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Preliminary Characterization of the Ubiquitin System in the Stallion Reproductive Tract |
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In men, ubiquitin (ubi) acts as a marker for abnormal sperm by binding to membranes of defective cells. Additionally, ubiquitination of spermatogenic cells and/or sperm may initiate or accompany programmed cell death, thus providing a novel mechanism for reductions in sperm numbers. We examined the ubi system in the reproductive tract of a seasonally reproducing mammal, the stallion. The hypotheses were that sperm ubiquitination is a marker for abnormal cells and that the ubi pathway aids in the regulation of both physiologic (seasonal) and pathologic decreases in sperm numbers and semen quality. Ubiquitinated sperm were present only in the epididymis, coincident with a strong accumulation of ubi in the apical cilia of the epididymal endothelium. Additionally, we identified ubi on the surface of a subset of largely morphologically abnormal sperm. Other ubi-conjugating enzymes also were detected. A flow cytometric ubi assay was performed on serial sperm samples collected from 2 fertile stallions. Lowest levels of sperm ubiquitination coincided with the onset of the horse breeding season and were highest in the non-breeding season. Additionally, flow cytometric and immunoblot analyses of sperm from 3 fertile and 1 subfertile stallion revealed significant differences in ubiquitination between the 2 groups. These data suggest that stallion sperm are differentially ubiquitinated during epididymal maturation and that abnormal sperm may be preferentially ubiquitinated. Our data also suggest that the ubi system may play a role in the seasonal regulation of sperm numbers. This study also provides preliminary evidence that unique ubiquitinated substrates may be present in the sperm of subfertile stallions. These substrates could prove useful for objective fertility analysis.
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