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Equine spermadhesin HSP-7 is a 14 kDa protein isolated from stallion seminal plasma and present on the surface of spermatozoa. HSP-7 displays carbohydrate and zona-pellucida binding properties, but the physiological role in equine fertilization is not well defined. HSP-7 has 98% amino acid sequence homology with the well-studied boar spermadhesin, AWN. Currently, these two proteins are considered to have the same reproductive function. Immunofluorescence studies presented here show that the stallion and boar spermadhesins are localized to different segments on spermatozoa. The variation in molecular compartmentalization of spermadhesin molecules in different species suggests that these structurally related proteins could be involved in independent events of fertilization. While the variation in HSP-7 abundance was not statistically significant between fertile and subfertile stallions, capacitated spermatozoa displayed a marked increase in HSP-7 when compared to neat sperm (P < 0.05). These results indicate that rather than aiding in capacitation, HSP-7 is exposed with capacitation and may have a more significant role in the acrosome reaction and sperm-oocyte recognition than previously documented. Mammalian spermatozoa are synthesized in the seminiferous epithelium and secreted into the lumen of the seminiferous tubules as functionally immature cells that are unable to fertilize an oocyte. Spermatozoa are highly differentiated haploid cells which transform into motile, functional cells during epididymal transit (Senger, 2005). The structure of spermatozoa consists of a head, neck, middle piece and principal piece (Figure 1). The head of spermatozoa includes the nucleus, acrosome and post-acrosomal lamina, which are surrounded by a plasma membrane. The shape of the spermatozoal nucleus defines the shape of the sperm head and therefore, equine spermatozoa are broad and flat. The rostral portion of the sperm nucleus is covered by the acrosome, which is a double-layered membrane with enzymatic activity. During the initial stage of fertilization, the equatorial segment, which lies at the base of the acrosome, is the site of physical interaction between the spermatozoal plasma membrane and the zona pellucida. Upon fusion with the oocyte, the acrosome reaction allows for the spermatozoa to penetrate the zona pellucida and gain access to the perivitelline space (Ellis et al., 2002). The plasma membrane is the outermost component surrounding a spermatozoon and consists of three distinct zones: lipid bilayer, phospholipid-water interface and glycocalyx. Integral and peripheral proteins are intermingled into the lipid bilayer with a negative net 2 charge, attracting and loosely binding proteins from the seminal plasma (McKinnon and Voss, 1992). The …

Characterization of the equine spermadhesin HSP-7 found on stallion spermatozoa as it relates to stallion fertility and sperm capacitation