Izumo Protein Understanding the Molecular

Izumo Protein

Understanding the molecular mechanics of fertilization has been a challenging task for biologists for the last two decades. The discovery of the ADAM group of genes, which were supposed to play a vital role in the fusion process, in the mid nineties, was widely regarded as a big breakthrough. However, shortly later, experiments with mice downplayed the importance of ADAM genes. Since then, the quest for understanding the fertilization process at the gene level has continued without much success until the research work of Naokazu Inoue, et al. (2005) that proved to be revolutionary in its outcome. The finding of the all important protein molecule named Izumo by the researchers has opened a whole new frontier in the field of reproductive biology and research in the science of contraception. This breakthrough is considered remarkable, as it would enable scientists to create contraceptive solutions that do not carry the risks and potential side effects of hormone-based contraceptives. A brief review would help us gain better insight into the field of reproductive biology.

Discussion

This research by Inoue et al. created a whole new understanding of the molecular mechanisms involved in the process of fertilization. Before this research was undertaken it was well-known that CD9, a protein molecule found in the surface of the eggs, was primarily responsible for the fusion of egg and sperm. Even though, as early as 1987, the main researcher of this paper had discovered an 'anti mice sperm monoclonal antibody' known as OBF13 which had significantly affected the fertilization rate in mice, it was only much later during this research that the unknown antigen which was bound to OBF13, was identified to be the crucial Izumo protein. The authors found the new antigen using fusion inhibiting antibodies and gene cloning techniques. Thus, this research is considered a breakthrough in that it resulted in finding the first protein molecule on the sperm, which is found to be indispensable for fertilization. [Inoue et. al, 2005]

For the research, the authors used the process of homologous recombination to produce Izumo deficient allele and 3 types of mice: wild type, heterozygous and homozygous knockout mice (Izumo -/- with no izumo protein). It is important to note that izumo is detectable only after the sperm undergoes acrosome reaction. [Nazet.al 2001] the researchers found that the expression of the izumo protein did not in any way affect the levels of other related proteins such as ADAM2, CD147, and sp56 that are also involved in fusion process. Nine wild type female mice were paired with izumo -/- males and the results showed not a single case of pregnancy. The research also clearly showed that izumo protein was essential for the process of fusion and izumo deficient sperms can still produce healthy embryos. The possibility that fusion might have occurred and some other factor prevented formation of pronucleus was totally outruled because all the 282 studied eggs failed to form pronucleus in the presence of izumo devoid sperms. Intracytoplasmic sperm injection (ICSI) of izumo -/- (izumo knockout sperm) did however proceed to create normal embryos suggesting clearly that the role of Izumo is clearly to do with fusion process and not beyond that. [Inoue et.al 2005]

The absence of Hoechst 33342, an antibody found only on fused sperms, also confirmed that none of the izumo -/- sperms fused with the eggs. The researchers also found that eggs had many more sperms on their zona pellucida (ZP) in the presence of Izumo -/- sperms than with izumo +/- sperms indicating the inability of Izumo -/- sperms to trigger the normal ZP reaction indicating no fusion. The researchers also attested that human izumo protein was essential for fusion by conducting an experiment of human hamster fusion using zona free hamster. The failure of human hamster fusion in the presence of anti-human izumo antibody clearly showed that Human izumo protein is essential for fertilization.

This study however has raised some new questions. The difference in fertilization capacities between wild type and Izumo +/- type was not clearly discussed. Also, the fact that ICSI of izumo -/- produces twice as many litters as did the izumo +/- type implicates the possible role of other proteins, the expression of which is affected by the allele. That is to say that the mutant knockout sperm might have up-regulated or down regulated the expression of this unknown protein which may have aided the fertilization process as indicated by the statistics. [Katie Winter] contraceptive vaccine targeting the sperm is very much in prospect but much work is yet to be accomplished. A CD46 gene knockout as tried by the author in an earlier study did not result in defective sperm egg fusion. [Inoue et.al 2003] Similarly, ADAM1, ADAM2, ADAM3 gene knockout did not result in fusion defects but only exhibited problems in sperm zona binding. [Cho et, al 1998] a recent development of an immuno-contraceptive antigen for mice proved to be unsuccessful in spite of its success in vitro. [Wang et.al, 2008]

Conclusion