Identification of luminal and secreted proteins in bull epididymis

The epididymis plays a major role in the acquisition of sperm fertility. In order to shed light on specific features of epididymal function in mammalian species, we characterized the luminal proteins (luminal proteome) and secreted proteins (secretome) in the bovine epididymis. We identified 172 different luminal proteins in 9 distinct epididymal regions. The concentration and secretory activity of luminal proteins were quantified throughout the epididymis. Among the most abundant secreted proteins, we found lipocalin 5, (LCN5), NADP(+)dependent prostaglandin dehydrogenase (PTGDS), Niemann-Pick disease type C2 protein (NPC2), glutathione peroxidase type 5 (GPX 5), clusterin (CLU), hexosaminidase B (HEXB) and galactosidase (GLB1), each of which is released in distinct epididymal regions. Gelsolin, (GSN) previously not described in mammalian epididymal fluid, appeared to be a major protein secreted exclusively in the distal region of the bovine epididymis, where fully mature spermatozoa are stored. Although the major epididymal proteins are conserved between mammalian species, this study highlights the specificity and mechanisms of protein processing of epididymal secretion in the bull. In addition, this study provides a major insight into the sequential changes occurring in the sperm environment while gaining fertilizing capacity and could provide new information for the future identification of potential fertility markers.