Abstract
BackgroundCystatin C (CST3), a cysteine protease inhibitor in seminal plasma, is expressed in animal uteri. However, its expression in the human female reproductive tract and its effect on human sperm capacitation are unclear.MethodsThe cellular localization of CST3 was observed using immunohistochemistry. The binding of CST3 to sperm was examined using immunocytochemistry. Sperm motility parameters were analyzed using computer-assisted sperm analysis. Sperm capacitation was evaluated by analyzing cholesterol content, protein tyrosine phosphorylation levels, and the acrosome reaction.ResultsImmunohistochemical staining demonstrated that CST3 is prominently expressed in the female reproductive tract, including the epithelial lining and cervix and endometrium fluids, particularly at times near ovulation. It can bind to human sperm on the post-acrosomal head region and the mid and principal piece of the tail. CST3 enhances sperm motility and inhibits the signal initiating sperm capacitation, i.e., efflux of cholesterol from the sperm plasma membrane and a late sperm capacitation event, i.e., the increase in the sperm protein tyrosine phosphorylation. The suppressive trend on sperm acrosome reaction further supports CST3’s ability to inhibit sperm capacitation.ConclusionsThese findings suggest that cervical CST3 may prevent precocious capacitation and acrosome reaction, thus preserving sperm fertilizing ability before it reaches the fallopian tube. Additionally, CST3 may help sperm enter the upper reproductive tract by enhancing sperm motility.
Highlights
Cystatin C (CST3), a cysteine protease inhibitor in seminal plasma, is expressed in animal uteri
CST3 was detected in cervical epithelial cells, glands, and mucus (Figs 1D and E), during the late-proliferative phase (Fig. 1E)
CST3 was detected in uterine fluid, indicating that it could be expressed and secreted by the endometrium (Fig. 2A) and in the fallopian tube epithelial cells, its expression levels were lower there (Fig. 2B)
Summary
Cystatin C (CST3), a cysteine protease inhibitor in seminal plasma, is expressed in animal uteri. CST3 is prominently expressed in the male reproductive tract with high levels in seminal plasma, approximately 37fold higher than in serum [4, 5]. Capacitation is a physiologically complex change and may be initiated by cholesterol removal from the sperm plasma membrane, leading to changes in the membrane’s structure and fluidity and increasing the sperm’s calcium and bicarbonate ion permeability [14, 15].
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