Spermatogenesis (sperm cell development) is a multi-step process beginning with mitotic and meiosis cell divisions that produce immature sperm cells known as spermatids. Maturation of the spermatids occur in the final stage of spermatogenesis referred to as spermiogenesis. During spermiogenesis, DNA packaging in the spermatid undergoes changes resulting in the replacement of most of the histones with arginine-rich peptides called protamines. Protamines undergo post translational modifications (PTMs) before binding to DNA. One of such modifications is phosphorylation. When newly synthesized, protamines are phosphorylated and are then later dephosphorylated. It is thought that the initial phosphorylation of protamines facilitates the correct binding of protamine to DNA while dephosphorylation of protamines is necessary for correct sperm chromatin condensation as well as facilitate other PTMs such as the formation of disulfide bonds that also help in generating the highly compact sperm chromatin structure. The occurrence of phosphorylated protamines in mature sperm has been observed. The presence of phosphorylated protamines in mature sperm may arise due to partial or complete failure of protamine dephosphorylation during sperm maturation. It is thought that alteration of the phosphorylation/dephosphorylation process of protamines impacts DNA packaging in sperm cells negatively and thus, allows greater access to radical damaging species to accumulate and degrade sperm DNA. Here, we utilized reconstituted sperm chromatin structures made with hexa-arginine as well as phosphorylated hexa-arginine to investigate the DNA-binding capacity and free radical-induced damage to condensed DNA nanoparticles. Our results indicate that insertion of a phosphorylated amino acid residue greatly reduces packaging density while allowing an increased accumulation of radical damage to the DNA.  Our findings are a preliminary view towards a better understanding of the impact of residual phosphorylation in protamines on DNA packaging in mature sperm cells.

Impact of phosphorylation on packaging and damage of reconstituted arginine peptide-condensed DNA

ACS Fall 2021

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