Genome-wide occupancy reveals the localization of H1T2 (H1fnt) to repeat regions and a subset of transcriptionally active chromatin domains in rat spermatids

Vasantha Shalini,Rao M R Satyanarayana,Anusha Rengarajan,Anjhana C Ravikkumar,Utsa Bhaduri

doi:10.1186/s13072-020-00376-2

Vasantha Shalini, Rao M R Satyanarayana + Show 3 more

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https://doi.org/10.1186/s13072-020-00376-2

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Abstract

BackgroundH1T2/H1FNT is a germ cell-specific linker histone variant expressed during spermiogenesis specifically in round and elongating spermatids. Infertile phenotype of homozygous H1T2 mutant male mice revealed the essential function of H1T2 for the DNA condensation and histone-to-protamine replacement in spermiogenesis. However, the mechanism by which H1T2 imparts the inherent polarity within spermatid nucleus including the additional protein partners and the genomic domains occupied by this linker histone are unknown.ResultsSequence analysis revealed the presence of Walker motif, SR domains and putative coiled-coil domains in the C-terminal domain of rat H1T2 protein. Genome-wide occupancy analysis using highly specific antibody against the CTD of H1T2 demonstrated the binding of H1T2 to the LINE L1 repeat elements and to a significant percentage of the genic regions (promoter-TSS, exons and introns) of the rat spermatid genome. Immunoprecipitation followed by mass spectrometry analysis revealed the open chromatin architecture of H1T2 occupied chromatin encompassing the H4 acetylation and other histone PTMs characteristic of transcriptionally active chromatin. In addition, the present study has identified the interacting protein partners of H1T2-associated chromatin mainly as nucleo-skeleton components, RNA-binding proteins and chaperones.ConclusionsLinker histone H1T2 possesses unique domain architecture which can account for the specific functions associated with chromatin remodeling events facilitating the initiation of histone to transition proteins/protamine transition in the polar apical spermatid genome. Our results directly establish the unique function of H1T2 in nuclear shaping associated with spermiogenesis by mediating the interaction between chromatin and nucleo-skeleton, positioning the epigenetically specialized chromatin domains involved in transcription coupled histone replacement initiation towards the apical pole of round/elongating spermatids.

Highlights

H1T2/H1FNT is a germ cell-specific linker histone variant expressed during spermiogenesis in round and elongating spermatids
It is interesting to note that while the general property of the C-terminal domain of linker histone is to stabilize higher order chromatin folding, functional sub-domains which are present in the C-terminal tail domain (CTD) of rodent specific H1T2 are absent in higher mammals
Earlier studies have demonstrated the nucleosomal retention and bivalent histone marks (H3K4me3/H3K27me3) to be significantly enriched at these loci of developmental importance, including HOX, SOX, FOX, TBX, PAX, CDX, and GATA family transcription factors [33, 59]. All together these results suggest a prominent role of the linker histone H1T2 in organizing specific spermatid chromatin domains to potentially regulate transcription of several developmentally important embryonic genes

Summary

Introduction

H1T2/H1FNT is a germ cell-specific linker histone variant expressed during spermiogenesis in round and elongating spermatids. Spermiogenesis is divided into different steps based on the three developmental processes involving condensation of the nucleus followed by the formation of the flagellum and the acrosome. During the steps 1–7, the early round spermatids are characterized by a round nucleus, with the beginning of acrosome and axoneme assembly. These early round spermatids actively transcribe many of the mRNAs necessary for the whole process of spermiogenesis, while many of these mRNAs are subjected to delayed translation until the later stages of spermiogenesis and sperm function. The step 8 comprises the acrosome polarization to one side of the nucleus and the initiation of spermatid elongation followed by the assembly of the accessory structures needed for flagella function. The final stage of spermiogenesis is the spermiation, a process in which elongating spermatids undergo extensive chromatin remodeling, which have been well studied in mouse [3]

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