Abstract
Spermatozoa (SPZ) are motile cells, characterized by a cargo of epigenetic information including histone post-translational modifications (histone PTMs) and non-coding RNAs. Specific histone PTMs are present in developing germ cells, with a key role in spermatogenic events such as self-renewal and commitment of spermatogonia (SPG), meiotic recombination, nuclear condensation in spermatids (SPT). Nuclear condensation is related to chromatin remodeling events and requires a massive histone-to-protamine exchange. After this event a small percentage of chromatin is condensed by histones and SPZ contain nucleoprotamines and a small fraction of nucleohistone chromatin carrying a landascape of histone PTMs. Circular RNAs (circRNAs), a new class of non-coding RNAs, characterized by a nonlinear back-spliced junction, able to play as microRNA (miRNA) sponges, protein scaffolds and translation templates, have been recently characterized in both human and mouse SPZ. Since their abundance in eukaryote tissues, it is challenging to deepen their biological function, especially in the field of reproduction. Here we review the critical role of histone PTMs in male germ cells and the profile of circRNAs in mouse and human SPZ. Furthermore, we discuss their suggested role as novel epigenetic biomarkers to assess sperm quality and improve artificial insemination procedure.
Highlights
The large storage of material in the oocyte and the extensive chromatin condensation in spermatozoa (SPZ) are events characterizing female and male gametes, respectively
After this event a small percentage of chromatin is condensed by histones and SPZ contain nucleoprotamines and a small fraction of nucleohistone chromatin carrying a landascape of histone post-translational modification (PTM)
The enzymatic process is catalyzed by histone acetyl transferase (HAT), which requires acetyl-CoA as coenzyme for the transfer of acetyl group (-COCH3) to the lysine acceptor, while-COCH3 removal is catalyzed by histone deacetylase (HDAC)
Summary
Histone acetylation mainly occurs on the N-terminal tails of the histone core (H2A, H2B, H3 and H4). The enzymatic process is catalyzed by histone acetyl transferase (HAT), which requires acetyl-CoA as coenzyme for the transfer of acetyl group (-COCH3) to the lysine acceptor, while-COCH3 removal is catalyzed by histone deacetylase (HDAC). HATs and HDACs are members of a large enzyme family. Typical lysine (K) acetylation sites are K5, K8, K12, K16 and K20 of histone H4; K9, K14, K18 and K23 of histone H3; K5, K12, K15 and K20 of histone H2B; K5 and K9 of histone H2A. Histone acetylation is primarily (not always) linked to transcriptionally active chromatin, whereas histone diacetylation leads to inhibition of transcriptional activity. I.e.,: Histone H4 acetylated in lysine 16 is designated as H4K16ac
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