Abstract
Testis-specific histone variants are crucial to promote open chromatin structure to enable nucleosome disassembly in the final stages of spermiogenesis. However, even after histone replacement, mature sperm retain a proportion of these variants, the function of which is unknown. The present study aimed to understand the functional relevance of the retained H2B and H2A variants, TH2B and TH2A. While no literature is available on the phenotype of TH2A knockouts, TH2B/TH2A double knockout male mice are reported to be infertile. In this study, ChIP-seq analysis was done for TH2B and TH2A to understand the epigenomics of the retained TH2B and TH2A, using murine caudal sperm. Distribution across genomic partitions revealed ∼35% of the TH2B peaks within ±5 kb of TSS whereas TH2A peaks distribution was sparse at TSS. Gene Ontology revealed embryo development as the most significant term associated with TH2B. Also, based on genomic regions, TH2B was observed to be associated with spindle assembly and various meiosis-specific genes, which is an important finding as TH2A/TH2B DKO mice have been reported to have defective cohesin release. A comparison of mouse and human TH2B-linked chromatin revealed 26% overlap between murine and human TH2B-associated genes. This overlap included genes crucial for embryogenesis. Most importantly, heterogeneity in the epigenetic landscape of TH2A and TH2B was seen, which is intriguing as TH2B and TH2A are well reported to be present in the same nucleosomes to promote open chromatin. Additionally, unlike TH2B, TH2A was enriched on the mitochondrial chromosome. TH2A was found to be associated with Nuclear insertion of Mitochondrial DNA sequences (NUMTs) in sperm. A comprehensive analysis of these observations indicates novel functions for the sperm-retained TH2B and TH2A.
Highlights
Mature sperm and round spermatids harbor the same haploid genome but different epigenomes
Developmental limitations of embryos resulting from somatic cell nuclear transfer (SCNT) and round spermatid injection (ROSI) as compared to sperm borne embryos reveal the importance of the unique sperm chromatin in efficient embryogenesis (Kimura and Yanagimachi, 1995; Kishigami et al, 2004; Teperek et al, 2016)
Since mice is an experimental model for mammalian studies, to understand the evolutionary conservation of the variants TH2B and TH2A, their homology in protein sequences was assessed by pairwise global alignment of TH2B and TH2A in human and mouse
Summary
Mature sperm and round spermatids harbor the same haploid genome but different epigenomes. Developmental limitations of embryos resulting from somatic cell nuclear transfer (SCNT) and round spermatid injection (ROSI) as compared to sperm borne embryos reveal the importance of the unique sperm chromatin in efficient embryogenesis (Kimura and Yanagimachi, 1995; Kishigami et al, 2004; Teperek et al, 2016). That embryonically important loci like Homeobox (HOX) cluster escapes epigenetic reprogramming during spermiogenesis underpins the importance of sperm epigenetics (Arpanahi et al, 2009; Hammoud et al, 2009; Erkek et al, 2013; Teperek et al, 2016; Murphy et al, 2018; Yamaguchi et al, 2018; Singh et al, 2021). The loci important for embryonic development and sperm function are marked by nucleosomes containing modified histones like H3K4me and H3K27me. The chromatin states of these respective loci are correlative to their expression in early embryos (Brykczynska et al, 2010; Jung et al, 2017; Oikawa et al, 2020)
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