Seasonal changes in the proteome of cryopreserved bull semen supernatant.

Seasonal variability in cattle fertility may be attributed to male reproduction, including the quality of semen produced by males and its usefulness after cryopreservation. The exact molecular mechanisms responsible for such seasonal variation are not entirely known. The aim of our study was to evaluate the changes in the proteome of cryopreserved bull (Bos taurus) semen supernatant throughout the year. The quality of cryopreserved semen collected from the same Limousin bulls during spring, summer, autumn and winter (n = 5 in each group) was assessed by measuring sperm motility parameters using the CASA system and sperm viability and the level of sperm reactive oxygen species using flow cytometry. Two-dimensional difference in-gel electrophoresis analysis coupled with the MALDI-TOF mass spectrometry was used to detect seasonal changes in the proteome of cryopreserved bull semen supernatant. We found seasonal variability in the percentage of sperm motility (P < 0.05), which was the highest in autumn and winter and the lowest in spring and summer. There was an effect of season on sperm viability (P < 0.05), with the highest percentage of dead sperm recorded in summer. There was no effect of season on sperm levels of oxidation. Proteomic analysis identified 23 protein spots, representing 10 proteins that changed during the year (P < 0.05). Albumin, clusterin, spermadhesin 1 and platelet-activating factor acetylhydrolase precursor were most abundant during winter, which presumably reflected correct lipid composition and morphology of spermatozoa, as well as involvement in protection against premature capacitation. Moreover, osteopontin and nucleobindin 1 could also be connected to increased semen quality in this season. C-C motif chemokine 2 precursor, epididymal-specific lipocalin-5, peptidyl-prolyl cis-trans isomerase and seminal ribonuclease were most abundant during summer and probably reflected disturbances during spermatogenesis and sperm maturation. In summary, our study has shown that cryopreserved bull semen quality parameters were higher in winter than in summer. The identification of proteins connected to the variability of semen quality during the year have provided new insight into understanding the mechanisms underlying the seasonality of cattle breeding.