Syndecan-4 cooperates with α5β1 integrin in human spermatozoa during the fibronectin-induced acrosomal reaction

Syndecan-4 is a transmembrane heparan sulfate proteoglycan that activates signaling pathways, either independently or by binding to growth factors or integrins. Syndecan-4 interacts with protein kinase C alpha (PKCα) and scaffolds for other proteins to modulate cell adhesion and migration. Although syndecan-4 plays crucial roles in somatic cells, its function in human spermatozoa remains largely unknown. In this study, we investigated the expression, localization, and functional role of syndecan-4 in human spermatozoa during the fibronectin-induced acrosome reaction. We detected syndecan-4 expression in noncapacitated and capacitated spermatozoa using immunoblotting and flow cytometry. Immunofluorescence analysis revealed a dynamic redistribution of syndecan-4 to the anterior acrosomal region after 18 h of incubation in capacitating medium. Functional assays using heparin and anti-syndecan-4 antibodies revealed a significant decrease in the percentage of acrosome-reacted spermatozoa upon fibronectin (FN) stimulation, indicating that syndecan-4 is required for this process. Colocalization and coexpression analyses revealed that syndecan-4 partially overlapped with α5β1 integrin, a known FN receptor, suggesting a cooperative function between them. Furthermore, sperm treatment with FN induced PKCα phosphorylation, and coimmunoprecipitation assays confirmed the formation of a syndecan-4/PKCα complex, which was inhibited by α5β1 blockade. Our findings demonstrate that syndecan-4 is functionally involved in FN-induced AR in human spermatozoa and cooperates with α5β1 integrin to regulate PKCα activation, revealing a novel signaling mechanism in sperm physiology.