Sperm Surface Proteins After Capacitation

Abstract

ABSTRACT: Several integrins recognize as ligands proteins containing the Arg‐Gly‐Asp (RGD) sequence, such as fibronectin, vitronectin, and laminin. It has been previously demonstrated that oligopeptides containing the RGD sequence competitively inhibit both the adhesion of hamster and human sperm to zona‐free hamster eggs and their subsequent penetration. In addition, the appearance of fibronectin on the surface of living human spermatozoa after capacitation has been demonstrated. In this work, it is shown that spermatozoa incubated overnight under capacitating conditions, but not fresh spermatozoa, also display the RGD‐containing proteins vitronectin and laminin. Whereas the expression of fibronectin does not appear to be localized to any specific region of the sperm surface, laminin is present solely on the sperm tail, and vitronectin was detected mostly as an equatorial band on the sperm head. The percent of capacitated spermatozoa within each ejaculate reacting with antivitronectin antibodies (51% to 94%) was similar to that observed with antifibronectin antibodies (72% to 100%) in a series of fertile donors, and in a series of infertile men (7% to 98% for vitronectin versus 5% to 100% for fibronectin). In contrast, the percent of spermatozoa displaying laminin was lower, ranging from 2% to 42% for fertile donors and from 5% to 34% for infertile donors, and was unrelated to the expression of fibronectin or vitronectin. The time of appearance of both fibronectin and vitronectin when spermatozoa were incubated under capacitating conditions varied for different sperm donors, suggesting a difference in the process of their expression between different men. The specificity of antivitronectin antibody binding to human spermatozoa was demonstrated by competitive inhibition with purified human vitronectin. That there was no immunologic reaction between the antivitronectin antibodies used and fibronectin was demonstrated both by the failure of free fibronectin to inhibit antivitronectin antibody binding to spermatozoa, and by Dot blot analysis. A partial cross‐reaction of the polyclonal antivitronectin antibody with fibronectin was shown by Western blot analysis, but this phenomenon was not present when the monoclonal antivitronectin antibody was used. In addition, both fibronectin and vitronectin could be extracted from capacitated spermatozoa solubilized in Chaps buffer, as shown by Dot blot and Western blot analysis. These observations suggest that vitronectin and fibronectin expressed on the surface of capacitated human spermatozoa could act as a ligand for specific receptors on the egg, and play a role in sperm—oolemmal adhesion.