Integrins are not involved in the process of human sperm-oolemmal fusion

Background: We investigated whether integrins are required for the human sperm-oocyte binding and fusion processes. Methods: The expression of several integrin subunits at the human oocyte plasma membrane was investigated using immunofluorescence microscopy, and the functional role of integrins expressed at the human oocyte surface in sperm-oocyte interaction was studied using a zona-free human oocyte binding and fusion assay. A total of 144 unfertilized oocytes were stained with anti-integrin antibodies and 147 zona-free unfertilized oocytes were inseminated in the presence of various anti-integrin antibodies that were expressed in oocyte plasma membrane. Results: The antibodies of six alpha integrin subunits (alpha(2), alpha(3), alpha(5), alpha(6), alpha(V), alpha(M)) and six beta integrin subunits (beta(1), beta(2), beta(3), beta(4), beta(5), beta(6)) were bound to the surface of fixed unfertilized oocytes. In contrast, the presence of alpha(1) and alpha(4) subunits could not be verified. The human sperm-oocyte binding was only partially inhibited by blocking antibodies of alpha(2), alpha(3), alpha(5), alpha(6), alpha(V), alpha(M), beta(1), beta(2) and beta(3) with a maximum of 55% inhibition, but antibodies of beta(4), beta(5) and beta(6) showed no effect on sperm-oolemmal binding. A similar reduction of the number of fused sperm was observed. However, the ratio of fused sperm to total sperm (bound and fused) was not impaired by all integrin antibodies, suggesting that integrins had no role in the sperm-oolemmal fusion process. Conclusions: These results suggest that one of the binding mechanisms can be inhibited by integrin antibodies but that this mechanism does not play an essential role in the human sperm-oolemmal binding and fusion processes. The other mechanisms, insensitive to integrins, may involve both binding and fusion processes in human oocytes.