Normal fertilization occurs with eggs lacking the integrin alpha 6 beta 1 and is CD9-dependent

Previous results, based on inhibition of fertilization by an anti-alpha 6 i ntegrin mAb (GoH3), suggest that the alpha 6 beta 1 integrin on mouse eggs functions as the receptor for sperm (Almeida, E.A., A.P. Huovila, A.E. Suth erland. L.E. Stephens PG. Calarco, L.M. Shaw A.M. Mercurio, A. Sonnenberg, P. Primakoff, D.G. Myles, and J.M. White. 1995. Cell. 81:1095-1103). Becaus e the egg surface tetraspanin CD9 is essential for gamete fusion (Kaji, K., S. Oda, T. Shikano, T. Ohnuki, Y. Uematsu. J. Sakagami, N. Tada, S. Miyaza ki, and A. Kudo. 2000. Nat. Genet. 24:279-282; Le Naour. F.,E. Rubinstein, C. Jasmin, M. Prenant, and C. Boucheix. 2000. Science. 287:319-321; Miyado, Ii., G. Yamada, S. Yamada, H. Hasuwa, Y. Nakamura, F. Ryu Ii. Suzuki, K. K osai, K. Inoue, A. Ogura, M. Okabe, and E. Mekada. 2000. Science. 287:321-3 24) and CD9 is known to associate with integrins, recent models of gamete f usion have posited that egg CD9 acts in association with alpha 6 beta 1 in fusion (Chen, M.S., K.S. Tung, S.A. Coonrod, Y. Takahashi, D. Bigler, A. Ch ang, Y. Yamashita, P.W. Kincade, J.C. Herr, and J.M. White. 1999. Proc. Nat l. Acad. Sci. USA. 96:11830-11835, Kaji, K., S. Oda, T. Shikano, T. Ohnuki, Y. Uematsu, J. Sakagami, N. Tada, S. Miyazaki, and A. Kudo. 2000. Nat. Gen et. 24:279-282; Le Naour, E, E. Rubinstein, C. Jasmin, M. Prenant, and C. B oucheix. 2000. Science. 287:319-321; Miyado, K., G. Yamada, S. Yamada, H. H asuwa, Y. Nakamura. F. Ryu, K. Suzuki, K. Kosai, K. Inoue, A. Ogura, M. Oka be, and E. Mekada. 2000. Science. 287:321-324). Using eggs from cultured ov aries of mice lacking the alpha 6 integrin subunit, we found that the ferti lization rate, fertilization index, and sperm binding were not impaired com pared with wild-type or heterozygous controls. Furthermore, a reexamination of antibody inhibition, using an assay that better simulates in vivo ferti lization conditions, revealed no inhibition of fusion by the GoH3 mAb. We a lso found that an anti-CD9 mAb completely blocks sperm fusion with either w ild-type eggs or eggs lacking alpha 6 beta 1. Based on these results, we co nclude that the alpha 6 beta 1 integrin is not essential for sperm-egg fusi on, and we suggest a new model in which CD9 acts by itself, or interacts wi th egg protein(s) other than alpha 6 beta 1,to function in sperm-egg fusion .