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The immunoglobulin superfamily protein Izumo is required for sperm to fuse with eggs


Representing the 60 trillion cells that build a human body, a sperm and an egg meet, recognize each other, and fuse to form a new generation of life. The factors involved in this important membrane fusion event, fertilization, have been sought for a long time1. Recently, CD9 on the egg membrane was found to be essential for fusion2,3,4, but sperm-related fusion factors remain unknown. Here, by using a fusion-inhibiting monoclonal antibody5 and gene cloning, we identify a mouse sperm fusion-related antigen and show that the antigen is a novel immunoglobulin superfamily protein. We have termed the gene Izumo and produced a gene-disrupted mouse line. Izumo-/- mice were healthy but males were sterile. They produced normal-looking sperm that bound to and penetrated the zona pellucida but were incapable of fusing with eggs. Human sperm also contain Izumo and addition of the antibody against human Izumo left the sperm unable to fuse with zona-free hamster eggs.

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Figure 1: Identification and characterization of Izumo.
Figure 2: Targeted disruption of Izumo gene.
Figure 3: Male infertility caused by Izumo disruption.
Figure 4: Involvement of Izumo in a xeno-species fusion system.

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We thank K. Toshimori for providing anti-MN9 antibody; K. Yamagata for discussions; G. L. Gerton and S. Moss for critically reviewing the draft; and Y. Maruyama, A. Kawai and Y. Koreeda for technical assistance with gene disruption. This work was supported by grant-in-aid for Scientific Research and the 21st Century COE program from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.

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Correspondence to Masaru Okabe.

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The authors declare that they have no competing financial interests.

Supplementary information

Supplementary Figure S1

a. Transgene to express mouse Izumo under the control of calmegin promoter. The locations of primers A to E were indicated in this figure. b. lane 1; Izumo +/- mouse with intrinsic Izumo, lane 2 and 3; Izumo -/- mouse with transgenically expressed Izumo and Izumo His-tag, respectively. c. Litter size obtained by mating male mice with C57BL/6 wild-type mice. The group numbers are equal to those shown in b. The numbers in parentheses indicate the numbers of matings. (PPT 11631 kb)

Supplementary Movie

First half. Mouse eggs 6 hours after insemination of Izumo +/- sperm. The eggs were fertilized and at the beginning of pronuclear stage after extruded their second polar body. Note that excess sperm bound to zona pellucida were already detached from probably due to the zona reaction. Second half. Mouse eggs 6 hours after insemination of Izumo -/- sperm. Unlike Izumo +/-, the eggs inseminated with Izumo -/- sperm remained unfertilized without extrusion of their second polar body and had many sperm on their zona pellucida, probably due to the absence of zona-reaction. Note that the Izumo -/- sperm has normal zona-binding ability and normal motility as wild-type sperm. (MOV 3356 kb)

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Inoue, N., Ikawa, M., Isotani, A. et al. The immunoglobulin superfamily protein Izumo is required for sperm to fuse with eggs. Nature 434, 234–238 (2005).

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