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Expression and putative function of fibronectin and its receptor (integrin ₅β₁) in male and female gametes during bovine fertilization in vitro

Departments of¹ Reproduction, Obstetrics and Herd Health² Virology, Parasitology and Immunology, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133, 9820 Merelbeke, Belgium³ Ghent Research Group on Nanomedicines, Faculty of Pharmaceutical Sciences, Ghent University, Harelbekestraat 72, 9000 Gent, Belgium

Correspondence should be addressed to M Thys; Email: mirjan.thys{at}ugent.be

Fibronectin (Fn) is a 440 kDa glycoprotein assumed to participate in sperm–egg interaction in human. Recently, it has been demonstrated that Fn – when present during bovine IVF – strongly inhibits sperm penetration. The present study was conducted firstly to evaluate the expression of Fn and its integrin receptor (₅β₁) on male and female bovine gametes using indirect immunofluorescence and secondly, to determine the function of Fn during bovine IVF. Endogenous Fn was detected underneath the zona pellucida (ZP) and integrin ₅ on the oolemma of cumulus-denuded oocytes. Bovine spermatozoa displayed integrin ₅ at their equatorial segment after acrosome reaction. We established that the main inhibitory effect of exogenously supplemented Fn was located at the sperm–oolemma binding, with a (concurrent) effect on fusion, and this can probably be attributed to the binding of Fn to spermatozoa at the equatorial segment, as shown by means of Alexa Fluor 488-conjugated Fn. Combining these results, the inhibitory effect of exogenously supplemented Fn seemed to be exerted on the male gamete by binding to the exposed integrin ₅β₁ receptor after acrosome reaction. The presence of endogenous Fn underneath the ZP together with integrin ₅ expression on oolemma and acrosome-reacted (AR) sperm cell surface suggests a ‘velcro’ interaction between the endogenous Fn ligand and corresponding receptors on both (AR) sperm cell and oolemma, initiating sperm–egg binding.