SERPINE2, one of the potent serine protease inhibitors that modulates the activity of plasminogen activator and thrombin, is implicated in many biological processes. In the present study, we purified SERPINE2 from mouse seminal vesicle secretion (SVS), using liquid chromatography and identified it by liquid chromatography/tandem mass spectrometry, and it showed potent inhibitory activity against the urokinase-type plasminogen activator. SERPINE2 was expressed predominantly in seminal vesicles among murine male reproductive tissues. It was immunolocalized to the SVS and mucosal epithelium of the seminal vesicle, epididymis, coagulating gland, and vas deferens. In the testes, SERPINE2 was immunostained in spermatogonia, spermatocytes, spermatids, Leydig cells, and spermatozoa. SERPINE2 was also detected on the acrosomal cap of testicular and epididymal sperm and was suggested to be an intrinsic sperm surface protein. The purified SERPINE2 protein could bind to epididymal sperm. A prominent amount of SERPINE2 was detected on ejaculated and oviductal spermatozoa. Nevertheless, SERPINE2 was detected predominantly on uncapacitated sperm, indicating that SERPINE2 is lost before initiation of the capacitation process. Moreover, SERPINE2 could inhibit in vitro bovine serum albumin-induced sperm capacitation and prevent sperm binding to the egg, thus blocking fertilization. It acts through preventing cholesterol efflux, one of the initiation events of capacitation, from the sperm. These findings suggest that the SERPINE2 protein may play a role as a sperm decapacitation factor.

We report a secreted serine protease inhibitor Kazal-type-like (SPINKL) protein. The SPINKL protein was purified from mouse seminal vesicle secretions through a series of steps, including ion-exchange chromatography on a diethylaminoethyl-Sephacel column, gel filtration on a Sephadex G-75 column, and ion-exchange HPLC on a Q strong anion exchange column. Further analysis identified several SPINKL proteins with various N-linked carbohydrates. The SPINKL protein has six conserved cysteine residues that are nearly identical to those of members of the SPINK protein family. It was noted that the SPINKL protein showed no inhibitory activities against common serine proteases such as trypsin, chymotrypsin, subtilisin, or elastase. Spinkl mRNA and SPINKL proteins were found to be primarily expressed in seminal vesicles. Immunohistochemistry revealed that the SPINKL protein occurred in the luminal fluid and mucosal epithelium of the seminal vesicles and was regulated by testosterone. The SPINKL protein was able to bind onto sperm and enhance sperm motility. Also, it was able to suppress BSAstimulated sperm capacitation and block sperm-oocyte interactions in vitro, suggesting that SPINKL may be a decapacitation factor.

The Ly-6 protein family refers to a group of glycophosphatidyl inositol-anchored membrane proteins with ten conserved cysteines. They are thought to be involved in cellular adhesion and signaling. Recently, a subfamily of secreted Ly-6 proteins has been identified. In the present study, we report a secreted Ly-6 protein, secreted seminal vesicle Ly-6 protein 1 (SSLP-1) purified from mouse seminal vesicles using a series of steps including ion-exchange chromatography on a diethylaminoethyl (DEAE)-Sephacel column, gel filtration on a Sephadex G-75 column, and ion-exchange HPLC on a sulfopropyl column. Further analysis demonstrated it to be a novel, previously unnamed, 17 kDa glycoprotein. N-glycosidase F treatment revealed a core protein with a molecular mass of 8720 Da. By Basic Local Alignment Search Tool Protein analysis, we found that SSLP-1 had ten conserved cysteine residues identical with other secreted Ly-6 proteins. The gene Gm191, which is located on chromosome 9, encodes SSLP-1. By Northern blotting with 21 different mouse tissues, we found that Sslp-1 mRNA was predominantly expressed in the seminal vesicle. Immunohistochemistry revealed SSLP-1 protein in the luminal fluid and mucosal epithelium of the seminal vesicles. The amount of Sslp-1 mRNA and SSLP-1 protein in the seminal vesicle was regulated by testosterone and correlated with the stage of animal maturation. The tissue-specific expression pattern suggests that SSLP-1 may play a physiological role in male mouse reproduction.

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