An aberrant protamine ratio is associated with decreased H4ac levels in murine and human sperm

Protamine 2 (Prm2/PRM2), together with Protamine 1 (Prm1/PRM1), constitute the two protamines found in both murine and human sperm. During spermiogenesis in haploid male germ cells, chromatin undergoes significant condensation, a phase in which most histones are replaced by a species-specific ratio of these two protamines. Altered PRM1/PRM2 ratios are associated with subfertility and infertility in both male mice and men. Notably, during histone-to-protamine exchange, a small fraction of histones remains (ranging from 1% to 15%) bound to DNA. The regulatory roles of these residual histones, governed by post-translational modifications (PTMs), play a pivotal role in spermatogenesis, particularly in chromatin remodelling and epigenetic regulation of genes during sperm differentiation or even in early embryogenesis. In this study, utilizing a Prm2-deficient mouse model and conducting an analysis of sperm samples from men exhibiting either normozoospermia or atypical spermiograms, we observed alterations in the methylation and acetylation profiles of histones H3 and H4. Subsequent in-depth analysis revealed that discrepancies in protamine ratios do not significantly influence the PTMs of histones in testicular sperm. In murine epididymal sperm, altered protamine ratios are associated with reduced acetylation of histone H4 (H4ac), a phenomenon similarly observed in ejaculated sperm from men. In particular, H4K5ac and H4K12ac were identified as the two modifications that appear to decrease as a result of reduced Prm2/PRM2 levels. Our findings reveal that Protamine 2 is necessary for the maintenance of specific histone PTMs, such as acetylation, which is essential for proper spermatogenesis and particularly for chromatin remodelling.