No abstract

MN9, a monoclonal antibody raised against mouse spermatozoa, specifically recognizes the equatorial segment of sperm head in several mammalian species, including humans. Colloidal gold-immuno-electron microscopy of mouse spermatozoa has shown that the antigen is localized in the space between the outer and inner acrosome membranes and on the acrosome membranes at the equatorial segment. Immunoblotting after electrophoresis of spermatozoa from the cauda epididymidis has identified two immunoreactive bands: 38 kDa and 48 kDa in mouse, and 48 kDa in rat. During spermiogenesis in rat, this antigen is transported to the equatorial segment via a unique pathway, first appearing in some cisternae of the endoplasmic reticulum and in the Golgi apparatus of spermatids at around step 3. The antigen can further be found on the vesicles at the trans-side of the Golgi apparatus, in the matrix of the head cap, and on the head cap membrane in step-4 to step-7 spermatids. The antigen appears to be concentrated at the equatorial segment during late spermiogenesis. Neither the (pro-)acrosomic granule nor the surrounding membrane are required in this pathway. This pathway can be termed the 'Golgi-head cap tract'.

Using a monoclonal antibody T21, we reported that a mouse sperm maturation-associated antigen sialoglycoprotein of 54000 daltons (54K sialoglycoprotein) was secreted at the distal caput to proximal corpus epididymidis and that the 54K sialoglycoprotein had a hidden determinant (cryptodeterminant), which could be eliminated by sialidase treatment (Toshimori et al. (1988): Histochemistry 90: 195-200; (1990a): Biol Reprod 42:151-160; (1990b): Arch Histol Cytol 53:339-349). This study evaluated the mouse sperm susceptibility to phagocytosis by macrophage in vitro. Comparisons were made between sperm from the caput epididymidis (caput sperm) incubated in modified Krebs Ringer's solution (MKR) and caput sperm incubated in MKR containing cauda fluid, and between sialylated (sialidaseuntreated) sperm from the corpus and cauda epididymidis (corpuslcauda sperm) and desialylated (sialidase-treated) corpuslcauda sperm. The results showed that macrophages were least actively engaged in phagocytosis for caput sperm incubated in MKR containing cauda fluid, and most active for desialylated corpuslcauda sperm. Incubation of caput sperm in MKR containing cauda fluid revealed that the 54K sialoglycoprotein in cauda fluid could be bound to the flagellar surface of caput sperm. These results together with previous findings strongly suggest that the 54K sialoglycoprotein bound to immature sperm during maturation in the epididymis is implicated in the protection of sperm from phagocytosis with the aid of sialic acid residues.Key Words: Mouse; Sperm; Epididymal maturation; Sialic acid; Phagocytosis. INTRODUCTIONA major function of the mammalian epididymal epithelium is to synthesize and secrete glycoproteins, and therefore both the synthetic activity and the nature of the secretory products vary along the excurrent duct, resulting in changes in composition of the duct fluid [26]. These secretory products of the epididymis are believed to be involved in the changes that spermatozoa undergo during maturation which causes them to gain functions essential for fertilizing ability such as zona pellucida binding and forward motility [3,4, 401. In general, cell surface negative charge is chiefly due to sialic acid residues [7,17, 281, and the bound sialic acid residues have significant biological functions, such as masking of antigens (anti-recognition) and cell-cell recognition in some cells [24,27, 281. In the epididymis, sialic acid residues are reported to be secreted by the epithelium as a terminal sugar of sialoglycoproteins and bound to sperm surface [ l , 6, 12-14, 18, 23, 25, 35, 36, 381. In previous studies using a monoclonal antibody T21, we reported evidences that a sperm maturation-associated antigen sialoglycoprotein of 54000 daltons (54K sialoglycoprotein) has a hidden determinant (cryptodeterminant) [3 1, 32, 331. From these evidences, we reported a hypothetical interaction of the 54K sialoglycoprotein with sperm flagellar plasma membrane, especially in consideration of the location of sialic acid residues on the molecule [3...