The main aim of this work is to gain insight into the mechanisms by which freezing-thawing alters the nucleoprotein structure of boar sperm. For this purpose, the freezing-thawing-related changes of structure and location of histones-DNA domains in the boar sperm head were analyzed through Western blot and immunocytochemistry. Afterwards, it was analyzed whether freezing-thawing induced changes in tyrosine phosphorylation levels of both protamine 1 and histone H1, through Western blot analyses in samples previously subjected to immunoprecipitation. This analysis was completed with the determination of the changes induced by freezing-thawing on the overall levels of sperm-head disulfide bonds through analysis of free-cysteine radicals levels. Freezing-thawing induced significant changes in the histones-DNA structures, which were manifested in the appearance of a freezing-thawing-linked histone H1-DNA aggregate of about a 35-kDa band and in the spreading of histone H1-positive markings from the caudal area of the sperm head to more cranial zones. Freezing-thawing did not have any significant effect on the tyrosine phosphorylation levels of either protamine 1 or histone H1. However, thawed samples showed a significant (P < 0.05) increase in the free cysteine radical content (from 3.1 ± 0.5 nmol/μg protein in fresh samples to 6.7 ± 0.8 nmol/μg protein). In summary, our results suggest that freezing-thawing causes significant alterations in the nucleoprotein structure of boar sperm head by mechanism/s linked with the rupture of disulfide bonds among the DNA. These mechanisms seem to be unspecific, affecting both the protamines-DNA unions and the histones-DNA bonds in a similar way. Furthermore, results suggest that the boar-sperm nuclear structure is heterogeneous suggesting the existence of a zonated pattern, differing in their total DNA density and the compactness of the precise nucleoprotein structures present in each zone.
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