The birds and the bees: Juno and Izumo

The recent identification of the two molecules that trigger fertilisation in mice is a key moment in Biology. Sexual reproduction is thought to be over 1bn years old and is the cornerstone of genetic variation, since asexual reproduction provides a limited basis for generating genetic diversity. (In simple terms, the gene pool is much bigger if you reproduce by sexual means). The search for these molecules has been going on for some time and it worth thinking for a moment about the problem. The molecules (if indeed there are just two) will be different in all species, since crossing the species barrier is an important aspect of sexual reproduction. (However, they may show similarities which define their overall biochemical characteristics). So are model organism studies valid and is the mouse an adequate model organism? Then then there is the ethical issue of obtaining sperm and eggs from human donors in order to isolate and characterise the molecules in humans. Is this acceptable? Is it even possible from a technological standpoint?

The Izumo1 protein from mouse sperm was identified in 2005 by a Japanese team from Osaka. The name refers to a wedding shrine in Japanese, and is essential for sperm egg interaction in both mice and humans. I immediately used BLAST to look at the sequence identity between Murine (mouse) and human versions of both  Izumo1 and Juno. Juno, as some of you may know is the Roman goddess of fertility and marriage. The Juno proteins from mice and man are just under 70% identical (although this isn't definitive since the folr4 gene [the previous name forJuno] has isoforms and I haven't read the papers in detail yet) and the Izumo proteins are around 50% identical (the same caveat applies here). So my first though is that the core functions are similar and there is sufficient difference to block cross fertilization. So far so good. 

How did the group at the Sanger Institute (see the interview on you tube) identify Juno? Gavin Wright's group at the Sanger have been developing an elegant method for the identification of weak affinity receptor ligand interactions. This may seem a contradiction: why would a highly specific interaction be weak and often transient? Most previous methods have assumed that such key interactions are strong and stable. I shall return to this issue later. The method also recognises that cell surface molecules are amongst the most challenging from a biochemical isolation perspective and so libraries of expressed molecules on cells are screened by a technique called AVEXIS. The extracellular component (ectodomain) of a particular protein is coupled to a plastic dish and cells are passed over the immobilized protein until they bind. These interactions can then be identified (for details see the Wright lab link). 

How do we know that the Wright lab are correct? The key pieces of data are the ability of specific antibodies targeted at Izumo1 and Juno to block sperm and egg interactions. However, a key finding is that around 30 minutes after the interaction of I and J, J has disappeared, which is consistent with the observation that one fertilisation event blocks all subsequent sperm binding interactions. This explains the block to polyspermy and opens the doors to an investigation of the mechanism behind the loss of J after 30 minutes. This discovery and its relationship to this phenomenon in other species will open a new window on evolutionary biology and suggests some important avenues for the Yin and Yang of childbirth: infertility and contraception.