Comparative study of sperm protamine gene expression and chromatin integrity in Nellore and Angus bulls

Methodologies used to evaluate bull fertility often overlook sperm chromatin, a nuclear structure critical for reproductive success due to its DNA composition, which is tightly bound to nucleoproteins called protamines. In mammals, the ratio between Protamine 1 (PRM1) and Protamine 2 (PRM2) plays a pivotal role in male fertility, with imbalances linked to infertility in species such as humans and mice. While bull sperm chromatin was previously believed to contain only PRM1, recent findings have confirmed the presence of PRM2, prompting questions about the impact of the PRM1:PRM2 ratio on bull fertility. The present study hypothesizes that bull infertility may be associated with impaired protamination, resulting in imbalances between PRM1 and PRM2 gene expression. The objective was to compare the chromatin structure through protamine deficiency (CMA3), DNA susceptibility to fragmentation, and PRM1 and PRM2 gene expression among bulls of differing in vivo fertility levels (high fertility and low fertility) and breeds (Nellore and Angus). Sperm from 14 Nellore and 20 Angus bulls with high and low in vivo fertility were analyzed for protamine deficiency, susceptibility to DNA fragmentation, and PRM1 and PRM2 gene expression. Additionally, PRM1:PRM2 ratios were calculated for each experimental group. Results revealed solely a breed effect. Nellore sperm exhibited lower susceptibility to DNA fragmentation and higher PRM1 and PRM2 total transcripts compared to Angus. Conversely, Nellore sperm also demonstrated greater susceptibility to protamine deficiency. Protamine composition may not explain fertility differences, it could play a significant role in adaptive mechanisms across bull breeds.