Sialidases on Mammalian Sperm Mediate Deciduous Sialylation during Capacitation

Background: Mammalian sperm lose sialic acids during capacitation through unknown mechanisms. Results: Sialidases Neu1 and Neu3 are present on sperm. Their activity is required for capacitation and zona pellucida binding. Conclusion: Sperm sialidases modulate sperm surface sialic acids en route to fertilization. Significance: Understanding the mechanism of deciduous sialylation in sperm provides novel insights into sperm function and glycan-mediated fertility.
Sialic acids (Sias) mediate many biological functions, including molecular recognition during development, immune response, and fertilization. A Sia-rich glycocalyx coats the surface of sperm, allowing them to survive as allogeneic cells in the female reproductive tract despite female immunity. During capacitation, sperm lose a fraction of their Sias. We quantified shed Sia monosaccharides released from capacitated sperm and measured sperm sialidase activity. We report the presence of two sialidases (neuraminidases Neu1 and Neu3) on mammalian sperm. These are themselves shed from sperm during capacitation. Inhibiting sialidase activity interferes with sperm binding to the zona pellucida of the ovum. A survey of human sperm samples for the presence of sialidases NEU1 and NEU3 identified a lack of one or both sialidases in sperm of some male idiopathic infertility cases. The results contribute new insights into the dynamic remodeling of the sperm glycocalyx prior to fertilization.