Bactericidal/Permeability‐Increasing Protein Is Associated With the Acrosome Region of Rodent Epididymal Spermatozoa

Abstract

To elucidate the molecular mechanisms involved in sperm maturation during epididymal transit, we intended to isolate secretory molecules that are region-specifically expressed along the epididymis and secreted into the lumen of epididymal ducts. By using differential display screening and DNA sequence analyses, we isolated a rat bactericidal/permeability-increasing protein (BPI) possessing a signal sequence at its N-terminal, which was expressed in the caput region of epididymis, but not in the caudal region. Reverse transcription polymerase chain reaction analysis and in situ hybridization showed that rat BPI messenger RNA (mRNA) was highly expressed in caput epididymal epithelium and that its expression level was developmentally up-regulated. Confocal laser scanning microscopy with the anti-BPI antibody revealed that in both rats and mice, BPI protein was detected on granulelike structures in the lumen of both caput and cauda epididymal ducts, as well as at the sperm surface covering the acrosome region in spermatozoa freshly isolated from epididymis. Acrosome reaction induced by calcium ionophore A23187 in vitro brought about the disappearance of BPI on mouse spermatozoa. These data suggested that BPI, which is synthesized in caput epididymis and secreted into the lumen, is associated with not only the granulelike structures, but also the sperm surface covering the acrosome region, and that BPI bound to the acrosome region is extinguished by acrosome reaction. Possibly BPI bound to the sperm surface covering the acrosome region in rodent spermatozoa is involved in sperm maturation or fertilization.

More Than a Simple Lock and Key Mechanism: Unraveling the Intricacies of Sperm-Zona Pellucida Binding

“…It has been suggested that these epididymal granules facilitate the transfer of proteins to the sperm surface during their transit of the organ , Saez, et al, 2003, Yano, et al, 2010. This is in keeping with the demonstration that biotinylated proteins are able to be transferred between epididymosomes and the sperm surface (Saez, et al, 2003).…”

Fertilization: a sperm’s journey to and interaction with the oocyte

“…Cholesterol is particularly abundant in seminal plasma (19) and has an inhibitory effect on sperm capacitation. During capacitation, cholesterol and other sterols are removed from the sperm surface, and non-covalently attached glycoproteins acquired in the epididymis are released from the sperm surface (20). Together, these modifications create a more fluid membrane environment, making the sperm competent for subsequent fertilization cues.…”

Bactericidal/permeability-increasing protein in the reproductive system of male mice may be involved in the sperm–oocyte fusion

“…For instance, it has been demonstrated that cystatinrelated epididymal spermatogenic protein has both anti-microbial functions and a role in the sperm-oocyte fusion process. BPI is located in the matrix of the acrosome and attaches to the spermatozoa plasma membrane surface of the acrosomal region (Yano et al 2010). Data from our sperm-oocyte fusion assay suggest that both the N-and C-terminal domains of BPI may be directly involved in the sperm-oocyte fusion process.…”

“…Immunofluorescence analysis showed BPI localization in the seminiferous tubule, caput epididymis epithelium, and sperm. Although it has been reported that BPI protein is associated with the acrosome region of mice epididymal spermatozoa (Yano et al 2010), BPI expression was not seen in the epithelia of the corpus and cauda epididymis. Results from sperm-oocyte fusion assays indicated that anti-BPI antiserum may influence the fusion of sperm and oocytes, although the mechanism by which this occurs is still unclear and needs further investigation.…”

“…We need to do more research to confirm whether BPI has a function of lipid transport in Leydig cells. In addition, it has been reported that BPI is localized in the matrix of the acrosome and attaches to the spermatozoa plasma membrane surface in the acrosomal region (Yano et al 2010), suggesting that BPI may play a role in fertilization in addition to its anti-microbial function.…”