Sperm protamine deficiency correlates with sperm DNA damage in Bos indicusbulls

The primary purpose of spermatozoa is to deliver the paternal DNA to the oocyte at fertilization. During the complex events of fertilization, if the spermatozoon penetrating the oocyte contains compromised or damaged sperm chromatin, the subsequent progression of embryogenesis and foetal development may be affected. Variation in sperm DNA damage and protamine content in ejaculated spermatozoa was reported in the cattle, with potential consequences to bull fertility. Protamines are sperm-specific nuclear proteins that are essential to packaging of the condensed paternal genome in spermatozoa. Sperm DNA damage is thought to be repaired during the process of protamination. This study investigates the potential correlation between sperm protamine content, sperm DNA damage and the subsequent relationships between sperm chromatin and commonly measured reproductive phenotypes. Bos indicus sperm samples (n=133) were assessed by two flow cytometric methods: the sperm chromatin structure assay (SCSA) and an optimized sperm protamine deficiency assay (SPDA). To verify the SPDA assay for bovine sperm protamine content, samples collected from testis, caput and cauda epididymidis were analyzed. As expected, mature spermatozoa in the cauda epididymidis had higher protamine content when compared with sperm samples from testis and caput epididymidis (p<0.01). The DNA fragmentation index (DFI), determined by SCSA, was positively correlated (r=0.33 +/- 0.08, p<0.05) with the percentage of spermatozoa that showed low protamine content using SPDA. Also, DFI was negatively correlated (r=-0.21 +/- 0.09, p<0.05) with the percentage of spermatozoa with high protamine content. Larger scrotal circumference contributes to higher sperm protamine content and lower content of sperm DNA damage (p<0.05). In conclusion, sperm protamine content and sperm DNA damage are closely associated. Protamine deficiency is likely to be one of the contributing factors to DNA instability and damage, which can affect bull fertility.