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Expression and putative function of fibronectin and its receptor (integrin alpha5 beta1) in male and female gametes during bovine fertilization in vitro

M Thys, Reproduction, Obstetrics and Herd Health, Faculty of Veterinary Medicine - Ghent University, Merelbeke, 9820, Belgium
H Nauwynck, Virology, Parasitology and Immunology, Faculty of Veterinary Medicine - Ghent University, Merelbeke, Belgium
D Maes, Reproduction, Obstetrics and Herd Health, Faculty of Veterinary Medicine, Merelbeke, Belgium
M Hoogewijs, Reproduction, Obstetrics and Herd Health, Faculty of Veterinary Medicine, Merelbeke, Belgium
D Vercauteren, Ghent Research Group on Nanomedicines, Faculty of Pharmaceutical Sciences - Ghent University, Gent, Belgium
H Favoreel, Virology, Parasitology and Immunology, Faculty of Veterinary Medicine - Ghent University, Merelbeke, Belgium
A Van Soom, Department of Reproduction, Ghent University, Merelbeke, Belgium

Correspondence: Mirjan Thys, Email: mirjan.thys{at}ugent.be

Abstract

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 in vitro fertilization (IVF) - strongly inhibits sperm penetration. The present study was conducted 1) to evaluate the expression of Fn and its integrin receptor (5β1) on male and female bovine gametes using indirect immunofluorescence and 2) 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 AlexaFluor488-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 sperm cell surface, suggests a ‘velcro’ interaction between the endogenous Fn ligand and corresponding receptors on both (acrosome reacted) sperm cell and oolemma, initiating sperm-egg binding.