility expert Dr Allan Pacey, senior
lecturer in reproduction and developmental medicine at the University of
Sheffield, said: "The identification of the Juno protein opens up many
exciting prospects. Perhaps the most obvious biomedical application of this
finding is whether screening for this protein (or its gene in a blood sample)
could be used as a test of fertility.
"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.
"The second, and perhaps most likely application,
is whether scientists could devise drugs or vaccines that could block the way
this protein works or how the sperm protein Izumo interacts with it. This could
lead to a new and novel non-hormonal contraceptive for both humans and other
species of mammals.”
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