Histone chaperone HIRA complex regulates retrotransposons in embryonic stem cells

Pluripotency state transition of embryonic stem cells requires the turnover of histone chaperone FACT on chromatin

The differentiation of embryonic stem cells (ESCs) begins with the transition from the naive to the primed state. The formative state was recently established as a critical intermediate between the two states. Here, we demonstrate the role of the histone chaperone FACT in regulating the naive-to-formative transition. We found that the Q265K mutation in the FACT subunit SSRP1 increased the binding of FACT to histone H3-H4, impaired nucleosome disassembly in vitro, and reduced the turnover of FACT on chromatin in vivo. Strikingly, mouse ESCs harboring this mutation showed elevated naive-to-formative transition. Mechanistically, the SSRP1-Q265K mutation enriched FACT at the enhancers of formative-specific genes to increase targeted gene expression. Together, these findings suggest that the turnover of FACT on chromatin is crucial for regulating the enhancers of formative-specific genes, thereby mediating the naive-to-formative transition. This study highlights the significance of FACT in fine-tuning cell fate transition during early development.

Regulation of endogenous retroviruses in murine embryonic stem cells and early embryos

Endogenous retroviruses (ERVs) are important components of transposable elements that constitute ∼40% of the mouse genome. ERVs exhibit dynamic expression patterns during early embryonic development and are engaged in numerous biological processes. Therefore, ERV expression must be closely monitored in cells. Most studies have focused on the regulation of ERV expression in mouse embryonic stem cells (ESCs) and during early embryonic development. This review touches on the classification, expression, and functions of ERVs in mouse ESCs and early embryos and mainly discusses ERV modulation strategies from the perspectives of transcription, epigenetic modification, nucleosome/chromatin assembly, and post-transcriptional control.

Dot1l cooperates with Npm1 to repress endogenous retrovirus MERVL in embryonic stem cells

Dot1l is a histone methyltransferase without a SET domain and is responsible for H3K79 methylation, which marks active transcription. In contradiction, Dot1l also participates in silencing gene expression. The target regions and mechanism of Dot1l in repressing transcription remain enigmatic. Here, we show that Dot1l represses endogenous retroviruses in embryonic stem cells (ESCs). Specifically, the absence of Dot1l led to the activation of MERVL, which is a marker of 2-cell-like cells. In addition, Dot1l deletion activated the 2-cell-like state and predisposed ESCs to differentiate into trophectoderm lineage. Transcriptome analysis revealed activation of 2-cell genes and meiotic genes by Dot1l deletion. Mechanistically, Dot1l interacted with and co-localized with Npm1 on MERVL, and depletion of Npm1 similarly augmented MERVL expression. The catalytic activity and AT-hook domain of Dot1l are important to suppress MERVL. Notably, Dot1l-Npm1 restricts MERVL by regulating protein level and deposition of histone H1. Furthermore, Dot1l is critical for Npm1 to efficiently interact with histone H1 and inhibit ubiquitination of H1 whereas Npm1 is essential for Dot1l to interact with MERVL. Altogether, we discover that Dot1l represses MERVL through chaperoning H1 by collaborating with Npm1. Importantly, our findings shed light on the non-canonical transcriptional repressive role of Dot1l in ESCs.

Histones and their chaperones: Adaptive remodelers of an ever-changing chromatinic landscape

Chromatin maintenance and remodeling are processes that take place alongside DNA repair, replication, or transcription to ensure the survival and adaptability of a cell. The environment and the needs of the cell dictate how chromatin is remodeled; particularly where and which histones are deposited, thus changing the canonical histone array to regulate chromatin structure and gene expression. Chromatin is highly dynamic, and histone variants and their chaperones play a crucial role in maintaining the epigenetic regulation at different genomic regions. Despite the large number of histone variants reported to date, studies on their roles in physiological processes and pathologies are emerging but continue to be scarce. Here, we present recent advances in the research on histone variants and their chaperones, with a focus on their importance in molecular mechanisms such as replication, transcription, and DNA damage repair. Additionally, we discuss the emerging role they have in transposable element regulation, aging, and chromatin remodeling syndromes. Finally, we describe currently used methods and their limitations in the study of these proteins and highlight the importance of improving the experimental approaches to further understand this epigenetic machinery.