The main role of the spermatozoa can be summarized as the delivery of the paternal genome to the oocyte and central to this is the compaction and protection of its DNA. Poor chromatin compaction is a major issue associated with both natural and artificial pregnancies.
The accepted theory behind poor chromatin compaction is an alteration in the Histone/Protamine1/Protamine2 ratios. Thus, Balhorn has shown that poor nuclear condensation is associated with decreased levels of protamine 2. However, other investigations into the levels of these proteins in poor quality sperm have reached alternate conclusions regarding these relationships. As such, there has been little consensus to solely support this idea.
To further understand chromatin condensation, we isolated spermatozoa with both good and poor chromatin compaction. The sperm nucleus was purified and protein composition compared using quantitative proteomics. In total, 342 proteins were identified, and 20 were found to be more abundant (p<0.05, min 1.5-fold change) in the poor quality population. In contrast no proteins were found to be lower within a poorly compacted nucleus. Immunoblotting confirmed higher levels of two of these proteins (Topoisomerase 2A and an ODF2 isoform) in poor quality populations. Of particular interest, the level of detected histones (H4, H3.3, H1T, H2A/B variants) and protamine 2 remained unchanged. This suggests that poor chromatin compaction is highly correlated with retained/excess nucleoplasm. There may be two explanations for this. Firstly, it is possible that chromatin compaction is unrelated to the Histone/Protamine ratio, but rather excess nucleoplasm may hinder condensation. Alternatively, it may be that the mechanism responsible for poor chromatin compaction is highly related to nucleoplasm removal.