S100 A9 is expressed and secreted by the oviduct epithelium, interacts with gametes and affects parameters of human sperm capacitation in vitro

Our previous findings demonstrate that some oviductal secretion proteins bind to gametes and affect sperm physiology and gamete interaction. One of these proteins possesses an estimated molecular weight of 14 kDa. The objective of this study was to isolate and identify this 14 kDa protein, to localize it in the human oviduct, to detect gamete binding sites for the protein, and to evaluate its effects on sperm capacitation parameters and gamete interaction. Explants from the human oviductal tissues of premenopausal women were cultured in the presence of [35S]‐Methionine‐proteins ([35S]‐Met‐proteins). De novo synthesized secreted [35S]‐Met‐proteins were isolated from the culture media by affinity chromatography using their sperm membrane binding ability and analysed by sodium dodecyl sulfate‐polyacrylamide gel electrophoresis. Using liquid chromatography‐tandem mass spectrometry peptide sequencing, human S100 A9 was identified as one of the isolated proteins from the 14 kDa protein band. S100 A9 was detected in oviduct epithelium and oviduct secretion using immunohistochemistry and a Western blot. S100 A9 binding to human oocytes and spermatozoa was assessed by indirect immunofluorescence. The acrosome reaction (AR) affected S100 A9 ability to bind sperm cells. The presence of S100 A9 significantly increased both the induced AR and the sperm protein tyrosine phosphorylation, with respect to controls. However, the protein did not affect sperm‐zona pellucida interaction. Results indicate that S100 A9 is present in the human oviduct and that it modulates parameters of sperm capacitation in vitro. Hence, the protein might contribute to the regulation of the reproductive process in the oviductal microenvironment. The present study indicated that S100 A9 is a protein present in the human oviductal fluid, and it is expressed and secreted by oviduct tissue. S100 A9 binds to female and male gametes and affects human sperm capacitation parameters in vitro . S100 A9 might contribute to the regulation of the reproductive process in the oviductal microenvironment.