H3K79 methylation directly precedes the histone‐to‐protamine transition in mammalian spermatids and is sensitive to bacterial infections

Abstract

SUMMARYIn both mammalian and Drosophila spermatids, the completely histone-based chromatin structure is reorganized to a largely protamine-based structure. During this histone-to-protamine switch, transition proteins are expressed, for example TNP1 and TNP2 in mammals and Tpl94D in Drosophila. Recently, we demonstrated that in Drosophila spermatids, H3K79 methylation accompanies histone H4 hyperacetylation during chromatin reorganization. Preceding the histone-to-protamine transition, the H3K79 methyltransferase Grappa is expressed, and the predominant isoform bears a C-terminal extension. Here, we show that isoforms of the Grappa-equivalent protein in humans, rats and mice, that is DOT1L, have a C-terminal extension. In mice, the transcript of this isoform was enriched in the post-meiotic stages of spermatogenesis. In human and mice spermatids, di-and tri-methylated H3K79 temporally overlapped with hyperacetylated H4 and thus accompanied chromatin reorganization. In rat spermatids, trimethylated H3K79 directly preceded transition protein loading on chromatin. We analysed the impact of bacterial infections on spermatid chromatin using a uropathogenic Escherichia coli-elicited epididymo-orchitis rat model and showed that these infections caused aberrant spermatid chromatin. Bacterial infections led to premature emergence of trimethylated H3K79 and hyperacetylated H4. Trimethylated H3K79 and hyperacetylated H4 simultaneously occurred with transition protein TNP1, which was never observed in spermatids of mock-infected rats. Upon bacterial infection, only histone-based spermatid chromatin showed abnormalities, whereas protaminecompacted chromatin seemed to be unaffected. Our results indicated that H3K79 methylation is a histone modification conserved in Drosophila, mouse, rat and human spermatids and may be a prerequisite for proper chromatin reorganization.

Murine and Human Spermatids Are Characterized by Numerous, Newly Synthesized and Differentially Expressed Transcription Factors and Bromodomain-Containing Proteins

“…Hence, these histone modifications might be involved in both transcription and the histone-to-protamine transition. The data presented here are contradictory to our recently published data [2] and to other published data showing that histones are unacetylated in most round spermatids [25]. This difference in our current observations could be explained by the use of 4% F-PBS to fix testes, which, as mentioned above, in many cases allows a more sensitive immunohistochemical detection of proteins.…”

“…In mice, active RNA polymerase II was observed in each germ cell up to steps 8-9 spermatids but not in later spermatids ( Fig. 2A) [2]. TAF2 was detected in the nuclei of spermatocytes and spermatids up to step 9 (Fig.…”

“…In mice, transcripts of 1652 genes (about 20% of which are turned on specifically in the testis) markedly increase during or after meiosis [1]. Large amounts of active RNA polymerase II are detected in haploid round spermatids up to steps 8-9 in mice and up to step 3 in humans [2]. The proteins needed in later spermatids are made from translationally repressed and stored mRNAs synthesized in round spermatids at the latest [3].…”

“…S4A) [2]. TAF2 seemed to be weakly expressed in spermatocyte nuclei up to step 5 spermatids (Supplemental Fig.…”

The Impact of Bacterial Infections on Human Spermatozoa

“…Dottermusch-Heidel et al (92) reported that the bacterial infections lead to the premature emergence of trimethylated H3K79 and hyperacetylated H4, which simultaneously occur with the transition protein TNP1. In contrast, they were never observed in the spermatids of infected rats.…”