Relative Levels of Semen Platelet Activating Factor‐Receptor (PAFr) and Ubiquitin in Yearling Bulls With High Content of Semen White Blood Cells: Implications for Breeding Soundness Evaluation

Abstract

High content of the platelet activating factor (PAF) and its plasma membrane receptor (PAFr) in semen is thought to benefit fertility in farm animals and humans. We used flow cytometric, biochemical, and immunocytochemical analysis to examine PAFr levels alone (Trial 1, n = 156 bulls) or in a dual assay with sperm defect marker ubiquitin (UBI; Trial 2, n = 88 bulls), in semen samples from 160 yearling bulls undergoing Breeding Soundness Evaluations (BSE). In both trials, we observed increased PAFr levels in semen samples with high content of white blood cells (WBC). Consequently, PAFr levels within such semen samples correlated negatively with several subjective parameters of BSE, including palpation, satisfaction of evaluation, and scrotal circumference. Due to a high WBC content, increased semen sample dilution had to be applied for microscopic evaluation. There was a negative correlation between semen PAFr and conventional sperm morphology, while the increased levels of PAFr correlated positively with sperm UBI content. Immunofluorescence microcopy revealed high expression of PAFr on the surface of leukocytes and morphologically normal spermatozoa, while reduced immunoreactivity was observed in defective spermatozoa immunoreactive to anti-UBI antibodies. A single PAFr band of appropriate mass was observed in Western blots of ejaculated spermatozoa, while testicular and epididymal spermatozoa also displayed several larger bands indicative of posttranslational processing or modification. Collectively, these data suggest that high levels of semen PAFr in young bulls are indicative of semen contamination with WBC. In the future, objective protein marker-based semen analyses in young bulls will likely require additional parameters distinguishing between marker expression in the spermatozoa and in the contaminating WBC. While identification of high sperm PAFr levels may support fertility, this assay alone is not reliable, due to the expression of PAFr in WBC that contaminate semen samples.