Alterations in the abundance of protamine proteoforms related to sperm chromatin packaging, obesity, and age in normozoospermic men

Protamines are considered among the most relevant sperm proteins because of their functional implications on paternal genome packaging and protection. Although the proteomic evaluation of protamines is technically challenging, mass spectrometry-based studies have shown a complex population of protamine proteoforms in the human sperm. This includes intact, truncated, and modified forms for protamine 1 (P1) and mature and immature components of the protamine 2 (P2) family. However, it is still unknown whether global or specific protamine proteoform levels may be unbalanced under conditions that may impair paternal chromatin maturity and epigenetic information. In this study, protamines from normozoospermic men stratified according to body mass index, age, and chromatin maturity (assessed through the P1/P2 ratio derived from acid-urea electrophoresis) were evaluated using a refined top-down mass spectrometry protocol for protamine proteoform quantification and comparative analysis. Accumulation of the P2 immature forms HPS1 and HPI2 was significantly associated with abnormally high P1/P2 ratios, suggesting either impaired eviction of P2 immature forms or defective P2 processing during spermatogenesis in these men clinically classified as normozoospermic. When considering weight and age as factors, P1 was the only affected protamine. Sperm from obese men, which were found to be exposed to high levels of oxidative damage derived from lipid peroxidation, showed mass shift(s) of +61 Da from the unmodified P1 protein sequence. Men of advanced age showed a specific loss of diphosphorylated P1, mainly on Ser 11 and 22. Our results allow the hypothesis that protamine proteoforms in the male gamete act as additional layers of epigenetic information, the alteration of which might be related to some cases of impaired sperm function.